Hormonal stimulation of myoblast differentiation in the absence of DNA synthesis

The role of DNA synthesis in the final stages of muscle cell differentiation has been a subject of controversy for more than a decade. In an attempt to resolve disagreements over the necessity for a unique (or "quantal") mitosis just prior to the conversion of proliferating myoblasts to form postmitotic myotubes, we have studied the effects of insulin and somatomedin on the stimulation of myoblast differentiation with or without DNA synthesis. Under conditions in which at least 95% of [3H]thymidine incorporation was blocked by cytosine arabinoside, there was a 5- to 10-fold increase in the extent of differentiation (determined as fusion or creatine kinase elevation) on addition of insulin or multiplication-stimulating activity. The effect of the hormones was on myoblast differentiation, not enzyme induction; insulin did not cause any increase in creatine kinase when it was added to performed myotubes. These studies were done using two different cell types, Yaffe's L6 cell line and Japanese quail myoblasts in serum-free media; we obtained similar results in both. Our results are not compatible with the view that a quantal mitosis is required at a late stage of muscle cell differentiation.