Liver X receptors regulate dendritic cell phenotype and function through blocked induction of the actin-bundling protein fascin

Regulatory mechanism underlying liver X receptor effects on the tumor microenvironment, inflammation and tumorigenesis

INTRODUCTION

Liver X receptors (LXRs) have emerged as novel targets for tumor treatment. LXRs within the tumor microenvironment show the capacity to impact tumorigenesis and tumor development by regulating the infiltration of immune cells and release of cytokines to moderate inflammation.

AREAS COVERED

In this review, we present a systematic description of recent progress in understanding the impact of LXRs on the tumor microenvironment and tumorigenesis. We also summarize the antitumor effects mediated by LXRs via their regulation of cytokine expression. Additionally, we discuss the limitations of LXR research in tumor studies to date.

EXPERT OPINION

Previous studies have demonstrated abnormal LXR expression in tumor tissues, and activation of LXRs has been shown to inhibit tumorigenesis and promote apoptosis in tumor cells. However, LXRs can also affect tumorigenesis by regulating immune cell functions within the tumor immune microenvironment. By summarizing the impact of LXRs on immune cells, we provide new insights into the multifaceted nature of LXRs as antitumor targets.

LXR signaling controls homeostatic dendritic cell maturation

Dendritic cells (DCs) mature in an immunogenic or tolerogenic manner depending on the context in which an antigen is perceived, preserving the balance between immunity and tolerance. Whereas the pathways driving immunogenic maturation in response to infectious insults are well-characterized, the signals that drive tolerogenic maturation during homeostasis are still poorly understood. We found that the engulfment of apoptotic cells triggered homeostatic maturation of type 1 conventional DCs (cDC1s) within the spleen. This maturation process could be mimicked by engulfment of empty, nonadjuvanted lipid nanoparticles (LNPs), was marked by intracellular accumulation of cholesterol, and was highly specific to cDC1s. Engulfment of either apoptotic cells or cholesterol-rich LNPs led to the activation of the liver X receptor (LXR) pathway, which promotes the efflux of cellular cholesterol, and repressed genes associated with immunogenic maturation. In contrast, simultaneous engagement of TLR3 to mimic viral infection via administration of poly(I:C)-adjuvanted LNPs repressed the LXR pathway, thus delaying cellular cholesterol efflux and inducing genes that promote T cell-mediated immunity. These data demonstrate that conserved cellular cholesterol efflux pathways are differentially regulated in tolerogenic versus immunogenic cDC1s and suggest that administration of nonadjuvanted cholesterol-rich LNPs may be an approach for inducing tolerogenic DC maturation.

Effects of Oxysterols on Immune Cells and Related Diseases

Oxysterols are the products of cholesterol oxidation. They have a wide range of effects on several cells, organs, and systems in the body. Oxysterols also have an influence on the physiology of the immune system, from immune cell maturation and migration to innate and humoral immune responses. In this regard, oxysterols have been involved in several diseases that have an immune component, from autoimmune and neurodegenerative diseases to inflammatory diseases, atherosclerosis, and cancer. Here, we review data on the participation of oxysterols, mainly 25-hydroxycholesterol and 7α,25-dihydroxycholesterol, in the immune system and related diseases. The effects of these oxysterols and main oxysterol receptors, LXR and EBI2, in cells of the immune system (B cells, T cells, macrophages, dendritic cells, oligodendrocytes, and astrocytes), and in immune-related diseases, such as neurodegenerative diseases, intestinal diseases, cancer, respiratory diseases, and atherosclerosis, are discussed.

Liver X receptor beta deficiency attenuates autoimmune-associated neuroinflammation in a T cell-dependent manner

The initiation and progression of autoimmune disorders such as multiple sclerosis (MS) is linked to aberrant cholesterol metabolism and overt inflammation. Liver X receptors (LXR) are nuclear receptors that function at the crossroads of cholesterol metabolism and immunity, and their activation is considered a promising therapeutic strategy to attenuate autoimmunity. However, despite clear functional heterogeneity and cell-specific expression profiles, the impact of the individual LXR isoforms on autoimmunity remains poorly understood. Here, we show that LXRα and LXRβ have an opposite impact on immune cell function and disease severity in the experimental autoimmune encephalomyelitis model, an experimental MS model. While Lxrα deficiency aggravated disease pathology and severity, absence of Lxrβ was protective. Guided by flow cytometry and by using cell-specific knockout models, reduced disease severity in Lxrβ-deficient mice was primarily attributed to changes in peripheral T cell physiology and occurred independent from alterations in microglia function. Collectively, our findings indicate that LXR isoforms play functionally non-redundant roles in autoimmunity, potentially having broad implications for the development of LXR-based therapeutic strategies aimed at dampening autoimmunity and neuroinflammation.

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.

Immunologic Aspects of Dyslipidemia: a Critical Regulator of Adaptive Immunity and Immune Disorders

Dyslipidemia is a major cause of cardiovascular diseases which represent a leading cause of death in humans. Diverse immune cells are known to be involved in the pathogenesis of cardiovascular diseases such as atherosclerosis. Conversely, dyslipidemia is known to be tightly associated with immune disorders in humans, as evidenced by a higher incidence of atherosclerosis in patients with autoimmune diseases including psoriasis, rheumatoid arthritis, and systemic lupus erythematosus. Given that the dyslipidemia-related autoimmune diseases are caused by autoreactive T cells and B cells, dyslipidemia seems to directly or indirectly regulate the adaptive immunity. Indeed, accumulating evidence has unveiled that proatherogenic factors can impact the differentiation and function of CD4⁺ T cells, CD8⁺ T cells, and B cells. This review discusses an updated overview on the regulation of adaptive immunity by dyslipidemia and proposes a potential therapeutic strategy for immune disorders by targeting lipid metabolism.

Immune metabolism: a bridge of dendritic cells function

An increasing number of researches have shown that cell metabolism regulates cell function. Dendritic cells (DCs), a professional antigen presenting cells, connect innate and adaptive immune responses. The preference of DCs for sugar or lipid affects its phenotypes and functions. In many diseases such as atherosclerosis (AS), diabetes mellitus and tumor, altered glucose or lipid level in microenvironment makes DCs exert ineffective or opposite immune roles, which accelerates the development of these diseases. In this article, we review the metabolism pathways of glucose and cholesterol in DCs, and the effects of metabolic changes on the phenotype and function of DCs. In addition, we discuss the effects of changes in glucose and lipid levels on DCs in the context of different diseases for better understanding the relationship between DCs and diseases. The immune metabolism of DCs may be a potential intervention link to treat metabolic-related immune diseases.

