IGF-I does not prevent myotube atrophy caused by proinflammatory cytokines despite activation of Akt/Foxo and GSK-3β pathways and inhibition of atrogin-1 mRNA

Myofibrillar protein loss occurring in catabolic situations is considered to be mediated by the release of proinflammatory cytokines and associated with a decrease in circulating and muscle levels of insulin-like growth factor I (IGF-I). In this paper, we investigated whether the C 2 C 12 myotube atrophy caused in vitro by TNF-α/IFN-γ cytokines might be reversed by exogenous IGF-I. Our results showed that, despite the presence of TNF-α/IFN-γ, IGF-I retained its full ability to induce the phosphorylation of Akt, Foxo3a, and GSK-3β (respectively, 16-fold, 9-fold, and 2-fold) together with a decrease in atrogin-1 mRNA (−39%, P < 0.001). Although this ubiquitin ligase has been reported to accelerate the degradation of MyoD, a myogenic transcription factor driving the transcription of myosin heavy chain (MHC), IGF-I failed to blunt the reduction of MyoD and MHC caused by TNF-α/IFN-γ. Moreover, IGF-I only very slightly attenuated the myotube atrophy induced by TNF-α/IFN-γ (TNF-α/IFN-γ 15.48 μm alone vs. TNF-α/IFN-γ/IGF-I 16.97 μm, P < 0.001). In conclusion, our data show that IGF-I does not reverse the myotube atrophy induced by TNF-α/IFN-γ despite the phosphorylation of Foxo and GSK-3β and the downregulation of atrogin-1 mRNA. Our study suggests therefore that factors other than IGF-I decrease are responsible for the muscle atrophy caused by proinflammatory cytokines.