ATP-binding cassette transporter A1 (ABCA1) affects total body sterol metabolism

BACKGROUND AND AIMS: Members of the family of ABC transporters are involved in different processes of sterol metabolism, and ABCA1 was recently identified as a key regulator of high-density lipoprotein (HDL) metabolism. Our aim was to further analyze the role of ABCA1 in cholesterol metabolism. METHODS: ABCA1-deficient mice (ABCA1-/-) and wild-type mice were compared for different aspects of sterol metabolism. Intestinal cholesterol absorption was determined by a dual stable isotope technique, and analysis of fecal, plasma, and tissue sterols was performed by gas chromatography/mass spectrometry. Key regulators of sterol metabolism were investigated by Northern and Western blot analyses or enzyme activity assays. RESULTS: ABCA1-disrupted sv129/C57BL/6 hybrid mice showed a significant reduction in intestinal cholesterol absorption. The decrease in cholesterol absorption was followed by an enhanced fecal loss of neutral sterols, whereas fecal bile acid excretion was not affected. Total body cholesterol synthesis was significantly increased, with enhanced 3-hydroxy-3-methyglutaryl-coenzyme A (HMG-CoA) reductase observed in adrenals and spleen. In addition, ABCA1-/- mice showed markedly increased concentrations of cholesterol precursors in the plasma, lung, intestine, and feces. Reduced HMG-CoA reductase messenger RNA and enzyme activity in the liver suggest that enhanced cholesterol synthesis in ABCA1-/- mice occurs in peripheral tissues rather than the liver. CONCLUSIONS: The metabolism of cholesterol and cholesterol precursors is markedly affected by a lack of ABCA1 function.     

Increased gallstone risk in humans conferred by common variant of hepatic ATP-binding cassette transporter for cholesterol

Genomewide scans of inbred strains of mice have linked the genes encoding the hepatocanalicular cholesterol transporter ABCG5/G8 to gallstone formation. Five nonsynonymous coding single-nucleotide polymorphisms (SNPs) in the orthologous human genes are associated with differences in serum cholesterol and plant sterol levels. We now tested these ABCG5/G8 SNPs for linkage and association with gallstone susceptibility in humans. Prospectively, we collected data from 178 white individuals with gallbladder stones or history of cholecystectomy in 84 families and from 70 stone-free controls, as confirmed by abdominal ultrasound. We performed nonparametric linkage (NPL) analysis of affected sib pairs (ASPs) and association tests of cases and controls. In ASPs, gallstones were strongly linked to the D19H variant of the ABCG8 gene (NPL score = 7.1; P = 4.6 x 10(-13)). The risk of gallstones in carriers of the 19H allele was significantly increased in randomly selected cases from the ASP cohort compared to the stone-free controls (OR = 3.018; P = 0.017). Consistent with the mouse model, heterozygosity for the lithogenic ABCG8 allele was associated with gallstones in humans; 21.4% of gallstone patients carried the heterozygous D19H genotype, compared with 8.6% of controls (OR = 2.954; P = 0.026). CONCLUSION: The linkage and association studies identified the cholesterol transporter ABCG5/G8 as a genetic determinant of gallstone formation, or LITH gene, in humans. The function of this transporter and the results of the genetic study taken together indicate that in gallstone-susceptible carriers of the ABCG8 19H allele, cholesterol cholelithiasis is secondary to increased hepatobiliary cholesterol secretion.     

Insulin down-regulates specific activity of ATP-binding cassette transporter A1 for high density lipoprotein biogenesis through its specific phosphorylation

Insulin resistance/hyperinsulinism is one of the major risks for atherosclerotic vascular diseases and low HDL may be involved in pathogenesis. We examined direct effects of insulin on HDL biosynthesis focusing on the activity of ATP-binding cassette transporter A1 (ABCA1) in culture cells and in experimental animals. Insulin impairs HDL biosynthesis through modulation of ABCA1 activity by two different mechanisms. Insulin enhances degradation of ABCA1. However, even after this effect was cancelled by blocking its specific signal, insulin still reduces HDL biogenesis. This effect was found due to phosphorylation of ABCA1 that leads to decrease of its specific activity. We identified a novel insulin-specific phosphorylation site Tyr1206 of ABCA1 to regulate its specific activity. The observation in a rat model of insulin resistance was consistent with these results. The findings demonstrate a new mechanism for regulation of ABCA1 activity and provide new insights into the link between development of atherosclerosis, and insulin resistance/hyperinsulinism.     

