Constitutive role of GADD34 and CReP in cancellation of phospho‐eIF2α‐dependent translational attenuation and insulin biosynthesis in pancreatic β cells

Abstract

Insulin biosynthesis has been well characterized with respect to transcriptional and post-translational regulation. However, the relationship between translational regulation of insulin and protein quality control in the endoplasmic reticulum (ER) remains to be clarified. Here we carried out forced expression of insulin in non-insulin-producing cells and compared activation level of ER stress-responsive molecules between insulin-producing cells and non-insulinproducing cells under normal culture condition or ER stress condition. Forced expression of insulin in non-insulin-producing cells caused severe ER stress with striking translational attenuation through phosphorylation of eIF2a by activation of protein kinase RNA-like endoplasmic reticulum kinase (PERK), resulting in inhibition of insulin production at the protein level. We also found that GADD34 and CReP are highly expressed in the cells that endogenously produce insulin and that eIF2a shows constitutively low phosphorylation level in these cells although PERK is constitutively activated under both normal culture conditions and physiological conditions in the same cells. Inhibition of eIF2a phosphatase further decreased insulin level in pancreatic b cells. These findings suggest that eIF2a phosphorylation level is kept low by GADD34-and/or CReP-regulated phosphatases in pancreatic b cells and that cancellation of phospho-eIF2a-dependent translational inhibition by the molecular mechanism contributes to mass production of insulin in pancreatic b cells.

Endoplasmic Reticulum (ER) Stress and Endocrine Disorders

“…Perk KO mice display early-onset diabetes, which suggests the PERK pathway has a cytoprotective role by attenuating proinsulin biosynthesis, but in contrast, it has been demonstrated that GADD34 and CReP constitutively suppress phosphorylation of eIF2α and enhance proinsulin production [107]. In contrast to the cytoprotective characteristics of IRE1α described above, sustained exposure to high blood glucose was shown to trigger RIDD-mediated proinsulin mRNA decay in pancreatic β-cells, which leads to a vicious cycle of glucotoxicity [77,108].…”

Living Dangerously: Protective and Harmful ER Stress Responses in Pancreatic β-Cells

“…The PERK/eIF2α pathway exerts tonic negative pressure on proinsulin translation ( 64 ). PERK is constitutively activated in insulin-producing cells under physiologic conditions, but eIF2α phosphorylation is kept low by abundant GADD34 and CReP phosphatases, illustrating a regulatory balance under tension that can tip in either direction depending on conditions ( 64 ). Consistent with excess PERK activity reducing insulin production, PERK overexpression in mice decreased pancreatic insulin content and in vivo GSIS ( 26 ).…”

Identification of Serum Anti-GADD34 Antibody as a Common Marker of Diabetes Mellitus and Parkinson Disease

“…GADD34 -/-mouse embryonic fibroblasts show delayed recovery from the shutoff of protein synthesis by ER stresses [45]. Mice defective in the elF2α phosphorylation develop DM, and active eIF2α is indispensable for insulin synthesis after ER stress [13,46]. Therefore, GADD34 and its substrate eIF2α …”

“…On the unfolded protein response, translation factor eIF2α is phosphorylated by activated PERK, and the phosphorylated eIF2α induces global repression of protein synthesis except transcription factor ATF4, which then transactivates autophagyinducing Atg12, apoptosis/autophagy-inducing CHOP and GADD34 [11]. Increased GADD34 can dephosphorylate eIF2α to resume protein synthesis including insulin [12,13]. Phosphorylated PERK and phosphorylated eIF2α were co-localized with SNCA in Lewy bodies of dopaminergic neurons of patients with PD [14].…”