Role of ABCG1 and other ABCG family members in lipid metabolism

Approaches to measuring cholesterol absorption in humans

Under optimal conditions, plasma cholesterol homeostasis is maintained by a variety of mechanisms, balancing input and output, thereby preventing the net accumulation of cholesterol in circulation and tissues. Among these mechanisms, intestinal cholesterol absorption has recently re-emerged as a potentially important contributor to cholesterol homeostasis. However, its regulation has been difficult to study in humans because of technical limitations in methodologies. In this review the major methods available for measuring cholesterol absorption including those that utilize cholesterol balance, single dose isotopic feeding, dual isotope plasma ratio, continuous isotope feeding, intestinal perfusion, stable isotopes and serum plant sterols or cholestanol to cholesterol ratios are reviewed and contrasted. Emphasis is placed on the strengths, technical and interpretational limitations and their applicability for use in metabolic, small-scale outpatient, population and large-scale intervention studies.

Functional expression and characterization of the human ABCG1 and ABCG4 proteins: indications for heterodimerization

The closely related human ABC half-transporters, ABCG1 and ABCG4, have been suggested to play an important role in cellular lipid/sterol regulation but no experimental data for their expression or function are available. We expressed ABCG1 and ABCG4 and their catalytic site mutant variants in insect cells, generated specific antibodies, and analyzed their function in isolated membrane preparations. ABCG1 had a high basal ATPase activity, further stimulated by lipophilic cations and significantly inhibited by cyclosporin A, thyroxine or benzamil. ABCG4 had a lower basal ATPase activity which was not modulated by any of the tested compounds. The catalytic site (K-M) mutants had no ATPase activity. Since dimerization is a requirement for half-transporters, we suggest that both ABCG1 and ABCG4 function as homodimers. Importantly, we also found that co-expression of the ABCG4-KM mutant selectively abolished the ATPase activity of the ABCG1 and therefore they most probably also heterodimerize. The heterologous expression, specific recognition, and functional characterization of these transporters should help to delineate their physiological role and mechanism of action.

Tissue-specific autoregulation of the LXRalpha gene facilitates induction of apoE in mouse adipose tissue

The functions of the liver X receptors (LXRs) are not well documented in adipose tissue. We demonstrate here that expression of the LXRalpha gene is highly induced in vivo and in vitro in mouse and human adipocytes in the presence of the synthetic LXR agonist T0901317. This autoregulation is caused by an identified LXR-responsive element motif in the mouse LXRalpha promoter, which is conserved in the human LXRalpha promoter. Using different LXR-deficient mice, we demonstrate that the basal expression level of LXRalpha is increased in LXRbeta(-/-) mice, whereas the basal expression level of LXRbeta is unchanged in LXRalpha(-/-) mice. The two LXRs can compensate for each other in mediating ligand-activated regulation of LXR target genes involved in lipid homeostasis in adipose tissue. Sterol regulatory element binding protein-1 (SREBP-1), ATP binding cassette transporter A1 (ABCA1), ABCG1, as well as apolipoprotein E (apoE) are induced in vivo by T0901317 in wild-type, LXRalpha(-/-) or LXRbeta(-/-) mice but not in LXRalpha(-/-)beta(-/-) mice. Although SREBP-1 and ABCG1 are induced in liver, muscle, and adipose tissue, the apoE, glucose transporter-4 (GLUT4), and LXRalpha genes are specifically induced only in adipose tissue. We suggest that an important aspect of LXRalpha autoregulation in adipose tissue may be to increase the level of LXRalpha over a threshold level necessary to induce the expression of certain target genes.

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.

Cholesterol absorption and hepatic acyl-coenzyme A:cholesterol acyltransferase activity play major roles in lipemic response to dietary cholesterol and fat in laboratory opossums

