Thrombin receptor expression in normal and atherosclerotic human arteries

Activation of coagulation and fibrinolysis despite heparinization during successful elective coronary angioplasty

Our aim was to assess whether the vessel wall trauma induced by balloon inflation during successful elective PTCA results in activation of coagulation and fibrinolysis detectable in circulating blood. In the pilot group (10 patients), when blood was collected under heparinization with adequate anti-Factor Xa activity, catheter-induced thrombin generation was not detected and results obtained from local coronary arterial versus systemic samples did not differ. Locally, von Willebrand factor antigen increased from 73.5 +/- 8.8% to 77.8 +/- 13.1% (p < 0.05) at 5 min after PTCA. In the study group with its 21 patients having adequate heparinization fibrinogen decreased when blood was collected from aorta 15 min after PTCA. In 30% of the patients having the largest calculated area of vessel damage, thrombin-antithrombin III (TAT) complex and prothrombin fragments (F1+2) spiked by at least 25% during PTCA. In all patients the mean TAT values did not increase, but F1+2 (from 0.56 +/- 0.36 to 0.63 +/- 0.39 nmol/l, mean +/- SD, p < 0.05) and D-dimer (from 268 +/- 37 to 325 +/- 45 ng/ml, p < 0.05) rose between 15 to 30 min after PTCA. In conclusion, in every third patient thrombin generation occurs after successful elective PTCA, implying a need for a tighter control than heparin provides.

Thrombin receptor activation elicits rapid protein tyrosine phosphorylation and stimulation of the raf-1/MAP kinase pathway preceding delayed mitogenesis in cultured rat aortic smooth muscle cells: evidence for an obligate autocrine mechanism promoting cell proliferation induced by G-protein-coupled receptor agonist

Treatment of quiescent rat aortic smooth muscle cells with either alpha-thrombin or a thrombin receptor-derived agonist peptide (SFLLRNP) resulted in pronounced increases in [3H]thymidine incorporation that were concentration dependent and reached a maximum of approximately 15-fold above serum-starved controls. However, in contrast to FBS, PDGF-BB, or basic fibroblast growth factor (bFGF), that initiated DNA synthesis promptly after 16-19 h, thymidine incorporation in response to thrombin was delayed by an additional 3-6 h. Delayed mitogenesis correlated with the appearance of a potent mitogenic activity in conditioned media samples obtained from thrombin-stimulated rat aortic smooth muscle cells, as assayed using Swiss 3T3 fibroblasts. This activity was not inhibited by neutralizing antibodies directed against PDGF or bFGF. Furthermore, in the Swiss 3T3 cells, simple addition of either alpha-thrombin or SFLLRNP failed to elicit a significant mitogenic response. In signal transduction studies, both thrombin and SFLLRNP treatment led to rapid tyrosine phosphorylation of proteins with apparent molecular masses of 42, 44, 75, 120, and 190 kD, respectively, as assessed by antiphosphotyrosine immunoblotting. The overall pattern of protein tyrosine phosphorylation was distinct from that observed after PDGF-BB addition. Activation of Raf-1 and the mitogen-activated protein (MAP) kinases p44mapk and p42mapk was also observed. However, the time course and duration of Raf-1/MAP kinase activation after thrombin stimulation were similar to those elicited by PDGF-BB. Taken together, our results indicate that thrombin-stimulated vascular smooth muscle proliferation is delayed and requires the de novo expression of one or more autocrine mitogens. In addition, the rapid induction of discrete intracellular signaling mechanisms by thrombin, including the Raf-1/MAP kinase pathway, appears to be insufficient alone to promote vascular smooth muscle cell mitogenesis.

Local adenoviral-mediated expression of recombinant hirudin reduces neointima formation after arterial injury

Catalytically active thrombin, acting locally, is thought to mediate neointima formation after arterial injury. We constructed an adenovirus vector, AdHV-1.2, containing a complementary DNA for the thrombin inhibitor hirudin. AdHV-1.2 directed the synthesis and secretion of biologically active hirudin from vascular cells in vitro. In vivo gene transfer of hirudin into smooth muscle cells of injured rat carotid arteries resulted in peak secretion of at least 34+/-23 pg hirudin per vessel per 24 hours, and resulted in a significant (P<0.05) 35% reduction in neointima formation. Systemic partial thromboplastin times were not affected by local hirudin expression. These results support the hypothesis that local thrombin activity contributes to neointima formation after arterial injury and suggest that local delivery of a highly specific antithrombin may constitute an effective intervention for arterial proliferative disease.

Restricted expression of homeobox genes distinguishes fetal from adult human smooth muscle cells

Smooth muscle cell plasticity is considered a prerequisite for atherosclerosis and restenosis following angioplasty and bypass surgery. Identification of transcription factors that specify one smooth muscle cell phenotype over another therefore may be of major importance in understanding the molecular basis of these vascular disorders. Homeobox genes exemplify one class of transcription factors that could govern smooth muscle cell phenotypic diversity. Accordingly, we screened adult and fetal human smooth muscle cell cDNA libraries with a degenerate oligonucleotide corresponding to a highly conserved region of the homeodomain with the idea that homeobox genes, if present, would display a smooth muscle cell phenotype-dependent pattern of expression. No homeobox genes were detected in the adult human smooth muscle cell library; however, five nonparalogous homeobox genes were uncovered from the fetal library (HoxA5, HoxA11, HoxB1, HoxB7, and HoxC9). Northern blotting of adult and fetal tissues revealed low and restricted expression of all five homeobox genes. No significant differences in transcripts of HoxA5, HoxA11, and HoxB1 were detected between adult or fetal human smooth muscle cells in culture. HoxB7 and HoxC9, however, showed preferential mRNA expression in fetal human smooth muscle cells that appeared to correlate with the age of the donor. This phenotype-dependent expression of homeobox genes was also noted in rat pup versus adult smooth muscle cells. While similar differences in gene expression have been reported between subsets of smooth muscle cells from rat vessels of different-aged animals or clones of rat smooth muscle, our findings represent a demonstration of a transcription factor distinguishing two human smooth muscle cell phenotypes.

