Somatostatin and Somatostatin Receptors

Abstract

The biological effects of somatostatin (SST) were first encountered unexpectedly in the late 1960s in two unrelated studies,
one by Krulich et al. (1968) who reported on a growth hormone (GH)-releasing inhibitory substance from hypothalamic extracts,
and the other, by Hellman and Lernmark (1969), on the presence of a potent insulin inhibitory factor from the extracts of
pigeon pancreatic islets. However, the inhibitory substance was not officially identified until 1973 by Guillemin’s group
(Brazeau et al. 1973). In both synthetic and naturally occurring forms, this tetradecapeptide, originally coined as somatotropin
release-inhibitory factor (SRIF, SST-14) was shown by Brazeau et al. to be the substance controlling hypothalamic GH release.
This single achievement not only pioneered SST research but was also duly recognized, as Guillemin shared the 1977 Nobel Prize
in Medicine. The following years bequeathed an exponential increase in SST-related studies. It soon became clear that SST-synthesis
was not restricted to the hypothalamus. Its production is widely distributed throughout the central nervous system (CNS),
peripheral neurons, the gastrointestinal tract, and the pancreatic islets of Langerhans (Luft et al. 1974; Arimura et al.
1975; Dubois 1975; Hokfelt et al. 1975; Orci et al. 1975; Pelletier et al. 1975; Polak et al. 1975; Patel and Reichlin 1978).
In fact, SST-like immunoreactivity can be found throughout various tissues of vertebrates and invertebrates, including the
plant kingdom (Patel 1992; Tostivint et al. 2004). Given its broad anatomical distribution, it is no wonder that SST produces
a wide spectrum of biological effects. Generally regarded as an inhibitory factor, SST can function either locally on neighboring
cells or distantly through the circulation, to regulate such physiological processes as glandular secretion, neurotransmission,
smooth muscle contractility, nutrient absorption, and cell division (Reichlin 1983a, b; Patel 1992, 1999; Patel et al. 2001;
Barnett 2003).

Full text

Somatostatin and Somatostatin Receptors
Ujendra Kumar and Michael Grant
1 Introduction
The biological effects of somatostatin (SST) were first encountered unexpectedly in
the late 1960s in two unrelated studies, one by Krulich et al. (1968) who reported on
a growth hormone (GH)-releasing inhibitory substance from hypothalamic extracts,
and the other, by Hellman and Lernmark (1969), on the presence of a potent insulin
inhibitory factor from the extracts of pigeon pancreatic islets. However, the inhibitory
substance was not officially identified until 1973 by Guillemin’s group (Brazeau
et al. 1973). In both synthetic and naturally occurring forms, this tetradecapeptide,
originally coined as somatotropin release-inhibitory factor (SRIF, SST-14) was
shown by Brazeau et al. to be the substance controlling hypothalamic GH release.
This single achievement not only pioneered SST research but was also duly
recognized, as Guillemin shared the 1977 Nobel Prize in Medicine. The following
years bequeathed an exponential increase in SST-related studies. It soon became
clear that SST-synthesis was not restricted to the hypothalamus. Its production is
widely distributed throughout the central nervous system (CNS), peripheral neurons,
the gastrointestinal tract, and the pancreatic islets of Langerhans (Luft et al. 1974;
Arimura et al. 1975; Dubois 1975; Hokfelt et al. 1975; Orci et al. 1975; Pelletier et al.
1975; Polak et al. 1975; Patel and Reichlin 1978). In fact, SST-like immunoreactivity
can be found throughout various tissues of vertebrates and invertebrates, including
the plant kingdom (Patel 1992; Tostivint et al. 2004). Given its broad anatomical
distribution, it is no wonder that SST produces a wide spectrum of biological effects.
Generally regarded as an inhibitory factor, SST can function either locally on
neighboring cells or distantly through the circulation, to regulate such physiological
U. Kumar (*)
Faculty of Pharmaceutical Sciences, Department of Pharmacology and Toxicology,
University of British Columbia, Vancouver, BC, V6T 1Z3, Canada
e-mail: ujkumar@interchange.ubc.ca
M. Grant
Department of Medicine, McGill University, Montreal, QC, Canada
Results Probl Cell Differ, DOI 10.1007/400_2009_29 137
© Springer-Verlag Berlin Heidelberg 2010

138 U. Kumar and M. Grant
processes as glandular secretion, neurotransmission, smooth muscle contractility,
nutrient absorption, and cell division (Reichlin 1983a, b; Patel 1992, 1999; Patel et al.
2001; Barnett 2003).
These extraordinary efforts into the biology of SST could not have been possible
if it were not for the availability of stable and potent analogs, given that SST has an
extremely short plasma half-life (Weckbecker et al. 2003). In 1978, while working
at Sandoz (later Novartis, Basel, Switzerland), Vale et al. reported on the first SST-
analog, an octapeptide with full SST-like biological activity, derived from a cyclic
cysteine-bridged hexapeptide backbone (Vale et al. 1978). Further modification of
the peptide by the introduction of two d-amino acid isomers and l-threoninol at the
C-terminal, provided increased metabolic stability even in the midst of aggressive
media such as gastric juices at elevated temperatures (Bauer et al. 1982). This
improved octapeptide was coded as SMS 201-995 or octreotide, and developed
under the name Sandostatin®. In 1988, Sandostatin gained its first FDA approval for
the symptomatic treatment of gastroenteropancreatic tumors. Today, the inhibitory
actions of SST-analogs are applied in several clinical scenarios, including, the sup-
pression of tumoural hormone hypersecretion (acromegaly, neuroendocrine tumors
(NETs), pancreatic tumors, carcinoid tumors), gastrointestinal bleeding, dumping
syndrome, and pancreatitis (Lamberts et al. 1996; Weckbecker et al. 2003).
Despite the marked achievement in the development of octreotide, it was not until
the early 1990s, that the structure of the first SST receptor (SSTR) emerged by
molecular cloning (Yamada et al. 1992). Subsequent cloning revealed five distinct
SSTR genes which was a greater number than predicted from pharmacological and
biochemical criteria at the time (Patel et al. 1995; Reisine and Bell 1995; Patel 1997).
The conceptualization behind the development and successful deployment of
octreotide, was only later identified by its preferential-binding to SSTR2 (Reisine and
Bell 1995; Patel 1997; Weckbecker et al. 2003), as many tumors express this receptor-
subtype (Lamberts et al. 2002; Hofland and Lamberts 2003; Gardette et al. 2004).
Rational approaches to developing peptide and nonpeptide analogs that bind more
selectively soon followed (Weckbecker et al. 2003). However, it soon became clear
that SSTRs would often show overlapping patterns of distribution in a tissue specific
manner, which raised questions on the relevance of receptor-coexpression and the
importance of target specificity, given the similarities in receptor-signaling. Nevertheless,
reports have surfaced describing activation of putative second messengers as well as
differential cellular and physiological responses in cells bearing more than one
receptor-subtype when treated with SST agonists (Shimon et al. 1997a, b; Cattaneo
et al. 2000; Jaquet et al. 2000; Danila et al. 2001; Saveanu et al. 2001; Tulipano et al.
2001; Bruns et al. 2002; Florio et al. 2003b; Ren et al. 2003; Zatelli et al. 2004, 2005b;
Ben-Shlomo et al. 2005; Jaquet et al. 2005; Saveanu et al. 2006; Fedele et al. 2007).
Recently, the trafficking and desensitization of SSTR2 following its selective activation
was shown to be affected when SSTR5 was coexpressed; however the mechanism for
this behavior had yet to be described (Sharif et al. 2007).
All five SSTR subtypes are members of the superfamily of G-protein coupled
receptors (GPCRs) (Patel 1999; Olias et al. 2004). An abounding amount of reports
has challenged the age-old notion that GPCRs exist and function as monomeric entities

Somatostatin and Somatostatin Receptors 139
at the cell surface. It is now clear that many (Franco et al. 2003; Kroeger et al. 2003;
Bai 2004; Breitwieser 2004; Hansen and Sheikh 2004; Terrillon and Bouvier 2004;
Prinster et al. 2005; Milligan 2008) but not all GPCRs function exclusively as dimers
(Patel et al. 2002; Gripentrog et al. 2003; Grant et al. 2004b; James et al. 2006; Meyer
et al. 2006; Bayburt et al. 2007; Rasmussen et al. 2007; Whorton et al. 2007, 2008).
SSTRs are no exception, as several laboratories have demonstrated their ability to form
both homo- and heterodimers, with members of the same or distantly related receptor-
families (Rocheville et al. 2000a, b; Pfeiffer et al. 2001, 2002; Patel et al. 2002; Grant
et al. 2004a, b; Baragli et al. 2007; Duran-Prado et al. 2007; Grant et al., 2008a; Watt
et al. 2008). This chapter describes the functional and pharmacological properties of
SST and SSTR subtypes and their possible clinical implication.
2 Somatostatin Processing
It has been over three decades since the discovery of SSTs - a family of cyclopeptide
hormones -which are mainly produced by normal endocrine, gastrointestinal,
immune, and neuronal cells (Brazeau et al. 1973; Reichlin, 1983a, b; Patel 1992,
1999; Patel et al. 2001; Barnett 2003). SST is synthesized as two bioactive products,
the form originally identified in the hypothalamus consisting of 14 amino acids,
SST-14, and its congener, SST-28, subsequently discovered to contain an extension
at the N-terminus (Pradayrol et al. 1980) (Fig. 1a and b). Elucidation of the
biosynthesis of both forms of SST, like other protein hormones (Hook et al. 1994),
Fig. 1 Somatostatin processing. (a) Prosomatostatin is processed into two bioactive forms,
SST-14 and SST-28. (b) Amino acid sequence of somatostatin isoforms depicting the cysteine
bridge maintaining its cyclic structure and the pharmacophore

140 U. Kumar and M. Grant
revealed a larger inactive precursor molecule, preprosomatostatin (PPSST), which
is processed by post-translational enzymatic cleavage to yield the active polypeptides
(Goodman et al. 1980; Joseph-Bravo et al. 1980; Oyama et al. 1980; Patzelt et al.
1980; Shields 1980; Goodman et al. 1981; Zingg and Patel 1982). In the early
1980s, development of recombinant DNA technology, allowed for the isolation and
cloning of human and rat cDNAs encoding PPSST (Goodman et al. 1982; Shen et al.
1982; Funckes et al. 1983). This work revealed the sequence and structure of
PPSST, a polypeptide consisting of 116 amino acids. Enzymes implicated in the
processing of PSST belong to the subtilisin/kexin-related Ca2+-dependent class of
serine proteinases, collectively termed precursor convertases (Seidah and Chretien
1999; Zhou et al. 1999). Although there are seven known mammalian precursor
convertases, a select few have been shown important in PSST processing
(Mouchantaf et al. 2001, 2004a, b). Processing of PSST primarily occurs at the
C-terminal end generating the two bioactive forms. SST-14 is generated by dibasic
cleavage at an Arg-Lys residue, whereas endoproteolysis of a monobasic Arg site
produces SST-28 (Patel and O’Neil 1988; Bersani et al. 1989). In addition, a
secondary monobasic site was determined in PSST, cleavage of which results in the
generation of a 10-amino acid peptide termed antrin (PSST1-10), named after its
initial discovery in the gastric antrum, for which it showed the highest concentration
(Benoit et al. 1987). Although antrin has no known function, it has been isolated in
all SST-producing tissues (Ravazzola et al. 1989; Rabbani and Patel 1990)
Due to differential processing of PSST, various mixtures of SST-14 and SST-28
are produced in mammalian tissues (Patel et al. 1981). SST-14 is largely present in
pancreatic islets, stomach, and neural tissues; it is the prominent form in the retina,
peripheral nerves, and enteric neurons (Patel et al. 1981). In the brain, SST-28
accounts for approximately 20–30% of total SST-like immunoreactivity. In the
periphery, SST-28 synthesis predominates in intestinal mucosal cells as the terminal
biosynthetic product following PSST processing (Patel et al. 1981; Baskin and
Ensinck 1984). Although only SST-14 and SST-28 are the known biologically
active forms of PSST, other products have been identified in circulation following
processing;however their biological function remains uncertain as they are devoid
of any known activity (Patel et al. 1981; Shoelson et al. 1986; Patel and O’Neil
1988; Ensinck et al. 1989; Ravazzola et al. 1989; Rabbani and Patel 1990).
3 Somatostatin Distribution
The production of SST occurs at high densities in cells throughout the CNS, the
peripheral nervous system, the endocrine pancreas, and the gut, in addition to small
numbers in the thyroid, adrenals, submandibular glands, kidney, prostate, placenta,
blood vessel walls, and immune cells (Arimura et al. 1975; Dubois 1975; Hokfelt
et al. 1975; Pelletier et al. 1975; Polak et al. 1975; Patel and Reichlin 1978; Finley
et al. 1981; Reichlin 1983a, b; Johansson et al. 1984; Fuller and Verity 1989;
Aguila et al. 1991; Patel 1992). Within the CNS, neurons and fibers positive for

Somatostatin and Somatostatin Receptors 141
SST are abundantly dotted, the notable exception being the cerebellum (Finley et al.
1981; Johansson et al. 1984). More specifically, brain regions such as the
hypothalamus, the deep layers of the cortex, the limbic system, and all levels of the
major sensory pathway are rich in SST-producing neurons (Kumar 2007). In a sub-
population of C cells in the thyroid, SST coexists with calcitonin (Reichlin 1983a,
b). At least in rats, total body SST can be divided as follows: gut accounts for the
majority of SST, approximately 65%, the brain for approximately 25%, the pan-
creas for approximately 5%, while the remaining organs account for the residual
5% (Patel and Reichlin 1978).
4 Somatostatin Physiology
The physiological role of hypothalamic SST is well established [reviewed in (Patel
1992, 1999; Barnett 2003)]. As early as week 10 of gestation, SST is detected in
the fetal hypothalamus (Bugnon et al. 1978). It is there that its release regulates the
secretion of GH from the pituitary, the counterbalance being a growth hormone-
releasing hormone (GHRH), which is detected in the hypothalamus at week 18 of
gestation (Bresson et al. 1984). In adults, GH secretion occurs at a basal rate
throughout the day. The major role of hypothalamic SST is the tonic inhibition of
both basal and GHRH-stimulated secretion of GH from anterior pituitary soma-
totrophs (Barinaga et al. 1985). Somatostatinergic neurons emanate from the ante-
rior hypothalamus and project to the median eminence, where SST is released into
hypophyseal portal vessels to interact with pituitary somatotrophs (Patel 1992;
Barnett 2003). SST and GHRH pathways interact with each other at both their point
of convergence at the level of the pituitary and through direct neural connections
within the hypothalamus (Horvath et al. 1989). Thus, SST inhibits the secretion of
GH via a direct interaction on the pituitary and indirectly through suppression of
GHRH release (Katakami et al. 1988; Tannenbaum et al. 1990). Two secretory
feedback loops exist that modulate SST release: the short loop, where SST is nega-
tively regulated by GHRH (Katakami et al. 1988) but subject to positive-feedback
by GH (Berelowitz et al. 1981a); the long loop, where insulin growth factor type 1
(IGF-I) produced by GH acting on the liver, provides a positive influence
(Berelowitz et al. 1981b). This mechanism in regulating GH release is further sup-
ported in SST knockout mice, as nadir GH levels are consistently higher in these
animals compared to their wild-type counterparts (Low et al. 2001). In addition,
secretion of hypothalamic SST can be further promoted by dopamine, substance P,
neurotensin, glucagon, hypoglycemia, various amino acids, acetylcholine, a2-
adrenergic agonists, vasoactive intestinal polypeptide (VIP), and cholecystokinin;
it is however inhibited by glucose (Chihara et al. 1979; Berelowitz et al. 1982;
Reichlin 1983b). Similar mechanisms also exist in the hypothalamic control of
thyroid-stimulating hormone (TSH) secretion (Siler et al. 1974; Vale et al. 1975;
Arimura and Schally 1976; Ferland et al. 1976; Tanjasiri et al. 1976; Rodriguez-
Arnao et al. 1981; Reichlin 1983a, b; Samuels et al. 1992; James et al. 1997).

142 U. Kumar and M. Grant
In addition to its actions on the pituitary, SST functions as a neurotransmitter in
the brain with effects on cognition, locomotor, sensory, and autonomic functions
(Reichlin, 1983a, b; Patel 1992; Epelbaum et al. 1994; Barnett 2003) (Fig. 2). SST
inhibits the release of dopamine from the midbrain, the secretions of norepinephrine,
thyroid-releasing hormone,and corticotrophin-releasing hormone including its own
secretion from the hypothalamus. As previously indicated, it inhibits both the basal
and stimulated secretion of GH and TSH, but has no effects on the release of lutein-
izing hormone, follicle-stimulating hormone, prolactin, or adrenal corticotrophin
hormone under normal physiological conditions. SST has direct effects on the thy-
roid by inhibiting the release of T4, T3, and calcitonin from thyroid parafollicular
cells stimulated by TSH. It acts on the adrenals to inhibit angiotensin II stimulated
aldosterone secretion and acetylcholine stimulated medullary catecholamine secre-
tion. SST inhibits the secretion of renin in the kidneys when stimulated by hypovo-
lemia, including the inhibition of antidiuretic hormone-mediated water absorption.
Within the gastrointestinal tract, virtually every gut hormone has been shown to be
inhibited by SST including gastric acid, pepsin, bile, and colonic fluid. SST also has
a generalized suppressive effect on the motor activity within the gastrointestinal
tract, such that it inhibits gastric emptying, gallbladder contraction, and small intes-
tinal segmentation. In the pancreas, SST is an endogenous islet hormone. Its actions
on the pancreas were first noted within the year of its discovery by two groups,
Fig. 2 Schematic depicting the hormonal actions of somatostatin

Somatostatin and Somatostatin Receptors 143
following infusion in humans and baboons (Alberti et al. 1973; Koerker et al. 1974).
SST regulates the secretion of hormones from several tissues, including neurotrans-
mission. When synthesized and released from d cells of pancreatic islets, SST causes
suppression of the synthesis and secretion of both insulin and glucagon, including
the inhibition of pancreatic polypeptide (German et al. 1990; Zhang et al. 1991;
Philippe 1993; Nelson-Piercy et al. 1994; Redmon et al. 1994; Kendall et al. 1995;
Kleinman et al. 1995; Ballian et al. 2006) (Fig. 2). SST is also known to block the
release of several growth factors and cytokines (Blum et al. 1992; Hayry et al. 1993;
Elliott et al. 1994). More recently, the antisecretory properties of SST were demon-
strated to affect ghrelin release (Barkan et al. 2003). Additional effects of SST
include vasoconstriction and an antiproliferative effect on immune, intestinal
mucosal, cartilage, and bone precursor cells (Weiss et al. 1981; Reichlin, 1983a, b;
Patel 1992; Karalis et al. 1994; Aguila et al. 1996; Takeba et al. 1997). Interestingly,
a down regulation in SST and SSTRs expression has been associated with
Alzheimer’s disease (Kumar 2005). The brains of mice which were deficient in SST
showed a greater accumulation of Ab42, the main contributor to Alzheimer’s disease,
due to a decrease in neprilysin activity (Saito et al. 2005). When either SST was
administered or neprilysin was directly activated, decreases in the aggregation of
Ab42 was observed (Saito et al. 2005). In addition, SST release is impaired in the
presence of Ab (Geci et al. 2007). Although the role of SST in Huntington’s disease
is controversial, it is believed that SST positive neurons are selectively spared in
disease. In an experimental model of Huntington’s disease, selective sparing of SST
positive neurons has been shown and blocking SST by using SST antisense oligonu-
cleotides potentiates neuronal cell death in quinolinic acid and NMDA induced
excitotoxicity (Kumar et al. 1997; Kumar 2004, 2008). These studies strongly link
SST with pathophysiology of Huntington’s disease.
5 Somatostatin Regulation
Given its widespread distribution and interaction with various bodily systems, it is
no wonder that SST can be regulated by a broad array of secretagogues - from ions
and nutrients to neuropeptides, neurotransmitters, hormones, growth factors, and
cytokines (Reichlin, 1983a, b; Patel 1992; Patel et al. 2001; Barnett 2003). For
instance, membrane depolarization stimulates SST release from both neurons and
peripheral SST-secreting cells. However, the effects of nutrients such as glucose,
amino acids, and lipids, on SST secretion appears to be tissue-specific, a predomi-
nant feature in the triggering of SST release from d cells in pancreatic islets.
Contrarily, the secretion of hypothalamic SST is inhibited by glucose but insensi-
tive to aminogenic agents. On the other hand, gut SST is promoted by luminal but
not circulating nutrients. The effects of glucocorticoids are distinct, however, and
employ a biphasic effect on SST secretion: stimulatory at low doses and inhibitory
at high doses. Almost every neurotransmitter or neuropeptide tested has been
shown to exert some sort of effect on SST secretion with a certain degree of tissue