Partially inbred lines of laboratory opossums differ considerably in their low-density lipoprotein (LDL) cholesterol responses to dietary cholesterol and fat. Genetic analysis suggested that a single major gene is responsible for the variation in LDL cholesterol on the high cholesterol and high fat (HCHF) diet. We measured cholesterol absorption and acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in intestine and liver to narrow the search for the major gene. We measured plasma lipoproteins and percent cholesterol absorption by the fecal isotope ratio method in high and low responding lines of opossums on basal and HCHF diets. We also measured lipids in liver and ACAT activity in liver and intestine on the HCHF diet. High and low lines exhibited no differences in percent cholesterol absorption on the basal diet. However, high responding opossums had significantly higher percent cholesterol absorption, hepatic free and esterified cholesterol, and hepatic ACAT activity than low responding opossums on the HCHF diet. Hepatic ACAT activity but not the intestinal ACAT activity was associated with hepatic cholesterol concentration and percent cholesterol absorption. Cholesterol absorption is a major determinant of diet-induced hyperlipidemia in opossums. Hepatic ACAT activity but not the intestinal ACAT may also play a role in diet-induced hyperlipidemia in opossums.

Comparative analysis of ATP-binding cassette (ABC) transporter gene expression levels in peripheral blood leukocytes and in liver with hepatocellular carcinoma

ATP-binding cassette (ABC) transporters comprise a superfamily of similar proteins involved in transmembrane transport of various substances. ABC transporter family members in the liver participate in bile formation and lipid metabolism. In order to assess whether peripheral blood leukocytes (PBL) are available as a surrogate for determination of the expression of ABC transporter genes in the liver, we compared ABC transporter gene expression levels in PBL with those in liver tissues from patients with hepatocellular carcinoma (HCC). We measured ABCA1, A2, B1-B4, C1 degrees C5, G1 and G2 gene expression levels in PBL, and cancerous and non-cancerous portions of liver from patients with hepatocellular carcinoma by means of real time reverse-transcription (RT)-PCR. We could not detect ABCC5 expression in any tissue of the liver. Close correlations between ABCA2, C1 and G1 in PBL and in non-tumor tissues of the liver were found. Compared with the non-tumor part, HCC tissue expressed lower levels of ABCA1, B4 and G2. We think monitoring of ABCA2, C1 and G1 gene expression levels in PBL will be useful for selection of anti-cancer agents and monitoring of drug resistance of HCC. Administration of chemotherapeutic agents which are substrates of ABCA1, B4 and G2 should be effective for the treatment of HCC.

Macrophage apolipoprotein A-I expression protects against atherosclerosis in ApoE-deficient mice and up-regulates ABC transporters

The antiatherogenic effect of high-density lipoprotein (HDL) and its major protein component apolipoprotein A-I (apoA-I) has been largely attributed to their key roles in reverse cholesterol transport (RCT) and cellular cholesterol efflux. Substantial evidence shows that overexpression of human apoA-I reduces atherosclerosis in animal models. However, it is uncertain whether this protection is due to an increase in plasma HDL level or to a local effect in the artery wall. To test the hypothesis that expression of human apoA-I in macrophages can promote RCT in the artery wall, we used a retroviral construct expressing human apoA-I cDNA (MFG-HAI) to transduce ApoE(-/-) bone marrow cells and then transplanted these cells into ApoE(-/-) mice with preexisting atherosclerosis. ApoE(-/-) mice reconstituted with MFG-HAI marrow had a significant reduction (30%) in atherosclerotic lesions in the proximal aorta compared to control mice that received marrow expressing MFG parental virus. Peritoneal macrophages isolated from MFG-HAI mice showed a four- to fivefold increase in mRNA expression levels of both ATP-binding cassette (ABC) A1 and ABCG1 compared to controls. Our data demonstrate that gene transfer-mediated expression of human apoA-I in macrophages can compensate in part for apoE deficiency and delay the progression of atherosclerotic lesions by stimulating ABC-dependent cholesterol efflux and RCT.

