Abnormal liver development and resistance to 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in mice carrying a mutation in the DNA-binding domain of the aryl hydrocarbon receptor

Using the Key Characteristics Framework to Unlock the Mysteries of Aryl Hydrocarbon Receptor-Mediated Effects on the Immune System

Initially discovered for its role mediating the deleterious effects of environmental contaminants, the aryl hydrocarbon receptor (AHR) is now known to be a crucial regulator of the immune system. The expanding list of AHR ligands includes synthetic and naturally derived molecules spanning pollutants, phytochemicals, pharmaceuticals, and substances derived from amino acids and microorganisms. The consequences of engaging AHR vary, depending on factors such as the AHR ligand, cell type, immune challenge, developmental state, dose, and timing of exposure relative to the immune stimulus. This review frames this complexity using the recently identified key characteristics of agents that affect immune system function (altered cell signaling, proliferation, differentiation, effector function, communication, trafficking, death, antigen presentation and processing, and tolerance). The use of these key characteristics provides a scaffold for continued discovery of how AHR and its myriad ligands influence the immune system, which will help harness the power of this enigmatic receptor to prevent or treat disease.

Structural basis for the ligand-dependent activation of heterodimeric AHR-ARNT complex

The aryl hydrocarbon receptor (AHR) possesses an extraordinary capacity to sense and respond to a wide range of small-molecule ligands, ranging from polycyclic aromatic hydrocarbons to endogenous compounds. Upon ligand binding, AHR translocates from the cytoplasm to nucleus, forming a transcriptionally active complex with aryl hydrocarbon receptor nuclear translocator (ARNT), for DNA binding and initiation of gene expression programs that include cellular detoxification pathways and immune responses. Here, we examine the molecular mechanisms governing AHR's high-affinity binding and activation by a diverse group of ligands. Crystal structures of the AHR-ARNT-DNA complexes, bound with each of six established AHR ligands, including Tapinarof, 6-formylindolo[3,2-b]carbazole (FICZ), benzo[a]pyrene (BaP), β-naphthoflavone (BNF), Indigo and Indirubin, reveal an unconventional mode of subunit assembly with intimate association between the PAS-B domains of AHR and ARNT. AHR's PAS-B domain utilizes eight conserved residues whose dynamic rearrangements account for the ability to bind to ligands through hydrophobic and π-π interactions. Our findings further reveal the structural underpinnings of a ligand-driven activation mechanism, whereby a segment of the AHR protein undergoes a structural transition from chaperone engagement to ARNT heterodimer stabilization, to generate the transcriptionally competent assembly. Our results provide key information for the future development of AHR-targeting drugs.

Optimal treatments and outcome measures of palmoplantar pustulosis: A systematic review and network meta-analysis-based comparison of treatment efficacy

Few studies have made direct comparisons between treatments for palmoplantar pustulosis (PPP); therefore, it is difficult to select the best treatment for each patient. To determine the best therapy and to compare reported measures of efficacy in clinical trials of systemic treatments for PPP in this systematic review and network meta-analysis. Six databases were used to perform database search on 10 July 2022. Randomized controlled trials (RCTs) were identified through a systematic literature search. The titles and abstracts of articles were initially screened for inclusion by two authors independently using our predetermined criteria. The full texts of selected articles were then independently assessed for inclusion in a blinded fashion. Disagreement between the authors was resolved by consensus. Data were abstracted in duplicate. Random-effects model was accepted to perform network meta-analysis. Assessed Grading of Recommendations Assessment, Development and Evaluation certainty of evidence were performed according to the PRISMA guidelines. The analysis was completed in July 2022. The primary outcome was the change of PPP Area and Severity Index (PPPASI) from baseline and the secondary outcome was the achievement of PPPASI-50 response. Seven RCTs with 567 patients were included. Guselkumab 100 mg was the one with the highest probability of reaching the proposed outcomes (mean difference [MD], -8.00; 95% confidence interval [CI], 4.88-11.11), while the achievement of PPPASI-50 response did not show a significant difference (odds ratio [OR], 3.79; 95% CI, 0.51-28.37). Guselkumab 200 mg was next to 100 mg of reaching the proposed outcomes (MD, -4.71; 95% CI, 2.12-7.30), while the achievement of PPPASI-50 response did not show a significant difference (OR, 2.34; 95% CI, 0.48-11.43). Network meta-analysis showed guselkumab 100 mg was the treatment with the highest probability of reaching both PPPASI and PPPASI-50 outcomes. Absolute PPPASI may be more appropriate as an outcome than PPPASI-50.

Dynamic relationship between the aryl hydrocarbon receptor and long noncoding RNA balances cellular and toxicological responses

The aryl hydrocarbon receptor (AhR) is a cytosolic transcription factor activated by endogenous ligands and xenobiotic chemicals. Once the AhR is activated, it translocates to the nucleus, dimerizes with the AhR nuclear translator (ARNT) and binds to xenobiotic response elements (XRE) to promote gene transcription, notably the cytochrome P450 CYP1A1. The AhR not only mediates the toxic effects of environmental chemicals, but also has numerous putative physiological functions. This dichotomy in AhR biology may be related to reciprocal regulation of long non-coding RNA (lncRNA). lncRNA are defined as transcripts more than 200 nucleotides in length that do not encode a protein but are implicated in many physiological processes such as cell differentiation, cell proliferation, and apoptosis. lncRNA are also linked to disease pathogenesis, particularly the development of cancer. Recent studies have revealed that AhR activation by environmental chemicals affects the expression and function of lncRNA. In this article, we provide an overview of AhR signaling pathways activated by diverse ligands and highlight key differences in the putative biological versus toxicological response of AhR activation. We also detail the functions of lncRNA and provide current data on their regulation by the AhR. Finally, we outline how overlap in function between AhR and lncRNA may be one way in which AhR can be both a regulator of endogenous functions but also a mediator of toxicological responses to environmental chemicals. Overall, more research is still needed to fully understand the dynamic interplay between the AhR and lncRNA.

PCB126 exposure during pregnancy alters maternal and fetal gene expression

Polychlorinated biphenyls (PCBs) are organic pollutants that can have lasting impacts on offspring health. Here, we sought to examine maternal and fetal gene expression differences of aryl hydrocarbon receptor (AHR)-regulated genes in a mouse model of prenatal PCB126 exposure. Female mice were bred and gavaged with 1 µmole/kg bodyweight PCB126 or vehicle control on embryonic days 0 and 14, and maternal and fetal tissues were collected on embryonic day 18.5. Total RNAs were isolated, and gene expression levels were analyzed in both maternal and fetal tissues using the NanoString nCounter system. Interestingly, we found that the expression levels of cytochrome P450 (Cyp)1a1 and Cyp1b1 were significantly increased in response to PCB exposure in the tested maternal and fetal tissues. Furthermore, PCB exposure altered the expression of several other genes related to energy balance, oxidative stress, and epigenetic regulation in a manner that was less consistent across tissue types. These results indicate that maternal PCB126 exposure significantly alters gene expression in both developing fetuses and pregnant dams, and such changes vary in intensity and expressivity depending on tissue type. The altered gene expression may provide insights into pathophysiological mechanisms by which in utero PCB exposures contribute to PCB-induced postnatal metabolic diseases.

Fifty Years of Aryl Hydrocarbon Receptor Research as Reflected in the Pages of Drug Metabolism and Disposition

The induction of multiple drug-metabolizing enzymes by halogenated and polycyclic aromatic hydrocarbon toxicants is mediated by the aryl hydrocarbon receptor (AHR). This fascinating receptor also has natural dietary and endogenous ligands, and much is now appreciated about the AHR's developmental and physiologic roles, as well as its importance in cancer and other diseases. The past several years has witnessed increasing emphasis on understanding the multifaceted roles of the AHR in the immune system. Most would agree that the "discovery" of the AHR occurred in 1976, with the report of specific binding of a high affinity radioligand in mouse liver, just three years after the launch of the journal Drug Metabolism and Disposition (DMD) in 1973. Over the ensuing 50 years, the AHR and DMD have led parallel and often intersecting lives. The overall goal of this mini-review is to provide a decade-by-decade overview of major historical landmark discoveries in the AHR field and to highlight the numerous contributions made by publications appearing in the pages of DMD. It is hoped that this historical tour might inspire current and future research in the AHR field. SIGNIFICANCE STATEMENT: With the launch of Drug Metabolism and Disposition (DMD) in 1973 and the discovery of the aryl hydrocarbon receptor (AHR) in 1976, the journal and the receptor have led parallel and often intersecting lives over the past 50 years. Tracing the history of the AHR can reveal how knowledge in the field has evolved to the present and highlight the important contributions made by discoveries reported in DMD. This may inspire additional DMD papers reporting future AHR landmark discoveries.

