DEXAMETHASONE-MEDIATED INDUCTION OF MMTV-MYF5 IN DD3 MYOBLASTS INCREASES ENDOGENOUS MYOGENIN EXPRESSION BUT DOES NOT TRANSACTIVATE MYF5

DD3 cells are a myoblast line generated by stable transfection of C3H1 0T1/2 fibroblasts with the bovine myf5 cDNA (bmyf) fused to the dexame thasone-inducible MMTV promoter. After treating proliferating cells wi th dexamethasone, bmyf transcripts were induced approximately fivefold within 1.5-2.5 h. Induction of bmyf was followed 4 h later by a simil ar increase in myogenin transcripts which was dependent on protein syn thesis. The elevated level of myogenin transcripts in dividing cells w as comparable to that observed in DD3 myoblasts after differentiation. The 4-h lag before the myogenin response suggested involvement of an intermediate factor. However, one possible factor, myocyte-specific en hancer binding factor (MEF-2) (a known enhancer of myogenin promoter a ctivity) was neither detectable nor inducible by dexamethasone in prol iferating cells. myoD transcripts were barely detectable and uninducib le in proliferating cells but strongly upregulated during differentiat ion. There was also a transient twofold increase in mrf4 transcripts b y dexamethasone treatment in dividing cells, while no changes were det ected in the levels of Id, E12, or TnC messages. The mouse myf5 gene w as silent and uninducible in DD3 cells-under proliferating and differe ntiating conditions. We conclude that ectopic expression of MMTV-bmyf led to activation of three of the endogenous myogenic factor genes of which only myogenin showed a rapid and sustained response to dexametha sone-mediated induction of bmyf in dividing cells. (C) 1994 Academic P ress,Inc.