Vascular endothelial (VE)-cadherin: only an intercellular glue?

Vascular permeability induced by VEGF family members in vivo: role of endogenous PAF and NO synthesis

We previously reported that vascular endothelial growth factor (VEGF) increases vascular permeability through the synthesis of endothelial platelet-activating factor (PAF), while others reported the contribution of nitric oxide (NO). Herein, we addressed the contribution of VEGF receptors and the role played by PAF and NO in VEGF-induced plasma protein extravasation. Using a modified Miles assay, intradermal injection in mice ears of VEGF-A(165), VEGF-A(121), and VEGF-C (1 microM) which activate VEGFR-2 (Flk-1) receptor increased vascular permeability, whereas a treatment with VEGFR-1 (Flt-1) analogs; PlGF and VEGF-B (1 microM) had no such effect. Pretreatment of mice with PAF receptor antagonist (LAU8080) or endothelial nitric oxide synthase (eNOS) inhibitor (L-NAME) abrogated protein extravasation mediated by VEGF-A(165). As opposed to PAF (0.01-1 microM), treatment with acetylcholine (ACh; up to 100 microM; inducer of NO synthesis) or sodium nitroprusside (SNP; up to 1 microM; NO donor) did not induce protein leakage. Simultaneous pretreatment of mice with eNOS and protein kinase A (PKA) inhibitors restored VEGF-A(165) vascular hyperpermeability suggesting that endogenous NO synthesis leads to PKA inhibition, which support maintenance of vascular integrity. Our data demonstrate that VEGF analogs increase vascular permeability through VEGFR-2 activation, and that both endogenous PAF and NO synthesis contribute to VEGF-A(165)-mediated vascular permeability. However, PAF but not NO directly increases vascular permeability per se, thereby, suggesting that PAF is a direct inflammatory mediator, whereas NO serves as a cofactor in VEGF-A(165) proinflammatory activities.

Inhibition of endothelial cell proliferation, in vitro angiogenesis, and the down-regulation of cell adhesion-related genes by genistein. Combined with a cDNA microarray analysis

Antiangiogenesis is presently one of the powerful strategies for treating cancer, and endothelial cells play a pivotal role in the process of angiogenesis. Genistein, a tyrosine kinase inhibitor, a major isoflavone plentiful in soybeans, is known to inhibit both tumor growth and angiogenesis. However, the precise molecular mechanism(s) by which genistein affects endothelial cells has yet to be elucidated. In the present study, a cDNA microarray was performed to investigate the targeted genes of human umbilical vein endothelial cells (HUVECs) affected by 10 microM genistein. As a result, a total of 256 genes showed an altered expression of more than twofold. Among them were the genes related to cell proliferation, adhesion, transcription, translation, metabolism, cytoskeleton, apoptosis, kinases, and functionally unknown. The down-regulation of mRNA or the protein expression of cell adhesion-related genes, including VE-cadherin, gap junction protein alpha 1 (connexin 43), integrin alpha V, and multimerin, were confirmed by reverse transcriptase-polymerase chain reaction (RT-PCR) or by immunofluorescence staining. The impaired cell-cell adhesion by genistein was also observed by electron microscopy. In addition, the antiangiogenesis role of genistein was confirmed on Matrigel using inverted microscopy and electron microscopy. In conclusion, genistein affects endothelial cells as a negative mediator of proliferation and angiogenesis in vitro, partially by down-regulating cell adhesion-related genes and impairing cell adhesions.

Apoptotic signaling induces hyperpermeability following hemorrhagic shock

Hemorrhagic shock (HS) disrupts the endothelial cell barrier, resulting in microvascular hyperpermeability. Recent studies have also demonstrated that activation of the apoptotic signaling cascade is involved in endothelial dysfunction, which may result in hyperpermeability. Here we report involvement of the mitochondrial "intrinsic" pathway in microvascular hyperpermeability following HS in rats. HS resulted in the activation of the mitochondrial intrinsic pathway, as is evident from an increase in the proapoptotic Bcl-2 family member BAK, release of mitochondrial cytochrome c into the cytoplasm, and activation of caspase-3. This, along with the in vivo transfection of the proapoptotic peptide BAK (BH3), resulted in hyperpermeability (as visualized by intravital microscopy), release of mitochondrial cytochrome c into the cytoplasm, and activation of caspase-3. Conversely, transfection of the BAK (BH3) mutant had no effect on hyperpermeability. Together, these results demonstrate involvement of the mitochondrial intrinsic apoptotic pathway in HS-induced hyperpermeability and that the attenuation of this pathway may provide an alternative strategy in preserving vascular barrier integrity.

Endothelial colony forming units: are they a reliable marker of endothelial progenitor cell numbers?

BACKGROUND

Flow cytometry and cell culture, the two main laboratory techniques employed for counting endothelial progenitor cells (EPCs), have serious limitations. Mononuclear cells cultured in media favouring endothelial growth allow cells to replicate and differentiate/mature. EPCs under these circumstances tend to form groups of cells called endothelial colony forming units (EC-CFUs). EC-CFUs are widely accepted as a surrogate as an estimate of EPC number and function in cell culture. However, some important limitations may restrict the assumption that EC-CFUs reflect EPC numbers accurately.

OUR FINDINGS

Our own experience of EPC culture in atrial fibrillation has demonstrated that: 1) the size of EC-CFUs and proportion of single cells fluctuate significantly, even on the same culture plate; 2) the ability of EPCs to migrate towards one another to form EC-CFUs varies; and 3) the rate of EPC differentiation and proliferation may significantly affect the number of EC-CFUs, despite similarities in EPC counts on separate plates. In contrast, the count of differentiated cultured EPCs by flow cytometry with specific mature endothelial markers (e.g. CD146, vascular endothelial (VE) cadherin) is a potentially more objective alternative.

SUMMARY

Endothelial CFU counts represent the cumulative characteristics of EPC quantity and their functional characteristics, and cannot be reliably used for the estimation of EPC numbers in peripheral blood or the bone marrow. Until stronger definition(s) of bone marrow or peripheral blood population(s) of EPCs are developed, flow cytometry may be the more optimal technique for EPC quantification.

Vascular endothelial cadherin regulates vascular permeability: Implications for ovarian hyperstimulation syndrome

CONTEXT

Ovarian hyperstimulation syndrome (OHSS) is an iatrogenic complication of treatment with fertility drugs. It is characterized by increased vascular permeability and simultaneous overexpression of vascular endothelial growth factor (VEGF) in ovarian cells.

OBJECTIVE

We tested the hypothesis that the endothelium and endothelial cell-to-cell junctions are downstream targets of VEGF during OHSS pathogenesis. We investigated the potential involvement of vascular endothelial (VE)-cadherin, an interendothelial adhesion molecule, in the capillary hyperpermeability in OHSS.

DESIGN

Human endothelial cells from umbilical veins (HUVEC) were used as an in vitro model of OHSS.

