Pathophysiological Actions of Protease Activated Receptors (Pars)

Pathophysiological Actions of Protease Activated Receptors (Pars)

REVIEW Department of Pharmacology1, L. M. College of Pharmacy, Navrangpura, Ahmedabad; Department of Pharmacology2,C.U. Shah College of Pharmacy and Research, Wadhwan City, Dist. Surendranagar, India Pathophysiological actions of protease activated receptors (PARs) N. M. Pandya2, 3, S. M Jain1, D. D. Santani1 Received September 27, 2005, accepted October 25, 2006 Dr. Nilesh Pandya, Department of Pharmacology, C. U. Shah College of Pharmacy & Research, Wadhwan City-363030, Dist. Surendranagar, India [email protected], [email protected] Pharmazie 62: 163–169 (2007) doi: 10.1691/ph.2007.3.6722 Serine proteases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteases have been considered to act pri- marily 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 subfam- ily of G protein-coupled receptors, termed protease-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteases act as signaling molecules that specifi- cally regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and acti- vate 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 proteases 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. 1. Introduction Hollenberg 2002). Thus, PARs are not activated like “classical” receptors because the specific receptor-activat- PARs belong to a new subfamily of G protein coupled ing ligand is part of the receptor, whereas the circulating receptors (GPCRs) with seven transmembrane domains ac- agonist is a relatively nonspecific serine protease that does tivated via proteolytic cleavage by serine proteases (Derry not behave like a traditional hormonal regulator akin to 1998; Coughlin 2000; Macfarlane 2001; Hollenberg 2002). insulin. PAR 1,PAR3, and PAR4 are targets for thrombin, trypsin, Taken together, data obtained using the enzyme activators or cathepsin G (Vu 1991; Ishihara 1997; Kahn 1997; Xu themselves (trypsin, thrombin), and using PAR gene-defi- 1998). In contrast, PAR2 is resistant to thrombin, but can cient mice provide compelling evidence that PARs play a be activated by trypsin, mast cell tryptase, factor Xa, acro- critical role in the regulation of various physiological and sin, gingipain, and neuronal serine proteases (Nystedt pathophysiological functions in mammals, including hu- 1995a, b; Molino 1997; Lourbakos 1998; Steinhoff 1999; mans. This review focuses on the biology and signaling Camerer 2000; Smith 2000; Lourbakos 2001). Interest- properties of PARs in various mammalian tissues and ingly, PARs are activated by a unique mechanism: pro- highlights the current knowledge about the role of PARs. teases activate PARs by proteolytic cleavage within the Protease signaling in tissues depends on the generation extracellular N-terminus of their receptors, thereby expos- and release of proteases, availability of cofactors, presence ing a novel “cryptic” receptor-activating N-terminal se- of protease inhibitors, and activation and inactivation of quence that, remaining tethered, binds to and activates the PARs. Many proteases that activate PARs are produced receptor within the same receptor (Vu 1991; Nystedt during tissue damage, and PARs make important contribu- 1995a, b). Specific residues (about six amino acids) within tions to tissue responses to injury, including hemostasis, this tethered ligand domain are believed to interact with repair, cell survival, inflammation, and pain. Drugs that extracellular loop 2 and other domains of the cleaved re- mimic or interfere with these processes are attractive thera- ceptor (Verrall 1997), resulting in activation. This intramo- pies: selective agonists of PARs may facilitate healing, re- lecular activation process is followed by coupling to G pair, and protection, whereas protease inhibitors and PAR proteins and the triggering of a variety of downstream sig- antagonists can impede exacerbated inflammation and nal transduction pathways (Dery 1998; Macfarlane 2001; pain. Major future challenges will be to understand the Pharmazie 62 (2007) 3 163 REVIEW role of proteases and PARs in physiological control me- by generalized inflammation and necrosis. PAR2 is up chanisms and human diseases and to develop selective regulated in a model of coronary ischaemia and reper- agonists and antagonists that can be used to probe func- fusion injury in rats, and administration of a PAR2 