Inhibition of cultured cell growth by vascular endothelial cadherin (cadherin-5/VE-cadherin)

VE-cadherin: the major endothelial adhesion molecule controlling cellular junctions and blood vessel formation

Vascular endothelial (VE)-cadherin is a strictly endothelial specific adhesion molecule located at junctions between endothelial cells. In analogy of the role of E-cadherin as major determinant for epithelial cell contact integrity, VE-cadherin is of vital importance for the maintenance and control of endothelial cell contacts. Mechanisms that regulate VE-cadherin-mediated adhesion are important for the control of vascular permeability and leukocyte extravasation. In addition to its adhesive functions, VE-cadherin regulates various cellular processes such as cell proliferation and apoptosis and modulates vascular endothelial growth factor receptor functions. Consequently, VE-cadherin is essential during embryonic angiogenesis. This review will focus on recent new developments in understanding the role of VE-cadherin in controlling endothelial cell contacts and influencing endothelial cell behavior by various outside-in signaling processes.

Conversion of cultured monocytes/macrophages into endothelial-like cells through direct contact with endothelial cells

When culturing human umbilical vein endothelial cells in a culture medium containing 4% human serum albumin, it was possible to maintain the epithelioid morphology and function for several months without subculturing. When coculturing endothelial cells and labeled monocytes/macrophages (Mo/Phi) that were collected from peripheral blood and allowed to engulf fluorescent latex beads, some Mo/Phi changed their shapes and became epithelioid cells that were indistinguishable from vascular endothelial cells. This transformation started within several hours of coculturing. At 7 days after the start of coculturing, more than half of the labeled cells were identified as endothelial-like cells morphologically. Furthermore, morphologically altered Mo/Phi did not express Mo/Phi-specific antigens, ie, the MHC Class II molecule and CD68, but expressed VE cadherin and vWF, which are specific antigens for endothelial cells, and labeled cells that changed into endothelial-like cells no longer engulfed fluorescent latex beads. This strongly suggests that peripheral blood monocytes differentiate into endothelial-like cells.

Cellular and multicellular form and function

Engineering artificial tissue constructs requires the appropriate spatial arrangement of cells within scaffolds. The introduction of microengineering tools to the biological community has provided a valuable set of techniques to manipulate the cellular environment, and to examine how cell structure affects cellular function. Using micropatterning techniques, investigators have found that the geometric presentation of cell-matrix adhesions are important regulators of various cell behaviors including cell growth, proliferation, differentiation, polarity and migration. Furthermore, the presence of neighboring cells in multicellular aggregates has a significant impact on the proliferative and differentiated state of cells. Using microengineering tools, it will now be possible to manipulate the various environmental factors for practical applications such as engineering tissue constructs with greater control over the physical structure and spatial arrangement of cells within their surrounding microenvironment.

Endothelial adherens and tight junctions in vascular homeostasis, inflammation and angiogenesis

Endothelial cells lining the vessel wall are connected by adherens, tight and gap junctions. These junctional complexes are related to those found at epithelial junctions but with notable changes in terms of specific molecules and organization. Endothelial junctional proteins play important roles in tissue integrity but also in vascular permeability, leukocyte extravasation and angiogenesis. In this review, we will focus on specific mechanisms of endothelial tight and adherens junctions.

HIF-2alpha specifically activates the VE-cadherin promoter independently of hypoxia and in synergy with Ets-1 through two essential ETS-binding sites

The mechanisms that are responsible for the restricted pattern of expression of the VE-cadherin gene in endothelial cells are not clearly understood. Regulation of expression is under the control of an approximately 140 bp proximal promoter that provides basal, non-endothelial specific expression. A larger region contained within the 2.5 kb genomic DNA sequence located ahead of the transcription start is involved in the specific expression of the gene in endothelial cells. We show here that the VE-cadherin promoter contains several putative hypoxia response elements (HRE) which are able to bind endothelial nuclear factors under normoxia. The VE-cadherin gene is not responsive to hypoxia but hypoxia-inducible factor (HIF)-2alpha specifically activates the promoter while HIF-1alpha does not. The HRE, that are involved in this activity have been identified. Further, we show that HIF-2alpha cooperates with the Ets-1 transcription factor for activation of the VE-cadherin promoter and that this synergy is dependent on the binding of Ets-1 to DNA. This cooperative action of HIF-2alpha with Ets-1 most probably participates to the transcriptional regulation of expression of the gene in endothelial cells. This mechanism may also be involved in the expression of the VE-cadherin gene by tumor cells in the process of vascular mimicry.

Plasma membrane assays and three-compartment image cytometry for high content screening

High throughput image cytometers analyze individual cells in digital photomicrographs by first assigning pixels within each image to plasma membrane, cytoplasm, nucleus, or other regions. In this study, we report on a novel algorithm that: 1) identifies plasma membrane regions to measure changes in plasma membrane-associated proteins (protein kinase C [PKC] alpha, N-cadherin, E-cadherin, vascular endothelium [VE]-cadherin, and pan-cadherin) that regulate cell division, migration, and adhesion and 2) delineates the cell for generalized three-compartment image cytometry. Validation assays were performed for these proteins on cells cultured in 96-well plates and also for tissue sections obtained from transgenic and chemical carcinogenic models of skin cancer. The algorithm successfully quantified phorbol 12-myristate 13-acetate (PMA)-induced plasma membrane localization of PKCalpha in HeLa cells (Z' of 0.88). Additionally, PMA activated translocation to the plasma membrane at P < .01 of N-cadherin (in HeLa cells), E-cadherin (in A431 cells), and VE-cadherin (in human dermal microvascular endothelial cells), suggesting a relationship between PKCalpha activity and cadherin localization. For VE-cadherin, a Z' of 0.52 was obtained between serum-free medium, which increased VE-cadherin, and EGTA, which diminished VE-cadherin at the plasma membrane. For sections obtained from the transgenic skin cancer model, analysis of images with the plasma membrane algorithm revealed that tumor cells exhibited cadherin expression that was just 34% of that expressed by surrounding normal tissue; furthermore, tumor cells expressed elevated DNA content, consistent with development of aneuploidy. In contrast, increased DNA content did not occur for tumor cells produced by chemical carcinogenesis. The results demonstrate that this new algorithm for plasma membrane image cytometry enables statistically significant analyses in a variety of applications in both cultured cells and tissue sections.

