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Published in final edited form as:
Life Sci. 2010 July 17; 87(3-4): 69–82. doi:10.1016/j.lfs.2010.06.001.
PECAM-1: Conflicts of Interest in Inflammation
Jamie R. Privratskya,b, Debra K. Newmana,b,c,d, and Peter J. Newmana,b,d,e
aBlood Research Institute BloodCenter of Wisconsin Milwaukee, WI 53201
bDepartment
of Pharmacology, Medical College of Wisconsin Milwaukee, WI 53226
cDepartment
of Microbiology and Molecular Genetics, Medical College of Wisconsin Milwaukee,
WI 53226
dDepartment
of Cellular Biology and Anatomy, Medical College of Wisconsin Milwaukee, WI
53226
eDepartment
of The Cardiovascular Research Center Medical College of Wisconsin Milwaukee,
WI 53226
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Abstract
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Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a cell adhesion and signaling
receptor that is expressed on hematopoietic and endothelial cells. PECAM-1 is vital to the
regulation of inflammatory responses, as it has been shown to serve a variety of pro-inflammatory
and anti-inflammatory functions. Pro-inflammatory functions of PECAM-1 include the facilitation
of leukocyte transendothelial migration and the transduction of mechanical signals in endothelial
cells emanating from fluid shear stress. Anti-inflammatory functions include the dampening of
leukocyte activation, suppression of pro-inflammatory cytokine production, and the maintenance
of vascular barrier integrity. Although PECAM-1 has been well-characterized and studied, the
mechanisms through which PECAM-1 regulates these seemingly opposing functions, and how
they influence each other, are still not completely understood. The purpose of this review,
therefore, is to provide an overview of the pro- and anti-inflammatory functions of PECAM-1 with
special attention paid to mechanistic insights that have thus far been revealed in the literature in
hopes of gaining a clearer picture of how these opposing functions might be integrated in a
temporal and spatial manner on the whole organism level. A better understanding of how
inflammatory responses are regulated should enable the development of new therapeutics that can
be used in the treatment of acute and chronic inflammatory disorders.
Keywords
PECAM-1; CD31; adhesion molecules; inflammation
© 2010 Elsevier Inc. All rights reserved.
Address correspondence to: Jamie Privratsky Blood Research Institute BloodCenter of Wisconsin P.O. Box 2178 638 N. 18th Street
Milwaukee, WI 53201 Phone: (414) 937-3825 Fax: (414) 937-6284 jamie.privratsky@bcw.edu .
Conflict of Interest PJN is a member of the Scientific Advisory Board of the New York Blood Center and Children’s Hospital of
Boston.
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Introduction
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Inflammation is a multi-faceted reaction to tissue injury and/or infection. Inflammatory
responses are protective; however, aberrant inflammation, whether unabated or unresolved,
underlies many of the most common diseases in Western societies. Consequently, a better
understanding of the biology of inflammation, and the key players involved, are vitally
important to the development of treatments that prevent the undesired sequelae of
inflammatory responses.
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The cardinal signs of inflammation were first characterized by Celsus in the first century
A.D. as rubor (redness), tumor (swelling), calor (heat), and dolor (pain) (Celsus 1935).
These cardinal signs are largely the result of two main components of inflammatory
responses: (1) increased vascular permeability and (2) the emigration, accumulation, and
activation of leukocytes (Lawrence et al. 2002). The modulation of vascular permeability
and the recruitment of leukocytes rely on cellular adhesion molecule (CAM)-mediated
intercellular communication amongst adjacent endothelial cells and between endothelial
cells and leukocytes. CAM-mediated interactions allow leukocytes to home to the site of
inflammation, they influence the release of inflammatory mediators that activate both cell
types, and they are important for the maintenance of vascular barrier function.
Consequently, CAM-mediated interactions are vitally important to the initial activation,
maintenance, and subsequent resolution of inflammation. PECAM-1 is one such adhesion
molecule that has historically been implicated in the regulation of inflammatory responses.
This review will focus on the biological properties of PECAM-1 that are pertinent to its proand anti-inflammatory functions.
The biology of PECAM-1
PECAM-1 is a member of the immunoglobulin (Ig)-superfamily of cell adhesion molecules.
It is expressed on most cells of the hematopoietic lineage including platelets, monocytes,
neutrophils, and lymphocyte subsets (Newman 1997; Newman 1999; Newman and Newman
2003). PECAM-1 is also highly expressed on endothelial cells, where it is a major
constituent of the endothelial cell intercellular junction in confluent vascular beds (Muller et
al. 1989; Albelda et al. 1990; Newman et al. 1990; Newman 1997).
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PECAM-1 is a type I transmembrane glycoprotein that consists of an extracellular region
composed of six Ig-like homology domains, a 19-residue transmembrane domain, and a 118
residue cytoplasmic tail (Newman and Newman 2003). The biological properties of
PECAM-1 in cellular adhesion and signaling have been mapped to specific regions of the
PECAM-1 molecule. Extracellular Ig-homology domain 1 contains residues important for
mediating homophilic PECAM-1/PECAM-1 interactions (Fig. 1) (Sun et al. 1996; Newton
et al. 1997). Most heterophilic binding interactions are thought to be mediated by amino acid
residues located in Ig-homology domains 5 and 6 (Fig. 1). The only heterophilic binding
partner of PECAM-1 that has thus far been shown to be physiologically relevant is the
neutrophil-specific antigen CD177 (NB1) (Sachs et al. 2007). Other perhaps more
controversial heterophilic binding partners of PECAM-1 include glycosaminoglycans
(GAG) (Delisser et al. 1993; Sun et al. 1998), the integrin αVβ3 (Piali et al. 1995; Buckley et
al. 1996; Sun et al. 1996), and CD38 on lymphocytes (Deaglio et al. 1998).
The cytoplasmic tail of PECAM-1 contains residues that serve as potential sites for
palmitoylation, phosphorylation, and the docking of cytosolic signaling molecules (Newman
and Newman 2003). The best characterized feature of the PECAM-1 cytosolic domain is
two Immunoreceptor Tyrosine-based Inhibitory Motifs (ITIMs) that encompass Tyr663 and
Tyr686 of human PECAM-1 (Fig. 1), which when phosphorylated, recruit Src homology 2
(SH2) domain-containing proteins, the best characterized of which is the SH2 domainLife Sci. Author manuscript; available in PMC 2011 July 17.
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containing protein tyrosine phosphatase SHP-2 (Newman and Newman 2003). Other SH2
domain-containing proteins that have been reported to associate with phosphorylated
PECAM-1 ITIMs, include members of the Src family of tyrosine kinases (SFK) (Lu et al.
1997; Masuda et al. 1997; Osawa et al. 1997), SHP-1 (Hua et al. 1998; Henshall et al. 2001),
SH2 domain-containing inositol 5’-phosphatase (SHIP) (Pumphrey et al. 1999), and
phospholipase Cγ1 (PLCγ1) (Pumphrey et al. 1999). Another residue in the PECAM-1
cytoplasmic domain that is subject to post-translational modification is Cys595, which, when
palmitoylated, can target PECAM-1 to membrane microdomains (Fig. 1) where it can act as
a regulator of cell signaling and apoptosis (Sardjono et al. 2006).
Due to its expression on vascular and hematopoietic cells, and its signaling and adhesive
capabilities, PECAM-1 is primed to serve a vital role in inflammation. Indeed, much work in
recent years has implicated PECAM-1 as both a positive and negative regulator of
inflammatory responses.
Pro-inflammatory roles for PECAM-1: facilitation of leukocyte
transendothelial migration
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After PECAM-1 was cloned and characterized in 1990 (Newman et al. 1990; Stockinger et
al. 1990; Simmons et al. 1990), many of the early studies of the biological functions of
PECAM-1 were focused on its pro-inflammatory role in leukocyte diapedesis. The first
indications that PECAM-1 helped to promote leukocyte transendothelial migration were
demonstrated in two 1993 reports showing that PECAM-1-specific antibodies blocked both
leukocyte transmigration across endothelial monolayers in vitro (Muller et al. 1993) and
leukocyte accumulation at sites of inflammation in vivo (Vaporciyan et al. 1993). These
studies set the stage for a large body of literature further investigating the mechanism by
which PECAM-1 promotes leukocyte transmigration. As such, many of the established proinflammatory functions of PECAM-1 (summarized in Table 1) center around its ability to
support leukocyte emigration out of the vasculature and into inflammatory sites (Fig. 2).