144 U. Kumar and M. Grant
specificity. In particular, glucagon, GH-releasing hormone, neurotensin, cortico-
trophin-releasing hormone, calcitonin gene-related peptide, and bombesin are
potent stimulators of SST secretion, while opiate and GABA are inhibitors (Patel
1992; Epelbaum et al. 1994; Patel et al. 2001). With regard to the hormones inves-
tigated, thyroid, GH, IGF-I, and insulin augment SST release from the hypothala-
mus (Patel 1992; Patel et al. 2001; Barnett 2003); insulin, leptin, and epinephrine
inhibits its release from the pancreas and hypothalamus respectively (Patel 1992;
Patel et al. 2001; Barnett 2003). Inflammatory mediators have also shown differen-
tial effects on SST secretion: IL-1, IL-6, IL-10, INF-g, and TNF-a stimulate SST
release while TGF-b inhibits it (Scarborough et al. 1989; Quintela et al. 1997;
Elliott 2004).
In addition to modulating SST secretion, many of the same agents also regulate
gene expression. For instance, various members of the growth factor and cytokine
family - glucocorticoids, testosterone, estradiol, insulin, leptin,TGF-b, and NMDA
receptor agonists - affect steady state SST mRNA levels (Patel 1992, 1999; Patel
et al. 2001). The typical transcriptional unit of a mammalian SST gene consists of
two exons separated by an intron (Patel et al. 2001; Vallejo 2004). Several
intracellular mediators are known to affect SST gene function and include, cAMP,
cGMP, nitric oxide, Ca2+, and activators of protein kinase C (Kanatsuka et al.
1981; Frankel et al. 1982; Montminy et al. 1986; Patel et al. 1991; Aguila 1994).
Immediately upstream of the start transcription site is a variant of the TATA box
element, followed by a cAMP response element (CRE), two glucocorticoid
response element (GRE), nonconsensus sequences, and an insulin response
element. Tissue-specific promoter elements are also present that work in concert
with the CRE to impart high levels of constitutive gene activity. Finally, two
silencer elements located within the promoter mediate repression of SST gene
transcription (Patel et al. 2001; Vallejo 2004).
6 A New Member in the Somatostatin Family?
A little over a decade ago, cDNA encoding a peptide was cloned from rat brain tissue
with structural similarity to SST (Tostivint et al. 1996). This new peptide termed
cortistatin (CST), due to its predominantly cortical expression, is synthesized from
a larger precursor molecule, preprocortistatin. Enzymatic cleavage gives rise to two
products, CST-14 and CST-29. Of the fourteen amino acids pertaining to CST-14,
eleven are identical to SST-14. A human form was also identified, but unlike the rat
homolog, it contains seventeen residues (hCST-17) (Fukusumi et al. 1997). However,
unlike SST, CST has potent sleep-promoting activities when infused into rat brain
ventricles, a property achieved by its antagonizing effect on the neurotransmitter
acetylcholine on cortical excitability (de Lecea et al. 1996). Recently, CST mRNA
has been demonstrated in various peripheral organs and hence,it is not restricting its
expression to the CNS (Papotti et al. 2003; Dalm et al. 2004; Xidakis et al. 2007).
Furthermore, a biological relevance for CST outside the CNS has been recently

Somatostatin and Somatostatin Receptors 145
confirmed, as similar observations have been obtained in comparison to SST-analogs
in measures of endocrine function (Gottero et al. 2004).
7 Somatostatin Receptors
The identification of high-affinity plasma membrane SSTRs, was first described in
1978, using the rat pituitary GH4C1 cell line by whole-cell binding analysis
(Schonbrunn and Tashjian 1978). However, it was soon apparent that more than one
class of SSTR existed, based upon differential binding affinities and potencies for
SST-14 and SST-28 in brain, pituitary, and islet cells (Mandarino et al. 1981;
Srikant and Patel 1981). These studies including one by Tran et al. further catego-
rized SSTRs into two subclasses based on their affinity for the then available SST-
analogs octreotide and seglitide: SRIF I, that bound SST-analogs and SRIF II, the
group that was insensitive to these compounds (Tran et al. 1985; Reisine and Bell
1995). Using a variety of techniques such as binding analysis, covalent crosslinking,
and purification of solubilized receptor including in vivo and in vitro autoradiography,
the expression of SSTRs was demonstrated at various densities in the brain,
gut, pituitary, thyroid, adrenals, endocrine and exocrine pancreas, kidneys, and
immune cells (Patel et al. 1995; Reisine and Bell 1995; Patel 1997, 1999; Olias
et al. 2004). Several tumor cell lines have also demonstrated to be rich sources of
SSTRs and include AtT-20 mouse pituitary tumor cells, hamster insulinoma and
Rin m5F islet tumor cells, AR42J and Mia PaCa pancreatic tumor cells and human
breast cancer, neuroblastoma, glioma, leukemic, and myeloma cell lines (Patel
et al. 1995; Reisine and Bell 1995; Patel 1997, 1999; Kumar 2005). Photoaffinity
labeling and purification studies, revealed the existence of several SSTR species in
the range of 32–85 kDa in a tissue-specific manner (Patel et al. 1990, 1995; Reisine
and Bell 1995).
Fourteen years following the discovery of high-affinity SSTR binding sites on
whole-cell membranes (Schonbrunn and Tashjian 1978), the first SSTR sequence
was resolved by molecular cloning (Yamada et al. 1992). It was not long before the
identity of five distinct SSTR genes became available (Patel et al. 1995; Reisine and
Bell 1995; Patel 1997, 1999; Olias et al. 2004). Using the mRNA from human
islets, the first two SSTRs were cloned and termed SSTR1 and SSTR2 (Yamada et al.
1992). The sequences of the remaining SSTRs were soon elucidated (SSTR3,
SSTR4 and SSTR5) as identified in human and rodent tissue (Patel et al. 1995,
1996; Patel 1997). SSTRs encoded from the human genome are all nonallelic, and
map to separate loci on different chromosomes. With the exception of SSTR2,
which gives rise to two spliced variants, SSTR2A and SSTR2B, SSTRs are intron-
less. SSTR2A and SSTR2B differ only in the length of their carboxy-terminal seg-
ments (C-terminus). All SSTR subtypes display seven a helical transmembrane
(TM) segments typified by GPCR topology (Fig. 3). GPCRs are grouped into three
distinct families, A, B, and C on the basis of their sequence similarity. Family A,
the largest group, also known as the rhodopsin-like family, includes rhodopsin, the

146 U. Kumar and M. Grant
adrenergic receptors, the olfactory, and many other nonolfactory members including
the SSTR family. Family B consists of approximately two dozen members
including the gastrointestinal peptide hormone receptor family (secretin, glucagon,
vasoactive intestinal peptide, and growth-hormone releasing hormone), corticotrophin-
releasing hormone, calcitonin, and parathyroid hormone receptors. Family C contains
only a few members including, the metabotropic glutamate receptor family,
the GABAB receptor, and the calcium-sensing and taste receptors. This family of
GPCRs is typified by a large extracellular amino terminus, which appears critical
for ligand binding.
SSTRs range in size from 356 to 391 amino acid residues and have an overall
sequence identity of 39–57%, with most of their divergence presented in the amino-
and C-terminal segments (Reisine and Bell 1995; Patel et al. 1996; Patel 1997,
1999). A highly conserved motif, YANSCANPI/VLY, in the seventh TM has been
identified in all SSTR subtypes in every species, and serves as a signature sequence
for this family of receptors. N-linked glycosylation sites have been identified within
the amino-terminus and second extracellular loop (ECL) of all five human (h)
SSTRs. Several putative phosphorylation recognition sites have been identified in
the C-terminus, second and third intracellular loops (ILs) for protein kinase A,
protein kinase C, and calmodulin kinase II for all hSSTRs. Interestingly, hSSTR3
is the only hSSTR that does not contain a cysteine residue downstream from the
seventh TM for purposes of palmitylation and hence membrane anchoring however,
it does possess an unusually long C-terminus, which may be a characteristic of its
unique signaling properties (Sharma et al. 1996, 1999; Sharma and Srikant 1998b).
In addition to these classical GPCR features (Pierce et al. 2002; Qanbar and
Bouvier 2003), various others have been identified including a PDZ (postsynaptic
density-95/discs large/ZO-1) recognition domain in the C-terminus of all SSTR
subtypes (Kreienkamp et al. 2004). Several PDZ interacting proteins have been
Fig. 3 Schematic representation of the structure of SSTR2A. Possible glycosylation and
phosphorylation sites including a palmitoyl membrane-anchorage site are shown

Somatostatin and Somatostatin Receptors 147
discovered, specific to each of the five subtypes, presumably implicated in the
chaperoning, scaffolding, and transport of SSTRs (Kreienkamp et al. 2004).
8 Development of Somatostatin Receptor Ligands
All five hSSTR subtypes bind SST-14 and SST-28 with nanomolar affinity; the
exception is hSSTR5, which binds SST-28 with a 5- to 10-fold higher affinity than
SST-14 (see Table 1). CST also interacts with all five SSTRs with nanomolar
affinities (Spier and de Lecea 2000). Administration of SST produces a wide
spectrum of effects that occur mainly at the site of injection and are short-lived.
This is the result of peptidases found in blood and tissues (Benuck and Marks 1976;
Marks et al. 1976), making the circulation half-life of SST extremely short
(1.1–3 min) (Schusdziarra et al. 1977). Not surprisingly, circulating SST levels are
relatively low, ranging between 14 and 32.5 pg ml−1 (Peeters et al. 1981; Penman et al.
1981; Tsuda et al. 1981; Vasquez et al. 1982; Skamene and Patel 1984; Shoelson
et al. 1986; Gyr et al. 1987; Ensinck et al. 1989). An intense investigation has
surrounded the development of compounds with selective actions and metabolic
stability to be used in both investigational and clinical settings (Lamberts et al.
1991; Reisine and Bell 1995; Weckbecker et al. 2003). Various hexa- and
octapeptide derivatives were synthesized, the most potent of which maintained the
b-turn of the original SST molecule - the biologically active core or pharmacophore.
Structure-function studies determined that amino acid residues Phe7, Trp8, Lys9, and
Thr10, are necessary for biological activity, although residues Phe7 and Thr10 could
undergo minor substitution. The first FDA approved SST analog SMS 201-995
(octreotide, Sandostatin®), an octapeptide, BIM 23014 (lanreotide, Somatuline®),
eventually followed. These analogs are prepared in long-acting formulations for
diagnosis and treatment of various disorders including, gastrointestinal, islet cell,
gut, and pituitary tumors (Lamberts et al. 1991, 1996; Weckbecker et al. 2003).
Both lanreotide and octreotide exhibit high-affinity binding to SSTR2 and
intermediate binding to SSTR3 and SSTR5 (see Table 1). In 2005, RC160
(vapreotide, Sanvar®), an SST-analog with similar binding affinities to SSTR2, 3,
and 5 like lanreotide and octreotide, but moderate affinity to SSTR4 (Patel 1999),
was granted approval for indication of acute oesophageal variceal bleeding
secondary to portal hypertension (Patch and Burroughs 2002). In an attempt to
reduce size but maintain metabolic stability, an SST mimic was achieved based on
a cyclohexapeptide template termed MK-678 (seglitide), showing slightly higher-
affinity and selectivity to SSTR2 than SSTR3 and SSTR5. As previously mentioned,
SSTR2 through -5 can be categorized as group SRIF I, based upon their ability to
bind octapeptide analogs; however, analogs that bind receptors in group SRIF II
(SSTR1 and SSTR4), would only become available in the mid 90s (Liapakis et al.
1996). The analog Des-AA1,2,5[D-Trp8 IAMP9] SST (SCH-275), was reported to
have high-affinity for SSTR1 and moderate affinity for SSTR4 (Liapakis et al.
1996; Patel 1997) (see Table 1). Recently, the highly potent and stable

148 U. Kumar and M. Grant
cyclohexapeptide SOM-230 (pasireotide), designed by Novartis, is a near-universal
agonist, the first of its kind, demonstrating high-affinity for SSTR1, 2, 3, and 5
(Bruns et al. 2002; Weckbecker et al. 2002). SOM-230 has demonstrated to be
affective in regulating pituitary control in rats, dogs, and monkeys including its
control in patients with acromegaly and Cushing disease (Bruns et al. 2002;
Weckbecker et al. 2002, 2003; Labeur et al. 2006). Despite the achievement of
SSTR-analogs with group selectivity, there has been moderate success in the
development of peptide analogs with receptor-specificity. Several analogs have
been devised however; their specificity in targeting receptor-subtype ranges
between 20-and 50-fold (Patel 1999; Weckbecker et al. 2003; Olias et al. 2004).
A breakthrough in SST agonist design came from the Merck Research Group, using
the backbone of peptide agonists for molecular modeling; they constructed subtype-
selective nonpeptide agonists by combinatorial chemistry (Rohrer et al. 1998). Of
the five nonpeptide agonists, three of the compounds, L-797,591, L-779,976, and
L-803,087 display high-selectivity and low nanomolar binding affinity for SSTR1,
SSTR2, and SSTR4 respectively. The compound L-796,778 binds to SSTR3 with
approximately 50-fold selectivity while the SSTR5 subtype-agonist, L-817,818,
displays dual selectivity for SSTR1 (see Table 1).
With respect to the development of SST antagonists, the field has been lagging.
The first SST peptide antagonist developed, CYN-154806, a cyclic octapeptide,which
displayed high-affinity for SSTR2 however, exhibited intermediate affinity for
SSTR5 (Bass et al. 1996; Feniuk et al. 2000). Unfortunately, follow up studies
Table 1 Binding-affinities of endogenous, synthetic and nonpeptide somatostatin agonists.
Adapted from (Florio 2008; Patel 1999)
Binding constants (nM)
Agonists
Receptors
SSTR1 SSTR2 SSTR3 SSTR4 SSTR5
Endogenous
SST-14 0.1–2.26 0.2–1.3 0.3–1.6 0.3–1.8 0.2–0.9
SST-28 0.1–2.2 0.2–4.1 0.3–6.1 0.3–7.9 0.05–0.4
hCST-17 7 0.6 0.6 0.5 0.4
Synthetic
Octreotide 290–1140 0.4–2.1 4.4–34.5 >1000 5.6–32
Lanreotide 500–2330 0.5–1.8 43–107 66–2100 0.6–14
Vapreotide >1000 5.4 31 45 0.7
SCH-275
SOM-230
3.2–4.3
9.3
>1000
1
>1000
15
4.3–874
100
>1000
0.16
Nonpeptide
L-797,591 1.4 >1000 >1000 170 >1000
L-779,976 >1000 0.05 729 310 >1000
L-796,778 >1000 >10000 24 >1000 >1000
L-803,087 199 >1000 >1000 0.7 >1000
L-817,818 3.3 52 64 82 0.4

Somatostatin and Somatostatin Receptors 149
demonstrated near full agonism in a cAMP-accumulation assay (Nunn et al. 2003).
Using the same backbone design as CYN-154806, a high-affinity SSTR3 antago-
nist was developed that inhibited the effects of SST-14 in a functional assay for
cAMP (Reubi et al. 2000). In a unique design, open-chain octapeptide antagonists
BIM 23056, BIM 23627, and BIM 23454 were selected for their preferentially
binding to SSTR5 and SSTR2 respectively; however, both compounds do show
partial affinities for the other subtypes to various degrees (Shimon et al. 1997b;
Tulipano et al. 2002). The first high-affinity nonpeptide antagonist was designed for
SSTR3 with greater than 1000-fold selectivity (Poitout et al. 2001). An SSTR1
nonpeptide antagonist SRA880, was recently characterized in vitro to have modest
selectivity of up to 100-fold (Hoyer et al. 2004).
The high density of SSTRs on tumor cells, particularly the SSTR2 subtype,
allowed the possibility of visualizing them and their metastases by scintigraphy.
Studies involving SSTR scintigraphy were initially performed using 123I-3Tyr-
octreotide in the late 80s (de Herder and Lamberts 2005). The use of iodinated
SST-analogs are no longer in practice for radiodiagnostic purposes; instead, octre-
otide and lanreotide have been suited with metal chelators for stable coupling to
various a- and b-emitting isotopes such as [111In-DTPA0]octreotide, [90Y-DOTA0Tyr3]
octreotide, [177Lu-DOTA0Tyr3]octreotate, [111In-DOTA0]lanreotide, and [90Y-DOTA0]
lanreotide (de Herder and Lamberts 2005). Although [111In-DTPA0]octreotide is the
standard in the radioimaging of NETs, the 90Yttrium (90Y) and 177Lutetium (177Lu)
conjugated derivatives have been effectively demonstrated in peptide receptor
radionuclide therapy (PRRT) of patients with inoperable or metastasized NETs
(Van Essen et al. 2007).
9 Somatostatin Receptor Localization
The expression of SSTR subtypes has been well characterized in human and rodent
tissue including various tumors and tumor cell lines by a multitude of techniques
such as Northern blot, RT-PCR, ribonuclease protection assay, in situ hybridization,
and immunocytochemistry and has been extensively reviewed elsewhere (Patel et al.
1995, 1996; Reisine and Bell 1995; Patel 1997, 1999; Barnett 2003; Moller et al.
2003; Gardette et al. 2004; Kreienkamp et al. 2004). The distribution is widespread,
with localization throughout the CNS, periphery, often overlapping in subtype
expression depending on tissue- and species-type. The mRNA expression of
SSTR1-5 in the rat has been localized to brain regions such as the cerebral cortex,
striatum, hippocampus, amygdale, olfactory bulb, and preoptic area (Bruno et al.
1993). Comparing the individual expression patterns of each receptor-subtype
revealed SSTR1 to predominate in the brain, with expression in the pituitary, islets,
and adrenals. SSTR2 is also abundantly expressed throughout the brain, including
the pituitary, islets, and adrenals. SSTR3 is densely expressed in the cerebellum but
in moderate amounts throughout the rest of the brain. However, it is highly expressed
in the spleen, kidneys, and the liver. Compared to the other SSTR subtypes, SSTR4

150 U. Kumar and M. Grant
is poorly expressed in the brain. It is however abundant in the heart and occurs at
moderate levels in the lungs and islets. SSTR5 is sparsely expressed throughout the
brain but is especially prominent in the pituitary, intestine, and islets.
As previously mentioned, coexpression of SSTR subtypes is often seen in various
degrees depending on tissue and cell type (Patel et al. 1995, 1996; Reisine and Bell
1995; Patel 1997, 1999; Barnett 2003; Moller et al. 2003; Gardette et al. 2004;
Kreienkamp et al. 2004). Overlapping patterns of SSTR distribution have been dem-
onstrated throughout the CNS. Colocalization of SSTR1 and SSTR2 mRNA can be
found in GHRH-producing arcuate neurons (Tannenbaum et al. 1998). In the adult
human pituitary, SSTR1, 2, 3, and 5 are expressed whereas all five are found in the
pituitary of rats (Bruno et al. 1993; Day et al. 1995; O’Carroll and Krempels 1995;
Panetta and Patel 1995). Although the five receptors have been identified in the
pituitary, the primary subtypes expressed are SSTR5 and SSTR2 (Day et al. 1995;
Kimura et al. 1998). In the periphery, human pancreatic islets were shown to express
all five subtypes but colocalization was strongly identified for SSTR1 and SSTR5 in
insulin-secreting b-cells - to a lesser extent for SSTR1 and SSTR2 (Kumar et al.
1999). The same authors also reported on the colocalization of SSTR2 and SSTR5
in glucagon-producing a-cells, an occurrence that was only identified in up to a third
of the population. Elsewhere, in rat testis, SSTR1-3 displayed overlapping distribu-
tion patterns in Sertoli and germ cells, a property that was dependent on the stage of
the seminiferous epithelium cycle (Zhu et al. 1998).
The expression of SSTRs in neoplastic tissue has been in the forefront of current
day investigation, as their densities are found to be much higher than in normal
tissue. The first evidence of the expression of SSTRs in human tumors appeared as
early as 1984 in GH-secreting pituitary adenomas (Reubi and Landolt 1984). The
identification of differential SSTR subtypes initially appeared in 1987 from autora-
diographic studies on NETs (Reubi et al. 1987). Since then, many tumors have been
shown to express SSTRs, a characteristic often exploited for both diagnostic and
treatment purposes (Patel et al. 1995, 1996; Reisine and Bell 1995; Patel 1997,
1999; Barnett 2003; Moller et al. 2003; Gardette et al. 2004; Kreienkamp et al.
2004; Reubi et al. 2004). SSTRs are often highly expressed in NETs, in particular,
GH-secreting pituitary adenomas and gastroenteropancreatic tumors. Several other
tumors known to express SSTRs include neoplasias of the brain, breast carcinomas,
lymphomas, renal cell cancers, mesenchymal tumors, prostatic, ovarian, gastric,
hepatocellular, and nasopharyngeal carcinomas. Although the most prevalent sub-
type expressed in human tumors is hSSTR2, the appearance of other subtypes is
often found - a property originally identified in pituitary adenomas (Greenman and
Melmed 1994a, b; Miller et al. 1995; Panetta and Patel 1995; Schaer et al. 1997)
and gastroenteropancreatic tumors (Jais et al. 1997; Schaer et al. 1997; Wulbrand
et al. 1998). Over the past several years, a profusion of studies have been published
addressing the variable expression of SSTR subtypes in a large variety of cancers
(Patel et al. 1995, 1996; Reisine and Bell 1995; Patel 1997, 1999; Barnett 2003;
Moller et al. 2003; Gardette et al. 2004; Kreienkamp et al. 2004; Reubi et al. 2004).
The knowledge gained by these studies has been instrumental not only for investigational
purposes but also to decipher the use of SST-analogs in both diagnostic (tumor

