Liver X receptor (LXR) regulation of the LXRalpha gene in human macrophages

Targeting the ceramidase ACER3 attenuates cholestasis in mice by mitigating bile acid overload via unsaturated ceramide-mediated LXRβ signaling transduction

Bile acid overload critically drives the pathogenesis of cholestatic liver injury (CLI). While ceramide metabolism has garnered increasing interest in liver research, the role of ceramides in CLI remains unclear. This study investigates the function of alkaline ceramidase 3 (ACER3)-catalyzed hydrolysis of unsaturated ceramides in CLI. Using clinical specimens, this work finds that ACER3 expression is upregulated in the cholestatic liver and positively correlated with the severity of CLI in patients. Acer3 ablation increases ceramide(d18:1/18:1) and attenuates bile duct ligation-induced CLI in female mice with reduced hepatic necrosis, inflammation, and fibrosis. However, it does not significantly impact CLI in male mice. Moreover, ceramide(d18:1/18:1) treatment attenuates CLI in wild-type female mice. Similarly, ACER3 knockdown and ceramide(d18:1/18:1) treatment prevent lithocholic-acid-induced cell death in human-liver-derived HepG2 cells. Mechanistically, ceramide(d18:1/18:1) binds the ligand binding domain of the liver X receptor β, acting as an agonist to activate its transcriptional functions. This activation upregulates sulfotransferase 2A1-catalyzed bile acid sulfation, normalizes bile acid metabolism, and restores lipogenesis, thereby reducing bile acid overload in hepatocytes to attenuate CLI. Our findings uncover the role of ceramide(d18:1/18:1)-liver X receptor β signaling in mitigating bile acid overload in the cholestatic liver, offering mechanistic insights and suggesting therapeutic potential for targeting ACER3 and ceramide(d18:1/18:1) for CLI.

Associations between liver X receptor polymorphisms and blood lipids: A systematic review and meta-analysis

Genetic susceptibility to dyslipidaemia remains incompletely understood. The liver X receptors (LXRs), members of the nuclear receptor superfamily of ligand dependent transcription factors, are homeostatic regulators of lipid metabolism. Multiple single nucleotide polymorphisms (SNPs)have been identified previously in the coding and regulatory regions of the LXRs. The aim of this systematic review and meta-analysis was to summarise associations between SNPs of LXRs (α and β isoforms) with blood lipid and lipoprotein traits. Five databases (PubMed, Ovid Embase, Scopus, Web of Science, and the Cochrane Library) were systematically searched for population-based studies that assessed associations between one or more blood lipid/lipoprotein traits and LXR SNPs. Of seventeen articles included in the qualitative synthesis, ten were eligible for meta-analysis. Nine LXRα SNPs and five LXRβ SNPs were identified, and the three most studied LXRα SNPs were quantitatively summarised. Carriers of the minor allele A of LXRα rs12221497 (-115G>A) had higher triglyceride levels than GG homozygotes (0.13 mmol/L; 95%CI: [0.03, 0.23], P = 0.01). Heterozygote carriers of LXRα rs2279238 (297C/T) had higher total cholesterol levels (0.12 mmol/L; (95%CI: [0.01, 0.23], P = 0.04) than either CC or TT homozygotes. For LXRα rs11039155 (-6G>A), no significant differences in blood levels of either triglyceride (P = 0.39) or HDL-C (P = 0.98) were detected between genotypes in meta-analyses. In addition, there were no strong associations for other SNPs of LXRα and LXRβ. This study provides the evidence of an association between LXRα, but not LXRβ, SNPs and blood-lipid traits. Systematic review registration: PROSPERO No. CRD42021246158.

Responsiveness of PNPLA3 and lipid-related transcription factors is dependent upon fatty acid profile in primary bovine hepatocytes

Knockdown of patatin-like phospholipase domain-containing protein 3 (PNPLA3) increased triglycerides (TG) in primary bovine hepatocytes, suggesting that PNPLA3 plays a causal role in hepatic TG clearing. In vivo, PNPLA3 abundance across the periparturient period is inversely related to hepatic TG accumulation and circulating fatty acid (FA) concentrations. The purpose of this research was to determine if PNPLA3, as well as other lipases, transcription factors, or FA-mediated genes, are regulated by FA mimicking liver lipid accumulation (ACCUM) and liver lipid clearing (RECOV) or singular FA physiologically found in dairy cows at 0.5 mM of circulating RECOV (iRECOV). Abundance of PNPLA3 tended to decrease with ACCUM and increased quadratically with RECOV (P ≤ 0.10), differing from PNPLA3 expression, but consistent with previous in vivo research. Adipose TG lipase abundance, but not other lipase abundances, was quadratically responsive to both ACCUM and RECOV (P ≤ 0.005). Abundance of PNPLA3 and SREBP1c and expression of LXRA responded similarly to iRECOV, with C18:0 tending to decrease abundance (P ≤ 0.07). Results indicate that bovine PNPLA3 is translationally regulated by FA and although a LXRA-SREBP1c pathway mediation is possible, the mechanism warrants further investigation.

Smooth muscle 22 alpha protein inhibits VSMC foam cell formation by supporting normal LXRα signaling, ameliorating atherosclerosis

Vascular smooth muscle cells (VSMCs) are indispensable components in foam cell formation in atherosclerosis. However, the mechanism behind foam cell formation of VSMCs has not been addressed. We found a potential association between deletion of smooth muscle (SM) 22α and deregulated nuclear receptors liver X receptors (LXRs)/retinoid X receptor (RXR) signaling in mice. Here, we investigated the roles of SM22α in LXRα-modulated cholesterol homeostasis, and explore possible mechanisms underlying this process. We identified that the depletion of SM22α was a primary event driving VSMC cholesterol accumulation and the development of atherosclerosis in mice. Proteomic and lipidomic analysis validated that downregulation of SM22α was correlated with reduced expression of LXRα and ATP-binding cassette transporter (ABCA) 1 and increased cholesteryl ester in phenotypically modulated VSMCs induced by platelets-derived growth factor (PDGF)-BB. Notably, LXRα was mainly distributed in the cytoplasm rather than the nucleus in the neointimal and Sm22α^-/- VSMCs. Loss of SM22α inhibited the nuclear import of LXRα and reduced ABCA1-mediated cholesterol efflux via promoting depolymerization of actin stress fibers. Affinity purification and mass spectrometry (AP-MS) analysis, co-immunoprecipitation and GST pull-down assays, confocal microscopy, and stochastic optical reconstruction microscopy (STORM) revealed that globular-actin (G-actin), monomeric actin, interacted with and retained LXRα in the cytoplasm in PDGF-BB-treated and Sm22α^-/- VSMCs. This interaction blocked LXRα binding to Importin α, a karyopherin that mediates the trafficking of macromolecules across the nuclear envelope, and the resulting reduction of LXRα transcriptional activity. Increasing SM22α expression restored nuclear localization of LXRα and removed cholesterol accumulation via inducing actin polymerization, ameliorating atherosclerosis. Our findings highlight that LXRα is a mechanosensitive nuclear receptor and that the nuclear import of LXRα maintained by the SM22α-actin axis is a potential target for blockade of VSMC foam cell formation and development of anti-atherosclerosis.

Nicotinamide Prevents Apolipoprotein B-Containing Lipoprotein Oxidation, Inflammation and Atherosclerosis in Apolipoprotein E-Deficient Mice

The potential of nicotinamide (NAM) to prevent atherosclerosis has not yet been examined. This study investigated the effect of NAM supplementation on the development of atherosclerosis in a mouse model of the disease. The development of aortic atherosclerosis was significantly reduced (NAM low dose: 45%; NAM high dose: 55%) in NAM-treated, apolipoprotein (Apo)E-deficient mice challenged with a Western diet for 4 weeks. NAM administration significantly increased (1.8-fold) the plasma concentration of proatherogenic ApoB-containing lipoproteins in NAM high-dose (HD)-treated mice compared with untreated mice. However, isolated ApoB-containing lipoproteins from NAM HD mice were less prone to oxidation than those of untreated mice. This result was consistent with the decreased (1.5-fold) concentration of oxidized low-density lipoproteins in this group. Immunohistochemical staining of aortas from NAM-treated mice showed significantly increased levels of IL-10 (NAM low-dose (LD): 1.3-fold; NAM HD: 1.2-fold), concomitant with a significant decrease in the relative expression of TNFα (NAM LD: −44%; NAM HD: −57%). An improved anti-inflammatory pattern was reproduced in macrophages cultured in the presence of NAM. Thus, dietary NAM supplementation in ApoE-deficient mice prevented the development of atherosclerosis and improved protection against ApoB-containing lipoprotein oxidation and aortic inflammation.

