ATP-binding cassette transporter A1 and cholesterol trafficking

Administration of tyrosyl radical-oxidized HDL inhibits the development of atherosclerosis in apolipoprotein E-deficient mice

OBJECTIVE

Tyrosyl radical-oxidized HDL (tyrHDL) increases the ability of cells to donate cholesterol to apolipoprotein (apo) A-I for HDL particle formation. We tested whether treatment with tyrHDL raises endogenous HDL cholesterol levels and decreases atherosclerosis development in apoE-deficient mice.

METHODS AND RESULTS

Tyrosyl radical oxidation of mouse HDL induced formation of apoAI-AII heterodimers and enhanced the ability of mouse HDL to deplete cultured fibroblasts of their regulatory pool of cholesterol. 125I-labeled HDL and tyrHDL delivered intraperitoneally were cleared at similar rates from plasma of chow-fed apoE-deficient mice. ApoE-deficient mice injected intraperitoneally twice weekly with 150 microg tyrHDL from age 10 to 18 weeks showed a maximum 2.3-fold increase in endogenous HDL cholesterol levels, which fell toward the end of the treatment period. tyrHDL treatment resulted in 37% less aortic lesion development than in control HDL-treated mice (P<0.001) and 67% less than in saline-injected animals (P<0.001).

CONCLUSIONS

Administration of tyrHDL for 8 weeks resulted in significantly less atherosclerosis development in apoE-deficient mice than injection of HDL or saline. Molecules increasing mobilization of cellular cholesterol to apoAI for HDL particle formation would be expected to decrease atherosclerosis without necessarily causing sustained increases in circulating HDL cholesterol levels.

Dyslipidaemia

The lowering of serum cholesterol is increasingly recognised as essential in the prevention of coronary heart disease and other atherosclerotic disease. The success of statin trials and the need to deploy these drugs effectively in the population has led increasingly to the identification of many people whose serum cholesterol, triglycerides, and HDL-cholesterol require clinical assessment, and frequently treatment. Lipid disorders are mainly straightforward, but some are complex or resistant to simple treatment strategies. I have reviewed the clinical manifestations of disordered lipid metabolism (dyslipidaemia) and its management.

Impaired ABCA1-dependent lipid efflux and hypoalphalipoproteinemia in human Niemann-Pick type C disease

The cholesterol trafficking defect in Niemann-Pick type C (NPC) disease leads to impaired regulation of cholesterol esterification, cholesterol synthesis, and low density lipoprotein receptor activity. The ATP-binding cassette transporter A1 (ABCA1), which mediates the rate-limiting step in high density lipoprotein (HDL) particle formation, is also regulated by cell cholesterol content. To determine whether the Niemann-Pick C1 protein alters the expression and activity of ABCA1, we determined the ability of apolipoprotein A-I (apoA-I) to deplete pools of cellular cholesterol and phospholipids in human fibroblasts derived from NPC1+/+, NPC1+/-, and NPC1-/- subjects. Efflux of low density lipoprotein-derived, non-lipoprotein, plasma membrane, and newly synthesized pools of cell cholesterol by apoA-I was diminished in NPC1-/- cells, as was efflux of phosphatidylcholine and sphingomyelin. NPC1+/- cells showed intermediate levels of lipid efflux compared with NPC1+/+ and NPC1-/- cells. Binding of apoA-I to cholesterol-loaded and non-cholesterol-loaded cells was highest for NPC1+/- cells, with NPC1+/+ and NPC1-/- cells showing similar levels of binding. ABCA1 mRNA and protein levels increased in response to cholesterol loading in NPC1+/+ and NPC1+/- cells but showed low levels at base line and in response to cholesterol loading in NPC1-/- cells. Consistent with impaired ABCA1-dependent lipid mobilization to apoA-I for HDL particle formation, we demonstrate for the first time decreased plasma HDL-cholesterol levels in 17 of 21 (81%) NPC1-/- subjects studied. These results indicate that the cholesterol trafficking defect in NPC disease results in reduced activity of ABCA1, which we suggest is responsible for the low HDL-cholesterol in the majority of NPC subjects and partially responsible for the overaccumulation of cellular lipids in this disorder.

