Pecam-1 is a modulator of stat family member phosphorylation and localization: lessons from a transgenic mouse

CD31 as a probable responding and gate-keeping protein of the blood-brain barrier and the risk of Alzheimer's disease

Several studies have shown that an abnormal vascular-immunity link could increase Alzheimer's disease (AD) risk; however, the mechanism is unclear. CD31, also named platelet endothelial cell adhesion molecule (PECAM), is a surface membrane protein of both endothelial and immune cells and plays important roles in the interaction between the vascular and immune systems. In this review, we focus on research regarding CD31 biological actions in the pathological process that may contribute to AD based on the following rationales. First, endothelial, leukocyte and soluble forms of CD31 play multi-roles in regulating transendothelial migration, increasing blood-brain barrier (BBB) permeability and resulting in neuroinflammation. Second, CD31 expressed by endothelial and immune cells dynamically modulates numbers of signaling pathways, including Src family kinases, selected G proteins, and β-catenin which in turn affect cell-matrix and cell-cell attachment, activation, permeability, survival, and ultimately neuronal cell injury. In endothelia and immune cells, these diverse CD31-mediated pathways act as a critical regulator in the immunity-endothelia-brain axis, thereby mediating AD pathogenesis in ApoE4 carriers, which is the major genetic risk factor for AD. This evidence suggests a novel mechanism and potential drug target for CD31 in the background of genetic vulnerabilities and peripheral inflammation for AD development and progression.

Temporal modulation of calcium sensing in hematopoietic stem cells is crucial for proper stem cell expansion and engraftment

Hematopoietic stem cells (HSCs) are known to reside in a bone marrow (BM) niche, which is associated with relatively higher calcium content. HSCs sense and respond to calcium changes. However, how calcium-sensing components modulate HSC function and expansion is largely unknown. We investigated temporal modulation of calcium sensing and Ca²⁺ homeostasis during ex vivo HSC culture and in vivo. Murine BM-HSCs, human BM, and umbilical cord blood (UCB) mononuclear cells (MNCs) were treated with store-operated calcium entry (SOCE) inhibitors SKF 96365 hydrochloride (abbreviated as SKF) and 2-aminoethoxydiphenyl borate (2-APB). Besides, K⁺ channel inhibitor TEA chloride (abbreviated as TEA) was used to compare the relationship between calcium-activated potassium channel activities. Seven days of SKF treatment induced mouse and human ex vivo BM-HSC expansion as well as UCB-derived primitive HSC expansion. SKF treatment induced the surface expression of CaSR, CXCR4, and adhesion molecules on human hematopoietic stem and progenitor cells. HSCs expanded with SKF successfully differentiated into blood lineages in recipient animals and demonstrated a higher repopulation capability. Furthermore, modulation of SOCE in the BM-induced HSC content and differentially altered niche-related gene expression profile in vivo. Intriguingly, treatments with SOCE inhibitors SKF and 2-APB boosted the mouse BM mesenchymal stem cell (MSC) and human adipose-derived MSCs proliferation, whereas they did not affect the endothelial cell proliferation. These findings suggest that temporal modulation of calcium sensing is crucial in expansion and maintenance of murine HSCs, human HSCs, and mouse BM-MSCs function.

Toxic damage increases angiogenesis and metastasis in fibrotic livers via PECAM-1

Excessive ethanol consumption is one of the main causes of liver fibrosis. However, direct effects of ethanol exposure on endothelial cells and their contribution to fibrogenesis and metastasis were not investigated. Therefore we analysed whether ethanol directly affects endothelial cells and if this plays a role during fibrogenesis and metastasis in the liver. Murine and human endothelial cells were exposed to ethanol for up to 72 hours. In vitro, effects on VEGF, HIF-1alpha, PECAM-1, and endothelial cell functions were analysed. In vivo, effects of continuous liver damage on blood vessel formation and metastasis were analysed by PECAM-1 immunohistochemistry. Ethanol increased HIF-1alpha and VEGF levels in murine and human endothelial cells. This resulted in enhanced intracellular signal transduction, and PECAM-1 expression as well as tube formation and wound healing. In vivo, toxic liver damage increased angiogenesis during fibrogenesis. Metastasis was also enhanced in fibrotic livers and located to PECAM-1 positive blood vessels compared to nonfibrotic mice. In conclusion, ethanol had strong effects on endothelial cells, which—at least in part—led to a profibrotic and prometastatic environment mediated by PECAM-1. Blockade of increased PECAM-1 expression could be a promising tool to inhibit fibrogenesis and metastasis in the liver.

