Synthetic LXR ligand inhibits the development of atherosclerosis in mice

Diet-dependent cardiovascular lipid metabolism controlled by hepatic LXRalpha

The high-cholesterol/high-fat Western diet has abetted an epidemic of atherosclerotic cardiovascular disease, the leading cause of death in industrialized nations. Liver X receptors (LXRs) are oxysterol sensors that are required for normal cholesterol and triglyceride homeostasis, yet synthetic LXR agonists produce undesirable hypertriglyceridemia. Here we report a previously unrecognized role for hepatic LXRalpha in the links between diet, serum lipids, and atherosclerosis. A modest increase in hepatic LXRalpha worsened serum lipid profiles in LDL-receptor null mice fed normal chow but had the opposite effect on lipids and afforded strong protection against atherosclerosis on a Western diet. The beneficial effect of hepatic LXRalpha was abrogated by a synthetic LXR agonist, which activated SREBP-1c and its target genes. Thus, the interplay between diet and hepatic LXRalpha is a critical determinant of serum lipid profiles and cardiovascular risk, and selective modulation of LXR target genes in liver can ameliorate hyperlipidemia and cardiovascular disease.

Liver X receptor agonists inhibit tissue factor expression in macrophages

Exposure of blood to tissue factor (TF) rapidly initiates the coagulation serine protease cascades. TF is expressed by macrophages and other types of cell within atherosclerotic lesions and plays an important role in thrombus formation after plaque rupture. Macrophage TF expression is induced by pro-inflammatory stimuli including lipopolysaccharide (LPS), interleukin-1beta and tumor necrosis factor-alpha. Here we demonstrate that activation of liver X receptors (LXRs) LXRalpha and LXRbeta suppresses TF expression. Treatment of mouse peritoneal macrophages with synthetic LXR agonist T0901317 or GW3965 reduced TF expression induced by pro-inflammatory stimuli. LXR agonists also suppressed TF expression and its activity in human monocytes. Human and mouse TF promoters contain binding sites for the transcription factors AP-1, NFkappaB, Egr-1 and Sp1, but no LXR-binding sites could be found. Cotransfection assays with LXR and TF promoter constructs in RAW 264.7 cells revealed that LXR agonists suppressed LPS-induced TF promoter activity. Analysis of TF promoter also showed that inhibition of TF promoter activity by LXR was at least in part through inhibition of the NFkappaB signaling pathway. In addition, in vivo, LXR agonists reduced TF expression within aortic lesions in an atherosclerosis mouse model as well as in kidney and lung in mice stimulated with LPS. These findings indicate that activation of LXR results in reduction of TF expression, which may influence atherothrombosis in patients with vascular disease.

Lipoproteins, macrophage function, and atherosclerosis: beyond the foam cell?

Atherogenesis requires and is highly influenced by the interaction between lipoproteins and macrophages. Most of the focus to date has been on the ability of atherogenic lipoproteins (such as low-density lipoproteins, LDL) to promote and of anti-atherogenic lipoproteins (such as high-density lipoproteins, HDL) to prevent the development of the cholesteryl ester-enriched macrophage-derived foam cell. However, lipoprotein-macrophage interactions have the potential to modulate macrophage function in a variety of additional ways that may impact on atherosclerosis. These include modulating cellular cholesterol and oxysterol content, providing fatty acids as ligands for PPARs, and acting as ligands for macrophage scavenger and Toll-like receptors. We suggest that atherogenic lipoproteins promote and anti-atherogenic lipoproteins inhibit atherogenesis by modulating macrophage function in a variety of ways beyond cholesteryl ester accumulation and foam cell formation.

Liver x receptor agonists inhibit cytokine-induced osteopontin expression in macrophages through interference with activator protein-1 signaling pathways

Osteopontin (OPN) is a proinflammatory cytokine and adhesion molecule implicated in the chemoattraction of monocytes and in cell-mediated immunity. We have recently reported that genetic OPN-deficiency attenuates the development of atherosclerosis in apoE-/- mice identifying OPN as potential target for pharmacological intervention in atherosclerosis. Synthetic agonists for the Liver X Receptor (LXR), members of the nuclear hormone receptor superfamily, prevent the development of atherosclerosis by regulating cholesterol homeostasis and suppressing inflammatory gene expression in macrophages. We demonstrate here that LXR ligands inhibit cytokine-induced OPN expression in macrophages. Two synthetic LXR ligands, T0901317 and GW3965, inhibited TNF-alpha, IL-1beta, INF-gamma and lipopolysaccharide induced OPN mRNA and protein expression in RAW 264.7 macrophages. Transient transfection experiments revealed that LXR ligands suppress cytokine-induced OPN promoter activity. Deletion analysis, heterologous promoter assays, and site-directed mutagenesis identified an activator protein-1 (AP-1) consensus site at -76 relative to the initiation site that supports OPN transcription in macrophages and mediates the effects of LXR ligands to inhibit OPN transcription. Electrophoretic mobility shift and chromatin immunoprecipitation assays indicated that LXR agonists inhibit cytokine-induced c-Fos and phospho-c-Jun binding to this AP-1 site. Cytokine-induced c-Fos and phospho-c-Jun protein expression was inhibited by LXR ligands and overexpression of c-Fos and c-Jun reversed the inhibitory effect of LXR ligands on OPN promoter activity in transactivation assays. Finally, treatment of C57BL/6J mice with LXR ligands inhibited OPN expression in peritoneal macrophages indicating that the observed effects of LXR ligands to inhibit OPN expression are applicable in vivo. These observations identify the regulation of macrophage OPN expression as a mechanism whereby LXR ligands may impact macrophage inflammatory responses and atherosclerosis. The full text of this article is available online at http://circres.ahajournals.org.

HDL as a target in the treatment of atherosclerotic cardiovascular disease

Lipid abnormalities are among the key risk factors for cardiovascular disease. Indeed, lipid-modifying drugs - in particular, the statins, which primarily lower plasma levels of low-density lipoprotein (LDL) cholesterol - considerably reduce the risk of cardiovascular events, leading to their widespread use. Nevertheless, it seems that there might be limits to the degree of benefit that can be achieved by lowering LDL-cholesterol levels alone, which has led to increased interest in targeting other lipid-related risk factors for cardiovascular disease, such as low levels of high-density lipoprotein (HDL) cholesterol. In this article, we first consider the mechanisms that underlie the protective effect of HDL cholesterol, and then discuss several strategies that have recently emerged to increase levels of HDL cholesterol to treat cardiovascular disease, including nuclear receptor modulation, inhibition of cholesteryl ester transfer protein and infusion of apolipoprotein/phospholipid complexes.

5beta-Cholane activators of the farnesol X receptor

The farnesoid X receptor (FXR) is activated by bile acids, natural agonists for this nuclear receptor. FXR-target genes play important roles in cholesterol and lipid metabolism. We have found that a series of 5beta-cholanic acid derivatives, even though without a hydroxyl group or any other substituent on the steroidal rings, can activate FXR more potently than hydroxylated bile acids in a reporter gene assay. The most potent compound among these derivatives, N-methyl-5beta-glycocholanic acid (NMGCA), induces the formation of receptor/coactivator complex in a gel-shift assay and also increases the expression of FXR target genes in human hepatoma HepG2 cells. Furthermore, in rats, NMGCA causes hypolipidemic effects as well as induction of the FXR target genes in liver. Our results suggest that NMGCA and its derivatives are important FXR activators in the study of the physiological functions of FXR and are potentially useful as pharmaceutical agents for treatment of cholesterol and lipid-related diseases.

