Loss of NR2E3 represses AHR by LSD1 reprogramming, is associated with poor prognosis in liver cancer.

The aryl hydrocarbon receptor (AHR) plays crucial roles in inflammation, metabolic disorder, and cancer. However, the molecular mechanisms regulating AHR expression remain unknown. Here, we found that an orphan nuclear NR2E3 maintains AHR expression, and forms an active transcriptional complex with transcription factor Sp1 and coactivator GRIP1 in MCF-7 human breast and HepG2 liver cancer cell lines. NR2E3 loss promotes the recruitment of LSD1, a histone demethylase of histone 3 lysine 4 di-methylation (H3K4me2), to the AHR gene promoter region, resulting in repression of AHR expression. AHR expression and responsiveness along with H3K4me2 were significantly reduced in the livers of Nr2e3rd7 (Rd7) mice that express low NR2E3 relative to the livers of wild-type mice. SP2509, an LSD1 inhibitor, fully restored AHR expression and H3K4me2 levels in Rd7 mice. Lastly, we demonstrated that both AHR and NR2E3 are significantly associated with good clinical outcomes in liver cancer. Together, our results reveal a novel link between NR2E3, AHR, and liver cancer via LSD1-mediated H3K4me2 histone modification in liver cancer development.

Cardiac LXRα protects against pathological cardiac hypertrophy and dysfunction by enhancing glucose uptake and utilization.

Pathological cardiac hypertrophy is characterized by a shift in metabolic substrate utilization from fatty acids to glucose, but the molecular events underlying the metabolic remodeling remain poorly understood. Here, we investigated the role of liver X receptors (LXRs), which are key regulators of glucose and lipid metabolism, in cardiac hypertrophic pathogenesis. Using a transgenic approach in mice, we show that overexpression of LXRα acts to protect the heart against hypertrophy, fibrosis, and dysfunction. Gene expression profiling studies revealed that genes regulating metabolic pathways were differentially expressed in hearts with elevated LXRα. Functionally, LXRα overexpression in isolated cardiomyocytes and murine hearts markedly enhanced the capacity for myocardial glucose uptake following hypertrophic stress. Conversely, this adaptive response was diminished in LXRα-deficient mice. Transcriptional changes induced by LXRα overexpression promoted energy-independent utilization of glucose via the hexosamine biosynthesis pathway, resulting in O-GlcNAc modification of GATA4 and Mef2c and the induction of cytoprotective natriuretic peptide expression. Our results identify LXRα as a key cardiac transcriptional regulator that helps orchestrate an adaptive metabolic response to chronic cardiac stress, and suggest that modulating LXRα may provide a unique opportunity for intervening in myocyte metabolism.

Loss of the liver X receptor LXRα/ß in peripheral sensory neurons modifies energy expenditure.

Peripheral neural sensory mechanisms play a crucial role in metabolic regulation but less is known about the mechanisms underlying vagal sensing itself. Recently, we identified an enrichment of liver X receptor alpha and beta (LXRα/ß) in the nodose ganglia of the vagus nerve. In this study, we show mice lacking LXRα/ß in peripheral sensory neurons have increased energy expenditure and weight loss when fed a Western diet (WD). Our findings suggest that the ability to metabolize and sense cholesterol and/or fatty acids in peripheral neurons is an important requirement for physiological adaptations to WDs.

Liver X receptors alpha and beta promote myelination and remyelination in the cerebellum.

