ATP Binding Cassette Subfamily G Member 1 (ABCG1, formerly called “white” and/or ABCG8) expression is induced upon activation of liver X receptors (LXRα/NR1H3 or LXRβ/NR1H2) (Sabol SL et al. 2005; Beyea MM et al. 2007). The ABCG1 gene has been mapped to chromosome 21q22.3 and multiple human ABCG1 transcripts have been detected resulting from different transcription initiation sites and alternative mRNA splicing (Croop JM et al 1997; Langmann T et al. 2000; Lorkowski S et al. 2001; Kennedy MA et al. 2001). Induction of ABCG1 expression by NR1H2,3 agonists likely involves the presence of multiple LXR response elements (LXRE) through the promoter region of the ABCG1 gene (Kennedy MA et al. 2001; Sabol SL et al. 2005; Uehara Y et al. 2007). Studies in ABCG1 knockout mice revealed that ABCG1 is primarily expressed in macrophages, endothelial cells, and lymphocytes. However, it is also found in Kupffer cells and hepatocytes (Kennedy MA et al. 2005). ABCG1 exhibits a tissue specific expression pattern with high expression levels of ABCG1 found in lung, brain, spleen, adrenal glands, heart and liver (Croop JM et al 1997; Klucken J et al. 2000). ABCG1 plays an important role in cholesterol efflux (Kennedy MA et al. 2005; Wang N et al. 2004). In contrast to ABCA1, which transports cholesterol to lipid-poor apolipoprotein acceptors, ABCG1 transports cholesterol to preformed high-density lipoprotein (HDL) particles. A synergistic relationship between ABCA1 and ABCG1 has been proposed, whereby ABCA1 promotes lipidation of lipid-poor apoproteins and thus generating HDL-like acceptors for ABCG1-mediated cholesterol efflux (Gelissen IC et al. 2006). Beyond a role in cellular lipid homeostasis, ABCG1 participates in glucose and lipid metabolism by controlling the secretion and activity of insulin and lipoprotein lipase (Sturek JM et al. 2010; Olivier M et al. 2012; Hardy LM et al. 2017).

G-protein pathway suppressor 2 (GPS2) was identified as a co-regulator required for NR1H2, NR1H3-induced transcription of the ABCG1 gene in human hepatic HepG2 and macrophage THP-1 cells (Jakobsson T et al. 2009).

The ATP binding cassette transporter G1 (ABCG1, formerly called “white” and/or ABC8) exhibits a tissue-specific expression pattern with high expression levels found in macrophage/microglia, lung, brain, spleen, adrenal gland, heart and liver (Croop JM et al 1997; Klucken J et al. 2000). The ABCG1 gene is transcriptionally activated by cholesterol-loading and agonists of liver X receptors (LXRα/NR1H3 and LXRβ/NR1H2) and retinoid X receptors (RXRs) and has been implicated in the efflux of cholesterol to high density lipoprotein (HDL) (Venkateswaran A et al. 2000; Kennedy MA et al. 2001; Ayaori M et al. 2012; Sabol SL et al. 2005; Jakobsson T et al. 2009). Beyond a role in cellular lipid homeostasis, ABCG1 participates in glucose and lipid metabolism by controlling the secretion and activity of insulin and lipoprotein lipase (Olivier M et al. 2012; Sturek JM et al. 2010; Hardy LM et al. 2017). ABCG1 gene has been mapped to chromosome 21q22.3 and multiple human ABCG1 transcripts have been detected resulting from different transcription initiation and alternative mRNA splicing (Croop JM et al 1997; Langmann T et al. 2000; Lorkowski S et al. 2001; Kennedy MA et al. 2001). Although discrepancies were initially found among reports in the literature regarding the structure of the ABCG1 gene, it is now established that the ABCG1 gene is composed of 23 exons encoding a protein that forms a half transporter with 6 transmembrane spanning domains and a single intracellular nucleotide binding domain (NBD) (Langmann T et al. 2000; Kennedy MA et al. 2001; Hardy LM et al. 2017). This NBD domain contains highly conserved Walker A and Walker B motifs and is required for the binding and the hydrolysis of ATP which might provide required energy to transport substrates across the membrane (Cserepes J et al. 2004; Hirayama H et al. 2013; Vaughan AM & Oram JF 2005). Induction of ABCG1 expression by LXR agonists likely involves the presence of multiple LXR response elements (LXRE) throughout the ABCG1 gene (Kennedy MA et al. 2001; Sabol SL et al. 2005; Uehara Y et al. 2007). Electromobility shift assays demonstrated that NR1H3 and RXR alpha bind to two LXREs in intron 7 (Kennedy MA et al. 2001). Another set of two functional LXREs, LXRE-A and LXRE-B, was identified in the first and second introns of the human ABCG1 gene (Sabol SL et al. 2005). Further, studies of the transcriptional activity of truncated human ABCG1 promoter constructs showed that the NR1H2,3:RXR response region (or LXRE) is located in the human ABCG1 promoter A (LXRE-A) between -303 and -233 (Uehara Y et al. 2007).

