Thrombin receptor expression in normal and atherosclerotic human arteries

Prevention of neointimal formation by a serine protease inhibitor, FUT-175, after carotid balloon injury in rats

BACKGROUND AND PURPOSE

In vivo and vitro studies revealed the activation of thrombin and the complement system in vascular lesion formation during the process of atherosclerosis, along with pathological proliferation of smooth muscle cells. We examined the effect of the synthetic serine protease inhibitor FUT-175 (developed as a potent inhibitor of thrombin and the complement system) on vascular lesions using balloon dilatation-induced neointimal formation in the carotid artery of rats.

METHODS

Sprague-Dawley (SD) rats underwent balloon dilatation injury of the left carotid artery to induce neointimal formation. Three groups of these rats (n=8, each) were treated with daily intraperitoneal injections of 1 of the following doses of FUT-175: 0.5, 1.0, or 2.0 mg/d in 1 mL of saline for 7 consecutive days. The control group (n=8) was similarly treated with 1 mL of saline for 7 days. The injections were started immediately after balloon injury. Two weeks after the injury, the left carotid arteries were perfusion-fixed, and the areas of the neointimal and medial layer were analyzed under a microscope.

RESULTS

A morphometric analysis revealed that there were significant differences in the intima-media ratio between the 4 groups treated with vehicle (saline) or a low, medium, or high dose of FUT-175 (1.45+/-0.11, 1.08+/-0.06, 0.71+/-0.04, or 0.32+/-0.04, respectively). This suppression was achieved in a dose-dependent manner by the administration of FUT-175 after balloon injury. In the histological study, it was demonstrated that FUT-175 suppresses the production of platelet-derived growth factor (PDGF)-BB in the neointima and the medial smooth muscle cell layer.

CONCLUSIONS

After balloon injury activated proteases that were inhibited by FUT-175 were demonstrated to have an essential role in the development of the pathological thickening of the arterial wall.

Pro- and anti-inflammatory actions of thrombin: a distinct role for proteinase-activated receptor-1 (PAR1)

1. Thrombin has well characterized pro-inflammatory actions that have recently been suggested to occur via activation of its receptor, proteinase-activated receptor-1 (PAR1). 2. In the present study, we have compared the effects of thrombin to those of two peptides that selectively activate the PAR1 receptor, in a rat hindpaw oedema model. We have also examined whether or not thrombin can exert anti-inflammatory activity in this model. 3. Both thrombin and the two PAR1 activating peptides induced significant oedema in the rat hindpaw following subplantar injection. 4. The oedema induced by thrombin was abolished by pre-incubation with hirudin, and was markedly reduced in rats in which mast cells were depleted through treatment with compound 48/80 and in rats pretreated with indomethacin. In contrast, administration of the PAR1 activating peptides produced an oedema response that was not reduced by indomethacin and was only slightly reduced in rats pretreated with compound 48/80. 5. Co-administration of thrombin together with a PAR1 activating receptor resulted in a significantly smaller oedema response than that seen with the PAR1 activating peptide alone. This anti-inflammatory effect of thrombin was abolished by pre-incubation with hirudin. 6. These results demonstrate that the pro-inflammatory effects of thrombin occur through a mast-cell dependent mechanism that is, at least in part, independent of activation of the PAR1 receptor. Moreover, thrombin is able to exert anti-inflammatory effects that are also unrelated to the activation of PAR1.

Time course of recovery of endothelial cell surface thrombin receptor (PAR-1) expression

We studied dynamics of cell surface expression of proteolytically activated thrombin receptor (PAR-1) in human pulmonary artery endothelial cells (HPAEC). PAR-1 activation was measured by changes in cytosolic calcium concentration ([Ca2+]i) and HPAEC retraction response (determined by real-time transendothelial monolayer electrical resistance). [Ca2+]i increase in response to thrombin was abolished by preexposure to 25 nM thrombin for >60 min, indicating PAR-1 desensitization, but preexposure to 25 nM thrombin for only 30 min or to 10 nM thrombin for up to 2 h did not desensitize PAR-1. Exposure to 10 or 25 nM thrombin decreased monolayer electrical resistance 40-60%. Cells preexposed to 10 nM thrombin, but not those preexposed to 25 nM thrombin, remained responsive to thrombin 3 h later. Loss of cell retractility was coupled to decreased cell surface PAR-1 expression as determined by immunofluorescence. Cell surface PAR-1 disappeared upon short-term (30 min) thrombin exposure but reappeared within 90 min after incubation in thrombin-free medium. Exposure to 25 nM thrombin for >60 min prevented rapid cycloheximide-insensitive PAR-1 reappearance. Cycloheximide-sensitive recovery of cell surface PAR-1 expression required 18 h. Therefore, both duration and concentration of thrombin exposure regulate the time course of recovery of HPAEC surface PAR-1 expression. The results support the hypothesis that initial recovery of PAR-1 surface expression in endothelial cells results from a rapidly mobilizable PAR-1 pool, whereas delayed recovery results from de novo PAR-1 synthesis. We conclude that thrombin itself regulates endothelial cell surface PAR-1 expression and that decreased surface expression interferes with thrombin-induced endothelial cell activation responses.

Differential regulation of protease activated receptor-1 and tissue plasminogen activator expression by shear stress in vascular smooth muscle cells

Recent studies have demonstrated that vascular smooth muscle cells are responsive to changes in their local hemodynamic environment. The effects of shear stress on the expression of human protease activated receptor-1 (PAR-1) and tissue plasminogen activator (tPA) mRNA and protein were investigated in human aortic smooth muscle cells (HASMCs). Under conditions of low shear stress (5 dyn/cm2), PAR-1 mRNA expression was increased transiently at 2 hours compared with stationary control values, whereas at high shear stress (25 dyn/cm2), mRNA expression was decreased (to 29% of stationary control; P<0.05) at all examined time points (2 to 24 hours). mRNA half-life studies showed that this response was not due to increased mRNA instability. tPA mRNA expression was decreased (to 10% of stationary control; P<0.05) by low shear stress after 12 hours of exposure and was increased (to 250% of stationary control; P<0.05) after 24 hours at high shear stress. The same trends in PAR-1 mRNA levels were observed in rat smooth muscle cells, indicating that the effects of shear stress on human PAR-1 were not species-specific. Flow cytometry and ELISA techniques using rat smooth muscle cells and HASMCs, respectively, provided evidence that shear stress exerted similar effects on cell surface-associated PAR-1 and tPA protein released into the conditioned media. The decrease in PAR-1 mRNA and protein had functional consequences for HASMCs, such as inhibition of [Ca2+] mobilization in response to thrombin stimulation. These data indicate that human PAR-1 and tPA gene expression are regulated differentially by shear stress, in a pattern consistent with their putative roles in several arterial vascular pathologies.

Increased endothelial NOS in lambs with increased pulmonary blood flow and pulmonary hypertension

Altered pulmonary vascular reactivity is a source of morbidity and mortality for children with congenital heart defects and increased pulmonary blood flow. Nitric oxide (NO) is an important mediator of pulmonary vascular reactivity. The objective of this study was to characterize potential early alterations in expression, localization, and activity of endothelial NO synthase (eNOS) induced by increased pulmonary blood flow and pulmonary hypertension. Utilizing aortopulmonary vascular graft placement in the fetal lamb, we have established a unique animal model of pulmonary hypertension that mimics congenital heart disease with increased pulmonary blood flow. Ten fetal lambs underwent in utero placement of an aortopulmonary vascular graft (shunt). RNase protection assays and Western blotting were performed on lung tissue prepared from 4-wk-old shunt lambs and age-matched controls. eNOS mRNA (2.4:1, P < 0.05) and protein (2. 08:1, P < 0.05) were increased in lungs of shunt lambs. In situ hybridization and immunohistochemistry revealed that the increase was confined to the endothelium of pulmonary arteries. eNOS protein (1.55:1, P < 0.05) and tissue cGMP concentrations (2.1:1, P < 0.05) were also increased in isolated fifth-generation pulmonary arteries of shunt lambs. In addition, total lung eNOS activity was increased (2.9:1, P < 0.05). Thus we report a previously undescribed, early upregulation of eNOS gene expression and activity in lambs with increased pulmonary blood flow and pulmonary hypertension.

