LXR/ApoE Activation Restricts Innate Immune Suppression in Cancer.

Therapeutic harnessing of adaptive immunity via checkpoint inhibition has transformed the treatment of many cancers. Despite unprecedented long-term responses, most patients do not respond to these therapies. Immunotherapy non-responders often harbor high levels of circulating myeloid-derived suppressor cells (MDSCs)-an immunosuppressive innate cell population. Through genetic and pharmacological approaches, we uncovered a pathway governing MDSC abundance in multiple cancer types. Therapeutic liver-X nuclear receptor (LXR) agonism reduced MDSC abundance in murine models and in patients treated in a first-in-human dose escalation phase 1 trial. MDSC depletion was associated with activation of cytotoxic T lymphocyte (CTL) responses in mice and patients. The LXR transcriptional target ApoE mediated these effects in mice, where LXR/ApoE activation therapy elicited robust anti-tumor responses and also enhanced T cell activation during various immune-based therapies. We implicate the LXR/ApoE axis in the regulation of innate immune suppression and as a target for enhancing the efficacy of cancer immunotherapy in patients.

Cholesterol Accumulation in CD11c+ Immune Cells Is a Causal and Targetable Factor in Autoimmune Disease.

Liver X receptors (LXRs) are regulators of cholesterol metabolism that also modulate immune responses. Inactivation of LXR α and ß in mice leads to autoimmunity; however, how the regulation of cholesterol metabolism contributes to autoimmunity is unclear. Here we found that cholesterol loading of CD11c+ cells triggered the development of autoimmunity, whereas preventing excess lipid accumulation by promoting cholesterol efflux was therapeutic. LXRß-deficient mice crossed to the hyperlipidemic ApoE-deficient background or challenged with a high-cholesterol diet developed autoantibodies. Cholesterol accumulation in lymphoid organs promoted T cell priming and stimulated the production of the B cell growth factors Baff and April. Conversely, B cell expansion and the development of autoantibodies in ApoE/LXR-ß-deficient mice was reversed by ApoA-I expression. These findings implicate cholesterol imbalance as a contributor to immune dysfunction and suggest that stimulating HDL-dependent reverse cholesterol transport could be beneficial in the setting of autoimmune disease.

LXR directly regulates glycosphingolipid synthesis and affects human CD4+ T cell function.

The liver X receptor (LXR) is a key transcriptional regulator of cholesterol, fatty acid, and phospholipid metabolism. Dynamic remodeling of immunometabolic pathways, including lipid metabolism, is a crucial step in T cell activation. Here, we explored the role of LXR-regulated metabolic processes in primary human CD4+ T cells and their role in controlling plasma membrane lipids (glycosphingolipids and cholesterol), which strongly influence T cell immune signaling and function. Crucially, we identified the glycosphingolipid biosynthesis enzyme glucosylceramide synthase as a direct transcriptional LXR target. LXR activation by agonist GW3965 or endogenous oxysterol ligands significantly altered the glycosphingolipid:cholesterol balance in the plasma membrane by increasing glycosphingolipid levels and reducing cholesterol. Consequently, LXR activation lowered plasma membrane lipid order (stability), and an LXR antagonist could block this effect. LXR stimulation also reduced lipid order at the immune synapse and accelerated activation of proximal T cell signaling molecules. Ultimately, LXR activation dampened proinflammatory T cell function. Finally, compared with responder T cells, regulatory T cells had a distinct pattern of LXR target gene expression corresponding to reduced lipid order. This suggests LXR-driven lipid metabolism could contribute to functional specialization of these T cell subsets. Overall, we report a mode of action for LXR in T cells involving the regulation of glycosphingolipid and cholesterol metabolism and demonstrate its relevance in modulating T cell function.

Absence of nuclear receptors LXRs impairs immune response to androgen deprivation and leads to prostate neoplasia.

