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.
Quinn
L S
LS
Department of Biological Structure, University of Washington, Seattle 98195.
Ehsan
M
M
Steinmetz
B
B
Kaleko
M
M
D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects; D002455:Cell Division / Q000187:drug effects; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D007328:Insulin / Q000494:pharmacology; D018969:Insulin-Like Growth Factor Binding Proteins; D008024:Ligands; D009132:Muscles / Q000166:cytology* / Q000378:metabolism*; D017526:Receptor, IGF Type 1 / Q000378:metabolism*; D013002:Somatomedins / Q000494:pharmacology
D002352:Carrier Proteins; D007328:Insulin; D018969:Insulin-Like Growth Factor Binding Proteins; D008024:Ligands; D013002:Somatomedins; D017526:Receptor, IGF Type 1