Thrombin-Activated Human Endothelial Cells Support Monocyte Adhesion In Vitro Following Expression of Intercellular Adhesion Molecule-1 (ICAM-1; CD54) and Vascular Cell Adhesion Molecule-1 (VCAM-1; CD106)

Thrombin, a central molecule in coagulation, is also involved in inflammation. Notably, thrombin induces endothelial neutrophil adhesion, P- and E-selectin expression, and chemokine production. We show here that thrombin induces expression of intercellular adhesion molecule-1 (ICAM-1; CD54) and vascular cell adhesion molecule-1 (VCAM-1; CD106) on human umbilical vein endothelial cells (HUVECs) associated with increased adhesion of monocytes. Thrombin increased mRNA steady-state levels and expression of ICAM-1 over 24 hours. Thrombin-induced VCAM-1 expression exhibited unusual kinetics, reaching maximum levels after 6 to 12 hours, but decreasing to near baseline after 24 hours. Thrombin activity on HUVECs was mediated through interaction with its specific receptor, because ICAM-1 and VCAM-1 expression were similarly induced by the 14-amino acid thrombin receptor-activating peptide. Thrombin-induced ICAM-1 and VCAM-1 expression was significantly inhibited by hirudin, but not by interleukin-1 receptor antagonist or anti-tumor necrosis factor  monoclonal antibody (MoAb). Thrombin-activated HUVECs significantly increased greater numbers of adhering THP-1 macrophagic cells, peripheral blood mononuclear cells, or purified monocytes than unstimulated HUVECs. This adhesion was inhibited by anti-CD18 and anti-CD49d MoAb, demonstrating that thrombin-induced ICAM-1 and VCAM-1 were functional. These results show that, in addition to selectins, thrombin directly induces a cytokine-independent expression of adhesion molecules of the Ig superfamily on HUVECs that may support firm leukocyte attachment during inflammation.

© 1998 by The American Society of Hematology.

Thrombin-Activated Human Endothelial Cells Support Monocyte Adhesion In Vitro Following Expression of Intercellular Adhesion Molecule-1 (ICAM-1; CD54) and Vascular Cell Adhesion Molecule-1 (VCAM-1; CD106)

Thrombin, a central molecule in coagulation, is also involved in inflammation. Notably, thrombin induces endothelial neutrophil adhesion, P- and E-selectin expression, and chemokine production. We show here that thrombin induces expression of intercellular adhesion molecule-1 (ICAM-1; CD54) and vascular cell adhesion molecule-1 (VCAM-1; CD106) on human umbilical vein endothelial cells (HUVECs) associated with increased adhesion of monocytes. Thrombin increased mRNA steady-state levels and expression of ICAM-1 over 24 hours. Thrombin-induced VCAM-1 expression exhibited unusual kinetics, reaching maximum levels after 6 to 12 hours, but decreasing to near baseline after 24 hours. Thrombin activity on HUVECs was mediated through interaction with its specific receptor, because ICAM-1 and VCAM-1 expression were similarly induced by the 14-amino acid thrombin receptor-activating peptide. Thrombin-induced ICAM-1 and VCAM-1 expression was significantly inhibited by hirudin, but not by interleukin-1 receptor antagonist or anti-tumor necrosis factor  monoclonal antibody (MoAb). Thrombin-activated HUVECs significantly increased greater numbers of adhering THP-1 macrophagic cells, peripheral blood mononuclear cells, or purified monocytes than unstimulated HUVECs. This adhesion was inhibited by anti-CD18 and anti-CD49d MoAb, demonstrating that thrombin-induced ICAM-1 and VCAM-1 were functional. These results show that, in addition to selectins, thrombin directly induces a cytokine-independent expression of adhesion molecules of the Ig superfamily on HUVECs that may support firm leukocyte attachment during inflammation.

© 1998 by The American Society of Hematology.

Thrombin stimulates fibroblast procollagen production via proteolytic activation of protease-activated receptor 1

Thrombin is a multifunctional serine protease that has a crucial role in blood coagulation. It is also a potent mesenchymal cell mitogen and chemoattractant and might therefore have an important role in the recruitment and local proliferation of mesenchymal cells at sites of tissue injury. We hypothesized that thrombin might also affect the deposition of connective tissue proteins at these sites by directly stimulating fibroblast procollagen production. To address this hypothesis, the effect of thrombin on procollagen production and gene expression by human foetal lung fibroblasts was assessed over 48 h. Thrombin stimulated procollagen production at concentrations of 1 nM and above, with maximal increases of between 60% and 117% at 10 nM thrombin. These effects of thrombin were, at least in part, due to increased steady-state levels of alpha1(I) procollagen mRNA. They could furthermore be reproduced with thrombin receptor-activating peptides for the protease-activated receptor 1 (PAR-1) and were completely abolished when thrombin was rendered proteolytically inactive with the specific inhibitors d-Phe-Pro-ArgCH2Cl and hirudin, indicating that thrombin is mediating these effects via the proteolytic activation of PAR-1. These results suggest that thrombin might influence the deposition of connective tissue proteins during normal wound healing and the development of tissue fibrosis by stimulating fibroblast procollagen production.

Stimulation of human smooth muscle cell proliferation by thrombin involves increased synthesis of platelet-derived growth factor

OBJECTIVES

Thrombin generated at sites of vascular injury not only participates in the coagulation cascade but can also signal other events related to cell mitogenesis and migration. In this report, we investigated the effects of thrombin on the proliferation of human arterial smooth muscle cells (SMCs) in culture and its interaction with platelet-derived growth factor (PDGF).

MATERIAL AND METHODS

Human arterial SMCs originated from a renal artery were grown in cell culture. The effect of thrombin on DNA synthesis was evaluated by 3H-thymidine incorporation. The effect of thrombin on inositol-phosphate formation by SMCs was also analyzed as well as the binding of PDGF AA and BB to these cells. PDGF secretion was analyzed by radioimmunoassay (RIA).

RESULTS

Exposure of cultured human SMCs to thrombin caused an increased rate of DNA synthesis in a dose-response manner, with a maximal stimulatory effect at a concentration of 2.0 U/mL. Thrombin was found to increase the accumulation of inositol phosphates and to increase the production of PDGF as measured by RIA. Exposure of cells to 2.0 U/mL thrombin resulted in a strong decrease in PDGF AA binding to PDGF receptors and did not change PDGF BB binding, probably indicating that PDGF alpha-receptors could be occupied by endogenously produced PDGF A.

CONCLUSION

Thrombin stimulates human vascular SMC proliferation in a dose-response way, in part by the formation of inositol phosphates. The mechanism responsible for this effect involves, at least in part, an increased endogenous synthesis of PDGF.

Atypical protease-activated receptor mediates endothelium-dependent relaxation of human coronary arteries

Protease-activated receptors (PARs) are a family of G protein-coupled receptors activated by a tethered ligand sequence within the amino terminal that are revealed by site-specific proteolysis. The thrombin-sensitive PAR-1 and trypsin-activated PAR-2 mediate endothelium-dependent vascular relaxation in a number of species. Because both thrombin and trypsin-like enzymes have been implicated in coronary artery disease, the purpose of this study was to investigate whether similar receptors are present in human coronary arteries. Thrombin (0.001 to 0.1 U/mL) and trypsin (0.001 to 1 U/mL) caused concentration- and endothelium-dependent relaxations of human coronary artery ring segments suspended in organ chambers for isometric tension recording and contracted with the thromboxane A2 mimetic U46619. These relaxations were dependent on the catalytic activity of each enzyme and were inhibited by the NO synthase inhibitor NG-nitro-L-arginine (100 micromol/L) and the NO scavenger oxyhemoglobin (20 micromol/L). The synthetic PAR-1 tethered ligand sequence SFLLRN-NH2 (0.01 to 10 micromol/L) also caused endothelium-dependent relaxation of U46619-contracted human coronary arteries; however, the equivalent PAR-2 ligand SLIGKV-NH2 caused almost no relaxation. In addition, desensitization to either thrombin or trypsin resulted in cross-desensitization to the other enzyme but had only a minimal affect on the response to SFLLRN-NH2. Therefore, we conclude that human coronary artery endothelial cells possess a PAR-1-like receptor that is potently activated by thrombin, trypsin, and SFLLRN-NH2 to cause NO-mediated vascular relaxation. Once cleaved, this receptor is recycled in a truncated form, able to respond to exogenous application of only its tethered ligand sequence, suggesting the presence of another endogenous activator possibly acting independently of receptor cleavage.

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Antibody to thrombin receptor inhibits neointimal smooth muscle cell accumulation without causing inhibition of platelet aggregation or altering hemostatic parameters after angioplasty in rat

An antibody was raised in rabbits against SFFLRNPSEDTFEQF peptide, which is an NH2-terminal peptide of the thrombin-cleaved rat thrombin receptor. In vitro, the antibody inhibited rat smooth muscle cell proliferation but had no effect on rat platelet aggregation or clotting time. These data indicate that the antibody is a specific blocker of the thrombin receptor-signaling pathway in rat smooth muscle cells but does not work as a blocker in rat platelets, suggesting the existence of a second thrombin receptor in the platelets. Using an in vivo balloon catheter-induced injury model in rats, we examined the effect of the anti-rat thrombin receptor IgG on intimal smooth muscle cell accumulation 2 weeks after angioplasty. Analysis of the ratio of intimal to medial cross-sectional areas showed that injection of immune IgG resulted in 43.7% and 53.1% reduction (P<0.01) of neointimal smooth muscle cell accumulation compared with saline and nonimmune IgG treatment, respectively. Moreover, the injection of immune IgG caused a significant decrease of thrombin receptor mRNA expression and also 40.5% and 43.0% decreases (P<0.01) of the proliferating cell nuclear antigen (PCNA) index in the intima compared with the PCNA index after saline and nonimmune IgG treatment, respectively. The suppression of the PCNA index was also observed in the immune IgG-treated group at an early stage after angioplasty. These results suggest that thrombin receptor activation is involved in the proliferation and accumulation of neointimal smooth muscle cells induced by balloon injury.