AP tion and treat disease. markedly protects the heart from injury. PAR2 activation also improves efficiency of ischaemic preconditioning and reduces cardiac inflammation in the rat heart (Zhang 2. PARs and wound healing 1995). Several findings suggest that PAR1 could play a role in wound healing. Activation of PAR stimulates prolifera- 1 4. PARs and cancer tion of keratinocytes (Thomas 1977; Corvera 1997) and fibroblasts (Noorbakhsh 2003) and also induces angiogen- The microenvironment of tumors is replete with proteases esis (Seeliger 2003). Plasmin, thrombin, and PAR1 AP that can activate PARs, and tumor cells themselves ex- (protease activated receptor 1 activated protein) induce ex- press PARs. Malignant cells secrete thrombin and trypsin, pression of cysteine-rich angiogenic protein Cyr-61, a which can affect proliferation and mediate metastatic pro- growth factor-like gene implicated in angiogenesis and cesses such as cellular invasion, extracellular matrix degra- wound healing, in fibroblasts from wild type but not dation, angiogenesis, and tissue remodeling. PAR 1-deficient mice (Minami 2004). Moreover, the topical PAR 1 and PAR2 are expressed by a wide range of tumor application of thrombin and PAR1 AP to incisional cells (Schultheiss 1997; Steinhoff 1999; Naldini 2000; wounds in rats improves wound strength and increases an- Covic 2000; Hirano 2002). In breast tumor tissues, PAR1 giogenesis, thereby promoting wound healing (Andrede- and PAR2 are expressed in the tumor cells, mast cells, Gorden 1999). macrophages, endothelial cells, and vascular smooth mus- cle cells of the metastatic tumor microenvironment (Hira- no 2002). In particular, there is an up-regulation of PAR 3. PARs and cardiovascular disease 1 and PAR2 in proliferating stromal fibroblasts surrounding The inappropriate aggregation of platelets makes an im- the carcinoma cells. In pulmonary tumor alveolar walls, portant contribution to occlusive vascular disorders such the expression of PAR1 and PAR2 mRNA is increased by as stroke, angina, and myocardial infarction, where accu- 10- and 16-fold, respectively, compared with normal al- mulation of atherosclerotic plaques promotes platelet- veolar tissues (Suidan 1996). Expression of trypsin is also mediated thrombus formation. Thrombin is a major media- detected in this tumor tissue. Exposure of certain tumor tor of platelet aggregation and fibrin deposition, and cells to thrombin and PAR1 AP also enhances their adhe- thrombin inhibitors are useful antithrombotic agents. One sion to platelets, fibronectin, and von Willebrand factor in drawback of such inhibitors is their disruption of the nor- vitro and promotes pulmonary metastasis when cells are mal hemostatic mechanism, and a selective suppression of administered to mice in vivo (Shapiro 2000; Wang 2003). the effects of thrombin on platelets may be advantageous. Moreover, over expression of PAR1 in certain tumor cells A difficulty of studying the role of PARs in thrombus for- can enhance their metastatic potential in animal models. mation is that platelets express several thrombin receptors, In colon cancer cell lines, activation of PAR1 induces a which vary among species. However, despite the redun- marked mitogenic response, which is dependent on activa- dancy and interspecies differences, recent observations in tion of MAP kinase ERK1/2, and also stimulate motility cynomologus monkeys, whose platelets, like those of hu- of wounded cells (Riewald 2002). Together, these results mans, express PAR1 and PAR4, suggest that PAR1 antago- suggest that antagonists of PAR1 may be useful treatments nists may be useful therapeutic agents in humans (Carmeliet for proliferation and metastasis of certain tumors. How- 2001). Thus the PAR1 antagonist RWJ-58259 markedly ever, PAR1 AP has also reported to inhibit migration and inhibits thrombus formation and vessel occlusion in a invasion of breast cancer cell lines when applied as a con- model of electrolytic injury of the carotid artery in cyno- centration gradient in the direction of movement (Macey mologus monkeys. Despite the fact that platelets from 1998). PAR2 may also contribute to tumor formation and these animals possess the dual PAR1/PAR4 system, antag- metastasis since PAR2 AP stimulates proliferation of colon onism of a single receptor is effective, raising the possibi- tumor cell lines

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