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

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

Angiogenesis: The VE-Cadherin Switch

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

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

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

Soluble VE-cadherin in rheumatoid arthritis patients correlates with disease activity: evidence for tumor necrosis factor α-induced VE-cadherin cleavage

OBJECTIVE

Rheumatoid arthritis (RA) is a chronic, systemic inflammatory disorder that principally attacks synovial joints. However, accelerated atherosclerosis and increased cardiovascular morbidity and mortality are major clinical consequences of endothelial dysfunction in RA patients. Tumor necrosis factor α (TNFα) is the major mediator of inflammation in RA, related to vascular injury by targeting VE-cadherin, an endothelium-specific adhesion molecule of vital importance for endothelium integrity and angiogenesis. We undertook this study to examine the mechanisms regulating VE-cadherin processing by TNFα and their occurrence in RA.

METHODS

Human umbilical vein endothelial cells were used in primary culture and treated with recombinant TNFα to study VE-cadherin cleavage. Cell lysates and conditioned media were analyzed by Western blotting for VE-cadherin cytoplasmic domain and extracellular domain (VE-90) generation, respectively. VE-90 was analyzed at baseline and at the 1-year followup in sera from 63 RA patients (from the Very Early Rheumatoid Arthritis cohort) with disease duration of <6 months.

RESULTS

TNFα induced a time-dependent shedding of VE-90 in cell media. This effect was prevented by tyrosine kinase inhibitors (genistein and PP2) or by knocking down Src kinase. In contrast, tyrosine phosphatase blockade enhanced VE-cadherin cleavage, confirming the requirement of tyrosine phosphorylation processes. In addition, using the matrix metalloproteinase (MMP) activator APMA and the MMP inhibitor GM6001, we demonstrated that MMPs are involved in TNFα-induced VE-cadherin cleavage. Of major importance, VE-90 was detected in sera from the 63 RA patients and was positively correlated with the Disease Activity Score at baseline and after 1-year followup.

CONCLUSION

These findings provide the first evidence of VE-cadherin proteolysis upon TNFα stimulation and suggest potential clinical relevance of soluble VE-cadherin in management of RA.

Steroidogenic properties of phorbol ester and a Ca2+ ionophore in bovine adrenocortical cell suspensions

When added independently to bovine adrenocortical fasciculata cell suspensions, 12 tetradecanoyl-phorbol-13 acetate (TPA) and the Ca2+ ionophore A23187 activated net cortisol production in a time and dose-dependent manner during one hour incubation. When added together (each at 1 microM concentration), the drugs appeared synergistic and mimicked the steroidogenic effect of suboptimal concentration of angiotensin II or acetylcholine on these cells, with no detectable variation of cellular cyclic nucleotide levels. In addition, the drug mixture markedly enhanced the steroidogenic effect of acetylcholine. These observations suggest that Ca2+-activated, phospholipid dependent protein kinase, which is present in adrenal cortex, might be considered as a possible target in the mechanism of action of steroidogenic agents such as angiotensin and acetylcholine, acting in adrenocortical cell through cyclic AMP independent processes.

Adrenocortical cytochrome P-450 responsible for cholesterol side chain cleavage (P-450scc) is phosphorylated by the calcium-activated, phospholipid-sensitive protein kinase (protein kinase C)

Purified bovine adrenocortical cytochrome P-450scc (specific for cholesterol side chain cleavage in the inner mitochondrial membrane) was selectively phosphorylated in vitro by a Ca2+-activated, phospholipid-sensitive protein kinase (protein kinase C) preparation, whereas cyclic AMP dependent and two cyclic nucleotide independent kinases were ineffective. Cytochrome P-450scc incorporated a maximum of 4 mol of phosphate in the presence of protein kinase C within 15 min at 30 degrees C, with apparent Km and Vmax of 0.14 mumol and 0.76 pmol/min, respectively. Serine and threonine were the two target aminoacids phosphorylated in a ratio of about 1:1. In the presence of 1 microM Ca2+, a mixture of phosphatidylserine and diolein (or a potent tumor promoter phorbol ester) was required for optimal cytochrome P-450scc phosphorylation. In addition, purified inner mitochondrial membrane preparations from adrenocortical mitochondria were found to contain protein kinase C activity. These findings, together with the previous demonstration that activators of protein kinase C such as a potent phorbol ester activates steroidogenesis of intact adrenocortical cells, suggest that phosphorylation of P-450scc should be examined for its possible role in the regulation of adrenocortical functions.