E-cadherin homophilic ligation inhibits cell growth and epidermal growth factor receptor signaling independently of other cell interactions

E-cadherin function leads to the density-dependent contact inhibition of cell growth. Because cadherins control the overall state of cell contact, cytoskeletal organization, and the establishment of many other kinds of cell interactions, it remains unknown whether E-cadherin directly transduces growth inhibitory signals. To address this question, we have selectively formed E-cadherin homophilic bonds at the cell surface of isolated epithelial cells by using functionally active recombinant E-cadherin protein attached to microspheres. We find that E-cadherin ligation alone reduces the frequency of cells entering the S phase, demonstrating that E-cadherin ligation directly transduces growth inhibitory signals. E-cadherin binding to beta-catenin is required for cell growth inhibition, but beta-catenin/T-cell factor transcriptional activity is not involved in growth inhibition resulting from homophilic binding. Neither E-cadherin binding to p120-catenin nor beta-catenin binding to alpha-catenin, and thereby the actin cytoskeleton, is required for growth inhibition. E-cadherin ligation also inhibits epidermal growth factor (EGF) receptor-mediated growth signaling by a beta-catenin-dependent mechanism. It does not affect EGF receptor autophosphorylation or activation of ERK, but it inhibits transphosphorylation of Tyr845 and activation of signal transducers and activators of transcription 5. Thus, E-cadherin homophilic binding independent of other cell contacts directly transduces growth inhibition by a beta-catenin-dependent mechanism that inhibits selective signaling functions of growth factor receptors.

Vascular endothelial cadherin-mediated cell-cell adhesion regulated by a small GTPase, Rap1

Vascular endothelial cadherin (VE-cadherin), which belongs to the classical cadherin family, is localized at adherens junctions exclusively in vascular endothelial cells. Biochemical and biomechanical cues regulate the VE-cadherin adhesive potential by triggering the intracellular signals. VE-cadherin-mediated cell adhesion is required for cell survival and endothelial cell deadhesion is required for vascular development. It is therefore crucial to understand how VE-cadherin-based cell adhesion is controlled. This review summarizes the inter-endothelial cell adhesions and introduces our recent advance in Rap1-regulated VE-cadherin adhesion. A further analysis of the VE-cadherin recycling system will aid the understanding of cell adhesion/deadhesion mechanisms mediated by VE-cadherin in response to extracellular stimuli during development and angiogenesis.

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.

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.

Cdc42 regulates adherens junction stability and endothelial permeability by inducing alpha-catenin interaction with the vascular endothelial cadherin complex

The endothelial adherens junctions (AJs) consist of trans-oligomers of membrane spanning vascular endothelial (VE)-cadherin proteins, which bind beta-catenin through their cytoplasmic domain. beta-Catenin in turn binds alpha-catenin and connects the AJ complex with the actin cytoskeleton. We addressed the in vivo effects of loss of VE-cadherin interactions on lung vascular endothelial permeability and the role of specific Rho GTPase effectors in regulating the increase in permeability induced by AJ destabilization. We used cationic liposomes encapsulating the mutant of VE-cadherin lacking the extracellular domain (DeltaEXD) to interfere with AJ assembly in mouse lung endothelial cells. We observed that lung vascular permeability (quantified as microvessel filtration coefficient [K(f,c)]) was increased 5-fold in lungs expressing DeltaEXD. This did not occur to the same degree on expression of the VE-cadherin mutant, DeltaEXDDeltabeta, lacking the beta-catenin-binding site. The increased vascular permeability was the result of destabilization of VE-cadherin homotypic interaction induced by a shift in the binding of beta-catenin from wild-type VE-cadherin to the expressed DeltaEXD mutant. Because DeltaEXD expression in endothelial cells activated the Rho GTPase Cdc42, we addressed its role in the mechanism of increased endothelial permeability induced by AJ destabilization. Coexpression of dominant-negative Cdc42 (N17Cdc42) prevented the increase in K(f,c) induced by DeltaEXD. This was attributed to inhibition of the association of alpha-catenin with the DeltaEXD-beta-catenin complex. The results demonstrate that Cdc42 regulates AJ permeability by controlling the binding of alpha-catenin with beta-catenin and the consequent interaction of the VE-cadherin/catenin complex with 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.

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.

Association of Csk to VE-cadherin and inhibition of cell proliferation

Vascular endothelial cadherin (VE-cadherin) mediates contact inhibition of cell growth in quiescent endothelial cell layers. Searching for proteins that could be involved in VE-cadherin signaling, we found the cytosolic C-terminal Src kinase (Csk), a negative regulator of Src family kinases. We show that Csk binds via its SH2 domain to the phosphorylated tyrosine 685 of VE-cadherin. VE-cadherin recruits Csk to cell contacts and both proteins can be co-precipitated from cell lysates of transfected cells and endothelial cells. Association of VE-cadherin and Csk in endothelial cells increased with increasing cell density. CHO cells expressing the tyrosine replacement mutant VE-cadherin-Y685F grow to higher cell densities than cells expressing wild-type VE-cadherin. Overexpression of Csk in these cells under an inducible promoter inhibits cell proliferation in the presence and absence of VE-cadherin, but not in the presence of VE-cadherin-Y685F. Reduction of Csk expression by RNA interference enhances endothelial cell proliferation. Our results suggest that the phosphorylated tyrosine residue 685 of VE-cadherin and probably the binding of Csk to this site are involved in inhibition of cell growth triggered by cell density.