The identification of new pathways governing myelination provides innovative avenues for remyelination. Liver X receptors (LXRs) α and ß are nuclear receptors activated by oxysterols that originated from the oxidation of cholesterol. They are crucial for cholesterol homeostasis, a major lipid constituent of myelin sheaths that are formed by oligodendrocytes. However, the role of LXRs in myelin generation and maintenance is poorly understood. Here, we show that LXRs are involved in myelination and remyelination processes. LXRs and their ligands are present in oligodendrocytes. We found that mice invalidated for LXRs exhibit altered motor coordination and spatial learning, thinner myelin sheaths, and reduced myelin gene expression. Conversely, activation of LXRs by either 25-hydroxycholesterol or synthetic TO901317 stimulates myelin gene expression at the promoter, mRNA, and protein levels, directly implicating LXRα/ß in the transcriptional control of myelin gene expression. Interestingly, activation of LXRs also promotes oligodendroglial cell maturation and remyelination after lysolecithin-induced demyelination of organotypic cerebellar slice cultures. Together, our findings represent a conceptual advance in the transcriptional control of myelin gene expression and strongly support a new role of LXRs as positive modulators in central (re)myelination processes.

Liver X receptor regulates hepatic nuclear O-GlcNAc signaling and carbohydrate responsive element-binding protein activity.

Liver X receptor (LXR)α and LXRß play key roles in hepatic de novo lipogenesis through their regulation of lipogenic genes, including sterol regulatory element-binding protein (SREBP)-1c and carbohydrate responsive element-binding protein (ChREBP). LXRs activate lipogenic gene transcription in response to feeding, which is believed to be mediated by insulin. We have previously shown that LXRs are targets for glucose-hexosamine-derived O-linked ß-N-acetylglucosamine (O-GlcNAc) modification enhancing their ability to regulate SREBP-1c promoter activity in vitro. To elucidate insulin-independent effects of feeding on LXR-mediated lipogenic gene expression in vivo, we subjected control and streptozotocin-treated LXRα/ß(+/+) and LXRα/ß(-/-) mice to a fasting-refeeding regime. We show that under hyperglycemic and hypoinsulinemic conditions, LXRs maintain their ability to upregulate the expression of glycolytic and lipogenic enzymes, including glucokinase (GK), SREBP-1c, ChREBPα, and the newly identified shorter isoform ChREBPß. Furthermore, glucose-dependent increases in LXR/retinoid X receptor-regulated luciferase activity driven by the ChREBPα promoter was mediated, at least in part, by O-GlcNAc transferase (OGT) signaling in Huh7 cells. Moreover, we show that LXR and OGT interact and colocalize in the nucleus and that loss of LXRs profoundly reduced nuclear O-GlcNAc signaling and ChREBPα promoter binding activity in vivo. In summary, our study provides evidence that LXRs act as nutrient and glucose metabolic sensors upstream of ChREBP by modulating GK expression, nuclear O-GlcNAc signaling, and ChREBP expression and activity.

A systems biology approach reveals the physiological origin of hepatic steatosis induced by liver X receptor activation.

Liver X receptor (LXR) agonists exert potent antiatherosclerotic actions but simultaneously induce excessive triglyceride (TG) accumulation in the liver. To obtain a detailed insight into the underlying mechanism of hepatic TG accumulation, we used a novel computational modeling approach called analysis of dynamic adaptations in parameter trajectories (ADAPT). We revealed that both input and output fluxes to hepatic TG content are considerably induced on LXR activation and that in the early phase of LXR agonism, hepatic steatosis results from only a minor imbalance between the two. It is generally believed that LXR-induced hepatic steatosis results from increased de novo lipogenesis (DNL). In contrast, ADAPT predicted that the hepatic influx of free fatty acids is the major contributor to hepatic TG accumulation in the early phase of LXR activation. Qualitative validation of this prediction showed a 5-fold increase in the contribution of plasma palmitate to hepatic monounsaturated fatty acids on acute LXR activation, whereas DNL was not yet significantly increased. This study illustrates that complex effects of pharmacological intervention can be translated into distinct patterns of metabolic regulation through state-of-the-art mathematical modeling.

Anti-inflammatory actions of acanthoic acid-related diterpenes involve activation of the PI3K p110γ/δ subunits and inhibition of NF-κB.