Lack of mitochondrial NADP(H)-transhydrogenase expression in macrophages exacerbates atherosclerosis in hypercholesterolemic mice

The atherosclerosis prone LDL receptor knockout mice (Ldlr-/-, C57BL/6J background) carry a deletion of the NADP(H)-transhydrogenase gene (Nnt) encoding the mitochondrial enzyme that catalyzes NADPH synthesis. Here we hypothesize that both increased NADPH consumption (due to increased steroidogenesis) and decreased NADPH generation (due to Nnt deficiency) in Ldlr-/- mice contribute to establish a macrophage oxidative stress and increase atherosclerosis development. Thus, we compared peritoneal macrophages and liver mitochondria from three C57BL/6J mice lines: Ldlr and Nnt double mutant, single Nnt mutant and wild-type. We found increased oxidants production in both mitochondria and macrophages according to a gradient: double mutant > single mutant > wild-type. We also observed a parallel up-regulation of mitochondrial biogenesis (PGC1a, TFAM and respiratory complexes levels) and inflammatory (iNOS, IL6 and IL1b) markers in single and double mutant macrophages. When exposed to modified LDL, the single and double mutant cells exhibited significant increases in lipid accumulation leading to foam cell formation, the hallmark of atherosclerosis. Nnt deficiency cells showed up-regulation of CD36 and down-regulation of ABCA1 transporters what may explain lipid accumulation in macrophages. Finally, Nnt wild-type bone marrow transplantation into LDLr-/- mice resulted in reduced diet-induced atherosclerosis. Therefore, Nnt plays a critical role in the maintenance of macrophage redox, inflammatory and cholesterol homeostasis, which is relevant for delaying the atherogenesis process.

Luteolin improves hypercholesterolemia and glucose intolerance through LXRα-dependent pathway in diet-induced obese mice

Luteolin, a naturally derived flavonoid, exerts beneficial effects such as antitumor, antioxidant, and anti-inflammatory effects. However, the molecular mechanism underlying the effect of luteolin in hypercholesterolemia remains unclear. In this study, we demonstrated that luteolin upregulated the expression of liver X receptor (LXR) α, ATP-binding cassette transporter G1 (ABCG1), and scavenger receptor class B member 1 (SRB1), which play a major role in cholesterol efflux, in HepG2 hepatocytes. Luteolin-stimulated expression of ABCG1 and SRB1 was reversed by inhibitory compound of LXRα. Luteolin administration also upregulated the expression of ABCG1, and SRB1 as well as cholesterol 7 α-hydroxylase (Cyp7α1) in the liver of diet-induced obese mice. Luteolin decreased the level of blood cholesterol and non-high-density lipoprotein cholesterol in obese mice. In addition, luteolin ameliorated glucose intolerance and reduced expression of gluconeogenesis-associated enzymes in an LXRα-dependent manner. PRACTICAL APPLICATIONS: Luteolin is known to possess various pharmacological activities. This research revealed that luteolin ameliorates hypercholesterolemia and glucose intolerance in diet-induced obesity. The results indicate that the potential properties of luteolin in cholesterol metabolism could be explained, at least in part, as being due to upregulated expression of ABCG1, and SRB1 through activation of liver X receptor, LXRα signaling pathway in HepG2 cells.

Peroxisome proliferator-activated receptor gamma (PPARγ) activation and metabolism disturbance induced by bisphenol A and its replacement analog bisphenol S using in vitro macrophages and in vivo mouse models

Bisphenol A (BPA) and its replacement analog, bisphenol S (BPS), have been proposed as environmental obesogen to disrupt the lipid metabolism through regulating peroxisome proliferator-activated receptor gamma (PPARγ) receptor. However, there is a dearth of information on whether this biological effect can occur in human macrophage, a cell type which closely interacts with adipocytes and hepatocytes to control lipid metabolism. Here, we for the first time investigate the activity of BPA and BPS on PPARγ pathway in human macrophages. The results demonstrated that BPA and BPS served as activators of PPARγ in human macrophage cell line, and significantly induced the expression of lipid metabolism-related genes, including fatty acid binding protein 4 (FABP4), cluster of differentiation 36 (CD36) and nuclear receptor subfamily 1 group H member 3 (NR₁H₃). In PPARγ knockout cells, expression of these genes was down-regulated, suggesting that these genes are dependent on PPARγ. The underlying mechanisms were further investigated using an in vivo mouse model, and the results confirmed the induction of PPARγ and its respective target genes in mice following exposure to BPA or BPS. Moreover, the observed alteration of PPARγ expression highly correlated with the disturbance of metabolism profiles in liver tissues as detected by ¹H Nuclear Magnetic Resonance (NMR)-based metabonomics. Overall, this study provided the first evidence that BPA and BPS activated PPARγ and its target genes in human macrophages, and provided comprehensive information to confirm that BPA and BPS disturb the metabolism through targeting PPARγ via both in vitro assays and in vivo animal models.

Liver X receptor-α activation enhances cholesterol secretion in lactating mammary epithelium

Liver X receptors (LXRs) are ligand-dependent transcription factors activated by cholesterol metabolites. These receptors induce a suite of target genes required for de novo synthesis of triglycerides and cholesterol transport in many tissues. Two different isoforms, LXRα and LXRβ, have been well characterized in liver, adipocytes, macrophages, and intestinal epithelium among others, but their contribution to cholesterol and fatty acid efflux in the lactating mammary epithelium is poorly understood. We hypothesize that LXR regulates lipogenesis during milk fat production in lactation. Global mRNA analysis of mouse mammary epithelial cells (MECs) revealed multiple LXR/RXR targets upregulated sharply early in lactation compared with midpregnancy. LXRα is the primary isoform, and its protein levels increase throughout lactation in MECs. The LXR agonist GW3965 markedly induced several genes involved in cholesterol transport and lipogenesis and enhanced cytoplasmic lipid droplet accumulation in the HC11 MEC cell line. Importantly, in vivo pharmacological activation of LXR increased the milk cholesterol percentage and induced sterol regulatory element-binding protein 1c (Srebp1c) and ATP-binding cassette transporter a7 (Abca7) expression in MECs. Cumulatively, our findings identify LXRα as an important regulator of cholesterol incorporation into the milk through key nodes of de novo lipogenesis, suggesting a potential therapeutic target in women with difficulty initiating lactation.

Retinal Vascular Abnormalities and Microglia Activation in Mice with Deficiency in Cytochrome P450 46A1-Mediated Cholesterol Removal

CYP46A1 is the cytochrome P450 enzyme that converts cholesterol to 24-hydroxycholesterol, a cholesterol elimination product and a potent liver X receptor (LXR) ligand. We conducted retinal characterizations of Cyp46a1^-/- mice that had normal fasting blood glucose levels but up to a 1.8-fold increase in retinal cholesterol. The retina of Cyp46a1^-/- mice exhibited venous beading and tortuosity, microglia/macrophage activation, and increased vascular permeability, features commonly associated with diabetic retinopathy. The expression of Lxrα and Lxrβ was increased in both the whole Cyp46a1^-/- retina and retinal macroglia/macrophages. The LXR-target genes were affected as well, primarily in activated microglial cells and macrophages. In the latter, the LXR-transactivated genes (Abca1, Abcg1, Apod, Apoe, Mylip, and Arg2) were up-regulated; similarly, there was an up-regulation of the LXR-transrepressed genes (Ccl2, Ptgs2, Cxcl1, Il1b, Il6, Nos2, and Tnfa). For comparison, gene expression was investigated in bone marrow-derived macrophages from Cyp46a1^-/- mice as well as retinal and bone marrow-derived macrophages from Cyp27a1^-/- and Cyp27a1^-/-Cyp46a1^-/- mice. CYP46A1 expression was detected in retinal endothelial cells, and this expression was increased in the proinflammatory environment. Retinal Cyp46a1^-/- phosphoproteome revealed altered phosphorylation of 30 different proteins, including tight junction protein zonula occludens 1 and aquaporin 4. Collectively, the data obtained establish metabolic and regulatory significance of CYP46A1 for the retina and suggest pharmacologic activation of CYP46A1 as a potential therapeutic approach to dyslipidemia-induced retinal damage.