ABCA1 gene polymorphisms and their associations with coronary artery disease and plasma lipids in males from three ethnic populations in Singapore

Mutations in the ATP-binding cassette transporter ABCA1 underlie Tangier disease and familial hypoalphaliproteinemia (FHA), disorders that are characterised by reduced high-density lipoprotein-cholesterol (HDL-C) concentration and cholesterol efflux, and increased coronary artery disease (CAD). We explored if polymorphisms in the ABCA1 gene are associated with CAD and variations in plasma lipid levels, especially HDL-C, and whether the associations may depend on ethnicity. Male cases and controls from the Singapore Chinese, Malay and Indian populations were genotyped for five ABCA1 single nucleotide polymorphisms. Various single-locus frequency distribution differences between cases and controls were detected in different ethnic groups: the promoter -14C>T in Indians, exon 18 M883I in Malays, and 3'-untranslated (UTR) region 8994A>G in Chinese. For the Malay population, certain haplotypes carrying the I825- A (exon 17) and M883- G alleles were more frequent among cases than controls, whereas the converse was true for the alternative configuration of V825- G and I883- A, and this association was reinforced in multi-locus disequilibrium analysis that utilized genotypic data. In the healthy controls, associations were found for -14C>T genotypes with HDL-C in Chinese; 237indelG (5'UTR) with apolipoprotein A1 (apoA1) in Malays and total cholesterol (TC) in Indians; M883I with lipoprotein(a) [Lp(a)] in Malays and apolipoprotein B (apoB) in Chinese; and 8994A>G with Lp(a) in Malays, and TC, low-density lipoprotein-cholesterol (LDL-C) as well as apoB in Indians. While genotype-phenotype associations were not reproduced across populations and loci, V825I and M883I were clearly associated with CAD status in Malays with no effects on HDL-C or apoA1.

ABCA1 redistributes membrane cholesterol independent of apolipoprotein interactions

ATP binding cassette transporter A1 (ABCA1) mediates the transport of phospholipids and cholesterol from cells to lipid-poor HDL apolipoproteins. Cholesterol loading of cells induces ABCA1, implicating cholesterol as its major physiologic substrate. It is believed, however, that ABCA1 is primarily a phospholipid transporter and that cholesterol efflux occurs by diffusion to ABCA1-generated phospholipid-rich apolipoproteins. Here we show that overexpression of ABCA1 in baby hamster kidney cells in the absence of apolipoproteins redistributed membrane cholesterol to cell-surface domains accessible to treatment with the enzyme cholesterol oxidase. The cholesterol removed by apolipoprotein A-I (apoA-I), but not by HDL phospholipids, was derived exclusively from these domains. ABCA1 overexpression also increased cholesterol esterification, which was prevented by addition of apoA-I, suggesting that some of the cell-surface cholesterol not removed by apolipoproteins is transported to the intracellular esterifying enzyme acyl-CoA:cholesterol acyltransferase. ABCA1 expression was essential for cholesterol efflux even when apolipoproteins had already acquired phospholipids during prior exposure to ABCA1-expressing cells. These studies show that ABCA1 redistributes cholesterol to cell-surface domains, where it becomes accessible for removal by apolipoproteins, consistent with a direct role of ABCA1 in cholesterol transport.

Study of ABCA1 function in transgenic mice

The ATP-binding cassette transporter A1 (ABCA1), identified in 1999 as the gene defective in Tangier disease, promotes efflux of cellular cholesterol from macrophages and other peripheral tissues to apolipoprotein acceptors. These ABCA1-mediated processes are anticipated to have antiatherogenic properties, prompting the development of pharmacological agents that increase ABCA1 gene expression as well as the establishment of ABCA1-transgenic mouse lines. Preliminary studies of ABCA1-Tg mice seem to validate the selection of this transporter as a therapeutic target for the treatment of low HDL syndromes and cardiovascular disease but have also raised new questions regarding the function of ABCA1. In particular, the relative contribution of hepatic and peripheral ABCA1 to plasma HDL levels and to reverse cholesterol transport, as well as the potential role of ABCA1 in modulating the plasma concentrations of the apolipoprotein B-containing lipoproteins and protecting against atherosclerosis, seem to be promising areas of investigation. The present review summarizes the most recent studies and discusses insights provided by these transgenic mouse models.

