Inhibition of Thrombin-mediated Cellular Effects by Triabin, a Highly Potent Anion-binding Exosite Thrombin Inhibitor

Triabin, a 17 kDa protein from the saliva of the assassin bug Triatoma pallidipennis is a potent thrombin inhibitor interfering with the anion-binding exosite of the enzyme. The recombinant protein, produced by the baculovirus/insect cell system, was used to study the inhibitory effect on thrombin-mediated cellular responses. The thrombin (1 nM)-stimulated aggregation of washed human platelets and the rise in cytoplasmic calcium in platelets were inhibited by triabin at nanomolar concentrations. In contrast, the rise in calcium induced by the thrombin receptor-activating peptide (10 μM) was not suppressed by triabin. In isolated porcine pulmonary arteries, preconstricted with PGF2γ, thrombin (2 nM) elicited an endothelium-dependent relaxation which was inhibited by triabin in the same concentration range as found for the inhibition of platelet aggregation. Higher concentrations of triabin were required to diminish the contractile response of endothelium- denuded pulmonary vessels to thrombin (10 nM). In cultured bovine coronary smooth muscle cells, the mitogenic activity of thrombin (3 nM), measured by [3H]thymidine incorporation, was also suppressed by triabin. In all these assays, the inhibitory effect of triabin was dependent on the thrombin concentration used.

These studies suggest that the new anion-binding exosite thrombin inhibitor triabin is one of the most potent inhibitors of thrombin-mediated cellular effects.

Factor Xa Acts as a PDGF-Independent Mitogen in Human Vascular Smooth Muscle Cells

This study investigates the mitogenic effect of the coagulation factor Xa in smooth muscle cells (SMC) from human saphenous vein and the procoagulant activity of these cells. Factor Xa elicited a concentrationdependent increase in [3H]thymidine incorporation. This mitogenic effect of factor Xa was inhibited by DX-9065a and BABCH, indicating the requirement of proteolytic activity of the enzyme. Factor Xa activated the MAP kinases ERK1/2 concentration-and time-dependently. PDGF-neutralizing antibodies neither inhibited the increase in [3H]thymidine incorporation nor ERK-1/2 phosphorylation in factor Xa-stimulated cells, suggesting that factor Xa-induced signaling and mitogenic activity in human venous SMC are independent of PDGF. Exposure of SMC to recalcified plasma resulted in a significant thrombin generation which was inhibited by anti-tissue factor antibody, tissue factor pathway inhibitor, inactivated factor VIIa and DX-9065a. These data indicate that interaction of SMC with the clotting system may contribute to venous graft disease, i.e. thrombus formation and intimal hyperplasia.

Regulation of human vascular protease-activated receptor-3 through mRNA stabilization and the transcription factor nuclear factor of activated T cells (NFAT)

Thrombin promotes vascular smooth muscle cell (SMC) proliferation and inflammation via protease-activated receptor (PAR)-1. A further thrombin receptor, PAR-3, acts as a PAR-1 cofactor in some cell-types. Unlike PAR-1, PAR-3 is dynamically regulated at the mRNA level in thrombin-stimulated SMC. This study investigated the mechanisms controlling PAR-3 expression. In human vascular SMC, PAR-3 siRNA attenuated thrombin-stimulated interleukin-6 expression and extracellular signal-regulated kinases 1/2 phosphorylation, indicating PAR-3 contributes to net thrombin responses in these cells. Thrombin slowed the decay of PAR-3 but not PAR-1 mRNA in the presence of actinomycin D and induced cytosolic shuttling and PAR-3 mRNA binding of the mRNA-stabilizing protein human antigen R (HuR). HuR siRNA prevented thrombin-induced PAR-3 expression. By contrast, forskolin inhibited HuR shuttling and destabilized PAR-3 mRNA, thus reducing PAR-3 mRNA and protein expression. Other cAMP-elevating agents, including the prostacyclin-mimetic iloprost, also down-regulated PAR-3, accompanied by decreased HuR/PAR-3 mRNA binding. Iloprost-induced suppression of PAR-3 was reversed with a myristoylated inhibitor of protein kinase A and mimicked by phorbol ester, an inducer of cyclooxygenase-2. In separate studies, iloprost attenuated PAR-3 promoter activity and prevented binding of nuclear factor of activated T cells (NFAT2) to the human PAR-3 promoter in a chromatin immunoprecipitation assay. Accordingly, PAR-3 expression was suppressed by the NFAT inhibitor cyclosporine A or NFAT2 siRNA. Thus human PAR-3, unlike PAR-1, is regulated post-transcriptionally via the mRNA-stabilizing factor HuR, whereas transcriptional control involves NFAT2. Through modulation of PAR-3 expression, prostacyclin and NFAT inhibitors may limit proliferative and inflammatory responses to thrombin after vessel injury.