Prothrombin cleavage by human vascular smooth muscle cells: a potential alternative pathway to the coagulation cascade

Thrombin is a potent mitogen for human vascular smooth muscle cells (HVSMC) and its enzymatic activity is required for this function. The present study demonstrates that prothrombin is also mitogenic for HVSMC due to the generation of enzymatically active thrombin which occurs upon incubation of prothrombin with the cells. Analysis by SDS-PAGE, immunoblotting, and amino acid sequencing revealed that prothrombin incubated with HVSMC undergoes limited proteolysis. Prethrombin 1 was formed through cleavage at R155-S156. Cleavage at R271-T272 generated fragment 1.2 and prethrombin 2 whilst cleavage at R284-T285 yielded truncated prothrombin 2 (prethrombin 2'). However, cleavage at R320-I321 which, during prothrombin activation produces two-chain alpha-thrombin, was not detectable. Studies on HVSMC-conditioned medium revealed that a similar pattern of prothrombin cleavage occurred by a cell-secreted factor(s). Amidolytic activity analysis indicated that 1-3% catalytically active thrombin-like activity was generated upon incubation of prothrombin with HVSMC-conditioned medium. By treating conditioned medium with various classes of proteinase inhibitors or hirudin, it was determined that prothrombin is cleaved by a cell-derived serine proteinase-like factor(s) at R271-S272 and by alpha-thrombin at R155-S156 and R284-T285. Antibodies neutralising the activity of either urokinase, tissue plasminogen activator, or factor Xa failed to alter the prothrombin cleaving activity of conditioned medium. This activity which may catalyse an alternative pathway for the generation of thrombin, was eluted from a gel filtration column as a single peak with apparent molecular mass of 30-40 kDa.

Role of insulin and proinsulin in diabetic vascular disease

Associations between loss of glucose tolerance, insulin resistance, and ischemic heart disease (IHD) are of great current concern. Considerable controversy and uncertainty relates to the mechanism(s) that underlies these associations. Whilst there is some evidence in prospective studies of an association between hyperinsulinemia and future IHD, it is by no means strong or consistent between different studies. Hypertriglyceridemia is another possible factor involved in the linkage between glucose intolerance and IHD. There is good evidence for an affect of plasma nonesterified fatty acids (NEFA) to increase hepatic output of VLDL. Insulin, contrary to some suggestions, acts to lower plasma VLDL by actions directly on hepatic output and activation of adipose tissue lipoprotein lipase, and indirectly via the hormones affect of lowering plasma NEFA. Glycosylation and oxidation of lipoproteins may enhance their atherogenic potential. It is highly probable that procoagulant changes are also important processes predisposing to IHD. Associations between plasminogen activator inhibitor-1 and insulin, intact and 32,33 split proinsulin hypertriglyceridemia, and insulin resistance have been reported, but a unifying hypothesis explaining these links remains elusive. Epidemiological studies now repeated in a number of centers have shown links between infant mortality and birth weight and risk of IHD, and between birth weight and risk of impaired glucose tolerance and non-insulin-dependent diabetes mellitus (NIDDM). It has been proposed, therefore, that impairment of fetal and infant growth may underlie the associations between loss of glucose tolerance and risk of IHD. Animal models form the basis of much current research to test this concept.

Thrombin regulates expression of monocyte chemoattractant protein-1 in vascular smooth muscle cells

Thrombin, a serine protease generated at sites of vascular injury, plays a role in the pathogenesis of atherosclerosis and restenosis after angioplasty. Adherence of monocytes to the endothelium and migration into the subendothelial space is an important early event in the pathogenesis of atherosclerosis. Monocyte chemoattractant protein 1 (MCP-1) may be an important mediator of monocyte recruitment to the tissue in this and other diseases. We have characterized the expression of MCP-1 in vascular smooth muscle cells (VSMCs) isolated from human renal artery and studied its regulation by thrombin. Serum-deprived cells release monocyte chemotactic activity that is neutralized (80%) by an MCP-1 antibody. The antibody recognized a 13- and 15-kD protein in smooth muscle cell-conditioned medium. Thrombin stimulates MCP-1 gene expression in a concentration- and time-dependent manner. An increase over basal levels was observed with concentrations of thrombin as low as 0.05 U/mL. The maximal effect occurred at 5 U/mL. The stimulatory effect was detected within 1 hour, reached a maximum at 3 hours, and was still present at 8 to 24 hours after the addition of thrombin. A concentration- and time-dependent effect of thrombin on MCP-1 gene expression was also found in rat VSMCs. The thrombin protease inhibitor hirudin blocked thrombin-induced MCP-1 expression. Thrombin stimulated the release of MCP-1 protein in conditioned medium of human VSMCs as measured by radioimmunoassay and chemotactic assay. Thrombin also increased monocyte chemotactic activity in short-term organ cultures of rat aortic rings and in first passage cells.(ABSTRACT TRUNCATED AT 250 WORDS)

New tripeptidic thrombin inhibitors. Influence of P2 and P3 residues on activity and selectivity

Structural variations of P2 and P3 residues in tripeptidic boroarginine thrombin inhibitors led to compounds with similar potency than reference compound DuP 714, but with enhanced selectivity for thrombin compared to plasmin.

