RAPID ACTIVATION OF INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-5 GENE-TRANSCRIPTION DURING MYOBLAST DIFFERENTIATION

Insulin-like growth factor binding proteins (IGFBPs) comprise a family of secreted proteins that bind insulin-like growth factors-I and -II (IGF-I and -II) with high affinity and potentially modulate their biol ogical effects. We have demonstrated previously that IGFBP-5, the most conserved of the six known IGFBPs, is expressed in muscle cells in th e developing embryo and during the terminal differentiation of several myogenic cell lines, In this study we show that an IGF-I analog that binds minimally to IGFBPs potently enhances the differentiation of the stringently controlled inducible C2 myoblast (C21) cell line and iden tify IGFBP-5 as the sole IGFBP secreted during C21 differentiation, We find that induction of IGFBP-5 mRNA and protein is coincident with th e onset of myogenin gene expression and occurs secondary to the rapid activation of IGFBP-5 gene transcription. By transient gene transfer e xperiments we demonstrate that a 1004 base pair segment of the IGFBP-5 promoter is very active in directing expression of the reporter gene luciferase in C21 myoblasts, A promoter fragment containing only 156 n ucleotides of 5'-flanking DNA retained more than 70% of maximal activi ty and mediated at least part of the differentiation-dependent rise in IGFBP-5 gene transcription, Within this active segment are several po tential binding sites for muscle-enriched transcription factors. Our r esults show that induction of IGFBP-5 expression is an early event in the myogenic differentiation of the C21 cell line and suggest that one function of this IGFBP is to modulate IGF-induced differentiation, C2 1 cells are thus an excellent in vitro model for elucidating the devel opmental factors that control IGFBP-5 gene transcription and action in skeletal muscle.