Decrease in transforming growth factor-β binding and action during differentiation in muscle cells

We report here the effects of differentiation on the binding and action of transforming growth factor-β (TGF-β) in three lines of myogenic cells. In two lines (IL6-A1 and C2) which irreversibly differentiate by fusing to form postmitotic myotubes, there is a virtual disappearance of TGF-β binding sites as differentiation occurs. Analyses of the binding curves by the method of Scatchard indicates that there is little or no change in affinity but a substantial decrease in the number of binding sites. In L6-A1 cells, responsiveness to TGF-β decreases in parallel to the loss of receptors. The decreases in TGF-β binding and activity with differentiation are not paralleled by similar changes in another growth factor, insulin-like growth factor-I, which exhibits little change in binding and only a modest decrease in activity as L6-A1 myoblasts differentiate to form myotubes. In a third cell line (BC₃H1), which exhibits reversible differentiation without fusion, there is little or no change in TGF-β binding as the cells differentiate. Comparisons with reported decreases in binding of fibroblast and epidermal growth factors indicates that there are substantial differences in growth factor binding and actions as muscle cells differentiate, but it is not possible to make the simple generalization that differentiation is accompanied by a decrease in binding of all growth factors.