Liver X Receptor Activation Impairs Neutrophil Functions and Aggravates Sepsis

BACKGROUND

Liver X receptors (LXRs) are nuclear receptors activated by oxidized lipids and were previously implicated in several metabolic development and inflammatory disorders. Although neutrophils express both LXR-α and LXR-β, the consequences of their activation, particularly during sepsis, remain unknown.

METHODS

We used the model of cecal ligation and puncture (CLP) to investigate the role of LXR activation during sepsis.

RESULTS

In this study, we verified that LXR activation reduces neutrophil chemotactic and killing abilities in vitro. Mice treated with LXR agonists showed higher sepsis-induced mortality, which could be associated with reduced neutrophil infiltration at the infectious foci, increased bacteremia, systemic inflammatory response, and multiorgan failure. In contrast, septic mice treated with LXR antagonist showed increased number of neutrophils in the peritoneal cavity, reduced bacterial load, and multiorgan dysfunction. More important, neutrophils from septic patients showed increased ABCA1 messenger ribonucleic acid levels (a marker of LXR activation) and impaired chemotactic response toward CXCL8 compared with cells from healthy individuals.

CONCLUSIONS

Therefore, our findings suggest that LXR activation impairs neutrophil functions, which might contribute to poor sepsis outcome.

Fascin in Cell Migration: More Than an Actin Bundling Protein

Simple Summary

Cell migration is an essential biological process that regulates both development and diseases, such as cancer metastasis. Therefore, understanding the factors that promote cell migration is crucial. One of the factors known to regulate cell migration is the actin-binding protein, Fascin. Fascin is typically thought to promote cell migration through bundling actin to form migratory structures such as filopodia and invadapodia. However, Fascin has many other functions in the cell that may contribute to cell migration. How these novel functions promote cell migration and are regulated is still not well understood. Here, we review the structure of Fascin, the many functions of Fascin and how they may promote cell migration, how Fascin is regulated, and Fascin’s role in diseases such as cancer metastasis.

Abstract

Fascin, an actin-binding protein, regulates many developmental migrations and contributes to cancer metastasis. Specifically, Fascin promotes cell motility, invasion, and adhesion by forming filopodia and invadopodia through its canonical actin bundling function. In addition to bundling actin, Fascin has non-canonical roles in the cell that are thought to promote cell migration. These non-canonical functions include regulating the activity of other actin-binding proteins, binding to and regulating microtubules, mediating mechanotransduction to the nucleus via interaction with the Linker of the Nucleoskeleton and Cytoskeleton (LINC) Complex, and localizing to the nucleus to regulate nuclear actin, the nucleolus, and chromatin modifications. The many functions of Fascin must be coordinately regulated to control cell migration. While much remains to be learned about such mechanisms, Fascin is regulated by post-translational modifications, prostaglandin signaling, protein–protein interactions, and transcriptional means. Here, we review the structure of Fascin, the various functions of Fascin and how they contribute to cell migration, the mechanisms regulating Fascin, and how Fascin contributes to diseases, specifically cancer metastasis.

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.

[Knocking down fascin inhibits cervical cancer cell proliferation and tumorigenesis in nude mice]

OBJECTIVE

To study the effect of knocking down fascin on cervical cancer cell proliferation and tumorigenicity in nude mice.

METHODS

Cervical cancer CaSki cells were infected with a lentiviral vector carrying fascin siRNA or with a negative control lentivirus, and fascin mRNA and protein expressions in the cells were detected using qRT-PCR and Western blotting. MTT assay was used to determine the proliferation of CaSki cells with fascin knockdown. CaSki cells transfected with fascin siRNA or the control lentiviral vector and non-transfected CaSki cells were inoculated subcutaneously in nude mice, and the volume and weight of the transplanted tumor were measured; Western blotting was used to detect the expressions of proliferating cell nuclear antigen (PCNA), survivin, cyclin dependent kinase 4 (CDK4) and p21 proteins in the tumor xenograft.

RESULTS

Infection with the lentiviral vector carrying fascin siRNA, but not the negative control vector, caused significant reductions in the expression levels of fascin mRNA and protein in CaSki cells (P < 0.05). Fascin knockdown resulted in significantly reduced proliferation of CaSki cells in vitro (P < 0.05). The nude mice inoculated with CaSki cells with fascin knockdown showed reduced tumor volume and weight, lowered levels of PCNA, survivin and CDK4, and increased expression of p21 protein in the tumor xenograft compared with the control mice. The negative control lentivirus did not affect the proliferation or tumorigenicity of CaSki cells in nude mice or the expression levels of PCNA, survivin, CDK4 or p21 proteins in the xenografts.

CONCLUSIONS

Knocking down fascin can inhibit the growth and tumorigenicity of cervical cancer cells in nude mice.

Nuclear Receptor Nur77 Deficiency Alters Dendritic Cell Function

Dendritic cells (DCs) are the professional antigen-presenting cells of the immune system. Proper function of DCs is crucial to elicit an effective immune response against pathogens and to induce antitumor immunity. Different members of the nuclear receptor (NR) family of transcription factors have been reported to affect proper function of immune cells. Nur77 is a member of the NR4A subfamily of orphan NRs that is expressed and has a function within the immune system. We now show that Nur77 is expressed in different murine DCs subsets in vitro and ex vivo, in human monocyte-derived DCs (moDCs) and in freshly isolated human BDCA1⁺ DCs, but its expression is dispensable for DC development in the spleen and lymph nodes. We show, by siRNA-mediated knockdown of Nur77 in human moDCs and by using Nur77^-/- murine DCs, that Nur77-deficient DCs have enhanced inflammatory responses leading to increased T cell proliferation. Treatment of human moDCs with 6-mercaptopurine, an activator of Nur77, leads to diminished DC activation resulting in an impaired capacity to induce IFNγ production by allogeneic T cells. Altogether, our data show a yet unexplored role for Nur77 in modifying the activation status of murine and human DCs. Ultimately, targeting Nur77 may prove to be efficacious in boosting or diminishing the activation status of DCs and may lead to the development of improved DC-based immunotherapies in, respectively, cancer treatment or treatment of autoimmune diseases.