Reduced E-cadherin expression contributes to the loss of p27kip1-mediated mechanism of contact inhibition in thyroid anaplastic carcinomas

In the present study, we have characterized several human thyroid cancer cell lines of different histotypes for their responsiveness to contact inhibition. We found that cells derived from differentiated carcinoma (TPC-1, WRO) arrest in G(1) phase at confluence, whereas cells derived from anaplastic carcinoma (ARO, FRO and FB1) continue to grow after reaching confluence. Furthermore, we provide experimental evidence that the axis, E-cadherin/beta-catenin/p27(Kip1), represents an integral part of the regulatory mechanism that controls proliferation at a high cell density, whose disruption may play a key role in determining the clinical behaviour of thyroid cancer. This conclusion derives from the finding that: (i) the expression of p27(Kip1) is enhanced at high cell density only in cells responsive to contact inhibition (TPC-1, WRO), but not in contact-inhibition resistant cells (ARO, FRO or FB1 cells); (ii) the increase in p27(Kip1) also resulted in increased levels of p27(Kip1) bound to cyclin E-Cdk2 complex, a reduction in cyclin E-Cdk2 activity and dephosphorylation of the retinoblastoma protein; (iii) antisense inhibition of p27(Kip1) upregulation at high cell density in confluent-sensitive cells completely prevents the confluence-induced growth arrest; (iv) proper expression and/or membrane localization of E-cadherin is observed only in cells responsive to contact inhibition (TPC-1, NPA, WRO) but not in unresponsive cells (ARO, FRO or FB1); (v) disruption of E-cadherin-mediated cell-cell contacts at high cell density induced by an anti-E-cadherin neutralizing antibody, inhibits the induction of p27(kip1) and restores proliferation in contact-inhibited cells; (vi) re-expression of E-cadherin into cells unresponsive to contact inhibition (ARO, FB1) induces a p27(kip1) expression and growth arrest. In summary, our data indicate that the altered response to contact inhibition exhibited by thyroid anaplastic cancer cells is due to the failure to upregulate p27(Kip1) in response to cell-cell interactions.

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.

Induction of VE-cadherin in rat placental trophoblasts by VEGF through a NO-dependent pathway

Vascular endothelial-cadherin (VE-cadherin), a calcium-dependent homotypic adhesion molecule, contributes to endothelial assembly and VEGF-mediated survival during angiogenesis. In human term placentas, villous vessels and extravillous cytotrophoblasts express VE-cadherin. Therefore, the purpose of this study was to examine if VEGF modulated placental development by increasing the expression of VE-cadherin in rat placentas. Placental tissues from rats on gestation days 14 (G14), 18 (G18) and 21 (G21) were used. Western blot analysis and immunohistochemistry were performed to detect the protein abundance and the distribution of VE-cadherin. A nitric oxide analyzer was used to measure the released nitric oxide (NO) from placental explant culture. With the progression of pregnancy, the abundance of VE-cadherin and the intensity of the immunoreactive staining for VE-cadherin in endovascular trophoblasts and labyrinth trophoblasts were decreased. In explant culture, VEGF (0.01-1.0 ng/ml) increased the protein abundance of VE-cadherin. SNP (an NO donor) or L-arginine (substrate for eNOS) induced the expression of VE-cadherin with the increase of NO production. L-NAME (a NOS inhibitor) reduced the VEGF-increased expression and L-arginine reversed the inhibitory effect of L-NAME. In conclusion, VEGF plays an important role in placental development by the induction of VE-cadherin in trophoblasts, which, in part, maintains the survival of labyrinth trophoblast in rat placentas.

Mechanotransduction at cell-matrix and cell-cell contacts

Mechanical forces play an important role in the organization, growth, maturation, and function of living tissues. At the cellular level, many of the biological responses to external forces originate at two types of specialized microscale structures: focal adhesions that link cells to their surrounding extracellular matrix and adherens junctions that link adjacent cells. Transmission of forces from outside the cell through cell-matrix and cell-cell contacts appears to control the maturation or disassembly of these adhesions and initiates intracellular signaling cascades that ultimately alter many cellular behaviors. In response to externally applied forces, cells actively rearrange the organization and contractile activity of the cytoskeleton and redistribute their intracellular forces. Recent studies suggest that the localized concentration of these cytoskeletal tensions at adhesions is also a major mediator of mechanical signaling. This review summarizes the role of mechanical forces in the formation, stabilization, and dissociation of focal adhesions and adherens junctions and outlines how integration of signals from these adhesions over the entire cell body affects how a cell responds to its mechanical environment. This review also describes advanced optical, lithographic, and computational techniques for the study of mechanotransduction.

The pathophysiologic role of VEGF in hematologic malignancies: therapeutic implications

Besides its role as an essential regulator of physiologic and pathologic angiogenesis, vascular endothelial growth factor (VEGF) triggers growth, survival, and migration of leukemia and multiple myeloma cells; plays a pivotal role in hematopoiesis; inhibits maturation of dendritic cells; and increases osteoclastic bone-resorbing activity as well as osteoclast chemotaxis. Dysregulation of VEGF expression and signaling pathways therefore plays an important role in the pathogenesis and clinical features of hematologic malignancies, in particular multiple myeloma. Direct and indirect targeting of VEGF and its receptors therefore may provide a potent novel therapeutic approach to overcome resistance to therapies and thereby improve patient outcome.

VE-cadherin expression and clustering maintain low levels of survivin in endothelial cells

Survivin is strongly expressed in embryonic organs and in tumor cells but is low or absent in differentiated normal tissues. Resting endothelium expresses low levels of survivin but can up-regulate its synthesis on activation to proliferate. The mechanisms responsible for survivin down-regulation in resting conditions are still unknown. We report here that confluence and vascular endothelial-cadherin (VE-cadherin) expression induce contact inhibition of cell growth and survivin down-regulation in the endothelium. Using beta-catenin null and positive isogenic endothelial cell lines we found that the effect requires beta-catenin expression and its association to VE-cadherin cytoplasmic tail. Furthermore, in allantois organ cultures, survivin expression is up-regulated in areas of growing vessels where VE-cadherin is partially dismantled from junctions or in VE-cadherin -/- specimens. Overall, these data indicate that VE-cadherin and beta-catenin may negatively regulate survivin synthesis in endothelial cells. Consistently, in epidermal and pancreatic cell lines or ovarian tumors, epithelial-cadherin (E-cadherin) and survivin expression is inversely related, suggesting a non-cell-specific role of cadherins in reducing survivin synthesis.

