Functional properties of human vascular endothelial cadherin (7B4/cadherin-5), an endothelium-specific cadherin

Stepping out of the flow: capillary extravasation in cancer metastasis

In order for cancer cells to successfully colonize a metastatic site, they must detach from the primary tumor using extracellular matrix-degrading proteases, intravasate and survive in the circulation, evade the immune response, and extravasate the vasculature to invade the target tissue parenchyma, where metastatic foci are established. Though many of the steps of metastasis are widely studied, the precise cellular interactions and molecular alterations associated with extravasation are unknown, and further study is needed to elucidate the mechanisms inherent to this process. Studies of leukocytes localized to inflamed tissue during the immune response may be used to elucidate the process of cancer extravasation, since leukocyte diapedesis through the vasculature involves critical adhesive interactions with endothelial cells, and both leukocytes and cancer cells express similar surface receptors capable of binding endothelial adhesion molecules. Thus, leukocyte extravasation during the inflammatory response has provided a model for transendothelial migration (TEM) of cancer cells. Leukocyte extravasation is characterized by a process whereby rolling mediated by cytokine-activated endothelial selectins is followed by firmer adhesions with beta1 and beta2 integrin subunits to an activated endothelium and subsequent diapedesis, which most likely involves activation of Rho GTPases, regulators of cytoskeletal rearrangements and motility. It is controversial whether such selectin-mediated rolling is necessary for TEM of cancer cells. However, it has been established that similar stable adhesions between tumor and endothelial cells precede cancer cell transmigration through the endothelium. Additionally, there is support for the preferential attachment of tumor cells to the endothelium and, accordingly, site-specific metastasis of cancer cells. Rho GTPases are critical to TEM of cancer cells as well, and some progress has been made in understanding the specific roles of the Rho GTPase family, though much is still unknown. As the mechanisms of cancer TEM are elucidated, new approaches to study and target metastasis may be utilized and developed.

Hemogenic endothelial progenitor cells isolated from human umbilical cord blood

Hemogenic endothelium has been identified in embryonic dorsal aorta and in tissues generated from mouse embryonic stem cells, but to date there is no evidence for such bipotential cells in postnatal tissues or blood. Here we identify a cell population from human umbilical cord blood that gives rise to both endothelial cells and hematopoietic progenitors in vitro. Cord blood CD34+/CD133+ cells plated at high density in an endothelial basal medium formed an endothelial monolayer and a nonadherent cell population after 14-21 days. AML-1, a factor required for definitive hematopoiesis, was detected at low levels in adherent cells and at high levels in nonadherent cells. Nonadherent cells coexpressed the endothelial marker vascular endothelial (VE)-cadherin and the hematopoietic marker CD45, whereas adherent cells were composed primarily of VE-cadherin+/CD45- cells and a smaller fraction of VE-cadherin+/CD45+ cells. Both nonadherent and adherent cells produced hematopoietic colonies in methylcellulose, with the adherent cells yielding more colony-forming units (CFU)-GEMM compared with the nonadherent cells. To determine whether the adherent endothelial cells were producing hematopoietic progenitors, single cells from the adherent population were expanded in 96-well dishes for 14 days. The clonal populations expressed VE-cadherin, and a subset expressed AML-1, epsilon-globin, and gamma-globin. Three of 17 clonal cell populations gave rise to early CFU-GEMM hematopoietic progenitors and burst-forming unit-erythroid progenitors. These results provide evidence for hemogenic endothelial cells in human umbilical cord blood.

Mechanisms of HER2-induced endothelial cell retraction

BACKGROUND

HER2 overexpression imparts a metastatic advantage in breast cancer. We have shown that HER2 signaling in breast cancer cells induces adjacent endothelial cell (EC) retraction, disrupting endothelial integrity. Because endothelial integrity is dependent on the adherens junctions, we postulated that the mechanism of tumor cell-induced EC retraction involves dissociation of catenin proteins from vascular endothelial (VE) cadherin. In this study, we report a loss of VE-cadherin in tumor-associated EC. We also tested for a change of catenin dissociation from VE-cadherin by manipulating HER2 signaling in tumor cells.

METHODS

We tested confluent monolayers of human EC for downregulation of VE cadherin and dissociation of catenins from VE cadherin after exposure to breast cancer cells or conditioned media. Using immunoprecipitation, we quantitated the remaining complexed catenins to VE-cadherin in tumor-associated EC after different treatments to manipulate HER2 signaling.

RESULTS

Treatment of EC with conditioned media from MCF-7 cells expressing HER2 induced a loss of VE-cadherin expression, and time-dependent dissociation of catenins from VE cadherin. Catenin dissociation from VE-cadherin was enhanced by Heregulin beta1 (P < .05) stimulation and decreased by trastuzumab (P < .05) blockade of HER2 signaling in cancer cells. An increase in EC phosphoSrc (Tyr 416) was seen by 8 hours.

CONCLUSIONS

Our data suggest that HER2 induction of EC retraction involves both down-regulation of VE-cadherin and dissociation of catenins. HER2 signaling appears to regulate this potential metastatic mechanism. Further, Src phosphorylation suggests that this pathway may be involved in this mechanism.

Long-term effects of subcutaneously injected 2,3,7,8-tetrachlorodibenzo-p-dioxin on the liver of rhesus monkeys

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) accumulates and remains stable in the fatty tissues and liver of rodents for a long time. Considering the pronounced difference between species, long-term, low dose hepatic effects of TCDD were investigated after subcutaneous administration of TCDD into rhesus monkeys during pregnancy. Macroscopic and histopathological examination of the liver carried out 4 y after TCDD administration demonstrated intrahepatic focal fatty changes, infarction, hemorrhage, microthrombi-formation, sinusoidal ectasia, small hepatocyte hyperplasia, and increased number of alpha-smooth muscle actin (alpha-SMA)-positive cells. An electron microscopic study disclosed sinusoidal endothelial cell degeneration and injury in the liver of TCDD-treated monkeys. Western blot analysis showed downregulation of aryl hydrocarbon receptor (AhR) protein expression and decreased level of vascular endothelial (VE) cadherin but increased expression levels of CYP1A1 and transforming growth factor beta (TGF-beta) protein in the liver tissues. These changes observed in TCDD-exposed monkeys indicated sinusoidal endothelial cell injury and impairment in intrasinusoidal microcirculation. Infarction, focal fatty change, and microthrombi-formation are considered to be closely associated with intrahepatic circulatory impairment. Increased number of alpha-SMA-positive cells and decreased level of VE cadherin expression in the liver tissues might also be associated with sinusoidal endothelial cell injury. In addition, downregulation of AhR expression and increased CYP1A1 protein levels in the liver were consistent with persistent effects of TCDD. Although it has been reported that TCDD induced endothelial cell injury, this is the first report to describe vascular disorders and protein expression in the liver after injection with TCDD in a primate model.

Paracrine VEGF/VE-cadherin action on ovarian cancer permeability

Ascites formation associated with neoplasms is most likely due to increased vascular permeability, a process in which vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) plays a pivotal role. We hypothesized that tumor-derived VEGF/VPF modulates ascites formation through a paracrine effect on both tumor and peritoneal vessels. We investigated human vascular endothelial permeability using a newly developed dual-chamber permeability assay by co-culturing human umbilical vein cells with and without ovarian cancer cell lines (OVCAR-3, Hey-A8, and OCC-1) in the presence or absence of a human VEGF monoclonal antibody and VE-cadherin function-blocking antibody. This method permits determination of mechanisms by which substances released from neoplasms and other sources of vascular endothelial cell secretagogues modulate vascular permeability and likely other pathologic states.

Cell-cell adhesion in lung endothelium

Homotypic cell-cell adhesion is essential for tissue and organ development, remodeling, regeneration, and physiological function. Whereas a significant number of homotypic cell-cell adhesion molecules have been identified, much more is known about those concentrated in epithelia than in endothelia. Among the endothelial cell-cell adhesion molecules, very little is known that is specific to endothelium in the pulmonary and bronchial circulations. This review focuses primarily on homotypic cell-cell adhesion molecules that are or are likely to be important in lung endothelium.

