Apolipoprotein E (APOE), a 34-kD glycoprotein, is involved in lipoprotein clearance by serving as a ligand for the low-density lipoprotein (LDL) receptor family. APOE is primarily lipidated via the ATP-binding cassette transporter A1 (ABCA1), and both are under transcriptional regulation by the liver X receptor α (LXRα or NR1H3) and LXRβ (NR1H2) (Laffitte BA et al. 2001; Beyea MM et al. 2007). The ligand-activated NR1H2 and NR1H3, whose natural ligands are oxysterols, function as obligate heterodimers with retinoid X receptor (RXR) to regulate the expression of target genes through binding to LXR response elements (LXREs) within the regulatory region of their target genes. Both NR1H2:RXRα and NR1H3 :RXRα heterodimers were reported to regulate APOE transcription directly through interaction with conserved LXREs found within tissue-specific enhancer regions (multienhancers ME.1 and ME.2) that confer APOE expression in adipose tissue and macrophages (Shih SJ et al. 2000; Laffitte BA et al. 2001). A low-affinity LXRE was also found in the promoter region of the APOE gene (Laffitte BA et al. 2001). Further, oxysterol-binding protein related protein 1S (ORP1S) was shown to associate with NR1H2 and NR1H3 in the nucleus (Lee S et al. 2012). ORP1S promoted the binding of the receptors to LXREs and specifically enhanced NR1H2,3-dependent transcription of APOE via the ME.1 and ME.2 of the APOE gene (Lee S et al. 2012).

When the low-density lipoprotein receptor (LDLR) is missing, saturated or inhibited, chylomicron remnants (CRs) containing all-trans-retinyl esters (atREs) bind apolioprotein E (apoE). ApoE, secreted by hepatocytes, acts as a high-affinity ligand for the LDL-related receptor protein (LRP) family. CR:atREs:apoE then binds to cell-surface heparan sulfate proteoglycan (HSPG), abundant in the space of Disse. HSPG/apoE binding plays a critical role in the capture of CR:atREs, ready for internalization via LRPs (Futamura et al. 2005, Yamauchi et al. 2008).

The intracellular fragment of ERBB4, ERBB4s80 (E4ICD) binds to the promoter region of the APOE gene, about 2 kb upstream from the transcription start site. The ERBB4s80 binding site overlaps with HNF4 and ETS1 binding sites (Wali et al. 2014).

The apolipoprotein E (APOE) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. APOE, a 34-kD glycoprotein, is involved in lipoprotein clearance by serving as a ligand for the low-density lipoprotein (LDL) receptor family. APOE is primarily lipidated via the ATP-binding cassette transporter 1 (ABCA1), and both are under transcriptional regulation by the liver X receptor α (LXRα or NR1H3) and LXRβ (NR1H2) whose natural ligands are oxysterols such as 24(S),25-epoxycholesterol (24(S),25-epoxy) (Laffitte BA et al. 2001; Beyea MM et al. 2007). The endogenous and synthetic agonists of NR1H2 or NR1H3 increased expression of APOE in human and murine macrophages, and murine adipocytes but not in liver (Laffitte BA et al. 2001; Mak PA et al 2002; Beyea MM et al. 2006). This tissue-specific regulation is conferred by the presence of LXR response elements (LXREs) in multienhancer regions ME.1 and ME.2 downstream of the APOE gene that are revealed only in adipose tissue and macrophages (Shih SJ et al. 2000). In addition, ligand-activated NR1H2 and NR1H3 lead to a dramatic increase in APOE mRNA and protein expression as well as secretion of APOE in a human astrocytoma cell line (CCF-STTG1 cells) to impact cholesterol efflux (Liang Y et al. 2004; Abildayeva K et al. 2006). In the central nervous system, APOE is considered a major apoprotein acceptor for the efflux of cholesterol in the formation of high-density lipoprotein (HDL)-like particles necessary for intercellular lipid trafficking, and is implicated in various neurodegenerative diseases, such as Alzheimer’s (reviewed in Hirsch-Reinshagen V & Wellington CL 2007).