Endothelial cell-to-cell junctions: molecular organization and role in vascular homeostasis

Intercellular junctions mediate adhesion and communication between adjoining endothelial and epithelial cells. In the endothelium, junctional complexes comprise tight junctions, adherens junctions, and gap junctions. The expression and organization of these complexes depend on the type of vessels and the permeability requirements of perfused organs. Gap junctions are communication structures, which allow the passage of small molecular weight solutes between neighboring cells. Tight junctions serve the major functional purpose of providing a "barrier" and a "fence" within the membrane, by regulating paracellular permeability and maintaining cell polarity. Adherens junctions play an important role in contact inhibition of endothelial cell growth, paracellular permeability to circulating leukocytes and solutes. In addition, they are required for a correct organization of new vessels in angiogenesis. Extensive research in the past decade has identified several molecular components of the tight and adherens junctions, including integral membrane and intracellular proteins. These proteins interact both among themselves and with other molecules. Here, we review the individual molecules of junctions and their complex network of interactions. We also emphasize how the molecular architectures and interactions may represent a mechanistic basis for the function and regulation of junctions, focusing on junction assembly and permeability regulation. Finally, we analyze in vivo studies and highlight information that specifically relates to the role of junctions in vascular endothelial cells.

Transference of recombinant VE-cadherin cytoplasmic domain alters endothelial junctional integrity and porcine microvascular permeability

VE-cadherin constitutes endothelial adherens junctions through a homophilic binding of its extracellular domain and by the anchoring of its intracellular domain to actin cytoskeleton via catenins. The aim of this study was to determine the functional importance of VE-cadherin-cytoskeleton association in the maintenance of endothelial junctional integrity. A recombinant VE-cadherin cytoplasmic domain (rVE-cad CPD) was expressed in E. coli and purified through Ni-NTA spin columns. Immunoprecipitation assays showed that rVE-cad CPD was able to bind beta-catenin in vitro and to compete with endogenous VE-cadherin for binding of beta-catenin in human umbilical vein endothelial cells. A significant increase in the transendothelial flux of albumin was observed in the endothelial cell monolayers transfected with rVE-cad CPD. Importantly, transfection of rVE-cad CPD into intact isolated coronary venules markedly elevated the albumin permeability of the venular endothelium. In addition, immunofluorescence microscopic analysis revealed a conformational change of VE-cadherin from a uniform, continuous distribution along the cell membrane under control conditions to a diffuse, stitch-like pattern after rVE-cad CPD transfection. The effects were likely due to an attenuated anchorage of endogenous VE-cadherin to the cytoskeleton, as evidenced by a decreased partitioning of VE-cadherin in the detergent-insoluble cytoskeletal pool. The results suggest that the intracellular association of VE-cadherin with beta-catenin-linked cytoskeleton is essential to the maintenance of endothelial junctional integrity and microvascular permeability.

Activated Ras induces a proangiogenic phenotype in primary endothelial cells

Angiogenic factors alter endothelial cell phenotype to promote the formation of new blood vessels, a process critical for a number of normal and pathological conditions. Ras is required for the induction of the angiogenic phenotype in response to vascular endothelial growth factor (VEGF). However, VEGF generates many signals, several of which are not dependent upon Ras activation. Our current study investigates the sufficiency of Ras activation for driving angiogenic responses. An activated Ras(V12) mutant induces prominent membrane ruffling, branching morphogenesis on three-dimensional collagen, DNA synthesis, and cell migration in primary endothelial cells. An upregulation of PI3K/AKT, Erk, and Jnk signaling pathways accompany these phenotypic changes. The inhibition of Erk blocked cell proliferation, but only partially attenuated migration. Blocking PI3K had no effect on DNA synthesis, but caused a modest reduction in cell migration. Lastly, Jnk played a significant role in both the proliferation and migration response. These effects of Ras(V12) are not the result of increased autocrine secretion of VEGF. These data suggest that the acquisition of activating Ras mutations can lead to a proangiogenic conversion in the phenotype of primary endothelial cells. Furthermore, these data raise the possibility that chronic Ras activation in endothelial cells may be sufficient to promote angiogenesis and the development of vascular anomalies.

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.

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.

Dismantling of cadherin-mediated cell-cell contacts modulates smooth muscle cell proliferation

Proliferation of vascular smooth muscle cells (VSMCs) contributes to intimal thickening during atherosclerosis and restenosis. The cadherins are transmembrane proteins, which form cell-cell contacts and may regulate VSMC proliferation. In this study, N-cadherin protein concentration was significantly reduced by stimulation of proliferation with fetal calf serum (FCS) and platelet-derived growth factor-BB (PDGF-BB) in human saphenous vein VSMCs. Furthermore, overexpression of a truncated N-cadherin, which acts as a dominant-negative increased VSMC proliferation. The amount of an extracellular fragment of N-cadherin (approximately 90 kDa) in the media after 24 hours was increased by 12-fold by FCS and 11-fold by PDGF-BB, suggesting that N-cadherin levels are regulated by proteolytic shedding. Incubation with a synthetic metalloproteinase inhibitor or adenoviral overexpression of the endogenous tissue inhibitors of metalloproteinases (TIMPs) demonstrated that metalloproteinase activity was responsible in part for this proteolysis. Although total levels of beta-catenin protein were not affected, beta-catenin was translocated to the nucleus after stimulation with FCS and PDGF-BB. Our data indicates cadherin-mediated cell-cell contacts modulate proliferation in VSMCs. Furthermore, disruption of N-cadherin cell-cell contacts mediated in part by metalloproteinase activity occurs during VSMC proliferation, releasing beta-catenin and possibly inducing beta-catenin-mediated intracellular signaling.

