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 skeletal myoblast differentiation. While low concentrations of IGF stimulate myoblast differentiation, high concentrations of IGF induce a progressive decrease in myoblast differentiation. The mechanism of this inhibition is unknown. Using a retroviral expression vector, we developed 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) myoblasts, which were infected with a virus lacking the type‐1 IGF receptor sequence. Overexpression of the type‐1 IGF receptor caused the IGF dose‐response curves of stimulation and progressive inhibition of differentiation to shift to the left. Additionally, at high insulin and IGF‐I concentrations, complete inhibition of P2‐LISN myoblast differentiation occurred. These results suggest that inhibition of differentiation at high ligand concentrations was not due to the primary involvement of other species of receptors for IGF. Type‐1 IGF receptor downregulation as a mechanism for inhibition of differentiation was also ruled out since P2‐LISN myoblasts constitutively expressed high levels of type‐1 IGF receptors. Additionally, inhibition of differentiation at high concentrations of IGF‐I was not correlated with overt stimulation of proliferation or with IGF binding protein (IGF‐BP) release into the culture medium. These results indicate that the type‐1 IGF receptor mediates two conflicting signal pathways in myogenic cells, differentiation‐inducing and differentiation‐inhibitory, which predominate at different ligand concentrations. © 1993 Wiley‐Liss, Inc.