FICZ, a tryptophan photoproduct, suppresses pulmonary eosinophilia and Th2-type cytokine production in a mouse model of ovalbumin-induced allergic asthma

Activation of the aryl hydrocarbon receptor improves allergen-specific immunotherapy of murine allergic airway inflammation: a novel adjuvant option?

Background

Allergen-specific immunotherapy (AIT) is able to restore immune tolerance to allergens in allergic patients. However, some patients do not or only poorly respond to current treatment protocols. Therefore, there is a need for deeper mechanistic insights and further improvement of treatment strategies. The relevance of the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor, has been investigated in several inflammatory diseases, including allergic asthma. However, its potential role in AIT still needs to be addressed.

Methods

A murine model of AIT in ovalbumin-induced allergic airway inflammation was performed in AhR-deficient (AhR-/-) and wild-type mice. Furthermore, AIT was combined with the application of the high-affinity AhR agonist 10-chloro-7H-benzimidazo[2,1-a]benzo[de]iso-quinolin-7-one (10-Cl-BBQ) as an adjuvant to investigate the effects of AhR activation on therapeutic outcome.

Results

Although AhR-/- mice suffer stronger allergic responses than wild-type mice, experimental AIT is comparably effective in both. Nevertheless, combining AIT with the administration of 10-Cl-BBQ improved therapeutic effects by an AhR-dependent mechanism, resulting in decreased cell counts in the bronchoalveolar fluid, decreased pulmonary Th2 and Th17 cell levels, and lower sIgE levels.

Conclusion

This study demonstrates that the success of AIT is not dependent on the AhR. However, targeting the AhR during AIT can help to dampen inflammation and improve tolerogenic vaccination. Therefore, AhR ligands might represent promising candidates as immunomodulators to enhance the efficacy of AIT.

Homeostatic activation of aryl hydrocarbon receptor by dietary ligands dampens cutaneous allergic responses by controlling Langerhans cells migration

Dietary compounds can affect the development of inflammatory responses at distant sites. However, the mechanisms involved remain incompletely understood. Here, we addressed the influence on allergic responses of dietary agonists of aryl hydrocarbon receptor (AhR). In cutaneous papain-induced allergy, we found that lack of dietary AhR ligands exacerbates allergic responses. This phenomenon was tissue-specific as airway allergy was unaffected by the diet. In addition, lack of dietary AhR ligands worsened asthma-like allergy in a model of 'atopic march.' Mice deprived of dietary AhR ligands displayed impaired Langerhans cell migration, leading to exaggerated T cell responses. Mechanistically, dietary AhR ligands regulated the inflammatory profile of epidermal cells, without affecting barrier function. In particular, we evidenced TGF-β hyperproduction in the skin of mice deprived of dietary AhR ligands, explaining Langerhans cell retention. Our work identifies an essential role for homeostatic activation of AhR by dietary ligands in the dampening of cutaneous allergic responses and uncovers the importance of the gut-skin axis in the development of allergic diseases.

Cyprinid herpesvirus 2 infection changes microbiota and metabolites in the gibel carp (Carassius auratus gibelio) midgut

Cyprinid herpesvirus 2 (CyHV-2) infects gibel carp (Carassius auratus gibelio) and causes severe losses. Microbiota in animal guts involves nutrition intake, development, immunity, and disease resistance. However, the relationship between gibel carp gut microbiota and CyHV-2 infection is not well known. Herein, we analyzed the gut microbiota composition and metabolite profiles in CyHV-2-infected and -uninfected fish using high-throughput sequencing and gas chromatography/mass spectrometry. Results showed that CyHV-2 infection significantly changed gut microbiota and metabolite profiles (p < 0.05). High-throughput sequencing demonstrated that the relative abundance of Aeromonas in the midgut increased dramatically while Cetobacterium decreased. Time-course analysis showed that the number of Aeromonas in the midgut of infected fish increased more than 1,000 times within 5 days post infection. Metabolome analysis illustrated that CyHV-2 infection significantly altered 24 metabolites in the midgut of gibel carp, annotating to the anomaly of digestion and metabolisms of amino acids, carbohydrates, and lipids, such as tryptophan (Trp) metabolism. The Mantel test demonstrated that gut microbiota and metabolite profiles were well related (r = 0.89). Furthermore, Trp metabolism responded to CyHV-2 infection closely was taken as one example to prove the correlation among CyHV-2 infection, metabolites and microbiota in the midgut, and host immunity. Results showed that modulating Trp metabolism could affect the relative abundance of Aeromonas in the midgut of fish, transcription of antiviral cytokines, and CyHV-2 infection. Therefore, we can conclude that CyHV-2 infection significantly perturbed the gut microbiome, disrupted its' metabolic functions, and caused the proliferation of the opportunistic pathogen Aeromonas. This study also suggests that modulation of the gut microbiome will open a therapeutic opportunity to control CyHV-2 infection in gibel carp.

Differential Regulation of the Asthmatic Phenotype by the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that regulates the metabolism of xenobiotics. There is growing evidence that the AhR is implicated in physiological processes such proliferation, differentiation, and immune responses. Recently, a role of the AhR in regulating allergic asthma has been suggested, but whether the AhR also regulates other type of asthma, particularly occupational/irritant-induced asthma, remains unknown. Using AhR-deficient (Ahr^−/−) mice, we compared the function of the AhR in the response to ovalbumin (OVA; allergic asthma) vs. chlorine (Cl₂; irritant-induced asthma) exposure. Lung inflammation and airway hyperresponsiveness were assessed 24h after exposure to Cl₂ or OVA challenge in Ahr^−/− and heterozygous (Ahr^+/−) mice. After OVA challenge, absence of AhR was associated with significantly enhanced eosinophilia and lymphocyte influx into the airways of Ahr^−/− mice. There were also increased levels of interleukin-4 (IL-4) and IL-5 in the airways. However, OVA-induced airway hyperresponsiveness was not affected. In the irritant-induced asthma model caused by exposure to Cl₂, the AhR did not regulate the inflammatory response. However, absence of AhR reduced Cl₂-induced airway hyperresponsiveness. Collectively, these results support a differential role for the AhR in regulating asthma outcomes in response to diverse etiological agents.