Src kinase phosphorylates vascular endothelial-cadherin in response to vascular endothelial growth factor: identification of tyrosine 685 as the unique target site

Src-family tyrosine kinases are regulatory proteins that play a pivotal role in the disorganization of cadherin-dependent cell-cell contacts. We previously showed that Src was associated with vascular endothelial (VE)-cadherin and that tyrosine phosphorylation level of VE-cadherin was dramatically increased in angiogenic tissues as compared to quiescent tissues. Here, we examined whether VE-cadherin was a direct substrate for Src in vascular endothelial growth factor (VEGF)-induced VE-cadherin phosphorylation, and we identified the target tyrosine sites. Co-transfections of Chinese hamster ovary cells (CHO) cells with VE-cadherin and constitutively active Src (Y530F) resulted in a robust tyrosine phosphorylation of VE-cadherin that was not detected with kinase-dead Src (K298M). In an in vitro Src assay, the VE-cadherin cytoplasmic domain is directly phosphorylated by purified Src as well as the tyrosine residue 685 (Tyr)685-containing peptide RPSLY(685)AQVQ. VE-cadherin peptide mapping from human umbilical vein endothelial cells stimulated by VEGF and VE-cadherin-CHO cells transfected with active Src revealed that Y685 was the unique phosphorylated site. The presence of PhosphoY685 was confirmed by its ability to bind to C-terminal Src kinase-SH2 domain in a pull-down assay. Finally, we found that in a VEGF-induced wound-healing assay, cadherin adhesive activity was impaired by Src kinase inhibitors. These data identify that VEGF-induced-VE-cadherin tyrosine phosphorylation is mediated by Src on Y685, a process that appears to be critical for VEGF-induced endothelial cell migration.

Angiogenesis: The VE-Cadherin Switch

Because angiogenesis is a key step in a number of pathologic processes, including tumor growth and atherosclerosis, many research studies have investigated the regulatory signals active at various stages of vascular invasion. The differential activities of the endothelial junction protein vascular endothelial (VE)-cadherin reflect the versatile behavior of endothelial cells between vascular quiescence and angiogenesis. VE-cadherin function and signaling are deeply modified in proliferating cells, and this conversion is accompanied by phosphorylation of the protein on tyrosine residues and enhanced transcription of its gene. Recent advances in the complex interplay between protein tyrosine kinases and phosphatases regulating VE-cadherin phosphorylation and function are discussed in this review.

Endothelial effects of 3-hydroxyglutaric acid: implications for glutaric aciduria type I

Infants with glutaric aciduria type 1 (GA1) are subject to intracranial vascular dysfunction. Here, we demonstrate that the disease-specific metabolite 3-hydroxyglutaric acid (3-OH-GA) inhibits basal and vascular endothelial growth factor (VEGF)-induced endothelial cell migration. 3-OH-GA affects the morphology of VEGF-induced endothelial tubes in vitro because of partial disintegration of endothelial cells. These effects correlate with Ve-cadherin loss. Remarkably, 3-OH-GA treatment of human dermal microvascular endothelial cells leads to disruption of actin cytoskeleton. Local application of 3-OH-GA alone or in combination with VEGF in chick chorioallantoic membrane induces abnormal vascular dilatation and hemorrhage in vivo. The study demonstrates that 3-OH-GA reduces endothelial chemotaxis and disturbs structural vascular integrity in vitro and in vivo. These data may provide insight in the mechanisms of 3-OH-GA-induced vasculopathic processes and suggest N-methyl-D-aspartate receptor-dependent and -independent pathways in the pathogenesis of GA1.

The complexus adhaerens of mammalian lymphatic endothelia revisited: a junction even more complex than hitherto thought

The significance of a special kind of VE-cadherin-based, desmoplakin- and plakoglobin-containing adhering junction, originally identified in certain endothelial cells of the mammalian lymphatic system (notably the retothelial cells of the lymph node sinus and a subtype of lining endothelial cells of peripheral lymphatic vessels), has been widely confirmed and its importance in the formation of blood and lymph vessels has been demonstrated in vivo and in vitro. We have recently extended the molecular and structural characterization of the complexus adhaerens and can now report that it represents a rare and special combination of components known from three other major types of cell junction. It comprises zonula adhaerens proteins (VE-cadherin, alpha- and beta-catenin, protein p120(ctn), and afadin), desmosomal plaque components (desmoplakin and plakoglobin), and tight-junction proteins (claudin-5 and ZO-1) and forms junctions that vary markedly in size and shape. The special character and the possible biological roles of the complexus adhaerens and its unique ensemble of molecules in angiogenesis, immunology, and oncology are discussed. The surprising finding of claudin-5 and protein ZO-1 in substructures of retothelial cell-cell bridges, i.e. structures that do not separate different tissues or cell layer compartments, suggests that such tight-junction molecules are involved in functions other than the "fence" and "barrier" roles of zonulae occludentes.

Platelet-activating factor increases VE-cadherin tyrosine phosphorylation in mouse endothelial cells and its association with the PtdIns3'-kinase

Platelet-activating factor (PAF), a potent inflammatory mediator, is involved in endothelial permeability. This study was designed to characterize PAF receptor (PAF-R) expression and its specific contribution to the modifications of adherens junctions in mouse endothelial cells. We demonstrated that PAF-R was expressed in mouse endothelial cells and was functionally active in stimulating p42/p44 MAPK and phosphatidylinositol 3-kinase (PtdIns3'-kinase)/Akt activities. Treatment of cells with PAF induced a rapid time- and dose-dependent (10(-7) to 10(-10) M) increase in tyrosine phosphorylation of a subset of proteins ranging from 90 to 220 kDa, including the VE-cadherin, the latter effect being prevented by the tyrosine kinase inhibitors herbimycin A and bis-tyrphostin. We demonstrated that PAF promoted formation of multimeric aggregates of VE-cadherin with PtdIns3'-kinase, which was also inhibited by herbimycin and bis-tyrphostin. Finally, we show by immunostaining of endothelial cells VE-cadherin that PAF dissociated adherens junctions. The present data provide the first evidence that treatment of endothelial cells with PAF promoted activation of tyrosine kinases and the VE-cadherin tyrosine phosphorylation and PtdIns3'-kinase association, which ultimately lead to the dissociation of adherens junctions. Physical association between PtdIns3'-kinase, serving as a docking protein, and VE-cadherin may thus provide an efficient mechanism for amplification and perpetuation of PAF-induced cellular activation.

Epac1 regulates integrity of endothelial cell junctions through VE-cadherin

We have previously shown that Rap1 as well as its guanine nucleotide exchange factor Epac1 increases cell-cell junction formation. Here, we show that activation of Epac1 with the exchange protein directly activated by cAMP (Epac)-specific cAMP analog 8CPT-2'O-Me-cAMP (007) resulted in a tightening of the junctions and a decrease in the permeability of the endothelial cell monolayer. In addition, 007 treatment resulted in the breakdown of actin stress fibers and the formation of cortical actin. These effects were completely inhibited by siRNA against Epac1. In VE-cadherin knock-out cells Epac1 did not affect cell permeability, whereas in cells re-expressing VE-cadherin this effect was restored. Finally, the effect of Epac activation on the actin cytoskeleton was independent of junction formation. From these results we conclude that in human umbilical vein endothelial cells Epac1 controls VE-cadherin-mediated cell junction formation and induces reorganization of the actin cytoskeleton.

Distribution of adherens junction mediated by VE-cadherin complex in rat spleen sinus endothelial cells

The splenic sinus endothelium regulates the passage of blood cells through the splenic cord. The goal of the present study was to assess the localization of vascular endothelial (VE)-cadherin, beta-catenin, and p120-catenin in the sinus endothelial cells of rat spleen and to characterize the presence and distribution of adherens junction formation mediated by the cadherin-catenin complex. Immunofluorescent microscopy of tissue cryosections demonstrated that VE-cadherin, beta-catenin, and p120-catenin were localized in the junctional regions of adjacent endothelial cells. Double-staining immunofluorescent microscopy for VE-cadherin and beta-catenin revealed colocalization at junctional regions. Transmission electron microscopy of thin sections of sinus endothelial cells treated with Triton X-100 clearly showed adherens junctions within the plasma membrane. Adherens junctions were located at various levels in the lateral membranes of adjacent endothelial cells regardless of the presence or absence of underlying ring fibers. Immunogold electron microscopy revealed VE-cadherin, beta-catenin, and p120-catenin in the juxtaposed junctional membranes of adjacent sinus endothelial cells. Double-staining immunogold microscopy for VE-cadherin and beta-catenin and for VE-cadherin and p120-catenin demonstrated colocalization to the junctional membranes of adjacent endothelial cells. Immunolabeling was evident at various levels in the lateral junctional membranes and was intermittently observed in the sinus endothelium. These data suggest that adherens junctions, whose formation appears to be mediated by VE-cadherin-catenin complexes, probably regulate the passage of blood cells through the spleen.

