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Paradoxical decrease in myf-5 messenger RNA levels during induction of myogenic differentiation by insulin-like growth factors.

Having previously demonstrated that the insulin-like growth factors (IGFs) induce expression of the myogenin gene, we have now extended our investigation of the induction of myogenesis by the IGFs to a second member of the MyoD family, myf-5. This is the only myogenesis gene other than myogenin expressed early in the differentiation of L6 myoblasts, so its regulation was of particular interest because of our observations on myogenin. In contrast to myogenin, myf-5 mRNA was detectable in proliferating myoblasts, but the steady state levels of myf-5 mRNA fell strikingly for 48 h after the cells were switched to low serum medium containing IGF-II in both murine cell lines and myoblasts cultured from human muscle. In spite of this decrease, translation of myf-5 mRNA appeared essential during the early stages of stimulation of myogenesis by the IGFs; an antisense oligodeoxynucleotide complementary to the first five codons of myf-5 blocked the increase in myogenin mRNA and inhibited morphological (cell fusion) and biochemical (creatine kinase elevation) aspects of myogenesis. We conclude that expression of myf-5 is essential for the initial induction of myogenin by the IGFs, but that subsequent elevation of myogenin expression is independent of myf-5, possibly resulting from autoinduction of the myogenin gene. The functional significance of the dramatic decrease in myf-5 mRNA levels during differentiation is not obvious.