title: Central insulin signaling is attenuated by long-term insulin exposure via insulin receptor substrate-1 serine phosphorylation, proteasomal degradation, and lysosomal insulin receptor degradation.
abstract: Central insulin signaling is critical for the prevention of insulin resistance. Hyperinsulinemia contributes to insulin resistance, but it is not yet clear whether neurons are subject to cellular insulin resistance. We used an immortalized, hypothalamic, clonal cell line, mHypoE-46, which exemplifies neuronal function and expresses the components of the insulin signaling pathway, to determine how hyperinsulinemia modifies neuronal function. Western blot analysis indicated that prolonged insulin treatment of mHypoE-46 cells attenuated insulin signaling through phospho-Akt. To understand the mechanisms involved, time-course analysis was performed. Insulin exposure for 4 and 8 h phosphorylated Akt and p70-S6 kinase (S6K1), whereas 8 and 24 h treatment decreased insulin receptor (IR) and IR substrate 1 (IRS-1) protein levels. Insulin phosphorylation of S6K1 correlated with IRS-1 ser1101 phosphorylation and the mTOR-S6K1 pathway inhibitor rapamycin prevented IRS-1 serine phosphorylation. The proteasomal inhibitor epoxomicin and the lysosomal pathway inhibitor 3-methyladenine prevented the degradation of IRS-1 and IR by insulin, respectively, and pretreatment with rapamycin, epoxomicin, or 3-methyladenine prevented attenuation of insulin signaling by long-term insulin exposure. Thus, a sustained elevation of insulin levels diminishes neuronal insulin signaling through mTOR-S6K1-mediated IRS-1 serine phosphorylation, proteasomal degradation of IRS-1 and lysosomal degradation of the IR.
authors.affiliation: Department of Physiology, University of Toronto, Toronto, Ontario, Canada.
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Mayer
authors.forename: Denise D
authors.initials: DD
authors.lastname: Belsham
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D002490:Central Nervous System / Q000187:drug effects* / Q000378:metabolism* / Q000502:physiology; D015536:Down-Regulation / Q000187:drug effects; D007328:Insulin / Q000008:administration & dosage / Q000378:metabolism* / Q000494:pharmacology*; D055504:Insulin Receptor Substrate Proteins / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins / Q000378:metabolism; D008247:Lysosomes / Q000187:drug effects / Q000378:metabolism; D053858:Metabolic Networks and Pathways / Q000187:drug effects / Q000502:physiology; D051379:Mice; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D051058:Oncogene Protein v-akt / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D011499:Protein Processing, Post-Translational / Q000187:drug effects; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011972:Receptor, Insulin / Q000378:metabolism*; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000378:metabolism; D012694:Serine / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases; D013997:Time Factors
chemical_list: D007328:Insulin; D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; C526295:Irs1 protein, mouse; D012694:Serine; C546843:mTOR protein, mouse; D011972:Receptor, Insulin; D051058:Oncogene Protein v-akt; D017346:Protein Serine-Threonine Kinases; D038744:Ribosomal Protein S6 Kinases, 90-kDa; C512964:Rps6ka1 protein, mouse; D058570:TOR Serine-Threonine Kinases; D046988:Proteasome Endopeptidase Complex