The therapeutic potential of diet on immune-related diseases: based on the regulation on tryptophan metabolism

Tryptophan (TRP), as an essential amino acid, plays crucial roles in maintaining immune homeostasis due to its complex metabolism pathway, including the microbial metabolism, 5-hydroxytryptamine and kynurenine pathways (KP). Metabolites from these pathways can act antioxidant and endogenous ligand of aryl hydrocarbon receptor (including microbiota metabolites: indole, indole aldehyde, indole acetic acid, indole acrylic acid, indole lactate, indole pyruvate acid, indole propionic acid, skatole, tryptamine, and indoxyl sulfate; and KP metabolites: kynurenine, kynurenic acid, 3-hydroxyanthranilic acid, xanthurenic acid, and cinnabarinic acid) for regulating immune response. In immune-related diseases, the production of pro-inflammatory cytokine activates indoleamine-2,3-dioxygenase, a rate-limiting enzyme of KP, leading to abnormal TRP metabolism in vivo. Many recent studies found that TRP metabolism could be regulated by diet, and the diet regulation on TRP metabolism could therapy related diseases. Accordingly, this review provides a critical overview of the relationships among diet, TRP metabolism and immunity with the aim to seek a treatment opportunity for immune-related diseases.

The Aryl Hydrocarbon Receptor Attenuates Acute Cigarette Smoke-Induced Airway Neutrophilia Independent of the Dioxin Response Element

Cigarette smoke is a prevalent respiratory toxicant that remains a leading cause of death worldwide. Cigarette smoke induces inflammation in the lungs and airways that contributes to the development of diseases such as lung cancer and chronic obstructive pulmonary disease (COPD). Due to the presence of aryl hydrocarbon receptor (AhR) ligands in cigarette smoke, activation of the AhR has been implicated in driving this inflammatory response. However, we have previously shown that the AhR suppresses cigarette smoke-induced pulmonary inflammation, but the mechanism by which the AhR achieves its anti-inflammatory function is unknown. In this study, we use the AhR antagonist CH-223191 to inhibit AhR activity in mice. After an acute (3-day) cigarette smoke exposure, AhR inhibition was associated with significantly enhanced neutrophilia in the airways in response to cigarette smoke, mimicking the phenotype of AhR-deficient mice. We then used genetically-modified mouse strains which express an AhR that can bind ligand but either cannot translocate to the nucleus or bind its cognate response element, to show that these features of the AhR pathway are not required for the AhR to suppress pulmonary neutrophilia. Finally, using the non-toxic endogenous AhR ligand FICZ, we provide proof-of-concept that activation of pulmonary AhR attenuates smoke-induced inflammation. Collectively, these results support the importance of AhR activity in mediating its anti-inflammatory function in response to cigarette smoke. Further investigation of the precise mechanisms by which the AhR exerts is protective functions may lead to the development of therapeutic agents to treat people with chronic lung diseases that have an inflammatory etiology, but for which few therapeutic options exist.

The Aryl Hydrocarbon Receptor in Asthma: Friend or Foe?

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that has emerged as an important player in asthma control. AhR is responsive to environmental molecules and endogenous or dietary metabolites and regulates innate and adaptive immune responses. Binding of this receptor by different ligands has led to seemingly opposite responses in different asthma models. In this review, we present two sides of the same coin, with the beneficial and deleterious roles of AhR evaluated using known endogenous or exogenous ligands, deficient mice or antagonists. On one hand, AhR has an anti-inflammatory role since its activation in dendritic cells blocks the generation of pro-inflammatory T cells or shifts macrophages toward an anti-inflammatory M2 phenotype. On the other hand, AhR activation by particle-associated polycyclic aromatic hydrocarbons from the environment is pro-inflammatory, inducing mucus hypersecretion, airway remodelling, dysregulation of antigen presenting cells and exacerbates asthma features. Data concerning the role of AhR in cells from asthmatic patients are also reviewed, since AhR could represent a potential target for therapeutic immunomodulation.

Age-dependent vulnerability of the ovary to AhR-mediated TCDD action before puberty: Evidence from mouse models

In female mammals, puberty and fertility are regulated by the synthesis of estradiol (E2) by the ovaries at the infantile stage and at the approach of puberty, a process which may be affected by endocrine disrupting chemicals (EDC)s acting through the Aryl hydrocarbon receptor (AhR). However, there is no information on AhR-mediated regulation of ovarian estrogenic activity during these developmental periods. Here, we assessed in mouse models, the intrinsic and exogenous ligand-induced AhR action on E2 synthesis at the infantile stage (14 days postnatal (dpn)) and at the approach of puberty (28 dpn). Intrinsic AhR pathway became activated in the ovary at the approach of puberty, as suggested by the decreased intra-ovarian expression in prototypical and steroidogenesis-related AhR targets and E2 contents in Ahr knockout (Ahr^-/-) mice versus Ahr^+/+ mice exclusively at 28 dpn. Accordingly, AhR nuclear localization in granulosa cells, reflecting its activity in cells responsible for E2 synthesis, was much lower at 14 dpn than at 28 dpn in C57BL/6 mice. However, AhR signaling could be activated by exogenous ligands at both ages, as revealed by FICZ- and TCDD-induced Ahrr and Cyp1a1 expression in C57BL/6 mice. Nevertheless, TCDD impacted ovarian estrogenic activity only at 28 dpn. This age-related AhR action may be ligand-dependent, since FICZ had no effect on E2 synthesis at 28 dpn. In conclusion, AhR would not regulate ovarian estrogenic activity before the approach of puberty. Its activation by EDCs may be more detrimental to reproductive health at this stage than during infancy.

Aryl Hydrocarbon Receptor: Its Regulation and Roles in Transformation and Tumorigenesis

AhR is an environmental response gene that mediates cellular responses to a variety of xenobiotic compounds that frequently function as AhR ligands. Many AhR ligands are classified as carcinogens or pro-carcinogens. Thus, AhR itself acts as a major mediator of the carcinogenic effect of many xenobiotics in vivo. In this concise review, mechanisms by which AhR trans-activates downstream target gene expression, modulates immune responses, and mediates malignant transformation and tumor development are discussed. Moreover, activation of AhR by post-translational modifications and crosstalk with other transcription factors or signaling pathways are also summarized.

Synthesis and biological evaluation of FICZ analogues as agonists of aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand activated transcription factor involved in multiple biological processes including immune cell differentiation, intestinal function and inflammation. Based on the scaffold of naturally occurring AhR ligand 6-formylindolo (3,2-b) carbazole (FICZ, 2), a series of analogues has been designed, synthesized and evaluated by cell-based assays. The structure-activity relationships study has successfully led to the discovery of compound 11e with extremely potent activity.

