Homeostatic role of IRE1a-XBP1-PDI1 in hepatic lipid metabolism

IRE1a-XBP1-PDI1 在肝脂质代谢中的稳态作用

• 批准号: 8888815
• 负责人: RANDAL J. KAUFMAN
• 金额: $ 58.5万
• 依托单位: SANFORD BURNHAM PREBYS MEDICAL DISCOVERY INSTITUTE
• 依托单位国家: 美国
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-04-01 至 2020-03-31
• 项目状态: 已结题
• 来源: https://reporter.nih.gov/project-details/8888815
• 关键词: Acute; Affect; Animals; Apolipoproteins B; Attenuated; Biochemistry; Cirrhosis; Consumption; Cytosol; Data; Development; Diabetes Mellitus; Diagnostic; Diet; Disease; Dysplasia; Endoplasmic Reticulum; Environment; Epidemic; Etiology; Fatty Liver; Fibrosis; Fructose; Genes; Genetic Models; Genetic Transcription; Grant; Health; Health Care Costs; Hepatic; Hepatocyte; Homeostasis; Human; Hyperlipidemia; Inflammation; Inositol; Insulin Resistance; Intervention; Knockout Mice; Link; Lipids; Lipoproteins; Liver; Liver diseases; Mass Spectrum Analysis; Mediating; Messenger RNA; Metabolic Diseases; Metabolic stress; Mitochondria; Modality; Molecular Chaperones; Molecular Profiling; Mus; Nutrient; Nutritional; Nutritional Study; Obesity; Pathogenesis; Pathway interactions; Physiology; Plasma; Play; Population; Primary carcinoma of the liver cells; Production; Prognostic Marker; Protein Biosynthesis; Protein Disulfide Isomerase; Proteins; Proteomics; RNA Splicing; Reactive Oxygen Species; Research; Role; Signal Transduction; Site; Smooth Endoplasmic Reticulum; Stimulus; Stress; Testing; Therapeutic Intervention; Time; Triglycerides; Very low density lipoprotein; alternative treatment; arm; base; cardiovascular disorder risk; glycosylation; improved; in vivo; insight; intrahepatic; lipid metabolism; lipid transport; mRNA Expression; microsomal triglyceride transfer protein; non-alcoholic fatty liver; nonalcoholic steatohepatitis; novel; novel therapeutic intervention; particle; prevent; protein folding; protein misfolding; public health relevance; response

 DESCRIPTION (provided by applicant): Approximately 20% of the US population suffers from non-alcoholic fatty liver disease (NAFLD) that can progress to hepatosteatosis, fibrosis, cirrhosis and hepatocellular carcinoma. Hepatic secretion of VLDL plays an essential role in regulating intrahepatic and intravascular lipid homeostasis. The overproduction of hepatic VLDL characterizes the pathogenesis of hyperlipidemia in obesity and diabetes and contributes significantly to an increased risk of cardiovascular disease. Despite the significant impact of abnormalities in VLDL secretion on human health, fundamental cellular mechanisms that drive VLDL assembly are yet to be understood. Hepatic VLDL assembly and secretion is significantly reduced by protein misfolding in the endoplasmic reticulum (ER) and increased by the adaptive unfolded protein response (UPR). The proposed research will elucidate the mechanistic role of the UPR in maintaining triglyceride (TG) levels in the ER lumen for VLDL assembly. We will vigorously test the hypothesis that changes in the ER protein-folding environment of the hepatocyte activate inositol-requiring kinase1 (IRE1) to splice Xbp1 mRNA to generate XBP1s in response to a nutritional stimulus are key in the etiology of NAFLD and abnormal lipid secretion and hyperlipidemia. Our hypothesis is based on three recent fundamental findings regarding assembly of VLDL. First, we have demonstrated that nutritional excess alters lipid homeostasis and activates the UPR in hepatocytes. Aim 1 will uncover the mechanism that links metabolic stress with protein misfolding in the ER. Aims 2 and 3 focus on how XBP1s is essential role to maintain ER lipid homeostasis. We have demonstrated that XBP1s provides two independent functions for assembly of TG-rich VLDL. First, XBP1s activates transcription of protein disulfide isomerase (Pdi1) to maintain microsomal TG transfer protein (MTP) in an active form to promote lipid assembly with apolipoprotein B (ApoB). Aim 2 will study how nutritional stress reduces MTP activity in a PDI1-dependent manner. Aim 3 will identify the second mechanism by which XBP1s is essential to promote TG-rich VLDL assembly in an MTP-independent manner using a combination of mRNA-Seq and mass spectrometry approaches. Finally, and significantly, our data show that PDI1 promotes oxidative folding of ApoB, thus for the first time uncovering a protein folding substrate for PDI1 in vivo. Aim 4 will use hepatocyte-specific Pdi1-null mice to study how PDI1 promotes ApoB100 oxidative folding and alters interactions with the protein-folding environment (i.e., molecular chaperones). Results from the proposed studies will elucidate the mechanisms by which IRE1/XBP1 maintains hepatocyte lipid homeostasis by partitioning TG into the ER lumen for VLDL assembly. The findings will provide fundamental understanding of mechanisms that govern VLDL maturation, which will facilitate the development of novel therapeutic approaches to treat hepatosteatosis and hyperlipidemia.

