A central enzyme in the pathway of de novo lipogenesis, fatty acid synthase (FAS) 1 catalyzes all of the steps in the conversion of malonyl-CoA to palmitate. Expression of the FAS gene is controlled primarily at the level of transcription and is responsive to both hormonal and nutritional signals (1, 2). Previous work has shown that sterol regulatory element-binding proteins (SREBPs) play a critical role in the transcriptional regulation of a number of genes in the lipogenic pathway, including FAS, steroyl-CoA desaturase (SCD-1), and acetyl-CoA carboxylase (ACC) (3-8). Three SREBP isoforms have been described: SREBP-1a and Ϫ1c (also called ADD1), which are derived from the same gene through alternative splicing, and SREBP-2, which is encoded by a separate gene (9, 10). Although their transcriptional targets overlap significantly, studies suggest that SREBP-1 preferentially activates genes involved in lipogenesis, whereas SREBP-2 preferentially activates genes in the cholesterol biosynthetic pathway (11-14). SREBPs have been shown to regulate FAS expression through direct interaction with the FAS promoter at multiple sites (7, 15). Overexpression of nuclear SREBP-1 is sufficient to induce expression of the FAS gene in cultured cells as well as transgenic mice (5,8). Recent work has also implicated the nuclear receptors LXR␣ and LXR␤ in the control of lipogenesis. Both LXRs bind to DNA and regulate transcription of target genes in a heterodimeric complex with RXR (16). Although early studies on LXRs focused on their role in cholesterol metabolism, mice carrying a targeted disruption in the LXR␣ gene were noted to be deficient in expression of FAS, SCD-1, ACC, and SREBP-1, consistent with a role in lipogenesis as well (17). Further support for this idea came with the observation that the administration of the synthetic LXR ligand T1317 to mice triggers induction of the lipogenic pathway and raises plasma triglyceride levels (18). The demonstration that the SREBP-1c promoter is a direct target for regulation by LXR/RXR heterodimers provided a straightforward explanation for the ability of LXR ligands to induce hepatic lipogenesis (19,20). Until now, the effects of LXR activation on the expression of lipogenic genes, including FAS, have been presumed to be entirely indirect.We demonstrate here that the FAS promoter is a direct target for regulation by the LXR/RXR heterodimer as well as SREBPs. This novel mechanism for the regulation of FAS expression and lipogenesis by LXRs has implications for the development of LXR agonists as modulators of human lipid metabolism. EXPERIMENTAL PROCEDURESReagents and Plasmids-Expression plasmids for RXR␣ and LXR␣, and nuclear SREBP-1a, -1c, and -2 have been described (21,22). GW3965 (23) and T0901317 (18) were provided by Timothy M. Willson (GlaxoSmithKline). Ligands were dissolved in Me 2 SO prior to use in cell culture. The Ϫ1594, Ϫ700, Ϫ150, and Ϫ135 rat FAS promoter luciferase reporter constructs were described previously (3). Mutations were

