Protein Kinase B/AKT 1 Plays a Pivotal Role in Insulin-like Growth Factor-1 Receptor Signaling Induced 3T3-L1 Adipocyte Differentiation

During 3T3-L1 preadipocyte differentiation induction, the insulin-stimulated insulin-like growth factor-1 (IGF-1) receptor signal is responsible for the induction of adipocyte differentiation. Treatment with inhibitors of phosphatidylinositol 3-kinase, LY294002 or wortmannin, leads to the complete blockade of adipocyte differentiation in 3T3-L1 preadipocytes. Of the three factors (1-methyl-3-isobutylxanthine, dexamethasone, and insulin) inducing 3T3-L1 preadipocyte differentiation, only insulin was able to activate the phosphatidylinositol 3-kinase-protein kinase B/Akt signal cascade. In 3T3-L1 preadipocytes, protein kinase B/Akt 1 RNA interference not only suppressed the expression of protein kinase B/Akt 1 but also blocked hormone-induced adipocyte differentiation. In these protein kinase B/Akt 1 RNA interference cells, the signal molecules upstream of protein kinase B/Akt 1, such as IGF-1 receptor and insulin receptor substrate-1, were normally activated by insulin stimulation, whereas insulin-stimulated phosphorylation of forkhead transcription factor (FKHR), which is a downstream molecule of PKB/Akt 1, was blocked. Thus, protein kinase B/Akt 1 is an important signal mediator in IGF-1 receptor signal cascade for inducing adipocyte differentiation.