Thrombin receptors in vascular smooth muscle cells - function and regulation by vasodilatory prostaglandins

The vast majority of thrombin (>95%) is generated after clotting is completed, suggesting that thrombin formation serves purposes beyond coagulation, such as tissue repair after vessel injury. Two types of vascular thrombin binding sites exist: protease-activated receptors (PARs) and thrombomodulin (TM). Their expression is low in contractile vascular smooth muscle cells (SMC), the dominating subendothelial cell population, but becomes markedly up-regulated upon injury. In human SMC, PAR-1, PAR-3, and PAR-4 mediate thrombin-induced proliferation, migration and matrix biosynthesis as well as generation of inflammatory and growth-promoting mediators. Thrombin-responsive PARs are transcriptionally down-regulated in human vascular SMC by vasodilatory prostaglandins (PGI2/PGE2). For PAR-1 and PAR-3 this mechanism involves cAMP-dependent inactivation of the transcription factor NFAT. The human PAR-4 promoter does not possess NFAT recognition motifs suggesting involvement of other cAMP-regulated effectors. Unlike PARs, TM is induced in SMC exposed to vasodilatory prostaglandins. Enhanced thrombin binding to TM might ameliorate PAR-mediated SMC stimulation. Also expressed in human SMC is the endothelial protein C receptor (EPCR), which serves as an anchor to facilitate generation of activated protein C (aPC) by TM-bound thrombin. Whether prostaglandins affect aPC-generation is not known. In SMC, thrombin and aPC act synergistically via PAR-1 to modify tissue remodelling, in contrast to their antagonistic interaction in the coagulation pathways. Overall, this will contribute to plaque stability and wound healing. The processes outlined here are likely to become clinically relevant after up-regulation of vascular cyclooxygenase2, the rate limiting step in vascular PGE2/PGI2 biosynthesis, such as in advanced atherosclerosis and acute coronary syndromes.

Human vascular smooth muscle cells express functionally active endothelial cell protein C receptor

The endothelial cell protein C receptor (EPCR) is expressed on endothelial cells and regulates the protein C anticoagulant pathway via the thrombin-thrombomodulin complex. Independent of its anticoagulant activity, activated protein C (APC) can directly signal to endothelial cells and upregulate antiapoptotic and antiinflammatory genes. Here we show that vascular smooth muscle cells (SMCs) also express EPCR. EPCR protein on SMCs was detected by flow cytometry and Western blotting. EPCR mRNA was identified by quantitative RT-PCR. To examine the functionality of EPCR, intracellular signaling in APC-stimulated SMCs was analyzed by determination of intracellular free calcium transients using confocal laser scanning microscopy. Phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK-1/2) was detected by immunoblotting. APC-induced ERK-1/2 phosphorylation was inhibited by an anti-EPCR antibody and by a cleavage site blocking anti-PAR-1 antibody, indicating that binding of APC to EPCR and cleavage of protease-activated receptor-1 (PAR-1) were involved. APC elicited an increase in [(3)H]-thymidine incorporation. The mitogenic effect of APC was significantly enhanced in the presence of thrombin. EPCR expression was also detected in SMCs in the fibrous cap of human carotid artery plaques. The present data demonstrate functionally active EPCR in SMCs and suggest that EPCR-bound APC might modulate PAR-1-mediated responses of SMCs to vascular injury.