CD44 regulation of endothelial cell proliferation and apoptosis via modulation of CD31 and VE-cadherin expression

CD44 has been implicated in a diverse array of cell behaviors and in a diverse range of signaling pathway activations under physiological and pathophysiological conditions. We have documented a role for CD44 in mediating vascular barrier integrity via regulation of PECAM-1 (CD31) expression. We now report our findings on the roles of CD44 in modulating proliferation and apoptosis of microvascular endothelial cells via its modulation of CD31 and VE-cadherin expression and the Hippo pathway. In this report, we demonstrate persistent increased proliferation and reduced activations of both effector and initiator caspases in high cell density, postconfluent CD44 knock-out (CD44KO), and CD31KO cultures. We found that reconstitution with murine CD44 or CD31 restored the proliferative and caspase activation rates to WT levels. Moreover, we have confirmed that the CD31 ecto-domain plays a key role in specific caspase cascades as well as cell adhesion-mediated cell growth and found that CD31 deficiency results in a reduction in VE-cadherin expression. Last, we have shown that both CD44KO and CD31KO endothelial cells exhibit a reduced VE-cadherin expression correlating with increased survivin expression and YAP nuclear localization, consistent with inactivation of the Hippo pathway, resulting in increased proliferation and decreased apoptosis. These findings support the concept that CD44 mediates several of its effects on endothelia through modulation of adhesion protein expression, which, in addition to its known modulation of junctional integrity, matrix metalloproteinase levels and activation, interactions with cortical membrane proteins, and selected signaling pathways, plays a key role as a critical regulator of vascular function.

Residues within a lipid-associated segment of the PECAM-1 cytoplasmic domain are susceptible to inducible, sequential phosphorylation

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptors inhibit cellular responsiveness to immunoreceptor tyrosine-based activation motif (ITAM)-linked receptors. Although tyrosine phosphorylation is central to the initiation of both inhibitory ITIM and stimulatory ITAM signaling, the events that regulate receptor phosphorylation are incompletely understood. Previous studies have shown that ITAM tyrosines engage in structure-inducing interactions with the plasma membrane that must be relieved for phosphorylation to occur. Whether ITIM phosphorylation is similarly regulated and the mechanisms responsible for release from plasma membrane interactions to enable phosphorylation, however, have not been defined. PECAM-1 is a dual ITIM-containing receptor that inhibits ITAM-dependent responses in hematopoietic cells. We found that the PECAM-1 cytoplasmic domain is unstructured in an aqueous environment but adopts an α-helical conformation within a localized region on interaction with lipid vesicles that mimic the plasma membrane. The lipid-interacting segment contains the C-terminal ITIM tyrosine and a serine residue that undergo activation-dependent phosphorylation. The N-terminal ITIM is excluded from the lipid-interacting segment, and its phosphorylation is secondary to phosphorylation of the membrane-interacting C-terminal ITIM. On the basis of these findings, we propose a novel model for regulation of inhibitory signaling by ITIM-containing receptors that relies on reversible plasma membrane interactions and sequential ITIM phosphorylation.

Platelet-endothelial cell adhesion molecule-1 regulates endothelial NO synthase activity and localization through signal transducers and activators of transcription 3-dependent NOSTRIN expression

BACKGROUND

NO produced by the endothelial NO synthase (eNOS) is an important regulator of cardiovascular physiological and pathological features. eNOS is activated by numerous stimuli, and its activity is tightly regulated. Platelet-endothelial cell adhesion molecule-1 (PECAM-1) has been implicated in regulating eNOS activity in response to shear stress. The current study was conducted to determine the role of PECAM-1 in the regulation of basal eNOS activity.

METHODS AND RESULTS

We demonstrate that PECAM-1-knockout ECs have increased basal eNOS activity and NO production. Mechanistically, increased eNOS activity is associated with a decrease in the inhibitory interaction of eNOS with caveolin-1, impaired subcellular localization of eNOS, and decreased eNOS traffic inducer (NOSTRIN) expression in the absence of PECAM-1. Furthermore, we demonstrate that activation of blunted signal transducers and activators of transcription 3 (STAT3) in the absence of PECAM-1 results in decreased NOSTRIN expression via direct binding of the signal transducers and activators of transcription 3 to the NOSTRIN promoter.

CONCLUSIONS

Our results reveal an elegant mechanism of eNOS regulation by PECAM-1 through signal transducers and activators of transcription 3-mediated transcriptional control of NOSTRIN.

Mechanisms of PECAM-1-mediated cytoprotection and implications for cancer cell survival

Defects in apoptotic pathways can promote cancer development and cause cancers to become resistant to chemotherapy. The cell adhesion and signaling molecule PECAM-1 has been shown to potently suppress apoptosis in a variety of cellular systems. PECAM-1 expression has been reported on a variety of human malignancies-especially hematopoietic and vascular cell cancers-but the significance of this expression has not been fully explored. The ability of PECAM-1 to inhibit apoptosis makes it an attractive candidate as a molecule that may promote cancer development and/or confer resistance to chemotherapeutic treatment. The exact mechanisms by which PECAM-1 mediates its cytoprotection have not been fully defined, but its anti-apoptotic effects have been shown to require both homophilic binding and intracellular signaling via its immunoreceptor tyrosine-based inhibitory motif (ITIM) domains. In this review, we will discuss the data regarding PECAM-1's anti-apoptotic effects and ways in which this cytoprotection may be clinically relevant to the development and/or treatment of hematologic malignancies that express this vascular cell-specific surface molecule.