Transcription of the APOE gene, encoding Apolipoprotein E, involved in binding and internalization of lipoprotein particles, is stimulated by the intracellular fragment of ERBB4, ERBB4s80 (E4ICD) (Wali et al. 2014).

DEK is recruited to the APOE gene promoter via its interaction with the TFAP2A (AP-2 alpha) homodimer. In the presence of DEK, TFAP2A associates with the APOE promoter more tightly (Campillos et al. 2003). Binding of TFAP2A to the APOE gene promoter may be stimulated by PKA-mediated phosphorylation of TFAP2A (Garcia et al. 1999).

In the body, this binding involved apoE synthesized by hepatocytes and concentrated in the space of Disse, an extracellular compartment adjacent to the hepatocytes to which blood-borne lipoprotein particles have free access (Ji et al. 1994).

The complex of TFAP2A homodimer and DEK stimulates transcription of the APOE gene (Campillos et al. 2003).

Copper chaperone of superoxide dismutase (CCS) transfers a copper(I) atom to a SOD1 monomer that already contains a Zn atom (Culotta et al. 1997, Casareno et al. 1998, Rae et al. 2001, Brown et al. 2004, Banci et al. 2012). The reaction proceeds by a two step mechanism in which SOD1 first forms heterodimers with CCS (Rae et al. 2001, Banci et al. 2012).

As inferred from the cytosolic reaction and from the mouse mitochondrial reaction, Copper chaperone of superoxide dismutase (CCS) transfers a copper(I) atom to a SOD1 monomer that already contains a Zn atom. The reaction proceeds by a two step mechanism in which SOD1 first forms heterodimers with CCS. The amounts of CCS and SOD1 in the intermembrane space appear to be regulated by the concentration of oxygen. Mutations in SOD1 are responsible for familial amyotrophic lateral sclerosis (fALS) and cause unregulated localization and aggregation of SOD1 in the intermembrane space (reviewed in Kawamata and Manfredi 2010).

The phospholipid transfer protein (PLTP) gene is transcribed to yield mRNA and the mRNA is translated to yield protein. PLTP is implicated in cholesterol and phospholipid transfer from triglyceride-rich lipoproteins to HDL during lipolysis by lipoprotein lipase, and in HDL remodeling (formation of β-HDL and large HDL) (Albers JJ et al. 2012; Jiang XC 2018). Beyond its impact on lipoprotein metabolism, PLTP has been reported to modulate inflammation and immune responses (Audo R et al. 2018). PLTP is expressed ubiquitously (Day JR et al. 1994). The highest expression levels in human tissues were observed in ovary, thymus, placenta, and lung (Day JR et al. 1994). Taking into account the organ size involved, liver and small intestine appear to be important sites of PLTP expression (Day JR et al. 1994; Jiang XC et al. 2012). It was also shown that PLTP is highly expressed in macrophages and in atherosclerotic lesions suggesting a potential role for this enzyme in lipid-loaded macrophages (Desrumaux CM et al. 2003; O'Brien KD et al. 2003; Laffitte BA et al. 2003; Vikstedt R et al. 2007). PLTP produced by macrophages may contribute to the optimal function of the ABCA1-mediated cholesterol efflux from macrophages to HDL (Oram JF et al. 2003; Lee-Rueckert M et al. 2006). PLTP is a direct target gene of liver X receptors (LXRα (NR1H3) and LXRβ (NR1H2)) which form functional heterodimers with the retinoid X receptor (RXR) (Laffitte BA et al. 2003; Mak PA et al. 2002; Cao G et al. 2002). NR1H2 & NR1H3 act as cellular sensors of sterol levels and are transcriptionally activated by oxidized forms of cholesterol, oxysterols (Janowski BA et al. 1996). LXR agonists induced the expression of PLTP in various tissues of mice treated with synthetic LXR agonists, T0901317 or GW3965, in a coordinate manner with known LXR target genes (Cao G et al. 2002; Laffitte BA et al. 2003). PLTP expression was also highly induced by LXR (NR1H2 and NR1H3) and retinoid X receptor (RXR) agonists in murine peritoneal and human THP-1 macrophages (Mak PA et al. 2002; Laffitte BA et al. 2003). Regulation of PLTP by NR1H2 or NR1H3 ligands was abolished in animals or cells lacking both NR1H3 (LXRα) and NR1H2 (LXRβ) (Laffitte BA et al. 2003). Further, administration of the synthetic NR1H2, NR1H3 ligand T0901317 to mice elevated HDL cholesterol and phospholipid and generated enlarged HDL particles that were enriched in cholesterol, ApoAI, ApoE, and phospholipid (Cao G et al. 2002). This occured alongside with the increased plasma PLTP activity and liver PLTP mRNA levels (Cao G et al. 2002). Similar findings were reported for the regulation of PLTP levels in vivo by another synthetic NR1H2, NR1H3 ligand GW3965 (Laffitte BA et al. 2003). These data suggest that NR1H2, NR1H3 and their ligands may modulate plasma lipoprotein metabolism through control of PLTP activity (Laffitte BA et al. 2003; Mak PA et al. 2002; Cao G et al. 2002).