Oxysterols trigger ABCA1-mediated basolateral surfactant efflux

Surfactant is an apically-secreted surface-active material containing primarily disaturated phosphatidylcholine (DSPtdCho) that is released from alveolar epithelia into the alveolus. Surfactant deficiency is an important aspect of inflammatory lung disease and may result from extravasation of serum lipoproteins into the alveolus. We investigated whether one bioactive component of modified lipoproteins, oxysterols, might reduce surfactant PtdCho availability by altering its trafficking. The oxysterol, 22-hydroxycholesterol (22HC), in combination with its obligate partner, 9 cis-retinoic acid (RA), decreased surfactant PtdCho levels, in part, by stimulating basolateral phospholipid export in murine lung epithelia. 22HC/RA stimulated basolateral PtdCho efflux in cells via transcriptional activation of the ATP-binding cassette transporter 1 (ABCA1) gene. This effect was mediated by a DR-4 locus within the ABCA1 promoter. ABCA1 knockdown studies using ABCA1 siRNA or the ABCA1 inhibitor, glyburide, selectively attenuated 22HC/RA-driven basolateral PtdCho efflux. 22HC/RA significantly increased export of PtdCho molecular species containing saturated (16:0) fatty-acyl species typical of DSPtdCho. Overexpression of ABCA1 mimicked 22HC/RA effects by increasing cellular PtdCho efflux, whereas mutagenesis of ABCA1 at Trp590 attenuated PtdCho release. The results indicate the existence of an oxysterol-activated basolateral exit pathway for surfactant that might impact the availability of phospholipid destined for apical secretion.

Assessing low levels of high-density lipoprotein cholesterol as a risk factor in coronary heart disease

Clinical data show that a 1% increase in serum concentrations of high-density lipoprotein cholesterol (HDL-C) can decrease cardiovascular risk by 2% to 3%. Therefore, mechanisms affecting the level and functionality of high-density lipoprotein (HDL) and its constituents are being investigated as targets for the rational development of drugs to prevent or treat cardiovascular disease. High-density lipoprotein-related research may also increase our understanding of the link between atherosclerosis and metabolic disorders. This report and update of the HDL Working Group discusses HDL metabolism and reverse cholesterol transport, impaired HDL as a marker and a cause of proatherogenic states, and experimental and current approaches to HDL-related therapy.

Reduced atherosclerosis in hormone-sensitive lipase transgenic mice overexpressing cholesterol acceptors

Macrophage-specific overexpression of cholesteryl ester hydrolysis in hormone-sensitive lipase transgenic (HSL Tg) female mice paradoxically increases cholesterol esterification and cholesteryl ester accumulation in macrophages, and thus susceptibility to diet-induced atherosclerosis compared to nontransgenic C57BL/6 mice. The current studies suggest that whereas increased cholesterol uptake could contribute to transgenic foam cell formation, there are no differences in cholesterol synthesis and the expression of cholesterol efflux mediators (ABCA1, ABCG1, apoE, PPARgamma, and LXRalpha) compared to wild-type macrophages. HSL Tg macrophages exhibit twofold greater efflux of cholesterol to apoA-I in vitro, suggesting the potential rate-limiting role of cholesteryl ester hydrolysis in efflux. However, macrophage cholesteryl ester levels appear to depend on the relative efficacy of alternate pathways for free cholesterol in either efflux or re-esterification. Thus, increased atherosclerosis in HSL Tg mice appears to be due to the coupling of the efficient re-esterification of excess free cholesterol to its limited removal mediated by the cholesterol acceptors in these mice. The overexpression of cholesterol acceptors in HSL-apoA-IV double-transgenic mice increases plasma HDL levels and decreases diet-induced atherosclerosis compared to HSL Tg mice, with aortic lesions reduced to sizes in nontransgenic littermates. The results in vivo are consistent with the effective efflux from HSL Tg macrophages supplemented with HDL and apoA-I in vitro, and highlight the importance of cholesterol acceptors in inhibiting atherosclerosis caused by imbalances in the cholesteryl ester cycle.

The genetic polymorphism of drug transporters: functional analysis approaches

Evidence is accumulating to strongly suggest that drug transporters are one of the determining factors governing the pharmacokinetic profile of drugs. To date, a variety of drug transporters have been cloned and classified as solute carriers and ATP-binding cassette transporters. Such drug transporters are expressed in various tissues such as the intestine, brain, liver, and kidney, and play critical roles in the absorption, distribution and excretion of drugs. However, at the present time, information is limited regarding the genetic polymorphism of drug transporters and its impact on their function. In this context, we have undertaken the functional analyses of the polymorphisms identified in drug transporter genes. This article aims to provide an overview on the functional aspects of the non-synonymous polymorphisms of drug transporters and to present standard methods for the evaluation of the effect of polymorphisms on their function.