Chronic inflammation is now a well-known precursor for cancer development. Infectious prostatitis are the most common causes of prostate inflammation, but emerging evidence points the role of metabolic disorders as a potential source of cancer-related inflammation. Although the widely used treatment for prostate cancer based on androgen deprivation therapy (ADT) effectively decreases tumor size, it also causes profound alterations in immune tumor microenvironment within the prostate. Here, we demonstrate that prostates of a mouse model invalidated for nuclear receptors liver X receptors (LXRs), crucial lipid metabolism and inflammation integrators, respond in an unexpected way to androgen deprivation. Indeed, we observed profound alterations in immune cells composition, which was associated with chronic inflammation of the prostate. This was explained by the recruitment of phagocytosis-deficient macrophages leading to aberrant hyporesponse to castration. This phenotypic alteration was sufficient to allow prostatic neoplasia. Altogether, these data suggest that ADT and inflammation resulting from metabolic alterations interact to promote aberrant proliferation of epithelial prostate cells and development of neoplasia. This raises the question of the benefit of ADT for patients with metabolic disorders.

LXR Alpha Restricts Gammaherpesvirus Reactivation from Latently Infected Peritoneal Cells.

Gammaherpesviruses are ubiquitous viruses that establish lifelong infections. Importantly, these viruses are associated with numerous cancers and lymphoproliferative diseases. While risk factors for developing gammaherpesvirus-driven cancers are poorly understood, it is clear that elevated viral reactivation from latency often precedes oncogenesis. Here, we demonstrate that the liver X receptor alpha isoform (LXRα) restricts gammaherpesvirus reactivation in an anatomic-site-specific manner. We have previously demonstrated that deficiency of both LXR isoforms (α and ß) leads to an increase in fatty acid and cholesterol synthesis in primary macrophage cultures, with a corresponding increase in gammaherpesvirus replication. Interestingly, expression of fatty acid synthesis genes was not derepressed in LXRα-deficient hosts, indicating that the antiviral effects of LXRα are independent of lipogenesis. Additionally, the critical host defenses against gammaherpesvirus reactivation, virus-specific CD8+ T cells and interferon (IFN) signaling, remained intact in the absence of LXRα. Remarkably, using a murine gammaherpesvirus 68 (MHV68) reporter virus, we discovered that LXRα expression dictates the cellular tropism of MHV68 in the peritoneal cavity. Specifically, LXRα-/- mice exhibit reduced latency within the peritoneal B cell compartment and elevated latency within F4/80+ cells. Thus, LXRα restricts gammaherpesvirus reactivation through a novel mechanism that is independent of the known CD8+ T cell-based antiviral responses or changes in lipid synthesis and likely involves changes in the tropism of MHV68 in the peritoneal cavity.IMPORTANCE Liver X receptors (LXRs) are nuclear receptors that mediate cholesterol and fatty acid homeostasis. Importantly, as ligand-activated transcription factors, LXRs represent potential targets for the treatment of hypercholesterolemia and atherosclerosis. Here, we demonstrate that LXRα, one of the two LXR isoforms, restricts reactivation of latent gammaherpesvirus from peritoneal cells. As gammaherpesviruses are ubiquitous oncogenic agents, LXRs may represent a targetable host factor for the treatment of poorly controlled gammaherpesvirus infection and associated lymphomagenesis.

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.

Stress as an immunomodulator: liver X receptors maybe the answer.

Stress is a reflex response, both psychological and physiological, of the body to a difficult situation that requires adaptation. Stress is at the intersection of the objective event and the subjective event. The physiological mechanisms involved in chronic stress are numerous and can contribute to a wide variety of disorders, in all systems including the immune system. Stress modifies the Th1/Th2 balance via the HPA axis and a set of immune mediators. This will make the body more vulnerable to external infections in a scientific way while others claim the opposite, stress could be considered immune stimulatory. The development of synthetic LXR ligands such as T0901317 and GW3965 as well as an understanding of the direct involvement of these receptors in the regulation of proopiomelanocortin (POMC) gene expression and indirectly by producing a variety of cytokines in a stressor response, will open in the near future new therapeutic methods against the undesirable effects of stress on the behavior of the immune system.

The role of monocytosis and neutrophilia in atherosclerosis.