The gene expression of PLTP can be also regulated by other members of the nuclear receptor family of transcription factors: farnesoid X receptor (FXR), and peroxisome proliferator-activated receptor α (PRARα ) (Laffitte BA et al. 2003; Mak PA et al. 2002; Tu AY & Albers JJ 2001).

Phospholipid transfer protein (PLTP) gene expression can be transcriptionally activated by liver X receptors (LXRα (NR1H3) and LXRβ (NR1H2)) that belong to the nuclear receptor superfamily (Mak PA et al. 2002; Cao G et al. 2002; Laffitte BA et al. 2003). NR1H2 and NR1H3 act as cellular sensors of sterol levels and are transcriptionally activated by oxidized forms of cholesterol, oxysterols (Janowski BA et al. 1996). Synthetic LXR agonists, T0901317 or GW3965, induced the expression of PLTP in various tissues of mice in a coordinate manner with known LXR target genes (Cao G et al. 2002; Laffitte BA et al. 2003). PLTP expression was also highly induced by LXR (NR1H2 and NR1H3) and retinoid X receptor (RXR) agonists in murine peritoneal and human THP-1 macrophages (Mak PA et al. 2002; Laffitte BA et al. 2003). The ability of synthetic and oxysterol ligands to regulate PLTP mRNA in macrophages and liver was lost in animals lacking both NR1H3 (LXRα) and NR1H2 (LXRβ), confirming the critical role of these receptors (Laffitte BA et al. 2003). Once activated, NR1H2 or 3 recognize an LXR response element (LXRE) sequence containing a variant direct-repeat-4 (DR4) motif in the promoter regions of target genes. The PLTP promoter contains a high-affinity LXRE that was found to bind NR1H3:RXR heterodimers in vitro, and was activated by NR1H3:RXR in transient-transfection studies (Mak PA et al. 2002; Laffitte BA et al. 2003).

In addition to NR1H2 or NR1H3, PLTP expression is regulated by other nuclear receptor RXR heterodimers, including peroxisome proliferator-activated receptor α (PPARα):RXR and farnesoid X receptor (FXR):RXR (Mak PA et al. 2002; Tu AY & Albers JJ 2001).

