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

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-insulin-producing 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 eIF2α 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 eIF2α 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 eIF2α phosphatase further decreased insulin level in pancreatic β cells. These findings suggest that eIF2α phosphorylation level is kept low by GADD34- and/or CReP-regulated phosphatases in pancreatic β cells and that cancellation of phospho-eIF2α-dependent translational inhibition by the molecular mechanism contributes to mass production of insulin in pancreatic β cells.