PAR-4 Background Information Thrombin receptor (also designated protease-activated receptor-1 or PAR-1), PAR-2, PAR-3 and PAR-4 compose a distinct class of G protein-coupled rec-eptors activated by proteolysis. Cleavage of these receptors by proteases occurs within the amino-terminal extracellular domain. Thrombin, a serine protease involved in platelet aggregation and blood coagulation, activates the thrombin receptor, resulting in elevated intracellular calcium levels in platelets. Thrombin also cleaves PAR-3 in vitro, suggesting that PAR-3 may be involved in thrombosis or mitogenesis. Thrombin receptor and PAR-4 appear to account for most thrombin signaling in platelets. Activation of PAR-2 in vitro is induced by trypsin, suggesting that PAR-2 is not an alternative thrombin receptor. Cytokines including TNF-å and IL-1∫ increase PAR-2 expression, indicating PAR-2 involvement in the acute inflammatory response.