Immunoselective targeting of an anti-thrombin agent to the surface of cytokine-activated vascular endothelial cells

An immunoconjugate was designed to target hirudin, a potent and specific inhibitor of thrombin, to the surface of activated endothelial cells. Hirudin was covalently cross-linked to the monoclonal antibody H18/7 that recognizes the extracellular domain of E-selectin (CD62E), an endothelium-leukocyte adhesion molecule that is expressed only on cytokine-activated endothelium. The hirudin-H18/7 immunoconjugate selectively bound to interleukin-1-activated but not to unactivated cultured human umbilical vein endothelial cells with a temporal profile similar to that of inducible cell-surface procoagulant activity. When bound to activated endothelial cells, the hirudin-H18/7 immunoconjugate significantly inhibited endogenous thrombin activity generated from coincubated human plasma and fibrin clot formation on the monolayer surface. Cellular responses that are mediated via the thrombin receptor, such as increases in cytoskeletal F-actin content, also were significantly downregulated, and monolayers were protected from thrombin-induced disruption by this treatment. The ability to selectively antagonize thrombin-dependent processes at the endothelium-blood interface may provide new insights into complex pathophysiological processes, such as thrombosis, inflammation, and atherogenesis. These studies also demonstrate the general feasibility of selective targeting of therapeutic agents to endothelial cells based on recognition of an activation-dependent surface phenotype.

Contractile actions of thrombin receptor-derived polypeptides in human umbilical and placental vasculature: evidence for distinct receptor systems

1. We studied the structure-activity profiles of four thrombin receptor-derived polypeptides (TRPs) (P5, SFLLR; P5-NH2, SFLLR-NH2; P7, SFLLRNP; P7-NH2, SFLLRN) in contractile human placental artery (PA), umbilical artery (UA) and umbilical vein (UV) preparations and in a human platelet aggregation assay. 2. The contractile actions of the TRPs in the two arterial preparations were endothelium-independent, whereas in the UV tissue a contractile response was observed only in an endothelium-denuded preparation; no endothelium-mediated relaxation responses were observed in any of the vascular preparations. 3. In the three vascular preparations, the contractile responses required extracellular calcium and were attenuated by the tyrosine kinase inhibitor, genistein. 4. The relative contractile orders of potencies of the TRPs in the three vascular preparations were distinct from each other (PA: P7-NH2 > P7 > P5-NH2 > P5; UA: P7-NH2 > or = P5-NH2 approximately = P7 > > P5; UV: P5-NH2 > > P7-NH2 = P7 > > P5) and these were in turn distinct from the potency order observed in the platelet aggregation assay (P5-NH2 > or = P7-NH2 > P7 > > P5). 5. Despite the markedly dissimilar TRP potency orders in the placental artery and umbilical vein preparations, the cDNA sequences for the thrombin receptor obtained by polymerase chain reaction cloning of cDNA from the two tissue sources were identical. 6. We conclude that the four tissues studied possess functionally distinct thrombin receptor systems that interact in a distinct way with agonist peptides. In view of the identity of the thrombin receptor cDNA in the two tissues displaying the most dissimilar structure-activity profiles, we suggest that in different tissues, differences in post-translational receptor processing or differences in receptor-effector coupling interactions may result in unique thrombin receptor systems that can display distinct structure-activity profiles.

Porcine aortic smooth muscle cells secrete a serine protease for insulin-like growth factor binding protein-2

Porcine aortic smooth muscle cells secrete two forms of insulin-like growth factor (IGF) binding proteins (IGFBP-2 and -4), and both forms have been shown to modulate IGF-I actions in this cell type. Recently, we showed that IGFBP-4 inhibited IGF-I action and that the cells produced a protease that cleaved IGFBP-4 into non-IGF binding fragments. After the cleavage of IGFBP-4, the cellular DNA synthesis response to IGF-I was enhanced. This study reports that these cells also secrete a protease for IGFBP-2. Like the IGFBP-4 protease, this protease is also secreted constitutively, but unlike the IGFBP-4 protease, its secretion is enhanced if the cells are serum-deprived for 24 hours before the collection of conditioned medium. The protease cleaved IGFBP-2 into 25- and 16-kD fragments, which had reduced IGF-I binding activity. Protease activity was enhanced by coincubation with IGF-I or IGF-II, and IGF-II was more potent than IGF-I. The protease is a serine protease, since its activity can be inhibited by 3,4-dichloroisocoumarin and aprotinin. It is also inhibited by EDTA, and its activity can be restored with calcium but not zinc. The heparin-binding serpins, specifically, heparin cofactor II and antithrombin III, are inhibitory. Heparin alone also had activity, and the combination of antithrombin III plus heparin caused complete inhibition. The conditioned medium also contained proteolytic activities for IGFBP-4 and -5 but it did not cleave IGFBP-1 and -3.(ABSTRACT TRUNCATED AT 250 WORDS)

The functional thrombin receptor is associated with the plasmalemma and a large endosomal network in cultured human umbilical vein endothelial cells

The functional thrombin receptor, normally expressed by endothelial cells and platelets, is a member of the G protein-coupled, seven membrane-spanning-domain receptor family and is thought to be responsible for most, if not all, the cell stimulatory effects of thrombin. Upon binding, thrombin cleaves the receptor's N-terminal ectodomain, unmasking a new N terminus, which by itself activates the receptor. Using antibodies to different domains of the human thrombin receptor, we have localized the receptor in cultured human umbilical vein endothelial cells by indirect immunofluorescence and immunoelectron microscopy. We found the receptor expressed on the plasmalemma of cultured endothelial cells in individual units rather than in clusters, at lower concentration than, and at different sites from, thrombomodulin. We also found the receptor associated with a distinct, intracellular, transferrin receptor-containing, tubulovesicular network. The thrombin receptor-positive structure spread from the perinuclear region to the periphery of the cells, exhibiting a number of varicosities interconnected by branching tubular elements, strikingly similar to an image recently described for a continuous endosomal reticulum. Our results provide morphological evidence for the presence of the functional thrombin receptor at relative low density on the surface of cultured endothelial cells (compared to thrombomodulin) and in relatively large quantities inside the cells, associated with an endosomal compartment.

