Résumé :
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Oxidative stress-induced mitochondrial dysfunction has been shown to play a crucial role in the pathogenesis of a wide range of diseases including muscle disorders. Protecting mitochondrial function, therefore, is vital for cells to survive. In this study, we demonstrate that melatonin, the main secretory product of the pineal gland, readily rescued mitochondria from oxidative stress-induced dysfunction and effectively prevented subsequent apoptotic/necrotic events and death in C57BL/6J myotubes. In particular, melatonin potently prevented myotube death induced by tert-butylhydroperoxide (t-BHP) in a concentration-dependent manner (10-4-10-6 M). This protective effect was more potent than that of N-acetyl-L-cystein, a well known antioxidant that increases cellular pools of free-radical scavengers. Moreover, melatonin maintained plasma membrane integrity (t-BHP-induced membrane blebbing) after t-BHP exposure and prevented t-BHP-induced fissions of the long mitochondrial filaments and inhibited mitochondrial swelling that was clearly visible after t-BHP treatment. To determine if the mitochondrial protection provided by melatonin was due to the inhibition of the formation of reactive oxygen species (ROS), intracellular ROS levels were measured using fluorescence imaging. Application of t-BHP produced a rapid and significant increase in free-radical generation in myotubes. This effect was concentration dependently prevented by pretreatment of the myotubes with melatonin. Considering that t-BHP cytotoxicity was also prevented by cyclosporin A, a mitochondrial permeability transition pore (mPTP) inhibitor, we investigated the effect of melatonin on mPTP. Melatonin prevented t-BHP-induced mitochondrial depolarization and protected the pyridine nucleotides and glutathione (two regulators of mPTP opening under conditions of oxidative stress) against t-BHP-induced stress. Using isolated mitochondria, we found that melatonin (10-8-10-6 M) desensitized the mPTP to Ca2+ and prevented t-BHP-induced mitochondrial swelling, pyridine nucleotides and glutathione oxidation, and enhanced mitochondrial function. In conclusion, our findings suggest that inhibition of the mPTP may essentially contribute to the protective effect of melatonin against oxidative stress in myotubes.
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