AhR: far more than an environmental sensor

Zearalenone and its metabolite exposure directs oestrogen metabolism towards potentially carcinogenic metabolites in human breast cancer MCF-7 cells

Zearalenone (ZEN) is produced by Fusarium species contaminating various agriculture crops. In this study, the effects of ZEN and its metabolites α-zearalenol (α-ZEL), and β-zearalenol (β-ZEL) on the formation of carcinogenic oestrogen-catechols in MCF-7 cells were investigated. To assess the effects of mycoestrogens on the activity of cytochrome P450 1A1 and CYP1B1, the rate of ethoxyresorufin O-deethylation (EROD-assay) was measured. The effects of mycoestrogens on the expression of CYP 1A1, CYP 1B1, aryl-hydrocarbon receptor (AhR), and oestrogen receptor alpha (ERα) were determined by qPCR. The catechol-O-methyltransferase (COMT) activity was measured as the ratio of the methoxy metabolites of oestradiol. Results show that mycoestrogens inhibited significantly the CYP1-dependent EROD activities. In the presence of selective inhibitors, mycoestrogens reduced CYP 1A1 and enhanced CYP 1B1 activity. Quantitative PCR analyses demonstrated the upregulation of AhR and confirmed the selective effect of mycoestrogens on CYP1 expression levels and the decline of the CYP 1A1/CYP 1B1 ratio. Mycoestrogens increased the ratio of 4-MeOE to 2-MeOE2 formation significantly (P < 0.05). Our results suggest that the tested mycoestrogens increase the production of CYP1B1-mediated oestrogen catechol metabolites, directing the biotransformation of E2 towards 4-OHE2, which has been identified earlier as a crucial factor in oestrogen-induced tumour initiation.

Gestational and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin primes cortical microglia to tissue injury

Recent studies have shown that the aryl hydrocarbon receptor (AhR) is expressed in the brain's native immune cells, known as microglia. However, while the impact of exposure to AhR ligands is well studied in the peripheral immune system, the impact of such exposure on immune function in the brain is less well defined. Microglia serve dual roles in providing synaptic and immunological support for neighboring neurons and in mediating responses to environmental stimuli, including exposure to environmental chemicals. Because of their dual roles in regulating physiological and pathological processes, cortical microglia are well positioned to translate toxic stimuli into defects in cortical function via aberrant synaptic and immunological functioning, mediated either through direct microglial AhR activation or in response to AhR activation in neighboring cells. Here, we use gene expression studies, histology, and two-photon in vivo imaging to investigate how developmental exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a high-affinity and persistent AhR agonist, modulates microglial characteristics and function in the intact brain. Whole cortical RT-qPCR analysis and RNA-sequencing of isolated microglia revealed that gestational and lactational TCDD exposure produced subtle, but durable, changes in microglia transcripts. Histological examination and two-photon in vivo imaging revealed that while microglia density, distribution, morphology, and motility were unaffected by TCDD exposure, exposure resulted in microglia that responded more robustly to focal tissue injury. However, this effect was rectified with depletion and repopulation of microglia. These results suggest that gestational and lactational exposure to AhR ligands can result in long-term priming of microglia to produce heightened responses towards tissue injury which can be restored to normal function through microglial repopulation.

Tryptophan Metabolites at the Crossroad of Immune-Cell Interaction via the Aryl Hydrocarbon Receptor: Implications for Tumor Immunotherapy

The Aryl hydrocarbon receptor (AhR) is a critical regulator of both innate and adaptive immune responses, with potent immunomodulatory effects that makes this receptor an attractive molecular target for novel therapeutics. Accumulating evidence indicates that diverse—both host’s and microbial—tryptophan metabolites profoundly regulate the immune system in the host via AhR, promoting either tolerance or immunity, largely as a function of the qualitative and quantitative nature of the metabolites being contributed by either source. Additional findings indicate that host and microbiota-derived tryptophan metabolic pathways can influence the outcome of immune responses to tumors. Here, we review recent studies on the role and modalities of AhR activation by various ligands, derived from either host-cell or microbial-cell tryptophan metabolic pathways, in the regulation of immune responses. Moreover, we highlight potential implications of those ligands and pathways in tumor immunotherapy, with particular relevance to checkpoint-blockade immune intervention strategies.

