ABCG1 Levels Research Articles

Coffee Consumption Enhances High-Density Lipoprotein-Mediated Cholesterol Efflux in Macrophages

Association of habitual coffee consumption with coronary heart disease morbidity and mortality has not been established. We hypothesized that coffee may enhance reverse cholesterol transport (RCT) as the antiatherogenic properties of high-density lipoprotein (HDL). This study was to investigate whether the phenolic acids of coffee and coffee regulates RCT from macrophages in vitro, ex vivo and in vivo. Caffeic acid and ferulic acid, the major phenolic acids of coffee, enhanced cholesterol efflux from THP-1 macrophages mediated by HDL, but not apoA-I. Furthermore, these phenolic acids increased both the mRNA and protein levels of ATP-binding cassette transporter (ABC)G1 and scavenger receptor class B type I (SR-BI), but not ABCA1. Eight healthy volunteers were recruited for the ex vivo study, and blood samples were taken before and 30 minutes after consumption of coffee or water in a crossover study. The mRNA as well as protein levels of ABCG1, SR-BI, and cholesterol efflux by HDL were increased in the macrophages differentiated under autologous sera obtained after coffee consumption compared to baseline sera. Finally, effects of coffee and phenolic acid on in vivo RCT were assessed by intraperitoneally injecting [(3)H]cholesterol-labeled acetyl low-density lipoprotein-loaded RAW264.7 cells into mice, then monitoring appearance of (3)H tracer in plasma, liver, and feces. Supporting in vitro and ex vivo data, ferulic acid was found to significantly increase the levels of (3)H tracer in feces. Coffee intake might have an antiatherogenic property by increasing ABCG1 and SR-BI expression and enhancing HDL-mediated cholesterol efflux from the macrophages via its plasma phenolic acids.

The expression of ATP-binding cassette transporter A1 in Chinese overweight and obese patients

ATP-binding cassette transporters (ABCA1, ABCG1) and scavenger receptor class B type I (SR-BI) are the three most important cellular cholesterol transporters/receptor in regulating cholesterol efflux. We have investigated whether the expression of these transporters/receptor is altered in overweight and obese patients, and the potential association with the circulating adipokines. Adiponectin, leptin and resistin were assayed in two groups of healthy controls, overweight and obese patients. The expression of ABCA1, ABCG1 and SR-BI in monocytes was measured. Cholesterol efflux from monocyte-derived macrophages was also determined. Circulating adiponectin was decreased, whereas leptin and resistin were increased in overweight and obese patients compared with healthy controls. ABCA1 expression was significantly decreased in overweight and obese patients (P<0.01), whereas the levels of ABCG1 and SR-BI were comparable between the two groups. Adiponectin was correlated with ABCA1 (r=0.44, P<0.001), but not with ABCG1 and SR-BI. No associations between leptin, resistin and the expression of ABCA1, ABCG1 and SR-BI were found. Cholesterol efflux from monocyte-derived macrophages to apolipoprotein AI or to autologous serum was significantly impaired in overweight and obese patients, which were correlated with ABCA1 expression (r=0.47, P<0.001; r=0.43, P<0.001, respectively). The expression of ABCA1 in monocytes is reduced in overweight and obese patients, and the reduction in ABCA1 is associated with the impairment of cholesterol efflux from monocyte-derived macrophages.

Discovery and implementation of transcriptional biomarkers of synthetic LXR agonists in peripheral blood cells

