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      Endoplasmic reticulum stress, degeneration of pancreatic islet β-cells, and therapeutic modulation of the unfolded protein response in diabetes

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          Abstract

          Background

          Myriad challenges to the proper folding and structural maturation of secretory pathway client proteins in the endoplasmic reticulum (ER) — a condition referred to as “ER stress” — activate intracellular signaling pathways termed the unfolded protein response (UPR).

          Scope of review

          Through executing transcriptional and translational programs the UPR restores homeostasis in those cells experiencing manageable levels of ER stress. But the UPR also actively triggers cell degeneration and apoptosis in those cells that are encountering ER stress levels that exceed irremediable thresholds. Thus, UPR outputs are “double-edged”. In pancreatic islet β-cells, numerous genetic mutations affecting the balance between these opposing UPR functions cause diabetes mellitus in both rodents and humans, amply demonstrating the principle that the UPR is critical for the proper functioning and survival of the cell.

          Major conclusions

          Specifically, we have found that the UPR master regulator IRE1α kinase/endoribonuclease (RNase) triggers apoptosis, β-cell degeneration, and diabetes, when ER stress reaches critical levels. Based on these mechanistic findings, we find that novel small molecule compounds that inhibit IRE1α during such “terminal” UPR signaling can spare ER stressed β-cells from death, perhaps affording future opportunities to test new drug candidates for disease modification in patients suffering from diabetes.

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          Most cited references 47

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          Protein folding in the cell.

           M Gething,  R Sambrook (1992)
          In the cell, as in vitro, the final conformation of a protein is determined by its amino-acid sequence. But whereas some isolated proteins can be denatured and refolded in vitro in the absence of other macromolecular cellular components, folding and assembly of polypeptides in vivo involves other proteins, many of which belong to families that have been highly conserved during evolution.
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            Five stages of evolving beta-cell dysfunction during progression to diabetes.

            This article proposes five stages in the progression of diabetes, each of which is characterized by different changes in beta-cell mass, phenotype, and function. Stage 1 is compensation: insulin secretion increases to maintain normoglycemia in the face of insulin resistance and/or decreasing beta-cell mass. This stage is characterized by maintenance of differentiated function with intact acute glucose-stimulated insulin secretion (GSIS). Stage 2 occurs when glucose levels start to rise, reaching approximately 5.0-6.5 mmol/l; this is a stable state of beta-cell adaptation with loss of beta-cell mass and disruption of function as evidenced by diminished GSIS and beta-cell dedifferentiation. Stage 3 is a transient unstable period of early decompensation in which glucose levels rise relatively rapidly to the frank diabetes of stage 4, which is characterized as stable decompensation with more severe beta-cell dedifferentiation. Finally, stage 5 is characterized by severe decompensation representing a profound reduction in beta-cell mass with progression to ketosis. Movement across stages 1-4 can be in either direction. For example, individuals with treated type 2 diabetes can move from stage 4 to stage 1 or stage 2. For type 1 diabetes, as remission develops, progression from stage 4 to stage 2 is typically found. Delineation of these stages provides insight into the pathophysiology of both progression and remission of diabetes.
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              Oxidative protein folding in eukaryotes

              The endoplasmic reticulum (ER) provides an environment that is highly optimized for oxidative protein folding. Rather than relying on small molecule oxidants like glutathione, it is now clear that disulfide formation is driven by a protein relay involving Ero1, a novel conserved FAD-dependent enzyme, and protein disulfide isomerase (PDI); Ero1 is oxidized by molecular oxygen and in turn acts as a specific oxidant of PDI, which then directly oxidizes disulfide bonds in folding proteins. While providing a robust driving force for disulfide formation, the use of molecular oxygen as the terminal electron acceptor can lead to oxidative stress through the production of reactive oxygen species and oxidized glutathione. How Ero1p distinguishes between the many different PDI-related proteins and how the cell minimizes the effects of oxidative damage from Ero1 remain important open questions.
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                Author and article information

                Contributors
                Journal
                Mol Metab
                Mol Metab
                Molecular Metabolism
                Elsevier
                2212-8778
                06 September 2019
                September 2019
                06 September 2019
                : 27
                : Suppl
                : S60-S68
                Affiliations
                [1 ]Department of Medicine, University of California, San Francisco San Francisco, CA, 94143-2520, USA
                [2 ]Diabetes Center, University of California, San Francisco San Francisco, CA, 94143-2520, USA
                [3 ]Lung Biology Center, University of California, San Francisco San Francisco, CA, 94143-2520, USA
                [4 ]Quantitative Biosciences Institute, University of California, San Francisco San Francisco, CA, 94143-2520, USA
                Author notes
                []Corresponding author. Department of Medicine, CA, 94143-2520, USA. Fax: +415 514 9656. frpapa@ 123456medicine.ucsf.edu
                Article
                S2212-8778(19)30573-3
                10.1016/j.molmet.2019.06.012
                6768499
                31500832
                © 2019 Published by Elsevier GmbH.

                This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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