Ambient glucose levels qualify the potency of insulin myogenic actions by regulating SIRT1 and FoxO3a in C₂C₁₂ myocytes

Nutrition availability is one of the major environmental signals influencing cell fate, such as proliferation, differentiation, and apoptosis, often functioning in concert with other humoral factors, including insulin. Herein, we show that low-serum-induced differentiation of C₂C ₁₂ myocytes is significantly hampered under low glucose (LG; 5 mM) compared with high glucose (HG; 22.5 mM) conditions, concurrently with nuclear accumulation of SIRT1, an NAD⁺-dependent deacetylase, and FoxO3a, both of which are implicated in the negative regulation of myogenesis. Intriguingly, insulin appears to exert opposite actions, depending on glucose availability, with regard to the regulation of SIRT1 and FoxO3a abundance, which apparently contributes to modulating the potency of insulin's myogenic action. Namely, insulin exerts a potent myogenic effect in the presence of sufficient glucose, whereas insulin is unable to exert its myogenic action under LG conditions, since insulin evokes massive upregulation of both SIRT1 and FoxO3a in the absence of sufficient ambient glucose. In addition, the hampered differentiation state under LG is significantly restored by sirtinol, a SIRT1 inhibitor, whereas insulin abolished this sirtinol-dependent restoration, indicating that insulin can function as a negative as well as a positive myogenic factor depending on glucose availability. Taken together, our data reveal the importance of ambient glucose levels in the regulation of myogenesis and also in the determination of insulin's myogenic potency, which is achieved, at least in part, through regulation of the cellular contents and localization of SIRT1 and FoxO3a in differentiating C₂C₁₂ myocytes.