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ORIGINAL PAPER
Endoglin in angiogenesis and vascular diseases
Peter ten Dijke Æ Marie-Jose
´
Goumans Æ
Evangelia Pardali
Received: 30 January 2008 / Accepted: 31 January 2008 / Published online: 19 February 2008
Ó Springer Science+Business Media B.V. 2008
Abstract Endoglin is a transmembrane auxillary receptor
for transforming growth factor-b (TGF-b) that is predom-
inantly expressed on proliferating endothelial cells.
Endoglin deficient mice die during midgestation due to
cardiovascular defects. Mutations in endoglin and activin
receptor-like kinase 1 (ALK1), an endothelial specific
TGF-b type I receptor, have been linked to hereditary
hemorrhagic telangiectasia (HHT), an autosomal dominant
vascular dysplasia characterized by telangiectases and
arteriovenous malformations. Endoglin heterozygote mice
develop HHT-like vascular abnormalities, have impaired
tumor and post-ischemic angiogenesis and demonstrate an
endothelial nitric oxide synthase-dependent deterioration in
the regulation of vascular tone. In pre-eclampsia, placenta-
derived endoglin has been shown to be strongly upregu-
lated and high levels of soluble endoglin are released into
the circulation. Soluble endoglin was found to cooperate
with a soluble form of vascular endothelial growth factor
receptor 1 in the pathogenesis of pre-eclampsia by inducing
endothelial cell dysfunction. Endoglin is highly expressed
in tumor-associated endothelium, and endoglin antibodies
have been successfully used to target activated endothelial
cells and elicit anti-angiogenic effects in tumor mouse
models. These exciting advances provide opportunities for
the development of new therapies for diseases with vas-
cular abnormalities.
Keywords Angiogenesis BMP Endothelial cells
Hereditary hemorrhagic telangiectasia
Signal transduction Smad TGF-b
Introduction
Endoglin (also known as CD105) was initially identified by
a monoclonal antibody (44G4) raised against a pre-B
lymphoblastic HOON cell line [1]. Its cDNA was isolated
in 1990 and predicted that the encoded endoglin protein is a
type I integral membrane glycoprotein (Fig. 1)[2]. This
and subsequent studies showed that endoglin is highly
expressed on proliferating vascular endothelial cells [2–5].
Since its identification as an accessory receptor for TGF-b
[6] and the link between endoglin haplo-insufficiency and
HHT [7], its role in (tumor) angiogenesis [7–12] and the
pathological role of soluble endoglin in pre-eclampsia [13],
the interest in endoglin has strongly increased. It is now
evident that endoglin has a pivotal function in vascular
development and disease [14].
TGF-b1 is the prototype of a family of multifunctional
proteins, which includes three TGF-b isoforms (i.e., TGF-
b1, -b2 and -b3), activins and bone morphogenetic proteins
(BMPs), which are involved in many different patho-
physiological processes, including development, wound
healing, cancer, fibrosis, vascular, and immune diseases
[14, 15]. The importance of TGF-b signaling pathway in
vascular morphogenesis was revealed by the targeted
inactivation of TGF-b
signaling components in mice,
which die at midgestation during embryogenesis due to
disrupted vasculogenesis in the yolk sac [14]. TGF-b
family members signal via two related single transmem-
brane spanning type I and type II receptors endowed with
serine/threonine kinase activity [16, 17]. Each ligand has a
P. ten Dijke (&) M.-J. Goumans E. Pardali
Department of Molecular Cell Biology, Leiden University
Medical Center, Building 2, Room R-02-022, Postzone S-1-P,
Postbus 9600, 2300 RC Leiden, The Netherlands
e-mail: p.ten_dijke@lumc.nl
123
Angiogenesis (2008) 11:79–89
DOI 10.1007/s10456-008-9101-9
specific set of type II and type I receptors with which it
interacts. In most cases TGF-b interacts with TGF-b type II
receptor (TbRII) and TGF-b type I receptor (TbRI), also
termed activin receptor-like kinase 5 (ALK5) [18]. In
endothelial cells TGF-b can also signal via ALK1 [19].
