Activation of protein kinase B induced by H2O2 and heat shock through distinct mechanisms dependent and independent of phosphatidylinositol 3-kinase

Protein kinase B (PKB) is a downstream target of phosphatidylinositol (PI) 3-kinase in the signaling pathway of growth factors, and is activated by ce llular stress such as H2O2 and heat shock. To study the mechanism of the st ress-induced activation of PKB, PI 3-kinase products were measured in stres s-stimulated cells. Both PI 3,4-bisphosphate and PI 3,4,5-trisphosphate inc reased in H2O2-treated cells, and the elevation of these phospholipids and activation of PRE were concurrently blocked by wortmannin, a potent inhibit or of PI 3-kinase. In heat-shocked cells, the level of PI 3,4-bisphosphate did not change while that of PI 3,4,5-trisphosphate increased slightly, and an association between PKB molecules was observed. Two active PKB fraction s, presumably monomeric and oligomeric forms, were resolved from heat-shock ed cells by gel filtration column chromatography. Activation of the former was suppressed by pretreatment with wortmannin, whereas the generation and activation of the latter were not blocked by the PI 3-kinase inhibitor. Onl y the monomeric form, but not the oligomeric form, was recovered from H2O2- treated cells,and its activation was prevented by wortmannin. These results indicate that PKB is activated by two distinct mechanisms that are depende nt and independent of PI 3-kinase in stress-stimulated cells.