Impaired methylation alters lipid droplet dynamics in liver and adipose tissue: Role in hepatic steatosis

甲基化受损改变肝脏和脂肪组织中的脂滴动力学：在肝脂肪变性中的作用

• 批准号: 10427223
• 负责人: Kusum K. Kharbanda
• 金额: --
• 依托单位: OMAHA VA MEDICAL CENTER
• 依托单位国家: 美国
• 财政年份: 2019
• 资助国家: 美国
• 起止时间: 2019-01-01 至 2023-12-31
• 项目状态: 已结题
• 来源: https://reporter.nih.gov/project-details/10427223
• 关键词: Adipocytes; Adipose tissue; Affect; Alcoholic Hepatitis; Alcoholic Liver Cirrhosis; Alcoholic Liver Diseases; Alcoholism; Alcohols; Animal Model; Benign; Betaine; Biogenesis; Catabolism; Catalysis; Characteristics; Chronic; Cirrhosis; Consumption; Data; Defect; Development; Digestion; Disease Progression; Economic Burden; Enzymes; Ethanol; Fatty Acids; Fatty Liver; Fatty acid glycerol esters; Fibrosis; Generations; Healthcare Systems; Hepatic; Hepatocyte; Hospitalization; Impairment; Lecithin; Lipase; Lipid Binding; Lipids; Lipolysis; Liver; Malignant Neoplasms; Mediating; Metabolic; Metabolism; Methylation; Military Personnel; Modeling; Nonesterified Fatty Acids; Ocular orbit; Organelles; Phosphatidylethanolamine; Phosphatidylethanolamine N-Methyltransferase; Phospholipids; Play; Prevention; Production; Proteins; Reaction; Reporting; Resistance; Risk; Role; Services; Structure; Supplementation; Surface; Testing; Therapeutic Intervention; Triglycerides; Very low density lipoprotein; Veterans; alcohol effect; alcohol misuse; alcohol use initiation; base; chronic liver disease; experience; fatty liver disease; insight; methyl group; military veteran; monolayer; non-alcoholic fatty liver disease; novel; prevent; problem drinker; targeted treatment; treatment strategy; uptake

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 benign and reversible early stage of ALD. However, fat accumulation is regarded as the “first hit” that leaves the liver more vulnerable to multiple hits leading to ALD progression, and is therefore, a prime target for therapeutic intervention. Our long-term objective is to (i) understand the mechanisms of alcoholic steatosis development and (ii) formulate strategies for treatment/prevention of this and other fatty liver diseases that present with similar histopathological and disease progression characteristics such as non-alcoholic fatty liver disease. We have shown that alcohol-impaired activity of a major liver enzyme, phosphatidylethanolamine methyltransferase (PEMT), inhibits very-low-density lipoprotein (VLDL) secretion, contributing to the development of hepatic steatosis. PEMT catalyzes the methylation of phosphatidylethanolamine (PE) to generate phosphatidylcholine (PC), which is preferentially used in the assembly of VLDL and is necessary for its normal secretion. We have further shown that treatment with betaine, (a methyl donor) increases PC generation by normalizing PEMT activity. This, in turn, normalizes VLDL secretion rate and thus prevents 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 must be hydrolyzed to provide substrates for VLDL biogenesis. LDs are surrounded by a monolayer of phospholipids; PC is the most abundant class followed by PE. Further, an orbit of proteins determines the metabolic fate of LD lipid stores. Reduction in LD monolayer PC:PE ratio promotes fusion of these organelles to form supersized structures that are more resistant to lipolysis. Furthermore, a lower PC:PE ratio enhances the binding of the LD-associated proteins known to protect this organelle from lipase digestion. We have evidence that phospholipid and protein composition of hepatic LD changes with the initiation of alcohol-induced hepatic steatosis. Importantly, ethanol-induced hepatic LD accumulation occurs in conjunction with impaired VLDL production and enhanced adipose lipolysis-generated free fatty acid delivery and uptake. However, little is known about why alcohol promotes LD accumulation in hepatocytes but enhances adipocyte LD shrinkage. Based on these considerations, we present a novel hypothesis that alcohol impaired methylation contributes to the development of hepatic steatosis by inhibiting liver LD lipolysis and promoting adipose LD breakdown. We propose that liver phospholipid methylation defect lowers the LD monolayer PC:PE ratio to generate enlarged structures with altered composition of LD-associated proteins. These changes together hinder the lipolysis of the hepatic LD triglyceride stores to disrupt VLDL biogenesis resulting in fat accumulation. Conversely, adipose methylation defects activate lipases that promote LD lipolysis and fatty acid export. We further postulate that betaine supplementation reverses alcoholic steatosis by normalizing hepatic LD monolayer PC:PE ratio and VLDL biogenesis, and by regulating adipose LD lipolysis, hepatic fatty acid uptake and metabolism. To test our hypothesis, we propose the following Specific Aims: Specific Aim 1: To characterize how ethanol alters the phospholipid and protein composition of LDs. Specific Aim 2: To examine the effect of alcohol on LD lipolysis for mobilizing triglyceride stores. Specific Aim 3: To determine the effect of betaine on alcohol-induced alterations in LD dynamics. Completion of these studies will provide insight into the importance of maintaining essential methylation reactions in regulating the dynamics of hepatic and adipose LDs and preventing the development of alcoholic steatosis and other chronic liver diseases including non-alcoholic fatty liver disease.