Activins signal via activin type II receptors (ActRII) and
ALK4 [20, 21], and BMPs transduce their effects through
BMP type II receptor (BMPRII) and ActRIIs and ALK1, -
2, -3 and -6 (Fig. 2)[22–25]. Upon ligand-induced heter-
omeric complex formation, the type II constitutively active
kinase trans-phosphorylates the type I receptor on serine
and threonine residues located in the juxtamembrane
region [26]. The activated type I receptor propagates the
signal into the cell by phosphorylating specific receptor-
regulated (R-) Smads at two carboxy-terminal serine resi-
dues [27, 28]. Whereas TGF-b and activins in most cases
signal via R-Smad2 and Smad3, BMPs activate R-Smad1,
Smad5 and Smad8 [29–32]. Activated R-Smads form het-
eromeric complexes with common mediator (Co-)Smad,
i.e., Smad4 in mammals, which accumulate in the nucleus.
There they can bind to DNA (in)directly and act as tran-
scription factor complexes together with other transcription
factors and co-activators and co-repressors [32–34].
Endoglin is a TGF-b type III auxiliary receptor (TbRIII)
[6] that is not directly involved in signaling, but modulates
signaling responses of multiple members of the TGF-b
family [35]. In addition, endoglin is involved in TGF-b
independent signaling [36]. Moreover, two endoglin splice
variants exist which can exert opposite effects [37, 38].
Furthermore, a soluble form of endoglin (sEnd) has been
found, most likely generated by proteolytic shedding, which
antagonizes the membrane bound form [13]. Taken toge-
ther, multiple layers of complexity exist by which the
function of endoglin is regulated. In this review, we discuss
the latest advances in our understanding of its mechanism of
action and function in angiogenesis and vascular diseases.
Structure
Human endoglin is a homo-dimeric protein of 658 amino
acid residues that contains an extracellular domain, a single
transmembrane domain and a short intracellular domain
(Fig. 1)[2]. Endoglin is structurally related to betaglycan
[39, 40], another TbRIII, with high similarity in the trans-
membrane and intracellular regions. Both endoglin and
betaglycan are glycoproteins with N-linked and O-linked
glycans; however, endoglin lacks the glycosamino-glycan
(GAG) chains that are characteristic for betaglycan. Both
type III receptors can form heteromeric complexes with
TbRII, but whereas endoglin requires TbRII for binding to
TGF-b1 and -b3, betaglycan alone can bind all three TGF-b
isoforms [41, 42]. Endoglin and betaglycan are usually
expressed as homodimers on the cell surface, which in the
case of endoglin are linked by disulfide bridges. Heteromeric
complexes between endoglin and betaglycan have been
observed in microvascular endothelial cells [43]. Besides the
membrane-bound form, both type III receptors can occur in
soluble form. High circulating levels of sEnd are detected in
patients with cancer [44, 45] or pre-eclampsia [13, 46].
Betaglycan can be shed by membrane-type metalloprotease
1 (MT1-MMP [47] and sEnd is likely also generated by
proteolytic cleavage of the membrane-bound form.
The extracellular domain of human endoglin contains an
Arg-Gly-Asp (RGD) tripeptide sequence that is an inter-
action motif for integrins. However, this sequence is not
conserved in mouse pig and rat [2, 48]. In its extracellular
A
B
Fig. 1 Schematic representation of endoglin structure. (a) Endoglin
is a disulphide-linked dimeric protein with large extracellular domain,
single transmembrane domain and short intracellular region. The
extracellular domain consists of an orphan domain and a ZP domain.
