title: Insulin receptor substrate-2 proteasomal degradation mediated by a mammalian target of rapamycin (mTOR)-induced negative feedback down-regulates protein kinase B-mediated signaling pathway in beta-cells.
abstract: Regulation of insulin receptor substrate (IRS)-2 expression is critical to beta-cell survival, but the mechanisms that control this are complex and undefined. Here in pancreatic beta-cells (INS-1), chronic exposure (>8 h) to 15 mm glucose and/or 5 nm IGF-1, increased Ser/Thr phosphorylation of IRS-2, which correlated with decreased IRS-2 levels. This glucose/IGF-1-induced decrease in IRS-2 levels was prevented by the proteasomal inhibitor, lactacystin. In addition, the glucose/IGF-1-induced increase in Ser/Thr phosphorylation of IRS-2 and the subsequent decrease in INS-1 cell IRS-2 protein levels was thwarted by the mammalian target of rapamycin(mTOR) inhibitor, rapamycin. Moreover, adenoviral-mediated expression of constitutively active mTOR (mTORDelta) further increased glucose/IGF-1-induced Ser/Thr phosphorylation of IRS-2 and decreased IRS-2 protein levels, whereas adenoviral-mediated expression of "kinase-dead" mTOR (mTOR-KD) conversely reduced Ser/Thr phosphorylation of IRS-2 and maintained IRS-2 protein levels. In adenoviral-infected beta-cells expressing mTORDelta, the decrease in IRS-2 protein levels was also prevented by rapamycin or lactacystin, further indicating a proteasomal mediated degradation of IRS-2 mediated via mTOR-induced Ser/Thr phosphorylation of IRS-2. Finally, we found that chronic activation of mTOR leading to decreased levels of IRS-2 in INS-1 cells led to a significant decrease in PKB activation and consequently increased beta-cell apoptosis. Thus, chronic activation of mTOR by glucose (and/or IGF-1) in beta-cells leads to increased Ser/Thr phosphorylation of IRS-2 that targets it for proteasomal degradation, resulting in decreased IRS-2 expression and increased beta-cell apoptosis. This may be a contributing mechanism as to how beta-cell mass is decreased by chronic hyperglycemia in the pathogenesis of type-2 diabetes.
authors.affiliation: Pacific Northwest Research Institute, and Department of Pharmacology, University of Washington, Seattle, Washington 98122, USA.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Briaud
authors.forename: Lorna M
authors.initials: LM
authors.lastname: Dickson
authors.forename: Melissa K
authors.initials: MK
authors.lastname: Lingohr
authors.forename: Jill F
authors.initials: JF
authors.lastname: McCuaig
authors.forename: John C
authors.initials: JC
authors.lastname: Lawrence
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Rhodes
mesh_terms: D000818:Animals; D002460:Cell Line; D003924:Diabetes Mellitus, Type 2 / Q000473:pathology; D024202:Electrophoretic Mobility Shift Assay; D005455:Fluorescent Antibody Technique; D005947:Glucose / Q000008:administration & dosage; D006868:Hydrolysis; D055504:Insulin Receptor Substrate Proteins; D007334:Insulin-Like Growth Factor I / Q000008:administration & dosage; D047908:Intracellular Signaling Peptides and Proteins; D007515:Islets of Langerhans / Q000378:metabolism; D008322:Mammals; D010750:Phosphoproteins / Q000378:metabolism*; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*
chemical_list: D055504:Insulin Receptor Substrate Proteins; D047908:Intracellular Signaling Peptides and Proteins; D010750:Phosphoproteins; D007334:Insulin-Like Growth Factor I; D046988:Proteasome Endopeptidase Complex; D005947:Glucose