INTERVENTION

Cell cultures were treated with varying doses of estradiol (E2), human chorionic gonadotropin (hCG), VEGF, and antihuman VEGF antibodies, either alone or in combination, and the effect on VE-cadherin release was evaluated at different time points. Permeability assays were performed using fluoresceinisothiocyanate-labeled albumin, and actin filaments rearrangement was evaluated by fluorescent microscopy.

RESULTS

Culturing of HUVEC with high doses of E2 produced no significant changes in VE-cadherin concentration, but hCG and VEGF produced a significant increase in VE-cadherin release. Time-course experiments showed that VE-cadherin was secreted 12 h after VEGF addition. Antihuman VEGF antibodies prevented these changes. Permeability assays demonstrated that, although E2 did not alter the arrangement of HUVEC in vitro, hCG and VEGF caused changes in the actin fibers indicative of increased capillary permeability. VEGF also induced an increase in paracellular permeability of HUVEC at the same doses used in the previous experiments.

CONCLUSIONS

Adhesion molecules like VE-cadherin may play a role in the development and progression of increased capillary permeability in severe OHSS.

Tumor necrosis factor-alpha damages tumor blood vessel integrity by targeting VE-cadherin

BACKGROUND

Isolated limb perfusion using high-dose human tumor necrosis factor-alpha with melphalan is effective therapy for bulky extremity in-transit melanoma and sarcoma.

OBJECTIVE

While it is widely accepted that melphalan is a DNA alkylating agent, the mechanism of selective antitumor effect of tumor necrosis factor-alpha is unclear.

METHODS AND RESULTS

Electron microscopic analyses of human melanoma biopsies, pre- and post-melphalan perfusion, showed that the addition of tumor necrosis factor-alpha caused gapping between endothelial cells by 3 to 6 hours post-treatment followed by vascular erythrostasis in treated tumors. In human melanoma xenografts raised in mice, tumor necrosis factor-alpha selectively increased tumor vascular permeability by 3 hours and decreased tumor blood flow by 6 hours post-treatment relative to treated normal tissue. In an in vitro tumor endothelial cell model, tumor necrosis factor-alpha caused vascular endothelial adherens junction protein, VE-cadherin, to relocalize within the cell membrane away from cell-cell junctions leading to gapping between endothelial cells by 3 to 6 hours post-treatment. Phosphotyrosinylation was a prerequisite for movement of VE-cadherin away from endothelial cell junctions and for gapping between endothelial cells. Clinical isolated limb perfusion tumor specimens, at 3 hours postperfusion, showed a discontinuous and irregular pattern of VE-cadherin expression at endothelial cell junctions when compared with normal (skin) or pretreatment tumor tissue.

CONCLUSIONS

Together, the data suggest that tumor necrosis factor-alpha selectively damages the integrity of tumor vasculature by disrupting VE-cadherin complexes at vascular endothelial cell junctions leading to gapping between endothelial cells, causing increased vascular leak and erythrostasis in tumors. VE-cadherin appears to be a potentially good target for selective antitumor therapy.

Gingipains from Porphyromonas gingivalis W83 synergistically disrupt endothelial cell adhesion and can induce caspase-independent apoptosis

We have shown previously that gingipains from Porphyromonas gingivalis W83 can induce cell detachment, cell adhesion molecule (CAM) cleavage, and apoptosis in endothelial cells; however, the specific roles of the individual gingipains are unclear. Using purified gingipains, we determined that each of the gingipains can cleave CAMs to varying degrees with differing kinetics. Kgp and HRgpA work together to quickly detach endothelial cells. Interestingly, in the absence of active caspases, both gingipain-active W83 extracts and purified HRgpA and RgpB induce apoptotic morphology, suggesting that the gingipains can induce both caspase-dependent and caspase-independent apoptosis. Using z-VAD-FMK to inhibit Kgp activity and leupeptin to inhibit Rgp activity in gingipain-active W83 extracts, we investigated the relative significance of the synergistic role of the gingipains. z-VAD-FMK or leupeptin delayed, but did not inhibit, cell detachment induced by gingipain-active W83 extracts or purified gingipains. There was partial cleavage of N-cadherin and cleavage of VE-cadherin was not inhibited. Degradation of integrin beta1 was inhibited only in the presence of z-VAD-FMK. These results further clarify the role P. gingivalis plays in tissue destruction occurring in the periodontal pocket.

The chromogranin A peptide vasostatin-I inhibits gap formation and signal transduction mediated by inflammatory agents in cultured bovine pulmonary and coronary arterial endothelial cells

The proinflammatory agent tumour necrosis factor alpha (TNFalpha) is one of several agents causing vascular leakage. The N-terminal domain of CgA, vasostatin-I (CgA1-76), has recently been reported to inhibit TNFalpha induced gap formation in human umbilical venous endothelial cells. Here we report on the effect of recombinant human CgA1-78, vasostatin-I, on TNFalpha induced gap formation in two model systems of vascular leakage in arterial endothelial cells of bovine pulmonary (BPAEC) and coronary (BCAEC) origin. Vasostatin-I inhibited the TNFalpha induced gap formation in both models, being inactive in the unstimulated cells. The phosphorylation of p38MAP kinase in TNFalpha activated BPAEC was markedly attenuated in the presence of vasostatin-I and the inhibitory effect corresponded to that of the specific p38MAPK inhibitor SB203580. Vasostatin-I also inhibited the phosphorylation of p38MAPK induced by both thrombin and pertussis toxin in these cells. The results demonstrate that vasostatin-I has inhibitory effects on TNFalpha-induced disruption of confluent layers of cultured pulmonary and coronary arterial endothelial cells. This suggests that vasostatin-I may affect endothelial barrier dysfunction also in arterial vascular beds. Furthermore, the inhibitory activity of vasostatin-I may be associated with the p38MAPK signalling cascade via a pertussis toxin sensitive, presumably Galphai coupled mechanism.

Regulation of transendothelial migration of colon cancer cells by E-selectin-mediated activation of p38 and ERK MAP kinases

The invasive properties of cancer cells depend on their intrinsic motile potential and on their ability to breach the endothelial barrier. In the present work, we investigated the mechanisms by which adhesion of colon cancer cells to E-selectin expressed by endothelial cells regulates the barrier function of these cells and modulates transmigration of cancer cells. We found that the stimulation of E-selectin by activating antibodies or the adhesion of HT-29 cells results in an increase in the activity of extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinases. In turn, the activation of p38 and ERK enhances transendothelial permeability and migration of HT-29 cells. We also obtained evidence suggesting that p38-mediated increase in transendothelial migration of cancer cells depends on a myosin light chain phosphorylation-mediated formation of stress fibres. On the other hand, the activation of ERK by E-selectin modulates the opening of interendothelial spaces by initiating the activation of Src kinase activities and the dissociation of the VE-cadherin/beta-catenin complex. We conclude that activation of E-selectin by adhering cancer cells is an important process that regulates the extravasation of colon cancer cells by initiating p38- and ERK-dependent mechanisms that both contribute to regulate the integrity of the endothelial layer.