Nuclear receptors as targets for drug development: regulation of cholesterol and bile acid metabolism by nuclear receptors

Nuclear receptors are ligand-dependent transcription factors that recently have been shown to play important roles in the metabolism of cholesterol and bile acids. Cholesterol homeostasis is maintained by de novo synthesis, absorption from diet, catabolism to bile acids and other steroids, and excretion into bile. Dysregulation of this mechanism leads to atherosclerosis and its life-threatening coronary and cerebrovascular sequelae. Conversion of cholesterol to bile acids in the liver is positively regulated by liver X receptor (LXR) alpha, a nuclear receptor for oxysterols. LXRalpha and LXRbeta, a second oxysterol receptor, regulate intestinal absorption and biliary excretion of cholesterol by inducing target gene expression. LXRs stimulate reverse cholesterol transport from peripheral tissues and exhibit antiatherogenic activity. Farnesoid X receptor (FXR), a bile acid receptor, represses bile acid synthesis and import in hepatocytes, stimulates bile acid export from cells, and protects hepatocytes from bile acid toxicity. Pregnane X receptor (PXR) and vitamin D receptor (VDR) respond to secondary bile acids and induce their catabolism. Thus, nuclear receptors play important roles in regulation of cholesterol and bile acid metabolism.

Differential inhibition of macrophage foam-cell formation and atherosclerosis in mice by PPARalpha, beta/delta, and gamma

PPARalpha, beta/delta, and gamma regulate genes involved in the control of lipid metabolism and inflammation and are expressed in all major cell types of atherosclerotic lesions. In vitro studies have suggested that PPARs exert antiatherogenic effects by inhibiting the expression of proinflammatory genes and enhancing cholesterol efflux via activation of the liver X receptor-ABCA1 (LXR-ABCA1) pathway. To investigate the potential importance of these activities in vivo, we performed a systematic analysis of the effects of PPARalpha, beta, and gamma agonists on foam-cell formation and atherosclerosis in male LDL receptor-deficient (LDLR(-/-)) mice. Like the PPARgamma agonist, a PPARalpha-specific agonist strongly inhibited atherosclerosis, whereas a PPARbeta-specific agonist failed to inhibit lesion formation. In concert with their effects on atherosclerosis, PPARalpha and PPARgamma agonists, but not the PPARbeta agonist, inhibited the formation of macrophage foam cells in the peritoneal cavity. Unexpectedly, PPARalpha and PPARgamma agonists inhibited foam-cell formation in vivo through distinct ABCA1-independent pathways. While inhibition of foam-cell formation by PPARalpha required LXRs, activation of PPARgamma reduced cholesterol esterification, induced expression of ABCG1, and stimulated HDL-dependent cholesterol efflux in an LXR-independent manner. In concert, these findings reveal receptor-specific mechanisms by which PPARs influence macrophage cholesterol homeostasis. In the future, these mechanisms may be exploited pharmacologically to inhibit the development of atherosclerosis.

PPARs in atherosclerosis: the clot thickens

Cardiovascular disease is the leading cause of morbidity and mortality in Western countries. Previous studies have highlighted the beneficial effects of PPARgamma activators on cardiovascular disease; however, the role of other PPAR family members in atherosclerosis is less clear. A report in this issue of the JCI expands our understanding of PPARs in vascular biology and highlights the potential use of multiple PPAR agonists to limit lipid accumulation in macrophages.

Nuclear receptors in macrophage biology: at the crossroads of lipid metabolism and inflammation

Macrophages are essential modulators of lipid metabolism and the innate immune system. Lipid and inflammatory pathways induced in activated macrophages are central to the pathogenesis of human diseases including atherosclerosis. Recent work has shown that expression of genes involved in lipid uptake and cholesterol efflux in macrophages is controlled by peroxisome proliferator-activated receptors (PPARs) and liver X receptors (LXRs). Other studies have implicated these same receptors in the modulation of macrophage inflammatory gene expression. Together, these observations position PPARs and LXRs at the crossroads of lipid metabolism and inflammation and suggest that these receptors may serve to integrate these pathways in the control of macrophage gene expression. In this review, we summarize recent work that has advanced our understanding of the roles of PPARs and LXRs in macrophage biology and discuss the implication of these results for cardiovascular physiology and disease.

The fatty acid-binding protein, aP2, coordinates macrophage cholesterol trafficking and inflammatory activity. Macrophage expression of aP2 impacts peroxisome proliferator-activated receptor gamma and IkappaB kinase activities

Fatty acid-binding proteins are cytosolic fatty acid chaperones, and the adipocyte isoform, aP2, plays an important role in obesity and glucose metabolism. Recently, this protein has been detected in macrophages where it strongly contributes to the development of atherosclerosis. Here, we investigated the role of aP2 in macrophage biology and the molecular mechanisms underlying its actions. We demonstrate that aP2-deficient macrophages display defects in cholesterol accumulation and alterations in pro-inflammatory responsiveness. Deficiency of aP2 alters the lipid composition in macrophages and enhances peroxisome proliferator-activated receptor gamma activity, leading to elevated CD36 expression and enhanced uptake of modified low density lipoprotein. The increased peroxisome proliferator-activated receptor gamma activity in aP2-deficient macrophages is also accompanied by a significant stimulation of the liver X receptor alpha-ATP-binding cassette transporter A1-mediated cholesterol efflux pathway. In parallel, aP2-deficient macrophages display reduced IkappaB kinase and NF-kappaB activity, resulting in suppression of inflammatory function including reduced cyclooxygenase-2 and inducible nitric-oxide synthase expression and impaired production of inflammatory cytokines. Our results demonstrate that aP2 regulates two central molecular pathways to coordinate macrophage cholesterol trafficking and inflammatory activity.

The flip side: Identifying small molecule regulators of nuclear receptors

Members of the nuclear hormone receptor superfamily function as ligand-activated transcription factors to regulate genetic networks controlling cell growth and differentiation, inflammatory responses, and metabolism. The ability to modulate nuclear receptor-dependent gene expression with small molecules has made the superfamily a favored target for drug discovery. Not surprisingly, small molecules that regulate receptor activity are currently used to treat a number of human disorders. Over the last 10 years, the availability of a common platform of functional assays suitable for any nuclear receptor has facilitated the identification of endogenous and synthetic ligands that have been used as tools to uncover previously unanticipated endocrine signaling pathways. Recent progress in understanding the molecular basis for ligand-dependent gene regulation suggests that a new era of "designer" ligands with tissue- and/or gene-selective activity will quickly be upon us.

Macrophage Liver X Receptor Is Required for Antiatherogenic Activity of LXR Agonists

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, LXRα and LXRβ (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.

Conclusions— These data substantiate that drugs targeting macrophage LXR activity may offer therapeutic benefit in the treatment of atherosclerotic cardiovascular disease.