The role of interleukin 22 in multiple sclerosis and its association with c-Maf and AHR

The aim of this paper was to summarise knowledge of IL-22 involvement in multiple sclerosis (MS) and the possible link between IL-22 and two transcription factors - AHR and c-Maf. The conclusion is that despite numerous studies, the exact role of IL-22 in the pathogenesis of MS is still unknown. The expression and function of c-Maf in MS have not been studied. It seems that the functions of c-Maf and AHR are at least partly connected with IL-22, as both directly or indirectly influence the regulation of IL-22 expression. This possible connection has never been studied in MS.

The tryptophan derivative 6-formylindolo[3,2-b]carbazole, FICZ, a dynamic mediator of endogenous aryl hydrocarbon receptor signaling, balances cell growth and differentiation

The aryl hydrocarbon receptor (AHR) is not essential to survival, but does act as a key regulator of many normal physiological events. The role of this receptor in toxicological processes has been studied extensively, primarily employing the high-affinity ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, regulation of physiological responses by endogenous AHR ligands remains to be elucidated. Here, we review developments in this field, with a focus on 6-formylindolo[3,2-b]carbazole (FICZ), the endogenous ligand with the highest affinity to the receptor reported to date. The binding of FICZ to different isoforms of the AHR seems to be evolutionarily well conserved and there is a feedback loop that controls AHR activity through metabolic degradation of FICZ via the highly inducible cytochrome P450 1A1. Several investigations provide strong evidence that FICZ plays a critical role in normal physiological processes and can ameliorate immune diseases with remarkable efficiency. Low levels of FICZ are pro-inflammatory, providing resistance to pathogenic bacteria, stimulating the anti-tumor functions, and promoting the differentiation of cancer cells by repressing genes in cancer stem cells. In contrast, at high concentrations FICZ behaves in a manner similar to TCDD, exhibiting toxicity toward fish and bird embryos, immune suppression, and activation of cancer progression. The findings are indicative of a dual role for endogenously activated AHR in barrier tissues, aiding clearance of infections and suppressing immunity to terminate a vicious cycle that might otherwise lead to disease. There is not much support for the AHR ligand-specific immune responses proposed, the differences between FICZ and TCDD in this context appear to be explained by the rapid metabolism of FICZ.

Chemistry and Properties of Indolocarbazoles

The indolocarbazoles are an important class of nitrogen heterocycles which has evolved significantly in recent years, with numerous studies focusing on their diverse biological effects, or targeting new materials with potential applications in organic electronics. This review aims at providing a broad survey of the chemistry and properties of indolocarbazoles from an interdisciplinary point of view, with particular emphasis on practical synthetic aspects, as well as certain topics which have not been previously accounted for in detail, such as the occurrence, formation, biological activities, and metabolism of indolo[3,2- b]carbazoles. The literature of the past decade forms the basis of the text, which is further supplemented with older key references.

Protective role of 6-formylindolo[3,2-b]carbazole (FICZ), an endogenous ligand for arylhydrocarbon receptor, in chronic mite-induced dermatitis

BACKGROUND

Chronic eczema such as atopic dermatitis imposes significant socio-econo-psychologic burdens on the affected individuals. In addition to conventional topical treatments, phototherapy is recommended for patients with extensive lesions. Although immunosuppression is believed to explain its primary effectiveness, the underlying mechanisms of phototherapy remain unsolved. Ultraviolet irradiation generates various tryptophan photoproducts including 6-formylindolo[3,2-b]-carbazole (FICZ). FICZ is known to be a potent endogenous agonist for aryl hydrocarbon receptor (AHR); however, the biological role of FICZ in chronic eczema is unknown.

OBJECTIVE

To investigate the effect of FICZ on chronic eczema such as atopic dermatitis.

METHODS

We stimulated HaCaT cells and normal human epidermal keratinocytes (NHEKs) with or without FICZ and then performed quantitative reverse transcriptase polymerase chain reaction, immunofluorescence, and siRNA treatment. We used the atopic dermatitis-like NC/Nga murine model and treated the mice for 2 weeks with either Vaseline® as a control, FICZ ointment, or betamethasone 17-valerate ointment. The dermatitis score, transepidermal water loss, histology, and expression of skin barrier genes and proteins were evaluated.

RESULTS

FICZ significantly upregulated the gene expression of filaggrin in both HaCaT cells and NHEKs in an AHR-dependent manner, but did not affect the gene expression of other barrier-related proteins. In addition, FICZ improved the atopic dermatitis-like skin inflammation, clinical scores, and transepidermal water loss in NC/Nga mice compared with those of control mice. On histology, FICZ significantly reduced the epidermal and dermal thickness as well as the number of mast cells. Topical FICZ also significantly reduced the gene expression of Il22.

CONCLUSION

These findings highlight the beneficial role of FICZ-AHR and provide a new strategic basis for developing new drugs for chronic eczema.

Glyteer, Soybean Tar, Impairs IL-4/Stat6 Signaling in Murine Bone Marrow-Derived Dendritic Cells: The Basis of Its Therapeutic Effect on Atopic Dermatitis

Atopic dermatitis (AD) is a common inflammatory skin disease. Recent studies have revealed the involvement of T helper (Th)2 cytokines including Interleukin 4 (IL-4) in the pathogenesis of AD. Since epidermal Langerhans cells (LCs) and dermal myeloid dendritic cells (DCs) produce CCL17 and CCL22 that chemoattract Th2 cells, interfering with CCL17 and CCL22 production from LCs and dermal myeloid DCs may be beneficial in the treatment of AD. To investigate this, we stimulated murine bone marrow-derived DCs (BMDCs) with IL-4. IL-4 stimulation produced Ccl17 and Ccl22, which was attenuated by soybean tar Glyteer, a known aryl hydrocarbon receptor (Ahr) activator. Notably, Glyteer treatment blocked the nuclear translocation of Stat6 induced by IL-4 stimulation, suggesting that this treatment impairs the IL-4/Stat6 signaling pathway in BMDCs. Unexpectedly, Glyteer treatment did not potently upregulate the expression of Cyp1a1, a specific Ahr-responsive gene, suggesting that its inhibitory machinery for Ccl17 and Ccl22 expression is likely to operate in an Ahr-independent manner. These findings indicate that Glyteer may exhibit therapeutic potential for AD by downregulating the CCL17 and CCL22 production from DCs in a Th2-deviated microenvironment.

Hypoxia and Mucosal Inflammation

Sites of inflammation are defined by significant changes in metabolic activity. Recent studies have suggested that O2 metabolism and hypoxia play a prominent role in inflammation so-called "inflammatory hypoxia," which results from a combination of recruited inflammatory cells (e.g., neutrophils and monocytes), the local proliferation of multiple cell types, and the activation of multiple O2-consuming enzymes during inflammation. These shifts in energy supply and demand result in localized regions of hypoxia and have revealed the important function off the transcription factor HIF (hypoxia-inducible factor) in the regulation of key target genes that promote inflammatory resolution. Analysis of these pathways has provided multiple opportunities for understanding basic mechanisms of inflammation and has defined new targets for intervention. Here, we review recent work addressing tissue hypoxia and metabolic control of inflammation and immunity.

