Insulin's effects primarily are initiated by insulin binding to its plasma membrane receptor and the sequential tyrosine phosphorylation of the insulin receptor and intracellular substrates, such as insulin receptor substrate-1 (IRS-1). However, studies suggest some insulin effects, including those at the nucleus, may not be regulated by this pathway. 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 expression in Chinese hamster ovary (CHO) cells expressing only neomycin-resistant plasmid (CHO), overexpressing wild type human insulin receptor (CHO) or ATP binding site-mutated insulin receptors (CHO). Insulin binding in CHO cells was markedly lower than that in other cell types. 10 nM insulin significantly increased tyrosine phosphorylation of insulin receptor and IRS-1 in CHO cells. Phosphorylation of insulin receptor and IRS-1 in CHO and CHO cells was not detected in the presence or absence of insulin. Similarly, insulin increased phosphatidylinositol 3-kinase activity only in CHO cells. As determined by Northern blot, nuclear run-on analysis, and in situ hybridization, insulin induced c-fos mRNA expression, through transcription, in CHO cells but not in CHO and CHO cells, consistent with previous reports. In contrast, all three cell types showed a similar insulin dose-dependent increase of egr-1 mRNA expression through transcription. These data indicated that insulin-induced egr-1 mRNA expression did not correlate with the levels of insulin binding to insulin receptor or phosphorylation of insulin receptor and IRS-1. These results suggest that different mechanisms are involved in induction of c-fos and egr-1 mRNA expression by insulin, the former by the more classic insulin receptor tyrosine kinase pathway and the latter by a yet to be determined alternative signal transduction pathway.

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