Impaired phospholipid methylation results in decreased lipid droplet lipolysis: Role in hepatic steatosis

项目摘要/摘要 脂肪肝(脂肪变性)的发展是酒精性肝病(ALD)的早期表现，可以 酒精持续滥用导致酒精性肝炎和肝硬变的进展。肝脏脂肪变性是一种可逆的 这是ALD的早期阶段，因此是治疗干预的主要靶点。我们的长期目标是 (I)了解酒精性脂肪变性的发病机制；及(Ii)制订策略，以 治疗/预防本病和其他具有相似组织病理学和进展史的脂肪肝疾病。 我们以前已经证明，磷脂酰乙醇胺甲基转移酶(PEMT)的减少会损害 极低密度脂蛋白(VLDL)分泌导致酒精性脂肪变性。PEMT催化了 磷脂酰乙醇胺(PE)甲基化生成磷脂酰胆碱(PC)，这是 正常的极低密度脂蛋白组装和分泌。我们进一步表明，甜菜碱是一种甲基供体， 使PEMT催化的PC合成正常化，促进极低密度脂蛋白的分泌，防止酒精性脂肪变性。 已证实肝脏胞浆脂滴在极低密度脂蛋白中起着不可或缺的作用 生物发生学。这是因为极低密度脂蛋白的组装受到储存在这些低密度脂蛋白中的甘油三酯的可用性的调节 它们必须被水解以提供极低密度脂蛋白组装的底物。LD被单层的 磷脂；PC是最丰富的一类，其次是PE和其他。此外，蛋白质的轨道 决定着LD脂类储存的代谢命运。 基于这些考虑，我们提出了一个新的假设，即磷脂甲基化受损 通过损害LD的脂解作用，促进肝脏脂肪变性的发展。我们建议乙醇- PEMT诱导的PC损伤降低了LD单层中PC/PE的比例。这促进了 随着LD相关蛋白的补体发生显著变化而产生扩大的LD。这些 这些变化共同影响LD甘油三酯储存的脂解作用，扰乱了 极低密度脂蛋白导致肝脏脂肪变性。我们进一步假设饮食中补充甜菜碱可逆转 酒精性脂肪变性通过使LD-PC/PE比值正常化和极低密度脂蛋白的生物发生。 为了验证我们的假设，我们提出了以下具体目标： 具体目的1：研究乙醇如何改变肝脏低密度脂蛋白的磷脂和蛋白质组成。 具体目的2：检查酒精对脂肪分解的影响，以动员甘油三酯储存 极低密度脂蛋白分泌。 具体目标3：确定甜菜碱对酒精诱导的LD动力学改变的影响。 这些研究的完成将使我们深入了解维持必要甲基化的重要性 PEMT催化的调节脂滴动力学和预防血管病变进展的反应 酒精性脂肪变性和其他慢性肝病，包括非酒精性肝病。

PROJECT SUMMARY/ABSTRACT The development of fatty liver (steatosis) is an early manifestation of alcoholic liver disease (ALD) that can progress to alcoholic hepatitis and cirrhosis with continued alcohol misuse. Hepatic steatosis is a reversible early stage of ALD and is, therefore, a prime target for therapeutic intervention. Our long term objectives are to (i) understand the mechanisms of alcoholic steatosis development and (ii) formulate strategies for treatment/prevention of this and other fatty liver diseases with similar histopathology and progression history. We have previously shown that reduction in phosphatidylethanolamine methyltransferase (PEMT) impairs very-low-density lipoproteins (VLDL) secretion contributing to alcoholic steatosis. PEMT catalyzes the methylation of phosphatidylethanolamine (PE) to generate phosphatidylcholine (PC), which is necessary for normal VLDL assembly and secretion. We have further shown that treatment with betaine, a methyl donor, normalizes PEMT-catalyzed PC synthesis to promote VLDL secretion and prevent alcoholic steatosis. It has been demonstrated that hepatic cytoplasmic lipid droplets (LDs) play an integral role in VLDL biogenesis. This is because VLDL assembly is regulated by the availability of triglycerides stored in these LDs which have to be hydrolyzed to provide substrates for VLDL assembly. LDs are surrounded by a monolayer of phospholipids; PC is the most abundant class followed by PE and others. Further, an orbit of proteins determines the metabolic fate of LD lipid stores. Based on these considerations, we present a novel hypothesis that impaired phospholipid methylation contributes to the development of hepatic steatosis by impairing LD lipolysis. We propose that the ethanol- induced impairment in PEMT-catalyzed PC decreases the PC:PE ratio in the LD monolayer. This promotes generation of enlarged LDs with significant alterations in the complement of LD-associated proteins. These changes together affect the lipolysis of the LD triglyceride stores disrupting the assembly and secretion of VLDL resulting in liver steatosis. We further hypothesize that dietary betaine supplementation reverses alcoholic steatosis by normalizing LD PC:PE ratio and VLDL biogenesis. To test our hypothesis, we propose the following Specific Aims: Specific Aim 1: To characterize how ethanol alters the phospholipid and protein composition of hepatic LDs. Specific Aim 2: To examine the effect of alcohol on LD lipolysis for mobilization of triglyceride stores for VLDL secretion. Specific Aim 3: To determine the effects of betaine on alcohol-induced alterations in LD dynamics. Completion of these studies will provide insight into the importance of maintaining the essential methylation reaction catalyzed by PEMT in regulating the dynamics of lipid droplet and preventing the development of alcoholic steatosis and other chronic liver diseases including non-alcoholic liver disease.