Platelet Activation by von Willebrand Factor Requires Coordinated Signaling through Thromboxane A₂ and FcγIIA Receptor

Interaction of von Willebrand Factor with glycoprotein Ib-IX-V induces platelet activation through a still poorly defined mechanism. Previous studies have suggested a possible role for the low affinity receptor for immunoglobulin, FcγRIIA, in GPIb-IX-V signaling. Here we show that binding of vWF to platelets induces the tyrosine phosphorylation of FcγRIIA by a Src kinase. Treatment of platelets with the anti-FcγRIIA monoclonal antibody IV.3 specifically inhibits vWF-induced but not thrombin-induced pleckstrin phosphorylation and serotonin secretion. Moreover, vWF fails to induce pleckstrin phosphorylation in mouse platelets, lacking FcyRIIA, and serotonin secretion is impaired. Pleckstrin phosphorylation and serotonin secretion in human platelets stimulated with vWF are blocked by the cyclooxygenase inhibitor acetylsalicylic acid. However, release of arachidonic acid and synthesis of TxA₂ induced by vWF are not affected by the anti-FcγRIIA monoclonal antibody IV.3. Similarly, vWF-induced tyrosine phosphorylation of FcγRIIA, as well as of Syk and PLCγ2, occurs normally in aspirinized platelets. Inhibition of the tyrosine kinase Syk by piceatannol does not affect vWF-induced tyrosine phosphorylation of FcγRIIA but prevents phosphorylation of PLCγ2. Pleckstrin phosphorylation and platelet secretion induced by vWF, but not by thrombin, are also inhibited by piceatannol. Pleckstrin phosphorylation is also sensitive to the phosphatidylinositol 3-kinase inhibitor wortmannin. These results indicate that PLCγ2 plays a central role in platelet activation by vWF and that the stimulation of this enzyme requires coordinated signals through endogenous TxA₂ and FcγRIIA.