Opposing early inhibitory and late stimulatory effects of insulin-like growth factor-I on myogenin gene transcription

Insulinlike growth factors (IGFs) stimulate skeletal muscle cell differentiation in association with an increase in the mRNA of myogenin, a member of the MyoD family of skeletal muscle–specific transcription factors that plays an essential role in the differentiation process. However, this is a relatively late effect, requiring treatment periods of >24 h. In contrast, IGFs initially inhibit skeletal muscle cell differentiation, associated with a marked reduction in myogenin mRNA. The mechanisms by which IGF‐I initially inhibits and subsequently stimulates myogenin expression are unknown. In the first 24 h, we find that IGF‐I inhibits myogenin gene transcription by >80% but has no effect on myogenin mRNA stability. Similarly, in the first 24 h, IGF‐I markedly inhibits myogenin promoter activity; the sequence −145 to −9 of the myogenin gene is sufficient to confer this inhibitory effect of IGF‐I. In contrast, 48 h of treatment with IGF‐I results in an increase in myogenin promoter activity that parallels the increase in myogenin steady‐state mRNA. This increase in promoter activity is completely prevented in constructs lacking the sequence −1,565 to −375 of the myogenin gene. These data indicate that the early inhibitory and late stimulatory effects of IGF‐I on myogenin expression are mediated at the level of transcription, and that these time‐dependent, opposing effects of IGF‐I on myogenin transcription are mediated by distinct regions of the myogenin gene. To our knowledge, this is the first demonstration of a gene whose promoter activity is initially inhibited and subsequently stimulated by IGF‐I. J. Cell. Biochem. 78:617–626, 2000. © 2000 Wiley‐Liss, Inc.