Regulation of G protein-coupled receptor activities by the platelet-endothelial cell adhesion molecule, PECAM-1

It is becoming increasingly evident that the cell-cell junction is a major signaling center. Here we show that the Galphaq/11 subunit of heterotrimeric G proteins forms a complex with platelet-endothelial cell adhesion molecule 1 (PECAM-1), a junctional protein that has been shown to be involved in mechanosignaling in endothelial cells. To understand the role of PECAM-1 in this complex, we determined the critical regions of PECAM-1 involved in this interaction. By expressing truncated forms of PECAM-1 in human embryonic kidney (HEK293) cells, we found that the cytoplasmic domain of PECAM-1 is not required for its association with Galphaq/11. Domain swapping of PECAM-1 with intracellular cell adhesion molecule 1 (ICAM-1), a protein that does not form a complex with Galphaq/11, provides evidence that the extracellular domain of PECAM-1 is critical for this interaction. This result also suggests that PECAM-1 does not directly interact with Galphaq/11. Coexpression of bradykinin receptor B2 (BKRB2), a Galphaq/11-coupled receptor, with PECAM-1 enhances formation of the PECAM-1-Galphaq/11 complex, suggesting an interaction between PECAM-1 and BKRB2. Co-immunoprecipitation experiments indicate that these two molecules indeed form a complex when expressed in HEK293 cells. Activation of ERK1/2 by bradykinin in HUVEC is enhanced when PECAM-1 expression is inhibited by transfection of small interference RNA against PECAM-1. Taken together, our results provide evidence of interaction of PECAM-1 with BKRB2 and of its possible role in regulating G protein-coupled receptor (GPCR) and G protein functions.

A novel role for PECAM-1 (CD31) in regulating haematopoietic progenitor cell compartmentalization between the peripheral blood and bone marrow

Although the expression of PECAM-1 (CD31) on vascular and haematopoietic cells within the bone marrow microenvironment has been recognized for some time, its physiological role within this niche remains unexplored. In this study we show that PECAM-1 influences steady state hematopoietic stem cell (HSC) progenitor numbers in the peripheral blood but not the bone marrow compartment. PECAM-1^−/− mice have higher levels of HSC progenitors in the blood compared to their littermate controls. We show that PECAM-1 is required on both progenitors and bone marrow vascular cells in order for efficient transition between the blood and bone marrow to occur. We have identified key roles for PECAM-1 in both the regulation of HSC migration to the chemokine CXCL12, as well as maintaining levels of the matrix degrading enzyme MMP-9 in the bone marrow vascular niche. Using intravital microscopy and adoptive transfer of either wild type (WT) or PECAM-1^−/− bone marrow precursors, we demonstrate that the increase in HSC progenitors in the blood is due in part to a reduced ability to migrate from blood to the bone marrow vascular niche. These findings suggest a novel role for PECAM-1 as a regulator of resting homeostatic progenitor cell numbers in the blood

PECAM-1: a multifaceted regulator of megakaryocytopoiesis

PECAM-1 (CD31) knockout (KO) mice exhibit excessive megakaryocytopoiesis accompanied by increased numbers of megakaryocytes associated with the stromal niche rather than the vascular niche. During earlier stages of megakaryocytopoiesis in KO marrow, an expanded Lin(-)Sca-1(+) c-kit(+) hematopoietic stem cell (HSC) population and increased quiescent Lin(-) progenitor pool were identified. During the later stages of megakaryocytopoiesis, CD31KO megakaryocytes exhibited abnormal adhesion/transmigration behaviors. Lastly, KO animals exhibited excessive splenic extramedullary megakaryocytopoiesis, which likely compensates for the impaired marrow megakaryocytopoiesis, resulting in normal peripheral platelet number. Thus, PECAM-1 modulates megakaryocytopoiesis in a hierarchic manner, functioning as a thermostat to "fine-tune" megakaryocytopoiesis.

PECAM-1 modulates thrombin-induced tissue factor expression on endothelial cells

Platelet endothelial cell adhesion molecule-1 (PECAM-1) (CD31) is known to inhibit platelet function and thrombus formation. The mechanisms involved in PECAM-1's roles as a modulator of hemostasis are still not completely understood. We examined the role of PECAM-1 as a regulator of tissue factor (TF) expression, a known important inducer of thrombosis. Wildtype and CD31KO mice underwent transient (30 min) renal ischemia followed by 24 h re-perfusion and their kidneys assessed for apoptosis, fibrin formation, and tissue factor expression. CD31KO mice exhibited increased tubular epithelial and endothelial apoptosis, increased fibrin deposition, and tissue factor expression. Human umbilical vein endothelial cells (HUVEC) transfected with antisense (AS) PECAM-1 oligonucleotides to downregulate PECAM-1 expression, exhibited greater induction of TF mRNA and protein expression as well as increased expression and nuclear localization of the transcription factor Egr-1 compared to scrambled AS PECAM-1 (Scr)-treated HUVEC following thrombin stimulation. TF induction was found to be mediated through thrombin receptor PAR-1 and the Galphai/o subunit of G-protein, confirmed by PAR-1 antagonist and pertussis toxin inhibition respectively. Thrombin-mediated TF induction was dependent on Rho Kinase activity, phosphorylation of p38(MAPK) and p85 & Akt dephosphorylation. The inverse correlation of PI3K-Akt phosphorylation with p38 (MAPK) phosphorylation was confirmed by pharmacological inhibition. These studies suggest that PECAM-1 is involved in regulating a signaling pathway, affecting PI3K and Akt activation, p38 (MAPK) phosphorylation, which in turn, affects Egr-1 expression and nuclear translocation, ultimately affecting TF expression. These findings provide new insights into the action of PECAM-1 as a modulator of thrombosis.