Indole-3-carbinol, a plant nutrient and AhR-Ligand precursor, supports oral tolerance against OVA and improves peanut allergy symptoms in mice

In general, dietary antigens are tolerated by the gut associated immune system. Impairment of this so-called oral tolerance is a serious health risk. We have previously shown that activation of the ligand-dependent transcription factor aryl hydrocarbon receptor (AhR) by the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects both oral tolerance and food allergy. In this study, we determine whether a common plant-derived, dietary AhR-ligand modulates oral tolerance as well. We therefore fed mice with indole-3-carbinole (I3C), an AhR ligand that is abundant in cruciferous plants. We show that several I3C metabolites were detectable in the serum after feeding, including the high-affinity ligand 3,3´-diindolylmethane (DIM). I3C feeding robustly induced the AhR-target gene CYP4501A1 in the intestine; I3C feeding also induced the aldh1 gene, whose product catalyzes the formation of retinoic acid (RA), an inducer of regulatory T cells. We then measured parameters indicating oral tolerance and severity of peanut-induced food allergy. In contrast to the tolerance-breaking effect of TCDD, feeding mice with chow containing 2 g/kg I3C lowered the serum anti-ovalbumin IgG1 response in an experimental oral tolerance protocol. Moreover, I3C feeding attenuated symptoms of peanut allergy. In conclusion, the dietary compound I3C can positively influence a vital immune function of the gut.

Is chronic AhR activation by rapidly metabolized ligands safe for the treatment of immune-mediated diseases?

There is a long standing perception that AhR ligands are automatically disqualified from pharmaceutical development due to their induction of Cyp1a1 as well as their potential for causing "dioxin-like" toxicities. However, recent discoveries of new AhR ligands with potential therapeutic applications have been reported, inviting reconsideration of this policy. One area of exploration is focused on the activation of AhR to promote the generation of regulatory T cells, which control the intensity and duration of immune responses. Rapidly metabolized AhR ligands (RMAhRLs), which do not bioaccumulate in the same manner as 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) have been discovered that induce Tregs and display impressive therapeutic efficacy in a broad range of preclinical models of immune-mediated diseases. Given the promise of these RMAhRLs, is the bias against AhR activators still valid? Can RMAhRLs be given chronically to maintain therapeutic levels of AhR activation without producing the same toxicity profile as dioxin-like compounds? Based on our review of the data, there is little evidence to support the indiscriminate exclusion of AhR activators/Cyp1a1 inducers from early drug developmental pipelines. We also found no evidence that short-term treatment with RMAhRLs produce "dioxin-like toxicity" and, in fact, were well tolerated. However, safety testing of individual RMAhRLs under therapeutic conditions, as performed with all promising new drugs, will be needed to reveal whether or not chronic activation of AhR leads to unacceptable adverse outcomes.

AhR activation by 6-formylindolo[3,2-b]carbazole and 2,3,7,8-tetrachlorodibenzo-p-dioxin inhibit the development of mouse intestinal epithelial cells

The intestinal epithelium plays a central role in immune homeostasis in the intestine. AhR, a ligand-activated transcription factor, plays an important role in diverse physiological processes. The intestines are exposed to various exogenous and endogenous AhR ligands. Thus, AhR may regulate the intestinal homeostasis, directly acting on the development of intestinal epithelial cells (IEC). In this study, we demonstrated that 6-formylindolo[3,2-b]carbazole (FICZ) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibited the in vitro development of mouse intestinal organoids. The number of Paneth cells in the small intestine and the depth of crypts of the small and large intestines were reduced in mice administrated with FICZ. Immunohistochemical and flow cytometric assays revealed that AhR was highly expressed in Lgr5(+) stem cells. FICZ inhibited Wnt signaling lowering the level of β-catenin protein. Gene expression analyses demonstrated that FICZ increased expression of Lgr5, Math1, BMP4, and Indian Hedgehog while inhibiting that of Lgr4.

Bronchial epithelial innate and adaptive immunity signals are induced by polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons including Benzo[a]pyrene have been recognised as important pollutant chemicals with the potential to influence the respiratory system in disease. Airway epithelial cells are an integral component of how immune responses are directed as a consequence of exposure to inhaled material. It was aim of this study to examine how such cells respond to PAH exposure and to characterise the immune response. Human primary bronchial epithelial cells (HPBECs) were exposed to Benzo[a]pyrene, Benzo[e]pyrene, Fluoranthene and Benzo[b]fluoranthene for 24 h and a repeat exposure up to 7 days, and examined for global gene expression using RNA-Seq. In addition to increased expression of CYP1A1 and other AHR dependent changes, we identified significant increases in innate and adaptive immune signals including, IL-1A, IL-19, SERPINB2, STAT6, HLA-DMB and HLA-DRA. We also observed increased expression of HMOX1 and NQO1, genes involved in the response to oxidative stress. Immune system related gene expression was differentially induced by each compound with Benzo[a]pyrene and Benzo[b]fluoranthene demonstrating the most potent responses. Differential induction paralleled the level to which AHR dependent gene expression and oxidative stress markers were induced. We also observed similar levels of gene expression when cells were exposed to organic extracts from diesel exhaust particles. In conclusion, hazard characterisation of responses to PAH exposure in HPBECs highlights specific responses of both innate and adaptive immunity.

Evidence for New Light-Independent Pathways for Generation of the Endogenous Aryl Hydrocarbon Receptor Agonist FICZ

Activation of the aryl hydrocarbon receptor (AhR), a conserved transcription factor best known as a target for highly toxic halogenated substances such as dioxin, under normal xenobiotic-free conditions is of considerable scientific interest. We have demonstrated previously that a photoproduct of tryptophan, 6-formylindolo[3,2-b]carbazole (FICZ), fulfills the criteria for an endogenous ligand for this receptor and proposed that this compound is the enigmatic mediator of the physiological functions of AhR. Here, we describe novel light-independent pathways by which FICZ can be formed. The oxidant H2O2 was shown to convert tryptophan to FICZ on its own in the absence of light. The enzymatic deamination of tryptamine yielded indole-3-acetaldehyde (I3A), which then rearranged to FICZ and its oxidation product, indolo[3,2-b]carbazole-6-carboxylic acid (CICZ). Indole-3-pyruvate (I3P) also produced I3A, FICZ, and CICZ. Malassezia yeast species, which constitute a part of the normal skin microbiota, produce a number of AhR activators from tryptophan. We identified both FICZ and CICZ among those products. Formation of FICZ from tryptophan or I3P produces a complex mixture of indole derivatives, some of which are CYP1A1 inhibitors. These can hinder the cellular clearance of FICZ and thereby increase its power as an AhR agonist. We present a general molecular mechanism involving dehydrogenations and oxidative coupling for the formation of FICZ in which I3A is the important precursor. In conclusion, our results suggest that FICZ is likely to be formed systemically.

