1. Academic Validation
  2. Proteasomal degradation of WT proinsulin in pancreatic beta cells

Proteasomal degradation of WT proinsulin in pancreatic beta cells

  • J Biol Chem. 2022 Aug 19;298(10):102406. doi: 10.1016/j.jbc.2022.102406.
Xiaoxi Xu 1 Anoop Arunagiri 2 Leena Haataja 2 Maroof Alam 2 Shuhui Ji 3 Ling Qi 4 Billy Tsai 5 Ming Liu 6 Peter Arvan 7
Affiliations

Affiliations

  • 1 The Division of Metabolism, Endocrinology & Diabetes, University of Michigan Medical Center, Ann Arbor, Michigan, USA; Department of Endocrinology and Metabolism, Tianjin Medical University General Hospital, Tianjin, China.
  • 2 The Division of Metabolism, Endocrinology & Diabetes, University of Michigan Medical Center, Ann Arbor, Michigan, USA.
  • 3 Department of Endocrinology and Metabolism, Tianjin Medical University General Hospital, Tianjin, China.
  • 4 Department of Molecular & Integrative Physiology, University of Michigan Medical School, Ann Arbor, Michigan, USA.
  • 5 Department of Cell and Developmental Biology, University of Michigan Medical School, Ann Arbor, Michigan, USA.
  • 6 Department of Endocrinology and Metabolism, Tianjin Medical University General Hospital, Tianjin, China. Electronic address: mingliu@tmu.edu.cn.
  • 7 The Division of Metabolism, Endocrinology & Diabetes, University of Michigan Medical Center, Ann Arbor, Michigan, USA. Electronic address: parvan@umich.edu.
Abstract

Preproinsulin entry into the endoplasmic reticulum yields proinsulin, and its subsequent delivery to the distal secretory pathway leads to processing, storage, and secretion of mature Insulin. Multiple groups have reported that treatment of pancreatic beta cell lines, rodent pancreatic islets, or human islets with Proteasome inhibitors leads to diminished proinsulin and Insulin protein levels, diminished glucose-stimulated Insulin secretion, and changes in beta-cell gene expression that ultimately lead to beta-cell death. However, these studies have mostly examined treatment times far beyond that needed to achieve acute proteasomal inhibition. Here, we report that although proteasomal inhibition immediately downregulates new proinsulin biosynthesis, it nevertheless acutely increases beta-cell proinsulin levels in pancreatic beta cell lines, rodent pancreatic islets, and human islets, indicating rescue of a pool of recently synthesized WT INS gene product that would otherwise be routed to proteasomal disposal. Our pharmacological evidence suggests that this disposal most likely reflects ongoing endoplasmic reticulum-associated protein degradation. However, we found that within 60 min after proteasomal inhibition, intracellular proinsulin levels begin to fall in conjunction with increased phosphorylation of eukaryotic initiation factor 2 alpha, which can be inhibited by blocking the general control nonderepressible 2 kinase. Together, these data demonstrate that a meaningful subfraction of newly synthesized INS gene product undergoes rapid proteasomal disposal. We propose that free Amino acids derived from proteasomal proteolysis may potentially participate in suppressing general control nonderepressible 2 kinase activity to maintain ongoing proinsulin biosynthesis.

Keywords

ERAD; cell stress; endoplasmic reticulum; insulin production; secretory pathway.

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