In order to home to the site of injury or infection, leukocytes go through the well-established
leukocyte adhesion cascade, which ends when they transmigrate across the endothelium in
order to enter extravascular tissues in a process termed leukocyte diapedesis (Muller 2002).
PECAM-1 is known to exert effects on both leukocytes and endothelial cells at both early
and late stages of the leukocyte adhesion cascade (Nourshargh et al. 2006; Woodfin et al.
2007). The first part of this section will discuss how leukocyte PECAM-1 helps to promote
leukocyte emigration and transmigration.
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The role of leukocyte PECAM-1 in leukocyte emigration
Upstream of the leukocyte adhesion cascade, PECAM-1 on leukocytes is reported to
promote chemokine-mediated directional migration of leukocytes to inflammatory sites (Wu
et al. 2005). Chemokine gradients serve to direct leukocytes to their destination by activating
integrins on the leukocyte surface and by promoting actin cycling and polymerization events
at localized sites within the cell (Baggiolini 1998). Consequently, leukocytes that express
PECAM-1 are better able to be recruited to the site of inflammation.
After leukocytes firmly adhere to the area of the endothelium to which they are recruited,
they then squeeze through endothelial junctions and traverse the perivascular basement
membrane to enter the inflamed tissue, both of which are processes that PECAM-1 is known
to promote. One of the well-established mechanisms by which PECAM-1 promotes these
processes is through homophilic PECAM-1/PECAM-1 adhesive interactions between
leukocytes and endothelial cells as they traverse the endothelial cell-cell junction (Muller et
al. 1993; Liao et al. 1995; Liao et al. 1997; Nakada et al. 2000). Though GAGs and the
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integrin αvβ3 expressed on endothelial cells have been reported as heterotypic ligands for
leukocyte PECAM-1 (Delisser et al. 1993; Piali et al. 1995; Buckley et al. 1996), their
physiological relevance remains in question (Sun et al. 1996; Sun et al. 1998).
Physiologically relevant heterotypic ligands for endothelial PECAM-1 have, however, been
shown to be expressed on leukocytes (Sachs et al. 2007), and as such, they will be discussed
in the next section.
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Homophilic PECAM-1/PECAM-1 interactions are not only thought to play an important
adhesive role, but they are also thought to trigger signaling events that lead to the activation
of integrins on leukocytes. Leukocyte integrins reported to be activated downstream of
PECAM-1 ligation include the β1- and β2-integrins in T cell subsets (Tanaka et al. 1992), β1integrins in macrophages (Vernon-Wilson et al. 2006; Vernon-Wilson et al. 2007), β2integrins in natural killer (NK) cells (Berman et al. 1996), and the αMβ2 (Mac-1, CD11b/
CD18) integrin in monocytes and neutrophils (Berman and Muller 1995). Additionally,
homophilic PECAM-1 interactions trigger the upregulation of the integrin α6β1 – a receptor
for laminin, a major component of the basal lamina (Dangerfield et al. 2002) – on
neutrophils, which enables them to traverse the perivascular basement membrane
(Dangerfield et al. 2002; Wang et al. 2005). In fact, leukocytes that lack PECAM-1 are not
able to efficiently transmigrate and are restrained between the endothelium and perivascular
basement membrane (Liao et al. 1995; Duncan et al. 1999; Thompson et al. 2001; Woodfin
et al. 2009). Thus, one of the main functions of PECAM-1 in leukocytes is to activate
integrins downstream of homophilic ligation, which enables the leukocyte to completely
transmigrate through the endothelium and subendothelial matrix.
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Though the process by which PECAM-1 promotes integrin activation in leukocytes is still
poorly understood, there have been hints at potential mechanisms. Antibody-mediated
ligation of PECAM-1 in Jurkat T cells was reported to activate Rap1, a Ras family GTPase,
and this activation required the presence of functional ITIMs in the cytoplasmic tail of
PECAM-1 (Reedquist et al. 2000). Following activation, Rap1 is known to be vital to the
control of cell-adhesive functions as it promotes phagocytosis, cell migration and spreading,
and inside-out integrin activation (Caron 2003). In a second study, a functional association
between PECAM-1 and phosphoinositide-3 kinase (PI3K) was reported in neutrophils, and
blocking the PI3K pathway with pharmacologic inhibitors downstream of PECAM-1
ligation prevented neutrophil adhesion to fibronectin or to fibroblasts transfected with
ICAM-1 (Pellegatta et al. 1998). Additionally, it was demonstrated that PECAM-1
monomers, dimers, and oligomers exist on the cell surface in a dynamic equilibrium, and
that PECAM-1 best serves as a positive activator of integrins when oligomerized (Zhao and
Newman 2001). Finally, PECAM-1 was shown to inhibit membrane repolarization mediated
by the ether-a-go-go related gene (ERG) voltage gated potassium channel, which was
correlated with enhanced β1-integrin-mediated firm adhesion of phagocytes to apoptotic
cells (Vernon-Wilson et al. 2007). Thus, it seems that PECAM-1 promotes integrin
activation through: (1) the regulation of other integrin modulators, (2) its association with
other PECAM-1 molecules within the plane of the membrane, and/or (3) the modulation of
cell membrane potential.
Taken together, these studies support an important pro-inflammatory role for leukocyte
PECAM-1 in leukocyte extravasation to sites of injury and inflammation, as it helps to
promote integrin activation in leukocytes downstream of homophilic adhesive interactions
that involve endothelial PECAM-1. PECAM-1-dependent integrin activation then enables
leukocytes to migrate through both the endothelial junction and subendothelial basement
membrane to enter extravascular sites.
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The role of endothelial PECAM-1 in leukocyte emigration
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Much like in leukocytes, adhesive interactions are a main mechanism through which
endothelial PECAM-1 helps to facilitate leukocyte transmigration. Homophilic PECAM-1/
PECAM-1 interactions between endothelial cells and leukocytes were thought to be the only
physiologically relevant binding interaction in which endothelial PECAM-1 participated
until the recent discovery of CD177 as a bona fide neutrophil heterophilic binding partner of
endothelial PECAM-1 (Sachs et al. 2007). The heterophilic interaction between neutrophil
CD177 and endothelial PECAM-1 is functionally relevant as blocking antibodies directed
against either CD177 or Ig-domain 6 of endothelial PECAM-1 were able to significantly
inhibit neutrophil transmigration toward chemotactic gradients, and CD177-positive
neutrophils migrated more rapidly than CD177-negative neutrophils (Sachs et al. 2007). It
should be noted, however, that CD177 is only expressed on ~45-65% of any individual’s
neutrophils, and that 3-5% of people do not express CD177 at all, with no readily apparent
inflammatory defect (Stroncek et al. 1996; Matsuo et al. 2000). Since CD177 has been
shown to be upregulated in severe bacterial infections (Gohring et al. 2004), after
granulocyte-colony stimulating factor (G-CSF) stimulation (Gohring et al. 2004), and in
pathological conditions in newborns (Wolff et al. 2006), it remains to be determined whether
the CD177/PECAM-1 transmigration pathway is utilized preferentially in certain
inflammatory states by neutrophils that express CD177. At any rate, endothelial PECAM-1
facilitates leukocyte transmigration through two different adhesive mechanisms: (1)
homophilic interactions with leukocyte PECAM-1 on neutrophils, monocytes, and
lymphocytes, and (2) heterophilic interactions with neutrophil CD177. The identification of
CD177 on neutrophils as a heterotypic binding partner of endothelial PECAM-1 raises the
intriguing possibility that distinct heterotypic binding partners on other leukocyte subsets act
to support transendothelial migration. Additionally, it remains to be determined how ligation
of endothelial PECAM-1 by neutrophil CD177 modulates signaling within endothelial cells
to promote leukocyte transmigration, and also if heterotypic CD177/PECAM-1 interactions
promote integrin activation in leukocytes as do homotypic interactions (Tanaka et al. 1992;
Berman and Muller 1995; Berman et al. 1996; Dangerfield et al. 2002; Wang et al. 2005).
These remain active areas of investigation.
