The endothelial cell protein C receptor: Its role in thrombosis

Abstract

The protein C anticoagulant pathway plays a crucial role as a regulator of the blood clotting cascade. Protein C is activated on the vascular endothelial cell membrane by the thrombin-thrombomodulin complex. The endothelial protein C receptor binds protein C and further enhances protein C activation. Once formed, activated protein C down-regulates thrombin formation by inactivating factors Va and VIIIa and exerts cytoprotective effects through endothelial protein C receptor binding. An adequate generation of activated protein C depends on the precise assembly, on the surface of the endothelial cells, of thrombin, thrombomodulin, protein C, and endothelial protein C receptor. Therefore, any change in the efficiency of this assembly may cause a reduction or increase in activated protein C generation and modulate the risk of thrombosis. This review highlights the role of the endothelial protein C receptor in disease and discusses the association of its mutations with the risk of thrombosis.

Full text

Review Article
The endothelial cell protein C receptor: Its role in thrombosis
Silvia Navarro
a
, Elena Bonet
a,b
, Amparo Estellés
a
, Ramón Montes
c
, José Hermida
c
, Laura Martos
a
,
Francisco España
a
, Pilar Medina
a,
⁎
a
Hemostasis and Thrombosis Unit, Research Center, La Fe University Hospital, Valencia, Spain
b
Clinical Pathology Service, La Fe University Hospital, Valencia, Spain
c
Division of Cardiovascular Sciences, Laboratory of Thrombosis and Haemostasis, Center for Applied Medical Research (CIMA), University of Navarra, Pamplona, Spain
abstractarticle info
Article history:
Received 13 May 2011
Received in revised form 14 July 2011
Accepted 1 August 2011
Available online 7 September 2011
Keywords:
activated protein C
endothelial protein C receptor
factor VII
venous thrombosis
myocardial infarction
The protein C anticoagulant pathway plays a crucial role as a regulator of the blood clotting cascade. Protein C
is activated on the vascular endothelial cell membrane by the thrombin-thrombomodulin complex. The
endothelial protein C receptor binds protein C and further enhances protein C activation. Once formed,
activated protein C down-regulates thrombin formation by inactivating factors Va and VIIIa and exerts
cytoprotective effects through endothelial protein C receptor binding. An adequate generation of activated
protein C depends on the precise assembly, on the surface of the endothelial cells, of thrombin,
thrombomodulin, protein C, and endothelial protein C receptor. Therefore, any change in the efficiency of
this assembly may cause a reduction or increase in activated protein C generation and modulate the risk of
thrombosis. This review highlights the role of the endothelial protein C receptor in disease and discusses the
association of its mutations with the risk of thrombosis.
© 2011 Elsevier Ltd. All rights reserved.
Contents
Introduction ................................................................ 410
EPCR structure ............................................................... 411
Soluble EPCR and disease .......................................................... 411
EPCR and FVII/FVIIa ............................................................. 412
Anti-EPCR autoantibodies .......................................................... 412
EPCR polymorphisms and thrombosis .................................................... 412
EPCR and venous thromboembolism ................................................... 412
EPCR and arterial thrombosis ...................................................... 413
Summary .................................................................. 414
Conflict of interest statement ........................................................ 414
Acknowledgment .............................................................. 414
References ................................................................. 414
Introduction
The protein C (PC) anticoagulant pathway plays a crucial role in
the regulation of fibrin formation via proteolytic inactivation of the
procoagulant cofactors Factor (F) Va and FVIIIa [1–3]. PC is a vitamin
K-dependent plasma glycoprotein that circulates in plasma as an
inactive zymogen, which is activated to activated PC (APC) [4] on the
surface of endothelial cells by the thrombin-thrombomodulin (TM)
complex. Another endothelial cell-specific protein, which is involved
in the PC anticoagulant pathway, is the endothelial cell PC receptor
(EPCR). EPCR binds PC to on the endothelial cell surface. This binding
Thrombosis Research 128 (2011) 410–416
Abbreviations: PC, protein C; F, factor; APC, activated protein C; TM, thrombomo-
dulin; EPCR, endothelial protein C receptor; TNF-α, tumor necrosis factor-α;PROCR,
endothelial protein C receptor gene; aa, amino acid; UTR, untranslated region; sEPCR,
soluble EPCR; TF, tissue factor; PAR-1, protease-activated receptor-1; MAPK, mitogen
activated protein kinase; VTE, venous thromboembolism; SNPs, single nucleotide
polymorphisms; T, thrombin; PS, protein S.