Combretastatin A4 phosphate induces rapid regression of tumor neovessels and growth through interference with vascular endothelial-cadherin signaling

The molecular and cellular pathways that support the maintenance and stability of tumor neovessels are not well defined. The efficacy of microtubule-disrupting agents, such as combretastatin A4 phosphate (CA4P), in inducing rapid regression of specific subsets of tumor neovessels has opened up new avenues of research to identify factors that support tumor neoangiogenesis. Herein, we show that CA4P selectively targeted endothelial cells, but not smooth muscle cells, and induced regression of unstable nascent tumor neovessels by rapidly disrupting the molecular engagement of the endothelial cell-specific junctional molecule vascular endothelial-cadherin (VE-cadherin) in vitro and in vivo in mice. CA4P increases endothelial cell permeability, while inhibiting endothelial cell migration and capillary tube formation predominantly through disruption of VE-cadherin/beta-catenin/Akt signaling pathway, thereby leading to rapid vascular collapse and tumor necrosis. Remarkably, stabilization of VE-cadherin signaling in endothelial cells with adenovirus E4 gene or ensheathment with smooth muscle cells confers resistance to CA4P. CA4P synergizes with low and nontoxic doses of neutralizing mAbs to VE-cadherin by blocking assembly of neovessels, thereby inhibiting tumor growth. These data suggest that the microtubule-targeting agent CA4P selectively induces regression of unstable tumor neovessels, in part through disruption of VE-cadherin signaling. Combined treatment with anti-VE-cadherin agents in conjunction with microtubule-disrupting agents provides a novel synergistic strategy to selectively disrupt assembly and induce regression of nascent tumor neovessels, with minimal toxicity and without affecting normal stabilized vasculature.

Characterization of cytoskeletal and junctional proteins expressed by cells cultured from human arachnoid granulation tissue

BACKGROUND

The arachnoid granulations (AGs) are projections of the arachnoid membrane into the dural venous sinuses. They function, along with the extracranial lymphatics, to circulate the cerebrospinal fluid (CSF) to the systemic venous circulation. Disruption of normal CSF dynamics may result in increased intracranial pressures causing many problems including headaches and visual loss, as in idiopathic intracranial hypertension and hydrocephalus. To study the role of AGs in CSF egress, we have grown cells from human AG tissue in vitro and have characterized their expression of those cytoskeletal and junctional proteins that may function in the regulation of CSF outflow.

METHODS

Human AG tissue was obtained at autopsy, and explanted to cell culture dishes coated with fibronectin. Typically, cells migrated from the explanted tissue after 7-10 days in vitro. Second or third passage cells were seeded onto fibronectin-coated coverslips at confluent densities and grown to confluency for 7-10 days. Arachnoidal cells were tested using immunocytochemical methods for the expression of several common cytoskeletal and junctional proteins. Second and third passage cultures were also labeled with the common endothelial markers CD-31 or VE-cadherin (CD144) and their expression was quantified using flow cytometry analysis.

RESULTS

Confluent cultures of arachnoidal cells expressed the intermediate filament protein vimentin. Cytokeratin intermediate filaments were expressed variably in a subpopulation of cells. The cultures also expressed the junctional proteins connexin43, desmoplakin 1 and 2, E-cadherin, and zonula occludens-1. Flow cytometry analysis indicated that second and third passage cultures failed to express the endothelial cell markers CD31 or VE-cadherin in significant quantities, thereby showing that these cultures did not consist of endothelial cells from the venous sinus wall.

CONCLUSION

To our knowledge, this is the first report of the in vitro culture of arachnoidal cells grown from human AG tissue. We demonstrated that these cells in vitro continue to express some of the cytoskeletal and junctional proteins characterized previously in human AG tissue, such as proteins involved in the formation of gap junctions, desmosomes, epithelial specific adherens junctions, as well as tight junctions. These junctional proteins in particular may be important in allowing these arachnoidal cells to regulate CSF outflow.

p120-Catenin regulates clathrin-dependent endocytosis of VE-cadherin

VE-cadherin is an adhesion molecule critical to vascular barrier function and angiogenesis. VE-cadherin expression levels are regulated by p120 catenin, which prevents lysosomal degradation of cadherins by unknown mechanisms. To test whether the VE-cadherin cytoplasmic domain mediates endocytosis, and to elucidate the nature of the endocytic machinery involved, the VE-cadherin tail was fused to the interleukin (IL)-2 receptor (IL-2R) extracellular domain. Internalization assays demonstrated that the VE-cadherin tail dramatically increased endocytosis of the IL-2R in a clathrin-dependent manner. Interestingly, p120 inhibited VE-cadherin endocytosis via a mechanism that required direct interactions between p120 and the VE-cadherin cytoplasmic tail. However, p120 did not inhibit transferrin internalization, demonstrating that p120 selectively regulates cadherin internalization rather than globally inhibiting clathrin-dependent endocytosis. Finally, cell surface labeling experiments in cells expressing green fluorescent protein-tagged p120 indicated that the VE-cadherin-p120 complex dissociates upon internalization. These results support a model in which the VE-cadherin tail mediates interactions with clathrin-dependent endocytic machinery, and this endocytic processing is inhibited by p120 binding to the cadherin tail. These findings suggest a novel mechanism by which a cytoplasmic binding partner for a transmembrane receptor can serve as a selective plasma membrane retention signal, thereby modulating the availability of the protein for endo-lysosomal processing.

Assessing microvessels and angiogenesis in human breast cancer, using VE-cadherin

AIMS

Vascular endothelial (VE)-cadherin, also known as cadherin-5 and CD144, is an adhesion molecule uniquely expressed in endothelial cells. We hypothesized that VE-cadherin may be a useful marker for assessing microvessels and angiogenesis in human breast cancer and sought to determine whether a correlation exists between levels of VE-cadherin, angiogenic markers factor VIII and platelet endothelial cell adhesion molecule (PECAM)-1 and patient outcome in breast cancer.

METHODS AND RESULTS

Frozen sections from breast cancer primary tumours (tumour n = 114, background n = 30) were immunostained with VE-cadherin, factor VIII and PECAM-1 antibodies and microvessel number was assessed. RNA was reverse transcribed and analysed by quantitative polymerase chain reaction (Q-PCR). VE-cadherin immunostaining showed a significant difference in microvessel number in tumour compared with background. There was no significant difference in the number of microvessels stained with PECAM-1 or factor VIII; there was increased staining of other structures within the sample and higher general background staining. Q-PCR revealed elevated levels of VE-cadherin and PECAM-1 in tumour samples compared with background tissue and in patients with a poor prognosis, as determined by the Nottingham Prognostic Index. There was no difference in levels with factor VIII. Both VE-cadherin and PECAM-1 had significantly reduced expression in lobular compared with ductal carcinomas: there was no difference with factor VIII.

CONCLUSION

Higher levels of angiogenic marker molecules in breast cancer may have an association with poor prognosis in patients. Moreover, VE-cadherin appears to be a preferable marker for such analysis.

17 beta-estradiol transiently disrupts adherens junctions in endothelial cells

Interendothelial junctions are important regulators of endothelial cell functions such as migration and proliferation, major features in angiogenesis, and endothelial cell monolayer wound healing. 17beta-estradiol regulates these functions in vivo and in vitro and also increases endothelial monolayer permeability as it results from impaired monolayer integrity and intercellular adhesion. We hypothesized that 17beta-estradiol affects these cell adhesion-dependent functions in endothelial cells by targeting the adherens junction complex. Here, we show that 17beta-estradiol increases uterine microvascular endothelial cell monolayer permeability and transiently redistributes interendothelial junction-forming proteins in endothelial cells. Concomitantly, adherens junction proteins are disconnected from the cytoskeleton and alpha-catenin, which links VE-cadherin to the cytoskeleton, is redistributed from the membrane and the adherens junction complex. Furthermore, 17beta-estradiol increased tyrosine phosphorylation of the adherens junction complex. These effects were inhibited by the estrogen receptor antagonist ICI 182,780 but could be provoked using non-cell membrane-permeable 17beta-estradiol-BSA in all cells tested, including EA.hy 926 cells, which have been shown unable to stimulate 17beta-estradiol-dependent gene transcription. Additionally, 17beta-estradiol treatment enhanced the angiogenic effect of vascular endothelial growth factor in an in vitro angiogenesis model, as a potential implication of the adherens junction disruption. Cotreatment with the Src-family kinase inhibitor PP2 prevented the redistribution and phosphorylation of the adherens junction proteins. Taken together, our data show that adherens junctions in endothelial cells are a downstream target of membrane-associated 17beta-estradiol signaling, possibly through Src-family kinases.