Somatostatin and Somatostatin Receptors 151
imaging by radio-labeled analogs) (Kwekkeboom et al. 2004) and therapeutic
applications.
10 Regulation of Somatostatin Receptor Genes
One major factor affecting the potency of SST is the expression of cell surface
receptors. Hormones have been shown to have a profound impact on SSTR gene
regulation. For instance, oestrogen induces SST binding sites in cultured rat prol-
actinoma cells via upregulation of SSTR2 and SSTR3 (Visser-Wisselaar et al.
1997). Similarly, both in vitro and in vivo studies have demonstrated the ability of
oestrogen to induce the transcription and upregulation of SSTR2 and SSTR3 in rat
pituitary cells (Djordjijevic et al. 1998; Kimura et al. 1998). While SSTR1 tran-
scripts were found to be upregulated by both oestrogen (Kimura et al. 1998) and
testosterone (Senaris et al. 1996) in the rat pituitary, SSTR5 mRNA was in fact
downregulated (Kimura et al. 1998). In MCF-7 breast cancer cells, oestrogen was
reported to simulate SSTR2 gene expression (Xu et al. 1996). In two other breast
cancer cell lines T47D and ZR75-1, oestrogen was found to increase and decrease
SSTR binding sites respectively (Van Den Bossche et al. 2004). Investigation on the
subtypes involved by Western blotting revealed an upregulation of SSTR2 in T47D
cells and a downregulation of SSTR5 in ZR75-1 cells following oestrogen treat-
ment (Van Den Bossche et al. 2004). In mouse TtT-97 thyrotrophic tumor cells,
thyroid hormone increases the synthesis of SSTR1 and SSTR5 transcripts (James
et al. 1997). The effects of glucocorticoids on SSTR gene expression are somewhat
unique: transient exposure induces SSTR1 and SSTR2 mRNA, while prolonged
exposure inhibits transcription (Xu et al. 1995). Other factors affecting SSTR gene
transcription include cAMP, gastrin, epidermal growth factor, and even SST itself
(Patel et al. 1993; Bruno et al. 1994a; Vidal et al. 1994). Finally, food deprivation
and even diabetes in rat models have shown decreases in mRNA transcripts for
SSTR1, 2, and 3 in the pituitary and SSTR5 in the hypothalamus (Bruno et al.
1994b). Investigation on the promoters of each subtype and specific elements
involved in their regulation has been described (Meyerhof 1998; Patel 1999; Moller
et al. 2003; Olias et al. 2004).
11 Somatostatin Receptor Signaling
The signal transduction pathway of SSTRs is rather complex, for most part,and it
begins by activation of G-proteins. In the classical model of GPCR activation,
agonist-binding induces a conformational change that transcends to G-proteins
resulting in their activation [reviewed in (Pierce et al. 2002; Lefkowitz 2004)].
The G-protein is comprised of three subunits: the a subunit (Ga) and the b ,and g
subunits (Gbg) which are tightly bound. There are fifteen a subunits, five b subunits

152 U. Kumar and M. Grant
and fourteen g subunits known to date. Activation of the G-protein heterotrimer is
preceded by the nucleotide exchange of GDP for GTP, resulting in the dissociation
of the complex and allowing the Ga and Gbg subunits to be free to propagate their
signal. The G-proteins are generally referred to by their Ga subunits and therefore,
can be classified under four categories based on function: Gas, stimulate adenylate
cyclase; Gai/o, inhibit adenylate cyclase and activate inwardly rectifying potassium
channels; Gaq, activate phospholipase Cb; and G12, activate Rho guanine-nucleotide
exchange factors.
Binding of SSTRs by SST ligands modulates the activates of several key
enzymes, including adenylate cyclase, phosphotyrosine phosphatases (PTPases),
and mitogen-activated protein kinase (MAPK) along with changes in the intracel-
lular levels of calcium and potassium ions, as typified by activation of calcium and
potassium channels, including the regulation of the sodium/proton antiporter
(Fig. 4) (Patel et al. 1995, 1996; Reisine and Bell 1995; Patel 1997, 1999; Csaba
and Dournaud 2001; Barnett 2003; Moller et al. 2003; Olias et al. 2004). The type
of signal that prevails is dependent on several factors such as the SSTR subtype(s)
expressed, signaling elements, SSTR internalization, desensitization, and/or receptor
crosstalk. The ability of SST to block regulated secretion from various cell types
is typified in part by its effects on the synthesis and release of two important
mediators, cAMP and calcium respectively. Adenylate cyclase was the first effector
Fig. 4 Schematic representation of SSTR signaling pathways. SSTR signaling cascades leading
to the modulation of hormone secretion, cell growth and apoptosis are shown

Somatostatin and Somatostatin Receptors 153
enzyme to be identified and regulated by SSTRs (Patel et al. 1994). All five SSTR
subtypes negatively couple to the enzyme by activating pertussis-toxin (PTX) sen-
sitive Gai G-proteins, a property observed in various cell types (Meyerhof 1998).
In an attempt to elucidate the most relevant G-protein subtypes involved in SSTR-
mediated inhibition of adenylate cyclase, the subtypes Gai1, Gai2 ,and Gai3 were
identified, as determined by targeted-disruption using either antiserum or G-protein
antisense plasmids (Tallent and Reisine 1992; Liu et al. 1994; Gu and Schonbrunn
1997). SSTRs are coupled to several types of potassium channels and include the
delayed rectifier, inward rectifier, ATP-sensitive potassium channels, and large-
conductance calcium-activated BK channels (de Weille et al. 1989; Wang et al.
1989; Sims et al. 1991; White et al. 1991; Akopian et al. 2000). The G-protein
subtype Gai3 and possibly its interacting bg dimer pair are implicated in potassium
channel regulation (Takano et al. 1997). SSTRs have also been shown to directly
modulate high-voltage-dependent calcium channels via Gao2 (Ikeda and Schofield
1989; Kleuss et al. 1991). SSTRs may also inhibit calcium currents by activation of
cGMP protein kinase, through the induction of cGMP to regulate channel phospho-
rylation (Meriney et al. 1994). Aside from regulating channels to control ion flux,
SSTRs have also been shown to couple to Na+/H+ exchangers (NHEs) (Barber et al.
1989; Hou et al. 1994; Smalley et al. 1998; Ye et al. 1999; Lin et al. 2003) to modu-
late such features as cell adhesion, migration, and proliferation (Putney et al. 2002).
SSTR1 was the first subtype to specifically-regulate NHE-1, decreasing the extra-
cellular acidification rate (ECAR) when transfected in either fibroblast Ltk− or
HEK-293 cells (Hou et al. 1994). It was later determined that SSTR3 and SSTR4
also contribute, but not SSTR2 and SSTR5 (Lin et al. 2003).
SSTRs activate a number of phosphatases that have been implicated in cell
growth [reviewed in (Patel 1999; Csaba and Dournaud 2001; Moller et al. 2003;
Olias et al. 2004)]. For instance, the SH2 domain containing tyrosine phosphatases,
SHP-1 and SHP-2, which play a role in cell growth and differentiation, are known
to be recruited by various SSTR subtypes. Both phosphatases are rapidly recruited
to the membrane of breast cancer cells upon stimulation with SST (Srikant and
Shen 1996). More specifically, SHP-1 has been demonstrated to coprecipitate with
SSTR2 in a constitutive manner, suggesting its importance in the formation of sig-
naling complexes (Lopez et al. 1997; Hortala et al. 2003). Furthermore, the activa-
tion of SHP-1 was shown to be highly dependent on the recruitment of SHP-2
through phosphorylation of tyrosine residues present at the C-terminal portion of
the receptor, impressing the importance of both phosphatases in SSTR signaling
(Ferjoux et al. 2003). A receptor-like PTP known as PTPh, has also been demon-
strated to be an important player in the cytostatic effects of SST, particularly since
its expression is necessary for the control of thyroid tumor and human glioma cells
(Florio et al. 1997, 2001; Massa et al. 2004a, b). Recently, a signaling complex
involving JAK2, SHP-2, and c-src was demonstrated in the SSTR1-mediated acti-
vation of PTPh (Arena et al. 2007). In addition to tyrosine phosphatases, the activation
of serine/threonine phosphatases has also been demonstrated to be recruited by SSTRs.
Modulation of the N- and L-type calcium channels and potassium channels have
been shown to be dependent on the activation of phosphatase 2A (PP2A) and cal-

154 U. Kumar and M. Grant
cineurin (PP2B) in several cell types such as sympathetic neurons, pancreatic alpha
cells, and pituitary tumor cells. Evidence of the importance of PP2B recruitment by
SSTR activation comes from studies on the regulation of neurotransmitter release
and exocytosis in sympathetic neurons and pancreatic alpha cells respectively
(White et al. 1991; Zhu and Yakel 1997; Gromada et al. 2001).
Several important yet recently identified signaling cascades found downstream
of SSTR activation are the MAPKs (Patel 1999; Bousquet et al. 2001; Csaba and
Dournaud 2001; Moller et al. 2003; Weckbecker et al. 2003; Olias et al. 2004).
SSTR activation (Florio et al. 1999, 2001, 2003a; Lahlou et al. 2003) or inhibition
(Dent et al. 1997; Cattaneo et al. 1999; Douziech et al. 1999) of MAPKs has been
demonstrated to be mediated by PTPases. In case of SSTR5, the inhibition of
MAPKs was related to the activation of a cGMP-dependent pathway when
expressed in CHO-K1 cells (Cordelier et al. 1997). In addition to PTPases, recruit-
ment of phosphoinositide-3 kinase has also been shown fundamental in the activa-
tion of MAPKs. For instance, studies involving human SSTR4 (Smalley et al.
1999), rat SSTR2B (Sellers et al. 2000) or mouse SSTR2A (Lahlou et al. 2003)
have underscored its relevance.
12 Relevance of SSTRs in Cancer
As previously mentioned, a number of cancer cells overexpress SSTRs, with more
than one subtype often being expressed. As early as the 1980s, the antiproliferative
effects of the SST-analog octreotide were being recognized for treatment of hyper-
secreting tumors of the pancreas, intestine, and pituitary (Lamberts et al. 1991,
1996; Weckbecker et al. 1993). It was noted that not only was treatment blocking
hormone secretion but it was also causing variable tumor shrinkage through a dis-
tinct antiproliferative effect. The antiproliferative effects of SST was demonstrated
in normal dividing cells such as intestinal mucosal cells, activated lymphocytes,
inflammatory cells, as well as in experimental tumor models for example solid
tumors of transplanted rat mammary carcinomas and finally cultured cells derived
from both endocrine and epithelial tumors (pituitary, thyroid, breast, prostate,
colon, pancreas, lung, and brain) (Patel 1999; Csaba and Dournaud 2001; Lamberts
et al. 2002; Moller et al. 2003; Weckbecker et al. 2003; Olias et al. 2004; Zatelli et al.
2006). The antiproliferative effects of SST on normal or transformed cells can be
directed by cell growth arrest and/or apoptosis; several SSTR signaling pathways
have been implicated (Fig. 3).
A large body of evidence implicates PTPs as central mediators in the antiprolif-
erative effects of SSTRs. SSTR subtypes activate different PTPs, resulting in varying
effects on downstream effectors such as MAPKs, ultimately regulating the
induction of cyclin-dependent kinase inhibitors p27Kip1 and p21Cip1/Waf1. In the mid
80s, a role for PTPases in SST-mediated antiproliferation of pancreatic cancer cells
was postulated on the basis of an inhibition toward epidermal growth factor receptor
phosphorylation patterns (Hierowski et al. 1985). All five receptor subtypes display

Somatostatin and Somatostatin Receptors 155
some capacity to activate PTPs (Buscail et al. 1994; Florio et al. 1994; Sharma
et al. 1996; Reardon et al. 1997; Sharma et al. 1999), whether the PTPases are
cytosolic (Srikant and Shen 1996; Reardon et al. 1997; Bousquet et al. 1998; Florio
et al. 2000) or membrane localized as demonstrated with PTP (Florio et al. 1997,
2001; Massa et al. 2004a, b). An increase in the PTP SHP-1 has been reported in
several different cancer cell lines including pituitary adenomas, pancreatic cancers,
medullary, breast, and prostate carcinomas following SST treatment (Douziech
et al. 1999; Thangaraju et al. 1999; Ferjoux et al. 2000; Zapata et al. 2002; Zatelli
et al. 2005a; Theodoropoulou et al. 2006). Consequently, SHP-1 activation is a
critical factor in SSTR2-mediated cell growth arrest (Lopez et al. 1997; Bousquet
et al. 1998; Theodoropoulou et al. 2006). In fact, a multieffector complex between
c-src and SHP-2 was determined central in the recruitment and activation of SHP-1
following stimulation of SSTR2 (Ferjoux et al. 2003). Ultimately, SHP-1 activation
results in growth factor receptor signaling inhibition by dephosphorylating its sub-
strates (Lopez et al. 1997; Bousquet et al. 1998). Other inhibitory pathways of
SHP-1 include the activation of nNOS by its dephosphorylation, resulting in an
increase in cGMP formation and subsequent induction of p27Kip1 and cell cycle
arrest (Lopez et al. 2001).
In a similar vein, activation of SSTR1 has also been shown to recruit SHP-2
and c-src for its antiproliferative activity (Reardon et al. 1997; Florio et al.,
1999). Activation of SHP-2 by SSTR1 was reported to orchestrate antiprolifera-
tion by mediating the dephosphorylation of growth factor receptors for EGF,
insulin, and platelet derived growth factor (PDGF), with the consequent inhibi-
tion of Ras and MAPK activity (Cattaneo et al. 2000). However, unlike SSTR2,
the final effector PTP for SSTR1 is not SHP-1 but the membrane-bound PTPη
(Florio et al. 1997, 2001; Massa et al. 2004a, b; Arena et al. 2007). Recently,
SSTR2 was shown to inhibit the activity of phosphatidyl inositol 3 kinase (PI3K),
thereby, preventing the activation of AKT in both pituitary and insulinoma tumor
cells (Theodoropoulou et al. 2006; Grozinsky-Glasberg et al. 2008c). The PI3K/
Akt signaling pathway has been demonstrated to promote the survival, prolifera-
tion, angiogenisis, and motility of tissue invasion of cancer cells and therefore,
provides an important target in tumor control (Altomare and Testa 2005).
Although typically involved in cell growth and proliferation (Dhanasekaran et al.
1995), the activation of the MAPKs as demonstrated via distinct SSTRs can be
associated with cell growth inhibition (Florio et al. 1999; Sellers et al. 2000;
Alderton et al. 2001; Lahlou et al. 2003). Stimulation of SSTR2 was shown to
inhibit the proliferation of CHO-K1 cells by activating two members of the
MAPK family - extracellular-regulated kinase-1 and -2 (ERK1/2) and p38 - and
upstream the activation of the cyclin-dependent protein kinase inhibitor p21cip1/
WAF1 (Sellers et al. 2000; Alderton et al. 2001). Similar findings were also
reported upon activation of SSTR1 (Florio et al. 1999). Contrarily, the antiprolif-
erative actions of SSTR5 do not require activation but rather inhibition of MAPKs
(Cordelier et al. 1997 ). Pathways suggested to be implicated in SSTR5-mediated
antiproliferation include one involving phospholipase C/inositol phospholipid/
Ca2+ (Buscail et al. 1995) and the other involving the induction of the retinoblas-

156 U. Kumar and M. Grant
toma tumor suppressor protein (Rb) and p21 (Sharma et al. 1999). In rare
instances, SST may actually stimulate cell growth - an anomaly shown to occur
by MAPK activation via human SSTR4 (Sellers et al. 2000).
In addition to the cytostatic effects of SST, apoptosis or programmed cell death
has also been observed to contribute to the antiproliferative response following
treatment. Apoptosis was first demonstrated in AtT-20 and MCF-7 cells when
treated with octreotide (Pagliacci et al. 1991; Srikant 1995; Sharma and Srikant
1998a). In MCF-7 cells, SHP-1 is necessary for SSTR-mediated apoptotic signaling
(Sharma and Srikant 1998a; Liu et al. 2000). Because both cell types express more
than one SSTR, it is not possible to assign the subtype that may be contributing to
apoptosis. When CHO-K1 cells were individually transfected with each receptor-
subtype, apoptosis was uniquely triggered by human SSTR3 (Sharma et al. 1996).
The events preceding apoptosis following hSSTR3 activation include activation of
tumor suppressor protein p53 and proapoptotic protein Bax (Sharma et al. 1996).
However, recent reports have described p53-independent apoptosis via SSTR2 in
HL-60, human pancreatic adenocarcinoma,and human somatotroph tumor cells
(Teijeiro et al. 2002; Guillermet et al. 2003; Ferrante et al. 2006).
Unlike the direct effects of SST on tumor cell proliferation mentioned above,
SST can indirectly control tumor growth and development by inhibiting angiogen-
esis. Antiangiogenic activity was first described by Woltering et al. using a chicken
corioallanthoic membrane model, a property that was further supported by the find-
ings conducted in vitro and in vivo with SST and its analogs (Barrie et al. 1993;
Danesi and Del Tacca 1996; Danesi et al. 1997; Albini et al. 1999; Dasgupta and
Mukherjee 2000; Garcia de la Torre et al. 2002; Koizumi et al. 2002; Zalatnai and
Timar 2002; Florio et al. 2003a; Murray et al. 2004). Three different pathways have
been proposed and may operate concurrently to achieve the antiangiogenic activity
of SSTRs. First, activation of SSTRs may directly inhibit the proliferation, migra-
tion, and invasion of endothelial cells to the tumor. Second, SST may regulate the
secretion of angiogenic promoting factors such as vascular endothelial growth factor
(VEGF) and basic fibroblast growth factor (bFGF). Third, SST may modulate the
activation of monocytes, cells which are important in the immune response,but
whose migration in the peritumoral region can produce proangiogenic factors
resulting in neovascularization (Florio 2008). SST may also indirectly regulate
tumor growth by inhibiting the synthesis and/or secretion of growth factors and
hormones such as EGF, transforming growth factor, insulin, prolactin, GH, and
IGF-I (Susini and Buscail 2006).
The first conclusive evidence that SST-analogs can have antiproliferative proper-
ties in the clinic came from a multicenter randomized trial recruiting 32 acromeg-
alic patients with hypersecreting pituitary adenomas (Thapar et al. 1997). These
patients demonstrated an 83% reduction in mean growth fraction when compared
to untreated controls, suggesting that octreotide had exerted an antineoplastic effect
on somatotroph adenomas. In a separate study by Losa et al. the Ki-67 index, a
nuclear protein expressed only in dividing cells, was significantly lower in the GH
hypersecreting adenomas of patients pretreated with octreotide compared to
untreated controls (Losa et al. 2001). Many trials have since been undertaken dem-

Somatostatin and Somatostatin Receptors 157
onstrating the effects of SST-analog therapy on tumor shrinkage in acromegalic
patients. Typically, patients receiving SST-analogs as primary therapy show reduc-
tions of up to 50% in tumor volume (Bevan 2005; Melmed et al. 2005). With regard
to the antiproliferative effects of SST-analogs in the treatment of patients with other
types of tumors, evidence is scanty. In approximately half of patients with gastroin-
testinal NETs, stabilization of tumor growth was apparent for duration of
8–16 months; however, tumor shrinkage was achieved in only 10–20% of cases
(Eriksson and Oberg 1999). In a study with patients diagnosed with malignant
gastrinoma, treatment with the long-acting formulation of octreotide demonstrated
an antiproliferative response in approximately 50% of the subjects (Shojamanesh et al.
2002). Although SST-analogs are effective in the symptomatic treatment of NETs,
a family of tumors which originate from various endocrine glands including the
pituitary, parathyroid, adrenals, endocrine islets, in addition to exocrine cells from
the digestive and respiratory tracts (Grozinsky-Glasberg et al. 2008a, b), SST-
analog therapy is rarely curative.
13 Agonist-Regulation of Somatostatin Receptors
The initial responses following activation of SSTRs diminish with continued expo-
sure to SST (Patel 1997, 1999; Csaba and Dournaud 2001; Moller et al. 2003; Olias
et al. 2004). This feature is shared by many GPCRs and is a requirement for termi-
nating signaling. This process can be divided into two general steps: desensitization
and internalization. Desensitization is the result of rapid attenuation of receptor
function, usually by phosphorylation of its c-tail, causing uncoupling of the receptor
from its respective G-protein. This property can be mediated by second-messenger
kinases, such as protein kinase A or protein kinase C, or through a distinct family of
G-protein-coupled receptor kinases termed as GRKs and is typically followed by
internalization (Pierce et al. 2002; Premont and Gainetdinov 2007). Internalization
is a process by which the receptor is redistributed away from the surface and brought
into the cell, also known as endocytosis (Claing et al. 2002; Pierce et al. 2002). The
process of internalization can be divided into three different pathways: clathrin-
coated pits, caveolae, and uncoated vesicles (Claing et al. 2002; Pierce et al. 2002).
The least understood method by which GPCRs internalize involves caveolae. This
mechanism involves membrane invaginations that are rich in both caveolin and cho-
lesterol (Nichols 2003). Several GPCRs have been demonstrated to undergo caveo-
lae-dependent internalization and include the endothelin and vasoactive intestinal
peptide receptors (Claing et al. 2000) in addition to the chemokine receptors (Neel
et al. 2005). The most investigated and best understood mechanism involved in
GPCR internalization is the b-arrestin-dependent mediated pathway, which occurs
via clathrin-coated vesicles (Claing et al. 2002; Pierce et al. 2002; Lefkowitz and
Shenoy 2005). There are two subtypes of b-arrestin, b-arrestin-1 and b-arrestin-2,
both of which are ubiquitously expressed. A third type of arrestin known as visual
arrestin is exclusively expressed in the retina where it was originally identified