Identification and characterization of rodent ABCA1 in isolated type II pneumocytes

ATP-binding cassette transporter A1 (ABCA1) promotes transfer of cholesterol and phospholipid from cells to lipid-free serum apolipoproteins. ABCA1 mRNA and protein expression in primary cultures of rodent type II cells was sensitive to upregulation with 5 microM 9-cis-retinoic acid (9cRA) and 6.2 microM 22-hydroxycholesterol (22-OH). The increase in ABCA1 protein levels was time dependent and was maximal after 16 h of exposure to 9cRA + 22-OH. Inducible ABCA1 was also found in transformed cell lines of lung origin: WI38/VA13, A549, and NIH-H441 cells. Stimulation of ABCA1 in rat type II cells by 9cRA + 22-OH resulted in a four- or fivefold enhancement of efflux of radioactive phospholipid or cholesterol, respectively, from the pneumocytes to apolipoprotein AI (apo AI), whereas cAMP (0.3 mM) had no effect. ABCA1-mediated lipid efflux to apo AI was independent of the surfactant secretion pathway, inasmuch as upregulation of ABCA1 resulted in a reduction of secretagogue-stimulated surfactant phospholipid release. These studies demonstrate the presence of functional ABCA1 in type II cells from the lung.

Changes in serum concentrations of noncholesterol sterols and lipoproteins in healthy subjects do not depend on the ratio of plant sterols to stanols in the diet

Consumption of plant sterols or stanols increases their respective serum concentrations, whereas plant sterols might reduce serum concentrations of plant stanols and vice versa. This suggests that changes in serum plant sterol and stanol concentrations depend on the ratio of plant sterols to stanols in the diet. To examine this in more detail, healthy men (n = 15) and women (n = 29) consumed in random order for 3 wk 1.5 g/d of plant sterols plus 0.5 g of plant stanols (high sterol margarine), 1 g of each (low sterol margarine) or control margarine. Sterols and stanols were provided as fatty acid esters. Compared with the control period, serum cholesterol-standardized campesterol and sitosterol concentrations increased by 33 (P < 0.001) and 19% (P < 0.002), respectively, during the high sterol period, but by only 20 (P < 0.001) and 11% (P = 0.001), respectively, during the low sterol period. During the high sterol period, these values for campestanol and sitostanol were 18 (P = 0.063) and 1% (P = 0.630), and during the low sterol period 25 (P = 0.105) and 7% (P = 0.163), respectively. Effects on LDL cholesterol were similar. We therefore conclude that changes in serum plant sterol and stanol concentrations are not greatly affected by the simultaneous consumption of plant sterols and plant stanols, but are proportional to intakes. Furthermore, both mixtures were equally effective in lowering serum LDL cholesterol concentrations.

Membrane topology of ABC-type macrolide antibiotic exporter MacB in Escherichia coli

MacB is an ABC-type membrane protein that exports only macrolide compounds containing 14- and 15-membered lactones, cooperating with a membrane fusion protein, MacA, and a multifunctional outer membrane channel, TolC. We determined the membrane topology of MacB by means of site-specific competitive chemical modification of single cysteine mutants. As a result, it was revealed that MacB is composed of four transmembrane (TM) segments with a cytoplasmic N-terminal nucleotide binding domain of about 270 amino acid residues and a periplasmic large hydrophilic polypeptide between TM segments 1 and 2 of about 200 amino acid residues.

Differential effects of scavenger receptor BI deficiency on lipid metabolism in cells of the arterial wall and in the liver

Scavenger receptor class B, type I (SRBI) is a key regulator of high density lipoprotein (HDL) metabolism. It facilitates the efflux of cholesterol from cells in peripheral tissues to HDL and mediates the selective uptake of cholesteryl esters from HDL in the liver. We investigated the effects of SRBI deficiency in the arterial wall and in the liver using SRBI-deficient mice and wild-type littermates fed a Western-type diet. The SRBI-deficient mice showed massive accumulation of cholesterol-rich HDL in the circulation, reflecting impaired delivery to the liver. Strikingly, SRBI deficiency did not alter hepatic cholesterol (ester) content nor did it affect the expression of key regulators of hepatic cholesterol homeostasis, including HMG-CoA reductase, the low density lipoprotein receptor, and cholesterol 7alpha-hydroxylase. However, a approximately 40% reduction in biliary cholesterol content was observed, and the expression of ABCG8 and ABCG5, ATP half-transporters implicated in the transport of sterols from the liver to the bile, was attenuated by 70 and 35%, respectively. In contrast to the situation in the liver, SRBI deficiency did result in lipid deposition in the aorta and atherosclerosis. Vascular mRNA analysis showed increased expression of inflammatory markers as well as of genes involved in cellular cholesterol homeostasis. Our data show that, although hepatic cholesterol homeostasis is maintained upon feeding a Western-type diet, SRBI deficiency is associated with de-regulation of cholesterol homeostasis in the arterial wall that results in an increased susceptibility to atherosclerosis.