Microenvironmental derived factors modulating dendritic cell function and vaccine efficacy: the effect of prostanoid receptor and nuclear receptor ligands

Dendritic cells (DCs) are widely used in DC-based immunotherapies because of their capacity to steer immune responses. So far treatment success is limited and more functional knowledge on how DCs initiate and stably drive specific responses is needed. Many intrinsic and extrinsic factors contribute to how DCs skew the immune response towards immunity or tolerance. The origin and type of DC, its maturation status, but also factors they encounter in the in vitro or in vivo microenvironment they reside in during differentiation and maturation affect this balance. Treatment success of DC vaccines will, therefore, also depend on the presence of these factors during the process of vaccination. Identification and further knowledge of natural and pharmacological compounds that modulate DC differentiation and function towards a specific response may help to improve current DC-based immunotherapies. This review focuses on factors that could improve the efficacy of DC vaccines in (pre-)clinical studies to enhance DC-based immunotherapy, with a particular emphasis on compounds acting on prostanoid or nuclear receptor families.

ABCA1, apoA-I, and BTN3A1: A Legitimate Ménage à Trois in Dendritic Cells

Human Vγ9Vδ2 T cells have the capacity to detect supra-physiological concentrations of phosphoantigens (pAgs) generated by the mevalonate (Mev) pathway of mammalian cells under specific circumstances. Isopentenyl pyrophosphate (IPP) is the prototypic pAg recognized by Vγ9Vδ2 T cells. B-cell derived tumor cells (i.e., lymphoma and myeloma cells) and dendritic cells (DCs) are privileged targets of Vγ9Vδ2 T cells because they generate significant amounts of IPP which can be boosted with zoledronic acid (ZA). ZA is the most potent aminobisphosphonate (NBP) clinically available to inhibit osteoclast activation and a very potent inhibitor of farnesyl pyrophosphate synthase in the Mev pathway. ZA-treated DCs generate and release in the supernatants picomolar IPP concentrations which are sufficient to induce the activation of Vγ9Vδ2 T cells. We have recently shown that the ATP-binding cassette transporter A1 (ABCA1) plays a major role in the extracellular release of IPP from ZA-treated DCs. This novel ABCA1 function is fine-tuned by physical interactions with IPP, apolipoprotein A-I (apoA-I), and butyrophilin-3A1 (BTN3A1). The mechanisms by which soluble IPP induces Vγ9Vδ2 T-cell activation remain to be elucidated. It is possible that soluble IPP binds to BTN3A1, apoA-I, or other unknown molecules on the cell surface of bystander cells like monocytes, NK cells, Vγ9Vδ2 T cells, or any other cell locally present. Investigating this scenario may represent a unique opportunity to further characterize the role of BTN3A1 and other molecules in the recognition of soluble IPP by Vγ9Vδ2 T cells.

Time-dependent LXR/RXR pathway modulation characterizes capillary remodeling in inflammatory corneal neovascularization

Inflammation in the normally immune-privileged cornea can initiate a pathologic angiogenic response causing vision-threatening corneal neovascularization. Inflammatory pathways, however, are numerous, complex and are activated in a time-dependent manner. Effective resolution of inflammation and associated angiogenesis in the cornea requires knowledge of these pathways and their time dependence, which has, to date, remained largely unexplored. Here, using a model of endogenous resolution of inflammation-induced corneal angiogenesis, we investigate the time dependence of inflammatory genes in effecting capillary regression and the return of corneal transparency. Endogenous capillary regression was characterized by a progressive thinning and remodeling of angiogenic capillaries and inflammatory cell retreat in vivo in the rat cornea. By whole-genome longitudinal microarray analysis, early suppression of VEGF ligand-receptor signaling and inflammatory pathways preceded an unexpected later-phase preferential activation of LXR/RXR, PPARα/RXRα and STAT3 canonical pathways, with a concurrent attenuation of LPS/IL-1 inhibition of RXR function and Wnt/β-catenin signaling pathways. Potent downstream inflammatory cytokines such as Cxcl5, IL-1β, IL-6 and Ccl2 were concomitantly downregulated during the remodeling phase. Upstream regulators of the inflammatory pathways included Socs3, Sparc and ApoE. A complex and coordinated time-dependent interplay between pro- and anti-inflammatory signaling pathways highlights a potential anti-inflammatory role of LXR/RXR, PPARα/RXRα and STAT3 signaling pathways in resolving inflammatory corneal angiogenesis.

Liver X Receptor Nuclear Receptors Are Transcriptional Regulators of Dendritic Cell Chemotaxis

The liver X receptors (LXRs) are ligand-activated nuclear receptors with established roles in the maintenance of lipid homeostasis in multiple tissues. LXRs exert additional biological functions as negative regulators of inflammation, particularly in macrophages. However, the transcriptional responses controlled by LXRs in other myeloid cells, such as dendritic cells (DCs), are still poorly understood. Here we used gain- and loss-of-function models to characterize the impact of LXR deficiency on DC activation programs. Our results identified an LXR-dependent pathway that is important for DC chemotaxis. LXR-deficient mature DCs are defective in stimulus-induced migration in vitro and in vivo Mechanistically, we show that LXRs facilitate DC chemotactic signaling by regulating the expression of CD38, an ectoenzyme important for leukocyte trafficking. Pharmacological or genetic inactivation of CD38 activity abolished the LXR-dependent induction of DC chemotaxis. Using the low-density lipoprotein receptor-deficient (LDLR^-/-) LDLR^-/- mouse model of atherosclerosis, we also demonstrated that hematopoietic CD38 expression is important for the accumulation of lipid-laden myeloid cells in lesions, suggesting that CD38 is a key factor in leukocyte migration during atherogenesis. Collectively, our results demonstrate that LXRs are required for the efficient emigration of DCs in response to chemotactic signals during inflammation.

