The liver X receptors were originally cloned based upon their nucleotide sequence homology to other nuclear receptors. Originally designated as "orphan" receptors, because no naturally occurring ligands had been identified, they were later shown to bind cholesterol metabolites, termed oxysterols, which activated the receptors at physiological concentrations. Several cholesterol metabolites have been identified as being able to activate the LXRs such as 22(R)-hydroxycholesterol, 27-hydroxycholesterol, 24(S),25-epoxycholesterol, and 24(S)-hydroxycholesterol.
There are two forms of the LXRs: LXRα and LXRβ. Both isoforms are highly related sharing 78% amino acid identity in their DNA-binding and ligand-binding domains. LXRα is highly expressed in tissues known to play important roles in overall lipid metabolism such as the liver, adipose tissue, small intestine, spleen, macrophages, and kidney. LXRβ is ubiquitously expressed. The LXRs form permissive heterodimers with the RXRs and as such can regulate gene expression either upon binding LXR ligands such as oxysterols or RXR ligands such as 9-cis-retinoic acid. The LXR/RXR heterodimers bind to LRX-responsive elements (LXREs) in target DNA that consists of direct repeats (DRs) with the core sequence AGGTCA separated by 4 nucleotides (designated DR4). In the absence of ligand the LXR/RXR heterodimer resides in the nucleus bound to LXREs in a complex with transcriptional co-repressors such as silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) and nuclear receptor co-repressor (N-CoR). Upon ligand binding there is a conformational change in the complex that facilitates a co-repressor for co-activator complex exchange and transcriptional activation of target genes.
Conclusive evidence regarding the regulation of and by the LXRs was made possible by the use of LXR knock-out mice. In LXRα–/– mice there is marked cholesteryl ester accumulation when these mice are fed a cholesterol-rich diet. These initial studies resulted in the identification of cholesterol 7α-hydroxylase (CYP7A1), the rate limiting enzyme in bile acid synthesis, as the first known target of LXRs. Because the LXRs bind oxysterols they are important in the regulation of whole body cholesterol levels. The function of LXRs in the liver is to mediate cholesterol metabolism by inducing the expression of SREBP-1c. SREBP-1c is a transcription factor involved in the control of the expression of numerous genes including several involved in cholesterol synthesis. The LXRs are also involved in the process of reverse cholesterol transport which is the process by which the lipoprotein particle, HDL, carries cholesterol from the peripheral tissues to the liver. For information of the process of reverse cholesterol transport visit the Lipoproteins page. Activation of LXRs in vivo using high-affinity synthetic ligands results in increased levels of plasma HDLs and an increase in cholesterol excretion. The ability of LXRs to exert thes effects on cholesterol excretion is due to their ability to regulate the expression of members of the ATP-binding cassette (ABC) family of membrane transporters (see the Membranes page for more information on ABC transporters). The expression levels of ABCA1, ABCG5, and ABCG8 are all increased by activation of the LXRs. The ABCA1 transporter is involved in the efflux of cholesterol and phospholipids to lipid-poor lipoproteins. Deficiencies in ABCA1 are the cause of Tangier disease, a disorder characterized by an accumulation of cholesteryl esters in reticuloendothelial tissues (e.g. tonsils, lymph nodes, spleen, thymus, and bone marrow). In the liver ABCA1 promotes the formation of pre-β HDL (a lipoprotein particle composed entirely of ApoA-I and phospholipids). This latter function of ABCA1 demonstrates its important role in hepatic production of HDLs. The role of LXRs in reducing intestinal uptake of dietary cholesterol is mediated primarily via the induction of the ABCG5 and ABCG8 genes. The ABCG5 and ABCG8 proteins form an obligate heterodimer pair that function to limit plant sterol and cholesterol uptake by the gut and mediate cholesterol efflux from the liver into the bile. Mutations in either ABCG5 or ABCG8 result in a rare genetic disorder identified as sitosterolemia (also called phytosterolemia).
