Loss of AMP-Activated Protein Kinase-α2 Impairs the Insulin-Sensitizing Effect of Calorie Restriction in Skeletal Muscle.

Whether the well-known metabolic switch AMP-activated protein kinase (AMPK) is involved in the insulin-sensitizing effect of calorie restriction (CR) is unclear. In this study, we investigated the role of AMPK in the insulin-sensitizing effect of CR in skeletal muscle. Wild-type (WT) and AMPK-α2^-/- mice received ad libitum (AL) or CR (8 weeks at 60% of AL) feeding. CR increased the protein level of AMPK-α2 and phosphorylation of AMPK-α2. In WT and AMPK-α2^-/- mice, CR induced comparable changes of body weight, fat pad weight, serum triglycerides, serum nonesterified fatty acids, and serum leptin levels. However, decreasing levels of fasting/fed insulin and fed glucose were observed in WT mice but not in AMPK-α2^-/- mice. Moreover, CR-induced improvements of whole-body insulin sensitivity (evidenced by glucose tolerance test/insulin tolerance test assays) and glucose uptake in skeletal muscle tissues were abolished in AMPK-α2^-/- mice. Furthermore, CR-induced activation of Akt-TBC1D1/TBC1D4 signaling, inhibition of mammalian target of rapamycin -- S6K1 -- insulin receptor substrate-1 pathway, and induction of nicotinamide phosphoribosyltransferase -- NAD⁺ --sirtuin-1 cascade were remarkably impaired in AMPK-α2^-/- mice. CR serum increased stability of AMPK-α2 protein via inhibiting the X chromosomelinked ubiquitin-specific protease 9--mediated ubiquitylation of AMPK-α2. Our results suggest that AMPK may be modulated by CR in a ubiquitylation-dependent manner and acts as a chief dictator for the insulin-sensitizing effects of CR in skeletal muscle. [ABSTRACT FROM AUTHOR]