Thrombin and histamine activate phospholipase C in human endothelial cells via a phorbol ester-sensitive pathway

Histamine and diabetic nephropathy: an up-to-date overview

The classification of diabetic nephropathy (DN) as a vascular complication of diabetes makes the possible involvement of histamine, an endogenous amine that is well known for its vasoactive properties, an interesting topic for study. The aim of the present review is to provide an extensive overview of the possible involvement of histamine in the onset and progression of DN. The evidence collected on the role of histamine in kidney function together with its well-known pleiotropic action suggest that this amine may act simultaneously on glomerular hyperfiltration, tubular inflammation, fibrosis development and tubular hypertrophy.

Involvement of the H1 Histamine Receptor, p38 MAP Kinase, Myosin Light Chains Kinase, and Rho/ROCK in Histamine-Induced Endothelial Barrier Dysfunction

OBJECTIVE

The mechanisms by which histamine increases microvascular permeability remain poorly understood. We tested the hypothesis that H1 receptor activation disrupts the endothelial barrier and investigated potential downstream signals.

METHODS

We used confluent EC monolayers, assessing TER as an index of barrier function. HUVEC, HCMEC, and HDMEC were compared. Receptor expression was investigated using Western blotting, IF confocal microscopy and RT-PCR. Receptor function and downstream signaling pathways were tested using pharmacologic antagonists and inhibitors, respectively.

RESULTS

We identified H1-H4 receptors on all three EC types. H1 antagonists did not affect basal TER but prevented the histamine-induced decrease in TER. Blockade of H2 or H3 attenuated the histamine response only in HDMEC, while inhibition of H4 attenuated the response only in HUVEC. Combined inhibition of both PKC and PI3K caused exaggerated histamine-induced barrier dysfunction in HDMEC, whereas inhibition of p38 MAP kinase attenuated the histamine response in all three EC types. Inhibition of RhoA, ROCK, or MLCK also prevented the histamine-induced decrease in TER in HDMEC.

CONCLUSION

The data suggest that multiple signaling pathways contribute to histamine-induced endothelial barrier dysfunction via the H1 receptor.

Podocyte dysfunction in atypical haemolytic uraemic syndrome

Genetic or autoimmune defects that lead to dysregulation of the alternative pathway of complement have been associated with the development of atypical haemolytic uraemic syndrome (aHUS), which is characterized by thrombocytopenia, haemolytic anaemia and acute kidney injury. The relationship between aHUS, podocyte dysfunction and the resultant proteinuria has not been adequately investigated. However, the report of mutations in diacylglycerol kinase ε (DGKE) as a cause of recessive infantile aHUS characterized by proteinuria, highlighted podocyte dysfunction as a potential complication of aHUS. DGKE deficiency was originally thought to trigger aHUS through pathogenetic mechanisms distinct from complement dysregulation; however, emerging findings suggest an interplay between DGKE and complement systems. Podocyte dysfunction with nephrotic-range proteinuria can also occur in forms of aHUS associated with genetic or autoimmune complement dysregulation without evidence of DGKE mutations. Furthermore, proteinuric glomerulonephritides can be complicated by aHUS, possibly as a consequence of podocyte dysfunction inducing endothelial injury and prothrombotic abnormalities.

The Bcl-2 homolog Nrz inhibits binding of IP3 to its receptor to control calcium signaling during zebrafish epiboly

Members of the Bcl-2 protein family regulate mitochondrial membrane permeability and also localize to the endoplasmic reticulum where they control Ca(2+) homeostasis by interacting with inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs). In zebrafish, Bcl-2-like 10 (Nrz) is required for Ca(2+) signaling during epiboly and gastrulation. We characterized the mechanism by which Nrz controls IP3-mediated Ca(2+) release during this process. We showed that Nrz was phosphorylated during early epiboly, and that in embryos in which Nrz was knocked down, reconstitution with Nrz bearing mutations designed to prevent its phosphorylation disrupted cyclic Ca(2+) transients and the assembly of the actin-myosin ring and led to epiboly arrest. In cultured cells, wild-type Nrz, but not Nrz with phosphomimetic mutations, interacted with the IP3 binding domain of IP3R1, inhibited binding of IP3 to IP3R1, and prevented histamine-induced increases in cytosolic Ca(2+). Collectively, these data suggest that Nrz phosphorylation is necessary for the generation of IP3-mediated Ca(2+) transients and the formation of circumferential actin-myosin cables required for epiboly. Thus, in addition to their role in apoptosis, by tightly regulating Ca(2+) signaling, Bcl-2 family members participate in the cellular events associated with early vertebrate development, including cytoskeletal dynamics and cell movement.

Tight junction-related barrier contributes to the electrophysiological asymmetry across vocal fold epithelium

Electrophysiological homeostasis is indispensable to vocal fold hydration. We investigate tight junction (TJ)-associated components, occludin and ZO-1, and permeability with or without the challenge of a permeability-augmenting agent, histamine. Freshly excised ovine larynges are obtained from a local abattoir. TJ markers are explored via reverse transcriptase polymerase chain reaction (RT-PCR). Paracellular permeabilities are measured in an Ussing system. The gene expression of both TJ markers is detected in native ovine vocal fold epithelium. Luminal histamine treatment significantly decreases transepithelial resistance (TER) (N = 72, p<0.01) and increases penetration of protein tracer (N = 35, p<0.001), respectively, in a time-, and dose-dependent fashion. The present study demonstrates that histamine compromises TJ-related paracellular barrier across vocal fold epithelium. The detection of TJ markers indicates the existence of typical TJ components in non-keratinized, stratified vocal fold epithelium. The responsiveness of paracellular permeabilities to histamine would highlight the functional significance of this TJ-equivalent system to the electrophysiological homeostasis, which, in turn, regulates the vocal fold superficial hydration.

Chronic exposure to high fatty acids impedes receptor agonist-induced nitric oxide production and increments of cytosolic Ca2+ levels in endothelial cells

Dyslipidemia is a common metabolic disorder in diabetes. Nitric oxide (NO) production from endothelium plays the primary role in endothelium-mediated vascular relaxation and other endothelial functions. Therefore, we investigated the effects of elevated free fatty acids (FFA) on the stimulation of NO production by phospholipase C (PLC)-activating receptor agonists (potent physiological endothelium-dependent vasodilators) and defined the possible alterations of signaling pathways implicated in this scenario. Exposure of bovine aortic endothelial cells (BAECs) to high concentrations of a mixture of fatty acids (oleate and palmitate) for 5 or 10 days significantly reduced NO production evoked by receptor agonists (bradykinin or ATP) in a time- and dose-dependent manner. Such defects were not associated with alterations of either endothelial NO synthase mass or inositol phospholipid contents but were probably due to reduced elevations of intracellular free Ca(2)(+) levels ([Ca(2)(+)](i)) under these conditions. Exposure of BAECs to FFA significantly attenuated agonist-induced [Ca(2)(+)](i) increases by up to 54% in a dose- and time-dependent manner. Moreover, bradykinin receptor affinity on the cell surface was significantly decreased by high concentrations of FFA. The morphology of BAECs was altered after 10-day culture with high FFA. Co-culture with protein kinase C (PKC) inhibitors or antioxidants was able to reverse the impairments of receptor agonist-induced NO production and [Ca(2)(+)](i) rises as well as the alteration of receptor affinity in BAECs exposed to FFA. These data indicate that chronic exposure to high FFA reduces NO generation in endothelial cells probably by impairing PLC-mediated Ca(2)(+) signaling pathway through activation of PKC and excess generation of oxidants.