Potential Mechanisms for a Proinflammatory Vascular Cytokine Response to Coagulation Activation

We have previously shown that an anticoagulant could attenuate inflammation in animal models of sepsis with disseminated intravascular coagulation (DIC) and that coagulation activation of human whole blood ex vivo results in a proinflammatory cytokine response. The current studies were performed to better understand mechanisms for the blood cell cytokine response and extend the investigation of such a response to endothelial cells as likely contributors to a vascular inflammatory response. Utilizing cell separation techniques, it was determined that the whole blood IL-8 response to coagulation activation or thrombin, specifically, was mediated by CD14+ monocytes. Moreover, thrombin was observed to stimulate both IL-8 and IL-6 production in cultured mononuclear cells. Analyses of the effects of coagulation activation and thrombin were extended to cultured human endothelial cells, and a similar cytokine response was observed. Thrombin catalytic activity appeared essential, since hirudin reduced thrombin-stimulated proinflammatory cytokine production in cultured monocytes and endothelial cells and prothrombin only weakly mimicked the thrombin response. The endothelial cell IL-8 and IL-6 response to thrombin could be mimicked by the thrombin receptor agonist peptide (TRAP), implicating a functional role of the classic thrombin receptor. Altogether, the results facilitate a better understanding of potential proinflammatory vascular responses to coagulation activation.

Changes in the expression of protease-activated receptor 1 and protease nexin-1 mRNA during rat nervous system development and after nerve lesion

HDs racI Thrombin causes profound metabolic and morphological changes in cultured neural cells via activation of the thrombin receptor, also called protease-activated receptor 1 (PAR1). PAR1 mRNA is present in the rat brain, but the role of this receptor in the nervous system remains elusive. The expression of PAR1 and the potent thrombin inhibitor protease nexin-1 (PN-1) was investigated in the developing rat brain and spinal cord and after peripheral nerve lesion. As seen by in situ hybridization, the PAR1 mRNA signal in the late embryonic and early postnatal nervous system was widespread, but generally of low intensity whereas in the adult it was more pronounced and confined to particular neuronal cells. These include the mesencephalic dopaminergic neurons, several thalamic and brainstem nuclei, the mitral cells in the olfactory bulb and the Purkinje cells in the cerebellum. In the spinal cord, PAR1 mRNA was abundant in motoneurons and a particularly high expression was detected in the preganglionic neurons of the autonomic nervous system. High PAR1 mRNA expression was also found in the dorsal root ganglia. Interestingly, strong immunoreactivity for the protease inhibitor PN-1 was present in spinal motoneuron cell bodies, although its transcript was undetectable there. In response to sciatic nerve transection, the signal intensity of PAR1 mRNA as seen by Northern analysis increased in the proximal and the distal part of the lesioned nerve and in the denervated muscle, whereas the PN-1 mRNA signal strongly increased only in the distal part of the nerve but remained unchanged in the proximal part and in the muscle. After facial nerve transection, PAR1 mRNA expression substantially decreased in facial motoneurons. No PAR1 transcript was detected in reactive astrocytes. Similar to PAR1, PN-1 mRNA which was expressed in interneurons within the facial nucleus was also decreased following facial nerve transection.

Expression of the thrombin receptor in developing bone and associated tissues

Thrombin, a serine protease with a central role in thrombosis and hemostasis, is also a specific agonist for a variety of cellular responses in osteoblasts and stimulates bone resorption in organ culture. Cultured osteoblast-like cells express the proteolytically activated thrombin receptor, but the significance of this finding in vivo remains unknown. Immunohistochemistry was used to investigate the normal tissue distribution of the proteolytically activated thrombin receptor in developing rat bones and associated tissues. In hind limbs, the receptor was first observed on embryonic day 16 and became more abundant within the limb as gestation progressed. Thrombin receptor staining was detected on osteoblasts, macrophages, muscle cells, and endothelial cells, but not osteoclasts. Similarly, osteoblasts in developing calvariae stained positively for the thrombin receptor. The pattern of receptor expression by primary osteoblast cultures and freshly isolated macrophages and osteoclasts corresponded to that observed in vivo. The observed pattern of thrombin receptor expression in bone cells supports the hypothesis that cell-mediated thrombin-induced bone resorption is mediated by osteoblasts.

The response of smooth muscle cells to alpha-thrombin depends on its arterial origin: comparison among different species

BACKGROUND

Thrombin plays a pivotal role in the pathogenesis of arterial thrombosis and exerts a wide spectrum of effects on the cells of vessel walls.

METHODS

In this paper we focus on the direct role of thrombin as a mitogen for smooth muscle cells (SMCs) derived from different vessels from the vascular tree (coronary artery, aorta, carotid artery and pulmonary artery) of different species (human and pig). All cell populations examined responded mitogenically to alpha-thrombin, however the extent of this response was dependent on both vascular origin and the species from which vessels were derived.

RESULTS

Thrombin (1-100 nmol L-1) induced DNA synthesis ranging from 1.5- to 4-fold baseline depending on cell type. Porcine coronary SMCs showed the highest response to thrombin (100 nmol L-1) in terms of protein and DNA synthesis (4.0 +/- 0.2-fold) and cell division (53.4 +/- 8.8%) among the tested cells. In these cells recombinant (r)-hirudin, a specific thrombin inhibitor, exhibited maximal effectiveness to block the mitogenic effect of thrombin. Human SMC response ([3H]-thymidine incorporation) to either sera or thrombin was lower than that of porcine cells. In contrast, c-fos mRNA levels induced by thrombin in human SMCs were higher than those induced in porcine cells. In human cells, thrombin elicited an overexpression of c-fos and a lower rate of [3H]-thymidine incorporation than in porcine cells. Insulin-like growth factor I but not insulin showed additive mitogenic effects with thrombin in human coronary SMCs. The response of these cells to thrombin from different sources was a function of thrombin specific activity.

CONCLUSION

These results suggest that the cell system chosen to check thrombin mitogenicity not only determines thrombin quantitative effects but also may affect the effectiveness of an inhibitor to block its biological activity.

The cleaved peptide of the thrombin receptor is a strong platelet agonist

Thrombin cleaves its G-protein-linked seven-transmembrane domain receptor, thereby releasing a 41-aa peptide and generating a new amino terminus that acts as a tethered ligand for the receptor. Peptides corresponding to the new amino terminal end of the proteolyzed seven-transmembrane domain thrombin receptor [TR42-55, SFLLRNPNDKYEPF, also known as TRAP (thrombin receptor-activating peptide)], previously have been demonstrated to activate the receptor. In this study, we demonstrate that the 41-aa cleaved peptide, TR1-41 (MGPRRLLLVAACFSLCGPLLSARTRARRPESKATNATLDPR) is a strong platelet agonist. TR1-41 induces platelet aggregation. In whole-blood flow cytometric studies, TR1-41 was shown to be more potent than TR42-55 and almost as potent as thrombin, as determined by the degree of increase in: (i) platelet surface expression of P-selectin (reflecting alpha granule secretion); (ii) exposure of the fibrinogen binding site on the glycoprotein (GP) IIb-IIIa complex; and (iii) fibrinogen binding to the activated GPIIb-IIIa complex. As determined by experiments with inhibitors [prostaglandin I2, staurosporine, wortmannin, the endothelium-derived relaxing factor congener S-nitroso-N-acetylcysteine (SNAC), EDTA, EGTA, and genestein], and with Bernard-Soulier or Glanzmann's platelets, we demonstrated that TR1-41-induced platelet activation is: (i) inhibited by cyclic AMP; (ii) mediated by protein kinase C, phosphatidyl inositol-3-kinase, myosin light chain kinase, and intracellular protein tyrosine kinases; (iii) dependent on extracellular calcium; and (iv) independent of the GPIb-IX and GPIIb-IIIa complexes. TR1-41-induced platelet activation was synergistic with TR42-55. In summary, the cleaved peptide of the seven-transmembrane domain TR (TR1-41) is a strong platelet agonist.

Differential transcriptional regulation of the human thrombin receptor gene by the Sp family of transcription factors in human endothelial cells

The mitogenic effects of thrombin are mediated by a G-protein-coupled receptor. Because the effects of thrombin are strongly influenced by the expression of its receptor, an understanding of its regulatory mechanisms is essential. To identify mechanisms of human thrombin receptor (HTR) gene regulation, a series of HTR-promoter-luciferase constructs were made and transfected into human microvascular endothelial cells for analysis. Deletion from bp -303 to -164 abolished reporter gene expression. Dimethyl sulphate treatment in vivo and DNase I footprinting in vitro demonstrated that a cluster of three GC box consensus sites was occupied, and electrophoretic mobility-shift assays established that Sp1 and Sp3 both bind to this 3' GC box cluster. We mutated each of the three GC boxes individually and all three collectively within this 3' cluster. Basal promoter activity was decreased to 46%, 78% and 29% of control for each of the GC boxes mutated individually, and to 6% when the three were mutated collectively. To test the individual abilities of Sp1 and Sp3 to activate or repress HTR transcription, we conducted co-transfection experiments with wild-type or mutated HTR-promoter-luciferase constructs. Co-transfection with Sp1 significantly augmented wild-type HTR promoter activity. Sp3 alone did not affect activity, and inhibited Sp1-mediated activation. Competition for shared binding sites by Sp1 and Sp3 might differentially regulate HTR expression in vascular endothelial cells.

