Effect of insulin and linoleic acid on satellite cell differentiation.

Differentiation of rat skeletal muscle satellite cells was studied in vitro. Linoleic acid and insulin, two unrelated compounds that reportedly stimulate differentiation of other types of myogenic cells, were used to examine the regulation of differentiation in satellite cell cultures. As in cultures of chick embryo muscle cells, linoleic acid stimulated fusion but only at low serum concentrations or in defined medium without fibroblast growth factor (FGF). The effects of insulin on differentiation were quite variable, however; at very low cell densities no stimulatory effect was observed. In intermediate and, to a lesser extent, high density satellite cell cultures, the addition of insulin at concentrations between .01 and 1.0 microM stimulated satellite cell fusion. Whenever increases in fusion were observed, however, a parallel increase in cell number was also found. A closer examination of the relationship between differentiation and the presence or absence of mitogenic agents in the medium suggested that a mitogenic signal and the resultant proliferation of cells prevented differentiation. Subsequent experiments indicated that fusion could be induced by lower serum concentration or by removal of FGF, as long as linoleic acid was present in the medium. Therefore, proliferation and differentiation appear to be antagonistic processes in cultured satellite cells. If the rate of proliferation is depressed, either by mitogen removal or by increasing cell density, differentiation is favored. Differentiation can, therefore, be regulated and applied to in vitro studies of satellite cell activity.