Cellular consequences of thrombin-receptor activation

Protease-activated receptors in the brain: receptor expression, activation, and functions in neurodegeneration and neuroprotection

Protease-activated receptors (PARs) are G protein-coupled receptors that regulate the cellular response to extracellular serine proteases, like thrombin, trypsin, and tryptase. The PAR family consists of four members: PAR-1, -3, and -4 as thrombin receptors and PAR-2 as the trypsin/tryptase receptor, which are abundantly expressed in the brain throughout development. Recent evidence has supported the direct involvement of PARs in brain development and function. The expression of PARs in the brain is differentially upregulated or downregulated under pathological conditions in neurodegenerative disorders, like Parkinson's disease, Alzheimer's disease, multiple sclerosis, stroke, and human immunodeficiency virus-associated dementia. Activation of PARs mediates cell death or cell survival in the brain, depending on the amplitude and the duration of agonist stimulation. Interference or potentiation of PAR activation is beneficial in animal models of neurodegenerative diseases. Therefore, PARs mediate either neurodegeneration or neuroprotection in neurodegenerative diseases and represent attractive therapeutic targets for treatment of brain injuries. Here, we review the abnormal expression of PARs in the brain under pathological conditions, the functions of PARs in neurodegenerative disorders, and the molecular mechanisms involved.

Thrombin-induced VEGF expression in human retinal pigment epithelial cells

PURPOSE

The purpose of the present study was to investigate the effects of thrombin and thrombin in combination with other proangiogenic factors on VEGF expression in hRPE cells.

METHODS

hRPE cells were stimulated with thrombin TNF-alpha, monocytes, and TGF-beta2. After stimulation, conditioned medium and lysed cells were subjected to ELISA, Western blot analysis, immunocytochemistry, and RT-PCR analyses. Inhibitors specific for various signal transduction pathways were used to determine the signaling pathways involved.

RESULTS

Treatment of RPE cells with thrombin resulted in dose- and time-dependent increases in VEGF mRNA levels and protein production. hRPE VEGF expression is predominantly protease-activated receptor (PAR)-1 dependent. Approximately 80% of thrombin-induced VEGF secretion was abrogated by inhibitors of MAPK/ERK kinase (MEK), p38, c-Jun NH2-terminal kinase (JNK), protein tyrosine kinase (PTK), phosphatidylinositol 3-kinase (PI3K), protein kinase C (PKC), nuclear factor-kappaB (NF-kappaB), and reactive oxygen species (ROS). Analyses of VEGF protein production and mRNA synthesis revealed that VEGF induction by thrombin plus TNF-alpha or coculture with monocytes was additive, whereas that by co-incubation with TGF-beta2 was synergistic. The costimulated VEGF production by TGF-beta2 plus thrombin was an average of three times higher than the sum of that induced by each agent alone. Furthermore, BAPTA [bis-(o-aminophenoxy)ethane-N,N',N'-tetraacetic acid], a calcium chelator, blocked the VEGF secretion induced by thrombin and thrombin plus TGF-beta2 by 65% and 20%, respectively, but had no effect on that induced by TGF-beta2 alone.

CONCLUSIONS

Thrombin alone and in combination with TNF-alpha, monocytes, and TGF-beta2 potently stimulated VEGF expression in hRPE cells via multiple signaling pathways. The thrombin-induced calcium mobilization may play an important permissive role in maximizing TGF-beta2-induced VEGF expression in RPE cells.

Possible involvement of protease-activated receptor-1 in the regulation of morphine-induced dopamine release and hyperlocomotion by the tissue plasminogen activator-plasmin system

We have previously demonstrated that tissue plasminogen activator (tPA)-plasmin system participates in the rewarding effect of morphine, by regulating dopamine release in the nucleus accumbens (NAc). However, it is unclear how plasmin increases the morphine-induced release of dopamine and hyperlocomotion. In the present study we investigated whether protease activated receptor-1 (PAR-1) is involved in the regulation of acute morphine-induced dopamine release by the tPA-plasmin system. Morphine significantly but transiently increased extracellular tPA activity in the NAc, which was completely blocked by naloxone. Microinjection of a PAR-1 antagonist, (tyr(-1))-thrombin receptor activating peptide 7, into the NAc significantly reduced morphine-induced dopamine release in the NAc and hyperlocomotion although the treatment had no effect on basal dopamine release and spontaneous locomotor activity. Furthermore, the PAR-1 antagonist blocked the ameliorating effect of plasmin on the defect of morphine-induced dopamine release in the NAc of tPA-deficient mice. In contrast, intracerebroventricular injection of the PAR-1 antagonist had no effect on the antinociceptive effects of morphine in mice. These results suggest that PAR-1 is a target for the tPA-plasmin system in the regulation of acute morphine-induced dopamine release in the NAc.

α-Thrombin inhibits DNA synthesis in rat hepatocytes but not in hepatoma cells by receptor activation and proteolysis

Prothrombin is a plasma protein, which after tissue injury is converted to alpha-thrombin and is mainly involved in blood clot formation. It has also been shown to have a mitogenic effect on primary endothelial cells, vascular smooth muscle cells, fibroblasts and some tumor cells, but is an inhibitor of rat hepatocyte DNA synthesis on fibronectin matrix in cell culture. We now report that prothrombin is converted to alpha-thrombin by primary cultures of normal adult rat hepatocytes and alpha-thrombin is also a potent inhibitor of hepatocytes DNA synthesis. In contrast, rat hepatoma cells cultured under similar conditions were resistant to alpha-thrombin mediated DNA synthesis inhibition. The inhibitory effect of alpha-thrombin on DNA synthesis was antagonized by hirudin and antithrombin, two specific alpha-thrombin inhibitors or by the presence of collagen-I matrix. A thrombin receptor activating peptide (TRAP6) also inhibited EGF-mediated rat hepatocyte DNA synthesis, suggesting a role of the thrombin receptors in this process. Matrix fibronectin was degraded by alpha-thrombin. However, no appreciable cell detachment was observed. These results suggest a role of alpha-thrombin as a potent growth inhibitor of normal hepatocytes, possibly through control of fibronectin or other matrix protein(s).

Himbacine derived thrombin receptor antagonists: discovery of a new tricyclic core

The synthesis and biological activity of a novel series of thrombin receptor antagonists is described. This series of compounds showed excellent in vitro and in vivo potency. The most potent compound 40 had an IC(50) of 7.6 nM and showed robust inhibition of platelet aggregation in a cynomolgus monkey model after oral administration.

Coagulation factor Xa drives tumor cells into apoptosis through BH3-only protein Bim up-regulation

Coagulation Factor (F)Xa is a serine protease that plays a crucial role during blood coagulation by converting prothrombin into active thrombin. Recently, however, it emerged that besides this role in coagulation, FXa induces intracellular signaling leading to different cellular effects. Here, we show that coagulation factor (F)Xa drives tumor cells of epithelial origin, but not endothelial cells or monocytes, into apoptosis, whereas it even enhances fibroblast survival. FXa signals through the protease activated receptor (PAR)-1 to activate extracellular-signal regulated kinase (ERK) 1/2 and p38. This activation is associated with phosphorylation of the transcription factor CREB, and in tumor cells with up-regulation of the BH3-only pro-apoptotic protein Bim, leading to caspase-3 cleavage, the main hallmark of apoptosis. Transfection of tumor cells with dominant negative forms of CREB or siRNA for either PAR-1, Bim, ERK1 and/or p38 inhibited the pro-apoptotic effect of FXa. In fibroblasts, FXa-induced PAR-1 activation leads to down-regulation of Bim and pre-treatment with PAR-1 or Bim siRNA abolishes proliferation. We thus provide evidence that beyond its role in blood coagulation, FXa plays a key role in cellular processes in which Bim is the central player in determining cell survival.

Soluble fibrin formation in the mesangial area of IgA nephropathy

BACKGROUND

Fibrin monomer and its derivatives in blood are found in an early stage of thrombosis. When they are produced in blood, they form complexes with fibrinogen, and they exist as soluble complexes named soluble fibrin (SF). As final insoluble products, cross-linked fibrin (XFb) is often observed in mesangial areas in active types of human glomerulonephritis. To clarify the mechanisms of mesangial SF production and its relationship to XFb deposition in IgA nephropathy (IgAN), an immunohistochemical study was conducted.

METHODS

Nineteen patients with IgAN were studied. XFb was detected in renal biopsy specimens using anti-d-dimer antibody combined with plasmin exposure. SF was detected with a monoclonal antibody (IF-43), and factor V was detected with a specific rabbit antibody. The relationships of SF staining to the disease activity index, XFb deposition, and factor V staining was evaluated.

RESULTS

XFb, factor V, and SF were observed in the mesangium in 14, 11, and 8, respectively, of a total of 19 specimens. SF had frequent staining in the proliferating areas, showing a significant relationship to XFb or factor V (P < 0.05). Furthermore, XFb, factor V, and SF depositions were markedly correlated with disease activity (P < 0.001 in each case).