Increased sensitivity to chemically induced colitis in mice harboring a DNA-binding deficient aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR), a transcription factor best known for mediating toxic responses of environmental pollutants, also integrates metabolic signals to promote anti-inflammatory responses, intestinal homeostasis, and maintain barrier integrity. AHR regulates its target genes through direct DNA-binding to aryl hydrocarbon response elements (AHREs) but also through tethering to other transcription factors in a DNA-binding independent manner. However, it is not known if AHR's anti-inflammatory role in the gut requires its ability to bind to AHREs. To test this, we determined the sensitivity of Ahrdbd/dbd mice, a genetically modified mouse line that express an AHR protein incapable of binding to AHREs, to dextran sulfate sodium (DSS)-induced colitis. Ahrdbd/dbd mice exhibited more severe symptoms of intestinal inflammation than Ahr+/+ mice. None of the Ahrdbd/dbd mice survived after the 5-day DSS followed by 7-day washout period. By day 6, the Ahrdbd/dbd mice had severe body weight loss, shortening of the colon, higher disease index scores, enlarged spleens, and increased expression of several inflammation genes, including interleukin 1b (Il-1b), Il-6, Il-17, C-x-c motif chemokine ligand 1 (Cxcl1), Cxcl2, Prostaglandin-endoperoxide synthase (Ptgs2), and lipocalin-2. Our findings show that AHR's DNA-binding domain and ability to bind to AHREs are required to reduce inflammation, maintain a healthy intestinal environment, and protect against DSS-induced colitis.

The Aryl Hydrocarbon Receptor Modulates T Follicular Helper Cell Responses to Influenza Virus Infection in Mice

T follicular helper (Tfh) cells support Ab responses and are a critical component of adaptive immune responses to respiratory viral infections. Tfh cells are regulated by a network of signaling pathways that are controlled, in part, by transcription factors. The aryl hydrocarbon receptor (AHR) is an environment-sensing transcription factor that modulates many aspects of adaptive immunity by binding a range of small molecules. However, the contribution of AHR signaling to Tfh cell differentiation and function is not known. In this article, we report that AHR activation by three different agonists reduced the frequency of Tfh cells during primary infection of C57BL/6 mice with influenza A virus (IAV). Further, using the high-affinity and AHR-specific agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin, we show that AHR activation reduced Tfh cell differentiation and T cell-dependent B cell responses. Using conditional AHR knockout mice, we demonstrated that alterations of Tfh cells and T cell-dependent B cell responses after AHR activation required the AHR in T cells. AHR activation reduced the number of T follicular regulatory (Tfr) cells; however, the ratio of Tfr to Tfh cells was amplified. These alterations to Tfh and Tfr cells during IAV infection corresponded with differences in expression of BCL6 and FOXP3 in CD4⁺ T cells and required the AHR to have a functional DNA-binding domain. Overall, these findings support that the AHR modulates Tfh cells during viral infection, which has broad-reaching consequences for understanding how environmental factors contribute to variation in immune defenses against infectious pathogens, such as influenza and severe acute respiratory syndrome coronavirus.

The Ahr2-Dependent wfikkn1 Gene Influences Zebrafish Transcriptome, Proteome, and Behavior

The aryl hydrocarbon receptor (AHR) is required for vertebrate development and is also activated by exogenous chemicals, including polycyclic aromatic hydrocarbons (PAHs) and TCDD. AHR activation is well-understood, but roles of downstream molecular signaling events are largely unknown. From previous transcriptomics in 48-hours post fertilization (hpf) zebrafish exposed to several PAHs and TCDD, we found wfikkn1 was highly co-expressed with cyp1a (marker for AHR activation). Thus, we hypothesized wfikkn1's role in AHR signaling, and showed that wfikkn1 expression was Ahr2 (zebrafish ortholog of human AHR)-dependent in developing zebrafish exposed to TCDD. To functionally characterize wfikkn1, we made a CRISPR-Cas9 mutant line with a 16-bp deletion in wfikkn1's exon, and exposed wildtype and mutants to DMSO or TCDD. 48-hpf mRNA sequencing revealed over 700 genes that were differentially expressed (p < 0.05, log2FC > 1) between each pair of treatment combinations, suggesting an important role for wfikkn1 in altering both the 48-hpf transcriptome and TCDD-induced expression changes. Mass spectrometry-based proteomics of 48-hpf wildtype and mutants revealed 325 significant differentially expressed proteins. Functional enrichment demonstrated wfikkn1 was involved in skeletal muscle development and played a role in neurological pathways after TCDD exposure. Mutant zebrafish appeared morphologically normal but had significant behavior deficiencies at all life stages, and absence of Wfikkn1 did not significantly alter TCDD-induced behavior effects at all life stages. In conclusion, wfikkn1 did not appear to be significantly involved in TCDD's overt toxicity but is likely a necessary functional member of the AHR signaling cascade.

The aryl hydrocarbon receptor as a model PAS sensor

Proteins containing PER-ARNT-SIM (PAS) domains are commonly associated with environmental adaptation in a variety of organisms. The PAS domain is found in proteins throughout Archaea, Bacteria, and Eukarya and often binds small-molecules, supports protein-protein interactions, and transduces input signals to mediate an adaptive physiological response. Signaling events mediated by PAS sensors can occur through induced phosphorelays or genomic events that are often dependent upon PAS domain interactions. In this perspective, we briefly discuss the diversity of PAS domain containing proteins, with particular emphasis on the prototype member, the aryl hydrocarbon receptor (AHR). This ligand-activated transcription factor acts as a sensor of the chemical environment in humans and many chordates. We conclude with the idea that since mammalian PAS proteins often act through PAS-PAS dimers, undocumented interactions of this type may link biological processes that we currently think of as independent. To support this idea, we present a framework to guide future experiments aimed at fully elucidating the spectrum of PAS-PAS interactions with an eye towards understanding how they might influence environmental sensing in human and wildlife populations.

Rodent genetic models of Ah receptor signaling

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription factor that is a member of the PER-ARNT-SIM superfamily of environmental sensors. This receptor has been a molecule of interest for many years in the field of toxicology, as it was originally discovered to mediate the toxic effects of certain environmental pollutants like benzo(a)pyrene and 2,3,7,8-tetrachlorodibenzo-p-dioxin. While all animals express this protein, there is naturally occurring variability in receptor size and responsiveness to ligand. This naturally occurring variation, particularly in mice, has been an essential tool in the discovery and early characterization of the AHR. Genetic models including congenic mice and induced mutations at the Ahr locus have proven invaluable in further understanding the role of the AHR in adaptive metabolism and TCDD-induced toxicity. The creation and examination of Ahr null mice revealed an important physiological role for the AHR in vascular and hepatic development and mediation of the immune system. In this review, we attempt to provide an overview to many of the AHR models that have aided in the understanding of AHR biology thus far. We describe the naturally occurring polymorphisms, congenic models, induced mutations at the Ahr locus and at the binding partner Ah Receptor Nuclear Translocator and chaperone, Ah receptor associated 9 loci in mice, with a brief description of naturally occurring and induced mutations in rats.

Generation of an Allelic Series at the Ahr Locus Using an Edited Recombinant Approach

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor and a member of the PER-ARNT-SIM (PAS) superfamily of environmental sensors. The AHR is involved in a series of biological processes including adaptive metabolism of xenobiotics, toxicity of certain environmental pollutants, vascular development, fertility, and immune function. Mouse models, including the Ahr null and Ahr conditional null (Ahrfx) mice, are widely used for the study of AHR-mediated biology and toxicity. The Ahr conditional null mouse harbors the low-affinity Ahrd allele that exhibits approximately a 10-fold lower binding affinity for certain xenobiotic AHR ligands than the widely used C57BL/6 mouse that harbors the higher affinity Ahrb1 allele. Here, we report a novel mouse model that introduces a V375A polymorphism that converts the low-affinity allele into a high-affinity allele, offering a more sensitive conditional model. In the generation of this novel conditional allele, two additional mutants arose, including a 3-bp deletion in the PAS-B domain (AhrNG367R) and an early termination codon in the PAS-B domain (AhrTer383). The AhrNG367R allele presents as a phenocopy of the null and the AhrTer383 allele presents as an antimorph when assessing for the ductus venosus and liver lobe weight endpoints. These new models represent a series of tools that will be useful in further characterizing AHR biology.