Lack of correlation between Tc-99m-sestaMIBI uptake and cadherin expression in infiltrating ductal breast carcinoma as prognostic indicators

Despite using various kinds of prognostic indicators, it is still not possible to predict the biological behavior of breast cancer in all patients. Tc-99m-sestaMIBI (MIBI) uptake determined by breast scintigraphy and cadherin expression of tumor tissue revealed by immunohistochemistry are suggested as potential agents for this purpose. We hypothesize that there can be a correlation between MIBI whose cellular mitochondrial content is claimed to play a significant role in its tumor uptake and cadherin whose downregulation causes an increase in mitochondrial activity in human mammary carcinoma cell lines. The aim of this study was to assess the relationship between the degree of MIBI tumor uptake and cadherin expression in infiltrating ductal breast carcinoma. Correlation with response to chemotherapy and some known prognostic factors of breast cancer such as tumor size, number of metastatic axillary lymph nodes and microscopic grading was also done. Fourteen patients who underwent scintimammography and subsequent surgical excisional biopsy that revealed infiltrating ductal carcinoma were enrolled in this study. Statistical analysis did not show any correlation between MIBI uptake and cadherin expression (p > 0.05). Also, no statistically significant correlation was noted between MIBI uptake and tumor size, number of metastatic lymph nodes, microscopic grade, stage of the disease or response to chemotherapy. Similarly, there was no statistically significant correlation between cadherin expression and tumor size, number of metastatic lymph nodes, microscopic grade, stage of the disease or chemotherapy response. The results of this study imply that there is no correlation between MIBI tumor uptake and cadherin expression with neither of them good enough to be used as prognostic indicators for breast cancer.

Contact inhibition of VEGF-induced proliferation requires vascular endothelial cadherin, beta-catenin, and the phosphatase DEP-1/CD148

Confluent endothelial cells respond poorly to the proliferative signals of VEGF. Comparing isogenic endothelial cells differing for vascular endothelial cadherin (VE-cadherin) expression only, we found that the presence of this protein attenuates VEGF-induced VEGF receptor (VEGFR) 2 phosphorylation in tyrosine, p44/p42 MAP kinase phosphorylation, and cell proliferation. VE-cadherin truncated in beta-catenin but not p120 binding domain is unable to associate with VEGFR-2 and to induce its inactivation. beta-Catenin-null endothelial cells are not contact inhibited by VE-cadherin and are still responsive to VEGF, indicating that this protein is required to restrain growth factor signaling. A dominant-negative mutant of high cell density-enhanced PTP 1 (DEP-1)//CD148 as well as reduction of its expression by RNA interference partially restore VEGFR-2 phosphorylation and MAP kinase activation. Overall the data indicate that VE-cadherin-beta-catenin complex participates in contact inhibition of VEGF signaling. Upon stimulation with VEGF, VEGFR-2 associates with the complex and concentrates at cell-cell contacts, where it may be inactivated by junctional phosphatases such as DEP-1. In sparse cells or in VE-cadherin-null cells, this phenomenon cannot occur and the receptor is fully activated by the growth factor.

Viable Saccharomyces cerevisiae cells at high concentrations cause early growth arrest of non-Saccharomyces yeasts in mixed cultures by a cell-cell contact-mediated mechanism

The growth of Kluyveromyces thermotolerans and Torulaspora delbrueckii was examined in mixed cultures with Saccharomyces cerevisiae in YPD modified for wine fermentations. Although the three yeasts had similar maximum specific growth rates in these fermentations, K. thermotolerans and T. delbrueckii arrested growth earlier than S. cerevisiae, thereby obtaining lower stationary phase cell concentrations than S. cerevisiae. Various single and mixed culture fermentations with the three yeasts were carried out in order to find an explanation for this phenomenon. The early growth arrests of K. thermotolerans and T. delbrueckii were absent in single cultures of the two yeasts, and they seemed to be due neither to nutrient limitations nor to the presence of growth-inhibitory compounds. Rather, they seemed to be due to a cell-cell contact mechanism dependent on the presence of viable S. cerevisiae cells at high concentrations. These results contribute to an increased understanding of why K. thermotolerans and T. delbrueckii arrest growth before S. cerevisiae during wine fermentations.

Effects of cadmium on E-cadherin and VE-cadherin in mouse lung

Exposure to Cd(2+) via inhalation or intratracheal instillation results in pulmonary edema, which is followed by the influx of leukocytes, the proliferation of type II pneumocytes and eventual scarring and fibrotic changes. While the general toxic effects of Cd(2+) in the lung have been well characterized, the specific molecular mechanisms underlying these effects have yet to be elucidated. Previously we have shown that Cd(2+) can disrupt the adhering junctions between various types of epithelial and endothelial cells in culture, most likely by perturbing the function of the Ca(2+) dependent cell adhesion molecules E-cadherin and VE-cadherin respectively. The objectives of this study were to determine whether respiratory exposure to Cd(2+) can alter the localization of E-cadherin and VE-cadherin in the lung, and to determine whether this effect may play a role in the acute pneumotoxic response to Cd(2+). Male CF-1 mice were exposed to CdCl(2) (0, 16.25, 32.5, 65 or 130 nmoles in 50 microl saline) via intratracheal instillation. After 24 hours, the lungs were removed and either subjected to bronchoalveolar lavage or analyzed for histopathologic changes. The results showed that Cd(2+) caused an increase in lung weight and in the protein content of the lavage fluid. These effects were accompanied by a pronounced decrease in the amount of E-cadherin in epithelial cells of the alveoli and small bronchioles and of VE-cadherin in vascular endothelial cells. Assessment of cell membrane integrity with ethidium homodimer-1 showed no evidence of severe injury or death in alveolar epithelial cells. These findings suggest that E-cadherin and VE-cadherin may be important early targets of Cd(2+) toxicity in the lung.