The effect of acanthoic acid analogs on the response to proinflammatory challenge was investigated. Some pimarane diterpenes are known activators of the LXRαß nuclear receptors, but we show here that they also exert a rapid, potent, and selective activation of the p110γ and p110δ subunits of PI3K. Combination of these effects results in an important attenuation of the global transcriptional response to LPS in macrophages. PI3K/Akt activation leads to inhibition of the LPS-dependent stimulation of IKK/NF-κB and p38 and ERK MAPKs. Macrophages from LXRαß-deficient mice exhibited an inhibition of these pathways similar to the corresponding wild-type cells. Silencing or inhibition of p110γ/δ suppressed the effect of these diterpenes (DTPs) on IKK/NF-κB and MAPKs signaling. Taken together, these data show a multitarget anti-inflammatory mechanism by these DTPs including a selective activation of PI3K isoenzymes.

Macrophage-independent regulation of reverse cholesterol transport by liver X receptors.

OBJECTIVE: The ability of high-density lipoprotein (HDL) particles to accept cholesterol from peripheral cells, such as lipid-laden macrophages, and to transport cholesterol to the liver for catabolism and excretion in a process termed reverse cholesterol transport (RCT) is thought to underlie the beneficial cardiovascular effects of elevated HDL. The liver X receptors (LXRs; LXRα and LXRß) regulate RCT by controlling the efflux of cholesterol from macrophages to HDL and the excretion, catabolism, and absorption of cholesterol in the liver and intestine. Importantly, treatment with LXR agonists increases RCT and decreases atherosclerosis in animal models. Nevertheless, LXRs are expressed in multiple tissues involved in RCT, and their tissue-specific contributions to RCT are still not well defined. APPROACH AND RESULTS: Using tissue-specific LXR deletions together with in vitro and in vivo assays of cholesterol efflux and fecal cholesterol excretion, we demonstrate that macrophage LXR activity is neither necessary nor sufficient for LXR agonist-stimulated RCT. In contrast, the ability of LXR agonists primarily acting in the intestine to increase HDL mass and HDL function seems to underlie the ability of LXR agonists to stimulate RCT in vivo. CONCLUSIONS: We demonstrate that activation of LXR in macrophages makes little or no contribution to LXR agonist-stimulated RCT. Unexpectedly, our studies suggest that the ability of macrophages to efflux cholesterol to HDL in vivo is not regulated by macrophage activity but is primarily determined by the quantity and functional activity of HDL.

LXRα fuels fatty acid-stimulated oxygen consumption in white adipocytes.

Liver X receptors (LXRs) are transcription factors known for their role in hepatic cholesterol and lipid metabolism. Though highly expressed in fat, the role of LXR in this tissue is not well characterized. We generated adipose tissue LXRα knockout (ATaKO) mice and showed that these mice gain more weight and fat mass on a high-fat diet compared with wild-type controls. White adipose tissue (WAT) accretion in ATaKO mice results from both a decrease in WAT lipolytic and oxidative capacities. This was demonstrated by decreased expression of the ß2- and ß3-adrenergic receptors, reduced level of phosphorylated hormone-sensitive lipase, and lower oxygen consumption rates (OCRs) in WAT of ATaKO mice. Furthermore, LXR activation in vivo and in vitro led to decreased adipocyte size in WAT and increased glycerol release from primary adipocytes, respectively, with a concomitant increase in OCR in both models. Our findings show that absence of LXRα in adipose tissue results in elevated adiposity through a decrease in WAT oxidation, secondary to attenuated FA availability.

Calcium signaling via Orai1 is essential for induction of the nuclear orphan receptor pathway to drive Th17 differentiation.

