ATP-binding cassette transporters G1 and G4 mediate cellular cholesterol efflux to high-density lipoproteins

The mechanisms responsible for the inverse relationship between plasma high-density lipoprotein (HDL) levels and atherosclerotic cardiovascular disease are poorly understood. The ATP-binding cassette transporter A1 (ABCA1) mediates efflux of cellular cholesterol to lipid-poor apolipoproteins but not to HDL particles that constitute the bulk of plasma HDL. We show that two ABC transporters of unknown function, ABCG1 and ABCG4, mediate isotopic and net mass efflux of cellular cholesterol to HDL. In transfected 293 cells, ABCG1 and ABCG4 stimulate cholesterol efflux to both smaller (HDL-3) and larger (HDL-2) subclasses but not to lipid-poor apoA-I. Treatment of macrophages with an liver X receptor activator results in up-regulation of ABCG1 and increases cholesterol efflux to HDL. RNA interference reduced the expression of ABCG1 in liver X receptor-activated macrophages and caused a parallel decrease in cholesterol efflux to HDL. These studies indicate that ABCG1 and ABCG4 promote cholesterol efflux from cells to HDL. ABCG1 is highly expressed in macrophages and probably mediates cholesterol efflux from macrophage foam cells to the major HDL fractions, providing a mechanism to explain the relationship between HDL levels and atherosclerosis risk.     

Relation between hepatic expression of ATP-binding cassette transporters G5 and G8 and biliary cholesterol secretion in mice

BACKGROUND/AIM: Mutations in genes encoding the ATP-binding cassette (ABC)-transporters ABCG5 and ABCG8 underlie sitosterolemia, which is characterized by elevated plasma levels of phytosterols due to increased intestinal absorption and impaired biliary secretion of sterols. The aim of our study was to correlate the expression levels of Abcg5 and Abcg8 to biliary cholesterol secretion in various (genetically-modified) mouse models. METHODS: Bile was collected from genetically-modified mice fed a chow diet, or from mice fed either a chow diet, or chow supplemented with either 1% diosgenin, 0.1% simvastatin, or a synthetic liver X receptor agonist, for determination of biliary lipids. Livers and small intestines were harvested and expression levels of Abcg5, Abcg8 and Abcb4 were determined by real-time polymerase chain reaction. RESULTS: Intestinal expression of Abcg5 and Abcg8 did not show much variation between the various models. In contrast, a linear correlation between hepatic expression levels of Abcg5 and Abcg8 and biliary cholesterol secretion rates was found. This relation was independent of Abcb4-mediated phospholipid secretion. However, in diosgenin-fed mice showing cholesterol hypersecretion, hepatic Abcg5 and Abcg8 expression levels remained unchanged. CONCLUSIONS: Our results strongly support a role for Abcg5 and Abcg8 in regulation of biliary cholesterol secretion, but also indicate the existence of a largely independent route of cholesterol secretion.     

ATP-Binding cassette transporter A1 modulates apolipoprotein A-I transcytosis through aortic endothelial cells

High-density lipoproteins and their major protein constituent apolipoprotein A-I (apoA-I) possess diverse atheroprotective properties. Most of them must be exerted within the arterial wall. Actually, high-density lipoproteins are the most abundant lipoproteins within the arterial intima. We have recently reported that apoA-I is transcytosed through aortic endothelial cells. In the present study, we evaluate the role of ATP-binding cassette transporter A1 (ABCA1) and scavenger receptor BI (SR-BI) in this process. Using pharmacological interventions and RNA interference, we investigated whether ABCA1 and SR-BI modulate apoA-I binding, internalization and transcytosis in endothelial cells. Upregulation of ABCA1 with oxysterols increased apoA-I binding and internalization. Trapping ABCA1 on the cell surface with cyclosporin A enhanced apoA-I binding but decreased its internalization and transcytosis. In addition, apoA-I binding, internalization, and transcytosis were reduced by at least 50% after silencing ABCA1 but not after knocking down SR-BI. The integrity of the endothelial cell monolayer was affected neither by cyclosporin A treatment nor by ABCA1 silencing, as controlled by measuring inulin permeability. Finally, in ABCA1-GFP-expressing cells, fluorescently labeled apoA-I colocalized intracellularly with ABCA1-GFP. However, apoA-I-containing vesicles did not colocalize with the late endosome marker LAMP-1 (lysosome-associated membrane protein-1). In conclusion, ABCA1, but not SR-BI, modulates the transcytosis of apoA-I through endothelial cells.     

