A novel role of vascular endothelial cadherin in modulating c-Src activation and downstream signaling of vascular endothelial growth factor

Role of c-Src and reactive oxygen species in cardiovascular diseases

Oxidative stress, caused by the over production of oxidants or inactivity of antioxidants, can modulate the redox state of several target proteins such as tyrosine kinases, mitogen-activated protein kinases and tyrosine phosphatases. c-Src is one such non-receptor tyrosine kinase which activates NADPH oxidases (Noxs) in response to various growth factors and shear stress. Interaction between c-Src and Noxs is influenced by cell type and primary messengers such as angiotensin II, which binds to G-protein coupled receptor and activates the intracellular signaling cascade. c-Src stimulated activation of Noxs results in elevated release of intracellular and extracellular reactive oxygen species (ROS). These ROS species disturb vascular homeostasis and cause cardiac hypertrophy, coronary artery disease, atherosclerosis and hypertension. Interaction between c-Src and ROS in the pathobiology of cardiac fibrosis is hypothesized to be influenced by cell type and stimuli. c-Src and ROS have a bidirectional relationship, thus increased ROS levels due to c-Src mediated activation of Noxs can further activate c-Src by promoting the oxidation and sulfenylation of critical cysteine residues. This review highlights the role of c-Src and ROS in mediating downstream signaling pathways underlying cardiovascular diseases. Furthermore, due to the central role of c-Src in activation of various signaling proteins involved in differentiation, migration, proliferation, and cytoskeletal reorganization of vascular cells, it is presented as therapeutic target for treating cardiovascular diseases except cardiac fibrosis.

A JAM-A-tetraspanin-αvβ5 integrin complex regulates contact inhibition of locomotion

Contact inhibition of locomotion (CIL) is a process that regulates cell motility upon collision with other cells. Improper regulation of CIL has been implicated in cancer cell dissemination. Here, we identify the cell adhesion molecule JAM-A as a central regulator of CIL in tumor cells. JAM-A is part of a multimolecular signaling complex in which tetraspanins CD9 and CD81 link JAM-A to αvβ5 integrin. JAM-A binds Csk and inhibits the activity of αvβ5 integrin-associated Src. Loss of JAM-A results in increased activities of downstream effectors of Src, including Erk1/2, Abi1, and paxillin, as well as increased activity of Rac1 at cell-cell contact sites. As a consequence, JAM-A-depleted cells show increased motility, have a higher cell-matrix turnover, and fail to halt migration when colliding with other cells. We also find that proper regulation of CIL depends on αvβ5 integrin engagement. Our findings identify a molecular mechanism that regulates CIL in tumor cells and have implications on tumor cell dissemination.

Expression Profile of CD157 Reveals Functional Heterogeneity of Capillaries in Human Dermal Skin

CD157 acts as a receptor, regulating leukocyte trafficking and the binding of extracellular matrix components. However, the expression pattern and the role of CD157 in human blood (BEC) and the lymphatic endothelial cells (LEC) of human dermal microvascular cells (HDMEC), remain elusive. We demonstrated constitutive expression of CD157 on BEC and LEC, in fetal and juvenile/adult skin, in situ, as well as in isolated HDMEC. Interestingly, CD157 epitopes were mostly localized on BEC, co-expressing high levels of CD31 (CD31High), as compared to CD31Low BEC, whereas the podoplanin expression level on LEC did not affect CD157. Cultured HDMEC exhibited significantly higher numbers of CD157-positive LEC, as compared to BEC. Interestingly, separated CD157− and CD157+ HDMEC demonstrated no significant differences in clonal expansion in vitro, but they showed distinct expression levels of cell adhesion molecules, before and after cytokine stimulation in vitro. In particular, we proved the enhanced and specific adherence of CD11b-expressing human blood myeloid cells to CD157+ HDMEC fraction, using an in vitro immune-binding assay. Indeed, CD157 was also involved in chemotaxis and adhesion of CD11b/c monocytes/neutrophils in prevascularized dermo–epidermal skin substitutes (vascDESS) in vivo. Thus, our data attribute specific roles to endothelial CD157, in the regulation of innate immunity during inflammation.

Growth Factor Deregulation and Emerging Role of Phosphatases in Diabetic Peripheral Artery Disease

Peripheral artery disease is caused by atherosclerosis of lower extremity arteries leading to the loss of blood perfusion and subsequent critical ischemia. The presence of diabetes mellitus is an important risk factor that greatly increases the incidence, the progression and the severity of the disease. In addition to accelerated disease progression, diabetic patients are also more susceptible to develop serious impairment of their walking abilities through an increased risk of lower limb amputation. Hyperglycemia is known to alter the physiological development of collateral arteries in response to ischemia. Deregulation in the production of several critical pro-angiogenic factors has been reported in diabetes along with vascular cell unresponsiveness in initiating angiogenic processes. Among the multiple molecular mechanisms involved in the angiogenic response, protein tyrosine phosphatases are potent regulators by dephosphorylating pro-angiogenic tyrosine kinase receptors. However, evidence has indicated that diabetes-induced deregulation of phosphatases contributes to the progression of several micro and macrovascular complications. This review provides an overview of growth factor alterations in the context of diabetes and peripheral artery disease, as well as a description of the role of phosphatases in the regulation of angiogenic pathways followed by an analysis of the effects of hyperglycemia on the modulation of protein tyrosine phosphatase expression and activity. Knowledge of the role of phosphatases in diabetic peripheral artery disease will help the development of future therapeutics to locally regulate phosphatases and improve angiogenesis.

Erythropoietin maintains VE-cadherin expression and barrier function in experimental diabetic retinopathy via inhibiting VEGF/VEGFR2/Src signaling pathway

AIMS

To explore the mechanisms of erythropoietin (EPO)'s protection on inner blood-retinal barrier (iBRB) in experimental diabetic retinopathy.

MATERIAL AND METHODS

Male SD rats were rendered diabetic with streptozotocin, followed by intravitreal injection of EPO. The permeability of iBRB was examined with fluorescein isothiocyanate (FITC)-dextran. Human retinal microvascular endothelial cells (HRMECs) and human umbilical vein endothelial cells (HUVECs) were treated with glyoxal and studied for cell viability and barrier function. The expressions of vascular endothelial (VE)-cadherin, Src kinase, vascular endothelial growth factor (VEGF) and VEGF receptor-2 (VEGFR2) were analyzed with Western blot, ELISA, qPCR, or immunofluorescence.

KEY FINDINGS

VE-cadherin in rat retinas was down-regulated with diabetes progression. EPO treatment could increase VE-cadherin expression at week 8 and week 16. The expressions of p-Src and p-VE-cadherin were increased at week 2, while decreased at week 8 of diabetes; which were prevented by EPO. The leakage of FITC-dextran in 8-week diabetic rat retinas was ameliorated by EPO. In vitro results showed the expressions of VEGF, p-Src and p-VE-cadherin were increased significantly, accompanied with the decreased barrier function, which were prevented by EPO. Ranibizumab and CGP77675 also inhibited the glyoxal-induced phosphorylation of Src and VE-cadherin. Cellular fractionation showed EPO mitigated the VE-cadherin internalization in glyoxal-treated cells.

SIGNIFICANCE

EPO maintained the expression of VE-cadherin in experimental diabetic retinopathy by inhibiting its phosphorylation and internalization through VEGF/VEGFR2/Src pathway, thus improved the integrity of iBRB.