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A prominent mechanism by which endothelial PECAM-1 helps to support leukocyte
transmigration was revealed in a series of elegant studies that identified PECAM-1 as a main
constituent of a recycling compartment on endothelial cells, termed the lateral border
recycling compartment (LBRC). The LBRC is a surface-connected membrane network,
distinct from caveolae or vesiculo-vacuolar organelles (VVO), that is located at the borders
between adjacent endothelial cells (Mamdouh et al. 2003). This membrane network contains
PECAM-1, CD99, and junctional adhesion molecule (JAM)-A, and is recycled and targeted
to the region of the cell where paracellular or transcellular migration is occurring (Mamdouh
et al. 2003; Mamdouh et al. 2009). Interestingly, when monocyte PECAM-1 was blocked
with an anti-PECAM-1 blocking antibody, monocytes adhered to endothelial cells and
moved to endothelial junctions normally but were unable to transmigrate, and endothelial
PECAM-1 was not targeted to the zone around the monocyte (Mamdouh et al. 2003).
Consequently, homophilic PECAM-1/PECAM-1 interactions between leukocytes and
endothelial cells are important for triggering targeted PECAM-1- and LBRC-recycling,
which is required for leukocyte transmigration (Mamdouh et al.2003). Later work by this
same group demonstrated that both PECAM-1-dependent and PECAM-1-independent
leukocyte transmigration is dependent on LBRC recycling mediated by endothelial
microtubules and kinesin family molecular motors (Mamdouh et al. 2008). Interestingly,
only Y663 of the PECAM-1 cytoplasmic ITIM, but not Y686 nor ITIM-mediated recruitment
of SHP-2, is essential for PECAM-1 to efficiently enter and exit the LBRC and to support
targeted recycling of the LBRC (Dasgupta et al. 2009).
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Although endothelial cell expression of PECAM-1 is clearly important for leukocyte
transendothelial migration, it is not clear whether signal transduction events mediated by the
cytoplasmic domain of endothelial PECAM-1 are required for these processes. Endothelial
PECAM-1 engagement may trigger signaling pathways that lead to a rise in intracellular
calcium ion concentration (Gurubhagavatula et al. 1998; O’Brien et al. 2001), which helps
initiate signaling pathways that open up intercellular junctions and allow leukocytes to
transit across the endothelial monolayer (Huang et al. 1993). On the other hand, O’Brien, et
al. reported that leukocyte transmigration might not be dependent on signaling by
endothelial PECAM-1 as endothelial-like REN cells expressing mutant forms of PECAM-1
lacking the entire cytoplasmic domain or portions of the cytoplasmic domain known to be
important for PECAM-1-mediated signaling in other cells were still able to support
leukocyte transmigration (O’Brien et al. 2003). Taken together, this study, along with the
study by Dasgupta, et al. described in the preceding paragraph, indicate that ITIM-mediated
recruitment of SHP-2 in endothelial cells does not appear to be required for leukocyte
transmigration (O’Brien et al. 2003; Dasgupta et al. 2009). Since PECAM-1-dependent
leukocyte transmigration requires Y663-mediated targeted recycling of PECAM-1 and the
LBRC (Mamdouh et al. 2003; Mamdouh et al. 2008; Mamdouh et al. 2009), however, it is
likely that this residue or other residues within the cytoplasmic tail of PECAM-1 serve as
docking sites for currently uncharacterized signaling partners and/or as targeting motifs that
result in the trafficking of PECAM-1 to the LBRC. These remain active areas of
investigation.
Whereas the studies described above demonstrate an important role for PECAM-1 in
leukocyte diapedesis, it is important to point out that the requirement for PECAM-1 in this
process is stimulus dependent. In vivo, PECAM-1 has been shown to be required for
leukocyte transmigration in response to certain stimuli, such as IL-1β, but not TNFα nor
certain chemokines (Thompson et al. 2001; Woodfin et al. 2009). It has been suggested that
stimuli such as IL-1β, which mainly activate endothelial cells as opposed to leukocytes,
render leukocytes dependent on transmigration mediated by ICAM-1, JAM-A, and
PECAM-1 in a sequential manner (Nourshargh et al. 2006; Woodfin et al. 2007; Woodfin et
al. 2009). Other stimuli, such as TNFα and chemokines, may bypass the need for these
adhesive interactions by directly activating the leukocyte and allowing it to transmigrate
through junctions (Woodfin et al. 2009). The stimulus dependence of the requirement for
PECAM-1 in leukocyte transendothelial migration lends further support to the importance of
PECAM-1-dependent adhesive interactions for integrin activation in leukocytes, as it
appears that certain stimuli cannot fully activate leukocyte integrins and thus rely on
PECAM-1-mediated integrin activation to support complete leukocyte transendothelial
migration.
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The evidence for PECAM-1’s role in leukocyte transmigration set the precedent for later
studies demonstrating that PECAM-1 blocking reagents might be clinically beneficial for
blockade of leukocyte emigration in inflammatory diseases (Table 1). Antibodies directed
against PECAM-1 blocked accumulation of neutrophils in (1) the peritoneum following
glycogen-induced peritonitis in mice and rats, (2) the lung following IgG immune complex
deposition in rats, and (3) human skin grafts transplanted onto immunodeficient mice
(Vaporciyan et al. 1993;Bogen et al. 1994). Additionally, PECAM-1 blocking reagents
attenuated disease progression in a model of endotoxin-induced keratitis (Khatri et al. 2002);
decreased disease burden in dextran sulfate sodium (DSS)-induced colitis (Rijcken et al.
2007), which is a murine model of inflammatory bowel disease; significantly reduced
ischemia-reperfusion injury in rats by preventing the accumulation of neutrophils in the
myocardium following ischemic injury (Gumina et al. 1996); attenuated the severity of
experimental autoimmune encephalitis (EAE) following short-term administration (Reinke
et al. 2007); and significantly eliminated cartilage and bone destruction in collagen
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antibody-induced arthritis (Dasgupta et al. 2010). Taken together, these studies indicate the
important pro-inflammatory role of PECAM-1 in leukocyte emigration to sites of injury and
inflammation, while potentially demonstrating the efficacy of PECAM-1 blocking reagents
in inflammatory disorders.
Anti-inflammatory roles for PECAM-1
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On the basis of its role in leukocyte transmigration alone, it would seem that PECAM-1
mainly functions to support the inflammatory process; however, it is becoming apparent that
perhaps a more dominant function of PECAM-1 is to suppress inflammatory responses.
More specifically, PECAM-1 has been found to dampen inflammation in a variety of
clinically-relevant acute and chronic inflammatory conditions in C57BL/6 mice
(summarized in Table 2), including collagen-induced arthritis (Tada et al. 2003;Wong et al.
2005), late-stage autoimmunity (Wilkinson et al. 2002), autoimmune encephalitis (Graesser
et al. 2002), lipopolysaccharide (LPS)-induced endotoxic shock (Maas et al. 2005;Carrithers
et al. 2005), atherogenic diet-induced steatohepatitis (Goel et al. 2007), and atherosclerosis
(Goel et al. 2008). PECAM-1 is thought to exert its anti-inflammatory effects through three
main mechanisms, including: (1) raising the threshold for leukocyte activation as a
consequence of its function as an inhibitory receptor (Newton-Nash and Newman
1999;Newman et al. 2001;Wilkinson et al. 2002;Wong et al. 2002;Rui et al. 2007), (2)
helping to maintain and restore the vascular barrier (Graesser et al. 2002;Carrithers et al.
2005;Maas et al. 2005), and (3) dampening production of pro-inflammatory cytokines (Tada
et al. 2003;Carrithers et al. 2005;Maas et al. 2005;Goel et al. 2007). These three mechanisms
will be discussed below.
The inhibitory role of PECAM-1 in leukocyte activation
Some of the earliest studies that implicated PECAM-1 as a possible inflammationdampening receptor were centered around the ability of PECAM-1 to raise the threshold for
leukocyte activation through its cytoplasmic ITIMs. The first of these studies demonstrated
that PECAM-1 is able to dampen T cell activation through attenuation of calcium
mobilization from intracellular stores (Newton-Nash and Newman 1999). It was later
revealed that reduction of calcium mobilization in B cells by PECAM-1 cross-linking
required the PECAM-1 ITIMs and the presence of SHP-2 (Newman et al. 2001). These
studies helped to demonstrate that PECAM-1 could indeed function as an inhibitory receptor
and were the first to provide evidence for inclusion of PECAM-1 in the ITIM family. In
support of PECAM-1 as an inhibitory receptor in lymphocytes, PECAM-1−/− mice exhibit
aberrant proliferation and activation of B cells, which correlates with development of
autoimmune disease in older mice (Wilkinson et al. 2002).
