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是必不可少的，这种平衡的扰动可导致多种人类疾病，包括代谢综合征。细胞脂质通常以脂滴形式储存为甘油三酯，并在营养缺乏期间水解成脂肪酸用于能量供应。最近，巨自噬（简称自噬）已经成为介导甘油三酯的分解和脂滴的清除（即，“巨噬细胞”）。自噬确保了在营养缺乏的条件下大分子的再循环，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。由于其在消除细胞“废物”和消耗能源方面的显着效率，自噬越来越被视为可用于治疗各种人类疾病的途径。最近发现的巨噬细胞已经提出了一些基本的问题，包括是否有分子途径致力于这种选择性类型的自噬，以及这个过程的功效是否受到饮食和衰老的影响。在这个建议中，我们探讨的可能性，脂质酶磷脂酶D1（PLD 1）是一个积极的调节巨噬细胞。PLD 1将磷脂酰胆碱水解成生物活性的脂质磷脂酸，其调节信号传导和膜运输过程。我们发表的数据表明，缺乏PLD 1的小鼠，这是可行的，表现出缺陷的自噬在肝脏中，基于发生更少和更小的自噬体在突变肝细胞。值得注意的是，这种表型是由饥饿条件诱导的，并且与蛋白质聚集体的积累无关，如通常在缺乏基因如Atg 5或Atg 7的突变小鼠中观察到的，所述基因对于基础和饥饿诱导的自噬都是必需的。在机制水平上，我们的研究结果表明，PLD 1衍生的磷脂酸的一个关键功能是促进这些细胞器与内体/溶酶体的融合，以促进自噬体的成熟。连同显示脂滴在来自饥饿小鼠的Pld 1敲除肝脏以及敲除成纤维细胞中更突出的初步观察，我们的结果强烈地表明，PLD 1与饥饿诱导的巨噬细胞有关。在这项提案中，我们计划使用肝细胞中选择性缺乏PLD 1的小鼠直接测试这一假设。我们还评估了“压力源”，如高脂肪饮食和年龄，对巨噬细胞功效的影响，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噬细胞在脂质代谢控制中的作用，并为未来的工作提供基础，测试刺激PLD 1通路是否可以用于促进人类条件下的巨噬细胞，如脂肪肝疾病。

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.