Acute 2,3,7,8-Tetrachlorodibenzo-p-dioxin exposure in adult mice does not alter the morphology or inflammatory response of cortical microglia

Microglia, the immune cells of the brain, have a canonical role in regulating responses to neurological disease or injury, but have also recently been implicated as regulators of neurophysiological processes such as learning and memory. Given these dual immune and physiological roles, microglia are a likely mechanism by which external toxic stimuli are converted into deficits in neuronal circuitry and subsequently function. However, while it is well established that exposure to environmental toxicants negatively affects the peripheral immune system, it remains unknown whether and how such exposure causes neuroinflammation which, in turn, may negatively impact microglial functions in vivo. Here, we examined how acute 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in adulthood, which negatively impacts immune cells in the periphery, affects microglial characteristics in the cortex of the mouse. We found that microglia density, distribution, morphology, inflammatory signaling, and response to a secondary, pathological activation were unaffected by acute TCDD exposure. These results suggest that acute, peripheral TCDD exposure in adulthood is not sufficient to induce an overt inflammatory phenotype in cortical microglia.

Interaction of 7-Alkoxycoumarins with the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a transcription factor activated by a vast array of natural and synthetic ligands. It plays a pivotal role in numerous physiological and pathological responses, such as cell proliferation and differentiation, induction of xenobiotic metabolizing enzymes, response to environmental toxins, and several others. In this study, we investigated the ability of some natural compounds (oxyprenylated ferulic acid and umbelliferone derivatives) and their semisynthetic analogues (e.g., differently substituted 7-alkoxycoumarins) to activate AhR, using a reporter luciferase assay. Among them, we found that 7-isopentenyloxycoumarin was the best AhR activator. Boropinic acid, 7-but-2'-enyloxycoumarin, 7-(2',2'-dimethyl-n-propyloxy)coumarin, 7-benzyloxycoumarin, and 7-(3'-hydroxymethyl-3'-methylallyloxy)coumarin were also active, although to a lesser extent. All the compounds were also analyzed for their ability to inhibit AhR activation, using a reference ligand, 6-formylindolo[3,2-b]carbazole. Data recorded in the present investigation pointed out the importance of a 3,3-dimethylallyloxy side chain attached to the coumarin ring core as a key moiety for AhR activation.

The Proteasome Inhibitor Bortezomib Controls Indoleamine 2,3-Dioxygenase 1 Breakdown and Restores Immune Regulation in Autoimmune Diabetes

Bortezomib (BTZ) is a first-in-class proteasome inhibitor approved for the therapy of multiple myeloma that also displays unique regulatory activities on immune cells. The enzyme indoleamine 2,3-dioxygenase 1 (IDO1) is a tryptophan metabolizing enzyme exerting potent immunoregulatory effects when expressed in dendritic cells (DCs), the most potent antigen-presenting cells capable of promoting either immunity or tolerance. We previously demonstrated that, in inflammatory conditions, IDO1 is subjected to proteasomal degradation in DCs, turning these cells from immunoregulatory to immunostimulatory. In non-obese diabetic (NOD) mice, an experimental model of autoimmune diabetes, we also identified an IDO1 defect such that the DCs do not develop tolerance toward pancreatic islet autoantigens. We found that BTZ rescues IDO1 protein expression in vitro in a particular subset of DCs, i.e., plasmacytoid DCs (pDCs) from NOD mice. When administered in vivo to prediabetic mice, the drug prevented diabetes onset through IDO1- and pDC-dependent mechanisms. Although the drug showed no therapeutic activity when administered alone to overtly diabetic mice, its combination with otherwise suboptimal dosages of autoimmune-preventive anti-CD3 antibody resulted in disease reversal in 70% diabetic mice, a therapeutic effect similar to that afforded by full-dosage anti-CD3. Thus, our data indicate a potential for BTZ in the immunotherapy of autoimmune diabetes and further underline the importance of IDO1-mediated immune regulation in such disease.