Prostaglandin E2 Inhibits Prostate Cancer Progression by Countervailing Tumor Microenvironment-Induced Impairment of Dendritic Cell Migration through LXRα/CCR7 Pathway

Migration and homing of dendritic cells (DCs) to lymphoid organs are quite crucial for T cell-induced immune response against tumor. However, tumor microenvironment can make some tumor cells escape immune response by impairing DC migration. Prostaglandin E2 (PGE2) plays important roles in initiating and terminating inflammatory responses. In this study, we investigated whether PGE2 could inhibit murine prostate cancer progression by countervailing tumor microenvironment-induced impairment of dendritic cell migration. We found that murine prostate cancer cell line RM-1-conditioned medium impaired chemotactic movement of marrow-derived DCs and splenic cDCs toward CC chemokine receptor-7 (CCR7) ligand CCL19 in vitro and migration to draining lymph gland in vivo. Meanwhile, it also induced LXRα activation and CCR7 inhibition on maturing DCs. However, the treatment of PGE2 rescued this impairment of DC migration with upregulation of CCR7 and inhibition of LXRα. Further, it was observed that PGE2 also increased MMP9 expression and activated Notch1 signaling on DCs. In RM-1-bearing mouse model, PGE2 treatment was identified to inhibit tumor growth and induce more tumor-infiltrating T cells and CD11c dendritic cells in tumor sites. Therefore, our findings may demonstrate a new perspective for therapeutic interventions on prostate cancer immunoescape.

CD40-CD40L cross-talk drives fascin expression in dendritic cells for efficient antigen presentation to CD4+ T cells

Fascin is an actin-bundling protein that, among immune cells, is restricted to expression in dendritic cells (DCs). Previous reports have suggested that fascin plays an important role in governing DC antigen presentation to CD4+ T cells. However, no report has clearly linked the receptor-ligand engagement that can direct downstream regulation of fascin expression. In this study, bone marrow-derived DCs from wild-type versus CD40-knockout C57BL/6 mice were used to elucidate the mechanisms of fascin expression and activity upon CD40-CD40 ligand (CD40L) engagement. These investigations now show that CD40 engagement governs fascin expression in DCs to promote CD4+ T-cell cytokine production. Absence of CD40 signaling resulted in diminished fascin expression in DCs and was associated with impaired CD4+ T-cell responses. Furthermore, the study found that loss of CD40-CD40L engagement resulted in reduced DC-T-cell contacts. Rescue by ectopic fascin expression in CD40-deficient DCs was able to re-establish sustained contacts with T cells and restore cytokine production. Taken together, these results show that cross-talk through CD40-CD40L signaling drives elevated fascin expression in DCs to support acquisition of full T-cell responses.

Dietary and metabolic modulators of hepatic immunity

The liver is the central metabolic organ of the organism and is thus constantly exposed to gut-derived dietary and microbial antigens. The liver maintains homoeostatic tolerance to these mostly harmless antigens. However, the liver also functions as a barrier organ to harmful pathogens and is thus permissive to liver inflammation. The regulation of the delicate balance between liver tolerance and liver inflammation is of vital importance for the organism. In recent years, a general role for dietary components and metabolites as immune mediators has been emerging. However, although the liver is exposed to a great deal of metabolic mediators, surprisingly, little is known about their actual role in the regulation of hepatic immune responses. Here, we will explore the possible impacts of metabolic mediators for homoeostatic and pathological immunity in the liver, by highlighting selected examples of metabolic immune regulation in the liver.

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 β 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. LXRβ-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-β-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.

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.

The ATP-binding cassette transporter A1 regulates phosphoantigen release and Vγ9Vδ2 T cell activation by dendritic cells

Vγ9Vδ2 T cells are activated by phosphoantigens, such as isopentenyl pyrophosphate (IPP), which is generated in the mevalonate pathway of antigen-presenting cells. IPP is released in the extracellular microenvironment via unknown mechanisms. Here we show that the ATP-binding cassette transporter A1 (ABCA1) mediates extracellular IPP release from dendritic cells (DC) in cooperation with apolipoprotein A-I (apoA-I) and butyrophilin-3A1. IPP concentrations in the supernatants are sufficient to induce Vγ9Vδ2 T cell proliferation after DC mevalonate pathway inhibition with zoledronic acid (ZA). ZA treatment increases ABCA1 and apoA-I expression via IPP-dependent LXRα nuclear translocation and PI3K/Akt/mTOR pathway inhibition. These results close the mechanistic gap in our understanding of extracellular IPP release from DC and provide a framework to fine-tune Vγ9Vδ2 T cell activation via mevalonate and PI3K/Akt/mTOR pathway modulation.

γδT cells are activated by phosphoantigens, and ABCA1 is involved in cholesterol transport. Here the authors link these ideas to show that ABCA1, apoA-I and BTN3A1 regulate extracellular phosphoantigen release by dendritic cells, and implicate ABCA1 in mevalonate-mediated activation of Vγ9Vδ2 T cells.

The Tax-Inducible Actin-Bundling Protein Fascin Is Crucial for Release and Cell-to-Cell Transmission of Human T-Cell Leukemia Virus Type 1 (HTLV-1)

The delta-retrovirus Human T-cell leukemia virus type 1 (HTLV-1) preferentially infects CD4⁺ T-cells via cell-to-cell transmission. Viruses are transmitted by polarized budding and by transfer of viral biofilms at the virological synapse (VS). Formation of the VS requires the viral Tax protein and polarization of the host cytoskeleton, however, molecular mechanisms of HTLV-1 cell-to-cell transmission remain incompletely understood. Recently, we could show Tax-dependent upregulation of the actin-bundling protein Fascin (FSCN-1) in HTLV-1-infected T-cells. Here, we report that Fascin contributes to HTLV-1 transmission. Using single-cycle replication-dependent HTLV-1 reporter vectors, we found that repression of endogenous Fascin by short hairpin RNAs and by Fascin-specific nanobodies impaired gag p19 release and cell-to-cell transmission in 293T cells. In Jurkat T-cells, Tax-induced Fascin expression enhanced virus release and Fascin-dependently augmented cell-to-cell transmission to Raji/CD4⁺ B-cells. Repression of Fascin in HTLV-1-infected T-cells diminished virus release and gag p19 transfer to co-cultured T-cells. Spotting the mechanism, flow cytometry and automatic image analysis showed that Tax-induced T-cell conjugate formation occurred Fascin-independently. However, adhesion of HTLV-1-infected MT-2 cells in co-culture with Jurkat T-cells was reduced upon knockdown of Fascin, suggesting that Fascin contributes to dissemination of infected T-cells. Imaging of chronically infected MS-9 T-cells in co-culture with Jurkat T-cells revealed that Fascin’s localization at tight cell-cell contacts is accompanied by gag polarization suggesting that Fascin directly affects the distribution of gag to budding sites, and therefore, indirectly viral transmission. In detail, we found gag clusters that are interspersed with Fascin clusters, suggesting that Fascin makes room for gag in viral biofilms. Moreover, we observed short, Fascin-containing membrane extensions surrounding gag clusters and clutching uninfected T-cells. Finally, we detected Fascin and gag in long-distance cellular protrusions. Taken together, we show for the first time that HTLV-1 usurps the host cell factor Fascin to foster virus release and cell-to-cell transmission.

