Endoplasmic reticulum–associated degradation (ERAD) has a critical role in supporting glucose-stimulated insulin secretion in pancreatic B-cells

Abstract

The molecular underpinnings of b-cell dysfunction and death leading to diabetes are not fully elucidated. The objective of the current study was to investigate the role of endoplasmic reticulum–associated degradation (ERAD) in pancreatic b-cells. Chemically induced ERAD deficiency in the rat insulinoma cell line INS-1 markedly reduced glucose-stimulated insulin secretion (GSIS). The mechanistic basis for this effect was studied in cells and mice lacking ERAD as a consequence of genetic ablation of the core ERAD protein SEL1L. Targeted disruption of SEL1L in INS-1 cells and in mouse pancreatic b-cells impaired ERAD and led to blunted GSIS. Additionally, mice with SEL1L deletion in b-cells were chronically hyperglycemic after birth and increasingly glucose intolerant over time. SEL1L absence caused an entrapment of proinsulin in the endoplasmic reticulum compartment in both INS-1 cells and mouse pancreatic b-cells. Both folding-competent and folding-deficient proinsulin can physiologically interact with and be efficiently degraded by HRD1, the E3 ubiquitin ligase subunit of the ERAD complex. GSIS impairment in insulinoma cells was accompanied by a reduced intracellular Ca 2+ ion level, overproduction of reactive oxygen species, and lowered mitochondrial membrane potential. Together, these findings suggest that ERAD plays a pivotal role in supporting pancreatic b-cell function by targeting wild-type and folding-deficient proinsulin for proteosomal degradation. ERAD deficiency may contribute to the development of diabetes by affecting proinsulin processing in the ER, intracellular Ca 2+ concentration, and mitochondrial function.