Role of the thrombin receptor in restenosis and atherosclerosis

Thrombus generation is central to thrombosis at vascular lesion sites, including post-PCTA acute reocclusion and chronic restenosis. Thrombin stimulates platelet activation, monocyte and neutrophil chemotaxis, and endothelial production of prothrombotic factors. The varied physiologic effects of thrombin are due to the widespread presence of thrombin receptors in many cell types. The receptor is uniquely activated: thrombin binds to the receptor at the thrombin anion-binding exosite, the receptor ligand ("tethered ligand") apparently being a sequence of 6 amino acids (SFLLRN). Thus, peptides corresponding to the sequence of the tethered ligand can stimulate almost all functions of native thrombin itself. Several intracellular signaling pathways have been identified as important in the restenosis process: the G protein-related pathway, cyclic adenosine monophosphate (cAMP) mediator pathway, and tyrosine kinase activation pathway. In situ hybridization has demonstrated an increase in thrombin receptor mRNA throughout the period of neointimal and vascular lesion development. The mechanism of this increase is unknown, but may be mediated by multiple inflammatory modulators. Several strategies have been tested in animal models for inhibiting thrombin: (1) Hirudin not only prevents thrombin from cleaving fibrinogen, but also prevents thrombin receptor activation. (2) Thrombin receptor antagonist peptides block platelet aggregation effects of thrombin. (3) Mono- and polyclonal antibodies inhibit thrombin receptor activation. (4) Antisense oligonucleotides block thrombin receptor expression.

Role of thrombosis in atherosclerosis and its complications

The endothelium is intact but activated and dysfunctioning during the early phase of atherogenesis. Owing to increased endothelial permeability, many blood-derived components, including hemostatic factors, are present in early as well as advanced atherosclerotic lesions. Insudated fibrin(ogen) and related degradation products and thrombin could contribute to atherogenesis by their chemotactic (attracting monocytes/macrophages) and mitogenic (stimulating cell proliferation) properties. All key cells in plaque may express thrombin receptors, indicating that thrombin may play a role in the genesis of uncomplicated atherosclerosis by mediating inflammatory and proliferative processes. Later, endothelial denudation with platelet adherence occurs over mature plaques. Then, incorporation of microthrombi and probably platelet/thrombus-derived growth factors are critical for the progressive growth of the smooth muscle cell-related plaque component. Besides transendothelial influx and incorporation of mural thrombi, blood products in atherosclerotic plaques may originate from hemorrhage through a ruptured plaque surface or from fragile newly formed vessels (neovascularization) frequently found at the base of advanced plaques. Rupture-related plaque progression due to luminal thrombosis and/or plaque hemorrhage is the most important mechanism underlying the unpredictable rapid progression of coronary lesions responsible for acute coronary syndromes. Both platelets and fibrin play a role in the dynamic thrombotic response to plaque rupture.(ABSTRACT TRUNCATED AT 250 WORDS)

Contraction of human brain endothelial cells induced by thrombogenic and fibrinolytic factors. An in vitro cell culture model

BACKGROUND AND PURPOSE

Vasogenic brain edema is a frequent complication of ischemic stroke. The mechanism of the blood-brain barrier opening that underlies the edema formation is poorly understood. In the present study we examined the response of endothelial cells cultured from adult human brain to thrombogenic and fibrinolytic factors that possibly accumulate in the occluded vascular segments in ischemic stroke.

METHODS

The changes in the morphology of cultured human brain microvascular endothelial cells were observed by phase-contrast light microscopy and quantified with computerized morphometry.

RESULTS

Active proteases (eg, thrombin, plasmin, urokinase) as well as heparin and protamine, but not fibrinogen and antithrombin III, produced significant changes in endothelial cell morphology. Two shape patterns of contraction were observed: protamine treatment resulted in rounded cells with a decrease in both cell perimeter and area, whereas all other agents induced spiderlike cell morphology with increased perimeter and reduced area. The rate of contraction was dose dependent, and at comparable enzyme concentrations plasmin produced faster contraction than thrombin. The observed changes were reversed 3 hours after abrogating the treatment.

CONCLUSIONS

In an in vitro model we have demonstrated that factors involved in thrombus formation and dissolution induce endothelial cell contraction, which could affect focally the permeability of the blood-brain barrier by opening paracellular avenues between endothelial cells in vivo. Thus, the genesis of brain edema in thromboembolic stroke or occasionally during fibrinolytic therapy can be attributed in part to the contact of these factors with the microvascular endothelium.

