Loss of Protease-Activated Receptor 4 Prevents Inflammation Resolution and Predisposes the Heart to Cardiac Rupture After Myocardial Infarction.

BACKGROUND: Cardiac rupture is a major lethal complication of acute myocardial infarction (MI). Despite significant advances in reperfusion strategies, mortality from cardiac rupture remains high. Studies suggest that cardiac rupture can be accelerated by thrombolytic therapy, but the relevance of this risk factor remains controversial. METHODS: We analyzed protease-activated receptor 4 (Par4) expression in mouse hearts with MI and investigated the effects of Par4 deletion on cardiac remodeling and function after MI by echocardiography, quantitative immunohistochemistry, and flow cytometry. RESULTS: Par4 mRNA and protein levels were increased in mouse hearts after MI and in isolated cardiomyocytes in response to hypertrophic and inflammatory stimuli. Par4-deficient mice showed less myocyte apoptosis, reduced infarct size, and improved functional recovery after acute MI relative to wild-type (WT). Conversely, Par4-/- mice showed impaired cardiac function, greater rates of myocardial rupture, and increased mortality after chronic MI relative to WT. Pathological evaluation of hearts from Par4-/- mice demonstrated a greater infarct expansion, increased cardiac hemorrhage, and delayed neutrophil accumulation, which resulted in impaired post-MI healing compared with WT. Par4 deficiency also attenuated neutrophil apoptosis in vitro and after MI in vivo and impaired inflammation resolution in infarcted myocardium. Transfer of Par4-/- neutrophils, but not of Par4-/- platelets, in WT recipient mice delayed inflammation resolution, increased cardiac hemorrhage, and enhanced cardiac dysfunction. In parallel, adoptive transfer of WT neutrophils into Par4-/- mice restored inflammation resolution, reduced cardiac rupture incidence, and improved cardiac function after MI. CONCLUSIONS: These findings reveal essential roles of Par4 in neutrophil apoptosis and inflammation resolution during myocardial healing and point to Par4 inhibition as a potential therapy that should be limited to the acute phases of ischemic insult and avoided for long-term treatment after MI.

Thrombin receptor PAR4 drives canonical NLRP3 inflammasome signaling in the heart.

The deleterious effects of diabetes in the heart are increasingly attributed to inflammatory signaling through the NLRP3 (NOD, LRR and PYD domains-containing protein 3) inflammasome. Thrombin antagonists reduce cardiac remodeling and dysfunction in diabetic mice, in part by suppressing fibrin-driven inflammation. The role of cellular thrombin receptor subtypes in this context is not known. We sought to determine the causal involvement of protease-activated receptors (PAR) in inflammatory signaling of the diabetic heart. Mice with diet-induced diabetes showed increased abundance of pro-caspase-1 and pro-interleukin (IL)-1ß in the left ventricle (LV), indicating transcriptional NLRP3 inflammasome priming, and augmented cleavage of active caspase-1 and IL-1ß, pointing to canonical NLRP3 inflammasome activation. Caspase-11 activation, which mediates non-canonical NLRP3 inflammasome signaling, was not augmented. Formation of the plasma membrane pore-forming protein N-terminal gasdermin D (GDSMD), a prerequisite for IL-1ß secretion, was also higher in diabetic vs. control mouse LV. NLRP3, ASC and IL-18 expression did not differ between the groups, nor did expression of PAR1 or PAR2. PAR3 was nearly undetectable. LV abundance of PAR4 by contrast increased with diabetes and correlated positively with active caspase-1. Genetic deletion of PAR4 in mice prevented the diet-induced cleavage of caspase-1, IL-1ß and GDSMD. Right atrial appendages from patients with type 2 diabetes also showed higher levels of PAR4, but not of PAR1 or PAR2, than non-diabetic atrial tissue, along with increased abundance of cleaved caspase-1, IL-1ß and GSDMD. Human cardiac fibroblasts maintained in high glucose conditions to mimic diabetes also upregulated PAR4 mRNA and protein, and increased PAR4-dependent IL-1ß transcription and secretion in response to thrombin, while PAR1 and PAR2 expressions were unaltered. In conclusion, PAR4 drives caspase-1-dependent IL-1ß production through the canonical NLRP3 inflammasome pathway in the diabetic heart, providing mechanistic insights into diabetes-associated cardiac thromboinflammation. The emerging PAR4-selective antagonists may provide a feasible approach to prevent cardiac inflammation in patients with diabetes.

