Laboratory of Experimental and Molecular Hepatology, Division of Gastroenterology and Hepatology, Department of Internal Medicine, Medical University of Graz, Graz, Austria.
Hepatology. 2012 Jul;56(1):270-80. doi: 10.1002/hep.25601. Epub 2012 May 29.
Nonalcoholic fatty liver disease (NAFLD) is characterized by triglyceride (TG) accumulation and endoplasmic reticulum (ER) stress. Because fatty acids (FAs) may trigger ER stress, we hypothesized that the absence of adipose triglyceride lipase (ATGL/PNPLA2)-the main enzyme for intracellular lipolysis, releasing FAs, and closest homolog to adiponutrin (PNPLA3) recently implicated in the pathogenesis of NAFLD-protects against hepatic ER stress. Wild-type (WT) and ATGL knockout (KO) mice were challenged with tunicamycin (TM) to induce ER stress. Serum biochemistry, hepatic TG and FA profiles, liver histology, and gene expression for markers of hepatic lipid metabolism, ER stress, and inflammation were explored. Moreover, cell-culture experiments were performed in Hepa1.6 cells after the knockdown of ATGL before FA and TM treatment. TM increased hepatic TG accumulation in ATGL KO, but not in WT, mice. Lipogenesis and β-oxidation were repressed at the gene-expression level (sterol regulatory element-binding transcription factor 1c, fatty acid synthase, acetyl coenzyme A carboxylase 2, and carnitine palmitoyltransferase 1 alpha) in both WT and ATGL KO mice. Genes for very-low-density lipoprotein (VLDL) synthesis (microsomal triglyceride transfer protein and apolipoprotein B) were down-regulated by TM in WT and even more in ATGL KO mice, which displayed strongly reduced serum VLDL cholesterol levels. Notably, ER stress markers glucose-regulated protein, C/EBP homolog protein, spliced X-box-binding protein, endoplasmic-reticulum-localized DnaJ homolog 4, and inflammatory markers Tnfα and iNos were induced exclusively in TM-treated WT, but not ATGL KO, mice. Total hepatic FA profiling revealed a higher palmitic acid/oleic acid (PA/OA) ratio in WT mice, compared to ATGL KO mice, at baseline. Phosphoinositide-3-kinase inhibitor-known to be involved in FA-derived ER stress and blocked by OA-was increased in TM-treated WT mice only. In line with this, in vitro OA protected hepatocytes from TM-induced ER stress.
Lack of ATGL may protect from hepatic ER stress through alterations in FA composition. ATGL could constitute a new therapeutic strategy to target ER stress in NAFLD.
非酒精性脂肪性肝病（NAFLD）的特征是甘油三酯（TG）积累和内质网（ER）应激。因为脂肪酸（FAs）可能引发 ER 应激，我们假设缺乏脂肪甘油三酯脂肪酶（ATGL/PNPLA2）-细胞内脂肪分解的主要酶，释放 FAs，并且与最近被牵连到 NAFLD 发病机制中的脂联素（PNPLA3）最接近-可防止肝 ER 应激。野生型（WT）和 ATGL 敲除（KO）小鼠用衣霉素（TM）挑战以诱导 ER 应激。研究了血清生化、肝 TG 和 FA 谱、肝组织学以及肝脂质代谢、ER 应激和炎症标志物的基因表达。此外，在 FA 和 TM 处理前用 ATGL 敲低进行 Hepa1.6 细胞的细胞培养实验。TM 增加了 ATGL KO 小鼠但没有 WT 小鼠的肝 TG 积累。脂肪生成和β-氧化在 WT 和 ATGL KO 小鼠的基因表达水平（固醇调节元件结合转录因子 1c、脂肪酸合酶、乙酰辅酶 A 羧化酶 2 和肉碱棕榈酰转移酶 1α）受到抑制。WT 和 ATGL KO 小鼠的极低密度脂蛋白（VLDL）合成基因（微粒体甘油三酯转移蛋白和载脂蛋白 B）均受 TM 下调，ATGL KO 小鼠的血清 VLDL 胆固醇水平明显降低。值得注意的是，只有在 TM 处理的 WT 小鼠中，而不是 ATGL KO 小鼠中，才诱导 ER 应激标志物葡萄糖调节蛋白、C/EBP 同源蛋白、剪接 X 盒结合蛋白、内质网定位的 DnaJ 同源物 4 和炎症标志物 Tnfα 和 iNos。总的肝 FA 分析显示，WT 小鼠的棕榈酸/油酸（PA/OA）比值高于 ATGL KO 小鼠，基线时。PI3K 抑制剂-已知参与 FA 衍生的 ER 应激并被 OA 阻断-仅在 TM 处理的 WT 小鼠中增加。与此一致，体外 OA 可保护肝细胞免受 TM 诱导的 ER 应激。
缺乏 ATGL 可能通过 FA 组成的改变来保护肝 ER 应激。ATGL 可能成为治疗 NAFLD 中 ER 应激的新策略。