Microtubule-associated protein 6 mediates neuronal connectivity through Semaphorin 3E-dependent signalling for axonal growth

Structural microtubule associated proteins (MAPs) stabilize microtubules, a property that was thought to be essential for development, maintenance and function of neuronal circuits. However, deletion of the structural MAPs in mice does not lead to major neurodevelopment defects. Here we demonstrate a role for MAP6 in brain wiring that is independent of microtubule binding. We find that MAP6 deletion disrupts brain connectivity and is associated with a lack of post-commissural fornix fibres. MAP6 contributes to fornix development by regulating axonal elongation induced by Semaphorin 3E. We show that MAP6 acts downstream of receptor activation through a mechanism that requires a proline-rich domain distinct from its microtubule-stabilizing domains. We also show that MAP6 directly binds to SH3 domain proteins known to be involved in neurite extension and semaphorin function. We conclude that MAP6 is critical to interface guidance molecules with intracellular signalling effectors during the development of cerebral axon tracts.

Loss of the structural microtubule-associated protein 6 (MAP6) leads to neuronal differentiation defects that are independent of MAP6's microtubule-binding properties. Here the authors establish a functional link between MAP6 and Semaphorin 3E signalling for proper formation of the fornix of the brain.

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

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

Hormonal regulation of mitogen-activated protein kinase activity in bovine adrenocortical cells: cross-talk between phosphoinositides, adenosine 3',5'-monophosphate, and tyrosine kinase receptor pathways

Angiotensin-II (AII), which stimulates steroidogenesis in bovine adrenocortical (BAC) cells through the phosphoinositides pathway, activates p42-p44 mitogen-activated protein kinases (MAPKs) after 5 min of treatment (EC50 = 0.1 nM). This activation is 1) completely inhibited by the AII receptor AT1 subtype antagonist Dup 753 (10 microM), but unaffected by the AT2 antagonist PD 123177; 2) not reproduced by the AT2 agonist CGP 42112A; 3) insensitive to pretreatment with pertussis toxin; and 4) abolished by a 48-h preexposure of the cells to the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA; 1 microM), which down-regulates protein kinase-C activity. Fibroblast growth factor-2, a potent mitogen for BAC cells, which acts through its tyrosine kinase receptor, also activates MAPK (EC50 = 0.3 in a TPA-insensitive manner, while exhibiting no detectable effect on BAC cell steroidogenesis. In contrast, ACTH, which stimulates steroidogenesis via cAMP and inhibits BAC cell proliferation, does not stimulate MAPK. Indeed, ACTH completely blocks (IC50 = 0.01 nM) the stimulation of MAPK by AII, fibroblast growth factor-2, or TPA. Therefore, bovine adrenocortical cells provide an example of positive and negative hormonal regulation of MAPK activity through a cross-talk between the inositide-, cAMP-, and growth factor-activated tyrosine kinase pathways.

A single form of protein kinase C is expressed in bovine adrenocortical tissue, as compared to four chromatographically resolved isozymes in rat brain

Protein kinase C purified to apparent homogeneity from rat brain was resolved into four active moieties following chromatography over a hydroxyapatite high resolution system. By contrast, the same procedure applied to bovine adrenocortical protein kinase C revealed that a single protein kinase C isoform could be detected in this tissue, with a chromatographic behavior identical to that of one of the brain isoenzymes. Although the isolated protein kinase C isozymes were all activated to various degrees in the presence of phospholipids and calcium, quantitative differences were observed in their catalytic properties, especially with regard to their sensitivity to diacylglycerol and TPA and to their relative affinity for different protein substrates. These observations confirmed at the protein level the heterogeneity of protein kinase C predicted on the basis of cDNA cloning studies. They also suggest that the expression of a specific set of protein kinase C isoenzyme(s) in a given cell type deserves further attention, since it may reflect a functional significance with regard to the regulation of specific cellular processes.