Transmission of human cytomegalovirus from infected uterine microvascular endothelial cells to differentiating/invasive placental cytotrophoblasts

Analysis of placentas infected with human cytomegalovirus (CMV) suggested that viral transmission could involve differentiating/invasive cytotrophoblasts in villi that attach the placenta to the uterine wall. To parse the cellular components in this process, we developed a coculture system of polarized uterine microvascular endothelial cell (UtMVEC) infection with an endothelial cell-tropic pathogenic strain of CMV. Then we evaluated the potential role of neutrophils and endothelial cells in the spread of infection to differentiating cytotrophoblasts. As shown by immunocytochemistry and analysis of viral replication, CMV preferentially infected endothelial cells via apical membranes and disrupted cell junction proteins, thereby altering paracellular permeability and cell polarity. Neutralizing antibodies to CMV glycoprotein B, an envelope component that facilitates virion penetration, blocked plaque formation in polarized UtMVEC. Neutrophils transmitted CMV infection to UtMVEC, which in turn infected cytotrophoblasts. However, neutrophils did not directly infect cytotrophoblasts. These findings implicate endothelial cells from the uterine microvasculature as a potential source for CMV infection of endovascular cytotrophoblasts of the anchoring villi. Possibly the cytokine/chemokine milieu in the pregnant uterus could attract immune cells that infect endothelial cells in hybrid fetal-maternal vessels. In turn, these cells could infect endovascular cytotrophoblasts, one possible initiation point of a cascade that results in retrograde placental CMV infection.

Molecular profiling of angiogenesis markers

The goal of this study was to develop a sensitive, simple, and widely applicable assay to measure copy numbers of specific mRNAs using real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), and identify a profile of gene expression closely associated with angiogenesis. We measured a panel of nine potential angiogenesis markers from a mouse transgenic model of prostate adenocarcinoma (TRAMP) and a mouse skin model of vascular endothelial growth factor (VEGF)-driven angiogenesis. In both models, expression of VEGF correlated with expression of mRNAs encoding other angiogenic cytokines (angiopoietin-1 and angiopoietin-2), endothelial cell receptor tyrosine kinases (Flt-1, KDR, Tie-1), and endothelial cell adhesion molecules (VE-cadherin, PECAM-1). Relative to control, in dermis highly stimulated by VEGF, the Ang-2 mRNA transcript numbers increased 35-fold, PECAM-1 and VE-cadherin increased 10-fold, Tie-1 increased 8-fold, KDR and Flt-1 each increased 4-fold, and Ang-1 increased 2-fold. All transcript numbers were correspondingly reduced in skin with less VEGF expression, indicating a relationship of each of these seven markers with VEGF. Thus, this study identifies a highly efficient method for precise quantification of a panel of seven specific mRNAs that correlate with VEGF expression and VEGF-induced neovascularization, and it provides evidence that real-time quantitative RT-PCR offers a highly sensitive strategy for monitoring angiogenesis.

Mechanisms of normal and tumor-derived angiogenesis

Often those diseases most evasive to therapeutic intervention usurp the human body's own cellular machinery or deregulate normal physiological processes for propagation. Tumor-induced angiogenesis is a pathological condition that results from aberrant deployment of normal angiogenesis, an essential process in which the vascular tree is remodeled by the growth of new capillaries from preexisting vessels. Normal angiogenesis ensures that developing or healing tissues receive an adequate supply of nutrients. Within the confines of a tumor, the availability of nutrients is limited by competition among actively proliferating cells, and diffusion of metabolites is impeded by high interstitial pressure (Jain RK. Cancer Res 47: 3039-3051, 1987). As a result, tumor cells induce the formation of a new blood supply from the preexisting vasculature, and this affords tumor cells the ability to survive and propagate in a hostile environment. Because both normal and tumor-induced neovascularization fulfill the essential role of satisfying the metabolic demands of a tissue, the mechanisms by which cancer cells stimulate pathological neovascularization mimic those utilized by normal cells to foster physiological angiogenesis. This review investigates mechanisms of tumor-induced angiogenesis. The strategies used by cancer cells to develop their own blood supply are discussed in relation to those employed by normal cells during physiological angiogenesis. With an understanding of blood vessel growth in both normal and abnormal settings, we are better suited to design effective therapeutics for cancer.

Vascular endothelial growth factor induces SHC association with vascular endothelial cadherin: a potential feedback mechanism to control vascular endothelial growth factor receptor-2 signaling

Vascular endothelial (VE)-cadherin is endothelium specific, mediates homophilic adhesion, and is clustered at intercellular junctions. VE-cadherin is required for normal development of the vasculature in the embryo and for angiogenesis in the adult. Here, we report that VE-cadherin is associated with VE growth factor (VEGF) receptor-2 (VEGFR-2) on the exposure of endothelial cells to VEGF. The binding parallels receptor phosphorylation on tyrosine residues, which is maximal at 5 minutes and then declines within 30 minutes. Tyrosine phosphorylation of VE-cadherin was maximal at 30 minutes after the addition of the growth factor. At this time point, the protein could be coimmunoprecipitated with the adaptor protein Shc. Pull-down experiments with different Shc domains and mutants of the VE-cadherin cytoplasmic tail have shown that Shc binds to the carboxy-terminal domain of the VE-cadherin tail through its Src homology 2 domain (SH2). We found that Shc phosphorylation lasts longer in endothelial cells carrying a targeted null mutation in the VE-cadherin gene than in VE-cadherin-positive cells. These data suggest that VE-cadherin expression exerts a negative effect on Shc phosphorylation by VEGFR-2. We speculate that VE-cadherin binding to Shc promotes its dephosphorylation through associated phosphatases.

