Sean B. Joseph

LXR alpha is a nuclear receptor that has previously been shown to regulate the metabolic conversion of cholesterol to bile acids. Here we define a role for this transcription factor in the control of cellular cholesterol efflux. We demonstrate that retroviral expression of LXR alpha in NIH 3T3 fibroblasts or RAW264.7 macrophages and/or treatment of these cells with oxysterol ligands of LXR results in 7- to 30-fold induction of the mRNA encoding the putative cholesterol/phospholipid transporter ATP-binding cassette (ABC)A1. In contrast, induction of ABCA1 mRNA in response to oxysterols is attenuated in cells that constitutively express dominant-negative forms of LXR alpha or LXR beta that lack the AF2 transcriptional activation domain. We further demonstrate that expression of LXR alpha in NIH 3T3 fibroblasts and/or treatment of these cells with oxysterols is sufficient to stimulate cholesterol efflux to extracellular apolipoprotein AI. The ability of oxysterol ligands of LXR to stimulate efflux is dramatically reduced in Tangier fibroblasts, which carry a loss of function mutation in the ABCA1 gene. Taken together, these results indicate that cellular cholesterol efflux is controlled, at least in part, at the level of transcription by a nuclear receptor-signaling pathway. They suggest a model in which activation of LXRs by oxysterols in response to cellular sterol loading leads to induction of the ABCA1 transporter and the stimulation of lipid efflux to extracellular acceptors. These findings have important implications for our understanding of mammalian cholesterol homeostasis and suggest new opportunities for pharmacological regulation of cellular lipid metabolism.

The nuclear receptors LXRalpha and LXRbeta have been implicated in the control of cholesterol and fatty acid metabolism in multiple cell types. Activation of these receptors stimulates cholesterol efflux in macrophages, promotes bile acid synthesis in liver, and inhibits intestinal cholesterol absorption, actions that would collectively be expected to reduce atherosclerotic risk. However, synthetic LXR ligands have also been shown to induce lipogenesis and hypertriglyceridemia in mice, raising questions as to the net effects of these compounds on the development of cardiovascular disease. We demonstrate here that the nonsteroidal LXR agonist GW3965 has potent antiatherogenic activity in two different murine models. In LDLR(-/-) mice, GW3965 reduced lesion area by 53% in males and 34% in females. A similar reduction of 47% was observed in male apoE(-/-) mice. Long-term (12-week) treatment with LXR agonist had differential effects on plasma lipid profiles in LDLR(-/-) and apoE(-/-) mice. GW3965 induced expression of ATP-binding cassettes A1 and G1 in modified low-density lipoprotein-loaded macrophages in vitro as well as in the aortas of hyperlipidemic mice, suggesting that direct actions of LXR ligands on vascular gene expression are likely to contribute to their antiatherogenic effects. These observations provide direct evidence for an atheroprotective effect of LXR agonists and support their further evaluation as potential modulators of human cardiovascular disease.

The nuclear receptors LXRalpha and LXRbeta have been implicated in the control of lipogenesis and cholesterol homeostasis. Ligand activation of these receptors in vivo induces expression of the LXR target gene SREBP-1c and increases plasma triglyceride levels. Expression of fatty acid synthase (FAS), a central enzyme in de novo lipogenesis and an established target of the SREBP-1 pathway, is also induced by LXR ligands. The effects of LXR ligands on FAS expression have been proposed to be entirely secondary to the induction of SREBP-1c. We demonstrate here that LXRs regulate FAS expression through direct interaction with the FAS promoter as well as through activation of SREBP-1c expression. Induction of FAS expression in HepG2 cells by LXR ligands is reduced, but not abolished, under conditions where SREBP processing is suppressed. Moreover, LXR ligands induce FAS expression in CHO-7 cells without altering expression of SREBP-1. We demonstrate that in addition to tandem SREBP sites, the FAS promoter contains a high affinity binding site for the LXR/RXR heterodimer that is conserved in diverse animal species including birds, rodents, and humans. The LXR and SREBP binding sites independently confer LXR responsiveness on the FAS promoter, and maximal induction requires both transcription factors. Transient elevation of plasma triglyceride levels in mice treated with a synthetic LXR agonist correlates with transient induction of hepatic FAS expression. These results indicate that the LXR signaling pathway modulates FAS expression through distinct but complementary mechanisms and suggest that the FAS gene may be a critical target in the control of lipogenesis by LXRs.