Thrombin, phorbol ester, and cAMP regulate thrombin receptor protein and mRNA expression by different pathways

Human mesangial cells have been used to study the regulation of thrombin receptor protein and mRNA expression during cross-talk between different signal transduction pathways. Persistent activation of thrombin receptor by thrombin led to homologous down-regulation of thrombin receptor protein. However, thrombin receptor mRNA expression was not affected, suggesting that increased receptor degradation is responsible for homologous down-regulation. Chronic activation of protein kinase C by phorbol 12-myristate 13-acetate (PMA) and of adenylylcyclase by prostaglandin E1 (PGE1) resulted in heterologous down-regulation of thrombin receptor protein. In contrast to thrombin, PMA and PGE1 reduced in parallel thrombin receptor mRNA levels to 51% and 24% of control, respectively, indicating that heterologous down-regulation of thrombin receptor protein is, at least in part, due to inhibition of receptor mRNA expression. The mechanisms of heterologous down-regulation of thrombin receptor protein have been studied in detail and compared to homologous down-regulation. PMA-induced down-regulation was completely blocked by GF 109 203 X, an inhibitor of protein kinase C. However, the loss of thrombin receptor induced by thrombin was not prevented by GF 109 203 X, indicating that homologous regulation is not dependent on protein kinase C activation. The heterologous effect of PGE1 was mimicked by 8-bromo-cAMP, isobutylmethylxanthine, and forskolin, suggesting that an increase in intracellular cAMP level is involved in heterologous regulation. Interestingly, heterologous down-regulation induced by PGE1 seems not to require previous internalization of thrombin receptor. These data indicate that thrombin receptor protein and mRNA expression can be regulated in homologous and heterologous ways by different mechanisms.

Constitutive expression and modulation of the functional thrombin receptor in the human kidney

Thrombin exerts procoagulant effects and has also many cellular effects mediated by cell surface receptors. A functional thrombin receptor from human platelets has been cloned and sequenced. In the present study, by reverse transcription and polymerase chain reaction, using specific primers designed from the thrombin receptor cDNA sequence, we show that the mRNA encoding for this receptor can be amplified from freshly isolated human glomeruli obtained by microdissection of normal kidney cortex. By immunohistochemistry using a specific monoclonal antibody, ATAP2, directed against the extracellular N-terminus of this receptor, we find that this functional thrombin receptor is constitutively expressed in the normal human kidney. The three glomerular cell types, endothelial, mesangial, and epithelial cells, were positively stained, as were the endothelial cells of renal arteries, arterioles, venules, and peritubular capillaries. Occasionally, interstitial cells and smooth muscle cells in the media of renal arteries were also stained. Proximal and distal tubular cells were not stained. By in situ hybridization, using a digoxigenin-labeled cDNA probe specific for thrombin receptor, the thrombin receptor mRNA was found to have the same distribution as the thrombin receptor protein detected by immunohistochemistry. A lighter staining of glomerular endocapillary cells was observed in cases of thrombotic microangiopathy and extracapillary glomerulonephritis, two renal diseases associated with in situ thrombin generation and fibrin formation. In one case of thrombotic microangiopathy, we observed an increase in thrombin receptor mRNA. This suggests that thrombin receptor protein is not always correlated with thrombin receptor mRNA level. Internalization and degradation of thrombin receptor protein have been demonstrated in vitro and could also occur after activation in vivo. This is the first demonstration of the constitutive expression of the functional thrombin receptor in the human kidney. These results suggest that thrombin may exert glomerular and vascular effects within the kidney in normal and in pathological conditions.

Characterization of thrombin receptor expression during vascular lesion formation

Blood vessels respond to injury by initiating cell proliferation and migration that result in vascular lesion formation. To determine the roles of thrombin and the thrombin receptor in this process, we characterized thrombin receptor expression in normal and injured arteries, thrombin receptor-mediated smooth muscle cell mitogenesis, and the regulation of thrombin receptor mRNA expression in vitro. Thrombin receptor mRNA was not detected in normal rat or baboon arteries by in situ hybridization. Immunohistochemistry using an antithrombin receptor antibody (TR-R9), directed against the thrombin cleavage site of the rat aortic smooth muscle cell thrombin receptor, revealed low-level staining for thrombin receptor protein in endothelial cells and smooth muscle cells of normal arteries. In contrast, balloon catheter injury increased thrombin mRNA expression in medial smooth muscle cells within 6 hours. This increased thrombin receptor expression continued within the media and in neointimal cells throughout vascular lesion formation, predominantly in areas of active cell proliferation. In vitro, alpha-thrombin stimulates rat aortic smooth muscle cell proliferation in a concentration-dependent manner. That thrombin receptor activation is required for the mitogenic response was confirmed by demonstrating that the polyclonal antibody TR-R9 inhibits thrombin-induced cell proliferation. Thrombin receptor mRNA synthesis was induced by both basic fibroblast growth factor (maximal stimulation of 1.8-fold at 1 hour) and platelet-derived growth factor (maximal stimulation of 2.4-fold at 8 and 24 hours) in quiesced cultured rat aortic smooth muscle cells. In summary, upregulation of smooth muscle cell thrombin receptor expression occurs very early after vascular injury and continues throughout neointimal development.(ABSTRACT TRUNCATED AT 250 WORDS)

Thrombin induces endothelial cell growth via both a proteolytic and a non-proteolytic pathway

