脂蛋白血管外循环与胆固醇逆向转运

胆固醇逆向转运是机体周围组织细胞胆固醇转运至肝脏转化排泄的重要生理过程，它在维持机体胆固醇平衡和防止动脉粥样硬化的发生和发展过程中起着重要的作用。脂蛋白是胆固醇的载体，血浆脂蛋白可以透过血管壁进入间质组织，通过与组织细胞直接接触接受细胞释出的胆固醇，再经淋巴液或直接返回血浆，并运至肝脏代谢。因此，脂蛋白血管外循环是胆固醇逆向转运重要组成部分。研究脂蛋白血管外循环的过程、特征以及代谢变化的规律，对心血管病的防治具有重要的理论和实践价值。     

过氧化体增殖物激活型受体α的激活与胆固醇逆向转运

胆固醇逆向转运是周围细胞胆固醇转运至肝脏转化、清除的重要生理过程。研究证实，胆固醇逆向转运实际上是高密度脂蛋白在多种生物活性分子参与下，由新生前β-高密度脂蛋白到成熟α-高密度脂蛋白递变的过程。过氧化体增殖物激活型受体α是一种核内受体转录因子，具有多种生物学效应，其激活后可影响决定高密度脂蛋白结构和功能的基因表达，从而调节胆固醇逆向转运过程。     

高密度脂蛋白的代谢相关基因表达产物与动脉粥样硬化

高密度脂蛋白胆固醇水平降低是动脉粥样硬化性心脏病独立和重要的危险因素。血浆高密度脂蛋白胆固醇水平不仅决定于它的生成速率,更重要的是取决于它的代谢水平。一系列的基因及其相关产物参与了高密度脂蛋白参与的胆固醇逆向转运过程,包括与升高血浆高密度脂蛋白胆固醇水平的基因及其产物如三磷酸腺苷结合盒转运A1、磷脂酰胆碱胆固醇酰基转移酶、磷脂转运蛋白和脂蛋白脂酶等,以及降低血浆高密度脂蛋白胆固醇水平的基因和产物如清道夫受体B1、胆固醇酯转运蛋白、肝脂肪酶和内皮细胞脂肪酶等。而高密度脂蛋白代谢与动脉粥样硬化的关系也是多方面的,不能仅由血浆高密度脂蛋白胆固醇水平来明确推断对动脉粥样硬化的影响,降低血浆高密度脂蛋白胆固醇水平与动脉粥样硬化有一定的关联但不是必然的联系。     

载脂蛋白A5对冠心病的影响

临床及流行病学研究表明血清高密度脂蛋白胆固醇(HDL-C)水平与冠心病的发生、发展密切相关[1],HDL-C参与体内胆固醇逆向转运,将外周细胞堆积的胆固醇转运至肝脏转化清除,具有抗动脉粥样硬化的作用,HDL-C水平降低是冠心病发生发展的独立危险因素.载脂蛋白是一类能与血浆脂质结合的蛋白质,对血浆脂蛋白代谢起决定性作用,载脂蛋白(apo)A5是新近发现的新型载脂蛋白,与TG代谢密切相关.     

胆固醇酯转运蛋白与动脉粥样硬化

胆固醇酯转运蛋白是胆固醇逆向运输的关键蛋白质，是血浆高密度脂蛋白水平的主要决定因素之一。关于胆固醇酯转运蛋白与动脉粥样硬化的关系目前尚有争论，现有研究资料表明它具有促动脉粥样硬化和抗动脉粥样硬化的双重作用。多数胆固醇酯转运蛋白基因的突变导致胆固醇酯转运蛋白水平下降，高密度脂蛋白水平上升，而使动脉粥样硬化风险降低；但在高密度脂蛋白水平正常或较低的个体，胆固醇酯转运蛋白水平的下降去使冠状动脉疾病的发病率升高，转基因动物的研究表明，胆固醇酯转运蛋白对动脉粥样硬化的影响与脂蛋白代谢背景有关。     

高密度脂蛋白逆转运胆固醇的分子生物学基础

高密度脂蛋白(HDL)能够将胆固醇从泡沫细胞中转运到肝脏,代谢转化为胆汁排出体外,进而产生抗动脉粥样硬化作用,称之为HDL的胆固醇逆转运(RCT)。因此,如何提高HDL浓度并促进HDL的功能,充分发挥其抗动脉粥样硬化的功能,成为近年来研究的热点。但研究显示单纯升高HDLC并未发现有明显的临床效果,揭示了HDL功能的复杂性。因此有必要进行系统的回顾HDL的分子结构、合成、代谢等,重新认识其RCT功能的分子生物学基础,为进一步研究HDL的RCT功能提供理论支撑。     