CONCLUSIONS

These findings suggest that SF is formed in the mesangial area in active IgA nephropathy accompanied by mesangial proliferation, in particular, in its early stage.

Roles of coagulation pathway and factor Xa in rat mesangioproliferative glomerulonephritis

Tissue factor initiates the extrinsic coagulation pathway by activating coagulation factor X to factor Xa, and factor V is a cofactor for the prothrombin activation by factor Xa. As factor Xa is known to promote the proliferation of mesangial cells in culture, the roles of the coagulation pathway and factor Xa were studied in an animal model of mesangioproliferative glomerulonephritis (MsPGN). MsPGN was induced in Wistar rats by an intravenous injection of anti-Thy 1.1 monoclonal antibody, OX-7. To clarify the role of factor Xa in MsPGN, a specific factor Xa inhibitor, DX-9065a, was injected intravenously at 2.5 or 10 mg/kg at the same time as OX-7, and kidney involvement was assessed by immunohistological analyses. We also examined p44/42 mitogen-activated protein (MAP) kinase activation. Time-course study revealed that expressions of tissue factor, factor V, and protease-activated receptor 2 (PAR2) were peaked on day 3, followed by factor X accumulation and mesangial proliferation. DX-9065a treatment significantly ameliorated proteinuria in a dose-dependent manner on day 8. Histological analyses showed a significant reduction in the size of glomeruli, the total number of glomerular cells, and crescent formation by DX-9065a treatment. Macrophage infiltration, which was rapidly observed on day 1 in disease control rats was not inhibited on days 1-3 by DX-9065a treatment, however it was suppressed on days 5-8. The deposition of fibrin, the number of PCNA-positive cells, and phosphorylation of p44/42 MAP kinase were markedly increased in the disease control group, whereas they were significantly reduced in the treatment group. Tissue factor and factor V induction may accelerate MsPGN through the activation and accumulation of factor X via proinflammatory and procoagulant mechanisms, and the inhibition of factor Xa would be a promising method to regulate the disease process.

Mechanism of endothelial nitric oxide synthase phosphorylation and activation by thrombin

Thrombin has been shown to activate endothelial NO synthase (eNOS) leading to endothelium-dependent vasorelaxation. In addition to its activation by Ca2+/calmodulin, eNOS has several regulatory sites. Ser1179 phosphorylation of eNOS by the phosphatidylinositol 3-kinase-dependent Akt stimulates its catalytic activity. In this study, we have elucidated the signaling mechanism of thrombin-induced phosphorylation of eNOS in the regulation of NO production. Immunoblot analysis showed that thrombin rapidly phosphorylates eNOS at Ser1179 in cultured bovine aortic endothelial cells. Also, thrombin was unable to stimulate eNOS if the Ser1179 was mutated to Ala. Akt is phosphorylated in response to thrombin at Ser473 at a later time point than eNOS. In this regard, a phosphatidylinositol 3-kinase inhibitor, LY294002, blocked Akt phosphorylation without affecting eNOS phosphorylation and cGMP production by thrombin. The Ca2+ ionophore A23187 stimulated eNOS phosphorylation, as well as cGMP production, and pretreatment with intracellular or extracellular Ca2+ chelators inhibited thrombin-induced eNOS phosphorylation and cGMP production. Moreover, infection of bovine aortic endothelial cell with adenovirus encoding dominant-negative mutants of protein kinase C (PKC) and PKC or pretreatment of bovine aortic endothelial cells with PKC inhibitors revealed that PKC is indispensable for thrombin-induced eNOS phosphorylation and activation. From these data, we concluded that thrombin induces the Ser1179 phosphorylation-dependent eNOS activation through a Ca2+-dependent, PKC-sensitive, but phosphatidylinositol 3-kinase/Akt-independent pathway.

Innate immunity and protective neuroinflammation: new emphasis on the role of neuroimmune regulatory proteins

Brain inflammation due to infection, hemorrhage, and aging is associated with activation of the local innate immune system as expressed by infiltrating cells, resident glial cells, and neurons. The innate immune response relies on the detection of "nonself" and "danger-self" ligands behaving as "eat me signals" by a plethora of pattern recognition receptors (PRRs) expressed by professional and amateur phagocytes to promote the clearance of pathogens, toxic cell debris (amyloid fibrils, aggregated synucleins, prions), and apoptotic cells accumulating within the brain parenchyma and the cerebrospinal fluid (CSF). These PRRs (e.g., complement, TLR, CD14, scavenger receptors) are highly conserved between vertebrates and invertebrates and may represent the most ancestral innate scavenging system involved in tissue homeostasis. However, in some diseases, these protective mechanisms lead to neurodegeneration on the ground that several innate immune molecules have neurocytotoxic activities. The response is a "double-edged sword" representing a fine balance between protective and detrimental effects. Several key regulatory mechanisms have now been evidenced in the control of CNS innate immunity, and these could be harnessed to explore novel therapeutic avenues. We will herein provide new emphasis on the role of neuroimmune regulatory proteins (NIRegs), such as CD95L, TNF, CD200, CD47, sialic acids, CD55, CD46, fH, C3a, HMGB1, which are involved in silencing innate immunity at the cellular and molecular levels and suppression of inflammation. For instance, NIRegs may play an important role in controlling lymphocyte/macrophage/microglia hyperinflammatory responses, while sparing host defense and repair mechanisms. Moreover, NIRegs have direct beneficial effects on neurogenesis and contributing to brain tissue remodeling.

Thrombin inhibits aquaporin 4 expression through protein kinase C-dependent pathway in cultured astrocytes

Aquaporin 4 (AQP4) is a key molecule for maintaining water balance in the central nervous system, and its dysfunction might cause brain edema. However, little is known about the regulation of AQP4 expression. Because thrombin has been implicated in brain edema formation, the purpose of this study is to determine whether thrombin affects expression of AQP4 in astrocytes. Here, the effect of thrombin on AQP4 expression in vitro was evaluated using Western blot analysis and RT-PCR. Meanwhile, we investigated whether the effect of thrombin on AQP4 expression was due to protease-activated receptor 1 (PAR-1). In addition, we examined the role of protein kinase C (PKC) in the effect of thrombin on AQP4 expression using Western blot analysis. We found that thrombin did not affect cell viability at concentrations of 0.05, 0.5, 5, or 50 nM but killed astrocytes at concentrations of 500 nM, with approx 72% of astrocytes surviving at 500 nM thrombin. Our data showed that AQP4 protein expression achieved only 28% of controls in 500 nM thrombin treatment, even if astrocytes survived approx 72% of controls at 500 nM thrombin. Thrombin significantly inhibited AQP4 in a time- and dose dependent manner in vitro (p<0.05). Cathepsin-G, a thrombin PAR-1 inhibitor, reversed significantly (p<0.05) the effect of thrombin on AQP4 mRNA and protein expression in astrocytes. We also observed that PKC inhibitor H-7 or prolonged pretreatment with TPA can rapidly increase AQP4 expression (p<0.05). Thrombin might inhibit AQP4 expression in rat astrocytes, and this effect is possibly mediated by the PKC pathway.

The rewards of nicotine: regulation by tissue plasminogen activator-plasmin system through protease activated receptor-1

Nicotine, a primary component of tobacco, is one of the most abused drugs worldwide. Approximately four million people die each year because of diseases associated with tobacco smoking. Mesolimbic dopaminergic neurons mediate the rewarding effects of abused drugs, including nicotine. Here we show that the tissue plasminogen activator (tPA)-plasmin system regulates nicotine-induced reward and dopamine release by activating protease activated receptor-1 (PAR1). In vivo microdialysis revealed that microinjection of either tPA or plasmin into the nucleus accumbens (NAc) significantly potentiated whereas plasminogen activator inhibitor-1 reduced the nicotine-induced dopamine release in the NAc in a dose-dependent manner. Nicotine-induced dopamine release was markedly diminished in tPA-deficient (tPA-/-) mice, and the defect of dopamine release in tPA-/- mice was restored by microinjection of either exogenous tPA or plasmin into the NAc. Nicotine increased tPA protein levels and promoted the release of tPA into the extracellular space in the NAc. Immunohistochemistry revealed that PAR1 immunoreactivity was localized to the nerve terminals positive for tyrosine hydroxylase in the NAc. Furthermore, we demonstrated that plasmin activated PAR1 and that nicotine-induced place preference and dopamine release were diminished in PAR1-deficient (PAR1-/-) mice. Targeting the tPA-plasmin-PAR1 system would provide new therapeutic approaches to the treatment of nicotine dependence.

