121-OR: Activation of FoxO1 Induces Akt Phosphorylation and Mimics the Metabolic Functions of Insulin without Proliferative Effects on 3T3-L1 Adipocytes

Abstract

Objective: Insulin promotes glucose uptake, lipogenesis, cell proliferation and cell growth mainly via Akt phosphorylation. FoxO1 is a substrate of Akt, which is inactivated when phosphorylated by Akt. Via this mechanism, FoxO1 antagonizes insulin. On the other hand, constitutive activation of FoxO1 reportedly enhances Akt phosphorylation. Thus, FoxO1 activation, via Akt phosphorylation, may mimic the functions of insulin. Methods: We overexpressed constitutively-active FoxO1 (CA-FoxO1) and FoxO1Δ256, a dominant-negative mutant, in 3T3-L1 adipocytes by adenoviruses and investigated the effects of these proteins on insulin’s functions. Results: CA-FoxO1 enhanced Akt phosphorylation, glucose uptake (275%, P<0.01), GLUT4 translocation and lipogenesis (176%, P<0.05) in the non-insulin stimulated state, while DNA synthesis was decreased (44%, P<0.01) in the insulin-stimulated state. Surprisingly, FoxO1Δ256 also enhanced Akt phosphorylation, glucose uptake (154%, P<0.05) and lipogenesis (268%, P<0.01) in the basal state. In contrast to CA-FoxO1, FoxO1Δ256 increased DNA synthesis (196%, P<0.01). FoxO1Δ256 did not exert a dominant negative action even on gene expressions related to adipocyte differentiation, de novo lipogenesis and cell cycle regulation. Akt phosphorylation was more strongly induced by CA-FoxO1 than by 3 nM insulin, while PI 3-kinase activation by FoxO1 was weaker than that induced by 3 nM insulin, indicating the mechanism of Akt phosphorylation by FoxO1 to mainly be downstream from PI 3-kinase. Expressions of IR-β, p110α, Rictor and TRB3 were not significantly affected. Conclusion: Activation of FoxO1 selectively mimics the metabolic functions of insulin, without its proliferative effects. Contrary to previously reported results, FoxO1Δ256 does not always exert a dominant negative action. Disclosure T. Ohno: None. H. Ono: None. K. Takeda: None. K. Yokote: None.