Hassler Justin R, Scheuner Donalyn L, Wang Shiyu, Han Jaeseok, Kodali Vamsi K, Li Philip, Nguyen Julie, George Jenny S, Davis Cory, Wu Shengyang P, Bai Yongsheng, Sartor Maureen, Cavalcoli James, Malhi Harmeet, Baudouin Gregory, Zhang Yaoyang, Yates John R, Itkin-Ansari Pamela, Volkmann Niels, Kaufman Randal J
Degenerative Diseases Program, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, California, United States of America; Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan, United States of America.
Department of Biological Chemistry, University of Michigan Medical Center, Ann Arbor, Michigan, United States of America; Lilly Research Laboratories, Eli Lilly & Company, Lilly Corporate Center, Indianapolis, Indiana, United States of America.
PLoS Biol. 2015 Oct 15;13(10):e1002277. doi: 10.1371/journal.pbio.1002277. eCollection 2015 Oct.
Although glucose uniquely stimulates proinsulin biosynthesis in β cells, surprisingly little is known of the underlying mechanism(s). Here, we demonstrate that glucose activates the unfolded protein response transducer inositol-requiring enzyme 1 alpha (IRE1α) to initiate X-box-binding protein 1 (Xbp1) mRNA splicing in adult primary β cells. Using mRNA sequencing (mRNA-Seq), we show that unconventional Xbp1 mRNA splicing is required to increase and decrease the expression of several hundred mRNAs encoding functions that expand the protein secretory capacity for increased insulin production and protect from oxidative damage, respectively. At 2 wk after tamoxifen-mediated Ire1α deletion, mice develop hyperglycemia and hypoinsulinemia, due to defective β cell function that was exacerbated upon feeding and glucose stimulation. Although previous reports suggest IRE1α degrades insulin mRNAs, Ire1α deletion did not alter insulin mRNA expression either in the presence or absence of glucose stimulation. Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions. In contrast, Ire1α deletion in β cells increased the expression of over 300 mRNAs encoding functions that cause inflammation and oxidative stress, yet only a few of these accumulated during high glucose. Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion. The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.
尽管葡萄糖能独特地刺激β细胞中胰岛素原的生物合成，但令人惊讶的是，其潜在机制却鲜为人知。在此，我们证明葡萄糖可激活未折叠蛋白反应转导因子肌醇需求酶1α（IRE1α），从而在成年原代β细胞中启动X盒结合蛋白1（Xbp1）mRNA的剪接。通过mRNA测序（mRNA-Seq），我们发现非常规的Xbp1 mRNA剪接对于增加和减少数百种mRNA的表达是必需的，这些mRNA分别编码扩展蛋白质分泌能力以增加胰岛素生成以及保护细胞免受氧化损伤的功能。在他莫昔芬介导的Ire1α缺失后2周，小鼠出现高血糖和低胰岛素血症，这是由于β细胞功能缺陷所致，喂食和葡萄糖刺激会使这种缺陷加剧。尽管先前的报道表明IRE1α会降解胰岛素mRNA，但无论有无葡萄糖刺激，Ire1α缺失均未改变胰岛素mRNA的表达。相反，Ire1α缺失导致的β细胞功能障碍主要是由于胰岛素原mRNA翻译减少，这主要是因为葡萄糖刺激诱导信号识别颗粒（SRP）、SRP受体、易位子、信号肽酶复合物所需的十几个基因以及其他100多个具有许多其他细胞内功能的基因存在缺陷。相比之下，β细胞中Ire1α的缺失增加了300多种编码导致炎症和氧化应激功能的mRNA的表达，但其中只有少数在高糖条件下积累。抗氧化剂治疗可显著降低β细胞特异性Ire1α缺失小鼠的葡萄糖不耐受以及炎症和氧化应激标志物。结果表明，葡萄糖激活IRE1α介导的Xbp1剪接，以扩展β细胞的分泌能力，从而增加胰岛素原的合成，并限制导致β细胞功能障碍的氧化应激。