158 U. Kumar and M. Grant
(Pierce et al. 2002; Lefkowitz and Shenoy 2005). This process of internalization is
initiated by the recruitment of b-arrestin to the phosphorylated portion of the receptor
(Lohse et al. 1990). This in turn engages the receptor into the clathrin-coated pit
machinery, as b-arrestin is known to interact with several components involved in
this process including the heavy chain of clathrin itself, the b2-adaptin subunit of the
clathrin adaptor protein AP-2, the small guanosine triphosphatase ARF6, and its
guanine nucleotide exchange factor ARNO, the N-ethylmaleimide-sensitive fusion
protein (NSF) in addition to constituents of the inner leaflet of the cell membrane
itself (Claing et al. 2002; Pierce et al. 2002; Lefkowitz and Shenoy 2005). The final
step to internalization requires the actions of a GTPase known as dynamin, as it is
responsible for pinching off the pits to generate endosomes. There are two general
types of b-arrestin-mediated internalization that depend on its avidity for the recep-
tor: class A, b-arrestins bind transiently to the receptor, target it to clathrin-coated
pits, and dissociate during receptor-internalization; class B, b-arrestin remains
tightly bound to the receptor and does so throughout the internalization process for
extended periods of time, from which the receptor can be sorted to lysosomes where
it is degraded. The end result is that class A receptors such as the b2-adrenergic
receptors, are recycled more quickly to the cell surface, as their fate is not tied to
b-arrestin sorting; whereas class B receptors, for example, the V2R vasopressin
receptors, are slowly recycled and often end up being degraded (Claing et al. 2002;
Pierce et al. 2002; Lefkowitz and Shenoy 2005).
In the early 1980s, it was appreciated that SSTRs can undergo agonist-induced
uncoupling from their G-proteins, a property demonstrated in AtT-20 cells (Reisine
and Axelrod 1983). It wasn’t long before agonist-induced internalization was docu-
mented and shown to occur in cells from the rat anterior pituitary and islet, mouse
AtT-20 cells, and human pituitary and islet tumor cells (Morel et al. 1985; Amherdt
et al. 1989; Hofland et al. 1995, 1999). However, a rather unusual occurrence devel-
oped following prolonged agonist exposure (24–48 h) in GH4C1 and Rin m5f cells;
SSTRs were found to increase at the cell surface (Presky and Schonbrunn 1988;
Sullivan and Schonbrunn 1988). Although the underlying mechanisms are still
unclear, agonist-induced upregulation has been observed by several other GPCRs
(Presky and Schonbrunn 1988; Cox et al. 1995; Hukovic et al. 1996; Ng et al. 1997;
Tannenbaum et al. 2001) and may play a role in long-term drug therapy. A hunt for
the specific receptor-subtypes mitigating these events is underway. The results are
confounding, as studies have revealed differences that were not only dependent on
receptor-subtype but also on the species from which the receptor is derived. Despite
these differences, two important conclusions can be made based upon the subgroup
of SSTRs examined: SRIF1 receptors (SSTR2, SSTR3 and SSTR5) internalize
readily following agonist treatment, whereas SRIF2 receptors (SSTR1 and SSTR4)
are rather resistant to agonist-induced internalization.
Initial evidence for the desensitization and internalization of SSTR2 following
agonist treatment in vivo came from rat brain slices (Boudin et al. 2000). Around the
same time, it was also observed that SSTR2 internalized when activated in primary
cultured hippocampal neurons using fluorescently-labeled SST ligands (Stroh et al.
2000). Stereotactic injections of the SST-analog octreotide in the rat parietal cortex

Somatostatin and Somatostatin Receptors 159
(Csaba et al. 2001) and endopiriform nucleus (Csaba and Dournaud 2001) demon-
strated that SSTR2 internalized via a clathrin-mediated pathway. Similar mecha-
nisms were also described for the internalization of SSTR2 in vivo, as demonstrated
by studies in the rat forebrain (Schreff et al. 2000), dorsolateral septum (Csaba et al.
2002), and arcuate nucleus of the hypothalamus (Csaba et al. 2003). When trans-
fected in either CHO-K1, HEK-293, or even pancreatic b-cells, human and rat
SSTR2 internalize in response to SST stimulation (Hukovic et al. 1996; Roosterman
et al. 1997; Roth et al. 1997b; Cescato et al. 2006), via a clathrin-dependent pathway.
Furthermore, endocytosis of SSTR2 was also demonstrated in glioma and neurob-
lastoma cells that endogenously express the receptor (Koenig et al. 1997; Krisch
et al. 1998). In other cell types, the densitization, internalization, and phosphorylation
of rat SSTR2 was observed (Hipkin et al. 1997, 2000; Roosterman et al. 1997). The
phosphorylation of SSTR2 was related to its internalization of clathrin-coated pits
and shown to occur at both the C-terminal portion and the IL of SSTR2. Both protein
kinase A and protein kinase C play a role in the phosphorylation and internalization
of SSTR2 (Hipkin et al. 2000; Oomen et al. 2001). Interestingly, although b-arrestin
subtype-1 was found to desensitize mouse SSTR2 transfected in CHO-K1 cells, it
was not implicated in its internalization. Recently, both GRK2 and b-arrestin sub-
type-2 were shown to be actively involved in the phosphorylation and clathrin-
mediated internalization of the receptor when expressed in HEK-293 cells,
respectively (Tulipano et al. 2004). The same authors also described a region in the
C-terminal portion of the receptor as a recognition site for GRK2 phosphorylation.
SSTR2 can therefore be classified a class B receptor, as SSTR2 forms stable associa-
tions with b-arrestin throughout its sequestration and localization in endosomes
(Tulipano et al. 2004).
The regulation of SSTR3 is very similar to that of SSTR2. Both human and rat
forms rapidly internalize following agonist stimulation in various transfected cell
lines (Hukovic et al. 1996; Roosterman et al. 1997; Roth et al. 1997b; Cescato et al.
2006). The receptor is phosphorylated at the C-terminus, a critical determinant
for agonist-induced internalization (Roth et al. 1997a; Tulipano et al. 2004).
Internalization follows a clathrin-mediated pathway, a property dependent on the
recruitment of b-arrestin (Kreuzer et al. 2001; Tulipano et al. 2004). Desensitization
of the receptor follows a slow recovery rate, as demonstrated by its effector cou-
pling to adenylate cyclase (Roosterman et al. 1997; Roth et al. 1997b). This could
be explained by the high avidity of b-arrestin binding to the receptor, however; both
proteins are found colocalized in intracellular endocytic compartments for rela-
tively short time periods (Kreuzer et al. 2001; Tulipano et al. 2004). Given that the
receptor is more prone to degradation unlike SSTR2, it is more probably that
sequestration to lysosomes dictates its slow recovery (Tulipano et al. 2004).
The final receptor in the SRIF1 class, SSTR5, undergoes differential regulation
in a species-specific manner. For instance, human SSTR5 is rapidly internalized
following activation with either SST-14 or SST-28 when expressed in CHO-K1
cells (Hukovic et al. 1996, 1998; Cescato et al. 2006). Desensitization has also been
observed as demonstrated by a reduced ability to couple to adenylate cyclase
following prestimulation, a property highly dependent on the structural domains

160 U. Kumar and M. Grant
present at the C-terminus (Hukovic et al. 1998). The loss of cell surface receptors
for rat SSTR5 is rather moderate compared to its human counterpart, as a rapid
recycling rate has been described for this difference (Stroh et al. 2000). Similar to
human SSTR5, the rat homolog also undergoes agonist-regulated desensitization
(Roosterman et al. 1997; Roth et al. 1997b; Stroh et al. 2000). More recently, the
interaction of b-arrestin with SSTR5 has been described (Tulipano et al. 2004;
Grant et al. 2008b), and although rat SSTR5 can be categorized as a class A recep-
tor as determined by its transient association with b-arrestin (Tulipano et al. 2004),
its human homolog does not show any interaction (Grant et al., 2008b). The species-
related differences in the regulation of human and rat SSTR5 may in part be
explained by their differential association with b-arrestin.
As previously mentioned, the SRIF2 class of SSTRs (SSTR1 and SSTR4) is
generally resistant to internalization but not desensitization by agonist. For instance,
when transfected in CHO-K1 cells, rat SSTR1 is quickly phosphorylated but slowly
sequestered within cells (Liu and Schonbrunn 2001). Similarly, activation of
endogenously expressed SSTR1 in both neurons of the hippocampus and cortex of
rat did not cause its internalization (Stroh et al. 2000). The upregulation of SSTR
binding sites in GH4C1 cells was attributed to the presence of SSTR1, as these cells
predominantly express this subtype (Presky and Schonbrunn 1988). A similar
occurrence was observed for hSSTR1 when expressed in CHO-K1 cells, where
upregulation rather than downregulation predominates followed prolonged agonist
exposure (Hukovic et al. 1996). Further examination revealed that the upregulation
of hSSTR1 was not dependent on de novo synthesis, but rather on dephosphoryla-
tion of amino acid residues present at the C-terminus and the recruitment of pools
of intracellular receptor (Hukovic et al. 1999). However, when hSSTR1 was
expressed in COS-7 cells, only a small fraction of receptor-bound ligand was inter-
nalized with the majority of receptors remaining within or just beneath the cell
membrane (Nouel et al. 1997).
Contrary to human SSTR1, hSSTR4 does show moderate levels of internaliza-
tion when expressed in CHO-K1 cells; however compared to hSSTR1, prolonged
treatment with agonist does induce its upregulation (Hukovic et al. 1996). The low
level of internalized hSSTR4 observed following agonist stimulation was attributed
to a rapid recycling rate (Smalley et al. 2001). Species-related differences have
been documented between the regulation of human and rat SSTR4 homologues. For
instance, rat SSTR4 does not internalize when transfected in either HEK-293 or rat
insulinoma cells following agonist-activation (Smalley et al. 2001). Interestingly,
internalization is apparent when part of the C-terminal portion of the receptor is
removed, suggesting a negative-regulatory motif involved in controlling the inter-
nalization of rat SSTR4 (Roosterman et al. 1997; Roth et al. 1997b). Further inves-
tigation using rat SSTR4 mutants, revealed threonine 331 as the residue most
accountable for inhibiting internalization (Kreienkamp et al. 1998). Taken together,
the in vitro analysis of rat SSTR4 is in good agreement with in vivo results, as
intracerebroventricular administration of SST-14 does not promote its sequestration
(Schreff et al. 2000). Despite the variability in the trafficking of SSTR1 and SSTR4,

Somatostatin and Somatostatin Receptors 161
it is clear that neither of them depend on interaction with b-arrestin for internaliza-
tion (Tulipano et al. 2004).
14 Dimerization of SSTRs
Physical evidence for the dimerization of SSTRs was first introduced in 2000 by
Rocheville et al. using a combination of pharmacological, biochemical, and
biophysical techniques (Rocheville et al. 2000a; Rocheville et al. 2000b). In these
studies, it was determined that human SSTR5 dimers could be stabilized following
agonist treatment in a dose-dependent fashion. Furthermore, using a functional
complementation technique with a signaling deficient variant of SSTR5, receptor-
activation could be restored when SSTR1 was introduced, presumably due to
heterodimerization (Rocheville et al. 2000b). Heterodimerization was suggested to
be a specific process, as signaling by the SSTR5 variant could not be reconstituted
by SSTR4 expression. Human SSTR1 is known to be resistant to agonist-mediated
internalization; however, in cells coexpressing both SSTR1 and SSTR5, internalization
could be observed (Rocheville et al. 2000b). In a related study, the homo- and
heterodimerization of SSTR1 and SSTR5 were specifically shown in live cells
using a combination of RET techniques (Patel et al. 2002). In these studies,
although SSTR5 was demonstrated to form both homo- and heterodimers with
SSTR1 in an agonist-regulated fashion, SSTR1 remained as a monomer when
expressed alone despite its activation with agonist. This was the first study which
demonstrated using RET techniques that not all GPCRs require dimerization to
function, as several other groups have since shown (Gripentrog et al. 2003; Meyer
et al. 2006; Whorton et al. 2007). In a follow-up study, the heterodimerization of
human SSTR1 and SSTR5 was shown as being subtype specific, that is, the
interaction was preferentially regulated by the ligand-binding of SSTR5 and not
SSTR1 (Grant et al. 2004b). This intriguing observation that human SSTR1 is
incapable of forming homo- or heterodimers in either an active or inactive state
(Patel et al. 2002; Grant et al. 2004b), appears to correlate with its resistance to
internalize and upregulate on prolonged agonist treatment (Hukovic et al. 1996,
1999). Swapping the carboxyl-terminal tails of SSTR1 with that of SSTR5,
reconstitutes the ability of this receptor to internalize and dimerize following
stimulation (Grant et al. 2004b). The importance of the carboxyl-terminal tail in
GPCR dimerization has been demonstrated earlier: on investigation of the d-opioid
receptor-trafficking (Cvejic and Devi 1997), in the masking of an ER retention
motif on heterodimerization of the g-aminobutyric acid receptor-subtypes (GABABR)
GABABR1 and GABABR2 (White et al. 1998; Kuner et al. 1999), and in generation
of the m- and d-opioid receptor heterodimer (Fan et al. 2005). Similarly, in line with
the results of Fan et al. the heterodimerization between m- and d-opioid receptors
could also be modulated by uncoupling of G-protein from the receptors (Law et al.
2005). Given that the carboxyl-terminal tails of GPCRs are important for G-protein
coupling, a mechanism for the heterodimerization of this receptor pair can be described.

162 U. Kumar and M. Grant
Furthermore, a detailed account on the heterodimerization of the adenosine A2A and
the dopamine D2 receptors was shown to occur between the carboxyl-terminal tail
of A2A and the third IL of D2 (Canals et al. 2003). More specifically, the interaction
was dependent on arginine-rich residues in the IL of the D2 receptor with either two
aspartate residues or a phosphorylated serine residue in the carboxyl-terminal
portion of A2A (Ciruela et al. 2004).
The inhibition of adenylate cyclase and cAMP synthesis, a typical hallmark of
SSTR activation, was shown as being more efficient following formation of SSTR1/
SSTR5 heterodimers (Grant et al. 2004b). More specifically, an approximate
50-fold increase in signaling efficiency was seen with the drug octreotide (SMS
201-995) in cells coexpressing both SSTR1 and SSTR5 compared to SSTR5 alone
(Grant et al. 2004b), despite its absence in affinity to SSTR1 (Patel 1999). However,
although the signaling efficiency was increased, the actual efficacy was decreased
- suggesting that the formation of the heterodimer results in a new receptor with
distinct signaling characteristics (Grant et al. 2004b). This alteration in maximum
coupling efficacy could have functional implications, as human prolactinomas
show poor responses to octreotide treatment. These tumors originate from the pitui-
tary and hypersecrete the hormone prolactin. Coincidently, prolactinomas predomi-
nantly express SSTR1 and SSTR5 (Shimon et al. 1997a; Jaquet et al. 1999). In
cultured studies of human excised prolactinomas, tumors that displayed increased
expression of SSTR1 responded poorly to treatment with octreotide in controlling
prolactin release, as opposed to those showing lower expression levels regardless of
SSTR5 expression (Jaquet et al. 1999). Interestingly, both SSTR1 and SSTR5 are
highly coexpressed in b-cells of the human pancreas (Kumar et al. 1999), suggesting
a possible role for heterodimerization in the control of insulin secretion.
Many tumors often express SSTR2, especially those of neuroendocrine origin
(Lamberts et al. 2002; de Herder et al. 2003), which makes this receptor subtype an
appropriate target to investigate. Using both coimmunoprecipition and RET
techniques, it was determined that hSSTR2 exists at the cell surface as a preformed
homodimer (Grant et al. 2004a; Duran-Prado et al. 2007). Surprisingly, treatment
with agonist causes it to dissociate into monomers (Grant et al. 2004b; Duran-Prado
et al. 2007). Although ligand-induced dissociation has been reported in the regulation
of other GPCR combinations (Cvejic and Devi 1997; Gines et al. 2000; Cheng and
Miller 2001; Pfeiffer et al. 2001; Latif et al. 2002; Berglund et al. 2003; Law et al.
2005), few have shown functional relevance for their occurrence. In the report by
Cvejic and Devi, dissociation of d-opioid receptor dimers was reported essential for
proper receptor-internalization. However, regulated dimerization and not dissociation
of the platelet activating factor receptor and the thyrotropin-releasing hormone
receptor, was shown to increase internalization (Perron et al. 2003; Song and Hinkle
2005). Further investigation on the dissociation of SSTR2 dimers, like the d-opioid
receptor, led us to conclude its importance in receptor-internalization, as crosslinking
SSTR2 dimers to prevent dissociation, drastically impaired its internalization rate
(Grant et al. 2004a). Interestingly, in the report by Duran-Prado et al. dissociation of
porcine SSTR2 dimers was also determined to be a feature occurring prior to its

Somatostatin and Somatostatin Receptors 163
internalization (Duran-Prado et al. 2007), possibly suggesting a common characteristic
for this subtype amongst all species.
Two other members of the SSTR family were shown to dimerize in the labora-
tory of S. Schulz, namely SSTR2 and SSTR3 (Pfeiffer et al. 2001). In their inves-
tigations, rodent SSTR2 and SSTR3 were demonstrated to form constitutive
homodimers and heterodimers when coexpressed in HEK 293 cells. Interestingly,
in cells coexpressing SSTR2 and SSTR3, the SSTR3-selective agonist L-796,778
displayed marked reductions in binding affinity, suggesting negative cooperativity
of SSTR2 on SSTR3. Furthermore, GTP binding, inhibition of adenylate cyclase,
and phosphorylation of ERK1/2 by the heterodimer reflected the characteristics of
SSTR2 when expressed alone in the same cells. However, unlike SSTR2, the
SSTR2/SSTR3 heterodimer displayed a strong resistance to agonist-induced desen-
sitization (Pfeiffer et al. 2001). The physiological relevance of these observations
remains unclear; however, both receptors colocalize in tissues such as the pancreas,
the anterior lobe of the pituitary (Pfeiffer et al. 2001), and in medullablastoma
tumoural cells (Cervera et al. 2002). The SSTR2-mediated inactivation of SSTR3
may explain the absence of SSTR3 binding sites in the cerebellum of developing
rats, as mRNA levels for both SSTR2 and SSTR3 are highly expressed in early
development (Viollet et al. 1997).
In a recent report by Grant et al. SSTR2 and SSTR5 were demonstrated to
physically interact - a property that was regulated by the binding of agonist (Grant et al.,
2008a). Interestingly, heterodimerization was not modulated by treatment with the
endogenous pan-agonist SST-14, but instead was induced by a selective agonist for
SSTR2 and not SSTR5 (Grant et al., 2008a). This is contrary to regulation of the
hSSTR1/hSSTR5 heterodimer, where treatment with SST-14 enhances its formation
(Rocheville et al. 2000b; Patel et al. 2002; Grant et al. 2004b). Although concurrent
stimulation had been shown as a requirement in the stabilization of heterodimers
between members of other family A GPCR subfamilies (Gines et al. 2000; Mellado
et al. 2001; Yoshioka et al. 2002; Rodriguez-Frade et al. 2004; Kearn et al. 2005;
Jiang et al. 2006; Pello et al. 2008), several heteromeric interactions were found to be
equally fostered following activation of just one of the receptor protomers (Rocheville
et al. 2000a; McGraw et al. 2006; Baragli et al. 2007). The SSTR2/SSTR5 heterodimer
demonstrated an approximate 10-fold greater efficiency for G-protein coupling and an
enhanced ability to activate MAPK (Grant et al., 2008a). More importantly, the
heterodimer conferred an extended growth inhibitory response that was related to an
increased induction of the cyclin-dependent kinase inhibitor p27Kip1.
An interesting observation that was also addressed in the Grant et al. study was
that heterodimerization altered the sequestration of b-arrestin and recycling of
SSTR2 (Grant et al., 2008a). GPCRs often require the interaction of b-arrestins to
internalize following their stimulation. This process is typically promoted by phos-
phorylation of the carboxyl-terminal portion of the receptor by a G-protein coupled
receptor kinase (GRK). b-arrestins are responsible for the recruitment of several
factors implicated in the internalization machinery including clathrin, AP-2, and
ARF6, to name a few (Claing et al. 2002; Pierce et al. 2002; Lefkowitz and Shenoy
2005). There are two main types of b-arrestin mediated internalization, class A and

164 U. Kumar and M. Grant
class B, that are primarily dependent on the avidity of b-arrestin to the receptor.
Class A GPCRs form transient interactions with b-arrestin during internalization,
whilst class B GPCRs form stable interactions during and following their sequestra-
tion. The avidity of b-arrestin to the receptor has direct effects on receptor recycling
rates, as class A GPCRs recycle back to the cell surface more efficiently than class
B, which is often sorted to the lysosomal compartment for degradation. Of the
SSTRs investigated, only SSTR2 and SSTR3 were shown to form stable interac-
tions with b-arrestin, indicative of a class B-dependent subtype (Tulipano et al.
2004). Heterodimerization of SSTR2 and SSTR5 as induced by activation with a
SSTR2-selective agonist, caused a transient interaction of b-arrestin with SSTR2
resulting in a rapid recycling rate, indicative of a class A GPCR (Grant et al.,
2008a). Similarly, selective activation of SSTR2 and not concurrent stimulation of
both SSTR2 and SSTR5, had been shown to reduce desensitization and increase the
recycling rate of SSTR2 in AtT-20 cells - the murine anterior pituitary-derived cell
line that endogenously expresses SSTR2 and SSTR5 (Sharif et al. 2007).
SST-analogs are frequently administered as first line treatment in acromegaly,
caused by GH hypersecreting pituitary adenomas to regulate endocrine function
(Tichomirowa et al. 2005). Over 90% of patients on SST-analogs show decreases in
circulating GH levels, while approximately 70% of those achieve biochemical
normalization. As previously mentioned, SST-analog therapy frequently results in
tumor shrinkage in roughly 50% of patients (Lamberts et al. 2002; Bevan 2005;
Melmed et al. 2005; Ferrante et al. 2006; Zatelli et al. 2006; Resmini et al. 2007). It
is known that GH secreting pituitary adenomas seldomundergodesensitization to
treatment, as acromegalic patients rarely show any signs of tachyphylaxis despite
years of SST-analog therapy. Interestingly, this property is specific to tumors of the
pituitary, as neither islet cell nor most other NETs share this feature; prolonged
administration usually results in desensitization and relapse, as symptoms invariably
return (Lamberts et al. 1996; de Herder et al. 2003; Hofland and Lamberts 2003).
Incidentally, the two SSTRs predominantly expressed in GH hypersecreting pituitary
adenomas are SSTR2 and SSTR5 (Jaquet et al. 2000; Park et al. 2004), therefore,
heterodimerization of these two receptors - as induced by treatment with SST-analogs
- which share higher affinities for SSTR2, could account for this behavior.
SSTRs have not only been shown to form dimers within their family but also with
other related members, such as the dopamine and opioid receptor families. For
instance, when expressed in CHO-K1 cells, human SSTR5 and human dopamine
(D2R) could be triggered to heterodimerize when activated by either a dopamine- or
a SST-related agonist (Rocheville et al. 2000a). Furthermore, heterodimerization
provided positive cooperativity to SST binding, a property that was also related to
enhanced receptor-signaling. Immunohistochemical analysis made evident the pos-
sibility of identifying these heterodimers under normal physiological conditions, as
colocalization of both SSTR5 and D2R were shown in a subset of neurons from both
the cortex and striatum of the rodent (Rocheville et al. 2000a). Recently, a physical
interaction between human SSTR2 and D2R was documented and shown to be regulated
by agonist-binding (Baragli et al. 2007). Interestingly, unlike the SSTR5/D2R het-
erodimer, positive cooperativity was a property observed for D2R, as the binding affinity
of dopamine was markedly enhanced by agonist-bound SSTR2 (Baragli et al. 2007).