Platelet endothelial cell adhesion molecule-1 and mechanotransduction in vascular endothelial cells

Endothelial cells are known to respond to mechanical forces such as fluid shear stress and cyclic stretch, but elucidating the mechanism for mechanosensing has been difficult. Experimental data indicate that there are probably several sensing mechanisms. We have recently proposed a novel mechanoresponse mechanism that involves platelet endothelial cell adhesion molecule-1 (PECAM-1). When endothelial cells are stimulated by fluid shear stress, PECAM-1 is tyrosine phosphorylated and activates the extracellular signal-regulated kinase 1 and 2 (ERK1/2) signalling cascade. The same signalling events occurred when we applied pulling force directly on PECAM-1 on the endothelial cell surface using magnetic beads coated with antibodies against the external domain of PECAM-1. These results appear to indicate that PECAM-1 is a mechanotransduction molecule. To our knowledge, this is the first mammalian molecule that is shown to respond to mechanical force directly exerted to it.

Src protein tyrosine kinase family and acute inflammatory responses

Acute inflammatory responses are one of the major underlying mechanisms for tissue damage of multiple diseases, such as ischemia-reperfusion injury, sepsis, and acute lung injury. By use of cellular and molecular approaches and transgenic animals, Src protein tyrosine kinase (PTK) family members have been identified to be essential for the recruitment and activation of monocytes, macrophages, neutrophils, and other immune cells. Src PTKs also play a critical role in the regulation of vascular permeability and inflammatory responses in tissue cells. Importantly, animal studies have demonstrated that small chemical inhibitors for Src PTKs attenuate tissue injury and improve survival from a variety of pathological conditions related to acute inflammatory responses. Further investigation may lead to the clinical application of these inhibitors as drugs for ischemia-reperfusion injury (such as stroke and myocardial infarction), sepsis, acute lung injury, and multiple organ dysfunction syndrome.

The role of VEGF receptors in angiogenesis; complex partnerships

Vascular endothelial growth factors (VEGFs) regulate blood and lymphatic vessel development and homeostasis but also have profound effects on neural cells. VEGFs are predominantly produced by endothelial, hematopoietic and stromal cells in response to hypoxia and upon stimulation with growth factors such as transforming growth factors, interleukins or platelet-derived growth factor. VEGFs bind to three variants of type III receptor tyrosine kinases, VEGF receptor 1, 2 and 3. Each VEGF isoform binds to a particular subset of these receptors giving rise to the formation of receptor homo- and heterodimers that activate discrete signaling pathways. Signal specificity of VEGF receptors is further modulated upon recruitment of coreceptors, such as neuropilins, heparan sulfate, integrins or cadherins. Here we summarize the knowledge accumulated since the discovery of these proteins more than 20 years ago with the emphasis on the signaling pathways activated by VEGF receptors in endothelial cells during cell migration, growth and differentiation.

Integrins regulate VE-cadherin and catenins: dependence of this regulation on Src, but not on Ras

Adhesions of cells to extracellular matrix and adjacent cells are mediated by integrins and VE-cadherin, respectively. Although these adhesion processes play crucial roles in vascular cell migration and angiogenesis, it remains unclear as to how they are coordinated to regulate cellular functions. We report here that integrin engagement by treating bovine endothelial aortic cell monolayers with beads coated with fibronectin (Fn) led to disruption of the VE-cadherin-containing adherens junctions. This disruption was accompanied by increases of tyrosine phosphorylation of beta-catenin, gamma-catenin, and p120ctn, as well as the dissociation of alpha-catenin and gamma-catenin from VE-cadherin. We applied a membrane-targeted Src reporter based on the fluorescence resonance energy transfer technique to visualize the dynamic Src activation at subcellular levels in live cells. The integrin engagement induced by Fn-coated beads caused the activation of Src around the beads and at adherens junctions, which are subsequently disrupted. The inhibition of Src with PP1 blocked the effects of integrin engagement on adherens junctions. Although Ras can also modulate adherens junctions, the resulting patterns of phosphorylation and association of junction proteins were distinct from those induced by integrin engagement. The inhibition of Ras by RasN17 did not rescue the disruption of adherens junctions induced by integrin engagement or by Src activation. Integrin engagement by Fn-coated beads also induced a significant alteration of cortical actin filaments at adherens junctions. The results indicate that integrin engagement disrupts VE-cadherin-containing adherens junctions via the activation of Src, but not Ras, possibly as a result of modulation of the actin network.

Src protein tyrosine kinase family and acute lung injury

Acute inflammatory responses are one of the major underlying mechanisms for tissue damage of multiple diseases, such as sepsis and acute lung injury. Inflammatory mediators released from a variety of cells in response to acute inflammations can interact with immune cells, microvascular endothelial cells and other tissue cells, to elicit a series of intracellular signaling reactions where activation of Src protein tyrosine kinase (PTK) family members is involved. Using cellular and molecular approaches and transgenic animals, Src PTK family members have been identified to be essential for the recruitment and activation of monocytes, macrophages, neutrophils and other immune cells. Src PTK family members also play a critical role in the regulation of vascular permeability and inflammatory responses in tissue cells. Importantly, animal studies have demonstrated that small chemical inhibitors for Src PTKs attenuated acute lung injury. Further investigation may lead to the clinical application of these inhibitors as drugs for acute lung injury.

Transport across the endothelium: regulation of endothelial permeability

An important function of the endothelium is to regulate the transport of liquid and solutes across the semi-permeable vascular endothelial barrier. Two cellular pathways controlling endothelial barrier function have been identified. The transcellular pathway transports plasma proteins of the size of albumin or greater via the process of transcytosis in vesicle carriers originating from cell surface caveolae. Specific signalling cues are able to induce the internalisation of caveolae and their movement to the basal side of the endothelium. Caveolin-1, the primary structural protein required for the formation of caveolae, is also important in regulating vesicle trafficking through the cell by controlling the activity and localisation of signalling molecules that mediate vesicle fission, endocytosis, fusion and finally exocytosis. An important function of the transcytotic pathways is to regulate the delivery of albumin and immunoglobulins, thereby controlling tissue oncotic pressure and host-defence. The paracellular pathway induced during inflammation is formed by gaps between endothelial cells at the level of adherens and tight junctional complexes. Paracellular permeability is increased by second messenger signalling pathways involving Ca2+ influx via activation of store-operated channels, protein kinase Calpha (PKCalpha), and Rho kinase that together participate in the stimulation of myosin light chain phosphorylation, actin-myosin contraction, and disruption of the junctions. In this review of the field, we discuss the current understanding of the signalling pathways regulating paracellular and transcellular endothelial permeability.

Regulation of growth factor signaling and cell cycle progression by cell adhesion and adhesion-dependent changes in cellular tension

The proliferation of most non-transformed cell types requires cell adhesion and cellular tension as well as exposure to mitogenic growth factors. Integrins and cadherins provide the adhesion signals, which ultimately allow for the cytoskeletal changes that control cellular tension. This review discusses the roles of integrins, cadherins, and the actin cytoskeleton as mediators of the mechanical tension critical for growth factor-dependent signaling and cell cycle progression.