Rate, extent and concentration dependence of histamine-evoked Weibel-Palade body exocytosis determined from individual fusion events in human endothelial cells

The rate, concentration dependence and extent of histamine-evoked Weibel-Palade body (WPB) exocytosis were investigated with time-resolved fluorescence microscopy in cultured human umbilical vein endothelial cells expressing WPB-targeted chimeras of enhanced green fluorescent protein (EGFP). Exocytosis of single WPBs was characterized by an increase in EGFP fluorescence, morphological changes and release of WPB contents. The fluorescence increase was due to a rise of intra-WPB pH from resting levels, estimated as pH 5.45+/-0.26 (s.d., n=144), to pH 7.40. It coincided with uptake of extracellular Alexa-647, indicating the formation of a fusion pore, prior to loss of fluorescent contents. Delays between the increase in intracellular free calcium ion concentration evoked by histamine and the first fusion event were 10.0+/-4.42 s (n=9 cells) at 0.3 microM histamine and 1.57+/-0.21 s (n=15 cells) at 100 microM histamine, indicating the existence of a slow process or processes in histamine-evoked WPB exocytosis. The maximum rates of exocytosis were 1.20+/-0.16 WPB s(-1) (n=9) at 0.3 microM and 3.66+/-0.45 WPB s(-1) at 100 microM histamine (n=15). These occurred 2-5 s after histamine addition and declined to lower rates with continued stimulation. The initial delays and maximal rate of exocytosis were unaffected by removal of external Ca2+ indicating that the initial burst of secretion is driven by Ca2+ release from internal stores, but sustained exocytosis required external Ca2+. Data were compared to exocytosis evoked by a maximal concentration of the strong secretagogue ionomycin (1 microM), for which there was a delay between calcium elevation and secretion of 1.67+/-0.24 s (n=6), and a peak fusion rate of approximately 10 WPB s(-1).

Endothelial thrombomodulin induces Ca2+ signals and nitric oxide synthesis through epidermal growth factor receptor kinase and calmodulin kinase II

Endothelial membrane-bound thrombomodulin is a high affinity receptor for thrombin to inhibit coagulation. We previously demonstrated that the thrombin-thrombomodulin complex restrains cell proliferation mediated through protease-activated receptor (PAR)-1. We have now tested the hypothesis that thrombomodulin transduces a signal to activate the endothelial nitric-oxide synthase (NOS3) and to modulate G protein-coupled receptor signaling. Cultured human umbilical vein endothelial cells were stimulated with thrombin or a mutant of thrombin that binds to thrombomodulin and has no catalytic activity on PAR-1. Thrombin and its mutant dose dependently activated NO release at cell surface. Pretreatment with anti-thrombomodulin antibody suppressed NO response to the mutant and to low thrombin concentration and reduced by half response to high concentration. Thrombin receptor-activating peptide that only activates PAR-1 and high thrombin concentration induced marked biphasic Ca2+ signals with rapid phosphorylation of PLC(beta3) and NOS3 at both serine 1177 and threonine 495. The mutant thrombin evoked a Ca2+ spark and progressive phosphorylation of Src family kinases at tyrosine 416 and NOS3 only at threonine 495. It activated rapid phosphatidylinositol-3 kinase-dependent NO synthesis and phosphorylation of epidermal growth factor receptor and calmodulin kinase II. Complete epidermal growth factor receptor inhibition only partly reduced the activation of phospholipase Cgamma1 and NOS3. Prestimulation of thrombomodulin did not affect NO release but reduced Ca2+ responses to thrombin and histamine, suggesting cross-talks between thrombomodulin and G protein-coupled receptors. This is the first demonstration of an outside-in signal mediated by the cell surface thrombomodulin receptor to activate NOS3 through tyrosine kinase-dependent pathway. This signaling may contribute to thrombomodulin function in thrombosis, inflammation, and atherosclerosis.

Chronic exposure to high glucose impairs bradykinin-stimulated nitric oxide production by interfering with the phospholipase-C-implicated signalling pathway in endothelial cells: evidence for the involvement of protein kinase C

AIMS/HYPOTHESIS

Overwhelming evidence indicates that endothelial cell dysfunction in diabetes is characterised by diminished endothelium-dependent relaxation, but the matter of the underlying molecular mechanism remains unclear. As nitric oxide (NO) production from the endothelium is the major player in endothelium-mediated vascular relaxation, we investigated the effects of high glucose on NO production, and the possible alterations of signalling pathways implicated in this scenario.

METHODS

NO production and intracellular Ca(2+) levels ([Ca(2+)](i)) were assessed using the fluorescent probes 4,5-diaminofluorescein diacetate and fura-2 respectively.

RESULTS

Exposure of cultured bovine aortic endothelial cells to high glucose for 5 or 10 days significantly reduced NO production induced by bradykinin (but not by Ca(2+) ionophore) in a time- and dose-dependent manner. This was probably due to an attenuation in bradykinin-induced elevations of [Ca(2+)](i) under these conditions, since a close correlation between [Ca(2+)](i) increases and NO generation was observed in intact bovine aortic endothelial cells. Both bradykinin-promoted intracellular Ca(2+) mobilisation and extracellular Ca(2+) entry were affected. Moreover, bradykinin-induced formation of Ins(1,4,5)P(3), a phospholipase C product leading to increases in [Ca(2+)](i), was also inhibited following high glucose culture. This abnormality was not attributable to a decrease in inositol phospholipids, but possibly to a reduction in the number of bradykinin receptors. The alterations in NO production, the increases in [Ca(2+)](i), and the bradykinin receptor number due to high glucose could be largely reversed by protein kinase C inhibitors and D: -alpha-tocopherol (antioxidant).

CONCLUSIONS/INTERPRETATION

Chronic exposure to high glucose reduces NO generation in endothelial cells, probably by impairing phospholipase-C-mediated Ca(2+) signalling due to excess protein kinase C activation. This defect in NO release may contribute to the diminished endothelium-dependent relaxation and thus to the development of cardiovascular diseases in diabetes.

Phospholipase D regulates calcium oscillation frequency and nuclear factor-kappaB activity in histamine- stimulated human endothelial cells

Histamine stimulates [Ca(2+)](i) oscillations in human aortic endothelial cells (HAEC), the frequency of which regulates the activity of nuclear factor-kappaB (NF-kappaB). This study was performed to determine whether phospholipase D (PLD) is involved in this signaling pathway. At a concentration of 1 microM, which stimulates [Ca(2+)](i) oscillations in this cell type, histamine initiated a twofold increase in [(32)P]phosphatidybutanol (PBt), an index of PLD activity as early as 5 min after stimulation. During established [Ca(2+)](i) oscillations induced by 1 microM histamine, 0.3% n-butanol, which "functionally" redirects phosphatidic acid formed by PLD to PBt, decreased [Ca(2+)](i) oscillation frequency by approximately 50% and produced a similar reduction in NF-kappaB activity. In the presence of the inositol 1,4,5-trisphosphate receptor blocker xestospongin C, which itself decreases the frequency of histamine-stimulated [Ca(2+)](i) oscillations, n-butanol produced a further decrease in oscillation frequency that was not associated with an additional reduction in NF-kappaB activity. This study shows that activation of PLD by histamine regulates [Ca(2+)](i) oscillation frequency and NF-kappaB activity in HAEC.

Thrombin-mediated permeability of human microvascular pulmonary endothelial cells is calcium dependent

BACKGROUND

In response to inflammation, endothelial cytoskeleton rearrangement, cell contraction, and intercellular gap formation contribute to a loss of capillary barrier integrity and resultant interstitial edema formation. The intracellular signals controlling these events are thought to be dependent on intracellular calcium concentration ([Ca2+]i). We hypothesized that, in human pulmonary microvascular endothelial cells, a thrombin-induced increase in permeability to albumin would be dependent on Ca2+i and subsequent actin cytoskeleton rearrangements.

METHODS

Human lung microvascular endothelial cells, grown on 0.4 micromol/L pore membranes, were activated with 10 nmol/L human thrombin in Hank's balanced salt solution/0.5% fetal bovine serum. Select cultures were pretreated (45 minutes) with 4 micromol Fura-2/AM to chelate Ca2+i. Permeability was assessed as diffusion of bovine serum albumin/biotin across the monolayer. Similarly treated cells were stained with rhodamine-phalloidin to demonstrate actin cytoskeletal morphology. Separately, cells loaded 2 micromol Fura-2/AM were assessed at OD340/380nm after thrombin exposure to detect free Ca2+i.

