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.

Academic Article

Abstract

  • 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 (CHONEO), overexpressing wild type human insulin receptor (CHOHIRc) or ATP binding site-mutated insulin receptors (CHOA1018K). Insulin binding in CHONEO cells was markedly lower than that in other cell types. 10 nM insulin significantly increased tyrosine phosphorylation of insulin receptor and IRS-1 in CHOHIRc cells. Phosphorylation of insulin receptor and IRS-1 in CHONEO and CHOA1018K cells was not detected in the presence or absence of insulin. Similarly, insulin increased phosphatidylinositol 3-kinase activity only in CHOHIRc cells. As determined by Northern blot, nuclear run-on analysis, and in situ hybridization, insulin induced c-fos mRNA expression, through transcription, in CHOHIRc cells but not in CHONEO and CHOA1018K 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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    Keywords

  • Adenosine Triphosphate, Animals, Binding Sites, CHO Cells, Cell Nucleus, Cricetinae, Cricetulus, DNA-Binding Proteins, Early Growth Response Protein 1, Gene Expression, Genes, Immediate-Early, Humans, Immediate-Early Proteins, In Situ Hybridization, Insulin, Interleukin 1 Receptor Antagonist Protein, Kinetics, Microscopy, Electron, Mutagenesis, Site-Directed, Phosphorylation, RNA, Messenger, Receptor, Insulin, Sialoglycoproteins, Signal Transduction, Transcription Factors, Transcription, Genetic, Zinc Fingers

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    Author List

  • Harada S; Smith RM; Smith JA; Shah N; Hu DQ; Jarett L

    • Start Page

  • 26632

    • End Page

  • 26638

    • Volume

  • 270

    • Issue

  • 44