INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-5 MODULATES MUSCLE DIFFERENTIATION THROUGH AN INSULIN-LIKE GROWTH FACTOR-DEPENDENT MECHANISM

The insulin-like growth factor binding proteins (IGFBPs) are a family of six secreted proteins which bind. to and modulate the actions of in sulin-like growth factors-I and -II (IGF-I and -II). IGFBP-5 is more c onserved than other IGFBPs characterized to date, and is expressed in adult rodent muscle and in the developing myotome. We have shown previ ously that C2 myoblasts secrete IGFBP-5 as their sole IGFBP. Here we u se these cells to study the function of IGFBP-5 during myogenesis, a p rocess stimulated by IGFs. We stably transfected C2 cells with IGFBP-5 cDNAs under control of a constitutively active promoter. Compared wit h vector-transfected control cells, C2 myoblasts expressing the IGFBP- 5 transgene in the sense orientation exhibit increased IGFBP-5 levels in the extracellular matrix during proliferation, and subsequently fai l to differentiate normally, as assessed by both morphological and bio chemical criteria. Compared to controls, IGFBP-5 sense myoblasts show enhanced survival in low serum medium, remaining viable for at least f our weeks in culture. By contrast, myoblasts expressing the IGFBP-5 an tisense transcript differentiate prematurely and more extensively than control cells. The inhibition of myogenic differentiation by high lev el expression of IGFBP-5 could be overcome by exogenous IGFs, with des (1-3) IGF-I, an analogue with decreased affinity for IGFBP-5 but norm al affinity for the IGF-I receptor, showing the highest potency. These results are consistent with a model in which IGFBP-5 blocks IGF-stimu lated myogenesis, and indicate that sequestration of IGFs in the extra cellular matrix could be a possible mechanism of action. Our observati ons also suggest that IGFBP-5 normally inhibits muscle differentiation , and imply a role for IGFBP-5 in regulating IGF action during myogeni c development in vivo.