Résumé :
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Glucocorticoids are currently the only drugs recognized to benefit Duchenne muscular dystrophy (DMD) patients. The mechanisms that underlie the beneficial effects still remain incompletely understood. In agreement with previous observations, we show here that treatment of myotubes with the glucocorticoid 6-alpha-methylprednisolone-21 sodium succinate (PDN) results in enhanced expression of utrophin A without concomitant increases in mRNA levels, thereby suggesting that translational regulation contributes to the increase. Previously, we established that the utrophin A 5’UTR contains an Internal Ribosome Entry Site (IRES) that is activated in vivo during muscle regeneration (J Biol Chem. 280:32997-3005, 2005). In the present study, using monocistronic and bicistronic reporter assays, we demonstrate that activity of this IRES is enhanced by PDN treatment. Analysis of polysomes from PDN treated cells show an increase in polysome association of endogenous utrophin A mRNAs and reporter mRNAs harbouring the utrophin A 5’UTR, while global translation rates were found to be depressed. Additional experiments identified a distinct region within the utrophin A 5’UTR that contains the inducible IRES activity and displays enhanced binding to multiple proteins following PDN treatment. Together, these studies demonstrate that a translational regulatory mechanism involving increased IRES activation mediates, at least partially, the enhanced expression of utrophin A in muscle cells treated with glucocorticoids. As part of this work we have also created several lines of transgenic mice harbouring the utrophin A IRES reporter in order to assess the tissue distribution of utrophin A IRES activity and its regulation in response to both PDN treatment and muscle regeneration in vivo. In addition, we are also investigating the involvement of the utrophin 3’UTR in regulating utrophin IRES activity. These studies may contribute to the development of drugs that specifically target the utrophin UTRs to drive increased expression of utrophin in the muscle fibers of DMD patients.
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