Abstract

Studies in nonmuscle cells have demonstrated that Ca²⁺/calmodulin-dependent protein kinase kinases (CaMKKs) are upstream regulators of AMP-activated protein kinase (AMPK) and Akt. In skeletal muscle, activation of AMPK and Akt has been implicated in the regulation of glucose uptake. The objective of this study was to determine whether CaMKKα regulates skeletal muscle glucose uptake, and whether it is dependent on AMPK and/or Akt activation. Expression vectors containing constitutively active CaMKKα (caCaMKKα) or empty vector were transfected into mouse muscles by in vivo electroporation. After 2 weeks, caCaMKKα was robustly expressed and increased CaMKI (Thr^177/180) phosphorylation, a known CaMKK substrate. In muscles from wild-type mice, caCaMKKα increased in vivo [³H]-2-deoxyglucose uptake 2.5-fold and AMPKα1 and -α2 activities 2.5-fold. However, in muscles from AMPKα2 inactive mice (AMPKα2i), caCaMKKα did not increase AMPKα1 or -α2 activities, but it did increase glucose uptake 2.5-fold, demonstrating that caCaMKKα stimulates glucose uptake independent of AMPK. Akt (Thr³⁰⁸) phosphorylation was not altered by CaMKKα, and caCaMKKα plus insulin stimulation did not increase the insulin-induced phosphorylation of Akt (Thr³⁰⁸). These results suggest that caCaMKKα stimulates glucose uptake via insulin-independent signaling mechanisms. To assess the role of CaMKK in contraction-stimulated glucose uptake, isolated muscles were treated with or without the CaMKK inhibitor STO-609 and then electrically stimulated to contract. Contraction increased glucose uptake 3.5-fold in muscles from both wild-type and AMPKα2i mice, but STO-609 significantly decreased glucose uptake (∼24%) only in AMPKα2i mice. Collectively, these results implicate CaMKKα in the regulation of skeletal muscle glucose uptake independent of AMPK and Akt activation.