Orai1 is the pore subunit of Ca(2+) release-activated Ca(2+) (CRAC) channels that stimulate downstream signaling pathways crucial for T cell activation. CRAC channels are an attractive therapeutic target for alleviation of autoimmune diseases. Using high-throughput chemical library screening targeting Orai1, we identified a novel class of small molecules that inhibit CRAC channel activity. One of these molecules, compound 5D, inhibited CRAC channel activity by blocking ion permeation. When included during differentiation, Th17 cells showed higher sensitivity to compound 5D than Th1 and Th2 cells. The selectivity was attributable to high dependence of promoters of retinoic-acid-receptor-related orphan receptors on the Ca(2+)-NFAT pathway. Blocking of CRAC channels drastically decreased recruitment of NFAT and histone modifications within key gene loci involved in Th17 differentiation. The impairment in Th17 differentiation by treatment with CRAC channel blocker was recapitulated in Orai1-deficient T cells, which could be rescued by exogenous expression of retinoic-acid-receptor-related orphan receptors or a constitutive active mutant of NFAT. In vivo administration of CRAC channel blockers effectively reduced the severity of experimental autoimmune encephalomyelitis by suppression of differentiation of inflammatory T cells. These results suggest that CRAC channel blockers can be considered as chemical templates for the development of therapeutic agents to suppress inflammatory responses.

Reciprocal regulation of hepatic and adipose lipogenesis by liver X receptors in obesity and insulin resistance.

Liver X receptors (LXRs) regulate lipogenesis and inflammation, but their contribution to the metabolic syndrome is unclear. We show that LXRs modulate key aspects of the metabolic syndrome in mice. LXRαß-deficient-ob/ob (LOKO) mice remain obese but show reduced hepatic steatosis and improved insulin sensitivity compared to ob/ob mice. Impaired hepatic lipogenesis in LOKO mice is accompanied by reciprocal increases in adipose lipid storage, reflecting tissue-selective effects on the SREBP, PPARγ, and ChREBP lipogenic pathways. LXRs are essential for obesity-driven SREBP-1c and ChREBP activity in liver, but not fat. Furthermore, loss of LXRs in obesity promotes adipose PPARγ and ChREBP-ß activity, leading to improved insulin sensitivity. LOKO mice also exhibit defects in ß cell mass and proliferation despite improved insulin sensitivity. Our data suggest that sterol sensing by LXRs in obesity is critically linked with lipid and glucose homeostasis and provide insight into the complex relationships between LXR and insulin signaling.

Nuclear receptor LXR: a new partner for sodium-dependent phosphate cotransporters.

New pharmaceutical research approaches are focusing on trying to alleviate the perturbed phosphate (Pi) homeostasis associated with the onset of chronic kidney disease; this includes activation of some of the nuclear receptors. We have recently reported the down regulation of the intestinal and renal sodium-phosphate (NaPi) cotransporters by the liver X receptor (LXR) agonists, and the consequent decrease of the serum Pi levels. In this review, we describe our current knowledge of the different proteins involved in the renal and intestinal actions of LXR.

Essential fatty acids deficiency promotes lipogenic gene expression and hepatic steatosis through the liver X receptor.

BACKGROUND & AIMS: Nutrients influence non-alcoholic fatty liver disease. Essential fatty acids deficiency promotes various syndromes, including hepatic steatosis, through increased de novo lipogenesis. The mechanisms underlying such increased lipogenic response remain unidentified. METHODS: We used wild type mice and mice lacking Liver X Receptors to perform a nutrigenomic study that aimed at examining the role of these transcription factors. RESULTS: We showed that, in the absence of Liver X Receptors, essential fatty acids deficiency does not promote steatosis. Consistent with this, Liver X Receptors are required for the elevated expression of genes involved in lipogenesis in response to essential fatty acids deficiency. CONCLUSIONS: This work identifies, for the first time, the central role of Liver X Receptors in steatosis induced by essential fatty acids deficiency.

Thyroid hormone-responsive SPOT 14 homolog promotes hepatic lipogenesis, and its expression is regulated by liver X receptor α through a sterol regulatory element-binding protein 1c-dependent mechanism in mice.