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PECAM-1 inhibitory receptor functions are not restricted to lymphocytes, but appear to
apply to mast cells and macrophages as well. PECAM-1 suppresses mast cell activation,
which prevents systemic and local IgE-dependent anaphylactic reactions when animals are
challenged with allergic stimuli (Wong et al. 2002). In macrophages, ligation of PECAM-1
with a CD38-Fc fusion protein (a reported heterotypic ligand for PECAM-1 on
lymphocytes) was reported to negatively regulate Toll-like receptor (TLR) 4 signaling,
likely through ITIM/SHP-2 interactions (Rui et al. 2007). Taken together, these studies
provide compelling evidence that PECAM-1 is able to negatively regulate pro-inflammatory
activation in lymphocytes, mast cells, and macrophages, likely through ITIM-mediated
inhibitory signaling (Fig. 3).
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PECAM-1 helps to maintain vascular barrier function
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The first indication that PECAM-1 was important for maintenance of the vascular barrier
was demonstrated by Ferrero, et al., who showed that a blocking antibody directed against
PECAM-1 increased the permeability of endothelial monolayers in vitro and of multiple
vascular beds in vivo (Ferrero et al. 1995). It was further demonstrated that PECAM-1 helps
to promote vascular barrier function in response to a wide range of inflammatory stimuli. In
EAE, which is an established rodent model of the human disease multiple sclerosis, mice
expressing PECAM-1 had less inflammatory cell infiltration into the brain parenchyma, and
this phenotype correlated with enhanced barrier function of PECAM-1-expressing
endothelial monolayers (Graesser et al.2002). Expression of PECAM-1 also hastened
restoration of the vascular barrier in LPS-induced endotoxemia, a well-established mouse
model of sepsis (Carrithers et al. 2005; Maas et al. 2005). Additionally, PECAM-1-deficient,
relative to PECAM-1-expressing, endothelial monolayers are hyperpermeable to histamine
both in vivo (Graesser et al. 2002) and in vitro (Graesser et al. 2002; Biswas et al. 2006).
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The precise mechanism(s) by which PECAM-1 helps to preserve and restore vascular
integrity are still poorly understood. It is possible that PECAM-1 helps to maintain vascular
barrier function through modulation of other primary regulators of permeability. One
mechanism might be through interactions with catenins, which are proteins known to
enhance barrier function (Komarova et al. 2007). PECAM-1 has been reported to bind,
maintain the de-phosphorylated (barrier protective) state, and enhance the stability of
catenins, which has been correlated with PECAM-1-mediated vascular barrier enhancement
(Matsumura et al. 1997; Biswas et al. 2003; Biswas et al. 2005; Biswas et al. 2006).
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An alternative, and potentially attractive mechanism for PECAM-1-mediated barrier
maintenance is via modulation of signaling by sphingosine-1-phosphate (S1P), which is
known to promote vascular barrier function through adherens junction assembly (Komarova
et al. 2007). Sphingosine kinase (Sphk) is the upstream kinase that produces active S1P
(Rosen and Goetzl 2005). PECAM-1 has been shown to interact with Sphk in transfected
cells and modulate its function (Fukuda et al. 2004; Limaye et al. 2005), suggesting that
PECAM-1 might regulate S1P signaling upstream of S1P binding to its cell surface
receptors. Localization of S1P receptors to membrane microdomains has been found to be
important for their signaling functions (Igarashi and Michel 2000). PECAM-1 might
regulate S1P function at the receptor level as it has been shown to modulate signaling from
receptors localized to membrane microdomains (Sardjono et al. 2006; Lee et al. 2006).
Downstream of its cell surface receptor, S1P was reported to induce the phosphorylation of
PECAM-1 in a Gαi- and SFK-dependent manner, though S1P-mediated phosphorylation of
PECAM-1 was not correlated with enhanced vascular barrier function (Huang et al. 2008).
Due to the importance of S1P in vascular barrier function and the ability of S1P and
PECAM-1 to regulate each other, further clarity is needed to determine whether PECAM-1
can indeed promote barrier maintenance through S1P signaling.
One other interesting mechanism by which PECAM-1 could promote barrier function is
through Rap1. Rap1 is important for restoring barrier function in endothelial cells through
accelerated assembly of endothelial cell-cell junctions (Wittchen et al. 2005). If PECAM-1
promotes activation of Rap1 in endothelial cells, as it does in T cells (Reedquist et al. 2000),
junctional assembly would be accelerated resulting in enhanced barrier function. Further
work needs to be done to determine whether any or all of these mechanisms are important
for PECAM-1-mediated vascular barrier maintenance. Identifying the pertinent biological
properties of PECAM-1 (i.e. homophilic adhesion, localization to lipid rafts, ITIM-mediated
signaling) that help to promote vascular barrier maintenance will go a long way toward
answering these questions.
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PECAM-1 dampens cytokine production
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Studies conducted in our laboratory and by Carrithers, et al. established that PECAM-1
helps to suppress the production of pro-inflammatory cytokines following endotoxin
exposure (Maas et al. 2005; Carrithers et al. 2005). It was subsequently demonstrated that
PECAM-1 also suppresses cytokine production in two other mouse models of inflammation,
namely nonalcoholic steatohepatitis (Goel et al. 2007) and collagen-induced arthritis (Tada
et al. 2003). In the latter study, lymphocytes expressing PECAM-1 produced lower levels of
the pro-inflammatory cytokine, IFNγ, following stimulation with collagen than did
PECAM-1−/− lymphocytes (Tada et al. 2003). Taken together, these studies firmly establish
that expression of PECAM-1 is important for dampening levels of multiple proinflammatory cytokines on the cellular and whole animal level.
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The mechanism by which PECAM-1 regulates cytokine production is still poorly
understood. Rui, et al. have been the only group to reveal a potential mechanism through
which PECAM-1 dampens cytokine production. They reported that PECAM-1 ligation in T
cells with a CD38 fusion protein was able to inhibit activation of the JNK, NF-κB, and
IRF-3 pathways, which was correlated with dampened cytokine production (Rui et al. 2007).
Since these signaling pathways regulate a variety of signaling pathways within cells, greater
clarity is needed as to the mechanism by which PECAM-1 might inhibit these pathways. It
will also be important in future studies to determine the main cellular source of proinflammatory cytokines that PECAM-1 inhibits to exert its anti-inflammatory effects.
Studies using leukocytes and endothelial cells isolated from PECAM-1+/+ and PECAM-1−/−
mice will go a long way toward clarifying PECAM-1’s role in regulating cytokine
production.
Endothelial PECAM-1 exerts the predominant protective effects in inflammation
One additional insight into the protective effects of PECAM-1 in inflammation has been
gleaned from studies of bone marrow chimeric mice that express PECAM-1 selectively on
either endothelial cells or leukocytes. These studies have revealed that endothelial, but not
leukocyte, PECAM-1, is largely sufficient for protection against excessive inflammation in
the inflammatory disease models EAE (Graesser et al. 2002) and LPS-induced endotoxemia
(Maas et al. 2005). The specific mechanism(s) by which endothelial PECAM-1 is protective
in these disease models, however, is not completely understood. PECAM-1 is likely to be
protective in endothelial cells during inflammation due to its ability to (1) inhibit cytokine
production, (2) maintain vascular integrity, and/or (3) inhibit pro-inflammatory signaling.
Since the first two mechanisms have been previously discussed, we will now focus our
attention on the third.
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One mechanism that helps to explain some of the anti-inflammatory effects of endothelial
PECAM-1 is PECAM-1-mediated promotion of signal transducer and activator of
transcription 3 (STAT3) signaling in endothelial cells (Carrithers et al. 2005). STAT3 is
known to be important in the regulation of the acute phase response during inflammation
(Alonzi et al. 2001), and although it can mediate transcription of both pro- and antiinflammatory genes, studies in cell-type specific knockout mice suggest that its antiinflammatory effects might predominate in models of inflammation (Takeda et al. 1999;
Kano et al. 2003). Expression of PECAM-1 was correlated with enhanced phosphorylation
of STAT3 in both endothelial cells and splenocytes from mice (Carrithers et al. 2005). On
this basis, it has been proposed that binding of SHP-2 to the PECAM-1 ITIMs sequesters
SHP-2 away from STAT3, which prevents SHP-2-mediated STAT3 dephosphorylation and
prolongs activation of STAT3 (Carrithers et al. 2005). Consequently, endothelial cells
expressing PECAM-1 are postulated to have more STAT3 mediated anti-inflammatory
signaling. This mechanism bears further examination in mice, however, since the
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predominant isoforms of PECAM-1 that are expressed in murine tissues, including
endothelial cells, lack exon 14 (contains the second cytoplasmic ITIM) (Sheibani et al. 1997;
Sheibani et al. 1999; Wang and Sheibani 2002), and thus are not likely to be able to
efficiently recruit SHP-2 (Wang and Sheibani 2006; Dimaio and Sheibani 2008).
