Liver X receptor agonism promotes articular inflammation in murine collagen-induced arthritis

Modulation of the K/BxN arthritis mouse model and the effector functions of human fibroblast-like synoviocytes by liver X receptors

The role of liver X receptors (LXR) in rheumatoid arthritis (RA) remains controversial. We studied the effect of LXR agonists on fibroblast-like synoviocytes (FLS) from RA patients and the K/BxN arthritis model in LXRα and β double-deficient (Nr1h2/3-/-) mice. Two synthetic LXR agonists, GW3965 and T0901317, were used to activate LXRs and investigate their effects on cell growth, proliferation and matrix metalloproteinases, and chemokine production in cultured FLS from RA patients. The murine model K/BxN serum transfer of inflammatory arthritis in Nr1h2/3-/- animals was used to investigate the role of LXRs on joint inflammation in vivo. LXR agonists inhibited the FLS proliferative capacity in response to TNF, the chemokine-induced migration, the collagenase activity in FLS supernatant and FLS CXCL12 production. In the K/BxN mouse model, Nr1h2/3-/- animals showed aggravated arthritis, histological inflammation, and joint destruction, as well as an increase in synovial metalloproteases and expression of proinflammatory mediators such as IL-1β and CCL2 in joints compared with wild type animals. Taken together, these data underscore the importance of LXRs in modulating the joint inflammatory response and highlight them as potential therapeutic targets in RA.

Cellular metabolism regulates the differentiation and function of T-cell subsets

T cells are an important component of adaptive immunity and protect the host from infectious diseases and cancers. However, uncontrolled T cell immunity may cause autoimmune disorders. In both situations, antigen-specific T cells undergo clonal expansion upon the engagement and activation of antigens. Cellular metabolism is reprogrammed to meet the increase in bioenergetic and biosynthetic demands associated with effector T cell expansion. Metabolites not only serve as building blocks or energy sources to fuel cell growth and expansion but also regulate a broad spectrum of cellular signals that instruct the differentiation of multiple T cell subsets. The realm of immunometabolism research is undergoing swift advancements. Encapsulating all the recent progress within this concise review in not possible. Instead, our objective is to provide a succinct introduction to this swiftly progressing research, concentrating on the metabolic intricacies of three pivotal nutrient classes-lipids, glucose, and amino acids-in T cells. We shed light on recent investigations elucidating the roles of these three groups of metabolites in mediating the metabolic and immune functions of T cells. Moreover, we delve into the prospect of "editing" metabolic pathways within T cells using pharmacological or genetic approaches, with the aim of synergizing this approach with existing immunotherapies and enhancing the efficacy of antitumor and antiinfection immune responses.

Inhibition of LXR controls the polarization of human inflammatory macrophages through upregulation of MAFB

Monocyte-derived macrophages contribute to pathogenesis in inflammatory diseases and their effector functions greatly depend on the prevailing extracellular milieu. Whereas M-CSF primes macrophages for acquisition of an anti-inflammatory profile, GM-CSF drives the generation of T cell-stimulatory and pro-inflammatory macrophages. Liver X Receptors (LXRα and LXRβ) are nuclear receptors that control cholesterol metabolism and regulate differentiation of tissue-resident macrophages. Macrophages from rheumatoid arthritis and other inflammatory pathologies exhibit an enriched LXR pathway, and recent reports have shown that LXR activation raises pro-inflammatory effects and impairs the acquisition of the anti-Inflammatory profile of M-CSF-dependent monocyte-derived macrophages (M-MØ). We now report that LXR inhibition prompts the acquisition of an anti-inflammatory gene and functional profile of macrophages generated within a pathological environment (synovial fluid from Rheumatoid Arthritis patients) as well as during the GM-CSF-dependent differentiation of human monocyte-derived macrophages (GM-MØ). Mechanistically, inhibition of LXR results in macrophages with higher expression of the v-Maf Avian Musculoaponeurotic Fibrosarcoma Oncogene Homolog B (MAFB) transcription factor, which governs the macrophage anti-inflammatory profile, as well as over-expression of MAFB-regulated genes. Indeed, gene silencing experiments on human macrophages evidenced that MAFB is required for the LXR inhibitor to enhance the anti-inflammatory nature of human macrophages. As a whole, our results demonstrate that LXR inhibition prompts the acquisition of an anti-inflammatory transcriptional and functional profile of human macrophages in a MAFB-dependent manner, and propose the use of LXR antagonists as potential therapeutic alternatives in macrophage re-programming strategies during inflammatory responses.

Activation of LXR Nuclear Receptors Impairs the Anti-Inflammatory Gene and Functional Profile of M-CSF-Dependent Human Monocyte-Derived Macrophages

Liver X Receptors (LXR) control cholesterol metabolism and exert anti-inflammatory actions but their contribution to human macrophage polarization remains unclear. The LXR pathway is enriched in pro-inflammatory macrophages from rheumatoid arthritis as well as in tumors-associated macrophages from human tumors. We now report that LXR activation inhibits the anti-inflammatory gene and functional profile of M-CSF-dependent human macrophages, and prompts the acquisition of a pro-inflammatory gene signature, with both effects being blocked by an LXR inverse agonist. Mechanistically, the LXR-stimulated macrophage polarization shift correlates with diminished expression of MAFB and MAF, which govern the macrophage anti-inflammatory profile, and with enhanced release of activin A. Indeed, LXR activation impaired macrophage polarization in response to tumor-derived ascitic fluids, as well as the expression of MAF- and MAFB-dependent genes. Our results demonstrate that LXR activation limits the anti-inflammatory human macrophage polarization and prompts the acquisition of an inflammatory transcriptional and functional profile.

