Role of Star Proteins in Bile Acid and Cholesterol Metabolism

描述（由申请人提供）：通过胆固醇摄取、合成、转运、酯化、分泌和降解的协调调节来维持胆固醇稳态。最近的进展使人们对胆固醇的分类和运输有助于维持胆固醇稳态的方式有了更全面的了解。然而，许多细胞内胆固醇结合蛋白在体内各种细胞中的作用及其对胆固醇代谢/降解的影响尚不清楚。本应用程序的总体目的是通过体外和体内模型阐明两个START结构域蛋白（StarD4, StarD5）在肝细胞和巨噬细胞胆固醇和胆汁酸稳态中的作用。P.I.s的实验室最近表明，胆固醇转运到线粒体内膜代表了通过“酸性”途径控制胆汁酸合成的速率决定步骤，并且“酸性”途径能够控制已知的调节氧甾醇，27-羟基胆固醇，25-羟基胆固醇和至少一种新型硫酸化氧甾醇的核水平。除了StarD1外，还有一个含有StarD1同源结构域的蛋白家族，可能能够在细胞内结合和运输甾醇。这个家族的所有蛋白都有一个类似的脂质结合结构域，称为StarD1相关脂质转移（START）结构域。最近，他们还表明：1)StarD1、27-羟基胆固醇和StarD5水平的协同反应；2)细胞内StarD5蛋白从高尔基体向质膜的再分配是对胆固醇的反应；3)胆固醇是StarD4和StarD5的配体；后者还能结合25-羟基胆固醇；4) StarD4过表达后胆汁酸合成和胆固醇酯形成增加。本申请拟开展的研究将采用体外和体内模型研究StarD4、StarD5在肝细胞和巨噬细胞中的调控及生理功能。具体来说，我们提出了以下研究：1)表征巨噬细胞和肝细胞（StarD4）中StarD4和StarD5对胆固醇的调节和亚细胞运动；2)研究StarD4和StarD5在巨噬细胞和肝细胞中的功能（StarD4）；3)描述StarD4在体内胆固醇稳态中的生理作用。了解肝细胞和巨噬细胞中胆固醇稳态的调节机制，对了解人类动脉硬化和心脏病的发生发展具有重要意义。公共卫生相关性：胆汁酸是肠道吸收和溶解胆固醇和脂肪所必需的。此外，胆汁酸在胆固醇稳态中起着至关重要的作用。胆汁酸是由胆固醇制成的。因为胆固醇不溶于水，它与蛋白质结合在细胞内移动。我们建议研究细胞内结合和运输胆固醇的新蛋白质，以及调节胆汁酸生物合成途径的其他蛋白质。这项研究的成功完成将为我们对胆汁酸生物合成和胆固醇稳态调控的分子机制提供新的见解。

DESCRIPTION (provided by applicant): Cholesterol homeostasis is maintained through the coordinated regulation of cholesterol uptake, synthesis, transport, esterification, secretion, and degradation. Recent advances have led to a more complete understanding of the way cholesterol's sorting and transport contribute to the maintenance of cholesterol homeostasis. However, the role of many intracellular cholesterol- binding proteins within various cells in the body, and their effect on cholesterol metabolism/degradation, remains unknown. The overall aim of this application is to elucidate the role of two START domain proteins (StarD4, StarD5) play in cholesterol and bile acid homeostasis in hepatocytes and macrophages using both in vitro and in vivo models. The P.I.s' labs have recently shown that cholesterol transport to the inner mitochondrial membrane represents the rate-determining step controlling bile acid synthesis via the 'acidic' pathway, and that the 'acidic' pathway is able to control nuclear levels of the known regulatory oxysterols, 27-hydroxycholesterol, 25-hydroxycholesterol, and at least one novel sulfated oxysterol. In addition to StarD1, there exists a family of proteins containing a StarD1 homologue domain that potentially is capable of binding and transporting sterols within cells. All proteins in this family have a similar structural lipid-binding domain referred to as the StarD1- related lipid transfer (START) domain. Most recently, they have also shown the following: 1) a coordinated response in the levels of StarD1, 27-hydroxycholesterol, and StarD5; 2) an intracellular redistribution of StarD5 protein from the Golgi to the plasma membrane in response to cholesterol; 3) that cholesterol is a ligand for both StarD4 and StarD5; the later also binds 25- hydroxycholesterol; and, 4) an increase in bile acid synthesis and cholesterol ester formation following StarD4 overexpression. In the studies proposed in this application, in vitro and in vivo models will be used to study the regulation, and physiological function of StarD4, and StarD5 in hepatocytes and in macrophages. Specifically, studies are proposed to: 1) characterize the regulation and subcellular movement of StarD4 and StarD5 in macrophages and hepatocytes (StarD4) in response to cholesterol; 2) characterize the function of StarD4, and StarD5 in macrophages and hepatocytes (StarD4); 3) characterize the physiological role(s) of StarD4 in cholesterol homeostasis in vivo. An understanding of the mechanisms of regulation of cholesterol homeostasis in hepatocytes and macrophages is very relevant to the understanding of the development of arteriosclerosis and heart disease in humans. PUBLIC HEALTH RELEVANCE: Bile acids are required for intestinal absorption and solubilization of cholesterol and fats. In addition, bile acids play an additional crucial role in cholesterol homeostasis. Bile acids are made from cholesterol. Because cholestrol is not soluble in water, it binds to proteins to move within the cell. We propose to study novel proteins that bind and transport cholesterol within the cell and, among other things, regulate the bile acid biosynthetic pathway. The successful completion of this study will provide us with new insights into the molecular mechanisms involved in the regulation of bile acid biosynthesis and cholesterol homeostasis.