G-protein pathway suppressor 2 (GPS2) was identified as a co-regulator required for NR1H2,3-induced transcription of the ABCG1 gene in human hepatic HepG2 and macrophage THP-1 cells (Jakobsson T et al. 2009). In macrophages, silencing of GPS2 by RNA interference reduced ABCG1 expression and diminished ABCG1-mediated cholesterol efflux. Chromatin immunoprecipitation analysis and 2-hybrid and protein-protein interaction assays revealed that GPS2 interacted with NR1H2,3:RXR heterodimer at the LXRE of the ABCG1 promoter (Jakobsson T et al. 2009). Chromosome conformation capture assays using the human hepatoma cell line, Huh7, transfected with GPS2-targeting siRNAs showed that GPS2 was required for intrachromosomal communication of the ABCG1 promoter and enhancer triggered by NR1H2,3 activation (Jakobsson T et al. 2009). Further, ligand activation of NR1H2,3 induced two functionally coupled GPS2-dependent processes: (1) receptor recruitment to an ABCG1 promoter/enhancer unit and (2) lysine-specific histone demethylase 1 (KDM1)-dependent H3K9 demethylation (Jakobsson T et al. 2009). The model suggests that the H3K9 methylation imposes a chromatin barrier at certain genomic loci (e.g., ABCG1) that prevents nuclear receptors (NR) (e.g., NR1H2,3) from high-affinity DNA binding (as detected by ChIP assays). Ligand activation in vivo triggers recruitment of KDMs to NRs (in the case of NR1H2,3 via GPS2), thereby facilitating H3K9 demethylation and high-affinity DNA binding (Jakobsson T et al. 2009).

In an ATP-dependent reaction, ABCG1 mediates the movement of intracellular cholesterol to the extracellular face of the plasma membrane. In a tissue culture model system, the active form of ABCG1 is predominantly a tetramer (Vuaghan and Oram 2005). The number of lipid molecules transported per ATP consumed is not known.

Extracellular discoidal HDL particles interact with cholesterol-rich membrane patches formed through the action of ABCG1 (Vaughan and Oram 2005). In the body this reaction is a key step in the process of reverse cholesterol transport, by which excess cholesterol is recovered from cells such a macrophages and transported ultimately to the liver. At a molecular level, it is one of the steps in the transformation of discoidal (small nascent) HDL particles into spherical ones, distinct from the similar reaction in which cholesterol is transferred to lipid-free apoA-I protein (Oram and Vaughan 2006; Kontush and Chapman 2006).

The ATP-binding cassette transporter A1 (ABCA1) gene is transcribed to yield mRNA.