Regulation and mechanisms of ATP-binding cassette transporter A1-mediated cellular cholesterol efflux

ATP-binding cassette transporter A1 (ABCA1) plays a major role in cholesterol homeostasis and HDL metabolism. ABCA1 mediates cellular cholesterol and phospholipid efflux to lipid-poor apolipoproteins, and upregulation of ABCA1 activity is antiatherogenic. ApoA-I, the major apolipoprotein component of HDL, promotes ABCA1-mediated cholesterol and phospholipid efflux, probably by directly binding to ABCA1. ABCA1 gene expression is markedly increased in cholesterol-loaded cells as a result of activation of LXR/RXR. ABCA1 protein turnover is rapid. ABCA1 contains a PEST--proline (P), glutamate (E), serine (S), and threonine (T)--sequence in the intracellular segment that mediates ABCA1 degradation by a thiol protease, calpain. ApoA-I and apoE stabilize ABCA1 in a novel mode of regulation by decreasing PEST sequence-mediated calpain proteolysis. ABCA1-mediated cholesterol and phospholipid efflux are distinctly regulated and affected by the activity of other gene products. Stearyol CoA desaturase decreases ABCA1-mediated cholesterol efflux but not phospholipid efflux, likely by decreasing the cholesterol pool available to ABCA1. This and other evidence suggest that ABCA1 promotes cholesterol and phospholipid efflux, probably by directly transporting both lipids as substrates.     

Cholesterol efflux via ATP-binding cassette transporter A1 (ABCA1) and cholesterol uptake via the LDL receptor influences cholesterol-induced impairment of beta cell function in mice

Aims/hypothesis  

Cellular cholesterol accumulation is an emerging mechanism for beta cell dysfunction in type 2 diabetes. Absence of the cholesterol transporter ATP-binding cassette transporter A1 (ABCA1) results in increased islet cholesterol and impaired insulin secretion, indicating that impaired cholesterol efflux leads to beta cell dysfunction. In this study, we aimed to determine the role of the LDL receptor (LDLr) in islet cholesterol uptake and to assess the contributions of cholesterol uptake compared with efflux to islet cholesterol levels.     

The ATP-binding cassette transporter subfamily A member 1 (ABC-A1) and type 2 diabetes: an association beyond HDL cholesterol

Recent findings from several groups demonstrate that ABC-A1 participates in the pathogenesis of the metabolic syndrome and type 2 diabetes. A variant of the ABC-A1 gene (R230C) is associated with the metabolic syndrome and its co-morbidities in Mexicans. Its presence is associated with an increased risk for obesity, the metabolic syndrome and type 2 diabetes. R230C is found exclusively in Amerindian and Amerindian-derived populations. Moreover, animal models confirm the participation of ABC-A1 in the pathogenesis of diabetes. Mice lacking AbcA1 specifically in beta cells had glucose intolerance at 8 weeks of age. The absence of ABC-A1 led to cholesterol accumulation within the beta cell plasma membrane, suggesting that cholesterol may play a role in the insulin secretory pathway. In conclusion, ABC-A1 may be more than a determinant of HDL-cholesterol. It may provide a link between components of the metabolic syndrome and atherosclerosis.     

Roles of ATP-binding cassette transporter A7 in cholesterol homeostasis and host defense system

ATP-binding cassette transporter (ABC) A7 is an ABC family protein that is a so-called full-size ABC transporter, highly homologous to ABCA1, which mediates the biogenesis of high-density lipoprotein (HDL) with cellular lipid and helical apolipoproteins. ABCA7 mediates the formation of HDL when exogenously transfected and expressed; however, endogenous ABCA7 was shown to have no significant impact on the generation of HDL and was found to be associated with phagocytosis regulated by sterol regulatory element binding protein 2. Since phagocytosis is one of the fundamental functions of animal cells as an important responsive reaction to infection, injury and apoptosis, ABCA7 seems to be one of the key molecules linking sterol homeostasis and the host defense system. In this context, HDL apolipoproteins were shown to enhance phagocytosis by stabilizing ABCA7 against calpain-mediated degradation and increasing its activity, shedding light on a new aspect of the regulation of the host-defense system.     