Uraemic solutes as therapeutic targets in CKD-associated cardiovascular disease

Chronic kidney disease (CKD) is characterized by the retention of a myriad of solutes termed uraemic (or uremic) toxins, which inflict damage to several organs, including the cardiovascular system. Uraemic toxins can induce hallmarks of cardiovascular disease (CVD), such as atherothrombosis, heart failure, dysrhythmias, vessel calcification and dysregulated angiogenesis. CVD is an important driver of mortality in patients with CKD; however, reliance on conventional approaches to managing CVD risk is insufficient in these patients, underscoring a need to target risk factors that are specific to CKD. Mounting evidence suggests that targeting uraemic toxins and/or pathways induced by uraemic toxins, including tryptophan metabolites and trimethylamine N-oxide (TMAO), can lower the risk of CVD in patients with CKD. Although tangible therapies resulting from our growing knowledge of uraemic toxicity are yet to materialize, a number of pharmacological and non-pharmacological approaches have the potential to abrogate the effects of uraemic toxins, for example, by decreasing the production of uraemic toxins, by modifying metabolic pathways induced by uraemic toxins such as those controlled by aryl hydrocarbon receptor signalling and by augmenting the clearance of uraemic toxins.

The role of MEG3 in the proliferation of palatal mesenchymal cells is related to the TGFβ/Smad pathway in TCDD inducing cleft palate

Cleft palate (CP) is a common birth defect with a high incidence of occurrence in humans. The 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) is a highly toxic halogenated aromatic hydrocarbon, with a strong CP effect on mice. Increasing recent evidences have shown that long-noncoding RNAs (lncRNAs) play an important role in several diseases, including CP. However, there is a paucity of studies on the role of lncRNA MEG3 in the occurrence and development of TCDD-induced CP. In this study, the relationship between MEG3 and the proliferation of palatal mesenchymal cells and the underlying molecular mechanism were studied by establishing fetal CP with TCDD (64 μg/kg) in C57BL/6N mice. The results revealed that MEG3 was highly expressed during the critical period of CP formation and that the fetal mesenchymal proliferation was significantly inhibited at certain critical periods in the mice receiving TCDD. In addition, we noted a possibility of a crosstalk between MEG3 and the TGF-β/Smad pathway, such that the inhibition of the TGF-β/Smad pathway was induced by TCDD. Cumulatively, our study suggests that TCDD-induced CP may be caused by MEG3 inhibition of the proliferation of palatal mesenchymal cells involving the TGFβ/Smad pathway, which may provide a novel perspective to understand the pathogenesis of CP.

Genetics-Based Approach to Identify Novel Genes Regulated by the Aryl Hydrocarbon Receptor in Mouse Liver

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor in the Per-Arnt-Sim superfamily of environmental sensors that is linked to several metabolic diseases, including nonalcoholic fatty liver disease. Much remains unknown regarding the impact of genetic variation in AHR-driven disease, as past studies have focused on a small number of inbred strains. Recently, the presence of a wide range of interindividual variability amongst humans was reported in response to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the prototypical ligand of the AHR. In this study, a panel of 14 diverse mouse strains was exposed to TCDD for 10 days to characterize the AHR-mediated response across genetic backgrounds. Responses to TCDD are heavily dependent on genetic background. Although mice carry 1 of 4 Ahr alleles known to impact the affinity to AHR-ligands, we observed significant intra-allelic variability suggesting the presence of novel genetic modifiers of AHR signaling. A regression-based approach was used to scan for genes regulated by the AHR and/or associated with TCDD-induced phenotypes. The approach identified 7 genes, 2 of which are novel, that are likely regulated by the AHR based on association with hepatic TCDD burden (p ≤ .05). Finally, we identified 1 gene, Dio1, which was associated with change in percent body fat across the diverse set of strains (p ≤ .05). Overall, the results in this study exemplify the power of genetics-based approaches in identifying novel genes that are putatively regulated by the AHR.

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 reduces LC3II expression and controls endoplasmic reticulum stress

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor whose physiological function is poorly understood. The AhR is highly expressed in barrier organs such as the skin, intestine, and lung. The lungs are continuously exposed to environmental pollutants such as cigarette smoke (CS) that can induce cell death mechanisms such as apoptosis, autophagy, and endoplasmic reticulum (ER) stress. CS also contains toxicants that are AhR ligands. We have previously shown that the AhR protects against apoptosis, but whether the AhR also protects against autophagy or ER stress is not known. Using cigarette smoke extract (CSE) as our in vitro surrogate of environmental tobacco exposure, we first assessed the conversion of LC3I to LC3II, a classic feature of both autophagic and ER stress-mediated cell death pathways. LC3II was elevated in CSE-exposed lung structural cells [mouse lung fibroblasts (MLFs), MLE12 and A549 cells] when AhR was absent. However, this heightened LC3II expression could not be explained by increased expression of key autophagy genes (Gabarapl1, Becn1, Map1lc3b), upregulation of upstream autophagic machinery (Atg5-12, Atg3), or impaired autophagic flux, suggesting that LC3II may be autophagy independent. This was further supported by the absence of autophagosomes in Ahr^-/- lung cells. However, Ahr^-/- lung cells had widespread ER dilation, elevated expression of the ER stress markers CHOP and GADD34, and an accumulation of ubiquitinated proteins. These findings collectively illustrate a novel role for the AhR in attenuating ER stress by a mechanism that may be autophagy independent.

Gene and Protein Expression in Subjects With a Nystagmus-Associated AHR Mutation

Recently, a consanguineous family was identified in Israel with three children affected by Infantile Nystagmus and Foveal Hypoplasia, following an autosomal recessive mode of inheritance. A homozygous stop mutation c.1861C > T; p.Q621^∗ in the aryl hydrocarbon receptor (AHR) gene (AHR; MIM 600253) was identified that co-segregated with the disease in the larger family. AHR is the first gene to be identified causing an autosomal recessive Infantile Nystagmus-related disease in humans. The goal of this study is to delineate the molecular basis of this newly discovered human genetic disorder associated with a rare AHR gene mutation. The gene and protein expression levels of AHR and selected AHR targets from leukocyte cultures of healthy subjects and the patients were analyzed. We observed significant variation between mRNA and protein expression of CYP1A1, CYP1B1, and TiPARP under rest and AHR-induced conditions. The CYP1A1 enzymatic activity in induced leukocytes also differs significantly between the patients and healthy volunteers. Intriguingly, the heterozygous subjects demonstrate CYP1A1 and TiPARP gene and protein expression similar to homozygous patients. In contrast, CYP1B1 inducibility and expression vary between hetero- and homozygous subjects. Similarity and differences in gene and protein expression between heterozygotes and homozygous patients can give us a hint as to which metabolic pathway/s might be involved in the Nystagmus etiology. Thus, we have a unique human model for AHR deficiency that will allow us the opportunity to study the biochemical basis of this rare human mutation, as well as the involvement of AHR in other physiological processes.

ahr2, But Not ahr1a or ahr1b, Is Required for Craniofacial and Fin Development and TCDD-dependent Cardiotoxicity in Zebrafish

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that binds environmental toxicants and regulates gene expression. AHR also regulates developmental processes, like craniofacial development and hematopoiesis, in the absence of environmental exposures. Zebrafish have 3 paralogs of AHR: ahr1a, ahr1b, and ahr2. Adult zebrafish with mutations in ahr2 exhibited craniofacial and fin defects. However, the degree to which ahr1a and ahr1b influence ahr2 signaling and contribute to fin and craniofacial development are not known. We compared morphology of adult ahr2 mutants and ahr1a;ahr1b single and double mutant zebrafish. We found that ahr1a;ahr1b single and double mutants were morphologically normal whereas ahr2 mutant zebrafish demonstrated fin and craniofacial malformations. At 5 days post fertilization, both ahr1a;ahr1b and ahr2 mutant larvae were normal, suggesting that adult phenotypes are due to defects in maturation or maintenance. Next, we analyzed the function of zebrafish AHRs activated by environmental ligands. The prototypical AHR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), induces toxicity in humans and rodents via AHR and causes cardiotoxicity in zebrafish embryos. It has been shown that embryos with mutations in ahr2 are resistant to TCDD toxicity, yet it is unclear whether ahr1 receptors are required. Furthermore, though AHR was shown to interact with estrogen receptor alpha following TCDD treatment, it is not known whether this interaction is constitutive or context-dependent. To determine whether estrogen receptors are constitutive cofactors for AHR signaling, we used genetic and pharmacologic techniques to analyze TCDD-dependent toxicity in estrogen receptor and ahr mutant embryos. We found that embryos with mutations in ahr1a;ahr1b or estrogen receptor genes are susceptible to TCDD toxicity whereas ahr2 mutant embryos are TCDD-resistant. Moreover, pharmacologic blockade of nuclear estrogen receptors failed to prevent TCDD toxicity. These findings suggest that ahr1 genes do not have overlapping functions with ahr2 in fin and craniofacial development or TCDD-dependent toxicity, and that estrogen receptors are not constitutive partners of ahr2.