Protease-activated receptor 4 protects mice from Coxsackievirus B3 and H1N1 influenza A virus infection.

PAR4 is expressed by a variety of cells, including platelets, cardiac, lung and immune cells. We investigated the contribution of PAR4 to viral infections of the heart and lung. Toll-like receptor (TLR) 3-dependent immune responses were analyzed after co-stimulation of PAR4 in murine bone-marrow derived macrophages, embryonic fibroblasts and embryonic cardiomyocytes. In addition, we analyzed Coxsackievirus B3 (CVB3) or H1N1 influenza A virus (H1N1 IAV) infection of PAR4-/- (ΔPAR4) and wild-type (WT) mice. Lastly, we investigated the effect of platelet inhibition on H1N1 IAV infection. In vitro experiments revealed that PAR4 stimulation enhances the expression of TLR3-dependent CXCL10 expression and decreases TLR3-dependent NFκB-mediated proinflammatory gene expression. Furthermore, CVB3-infected ΔPAR4 mice exhibited a decreased anti-viral response and increased viral genomes in the heart leading to more pronounced CVB3 myocarditis compared to WT mice. Similarly, H1N1 IAV-infected ΔPAR4 mice had increased immune cell numbers and inflammatory mediators in the lung, and increased mortality compared with infected WT controls. The study showed that PAR4 protects mice from viral infections of the heart and lung.

TERT and Akt Are Involved in the Par-4-Dependent Apoptosis of Islet ß Cells in Type 2 Diabetes.

Islet ß cell apoptosis plays an important role in type 2 diabetes. We previously reported that Par-4-mediated islet ß cell apoptosis is induced by high-glucose/fatty acid levels. In the present study, we show that Par-4, which is induced by high-glucose/fatty acid levels, interacts with and inhibits TERT in the cytoplasm and then translocates to the nucleus. Par-4 also inhibited Akt phosphorylation, leading to islet ß cell apoptosis. We inhibited Par-4 in islet ß cells under high-glucose/fatty acid conditions and knocked out Par-4 in diabetic mice, which led to the up-regulation of TERT and an improvement in the apoptosis rate. We inhibited Akt phosphorylation in islet ß cells and diabetic mice, which led to aggressive apoptosis. In addition, the biological film interference technique revealed that Par-4 bound to TERT via its NLS and leucine zipper domains. Our research suggests that Par-4 activation and binding to TERT are key steps required for inducing the apoptosis of islet ß cells under high-glucose/fatty acid conditions. Inhibiting Akt phosphorylation aggravated apoptosis by activating Par-4 and inhibiting TERT, and Par-4 inhibition may be an attractive target for the treatment of islet ß cell apoptosis.

Protease-activated receptor 4 deficiency offers cardioprotection after acute ischemia reperfusion injury.

Protease-activated receptor (PAR)4 is a low affinity thrombin receptor with less understood function relative to PAR1. PAR4 is involved in platelet activation and hemostasis, but its specific actions on myocyte growth and cardiac function remain unknown. This study examined the role of PAR4 deficiency on cardioprotection after myocardial ischemia-reperfusion (IR) injury in mice. When challenged by in vivo or ex vivo IR, PAR4 knockout (KO) mice exhibited increased tolerance to injury, which was manifest as reduced infarct size and a more robust functional recovery compared to wild-type mice. PAR4 KO mice also showed reduced cardiomyocyte apoptosis and putative signaling shifts in survival pathways in response to IR. Inhibition of PAR4 expression in isolated cardiomyocytes by shRNA offered protection against thrombin and PAR4-agonist peptide-induced apoptosis, while overexpression of wild-type PAR4 significantly enhanced the susceptibility of cardiomyocytes to apoptosis, even under low thrombin concentrations. Further studies implicate Src- and epidermal growth factor receptor-dependent activation of JNK on the proapoptotic effect of PAR4 in cardiomyocytes. These findings reveal a pivotal role for PAR4 as a regulator of cardiomyocyte survival and point to PAR4 inhibition as a therapeutic target offering cardioprotection after acute IR injury.