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Alternatively, Cepinskas, et al. reported that PECAM-1 engagement, induced by either
antibody mediated cross-linking or leukocyte transmigration, resulted in decreased levels of
NF-κB in the nuclei of endothelial cells, which led the authors to propose that inhibition of
NF-κB translocation to the nucleus by PECAM-1 initiates a negative feedback loop that
prevents excessive leukocyte recruitment to sites of inflammation by dampening the NF-κBdependent expression of pro-inflammatory adhesion molecules on the endothelial cell
surface (Cepinskas et al. 2003). In a series of extensive investigations, however, we have
been unable to confirm this mechanism of PECAM-1-mediated anti-inflammatory signaling
(Privratsky et al. 2010). Using a variety of antibody reagents to cross-link PECAM-1 in
primary endothelial cells, we found that neither engagement nor cross-linking of PECAM-1
has an inhibitory effect on NF-κB activity, as determined by Western blot analysis for
phosphorylated and total IκBα, immunofluorescence for detection of nuclear NF-κB, or
electrophoretic mobility shift assays to detect binding of NF-κB to target oligonucleotides
(Privratsky et al. 2010). We also found that higher levels of PECAM-1 expression do not
correlate with lower levels of NF-κB transcriptional activity in cytokine-stimulated HEK293
cells containing an NF-κB luciferase reporter plasmid, nor do they prevent the upregulation
of ICAM-1, an NF-κB target gene, in cytokine-stimulated endothelial cells (Privratsky et al.
2010). Taken together, these results suggest that the anti-inflammatory effects of PECAM-1
in endothelial cells, at least in the case of inflammatory mediators such as LPS and
cytokines, are likely not due to inhibition of NF-κB transcriptional activity.
Even though engagement of endothelial PECAM-1 during leukocyte transmigration does not
appear to inhibit NF-κB activity, it likely does send inhibitory signals to prevent excessive
endothelial activation. Couty, et al. demonstrated that PECAM-1 ligation with monoclonal
antibodies counteracted ICAM-1 ligation-induced endothelial activation and cytoskeletal
rearrangement, which is thought to promote junctional opening and leukocyte transit (Couty
et al. 2007). These results would suggest that as leukocytes transmigrate through the
junction, PECAM-1 becomes engaged, which then signals for the endothelial cell to close
the junction and return to the basal state (Couty et al. 2007). This attractive hypothesis also
has implications for PECAM-1-mediated vascular barrier protection as cytoskeletal
rearrangements are prominent during barrier disruption.
The interplay of the pro- and anti-inflammatory functions of PECAM-1
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Though seemingly opposing pro- and anti-inflammatory roles for PECAM-1 have been
established, which of these roles dominates is likely to depend on the context of the cells,
organs, inflammatory stimulus, and animal model used. Studies examining PECAM-1’s role
in atherosclerotic development, how strain-specific differences confer PECAM-1independent leukocyte transmigration, and the biological outcomes resulting from the
differential expression of alternatively-spliced PECAM-1 isoforms in mouse and human
tissues have provided some insights into how the pro- and anti-inflammatory properties
influence each other spatially and on the whole animal level.
PECAM-1 as site-specific regulator of atherosclerotic lesion development
Though most studies demonstrating a pro-inflammatory role for PECAM-1 have centered
around the process of leukocyte emigration, there is a growing body of literature in the field
of atherosclerosis research revealing that PECAM-1 promotes development of inflammatory
responses through another mechanism, that being as a mechanosensor that helps to activate
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endothelial cells in response to mechanical stimulation (Fig. 2). One of the main components
of atherosclerotic lesion development is the response of the vessel to the flowing blood
within it. The endothelium is constantly subjected to mechanical forces such as stretch,
cyclic mechanical strain, and fluid shear stress (Lehoux et al. 2006). Signals emanating from
these forces are thought to be transmitted by the cytoskeleton from the apical surface of the
endothelial cell to points of attachment at cell-cell and cell-matrix junctions (Tzima et al.
2005). Such signals can be pro-inflammatory by inducing expression of adhesion molecules
that support leukocyte adhesion and transmigration, and the nature of the mechanical forces
to which endothelial cells are exposed determine whether signal transduction is initiated or
not. Endothelial cells adapt to unidirectional, or laminar, shear stresses and therefore fail to
activate pro-inflammatory signaling pathways (Davies 1997;Tzima et al. 2005). As such,
unidirectional or laminar shear stress is thought to be atheroprotective (Davies 1997). In
contrast, in regions of oscillatory or disturbed flow, which occur at vessel bifurcations and in
regions of high curvature, endothelial cells are unable to adapt to shear stresses and therefore
activate pro-inflammatory signaling pathways without compensatory downregulation
(Mohan et al. 1997). As a result, atherosclerotic lesions tend to develop preferentially in
these regions of low or disturbed shear stress (Davies 1997).
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Much work has gone into identifying the cell surface receptors that transmit mechanical
signals. Few “mechano-responsive” receptors have been identified, however, and the
mechanisms by which these receptors transmit signals are largely unknown. It is thought
that components of the cell-cell and cell-matrix junctions are candidates to transduce these
signals, and due to PECAM-1’s localization to cell-cell junctions in endothelial cells, it
emerged as a prime candidate. The first studies examining PECAM-1 as a mechanosensor
reported that PECAM-1 becomes tyrosine phosphorylated by SFK in response to mechanical
force (Osawa et al. 1997), which allows it to recruit SHP-2 and subsequently activate
extracellular signal-regulated kinase (ERK) (Osawa et al. 2002). Further proof that
PECAM-1 is part of a mechanosensory complex that responds to shear stress on endothelial
cells was demonstrated by Tzima, et al (Tzima et al. 2005). The authors of this study found
that VE-cadherin and PECAM-1 cooperate in endothelial cells to induce PI3K/Akt-mediated
integrin activation, align actin filaments, and activate NF-κB following shear stress (Tzima
et al. 2005). Responsiveness to flow in endothelial cells was further shown to be dependent
on (1) PECAM-1, which transmits mechanical force, (2) VE-cadherin, which functions as an
adaptor, and (3) VEGFR2, which activates PI3K (Tzima et al. 2005). PI3K subsequently
activates NF-κB, causing pro-inflammatory gene induction. Accordingly, mice that express
PECAM-1 are able to transduce signals in response to mechanical force and activate NF-κB,
which results in the transcription of pro-inflammatory genes at regions of disturbed flow
(Tzima et al. 2005) and subsequently induces vascular remodeling (Chen and Tzima 2009).
PECAM-1-mediated promotion of atherosclerotic lesion development was further confirmed
in two other independent studies (Harry et al. 2008; Stevens et al. 2008).
In contrast, another study has demonstrated that PECAM-1 can have site-specific
atheroprotective effects during the development of atherosclerosis. LDL receptor knockout
(LDL−/−)/PECAM-1+/+ mice that are fed a high fat diet were shown to have significantly
decreased atherosclerotic lesion area in the total aorta with preferential protection in the
aortic sinus, descending aorta, and the branching arteries of the aortic arch compared to
LDL−/−/PECAM-1−/− mice (Goel et al. 2008), Interestingly, in support of the studies
described above (Tzima et al. 2005; Harry et al. 2008; Stevens et al. 2008), expression of
PECAM-1 did, however, promote atherosclerotic development in the inner curvature of the
aortic arch, an area associated with disturbed flow (Goel et al. 2008).
Taken together, the results of these studies indicate that PECAM-1 can have
atheroprotective effects in certain areas of the vasculature (Goel et al. 2008), but pro-
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atherosclerotic effects in other areas of the vasculature (Tzima et al. 2005; Harry et al. 2008;
Stevens et al. 2008; Goel et al. 2008). These seemingly contradictory results can likely be
explained by the different biological functions of PECAM-1. Pro-atherosclerotic effects of
PECAM-1 in the inner curvature of the aortic arch support the concept that PECAM-1 acts
as a mechanotransducer that contributes to the development of atherosclerosis in regions of
disturbed shear stress (Tzima et al. 2005; Harry et al. 2008; Stevens et al. 2008; Goel et al.