⁎Corresponding author at: Cen tro de Investigación, Hospita l Universitario y
Politécnico La Fe. Av. Campanar 21, 46009 Valencia, Spain. Tel.: + 34 963862797; fax:
+34 961973018.
E-mail address: medina_pil@gva.es (P. Medina).
0049-3848/$ –see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.thromres.2011.08.001
Contents lists available at ScienceDirect
Thrombosis Research
journal homepage: www.elsevier.com/locate/thromres

enhances the rate of PC activation by the thrombin-TM complex by 5-
to 20-fold [5] by decreasing the K
m
of PC for its activation by the
thrombin-TM complex. The relevance of the anticoagulant role of
EPCR in vivo has been well documented: EPCR blockade in baboons
notably reduced the amount of APC generated upon thrombin
administration [6], and an anti-EPCR monoclonal antibody accelerat-
ed thrombus development in a murine model of thrombosis [7].
TM is uniformly distributed on all endothelial cells, which results
in relatively low TM concentrations in large vessels. This would result
in inefficient activation of PC in these vessels [8]. This effect is
counterbalanced by EPCR, which is highly expressed on the
endothelium of large vessels and is present in trace levels in most
capillary beds [9]. This effective location may ensure efficacious PC
activation on the surface of large vessels where the ratio of the blood
volume to the surface area is high.
Once activated, APC may either dissociate from EPCR, bind to its
cofactor protein S [1,3], and exhibit its anticoagulant functions, or it
may remain bound to EPCR and display cell-signaling cytoprotective
activities [10–16] (Fig. 1a).
The clinical relevance of the PC pathway is evident from reports
showing a clear association between deficiencies of PC [2,17,18] and
protein S [19–21] or reduced APC levels [22–24] with thrombosis. In
fact, the deficiencies of PC and protein S or the FV Leiden mutation are
present in more than 50% of patients with inherited thrombophilia
[25].
EPCR is widely expressed on the surface of the endothelium of
large vessels. Inflammatory stimuli-like tumor necrosis factor-α
(TNF-α) or an atherosclerotic setting have been shown to reduce its
expression [26,27]. The presence of EPCR on the surface of other cells
has also been reported: trophoblasts, monocytes, neutrophils and
eosinophils [28,29], brain endothelial cells [30], lymphocytes [31],
osteoblasts [32], gastric epithelial cells [33], chondrocytes [34],
tenocytes [35], epidermal keratinocytes [36], and human vascular
smooth muscle cells [37]. More recently, EPCR was reported to be
expressed in murine CD8
+
dendritic cells [38], hematopoietic stem
cells [39], and cardiomyocytes [40].
EPCR structure
The human EPCR gene (PROCR) spans approximately 8 kb, is
located on chromosome 20 at position q11.2, and consists of 4 exons
[26,41–43]. Exon 1 [138 bp; 1 to 24 amino acids (aa)] encodes the 5′-
untranslated region (UTR), the signal peptide, and 7 additional
residues. Exons 2 (252 bp; 24 to 108 aa) and 3 (279 bp; 108 to 201
aa) encode most of the extracellular region of EPCR. Exon 4 (659 bp;
201 to 238 aa) encodes an additional 10 residues of the extracellular
region of EPCR, the transmembrane domain, the cytoplasmic tail, and
the 3′UTR [42].
EPCR is a 46-kD type I transmembrane protein, constitutively
expressed on the luminal surface of endothelium by endothelial cells,
and is structurally similar to the major histocompatibility class 1/CD1
family proteins involved in the immune and inflammation responses
[26]. The crystal structure of EPCR showed that a tightly bound
phospholipid resides in the groove typically involved in antigen
presentation, and its extraction results in loss of PC binding, which can
be restored by lipid reconstitution [44]. The crystal structure, solved
alone and in complex with the phospholipid-binding domain of
protein C, revealed that most of the residues contacting the lipid in
EPCR are identical to or highly conserved in CD1d, which may help to
further understand the role of EPCR in immune regulation. Finally, it
showed that the PC binding site is outside this conserved groove and
is distal from the membrane-spanning domain [44].
The EPCR protein is comprised of 238 aa with a signal sequence at
the amino-terminal end (17 aa); an extracellular domain composed of
2 alpha domains (α1 and α2), 4 potential N-glycosylation sites, and 3
Cys residues; a transmembrane domain near the carboxy-terminal
(25 aa); and a short cytosolic domain (3 aa) [41,45,46].