Thrombin receptor deficiency leads to a high bone mass phenotype by decreasing the RANKL/OPG ratio.

Thrombin and its receptor (TR) are, respectively, expressed in osteoclasts and osteoblasts. However, their physiological roles on bone metabolism have not been fully elucidated. Here we investigated the bone microarchitecture by micro-computed tomography (µCT) and demonstrated increased trabecular and cortical bone mass in femurs of TR KO mice compared to WT littermates. Trabecular thickness and connectivity were significantly enhanced. The physiological role of TR on both inorganic and organic phases of bone is illustrated by a significant increase in BMD and a decrease in urinary deoxypyridinoline (DPD) crosslink concentration in TR KO mice. Moreover, TR KO cortical bone expanded and had a higher polar moment of inertia (J), implying stronger bone. Bone histomorphometry illustrated unaltered osteoblast and osteoclast number and surface in femoral metaphyses, indicating that thrombin/TR regulates osteoblasts and osteoclasts at functional levels. Serum analysis showed a decrease in RANKL and an increase in osteoprotegerin (OPG) levels and reflected a reduced RANKL/OPG ratio in the TR KO group. In vitro experiments using MC3T3 pre-osteoblasts demonstrated a TR-dependent stimulatory effect of thrombin on the RANKL/OPG ratio. This effect was blocked by TR antagonist and p42/p44-ERK inhibitor. In addition, thrombin also intensified p42/p44-ERK expression and phosphorylation. In conclusion, the thrombin/TR system maintains normal bone remodeling by activating RANKL and limiting OPG synthesis by osteoblasts through the p42/44-ERK signaling pathway. Consequently, TR deficiency inhibits osteoclastogenesis, resulting in a high bone mass phenotype.

Coagulation-driven platelet activation reduces cholestatic liver injury and fibrosis in mice.

BACKGROUND: The coagulation cascade has been shown to participate in chronic liver injury and fibrosis, but the contribution of various thrombin targets, such as protease activated receptors (PARs) and fibrin(ogen), has not been fully described. Emerging evidence suggests that in some experimental settings of chronic liver injury, platelets can promote liver repair and inhibit liver fibrosis. However, the precise mechanisms linking coagulation and platelet function to hepatic tissue changes following injury remain poorly defined. OBJECTIVES: To determine the role of PAR-4, a key thrombin receptor on mouse platelets, and fibrin(ogen) engagement of the platelet αII b ß3 integrin (αIIb ß3 ) in a model of cholestatic liver injury and fibrosis. METHODS: Biliary and hepatic injury was characterized following 4 week administration of the bile duct toxicant α-naphthylisothiocyanate (ANIT) (0.025%) in PAR-4-deficient mice, mice expressing a mutant form of fibrin(ogen) incapable of binding integrin αII b ß3 (Fibγ(Δ5) ), and wild-type mice. RESULTS: Elevated plasma thrombin-antithrombin and serotonin levels, hepatic fibrin deposition, and platelet accumulation in liver accompanied hepatocellular injury and fibrosis in ANIT-treated wild-type mice. PAR-4 deficiency reduced plasma serotonin levels, increased serum bile acid concentration, and exacerbated ANIT-induced hepatocellular injury and peribiliary fibrosis. Compared with PAR-4-deficient mice, ANIT-treated Fibγ(Δ5) mice displayed more widespread hepatocellular necrosis accompanied by marked inflammation, robust fibroblast activation, and extensive liver fibrosis. CONCLUSIONS: Collectively, the results indicate that PAR-4 and fibrin-αII b ß3 integrin engagement, pathways coupling coagulation to platelet activation, each exert hepatoprotective effects during chronic cholestasis.

Redundancy and interaction of thrombin- and collagen-mediated platelet activation in tail bleeding and carotid thrombosis in mice.

