Stimulation and inhibition of myoblast differentiation by hormones

The growth and differentiation of L6 myoblasts are subject to control by two proteins secreted by cells of the Buffalo rat liver line. The first of these, rat insulinlike growth factor-II (formerly designated multiplication stimulating activity) is a potent stimulator of myoblast proliferation and differentiation, as well as associated processes such as amino acid uptake and incorporation into protein, RNA synthesis, and thymidine incorporation into DNA. In addition, this hormone causes a significant decrease in the rate of protein degradation. All of these actions seem to be attributable to a single molecular species, although their time courses and sensitivity to the hormone differ substantially. The second protein, the differentiation inhibitor (DI), is a nonmitogenic inhibitor of all tested aspects of myoblast differentiation, including fusion and the elevation of creatine kinase. Indirect immunofluorescence experiments demonstrated that DI also blocks accumulation of myosin heavy chain and myomesin. Upon removal of DI after 72 h incubation, all of these effects were reversed and normal myotubes containing the usual complement of muscle-specific proteins were formed. Thus, this system makes it possible to achieve specific stimulation or inhibition of muscle cell differentiation by addition of purified proteins to cloned cells in serum-free medium.