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，由内皮细胞蛋白GPIHBP 1锚定到毛细血管腔表面的脂蛋白脂肪酶（LPL）将循环TG水解为游离脂肪酸，由外周组织吸收。LPL活性受到严格调节，以使循环TG能够在禁食期间进入心脏和骨骼肌以产生能量，并进入白色脂肪组织（WAT）以在进食状态下储存。然而，在禁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。ANGPTL 8（lipasin）是最近描述的TG代谢调节剂，其在肝脏和脂肪组织中特异性表达。在将ANGPTL 8分泌到循环中的肝脏中，ANGPTL 8的表达通过禁食而减少，并且通过进食而显著增加。在小鼠中，ANGPTL 8缺乏导致低血糖，而其肝脏过表达导致高血糖。尽管在治疗血脂异常方面具有巨大的治疗潜力，但ANGPTL 8的功能和作用机制仍然难以捉摸。基于我们坚实的初步数据，我们建议研究ANGPTL 8的功能作用及其调节TG代谢的机制。在目的1中，我们将通过在ANGPTL 8 KO小鼠、注射ANGPTL 8单克隆TG降低抗体的野生型小鼠以及肝脏和脂肪特异性KO小鼠中用同位素标记的TG进行通量组学分析来确定ANGPTL 8在调节TG运输中的功能作用。在目标2中，我们将阐明ANGPTL 8介导的GPIHBP 1结合LPL抑制的机制。通过使用一种新的细胞培养系统来检测内皮细胞表面上与GPIHBP 1复合的LPL，我们将检测ANGPTL 8、ANGPTL 3和LPL-GPIHBP 1复合物之间的相互作用。完成后，我们将揭示ANGPTL 8调节LPL的分子机制，以指导TG运输到特定的组织在禁食周期，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.