OBJECTIVE: Current antiplatelet strategies to prevent myocardial infarction and stroke are limited by bleeding risk. A better understanding of the roles of distinct platelet-activating pathways is needed. We determined whether platelet activation by 2 key primary activators, thrombin and collagen, plays distinct, redundant, or interacting roles in tail bleeding and carotid thrombosis in mice. APPROACH AND RESULTS: Platelets from mice deficient for the thrombin receptor protease-activated receptor-4 (Par4) and the collagen receptor glycoprotein VI protein (GPVI) lack responses to thrombin and collagen, respectively. We examined tail bleeding and FeCl3-induced carotid artery occlusion in mice lacking Par4, GPVI, or both. We also examined a series of Par mutants with increasing impairment of thrombin signaling in platelets. Ablation of thrombin signaling alone by Par4 deficiency increased blood loss in the tail bleeding assay and impaired occlusive thrombus formation in the carotid occlusion assay. GPVI deficiency alone had no effect. Superimposing GPVI deficiency on Par4 deficiency markedly increased effect size in both assays. In contrast to complete ablation of thrombin signaling, 9- and 19-fold increases in EC50 for thrombin-induced platelet activation had only modest effects. CONCLUSIONS: The observation that loss of Par4 uncovered large effects of GPVI deficiency implies that Par4 and GPVI made independent, partially redundant contributions to occlusive thrombus formation in the carotid and to hemostatic clot formation in the tail under the experimental conditions examined. At face value, these results suggest that thrombin- and collagen-induced platelet activation can play partially redundant roles, despite important differences in how these agonists are made available to platelets.

Platelet ITAM signaling is critical for vascular integrity in inflammation.

Platelets play a critical role in maintaining vascular integrity during inflammation, but little is known about the underlying molecular mechanisms. Here we report that platelet immunoreceptor tyrosine activation motif (ITAM) signaling, but not GPCR signaling, is critical for the prevention of inflammation-induced hemorrhage. To generate mice with partial or complete defects in these signaling pathways, we developed a protocol for adoptive transfer of genetically and/or chemically inhibited platelets into thrombocytopenic (TP) mice. Unexpectedly, platelets with impaired GPCR signaling, a crucial component of platelet plug formation and hemostasis, were indistinguishable from WT platelets in their ability to prevent hemorrhage at sites of inflammation. In contrast, inhibition of GPVI or genetic deletion of Clec2, the only ITAM receptors expressed on mouse platelets, significantly reduced the ability of platelets to prevent inflammation-induced hemorrhage. Moreover, transfusion of platelets without ITAM receptor function or platelets lacking the adapter protein SLP-76 into TP mice had no significant effect on vascular integrity during inflammation. These results indicate that the control of vascular integrity is a major function of immune-type receptors in platelets, highlighting a potential clinical complication of novel antithrombotic agents directed toward the ITAM signaling pathway.

Roles and interactions among protease-activated receptors and P2ry12 in hemostasis and thrombosis.

Toward understanding their redundancies and interactions in hemostasis and thrombosis, we examined the roles of thrombin receptors (protease-activated receptors, PARs) and the ADP receptor P2RY12 (purinergic receptor P2Y G protein-coupled 12) in human and mouse platelets ex vivo and in mouse models. Par3(-/-) and Par4(+/-) mouse platelets showed partially decreased responses to thrombin, resembling those in PAR1 antagonist-treated human platelets. P2ry12(+/-) mouse platelets showed partially decreased responses to ADP, resembling those in clopidogrel-treated human platelets. Par3(-/-) mice showed nearly complete protection against carotid artery thrombosis caused by low FeCl(3) injury. Par4(+/-) and P2ry12(+/-) mice showed partial protection. Increasing FeCl(3) injury abolished such protection; combining partial attenuation of thrombin and ADP signaling, as in Par3(-/-):P2ry12(+/-) mice, restored it. Par4(-/-) mice, which lack platelet thrombin responses, showed still better protection. Our data suggest that (i) the level of thrombin driving platelet activation and carotid thrombosis was low at low levels of arterial injury and increased along with the contribution of thrombin-independent pathways of platelet activation with increasing levels of injury; (ii) although P2ry12 acts downstream of PARs to amplify platelet responses to thrombin ex vivo, P2ry12 functioned in thrombin/PAR-independent pathways in our in vivo models; and (iii) P2ry12 signaling was more important than PAR signaling in hemostasis models; the converse was noted for arterial thrombosis models. These results make predictions being tested by ongoing human trials and suggest hypotheses for new antithrombotic strategies.

