Thrombin-induced rapid geranylgeranylation of RhoA as an essential process for RhoA activation in endothelial cells

RhoA plays a critical signaling role in thrombin-induced endothelial dysfunction. The possible thrombin regulation of geranylgeranylation, a lipid modification, of unprocessed RhoA and the significance of the geranylgeranylation in RhoA activation in endothelial cells (ECs) are not well understood. The amounts of the unprocessed and geranylgeranylated forms of RhoA in non-stimulated cultured human aortic ECs were 31 +/- 8 and 69 +/- 8% total cellular RhoA, respectively (n = 6, p < 0.0001), as determined by the Triton X-114 partition method. Thrombin-induced rapid conversion of most of the unprocessed RhoA into the geranylgeranylated form within 1 min through stimulating geranylgeranyltransferase I (GGTase I) activity. Thrombin-induced rapid geranylgeranylation was inhibited by acute short term (3 min) pretreatment with atorvastatin as well as by an inhibitor of GGTase I (GGTI-286). Thrombin also rapidly stimulated GTP loading of RhoA, which was blocked by acute pretreatment with either atorvastatin or GGTI-286. These observations indicate the dependence of thrombin stimulation of RhoA on the rapid geranylgeranylation of unprocessed RhoA. Importantly, the addition of geranylgeranylpyrophosphate to ECs pretreated with atorvastatin quickly reversed the atorvastatin inhibition of thrombin stimulation of RhoA. These results suggest that geranylgeranylation of unprocessed RhoA may limit thrombin-induced full activation of RhoA in ECs. Cytoskeleton analysis demonstrated that atorvastatin and GGTI-286 inhibited thrombin-induced stress fiber formation. We provide the evidence that, in thrombin-stimulated ECs, the unprocessed form of RhoA is rapidly geranylgeranylated to become the mature form, which then is converted into GTP-bound active RhoA.     

Fluvastatin inhibits regulated secretion of endothelial cell von Willebrand factor in response to diverse secretagogues

Regulated secretion of EC (endothelial cell) vWF (von Willebrand factor) is part of the haemostatic response. It occurs in response to secretagogues that raise intracellular calcium or cAMP. Statins are cholesterol-lowering drugs used for the treatment of cardiovascular disease. We studied the effect of fluvastatin on regulated secretion of vWF from HUVEC (human umbilical-vein ECs). Secretion in response to thrombin, a protease-activated receptor-1 agonist peptide, histamine, forskolin and adrenaline (epinephrine) was inhibited. This inhibition was reversed by mevalonate or geranylgeranyl pyrophosphate, and mimicked by a geranylgeranyl transferase inhibitor, demonstrating that the inhibitory mechanism includes inhibition of protein geranylgeranylation. To investigate this mechanism further, calcium handling and NO (nitric oxide) regulation were studied in fluvastatin-treated HUVEC. Intracellular calcium mobilization did not correlate with vWF secretion. Fluvastatin increased eNOS [endothelial NOS (NO synthase)] expression, but NOS inhibitors failed to reverse the effect of fluvastatin on vWF secretion. Exogenous NO did not inhibit thrombin-induced vWF secretion. Many small GTPases are geranylgeranylated and some are activated by secretagogues. We overexpressed DN (dominant negative) Rho GTPases, RhoA, Rac1 and Cdc42 (cell division cycle 42), in HUVEC. DNCdc42 conferred inhibition of thrombin- and forskolin-induced vWF secretion. We conclude that, via inhibition of protein geranylgeranylation, fluvastatin is a broadspectrum inhibitor of regulated vWF secretion. Geranylgeranylated small GTPases with functional roles in regulated secretion, such as Cdc42, are potential targets for the inhibitory activity of fluvastatin.     