Lack of antitumor activity of recombinant endostatin in a human neuroblastoma xenograft model

Patients with metastatic neuroblastoma are rarely curable with currently available therapy, and the search for new treatment options, which include the use of inhibitors of tumor angiogenesis, is warranted. Here, we have evaluated the efficacy of one of the most promising natural inhibitors of angiogenesis described to date, endostatin, in a human neuroblastoma xenograft model in nude mice. Murine endostatin cDNA was cloned in a bacterial expression vector, expressed as a polyHis-Endostatin fusion protein and purified on Ni2+-NTA beads. The in vitro activity of soluble endostatin was confirmed on bovine capillary endothelial cells and human umbilical vein endothelial cells. The human neuroblastoma cell line SKNAS was injected subcutaneously in the flank of nude mice and administration of the recombinant angiogenesis inhibitor started when tumors reached the size of 100 microm3. Twenty mg/kg of recombinant precipitated endostatin or PBS was subcutaneously injected daily for 12 days. Serum endostatin levels were measured using a competitive enzyme immunoassay. Tumor growth was only slowed down in endostatin-treated mice when compared to control mice, and no statistically significant difference in serum levels of endostatin was observed between endostatin-treated and control groups. The lack of correlation between serum concentration and tumor response raises concern regarding the mechanism of action of endostatin.

Phosphorylation of basic fibroblast growth factor by a protein kinase associated with the outer surface of a target cell

A protein kinase capable of phosphorylating basic fibroblast growth factor (FGF) can be localized on the outer cell surface of human hepatoma cells (SK-Hep cells). The addition of [gamma-32P]ATP, but not H3(32)PO4, results in a rapid (less than 10 min) incorporation of 32P into exogenously added basic FGF. The reaction is time and concentration dependent (apparent Km, 170 nM) and is stimulated by the addition of cAMP (EC50, 0.5 microM), but not the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate. There is also no tyrosine protein kinase detected on the cell surface. The inhibition of basic FGF binding to its low and/or high affinity sites decreases the phosphorylation of basic FGF by the ecto-protein kinase. Accordingly, pretreatment of cells with heparinase for 30 min or coincubation with heparin (0.1-10 micrograms/ml) decreases phosphorylation in a dose-dependent manner. Furthermore, the addition of a nonphosphorylatable peptide analog of basic FGF ([Val112] basic FGF-(106-146)NH2) that can compete with basic FGF binding to cells prevents the phosphorylation of basic FGF. Together, these observations suggest that 1) exogenous basic FGF must associate with its low and/or high affinity binding sites to be phosphorylated, and 2) the kinase is cAMP dependent and associated with the outer cell surface, and support the hypothesis that phosphorylation may regulate the activity and/or bioavailability of the growth factor.

Further characterization of four lipocortins from human peripheral blood mononuclear cells

Four calcium and phospholipid binding proteins purified from mononuclear cells were characterized for PKC and EGF phosphorylation, actin binding capacity, and partial tissue distribution. Those named 35K, 32K, and 73K are equivalent, respectively, to lipocortin III, endonexin II and the 67 kDa calelectrin; 36K is a fragment of 73K. After purification, 35K and 73K were phosphorylated by protein kinase C in vitro but 36K nor 32K were not. None were phosphorylated by the epidermal growth factor receptor kinase in vitro; 73K bound F-actin in a calcium-dependent manner, whereas 35K, 36K, and 32K did not. Using Western blotting analysis, 32K and 73K were detected in high amounts in human lymphocytes, monocytes, liver, and placenta and in rat adrenal medulla; but 32K was not detected in polymorphonuclear cells, and 36K and 35K were detected in high amounts only, respectively, in human blood lymphocytes and polymorphonuclear cells. Thus, 32K and 73K appear to have a wide tissue distribution, whereas 35K has a much more restricted distribution.