The control of lipid and glucose metabolism is closely linked. The nuclear receptors liver X receptor (LXR)␣ and LXR␤ have been implicated in gene expression linked to lipid homeostasis; however, their role in glucose metabolism is not clear. We demonstrate here that the synthetic LXR agonist GW3965 improves glucose tolerance in a murine model of diet-induced obesity and insulin resistance. Analysis of gene expression in LXR agonist-treated mice reveals coordinate regulation of genes involved in glucose metabolism in liver and adipose tissue. In the liver, activation of LXR led to the suppression of the gluconeogenic program including down-regulation of peroxisome proliferator-activated receptor ␥ coactivator-1␣ (PGC-1), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase expression. Inhibition of gluconeogenic genes was accompanied by an induction in expression of glucokinase, which promotes hepatic glucose utilization. In adipose tissue, activation of LXR led to the transcriptional induction of the insulin-sensitive glucose transporter, GLUT4. We show that the GLUT4 promoter is a direct transcriptional target for the LXR͞retinoid X receptor heterodimer and that the ability of LXR ligands to induce GLUT4 expression is abolished in LXR null cells and animals. Consistent with their effects on GLUT4 expression, LXR agonists promote glucose uptake in 3T3-L1 adipocytes in vitro. Thus, activation of LXR alters the expression of genes in liver and adipose tissue that collectively would be expected to limit hepatic glucose output and improve peripheral glucose uptake. These results outline a role for LXRs in the coordination of lipid and glucose metabolism.L iver X receptor (LXR)␣ and LXR␤ have emerged as important regulators of lipid and lipoprotein metabolism. The LXRs are activated by physiological concentrations of oxidized derivatives of cholesterol such as 22(R)-hydroxycholesterol, 27-hydroxycholesterol, and 24(S),25-epoxycholesterol (1-3). LXR␣ is expressed at particularly high levels in liver, adipose tissue, and macrophages, whereas LXR␤ is expressed ubiquitously. These ligand-activated transcription factors form obligate heterodimers with the retinoid X receptor (RXR) and regulate the expression of target genes containing LXR response elements (LXREs). All the LXREs identified thus far are DR-4 hormone response elements (direct repeat of the consensus AGGTCA separated by four nucleotides) (4).To date, more than a dozen LXR target genes have been identified (5). In the liver, LXRs regulate expression of a number of proteins involved in cholesterol and fatty acid metabolism, including CYP7A and sterol regulatory binding element protein 1c (SREBP-1c) (6, 7). In macrophages and other peripheral cells, LXRs have been implicated in the reverse cholesterol transport pathway. LXRs control the transcription of several genes involved in cellular cholesterol efflux including ATP-binding cassette (ABC)A1, ABCG1, and apolipoprotein E (8-11). LXRs also seem to influence lipoprotein metabolism through the c...

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.

Previous work has implicated the nuclear receptors liver X receptor ␣ (LXR␣) and LXR␤ in the regulation of macrophage gene expression in response to oxidized lipids. Macrophage lipid loading leads to ligand activation of LXRs and to induction of a pathway for cholesterol efflux involving the LXR target genes ABCA1 and apoE. We demonstrate here that autoregulation of the LXR␣ gene is an important component of this lipid-inducible efflux pathway in human macrophages. Oxidized low-density lipoprotein, oxysterols, and synthetic LXR ligands induce expression of LXR␣ mRNA in human monocyte-derived macrophages and human macrophage cell lines but not in murine peritoneal macrophages or cell lines. This is in contrast to peroxisome proliferator-activated receptor ␥ (PPAR␥)-specific ligands, which stimulate LXR␣ expression in both human and murine macrophages. We further demonstrate that LXR and PPAR␥ ligands cooperate to induce LXR␣ expression in human but not murine macrophages. Analysis of the human LXR␣ promoter led to the identification of multiple LXR response elements. Interestingly, the previously identified PPAR response element (PPRE) in the murine LXR␣ gene is not conserved in humans; however, a different PPRE is present in the human LXR 5-flanking region. These results have implications for cholesterol metabolism in human macrophages and its potential to be regulated by synthetic LXR and/or PPAR␥ ligands. The ability of LXR␣ to regulate its own promoter is likely to be an integral part of the macrophage physiologic response to lipid loading.Oxidized lipid signaling in macrophages is central to the pathogenesis of atherosclerosis (20,24). Exposure of macrophages and other vascular cells to oxidized low-density lipoprotein (oxLDL) leads to complex changes in gene expression that are collectively thought to influence the development of the atherosclerotic lesion. Mounting evidence suggests that nuclear receptor signaling pathways mediate many of the effects of oxidized lipids on cellular gene expression. Macrophage uptake of oxLDL has the potential to provide the cell with oxidized fatty acid ligands of peroxisome proliferator-activated receptor ␥ (PPAR␥) as well as oxysterol ligands of liver X receptor ␣ (LXR␣) and LXR␤ (8,12,13).LXR␣ and LXR␤ have been identified as key regulators of lipid homeostasis in multiple cell types (18). Targeted disruption of the Lxr␣ gene in mice uncovered roles for this receptor in the regulation of both hepatic bile acid synthesis and intestinal cholesterol absorption (16,19). The observation that sterol regulatory element-binding protein 1-c is a target for LXRs suggests that LXRs may be involved in the control of lipogenesis (6,17,21). Recent work has also implicated LXRs in the control of gene expression in response to macrophage lipid loading. Multiple genes potentially involved in the cellular cholesterol efflux pathway, including the putative cholesterol/ phospholipid transporter ABCA1 (5,19,22,28), ABCG1 (29), and apolipoprotein E (apoE) (11), have been identified as transcr...