Tyrosine Phosphorylation of VE-cadherin Prevents Binding of p120- and β-Catenin and Maintains the Cellular Mesenchymal State

In several pathological conditions, epithelial cells demonstrate a breakdown of barrier function and acquire an invasive phenotype. Endothelial cells in particular are maintained in a mesenchymal state during the cell invasion phase of angiogenesis. We show here that tyrosine phosphorylation of the adherens junction protein VE-cadherin at two critical tyrosines, Tyr-658 and Tyr-731, via tyrosine kinase activation or phosphatase inactivation was sufficient to prevent the binding of p120- and beta-catenin, respectively, to the cytoplasmic tail of VE-cadherin. In fact, phosphorylation at either site led to the inhibition of cell barrier function. Cells expressing wild-type VE-cadherin showed decreased cell migration compared with cells lacking VE-cadherin, whereas expression of VE-cadherin with a simple phosphomimetic tyrosine-to-glutamic acid mutation of Y658E or Y731E was sufficient to restore the migratory response. These findings demonstrate that a single phosphorylation event within the VE-cadherin cytoplasmic tail is sufficient to maintain cells in a mesenchymal state.

Anthrax lethal toxin induces endothelial barrier dysfunction

Hemorrhage and pleural effusion are prominent pathological features of systemic anthrax infection. We examined the effect of anthrax lethal toxin (LT), a major virulence factor of Bacillus anthracis, on the barrier function of primary human lung microvascular endothelial cells. We also examined the distribution patterns of cytoskeletal actin and vascular endothelial-cadherin (VE-cadherin), both of which are involved in barrier function regulation. Endothelial monolayers cultured on porous membrane inserts were treated with the LT components lethal factor (LF) and protective antigen (PA) individually, or in combination. LT induced a concentration- and time-dependent decrease in transendothelial electrical resistance that correlated with increased permeability to fluorescently labeled albumin. LT also produced a marked increase in central actin stress fibers and significantly altered VE-cadherin distribution as revealed by immunofluorescence microscopy and cell surface enzyme-linked immunosorbent assay. Treatment with LF, PA, or the combination of an inactive LF mutant and PA did not alter barrier function or the distribution of actin or VE-cadherin. LT-induced barrier dysfunction was not dependent on endothelial apoptosis or necrosis. The present findings support a possible role for LT-induced barrier dysfunction in the vascular permeability changes accompanying systemic anthrax infection.

Proline-rich Tyrosine Kinase 2 (Pyk2) Mediates Vascular Endothelial-Cadherin-based Cell-Cell Adhesion by Regulating β-Catenin Tyrosine Phosphorylation

Vascular endothelial-cadherin (VE-cadherin) controls endothelial cell-cell adhesion and preserves endothelial integrity. In order to maintain endothelial barrier function, VE-cadherin function is tightly regulated through mechanisms that involve protein phosphorylation and cytoskeletal dynamics. Here, we show that loss of VE-cadherin function results in intercellular gap formation and a drop in electrical resistance of monolayers of primary human endothelial cells. Detailed analysis revealed that loss of endothelial cell-cell adhesion, induced by VE-cadherin-blocking antibodies, is preceded by and dependent on a rapid activation of Rac1 and increased production of reactive oxygen species. Moreover, VE-cadherin-associated beta-catenin is tyrosine-phosphorylated upon loss of cell-cell contact. Finally, the redox-sensitive proline-rich tyrosine kinase 2 (Pyk2) is activated and recruited to cell-cell junctions following the loss of VE-cadherin homotypic adhesion. Conversely, the inhibition of Pyk2 activity in endothelial cells by the expression of CRNK (CADTK/CAKbeta-related non-kinase), an N-terminal deletion mutant that acts in a dominant negative fashion, not only abolishes the increase in beta-catenin tyrosine phosphorylation but also prevents the loss of endothelial cell-cell contact. These results implicate Pyk2 in the reduced cell-cell adhesion induced by the Rac-mediated production of ROS through the tyrosine phosphorylation of beta-catenin. This signaling is initiated upon loss of VE-cadherin function and is important for our insight in the modulation of endothelial integrity.

Antiangiogenic gene therapy: disruption of neovascular networks mediated by inducible caspase-9 delivered with a transcriptionally targeted adenoviral vector

The activation of an inducible caspase (iCaspase-9) mediates apoptosis of neovascular endothelial cells, and overcomes the prosurvival effect of vascular endothelial growth factor or basic fibroblast growth factor. The potential utilization of direct activation of caspases as an antiangiogenic strategy for treatment of angiogenesis-dependent diseases (eg cancer) requires expression of the inducible caspase primarily in the tumor endothelium. The objective of this work was to develop and characterize a transcriptionally targeted adenoviral vector that mediates expression of iCaspase-9 specifically in neovascular endothelial cells. We observed that adenoviral vectors containing the human VEGFR2 promoter induced reporter gene expression primarily in proliferating human dermal microvascular endothelial cells (HDMEC). HDMEC transduced with recombinant adenoviral vectors containing iCaspase-9 under regulation of the VEGFR2 promoter (Ad-hVEGFR2-iCaspase-9) and exposed to a cell-permeable dimerizer drug (AP20187), presented higher caspase-3 activity and apoptosis than controls (P < or = 0.05). Using the SCID Mouse Model of Human Angiogenesis, we observed that local delivery of Ad-hVEGFR2-iCaspase-9 followed by intraperitoneal injection of AP20187 resulted in endothelial cell apoptosis and local ablation of microvessels. We believe that this constitutes the first report of a transcriptionally targeted antiangiogenic adenoviral vector that mediates neovascular disruption upon activation of a caspase-based artificial death switch.

Gingipains from Porphyromonas gingivalis W83 induce cell adhesion molecule cleavage and apoptosis in endothelial cells

The presence of Porphyromonas gingivalis in the periodontal pocket and the high levels of gingipain activity detected in gingival crevicular fluid could implicate a role for gingipains in the destruction of the highly vascular periodontal tissue. To explore the effects of these proteases on endothelial cells, we exposed bovine coronary artery endothelial cells and human microvascular endothelial cells to gingipain-active extracellular protein preparations and/or purified gingipains from P. gingivalis. Treated cells exhibited a rapid loss of cell adhesion properties that was followed by apoptotic cell death. Cleavage of N- and VE-cadherin and integrin beta1 was observed in immunoblots of cell lysates. There was a direct correlation between the kinetics of cleavage of N- and VE-cadherin and loss of cell adhesion properties. Loss of cell adhesion, as well as N- and VE-cadherin and integrin beta1 cleavage, could be inhibited or significantly delayed by preincubation of P. gingivalis W83 gingipain-active extracellular extracts with the cysteine protease inhibitor Nalpha-p-tosyl-l-lysine chloromethylketone. Furthermore, purified gingipains also induced endothelial cell detachment and apoptosis. Apoptosis-associated events, including annexin V positivity, caspase-3 activation, and cleavage of the caspase substrates poly(ADP-ribose) polymerase and topoisomerase I (Topo I), were observed in endothelial cells after detachment. All of the effects observed were correlated with the different levels of cysteine-dependent proteolytic activity of the extracts tested. Taken together, these results indicate that gingipains from P. gingivalis can alter cell adhesion molecules and induce endothelial cell death, which could have implications for the pathogenicity of this organism.

