Gi-dependent and -independent mechanisms downstream of the P2Y12 ADP-receptor

The P2Y12 ADP receptor is one of the major regulators of platelet activation and the target of antithrombotic thienopyridines (ticlopidine and clopidogrel). It has been recently cloned but the signaling pathways triggered by this receptor are still poorly documented. Here, we show that stimulation of the human P2Y12 receptor stably expressed in Chinese hamster ovary cells activates two major intracellular signaling mechanisms leading either to cell proliferation or to actin cytoskeleton reorganization. Both effects were blocked by the active metabolite of clopidogrel, a specific antagonist of P2Y12. The P2Y12-mediated stimulation of proliferation required the pertussis toxin-sensitive activation of PI3-kinase/Akt upstream of MAP-kinases. A partial contribution of a transactivation mechanism, through the tyrosine kinase receptor platelet-derived growth factor (PDGF)-R-beta, was also observed. Conversely, the P2Y12-mediated reorganization of the actin cytoskeleton was Gi-independent, requiring activation of RhoA and Rho-kinase. Our results provide new insights into the molecular basis of P2Y12-mediated intracellular signaling. These data may prove to be useful for a better understanding of the physiological role of P2Y12, particularly in platelets and glial cells which express this important therapeutic target.