Akai Ryoko, Hosoda Akira, Yoshino Mayuko, Iwawaki Takao
Iwawaki Laboratory, Education and Research Support Center, Graduate School of Medicine, Gunma University, 3-39-22 Showa-machi, Maebashi, Gunma, 371-8511, Japan.
Iwawaki Initiative Research Unit, Advanced Science Institute, RIKEN, 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan.
Genes Cells. 2015 Nov;20(11):871-86. doi: 10.1111/gtc.12279. Epub 2015 Aug 28.
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.
胰岛素生物合成在转录和翻译后调控方面已得到充分表征。然而，胰岛素的翻译调控与内质网（ER）中的蛋白质质量控制之间的关系仍有待阐明。在此，我们在非胰岛素产生细胞中进行胰岛素的强制表达，并比较正常培养条件或内质网应激条件下胰岛素产生细胞和非胰岛素产生细胞中内质网应激反应分子的激活水平。在非胰岛素产生细胞中强制表达胰岛素会导致严重的内质网应激，并通过蛋白激酶RNA样内质网激酶（PERK）激活使eIF2α磷酸化，从而显著抑制翻译，导致蛋白质水平的胰岛素产生受到抑制。我们还发现，GADD34和CReP在天然产生胰岛素的细胞中高度表达，并且在这些细胞中eIF2α显示出组成性低磷酸化水平，尽管在相同细胞的正常培养条件和生理条件下PERK均组成性激活。抑制eIF2α磷酸酶会进一步降低胰腺β细胞中的胰岛素水平。这些发现表明，胰腺β细胞中GADD34和/或CReP调节的磷酸酶使eIF2α磷酸化水平保持较低，并且通过该分子机制消除磷酸化eIF2α依赖性的翻译抑制有助于胰腺β细胞中胰岛素的大量产生。