Expression of the LXRalpha protein in human atherosclerotic lesions

OBJECTIVE

Liver X-activated receptor alpha (LXRalpha) regulates multiple genes controlling cholesterol metabolism and transport. To clarify its role in atherogenesis, we established a monoclonal antibody recognizing native human LXRalpha protein and studied the expression pattern in human atherosclerotic lesions.

METHODS AND RESULTS

A novel monoclonal antibody PPZ0412 was raised against the ligand-binding domain of LXRalpha, which can be used for immunostaining of human LXRalpha protein. LXRalpha protein was detected in the nucleus of macrophages in the liver, spleen, or lung and also in hepatocytes and adipocytes. In atherosclerotic lesions, the LXRalpha protein was detected in macrophages positive for scavenger receptor class A and/or CD68.

CONCLUSIONS

In the human body, the LXRalpha protein is highly expressed in macrophage lineage cells and foam cells in atherosclerotic lesions and is identified as a target for intervention in atherosclerotic disease.

A nuclear strike against Listeria--the evolving life of LXR

LXRs are members of the nuclear receptor superfamily and function as master regulators of cholesterol metabolism. In the macrophage, they control cholesterol efflux and inhibit the transcription factor NF-kappaB-mediated proinflammatory responses. In this issue of Cell, discover surprising, protective functions for LXRalpha in innate immunity.

Liver X receptor agonists suppress vascular smooth muscle cell proliferation and inhibit neointima formation in balloon-injured rat carotid arteries

The liver X receptors alpha and beta (LXRalpha and LXRbeta) are important regulators of cholesterol homeostasis in liver and macrophages. Synthetic LXR ligands prevent the development of atherosclerosis in murine models; however, the potential functional relevance of LXRs in vascular smooth muscle cells (VSMCs) has not been investigated. In the present study, we demonstrate that LXRs are expressed and functional in primary human coronary artery VSMCs (CASMCs). LXR ligands inhibited mitogen-induced VSMC proliferation and G1-->S phase progression of the cell cycle. Inhibition of G1 exit by LXR ligands was accompanied by a dose-dependent inhibition of retinoblastoma protein (Rb) phosphorylation, which functions as the key switch for G1-->S cell cycle progression. LXR ligands suppressed mitogen-induced degradation of the cyclin-dependent kinase inhibitor p27Kip1, attenuated cyclin D1 and cyclin A expression, and inhibited the expression of S phase-regulatory minichromosome maintenance protein 6. Stabilization of p27kip1 by LXR ligands was mediated by supressing the transcriptional activation of the S phase kinase-associated protein 2 (Skp2), an F-box protein that targets p27Kip1 for degradation. Inhibition of Rb phosphorylation and G1-->S cell cycle progression by LXR ligands was reversed in VSMCs overexpressing Skp2, indicating that Skp2 as an upstream regulator of p27Kip1 degradation plays a central role in LXR ligand-mediated inhibition of VSMC proliferation. Furthermore, adenovirus-mediated overexpression of the S phase transcription factor E2F, which is released after Rb phosphorylation, reversed the inhibitory effect of LXR ligands on VSMC proliferation and S phase gene expression, suggesting that the primary mechanisms by which LXR ligands inhibit VSMC proliferation occur upstream of Rb phosphorylation. Finally, neointima formation in a model of rat carotid artery balloon injury was significantly attenuated after treatment with the LXR ligand T1317 compared with vehicle-treated animals. These data demonstrate that LXR ligands inhibit VSMC proliferation and neointima formation after balloon injury and suggest that LXR ligands may constitute a novel therapy for proliferative vascular diseases. The full text of this article is available online at http://circres.ahajournals.org.

Diet, fatty acids, and regulation of genes important for heart disease

Diets rich in omega-3 polyunsaturated fatty acids (n-3 PUFAs), such as alpha-linoleic acid, eicosapentaenoic acid, and docosahexaenoic acid, are associated with decreased incidence and severity of coronary heart disease. Similarly, conjugated linoleic acids (CLAs), which are found in meat and dairy products, have beneficial effects against atherosclerosis, diabetes, and obesity. The effects of n3-PUFAs and CLAs are in contrast to fatty acids with virtually identical structures, such as linoleic acid and arachidonic acid (ie, n-6 PUFAs). This article discusses the possibility that cognate receptors exist for fatty acids or their metabolites that are able to regulate gene expression and coordinately affect metabolic or signaling pathways associated with coronary heart disease. Three nuclear receptors are emphasized as fatty acid receptors that respond to dietary and endogenous ligands: peroxisome proliferator activated receptors, retinoid X receptors, and liver X receptors.

Liver X receptor-α mediates cholesterol efflux in glomerular mesangial cells

Lipid-mediated injury plays an important role in the pathogenesis of many renal diseases including diabetic nephropathy. Liver X receptor-α (LXRα) is an intracellular sterol sensor that regulates expression of genes controlling cholesterol absorption, excretion, catabolism, and cellular efflux. The present study was aimed at examining the role of LXRα in cholesterol metabolism in glomerular mesangial cells. A 1,561-bp fragment of full-length rabbit LXR cDNA was cloned. The deduced protein sequence exhibited 92.4 and 89.2% identity to human and mouse LXRα, respectively. Tissue distribution studies showed that rabbit LXRα was expressed in the liver, spleen, and kidney. In situ hybridization and RT-PCR assays further indicated that LXRα mRNA was widely expressed in the kidney and present in every nephron segment including the glomeruli. To determine intrarenal regulation of LXRα, rabbits were treated with thiazolidinedione (TZD) peroxisome proliferator-activated receptor-γ (PPARγ) agonists, which have been previously shown to enhance LXRα expression via PPARγ and increase cholesterol efflux in macrophages. The results showed that glomerular LXRα expression was markedly induced by TZDs. In cultured rabbit mesangial cells, LXRα mRNA and protein were detected by RT-PCR and immunoblotting. Treatment of mesangial cells with a specific LXRα agonist, TO-901317, significantly increased basal and apolipoprotein AI-mediated cholesterol efflux and markedly enhanced the promoter activity of an LXRα target gene, ATP-binding cassette transporter A1 (ABCA1). In conclusion, LXRα is expressed in renal glomeruli and functionally present in mesangial cells where its activation mediates cholesterol efflux via ABCA1. These data suggest that LXRα may be a potential therapeutic target for treating lipid-related renal glomerular disease.

LXR activation and cholesterol efflux from a lipoprotein depot in vivo

Activation of LXR in cultured cells results in enhancement of cholesterol efflux to apo Al. To study cholesterol efflux, in vivo cationized LDL was injected into the rectus femoris muscle of mice to create a lipoprotein depot. LXR ligand TO901317, 10 mg/kg, was given by gavage for 8 days, starting 4 days after injection of the lipoprotein. The rate of cholesterol efflux from the depot was compared in treated and control mice. Administration of the ligand resulted in a 70% increase in plasma cholesterol and 40% in phospholipids, but HDL-cholesterol and HDL-phospholipids increased by 43% and 24% only. Efflux of the injected cholesterol from the lipoprotein depot of treated mice was not enhanced but even somewhat delayed. This impairment was unexpected and its cause could be multifactorial. A plausible explanation seems that induced hypercholesterolemia, and a decrease in HDL-cholesterol to total cholesterol ratio, delayed the clearance.

