Phosphatidylinositol-3,4,5-trisphosphate stimulates Ca2 + elevation and Akt phosphorylation to constitute a major mechanism of thromboxane A2 formation in human platelets

Abstract

Phosphatidylinositol trisphosphate (PIP3) has been implicated in many platelet functions however many of the mechanisms need clarification. We have used cell permeable analogues of PIP3,1-O-(1,2-di-palmitoyl-sn-glyero-3-O-phosphoryl)-D-myo-inositol-3,4,5-trisphosphate (DiC16-PIP3) or 1-O-(1,2-di-octanoyl-sn-glyero-3-O-phosphoryl)-D-myo-inositol-3,4,5-trisphosphate (DiC8-PIP3) to study their effects on activation on washed human platelets. Addition of either DiC8- or DiC16-PIP3 to human platelets induced aggregation in the presence of extracellular Ca2+. This was reduced by the presence of indomethacin, the phospholipase C inhibitor U73122 and apyrase. DiC8-PIP3 induced the phosphorylation of Akt-Ser473 which was reduced by the Akt inhibitor IV, wortmannin and EGTA (suggesting a dependence on Ca2+ entry). In Fura2 loaded platelets DiC8-PIP3 was effective at increasing intracellular Ca2+ in a distinct and transient manner that was reduced in the presence of indomethacin, U73122 and 2-aminoethyl diphenylborinate (2APB). Ca2+ elevation was reduced by the non-SOCE inhibitor LOE908 and also by the SOCE inhibitor BTP2. DiC8-PIP3 induced the release of Ca2+ from stores which was not affected by the proton dissipating agent bafilomycin A1 and was more potent than the two-pore channel agonist DiC8-PI[3,5]P2 suggesting release from an endoplasmic reticulum type store. DiC8-PIP3 weakly induced the tyrosine phosphorylation of Syk but not of PLCγ2. Finally like thrombin DiC8-PIP3 induced the formation of thromboxane B2 that was inhibited by the Akt inhibitor IV. These studies suggest that PIP3 via Ca2+ elevation and Akt phosphorylation forms a central role in thromboxane A2 formation and the amplification of platelet activation.