β-Cell Dysfunctional ERAD/Ubiquitin/Proteasome System in Type 2 Diabetes Mediated by Islet Amyloid Polypeptide-Induced UCH-L1 Deficiency

OBJECTIVE-The islet in type 2 diabetes is characterized by beta-cell apoptosis, beta-cell endoplasmic reticulum stress, and islet amyloid deposits derived from islet amyloid polypeptide (TAPP). Toxic oligomers of IAPP form intracellularly in beta-cells in humans with type 2 diabetes, suggesting impaired clearance of misfolded proteins. In this study, we investigated whether human-IAPP (h-IAPP) disrupts the endoplasmic reticulum associated degradation/ubiquitin/proteasome system. RESEARCH DESIGN AND METHODS-We used pancreatic tissue from humans with and without type 2 diabetes, isolated islets from h-IAPP transgenic rats, isolated human islets, and INS 832/13 cells transduced with adenoviruses expressing either h-IAPP or a comparable expression of rodent-IAPP. Immunofluorescence and Western blotting were used to detect polyubiquitinated proteins and ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) protein levels. Proteasome activity was measured in isolated rat and human islets. UCH-L1 was knocked down by small-interfering RNA in INS 832/13 cells and apoptosis was evaluated. RESULTS-We report accumulation of polyubiquinated proteins and UCH-L1 deficiency in beta-cells of humans with type 2 diabetes. These findings were reproduced by expression of oligomeric h-IAPP but not soluble rat-IAPP. Downregulation of UCH-L1 expression and activity to reproduce that caused by h-IAPP in beta-cells induced endoplasmic reticulum stress leading to apoptosis. CONCLUSIONS-Our results indicate that defective protein degradation in beta-cells in type 2 diabetes can, at least in part, be attributed to misfolded h-IAPP leading to UCH-L1 deficiency, which in turn further compromises beta-cell viability. Diabetes 60: 227-238, 2011