The roles of proteinase-activated receptors in the vascular physiology and pathophysiology

Proteinase-activated receptors (PARs) belong to a family of G protein-coupled receptors, thus mediating the cellular effects of proteinases. In the vascular system, thrombin and other proteinases in the coagulation-fibrinolysis system are considered to be the physiologically relevant agonists, whereas PARs are among the most important mechanisms mediating the interaction between the coagulation-fibrinolysis system and the vascular wall. Under physiological conditions, PARs are mainly expressed in endothelial cells, and participate in the regulation of vascular tone, mostly by inducing endothelium-dependent relaxation. PARs in endothelial cells are also suggested to contribute to a proinflammatory phenotypic conversion and an increase in the permeability of vascular lesions. In smooth muscle cells, PARs mediate contraction, migration, proliferation, hypertrophy, and production of the extracellular matrix, thereby contributing to the development of vascular lesions and the pathophysiology of such vascular diseases as atherosclerosis. However, the expression of PARs in the smooth muscle of normal arteries is limited. The upregulation of PARs in the smooth muscle is thus considered to be a key step for PARs to participate in the pathogenesis of vascular lesions. Elucidating the molecular mechanism regulating the PARs expression is therefore important to develop new strategies for the prevention and treatment of vascular diseases.

Covalent antithrombin-heparin complexes

Unfractionated heparin (UFH) and low molecular weight heparin (LMWH) have been utilized as primary anticoagulants for thrombosis prophylaxis and treatment. However, a number of biophysical and safety limitations have led to development of new anticoagulants. Covalent antithrombin-heparin (ATH) complexes may address many of these issues. Early ATH products were prepared that had increased intravenous half-lives relative to UFH but lacked any improvement in anti-factor Xa activity or had no catalytic activity or reactivity against thrombin. However, a recent conjugate developed by Chan et al. has displayed a number of superior properties. Chan et al. ATH has an increased direct thrombin inhibition rate and can catalyze coagulant enzyme inhibition by exogenous antithrombin with very high specific activity. Unlike UFH, clot-bound thrombin is readily inhibited by ATH and, at similar antithrombotic efficacy, the ATH has improved bleeding profiles compared to heparins. Given the preclinical findings, Chan et al. ATH may warrant clinical trial testing for control of clot propagation.

Involvement of Tissue Plasminogen Activator-Plasmin System in Depolarization-Evoked Dopamine Release in the Nucleus Accumbens of Mice

Tissue plasminogen activator (tPA), a serine protease, catalyzes the conversion of plasminogen to plasmin. In the present study, we investigated the role of the tPA-plasmin system in depolarization-evoked dopamine (DA) and acetylcholine (ACh) release in the nucleus accumbens (NAc) and hippocampus, respectively, of mice, by using in vivo microdialysis. Microinjection of either tPA or plasmin significantly potentiated 40 mM KCl-induced DA release without affecting basal DA levels. In contrast, plasminogen activator inhibitor-1 dose-dependently reduced 60 mM KCl-induced DA release. The 60 mM KCl-evoked DA release in the NAc was markedly diminished in tPA-deficient (tPA-/-) mice compared with wild-type mice, although basal DA levels did not differ between the two groups. Microinjections of either exogenous tPA (100 ng) or plasmin (100 ng) into the NAc of tPA-/-mice restored 60 mM KCl-induced DA release, as observed in wild-type mice. In contrast, there was no difference in either basal or 60 mM KCl-induced ACh release in the hippocampus between wild-type and tPA-/-mice. Our findings suggest that the tPA-plasmin system is involved in the regulation of depolarization-evoked DA release in the NAc.

Modelling thrombin generation in human ovarian follicular fluid

A mathematical model is constructed to study thrombin production in human ovarian follicular fluid. The model results show that the amount of thrombin that can be produced in ovarian follicular fluid is much lower than that in blood plasma, failing to reach the level required for fibrin formation, and thereby supporting the hypothesis that in follicular fluid thrombin functions to initiate cellular activities via intracellular signalling receptors. It is also concluded that the absence of the amplification pathway to thrombin production in follicular fluid is a major factor in restricting the amount of thrombin that can be produced. Titration of the initial concentrations of the various reactants in the model lead to predictions for the amount of tissue factor and phospholipid that is required to maintain thrombin production in the follicle, as well as to the conclusion that tissue factor pathway inhibitor has little effect on the time that thrombin generation is sustained. Numerical experiments to determine the effect of factor V, which is at a much reduced level in follicular fluid compared to plasma, and thrombomodulin, illustrate the importance for further experimental work to determine values for several parameters that have yet to be reported in the literature.

Unraveling thrombin's true microglia-activating potential: markedly disparate profiles of pharmaceutical-grade and commercial-grade thrombin preparations

Microglia are the resident immune cells of the CNS. Brain injury triggers microglial activation, leading to proliferation, changes in antigenic profile, NO production and cytokine release. It is widely believed that serum factors inundating the injured tissue can prompt this activation, leading to long-term phenotypic changes. We and others have recently reported that commercial-grade preparations of thrombin, a serine protease known for its central function in blood coagulation, activate microglial cells. Recent findings, however, have called into question the involvement of thrombin itself in the induction of microglial cytokine release and led us to systematically re-investigate the ability of the protease to induce a broad spectrum of microglial activation parameters. We used a pharmaceutical-grade recombinant human thrombin (rh-thr) and compared it with a commercial-grade plasma-derived bovine thrombin (pb-thr) preparation that has been used extensively in the literature, including in our own earlier report. We investigated the effect of these two thrombin preparations on proliferation, NO production, interleukin-6 and tumour necrosis factor-alpha release, intracellular calcium signaling and cell surface expression of CD95 (Fas) and CD40. Pb-thr induced robust responses in all variables tested. In contrast, rh-thr triggered calcium signals and induced small but significant changes in the expression of cell surface antigens, but had no effect on proliferation, NO production or cytokine release. Control studies assured equivalent thrombin potencies and excluded both species-specific effects and endotoxin (lipopolysaccharide) contamination as possible causes of the disparity. Our results indicate a substantially more restricted role for thrombin itself in microglial activation than previously appreciated, but point to several potentially important co-stimulatory effects. In addition, these results suggest that previous studies examining thrombin's activation of microglia should be cautiously re-interpreted.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Discovery and synthesis of a novel series of quinoline-based thrombin receptor (PAR-1) antagonists

The design, synthesis, and SAR studies of a structurally novel series of highly potent thrombin receptor (PAR-1) antagonists are described. Compound 30 is a highly potent thrombin receptor antagonist (IC(50)=6.3 nM), a related compound 36 showing efficacy in a monkey ex vivo study.

Adenylyl cyclase-cAMP system inhibits thrombin-induced HSP27 in vascular smooth muscle cells

We previously reported that thrombin stimulates the induction of heat shock protein (HSP) 27 via p38 mitogen-activated protein (MAP) kinase activation in aortic smooth muscle A10 cells. In the present study, we investigated the effect of the adenylyl cyclase-cAMP system on the thrombin-stimulated induction of HSP27 in A10 cells. Forskolin, a direct activator of adenylyl cyclase, reduced the thrombin-induced p38 MAP kinase phosphorylation, and significantly suppressed the thrombin-stimulated accumulation of HSP27. However, dideoxyforskolin, a forskolin derivative that does not activate cAMP, failed to suppress the HSP27 accumulation. Furthermore, dibutyryl-cAMP (DBcAMP), a permeable analog of cAMP, significantly suppressed the accumulation of HSP27. On the other hand, calphostin C, an inhibitor of protein kinase C (PKC), reduced the thrombin-induced p38 MAP kinase phosphorylation, and significantly suppressed the thrombin-stimulated accumulation of HSP27. Moreover, forskolin reduced the p38 MAP kinase phosphorylation induced by the 12-O-tetradecanoylphorbol-13-acetate (TPA), a PKC-activating phorbol ester, and significantly suppressed the TPA-stimulated accumulation of HSP27. These results indicate that adenylyl cyclase-cAMP system has an inhibitory role in thrombin-stimulated HSP27 induction in aortic smooth muscle cells, and the effect seems to be exerted on the thrombin-induced PKC- p38 MAP kinase signaling pathway.

Role of coagulation factor Xa and protease-activated receptor 2 in human mesangial cell proliferation

BACKGROUND

Fibrin deposition and mesangial cell proliferation are frequently observed in the active type of mesangioproliferative glomerulonephritis. Coagulation factors, such as factor V and factor Xa are colocalized with fibrin in the mesangial areas in active type of IgA nephropathy with mesangial cell proliferation. In this study, therefore, we studied the role of factor Xa and its receptor, protease-activated receptor 2 (PAR2) in mesangial cell proliferation and fibrin deposition, and examined ant-proliferative effects of a specific factor Xa inhibitor, DX-9065a, in cultured human mesangial cells.

METHODS

To examine the effect of DX-9065a on the factor Xa-induced proliferation of cultured human mesangial cells, we measured thymidine incorporation and cell numbers. We also examined the effect of DX-9065a on extracellular regulated kinase (ERK) activation and fibrin production induced by factor Xa in human mesangial cells.

RESULTS

Factor Xa increased [(3)H]-thymidine incorporation and cell numbers in a dose-dependent manner in mesangial cells, which was inhibited by DX-9065a. DX-9065a also suppressed factor Xa-triggered fibrin deposition on mesangial cell surface. Factor Xa induced the activation of ERK in mesangial cells and this activation was also completely inhibited by DX-9065a, but not inhibited by PAR1 antagonist. Factor Xa-induced cell proliferation and ERK activation were inhibited by PD98059.

