PHOSPHATIDYLINOSITOL 3,4,5-TRISPHOSPHATE TRIGGERS PLATELET-AGGREGATION BY ACTIVATING CA2+ INFLUX

Exogenous phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P-3] stimulates the aggregation of washed rabbit platelets in a Ca2+- and d ose-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 mu M. Other D-3 and D-4 phosphoinositides exam ined, including phosphatidylinositol 3,4-bisphosphate [PtdIns(3,4)P-2] , phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P-2], and phosphat idylinositol 3-monophosphate [PtdIns(3)P], fail to exert appreciable p latelet activation at comparable concentrations. In addition, PtdIns(3 ,4,5)P-3 can reverse the inhibitory effect of wortmannin on thrombin-i nduced platelet aggregation. Taken together with the observation that PtdIns(3,4,5)P-3 is readily incorporated into cell membranes, these fi ndings reaffirm the second messenger role of PtdIns(3,4,5)P-3 in throm bin receptor activation. The existence of a PtdIns(3,4,5)P-3-dependent Ca2+ entry system on platelet membranes is supported by the partial i nhibition of thrombin-induced Ca2+ influx by wortmannin. Evidence sugg ests that this system differs from receptor-operated nonselective Ca2 channels.; However, the mechanism by which PtdIns(3,4,5)P-3 facilitat es Ca2+ entry remains unclear. Although PtdIns(3,4,5)P-3 has been know n to stimulate phospholipase C-gamma (PLC-gamma), internal Ca2+ mobili zation does not play a significant role in the cytosolic Ca2+ increase in response to PtdIns(3,4,5)P-3 stimulation. Collectively, these data provide a putative link between PtdIns(3,4,5)P-3 and Ca2+ signaling, which may, in part, account for the regulatory function of PtdIns(3,4, 5)P-3 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.