Liver X receptors conserve the therapeutic target potential for the treatment of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic multi-system autoimmune disease with extremely complex pathogenesis. Significantly altered lipid paradox related to the inflammatory burden is reported in RA patients, inducing 50% higher cardiovascular risks. Recent studies have also demonstrated that lipid metabolism can regulate many functions of immune cells in which metabolic pathways have altered. The nuclear liver X receptors (LXRs), including LXRα and LXRβ, play a central role in regulating lipid homeostasis and inflammatory responses. Undoubtedly, LXRs have been considered as an attractive therapeutic target for the treatment of RA. However, there are some contradictory effects of LXRs agonists observed in previous animal studies where both pro-inflammatory role and anti-inflammatory role were revealed for LXRs activation in RA. Therefore, in addition to updating the knowledge of LXRs as the prominent regulators of lipid homeostasis, the purpose of this review is to summarize the effects of LXRs agonists in RA-associated immune cells, to explore the underlying reasons for the contradictory therapeutic effects of LXRs agonists observed in RA animal models, and to discuss future strategy for the treatment of RA with LXRs modulators.

Suppression of up-regulated LXRα by silybin ameliorates experimental rheumatoid arthritis and abnormal lipid metabolism

BACKGROUND

As dysregulation of immunometabolism plays a key role in the immunological diseases, dyslipidemia frequently observed in rheumatoid arthritis (RA) patients (60%) is associated with the disease activity and has been considered as the potential target of anti-inflammatory strategy. However, targeting of metabolic events to develop novel anti-inflammatory therapeutics are far from clear as well as the mechanism of dyslipidemia in RA.

PURPOSE

To explore the therapeutic potential and mechanisms of silybin again RA through the regulation of lipid metabolism.

METHODS

Adjuvant-induced arthritis (AIA) rat model was used to examine the effects of silybin on modulating dysregulated lipid metabolism and arthritis. Metabolomics, docking technology, and biochemical methods such as western blots, qRT-PCR, immunofluorescence staining were performed to understanding the underlying mechanisms. Moreover, knock-down of LXRα and LXRα agonist were used on LO2 cell lines to understand the action of silybin.

RESULTS

We are the first to demonstrate that silybin can ameliorate dyslipidemia and arthritis in AIA rats. Overexpression of LXRα and several key lipogenic enzymes regulated by LXRα, including lipoprotein lipase (LPL), cholesterol 7α and 27α hydroxylase (CYP7A, CYP27A), adipocyte fatty acid-binding protein (aP2/FABP4) and fatty acid translocase (CD36/FAT), were observed in AIA rats, which mostly accounted for dyslipidemia during arthritis development. Metabolomics, docking technology, and biochemical results indicated that anti-arthritis effects of silybin related to suppressing the up-regulated LXRα and abnormal lipid metabolism. Notably, activation of LXRα could potentiate cell inflammatory process induced by LPS through the regulation of NF-κB pathway, however, suppression of LXRα agonism by siRNA or silybin reduced the nuclear translocation of NF-κB as well as the induction of downstream cytokines, indicating LXRα agonism is the important factor for the arthritis development and could be a potential target.

CONCLUSION

The up-regulation of LXRα can activate lipogenesis enzymes to worsen the inflammatory process in AIA rats as well as the development of dyslipidemia, therefore, rectifying lipid disorder via suppression of LXRα agonism pertains the capacity of drug target, which enables to discover and develop new drugs to treat rheumatoid arthritis with dyslipidaemia.

Liver X Receptors: Regulators of Cholesterol Metabolism, Inflammation, Autoimmunity, and Cancer

The interplay between cellular stress and immune response can be variable and sometimes contradictory. The mechanisms by which stress-activated pathways regulate the inflammatory response to a pathogen, in autoimmunity or during cancer progression remain unclear in many aspects, despite our recent knowledge of the signalling and transcriptional pathways involved in these diseases. In this context, over the last decade many studies demonstrated that cholesterol metabolism is an important checkpoint for immune homeostasis and cancer progression. Indeed, cholesterol is actively metabolized and can regulate, through its mobilization and/or production of active derivatives, many aspects of immunity and inflammation. Moreover, accumulation of cholesterol has been described in cancer cells, indicating metabolic addiction. The nuclear receptors liver-X-receptors (LXRs) are important regulators of intracellular cholesterol and lipids homeostasis. They have also key regulatory roles in immune response, as they can regulate inflammation, innate and adaptive immunity. Moreover, activation of LXRs has been reported to affect the proliferation and survival of different cancer cell types that show altered metabolic pathways and accumulation of cholesterol. In this minireview we will give an overview of the recent understandings about the mechanisms through which LXRs regulate inflammation, autoimmunity, and cancer, and the therapeutic potential for future treatment of these diseases through modulation of cholesterol metabolism.