Natriuretic peptides differentially attenuate thrombin-induced barrier dysfunction in pulmonary microvascular endothelial cells

Previous studies have described a protective effect of atrial natriuretic peptide (ANP) against agonist-induced permeability in endothelial cells derived from various vascular beds. In the current study, we assessed the effects of the three natriuretic peptides on thrombin-induced barrier dysfunction in rat lung microvascular endothelial cells (LMVEC). Both ANP and brain natriuretic peptide (BNP) attenuated the effect of thrombin on increased endothelial monolayer permeability and significantly enhanced the rate of barrier restoration. C-type natriuretic peptide (CNP) had no effect on the degree of thrombin-induced monolayer permeability, but did enhance the restoration of the endothelial barrier, similar to ANP and BNP. In contrast, the non-guanylyl cyclase-linked natriuretic peptide receptor specific ligand, cyclic-atrial natriuretic factor (c-ANF), delayed the rate of barrier restoration following exposure to thrombin. All three natriuretic peptides promoted cGMP production in the endothelial cells; however, 8-bromo-cGMP alone did not significantly affect thrombin modulation of endothelial barrier function. ANP and BNP, but not CNP or c-ANF, blunted thrombin-induced RhoA GTPase activation. We conclude that ANP and BNP protect against thrombin-induced barrier dysfunction in the pulmonary microcirculation by a cGMP-independent mechanism, possibly by attenuation of RhoA activation.     

Inhibition of platelet-surface-bound proteins during coagulation under flow I: TFPI

Blood coagulation is a self-repair process regulated by activated platelet surfaces, clotting factors, and inhibitors. Tissue factor pathway inhibitor (TFPI) is one such inhibitor, well known for its inhibitory action on the active enzyme complex comprising tissue factor (TF) and activated clotting factor VII. This complex forms when TF embedded in the blood vessel wall is exposed by injury and initiates coagulation. A different role for TFPI, independent of TF:VIIa, has recently been discovered whereby TFPI binds a partially cleaved form of clotting factor V (FV-h) and impedes thrombin generation on activated platelet surfaces. We hypothesized that this TF-independent inhibitory mechanism on platelet surfaces would be a more effective platform for TFPI than the TF-dependent one. We examined the effects of this mechanism on thrombin generation by including the relevant biochemical reactions into our previously validated mathematical model. Additionally, we included the ability of TFPI to bind directly to and inhibit platelet-bound FXa. The new model was sensitive to TFPI levels and, under some conditions, TFPI could completely shut down thrombin generation. This sensitivity was due entirely to the surface-mediated inhibitory reactions. The addition of the new TFPI reactions increased the threshold level of TF needed to elicit a strong thrombin response under flow, but the concentration of thrombin achieved, if there was a response, was unchanged. Interestingly, we found that direct binding of TFPI to platelet-bound FXa had a greater anticoagulant effect than did TFPI binding to FV-h alone, but that the greatest effects occurred if both reactions were at play. The model includes activated platelets’ release of FV species, and we explored the impact of varying the FV/FV-h composition of the releasate. We found that reducing the zymogen FV fraction of this pool, and thus increasing the fraction that is FV-h, led to acceleration of thrombin generation.     

Wheat germ agglutinin inhibits thrombin-induced rises in cytosolic free calcium and prostacyclin synthesis by human umbilical vein endothelial cells

To characterize the endothelial cell surface membrane glycoproteins that mediate thrombin stimulation of PGI2 synthesis by human umbilical vein endothelial cells (HUVEC), HUVEC were stimulated with thrombin in the presence or absence of different lectins. Of the lectins tested, only wheat germ agglutinin (WGA) inhibited thrombin-induced rises in cytosolic free calcium [( Ca2+]i), measured using Quin 2-loaded HUVEC and PGI2 production measured by radioimmunoassay. However, WGA by itself had no influence on baseline [Ca2+]i or PGI2 production and did not inhibit histamine-induced rises in [Ca2+]i. The inhibition of thrombin-induced rises in [Ca2+]i and PGI2 production by WGA was dose dependent, with half-maximal inhibition occurring at 2 micrograms/ml. WGA also inhibited thrombin-induced release of 3H-arachidonic acid. These effects of WGA were reversed by N-acetyl-glucosamine (GlcNAc) and N-acetyl-neuraminic acid, which bind specifically to WGA, but not by unrelated sugars. Succinylated WGA (succ-WGA), a chemically modified derivative of WGA that binds to GlcNAc but, unlike native WGA, not to sialoglycoproteins, did not inhibit thrombin-induced rises in [Ca2+]i and PGI2 production. These results suggest that thrombin induces rises in [Ca2+]i and PGI2 production by interacting with an endothelial surface membrane sialoglycoprotein.     