BackgroundLXRs (Liver X Receptor α and β) are nuclear receptors that act as ligand-activated transcription factors. LXR activation causes upregulation of genes involved in reverse cholesterol transport (RCT), including ABCA1 and ABCG1 transporters, in macrophage and intestine. Anti-atherosclerotic effects of synthetic LXR agonists in murine models suggest clinical utility for such compounds.ObjectiveBlood markers of LXR agonist exposure/activity were sought to support clinical development of novel synthetic LXR modulators.MethodsTranscript levels of LXR target genes ABCA1 and ABCG1 were measured using quantitative reverse transcriptase/polymerase chain reaction assays (qRT-PCR) in peripheral blood from mice and rats (following a single oral dose) and monkeys (following 7 daily oral doses) of synthetic LXR agonists. LXRα, LXRβ, ABCA1, and ABCG1 mRNA were measured by qRT-PCR in human peripheral blood mononuclear cells (PBMC), monocytes, T- and B-cells treated ex vivo with WAY-252623 (LXR-623), and protein levels in human PBMC were measured by Western blotting. ABCA1/G1 transcript levels in whole-blood RNA were measured using analytically validated assays in human subjects participating in a Phase 1 SAD (Single Ascending Dose) clinical study of LXR-623.ResultsA single oral dose of LXR agonists induced ABCA1 and ABCG1 transcription in rodent peripheral blood in a dose- and time-dependent manner. Induction of gene expression in rat peripheral blood correlated with spleen expression, suggesting LXR gene regulation in blood has the potential to function as a marker of tissue gene regulation. Transcriptional response to LXR agonist was confirmed in primates, where peripheral blood ABCA1 and ABCG1 levels increased in a dose-dependent manner following oral treatment with LXR-623. Human PBMC, monocytes, T- and B cells all expressed both LXRα and LXRβ, and all cell types significantly increased ABCA1 and ABCG1 expression upon ex vivo LXR-623 treatment. Peripheral blood from a representative human subject receiving a single oral dose of LXR-623 showed significant time-dependent increases in ABCA1 and ABCG1 transcription.ConclusionPeripheral blood cells express LXRα and LXRβ, and respond to LXR agonist treatment by time- and dose-dependently inducing LXR target genes. Transcript levels of LXR target genes in peripheral blood are relevant and useful biological indicators for clinical development of synthetic LXR modulators.

Effects of combined PPARγ and PPARα agonist therapy on reverse cholesterol transport in the Zucker diabetic fatty rat

We investigated the effects of the combined therapy of PPARgamma and PPARalpha agonists on HDL metabolism, especially concerning reverse cholesterol transport (RCT), using Zucker diabetic fatty rats (ZDF/Crl-Lepr fa rats) that are the rodent model for type 2 diabetes with obesity, hyperlipidaemia and insulin resistance. The ZDF rats were divided into four medicated groups as follows: pioglitazone as a PPARgamma agonist (5 mg/kg/day; P group, n = 6), fenofibrate as a PPARalpha agonist (30 mg/kg/day; F group, n = 6), both these medications (P + F group, n = 6) and no treatment (UNT group, n = 6). Non-diabetic rats (ZDF/GmiCrl-lean, CON group, n = 6) served as controls. We evaluated HDL particle size and messenger RNA (mRNA) levels of the following factors: liver X receptor alpha (L x R alpha), ATP-binding cassette A1 (ABCA1) and ABCG1 which are regulated by PPARs and are related to early stage RCT. The significant increase in HDL particle size was demonstrated in rats of the F and P + F groups, although changes in plasma HDL-cholesterol levels were not significant. In accordance with this finding, mRNA levels of ABCG1 in the liver increased significantly. These findings suggest the efficacy of combined therapy with PPARgamma and PPARalpha in improving lipid metabolism, partly through the enhanced RCT, and insulin resistance in type 2 diabetes mellitus.

ABCG1 influences the brain cholesterol biosynthetic pathway but does not affect amyloid precursor protein or apolipoprotein E metabolism in vivo

Cholesterol homeostasis is of emerging therapeutic importance for Alzheimer's disease (AD). Agonists of liver-X-receptors (LXRs) stimulate several genes that regulate cholesterol homeostasis, and synthetic LXR agonists decrease neuropathological and cognitive phenotypes in AD mouse models. The cholesterol transporter ABCG1 is LXR-responsive and highly expressed in brain. In vitro, conflicting reports exist as to whether ABCG1 promotes or impedes Abeta production. To clarify the in vivo roles of ABCG1 in Abeta metabolism and brain cholesterol homeostasis, we assessed neuropathological and cognitive outcome measures in PDAPP mice expressing excess transgenic ABCG1. A 6-fold increase in ABCG1 levels did not alter Abeta, amyloid, apolipoprotein E levels, cholesterol efflux, or cognitive performance in PDAPP mice. Furthermore, endogenous murine Abeta levels were unchanged in both ABCG1-overexpressing or ABCG1-deficient mice. These data argue against a direct role for ABCG1 in AD. However, excess ABCG1 is associated with decreased levels of sterol precursors and increased levels of SREBP-2 and HMG-CoA-reductase mRNA, whereas deficiency of ABCG1 leads to the opposite effects. Although functions for ABCG1 in cholesterol efflux and Abeta metabolism have been proposed based on results with cellular model systems, the in vivo role of this enigmatic transporter may be largely one of regulating the sterol biosynthetic pathway.

