ANGPTL8: function and mechanism of action

血浆甘油三酯(TG)升高会增加心血管疾病的风险，心血管疾病是美国发病率和死亡率的主要原因。作为血浆甘油三酯水平的主要决定因素，脂蛋白脂酶(LPL)由内皮细胞蛋白GPIHBP1锚定在毛细血管管腔表面，将循环中的甘油三酯水解为游离脂肪酸，被周围组织摄取。LPL的活性受到严格的调控，使循环中的甘油三酯能够在禁食期间进入心肌和骨骼肌产生能量，并进入白色脂肪组织(WAT)以便在进食状态下储存。然而，在快速反馈周期中调节组织特异性LPL活性的分子机制在很大程度上是未知的。ANGPTL8(Lipasin)是新近发现的一种甘油三酯代谢调节剂，在肝脏和脂肪组织中特异表达。在肝脏中，将ANGPTL8分泌到循环中，禁食会减少ANGPTL8的表达，而喂食会显著增加ANGPTL8的表达。在小鼠中，ANGPTL8缺乏会导致低甘油三酯血症，而它在肝脏的过度表达会导致高甘油三酯血症。尽管ANGPTL8的S在治疗血脂异常方面具有巨大的治疗潜力，但其功能和作用机制仍不清楚。基于我们坚实的初步数据，我们建议研究ANGPTL8的功能作用及其调节甘油三酯代谢的机制。在目标1中，我们将通过用同位素标记的TG在ANGPTL8 KO小鼠、注射ANGPTL8单抗的野生型小鼠以及肝脏和脂肪特异性KO小鼠中进行流动组学分析，来确定ANGPTL8在调节TG运输中的功能。在目标2中，我们将阐明ANGPTL8介导的抑制GPIHBP1结合的LPL的机制。通过使用一种新的细胞培养系统来检测LPL与GPIHBP1在内皮细胞表面的复合体，我们将研究ANGPTL8、Angptl3和LPL-GPIHBP1复合体之间的相互作用。完成后，我们将揭示ANGPTL8调节LPL的分子机制，以在快速反馈周期中将TG运输到特定组织，这是一个基本重要的生理过程。

Elevated plasma triglycerides (TG) increase the risk of cardiovascular disease, a leading cause of morbidity and mortality in the United States. As the primary determinant of plasma TG levels, lipoprotein lipase (LPL), anchored by the endothelial cell protein GPIHBP1 to the capillary luminal surface, hydrolyzes circulating TG into free fatty acids that are taken up by peripheral tissues. LPL activity is tightly regulated to enable circulating TG be routed into cardiac and skeletal muscles to generate energy during fasting and into white adipose tissue (WAT) for storage in the fed state. However, the molecular mechanisms for regulating tissue-specific LPL activity during the fed-fast cycle are largely unknown. ANGPTL8 (lipasin) is a recently described TG metabolism regulator, which is specifically expressed in liver and adipose tissues. In the liver, which secretes ANGPTL8 into the circulation, ANGPTL8 expression is reduced by fasting and dramatically increased by feeding. In mice, ANGPTL8 deficiency causes hypotriglyceridemia, whereas its hepatic overexpression causes hypertriglyceridemia. Despite having great therapeutic potential in treating dyslipidemia, ANGPTL8’s functions and mechanism of action remain elusive. Based on our solid preliminary data, we propose to study the functional roles of ANGPTL8 and the mechanism by which it regulates TG metabolism. In Aim 1, we will determine functional roles of ANGPTL8 in regulating TG trafficking, by performing fluxomics analysis with isotope-labeled TG in ANGPTL8 KO mice, wild-type mice with injection of an ANGPTL8 monoclonal TG- lowering antibody, as well as liver and adipose-specific KO mice. In Aim 2, we will elucidate mechanisms of ANGPTL8-mediated inhibition of GPIHBP1-bound LPL. By using a novel cell-culture system to examine LPL complexed with GPIHBP1 on the endothelial cell surface, we will examine the interactions among ANGPTL8, ANGPTL3, and LPL-GPIHBP1 complexes. Upon completion, we will have revealed the molecular mechanism by which ANGPTL8 regulates LPL to direct TG trafficking to specific tissues during the fed-fast cycle, a physiological process of fundamental importance.