Cysteinyl leukotriene 2 receptor and protease-activated receptor 1 activate strongly correlated early genes in human endothelial cells

Cysteinyl leukotrienes (cysLT), i.e., LTC4, LTD4, and LTE4, are lipid mediators derived from the 5-lipoxygenase pathway, and the cysLT receptors cysLT1-R/cysLT2-R mediate inflammatory tissue reactions. Although endothelial cells (ECs) predominantly express cysLT2-Rs, their role in vascular biology remains to be fully understood. To delineate cysLT2-R actions, we stimulated human umbilical vein EC with LTD4 and determined early induced genes. We also compared LTD4 effects with those induced by thrombin that binds to protease-activated receptor (PAR)-1. Stringent filters yielded 37 cysLT2-R- and 34 PAR-1-up-regulated genes (>2.5-fold stimulation). Most LTD4-regulated genes were also induced by thrombin. Moreover, LTD4 plus thrombin augmented gene expression when compared with each agonist alone. Strongly induced genes were studied in detail: Early growth response (EGR) and nuclear receptor subfamily 4 group A transcription factors; E-selectin; CXC ligand 2; IL-8; a disintegrin-like and metalloprotease (reprolysin type) with thrombospondin type 1 motif 1 (ADAMTS1); Down syndrome critical region gene 1 (DSCR1); tissue factor (TF); and cyclooxygenase 2. Transcripts peaked at approximately 60 min, were unaffected by a cysLT1-R antagonist, and were superinduced by cycloheximide. The EC phenotype was markedly altered: LTD4 induced de novo synthesis of EGR1 protein and EGR1 localized in the nucleus; LTD4 up-regulated IL-8 formation and secretion; and LTD4 raised TF protein and TF-dependent EC procoagulant activity. These data show that cysLT2-R activation results in a proinflammatory EC phenotype. Because LTD4 and thrombin are likely to be formed concomitantly in vivo, cysLT2-R and PAR-1 may cooperate to augment vascular injury.

Regulation of thrombomodulin expression in human vascular smooth muscle cells by COX-2-derived prostaglandins

There is concern that cyclooxygenase (COX)-2 inhibitors may promote atherothrombosis by inhibiting vascular formation of prostacyclin (PGI2) and an increased thrombotic risk of COX-2 inhibitors has been reported. It is widely accepted that the prothrombotic effects of COX-2 inhibitors can be explained by the removal of platelet-inhibitory PGI2. Using microarray chip technology, we have previously demonstrated that thrombomodulin (TM) mRNA is upregulated in cultured human coronary artery smooth muscle cells by the stable prostacyclin mimetic iloprost. This study is the first to demonstrate a stimulation of the expression of functionally active thrombomodulin in human smooth muscle cells by prostaglandins, endogenously formed via the COX-2 pathway. Because TM is an important inhibitor of blood coagulation, these findings provide a novel platelet-independent mechanism to explain the prothrombotic effects of COX-2 inhibitors. The full text of this article is available online at http://circres.ahajournals.org.

Evidence for functionally active protease-activated receptor-3 (PAR-3) in human vascular smooth muscle cells

The present study investigates whether vascular smooth muscle cells of the human saphenous vein (SMC) express a functionally active protease-activated receptor-3 (PAR-3). PAR-3 mRNA was detected by RT-PCR. In the presence of thrombin, a rapid and transient increase in PAR-3 mRNA was observed. Stimulation of SMC with thrombin or the synthetic PAR-3-activating peptide, TFRGAP, resulted in transient mobilization of intracellular calcium. After a preceding challenge with thrombin, the calcium signal to TFRGAP was abolished, suggesting cleavage and subsequent desensitization of PAR-3 by thrombin. Activation of PAR-3 by TFRGAP elicited a time-dependent activation of the extracellular-signal-regulated kinase (ERK)-1/2 with a maximum response 10-20 min after stimulation. At 200 microM, TFRGAP increased [3H]-thymidine incorporation into cellular DNA about two-fold. These data indicate that PAR-3 is expressed in human SMC and triggers intracellular signaling. Thus, in the SMC PAR-3 might contribute to thrombin-induced responses.

