INSULIN-LIKE-GROWTH-FACTOR-I INDUCTION OF C-MYC EXPRESSION IN BOVINE FIBROBLASTS CAN BE BLOCKED BY ANTECEDENT INSULIN-RECEPTOR ACTIVATION

We previously reported that preexposure of cultured bovine fibroblasts to insulin at low concentrations inhibits subsequent insulin like gro wth factor I (IGF-I)-stimulated DNA synthesis. This insulin-induced de sensitization to IGF-I is mediated by specific insulin receptors on bo vine fibroblasts and occurs distally to IGF-I receptor engagement and activation. fa the present study, ave use this model system to determi ne insulin and IGF-I receptor interplay in the regulation of proto-onc ogenes involved in mitogenesis. Insulin (10 nM), IGF-I (10 nM), and 10 % fetal bovine serum were each capable of stimulating rapid. and trans ient c-fos and c-myc mRNA expression in bovine fibroblasts, Expression of c-myc was most responsive to mitogenic stimuli; IGF-I and serum ha d equivalent potency resulting in similar to 14-fold increases in c-my c mRNA expression, while insulin produced 3- to 5-fold increases. Max mRNA, which encodes the partner protein for Myc, was constitutively ex pressed and levels did not change with treatment or with time. When bo vine fibroblasts were pretreated with 10 nM insulin for 45 h, washed, and then stimulated with 10 nM IGF-I, alterations in c-fos mRNA expres sion were moderate, in contrast, insulin pretreatment completely Block ed IGF-I induction of c-myc expression. This Block was averted if a sp ecific inhibitor of intracellular signaling through the phosphatidylin ositol 3-kinase pathway was present during the incubation period with insulin. These data indicate significant insulin/IGF-I receptor interp lay in normal bovine fibroblasts and suggest that insulin receptor-ini tiated signaling can profoundly alter proto-oncogene expression induce d by growth factors sharing components of a common intracellular signa ling network. (C) 1998 Academic Press.