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

脂肪肝（脂肪变性）的发展是酒精性肝病（ALD）的早期表现， 进展为酒精性肝炎和肝硬化，并持续滥用酒精。肝脂肪变性是一种良性的， ALD的早期可逆性。然而，脂肪堆积被视为离开肝脏更多的“第一击” 容易受到多次打击，导致ALD进展，因此是治疗的主要目标。 干预我们的长期目标是（i）了解酒精性脂肪变性发展的机制 和（ii）制定治疗/预防这种和其他脂肪肝疾病的策略， 相似的组织病理学和疾病进展特征，如非酒精性脂肪肝。 我们已经表明，酒精损害的主要肝酶，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）分泌，有助于 肝脂肪变性的发展。PEMT催化磷脂酰乙醇胺（PE）甲基化， 产生磷脂酰胆碱（PC），其优先用于VLDL的组装，并且是必需的， 正常的分泌物。我们进一步表明，用甜菜碱（一种甲基供体）处理可增加PC 通过使PEMT活性正常化来生成。这反过来又使VLDL分泌率正常化，从而防止 酒精性脂肪变性 肝细胞质脂滴（LDs）在VLDL形成中起重要作用 生物起源这是因为极低密度脂蛋白的组装是由储存在这些低密度脂蛋白中的甘油三酯的可用性来调节的 其必须被水解以提供VLDL生物合成的底物。LD被单层的 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 以形成更能抵抗脂肪分解的超大结构。此外，较低的PC：PE比率增强了 已知LD相关蛋白的结合可保护该细胞器免受脂肪酶消化。我们有 肝LD的磷脂和蛋白质组成随着酒精诱导的肝硬化的开始而改变的证据 肝脂肪变性重要的是，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增强脂肪分解产生的游离脂肪酸的输送和摄取。不过小 关于为什么酒精促进肝细胞中LD的积累但增强脂肪细胞LD的收缩是已知的。 基于这些考虑，我们提出了一个新的假设，即酒精损伤的甲基化有助于 通过抑制肝脏LD脂解和促进脂肪LD分解来发展肝脂肪变性。我们 提出肝磷脂甲基化缺陷降低了LD单层PC：PE比率， LD相关蛋白组成改变的结构。这些变化共同阻碍了脂肪分解， 肝脏LD甘油三酯储存破坏VLDL生物合成，导致脂肪积累。相反，脂肪 甲基化缺陷激活促进LD脂解和脂肪酸输出的脂肪酶。我们进一步假设， 甜菜碱补充通过使肝LD单层PC：PE比率正常化来逆转酒精性脂肪变性， VLDL的生物合成，并通过调节脂肪LD的脂解，肝脂肪酸的摄取和代谢。 为了验证我们的假设，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。 具体目的2：检查酒精对LD脂解作用的影响，以动员甘油三酯储存。 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需甲基化的重要性 在调节肝脏和脂肪LDs的动力学和防止酒精中毒发展方面的反应 脂肪变性和其它慢性肝病，包括非酒精性脂肪性肝病。

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.