2008). In these areas of disturbed flow and shear, the function of PECAM-1 as a
mechanotransducer appears to be required, perhaps to both induce NF-κB-dependent
expression of pro-inflammatory adhesion molecules and enable leukocyte transmigration
into the affected region, both of which would be pro-inflammatory and pro-atherosclerotic.
In contrast, in other regions of the vasculature (descending aorta, aortic sinus, branching
vessels), where PECAM-1 was demonstrated to have atheroprotective effects (Goel et al.
2008), the role of PECAM-1 in mechanotransduction and leukocyte transmigration do not
appear to be as important and its anti-inflammatory properties (i.e. the maintenance of
vascular barrier function and dampening of pro-inflammatory cytokine production)
predominate, which slows the development of atherosclerotic lesions. Future work will need
to be aimed at reconciling these differences, and definitively determining which biological
function of PECAM-1 predominates at what time and in which area. It is also interesting to
note that certain single nucleotide polymorphisms (SNP) within PECAM-1 have been
associated with a higher risk of coronary artery disease, which is a downstream consequence
of atherosclerosis (Wei et al. 2004). It will be interesting to determine whether these SNPs
change the biological properties of PECAM-1 such that it is better able to transduce
mechanical signals or less well able to exert anti-inflammatory effects. It is also currently
not known how PECAM-1 converts mechanical signals into cellular signals, or how it
couples to VE-cadherin and VEGFR2 signaling. One potential mechanism could be through
association with G-protein coupled receptors (GPCR), such as the bradykinin receptor B2
(Yeh et al. 2008; Otte et al. 2009), through which PECAM-1 could modulate VE-cadherin/
VEGFR2 signaling.
Strain-specific differences confer PECAM-1-independent leukocyte transendothelial
migration
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The important roles that PECAM-1 plays in recruiting leukocytes to sites of inflammation
and supporting their extravasation support the hypothesis that a PECAM-1-deficient mouse
might exhibit severe defects in leukocyte trafficking. Interestingly, when Duncan, et al.
generated the first PECAM-1 knockout mice on the C57BL/6 genetic background, they
unexpectedly found that normal numbers of leukocytes were recovered from sites of
inflammation in these mice, with the only phenotype being trapping of leukocytes at the
perivascular basement membrane (Duncan et al. 1999). To further investigate these
unexpected findings, Schenkel, et al. backcrossed C57BL/6 PECAM-1 knockout mice onto
the FVB/n strain, and they showed that these mice did display defects in leukocyte
emigration following thioglycollate-induced peritonitis and croton oil-induced topical
dermatitis (Schenkel et al. 2004). They further demonstrated that leukocyte emigration could
be blocked by anti-PECAM-1 reagents in not only FVB/n, but also SJL and Swiss Webster
mice, whereas emigration could not be blocked by these same reagents in C57BL/6
PECAM-1−/− mice (Schenkel et al. 2004). As such, C57BL/6 PECAM-1−/− mice are unique
among mouse strains in their ability to compensate for loss of PECAM-1 function in
leukocyte transmigration (Schenkel et al. 2004). QTL mapping between PECAM-1−/− FVB
and C57BL/6 mice has subsequently identified a single locus on chromosome 2 that confers
PECAM-1-independent leukocyte transmigration in C57BL/6 mice, though the specific
gene(s) remain to be determined (Seidman et al. 2009).
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It is interesting to point out that virtually all of the studies thus far describing an antiinflammatory and protective effect of PECAM-1 (Table 2) have been performed in C57BL/6
mice, in which leukocyte transendothelial migration has been found to be largely
PECAM-1-independent. These studies include evaluation of the effect of PECAM-1
deficiency on LPS-induced endotoxemia (Maas et al. 2005), EAE (Graesser et al. 2002),
collagen-induced arthritis (Tada et al. 2003), and atherogenic diet-induced steatohepatitis
(Goel et al. 2007), and atherosclerosis (Goel et al. 2008). It can be hypothesized that use of
this particular strain of mice has enabled investigators to observe the anti-inflammatory
effects of PECAM-1 without those effects being influenced by PECAM-1’s proinflammatory promotion of leukocyte transmigration (Fig. 4a). In other strains of mice,
wherein PECAM-1 is required for leukocyte transendothelial migration, it might be
expected that the anti-inflammatory effects of PECAM-1 would be offset by its proinflammatory effects on leukocyte transmigration (Fig. 4b). Consequently, the ability of
PECAM-1 to suppress cytokine production and maintain vascular integrity in response to
inflammatory insult in C57BL/6 mice, thus lessening the severity of disease and giving the
impression that PECAM-1 is mainly anti-inflammatory on the whole animal level (Fig. 4b),
may not generalize to all strains of mice. It will be interesting in future studies to determine
whether the predominant anti-inflammatory effect of PECAM-1, which is seen in C57BL6
mice, is also observed when mice of other genetic strains are exposed to similar
inflammatory disease models.
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Isoform-specific functions of PECAM-1
It is now becoming apparent that some of the contrasting functions of PECAM-1 in
inflammation might be due to differential, cell type-specific expression of alternatively
spliced PECAM-1 isoforms as alternative splicing of the PECAM-1 cytoplasmic domain can
affect inflammatory events, including angiogenesis, leukocyte-endothelial cell adhesion,
leukocyte diapedesis, endothelial junctional stability, and cell survival in both murine and
human cells (Sheibani et al. 1997; Sheibani et al. 2000; Wang et al. 2003a; Wang and
Sheibani 2006; Kondo et al. 2007; Dimaio and Sheibani 2008; Bergom et al. 2008). The
PECAM-1 gene consists of 16 exons, with the cytoplasmic domain being encoded from the
end of exon 9 through exon 16 (Kirschbaum et al. 1994). Alternative splicing of the
PECAM-1 cytoplasmic and transmembrane domains results in the production of numerous
PECAM-1 isoforms, including a soluble form (Goldberger et al. 1994) and various isoforms
that lack one or more cytoplasmic exons (Kirschbaum et al. 1994; Baldwin et al. 1994; Yan
et al. 1995; Sheibani et al. 1997; Sheibani et al. 1999; Sheibani et al. 2000; Robson et al.
2001; Wang and Sheibani 2002; Wu and Sheibani 2003; Wang et al. 2003a; Wang et al.
2003b; Wang et al. 2004; Bergom et al. 2008; Dimaio and Sheibani 2008).
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In humans, full length PECAM-1 is by far the predominant isoform expressed in all cells
(Wang et al. 2003b), whereas PECAM-1 mRNA in mice tends to undergo more extensive
alternative splicing with Δ14,15 (loss of exons 14 and 15) being the predominant isoform
expressed in most cells (Sheibani et al. 1999). The reader is referred to a previous review
that describes the production of PECAM-1 isoforms encoding differing C-terminal
sequences and the tissue distribution of alternatively spliced PECAM-1 isoforms (Newman
and Newman 2003).
The first biological function of PECAM-1 reported to be affected by alternative splicing was
cell-cell adhesion. Thus, transfection of mouse PECAM-1 isoforms into mouse fibroblast Lcells was found to alter the adhesive properties of L-cell fibroblasts such that PECAM-1
participated in heterophilic binding interactions when exon 14 is present, but only
homophilic binding interactions when exon 14 was absent (Yan et al. 1995). Alternatively
spliced human PECAM-1 lacking exon 14 also modified the adhesive properties of
hematopoietic cell lines (Wang et al. 2003a), leading the authors to propose that stimulusLife Sci. Author manuscript; available in PMC 2011 July 17.
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specific isoform switching might provide a mechanism by which PECAM-1-expressing
cells, especially leukocytes, can regulate their adhesive properties.