The RGD binding motif that is present in human endoglin is
indicated. Interaction partners and the (potential) regulatory effects
that are mediated through these interactions are indicated (b). The two
splice forms of endoglin, i.e., long (L) and short (S) differ in their
intracellular regions. The intracellular region lacks an enzymatic
motif but is rich in serine and threonine residues that can be
phosphorylated by TbRII, ALK1 and ALK5. At the C-terminus of L-
endoglin a PDZ interaction motif (SMA) is present. Abbreviations:
ALK, activin receptor-like kinase; eNOS, endothelial nitric oxide
synthase; MAPK, mitogen activated protein kinase; TbR, TGF-b
receptor; ZP, Zona Pellucida
80 Angiogenesis (2008) 11:79–89
123
region endoglin harbors a zona pellucida (ZP) domain of
approximately 260 amino acid residues, a feature shared
with betaglycan [49]. The ZP domain is involved in en-
doglin oligomerization and in ligand (in)dependent
heteromeric interactions with the TGF-b receptors TbRII
and ALK5 [50]. Using single-particle electron microscopy,
the three-dimensional structure of the extracellular domain
of endoglin was determined at 25 A
˚
resolution. The two
monomers are positioned in an anti-parallel fashion with
each monomer consisting of three well-defined domains
[51], i.e., one domain with unknown structural homology
and a ZP domain with bipartite structure (Fig. 1).
The intracellular domain of endoglin lacks an enzymatic
motif but contains many serine and threonine residues of
which certain residues are phosphorylated by TGF-b
receptor kinases (see below) [52]. At its C-terminus en-
doglin contains a PDZ interaction motif (SerSerMetAla).
There are two splice isoforms, termed Long (L)- and Short
(S)-endoglin, which differ in their intracellular part; L and
S have cytoplasmic tails of 47 and 14 amino acids,
respectively, and have only 7 juxtamembrane amino acids
in common [53]. The L form is most abundantly expressed
and is also the predominant form in endothelial cells while
the S-form is expressed in liver and lung at significant
levels [37]. Both forms can bind TGF-b [53], and differ-
ently regulate TGF-b-induced responses. L-endoglin
enhanced the TGF-b/ALK1 pathway, while S-endoglin
promoted the TGF-b/ALK5 route upon ectopic expression
in myoblasts [38]. Forced expression of S-endoglin in
vascular endothelium in mice results in reduced tumor
growth and neovascularization and suggest that S-endoglin
(in contrast to L-endoglin) has anti-angiogenic activity.
S-endoglin may elicit this effect by inhibiting the formation
of L–L homodimers as L-and S-forms have been shown to
form heterodimers [37].
Cellular and tissue distribution
Whereas endoglin is expressed at low to non-detectable
levels in resting endothelial cells within normal tissues, it is
highly expressed in vascular endothelial cells in sites of
Fig. 2 TGF-b family signaling pathways in endothelial cells.
Signaling by TGF-b family members, which includes TGF-bs,
activins and BMPs, occurs via specific cell surface type I and type
II receptors that are endowed with serine/threonine kinase activity.
Accessory receptors endoglin and betaglycan modulate TGF-b family
signaling via type I and type II receptors. Soluble endoglin and
betaglycan can sequester ligand and thereby inhibit receptor binding.
In most cells TGF-b signals via TbRII and ALK5. In endothelial cells
(depicted here) it signals also via another type I receptor ALK1.
Activins signal via ActRII and ALK4. BMPs signal via BMPRII and
ActRII and type I receptors ALK1, ALK2, ALK3 and ALK6. The
type I receptors act downstream of type II receptor and determine the
signaling specificity of the receptor complex. Activated type I
receptors initiate intracellular signaling by phosphorylating specific
R-Smads. Activation of ALK1, ALK23, ALK3 and ALK6 leads to
phosphorylation of Smad1, Smad5 and Smad8, and Smad2 and
Smad3 are phosphorylated by ALK4, ALK5 and ALK7. Activated R-
Smads assemble with Smad4 in heteromeric complexes that accu-
mulate in the nucleus. There these complexes regulate specific gene
expression responses by binding to DNA together with other DNA
binding transcription factors. Abbreviatons: ActR, activin receptor;
BMP, bone morphogenetic protein; BMPR, BMP receptor; sEnd,
soluble endoglin; transforming growth factor-b;TbR, TGF-b recep-
tor; TF, transcription factor
Angiogenesis (2008) 11:79–89 81
123
active angiogenesis during embryogenesis, in inflamed
tissues, and within and surrounding tumors [3–5]. After
ischemia-reperfusion injury and myocardial infarction,
endoglin expression is upregulated in the ischemic area and
border zone [54]. Besides hypoxia, TGF-b, BMP9, and
constitutively active (ca)ALK1 also potently stimulate
endoglin expression [55–57], whereas TNFa inhibits en-
doglin expression in endothelial cells [58]. The endoglin
promoter contains multiple Sp1 binding sites that are
shown to play a critical role for basal transcription [59].