Soluble EPCR and disease
A soluble form of EPCR (sEPCR), which lacks the transmembrane
and cytoplasmic tail domain, is present in normal human plasma [47].
sEPCR is generated in vitro through proteolytic cleavage by metallo-
protease activity inducible by thrombin and other inflammatory
mediators, a process called shedding. This metalloprotease has been
identified as the TNF-αconverting enzyme or ADAM17 [48] and
cleaves EPCR between aa's 192 and 200. Moreover, ADAM17 promotes
the release of pro-inflammatory and adhesion molecules [48,49], and
TNF-αsignificantly decreases the expression of EPCR and TM in
several human endothelial cells [50]. A variety of mediators, including
interleukin-1, hydrogen peroxide, phorbol esters, and thrombin
dramatically increase EPCR shedding from the endothelium [51,52].
Like the membrane-bound form, sEPCR binds PC and APC with similar
affinity. However, its binding to APC inhibits its anticoagulant and
anti-inflammatory properties, and its binding to PC prevents PC
activation by the thrombin-TM complex [53]. This would suggest that
sEPCR may display procoagulant activity. sEPCR may also bind to
activated neutrophils in a process that involves proteinase-3 and Mac-
1. There is a controversy whether or not this interaction exerts an
anti-inflammatory effect. On the one hand, it was proposed that
binding of sEPCR to proteinase-3/Mac-1 may reduce leukocyte-
Fig. 1. Endothelial protein C receptor (EPCR) functions. 1a. Protein C (PC) is activated by
the thrombin (T)-thrombomodulin (TM)-EPCR ternary complex on the endothelial cell
surface. Upon activation, activated PC (APC) may dissociate from the complex, bind to
protein S (PS) and display its anticoagulant functions, or may remain bound to EPCR
and display its cytoprotective activities through cell signaling mechanisms, most of
them via protease-activated receptor-1 (PAR-1). 1b. Factor (F) VII and FVIIa also bind to
EPCR. Binding of FVII prevents the FXa-dependent generation of FVIIa, which may
represent a new anticoagulant role for EPCR. On the other hand, FVIIa, upon binding to
EPCR on endothelial cells, activates PAR1-mediated cell signaling and provides a
barrier-protective effect.
411S. Navarro et al. / Thrombosis Research 128 (2011) 410–416

endothelial cells interactions, thus modulating inflammation and
preventing endothelium damage [54,55]. However, it has also been
reported that proteinase-3 is able to proteolyze EPCR, thus triggering
an additional mechanism by which anticoagulant and cell protective
pathways may be down-regulated during inflammation [56]. In sum,
the physiological significance of sEPCR in vivo is not well known.
sEPCR levels showed a bimodal distribution in a healthy
population. Approximately 15% to 20% of the general population has
plasma sEPCR levels between 200 and 800 ng/mL, whereas the
remainder have levels below 200 ng/mL [57]. Plasma sEPCR levels
responded to anticoagulant treatment, suggesting that sEPCR may be
a marker for a hypercoagulable state [58].
sEPCR levels increase in a wide variety of pathophysiological
conditions, and may reflect endothelial dysfunction, and contribute to
a procoagulant phenotype and increased risk of thrombosis. Accordingly,
sEPCR levels increase in patients with sepsis, a disorder that results from
a complex dysregulation of hemostatic mechanisms, with activation of
procoagulant pathways and impairment of the fibrinolytic system and
natural anticoagulant pathways, especially the PC pathway. In fact, as
previously explained, the anti-inflammatory properties of APC seem to
be beneficial in the treatment of sepsis [59,60]. sEPCR level also increases
in systemic lupus erythematosus [61,62], a potentially fatal autoimmune
disease affecting multiple organ systems, and it is associated with
thrombotic manifestations, inflammation, and widespread activation of
the vascular endothelium.
It has been shown that 50–80% of plasma sEPCR variations are
under genetic control [63–65] and that most subjects with elevated
sEPCR levels carry the H3 haplotype, one of the EPCR haplotypes
described, but this will be discussed in detail later.