Expression of the thrombin receptor in human liver: up-regulation during acute and chronic injury

Thrombin is generated during tissue damage in several organs, including the liver, and participates in the process of tissue repair through proteolytic activation of a specific thrombin receptor (TR). The aim of this study was to investigate TR expression in human liver by immunohistochemistry and in situ hybridization. In normal liver, immunostaining for TR was present in the endothelial lining of the hepatic sinusoids. During chronic hepatitis, several cells expressing the TR were detected in the inflammatory infiltrate of portal tracts. In cirrhosis with chronic active hepatitis, expression of the TR was also present in mesenchymal cells of fibrous septa. TR expression was markedly up-regulated during fulminant hepatitis, with the highest expression in mesenchymal cells in areas of regeneration. Up-regulation of TR expression was associated with increased levels of TR messenger RNA (mRNA), as assessed by in situ hybridization and RNAse protection assay of liver RNA. Immunostaining of serial sections using specific cellular markers showed that different nonparenchymal cells contribute to TR expression during liver injury. TR expression was also shown in cultured human hepatic stellate cells, with increasing signal comparing activated versus quiescent cells. Because thrombin is rapidly generated after tissue damage, regulated TR expression may be involved in tissue remodeling and/or scarring during liver damage.

Atherogenic Lipoproteins Support Assembly of the Prothrombinase Complex and Thrombin Generation: Modulation by Oxidation and Vitamin E

The importance of lipoproteins in the etiology of atherosclerosis is well established. Evidence is now accumulating to implicate thrombin in the pathogenesis of atherosclerosis. We have investigated whether atherogenic lipoproteins can support thrombin generation. In the absence of platelets or endothelial cells, both very low-density lipoprotein (VLDL) and oxidized low-density lipoprotein (LDL) support assembly of the prothrombinase complex and generation of thrombin. Thrombin generation (per μg of apolipoprotein) supported by VLDL was 19.4-fold greater than that supported by high-density lipoprotein (HDL), P < .00001, and 11.7-fold greater than that supported by LDL, P < .00001. Oxidation of LDL increased lipoprotein-supported thrombin generation 12-fold compared to unmodified LDL, P < .0001. We have shown that the phenomenon of lipoprotein-supported thrombin generation is mediated predominantly by specific phospholipids and is enhanced by oxidation of these phospholipids. The addition of vitamin E (α-tocopherol) markedly reduced the increase in thrombin generation observed after oxidation of LDL (822 ± 57 v 138 ± 47 nmol/L;P < .0001). These effects suggest that lipoproteins are important in the production of thrombin and that vitamin E may confer protection from the detrimental effects of lipoprotein oxidation by limiting thrombin formation. These results suggest that atherogenic lipoproteins are linked to the development of atherosclerosis in part by their capacity to support thrombin generation.

Atherogenic Lipoproteins Support Assembly of the Prothrombinase Complex and Thrombin Generation: Modulation by Oxidation and Vitamin E

The importance of lipoproteins in the etiology of atherosclerosis is well established. Evidence is now accumulating to implicate thrombin in the pathogenesis of atherosclerosis. We have investigated whether atherogenic lipoproteins can support thrombin generation. In the absence of platelets or endothelial cells, both very low-density lipoprotein (VLDL) and oxidized low-density lipoprotein (LDL) support assembly of the prothrombinase complex and generation of thrombin. Thrombin generation (per μg of apolipoprotein) supported by VLDL was 19.4-fold greater than that supported by high-density lipoprotein (HDL), P < .00001, and 11.7-fold greater than that supported by LDL, P < .00001. Oxidation of LDL increased lipoprotein-supported thrombin generation 12-fold compared to unmodified LDL, P < .0001. We have shown that the phenomenon of lipoprotein-supported thrombin generation is mediated predominantly by specific phospholipids and is enhanced by oxidation of these phospholipids. The addition of vitamin E (α-tocopherol) markedly reduced the increase in thrombin generation observed after oxidation of LDL (822 ± 57 v 138 ± 47 nmol/L;P < .0001). These effects suggest that lipoproteins are important in the production of thrombin and that vitamin E may confer protection from the detrimental effects of lipoprotein oxidation by limiting thrombin formation. These results suggest that atherogenic lipoproteins are linked to the development of atherosclerosis in part by their capacity to support thrombin generation.

Receptor-activating peptides distinguish thrombin receptor (PAR-1) and protease activated receptor 2 (PAR-2) mediated hemodynamic responses in vivo

Vascular expression and cellular functions of the thrombin receptor (PAR-1) and protease activated receptor 2 (PAR-2) suggest similar but distinct vascular regulatory roles. The vascular actions of PAR-1 and PAR-2 in vivo were differentiated by monitoring mean arterial pressure (MAP) and heart rate (HR) of anesthetized mice in response to intravenous SFLLRN (0.1, 0.3, and 1 µmol/kg) and SLIGRL (0.1, 0.3, and 1 µmol/kg), the respective receptor-activating sequences for PAR-1 and PAR-2, and TFLLRNPNDK (0.3, 1, and 3 µmol/kg), a synthetic peptide selective for PAR-1. All peptides dose dependently decreased MAP (order of potency: SLIGRL >> SFLLRN >> TFLLRNPNDK). SLIGRL induced a more prolonged hypotension with a slow return to baseline, whereas SFLLRN- and TFLLRNPNDK-induced hypotension was followed by a rapid return towards baseline and a sustained moderate hypotension. SFLLRN and TFLLRNPNDK, but not SLIGRL, decreased HR. N omega -Nitro-L-arginine methyl ester HCl (L-NAME), an inhibitor of nitric oxide synthesis, attenuated the cumulative hypotensive response to SLIGRL but had no effect on the SFLLRN and TFLLRNPNDK hypotension. However, L-NAME revealed a rebound hypertension in response to SFLLRN and TFLLRNPNDK but not SLIGRL. In conclusion, activation of either PAR-1 or PAR-2 in vivo results in hypotension. In addition, only PAR-1 activation induced hypertension following L-NAME, reflecting concurrent PAR-1-mediated vasoconstriction. Thus, these different hemodynamic responses in vivo suggest distinct physiological or pathophysiological roles for PAR-1 and PAR-2 in local vascular regulation.

Key words: protease activated receptor, thrombin receptor, protease activated receptor 2 (PAR-2), arterial pressure.

Aggregating human platelets stimulate the expression of thrombin receptors in cultured vascular smooth muscle cells via the release of transforming growth factor-beta1 and platelet-derived growth factorAB

BACKGROUND

Thrombin and the thrombin receptor have been implicated in the proliferation of vascular smooth muscle cells (VSMCs) observed after angioplasty and in atherosclerosis. Because thrombin receptor activation is an irreversible proteolytic event, the marked upregulation of the smooth muscle cell thrombin receptor after vascular injury may account for the maintained mitogenic activity of thrombin. The present study was designed to determine whether aggregating platelets stimulate thrombin receptor expression in cultured VSMCs and, if so, to identify the mediators.

METHODS AND RESULTS

Thrombin receptor expression was assessed by Northern and Western blot analyses and functionally by measuring the release of 6-keto prostaglandin F1alpha. Platelet-derived products (PDPs) released by aggregating human platelets enhanced thrombin receptor mRNA levels in a time- and concentration-dependent manner, an effect that was potentiated by transient acidification of PDPs, which release bioactive transforming growth factor (TGF)-beta1, and that was slightly inhibited by ketanserin. Among several factors known to be released by aggregating platelets, only TGF-beta1, platelet-derived growth factorAB (PDGF(AB)), and serotonin mimicked the PDP effect. The level of membrane thrombin receptor protein was increased in TGF-beta1-treated VSMCs. Pretreatment of VSMCs with either acidified PDP, or TGF-beta1 increased the alpha-thrombin-stimulated release of 6-keto prostaglandin F1alpha. This effect was blunted by incubating acidified PDP with either a TGF-beta- or a PDGF-neutralizing antibody.

CONCLUSIONS

Aggregating human platelets stimulate the expression of thrombin receptors in VSMCs through the release of TGF-beta1, PDGF(AB), and, to a lesser extent, serotonin. The upregulation of the thrombin receptor by products released by aggregating platelets may sustain the mitogenic activity of thrombin in the vascular wall at sites of injury.