Binding of 125I-thrombin to human umbilical vein endothelial cells (HUVECs) was specifically displaced by the synthetic tetradecapeptide SFLLRNPNDKYEPF, named thrombin receptor agonist peptide (TRAP), which has recently been described as a peptide mimicking the new N-terminus created by cleavage of the thrombin receptor, and F-14, a tetradecapeptide representing residues 365-378 of the human alpha-thrombin B chain. Binding of 125I-TRAP to HUVECs was time-dependent, reversible and saturable, showing high affinity (KD = 1.5 +/- 0.4 microM) and high binding capacity (Bmax. = 7.1 +/- 0.6 x 10(6) sites/cell) (n = 3). Unlabelled thrombin and TRAP competitively and selectively inhibited the specific binding of 125I-TRAP with IC50 values of 5.8 +/- 0.7 nM and 2.8 +/- 0.4 microM respectively, whereas F-14 remained ineffective at displacing 125I-TRAP from its binding sites, suggesting the presence of at least two different types of thrombin-binding sites on HUVECs. TRAP was a potent mitogen for HUVECs in culture. Both TRAP and alpha-thrombin stimulated the proliferation of HUVECs with half-maximum mitogenic responses between 1 and 10 nM. F-14 also promoted HUVEC growth. The mitogenic effects of F-14 and TRAP were additive. N alpha-(2-Naphthylsulphonylglycyl)-DL-p-amidinophenylalanylpiper idine (NAPAP) and hirudin (two specific inhibitors of the enzyme activity of thrombin) specifically inhibited thrombin-induced HUVEC growth (IC50 values 400 +/- 60 and 52 +/- 8 nM respectively) but remained without effect on the mitogenic effect of TRAP or F-14. This demonstrated that the mitogenic effect of alpha-thrombin for HUVECs was intimately linked to its esterolytic activity but also showed that thrombin can stimulate HUVEC growth via another non-enzymic pathway. This hypothesis was further reinforced by the fact that F-14-induced proliferation of HUVECs remained unaltered by two antibodies directed against TRAP or the cleavage site on the extracellular portion of the thrombin receptor, which both strongly reduced thrombin-induced proliferation of HUVECs. Thrombin-, TRAP- or F-14-induced HUVEC proliferation was strongly inhibited by a neutralizing monoclonal antibody directed against basic fibroblast growth factor (bFGF), suggesting that thrombin regulates the autocrine release of bFGF in HUVECs.

Synthesis and characterization of photoactivatable peptide agonists of the human thrombin receptor

Chemical synthesis and biochemical analysis of modified agonist peptides of the human thrombin receptor derived from the sequence SFLLRNP containing photoactivatable azido groups and biotin for sensitive detection is described. Substitution of leucine in position three with p-azidophenylalanine and extension of the C-terminus with a KGGK spacer containing biotin covalently linked to the side chain of the C-terminal lysine residue resulted in an active receptor agonist as determined by intracellular Ca2+ mobilization in human erythroleukemia (HEL) cells. In contrast, substitution of phenylalanine in position two with p-azidophenylalanine reduced agonist activity significantly.

Molecular mapping of the heparin-binding exosite of thrombin

Thrombin contains electropositive patches at opposite poles of the molecule which represent potential exosites for the binding of macromolecular ligands. The function of anion-binding exosite I, the fibrin(ogen) recognition site, has been well described. Anion-binding exosite II, located near the carboxyl terminus of the molecule, has been proposed to bind heparin on the basis of chemical modification studies. To define the functional heparin-binding site on thrombin, purified recombinant alpha-thrombins were prepared with glutamic acid substitution for selected basic amino acid residues in exosite II or exosite I. Heparin affinity was assessed by NaCl gradient elution from heparin-agarose, and second-order rate constants for inhibition by antithrombin III were determined in the absence and presence of heparin. Affinity for heparin-agarose was reduced markedly by selected mutations in exosite II (R89E, R245E, K248E, and K252E, numbered from the amino terminus of the B chain) but not by other mutations in exosite II (K174E, K247E) or by mutations in exosite I (R68E, K154E). All recombinant thrombins had similar rate constants for inhibition by antithrombin III without heparin. However, affinity for heparin-agarose correlated directly with the rate of inhibition by antithrombin III with heparin. These results demonstrate that selected mutations in anion-binding exosite II define a functional heparin-binding site and support the template mechanism of heparin action.

A novel biologic activity of thrombin: stimulation of monocyte chemotactic protein production

Thrombin is a serine protease that is released at sites of vascular injury and exerts a variety of biologic effects on different cell types. Thrombin is postulated to play a role in the pathogenesis of a number of diseases including atherosclerosis, since it activates vascular smooth muscle and endothelial cells. Thrombin mediates these effects through a specific receptor that is upregulated in vascular cells in atherosclerosis. Atherosclerosis and glomerulosclerosis are characterized by the presence of monocyte-macrophages in the lesions. Monocyte chemotactic protein (MCP-1) is believed to be an important mediator of monocyte recruitment to the tissue and can be induced in a broad variety of cells including mesangial cells. We studied the effect of thrombin on MCP-1 production and gene expression in well-characterized human mesangial cells, vascular pericytes that play a central role in fibrosis of the glomerular microvascular bed. alpha thrombin stimulates MCP-1 production and gene expression in mesangial cells in a dose- and time-dependent manner. Experiments with diisopropylfluorophosphate thrombin and gamma thrombin demonstrate that this thrombin effect requires both receptor binding as well as catalytic activity, features consistent with the known properties of the recently characterized and cloned thrombin receptor. Moreover, a human thrombin receptor activating peptide (TRAP1-7) also stimulates MCP-1 production. Northern blot analysis demonstrated that mesangial cells express an mRNA transcript that hybridizes with labeled human thrombin receptor cDNA. These data describe a novel biologic activity of thrombin and suggest an additional mechanism by which this coagulation factor may participate in the progression of glomerulosclerosis, and by analogy, atherosclerosis.

The thrombin receptor

1. The thrombin receptor has now been cloned and found to be a member of the G-protein-coupled seven-transmembrane domain receptor family. 2. The receptor has been detected directly in platelets, endothelial cells and smooth muscle cells and studies using receptor-derived peptides have demonstrated that this receptor may be the one responsible for many of the actions of thrombin in platelets, endothelial cells, smooth muscle cells, fibroblasts, mesangial cells and neural cells. 3. The receptor appears to be activated by the novel mechanism of cleavage by thrombin to yield a new N-terminus which then interacts with the receptor as a tethered ligand to initiate cell activation.