ACTH-regulated expression of vascular endothelial growth factor in the adult bovine adrenal cortex: a possible role in the maintenance of the microvasculature

Endothelial cells lining vessels of endocrine tissues are fenestrated. Interactions with the local environment via either soluble factors or cell-cell interactions appear to govern this terminal endothelial differentiation. Adrenocorticotropin (ACTH) has previously been reported to modulate endothelial fenestration in the rat adrenal cortex. Since vascular endothelial growth factor (VEGF) has been characterized as a potent inducer of endothelial fenestration, we aimed to characterize the status of VEGF expression in the bovine adult adrenal cortex and asked whether ACTH may regulate VEGF expression. By immunohistochemical analysis, we observed VEGF expression in steroidogenic cells from both zona glomerulosa and zona fasciculata of the bovine adrenal cortex. Double-labeling experiments performed on isolated cells in primary culture revealed VEGF immunoreactivity, essentially colocalized with the Golgi apparatus. The expression of two predominant VEGF isoforms, VEGF(121) and VEGF(165), was observed by RT-PCR analysis. ACTH (10 nM) was found to rapidly (within 2-4 h) increase the abundance of these VEGF transcripts, as assessed by both RT-PCR and Northern blot analysis. In parallel, ACTH significantly induced VEGF secretion into the medium of fasciculata cells in primary culture. Thus, our data are consistent with the involvement of ACTH, through its regulation of VEGF expression, in the maintenance of the adult adrenal cortex endothelium.

Isolation of two 67 kDa calcium-binding proteins from pig lung differing in affinity for phospholipids and in anti-phospholipase A2 activity

Two 67 kDa proteins adsorbed to membranes in the presence of Ca2+ have been purified to homogeneity from pig lung using conventional procedures, followed by calcium-dependent affinity chromatography on polyacrylamide-immobilized phosphatidylserine. The two proteins were, respectively, excluded (67E) and retained (67R) on the column in the presence of Ca2+. On the basis of amino acid composition and isoelectric point, 67R was identified as 67 kDa calelectrin/calcimedin, whereas 67E could be differentiated from albumin, calregulin, 67 kDa fragment of protein kinase C and surfactant-associated proteins. Only 67R was slightly phosphorylated by protein kinase C, reacted with an antibody raised against 32.5 kDa endonexin and inhibited pig pancreas phospholipase A2 in a way similar to that of lipocortin or endonexin. These data bring further support to the view that inhibition of phospholipase A2 by lipocortin or other related proteins involves interaction with the lipid/water interface. They also provide evidence for a new kind of Ca2+-binding protein (67E), whose role still remains to be determined.

Differential expression of VEGF receptors in adrenal atrophy induced by dexamethasone: a protective role of ACTH

Although ACTH is important to adrenal growth and steroidogenesis, its role in vascular development and function has not been established in vivo. In the present study, we demonstrate the expression of mRNA for all four VEGF isoforms (mVEGF(120,144,164,188)) and for Flk-1/KDR and Flt-1 receptors in the mouse adrenal in vivo. Suppression of the pituitary adrenocortical axis by dexamethasone (0.5 mg x 100 g body wt(-1) x day(-1) during 6 days) induced a decrease in corticosterone levels, adrenal weights by 50% (P < 0.001), VEGF(188) mRNA, and Flk-1/KDR mRNA, whereas Flt-1 remained consistent during steroid treatment. A daily injection of ACTH-(1-39) restored the transcript for Flk-1/KDR and both VEGF(188) and plasma corticosterone to control levels. To gain further insights into the effects of ACTH, cultured endothelial cells (ECs) were treated with forskolin, which increases cAMP, the second messenger in ACTH action. We demonstrate that Flk-1/KDR protein expression was markedly increased by forskolin within 24-48 h of treatment in a dose-dependent manner (0.1-10 microM). The biological effect of ACTH on ECs was then tested by use of coincubations of fasciculata cells and ECs in 3D-collagen assay. Within 5-7 days of culture, ECs organized into multicellular structures that resemble networks of microvasculature, which characterize angiogenesis in vitro.

Evidence for post-translational processing of vascular endothelial (VE)-cadherin in brain tumors: towards a candidate biomarker