T0901317 or GW3965, two synthetic agonists of liver X-receptors (LXRα, NR1H3 and LXRβ, NR1H2) or cholesterol-loading significantly induced the expression of ABCA1 mRNA in mouse RAW 264.7 and human THP1 macrophage cell lines (Costet P et al. 2000; Venkateswaran A et al. 2000; Whitney KD et al. 2001; Jakobsson T et al. 2009). Similar regulation of ABCA1 mRNA expression by NR1H2, 3 agonists was observed in human peripheral blood-derived monocytes (Larrede S et al. 2009). Treatment with T0901317 increased expression of ABCA1 mRNA in variety of cells and tissues isolated from wild type but not LXR-/- mice (lacking both NR1H3 and NR1H2) (Repa JJ et al. 2000; Wagner BL et al. 2003). At the same time, NR1H2, 3 repressed basal expression of ABCA1 in a tissue-specific manner, occurring in macrophages and intestinal mucosa but not in several other mouse tissues (Wagner BL et al. 2003). Treatment of human THP-1 macrophages with endogenous (25-hydroxycholesterol) or synthetic (T0901317) ligands of NR1H2,3 stimulated both transcriptional and posttranscriptional events to enhance ABCA1 expression (Ignatova ID et al. 2013). NR1H2,3-induced expression of ABCA1 is thought to promote ABCA1-mediated cellular cholesterol transport across the plasma membrane to lipid-poor apolipoproteins, such as ApoA1 and ApoE in the generation of nascent high-density lipoproteins (HDL) particles (Ignatova ID et al. 2013; Vedhachalam C et a. 2007). Loss of ABCA1 in humans results in Tangier disease, a condition in which patients have extremely low levels of circulating HDL, massive accumulation of cholesterol in macrophages, and an increased risk for developing atherosclerosis (Rust S et al. 1999).

Multiple microRNAs have been identified as regulators of ABCA1 mRNA levels (Horie T et al. 2010; Sun D et al. 2012; de Aguiar Vallim TQ et al. 2013).

In the unliganded state, NR1H2,3 (LXR):RXR heterodimers are bound to DNA response elements in association with co-repressor complexes resulting in repression of target genes such as the ATP-binding cassette transporter (ABCA1) gene (Wagner BL et al. 2003; Jakobsson T et al. 2009). Ligand binding to NR1H2,3 induces conformational changes leading to release of co-repressor complexes and recruitment of co-activator complexes and transcription of target genes. A mammalian two-hybrid analysis, using GAL4 fusions of the receptor interacting domains (ID) from the nuclear receptor corepressor (NCOR1) and the silencing mediator of retinoic acid and thyroid hormone receptors (SMRT, also known as NCOR2) transiently co-expressed with VP-16 fusions of NR1H3 or NR1H2 ligand binding domains in the monkey kidney fibroblasts CV-1 cells showed that in the absence of ligand, both NR1H2 and NR1H3 interacted with the corepressor IDs of NCOR and SMRT (Wagner BL et al. 2003). Biochemical work has identified a core complex consisting of NCOR, histone deacetylase 3 (HDAC3), transducin β-like proteins (TBL1, TBLR1), and G protein pathway suppressor 2 (GPS2) (Zhang J et al. 2002). Chromatin immunoprecipitation (ChIP) assays in HepG2 cells revealed that, in the absence of GW3965, a synthetic NR1H2,3 agonist, NCOR and HDAC3 were associated with the ABCA1 promoter, while agonist treatment caused their dissociation and induced recruitment of histone acetyltransferase (HAT) CBP and RNA polymerase II (Jakobsson T et al. 2009). TBLR1 was also present at the promoter and unaffected by the ligand status. GPS2 was found to occupy the ABCA1 promoter in the absence of ligand but was released upon GW3965 treatment, while NR1H2,3 (LXR) recruitment was observed already in the absence of ligand and further enhanced upon ligand activation (Jakobsson T et al. 2009). The inclusion of anti-RXR antibody in the re-ChIP assays demonstrates that GPS2 associates with the LXR:RXR heterodimer. Importantly, similar recruitment patterns were obtained in human THP-1 macrophages. Thus, at the ABCA1 promoter, NR1H2,3 ligand triggers exchange of a GPS corepressor complex (containing NCoR, HDAC3, TBLR1) for the coactivator complex devoid of GPS2 (Jakobsson T et al. 2009).