CD31/PECAM-1 genotyping and haplotype analyses show population diversity

Using direct sequencing of complementary DNA products, the sequences of human CD31 from exon 1 through exon 16 of 179 individuals (139 unrelated) were systematically examined. Of the 14 biallelic single nucleotide polymorphic sites detected, 7 polymorphic sites involved amino acid substitution. These 14 polymorphic sites yielded 18 observed CD31 alleles and 9 predicted CD31 polypeptide sequences. Based on molecular haplotyping and family pedigree analysis, linkage disequilibrium among some single nucleotide polymorphic sites was observed. Single nucleotide polymorphism frequencies between populations were also measured using dot-blot hybridization with DNA or peptide nucleic acid probes.

An immunoreceptor tyrosine activation motif in the mouse mammary tumor virus envelope protein plays a role in virus-induced mammary tumors

Mouse mammary tumor virus (MMTV) induces breast cancer with almost 100% efficiency in susceptible strains through insertional activation of protooncogenes, such as members of the wnt and fibroblast growth factor (fgf) families. We previously showed that expression of the MMTV envelope protein (Env) in normal immortalized mammary epithelial cells grown in three-dimensional cultures caused their morphological transformation, and that this phenotype depended on an immunoreceptor tyrosine-based activation motif (ITAM) present in Env and signaling through the Syk tyrosine kinase (E. Katz, M. H. Lareef, J. C. Rassa, S. M. Grande, L. B. King, J. Russo, S. R. Ross, and J. G. Monroe, J. Exp. Med. 201:431-439, 2005). Here, we examined the role of the Env protein in virus-induced mammary tumorigenesis in vivo. Similar to the effect seen in vitro, Env expression in the mammary glands of transgenic mice bearing either full-length wild-type provirus or only Env transgenes showed increased lobuloalveolar budding. Introduction of the ITAM mutation into the env of an infectious, replication-competent MMTV or into MMTV/murine leukemia virus pseudotypes had no effect on incorporation of Env into virus particles or on in vitro infectivity. Moreover, replication-competent MMTV bearing the ITAM mutation in Env infected lymphoid and mammary tissue at the same level as wild-type MMTV and was transmitted through milk. However, mammary tumor induction was greatly attenuated, and the pattern of oncogene activation was altered. Taken together, these studies indicate that the MMTV Env protein participates in mammary epithelial cell transformation in vivo and that this requires a functional ITAM in the envelope protein.

Fer and Fps/Fes participate in a Lyn-dependent pathway from FcepsilonRI to platelet-endothelial cell adhesion molecule 1 to limit mast cell activation

Mast cells express the high affinity IgE receptor FcepsilonRI, which upon aggregation by multivalent antigens elicits signals that cause rapid changes within the mast cell and in the surrounding tissue. We previously showed that FcepsilonRI aggregation caused a rapid increase in phosphorylation of both Fer and Fps/Fes kinases in bone marrow-derived mast cells. In this study, we report that FcepsilonRI aggregation leads to increased Fer/Fps kinase activities and that Fer phosphorylation downstream of FcepsilonRI is independent of Syk, Fyn, and Gab2 but requires Lyn. Activated Fer/Fps readily phosphorylate the C terminus of platelet-endothelial cell adhesion molecule 1 (Pecam-1) on immunoreceptor tyrosine-based inhibitory motifs (ITIMs) and a non-ITIM residue (Tyr(700)) in vitro and in transfected cells. Mast cells devoid of Fer/Fps kinase activities display a reduction in FcepsilonRI aggregation-induced tyrosine phosphorylation of Pecam-1, with no defects in recruitment of Shp1/Shp2 phosphatases observed. Lyn-deficient mast cells display a dramatic reduction in Pecam-1 phosphorylation at Tyr(685) and a complete loss of Shp2 recruitment, suggesting a role as an initiator kinase for Pecam-1. Consistent with previous studies of Pecam-1-deficient mast cells, we observe an exaggerated degranulation response in mast cells lacking Fer/Fps kinases at low antigen dosages. Thus, Lyn and Fer/Fps kinases cooperate to phosphorylate Pecam-1 and activate Shp1/Shp2 phosphatases that function in part to limit mast cell activation.