3,3'-Diindolylmethane Inhibits Flt3L/GM-CSF-induced-bone Marrow-derived CD103(+) Dendritic Cell Differentiation Regulating Phosphorylation of STAT3 and STAT5

The intestinal immune system maintains oral tolerance to harmless antigens or nutrients. One mechanism of oral tolerance is mediated by regulatory T cell (Treg)s, of which differentiation is regulated by a subset of dendritic cell (DC)s, primarily CD103⁺ DCs. The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, plays an important role in regulating immunity. The intestines are exposed to various AhR ligands, including endogenous metabolites and phytochemicals. It was previously reported that AhR activation induced tolerogenic DCs in mice or in cultures of bone marrow-derived DCs. However, given the variety of tolerogenic DCs, which type of tolerogenic DCs is regulated by AhR remains unknown. In this study, we found that AhR ligand 3,3'-diindolylmethane (DIM) inhibited the development of CD103⁺ DCs from mouse bone marrow cells stimulated with Flt3L and GM-CSF. DIM interfered with phosphorylation of STAT3 and STAT5 inhibiting the expression of genes, including Id2, E2-2, IDO-1, and Aldh1a2, which are associated with DC differentiation and functions. Finally, DIM suppressed the ability of CD103⁺ DCs to induce Foxp3⁺ Tregs.

The mannose receptor negatively modulates the Toll-like receptor 4-aryl hydrocarbon receptor-indoleamine 2,3-dioxygenase axis in dendritic cells affecting T helper cell polarization

BACKGROUND

Dendritic cells (DCs) are key players in the induction and re-elicitation of TH2 responses to allergens. We have previously shown that different C-type lectin receptors on DCs play a major role in allergen recognition and uptake. In particular, mannose receptor (MR), through modulation of Toll-like receptor (TLR) 4 signaling, can regulate indoleamine 2,3-dioxygenase (IDO) activity, favoring TH2 responses. Interestingly, the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor with an emerging role in immune modulation, has been implicated in IDO activation in response to TLR stimulation.

OBJECTIVE

Here we investigated how allergens and lectins modulate the TLR4-AhR-IDO axis in human monocyte-derived DCs.

METHODS

Using a combination of genomics, proteomics, and immunologic studies, we investigated the role of MR and AhR in IDO regulation and its effect on T helper cell differentiation.

RESULTS

We have demonstrated that LPS induces both IDO isoforms (IDO1 and IDO2) in DCs, with partial involvement of AhR. Additionally, we found that, like mannan, different airborne allergens can effectively downregulate TLR4-induced IDO1 and IDO2 expression, most likely through binding to the MR. Mannose-based ligands were also able to downregulate IL-12p70 production by DCs, affecting T helper cell polarization. Interestingly, AhR and some components of the noncanonical nuclear factor κB pathway were shown to be downregulated after MR engagement, which could explain the regulatory effects of MR on IDO expression.

CONCLUSION

Our work demonstrates a key role for MR in the modulation of the TLR4-AhR-IDO axis, which has a significant effect on DC behavior and the development of immune responses against allergens.

Aryl Hydrocarbon Receptor Protects Lungs from Cockroach Allergen-Induced Inflammation by Modulating Mesenchymal Stem Cells

Exposure to cockroach allergen leads to allergic sensitization and increased risk of developing asthma. Aryl hydrocarbon receptor (AhR), a receptor for many common environmental contaminants, can sense not only environmental pollutants but also microbial insults. Mesenchymal stem cells (MSCs) are multipotent progenitor cells with the capacity to modulate immune responses. In this study, we investigated whether AhR can sense cockroach allergens and modulate allergen-induced lung inflammation through MSCs. We found that cockroach allergen-treated AhR-deficient (AhR(-/-)) mice showed exacerbation of lung inflammation when compared with wild-type (WT) mice. In contrast, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an AhR agonist, significantly suppressed allergen-induced mouse lung inflammation. MSCs were significantly reduced in cockroach allergen-challenged AhR(-/-) mice as compared with WT mice, but increased in cockroach allergen-challenged WT mice when treated with TCDD. Moreover, MSCs express AhR, and AhR signaling can be activated by cockroach allergen with increased expression of its downstream genes cyp1a1 and cyp1b1. Furthermore, we tracked the migration of i.v.-injected GFP(+) MSCs and found that cockroach allergen-challenged AhR(-/-) mice displayed less migration of MSCs to the lungs compared with WT. The AhR-mediated MSC migration was further verified by an in vitro Transwell migration assay. Epithelial conditioned medium prepared from cockroach extract-challenged epithelial cells significantly induced MSC migration, which was further enhanced by TCDD. The administration of MSCs significantly attenuated cockroach allergen-induced inflammation, which was abolished by TGF-β1-neutralizing Ab. These results suggest that AhR plays an important role in protecting lungs from allergen-induced inflammation by modulating MSC recruitment and their immune-suppressive activity.

Endogenous ligands of the aryl hydrocarbon receptor regulate lung dendritic cell function

The aryl hydrocarbon receptor (AhR) is a transcription factor that has been extensively studied as a regulator of toxicant metabolism. However, recent evidence indicates that the AhR also plays an important role in immunity. We hypothesized that the AhR is a novel, immune regulator of T helper type 2 (Th2) -mediated allergic airway disease. Here, we report that AhR-deficient mice develop increased allergic responses to the model allergen ovalbumin (OVA), which are driven in part by increased dendritic cell (DC) functional activation. AhR knockout (AhR(-/-) ) mice sensitized and challenged with OVA develop an increased inflammatory response in the lung compared with wild-type controls, with greater numbers of inflammatory eosinophils and neutrophils, greater T-cell proliferation, greater production of Th2 cytokines, and higher levels of OVA-specific IgE and IgG1. Lung DCs from AhR(-/-) mice stimulated antigen-specific proliferation and Th2 cytokine production by naive T cells in vitro. Additionally, AhR(-/-) DCs produced higher levels of tumour necrosis factor-α and interleukin-6, which promote Th2 differentiation, and expressed higher cell surface levels of stimulatory MHC Class II and CD86 molecules. Overall, loss of the AhR was associated with enhanced T-cell activation by pulmonary DCs and heightened pro-inflammatory allergic responses. This suggests that endogenous AhR ligands are involved in the normal regulation of Th2-mediated immunity in the lung via a DC-dependent mechanism. Therefore, the AhR may represent an important target for therapeutic intervention in allergic airways inflammation.