The human VE-cadherin promoter is subjected to organ-specific regulation and is activated in tumour angiogenesis

Vascular endothelial (VE)-cadherin is exclusively expressed at interendothelial junctions of normal and tumour vessels. In this report, we characterized the transcriptional activity of the human VE-cadherin promoter. Transient transfection assays revealed that sequences at positions --1135/-744 and -166/-5 base pairs are critical for promoter activity in endothelial cells. We show that specific sequences in the proximal region interact with Ets and Sp1 family members. Transgenic mice were created and the human VE-cadherin promoter was able to confer correct temporal and spatial expression on the LacZ gene in embryos. In adults, the transgene was specifically and strongly expressed in the lung, heart, ovary, spleen and kidney glomeruli, whereas expression was weak or absent in the vasculature of other organs, including the brain, thymus, liver and skeletal muscle. Neovessels in tumour grafts and Matrigel implants harboured strong stainings, indicating that promoter activity is enhanced in angiogenic situations. Furthermore, Matrigel and transfection assays showed that VE-cadherin promoter is subjected to bFGF induction. Transgene expression was also noticed in extravascular sites of the central nervous system, suggesting that silencer elements may be located elsewhere in the gene. These results are a first step towards addressing the organ- and tumour-specific regulation of the VE-cadherin gene.

Identification of a transiently exposed VE-cadherin epitope that allows for specific targeting of an antibody to the tumor neovasculature

VE-cadherin is an adhesion molecule localized at the adherens junctions of endothelial cells. It is crucial for the proper assembly of vascular structures during angiogenesis and maintaining vascular integrity. We have studied 3 monoclonal antibodies (mAbs) against murine VE-cadherin that inhibit angiogenesis and tumor growth. Two of these, BV13 and 10G4, also disrupted normal vessels, resulting in severe vascular leakage, whereas the third, E4G10, did not. The goal of the current report was to identify the epitope of E4G10 and distinguish it from those of the disruptive mAbs. We mapped the epitope of E4G10 to within the first 10 amino acids of mature VE-cadherin and demonstrated that conserved tryptophan residues in this sequence are required for VE-cadherin-mediated trans-adhesion. The disruptive mAbs target a different epitope within amino acids 45 to 56, which structural homology modeling suggests is not involved in trans-adhesion. From our studies, we hypothesize that E4G10 can only bind the neovasculature, where VE-cadherin has not yet engaged in trans-adhesion and its epitope is fully exposed. Thus, E4G10 can inhibit junction formation and angiogenesis but is unable to target normal vasculature because its epitope is masked. In contrast, BV13 and 10G4 bind an epitope that is accessible regardless of VE-cadherin interactions, leading to the disruption of adherens junctions. Our findings establish the immediate N-terminal region of VE-cadherin as a novel target for inhibiting angiogenesis.

Vascular endothelial-cadherin tyrosine phosphorylation in angiogenic and quiescent adult tissues

Vascular endothelial-cadherin (VE-cadherin) plays a key role in angiogenesis and in vascular permeability. The regulation of its biological activity may be a central mechanism in normal or pathological angiogenesis. VE-cadherin has been shown to be phosphorylated on tyrosine in vitro under various conditions, including stimulation by VEGF. In the present study, we addressed the question of the existence of a tyrosine phosphorylated form of VE-cadherin in vivo, in correlation with the quiescent versus angiogenic state of adult tissues. Phosphorylated VE-cadherin was detected in mouse lung, uterus, and ovary but not in other tissues unless mice were injected with peroxovanadate to block protein phosphatases. Remarkably, VE-cadherin tyrosine phosphorylation was dramatically increased in uterus and ovary, and not in other organs, during PMSG/hCG-induced angiogenesis. In parallel, we observed an increased association of VE-cadherin with Flk1 (VEGF receptor 2) during hormonal angiogenesis. Additionally, Src kinase was constitutively associated with VE-cadherin in both quiescent and angiogenic tissues and increased phosphorylation of VE-cadherin-associated Src was detected in uterus and ovary after hormonal treatment. Src-VE-cadherin association was detected in cultured endothelial cells, independent of VE-cadherin phosphorylation state and Src activation level. In this model, Src inhibition impaired VEGF-induced VE-cadherin phosphorylation, indicating that VE-cadherin phosphorylation was dependent on Src activation. We conclude that VE-cadherin is a substrate for tyrosine kinases in vivo and that its phosphorylation, together with that of associated Src, is increased by angiogenic stimulation. Physical association between Flk1, Src, and VE-cadherin may thus provide an efficient mechanism for amplification and perpetuation of VEGF-stimulated angiogenic processes.

VE-cadherin: adhesion at arm's length

VE-cadherin was first identified in the early 1990s and quickly emerged as an important endothelial cell adhesion molecule. The past decade of research has revealed key roles for VE-cadherin in vascular permeability and in the morphogenic events associated with vascular remodeling. The details of how VE-cadherin functions in adhesion became apparent with structure-function analysis of the cadherin extracellular domain and with the identification of the catenins, a series of cytoplasmic proteins that bind to the cadherin tail and mediate interactions between cadherins and the cytoskeleton. Whereas early work focused on the armadillo family proteins beta-catenin and plakoglobin, more recent investigations have identified p120-catenin (p120(ctn)) and a related group of armadillo family members as key binding partners for the cadherin tail. Furthermore, a series of new studies indicate a key role for p120(ctn) in regulating cadherin membrane trafficking in mammalian cells. These recent studies place p120(ctn) at the hub of a cadherin-catenin regulatory mechanism that controls cadherin plasma membrane levels in cells of both epithelial and endothelial origin.

Src blockade stabilizes a Flk/cadherin complex, reducing edema and tissue injury following myocardial infarction

Ischemia resulting from myocardial infarction (MI) promotes VEGF expression, leading to vascular permeability (VP) and edema, a process that we show here contributes to tissue injury throughout the ventricle. This permeability/edema can be assessed noninvasively by MRI and can be observed at the ultrastructural level as gaps between adjacent endothelial cells. Many of these gaps contain activated platelets adhering to exposed basement membrane, reducing vessel patency. Following MI, genetic or pharmacological blockade of Src preserves endothelial cell barrier function, suppressing VP and infarct volume, providing long-term improvement in cardiac function, fibrosis, and survival. To our surprise, an intravascular injection of VEGF into healthy animals, but not those deficient in Src, induced similar endothelial gaps, VP, platelet plugs, and some myocyte damage. Mechanistically, we show that quiescent blood vessels contain a complex involving Flk, VE-cadherin, and beta-catenin that is transiently disrupted by VEGF injection. Blockade of Src prevents disassociation of this complex with the same kinetics with which it prevents VEGF-mediated VP/edema. These findings define a molecular mechanism to account for the Src requirement in VEGF-mediated permeability and provide a basis for Src inhibition as a therapeutic option for patients with acute MI.

Elevated Concentration of Soluble Vascular Endothelial Cadherin Is Associated With Coronary Atherosclerosis

BACKGROUND

Vascular endothelial (VE)-cadherin, a Ca(2+)-dependent cell adhesion molecule, is expressed in atherosclerotic lesions by endothelial cells and is associated with neovascularization, although the relationship between circulating VE-cadherin and coronary artery disease has not been studied.