Vascular permeability in retinopathy is regulated by VEGFR2 Y949 signaling to VE-cadherin

Edema stemming from leaky blood vessels is common in eye diseases such as age-related macular degeneration and diabetic retinopathy. Whereas therapies targeting vascular endothelial growth factor A (VEGFA) can suppress leakage, side-effects include vascular rarefaction and geographic atrophy. By challenging mouse models representing different steps in VEGFA/VEGF receptor 2 (VEGFR2)-induced vascular permeability, we show that targeting signaling downstream of VEGFR2 pY949 limits vascular permeability in retinopathy induced by high oxygen or by laser-wounding. Although suppressed permeability is accompanied by reduced pathological neoangiogenesis in oxygen-induced retinopathy, similarly sized lesions leak less in mutant mice, separating regulation of permeability from angiogenesis. Strikingly, vascular endothelial (VE)-cadherin phosphorylation at the Y685, but not Y658, residue is reduced when VEGFR2 pY949 signaling is impaired. These findings support a mechanism whereby VE-cadherin Y685 phosphorylation is selectively associated with excessive vascular leakage. Therapeutically, targeting VEGFR2-regulated VE-cadherin phosphorylation could suppress edema while leaving other VEGFR2-dependent functions intact.

The number of people with impaired vision and blindness is increasing in Western society due to the aging population and the increased prevalence of diabetes. This has led to eye diseases, such as age-related macular degeneration and diabetic retinopathy becoming more common. In both these eye diseases, new blood vessels grow in the retina – the light-sensitive part of the eye – to bring oxygen and nutrients to the tissue. However, these new blood vessels are leaky and allow molecules to leave the bloodstream and enter the retinal tissue. This causes the retina to swell and impair a person’s vision. The leaky blood supply also reduces the amount of oxygen that gets to the tissue, resulting in further damage to the retina.

When tissues experience low levels of oxygen, cells start making a protein called vascular endothelial growth factor (or VEGF for short). Whilst this protein is important for helping form new blood vessels, it also makes these vessels leaky. Current treatments for age-related macular degeneration and diabetic retinopathy decrease swelling in the eye by blocking the action of VEGF. However, these treatments also cause existing blood vessels and nerve cells to die, leading to irreversible damage. Now, Smith et al. have set out to find whether the effects of VEGF can be blocked without causing further damage to existing cells.

To investigate this possibility, the eyes and retinas of mice were treated with a laser or exposed to changing oxygen levels to create injuries that resembled human age-related macular degeneration and diabetic retinopathy. Each of the tested mice had specific mutations in proteins known to interact with VEGF. Fluorescent particles were injected into the bloodstream of the mice to assess how these different mutations affected blood vessel leakage: if fluorescent particles could no longer be detected outside the blood vessels, this suggested that the mutation had stopped the vessels from leaking. Further experiments showed these specific mutations affected leakage and did not prevent new blood vessels from forming.

In the future it will be important to see if drugs, rather than mutations, can also decrease the leakiness of blood vessels in the retina. Such chemical compounds could then be tested in mouse experiments. If successful, these drugs might be used to treat patients with age-related macular degeneration and diabetic retinopathy.

VE-cadherin regulates migration inhibitory factor synthesis and release

OBJECTIVE

Vascular endothelial (VE)-cadherin-mediated adherens junction is critical to maintain endothelial integrity. Besides its role of homophilic intercellular adhesion, VE-cadherin also has a role of outside-in signaling with functional consequences for vascular physiology. However, the nature of these signals remains not completely understood.

MATERIALS AND METHODS

Human umbilical vein endothelial cells (HUVECs) were used in cell culture experiments. Confluent HUVECs were treated with VE-cadherin function-blocking antibodies BV9 (50 μg/ml) or IgG control. Antibody array was used to screen for cytokine/chemokine in supernatant. For VE-cadherin knockdown, siRNA transfection was used. ELISA, Western blot, and qRT-PCR were used to confirm the expression of screened cytokine/chemokine. To explore the possible mechanisms, Scr phosphorylation was detected and Scr inhibitor PP2 (1 μM) was used. To investigate in vivo relevance of the findings, BV9 and the indicated neutralizing antibodies were injected into mice and then lung vascular leak and inflammation were examined by Evans blue assay and lung tissue H&E, respectively.

RESULTS

Using a non-biased, high-throughout human cytokine/chemokine antibody array, we first found that disruption of VE-cadherin-mediated adhesion by function-blocking antibody BV9 triggered the release of migration inhibitory factor (MIF). This VE-cadherin-mediated release of MIF further confirmed by ELISA with both VE-cadherin blocking antibody and siRNA technique was due to enhanced expression of MIF mRNA, which was mediated by Src kinase activation. In addition, in vivo lung vascular leak induced by VE-cadherin function-blocking antibody was partly alleviated by neutralizing MIF.

CONCLUSIONS

VE-cadherin regulates MIF synthesis and release via Src kinase. Our data provide additional evidence to the concept that VE-cadherin transfers intracellular signals to coordinate the state of cell-cell adhesion with gene expression.

Electroacupuncture facilitates implantation by enhancing endometrial angiogenesis in a rat model of ovarian hyperstimulation

Controlled ovarian hyperstimulation (COH) impairs the synchronized development of endometrium and embryo, resulting in the failure of embryo implantation. Here, we investigated what effects electroacupuncture had on embryo implantation in COH rats. Female rats were randomly assigned to four groups: normal (N), model (M), electroacupuncture (EA), and electroacupuncture pretreatment (PEA). Rats in groups M, EA, PEA were injected with pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin to establish the COH model. Rats in group EA received electroacupuncture treatment from the PMSG injection day to the 3rd day of pregnancy (D3), while those in group PEA received electroacupuncture treatment for 3 days before the PMSG day and continuing to D3. Furthermore, another 30 female rats who received the same treatment as the rats in group PEA were injected with siVEGFR2 into uterine lumen. The endometrial microvascular density (MVD) and the expression levels of vascular endothelial growth factor-A, angiopoietin-1, and fibroblast growth factor-2 were significantly lower in groups M than in groups N and PEA. The percentage of dolichos biflorus agglutinin positive uterine natural killer cells in groups N, EA and PEA was higher than that in group M. After the siVEGFR2 injection, the protein expression levels of vascular endothelial growth factor receptor 2 (VEGFR2), PI3K, p-AKT and p-ERK, the embryo number and the MVD were significantly reduced. In conclusion, electroacupuncture can facilitate embryo implantation in COH rats by activating the VEGFR2/PI3K/AKT and VEGFR2/ERK signaling pathways which have a positive relationship with endometrial angiogenesis.

