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 β-Cells

• Published in: The Journal of biological chemistry Vol. 280; no. 3; pp. 2282 - 2293
• Main Authors: Briaud, Isabelle, Dickson, Lorna M., Lingohr, Melissa K., McCuaig, Jill F., Lawrence, John C., Rhodes, Christopher J.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 21.01.2005
• Subjects: Animals; Cell Line; Diabetes Mellitus, Type 2 - pathology; Electrophoretic Mobility Shift Assay; Fluorescent Antibody Technique; Glucose - administration & dosage; Hydrolysis; Insulin Receptor Substrate Proteins; Insulin-Like Growth Factor I - administration & dosage; Intracellular Signaling Peptides and Proteins; Islets of Langerhans - metabolism; Mammals; Phosphoproteins - metabolism; Phosphorylation; Proteasome Endopeptidase Complex - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M412179200

Regulation of insulin receptor substrate (IRS)-2 expression is critical to β-cell survival, but the mechanisms that control this are complex and undefined. Here in pancreatic β-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 (mTORΔ) 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 β-cells expressing mTORΔ, 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 β-cell apoptosis. Thus, chronic activation of mTOR by glucose (and/or IGF-1) in β-cells leads to increased Ser/Thr phosphorylation of IRS-2 that targets it for proteasomal degradation, resulting in decreased IRS-2 expression and increased β-cell apoptosis. This may be a contributing mechanism as to how β-cell mass is decreased by chronic hyperglycemia in the pathogenesis of type-2 diabetes.