Activation of protein kinase C by phosphatidylinositol 3,4,5-trisphosphate.

Phosphatidylinositol 3-kinase (PI 3-kinase) was partially purified from rat liver cytosol and used to synthesize phosphatidylinositol 3,4,5-trisphosphate (PIP3), using phosphatidylinositol 4,5-bisphosphate (PIP2) as a substrate. Purified PIP3 (free of chromatographic oxalate) activated protein kinase C (PKC) in the presence of phosphatidylserine and calcium (PKC -cofactors) in a concentration-dependent manner. In the absence of these cofactors, effect of PIP3 was not observed. Comparison of the effects of PIP3 and PIP2 on PKC activity indicates that PIP3 is a more potent PKC-activator than PIP2. The affinity of PKC to PIP3 was 4 fold higher than that to PIP2 (KPIP3 = 0.022 and KPIP2 = 0.087 mol %), while its maximal velocity (Vmax) was similar to that of PIP2-stimulated PKC activity (0.4 - 0.5 mumol/mg/min). These results suggest a physiological role for PIP3 in signal transduction, and support the previous finding (Chauhan et al. (1991) Arch. Biochem. Biophys. 287,283) that PKC-activation by phosphoinositides increases with the state of phosphorylation of these lipids. We propose that PIP3 by activating PKC may initiate a cascade of events from PIP3-->PKC- activation-->effects on other protein kinases such as MAP-kinase-->gene expression.