The aryl hydrocarbon receptor in barrier organ physiology, immunology, and toxicology

The aryl hydrocarbon receptor (AhR) is an evolutionarily old transcription factor belonging to the Per-ARNT-Sim-basic helix-loop-helix protein family. AhR translocates into the nucleus upon binding of various small molecules into the pocket of its single-ligand binding domain. AhR binding to both xenobiotic and endogenous ligands results in highly cell-specific transcriptome changes and in changes in cellular functions. We discuss here the role of AhR for immune cells of the barrier organs: skin, gut, and lung. Both adaptive and innate immune cells require AhR signaling at critical checkpoints. We also discuss the current two prevailing views-namely, 1) AhR as a promiscuous sensor for small chemicals and 2) a role for AhR as a balancing factor for cell differentiation and function, which is controlled by levels of endogenous high-affinity ligands. AhR signaling is considered a promising drug and preventive target, particularly for cancer, inflammatory, and autoimmune diseases. Therefore, understanding its biology is of great importance.

The impact of diet on asthma and allergic diseases

The incidence of allergic diseases is increasing, both in developed and developing countries, concomitantly with the rise in living standards and the adoption of a 'western lifestyle'. For two decades, the hygiene hypothesis - which proposes that the lack of early childhood exposure to infectious agents increases susceptibility to allergic diseases in later life - provided the conceptual framework for unravelling the mechanisms that could account for the increased incidence of allergic diseases. In this Review, we discuss recent evidence that highlights the role of diet as a key factor influencing immune homeostasis and the development of allergic diseases through a complex interplay between nutrients, their metabolites and immune cell populations. Although further investigations are still required to understand these complex relationships, recent data have established a possible connection between metabolic homeostasis and allergic diseases.

Differential regulation of Th17 and T regulatory cell differentiation by aryl hydrocarbon receptor dependent xenobiotic response element dependent and independent pathways

The aryl hydrocarbon receptor (AHR) is regarded as an environmental sensor and has been shown to link environmental stresses with chronic inflammatory and autoimmune diseases. The AHR can be activated to regulate both the X/DRE (xenobiotic or dioxin response elements) as well as a non-X/DRE mediated pathway. Selective AHR modulators (SAhRMs) are recently identified compounds that activate non-X/DRE mediated pathway without activating the X/DRE-driven responses. Here, we have used 3 classes of AHR ligands; agonist, antagonist, and a SAhRM, to delineate the role of these AHR-driven pathways in T helper 17 (Th17)/T regulatory (Treg) regulation. We show that Th17 differentiation is primarily dependent on X/DRE-driven responses, whereas Treg differentiation can be suppressed by inhibiting non-X/DRE pathway. Using a model of Citrobacter rodentium infection, we further show that AHR agonist enhances Th17 production and promoted resolution of infection, whereas a SAhRM inhibited Th17 mediated responses with reduced resolution of infection. These data indicate that Th17/Treg function may be differentially regulated by SAhRMs that differentially activate the X/DRE and non-X/DRE mediated pathways, and point to a therapeutic strategy to leverage AHR function in the treatment of chronic inflammatory and autoimmune disease.

The aryl hydrocarbon receptor: multitasking in the immune system

The aryl hydrocarbon receptor (AhR), for many years almost exclusively studied by the pharmacology/toxicology field for its role in mediating the toxicity of xenobiotics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), has more recently attracted the attention of immunologists. The evolutionary conservation of this transcription factor and its widespread expression in the immune system point to important physiological functions that are slowly being unraveled. In particular, the emphasis is now shifting from the role of AhR in the xenobiotic pathway toward its mode of action in response to physiological ligands. In this article, we review the current understanding of the molecular interactions and functions of AhR in the immune system in steady state and in the presence of infection and inflammation, with a focus on barrier organs such as the skin, the gut, and the lung.

Administration of mycobacterial Ag85A and IL-17A fusion protein attenuates airway inflammation in a murine model of asthma

Interleukin (IL)-17A contributes to the development of asthma, especially in severe asthma which has characteristic neutrophil infiltration in airways. However, IL-17A-blocking antibody could escalate T helper (Th) 2 cytokines, such as IL-13, IL-4 in murine models. We aimed at determining the effect of mycobacterial Ag85A and IL-17A fusion protein—Ag85A-IL-17A on airway inflammation in a murine model of asthma. IL-17A recombinant protein fused mycobacterial immunodominant antigen Ag85A was constructed, expressed and purified. The fusion protein was then administrated into BALB/c mice and its anti-inflammatory effects in the infiltration of inflammatory cells, Th2/Th17 cytokines in BALF, histopathological changes of lung tissues as well as chemokines in lung tissues were evaluated in the murine model of asthma. We found that administration of mycobacterial Ag85A and IL-17A fusion protein induced IL-17A specific immunoglobulin (Ig)G in sera and significantly decreased IL-17A and IL-6 levels in bronchoalveolar lavage fluid (BALF). Ag85A-IL-17A vaccinated mice also showed marked reduction in the infiltration of inflammatory cells in peribronchiolar region and significant decrease in total cells, eosinophil cells and neutrophil cells in BALF. The increased levels of IL-13 and IL-4 in BALF of ovalbumin-sensitized mice were significantly reduced by the administration of Ag85A-IL-17A. Furthermore, CD3+CD4+IL-13+ splenocytes stimulated with OVA and CXCL1 mRNA, CCL2 mRNA and GATA-3 mRNA expressed in lung tissues were decreased markedly in Ag85A-IL-17A vaccinated group. Our results demonstrate remarkable antiallergic effects of Ag85A-IL-17A in a murine model of asthma and it may have protective effects on allergic asthma.

Synthesis and biological evaluation of 2,3'-diindolylmethanes as agonists of aryl hydrocarbon receptor

Recent studies suggest that arylhydrocarbon receptor (AhR) may be a target for a number of diseases. Natural product malassezin is a AhR agonist with an interesting 2,3'-diindolylmethane skeleton. We have prepared a series of analogues of natural product malassezin using our recently developed method and tested the activity of these analogues against AhR in a cell-based assay. We found that a methyl substituent at 1'-N can significantly increase the activity and the 2-formyl group is not critical for some diindolylmethanes.