Two natural eudesmane-type sesquiterpenes from Laggera alata inhibit angiogenesis and suppress breast cancer cell migration through VEGF- and Angiopoietin 2-mediated signaling pathways

Eudesmane-type sesquiterpenes are natural sesquiterpenes with anti-inflammatory properties, but their anti-angiogenic activities are not known. The present study demonstrated that 5α-hydroxycostic acid and hydroxyisocostic acid, two eudesmane-type sesquiterpenes (ETSs), isolated from the herb Laggera alata, possessed anti-angiogenic effects. Under non-toxic dosage, ETSs suppressed VEGF‑induced proliferation in human umbilical vein endothelial cells (HUVECs) and vessel formation in zebrafish embryos. Moreover, ETSs inhibited VEGF-stimulated HUVEC migration, stress fibers and tube formation. Results from real‑time PCR analysis involving in vivo and in vitro experiments indicated that pro-angiogenic-related mRNA levels were downregulated, including VEGFA, VEGFR2 and Tie2 genes after ETS treatments. Western blot analysis showed that ETSs suppressed VEGF-stimulated VEGFR2 phosphorylation and activation of its downstream molecules, such as Src/AKT/eNOS, FAK, PLCγ/ERK1/2 and p38. Moreover, the VEGF-stimulation of angiopoietin 2 (Ang2) mRNA level increase was significantly downregulated in the presence of ETSs. ETSs inhibited Ang2-induced phosphorylation of the receptor Tie2 in HUVECs, which indicated that ETSs not just suppressed VEGF/VEGFR2 axis, but also the Ang2/Tie2 one. Furthermore, the wound-healing assay revealed that ETSs reduced the migration of Ang2-stimulated human breast cancer (MCF-7) cells. Mechanistically, the anti-migration effect of ETSs correlated with the blockade of Ang2-induced E-cadherin loss and AKT activation. Collectively, the present study suggests that ETSs possess anti-angiogenic ability by interfering the VEGF- and Ang2-related pathways, and they may be good drug candidates.

Modulation of long-term endothelial-barrier integrity is conditional to the cross-talk between Akt and Src signaling

Although numerous studies have implicated Akt and Src kinases in vascular endothelial growth factor (VEGF) and Angiopoietin-1 (Ang-1)-induced endothelial-barrier regulation, a link between these two pathways has never been demonstrated. We determined the long-term effects of Akt inhibition on Src activity and vice versa, and in turn, on the human microvascular endothelial cell (HMEC) barrier integrity at the basal level, and in response to growth factors. Our data showed that Akt1 gene knockdown increases gap formation in HMEC monolayer at the basal level. Pharmacological inhibition of Akt, but not Src resulted in exacerbated VEGF-induced vascular leakage and impaired Ang-1-induced HMEC-barrier protection in vitro at 24 hr. Whereas inhibition of Akt had no effect on VEGF-induced HMEC gap formation in the short term, inhibition of Src blunted this process. In contrast, inhibition of Akt disrupted the VEGF and Ang-1 stabilized barrier integrity in the long-term while inhibition of Src did not. Interestingly, both long-term Akt inhibition and Akt1 gene knockdown in HMECs resulted in increased Tyr416 phosphorylation of Src. Treatment of HMECs with transforming growth factor-β1 (TGFβ1) that inhibited Akt Ser473 phosphorylation in the long-term, activated Src through increased Tyr416 phosphorylation and decreased HMEC-barrier resistance. The effect of TGFβ1 on endothelial-barrier breakdown was blunted in Akt1 deficient HMEC monolayers, where endothelial-barrier resistance was already impaired compared to the control. To our knowledge, this is the first report demonstrating a direct cross-talk between Akt and Src in endothelial-barrier regulation.

Modulation of VEGF receptor 2 signaling by protein phosphatases

Phosphorylation of serines, threonines, and tyrosines is a central event in signal transduction cascades in eukaryotic cells. The phosphorylation state of any particular protein reflects a balance of activity between kinases and phosphatases. Kinase biology has been exhaustively studied and is reasonably well understood, however, much less is known about phosphatases. A large body of evidence now shows that protein phosphatases do not behave as indiscriminate signal terminators, but can function both as negative or positive regulators of specific signaling pathways. Genetic models have also shown that different protein phosphatases play precise biological roles in health and disease. Finally, genome sequencing has unveiled the existence of many protein phosphatases and associated regulatory subunits comparable in number to kinases. A wide variety of roles for protein phosphatase roles have been recently described in the context of cancer, diabetes, hereditary disorders and other diseases. In particular, there have been several recent advances in our understanding of phosphatases involved in regulation of vascular endothelial growth factor receptor 2 (VEGFR2) signaling. The receptor is the principal signaling molecule mediating a wide spectrum of VEGF signal and, thus, is of paramount significance in a wide variety of diseases ranging from cancer to cardiovascular to ophthalmic. This review focuses on the current knowledge about protein phosphatases' regulation of VEGFR2 signaling and how these enzymes can modulate its biological effects.

Akt1 promotes stimuli-induced endothelial-barrier protection through FoxO-mediated tight-junction protein turnover

Vascular permeability regulated by the vascular endothelial growth factor (VEGF) through endothelial-barrier junctions is essential for inflammation. Mechanisms regulating vascular permeability remain elusive. Although 'Akt' and 'Src' have been implicated in the endothelial-barrier regulation, it is puzzling how both agents that protect and disrupt the endothelial-barrier activate these kinases to reciprocally regulate vascular permeability. To delineate the role of Akt1 in endothelial-barrier regulation, we created endothelial-specific, tamoxifen-inducible Akt1 knockout mice and stable ShRNA-mediated Akt1 knockdown in human microvascular endothelial cells. Akt1 loss leads to decreased basal and angiopoietin1-induced endothelial-barrier resistance, and enhanced VEGF-induced endothelial-barrier breakdown. Endothelial Akt1 deficiency resulted in enhanced VEGF-induced vascular leakage in mice ears, which was rescued upon re-expression with Adeno-myrAkt1. Furthermore, co-treatment with angiopoietin1 reversed VEGF-induced vascular leakage in an Akt1-dependent manner. Mechanistically, our study revealed that while VEGF-induced short-term vascular permeability is independent of Akt1, its recovery is reliant on Akt1 and FoxO-mediated claudin expression. Pharmacological inhibition of FoxO transcription factors rescued the defective endothelial barrier due to Akt1 deficiency. Here we provide novel insights on the endothelial-barrier protective role of VEGF in the long term and the importance of Akt1-FoxO signaling on tight-junction stabilization and prevention of vascular leakage through claudin expression.

Tyrosine Phosphatase PTPRJ/DEP-1 Is an Essential Promoter of Vascular Permeability, Angiogenesis, and Tumor Progression

The protein tyrosine phosphatase PTPRJ/DEP-1 has been implicated in negative growth regulation in endothelial cells, where its expression varies at transitions between proliferation and contact inhibition. However, in the same cells, DEP-1 has also been implicated in VEGF-dependent Src activation, permeability, and capillary formation, suggesting a positive role in regulating these functions. To resolve this dichotomy in vivo, we investigated postnatal angiogenesis and vascular permeability in a DEP-1-deficient mouse. In this study, we report that DEP-1 is required for Src activation and phosphorylation of its endothelial cell-specific substrate, VE-cadherin, after systemic injection of VEGF. Accordingly, VEGF-induced vascular leakage was abrogated in the DEP-1-deficient mice. Furthermore, capillary formation was impaired in murine aortic tissue rings or Matrigel plugs infused with VEGF. In the absence of DEP-1, angiogenesis triggered by ischemia or during tumor formation was defective, which in the latter case was associated with reduced tumor cell proliferation and increased apoptosis. Macrophage infiltration was also impaired, reflecting reduced vascular permeability in the tumors or a possible cell autonomous effect of DEP-1. Consequently, the formation of spontaneous and experimental lung metastases was strongly decreased in DEP-1-deficient mice. In clinical specimens of cancer, less vascularized tumors exhibited lower microvascular expression of DEP-1. Altogether, our results established DEP-1 as an essential driver of VEGF-dependent permeability, angiogenesis, and metastasis, suggesting a novel therapeutic route to cancer treatment. Cancer Res; 76(17); 5080-91. ©2016 AACR.