Differential leukotriene receptor expression and calcium responses in endothelial cells and macrophages indicate 5-lipoxygenase-dependent circuits of inflammation and atherogenesis

OBJECTIVE

Inflammatory infiltrates and atherosclerotic lesions emerge when monocytes adhere to endothelial cells (ECs), migrate into the subendothelial space, and become macrophages (MPhi(s)). Leukotrienes (LTs), products of 5-lipoxygenase, are powerful inflammatory mediators. 5-lipoxygenase+ MPhi(s) have been shown to increase during atherogenesis, and LT receptor (LT-R) transcripts were identified in diseased arteries. To investigate LT-Rs in cells involved in inflammation and atherogenesis, we used the in vitro models of human umbilical vein ECs (HUVECs) and monocyte-derived MPhi(s).

METHODS AND RESULTS

HUVECs primarily expressed transcripts of the cysteinyl (cys) LT2-R, which was strongly upregulated by interleukin-4. By contrast, MPhi(s) predominantly expressed transcripts of the cysLT1-R. Calcium responses toward LTs revealed differential cysLT-R utilization by both cell types: HUVECs responded to both cysLTs, whereas MPhi(s) preferentially responded to LTD4; HUVECs, but not MPhi(s), were resistant toward a cysLT1-R antagonist, montelukast; cysLTs generated regular calcium oscillations in HUVECs that lasted >60 minutes, resulting in >500 oscillations per cell. By contrast, calcium elevations in MPhi(s) returned to baseline within seconds and were nonoscillatory.

CONCLUSIONS

Our data raise the possibility that MPhi-derived LTs differentially activate cysLT2-Rs via paracrine stimulation and cysLT1-Rs via autocrine and paracrine stimulation during inflammation and atherogenesis.

Factor Xa releases matrix metalloproteinase-2 (MMP-2) from human vascular smooth muscle cells and stimulates the conversion of pro-MMP-2 to MMP-2: role of MMP-2 in factor Xa-induced DNA synthesis and matrix invasion

Pro-matrix metalloproteinase-2 (pro-MMP-2) is expressed in vascular smooth muscle cells (SMCs). We report that activated coagulation factor X (FXa) induces the release of MMP-2 (65 kDa) from human SMCs. In addition, FXa cleaves pro-MMP-2 (72 kDa) into MMP-2. Pro-MMP-2 and MMP-2 were determined by gelatin zymography. MMP-2 was generated in conditioned medium containing pro-MMP-2 in a concentration-dependent fashion by FXa (3 to 100 nmol/L). FX at concentrations up to 300 nmol/L was ineffective. The conversion of pro-MMP-2 to MMP-2 was inhibited by a selective FXa inhibitor (DX-9065a) at 3 to 10 micromol/L. There was a concentration-dependent induction of an intermediate MMP-2 form (68 kDa) in lysates of FXa-treated cells. This indicates that cellular mechanisms are involved in FXa-induced conversion of pro-MMP-2. As a possible biological consequence of MMP-2 activation by FXa, DNA synthesis and matrix invasion of SMCs were determined. Both were stimulated by FXa and inhibited by the selective FXa inhibitor DX-9065a and the MMP inhibitor GM 6001 but not by hirudin or aprotinin. It is concluded that stimulation of SMCs by FXa increases the levels of MMP-2 in the extracellular space and that two different mechanisms are involved: release of active MMP-2 and cleavage of secreted pro-MMP-2. Both might contribute to the mitogenic potency of FXa and FXa-stimulated matrix invasion of SMCs.

Cellular effects of factor Xa on vascular smooth muscle cells--inhibition by heparins?