The role of AhR in autoimmune regulation and its potential as a therapeutic target against CD4 T cell mediated inflammatory disorder

AhR has recently emerged as a critical physiological regulator of immune responses affecting both innate and adaptive systems. Since the AhR signaling pathway represents an important link between environmental stimulators and immune-mediated inflammatory disorder, it has become the object of great interest among researchers recently. The current review discusses new insights into the mechanisms of action of a select group of inflammatory autoimmune diseases and the ligand-activated AhR signaling pathway. Representative ligands of AhR, both exogenous and endogenous, are also reviewed relative to their potential use as tools for understanding the role of AhR and as potential therapeutics for the treatment of various inflammatory autoimmune diseases, with a focus on CD4 helper T cells, which play important roles both in self-immune tolerance and in inflammatory autoimmune diseases. Evidence indicating the potential use of these ligands in regulating inflammation in various diseases is highlighted, and potential mechanisms of action causing immune system effects mediated by AhR signaling are also discussed. The current review will contribute to a better understanding of the role of AhR and its signaling pathway in CD4 helper T cell mediated inflammatory disorder. Considering the established importance of AhR in immune regulation and its potential as a therapeutic target, we also think that both further investigation into the molecular mechanisms of immune regulation that are mediated by the ligand-specific AhR signaling pathway, and integrated research and development of new therapeutic drug candidates targeting the AhR signaling pathway should be pursued urgently.

Differential consequences of two distinct AhR ligands on innate and adaptive immune responses to influenza A virus

Immune modulation by the aryl hydrocarbon receptor (AhR) has been primarily studied using 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD). Recent reports suggest another AhR ligand, 6-formylindolo[3,2-b]carbazole (FICZ), exhibits distinct immunomodulatory properties, but side-by-side comparisons of these 2 structurally distinct, high-affinity ligands are limited. In this study, the effects of in vivo AhR activation with TCDD and FICZ were directly compared in a mouse model of influenza virus infection using 3 key measures of the host response to infection: pulmonary neutrophilia, inducible nitric oxide synthase (iNOS) levels, and the virus-specific CD8(+) T-cell response. By this approach, the consequences of AhR activation on innate and adaptive immune responses to the same antigenic challenge were compared. A single dose of TCDD elicited AhR activation that is sustained for the duration of the host's response to infection and modulated all 3 responses to infection. In contrast, a single dose of FICZ induced transient AhR activation and had no effect on the immune response to infection. Micro-osmotic pumps and Cyp1a1-deficient mice were utilized to augment FICZ-mediated AhR activation in vivo, in order to assess the effect of transient versus prolonged AhR activation. Prolonged AhR activation with FICZ did not affect neutrophil recruitment or pulmonary iNOS levels. However, FICZ-mediated AhR activation diminished the CD8(+) T-cell response in Cyp1a1-deficient mice in a similar manner to TCDD. These results demonstrate that immunomodulatory differences in the action of these 2 ligands are likely due to not only the duration of AhR activation but also the cell types in which the receptor is activated.

Toward understanding the role of aryl hydrocarbon receptor in the immune system: current progress and future trends

The immune system is regulated by distinct signaling pathways that control the development and function of the immune cells. Accumulating evidence suggest that ligation of aryl hydrocarbon receptor (Ahr), an environmentally responsive transcription factor, results in multiple cross talks that are capable of modulating these pathways and their downstream responsive genes. Most of the immune cells respond to such modulation, and many inflammatory response-related genes contain multiple xenobiotic-responsive elements (XREs) boxes upstream. Active research efforts have investigated the physiological role of Ahr in inflammation and autoimmunity using different animal models. Recently formed paradigm has shown that activation of Ahr by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 3,3′-diindolylmethane (DIM) prompts the differentiation of CD4⁺Foxp3⁺ regulatory T cells (Tregs) and inhibits T helper (Th)-17 suggesting that Ahr is an innovative therapeutic strategy for autoimmune inflammation. These promising findings generate a basis for future clinical practices in humans. This review addresses the current knowledge on the role of Ahr in different immune cell compartments, with a particular focus on inflammation and autoimmunity.

Uremic toxin indoxyl 3-sulfate regulates the differentiation of Th2 but not of Th1 cells to lessen allergic asthma

Immune system dysfunctions including the increased Th1/Th2 ratio are common in chronic kidney disease (CKD) patients, and a wide variety of skin diseases including Th1-mediated uremic pruritis are associated with CKD. Although there are more than 90 uremic toxins reported, it is yet to be known which uremic solute is associated with the unbalanced Th1/Th2 ratio and how it works. Indoxyl 3-sulfate (I3S), one of uremic toxins and a potent aryl hydrocarbon receptor (AhR) ligand, accumulates in blood and tissues, increasing up to 81.04 μM in CKD patients, compared with 1.03 μM in healthy subjects. I3S activates NF-κB and AhR. Thus, we investigated roles of I3S in the differentiation of Th1 and Th2 cells. I3S inhibited Th2 differentiation but showed little or no effect on Th1 differentiation. I3S suppressed Th2-mediated ovalbumin-induced allergic asthma in mice and decreased the frequency of IL-4 producing CD4 T cells in the lungs. I3S inhibited phosphorylation of STAT5 and STAT6, transcription factors associated with Th2 differentiation. Effects of I3S on Th2 differentiation were suppressed by α-naphtoflavone, an AhR antagonist, indicating that I3S regulates Th2 differentiation AhR-dependently.

Role of the aryl hydrocarbon receptor (AhR) in lung inflammation

Millions of individuals worldwide are afflicted with acute and chronic respiratory diseases, causing temporary and permanent disabilities and even death. Oftentimes, these diseases occur as a result of altered immune responses. The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, acts as a regulator of mucosal barrier function and may influence immune responsiveness in the lungs through changes in gene expression, cell-cell adhesion, mucin production, and cytokine expression. This review updates the basic immunobiology of the AhR signaling pathway with regards to inflammatory lung diseases such as asthma, chronic obstructive pulmonary disease, and silicosis following data in rodent models and humans. Finally, we address the therapeutic potential of targeting the AhR in regulating inflammation during acute and chronic respiratory diseases.

Platinum-catalyzed tandem indole annulation/arylation for the synthesis of diindolylmethanes and indolo[3,2-b]carbazoles

Various diindolylmethanes were prepared from propargylic ethers and substituted indoles via a platinum-catalyzed tandem indole annulation/arylation cascade. The resulting diindolylmethanes could be converted to natural product malassezin by formylation or indolo[3,2-b]carbazoles by cyclization.

