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pathways and inhibition of atrogin-1 mRNA
Divisions of 1Diabetology and Nutrition and 2Cardiology, Université catholique de Louvain, Brussels, Belgium
Submitted 21 February 2006 ; accepted in final form 14 August 2006
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 C2C12 myotube atrophy caused in vitro by TNF-
/IFN-
cytokines might be reversed by exogenous IGF-I. Our results showed that, despite the presence of TNF-
/IFN-
, IGF-I retained its full ability to induce the phosphorylation of Akt, Foxo3a, and GSK-3
(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-
/IFN-
. Moreover, IGF-I only very slightly attenuated the myotube atrophy induced by TNF-
/IFN-
(TNF-
/IFN-
15.48 µm alone vs. TNF-
/IFN-
/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-
/IFN-
despite the phosphorylation of Foxo and GSK-3
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.
growth factors; ubiquitin-proteasome system; tumor necrosis factor-
; transduction pathway; muscle atrophy; glycogen synthase kinase-3
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