RESULTS

Intracellular Ca2+ levels increased 15-fold (2 seconds) and fell to baseline (10 minutes) after thrombin. Permeability increased 10-fold (30 minutes), and a shift from cortical to actin stress fiber morphology was observed. Chelation of Ca2+i diminished permeability to baseline and reduced the percentage of cells exhibiting stress fiber formation.

CONCLUSION

Thrombin stimulates pulmonary capillary leak by affecting the barrier function of activated pulmonary endothelial cells. These data demonstrate a thrombin-stimulated increase in monolayer permeability, and cytoskeletal F-actin stress fibers were, in part, regulated by endothelial Ca2+i. This early, transient rise in Ca2+i likely activates downstream pathways that more directly affect the intracellular endothelial structural changes that control vascular integrity.

Cellular regulation of endothelial nitric oxide synthase

Renal function is highly dependent on endothelium-derived nitric oxide (NO). Several renal disorders have been linked to impaired NO bioavailability. The enzyme that is responsible for the synthesis of NO within the renal endothelium is endothelial NO synthase (eNOS). eNOS-mediated NO generation is a highly regulated cellular event, which is induced by calcium-mobilizing agonists and fluid shear stress. eNOS activity is regulated at the transcriptional level but also by a variety of modifications, such as acylation and phosphorylation, by its cellular localization, and by protein-protein interactions. The present review focuses on the complex regulation of eNOS within the endothelial cell.

Microtubule dysfunction by posttranslational nitrotyrosination of alpha-tubulin: a nitric oxide-dependent mechanism of cellular injury

NO2Tyr (3-Nitrotyrosine) is a modified amino acid that is formed by nitric oxide-derived species and has been implicated in the pathology of diverse human diseases. Nitration of active-site tyrosine residues is known to compromise protein structure and function. Although free NO2Tyr is produced in abundant concentrations under pathological conditions, its capacity to alter protein structure and function at the translational or posttranslational level is unknown. Here, we report that free NO2Tyr is transported into mammalian cells and selectively incorporated into the extreme carboxyl terminus of alpha-tubulin via a posttranslational mechanism catalyzed by the enzyme tubulin-tyrosine ligase. In contrast to the enzymatically regulated carboxyl-terminal tyrosination/detyrosination cycle of alpha-tubulin, incorporation of NO2Tyr shows apparent irreversibility. Nitrotyrosination of alpha-tubulin induces alterations in cell morphology, changes in microtubule organization, loss of epithelial-barrier function, and intracellular redistribution of the motor protein cytoplasmic dynein. These observations imply that posttranslational nitrotyrosination of alpha-tubulin invokes conformational changes, either directly or via allosteric interactions, in the surface-exposed carboxyl terminus of alpha-tubulin that compromises the function of this critical domain in regulating microtubule organization and binding of motor- and microtubule-associated proteins. Collectively, these observations illustrate a mechanism whereby free NO2Tyr can impact deleteriously on cell function under pathological conditions encompassing reactive nitrogen species production. The data also yield further insight into the role that the alpha-tubulin tyrosination/detyrosination cycle plays in microtubule function.

Thrombin-, bradykinin-, and arachidonic acid-induced Ca2+ signaling in Ehrlich ascites tumor cells

Stimulation of single Ehrlich ascites tumor cells with agonists (bradykinin, thrombin) and with arachidonic acid (AA) induces increases in the free intracellular Ca2+ concentration ([Ca2+]i) in the presence and absence of extracellular Ca2+, measured using the Ca2+-sensitive probe fura 2. Sequential stimulation with two agonists elicits sequential increases in [Ca2+]i, unlike addition of the same agonist twice. Bradykinin and thrombin have additive effects on [Ca2+]i in Ca2+-free medium. The phosphoinositidase C inhibitor U-73122 inhibits the agonist-induced increases in [Ca2+]i, whereas ryanodine has no effect. Pretreatment of cells in Ca2+-free medium with thapsigargin abolishes the bradykinin-induced increase in [Ca2+]i but not the response to thrombin. The AA-induced response is not inhibited by U-73122 and cannot be mimicked by the inactive structural analog trifluoromethylarachidonyl ketone. Pretreatment of the cells with 50 microM AA (but not with 10 microM AA) abolishes the agonist-induced increase in [Ca2+]i. Thus bradykinin, thrombin, and AA induce increases in [Ca2+]i in Ehrlich cells due to Ca2+ entry and release from intracellular stores. Thrombin causes release of Ca2+ from an intracellular store that is insensitive to bradykinin and is not depleted by thapsigargin but is depleted by AA.

Mechanism of endothelium-dependent relaxation induced by thrombin in the pig coronary artery

The mechanism of thrombin-induced endothelium-dependent relaxation was investigated using fura-2 front-surface fluorometry. Thrombin induced an endothelium-dependent relaxation during U46619-induced contractions in pig coronary arterial strips. The relaxation consisted of two components: the early phasic component with a transient decrease in [Ca2+]i of smooth muscle and the subsequent sustained tonic component without [Ca2+]i decrease. The phasic relaxation was inhibited by a combination of N(omega)-nitro-L-arginine and K+-depolarization, while the tonic component was inhibited by either indomethacin or K+-depolarization. Thrombin induced a transient [Ca2+]i increase and nitric oxide (NO) production in pig aortic valvular endothelial cells, which expressed NO synthase as determined by reverse transcription and polymerase chain reaction. Thus, it was concluded that NO and hyperpolarizing factor were involved in the phasic component of thrombin-induced relaxation and that hyperpolarizing factor and prostacyclin were involved in the tonic component.

Endothelial cell intracellular Ca2+ concentration is increased upon breast tumor cell contact and mediates tumor cell transendothelial migration

Tumor cell extravasation is a determinant step in the process of hematogenous metastasis. The signal transduction pathways involved in the interactions between tumor cells and the vascular endothelium during transendothelial migration are still undefined. In the present study, we have investigated the influence of human breast adenocarcinoma cells (MCF7) on human umbilical vein endothelial cell (HUVEC) intracellular Ca2+ concentration ([Ca2+]i). We show that the contact between MCF7 cells and a confluent HUVEC monolayer induces an immediate and transient increase in HUVEC [Ca2+]i. This [Ca2+]i rise could not be elicited by tumor cell-conditioned medium, isolated tumor cell membranes, inert beads or normal breast epithelial cells, demonstrating the involvement of specific recognition mechanisms between MCF7 cells and HUVEC. Depletion of HUVEC intracellular Ca2+ stores by the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin as well as the selective depletion of inositol 1,4,5-triphosphate (IP3)-sensitive Ca2+ stores by prior activation of HUVEC using histamine resulted in a complete inhibition of tumor cell-induced [Ca2+]i elevation. Similar results were obtained when HUVEC monolayers were treated with the tyrosine kinase inhibitor herbimycin A, suggesting a role for tyrosine kinase-associated cell surface receptors in tumor cell-endothelial cell interactions. The depletion of HUVEC intracellular Ca2+ stores by thapsigargin was also shown to delay MCF7-induced endothelial cell disjunction, to prevent their spreading on the subendothelial extracellular matrix and transendothelial migration in vitro. These results suggest that transient changes in endothelial [Ca2+]i may govern multiple steps of tumor cell extravasation.

