Thrombin and PAR-1 activating peptide increase iNOS expression in cytokine-stimulated C6 glioma cells

A Novel Compound Targeting Protease Receptor 1 Activators for the Treatment of Glioblastoma

Data from human biopsies, in-vitro and in-vivo models, strongly supports the role of thrombin, and its protease-activated receptor (PAR1) in the pathology and progression of glioblastoma (GBM), a high-grade glial tumor. Activation of PAR1 by thrombin stimulates vasogenic edema, tumor adhesion and tumor growth. We here present a novel six amino acid chloromethyl-ketone compound (SIXAC) which specifically inhibits PAR1 proteolytic activation and counteracts the over-activation of PAR1 by tumor generated thrombin. SIXAC effects were demonstrated in-vitro utilizing 3 cell-lines, including the highly malignant CNS-1 cell-line which was also used as a model for GBM in-vivo. The in-vitro effects of SIXAC on proliferation rate, invasion and thrombin activity were measured by XTT, wound healing, colony formation and fluorescent assays, respectively. The effect of SIXAC on GBM in-vivo was assessed by measuring tumor and edema size as quantified by MRI imaging, by survival follow-up and brain histopathology. SIXAC was found in-vitro to inhibit thrombin-activity generated by CNS-1 cells (IC50 = 5 × 10-11M) and significantly decrease proliferation rate (p < 0.03) invasion (p = 0.02) and colony formation (p = 0.03) of these cells. In the CNS-1 GBM rat animal model SIXAC was found to reduce edema volume ratio (8.8 ± 1.9 vs. 4.9 ± 1, p < 0.04) and increase median survival (16 vs. 18.5 days, p < 0.02 by Log rank Mental-Cox test). These results strengthen the important role of thrombin/PAR1 pathway in glioblastoma progression and suggest SIXAC as a novel therapeutic tool for this fatal disease.

NOS Expression and NO Function in Glioma and Implications for Patient Therapies

SIGNIFICANCE

Gliomas are central nervous system tumors that primarily occur in the brain and arise from glial cells. Gliomas include the most common malignant brain tumor in adults known as grade IV astrocytoma, or glioblastoma (GBM). GBM is a deadly disease for which the most significant advances in treatment offer an improvement in survival of only ∼2 months.

CRITICAL ISSUES

To develop novel treatments and improve patient outcomes, we and others have sought to determine the role of molecular signals in gliomas. Recent Advances: One signaling molecule that mediates important biologies in glioma is the free radical nitric oxide (NO). In glioma cells and the tumor microenvironment, NO is produced by three isoforms of nitric oxide synthase (NOS), NOS1, NOS2, and NOS3. NO and NOS affect glioma growth, invasion, angiogenesis, immunosuppression, differentiation state, and therapeutic resistance.

FUTURE DIRECTIONS

These multifaceted effects of NO and NOS on gliomas both in vitro and in vivo suggest the potential of modulating the pathway for antiglioma patient therapies. Antioxid. Redox Signal. 26, 986-999.

TSPO-ligands prevent oxidative damage and inflammatory response in C6 glioma cells by neurosteroid synthesis

Translocator protein 18kDa (TSPO) is predominantly located in the mitochondrial outer membrane, playing an important role in steroidogenesis, inflammation, cell survival and proliferation. Its expression in central nervous system, mainly in glial cells, has been found to be upregulated in neuropathology, and brain injury. In this study, we investigated the anti-oxidative and anti-inflammatory effects of a group of TSPO ligands from the N,N-dialkyl-2-phenylindol-3-ylglyoxylamide class (PIGAs), highlighting the involvement of neurosteroids in their pharmacological effects. To this aim we used a well-known in vitro model of neurosteroidogenesis: the astrocytic C6 glioma cell line, where TSPO expression and localization, as well as cell response to TSPO ligand treatment, have been established. All PIGAs reduced l-buthionine-(S,R)-sulfoximine (BSO)-driven cell cytotoxicity and lipid peroxidation. Moreover, an anti-inflammatory effect was observed due to the reduction of inducible nitric oxide synthase and cyclooxygenase-2 induction in LPS/IFNγ challenged cells. Both effects were blunted by aminoglutethimide (AMG), an inhibitor of pregnenolone synthesis, suggesting neurosteroids' involvement in PIGA protective mechanism. Finally, pregnenolone evaluation in PIGA exposed cells revealed an increase in its synthesis, which was prevented by AMG pre-treatment. These findings indicate that these TSPO ligands reduce oxidative stress and pro-inflammatory enzymes in glial cells through the de novo synthesis of neurosteroids, suggesting that these compounds could be potential new therapeutic tools for the treatment of inflammatory-based neuropathologies with beneficial effects possibly comparable to steroids, but potentially avoiding the negative side effects of long-term therapies with steroid hormones.