TIMP-1 inhibits microvascular endothelial cell migration by MMP-dependent and MMP-independent mechanisms

It was reported over a decade ago that tissue inhibitor of metalloproteinases-1 (TIMP-1) suppresses angiogenesis in experimental models but the mechanism is still incompletely understood. This in vitro study focused on the molecular basis of TIMP-1-mediated inhibition of endothelial cell (EC) migration, a key step in the angiogenic process. Both recombinant human TIMP-1 and the synthetic MMP inhibitors, GM6001 and MMP-2-MMP-9 Inhibitor III, suppressed migration of human dermal microvascular endothelial cells (HDMVEC) in a dose-dependent fashion. The MMP-dependent inhibition of migration was associated with increased expression of the junctional adhesion proteins, VE-cadherin and PECAM-1, and VE-cadherin accumulation at cell-cell junctions. TIMP-1 also caused MMP-independent dephosphorylation of focal adhesion kinase (FAK) (pY397) and paxillin, which was associated with reduced number of F-actin stress fibers and focal adhesions. Moreover, TIMP-1 stimulated expression of PTEN that has been shown to reduce phosphorylation of FAK and inhibit cell migration. Our data suggest that TIMP-1 inhibits HDMVEC migration through MMP-dependent stimulation of VE-cadherin and MMP-independent stimulation of PTEN with subsequent dephosphorylation of FAK and cytoskeletal remodeling.

NADPH oxidase mediates vascular endothelial cadherin phosphorylation and endothelial dysfunction

Vascular endothelial activation is an early step during leukocyte/endothelial adhesion and transendothelial leukocyte migration in inflammatory states. Leukocyte transmigration occurs through intercellular gaps between endothelial cells. Vascular endothelial cadherin (VE-cadherin) is a predominant component of endothelial adherens junctions that regulates intercellular gap formation. We found that tumor necrosis factor (TNF) caused tyrosine phosphorylation of VE-cadherin, separation of lateral cell-cell junctions, and intercellular gap formation in human umbilical vein endothelial cell (HUVEC) monolayers. These events appear to be regulated by intracellular oxidant production through endothelial NAD(P)H (nicotinamide adenine dinucleotide phosphate) oxidase because antioxidants and expression of a transdominant inhibitor of the NADPH oxidase, p67(V204A), effectively blocked the effects of TNF on all 3 parameters of junctional integrity. Antioxidants and p67(V204A) also decreased TNF-induced JNK activation. Dominant-negative JNK abrogated VE-cadherin phosphorylation and junctional separation, suggesting a downstream role for JNK. Finally, adenoviral delivery of the kinase dead PAK1(K298A) decreased TNF-induced JNK activation, VE-cadherin phosphorylation, and lateral junctional separation, consistent with the proposed involvement of PAK1 upstream of the NADPH oxidase. Thus, PAK-1 acts in concert with oxidase during TNF-induced oxidant production and loss of endothelial cell junctional integrity.

Pro-angiogenic signaling by the endothelial presence of CEACAM1

Here, we demonstrate the expression of carcinoembryonic antigen-related cell adhesion molecule-1 (CEACAM1) in angiogenic sprouts but not in large mother blood vessels within tumor tissue. Correspondingly, only human microvascular endothelial cells involved in in vitro tube formation exhibit CEACAM1. CEACAM1-overexpressing versus CEACAM1-silenced human microvascular endothelial cells were used in migration and tube formation assays. CEACAM1-overexpressing microvascular endothelial cells showed prolonged survival and increased tube formation when they were stimulated with vascular endothelial growth factor (VEGF), whereas CEACAM1 silencing via small interfering RNA blocks these effects. Gene array and LightCycler analyses show an up-regulation of angiogenic factors such as VEGF, VEGF receptor 2, angiopoietin-1, angiopoietin-2, tie-2, angiogenin, and interleukin-8 but a down-regulation of collagen XVIII/endostatin and Tie-1 in CEACAM1-overexpressing microvascular endothelial cells. Western blot analyses confirm these results for VEGF and endostatin at the protein level. These results suggest that constitutive expression of CEACAM1 in microvascular endothelial cells switches them to an angiogenic phenotype, whereas CEACAM1 silencing apparently abrogates the VEGF-induced morphogenetic effects during capillary formation. Thus, strategies targeting the endothelial up-regulation of CEACAM1 might be promising for antiangiogenic tumor therapy.

Endothelial adhesion molecule ESAM binds directly to the multidomain adaptor MAGI-1 and recruits it to cell contacts

Endothelial cell-selective adhesion molecule (ESAM) is an immunoglobulin-like transmembrane protein associated with endothelial tight junctions (TJ). Based on a yeast two-hybrid screen, we have identified the membrane-associated guanylate kinase protein MAGI-1 as an intracellular binding partner of ESAM. MAGI-1 is a multidomain adaptor protein, which binds to transmembrane, cytoskeletal, and signaling molecules, and has been localized to tight junctions in epithelial cells. MAGI-1 associates with the very C-terminal sequence of ESAM most likely through a PDZ domain-mediated interaction. The direct interaction between ESAM and MAGI-1 was confirmed by pull-down experiments. The two proteins formed stable complexes in transfected Chinese hamster ovary (CHO) cells, which could be immunoisolated. We found MAGI-1 to be associated with cell-cell contacts in human umbilical vein endothelial cells (HUVECs) and in mouse endothelium, where it colocalizes with ESAM. In CHO cells, recruitment of MAGI-1 to cell contacts required the presence of ESAM. Hence, ESAM may be involved in anchoring MAGI-1 at endothelial tight junctions.

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.

Development of a Whole-Cell-Based Biosensor for Detecting Histamine as a Model Toxin

A novel whole-cell potentiometric biosensor for screening of toxins has been developed. The constructed biosensor consists of a confluent monolayer of human umbilical vein endothelial cells (HUVECs) attached to an ion-selective cellulose triacetate (CTA) membrane modified with a covalently attached RGD (arginine-glycine-aspartic acid) peptide sequence. When the HUVECs form a confluent monolayer, ion transport is almost completely inhibited, thereby reducing the response of the ion-selective electrode (ISE). When the monolayer is exposed to agents that increase its permeability (e.g., toxins), ions can diffuse through the membrane, and a potential response from the ISE is achieved. Histamine, a model toxin that increases the permeability of HUVEC monolayers, was used in this study. When the cell-based membranes are exposed to varying concentrations of histamine, the overall response increases with increasing histamine concentration. Thus, the measured potential is an indirect measurement of the histamine concentration. Further experiments were performed for a similar molecule, l-histidine, to test for selectivity. The cell permeability was unaffected by l-histidine, and the sensor response remained unchanged. This type of sensor should find multiple applications in medical, food, and environmental fields and in homeland security.