ZNF202 is inversely regulated with its target genes ABCA1 and apoE during macrophage differentiation and foam cell formation

The zinc finger protein ZNF202 is a transcriptional repressor that binds to promoter elements predominantly found in genes involved in lipid metabolism. Here we demonstrate that ZNF202 mRNA expression is inversely correlated with ATP binding cassette A1 (ABCA1), ABCG1, and apolipoprotein E (apoE) in human monocytes. Upregulation of ABCA1, ABCG1, and apoE expression during monocyte differentiation and foam cell formation was accompanied by a simultaneous downregulation of both ZNF202 mRNA isoforms m1 and m3. Conversely, deloading of macrophage foam cells with HDL3 caused upregulation of ZNF202 mRNA. To further characterize the transcriptional regulation of the ZNF202 gene, comparative genomic sequence analysis and reporter gene assays were performed. The ZNF202 core promoter region resides within 247 bp upstream of the transcription initiation site and is highly active in THP-1 monocytes, yet downregulated upon macrophage differentiation. Using site-directed mutagenesis, we show that two highly conserved transcription factor binding sites, a GC-box and an Ets-binding motif, are required for ZNF202 gene expression. Furthermore, electrophoretic mobility shift assays demonstrate in vitro binding of PU.1 and GC-box binding proteins to the ZNF202 proximal promoter. We conclude that the inversely correlated transcriptional activity of ZNF202 and its target genes during macrophage differentiation may reflect a direct regulatory interdependence and thus provide further evidence for ZNF202 as an important gatekeeper of lipid efflux.

ABCA1 is essential for efficient basolateral cholesterol efflux during the absorption of dietary cholesterol in chickens

The ATP-binding cassette transporter A1 (ABCA1) participates in the efflux of cholesterol from cells. It remains unclear whether ABCA1 functions to efflux cholesterol across the basolateral or apical membrane of the intestine. We used a chicken model of ABCA1 dysfunction, the Wisconsin hypoalpha mutant (WHAM) chicken, to address this issue. After an oral gavage of radioactive cholesterol, the percentage appearing in the bloodstream was reduced by 79% in the WHAM chicken along with a 97% reduction in the amount of tracer in high density lipoprotein. In contrast, the percentage of radioactive cholesterol absorbed from the lumen into the intestine was not affected by the ABCA1 mutation. Liver X receptor (LXR) agonists have been inferred to decrease cholesterol absorption through activation of ABCA1 expression. However, the LXR agonist T0901317 decreased cholesterol absorption equally in both wild type and WHAM chickens, indicating that the effect of LXR activation on cholesterol absorption is independent of ABCA1. The ABCA1 mutation resulted in accumulation of radioactive cholesterol ester in the intestine and the liver of the WHAM chicken (5.0- and 4.4-fold, respectively), whereas biliary lipid concentrations were unaltered by the WHAM mutation. In summary, ABCA1 regulates the efflux of cholesterol from the basolateral but not apical membrane in the intestine and the liver.