METHODS AND RESULTS

The plasma concentration of VE-cadherin was measured in peripheral blood (femoral artery) and the coronary sinus of 24 patients with acute myocardial infarction (AMI), 26 with stable angina pectoris (AP), 18 with old myocardial infarction (OMI), and 30 control subjects (Control) who had no coronary artery stenosis on angiography. For the patients with AMI, blood samples were obtained in the acute (day 1) and chronic (day 21) phases. The plasma concentration of VE-cadherin was measured by enzyme-linked immunosorbent assay. The correlation between the plasma VE-cadherin concentration and the Gensini score was also determined as an index of the severity of coronary atherosclerosis. The plasma concentrations of VE-cadherin (ng/ml) in both the peripheral and coronary sinus blood were higher in patients with AMI, AP, and OMI than in the control subjects, and were similar in the 3 groups with coronary artery disease (femoral artery: AMI 5.1+/-2.5, AP 4.7+/-2.4, OMI 4.5+/-3.3, Control 2.6+/-2.3; coronary sinus: AMI 5.6+/-2.6, AP 5.0+/-2.3, OMI 5.0+/-2.9, Control 2.4+/-2.1, respectively). Plasma VE-cadherin concentrations were higher in the coronary sinus than peripheral blood samples in patients with AMI (p<0.01), AP (p<0.01), and OMI (p<0.05). The plasma VE-cadherin concentration was the same in the acute and chronic phases in patients with AMI. In the 3 groups of patients with coronary disease, both the peripheral plasma VE-cadherin concentration and the coronary sinus concentration correlated with the Gensini score (r=0.32, p<0.01 and r=0.42, p<0.001, respectively). Multiple regression analysis revealed that the plasma VE-cadherin concentration predicted the Gensini score independently of sex, age, hypertension, diabetes mellitus, smoking, and the lipid profiles.

CONCLUSION

Increased secretion of VE-cadherin from the epicardial arteries is associated with the degree of coronary atherosclerosis, indicating the presence of atherosclerosis rather than disease activity.

VE-cadherin-p120 interaction is required for maintenance of endothelial barrier function

Interaction of p120 with juxtamembrane domain (JMD) of VE-cadherin has been implicated in regulation of endothelial cell-cell adhesion. We used a number of approaches to alter the level of p120 available for binding to VE-cadherin as a means to investigate the role of p120-VE-cadherin interaction in regulation of barrier function in confluent endothelial monolayers. Expression of an epitope-tagged fragment corresponding to JMD of VE-cadherin resulted in a decrease in endothelial barrier function as assessed by changes in albumin clearance and electrical resistance. Binding of JMD-Flag to p120 resulted in a decreased level of p120. In addition to decreasing p120 level, expression of JMD also decreased level of VE-cadherin. Expression of JMD also caused an increase in MLC phosphorylation and rearrangement of actin cytoskeleton, which, coupled with decreased cadherin, can contribute to loss of barrier function. Reducing p120 by siRNA resulted in a decrease in VE-cadherin, whereas increasing the level of p120 increased the level of VE-cadherin, demonstrating that p120 regulates the level of VE-cadherin. Overexpression of p120 was, however, associated with decreased barrier function and rearrangement of the actin cytoskeleton. Interestingly, expression of p120 was able to inhibit thrombin-induced increases in MLC phosphorylation, suggesting that p120 inhibits activation of Rho/Rho kinase pathway in endothelial cells. Excess p120 also prevented JMD-induced increases in MLC phosphorylation, correlating this phosphorylation with Rho/Rho kinase pathway. These findings show p120 plays a major role in regulating endothelial barrier function, as either a decrease or increase of p120 resulted in disruption of permeability across cell monolayers.

Vascular endothelial cadherin expression in human carotid atherosclerotic plaque and its relationship with plaque morphology and clinical data

OBJECTIVES

To determine the relationship between Vascular Endothelial (VE)-cadherin expression in carotid plaques, carotid plaque morphology and clinical findings of carotid disease.

MATERIALS AND METHODS

Fifty-three formalin-fixed, paraffin embedded specimens of human carotid atherosclerotic plaque obtained by endarterectomy and 20 normal postmortem arteries (control group) were studied. Thirty patients were symptomatic and 23 asymptomatic. The expression of VE-cadherin was examined by an avidin-biotin immunoperoxidase technique using specific monoclonal antibodies against this molecule. We used a scale for the estimation of the expression of the VE-cadherin, in which negative expression was indicated by 0, weak expression by 1, and strong expression by 2. In serial sections we also determined the cellular phenotype of atherosclerotic plaques: i.e. the endothelial cells (F8), macrophage (CD68) and smooth muscle cells. Possible relations between variables in statistical analysis were examined by the chi-square test or Fisher's exact test.

RESULTS

Expression of VE-cadherin was observed in small newly established vessels, particularly in areas with intense inflammatory infiltrations by macrophages and leucocytes. A strong expression of VE-cadherin was evident particularly in symptomatic instead in asymptomatic patients (43% vs. 13%, p=0.057), in high degree stenosis group (81% vs. 0%, p=0.005), and in patients with ischaemic infarct in brain scan (71% vs. 23%, p=0.021). On the other hand, there was no relation between molecule expression and plaque ultrasonic characteristics (echogenic or echolucent, p=0.499). Finally, there was a significant statistical correlation in the expression of VE-cadherin and the histological type of the plaque, namely fibrotic and complicated plaques. Strong VE-cadherin expression was observed in 64% of complicated plaques instead of 6.5% in fibrotic plaques (p=0.001).

CONCLUSION

An intense expression of VE-cadherin in carotid plaques is linked with plaque instability, high degree of stenosis and clinical events. This molecule seems to be a marker of progression of the atherosclerotic plaque.

Green tea catechins inhibit VEGF-induced angiogenesis in vitro through suppression of VE-cadherin phosphorylation and inactivation of Akt molecule

Studies have indicated that the consumption of green tea is associated with a reduced risk of developing certain forms of cancer and angiogenesis. The mechanism of inhibition of angiogenesis by green tea or its catechins, however, has not been well-established. Vascular endothelial (VE)-cadherin, an adhesive molecule located at the site of intercellular contact, is involved in cell-cell recognition during vascular morphogenesis. The extracellular domain of VE-cadherin mediates initial cell adhesion, whereas the cytosolic tail binding with beta-catenin is required for interaction with the cytoskeleton and junctional strength. Therefore, the cadherin-catenin adhesion system is implicated in cell recognition, differentiation, growth and migration of capillary endothelium. Using tube formation of human microvascular endothelial cells (HMVEC) in culture as an in vitro model of angiogenesis, we reported that vascular endothelial growth factor (VEGF)-induced tube formation is inhibited by anti-VE-cadherin antibody and dose-dependently by green tea catechins. We also demonstrated here that inhibition of tube formation by epigallocatechin gallate (EGCG), one of the green tea catechins, is in part mediated through suppression of VE-cadherin tyrosine phosphorylation and inhibition of Akt activation during VEGF-induced tube formation. These findings indicate that VE-cadherin and Akt, known downstream proteins in VEGFR-2-mediated cascade, are the new-targeted proteins by which green tea catechins inhibit angiogenesis.

VE-cadherin simultaneously stimulates and inhibits cell proliferation by altering cytoskeletal structure and tension

Engagement of vascular endothelial (VE)-cadherin leads to the cessation of proliferation commonly known as 'contact inhibition'. We show that VE-cadherin inhibits growth by mediating changes in cell adhesion to the extracellular matrix. Increasing cell-cell contact decreased cell spreading and proliferation, which was reversed by blocking engagement of VE-cadherin. Using a new system to prevent the cadherin-induced changes in cell spreading, we revealed that VE-cadherin paradoxically increased proliferation. Treating cells with inhibitors of PKC and MEK abrogated the stimulatory signal at concentrations that disrupted the formation of actin fibers across the cell-cell contact. Directly disrupting actin fibers, blocking actin-myosin-generated tension, or inhibiting signaling through Rho specifically inhibited the cadherin-induced proliferative signal. By progressively altering the degree to which cell-cell contact inhibited cell spreading, we show that cell-cell contact ultimately increased or decreased the overall proliferation rate of the population by differentially shifting the balance between the two opposing proliferative cues. The existence of opposing growth signals induced by VE-cadherin that are both mediated through crosstalk with cytoskeletal structure highlights the complex interplay of mechanical and chemical signals with which cells navigate in their physical microenvironment.

Involvement of proteinase-activated receptor-2 in mast cell tryptase-induced barrier dysfunction in bovine aortic endothelial cells

We report here a direct modulation by mast cell tryptase of endothelial barrier function through activation of proteinase-activated receptor-2 (PAR-2). In cultured bovine aortic endothelial cells (BAECs), tryptase, trypsin and PAR-2 activating peptide impaired the barrier function as determined by the permeability of protein-conjugated Evans blue. The tryptase-induced barrier dysfunction was completely blocked by U73122, and partially reversed by xestospongin C, calphostin C or Y27632. The intracellular Ca(2+) was elevated by tryptase. It was notable that ioxaglate, a contrast material that degranulates mast cells, markedly increased the permeability when applied to BAECs in combination with mast cells, an action that was blocked by nafamostat, a potent tryptase inhibitor. Immunofluorescence analysis showed that actin stress fibre formation and disruption of VE-cadherin were observed after exposure to tryptase or ioxaglate in combination with mast cells. Therefore, it is suggested that mast cell tryptase impairs endothelial barrier function through activation of endothelial PAR-2 in a manner dependent on the phospholipase C activity.