HDL apolipoproteins and ABCA1: partners in the removal of excess cellular cholesterol

It is widely believed that HDL protects against atherosclerosis by removing excess cholesterol from arterial cells. Lipid-poor HDL apolipoproteins promote efflux of cholesterol, phospholipids, and other lipophilic molecules from cells by an active process mediated by a cell-membrane transporter called the ATP binding cassette transporter A-1 (ABCA1). ABCA1 either directly or indirectly translocates phospholipids and cholesterol to the cell surface, where they appear to form lipid domains that interact with amphipathic alpha-helixes in apolipoproteins. This interaction solubilizes these lipids and generates nascent HDL particles that dissociate from the cell. Binding of apolipoproteins to ABCA1 may also enhance the activity of this lipid-transport pathway. Thus, the apolipoprotein/ABCA1 pathway efficiently clears cells of excess cholesterol that would otherwise accumulate as intracellular lipid droplets. ABCA1 expression is highly induced by cholesterol loading of cells and is also modulated by sterol-independent mechanisms at both the transcriptional and posttranslational level. Studies of human disease and animal models have shown that both an increased availability of apolipoproteins and an enhanced macrophage ABCA1 activity are atheroprotective. These findings implicate the apolipoprotein/ABCA1 pathway as an important therapeutic target for treating cardiovascular disease.

Recycling of apoprotein E is associated with cholesterol efflux and high density lipoprotein internalization

After receptor-mediated endocytosis of triglyceride-rich lipoproteins (TRL) into the liver, TRL particles are immediately disintegrated in peripheral endosomal compartments. Whereas core lipids and apoprotein B are delivered for degradation into lysosomes, TRL-derived apoE is efficiently recycled back to the plasma membrane. This is followed by apoE re-secretion and association of apoE with high density lipoproteins (HDL). Because HDL and apoE can independently promote cholesterol efflux, we investigated whether recycling of TRL-derived apoE in human hepatoma cells and fibroblasts could be linked to intracellular cholesterol transport. In this study we demonstrate that HDL(3) does not only act as an extracellular acceptor for recycled apoE but also stimulates the recycling of internalized TRL-derived apoE. Furthermore, radioactive pulse-chase experiments indicate that apoE recycling is accompanied by cholesterol efflux. Confocal imaging reveals co-localization of apoE and cholesterol in early endosome antigen 1-positive endosomes. During apoE re-secretion, HDL(3)-derived apoA-I is found in these early endosome antigen 1, cholesterol-containing endosomes. As shown by time-lapse fluorescence microscopy, apoE recycling involves the intracellular trafficking of apoA-I to pre-existing and TRL-derived apoE/cholesterol-containing endosomes in the periphery. Thus, these studies provide evidence for a new intracellular link between TRL-derived apoE, cellular cholesterol transport, and HDL metabolism.

The association of the R219K polymorphism in the ATP-binding cassette transporter 1 ( ABCA1) gene with coronary heart disease and hyperlipidaemia

The R219K polymorphism in the ATP-binding cassette transporter 1 gene ( ABCA1) has been associated with reduced severity of atherosclerosis, fewer coronary events, decreased triglycerides and a trend to increased HDL in men with coronary heart disease (CHD). This study examined the frequency and the effect on CHD and plasma lipids of the polymorphism in patients of both sexes attending a lipid out-patient clinic. The overall frequency of the K allele was 0.26. It was lower in patients with CHD (0.21) than in those without (0.27) but this was not statistically significant. Amongst patients with elevated Lp(a) the frequency of the K allele was significantly lower in those with CHD (0.16) than in those without (0.29). There were no statistically significant differences in total cholesterol, LDL, HDL, apoB or apoAI between carriers and non-carriers. When patients with probable secondary hypertriglyceridaemia (triglycerides >1000 mg/dl), type 2 diabetes and carriers of lipoprotein lipase polymorphisms associated with hypertriglyceridaemia were excluded, the K allele was significantly associated with reduced triglycerides but only in patients with apoE 3/3 genotype. In conclusion, we provide additional evidence that the R219K polymorphism in the ABCA1 gene either directly or as a result of linkage disequilibrium with additional functional variant(s), has a protective effect against CHD and is associated with lower plasma triglycerides in sub-groups of patients with hyperlipidaemia.