New insights into the role of the adenosine triphosphate-binding cassette transporters in high-density lipoprotein metabolism and reverse cholesterol transport

Four adenosine triphosphate-binding cassette (ABC) transporters-ABCA1, ABCG1, ABCG5, and ABCG8-have been identified and shown to modulate cholesterol and lipoprotein metabolism. Recent analyses of ABCA1 indicate that upregulation of ABCA1 in the liver and macrophages of transgenic mice is associated with increased plasma high-density lipoprotein (HDL) cholesterol levels, increased net flux of cholesterol to the liver, and reduced diet-induced atherosclerosis. In ABCA1 transgenic mice, the enhanced expression of hepatic ABCA1 transporters is associated with increased plasma HDL cholesterol levels, suggesting that the liver plays an important role in the levels of plasma HDL cholesterol. Overexpression of ABCG1 in the liver of mice using recombinant ABCG1 vectors results in decreased plasma HDL levels and indicates that ABCG1 can modulate plasma lipoprotein levels in vivo. The potential importance of ABCG1 in reverse cholesterol transport has not been definitively established. Studies in patients with sitosterolemia have identified 2 major new transporters, ABCG5 and ABCG8, that play a pivotal role in the regulation of intestinal cholesterol, plant, and shellfish absorption. Modulation of the expression of ABCG5 and ABCG8 represents an important new mechanism in the regulation of cholesterol absorption in the intestine. The ABC transporters currently represent excellent targets for the development of new drugs for the treatment of patients with increased risk of premature cardiovascular disease.

Phytosterols and human lipid metabolism: efficacy, safety, and novel foods

Plant sterols have been known for several decades to cause reductions in plasma cholesterol concentrations. These plant materials have been granted a conditional health claim in the United States regarding their effects in the prevention of cardiovascular disease and are being sold in functional foods in several countries in Europe as well as in the United States and Australia. It is generally suggested that daily consumption of approximately 2 g of plant sterols can lower cholesterol concentrations as part of a dietary prevention strategy. However, phytosterols have been added and tested for their cholesterol-lowering effects mainly in spreads. Consumption of these high-fat foods seemingly flies in the face of current recommendations for the promotion of heart health, which suggest lowering total fat and energy intake to maintain weight. Hence, new food formulations are being evaluated using phytosterols incorporated into low-fat and reduced-fat food items. The purpose of this review is to examine the cholesterol-lowering efficacy of plant sterols, focusing on novel food applications, their mechanism of action, and safety. These novel food formulations include new solubilization processes that lead to improved uses for plant sterols, as well as new foods into which phytosterols have been incorporated, such as breads, cereals, and beef. Such new foods and formulations should pave the way for greater use of phytosterols in heart health promotion, increasing the longer-term potential for the creation of innovative functional foods containing plant sterols and their derivatives.

Activation of peroxisome proliferator-activated receptor gamma and retinoid X receptor results in net depletion of cellular cholesteryl esters in macrophages exposed to oxidized lipoproteins

OBJECTIVE

Peroxisome proliferator-activated receptor gamma (PPARgamma), a ligand-activated transcription factor, has pleiotropic effects, including regulation of macrophage differentiation and lipid homeostasis. The PPARgamma ligands, thiazolidinediones (TZDs), attenuate atherosclerosis in mice by uncertain mechanisms. The objective of this study was to determine whether activation of PPARgamma or its obligate heterodimer, retinoid X receptor (RXR), modulates macrophage foam cell formation induced by oxidized (ox) lipoproteins.

METHODS AND RESULTS

Incubation of THP-1 macrophages with oxHTG-VLDL, oxREM, or oxLDL increased cellular cholesteryl ester over 6-fold. Preincubation with the TZD, ciglitazone, the RXR-specific ligand, 9-cis retinoic acid (9cRA) or the combination reduced CE mass accumulation by up to 65%. Ciglitazone and 9cRA increased CD36 mRNA (up to 4-fold); however, uptake of [125I]oxLDL was only modestly enhanced (up to 1.8-fold) becaues of a concomitant PPARgamma:RXR-induced decrease in SRAI/II activity (up to 40%). This suggested that PPARgamma:RXR activation inhibited cholesteryl ester accumulation by enhancing cholesterol efflux. Ciglitazone and 9cRA were found to increase the expression of ATP-binding cassette proteins A1 and G1, resulting in enhanced cholesterol efflux to lipoprotein-deficient serum, apoAI and HDL3.

CONCLUSIONS

PPARgamma and/or RXR activation inhibit foam cell formation through enhanced cholesterol efflux despite increased oxLDL uptake. These observations explain the reduced atherosclerosis in TZD-treated mice and may extend the therapeutic implications of these ligands.