载脂蛋白M参与前β高密度脂蛋白形成、胆固醇流向高密度脂蛋白,并具有抗动脉粥样硬化作用

高密度脂蛋白具有抗动脉粥样硬化的原因在于它能介导胆固醇的逆向转运，即把胆固醇从外周转运到肝脏降解和排泄。我们进行了载脂蛋白M缺陷鼠的研究（载脂蛋白M是高密度脂蛋白的组成成分之一），发现尽管高密度脂蛋白转化为前β高密度脂蛋白的能力减弱，胆固醇却依然可在高密度脂蛋白大颗粒（高密度脂蛋白1）中蓄积。载脂蛋白M缺陷鼠体内缺乏前β高密度脂蛋白，前β高密度脂蛋白是一类贫脂的载脂蛋白，是外周细胞胆固醇的关键接受体。体外实验中，与正常高密度脂蛋白相比，     

清道夫受体BI与动脉粥样硬化

清道夫受体B族I型(SR-BI)是细胞膜上首个被定义为高密度脂蛋白(HDL)受体的糖蛋白,其介导细胞选择性摄取高密度脂蛋白胆固醇(HDLC),影响细胞胆固醇平衡、炎症表型。肝脏SR-BI在胆固醇逆向转运中扮演重要角色,而与HDL的代谢及其抗动脉粥样硬化(As)作用密切相关,并且已发现人类存在SR-BI基因多态性。本文重点从SR-BI结构、功能、调节及其对As的作用进行综述。     

载脂蛋白M与冠心病及其危险因素的关系

载脂蛋白M(ApoM) 是一种新近发现的载脂蛋白,有研究表明,ApoM主要存在于高密度脂蛋白(HDL)中,对HDL的代谢起重要作用,参与胆固醇逆向转运过程,可能具有心脏保护和抗动脉粥样硬化(AS)作用[1].     

ABCA1在冠心病及其发病机制中的研究新进展

动脉粥样硬化(As)是冠心病的主要病因,而泡沫细胞又是As的主要病因,过多的胆固醇在巨噬细胞中积累形成泡沫细胞,因此减少胆固醇的积累从而减少泡沫细胞的形成可能成为治疗As有效的方法。ATP结合盒转运体A1(ABCA1)可使细胞内胆固醇和磷脂转运到载脂蛋白AⅠ(Apo AⅠ)形成高密度脂蛋白前体,使过多的胆固醇进入肝脏重新利用或经胆汁和粪便排出,这个过程就是胆固醇逆转运。ABCA1还能够抑制As的炎症反应,引起血管内皮细胞变化,参与氧化应激反应,可通过多种代谢通路影响As,其不同的基因型对As的影响也不相同。因此,ABCA1在As的发生发展中具有举足轻重的作用。     

Role of Phospholipid Transfer Protein in High-Density Lipoprotein– Mediated Reverse Cholesterol Transport

Reverse cholesterol transport (RCT) describes the process whereby cholesterol in peripheral tissues is transported to the liver where it is ultimately excreted in the form of bile. Given the atherogenic role of cholesterol accumulation within the vessel intima, removal of cholesterol through RCT is considered an anti-atherogenic process. The major constituents of RCT include cell membrane– bound lipid transporters, plasma lipid acceptors, plasma proteins and enzymes, and lipid receptors of liver cell membrane. One major cholesterol acceptor in RCT is high-density lipoprotein (HDL). Both the characteristics and level of HDL are critical determinants for RCT. It is known that phospholipid transfer protein (PLTP) impacts both HDL cholesterol level and biological quality of the HDL molecule. Recent data suggest that PLTP has a site-specific variation in its function. Moreover, the RCT pathway also has multiple steps both in the peripheral tissues and circulation. Therefore, PLTP may influence the RCT pathway at multiple levels. In this review, we focus on the potential role of PLTP in RCT through its impact on HDL homeostasis. The relationship between PLTP and RCT is expected to be an important area in finding novel therapies for atherosclerosis.     