Thrombin promotes endothelial cell alignment in Matrigel in vitro and angiogenesis in vivo

We have tested the effect of thrombin on endothelial cell tube formation in vitro and angiogenesis in vivo. Thrombin induces the differentiation of endothelial cells into capillary structures in a dose-dependent fashion (0.1-0.3 units thrombin/ml) on Matrigel, a laminin-rich reconstituted basement membrane matrix. At higher thrombin concentrations (1.0 unit/ml), a suppression of tube formation is evident, probably due to downregulation (desensitization) of the thrombin receptor. D-Phe-Pro-Arg-CH2Cl-thrombin is without effect when used alone, but it abolishes the tube-promoting effect of thrombin when used in combination with thrombin, indicating the involvement of the catalytic site of thrombin. Activation of protein kinase C (PKC) seems to be the transduction mechanism involved in the stimulation of tube formation by thrombin. Ro-318220 (3 micrograms/ml), a specific inhibitor of PKC, completely abolishes the stimulatory effect of thrombin. In the in vivo Matrigel system of angiogenesis, there is a 10-fold increase in endothelial cell infiltration in response to thrombin. These results provide evidence for the angiogenesis-promoting effect of thrombin in vivo and the induction by thrombin of the angiogenic phenotype of endothelial cells in vitro in the absence of other cell types such as smooth muscle cells, pericytes, and inflammatory cells.

Vascular dysfunction induced by elevated glucose levels in rats is mediated by vascular endothelial growth factor

The purpose of these experiments was to investigate a potential role for vascular endothelial growth factor (VEGF) in mediating vascular dysfunction induced by increased glucose flux via the sorbitol pathway. Skin chambers were mounted on the backs of Sprague-Dawley rats and 1 wk later, granulation tissue in the chamber was exposed twice daily for 7 d to 5 mM glucose, 30 mM glucose, or 1 mM sorbitol in the presence and absence of neutralizing VEGF antibodies. Albumin permeation and blood flow were increased two- to three-fold by 30 mM glucose and 1 mM sorbitol; these increases were prevented by coadministration of neutralizing VEGF antibodies. Blood flow and albumin permeation were increased approximately 2.5-fold 1 h after topical application of recombinant human VEGF and these effects were prevented by nitric oxide synthase (NOS) inhibitors (aminoguanidine and N(G)-monomethyl L-arginine). Topical application of a superoxide generating system increased albumin permeation and blood flow and these changes were markedly attenuated by VEGF antibody and NOS inhibitors. Application of sodium nitroprusside for 7 d or the single application of a calcium ionophore, A23187, mimicked effects of glucose, sorbitol, and VEGF on vascular dysfunction and the ionophore effect was prevented by coadministration of aminoguanidine. These observations suggest a potentially important role for VEGF in mediating vascular dysfunction induced by "hypoxia-like" cytosolic metabolic imbalances (reductive stress, increased superoxide, and nitric oxide production) linked to increased flux of glucose via the sorbitol pathway.

Vascular endothelial growth factor/vascular permeability factor augments nitric oxide release from quiescent rabbit and human vascular endothelium

BACKGROUND

Vascular endothelial growth factor (VEGF)/ vascular permeability factor (VPF) is an endothelial cell (EC) mitogen. This feature is considered central to the documented role of VEGF/VPF in promoting angiogenesis. More recent evidence suggests that VEGF/VPF may also serve a "maintenance" function, modulating various aspects of EC biology. In the present study, we sought to determine the extent to which VEGF/VPF may stimulate the release of NO from normal ECs.

METHODS AND RESULTS

VEGF/VPF produced a dose-dependent rise in NO concentration ([NO]) from vascular segments of rabbit thoracic aorta, pulmonary artery, and inferior vena cava. In comparison to stimulation with acetylcholine, the onset of increased [NO] after administration of VEGF/VPF was slower, reaching a maximum value after 8 minutes. Preincubation of the aortic segments with L-arginine raised by twofold both baseline [NO] and [NO] stimulated by addition of 2.5 micrograms/mL VEGF/VPF. Removal of CaCl2 from the Krebs solution, disruption of the endothelium, and administration of NG-monomethyl-L-arginine abrogated the stimulatory effect of 10 micrograms/mL VEGF/VPF. Similar findings were documented with an NO-specific polarographic electrode to measure NO released from cultured human umbilical vein ECs.

CONCLUSIONS

VEGF/VPF stimulates production of NO from rabbit and human ECs. This finding (1) constitutes inferential evidence for the presence of functional VEGF/VPF receptors on quiescent endothelium of the adult rabbit as well as human ECs and (2) supports the notion that putative maintenance functions of VEGF/VPF may include regulation of baseline synthesis and/or release of EC NO.

Histamine reduces ZO-1 tight-junction protein expression in cultured retinal microvascular endothelial cells

We examined ZO-1 protein content in cultured retinal vascular endothelial cells to test the hypothesis that histamine alters tight-junction-protein expression. Histamine (10(-9) -10(-4) M) causes a reversible concentration-dependent reduction of ZO-1 protein content, mediated by both H1 and H2 receptors. Histamine reduces ZO-1 expression within the time associated with increased paracellular permeability. Tight-junction-protein alterations may be a novel explanation for the mechanism by which vasoactive agents increase microvascular permeability.

Human immunoglobulins inhibit thrombin-induced Ca2+ movements and nitric oxide production in endothelial cells

Binding of natural antibodies to endothelial cell plays an important role in hyperacute xenograft rejection between discordant species. Human intravenous immunoglobulins (IVIg) delay this hyperacute rejection, but their mechanisms of action on endothelial cells have to be defined. Here we demonstrate that IVIg dose-dependently prevent thrombin from eliciting cytosolic Ca2+ movements and nitric oxide (NO) production in aortic endothelial cells from guinea pig. The Ca2+ response to thrombin was similarly affected by IVIg whether they were removed or not from the incubation medium before stimulation. Pretreatment by rat natural antibodies also suppress the thrombin-induced Ca2+ peak corresponding to Ca2+ release from intracellular stores but stimulate the subsequent sustained increase in [Ca2+]i and the release of NO. The action of human intravenous immunoglobulins seems to be selective for the thrombin receptor because they do not affect [Ca2+]i and NO responses to endothelin-1 or thapsigargin. However, these antibodies also suppress the first phase of the cytosolic Ca2+ response to ATP, which does not release NO under our experimental conditions. These observations raise the possibility that IVIg selectively interact with targets localized on plasma membrane of endothelial cells for controlling receptor-activated Ca2+ pathways and NO release.

Integrin alpha v beta 3 and phospholipase C regulate prostacyclin formation of endothelial cells caused by ancrod-generated fibrin

Ancrod-generated fibrin has been shown to stimulate prostacyclin synthesis of human umbilical vein endothelial cells (Chang et al., 1994, Biochem. Biophys. Res. Commun. 203, 1920). We further investigated its mechanism of action. The increment of 6-keto prostaglandin F1 alpha stimulated by ancrod-generated fibrin was almost completely inhibited when endothelial cells were either pretreated with 50 microM 8-(N,N'-diethylamino)octyl-3,4,5- trimethoxybenzoate (TMB-8) or preloaded with 15 microM 1,2-bis(2- aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA). 6-Keto prostaglandin F1 alpha production during 2 and 10 h incubation period was also inhibited by 1.2 mM ethyleneglycol-bis-(beta-aminoethylether)-N,N,N',N'-tetraace tic acid (EGTA) (41 +/- 12 and 53 +/- 17% inhibition, respectively). Further, ancrod-generated fibrin caused a rapid-onset increase in [3h]inositol monophosphate (IP1) formation in endothelial cells. This increase in IP1 was significantly inhibited by 1 mM Gly-Pro-Arg-Pro, 1 mM neomycin or 100 ng/ml pertussis toxin. At the same time, neomycin and pertussis toxin also significantly inhibited 6-keto prostaglandin F1 alpha synthesis of endothelial cells stimulated by ancrod-generated fibrin. Additionally, the increment of IP1 production as well as prostacyclin production were significantly inhibited by monoclonal antibodies directed against alpha v beta 3. These results suggest that intra- and extra-cellular Ca2+ participate in prostacyclin synthesis stimulated by ancrod-generated fibrin. Ancrod-generated fibrin stimulates pertussis toxin-sensitive G-protein regulated phosphoinositide breakdown, which is responsible for prostacyclin synthesis. This augmentation in prostacyclin synthesis and phosphoinositide breakdown caused by ancrod-generated fibrin area, at least in part, mediated by fibrin binding to integrin alpha v beta 3 on endothelial cells.