Apoptosis and tumor resistance conferred by Par-4.

Par-4 is a tumor suppressor protein with a pro-apoptotic function. Epigenetic silencing of Par-4 is seen in diverse tumors and Par-4 knockout mice develop spontaneous tumors in various tissues. Endogenous Par-4 is essential for sensitization of cells to diverse apoptotic stimuli, whereas ectopic expression of Par-4 can selectively induce apoptosis in cancer cells. The cancer-specific pro-apoptotic action of Par-4 resides in its centrally located SAC domain. This review emphasizes the role of Par-4/SAC in apoptosis and tumor resistance. SAC transgenic mice display normal development and life span, and, most importantly, are resistant to spontaneous, as well as oncogene-induced, autochthonous tumors. The tumor resistant phenotype and undetectable toxicity of SAC in vivo suggests the SAC domain possesses tremendous therapeutic potential.

Par-4 inhibits Akt and suppresses Ras-induced lung tumorigenesis.

The atypical PKC-interacting protein, Par-4, inhibits cell survival and tumorigenesis in vitro, and its genetic inactivation in mice leads to reduced lifespan, enhanced benign tumour development and low-frequency carcinogenesis. Here, we demonstrate that Par-4 is highly expressed in normal lung but reduced in human lung cancer samples. We show, in a mouse model of lung tumours, that the lack of Par-4 dramatically enhances Ras-induced lung carcinoma formation in vivo, acting as a negative regulator of Akt activation. We also demonstrate in cell culture, in vivo, and in biochemical experiments that Akt regulation by Par-4 is mediated by PKCzeta, establishing a new paradigm for Akt regulation and, likely, for Ras-induced lung carcinogenesis, wherein Par-4 is a novel tumour suppressor.

Roles of protease-activated receptors in a mouse model of endotoxemia.

Endotoxemia is often associated with extreme inflammatory responses and disseminated intravascular coagulation. Protease-activated receptors (PARs) mediate cellular responses to coagulation proteases, including platelet activation and endothelial cell reactions predicted to promote inflammation. These observations suggested that PAR activation by coagulation proteases generated in the setting of endotoxemia might promote platelet activation, leukocyte-mediated endothelial injury, tissue damage, and death. Toward testing these hypotheses, we examined the effect of PAR deficiencies that ablate platelet and endothelial activation by coagulation proteases in a mouse endotoxemia model. Although coagulation was activated as measured by thrombin-antithrombin (TAT) production and antithrombin III (ATIII) depletion, Par1(-/-), Par2(-/-), Par4(-/-), Par2(-/-):Par4(-/-), and Par1(-/-):Par2(-/-) mice all failed to show improved survival or decreased cytokine responses after endotoxin challenge compared with wild type. Thus, our results fail to support a necessary role for PARs in linking coagulation to inflammation or death in this model. Interestingly, endotoxin-induced thrombocytopenia was not diminished in Par4(-/-) mice. Thus, a mechanism independent of platelet activation by thrombin was sufficient to cause thrombocytopenia in our model. These results raise the possibility that decreases in platelet count in the setting of sepsis may not be caused by disseminated intravascular coagulation but instead report on a sometimes parallel but independent process.

Combined deficiency of protease-activated receptor-4 and fibrinogen recapitulates the hemostatic defect but not the embryonic lethality of prothrombin deficiency.

