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

Abstract

OBJECTIVEThe 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 β-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 METHODSWe 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.RESULTSWe report accumulation of polyubiquinated proteins and UCH-L1 deficiency in β-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 β-cells induced endoplasmic reticulum stress leading to apoptosis.CONCLUSIONSOur results indicate that defective protein degradation in β-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 β-cell viability.

A Novel “Molecular Tweezer” Inhibitor of α-Synuclein Neurotoxicity in Vitro and in Vivo

“…Inhibition of the UPS system has been reported not only for α-syn oligomers [42,43], but also with toxic assemblies of other amyloidogenic proteins, including Aβ [44] and islet amyloid polypeptide [45,46]. A recent report demonstrated that overexpression of polyglutamine results in transient UPS inhibition in mice, but the activity returned when large inclusions formed [47].…”

The role of caspase-8 in amyloid-induced beta cell death in human and mouse islets

“…heparan sulphate proteoglycans) [24]; inducing endoplasmic reticulum stress [25] and oxidative stress [26]; and disruption of the endoplasmic reticulum-associated degradation (ERAD)/ubiquitin/proteasome system [27] and autophagy/ lysosomal pathway [28]. Although several mechanisms, at least in vitro, are involved in hIAPP-induced beta cell apoptosis, some of these may share the same apoptotic signalling pathways.…”

The glucagon-like peptide-1 receptor agonist exenatide restores impaired pro-islet amyloid polypeptide processing in cultured human islets: implications in type 2 diabetes and islet transplantation

“…Several mechanisms have been shown to mediate amyloid-induced beta cell death, including membrane disruption by hIAPP aggregates [15,16], activation of the Fasmediated apoptosis and caspase pathways [7,8,17], interaction of hIAPP aggregates with components of beta cell membranes (such as heparan sulphate proteoglycans [18] or touch receptors [19]), endoplasmic reticulum stress [20,21], oxidative stress [22], activation of the cJUN N-terminal kinase (JNK) pathway [23] and disruption of the autophagy/lysosomal pathway [24]. Growing evidence suggests that small hIAPP aggregates (e.g.…”