Overexpression of P2Y2 purinoceptor in intimal lesions of the rat aorta

Extracellular nucleotides, particularly ATP, are involved in the modulation of arterial vasomotricity via P2 purinoceptors present on smooth muscle and endothelial cells. These nucleotides could also be implicated in the smooth muscle cell hyperplasia observed in intimal lesions. In this study, we tried to define the potential role of the P2Y2 (P2u) purinoceptor by studying its expression in normal and balloon-injured rat aortas. The cloning of a rat P2Y2 cDNA from a rat smooth muscle cell cDNA library made it possible to study P2Y2 expression both by Northern blot and in situ hybridization. Northern blot experiments indicated that P2Y2 mRNA was present in rat medial aortic smooth muscle and in cultured rat aortic smooth muscle cells. In situ hybridization indicated that P2Y2 mRNA was present in endothelial cells of the intima and in some smooth muscle cells scattered throughout the media of adult rat aortas, while almost all medial smooth muscle cells of rat embryo aorta expressed this receptor. In contrast with adult aortic media, the majority of neointimal smooth muscle cells found in aortic intimal lesions either 8 or 20 days after balloon injury were positive for P2Y2 mRNA. Moreover, a subpopulation of neointimal cells localized at the luminal surface could be identified by a higher P2Y2 expression than the underlying neointimal smooth muscle cells. These data showing a strong expression of the P2Y2 purinoceptor in the neointima of injured arteries suggest that extracellular nucleotides may be involved, via this receptor, in the intimal hyperplasia and/or chronic constriction observed at the lesion site, and consequently in the restenotic process.

Effects of locally administered argatroban on restenosis after balloon angioplasty: experimental and clinical study

1. This study was undertaken to evaluate the preventive effects of locally administered argatroban, a competitive inhibitor of thrombin-induced platelet activation, on restenosis after balloon angioplasty. 2. A hydrogel-coated balloon catheter was immersed three times in argatroban/saline solution (1 mg/mL) for 60 s, inflated to a pressure of 606 kPa and left in the rabbit common carotid artery for 1 min. The same procedure was performed, without drug, as a control. The pharmacokinetics of delivered argatroban in the arterial wall were assessed using [14C]-argatroban. Platelet deposition 2 h after balloon injury was quantified by fluorescence studies using antiplatelet antibody. Vascular smooth muscle cell (VSMC) proliferation 3 days after balloon injury was assessed by immunohistochemical staining for proliferative cell nuclear antigen (PCNA). In a clinical study, we divided 50 elective patients into two groups: argatroban and control. 3. In the experimental study, the mean quantities of argatroban at 0, 2 and 6 h after deflation were 24.63, 0.49 and 0.11 nmol/g wet weight of artery, respectively. Argatroban was undetected 24 h after deflation. Two hours after deflation, argatroban-treated arteries showed less platelet adhesion than saline-treated controls. The mean number of PCNA-positive cells was 16.9 and 43.8% in the argatroban and control groups, respectively (P < 0.01). In the clinical study, the mean late gain loss was 8.2 and 27.3% in the argatroban and control groups, respectively (P < 0.05). The mean late restenosis rate was 11.1 and 41.4% in the argatroban and control groups, respectively (P < 0.05). 4. These data suggest that blood coagulation plays a significant role in VSMC proliferation after balloon injury and that locally administered argatroban using hydrogel-coated balloon catheter may prevent post-percutaneous transluminal coronary angioplasty restenosis.

Aging decreases the production of PGI2 in rat aortic endothelial cells

It has been suggested that progressive pathophysiologic modifications of endothelium are associated with aging. Aging has been shown to influence some specific functions at the cellular level. In the present study, the effects of aging on levels of prostacyclin (PGI2) production were examined in cultured rat aortic endothelial cells from young (six-week-old) and old (100-week-old) Wistar rats. The level of PGI2 production from rat aortic endothelial cells decreased significantly with increasing age, suggesting decreased function of the endothelial cells. The production of PGI2 stimulated by thrombin was decreased in old rat aortic endothelial cells compared to young rat aortic endothelial cells, whereas there was no difference in the rate of intracellular calcium mobilization caused by thrombin. These data indicate that aging nonuniformly affects both basal and agonist-induced levels of PGI2 production in rat aortic endothelial cells, and that this diminution in PGI2 production may be related to the age-related potentiation of various thrombotic events.

Role of hemostatic risk factors for restenosis in peripheral arterial occlusive disease after transluminal angioplasty

In a prospective study, the role of various hemostatic factors known to be associated with thrombotic risk was investigated in 71 patients with peripheral arterial occlusive disease (PAOD, stages II through IV, Fontaine; aged 68 +/- 13 years). Laboratory investigations were done before; 1, 24, and 48 hours after; and 3 and 6 months after percutaneous transluminal angioplasty (PTA). Thirty of 71 (42.3%) patients developed restenosis (> 50% reduction of the lumen diameter) at the site of PTA within 6 months, verified by color-coded duplex sonography. Significantly increased levels of thrombin-antithrombin III complexes (P < .01), prothrombin fragments 1 + 2 (P < .01), and D-dimers (P < .01) were found 1 hour, as well as 24 to 48 hours, after PTA. Fibrinogen (P < .01) and von Willebrand factor (P < .01) were significantly higher 48 hours after PTA. Restenotic patients as a whole had higher plasma fibrinogen (3.46 +/- 1.12 versus 2.95 +/- 0.62 g/L, P < .01) and C-reactive protein (25.4 +/- 46.7 versus 7.9 +/- 6.9 mg/L, P < .05) at baseline, as well as higher fibrinogen (P < .05) and prothrombin fragments 1 + 2 (P < .01) during months 3 to 6 after PTA. There was a nonsignificant tendency for higher values of von Willebrand factor (206 +/- 98% versus 184 +/- 100%, P = .2) at baseline in patients with restenosis, whereas tissue plasminogen activator, plasminogen activator inhibitor, coagulation screening tests, blood cell counts, and serum lipids showed no significant difference between the two groups. The relative risk for developing restenosis within 6 months while having high fibrinogen (> 2.8 g/L) or C-reactive protein at baseline was 2.80 (95% CI: 1.30-6.02, P < .01) and 1.96 (95% CI: 1.07-3.58, P < .05), respectively. Patients with critical limb ischemia (stage III/IV, Fontaine) had significantly higher fibrinogen and von Willebrand factor at repeated points of time, as well as significantly higher C-reactive protein and lower creatinine clearance at entry. In the logistic regression risk factor analysis, baseline plasma fibrinogen, C-reactive protein concentration, and the severity of the arterial disease were significantly predictive of restenosis. Our results indicate that high procoagulant factors and persistent thrombin generation of the hemostatic system might promote restenosis, particularly in patients with extended atherosclerosis. This finding suggests that new treatment strategies should be taken under consideration for patients with PAOD and PTA.

Expression of thrombin receptors in human atherosclerotic coronary arteries leads to an exaggerated vasoconstrictory response in vitro

Alpha-thrombin can alter vascular tone by proteolytic cleavage of its cell-surface receptor, which exposes a tethered peptide sequence, Ser-Phe-Leu-Leu-Arg-Asn (SFLLRN) that activates the receptor. We investigated the effects of increasing severity of coronary atherosclerosis on SFLLRN-induced responses on 165 human coronary artery rings isolated fresh from 15 patients who underwent cardiac transplantation. In 40 coronary rings with minimal intimal proliferation, addition of 0.001-5 microM SFLLRN resulted in a dose- and endothelium-dependent relaxation reaching a maximum of -87.0 +/- 2.3% (mean +/- SEM) and median inhibitory concentration (IC50) of 0.1 microM. Increasing severity of atherosclerotic lesion, as determined by morphometric quantification of intimal thickening under light microscopy, resulted in graded decreases in both sensitivity and magnitude of the observed relaxation. The maximal relaxations in coronary arteries with mild and moderate intimal proliferation were -76.7 +/- 3.5% (mean +/- SEM of 41 rings) and -63.6 +/- 6.4% (mean +/- SEM of 22 rings), respectively. In the 21 coronary rings with severe intimal proliferation, no significant SFLLRN-induced relaxation was noted. Mechanical disruption of intimal endothelium abolished the SFLLRN-induced relaxation observed in the minimal to mild intimally thickened arteries, whereas in arteries with moderate and severe intimal thickening, a significant SFLLRN-induced contraction (19 +/- 10% and 43 +/- 7%, respectively) was observed. Similar endothelium-dependent relaxations in minimal atherosclerotic and endothelium-independent contraction in severe atherosclerotic coronary arteries were also observed with alpha-thrombin. These findings confirm a recent in situ hybridization and immunohistochemistry study reporting localization of cloned thrombin receptors only in endothelium of "normal appearing" human abdominal aortae and induced expression of thrombin receptors in intimal/medial regions of the atherosclerotic vessels and further demonstrate that similar expression of thrombin receptors in human atherosclerotic coronary arteries leads to an unmasking of a marked vasoconstrictory response.

Ventilation and oxygenation induce endothelial nitric oxide synthase gene expression in the lungs of fetal lambs

At birth, ventilation and oxygenation immediately decrease pulmonary vascular resistance (PVR) and increase pulmonary blood flow (PBF); more gradual changes occur over the next several hours. Nitric oxide, produced by endothelial nitric oxide synthase (eNOS), mediates these gradual changes. To determine how ventilation and oxygenation affect eNOS gene expression, 12 fetal lambs were ventilated for 8 h without changing fetal descending aortic blood gases or pH (rhythmic distension) or with 100% oxygen (O2 ventilation). Vascular pressures and PBF were measured. Total RNA, protein, and tissue sections were prepared from lung tissue for RNase protection assays, Western blotting, and in situ hybridization. O2 ventilation increased PBF and decreased PVR more than rhythmic distension (P < 0.05). Rhythmic distension increased eNOS mRNA expression; O2 ventilation increased eNOS mRNA expression more and increased eNOS protein expression (P < 0.05). To define the mechanisms responsible for these changes, ovine fetal pulmonary arterial endothelial cells were exposed to 1, 21, or 95% O2 or to shear stress. 95% O2 increased eNOS mRNA and protein expression (P < 0.05). Shear stress increased eNOS mRNA and protein expression (P < 0.05). Increased oxygenation but more importantly increased PBF with increased shear stress induce eNOS gene expression and contribute to pulmonary vasodilation after birth.