Localization of human ATP-binding cassette transporter 1 (ABC1) in normal and atherosclerotic tissues

The present study examines the expression of ATP-binding cassette transporter 1 (ABC1) mRNA in normal and atherosclerotic tissues by using in situ hybridization in an effort to better understand the function of this cholesterol transport protein. Samples of normal baboon tissues as well as human normal and atherosclerotic aortas were hybridized with (35)S-labeled ABC1 sense and antisense riboprobes. Widespread expression of ABC1 was observed generally in tissues containing inflammatory cells and lymphocytes. Other noninflammatory cells that were also sites of ABC1 synthesis included the ductal cells of the kidney medulla, Leydig cells in the testis, and glial cells in the baboon cerebellum. Although normal veins and arteries did not express ABC1 mRNA, it was found to be upregulated in the setting of atherosclerosis, where widespread expression was found in macrophages within atherosclerotic lesions. These results are consistent with the proposed role of ABC1 in cholesterol transport in inflammatory cells. The specific upregulation of ABC1 mRNA in the setting of atherosclerosis probably reflects the response of leukocytes to cholesterol loading. However, the presence of ABC1 in ductal cells of the kidney medulla and in the small intestine suggest a more general role for this protein in cholesterol transport in other cell types.     

Associations between serum high-density lipoprotein cholesterol or apolipoprotein AI levels and common genetic variants of the ABCA1 gene in Japanese school-aged children

ATP-binding cassette transporter A1 (ABCA1) plays an important role in apolipoprotein AI (apoAI)-mediated cholesterol efflux from peripheral cells. The mild changes in ABCA1 activity due to genomic variation might be associated with interindividual variations in serum high-density lipoprotein cholesterol (HDL-C) and apoAI levels, or primary hypoalphalipoproteinemia in the general population. In the present study, we analyzed the relationships between 5 single nucleotide polymorphisms (SNPs) and 2 insertion/deletion polymorphisms in the 5' flanking region and 5 missense polymorphisms of the ABCA1 gene and serum lipid levels in healthy school-aged children. We detected significant associations between the K219R and V771M polymorphisms, and HDL-C or apoAI levels. The present data support the proposition that the K219 allele is an anti-atherogenic allele with increased cholesterol efflux activity. Similarly, the M771 allele appears to be anti-atherogenic, although the frequency of the M771 allele is low.     

Relationships between common polymorphisms of adenosine triphosphate-binding cassette transporter A1 and high-density lipoprotein cholesterol and coronary heart disease in a population with type 2 diabetes mellitus

Patients with type 2 diabetes mellitus (T2D) have a high coronary risk partly because of low levels of high-density lipoprotein-cholesterol (HDL-C). The adenosine triphosphate-binding cassette transporter A1 (ABCA1) plays a key role in HDL metabolism. We studied the association of common single nucleotide polymorphisms (SNPs) in the ABCA1 gene with HDL-C levels and coronary risk in a cohort of subjects with T2D. We studied 5 SNPs: +69C>T, +378G>C, R219K, I883M, and R1587K. The C allele of +378G>C was significantly associated with lower HDL-C concentrations (P = .04); and the M allele of I883M, with higher HDL-C concentrations (P = .03). No significant association was found between these SNPs and the incidence of new coronary events. Nevertheless, cross-sectional data on entry showed that the frequency of K219 was lower in patients with previous coronary heart disease (angina pectoris and/or myocardial infarction) (odds ratio, OR [95% confidence interval, CI] = 0.80 [0.65-0.98], P = .03, after adjustment for multiple risk factors other than HDL-C). The frequency of K1587 was higher in patients with angina pectoris (OR [95% CI] = 1.27 [1.01-1.58], P = .04, after multiple adjustment). The TT genotype of the C69T SNP was less frequent in subjects with prior myocardial infarction (OR [95% CI] = 0.28 [0.13-0.61], P = .001, after multiple adjustment). These associations persisted after further adjustment for HDL-C levels. In conclusion, common genetic variations of ABCA1 had a moderate influence on HDL-C levels and/or coronary heart disease in patients with T2D. These 2 effects were independent.     

新发现的中国人ATP结合盒转运子A1基因M233V 单核苷酸多态性及其家系调查

目的对112例中国冠心病患者行ATP结合盒转运子A1（ABCA1）基因筛选以发现新的单核苷酸多态性（SNP）,并对先证者家系进行调查确定其意义。方法用聚合酶链反应（PCR）．单链构象多态性分析（SSCP）-DNA测序及限制性酶切发现ABCA1基因新的SNP,进一步对先证者家系的16位成员进行血脂检查、基因组DNA提取及限制性酶切分析。结果N233V是一个新发现的ABCA1基因SNP,此位点在外显子7,cDNA位置为A1092G,并导致233位氨基酸由甲硫氨酸转变为缬氨酸（N233V）。16位家族成员中发现5例存在N233V SNP位点改变,且均为AG型改变。结论中国人中存在新发现的ABCA1基因N233V SNP,家系调查显示N233V SNP中AA型分布频率最高,AG型次之,AG型SNP的发生具有家族倾向性。