Cell-cell signaling by direct contact increases cell proliferation via a PI3K-dependent signal

We report a novel mechanism of cellular growth control. Increasing the density of endothelial or smooth muscle cells in culture increased cell-cell contact and decreased cell spreading, leading to growth arrest. Using a new method to independently control cell-cell contact and cell spreading, we found that introducing cell-cell contact positively regulates proliferation, but that contact-mediated proliferation can be masked by changes in cell spreading: Round cells with many contacts proliferated less than spread cells with none. Physically blocking cell-cell contact or inhibiting PI3K signaling abrogated cell-cell induced proliferation, but inhibiting diffusible paracrine signaling did not. Thus, direct cell-cell contact induces proliferation in these cells.

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.

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.

VEGF receptor signal transduction

The family of vascular endothelial growth factors (VEGFs) currently includes VEGF-A, -B, -C, -D, -E, and placenta growth factor (PlGF). Several of these factors, notably VEGF-A, exist as different isoforms, which appear to have unique biological functions. The VEGF family proteins bind in a distinct pattern to three structurally related receptor tyrosine kinases, denoted VEGF receptor-1, -2, and -3. Neuropilins, heparan-sulfated proteoglycans, cadherins, and integrin alphavbeta3 serve as coreceptors for certain but not all VEGF proteins. Moreover, the angiogenic response to VEGF varies between different organs and is dependent on the genetic background of the animal. Inactivation of the genes for VEGF-A and VEGF receptor-2 leads to embryonal death due to the lack of endothelial cells. Inactivation of the gene encoding VEGF receptor-1 leads to an increased number of endothelial cells, which obstruct the vessel lumen. Inactivation of VEGF receptor-3 leads to abnormally organized vessels and cardiac failure. Although VEGF receptor-3 normally is expressed only on lymphatic endothelial cells, it is up-regulated on vascular as well as nonvascular tumors and appears to be involved in the regulation of angiogenesis. A large body of data, such as those on gene inactivation, indicate that VEGF receptor-1 exerts a negative regulatory effect on VEGF receptor-2, at least during embryogenesis. Recent data imply a positive regulatory role for VEGF receptor-1 in pathological angiogenesis. The VEGF proteins are in general poor mitogens, but binding of VEGF-A to VEGF receptor-2 leads to survival, migration, and differentiation of endothelial cells and mediation of vascular permeability. This review outlines the current knowledge about the signal transduction properties of VEGF receptors, with focus on VEGF receptor-2.

Transcriptional up-regulation of p27(Kip1) during contact-induced growth arrest in vascular endothelial cells

By plating porcine aortic endothelial cells at two different densities and thereby inducing two different time courses of contact-induced growth arrest, the temporal correlation between p27(Kip1) expression and cell cycle progression was investigated. When the quiescent cells were replated, they synchronously entered S phase with a peak at 20 h in both cases, while the cells plated at 25 and 80% of confluent densities exited the cell cycle by 96 and 48 h, respectively. Nuclear p27(Kip1) disappeared when the cells reentered the cell cycle and then recovered when the cells exited the cell cycle. The change in p27(Kip1) was associated with a concomitant change in Kip1 mRNA. The p27(Kip1) degradation activity did not increase in the cells reentering the cell cycle, nor did it decrease in the cells exiting the cell cycle. The Kip1 mRNA stability decreased in the growing cells and increased in the cells exiting the cell cycle and at confluence. A nuclear run-on assay revealed a close correlation between the Kip1 transcriptional activity and the level of Kip1 mRNA. We conclude that the cell-cell contact up-regulated the Kip1 gene transcription and increased the Kip1 mRNA stability, which was related to the recovery of p27(Kip1) protein during contact-induced growth arrest in endothelial cells.

Matrix-bound fibroblasts regulate angiogenesis by modulation of VE-cadherin

BACKGROUND

Vascular endothelial cadherin (VE-cadherin/cadherin-5) has previously been described as playing a specific role in angiogenesis due to its localisation at areas of intercellular contact, where it functions in maintenance of tubular architecture. Matrix-bound fibroblasts have been show to produce a number of factors that are important in inducing angiogenesis and to promote tubule-formation by human endothelial cells, an indicator of angiogenic potential.

RESULTS

Tubule formation stimulated by fibroblast-derived growth factors can be prevented by the addition of monoclonal antibody to VE-cadherin. In addition, fibroblasts-derived growth factors are able to modulate the expression and hence the regulation of this endothelial cell specific cadherin.

CONCLUSIONS

The change in VE-cadherin expression of human endothelial cells by fibroblast-derived growth factors may contribute to the regulation of angiogenesis.

Lithium inhibits cell cycle progression and induces stabilization of p53 in bovine aortic endothelial cells

Lithium affects development of various organisms and cell fate through the inhibition of glycogen synthase kinase-3 beta and induction of the Wnt/beta-catenin signaling pathway. In this study, we investigated the effects of lithium on primary bovine aortic endothelial cells (BAEC). Lithium treatment of BAEC induced beta-catenin stabilization but failed to activate the transcriptional activity of the beta-catenin/T-cell factor complex. Lithium caused a sustained G(2)/M cell cycle arrest without affecting cell viability. Reversibility of this cell cycle arrest occurred up to 3 days after treatment but was reduced thereafter. Lithium-treated BAEC exhibited a senescent-like morphology with an increase in cells positive for the senescence-associated-beta-galactosidase activity. Lithium also increased the expression of p21(Cip), a cyclin-dependent kinase inhibitor, both at the protein and RNA levels. No change in p21(Cip) mRNA stability was observed, whereas the transcriptional activity of a p21(Cip) promoter-luciferase construct containing p53 binding sites was increased after lithium treatment. Furthermore, lithium caused increased transcription of a reporter gene under the control of a promoter containing the p53 consensus binding sites both in transiently transfected BAEC and in a stably transfected fibroblast cell line. Lithium caused accumulation of p53 protein in BAEC without affecting p53 mRNA levels. Finally, up-regulation of p21(Cip) in response to lithium did not occur in mouse embryonic fibroblasts that were null for p53 alleles, confirming the dependence on a p53 pathway for this lithium effect. These findings demonstrate for the first time that lithium induces also stabilization of the tumor suppressor p53 and reveal a new mechanism that may contribute to the neuroprotective effects of lithium.