VE-cadherin facilitates BMP-induced endothelial cell permeability and signaling

Several vascular disorders, such as aberrant angiogenesis, atherosclerosis and pulmonary hypertension, have been linked to dysfunctional BMP signaling. Vascular hyperpermeability via distortion of endothelial cell adherens junctions is a common feature of these diseases, but the role of BMPs in this process has not been investigated. BMP signaling is initiated by binding of ligand to, and activation of, BMP type I (BMPRI) and type II (BMPRII) receptors. Internalization of VE-cadherin as well as c-Src kinase-dependent phosphorylation have been implicated in the loosening of cell–cell contacts, thereby modulating vascular permeability. Here we demonstrate that BMP6 induces hyperpermeabilization of human endothelial cells by inducing internalization and c-Src-dependent phosphorylation of VE-cadherin. Furthermore, we show BMP-dependent physical interaction of VE-cadherin with the BMP receptor ALK2 (BMPRI) and BMPRII, resulting in stabilization of the BMP receptor complex and, thereby, the support of BMP6-Smad signaling. Our results provide first insights into the molecular mechanism of BMP-induced vascular permeability, a hallmark of various vascular diseases, and provide the basis for further investigations of BMPs as regulators of vascular integrity, both under physiological and pathophysiological conditions.

Summary: We reveal the molecular mechanism by which BMP6 induces hyperpermeabilization of the endothelium. This provides first insights into the mechanism of BMP-dependent vascular integrity in normal physiology and disease.

Regulation of Endothelial Adherens Junctions by Tyrosine Phosphorylation

Endothelial cells form a semipermeable, regulated barrier that limits the passage of fluid, small molecules, and leukocytes between the bloodstream and the surrounding tissues. The adherens junction, a major mechanism of intercellular adhesion, is comprised of transmembrane cadherins forming homotypic interactions between adjacent cells and associated cytoplasmic catenins linking the cadherins to the cytoskeleton. Inflammatory conditions promote the disassembly of the adherens junction and a loss of intercellular adhesion, creating openings or gaps in the endothelium through which small molecules diffuse and leukocytes transmigrate. Tyrosine kinase signaling has emerged as a central regulator of the inflammatory response, partly through direct phosphorylation and dephosphorylation of the adherens junction components. This review discusses the findings that support and those that argue against a direct effect of cadherin and catenin phosphorylation in the disassembly of the adherens junction. Recent findings indicate a complex interaction between kinases, phosphatases, and the adherens junction components that allow a fine regulation of the endothelial permeability to small molecules, leukocyte migration, and barrier resealing.

Yes-associated protein regulates endothelial cell contact-mediated expression of angiopoietin-2

Angiogenesis is regulated by the dynamic interaction between endothelial cells (ECs). Hippo-Yes-associated protein (YAP) signalling has emerged as a key pathway that controls organ size and tissue growth by mediating cell contact inhibition. However, the role of YAP in EC has not been defined yet. Here, we show expression of YAP in the developing front of mouse retinal vessels. YAP subcellular localization, phosphorylation and activity are regulated by VE-cadherin-mediated-EC contacts. This VE-cadherin-dependent YAP phosphorylation requires phosphoinositide 3-kinase-Akt activation. We further identify angiopoietin-2 (ANG-2) as a potential transcriptional target of YAP in regulating angiogenic activity of EC in vitro and in vivo. Overexpression of YAP-active form in EC enhances angiogenic sprouting, and this effect is blocked by ANG-2 depletion or soluble Tie-2 treatment. These findings implicate YAP as a critical regulator in angiogenesis and provide new insights into the mechanism coordinating junctional stability and angiogenic activation of ECs.

Novel role of reactive oxygen species-activated Trp melastatin channel-2 in mediating angiogenesis and postischemic neovascularization

OBJECTIVE

Transient receptor potential melastatin-2 (TRPM2) channel is a nonselective cation channel that mediates influx of Ca(2+) and Na(+) with relative permeability of PCa:PNa ≈0.6 in response to cellular oxidative stress. As angiogenesis and ischemic neovascularization are both significantly dependent on oxidant signaling, here we investigated the possible role of vascular endothelial growth factor (VEGF)-induced reactive oxygen species production in activating TRPM2-dependent Ca(2+) signaling and in the mechanism of angiogenesis and ischemic neovascularization.

APPROACH AND RESULTS

We observed that VEGF stimulation rapidly induced the association of TRPM2 and cellular Src kinase with vascular endothelial-cadherin forming a signalplex at vascular endothelial-cadherin junctions in endothelial cells. Using endothelial cells isolated from TRPM2(-/-) mice or after small interfering RNA depletion of TRPM2, we demonstrated that TRPM2-activated Ca(2+) signaling was required for cellular Src kinase-induced phosphorylation of vascular endothelial-cadherin at Y658 and Y731, the crucial sites involved in vascular endothelial-cadherin internalization in response to VEGF. VEGF-induced reactive oxygen species generation activated TRPM2-induced Ca(2+) entry, whereas the reactive oxygen species-insensitive TRPM2 mutant (C1008→A) showed impaired Ca(2+) entry. Endothelial cells depleted of TRPM2 also displayed significantly perturbed migratory phenotype and impaired activation of cellular Src in response to VEGF. TRPM2(-/-) mice reconstituted with wild-type myeloid cells demonstrated aberrant angiogenesis and neovascularization in the hindlimb ischemia model as compared with wild-type mice.

CONCLUSIONS

VEGF-induced angiogenesis and postischemic neovascularization in mice required reactive oxygen species generation in endothelial cells and resultant TRPM2 activation. Thus, our findings provide novel insight into the role of TRPM2 in mechanism of angiogenesis and ischemic neovascularization.

SHP-2 binds to caveolin-1 and regulates Src activity via competitive inhibition of CSK in response to H2O2 in astrocytes

Reactive oxygen species (ROS) regulate diverse cellular functions by triggering signal transduction events, such as Src and mitogen-activated protein (MAP) kinases. Here, we report the role of caveolin-1 and Src homology 2 domain-containing protein tyrosine phosphatase 2 (SHP-2) in H₂O₂-induced signaling pathway in brain astrocytes. H₂O₂-mediated oxidative stress induced phosphorylation of caveolin-1 and association between p-caveolin-1 and SHP-2. SHP-2 specifically bound to wild-type caveolin-1 similarly to c-Src tyrosine kinase (CSK), but not to phosphorylation-deficient mutant of caveolin-1 (Y14A), and interfered with complex formation between caveolin-1 and CSK. In the presence of CSK siRNA, binding between caveolin-1 and SHP-2 was enhanced by H₂O₂ treatment, which led to reduced Src phosphorylation at tyrosine (Tyr) 530 and enhanced Src phosphorylation at Tyr 419. In contrast, siRNA targeting of SHP-2 facilitated H₂O₂-mediated interaction between caveolin-1 and CSK and enhanced Src phosphorylation at Tyr 530, leading to subsequent decrease in Src downstream signaling, such as focal adhesion kinase (FAK) and extracellular signal-related kinase (ERK). Our results collectively indicate that SHP-2 alters Src kinase activity by interfering with the complex formation between CSK and phosphotyrosine caveolin-1 in the presence of H₂O₂, thus functions as a positive regulator in Src signaling under oxidative stress in brain astrocytes.

Thioredoxin-interacting protein is a biomechanical regulator of Src activity: key role in endothelial cell stress fiber formation

RATIONALE

Fluid shear stress differentially regulates endothelial cell stress fiber formation with decreased stress fibers in areas of disturbed flow compared with steady flow areas. Importantly, stress fibers are critical for several endothelial cell functions including cell shape, mechano-signal transduction, and endothelial cell-cell junction integrity. A key mediator of steady flow-induced stress fiber formation is Src that regulates downstream signaling mediators such as phosphorylation of cortactin, activity of focal adhesion kinase, and small GTPases.