In addition to its key function as a clotting enzyme, factor Xa (FXa) also elicits cellular effects. In cultured human venous smooth muscle cells (SMCs), FXa induced a mitogenic response that was independent of thrombin and platelet-derived growth factor (PDGF). Unfractionated heparin (UFH) as well as low molecular weight heparin (LMWH) (enoxaparin) inhibited the mitogenic effects of FXa, thrombin and fetal calf serum (FCS), but did not reduce mitogenesis induced by PDGF. Similarly, both UFH and LMWH inhibited the activation of extracellular signal-regulated kinase (ERK-1/2) by FXa, thrombin and FCS, but not by PDGF. This indicates that heparins can influence cellular signaling in SMC via an antithrombin-II (AT-III)-independent mechanism. The inhibition of ERK-1/2 correlated with the inhibition of mitogenesis by the heparins. Thus, the inhibition of ERK-1/2 phosphorylation by heparins might predict an antimitogenai response in this system.

Evidence for functionally active protease-activated receptor-4 (PAR-4) in human vascular smooth muscle cells

1. This study investigates, whether in addition to the protease-activated receptor-1 (PAR-1), PAR-4 is present in vascular smooth muscle cells (SMC) of the human saphenous vein and whether this receptor is functionally active. PAR-1 and PAR-4 are stimulated by thrombin and by the synthetic peptides SFLLRN and GYPGQV, respectively. 2. mRNAs for both, PAR-1 and PAR-4, were detected in the SMC by using reverse transcriptase polymerase chain reaction (RT - PCR). 3. Treatment of the SMC with GYPGQV (200 microM) resulted in a transient increase in free intracellular calcium. This calcium signal was completely abolished after a preceding challenge with thrombin (10 nM), indicating homologous receptor desensitization. 4. Stimulation of the SMC with 10 nM thrombin or 200 microM SFLLRN caused a time-dependent activation of the extracellular signal-regulated kinases-1/2 (ERK-1/2) with a maximum at 5 min. In contrast, 100 nM thrombin as well as 200 microM of GYPGQV induced a prolonged phosphorylation of ERK-1/2 with a maximum at 60 min. These data suggest that PAR-1 and PAR-4 are activated by thrombin at distinct concentrations and with distinct kinetics. 5. GYPGQV stimulated [(3)H]-thymidine incorporation in SMC. At 500 microM, the peptide increased DNA synthesis 2.5 fold above controls. A comparable mitogenic effect was obtained after stimulation of the SMC by 10 nM thrombin or 100 microM SFLLRN, respectively. 6. These data indicate that a functionally active PAR-4 is present in SMC and, in addition to PAR-1, might contribute to thrombin-induced mitogenesis.

Factor Xa acts as a PDGF-independent mitogen in human vascular smooth muscle cells

This study investigates the mitogenic effect of the coagulation factor Xa in smooth muscle cells (SMC) from human saphenous vein and the procoagulant activity of these cells. Factor Xa elicited a concentration-dependent increase in [3H]thymidine incorporation. This mitogenic effect of factor Xa was inhibited by DX-9065a and BABCH, indicating the requirement of proteolytic activity of the enzyme. Factor Xa activated the MAP kinases ERK1/2 concentration- and time-dependently. PDGF-neutralizing antibodies neither inhibited the increase in [3H]thymidine incorporation nor ERK-1/2 phosphorylation in factor Xa-stimulated cells, suggesting that factor Xa-induced signaling and mitogenic activity in human venous SMC are independent of PDGF. Exposure of SMC to recalcified plasma resulted in a significant thrombin generation which was inhibited by anti-tissue factor antibody, tissue factor pathway inhibitor, inactivated factor VIIa and DX-9065a. These data indicate that interaction of SMC with the clotting system may contribute to venous graft disease, i.e. thrombus formation and intimal hyperplasia.

