CD44 Regulation of Endothelial Cell Proliferation and Apoptosis via Modulation of CD31 and VE-cadherin Expression

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

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.     

Dynamics of vascular endothelial-cadherin and beta-catenin localization by vascular endothelial growth factor-induced angiogenesis in human umbilical vein cells

The adherens junctional molecule, vascular endothelial cadherin (VE-cadherin), functions to maintain adherens junction stability and to suppress apoptosis of endothelial cells by forming a complex with vascular endothelial growth factor (VEGF) receptor 2 and members of the armadillo family of cytoplasmic proteins. In order to investigate the dynamics of the association of VE-cadherin with adherens junctions during the initial stages of angiogenesis, human umbilical cord endothelial cells (HUVECs) were stimulated with VEGF to undergo angiogenesis in type-I collagen three-dimensional culture. In confluent monolayers of HUVECs, VE-cadherin and its signaling partner, beta-catenin, as well as the paracellular transmembrane adhesion molecule platelet-endothelial cell adhesion molecule (PECAM-1), were all present in regions of cell-cell contact. Within 3 h of stimulation of angiogenesis, VE-cadherin and beta-catenin were lost from these regions. In contrast, the distribution pattern of PECAM-1 did not alter. After 6 h the majority of endothelial cells had migrated to form a network of capillary cords with cell-cell contacts that contained all three molecules. By metabolic labeling of HUVECs it was found that de novo synthesis of VE-cadherin was not essential for the formation of new adherens junctions. Coimmunoprecipitation and immunoblotting experiments showed that the VE-cadherin and beta-catenin remained associated after they were lost from adherens junctions. Detergent extraction of cells with Triton X-100 indicted that the majority of VE-cadherin and beta-catenin was Triton soluble, indicating that they are only weakly associated with the actin-based cytoskeleton.     

TGF-beta1 induces rearrangement of FLK-1-VE-cadherin-beta-catenin complex at the adherens junction through VEGF-mediated signaling

VEGF and TGF-beta1 induce angiogenesis but have opposing effects on vascular endothelial cells: VEGF promotes survival; TGF-beta1 induces apoptosis. We have previously shown that TGF-beta1 induces endothelial cell apoptosis via up-regulation of VEGF expression and activation of signaling through VEGF receptor-2 (flk-1). In context with TGF-beta1, VEGF signaling is transiently converted from a survival into an apoptotic one. VEGF promotes cell survival in part via activation of PI3K/Akt by a mechanism dependent on the formation of a multi-protein complex that includes flk-1 and the adherens junction proteins VE-cadherin and beta-catenin. Here we report that TGF-beta1 induces rearrangement of the adherens junction complex by separating flk-1 from VE-cadherin and increasing beta-catenin association with both flk-1 and VE-cadherin. This rearrangement is caused neither by changes in adherens junction mRNA or protein expression nor by post-translational modification, and requires VEGF signaling through flk-1. These results show that the adherens junction is an important regulatory component of TGF-beta1-VEGF interaction in endothelial cells.     

Cellular Inhibitor of Apoptosis Protein 1 (cIAP1) Stability Contributes to YM155 Resistance in Human Gastric Cancer Cells

YM155, which blocks the expression of survivin, a member of the inhibitor of apoptosis (IAP) family, induces cell death in a variety of cancer types, including prostate, bladder, breast, leukemia, and non-small lung cancer. However, the mechanism underlying gastric cancer susceptibility and resistance to YM155 is yet to be specified. Here, we demonstrate that cIAP1 stability dictates resistance to YM155 in human gastric cancer cells. Treatment of human gastric cancer cells with YM155 differentially induced cell death dependent on the stability of cIAP1 as well as survivin. Transfection with cIAP1 expression plasmids decreased cell sensitivity to YM155, whereas knockdown of endogenous cIAP1 using RNA interference enhanced sensitivity to YM155. In addition, double knockdown of survivin and cIAP1 significantly induced cell death in the YM155-resistant cell line, MKN45. We also showed that YM155 induced autoubiquitination and proteasome-dependent degradation of cIAP1. Surprisingly, survivin affected the stability of cIAP1 through binding, contributing to cell sensitivity to YM155. Thus, our findings reveal that YM155 sensitizes human gastric cancer cells to apoptotic cell death by degrading cIAP1, and furthermore, cIAP1 in gastric cancer cells may act as a PD marker for YM155 treatment.     