Hypoxia and TGF-b cooperate in endoglin promoter acti-
vation [60]. This appears to be mediated via a multi-protein
complex consisting of Smad3/4, Sp1 and HIF1a/b bound to
their cognate DNA binding elements.
Despite the fact that endoglin is considered to be an
endothelial specific marker, several other cell types have
been shown to express endoglin. For example, endoglin is
present on monocytes and upregulated during the mono-
cyte-macrophage transition [61]. Endoglin was also found
to have a crucial role in monocyte-mediated vascular repair
[54]. In addition, endoglin is expressed in syncytiotroph-
oblasts of term placenta [62] and in pre-eclampsia its
expression is highly elevated [13, 46]. While endoglin
in normal smooth muscle cells is low, its expression is
strongly upregulated in vascular smooth muscle cells
in atherosclerosis [63]. During development endoglin
is expressed in a subset of neural crest stem cells and is
required for myogenic differentiation [64]. Moreover, en-
doglin is expressed in adult bone marrow hematopoietic
stem cells (HSCs) [65] and is a functional marker that
defines long-term repopulating hematopoietic stem cells
[66]. Using the differentiation of ES cells into embryoid
bodies as an assay system, endoglin was shown to be
required for efficient myelopoiesis and definitive erythro-
poiesis [67]. In addition, overexpression or knockdown of
endoglin in hematopoietic stem cells revealed that, while
endoglin is not required for engraftment and reconstituting
capacities, it regulates adult erythroid development [68].
Furthermore, endoglin expression is also found in cer-
tain tumor cells, including primary and metastatic lesions
of melanoma [69] and ovary [70] and prostate cancer cells
[71]. In prostate cancer cells, endoglin suppresses cell
adhesion, motility and invasion by activating the TGF-b/
ALK2/Smad1 pathway [72, 73], in a manner that is remi-
niscent of TGF-b/ALK1/Smad1 signaling in endothelial
cells [19]. Consistent with the notion that endoglin
expression is attenuated during prostate cancer progression,
it has been postulated that endoglin has a tumor suppressor
role in prostate cancer. Endoglin is also expressed in epi-
dermal keratinocytes [74]. In a multistage model for mouse
skin carcinogenesis using wild type and endoglin hetero-
zygous mice, endoglin was shown to have a dual role by
inhibiting benign tumor formation, but accelerating the
malignant conversion in skin carcinogenesis. Endoglin
haplo-insufficieny may elicit these effects by enhancing the
tumor suppressing and inhibiting the tumor promoting
effects of TGF-b/ALK5 signaling in tumor cells [74].
Interplay with TGF-b and other signal transduction
pathways
Besides the TGF-b dependent interaction of endoglin with
TbRII [42], endoglin can also interact with TbRII and type
I receptors ALK1 and ALK5 in a ligand independent
manner and both extra- and intracellular domains contrib-
ute to this interaction [50, 75]. Endoglin can bind several
other ligands besides TGF-b, including activins and BMPs
and can interact with activin type II receptors [76]. Inter-
estingly, BMP9 can bind endoglin with high affinity
independently from type I or type II receptors (Fig. 2)[56].
In endothelial cells TGF-b can signal via two distinct
type I receptor pathways, i.e., ALK1 and ALK5 [18].
Whereas TGF-b/ALK1 signaling stimulates cell prolifera-
tion and migration of endothelial cells, TGF-b/ALK5
signaling inhibits these responses (Fig. 2)[19]. Recent
studies have revealed an intricate interplay between the two
signaling pathways, and with endoglin. ALK5/Smad2/3
inhibits ALK1-Smad1/5 signaling and vice versa, and
ALK5 is required for efficient TGF-b/ALK1 signaling [77].
