Recent studies have identified the liver X receptors (LXR␣ and LXR␤) as important regulators of cholesterol metabolism and transport. LXRs control transcription of genes critical to a range of biological functions including regulation of high density lipoprotein cholesterol metabolism, hepatic cholesterol catabolism, and intestinal sterol absorption. Although LXR activity has been proposed to be critical for physiologic lipid metabolism and transport, direct evidence linking LXR signaling pathways to the pathogenesis of cardiovascular disease has yet to be established. In this study bone marrow transplantations were used to selectively eliminate macrophage LXR expression in the context of murine models of atherosclerosis. Our results demonstrate that LXRs are endogenous inhibitors of atherogenesis. Additionally, elimination of LXR activity in bone marrow-derived cells mimics many aspects of Tangier disease, a human high density lipoprotein deficiency, including aberrant regulation of cholesterol transporter expression, lipid accumulation in macrophages, splenomegaly, and increased atherosclerosis. These results identify LXRs as targets for intervention in cardiovascular disease.

Objective-Complications of atherosclerotic cardiovascular disease due to elevated blood cholesterol levels are the major cause of death in the Western world. The liver X receptors, LXR␣ and LXR␤ (LXRs), are ligand-dependent transcription factors that act as cholesterol sensors and coordinately control transcription of genes involved in cholesterol and lipid homeostasis as well as macrophage inflammatory gene expression. LXRs regulate cholesterol balance through activation of ATP-binding cassette transporters that promote cholesterol transport and excretion from the liver, intestine, and macrophage. Although LXR agonists are known to delay progression of atherosclerosis in mouse models, their ability to abrogate preexisting cardiovascular disease by inducing regression and stabilization of established atherosclerotic lesions has not been addressed. Methods and Results-We demonstrate that LXR agonist treatment increases ATP-binding cassette transporter expression within preexisting atherosclerotic lesions, resulting in regression of these lesions as well as remodeling from vulnerable to stable lesions and a reduction in macrophage content. Further, using macrophage-selective LXR-deficient mice created by bone marrow transplantation, we provide the first evidence that macrophage LXR expression is necessary for the atheroprotective actions of an LXR agonist. Key Words: atherosclerosis Ⅲ LXRs Ⅲ nuclear receptors Ⅲ reverse cholesterol transport E levated plasma non-high-density lipoprotein cholesterol (non-HDL cholesterol) levels, a major risk factor in the development of cardiovascular disease, increase macrophage cholesterol accumulation and foam cell formation in the underlying epithelium of large arteries. Progressive recruitment of macrophages to this site, combined with their uptake of oxidized low-density lipoprotein (LDL) particles, leads to the development of atherosclerotic lesions. In addition to lipid loading, progression of early lesions to complex lesions (plaques) is a chronic inflammatory process in which monocyte-derived macrophages play a key role. 1 The liver X receptors LXR␣ and LXR␤ (LXRs) are members of the nuclear receptor family that control transcription of genes that coordinately regulate cholesterol transport and lipid metabolism in 3 major sites (macrophage, liver, and intestine) and play a critical role in the maintenance of cholesterol homeostasis. 2 Recently, we demonstrated that reconstitution of macrophages from mice deficient in LXR␣ and ␤ (LXR␣␤ Conclusions-These See page 10LXRs modify the atherogenic program in the macrophage through regulation of genes involved in both inflammation and cholesterol elimination pathways. LXR agonists negatively regulate macrophage inflammatory gene expression, 4 and recent data indicate that pathogens that contribute to the initiation and progression of atherosclerosis interfere with macrophage cholesterol metabolism by inhibition of the LXR signaling pathway. 5 Further, LXRs directly regulate the expression of ATP-binding cassette transporters ABCA...

Liver X receptors (LXRs) regulate the expression of genes involved in cholesterol and fatty acid homeostasis, including the genes for ATP-binding cassette transporter A1 (ABCA1) and sterol response element binding protein 1 (SREBP1). Loss of LXR leads to derepression of the ABCA1 gene in macrophages and the intestine, while the SREBP1c gene remains transcriptionally silent. Here we report that high-density-lipoprotein (HDL) cholesterol levels are increased in LXR-deficient mice, suggesting that derepression of ABCA1 and possibly other LXR target genes in selected tissues is sufficient to result in enhanced HDL biogenesis at the whole-body level. We provide several independent lines of evidence indicating that the repressive actions of LXRs are dependent on interactions with the nuclear receptor corepressor (NCoR) and the silencing mediator of retinoic acid and thyroid hormone receptors (SMRT). While dissociation of NCoR and SMRT results in derepression of the ABCA1 gene in macrophages, it is not sufficient for derepression of the SREBP1c gene. These findings reveal differential requirements for corepressors in the regulation of genes involved in cholesterol and fatty acid homeostasis and raise the possibility that these interactions may be exploited to develop synthetic ligands that selectively modulate LXR actions in vivo.

This article is available online at http://www.jlr.org early event in the development of atherosclerosis is the recruitment of macrophages to the subendothelial space of vessel walls and the uncontrolled uptake of oxidized or aggregated low density lipoprotein particles. Continued accumulation of oxidized or aggregated LDL by macrophages and an associated infl ammatory response leads to foam cell formation and the initiation of atherosclerosis ( 2 ).The liver X receptors LXR ␣ (NR1H3) and LXR ␤ (NR1H2), members of the nuclear hormone receptor superfamily of transcription factors, have been identifi ed as important regulators of cholesterol homeostasis in multiple cell types, including macrophages ( 3 ). Treatment of cells with oxysterols, natural LXR ligands derived from cholesterol, or synthetic LXR agonists promotes the effl ux of cholesterol by increasing expression of the ATP binding cassette transporters ABCA1 and ABCG1 and the apolipoprotein E (apoE) ( 4, 5 ). ABCA1, ABCG1, and apoE all participate in the transfer of intracellular and plasma membrane cholesterol to HDL, a process termed reverse cholesterol transport ( 6 ). Importantly, LXR agonists reduce atherosclerosis in animal models of cardiovascular disease, and upregulation of ABC transporters can be detected in the atherosclerotic lesions of treated animals ( 7,8 ). The ability of LXRs to inhibit pro-infl ammatory pathways may also contribute to the anti-atherogenic activities of these receptors ( 9 ). Previous work from our laboratory has demonstrated that LXR function in hema- The contribution of elevated cholesterol levels to the development of cardiovascular disease and atherosclerosis is well documented. Nevertheless the molecular signaling pathways that regulate cholesterol homeostasis at the blood vessel wall, particularly in response to elevated cholesterol levels, remain to be fully deciphered ( 1 ). A critical Abbreviations: ABCA1, ATP binding cassette transporter A1; ABCG1, ATP binding cassette transporter G1; apoE, apolipoprotein E; FPLC, fast-protein liquid chromatography; iNOS, inducible nitric oxide synthase; LXR, liver X receptor; MCP-1, monocycte chemotactic protein 1; NF ␤ , nuclear factor kappa ␤ ; SREBP, sterol-regulatory element binding protein; TNF-␣ , tumor necrosis factor-␣ .