Insight Into ABCG1-Mediated Cholesterol Efflux

Macrophages in the arterial intima take up modified LDL and become cholesterol-ester laden foam cells, which are the primary cell type in newly formed fatty streak lesions, and which play an important role throughout lesion progression and plaque vulnerability. Macrophages can unload their excess cellular cholesterol stores via lipid efflux, the first step in the protective reverse cholesterol transport pathway. In this pathway, efflux of cellular cholesterol to extracellular acceptors, such as HDL and lipid-poor apolipoproteins, targets this cholesterol for delivery to the liver for metabolism and direct excretion into the bile. The mechanisms of cellular cholesterol efflux are the focus of intensive research, and this field was advanced greatly by the discovery of ABCA1 as the Tangier disease gene in 1999.1 Tangier disease subjects have almost no plasma HDL, and their cells have a complete deficiency in cholesterol and phospholipid efflux to apolipoprotein A-I (apoAI) and a partial defect in lipid efflux to HDL.2 Thus, it was apparent that there is more than one pathway for lipid efflux to HDL. In 2001, Schmitz and colleagues demonstrated that ABCG1, another member of the ABC gene superfamily, was upregulated by cholesterol in human macrophages, and that an ABCG1 antisense oligonucleotide that reduced ABCG1 expression also decreased lipid efflux to HDL.3 The role of ABCG1 in lipid efflux was confirmed in 2004 when Wang and colleagues and Edwards and colleagues showed that ABCG1 transfected cells had increased cholesterol efflux to HDL.4,5 See page 1310 In the current issue of Arteriosclerosis, Thrombosis, and Vascular Biology , Wang et al provide novel insights both into the regulation of ABCG1 activity and into aspects of the mechanism of ABCG1-mediated lipid efflux,6 which differentiate it from the mechanism of ABCA1-mediated lipid efflux. Their essential discovery is that treatment of cells with …

Characterization of nascent HDL particles and microparticles formed by ABCA1-mediated efflux of cellular lipids to apoA-I

The nascent HDL created by ABCA1-mediated efflux of cellular phospholipid (PL) and free (unesterified) cholesterol (FC) to apolipoprotein A-I (apoA-I) has not been defined. To address this issue, we characterized the lipid particles released when J774 mouse macrophages and human skin fibroblasts in which ABCA1 is activated are incubated with human apoA-I. In both cases, three types of nascent HDL containing two, three, or four molecules of apoA-I per particle are formed. With J774 cells, the predominant species have hydrodynamic diameters of approximately 9 and 12 nm. These discoidal HDL particles have different FC contents and PL compositions, and the presence of acidic PL causes them to exhibit alpha-electrophoretic mobility. These results are consistent with ABCA1 located in more than one membrane microenvironment being responsible for the production of the heterogeneous HDL. Activation of ABCA1 also leads to the release of apoA-I-free plasma membrane vesicles (microparticles). These larger, spherical particles released from J774 cells have the same PL composition as the 12 nm HDL and contain CD14 and ganglioside, consistent with their origin being plasma membrane raft domains. The various HDL particles and microparticles are created concurrently, and there is no precursor-product relationship between them. Importantly, a large fraction of the cellular FC effluxed from these cells by ABCA1 is located in microparticles. Collectively, these results show that the products of the apoA-I/ABCA1 interaction include discoidal HDL particles containing different numbers of apoA-I molecules. The cellular PLs and cholesterol incorporated into these nascent HDL particles originate from different cell membrane domains.