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

摘要

Objective-the islet in type 2 diabetes is characterized by β-cell apoptosis, β-cell endoplasmic reticulum stress, and islet amyloid deposits derived from islet amyloid polypeptide (iapp). Toxic oligomers of iapp form intracellularly in p-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 degra-dation/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. Immunoflu-orescence and western blotting were used to detect polyubiquiti-nated proteins and ubiquitin carboxyl-terminal hydrolase li (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 p-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 p-cells induced endoplasmic reticulum stress leading to apoptosis. Conclusions-our results indicate that defective protein degradation in p-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 p-cell viability. Diabetes 60: 227-238, 2011