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，这是美国发病率和死亡率的主要原因。作为血浆TG水平的主要决定因素，脂蛋白脂肪酶（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TG水解为游离脂肪酸，被外周组织吸收。LPL活性受到严格调节，使循环TG在禁食期间进入心脏和骨骼肌产生能量，并在进食状态下进入白色脂肪组织（WAT）储存。然而，在快速进食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 （lippasin）是最近发现的一种TG代谢调节剂，在肝脏和脂肪组织中特异性表达。肝脏将ANGPTL8分泌到血液循环中，在肝脏中，ANGPTL8的表达在禁食时减少，在喂食时显著增加。在小鼠中，ANGPTL8缺乏导致低甘油三酯血症，而其肝脏过度表达导致高甘油三酯血症。尽管ANGPTL8在治疗血脂异常方面具有很大的治疗潜力，但其功能和作用机制尚不清楚。基于我们坚实的前期数据，我们拟研究ANGPTL8的功能作用及其调控TG代谢的机制。在Aim 1中，我们将通过对ANGPTL8 KO小鼠、注射ANGPTL8单克隆降TG抗体的野生型小鼠以及肝脏和脂肪特异性KO小鼠进行同位素标记TG的通量组学分析，确定ANGPTL8在调节TG运输中的功能作用。在Aim 2中，我们将阐明angptl8介导的抑制gpihbp1结合LPL的机制。通过使用一种新的细胞培养系统来检测内皮细胞表面与GPIHBP1复合物的LPL，我们将研究ANGPTL8、ANGPTL3和LPL-GPIHBP1复合物之间的相互作用。完成后，我们将揭示ANGPTL8调节LPL的分子机制，从而在快速进食周期中指导TG运输到特定组织，这是一个至关重要的生理过程。

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.