Vitamin A Storage and Metabolism

描述（由申请人提供）：肝脏是体内维生素A储存的主要组织部位，肝星状细胞（HSC）（也称为伊藤细胞、脂肪储存细胞或脂肪细胞）是肝脏内维生素A储存的主要细胞部位。肝脏中维生素A的储存可以起到缓冲作用，防止饮食中维生素A摄入不足或过量的不良后果。几乎所有肝脏维生素A都存在于造血干细胞中存在的大细胞质脂滴（ld）中。这些ld的数量、大小和维生素A（视黄醇酯）含量随着膳食维生素A摄入量的增加而增加，而在膳食摄入量不足时则减少。HSC ld是一种特殊的亚细胞细胞器，其唯一已知的生理功能是促进维生素a的储存。这与其他肝脏ld不同，例如肝细胞中的ld在中性脂质（甘油三酯和胆固醇）储存和代谢中具有更广泛的作用。HSC ld中相对较高的视黄醇酯含量及其对膳食维生素A摄入的反应使它们与存在于身体其他细胞类型中的其他ld非常不同。然而，关于HSC ld的发生或维持，以及对此所必需的生化和分子事件，人们知之甚少。我们的研究将建立对HSC LD形成和维持至关重要的生化和分子过程，以及这些过程如何与肝脏中维生素A的储存和动员联系起来。本项目将解决HSC LD生物化学和维生素A在肝脏中的储存和代谢的3个基本问题。目的1提出了在维生素A摄入过量、正常（对照）或不足的情况下小鼠HSC ld蛋白质组的表征和研究。Aim 2中提出的研究源于我们已发表的研究，该研究表明，缺乏卵磷脂：视黄醇酰基转移酶（LRAT）的小鼠在hsc中也缺乏lld，而在肝细胞中则不缺乏lld。LRAT是hsc中负责视黄醇酯形成的酶。这是非常令人惊讶的，因为对于一个控制饮食的啮齿动物来说，视黄醇酯占液滴中总脂质的比例不到40%。在目的2中，我们建议研究为什么LRAT是HSC LD形成所需要的。最后，在Aim 3中，我们将研究维生素A是如何从造血干细胞中动员的，以及这是否需要肝细胞合成的血清视黄醇结合蛋白（RBP）的直接参与。

DESCRIPTION (provided by applicant): The liver is the main tissue site of vitamin A storage in the body and the hepatic stellate cell (HSC) (also referred to as Ito cell, fat-storing cell or lipocyte) is the major cellular site of vitamin A storage within the liver. Liver vitamin A stores serve as a buffer to prevent the adverse consequences of both insufficient and excessive dietary vitamin A intake. Nearly all hepatic vitamin A is found in the large cytoplasmic lipid droplets (LDs) present in HSCs. The number, size and vitamin A (retinyl ester) content of these LDs increases in response to greater dietary vitamin A intake and decreases in times of insufficient dietary intake. HSC LDs are a specialized subcellular organelle that has as its' only known physiologic function to facilitate vitamin A storage. This is unlike other hepatic LDs, for instance the LDs in hepatocytes, which have a more generalized role in neutral lipid (triglyceride and cholesterol) storage and metabolism. The relatively large retinyl ester content of HSC LDs and their responsiveness to dietary vitamin A intake render them very distinct from other LDs that are present in other cell types of the body. Yet little is known about the genesis or maintenance of HSC LDs or the biochemical and molecular events that are essential for this. Our studies will establish the biochemical and molecular processes that are essential for HSC LD formation and maintenance and how these are linked to vitamin A storage and mobilization from the liver. This project will address 3 fundamental questions regarding HSC LD biochemistry and vitamin A storage and metabolism in the liver. Aim 1 proposes characterization and study of the proteome of mouse HSC LDs in times of excessive, normal (control) or insufficient vitamin A dietary intake. The studies proposed in Aim 2 grow out of our published research showing that that mice lacking lecithin:retinol acyltransferase (LRAT), the enzyme responsible for retinyl ester formation in HSCs, also lack LDs in HSCs but not hepatocytes. This is very surprising since, for a rodent maintained on a control diet, retinyl ester accounts for less than 40% of the total lipid present in the droplets. In Aim 2 we propose to investigate why LRAT is needed for HSC LD formation. Finally, in Aim 3 we will investigate how vitamin A is mobilized from HSCs and whether this requires direct involvement of serum retinol-binding protein (RBP) synthesized in hepatocytes.