Intracellular trafficking and regulation of canalicular ATP-binding cassette transporters

The bile canaliculus contains at least four ATP-binding cassette (ABC) proteins responsible for ATP-dependent transport of bile acids (spgp), nonbile acid organic anions (mrp2), organic cations (mdr1), and phosphatidylcholine (mdr2). Other ABC transporters (including mrp3) have also been partially localized to the canaliculus; however, their function has not been fully delineated. The specific amount and function of spgp and mrp2 in the canalicular membrane increases in response to taurocholate and cAMP. The mechanism involves increased recruitment of spgp and mrp2 from Golgi to the canalicular membrane by a microtubular and PI3 kinase-dependent vesicular trafficking system. Because the effects of taurocholate and cAMP summate, two distinct pathways are proposed. Mdr family members traffic either directly to the apical plasma membrane or, in the case of spgp, through a separate intracellular pool(s); in either case, there is no direct evidence for transcytosis of ABC transporters from Golgi to basolateral plasma membrane and subsequently to the canalicular plasma membrane. Direct transfer from Golgi to apical membrane was demonstrated by in vivo pulse labeling, in vitro membrane localization, and on-line video microscopy in WIFB9 cells that were stably transfected with mdr1-GFP. A critical role for 3'-phosphoinositide products of PI3 kinase was demonstrated in the intracellular trafficking of canalicular ABC transporters and for optimal transporter activity within the canalicular membrane. These studies suggest that many intracellular components, including ATP, Ca2+, numerous GTPases, microtubules, cytoplasmic motors, and other unknown factors, are required for physiologic regulation of ABC transporter traffic from Golgi to the canalicular membrane. Defects in this complex system are postulated to produce an "intrahepatic traffic jam" that results in defective ABC transporter function in the canalicular membrane and, consequently, in cholestasis.     

Genetic lecithin:cholesterol acyltransferase deficiency and cardiovascular disease

The lecithin:cholesterol acyltransferase (LCAT) enzyme is responsible for the synthesis of cholesteryl esters in human plasma and plays a critical role in high density lipoprotein (HDL) metabolism. Genetic LCAT deficiency is a rare metabolic disorder characterized by low HDL cholesterol levels. This paper reviews the genetic and biochemical features of LCAT deficiency, highlighting the absence of enhanced preclinical atherosclerosis in carriers, despite the remarkably low HDL cholesterol.     

ATP-binding cassette (ABC) transporters in atherosclerosis

Macrophages play a central role in the initiation and progression of atherosclerotic lesions. In the nascent lesion, macrophages transform into foam cells through the excessive accumulation of cholesteryl esters. Dysfunctional lipid homeostasis in macrophages and foam cells ultimately results in the breakdown of membrane integrity and cell death. Studies within the past 2 years have implicated a defined subset of multispan transmembrane proteins, the ATP-binding cassette (ABC) transporters, in macrophage lipid homeostasis. The recent finding that ABCA1, beyond its function as a major regulator of plasma high-density lipoprotein metabolism, exerts significant antiatherosclerotic activities has provided the first direct evidence for the role of an ABC transporter in the pathogenesis of atherosclerosis.     