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Isoform switching of PECAM-1 also has the potential to change the signaling properties of
PECAM-1-expressing cells, as only PECAM-1 isoforms that contain ITIMs encoded by
exons 13 and 14 are able to efficiently recruit and activate SHP-2 (Wang and Sheibani 2006;
Dimaio and Sheibani 2008; Bergom et al. 2008). This has cell-specific consequences, as
expression in heterologous Madin-Darby canine kidney (MDCK) cells of a mouse
PECAM-1 isoform containing exon 14, as opposed to one lacking exon 14, led to activation
of mitogen activated protein kinases (MAPK), extracellular signal-regulated kinases (ERK),
and the small GTPases Rac1 and Rap1, resulting in loss of cell-cell contacts, de-stabilization
of adherens junctions, and a change in the subcellular localization of cadherins and catenins
(Sheibani et al. 2000; Wang and Sheibani 2006). These effects have been proposed to
explain the more migratory phenotype of PECAM-1-expressing endothelial cells during
angiogenesis, as exon 14-positive PECAM-1 isoforms were found to be preferentially
expressed early in vascular development, and replaced later by isoforms lacking exon 14
(Sheibani et al. 1997; Sheibani et al. 2000; Wu and Sheibani 2003); or to modulate the
spatio-temporal disruption of adherens junctions downstream of PECAM-1 homophilic
interactions during leukocyte transmigration (Wang and Sheibani 2006). Expression of the
exon-14-containing murine PECAM-1 in an immortalized mouse brain endothelial cell line
(bEND), however, had minimal effects on the activation of MAPK/ERKs and resulted in a
less migratory phenotype, which was hypothesized to be due to SHP-2-mediated inhibition
of signaling following recruitment to the PECAM-1 ITIMs (Dimaio and Sheibani 2008).
Thus, it appears that isoform switching can change not only the adhesive properties of
PECAM-1, but through loss of functional domains that bind signaling partners, it can also
lead to the differential modulation of signaling pathways that can have quite divergent
effects. Future studies will be required to determine which signaling pathways become
activated versus suppressed by PECAM-1 in a cell-, isoform, and context-specific manner.
Additionally, since any individual cell is able to express multiple PECAM-1 isoforms, it is
likely that the sum biological effect of PECAM-1 on cellular signaling will reflect the
relative abundance of the various isoforms that are expressed (Dimaio and Sheibani 2008).
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The subcellular localization of PECAM-1 can also be influenced by isoform switching. For
example, mouse Δ15 PECAM-1 (contains exon 14) expressed in MDCK cells does not
localize to cell-cell junctions (Sheibani et al. 2000), whereas expression of this same isoform
in bEND cells results in predominantly junctional localization (Dimaio and Sheibani 2008).
These studies suggest that specific signaling domains of the mouse PECAM-1 cytoplasmic
domain are important for determining its subcellular localization, which is especially
relevant to endothelial cells where PECAM-1 tends to concentrate at the cell-cell junction
(Muller et al. 1989; Newman et al. 1990; Albelda et al. 1990). Other studies have
demonstrated, however, that the cytoplasmic domain of human PECAM-1 is not important
for junctional localization, but that homophilic-mediated adhesion mediated by extracellular
Ig domains determines its junctional localization (Sun et al. 2000; Bergom et al. 2008),
though these studies were performed in heterologous REN cells. Though controversial, it
nevertheless appears that species- and cell-specific effects modulate the subcellular
localization of PECAM-1 such that homophilic binding to neighboring PECAM-1 molecules
is sufficient to enable junctional localization in some cells and species, whereas additional
signaling and/or trafficking mediated by cytoplasmic residues is required in others. Since
endothelial cells are the relevant cell of interest for this line of investigation, studies that
have used non-endothelial cell lines to ascertain the biological functions of PECAM-1 in
endothelial cells might benefit by re-performing them in primary murine and human
endothelial cell lines.
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Concluding remarks
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PECAM-1 is a multi-functional adhesion and signaling molecule that displays both pro- and
anti-inflammatory effects. For pro-inflammatory effects, it has been well-established and
validated in the literature that PECAM-1 is very important for the process of leukocyte
transendothelial migration. PECAM-1 mediates leukocyte transmigration through adhesive
interactions, the activation of integrins, and the modulation of LBRC recycling, which is
important for both paracellular and transcellular leukocyte migration. The precise
mechanisms through which PECAM-1 promotes integrin activation and LBRC recycling are
still not completely understood. PECAM-1 has also been demonstrated to be a mechanotransducing molecule that enables endothelial cells to respond to changed in fluid shear
stress (Tzima et al. 2005). This likely has implications for atherosclerotic development in
areas of the vasculature exposed to disturbed shear stress as PECAM-1 can activate the proinflammatory transcription factor NF-κB downstream of mechanical activation (Tzima et al.
2005). How PECAM-1 actually senses mechanical force, and how this couples to signal
transduction, remains an active area of investigation.
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For anti-inflammatory roles, PECAM-1 is able to dampen leukocyte activation through
recruitment of inhibitory phosphatases to its cytoplasmic ITIMs (Newton-Nash and
Newman 1999; Wilkinson et al. 2002; Wong et al. 2002; Rui et al. 2007). PECAM-1 also
helps to dampen pro-inflammatory cytokine production and restore vascular barrier integrity
through as yet poorly understood mechanisms. Future studies will need to be aimed at
elucidating the specific mechanisms by which PECAM regulates these latter two processes.
On the whole organism level, the interplay of the pro- and anti-inflammatory functions of
PECAM-1 has implications for both the promotion of atherosclerotic lesion development
and for protection from their progression. It will be interesting in future studies to determine
which of PECAM-1’s biological functions predominate in both a temporal and spatial
manner, and whether isoform-specific expression of PECAM-1, and/or genetic differences
between mouse strains, influence these processes. Overall, a better understanding of how
PECAM-1 integrates its pro- and anti-inflammatory properties during inflammatory
responses will provide new insights into the biology of inflammation as well as reveal novel
therapeutic targets in the treatment of both acute and chronic inflammatory disorders.
Acknowledgments
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The authors would like to thank Benjamin Tourdot for his helpful comments during manuscript preparation. This
work was supported by Predoctoral Fellowship Award 0810167Z (to JRP) from the Midwest Affiliate of the
American Heart Association, and by grant HL-40926 (to PJN) from the National Heart, Lung, and Blood Institute
of the National Institutes of Health.
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Figure 1. The structure and function of PECAM-1
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PECAM-1 is a 130 kDa type I transmembrane glycoprotein belonging to the Ig-like
superfamily of adhesion molecules. The biological functions of PECAM-1 have been
mapped to specific regions of the PECAM-1 molecule as shown in the figure. Residues
important for mediating homophilic and heterophilic binding interactions are located within
Ig-domain 1 and Ig-domains 5 & 6, respectively. Localization of PECAM-1 to membrane
microdomains occurs after palmitoylation of residue Cys595. Two ITIMs encompass
residues Y663 and Y686 within the cytoplasmic tail and are able to serve as docking sites for
cytosolic signaling molecules when the tyrosines become phosphorylated.
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Figure 2. The pro-inflammatory functions of PECAM-1
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PECAM-1 serves a variety of pro-inflammatory functions in both leukocytes and endothelial
cells. 1. Upstream of the leukocyte adhesion cascade, PECAM-1 promotes chemokinedirected migration of leukocytes by enhancing actin polymerization and cycling. 2. The
binding interaction between CD177 on neutrophils and PECAM-1 on endothelial cells is the
only known heterotypic binding interaction that has been shown to be important for
PECAM-1-mediated leukocyte transmigration. 3. Homotypic binding interactions between
PECAM-1 on leukocytes and endothelial cells are thought to be essential for leukocyte
diapedesis as blocking these interactions prevents diapedesis. 4. Downstream of homotypic
binding interactions, PECAM-1 is able to activate integrins on leukocytes that are essential
for adhesion to endothelial cells and passage through the basement membrane. Postulated
mechanisms of PECAM-1-mediated integrin activation on leukocytes include, but are not
limited to, the activation of PI3K and the GTPase Rap1. 5. During both paracellular (shown
in figure) and transcellular (not shown) leukocyte transmigration, PECAM-1 is part of a
surface connected membrane compartment termed the LBRC that likely serves to provide
more surface area and/or unligated PECAM and other molecules to the leukocyte. 6.
PECAM-1 also is able to transmit mechanical signals in endothelial cells in response to fluid
forces. PECAM-1 becomes activated, and through poorly understood mechanisms, is able to
transduce signals to VE-cadherin, which then activates VEGFR2. This results in the
activation of integrins and the pro-inflammatory transcription factor NF-κB, which promotes
atherosclerotic lesion development.