Transitional States in Ligand-Dependent Transformation of the Aryl Hydrocarbon Receptor into Its DNA-Binding Form

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the biological and toxicological effects of an AhR lacking the entire PASB structurally diverse chemicals, including halogenated aromatic hydrocarbons. Ligand-dependent transformation of the AhR into its DNA binding form involves a ligand-dependent conformational change, heat shock protein 90 (hsp90), dissociation from the AhR complex and AhR dimerization with the AhR nuclear translocator (ARNT) protein. The mechanism of AhR transformation was examined using mutational approaches and stabilization of the AhR:hsp90 complex with sodium molybdate. Insertion of a single mutation (F281A) in the hsp90-binding region of the AhR resulted in its constitutive (ligand-independent) transformation/DNA binding in vitro. Mutations of AhR residues within the Arg-Cys-rich region (R212A, R217A, R219A) and Asp371 (D371A) impaired AhR transformation without a significant effect on ligand binding. Stabilization of AhR:hsp90 binding with sodium molybdate decreased transformation/DNA binding of the wild type AhR but had no effect on constitutively active AhR mutants. Interestingly, transformation of the AhR in the presence of molybdate allowed detection of an intermediate transformation ternary complex containing hsp90, AhR, and ARNT. These results are consistent with a stepwise transformation mechanism in which binding of ARNT to the liganded AhR:hsp90 complex results in a progressive displacement of hsp90 and conversion of the AhR into its high affinity DNA binding form. The available molecular insights into the signaling mechanism of other Per-ARNT-Sim (PAS) domains and structural information on hsp90 association with other client proteins are consistent with the proposed transformation mechanism of the AhR.

Gene co-regulation and co-expression in the aryl hydrocarbon receptor-mediated transcriptional regulatory network in the mouse liver

Four decades after its discovery, the aryl hydrocarbon receptor (AHR), a ligand-inducible transcription factor (TF) activated by the persistent environmental contaminant 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), remains an enigmatic molecule with a controversial endogenous role. Here, we have assembled a global map of the AHR gene regulatory network in female C57BL/6 mice orally gavaged with 30 µg/kg of TCDD from a combination of previously published gene expression and genome-wide TF-binding data sets. Using Kohonen self-organizing maps and subspace clustering, we show that genes co-regulated by common upstream TFs in the AHR network exhibit a pattern of co-expression. Directly bound, indirectly bound, and non-genomic AHR target genes exhibit distinct expression patterns, with the directly bound targets associated with highest median expression. Interestingly, among the directly bound AHR target genes, the expression level increases with the number of AHR-binding sites in the proximal promoter regions. Finally, we show that co-regulated genes in the AHR network activate distinct groups of downstream biological processes. Although the specific findings described here are restricted to hepatic effects under short-term TCDD exposure, this work describes a generalizable approach to the reconstruction and analysis of transcriptional regulatory cascades underlying cellular stress response, revealing network hierarchy and the nature of information flow from the initial signaling events to phenotypic outcomes. Such reconstructed networks can form the basis of a new generation of quantitative adverse outcome pathways.

Selective Ah receptor ligands mediate enhanced SREBP1 proteolysis to restrict lipogenesis in sebocytes

The aryl hydrocarbon receptor (AHR) mediates 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) induced toxicity that can lead to chloracne in humans. A characteristic of chloracne, in contrast to acne vulgaris, is shrinkage or loss of sebaceous glands. Acne vulgaris, on the other hand, is often accompanied by excessive sebum production. Here, we examined the role of AHR in lipid synthesis in human sebocytes using distinct classes of AHR ligands. Modulation of AHR activity attenuated the expression of lipogenic genes and key pro-inflammatory markers in the absence of canonical DRE-driven transcription of the AHR target gene CYP1A1. Furthermore, topical treatment with TCDD, which mediates DRE-dependent activity, and SGA360, which fails to induce DRE-mediated responses, both exhibited a decrease in the size of sebaceous glands and the number of sebocytes within each gland in the skin. To elucidate the mechanism of AHR-mediated repression of lipid synthesis, we demonstrated that selective AHR modulators, SGA360 and SGA315 increased the protein turnover of the mature sterol regulatory element-binding protein (mSREBP-1), the principal transcriptional regulator of the fatty acid synthesis pathway. Interestingly, selective AHR ligand treatment significantly activated the AMPK-dependent kinase (AMPK) in sebocytes. Moreover, we demonstrated an inverse correlation between the active AMPK and the mSREBP-1 protein, which is consistent with the previously reported role of AMPK in inhibiting cleavage of SREBP-1. Overall, our findings indicate a DRE-independent function of selective AHR ligands in modulating lipid synthesis in human sebocytes, which might raise the possibility of using AHR as a therapeutic target for treatment of acne.

Genome-Wide Transcriptional Analysis Reveals Novel AhR Targets That Regulate Dendritic Cell Function during Influenza A Virus Infection

Activation of the ligand inducible aryl hydrocarbon receptor (AhR) during primary influenza A virus infection diminishes host responses by negatively regulating the ability of dendritic cells (DC) to prime naive CD8⁺ T cells, which reduces the generation of CTL. However, AhR-regulated genes and signaling pathways in DCs are not fully known. In this study, we used unbiased gene expression profiling to identify differentially expressed genes and signaling pathways in DCs that are modulated by AhR activation in vivo. Using the prototype AhR agonist TCDD, we identified the lectin receptor Cd209a (DC-SIGN) and chemokine Ccl17 as novel AhR target genes. We further show the percentage of DCs expressing CD209a on their surface was significantly decreased by AhR activation during infection. Whereas influenza A virus infection increased CCL17 protein levels in the lung and lung-draining lymph nodes, this was significantly reduced following AhR activation. Targeted excision of AhR in the hematopoietic compartment confirmed AhR is required for downregulation of CCL17 and CD209a. Loss of AhR's functional DNA-binding domain demonstrates that AhR activation alone is necessary but not sufficient to drive downregulation. AhR activation induced similar changes in gene expression in human monocyte-derived DCs. Analysis of the murine and human upstream regulatory regions of Cd209a and Ccl17 revealed a suite of potential transcription factor partners for AhR, which may coregulate these genes in vivo. This study highlights the breadth of AhR-regulated pathways within DCs, and that AhR likely interacts with other transcription factors to modulate DC functions during infection.

Microbiota Metabolism Promotes Synthesis of the Human Ah Receptor Agonist 2,8-Dihydroxyquinoline

The aryl hydrocarbon receptor (AHR) is a major regulator of immune function within the gastrointestinal tract. Resident microbiota are capable of influencing AHR-dependent signaling pathways via production of an array of bioactive molecules that act as AHR agonists, such as indole or indole-3-aldehyde. Bacteria produce a number of quinoline derivatives, of which some function as quorum-sensing molecules. Thus, we screened relevant hydroxyquinoline derivatives for AHR activity using AHR responsive reporter cell lines. 2,8-Dihydroxyquinoline (2,8-DHQ) was identified as a species-specific AHR agonist that exhibits full AHR agonist activity in human cell lines, but only induces modest AHR activity in mouse cells. Additional dihydroxylated quinolines tested failed to activate the human AHR. Nanomolar concentrations of 2,8-DHQ significantly induced CYP1A1 expression and, upon cotreatment with cytokines, synergistically induced IL6 expression. Ligand binding competition studies subsequently confirmed 2,8-DHQ to be a human AHR ligand. Several dihydroxyquinolines were detected in human fecal samples, with concentrations of 2,8-DHQ ranging between 0 and 3.4 pmol/mg feces. Additionally, in mice the microbiota was necessary for the presence of DHQ in cecal contents. These results suggest that microbiota-derived 2,8-DHQ would contribute to AHR activation in the human gut, and thus participate in the protective and homeostatic effects observed with gastrointestinal AHR activation.

