LIGAND-DEPENDENT INHIBITION OF MYOBLAST DIFFERENTIATION BY OVEREXPRESSION OF THE TYPE-1 INSULIN-LIKE GROWTH-FACTOR RECEPTOR

The insulin-like growth factors (IGFs) have paradoxical effects on ske letal myoblast differentiation. While low concentrations of IGF stimul ate myoblast differentiation, high concentrations of IGF induce a prog ressive decrease in myoblast differentiation. The mechanism of this in hibition is unknown. Using a retroviral expression vector, we develope d a subline of mouse P2 mouse myoblasts (P2-LISN) which expressed 7.5 times higher levels of type-1 IGF receptors than control (P2-LNL6) myo blasts, which were infected with a virus lacking the type-1 IGF recept or sequence. Overexpression of the type-1 IGF receptor caused the IGF dose-response curves of stimulation and progressive inhibition of diff erentiation to shift to the left. Additionally, at high insulin and IG F-I concentrations, complete inhibition of P2-LISN myoblast differenti ation occurred. These results suggest that inhibition of differentiati on at high ligand concentrations was not due to the primary involvemen t of other species of receptors for IGF. Type-1 IGF receptor downregul ation as a mechanism for inhibition of differentiation was also ruled out since P2-LISN myoblasts constitutively expressed high levels of ty pe-1 IGF receptors. Additionally, inhibition of differentiation at hig h concentrations of IGF-I was not correlated with overt stimulation of proliferation or with IGF binding protein (IGF-BP) release into the c ulture medium. These results indicate that the type-1 IGF receptor med iates two conflicting signal pathways in myogenic cells, differentiati on-inducing and differentiation-inhibitory, which predominate at diffe rent ligand concentrations. (C) 1993 Wiley-Liss, Inc.