The Suicide Substrate Reaction Between Plasminogen Activator Inhibitor 1 and Thrombin Is Regulated by the Cofactors Vitronectin and Heparin

The interaction of thrombin with plasminogen activator inhibitor 1 (PAI-1) is shown to result in the simultaneous formation of both cleaved PAI-1 and a sodium dodecyl sulfate-stable thrombin-PAI-1 complex. The kinetics of this reaction can be described by a “suicide substrate” mechanism that includes a branched reaction pathway, which terminates in either the stable inhibitor-enzyme complex or the cleaved inhibitor plus free enzyme. Because of the branched pathway, approximately three moles of PAI-1 are needed to completely inhibit one mole of thrombin. Heparin and vitronectin enhance the rate of inhibition from 9.8 × 102 L mol−1 s−1 to 6.2 × 104 L mol−1 s−1 and 2.1 × 105 L mol−1 s−1, respectively, under optimal conditions. In addition to enhancing the rate of inhibition, both cofactors increase the apparent stoichiometry of the PAI-1–thrombin interaction, with cofactor concentration dependencies similar to the inhibition reaction. Thus, at 37°C approximately six cleavage reactions occur per inhibition reaction. Therefore, thrombin will efficiently inactivate PAI-1 in the presence of either vitronectin or heparin, unless a sufficient excess of the inhibitor is present. These results show that physiological cofactors are able to switch a protease-serpin inhibition reaction to a substrate reaction, depending on the local concentrations of each of the components.

The regulation of p42/p44 mitogen-activated protein kinases in the injured rat carotid artery

Arterial smooth muscle cell (SMC) proliferation is an important factor in the development of atherosclerotic plaques and restenotic lesions following arterial reconstruction. Basic fibroblast growth factor (bFGF), platelet-derived growth factor (PDGF), and thrombin are known to induce SMC proliferation and migration in vitro and in vivo. In cultured cells the proliferative responses to these mitogens depend on the activation of the p42/p44 mitogen-activated protein kinases (MAPKs), whereas the role of these kinases in vivo has yet to be established. We tested whether MAPK activity is induced following vessel injury and whether activity is dependent on the release of bFGF, PDGF, and thrombin. Following balloon injury of the left carotid of male Sprague-Dawley rats, arteries were removed and analyzed with respect to MAPK activity, BrdU-labeled nuclei, and/or luminal, medial, and intimal areas. MAPK activity is induced in the rat carotid artery following balloon-catheter injury with a maximum activation at 30 min with a return to just above baseline at 11 hr after injury. Intravenous administration of heparin or neutralizing antibodies to bFGF or PDGF prior to injury reduced SMC proliferation and neointimal lesional formation but did not affect the early induction of MAPK activity. Administration of a tissue factor inhibitor or thrombin inhibitor also did not affect MAPK activity, although it impaired the initiation of the coagulation cascade.

IN CONCLUSION

(1) MAPK is activated in a time-dependent manner in response to injury; (2) the antiproliferative effect of heparin in vivo is not mediated through the inhibition of MAPK activity induced 30 min after injury; (3) the activation of MAPK after 30 min is not dependent on PDGF, bFGF, or thrombin following vessel injury in the rat.

Thrombin Generation by Apoptotic Vascular Smooth Muscle Cells

Thrombin activation requires assembly of a prothrombinase complex of activated coagulation factors on an anionic phospholipid surface, classically provided by activated platelets. We have previously shown that anionic phosphatidylserine is exposed by rat vascular smooth muscle cells (VSMCs) undergoing apoptosis after serum withdrawal. In this study, using a chromogenic assay, we have shown thrombin generation by apoptotic VSMCs expressing c-myc (VSMC-myc) with an area under the thrombin-generation curve (AUC) of 305 ± 17 nmol⋅min/L and a peak thrombin (PT) of 154 ± 9 nmol/L. The thrombin-generating potential of the apoptotic VSMC-myc cells was greater than that of unactivated platelets (P = .003 for AUC; P = .0002 for PT) and similar to calcium-ionophore activated platelets (AUC of 332 ± 15 nmol⋅min/L, P = .3; PT of 172 ± 8 nmol/L, P = .2). Thrombin activation was also seen with apoptotic human VSMCs (AUC of 211 ± 8 nmol⋅min/L; PT of 103 ± 4 nmol/L) and was inhibited by annexin V (P < .0001 for AUC and PT). VSMC-myc cells maintained in serum generated less thrombin than after serum withdrawal (P = .0002 for AUC and PT). VSMCs derived from human coronary atherosclerotic plaques that apoptose even in serum also generated thrombin (AUC of 260 ± 2 nmol⋅min/L; PT of 128 ± 4 nmol/L). We conclude that apoptotic VSMCs possess a significant thrombin-generating capacity secondary to phosphatidylserine exposure. Apoptotic cells within atherosclerotic plaques may allow local thrombin activation, thereby contributing to disease progression.

Vascular thrombin receptor regulation in hypertensive rats

Thrombin has been implicated as an important mediator of vascular lesion formation in atherosclerosis and restenosis. To investigate a potential role for thrombin signaling in the vascular response to hypertension, we have studied thrombin receptor (TR) expression and regulation in hypertensive rats. Aortic TR mRNA was upregulated by angiotensin II (Ang II)-induced hypertension (10.7 +/- 2.5 times control, P < .02), which correlated with a 4-fold increase in thrombin-induced constriction in isolated endothelium-denuded aortic rings. The AT1 receptor antagonist losartan normalized blood pressure and TR mRNA. Conversely, lowering blood pressure to the same degree with hydralazine did not abolish the upregulation of TR mRNA expression. When low-renin low-Ang II hypertension was induced in Dahl salt-sensitive rats, there was no detectable increase in the expression of aortic thrombin receptor mRNA. Finally, treatment with a chimeric heparin-binding form of the recombinant human Cu/Zn superoxide dismutase caused complete inhibition of TR mRNA upregulation, suggesting that an increased rate of superoxide anion production is an important signaling mechanism. Thus, increased TR expression via a redox-sensitive mechanism in the aortic smooth muscle of rats treated with Ang II represents a novel in vivo mechanism through which the hypertensive effects of Ang II are mediated.

Failure of heparin to inhibit the expression of the thrombin receptor following endothelial injury of the rabbit carotid artery

The effect of heparin on thrombin receptor expression was evaluated in an experimental model of myointimal smooth muscle cell proliferation in rabbits. Myointimal hyperplasia was induced by an air-drying injury of the carotid artery and thrombin receptor expression following endothelial injury was measured by in situ hybridisation and immunohistochemistry. In healthy arteries, thrombin receptor mRNA and protein were detected in the endothelial cells only. In contrast, 14 days after endothelial injury, thrombin receptor mRNA expression increased in the smooth muscle cells present in the neointima, predominantly in areas of active cell proliferation. A 2-week subcutaneous treatment with heparin (10 mg/kg per day, s.c.) inhibited smooth muscle cell hyperplasia occurring in the intima following deendothelialization (80 +/- 7.8% inhibition, P < 0.001). The 14-day heparin treatment strongly reduced thrombin receptor gene and protein expression observed in the endothelial cells in healthy arteries but did not affect thrombin receptor expression which occurred in smooth muscle cells which have proliferated in the neointima as a consequence of endothelial injury. These results therefore establish that thrombin receptor expression during intimal hyperplasia is an heparin-insensitive event.

Thrombin activates NF-kappa B through thrombin receptor and results in proliferation of vascular smooth muscle cells: role of thrombin in atherosclerosis and restenosis

We investigated the role of thrombin in the pathogenesis in atherosclerosis and restenosis. First we examined the effect of thrombin on cultured human vascular smooth muscle cells (VSMC). We showed that thrombin acts as a mitogen on VSMC through thrombin receptor. The expression of thrombin receptor was increased in the cell lines of VSMC established from directional coronary atherectomy (DCA). This is more pronounced in the cells from patients with restenosis after PTCA. Next we investigated the signaling pathway from thrombin/thrombin receptor. Thrombin activates thrombin receptor resulting in the exposing of the agonist peptide domain (thrombin receptor agonist peptide, TRAP). The signal from thrombin/thrombin receptor activated protein C kinase, tyrosine kinase, and MAP kinase and resulted in NF-kappa B activation. Furthermore, treatment of the cells with antisense p65 oligodeoxynucleotides of NF-kappa B inhibited the thrombin-stimulated growth of VSMC in vitro. These results suggest that thrombin may have a role in the pathogenesis of atherosclerosis and restenosis after PTCA through the thrombin receptor.