Vessel abnormalities are among the most important features in malignant glioma. Vascular endothelial (VE)-cadherin is of major importance for vascular integrity. Upon cytokine challenge, VE-cadherin structural modifications have been described including tyrosine phosphorylation and cleavage. The goal of this study was to examine whether these events occurred in human glioma vessels. We demonstrated that VE-cadherin is highly expressed in human glioma tissue and tyrosine phosphorylated at site Y(685), a site previously found phosphorylated upon VEGF challenge, via Src activation. In vitro experiments showed that VEGF-induced VE-cadherin phosphorylation, preceded the cleavage of its extracellular adhesive domain (sVE, 90 kDa). Interestingly, metalloproteases (MMPs) secreted by glioma cell lines were responsible for sVE release. Because VEGF and MMPs are important components of tumor microenvironment, we hypothesized that VE-cadherin proteolysis might occur in human brain tumors. Analysis of glioma patient sera prior treatment confirmed the presence of sVE in bloodstream. Furthermore, sVE levels studied in a cohort of 53 glioma patients were significantly predictive of the overall survival at three years (HR 0.13 [0.04; 0.40] p ≤ 0.001), irrespective to histopathological grade of tumors. Altogether, these results suggest that VE-cadherin structural modifications should be examined as candidate biomarkers of tumor vessel abnormalities, with promising applications in oncology.

VE-cadherin Y685F knock-in mouse is sensitive to vascular permeability in recurrent angiogenic organs

Covalent modifications such as tyrosine phosphorylation are associated with the breakdown of endothelial cell junctions and increased vascular permeability. We previously showed that vascular endothelial (VE)-cadherin was tyrosine phosphorylated in vivo in the mouse reproductive tract and that Y685 was a target site for Src in response to vascular endothelial growth factor in vitro. In the present study, we aimed to understand the implication of VE-cadherin phosphorylation at site Y685 in cyclic angiogenic organs. To achieve this aim, we generated a knock-in mouse carrying a tyrosine-to-phenylalanine point mutation of VE-cadherin Y685 (VE-Y685F). Although homozygous VE-Y685F mice were viable and fertile, the nulliparous knock-in female mice exhibited enlarged uteri with edema. This phenotype was observed in 30% of females between 4 to 14 mo old. Histological examination of longitudinal sections of the VE-Y685F uterus showed an extensive disorganization of myometrium and endometrium with highly edematous uterine glands, numerous areas with sparse cells, and increased accumulation of collagen fibers around blood vessels, indicating a fibrotic state. Analysis of cross section of ovaries showed the appearance of spontaneous cysts, which suggested increased vascular hyperpermeability. Electron microscopy analysis of capillaries in the ovary showed a slight but significant increase in the gap size between two adjacent endothelial cell membranes in the junctions of VE-Y685F mice (wild-type, 11.5 ± 0.3, n = 78; and VE-Y685F, 12.48 ± 0.3, n = 65; P = 0.045), as well as collagen fiber accumulation around capillaries. Miles assay revealed that either basal or vascular endothelial growth factor-stimulated permeability in the skin was increased in VE-Y685F mice. Since edema and fibrotic appearance have been identified as hallmarks of initial increased vascular permeability, we conclude that the site Y685 in VE-cadherin is involved in the physiological regulation of capillary permeability. Furthermore, this knock-in mouse model is of potential interest for further studies of diseases that are associated with abnormal vascular permeability.

Hormonal regulation of focal adhesions in bovine adrenocortical cells: induction of paxillin dephosphorylation by adrenocorticotropic hormone

A study of bovine adrenocortical cell shape on adrenocorticotropic hormone (ACTH) challenge showed that the cells round up and develop arborized processes. This effect was found to be (1) specific for ACTH because angiotensin II and basic fibroblast growth factor have no effect; (2) mediated by a cAMP-dependent pathway because forskolin reproduces the effect of the hormone; (3) inhibited by sodium orthovanadate, a phosphotyrosine phosphatase inhibitor, but unchanged by okadaic acid, a serine/threonine phosphatase inhibitor; and (4) correlated with a complete loss of focal adhesions. Biochemical studies of the focal-adhesion-associated proteins showed that pp125fak, vinculin (110 kDa) and paxillin (70 kDa) were detected in the Triton X-100-insoluble fraction from adrenocortical cells. During cell adhesion on fibronectin as substratum, two major phosphotyrosine-containing proteins of molecular masses 125 and 68 kDa were immunodetected in the same fraction. A dramatic decrease in the extent of tyrosine phosphorylation of these proteins was observed within 60 min after treatment with ACTH. No change in pp125fak tyrosine phosphorylation nor in Src activity was detected. In contrast, paxillin was found to be tyrosine-dephosphorylated in a time-dependent manner in ACTH-treated cells. Sodium orthovanadate completely prevented the effect of ACTH. These observations suggest a possible role for phosphotyrosine phosphatases in hormone-dependent cellular regulatory processes.