Human T-cell leukemia virus type 1 (HTLV-1) is the only human retrovirus causing cancer and is transmitted via breast feeding, sexual intercourse, and cell-containing blood products. Efficient infection of CD4⁺ T-cells occurs via polarized budding of virions or via cell surface transfer of viral biofilms at a tight, specialized cell-cell contact, the virological synapse (VS). The viral protein Tax and polarization of the host cell cytoskeleton are crucial for formation of the VS, however, only little is known about the link between Tax and remodeling of the cytoskeleton to foster viral spread. The actin-bundling protein Fascin has evolved as a therapeutic target in several types of cancer. Here, we show that Fascin is also crucial for release and transmission of the tumorvirus HTLV-1. Since Fascin is a transcriptional target gene of Tax in T-cells, our work provides a link between Tax’s activity and virus transmission. Visualization of cell-cell contacts between infected and uninfected T-cells suggests a role of Fascin in viral transmission potentially by facilitating the transport of viral proteins to budding sites. Thus, Fascin is not only crucial for metastasis of tumors, but also for transmission of HTLV-1 and is a new cellular target to counteract HTLV-1.

Atherosclerosis: the interplay between lipids and immune cells

PURPOSE OF REVIEW

Cardiovascular disease is the leading cause of mortality worldwide. The underlying cause of the majority of cardiovascular disease is atherosclerosis. In the past, atherosclerosis was considered to be the result of passive lipid accumulation in the vessel wall. However, today's picture of the pathogenesis of atherosclerosis is much more complex, with a key role for immune cells and inflammation in conjunction with hyperlipidemia, especially elevated (modified) LDL levels. Knowledge on immune cells and immune responses in atherosclerosis has progressed tremendously over the past decades, and the same is true for the role of lipid metabolism and the different lipid components. However, it is largely unknown how lipids and the immune system interact. In this review, we will describe the effect of lipids on immune cell development and function, and the effects of immune cells on lipid metabolism.

RECENT FINDINGS

Recently, novel data have emerged that show that immune cells are affected, and behave differently in a hyperlipidemic environment. Moreover, immune cells have reported to be able to affect lipid metabolism.

SUMMARY

In this review, we will summarize the latest findings on the interactions between lipids and the immune system, and we will discuss the potential consequences of these novel insights for future therapies for atherosclerosis.

Regulation of Adaptive Immunity in Health and Disease by Cholesterol Metabolism

Four decades ago, it was observed that stimulation of T cells induces rapid changes in cellular cholesterol that are required before proliferation can commence. Investigators returning to this phenomenon have finally revealed its molecular underpinnings. Cholesterol trafficking and its dysregulation are now also recognized to strongly influence dendritic cell function, T cell polarization, and antibody responses. In this review, the state of the literature is reviewed on how cholesterol and its trafficking regulate the cells of the adaptive immune response and in vivo disease phenotypes of dysregulated adaptive immunity, including allergy, asthma, and autoimmune disease. Emerging evidence supporting a potential role for statins and other lipid-targeted therapies in the treatment of these diseases is presented. Just as vascular biologists have embraced immunity in the pathogenesis and treatment of atherosclerosis, so should basic and clinical immunologists in allergy, pulmonology, and other disciplines seek to encompass a basic understanding of lipid science.

Liver X receptors in immune cell function in humans

The liver X receptors (LXRs), LXRα and LXRβ, are transcription factors with well-established roles in the regulation of lipid metabolism and cholesterol homeostasis. In addition, LXRs influence innate and adaptive immunity, including responses to inflammatory stimuli, proliferation and differentiation, migration, apoptosis and survival. However, the majority of work describing the role of LXRs in immune cells has been carried out in mouse models, and there are a number of known species-specific differences concerning LXR function. Here we review what is known about the role of LXRs in human immune cells, demonstrating the importance of these receptors in the integration of lipid metabolism and immune function, but also highlighting the need for a better understanding of the species, isoform, and cell-type specific effects of LXR activation.

Human monocyte-derived dendritic cells turn into foamy dendritic cells with IL-17A

Interleukin 17A (IL-17A) is a proinflammatory cytokine involved in the pathogenesis of chronic inflammatory diseases. In the field of immunometabolism, we have studied the impact of IL-17A on the lipid metabolism of human in vitro-generated monocyte-derived dendritic cells (DCs). Microarrays and lipidomic analysis revealed an intense remodeling of lipid metabolism induced by IL-17A in DCs. IL-17A increased 2-12 times the amounts of phospholipids, cholesterol, triglycerides, and cholesteryl esters in DCs. Palmitic (16:0), stearic (18:0), and oleic (18:ln-9c) acid were the main fatty acid chains present in DCs. They were strongly increased in response to IL-17A while their relative proportion remained unchanged. Capture of extracellular lipids was the major mechanism of lipid droplet accumulation, visualized by electron microscopy and Oil Red O staining. Besides this foamy phenotype, IL-17A induced a mixed macrophage-DC phenotype and expression of the nuclear receptor NR1H3/liver X receptor-α, previously identified in the context of atherosclerosis as the master regulator of cholesterol homeostasis in macrophages. These IL-17A-treated DCs were as competent as untreated DCs to stimulate allogeneic naive T-cell proliferation. Following this first characterization of lipid-rich DCs, we propose to call these IL-17A-dependent cells "foamy DCs" and discuss the possible existence of foamy DCs in atherosclerosis, a metabolic and inflammatory disorder involving IL-17A.

