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)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良性的 可逆性ALD早期。然而，脂肪堆积被认为是让肝脏更多的“第一击”。 易受多次打击导致ALD进展，因此是治疗的主要靶点 干预。我们的长期目标是(I)了解酒精性脂肪变性的发生机制 和(2)制订治疗/预防这种和其他脂肪性肝病的战略 类似的组织病理学和疾病进展特征，如非酒精性脂肪性肝病。 我们已经证明，酒精损害了肝脏的一种主要酶--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 肝脏脂肪变性的发展。PEMT催化磷脂酰乙醇胺(PE)甲基化 产生磷脂酰胆碱(PC)，它优先用于极低密度脂蛋白的组装，是 它的正常分泌物。我们进一步表明，甜菜碱(一种甲基供体)治疗可以增加PC 通过使PEMT活动正常化而产生。这反过来又使极低密度脂蛋白的分泌速度正常化，从而防止 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 它必须被水解才能为极低密度脂蛋白的生物生成提供底物。LD被单层的 磷脂；PC是含量最丰富的一类，其次是PE。此外，蛋白质的轨道决定了 LD脂肪储存的代谢命运。降低LD单层PC/PE比例促进这些细胞器融合 形成更能抵抗脂肪分解的超大型结构。此外，较低的PC：PE比率可增强 与LD相关的蛋白质的结合，以保护这个细胞器免受脂肪酶消化。我们有 酒精性肝损伤后肝组织磷脂和蛋白质组成变化的证据 肝脏脂肪变性。重要的是，乙醇诱导的肝脏LD蓄积与受损的 极低密度脂蛋白的产生和脂肪分解增强产生的游离脂肪酸的输送和摄取。然而，几乎没有 已知为什么酒精会促进肝细胞中LD的积累，但会增加脂肪细胞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 通过抑制肝脏脂肪分解和促进脂肪分解来促进肝脏脂肪变性的发生。我们 提示肝磷脂甲基化缺陷降低LD单层PC/PE比值产生增大 LD相关蛋白组成发生变化的结构。这些变化共同阻碍了脂肪分解。 肝脏的低密度脂蛋白甘油三酯储存，以扰乱极低密度脂蛋白的生物合成，导致脂肪积累。相反，脂肪 甲基化缺陷会激活脂肪酶，从而促进LD脂肪分解和脂肪酸输出。我们进一步假定 补充甜菜碱可通过使肝脏单层PC/PE比值正常化来逆转酒精性脂肪变性 极低密度脂蛋白的生物发生，并通过调节脂肪脂肪分解，肝脏脂肪酸的摄取和代谢。 为了验证我们的假设，我们提出了以下具体目标： 具体目标1：研究乙醇如何改变低密度脂蛋白的磷脂和蛋白质组成。 具体目的2：研究酒精对脂肪分解动员甘油三酯储备量的影响。 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。 这些研究的完成将有助于深入了解维持必要甲基化的重要性 调节肝脏和脂肪乳酸脱氢酶动力学及预防酒精性疾病发生的反应 脂肪变性和其他慢性肝病，包括非酒精性脂肪性肝病。