Identification of the regions of PECAM-1 involved in β- and γ-catenin associations

Platelet endothelial cell adhesion molecule-1 (PECAM-1) binds tyrosine-phosphorylated beta-catenin and modulates beta-catenin localization and sequestration. The biological significance of this interaction, while still unclear, it has been postulated to be involved in modulating adherens junction dynamics in response to perturbants [J. Clin. Invest. 109 (2002) 383]. Here we demonstrate that tyrosine-phosphorylated beta-catenin, and to a lesser extent unphosphorylated beta-catenin, interact with a portion of the cytoplasmic domain of PECAM-1 encoded by exon 15. Using RT-PCR, we obtained products representing alternatively spliced PECAM-1 isoforms from mouse kidney total mRNA and generated PECAM-1-GST constructs expressing full length and naturally occurring alternatively spliced PECAM-1 variants. Co-precipitation assays revealed that the protein sequence encoded by exon 15 is necessary for beta-catenin binding. Transfections using deletion mutants confirmed the importance of the exon 15 sequence in this interaction. In contrast, gamma-catenin-PECAM-1 interactions are thought to be modulated by an as yet undefined PECAM-1 serine phosphorylation and appear to mediate dynamic PECAM-1 intermediate filament cytoskeletal interactions [J. Biol. Chem. 275 (2000) 21435]. Here we demonstrate that the PECAM-1-gamma-catenin interaction occurs via an exon 13-mediated process. GST-pull-down assays illustrated the importance of the exon 13 sequence in this interaction. Further, using site-directed mutagenesis of S(673) to C and D and S(669 and 670) to C, we confirmed the importance of S(673) and its phosphorylation state as a mediator of gamma-catenin-PECAM-1 binding. Our studies define the exons of the PECAM-1 cytoplasmic domain that is involved in mediating these PECAM-1-catenin family member interactions and will allow investigators to better define the biological functions resulting from these interactions.

Enhanced susceptibility to endotoxic shock and impaired STAT3 signaling in CD31-deficient mice

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31), an adhesion molecule expressed on hematopoietic and endothelial cells, mediates apoptosis, cell proliferation, and migration and maintains endothelial integrity in addition to its roles as a modulator of lymphocyte and platelet signaling and facilitator of neutrophil transmigration. Recent data suggest that CD31 functions as a scaffolding protein to regulate phosphorylation of the signal transducers and activators of transcription (STAT) family of signaling molecules, particularly STAT3 and STAT5. STAT3 regulates the acute phase response to innate immune stimuli such as lipopolysaccharide (LPS) and promotes recovery from LPS-induced septic shock. Here we demonstrate that CD31-deficient mice have reduced survival during endotoxic LPS-induced shock. As compared to wild-type controls, CD31-deficient mice showed enhanced vascular permeability; increased apoptotic cell death in liver, kidney, and spleen; and elevated levels of serum tumor necrosis factor alpha (TNF-alpha), interferon gamma (IFNgamma), MCP-1, MCP-5, sTNRF, and IL-6. In response to LPS in vivo and in vitro, splenocytes and endothelial cells from knockout mice had reduced levels of phosphorylated STAT3. These results suggest that CD31 is necessary for maintenance of endothelial integrity and prevention of apoptosis during septic shock and for STAT3-mediated acute phase responses that promote survival during septic shock.

[Crucial roles of PECAM-1 in shear stress sensing of vascular endothelial cells]

Fluid shear stress (FSS) induces many forms of responses, including phosphorylation of ERK in endothelial cells (ECs). We have earlier reported that FSS and hyper-osmotic shock rapidly induce tyrosine phosphorylation of PECAM-1 (CD31). The phosphorylated PECAM-1 acts as a plasma membrane anchoring site for SHP2, a protein tyrosine phosphatase involved in the signal transmission from receptor tyrosine kinases to ERK. Osmotic shock also induces transient ERK activation in ECs. The osmotic-shock-induced ERK activation but not p38 MAP kinase activation was dependent on the PECAM-1 engagement and was blocked by its downregulation. When magnetic beads coated with antibodies against the extracellular domain of PECAM-1 were attached to ECs and tugged by magnetic force, PECAM-1 associated with the beads was tyrosine phosphorylated. ERK was also phosphorylated in these cells. Binding of the beads by itself or pulling on the cell surface using poly-L-lysine coated beads did not induce phosphorylation of PECAM-1 and ERK. These results suggest that PECAM-1 is a mechanotransduction molecule.

