A new role for Fc gamma RIIA in the potentiation of human platelet activation induced by weak stimulation

The low affinity receptor for immunoglobulin G, Fc gamma RIIA, is expressed in human platelets, mediates heparin-induced thrombocytopenia and participates to platelet activation induced by von Willebrand factor. In this work, we found that stimulation of platelets with agonists acting on G-protein-coupled receptors resulted in the tyrosine phosphorylation of Fc gamma RIIA, through a mechanism involving a Re kinase. Treatment of platelets with the blocking monoclonal antibody IV3 against Fc gamma RIIA, but not with control IgG, inhibited platelet aggregation induced by TRAP1, TRAP4, the thromboxane analogue U46619, and low concentrations of thrombin. By contrast, platelet aggregation induced by high doses of thrombin was unaffected by blockade of Fc gamma RIIA. We also found that the anti-Fc gamma RIIA monoclonal antibody IV.3 inhibited pleckstrin phosphorylation and calcium mobilization induced by low, but not high, concentrations of thrombin. In addition, thrombin- or U46619-induced tyrosine phosphorylation of several substrates typically involved in Fc gamma RIIA-mediated signalling, such as Syk and PLC gamma 2, was clearly reduced by incubation with anti-Fc gamma RIIA antibody IV.3. Upon stimulation with thrombin, Fc gamma RIIA relocated in lipid rafts, and thrombin-induced tyrosine phosphorylation of Fc gamma RIIA occurred within these membrane domains. Controlled disruption of lipid rafts by depleting membrane cholesterol prevented tyrosine phosphorylation of Fc gamma RIIA and impaired platelet aggregation induced by U46619 or by low, but not high, concentrations of thrombin. These results indicate that Fc gamma RIIA can be activated in human platelets downstream G-protein-coupled receptors and suggest a novel general mechanism for the reinforcement of platelet activation induced by low concentrations of agonists. (c) 2005 Elsevier Inc. All rights reserved.