Biphasic concentration dependency of stimulation of myoblast differentiation by somatomedins

It is widely believed that mitogens inhibit in vitro differentiation of myoblasts to form postmitotic myotubes, but we and others have shown that the mitogenic hormones insulin and the insulin-like growth factors (IGFs) stimulate myoblast differentiation. We now report the results of concentration-dependency studies that resolve this disagreement. We found that the IGFs give a biphasic dose-response curve; at low concentrations, there is progressive stimulation of L6 myoblast differentiation; at higher concentrations, there is a progressive decrease. Similar results were obtained with IGF-II and insulin. When differentiation was maximally stimulated (by 1,280 ng/ml insulin), adding rat IGF-II gave decreases in differentiation similar to those reported for other mitogens. Two trivial explanations have been eliminated: stimulation of differentiation (at low concentrations) is not due to enhanced survival or growth of the cells, and inhibition (at higher concentrations) is not a toxic effect. In L6 cells, epidermal growth factor and fibroblast growth factor had no effect on proliferation or differentiation. We conclude that the effects of medium components on myoblast differentiation cannot be generalized to indicate inhibition by all mitogens; depending on the cell lines and concentrations used, certain mitogens may either stimulate or inhibit differentiation.