Control of liver autophagy by phosphatidic acid signaling

描述（由申请人提供）：脂质的适当储存和利用对于维持细胞能量稳态至关重要，这种平衡的扰动可能导致各种人类疾病，包括代谢综合征。细胞脂质通常以甘油三酯的形式以脂滴的形式储存，在营养剥夺时被水解成脂肪酸提供能量。近年来，巨噬（macroautophagy，简称自噬）已成为肝细胞和许多其他细胞类型中介导甘油三酯分解代谢和脂滴清除（即“巨噬”）的主要细胞途径。自噬确保在营养匮乏的条件下大分子的循环，以及消除有缺陷的细胞器、长寿命蛋白质和细胞内蛋白质聚集体。由于自噬在消除细胞“废物”和消耗能量方面的显著效率，它越来越被视为一种可用于治疗各种人类疾病的途径。最近对巨脂噬的发现提出了许多基本问题，包括是否存在专门用于这种选择性自噬的分子途径，以及这一过程的功效是否受到饮食和衰老的影响。在这一提议中，我们探讨了脂质酶磷脂酶D1 （PLD1）是大脂噬的积极调节剂的可能性。PLD1将磷脂酰胆碱水解成生物活性脂质磷脂酸，调节信号传导和膜运输过程。我们发表的数据表明，缺乏PLD1的小鼠在肝脏中表现出自噬缺陷，这是基于突变肝细胞中自噬体的减少和缩小。值得注意的是，这种表型是由饥饿条件诱导的，与蛋白质聚集体的积累无关，而在缺乏Atg5或Atg7等基因的突变小鼠中通常观察到，这些基因对于基础和饥饿诱导的自噬都是必不可少的。在机制水平上，我们的研究结果表明，pld1衍生的磷脂酸的一个关键功能是通过促进这些细胞器与核内体/溶酶体的融合来促进自噬体的成熟。初步观察显示，脂滴在饥饿小鼠的Pld1敲除肝脏中以及在敲除成纤维细胞中更为突出，我们的结果强烈表明Pld1与饥饿诱导的巨脂肪吞噬有关。在这项提议中，我们计划使用肝细胞中选择性缺乏PLD1的小鼠直接验证这一假设。我们还评估了“压力源”（如高脂肪饮食和年龄）对巨脂吞噬疗效的影响，并预测它们可能进一步揭示脂肪积累和脂质代谢的基因型特异性差异。我们预计我们的研究将阐明巨脂噬在脂质代谢控制中的作用，并为未来的工作测试是否可以利用刺激PLD1通路来促进人类条件下的巨脂噬，如脂肪肝疾病提供基础。

DESCRIPTION (provided by applicant): Proper storage and utilization of lipids is essential to maintain cellular energy homeostasis and perturbation of this balance can lead to a variety of human disorders, including metabolic syndrome. Cellular lipids are generally stored as triglycerides in the form of lipid droplets and are hydrolyzed into fatty acids for energy supply during nutrient deprivation. Recently, macroautophagy (simply called autophagy) has emerged as a major cellular pathway mediating the catabolism of triglycerides and the clearance of lipid droplets (i.e., "macrolipophagy") in hepatocytes and many other cell types. Autophagy ensures the recycling of macromolecules under conditions of nutritional scarcity, as well as the elimination of defective organelles, long- lived proteins, and intracellular protein aggregates. Because of its remarkable efficiency at eliminating cellular "waste" and consuming energy sources, autophagy is increasingly viewed as a pathway that can be exploited for the therapy of various human conditions. The recent discovery of macrolipophagy has raised a number of fundamental questions, including whether there are molecular pathways dedicated to this selective type of autophagy and whether the efficacy of this process is affected by diet and aging. In this proposal, we explore the possibility that the lipid enzyme phospholipase D1 (PLD1) is a positive modulator of macrolipophagy. PLD1 hydrolyzes phosphatidylcholine into the bioactive lipid phosphatidic acid, which regulates signaling and membrane trafficking processes. Our published data indicate that mice lacking PLD1, which are viable, exhibit defects in autophagy in the liver, based on the occurrence of both fewer and smaller autophagosomes in mutant hepatocytes. Remarkably, this phenotype is induced by starvation conditions and is not associated with the accumulation of protein aggregates, as typically observed in mutant mice lacking genes such as Atg5 or Atg7 which are essential for both basal and starvation-induced autophagy. At the mechanistic level, our results suggest that a key function of PLD1-derived phosphatidic acid is to promote the maturation of autophagosomes by facilitating the fusion of these organelles with endosomes/lysosomes. Together with preliminary observations showing that lipid droplets are more prominent in Pld1 knockout liver from starved mice as well as in knockout fibroblasts, our results strongly suggest that PLD1 is implicated in starvation- induced macrolipophagy. In this proposal, we plan to directly test this hypothesis using mice that lack PLD1 selectively in hepatocytes. We also assess the impact of "stressors", such as high fat diet and age, on the efficacy of macrolipophagy, with the prediction that they may further unmask genotype-specific differences in fat accumulation and lipid metabolism. We anticipate our studies will clarify the role of macrolipophagy in the control of lipid metabolism and provide the basis for future work testing whether stimulating the PLD1 pathway can be exploited to promote macrolipophagy in human conditions, such as fatty liver diseases.