Activation of Liver X Receptor α Sensitizes Mice to T-Cell Mediated Hepatitis

Autoimmune hepatitis (AIH) is an inflammatory disease of the liver. Liver X receptors (LXRs), including the α and β isoforms, are previously known for their anti-inflammatory activities. The goal of this study is to determine whether and how LXR plays a role in AIH. LXRα gain-of-function and loss-of-function mouse models were used, in conjunction with the concanavalin A (ConA) model of T-cell mediated hepatitis. We first showed that the hepatic expression of LXRα was decreased in the ConA model of hepatitis and in human patients with AIH. In the ConA model, we were surprised to find that activation of LXRα in the constitutively activated VP-LXRα whole-body knock-in (LXRα-KI) mice exacerbated ConA-induced AIH, whereas the LXRα-/- mice showed attenuated ConA-induced AIH. Interestingly, hepatocyte-specific activation of LXRα in the fatty acid binding protein-VP-LXRα transgenic mice did not exacerbate ConA-induced hepatitis. Mechanistically, the sensitizing effect of the LXRα-KI allele was invariant natural killer T (iNKT)-cell dependent, because the sensitizing effect was abolished when the LXRα-KI allele was bred into the NKT-deficient CD1d-/- background. In addition, LXRα-enhanced ConA-induced hepatitis was dependent on interferon gamma. In contrast, adoptive transfer of hepatic iNKT cells isolated from LXRα-KI mice was sufficient to sensitize CD1d-/- mice to ConA-induced AIH. Conclusion: Activation of LXRα sensitizes mice to ConA-induced AIH in iNKT and interferon gamma-dependent manner. Our results suggest that LXRα plays an important role in the development of AIH.

Oxysterols in Autoimmunity

Cholesterol is a member of the sterol family that plays essential roles in biological processes, including cell membrane stability and myelin formation. Cholesterol can be metabolized into several molecules including bile acids, hormones, and oxysterols. Studies from the last few decades have demonstrated that oxysterols are not only active metabolites but are further involved in the modulation of immune responses. Liver X Receptors (LXRs), nuclear receptors for oxysterols, are important for cholesterol homeostasis and regulation of inflammatory response but are still poorly characterized during autoimmune diseases. Here we review the current knowledge about the role of oxysterols during autoimmune conditions and focus on the implication of LXR-dependent and LXR-independent pathways. We further highlight the importance of these pathways in particular during central nervous system (CNS) autoimmunity and inflammatory bowel diseases (IBD) in both experimental models and human studies. Finally, we discuss our vision about future applications and research on oxysterols related to autoimmunity.

The Liver X Receptor Is Upregulated in Monocyte-Derived Macrophages and Modulates Inflammatory Cytokines Based on LXRα Polymorphism

Liver X receptors (LXRs) have emerged as important regulators of inflammatory gene expression. Previously, we had reported that an LXRα gene promoter polymorphism (-1830 T > C) is associated with systemic lupus erythematosus (SLE). Therefore, we assessed cytokine expression in relation to LXRα polymorphism in monocyte-derived macrophages from patients with SLE. Macrophages were obtained after 72 hours of culture of human monocytes supplemented with phorbol 12-myristate 13-acetate. Cells were transfected with LXRα promoter constructs. Additionally, peripheral blood mononuclear cell- (PBMC-) derived macrophages from the patients were evaluated for proinflammatory cytokines in relation to the genotypes of LXRα -1830 T > C. The expression of LXRα was increased in macrophages; levels of proinflammatory cytokines were decreased with LXRα expression. Production of proinflammatory cytokines varied depending on LXRα -1830 T > C genotype. In particular, expression of LXRα was decreased and that of proinflammatory cytokines was increased for LXRα -1830 TC genotype compared to that for TT genotype. The data were consistent in PBMC-derived macrophages from patients with SLE. Increased proinflammatory cytokines is related to TLR7 and TLR9 expression. These data suggest that the expression levels of LXRα, according to LXRα -1830 T > C genotype, may contribute to the inflammatory response by induction of inflammatory cytokines in SLE.

IL-27-Induced Type 1 Regulatory T-Cells Produce Oxysterols that Constrain IL-10 Production

The behaviors of lymphocytes, including CD4⁺ T helper cells, are controlled on many levels by internal metabolic properties. Lipid metabolites have recently been ascribed a novel function as immune response modulators and perturbation of steroids pathways modulates inflammation and potentially promotes a variety of diseases. However, the impact of lipid metabolism on autoimmune disease development and lymphocyte biology is still largely unraveled. In this line, oxysterols, oxidized forms of cholesterol, have pleiotropic roles on the immune response aside from their involvements in lipid metabolism. The oxysterols 25-hydroxycholesterol (25-OHC) and 7α,25-dihydroxycholesterol (7α,25-OHC) regulate antiviral immunity and immune cell chemotaxis. However, their physiological effects on adaptive immune response in particular on various subset CD4⁺ T lymphocytes are largely unknown. Here, we assessed oxysterol levels in subset of CD4⁺ T cells and demonstrated that 25-OHC and transcript levels of its synthesizing enzyme, cholesterol 25-hydroxylase, were specifically increased in IL-27-induced type 1 regulatory T (T_R1) cells. We further showed that 25-OHC acts as a negative regulator of T_R1 cells in particular of IL-10 secretion via liver X receptor signaling. Not only do these findings unravel molecular mechanisms accounting for IL-27 signaling but also they highlight oxysterols as pro-inflammatory mediators that dampens regulatory T cell responses and thus unleash a pro-inflammatory response.