Somatostatin and Somatostatin Receptors 165
In addition, combined treatment of SST-14 with either dopamine or the D2R agonist
quinpirole improved signaling efficiency. There have been several indications
suggesting a functional linkage between the somatostatinergic and dopaminergic
systems. For instance, dopamine enhances SSTR-mediated inhibition of adenylate
cyclase in rat striatum and the hippocampus (Rodriguez-Sanchez et al. 1997).
Additionally, SSTR2 has been shown to mediate striatal dopamine release (Hathway
et al. 1999). Although SSTRs are the primary targets in the medical treatment of
acromegaly caused by growth-hormone hypersecreting pituitary adenomas, the
dopamine agonist, cabergoline, provides effective control in 29–39% of patients
(Abs et al. 1998; Cozzi et al. 1998). Incidentally, combination treatment of SST and
dopamine agonists has been shown to be more effective than the activation of SST-
analogs alone (Marzullo et al. 1999). Incidentally, heterodimers between D2R and
SSTR2 were observed in cultured rat striatal neurons (Baragli et al. 2007). The
recent development of chimeric molecules that target both SSTR2 and D2R attest to
these findings and suggest an interaction between both receptors to account for their
behavior (Saveanu et al., 2002, 2006, 2008; Jaquet et al. 2005).
Finally, the SST-analog octreotide, has been observed to behave as an antagonist
in morphine-dependent individuals (Maurer et al. 1982) and patients undergoing
morphine withdrawal have presented with reduced vomiting following octreotide
administration (Bell et al. 1999). Since both receptors, SSTR2 and the m-opioid
receptor (mOR), have been shown to be colocalized in neurons of the locus coeru-
leus (Pfeiffer et al. 2002), a region of the brain implicated in opioid dependency and
withdrawal (Gold et al. 2003), it is not unreasonable to assume that heteromeric
interactions may exist. Indeed, when expressed in HEK-293 cells, heterodimeriza-
tion between SSTR2 and the mOR could be demonstrated (Pfeiffer et al. 2002).
Although it was determined that ligand binding profiles of either the SST-analog
L-779,976 or the mOR agonist DAMGO were unaltered by heterodimerization,
receptor regulations such as phosphorylation, desensitization, and internalization
were affected. For instance, binding of either L-779,976 or DAMGO to the het-
erodimer resulted in cross-phosphorylation of each receptor-subtype (Pfeiffer et al.
2002). Furthermore, this form of heterologous desensitization translated into a loss
of coupling to adenylate cyclase and a diminished MAPK signaling response.
Interestingly, cointernalization of SSTR2 and mOR was only observed following
stimulation of SSTR2 and not by activation of the mOR agonist DAMGO (Pfeiffer
et al. 2002). These results implicate SSTR agonists in the stabilization of this het-
erodimer. A similar finding was reported for the SSTR2/SSTR3 heterodimer; how-
ever, in this case, activation of SSTR2 resulted in its selective-internalization while
SSTR3 was maintained at the cell surface (Pfeiffer et al. 2001).
15 Conclusions
The history of SST has come a long way, from its initial discovery as a hypotha-
lamic regulator of GH secretion from the anterior pituitary, to its role in the anti-
proliferation of tumor growth. Intense investigation surrounds the development of

166 U. Kumar and M. Grant
new SST-analogs with the capability of either targeting a wider distribution of
SSTRs - as is the situation for SOM-230 - or chimeric molecules, that target in
addition to SSTRs, dopamine receptors,and coined dopastatins. Dopastatins are
an exiting new class of NET regulators that are currently under clinical investigation
(Ipsen, Paris, France). In addition, the application of radiolabeled SST-analogs
in PRRT has shown promise in the treatment of patients with inoperable or metas-
tasized NETs. Whatever the ligand, it is the receptor that is the target and with the
understanding of GPCR dimerization, a new dimension in SSTR drug discovery
may unfold. We have come a long way since the development of the first SST-
analog over 30 years ago, and yet the achievements in the field have proven
unrelenting.
Acknowledgments The work cited from the author’s laboratory was supported by grants from
the Canadian Institute of Health research (MOP 6196, MOP 74465), Canadian breast cancer
Foundation BC/Yukon Chapter, Michael Smith foundation of Health Research and Faculty of
Pharmaceutical Sciences.
References
Abs R, Verhelst J, Maiter D et al (1998) Cabergoline in the treatment of acromegaly: a study in
64 patients. J Clin Endocrinol Metab 83:374–378
Aguila MC (1994) Growth hormone-releasing factor increases somatostatin release and mRNA
levels in the rat periventricular nucleus via nitric oxide by activation of guanylate cyclase. Proc
Natl Acad Sci USA 91:782–786
Aguila MC, Dees WL, Haensly WE et al (1991) Evidence that somatostatin is localized and syn-
thesized in lymphoid organs. Proc Natl Acad Sci USA 88:11485–11489
Aguila MC, Rodriguez AM, Aguila-Mansilla HN et al (1996) Somatostatin antisense oligodeox-
ynucleotide-mediated stimulation of lymphocyte proliferation in culture. Endocrinology
137:1585–1590
Akopian A, Johnson J, Gabriel R et al (2000) Somatostatin modulates voltage-gated K(+) and Ca(2+)
currents in rod and cone photoreceptors of the salamander retina. J Neurosci 20:929–936
Alberti KG, Christensen NJ, Christensen SE et al (1973) Inhibition of insulin secretion by soma-
tostatin. Lancet 2:1299–1301
Albini A, Florio T, Giunciuglio D et al (1999) Somatostatin controls Kaposi’s sarcoma tumor
growth through inhibition of angiogenesis. FASEB J 13:647–655
Alderton F, Fan TP, Humphrey PP (2001) Somatostatin receptor-mediated arachidonic acid mobi-
lization: evidence for partial agonism of synthetic peptides. Br J Pharmacol 132:760–766
Altomare DA, Testa JR (2005) Perturbations of the AKT signaling pathway in human cancer.
Oncogene 24:7455–7464
Amherdt M, Patel YC, Orci L (1989) Binding and internalization of somatostatin, insulin, and
glucagon by cultured rat islet cells. J Clin Invest 84:412–417
Arena S, Pattarozzi A, Massa A et al (2007) An intracellular multi-effector complex mediates
somatostatin receptor 1 activation of phospho-tyrosine phosphatase eta. Mol Endocrinol
21:229–246
Arimura A, Schally AV (1976) Increase in basal and thyrotropin-releasing hormone (TRH)-
stimulated secretion of thyrotropin (TSH) by passive immunization with antiserum to somato-
statin in rats. Endocrinology 98:1069–1072
Arimura A, Sato H, Dupont A et al (1975) Somatostatin: abundance of immunoreactive hormone
in rat stomach and pancreas. Science 189:1007–1009

Somatostatin and Somatostatin Receptors 167
Bai M (2004) Dimerization of G-protein-coupled receptors: roles in signal transduction. Cell
Signal 16:175–186
Ballian N, Brunicardi FC, Wang XP (2006) Somatostatin and its receptors in the development of
the endocrine pancreas. Pancreas 33:1–12
Baragli A, Alturaihi H, Watt HL et al (2007) Heterooligomerization of human dopamine receptor
2 and somatostatin receptor 2 Co-immunoprecipitation and fluorescence resonance energy
transfer analysis. Cell Signal 19:2304–2316
Barber DL, McGuire ME, Ganz MB (1989) Beta-adrenergic and somatostatin receptors regulate
Na-H exchange independent of cAMP. J Biol Chem 264:21038–21042
Barinaga M, Bilezikjian LM, Vale WW et al (1985) Independent effects of growth hormone
releasing factor on growth hormone release and gene transcription. Nature 314:279–281
Barkan AL, Dimaraki EV, Jessup SK et al (2003) Ghrelin secretion in humans is sexually dimorphic,
suppressed by somatostatin, and not affected by the ambient growth hormone levels. J Clin
Endocrinol Metab 88:2180–2184
Barnett P (2003) Somatostatin and somatostatin receptor physiology. Endocrine 20:255–264
Barrie R, Woltering EA, Hajarizadeh H et al (1993) Inhibition of angiogenesis by somatostatin
and somatostatin-like compounds is structurally dependent. J Surg Res 55:446–450
Baskin DG, Ensinck JW (1984) Somatostatin in epithelial cells of intestinal mucosa is present
primarily as somatostatin 28. Peptides 5:615–621
Bass RT, Buckwalter BL, Patel BP et al (1996) Identification and characterization of novel soma-
tostatin antagonists. Mol Pharmacol 50:709–715
Bauer W, Briner U, Doepfner W et al (1982) SMS 201–995: a very potent and selective octapeptide
analogue of somatostatin with prolonged action. Life Sci 31:1133–1140
Bayburt TH, Leitz AJ, Xie G et al (2007) Transducin activation by nanoscale lipid bilayers
containing one and two rhodopsins. J Biol Chem 282:14875–14881
Bell JR, Young MR, Masterman SC et al (1999) A pilot study of naltrexone-accelerated detoxifi-
cation in opioid dependence. Med J Aust 171:26–30
Benoit R, Ling N, Esch F (1987) A new prosomatostatin-derived peptide reveals a pattern for
prohormone cleavage at monobasic sites. Science 238:1126–1129
Ben-Shlomo A, Wawrowsky KA, Proekt I et al (2005) Somatostatin receptor type 5 modulates
somatostatin receptor type 2 regulation of adrenocorticotropin secretion. J Biol Chem
280:24011–24021
Benuck M, Marks N (1976) Differences in the degradation of hypothalamic releasing factors by
rat and human serum. Life Sci 19:1271–1276
Berelowitz M, Firestone SL, Frohman LA (1981a) Effects of growth hormone excess and defi-
ciency on hypothalamic somatostatin content and release and on tissue somatostatin distribu-
tion. Endocrinology 109:714–719
Berelowitz M, Szabo M, Frohman LA et al (1981b) Somatomedin-C mediates growth hormone negative
feedback by effects on both the hypothalamus and the pituitary. Science 212:1279–1281
Berelowitz M, Dudlak D, Frohman LA (1982) Release of somatostatin-like immunoreactivity
from incubated rat hypothalamus and cerebral cortex. Effects of glucose and glucoregulatory
hormones. J Clin Invest 69:1293–1301
Berglund MM, Schober DA, Esterman MA et al (2003) Neuropeptide Y Y4 receptor homodimers
dissociate upon agonist stimulation. J Pharmacol Exp Ther 307:1120–1126
Bersani M, Thim L, Baldissera FG et al (1989) Prosomatostatin 1–64 is a major product of soma-
tostatin gene expression in pancreas and gut. J Biol Chem 264:10633–10636
Bevan JS (2005) Clinical review: The antitumoral effects of somatostatin analog therapy in
acromegaly. J Clin Endocrinol Metab 90:1856–1863
Blum AM, Metwali A, Mathew RC et al (1992) Granuloma T lymphocytes in murine schisto-
somiasis mansoni have somatostatin receptors and respond to somatostatin with decreased
IFN-gamma secretion. J Immunol 149:3621–3626
Boudin H, Sarret P, Mazella J et al (2000) Somatostatin-induced regulation of SST(2A) receptor
expression and cellsurface availability in central neurons: role of receptor internalization.
J Neurosci 20:5932–5939

168 U. Kumar and M. Grant
Bousquet C, Delesque N, Lopez F et al (1998) sst2 somatostatin receptor mediates negative regu-
lation of insulin receptor signaling through the tyrosine phosphatase SHP-1. J Biol Chem
273:7099–7106
Bousquet C, Puente E, Buscail L et al (2001) Antiproliferative effect of somatostatin and analogs.
Chemotherapy 47(Suppl 2):30–39
Brazeau P, Vale W, Burgus R et al (1973) Hypothalamic polypeptide that inhibits the secretion of
immunoreactive pituitary growth hormone. Science 179:77–79
Breitwieser GE (2004) G protein-coupled receptor oligomerization: implications for G protein
activation and cell signaling. Circ Res 94:17–27
Bresson JL, Clavequin MC, Fellmann D et al (1984) Ontogeny of the neuroglandular system
revealed with HPGRF 44 antibodies in human hypothalamus. Neuroendocrinology 39:68–73
Bruno JF, Xu Y, Song J et al (1993) Tissue distribution of somatostatin receptor subtype messen-
ger ribonucleic acid in the rat. Endocrinology 133:2561–2567
Bruno JF, Xu Y, Berelowitz M (1994a) Somatostatin regulates somatostatin receptor subtype
mRNA expression in GH3 cells. Biochem Biophys Res Commun 202:1738–1743
Bruno JF, Xu Y, Song J et al (1994b) Pituitary and hypothalamic somatostatin receptor subtype
messenger ribonucleic acid expression in the food-deprived and diabetic rat. Endocrinology
135:1787–1792
Bruns C, Lewis I, Briner U et al (2002) SOM230: a novel somatostatin peptidomimetic with broad
somatotropin release inhibiting factor (SRIF) receptor binding and a unique antisecretory
profile. Eur J Endocrinol 146:707–716
Bugnon C, Fellmann D, Bloch B (1978) Immunocytochemical study of the ontogenesis of
the hypothalamic somatostatin-containing neurons in the human fetus. Metabolism 27:
1161–1165
Buscail L, Delesque N, Esteve JP et al (1994) Stimulation of tyrosine phosphatase and inhibition
of cell proliferation by somatostatin analogues: mediation by human somatostatin receptor
subtypes SSTR1 and SSTR2. Proc Natl Acad Sci USA 91:2315–2319
Buscail L, Esteve JP, Saint-Laurent N et al (1995) Inhibition of cell proliferation by the somato-
statin analogue RC-160 is mediated by somatostatin receptor subtypes SSTR2 and SSTR5
through different mechanisms. Proc Natl Acad Sci USA 92:1580–1584
Canals M, Marcellino D, Fanelli F et al (2003) Adenosine A2A-dopamine D2 receptor-receptor
heteromerization: qualitative and quantitative assessment by fluorescence and biolumines-
cence energy transfer. J Biol Chem 278:46741–46749
Cattaneo MG, Scita G, Vicentini LM (1999) Somatostatin inhibits PDGF-stimulated Ras activa-
tion in human neuroblastoma cells. FEBS Lett 459:64–68
Cattaneo MG, Taylor JE, Culler MD et al (2000) Selective stimulation of somatostatin receptor
subtypes: differential effects on Ras/MAP kinase pathway and cell proliferation in human
neuroblastoma cells. FEBS Lett 481:271–276
Cervera P, Videau C, Viollet C et al (2002) Comparison of somatostatin receptor expression in
human gliomas and medulloblastomas. J Neuroendocrinol 14:458–471
Cescato R, Schulz S, Waser B et al (2006) Internalization of sst2, sst3, and sst5 receptors: effects
of somatostatin agonists and antagonists. J Nucl Med 47:502–511
Cheng ZJ, Miller LJ (2001) Agonist-dependent dissociation of oligomeric complexes of G pro-
tein-coupled cholecystokinin receptors demonstrated in living cells using bioluminescence
resonance energy transfer. J Biol Chem 276:48040–48047
Chihara K, Arimura A, Schally AV (1979) Effect of intraventricular injection of dopamine, nore-
prinephrine, acetylcholine, and 5-hydroxytryptamine on immunoreactive somatostatin release
into rat hypophyseal portal blood. Endocrinology 104:1656–1662
Ciruela F, Burgueno J, Casado V et al (2004) Combining mass spectrometry and pull-down tech-
niques for the study of receptor heteromerization. Direct epitope-epitope electrostatic interac-
tions between adenosine A2A and dopamine D2 receptors. Anal Chem 76:5354–5363
Claing A, Perry SJ, Achiriloaie M et al (2000) Multiple endocytic pathways of G protein-coupled
receptors delineated by GIT1 sensitivity. Proc Natl Acad Sci USA 97:1119–1124
Claing A, Laporte SA, Caron MG et al (2002) Endocytosis of G protein-coupled receptors: roles
of G protein-coupled receptor kinases and beta-arrestin proteins. Prog Neurobiol 66:61–79

Somatostatin and Somatostatin Receptors 169
Cordelier P, Esteve JP, Bousquet C et al (1997) Characterization of the antiproliferative signal
mediated by the somatostatin receptor subtype sst5. Proc Natl Acad Sci USA 94:9343–9348
Cox BA, Rosser MP, Kozlowski MR et al (1995) Regulation and functional characterization of a
rat recombinant dopamine D3 receptor. Synapse 21:1–9
Cozzi R, Attanasio R, Barausse M et al (1998) Cabergoline in acromegaly: a renewed role for
dopamine agonist treatment? Eur J Endocrinol 139:516–521
Csaba Z, Dournaud P (2001) Cellular biology of somatostatin receptors. Neuropeptides 35:1–23
Csaba Z, Bernard V, Helboe L et al (2001) In vivo internalization of the somatostatin sst2A recep-
tor in rat brain: evidence for translocation of cell-surface receptors into the endosomal recy-
cling pathway. Mol Cell Neurosci 17:646–661
Csaba Z, Simon A, Helboe L et al (2002) Neurochemical characterization of receptor-expressing
cell populations by in vivo agonist-induced internalization: insights from the somatostatin
sst2A receptor. J Comp Neurol 454:192–199
Csaba Z, Simon A, Helboe L et al (2003) Targeting sst2A receptor-expressing cells in the rat
hypothalamus through in vivo agonist stimulation: neuroanatomical evidence for a major role
of this subtype in mediating somatostatin functions. Endocrinology 144:1564–1573
Cvejic S, Devi LA (1997) Dimerization of the delta opioid receptor: implication for a role in
receptor internalization. J Biol Chem 272:26959–26964
Dalm VA, Van Hagen PM, de Krijger RR et al (2004) Distribution pattern of somatostatin and
cortistatin mRNA in human central and peripheral tissues. Clin Endocrinol (Oxf) 60:625–629
Danesi R, Del Tacca M (1996) The effects of the somatostatin analog octreotide on angiogenesis
in vitro. Metabolism 45:49–50
Danesi R, Agen C, Benelli U et al (1997) Inhibition of experimental angiogenesis by the somato-
statin analogue octreotide acetate (SMS 201–995). Clin Cancer Res 3:265–272
Danila DC, Haidar JN, Zhang X et al (2001) Somatostatin receptor-specific analogs: effects on
cell proliferation and growth hormone secretion in human somatotroph tumors. J Clin
Endocrinol Metab 86:2976–2981
Dasgupta P, Mukherjee R (2000) Lipophilization of somatostatin analog RC-160 with long chain
fatty acid improves its antiproliferative and antiangiogenic activity in vitro. Br J Pharmacol
129:101–109
Day R, Dong W, Panetta R et al (1995) Expression of mRNA for somatostatin receptor (sstr) types
2 and 5 in individual rat pituitary cells. A double labeling in situ hybridization analysis.
Endocrinology 136:5232–5235
de Herder WW, Lamberts SW (2005) Somatostatin analogs as radiodiagnostic tools. Rev Endocr
Metab Disord 6:23–27
de Herder WW, Hofland LJ, van der Lely AJ et al (2003) Somatostatin receptors in gastroentero-
pancreatic neuroendocrine tumours. Endocr Relat Cancer 10:451–458
de Lecea L, Criado JR, Prospero-Garcia O et al (1996) A cortical neuropeptide with neuronal
depressant and sleep-modulating properties. Nature 381:242–245
de Weille JR, Schmid-Antomarchi H, Fosset M et al (1989) Regulation of ATP-sensitive K+ chan-
nels in insulinoma cells: activation by somatostatin and protein kinase C and the role of cAMP.
Proc Natl Acad Sci USA 86:2971–2975
Dent P, Wang Y, Gu YZ et al (1997) S49 cells endogenously express subtype 2 somatostatin recep-
tors which couple to increase protein tyrosine phosphatase activity in membranes and down-
regulate Raf-1 activity in situ. Cell Signal 9:539–549
Dhanasekaran N, Heasley LE, Johnson GL (1995) G protein-coupled receptor systems involved
in cell growth and oncogenesis. Endocr Rev 16:259–270
Djordjijevic D, Zhang J, Priam M et al (1998) Effect of 17beta-estradiol on somatostatin receptor
expression and inhibitory effects on growth hormone and prolactin release in rat pituitary cell
cultures. Endocrinology 139:2272–2277
Douziech N, Calvo E, Coulombe Z et al (1999) Inhibitory and stimulatory effects of somatostatin
on two human pancreatic cancer cell lines: a primary role for tyrosine phosphatase SHP-1.
Endocrinology 140:765–777
Dubois MP (1975) Immunoreactive somatostatin is present in discrete cells of the endocrine pan-
creas. Proc Natl Acad Sci USA 72:1340–1343