Monocytosis and neutrophilia are frequent events in atherosclerosis. These phenomena arise from the increased proliferation of hematopoietic stem and multipotential progenitor cells (HSPCs) and HSPC mobilization from the bone marrow to other immune organs and circulation. High cholesterol and inflammatory signals promote HSPC proliferation and preferential differentiation to the myeloid precursors (i.e., myelopoiesis) that than give rise to pro-inflammatory immune cells. These cells accumulate in the plaques thereby enhancing vascular inflammation and contributing to further lesion progression. Studies in animal models of atherosclerosis showed that manipulation with HSPC proliferation and differentiation through the activation of LXR-dependent mechanisms and restoration of cholesterol efflux may have a significant therapeutic potential.

ENHO, RXRA, and LXRA polymorphisms and dyslipidaemia, related comorbidities and survival in haemodialysis patients.

BACKGROUND: The energy homeostasis-associated gene (ENHO), retinoid X receptor alpha gene (RXRA), and liver X receptor alpha gene (LXRA) are involved in adipogenic/lipogenic regulation. We investigated whether single-nucleotide polymorphisms in these genes (ENHO rs2281997, rs72735260; RXRA rs749759, rs10776909, rs10881578; LXRA rs2279238, rs7120118, rs11039155) are associated with dyslipidaemia, related comorbidities and survival of haemodialysis (HD) patients also tested for T-helper (Th) cell interleukin genes (IL). METHODS: The study was carried out in 873 HD patients. Dyslipidaemia was diagnosed by the recommendations of the Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines (2003); atherogenic dyslipidaemia was referred to if the TG/HDL cholesterol ratio was equal to or higher than 3.8. Genotyping of ENHO SNPs, LXRA SNPs, and IL12A rs568408 was carried out using HRM analysis. RXRA SNPs, IL12B rs3212227, and IL18 rs360719 were genotyped using PCR-RFLP analysis. The circulating adropin concentration was determined in 126 patients by enzyme-linked immunosorbent assay. Survival probability was analysed using the Kaplan-Meier method in 440 patients followed through 7.5 years. RESULTS: Dyslipidaemia by K/DOQI was diagnosed in 459 patients (91% revealed hyper-LDL- cholesterolaemia), atherogenic dyslipidaemia was diagnosed in 454 patients, and 231 patients were free of dyslipidaemia by both criteria. The variant allele (T) of ENHO rs2281997 was associated with the hyper-LDL cholesterolaemic pattern of dyslipidaemia by K/DOQI. The frequency of atherogenic dyslipidaemia was lower in T-allele bearers than in CC-genotype patients. The rs2281997 T allele was associated with lower cardiovascular mortality in HD patients showing atherogenic dyslipidaemia. ENHO, RXRA, and LXRA showed epistatic interactions in dyslipidaemia. Circulating adropin was lower in atherogenic dyslipidaemia than in non-atherogenic conditions. RXRA rs10776909 was associated with myocardial infarction. Bearers of LXRA rs2279238, rs7120118 or rs11039155 minor alleles showed higher mortality. ENHO SNP positions fell within the same DNase 1 hypersensitivity site expressed in the Th1 cell line. Epistatic interactions occurred between rs2281997 and Th1 IL SNPs (rs360719, rs568408). CONCLUSIONS: Atherogenic dyslipidaemia occurs in HD patients in whom ENHO encodes less adropin. ENHO, RXRA, and LXRA SNPs, separately or jointly, are associated with dyslipidaemia, myocardial infarction, and survival in HD patients. Differences in the availability of transcription binding sites may contribute to these associations.

Effect of short-term liver X receptor activation on epidermal barrier features in mild to moderate atopic dermatitis: A randomized controlled trial.