HDL (high-density lipoprotein) particles play a central role in the reverse transport of cholesterol, the process by which cholesterol in tissues other than the liver is returned to the liver for conversion to bile salts and excretion from the body and provided to tissues such as the adrenals and gonads for steroid hormone synthesis (Tall et al. 2008).
ABCG1 mediates the movement of intracellular cholesterol to the extracellular face of the plasma membrane where it is accessible to circulating HDL (Vaughan & Oram 2005). Spherical (mature) HDL particles can acquire additional molecules of free cholesterol (CHOL) and phospholipid (PL) from cell membranes.
At the HDL surface, LCAT (lecithin-cholesterol acyltransferase) associates strongly with HDL particles and, activated by apoA-I, catalyzes the reaction of cholesterol and phosphatidylcholine to yield cholesterol esterified with a long-chain fatty acid and 2-lysophosphatidylcholine. The hydrophobic cholesterol ester reaction product is strongly associated with the HDL particle while the 2-lysophosphatidylcholine product is released. Torcetrapib associates with a molecule of CETP and a spherical HDL particle to form a stable complex, thus trapping CETP and inhibiting CETP-mediated lipid transfer between HDL and LDL (Clark et al. 2006).
Spherical HDL particles can bind apoC-II, apoC-III and and apoE proteins.

The liver X receptors (LXRs), LXRα (NR1H3) and LXRβ (NR1H2), are nuclear receptors that are activated by endogenous oxysterols, oxidized derivatives of cholesterol (Janowski BA et al. 1996). When cellular oxysterols accumulate as a result of increasing concentrations of cholesterol, NR1H2,3 induce the transcription of genes that protect cells from cholesterol overload (Zhao C & Dahlman‑Wright K 2010; Ma Z et al. 2017). In peripheral cells such as macrophages, NR1H2 and NR1H3 increase cholesterol efflux by inducing expression of ATP-binding cassette subfamily A type 1 (ABCA1), ABCG1, and apolipoprotein APOE (Jakobsson T et al. 2009; Laffitte BA et al. 2001; Mak PA et al. 2002). In the intestine, LXR agonists decrease cholesterol absorption through induction of ABCA1, ABCG5, and ABCG8 (Repa JJ et al. 2000; Back SS et al. 2013). Cholesterol removal from non-hepatic peripheral cells, such as lipid-laden macrophages, and its delivery back to the liver for catabolism and excretion are processes collectively known as reverse cholesterol transport (RCT) (Francis GA 2010; Rosenson RS et al. 2012). This Reactome module describes the activation of several direct NR1H2,3 target genes that are closely associated with the RCT pathway, including genes encoding membrane lipid transporters, such ABCA1, ABCG1, ABCG5, ABCG8 and a cluster of apolipoprotein genes APOE, APOC1, APOC2 and APOC4 (Jakobsson T et al. 2009; Back SS et al. 2013; Mak PA et al. 2002).