Significance of AHR nuclear translocation sequence in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced cPLA2α activation and hydronephrosis

The aryl hydrocarbon receptor (AHR) plays a major role in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced toxicity phenotypes. TCDD bound to AHR elicits both genomic action in which target genes are transcriptionally upregulated and nongenomic action in which cytosolic phospholipase A₂α (cPLA₂α) is rapidly activated. However, how either of these actions, separately or in combination, induces toxicity phenotypes is largely unknown. In this study, we used AHR^nls/nls mice as a model in which AHR was mutated to lack nuclear translocation sequence (NLS), and AHR^d/- mice as the corresponding control. Using this model, we studied TCDD-induced alterations in cPLA₂α activation and related factors because of the pivotal roles of cPLA₂α both in AHR's nongenomic action and in regulation of causative genes of TCDD-induced hydronephrosis. Dams were orally administered TCDD at a dose of 300 µg/kg body weight on postnatal day 1, and pups subsequently exposed to TCDD via milk were examined for gene expression on PND 7 and for histological changes on PND 14. The activation of the AHR genomic action and hydronephrosis onset were observed in the control group but not in the AHR^nls/nls group. An ex vivo experiment using peritoneal macrophages exposed to 100 nM TCDD resulted in rapid activation of cPLA₂α, an indicator of the nongenomic action, only in the control group but not in the AHR^nls/nls group. These results indicated that an NLS is required for the AHR's genomic and nongenomic actions.

The Aryl Hydrocarbon Receptor and the Maintenance of Lung Health

Much of what is known about the Aryl Hydrocarbon Receptor (AhR) centers on its ability to mediate the deleterious effects of the environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin). However, the AhR is both ubiquitously-expressed and evolutionarily-conserved, suggesting that it evolved for purposes beyond strictly mediating responses to man-made environmental toxicants. There is growing evidence that the AhR is required for the maintenance of health, as it is implicated in physiological processes such as xenobiotic metabolism, organ development and immunity. Dysregulation of AhR expression and activity is also associated with a variety of disease states, particularly those at barrier organs such as the skin, gut and lungs. The lungs are particularly vulnerable to inhaled toxicants such as cigarette smoke. However, the role of the AhR in diseases such as chronic obstructive pulmonary disease (COPD)-a respiratory illness caused predominately by cigarette smoking-and lung cancer remains largely unexplored. This review will discuss the growing body of literature that provides evidence that the AhR protects the lungs against the damaging effects of cigarette smoke.

Risk for animal and human health related to the presence of dioxins and dioxin-like PCBs in feed and food

The European Commission asked EFSA for a scientific opinion on the risks for animal and human health related to the presence of dioxins (PCDD/Fs) and DL-PCBs in feed and food. The data from experimental animal and epidemiological studies were reviewed and it was decided to base the human risk assessment on effects observed in humans and to use animal data as supportive evidence. The critical effect was on semen quality, following pre- and postnatal exposure. The critical study showed a NOAEL of 7.0 pg WHO2005-TEQ/g fat in blood sampled at age 9 years based on PCDD/F-TEQs. No association was observed when including DL-PCB-TEQs. Using toxicokinetic modelling and taking into account the exposure from breastfeeding and a twofold higher intake during childhood, it was estimated that daily exposure in adolescents and adults should be below 0.25 pg TEQ/kg bw/day. The CONTAM Panel established a TWI of 2 pg TEQ/kg bw/week. With occurrence and consumption data from European countries, the mean and P95 intake of total TEQ by Adolescents, Adults, Elderly and Very Elderly varied between, respectively, 2.1 to 10.5, and 5.3 to 30.4 pg TEQ/kg bw/week, implying a considerable exceedance of the TWI. Toddlers and Other Children showed a higher exposure than older age groups, but this was accounted for when deriving the TWI. Exposure to PCDD/F-TEQ only was on average 2.4- and 2.7-fold lower for mean and P95 exposure than for total TEQ. PCDD/Fs and DL-PCBs are transferred to milk and eggs, and accumulate in fatty tissues and liver. Transfer rates and bioconcentration factors were identified for various species. The CONTAM Panel was not able to identify reference values in most farm and companion animals with the exception of NOAELs for mink, chicken and some fish species. The estimated exposure from feed for these species does not imply a risk.

Comparative In Vitro and In Silico Analysis of the Selectivity of Indirubin as a Human Ah Receptor Agonist

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that modulates gene expression following its binding and activation by structurally diverse chemicals. Species differences in AhR functionality have been observed, with the mouse AhR (mAhR) and human AhR (hAhR) exhibiting significant differences in ligand binding, coactivator recruitment, gene expression and response. While the AhR agonist indirubin (IR) is a more potent activator of hAhR-dependent gene expression than the prototypical ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), it is a significantly less potent activator of the mAhR. DNA binding analysis confirmed the greater potency/efficacy of IR in stimulating transformation/DNA binding of the hAhR in vitro and domain-swapping experiments demonstrated that the enhanced response to IR was primarily due to the hAhR ligand binding domain (LBD). Site-directed mutagenesis and functional analysis studies revealed that mutation of H326 and A349 in the mAhR LBD to the corresponding residues in the hAhR LBD significantly increased the potency of IR. Since these mutations had no significant effect on ligand binding, these residues likely contribute to an enhanced efficiency of transformation/DNA binding by IR-bound hAhR. Molecular docking to mAhR LBD homology models further elucidated the different roles of the A375V mutation in TCDD and IR binding, as revealed by [³H]TCDD competitive binding results. These results demonstrate the differential binding of structurally diverse ligands within the LBD of a given AhR and confirm that amino acid differences within the LBD of AhRs contribute to significant species differences in ligand response.

Aryl Hydrocarbon Receptor: A New Player of Pathogenesis and Therapy in Cardiovascular Diseases

The aryl hydrocarbon receptor (AhR) is a DNA binding protein that acts as a nuclear receptor mediating xenobiotic metabolism and environmental responses. Owing to the evolutionary conservation of this gene and its widespread expression in the immune and circulatory systems, AhR has for many years been almost exclusively studied by the pharmacological/toxicological field for its role in contaminant toxicity. More recently, the functions of AhR in environmental adaption have been examined in the context of the occurrence, development, and therapy of cardiovascular diseases. Increasing evidence suggests that AhR is involved in maintaining homeostasis or in triggering pathogenesis by modulating the biological responses of critical cell types in the cardiovascular system. Here, we describe the structure, distribution, and ligands of AhR and the AhR signaling pathway and review the impact of AhR on cardiovascular physiology. We also discuss the potential contribution of AhR as a new potential factor in the targeted treatment of cardiovascular diseases.

Ligand-mediated cytoplasmic retention of the Ah receptor inhibits macrophage-mediated acute inflammatory responses

The Ah receptor (AHR) has been shown to exhibit both inflammatory and anti-inflammatory activity in a context-specific manner. In vivo macrophage-driven acute inflammation models were utilized here to test whether the selective Ah receptor modulator 1-allyl-7-trifluoromethyl-1H-indazol-3-yl]-4-methoxyphenol (SGA360) would reduce inflammation. Exposure to SGA360 was capable of significantly inhibiting lipopolysaccharide (LPS)-mediated endotoxic shock in a mouse model, both in terms of lethality and attenuating inflammatory signaling in tissues. Topical exposure to SGA360 was also able to mitigate joint edema in a monosodium urate (MSU) crystal gout mouse model. Inhibition was dependent on the expression of the high-affinity allelic AHR variant in both acute inflammation models. Upon peritoneal MSU crystal exposure SGA360 pretreatment inhibited neutrophil and macrophage migration into the peritoneum. RNA-seq analysis revealed that SGA360 attenuated the expression of numerous inflammatory genes and genes known to be directly regulated by AHR in thioglycolate-elicited primary peritoneal macrophages treated with LPS. In addition, expression of the high-affinity allelic AHR variant in cultured macrophages was necessary for SGA360-mediated repression of inflammatory gene expression. Mechanistic studies revealed that SGA360 failed to induce nuclear translocation of the AHR and actually enhanced cytoplasmic localization. LPS treatment of macrophages enhanced the occupancy of the AHR and p65 to the Ptgs2 promoter, whereas SGA360 attenuated occupancy. AHR ligand activity was detected in peritoneal exudates isolated from MSU-treated mice, thus suggesting that the anti-inflammatory activity of SGA360 is mediated at least in part through AHR antagonism of endogenous agonist activity. These results underscore an important role of the AHR in participating in acute inflammatory signaling and warrants further investigations into possible clinical applications.

