Thrombin and histamine rapidly stimulate the phosphorylation of the myristoylated alanine-rich C-kinase substrate in human umbilical vein endothelial cells: evidence for distinct patterns of protein kinase activation

Histamine-induced production of interleukin-6 and interleukin-8 by human coronary artery endothelial cells is enhanced by endotoxin and tumor necrosis factor-alpha

In this study, we tested the synergy between histamine and LPS, and histamine and TNF-alpha, on endothelial cell production of interleukin-6 (IL-6), interleukin-8 (IL-8), and monocyte chemoattractant protein-1 (MCP-1). Human coronary artery endothelial cells (HCAEC) were cultured in vitro with histamine (0.1 to 1000 microM) in the presence or absence of LPS or TNF-alpha for 24 h, and the secreted IL-6, IL-8 and MCP-1 were quantified. Unactivated HCAEC produced minimal levels of IL-6, IL-8, or MCP-1. The incubation of HCAEC with histamine resulted in low level induction of IL-6 and IL-8 production, which was dose-dependent and attained a plateau at a concentration of 10 microM. On the other hand, histamine failed to induce MCP-1 production. Stimulation of HCAEC with LPS or TNF-alpha caused dose-dependent increase in cytokine production. In the presence of all stimulatory concentrations of LPS and TNF-alpha tested, histamine was shown to further enhance IL-6 and IL-8 production. The effect of histamine on endothelial cell production of cytokines was completely inhibited by the H-1 receptor antagonist, diphenhydramine, and not by the H-2 antagonist, famotidine. Electrophoretic mobility shift assays of nuclear proteins extracted from HCAEC treated with histamine and LPS, or histamine and TNF-alpha, revealed amplified translocation of NF-kappaB proteins to the nuclei. Since both LPS and TNF-alpha potentiated histamine-induced cytokine production, it is possible that these activators stimulate H-1 receptor expression and/or augment the signal transduction pathways leading to the expression of IL-6 and IL-8. These results indicate the importance of synergy between histamine and other inflammatory stimuli on endothelial cell activation and implicate their cooperative participation in vascular leak and inflammation.

Calcium antagonists ameliorate ischemia-induced endothelial cell permeability by inhibiting protein kinase C

BACKGROUND

Dihydropyridines block calcium channels; however, they also influence endothelial cells, which do not express calcium channels. We tested the hypothesis that nifedipine can prevent ischemia-induced endothelial permeability increases by inhibiting protein kinase C (PKC) in cultured porcine endothelial cells.

METHODS AND RESULTS

Ischemia was induced by potassium cyanide/deoxyglucose, and permeability was measured by albumin flux. Ion channels were characterized by patch clamp. [Ca2+]i was measured by fura 2. PKC activity was measured by substrate phosphorylation after cell fractionation. PKC isoforms were assessed by Western blot and confocal microscopy. Nifedipine prevented the ischemia-induced increase in permeability in a dose-dependent manner. Ischemia increased [Ca2+]i, which was not affected by nifedipine. Instead, ischemia-induced PKC translocation was prevented by nifedipine. Phorbol ester also increased endothelial cell permeability, which was dose dependently inhibited by nifedipine. The effects of non-calcium-channel-binding dihydropyridine derivatives were similar. Analysis of the PKC isoforms showed that nifedipine prevented ischemia-induced translocation of PKC-alpha and PKC-zeta. Specific inhibition of PKC isoforms with antisense oligodeoxynucleotides demonstrated a major role for PKC-alpha.

CONCLUSIONS

Nifedipine exerts a direct effect on endothelial cell permeability that is independent of calcium channels. The inhibition of ischemia-induced permeability by nifedipine seems to be mediated primarily by PKC-alpha inhibition. Anti-ischemic effects of dihydropyridine calcium antagonists could be due in part to their effects on endothelial cell permeability.

Signal transduction pathways involved in enterohemorrhagic Escherichia coli-induced alterations in T84 epithelial permeability

Enterohemorrhagic Escherichia coli (EHEC) infection is associated with watery diarrhea and can lead to complications, including hemorrhagic colitis and the hemolytic-uremic syndrome. The mechanisms by which these organisms produce diarrheal disease remain to be elucidated. Changes in T84 epithelial cell electrophysiology were examined following EHEC infection. T84 cell monolayers infected with EHEC O157:H7 displayed a time-dependent decrease in transepithelial resistance. Increases in the transepithelial flux of both [3H]mannitol and 51Cr-EDTA accompanied the EHEC-induced decreases in T84 resistance. Altered barrier function induced by EHEC occurred at the level of the tight junction since immunofluorescent staining of the tight-junction-associated protein ZO-1 was disrupted when examined by confocal microscopy. Decreased resistance induced by EHEC involved a protein kinase C (PKC)-dependent pathway as the highly specific PKC inhibitor, CGP41251, abrogated the EHEC-induced drop in resistance. PKC activity was also increased in T84 cells infected with EHEC. Calmodulin and myosin light chain kinase played a role in EHEC-induced resistance changes as inhibition of these effector molecules partially reversed the effects of EHEC on barrier function. These studies demonstrate that intracellular signal transduction pathways activated following EHEC infection link the increases in T84 epithelial permeability induced by this pathogen.