Expression of thymidine kinase driven by an endothelial-specific promoter inhibits tumor growth of Lewis lung carcinoma cells in transgenic mice

The possibility of inhibiting tumor growth by limiting angiogenesis has raised considerable interest. In this study, we examined the feasibility of inhibiting tumor growth by targeting a suicide gene in the endothelium. Toxicity must be directed solely to angiogenic cells. Therefore, we used the herpes simplex virus-thymidine kinase (TK) gene, in combination with the prodrug ganciclovir (GCV), which affects replicative cells. To test this strategy, we produced transgenic mice carrying the TK gene driven by the vascular endothelial (VE)-cadherin promoter. Lewis lung carcinoma cells were injected subcutaneously to establish tumors and to test the effect of GCV on tumor growth. In two independent transgenic lines, GCV treatment (75 mg/kg/day) resulted in a 66-71% reduction of tumor volume at day 20 postimplantation compared to wild-type mice (650 and 550 versus 1930 mm(3), P<0.02 and 0.01, respectively), whereas no significant difference was observed when vehicle alone was injected. Tumor growth inhibition was accompanied by a marked reduction in tumor vascular density (151 versus 276 vessels/mm(2), P<0.05) and an increase in tumor cell death, suggesting that tumor growth inhibition was caused by a reduction in tumor angiogenesis. Our data support the potential utility of endothelial targeting of suicide genes in cancer therapy.

Leukocyte-endothelial-cell interactions in leukocyte transmigration and the inflammatory response

Both the innate and adaptive immune responses are dependent on the migration of leukocytes across endothelial cells. The process of diapedesis, in which the leukocyte crawls between tightly apposed endothelial cells, is a unique and complex process. Several molecules concentrated at the junctions of endothelial cells, originally described as having a role in holding the endothelial monolayer together, have also been shown to have a role in the emigration of leukocytes. Several mechanisms have been proposed for 'loosening' the junctions between endothelial cells to enable leukocyte passage. These leukocyte-endothelial-cell adhesion molecules are probably involved in regulating the signaling as well as the adhesion events of diapedesis. In addition, this Review introduces a new and unified nomenclature for the junctional adhesion molecule (JAM) family.

Mechanisms of VE-cadherin processing and degradation in microvascular endothelial cells

VE-cadherin is an endothelial-specific cadherin that plays important roles in vascular morphogenesis and growth control. To investigate the mechanisms by which endothelial cells regulate cadherin cell surface levels, a VE-cadherin mutant containing the non-adhesive interleukin-2 (IL-2) receptor extracellular domain and the VE-cadherin cytoplasmic tail (IL-2R-VE-cadcyto) was expressed in microvascular endothelial cells. Expression of the IL-2R-VE-cadcyto mutant resulted in the internalization of endogenous VE-cadherin and in a dramatic decrease in endogenous VE-cadherin levels. The internalized VE-cadherin co-localized with early endosomes, and the lysosomal inhibitor chloroquine dramatically inhibited the down-regulation of VE-cadherin in cells expressing the IL-2R-VE-cadcyto mutant. Chloroquine treatment also resulted in the accumulation of a VE-cadherin fragment lacking the beta-catenin binding domain of the VE-cadherin cytoplasmic tail. The formation of the VE-cadherin fragment could be prevented by treating endothelial cells with proteasome inhibitors. Furthermore, inhibition of the proteasome prevented VE-cadherin internalization and inhibited the disruption of endothelial intercellular junctions by the IL-2RVE-cadcyto mutant. These results provide new insights into the mechanisms of VE-cadherin processing and degradation in microvascular endothelial cells.

Nitric oxide disrupts VE-cadherin complex in murine microvascular endothelial cells

Vascular endothelial cadherin (VE-cadherin), which is localized at adherent junctions, is involved in the control of vascular permeability. A growing body of evidence indicates that NO modulates the movement of fluid and proteins out of the vasculature. In this paper, we investigated whether NO can disrupt the VE-cadherin complex. We found that treatment with two NO donors (SIN-1 and SNAP) markedly reduced the amount of VE-cadherin in a murine microvascular endothelial cell line (H5V) as demonstrated by immunoprecipitation analysis, cellular ELISA, and Northern blot analysis. Beta- and gamma-catenins were also found to be affected by the two NO donors. Moreover, the disruption of the complex, induced by NO donors, correlated with increases in vascular permeability using both in vivo and in vitro models. These results clearly demonstrate a role for NO in vascular permeability.

Homoassociation of VE-cadherin follows a mechanism common to "classical" cadherins

Vascular endothelial cadherin (VE-cadherin/cadherin5) is specifically expressed in adherens junctions of endothelial cells and exerts important functions in cell-cell adhesion as well as signal transduction. To analyze the mechanism of VE-cadherin homoassociation, the ectodomains CAD1-5 were connected by linker sequences to the N terminus of the coiled-coil domain of cartilage matrix protein (CMP). The chimera VECADCMP were expressed in mammalian cells. The trimeric coiled-coil domain leads to high intrinsic domain concentrations and multivalency promoting self-association. Ca(2+)-dependent homophilic association of VECADCMP was detected in solid phase assays and cross-linking experiments. A striking analogy to homoassociation of type I ("classical") cadherins like E, N or P-cadherin was observed when interactions in VECADCMP and between these trimeric proteins were analyzed by electron microscopy. Ca(2+)-dependent ring-like and double ring-like arrangements suggest interactions between domains 1 and 2 of the ectodomains, which may be correlated with lateral and adhesive contacts in the adhesion process. Association to complexes composed of two VECADCMP molecules was also demonstrated by chemical cross-linking. No indication for an antiparallel association of VECAD ectodomains to hexameric complexes as proposed by Legrand et al. was found. Instead the data suggest that homoassociation of VE-cadherin follows the conserved mechanism of type I cadherins.

The Armadillo family protein p0071 is a VE-cadherin- and desmoplakin-binding protein

p0071, a member of the armadillo protein family, localizes to both adherens junctions and desmosomes in epithelial cells and exhibits homology to the adherens junction protein p120 and the desmosomal protein plakophilin-1. p0071 is also present at dermal microvascular endothelial intercellular junctions and colocalizes with VE-cadherin, an endothelium-specific cadherin that associates with both actin and intermediate filament networks. To define the role of p0071 in junction assembly, p0071 was tested for interactions with other components of the endothelial junctional complex. In transient expression assays, p0071 colocalized with and formed complexes with both VE-cadherin and desmoplakin. Deletion analysis using the yeast two-hybrid system revealed that the armadillo repeat domain of p0071 bound directly to VE-cadherin. Site-directed mutagenesis experiments demonstrated that p0071 and p120 bound to the same region on the cytoplasmic tail of VE-cadherin and that overexpression of p0071 could displace p120 from intercellular junctions. In contrast to VE-cadherin, desmoplakin was found to associate with the non-armadillo head domain of p0071. Cotransfections and triple-label immunofluorescence analysis revealed that VE-cadherin colocalization with desmoplakin in transfected COS cells required p0071, suggesting that p0071 may couple VE-cadherin to desmoplakin. Based on previous findings that both VE-cadherin and desmoplakin play central roles in vasculogenesis, these new results suggest that p0071 may play an important role in endothelial junction assembly and in the morphogenic events associated with vascular remodeling.