Tofacitinib restores the inhibition of reverse cholesterol transport induced by inflammation: understanding the lipid paradox associated with rheumatoid arthritis

BACKGROUND AND PURPOSE

Patients with active rheumatoid arthritis (RA) have increased cardiovascular mortality, paradoxically associated with reduced circulating lipid levels. The JAK inhibitor tofacitinib ameliorates systemic and joint inflammation in RA with a concomitant increase in serum lipids. We analysed the effect of tofacitinib on the lipid profile of hyperlipidaemic rabbits with chronic arthritis (CA) and on the changes in reverse cholesterol transport (RCT) during chronic inflammation.

EXPERIMENTAL APPROACH

CA was induced in previously immunized rabbits, fed a high-fat diet, by administering four intra-articular injections of ovalbumin. A group of rabbits received tofacitinib (10 mg·kg-1 ·day-1 ) for 2 weeks. Systemic and synovial inflammation and lipid content were evaluated. For in vitro studies, THP-1-derived macrophages were exposed to high lipid concentrations and then stimulated with IFNγ in the presence or absence of tofacitinib in order to study mediators of RCT.

KEY RESULTS

Tofacitinib decreased systemic and synovial inflammation and increased circulating lipid levels. Although it did not modify synovial macrophage density, it reduced the lipid content within synovial macrophages. In foam macrophages in culture, IFNγ further stimulated intracellular lipid accumulation, while the JAK/STAT inhibition provoked by tofacitinib induced lipid release by increasing the levels of cellular liver X receptor α and ATP-binding cassette transporter (ABCA1) synthesis.

CONCLUSIONS AND IMPLICATIONS

Active inflammation could be associated with lipid accumulation within macrophages of CA rabbits. JAK inhibition induced lipid release through RCT activation, providing a plausible explanation for the effect of tofacitinib on the lipid profile of RA patients.

The challenges and promise of targeting the Liver X Receptors for treatment of inflammatory disease

The Liver X Receptors (LXRs) are oxysterol-activated transcription factors that upregulate a suite of genes that together promote coordinated mobilization of excess cholesterol from cells and from the body. The LXRs, like other nuclear receptors, are anti-inflammatory, inhibiting signal-dependent induction of pro-inflammatory genes by nuclear factor-κB, activating protein-1, and other transcription factors. Synthetic LXR agonists have been shown to ameliorate atherosclerosis and a wide range of inflammatory disorders in preclinical animal models. Although this has suggested potential for application to human disease, systemic LXR activation is complicated by hepatic steatosis and hypertriglyceridemia, consequences of lipogenic gene induction in the liver by LXRα. The past several years have seen the development of multiple advanced LXR therapeutics aiming to avoid hepatic lipogenesis, including LXRβ-selective agonists, tissue-selective agonists, and transrepression-selective agonists. Although several synthetic LXR agonists have made it to phase I clinical trials, none have progressed due to unforeseen adverse reactions or undisclosed reasons. Nonetheless, several sophisticated pharmacologic strategies, including structure-guided drug design, cell-specific drug targeting, as well as non-systemic drug routes have been initiated and remain to be comprehensively explored. In addition, recent studies have identified potential utility for targeting the LXRs during therapy with other agents, such as glucocorticoids and rexinoids. Despite the pitfalls encountered to date in translation of LXR agonists to human disease, it appears likely that this accelerating field will ultimately yield effective and safe applications for LXR targeting in humans.

Differentiated all-trans retinoic acid response of naive CD4+CD25- cells isolated from rats with collagen-induced arthritis and healthy ones under in vitro conditions

Aim o the study

To compare the potential of CD4+CD25– cells, isolated from both healthy rats and rats with CIA (Collagen-Induced Arthritis), for differentiation into regulatory T cells in the presence of all-trans retinoic acid in order to learn more about the activation mechanisms and therapeutic potential of regulatory T cells.

Material and methods

Sorted CD4+CD25– cells were cultured in vitro with/without ATRA, and then the frequency of regulatory T cells and their ability to secrete IL-10 by CD4+ FOXP3+ cells was examined. Gene expression of the foxp3, rarα, rarβ, rxrβ, and ppar β/δ and protein expression of the Rarα, Rarβ, and Rxrβ in cells after stimulation with ATRA were also investigated.

