Aberrant regulation of MyoD1 contributes to the partially defective myogenic phenotype of BC₃H1 cells

Two skeletal muscle-specific regulatory factors, myogenin and MyoD1, share extensive homology within a myc similarity region and have each been shown to activate the morphologic and molecular events associated with myogenesis after transfection into nonmyogenic cells. The BC₃H1 muscle cell line expresses myogenin and other muscle-specific genes, but does not express MyoD1 during differentiation. BC₃H1 cells also do not upregulate α-cardiac actin or fast myosin light chain, nor do they form multinucleate myotubes during differentiation. In this study, we examined the basis for the lack of MyoD1 expression in BC₃H1 cells and investigated whether their failure to express MyoD1 is responsible for their defects in differentiation. We report that expression of an exogenous MyoD1 cDNA in BC₃H1 cells was sufficient to elevate the expression of α-cardiac actin and fast myosin light chain, and to convert these cells to a phenotype that forms multinucleate myotubes during differentiation. Whereas myogenin and MyoD1 positively regulated their own expression in transfected 10T1/2 cells, they could not, either alone or in combination, activate MyoD1 expression in BC₃H1 cells. Exposure of BC₃H1 cells to 5-azacytidine also failed to activate MyoD1 expression or to rescue the cell's ability to fuse. These results suggest that BC₃H1 cells may possess a defect that prevents activation of the MyoD1 gene by MyoD1 or myogenin. That an exogenous MyoD1 gene could rescue those aspects of the differentiation program that are defective in BC₃H1 cells also suggests that the actions of MyoD1 and myogenin are not entirely redundant and that MyoD1 may be required for activation of the complete repertoire of events associated with myogenesis.