Oxidized low-density lipoprotein-induced apoptotic dendritic cells as a novel therapy for atherosclerosis

Modulation of immune responses may form a powerful approach to treat atherosclerosis. It was shown that clearance of apoptotic cells results in tolerance induction to cleared Ags by dendritic cells (DCs); however, this seems impaired in atherosclerosis because Ag-specific tolerance is lacking. This could result, in part, from decreased emigration of DCs from atherosclerotic lesions because of the high-cholesterol environment. Nonetheless, local induction of anti-inflammatory responses by apoptotic cell clearance seems to dampen atherosclerosis, because inhibition of apoptotic cell clearance worsens atherosclerosis. In this study, we assessed whether i.v. administration of oxLDL-induced apoptotic DCs (apop(ox)-DCs) and, as a control, unpulsed apoptotic DCs could modulate atherosclerosis by inducing tolerance. Adoptive transfer of apop(ox)-DCs into low-density lipoprotein receptor knockout mice either before or during feeding of a Western-type diet resulted in increased numbers of CD103(+) tolerogenic splenic DCs, with a concomitant increase in regulatory T cells. Interestingly, both types of apoptotic DCs induced an immediate 40% decrease in Ly-6C(hi) monocyte numbers and a 50% decrease in circulating CCL2 levels, but only apop(ox)-DC treatment resulted in long-term effects on monocytes and CCL2 levels. Although initial lesion development was reduced by 40% in both treatment groups, only apop(ox)-DC treatment prevented lesion progression by 28%. Moreover, progressed lesions of apop(ox)-DC-treated mice showed a robust 45% increase in collagen content, indicating an enhanced stability of lesions. Our findings clearly show that apoptotic DC treatment significantly decreases lesion development, but only apop(ox)-DCs can positively modulate lesion progression and stability. These findings may translate into a safe treatment for patients with established cardiovascular diseases using patient-derived apop(ox)-DCs.

The oxysterol receptor LXRβ protects against DSS- and TNBS-induced colitis in mice

We examined the function of the oxysterol receptors (LXRs) in inflammatory bowel disease (IBD) through studying dextran sodium sulfate (DSS)- and 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in mice and by elucidating molecular mechanisms underlying their anti-inflammatory action. We observed that Lxr-deficient mice are more susceptible to colitis. Clinical indicators of colitis including weight loss, diarrhea and blood in feces appeared earlier and were more severe in Lxr-deficient mice and particularly LXRβ protected against symptoms of colitis. Addition of an LXR agonist led to faster recovery and increased survival. In contrast, Lxr-deficient mice showed slower recovery and decreased survival. In Lxr-deficient mice, inflammatory cytokines and chemokines were increased together with increased infiltration of immune cells in the colon epithelium. Activation of LXRs strongly suppressed expression of inflammatory mediators including TNFα. While LXRα had anti-inflammatory effects in CD11b(+) immune cell populations, LXRβ in addition had anti-inflammatory effects in colon epithelial cells. Lack of LXRβ also induced CD4(+)/CD3(+) immune cell recruitment to the inflamed colon. Expression of both LXRA and LXRB was significantly suppressed in inflamed colon from subjects with IBD compared with non-inflamed colon. Taken together, our observations suggest that the LXRs could provide interesting targets to reduce the inflammatory responses in IBD.

LXR-dependent and -independent effects of oxysterols on immunity and tumor growth

Oxysterols are involved in maintaining cellular cholesterol levels. Recently, oxysterols have been demonstrated to modulate the function of immune cells and tumor growth. These effects can be dependent on the activation of the oxysterol-binding liver X receptors (LXRs) or, as recently demonstrated for T and B cells, DCs and neutrophils, can be independent of LXR activation. LXR-dependent oxysterol effects can be ascribed to the activation of LXRα, LXRβ or LXRαβ isoforms, which induces transcriptional activation or trans-repression of target genes. The prevalent activation of one isoform seems to be cell-, tissue-, or context-specific, as shown in some pathologic processes, i.e., infectious diseases, atherosclerosis, and autoimmunity. Oxysterol-LXR signaling has recently been shown to inhibit antitumor immune responses, as well as to modulate tumor cell growth. Here, we review the mechanisms that link oxysterols to tumor growth, and discuss possible networks at the basis of LXR-dependent and -independent oxysterol effects on immune cells and tumor development.

Liver X receptor gene polymorphisms in tuberculosis: effect on susceptibility

Objectives

The Liver X receptors (LXRs), Liver X receptor A (LXRA) and Liver X receptor B (LXRB), regulate lipid metabolism and antimicrobial response. LXRs have a crucial role in the control of Mycobacterium tuberculosis (M.tb). Lacking LXRs mice is more susceptibility to infection M.tb, developing higher bacterial burdens and an increase in the size and number of granulomatous lesions. We aimed to assess the associations between single nucleotide polymorphisms (SNPs) in LXRs and risk of tuberculosis.

Methods

We sequenced the LXRs genes to detect SNPs and to examine genotypic frequencies in 600 patients and 620 healthy controls to investigate for associations with tuberculosis (TB) in the Chinese Han population. DNA re-sequencing revealed eight common variants in the LXRs genes.

Results

The G allele of rs1449627 and the T allele of rs1405655 demonstrated an increased risk of developing TB (p<0.001, p = 0.002), and the T allele of rs3758673, the T allele of rs2279238, and the C allele of rs1449626 in LXRA and the C allele of rs17373080, the G allele of rs2248949, and the C allele of rs1052677 in LXRB were protective against TB patients compared to healthy controls (p = 0.0002, p = 0.006, p<0.001, p = 0.004, p = 0.008, p = 0.003, respectively). All SNP genotypes were significantly associated with TB. An estimation of the frequencies of haplotypes revealed two potential risk haplotypes,GGCG in LXRB (p = 0.004,) and TTCG in LXRA (p<0.001, p = 0.004). Moreover, three protective haplotypes, TTAT and CCAT in LXRA and CATC in LXRB, were significantly “protective” (p = 0.008, p<0.001, p = 0.031) for TB. Furthermore, we determined that the LXRs SNPs were nominally associated with the clinical pattern of disease.