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Decoding of the UGA selenocysteine codon for selenoprotein translation requires the SECIS element, a stem-loop motif in the 3'-UTR of the mRNA carrying short or large apical loops. In previous structural studies, we derived a secondary structure model for SECIS RNAs with short apical loops. Work from others proposed that intra-apical loop base pairing can occur in those SECIS that possess large apical loops, yielding form 2 SECIS versus the form 1 with short loops. In this work, SECIS elements arising from eight different selenoprotein mRNAs were assayed by enzymatic and/or chemical probing showing that seven can adopt form 2. Further, database searches led to the discovery in drosophila and zebrafish of SECIS elements in the selenophosphate synthetase 2, type 1 deiodinase and SelW mRNAs. Alignment of SECIS sequences not only highlighted the predominance of form 2 but also made it possible to classify the SECIS elements according to the type of selenoprotein mRNA they belong to. Interestingly, the alignment revealed that an unpaired adenine, previously thought to be invariant, is replaced by a guanine in four SECIS elements. Tested in vivo, neither the A to G nor the A to U changes at this position greatly affected the activity while the most detrimental effect was provided by a C. The putative contribution of the various SECIS motifs to function and ligand binding is discussed.

Ligand activation of liver X receptors (LXRs) has been shown to impact both lipid metabolism and inflammation. One complicating factor in studies utilizing synthetic LXR agonists is the potential for pharmacologic and receptor-independent effects. Here, we describe an LXR gain-of-function system that does not depend on the addition of exogenous ligand. We generated transgenic mice expressing a constitutively active VP16-LXRα protein from the aP2 promoter. These mice exhibit increased LXR signaling selectively in adipose and macrophages. Analysis of gene expression in primary macrophages derived from two independent VP16-LXRα transgenic lines confirmed the ability of LXR to drive expression of genes involved in cholesterol efflux and fatty acid synthesis. Moreover, VP16-LXRα expression also suppressed the induction of inflammatory genes by lipopolysaccharide to a comparable degree as synthetic agonist. We further utilized VP16-LXRα-expressing macrophages to identify and validate new targets for LXRs, including the gene encoding ADP-ribosylation factor-like 7 (ARL7). ARL7 has previously been shown to transport cholesterol to the membrane for ABCA1-associated removal and thus may be integral to the LXR-dependent efflux pathway. We show that the ARL7 promoter contains a functional LXRE and can be transactivated by LXRs in a sequence-specific manner, indicating that ARL7 is a direct target of LXR. These findings provide further support for an important role of LXRs in the coordinated regulation of lipid metabolic and inflammatory gene programs in macrophages.

Macrophages are an important source of angiogenic activity in wound healing, cancer, and chronic inflammation. Vascular endothelial growth factor (VEGF), a cytokine produced by macrophages, is a primary inducer of angiogenesis and neovascularization in these contexts. VEGF expression by macrophages is known to be stimulated by low oxygen tension as well as by inflammatory signals. In this study, we provide evidence that Vegfa gene expression is also regulated by activation of liver X receptors (LXRs). VEGF mRNA was induced in response to synthetic LXR agonists in murine and human primary macrophages as well as in murine adipose tissue in vivo. The effects of LXR ligands on VEGF expression were independent of hypoxia-inducible factor HIF-1␣ activation and did not require the previously characterized hypoxia response element in the VEGF promoter. Rather, LXR/retinoid X receptor heterodimers bound directly to a conserved hormone response element (LXRE) in the promoter of the murine and human Vegfa genes. Both LXR␣ and LXR␤ transactivated the VEGF promoter in transient transfection assays. Finally, we show that induction of VEGF expression by inflammatory stimuli was independent of LXRs, because these effects were preserved in LXR null macrophages. These observations identify VEGF as an LXR target gene and point to a previously unrecognized role for LXRs in vascular biology.