BACKGROUND: Liver X receptors (LXRs) are involved in maintaining epidermal barrier and suppressing inflammatory responses in model systems. The LXR agonist VTP-38543 showed promising results in improving barrier function and inflammatory responses in model systems. OBJECTIVE: To assess the safety, tolerability, cellular and molecular changes, and clinical efficacy of the topical VTP-38543 in adults with mild to moderate atopic dermatitis (AD). METHODS: A total of 104 ambulatory patients with mild to moderate AD were enrolled in this randomized, double-blind, vehicle-controlled trial between December 2015 and September 2016. VTP-38543 cream in 3 concentrations (0.05%, 0.15%, and 1.0%) or placebo was applied twice daily for 28 days. Pretreatment and posttreatment skin biopsy specimens were obtained from a subset of 33 patients. Changes in SCORing of Atopic Dermatitis, Eczema Area and Severity Index, Investigator's Global Assessment, and tissue biomarkers (by real-time polymerase chain reaction and immunostaining) were evaluated. RESULTS: Topical VTP-38543 was safe and well tolerated. VTP-38543 significantly increased messenger RNA (mRNA) expression of epidermal barrier differentiation (loricrin and filaggrin, P = .02) and lipid (adenosine triphosphate-binding cassette subfamily G member 1 and sterol regulatory element binding protein 1c, P < .01) measures and reduced epidermal hyperplasia markers (thickness, keratin 16 mRNA). VTP-38543 nonsignificantly suppressed cellular infiltrates and down-regulated mRNA expression of several TH17/TH22-related (phosphatidylinositol 3, S100 calcium-binding protein A12) and innate immunity (interleukin 6) markers. CONCLUSION: Topical VTP-38543 is safe and well tolerated. Its application led to improvement in barrier differentiation and lipids. Longer-term studies are needed to clarify whether a barrier-based approach can induce meaningful suppression of immune abnormalities. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT02655679.

Liver X receptors agonists suppress NLRP3 inflammasome activation.

Inflammasomes are multiprotein complexes that control the production of IL-1ß and IL-18. NLRP3 inflammasome, the most characterized inflammasome, plays prominent roles in defense against infection, however aberrant activation is deleterious and leads to diseases. Therefore, its tight control offers therapeutic promise. Liver X receptors (LXRs) have significant anti-inflammatory properties. Whether LXRs regulate inflammasome remains unresolved. We thus tested the hypothesis that LXR's anti-inflammatory properties may result from its ability to suppress inflammasome activation. In this study, LXRs agonists inhibited the induction of IL-1ß production, caspase-1 cleavage and ASC oligomerization by NLRP3 inflammasome. The agonists also inhibited inflammasome-associated mtROS production. Importantly, the agonists inhibited the priming of inflammasome activation. In vivo data also showed that LXRs agonist prevented NLRP3-dependent peritonitis. In conclusion, LXRs agonists are identified to potently suppress NLRP3 inflammasome and the regulation of LXRs signaling is a potential therapeutic for inflammasome-driven diseases.

Administration of GDF3 Into Septic Mice Improves Survival via Enhancing LXRα-Mediated Macrophage Phagocytosis.

The defective eradication of invading pathogens is a major cause of death in sepsis. As professional phagocytic cells, macrophages actively engulf/kill microorganisms and play essential roles in innate immune response against pathogens. Growth differentiation factor 3 (GDF3) was previously implicated as an important modulator of inflammatory response upon acute sterile injury. In this study, administration of recombinant GDF3 protein (rGDF3) either before or after CLP surgery remarkably improved mouse survival, along with significant reductions in bacterial load, plasma pro-inflammatory cytokine levels, and organ damage. Notably, our in vitro experiments revealed that rGDF3 treatment substantially promoted macrophage phagocytosis and intracellular killing of bacteria in a dose-dependent manner. Mechanistically, RNA-seq analysis results showed that CD5L, known to be regulated by liver X receptor α (LXRα), was the most significantly upregulated gene in rGDF3-treated macrophages. Furthermore, we observed that rGDF3 could promote LXRα nuclear translocation and thereby, augmented phagocytosis activity in macrophages, which was similar as LXRα agonist GW3965 did. By contrast, pre-treating macrophages with LXRα antagonist GSK2033 abolished beneficial effects of rGDF3 in macrophages. In addition, rGDF3 treatment failed to enhance bacteria uptake and killing in LXRα-knockout (KO) macrophages. Taken together, these results uncover that GDF3 may represent a novel mediator for controlling bacterial infection.

Lipid metabolism as a mechanism of immunomodulation in macrophages: the role of liver X receptors.

