"Spontaneous" differentiation of skeletal myoblasts is dependent upon autocrine secretion of insulin-like growth factor-II

Differentiation of muscle cells to form postmitotic myotubes is usually viewed as being negatively controlled by medium components, sometimes designated "mitogens". However, we have found that a family of mitogenic agents, the insulin-like growth factors (IGFs), are potent stimulators of differentiation in myoblasts which act by inducing expression of the myogenin gene. We show here that this action of the IGFs occurs even when these growth factors are not added to the cell medium; upon transfer to low-serum "differentiation medium", myoblasts begin active expression of the IGF-II gene, at both the mRNA and protein levels. Furthermore, autocrine secretion of IGF-II is essential for the process of terminal differentiation of the cells. These conclusions are based upon four lines of evidence. (1) The rate of spontaneous differentiation in several sublines of myogenic cells correlates with their level of expression of IGF-II. (2) C2 and Sol 8 cells, which secrete high levels of IGF-II, are relatively insensitive to exogenous IGFs, in contrast to L6 lines, which exhibit lower levels of IGF-II gene expression. (3) An antisense oligodeoxyribonucleotide complementary to the first five codons of IGF-II inhibits myogenic differentiation in the absence but not in the presence of exogenous IGF-II. (4) Spontaneous differentiation in response to autocrine IGF-II involves the same mechanism that occurs in cells stimulated by the IGFs, i.e. elevation of expression of the myogenin gene.