Differential expression of nonmuscle myosin II isoforms in human atherosclerotic plaque

Intimal proliferation and functional changes involving vascular smooth muscle cells are key events in the development of atherosclerosis, including restenosis after percutaneous transluminal angioplasty. Nonmuscle myosin (NMM) is required for cytokinesis and has been shown in cultures of vascular smooth muscle cells to undergo changes of isoform expression depending on the stage of proliferation and differentiation. The purpose of this study was to examine the differential expression of the two most recently identified nonmuscle myosin heavy chain isoform II (NMMHC-II) isoforms A and B in atherosclerotic plaque. Primary atherosclerotic and restenotic atherectomy specimens and non-atherosclerotic controls, were analyzed by Western Blot analysis, immunohistochemistry and in situ hybridization. Nonmuscle myosin heavy chain isoform IIA (NMMHC-IIA) was equally expressed in all types of tissue specimens both at the protein and mRNA levels. In contrast, NMMHC-IIB protein was found in restenotic specimens and normal artery but was at very low levels in primary atherosclerotic plaque. By in situ hybridization NMMHC-IIB mRNA levels were significantly greater in restenotic versus primary atherosclerotic lesions. NMMHC-IIB expression is associated with vascular restenosis but is downregulated in stable atherosclerotic lesions, whereas NMMHC-IIA is expressed in both. These results indicate that these new myosin isoforms have different functions and should be regarded separately with respect to smooth muscle proliferation and restenosis. They should prove to be useful molecular markers for the study of atherosclerosis and restenosis.

Specific Inhibition of Thrombin-Induced Cell Activation by the Neutrophil Proteinases Elastase, Cathepsin G, and Proteinase 3: Evidence for Distinct Cleavage Sites Within the Aminoterminal Domain of the Thrombin Receptor

The aim of this study was to investigate the inhibitory effects of human leukocyte elastase (HLE), cathepsin G (Cat G), and proteinase 3 (PR3) on the activation of endothelial cells (ECs) and platelets by thrombin and to elucidate the underlying mechanisms. Although preincubation of ECs with HLE or Cat G prevented cytosolic calcium mobilization and prostacyclin synthesis induced by thrombin, these cell responses were not affected when triggered by TRAP42-55, a synthetic peptide corresponding to the sequence of the tethered ligand (Ser42-Phe55) unmasked by thrombin on cleavage of its receptor. Using IIaR-A, a monoclonal antibody directed against the sequence encompassing this cleavage site, flow cytometry analysis showed that the surface expression of this epitope was abolished after incubation of ECs with HLE or Cat G. Further experiments conducted with platelets indicated that not only HLE and Cat G but also PR3 inhibited cell activation induced by thrombin, although they were again ineffective when TRAP42-55 was the agonist. Similar to that for ECs, the epitope for IIaR-A disappeared on treatment of platelets with either proteinase. These results suggested that the neutrophil enzymes proteolyzed the thrombin receptor dowstream of the thrombin cleavage site (Arg41-Ser42) but left intact the TRAP42-55 binding site (Gln83-Ser93) within the extracellular aminoterminal domain. The capacity of these proteinases to cleave five overlapping synthetic peptides mapping the portion of the receptor from Asn35 to Pro85 was then investigated. Mass spectrometry studies showed several distinct cleavage sites, ie, two for HLE (Val72-Ser73 and Ile74-Asn75), three for Cat G (Arg41-Ser42, Phe55-Trp56 and Tyr69-Arg70), and one for PR3 (Val72-Ser73). We conclude that this singular susceptibility of the thrombin receptor to proteolysis accounts for the ability of neutrophil proteinases to inhibit cell responses to thrombin.

Effects of locally administration of argatroban using a hydrogel-coated balloon catheter on intimal thickening induced by balloon injury

The purpose of this study was to evaluate the inhibitory effects of locally delivered argatroban, a competitive inhibitor of thrombin-induced platelet activation, on intimal proliferation after balloon injury. A hydrogel-coated balloon catheter was immersed 3 times in an argatroban/saline solution (1, 0.1, or 0.01 mg/ml) for 60 s and inflated at 6 atm pressure for 1 min in the rabbit common carotid artery. Immersion in a saline solution without drug followed by the same procedure served as a control. Accumulation of argatroban in the vascular wall was confirmed by chemical determination using high-performance liquid chromatography (HPLC). The concentration of argatroban in the vessel wall immediately after deflation after balloon immersion in solutions of 1 and 0.1 mg/ml was 14.8 +/- 10.9 and 5.5 +/- 4.6 nmol/g wet weight of artery, respectively. Argatroban was not detected in arteries treated with a balloon that had been immersed in the 0.01 mg/ml argatroban/saline solution. Intima-media area ratios 20 days after balloon injury in the groups treated with 1 mg/ml (n = 8) and 0.1 mg/ml (n = 6) agratoban were significantly smaller than that in the groups treated with 0.01 mg/ml (n = 7) argatroban or saline (n = 8) (0.35 +/- 0.11, 0.50 +/- 0.17, 1.24 +/- 0.39, and 1.35 +/- 0.43, respectively; p < 0.001). These data suggest that locally administered argatroban dose-dependently inhibits intimal thickening in a rabbit model of carotid artery injury.

Binding of a thrombin receptor tethered ligand analogue to human platelet thrombin receptor

A thrombin receptor-radioligand binding assay was developed using [3H]A(pF-F)R(ChA)(hR)Y-NH2 ([3H]haTRAP), a high affinity thrombin receptor-activating peptide (TRAP), and human platelet membranes. Scatchard analysis of saturation binding data indicated that [3H]haTRAP bound to platelet membranes with a Kd of 15 nM and a Bmax of 5.2 pmol/mg of protein. The binding was reduced by GPPNHP, a nonmetabolizable GTP analogue. Various TRAPs and a TRAP antagonist, but not other receptor agonists, displaced [3H]haTRAP from the binding sites. SFLLRN-NH2, a thrombin receptor-tethered ligand analogue, and [3H]haTRAP exhibited competitive binding for the same binding sites. The relative affinity of these peptides for the binding site paralleled their EC50 or IC50 values for platelet aggregation. These data indicate that [3H]haTRAP binds specifically and saturably to the functioning G protein-linked thrombin (tethered ligand) receptor in human platelet membranes.

Intimal hyperplasia following vascular injury is not inhibited by an antisense thrombin receptor oligodeoxynucleotide

Thrombin is a multifunctional serine protease with central functions in hemostasis, but demonstration of its role in the initiation and maintenance of cell proliferation which occurs following vascular injury is still lacking. To determine the role played by thrombin and its receptor in neointimal accumulation of smooth muscle cells in a rabbit carotid artery model, we have used an 18 mer antisense phosphorothioate oligonucleotide (ODN) directed against the translation initiation region of the human thrombin receptor gene. The antisense ODN inhibited in a dose-dependent manner thrombin- or thrombin receptor activating peptide-induced human aortic smooth muscle cell proliferation. The growth-inhibitory effect of thrombin receptor antisense ODN was preventable by an excess of sense oligomer and specific for thrombin. The suppression of growth was accompanied by a marked decrease of the level of thrombin receptor expression as evidenced by [125I]-thrombin binding to smooth muscle cells. Under the same experimental conditions, the corresponding sense ODN was inactive. The effect of the antisense ODN on intimal smooth muscle hyperplasia in rabbit carotid arteries subjected to endothelial injury was then investigated. The topical application of the antisense (500 microg/artery) but not the sense ODN dissolved in F127 pluronic gel around the injured artery resulted, 2 weeks after the application, in a dramatic reduction of the expression of the thrombin receptor mRNA and protein levels as determined by in situ hybridization and immunohistochemistry. However, intimal smooth muscle cell accumulation as estimated by an intimal to medial cross-sectional area ratio was reduced only by 2.7% (vs. 10.3% for the sense ODN), whereas r-hirudin (200 microg/kg/day, s.c.), a potent direct thrombin inhibitor significantly reduced the formation of neointima in denuded carotid arteries (35.4% inhibition, P = 0.03).

Thrombin inhibits atrial natriuretic peptide receptor activity in cultured bovine endothelial cells

Thrombin and the atrial natriuretic peptide (ANP) possess a number of functionally antagonistic properties in vascular endothelial cells. Thus, regulatory interactions that modulate the activity of one or the other could have important sequelae with regard to cardiovascular homeostasis. Thrombin treatment effected a dose- and time-dependent reduction in ANP receptor activity (maximal 70% to 80% inhibition) in cultured bovine aortic endothelial cells. This resulted from a decrease in total receptor number as well as a modest reduction in the affinity of the receptor for its ligand. The inhibition was largely confined to the type C receptor population, in that thrombin had no effect on maximal type A receptor-linked cGMP accumulation. The protein kinase C-activating phorbol ester 12-O-tetradecanoylphorbol 13-acetate effected a similar reduction in binding activity; however, suppression of protein kinase C activity did not reverse the thrombin effect. Pretreatment of endothelial cells with cycloheximide did not completely prevent the thrombin-dependent inhibition, and thrombin did not effect a reduction in type C receptor mRNA levels, findings that argue for a postsynthetic inhibitory locus. The inhibition of receptor activity was effectively irreversible in that suspension of protein synthesis blocked the recovery of receptor density on the cell surface. Reduction in type C receptor density was accompanied by modest increases in the stability of ANP in the culture medium and enhancement of the cellular cGMP response to the peptide, particularly at low ligand concentrations. These findings demonstrate a potentially important interaction between these two agonist systems in regulating endothelial cell function within the vascular wall.