Reverse cholesterol transport in type 2 diabetes mellitus

High-density lipoprotein (HDL) plays an important protective role against atherosclerosis, and the anti-atherogenic properties of HDL include the promotion of cellular cholesterol efflux and reverse cholesterol transport (RCT), as well as antioxidant, anti-inflammatory and anticoagulant effects. RCT is a complex pathway, which transports cholesterol from peripheral cells and tissues to the liver for its metabolism and biliary excretion. The major steps in the RCT pathway include the efflux of free cholesterol mediated by cholesterol transporters from cells to the main extracellular acceptor HDL, the conversion of free cholesterol to cholesteryl esters and the subsequent removal of cholesteryl ester in HDL by the liver. The efficiency of RCT is influenced by the mobilization of cellular lipids for efflux and the intravascular remodelling and kinetics of HDL metabolism. Despite the increased cardiovascular risk in people with type 2 diabetes, current knowledge on RCT in diabetes is limited. In this article, abnormalities in RCT in type 2 diabetes mellitus and therapeutic strategies targeting HDL and RCT will be reviewed.     

Future Therapeutic Directions in Reverse Cholesterol Transport

Despite a robust inverse association between high-density lipoprotein (HDL) cholesterol levels and atherosclerotic cardiovascular disease, the development of new therapies based on pharmacologic enhancement of HDL metabolism has proven challenging. Emerging evidence suggests that static measurement of HDL levels has inherent limitations as a surrogate for overall HDL functionality, particularly with regard to the rate of flux through the macrophage reverse cholesterol transport (RCT) pathway. Recent research has provided important insight into the molecular underpinnings of RCT, the process by which excess cellular cholesterol is effluxed from peripheral tissues and returned to the liver for ultimate intestinal excretion. This review discusses the critical importance and current strategies for quantifying RCT flux. It also highlights therapeutic strategies for augmenting macrophage RCT via three conceptual approaches: 1) improved efflux of cellular cholesterol via targeting the macrophage; 2) enhanced cholesterol efflux acceptor functionality of circulating HDL; and 3) increased hepatic uptake and biliary/intestinal excretion.     

High density lipoproteins and arteriosclerosis. Role of cholesterol efflux and reverse cholesterol transport

High density lipoprotein (HDL) cholesterol is an important risk factor for coronary heart disease, and HDL exerts various potentially antiatherogenic properties, including the mediation of reverse transport of cholesterol from cells of the arterial wall to the liver and steroidogenic organs. Enhancement of cholesterol efflux and of reverse cholesterol transport (RCT) is considered an important target for antiatherosclerotic drug therapy. Levels and composition of HDL subclasses in plasma are regulated by many factors, including apolipoproteins, lipolytic enzymes, lipid transfer proteins, receptors, and cellular transporters. In vitro experiments as well as genetic family and population studies and investigation of transgenic animal models have revealed that HDL cholesterol plasma levels do not necessarily reflect the efficacy and antiatherogenicity of RCT. Instead, the concentration of HDL subclasses, the mobilization of cellular lipids for efflux, and the kinetics of HDL metabolism are important determinants of RCT and the risk of atherosclerosis.     

Cellular cholesterol flux studies: methodological considerations

Reverse cholesterol transport (RCT) is the process in which peripheral cells release cholesterol to an extracellular acceptor such as high-density lipoprotein (HDL) which then mediates cholesterol delivery to the liver for excretion. RCT represents a physiological mechanism by which peripheral tissues are protected against excessive accumulation of cholesterol. The first step in RCT is the interaction of the cell with lipoprotein particles, a process that results in both the cellular uptake and release of cholesterol. The various components of this cholesterol flux can be viewed as efflux, influx and net flux. Experimental protocols for measuring each of these components of cholesterol flux are very different, and a number of considerations are required to design experimental approaches for the quantitation of flux parameters. Although many flux studies have been conducted in the past, the recent discoveries of the scavenger receptor B1 (SR-B1) and ATP binding cassette 1 (ABCA1), which mediate the movement of cholesterol between cells and extracellular acceptors, has led to increased interest in studies of cellular cholesterol flux. The aim of this review is to present a discussion of the methodological considerations that should be evaluated during the design and analysis of cellular cholesterol flux experiments.     