 描述（由申请人提供）：大约 20% 的美国人患有非酒精性脂肪肝病 (NAFLD)，可进展为肝脂肪变性、纤维化、肝硬化 肝脏分泌 VLDL 在调节肝内和血管内脂质稳态中发挥着重要作用，肝脏 VLDL 的过量产生是肥胖和糖尿病中高脂血症的发病机制，并且会显着增加心血管疾病的风险。 VLDL 分泌异常对人类健康的影响，驱动 VLDL 组装的基本细胞机制尚不清楚，肝脏中的蛋白质错误折叠会显着减少 VLDL 组装和分泌。内质网 (ER) 并通过适应性未折叠蛋白反应 (UPR) 增加。拟议的研究将阐明 UPR 在维持 ER 腔内甘油三酯 (TG) 水平以进行 VLDL 组装方面的机制作用。肝细胞内质网蛋白折叠环境的变化激活肌醇需求激酶1α（IRE1α）来剪接Xbp1 mRNA以生成XBP1 对营养刺激的反应是 NAFLD 和脂质分泌异常和高脂血症的关键。我们的假设基于最近关于 VLDL 组装的三个基本发现。首先，我们已经证明营养过量会改变脂质稳态并激活 UPR。目标 1 将揭示代谢应激与 ER 中蛋白质错误折叠之间的联系机制。目标 2 和 3 重点关注 XBP1 如何在维持 ER 脂质方面发挥重要作用。我们已经证明，XBP1s 为富含 TG 的 VLDL 的组装提供两个独立的功能，首先，XBP1s 激活蛋白质二硫键异构酶 (Pdi1) 的转录，以维持微粒体 TG 转移蛋白 (MTP) 的活性形式，从而促进脂质与载脂蛋白的组装。 B (ApoB)。目标 2 将研究营养应激如何以 PDI1 依赖性方式降低 MTP 活性，目标 3 将确定第二种机制。 XBP1s 对于结合 mRNA-Seq 和质谱方法以不依赖 MTP 的方式促进富含 TG 的 VLDL 组装至关重要。最后，重要的是，我们的数据表明 PDI1 促进 ApoB 的氧化折叠，从而首次揭示了这一点。 Aim 4 将使用肝细胞特异性 Pdi1 缺失小鼠来研究 PDI1 如何促进 ApoB100 氧化。折叠并改变与蛋白质折叠环境（即分子伴侣）的相互作用。拟议研究的结果将阐明 IRE1/XBP1 通过将 TG 分配到 ER 腔中进行 VLDL 组装来维持肝细胞脂质稳态的机制。提供对控制 VLDL 成熟机制的基本了解，这将有助于开发治疗肝脂肪变性和高脂血症的新治疗方法。