INSULIN-LIKE GROWTH FACTOR-I INDUCES C-FOS MESSENGER-RIBONUCLEIC-ACID IN L6 RAT SKELETAL-MUSCLE CELLS

Insulin-like growth factor I (IGF-I) mediates growth promotion of GH on specific target cells. As other growth factor action has been associated with oncogene induction, we examined whether IGF-I activated cellular oncogenes in L6 rat muscle cells, known target cells of IGF-I stimulatory action. Quiescent cells were incubated in serum-free defined medium with IGF-I (Amgen recombinant analog, Thr59) from 5 min to 4 h. 32P-Labeled DNA was first prepared from IGF-I-treated cell poly(A) RNA using avian myeloblastosis virus reverse transcriptase. This labeled probe was hybridized against a blot containing 12 different immobilized oncogene DNAs. The v-fos DNA showed a positive hybridization signal, indicating the presence of c-fos mRNA sequences. Northern analysis with [32P]v-fos DNA revealed a major c-fos mRNA species (2.2 kilobases) induced by treatment of cells with IGF-I (100 ng/ml) or fetal calf serum (15%) for 45 min. We, therefore, measured c-fos induction by cytoplasmic RNA blot hybridization. IGF-I (6.25 ng/ml) stimulated c-fos mRNA 4-fold. Maximum stimulation (18-fold) was seen with 100 ng/ml IGF-I. Peak c-fos induction occurred after 30-min exposure to IGF-I and remained elevated for up to 2 h. Treatment of cells with a high dose of insulin (100 nM) also resulted in a modest (27%) increase in relative levels of c-fos mRNA. The results show that c-fos mRNA is induced by IGF-I in L6 cells. This novel observation suggests that IGF-I action at least in part may be mediated by c-fos, a cellular oncogene thought to play a critical role in cell differentiation and proliferation.