OBJECTIVE

Previously, we showed that thioredoxin-interacting protein (TXNIP, also VDUP1 [vitamin D upregulated protein 1] and TBP-2 [thioredoxin binding protein 2]) was regulated by fluid shear stress; TXNIP expression was increased in disturbed flow compared with steady flow areas. Although TXNIP was originally characterized for its role in redox and metabolic cellular functions, recent reports show important scaffold functions related to its α-arrestin structure. Based on these findings, we hypothesized that TXNIP acts as a biomechanical sensor that regulates Src kinase activity and stress fiber formation.

METHODS AND RESULTS

Using en face immunohistochemistry of the aorta and cultured endothelial cells, we show inverse relationship between TXNIP expression and Src activity. Specifically, steady flow increased Src activity and stress fiber formation, whereas it decreased TXNIP expression. In contrast, disturbed flow had opposite effects. We studied the role of TXNIP in regulating Src homology phosphatase-2 plasma membrane localization and vascular endothelial cadherin binding because Src homology phosphatase-2 indirectly regulates dephosphorylation of Src tyrosine 527 that inhibits Src activity. Using immunohistochemistry and immunoprecipitation, we found that TXNIP prevented Src homology phosphatase-2-vascular endothelial cadherin interaction.

CONCLUSIONS

In summary, these data characterize a fluid shear stress-mediated mechanism for stress fiber formation that involves a TXNIP-dependent vascular endothelial cadherin-Src homology phosphatase-2-Src pathway.

VE-cadherin and endothelial adherens junctions: active guardians of vascular integrity

VE-cadherin is a component of endothelial cell-to-cell adherens junctions, and it has a key role in the maintenance of vascular integrity. During embryo development, VE-cadherin is required for the organization of a stable vascular system, and in the adult it controls vascular permeability and inhibits unrestrained vascular growth. The mechanisms of action of VE-cadherin are complex and include reshaping and organization of the endothelial cell cytoskeleton and modulation of gene transcription. Here we review some of the most important pathways through which VE-cadherin modulates vascular homeostasis and discuss the emerging concepts in the overall biological role of this protein.

Molecular biology of atherosclerosis

At least 468 individual genes have been manipulated by molecular methods to study their effects on the initiation, promotion, and progression of atherosclerosis. Most clinicians and many investigators, even in related disciplines, find many of these genes and the related pathways entirely foreign. Medical schools generally do not attempt to incorporate the relevant molecular biology into their curriculum. A number of key signaling pathways are highly relevant to atherogenesis and are presented to provide a context for the gene manipulations summarized herein. The pathways include the following: the insulin receptor (and other receptor tyrosine kinases); Ras and MAPK activation; TNF-α and related family members leading to activation of NF-κB; effects of reactive oxygen species (ROS) on signaling; endothelial adaptations to flow including G protein-coupled receptor (GPCR) and integrin-related signaling; activation of endothelial and other cells by modified lipoproteins; purinergic signaling; control of leukocyte adhesion to endothelium, migration, and further activation; foam cell formation; and macrophage and vascular smooth muscle cell signaling related to proliferation, efferocytosis, and apoptosis. This review is intended primarily as an introduction to these key signaling pathways. They have become the focus of modern atherosclerosis research and will undoubtedly provide a rich resource for future innovation toward intervention and prevention of the number one cause of death in the modern world.

The HCMV gH/gL/UL128-131 complex triggers the specific cellular activation required for efficient viral internalization into target monocytes

We have established that HCMV acts as a specific ligand engaging and activating cellular integrins on monocytes. As a result, integrin signaling via Src activation leads to the functional activation of paxillin required for efficient viral entry and for the biological changes in monocytes needed for viral dissemination. These biological/molecular changes allow HCMV to use monocytes as "vehicles" for systemic spread and the establishment of lifelong persistence. However, it remains unresolved how HCMV specifically induces this observed monocyte activation. It was previously demonstrated that the HCMV gH/gL/UL128-131 glycoprotein complex facilitates viral entry into biologically relevant cell types. Nevertheless, the mechanism by which the gH/gL/UL128-131 complex promotes this process is unknown. We now show that only HCMV virions possessing the gH/gL/UL128-131 complex are capable of activating integrin/Src/paxillin-signaling in monocytes. In fibroblasts, this signaling is reversed, such that virus lacking the gH/gL/UL128-131 complex is the only virus able to induce the paxillin activation cascade. The presence of the gH/gL/UL128-131 complex also may have an inhibitory effect on integrin-mediated signaling pathway in fibroblasts. Furthermore, we demonstrate that the presence of the gH/gL/UL128-131 complex on the viral envelope, through its activation of the integrin/Src/paxillin pathway, is necessary for efficient HCMV internalization into monocytes and that appropriate actin and dynamin regulation is critical for this entry process. Importantly, productive infection in monocyte-derived macrophages was seen only in cells exposed to HCMV expressing the gH/gL/UL128-131 complex. From our data, the HCMV gH/gL/U128-131 complex emerges as the specific ligand driving the activation of the receptor-mediated signaling required for the regulation of the actin cytoskeleton and, consequently, for efficient and productive internalization of HCMV into monocytes. To our knowledge, our studies demonstrate a possible molecular mechanism for why the gH/gL/UL128-131 complex dictates HCMV tropism and why the complex is lost as clinical isolates are passaged in the laboratory.

Tyrosine phosphorylation of DEP-1/CD148 as a mechanism controlling Src kinase activation, endothelial cell permeability, invasion, and capillary formation

DEP-1/CD148 is a receptor-like protein tyrosine phosphatase with antiproliferative and tumor-suppressive functions. Interestingly, it also positively regulates Src family kinases in hematopoietic and endothelial cells, where we showed it promotes VE-cadherin-associated Src activation and endothelial cell survival upon VEGF stimulation. However, the molecular mechanism involved and its biologic functions in endothelial cells remain ill-defined. We demonstrate here that DEP-1 is phosphorylated in a Src- and Fyn-dependent manner on Y1311 and Y1320, which bind the Src SH2 domain. This allows DEP-1-catalyzed dephosphorylation of Src inhibitory Y529 and favors the VEGF-induced phosphorylation of Src substrates VE-cadherin and Cortactin. Accordingly, RNA interference (RNAi)-mediated knockdown of DEP-1 or expression of DEP-1 Y1311F/Y1320F impairs Src-dependent biologic responses mediated by VEGF including permeability, invasion, and branching capillary formation. In addition, our work further reveals that above a threshold expression level, DEP-1 can also dephosphorylate Src Y418 and attenuate downstream signaling and biologic responses, consistent with the quiescent behavior of confluent endothelial cells that express the highest levels of endogenous DEP-1. Collectively, our findings identify the VEGF-dependent phosphorylation of DEP-1 as a novel mechanism controlling Src activation, and show this is essential for the proper regulation of permeability and the promotion of the angiogenic response.

Fibroblast growth factor signaling potentiates VE-cadherin stability at adherens junctions by regulating SHP2

Background

The fibroblast growth factor (FGF) system plays a critical role in the maintenance of vascular integrity via enhancing the stability of VE-cadherin at adherens junctions. However, the precise molecular mechanism is not well understood. In the present study, we aimed to investigate the detailed mechanism of FGF regulation of VE-cadherin function that leads to endothelial junction stabilization.