Fat-specific protein 27 is a novel target gene of liver X receptor α

Fat-specific protein 27 (FSP27) is highly expressed in the fatty liver of genetically obese ob/ob mice and promotes hepatic triglyceride (TG) accumulation. The nuclear hormone receptor liver X receptor α (LXRα) also plays a critical role in the control of TG levels in the liver. The present study demonstrated transcriptional regulation of Fsp27a and Fsp27b genes by LXRα. Treatment with the LXR ligand T0901317 markedly increased Fsp27a and Fsp27b mRNAs in wild-type C57BL/6J and ob/ob mouse livers. A reporter assay indicated that two LXR-responsive elements (LXREs) are necessary for LXRα-dependent induction of Fsp27a and Fsp27b promoter activities. Furthermore, the LXRα/retinoid X receptor α complex is capable of directly binding to the two LXREs both in vitro and in vivo. These results suggest that LXRα positively regulates Fsp27a and Fsp27b expression through two functional LXREs. Fsp27a/b are novel LXR target genes in the ob/ob fatty liver.

Liver X Receptors Suppress Activity of Cholesterol and Fatty Acid Synthesis Pathways To Oppose Gammaherpesvirus Replication

Gammaherpesviruses are oncogenic pathogens that persist in ~95% of the adult population. Cellular metabolic pathways have emerged as important regulators of many viral infections, including infections by gammaherpesviruses that require several lipid synthetic pathways for optimal replication. Liver X receptors (LXRs) are transcription factors that are critical regulators of cellular fatty acid and cholesterol synthesis pathways. Not surprisingly, LXRs are attractive therapeutic targets in cardiovascular disease. Here we describe an antiviral role for LXRs in the context of gammaherpesvirus infection of primary macrophages. We show that type I interferon increased LXR expression following infection. Surprisingly, there was not a corresponding induction of LXR target genes. Rather, LXRs suppressed the expression of target genes, leading to decreased fatty acid and cholesterol synthesis, two metabolic pathways that support gammaherpesvirus replication. This report defines LXR-mediated restriction of cholesterol and lipid synthesis as an intrinsic metabolic mechanism to restrict viral replication in innate immune cells.IMPORTANCE Fatty acid and cholesterol synthesis pathways of the host play important roles in diverse biological systems. Importantly, these two metabolic pathways are also usurped by a number of viruses to facilitate viral replication. In this report, we show that suppression of these pathways by liver X receptors in primary macrophages creates an intrinsic antiviral state that attenuates gammaherpesvirus replication by limiting viral access to the two metabolic pathways.

Ginsenoside 25-OCH3-PPD Promotes Activity of LXRs To Ameliorate P2X7R-Mediated NLRP3 Inflammasome in the Development of Hepatic Fibrosis

Ginseng is widely used in energy drinks, dietary supplements, and herbal medicines, and its pharmacological actions are related with energy metabolism. As an important modulating energy metabolism pathway, liver X receptors (LXRs) can promote the resolving of hepatic fibrosis and inflammation. The present study aims to evaluate the regulation of 25-OCH₃-PPD, a ginsenoside isolated from Panax ginseng, against hepatic fibrosis and inflammation in thioacetamide (TAA)-stimulated mice by activating the LXRs pathway. 25-OCH₃-PPD decreases serum ALT/AST levels and improves the histological pathology of liver in TAA-induced mice; attenuates transcripts of pro-fibrogenic markers associated with hepatic stellate cell activation; attenuates the levels of pro-Inflammatory cytokines and blocks apoptosis happened in liver; inhibits NLRP3 inflammasome by affecting P2X7R activation; and regulates PI3K/Akt and LKB1/AMPK-SIRT1. 25-OCH₃-PPD also facilitates LX25Rs and FXR activities decreased by TAA stimulation. 25-OCH₃-PPD also decreases α-SMA via regulation of LXRs and P2X7R-NLRP3 in vitro. Our data suggest the possibility that 25-OCH₃-PPD promotes activity of LXRs to ameliorate P2X7R-mediated NLRP3 inflammasome in the development of hepatic fibrosis.

The Role of PPAR and Its Cross-Talk with CAR and LXR in Obesity and Atherosclerosis

The prevalence of obesity and atherosclerosis has substantially increased worldwide over the past several decades. Peroxisome proliferator-activated receptors (PPARs), as fatty acids sensors, have been therapeutic targets in several human lipid metabolic diseases, such as obesity, atherosclerosis, diabetes, hyperlipidaemia, and non-alcoholic fatty liver disease. Constitutive androstane receptor (CAR) and liver X receptors (LXRs) were also reported as potential therapeutic targets for the treatment of obesity and atherosclerosis, respectively. Further clarification of the internal relationships between these three lipid metabolic nuclear receptors is necessary to enable drug discovery. In this review, we mainly summarized the cross-talk of PPARs-CAR in obesity and PPARs-LXRs in atherosclerosis.

IL-36/LXR axis modulates cholesterol metabolism and immune defense to Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is a life-threatening pathogen in humans. Bacterial infection of macrophages usually triggers strong innate immune mechanisms, including IL-1 cytokine secretion. The newer member of the IL-1 family, IL-36, was recently shown to be involved in cellular defense against Mtb. To unveil the underlying mechanism of IL-36 induced antibacterial activity, we analyzed its role in the regulation of cholesterol metabolism, together with the involvement of Liver X Receptor (LXR) in this process. We report that, in Mtb-infected macrophages, IL-36 signaling modulates cholesterol biosynthesis and efflux via LXR. Moreover, IL-36 induces the expression of cholesterol-converting enzymes and the accumulation of LXR ligands, such as oxysterols. Ultimately, both IL-36 and LXR signaling play a role in the regulation of antimicrobial peptides expression and in Mtb growth restriction. These data provide novel evidence for the importance of IL-36 and cholesterol metabolism mediated by LXR in cellular host defense against Mtb.

The trisaccharide raffinose modulates epidermal differentiation through activation of liver X receptor

The epidermal barrier function requires optimal keratinocyte differentiation and epidermal lipid synthesis. Liver X receptor (LXR) α and β, are important transcriptional regulators of the epidermal gene expression. Here, we show that raffinose, a ubiquitously present trisaccharide in plants, activated the transcriptional activity of LXRα/β, which led to the induction of genes required for keratinocyte differentiation such as involucrin and filaggrin, and genes involved in lipid metabolism and transport including SCD1 and ABCA1 in both HaCaT and normal human epidermal keratinocytes. Raffinose induced the expression of JunD and Fra1, and their DNA binding in the AP1 motif in the promoters of involucrin and loricrin. Interestingly, LXR bound the AP1 motif upon raffinose treatment, and conversely, JunD and Fra1 bound the LXR response element in promoters of LXR target genes, which indicates the presence of a postive cross-talk between LXR and AP1 in the regualtion of these genes. Finally, the effect of raffinose in epidermal barrier function was confirmed by applying raffinose in an ointment formulation to the skin of hairless mice. These findings suggest that raffinose could be examined as an ingredient in functional cosmetics and therapeutic agents for the treatment of cutaneous disorders associated with abnormal epidermal barrier function.