The liver X receptors LXRα (NR1H3) and LXRβ (NR1H2) are members of the nuclear receptor superfamily and function as ligand-activated transcription factors. The natural ligands of NR1H2 and NR1H3 are oxysterols (e.g., 24(S),25-epoxycholesterol, 24(S)-hydroxycholesterol (OH), 25-OH, and 27-OH) that are produced endogenously by enzymatic reactions, by reactive oxygen species (ROS)-dependent oxidation of cholesterol and by the alimentary processes (reviewed in:Jakobsson T et al. 2012; Huang C 2014; Komati R et al. 2017). It has been shown that these oxysterols bind directly to the ligand-binding domain of LXRs with Kd values ranging from 0.1 to 0.4 microM. 24(S), 25-epoxycholesterol was found to be the most potent endogenous agonist (Janowski BA et al. 1999). NR1H3 (LXRα) and NR1H2 (LXRβ) showed similar affinities for these compounds (Janowski BA et al. 1999). In physiological conditions, oxysterols are formed in amounts proportional to cholesterol content in the cell and therefore the LXRs operate as cholesterol sensors to alter gene expression and protect the cells from cholesterol overload via: (1) inhibiting intestinal cholesterol absorption; (2) stimulating cholesterol efflux from cells to high-density lipoproteins through the ATP-binding cassette transporters ABCA1 and ABCG1: (3) activating the conversion of cholesterol to bile acids in the liver; and (4) activating biliary cholesterol and bile acid excretion (reviewed in: Wójcicka G et al. 2007; Baranowski M 2008; Laurencikiene J & Rydén M 2012; Edwards PA et al. 2002; Zelcer N & Tontonoz P 2006; Zhao C & Dahlman-Wright K 2010). In addition, LXR agonists enhance de novo fatty acid synthesis by stimulating the expression of a lipogenic transcription factor, sterol regulatory element-binding protein-1c (SREBP-1c), leading to the elevation of plasma triglycerides and hepatic steatosis (Wójcicka G et al. 2007; Baranowski M 2008; Laurencikiene J & Rydén M 2012). In addition to their function in lipid metabolism, NR1H2,3 have also been found to modulate immune and inflammatory responses in macrophages (Zelcer N & Tontonoz P 2006). The NR1H2 and NR1H3 molecules can be viewed as having four functional domains: (1) an amino-terminal ligand-independent activation function domain (AF-1), which may stimulate transcription in the absence of ligand; (2) a DNA-binding domain (DBD) containing two zinc fingers; (3) a hydrophobic ligand-binding domain (LBD) required for ligand binding and receptor dimerization; and, (4) a carboxy-terminal ligand-dependent transactivation sequence (also referred to as the activation function-2 (AF-2) domain) that stimulates transcription in response to ligand binding (Robinson-Rechavi M et al. 2003; Jakobsson T et al. 2012; Färnegardh M et al. 2003; Lin CY & Gustafsson JA 2015). Although both NR1H3 and NR1H2 are activated by the same ligands and are structurally similar, their tissue expression profiles are very different. NR1H3 is selectively expressed in specific tissues and cell types, such as the liver, intestine, adrenal gland, adipose tissue and macrophages, whereas NR1H2 is ubiquitously expressed (Nishimura M et al. 2004; Bookout AL et al. 2006). Upon activation NR1H2 or NR1H3 heterodimerizes with retinoid X receptors (RXR) and binds to LXR-response elements (LXREs) consisting of a direct repeat of the core sequence 5'-AGGTCA-3' separated by 4 nucleotides (DR4) in the DNA of target genes (Wiebel FF & Gustafsson JA 1997). An inverted repeat of the same consensus sequence with no spacer region(IR-0) and an inverted repeat of the same consensus sequence separated by a 1 bp spacer (IR-1) have also been shown to mediate LXR transactivation (Mak PA et al. 2002, Landrier JF et al. 2003). NR1H3 and NR1H2 have been shown to regulate gene expression via LXREs in the promoter regions of their target genes such as UDP glucuronosyltransferase 1 family, polypeptide A3 (UGT1A3) (Verreault M et al. 2006), fatty acid synthase (FAS) (Joseph SB et al. 2002a), carbohydrate response element binding protein (ChREBP, also known as MLX-interacting protein-like or MLXIPL) (Cha JY & Repa JJ 2007) and phospholipid transfer protein (PLTP) (Mak PA et al. 2002). LXREs have also been reported to be present in introns of target genes such as the ATP-binding cassette transporter G1 (ABCG1) (Sabol SL et al. 2005). NR1H3 has been shown to activate gene expression via the FXR-responsive element found in the proximal promoter of the human ileal bile acid-binding protein (FABP6) (Landrier JF et al. 2003). The NR1H2,3:RXR heterodimers are permissive, in that they can be activated by ligands for either NR1H2,3 (LXR) or RXR (Willy PJ et al. 1995).