The availability of the relevant mutant mouse lines provided an opportunity to test the doctrine that platelet activation and fibrin formation account for the importance of thrombin for hemostasis. Prothrombin-deficient mice that survive to birth exsanguinate in the perinatal period. By contrast, protease-activated receptor 4 (PAR4)-deficient mice, which have platelets that fail to respond to thrombin, survive to adulthood with only a mild bleeding diathesis, and fibrinogen-deficient mice show perinatal bleeding but those that survive this period can have a relatively normal life expectancy. We now report that mice that lacked both PAR4 and fibrinogen exsanguinated at birth like prothrombin-deficient mice. However, while approximately half of prothrombin-deficient embryos die during midgestation, mice lacking both PAR4 and fibrinogen developed normally. At face value, these results suggest that platelet activation and fibrin formation are together sufficient to account for the importance of thrombin for hemostasis but not for its importance for embryonic development.

Extracellular mediators in atherosclerosis and thrombosis: lessons from thrombin receptor knockout mice.

It is well appreciated that thrombin as well as other proteases can act as signaling molecules that specifically regulate cells by cleaving and activating members of a novel class of protease-activated receptors (PARs). The utility of gene knockout strategies to define and better comprehend the physiological role of specific proteins is perhaps best exemplified in the field of thrombin receptors. The development of PAR knockout mice has provided the unique opportunity to identify and characterize new members of this novel family of GPCRs, evaluate the interaction of PARs jointly expressed in common cells and tissues, and better understand the role of PARs in thrombosis, restenosis, vascular remodeling, angiogenesis, and inflammation. Presently, 4 members of the PAR family have been cloned and identified. In this review, we examine experimental evidence gleaned from PAR-/- mouse models as well as how the use of PAR-/- mice has provided insights toward understanding the physiological role of thrombin in cells of the vascular system and vascular pathology.

Genetic inactivation of Par4 results in hyperactivation of NF-kappaB and impairment of JNK and p38.

The Par4 gene was first identified in prostate cells undergoing apoptosis after androgen withdrawal. PAR4 was subsequently shown to interact with, and inhibit, atypical protein kinase C isoforms, functioning as a negative regulator of the NF-kappaB pathway. This may explain its pro-apoptotic function in overexpression experiments. To determine the physiological role of PAR4, we have derived primary embryonic fibroblasts (EFs) from Par4(-/-) mice. We show here that loss of PAR4 leads to a reduction in the ability of tumour necrosis factor-alpha (TNF-alpha) to induce apoptosis by increased activation of NF-kappaB. Consistent with recent reports demonstrating the antagonistic actions of NF-kappaB and c-Jun amino-terminal kinase (JNK) signalling, we have found that Par4(-/-) cells show a reduced activation of the sustained phase of JNK and p38 stimulation by TNF-alpha and interleukin 1. Higher levels of an anti-apoptotic JNK-inhibitor protein, X-chromosome-linked inhibitor of apoptosis, in Par4(-/-) EFs might explain the inhibition of JNK activation in these cells.

Proteinase-activated receptor-2 (PAR2): vascular effects of a PAR2-derived activating peptide via a receptor different than PAR2.

We studied the actions of the proteinase-activated receptor-2-activating peptide (PAR2-AP) trans-cinnamoyl-LIGRLO-amide (tc-LI) in femoral (FA), renal, and small mesenteric (MA) arterial vessels from C57BL/6 [PAR2 (+/+)] and PAR2 (-/-) mice. The actions of tc-LI were compared with those of the parent PAR2-AP Ser-Leu-Ile-Gly-Arg-Leu-amide (SLIGRL-amide; SLI-NH2). Either SLI-NH2 or tc-LI (0.1-10 microM) induced relaxation of either 9,11-dideoxy-9alpha,11alpha-methanoepoxy-prosta-5Z,13E-dien-1-oic acid (U46619)- or cirazoline-precontracted FA from PAR2 (+/+) in endothelium-intact preparations but did not relax vessels from PAR2 (-/-) mice. This FA relaxation by SLI-NH2 and by tc-LI was inhibited by 1) pretreatment with a combination of L-N(G)-nitroarginine methyl ester (L-NAME) and 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), 2) precontraction with 30 mM KCl, or 3) removal of the endothelium. In contrast, tc-LI caused an L-NAME/ODQ/indomethacin-resistant relaxation of MA from PAR2 (+/+) mice. In contrast with SLI-NH2, tc-LI (>30 microM) contracted arteries from both PAR2 (-/-) and PAR2 (+/+) mice. Pretreatment of tissues with a combination of cyclopiazonic acid plus caffeine reduced significantly tc-LI-induced contractions, whereas nifedipine, CdCl2, and Ca2+-free conditions did not. Inhibitors of vascular muscarinic, alpha1-adrenergic, neurokinin, thromboxane A2, histamine, angiotensin II, or endothelin-1 receptors failed to inhibit contractions by 50 microM tc-LI. At resting tension, SLI-NH2 (>10 microM) contracted all arteries in an endothelium-independent manner but only from PAR2 (+/+) mice. We conclude that the endothelium-dependent vasodilation initiated by SLI-NH2 and tc-LI, but not the endothelium-independent contraction initiated by tc-LI, are due to the activation of PAR2. Indeed, the data from PAR2 (-/-) mice indicate that tc-LI, in addition to activating PAR2, is an agonist of vascular smooth muscle contraction via a receptor different than PAR2.

