Molecular mechanism of thromboxane A2-induced platelet aggregation -: Essential role for P2TAC and α2A receptors

Thromboxane A, is a positive feedback Lipid mediator produced following platelet activation. The G(q)-coupled thromboxane A(2) receptor subtype, TP alpha, and G(i)-coupled TP beta subtype have been shown in human platelets. ADP-induced platelet aggregation requires concomitant signaling hom two P2 receptor subtypes, P2Y1 and P2T(AC), coupled to G(q) and G(i) respectively. me investigated whether the stable thromboxane A(2) mimetic, (15S)-hydroxy-9,11-epoxymethanoprosta-5Z,13E-dienoic acid (U46619), also causes platelet aggregation by concomitant signaling through G(q) and G(i), through cc-activation of TP alpha and TP beta receptor subtypes. Here we report that secretion blockade with Ro 31-8220, a protein kinase C inhibitor, completely inhibited U46619-induced, but not ADP- or thrombin-induced, platelet aggregation. Ro 31-8220 had no effect on U46619-induced intracellular calcium mobilization or platelet shape change. Furthermore, U46619-induced intracellular calcium mobilization and shape change were unaffected by A3P5P, a P2Y1 receptor-selective antagonist, and/or cyproheptadine, a 5-hydroxytryptamine subtype 2A receptor antagonist. Either Ro 31-8220 or AR-C66096, a P2T(AC) receptor selective antagonist, abolished U46619-induced inhibition of adenylyl cyclase. In addition, AR-C66096 drastically inhibited U46619-mediated platelet aggregation, which was further inhibited by yohimbine, an alpha(2A)-adrenergic receptor antagonist Furthermore, inhibition of U46619-induced platelet aggregation by Ro 31-8220 was relieved by activation of the G(i) pathway by selective activation of either the P2T(AC) receptor or the alpha(2A)-adrenersc receptor. We conclude that whereas thromboxane A(2) causes intracellular calcium mobilization and shape change independently, thromboxane A(2)-induced inhibition of adenylyl cyclase and platelet aggregation depends exclusively upon secretion of other agonists that stimulate G(i)-coupled receptors.