Reelin (RELN) (DeSilva et al. 1997) is an extracellular matrix serine protease highly expressed in the brain that plays a role in neural cell positioning during brain development by regulating their microtubule function and neuronal migration. It can bind to the extracellular domains of lipoprotein receptors VLDLR and APOER2 to induce tyrosine phosphorylation of disabled-1 (DAB1), an adaptor protein bound to the cytoplasmic tails of these receptors that is a phosphorylation target for a signaling cascade trigggered by RELN. This suggests VLDLR and APOER2 participate in transmitting the extracellular RELN signal to intracellular signaling processes initiated by DAB1 (Hiesberger et al. 1999). In post-mortem studies of schizophrenia sufferers, RELN was found to be significantly reduced (by up to 50%) in brain samples whereas DAB1 was observed to be at normal levels, suggesting a role for RELN in schizophrenia (Impagnatiello et al. 1998).

The ATP-binding cassette sub-family A member 1 (ABCA1) mRNA is translated to yield ABCA1 protein.

Synthetic agonists of liver X-receptors (LXRα, NR1H3 and LXRβ, NR1H2) or cholesterol-loading significantly induced the expression of ABCA1 protein in mouse RAW 264.7 and human THP-1 macrophage cell lines (Beyea MM et al. 2007; Ku CS et al. 2012). Similar regulation of ABCA1 protein expression by NR1H2, 3 agonists was observed in human peripheral blood-derived monocytes (Mauerer R et al. 2009). Treatment of THP-1 macrophages with endogenous (25-hydroxycholesterol) or synthetic (T0901317) ligands of NR1H2,3 stimulated both transcriptional and posttranscriptional pathways to enhance ABCA1 expression (Ignatova ID et al. 2013). Further, partial inhibition of oxidosqualene:lanosterol cyclase (OSC) stimulated synthesis of the NR1H2,3 agonist 24(S),25-epoxycholesterol (24(S),25-epoxy) and enhanced ABCA1-mediated cholesterol efflux in THP-1 cells (Beyea MM et al. 2007). NR1H3 and NR1H2-induced expression of ABCA1 is thought to promote cellular cholesterol transfer to lipid-poor apolipoproteins such as ApoA1 and ApoE in the formation of nascent HDL particles (Ignatova ID et al. 2013; Vedhachalam C et al. 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).

MicroRNAs miR-26 and miR-33 negatively regulate the translation of ABCA1 mRNA and thus repress the NR1H2, NR1H3-dependent cholesterol efflux from macrophages (Sun D et al. 2012; Rayner KJ et al. 2010). MicroRNA miR-144 also binds the ABCA1 3’UTR to prohibit translation and reduce ABCA1-mediated cholesterol efflux from hepatocytes (de Aguiar Vallim TQ et al. 2013)

SCARB1 (SR-BI) binds low density lipoprotein (LDL), acetylated LDL, oxidized LDL, high density lipoprotein (HDL) (Calvo et al. 1997, Murao et al. 1997, Rhainds et al. 1999, inferred from hamster in Acton et al. 1994). SCARB1 binds HDL via its protein moiety, including apolipoproteins A-I, A-II, CII, CIII and E (Bultel-Brienne et al. 2002, inferred from mouse in Xu, Laccotripe et al. 1997, Li et al. 2002). SCARB1 also binds serum amyloid A protein (Baranova et al. 2005), and lipopolysaccharide (LPS) (Vishnyakova et al. 2003). SCARB1 is expressed on the extracellular face of the plasma membrane of several types of polarized epithelial cells.

When the low-density lipoprotein receptor (LDLR) is missing, saturated or inhibited, chylomicron remnants (CRs) containing all-trans-retinyl esters (atREs) can be cleared from circulation by interaction with cell-surface heparan sulfate proteoglycan (HSPG) and secreted apolipoprotein E (apoE). This complex is then presented to LDL receptor-related proteins (LRPs; reviews May et al. 2007, Li et al. 2001, Hussain 2001) for internalization (Ji et al. 1993).

Spherical HDL particles can bind apoC-II, apoC-III and and apoE proteins. The sources of these proteins and their role or roles in HDL function under physiological conditions are not well understood, however (Kontush and Chapman 2006).