Forced overexpression of endoglin inhibits TGF-b/ALK5
signaling [78, 79] and TGF-b-induced growth inhibition
[42, 61, 80]. Conversely, inhibiting endoglin function by
specific knockdown or treatment with neutralizing anti-
bodies inhibits TGF-b/ALK1 signaling, and (in) directly
potentiates TGF-b/ALK5 signaling [80–83
]. Establishment
of endothelial cell lines from endoglin deficient mouse
embryos (mouse embryonic endothelial cells, MEECs)
demonstrated that endothelial cells deficient in endoglin do
not proliferate efficiently, possibly because TGF-b/ALK1
signaling is attenuated and TGF-b/ALK5 signaling is
enhanced. Surviving cells were found to have downregu-
lated ALK5 expression, possibly caused by an adaptive
response to overcome the increased TGF-b/ALK5 induced
growth arrest [80]. Similar results were obtained with pri-
mary cultures of outgrown endothelial cells from the blood
of HHT1 patients [84]. However, in another study using
MEECs derived from endoglin null embryos, it was dem-
onstrated that endoglin is not required for TGF-b-induced
Smad1/5 activation and that endoglin may regulate TGF-b
receptor affinity [85].
The intracellular region of endoglin can be phosphory-
lated by TbRII and type I receptors [52]. TbRII and ALK5
preferentially phosphorylate Ser634 and Ser635, which
greatly facilitate subsequent phosphorylation by caALK1
on threonine residues (Fig. 1). The PDZ-binding motif
82 Angiogenesis (2008) 11:79–89
123
appears to negatively regulate phosphorylation as its
removal greatly increases the serine phosphorylation level
by all three receptor kinases. The endoglin phosphorylation
status influences its subcellular distribution and may
modulate endoglin-mediated effects on endothelial cell
adhesion and proliferation.
Endoglin can localize to caveolae [86]. Interestingly,
caveolin-1, the major protein component in caveolae, was
recently shown to interact and functionally cooperate with
TGF-b/ALK1 signaling in endothelial cells [87]. Specific
knockdown of caveolin-1 or caveolae disruption by cho-
lesterol depletion attenuated ALK1 signaling. This in
contrast to TGF-b/ALK5 signaling for which caveolin-1
has an inhibitory effect [88]. Interestingly, cavaolin-1
knock-out mice develop vascular abnormalities with
increased endothelial permeability [89].
Several studies have suggested that endoglin has func-
tions that are independent from TGF-b family signaling
(Fig. 1). The endoglin-induced inhibition of apoptosis in
endothelial cells subjected to hypoxic stress has been
suggested to occur independently of TGF-b [55]. Endoglin
has also been shown to induce activation of MAP kinases
[78]. Endoglin interacts with zyxin and ZRP-1, LIM
domain containing proteins which are concentrated in focal
adhesions [90, 91]. Via these interactions endoglin may
control cell migration independently of TGF-b. The en-
doglin cytoplasmic tail interacts with b-arrestin2 in
endothelial cells [92]. This interaction results in endoglin
internalization in endosomal vesicles and was shown to
inhibit TGF-b-induced ERK activation and migration in
endothelial cells. Using the cytoplasmic tail as bait in yeast
two hybrid interaction screen, Tctex2b, a dynein light
chain (DLC) family member, was identified [93]. Tctex2b
was also found to interact with TbRII. Ectopic expression
of Tctex2b had an inhibitory effect on TGF-b-induced
transcriptional reporter activity. Endoglin, TbRII, and
Tctex2b colocalize in the plasmamembrane. DLC protein
family members are involved in retrograde protein trans-
port, but whether the function of endoglin is regulated and
directed by this remains to be investigated.