Molecular structure and characterization of a novel murine ABC transporter, Abca13

We report the isolation and structural characterization of the full-length gene and cDNA for a novel mouse ATP-binding cassette (ABC) transporter, Abca13. The mRNA, isolated from mouse kidney, is 6.7 kb in size and encodes a protein consisting of 2143 amino acids with a predicted molecular weight of 240 kDa. The Abca13 gene consists of 44 exons which span 360 kb of genomic sequence. Abca13 has been mapped to mouse chromosome 11.a2, revealing the human orthologue highly conserved on a syntenic region of human chromosome 7p12. The deduced mouse Abca13 protein shows highest amino acid sequence homology to human ABCA1 (50%), ABCA4 (50%), and ABCA12 (56%). Analysis of the putative Abca13 promoter region revealed potential transcription factor binding sites associated with myeloid- and lymphoid-derived cell types. mRNA transcript levels were highest in mouse submaxillary gland, epididymus, ovary, and thymus; with lower levels in a variety of other tissues. An alternative transcript was discovered in mouse kidney devoid of exon 11. The removal of exon 11 by post-transcriptional splicing causes a frameshift in the open reading frame and results in a premature termination codon. We hypothesize that the excision of exon 11 may serve as a regulatory mechanism in kidney, and perhaps other tissues as well. The molecular characterization of the mouse Abca13 gene will establish the foundation for future functional studies of the human ABCA13 transporter.

Apolipoprotein and apolipoprotein receptor genes, blood lipids and disease

PURPOSE OF REVIEW

Apolipoproteins and their receptors are the main controllers of lipid metabolism and, as such, have a major impact not only on the risk of cardiovascular disease but also on the development and degeneration of the central nervous system. Variations in the genes coding for these apolipoproteins and their receptors and the interaction with the environment determine individual susceptibility to metabolic disturbances, the response to dietary or pharmacological intervention and, finally, to disease.

RECENT FINDINGS

This review will focus on recent findings, such as the latest concepts regarding apolipoprotein E in neurodevelopment, the newly identified apolipoprotein A-V and its influence in triglyceride metabolism, and the improved understanding of apolipoprotein A-I and HDL metabolism in the light of the discovery of the ABC family of transporters. Other key aspects of lipoprotein metabolism and cardiovascular disease risk such as apolipoprotein B-100, the LDL receptor, apolipoprotein C-III or apolipoprotein (a) will be updated.

SUMMARY

Variations in these genes will be analysed in relation to plasma lipid levels, their interactions with diet, treatment or other environmental stimuli, and their influence on the risk of cardiovascular disease and neurological disorders.

ATP-binding cassette transporter A1, fatty acids, and cholesterol absorption

PURPOSE OF REVIEW

A significant advance in our understanding of the reverse cholesterol transport pathway occurred in 1999 with the identification of defects in the ATP-binding cassette transporter A1 gene as the cause of Tangier disease. Since this discovery, an overwhelming number of experiments have been conducted to further define the function of this gene. Among the concepts emerging from such studies is a possible role for the gene in cholesterol absorption. The present review summarizes the most recent of these studies, as well as the only report to describe the effects of fatty acids on ATP-binding cassette transporter A1 gene activity.

RECENT FINDINGS

From the one study conducted thus far, it appears that unsaturated fatty acids can reduce ATP-binding cassette transporter A1 gene activity by enhancing its degradation. Among the primary modulators of the gene's transcription is the liver X receptor, with liver X receptor-selective agonists significantly increasing expression of the gene. While some studies indicate that upregulation of the gene inhibits cholesterol absorption, the results of other studies suggest that it facilitates cholesterol absorption and the transfer of cholesterol into the bile. Preliminary evidence from studies with transgenic and knockout mice supports the concept that increasing ATP-binding cassette transporter A1 gene expression may be beneficial in the prevention of diet-induced atherosclerosis.

SUMMARY

Although there is substantial evidence from and studies to suggest that the ATP-binding cassette transporter A1 gene regulates intestinal cholesterol absorption, perhaps by mediating cholesterol efflux from the basolateral surface of enterocytes, it remains unclear whether or not this gene is the primary ATP-binding cassette transporter involved in the process.

Macrophage cholesterol transport: a critical player in foam cell formation

Mammalian cells have evolved complex feedback mechanisms to ensure sufficient supply of cholesterol and to prevent its excessive accumulation. During the process of atherosclerosis, these homeostatic mechanisms fail in macrophages. Uncontrolled cholesterol deposition is promoted by scavenger functions of the macrophages and the adaptive mechanisms elicited are not sufficient to process the lipid load. Consequently, a lipid-laden 'foam cell' is formed. In this review, we summarize key aspects of intracellular cholesterol processing in the special case of macrophages, including mechanisms of lipoprotein cholesterol uptake, fate of the internalized cholesterol and mechanisms implicated in cholesterol efflux. The importance of inflammatory cues, the cellular compartmentalization of cholesterol homeostatic responses and the increasing information on the transcriptional control of cholesterol balance are discussed.