Desmoplakin is required for microvascular tube formation in culture

Desmoplakin (DP) is a key component of cellular adhesion junctions known as desmosomes; however, recent investigations have revealed a novel location for DP in junctions separate from desmosomes termed complexus adherens junctions. These junctions are found at contact sites between endothelial cells that line capillaries. Few studies have focused on the function of DP in de novo capillary formation (vasculogenesis) and branching (angiogenesis) during tumorigenesis, embryonic development, cardiovascular development or wound healing. Only recently have investigations begun to determine the effect the loss of DP has on capillaries during embryogenesis (i.e. in DP-/- mice). Evidence shows that the loss of desmoplakin in vivo results in leaky capillaries and/or capillary malformation. Consequently, the goal of this study was to determine the function of DP in complexus adherens junctions during capillary formation. To accomplish this goal, we used siRNA technology to knock down desmoplakin expression in endothelial cells before they were induced to form microvascular tubes on matrigel. DP siRNA treated cells sent out filopodia and came in close contact with each other when plated onto matrigel; however, in most cases they failed to form tubes as compared with control endothelial cells. Interestingly, after siRNA degradation, endothelial cells were then capable of forming microvascular tubes. In depth analyses into the function of DP in capillary formation were not previously possible because the tools and experimental approaches only recently have become available (i.e. siRNA). Consequently, fully understanding the role of desmoplakin in capillary formation may lead to a novel approach for inhibiting vasculo- and angiogenesis in tumor formation.

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.

Vascular endothelial-cadherin regulates cytoskeletal tension, cell spreading, and focal adhesions by stimulating RhoA

Changes in vascular endothelial (VE)-cadherin-mediated cell-cell adhesion and integrin-mediated cell-matrix adhesion coordinate to affect the physical and mechanical rearrangements of the endothelium, although the mechanisms for such cross talk remain undefined. Herein, we describe the regulation of focal adhesion formation and cytoskeletal tension by intercellular VE-cadherin engagement, and the molecular mechanism by which this occurs. Increasing the density of endothelial cells to increase cell-cell contact decreased focal adhesions by decreasing cell spreading. This contact inhibition of cell spreading was blocked by disrupting VE-cadherin engagement with an adenovirus encoding dominant negative VE-cadherin. When changes in cell spreading were prevented by culturing cells on a micropatterned substrate, VE-cadherin-mediated cell-cell contact paradoxically increased focal adhesion formation. We show that VE-cadherin engagement mediates each of these effects by inducing both a transient and sustained activation of RhoA. Both the increase and decrease in cell-matrix adhesion were blocked by disrupting intracellular tension and signaling through the Rho-ROCK pathway. In all, these findings demonstrate that VE-cadherin signals through RhoA and the actin cytoskeleton to cross talk with cell-matrix adhesion and thereby define a novel pathway by which cell-cell contact alters the global mechanical and functional state of cells.

Mycobacteria induces pleural mesothelial permeability by down-regulating beta-catenin expression

Patients with pulmonary tuberculosis develop pleural effusions with a high protein content. Pleural mesothelial adherens junctions promote mesothelial cell-cell adhesion and contribute to pleural integrity. In the present study we have investigated the effect of mycobacterium (BCG) on mesothelial cell adherens junction proteins and pleural permeability. BCG enhanced pleural mesothelial cell (PMC) release of vascular endothelial growth factor (VEGF), and decreased electrical resistance across the PMC monolayer. Neutralizing antibodies to VEGF significantly restored the drop in PMC electrical resistance caused by BCG. BCG infection down regulated beta-catenin (adherens junction protein) expression and caused increased permeability across confluent mesothelial monolayer. Our results suggest that in TB pleurisy, mycobacteria cause VEGF release from mesothelial cells and leads to protein exudation by altering mesothelial adherens junction proteins.

The conditional inactivation of the beta-catenin gene in endothelial cells causes a defective vascular pattern and increased vascular fragility

Using the Cre/loxP system we conditionally inactivated beta-catenin in endothelial cells. We found that early phases of vasculogenesis and angiogenesis were not affected in mutant embryos; however, vascular patterning in the head, vitelline, umbilical vessels, and the placenta was altered. In addition, in many regions, the vascular lumen was irregular with the formation of lacunae at bifurcations, vessels were frequently hemorrhagic, and fluid extravasation in the pericardial cavity was observed. Cultured beta-catenin -/- endothelial cells showed a different organization of intercellular junctions with a decrease in alpha-catenin in favor of desmoplakin and marked changes in actin cytoskeleton. These changes paralleled a decrease in cell-cell adhesion strength and an increase in paracellular permeability. We conclude that in vivo, the absence of beta-catenin significantly reduces the capacity of endothelial cells to maintain intercellular contacts. This may become more marked when the vessels are exposed to high or turbulent flow, such as at bifurcations or in the beating heart, leading to fluid leakage or hemorrhages.

VCAM-1-mediated Rac signaling controls endothelial cell-cell contacts and leukocyte transmigration

Leukocyte adhesion is mediated totally and transendothelial migration partially by heterotypic interactions between the beta1- and beta2-integrins on the leukocytes and their ligands, Ig-like cell adhesion molecules (Ig-CAM), VCAM-1, and ICAM-1, on the endothelium. Both integrins and Ig-CAMs are known to have signaling capacities. In this study we analyzed the role of VCAM-1-mediated signaling in the control of endothelial cell-cell adhesion and leukocyte transendothelial migration. Antibody-mediated cross-linking of VCAM-1 on IL-1beta-activated primary human umbilical vein endothelial cells (pHUVEC) induced actin stress fiber formation, contractility, and intercellular gaps. The effects induced by VCAM-1 cross-linking were inhibited by C3 toxin, indicating that the small GTPase p21Rho is involved. In addition, the effects of VCAM-1 were accompanied by activation of Rac, which we recently showed induce intercellular gaps in pHUVEC in a Rho-dependent fashion. With the use of a cell-permeable peptide inhibitor, it was shown that Rac signaling is required for VCAM-1-mediated loss of cell-cell adhesion. Furthermore, VCAM-1-mediated signaling toward cell-cell junctions was accompanied by, and dependent on, Rac-mediated production of reactive oxygen species and activation of p38 MAPK. In addition, it was found that inhibition of Rac-mediated signaling blocks transendothelial migration of monocytic U937 cells. Together, these data indicate that VCAM-1-induced, Rac-dependent signaling plays a key role in the modulation of vascular-endothelial cadherin-mediated endothelial cell-cell adhesion and leukocyte extravasation.

