Hepatocyte Abca1, cholesterol trafficking, and lipid mobilization

摘要-此次R01更新旨在继续PI的创造性、生产性和有意义的研究计划， 自1996年以来，NIH一直享受着不间断的资金支持，重点是在 心血管疾病(CVD)和心脏代谢疾病(CMD)的发病机制。15年来，私家侦探 研究计划的重点是三磷酸腺苷结合盒转运体A1(ABCA1)和脂类和脂蛋白 利用组织/细胞特异性ABCA1基因敲除小鼠的代谢。肝细胞特异性ABCA1基因敲除的研究 (HSKO)小鼠提供了新的机制见解，了解肝脏ABCA1如何影响生产和 引起心血管疾病的三种主要血浆脂蛋白(极低密度脂蛋白、低密度脂蛋白、高密度脂蛋白)的分解代谢 CMD。尽管与对照组相比，血浆高密度脂蛋白大幅(~80%)下降，但高脂血症HSKO小鼠 维持巨噬细胞的反向胆固醇转运，并减少了主动脉根部动脉粥样硬化 到控制程序。HSKO小鼠还增加了血浆高密度脂蛋白进入粪便的转运，减少了 肝脏胰岛素信号和从头脂肪生成(DNL)，增加线粒体呼吸，增加低密度脂蛋白 受体的表达，并增加肝脏VLDL甘油三酯的分泌。这种独特的表型是 与溶酶体游离胆固醇(Fc)向细胞膜顺行转运减少有关 肝细胞，HSKO组和对照组小鼠的总肝脂质量无差异。在此基础上 我们的全球假设是，肝细胞ABCA1的靶向缺失促进了肝细胞的逆行 Fc从质膜到细胞内的重新分布，胰岛素介导的重新编程 合成代谢朝向协调的分解代谢程序，减少肝脏DNL，改善线粒体 代谢，增加甘油三酯的分泌，从而减少小鼠应激时的肝脏脂肪变性 高脂肪饮食。在缺乏肝脏ABCA1的情况下，这种代谢重新编程导致了一种新的形式 HSKO小鼠的选择性胰岛素抵抗，其中肝脏DNL受到抑制，但糖异生受到抑制 适当地被胰岛素下调。未来的研究将解决知识差距和 进展，包括：具体目标1)ABCA1缺失如何改变FC在 质膜、内质网和其他细胞器；特定目标2a)胰岛素信号和 内质网Fc相互作用激活Srebp1c；和特异性靶2b)ABCA1的丢失是否导致 增加线粒体Fc和/或溶血磷脂，改善线粒体呼吸。我们的建议 研究将为ABCA1建立一个更全面和更广泛的模型，作为代谢的中央控制点 动态平衡并推动我们之前5年的发现朝着新的方向发展，从而推动该领域的发展。 影响-我们的研究将继续产生关于肝脏脂肪生成和调节的新发现 ABCA1和Fc转运在重新编程肝脂代谢中的新作用，这是 CVD和CMD。

Abstract- This R01 renewal seeks to continue the PI's creative, productive, and significant research program, which has enjoyed uninterrupted NIH funding since 1996, focused on lipid/lipoprotein metabolism in the pathogenesis of cardiovascular disease (CVD) and cardiometabolic disease (CMD). For over 15 years, the PI's research program has focused on ATP binding cassette transporter A1 (Abca1) and lipid and lipoprotein metabolism using tissue/cell-specific Abca1 knockout mice. Studies in hepatocyte-specific Abca1 knockout (HSKO) mice provided novel mechanistic insights into how hepatic Abca1 impacts the production and catabolism of all three major classes of plasma lipoproteins (VLDL, LDL, HDL) that contribute to CVD and CMD. Despite the massive (~80%) drop in plasma HDL relative to control mice, hyperlipidemic HSKO mice maintained macrophage reverse cholesterol transport and had decreased aortic root atherosclerosis compared to controls. HSKO mice also had increased plasma HDL cholesterol transport into the feces, decreased hepatic insulin signaling and de novo lipogenesis (DNL), increased mitochondrial respiration, increased LDL receptor expression, and increased hepatic VLDL triglyceride secretion. This unique phenotype was associated with diminished antegrade trafficking of lysosomal free cholesterol (FC) to the plasma membrane of hepatocytes, with no differences in total hepatic lipid mass between HSKO and control mice. Based on this work, our global hypothesis is that targeted deletion of hepatocyte Abca1 enhances hepatocyte retrograde FC redistribution from the plasma membrane to intracellular compartments, reprogramming insulin-mediated anabolism towards a coordinated catabolic program that reduces hepatic DNL, improves mitochondrial metabolism, and increases TG secretion, thereby decreasing hepatic steatosis when mice are stressed with a high-fat diet. This metabolic reprogramming in the absence of hepatic Abca1 results in a novel form of selective insulin resistance in HSKO mice, in which hepatic DNL is suppressed, but gluconeogenesis is appropriately downregulated by insulin. Future studies will address gaps in knowledge and barriers to progress, including: Specific aim 1) how Abca1 deletion alters FC distribution (or trafficking) among the plasma membrane, endoplasmic reticulum, and other organelles; Specific aim 2a) how insulin signaling and endoplasmic reticulum FC interact to activate Srebp1c; and Specific aim 2b) whether loss of Abca1 leads to increased mitochondria FC and/or lysophospholipid that improves mitochondrial respiration. Our proposed studies will build a more comprehensive and expansive model for Abca1 as a central control point of metabolic homeostasis and propel our previous 5-year discoveries in new directions, thereby advancing the field. Impact- Our studies will continue generating new discoveries regarding regulation of hepatic lipogenesis and the emerging role of Abca1 and FC trafficking in reprogramming hepatic lipid metabolism, a key contributor to CVD and CMD.