Real-time reverse transcription-PCR expression profiling of the complete human ATP-binding cassette transporter superfamily in various tissues

BACKGROUND

ATP-binding cassette (ABC) transporters are involved in many physiologic processes, such as lipid transport, sterol homeostasis, immune mechanisms, and drug transport, and cause various human inherited diseases. Thus, the analysis of ABC transporter mRNA expression profiles for basic research, especially in the field of lipid metabolism, for clinical diagnosis, and for monitoring of drug effects is of great interest.

METHODS

We have developed a rapid, accurate, and highly sensitive real-time reverse transcription-PCR (RT-PCR) method for detection and quantification of all 47 currently known members of the ABC transporter superfamily. Our expression analysis is based on relative quantification using a calibration curve method. With our assay, expression monitoring of a large number of RNA samples in a 384-well format with only 50 ng of total RNA is possible.

RESULTS

In contrast to previous expression analyses of single ABC genes, our method allows the rapid and complete analysis of all ABC transporters in given RNA samples. We used our newly established expression panel to study the gene expression of all human ABC transporters in 20 different human tissues. As a result, we identified tissues with high transcriptional activity for ABC transporters. These organs are mainly involved in secretory function (adrenal gland), metabolic function (liver), barrier function (lung, trachea, small intestine), and tropic function (placenta, uterus).

CONCLUSIONS

Our RT-PCR assay allows rapid, high-throughput transcriptional profiling of the complete ABC transporter superfamily and thus provides a new enabling tool for research, clinical diagnosis of disease, and drug testing and development.

Rapid quantification of murine ABC mRNAs by real time reverse transcriptase-polymerase chain reaction

Several ATP-binding cassette (ABC) transporters are critically involved in cholesterol and phospholipid efflux, reverse cholesterol transport, and play an important role in the development of atherosclerosis. Quantification of ABC mRNA is important in studying the regulation of cellular cholesterol homeostasis and mechanisms related to the pathogenesis of atherosclerosis. We have developed a one-step real time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) method for measuring mRNA levels of ABCA1, ABCG1, and ABCA2 in murine tissues using the TaqMan(TM) technology. It has significant methodological benefits when compared to classic Northern blotting or semi-quantitative RT-PCR analysis. Using this method, we found high expression levels of ABCA1 in liver and macrophages, and of ABCG1 in the brain and macrophages. The expression of ABCA1 and ABCG1 were further induced in macrophages loaded with acLDL. In contrast, ABCA2 was expressed exclusively in the brain with low expression levels in the macrophages. This method provides a rapid, highly sensitive, specific, and reproducible quantification of ABC mRNA, and can be performed with nanograms of total RNA sample, thus making it a superior method for studying the regulation of ABC transporters in cholesterol efflux and its role in the pathogenesis of atherosclerosis in murine models.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

The breast cancer resistance protein protects against a major chlorophyll-derived dietary phototoxin and protoporphyria

The breast cancer resistance protein (BCRPABCG2) is a member of the ATP-binding cassette family of drug transporters and confers resistance to various anticancer drugs. We show here that mice lacking Bcrp1Abcg2 become extremely sensitive to the dietary chlorophyll-breakdown product pheophorbide a, resulting in severe, sometimes lethal phototoxic lesions on light-exposed skin. Pheophorbide a occurs in various plant-derived foods and food supplements. Bcrp1 transports pheophorbide a and is highly efficient in limiting its uptake from ingested food. Bcrp1(-/-) mice also displayed a previously unknown type of protoporphyria. Erythrocyte levels of the heme precursor and phototoxin protoporphyrin IX, which is structurally related to pheophorbide a, were increased 10-fold. Transplantation with wild-type bone marrow cured the protoporphyria and reduced the phototoxin sensitivity of Bcrp1(-/-) mice. These results indicate that humans or animals with low or absent BCRP activity may be at increased risk for developing protoporphyria and diet-dependent phototoxicity and provide a striking illustration of the importance of drug transporters in protection from toxicity of normal food constituents.