Thrombin receptor expression in rheumatoid and osteoarthritic synovial tissue

OBJECTIVE

To investigate the possibility that synovial cells might respond to thrombin in the inflamed human joint, using immunohistochemical detection of thrombin receptors.

METHODS

Frozen sections of synovial membrane from 20 patients with rheumatoid arthritis, 16 with osteoarthritis, and four normal controls were stained using a monoclonal antibody to the human thrombin receptor. Sections were also double stained for both receptors and non-specific esterase.

RESULTS

Receptor positive cells were present in rheumatoid synovia, with some cells also staining positively for non-specific esterase. In contrast, both osteoarthritic and normal synovia contained very few cells expressing receptors.

CONCLUSIONS

Thrombin may mediate important pathological changes during chronic inflammatory joint disease.

Cloning and identification of regulatory sequences of the human thrombin receptor gene

Thrombin, via activation of vascular endothelial and smooth muscle cell thrombin receptors, modulates vascular wall healing. To understand the mechanisms that regulate human thrombin receptor (HTR) expression, we cloned and characterized the HTR gene. The HTR gene consists of Exon I, which contains the 5'-regulatory region and 85 nucleotides of coding sequence; a approximately 15-kb intron; and Exon II, which contains the remainder of the coding sequence and the entire 3'-untranslated region. Multiple transcription initiation sites were identified by S1 mapping and ribonuclease protection assay. DNA sequence analysis indicated the presence of two SP-1-AP-2 consensus binding sequences, near or within the transcription initiation sites, and consensus binding sequences for numerous regulatory proteins that potentially modulate HTR expression. Functional analysis of the HTR promoter was performed by transfecting human microvascular endothelial cells with HTR promoter region-luciferase constructs. The highest level of expression was obtained with a 0.7-kb promoter sequence and was progressively less with fragments of 0.54, 1.16, 1.6, and approximately3.2 kb. The data presented in this report provide a foundation for further characterization of the HTR gene and the mechanisms that regulate its expression within the blood vessel wall.

Comparison of the effects of the thrombin inhibitor r-hirudin in four animal models of neointima formation after arterial injury

Thrombin has been implicated as a contributing factor to restenosis after vessel reopening procedures. We compared the ability of the direct thrombin inhibitor recombinant (r-) hirudin to reduce neointimal growth in different animal models of arterial injury. Carotid arteries of rats, rabbits, and hypercholesterolemic minipigs were injured by withdrawal of an inflated balloon catheter. In addition, we used a double-lesion model in rabbits, which involved balloon angioplasty of a preexisting lesion induced by carotid denudation 4 weeks earlier. r-Hirudin was given in all four animal models as a short-term application (bolus of 1 mg/kg i.v. immediately before injury, followed by infusion of 1 mg.kg-1.h-1 for 2 hours, and an injection of 6 mg/kg SC). Additionally, we investigated the effects of prolonged treatment (intravenous infusion for 3 and 14 days) in rats. Inhibition of thrombin was monitored by determination of activated partial thromboplastin time, and histomorphometric analysis of the arteries was performed after 2 (rats) or 4 (rabbits and minipigs) weeks. In rabbits, short-term r-hirudin treatment reduced neointimal area by 59% (single-injury model, P = .05) and 44% (double-injury model, P = .02). In rats and minipigs no inhibition of neointimal growth was observed after short-term r-hirudin application. A 3- or 14-day infusion of r-hirudin in rats, however, resulted in 25% (P = .007) and 27% (P = .003) reductions in neointimal area, respectively. In conclusion, there is considerable interspecies variation in the time frame of susceptibility for reduction of neointimal growth by inhibition of thrombin after arterial injury. These results demonstrate the importance of testing potential antirestenotic treatments in an array of different animal models.

Arterial smooth muscle cell heparan sulfate proteoglycans accelerate thrombin inhibition by heparin cofactor II

Heparin cofactor II (HCII) is a potent thrombin inhibitor in the presence of heparin and dermatan sulfate, glycosaminoglycans that accelerate the inhibition reaction. HCII is postulated to be an extravascular thrombin inhibitor that is stimulated physiologically by dermatan sulfate proteoglycans. To understand how thrombin activity may be downregulated within the artery wall, cultured monkey aorta smooth muscle cell (SMC) proteoglycans were tested for their ability to accelerate thrombin inhibition by HCII. Early confluent SMC monolayers increased thrombin-HCII inhibition rates 2-fold to 4-fold compared with reactions in cell-free control wells (7.3 +/- 0.5 versus 2.7 +/- 0.2 x 10(4) mol.L-1.min-1, with and without SMCs, respectively; n = 7 experiments). Extracellular matrix obtained by cell monolayer removal also accelerated the thrombin-HCII inhibition reaction 3-fold to 5-fold. Rate increases were abolished by Polybrene or protamine sulfate. Pretreatment of monolayers with heparitinase I (and of extracellular matrix with HNO2) to degrade heparan sulfate blocked the thrombin-HCII inhibition rate increase. In contrast, pretreatment with chondroitinase ABC in the presence of proteinase inhibitors had no effect. "Pericellular" (cell surface- and extracellular matrix-derived) SMC heparan sulfate proteoglycans (HSPGs) were purified and fractionated by charge on DEAE-Sephacel. At a concentration of 1 microgram/mL hexuronic acid, high-charge HSPG stimulated a 7-fold thrombin-HCII inhibition rate increase relative to reactions without proteoglycan, whereas low-charge HSPG induced a 2-fold rate increase. In comparison, an 18-fold rate increase was observed with 1 microgram/mL dermatan sulfate proteoglycan purified from SMC culture media. These results indicate that SMC HSPG could contribute significantly to thrombin inhibition by HCII in the artery wall.

Haemostatic factors and atherogenesis

It is increasingly realised that fibrin deposition and fibrin lysis are major factors in vascular pathology. In addition to thrombotic occlusion fibrin is a component of atherosclerotic lesions, but the increased interest in components of the haemostatic system was mainly triggered by clinical use of fribrinolytic agents, and the problems of re-stenosis following angioplasty. This review focuses on the main components of the fibrinolytic system--tissue plasminogen activator (tPA), urokinase (uPA) and plasminogen activator inhibitor (PAI-1)--and on thrombin. These factors are not only involved in fluid phase clotting and clot lysis; they react specifically with cells and matrix components. During the last 5 years, the main period under review, there have been numerous studies on their interactions with endothelial and smooth muscle cells in culture, in whole tissues and in vivo, and with arterial extracellular matrix of which a major component is fibrin. Plasminogen activators bind to cell surface receptors, influence cell migration and release active thrombin from fibrin. Thrombin emerges as a pluripotent factor which modulates many aspects of endothelial and smooth muscle cell behaviour, including release and synthesis of fibrinolytic components, and stimulation of cell proliferation.

Thrombin potently stimulates cytokine production in human vascular smooth muscle cells but not in mononuclear phagocytes

Thrombosis frequently occurs during atherogenesis and in response to vascular injury. Accumulating evidence supports a role for inflammation in the same situation. The present study therefore sought links between thrombosis and inflammation by determining whether thrombin, which is present in active form at sites of thrombosis, can elicit inflammatory functions of human monocytes and vascular smooth muscle cells (SMCs), two major constituents of advanced atheroma. Human alpha-thrombin (EC50, approximately equal to 500 pmol/L) potently induced interleukin (IL)-6 release from SMCs. The tethered-ligand thrombin receptor appeared to mediate this effect. Furthermore, alpha-thrombin also rapidly increased levels of mRNA encoding IL-6 and monocyte chemotactic protein-1 (MCP-1) in SMCs. In contrast, only alpha-thrombin concentrations of > or = 100 nmol/L could stimulate release of IL-6 or tumor necrosis factor-alpha (TNF alpha) in peripheral blood monocytes or monocyte-derived macrophages. Lipid loading of macrophages did not augment thrombin responsiveness. Likewise, only alpha-thrombin concentrations of > or = 100 nmol/L increased levels of IL-6, IL-1 beta, MCP-1, or TNF alpha mRNA in monocytes. Differential responses of SMCs and monocytes to thrombin extended to early agonist-mediated increases in [Ca2+]i. SMCs and endothelial cells, but not monocytes, contained abundant mRNA encoding the thrombin receptor and displayed cell surface thrombin receptor expression detected with a novel monoclonal antibody. Thus, the level of thrombin receptors appeared to account for the differential thrombin susceptibility of SMCs and monocytes. These data suggest that SMCs may be more sensitive than monocytes/macrophages to thrombin activation in human atheroma. Cytokines produced by thrombin-activated SMCs may contribute to ongoing inflammation in atheroma complicated by thrombosis or subjected to angioplasty.