PPAR- and LXR-dependent pathways controlling lipid metabolism and the development of atherosclerosis

The nuclear receptor superfamily is composed of transcription factors that positively and negatively regulate gene expression in response to the binding of a diverse array of lipid-derived hormones and metabolites. Intense efforts are currently being directed at defining the biological roles and mechanisms of action of liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs). LXRs have been found to play essential roles in the regulation of whole body cholesterol absorption and excretion, in the efflux of cholesterol from peripheral cells, and in the biosynthesis and metabolism of very low density lipoproteins. PPARs have been found to regulate diverse aspects of lipid metabolism, including fatty acid oxidation, fat cell development, lipoprotein metabolism, and glucose homeostasis. Intervention studies indicate that activation of PPARalpha, PPARgamma, and LXRs by specific synthetic ligands can inhibit the development of atherosclerosis in animal models. Here, we review recent studies that provide new insights into the mechanisms by which these subclasses of nuclear receptors act to systemically influence lipid and glucose metabolism and regulate gene expression within the artery wall.

Fatty acid regulation of gene transcription

Dietary fat has a dual role in human physiology: a) it functions as a source of energy and structural components for cells; b) it functions as a regulator of gene expression that impacts lipid, carbohydrate, and protein metabolism, as well as cell growth and differentiation. Fatty acid effects on gene expression are cell-specific and influenced by fatty acid structure and metabolism. Fatty acids interact with the genome through several mechanisms. They regulate the activity or nuclear abundance of several transcription factors, including PPAR, LXR, HNF-4, NFkappaB, and SREBP. Fatty acids or their metabolites bind directly to specific transcription factors to regulate gene transcription. Alternatively, fatty acids indirectly act on gene expression through their effects on a) specific enzyme-mediated pathways, such as cyclooxygenase, lipoxygenase, protein kinase C, or sphingomyelinase signal transduction pathways; or b) pathways that involve changes in membrane lipid/lipid raft composition that affect G-protein receptor or tyrosine kinase-linked receptor signaling. Further definition of these fatty acid-regulated pathways will provide insight into the role dietary fat plays in human health and the onset and progression of several chronic diseases, like coronary artery disease and atherosclerosis, dyslipidemia and inflammation, obesity and diabetes, cancer, major depressive disorders, and schizophrenia.

Convergence of lipid homeostasis through liver X and thyroid hormone receptors

Members of the nuclear receptor gene family act as biological rheostats to maintain metabolic homeostasis in response to endocrine and nutritional changes. The liver X (LXR) and thyroid hormone (TR) receptors have been shown to regulate overlapping but distinct metabolic pathways important for overall lipid homeostasis. Dyslipidemia is one out of four key determinants for cardiovascular risk and both LXRs and TRs may provide attractive targets for intervention of cardiovascular disease. In this review we will compare the two receptor systems to highlight similarities and differences in structure and function with implications for development of novel treatments for dyslipidemia and atherosclerosis.

The Liver X Receptor Ligand T0901317 Down-regulates APOA5 Gene Expression through Activation of SREBP-1c

Alterations in the expression of the recently discovered apolipoprotein A5 gene strongly affect plasma triglyceride levels. In this study, we investigated the contribution of APOA5 to the liver X receptor (LXR) ligand-mediated effect on plasma triglyceride levels. Following treatment with the LXR ligand T0901317, we found that APOA5 mRNA levels were decreased in hepatoma cell lines. The observation that no down-regulation of APOA5 promoter activity was obtained by LXR-retinoid X receptor (RXR) co-transfection prompted us to explore the possible involvement of the known LXR target gene SREBP-1c (sterol regulatory element-binding protein 1c). In fact, we found that co-transfection with the active form of SREBP-1c down-regulated APOA5 promoter activity in a dose-dependent manner. We then scanned the human APOA5 promoter sequence and identified two putative E-box elements that were able to bind specifically SREBP-1c in gel-shift assays and were shown to be functional by mutation analysis. Subsequent suppression of SREBP-1 mRNA through small interfering RNA interference abolished the decrease of APOA5 mRNA in response to T0901317. Finally, administration of T0901317 to hAPOA5 transgenic mice revealed a significant decrease of APOA5 mRNA in liver tissue and circulating apolipoprotein AV protein in plasma, confirming that the described down-regulation also occurs in vivo. Taken together, our results demonstrate that APOA5 gene expression is regulated by the LXR ligand T0901317 in a negative manner through SREBP-1c. These findings may provide a new mechanism responsible for the elevation of plasma triglyceride levels by LXR ligands and support the development of selective LXR agonists, not affecting SREBP-1c, as beneficial modulators of lipid metabolism.

Gene-selective modulation by a synthetic oxysterol ligand of the liver X receptor

Liver X receptors (LXRs) play key roles in the regulation of cholesterol homeostasis by limiting cholesterol accumulation in macrophages within arterial wall lesion sites by a mechanism that includes the upregulation of ATP binding cassette transporters. These atheroprotective properties distinguish LXRs as potential targets for pharmaceutical intervention in cardiovascular disease. Their associated activity for promoting lipogenesis and triglyceride accretion through the activation of sterol-response element binding protein 1c (SREBP-1c) expression, however, represents a potential proatherogenic liability. A newly characterized synthetic oxysterol, N,N-dimethyl-3beta-hydroxycholenamide (DMHCA), represents a gene-selective LXR modulator that mediates potent transcriptional activation of ABCA1 gene expression while exhibiting minimal effects on SREBP-1c both in vitro and in vivo in mice. DMHCA has the potential to stimulate cholesterol transport through the upregulation of LXR target genes, including ABCA1, in liver, small intestine, and peritoneal macrophages. Compared with known nonsteroidal LXR agonists, however, DMHCA exhibits only limited activity for increasing hepatic SREBP-1c mRNA and does not alter circulating plasma triglycerides. Cell-based studies also indicate that DMHCA enhances cholesterol efflux in macrophages and suggest a mechanism whereby this selective modulator can potentially inhibit cholesterol accumulation. DMHCA and related gene-selective ligands of LXR may have application to the study and treatment of atherosclerosis.

Effects of Pravastatin on the Expression of ATP-Binding Cassette Transporter A1

In vitro inhibition of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase causes the suppression of liver X receptor (LXR) activity. Because LXR regulates the expression of ATP-binding cassette transporter (ABC) A1, which is involved in the high-density lipoprotein-related reverse cholesterol transport pathway, we examined the effects of an HMG-CoA reductase inhibitor pravastatin on ABCA1 expression in vitro and in vivo. Pravastatin (10 microM) significantly reduced the transcript levels of murine ABCA1 gene by 35% in RAW264.7 macrophages under a lipoprotein-deficient condition. The inhibition was due to the decreased mevalonic acid production because addition of exogenous mevalonic acid restored ABCA1 mRNA levels. In addition, cholesterol and 22(R)-hydroxycholesterol thoroughly blunted the inhibition. Furthermore, pravastatin did not decrease ABCA1 mRNA and protein levels in HepG2 hepatocytes even in the absence of exogenous LXR agonists. Oral dosing of pravastatin (0.1% concentration in drinking water) for 24 h or 2 weeks to mice did not decrease ABCA1 mRNA and protein levels in the liver and leukocytes. Interestingly, pravastatin significantly increased both hepatic and leukocyte LXRalpha mRNA levels. Thus, although HMG-CoA reductase inhibitors suppress ABCA1 mRNA expression in the absence of LXR agonists, in vivo inhibition of HMG-CoA reductase is unlikely to cause such suppression.

