Platelet activation by von Willebrand Factor requires coordinated signaling through thromboxane A(2) and Fc gamma IIA receptor

Interaction of von Willebrand Factor with glycoprotein Ib-M-V induces plate let activation through a still poorly defined mechanism. Previous studies h ave suggested a possible role for the low affinity receptor for immunoglobu lin, Fc gamma RIIA, in GPIb-IX-V signaling. Here we show that binding of VW F to platelets induces the tyrosine phosphorylation of Fc gamma RIIA by a S rc kinase, Treatment of platelets with the anti-Fc gamma RIIA monoclonal an tibody IV.3 specifically inhibits vWF-induced but not thrombin-induced plec kstrin phosphorylation and serotonin secretion. Moreover, VWF fails to indu ce pleckstrin phosphorylation in mouse platelets, lacking Fc gamma RIIA, an d serotonin secretion is impaired. Pleckstrin phosphorylation and serotonin secretion in human platelets stimulated with vWF are blocked by the cycloo xygenase inhibitor acetylsalicylic acid. However, release of arachidonic ac id and synthesis of TxA(2) induced by vWF are not affected by the anti-Fc g amma RIIA monoclonal antibody IV.3, Similarly, vWF-induced tyrosine phospho rylation of Fc gamma RIIA, as well as of Syk and PLC gamma2, occurs normall y in aspirinized platelets. Inhibition of the tyrosine kinase Syk by piceat annol does not affect vWF-induced tyrosine phosphorylation of Fc gamma RIIA but prevents phosphorylation of PLC gamma2, Pleckstrin phosphorylation and platelet secretion induced by VWF, but not by thrombin, are also inhibited by piceatannol, Pleckstrin phosphorylation is also sensitive to the phosph atidylinositol 3-kinase inhibitor wortmannin. These results indicate that P LC gamma2 plays a central role in platelet activation by vWF and that the s timulation of this enzyme requires coordinated signals through endogenous T xA(2) and Fc gamma RIIA.