Insulin-like growth factor-I inhibits dexamethasone-induced proteolysis in cultured L6 myotubes through PI3K/Akt/GSK-3β and PI3K/Akt/mTOR-dependent mechanisms

We and others reported previously that IGF-I inhibits dexamethasone-induced proteolysis in cultured L6 myotubes. Recent evidence suggests that this effect of IGF-I at least in part reflects PI3K/Akt-mediated inhibition of Foxo transcription factors. The potential role of other mechanisms, downstream of PI3K/Akt, is not well understood. Here we tested the hypothesis that PI3K/Akt-mediated inactivation of GSK-3β and activation of mTOR contribute to the anabolic effects of IGF-I in dexamethasone-treated myotubes. Cultured L6 myotubes were treated with 1 μM dexamethasone in the absence or presence of 0.1 μg/ml of IGF-I and inhibitors of GSK-3β and mTOR. Protein degradation was measured by determining the release of trichloroacetic acid soluble radioactivity from myotubes that had been prelabeled with 3H-tyrosine for 48 h. IGF-I reduced basal protein breakdown rates and completely abolished the dexamethasone-induced increase in myotube proteolysis. These effects of IGF-I were associated with increased phosphorylation of Akt, GSK-3β, and the mTOR downstream targets p70S6K and 4E-BP1. The PI3K inhibitor LY294002 and the mTOR inhibitor rapamycin reversed the anabolic effect of IGF-I in dexamethasone-treated myotubes. In addition, the GSK-3β inhibitors LiCl and TDZD-8 reduced protein degradation in a similar fashion as IGF-I. Our results suggest that PI3K/Akt-mediated inactivation of GSK-3β and activation of mTOR contribute to the anabolic effects of IGF-I in dexamethasone-treated myotubes.