Wound closure in sheared endothelial cells is enhanced by modulation of vascular endothelial-cadherin expression and localization

We previously demonstrated that laminar shear stress enhances human coronary artery endothelial cell (HCAEC) wound closure via the mechanisms of cell spreading and migration. Because cell-cell junctional proteins such as vascular endothelial cell cadherin (VE-cadherin) are critical to cell-cell adhesion and motility, we tested the hypothesis that modulation of VE-cadherin expression under shear stress may be linked to this enhancement in wound closure. HCAEC monolayers were preconditioned to attain cellular alignment by shearing at 12 dynes/cm(2) for 18 hr in a parallel-plate flow chamber. Subsequently, they were divided into the following three groups: (i) control; (ii) treated with anti-cadherin-5 antibody; or (iii) treated with the calcium chelating agent EGTA. Next, the monolayers were wounded with a metal spatula and resheared at 20 dynes/cm(2) or left static. Time-lapse imaging was performed during the first 3 hr after imposition of these conditions. Immunocytochemistry or Western blot analyses for VE-cadherin expression were performed on all wounded monolayers. Deconvolution microscopy, three-dimensional cell-cell junctional reconstruction images, and histogram analyses of interendothelial junction signal intensities were performed on cells at the wound edge of a monolayer. Under shear, HCAEC demonstrated increased VE-cadherin immunofluorescence and protein expression despite an enhancement in wound closure compared with static conditions. In separate experiments, application with anti-cadherin-5 antibody or treatment with EGTA attenuated VE-cadherin expression and further enhanced wound closure compared with control shear and all static conditions. In addition, the pattern of VE-cadherin localization with these treatments became more intracellular and nuclear in appearance. These findings of changes in this junctional adhesion protein expression and localization may further our understanding of laminar shear stress-induced endothelial repair in the coronary circulation.

Role of interendothelial adhesion molecules in the control of vascular functions

The function of endothelium is the lining of the vessel wall and the control of vascular permeability, homeostasis and leukocyte emigration from the blood into the surrounding tissue. Different adhesion molecules expressed in a coordinated and regulated way control this function. In this review, we discuss adhesion molecules involved in endothelial junctions and their involvement in leukocyte transendothelial migration. Passage of the leukocyte across the endothelium appears to require delocalization of certain vascular adhesion molecules whereas other molecules interact directly with leukocyte ligands. Understanding of the function of vascular adhesion molecules is further complicated as they transduce signals to the endothelium and interact with the cytoskeleton and adaptor proteins.

VE-PTP and VE-cadherin ectodomains interact to facilitate regulation of phosphorylation and cell contacts

VE-cadherin is the essential adhesion molecule in endothelial adherens junctions, and the regulation of protein tyrosine phosphorylation is thought to be important for the control of adherens junction integrity. We show here that VE-PTP (vascular endothelial protein tyrosine phosphatase), an endothelial receptor-type phosphatase, co-precipitates with VE-cadherin, but not with beta-catenin, from cell lysates of transfected COS-7 cells and of endothelial cells. Co-precipitation of VE-cadherin and VE-PTP required the most membrane-proximal extracellular domains of each protein. Expression of VE-PTP in triple-transfected COS-7 cells and in CHO cells reversed the tyrosine phosphorylation of VE-cadherin elicited by vascular endothelial growth factor receptor 2 (VEGFR-2). Expression of VE-PTP under an inducible promotor in CHO cells transfected with VE-cadherin and VEGFR-2 increased the VE-cadherin-mediated barrier integrity of a cellular monolayer. Surprisingly, a catalytically inactive mutant form of VE-PTP had the same effect on VE-cadherin phosphorylation and cell layer permeability. Thus, VE-PTP is a transmembrane binding partner of VE-cadherin that associates through an extracellular domain and reduces the tyrosine phosphorylation of VE-cadherin and cell layer permeability independently of its enzymatic activity.

Regulation of endothelial barrier function and growth by VE-cadherin, plakoglobin, and beta-catenin

VE-cadherin is an endothelial-specific cadherin that plays a central role in vascular barrier function and angiogenesis. The cytoplasmic domain of VE-cadherin is linked to the cytoskeleton through interactions with the armadillo family proteins beta-catenin and plakoglobin. Growing evidence indicates that beta-catenin and plakoglobin play important roles in epithelial growth and morphogenesis. To test the role of these proteins in vascular cells, a replication-deficient retroviral system was used to express intercellular junction proteins and mutants in the human dermal microvascular endothelial cell line (HMEC-1). A mutant VE-cadherin lacking an adhesive extracellular domain disrupted endothelial barrier function and inhibited endothelial growth. In contrast, expression of exogenous plakoglobin or metabolically stable mutants of beta-catenin stimulated HMEC-1 cell growth, which suggests that the beta-catenin signaling pathway was active in HMEC-1 cells. This possibility was supported by the finding that a dominant-negative mutant of the transcription factor TCF-4, designed to inhibit beta-catenin signaling, also inhibited HMEC-1 cell growth. These observations suggest that intercellular junction proteins function as components of an adhesion and signaling system that regulates vascular barrier function and growth.

A monoclonal antibody to vascular endothelial-cadherin inhibits tumor angiogenesis without side effects on endothelial permeability

Vascular endothelial cadherin (VE-cadherin) is an endothelial-specific, trans-membrane protein that promotes homophilic cell adhesion. Inhibition of VE-cadherin by the blocking monoclonal antibody (mAb) BV13 inhibited angiogenesis and tumor growth in vivo. However, this effect was accompanied by a marked increase in lung and heart permeability. In the present paper, we characterize a different VE-cadherin mAb (BV14) that is able to inhibit angiogenesis without affecting vascular permeability. In vitro studies show that BV14, in contrast to BV13, did not increase paracellular permeability of endothelial monolayers and did not disrupt VE-cadherin clusters at junctions. However, both antibodies could inhibit formation of vascularlike structures in collagen gels and increase migration of endothelial cells into wounded areas. In vivo, BV14 and BV13 were equally active in inhibiting angiogenesis in the mouse cornea and in reducing the growth of hemangioma and C6 glioma. In contrast to BV13, BV14 did not change vascular permeability in all the organs tested and at any dose used. BV14 and BV13 bind to VE-cadherin extracellular repeats EC4 and EC1, respectively. We propose that, in resting vessels, where junctions are stable and well-structured, antibody binding to EC1 but not EC4 disrupts their organization and increases permeability. In contrast, in growing vessels, where endothelial cells are migrating and junctions are weaker, antibody binding to EC4 may be sufficient to disrupt cell-to-cell adhesion and inhibit assembly of new vascular structures.

The role of endothelial cell lateral junctions during leukocyte trafficking

An essential function of the inflammatory response is selective targeting of appropriate leukocyte types to a site of infection or injury. The past decade has witnessed an explosion in the level of detail concerning the identification and deciphering of the molecular mechanisms that capture leukocytes from flowing blood and promote leukocyte arrest on the vessel wall. In contrast, less information is known about the migration of adherent blood leukocytes through endothelial cell-to-cell borders (transendothelial migration, TEM) and into the underlying tissues. This article reviews the endothelial-dependent mechanisms that coordinate TEM in peripheral vasculature and highlights the role of certain lateral junctional proteins and protein complexes.

Reactive oxygen species mediate Rac-induced loss of cell-cell adhesion in primary human endothelial cells

The integrity of the endothelium is dependent on cell-cell adhesion, which is mediated by vascular-endothelial (VE)-cadherin. Proper VE-cadherin-mediated homotypic adhesion is, in turn, dependent on the connection between VE-cadherin and the cortical actin cytoskeleton. Rho-like small GTPases are key molecular switches that control cytoskeletal dynamics and cadherin function in epithelial as well as endothelial cells. We show here that a cell-penetrating, constitutively active form of Rac (Tat-RacV12) induces a rapid loss of VE-cadherin-mediated cell-cell adhesion in endothelial cells from primary human umbilical veins (pHUVEC). This effect is accompanied by the formation of actin stress fibers and is dependent on Rho activity. However,transduction of pHUVEC with Tat-RhoV14, which induces pronounced stress fiber and focal adhesion formation, did not result in a redistribution of VE-cadherin or an overall loss of cell-cell adhesion. In line with this observation, endothelial permeability was more efficiently increased by Tat-RacV12 than by Tat-RhoV14.

The loss of cell-cell adhesion, which is induced by Tat-RacV12, occurred in parallel to and was dependent upon the intracellular production of reactive oxygen species (ROS). Moreover, Tat-RacV12 induced an increase in tyrosine phosphorylation of a component the VE-cadherin-catenin complex, which was identified as α-catenin. The functional relevance of this signaling pathway was further underscored by the observation that endothelial cell migration, which requires a transient reduction of cell-cell adhesion, was blocked when signaling through ROS was inhibited.

In conclusion, Rac-mediated production of ROS represents a previously unrecognized means of regulating VE-cadherin function and may play an important role in the (patho)physiology associated with inflammation and endothelial damage as well as with endothelial cell migration and angiogenesis.

