A differential role of the platelet ADP receptors P2Y1 and P2Y12 in Rac activation

The dynamics of the actin cytoskeleton, largely controlled by the Rho family of small GTPases (Rho, Rac and Cdc42), is critical for the regulation of platelet responses such as shape change, adhesion, spreading and aggregation. Here, we investigated the role of adenosine diphosphate (ADP), a major co-activator of platelets, on the activation of Rac. ADP rapidly activated Rac in a dose-dependent manner and independently of GPIIb/IIIa and phosphoinositide 3-kinase. ADP alone, used as a primary agonist, activated Rac and its effector PAK via its P2Y(1) receptor, through a G(q)-dependent pathway and independently of P2Y(12). The P2Y12 receptor appeared unable to activate the GTPase per se as also observed for the adenosine triphosphate receptor P2X(1). Conversely, secreted ADP strongly potentiated Rac activation induced by Fc gamma RIIa clustering or TRAP via its P2Y(12) receptor, the target of antithrombotic thienopyridines. Stimulation of the alpha(2A)-adrenergic receptor/G(z) pathway by epinephrine was able to replace the P2Y(12)/G(i)-mediated pathway to amplify Rac activation by Fc gamma RIIa or by the thrombin receptor PAR-1. This co-activation appeared necessary to reach a full stimulation of Rac as well as PAK activation and actin polymerization and was blocked by a G-protein beta gamma subunits scavenger peptide.