Liver X receptor reduces proliferation of human oral cancer cells by promoting cholesterol efflux via up-regulation of ABCA1 expression

Liver X receptors (LXRs) contribute not only to maintain cholesterol homeostasis but also to control cell growth. However, the molecular mechanisms behind the LXR-mediated anti-proliferative effects are largely unknown. Here we show, by immunohistochemistry, that LXRα and LXRβ are differentially distributed in oral stratified squamous epithelia. By immunohistochemical and Western blot analyses, we also reveal that LXRα is abundantly expressed in human oral squamous cell carcinoma (HOSCC) tissues and cell lines. Cell counting, BrdU labeling and cell cycle assay indicated that LXR stimulation led to significant reduction of proliferation in HOSCC cells. Importantly, our study highlights, by using RNA interference, that the ATP-binding cassette transporter A1 (ABCA1)-accelerated cholesterol efflux is critical for the growth inhibitory action of LXRs in HOSCC cells. Moreover, we demonstrate that LXR activation reduces the growth of xenograft tumour of HOSCC cells in mice accompanied by the upregulation of ABCA1 expression and the decline of cholesterol levels in the tumour. These findings strongly suggested that targeting the LXR-regulated cholesterol transport, yielding in lowering intracellular cholesterol levels, could be a promising therapeutic option for certain types of cancers.

Liver X receptor and STAT1 cooperate downstream of Gas6/Mer to induce anti-inflammatory arginase 2 expression in macrophages

Mer signaling increases the transcriptional activity of liver X receptor (LXR) to promote the resolution of acute sterile inflammation. Here, we aimed to understand the pathway downstream of Mer signaling after growth arrest-specific protein 6 (Gas6) treatment that leads to LXR expression and transcriptional activity in mouse bone-marrow derived macrophages (BMDM). Gas6-induced increases in LXRα and LXRβ and expression of their target genes were inhibited in BMDM from STAT1^−/− mice or by the STAT1-specific inhibitor fludarabine. Gas6-induced STAT1 phosphorylation, LXR activation, and LXR target gene expression were inhibited in BMDM from Mer^−/− mice or by inhibition of PI3K or Akt. Gas6-induced Akt phosphorylation was inhibited in BMDM from STAT1^−/− mice or in the presence of fludarabine. Gas6-induced LXR activity was enhanced through an interaction between LXRα and STAT1 on the DNA promoter of Arg2. Additionally, we found that Gas6 inhibited lipopolysaccharide (LPS)-induced nitrite production in a STAT1 and LXR pathway-dependent manner in BMDM. Additionally, Mer-neutralizing antibody reduced LXR and Arg2 expression in lung tissue and enhanced NO production in bronchoalveolar lavage fluid in LPS-induced acute lung injury. Our data suggest the possibility that the Gas6-Mer-PI3K/Akt-STAT1-LXR-Arg2 pathway plays an essential role for resolving inflammatory response in acute lung injury.

Styrylphenylphthalimides as Novel Transrepression-Selective Liver X Receptor (LXR) Modulators

Anti-inflammatory effects of liver X receptor (LXR) ligands are thought to be largely due to LXR-mediated transrepression, whereas side effects are caused by activation of LXR-responsive gene expression (transactivation). Therefore, selective LXR modulators that preferentially exhibit transrepression activity should exhibit anti-inflammatory properties with fewer side effects. Here, we synthesized a series of styrylphenylphthalimide analogues and evaluated their structure-activity relationships focusing on LXRs-transactivating-agonistic/antagonistic activities and transrepressional activity. Among the compounds examined, 17l showed potent LXR-transrepressional activity with high selectivity over transactivating activity and did not show characteristic side effects of LXR-transactivating agonists in cells. This representative compound, 17l, was confirmed to have LXR-dependent transrepressional activity and to bind directly to LXRβ. Compound 17l should be useful not only as a chemical tool for studying the biological functions of LXRs transrepression but also as a candidate for a safer agent to treat inflammatory diseases.

Retinoic acid induces macrophage cholesterol efflux and inhibits atherosclerotic plaque formation in apoE-deficient mice

It has been suggested that retinoic acid (RA) has a potential role in the prevention of atherosclerotic CVD. In the present study, we used J774A.1 cell lines and primary peritoneal macrophages to investigate the protective effects of RA on foam cell formation and atherogenesis in apoE-deficient (apoE- / -) mice. A total of twenty male apoE- / - mice (n 10 animals per group), aged 8 weeks, were fed on a high-fat diet (HFD) and treated with vehicle or 9-cis-RA for 8 weeks. The atherosclerotic plaque area in the aortic sinus of mice in the 9-cis-RA group was 40·7 % less than that of mice in the control group (P< 0·01). Mouse peritoneal macrophages from the 9-cis-RA group had higher protein expression levels of ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1) than those from the control group. Serum total and LDL-cholesterol concentrations were lower in the 9-cis-RA group than in the control group (P< 0·05). In vitro studies showed that incubation of cholesterol-loaded J774A.1 macrophages with 9-cis-RA (0·1, 1 and 10 μmol/l) induced cholesterol efflux in a dose-dependent manner. The 9-cis-RA treatment markedly attenuated lipid accumulation in macrophages exposed to oxidised LDL. Moreover, treatment with 9-cis-RA significantly increased the protein expression levels of ABCA1 and ABCG1 in J774A.1 macrophages in a dose-dependent manner. Furthermore, 9-cis-RA dose-dependently enhanced the protein expression level of liver X receptor-α (LXRα), the upstream regulator of ABCA1 and ABCG1. Taken together, the present results show that 9-cis-RA suppresses foam cell formation and prevents HFD-induced atherogenesis via the LXRα-dependent up-regulation of ABCA1 and ABCG1.

Decreased cholesterol uptake and increased liver x receptor-mediated cholesterol efflux pathways during prostaglandin F2 alpha-induced and spontaneous luteolysis in sheep

In nonprimate species, it has been well established that prostaglandin F2 alpha (PGF2alpha) initiates luteolysis. Changes in intracellular cholesterol concentrations caused by modulation of cholesterol uptake and efflux may mediate PGF2alpha-induced luteolysis. These changes in cholesterol efflux and uptake are controlled, in part, by the liver x receptors (LXR) alpha (NR1H3) and beta (NR1H2), nuclear receptors that increase expression of genes necessary for cholesterol efflux or limiting cholesterol uptake. Therefore, we hypothesized that PGF2alpha reduces expression of cholesterol uptake and increases expression of cholesterol efflux genes, mediated in part by enhanced LXR activity. To test this hypothesis, an induced luteolysis model was used whereby ewes were treated during their midluteal phase with saline or PGF2alpha and corpora lutea (CL) collected 12, 24, or 48 h later for determination of mRNA and protein concentrations by quantitative real-time PCR and Western blot analysis, respectively. As a complementary approach, CL undergoing spontaneous luteolysis were compared to midluteal phase CL. The lipoprotein receptors responsible for cholesterol uptake were significantly decreased in both luteolysis models. Expression of the LXR target gene ATP binding cassette subfamily A1 (ABCA1), an important mediator of cholesterol efflux, was significantly increased in both experimental models. Chromatin immunoprecipitation confirmed that PGF2alpha treatment resulted in enhanced NR1H3 and NR1H2 binding to the ABCA1 promoter. Qualitative changes in lipid droplet distribution were also observed following PGF2alpha treatment. These data support the hypothesis that reduced cholesterol uptake and increased efflux mediate luteolysis in sheep, which is partially controlled by PGF2alpha stimulation of LXR activity.