Identification of a novel transcript of human PECAM-1 and its role in the transendothelial migration of monocytes and Ca2+ mobilization

Platelet-endothelial cell adhesion molecule-1 (PECAM-1) is an integral component of endothelial cells and has been implicated in the transendothelial migration (TEM) of circulating leukocytes mediated by its 1st and 2nd extracellular immunoglobulin (Ig)-like domains and regulation of intracellular Ca(2+) homeostasis with its 6th domain. Up-to-date, little is known about the role of the 5th extracellular (Ig)-like domain. We have discovered a novel human PECAM-1 transcript missing the entire 7th exon, which encodes the 5th extracellular (Ig)-like domain of PECAM-1. A synthetic peptide with sequence homology to the 5th domain of PECAM-1 (JHS-7 peptide) and a corresponding polyclonal antibody (JHS-7 Ab) were prepared and their potential role in transendothelial migration and Ca(2+) influx was measured. The JHS-7 peptide and the antibody exerted a dose dependent decrease (50-80%) in the transendothelial migration of freshly isolated human monocytes and a promonocytic cell line (U-937) in resting HUVECs and HUVECs activated with tumor necrosis factor-alpha. This was accompanied by an increase in Ca(2+) influx and decrease in refilling of the intracellular Ca(2+) stores in HUVECs. In summary, we have identified a novel PECAM-1 transcript (Deltaexon 7) and shown that the 5th (Ig)-like domain of PECAM-1 plays a role in monocyte TEM and Ca(2+) homeostasis.

Disruption of endothelial-cell caveolin-1alpha/raft scaffolding during development of monocrotaline-induced pulmonary hypertension

BACKGROUND

In the monocrotaline (MCT)-treated rat, there is marked stimulation of DNA synthesis and megalocytosis of pulmonary arterial endothelial cells (PAECs) within 3 to 4 days, followed by pulmonary hypertension (PH) 10 to 14 days later. Growing evidence implicates caveolin-1 (cav-1) in plasma membrane rafts as a negative regulator of promitogenic signaling. We have investigated the integrity and function of endothelial cell-selective cav-1alpha/raft signaling in MCT-induced PH.

METHODS AND RESULTS

Although PH and right ventricular hypertrophy developed by 2 weeks after MCT, a reduction in cav-1alpha levels in the lung was apparent within 48 hours, declining to approximately 30% by 2 weeks, accompanied by an increase in activation of the promitogenic transcription factor STAT3 (PY-STAT3). Immunofluorescence studies showed a selective loss of cav-1alpha and platelet endothelial cell adhesion molecule-1 in the PAEC layer within 48 hours after MCT but an increase in PY-STAT3. PAECs with cav-1alpha loss displayed high PY-STAT3 and nuclear immunostaining for proliferating cell nuclear antigen (PCNA). Biochemical studies showed a loss of cav-1alpha from the detergent-resistant lipid raft fraction concomitant with hyperactivation of STAT3. Moreover, cultured PAECs treated with MCT-pyrrole for 48 hours developed megalocytosis associated with hypo-oligomerization and reduction of cav-1alpha, hyperactivation of STAT3 and ERK1/2 signaling, and stimulation of DNA synthesis.

CONCLUSIONS

MCT-induced disruption of cav-1alpha chaperone and scaffolding function in PAECs likely accounts for diverse alterations in endothelial cell signaling in this model of PH.

PECAM-1: old friend, new partners

The maintenance of vascular function is of paramount importance to an organism's existence. PECAM-1 (CD31), first thought of as a marker for endothelia, has been shown to be an important scaffolding molecule involved in several signaling pathways. Recent studies have demonstrated an even wider range of functions for this versatile molecule including participation in maintenance of adherens junction integrity and permeability, organization of the intermediate filament cytoskeleton, regulation of catenin localization and transcriptional activities, participation in STAT isoform signaling, control of apoptotic events, and modulation of cardiac cushion development.

Embryonic Stem Cells Expressing Both Platelet Endothelial Cell Adhesion Molecule-1 and Stage-Specific Embryonic Antigen-1 Differentiate Predominantly into Epiblast Cells in a Chimeric Embryo1

We examined the expression of cell-surface markers on subpopulations of mouse embryonic stem (ES) cells to identify those that were associated with cells that had the highest pluripotency. Flow cytometry analysis revealed a wide variation in the expression of platelet endothelial cell adhesion molecule 1 (PECAM-1) and stage-specific embryonic antigen (SSEA)-1 in ES cells. Almost all SSEA-1+ cells expressed a high level of PECAM- 1, and reversible repopulation was observed between PECAM- 1+SSEA-1+ and PECAM-1+SSEA-1- cells. The ES cells carrying the lacZ gene were sorted into three subpopulations: PECAM- 1-SSEA-1-, PECAM-1+SSEA-1-, and PECAM-1+SSEA-1+. Quantitative reverse transcription-polymerase chain reaction revealed a low level of Oct3/4 mRNA expression and an elevation in differentiation maker gene expression in PECAM-1- cells. To compare the pluripotency of these three subpopulations, a single cell from each was injected into eight-cell embryo and ES cells identified at later stages by X-gal staining. At the blastocyst stage, PECAM-1+ SSEA-1+/- cells were found to have differentiated into epiblast cells in high numbers. In contrast, PECAM- 1- cell derivatives localized in the primitive endoderm or trophectoderm. At 6.0-7.0 days post coitum, many PECAM-1+SSEA- 1+ cells were found in the epiblast, but few beta-gal+ cells were detected in any regions of embryos that were injected with cells from the other two populations. These results showed that the expression levels of PECAM-1 and SSEA-1 in ES cells correlated closely with their pluripotency and/or their ability to incorporate into the epiblast of chimeric embryos.