ATP binding cassette transporters and drug resistance in breast cancer

Resistance to chemotherapy is a critical issue in the management of breast cancer patients. The nature of clinical drug resistance is likely to be multifactorial. However, in the last decade considerable attention has been dedicated to the role played by membrane transporter proteins belonging to the ATP binding cassette protein superfamily, and in particular by the MDR1 product P-glycoprotein (Pgp) and the multidrug resistance protein (MRP1). Heterogeneity of results is a common feature of studies evaluating the expression and prognostic role of these proteins, due to both methodological and biological factors. Nonetheless, Pgp and MRP1 are detected in a significant proportion of untreated breast cancers (on average 40 and 50% respectively, by immunohistochemistry), without a clear and consistent association with cancer stage. Exposure to chemotherapy increases the expression of both proteins. In vitro studies on primary cultures of breast cancer cells obtained at surgery consistently show an association between Pgp (protein) or MDR1 (mRNA) expression and resistance to chemotherapy. However, the correlation with clinical drug resistance is not as well defined. A stronger association of Pgp/MDR1 with response rates has been observed when expression or an increase in expression are detected immediately following chemotherapy. Correlations with prognosis appear more evident in studies using immunohistochemistry, in adjuvant and neoadjuvant settings. Evidence of clinical reversal of drug resistance by verapamil suggests a functional role of Pgp in drug resistance, although the significance of the evidence is generally weakened by poor trial designs. Future studies should take into account the multifactorial nature of drug resistance in breast cancer and use standardized approaches with adequate controls. Expression studies should be complemented by well-designed trials of drug-resistance reversal using target-specific chemosensitizing agents, and relating the results to the levels of expression of the target proteins.     

非高密度脂蛋白胆固醇水平与中国人群心血管病发病危险的相关性

目的 探讨我国35～64岁人群血清非高密度脂蛋白胆固醇(non-HDL-C)水平与不同心血管病事件发病危险的相关性.方法 采用前瞻性队列研究方法 ,应用Cox比例风险模型对1992年建立的中国多省市心血管病危险因素队列中35～64岁基线无心血管疾病且资料完整的29 937名研究对象的基线non-HDL-C水平和12年间急性冠心病事件(AGE)、缺血性卒中、出血性卒中和缺血性心血管病事件(ICVD)的发病危险进行分析.结果 (1)多因素Cox回归分析显示,调整年龄、性别、吸烟、糖尿病、体质指数、收缩压等传统危险因素后,ACE、缺血性卒中及ICVD事件发病的相对危险均随non-HDL-C水平的升高而增加.以non-HDL-C＜3.37 mmol/L(130 mg/dl)为参照组,3.37～4.13 mmol/L(130～159 mg/dl)、4.14～4.91 mmol/L(160～189 mg/dl)和≥4.92 mmol/L(190 mg/dl)组ACE、缺血性卒中及ICVD事件发病相对危险分别为:1.24(0.91～1.70)、1.78(1.25～2.53)、2.23(1. 48～3.35);1.34(1.07～1.68)、1.38(1.04～1.83)、1.38(0.97～1.94)和1.37(1.12～1.63)、1.52(1.22～1.90)、1.70(1.30～2.22).而non-HDL-C≥4.92 mmol/L(190 mg/dl)时,出血性卒中发病危险明显下降.(2)对极低密度脂蛋白胆固醇(VLDL-C)和低密度脂蛋白胆固醇进行联合分析显示:VLDL-C与ACE的发病危险相关性最强,其次为ICVD事件.出血性卒中的危险随VLDL-C升高呈下降趋势.结论 non-HDL-C可增加ACE、缺血性卒中和ICVD事件的发病危险.VLDL-C对ICVD事件的发病也具有一定的作用,其中对ACE的作用最为明显.     

High-density lipoprotein cholesterol and cardiovascular disease. Four prospective American studies

The British Regional Heart Study (BRHS) reported in 1986 that much of the inverse relation of high-density lipoprotein cholesterol (HDLC) and incidence of coronary heart disease was eliminated by covariance adjustment. Using the proportional hazards model and adjusting for age, blood pressure, smoking, body mass index, and low-density lipoprotein cholesterol, we analyzed this relation separately in the Framingham Heart Study (FHS), Lipid Research Clinics Prevalence Mortality Follow-up Study (LRCF) and Coronary Primary Prevention Trial (CPPT), and Multiple Risk Factor Intervention Trial (MRFIT). In CPPT and MRFIT (both randomized trials in middle-age high-risk men), only the control groups were analyzed. A 1-mg/dl (0.026 mM) increment in HDLC was associated with a significant coronary heart disease risk decrement of 2% in men (FHS, CPPT, and MRFIT) and 3% in women (FHS). In LRCF, where only fatal outcomes were documented, a 1-mg/dl increment in HDLC was associated with significant 3.7% (men) and 4.7% (women) decrements in cardiovascular disease mortality rates. The 95% confidence intervals for these decrements in coronary heart and cardiovascular disease risk in the four studies overlapped considerably, and all contained the range 1.9-2.9%. HDLC levels were essentially unrelated to non-cardiovascular disease mortality. When differences in analytic methodology were eliminated, a consistent inverse relation of HDLC levels and coronary heart disease event rates was apparent in BRHS as well as in the four American studies.     