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Figure 3. PECAM-1/SHP-2 interactions dampen leukocyte activation
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The PECAM-1 cytoplasmic tail contains ITIM tyrosines, which when phosphorylated, can
serve as docking sites for cytosolic signaling molecules. Most times, ITIMs recruit
phosphatases to counteract kinases that become activated by ITAMs. The best characterized
phosphatase that is recruited by PECAM-1 is SHP-2. PECAM-1/SHP-2 interactions have
been proposed to inhibit activation of B cell, T cell, mast cell, and macrophage functions as
represented in the figure.
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Figure 4. Proposed schematic for the interplay of the pro- and anti-inflammatory properties of
PECAM-1 in different genetic strains of mice
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(a) PECAM-1 is known to possess both pro- and anti-inflammatory properties as displayed
in the figure, which would be proposed to largely offset each other during inflammation. (b)
Most studies to date, however, have used the C57BL/6 genetic strain of mice and have
described a predominant anti-inflammatory effect of PECAM-1 in models of inflammation.
Loss of PECAM-1 in most strains of mice results in profound defects in leukocyte
transmigration, however, this is not true in C57BL/6 mice, where PECAM-1 plays only a
minor role in this process. As a result, the ability of PECAM-1 to suppress cytokine
production and/or maintain vascular integrity in response to inflammatory insult tends to
dominate in C57BL/6 mice, lessening their severity of their disease. It is hypothesized,
therefore, that examining inflammatory disease models in other strains of mice, where the
pro- and anti-inflammatory functions of PECAM-1 should largely offset each other, will not
show predominate anti-inflammatory effects of PECAM-1.
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Table 1
Pro-inflammatory functions of PECAM-1
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Inflammatory Model
Results
Mouse
strain
References
Neutrophil chemotaxis in Zigmund
chambers toward IL-8, KC, or fMLP
gradients
PECAM-1 promotes directionality, motility,
spreading, and stabilizes
F-actin polymerization in neutrophils
N/A
(Wu et al. 2005)
Endotoxin-induced keratitis
Antibody blockade of PECAM-1 inhibited
neutrophil recruitment and
prevented endotoxin-induced increases in
stromal thickness and
haze
BALB/c
(Khatri et al. 2002)
DSS-induced colitis
Antibody blockade of PECAM-1 reduced
leukocyte transmigration
and diminished disease severity
BALB/c
(Rijcken et al. 2007)
IL-1β-induced peritonitis
PECAM-1 helps neutrophils transmigrate
across the basement
membrane
C57BL/6J
(Duncan et al. 1999)
Transmigration of monocytes across
cultured endothelial monolayers
Antibody blockade of PECAM-1 prevents
transmigration through
unstimulated and cytokine-stimulated
monolayers but does not
prevent chemotaxis
N/A
(Muller et al. 1993)
Glycogen-induced peritonitis in rats
Antibody-blockade of PECAM-1 prevented
accumulation of
neutrophils into the peritoneal cavity
N/A
(Vaporciyan et al. 1993)
Glycogen-induced peritonitis in mice
Antibody-blockade of PECAM-1 prevented
accumulation of
neutrophils into the peritoneal cavity
AKR/J,
CD2F1
(Bogen et al. 1994)
IgG immune complex deposition in
lungs in rats
Antibody-blockade of PECAM-1 blocked
accumulation of neutrophils
into the alveolar compartment
N/A
(Vaporciyan et al. 1993)
Human skin grafts transplanted into
immunodeficient mice
Antibody-blockade of PECAM-1 blocked
accumulation of neutrophils
into the skin graft
Not listed
(Vaporciyan et al. 1993)
Intravital microscopy of cytokinestimulated
cremaster venules
PECAM-1 helps leukocytes transmigrate
through the cell-cell
junction and across the basement membrane
C57BL/6J
(Thompson et al. 2001;
Dangerfield et al. 2002; Wang
et al. 2005;
Woodfin et al. 2009)
Thioglycollate-induced peritonitis
PECAM-1 promotes leukocyte emigration to
the peritoneum
FVBn,
Swiss
Webster
(Schenkel et al. 2004)
Croton oil-induced topical dermatitis
PECAM-1 helps neutrophils extravasate
FVBn
(Schenkel et al. 2004)
Foreign body implant
PECAM-1+/+ mice have increased neutrophil
infiltration in and around
the implant compared to PECAM-1−/− mice
C57BL/6J
(Solowiej et al. 2003)
IL-1β- and immune complex-induced
lung injury
PECAM-1+/+ mice have greater numbers of
leukocytes in
brocheoalveolar lavage fluid compared to
PECAM-1−/− mice
C57BL/6J
(Albelda et al. 2004)
Fluid and shear stress in the aorta
ICAM-1 staining and nuclear NF-κB
translocation is increased in
PECAM-1+/+ aortas compared to PECAM-1−/−
aortas
C57BL/6J
(Tzima et al. 2005)
Atherosclerosis in LDL receptor−/− mice
PECAM-1 expression is correlated with more
plaques in
atherosusceptible regions of the aorta
C57BL/6J
(Harry et al. 2008;
Stevens et al. 2008)
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Inflammatory Model
Results
Mouse
strain
References
Leukocyte transmigration across
endothelial monolayers
PECAM-1 is part of the LBRC, a surface
connected membrane
compartment, that promotes both paracellular
and transcellular
migration
N/A
(Mamdouh et al. 2003;
Mamdouh et al. 2008;
Mamdouh et al. 2009;
Dasgupta et al. 2009)
EAE
Short-term administration of chimeric soluble
PECAM-1 fused to
human IgG-Fc (PECAM-Fc)decreased
migration of lymphocytes
across brain endothelial monolayers and
diminished the severity of
EAE when administered at the onset of
symptoms, though
chronically high levels of PECAM-Fc hastened
onset of EAE
SJL/J
(Reinke et al. 2007)
Collagen antibody-induced arthritis
PECAM-Fc treatment reduced inflammation
and attenuated bone
and cartilage destruction in symptomatic mice
DBA-1
(Dasgupta etal. 2010)
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Table 2
Anti-inflammatory functions of PECAM-1
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Inflammatory Model
Results
Mouse
strain
References
In vitro B lymphocyte activation assays
B cells expressing PECAM-1 have lower proliferation rates
in
response to BCR cross-linking and decreased antibody
production in
response to T-independent antigens
C57BL/6J
(Wilkinson et al.,
2002)
Spontaneous immune complexmediated
glomerulonephritis
Expression of PECAM-1 helps to prevent the spontaneous
development of autoantibodies and immune complexmediated
glomerulonephritis
C57BL/6J
(Wilkinson et al.,
2002)
In vitro T lymphocyte activation assays
Cross-linking of antibody bound PECAM-1 attenuates
calcium
mobilization following CD3 cross-linking
N/A
(Newton-Nash and
Newman, 1999)
IgE-dependent systemic and local
anaphylaxis
Expression of PECAM-1 is correlated with lower serum
histamine
concentrations and decreased tissue swelling and mast cell
degranulation following exposure to allergic stimuli
C57BL/6J
(Wong et al., 2002)
Collagen-induced arthritis
PECAM-1+/+ mice have delayed onset of arthritis with
decreased proinflammatory
cytokine production and leukocyte infiltration into joints
compared to PECAM-1−/− mice
C57BL/6J
(Tada et al.,
2003;Wong et al.,
2005)
Experimental autoimmune encephalitis
(EAE)
Expression of PECAM-1 delays the onset of EAE, decreases
parenchymal inflammatory cell infiltration and promotes
vascular
integrity
C57BL/6J
(Graesser et al.,
2002)
LPS-induced endotoxic shock
Expression of PECAM-1 is protective in LPS-induced
endotoxemia
and is correlated with decreased production of proinflammatory
cytokines and increased vascular integrity
C57BL/6J
(Carrithers et al.,
2005;Maas et al.,
2005)
In vitro cytokine production by
macrophages
Ligation and cross-linking of PECAM-1 by CD38-Fc fusion
protein
inhibits pro-inflammatory cytokine production in LPSstimulated
macrophages
N/A
(Rui et al., 2007)
Atherogenic-diet induced steatohepatitis
Expression of PECAM-1 delays onset and lessens severity of
atherogenic diet-induced steatohepatitis by decreasing
proinflammatory
cytokine production and leukocyte infiltration
C57BL/6J
(Goel et al., 2007)
Athersclerosis in LDL receptor−/− mice
Expression of PECAM-1 is correlated with decreased
atherosclerotic
lesion area in the total aorta with preferential protection in the
aortic
sinus, descending aorta, and the branching arteries of the
aortic arch
C57BL/6J
(Goel et al., 2008)
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