BACKGROUND Apolipoprotein A-I (ApoA-I) is involved in reverse cholesterol transport as a major component of HDL, but also conveys anti-thrombotic, anti-oxidative, anti-inflammatory and immune-regulatory properties that are pertinent to its protective roles in cardiovascular, inflammatory and malignant pathologies. Despite the pleiotropy in ApoA-I functions, the regulation of intracellular ApoA-I levels remains poorly explored. METHODS HepG2 hepatoma cells and primary mouse hepatocytes were used as in vitro models to study the impact of genetic and chemical inhibitors of autophagy and the proteasome on ApoA-I by immunoblot, immunofluorescence and electron microscopy. Different growth conditions were implemented in conjunction with mTORC inhibitors to model the influence of nutrient scarcity versus sufficiency on ApoA-I regulation. Hepatic ApoA-I expression was also evaluated in high fat diet-fed mice displaying blockade in autophagy. RESULTS Under nutrient-rich conditions, basal ApoA-I levels in liver cells are sustained by the balancing act of autophagy and of mTORC1-dependent de novo protein synthesis. ApoA-I proteolysis occurs through a canonical autophagic pathway involving Beclin1 and ULK1 and the receptor protein p62/SQSTM1 that targets ApoA-I to autophagosomes. However, upon aminoacid insufficiency, suppression of ApoA-I synthesis prevails, rendering mTORC1 inactivation dispensable for autophagy-mediated ApoA-I proteolysis. CONCLUSION These data underscore the major contribution of post-transcriptional mechanisms to ApoA-I levels which differentially involve mTORC1-dependent signaling to protein synthesis and autophagy, depending on nutrient availability. Given the established role of ApoA-I in HDL-mediated reverse cholesterol transport, this mode of ApoA-I regulation may reflect a hepatocellular response to the organismal requirement for maintenance of cholesterol and lipid reserves under conditions of nutrient scarcity.

中文翻译：

---

mTORC1依赖的蛋白质合成和自噬在载脂蛋白AI表达的调节中解耦。

背景技术载脂蛋白AI（ApoA-I）作为高密度脂蛋白（HDL）的主要成分参与胆固醇的逆向转运，但也传达与其在心血管疾病中的保护作用有关的抗血栓形成，抗氧化，抗炎和免疫调节特性。炎性和恶性病理。尽管ApoA-I功能具有多效性，但对细胞内ApoA-I水平的调节仍缺乏探索。方法以HepG2肝癌细胞和原代小鼠肝细胞为体外模型，通过免疫印迹，免疫荧光和电子显微镜研究自噬的遗传和化学抑制剂以及蛋白酶体对ApoA-I的影响。结合mTORC抑制剂实施了不同的生长条件，以模拟营养不足与充足对ApoA-I调节的影响。还对高脂饮食喂养的小鼠自噬受阻的肝脏ApoA-I表达进行了评估。结果在营养丰富的条件下，肝细胞中基础ApoA-I的水平通过自噬和mTORC1依赖的从头蛋白质合成的平衡作用得以维持。ApoA-I蛋白水解通过涉及Beclin1和ULK1以及将ApoA-I靶向自噬体的受体蛋白p62 / SQSTM1的规范性自噬途径发生。但是，在氨基酸不足时，普遍存在对ApoA-I合成的抑制作用，使mTORC1失活对于自噬介导的ApoA-I蛋白水解是可有可无的。结论这些数据强调了转录后机制对ApoA-I水平的主要贡献，后者取决于营养物质的可获得性，而mTORC1依赖的信号转导涉及蛋白质合成和自噬。