UNLABELLED: The protein, thyroid hormone-responsive SPOT 14 homolog (Thrsp), has been reported to be a lipogenic gene in cultured hepatocytes, implicating an important role of Thrsp in the pathogenesis of nonalcoholic fatty liver disease (NAFLD). Thrsp expression is known to be regulated by a variety of transcription factors, including thyroid hormone receptor, pregnane X receptor, and constitutive androstane receptor. Emerging in vitro evidence also points to a critical role of liver X receptor (LXR) in regulating Thrsp transcription in hepatocytes. In the present study, we showed that Thrsp was up-regulated in livers of db/db mice and high-fat-diet-fed mice, two models of murine NAFLD. Hepatic overexpression of Thrsp increased triglyceride accumulation with enhanced lipogenesis in livers of C57Bl/6 mice, whereas hepatic Thrsp gene silencing attenuated the fatty liver phenotype in db/db mice. LXR activator TO901317 induced Thrsp expression in livers of wild-type (WT) and LXR-ß gene-deficient mice, but not in LXR-α or LXR-α/ß double-knockout mice. TO901317 treatment significantly enhanced hepatic sterol regulatory element-binding protein 1c (SREBP-1c) expression and activity in WT mice, but failed to induce Thrsp expression in SREBP-1c gene-deficient mice. Sequence analysis revealed four LXR response-element-like elements and one sterol regulatory element (SRE)-binding site within a -2,468 ∼+1-base-pair region of the Thrsp promoter. TO901317 treatment and LXR-α overexpression failed to induce, whereas overexpression of SREBP-1c significantly increased Thrsp promoter activity. Moreover, deletion of the SRE site completely abolished SREBP-1c-induced Thrsp transcription. CONCLUSION: Thrsp is a lipogenic gene in the liver that is induced by the LXR agonist through an LXR-α-mediated, SREBP-1c-dependent mechanism. Therefore, Thrsp may represent a potential therapeutic target for the treatment of NAFLD.

A systems biology approach to the hepatic role of the oxysterol receptor LXR in the regulation of lipogenesis highlights a cross-talk with PPARα.

The Liver X Receptors (LXRs) α and ß and the Peroxisome Proliferator-Activated Receptor α (PPARα) are transcription factors that belong to class II nuclear receptors. They drive the expression of genes involved in hepatic lipid homeostasis and therefore are important targets for the prevention and treatment of nonalcoholic fatty liver disease (NAFLD). LXRs and PPARα are regulated by endogenous ligands, oxysterols and fatty acid derived molecules, respectively. In the liver, pharmacological activation of LXRs leads to the over-expression of genes involved in de novo lipogenesis, while PPARα is critical for fatty acid catabolism in nutrient deprivation. Even if these two nuclear receptors seemed to play opposite parts, recent studies have highlighted that PPARα also influence the expression of genes involved in fatty acids synthesis. In this study, we used pharmacological approaches and genetically engineered mice to investigate the cross-talk between LXRs and PPARα in the regulation of genes responsible for lipogenesis. We first investigated the effect of T0901317 and fenofibrate, two synthetic agonists of LXRs and PPARα, respectively. As expected, T0901317 and fenofibrate induce expression of genes involved LXR-dependent and PPARα-dependent lipogenic responses. Considering such overlapping effect, we then tested whether LXR agonist may influence PPARα driven response and vice versa. We show that the lack of PPARα does not influence the effects of T0901317 on lipogenic genes expression. However, PPARα deficiency prevents the up-regulation of genes involved in ω-hydroxylation that are induced by the LXR agonist. In addition, over-expression of lipogenic genes in response to fenofibrate is decreased in LXR knockout mice as well as the expression of PPARα target genes involved in fatty acid oxidation. Altogether, our work provides in vivo evidence for a central interconnection between nuclear receptors that drive hepatic lipid metabolism in response to oxysterol and fatty acids.