The morphologic change of endothelial cells by ancrod-generated fibrin is triggered by alpha v beta 3 integrin binding and the subsequent activation of a G-protein coupled phospholipase C

The mechanism of morphologic change of human cultured umbilical vein endothelial cells (HUVECs) caused by fibrin was investigated. Ancrod, a thrombin-like enzyme, did not cause morphologic alteration of HUVEC by itself at concentrations ranging from 0.01 to 10 U/ml. However, when 0.02 U/ml of ancrod was added to cultured HUVEC monolayers in the presence of citrated plasma, it caused pronounced morphologic change of HUVEC after 6-10 h incubation period. Gly-Pro-Arg-Pro (4 mg/ml), an inhibitor of fibrin polymerization, prevented the morphologic alteration, indicating that the morphologic alteration was caused by the polymerized fibrin. The morphologic change of HUVEC caused by ancrod-generated fibrin was not observed in the presence of an intracellular calcium mobilization inhibitor TMB-8 (50 microM), and the morphologic alteration was also less pronounced with BAPTA(15 microM)-loaded HUVECs and HUVECs pretreated with EGTA (1.2 mM). Ancrod (in Medium 199) itself did not stimulate phosphoinositide breakdown of HUVEC. However, when ancrod was present in plasma, it caused an increase of [3H]IP1 of HUVECs preloaded with [3H]myoinositol. This IP1 increment was inhibited by Gly-Pro-Arg-Pro. The increase of IP1 was significantly inhibited by the pretreatment of monoclonal antibodies 23C6 and 7E3 directed against alpha v beta 3 integrin. Neomycin (1 mM) and pertussis toxin (100 ng/ml), but not aspirin or mepacrine, blocked this enhanced phosphoinositide breakdown. The morphologic change was also prevented by the monoclonal antibodies, 23C6 and 7E3. These results suggest that both intra- and extra-cellular calcium participate in the event of morphologic change of HUVEC caused by ancrod-generated fibrin, and the morphologic change is mediated, at least in part, by fibrin binding to integrin alpha v beta 3 on HUVECs, causing the subsequent activation of the endogenous G-protein coupled phospholipase C.

Regulation of thrombin receptors on human umbilical vein endothelial cells

Activated thrombin receptors on human umbilical vein endothelial cells rapidly undergo homologous desensitization, leaving the cells unable to respond to thrombin. The present studies examine the fate of activated thrombin receptors on endothelial cells and the mechanisms that restore intact receptors to the cell surface. The results show that: 1) at biologically relevant concentrations, thrombin rapidly cleaves all of its receptors on the cell surface. 2) The cleaved receptors are cleared from the cell surface in a two-phase process, with 60% being internalized within 10 min, the remainder requiring several hours. 3) The restoration of intact, thrombin-responsive receptors on the cell surface initially occurs from an intracellular pool of receptors in a process that is independent of protein synthesis. 4) Recycling of cleaved receptors either does not occur on endothelial cells or is masked by receptor clearance. 5) Subconfluent endothelial cells re-express intact receptors on the cell surface at a slower rate than confluent cells. 6) The agonist peptide, SFLLRN, also causes receptor internalization, although at concentrations greater than those required for receptor activation and desensitization. These results are distinctly different from those observed with megakaryoblastic cell lines, where > 90% of the activated thrombin receptors are internalized rapidly, up to 40% of the cleaved receptors are recycled, and no intracellular pool of intact receptors has been detected. Since the primary structure of the thrombin receptor is the same in all the cell types studied, these results demonstrate that there can be substantial differences between cell types in the mechanisms involved in the clearance of activated receptors and the re-expression on the cell surface of intact receptors capable of responding to thrombin.

Thrombin decreases the urokinase receptor and surface-localized fibrinolysis in cultured endothelial cells

The endothelial cell (EC) urokinase receptor plays an important role in the localization and receptor-mediated activation of EC-bound plasminogen and hence surface-localized fibrinolysis. Thrombin induced a rapid (< 5 minute), time- (0 to 30 minutes) and dose- (0.1 to 8 U/mL) dependent decrease in the specific binding of 125I-labeled two-chain urokinase-type plasminogen activator (tcu-PA) or diisopropylfluoro-phosphate-tcu-PA to urokinase-type plasminogen activator receptor (u-PAR) in cultured ECs from various sources (range, 21% to 50%). The thrombin receptor activation peptide but not control peptide showed a similar but reduced decrease in the specific binding of 125I-labeled tcu-PA to u-PAR. Incubation of thrombin-treated cultures (10 to 12 hours) in complete medium restored 125I-labeled tcu-PA ligand binding to normal levels. u-PAR mRNA levels rapidly (1 hour) increased and peaked 10 to 12 hours after thrombin treatment as analyzed by reverse transcriptase-polymerase chain reaction. Decreased thrombin-induced 125I-labeled tcu-PA binding correlated with the time-dependent decrease in surface-localized plasmin generation, as measured by the direct activation of 125I-labeled Glu-plasminogen and quantification of the 20-kD light chains of 125I-labeled plasmin. After incubation with thrombin, plasmin generation was decreased 50% to 56% (125 to 152 fmol/3 to 3.5 x 10(4) cells). Isolation of metabolically labeled 35S-labeled u-PAR from the media of thrombin and phospholipase C-treated human aortic cultures yielded approximately 10- and approximately 12-fold more 55-kD M(r) and approximately 6-fold more 35-kD M(r) 35S-labeled u-PAR forms than control cultures, respectively. The u-PAR antigen forms (M(r), 54 kD) and the glycosyl-phosphatidylinositol-anchored protein CD59 (M(r), 20 kD) were also simultaneously identified by immunoprecipitation in the media of thrombin-treated cultures. This suggests that thrombin may release u-PAR and decrease u-PA ligand binding through a common pathway involving phospholipase C. These results establish a novel interrelation between thrombin and EC fibrinolysis and suggest that thrombin may also have an additional regulatory role in the net expression of surface-localized EC fibrinolytic activity.

Inhibition of endothelial nitric oxide synthase activity by protein kinase C

Nitric oxide (NO) is an important molecular messenger accounting for endothelium-derived relaxing factor. Recently, NO synthase (NOS) from cultured endothelial cells has been purified and molecularly cloned. To evaluate the effect of phosphorylation by protein kinase C (PKC) and cyclic AMP-dependent protein kinase (PKA) on endothelial constitutive NOS catalytic activity, we incubated purified endothelial NOS with PKC or PKA. Endothelial NOS was stoichiometrically phosphorylated by PKC and PKA. In intact bovine aortic endothelial cells (BAECs), NOS was phosphorylated by stimulation with 12-O-tetradecanoylphorbol-13-acetate (TPA). NOS activity measured by the conversion of [3H]arginine to [3H]citrulline in homogenates of BAECs treated with TPA or phorbol 12,13-dibutyrate was reduced by 30%, whereas dibutylyl cyclic AMP did not affect NOS activity. Moreover, we measured NO release from cultured BAECs by a chemiluminescence method to examine the effect of PKC and PKA on endothelial NOS activity. In cultured BAECs, ATP gamma S and A23187 induced NO release in time- and dose-dependent manners. Phorbol esters such as TPA and phorbol 12,13-dibutyrate dose dependently inhibited NO release stimulated by A23187 as well as ATP gamma S. Reduction of NO release by TPA was almost completely prevented by pretreatment with staurosporine, an inhibitor of PKC. NO release by A23187 was increased in PKC-downregulated BAECs. In contrast, dibutylyl cyclic AMP or 8-bromo cyclic GMP had no effect on NO release from BAECs induced by A23187 or ATP gamma S. These results indicate that phosphorylation of NOS by PKC is associated with a reduction of its catalytic activity in vascular endothelial cells.

