Disassociation of insulin action and Akt/FOXO signaling in skeletal muscle of older Akt-deficient mice.

The purpose of the present study was to determine the effect of Akt gene ablation on Akt/Forkhead Box O (FOXO) signaling and atrogene expression. This was accomplished by studying wild-type (WT) and isoform-specific Akt knockout (Akt1(-/-) and Akt2(-/-)) mice. The ability of insulin to promote Akt phosphorylation on Ser(473) was significantly lower in extensor digitorum longus (EDL) and soleus muscles from Akt1(-/-) and Akt2(-/-) mice compared with WT mice. Total Akt1 protein levels were significantly lower in EDL muscles of Akt2(-/-) mice compared with WT mice, a process that appears to be posttranscriptionally regulated as Akt1 mRNA levels were unchanged. The loss of Akt1 protein in EDL muscles of Akt2(-/-) mice does not appear to be due to insulin resistance because 4 mo of a high-fat diet failed to reduce Akt1 protein levels in muscles of WT mice. Although FOXO3a phosphorylation and atrogin-1 expression were unaltered in muscles of Akt1(-/-) and Akt2(-/-) mice, the expression of the atrogenes Bnip3 and gabarapl were significantly elevated in muscles of both Akt1 and Akt2 knockout mice. Finally, the expression of striated activator of Rho signaling was significantly increased in muscles of Akt2(-/-) mice compared with Akt1(-/-) and WT mice. Our results demonstrate that the ablation of Akt isoforms disassociates insulin action and Akt/FOXO signaling to atrogenes.

PKB signaling and atrogene expression in skeletal muscle of aged mice.

The purpose of this study was to determine if PKB signaling is decreased and contractile protein degradation is increased in extensor digitorum longus (EDL) and soleus (SOL) muscles from middle-aged (MA) and aged (AG) mice. We also examined the effect of age on atrogene expression in quadriceps muscle. PKB activity, as assessed by Thr(308) and Ser(473) phosphorylation, was significantly higher in EDL and SOL muscles from AG than MA mice. The age-related increase in PKB activity appears to be due to an increase in expression of the kinase, as PKB-α and PKB-ß levels were significantly higher in EDL and SOL muscles from AG than MA mice. The phosphorylation of forkhead box 3a (FOXO3a) on Thr(32), a PKB target, was significantly higher in EDL muscles from AG than MA mice. The rate of contractile protein degradation was similar in EDL and SOL muscles from AG and MA mice. Atrogin-1 and muscle-specific RING finger protein 1 (MuRF-1) mRNA levels did not change in muscles from AG compared with MA mice, indicating that ubiquitin-proteasome proteolysis does not contribute to sarcopenia. A significant decrease in Bcl-2 and 19-kDa interacting protein 3 (Bnip3) and GABA receptor-associated protein 1 (Gabarap1) mRNA was observed in muscles from AG compared with MA mice, which may contribute to age-related contractile dysfunction. In conclusion, the mechanisms responsible for sarcopenia are distinct from experimental models of atrophy and do not involve atrogin-1 and MuRF-1 or enhanced proteolysis. Finally, a decline in autophagy-related gene expression may provide a novel mechanism for impaired contractile function and muscle metabolism with advancing age.