Loss of liver-specific and sexually dimorphic gene expression by aryl hydrocarbon receptor activation in C57BL/6 mice

The aryl hydrocarbon receptor (AhR) is a highly conserved transcription factor that mediates a broad spectrum of species-, strain-, sex-, age-, tissue-, and cell-specific responses elicited by structurally diverse ligands including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Dose-dependent effects on liver-specific and sexually dimorphic gene expression were examined in male and female mice gavaged with TCDD every 4 days for 28 or 92 days. RNA-seq data revealed the coordinated repression of 181 genes predominately expressed in the liver including albumin (3.7-fold), α-fibrinogen (14.5-fold), and β-fibrinogen (17.4-fold) in males with corresponding AhR enrichment at 2 hr. Liver-specific genes exhibiting sexually dimorphic expression also demonstrated diminished divergence between sexes. For example, male-biased Gstp1 was repressed 3.0-fold in males and induced 4.5-fold in females, which were confirmed at the protein level. Disrupted regulation is consistent with impaired GHR-JAK2-STAT5 signaling and inhibition of female specific CUX2-mediated transcription as well as the repression of other key transcriptional regulators including Ghr, Stat5b, Bcl6, Hnf4a, Hnf6, Foxa1/2/3, and Zhx2. Attenuated liver-specific and sexually dimorphic gene expression was concurrent with the induction of fetal genes such as alpha-fetoprotein. The results suggest AhR activation causes the loss of liver-specific and sexually dimorphic gene expression producing a functionally "de-differentiated" hepatic phenotype.

Structural modeling of the AhR:ARNT complex in the bHLH-PASA-PASB region elucidates the key determinants of dimerization

Elucidation of the dimerization process of the aryl hydrocarbon receptor (AhR) with the AhR nuclear translocator (ARNT) is crucial for understanding the mechanisms underlying the functional activity of AhR, including mediation of the toxicity of environmental contaminants. In this work, for the first time a structural model of the AhR:ARNT dimer encompassing the entire bHLH-PASA-PASB domain region is proposed. It is developed by using a template-based modeling approach, relying on the recently available crystallographic structures of two dimers of homologous systems in the bHLH-PAS family of proteins: the CLOCK:BMAL1 and the HIF2α:ARNT heterodimers. The structural and energetic characteristics of the modeled AhR:ARNT protein-protein interface are determined by evaluating the variations in solvent accessible surface area, the total binding free energy and the per-residue free energy contributions obtained by the MM-GBSA method and the Energy Decomposition Analysis. The analyses of the intricate network of inter-domain interactions at the dimerization interfaces provide insights into the key determinants of dimerization. These are confirmed by comparison of the computational findings with the available experimental mutagenesis and functional analysis data. The results presented here on the AhR:ARNT dimer structure and interactions provide a framework to start analyzing the mechanism of AhR transformation into its functional DNA binding form.

When evolution is the solution to pollution: Key principles, and lessons from rapid repeated adaptation of killifish (Fundulus heteroclitus) populations

For most species, evolutionary adaptation is not expected to be sufficiently rapid to buffer the effects of human‐mediated environmental changes, including environmental pollution. Here we review how key features of populations, the characteristics of environmental pollution, and the genetic architecture underlying adaptive traits, may interact to shape the likelihood of evolutionary rescue from pollution. Large populations of Atlantic killifish (Fundulus heteroclitus) persist in some of the most contaminated estuaries of the United States, and killifish studies have provided some of the first insights into the types of genomic changes that enable rapid evolutionary rescue from complexly degraded environments. We describe how selection by industrial pollutants and other stressors has acted on multiple populations of killifish and posit that extreme nucleotide diversity uniquely positions this species for successful evolutionary adaptation. Mechanistic studies have identified some of the genetic underpinnings of adaptation to a well‐studied class of toxic pollutants; however, multiple genetic regions under selection in wild populations seem to reflect more complex responses to diverse native stressors and/or compensatory responses to primary adaptation. The discovery of these pollution‐adapted killifish populations suggests that the evolutionary influence of anthropogenic stressors as selective agents occurs widely. Yet adaptation to chemical pollution in terrestrial and aquatic vertebrate wildlife may rarely be a successful “solution to pollution” because potentially adaptive phenotypes may be complex and incur fitness costs, and therefore be unlikely to evolve quickly enough, especially in species with small population sizes.

Diversity as Opportunity: Insights from 600 Million Years of AHR Evolution

The aryl hydrocarbon receptor (AHR) was for many years of interest only to pharmacologists and toxicologists. However, this protein has fundamental roles in biology that are being revealed through studies in diverse animal species. The AHR is an ancient protein. AHR homologs exist in most major groups of modern bilaterian animals, including deuterostomes (chordates, hemichordates, echinoderms) and the two major clades of protostome invertebrates [ecdysozoans (e.g. arthropods and nematodes) and lophotrochozoans (e.g. molluscs and annelids)]. AHR homologs also have been identified in cnidarians such as the sea anemone Nematostella and in the genome of Trichoplax, a placozoan. Bilaterians, cnidarians, and placozoans form the clade Eumetazoa, whose last common ancestor lived approximately 600 million years ago (MYA). The presence of AHR homologs in modern representatives of all these groups indicates that the original eumetazoan animal possessed an AHR homolog. Studies in invertebrates and vertebrates reveal parallel functions of AHR in the development and function of sensory neural systems, suggesting that these may be ancestral roles. Vertebrate animals are characterized by the expansion and diversification of AHRs, via gene and genome duplications, from the ancestral protoAHR into at least five classes of AHR-like proteins: AHR, AHR1, AHR2, AHR3, and AHRR. The evolution of multiple AHRs in vertebrates coincided with the acquisition of high-affinity binding of halogenated and polynuclear aromatic hydrocarbons and the emergence of adaptive functions involving regulation of xenobiotic-metabolizing enzymes and roles in adaptive immunity. The existence of multiple AHRs may have facilitated subfunction partitioning and specialization of specific AHR types in some taxa. Additional research in diverse model and non-model species will continue to enrich our understanding of AHR and its pleiotropic roles in biology and toxicology.

Compendium of TCDD-mediated transcriptomic response datasets in mammalian model systems

Background

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is the most potent congener of the dioxin class of environmental contaminants. Exposure to TCDD causes a wide range of toxic outcomes, ranging from chloracne to acute lethality. The severity of toxicity is highly dependent on the aryl hydrocarbon receptor (AHR). Binding of TCDD to the AHR leads to changes in transcription of numerous genes. Studies evaluating the transcriptional changes brought on by TCDD may provide valuable insight into the role of the AHR in human health and disease. We therefore compiled a collection of transcriptomic datasets that can be used to aid the scientific community in better understanding the transcriptional effects of ligand-activated AHR.

Results

Specifically, we have created a datasets package – TCDD.Transcriptomics – for the R statistical environment, consisting of 63 unique experiments comprising 377 samples, including various combinations of 3 species (human derived cell lines, mouse and rat), 4 tissue types (liver, kidney, white adipose tissue and hypothalamus) and a wide range of TCDD exposure times and doses. These datasets have been fully standardized using consistent preprocessing and annotation packages (available as of September 14, 2015). To demonstrate the utility of this R package, a subset of “AHR-core” genes were evaluated across the included datasets. Ahrr, Nqo1 and members of the Cyp family were significantly induced following exposure to TCDD across the studies as expected while Aldh3a1 was induced specifically in rat liver. Inmt was altered only in liver tissue and primarily by rat-AHR.

Conclusions

Analysis of the “AHR-core” genes demonstrates a continued need for studies surrounding the impact of AHR-activity on the transcriptome; genes believed to be consistently regulated by ligand-activated AHR show surprisingly little overlap across species and tissues. Until now, a comprehensive assessment of the transcriptome across these studies was challenging due to differences in array platforms, processing methods and annotation versions. We believe that this package, which is freely available for download (http://labs.oicr.on.ca/boutros-lab/tcdd-transcriptomics) will prove to be a highly beneficial resource to the scientific community evaluating the effects of TCDD exposure as well as the variety of functions of the AHR.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3446-z) contains supplementary material, which is available to authorized users.

