Activation of protein kinase B and induction of adipogenesis by insulin in3T3-L1 preadipocytes - Contribution of phosphoinositide-3,4,5-trisphosphate versus phosphoinositide-3,4-bisphosphate

Ectopic expression of activated protein kinase B (PKB) induces the differen tiation of confluent 3T3-L1 preadipocytes into adipocytes. PKB is regulated by the lipid products of phosphoinositide 3-kinase (PI 3-kinase), phosphat idylinositol-3,4-bisphosphate [PI(3,4)P2], and phosphatidylinositol-3,4,5-t risphosphate [PI(3,4,5)P3]. However, the relative contribution of each 3-ph osphorylated phosphoinositide species in activating PKB remains unclear. Tr eatment of intact 3T3-L1 preadipocytes with synthetic 3-phosphorylated phos phoinositides revealed that only PI(3,4)P2 stimulated PKB activity. PKB was also activated by insulin, in a dose- and time-dependent manner. This acti vation was associated with an isolated rise in PI(3,4,5)P3, without any det ectable change in PI(3,4)P2, demonstrating that this lipid was sufficient t o activate PKB. Wortmannin and LY294002, inhibitors of PI 3-kinase, reduced insulin-dependent activation of PKB, whereas rapamycin, an inhibitor of p7 0 S6 kinase, had no effect. Platelet-derived growth factor (PDGF), which is not adipogenic, stimulated the production of both 3-phosphorylated phospho inositide species, and this was associated with a greater activation of PKB than that observed with insulin. A low dose of PDGF (1 ng/ml), which incre ased the production of only PI(3,4,5)P3 and mirrored the insulin effect, wa s unable to induce adipocyte differentiation. In summary, insulin and PDGF differ with respect to the accumulation of 3-phosphorylated phosphoinositid es and to PKB activation in 3T3-L1 preadipocytes, but these responses do no t themselves explain why insulin, but not PDGF, is adipogenic.