170 U. Kumar and M. Grant
Duran-Prado M, Bucharles C, Gonzalez BJ et al (2007) Porcine somatostatin receptor 2 displays
typical pharmacological sst2 features but unique dynamics of homodimerization and internali-
zation. Endocrinology 148:411–421
Elliott DE (2004) Expression and function of somatostatin and its receptors in immune cells. In:
Srikant CB (ed) Somatostatin. Kluwer, Norwell, pp 169–184
Elliott DE, Metwali A, Blum AM et al (1994) T lymphocytes isolated from the hepatic granulo-
mas of schistosome-infected mice express somatostatin receptor subtype II (SSTR2) messen-
ger RNA. J Immunol 153:1180–1186
Ensinck JW, Laschansky EC, Vogel RE et al (1989) Circulating prosomatostatin-derived peptides.
Differential responses to food ingestion. J Clin Invest 83:1580–1589
Epelbaum J, Dournaud P, Fodor M et al (1994) The neurobiology of somatostatin. Crit Rev
Neurobiol 8:25–44
Eriksson B, Oberg K (1999) Summing up 15 years of somatostatin analog therapy in neuroendo-
crine tumors: future outlook. Ann Oncol 10(Suppl 2):S31–S38
Fan T, Varghese G, Nguyen T et al (2005) A role for the distal carboxyl tails in generating the
novel pharmacology and G protein activation profile of mu and delta opioid receptor hetero-
oligomers. J Biol Chem 280:38478–38488
Fedele M, De Martino I, Pivonello R et al (2007) SOM230, a new somatostatin analogue, is highly
effective in the therapy of growth hormone/prolactin-secreting pituitary adenomas. Clin
Cancer Res 13:2738–2744
Feniuk W, Jarvie E, Luo J et al (2000) Selective somatostatin sst(2) receptor blockade with the
novel cyclic octapeptide, CYN-154806. Neuropharmacology 39:1443–1450
Ferjoux G, Bousquet C, Cordelier P et al (2000) Signal transduction of somatostatin receptors
negatively controlling cell proliferation. J Physiol Paris 94:205–210
Ferjoux G, Lopez F, Esteve JP et al (2003) Critical role of Src and SHP-2 in sst2 somatostatin
receptor-mediated activation of SHP-1 and inhibition of cell proliferation. Mol Biol Cell
14:3911–3928
Ferland L, Labrie F, Jobin M et al (1976) Physiological role of somatostatin in the control of
growth hormone and thyrotropin secretion. Biochem Biophys Res Commun 68:149–156
Ferrante E, Pellegrini C, Bondioni S et al (2006) Octreotide promotes apoptosis in human
somatotroph tumor cells by activating somatostatin receptor type 2. Endocr Relat Cancer
13:955–962
Finley JC, Maderdrut JL, Roger LJ et al (1981) The immunocytochemical localization of somato-
statin-containing neurons in the rat central nervous system. Neuroscience 6:2173–2192
Florio T (2008) Molecular mechanisms of the antiproliferative activity of somatostatin receptors
(SSTRs) in neuroendocrine tumors. Front Biosci 13:822–840
Florio T, Rim C, Hershberger RE et al (1994) The somatostatin receptor SSTR1 is coupled to
phosphotyrosine phosphatase activity in CHO-K1 cells. Mol Endocrinol 8:1289–1297
Florio T, Scorziello A, Thellung S et al (1997) Oncogene transformation of PC Cl3 clonal thyroid
cell line induces an autonomous pattern of proliferation that correlates with a loss of basal and
stimulated phosphotyrosine phosphatase activity. Endocrinology 138:3756–3763
Florio T, Yao H, Carey KD et al (1999) Somatostatin activation of mitogen-activated protein
kinase via somatostatin receptor 1 (SSTR1). Mol Endocrinol 13:24–37
Florio T, Thellung S, Arena S et al (2000) Somatostatin receptor 1 (SSTR1)-mediated inhibition
of cell proliferation correlates with the activation of the MAP kinase cascade: role of the phos-
photyrosine phosphatase SHP-2. J Physiol Paris 94:239–250
Florio T, Arena S, Thellung S et al (2001) The activation of the phosphotyrosine phosphatase eta
(r-PTP eta) is responsible for the somatostatin inhibition of PC Cl3 thyroid cell proliferation.
Mol Endocrinol 15:1838–1852
Florio T, Morini M, Villa V et al (2003a) Somatostatin inhibits tumor angiogenesis and growth via
somatostatin receptor-3-mediated regulation of endothelial nitric oxide synthase and mitogen-
activated protein kinase activities. Endocrinology 144:1574–1584
Florio T, Thellung S, Corsaro A et al (2003b) Characterization of the intracellular mechanisms
mediating somatostatin and lanreotide inhibition of DNA synthesis and growth hormone

Somatostatin and Somatostatin Receptors 171
release from dispersed human GH-secreting pituitary adenoma cells in vitro. Clin Endocrinol
(Oxf) 59:115–128
Franco R, Canals M, Marcellino D et al (2003) Regulation of heptaspanning-membrane-receptor
function by dimerization and clustering. Trends Biochem Sci 28:238–243
Frankel BJ, Heldt AM, Grodsky GM (1982) Effects of K+ and arginine on insulin, glucagon, and
somatostatin release from the in vitro perfused rat pancreas. Endocrinology 110:428–431
Fukusumi S, Kitada C, Takekawa S et al (1997) Identification and characterization of a novel
human cortistatin-like peptide. Biochem Biophys Res Commun 232:157–163
Fuller PJ, Verity K (1989) Somatostatin gene expression in the thymus gland. J Immunol
143:1015–1017
Funckes CL, Minth CD, Deschenes R et al (1983) Cloning and characterization of a mRNA-
encoding rat preprosomatostatin. J Biol Chem 258:8781–8787
Garcia de la Torre N, Wass JA, Turner HE (2002) Antiangiogenic effects of somatostatin ana-
logues. Clin Endocrinol (Oxf) 57:425–441
Gardette R, Petit F, Peineau S et al (2004) Molecular evolution of somatostatin genes. In: Srikant
CB (ed) Somatostatin. Kluwer, Norwell, pp 123–142
Geci C, How J, Alturaihi H et al (2007) Beta-amyloid increases somatostatin expression in cul-
tured cortical neurons. J Neurochem 101:664–673
German MS, Moss LG, Rutter WJ (1990) Regulation of insulin gene expression by glucose and
calcium in transfected primary islet cultures. J Biol Chem 265:22063–22066
Gines S, Hillion J, Torvinen M et al (2000) Dopamine D1 and adenosine A1 receptors form func-
tionally interacting heteromeric complexes. Proc Natl Acad Sci USA 97:8606–8611
Gold SJ, Han MH, Herman AE et al (2003) Regulation of RGS proteins by chronic morphine in
rat locus coeruleus. Eur J Neurosci 17:971–980
Goodman RH, Lund PK, Jacobs JW et al (1980) Pre-prosomatostatins. Products of cell-free trans-
lations of messenger RNAs from anglerfish islets. J Biol Chem 255:6549–6552
Goodman RH, Lund PK, Barnett FH et al (1981) Intestinal pre-prosomatostatin. Identification of
mRNA coding for a precursor by cell-free translations and hybridization with a cloned islet
cDNA. J Biol Chem 256:1499–1501
Goodman RH, Jacobs JW, Dee PC et al (1982) Somatostatin-28 encoded in a cloned cDNA
obtained from a rat medullary thyroid carcinoma. J Biol Chem 257:1156–1159
Gottero C, Prodam F, Destefanis S et al (2004) Cortistatin-17 and -14 exert the same endocrine
activities as somatostatin in humans. Growth Horm IGF Res 14:382–387
Grant M, Collier B, Kumar U (2004a) Agonist-dependent dissociation of human somatostatin
receptor 2 dimers: a role in receptor trafficking. J Biol Chem 279:36179–36183
Grant M, Patel RC, Kumar U (2004b) The role of subtype-specific ligand binding and the C-tail
domain in dimer formation of human somatostatin receptors. J Biol Chem 279:38636–38643
Grant M, Alturaihi H, Jaquet P et al (2008a) Cell growth inhibition and functioning of human
somatostatin receptor type 2 are modulated by receptor heterodimerization. Mol Endocrinol
22:2278–2292
Grant M, Alturaihi H, Jaquet P et al (2008) Cell growth inhibition and functioning of human
somatostatin receptor type 2 are modulated by receptor heterodimerization. Mol Endocrinol
22:2278–2292
Greenman Y, Melmed S (1994a) Heterogeneous expression of two somatostatin receptor subtypes
in pituitary tumors. J Clin Endocrinol Metab 78:398–403
Greenman Y, Melmed S (1994b) Expression of three somatostatin receptor subtypes in pituitary
adenomas: evidence for preferential SSTR5 expression in the mammosomatotroph lineage. J
Clin Endocrinol Metab 79:724–729
Gripentrog JM, Kantele KP, Jesaitis AJ et al (2003) Experimental evidence for lack of homodimer-
ization of the G protein-coupled human N-formyl peptide receptor. J Immunol
171:3187–3193
Gromada J, Hoy M, Buschard K et al (2001) Somatostatin inhibits exocytosis in rat pancreatic
alpha-cells by G(i2)-dependent activation of calcineurin and depriming of secretory granules.
J Physiol 535:519–532

172 U. Kumar and M. Grant
Grozinsky-Glasberg S, Grossman AB, Korbonits M (2008a) The role of somatostatin analogues
in the treatment of neuroendocrine tumours. Mol Cell Endocrinol 286:238–250
Grozinsky-Glasberg S, Shimon I, Korbonits M et al (2008b) Somatostatin analogues in the control
of neuroendocrine tumours: efficacy and mechanisms. Endocr Relat Cancer 15:701–720
Grozinsky-Glasberg S, Franchi G, Teng M et al (2008c) Octreotide and the mTOR inhibitor
RAD001 (everolimus) block proliferation and interact with the Akt-mTOR-p70S6K pathway
in a neuro-endocrine tumour cell Line. Neuroendocrinology 87:168–181
Gu YZ, Schonbrunn A (1997) Coupling specificity between somatostatin receptor sst2A and G
proteins: isolation of the receptor-G protein complex with a receptor antibody. Mol Endocrinol
11:527–537
Guillermet J, Saint-Laurent N, Rochaix P et al (2003) Somatostatin receptor subtype 2 sensitizes
human pancreatic cancer cells to death ligand-induced apoptosis. Proc Natl Acad Sci USA
100:155–160
Gyr K, Beglinger C, Kohler E et al (1987) Circulating somatostatin. Physiological regulator of
pancreatic function? J Clin Invest 79:1595–1600
Hansen JL, Sheikh SP (2004) Functional consequences of 7TM receptor dimerization. Eur J
Pharm Sci 23:301–317
Hathway GJ, Humphrey PP, Kendrick KM (1999) Evidence that somatostatin sst2 receptors medi-
ate striatal dopamine release. Br J Pharmacol 128:1346–1352
Hayry P, Raisanen A, Ustinov J et al (1993) Somatostatin analog lanreotide inhibits myocyte
replication and several growth factors in allograft arteriosclerosis. Faseb J 7:1055–1060
Hellman B, Lernmark A (1969) Inhibition of the in vitro secretion of insulin by an extract of
pancreatic alpha-1 cells. Endocrinology 84:1484–1488
Hierowski MT, Liebow C, du Sapin K et al (1985) Stimulation by somatostatin of dephosphoryla-
tion of membrane proteins in pancreatic cancer MIA PaCa-2 cell line. FEBS Lett
179:252–256
Hipkin RW, Friedman J, Clark RB et al (1997) Agonist-induced desensitization, internalization,
and phosphorylation of the sst2A somatostatin receptor. J Biol Chem 272:13869–13876
Hipkin RW, Wang Y, Schonbrunn A (2000) Protein kinase C activation stimulates the phosphor-
ylation and internalization of the sst2A somatostatin receptor. J Biol Chem 275:5591–5599
Hofland LJ, Lamberts SW (2003) The pathophysiological consequences of somatostatin receptor
internalization and resistance. Endocr Rev 24:28–47
Hofland LJ, van Koetsveld PM, Waaijers M et al (1995) Internalization of the radioiodinated
somatostatin analog [125I-Tyr3]octreotide by mouse and human pituitary tumor cells: increase
by unlabeled octreotide. Endocrinology 136:3698–3706
Hofland LJ, Breeman WA, Krenning EP et al (1999) Internalization of [DOTA degrees, 125I-
Tyr3] Octreotide by somatostatin receptor-positive cells in vitro and in vivo: implications for
somatostatin receptor-targeted radio-guided surgery. Proc Assoc Am Physicians 111:63–69
Hokfelt T, Efendic S, Hellerstrom C et al (1975) Cellular localization of somatostatin in endo-
crine-like cells and neurons of the rat with special references to the A1-cells of the pancreatic
islets and to the hypothalamus. Acta Endocrinol Suppl (Copenh) 200:5–41
Hook VY, Azaryan AV, Hwang SR et al (1994) Proteases and the emerging role of protease inhibi-
tors in prohormone processing. FASEB J 8:1269–1278
Hortala M, Ferjoux G, Estival A et al (2003) Inhibitory role of the somatostatin receptor SST2 on
the intracrine-regulated cell proliferation induced by the 210-amino acid fibroblast growth
factor-2 isoform: implication of JAK2. J Biol Chem 278:20574–20581
Horvath S, Palkovits M, Gorcs T et al (1989) Electron microscopic immunocytochemical evidence
for the existence of bidirectional synaptic connections between growth hormone-releasing
hormone- and somatostatin-containing neurons in the hypothalamus of the rat. Brain Res
481:8–15
Hou C, Gilbert RL, Barber DL (1994) Subtype-specific signaling mechanisms of somatostatin
receptors SSTR1 and SSTR2. J Biol Chem 269:10357–10362
Hoyer D, Nunn C, Hannon J et al (2004) SRA880, in vitro characterization of the first non-peptide
somatostatin sst(1) receptor antagonist. Neurosci Lett 361:132–135

Somatostatin and Somatostatin Receptors 173
Hukovic N, Panetta R, Kumar U et al (1996) Agonist-dependent regulation of cloned human
somatostatin receptor types 1–5 (hSSTR1–5): subtype selective internalization or upregula-
tion. Endocrinology 137:4046–4049
Hukovic N, Panetta R, Kumar U et al (1998) The cytoplasmic tail of the human somatostatin
receptor type 5 is crucial for interaction with adenylyl cyclase and in mediating desensitization
and internalization. J Biol Chem 273:21416–21422
Hukovic N, Rocheville M, Kumar U et al (1999) Agonist-dependent up-regulation of human
somatostatin receptor type 1 requires molecular signals in the cytoplasmic C-tail. J Biol Chem
274:24550–24558
Ikeda SR, Schofield GG (1989) Somatostatin blocks a calcium current in rat sympathetic ganglion
neurones. J Physiol 409:221–240
Jais P, Terris B, Ruszniewski P et al (1997) Somatostatin receptor subtype gene expression in
human endocrine gastroentero-pancreatic tumours. Eur J Clin Invest 27:639–644
James RA, Sarapura VD, Bruns C et al (1997) Thyroid hormone-induced expression of specific
somatostatin receptor subtypes correlates with involution of the TtT-97 murine thyrotrope
tumor. Endocrinology 138:719–724
James JR, Oliveira MI, Carmo AM et al (2006) A rigorous experimental framework for detecting
protein oligomerization using bioluminescence resonance energy transfer. Nat Methods
3:1001–1006
Jaquet P, Ouafik L, Saveanu A et al (1999) Quantitative and functional expression of somatostatin
receptor subtypes in human prolactinomas. J Clin Endocrinol Metab 84:3268–3276
Jaquet P, Saveanu A, Gunz G et al (2000) Human somatostatin receptor subtypes in acromegaly:
distinct patterns of messenger ribonucleic acid expression and hormone suppression identify
different tumoral phenotypes. J Clin Endocrinol Metab 85:781–792
Jaquet P, Gunz G, Saveanu A et al (2005) Efficacy of chimeric molecules directed towards multi-
ple somatostatin and dopamine receptors on inhibition of GH and prolactin secretion from
GH-secreting pituitary adenomas classified as partially responsive to somatostatin analog
therapy. Eur J Endocrinol 153:135–141
Jiang H, Betancourt L, Smith RG (2006) Ghrelin amplifies dopamine signaling by cross talk
involving formation of growth hormone secretagogue receptor/dopamine receptor subtype 1
heterodimers. Mol Endocrinol 20:1772–1785
Johansson O, Hokfelt T, Elde RP (1984) Immunohistochemical distribution of somatostatin-like
immunoreactivity in the central nervous system of the adult rat. Neuroscience 13:265–339
Joseph-Bravo P, Charli JL, Sherman T et al (1980) Identification of a putative hypothalamic
mRNA coding for somatostatin and of its product in cell-free translation. Biochem Biophys
Res Commun 94:1004–1012
Kanatsuka A, Makino H, Matsushima Y et al (1981) Effect of calcium on the secretion of soma-
tostatin and insulin from pancreatic islets. Endocrinology 108:2254–2257
Karalis K, Mastorakos G, Chrousos GP et al (1994) Somatostatin analogues suppress the inflam-
matory reaction in vivo. J Clin Invest 93:2000–2006
Katakami H, Downs TR, Frohman LA (1988) Inhibitory effect of hypothalamic medial preoptic
area somatostatin on growth hormone-releasing factor in the rat. Endocrinology
123:1103–1109
Kearn CS, Blake-Palmer K, Daniel E et al (2005) Concurrent stimulation of cannabinoid CB1 and
dopamine D2 receptors enhances heterodimer formation: a mechanism for receptor cross-talk?
Mol Pharmacol 67:1697–1704
Kendall DM, Poitout V, Olson LK et al (1995) Somatostatin coordinately regulates glucagon gene
expression and exocytosis in HIT-T15 cells. J Clin Invest 96:2496–2502
Kimura N, Tomizawa S, Arai KN et al (1998) Chronic treatment with estrogen up-regulates
expression of sst2 messenger ribonucleic acid (mRNA) but down-regulates expression of sst5
mRNA in rat pituitaries. Endocrinology 139:1573–1580
Kleinman R, Gingerich R, Ohning G et al (1995) The influence of somatostatin on glucagon and
pancreatic polypeptide secretion in the isolated perfused human pancreas. Int J Pancreatol
18:51–57

174 U. Kumar and M. Grant
Kleuss C, Hescheler J, Ewel C et al (1991) Assignment of G-protein subtypes to specific receptors
inducing inhibition of calcium currents. Nature 353:43–48
Koenig JA, Edwardson JM, Humphrey PP (1997) Somatostatin receptors in Neuro2A neuroblas-
toma cells: ligand internalization. Br J Pharmacol 120:52–59
Koerker DJ, Ruch W, Chideckel E et al (1974) Somatostatin: hypothalamic inhibitor of the endo-
crine pancreas. Science 184:482–484
Koizumi M, Onda M, Tanaka N et al (2002) Antiangiogenic effect of octreotide inhibits the
growth of human rectal neuroendocrine carcinoma. Digestion 65:200–206
Kreienkamp HJ, Roth A, Richter D (1998) Rat somatostatin receptor subtype 4 can be made sensi-
tive to agonist-induced internalization by mutation of a single threonine (residue 331). DNA
Cell Biol 17:869–878
Kreienkamp HJ, Liew CW, Bachner D et al (2004) Physiology of somatostatin receptors: from
genetics to molecular analysis. In: Srikant CB (ed) Somatostatin. Kluwer, Norwell, pp
185–202
Kreuzer OJ, Krisch B, Dery O et al (2001) Agonist-mediated endocytosis of rat somatostatin
receptor subtype 3 involves beta-arrestin and clathrin coated vesicles. J Neuroendocrinol
13:279–287
Krisch B, Feindt J, Mentlein R (1998) Immunoelectronmicroscopic analysis of the ligand-induced
internalization of the somatostatin receptor subtype 2 in cultured human glioma cells. J
Histochem Cytochem 46:1233–1242
Kroeger KM, Pfleger KD, Eidne KA (2003) G-protein coupled receptor oligomerization in neu-
roendocrine pathways. Front Neuroendocrinol 24:254–278
Krulich L, Dhariwal AP, McCann SM (1968) Stimulatory and inhibitory effects of purified
hypothalamic extracts on growth hormone release from rat pituitary in vitro. Endocrinology
83:783–790
Kumar U (2004) Characterization of striatal cultures with the effect of QUIN and NMDA.
Neurosci Res 49:29–38
Kumar U (2005) Expression of somatostatin receptor subtypes (SSTR1–5) in Alzheimer’s disease
brain: an immunohistochemical analysis. Neuroscience 134:525–538
Kumar U (2007) Colocalization of somatostatin receptor subtypes (SSTR1–5) with somatostatin,
NADPH-diaphorase (NADPH-d), and tyrosine hydroxylase in the rat hypothalamus. J Comp
Neurol 504:185–205
Kumar U (2008) Somatostatin in medium-sized aspiny interneurons of striatum is responsible for
their preservation in quinolinic acid and N-methyl-d-asparate-induced neurotoxicity. J Mol
Neurosci 35:345–354
Kumar U, Asotra K, Patel SC et al (1997) Expression of NMDA receptor-1 (NR1) and huntingtin
in striatal neurons which colocalize somatostatin, neuropeptide Y, and NADPH diaphorase: a
double-label histochemical and immunohistochemical study. Exp Neurol 145:412–424
Kumar U, Sasi R, Suresh S et al (1999) Subtype-selective expression of the five somatostatin
receptors (hSSTR1–5) in human pancreatic islet cells: a quantitative double-label immunohis-
tochemical analysis. Diabetes 48:77–85
Kuner R, Kohr G, Grunewald S et al (1999) Role of heteromer formation in GABAB receptor
function. Science 283:74–77
Kwekkeboom DJ, de Jong M, Krenning EP (2004) Somatostatin receptor imaging. In: Srikant CB
(ed) Somatostatin. Kluwer, Norwell, pp 203–214
Labeur M, Theodoropoulou M, Sievers C et al (2006) New aspects in the diagnosis and treatment
of Cushing disease. Front Horm Res 35:169–178
Lahlou H, Saint-Laurent N, Esteve JP et al (2003) sst2 Somatostatin receptor inhibits cell prolif-
eration through Ras-, Rap1-, and B-Raf-dependent ERK2 activation. J Biol Chem
278:39356–39371
Lamberts SW, Krenning EP, Reubi JC (1991) The role of somatostatin and its analogs in the
diagnosis and treatment of tumors. Endocr Rev 12:450–482
Lamberts SW, van der Lely AJ, de Herder WW et al (1996) Octreotide. N Engl J Med
334:246–254