Recently, a new mechanism to generate sEPCR has been reported,
consisting of the alternative mRNA splicing that generates a truncated
EPCR mRNA lacking the sequence encoding the transmembrane and
intra-cytoplasmic domains [66]. The resulting protein is not able to
anchor to the membrane and, as a consequence, it is secreted and can
be detected in plasma as sEPCR. The truncated sEPCR can bind PC and
APC, and its generation is particularly efficient in H3-carrying subjects
[66,67], and their endothelial cells might therefore be able to produce
a soluble receptor that might compete with the membrane-bound
EPCR for APC binding.
EPCR and FVII/FVIIa
It has been reported that EPCR may exhibit PC-independent
anticoagulant activities. FVIIa is a serine protease that binds to tissue
factor (TF) and initiates the coagulation cascade. The Gla domain of
FVIIa exhibits an important degree of homology with the Gla domain
of PC, and all the residues directly involved in the binding of PC to
EPCR are conserved in FVII [68–70]. A binding analysis has shown that
FVII, FVIIa, PC, and APC bind to EPCR with similar affinity[70–72],
suggesting that the interaction of FVII/FVIIa with EPCR on endothelial
cells may influence the activation of PC and APC-mediated cell
signaling. In addition, EPCR mediates the internalization of FVIIa
bound to it on the cell surface, indicating that it may play a role in
FVIIa clearance [71,73,74].
More recently, it has been shown that endothelial cells may down-
regulate the FXa-dependent generation of FVIIa through EPCR binding
[75]. This regulation is probably made by moving FVII from
phosphatidylserine-rich regions, suggesting a new anticoagulant
role for EPCR. Moreover, FVIIa, upon binding to the EPCR on the
endothelial cell surface, activates the endogenous protease-activated
receptor-1 (PAR-1) and induces PAR-1-mediated p44/42 mitogen-
activated protein kinase (MAPK) activation, thus providing a barrier-
protective effect [76] (Fig. 1b).
It must be noted that, very recently, FXa has been shown to bind to
EPCR, which opens new venues in serine-protease signaling [77].
Anti-EPCR autoantibodies
The presence of high titers of anti-EPCR autoantibodies has been
described in patients with antiphospholipid syndrome, fetal death
[78], deep vein thrombosis in the general population [79], and women
with acute myocardial infarction [80]. Also, a case report described a
patient with stroke and massive cutaneous necrosis who had high
titers of anti-EPCR autoantibodies [81]. Two of these autoantibodies
blocked the binding of PC to EPCR, and thus inhibited the generation
of APC on the endothelium. [78]. For this reason, anti-EPCR
autoantibodies may play a causative role in thrombosis, since low
APC levels have been associated with an increased risk of venous and
arterial thrombosis [24,82].
There is an association between elevated levels of the anti-EPCR
autoantibodies, high levels of coagulation activity estimated by D-
dimer levels, and levels of sEPCR [79], which could be related with
endothelial injury induced by these autoantibodies. Anyhow, the
mechanisms by which anti-EPCR autoantibodies confer a risk for
thrombotic events are not fully understood.
EPCR polymorphisms and thrombosis
As mentioned before, normal APC generation depends on the
precise assembly of thrombin and PC to their respective receptors, TM
and EPCR, on the surface of endothelial cells. Any change in the
efficiency of this coupling may cause altered APC generation and a
modification in the risk of thrombosis. In fact, several mutations and
polymorphisms have been reported in the EPCR gene, some of them
associated with the risk of venous or arterial thrombosis.
EPCR and venous thromboembolism
Merati et al. [83] described a 23-bp insertion in exon 3. This
mutation duplicates the preceding 23 bases and results in a STOP
codon downstream from the insertion point [84]. Although statistical
analysis did not reveal a significant association between the mutation
and the risk of thrombosis, expression studies in mammalian cells
showed that the truncated protein is not localized on the cell surface,
cannot be secreted in the culture medium, and does not bind APC,
suggesting that the insertion is a risk factor for arterial and venous
thrombosis. However, given its low population frequency (b1%)
[85,86], it is difficult to assess the effect of this mutation on the risk of
venous and arterial thrombosis. In fact, after combining data from 7
studies that genotyped a total of 2,508 venous thromboembolism
(VTE) patients and 2,617 controls [83–89], the prevalence of the
mutation was 0.48% and 0.38%, respectively.
Hermida et al. described the point mutation C2769T, which causes
a substitution of Arg to Cys at position 96 in the mature protein [90].In
vitro expression and characterization studies of the EPCR R96C variant
revealed no biochemical differences with the wild-type counterpart,
supporting no role of this mutation in VTE [90].