Evidence for proteinase-activated receptor-2 (PAR-2)-mediated mitogenesis in coronary artery smooth muscle cells

1. This study investigates, whether in addition to the thrombin receptor (PAR-1), the proteinase-activated receptor-2 (PAR-2) is present in vascular smooth muscle cells (SMC) and mediates mitogenesis. PAR-2 is activated by low concentrations of trypsin and the synthetic peptide SLIGRL. 2. Stimulation of bovine coronary artery SMC by trypsin (2 nM) caused a 3 fold increase in DNLA-synthesis. A similar effect was observed with 10 nM thrombin. Trypsin-induced mitogenesis was inhibited by soybean trypsin inhibitor, indicating that the proteolytic activity of the enzyme was required for its mitogenic effect. 3. The specific PAR-2-activating peptide SLIGRL or the PAR1-activating peptide SFFLRN did not elicit mitogenesis. 4. When the SMC were exposed to SLIGRL (40 nM), a homologous desensitization of cytosolic Ca2+ mobilization was found after subsequent stimulation with trypsin (40 nM) but not thrombin (15 nM). 5. Trypsin (2 nM) as well as SLIGRL (100 microm) activated the nuclear factor KB (NFkappaB) with a maximum response 2 h after stimulation of the SMC. This suggests that both agonists acted via a common receptor, PAR-2. Maximum activation of NFkappaB by thrombin (10 nM) was detected after 4-5 h. 6. These data suggest that PAR-2 is present in coronary SMC and mediates a mitogenic response. Activation of NFkappaB via either PAR-1 or PAR-2 does not predict mitogenesis.

Thromboxane A2 potentiates thrombin-induced proliferation of coronary artery smooth muscle cells

The activation of thrombin is the key event in clot formation after vascular injury. Thrombin itself, but also other clot-derived factors, such as thromboxane A2 (TXA2), are mitogenic for vascular smooth muscle cells. We have studied the possible interactions between thrombin and TXA2 in stimulation of coronary artery smooth muscle cell (SMC) proliferation. Thrombin (1 U/ml) caused a significant proliferatory response in SMC. U 46619, a stable TXA2 mimetic, had only a minor stimulating effect by its own but markedly potentiated the thrombin-induced mitogenesis. A possible mechanism for these potentiating effects is provided by the demonstration of a marked (6 fold) but transient (maximum after 20 min) increase in the expression of TXA2 receptor (TP receptor) mRNA in SMC by thrombin. Since a significant clot-related TXA2 generation was detected for at least 2 hours, the up-regulation of TP receptors by thrombin may represent a mechanism that is relevant for the in vivo situation of SMC proliferation after vessel injury.

Thrombin-induced mitogenesis in coronary artery smooth muscle cells is potentiated by thromboxane A2 and involves upregulation of thromboxane receptor mRNA

BACKGROUND

Previous studies have shown that thrombin is a potent though slow-acting mitogen for vascular smooth muscle cells (SMC). Because thrombin generation in vivo is accompanied by platelet activation, it has been suggested that platelet-derived factors might enhance thrombin-induced SMC proliferation. No information is available so far on the possible role of thromboxane A2.

METHODS AND RESULTS

Thrombin (1 U/mL) caused a threefold to fourfold increase of DNA synthesis in cultured bovine coronary artery SMC as assessed from [3H]thymidine incorporation. U 46619, a stable thromboxane A2 mimetic, had only a minor stimulating effect on its own but potentiated the thrombin effect sixfold to sevenfold above control (P<.05). These findings were paralleled by a 52+/-5% (P<.05) increase in cell number at 48 hours after addition of both mitogens as compared with 24+/-5% with thrombin alone and no change with U 46619 alone. Thromboxane A2 receptor mRNA was found to be upregulated sixfold 20 minutes after thrombin stimulation. Pretreatment of SMC with thrombin for 4 hours markedly increased U 46619-induced mitogen-activated protein kinase activity, indicating thrombin-induced upregulation of functional thromboxane receptors in SMC.

CONCLUSIONS

Thrombin-induced proliferation of SMC is markedly enhanced by thromboxane A2. This might result in an enhancement of SMC proliferation by platelet-derived thromboxane A2 in vivo.

