INSULIN-INDUCED EGR-1 EXPRESSION IN CHINESE-HAMSTER OVARY CELLS IS INSULIN-RECEPTOR AND INSULIN-RECEPTOR SUBSTRATE-1 PHOSPHORYLATION-INDEPENDENT - EVIDENCE OF AN ALTERNATIVE SIGNAL-TRANSDUCTION PATHWAY

Insulin's effects primarily are initiated by insulin binding to its pl asma membrane receptor and the sequential tyrosine phosphorylation of the insulin receptor and intracellular substrates, such as insulin rec eptor substrate-1 (IRS-1), However, studies suggest some insulin effec ts, including those at the nucleus, may not be regulated by this pathw ay, The present study compared the levels of insulin binding, insulin receptor and IRS-1 tyrosine phosphorylation, and phosphatidylinositol 3'-kinase activity to immediate early gene c-fos and egr-1 mRNA expres sion in Chinese hamster ovary (CHO) cells expressing only neomycin-res istant plasmid (CHONEO), overexpressing wild type human insulin recept or (CHOHIRc) or ATP binding site-mutated insulin receptors (CHOA1018K) . Insulin binding in CHONEO cells was markedly lower than that in othe r cell types, 10 nM insulin significantly increased tyrosine phosphory lation of insulin receptor and IRS-1 in CHOHIRc cells, Phosphorylation of insulin receptor and IRS-I in CHONEO and CHOA1018K cells was not d etected in the presence or absence of insulin, Similarly, insulin incr eased phosphatidylinositol S-kinase activity only in CHOHIRc cells, As determined by Northern blot, nuclear run-on analysis, and in situ hyb ridization, insulin induced c-fos mRNA expression, through transcripti on, in CHOHIRc cells but not in CHONEO and CHOA1018K cells, consistent with previous reports, In contrast, all three cell types showed a sim ilar insulin dose-dependent increase of egr-1 mRNA expression through transcription. These data indicated that insulin-induced egr-1 mRNA ex pression did not correlate with the levels of insulin binding to insul in receptor or phosphorylation of insulin receptor and IRS-1. These re sults suggest that different mechanisms are involved in induction of c -fos and egr-1 mRNA expression by insulin, the former by the more clas sic insulin receptor tyrosine kinase pathway and the latter by a yet t o be determined alternative signal transduction pathway.