CONCLUSION

There results suggest that factor Xa can induce mesangial cell proliferation through the activation of ERK via PAR2 in mesangial cells and that PAR2 may play a crucial role in the cell proliferation induced by factor Xa. Our results implicate that DX-9065a may be a promising agent to regulate proliferation of mesangial cellss and inhibit the coagulation process in mesangium.

Thrombin-mediated IL-10 up-regulation involves protease-activated receptor (PAR)-1 expression in human mononuclear leukocytes

Thrombin, the key enzyme of the coagulation cascade, exerts cellular effects through activation of the protease-activated receptors (PARs). Interleukin (IL)-10, besides its anti-inflammatory properties, is considered a major denominator of the immunosuppressive effect during human endotoxemia. We have recently shown that thrombin inhibits IL-12 production in human mononuclear cells and that such inhibition is accompanied by IL-10 up-regulation. To our knowledge, there are no data available to show that thrombin mediates IL-10 production by its interactions with PAR-1. We here report that human alpha-thrombin enhances IL-10 expression in human peripheral blood mononuclear cells and in established monocytic cell lines and that this up-regulation requires PAR-1 expression. The use of proteolytically inactive thrombin reveals that such enhancement requires thrombin proteolytic activity. Addition of PAR-1 agonist peptides, such as SFLLRN, results in a significant increase of IL-10 production. PAR-1 expression is required for thrombin-induced IL-10 production, as shown by experiments performed with antisense or sense PAR-1 oligonucleotides. Treatment with thrombin or SFLLRN of monocytic cell lines, such as U937 and Mono Mac-6, results in an increased IL-10 production. This suggests that the observed IL-10 up-regulation may be the result of a direct interaction with monocytes. The observation that thrombin-mediated up-regulation of IL-10 may require the expression of the PAR-1 receptor identifies a new, functional link between inflammation and coagulation. Our results may also contribute to better design therapeutic strategies to treat several disorders, characterized by the presence of inflammatory as well as coagulant responses.

Increased thrombin inhibition in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE), are inflammatory diseases of the central nervous system (CNS). Activated coagulation factors are associated with inflammation and are elevated in the plasma of animals with EAE. Thrombin is a key coagulation factor and its major endogenous inhibitors are antithrombin III (ATIII) in the plasma and protease nexin 1 (PN-1) in the brain. We measured the capacity of brain homogenates to inhibit exogenous thrombin and the CNS levels of ATIII and PN-1 during the course of EAE. Acute EAE was induced in SJL/J mice by immunization with mouse spinal cord homogenates. On Days 8, 13, and 22 post-immunization, inhibition of exogenous thrombin activity was measured by a recently developed fluorimetric assay. PN-1 and ATIII were assayed both by immunohistochemistry and by immunoblots in the brain and spinal cord. Total brain thrombin inhibitory activity increased (32%) in EAE mice at the peak of clinical disease (Day 13, P=0.04 compared to controls). Brain ATIII also increased at the peak of disease (2.5-fold higher than controls, P=0.0001), and correlated significantly with clinical scores at all stages of disease (r=0.72, P=0.0068). In contrast, PN-1 elevations were more pronounced at the preclinical stage on Day 8 (3-fold higher than controls, P=0.01) than on Day 13 (1.4-fold higher, P=0.005). Increased brain thrombin inhibition at the clinical peak of EAE probably reflects increased influx of plasma thrombin inhibitors. Early PN-1 changes represent a potential target for thrombin modulating drugs in EAE and MS.

Two types of protease-activated receptors (PAR-1 and PAR-2) mediate calcium signaling in rat retinal ganglion cells RGC-5

Protease-activated receptors (PARs), G-protein-coupled receptors, are widely expressed in various tissues, where they participate in physiological and pathological processes, such as hemostasis, proliferation, tissue repair, and inflammation. Recently, we found that PARs were upregulated in the rat retina following optic nerve crush injury. However, the role of PAR in retinal ganglion cells following optic nerve crush still remains unknown. Here, we studied PAR-mediated calcium signaling in retinal ganglion cells, RGC-5. Using reverse transcription-polymerase chain reaction, we demonstrate that RGC-5 cells mainly express PAR-1 and to a lower extent PAR-2, which was further confirmed by indirect immunofluorescence. Short-term stimulation of RGC-5 cells with thrombin (0.001-1 U/ml) and trypsin (1-100 nM) concentration-dependently induced a transient increase in intracellular calcium concentration ([Ca(2+)](i)). An increase in [Ca(2+)](i) was also induced by both TRag (PAR-1 activating peptide) and PAR-2 activating peptide (PAR-2 AP). The EC(50) values were 0.3 nM for thrombin, 12.0 nM for trypsin, 1.3 microM for TRag, and 1.6 microM for PAR-2 AP, respectively. Desensitization was studied using two successive pulses of agonists. The thrombin-induced calcium response was significantly reduced by PAR-1 desensitization caused by pre-challenging RGC-5 cells with thrombin or TRag, but not by PAR-2 desensitization. On the other hand, pretreatment with trypsin, TRag or PAR-2 AP desensitized the cells since the calcium response to a second exposure to trypsin was significantly reduced. Calcium source studies revealed that PAR-induced [Ca(2+)](i) rise mainly comes from intracellular stores in RGC-5 cells. Thus, we demonstrate that PAR-1 and PAR-2 are functionally expressed in retinal ganglion cells, mediating calcium mobilization mainly from intracellular stores.

Activation of protease-activated receptor-1 triggers astrogliosis after brain injury

We have studied the involvement of the thrombin receptor [protease-activated receptor-1 (PAR-1)] in astrogliosis, because extravasation of PAR-1 activators, such as thrombin, into brain parenchyma can occur after blood-brain barrier breakdown in a number of CNS disorders. PAR1-/- animals show a reduced astrocytic response to cortical stab wound, suggesting that PAR-1 activation plays a key role in astrogliosis associated with glial scar formation after brain injury. This interpretation is supported by the finding that the selective activation of PAR-1 in vivo induces astrogliosis. The mechanisms by which PAR-1 stimulates glial proliferation appear to be related to the ability of PAR-1 receptor signaling to induce sustained extracellular receptor kinase (ERK) activation. In contrast to the transient activation of ERK by cytokines and growth factors, PAR-1 stimulation induces a sustained ERK activation through its coupling to multiple G-protein-linked signaling pathways, including Rho kinase. This sustained ERK activation appears to regulate astrocytic cyclin D1 levels and astrocyte proliferation in vitro and in vivo. We propose that this PAR-1-mediated mechanism underlying astrocyte proliferation will operate whenever there is sufficient injury-induced blood-brain barrier breakdown to allow extravasation of PAR-1 activators.

Protease-activated receptor 1: a role in prostate cancer metastasis

Protease-activated receptor (PAR) 1, PAR3, and PAR4 are considered "thrombin receptors" because thrombin specifically cleaves the extracellular N-termini of the receptor to unmask a new amino acid terminus, which in turn acts as a peptide ligand by binding intramolecularly to the body of the receptor. Among those 3 family members, PAR1 is the predominant thrombin receptor. Although the thrombin-mediated regulation of clot formation has been studied extensively over the past decades, the possible role of thrombin in tumor metastasis via PAR1 has only recently received attention and is briefly discussed herein.

Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response

Serine proteinases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteinases have been considered to act primarily as degradative enzymes in the extracellular space. However, their biological actions in tissues and cells suggest important roles as a part of the body's hormonal communication system during inflammation and immune response. These effects can be attributed to the activation of a new subfamily of G protein-coupled receptors, termed proteinase-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteinases act as signaling molecules that specifically regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and activate signal transduction pathways resulting in the rapid transcription of genes that are involved in inflammation. For example, PARs are widely expressed by cells involved in immune responses and inflammation, regulate endothelial-leukocyte interactions, and modulate the secretion of inflammatory mediators or neuropeptides. Together, the PAR family necessitates a paradigm shift in thinking about hormone action, to include proteinases as key modulators of biological function. Novel compounds that can modulate PAR function may be potent candidates for the treatment of inflammatory or immune diseases.

Correlation of Protease-Activated Receptor-1 With Differentiation Markers in Squamous Cell Carcinoma of the Head and Neck and Its Implication in Lymph Node Metastasis

PURPOSE

Protease-activated receptor-1 (PAR-1) is a G-protein-coupled receptor that contributes to multiple signal transduction pathways. Although the functions of PAR-1 in many normal cells, such as platelets and astrocytes, have been well studied, its roles in cancer progression and metastasis have not been fully elucidated, and studies to date appear contradictory.

EXPERIMENTAL DESIGN

To clarify the function of PAR-1 in metastasis of squamous cell carcinoma of the head and neck (SCCHN), we examined PAR-1 expression in clinical specimens by immunohistochemistry and in SCCHN cell lines by immunoblotting. Furthermore, par-1 cDNA-transfected SCCHN cell lines were also used to verify PAR-1-mediated pathway.