Tetraspanins in intercellular adhesion of polarized epithelial cells: spatial and functional relationship to integrins and cadherins

The subcellular distribution of tetraspanin molecules and their functional relationship with integrins in cell-cell adhesion was studied in detail in different polarized epithelial cell models. CD9, CD81 and CD151 tetraspanins were localized at lateral cell-cell contact sites in a similar distribution to E-cadherin. Interestingly, CD9 was partially localized at the apical microvillae of Madin-Darby canine kidney cells forming multimolecular complexes distinct from those found on the basolateral membrane, suggesting the coexistence of differential tetraspanin webs with different subcellular localization. We found that tetraspanin-associated beta1 integrins at cell-to-cell contacts were in a low-affinity conformational state, and that their localization at intercellular contacts was independent of cadherin expression and adhesion. Furthermore, integrin-tetraspanin complexes were functionally relevant in cell-cell adhesion in a cadherin-independent manner, without requiring a conformational change of the integrin moiety. Nevertheless, the integrin alpha3beta1 was ligand-binding competent and this binding did not disrupt association to tetraspanins. Moreover, Chinese hamster ovary cells treated with anti-tetraspanin mAbs or activatory anti-beta1 integrin mAbs were able to develop tubule-like structures. Together, these data support tetraspanin association as a new regulatory mechanism of integrin function and suggest a role for tetraspanins-integrin complexes in providing the cell with the spatial cues necessary for their proper polarization.

Platelet endothelial cell adhesion molecule-1 and vascular endothelial cadherin cooperatively regulate fibroblast growth factor-induced modulations of adherens junction functions

Cellular adherens junctions are formed by cadherins linked to proteins of the catenin family. In endothelial cells, not only vascular endothelial cadherin but also platelet endothelial cell adhesion molecule-1 localizes into junctions and associates with beta-catenin. To explore a putative cooperation of platelet endothelial cell adhesion molecule-1 and vascular endothelial cadherin, we analyzed transfectants expressing either platelet endothelial cell adhesion (CD31 cells) or vascular endothelial cadherin (CD144 cells) or both molecules (CD31/CD144 cells), and, for comparison, human umbilical vein endothelial cells. Basic fibroblast growth factor completely dissociated vascular endothelial cadherin/beta-catenin complexes and robustly moved beta-catenin into the nucleus in CD144 cells, whereas in CD31/CD144 cells as well as in human umbilical vein endothelial cells, fibroblast growth factor only partially dissociated the junctional complex followed by a significantly reduced nuclear translocation of beta-catenin. In contrast, in CD31 cells, the subcellular distribution of beta-catenin remained unaffected by fibroblast growth factor. As a functional consequence, fibroblast growth factor induced a complete collapse of the F-actin network in CD144 cells, a limited rearrangement of F-actin fibers in CD31/CD144 cells and no F-actin rearrangement in CD31 cells. We also analyzed the effect of fibroblast growth factor-induced rearrangement of junctions on junction permeability for leukocytes: in line with our observation that vascular endothelial cadherin was required for cells to respond to fibroblast growth factor, only in CD31/CD144 cells, but not in CD31 cells, leukocyte transmigration was significantly enhanced by fibroblast growth factor. In conclusion platelet endothelial cell adhesion molecule-1 cooperates with vascular endothelial cadherin in a mutual fashion; platelet endothelial cell adhesion molecule-1 reduces and temporarily limits fibroblast growth factor-induced dissociation of vascular endothelial cadherin/beta-catenin complexes, but requires vascular endothelial cadherin to control leukocyte transmigration in dependence of fibroblast growth factor.

Endothelial adherens junctions

The principle of the molecular organization of adherens junctions follows a uniform pattern, which is found in epithelial, muscular, neuroneal as well as in endothelial cells and is highly conserved among species. Transmembrane molecules of the cadherin family link to catenins, which anchor the adhesion plaque to the cytoskeleton. The kind of cadherin used in adherens junctions is cell-type specific, vascular endothelial (VE)-cadherin is specific for endothelial cells. The assembly and disassembly of the cadherin/catenin complex is dynamic and regulated by growth factors. The functional status of adherens junctions controls endothelial cell-to-cell adhesion, cell scattering, vessel morphogenesis and has intracellular signaling properties, thereby playing an important role in vasculogenesis and angiogenesis.

Cell confluence-dependent remodeling of endothelial membranes mediated by cholesterol

The plasma membranes of endothelial cells reaching confluence undergo profound structural and functional modifications, including the formation of adherens junctions, crucial for the regulation of vascular permeability and angiogenesis. Adherens junction formation is accompanied by the tyrosine dephosphorylation of adherens junctions proteins, which has been correlated with the strength and stability of adherens junctions. Here we show that cholesterol is a critical determinant of plasma membrane remodeling in cultures of growing cow pulmonary aortic endothelial cells. Membrane cholesterol increased dramatically at an early stage in the formation of confluent cow pulmonary aortic endothelial cell monolayers, prior to formation of intercellular junctions. This increase was accompanied by the redistribution of caveolin from a high density to a low density membrane compartment, previously shown to require cholesterol, and increased binding of the annexin II-p11 complex to membranes, consistent with other studies indicating cholesterol-dependent binding of annexin II to membranes. Furthermore, partial depletion of cholesterol from confluent cells with methyl-beta-cyclodextrin both induced tyrosine phosphorylation of multiple membrane proteins, including adherens junctions proteins, and disrupted adherens junctions. Both effects were dramatically reduced by prior complexing of methyl-beta-cyclodextrin with cholesterol. Our results reveal a novel physiological role for cholesterol regulating the formation of adherens junctions and other plasma membrane remodeling events as endothelial cells reach confluence.