Liver X receptor activation reduces angiogenesis by impairing lipid raft localization and signaling of vascular endothelial growth factor receptor-2.

OBJECTIVE: Liver X receptors (LXRα, LXRß) are master regulators of cholesterol homeostasis. In the endothelium, perturbations of cell cholesterol have an impact on fundamental processes. We, therefore, assessed the effects of LXR activation on endothelial functions related to angiogenesis in vitro and in vivo. METHODS AND RESULTS: LXR agonists (T0901317, GW3965) blunted migration, tubulogenesis, and proliferation of human umbilical vein endothelial cells. By affecting endothelial cholesterol homeostasis, LXR activation impaired the compartmentation of vascular endothelial growth factor receptor-2 in lipid rafts/caveolae and led to defective phosphorylation and downstream signaling of vascular endothelial growth factor receptor-2 upon vascular endothelial growth factor-A stimulation. Consistently, the antiangiogenic actions of LXR agonists could be prevented by coadministration of exogenous cholesterol. LXR agonists reduced endothelial sprouting from wild-type but not from LXRα(-/-)/LXRß(-/-) knockout aortas and blunted the vascularization of implanted angioreactors in vivo. Furthermore, T0901317 reduced the growth of Lewis lung carcinoma grafts in mice by impairing angiogenesis. CONCLUSIONS: Pharmacological activation of endothelial LXRs reduces angiogenesis by restraining cholesterol-dependent vascular endothelial growth factor receptor-2 compartmentation and signaling. Thus, administration of LXR agonists could exert therapeutic effects in pathological conditions characterized by uncontrolled angiogenesis.

Anxiety in liver X receptor ß knockout female mice with loss of glutamic acid decarboxylase in ventromedial prefrontal cortex.

Anxiety disorders are the most prevalent mental disorders in adolescents in the United States. Female adolescents are more likely than males to be affected with anxiety disorders, but less likely to have behavioral and substance abuse disorders. The prefrontal cortex (PFC), amygdala, and dorsal raphe are known to be involved in anxiety disorders. Inhibitory input from the PFC to the amygdala controls fear and anxiety typically originating in the amygdala, and disruption of the inhibitory input from the PFC leads to anxiety, fear, and personality changes. Recent studies have implicated liver X receptor ß (LXRß) in key neurodevelopmental processes and neurodegenerative diseases. In the present study, we used elevated plus-maze, startle and prepulse inhibition, open field, and novel object recognition tests to evaluate behavior in female LXRß KO (LXRß(-/-)) mice. We found that the female LXRß(-/-) mice were anxious with impaired behavioral responses but normal locomotion and memory. Immunohistochemistry analysis revealed decreased expression of the enzyme responsible for GABA synthesis, glutamic acid decarboxylase (65+67), in the ventromedial PFC. Expression of tryptophan hydroxylase 2 in the dorsal raphe was normal. We conclude that the anxiogenic phenotype in female LXRß(-/-) mice is caused by reduced GABAergic input from the ventromedial PFC to the amygdala.

Pharmacological LXR activation reduces presence of SR-B1 in liver membranes contributing to LXR-mediated induction of HDL-cholesterol.