RESULTS

The metastatic tumors showed a lower percentage of PAR-1-positive cells (46%) and lower levels of PAR-1 expression (median weight index = 10) than node negative primary tumors (80% and median weight index = 60, respectively). In addition, expression level of PAR-1 positively correlated with levels of keratinocyte differentiation markers keratin-1, -10, and -11. Additional studies using sense and antisense par-1 cDNA-transfected SCCHN cell lines illustrated that the presence of PAR-1 was required for the expression of involucrin, a keratinocyte differentiation marker. PAR-1 expression also contributes to activation of the mitogen-activated protein kinase (MAPK) pathway. Blocking MAPK activation by a mitogen-activated protein/extracellular signal-regulated kinase inhibitor, not by a phosphatidylinositol 3'-kinase inhibitor, reduced level of involucrin, suggesting that regulation of involucrin by PAR-1 is partially through the MAPK signaling pathway.

CONCLUSIONS

Our study suggests that PAR-1 signaling induces differentiation markers in SCCHN cells, and its expression is conversely correlated with cervical lymph node metastasis.

Up-regulation of Flotillin-2 is associated with melanoma progression and modulates expression of the thrombin receptor protease activated receptor 1

Flotillin 2 (flot-2) is a highly conserved protein isolated from caveolae/lipid raft domains that tether growth factor receptors linked to signal transduction pathways. Flot-2 protein and mRNA were increased in tumorigenic and metastatic melanoma cell lines in vitro, and the immunostaining intensity increased substantially across a tissue array of melanocytic lesions. Flot-2 transfection transformed SB2 melanoma cells from nontumorigenic, nonmetastatic to highly tumorigenic and metastatic in a nude mouse xenograft model. SB2 cells stably transfected with the flot-2 cDNA (SB2-flot)-2 cells proliferated faster in the absence of serum, and their migration through Matrigel was additionally enhanced by thrombin. When SB2-flot-2 cells were compared with SB2-vector-control cells on a cancer gene pathway array, SB2-flot-2 cells had increased expression of protease activated receptor 1 (PAR-1) mRNA, a transmembrane, G-protein-coupled receptor involved in melanoma progression. PAR-1 and flot-2 were coimmunoprecipitated from SB2-flot-2 cells. Up-regulation of PAR-1 was additionally confirmed in SB2-flot-2 cells and melanoma cell lines. SB2-flot-2 cells transfected with flot-2-specific small-interfering RNAs made substantially less flot-2 and PAR-1 mRNA. In conclusion, flot-2 overexpression is associated with melanoma progression, with increased PAR-1 expression, and with transformation of SB2 melanoma cells to a highly metastatic line. Flot-2 binds to PAR-1, a known upstream mediator of major signal transduction pathways implicated in cell growth and metastasis, and may thereby influence tumor progression.

Thrombin and Its Protease-Activated Receptor-1 (PAR1) Participate in the Endothelial–Mesenchymal Transdifferentiation Process

The serine protease thrombin, independently of its participation in hemostasis and thrombosis, has been involved in tissue repair and remodeling, embryogenesis, angiogenesis, and development and progression of atherosclerosis. Many of these functions appear to be mediated by specific thrombin receptors, particularly the protease-activated receptor-1 (PAR1). In this study, we investigated whether both thrombin and PAR1 were present in the aortic wall of chicken embryos at days 11 and 12 of development. We found that PAR1 was limited to some cells of the intimal thickening and the inner media, whereas thrombin appeared distributed across the aortic wall. We also investigated whether PAR1 was present during endothelial-mesenchymal transdifferentiation (EMT) in vitro. A moderate immunoreactivity was detected in the monolayer of endothelial cells. In contrast, a strong cytoplasmic immunoreactivity was observed in the detaching and migrating cells and those that had acquired mesenchymal characteristics. This PAR1 expression was confirmed by flow cytometry. In this study, the addition of thrombin to arrested endothelial cell cultures was assessed. We found that thrombin stimulated endothelial cell spreading and migration, as no migrating cells were observed in serum-free medium (SFM) condition. Immunolocalization of PAR1 in the thrombin-treated cultures showed strong cytoplasmic immunoreactivity in the monolayers and in spreading and migrating cells, whereas in the SFM condition undetectable PAR1 immunoreactivity was observed. Flow cytometry of these cultures revealed an elevated expression of PAR1 in the presence of thrombin, in contrast to that detected in SFM and complete medium. These data indicate that both thrombin and PAR1 are involved in the remodeling of the aortic wall and intimal thickening formation, and in the endothelial-mesenchymal transdifferentiation process.

The thrombin peptide, TP508, enhances cytokine release and activates signaling events

The thrombin peptide, TP508, accelerates tissue repair and initiates a cascade of cellular events. We have previously shown that alpha-thrombin induces cytokine expression in human mononuclear cells. We, therefore, investigated the possibility that TP508 might activate cytokine production and intracellular signaling pathways associated with cytokine activation. Our results show that TP508 induces cytokine expression in human mononuclear cells. TP508 treatment enhances extracellular signal-regulated kinase (Erk1/2) activities in U937 cells, as well as Erk1/2 and p38 activation in Jurkat T cells. These data support the hypothesis that TP508 may accelerate tissue repair through the activation of the inflammatory response.

Protease nexin-1: a cellular serpin down-regulated by thrombin in rat aortic smooth muscle cells

Protease nexin-1 (PN-1), a potent inhibitor of serine proteases, is present in vascular cells and forms complexes with thrombin, plasminogen activators, and plasmin. We examined the effect of thrombin on PN-1 expression by rat aortic smooth muscle cells (RASMCs). PN-1 expression was determined by measuring protein and mRNA levels, using respectively immunoblotting and semi-quantitative reverse transcriptase polymerase chain reaction (PCR). Thrombin down-regulated PN-1 expression in a dose- and time-dependent manner. This effect was mediated via the interaction of thrombin with its receptor protease activated receptor (PAR-1) since the peptide thrombin receptor activating peptide (TRAP) reduced PN-1 expression. PN-1 secreted by smooth muscle cells remained essentially associated to cell-surface glycosaminoglycans and was released from the cell surface by heparin. A lower amount of PN-1 was released by heparin from TRAP-stimulated versus unstimulated cells and correlated with a decreased capacity to inhibit thrombin. In addition, the ability to generate peri-cellular plasmin was increased in cells with a low PN-1 expression. Pre-treatment of smooth muscle cells with cycloheximide abolished the reduction of PN-1 expression by thrombin. Furthermore, conditioned media from thrombin-treated cells reproduced the effect of thrombin, suggesting that thrombin acted via the induction of auto/paracrine mediator(s). We observed that fibroblast growth factor-2 (FGF-2)-neutralizing antibodies abolished thrombin effect whereas FGF-2 reproduced it, indicating that FGF-2 is one of the involved mediator. Together, these results indicate that (i) PN-1 modulates the activity of endogenous and exogenous serine proteases in RASMCs, (ii) thrombin down-regulates PN-1 expression and thus may increase its own activity on cells.

The microglia-activating potential of thrombin: the protease is not involved in the induction of proinflammatory cytokines and chemokines

The serine protease thrombin is known as a blood coagulation factor. Through limited cleavage of proteinase-activated receptors it can also control growth and functions in various cell types, including neurons, astrocytes, and microglia (brain macrophages). A number of previous studies indicated that thrombin induces the release of proinflammatory cytokines and chemokines from microglial cells, suggesting another important role for the protease beyond hemostasis. In the present report, we provide evidence that this effect is not mediated by any proteolytic or non-proteolytic mechanism involving thrombin proper. Inhibition of the enzymatic thrombin activity did not affect the microglial release response. Instead the cyto-/chemokine-inducing activity solely resided in a high molecular weight protein fraction that could be isolated in trace amounts even from apparently homogenous alpha- and gamma-thrombin preparations. High molecular weight material contained thrombin-derived peptides as revealed by mass spectrometry but was devoid of thrombin-like enzymatic activity. Separated from the high molecular weight fraction by fast protein liquid chromatography, enzymatically intact alpha- and gamma-thrombin failed to trigger any release. Our findings may force a revision of the notion that thrombin itself is a direct proinflammatory release signal for microglia. In addition, they could be relevant for the study of other cellular activities and their assignment to this protease.

alpha-thrombin rapidly induces tyrosine phosphorylation of a novel, 74-78-kDa stress response protein(s) in lung fibroblast cells

We demonstrated previously that exposure of CCL39 lung fibroblasts to alpha-thrombin rapidly inhibits interleukin 6-induced tyrosine phosphorylation of signal transducers and activators of transcription 3 (Stat3). While studying the cross-talk between alpha-thrombin and interleukin 6, we observed that the phospho-specific (tyrosine) anti-Stat3 antibody specifically cross-reacted with a 74-78-kDa protein(s) in alpha-thrombin-treated cells. In this study, we demonstrate that in alpha-thrombin-treated CCL39 cells, the 74-78-kDa protein(s) rapidly undergoes tyrosine phosphorylation. The phosphorylation by alpha-thrombin was detected as early as 5 min and reached a maximum at 15 min; however, low levels were present at 2 h. alpha-Thrombin receptor agonist peptide (SFLLRN) induced its tyrosine phosphorylation, suggesting that alpha-thrombin mediates the effects via protease-activated receptor type 1. Anti-Stat3 antibodies specific to different regions of Stat3 failed to recognize the 74-78-kDa protein(s), suggesting that it is unrelated to Stat3. Cell fractionation experiments showed that it is localized to the cytoplasm. Mass spectrometric analysis of the immunoprecipitated protein showed that the 74-78-kDa protein(s) is related to glucose-regulated protein 75 (GRP-75), a member of the heat shock/stress-response protein family. Consistent with these data, we observed tyrosine phosphorylation of GRP-75 in alpha-thrombin-treated cells. Exposure of cells to pervanadate, a stress-inducing agent, stimulated its tyrosine phosphorylation; however, cytokines and growth factors were ineffective. This is the first report of tyrosine phosphorylation of GRP-75-related stress protein(s) by alpha-thrombin and suggests that this pathway may contribute to the ability of alpha-thrombin to prevent apoptosis in cells exposed to stress or in the injured tissue.