LPR8 (apoER2) is the platelet low density lipoprotein (LDL) receptor. Mice lacking ApoE develop hypercholesterolemia and later atherosclerosis (Zhang et al. 1992). Similiar results are seen in familial hypercholesterolemia, where defective apoB/E receptors fail to remove LDL from the circulation.

The APOC1 gene is transcribed to yield mRNA and the mRNA is translated to yield protein.

Ligand-activated liver X receptors (LXRα, NR1H3 and LXRβ NR1H2) induce expression of a cluster of apolipoprotein genes APOE, APOC1, APOC2 and APOC4 in both human and mouse macrophages (Mak PA et al. 2002). Induction of APOC2 mRNA was attenuated or abolished in macrophages derived from LXR α/β-/- mice (Mak PA et al. 2002).

Apolipoprotein C4 (APOC4) is present in the APOE, APOC1, APOC4 and APOC2-gene cluster which is induced by natural and synthetic ligands of liver X receptors (LXRα, NR1H3 and LXRβ, NR1H2) in both human and mouse macrophages (Mak PA et al. 2002). The induction of all four mRNAs was greatly attenuated in macrophages derived from LXR α/β-/- mice (Mak PA et al. 2002). Cell reporter assays suggest that the LXR response elements (LXRE) in the multienhancer regions ME.1 and ME.2, which confer tissue-specific expression in macrophages and adipocytes (Shih SJ et al. 2000), are necessary for the expression of this gene cluster (Mak PA et al. 2002). These secreted apolipoproteins regulate lipid transport and catabolism.

Ligand-activated liver X receptors (LXRα, NR1H3 and LXRβ NR1H2) induce expression of a cluster of apolipoprotein genes APOE, APOC1, APOC2 and APOC4 in both human and mouse macrophages (Mak PA et al. 2002). Induction was attenuated or abolished in macrophages derived from LXR α/β-/- mice. Studies with reporter genes suggest that the LXR response element (LXRE) in the distal multienhancer regions ME.1 and ME.2 are necessary for the expression of this gene cluster (Mak PA et al. 2002). These secreted apolipoproteins regulate lipid transport and catabolism. In particular, APOC1 has been suggested to serve as an inhibitor of cholesteryl ester transfer protein (CETP) activity to impact cholesterol distribution among lipoprotein particles (Gautier T et al. 2000).

The APOC4 gene is transcribed to yield mRNA and the mRNA is translated to yield protein.

Ligand-activated liver X receptors (LXRα, NR1H3 and LXRβ NR1H2) induce expression of a cluster of apolipoprotein genes APOE, APOC1, APOC2 and APOC4 in both human and mouse macrophages (Mak PA et al. 2002). The induction of all four mRNAs was greatly attenuated in peritoneal macrophages derived from LXR α/β-/- mice (Mak PA et al. 2002). Cel reporter assays suggest that the LXR response elements (LXRE) in the multienhancer regions ME.1 and ME.2, which confer tissue-specific expression in macrophages and adipocytes (Shih SJ et al. 2000), are necessary for the expression of this gene cluster (Mak PA et al. 2002). These secreted apolipoproteins regulate lipid transport and catabolism.

The APOC2 gene is transcribed to yield mRNA and the mRNA is translated to yield protein.

Ligand-activated liver X receptors (LXRα, NR1H3 and LXRβ NR1H2) induce expression of a cluster of apolipoprotein genes APOE, APOC1, APOC2 and APOC4 in both human and mouse macrophages (Mak PA et al. 2002). The induction of all four mRNAs was greatly attenuated in peritoneal macrophages derived from LXR α/β-/- mice (Mak PA et al. 2002). Cel reporter assays suggest that the LXR response elements (LXRE) in the multienhancer regions ME.1 and ME.2, which confer tissue-specific expression in macrophages and adipocytes (Shih SJ et al. 2000), are necessary for the expression of this gene cluster (Mak PA et al. 2002). These secreted apolipoproteins regulate lipid transport and catabolism.