Synergistic effects of tumor necrosis factor-alpha and thrombin in increasing endothelial permeability

Because activation of the coagulation cascade and the generation of thrombin coexist with sepsis and the release of tumor necrosis factor (TNF)-alpha, we determined the effects of TNF-alpha on the mechanism of thrombin-induced increase in endothelial permeability. We assessed Ca(2+) signaling in human umbilical vein endothelial cells. In human umbilical vein endothelial cells exposed to TNF-alpha for 2 h, thrombin produced a rise in the intracellular Ca(2+) concentration ([Ca(2+)](i)) lasting up to 10 min. In contrast, thrombin alone produced a rise in [Ca(2+)](i) lasting for 3 min, whereas TNF-alpha alone had no effect on [Ca(2+)](i.) Thrombin-induced inositol 1,4,5-trisphosphate generation was not different between control and TNF-alpha-exposed cells. In the absence of extracellular Ca(2+), thrombin produced similar increases in [Ca(2+)](i) in both control and TNF-alpha-exposed cells. In TNF-alpha-exposed cells, the thrombin-induced Ca(2+) influx after intracellular Ca(2+) store depletion was significantly greater and prolonged compared with control cells. Increased Ca(2+) entry was associated with an approximately fourfold increase in Src activity and was sensitive to the Src kinase inhibitor PP1. After TNF-alpha exposure, thrombin caused increased tyrosine phosphorylation of junctional proteins and actin stress fiber formation as well as augmented endothelial permeability. These results suggest that TNF-alpha stimulation of endothelial cells results in amplification of the thrombin-induced Ca(2+) influx by an Src-dependent mechanism, thereby promoting loss of endothelial barrier function.     

Overexpression of the 78-kDa glucose-regulated protein/immunoglobulin-binding protein (GRP78/BiP) inhibits tissue factor procoagulant activity

Previous studies have demonstrated that overexpression of GRP78/BiP, an endoplasmic reticulum (ER)-resident molecular chaperone, in mammalian cells inhibits the secretion of specific coagulation factors. However, the effects of GRP78/BiP on activation of the coagulation cascade leading to thrombin generation are not known. In this study, we examined whether GRP78/BiP overexpression mediates cell surface thrombin generation in a human bladder cancer cell line T24/83 having prothrombotic characteristics. We report here that cells overexpressing GRP78/BiP exhibited significant decreases in cell surface-mediated thrombin generation, prothrombin consumption and the formation of thrombin-inhibitor complexes, compared with wild-type or vector-transfected cells. This effect was attributed to the ability of GRP78/BiP to inhibit cell surface tissue factor (TF) procoagulant activity (PCA) because conversion of factor X to Xa and factor VII to VIIa were significantly lower on the surface of GRP78/BiP-overexpressing cells. The additional findings that (i) cell surface factor Xa generation was inhibited in the absence of factor VIIa and (ii) TF PCA was inhibited by a neutralizing antibody to human TF suggests that thrombin generation is mediated exclusively by TF. GRP78/BiP overexpression did not decrease cell surface levels of TF, suggesting that the inhibition in TF PCA does not result from retention of TF in the ER by GRP78/BiP. The additional observations that both adenovirus-mediated and stable GRP78/BiP overexpression attenuated TF PCA stimulated by ionomycin or hydrogen peroxide suggest that GRP78/BiP indirectly alters TF PCA through a mechanism involving cellular Ca(2+) and/or oxidative stress. Similar results were also observed in human aortic smooth muscle cells transfected with the GRP78/BiP adenovirus. Taken together, these findings demonstrate that overexpression of GRP78/BiP decreases thrombin generation by inhibiting cell surface TF PCA, thereby suppressing the prothrombotic potential of cells.     