Mer signaling increases the abundance of the transcription factor LXR to promote the resolution of acute sterile inflammation

The receptor tyrosine kinase Mer plays a central role in inhibiting the inflammatory response of immune cells to pathogens. We aimed to understand the function of Mer signaling in the resolution of sterile inflammation in experiments with a Mer-neutralizing antibody or with Mer-deficient (Mer-/-) mice in a model of sterile, zymosan-induced acute inflammation. We found that inhibition or deficiency of Mer enhanced local and systemic inflammatory responses. The exacerbated inflammatory responses induced by the lack of Mer signaling were associated with reduced abundance of the transcription factors liver X receptor α (LXRα) and LXRβ and decreased expression of their target genes in peritoneal macrophages, spleens, and lungs. Similarly, treatment of mice with a Mer/Fc fusion protein, which prevents the Mer ligand Gas6 (growth arrest-specific protein 6) from binding to Mer, exacerbated the inflammatory response and decreased the abundance of LXR. Coadministration of the LXR agonist T0901317 with the Mer-neutralizing antibody inhibited the aggravating effects of the antibody on inflammation in mice. In vitro exposure of RAW264.7 cells or primary peritoneal macrophages to Gas6 increased LXR abundance in an Akt-dependent manner. Thus, we have elucidated a previously uncharacterized pathway involved in the resolution of acute sterile inflammation: Enhanced Mer signaling during the recovery phase increases the abundance and activity of LXR to inactivate the inflammatory response in macrophages.

RIP140 triggers foam-cell formation by repressing ABCA1/G1 expression and cholesterol efflux via liver X receptor

Receptor-interacting protein 140 (RIP140) is a multifunctional coregulator of lipid metabolism and inflammation. However, the potential role of RIP140 in atherosclerosis remains unknown. The present study investigated the impact of RIP140 on foam cell formation, a critical step in pathogenesis of atherosclerosis. The expression of RIP140 was increased in foam cells. RIP140 overexpression resulted in decreased cholesterol efflux in macrophages and their concomitant differentiation into foam cells. Moreover, RIP140 negatively regulated the macrophage expression of ATP-binding cassette transporters A1 and G1 (ABCA1/G1), by suppressing the expression and activity of liver X receptor (LXR). These findings shed light onto the contribution of RIP140 to the development and progression of atherosclerosis, and suggest a novel therapeutic target for the treatment of atherosclerosis.

Infliximab reverses suppression of cholesterol efflux proteins by TNF-α: a possible mechanism for modulation of atherogenesis

Tumor necrosis factor- (TNF-) α is a proinflammatory proatherogenic cytokine. Infliximab, an anti-TNF-α monoclonal antibody, is effective in treating rheumatoid arthritis. However, its impact on cardiovascular burden and lipid transport is unclear. The present study investigates the effect of TNF-α and infliximab on reverse cholesterol transport (RCT) proteins. Uptake of modified lipoproteins by macrophages in the vasculature leads to atherogenic foam cell formation. RCT is mediated by proteins including ATP binding cassette transporters A1 (ABCA1), G1 (ABCG1), liver X receptor- (LXR-) α, and 27-hydroxylase. RCT counteracts lipid overload by ridding cells of excess cholesterol. THP-1 human monocytes were incubated with either TNF-α alone or TNF-α with infliximab. Expression of proteins involved in cholesterol efflux was analyzed. TNF-α significantly reduced both ABCA1 and LXR-α mRNA (to 68.5 ± 1.59%, P < 0.05, and 41.2 ± 0.25%, P < 0.01, versus control set as 100%, resp.). Infliximab nullified the TNF-α effect. Results were confirmed by Western blot. Infliximab abolished the increase in foam cells induced by TNF-α. TNF-α treatment significantly reduces ABCA1 and LXR-α expression in monocytes, thus bringing about a proatherogenic state. The anti-TNF drug infliximab, commonly used in rheumatology, restored RCT proteins. This is the first report of an atheroprotective effect of infliximab on RCT in monocytes.

The role of the liver X receptor in chronic obstructive pulmonary disease

BACKGROUND

There is a need for novel anti-inflammatory therapies to treat COPD. The liver X receptor (LXR) is a nuclear hormone receptor with anti-inflammatory properties.

METHODS

We investigated LXR gene and protein expression levels in alveolar macrophages and whole lung tissue from COPD patients and controls, the effect of LXR activation on the suppression of inflammatory mediators from LPS stimulated COPD alveolar macrophages, and the effect of LXR activation on the induction of genes associated with alternative macrophage polarisation.

RESULTS

The levels of LXR mRNA were significantly increased in whole lung tissue extracts in COPD patients and smokers compared to non-smokers. The expression of LXR protein was significantly increased in small airway epithelium and alveolar epithelium in COPD patients compared to controls. No differences in LXR mRNA and protein levels were observed in alveolar macrophages between patient groups. The LXR agonist GW3965 significantly induced the expression of the LXR dependent genes ABCA1 and ABCG1 in alveolar macrophage cultures. In LPS stimulated alveolar macrophages, GW3965 suppressed the production of CXCL10 and CCL5, whilst stimulating IL-10 production.

CONCLUSIONS

GW3965 did not significantly suppress the production of TNFα, IL-1β, or CXCL8. Our major finding is that LXR activation has anti-inflammatory effects on CXC10, CCL5 and IL-10 production from alveolar macrophages.

Oxysterol generation and liver X receptor-dependent reverse cholesterol transport: not all roads lead to Rome

Cell cholesterol metabolism is a tightly regulated process, dependent in part on activation of nuclear liver X receptors (LXRs) to increase expression of genes mediating removal of excess cholesterol from cells in the reverse cholesterol transport pathway. LXRs are thought to be activated predominantly by oxysterols generated enzymatically from cholesterol in different cell organelles. Defects resulting in slowed release of cholesterol from late endosomes and lysosomes or reduction in sterol-27-hydroxylase activity lead to specific blocks in oxysterol production and impaired LXR-dependent gene activation. This block does not appear to be compensated by oxysterol production in other cell compartments. The purpose of this review is to summarize current knowledge about oxysterol-dependent activation by LXR of genes involved in reverse cholesterol transport, and what these defects of cell cholesterol homeostasis can teach us about the critical pathways of oxysterol generation for expression of LXR-dependent genes.

ATP-binding cassette transporter A1 expression is decreased in preeclamptic placentas

Preeclampsia is a pregnancy-specific multisystem disorder characterized by hypertension and proteinuria. Accentuated maternal hyperlipidemia, especially high serum levels of oxidized low-density lipoprotein (oxLDL), is one of the features of preeclampsia. We previously reported that lectin-like oxidized LDL receptor 1 (LOX-1) expression was decreased in preeclamptic placentas. Here, we show that decreased LOX-1 expression is associated with low expression of adenosine triphosphate-binding cassette transporter A1 (ABCA1) in the placenta. The ABCA1 mediates cellular efflux of cholesterol, and liver X receptors (LXRs) are its predominant transcriptional regulators. Both ABCA1 and LXR expressions were significantly lower in preeclamptic placentas than those in normal controls. Oxidized LDL upregulated ABCA1 expression, while LOX-1 blockade resulted in the alleviation of increasing ABCA1 messenger RNA in JAR cells. These results suggest that low LOX-1 expression may lead to insufficient oxLDL uptake, thereby contributing to reduced LXR activation and decreased ABCA1 expression in preeclamptic placentas.

Fucosterol is a selective liver X receptor modulator that regulates the expression of key genes in cholesterol homeostasis in macrophages, hepatocytes, and intestinal cells

Fucosterol, a sterol that is abundant in marine algae, has hypocholesterolemic activity, but the mechanism underlying its effect is not clearly understood. Because data suggest that fucosterol can increase plasma high-density lipoprotein concentrations, we investigated whether it could activate liver X receptors (LXRs), critical transcription factors in reverse cholesterol transport. Fucosterol dose-dependently stimulated the transcriptional activity of both LXR-α and -β in a reporter gene assay, responses that were attenuated by the LXR antagonist As(2)O(3). Fucosterol also activated co-activator recruitment in cell-free time-resolved fluorescence resonance energy transfer analysis. In THP-1-derived macrophages, it induced the transcriptional activation of ABCA1, ABCG1, and ApoE, key genes in reverse cholesterol transport, and thereby significantly increased the efflux of cholesterol. Fucosterol also regulated intestinal NPC1L1 and ABCA1 in Caco-2 cells. Notably, fucosterol did not induce cellular triglyceride accumulation in HepG2 cells, primarily because of its upregulation of Insig-2a, which delays nuclear translocation of SREBP-1c, a key hepatic lipogenic transcription factor. These results suggest that fucosterol is a dual-LXR agonist that regulates the expression of key genes in cholesterol homeostasis in multiple cell lines without inducing hepatic triglyceride accumulation.