PAR-1 deficiency protects against neuronal damage and neurologic deficits after unilateral cerebral hypoxia/ischemia

Cardiovascular and neurologic surgeries often involve a temporary reduction in cerebral blood flow. In these conditions, as well as during cerebral ischemia and traumatic brain injury, the temporary loss of oxygen and glucose initiates a cascade of cellular events that culminate in neuronal death and damage. Understanding the mechanisms that contribute to neuronal death after hypoxia/ischemia is critically important for treatment of such brain injury. Here, we use a model of combined cerebral hypoxia/ischemia (H/I) to examine the role of protease-activated receptor-1 (PAR-1) in hypoxic/ischemic neuronal damage. Our data show that PAR-1-deficient mice have smaller lesion volumes than wild-type controls after 45 minutes of H/I. The results of the genetic block of PAR-1 were corroborated using a PAR-1 antagonist, which decreased infarct volume in wild-type C57Bl6 mice. Examination of cellular responses to H/I reveals that PAR-1 -/- animals have less cellular death and diminished glial fibrillary acidic protein expression. Additionally, PAR-1 -/- mice exhibit less motor behavior impairment in rotorod and inverted wire-hang tests. These data suggest that PAR-1 contributes to hypoxic/ischemic brain injury and are consistent with other studies that implicate serine proteases and their receptors in neuropathology after cerebral insults.

Role of CL-100, a dual specificity phosphatase, in thrombin-induced endothelial cell activation

Using a cDNA microarray screening approach, we have identified seven novel thrombin-responsive genes in human umbilical vein endothelial cells that were verifiable by Northern blot analysis. Among them CL-100, a dual-specificity phosphatase also known as MAP kinase phosphatase-1 (MKP-1), showed greatest induction by thrombin. Steady-state levels of CL-100 mRNA induction by thrombin peaked at 1 h and declined rapidly (t1/2 approximately 45 min). Induction by thrombin was protease-activated receptor-1-mediated, protein synthesis-independent, and transcriptionally regulated. Metabolic labeling followed by immunoprecipitation verified that the thrombin-induced CL-100 mRNA was translated into protein. We found that both Src-kinase and p42/p44 ERK activity are critical for thrombin-induced CL-100 expression, whereas phosphatidylinositol 3-kinase and protein kinase C activity were not required. Antisense-mediated inhibition of CL-100 was shown to prolong thrombin-induced ERK activity in endothelial cells, concomitant with an inhibition in thrombin-induced PDGF-A (platelet-derived growth factor A) and PDGF-B gene expression and an up-regulation in thrombin-induced VCAM-1 and E-selectin gene expression. Inhibition of ERK activation by PD98059 in endothelial cells was shown to potentiate thrombin-induced expression of PDGF-B (approximately 3-fold) while inhibiting thrombin-induced VCAM-1 and E-selectin gene expression by 60 and 70%, respectively. These results suggested that induced expression of the CL-100 phosphatase and its subsequent regulation of ERK activity play a key regulatory role in the thrombin signaling pathway and in the transcriptional regulation of pathologically important "endothelial cell activation genes."

Thrombin reduces MuSK and acetylcholine receptor expression along with neuromuscular contact size in vitro

In the course of studies on thrombin and its inhibitor(s) in synaptic plasticity, we addressed the question of their roles in the formation of neuromuscular junctions (NMJ) and used a model of rat neuron-myotube cocultures. We report that the size of acetylcholinesterase (AChE) patches used as a marker of neuromuscular contacts was decreased in the presence of either thrombin or SFLLRN, the agonist peptide of the thrombin receptor PAR-1, whereas it was increased with hirudin, a specific thrombin inhibitor. In an attempt to relate these neuromuscular contact size variations to molecular changes, we studied muscle-specific tyrosine kinase receptor (MuSK), acetylcholine receptor (AChR) and rapsyn expression in the presence of thrombin. We showed that thrombin did not change rapsyn gene and protein expression. However, the expression of MuSK and surface AChR proteins was diminished in both myotube cultures and neuron-myotube cocultures. These reductions in protein expression were associated with a decrease in MuSK and AChR alpha-subunit gene expression in myotube cultures but not in neuron-myotube cocultures. Moreover, the expression of the AChR epsilon-subunit gene, specifically enhanced by neuron-released factors, was not modified by thrombin in neuron-myotube cocultures. This suggests that thrombin did not affect the expression of synaptic AChRs enhanced by neuron-released factors but rather reduced the level of extrasynaptic AChRs. Taken together, these results indicate that thrombin in balance with its inhibitor(s) could modulate the formation of neuromuscular contacts in vitro by affecting the expression of two essential molecules in NMJ postsynaptic differentiation, MuSK and AChR.

Design and Synthesis of Novel Biologically Active Thrombin Receptor Non-Peptide Mimetics Based on the Pharmacophoric Cluster Phe/Arg/NH2 of the Ser42-Phe-Leu-Leu-Arg46 Motif Sequence: Platelet Aggregation and Relaxant Activities

The identification of the thrombin receptor has promoted the interest for the development of new therapeutic agents capable of selectively inhibiting unwanted biological effects of thrombin on various cell types. In this study we have designed and synthesized two series of new thrombin receptor antagonists based on the thrombin receptor motif sequence S42FLLR46, one possessing two (Phe/Arg) pharmacophoric groups and the other possessing three (Phe/Arg/NH2). N-(6-Guanidohexanoyl)-N'-(phenylacetyl)piperazine (1), N-(phenylacetyl)-4-(6-guanidohexanoylamidomethyl)piperidine (2), and N-(phenylacetyl)-3-(6-guanidohexanoylamido)pyrrolidine (3) (group A) carry the two pharmacophoric side chains of Phe and Arg residues incorporated on three different templates (piperazine, 4-aminomethylpiperidine, and 3-aminopyrrolidine). Compounds with three pharmacophoric groups (group B) were built similarly to group A using the same templates with the addition of an extra methylamino group leading to (S)-N-(6-guanidohexanoyl)-N'-(2-amino-3-phenylpropionyl)piperazine (4), (S)-N-(2-amino-3-phenylpropionyl)-4-(6-guanidohexanoylamidomethyl)piperidine (5), and (S)-N-(2-amino-3-phenylpropionyl)-3-(6-guanidohexanoylamido)pyrrolidine (6). Compounds were able to inhibit thrombin-induced human platelet activation even at low concentrations. In particular, among compounds in group A, compound 3 was found to be the most powerful thrombin receptor activation inhibitor, showing an IC50 of approximately 0.11 mM on platelet aggregation assay. Among compounds in group B, compound 4 was the most powerful to inhibit thrombin-induced platelet aggregation, showing an IC50 of approximately 0.09 mM. All compounds were also found to act as agonists in the rat aorta relaxation assay. Interestingly, the order of potency of these compounds as agonists of the endothelial thrombin receptor was the inverse of the order of potency of the same compounds as antagonists of the platelet thrombin receptor. Such compounds that are causing vasodilation while simultaneously inhibiting platelet aggregation would be very useful in preventing the installation of atherosclerotic lesions and deserve further investigation as potential drugs for treating cardiovascular diseases. The above findings coupled with computational analysis molecular dynamics experiments support also our hypothesis that a cluster of phenyl, guanidino, and amino groups is responsible for thrombin receptor triggering and activation.

