Vitamin A Storage and Metabolism

说明(申请人提供)：肝脏是体内储存维生素A的主要组织部位，而肝星状细胞(HSC)(也称为Ito细胞、储脂细胞或脂肪细胞)是肝脏内储存维生素A的主要细胞部位。肝脏维生素A储存起到缓冲作用，防止饮食中维生素A摄入量不足和过量的不良后果。几乎所有的肝脏维生素A都存在于HSC中的大细胞质脂滴(LDs)中。这些LDS的数量、大小和维生素A(视黄酸酯)含量随着饮食中维生素A摄入量的增加而增加，而在饮食摄入量不足时减少。HSC LDS是一种特殊的亚细胞细胞器，具有“唯一已知的促进维生素A储存的生理功能”。这与其他肝脏低密度脂蛋白不同，例如，肝细胞中的低密度脂蛋白在中性脂肪(甘油三酯和胆固醇)的储存和代谢中具有更广泛的作用。HSC低密度脂蛋白相对较大的视黄酸酯含量以及它们对膳食维生素A摄取的反应性，使它们与存在于身体其他细胞类型中的其他低密度脂蛋白非常不同。然而，关于HSC LDS的发生或维持，以及对其至关重要的生化和分子事件，人们知之甚少。我们的研究将确定对HSC LD的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的储存和动员有关。该项目将解决有关HSC、LD生物化学和维生素A在肝脏中的存储和代谢的3个基本问题。目的1研究维生素A摄入过多、正常(对照)或不足时小鼠肝星状细胞低密度脂蛋白的蛋白质组特征。在Aim 2中提出的研究源于我们发表的研究表明，缺乏卵磷脂：视黄醇酰基转移酶(LRAT)的小鼠，HSC中负责形成视黄酸酯的酶也缺乏LDS，但肝细胞中没有。这是非常令人惊讶的，因为对于一只维持在控制饮食中的啮齿动物来说，视黄酯在液滴中存在的总脂肪中所占比例不到40%。在目标2中，我们建议调查为什么HSC LD的形成需要LRAT。最后，在目标3中，我们将研究如何从HSCs中动员维生素A，以及这是否需要在肝细胞中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.