Molecular cloning and characterization of the aryl hydrocarbon receptors and aryl hydrocarbon receptor nuclear translocators in the American alligator

Aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, binds to a variety of chemical compounds including various environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. This receptor regulates expression of target genes through dimerization with the AHR nuclear translocator (ARNT). Since AHR-ARNT signaling pathways differ among species, characterization of AHR and ARNT is important to assess the effects of environmental contamination and for understanding the molecular mechanism underlying the intrinsic function. In this study, we isolated the cDNAs encoding three types of AHR and two types of ARNT from a reptile, the American alligator (Alligator mississippiensis). In vitro reporter gene assays showed that all complexes of alligator AHR-ARNT were able to activate ligand-dependent transcription on a xenobiotic response element. We found that AHR-ARNT complexes had higher sensitivities to a ligand than AHR-ARNT2 complexes. Alligator AHR1B showed the highest sensitivity in transcriptional activation induced by indigo when compared with AHR1A and AHR2. Taken together, our data revealed that all three alligator AHRs and two ARNTs were functional in the AHR signaling pathway with ligand-dependent and isoform-specific transactivations in vitro.

Antioxidant Functions of the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a transcription factor belonging to the basic helix-loop-helix/PER-ARNT-SIM family. It is activated by a variety of ligands, such as environmental contaminants like polycyclic aromatic hydrocarbons or dioxins, but also by naturally occurring compounds and endogenous ligands. Binding of the ligand leads to dimerization of the AhR with aryl hydrocarbon receptor nuclear translocator (ARNT) and transcriptional activation of several xenobiotic phase I and phase II metabolizing enzymes. It is generally accepted that the toxic responses of polycyclic aromatic hydrocarbons, dioxins, and structurally related compounds are mediated by activation of the AhR. A multitude of studies indicate that the AhR operates beyond xenobiotic metabolism and exerts pleiotropic functions. Increasing evidence points to a protective role of the AhR against carcinogenesis and oxidative stress. Herein, I will highlight data demonstrating a causal role of the AhR in the antioxidant response and present novel findings on potential AhR-mediated antioxidative mechanisms.

Gene Expression Profiling Identifies Cell Proliferation and Inflammation as the Predominant Pathways Regulated by Aryl Hydrocarbon Receptor in Primary Human Fetal Lung Cells Exposed to Hyperoxia

Exposure to hyperoxia contributes to the development of bronchopulmonary dysplasia (BPD) in premature infants. We observed that aryl hydrocarbon receptor (AhR) signaling protects newborn mice and primary fetal human pulmonary microvascular endothelial cells (HPMECs) against hyperoxic injury. Additionally, a recent genome-wide transcriptome study in a newborn mouse model of BPD identified AhR as a key regulator of hyperoxia-induced gene dysregulation. Whether the AhR similarly deregulates genes in HPMEC is unknown. Therefore, the objective of this study was to characterize transcriptome level gene expression profile in AhR-sufficient and -deficient HPMEC exposed to normoxic and hyperoxic conditions. Global gene expression profiling was performed using Illumina microarray platform and selected genes were validated by real-time RT-PCR. AhR gene expression and hyperoxia independently affected the expression of 540 and 593 genes, respectively. Two-way ANOVA further identified 85 genes that were affected by an interaction between AhR expression and exposure to hyperoxia. Kyoto Encyclopedia of Genes and Genomes (KEGG), Gene Ontology, and Reactome pathway analysis identified cell proliferation, immune function, cytokine signaling, and organ development as the major pathways affected in AhR-deficient cells. The biological processes that were significantly enriched by hyperoxia included metabolic process, stress response, signal transduction, cell cycle, and immune regulation. Cell cycle was the predominant pathway affected by the combined effect of AhR knockdown and hyperoxia. Functional analysis of cell cycle showed that AhR-deficient cells had decreased proliferation compared with AhR-sufficient cells. These findings suggest that AhR modulates hyperoxic lung injury by regulating the genes that are necessary for cell proliferation and inflammation.

The aryl hydrocarbon receptor suppresses cigarette-smoke-induced oxidative stress in association with dioxin response element (DRE)-independent regulation of sulfiredoxin 1

The aryl hydrocarbon receptor (AhR) is a ubiquitously expressed receptor/transcription factor that mediates toxicological responses of environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Emerging evidence indicates that the AhR suppresses apoptosis and proliferation independent of exogenous ligands, including suppression of apoptosis by cigarette smoke, a key risk factor for chronic obstructive pulmonary disease (COPD). As cigarette smoke is a potent inducer of oxidative stress, a feature that may contribute to the development of COPD, we hypothesized that the AhR prevents smoke-induced apoptosis by regulating oxidative stress. Utilizing primary lung fibroblasts derived from AhR(+/+) and AhR(-/-) mice as well as A549 human lung adenocarcinoma cells deficient in AhR expression (A549-AhR(ko)), we first show that AhR(-/-) fibroblasts and A549-AhR(ko) epithelial cells have a significant increase in cigarette smoke extract (CSE)-induced oxidative stress compared to wild type. CSE induced a significant increase in the mRNA expression of key antioxidant genes, including Nqo1 and Srxn1, predominantly in AhR(+/+) fibroblasts, with significantly less induction in AhR(-/-) cells. The induction of Srxn1, but not Nqo1, was independent of dioxin-response element (DRE) binding as AhR(DBD/DBD) lung fibroblasts, which express an AhR incapable of binding the DRE, increased Srxn1 to a degree similar to wild-type cells in response to CSE. There was no difference in Nrf2 expression or activation based on AhR expression. Lung fibroblasts derived from COPD subjects have significantly less AhR protein expression compared with both never-smokers (Normal) and smokers (At Risk). Consequently, COPD-derived fibroblasts were less robust in their induction of both Nqo1 and Srxn1 mRNA after exposure to CSE, which also failed to activate the AhR in the COPD fibroblasts. Taken together, these results support a new role for the AhR in regulating antioxidant defense in lung structural cells, such that low AhR expression may facilitate the development or progression of COPD.

Male and female mice show significant differences in hepatic transcriptomic response to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Background

2,3,7,8–tetrachlorodibenzo-p-dixion (TCDD) is the most potent of the dioxin congeners, capable of causing a wide range of toxic effects across numerous animal models. Previous studies have demonstrated that males and females of the same species can display divergent sensitivity phenotypes to TCDD toxicities. Although it is now clear that most TCDD-induced toxic outcomes are mediated by the aryl hydrocarbon receptor (AHR), the mechanism of differential responses to TCDD exposure between sexes remains largely unknown. To investigate the differential sensitivities in male and female mice, we profiled the hepatic transcriptomic responses 4 days following exposure to various amounts of TCDD (125, 250, 500 or 1000 μg/kg) in adult male and female C57BL/6Kuo mice.

Results

Several key findings were revealed by our study. 1) Hepatic transcriptomes varied significantly between the sexes at all doses examined. 2) The liver transcriptome of males was more dysregulated by TCDD than that of females. 3) The alteration of “AHR-core” genes was consistent in magnitude, regardless of sex. 4) A subset of genes demonstrated sex-dependent TCDD-induced transcriptional changes, including Fmo3 and Nr1i3, which were significantly induced in livers of male mice only. In addition, a meta-analysis was performed to contrast transcriptomic profiles of various organisms and tissues following exposure to equitoxic doses of TCDD. Minimal overlap was observed in the differences between TCDD-sensitive or TCDD-resistant models.

Conclusions

Sex-dependent sensitivities to TCDD exposure are associated with a set of sex-specific TCDD-responsive genes. In addition, complex interactions between the aryl hydrocarbon and sex hormone receptors may affect the observable differences in sensitivity phenotypes between the sexes. Further work is necessary to better understand the roles of those genes altered by TCDD in a sex-dependent manner, and their association with changes to sex hormones and receptors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1840-6) contains supplementary material, which is available to authorized users.