Targeting high-density lipoprotein cholesterol in the management of cardiovascular disease

The association between low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol and the development of coronary heart disease (CHD) is well established. The management of CHD has traditionally focused on the reduction of LDL cholesterol or on the total lipid profile. Yet, treatment strategies for reducing risk in CHD also requires a focus on increasing low HDL cholesterol, as LDL cholesterol reduction alone is insufficient in preventing CHD events. This article reviews the strategies for targeting HDL cholesterol to optimize outcomes in CHD.     

Non-high-density lipoprotein cholesterol level as a predictor of cardiovascular disease mortality

BACKGROUND: Non-high-density lipoprotein cholesterol (non-HDL-C) contains all known and potential atherogenic lipid particles. Therefore, non-HDL-C level may be as good a potential predictor of risk for cardiovascular disease (CVD) as low-density lipoprotein cholesterol (LDL-C). OBJECTIVES: To determine whether non-HDL-C level could be useful in predicting CVD mortality and to compare the predictive value of non-HDL-C and LDL-C levels. METHODS: Data are from the Lipid Research Clinics Program Follow-up Study, a mortality study with baseline data gathered from 1972 through 1976, and mortality ascertained through 1995. A total of 2406 men and 2056 women aged 40 to 64 years at entry were observed for an average of 19 years, with CVD death as the main outcome measure. RESULTS: A total of 234 CVD deaths in men and 113 CVD deaths in women occurred during follow-up. Levels of HDL-C and non-HDL-C at baseline were significant and strong predictors of CVD death in both sexes. In contrast, LDL-C level was a somewhat weaker predictor of CVD death in both. Differences of 0.78 mmol/L (30 mg/dL) in non-HDL-C and LDL-C levels corresponded to increases in CVD risk of 19% and 15%, respectively, in men. In women, differences of 0.78 mmol/L (30 mg/dL) in non-HDL-C and LDL-C levels corresponded to increases in CVD risk of 11% and 8%, respectively. CONCLUSIONS: Non-HDL-C level is a somewhat better predictor of CVD mortality than LDL-C level. Screening for non-HDL-C level may be useful for CVD risk assessment.     

ATP-binding cassette proteins Common denominators between ion channels, transporters, and enzymes

Transporters and ion channels are highly specialized and functionally divergent molecules. However, these proteins may be less structurally diverse than previously appreciated. This is clearly apparent for one superfamily of molecules, the so-called ATP-binding cassette (ABC) proteins, which behave as ATP-dependent ion channels and/or transporters for a wide variety of substrates. ABC proteins also share common structural motifs with voltage-gated ion channels, transporters for glucose and neurotransmitters, and even adenylylcyclase. Beyond this, agents such as verapamil and forskolin, which inhibit and bind to one ABC protein (P-glycoprotein), may interact in homologous domains compared with some of these related proteins. Comparisons between these proteins are likely to provide a general understanding of pores in the lipid bilayer as well as specific properties that allow regulated movement and/or hydrolysis of selected substrates. This knowledge is important since certain ABC family members play a role in normal function and disease and provide novel therapeutic targets for drug development.     

Low high-density lipoprotein cholesterol and cardiovascular disease: risk reduction with statin therapy

A low level of high-density lipoprotein cholesterol (HDL-C) is a major risk factor for cardiovascular disease; however, patients with low levels of HDL-C without raised low-density lipoprotein cholesterol (LDL-C) levels are not currently eligible for lipid-lowering therapy. Many individuals with low levels of HDL-C have a combination of cardiovascular risk factors that include high LDL particle concentrations. Lowering LDL particle concentration and its surrogate measure, LDL-C, is an important approach to reducing cardiovascular risk. Statins are the most effective agents for lowering levels of LDL and can significantly increase levels of HDL-C. Extending statin therapy to patients with low levels of HDL-C but with LDL-C levels below target may have benefits for cardiovascular disease reduction in these patients.     