Tissue plasminogen activator, plasminogen activator inhibitor-1, and fibrin as indexes of clinical course in cardiac allograft recipients. An immunocytochemical study

BACKGROUND

Tissue-type plasminogen activator (TPA) is the principal activator of plasminogen. Since hemostasis in the microcirculation of allografts is a well-recognized complication of transplantation, we asked (1) whether the distribution and amount of cellular TPA in biopsies of transplanted human hearts are associated with fibrin deposits in and around the microcirculation, (2) whether such changes involve the physiological inhibitors of TPA and plasmin, and (3) whether the presence of these activators and inhibitors of fibrinolysis in tissue is correlated with clinical outcome.

METHODS AND RESULTS

We immunocytochemically quantified the presence of fibrin, plasmin, TPA, and the TPA inhibitor PAI-1 in 938 biopsies from 68 consecutive cardiac allografts over a 54-month period. The localization, distribution, and quantification of TPA in arteriolar smooth muscle cells revealed that 35 of the 68 allografts maintained vascular TPA reactivity consistent with time-zero biopsies of autologous donor hearts: this was designated as the normal TPA group. In contrast, 33 of the 68 allografts significantly lost vascular TPA reactivity compared with time-zero biopsies of autologous donor hearts: this was designated as the depleted TPA group. Analysis of sequential biopsies from both groups during 54 months revealed that the mean cumulative quantitative TPA value for the normal TPA group was 1.0 +/- 0.01, whereas the depleted TPA group value was 1.9 +/- 0.02 (P = .0001), and the mean cumulative quantitative fibrin value for the normal TPA group was 1.0 +/- 0.01, whereas the depleted TPA group value was 1.5 +/- 0.05 (P = .0001). Biopsies of allografts in the depleted TPA group contained endothelial reactivity for TPA-PAI-1 complexes, whereas biopsies from the normal TPA group did not. Plasmin-associated molecules were rarely identified in biopsies of the normal TPA group but were present in the depleted TPA group, and the fibrin-to-plasmin ratio in the normal TPA group always was less than the fibrin-to-plasmin ratio in biopsies from the depleted TPA group. Analysis of demographic and risk factors revealed no significant differences between patients in the normal and depleted TPA groups, but none of the 35 patients in the normal TPA group died or were retransplanted, and 13 of the 33 patients in the depleted TPA group died or required retransplantation (P = .0001).

CONCLUSIONS

Time-zero hearts (n = 68) and 34 of 38 stable allografts contained immunocytochemically detectable TPA only in vascular smooth muscle cells. Twenty-nine of 30 patients with normal TPA in their time-zero biopsies who subsequently developed a poor clinical outcome were found to have depleted TPA in biopsies evaluated during their first postoperative month and remained depleted throughout the study. Of 33 patients with depleted TPA, 39% died or required retransplantation. Depleted arteriolar TPA associated significantly with vascular and interstitial deposits of fibrin, plasmin, and endothelial TPA-PAI-1 complexes. These findings indicate that hemostatic and fibrinolytic pathways are activated in falling allografts, and they reveal evidence of depleted TPA before clinical or histopathological signs of failure. Patients with such allografts were found to be at high risk of death independently of other widely used clinical/laboratory parameters of prediction.

Antithrombin III inhibits thrombin-induced proliferation in human arterial smooth muscle cells

Thrombin has attracted increasing attention as a possible mitogen for vascular smooth muscle cells in lesion development both after vascular injury and in atherogenesis. In this study, the ability of antithrombin III to inhibit alpha-thrombin-induced DNA synthesis and cell proliferation in human arterial smooth muscle cells was analyzed. We demonstrate a concentration-dependent initiation of DNA synthesis and cell proliferation by alpha-thrombin. This effect was abolished when complex formation with antithrombin III was allowed before thrombin was added to the cell cultures. Addition of alpha-thrombin and antithrombin III simultaneously at the beginning of the incubation period also resulted in an inhibition of thrombin-induced DNA synthesis, but to a lower degree. The inhibitory activity of antithrombin III was enhanced in the presence of heparin, which on its own had no inhibitory effect on thrombin-induced DNA synthesis. In contrast, the mitogenic activity of alpha-thrombin could be inhibited by heparin in the presence of low concentrations of serum. This inhibition was dependent on the presence of antithrombin III in serum, since heparin lacked effect if antithrombin III was depleted from serum by immunoaffinity chromatography. Analysis of the enzymatic activity of thrombin showed that the influence on catalytic activity of thrombin corresponded to the mitogenic activity of thrombin in the presence of heparin, antithrombin III, and serum. The results suggest that the mitogenic activity of thrombin is regulated by antithrombin III. Therefore, antithrombin III may serve dual functions by inhibiting thrombin in the coagulation cascade and by neutralizing its growth-promoting effects on vascular smooth muscle cells.

