9-CIS-RETINOIC ACID REGULATES THE EXPRESSION OF THE MUSCLE DETERMINATION GENE MYF5

Myf5 is a member of the MyoD family, a set of four helix-loop-helix tr anscription factors that controls myogenic differentiation. The Myf5 g ene has both in vivo and in vitro expression patterns consistent with an involvement in the first events of myogenesis, such as acquisition and/or maintenance of myogenic ''determined'' phenotype. To date, very little is known about the mechanism underlying the tight regulation o f Myf5 expression. We report here that retinoic acid (RA) reduces the level of Myf5 message in both mouse C2 and rat L6 cell lines, probably at the transcriptional level, because Myf5 mRNA stability is not affe cted by RA. This repression is dose dependent, starting at 0.1 mum of all-trans RA, and is not abrogated by cycloheximide, suggesting a dire ct involvement of RA receptors in the control of Myf5 expression. Furt hermore, we compared the efficiency of natural (all-trans RA and 9-cis RA) or synthetic (TTNPB) retinoids that differentially activate the t wo families of RA receptors, RA receptors and retinoid X-receptors (9- cis RA). As 9-cis RA is about 10 times more efficient than all-trans R A in repressing Myf5, whereas TTNPB, which preferentially activates RA receptors, is far less potent, our data provide evidence for an impor tant role of ligand-bound retinoid X-receptors in the mediation of thi s inhibition.