Smoking and atherosclerotic cardiovascular disease in women with lower levels of serum cholesterol

This cohort study of Koreans examines the relationship between smoking on atherosclerotic cardiovascular disease (ASCVD) and whether serum levels of total cholesterol modify the impact of smoking on ASCVD.

A 10-year prospective cohort study was carried out on 234,399 Korean women, ranging 40–69 years of age who received health insurance from the National Health Insurance Corporation and had a medical evaluation in 1993. The main outcome measures were hospital admissions and deaths from ischemic heart disease (IHD), cerebrovascular disease (CVD), and total ASCVD.

At baseline, 13,696 (5.8%) were current smokers and 105,755 (45.1%) had a total cholesterol <200 mg/dl. Between 1994 and 2003, 4534 IHD (176/100,000 person year), 7961 CVD (310/100,000 person year), and 2418 other ASCVD events (94/100,000 person year) occurred. In multivariate Cox proportional hazard models controlling for age, hypertension, hypercholesterolemia, diabetes and alcohol drinking, current smoking increased the risk of IHD [hazard ratio (HR) = 1.7 (95% CI: 1.5–1.9)], CVD [HR = 1.6 (95% CI: 1.5–1.6)], and total ASCVD events [HR = 1.6 (95% CI: 1.5–1.7)]. Throughout the range of serum cholesterol levels, current smoking significantly increased the risk of myocardial infarction and CVD, but not angina pectoris. There was no evidence of an interaction between smoking and serum cholesterol (p for interaction = 0.469, 0.612, and 0.905 for IHD, CVD, and total ASCVD, respectively).

This study demonstrated that smoking was a major independent risk factor for IHD, CVD and ASCVD in Korean women. A low cholesterol level confers no protective benefit against smoking-related ASCVD.     

Skin tissue cholesterol (SkinTc) is related to angiographically-defined cardiovascular disease

Parallel changes associated with aging found in the vasculature and skin at necropsy, have prompted small preliminary studies to assess the relation between skin tissue cholesterol (SkinTc) and cardiovascular disease. While these studies have been suggestive, no formal investigation is available to test this association. It would, therefore, be valuable to determine whether a relation between SkinTc and angiographic narrowing actually exists, the latter representing one accepted measurement of coronary atherosclerosis. METHODS: Patients at three hospitals undergoing coronary angiography and not on lipid altering agents (n = 649) were examined for skinTc using a non-invasive method. Vessels were evaluated manually (number with stenosis > or = 50%). Clinical characteristics, current medication use, and Framingham global risk score were recorded. RESULTS: SkinTc was significantly higher in patients with angiographic disease (124 +/- 30 vs. 118 +/- 30, P = 0.02). After adjustment for traditional coronary artery disease risk factors, SkinTc provided 7% additional risk (per 10 SkinTc units) for angiographic disease. CONCLUSION: SkinTc, measured with a non-invasive method, is associated with the presence of coronary artery disease as determined by catheterization. Such an assay may eventually help stratify patient risk and target prevention efforts.     

Raising HDL cholesterol for cardiovascular disease prevention: Is this still feasible?

Plasma high-density lipoprotein cholesterol (HDLC) has received considerable attention as a potential therapeutic target to further reduce cardiovascular risk in the statin era. However, doubts about the clinical benefit achievable with treatments enhancing plasma HDLC levels have been raised by the premature termination of a large phase 3 trial with torcetrapib—the most potent and furthest developed HDLC-raising compound—resulting from excess mortality in patients receiving the drug. The causes of torcetrapib failure are unknown and may be related to the drug’s mode of action, off-target toxic effects, or a mixture of both. The failure of torcetrapib does not mean that the concept of targeting HDL in cardiovascular prevention is dead. Other HDLC-raising therapies, which act through disparate molecular mechanisms, are in various stages of preclinical and clinical development. The alternative is the direct administration of synthetic HDL, which has proven activity on atherosclerosis regression in coronary patients.