The Importance of Thrombin in Cerebral Injury and Disease

There is increasing evidence that prothrombin and its active derivative thrombin are expressed locally in the central nervous system. So far, little is known about the physiological and pathophysiological functions exerted by thrombin in the human brain. Extra-hepatic prothrombin expression has been identified in neuronal cells and astrocytes via mRNA measurement. The actual amount of brain derived prothrombin is expected to be 1% or less compared to that in the liver. The role in brain injury depends upon its concentration, as higher amounts cause neuroinflammation and apoptosis, while lower concentrations might even be cytoprotective. Its involvement in numerous diseases like Alzheimer’s, multiple sclerosis, cerebral ischemia and haemorrhage is becoming increasingly clear. This review focuses on elucidation of the cerebral thrombin expression, local generation and its role in injury and disease of the central nervous system.

Thrombin decreases expression of the glutamate transporter GLAST and inhibits glutamate uptake in primary cortical astrocytes via the Rho kinase pathway

Astrocyte glutamate transporters GLAST and GLT1 play a key role in regulating neuronal excitation and their levels are altered in patients with epilepsy, and after traumatic brain injury. The mechanisms which regulate their expression are not well understood. We tested the hypothesis that exposure of astrocytes to high levels of thrombin, as may occur after a compromise of the blood-brain barrier, would reduce astrocyte glutamate transporter levels. In isolated rat cortical astrocytes we examined the effects of thrombin on the expression and function of glutamate transporters, and the signaling pathways involved in these responses by using Western blotting and selective inhibitors. Thrombin induced a selective decrease in the expression of GLAST but not GLT1, with a corresponding decrease in the capacity of astrocytes to take up glutamate. Activation of the thrombin receptor PAR-1 with an activating peptide induced a similar decrease in the expression of GLAST and compromise of glutamate uptake. The downregulation of GLAST induced by thrombin was mediated by the mitogen activated protein kinases p38 MAPK, ERK and JNK, but inhibition of these kinases did not prevent the decrease in glutamate uptake induced by thrombin. In contrast, inhibition of the Rho kinase pathway using the specific inhibitor, Y27632, suppressed both the decrease in the expression of GLAST and the decrease in glutamate uptake induced by thrombin. In hippocampal astrocyte cultures, thrombin caused a decrease in both GLAST and GLT1. In tissue resected from brains of children with intractable epilepsy, we found a decrease in the integrity of the blood-brain barrier along with a reduction in immunoreactivity for both transporters which was associated with an increase in cleaved thrombin and reactive astrogliosis. The in vitro results suggest a specific mechanism by which thrombin may lead to a compromise of astrocyte function and enhanced synaptic excitability after the blood-brain barrier is compromised. The human in vivo results provide indirect support evidence linking the compromise of the blood-brain barrier to thrombin-induced reduction in glutamate transporter expression and an increase in neuronal excitation.

Platelets and protease-activated receptor-4 contribute to acetaminophen-induced liver injury in mice

Acetaminophen (APAP)-induced liver injury in humans is associated with robust coagulation cascade activation and thrombocytopenia. However, it is not known whether coagulation-driven platelet activation participates in APAP hepatotoxicity. Here, we found that APAP overdose in mice caused liver damage accompanied by significant thrombocytopenia and accumulation of platelets in the liver. These changes were attenuated by administration of the direct thrombin inhibitor lepirudin. Platelet depletion with an anti-CD41 antibody also significantly reduced APAP-mediated liver injury and thrombin generation, indicated by the concentration of thrombin-antithrombin (TAT) complexes in plasma. Compared with APAP-treated wild-type mice, biomarkers of hepatocellular and endothelial damage, plasma TAT concentration, and hepatic platelet accumulation were reduced in mice lacking protease-activated receptor (PAR)-4, which mediates thrombin signaling in mouse platelets. However, selective hematopoietic cell PAR-4 deficiency did not affect APAP-induced liver injury or plasma TAT levels. These results suggest that interconnections between coagulation and hepatic platelet accumulation promote APAP-induced liver injury, independent of platelet PAR-4 signaling. Moreover, the results highlight a potential contribution of nonhematopoietic cell PAR-4 signaling to APAP hepatotoxicity.

Thrombin Activity and Thrombin Receptor in Rat Glioblastoma Model: Possible Markers and Targets for Intervention?