Pulsatile to-fro flow induces greater and sustained expression of tissue factor RNA in HUVEC than unidirectional laminar flow

Tissue factor (TF) is expressed in atherosclerotic lesions. Since mechanical forces influence endothelial cell (EC) function and are thought to account for the unique distribution of atherosclerosis in areas exposed to disturbed flow, we hypothesized that disturbed to-fro flow (TFF) and unidirectional pulsatile forward flow (PFF) would have different effects on TF expression in EC. TF RNA expression in HUVEC exposed to mechanical stress in the presence or absence of chemical stimulation with thrombin was determined. TFF induced a significantly higher TF expression than PFF that was sustained for 8 h. Combination of mechanical and chemical stimuli induced significantly higher TF expression than only mechanical stresses, and this effect was synergistic in both TFF and PFF. The MAPK p38 inhibitor SB-203580 significantly inhibited TF expression induced by mechanical and chemical stimulations, but the MEK inhibitor PD-98059 did not inhibit TF induced by TFF. Immunoblotting revealed that ERK1/2 phosphorylation induced by TFF was sustained for 120 min, whereas that induced by PFF was not. We conclude that disturbed flow induced greater and sustained amplification of TF expression, and this synergistic effect may be regulated by p38 MAPK and ERK1/2. These results provide added insight into the mechanism of atherosclerosis in areas of disturbed flow.     

Inhibition of Rho and Rac Geranylgeranylation by Atorvastatin Is Critical for Preservation of Endothelial Junction Integrity

Background

Small GTPases (guanosine triphosphate, GTP) are involved in many critical cellular processes, including inflammation, proliferation, and migration. GTP loading and isoprenylation are two important post-translational modifications of small GTPases, and are critical for their normal function. In this study, we investigated the role of post-translational modifications of small GTPases in regulating endothelial cell inflammatory responses and junctional integrity.

Methods and Results

Confluent human umbilical vein endothelial cell (HUVECs ) treated with atorvastatin demonstrated significantly decreased lipopolysaccharide (LPS)-mediated IL-6 and IL-8 generation. The inhibitory effect of atorvastatin (Atorva) was attenuated by co-treatment with 100 µM mevalonate (MVA) or 10 µM geranylgeranyl pyrophosphate (GGPP), but not by 10 µM farnesyl pyrophosphate (FPP). Atorvastatin treatment of HUVECs produced a time-dependent increase in GTP loading of all Rho GTPases, and induced the translocation of small Rho GTPases from the cellular membrane to the cytosol, which was reversed by 100 µM MVA and 10 µM GGPP, but not by 10 µM FPP. Atorvastatin significantly attenuated thrombin-induced HUVECs permeability, increased VE-cadherin targeting to cell junctions, and preserved junction integrity. These effects were partially reversed by GGPP but not by FPP, indicating that geranylgeranylation of small GTPases plays a major role in regulating endothelial junction integrity. Silencing of small GTPases showed that Rho and Rac, but not Cdc42, play central role in HUVECs junction integrity.

Conclusions

In conclusion, our studies show that post-translational modification of small GTPases plays a vital role in regulating endothelial inflammatory response and endothelial junction integrity. Atorvastatin increased GTP loading and inhibited isoprenylation of small GTPases, accompanied by reduced inflammatory response and preserved cellular junction integrity.     