Somatostatin and Somatostatin Receptors 175
Lamberts SW, de Herder WW, Hofland LJ (2002) Somatostatin analogs in the diagnosis and treat-
ment of cancer. Trends Endocrinol Metab 13:451–457
Latif R, Graves P, Davies TF (2002) Ligand-dependent inhibition of oligomerization at the human
thyrotropin receptor. J Biol Chem 277:45059–45067
Law PY, Erickson-Herbrandson LJ, Zha QQ et al (2005) Heterodimerization of mu- and delta-
opioid receptors occurs at the cell surface only and requires receptor-G protein interactions. J
Biol Chem 280:11152–11164
Lefkowitz RJ (2004) Historical review: a brief history and personal retrospective of seven-trans-
membrane receptors. Trends Pharmacol Sci 25:413–422
Lefkowitz RJ, Shenoy SK (2005) Transduction of receptor signals by beta-arrestins. Science
308:512–517
Liapakis G, Hoeger C, Rivier J et al (1996) Development of a selective agonist at the somatostatin
receptor subtype sstr1. J Pharmacol Exp Ther 276:1089–1094
Lin CY, Varma MG, Joubel A et al (2003) Conserved motifs in somatostatin, D2-dopamine, and
alpha 2B-adrenergic receptors for inhibiting the Na-H exchanger, NHE1. J Biol Chem
278:15128–15135
Liu Q, Schonbrunn A (2001) Agonist-induced phosphorylation of somatostatin receptor subtype
1 (sst1). Relationship to desensitization and internalization. J Biol Chem 276:3709–3717
Liu YF, Jakobs KH, Rasenick MM et al (1994) G protein specificity in receptor-effector coupling.
Analysis of the roles of G0 and Gi2 in GH4C1 pituitary cells. J Biol Chem 269:
13880–13886
Liu D, Martino G, Thangaraju M et al (2000) Caspase-8-mediated intracellular acidification pre-
cedes mitochondrial dysfunction in somatostatin-induced apoptosis. J Biol Chem
275:9244–9250
Lohse MJ, Benovic JL, Codina J et al (1990) Beta-arrestin: a protein that regulates beta-adrenergic
receptor function. Science 248:1547–1550
Lopez F, Esteve JP, Buscail L et al (1997) The tyrosine phosphatase SHP-1 associates with the
sst2 somatostatin receptor and is an essential component of sst2-mediated inhibitory growth
signaling. J Biol Chem 272:24448–24454
Lopez F, Ferjoux G, Cordelier P et al (2001) Neuronal nitric oxide synthase: a substrate for SHP-1
involved in sst2 somatostatin receptor growth inhibitory signaling. FASEB J 15:2300–2302
Losa M, Ciccarelli E, Mortini P et al (2001) Effects of octreotide treatment on the proliferation
and apoptotic index of GH-secreting pituitary adenomas. J Clin Endocrinol Metab
86:5194–5200
Low MJ, Otero-Corchon V, Parlow AF et al (2001) Somatostatin is required for masculinization
of growth hormone-regulated hepatic gene expression but not of somatic growth. J Clin Invest
107:1571–1580
Luft R, Efendic S, Hokfelt T et al (1974) Immunohistochemical evidence for the localization of
somatostatin-like immunoreactivity in a cell population of the pancreatic islets. Med Biol
52:428–430
Mandarino L, Stenner D, Blanchard W et al (1981) Selective effects of somatostatin-14, -25 and
-28 on in vitro insulin and glucagon secretion. Nature 291:76–77
Marks N, Stern F, Benuck M (1976) Correlation between biological potency and biodegradation
of a somatostatin analogue. Nature 261:511–512
Marzullo P, Ferone D, Di Somma C et al (1999) Efficacy of combined treatment with lanreotide
and cabergoline in selected therapy-resistant acromegalic patients. Pituitary 1:115–120
Massa A, Barbieri F, Aiello C et al (2004a) The expression of the phosphotyrosine phosphatase
DEP-1/PTPeta dictates the responsivity of glioma cells to somatostatin inhibition of cell pro-
liferation. J Biol Chem 279:29004–29012
Massa A, Barbieri F, Aiello C et al (2004b) The phosphotyrosine phosphatase eta mediates soma-
tostatin inhibition of glioma proliferation via the dephosphorylation of ERK1/2. Ann N Y
Acad Sci 1030:264–274
Maurer R, Gaehwiler BH, Buescher HH et al (1982) Opiate antagonistic properties of an octapeptide
somatostatin analog. Proc Natl Acad Sci USA 79:4815–4817

176 U. Kumar and M. Grant
McGraw DW, Mihlbachler KA, Schwarb MR et al (2006) Airway smooth muscle prostaglandin-
EP1 receptors directly modulate beta2-adrenergic receptors within a unique heterodimeric
complex. J Clin Invest 116:1400–1409
Mellado M, Rodriguez-Frade JM, Vila-Coro AJ et al (2001) Chemokine receptor homo- or het-
erodimerization activates distinct signaling pathways. Embo J 20:2497–2507
Melmed S, Sternberg R, Cook D et al (2005) A critical analysis of pituitary tumor shrinkage dur-
ing primary medical therapy in acromegaly. J Clin Endocrinol Metab 90:4405–4410
Meriney SD, Gray DB, Pilar GR (1994) Somatostatin-induced inhibition of neuronal Ca2+ current
modulated by cGMP-dependent protein kinase. Nature 369:336–339
Meyer BH, Segura JM, Martinez KL et al (2006) FRET imaging reveals that functional neuroki-
nin-1 receptors are monomeric and reside in membrane microdomains of live cells. Proc Natl
Acad Sci USA 103:2138–2143
Meyerhof W (1998) The elucidation of somatostatin receptor functions: a current view. Rev
Physiol Biochem Pharmacol 133:55–108
Miller GM, Alexander JM, Bikkal HA et al (1995) Somatostatin receptor subtype gene expression
in pituitary adenomas. J Clin Endocrinol Metab 80:1386–1392
Milligan G (2008) A day in the life of a G protein-coupled receptor: the contribution to function
of G protein-coupled receptor dimerization. Br J Pharmacol 153(Suppl 1):S216–S229
Moller LN, Stidsen CE, Hartmann B et al (2003) Somatostatin receptors. Biochim Biophys Acta
1616:1–84
Montminy MR, Low MJ, Tapia-Arancibia L et al (1986) Cyclic AMP regulates somatostatin
mRNA accumulation in primary diencephalic cultures and in transfected fibroblast cells. J
Neurosci 6:1171–1176
Morel G, Leroux P, Pelletier G (1985) Ultrastructural autoradiographic localization of somatosta-
tin-28 in the rat pituitary gland. Endocrinology 116:1615–1620
Mouchantaf R, Kumar U, Sulea T et al (2001) A conserved alpha-helix at the amino terminus of
prosomatostatin serves as a sorting signal for the regulated secretory pathway. J Biol Chem
276:26308–26316
Mouchantaf R, Patel YC, Kumar U (2004a) Processing and intracellular targeting of somatostatin.
In: Srikant CB (ed) Somatostatin. Kluwer, Norwell, pp 16–27
Mouchantaf R, Watt HL, Sulea T et al (2004b) Prosomatostatin is proteolytically processed at the
amino terminal segment by subtilase SKI-1. Regul Pept 120:133–140
Murray RD, Kim K, Ren SG et al (2004) Central and peripheral actions of somatostatin on the
growth hormone-IGF-I axis. J Clin Invest 114:349–356
Neel NF, Schutyser E, Sai J et al (2005) Chemokine receptor internalization and intracellular traf-
ficking. Cytokine Growth Factor Rev 16:637–658
Nelson-Piercy C, Hammond PJ, Gwilliam ME et al (1994) Effect of a new oral somatostatin ana-
log (SDZ CO 611) on gastric emptying, mouth to cecum transit time, and pancreatic and gut
hormone release in normal male subjects. J Clin Endocrinol Metab 78:329–336
Ng GY, Varghese G, Chung HT et al (1997) Resistance of the dopamine D2L receptor to desen-
sitization accompanies the up-regulation of receptors on to the surface of Sf9 cells.
Endocrinology 138:4199–4206
Nichols B (2003) Caveosomes and endocytosis of lipid rafts. J Cell Sci 116:4707–4714
Nouel D, Gaudriault G, Houle M et al (1997) Differential internalization of somatostatin in COS-7
cells transfected with SST1 and SST2 receptor subtypes: a confocal microscopic study using
novel fluorescent somatostatin derivatives. Endocrinology 138:296–306
Nunn C, Langenegger D, Hurth K et al (2003) Agonist properties of putative small-molecule
somatostatin sst2 receptor-selective antagonists. Eur J Pharmacol 465:211–218
O’Carroll AM, Krempels K (1995) Widespread distribution of somatostatin receptor messenger
ribonucleic acids in rat pituitary. Endocrinology 136:5224–5227
Olias G, Viollet C, Kusserow H et al (2004) Regulation and function of somatostatin receptors. J
Neurochem 89:1057–1091
Oomen SP, Hofland LJ, Lamberts SW et al (2001) Internalization-defective mutants of somatosta-
tin receptor subtype 2 exert normal signaling functions in hematopoietic cells. FEBS Lett
503:163–167

Somatostatin and Somatostatin Receptors 177
Orci L, Baetens D, Dubois MP et al (1975) Evidence for the D-cell of the pancreas secreting
somatostatin. Horm Metab Res 7:400–402
Oyama H, O’Connell K, Permutt A (1980) Cell-mediated synthesis of somatostatin. Endocrinology
107:845–847
Pagliacci MC, Tognellini R, Grignani F et al (1991) Inhibition of human breast cancer cell (MCF-
7) growth in vitro by the somatostatin analog SMS 201–995: effects on cell cycle parameters
and apoptotic cell death. Endocrinology 129:2555–2562
Panetta R, Patel YC (1995) Expression of mRNA for all five human somatostatin receptors
(hSSTR1–5) in pituitary tumors. Life Sci 56:333–342
Papotti M, Tarabra E, Allia E et al (2003) Presence of cortistatin in the human pancreas. J
Endocrinol Invest 26:RC15–RC18
Park C, Yang I, Woo J et al (2004) Somatostatin (SRIF) receptor subtype 2 and 5 gene expression
in growth hormone-secreting pituitary adenomas: the relationship with endogenous srif activ-
ity and response to octreotide. Endocr J 51:227–236
Patch D, Burroughs A (2002) Vapreotide in variceal bleeding. J Hepatol 37:167–168
Patel YC (1992) General aspects of the biology and function of somatostatin. In: Weil C,
Muller EE, Thorner MO (eds) Basic and clinical aspects of neuroscience. Springer, Berlin,
pp 1–16
Patel YC (1997) Molecular pharmacology of somatostatin receptor subtypes. J Endocrinol Invest
20:348–367
Patel YC (1999) Somatostatin and its receptor family. Front Neuroendocrinol 20:157–198
Patel YC, O’Neil W (1988) Peptides derived from cleavage of prosomatostatin at carboxyl- and
amino-terminal segments. Characterization of tissue and secreted forms in the rat. J Biol Chem
263:745–751
Patel YC, Reichlin S (1978) Somatostatin in hypothalamus, extrahypothalamic brain, and periph-
eral tissues of the rat. Endocrinology 102:523–530
Patel YC, Wheatley T, Ning C (1981) Multiple forms of immunoreactive somatostatin: compari-
son of distribution in neural and nonneural tissues and portal plasma of the rat. Endocrinology
109:1943–1949
Patel YC, Murthy KK, CB EEE et al (1990) Mechanism of action of somatostatin: an overview of
receptor function and studies of the molecular characterization and purification of somatostatin
receptor proteins. Metabolism 39:63–69
Patel YC, Papachristou DN, Zingg HH et al (1991) Regulation of islet somatostatin secretion and
gene expression: selective effects of adenosine 3¢, 5¢-monophosphate and phorbol esters in
normal islets of Langerhans and in a somatostatin-producing rat islet clonal cell line 1027 B2.
Endocrinology 128:1754–1762
Patel YC, Greenwood M, Kent G et al (1993) Multiple gene transcripts of the somatostatin recep-
tor SSTR2: tissue selective distribution and cAMP regulation. Biochem Biophys Res Commun
192:288–294
Patel YC, Greenwood MT, Warszynska A et al (1994) All five cloned human somatostatin recep-
tors (hSSTR1–5) are functionally coupled to adenylyl cyclase. Biochem Biophys Res Commun
198:605–612
Patel YC, Greenwood MT, Panetta R et al (1995) The somatostatin receptor family. Life Sci
57:1249–1265
Patel YC, Greenwood M, Panetta R et al (1996) Molecular biology of somatostatin receptor sub-
types. Metabolism 45:31–38
Patel YC, Liu JL, Galanopoulou AS et al (2001) Production, action, and degradation of somato-
statin. In: Jefferson LS, Cherrington AD (eds) Handbook of physiology, the endocrine pan-
creas and regulation of metabolism. Oxford University Press, New York
Patel RC, Kumar U, Lamb DC et al (2002) Ligand binding to somatostatin receptors induces
receptor-specific oligomer formation in live cells. Proc Natl Acad Sci USA 99:3294–3299
Patzelt C, Tager HS, Carroll RJ et al (1980) Identification of prosomatostatin in pancreatic islets.
Proc Natl Acad Sci USA 77:2410–2414
Peeters TL, Depraetere Y, Vantrappen GR (1981) Simple extraction method and radioimmu-
noassay for somatostatin in human plasma. Clin Chem 27:888–891

178 U. Kumar and M. Grant
Pelletier G, Leclerc R, Arimura A et al (1975) Letter: immunohistochemical localization of soma-
tostatin in the rat pancreas. J Histochem Cytochem 23:699–702
Pello OM, Martinez-Munoz L, Parrillas V et al (2008) Ligand stabilization of CXCR4/delta-opi-
oid receptor heterodimers reveals a mechanism for immune response regulation. Eur J
Immunol 38:537–549
Penman E, Wass JA, Medbak S et al (1981) Response of circulating immunoreactive somatostatin
to nutritional stimuli in normal subjects. Gastroenterology 81:692–699
Perron A, Chen ZG, Gingras D et al (2003) Agonist-independent desensitization and internaliza-
tion of the human platelet-activating factor receptor by coumermycin-gyrase B-induced
dimerization. J Biol Chem 278:27956–27965
Pfeiffer M, Koch T, Schroder H et al (2001) Homo- and heterodimerization of somatostatin recep-
tor subtypes. Inactivation of sst(3) receptor function by heterodimerization with sst(2A). J Biol
Chem 276:14027–14036
Pfeiffer M, Koch T, Schroder H et al (2002) Heterodimerization of somatostatin and opioid recep-
tors cross-modulates phosphorylation, internalization, and desensitization. J Biol Chem
277:19762–19772
Philippe J (1993) Somatostatin inhibits insulin-gene expression through a posttranscriptional
mechanism in a hamster islet cell line. Diabetes 42:244–249
Pierce KL, Premont RT, Lefkowitz RJ (2002) Seven-transmembrane receptors. Nat Rev Mol Cell
Biol 3:639–650
Poitout L, Roubert P, Contour-Galcera MO et al (2001) Identification of potent non-peptide soma-
tostatin antagonists with sst(3) selectivity. J Med Chem 44:2990–3000
Polak JM, Pearse AG, Grimelius L et al (1975) Growth-hormone release-inhibiting hormone in
gastrointestinal and pancreatic D cells. Lancet 1:1220–1222
Pradayrol L, Jornvall H, Mutt V et al (1980) N-terminally extended somatostatin: the primary
structure of somatostatin-28. FEBS Lett 109:55–58
Premont RT, Gainetdinov RR (2007) Physiological roles of G protein-coupled receptor kinases
and arrestins. Annu Rev Physiol 69:511–534
Presky DH, Schonbrunn A (1988) Somatostatin pretreatment increases the number of somatosta-
tin receptors in GH4C1 pituitary cells and does not reduce cellular responsiveness to somato-
statin. J Biol Chem 263:714–721
Prinster SC, Hague C, Hall RA (2005) Heterodimerization of g protein-coupled receptors: specifi-
city and functional significance. Pharmacol Rev 57:289–298
Putney LK, Denker SP, Barber DL (2002) The changing face of the Na+/H+ exchanger, NHE1:
structure, regulation, and cellular actions. Annu Rev Pharmacol Toxicol 42:527–552
Qanbar R, Bouvier M (2003) Role of palmitoylation/depalmitoylation reactions in G-protein-
coupled receptor function. Pharmacol Ther 97:1–33
Quintela M, Senaris RM, Dieguez C (1997) Transforming growth factor-betas inhibit somatostatin
messenger ribonucleic acid levels and somatostatin secretion in hypothalamic cells in culture.
Endocrinology 138:4401–4409
Rabbani SN, Patel YC (1990) Peptides derived by processing of rat prosomatostatin near the amino-
terminus: characterization, tissue distribution, and release. Endocrinology 126:2054–2061
Rasmussen SG, Choi HJ, Rosenbaum DM et al (2007) Crystal structure of the human beta2 adren-
ergic G-protein-coupled receptor. Nature 450:383–387
Ravazzola M, Benoit R, Ling N et al (1989) Prosomatostatin-derived antrin is present in gastric D
cells and in portal blood. J Clin Invest 83:362–366
Reardon DB, Dent P, Wood SL et al (1997) Activation in vitro of somatostatin receptor subtypes
2, 3, or 4 stimulates protein tyrosine phosphatase activity in membranes from transfected Ras-
transformed NIH 3T3 cells: coexpression with catalytically inactive SHP-2 blocks responsive-
ness. Mol Endocrinol 11:1062–1069
Redmon JB, Towle HC, Robertson RP (1994) Regulation of human insulin gene transcription by
glucose, epinephrine, and somatostatin. Diabetes 43:546–551
Reichlin S (1983a) Somatostatin (second of two parts). N Engl J Med 309:1556–1563
Reichlin S (1983b) Somatostatin. N Engl J Med 309:1495–1501