High-grade gliomas constitute a group of aggressive CNS cancers that have high morbidity and mortality rates. Despite extensive research, current therapeutic approaches enable survival beyond 2 years in rare cases only. Thrombin and its main CNS target, protease-activated receptor-1, have been implicated in tumor progression and brain edema. Our aim was to study protease-activated receptor-1 (PAR-1) protein expression and thrombin-like activity levels in both in vitro and in vivo models of glioblastoma and correlate them with the volume of the surrounding edema. We measured the presence of PAR-1 protein using fluorescence immunohistochemistry and assessed thrombin activity in various glial and non-glial cell lines and in a CNS-1 glioma rat model using a thrombin-specific fluorescent assay. Thrombin activity was found to be highly elevated in various high-grade glioma cell lines as well as in non-glial malignant cell lines. In the CNS-1 glioma model, the level of PAR-1 fluorescence in the tumor was significantly elevated compared to adjacent regions of reactive gliosis or distant brain areas. The elevated level of thrombin activity observed in the high-grade glioma positively correlated with tumor-induced brain edema. In conclusion, thrombin is secreted from glioma cells and PAR-1 may be a new biological marker for high-grade gliomas.

New bioactive alkyl sulfates from Mediterranean tunicates

Chemical investigation of two species of marine ascidians, Aplidium elegans and Ciona edwardsii, collected in Mediterranean area, led to isolation of a series of alkyl sulfates (compounds 1-5) including three new molecules 1-3. Structures of the new metabolites have been elucidated by spectroscopic analysis. Based on previously reported cytotoxic activity of these type of molecules, compounds 1-3 have been tested for their effects on the growth of two cell lines, J774A.1 (BALB/c murine macrophages) and C6 (rat glioma) in vitro. Compounds 1 and 2 induced selective concentration-dependent mortality on J774A.1 cells.

Essential role of mitogen-activated protein kinase pathways in protease activated receptor 2-mediated nitric-oxide production from rat primary astrocytes

Protease-activated receptors (PARs) play important roles in the regulation of brain function such as neuroinflammation by transmitting the signal from proteolytic enzymes such as thrombin and trypsin. We and others have reported that a member of the family, PAR-2 is activated by trypsin, whose involvement in the neurophysiological process is increasingly evident, and is involved in the neuroinflammatory processes including morphological changes of astrocytes. In this study, we investigated the role of PAR-2 in the production of nitric oxide (NO) in rat primary astrocytes. Treatment of PAR-2 agonist trypsin increased NO production in a dose-dependent manner, which was mediated by the induction of inducible nitric-oxide synthase. The trypsin-mediated production of NO was mimicked by PAR-2 agonist peptide and reduced by either pharmacological PAR-2 antagonist peptide or by siRNA-mediated inhibition of PAR-2 expression, which suggests the critical role of PAR-2 in this process. NO production by PAR-2 was mimicked by PMA, a PKC activator, and was attenuated by Go6976, a protein kinase C (PKC) inhibitor. PAR-2 stimulation activated three subtypes of mitogen-activated protein kinases (MAPKs): extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK. NO production by PAR-2 was blocked by inhibition of ERK, p38, and JNK pathways. PAR-2 stimulation also activated nuclear factor-kappaB (NF-kappaB) DNA binding and transcriptional activity as well as IkappaBalpha phosphorylation. Inhibitors of NF-kappaB pathway inhibited PAR-2-mediated NO production. In addition, inhibitors of MAPK pathways prevented transcriptional activation of NF-kappaB reporter constructs. These results suggest that PAR-2 activation-mediated NO production in astrocytes is transduced by the activation of MAPKs followed by NF-kappaB pathways.

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.

Astrocytic control of synaptic NMDA receptors

Astrocytes express a wide range of G-protein coupled receptors that trigger release of intracellular Ca2+, including P2Y, bradykinin and protease activated receptors (PARs). By using the highly sensitive sniffer-patch technique, we demonstrate that the activation of P2Y receptors, bradykinin receptors and protease activated receptors all stimulate glutamate release from cultured or acutely dissociated astrocytes. Of these receptors, we have utilized PAR1 as a model system because of favourable pharmacological and molecular tools, its prominent expression in astrocytes and its high relevance to neuropathological processes. Astrocytic PAR1-mediated glutamate release in vitro is Ca2+ dependent and activates NMDA receptors on adjacent neurones in culture. Activation of astrocytic PAR1 in hippocampal slices induces an APV-sensitive inward current in CA1 neurones and causes APV-sensitive neuronal depolarization in CA1 neurones, consistent with release of glutamate from astrocytes. PAR1 activation enhances the NMDA receptor-mediated component of synaptic miniature EPSCs, evoked EPSCs and evoked EPSPs in a Mg2+-dependent manner, which may reflect spine head depolarization and consequent reduction of NMDA receptor Mg2+ block during subsequent synaptic currents. The release of glutamate from astrocytes following PAR1 activation may also lead to glutamate occupancy of some perisynaptic NMDA receptors, which pass current following relief of tonic Mg2+ block during synaptic depolarization. These results suggest that astrocytic G-protein coupled receptors that increase intracellular Ca2+ can tune synaptic NMDA receptor responses.