Protection against thrombosis in mice lacking PAR3.

The recent observation that knock-out of protease-activated receptor-4 (PAR4) ablates thrombin signaling in mouse platelets and protects against ferric chloride-induced thrombosis of mouse mesenteric arterioles suggests that thrombin's actions on platelets can play an important role in thrombosis. Complete ablation of thrombin signaling would be difficult to achieve in human beings because human platelets have 2 thrombin receptors that are each capable of mediating transmembrane signaling. However, it is possible that complete ablation of thrombin signaling in platelets is not necessary for an antithrombotic effect. In mouse platelets, PAR3 functions as a cofactor that binds thrombin and promotes productive cleavage of PAR4, and thrombin responses are decreased but not absent in Par3(-/-) platelets. We now report that Par3(-/-) mice were protected against ferric chloride-induced thrombosis of mesenteric arterioles and against thromboplastin-induced pulmonary embolism. Surprisingly, Par3(-/-) and Par4(-/-) mice showed similar degrees of protection in these models and similar prolongation of tail bleeding times. Thus, even a partial decrease in mouse platelet responsiveness to thrombin protected against thrombosis and impaired hemostasis in some settings. These results demonstrate the importance of PAR3's unusual cofactor function and underscore the relative importance of thrombin's actions on platelets in vivo. They also suggest that PAR inhibition might be explored for the prevention or treatment of thrombosis in human beings.

Protease-activated receptor 2 mediates eosinophil infiltration and hyperreactivity in allergic inflammation of the airway.

Trypsin and mast cell tryptase can signal to epithelial cells, myocytes, and nerve fibers of the respiratory tract by cleaving proteinase-activated receptor 2 (PAR2). Since tryptase inhibitors are under development to treat asthma, a precise understanding of the contribution of PAR2 to airway inflammation is required. We examined the role of PAR2 in allergic inflammation of the airway by comparing OVA-sensitized and -challenged mice lacking or overexpressing PAR2. In wild-type mice, immunoreactive PAR2 was detected in airway epithelial cells and myocytes, and intranasal administration of a PAR2 agonist stimulated macrophage infiltration into bronchoalveolar lavage fluid. OVA challenge of immunized wild-type mice stimulated infiltration of leukocytes into bronchoalveolar lavage and induced airway hyperreactivity to inhaled methacholine. Compared with wild-type animals, eosinophil infiltration was inhibited by 73% in mice lacking PAR2 and increased by 88% in mice overexpressing PAR2. Similarly, compared with wild-type animals, airway hyperreactivity to inhaled methacholine (40 micro g/ml) was diminished 38% in mice lacking PAR2 and increased by 52% in mice overexpressing PAR2. PAR2 deletion also reduced IgE levels to OVA sensitization by 4-fold compared with those of wild-type animals. Thus, PAR2 contributes to the development of immunity and to allergic inflammation of the airway. Our results support the proposal that tryptase inhibitors and PAR2 antagonists may be useful therapies for inflammatory airway disease.