Methods and Findings

In vitro studies demonstrated that the loss of FGF signaling disrupts the VE-cadherin-catenin complex at adherens junctions by increasing tyrosine phosphorylation levels of VE-cadherin. Among protein tyrosine phosphatases (PTPs) known to be involved in the maintenance of the VE-cadherin complex, suppression of FGF signaling reduces SHP2 expression levels and SHP2/VE-cadherin interaction due to accelerated SHP2 protein degradation. Increased endothelial permeability caused by FGF signaling inhibition was rescued by SHP2 overexpression, indicating the critical role of SHP2 in the maintenance of endothelial junction integrity.

Conclusions

These results identify FGF-dependent maintenance of SHP2 as an important new mechanism controlling the extent of VE-cadherin tyrosine phosphorylation, thereby regulating its presence in adherens junctions and endothelial permeability.

NHERF-2 maintains endothelial homeostasis

The Na(+)/H(+) exchanger regulatory factor-2 (NHERF-2) is an integral component of almost all endothelial cells (ECs), yet its endothelial function is not known. Here, we found that NHERF-2, is a key regulator of endothelial homeostasis because NHERF-2-silenced ECs proliferate at a much higher rate even in the absence of mitogens such as VEGF compared with control ECs. We further show that the hyperproliferation phenotype of NHERF-2-silenced EC is because of an accelerated cell cycle that is probably caused by a combination of the following factors: increased cytoplasmic calcium, increased expression of c-Myc, increased expression of cyclin D1, and reduced expression of p27. Using an experimental mouse model of human hemangioma, we found that the endothelial neoplasms derived from NHERF-2-silenced cells were much larger in volume than those derived from control cells. Thus, NHERF-2 is a negative regulator of endothelial proliferation and may have important roles in endothelial homeostasis and vascular modeling.

Signaling required for blood vessel maintenance: molecular basis and pathological manifestations

As our understanding of molecular mechanisms leading to vascular formation increases, vessel maintenance including stabilization of new vessels and prevention of vessel regression began to be considered as an active process that requires specific cellular signaling. While signaling pathways such as VEGF, FGF, and angiopoietin-Tie2 are important for endothelial cell survival and junction stabilization, PDGF and TGF-β signaling modify mural cell (vascular smooth muscle cells and pericytes) functions, thus they fortify vessel integrity. Breakdown of these signaling systems results in pathological hyperpermeability and/or genetic vascular abnormalities such as vascular malformations, ultimately progressing to hemorrhage and edema. Hence, blood vessel maintenance is fundamental to controlling vascular homeostasis and tissue functions. This paper discusses signaling pathways essential for vascular maintenance and clinical conditions caused by deterioration of vessel integrity.

R-Ras interacts with filamin a to maintain endothelial barrier function

The molecular mechanisms regulating vascular barrier integrity remain incompletely elucidated. We have previously reported an association between the GTPase R-Ras and repeat 3 of Filamin A (FLNa). Loss of FLNa has been linked to increased vascular permeability. We sought to determine whether FLNa's association with R-Ras affects endothelial barrier function. We report that in endothelial cells endogenous R-Ras interacts with endogenous FLNa as determined by co-immunoprecipitations and pulldowns with the FLNa-GST fusion protein repeats 1-10. Deletion of FLNa repeat 3 (FLNaΔ3) abrogated this interaction. In these cells FLNa and R-Ras co-localize at the plasma membrane. Knockdown of R-Ras and/or FLNa by siRNA promotes vascular permeability, as determined by TransEndothelial Electrical Resistance and FITC-dextran transwell assays. Re-expression of FLNa restored endothelial barrier function in cells lacking FLNa whereas re-expression of FLNaΔ3 did not. Immunostaining for VE-Cadherin in cells with knocked down R-Ras and FLNa demonstrated a disorganization of VE-Cadherin at adherens junctions. Loss of R-Ras and FLNa or blocking R-Ras function via GGTI-2133, a selective R-Ras inhibitor, induced vascular permeability and increased phosphorylation of VE-Cadherin (Y731) and Src (Y416). Expression of dominant negative R-Ras promoted vascular permeability that was blocked by the Src inhibitor PP2. These findings demonstrate that maintaining endothelial barrier function is dependent upon active R-Ras and association between R-Ras and FLNa and that loss of this interaction promotes VE-Cadherin phosphorylation and changes in downstream effectors that lead to endothelial leakiness.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Zebrafish as a model for hemorrhagic stroke

Blood vessels perform the fundamental role of providing conduits for the circulation of oxygen and nutrients and the removal of waste products throughout the body. Disruption of tissue perfusion by ischemia or hemorrhage of blood vessels has a range of devastating consequences including stroke. Stroke is a complex trait that includes both genetic and environmental risk factors. The zebrafish is an attractive model for the study of hemorrhagic stroke due to the conservation of the molecular mechanisms of blood vascular development among vertebrates and the experimental advantages that can be applied to zebrafish embryos and larva. This chapter will focus on the maintenance of vascular integrity and some of the seminal experimentation carried out in the zebrafish.

Dentin matrix protein 1 induces membrane expression of VE-cadherin on endothelial cells and inhibits VEGF-induced angiogenesis by blocking VEGFR-2 phosphorylation

Dentin matrix protein 1 (DMP1) is a member of the small integrin-binding ligand N-linked glycoprotein (SIBLING) family, a group of proteins initially described as mineralized extracellular matrices components. More recently, SIBLINGs have been implicated in several key steps of cancer progression, including angiogenesis. Although proangiogenic activities have been demonstrated for 2 SIBLINGs, the role of DMP1 in angiogenesis has not yet been addressed. We demonstrate that this extracellular matrix protein induced the expression of vascular endothelial cadherin (VE-cadherin), a key regulator of intercellular junctions and contact inhibition of growth of endothelial cells that is also known to modulate vascular endothelial growth factor receptor 2 (VEGFR-2) activity, the major high-affinity receptor for VEGF. DMP1 induced VE-cadherin and p27(Kip1) expression followed by cell-cycle arrest in human umbilical vein endothelial cells (HUVECs) in a CD44-dependent manner. VEGF-induced proliferation, migration, and tubulogenesis responses were specifically blocked on DMP1 pretreatment of HUVECs. Indeed, after VE-cadherin induction, DMP1 inhibited VEGFR-2 phosphorylation and Src-mediated signaling. However, DMP1 did not interfere with basic fibroblast growth factor-induced angiogenesis. In vivo, DMP1 significantly reduced laser-induced choroidal neovascularization lesions and tumor-associated angiogenesis. These data enable us to put DMP1 on the angiogenic chessboard for the first time and to identify this protein as a new specific inhibitor of VEGF-induced angiogenesis.

Increased levels of soluble vascular endothelial cadherin are associated with poor outcome in severe sepsis

Endothelial cells are thought to play a key role in sepsis pathogenesis: vascular endothelial damage occurs in severe sepsis and multiple organ dysfunction. Soluble vascular endothelial cadherin (VE-cadherin) levels were investigated in a prospective study involving 28 consecutive critically ill patients with or without severe sepsis who were admitted to surgical intensive care; 13 healthy age-matched volunteers were included as controls. Soluble VE-cadherin levels increased significantly in patients with severe sepsis compared with ill patients without severe sepsis and healthy controls. There was a significant linear correlation between soluble VE-cadherin levels and illness severity scores. Soluble VE-cadherin levels were significantly higher in patients who died compared with survivors. In vitro cell culture showed that serum from patients with severe sepsis greatly decreased VE-cadherin staining at intercellular junctions and total VE-cadherin expression in human umbilical cord vein endothelial cells. These findings suggest that endothelial cells play an important role in the poor outcome of patients with severe sepsis.