Up to 4 haplotypes of EPCR have been reported [63–65]: H1, H2,
H3, and H4; 3 of which contain 1 or more single-nucleotide
polymorphisms (SNPs) that are haplotype-specific(Fig. 2), while H2
contains the common allele of each SNP.
The H1 haplotype, tagged by the rare allele of 4678G/C (rs9574),
has been associated with increased circulating APC levels [64] and a
reduced risk of VTE in 2 independent studies [64,91]. H1 also reduced
the risk of thrombosis in carriers of FV Leiden [92]. In patients with the
FV Leiden mutation, the mean age at the first thrombosis was
significantly higher in H1H1 propositi than in non-carriers of the H1
haplotype. In contrast, 2 other groups found no association of the H1
haplotype with the risk of thrombosis [63,65]. A possible explanation
for the protective effect of EPCR H1 against VTE would be its
association with increased APC levels. In fact, it has been described
412 S. Navarro et al. / Thrombosis Research 128 (2011) 410–416

that a low level of APC in plasma is a strong, prevalent, independent
risk factor for VTE [93,94].
EPCR is essential for normal embryonic development and plays a
key role in preventing thrombosis at the maternal-embryonic
interface [95]. Recently, it has been shown that the EPCR H1 seems
to protect against recurrent pregnancy loss, particularly in carriers of
FV Leiden [96], but not in the absence of this thrombophilic defect
[97]. Presently it is unknown which SNP in EPCR H1 is responsible for
the reported protective effect. The H1 haplotype contains 10 specific
alleles, the 1451T (rs2069943), 1541A (rs2069944), 1880C
(rs2069945), 2532C (rs2069948), 2897A (rs945960), 3424C
(rs871480), 3997C (rs2069952), 4678C (rs9574), 5632G
(rs1415773), and 5663A (rs1415774) (Fig. 2). Therefore, any of
these nucleotides may be responsible for the observed association of
H1 with increased levels of plasma APC and reduced risk of VTE
[64,92]. Further studies are required to identify which polymorphism/
s is responsible for the observed associations, although some efforts
are being made [98].
The H3 haplotype, tagged bythe rare allele of 4600A/G (rs867186),is
associated with increased plasma levels of sEPCR, but its association
with the risk of VTE is controversial [63–65,99].Onestudy[63] reported
that carriers of the H3 haplotype have an increased risk of VTE in men
but not in women, whereas others [64,65,91,99] did not find a
significant association between EPCR H3 and the risk of thrombosis.
The presence of EPCR H3 and concomitant elevated sEPCR plasma levels
in carriers of the 2 dysfunctional PC variants, Arg-1Cys and Arg-1Leu, is
associated with severe thrombotic manifestations [100]. In addition, it
has been observed that EPCR H3 increases the risk of VTE in carriers of
the prothrombin 20210A mutation, probably due to its association with
increased sEPCR levels [101]. Furthermore, H3 carriers experienced the
first VTE episode at a young age [101]. Recently, the maternal EPCR H3
allele has been found to be a mild risk factor for iliac VTE during
pregnancy and puerperium [102]. Overall, the thrombogenicity of the
EPCR H3, evenif weak, does not seemanecdotal. First,the high incidence
of this polymorphism in the Caucasian Mediterranean population
(21.4%) and the fact that it may potentiate the prothrombotic effect of
other thrombophilias, like the prothrombin 20210A allele [101],
suggeststhat its contributiontowards a VTE event maynot be negligible.
Second, the H3 haplotype may be a risk factor not only for VTE but also
for pregnancy loss [103,104].
It has been suggested that high sEPCR levels associated with the H3
haplotype could be the mechanism that increases the thrombotic risk,
even if sEPCR levels have not been deeply studied in this regard and
some contradictory results exist [65,101]. Regarding the SNPs that
comprise the H3 haplotype, the 4600G (219Gly) allele arises as the
more obvious candidate responsible for the association of the H3
haplotype with increased sEPCR levels, in view of the fact that the
cleavage of the EPCR anchored in the endothelial cell membrane to
generate sEPCR occurs within the region of the protein encoded by
exon 4, near the 4,600 position. The 4600G variant predicts a
conformational change in the protein due to the Ser 219 to Gly
substitution, which may render an EPCR more susceptible to cleavage
by metalloproteases such as ADAM17 [48,63,64]. This hypothesis has
been supported by 2 independent studies [105,106]. And other
mechanism that could link the H3 haplotype and high levels of sEPCR
is the alternative splicing previously explained [66]. The mean
physiological sEPCR concentration is about 3 nM, well below the
concentration of circulating PC, which is about 70 nM, and the Kd of
the EPCR-PC/APC interactions is about 30 nM. It has been hypothe-
sized that in individuals with markedly increased sEPCR levels due to
the H3 haplotype (between 6 and 20 nM), the local concentration at
the endothelial cell surface may approach or exceed the Kd value of
the PC interaction, resulting in a decreased APC generation and
inhibition of the APC generated [63].