Conclusions

Our study data supported that LXRs play a fundamental role in the genetic susceptibility to TB and to different clinical patterns of disease. Thus, further investigation is required in larger populations and in additional areas.

Liver X receptor regulates mouse GM-CSF-derived dendritic cell differentiation in vitro

Liver X receptors (LXRs) are nuclear receptors that play an essential role in lipid and cholesterol metabolism. Emerging studies indicate a potential function for LXRs in regulating dendritic cell (DC)-dependent immune responses; however, the role of LXRs in DC differentiation is largely unknown. Here, we report that LXRα regulates the differentiation of mouse GM-CSF-derived DCs. Activation or overexpression of LXRα significantly enhanced myeloid DC differentiation from mouse bone marrow (BM) cells, while siRNA-mediated knockdown of LXRα suppressed DC differentiation. In addition, we demonstrated that LXR agonist-programmed DCs showed an increased capacity for stimulating T-cell proliferation. Mechanistic studies showed that activation of LXR could inhibit the phosphorylation of STAT3 and downregulate the expression of its target, S100A9, an important negative regulator of myeloid DC differentiation. We also found that Histone deacetylase (HDAC) inhibition interfered with the effect of LXR on STAT3 signaling via acetylation of STAT3. Chromatin immunoprecipitation assays confirmed that LXR activation and HDAC inhibition balanced the recruitment of STAT3 to the S100A9 promoter, which involved distinct post-translational modifications of STAT3. In conclusion, our observations demonstrated a novel role for LXRα in GM-CSF-derived DC differentiation and revealed the underlying mechanism.

Role of PPAR, LXR, and PXR in epidermal homeostasis and inflammation

Epidermal lipid synthesis and metabolism are regulated by nuclear hormone receptors (NHR) and in turn epidermal lipid metabolites can serve as ligands to NHR. NHR form a large superfamily of receptors modulating gene transcription through DNA binding. A subgroup of these receptors is ligand-activated and heterodimerizes with the retinoid X receptor including peroxisome proliferator-activated receptor (PPAR), liver X receptor (LXR) and pregnane X receptor (PXR). Several isotypes of these receptors exist, all of which are expressed in skin. In keratinocytes, ligand activation of PPARs and LXRs stimulates differentiation, induces lipid accumulation, and accelerates epidermal barrier regeneration. In the cutaneous immune system, ligand activation of all three receptors, PPAR, LXR, and PXR, has inhibitory properties, partially mediated by downregulation of the NF-kappaB pathway. PXR also has antifibrotic effects in the skin correlating with TGF-beta inhibition. In summary, ligands of PPAR, LXR and PXR exert beneficial therapeutic effects in skin disease and represent promising targets for future therapeutic approaches in dermatology. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Sterols and oxysterols in immune cell function

Intermediates in the cholesterol-biosynthetic pathway and oxysterol derivatives of cholesterol regulate diverse cellular processes. Recent studies have expanded the appreciation of their roles in controlling the functions of cells of the innate and adaptive immune systems. Here we review recent literature reporting on the biological functions of sterol intermediates and oxysterols, acting through transcription factors such as the liver X receptors (LXRs), sterol regulatory element-binding proteins (SREBPs) and the G protein-coupled receptor EBI2, in regulating the differentiation and population expansion of cells of the innate and adaptive immune systems, their responses to inflammatory mediators, their effects on the phagocytic functions of macrophages and their effects on antiviral activities and the migration of immune cells. Such findings have raised many new questions about the production of endogenous bioactive sterols and oxysterols and their mechanisms of action in the immune system.

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.

Activation of liver X receptor suppresses the production of the IL-12 family of cytokines by blocking nuclear translocation of NF-κBp50

There is now convincing evidence that liver X receptor (LXR) is an important modulator of the inflammatory response; however, its mechanism of action remains unclear. This study aimed to examine the effect of LXR on the IL-12 family of cytokines and examined the mechanism by which LXR exerted this effect. We first demonstrated that activation of murine-derived dendritic cells (DC) with a specific agonist to LXR enhanced expression of LXR following activation with LPS, suggesting a role in inflammation. Furthermore, we showed LXR expression to be increased in vivo in dextrane sulphate sodium-induced colitis. LXR activation also suppressed production of IL-12p40, IL-12p70, IL-27 and IL-23 in murine-derived DC following stimulation with LPS, and specifically targeted the p35, p40 and EBI3 subunits of the IL-12 cytokine family, which are under the control of the NF-κB subunit p50 (NF-κBp50). Finally, we demonstrated that LXR can associate with NF-κBp50 in DC and that LXR activation prevents translocation of the p50 subunit into the nucleus. In summary, our study indicates that LXR can specifically suppress the IL-12 family of cytokines though its association with NF-κBp50 and highlights its potential as a therapeutic target for chronic inflammatory diseases.

Temporal induction of immunoregulatory processes coincides with age-dependent resistance to viral-induced type 1 diabetes

The dilute plasma cytokine milieu associated with type 1 diabetes (T1D), while difficult to measure directly, is sufficient to drive transcription in a bioassay that uses healthy leukocytes as reporters. Previously, we reported disease-associated, partially IL-1 dependent, transcriptional signatures in both T1D patients and the BioBreeding (BB) rat model. Here, we examine temporal signatures in congenic BBDR.lyp/lyp rats that develop spontaneous T1D, and BBDR rats where T1D progresses only after immunological perturbation in young animals. After weaning, the BBDR temporal signature showed early coincident induction of transcription related to innate inflammation as well as IL-10- and TGF-β-mediated regulation. BBDR plasma cytokine levels mirrored the signatures showing early inflammation, followed by induction of a regulated state that correlated with failure of virus to induce T1D in older rats. In contrast, the BBDR.lyp/lyp temporal signature exhibited asynchronous dynamics, with delayed induction of inflammatory transcription and later, weaker induction of regulatory transcription, consistent with their deficiency in regulatory T cells. Through longitudinal analyses of plasma-induced signatures in BB rats and a human T1D progressor, we have identified changes in immunoregulatory processes that attenuate a preexisting innate inflammatory state in BBDR rats, suggesting a mechanism underlying the decline in T1D susceptibility with age.