EWS-FLI1 impairs aryl hydrocarbon receptor activation by blocking tryptophan breakdown via the kynurenine pathway

Ewing sarcoma (ES) is an aggressive pediatric tumor driven by the fusion protein EWS-FLI1. We report that EWS-FLI1 suppresses TDO2-mediated tryptophan (TRP) breakdown in ES cells. Gene expression and metabolite analyses reveal an EWS-FLI1-dependent regulation of TRP metabolism. TRP consumption increased in the absence of EWS-FLI1, resulting in kynurenine and kynurenic acid accumulation, both aryl hydrocarbon receptor (AHR) ligands. Activated AHR binds to the promoter region of target genes. We demonstrate that EWS-FLI1 knockdown results in AHR nuclear translocation and activation. Our data suggest that EWS-FLI1 suppresses autocrine AHR signaling by inhibiting TDO2-catalyzed TRP breakdown.

Distinct Roles for Intestinal Epithelial Cell-Specific Hdac1 and Hdac2 in the Regulation of Murine Intestinal Homeostasis

The intestinal epithelium responds to and transmits signals from the microbiota and the mucosal immune system to insure intestinal homeostasis. These interactions are in part conveyed by epigenetic modifications, which respond to environmental changes. Protein acetylation is an epigenetic signal regulated by histone deacetylases, including Hdac1 and Hdac2. We have previously shown that villin-Cre-inducible intestinal epithelial cell (IEC)-specific Hdac1 and Hdac2 deletions disturb intestinal homeostasis. To determine the role of Hdac1 and Hdac2 in the regulation of IEC function and the establishment of the dual knockout phenotype, we have generated villin-Cre murine models expressing one Hdac1 allele without Hdac2, or one Hdac2 allele without Hdac1. We have also investigated the effect of short-term deletion of both genes in naphtoflavone-inducible Ah-Cre and tamoxifen-inducible villin-Cre(ER) mice. Mice with one Hdac1 allele displayed normal tissue architecture, but increased sensitivity to DSS-induced colitis. In contrast, mice with one Hdac2 allele displayed intestinal architecture defects, increased proliferation, decreased goblet cell numbers as opposed to Paneth cells, increased immune cell infiltration associated with fibrosis, and increased sensitivity to DSS-induced colitis. In comparison to dual knockout mice, intermediary activation of Notch, mTOR, and Stat3 signaling pathways was observed. While villin-Cre(ER) Hdac1 and Hdac2 deletions led to an impaired epithelium and differentiation defects, Ah-Cre-mediated deletion resulted in blunted proliferation associated with the induction of a DNA damage response. Our results suggest that IEC determination and intestinal homeostasis are highly dependent on Hdac1 and Hdac2 activity levels, and that changes in the IEC acetylome may alter the mucosal environment.

An overview of detection techniques for monitoring dioxin-like compounds: latest technique trends and their applications

Dioxin-like compounds (DLCs) are considered as persistent bioaccumulative toxicants with a number of continuing issues in the fields of ecotoxicology and bioassay.

The cross-talk between opportunistic fungi and the mammalian host via microbiota's metabolism

An increased understanding of the importance of microbiota in shaping the host's immune and metabolic activities has rendered fungal interactions with their hosts more complex than previously appreciated. It is now clear that a three-way interaction between host, fungi, and microbiota dictates the types of host-fungus relationship. Indeed, microbial dysbiosis predisposes to a variety of chronic fungal infections and diseases at local and distant sites. By correlating changes in metabolite profiles with microbiota metagenomic composition, we have defined a functional node whereby certain bacteria species contribute to host-fungal symbiosis and mucosal homeostasis. A tryptophan catabolic pathway is exploited by commensal lactobacilli and the mammalian host to increase fitness in response to Candida albicans by inducing resistance and tolerance mechanisms of antifungal immunity. Much like lactobacilli in the gut, Firmicutes change significantly in the airways during aspergillosis. The aryl hydrocarbon receptor has a pivotal role in connecting tryptophan catabolism by microbial communities and the host's own pathway of tryptophan degradation through the enzyme indoleamine 2,3-dioxygenase 1. These data suggest that the study of the human microbiota in the trans-omics era, with a focus on metagenomics and metabonomics, is providing novel insights into the regulation of host immune responsiveness to fungi.