Phosphorylation of VE-cadherin controls endothelial phenotypes via p120-catenin coupling and Rac1 activation

To establish the role of vascular endothelial (VE)-cadherin in the regulation of endothelial cell functions, we investigated the effect of phosphorylation of a VE-cadherin site sought to be involved in p120-catenin binding on vascular permeability and endothelial cell migration. To this end, we introduced either wild-type VE-cadherin or Y658 phosphomimetic (Y658E) or dephosphomimetic (Y658F) VE-cadherin mutant constructs into an endothelial cell line (rat fat pad endothelial cells) lacking endogenous VE-cadherin. Remarkably, neither wild-type- nor Y658E VE-cadherin was retained at cell-cell contacts because of p120-catenin preferential binding to N-cadherin, resulting in the targeting of N-cadherin to cell-cell junctions and the exclusion of VE-cadherin. However, Y658F VE-cadherin was able to bind p120-catenin and to localize at adherence junctions displacing N-cadherin. This resulted in an enhanced barrier function and a complete abrogation of Rac1 activation and lamellipodia formation, thereby inhibiting cell migration. These findings demonstrate that VE-cadherin, through the regulation of Y658 phosphorylation, competes for junctional localization with N-cadherin and controls vascular permeability and endothelial cell migration.

Control of endothelial barrier function by regulating vascular endothelial-cadherin

PURPOSE OF REVIEW

Proper control of endothelial cell contacts is the basis for maintenance of the vascular barrier function. Loss of this function leads to leak of fluid and protein from the vasculature and extensive leaks cause shock and death. The endothelial barrier also controls the entry of leukocytes into tissue and it is believed that leukocytes target endothelial cell contacts to reach sites of inflammation.

RECENT FINDINGS

Within the last 2 years several new molecular players and molecular interactions have been identified that either help in stabilizing the endothelial contacts or mediate their opening if triggered by the appropriate stimuli. Novel signaling mechanisms have been identified that regulate endothelial cell contacts. Whether, how and to what extent the complex of the endothelial specific adhesion molecule vascular endothelial-cadherin and its associated catenins is involved in these processes will be a major focus of this article.

SUMMARY

Endothelial cell contacts are regulated by a complex interplay between various receptors and signaling mediators that control the plasticity of the cytoskeleton and the function of junctional adhesion molecules. Knowing and understanding the essential players of this network will allow development of agents that could prevent breakdown of the vascular permeability barrier in shock or that could block leukocyte extravasation and thereby antagonize inflammation.

Vascular endothelial growth factors and vascular permeability

Vascular endothelial growth factors (VEGFs) are key regulators of permeability. The principal evidence behind how they increase vascular permeability in vivo and in vitro and the consequences of that increase are addressed here. Detailed analysis of the published literature has shown that in vivo and in vitro VEGF-mediated permeability differs in its time course, but has common involvement of many specific signalling pathways, in particular VEGF receptor-2 activation, calcium influx through transient receptor potential channels, activation of phospholipase C gamma and downstream activation of nitric oxide synthase. Pathways downstream of endothelial nitric oxide synthase appear to involve the guanylyl cyclase-mediated activation of the Rho–Rac pathway and subsequent involvement of junctional signalling proteins such as vascular endothelial cadherin and the tight junctional proteins zona occludens and occludin linked to the actin cytoskeleton. The signalling appears to be co-ordinated through spatial organization of the cascade into a signalplex, and arguments for why this may be important are considered. Many proteins have been identified to be involved in the regulation of vascular permeability by VEGF, but still the mechanisms through which these are thought to interact to control permeability are dependent on the experimental system, and a synthesis of existing data reveals that in intact vessels the co-ordination of the pathways is still not understood.

Src-induced tyrosine phosphorylation of VE-cadherin is not sufficient to decrease barrier function of endothelial monolayers

Activation of Src family kinases (SFK) and the subsequent phosphorylation of VE-cadherin have been proposed as major regulatory steps leading to increases in vascular permeability in response to inflammatory mediators and growth factors. To investigate Src signaling in the absence of parallel signaling pathways initiated by growth factors or inflammatory mediators, we activated Src and SFKs by expression of dominant negative Csk, expression of constitutively active Src, or knockdown of Csk. Activation of SFK by overexpression of dominant negative Csk induced VE-cadherin phosphorylation at tyrosines 658, 685, and 731. However, dominant negative Csk expression was unable to induce changes in the monolayer permeability. In contrast, expression of constitutively active Src decreased barrier function and promoted VE-cadherin phosphorylation on tyrosines 658 and 731, although the increase in VE-cadherin phosphorylation preceded the increase in permeability by 4-6 h. Csk knockdown induced VE-cadherin phosphorylation at sites 658 and 731 but did not induce a loss in barrier function. Co-immunoprecipitation and immunofluorescence studies suggest that phosphorylation of those sites did not impair VE-cadherin ability to bind p120 and beta-catenin or the ability of these proteins to localize at the plasma membrane. Taken together, our data show that Src-induced tyrosine phosphorylation of VE-cadherin is not sufficient to promote an increase in endothelial cell monolayer permeability and suggest that signaling leading to changes in vascular permeability in response to inflammatory mediators or growth factors may require VE-cadherin tyrosine phosphorylation concurrently with other signaling pathways to promote loss of barrier function.

Fluid shear stress stimulates phosphorylation-dependent nuclear export of HDAC5 and mediates expression of KLF2 and eNOS

Fluid shear stress generated by steady laminar blood flow protects vessels from atherosclerosis. Krüppel-like factor 2 (KLF2) and endothelial nitric oxide synthase (eNOS) are fluid shear stress-responsive genes and key mediators in flow anti-inflammatory and antiatherosclerotic actions. However, the molecular mechanisms underlying flow induction of KLF2 and eNOS remain largely unknown. Here, we show a novel role of histone deacetylase 5 (HDAC5) in flow-mediated KLF2 and eNOS expression. We found for the first time that fluid shear stress stimulated HDAC5 phosphorylation and nuclear export in endothelial cells through a calcium/calmodulin-dependent pathway. Consequently, flow induced the dissociation of HDAC5 and myocyte enhancer factor-2 (MEF2) and enhanced MEF2 transcriptional activity, which leads to expression of KLF2 and eNOS. Adenoviral overexpression of a HDAC5 phosphorylation-defective mutant (Ser259/Ser498 were replaced by Ala259/Ala498, HDAC5-S/A), which shows resistance to flow-induced nuclear export, suppressed flow-mediated MEF2 transcriptional activity and expression of KLF2 and eNOS. Importantly, HDAC5-S/A attenuated the flow-inhibitory effect on monocyte adhesion to endothelial cells. Taken together, our results reveal that phosphorylation-dependent derepression of HDAC5 mediates flow-induced KLF2 and eNOS expression as well as flow anti-inflammation, and suggest that HDAC5 could be a potential therapeutic target for the prevention of atherosclerosis.