Thrombin-mediated focal adhesion plaque reorganization in endothelium: role of protein phosphorylation

Endothelial cell (EC) gap formation and barrier function are subject to dual regulation by (1) axial contractile forces, regulated by myosin light chain kinase activity, and (2) tethering forces, represented by cell-cell and cell-substratum adhesions. We examined whether focal adhesion plaque proteins (vinculin and talin) and focal adhesion kinase, p125FAK (FAK), represent target regulatory sites involved in thrombin-mediated EC barrier dysfunction. Histologically, thrombin produced dramatic rearrangement of EC actin, vinculin, and FAK in parallel with the evolution of gap formation and barrier dysfunction. Vinculin and talin were in vitro substrates for phosphorylation by EC PKC, a key effector enzyme involved in thrombin-induced EC barrier dysfunction. Although vinculin and talin were phosphorylated in situ under basal conditions in 32P-labeled EC, thrombin failed to alter the basal level of phosphorylation of these proteins. Phosphotyrosine immunoblotting showed that neither vinculin nor talin was significantly phosphorylated in situ on tyrosine residues in unstimulated ECs, and this was not further increased after thrombin. In contrast, both thrombin and the thrombin receptor-activating peptide (TRAP) produced an increase in FAK phosphotyrosine levels (corrected for immunoreactive FAK content) present in EC immunoprecipitates. Ionomycin, which produces EC barrier dysfunction in a myosin light chain kinase-independent manner, was used to increase intracellular Ca2+ and evaluate the Ca2+ sensitivity of this observation. In contrast to thrombin, ionomycin effected a dramatic decrease in the phosphotyrosine-to-immunoreactive FAK ratios, suggesting distinct effects of the two agents on FAK phosphorylation and function. These data indicate that modulation of cell tethering via phosphorylation of focal adhesion proteins is complex, agonist-specific, and may be a relevant mechanism of EC barrier dysfunction in permeability models that do not depend on an increase in myosin 20-kD regulatory light chain phosphorylation.

Thrombin-induced phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) protein in bovine pulmonary artery endothelial cells

Myristoylated alanine-rich C kinase substrates (MARCKS) is a prominent protein kinase C (PKC) substrate that is targeted to the plasma membrane by an aminoterminal myristoyl group. In its nonphosphorylated form, MARCKS cross-links Factin and binds calmodulin (CaM) reciprocally. However, upon phosphorylation by PKC, MARCKS release the actin or CaM MARCKS may therefore act as a CaM sink in resting cells and regulate CaM availability during cell activation. We have demonstrated previously that thrombin-induced myosin light chain (MLC) phosphorylation and increased monolayer permeability in bovine pulmonary artery endothelial cells (BPAEC) require both PKC-and CaM-dependent pathways. We therefore decided to investigate the phosphorylation of MARCKS in BPAEC to ascertain whether this occurs in a temporally relevant manner to participate in the thrombin-induced events. MARCKS is phosphorylated in response to thrombin with a time course similar to that seen with MLC. As expected, MARCKS is also phosphorylated by phorbol 12-myristate 13 acetate (PMA), a PKC activator, but with a slower onset and more prolonged duration. Bradykinin also enhances MARCKS phosphorylation in BPAEC, but histamine does not. MARCKS is distributed evently between the membrane and cytosol in BPAEC, and neither thrombin nor PMA caused significant translocation of the protein. Specific PKC inhibitors attenuated MARCKS phosphorylation by either thrombin or PMA. Since thrombin-induced MLC phosphorylation is also attenuated by these inhibitors, MARCKS may be involved in MLC kinase activation and subsequent BPAEC contraction. W7, a CaM antagonist, enhances the phosphorylation of MARCKS. This was expected since CaM binding to MARCKS has been shown to decrease MARCKS phosphorylation by PKC. On the other hand, tyrosine kinase inhibitors, genistein and tyrphostin, attenuate MARCKS phosphorylation but have no effect on MLC phosphorylation, suggesting that MARCKS may be phosphorylated by kinases other than PKC. Phosphorylation of MARCKS outside the PKC phosphorylation domain would not be expected to induce the release of CaM. These data provide support for the hypothesis that MARCKS may serve as a regulator of CaM availability in BPAEC.