Modulateurs du métabolisme du cholestérol et avenir du traitement de l’athérosclérose

Atherosclerosis is a disease characterized by lipid accumulation in the vascular wall leading to myocardial infarction or stroke. Hypercholesterolemia is an important risk factor and current treatments are largely based on cholesterol lowering. In spite of proven efficacy of existing drugs, like statins, cardiovascular diseases still remain the most common cause of death in industrialised countries. Many new molecular targets are being studied to improve atherosclerosis treatment and reduce the number of deaths. The action on these targets could lead to a decrease of blood cholesterol levels or produce a direct anti-atherosclerotic effect on the vascular wall. A cholesterol lowering effect could be achieved by reducing cholesterol synthesis (squalene synthase inhibitors), intestinal cholesterol absorption as well as intestinal and liver lipoprotein secretion (microsomal transfer protein inhibitors, acyl-coenzyme A-cholesterol acyltransferase inhibitors) or by increasing fecal excretion of bile acids (ileal sodium-dependent bile acid transporter inhibitors). An anti-atherosclerotic effect on the vascular wall could be achieved by reducing the inflammation via activation of peroxisome proliferator activated receptors, or, more particularly, could consist of decreased expression of adhesion molecules and chemoattractant proteins. Increasing the adenosine triphosphate-binding cassette A1 protein and inhibiting acyl-coenzyme A :cholesterol acyltransferase 1 activity could slow down formation of foam cells, which are a hallmark of atherosclerosis. Finally, the cholesterol fraction carried by high density lipoproteins, which is inversely correlated to cardiovascular risk, could be increased by cholesterol ester transfer protein inhibitors. All of these new classes of compounds are currently studied by pharmaceutical companies and are in different phases of development (preclinical or clinical).

Evidence for anti-inflammatory activity of statins and PPARα activators in human C-reactive protein transgenic mice in vivo and in cultured human hepatocytes in vitro

Inflammatory processes, aside from cholesterol, play a central role in atherogenesis. Human C-reactive protein (huCRP) signals systemic inflammation and independently predicts future cardiovascular risk. Cholesterol-lowering statins reduce atherosclerosis and plasma huCRP levels. Evidence is sought for a direct anti-inflammatory statin effect in vivo, independent of effects on plasma cholesterol and atherogenesis. The effect of atorvastatin and simvastatin on huCRP expression was studied in nonatherosclerotic huCRP transgenic mice and compared with another class of hypolipidemic drugs, peroxisome proliferator-activated receptor-alpha (PPARα) activators, notably fenofibrate and Wy14643. Like statins, PPARα activators combine antiatherosclerotic properties with huCRP-lowering effects. Dietary treatment with statins or PPARα activators decreased basal and interleukin-1β (IL-1β)-induced plasma huCRP levels independently of cholesterol lowering. These direct anti-inflammatory in vivo effects occurred at the transcriptional level and could be confirmed in cultured human liver slices and in human hepatoma cells transiently transfected with a huCRP promoter-driven luciferase reporter. A molecular rationale for the suppression of IL-1-induced huCRP transcription is provided by showing that statins and PPARα activators up-regulate IκBα protein expression. This results in a reduced nuclear translocation of p50-nuclear factor κ B (NFκB) and thereby decreased amounts of nuclear p50-NFκB∼CCAAT/enhancer binding protein beta (C/EBPβ) complexes, which determine the huCRP transcription rate. Our results provide conclusive evidence for a direct suppressive effect of statins and PPARα activators on huCRP expression independent of cholesterol lowering and atherogenesis. (Blood. 2004;103:4188-4194)

Raising HDL cholesterol without inducing hepatic steatosis and hypertriglyceridemia by a selective LXR modulator

Liver X receptors (LXRs) are ligand-activated transcription factors that belong to the nuclear receptor superfamily. LXRs activate transcription of a spectrum of genes that regulate reverse cholesterol transport, including the ATP binding cassette transporter A1 (ABCA1), and raise HDL cholesterol (HDL-C) levels. However, LXR agonists also induce genes that stimulate lipogenesis, including the sterol response element binding protein (SREBP1-c) and fatty acid synthetase (FAS). The induction of these genes in the liver cause increased hepatic triglyceride synthesis, hypertriglyceridemia, and hepatic steatosis. As LXR response elements have been identified in these promoters, it is not clear if these two processes can be separated. Herein, we demonstrate that plasma HDL-C elevation and intestinal ABCA1 induction can occur with relatively little induction of FAS and SREBP1-c in mouse liver via a selective LXR modulator GW3965. This is in contrast to the strong induction of hepatic lipogenic genes by the well-characterized LXR agonist T0901317 (T317). Consistent with the in vivo results, GW3965 is a very weak LXR activator compared with T317 in human hepatoma cells. GW3965-liganded LXR recruits selected coactivators less effectively than T317 and may explain in part the tissue selective gene induction. This demonstration that tissue and gene selective modulation is possible with selective LXR modulators has positive implications for the development of this class of antiatherosclerotic agents.

PPARs and LXRs: atherosclerosis goes nuclear

Atherosclerosis is the leading cause of mortality in the Western world, and new therapeutics to target the metabolic and inflammatory factors that underlie its pathogenesis are needed. Peroxisome proliferator-activated receptors and liver X receptors are lipid-activated nuclear receptors that regulate systemic glucose and lipid metabolism, and modulate inflammation within the vascular wall. New understanding of their functions in physiology and the development of high-affinity synthetic ligands highlight their potential as targets for the treatment of cardiovascular disease.

Atherosclerosis in the apolipoprotein-E-deficient mouse: a decade of progress

Arguably the most critical advancement in the elucidation of factors affecting atherogenesis has been the development of mouse models of atherosclerosis. Among available models, the apolipoprotein E-deficient (apoE-/-) mouse is particularly popular because of its propensity to spontaneously develop atherosclerotic lesions on a standard chow diet. A Medline search reveals over 645 articles dedicated to studies using this reliable and convenient "super" animal model since its inception (Piedrahita JA et al, Proc Natl Acad Sci U S A 1992;89:4471-4475; Plump AS et al, Cell 1992;71:343-353) with a more or less steady increase from year to year. This review will examine our present understanding of the pathology and progression of plaques in this animal and highlight some of the nutritional, pharmacological, and genetic studies that have enhanced this understanding.

Linking lipids, Alzheimer's and LXRs?