Increased gene expression after liposome-mediated arterial gene transfer associated with intimal smooth muscle cell proliferation. In vitro and in vivo findings in a rabbit model of vascular injury

Arterial gene transfer represents a novel strategy that is potentially applicable to a variety of cardiovascular disorders. Attempts to perform arterial gene transfer using nonviral vectors have been compromised by a low transfection efficiency. We investigated the hypothesis that cellular proliferation induced by arterial injury could augment gene expression after liposome-mediated gene transfer. Nondenuded and denuded rabbit arterial strips were maintained in culture for up to 21 d, after which transfection was performed with a mixture of the plasmid encoding firefly luciferase and cationic liposomes. In non-denuded arteries, the culture interval before transfection did not affect the gene expression. In contrast, denuded arteries cultured for 3-14 d before transfection yielded 7-13-fold higher expression (vs. day 0; P < 0.005). Transfection was then performed percutaneously to the iliac arteries of live rabbits with or without antecedent angioplasty. Gene expression increased when transfection was performed 3-7 d postangioplasty (P < 0.05). Proliferative activity of neointimal cells assessed in vitro by [3H]thymidine incorporation, and in vivo by immunostaining for proliferating cell nuclear antigen, increased and declined in parallel with gene expression. These findings thus indicate that the expression of liposome-mediated arterial gene transfer may be augmented in presence of ongoing cellular proliferation.

Thrombin in inflammation and healing: relevance to rheumatoid arthritis

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Disparate temporal expression of the prothrombin and thrombin receptor genes during mouse development

The protease thrombin is a potent agonist for platelet aggregation, mesenchymal cell proliferation, and endothelial production of growth factors and adhesion molecules. Thrombin also modulates neurite outgrowth in neuronal cultures. These apparently disparate responses to thrombin appear to be largely mediated by the recently cloned thrombin receptor. In the adult, thrombin is generated from its zymogen prothrombin at sites of vascular injury when circulating coagulation factors meet extravascular tissue factor. In this context thrombin's varied actions may mediate responses to wounding. Whether thrombin's actions on cells may also play a role in development is unknown. We examined the expression of thrombin receptor, prothrombin, and tissue factor by in situ hybridization in mouse development. Thrombin receptor mRNA was expressed widely in mesenchymal cell populations during early organogenesis (E9.5) and was particularly abundant in developing heart and blood vessels. Robust receptor expression was also noted in the germinal epithelium of the hindbrain. Thrombin receptor expression became more restricted with time and by the fetal growth stage (E16.5) was most readily detected in certain neurons, endocardial and endothelial cells, and within lung and liver. In contrast to the thrombin receptor, prothrombin mRNA was limited to the embryonic liver and was not detected until E12.5, well after the onset of receptor expression. mRNA for tissue factor, one important trigger for thrombin generation in the adult, was detected in embryonic epithelia from E9.5-12.5. In several instances, tissue factor-expressing epithelia were surrounded by thrombin receptor-expressing mesenchyme. These data suggest a possible role for the thrombin receptor in development. The finding of robust thrombin receptor expression before prothrombin mRNA was detected raises the question of whether other proteases or peptide ligands can activate the thrombin receptor.

Synthetic low-molecular weight thrombin inhibitors: molecular design and pharmacological profile

Thrombin is a multifunctional protein: in addition to its role in coagulation, thrombin has important biological effects on platelets, endothelial and smooth muscle cells, leukocytes, the heart and neurones. A detailed understanding of the structure of thrombin, of related serine proteases and of enzyme-inhibitor complexes has aided in the discovery of potent and selective new inhibitor molecules. Some of these novel thrombin inhibitors are active when administered orally and have shown remarkable efficacy as antithrombotic agents in animal models, offering a greater therapeutic potential than presently available drugs. This potential extends also to non-thrombotic indications where thrombin may be involved, namely inflammation, cancer and neurodegenerative diseases. The recent identification of specific thrombin receptors on different cells provides an alternative strategy for inhibiting thrombin's cellular actions, without necessarily compromising its role in haemostasis. In this review, Carlo Tapparelli and colleagues present a comprehensive update of these recent developments in the field of thrombin biology and pharmacology suggesting a new era of therapeutic drugs is on the horizon.

M(r) 6,400 aurin tricarboxylic acid directly activates platelets

ATA is a novel anticoagulant polymeric anionic aromatic compound that inhibits von Willebrand factor binding to platelet glycoprotein Ib and thereby prevents ristocetin- and shear stress-induced platelet aggregation. To investigate its mechanism of action, ATA fractions of homogeneous M(r) have been prepared by size exclusion chromatography. ATA fractions of M(r) > or = 2,500 are most effective at inhibiting vWF-mediated platelet aggregation, and ATA of M(r) = 2,500 also inhibits thrombin-induced platelet activation. Paradoxical results were observed in studies of ATA with M(r) = 6,400. This fraction of ATA stimulates aggregation of washed platelets or platelet-rich-plasma. The dose/response of aggregation shows a bell-shaped curve with maximal aggregation at approximately 2 micrograms/ml. Platelet aggregation is associated with phosphoinositide turnover and protein kinase C- and calcium-dependent protein phosphorylation. Platelet signalling responses to ATA are inhibited by platelet pretreatment with PGI2 or dibutyryl-cyclic AMP, but are unaffected by inhibiting platelet cyclooxygenase with aspirin. These results suggest that M(r) 6,400 ATA directly activates platelet phospholipase C to initiate platelet aggregation. This effect, unique to M(r) 6,400 ATA, could potentially mitigate ATA's beneficial anti-thrombotic effect on vWF-mediated platelet responses, and should be considered when analyzing results of experiments that utilize unfractionated ATA.

Peripheral blood lymphocytes express the platelet-type thrombin receptor

Northern blot analysis of human mononuclear cells indicated that the platelet thrombin receptor may be expressed by lymphocytes. In order to investigate this, we prepared affinity purified rabbit antibodies against the thrombin receptor which bound platelets and blocked thrombin activation. Using flow cytometry on peripheral blood cells, we found that the vast majority of NK cells (CD16/CD56 positive) and a fraction of CD3/CD4 positive T cells expressed the thrombin receptor. B cells, neutrophils and monocytes were negative. These data suggest that potentially thrombin may play a direct role in regulating NK and T cell function.