Lack of Association between Increased Carotid Intima-Media Thickening and Decreased HDL-Cholesterol in a Family with a Novel ABCA1 Variant, G2265T

Low HDL-cholesterol (HDL-C) concentrations are inversely correlated with cardiovascular disease, and previous studies have demonstrated that variants in the ATP-binding cassette transporter, ABCA1, are responsible for a proportion of HDL-C deficiency states. We identified a novel variant in ABCA1 in a kindred with decreased HDL-C. This variant was not identified in &gt;200 chromosomes of healthy individuals. The proband, a heterozygote for G2265T, developed premature coronary artery disease. In addition to low HDL-C, six biological family members heterozygous for the ABCA1 variant exhibited low HDL-C concentrations compared with unaffected family members (0.83 ± 0.32 vs 1.33 ± 0.36 mmol/L; P = 0.009). Despite the decreased HDL-C, carotid artery B-mode ultrasound studies failed to reveal increased intima-media thickening in affected individuals compared with age- and sex-matched controls. Although these data extend previous observations that a single defective ABCA1 allele may lead to decreased HDL-C, associated evidence of early atherosclerosis was not confirmed.

Green tea extract inhibits the lymphatic absorption of cholesterol and alpha-tocopherol in ovariectomized rats

Evidence indicates that green tea consumption lowers the serum level of cholesterol (CH). This study was conducted to determine whether green tea lowers the intestinal absorption of CH and other lipids in ovariectomized (OX) rats. OX rats with lymph duct cannulae were infused at 3.0 mL/h for 8 h via an intraduodenal catheter with a lipid emulsion containing (14)C-cholesterol ((14)C-CH) and alpha-tocopherol (alphaTP) without (GT0) or with green tea extract standardized to 42.9 mg (GT1) or 120.5 mg (GT2) of total catechins in PBS (pH 6.5). Green tea extracts dose-dependently reduced (P < 0.05) the lymphatic absorption of (14)C-CH. The cumulative absorptions of (14)C-CH in rats infused with GT0, GT1 and GT2 were 36.3 +/- 1.1, 20.7 +/- 4.3 and 4.8 +/- 4.1% dose, respectively. The percentage distribution of esterified CH did not differ between rats infused with GT0 and GT1 (80.2 +/- 2.3% vs. 79.0 +/- 1.7%), but was significantly lower in those given GT2 (69.1 +/- 6.8%). The absorption of alphaTP also was significantly reduced by GT1 (736.5 +/- 204.9 nmol, 20.8 +/- 5.8% dose) and GT2 (281.0 +/- 190.8 nmol, 7.9 +/- 5.4% dose) compared with GT0 (1048.8 +/- 174.9 nmol, 29.6 +/- 4.9% dose). The absorption of fat was significantly increased by GT1 (862.6 +/- 151.1 micromol) but lowered by GT2 (557.9 +/- 252.2 micromol) relative to GT0 (717.7 +/- 39.1 micromol). The findings provide direct evidence that green tea has a profound inhibitory effect on the intestinal absorption of CH and alphaTP in OX rats. Whether the inhibitory effect of green tea extract is attributable to a specific catechin(s) and other components in green tea remains to be determined.

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.

Does ABCG2 Need a Heterodimer Partner? Expression and Functional Evaluation of ABCG2 (Arg 482)*

Accumulating evidence suggests that several ATP-binding cassette (ABC) transporters mediate the elimination of anticancer drugs from cancer cells and thereby confer drug resistance. SN-38-selected PC-6/SN2-5H human lung carcinoma cells were shown to overexpress ABCG2 with the reduced intracellular accumulation of SN-38, the active metabolite of irinotecan. We have recently demonstrated that plasma membrane vesicles prepared from those cells transported SN-38 in an ATP-dependent manner, and it was suggested that ABCG2 is involved in the active extrusion of SN-38 from cancer cells. In the present study, we have cloned the cDNA of ABCG2 from PC-6/SN2-5H human lung carcinoma cells, expressed ABCG2 in Sf9 insect cells, and characterized its function. Sequence analysis has revealed that the cloned ABCG2 has an arginine at the amino acid position 482, as does the wild type. Expression of the cloned ABCG2 in Sf9 cell membranes was detected by immunoblotting with the BXP-21 antibody. Contrary to our expectation, however, ATPase activity in the cell membranes expressing ABCG2 was stimulated by neither SN-38 nor rhodamine 123. It is suggested that there is a partner protein of ABCG2 required for heterodimer formation to exhibit transport activity toward SN-38.