Coexpression of P2Y and P2U receptors on aortic endothelial cells. Comparison of cell localization and signaling pathways

Depending on the vascular bed considered, the actions of ATP on the endothelium are mediated by either P2Y or P2U receptors. The two types of receptors seem to coexist on bovine aortic endothelial cells, where they are both coupled to phospholipase C. In this study, we have investigated whether they are truly coexpressed on the same cells and whether their signaling pathways diverge beyond phospholipase C activation. Measurements of [Ca2+]i in single cells showed that almost all bovine aortic endothelial cells are responsive to both 2-methylthio-ATP (2MeSATP), an agonist of P2Y receptors, and UTP, an agonist of P2U receptors. UTP stimulated the release of prostacyclin from freshly isolated bovine aortic endothelial cells, even when they were exposed to cycloheximide at the time of their collection: this indicates that P2U receptors must already be expressed on endothelial cells in situ and do not appear during cell culture. The time course of inositol phosphate (InsP) accumulation and the relative proportion of Ins(1,4,5)P3, Ins(1,3,4,5)P4, and Ins(1,3,4)P3 were similar in cells stimulated by 2MeSATP or UTP. UTP and 2MeSATP both stimulated the hydrolysis of phosphatidylcholine by phospholipase D, as reflected by the release of [3H]choline from prelabeled cells. The responses to both agents were blocked after downregulation of protein kinase C, resulting from a prolonged exposure to phorbol 12-myristate 13-acetate: this blockade occurred at a step distal to phospholipase C activation. A single difference between the two pathways has been identified: the effect of 2MeSATP on InsP3 was significantly more inhibited after a short exposure to phorbol 12-myristate 13-acetate than that of UTP.(ABSTRACT TRUNCATED AT 250 WORDS)

The control of endothelin-1 secretion

1. The human endothelin-1 (ET-1) gene, which is located on chromosome 6, contains cis-regulatory elements in the 5'-flanking region including the TPA-responsive element, nuclear factor 1 binding element and GATA motif. 2. The expression of preproendothelin-1 (PPET-1) mRNA is regulated by a mechanism involving receptor mediated mobilization of intracellular Ca2+ and activation of protein kinase C in endothelial cells. 3. Activation of protein kinase C results in the synthesis of c-Jun protein and the rapid dephosphorylation of c-Jun protein. Consequently, the binding activity of c-Jun protein to the TPA-responsive element increases, and this causes the induction of PPET-1 mRNA. 4. The microtubular system seems to play some important roles in ET-1 secretion, especially in the process of transferring the synthesized ET-1 to the cell surface of the endothelial cells. 5. The secretion of ET-1 from endothelial cells is also regulated by intracellular Ca2+ released from the Ca2+ store and by Ca2+-calmodulin complex. The phosphorylation of the myosin light chain, elicited by myosin light chain kinase and activated by Ca2+-calmodulin complex, facilitates the formation of filamentous myosin and actin which probably participate in ET-1 secretion especially in transporting the ET-1-containing vesicles towards the cell membrane in the stimulated endothelial cells. 6. Many cultured cells, other than endothelial cells, also secret ET-1 into the culture medium and this secretion can be stimulated by a variety of agents.

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.

New perspectives in hypertension research. Potentials of vascular biology

The vessel wall was once considered to be a passive conduit responding to the circulating endocrine system. However, the emergence of molecular and vascular biology in hypertension research has redefined our understanding of the role of the vasculature as a vital organ in the pathogenesis of hypertension. It is now recognized that the vasculature can regulate its own tone by a variety of previously unknown autocrine and/or paracrine vasoactive systems. Recent evidence indicates that the process of vascular remodeling in hypertension appears to be mediated by locally generated factors within the vessel wall. This review examines the implications of this new paradigm in hypertension, focusing on five topics that have developed through the emergence of molecular vascular biology: the discovery and characterization of novel biologically active molecules synthesized by the vessel wall, the molecular mechanisms and consequences of vascular remodeling, the developmental biology of the blood vessel and the relation to pathobiology, the use of in vivo gene transfer to test hypotheses in vivo, and novel treatment strategies based on gene therapy of the vessel wall.

Vitamin E suppresses diacylglycerol (DAG) level in thrombin-stimulated endothelial cells through an increase of DAG kinase activity

The present study has examined the role of vitamin E, a natural lipid antioxidant, in the production of diacylglycerol (DAG) and phosphatidic acid (PA) in thrombin-stimulated human endothelial cells. Cells were labelled with [3H]myristate and the incorporation and distribution of [3H]myristate into cellular lipids was not affected by vitamin E. However, in response to thrombin stimulation, considerably more PA and less DAG were formed in cells enriched with vitamin E. The time-course of thrombin stimulation indicated that vitamin E attenuated the accumulation of sustained DAG levels with a concomitant increase in PA. Direct determination of DAG mass further confirmed that vitamin E suppresses the accumulation of DAG induced by thrombin. In the presence of ethanol, the formation of [3H]phosphatidylethanol (PEt) in [3H]myristate-labelled cells stimulated by thrombin was unaffected by vitamin E enrichment. DL-Propranolol, a PA phosphohydrolase inhibitor, caused an accumulation of PA, without affecting DAG formation in either vitamin E-treated and untreated cells. This indicated that the increase in PA and decrease in DAG in vitamin E-treated cells was not due to a stimulation of phospholipase D or an inhibition of PA phosphohydrolase. Determination of inositol phosphates formation in response to thrombin showed that the change of DAG levels elicited by vitamin E was independent of phospholipase C-induced hydrolysis of inositol phospholipids. In contrast, analysis of DAG kinase activity revealed that vitamin E enrichment enhanced the activity of the enzyme in both basal and thrombin-stimulated cells. Taken together, these data indicated that vitamin E caused an increased conversion of DAG to PA by activating DAG kinase activity without causing any change in the activities of phospholipase D, PA phosphohydrolase or phospholipase C.

Thrombin in inflammation and healing: relevance to rheumatoid arthritis

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Modulation of endothelial autacoid release by protein kinase C: feedback inhibition or non-specific attenuation of receptor-dependent cell activation?

Receptor-mediated elevations of intracellular Ca2+ in endothelial cells may be controlled by a negative feedback mechanism through activation of protein kinase C (PKC). To test this hypothesis, we studied the effects of an activation or inhibition of PKC on the release of nitric oxide (NO) and prostacyclin (PGI2) from cultured bovine and porcine aortic endothelial cells (EC). Preincubation with the PKC activators phorbol-12-myristate-13-acetate (PMA) (3-300 nM) or 1-oleyl-2-acetyl-glycerol (OAG) (30 microM) significantly attenuated the release of NO and PGI2 from EC stimulated with bradykinin (0.3-30 nM), whereas phorbol-12,13-didecanoate (PDD) (30-300 nM), which does not activate PKC, had no effect. UCN-01 (10 nM), a specific PKC inhibitor, significantly augmented the bradykinin-stimulated release of NO from EC. These effects were correlated with a reduced (PMA) or enhanced (UCN-01) elevation of intracellular Ca2+ in response to bradykinin in both types of EC. Neither the PKC activators nor the inhibitor had any effect on resting intracellular Ca2+ or basal endothelial autacoid release. Several isoforms of PKC (namely PKC alpha, PKC delta, PKC epsilon, and PKC zeta) were detected in bovine, human, and porcine EC by immunoblotting analysis with isotype-specific anti-PKC antibodies, which, except PKC epsilon, were predominantly located in the cytosol. Incubation of bovine EC with PMA elicited a significant increase in membrane-bound PKC alpha immunoreactivity, whereas there was no translocation of PKC alpha from the cytosolic to the membrane fraction with bradykinin. As determined by histone phosphorylation, PKC activity was similarly reduced in the cytosol, but increased in the membrane fraction of bovine EC exposed to PMA, whereas bradykinin had no significant effect. These findings indicate that endothelial autacoid release can be modulated by activators and inhibitors of PKC. However, stimulation of EC with bradykinin does not lead to a detectable activation of PKC, suggesting that PKC does not exert a negative feedback in the signal transduction pathway of this receptor-dependent agonist.