Protein kinase C modifications of VE-cadherin, p120, and beta-catenin contribute to endothelial barrier dysregulation induced by thrombin

The adherens junction is a multiprotein complex consisting of the transmembrane vascular endothelial cadherin (VEC) and cytoplasmic catenins (p120, beta-catenin, plakoglobin, alpha-catenin) responsible for the maintenance of endothelial barrier function. Junctional disassembly and modifications in cadherin/catenin complex lead to increased paracellular permeability of the endothelial barrier. However, the mechanisms of junctional disassembly remain unclear. In this study, we used the proinflammatory mediator thrombin to compromise the barrier function and test the hypothesis that phosphorylation-induced alterations of VEC, beta-catenin, and p120 regulate junction disassembly and mediate the increased endothelial permeability response. The study showed that thrombin induced dephosphorylation of VEC, which is coupled to disassembly of cell-cell contacts, but VEC remained in aggregates at the plasma membrane. The cytoplasmic catenins dissociated from the VEC cytoplasmic domain in thin membrane projections formed in interendothelial gaps. We also showed that thrombin induced dephosphorylation of beta-catenin and phosphorylation of p120. Thrombin-induced interendothelial gap formation and increased endothelial permeability were blocked by protein kinase C inhibition using chelerythrine and Gö-6976 but not by LY-379196. Chelerythrine also prevented thrombin-induced phosphorylation changes of the cadherin/catenin complex. Thus the present study links posttranslational modifications of VEC, beta-catenin, and p120 to the mechanism of thrombin-induced increase in endothelial permeability.

Characterization of serine palmitoyltransferase in normal human tissues

Sphingolipids serve as structural elements of cells and as lipid second messengers. They regulate cellular homeostasis, mitogenesis, and apoptosis. Sphingolipid signaling may also be important in various pathophysiologies such as vascular injury, inflammation, and cancer. Serine palmitoyltransferase (SPT) catalyzes the condensation of serine with palmitoyl-CoA, the first, rate-limiting step in de novo sphingolipid biosynthesis. This integral microsomal membrane protein consists of at least two subunits, SPT1 and SPT2. In this study we analyzed the expression of SPT1 and SPT2 in normal human tissues. Strong SPT1 and SPT2 expression was observed in pyramidal neurons in the brain, in colon epithelium, and in mucosal macrophages. However, SPT2 expression was more prominent than SPT1 in the colon mucosal macrophages, the adrenomedullary chromaffin cells and endothelium, and in the uterine endothelium. SPT2 was localized in both nuclei and cytoplasm of the adrenomedullary chromaffin cells, whereas SPT1 was primarily cytoplasmic. These observations link enhanced SPT expression to proliferating cells, such as the lung, stomach, intestinal epithelium, and renal proximal tubular epithelium, and to potentially activated cells such as neurons, chromaffin cells, and mucosal macrophages. A baseline expression of SPT, established by this study, may serve as a measure for aberrant expression in various disease states.

Fluid shear stress and the vascular endothelium: for better and for worse

As blood flows, the vascular wall is constantly subjected to physical forces, which regulate important physiological blood vessel responses, as well as being implicated in the development of arterial wall pathologies. Changes in blood flow, thus generating altered hemodynamic forces are responsible for acute vessel tone regulation, the development of blood vessel structure during embryogenesis and early growth, as well as chronic remodeling and generation of adult blood vessels. The complex interaction of biomechanical forces, and more specifically shear stress, derived by the flow of blood and the vascular endothelium raise many yet to be answered questions:How are mechanical forces transduced by endothelial cells into a biological response, and is there a "shear stress receptor"?Are "mechanical receptors" and the final signaling pathways they evoke similar to other stimulus-response transduction systems?How do vascular endothelial cells differ in their response to physiological or pathological shear stresses?Can shear stress receptors or shear stress responsive genes serve as novel targets for the design of diagnostic and therapeutic modalities for cardiovascular pathologies?The current review attempts to bring together recent findings on the in vivo and in vitro responses of the vascular endothelium to shear stress and to address some of the questions raised above.

Scanning the human genome with combinatorial transcription factor libraries

Despite the critical importance of transcription factors in mediating gene regulation, there exists no general, genome-wide tool that uses transcription factors to induce or silence a target gene or select for a particular phenotype. In the strategy described here, we prepared large combinatorial libraries of artificial transcription factors comprising three or six zinc-finger domains, and selected transcription factor-DNA interactions able to upregulate several genes in human cells. Selected transcription factors either induced the expression of an endothelial-specific differentiation marker, VE-cadherin, in non-endothelial cell lines or, when combined with a repression domain, knocked down expression. Potential binding sites for a number of these transcription factors were mapped along the promoter of CDH5, the gene encoding VE-cadherin. Transcription factor libraries represent a useful approach for studying and modulating gene function in cells and potentially in whole organisms.

Vascular endothelial growth factor modulates the transendothelial migration of MDA-MB-231 breast cancer cells through regulation of brain microvascular endothelial cell permeability

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor (VPF), has been shown to increase potently the permeability of endothelium and is highly expressed in breast cancer cells. In this study, we investigated the role of VEGF/VPF in breast cancer metastasis to the brain. Very little is known about the role of endothelial integrity in the extravasation of breast cancer cells to the brain. We hypothesized that VEGF/VPF, having potent vascular permeability activity, may support tumor cell penetration across blood vessels by inducing vascular leakage. To examine this role of VEGF/VPF, we used a Transwell culture system of the human brain microvascular endothelial cell (HBMEC) monolayer as an in vitro model for the blood vessels. We observed that VEGF/VPF significantly increased the penetration of the highly metastatic MDA-MB-231 breast cancer cells across the HBMEC monolayer. We found that the increased transendothelial migration (TM) of MDA-MB-231 cells resulted from the increased adhesion of tumor cells onto the HBMEC monolayer. These effects (TM and adhesion of tumor cells) were inhibited by the pre-treatment of the HBMEC monolayer with the VEGF/VPF receptor (KDR/Flk-1) inhibitor, SU-1498, and the calcium chelator 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethyl)ester. These treatments of the HBMEC monolayer also inhibited VEGF/VPF-induced permeability and the cytoskeletal rearrangement of the monolayer. These data suggest that VEGF/VPF can modulate the TM of tumor cells by regulating the integrity of the HBMEC monolayer. Taken together, these findings indicate that VEGF/VPF might contribute to breast cancer metastasis by enhancing the TM of tumor cells through the down-regulation of endothelial integrity.

Is the cadherin/catenin complex a functional unit of cell-cell actin-based adherens junctions in the rat testis?