Macrophages are immune myeloid cells with an extreme ability to modulate their phenotype in response to insults and/or pathogens. The immunomodulatory capacity of macrophages is also patent during development as they adapt their phenotype to the host tissue environment establishing the heterogeneous populations of tissue-resident macrophages. An important mechanism of immunomodulation in macrophages occurs through the regulation of transcriptional activity. Numerous transcription factors are associated with macrophage plasticity, among them, several nuclear receptors. The nuclear receptors Liver X Receptors (LXRα and LXRß) have also revealed as active players during macrophage adaptations in diverse scenarios. This review will address the different mechanisms by which LXRs contribute to immunomodulation in macrophages by connecting lipid metabolism and immunity through transcriptional regulation.

Integrating the roles of liver X receptors in inflammation and infection: mechanisms and outcomes.

Liver X receptors (LXRs) are transcription factors from the nuclear receptor family that can be pharmacologically activated by high-affinity agonists. LXR activation exerts a combination of metabolic and anti-inflammatory actions that result in the modulation of immune responses and in the amelioration of inflammatory disorders. In addition, LXR agonists modulate the metabolism of infected cells and limit the infectivity and/or growth of several pathogens. This review gives an overview of the recent advances in understanding the complexity of the mechanisms through which the LXR pathway controls inflammation and host-cell pathogen interaction.

Frontline Science: Activation of metabolic nuclear receptors restores periodontal tissue homeostasis in mice with leukocyte adhesion deficiency-1.

ß2 Integrins mediate neutrophil-endothelial adhesion and recruitment of neutrophils to sites of inflammation. The diminished expression of ß2 integrins in patients with mutations in the ITGB2 (CD18) gene (leukocyte adhesion deficiency-Type 1; LAD1) results in few or no neutrophils in peripheral tissues. In the periodontium, neutrophil paucity is associated with up-regulation of IL-23 and IL-17, which drive inflammatory bone loss. Using a relevant mouse model, we investigated whether diminished efferocytosis (owing to neutrophil scarcity) is associated with LAD1 periodontitis pathogenesis and aimed to develop approaches to restore the missing efferocytosis signals. We first showed that CD18-/- mice phenocopied human LAD1 in terms of IL-23/IL-17-driven inflammatory bone loss. Ab-mediated blockade of c-Mer tyrosine kinase (Mer), a major efferocytic receptor, mimicked LAD1-associated up-regulation of gingival IL-23 and IL-17 mRNA expression in wild-type (WT) mice. Consistently, soluble Mer-Fc reversed the inhibitory effect of efferocytosis on IL-23 expression in LPS-activated Mϕs. Adoptive transfer of WT neutrophils to CD18-/- mice down-regulated IL-23 and IL-17 expression to normal levels, but not when CD18-/- mice were treated with blocking anti-Mer Ab. Synthetic agonist-induced activation of liver X receptors (LXR) and peroxisome proliferator-activated receptors (PPAR), which link efferocytosis to generation of homeostatic signals, inhibited the expression of IL-23 and IL-17 and favorably affected the bone levels of CD18-/- mice. Therefore, our data link diminished efferocytosis-associated signaling due to impaired neutrophil recruitment to dysregulation of the IL-23-IL-17 axis and, moreover, suggest LXR and PPAR as potential therapeutic targets for treating LAD1 periodontitis.

[Liver X receptors define the immune response during general adaptation syndrome (GAS)]. / Les récepteurs hépatiques X définissent la réaction immunitaire pendant le syndrome général d'adaptation (SGA).

The impact of stressful conditions on immunity seems mixed and at times counterbalanced. Such inconsistencies can often be attributed to the fact that the notion of stress has a very wide meaning and covers a large number of different situations. Research on liver X receptors using both natural and synthetic ligands may help to solve this conflict. When an infectious agent is present in a stressed body, LXR activation is likely to be a key element in the regulation of POMC, IFN-γ, and IL-18; moreover, it is a unique anti-inflammatory mode of action. They concurrently stimulate a non-specific immune reaction as they suppress inflammatory and autoimmune processes.

Liver X Receptor Alpha Is Important in Maintaining Blood-Brain Barrier Function.

