Protease-Activated Receptor Signaling in Platelets Activates Cytosolic Phospholipase A 2α Differently for Cyclooxygenase-1 and 12-Lipoxygenase Catalysis

Abstract

Objective— The rate-limiting step in the biosynthesis of thromboxane A 2 (TxA 2 ) and 12-hydroxyeicosatetraenoic acid (12-HETE) by platelets is activation of cytosolic phospholipase A 2α (cPLA 2α ), which releases arachidonic acid, which is the substrate for cyclooxygenase-1 (COX-1) and 12-lipoxygenase. We evaluated signaling via the human platelet thrombin receptors, protease-activated receptor (PAR) 1 and PAR4, to the activation of cPLA 2α , which provides a substrate for the biosynthesis of TxA 2 and 12-HETE. Methods and Results— Stimulating washed human platelets resulted in delayed biosynthesis of 12-HETE, which continues after maximal formation of TxA 2 is completed, suggesting that 12-HETE is not formed by the same pool of arachidonic acid that provides a substrate to COX-1. PAR1-induced formation of TxA 2 was inhibited by the phosphatidylinositol kinase inhibitor LY294002, whereas this inhibitor did not block 12-HETE biosynthesis. Both 1-butanol and propranolol also blocked TxA 2 biosynthesis but did not inhibit 12-HETE formation. Conclusion— The concerted evidence indicates that the platelet thrombin receptors signal activation of cPLA 2α coupled to COX-1 by a pathway different from that signaling activation of the cPLA 2α coupled to 12-lipoxygenase.