NEGATIVE FEEDBACK-REGULATION OF INSULIN-LIKE GROWTH FACTOR-II GENE-EXPRESSION IN DIFFERENTIATING MYOBLASTS IN-VITRO

We have previously reported that autocrine secretion of insulin-like g rowth factor-II (IGF-II) plays a critical role in stimulating spontane ous myogenic differentiation in vitro. Myogenesis and IGF-II gene expr ession are both negatively controlled by high serum growth medium, and it is likely that serum inhibits terminal differentiation at least in part by blocking autocrine secretion of IGF-II. To investigate this p ossibility, we assessed the effects of various serum fractions and gro wth factors on endogenous IGF-II gene expression in rat L6A1 myoblasts . Unexpectedly, we found that IGF-I, IGF-II, and high concentrations o f insulin were potent inhibitors of IGF-II gene expression. This is th e first example we have seen in which IGFs regulate their own expressi on by a negative feedback mechanism. Feedback inhibition was not depen dent on the stimulation of cell proliferation by IGFs, and differentia ted L6A1 myotubes remained sensitive to this action of the IGFs. Resul ts with IGF analogs suggested that the inhibition of IGF-II gene expre ssion by IGFs was mediated by the type I IGF receptor and was strongly suppressed by LGA1-secreted IGF-binding proteins. Human primary myobl asts also exhibited feedback inhibition by the IGFs, whereas the rapid ly fusing mouse Sol 8 cell line did not. We conclude that IGF-II gene expression in differentiating L6A1 myoblasts is regulated by a negativ e feedback mechanism (unusual for the IGFs) that acts primarily throug h the type I IGF receptor and appears to be inhibited by IGF-binding p roteins secreted by L6A1 myoblasts in low serum differentiation medium .