The LXRs also play an important role in overall cholesterol metabolism in macrophages. This role of LXRs in macrophages is clinically important as the macrophage is a central cell in the development of atherosclerotic lesions. The role of macrophages in atherosclerosis stems from their ability to phagocytose oxidized and modified lipoproteins following which they release pro-inflammatory cytokines. When macrophages are present in the atherosclerotic lesion they are able to accumulate LXR ligands such as oxysterols from the oxidized lipoproteins. Within macrophages the primary function of LXRs is to maintain cholesterol homeostasis. When macrophages consume oxidized lipoproteins the LXR genes are activated by the oxysterols present which in turn results in the activation of the ABCA1 gene. Activation of ABCA1 in macrophages allows for increased efflux of cholesterol to HDLs increasing the rate of reverse cholesterol transport. Another ABC transporter activated in macrophages by LXRs is ABCG1. ABCG1 facilitates cholesterol efflux to HDLs that do not contain apoA-I which distinguishes its activity from that of ABCA1. LXRs also activate apoE gene expression in macrophages (but not in liver) as well as apoC-I, apoC-II, and apoC-IV. There is a clear correlation to the protective role of apoE in atherogenesis so impaired activation of this gene in macrophages likely plays a significant part in the development of atherosclerotic lesions. Indeed, in mouse models of athersclerosis it has been shown that activation of LXRs results in up to a 50% reduction in the size of lesions. Further proof of the anti-atherogenic potential of LXRs has been demonstrated in mice in which there has been a macrophage-specific knock-out of the LXRs. In these mice there is a marked increase in the size of atherosclerotic lesions.
Within the macrophage environment LXRs not only influence cholesterol homeostasis but also directly impact the processes of macrophage inflammatory signaling. Within the atherosclerotic lesion the inflammatory processes of the macrophage are enhanced. LXRs have been shown to repress the expression of several pro-inflammatory genes in the macrophage. These include the chemokines, monocyte chemoattractant protein-1 (MCP-1) and MCP-2, several genes whose products are involved in the production of bioactive molecules such as iNOS and COX2, and the inflammatory cytokines IL-1β and IL-6. Both LXRα and LXRβ are known to repress these inflammatory genes. Although there are numerous in vivo models of inflammation that are known to be represses by LXR activation the precise mechanism exerted by the LXRs is unclear since none of the repressed genes has had an LXRE identified in the proximal promoter region.
In addition to their important role in cholesterol homeostasis, the LXRs are critical regulators of overall hepatic lipogenesis. As in the role of LXRs in cholesterol homeostasis, exerted via the activation of the expression of SREBP-1c, the lipogenic action of the LXRs is also due to the activation of this master regulatory transcription factor. Activation of LXRs results in increased expression of acetyl-CoA carboxylase 1 (ACC1) and fatty acid synthase (FAS) both of which are required for de novo fatty acid synthesis. An additional enzyme involved in de novo fatty acid remodeling whose expression is indiced by LXRs is steroyl-CoA desaturase (SCD). SCD is the rate-limiting enzyme catalyzing the synthesis of monounsaturated fatty acids, primarily oleate (18:1) and palmitoleate (16:1). These two monounsaturated fatty acids represent the majority of monounsaturated fatty acids present in membrane phospholipids, triglycerides, and cholesterol esters. That LXR expression is critical to the regulation of these three enzymes of fatty acid synthesis is apparent from studies in the LXRα–/– mice where the reduction in SREBP-1c expression correlates to reduced expression of ACC1, FAS, and SCD. LXRs also induce the expression of genes involved in the remodeling of lipoprotein particles such as lipoprotein lipase, cholesterol ester transfer protein (CETP) and phospholipid transfer protein (PLTP).
The LXRs have also been shown to be important in the regulation of hepatic glucose metabolism. Activation of LXRs results in reduced expression of the hepatic gluconeogenesis genes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase. In contrast the hepatic glucokinase gene is induced by LXRs. The net effect of theses changes in hepatic glucose metabolism genes will be increased glucose utilization and reduced glucose output by the liver. The LXRβ gene has been shown to enhance glucose-dependent insulin secretion by the pancreas and in LXRβ–/– mice this process is impaired. The effects of LXR activation on adipose tissue glucose utilization has also been demonstrated by the presence of an LXRE in the insulin-dependent GLUT4 gene promoter.
Given that LXRs are important in the regulation of reverse cholesterol transport, exert an anti-inflammatory influence, and improve glucose tolerance suggests that they would be ideal targets for therapeutic purposes. However, the first generation synthetic LXR ligands activate both LXRs and lead to marked increases in hepatic lipogenesis and plasma triglyceride levels. These effects are due to the role of LXRs in activation of hepatic SREBP-1c and the resultant activation of each of its target genes as described above. Although it could be theoretically possible to enhance the reverse cholesterol effects of LXRs without targeting hepatic lipogenesis with the use of LXRβ-specific ligands since most of the hepatic responses are due to activation of LXRα, this will be a difficult challenge as the ligand binding pocket in both isoforms has been shown to be nearly identical. In addition, there are species-specific differences in overall LXR responses that need to be carefully considered meaning the use of animal models that more closely resemble humans in their metabolic pathways.