Protein tyrosine kinases regulate agonist-stimulated prostacyclin release but not von Willebrand factor secretion from human umbilical vein endothelial cells

The rapid synthesis and release of prostacyclin (PGI2) and the exocytotic secretion of von Willebrand Factor (vWF) elicited by activation of G-protein-coupled receptors on endothelium occur via signaling mechanisms which are incompletely defined. Activation of protein tyrosine kinases (PTKs) and modulation of the tyrosine-phosphorylation state of endogenous proteins have been implicated in several cellular processes including arachidonate release and exocytosis. In the present study we have examined the regulatory role of PTKs in agonist-stimulated release of PGI2 and vWF from human umbilical vein endothelial cells (HUVECs) using two chemically and mechanistically dissimilar PTK inhibitors (genistein and ST271). Genistein, but not the less active analogue daidzein, dose-dependently attenuated PGI2 release in response to thrombin and histamine (IC50 approx. 20 microM), and to the thrombin-receptor-activating peptide. A more potent inhibition of thrombin- and histamine-induced PGI2 synthesis was observed in cells exposed to ST271. In contrast, neither genistein nor ST271 modulated agonist-drive vWF secretion. At concentrations that abolished PGI2 release, genistein blocked thrombin- or histamine-evoked tyrosine phosphorylation of a 42 kDa protein. Ca2+ ionophore-induced PGI2 generation, but not vWF secretion, was also inhibited by both genistein and ST271, suggesting that these agents modulate PGI2 synthesis by acting at, or distal to, agonist-induced changes in intracellular CA2+ ([Ca2+]i). In fura-2-loaded HUVECs genistein partially reduced the histamine-induced peak [Ca2+]i but had no effect on the thrombin response. Ca(2+)-induced PGI2 release from electrically permeabilized HUVECs was abolished in the presence of ST271 or genistein, but not diadzein. The generation of PGI2 in response to exogenous arachidonic acid was not modulated by genistein or ST271, suggesting that PTK inhibitors do not directly inhibit cyclo-oxygenase activity. Taken together, these results suggest that PTKs regulate PGI2 synthesis and release in HUVECs by modulating, directly or indirectly, a CA(2+)-sensitive step upstream of cyclo-oxygenase.

Regulation of endothelial cell gap formation and barrier dysfunction: role of myosin light chain phosphorylation

Endothelial cell (EC) contraction results in intercellular gap formation and loss of the selective vascular barrier to circulating macromolecules. We tested the hypothesis that phosphorylation of regulatory myosin light chains (MLC) by Ca2+/calmodulin-dependent myosin light chain kinase (MLCK) is critical to EC barrier dysfunction elicited by thrombin. Thrombin stimulated a rapid (< 15 sec) increase in [Ca2+]i which preceded maximal MLC phosphorylation (60 sec) with a 6 to 8-fold increase above constitutive levels of phosphorylated MLC. Dramatic cellular shape changes indicative of contraction and gap formation were observed at 5 min with maximal increases in albumin permeability occurring by 10 min. Neither the Ca2+ ionophore, A23187, nor phorbol myristate acetate (PMA), a direct activator of protein kinase C (PKC), alone or in combination, produced MLC phosphorylation. The combination was synergistic, however, in stimulating EC contraction/gap formation and barrier dysfunction (3 to 4-fold increase). Down-regulation or inhibition of PKC activity attenuated thrombin-induced MLC phosphorylation (approximately 40% inhibition) and both thrombin- and PMA-induced albumin clearance (approximately 50% inhibition). Agents which augmented [cAMP]i partially blocked thrombin-induced MLC phosphorylation (approximately 50%) and completely inhibited both thrombin- and PMA-induced EC permeability (100% inhibition). Furthermore, cAMP produced significant reduction in the basal levels of constitutive MLC phosphorylation. Finally, MLCK inhibition (with either ML-7 or KT 5926) or Ca2+/calmodulin antagonism (with either trifluoperazine or W-7) attenuated thrombin-induced MLC phosphorylation and barrier dysfunction. These results suggest a model wherein EC contractile events, gap formation and barrier dysfunction occur via MLCK-dependent and independent mechanisms and are significantly modulated by both PKC and cAMP-dependent protein kinase A activities.