Thrombin and PAR-1-AP increase proinflammatory cytokine expression in C6 cells

BACKGROUND

In addition to a recognized role in the coagulation cascade, thrombin is known to have other functions via G protein-coupled receptors, including protease-activated receptor-1 (PAR-1). To investigate the relationship between PAR-1 activation and proinflammatory cytokine expression, we studied the responsiveness of C6 cells to thrombin and to the agonist PAP-1-activating peptide (PAR-1-AP).

MATERIALS AND METHODS

Cultured C6 rat glioma cells were stimulated with human alpha-thrombin or PAR-1-AP. To study mRNA expression changes, total RNA was isolated from the C6 cells, reverse transcribed, and amplified by real-time polymerase chain reaction. Three proinflammatory cytokines were studied: interleukin-6 (IL-6), interleukin-1beta (IL-1beta), and tumor necrosis factor-alpha (TNF-alpha). To measure cytokine release, cell-free supernatants were assayed using enzyme-linked immunosorbent assay (ELISA).

RESULTS

By quantitative real time reverse transcriptase polymerase chain reaction, thrombin (5 U/mL) exposure significantly increased mRNA expression of the proinflammatory cytokines: IL-6 (2.8 +/- 0.4, multiple of control), IL-1beta (4.8 +/- 1.6), and TNF-alpha (16.5 +/- 4.2). Effects on IL-6 mRNA expression were dose-dependent and matched by increments in IL-6 protein secretion. Effects of thrombin on IL-6 mRNA expression could be inhibited by hirudin. PAR-1-AP exposure also significantly increased mRNA expression of IL-6, IL-1beta and TNF-alpha. PAR-1 mRNA is expressed in C6 cells.

CONCLUSION

Both thrombin and its agonist, PAR-1-AP, significantly increased mRNA expression of pro-inflammatory cytokines in C6 glioma cells via PAR-1 activation.

Tissue factor mediates inflammation

The role of tissue factor (TF) in inflammation is mediated by blood coagulation. TF initiates the extrinsic blood coagulation that proceeds as an extracellular signaling cascade by a series of active serine proteases: FVIIa, FXa, and thrombin (FIIa) for fibrin clot production in the presence of phospholipids and Ca2+. TF upregulation resulting from its enhanced exposure to clotting factor FVII/FVIIa often manifests not only hypercoagulable but also inflammatory state. Coagulant mediators (FVIIa, FXa, and FIIa) are proinflammatory, which are largely transmitted by protease-activated receptors (PAR) to elicit inflammation including the expression of tissue necrosis factor, interleukins, adhesion molecules (MCP-1, ICAM-1, VCAM-1, selectins, etc.), and growth factors (VEGF, PDGF, bFGF, etc.). In addition, fibrin, and its fragments are also able to promote inflammation. In the event of TF hypercoagulability accompanied by the elevations in clotting signals including fibrin overproduction, the inflammatory consequence could be enormous. Antagonism to coagulation-dependent inflammation includes (1) TF downregulation, (2) anti-coagulation, and (3) PAR blockade. TF downregulation and anti-coagulation prevent and limit the proceeding of coagulation cascade in the generation of proinflammatory coagulant signals, while PAR antagonists block the transmission of such signals. These approaches are of significance in interrupting the coagulation-inflammation cycle in contribution to not only anti-inflammation but also anti-thrombosis for cardioprotection.

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.