High-density lipoproteins and endothelial function

Elevated plasma levels of HDL cholesterol or apolipoprotein A-I, the major protein moiety of HDL particles, are protective against coronary artery disease. HDL particles remove cholesterol from peripheral cells and transfer it to the liver for bile acid synthesis. The interaction between lipoproteins is not mediated through simple contact between 2 phospholipid membranes but involves specific protein-receptor interactions, charged phospholipid-phospholipid contact, and activation of cellular signaling pathways. These lead to regulation of genes or the modification of proteins involved in vasomotor function, platelet activation, thrombosis and thrombolysis, cell adhesion, apoptosis and cell proliferation, and cellular cholesterol homeostasis.     

New insights into the regulation of HDL metabolism and reverse cholesterol transport

The metabolism of high-density lipoproteins (HDL), which are inversely related to risk of atherosclerotic cardiovascular disease, involves a complex interplay of factors regulating HDL synthesis, intravascular remodeling, and catabolism. The individual lipid and apolipoprotein components of HDL are mostly assembled after secretion, are frequently exchanged with or transferred to other lipoproteins, are actively remodeled within the plasma compartment, and are often cleared separately from one another. HDL is believed to play a key role in the process of reverse cholesterol transport (RCT), in which it promotes the efflux of excess cholesterol from peripheral tissues and returns it to the liver for biliary excretion. This review will emphasize 3 major evolving themes regarding HDL metabolism and RCT. The first theme is that HDL is a universal plasma acceptor lipoprotein for cholesterol efflux from not only peripheral tissues but also hepatocytes, which are a major source of cholesterol efflux to HDL. Furthermore, although efflux of cholesterol from macrophages represents only a tiny fraction of overall cellular cholesterol efflux, it is the most important with regard to atherosclerosis, suggesting that it be specifically termed macrophage RCT. The second theme is the critical role that intravascular remodeling of HDL by lipid transfer factors, lipases, cell surface receptors, and non-HDL lipoproteins play in determining the ultimate metabolic fate of HDL and plasma HDL-c concentrations. The third theme is the growing appreciation that insulin resistance underlies the majority of cases of low HDL-c in humans and the mechanisms by which insulin resistance influences HDL metabolism. Progress in our understanding of HDL metabolism and macrophage reverse cholesterol transport will increase the likelihood of developing novel therapies to raise plasma HDL concentrations and promote macrophage RCT and in proving that these new therapeutic interventions prevent or cause regression of atherosclerosis in humans.     

High-density lipoproteins from human alcoholics exhibit impaired reverse cholesterol transport function

We have previously shown that chronic alcohol consumption leads to inhibition of sialylation of apolipoprotein E (apo E) that results in its impaired binding to high-density lipoprotein (HDL) molecule. Because apo E plays a major role in reverse cholesterol transport (RCT), we speculated that ethanol-mediated formation of HDL molecules without apo E may affect the RCT process. Therefore, we have investigated whether the RCT function of HDL is affected in chronic alcoholics with or without liver disease compared with nondrinkers. HDL was isolated from fasting plasma of normal subjects, n = 9 (nondrinkers), chronic alcoholics, n = 8 (ALC), and chronic alcoholics with liver disease, n = 6 (ALD). A portion of HDL sample from each subject was evaluated for its cholesterol efflux capacity from [3H]cholesterol oleate preloaded mouse macrophages. The remaining portion of each HDL sample was labeled with [3H]cholesterol oleate and evaluated for its ability to deliver cholesterol to the liver using HepG2 cells in culture. Cholesterol efflux capacity of HDLs was decreased by 83% (P < .0002) in alcoholics without liver disease and by 84% (P < .0006) in alcoholics with liver disease compared with the HDLs from nondrinkers. The capacities of HDLs to deliver cholesterol to the liver were decreased by 54% (P < .005) in alcoholics without liver disease and by 64% (P < .005) in alcoholics with liver disease compared with the HDLs from nondrinkers. The fact that further complications by liver disease in alcoholic subjects did not significantly exacerbate the extent of impairment in RCT function of HDL suggest that alcohol per se is responsible for its deleterious effects on RCT. Significantly, plasma HDL apo E concentration relative to that of apo A1 (apo E/apo A1 ratio) was also decreased by 31% to 32% (P < .0005) in alcoholics without or with liver disease compared with nondrinkers. It is therefore concluded that chronic alcohol consumption adversely affects the RCT function of HDL by altering its association with apo E due to ethanol-induced desialylation of apo E.