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

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

YM155 sensitizes non-small cell lung cancer cells to EGFR-tyrosine kinase inhibitors through the mechanism of autophagy induction

Resistance to epidermal growth factor receptor-tyrosine kinase inhibitors (EGFR-TKIs), such as erlotinib and gefitinib, is a major clinical problem in the treatment of patients with non-small cell lung cancer (NSCLC). YM155 is a survivin small molecule inhibitor and has been demonstrated to induce cancer cell apoptosis and autophagy. EGFR-TKIs have been known to induce cancer cell autophagy. In this study, we showed that YM155 markedly enhanced the sensitivity of erlotinib to EGFR-TKI resistant NSCLC cell lines H1650 (EGFR exon 19 deletion and PTEN loss) and A549 (EGFR wild type and KRAS mutation) through inducing autophagy-dependent apoptosis and autophagic cell death. The effects of YM155 combined with erlotinib on apoptosis and autophagy inductions were more obvious than those of YM155 in combination with survivin knockdown by siRNA transfection, suggesting that YM155 induced autophagy and apoptosis in the NSCLC cells partially depend on survivin downregulation. Meanwhile, we found that the AKT/mTOR pathway is involved in modulation of survivin downregulation and autophagy induction caused by YM155. In addition, YM155 can induce DNA damage in H1650 and A549 cell lines. Moreover, combining erlotinib further augmented DNA damage by YM155, which were retarded by autophagy inhibitor 3MA, or knockdown of autophagy-related protein Beclin 1, revealing that YM155 induced DNA damage is autophagy-dependent. Similar results were also observed in vivo xenograft experiments. Therefore, combination of YM155 and erlotinib offers a promising therapeutic strategy in NSCLC with EGFR-TKI resistant phenotype.     

Activation of MMP-2, cleavage of matrix proteins,and adherens junctions during a snake venom metalloproteinase-induced endothelial cell apoptosis

Snake venom metalloproteinases (SVMPs) are structurally and functionally similar to matrix metalloproteinases (MMPs). We have previously demonstrated that a SVMP, named gaminelysin, can induce endothelial cell apoptosis [Biochem J. 357 (2001) 719]. In this study, the action mechanism of graminelysin in causing endothelial cell apoptosis was further investigated. We showed that the apoptosis was initiated with cell shape change and extracellular matrix degradation and occurred before cell detachment. Cleaved forms of MMP-2 might act in concert with graminelysin to cause apoptosis. During apoptosis, adherens junctions, including VE-cadherin and beta- and gamma-catenin were cleaved and alpha-catenin was decreased. VE-cadherin and beta-catenin at cell periphery were decreased and the discontinuity in alignment was found as observed with immunofluorescence microscopy. This was accompanied with a diffuse beta-catenin staining in the cytoplasm and a decreased F-actin stress fibers in some rounded cells. The decrease of VE-cadherin and beta-catenin in Triton-insoluble fractions confirmed that the association of adherens junctions with actin cytoskeleton was altered during apoptosis. Graminelysin-induced cleavage in adherens junctions was paralleled with the changes in paracellular permeability. We also detected the activation of caspase-3 and the decrease of Bcl-2/Bax ratio during apoptosis. However, caspase inhibitors showed differential effects in blocking the cleavage of PARP, adherens junctions, and DNA fragmentation. Taken together, the data presented suggest that metalloproteinase can control cell fates via the degradation of matrix proteins, the change of cell shape, and the cleavage of adherens junctions.     

Targeting Survivin Enhances Chemosensitivity in Retinoblastoma Cells and Orthotopic Tumors

Treatments for retinoblastoma (Rb) vary depending on the size and location of the intraocular lesions and include chemotherapy and radiation therapy. We examined whether agents used to treat Rb induce a pro-survival phenotype associated with increased expression of survivin, a member of the inhibitor of apoptosis family of proteins. We document that exposure to carboplatin, topotecan or radiation resulted in elevated expression of survivin in two human Rb cell lines but not in normal retinal pigmented epithelial (RPE) cells. Cellular levels of survivin were attenuated in Rb cells exposed to an imidazolium-based survivin suppressant, Sepantronium bromide (YM155). Protein expression patterns of survivin in RPE cells were not altered following treatment protocols involving exposure to YM155. Including YM155 with chemotherapy or radiation increased levels of apoptosis in Rb cells but not in RPE cells. Intraocular luciferase expressing Rb tumors were generated from the Rb cell lines and used to evaluate the effects of carboplatin and YM155 on in-vivo survivin expression and tumor growth. Carboplatin induced expression of survivin while carboplatin combined with YM155 reduced survivin expression in tumor bearing eyes. The combination protocol was also most effective in reducing the rate of tumor regrowth. These results indicate that targeted inhibition of the anti-apoptotic protein survivin provides a therapeutic advantage for Rb cells and tumors treated with chemotherapy.     