Thrombin receptor activating peptides induce Ca2+ mobilization, barrier dysfunction, prostaglandin synthesis, and platelet-derived growth factor mRNA expression in cultured endothelium

Endothelial cell activation by thrombin is a key event in wound healing, inflammation, and hemostasis. To better define thrombin-endothelial cell interactions we synthesized several peptides of varying length corresponding to the initial 14 amino acid sequence of the cloned human platelet thrombin receptor after cleavage at an arginine41 site (R/SFLLRNPNDKYEPF). Thrombin receptor activating peptides (TRAPs) as short as 5 amino acids induced significant levels of PGI2 synthesis and expression of PDGF mRNA in human endothelium and produced dose-dependent cellular contraction and permeability of confluent human umbilical vein and bovine pulmonary artery endothelial monolayers. To explore whether TRAPs utilized similar signal transducing pathways as alpha-thrombin to accomplish endothelial cell activation, phospholipase C production of the Ca2+ secretagogue IP3 was measured and detected 10 seconds after either TRAP 7 or alpha-thrombin. Furthermore, TRAPs ranging from 5-14 residues induced significant dose-dependent increases in Fura-2 fluorescence indicative of Ca2+(1) mobilization. These results indicate that thrombin-mediated proteolytic cleavage of the human and bovine thrombin receptor initiates stimulus/coupling responses such phospholipase C activation, Ca2+ mobilization, and protein kinase C activation. The functional consequence of this cellular activation via the cleaved receptor is enhanced cellular contraction, barrier dysfunction, PGI2 synthesis, and expression of PDGF mRNA.

Hyperosmotic stress stimulates tissue plasminogen activator expression by a PKC-independent pathway

Shear, stretch, and the generation of oxygen radicals stimulate increases in tissue plasminogen activator (t-PA) mRNA levels and antigen production, suggesting that environmental stress may regulate t-PA gene expression. We have examined whether t-PA production is also responsive to a hyperosmotic environment. Endothelial and HeLa cells were treated with hyperosmotic medium, and t-PA mRNA and antigen secretion were measured. Endothelial cells incubated in hyperosmotic medium showed a dose-dependent decrease in cell volume and a 1.9 +/- 0.3- and 3.7 +/- 0.9-fold increase in t-PA secretion at 425 and 485 mosmol/kgH2O, respectively. HeLa cells showed a 3.3 +/- 0.6- and 5.1 +/- 1.2-fold increase at the same osmolalities. Increased secretion began between 8 and 16 h and continued through 24 h. Cultures returned to isosmotic medium after 8 h of treatment continued to release 98.1 +/- 7% of the maximum levels of t-PA for the next 16 h, despite the reversal of other responses to hyperosmotic environment. t-PA mRNA levels also increased between 8 and 16 h to five times control levels but returned to baseline by 24 h. No change in intracellular Ca2+ concentration, inositol 1,4,5-trisphosphate, or diacylglycerol content was detected, suggesting that a different intracellular signal pathway may be involved in the response to hyperosmolar stimulus. Thus environmental stress may be a general stimulatory signal through which t-PA production can be induced.

Involvement of protein kinase C in the control of microvascular permeability to colloidal carbon

The vasculature of the rat small intestine and attached mesentery was perfused in vitro with a gelatin-containing physiological salt solution (GPSS). The inclusion of colloidal carbon (CC) in the perfusate towards the end of the experimental period enabled the "leakiness" of microvessels in the villi to be determined, since "leaky" vessels trap CC in their walls. Addition to the perfusate of the inflammatory agonists platelet-activating factor (PAF, 5 x 10(-6) M) or 5-hydroxytryptamine (5-HT, 1 x 10(-4) M), or the microtubule-disrupting agents podophyllotoxin (5 x 10(-5) M), or colcemid (5 x 10(-5) M), or the protein kinase C (PKC) activator phorbol 12, 13-dibutyrate (PDB, 1 x 10(-6) M), caused significantly increased microvascular "blackening" as assessed by image analysis. 4 alpha-phorbol 12, 13-didecanoate (PDD, 1 x 10(-6) M) had no effect. Pretreatment with the PKC inhibitor Ro 31-8220 [corrected] (1 x 10(-6) M) significantly reduced the effects of PAF, 5-HT and PDB, but not those of podophyllotoxin or colcemid. These results suggest, therefore, that PKC is involved in the permeability-enhancing effects of PAF, 5-HT and PDB. Pretreatment with indomethacin (1 x 10(-6) M) as a cyclooxygenase inhibitor did not reduce the response to PDB, indicating that prostaglandin release is of minor importance in the PDB-induced increase in microvascular permeability.

Thrombin promotes angiogenesis by a mechanism independent of fibrin formation

The role of thrombin in angiogenesis was investigated in the chick chorioallantoic membrane (CAM) system. alpha-Thrombin promoted angiogenesis in a dose-dependent fashion and at 8.4 pmol/disk reached a maximum of 78% above the control. At a higher dose of alpha-thrombin (25 pmol/disk) the angiogenic effect declines and this can be explained by desensitization of the thrombin receptor. The promotion of angiogenesis by alpha-thrombin is specific as evidenced by the reversal of this effect by hirudin, which binds both the catalytic and the anion-binding exosite of thrombin or by heparin, which binds thrombin and accelerates its inactivation by antithrombin III. gamma-Thrombin, which is catalytically active but lacks the anion-binding exosite required for clotting activity, promotes angiogenesis in the CAM in the same fashion and to the same extent as alpha-thrombin, at doses up to 130 pmol/disk. Phenylalanyl-propyl-arginine chloromethyl ketone (P-PACK)-thrombin, the catalytically inactive analogue of alpha-thrombin that retains the anion-binding exosite, had no significant effect on angiogenesis in the CAM. When combined with alpha-thrombin, P-PACK-thrombin abolished the angiogenesis-promoting effect of alpha-thrombin. These results suggest that alpha-thrombin can promote angiogenesis in the CAM through interaction with its catalytic site without the requirement for fibrin formation.

Thrombin receptor peptide inhibits thrombin-induced increase in endothelial permeability by receptor desensitization

Thrombin, a potent activator of cellular responses, proteolytically cleaves, and thereby activates its receptor. In the present study, we compared the effects of the thrombin receptor 14-amino acid peptide (TRP-14; SFLLRNPNDKYEPF), which comprises the NH2 terminus after cleavage of the thrombin receptor, and of the native alpha-thrombin on endothelial monolayer permeability. Addition of TRP-14 (1-200 microM) to bovine pulmonary artery endothelial cells increased [Ca2+]i in a dose-dependent manner. The peak increase in [Ca2+]i in response to 100 microM TRP-14 or 0.1 microM alpha-thrombin was similar (i.e., 931 +/- 74 nM and 1032 +/- 80 nM, respectively), which was followed by a slow decrease with t1/2 values of 0.73 and 0.61 min, respectively. Extracellular Ca2+ chelation with 5 mM EGTA abolished the sustained increases in [Ca2+]i induced by either TRP-14 or alpha-thrombin. alpha-thrombin (0.1 microM) increased transendothelial [125I]albumin permeability, whereas TRP-14 (1-100 microM) had no effect. Coincubation of 100 microM TRP-14 with 1 microM DIP-alpha-thrombin also did not increase permeability over control values. Stimulation of BPAEC with 0.1 microM alpha-thrombin induced translocation of protein kinase C (PKC) from the cytosol to the plasma membrane indicative of PKC activation, whereas TRP-14 had no effect at any concentration. TRP-14 at 100 microM desensitized BPAEC to thrombin-induced increases in [Ca2+]i and transendothelial permeability. The Ca2+ desensitization was reversed after approximately 60 min, and this recovery paralleled the recovery of the permeability response. These findings indicate that the TRP-14-induced Ca2+ mobilization in the absence of PKC activation is insufficient to increase endothelial permeability. In contrast, the increase in endothelial permeability after alpha-thrombin occurred in conjunction with Ca2+ mobilization as well as PKC activation. TRP-14 pretreatment prevented the alpha-thrombin-induced increase in endothelial permeability secondary to desensitization of the Ca2+ signal. The results suggest that combined cytosolic Ca2+ mobilization mediated by TRP-14 and PKC activation mediated by a TRP-14-independent pathway are dual signals responsible for the thrombin-induced increase in vascular endothelial permeability.