Vascular development
Endoglin expression is elevated during alterations in vas-
cular structure as they occur during embryogenesis,
inflammation, and wound healing [94, 95]. The link
between mutations in the endoglin gene and the vascular
disorder HHT [7] also indicates an important role for en-
doglin in maintaining normal vascular architecture (see
discussion below). Mice deficient in endoglin die 10.5 days
p.c. and fail to form mature blood vessels in the yolk sac
[8–10
]. Vessels do form, but are dilated and fragile, and
easily rupture. The endoglin deficient embryos are much
more fragile and smaller than their wild type litter mates.
Most embryos lack vitelline vessels, which connect the
yolk sac with the embryo proper. This may also explain the
pericardial effusion, indicative of a circulation defect that
is observed in most mutant embryos. This phenotype of
endoglin deficient embryos is reminiscent of that of TGF-
b1, TbRII, ALK5, and ALK1 [14], and suggests a func-
tional link of endoglin with these other TGF-b receptors
during extra-embryonic vascular development. In contrast
to the lack of vascular networks in the yolk sac of endoglin
deficient mice, the vasculature in the embryo proper was
normal with the exception of the heart [8–10]. Endoglin
null mice were found to have several cardiac defects;
almost all endoglin deficient embryos demonstrated
enlarged cardiac ventricles and dilated outflow tracts and
the atrioventricular canal endocardium failed to undergo
mesenchymal transformation and to form cushions. These
defects in heart valve formation in the endoglin knockouts
might also be related to perturbation of TGF-b signaling.
TGF-b ligands and signaling receptors are also expressed
in the developing heart [96] and defects in heart looping
and morphogenesis have been reported in transgenic mice
with misexpressed TGF-b receptor [97].
TGF-b has an important role in vascular morphogenesis,
and is involved in endothelial cell function and differen-
tiation of pericytes and smooth muscle cells [98]. Endoglin
mutant mice also demonstrate defects in both endothelial
and smooth muscle cell function [8–10]. Its predominant
expression in endothelial cells suggests that the primary
defect is endothelial and that defects in smooth muscle cell
differentiation are secondary. Consistent with this notion,
endoglin deficient mice have decreased paracrine TGF-b
signaling from endothelial cells to neighboring mesothelial
cells in the yolk sac [99]. This may lead to defective
mesothelial cell differentiation into pericytes or vascular
smooth muscle cells. As a result the vessels are weak and
susceptible to damage and hemorrhage. However, recent
studies also demonstrate that endoglin may have a cell
autonomous role in vascular smooth muscle cells. Ectopic
expression of endoglin in neural crest cells resulted in
pronounced hemorrhage in cranial vessels and in the
pericardial cavity due to aberrant smooth muscle cells in
the vascular wall [64].
Mice heterozygotes for endoglin serve as a good model
for HHT [9]. They have dilated and fragile blood vessels,
telangiectases, and nosebleeds, which are the clinical
manifestations that are also observed in HHT patients.
Some strains of mice are more affected than others, sug-
gesting the existence of modifier genes. In addition, shear,
blood pressure, and inflammation products may contribute
to disease heterogeneity. This has also been suggested to be
the case for HHT patients [100]. Endoglin heterozygous
Angiogenesis (2008) 11:79–89 83
123
mice have reduced eNOS expression and NO synthesis-
dependent vasodilation is impaired [86, 101]. Endothelial
cells derived from endoglin heterozygous mice showed a
significantly attenuated proliferation, migration, increased
collagen production, and mitigated NO synthase expression
and VEGF secretion. Compared to wild type animals, mice
heterozygous for endoglin showed impaired angiogenesis
as observed by a delayed reperfusion following hindlimb
ischemia [11]. In addition, when matrigel plugs were
implanted in endoglin heterozygote mice to measure
endothelial cell outgrowth and invasion into the extracel-
lular matrix in vivo, it was found that they had significantly
less vascular structures compared to wild type mice [11].
Hereditary hemorrhagic telangiectasia (HHT)
HHT, also termed Osler–Weber–Rendu disease, is an
autosomal dominant vascular disorder with an incidence of
about 1 in 10,000 [100, 102]. Characteristic clinical features
include small dilated bloodvessels (telangiectases) and
arteriovenous malformations (AVMs) in the vasculature of
lung, liver, and brain. The disease manifests itself often
during puberty with spontaneous recurrent nose bleeds.