Chylomicron remnants (CRs) are "sieved" when they arrive at the liver by size, the appropriate sized remnants passing through the space of Disse. Once inside, CRs containing all-trans-retinyl esters (atREs) can be directly and rapidly taken up by liver parenchymal cells via the low-density lipoprotein receptor (LDLR) using apolipoprotein E (apoE) as a ligand. Internalization of remnants occur via endocytosis (see review D'Ambrosio et al. 2011). This reaction is inferred from uptake studies in mice (Yu et al. 2000). Defects in LDLR cause familial hypercholesterolemia (FH, MIM:143890), a common autosomal disease that affects about 1 in 500 people in most countries. Abnormal LDLR doesn't remove LDL from circulation resulting in high levels of LDL in blood, leading to early cardiovascular disease via atherosclerosis. The defect was first described by Brown and Goldstein (1974).

Natural and synthetic ligands of liver X receptors (LXRα, NR1H3 and LXRβ, NR1H2) induced expression of a cluster of apolipoprotein genes APOE, APOC1, APOC2 and APOC4 in both human and mouse macrophages (Mak PA et al. 2002). The induction of all four mRNAs was greatly attenuated in peritoneal macrophages derived from LXR α/β-/- mice (Mak PA et al. 2002). Cell reporter assays suggest that the LXR response elements (LXRE) in the multienhancer regions ME.1 and ME.2, which confer tissue-specific expression in macrophages and adipocytes (Shih SJ et al. 2000), are necessary for the expression of this gene cluster (Mak PA et al. 2002). These secreted apolipoproteins regulate lipid transport and catabolism. APOC2 is recognized as an activator of lipoprotein lipase (reviewed in Wolska A et al. 2017). Thus the genetic deficiency of APOC2 results in a phenotype that resembles lipoprotein lipase deficiency, and is aptly called hyperlipoproteinemia type IB. Individuals lacking APOC2 exhibit hyperchylomicronemia and hypertriglyceridemia (reviewed in Wolska A et al. 2017).

Lipoprotein lipase dimers (LPL:LPL) are tethered to heparan sulfate proteoglycans (HSPG) at endothelial cell surfaces (Fernandez-Borja et al. 1996; Peterson et al. 1992). Both syndecan 1 (Rosenberg et al. 1997) and perlecan (Goldberg 1996) HSPG molecules are capable of tethering LPL. The LPL enzyme catalyzes the hydrolysis and release of triacylglycerols (TG) associated with circulating chylomicrons to leave a CM remnant (CR). This reaction is annotated here as causing the hydrolysis and release of 50 molecules of TG. In vivo, the number is much larger, and TG depletion probably occurs in the course of multiple encounters between a chylomicron and endothelial LPL. This reaction is strongly activated by chylomicron-associated apo C-II protein both in vivo and in vitro (Jackson et al. 1986). Chylomicron-associated apoC-II protein inhibits LPL activity in vitro (Brown and Baginsky 1972), and recent studies have indicated a positive regulatory role for apoA-5 protein, though its molecular mechanism of action remains unclear (Marcais et al. 2005; Merkel and Heeren 2005). CRs can then be taken up by liver parenchymal cells in two ways; 1) directly by the LDL receptor or 2) apoE/HSPG-directed uptake by LDL receptor-related proteins.