Dysfunction of the blood-brain barrier (BBB) contributes significantly to the pathogenesis of several neuroinflammatory diseases, including multiple sclerosis (MS). Potential players that regulate BBB function are the liver X receptors (LXRs), which are ligand activated transcription factors comprising two isoforms, LXRα, and LXRß. However, the role of LXRα and LXRß in regulating BBB (dys)function during neuroinflammation remains unclear, as well as their individual involvement. Therefore, the goal of the present study is to unravel whether LXR isoforms have different roles in regulating BBB function under neuroinflammatory conditions. We demonstrate that LXRα, and not LXRß, is essential to maintain barrier integrity in vitro. Specific knockout of LXRα in brain endothelial cells resulted in a more permeable barrier with reduced expression of tight junctions. Additionally, the observed dysfunction was accompanied by increased endothelial inflammation, as detected by enhanced expression of vascular cell adhesion molecule (VCAM-1) and increased transendothelial migration of monocytes toward inflammatory stimuli. To unravel the importance of LXRα in BBB function in vivo, we made use of the experimental autoimmune encephalomyelitis (EAE) MS mouse model. Induction of EAE in a constitutive LXRα knockout mouse and in an endothelial specific LXRα knockout mouse resulted in a more severe disease score in these animals. This was accompanied by higher numbers of infiltrating leukocytes, increased endothelial VCAM-1 expression, and decreased expression of the tight junction molecule claudin-5. Together, this study reveals that LXRα is indispensable for maintaining BBB integrity and its immune quiescence. Targeting the LXRα isoform may help in the development of novel therapeutic strategies to prevent BBB dysfunction, and thereby neuroinflammatory disorders.

Saikosaponin A protects against dextran sulfate sodium-induced colitis in mice.

Ulcerative colitis, one of the most important inflammatory bowel diseases, affects millions of people worldwide. The aim of this study was to investigate the effects of Saikosaponin A on dextran sulfate sodium (DSS)-induced colitis in mice. The mice were treated with 2.5% DSS for 5 d to induce acute colitis. Saikosaponin A was given 3 d before and during DSS treatment by intragastric administration. The results showed that Saikosaponin A significantly inhibited DSS-induced body weight loss and shortening of colon length. DSS-induced colonic histological changes and MPO activity were also prevented by treatment of Saikosaponin A. The levels of TNF-α and IL-1ß were increased by DSS and dose-dependently inhibited by Saikosaponin A. Furthermore, Saikosaponin A significantly inhibited DSS-induced NF-κB activation and up-regulated the expression of LXRα. Taken together, our results indicated that Saikosaponin A had protective effects against DSS-induced colitis. Saikosaponin A protected DSS-induced colitis through inhibiting inflammatory response.

System analysis of cross-talk between nuclear receptors reveals an opposite regulation of the cell cycle by LXR and FXR in human HepaRG liver cells.

Transcriptional regulations exert a critical control of metabolic homeostasis. In particular, the nuclear receptors (NRs) are involved in regulating numerous pathways of the intermediate metabolism. The purpose of the present study was to explore in liver cells the interconnectedness between three of them, LXR, FXR, and PPARα, all three known to act on lipid and glucose metabolism, and also on inflammation. The human cell line HepaRG was selected for its best proximity to human primary hepatocytes. Global gene expression of differentiated HepaRG cells was assessed after 4 hours and 24 hours of exposure to GW3965 (LXR agonist), GW7647 (PPARα agonist), and GW4064 and CDCA (FXR synthetic and natural agonist, respectively). Our work revealed that, contrary to our expectations, NR specificity is largely present at the level of target genes, with a smaller than expected overlap of the set of genes targeted by the different NRs. It also highlighted the much broader activity of the synthetic FXR ligand compared to CDCA. More importantly, our results revealed that activation of FXR has a pro-proliferative effect and decreases the number of tetraploid (or binucleated) hepatocytes, while LXR inhibits the cell cycle progression, inducing hepatocyte differentiation and an increase in tetraploidism. Conclusion: these results highlight the importance of analyzing the different NR activities in a context allowing a direct confrontation of each receptor outcome, and reveals the opposite role of FXR and LXR in hepatocyte cells division and maturation.