Later in life the intensity of the nosebleeds increase and also
AVMs become larger and problematic. However, onset and
clinical manifestations of HHT are heterogeneous between
different individuals and even within families [100, 102].
Two HHT variants, i.e., HHT1 and HHT2, have been
described that have been linked to mutations in endoglin
and ALK1, respectively. HHT1 differs from HHT2 in an
earlier onset, stronger prevalence and higher frequency of
pulmonary AVMs. In HHT2 the gastrointestinal bleeding
and liver involvement appears to occur frequently than in
HHT1 families [100, 102]. Recently, another gene in the
TGF-b signaling pathway has been implicated in HHT;
Smad4 is mutated in a subset of HHT patients with juvenile
polyposis, lacking mutations in endoglin and ALK1 [103].
This syndrome is now termed JP-HHT.
Mutations in endoglin include deletions, insertions and
missense mutations, and splice site changes, and in about
80% of the cases lead to premature stop codons and trun-
cated endoglin proteins [100, 102]. It has been suggested
that these mutant proteins may achieve a dominant nega-
tive effect by disrupting normal endoglin function.
However, mutated endoglin proteins were found to be
expressed at low levels, mediated by nonsense mediated
mRNA decay. In addition, mutated proteins may be mis-
folded and not stable and/or do not reach the cell surface
and are thereby unable to form heterodimers with normal
endoglin. Indeed, patients with endoglin mutations have
significantly lower endoglin levels in peripheral blood
monocytes compared to control group [100]. Haplo-
insufficiency, therefore, likely provides the explanation for
the mechanism underlying HHT1. The lower levels of
endoglin expression leads to a dysfunction in TGF-b signal
transduction. Whether other non-TGF-b signaling path-
ways are affected as well by lowered endoglin expression
is not clear.
As already mentioned before, studies in mouse models
suggest that defective paracrine TGF-b signaling of endo-
thelial cells to adjacent pericytes/smooth muscle cells lead
to an inefficient maturation of these cells [99]. This makes
the vessels highly susceptible to rupture, a main charac-
teristic of HHT. Interestingly, HHT patients with endoglin
mutations have been found to have low circulating levels of
TGF-b [104]. Several mechanisms have been suggested
through which the AVMs develop. During development
there may be a loss of arterial and venous identity whereby
the normal separation between veins and arteries are dis-
rupted. In addition, defective vascular remodeling and
dilatation may occur following local inflammation. More-
over, cells that form the capillary endothelial bed that
separates the arteries and veins may be removed by
apoptosis in response to hypoxic stress [14].
Pre-eclampsia
Pre-eclampsia is characterized by the onset of high blood
pressure and significant amounts of protein in the urine in the
third trimester of pregnancy [105]. It affects both the fetus
and the mother and occurs in about 5% of the pregnancies.
Severe pre-eclampsia leads to appearance of Hemolysis
Elevated Liver enzymes and Low Platelets (HELLP) syn-
drome, seizures and/or fetal growth restriction, and can
result in death [105, 106]. The endothelial dysfunction in
pre-eclampsia is thought to be caused by circulating factors
that are released from the placenta [107, 108]. Circulating
levels of placenta-derived soluble form of vascular endo-
thelial growth factor receptor (VEGFR)1 (also known as
sFlt), which sequesters the pro-angiogenic proteins placenta
growth factor (PlGF) and VEGF, are increased before onset
and correlate with the severity of pre-eclampsia [109].
Interestingly, a 65 kDa sEnd is increased in sera of pregnant
women, and strongly elevated in pre-eclamptic patients.
SEnd levels also correlate with disease severity [46].
Whereas forced expression of soluble endoglin increased
vascular permeability and induced a modest increase in
blood pressure without significant proteinuria, ectopic
expression of both sFlt and sEnd in pregnant rats induced the
hallmarks of severe pre-eclampsia [13].