Thrombin-induced expression of the KC gene in cultured aortic endothelial cells. Involvement of proteolytic activity and protein kinase C

The KC gene, first identified in platelet-derived growth factor-stimulated BALB/c 3T3 cells, shares structural similarities with a new family of genes that code for secreted cytokines which appear to be involved in wound healing and inflammation. Thrombin is a coagulation system proteinase likely to be present in vivo at sites of tissue injury. This enzyme is known to stimulate multiple responses in cultured endothelial cells (EC), including the production of eicosanoids, the expression of growth factor genes and the adhesion of leukocytes. The present experiments were designed to examine the effect of thrombin on KC mRNA expression in EC and to explore the molecular mechanisms involved. Thrombin caused a marked concentration-dependent increase in the steady state level of KC mRNA in confluent porcine aortic EC. The level of KC mRNA reached a peak 2 h after thrombin treatment and returned to near control levels by 8 h. Thrombin that was pretreated with phenylmethylsulfonyl fluoride (PMSF) to block proteolytic activity did not stimulate KC gene expression. Trypsin (2 micrograms/ml) but not PSMF-trypsin also caused a substantial increase in the level of KC mRNA. We postulated a role for protein kinase C in thrombin-induced KC gene expression since previous work had demonstrated a similar EC response to phorbol esters. This hypothesis was further supported by the finding that thrombin-induced KC expression was suppressed by the C kinase inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpiperazine, but not by its structural analogue. The results of the present study demonstrate that thrombin augments KC mRNA expression by vascular EC in a process that requires intact proteinase activity. The activation of protein kinase C may be a necessary component of the intracellular signalling pathway involved in this response.     

Thrombin, TNF-alpha, and LPS exert overlapping but nonidentical effects on gene expression in endothelial cells and vascular smooth muscle cells

Thrombin, TNF-alpha, and LPS have each been implicated in endothelial cell and vascular smooth muscle cell (VSMC) activation. We wanted to test the hypothesis that these three agonists display mediator and/or cell type-specific properties. The addition of thrombin to human pulmonary artery endothelial cells resulted in an upregulation of PDGF-A, tissue factor (TF), ICAM-1, and urokinase-type plasminogen activator (u-PA), whereas TNF-alpha and LPS failed to induce PDGF-A. These effects were mimicked by protease-activated receptor-1 activation. In VSMC, thrombin induced expression of TF and PDGF-A but failed to consistently induce ICAM-1 or u-PA expression. In contrast, TNF-alpha and LPS increased expression of all four genes in this cell type. Inhibitor studies in endothelial cells demonstrated a critical role for PKC in mediating thrombin, TNF-alpha, and LPS induction of ICAM-1, TF, and u-PA and for p38 MAPK in mediating thrombin, TNF-alpha, and LPS induction of TF. Taken together, these results suggest that inflammatory mediators engage distinct signaling pathways and expression profiles in endothelial cells and VSMC. The data support the notion that endothelial cell activation is not an all-or-nothing phenomenon but rather is dependent on the nature of the extracellular mediator.     

Carotenoids enhance phosphorylation of Akt and suppress tissue factor activity in human endothelial cells

Enhanced production of tissue factor has been linked to development of cardiovascular disease due to endothelial activation, resulting in thrombosis of blood vessels. Epidemiological studies reported that diet-derived antioxidants might suppress and/or delay progression of cardiovascular disease. Detailed molecular level studies are needed to understand this effect with prevention as a goal. Water-dispersible forms of various carotenoids (beta-carotene, lutein and lycopene) from natural sources in microemulsion were used to study effects of carotenoids on tissue factor activity in human endothelial cells. All carotenoids studied suppressed tissue factor activity (P<.01) and gene expression in human endothelial cells. Our study also demonstrated that addition of Akt-specific inhibitor reversed the inhibitory effect of carotenoids on tissue factor activity, indicating that carotenoids enhanced phosphorylation of Akt and suppressed tissue factor activity in endothelial cells by this mechanism.     