Lipoprotein lipase dimers (LPL:LPL) are tethered to heparan sulfate proteoglycans (HSPG) at endothelial cell surfaces (Fernandez-Borja et al. 1996; Peterson et al. 1992). Both syndecan 1 (Rosenberg et al. 1997) and perlecan (Goldberg 1996) HSPG molecules are capable of tethering LPL. The LPL enzyme catalyzes the hydrolysis and release of triacylglycerols (TG) associated with circulating chylomicrons to leave a CM remnant (CR). This reaction is annotated here as causing the hydrolysis and release of 50 molecules of TG. In vivo, the number is much larger, and TG depletion probably occurs in the course of multiple encounters between a chylomicron and endothelial LPL. This reaction is strongly activated by chylomicron-associated apo C-II protein both in vivo and in vitro (Jackson et al. 1986). Chylomicron-associated apoC-II protein inhibits LPL activity in vitro (Brown and Baginsky 1972), and recent studies have indicated a positive regulatory role for apoA-5 protein, though its molecular mechanism of action remains unclear (Marcais et al. 2005; Merkel and Heeren 2005). CRs can then be taken up by liver parenchymal cells in two ways; 1) directly by the LDL receptor or 2) apoE/HSPG-directed uptake by LDL receptor-related proteins.

In addition to the main fibril peptide, mature amyloid fibrils have additional components. Serum amyloid P component (SAP) binds to all types of amyloid fibrils and is a universal constituent of amyloid deposits. SAP binding protects amyloid fibrils from proteolytic degradation (Tennent et al. 1995, Westermark 2005). SAP may function as a chaperone for amyloid formation (Coker et al. 2000).

Glycosaminoglycans (GAGs) and proteoglycans are found associated with all types of amyloid deposits (Alexandrescu 2005). Of the different types of GAG heparan sulfate and dermatan sulfate are the most prominent in amyloid deposits (Hirschfield & Hawkins, 2003). GAGs have been implicated in the nucleation of fibrils, they can also stabilize mature fibrils against dissociation (Yamaguchi et al. 2003) and proteolytic degradation (Gupta-Bansal et al. 1995).

Perlecan coimmunolocalizes with all types of amyloids (Snow & Wright 1989), accelerating fibril formation (Castillo et al. 1998), stabilizing them once formed (Castillo et al. 1997), and protecting them from proteolytic degradation (Gupta-Bansal et al. 1995).

APOE isoform 4 binds tightly to soluble ABeta peptide forming complexes that resist dissociation; it also binds to ABeta in its fibril form (Bales et al. 2002).

Very low-density lipoproteins (VLDLs) are produced in the liver to transport endogenous triglycerides, phospholipids, cholesterol, and cholesteryl esters in the hydrophilic environment of the bloodstream. They comprise triglycerides (50-60%), cholesterol (10-12%), cholesterol esters (4-6%), phospholipids (18-20%), and apolipoprotein B (8-12%). Of the protein content, two other apolipoproteins are constituents; apolipoprotein C-I (APOC around 20%) (Westerterp et al. 2007) and apolipoprotein C4 (APOC4, minor amount) (Kotite et al. 2003). After release from the liver, circulating VLDL particles can bind very low-density lipoprotein receptors (VLDLR) (Sakai et al. 1994) on extra-hepatic target cells and undergo endocytosis (Go & Mani 2012). VLDL uptake by VLDLR represents a minor contribution towards VLDL metabolism. The majority of VLDL is catalysed by lipoprotein lipase (LPL) which hydrolyses TAGs from VLDL, converting it to intermediate-density lipoprotein (IDL). IDL can be further hydrolysed by hepatic lipase to cholesterol-rich low-density lipoprotein (LDL).

VLDLR consists of five functional domains that resemble the LDL receptor. It is highly expressed in tissues that actively metabolise fatty acids as a source of energy. Binding of VLDLs to VLDLR appears to be inhibited by apolipoprotein C-I (APOC1), therby slowing the clearance of triglyceride-rich lipoproteins from the circulation (Westerterp et al. 2007). The APOE/C1/C4/C2 gene cluster is closely associated with plasma lipid levels, atherosclerotic plaque formation, and thereby implicated in the development of coronary artery disease and Alzheimer’s disease (Xu et al. 2015).

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).