In vitro studies on endothelial cell lines showed that
sEnd interferes with TGF-b signaling and eNOS activation
and thereby causes endothelial dysfunction [13]. The sol-
uble form of endoglin functions likely as a scavenger of
84 Angiogenesis (2008) 11:79–89
123
specific circulating TGF-b family ligands, such as TGF-b
or BMP9 [13, 14]. sEnd and sFlt act differently and may
therefore cooperate to induce the endothelial cell dys-
function [13]. As sEnd levels increase 2–3 months before
the onset of pre-eclampsia, sEnd (and sFlt and PlGF) levels
may be used as diagnostic marker to prioritize patients and
thereby prevent pre-eclampsia-induced death [46]. Inhibi-
tion of the putative protease involved in endoglin shedding
may be of therapeutic benefit in the treatment of pre-
eclampsia.
Whereas in pre-eclampsia there is an increase in sEnd,
patients suffering from an acute myocardial infarction were
found to have a significantly lower level of sEnd in their
serum [110]. Interestingly, patients that died had signifi-
cantly lower levels of sEnd than those that survived. Low
levels of sEnd may therefore be used as a prognostic
marker after acute myocardial infarction.
Anti-angiogenic activity of endoglin antibodies
Tumors depend on angiogenesis to grow beyond a few
cubic millimeters; the exchange of oxygen and nutrients
with carbon dioxide and waste products cannot rely any
longer on diffusion, but require blood vessels. Also
metastasis, the spreading and colonization of the primary
tumor to distant organs, requires tumor angiogenesis [111].
Anti-angiogenic therapy, in which proliferating endothelial
cells within and surrounding tumor associated vessels are
selectively targeted, has been shown (in combination with
chemotherapy) to induce tumor regression and inhibit
metastasis [112].
Endoglin is expressed in tumor associated endothelium
in many solid cancers, including breast, prostate, and cer-
vical cancer [113–116]. Intra-tumor microvessel density as
determined by anti-endoglin staining and circulating levels
of soluble endoglin have prognostic significance in cancer.
Antibodies that specifically detect endoglin have success-
fully been used for tumor imaging and endoglin antibodies
coupled with toxins [117] or radioactivity [118] have been
used with favorable outcome for vascular targeting [119,
120]. Monoclonal antibodies to endoglin can inhibit pro-
liferation of endothelial cells [121] and have been shown to
have anti-angiogenic effects [122]. Moreover, immuno-
therapy with the extracellular domain of porcine endoglin
was found to induce cytotoxic T lymphocyte (CTL)-med-
iated cytotoxicity and inhibits both endothelial cell
proliferation and tumor growth in mouse cancer models
[123, 124]. However, the suitability of endoglin as a target
for anti-angiogenic therapy for cancer has been questioned;
endoglin expression has also been observed in the endo-
thelial cells of normal tissues [125, 126]. Differences may
prevail between different antibodies, and testing of
different endoglin antibodies to select the best one for
antibody-based therapeutic approaches has been proposed
[119, 126].
The anti-angiogenic effects of sEnd may be used to
interfere with angiogenically active tumors. Besides en-
doglin, also the signaling TGF-b receptors can be targeted
to achieve anti-angiogenic effects. In this respect, it is of
interest to note that treatment of mice with a small mole-
cule ALK5 kinase inhibitor decreased the pericyte
coverage of the endothelial vessels, and in particular those
ones present in the tumor neovasculature. This activity was
used to stimulate a greater accumulation of anticancer
nanocarriers in tumors [127].
Concluding remarks
Endoglin plays a critical role in angiogenesis and dysreg-
ulation of its expression and/or activity has been implicated
in multiple vascular diseases, most notably HHT, pre-
eclampsia and tumor angiogenesis. While much needs to be
investigated on the molecular mechanisms that underlie its
role in vascular development and disease, the recent
advances provide ample opportunities for better diagnosis
and development of new therapies for diseases with vas-
cular abnormalities.
Acknowledgments Our studies on endoglin are supported by Dutch
Cancer Society (UL-2005-3371) and EU grants Angiotargeting and
Tumor-Host-Genomics.
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