Résumé :
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INTRODUCTION The control of muscle cell size is balanced between protein synthesis and degradation. The muscle specific ubiquitine ligase Atrogin-1 is associated to pathology ranking from immobilisation, to sepsis, cancer and HIV. OBJECTIVE Atrogin-1 accumulation is a universal and early response to the activation of muscle atrophy, necessary for rapid proteolysis of myofibers in vivo. However, the precise function of MAFbx in muscle wasting has been questionned. METHODS In order to idenify Atrogin-1 targets we devellopped stringent screening strategy of human skeletal muscle yeast two hybrid library using several baits, ie the hAtrogin-1 N-terminal protein fragment, and validation in primary and immotalized muscle cells. Purfied recombinant and reconstituted Atrogin-1 (wild type and mutated) E3-ligase complex was used in vitro to confirm substrat specificity and kinetics of proteasomal degradation. In vivo experiments were achieved in mice electroporated in tibialis anterior (TA). RESULTS We have provided direct biological and biochemical evidence that Atrogin-1 targets both MyoD and eIF3f (eukaryotic translation factor 3f) thereby inducing their ubiquitin dependent proteolysis in vitro, in cellulo and in vivo. Overexpression of Atrogin-1 suppresses MyoD-induced differentiation and inhibits myotube formation. More importantly, we showed that eIF3-f act as a translational enhancer that increases the efficiency of the structural muscle proteins synthesis leading to both in vitro and in vivo muscle hypertrophy DISCUSSION Our results demonstrate that in pathological muscles, Atrogin-1 dependant targeting of MyoD and eIF3f degradation prevent both transcription and translation of hypertrophy associated genes. Moreover, we strongly believe that Atrogin-1 dependant degradation of MyoD in activated satellite cells prevents regeneration of skeletal muscle fibers undergoing atrophy. CONCLUSION We provide here a detailed and coherant description of Atrogin-1 mediated inhibition of muscle protein synthesis during skelettal muscle atrophy (Tintignac LA et al., JBC 2005; Lagirand-Cantaloube1 J., et al EMBO 2008).
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