Vascular endothelial growth factor expression, beta-catenin tyrosine phosphorylation, and endothelial proliferative behavior: a pathway for transformation?

The hypothesis that tumor growth is angiogenesis dependent has been documented by a considerable body of direct and indirect experimental data and has generated intense basic and pharmaceutical-related interest. In contrast, the study of endothelial cell tumors has been modest by comparison. Hemangioma is the most common tumor of any kind seen in infancy and also, perhaps, the least understood. We compared a mouse hemangioma-derived cell line (EOMA) and primary human endothelial cells (HUVEC) for their proliferative behavior and molecular alterations. EOMA cells intrinsically expressed vascular endothelial growth factor (VEGF), which acts in an autocrine manner, resulting in an increase in CD1 expression and cell proliferation, both of which were inhibited by anti-VEGF neutralizing antibodies. Such an autocrine loop is supported by constitutive VEGF receptor (Flk-1) tyrosine phosphorylation, Flk-1 and Flt-1 nuclear localization, and mitogen-activated protein kinase activation. beta-catenin was also found to exhibit significant nuclear localization and constitutively associate with Flk-1 and Flt-1 in EOMA cells but much less so in HUVEC, and immunoprecipitated Flk-1 was able to phosphorylate purified beta-catenin in an immune complex kinase assay. EOMA cells were also noted to express reduced levels of N-cadherin and gamma-catenin compared with HUVEC. Interestingly, sequestration of endogenous VEGF in EOMA cultures resulted in a dramatic decrease in nuclear beta-catenin and a reduction in CD1 levels, whereas addition of exogenous VEGF elicited increased nuclear beta-catenin localization and increased CD1 levels in HUVEC. The possible contributions of VEGF signaling pathways, cell junction component expression levels, and phosphorylation states to endothelial cell transformation and proliferation are discussed.

PECAM-1 functions as a specific and potent inhibitor of mitochondrial-dependent apoptosis

Programmed cell death, or apoptosis, is a tightly regulated, naturally occurring process by which damaged or unwanted cells are removed. Dysregulated apoptosis has been implicated in a variety of pathophysiological conditions, including degenerative diseases, tissue remodeling, and tumorigenesis. The decision to live or die results from integration of numerous environmental signals transmitted by specific classes of cell surface receptors that bind hormones, growth factors, or components of the extracellular matrix. Here we show that platelet endothelial cell adhesion molecule-1 (PECAM-1), a homophilic-binding member of the immunoreceptor tyrosine-based inhibitory motif (ITIM) family of inhibitory receptors, functions prominently to inhibit apoptosis in naturally occurring vascular cells subjected to apoptotic stimuli. Murine endothelial cells and human T lymphocytes lacking PECAM-1 were found to be far more sensitive than their PECAM-1-expressing counterparts to multiple death signals that stimulate Bax, a multidomain, proapoptotic member of the Bcl-2 family that plays a central role in mitochondrial dysfunction-dependent apoptosis. In addition, PECAM-1 markedly suppressed Bax overexpression-induced cytochrome c release, caspase activation, and nuclear fragmentation. Amino acid substitutions within PECAM-1's extracellular homophilic binding domain, or within its cytoplasmic ITIM, completely abolished PECAM-1-mediated cytoprotection. Taken together, these data implicate PECAM-1 as a novel and potent suppressor of Bax-mediated apoptosis and suggest that members of the immunoglobulin gene (Ig) superfamily, like cell surface integrins, may also transmit survival signals into blood and vascular cells.

Signal transduction pathways mediated by PECAM-1: new roles for an old molecule in platelet and vascular cell biology

Recent studies of platelet endothelial cell adhesion molecule-1 (PECAM-1 [CD31])-deficient mice have revealed that this molecule plays an important role in controlling the activation and survival of cells on which it is expressed. In this review, we focus on the complex cytoplasmic domain of PECAM-1 and describe what is presently known about its structure, posttranslational modifications, and binding partners. In addition, we summarize findings that implicate PECAM-1 as an inhibitor of cellular activation via protein tyrosine kinase-dependent signaling pathways, an activator of integrins, and a suppressor of cell death via pathways that depend on damage to the mitochondria. The challenge of future research will be to bridge our understanding of the functional and biochemical properties of PECAM-1 by establishing mechanistic links between signals transduced by the PECAM-1 cytoplasmic domain and discrete cellular responses.