Somatostatin and Somatostatin Receptors 179
Reisine T, Axelrod J (1983) Prolonged somatostatin pretreatment desensitizes somatostatin’s
inhibition of receptor-mediated release of adrenocorticotropin hormone and sensitizes ade-
nylate cyclase. Endocrinology 113:811–813
Reisine T, Bell GI (1995) Molecular biology of somatostatin receptors. Endocr Rev 16:427–442
Ren SG, Taylor J, Dong J et al (2003) Functional association of somatostatin receptor subtypes 2
and 5 in inhibiting human growth hormone secretion. J Clin Endocrinol Metab
88:4239–4245
Resmini E, Dadati P, Ravetti JL et al (2007) Rapid pituitary tumor shrinkage with dissociation
between antiproliferative and antisecretory effects of a long-acting octreotide in an acromeg-
alic patient. J Clin Endocrinol Metab 92:1592–1599
Reubi JC, Landolt AM (1984) High density of somatostatin receptors in pituitary tumors from
acromegalic patients. J Clin Endocrinol Metab 59:1148–1151
Reubi JC, Hacki WH, Lamberts SW (1987) Hormone-producing gastrointestinal tumors contain
a high density of somatostatin receptors. J Clin Endocrinol Metab 65:1127–1134
Reubi JC, Schaer JC, Wenger S et al (2000) SST3-selective potent peptidic somatostatin receptor
antagonists. Proc Natl Acad Sci USA 97:13973–13978
Reubi JC, Waser B, Schaer JC (2004) Expression of somatostatin receptors in human tissue in
health and disease. In: Srikant CB (ed) Somatostatin. Kluwer, Norwell, pp 107–121
Rocheville M, Lange DC, Kumar U et al (2000a) Receptors for dopamine and somatostatin: for-
mation of hetero-oligomers with enhanced functional activity. Science 288:154–157
Rocheville M, Lange DC, Kumar U et al (2000b) Subtypes of the somatostatin receptor assemble
as functional homo- and heterodimers. J Biol Chem 275:7862–7869
Rodriguez-Arnao MD, Gomez-Pan A, Rainbow SJ et al (1981) Effects of prosomatostatin on
growth hormone and prolactin response to arginine in man. Comparison with somatostatin.
Lancet 1:353–356
Rodriguez-Frade JM, del Real G, Serrano A et al (2004) Blocking HIV-1 infection via CCR5 and
CXCR4 receptors by acting in trans on the CCR2 chemokine receptor. EMBO J 23:66–76
Rodriguez-Sanchez MN, Puebla L, Lopez-Sanudo S et al (1997) Dopamine enhances somatostatin
receptor-mediated inhibition of adenylate cyclase in rat striatum and hippocampus. J Neurosci
Res 48:238–248
Rohrer SP, Birzin ET, Mosley RT et al (1998) Rapid identification of subtype-selective agonists
of the somatostatin receptor through combinatorial chemistry. Science 282:737–740
Roosterman D, Roth A, Kreienkamp HJ et al (1997) Distinct agonist-mediated endocytosis of cloned rat
somatostatin receptor subtypes expressed in insulinoma cells. J Neuroendocrinol 9:741–751
Roth A, Kreienkamp HJ, Meyerhof W et al (1997a) Phosphorylation of four amino acid residues
in the carboxyl terminus of the rat somatostatin receptor subtype 3 is crucial for its desensitiza-
tion and internalization. J Biol Chem 272:23769–23774
Roth A, Kreienkamp HJ, Nehring RB et al (1997b) Endocytosis of the rat somatostatin receptors:
subtype discrimination, ligand specificity, and delineation of carboxy-terminal positive and
negative sequence motifs. DNA Cell Biol 16:111–119
Saito T, Iwata N, Tsubuki S et al (2005) Somatostatin regulates brain amyloid beta peptide
Abeta42 through modulation of proteolytic degradation. Nat Med 11:434–439
Samuels MH, Henry P, Ridgway EC (1992) Effects of dopamine and somatostatin on pulsatile
pituitary glycoprotein secretion. J Clin Endocrinol Metab 74:217–222
Saveanu A, Gunz G, Dufour H et al (2001) Bim-23244, a somatostatin receptor subtype 2- and
5-selective analog with enhanced efficacy in suppressing growth hormone (GH) from octre-
otide-resistant human GH-secreting adenomas. J Clin Endocrinol Metab 86:140–145
Saveanu A, Lavaque E, Gunz G et al (2002) Demonstration of enhanced potency of a chimeric
somatostatin-dopamine molecule, BIM-23A387, in suppressing growth hormone and prolactin
secretion from human pituitary somatotroph adenoma cells. J Clin Endocrinol Metab
87:5545–5552
Saveanu A, Gunz G, Guillen S et al (2006) Somatostatin and dopamine-somatostatin multiple
ligands directed towards somatostatin and dopamine receptors in pituitary adenomas.
Neuroendocrinology 83:258–263

180 U. Kumar and M. Grant
Saveanu A, Jaquet P, Brue T et al (2008) Relevance of coexpression of somatostatin and dopamine
D2 receptors in pituitary adenomas. Mol Cell Endocrinol 286:206–213
Scarborough DE, Lee SL, Dinarello CA et al (1989) Interleukin-1 beta stimulates somatostatin
biosynthesis in primary cultures of fetal rat brain. Endocrinology 124:549–551
Schaer JC, Waser B, Mengod G et al (1997) Somatostatin receptor subtypes sst1, sst2, sst3 and
sst5 expression in human pituitary, gastroentero-pancreatic and mammary tumors: comparison
of mRNA analysis with receptor autoradiography. Int J Cancer 70:530–537
Schonbrunn A, Tashjian H Jr (1978) Characterization of functional receptors for somatostatin in
rat pituitary cells in culture. J Biol Chem 253:6473–6483
Schreff M, Schulz S, Handel M et al (2000) Distribution, targeting, and internalization of the sst4
somatostatin receptor in rat brain. J Neurosci 20:3785–3797
Schusdziarra V, Dobbs RE, Harris V et al (1977) Immunoreactive somatostatin levels in plasma
of normal and alloxan diabetic dogs. FEBS Lett 81:69–72
Seidah NG, Chretien M (1999) Proprotein and prohormone convertases: a family of subtilases
generating diverse bioactive polypeptides. Brain Res 848:45–62
Sellers LA, Alderton F, Carruthers AM et al (2000) Receptor isoforms mediate opposing prolifera-
tive effects through gbetagamma-activated p38 or Akt pathways. Mol Cell Biol
20:5974–5985
Senaris RM, Lago F, Dieguez C (1996) Gonadal regulation of somatostatin receptor 1, 2 and 3
mRNA levels in the rat anterior pituitary. Brain Res Mol Brain Res 38:171–175
Sharif N, Gendron L, Wowchuk J et al (2007) Coexpression of somatostatin receptor subtype 5
affects internalization and trafficking of somatostatin receptor subtype 2. Endocrinology
148:2095–2105
Sharma K, Srikant CB (1998a) Induction of wild-type p53, Bax, and acidic endonuclease during
somatostatin-signaled apoptosis in MCF-7 human breast cancer cells. Int J Cancer
76:259–266
Sharma K, Srikant CB (1998b) G protein coupled receptor signaled apoptosis is associated with
activation of a cation insensitive acidic endonuclease and intracellular acidification. Biochem
Biophys Res Commun 242:134–140
Sharma K, Patel YC, Srikant CB (1996) Subtype-selective induction of wild-type p53 and apop-
tosis, but not cell cycle arrest, by human somatostatin receptor 3. Mol Endocrinol
10:1688–1696
Sharma K, Patel YC, Srikant CB (1999) C-terminal region of human somatostatin receptor 5 is
required for induction of Rb and G1 cell cycle arrest. Mol Endocrinol 13:82–90
Shen LP, Pictet RL, Rutter WJ (1982) Human somatostatin I: sequence of the cDNA. Proc Natl
Acad Sci USA 79:4575–4579
Shields D (1980) In vitro biosynthesis of somatostatin. Evidence for two distinct preprosomato-
statin molecules. J Biol Chem 255:11625–11628
Shimon I, Yan X, Taylor JE et al (1997a) Somatostatin receptor (SSTR) subtype-selective ana-
logues differentially suppress in vitro growth hormone and prolactin in human pituitary adeno-
mas. Novel potential therapy for functional pituitary tumors. J Clin Invest 100:2386–2392
Shimon I, Taylor JE, Dong JZ et al (1997b) Somatostatin receptor subtype specificity in human
fetal pituitary cultures. Differential role of SSTR2 and SSTR5 for growth hormone, thyroid-
stimulating hormone, and prolactin regulation. J Clin Invest 99:789–798
Shoelson SE, Polonsky KS, Nakabayashi T et al (1986) Circulating forms of somatostatinlike
immunoreactivity in human plasma. Am J Physiol 250:E428–E434
Shojamanesh H, Gibril F, Louie A et al (2002) Prospective study of the antitumor efficacy of long-
term octreotide treatment in patients with progressive metastatic gastrinoma. Cancer
94:331–343
Siler TM, Yen SC, Vale W et al (1974) Inhibition by somatostatin on the release of TSH induced
in man by thyrotropin-releasing factor. J Clin Endocrinol Metab 38:742–745
Sims SM, Lussier BT, Kraicer J (1991) Somatostatin activates an inwardly rectifying K+ conduct-
ance in freshly dispersed rat somatotrophs. J Physiol 441:615–637

Somatostatin and Somatostatin Receptors 181
Skamene A, Patel YC (1984) Infusion of graded concentrations of somatostatin in man: pharma-
cokinetics and differential inhibitory effects on pituitary and islet hormones. Clin Endocrinol
(Oxf) 20:555–564
Smalley KS, Feniuk W, Humphrey PP (1998) Differential agonist activity of somatostatin and
L-362855 at human recombinant sst4 receptors. Br J Pharmacol 125:833–841
Smalley KS, Feniuk W, Sellers LA et al (1999) The pivotal role of phosphoinositide-3 kinase in
the human somatostatin sst(4) receptor-mediated stimulation of p44/p42 mitogen-activated
protein kinase and extracellular acidification. Biochem Biophys Res Commun 263:239–243
Smalley KS, Koenig JA, Feniuk W et al (2001) Ligand internalization and recycling by human
recombinant somatostatin type 4 (h sst(4)) receptors expressed in CHO-K1 cells. Br J
Pharmacol 132:1102–1110
Song GJ, Hinkle PM (2005) Regulated dimerization of the thyrotropin-releasing hormone receptor
affects receptor trafficking but not signaling. Mol Endocrinol 19:2859–2870
Spier AD, de Lecea L (2000) Cortistatin: a member of the somatostatin neuropeptide family with
distinct physiological functions. Brain Res Brain Res Rev 33:228–241
Srikant CB (1995) Cell cycle dependent induction of apoptosis by somatostatin analog SMS
201–995 in AtT-20 mouse pituitary cells. Biochem Biophys Res Commun 209:400–406
Srikant CB, Patel YC (1981) Receptor binding of somatostatin-28 is tissue specific. Nature
294:259–260
Srikant CB, Shen SH (1996) Octapeptide somatostatin analog SMS 201–995 induces transloca-
tion of intracellular PTP1C to membranes in MCF-7 human breast adenocarcinoma cells.
Endocrinology 137:3461–3468
Stroh T, Jackson AC, Dal Farra C et al (2000) Receptor-mediated internalization of somatostatin
in rat cortical and hippocampal neurons. Synapse 38:177–186
Sullivan SJ, Schonbrunn A (1988) Distribution of somatostatin receptors in RINm5F insulinoma
cells. Endocrinology 122:1137–1145
Susini C, Buscail L (2006) Rationale for the use of somatostatin analogs as antitumor agents. Ann
Oncol 17:1733–1742
Takano K, Yasufuku-Takano J, Teramoto A et al (1997) Gi3 mediates somatostatin-induced acti-
vation of an inwardly rectifying K+ current in human growth hormone-secreting adenoma
cells. Endocrinology 138:2405–2409
Takeba Y, Suzuki N, Takeno M et al (1997) Modulation of synovial cell function by somatostatin
in patients with rheumatoid arthritis. Arthritis Rheum 40:2128–2138
Tallent M, Reisine T (1992) Gi alpha 1 selectively couples somatostatin receptors to adenylyl
cyclase in pituitary-derived AtT-20 cells. Mol Pharmacol 41:452–455
Tanjasiri P, Kozbur X, Florsheim WH (1976) Somatostatin in the physiologic feedback control of
thyrotropin secretion. Life Sci 19:657–660
Tannenbaum GS, McCarthy GF, Zeitler P et al (1990) Cysteamine-induced enhancement of
growth hormone-releasing factor (GRF) immunoreactivity in arcuate neurons: morphological
evidence for putative somatostatin/GRF interactions within hypothalamus. Endocrinology
127:2551–2560
Tannenbaum GS, Zhang WH, Lapointe M et al (1998) Growth hormone-releasing hormone neu-
rons in the arcuate nucleus express both Sst1 and Sst2 somatostatin receptor genes.
Endocrinology 139:1450–1453
Tannenbaum GS, Turner J, Guo F et al (2001) Homologous upregulation of sst2 somatostatin
receptor expression in the rat arcuate nucleus in vivo. Neuroendocrinology 74:33–42
Teijeiro R, Rios R, Costoya JA et al (2002) Activation of human somatostatin receptor 2 promotes
apoptosis through a mechanism that is independent from induction of p53. Cell Physiol
Biochem 12:31–38
Terrillon S, Bouvier M (2004) Roles of G-protein-coupled receptor dimerization. EMBO Rep
5:30–34
Thangaraju M, Sharma K, Liu D et al (1999) Interdependent regulation of intracellular acidifica-
tion and SHP-1 in apoptosis. Cancer Res 59:1649–1654

182 U. Kumar and M. Grant
Thapar K, Kovacs KT, Stefaneanu L et al (1997) Antiproliferative effect of the somatostatin ana-
logue octreotide on growth hormone-producing pituitary tumors: results of a multicenter ran-
domized trial. Mayo Clin Proc 72:893–900
Theodoropoulou M, Zhang J, Laupheimer S et al (2006) Octreotide, a somatostatin analogue,
mediates its antiproliferative action in pituitary tumor cells by altering phosphatidylinositol
3-kinase signaling and inducing Zac1 expression. Cancer Res 66:1576–1582
Tichomirowa MA, Daly AF, Beckers A (2005) Treatment of pituitary tumors: somatostatin.
Endocrine 28:93–100
Tostivint H, Lihrmann I, Bucharles C et al (1996) Occurrence of two somatostatin variants in the
frog brain: characterization of the cDNAs, distribution of the mRNAs, and receptor-binding
affinities of the peptides. Proc Natl Acad Sci USA 93:12605–12610
Tostivint H, Trabucchi M, Vallarino M et al (2004) Molecular evolution of somatostatin genes. In:
Srikant CB (ed) Somatostatin. Kluwer, Norwell
Tran VT, Beal MF, Martin JB (1985) Two types of somatostatin receptors differentiated by cyclic
somatostatin analogs. Science 228:492–495
Tsuda K, Sakurai H, Seino Y et al (1981) Somatostatin-like immunoreactivity in human peripheral
plasma measured by radioimmunoassay following affinity chromatography. Diabetes
30:471–474
Tulipano G, Bonfanti C, Milani G et al (2001) Differential inhibition of growth hormone secretion
by analogs selective for somatostatin receptor subtypes 2 and 5 in human growth-hormone-
secreting adenoma cells in vitro. Neuroendocrinology 73:344–351
Tulipano G, Soldi D, Bagnasco M et al (2002) Characterization of new selective somatostatin
receptor subtype-2 (sst2) antagonists, BIM-23627 and BIM-23454. Effects of BIM-23627 on
GH release in anesthetized male rats after short-term high-dose dexamethasone treatment.
Endocrinology 143:1218–1224
Tulipano G, Stumm R, Pfeiffer M et al (2004) Differential beta-arrestin trafficking and endosomal
sorting of somatostatin receptor subtypes. J Biol Chem 279:21374–21382
Vale W, Brazeau P, Rivier C et al (1975) Somatostatin. Recent Prog Horm Res 31:365–397
Vale W, Rivier J, Ling N et al (1978) Biologic and immunologic activities and applications of
somatostatin analogs. Metabolism 27:1391–1401
Vallejo M (2004) Somatostatin gene structure and regulation. In: Srikant CB (ed) Somatostatin.
Kluwer, Norwell, pp 1–16
Van Den Bossche B, D’Haeninck E, De Vos F et al (2004) Oestrogen-mediated regulation of
somatostatin receptor expression in human breast cancer cell lines assessed with 99mTc-
depreotide. Eur J Nucl Med Mol Imaging 31:1022–1030
Van Essen M, Krenning EP, De Jong M et al (2007) Peptide Receptor Radionuclide Therapy with
radiolabelled somatostatin analogues in patients with somatostatin receptor positive tumours.
Acta Oncol 46:723–734
Vasquez B, Harris V, Unger RH (1982) Extraction of somatostatin from human plasma on octa-
decylsilyl silica. J Clin Endocrinol Metab 55:807–809
Vidal C, Rauly I, Zeggari M et al (1994) Up-regulation of somatostatin receptors by epidermal
growth factor and gastrin in pancreatic cancer cells. Mol Pharmacol 46:97–104
Viollet C, Bodenant C, Prunotto C et al (1997) Differential expression of multiple somatostatin
receptors in the rat cerebellum during development. J Neurochem 68:2263–2272
Visser-Wisselaar HA, Van Uffelen CJ, Van Koetsveld PM et al (1997) 17-beta-estradiol-dependent
regulation of somatostatin receptor subtype expression in the 7315b prolactin secreting rat
pituitary tumor in vitro and in vivo. Endocrinology 138:1180–1189
Wang HL, Bogen C, Reisine T et al (1989) Somatostatin-14 and somatostatin-28 induce opposite
effects on potassium currents in rat neocortical neurons. Proc Natl Acad Sci USA
86:9616–9620
Watt HL, Kharmate G, Kumar U (2008) Biology of somatostatin in breast cancer. Mol Cell
Endocrinol 286:251–261
Weckbecker G, Raulf F, Stolz B et al (1993) Somatostatin analogs for diagnosis and treatment of
cancer. Pharmacol Ther 60:245–264

Somatostatin and Somatostatin Receptors 183
Weckbecker G, Briner U, Lewis I et al (2002) SOM230: a new somatostatin peptidomimetic with
potent inhibitory effects on the growth hormone/insulin-like growth factor-I axis in rats, pri-
mates, and dogs. Endocrinology 143:4123–4130
Weckbecker G, Lewis I, Albert R et al (2003) Opportunities in somatostatin research: biological,
chemical and therapeutic aspects. Nat Rev Drug Discov 2:999–1017
Weiss RE, Reddi AH, Nimni ME (1981) Somatostatin can locally inhibit proliferation and dif-
ferentiation of cartilage and bone precursor cells. Calcif Tissue Int 33:425–430
White RE, Schonbrunn A, Armstrong DL (1991) Somatostatin stimulates Ca(2+)-activated K+
channels through protein dephosphorylation. Nature 351:570–573
White JH, Wise A, Main MJ et al (1998) Heterodimerization is required for the formation of a
functional GABA(B) receptor. Nature 396:679–682
Whorton MR, Bokoch MP, Rasmussen SG et al (2007) A monomeric G protein-coupled receptor
isolated in a high-density lipoprotein particle efficiently activates its G protein. Proc Natl Acad
Sci USA 104:7682–7687
Whorton MR, Jastrzebska B, Park PS et al (2008) Efficient coupling of transducin to monomeric
rhodopsin in a phospholipid bilayer. J Biol Chem 283:4387–4394
Wulbrand U, Wied M, Zofel P et al (1998) Growth factor receptor expression in human gastroen-
teropancreatic neuroendocrine tumours. Eur J Clin Invest 28:1038–1049
Xidakis C, Mastrodimou N, Notas G et al (2007) RT-PCR and immunocytochemistry studies sup-
port the presence of somatostatin, cortistatin and somatostatin receptor subtypes in rat Kupffer
cells. Regul Pept 143:76–82
Xu Y, Berelowitz M, Bruno JF (1995) Dexamethasone regulates somatostatin receptor subtype
messenger ribonucleic acid expression in rat pituitary GH4C1 cells. Endocrinology
136:5070–5075
Xu Y, Song J, Berelowitz M et al (1996) Estrogen regulates somatostatin receptor subtype 2 mes-
senger ribonucleic acid expression in human breast cancer cells. Endocrinology
137:5634–5640
Yamada Y, Post SR, Wang K et al (1992) Cloning and functional characterization of a family of
human and mouse somatostatin receptors expressed in brain, gastrointestinal tract, and kidney.
Proc Natl Acad Sci USA 89:251–255
Ye WZ, Mathieu S, Marteau C (1999) Somatostatin inhibits the Na+/H+ exchange activity of rat
hepatocytes in short term primary culture. Cell Mol Biol (Noisy-le-grand) 45:1183–1189
Yoshioka K, Saitoh O, Nakata H (2002) Agonist-promoted heteromeric oligomerization between
adenosine A(1) and P2Y(1) receptors in living cells. FEBS Lett 523:147–151
Zalatnai A, Timar F (2002) In vitro antiangiogenic effect of sandostatin (octreotide) on the prolif-
eration of the placental vessels. Anticancer Res 22:4225–4227
Zapata PD, Ropero RM, Valencia AM et al (2002) Autocrine regulation of human prostate carci-
noma cell proliferation by somatostatin through the modulation of the SH2 domain containing
protein tyrosine phosphatase (SHP)-1. J Clin Endocrinol Metab 87:915–926
Zatelli MC, Piccin D, Bottoni A et al (2004) Evidence for differential effects of selective somato-
statin receptor subtype agonists on alpha-subunit and chromogranin a secretion and on cell
viability in human nonfunctioning pituitary adenomas in vitro. J Clin Endocrinol Metab
89:5181–5188
Zatelli MC, Piccin D, Tagliati F et al (2005a) SRC homology-2-containing protein tyrosine phos-
phatase-1 restrains cell proliferation in human medullary thyroid carcinoma. Endocrinology
146:2692–2698
Zatelli MC, Piccin D, Tagliati F et al (2005b) Dopamine receptor subtype 2 and somatostatin
receptor subtype 5 expression influences somatostatin analogs effects on human somatotroph
pituitary adenomas in vitro. J Mol Endocrinol 35:333–341
Zatelli MC, Piccin D, Ambrosio MR et al (2006) Antiproliferative effects of somatostatin analogs
in pituitary adenomas. Pituitary 9:27–34
Zhang HJ, Redmon JB, Andresen JM et al (1991) Somatostatin and epinephrine decrease insulin
messenger ribonucleic acid in HIT cells through a pertussis toxin-sensitive mechanism.
Endocrinology 129:2409–2414

184 U. Kumar and M. Grant
Zhou A, Webb G, Zhu X et al (1999) Proteolytic processing in the secretory pathway. J Biol Chem
274:20745–20748
Zhu Y, Yakel JL (1997) Calcineurin modulates G protein-mediated inhibition of N-type calcium
channels in rat sympathetic neurons. J Neurophysiol 78:1161–1165
Zhu LJ, Krempels K, Bardin CW et al (1998) The localization of messenger ribonucleic acids for
somatostatin receptors 1, 2, and 3 in rat testis. Endocrinology 139:350–357
Zingg HH, Patel YC (1982) Biosynthesis of immunoreactive somatostatin by hypothalamic
neurons in culture. J Clin Invest 70:1101–1109