Macrophage Migration Inhibitory Factor Is Induced by Thrombin and Factor Xa in Endothelial Cells

Macrophage migration inhibitory factor (MIF), a proinflammatory cytokine, has been shown to play a role in wound-healing processes. In this study, we investigated whether protease-activated receptor (PAR)-1 and PAR-2 mediated MIF expression in human endothelial cells. Thrombin, factor Xa (FXa), and trypsin induced MIF expression in human dermal microvascular endothelial cells and human umbilical vein endothelial cells, but other proteases, including kallikrein and urokinase, failed to do so. Thrombin-induced MIF mRNA expression was significantly reduced by the thrombin-specific inhibitor hirudin. Thrombin receptor activation peptide-6, a synthetic PAR-1 peptide, induced MIF mRNA expression, suggesting that PAR-1 mediates MIF expression in response to thrombin. The effects of FXa were blocked by antithrombin III, but not by hirudin, indicating that FXa might enhance MIF production directly rather than via thrombin stimulation. The synthetic PAR-2 peptide SLIGRL-NH(2) induced MIF mRNA expression, showing that PAR-2 mediated MIF expression in response to FXa. Concerning the signal transduction, a mitogen-activated protein kinase kinase inhibitor (PD98089) and a nuclear factor (NF)-kappaB inhibitor (SN50) suppressed the up-regulation of MIF mRNA in response to thrombin, FXa, and PAR-2 agonist stimulation, whereas a p38 inhibitor (SB203580) had little effect. These facts indicate that up-regulation of MIF by thrombin or FXa is regulated by p44/p42 mitogen-activated protein kinase-dependent pathways and NF-kappaB-dependent pathways. Moreover, we found that PAR-1 and PAR-2 mRNA expression in endothelial cells was enhanced by MIF. Furthermore, we examined the inflammatory response induced by PAR-1 and PAR-2 agonists injected into the mouse footpad. As shown by footpad thickness, an indicator of inflammation, MIF-deficient mice (C57BL/6) were much less sensitive to either PAR-1 or PAR-2 agonists than wild-type mice. Taken together, these results suggest that MIF contributes to the inflammatory phase of the wound healing process in concert with thrombin and FXa via PAR-1 and PAR-2.

Protease-activated receptors: contribution to physiology and disease

Proteases acting at the surface of cells generate and destroy receptor agonists and activate and inactivate receptors, thereby making a vitally important contribution to signal transduction. Certain serine proteases that derive from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast cell and neutrophil proteases), and from multiple other sources (e.g., epithelial cells, neurons, bacteria, fungi) can cleave protease-activated receptors (PARs), a family of four G protein-coupled receptors. Cleavage within the extracellular amino terminus exposes a tethered ligand domain, which binds to and activates the receptors to initiate multiple signaling cascades. Despite this irreversible mechanism of activation, signaling by PARs is efficiently terminated by receptor desensitization (receptor phosphorylation and uncoupling from G proteins) and downregulation (receptor degradation by cell-surface and lysosomal proteases). Protease signaling in tissues depends on the generation and release of proteases, availability of cofactors, presence of protease inhibitors, and activation and inactivation of PARs. Many proteases that activate PARs are produced during tissue damage, and PARs make important contributions to tissue responses to injury, including hemostasis, repair, cell survival, inflammation, and pain. Drugs that mimic or interfere with these processes are attractive therapies: selective agonists of PARs may facilitate healing, repair, and protection, whereas protease inhibitors and PAR antagonists can impede exacerbated inflammation and pain. Major future challenges will be to understand the role of proteases and PARs in physiological control mechanisms and human diseases and to develop selective agonists and antagonists that can be used to probe function and treat disease.

Selected Haemostatic Factors in Carotid Bifurcation Plaques of Patients Undergoing Carotid Endarterectomy

OBJECTIVE

To determine the concentration of selected haemostatic factors (HFs): thrombin-antithrombin complexes (TAT), antithrombin (AT), tissue plasminogen activator (t-PA), plasminogen activator inhibitor type 1 (PAI-1) and D-dimers in carotid bifurcation plaques and to compare plaque composition in different subgroups of patients (mainly those with symptomatic and asymptomatic carotid stenosis).

MATERIALS AND METHODS

Thirty-eight consecutive patients (20 symptomatic, 18 asymptomatic) undergoing carotid endarterectomy were enrolled in the study. The concentration of selected HFs in carotid plaques was measured using mainly enzyme immunoassay (ELISA). Simultaneously, the concentration of HFs in plasma was also obtained.

RESULTS

Symptomatic plaques contained significantly more TAT complexes (p=0.03). AT was found only in nine out of 38 carotid plaques and was present mainly in symptomatic carotid plaques (n=8/9)(p<0.006). No significant differences were found between symptomatic and asymptomatic carotid plaques with respect to t-PA, PAI-1 and D-dimers concentration. There was an increased concentration of TAT (p<0.001), t-PA (p<0.02) and D-dimers (p<0.02) in carotid plaques of diabetic patients. Patients with coexisting intermittent claudication had elevated levels of D-dimers in carotid plaques (p<0.02). The only positive correlation was demonstrated between the concentration of AT in plasma and carotid plaques (R=0.76; p=0.02).

CONCLUSIONS

All the evaluated HFs are the components of a carotid plaque. Symptomatic patients have increased concentration of TAT complexes in a carotid plaque. The symptomatic carotid plaque contains AT more frequently, which correlates positively with AT plasma levels. The most marked changes in the carotid plaque haemostatic composition (expressed by elevated levels of TAT, t-PA and D-dimers) have diabetic patients.

Increased Thrombin Activity after Allergen Challenge

In addition to its central role in hemostasis, thrombin may play a role in inflammation and remodeling. To investigate the contribution of thrombin to allergic airway inflammation in asthma, we used an enzymatic assay to determine thrombin activity in bronchoalveolar lavage fluid obtained from 19 subjects with atopic asthma before (Day 0) and 48 hours after (Day 2) segmental bronchoprovocation with antigen. Thrombin activity increased from 0 (0, 2.9) on Day 1 to 41.1 (0.3, 75.6) U x 10(-3)/ml on Day 2 (p = 0.002) and correlated with total protein levels in lavage fluid on Day 2 (r = 0.885, p < 0.001). After antigen challenge, thrombin activity also showed significant correlations with interleukin-5 (r = 0.66, p = 0.002), transforming growth factor beta1 (r = 0.70, p < 0.001), fibronectin (r = 0.85, p < 0.001) and tissue factor (r = 0.55, p = 0.03) levels in lavage fluid. Furthermore, Day 2, but not Day 0 lavage fluid, induced proliferation of human airway fibroblasts. This mitogenic effect was significantly reduced with hirudin, a specific thrombin inhibitor. Taken together, our findings suggest that allergen-driven airway inflammation in asthma is associated with enhanced potential for fibroblast proliferation that is related, at least in part, to increased thrombin activity. We propose that enhanced thrombin activity provides a potential link between allergic inflammation and initiation of airway remodeling.

Thrombin and phenotypic modulation of the endothelium

Thrombin signaling in the endothelium is linked to multiple phenotypic changes, including alterations in permeability, vasomotor tone, and leukocyte trafficking. The thrombin signal is transduced, at least in part, at the level of gene transcription. In this review, we focus on the role of thrombin signaling and transcriptional networks in mediating downstream gene expression and endothelial phenotype. In addition, we report the results of DNA microarrays in control and thrombin-treated endothelial cells. We conclude that (1) thrombin induces the upregulation and downregulation of multiple genes in the endothelium, (2) thrombin-mediated gene expression involves a multitude of transcription factors, and (3) future breakthroughs in the field will depend on a better understanding of the spatial and temporal dynamics of these transcriptional networks.

Chondroitin sulphate proteoglycans: preventing plasticity or protecting the CNS?

It is well established that axonal regeneration in the adult CNS is largely unsuccessful. Numerous axon-inhibitory molecules are now known to be present in the injured CNS, and various strategies for overcoming these obstacles and enhancing CNS regeneration have been experimentally developed. Recently, the use of chondroitinase-ABC to treat models of CNS injury in vivo has proven to be highly beneficial towards regenerating axons, by degrading the axon-inhibitory chondroitin sulphate glycosaminoglycan chains found on many proteoglycans in the astroglial scar. This enzyme has now been shown to restore synaptic plasticity in the visual cortex of adult rats by disrupting perineuronal nets, which contain high levels of chondroitin sulphate proteoglycans (CS-PGs) and are expressed postnatally around groups of certain neurons in the normal CNS. The findings suggest exciting prospects for enhancing growth and plasticity in the adult CNS; however, some protective roles of CS-PGs in the CNS have also been demonstrated. Clearly many questions concerning the mechanisms regulating expression of extracellular matrix molecules in CNS pathology remain to be answered.