Results

CD4+CD25– cells isolated from healthy animals or from animals with CIA are characterised by different potential of the differentiation into CD4+CD25+ FOXP3+ cells. Retinoic acid receptor Rxrβ is present in the CD4+CD25– cells isolated from rats with CIA.

Conclusions

We showed that although ATRA did not increase the frequency of Treg in culture, it significantly increased expression of rarβ and rxrβ only in lymphocytes taken from diseased animals and foxp3 expression only in healthy animals. Moreover, after ATRA stimulation, the frequency of Treg-produced IL-10 tended to be lower in diseased animals than in the healthy group. The results imply that the potential of naïve cell CD4 lymphocytes to differentiate into Tregs and their putative suppressive function is dependent on the donor’s health status.

Activation of LXR attenuates collagen-induced arthritis via suppressing BLyS production

B-lymphocyte stimulator (BLyS) plays a critical role in the pathogenesis and progression of rheumatoid arthritis (RA). Liver X receptor (LXR), a nuclear receptor, has an important anti-inflammatory effect. However, it is unclear whether the BLyS expression is regulated by LXR. In this study, we found that treatment with LXR agonist in collagen-induced arthritis (CIA) mice significantly attenuated arthritis progression, and markedly decreased BLyS production in serum and splenocytes as well as the production of serum IFNγ and TGFβ. Activation of LXR in B lymphocytes dramatically suppressed the basal and IFNγ/TGFβ-induced BLyS expression. Moreover, LXR agonist prominently suppressed the binding of NF-κB to BLyS promoter region, and decreased the promoter's transcriptional activity. Additionally, activation of LXR obviously repressed IFNγ-induced STAT1 activation and TGFβ-induced SMAD3 activation. These results indicated that downregulation of BLyS may be a novel mechanism by which LXR ameliorates RA, and LXR/BLyS pathway may serve as a novel target for the treatment of RA.

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.

Identification of interferon-γ as a new molecular target of liver X receptor

LXR (liver X receptor) is a ligand-activated transcription factor and plays an important role in regulation of lipid homoeostasis and inflammation. Several studies indicate that LXR inhibits IFN-γ (interferon γ)-induced biological responses; however, the influence of LXR on IFN-γ expression has not been fully elucidated. In the present study, we investigated the effects of LXR activation on IFN-γ expression at different levels. At the molecular level, we surprisingly observed that LXR ligand (T0901317) induced macrophage and T-cell IFN-γ protein expression which was associated with increased mRNA and secreted protein levels in culture medium. In contrast, selective inhibition of LXRα and/or LXRβ expression by siRNA reduced IFN-γ expression. Promoter analysis defined the multiple LXREs (LXR-responsive elements) in the proximal region of the IFN-γ promoter. EMSAs and ChIP indicated that LXR activation enhanced the binding of LXR protein to these LXREs. In vivo, T0901317 increased wild-type mouse serum IFN-γ levels and IFN-γ expression in the lung and lymph nodes. Functionally, we observed that administration of T0901317 to wild-type mice increased rates of survival and being tumour-free, and inhibited tumour growth when the animals were inoculated with LLC1 carcinoma. In contrast, these protective effects were substantially attenuated in IFN-γ-knockout (IFN-γ-/-) mice, suggesting that the induction of IFN-γ production plays a critical role in T0901317-inhibited tumour growth. Taken together, the results of the present study show that IFN-γ is another molecular target of LXR activation, and it suggests a new mechanism by which LXR inhibits tumour growth.

Activation of liver X receptors inhibits experimental fibrosis by interfering with interleukin-6 release from macrophages

OBJECTIVES

To investigate the role of liver X receptors (LXRs) in experimental skin fibrosis and evaluate their potential as novel antifibrotic targets.

METHODS

We studied the role of LXRs in bleomycin-induced skin fibrosis, in the model of sclerodermatous graft-versus-host disease (sclGvHD) and in tight skin-1 (Tsk-1) mice, reflecting different subtypes of fibrotic disease. We examined both LXR isoforms using LXRα-, LXRβ- and LXR-α/β-double-knockout mice. Finally, we investigated the effects of LXRs on fibroblasts and macrophages to establish the antifibrotic mode of action of LXRs.

RESULTS

LXR activation by the agonist T0901317 had antifibrotic effects in bleomycin-induced skin fibrosis, in the sclGvHD model and in Tsk-1 mice. The antifibrotic activity of LXRs was particularly prominent in the inflammation-driven bleomycin and sclGvHD models. LXRα-, LXRβ- and LXRα/β-double-knockout mice showed a similar response to bleomycin as wildtype animals. Low levels of the LXR target gene ABCA-1 in the skin of bleomycin-challenged and control mice suggested a low baseline activation of the antifibrotic LXR signalling, which, however, could be specifically activated by T0901317. Fibroblasts were not the direct target cells of LXRs agonists, but LXR activation inhibited fibrosis by interfering with infiltration of macrophages and their release of the pro-fibrotic interleukin-6.

CONCLUSIONS

We identified LXRs as novel targets for antifibrotic therapies, a yet unknown aspect of these nuclear receptors. Our data suggest that LXR activation might be particularly effective in patients with inflammatory disease subtypes. Activation of LXRs interfered with the release of interleukin-6 from macrophages and, thus, inhibited fibroblast activation and collagen release.