Liver X receptor and peroxisome proliferator-activated receptor as integrators of lipid homeostasis and immunity

Lipid metabolism has emerged as an important modulator of innate and adaptive immune cell fate and function. The lipid-activated transcription factors peroxisome proliferator-activated receptor (PPAR) α, β/δ, γ and liver X receptor (LXR) are members of the nuclear receptor superfamily that have a well-defined role in regulating lipid homeostasis and metabolic diseases. Accumulated evidence over the last decade indicates that PPAR and LXR signaling also influence multiple facets of inflammation and immunity, thereby providing important crosstalk between metabolism and immune system. Herein, we provide a brief introduction to LXR and PPAR biology and review recent discoveries highlighting the importance of PPAR and LXR signaling in the modulation of normal and pathologic states of immunity. We also examine advances in our mechanistic understanding of how nuclear receptors impact immune system function and homeostasis. Finally, we discuss whether LXRs and PPARs could be pharmacologically manipulated to provide novel therapeutic approaches for modulation of the immune system under pathologic inflammation or in the context of allergic and autoimmune disease.

Control of the immune system by oxysterols and cancer development

Oxysterols/oxysterol receptors have been shown to modulate several immune cell subsets, such as macrophages, T-cells and B-cells, neutrophils and dendritic cells (DCs). They participate in the control of several pathologic processes, that is, infectious diseases, atherosclerosis and autoimmunity. Moreover, some oxysterols have also been shown to favor tumor progression by dampening the antitumor immune response. The cellular responses generated by oxysterols depend on the engagement of Liver X Receptor (LXR) α and/or β isoforms, which induce activation of target genes or trans-repression of pro-inflammatory gene transcription. Recently, some reports have described a different mechanism of action of oxysterols, mediated by the engagement of G-Protein Coupled Receptors. Here, we summarize LXR-dependent and LXR-independent responses of oxysterols on immune cells with possible effects on tumor development.

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.

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.

ATP binding cassette transporter G1 deletion induces IL-17-dependent dysregulation of pulmonary adaptive immunity

Mice with genetic deletion of the cholesterol transporter ATP binding cassette G1 (ABCG1) have pulmonary lipidosis and enhanced innate immune responses in the airway. Whether ABCG1 regulates adaptive immune responses to the environment is unknown. To this end, Abcg1(+/+) and Abcg1(-/-) mice were sensitized to OVA via the airway using low-dose LPS as an adjuvant, and then challenged with OVA aerosol. Naive Abcg1(-/-) mice displayed increased B cells, CD4(+) T cells, CD8(+) T cells, and dendritic cells (DCs) in lung and lung-draining mediastinal lymph nodes, with lung CD11b(+) DCs displaying increased CD80 and CD86. Upon allergen sensitization and challenge, the Abcg1(-/-) airway, compared with Abcg1(+/+), displayed reduced Th2 responses (IL-4, IL-5, eosinophils), increased neutrophils and IL-17, but equivalent airway hyperresponsiveness. Reduced Th2 responses were also found using standard i.p. OVA sensitization with aluminum hydroxide adjuvant. Mediastinal lymph nodes from airway-sensitized Abcg1(-/-) mice produced reduced IL-5 upon ex vivo OVA challenge. Abcg1(-/-) CD4(+) T cells displayed normal ex vivo differentiation, whereas Abcg1(-/-) DCs were found paradoxically to promote Th2 polarization. Th17 cells, IL-17(+) γδT cells, and IL-17(+) neutrophils were all increased in Abcg1(-/-) lungs, suggesting Th17 and non-Th17 sources of IL-17 excess. Neutralization of IL-17 prior to challenge normalized eosinophils and reduced neutrophilia in the Abcg1(-/-) airway. We conclude that Abcg1(-/-) mice display IL-17-mediated suppression of eosinophilia and enhancement of neutrophilia in the airway following allergen sensitization and challenge. These findings identify ABCG1 as a novel integrator of cholesterol homeostasis and adaptive immune programs.

Virus-induced differential expression of nuclear receptors and coregulators in dendritic cells: implication to interferon production

We investigated mRNA expression of 49 nuclear hormone receptors (NRs) and 35 transcriptional coregulators in mouse bone marrow-derived dendritic cells (DCs) upon infection with Newcastle Disease virus (NDV) or murine cytomegalovirus (MCMV). These viruses regulated mRNA expression of some NRs among which NOR1 and LXRα were highly induced at mRNA and protein levels. Exogenous expression of the latter NRs repressed IRF3- or IRF7-induced transactivation of the interferon β promoter and NDV infection further potentiated their repressive effect. The viral infection also significantly regulated mRNA expression of some coregulators, including HDAC1. Toll-like receptor ligands regulated NR and coregulator mRNA expression similar to the viruses. Thus, NRs and coregulators are integral components of DC-organizing anti-viral response wherein NOR1 and LXRα participate in regulating interferon production.

Fascin1 promotes cell migration of mature dendritic cells

Dendritic cells (DCs) play central roles in innate and adaptive immunity. Upon maturation, DCs assemble numerous veil-like membrane protrusions, disassemble podosomes, and travel from the peripheral tissues to lymph nodes to present Ags to T cells. These alterations in morphology and motility are closely linked to the primary function of DCs, Ag presentation. However, it is unclear how and what cytoskeletal proteins control maturation-associated alterations, in particular, the change in cell migration. Fascin1, an actin-bundling protein, is specifically and greatly induced upon maturation, suggesting a unique role for fascin1 in mature DCs. To determine the physiological roles of fascin1, we characterized bone marrow-derived, mature DCs from fascin1 knockout mice. We found that fascin1 is critical for cell migration: fascin1-null DCs exhibit severely decreased membrane protrusive activity. Importantly, fascin1-null DCs have lower chemotactic activity toward CCL19 (a chemokine for mature DCs) in vitro, and in vivo, Langerhans cells show reduced emigration into draining lymph nodes. Morphologically, fascin1-null mature DCs are flatter and fail to disassemble podosomes, a specialized structure for cell-matrix adhesion. Expression of exogenous fascin1 in fascin1-null DCs rescues the defects in membrane protrusive activity, as well as in podosome disassembly. These results indicate that fascin1 positively regulates migration of mature DCs into lymph nodes, most likely by increasing dynamics of membrane protrusions, as well as by disassembling podosomes.