Deposition of the β-amyloid (Aβ) peptide is thought to underlie development of Alzheimer's disease (AD). This pathological linkage has spurred considerable interest in therapeutic strategies to reduce Aβ production. It is becoming increasingly clear that altered cholesterol homeostasis can modulate Aβ production and/or accumulation. In this review, we discuss the molecular pathology of AD, the cholesterol connection and recent data suggesting that the oxysterol receptor, liver X receptor LXR (NR1H2 and NR1H3), may modulate these events.

Cholesteryl ester transfer protein modulates the effect of liver X receptor agonists on cholesterol transport and excretion in the mouse

Human plasma, unlike mouse plasma, contains the cholesteryl ester transfer protein (CETP) that may influence the reverse cholesterol transport. Liver X receptor (LXR), an oxysterol-activated nuclear receptor induces CETP transcription via a direct repeat 4 element in the CETP gene promoter. The aim of the study was to assess in vivo the impact of LXR activation on CETP expression and its consequences on plasma lipid metabolism and hepatic and bile lipid content. Wild-type and humanized mice expressing CETP were treated for five days with T0901317 LXR agonist. This treatment produced marked rises in both hepatic CETP mRNA and plasma CETP activity levels. Interestingly, the LXR agonist-mediated, 2-fold rise in both total and HDL cholesterol levels in treated wild-type mice was not observed in CETPTg mice, and the accumulation of cholesterol in the liver of CETPTg mice was reversed by LXR agonist treatment. Moreover, LXR activation induced a 2-fold increase in hepatic LDL-receptor expression in wild-type and CETPTg mice, and it produced a significantly greater rise in biliary cholesterol concentration in CETPTg mice as compared with wild-type mice. In conclusion, induction of CETP constitutes a major determinant of the effect of LXR agonists on cholesterol transport and excretion.

ABCA1-dependent lipid efflux to apolipoprotein A-I mediates HDL particle formation and decreases VLDL secretion from murine hepatocytes

High levels of expression of the ATP binding cassette transporter A1 (ABCA1) in the liver and the need to over- or underexpress hepatic ABCA1 to impact plasma HDL levels in mice suggest a major role of the liver in HDL formation and in determining circulating HDL levels. Cultured murine hepatocytes were used to examine the role of hepatic ABCA1 in mediating the lipidation of apolipoprotein A-I (apoA-I) for HDL particle formation. Exogenous apoA-I stimulated cholesterol efflux to the medium from wild-type hepatocytes, but not from ABCA1-deficient (abca1(-/-)) hepatocytes. ApoA-I induced the formation of new HDL particles and enhanced the lipidation of endogenously secreted murine apoA-I in ABCA1-expressing but not abca1(-/-) hepatocytes. ABCA1-dependent cholesterol mobilization to apoA-I increased new cholesterol synthesis, indicating depletion of the regulatory pool of hepatocyte cholesterol during HDL formation. Secretion of triacylglycerol and apoB was decreased following apoA-I incubation with ABCA1-expressing but not abca1(-/-) hepatocytes. These results support a major role for hepatocyte ABCA1 in generating a critical pool of HDL precursor particles that enhance further HDL generation and passive cholesterol mobilization in the periphery. The results also suggest that diversion of hepatocyte cholesterol into the "reverse" cholesterol transport pathway diminishes cholesterol availability for apoB-containing lipoprotein secretion by the liver.

Transcription of the Vascular Endothelial Growth Factor Gene in Macrophages Is Regulated by Liver X Receptors

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-1alpha 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 LXRalpha and LXRbeta 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.

Nuclear receptor signaling in macrophages

Macrophages play diverse roles in host defense and in maintenance of homeostasis. Based on their ability to promote inflammatory responses, inappropriate macrophage function also contributes to numerous pathological processes, including atherosclerosis, rheumatoid arthritis and inflammatory bowel disease. Members of the nuclear receptor superfamily of ligand-dependent transcriptions factors have emerged as key regulators of inflammation and lipid homeostasis in macrophages. These include the glucocorticoid receptor (GR), which inhibits inflammatory programs of gene expression in response to natural corticosteroids and synthetic anti-inflammatory ligands such as dexamethasone. Also, in response to endogenous eicosanoids and oxysterols, respectively, peroxisome proliferator-activated receptors (PPARs) and liver X receptors (LXRs) regulate transcriptional programs involved in inflammatory responses and lipid homeostasis. Identification of their mechanisms of action should help guide the development of new therapeutic agents useful in the treatment of diseases in which macrophages play critical pathogenic roles.

Decoding Transcriptional Programs Regulated by PPARs and LXRs in the Macrophage

Macrophages play essential roles in immunity and homeostasis. As professional scavengers, macrophages phagocytose microbes and apoptotic and necrotic cells and take up modified lipoprotein particles. These functions require tightly regulated mechanisms for the processing and disposal of cellular lipids. Under pathological conditions, arterial wall macrophages become foam cells by accumulating large amounts of cholesterol, contributing to the development of atherosclerosis. Peroxisome proliferator–activated receptors (PPARs) and liver X receptors (LXRs) are members of the nuclear receptor superfamily of transcription factors that have emerged as key regulators of macrophage homeostasis. PPARs and LXRs control transcriptional programs involved in processes of lipid uptake and efflux, lipogenesis, and lipoprotein metabolism. In addition, PPARs and LXRs negatively regulate transcriptional programs involved in the development of inflammatory responses. This review summarizes recent efforts to decode the differential and overlapping roles of PPARs and LXRs in the context of macrophage lipid homeostasis and the control of inflammation.

Peroxisome proliferator-activated receptor gamma: implications for cardiovascular disease

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor belonging to the nuclear hormone receptor superfamily. PPARgamma is expressed by macrophages, endothelial cells, and vascular smooth muscle cells. It regulates gene expression of key proteins involved in lipid metabolism, vascular inflammation, and proliferation contributing to atherogenesis and postangioplasty restenosis. The discovery of synthetic ligands for PPARgamma has led to significant enhancement of our understanding of the mechanism of their ligand-dependent activation and subsequent biological effects, particularly with respect to the role of PPARgamma in vascular pathophysiology. The thiazolidinedione PPARgamma agonists not only improve insulin resistance in patients with type II diabetes but also exert a broad spectrum of antiatherogenic effects in vitro and in animal models of atherosclerosis. In this review, we summarize the important role of PPARgamma as a molecular target for thiazolidinediones and its implications for the control of vascular inflammation and proliferation for the cardiovascular system.

Crystal structure of the human liver X receptor beta ligand-binding domain in complex with a synthetic agonist

LXRbeta belongs to the nuclear hormone receptor superfamily of ligand-activated transcription factors. Its natural ligands are supposed to be oxidised derivatives of cholesterol. Stimulation of LXRbeta by agonists activates a number of genes that are involved in the regulation of lipid metabolism and cholesterol efflux from cells. Therefore, LXRbeta may represent a novel therapeutic target for the treatment of dyslipidemia and atherosclerosis.Here, we report the X-ray crystal structure of the LXRbeta ligand-binding domain in complex with a synthetic agonist, T-0901317. This compound occupies the ligand-binding pocket of the receptor, forms numerous lipophilic contacts with the protein and one crucial hydrogen bond to His435 and stabilises the agonist conformation of the receptor ligand-binding domain. The recruitment of the AF2-region of the protein is not achieved via direct polar interactions of the ligand with protein side-chains of this helical segment, but rather via few hydrophobic contacts and probably more importantly via indirect effects involving the pre-orientation of side-chains that surround the ligand-binding pocket and form the interface to the AF2-helix. On the basis of these results we propose a binding mode and a mechanism of action for the putative natural ligands, oxidised derivatives of cholesterol.

