Choi Woo-Gyun, Han Jaeseok, Kim Ji-Hyeon, Kim Mi-Jeong, Park Jae-Woo, Song Benbo, Cha Hee-Jeong, Choi Hye-Seon, Chung Hun-Taeg, Lee In-Kyu, Park Tae-Sik, Hatzoglou Maria, Choi Hueng-Sik, Yoo Hyun Ju, Kaufman Randal J, Back Sung Hoon
School of Biological Sciences, University of Ulsan, Ulsan, 44610 Republic of Korea.
Soonchunhyang Institute of Med-bio Science (SIMS), Soonchunhyang University, Cheonan-si, Choongchungnam-do, 31151 Republic of Korea.
Nutr Metab (Lond). 2017 Aug 1;14:48. doi: 10.1186/s12986-017-0202-6. eCollection 2017.
Dietary fructose can rapidly cause fatty liver in animals through de novo lipogenesis (DNL) and contribute to the development and severity of nonalcoholic fatty liver disease (NAFLD). In response to diverse cellular insults including endoplasmic reticulum (ER) and oxidative stress, phosphorylation of the eukaryotic translation initiation factor 2 alpha subunit (eIF2α) attenuates general translation initiation, allowing cells to conserve resources and initiate adaptive gene expression to restore homeostasis. The present study aimed to investigate the role of eIF2α phosphorylation in protecting against NAFLD induced by high fructose ingestion in a hepatocyte-specific eIF2α-phosphorylation-deficient mouse model.
Hepatocyte-specific non-phosphorylatable (S51A) eIF2α knock-in (, ) mice were generated by crossing mice with the WT transgene () with recombinase transgenic ( ) mice. Hepatocyte-specific eIF2α-phosphorylation-deficient 3-month-old mice or 12-month-old mice were fed a 60% high fructose diet (HFrD) for 16 or 5 wks, and the effects of eIF2α-phosphorylation deficiency on NADP/NADPH and GSSG/GSH levels, ROS-defense gene expression, oxidative damage, cell death, and fibrosis were observed.
Prolonged fructose feeding to mice caused dysregulation of the unfolded protein response (UPR) sensor activation and UPR gene expression, and then led to decreased expression of several ROS defense genes including glutathione biogenesis genes. Nonetheless, these changes were not sufficient to induce the death of eIF2α phosphorylation-sufficient hepatocytes. However, there was a substantial increase in hepatocyte death and liver fibrosis in fructose-fed middle-aged mice deficient in hepatocyte-specific eIF2α phosphorylation because of diminished antioxidant capacity due to reduced expression of antioxidant enzymes (GPX1 and HO-1) and lower NADPH and glutathione levels, as well as a possible increase in ROS-induced damage from infiltrating NOX2-expressing leukocytes; all this led to a vicious cycle of hepatocyte death and leukocyte infiltration.
Our findings suggest that eIF2α phosphorylation maintains NADPH and GSH levels and controls the expression of ROS-defense genes, thereby protecting hepatocytes from oxidative stresses induced by fructose metabolism.
膳食果糖可通过从头脂肪生成（DNL）迅速在动物体内导致脂肪肝，并促进非酒精性脂肪性肝病（NAFLD）的发生和严重程度。作为对包括内质网（ER）应激和氧化应激在内的多种细胞损伤的反应，真核翻译起始因子2α亚基（eIF2α）的磷酸化会减弱一般的翻译起始，使细胞能够保存资源并启动适应性基因表达以恢复体内平衡。本研究旨在探讨在肝细胞特异性eIF2α磷酸化缺陷小鼠模型中，eIF2α磷酸化在预防高果糖摄入诱导的NAFLD中的作用。
通过将携带WT转基因（）的小鼠与重组酶转基因（）小鼠杂交，培育出肝细胞特异性非磷酸化（S51A）eIF2α敲入（，）小鼠。对3个月大或12个月大的肝细胞特异性eIF2α磷酸化缺陷小鼠喂食60%的高果糖饮食（HFrD）16周或5周，观察eIF2α磷酸化缺陷对NADP/NADPH和GSSG/GSH水平、ROS防御基因表达、氧化损伤、细胞死亡和纤维化的影响。
对小鼠长期喂食果糖会导致未折叠蛋白反应（UPR）传感器激活和UPR基因表达失调，进而导致包括谷胱甘肽生物合成基因在内的几种ROS防御基因的表达降低。尽管如此，这些变化并不足以诱导eIF2α磷酸化充足的肝细胞死亡。然而，在果糖喂养的中年小鼠中，由于抗氧化酶（GPX1和HO-1）表达降低、NADPH和谷胱甘肽水平较低，抗氧化能力下降，以及来自浸润的表达NOX2的白细胞的ROS诱导损伤可能增加，导致肝细胞特异性eIF2α磷酸化缺陷的小鼠肝细胞死亡和肝纤维化显著增加；所有这些导致了肝细胞死亡和白细胞浸润的恶性循环。
我们的研究结果表明，eIF2α磷酸化维持NADPH和GSH水平，并控制ROS防御基因的表达，从而保护肝细胞免受果糖代谢诱导的氧化应激。