Role of CD31/platelet endothelial cell adhesion molecule-1 expression in in vitro and in vivo growth and differentiation of human breast cancer cells

Breast ductal carcinoma in situ is an intraductal proliferation of malignant epithelial cells that diffuse within the ductal system without stromal invasion. Our finding that a subset of these tumors express CD31/platelet endothelial cell adhesion molecule-1 suggests that breast cancer represents an informative model for studying the involvement of the molecule in the morphogenesis, differentiation, and diffusion of this disease. Transfection of CD31 in MDA-MB-231 cells caused reduction in growth, loss of CD44, and acquisition of a ductal morphology. The same effects were maintained in vivo, in which CD31(+) tumors grew with in situ-like aspects, papillary differentiation, and a secretory phenotype. CD44 was down-modulated, with the CD31(+) cells blocked in the G(1) phase. The morphology was highly similar to what was observed in some human CD31(+) ductal carcinomas in situ. MDA-MB-231 mock cells grew in solid sheets, lacking stromal material, and displaying high levels of CD44 and proliferation. CD31(+) cells acquired motility characteristics in in vitro assays, a finding confirmed in vivo by the diffusion of human tumor cells throughout the normal ducts residual in the murine mammary gland. In conclusion, CD31 expression reverts the undifferentiated morphology and aggressive behavior of MDA-MB-231 cells, indicating its active role in the morphogenesis of breast ductal in situ carcinomas.

PECAM-1 promotes beta-catenin accumulation and stimulates endothelial cell proliferation

Platelet endothelial cell adhesion molecule-1 (PECAM-1) binds tyrosine phosphorylated beta-catenin and modulates beta-catenin localization [J. Immunol. 158 (7) (1997) 3408; J. Cell Sci. 112 (Pt 18) (1999) 3005]. To elucidate functional consequences of this interaction, we studied endothelial cells from PECAM-1 knockout animals and compared them to PECAM-1 expressing endothelial cells [Mol. Biol. Cell 11 (9) (2000) 3109]. We noted an increase in the expression of beta-catenin protein in PECAM-1 expressing endothelial cells. Further, by immunofluorescence, beta-catenin localized to the cell membrane as well as to the nucleus in PECAM-1 positive endothelial cells, whereas cells not expressing PECAM-1 stained for beta-catenin only at the membrane. Additionally, we demonstrate that PECAM-1 lacking the majority of the cytoplasmic domain promotes significantly less accumulation of transcriptionally active beta-catenin than full-length PECAM-1. Finally, we note an increased proliferative rate in the PECAM-1 reconstituted cells compared to the endothelial cells lacking PECAM-1. Taken together, our data suggest that PECAM-1, an adhesion molecule, affects cell proliferation via accumulation of transcriptionally active beta-catenin.

Lack of platelet endothelial cell adhesion molecule-1 attenuates foreign body inflammation because of decreased angiogenesis

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a 130-kd member of the immunoglobulin superfamily of proteins, expressed on endothelial cells, leukocytes, and platelets. Antibody-blocking studies have implicated it in modulating leukocyte transmigration and angiogenesis. However, the generation of the PECAM-1 knockout mouse has shown that its function can be compensated for by similarly acting proteins because most acute inflammatory models proceed in a comparable manner in wild-type and knockout animals. We decided to examine the function of PECAM-1 in the chronic process of foreign body inflammation. We show that PECAM-1-deficient mice exhibit attenuated neutrophil infiltration in and around a subcutaneous polyvinyl acetyl implant. Bone marrow engraftment studies indicate that the lack of CD31 expression on the endothelium determines the diminished leukocyte accumulation in the knockout implants. Specifically, we find that decreased angiogenesis (as manifested by lower vessel density, decreased hemoglobin content, and less laminin deposition) correlates with lower neutrophil accumulation in the knockout animals. This study indicates that the absence of endothelial PECAM-1 results in decreased angiogenesis and therefore in diminished delivery of leukocytes to the foreign body implants.

Elevated glucose inhibits VEGF-A-mediated endocardial cushion formation: modulation by PECAM-1 and MMP-2

Atrioventricular (AV) septal defects resulting from aberrant endocardial cushion (EC) formation are observed at increased rates in infants of diabetic mothers. EC formation occurs via an epithelial-mesenchymal transformation (EMT), involving transformation of endocardial cells into mesenchymal cells, migration, and invasion into extracellular matrix. Here, we report that elevated glucose inhibits EMT by reducing myocardial vascular endothelial growth factor A (VEGF-A). This effect is reversed with exogenous recombinant mouse VEGF-A165, whereas addition of soluble VEGF receptor-1 blocks EMT. We show that disruption of EMT is associated with persistence of platelet endothelial cell adhesion molecule-1 (PECAM-1) and decreased matrix metalloproteinase-2 (MMP-2) expression. These findings correlate with retention of a nontransformed endocardial sheet and lack of invasion. The MMP inhibitor GM6001 blocks invasion, whereas explants from PECAM-1 deficient mice exhibit MMP-2 induction and normal EMT in high glucose. PECAM-1-negative endothelial cells are highly motile and express more MMP-2 than do PECAM-1-positive endothelial cells. During EMT, loss of PECAM-1 similarly promotes single cell motility and MMP-2 expression. Our findings suggest that high glucose-induced inhibition of AV cushion morphogenesis results from decreased myocardial VEGF-A expression and is, in part, mediated by persistent endocardial cell PECAM-1 expression and failure to up-regulate MMP-2 expression.