Activation of liver X receptor decreases atherosclerosis in Ldlr⁻/⁻ mice in the absence of ATP-binding cassette transporters A1 and G1 in myeloid cells

OBJECTIVE

Liver X receptor (LXR) activators decrease atherosclerosis in mice. LXR activators (1) directly upregulate genes involved in reverse cholesterol transport and (2) exert anti-inflammatory effects mediated by transrepression of nuclear factor-κB target genes. We investigated whether myeloid cell deficiency of ATP-binding cassette transporters A1 and G1 (ABCA1/G1), principal targets of LXR that promote macrophage cholesterol efflux and initiate reverse cholesterol transport, would abolish the beneficial effects of LXR activation on atherosclerosis.

APPROACH AND RESULTS

LXR activator T0901317 substantially reduced inflammatory gene expression in macrophages lacking ABCA1/G1. Ldlr(-/-) mice were transplanted with Abca1(-/-)Abcg1(-/-) or wild-type bone marrow (BM) and fed a Western-type diet for 6 weeks with or without T0901317 supplementation. Abca1/g1 BM deficiency increased atherosclerotic lesion complexity and inflammatory cell infiltration into the adventitia and myocardium. T0901317 markedly decreased lesion area, complexity, and inflammatory cell infiltration in the Abca1(-/-)Abcg1(-/-) BM-transplanted mice. To investigate whether this was because of macrophage Abca1/g1 deficiency, Ldlr(-/-) mice were transplanted with LysmCreAbca1(fl/fl)Abcg1(fl/fl) or Abca1(fl/fl)Abcg1(fl/fl) BM and fed Western-type diet with or without the more specific LXR agonist GW3965 for 12 weeks. GW3965 decreased lesion size in both groups, and the decrease was more prominent in the LysmCreAbca1(fl/fl)Abcg1(fl/fl) group.

CONCLUSIONS

The results suggest that anti-inflammatory effects of LXR activators are of key importance to their antiatherosclerotic effects in vivo independent of cholesterol efflux pathways mediated by macrophage ABCA1/G1. This has implications for the development of LXR activators that lack adverse effects on lipogenic genes while maintaining the ability to transrepress inflammatory genes.

The role of lipid-activated nuclear receptors in shaping macrophage and dendritic cell function: From physiology to pathology

Nuclear receptors are ligand-activated transcription factors linking lipid signaling to the expression of the genome. There is increasing appreciation of the involvement of this receptor network in the metabolic programming of macrophages and dendritic cells (DCs), essential members of the innate immune system. In this review we focus on the role of retinoid X receptor, retinoic acid receptor, peroxisome proliferator-associated receptor γ, liver X receptor, and vitamin D receptor in shaping the immune and metabolic functions of macrophages and DCs. We also provide an overview of the contribution of macrophage- and DC-expressed nuclear receptors to various immunopathologic conditions, such as rheumatoid arthritis, inflammatory bowel disease, systemic lupus erythematosus, asthma, and some others. We suggest that systematic analyses of the roles of these receptors and their activating lipid ligands in immunopathologies combined with complementary and focused translational and clinical research will be crucial for the development of new therapies using the many molecules available to target nuclear receptors.

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.

Targeting liver X receptors in inflammation

INTRODUCTION

The two oxysterol receptors, 'liver X receptors (LXRs)' LXRα and LXRβ, are amongst the emerging newer drug targets within the nuclear receptor family and targeting LXRs represents novel strategies needed for prevention and treatment of diseases where current therapeutics is inadequate.

AREAS COVERED

This review discusses the current understanding of LXR biology with an emphasis on the molecular aspects of LXR signalling establishing their potential as drug targets. Recent advances of their transcriptional mechanisms in inflammatory pathways and their physiological roles in inflammation and immunity are described.

EXPERT OPINION

The new discoveries of LXR-regulated inflammatory pathways have ignited new promises for LXRs as drug targets. The broad physiological roles of LXRs involve a high risk of unwanted side effects. Recent insights into LXR biology of the brain indicate a highly important role in neuronal development and a clinical trial testing an LXR agonist reported adverse neurological side effects. This suggests that drug development must focus on limiting the range of LXR signalling - possibly achieved through subtype, tissue specific, promoter specific or pathway specific activation of LXRs where a successful candidate drug must be carefully studied for its effect in the central nervous system.

The liver X receptor pathway is highly upregulated in rheumatoid arthritis synovial macrophages and potentiates TLR-driven cytokine release

OBJECTIVES

Macrophages are central to the inflammatory processes driving rheumatoid arthritis (RA) synovitis. The molecular pathways that are induced in synovial macrophages and thereby promote RA disease pathology remain poorly understood.

METHODS

We used microarray to characterise the transcriptome of synovial fluid (SF) macrophages compared with matched peripheral blood monocytes from patients with RA (n=8).