Sensitization of adrenocortical cell adenylate cyclase activity to ACTH by angiotensin II and activators of protein kinase C

Exposure of bovine adrenocortical cells to optimal concentrations of angiotensin II (A II) resulted in an almost 2-fold enhancement of cellular cAMP accumulation in response to steroidogenic concentrations of ACTH. This effect was dose-dependent and transient, with a maximum after 4-6 min of treatment with A II. Activators of protein kinase C such as 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and 1,2-dioctanoyl-sn-glycerol mimicked that effect in a sustained fashion. The ACTH-sensitized state of the adrenocortical adenylate cyclase system induced by TPA exhibited also an enhanced response to forskolin. On the other hand, previous treatment of the cells by pertussis toxin suppressed any further effect of TPA. It is suggested that, following A II exposure, the Gi inhibitory components of the adrenocortical cell adenylate cyclase system may be inactivated, leading to increased response to ACTH. This process may involve protein kinase C activation, subsequent to intracellular generation of lipidic messengers resulting from accelerated phosphoinositide breakdown induced by angiotensin.

VE-cadherin cleavage in sleep apnoea: new insights into intermittent hypoxia-related endothelial permeability

BACKGROUND

Obstructive sleep apnoea (OSA) causes intermittent hypoxia that in turn induces endothelial dysfunction and atherosclerosis progression. We hypothesised that VE-cadherin cleavage, detected by its released extracellular fragment solubilised in the blood (sVE), may be an early indicator of emergent abnormal endothelial permeability. Our aim was to assess VE-cadherin cleavage in OSA patients and in in vivo and in vitro intermittent hypoxia models to decipher the cellular mechanisms and consequences.

METHODS

Sera from seven healthy volunteers exposed to 14 nights of intermittent hypoxia, 43 OSA patients and 31 healthy control subjects were analysed for their sVE content. Human aortic endothelial cells (HAECs) were exposed to 6 h of intermittent hypoxia in vitro, with or without an antioxidant or inhibitors of hypoxia-inducible factor (HIF)-1, tyrosine kinases or vascular endothelial growth factor (VEGF) pathways. VE-cadherin cleavage and phosphorylation were evaluated, and endothelial permeability was assessed by measuring transendothelial electrical resistance (TEER) and fluorescein isothiocyanate (FITC)-dextran flux.

RESULTS

sVE was significantly elevated in sera from healthy volunteers submitted to intermittent hypoxia and OSA patients before treatment, but conversely decreased in OSA patients after 6 months of continuous positive airway pressure treatment. OSA was the main factor accounting for sVE variations in a multivariate analysis. In in vitro experiments, cleavage and expression of VE-cadherin increased upon HAEC exposure to intermittent hypoxia. TEER decreased and FITC-dextran flux increased. These effects were reversed by all of the pharmacological inhibitors tested.

CONCLUSIONS

We suggest that in OSA, intermittent hypoxia increases endothelial permeability in OSA by inducing VE-cadherin cleavage through reactive oxygen species production, and activation of HIF-1, VEGF and tyrosine kinase pathways.

Soluble vascular endothelial-cadherin and auto-antibodies to human vascular endothelial-cadherin in human diseases: Two new biomarkers of endothelial dysfunction

Vascular endothelial-cadherin is the most important transmembrane component of endothelial adherens junctions, exclusively expressed by endothelial cells in all types of vessels. Targeting either the extracellular domain or the cytoplasmic tail deleteriously affects the junctional strength and leads to vascular permeability. Recently, cytokine-induced phosphorylation of the vascular endothelial-cadherin cytoplasmic domain was reported to trigger cleavage of its extracellular domain, producing the soluble form of the protein – soluble vascular endothelial-cadherin. Hence, the presence of soluble vascular endothelial-cadherin or auto-antibodies to human vascular endothelial-cadherin in human serum could signalize the presence of vascular abnormalities. This systematic review covers many human studies reporting increased levels of soluble vascular endothelial-cadherin, as well as auto-antibodies to human vascular endothelial-cadherin, which could be promising biomarkers of endothelial dysfunction in a large panel of diseases.