Alpha-thrombin-mediated phosphatidylinositol 3-kinase activation through release of Gbetagamma dimers from Galphaq and Galphai2

Chinese hamster embryonic fibroblasts (IIC9 cells) express the Galpha subunits Galphas, Galphai2, Galphai3, Galphao, Galpha(q/11), and Galpha13. Consistent with reports in other cell types, alpha-thrombin stimulates a subset of the expressed G proteins in IIC9 cells, namely Gi2, G13, and Gq as measured by an in vitro membrane [35S]guanosine 5'-O-(3-thio)triphosphate binding assay. Using specific Galpha peptides, which block coupling of G-protein receptors to selective G proteins, as well as dominant negative xanthine nucleotide-binding Galpha mutants, we show that activation of the phosphatidylinositol 3-kinase/Akt pathway is dependent on Gq and Gi2. To examine the role of the two G proteins, we examined the events upstream of PI 3-kinase. The activation of the PI 3-kinase/Akt pathway by alpha-thrombin in IIC9 cells is blocked by the expression of dominant negative Ras and beta-arrestin1 (Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2000) J. Biol. Chem. 275, 18046-18053, and Goel, R., Phillips-Mason, P. J., Raben, D. M., and Baldassare, J. J. (2002) J. Biol. Chem. 277, 18640-18648), indicating a role for Ras and beta-arrestin1. Interestingly, inhibition of Gi2 and Gq activation blocks Ras activation and beta-arrestin1 membrane translocation, respectively. Furthermore, expression of the Gbetagamma sequestrant, alpha-transducin, inhibits both Ras activation and membrane translocation of beta-arrestin1, suggesting that Gbetagamma dimers from Galphai2 and Galphaq activate different effectors to coordinately regulate the PI 3-kinase/Akt pathway.

Loss of activator protein-2alpha results in overexpression of protease-activated receptor-1 and correlates with the malignant phenotype of human melanoma

Increasing evidence implicates the protease-activated receptor-1 (PAR-1) as a contributor to tumor invasion and metastasis of human melanoma. Here we demonstrate that the metastatic potential of human melanoma cells correlates with overexpression of PAR-1. We also provide evidence that an inverse correlation exists between the expression of activator protein-2alpha (AP-2) and the expression of PAR-1 in human melanoma cells. Reexpression of AP-2 in WM266-4 melanoma cells, which are AP-2-negative, resulted in decreased mRNA and protein expression of PAR-1. The promoter of the PAR-1 gene contains multiple putative consensus elements for the transcription factors AP-2 and specificity protein 1 (Sp1). Chromatin immunoprecipitation analysis of the PAR-1 promoter regions bp -365 to -329 (complex 1) and bp -206 to -180 (complex 2) demonstrated that Sp1 was predominantly bound to the PAR-1 promoter in metastatic cells, whereas AP-2 was bound to the PAR-1 promoter in nonmetastatic cells. In vitro analysis of complex 1 demonstrated that AP-2 and Sp1 bound to this region in a mutually exclusive manner. Transfection experiments with full-length and progressive deletions of the PAR-1 promoter luciferase constructs demonstrated that metastatic melanoma cells had increased PAR-1 promoter activity compared with low and nonmetastatic melanoma cells. Our data show that exogenous AP-2 expression decreased promoter activity, whereas transient expression of Sp1 further increased expression of the reporter gene. Mutational analysis of complex 1 within PAR-1 luciferase constructs further demonstrated that the regulation of PAR-1 was mediated through interactions with AP-2 and Sp1. Our data suggest that loss of AP-2 in metastatic cells alters the AP-2/Sp1 ratio, resulting in overexpression of PAR-1. Taken together, our results provide strong evidence that loss of AP-2 correlates with overexpression of PAR-1, which in turn contributes to the acquisition of the malignant phenotype of human melanoma.

Mitogen-Activated Protein Kinases in Endothelial Pathophysiology

Endothelial cells continuously respond to extracellular stimuli such as chemical signals produced by circulating blood elements or mechanical forces such as shear stress. Proinflammatory cytokines, mitogens, reactive oxygen species, and shear stress trigger signal molecules to initiate multiple intracellular pathways, which often converge at mitogen-activated protein (MAP) kinase activation. The MAP kinase superfamily represents a burgeoning area of clinical investigation for treatment of various inflammatory and oncologic diseases and plays an essential role in mediating response to infection, ischemia/reperfusion injury, and vessel healing and remodeling through regulation of such diverse phenomena as endothelial cell proliferation, migration, apoptosis, and endothelial barrier function. The downstream effects of MAP kinase activation include modulation of gene expression via up-regulation of various transcription factors. In addition to these sustained effects, MAP kinases coordinate more immediate responses that affect dynamic cytoskeletal rearrangements necessary for cell migration and regulation of barrier function. This review discusses the important regulatory roles of MAP kinases in the vital physiologic functions of endothelium, focusing mainly on the role of MAP kinases in the maintenance of endothelial barrier.

Inhibition of interleukin-12 expression by alpha-thrombin in human peripheral blood mononuclear cells: a potential mechanism for modulating Th1/Th2 responses

In addition to its central role in blood coagulation and hemostasis, human alpha-thrombin is a powerful regulator of inflammatory responses and is known to affect cell-mediated immunity. Interleukin (IL)-12 is a strong promoter of the development of Th1-type lymphocytes and its downregulation implies a positive feedback mechanism for development of Th2 responses. We have previously shown that thrombin enhances the release of IL-6, a Th2-related cytokine, in human peripheral blood mononuclear cells (PBMC). Here we show that thrombin downregulates IL-12 production at both protein and mRNA levels in human PBMC. The inhibition of IL-12 production was accompanied by an enhanced release of IL-10, which inhibits Th1-related processes and promotes Th2-type responses. The use of proteolytically inactive thrombin and of the specific thrombin receptor agonist peptide, SFLLRN, reveals that this downregulation is thrombin-specific and requires thrombin proteolytic activity. In addition, activation of coagulation inhibits IL-12 production in whole blood cultures, confirming the tight relationship between the coagulation pathway, where thrombin is a key enzyme, and inflammation. Decreased IL-12 production appears to be related also to IL-10 production, since the addition of an anti-IL-10 monoclonal antibody to thrombin-treated PBMC resulted in a partial restoration of IL-12 production. In conclusion, the observation that thrombin significantly affects the production of IL-12, as well as of IL-10, implies a concerted role orchestrated by thrombin in PBMC that could be crucial to effective immunity and inflammation.

Atorvastatin limits the pro-inflammatory response of rat aortic smooth muscle cells to thrombin

Thrombin, a serine protease, plays an important role in the progression of atherosclerosis. How atorvastatin could limit the pro-inflammatory response to thrombin was studied in cultured rat aortic smooth muscle cells. The variations in expression of interleukin-6, heme oxygenase-1, p(22phox) and Mox-1 mRNAs were evaluated by reverse transcriptase-polymerase chain reaction (RT-PCR). Interleukin-6 release was determined using the B9 cell assay. Nuclear factor-kappa B (NF-kappaB) translocation was analysed by electrophoretic mobility shift assay (EMSA) and RhoA protein translocation by Western blot. Thrombin activated interleukin-6 secretion and mRNA expression in smooth muscle cells in a dose-dependent manner. The greatest effect on mRNA expression was obtained after 1 h of stimulation. Preincubation (72 h) of the cells with various concentrations of atorvastatin prevented this effect. Simultaneous addition of mevalonate overcame this statin effect. Thrombin was without effect on p(22phox) and heme oxygenase-1 mRNA expression but, after 3 h of stimulation, induced a two-fold increase in that of Mox-1. Preincubation with atorvastatin dose-dependently downregulated this Mox-1 mRNA expression. In addition, thrombin induced NF-kappaB translocation and membrane translocation of RhoA in smooth muscle cells which were both prevented by pre-treatment of the cells by atorvastatin. These data demonstrate the ability of atorvastatin to prevent the induction by thrombin of a pro-inflammatory phenotype in smooth muscle cells.

Thrombin downregulates muscle acetylcholine receptors via an IP3 signaling pathway by activating its G-protein-coupled protease-activated receptor-1

Regulation of thrombin activity may be required during skeletal muscle differentiation since the thrombin tissue inhibitor protease nexin-1 appears at the myotube stage before being localized at the neuromuscular synapse. Here, we have used a model of rat fetal myotube primary cultures to study the effect of thrombin on acetylcholine receptor (AChR) expression, which is enhanced at the myotube stage. Our results show that thrombin decreases both the number of surface AChRs (AChRn) and AChR alpha-subunit gene expression. Using the agonist peptide SFLLRN, we establish that the AChRn decrease is mediated by the G protein-coupled thrombin receptor "protease-activated receptor-1" (PAR-1). Moreover, the specific thrombin inhibitor hirudin increases AChRn by inhibiting the thrombin intrinsically present in the cultures. We further demonstrate that the activation of PAR-1 by thrombin induces intracellular calcium movements that are blocked by 2-APB, an inhibitor of inositol 1,4,5-triphosphate (IP3)-induced calcium release. These calcium signals are more intense in nuclei than in the cytoplasm and are consistent with the intracellular distribution of IP3 receptor that we find in the cytoplasm in a cross-striated pattern and at a high level in the nuclear envelope zone. Finally, we show that the blockade of these IP3-induced calcium signals by 2-APB prevents the AChRn decrease induced by thrombin. Our results thus demonstrate that thrombin downregulates AChR expression by activating PAR-1 and that this effect is mediated via an IP3 signaling pathway.