Thrombin induces expression of cytokine-induced SH2 protein (CIS) in rat brain astrocytes: involvement of phospholipase A2, cyclooxygenase, and lipoxygenase

Previously we have reported that thrombin induces inflammatory mediators in brain glial cells (Ryu et al. 2000. J Biol Chem 275:29955). In the present study, we found that thrombin induced a negative regulator of a cytokine signaling molecule, cytokine-induced SH2 protein (CIS), in rat brain astrocytes. In response to thrombin, CIS expression was increased at both the mRNA and protein levels. Although STAT5 is known to regulate CIS expression, thrombin did not activate STAT5, and inhibitors of JAK2 (AG490) and JAK3 (WHI-P97 and WHI-P154) had little effect on thrombin-induced CIS expression. In contrast, cytosolic phospholipase A(2) (cPLA(2)), cyclooxygenase (COX), and lipoxygenase (LO) play a role in CIS expression, since inhibitors of cPLA(2), cyclooxygenase (COX), and LO significantly reduced CIS expression. Reactive oxygen species (ROS) scavengers (N-acetyl-cysteine [NAC] and trolox) reduced thrombin-induced CIS expression, and inhibitors of COX and LO reduced ROS produced by thrombin. Furthermore, prostaglandin E(2) (PGE(2)) and leukotriene B(4) (LTB(4)), products of COX and LO, respectively, potentiated thrombin-induced CIS expression, indicating that ROS, and PGE(2) and LTB(4) generated by COX and LO, mediate CIS expression. Since interferon-gamma (IFN-gamma)-induced GAS-luciferase activity and tyrosine phosphorylation of STAT1 and STAT3 were lower in CIS-transfected cells compared to control vector-transfected cells, CIS could have anti-inflammatory activity. These data suggest that thrombin-stimulation of ROS and prostaglandin and leukotriene production via the cPLA(2), COX and LO pathways results in CIS expression. More importantly, CIS expression may be a negative feedback mechanism that prevents prolonged inflammatory responses.

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.

Thrombin induces nitric-oxide synthase via Galpha12/13-coupled protein kinase C-dependent I-kappaBalpha phosphorylation and JNK-mediated I-kappaBalpha degradation

An imbalance between thrombin and antithrombin III contributed to vascular hyporeactivity in sepsis, which can be attributed to excess NO production by inducible nitric-oxide synthase (iNOS). In view of the importance of the thrombin-activated coagulation pathway and excess NO as the culminating factors in vascular hyporeactivity, this study investigated the effects of thrombin on the induction of iNOS and NO production in macrophages. Thrombin induced iNOS protein in the Raw264.7 cells, which was inhibited by a thrombin inhibitor, LB30057. Thrombin increased NF-kappaB DNA binding, whose band was supershifted with anti-p65 and anti-p50 antibodies. Thrombin elicited the phosphorylation and degradation of I-kappaBalpha prior to the nuclear translocation of p65. The NF-kappaB-mediated iNOS induction was stimulated by the overexpression of activated mutants of Galpha(12/13) (Galpha(12/13)QL). Protein kinase C depletion inhibited I-kappaBalpha degradation, NF-kappaB activation, and iNOS induction by thrombin or the iNOS induction by Galpha(12/13)QL. JNK, p38 kinase, and ERK were all activated by thrombin. JNK inhibition by the stable transfection with a dominant negative mutant of JNK1 (JNK1(-)) completely suppressed the NF-kappaB-mediated iNOS induction by thrombin. Conversely, the inhibition of p38 kinase enhanced the expression of iNOS. In addition, JNK and p38 kinase oppositely controlled the NF-kappaB-mediated iNOS induction by Galpha(12/13)QL. Hence, iNOS induction by thrombin was regulated by the opposed functions of JNK and p38 kinase downstream of Galpha(12/13). In the JNK1(-) cells, thrombin did not increase either the NF-kappaB binding activity or I-kappaBalpha degradation despite I-kappaBalpha phosphorylation. These results demonstrated that thrombin induces iNOS in macrophages via Galpha(12) and Galpha(13), which leads to NF-kappaB activation involving the protein kinase C-dependent phosphorylation of I-kappaBalpha and the JNK-dependent degradation of phosphorylated I-kappaBalpha.

Thrombin signaling in the brain: the role of protease-activated receptors

Signaling by the protease thrombin has started to be appreciated in cell biology, especially since the gene for protease-activated receptor-1 (PAR-1) has been cloned. Apart from the central role of thrombin in blood coagulation and wound healing, thrombin also regulates cellular functions in a large variety of cells through PAR-1, PAR-3 and PAR-4. Receptors are activated by a proteolytic cleavage mechanism via G protein-coupled signaling pathways. Accumulating evidence shows that thrombin changes the morphology of neurons and astrocytes, induces glial cell proliferation, and even exerts, depending on the concentration applied, either cytoprotective or cytotoxic effects on neural cells. These effects may be mediated, through either distinct or overlapping signal transduction cascades, by activation of PARs. This review focuses on the underlying signaling events initiated by thrombin in neuronal and glial cells, to summarize our understanding of the intracellular signaling machinery linking thrombin receptors to their potential physiological and pathological functions in the CNS.