Aryl hydrocarbon receptor control of adaptive immunity

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that belongs to the family of basic helix-loop-helix transcription factors. Although the AhR was initially recognized as the receptor mediating the pathologic effects of dioxins and other pollutants, the activation of AhR by endogenous and environmental factors has important physiologic effects, including the regulation of the immune response. Thus, the AhR provides a molecular pathway through which environmental factors modulate the immune response in health and disease. In this review, we discuss the role of AhR in the regulation of the immune response, the source and chemical nature of AhR ligands, factors controlling production and degradation of AhR ligands, and the potential to target the AhR for therapeutic immunomodulation.

A common environmental pollutant, 4-nonylphenol, promotes allergic lung inflammation in a murine model of asthma

BACKGROUND

Exposure to environmental hormones, such as alkylphenols, has been suggested to be associated with the development of asthma, but the mechanism of action remains unclear.

OBJECTIVE

This study examined the effect of 4-nonylphenol (NP), one of the most important alkylphenols, on conventional dendritic cells (cDCs) and adaptive T-cell responses. It also explored the role of aryl hydrocarbon receptor (AhR) in NP's effect.

METHODS

NP-conditioned bone marrow-derived DCs (BM-DCs) and splenic CD11c(+) cDCs were assessed regarding function in a murine model under conditions relevant to route and level of exposure in humans.

RESULTS

Our results showed that splenic cDCs from NP-exposed mice have potent Th2-skewing ability and secrete increased levels of IL-6 and TNF-α, but not IL-10 and IL-12, at baseline and after stimulation with LPS. Further, bone marrow-derived DCs were cultured in the presence of NP and showed similar cytokine pattern and influenced the antigen-specific T cells secreting significantly less IFN-γ. Importantly, NP-exposed mice developed more severe OVA-induced allergic lung inflammation compared with control group. Interestingly, in a congenic strain of mice carrying low-affinity, ligand-binding mutant AhR (AhR(d) ), NP's effect on DC functions and lung inflammation was not observed in vitro and in vivo.

CONCLUSION

These results suggested that NP may disturb physiologic function of DCs through, in part, AhR-dependent mechanisms, supporting the importance of NP exposure on the regulation of DC functions and allergic inflammation.

Indoxyl 3-sulfate stimulates Th17 differentiation enhancing phosphorylation of c-Src and STAT3 to worsen experimental autoimmune encephalomyelitis

Although AhR activation regulates CD4T cell differentiation, how it works has yet to be elucidated. In the present study, using in vitro Th17 differentiation model, we examined effects of AhR activation by indoxyl 3-sulfate (I3S), a uremic toxin, on Th17 differentiation and investigated underlying mechanisms. I3S increased expression of RORγt, the master transcription factor for Th17 differentiation, and stimulated Th17 differentiation, in a comparative manner as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a prototypical AhR ligand. Activation of STAT3, which is phosphorylated by the IL-6 signaling pathways and thus is necessary for Th17 differentiation, was strongly stimulated by I3S and TCDD. Phosphorylation of c-Src, which was shown to be activated by AhR ligands, was also increased by I3S and TCDD, and blocking of c-Src activity by 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d]pyrimidine (PP2) inhibited phosphorylation of both c-Src and STAT3, raising a possibility that stimulatory activities of I3S and TCDD on Th17 differentiation could be exerted via increased phosphorylation of c-Src, which in turn stimulates STAT3 activation. Finally, we found that I3S worsened experimental autoimmune encephalomyelitis (EAE), which is primarily mediated by Th17 cells, enhancing the frequency of IL-17-producing cells in draining lymph nodes.

Coal tar induces AHR-dependent skin barrier repair in atopic dermatitis

Topical application of coal tar is one of the oldest therapies for atopic dermatitis (AD), a T helper 2 (Th2) lymphocyte-mediated skin disease associated with loss-of-function mutations in the skin barrier gene, filaggrin (FLG). Despite its longstanding clinical use and efficacy, the molecular mechanism of coal tar therapy is unknown. Using organotypic skin models with primary keratinocytes from AD patients and controls, we found that coal tar activated the aryl hydrocarbon receptor (AHR), resulting in induction of epidermal differentiation. AHR knockdown by siRNA completely abrogated this effect. Coal tar restored filaggrin expression in FLG-haploinsufficient keratinocytes to wild-type levels, and counteracted Th2 cytokine-mediated downregulation of skin barrier proteins. In AD patients, coal tar completely restored expression of major skin barrier proteins, including filaggrin. Using organotypic skin models stimulated with Th2 cytokines IL-4 and IL-13, we found coal tar to diminish spongiosis, apoptosis, and CCL26 expression, all AD hallmarks. Coal tar interfered with Th2 cytokine signaling via dephosphorylation of STAT6, most likely due to AHR-regulated activation of the NRF2 antioxidative stress pathway. The therapeutic effect of AHR activation herein described opens a new avenue to reconsider AHR as a pharmacological target and could lead to the development of mechanism-based drugs for AD.

Quercetin, resveratrol, and curcumin are indirect activators of the aryl hydrocarbon receptor (AHR)

Several polyphenols have been shown to activate the aryl hydrocarbon receptor (AHR) in spite of the fact that they bind to the receptor with low affinity. The aim of this study was to investigate whether quercetin (QUE), resveratrol (RES), and curcumin (CUR) interfere with the metabolic degradation of the suggested endogenous AHR ligand 6-formylindolo[3,2-b]carbazole (FICZ) and thereby indirectly activate the AHR. Using recombinant human enzyme, we confirmed earlier reported inhibitory effects of the polyphenols on cytochrome P4501A1 (CYP1A1) activity, and inhibition of metabolic clearance of FICZ was documented in FICZ-treated immortalized human keratinocytes (HaCaT). CYP1A1 activity was induced in HaCaT cells by all three compounds, and when they were added together with FICZ, a prolonged activation was observed after a dose-dependent inhibition period. The same pattern of responses was seen at the transcriptional level as determined with a CYP1A1 reporter assay in human liver hepatoma (HepG2) cells. To test the ability of the polyphenols to activate the AHR in the absence of FICZ, the cells were treated in medium, which in contrast to commercial batches of medium did not contain background levels of FICZ. Importantly, AHR activation was only observed in the commercial medium. Taken together, these findings suggest that QUE, RES, and CUR induce CYP1A1 in an indirect manner by inhibiting the metabolic turnover of FICZ. Humans are exposed to these compounds through the diet and nutritional supplements, and we propose that altered systemic levels of FICZ caused by such compounds may have physiological consequences.