RESULTS

Using in silico pathway mapping, we found that pathways downstream of the cholesterol activated liver X receptors (LXRs) and those associated with Toll-like receptor (TLR) signalling were upregulated in SF macrophages. Macrophage differentiation and tumour necrosis factor α promoted the expression of LXRα. Furthermore, in functional studies we demonstrated that activation of LXRs significantly augmented TLR-driven cytokine and chemokine secretion.

CONCLUSIONS

The LXR pathway is the most upregulated pathway in RA synovial macrophages and activation of LXRs by ligands present within SF augments TLR-driven cytokine secretion. Since the natural agonists of LXRs arise from cholesterol metabolism, this provides a novel mechanism that can promote RA synovitis.

Increased synovial expression of nuclear receptors correlates with protection in pristane-induced arthritis: a possible novel genetically regulated homeostatic mechanism

OBJECTIVE

To use microarray analyses of gene expression to characterize the synovial molecular pathways regulated by the arthritis regulatory locus Cia25 and to determine how it operates to control disease severity and joint damage.

METHODS

Synovial tissues from DA rats and DA.ACI(Cia25) rats obtained 21 days after induction of pristane-induced arthritis were used for RNA extraction and hybridization to Illumina RatRef-12 Expression BeadChips (22,228 genes). Genes with a P value≤0.01 and a fold difference in expression≥1.5 between DA rats and DA.ACI(Cia25) rats were considered significant.

RESULTS

Interleukin-1β (IL-1β) (7.4-fold), IL-6 (67-fold), Ccl2, Cxcl10, Mmp3, Mmp14, and innate immunity genes were expressed at increased levels in DA rats and at significantly lower levels in DA.ACI(Cia25) congenic rats. DA.ACI(Cia25) rats had increased expression of 10 nuclear receptor (NR) genes, including those known to interfere with NF-κB activity and cytokine expression, such as Lxra, Pparg, and Rxrg. DA.ACI(Cia25) rats also had increased expression of NR targets, suggesting increased NR activity. While Vdr was not differentially expressed, a Vdr expression signature was detected in congenic rats, along with up-regulation of mediators of vitamin D synthesis.

CONCLUSION

This is the first description of the association between increased synovial levels of NRs and arthritis protection. The expression of NRs was inversely correlated with the expression of key mediators of arthritis, suggesting reciprocally opposing effects either via NF-κB or at the genomic level in the synovial tissue. We consider that the NR signature may have an important role in maintaining synovial homeostasis and an inflammation-free tissue. These processes are regulated by the Cia25 gene and suggest a new function for this gene.

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.

Liver X receptor regulates rheumatoid arthritis fibroblast-like synoviocyte invasiveness, matrix metalloproteinase 2 activation, interleukin-6 and CXCL10

Fibroblast-like synoviocyte (FLS) invasiveness correlates with articular damage in rheumatoid arthritis (RA), yet little is known about its regulation. In this study we aimed to determine the role of the nuclear receptor liver X receptor (LXR) in FLS invasion. FLS were isolated from synovial tissues obtained from RA patients and from DA rats with pristane-induced arthritis. Invasion was tested on Matrigel-coated chambers in the presence of the LXR agonist T0901317, or control vehicle. FLS were cultured in the presence or absence of T0901317, and supernatants were used to quantify matrix metalloproteinase 1 (MMP-1), MMP-2, MMP-3, interleukin-6 (IL-6), tumor necrosis factor-α and C-X-C motif chemokine ligand 10 (CXCL10). Nuclear factor-κB (NF-κB) (p65) and Akt activation, actin cytoskeleton, cell morphology and lamellipodia formation were also determined. The LXR agonist T0901317 significantly reduced DA FLS invasion by 99% (P ≤ 0.001), and RA FLS invasion by 96% (P ≤ 0.001), compared with control. T0901317-induced suppression of invasion was associated with reduced production of activated MMP-2, IL-6 and CXCL10 by RA FLS, and with reduction of actin filament reorganization and reduced polarized formation of lamellipodia. T0901317 also prevented both IL-1β-induced and IL-6-induced FLS invasion. NF-κB (p65) and Akt activation were not significantly affected by T0901317. This is the first description of a role for LXR in the regulation of FLS invasion and in processes and pathways implicated both in invasion as well as in inflammatory responses. These findings provide a new rationale for considering LXR agonists as therapeutic agents aimed at reducing both inflammation and FLS-mediated invasion and destruction in RA.

Liver X receptor biology and pharmacology: new pathways, challenges and opportunities

Nuclear receptors (NRs) are master regulators of transcriptional programs that integrate the homeostatic control of almost all biological processes. Their direct mode of ligand regulation and genome interaction is at the core of modern pharmacology. The two liver X receptors LXRα and LXRβ are among the emerging newer drug targets within the NR family. LXRs are best known as nuclear oxysterol receptors and physiological regulators of lipid and cholesterol metabolism that also act in an anti-inflammatory way. Because LXRs control diverse pathways in development, reproduction, metabolism, immunity and inflammation, they have potential as therapeutic targets for diseases as diverse as lipid disorders, atherosclerosis, chronic inflammation, autoimmunity, cancer and neurodegenerative diseases. Recent insights into LXR signaling suggest future targeting strategies aiming at increasing LXR subtype and pathway selectivity. This review discusses the current status of our understanding of LXR biology and pharmacology, with an emphasis on the molecular aspects of LXR signaling that constitute the potential of LXRs as drug targets.