Much controversy exists regarding the presence of the cadherin/catenin complex and its intracellular attachment site in the testis, which is the functional unit for actin-based cell-cell adherens junctions (AJs) in multiple epithelia. Furthermore, whether germ and Sertoli cells are equipped with the necessary AJ-associated signaling molecules to regulate this cadherin/catenin complex during spermatogenesis is not known. In the present study, it was shown that both Sertoli and germ cells indeed express N-cadherin, E-cadherin, alpha-catenin, beta-catenin, and p120(ctn) by semiquantitative reverse transcription-polymerase chain reaction and immunoblotting. Furthermore, the assembly of AJs between Sertoli and germ cells was associated with a transient induction in the steady-state mRNA and protein levels of cadherins and catenins. These analyses reveal, to our knowledge for the first time, that the testis may indeed be using the cadherin/catenin complex as one of the functional units to regulate AJ dynamics between Sertoli and germ cells in addition to alpha(6)beta(1) integrin and the nectin/afadin complex. To further confirm the existence of such a complex between Sertoli and germ cells, immunoprecipitation experiments were performed using Sertoli-germ cell lysates during AJ assembly. An anti-N-cadherin antibody can pull out beta-catenin, whereas N-cadherin can also be pulled out using an anti-beta-catenin antibody. To further expand and validate these in vitro biochemical studies, immunofluorescent histochemistry was performed, which colocalized N-cadherin and beta-catenin to the same site of Sertoli-Sertoli and Sertoli-germ cell AJs, possibly ectoplasmic specializations near the basal compartment, at the lower third of the seminiferous epithelium in vivo as well as between Sertoli cells cultured in vitro. Furthermore, studies by cross-linking using dithiobis(succinimidylpropionate) confirmed that the cadherin/catenin complex between Sertoli cells as well as between Sertoli and germ cells indeed structurally linked to actin but not to vimentin (an intermediate filament protein) or to tubulin (a microtubule protein). These results thus unequivocally demonstrate that the cadherin/catenin complex, which can be up-regulated by testosterone, is indeed present between Sertoli and germ cells and is used for the assembly of functional AJs.

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.

Enforced expression of tissue inhibitor of matrix metalloproteinase-3 affects functional capillary morphogenesis and inhibits tumor growth in a murine tumor model

Homeostasis of the extracellular matrix is a delicate balance between degradation and remodeling, the balance being maintained by the interaction of activated matrix metalloproteinases (MMPs) and specific tissue inhibitors of matrix metalloproteinases (TIMPs). Up-regulation of MMP activity, favoring proteolytic degradation of the basement membrane and extracellular matrix, has been linked to tumor growth and metastasis, as well as tumor-associated angiogenesis, whereas inhibition of MMP activity appears to restrict these processes. We have used retroviral-mediated gene delivery to effect sustained autocrine expression of TIMP-3 in murine neuroblastoma and melanoma tumor cells in order to further examine the ability of TIMPs to inhibit angiogenesis in vivo. Growth of both histologic types of gene-modified tumor cells in severe combined immunodeficiency (SCID) mice was significantly restricted when compared with controls. Grossly, these tumors were small and had few feeding vessels. Histologic evaluation revealed that although tumors overexpressing TIMP-3 had an increased number of CD31(+) endothelial cells, these endothelial cells had not formed functional tubules, as evidenced by decreased vessel continuity and minimal pericyte recruitment. This effect appears to be mediated, in part, by decreased expression of vascular endothelial (VE)-cadherin by endothelial cells in the presence of TIMP-3 as seen both in an in vitro assay and in TIMP-3-overexpressing tumors. Taken together, these results demonstrate that overexpression of TIMP-3 can inhibit angiogenesis and associated tumor growth, and that the antiangiogenic effects of TIMP-3 appear to be mediated through the inhibition of functional capillary morphogenesis.

Protein kinase signaling in the modulation of microvascular permeability

The permeability of exchange microvessels is regulated through complex interactions between signaling molecules and structural proteins in the endothelium. Endothelial barrier integrity is maintained by adhesive interactions occurring at the cell-cell and cell-matrix contacts via junctional proteins and focal adhesion complexes that are anchored to the cytoskeleton. Cyclic AMP (cAMP) and cAMP-dependent kinase counteract with the nitric oxide (NO)-cyclic GMP (cGMP) pathway to protect the basal barrier function. Upon stimulation by physical stress, growth factors, or inflammatory agents, endothelial cells undergo a series of intracellular signaling reactions involving activation of protein kinase C (PKC), protein kinase G (PKG), mitogen-activated protein kinases (MAPK), and/or protein tyrosine kinases. The phosphorylation cascades trigger biochemical and conformational changes in the barrier structure and ultimately lead to an opening of the paracellular pathway. In particular, myosin light chain kinase (MLCK) activation and subsequent myosin light chain (MLC) phosphorylation in endothelial cells directly result in cell contraction and shape changes. The phosphorylation of beta-catenin may cause disorganization of adherens junctions or dissociation of vascular endothelial (VE)-cadherin-catenin complex from its cytoskeletal anchor, leading to loose or opened intercellular junctions. Additionally, focal adhesion kinase (FAK) phosphorylation-coupled focal adhesion assembly and redistribution provide an anchorage support for the conformational changes occurring in the cells and at the cell junctions. The Src family tyrosine kinases may serve as common signals that coordinate these molecular events to facilitate the paracellular transport of macromolecules. The critical roles of protein kinases in endothelial hyperpermeability implicate the therapeutic significance of protein kinase inhibitors in the prevention and treatment of diseases and injuries that are associated with microvascular barrier dysfunction.

ADAM15 Is an Adherens Junction Molecule Whose Surface Expression Can Be Driven by VE-Cadherin

ADAM15 belongs to the family of proteins containing disintegrin and metalloprotease domains (ADAM) that have been implicated in cell adhesion via integrin binding and shedding of cell surface molecules. Here we provide the first report on the localization of an ADAM in adherens junctions. We show that ADAM15 colocalizes with a cell adhesion molecule, vascular endothelial (VE)-cadherin, which mediates endothelial cell adherens junction formation. In contrast, the distribution of ADAM15 correlates poorly with the localization in cell contacts of one of its proposed ligands, the beta1-integrin. Furthermore, ADAM15 accumulation in cell-cell contacts is preceded by VE-cadherin-mediated adherens junction formation. To investigate the dependence of ADAM15 surface expression on adherens junction formation, we coexpressed VE-cadherin with ADAM15 and an ADAM15 green fluorescence protein (GFP) fusion protein in Chinese hamster ovary cells. VE-cadherin coexpression results in the translocation of ADAM15-GFP to the cell periphery. Analysis of cell surface levels of ADAM15 and ADAM15-GFP, with or without VE-cadherin coexpression, clearly demonstrates that VE-cadherin can drive surface expression of ADAM15. Our data suggest that ADAM15 may be a novel component of adherens junctions and thus could play a role in endothelial functions that are mediated by these cell contacts.

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.

VEGF receptor 2 and the adherens junction as a mechanical transducer in vascular endothelial cells

Blood-flow interactions with the vascular endothelium represents a specialized example of mechanical regulation of cell function that has important physiological and pathophysiological cardiovascular consequences. Yet, the mechanisms of mechanostransduction are not understood fully. This study shows that shear stress induces a rapid induction as well as nuclear translocation of the vascular endothelial growth factor (VEGF) receptor 2 and promotes the binding of the VEGF receptor 2 and the adherens junction molecules, VE-cadherin and beta-catenin, to the endothelial cytoskeleton. These changes are accompanied by the formation of a complex containing the VEGF receptor 2-VE-cadherin-beta-catenin. In endothelial cells lacking VE-cadherin, shear stress did not augment nuclear translocation of the VEGF receptor 2 and phosphorylation of Akt1 and P38 as well as transcriptional induction of a reporter gene regulated by a shear stress-responsive promoter. These results suggest that VEGF receptor 2 and the adherens junction act as shear-stress cotransducers, mediating the transduction of shear-stress signals into vascular endothelial cells.