eIF4G1 and carboxypeptidase E axis dysregulation in O- GlcNAc transferase-deficient pancreatic β-cells contributes to hyperproinsulinemia in mice

J Biol Chem. 2019 Aug 30;294(35):13040-13050. doi: 10.1074/jbc.RA119.008670. Epub 2019 Jul 12.

Abstract

An early hallmark of type 2 diabetes is a failure of proinsulin-to-insulin processing in pancreatic β-cells, resulting in hyperproinsulinemia. Proinsulin processing is quite sensitive to nutrient flux, and β-cell-specific deletion of the nutrient-sensing protein modifier OGlcNAc transferase (βOGTKO) causes β-cell failure and diabetes, including early development of hyperproinsulinemia. The mechanisms underlying this latter defect are unknown. Here, using several approaches, including site-directed mutagenesis, Click O-GlcNAc labeling, immunoblotting, and immunofluorescence and EM imaging, we provide the first evidence for a relationship between the O-GlcNAcylation of eukaryotic translation initiation factor 4γ1 (eIF4G1) and carboxypeptidase E (CPE)-dependent proinsulin processing in βOGTKO mice. We first established that βOGTKO hyperproinsulinemia is independent of age, sex, glucose levels, and endoplasmic reticulum-CCAAT enhancer-binding protein homologous protein (CHOP)-mediated stress status. Of note, OGT loss was associated with a reduction in β-cell-resident CPE, and genetic reconstitution of CPE in βOGTKO islets rescued the dysfunctional proinsulin-to-insulin ratio. We show that although CPE is not directly OGlcNAc modified in islets, overexpression of the suspected OGT target eIF4G1, previously shown to regulate CPE translation in β-cells, increases islet CPE levels, and fully reverses βOGTKO islet-induced hyperproinsulinemia. Furthermore, our results reveal that OGT O-GlcNAc-modifies eIF4G1 at Ser-61 and that this modification is critical for eIF4G1 protein stability. Together, these results indicate a direct link between nutrient-sensitive OGT and insulin processing, underscoring the importance of post-translational O-GlcNAc modification in general cell physiology.

Keywords: Chop; ER-stress; O-GlcNAcylation; O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT); diabetes; eIF4G1; hyperproinsulinemia; insulin synthesis; islet; post-translational modification (PTM).

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Animals
  • Carboxypeptidase H / metabolism*
  • Diabetes Mellitus / metabolism*
  • Disease Models, Animal
  • Eukaryotic Initiation Factor-4G / metabolism*
  • Insulin-Secreting Cells / metabolism*
  • Mice
  • Mice, Knockout
  • N-Acetylglucosaminyltransferases / deficiency
  • N-Acetylglucosaminyltransferases / metabolism*

Substances

  • Eif4g1 protein, mouse
  • Eukaryotic Initiation Factor-4G
  • N-Acetylglucosaminyltransferases
  • O-GlcNAc transferase
  • Carboxypeptidase H

Supplementary concepts

  • Hyperproinsulinemia