Heparin has an inhibitory effect on endothelin-1 synthesis and release by endothelial cells

We studied the inhibitory effects of heparin on basal and agonist-induced endothelin-1 biosynthesis and release from cultured bovine endothelial cells. Heparin dose-dependently and similarly inhibited endothelin-1 release, inositol trisphosphate production, and intracellular free Ca2+ levels stimulated by thrombin. Hirudin fragment had an inhibitory effect on thrombin-induced endothelin-1 release, whereas anti-thrombomodulin antibody had no effect. Heparin completely blocked phorbol ester-induced endothelin-1 release, whereas it had a partial inhibitory effect on endothelin-1 release stimulated by angiotensin and vasopressin. Northern blot analysis using complementary DNA for bovine preproendothelin-1 as a probe revealed that heparin reduced not only the basal but also the stimulated expression of preproendothelin-1 messenger RNA by thrombin and phorbol ester. These data suggest that heparin, in addition to its antithrombin effect, has an inhibitory effect on the biosynthesis and release of endothelin-1, possibly by inhibiting protein kinase C-dependent pathway.

Thrombin stimulates release of endothelin and vasopressin, but not substance P, from isolated rabbit tracheal epithelial cells

Release of endothelin, vasopressin and substance P from isolated rabbit tracheal epithelial cells was monitored after incubation for 2 h with thrombin (10 U/ml) in the presence and absence of cycloheximide (100 micrograms/ml) and 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC, 0.6 mM). Thrombin stimulated the release of endothelin and vasopressin. Inhibition of the thrombin-stimulated release of endothelin and vasopressin by cycloheximide (44 and 56%, respectively) and NCDC (59 and 88%, respectively) indicates that the release of these peptides is at least partially dependent on the ability of thrombin to stimulate protein synthesis and activate the phospholipase C pathway. Substance P, which is also present in tracheal epithelial cells, was not released by thrombin.

Epidermal growth factor stimulates vascular endothelial growth factor production by human malignant glioma cells: a model of glioblastoma multiforme pathophysiology

Hypervascularity, focal necrosis, persistent cerebral edema, and rapid cellular proliferation are key histopathologic features of glioblastoma multiforme (GBM), the most common and malignant of human brain tumors. By immunoperoxidase and immunofluorescence, we definitively have demonstrated the presence of vascular endothelial growth factor (VEGF) and epidermal growth factor receptor (EGFr) in five out of five human glioma cell lines (U-251MG, U-105MG, D-65MG, D-54MG, and CH-235MG) and in eight human GBM tumor surgical specimens. In vitro experiments with glioma cell lines revealed a consistent and reliable relation between EGFr activation and VEGF production; namely, EGF (1-20 ng/ml) stimulation of glioma cells resulted in a 25-125% increase in secretion of bioactive VEGF. Conditioned media (CM) prepared from EGF-stimulated glioma cell lines produced significant increases in cytosolic free intracellular concentrations of Ca2+ ([Ca2+]i) in human umbilical vein endothelial cells (HUVECs). Neither EGF alone or CM from glioma cultures prepared in the absence of EGF induced [Ca2+]i increases in HUVECs. Preincubation of glioma CM with A4.6.1, a monoclonal antibody to VEGF, completely abolished VEGF-mediated [Ca2+]i transients in HUVECs. Likewise, induction by glioma-derived CM of von Willebrand factor release from HUVECs was completely blocked by A4.6.1 pretreatment. These observations provide a key link in understanding the basic cellular pathophysiology of GBM tumor angiogenesis, increased vascular permeability, and cellular proliferation. Specifically, EGF activation of EGFr expressed on glioma cells leads to enhanced secretion of VEGF by glioma cells. VEGF released by glioma cells in situ most likely accounts for pathognomonic histopathologic and clinical features of GBM tumors in patients, including striking tumor angiogenesis, increased cerebral edema and hypercoagulability manifesting as focal tumor necrosis, deep vein thrombosis, or pulmonary embolism.

Cellular mechanism of thrombin on endothelin-1 biosynthesis and release in bovine endothelial cell

We have studied the cellular mechanism responsible for induction of preproendothelin (preproET)-1 mRNA and release of ET-1 by thrombin in cultured bovine endothelial cells (ECs). Thrombin induced an immediate and dose-dependent formation of inositol-1,4,5-trisphosphate (IP3) with a concomitant increase in intracellular Ca2+ concentration ([Ca2+]i). The thrombin-induced ET-1 release was abolished either by a phospholipase C inhibitor, a protein kinase C (PKC) inhibitor, or an intracellular Ca(2+)-chelator, whereas a Ca(2+)-channel antagonist was ineffective. A selective thrombin inhibitor (argatroban) decreased IP3 formation and the increase in [Ca2+]i and ET-1 release stimulated by thrombin. Northern blot analysis revealed that thrombin-induced expression of preproET-1 mRNA was inhibited completely by a PKC inhibitor and partially by argatroban. These data suggest that thrombin is involved in the mechanism of preproET-1 mRNA expression and subsequent ET-1 release, possibly through activation PKC and mobilization of intracellular Ca2+ resulting from the receptor-mediated phosphoinositide breakdown in ECs.

Lysophosphatidylcholine inhibits surface receptor-mediated intracellular signals in endothelial cells by a pathway involving protein kinase C activation

Lysophosphatidylcholine (lysoPC) transferred from oxidatively modified low density lipoprotein (Ox-LDL) to the endothelial surface membrane has been shown to produce a selective unresponsiveness to cell surface receptor-regulated endothelium-dependent relaxation (EDR) in the rabbit aorta. To determine its mechanism we examined the effects of lysoPC on endothelial surface receptor-mediated transmembrane signals. Incubation for 1 minute with palmitoyl lysoPC (5-10 microM) decreased thrombin (Th, 2 units/ml)- or histamine (His, 0.1 mM)-stimulated inositol 1,4,5-trisphosphate (IP3) production in primary cultures of human umbilical vein endothelial cells (HUVECs). LysoPC also decreased Th- or His-induced intracellular calcium ([Ca2+]i, fura 2) elevation. Pretreatment with protein kinase C (PKC) inhibitors staurosporine (100 nM) or H-7 (50 microM) prevented the inhibitory actions of lysoPC, but HA-1004 had no effect. Incubation for 5 minutes with phorbol 12-myristate 13-acetate (PMA, 100 nM) produced the inhibitory actions on the Th- or His-induced intracellular signals, which closely mimic those exhibited by lysoPC. However, the inhibitory effect of lysoPC was lost in cells that were depleted of PKC by pretreatment for 24 hours with 100 nM PMA. Furthermore, incubation of the cells for 1 minute with lysoPC stimulated PKC activity in the membrane fraction. In organ chamber experiments with porcine coronary artery rings, pretreatment with staurosporine (20 nM) attenuated lysoPC-induced impairment of EDR in response to Th. These results indicate that lysoPC, which accumulates in Ox-LDL and atherosclerotic arterial walls, inhibits the early transmembrane signaling pathway in endothelial cells, and PKC activation could at least partially be involved in the negative regulation by lysoPC.(ABSTRACT TRUNCATED AT 250 WORDS)