Prolonged peak elevations in cytoplasmic free calcium ions, derived from intracellular stores, correlate with the extent of thrombin-stimulated exocytosis in single human umbilical vein endothelial cells

We have used indo-1-loaded human endothelial cells (EC) in monolayer culture and quantitative laser scanning fluorescence microscopy techniques to investigate the magnitude and duration of the change in cytoplasmic free calcium ([Ca2+]i) required for thrombin-stimulated von Willebrand factor (vWF) secretion in individual EC. Both alpha-thrombin and a 14 amino acid thrombin receptor activating peptide stimulate an increase in EC [Ca2+]i that is agonist dose dependent. Low-dose agonist treatment generates asynchronous oscillations (i.e., repetitive spikes < 80 sec duration) in [Ca2+]i. Stimulation with higher agonist concentrations generates a prolonged single peak elevation in [Ca2+]i. Both the number of cells displaying prolonged [Ca2+]i peaks and the mean amplitude of the peaks increase as a function of agonist concentration. Higher doses of agonist also cause sustained elevations in [Ca2+]i that depend upon extracellular Ca2+. Oscillations in [Ca2+]i are not sufficient to stimulate significant vWF secretion, and sustained elevations in [Ca2+]i are not required for maximal secretion. Both the number of cells displaying prolonged peaks and the mean peak amplitude correlate with increasing levels of vWF secretion from the culture. We have used the expression of P-selectin, a secretory granule membrane protein, as a marker for measuring thrombin-induced exocytosis in individual EC. Both the number of secreting cells and the amount of secretion per cell increase as a function of thrombin concentration. The graded responses in [Ca2+]i amplitudes and the graded exocytotic response may be causally related.

The control of production and release of haemostatic factors in the endothelial cell

Endothelial cell products contribute to many aspects of the regulation of haemostasis. They include potent inhibitors of platelet aggregation (prostacyclin and nitric oxide) rapidly released in response to agonists such as thrombin. Similar agonists also induce the formation of platelet-activating factor by endothelium. Endothelial cell surface ectonucleotidase enzymes control the catabolism of platelet-active adenine nucleotides. The main source of the circulating coagulant cofactor von Willebrand factor is the endothelium, where it is stored in granules for agonist-triggered exocytosis and also secreted constitutively. Surface anticoagulant activities are due to the presence of antithrombin and thrombomodulin. Endothelial cells also secrete plasminogen activator and its inhibitor. Many of these reactions are significantly modulated by exposure of endothelium to cytokines or bacterial endotoxin, the most striking example being the new synthesis and surface expression of the procoagulant tissue factor (thromboplastin).

Role of ADP-ribosylation in endothelial signal transduction and prostacyclin production

ADP-ribosylation of proteins by the enzymatic transfer of ADP-ribose from NAD has been implicated in a number of biological processes. We report that inhibitors of ADP-ribosylation, most notably the novel inhibitor of arginine specific cellular mono(ADP-ribosyl) transferase, meta-iodobenzylguanidine (MIBG) as well as nicotinamide, L-arginine methyl ester (LAME) and guanyltyramine, inhibit histamine-induced endothelial production of inositol phosphates, release of arachidonic acid and production of prostacyclin (PGI2). Those same responses were unaffected by MIBG when triggered by thrombin or leukotriene C4. These findings suggest that ADP-ribosylation serves a role in histamine-induced production of prostacyclin and imply differences in transduction pathways employed by the different agonists.

Calcium/calmodulin transduces thrombin-stimulated secretion: studies in intact and minimally permeabilized human umbilical vein endothelial cells

Thrombin stimulates cultured endothelial cells (EC) to secrete stored von Willebrand factor (vWF), but the signal transduction pathways are poorly defined. Thrombin is known to elevate the concentration of intracellular calcium ([Ca2+]i) and to activate protein kinase C (PKC) in EC. Since both calcium ionophores and phorbol esters release vWF, both second messenger pathways have been postulated to participate in vWF secretion in response to naturally occurring agonists. We find that in intact human EC, vWF secretion stimulated by either thrombin or by a thrombin receptor activating peptide, TR(42-55), can be correlated with agonist-induced elevations of [Ca2+]i. Further evidence implicating calcium in the signal transduction pathway is suggested by the finding that MAPTAM, a cell-permeant calcium chelator, in combination with the extracellular calcium chelator EGTA, can inhibit thrombin-stimulated secretion. In contrast, the observation that staurosporine (a pharmacological inhibitor of PKC) blocks phorbol ester- but not thrombin-stimulated secretion provides evidence against PKC-mediated signal transduction. To examine further the signal transduction pathway initiated by thrombin, we developed novel conditions for minimal permeabilization of EC with saponin (4-8 micrograms/ml for 5-15 min at 37 degrees C) which allow the introduction of small extracellular molecules without the loss of large intracellular proteins and which retain thrombin-stimulated secretion. These minimally permeabilized cells secrete vWF in response to exogenous calcium, and EGTA blocks thrombin-induced secretion. Moreover, in these cells, thrombin-stimulated secretion is blocked by a calmodulin-binding inhibitory peptide but not by a PKC inhibitory peptide. Taken together, these findings demonstrate that thrombin-stimulated vWF secretion is transduced by a rise in [Ca2+]i and provide the first evidence for the role of calmodulin in this process.