Aryl Hydrocarbon Receptor Activation Synergistically Induces Lipopolysaccharide-Mediated Expression of Proinflammatory Chemokine (c-c motif) Ligand 20

The Ah receptor (AHR) is directly involved in the regulation of both innate and adaptive immunity. However, these activities are poorly understood at the level of gene regulation. The chemokine (c-c motif) ligand 20 (CCL20) plays a nonredundant role in the chemoattraction of C-C motif receptor 6 expressing cells (eg, T cells and others). A survey of promoter regions of chemokine genes revealed that there are several putative dioxin responsive elements in the mouse Ccl20 promoter. The addition of an AHR agonist along with lipopolysaccharide (LPS) to cultured primary peritoneal macrophages results in synergistic induction of both Ccl20 mRNA and protein, compared with each compound alone. Through the use of macrophage cultures derived from Ahr(-) (/) (-) and Ahr(nls/nls) mice, it was established that expression of the AHR and its ability to translocate into the nucleus are necessary for AHR ligand-mediated synergistic induction of Ccl20. Gel shift analysis determined that a potent tandem AHR binding site ~3.1 kb upstream from the transcriptional start site can efficiently bind the AHR/ARNT (aryl hydrocarbon receptor/AHR nuclear translocator) heterodimer upon activation with a number of AHR agonists. Furthermore, studies reveal that LPS increases AHR levels on the Ccl20 promoter while decreasing HDAC1 occupancy. The level of Ccl20 constitutive expression in the colon is greatly attenuated in Ahr(-) (/) (-) mice. These studies suggest that the presence of AHR ligands during localized inflammation may augment chemokine expression, thus participating in the overall response to pathogens.

Changing the face of kynurenines and neurotoxicity: therapeutic considerations

Kynurenines are the products of tryptophan metabolism. Among them, kynurenine and kynurenic acid are generally thought to have neuroprotective properties, while 3-hydroxykynurenine, 3-hydroxyanthranilic acid and quinolinic acid are considered neurotoxic. They participate in immunoregulation and inflammation and possess pro- or anti-excitotoxic properties, and their involvement in oxidative stress has also been suggested. Consequently, it is not surprising that kynurenines have been closely related to neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis and multiple sclerosis. More information about the less-known metabolites, picolinic and cinnabarinic acid, evaluation of new receptorial targets, such as aryl-hydrocarbon receptors, and intensive research on the field of the immunomodulatory function of kynurenines delineated the high importance of this pathway in general homeostasis. Emerging knowledge about the kynurenine pathway provides new target points for the development of therapeutical solutions against neurodegenerative diseases.

Cross-species transcriptomic analysis elucidates constitutive aryl hydrocarbon receptor activity

Background

Research on the aryl hydrocarbon receptor (AHR) has largely focused on variations in toxic outcomes resulting from its activation by halogenated aromatic hydrocarbons. But the AHR also plays key roles in regulating pathways critical for development, and after decades of research the mechanisms underlying physiological regulation by the AHR remain poorly characterized. Previous studies identified several core genes that respond to xenobiotic AHR ligands across a broad range of species and tissues. However, only limited inferences have been made regarding its role in regulating constitutive gene activity, i.e. in the absence of exogenous ligands. To address this, we profiled transcriptomic variations between AHR-active and AHR-less-active animals in the absence of an exogenous agonist across five tissues, three of which came from rats (hypothalamus, white adipose and liver) and two of which came from mice (kidney and liver). Because AHR status alone has been shown sufficient to alter transcriptomic responses, we reason that by contrasting profiles amongst AHR-variant animals, we may elucidate effects of the AHR on constitutive mRNA abundances.

Results

We found significantly more overlap in constitutive mRNA abundances amongst tissues within the same species than from tissues between species and identified 13 genes (Agt, Car3, Creg1, Ctsc, E2f6, Enpp1, Gatm, Gstm4, Kcnj8, Me1, Pdk1, Slc35a3, and Sqrdl) that are affected by AHR-status in four of five tissues. One gene, Creg1, was significantly up-regulated in all AHR-less-active animals. We also find greater overlap between tissues at the pathway level than at the gene level, suggesting coherency to the AHR signalling response within these processes. Analysis of regulatory motifs suggests that the AHR mostly mediates transcriptional regulation via direct binding to response elements.

Conclusions

These findings, though preliminary, present a platform for further evaluating the role of the AHR in regulation of constitutive mRNA levels and physiologic function.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1053) contains supplementary material, which is available to authorized users.

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.

Cardiac myocyte-specific AHR activation phenocopies TCDD-induced toxicity in zebrafish

Exposure of zebrafish embryos to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) activates the zebrafish aryl hydrocarbon receptor 2 (AHR) to produce developmental and cardiovascular toxicity. AHR is found in the heart; however, AHR activation by TCDD is not confined to the heart and occurs throughout the organism. In order to understand the cause of cardiotoxicity, we constructed a constitutively active AHR (caAHR) based on the zebrafish AHR2 and expressed it specifically in cardiomyocytes. We show that AHR activation within the cardiomyocytes can account for the heart failure induced by TCDD. Expression of the caAHR within the heart produced cardiac malformations, loss of circulation, and pericardial edema. The heart-specific activation of AHR reproduced several other well-characterized endpoints of TCDD toxicity outside of the cardiovascular system, including defects in swim bladder and craniofacial development. This work identifies a single cellular site of TCDD action, the myocardial cell, that can account for the severe cardiovascular collapse observed following early life stage exposure to TCDD, and contributes to other forms of toxicity.

Aryl hydrocarbon receptor-dependent regulation of miR-196a expression controls lung fibroblast apoptosis but not proliferation

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor implicated in the regulation of apoptosis and proliferation. Although activation of the AhR by xenobiotics such as dioxin inhibits the cell cycle and control apoptosis, paradoxically, AhR expression also promotes cell proliferation and survival independent of exogenous ligands. The microRNA (miRNA) miR-196a has also emerged as a regulator of proliferation and apoptosis but a relationship between the AhR and miR-196a is not known. Therefore, we hypothesized that AhR-dependent regulation of endogenous miR-196a expression would promote cell survival and proliferation. Utilizing lung fibroblasts from AhR deficient (AhR(-/-)) and wild-type (AhR(+/+)) mice, we show that there is ligand-independent regulation of miRNA, including low miR-196a in AhR(-/-) cells. Validation by qRT-PCR revealed a significant decrease in basal expression of miR-196a in AhR(-/-) compared to AhR(+/+) cells. Exposure to AhR agonists benzo[a]pyrene (B[a]P) and FICZ as well as AhR antagonist CH-223191 decreased miR-196a expression in AhR(+/+) fibroblasts concomitant with decreased AhR protein levels. There was increased proliferation only in AhR(+/+) lung fibroblasts in response to serum, corresponding to a decrease in p27(KIP1) protein, a cyclin-dependent kinase inhibitor. Increasing the cellular levels of miR-196a had no effect on proliferation or expression of p27(KIP1) in AhR(-/-) fibroblasts but attenuated cigarette smoke-induced apoptosis. This study provides the first evidence that AhR expression is essential for the physiological regulation of cellular miRNA levels- including miR-196a. Future experiments designed to elucidate the functional relationship between the AhR and miR-196a may delineate additional novel ligand-independent roles for the AhR.

New insights into the role of the aryl hydrocarbon receptor in the function of CD11c⁺ cells during respiratory viral infection

The aryl hydrocarbon receptor (AHR) has garnered considerable attention as a modulator of CD4(+) cell lineage development and function. It also regulates antiviral CD8(+) T-cell responses, but via indirect mechanisms that have yet to be determined. Here, we show that during acute influenza virus infection, AHR activation skews dendritic-cell (DC) subsets in the lung-draining lymph nodes, such that there are fewer conventional CD103(+) DCs and CD11b(+) DCs. Sorting DC subsets reveals AHR activation reduces immunostimulatory function of CD103(+) DCs in the mediastinal lymph nodes, and decreases their frequency in the lung. DNA-binding domain Ahr mutants demonstrate that alterations in DC subsets require the ligand-activated AHR to contain its inherent DNA-binding domain. To evaluate the intrinsic role of AHR in DCs, conditional knockouts were created using Cre-LoxP technology, which revealed that AHR in CD11c(+) cells plays a key role in controlling the acquisition of effector CD8(+) T cells in the infected lung. However, AHR within other leukocyte lineages contributes to diminished naïve CD8(+) T-cell activation in the draining lymphoid nodes. These findings indicate DCs are among the direct targets of AHR ligands in vivo, and AHR signaling modifies host responses to a common respiratory pathogen by affecting the complex interplay of multiple cell types.