VE-cadherin regulates endothelial actin activating Rac and increasing membrane association of Tiam

Previously published reports support the concept that, besides promoting homotypic intercellular adhesion, cadherins may transfer intracellular signals. However, the signaling pathways triggered by cadherin clustering and their biological significance are still poorly understood. We report herein that transfection of VE-cadherin (VEC) cDNA in VEC null endothelial cells induces actin rearrangement and increases the number of vinculin positive adhesion plaques. VEC expression augments the level of active Rac but decreases active Rho. Microinjection of a dominant negative Rac mutant altered stress fiber organization, whereas inhibition of Rho was ineffective. VEC expression increased protein and mRNA levels of the Rac-specific guanosine exchange factor Tiam-1 and induced its localization at intercellular junctions. In addition, in the presence of VEC, the amounts of Tiam, Rac, and the Rac effector PAK as well as the level of PAK phosphorylation were found increased in the membrane/cytoskeletal fraction. These observations are consistent with a role of VEC in localizing Rac and its signaling partners in the same membrane compartment, facilitating their reciprocal interaction. Through this mechanism VEC may influence the constitutive organization of the actin cytoskeleton.

Antiphospholid antibodies regulate the expression of trophoblast cell adhesion molecules

OBJECTIVE

To examine the effect of antiphospholipid antibodies on trophoblast expression of adhesion molecules.

DESIGN

Primary cytotrophoblast cell cultures.

SETTING

Department of Obstetrics and Gynecology, Catholic University, Rome, Italy.

PATIENT(S)

Five normal pregnant women underwent uncomplicated vaginal delivery at 36 weeks of gestation.

INTERVENTION(S)

IgG antibodies were isolated from a patient with antiphospholipid syndrome and from a normal control subject, using protein-G Sepharose columns. Cytotrophoblast cells were dispersed in bicarbonate buffer containing trypsin and DNAse I.

MAIN OUTCOME MEASURE(S)

We investigated the effects of antiphospholipid antibodies on trophoblast adhesion molecules (alpha1 and alpha5 integrins, E and VE cadherins), both at the protein and mRNA levels.

RESULT(S)

The alpha1 and alpha5 integrins were present in trophoblast cells from 24 hours of culture. Treatment with IgG that were obtained from the patient with antiphospholipid syndrome significantly decreased alpha1 integrin and increased alpha5 integrin at both the mRNA and protein levels. Furthermore, IgG with antiphospholipid antibodies activities induced VE-cadherin down-regulation and the E-cadherin up-regulation at protein and mRNA levels compared with control IgG or untreated cells.

CONCLUSION(S)

The results suggest that the inadequate trophoblastic invasion, induced by antiphospholipid antibodies, can be the result of abnormal trophoblast adhesion molecules expression.

An octapeptide in the juxtamembrane domain of VE-cadherin is important for p120ctn binding and cell proliferation

The cadherins are a family of adhesive proteins involved in cell-cell homophilic interactions. VE-cadherin, expressed in endothelial cells, is involved in morphogenesis, regulation of permeability, and cellular proliferation. The cytoplasmic tails of cadherins contain two major domains, the juxtamembrane domain that plays a role in the intercellular localization of the protein and also serves for binding of p120ctn, and a C-terminal domain that associates with beta- or gamma-catenin. A highly conserved region present in the juxtamembrane domain of the cadherins has been shown to be necessary for p120ctn binding in E-cadherin. Using a mutant VE-cadherin lacking a highly conserved octapeptide, we demonstrated that it is required for p120ctn binding to VE-cadherin as determined by immunoprecipitation and colocalization studies. By immunofluorescence, this mutant protein has a topographical distribution similar to that of the wild-type VE-cadherin and, therefore, we conclude that the topographical distribution of VE-cadherin is independent of this motif. In addition, although cell-cell association is present in cells expressing this mutant form of VE-cadherin, we found that the strength of adhesion is decreased. Finally, our results for the first time demonstrate that the interaction of VE-cadherin with p120 catenin plays an important role in cellular growth, suggesting that the binding of p120 catenin to cadherins may regulate cell proliferation.

Endothelial cell heterogeneity and organ specificity

Endothelial cells consist of a heterogeneous population covering the entire inner surface of blood vessels. This review will focus on the factors influencing this heterogeneity including: (1) morphological and functional differences between large and small vessels and between cells derived from various microvascular endothelial beds; (2) the microenvironment and extracellular matrix modulating the phenotype; (3) different response to growth factors; (4) organ specificity reflecting the cumulative expression of post-translation modifications and also the expression of unique genes under the control of organ-specific regulatory elements; and (5) pathological conditions, such as tumor growth, which is accompanied by the development of a characteristic tumor vasculature and tumors formed by endothelial cells.

Rate of endothelial expansion is controlled by cell:cell adhesion

Procedures used to alleviate blood vessel occlusion result in varying degrees of damage to the vascular wall and endothelial denudation. The presence of intact, functioning endothelium is thought to be important in controlling smooth muscle cell growth, and limiting the intimal thickening which results from damage to the vessel wall. Recovery of the endothelium is commonly slow and incomplete, due in part to endothelial lateral cell:cell adhesion, which limits cell migration and proliferation. We have investigated the effect of fibroblast growth factor 2 and vascular/endothelial growth factor on the relationship between the temporal distribution of the junctional adhesion proteins, platelet/endothelial cell adhesion molecule, vascular/endothelial cadherin and plakoglobin, and cellular migration and proliferation in an in vitro model of endothelial expansion. We found that whereas cell:cell junctions were initially disturbed to similar extents by single applications of the growth factors, outward cell migration and proliferation rates were inversely correlated with the speed at which cell:cell junctions were re-established. This occurred very rapidly with vascular/endothelial growth factor treatment and more slowly with fibroblast growth factor-2, resulting in more extensive outward migration and proliferation in response to the latter. Platelet/endothelial cell adhesion molecule and vascular/endothelial cadherin appeared to be associated with cell:cell junctional control of migration and proliferation, while plakoglobin did not contribute. It was concluded that the rate of endothelial expansion in response to growth factors, is limited by the rate of re-association of junctional complexes following initial disruption.

Identification of CD146 as a component of the endothelial junction involved in the control of cell-cell cohesion

CD146 is a cell-surface molecule belonging to the immunoglobulin superfamily and expressed in all types of human endothelial cells. Confocal and electron microscopic analysis of confluent human umbilical vein endothelial cells (HUVECs) were used to demonstrate that CD146 is a component of the endothelial junction. Double immunolabeling with vascular endothelial cadherin showed that CD146 is localized outside the adherens junction. Moreover, CD146 expression is not restricted to the junction, since part of the labeling was detectable at the apical side of the HUVECs. Interestingly, cell-surface expression of CD146 increased when HUVECs reached confluence. In addition, the paracellular permeability of CD146-transfected fibroblast cells was decreased compared with that of control cells. Finally, CD146 colocalized with actin, was partly resistant to Triton X-100 extraction, and had its expression altered by actin-disrupting agents, indicating that CD146 is associated with the actin cytoskeleton. These results show the regulated expression of CD146 at areas of cell-cell junction and strongly suggest involvement of CD146 as a mediator of cell-cell interaction.

Albumin-derived advanced glycation end-products trigger the disruption of the vascular endothelial cadherin complex in cultured human and murine endothelial cells

Endothelial cell (EC) junctions regulate in large part the integrity and barrier function of the vascular endothelium. Advanced glycation end-products (AGEs), the irreversibly formed reactive derivatives of non-enzymic glucose-protein condensation reactions, are strongly implicated in endothelial dysfunction that distinguishes diabetes- and aging-associated vascular complications. The aim of the present study was to determine whether AGEs affect EC lateral junction proteins, with particular regard to the vascular endothelial cadherin (VE-cadherin) complex. Our results indicate that AGE-modified BSA (AGE-BSA), a prototype of advanced glycated proteins, disrupts the VE-cadherin complex when administered to ECs. AGE-BSA, but not unmodified BSA, was found to induce decreases in the levels of VE-cadherin, beta-catenin and gamma-catenin in the complex and in total cell extracts, as well as a marked reduction in the amount of VE-cadherin present at the cell surface. In contrast, the level of platelet endothelial cell adhesion molecule-1 (PECAM-1), which is located at lateral junctions, was not altered. Supplementation of the cellular antioxidative defences abolished these effects. Finally, the loss of components of the VE-cadherin complex was correlated with increases in vascular permeability and in EC migration. These findings suggest that some of the AGE-induced biological effects on the endothelium could be mediated, at least in part, by the weakening of intercellular contacts caused by decreases in the amount of VE-cadherin present.