Anti-HLA I antibodies induce VEGF production by endothelial cells, which increases proliferation and paracellular permeability

Anti-human leukocyte antigen class I (HLA I) antibodies were shown to activate several protein kinases in endothelial cells (ECs), which induces proliferation and cell survival. An important phenomenon in antibody-mediated rejection is the occurrence of interstitial edema. We investigated the effect of anti-HLA I antibodies on endothelial proliferation and permeability, as one possible underlying mechanism of edema formation. HLA I antibodies increased the permeability of cultured ECs isolated from umbilical veins. Anti-HLA I antibodies induced the production of vascular endothelial growth factor (VEGF) by ECs, which activated VEGF receptor 2 (VEGFR2) in an autocrine manner. Activated VEGFR2 led to a c-Src-dependent phosphorylation of vascular endothelial (VE)-cadherin and its degradation. Aberrant VE-cadherin expression resulted in impaired adherens junctions, which might lead to increased endothelial permeability. This effect was only observed after cross-linking of HLA I molecules by intact antibodies. Furthermore, our results suggest that increased endothelial proliferation following anti-HLA I treatment occurs via autocrine VEGFR2 activation. Our data indicate the ability of anti-HLA I to induce VEGF production in ECs. Transactivation of VEGFR2 leads to increased EC proliferation and paracellular permeability. The autocrine effect of VEGF on endothelial permeability might be an explanation for the formation of interstitial edema after transplantation.

Phosphorylation of endothelial nitric oxide synthase by atypical PKC zeta contributes to angiopoietin-1-dependent inhibition of VEGF-induced endothelial permeability in vitro

Vascular endothelial growth factor (VEGF) is a potent angiogenic cytokine that also increases vascular permeability. Nitric oxide (NO) released from endothelial cells, after activation of endothelial NO synthase (eNOS), contributes to proangiogenic and permeability effects of VEGF. Angiopoietin-1 (Ang-1), via Tie2 receptors, shares many of the proangiogenic properties of VEGF on endothelial cells. However, in contrast to VEGF, Ang-1 protects blood vessels from increased plasma leakage, which contributes to their stabilization. Because eNOS-derived NO is central to increased permeability in response to VEGF, we investigated whether Ang-1 interferes with VEGF signaling to eNOS. We demonstrate that Ang-1 stimulation of endothelial cells inhibits VEGF-induced NO release and transendothelial permeability. In contrast to VEGF stimulation, Ang-1 causes a marked protein kinase C (PKC)-dependent increase in phosphorylation of eNOS on the inhibitory Thr(497). Furthermore, using pharmacologic inhibitors, overexpression studies, and small interfering RNA-mediated gene silencing, we demonstrate that atypical PKC zeta is responsible for phosphorylation of eNOS on Thr(497) in response to Ang-1. In addition, PKC zeta knockdown abrogates the capacity of Ang-1 to inhibit VEGF-induced NO release and endothelial permeability. Thus, inhibition of NO production by Ang-1, via phosphorylation of eNOS on Thr(497) by PKC zeta, is responsible, at least in part, for inhibition of VEGF-stimulated endothelial permeability by Ang-1.

Sustained activation of XBP1 splicing leads to endothelial apoptosis and atherosclerosis development in response to disturbed flow

X-box binding protein 1 (XBP1) is a key signal transducer in endoplasmic reticulum stress response, and its potential role in the atherosclerosis development is unknown. This study aims to explore the impact of XBP1 on maintaining endothelial integrity related to atherosclerosis and to delineate the underlying mechanism. We found that XBP1 was highly expressed at branch points and areas of atherosclerotic lesions in the arteries of ApoE(-/-) mice, which was related to the severity of lesion development. In vitro study using human umbilical vein endothelial cells (HUVECs) indicated that disturbed flow increased the activation of XBP1 expression and splicing. Overexpression of spliced XBP1 induced apoptosis of HUVECs and endothelial loss from blood vessels during ex vivo cultures because of caspase activation and down-regulation of VE-cadherin resulting from transcriptional suppression and matrix metalloproteinase-mediated degradation. Reconstitution of VE-cadherin by Ad-VEcad significantly increased Ad-XBP1s-infected HUVEC survival. Importantly, Ad-XBP1s gene transfer to the vessel wall of ApoE(-/-) mice resulted in development of atherosclerotic lesions after aorta isografting. These results indicate that XBP1 plays an important role in maintaining endothelial integrity and atherosclerosis development, which provides a potential therapeutic target to intervene in atherosclerosis.

Noonan syndrome cardiac defects are caused by PTPN11 acting in endocardium to enhance endocardial-mesenchymal transformation

Noonan syndrome (NS), the most common single-gene cause of congenital heart disease, is an autosomal dominant disorder that also features proportionate short stature, facial abnormalities, and an increased risk of myeloproliferative disease. Germline-activating mutations in PTPN11, which encodes the protein tyrosine phosphatase SHP2, cause about half of NS cases; other causative alleles include KRAS, SOS1, and RAF1 mutants. We showed previously that knock-in mice bearing the NS mutant Ptpn11(D61G) on a mixed 129S4/SvJae X C57BL6/J background exhibit all major NS features, including a variety of cardiac defects, with variable penetrance. However, the cellular and molecular mechanisms underlying NS cardiac defects and whether genetic background and/or the specific NS mutation contribute to the NS phenotype remained unclear. Here, using an inducible knock-in approach, we show that all cardiac defects in NS result from mutant Shp2 expression in the endocardium, not in the myocardium or neural crest. Furthermore, the penetrance of NS defects is affected by genetic background and the specific Ptpn11 allele. Finally, ex vivo assays and pharmacological approaches show that NS mutants cause cardiac valve defects by increasing Erk MAPK activation, probably downstream of ErbB family receptor tyrosine kinases, extending the interval during which cardiac endocardial cells undergo endocardial-mesenchymal transformation. Our data provide a mechanistic underpinning for the cardiac defects in this disorder.

Synapses are regulated by the cytoplasmic tyrosine kinase Fer in a pathway mediated by p120catenin, Fer, SHP-2, and beta-catenin

Localization of presynaptic components to synaptic sites is critical for hippocampal synapse formation. Cell adhesion–regulated signaling is important for synaptic development and function, but little is known about differentiation of the presynaptic compartment. In this study, we describe a pathway that promotes presynaptic development involving p120catenin (p120ctn), the cytoplasmic tyrosine kinase Fer, the protein phosphatase SHP-2, and β-catenin. Presynaptic Fer depletion prevents localization of active zone constituents and synaptic vesicles and inhibits excitatory synapse formation and synaptic transmission. Depletion of p120ctn or SHP-2 similarly disrupts synaptic vesicle localization with active SHP-2, restoring synapse formation in the absence of Fer. Fer or SHP-2 depletion results in elevated tyrosine phosphorylation of β-catenin. β-Catenin overexpression restores normal synaptic vesicle localization in the absence of Fer or SHP-2. Our results indicate that a presynaptic signaling pathway through p120ctn, Fer, SHP-2, and β-catenin promotes excitatory synapse development and function.

Biosynthesis and expression of VE-cadherin is regulated by the PI3K/mTOR signaling pathway

Vascular endothelial (VE)-cadherin is an essential protein of adherens junctions of endothelial cells and plays a pivotal role in vascular homeostasis. Mammalian target of rapamycin complex 2 (mTORC2) deficient mice display defects in fetal vascular development. Blocking mTOR or the upstream kinase phosphoinositide 3-kinase (PI3K) led to a dose-dependently decrease of the VE-cadherin mRNA and protein expression. Immunofluorescent staining showed a strongly decreased expression of VE-cadherin at the interface of human umbilical endothelial cells (HUVECs) followed by intercellular gap formation. Herewith, we demonstrated that the expression of VE-cadherin is dependent on mTOR and PI3K signaling.