Liver X receptors bridge hepatic lipid metabolism and inflammation

Liver X receptors (LXRs) are members of the superfamily of metabolic nuclear receptors, which play central roles in the regulation of cholesterol absorption, efflux, transportation and excretion and many other processes correlating with lipid metabolism. LXRs can also regulate inflammation in vitro and in vivo. Accumulating evidence demonstrates that LXR are involved in the metabolism and inflammation in human diseases. Nonalcoholic fatty liver disease (NAFLD) is classically associated with lipid metabolic disorders and inflammatory responses, especially in the nonalcoholic steatohepatitis (NASH) phase. The effects of LXRs on cholesterol metabolism and inflammation make them attractive as a potential target for the treatment of NAFLD. Since the ability to synthesize triglycerides may be protective in obesity and fatty liver, the hepatic lipogenesis by LXRs should not rule out the possibility of the use of LXRs in NAFLD.

Genome-wide landscape of liver X receptor chromatin binding and gene regulation in human macrophages

Background

The liver X receptors (LXRs) are oxysterol sensing nuclear receptors with multiple effects on metabolism and immune cells. However, the complete genome-wide cistrome of LXR in cells of human origin has not yet been provided.

Results

We performed ChIP-seq in phorbol myristate acetate-differentiated THP-1 cells (macrophage-type) after stimulation with the potent synthetic LXR ligand T0901317 (T09). Microarray gene expression analysis was performed in the same cellular model. We identified 1357 genome-wide LXR locations (FDR < 1%), of which 526 were observed after T09 treatment. De novo analysis of LXR binding sequences identified a DR4-type element as the major motif. On mRNA level T09 up-regulated 1258 genes and repressed 455 genes. Our results show that LXR actions are focused on 112 genomic regions that contain up to 11 T09 target genes per region under the control of highly stringent LXR binding sites with individual constellations for each region. We could confirm that LXR controls lipid metabolism and transport and observed a strong association with apoptosis-related functions.

Conclusions

This first report on genome-wide binding of LXR in a human cell line provides new insights into the transcriptional network of LXR and its target genes with their link to physiological processes, such as apoptosis.

The gene expression microarray and sequence data have been submitted collectively to the NCBI Gene Expression Omnibus http://www.ncbi.nlm.nih.gov/geo under accession number GSE28319.

Nuclear receptors take center stage in Th17 cell-mediated autoimmunity

Liver X receptors (LXRs) are nuclear receptors involved in cholesterol homeostasis. Notably, they are also expressed by T cells and are involved in regulating T cell proliferation and differentiation. In this issue of the JCI, Cui et al. have elucidated the molecular mechanism underlying the effects of LXR activation on a subset of T cells known as Th17 cells in mice and humans. Specifically, they showed that LXR-induced Srebp-1 inhibits Il17 transcription by binding to the Il17 promoter through interaction with the aryl hydrocarbon receptor (Ahr), a transcription factor known to enhance Th17 cell responses.

Liver X receptor (LXR) mediates negative regulation of mouse and human Th17 differentiation

Th17 cells are a subset of CD4+ T cells with an important role in clearing certain bacterial and fungal pathogens. However, they have also been implicated in autoimmune diseases such as multiple sclerosis. Exposure of naive CD4+ T cells to IL-6 and TGF-β leads to Th17 cell differentiation through a process in which many proteins have been implicated. We report here that ectopic expression of liver X receptor (LXR) inhibits Th17 polarization of mouse CD4+ T cells, while LXR deficiency promotes Th17 differentiation in vitro. LXR activation in mice ameliorated disease in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis, whereas LXR deficiency exacerbated disease. Further analysis revealed that Srebp-1, which is encoded by an LXR target gene, mediated the suppression of Th17 differentiation by binding to the E-box element on the Il17 promoter, physically interacting with aryl hydrocarbon receptor (Ahr) and inhibiting Ahr-controlled Il17 transcription. The putative active site (PAS) domain of Ahr and the N-terminal acidic region of Srebp-1 were essential for this interaction. Additional analyses suggested that similar LXR-dependent mechanisms were operational during human Th17 differentiation in vitro. This study reports what we believe to be a novel signaling pathway underlying LXR-mediated regulation of Th17 cell differentiation and autoimmunity.

Liver X receptors and immune regulation

Recent studies suggest that homeostasis of lipid metabolism is crucial for the function of various immune cells. Oxygenated derivatives of cholesterol (oxysterols) are well-known regulators of lipid metabolism and have diverse functions, such as inhibition of cholesterol synthesis, efflux of intracellular cholesterol, synthesis of cholesterol esters, and activation of liver X receptors (LXRs). In this review, we introduce novel roles of the oxysterol receptors LXRs in the immune system, including regulation of inflammatory responses, T cell expansion, immunoglobulin production, and antitumor responses. We also discuss lipid-mediated signaling as a potential target for treatment of immune diseases.