OBJECTIVE: Pharmacological LXR activation has anti-atherosclerotic actions in animal models. Part of these beneficial effects may be explained by accelerated reverse cholesterol transport since both plasma high density lipoprotein (HDL) cholesterol and fecal neutral sterol secretion are higher upon LXR activation. Mechanisms underlying these LXR-mediated effects have not been fully elucidated. METHODS: We investigated the roles of the isoforms LXRα and LXRß and the HDL cholesterol uptake receptor SR-B1 in modulation of cholesterol metabolism upon treatment of mice with the LXR ligand T0901317. RESULTS: HDL cholesterol was maximally 60% increased in a time-dependent fashion due to appearance of more and larger HDL particles. Fecal neutral sterol secretion was maximally induced after 1 week treatment. T0901317 treatment induced fecal neutral sterol secretion by ~300% in wild-type but not in Lxrα deficient mice. Surprisingly, LXR activation reduced SR-B1 protein amount in hepatic membranes, suggesting that this might contribute to elevated HDL cholesterol. However, T0901317 still elevated plasma HDL cholesterol in Sr-b1 deficient mice, suggesting that SR-B1 is not the only step involved in LXR-mediated induction of plasma HDL cholesterol. In addition, SR-B1 is not essential for LXR-induced cholesterol removal from the body. CONCLUSION: Induction of fecal neutral sterol secretion by T0901317 critically depends on LXRα but not on LXRß. LXR activation reduces SR-B1 in hepatic membranes, probably partly contributing to elevated HDL cholesterol. SR-B1 is not required to enhance fecal neutral sterol secretion.

Central diabetes insipidus associated with impaired renal aquaporin-1 expression in mice lacking liver X receptor ß.

The present study demonstrates a key role for the oxysterol receptor liver X receptor ß (LXRß) in the etiology of diabetes insipidus (DI). Given free access to water, LXRß(-/-) but not LXRα(-/-) mice exhibited polyuria (abnormal daily excretion of highly diluted urine) and polydipsia (increased water intake), both features of diabetes insipidus. LXRß(-/-) mice responded to 24-h dehydration with a decreased urine volume and increased urine osmolality. To determine whether the DI was of central or nephrogenic origin, we examined the responsiveness of the kidney to arginine vasopressin (AVP). An i.p. injection of AVP to LXRß(-/-) mice revealed a partial kidney response: There was no effect on urine volume, but there was a significant increase of urine osmolality, suggesting that DI may be caused by a defect in central production of AVP. In the brain of WT mice LXRß was expressed in the nuclei of magnocellular neurons in the supraoptic and paraventricular nuclei of the hypothalamus. In LXRß(-/-) mice the expression of AVP was markedly decreased in the magnocellular neurons as well as in urine collected over a 24-h period. The persistent high urine volume after AVP administration was traced to a reduction in aquaporin-1 expression in the kidney of LXRß(-/-) mice. The LXR agonist (GW3965) in WT mice elicited an increase in urine osmolality, suggesting that LXRß is a key receptor in controlling water balance with targets in both the brain and kidney, and it could be a therapeutic target in disorders of water balance.

Genome-wide profiling of liver X receptor, retinoid X receptor, and peroxisome proliferator-activated receptor α in mouse liver reveals extensive sharing of binding sites.

The liver X receptors (LXRs) are nuclear receptors that form permissive heterodimers with retinoid X receptor (RXR) and are important regulators of lipid metabolism in the liver. We have recently shown that RXR agonist-induced hypertriglyceridemia and hepatic steatosis in mice are dependent on LXRs and correlate with an LXR-dependent hepatic induction of lipogenic genes. To further investigate the roles of RXR and LXR in the regulation of hepatic gene expression, we have mapped the ligand-regulated genome-wide binding of these factors in mouse liver. We find that the RXR agonist bexarotene primarily increases the genomic binding of RXR, whereas the LXR agonist T0901317 greatly increases both LXR and RXR binding. Functional annotation of putative direct LXR target genes revealed a significant association with classical LXR-regulated pathways as well as peroxisome proliferator-activated receptor (PPAR) signaling pathways, and subsequent chromatin immunoprecipitation-sequencing (ChIP-seq) mapping of PPARα binding demonstrated binding of PPARα to 71 to 88% of the identified LXR-RXR binding sites. The combination of sequence analysis of shared binding regions and sequential ChIP on selected sites indicate that LXR-RXR and PPARα-RXR bind to degenerate response elements in a mutually exclusive manner. Together, our findings suggest extensive and unexpected cross talk between hepatic LXR and PPARα at the level of binding to shared genomic sites.