Phosphatidylinositol 3,4,5-trisphosphate Triggers Platelet Aggregation by Activating Ca2+ Influx

Exogenous phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] stimulates the aggregation of washed rabbit platelets in a Ca2+- and dose-dependent manner. This aggregation is reversible at low PtdIns(3,4,5)P3 levels, but becomes irreversible when the concentration exceeds a threshold of about 20 microM. Other D-3 and D-4 phosphoinositides examined, including phosphatidylinositol 3, 4-bisphosphate [PtdIns(3,4)P2], phosphatidylinositol 4, 5-bisphosphate [PtdIns(4,5)P2], and phosphatidylinositol 3-monophosphate [PtdIns(3)P], fail to exert appreciable platelet activation at comparable concentrations. In addition, PtdIns(3,4, 5)P3 can reverse the inhibitory effect of wortmannin on thrombin-induced platelet aggregation. Taken together with the observation that PtdIns(3,4,5)P3 is readily incorporated into cell membranes, these findings reaffirm the second messenger role of PtdIns(3,4,5)P3 in thrombin receptor activation. The existence of a PtdIns(3,4,5)P3-dependent Ca2+ entry system on platelet membranes is supported by the partial inhibition of thrombin-induced Ca2+ influx by wortmannin. Evidence suggests that this system differs from receptor-operated nonselective Ca2+ channels. However, the mechanism by which PtdIns(3,4,5)P3 facilitates Ca2+ entry remains unclear. Although PtdIns(3,4,5)P3 has been known to stimulate phospholipase C-gamma (PLC-gamma), internal Ca2+ mobilization does not play a significant role in the cytosolic Ca2+ increase in response to PtdIns(3,4,5)P3 stimulation. Collectively, these data provide a putative link between PtdIns(3,4,5)P3 and Ca2+ signaling, which may, in part, account for the regulatory function of PtdIns(3,4,5)P3 during platelet aggregation. Moreover, this study bears out the notion that individual PI 3-kinase lipid products play distinct roles in the regulation of cellular functions.