Pharmacological modulation of cholesteryl ester transfer protein, a new therapeutic target in atherogenic dyslipidemia

In mediating the transfer of cholesteryl esters (CE) from antiatherogenic high density lipoprotein (HDL) to proatherogenic apolipoprotein (apo)-B-containing lipoprotein particles (including very low density lipoprotein [VLDL], VLDL remnants, intermediate density lipoprotein [IDL], and low density lipoprotein [LDL]), the CE transfer protein (CETP) plays a critical role not only in the reverse cholesterol transport (RCT) pathway but also in the intravascular remodeling and recycling of HDL particles. Dyslipidemic states associated with premature atherosclerotic disease and high cardiovascular risk are characterized by a disequilibrium due to an excess of circulating concentrations of atherogenic lipoproteins relative to those of atheroprotective HDL, thereby favoring arterial cholesterol deposition and enhanced atherogenesis. In such states, CETP activity is elevated and contributes significantly to the cholesterol burden in atherogenic apoB-containing lipoproteins. In reducing the numbers of acceptor particles for HDL-derived CE, both statins (VLDL, VLDL remnants, IDL, and LDL) and fibrates (primarily VLDL and VLDL remnants) act to attenuate potentially proatherogenic CETP activity in dyslipidemic states; simultaneously, CE are preferentially retained in HDL and thereby contribute to elevation in HDL-cholesterol content. Mutations in the CETP gene associated with CETP deficiency are characterized by high HDL-cholesterol levels (>60 mg/dL) and reduced cardiovascular risk. Such findings are consistent with studies of pharmacologically mediated inhibition of CETP in the rabbit, which argue strongly in favor of CETP inhibition as a valid therapeutic approach to delay atherogenesis. Consequently, new organic inhibitors of CETP are under development and present a potent tool for elevation of HDL in dyslipidemias involving low HDL levels and premature coronary artery disease, such as the dyslipidemia of type II diabetes and the metabolic syndrome. The results of clinical trials to evaluate the impact of CETP inhibition on premature atherosclerosis are eagerly awaited.

Oxysterols and oxysterol binding proteins: role in lipid metabolism and atherosclerosis

Oxidized derivatives of cholesterol have been investigated actively for decades in the context of the oxidative hypothesis of atherosclerosis. Oxysterols arise in our tissues as a result of enzymatic or non-enzymatic oxidation reactions and are also obtained from dietary sources. Even though these compounds are found enriched in the atherosclerotic lesions in arterial walls, the plasma concentrations of oxysterols cannot, in the light of current knowledge, be regarded as a risk factor for atherosclerotic disease. However, oxysterols may still have important local effects in the arterial wall as factors that regulate the cellular lipid homeostasis and possibly the maturation of the lesions. Work during the past few years has revealed that oxysterols have a potential as signaling molecules that may play important roles in lipid metabolism, especially the reverse cholesterol transport process. This finding has recently moved oxysterols and the protein mediators of their biological effects, liver X receptors and cytosolic oxysterol binding proteins, into the center stage of atherosclerosis research.

Crosstalk between LXR and toll-like receptor signaling mediates bacterial and viral antagonism of cholesterol metabolism

The liver X receptors (LXR) alpha and beta are regulators of cholesterol metabolism and determinants of atherosclerosis susceptibility. Viral and bacterial pathogens have long been suspected to be modulators of atherogenesis; however, mechanisms linking innate immunity to cholesterol metabolism are poorly defined. We demonstrate here that pathogens interfere with macrophage cholesterol metabolism through inhibition of the LXR signaling pathway. Activation of Toll-like receptors (TLR) 3 and 4 by microbial ligands blocks the induction of LXR target genes including ABCA1 in cultured macrophages as well as in aortic tissue in vivo. As a consequence of these transcriptional effects, TLR3/4 ligands strongly inhibit cholesterol efflux from macrophages. Crosstalk between LXR and TLR signaling is mediated by IRF3, a specific effector of TLR3/4 that inhibits the transcriptional activity of LXR on its target promoters. These findings highlight a common mechanism whereby bacterial and viral pathogens may modulate macrophage cholesterol metabolism and cardiovascular disease.

Retinoic acid receptor-mediated induction of ABCA1 in macrophages

ABCA1, the mutant molecule in Tangier Disease, mediates efflux of cellular cholesterol to apoA-I and is induced by liver X receptor (LXR)/retinoid X receptor (RXR) transcription factors. Retinoic acid receptor (RAR) activators (all-trans-retinoic acid [ATRA] and TTNPB) were found to increase ATP-binding cassette transporter 1 (ABCA1) mRNA and protein in macrophages. In cellular cotransfection assays, RARgamma/RXR activated the human ABCA1 promoter, via the same direct repeat 4 (DR4) promoter element as LXR/RXR. Chromatin immunoprecipitation analysis in macrophages confirmed the binding of RARgamma/RXR to the ABCA1 promoter DR4 element in the presence of ATRA, with weaker binding of RARalpha/RXR, and no binding of RARbeta/RXR. However, in macrophages from RARgamma(-/-) mice, TTNPB still induced ABCA1, in association with marked upregulation of RARalpha, suggesting that high levels of RARalpha can compensate for the absence of RARgamma. Dose-response experiments with ATRA in mouse primary macrophages showed that other LXR target genes were weakly induced (ABCG1 and SREBP-1c) or not induced (apoE and LXRalpha). The more specific RAR activator TTNPB did not induce SREBP-1c in mouse primary macrophages or liver. These studies indicate a direct role of RARgamma/RXR in induction of macrophage ABCA1.

Miniaturization of Cell-Based β-Lactamase-Dependent FRET Assays to Ultra-High Throughput Formats to Identify Agonists of Human Liver X Receptors

Activation of liver X receptors (LXRs) induces reverse cholesterol transport and increases high-density lipoprotein cholesterol in vivo. Here, we describe novel, functional, homogeneous cell-based fluorescence resonance energy transfer assays for identifying agonists of LXRs using beta-lactamase as the reporter gene. Stable Chinese hamster ovary cell lines expressing LXRalpha-GAL4 or LXRbeta-GAL4 fusion proteins that regulate beta-lactamase transcription from upstream 7 x UAS GAL4 DNA binding sequences were generated and characterized. Synthetic and natural ligands of LXR dose-dependently activated the expression of beta-lactamase in a subtype-specific manner. These assays were used to demonstrate that a 1-pyridyl hydantoin small molecule LXR synthetic ligand specifically activates LXRalpha receptors. The beta-lactamase assays were optimized for cell density, dimethyl sulfoxide sensitivity, and time of agonist stimulation. Clonal LXRbeta-GAL4-beta-lactamase cells were miniaturized into an ultra high throughput (3456-well nanoplates) screening format.