Recently, a study that looked for genetic determinants for PC levels
has shown that the EPCR H3 is associated with higher levels of plasma
PC [107,108]. Additionally, the H3 haplotype has also been associated
with higher levels of FVII [109,110], which could hypothetically confer
its risk of thrombosis. An alternative explanation for the thrombo-
genicity that the H3 haplotype may induce is that the increased
shedding of EPCR could reduce the amount of EPCR at the endothelial
surface. In favor of this argument is the fact that inducing EPCR
shedding in cells bearing the H3 haplotype notably reduced their
ability to sustain PC activation as compared with non-H3 cells [106].
Finally, the EPCR H4 was reported to be associated with a slight
increase in the risk of VTE [65], although no further studies have
confirmed these results.
EPCR and arterial thrombosis
In accordance to the risk of VTE, the role of the EPCR 23-bp insertion
[83] in the risk of arterial disease is hard to ascertain mainly due to the
low prevalence of this mutation in the population. Actually, the
combination of the data from the 3 reported studies that genotyped a
total of 669 patients with myocardial infarction and 372 controls
[83,84,111] revealed prevalences of 1.20% and 0.27%, respectively.
With regard to EPCR haplotypes and arterial disease, the results are
also controversial. In the widest study performed so far, no association
between the H3 haplotype and risk of coronary heart disease, stroke,
or mortality was found [107]. In contrast, H1 was associated with
lymphoid EPCR mRNA expression and with increased risk of incident
stroke, all-cause mortality, and decreased healthy survival during
follow-up [107]. These results are in discrepancy with the other wide
study available, which showed that, surprisingly, both H1 and H3
haplotypes were associated with a reduction in the risk of premature
myocardial infarction, and that these effects were additive [112]. The
protective mechanism by which the H1 haplotype would reduce the
risk of premature myocardial infarction may be related to an increase
Fig. 2. The 4 haplotypes of the EPCR gene. Numbering according to Simmonds RE and Lane DA [42]. Circled letters correspond to specific alleles for each haplotype. Bold numbers
indicate supposed numbering for these single-nucleotide polymorphisms according to this numbering since the sequence described by Simmonds RE and Lane DA do not reach these
positions.
413S. Navarro et al. / Thrombosis Research 128 (2011) 410–416

in the circulating APC levels, which are characteristic of the carriers of
the H1 haplotype [64,92]. The protection conferred by the H3
haplotype would be more difficult to explain as increased sEPCR
levels would promote neither the activation of protein C nor the APC-
dependent anticoagulant and cytoprotective actions. The rest of the
studies available are not wide enough to withdraw powerful
conclusions. One study [105] suggested that individuals carrying
one EPCR H3 allele were protected from myocardial infarction,
provided that they were neither diabetic nor showing symptoms of
metabolic syndrome, since in patients with these pathologies, the H3
allele would increase the risk. Finally, there is another study which
suggested that elevated sEPCR levels, linked to the H3 haplotype,
might increase the risk of stroke at pediatric age [113].
Summary
EPCR plays an important role in the regulation of coagulation and
in the APC-evoked cell signaling. Whether or not functional poly-
morphisms in the EPCR gene increase or decrease the risk of
thrombosis, alone or in combination with other risk factors, has not
been undoubtedly demonstrated and requires further investigation.
Conflict of interest statement
The authors state that they have no conflict of interest.
Acknowledgment
This work was supported by the PN de I+ D + I 2008–20011 of
Instituto de Salud Carlos III-Fondo de Investigación Sanitaria and FEDER
(PS09/00610, PI10/01432 and Red Temática de Investigación RECAVA
RD06/0014/0004 and RD06/0014/0008), Conselleria de Educación-
Generalitat Valenciana (Prometeo/2011/027), and Fundación para la
Investigación del Hospital Universitario La Fe (2007–0185), Spain. Pilar
Medina is a Miguel Servet Researcher (ISCIII CP09/00065).
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