Résumé :
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Muscle atrophy associated with a number of systemic diseases such as cancer constitutes a major health problem, due to its contribution to the deterioration of patient status and its impact on mortality. The primary cause of rapid muscle mass loss is accelerated protein breakdown associated with decreased synthesis. Available therapies have limited effects on muscle wasting and restoration of muscle function. Our project focuses on the role of sphingolipid metabolites in cancer-induced muscle atrophy, and on the possibilities of therapeutical interventions based on the modulation of sphingolipid metabolism. The pro-inflammatory cytokine TNFalpha, the involvement of which in cancer cachexia is well-recognized, can increase ceramide levels by activating both de novo synthesis and recruitment of sphingomyelinases. By using L6 myotubes treated with TNFalpha as an in vitro model of atrophy, we observed that myriocin, an inhibitor of de novo ceramide synthesis, and the neutral sphingomyelinase inhibitors GW4869 and 3-O-Methyl-sphingomyelin, are able to counteract the negative effects of TNFalpha on creatine kinase activity, size and myosin content of myotubes, suggesting that ceramides are involved in TNFalpha atrophic effects. We then investigated the effects of ceramide synthesis inhibitors on the mTOR/Akt pathway, a key player in the control of protein synthesis/breakdown. The inhibitors increased phosphorylation of mTOR substrates S6K1 and 4E-BP1 that critically regulate protein translation, and activated Akt, a negative regulator of proteolysis. In addition, myriocin and GW4869 increased the expression of phospholipase D1 (PLD1), an activator of mTOR pathway. These data suggest that ceramides can act by inhibiting PLD1 and thereby down-regulating mTOR pathway. As a confirmation, phosphatidic acid, the product of PLD activity, rescued myotubes from TNFalpha-induced atrophy. In vivo experiments confirmed that inhibition of ceramide synthesis protects mice from C26 tumor-induced muscle atrophy. These findings provide evidence that sphingolipid messengers are involved in muscle wasting, and suggest that drugs, or modifications of the dietary fatty acid supply, that affect the sphingolipid pathway have interesting therapeutical potentialities for the preservation of muscle mass and the improvement of cancer patients condition.
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