脂肪肝（脂肪变性）是酒精性肝病 (ALD) 的早期表现，可导致 持续滥用酒精会导致酒精性肝炎和肝硬化 肝脂肪变性是一种良性的、慢性的疾病。 ALD的早期阶段是可逆的，但脂肪堆积被认为是让肝脏遭受更多打击的“第一击”。 易受多重打击导致 ALD 进展，因此是治疗的主要目标 我们的长期目标是 (i) 了解酒精性脂肪变性的发展机制。 (ii) 制定治疗/预防这种疾病和其他脂肪肝疾病的策略 类似的组织病理学和疾病进展特征，例如非酒精性脂肪肝。 我们已经证明，酒精会损害主要肝酶磷脂酰乙醇胺的活性 甲基转移酶（PEMT），抑制极低密度脂蛋白（VLDL）的分泌，有助于 PEMT 催化磷脂酰乙醇胺 (PE) 甲基化，从而形成肝脂肪变性。 生成磷脂酰胆碱 (PC)，它优先用于 VLDL 的组装，并且是 我们进一步表明，用甜菜碱（甲基供体）治疗会增加 PC 的分泌。 通过使 PEMT 活性正常化，从而使 VLDL 分泌率正常化，从而防止产生 VLDL。 酒精性脂肪变性。 已证明肝细胞质脂滴 (LD) 在 VLDL 中发挥着不可或缺的作用 这是因为 VLDL 的组装受到这些 LD 中储存的甘油三酯的调节。 LD 必须被水解以提供 VLDL 生物发生的底物，其周围是单层的 LD。 磷脂；PC 是最丰富的类别，其次是 PE。此外，蛋白质的轨道决定了。 LD 脂质储存的代谢命运 LD 单层 PC:PE 比例的降低促进了这些细胞器的融合。 形成更耐脂解的超大结构此外，较低的 PC:PE 比例可增强性能。 已知 LD 相关蛋白的结合可以保护该细胞器免受脂肪酶的消化。 有证据表明肝脏 LD 的磷脂和蛋白质组成随着酒精诱导的开始而变化 重要的是，乙醇诱导的肝脏 LD 积累与受损有关。 VLDL 的产生和增强的脂肪分解产生的游离脂肪酸的输送和吸收然而很少。 众所周知，为什么酒精会促进肝细胞中 LD 的积累，但会增强脂肪细胞 LD 的收缩。 基于这些考虑，我们提出了一个新的假设，即酒精损害甲基化有助于 通过抑制肝脏 LD 脂解作用和促进脂肪 LD 分解来预防肝脂肪变性。 提出肝磷脂甲基化缺陷降低 LD 单层 PC:PE 比率以产生放大 LD 相关蛋白组成发生改变的结构，这些变化共同阻碍了 LD 的脂肪分解。 肝脏 LD 甘油三酯储存会破坏 VLDL 生物合成，导致脂肪堆积。 甲基化缺陷激活脂肪酶，促进 LD 脂肪分解和脂肪酸输出。 补充甜菜碱可通过使肝脏 LD 单层 PC:PE 比率正常化来逆转酒精性脂肪变性 VLDL的生物发生，并通过调节脂肪LD的脂解、肝脏脂肪酸的摄取和代谢。 为了检验我们的假设，我们提出以下具体目标： 具体目标 1：表征乙醇如何改变 LD 的磷脂和蛋白质组成。 具体目标 2：检查酒精对 LD 脂肪分解以动员甘油三酯储存的影响。 具体目标 3：确定甜菜碱对酒精引起的 LD 动态变化的影响。 完成这些研究将深入了解维持必需甲基化的重要性 调节肝脏和脂肪 LD 动态并预防酒精发展的反应 脂肪变性和其他慢性肝病，包括非酒精性脂肪肝。