Liver X receptors, atherosclerosis and inflammation

Liver X receptors (LXRs) belong to the nuclear receptor superfamily of ligand-dependent transcription factors. LXRs are activated by oxysterols, metabolites of cholesterol, and therefore act as intracellular sensors of this lipid. There are two LXR genes (α and β) that display distinct tissue/cell expression profiles. LXRs interact with regulatory sequences in target genes as heterodimers with retinoid X receptor. Such direct targets of LXR actions include important genes implicated in the control of lipid homeostasis, particularly reverse cholesterol transport. In addition, LXRs attenuate the transcription of genes associated with the inflammatory response indirectly by transrepression. In this review, we describe recent evidence that both highlights the key roles of LXRs in atherosclerosis and inflammation and provides novel insights into the mechanisms underlying their actions. In addition, we discuss the major limitations of LXRs as therapeutic targets for the treatment of atherosclerosis and how these are being addressed.

Transcription of liver X receptor is down-regulated by 15-deoxy-Δ(12,14)-prostaglandin J(2) through oxidative stress in human neutrophils

Liver X receptors (LXRs) are ligand-activated transcription factors of the nuclear receptor superfamily. They play important roles in controlling cholesterol homeostasis and as regulators of inflammatory gene expression and innate immunity, by blunting the induction of classical pro-inflammatory genes. However, opposite data have also been reported on the consequences of LXR activation by oxysterols, resulting in the specific production of potent pro-inflammatory cytokines and reactive oxygen species (ROS). The effect of the inflammatory state on the expression of LXRs has not been studied in human cells, and constitutes the main aim of the present work. Our data show that when human neutrophils are triggered with synthetic ligands, the synthesis of LXRα mRNA became activated together with transcription of the LXR target genes ABCA1, ABCG1 and SREBP1c. An inflammatory mediator, 15-deoxy-Δ^12,14-prostaglandin J₂ (15dPGJ₂), hindered T0901317-promoted induction of LXRα mRNA expression together with transcription of its target genes in both neutrophils and human macrophages. This down-regulatory effect was dependent on the release of reactive oxygen species elicited by 15dPGJ₂, since it was enhanced by pro-oxidant treatment and reversed by antioxidants, and was also mediated by ERK1/2 activation. Present data also support that the 15dPGJ₂-induced serine phosphorylation of the LXRα molecule is mediated by ERK1/2. These results allow to postulate that down-regulation of LXR cellular levels by pro-inflammatory stimuli might be involved in the development of different vascular diseases, such as atherosclerosis.

Taurine is a liver X receptor-α ligand and activates transcription of key genes in the reverse cholesterol transport without inducing hepatic lipogenesis

SCOPE

Taurine, which is abundant in seafood, has antiatherogenic activities in both animals and humans; however, its molecular target has been elusive. We examined whether taurine could activate liver X receptor-α (LXR-α), a critical transcription factor in the regulation of reverse cholesterol transport in macrophages.

METHODS AND RESULTS

Taurine bound directly to LXR-α in a reporter gene assay, time-resolved fluorescence resonance energy transfer analysis, and limited protease digestion experiment. Macrophage cells incubated with taurine showed reduced cellular cholesterol and induced medium cholesterol in a dose-dependent manner with the induction of ATP-binding cassette transporter A1 and G gene and protein expression. In hepatocytes, taurine significantly induced Insig-2a levels and delayed nuclear translocation of the sterol regulatory element-binding protein 1 (SREBP-1) protein, resulting in a dose-dependent reduction in the cellular lipid levels without inducing the expression of fatty acid synthesis genes.

CONCLUSION

Taurine is a direct LXR-α ligand, represses cholesterol accumulation, and modulates the expression of genes involved in reverse cholesterol transport in macrophages, without inducing hepatic lipogenesis. The induction of Insig-2a suppressed the nuclear translocation of SREBP-1c.

Heme and haemoglobin direct macrophage Mhem phenotype and counter foam cell formation in areas of intraplaque haemorrhage

PURPOSE OF REVIEW

Several studies have recently shown that haemoglobin drives a novel macrophage subset that is protected from foam cell formation.

RECENT FINDINGS

In a previously overlooked area, two centres have independently shown that heme and haemoglobin drive an atheroprotective macrophage subset. We compare and contrast the approaches and findings of the laboratories and discuss some of the underlying biology and implications, concentrating on the aspects of lipidological relevance.

SUMMARY

Treatments based on direct heme-mimetics or other agonists of this pathway have enormous potential for linked antioxidant protection via heme oxygenase 1 and reduced foam cell formation via liver X receptor, a potent combination for treating atherosclerosis.

Alterations of LXRα and LXRβ expression in the hypothalamus of glucose-intolerant rats

Liver X receptor (LXR) α and β are nuclear receptors that are crucial for the regulation of carbohydrate and lipid metabolism. Activation of LXRs in the brain facilitates cholesterol clearance and improves cognitive deficits, thus they are considered as promising drug targets to treat diseases such as atherosclerosis and Alzheimer's disease. Nevertheless, little is known about the function and localization of LXRs in the brain. Here, we studied the expression of LXR in the brains of rats that received free access to 10% (w/v) fructose group (FG) in their beverages or water control drinks (control group (CG)). After 6 weeks rats in the FG presented with hypertriglyceridemia, hyperinsulinemia, and became glucose intolerant, suggesting a progression toward type 2 diabetes. We found that hypothalamic LXR expression was altered in fructose-fed rats. Rats in the FG presented with a decrease in LXRβ levels while showing an increase in LXRα expression in the hypothalamus but not in the hippocampus, cerebellum, or neocortex. Moreover, both LXRα and β expression correlated negatively with insulin and triglyceride levels. Interestingly, LXRβ showed a negative correlation with the area under the curve during the glucose tolerance test in the CG and a positive correlation in the FG. Immunocytochemistry revealed that the paraventricular and ventromedial nuclei express mainly LXRα whereas the arcuate nucleus expresses LXRβ. Both LXR immunosignals were found in the median preoptic area. This is the first study showing a relationship between glucose and lipid homeostasis and the expression of LXRs in the hypothalamus, suggesting that LXRs may trigger neurochemical and neurophysiological responses for the control of food intake and energy expenditure through these receptors.

Plant sterol oxidation products--analogs to cholesterol oxidation products from plant origin?

Cholesterol and plant sterols are lipids which are abundantly present in a western type diet of animal and plant origin, respectively. The daily intake averages 300 mg/day each. Over the past decades, a steadily increasing consumption of plant sterol enriched dairy products (2-3 g/day) took place to lower circulating LDL cholesterol concentrations. Like all unsaturated components, plant sterols can be attacked by reactive oxygen species resulting in plant sterol oxidation products (POPs). The most widespread methods for POP determination are high-performance liquid chromatography and gas-liquid chromatography. Yet, based on the low plasma POP concentrations in normophytosterolemic subjects (POPs: ∼0.3-4.5 ng/mL), a reliable quantification yielding an appropriate limit of detection remains a challenge. While the more abundantly present cholesterol oxidation products (COPs) have elaborately been studied, research on the metabolism and biological effects of POPs is only emerging. In relation to atherogenity, biological effects including modulation of cholesterol homeostasis, membrane functioning, and inflammation are attributed to POPs. Although mostly supra-physiological concentrations are applied in in vitro assays, anti-tumor activity, cytotoxicity and estrogen-competition have been attributed to specific POPs. However, it is not obvious, if and how POPs may exert in vivo adverse or beneficial health effects similar to those attributed to COPs. In the field of nutritional science, standardized methods for the determination of POPs are required to perform relevant biological studies and to assess their presence in complex foods or biological tissues and fluids. The aim of this review is to provide an overview and evaluation of the published methods and an update on the biological effects attributed to POPs.

