Objective: FoxO1 is a substrate of serine/threonine-kinase Akt. Although FoxO1 is phosphorylated by Akt and inactivated by insulin stimulation, constitutive activation of FoxO1 is reported to enhance Akt phosphorylation. Therefore, constitutive activation of FoxO1 may mimic insulin’s functions via Akt phosphorylation.
Methods: Constitutively-active FoxO1 (CA-FoxO1) in 3T3-L1 adipocytes is overexpressed by adenovirus and Changes in insulin signaling and downstream actions was studied.
Results: CA-FoxO1 induced Akt phosphorylation in differentiated 3T3-L1 adipocytes. Regarding downstream actions, CA-FoxO1 significantly enhanced glucose uptake (148.6% compared to controls, P =0.030), GLUT4 translocation (297.6%, P <0.01), lipogenesis (175.9%, P =0.030) and tended to enhance glycogen synthesis (128.7%, P =0.154), while in contrast, under insulin-stimulated condition, significantly suppressed protein synthesis (56.5%, P <0.01) and DNA synthesis (44.4%, P <0.01). Regarding insulin signaling, CA-FoxO1 enhanced phosphorylations of GSK-3, PRAS40, and p70S6K. The enhancement of PI 3-kinase activity by 3nM insulin is larger than that by CA-FoxO1, while Akt phosphorylation by 3nM insulin is smaller than that by CA-FoxO1, suggesting the mechanism of FoxO1-Akt feedback is downstream of PI 3-kinase. In contrast to previous papers, protein amounts of IR-β, p110α subunit of PI 3-kinase, or Rictor were not significantly changed by CA-FoxO1.
Conclusion: Constitutive activation of FoxO1 induced Akt phosphorylation even in differentiated 3T3-L1 adipocytes, but FoxO1 activation mimicked only metabolic functions of glucose but not proliferative or oncogenic functions of insulin. This selective mimicking of insulin by FoxO1 activation in adipocytes may improve glucose metabolism with a low risk of tumorigenesis. The mechanisms of FoxO1-Akt feedback is suggested not to be the increase in IR-β or p110α subunit of PI 3-kinase as previously reported, but to be mainly downstream of PI 3-kinase.
T. Ohno: None. H. Ono: None. K. Yokote: Research Support; Self; Astellas Pharma Inc., Daiichi Sankyo, Eli Lilly Japan K.K., Kowa Company, Ltd., Kyowa Hakko Kirin Co., Ltd., Merck Sharp & Dohme Corp., Mitsubishi Tanabe Pharma Corporation, Mochida Pharmaceutical Co., Ltd., Nippon Boehringer Ingelheim Co. Ltd., Novartis Pharma K.K., Novo Nordisk Inc., Ono Pharmaceutical Co., Ltd., Pfizer Japan Inc., Sanofi K.K., Shionogi & Co., Ltd., Sumitomo Dainippon Pharma Co., Ltd., Taisho Pharmaceutical Co., Ltd., Takeda Pharmaceutical Company Limited, Teijin Pharma Limited. Speaker’s Bureau; Self; Abbott, Astellas Amgen, AstraZeneca K.K., Bayer Inc., FUJIFILM Pharmaceuticals U.S.A., Inc., Kaken Pharmaceutical Co., Ltd., Sanwa Kagaku Kenkyusho.