Oxysterol and 9-cis-retinoic acid stimulate the group IIA secretory phospholipase A2 gene in rat smooth-muscle cells

The inflammation that occurs during rheumatoid arthritis or atherosclerosis is characterized by the release of large amounts of sPLA(2) (group IIA secretory phospholipase A(2)). We have shown previously that the sPLA(2) promoter in SMC (smooth-muscle cells) is activated by interleukin-1beta and cAMP-signalling pathways, through the interplay of multiple transcription factors [Antonio, Brouillet, Janvier, Monne, Bereziat, Andreani, and Raymondjean (2002) Biochem. J. 368, 415-424]. In the present study, we have investigated the regulation of sPLA(2) gene expression in rat aortic SMCs by oxysterols. We found that oxysterol ligands that bind to the LXR (liver X receptor), including 25-HC (25-hydroxycholesterol) and 22( R )-HC, cause the accumulation of sPLA(2) mRNA and an increased enzyme activity. Transient transfection experiments demonstrated that the sPLA(2) promoter is synergistically activated by 22( R )-HC in combination with 9- cis -retinoic acid, a ligand for the LXR heterodimeric partner RXR (retinoid X receptor). Promoter activity was also increased in a sterol-responsive fashion when cells were co-transfected with LXRalpha/RXRalpha or LXRbeta/RXRalpha. Mutagenesis studies and gel mobility-shift assays revealed that LXR/RXR heterodimers regulate sPLA(2) transcription directly, by interacting with a degenerated LXRE (LXR response element) at position [-421/-406] of the sPLA(2) promoter. Chromatin immunoprecipitation revealed the in vivo occupancy of LXR on the sPLA(2) promoter. In addition, the orphan nuclear receptor LRH-1 (liver receptor homologue-1) potentiated the sterol-dependent regulation of the sPLA(2) promoter by binding to an identified promoter element (TCAAGGCTG). Finally, we have demonstrated that oxysterols act independent of interleukin-1beta and cAMP pathways to activate the sPLA(2) promoter. In the present study, we have identified a new pathway activating sPLA(2) gene expression in SMCs.

Molecular determinants of LXRalpha agonism

Liver X receptors (LXRs) are nuclear receptors that participate in the regulation of cholesterol, bile acid, and glucose metabolism. Despite the identification of the natural oxysterol and nonsteroidal ligands for LXRalpha, little is known about the structure of the LXRalpha ligand-binding domain (LBD). We constructed a three-dimensional (3D) homology model of the LBD of LXRalpha based on the crystal structure of the retinoic acid receptor gamma (RARgamma) and all-trans retinoic acid complex. We combined molecular modeling and classical structure-function techniques to define the interactions between the LBD and three structurally diverse ligands, 22(R)-hydroxycholesterol (22RHC), N-(2,2,2-trifluoro-ethyl)-N-[4-(2,2,2-trifluoro-1-hydroxy-1-trifluoromethyl-ethyl)-phenyl]-benzenesulfonamide (T0901317) and (3-[3-[(2-chloro-3-trifluoromethyl-benzyl)-(2,2-diphenyl-ethyl)-amino]-propoxy]-phenyl)-acetic acid (GW3965). Sixteen individual amino acid point mutations were made in the predicted ligand-binding cavity of the LBD, and each of these mutant receptors was assessed for their ability to be activated by these three ligands. The majority of individual mutations resulted in lack of activation by all three ligands. Two residues were identified that resulted in a significant increase in basal activity while retaining responsiveness to the ligands. Interestingly, a number of residues were identified that appear to be selective in their response to a particular ligand, indicating that these three ligands recognize distinct structural components within the ligand-binding cavity. These data, together with our docking study, enable us to identify the amino acids that coordinate the interaction of both steroidal and non-steroidal ligands in the ligand-binding pocket of LXRalpha.

The LXR ligand T0901317 induces severe lipogenesis in the db/db diabetic mouse

Liver X receptor (LXR) ligands are currently being evaluated as potential therapeutic agents for the treatment of low HDL. The LXR ligand T0901317 elevates ATP binding cassette transporter A1 (ABCA1) and HDL levels in animal models and induces moderate lipogenesis through upregulation of sterol regulatory element binding protein 1c (SREBP1c). Because insulin may also regulate lipogenesis through SREBP1c and fatty acid synthase (FAS), we investigated the effect of an LXR ligand in hyperinsulinemic mice. Administration of T0901317 to male db/db mice for 12 days resulted in a more severe hypertriacylglycerolemia and hepatic triacylglycerol accumulation than observed in nondiabetic mice. The LXR target genes ABCA1, SREBP1c, FAS, and stearoyl-CoA desaturase 1 were upregulated by T0901317 treatment in both diabetic db/db and nondiabetic C57BLKS mice. Changes in lipogenic gene expression were independent of mouse strain, indicating that the severe lipogenesis observed in LXR ligand-treated db/db mice was not due to additive effects of insulin on lipogenic gene expression. Phosphoenolpyruvate carboxykinase expression was suppressed, suggesting that a shift from gluconeogenesis toward lipogenesis could partially explain our observations in db/db mice. Our data suggest that LXR ligands that have effects on both fatty acid and carbohydrate metabolism should be carefully evaluated in obesity, insulin, and leptin resistance.

Transdifferentiation of mouse aortic smooth muscle cells to a macrophage-like state after cholesterol loading

Mouse aortic smooth muscle cells (SMCs) were loaded for 72 h with cholesterol by using cholesterol:methyl-beta-cyclodextrin complexes, leading to approximately 2-fold and approximately 10-fold increases in the contents of total cholesterol and cholesteryl ester, respectively. Foam-cell formation was demonstrated by accumulation of intracellular, Oil Red O-stained lipid droplets. Immunostaining showed decreased protein levels of smooth muscle alpha-actin and alpha-tropomyosin and increased levels of macrophage markers CD68 and Mac-2 antigen. Quantitative real-time RT-PCR revealed that after cholesterol loading, the expression of SMC-related genes alpha-actin, alpha-tropomyosin, myosin heavy chain, and calponin H1 decreased (to 11.5 +/- 0.5%, 29.3 +/- 1.4%, 23.8 +/- 1.4%, and 3.8 +/- 0.5% of unloaded cells, respectively; P < 0.05 for all), whereas expression of macrophage-related genes CD68, Mac-2, and ABCA1 mRNA increased (to 709 +/- 84%, 330 +/- 11%, and 207 +/- 13% of unloaded cells, respectively; P < 0.05 for all), thereby demonstrating that the protein changes were regulated at the mRNA level. Furthermore, these changes were accompanied by a gain in macrophage-like function as assessed by phagocytotic activity. Expression of vascular cell adhesion molecule 1 and monocyte chemoattractant protein 1, known responders to inflammation, were not changed. In conclusion, cholesterol loading of SMC causes phenotypic changes regulated at the mRNA level that result in a transdifferentiation to a macrophage-like state. This finding suggests that not all foam cells in lesions may have a macrophage origin, despite what is indicated by immunostaining for macrophage-related markers. Furthermore, inflammatory changes in foam cells observed in vivo may not be simple consequences of cholesterol accumulation.