Nuclear hormone receptors enable macrophages and dendritic cells to sense their lipid environment and shape their immune response

A key issue in the immune system is to generate specific cell types, often with opposing activities. The mechanisms of differentiation and subtype specification of immune cells such as macrophages and dendritic cells are critical to understand the regulatory principles and logic of the immune system. In addition to cytokines and pathogens, it is increasingly appreciated that lipid signaling also has a key role in differentiation and subtype specification. In this review we explore how intracellular lipid signaling via a set of transcription factors regulates cellular differentiation, subtype specification, and immune as well as metabolic homeostasis. We introduce macrophages and dendritic cells and then we focus on a group of transcription factors, nuclear receptors, which regulate gene expression upon receiving lipid signals. The receptors we cover are the ones with a recognized physiological function in these cell types and ones which heterodimerize with the retinoid X receptor. These are as follows: the receptor for a metabolite of vitamin A, retinoic acid: retinoic acid receptor (RAR), the vitamin D receptor (VDR), the fatty acid receptor: peroxisome proliferator-activated receptor γ (PPARγ), the oxysterol receptor liver X receptor (LXR), and their obligate heterodimeric partner, the retinoid X receptor (RXR). We discuss how they can get activated and how ligand is generated and eliminated in these cell types. We also explore how activation of a particular target gene contributes to biological functions and how the regulation of individual target genes adds up to the coordination of gene networks. It appears that RXR heterodimeric nuclear receptors provide these cells with a coordinated and interrelated network of transcriptional regulators for interpreting the lipid milieu and the metabolic changes to bring about gene expression changes leading to subtype and functional specification. We also show that these networks are implicated in various immune diseases and are amenable to therapeutic exploitation.

Role of LXR in trout adipocytes: target genes, hormonal regulation, adipocyte differentiation and relation to lipolysis

In the present study, we describe an initial approach to investigate the role of LXR in fish adipose tissue. Rainbow trout (Oncorhynchus mykiss) isolated adipocytes were incubated with LXR agonists, unsaturated fatty acids, tumour necrosis factor-α (TNFα), insulin or growth hormone (GH) for 6h and LXR expression was analyzed. Lipolysis was measured after incubation with one of the LXR agonists and LXR expression was compared with levels of lipolysis. LXR expression was also analyzed during the differentiation of adipocytes in culture. The incubations with agonists in isolated adipocytes indicated that ATP-binding cassette transporter A1 (ABCA1) is an LXR target gene, but lipoprotein lipase (LPL), fatty acid synthase (FAS), hormone-sensitive lipase (HSL) and peroxisome proliferator-activated receptor (PPARs) are not. LXR agonists also induced LXR expression and raised lipolysis levels. Besides, LXR expression was upregulated in parallel with basal lipolysis. LXR mRNA expression was regulated by unsaturated fatty acids, insulin, TNFα and GH in isolated adipocytes. Besides, LXR showed an upregulation during adipocyte differentiation. All these data indicate that LXR is involved in orchestrating the transcriptional regulatory network in trout adipocyte lipid metabolism, specifically, in cholesterol transport, adipocyte differentiation and lipolysis.

Asymmetric positive feedback loops reliably control biological responses

Positive feedback is a common mechanism enabling biological systems to respond to stimuli in a switch-like manner. Such systems are often characterized by the requisite formation of a heterodimer where only one of the pair is subject to feedback. This ASymmetric Self-UpREgulation (ASSURE) motif is central to many biological systems, including cholesterol homeostasis (LXRα/RXRα), adipocyte differentiation (PPARγ/RXRα), development and differentiation (RAR/RXR), myogenesis (MyoD/E12) and cellular antiviral defense (IRF3/IRF7). To understand why this motif is so prevalent, we examined its properties in an evolutionarily conserved transcriptional regulatory network in yeast (Oaf1p/Pip2p). We demonstrate that the asymmetry in positive feedback confers a competitive advantage and allows the system to robustly increase its responsiveness while precisely tuning the response to a consistent level in the presence of varying stimuli. This study reveals evolutionary advantages for the ASSURE motif, and mechanisms for control, that are relevant to pharmacologic intervention and synthetic biology applications.

The oxysterol 27-hydroxycholesterol regulates α-synuclein and tyrosine hydroxylase expression levels in human neuroblastoma cells through modulation of liver X receptors and estrogen receptors--relevance to Parkinson's disease

Loss of dopaminergic neurons and α-synuclein accumulation are the two major pathological hallmarks of Parkinson's disease. Currently, the mechanisms governing depletion of dopamine content and α-synuclein accumulation are not well understood. We showed that the oxysterol 27-hydroxycholesterol (27-OHC) reduces the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis, and increases α-synuclein levels in SH-SY5Y cells. However, the cellular mechanisms involved in 27-OHC effects were not elucidated. In this study, we demonstrate that 27-OHC regulates TH and α-synuclein expression levels through the estrogen receptors (ER) and liver X receptors (LXR). We specifically show that inhibition of ERβ mediates 27-OHC-induced decrease in TH expression, an effect reversed by the ER agonist estradiol. We also show that 27-OHC and the LXR agonist GW3965 increase α-synuclein while the LXR antagonist 5α-6α-epoxycholesterol-3-sulfate significantly attenuated the 27-OHC-induced increase in α-synuclein expression. We further demonstrate that LXRβ positively regulates α-synuclein expression and 27-OHC increases LXRβ-mediated α-synuclein transcription. Our results demonstrate the involvement of two distinct pathways that are involved in the 27-OHC regulation of TH and α-synuclein levels. Concomitant activation of ERβ and inhibition of LXRβ prevent 27-OHC effects and may therefore reduce the progression of Parkinson's disease by precluding TH reduction and α-synuclein accumulation.

Positive Cross-Talk Between Hypoxia Inducible Factor-1α and Liver X Receptor α Induces Formation of Triglyceride-Loaded Foam Cells

Objective—

Atherosclerosis is a chronic and progressive inflammatory disease of the arteries that is characterized by subendothelial accumulation of lipid-rich macrophages, called foam cells. We sought to identify the molecular details of cross-talk between liver X receptor α (LXRα) and hypoxia-inducible factor 1α (HIF-1α) for the formation of triglyceride-rich foam cells under hypoxic conditions.

Methods and Results—

We first observed that expression of LXRα and its target lipogenic genes was time-dependently induced in human primary macrophages and RAW 264.7 cells under hypoxia. Similarly, TO901317, an activator of LXRα, enhanced the expression level and the transcriptional activity of HIF-1α. Second, we demonstrated that LXRα increased HIF-1α protein stability through a physical interaction between the ligand binding domain of LXRα and the oxygen-dependent degradation domain of HIF-1α. Third, we found that the activation of HIF-1α or LXRα synergistically induced triglyceride accumulation in macrophages. Finally, we showed that LXRα and HIF-1α were codistributed in the macrophages of atherosclerotic lesions of patients.

Conclusion—

These results suggest that the positive feed-forward regulation of transcriptional activity and protein stability of LXRα and HIF-1α has an important impact in foam cell formation.

Liver X receptors regulate cholesterol homeostasis in oligodendrocytes

Cholesterol synthesis and transport in oligodendrocytes are essential for optimal myelination and remyelination in pathological conditions such as multiple sclerosis. However, little is known about cholesterol homeostasis in the myelin-forming oligodendrocytes. Liver X receptors (LXRs) are nuclear oxysterol receptors that regulate genes involved in cholesterol homeostasis and may therefore play an important role in de- and remyelination. We investigated whether LXRs regulate cholesterol homeostasis in oligodendrocytes. mRNA expression of genes encoding LXR-α and LXR-β and their target genes (ABCA1, ABCG1, ABCG4, apoE, and LDLR) was detected in oligodendrocytes derived from both neonatal and adult rats using quantitative real-time PCR. The expression of LXR-β and several target genes was increased during oligodendrocyte differentiation. We further demonstrated that treatment of primary neonatal rat oligodendrocytes with the synthetic LXR agonist T0901317 induced the expression of several established LXR target genes, including ABCA1, ABCG1, apoE, and LDLR. Treatment of oligodendrocytes with T0901317 resulted in an enhanced cholesterol efflux in the presence of apolipoprotein A-I or high-density lipoprotein particles. These data show that LXRs are involved in regulating cholesterol homeostasis in oligodendrocytes.

