INSULIN-LIKE-GROWTH-FACTOR-I (IGF-I) RECEPTOR OVEREXPRESSION ABOLISHES THE IGF REQUIREMENT FOR DIFFERENTIATION AND INDUCES A LIGAND-DEPENDENT TRANSFORMED PHENOTYPE IN C2 INDUCIBLE MYOBLASTS

Insulin-like growth factors (IGFs) stimulate both proliferation and di fferentiation of myogenic cell lines, and these actions are mostly med iated through the type I IGF receptor (type I IGF-R). To further inves tigate the role of this receptor in phenotypic characteristics of C2 m urine myoblasts, we overexpressed the human type I IGF-R in the induci ble clone of C2 cells, which requires IGFs in the differentiation medi um to undergo terminal differentiation. Inducible myoblasts were trans fected with either the eukaryotic expression vector pNTK or pNTK conta ining the human type I IGF-R complementary DNA, and we isolated two cl ones named Ind-Neo and Ind-R, respectively. Binding and autophosphoryl ation experiments indicate that Ind-R cells express about 10 times as much type I IGF-R compared with Ind-Neo control cells and that the tra nsfected type I IGF-R is functional in Ind-R cells. We show that overe xpression of the human type I IGF-R makes inducible myoblasts able to differentiate spontaneously, as assessed by expression of the myogenic transcription factors MyoD and myogenin, detection of the muscle-spec ific protein troponin T, and myotube formation. Moreover, when exposed to IGF-I, Ind-R cells lose contact inhibition, grow in the presence o f a low level of growth factors and form colonies in soft agar, which is characteristic of a ligand-dependent transformed phenotype. It emer ges from this study that 1) the type I IGF-R is strongly involved in t he phenotypic differences between inducible and permissive cells with respect to the differentiation program; and 2) overexpression causes t his receptor to act as a ligand-dependent transforming protein in musc le cells. We suggest that type I IGF-R abundance and level of activati on may determine the efficiency of the autocrine mode of action of IGF s and discriminate their biological functions.