Synthesis of a nitric oxide-like factor from L-arginine by rat serosal mast cells: stimulation of guanylate cyclase and inhibition of platelet aggregation

Rat serosal mast cells were tested for their ability to generate a nitric oxide-like factor by two bioassay systems: inhibition of platelet aggregation and stimulation of mast cell guanylate cyclase. Incubation of rat serosal mast cells with human washed platelets resulted in an inhibition of thrombin-induced platelet aggregation proportional to the number of cells. The inhibition was potentiated by superoxide dismutase (SOD) and reversed by oxyhaemoglobin (oxyHb). The inhibitory activity of mast cells was also prevented by NG-monomethyl-L-arginine (MeArg), an effect reversed by co-incubation with L-Arg but not D-Arg. When mast cells alone were stirred at 1,000 rpm, a time-dependent increase in the levels of their cGMP but not cAMP was observed. This increase was reduced by pretreatment with MeArg. The inhibitory effect of MeArg was reversed by L-Arg but not D-Arg. These results demonstrate that rat mast cells release a factor with the same pharmacological profile as NO, and that this NO-like factor is derived from L-arginine.     

Thrombin-induced activation of cultured rodent microglia

Microglia are the resident immune cells of the CNS. Upon brain damage, these cells are rapidly activated and function as tissue macrophages. The first steps in this activation still remain unclear, but it is widely believed that substances released from damaged brain tissue trigger this process. In this article, we describe the effects of the blood coagulation factor thrombin on cultured rodent microglial cells. Thrombin induced a transient Ca(2+) increase in microglial cells, which persisted in Ca(2+)-free media. It was blocked by thapsigargin, indicating that thrombin caused a Ca(2+) release from internal stores. Preincubation with pertussis toxin did not alter the thrombin-induced [Ca(2+)](i) signal, whereas it was blocked by hirudin, a blocker of thrombin's proteolytic activity. Incubation with thrombin led to the production of nitric oxide and the release of the cytokines tumor necrosis factor-alpha, interleukin-6, interleukin-12, the chemokine KC, and the soluble tumor necrosis factor-alpha receptor II and had a significant proliferative effect. Our findings indicate that thrombin, a molecule that enters the brain at sites of injury, rapidly triggered microglial activation.     

Protein kinase C and cyclic AMP modulate thrombin-induced platelet-activating factor synthesis in human endothelial cells.

Stimulation of human endothelial cells (EC) by thrombin elicits a rapid increase of intracellular free Ca2+ [(Ca2+]i), platelet-activating factor (PAF) production and 1-O-alkyl-2-lyso-sn-glycero-3- phosphocholine (lyso-PAF): acetyl-CoA acetyltransferase (EC 2.3.1.67) activity. The treatment of EC with thrombin leads to a 90% decrease in the cytosolic protein kinase C (PKC) activity; this dramatic decline is accompanied by an increase of the enzymatic activity in the particulate fraction. The role of PKC in thrombin-mediated PAF synthesis has been assessed: (1) by the blockade of PKC activity with partially selective inhibitors (palmitoyl-carnitine, sphingosine and H-7); (2) by chronic exposure of EC to phorbol 12-myristate 13-acetate (PMA), which results in down-regulation of PKC. In both cases, a strong inhibition of thrombin-induced PAF production is observed, suggesting obligatory requirement of PKC activity for PAF synthesis. It is suggested that PKC regulates EC phospholipase A2 (PLA2) activity as thrombin-induced arachidonic acid (AA) release is 90% inhibited in PKC-depleted cells. Brief exposure of EC to PMA strongly inhibits thrombin-induced [Ca2+]i rise, acetyltransferase activation and PAF production, suggesting that, in addition to the positive forward action, PKC provides a negative feedback control over membrane signalling pathways involved in the thrombin effect on EC. Forskolin and iloprost, two agents that increase the level of cellular cAMP in EC, are very effective in inhibiting thrombin-evoked cytosolic Ca2+ rise, acetyltransferase activation and PAF production; this suggests that endogenously generated prostacyclin (PGI2) may modulate the synthesis of PAF in human endothelial cells.