Lysosomal acid lipase deficiency impairs regulation of ABCA1 gene and formation of high density lipoproteins in cholesteryl ester storage disease

ATP-binding cassette transporter A1 (ABCA1) mediates the rate-limiting step in high density lipoprotein (HDL) particle formation, and its expression is regulated primarily by oxysterol-dependent activation of liver X receptors. We previously reported that ABCA1 expression and HDL formation are impaired in the lysosomal cholesterol storage disorder Niemann-Pick disease type C1 and that plasma HDL-C is low in the majority of Niemann-Pick disease type C patients. Here, we show that ABCA1 regulation and activity are also impaired in cholesteryl ester storage disease (CESD), caused by mutations in the LIPA gene that result in less than 5% of normal lysosomal acid lipase (LAL) activity. Fibroblasts from patients with CESD showed impaired up-regulation of ABCA1 in response to low density lipoprotein (LDL) loading, reduced phospholipid and cholesterol efflux to apolipoprotein A-I, and reduced α-HDL particle formation. Treatment of normal fibroblasts with chloroquine to inhibit LAL activity reduced ABCA1 expression and activity, similar to that of CESD cells. Liver X receptor agonist treatment of CESD cells corrected ABCA1 expression but failed to correct LDL cholesteryl ester hydrolysis and cholesterol efflux to apoA-I. LDL-induced production of 27-hydroxycholesterol was reduced in CESD compared with normal fibroblasts. Treatment with conditioned medium containing LAL from normal fibroblasts or with recombinant human LAL rescued ABCA1 expression, apoA-I-mediated cholesterol efflux, HDL particle formation, and production of 27-hydroxycholesterol by CESD cells. These results provide further evidence that the rate of release of cholesterol from late endosomes/lysosomes is a critical regulator of ABCA1 expression and activity, and an explanation for the hypoalphalipoproteinemia seen in CESD patients.

Critical role of astroglial apolipoprotein E and liver X receptor-α expression for microglial Aβ phagocytosis

Liver X receptors (LXRs) regulate immune cell function and cholesterol metabolism, both factors that are critically involved in Alzheimer's disease (AD). To investigate the therapeutic potential of long-term LXR activation in amyloid-β (Aβ) peptide deposition in an AD model, 13-month-old, amyloid plaque-bearing APP23 mice were treated with the LXR agonist TO901317. Postmortem analysis demonstrated that TO901317 efficiently crossed the blood-brain barrier. Insoluble and soluble Aβ levels in the treated APP23 mice were reduced by 80% and 40%, respectively, compared with untreated animals. Amyloid precursor protein (APP) processing, however, was hardly changed by the compound, suggesting that the observed effects were instead mediated by Aβ disposal. Despite the profound effect on Aβ levels, spatial learning in the Morris water maze was only slightly improved by the treatment. ABCA1 (ATP-binding cassette transporter 1) and apolipoprotein E (ApoE) protein levels were increased and found to be primarily localized in astrocytes. Experiments using primary microglia demonstrated that medium derived from primary astrocytes exposed to TO901317 stimulated phagocytosis of fibrillar Aβ. Conditioned medium from TO901317-treated ApoE(-/-) or LXRα(-/-) astrocytes did not increase phagocytosis of Aβ. In APP23 mice, long-term treatment with TO901317 strongly increased the association of microglia and Aβ plaques. Short-term treatment of APP/PS1 mice with TO901317 also increased this association, which was dependent on the presence of LXRα and was accompanied by increased ApoE lipidation. Together, these data suggest that astrocytic LXRα activation and subsequent release of ApoE by astrocytes is critical for the ability of microglia to remove fibrillar Aβ in response to treatment with TO901317.

Liver x receptor: a novel therapeutic target

The liver X receptors α and β are orphan nuclear receptors that are key regulators in maintaining cholesterol homeostasis. Originally they were found to play an important role in reverse cholesterol transport, a pathway for the removal of excess cellular cholesterol. However several groups have now shown that the liver X receptors also functions in lipid and carbohydrate metabolism, cellular differentiation, apoptosis and many immune responses. Tissue distribution of the two paralogues differs with liver X receptor β ubiquitously expressed, while liver X receptor α is confined to the liver, kidney, intestine, spleen, adipose tissue, macrophages and skeletal muscle. The endogenous ligands for the liver X receptors are certain oxidized derivatives of cholesterol, the oxysterols. Upon activation by oxysterols, the receptors form obligate heterodimers with retinoid X receptors α, β and γ; and become competent to activate the transcription of target genes.

Cross-talk between the androgen receptor and the liver X receptor: implications for cholesterol homeostasis

High cholesterol levels are associated with prostate cancer development. Androgens promote cholesterol accumulation by activating the sterol-regulatory element-binding protein isoform 2 (SREBP-2) transcription factor. However, SREBP-2 is in balance with the liver X receptor (LXR; NR1H2/NR1H3), a transcription factor that prevents cholesterol accumulation. Here, we show that LXR activity is down-regulated by the androgen receptor (AR; NR3C4). In turn, this reduces LXR target gene expression. This antagonism on LXR is also exerted by other steroid hormone receptors, including the estrogen, glucocorticoid, and progesterone receptors. This suggests a generalizable mechanism, but the AR does not affect LXR mRNA levels, protein degradation, or DNA binding. We also found that the AR does not require protein synthesis to influence LXR, suggesting a direct antagonism. However, the AR does not directly bind LXR. The AR N-terminal domain (involved in transactivation), but not its DNA-binding domain, is required to suppress LXR activity, suggesting coactivator competition. Overall, this androgen-mediated antagonism of LXR complements SREBP-2 activation, providing a more complete picture as to how androgens increase cellular cholesterol levels in a prostate cancer setting. Given the cross-talk between other steroid hormone receptors and LXR, hormonal regulation of cholesterol via LXR may occur in a variety of cellular contexts.

A novel role for c-Jun N-terminal kinase and phosphoinositide 3-kinase in the liver X receptor-mediated induction of macrophage gene expression

Liver X receptors (LXRs) are ligand-dependent transcription factors that are activated by metabolites of cholesterol, oxysterols, and a number of synthetic agonists. LXRs play potent anti-atherogenic roles in part by stimulating the efflux of cholesterol from macrophage foam cells. The LXR-induced expression of ATP-binding cassette transporter (ABC)-A1 and Apolipoprotein E (ApoE) in macrophages is essential for the stimulation of cholesterol efflux and the prevention of atherosclerotic development. Unfortunately, the signaling pathways underlying such regulation are poorly understood and were therefore investigated in human macrophages. The expression of ApoE and ABCA1 induced by synthetic or natural LXR ligands [TO901317, GW3965, and 22-(R)-hydroxycholesterol (22-(R)-HC), respectively] was attenuated by inhibitors of c-Jun N-terminal kinase (JNK) (curcumin and SP600125) and phosphoinositide 3-kinase (PI3K) (LY294002). Similar results were obtained with ABCG1 and LXR-α, two other LXR target genes. LXR agonists activated several components of the JNK pathway (SEK1, JNK and c-Jun) along with AKT, a downstream target for PI3K. In addition, dominant negative mutants of JNK and PI3K pathways inhibited the LXR-agonists-induced activity of the ABCA1 and LXR-α gene promoters in transfected cells. LXR agonists also induced the binding of activator protein-1 (AP-1), a key transcription factor family regulated by JNK, to recognition sequences present in the regulatory regions of the ApoE and ABCA1 genes. These studies reveal a novel role for JNK and PI3K/AKT signaling in the LXR-regulated expression in macrophages of several key genes implicated in atherosclerosis.