Activation of NFkappaB is essential but not sufficient to stimulate mitogenesis of vascular smooth muscle cells

This study investigates the role of the transcription factor NFkappaB in thrombin- and thrombin receptor activating peptide (TRAP, SFLLRNPNDKYEPYF)-induced mitogenesis of cultured bovine coronary artery smooth muscle cells (SMC). Stimulation of resting cells by thrombin (10 nM) or TRAP (10-100 microM) resulted in a comparable time-dependent activation of NFkappaB as detected by Western blotting and electrophoretic mobility shift assay (EMSA) of nuclear extracts. The NFkappaB activation was antagonized by N-acetyl-L-cysteine (20 mM) and pentoxifylline (0.5 mM). Thrombin caused a 3-4-fold increase in [3H]thymidine incorporation within 24 h which was prevented by inhibitors of NFkappaB activation. In contrast, TRAP did not cause any mitogenic response. These results demonstrate that activation of NFkappaB is an essential but not a sufficient signal for SMC mitogenesis.

Inhibition of thrombin-mediated cellular effects by triabin, a highly potent anion-binding exosite thrombin inhibitor

Triabin, a 17 kDa protein from the saliva of the assassin bug Triatoma pallidipennis is a potent thrombin inhibitor interfering with the anion-binding exosite of the enzyme. The recombinant protein, produced by the baculovirus/insect cell system, was used to study the inhibitory effect on thrombin-mediated cellular responses. The thrombin (1 nM)-stimulated aggregation of washed human platelets and the rise in cytoplasmic calcium in platelets were inhibited by triabin at nanomolar concentrations. In contrast, the rise in calcium induced by the thrombin receptor-activating peptide (10 microM) was not suppressed by triabin. In isolated porcine pulmonary arteries, preconstricted with PGF 2 alpha thrombin (2 nM) elicited an endothelium-dependent relaxation which was inhibited by triabin in the same concentration range as found for the inhibition of platelet aggregation. Higher concentrations of triabin were required to diminish the contractile response of endotheliumdenuded pulmonary vessels to thrombin (10 nM). In cultured bovine coronary smooth muscle cells, the mitogenic activity of thrombin (3 nM), measured by [3H]thymidine incorporation, was also suppressed by triabin. In all these assays, the inhibitory effect of triabin was dependent on the thrombin concentration used. These studies suggest that the new anion-binding exosite thrombin inhibitor triabin is one of the most potent inhibitors of thrombin-mediated cellular effects.

Relaxant and contractile responses of porcine pulmonary arteries to thrombin and thrombin receptor activating peptides

The vascular effects of thrombin and thrombin receptor activating peptides (TRAP) were studied on isolated rings from porcine pulmonary arteries. In prostaglandin F2 alpha (PGF2 alpha)-precontracted vessels with intact endothelium, both thrombin- and TRAP-induced nitric oxide-mediated relaxation, whereas in endothelium-denuded vessels thrombin and TRAP elicited concentration-dependent contractile responses. The first phasic component of contraction was associated with increased generation of inositol 1,4,5-triphosphate and the tonic component seemed to be due to the activation of protein kinase C. Both peptides (TRAP-6 with 6 and TRAP-14 with 14 amino acid residues) did not differ in their intrinsic activity; like thrombin, both peptides elicited dualistic vascular effects but their potency was more than three orders of magnitude less than that of thrombin.

Involvement of inositol 1,4,5-triphosphate and protein kinase C in thrombin-induced contraction of porcine pulmonary artery

The role of the intracellular messengers inositol 1,4,5-triphosphate (IP3) and protein kinase C (PKC) in the thrombin (3 U/mL)-induced contraction of endothelium-denuded porcine pulmonary arteries was investigated. Thrombin induced a sustained contractile response with an initial transient increase in IP3 to about 160% of the unstimulated control. Omission of extracellular Ca2+ or preincubation with verapamil (10 mumol/L) reduced the maximum of contraction without significantly affecting the thrombin-induced increase in IP3. To evaluate the role of PKC for the contractile response, the PKC was activated directly by phorbol 12,13-dibutyrate (PDBu, 50 nmol/L). The phorbol ester produced a slowly increasing tonic contraction without any changes in the basal IP3 level. There was a moderate inhibition of PDBu-induced contractions in Ca(2+)-free solution, while they were not inhibited after preincubation with verapamil. Preincubation with the PKC inhibitor staurosporine (50 nmol/L) significantly reduced the PDBu-induced contraction (by about 80%). In thrombin-stimulated vessels staurosporine only inhibited the tonic phase of the contractile response whereas the increase in IP3 and the phasic component of contraction were still evident. These results suggest that IP3 and PKC are involved in the thrombin-induced contraction. The phasic component of contraction is associated with the generation of IP3; the tonic component might be due to the activation of PKC.