Biochemical processing of E-cadherin under cellular stress

The proteolytic cleavage pathways of E-cadherin endogenously expressed in MDCK (Madin-Darby canine kidney) cells were characterized in cells treated with antimycin A and deoxyglucose to examine transmembrane protein processing under cellular stress. E-cadherin is a type I transmembrane protein which operates as the cell adhesion molecule component of the adherens junction, a complex of proteins involved in epithelial tissue development and integrity. We now demonstrate that treatment of MDCK cells with antimycin A and deoxyglucose activates caspase mediated pathways that cleave E-cadherin. E-cadherin is cleaved into two major fragments, with the sizes predicted by the location of a caspase-3 cleavage consensus sequence. Cleavage of E-cadherin and deposition of the C-terminal fragment into the cytoplasm are inhibited by the caspase inhibitor DEVD-CHO. Thus, a major mechanism for E-cadherin cleavage and dissolution of the adherens junction under antimycin/deoxyglucose treatment is caspase mediated, initiated by activation of an apoptosis pathway.     

Remodeling of endothelial adherens junctions by Kaposi's sarcoma-associated herpesvirus

Vascular endothelial cadherin (VE-cadherin) connects neighboring endothelial cells (ECs) via interendothelial junctions and regulates EC proliferation and adhesion during vasculogenesis and angiogenesis. The cytoplasmic domain of VE-cadherin recruits α- and β-catenins and γ-catenin, which interact with the actin cytoskeleton, thus modulating cell morphology. Dysregulation of the adherens junction/cytoskeletal axis is a hallmark of invasive tumors. We now demonstrate that the transmembrane ubiquitin ligase K5/MIR-2 of Kaposi's sarcoma-associated herpesvirus targets VE-cadherin for ubiquitin-mediated destruction, thus disturbing EC adhesion. In contrast, N-cadherin levels in K5-expressing cells were increased compared to those in control cells. Steady-state levels of α- and β-catenins and γ-catenin in K5-expressing ECs were drastically reduced due to proteasomal destruction. Moreover, the actin cytoskeleton was rearranged, resulting in the dysregulation of EC barrier function as measured by electric cell-substrate impedance sensing. Our data represent the first example of a viral protein targeting adherens junction proteins and suggest that K5 contributes to EC proliferation, vascular leakage, and the reprogramming of the EC proteome during Kaposi's sarcoma tumorigenesis.     

Endothelial cadherins in cancer

Cadherins are cell adhesion receptors that play important roles in embryogenesis and tissue homoeostasis. Endothelial cells express various members of the cadherin superfamily, in particular vascular endothelial (VE-) cadherin, which is the main adhesion receptor of endothelial adherens junctions and neural (N-) cadherin, which is normally localized outside the junctions and may mediate adhesion between endothelial cells and non-endothelial cells. Dysregulation of cadherin expression has been implicated in tumor progression, in particular the loss of epithelial (E-) cadherin expression or function and the gain of N-cadherin. Moreover, more recently, aberrant expression of VE-cadherin was observed in certain cancer types. In breast carcinoma, VE-cadherin was shown to promote tumor cell proliferation and invasion through enhancing TGF-β signaling. Thus, in breast cancer, the cadherin switch involves another player, vascular endothelial cadherin, which is part of an intricate interplay of classical cadherins in breast cancer progression.     

Localization of VE-cadherin in plasmalemmal cholesterol rich microdomains and the effects of cholesterol depletion on VE-cadherin mediated cell–cell adhesion

VE-cadherin is the predominant adhesion molecule in vascular endothelial cells being responsible for maintenance of the endothelial barrier function by forming adhesive contacts (adherens junctions) to neighbouring cells. We found by use of single molecule fluorescence microscopy that VE-cadherin is localised in preformed clusters when not inside adherens junctions. These clusters depend on the integrity of the actin cytoskeleton and are localised in cholesterol rich microdomains of mature endothelial cells as found by membrane fractionation. The ability to form and maintain VE-cadherin based junctions was probed using the laser tweezer technique, and we found that cholesterol depletion has dramatical effects on VE-cadherin mediated adhesion. While a 30% reduction of the cholesterol-level results in an increase of adhesion, excessive cholesterol depletion by about 60% leads to an almost complete loss of VE-cadherin function. Nevertheless, the cadherin concentration in the membrane and the single molecule kinetic parameters of the cadherin are not changed. Our results suggest that the actin cytoskeleton, junction-associated proteins and protein–lipid assemblies in cholesterol-rich microdomains mutually stabilise each other to form functional adhesion contacts.     