Inositol 1,4,5-triphosphate and protein kinase C are involved in thrombin- and trap-induced vascular smooth muscle contraction

Thrombin (30 nmol/l) as well as the thrombin receptor activating peptide (TRAP), 10 mumol/l) induce a sustained contraction of endothelium-denuded porcine pulmonary arteries. The first phasic component of contraction is associated with the generation of IP3 which precedes the development of contractile force. Since the PKC inhibitor staurosporine (50 nmol/l) completely inhibits the tonic contraction this component of contraction seems to be due to the activation of protein kinase C (PKC). The thrombin- and TRAP-induced vasoconstriction strongly depends on extracellular calcium; the remaining thrombin- or TRAP-induced contraction in Ca(2+)-free medium seems to be attributed to the IP3-mediated release of calcium from intracellular stores.

ADP-, PAF- and adrenaline-induced platelet aggregation and thromboxane formation are not affected by a thromboxane receptor antagonist at physiological external Ca++ concentrations

Thromboxane (TX) receptor antagonists are of considerable clinical interest in prevention of acute thrombembolic vessel occlusion. This study demonstrates that the selective TX receptor antagonist, daltroban, at a concentration (10 microM) that does not inhibit TX synthesis, markedly inhibits ADP-, PAF- and adrenaline-induced platelet secretion and TX formation. With the exception of ADP-induced platelet secretion, these actions are only detectable in citrated platelet-rich plasma but not in plasma anticoagulated by hirudin. Since TX antagonists are supposed to act at physiological external Ca++ concentrations in the clinics, it is questionable whether in vitro studies in Ca(++)-deprived media are the optimum model to evaluate the clinical potential of these compounds.

Contractile effects of thrombin in porcine pulmonary arteries and the influence of thrombin inhibitors

Thrombin catalyzes not only the conversion of fibrinogen to fibrin but also activates several receptor-mediated cell responses. In ring segments of porcine pulmonary arteries the contractile effect of thrombin was studied in the presence and absence of endothelium. The integrity of endothelium was assessed by the bradykinin-induced relaxation of PGF2 alpha (3 mumol/l)-precontracted vessels which was absent after mechanical removal of endothelium. Thrombin at 0.1 to 10 U/ml (i.e. about 1-100 nmol/l) caused a sustained contraction in endothelium-denuded arteries with a maximum at 20-30 min. In vessels with intact endothelium a significant increase in tension up to 1 U/ml was observed preceded by a transient relaxant response. The contractile effect in vessels with intact endothelium was comparatively weaker. This is probably due to the release of EDRF from endothelial cells since blockade of EDRF synthesis by NG-nitro-L-arginine augmented the thrombin-induced contractions in arteries with intact endothelium. Indomethacin did not alter the contractile effect. However, in vessels with endothelium and in endothelium-denuded vessels the contractions were reduced when extracellular calcium was omitted. Verapamil (10 mumol/l) significantly diminished the contractile effect only in endothelium-denuded vessels. On preincubation of endothelium-denuded arterial ring segments with myo-[2-3H]inositol the addition of thrombin (10 U/ml) caused an accumulation of [3H]inositol-1,4,5-triphosphate (IP3). A maximum was observed after 2 min preceding the maximum increase in contraction. Measurement of thrombin-induced endogenous IP3 generation by radioreceptor assay yielded the same results. The thrombin-induced contractile effect requires the proteolytic activity of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)