Impaired fertility and spermiogenetic disorders with loss of cell adhesion in male mice expressing an interfering Rap1 mutant

The guanosine trisphosphatase Rap1 serves as a critical player in signal transduction, somatic cell proliferation and differentiation, and cell-cell adhesion by acting through distinct mechanisms. During mouse spermiogenesis, Rap1 is activated and forms a signaling complex with its effector, the serine-threonine kinase B-Raf. To investigate the functional role of Rap1 in male germ cell differentiation, we generated transgenic mice expressing an inactive Rap1 mutant selectively in differentiating spermatids. This expression resulted in a derailment of spermiogenesis due to an anomalous release of immature round spermatids from the seminiferous epithelium within the tubule lumen and in low sperm counts. These spermiogenetic disorders correlated with impaired fertility, with the transgenic males being severely subfertile. Because mutant testis exhibited perturbations in ectoplasmic specializations (ESs), a Sertoli-germ cell-specific adherens junction, we searched for expression of vascular endothelial cadherin (VE-cadherin), an adhesion molecule regulated by Rap1, in spermatogenic cells of wild-type and mutant mice. We found that germ cells express VE-cadherin with a timing strictly related to apical ES formation and function; immature, VE-cadherin-positive spermatids were, however, prematurely released in the transgenic testis. In conclusion, interfering with Rap1 function during spermiogenesis leads to reduced fertility by impairment of germ-Sertoli cell contacts; our transgenic mouse provides an in vivo model to study the regulation of ES dynamics.     

Annexin 2 regulates endothelial morphogenesis by controlling AKT activation and junctional integrity

Sprouting angiogenesis is a multistep process that involves endothelial cell activation, basement membrane degradation, proliferation, lumen formation, and stabilization. In this study, we identified annexin 2 as a regulator of endothelial morphogenesis using a three-dimensional in vitro model where sprouting angiogenesis was driven by sphingosine 1-phosphate and angiogenic growth factors. We observed that sphingosine 1-phosphate triggered annexin 2 translocation from the cytosol to the plasma membrane and its association with vascular endothelial (VE)-cadherin. In addition, annexin 2 depletion attenuated Akt activation, which was associated with increased phosphorylation of VE-cadherin and endothelial barrier leakage. Disrupting homotypic VE-cadherin interactions with EGTA, antibodies to the extracellular domain of VE-cadherin, or gene silencing all resulted in decreased Akt (but not Erk1/2) activation. Furthermore, expression of constitutively active Akt restored reduced endothelial sprouting responses observed with annexin 2 and VE-cadherin knockdown. Collectively, we report that annexin 2 regulates endothelial morphogenesis through an adherens junction-mediated pathway upstream of Akt.     

VE-cadherin links tRNA synthetase cytokine to anti-angiogenic function

A natural fragment of an enzyme that catalyzes the first step of protein synthesis-human tryptophanyl-tRNA synthetase (T2-TrpRS) has potent anti-angiogenic activity. A cellular receptor through which T2-TrpRS exerts its anti-angiogenic activity has not previously been identified. Here T2-TrpRS was shown to bind at intercellular junctions of endothelial cells (ECs). Using genetic knock-outs, binding was established to depend on VE-cadherin, a calcium-dependent adhesion molecule, which is selectively expressed in ECs, concentrated at adherens junctions, and is essential for normal vascular development. In contrast, T2-TrpRS binding to EC junctions was not dependent on platelet endothelial cell adhesion molecule type-1, another adhesion molecule found at EC junctions. Pull-down assays confirmed direct complex formation between T2-TrpRS and VE-cadherin. Binding of T2-TrpRS inhibited VEGF-induced ERK activation and EC migration. Thus, a VE-cadherin-dependent pathway is proposed to link T2-TrpRS to inhibition of new blood vessel formation.