Complete structural characterisation of the human aryl hydrocarbon receptor gene

In Silico Exploration of AHR-HIF Pathway Interplay: Implications for Therapeutic Targeting in ccRCC

The oxygen-sensing pathway is a crucial regulatory circuit that defines cellular conditions and is extensively exploited in cancer development. Pathogenic mutations in the von Hippel-Lindau (VHL) tumour suppressor impair its role as a master regulator of hypoxia-inducible factors (HIFs), leading to constitutive HIF activation and uncontrolled angiogenesis, increasing the risk of developing clear cell renal cell carcinoma (ccRCC). HIF hyperactivation can sequester HIF-1β, preventing the aryl hydrocarbon receptor (AHR) from correctly activating gene expression in response to endogenous and exogenous ligands such as TCDD (dioxins). In this study, we used protein-protein interaction networks and gene expression profiling to characterize the impact of VHL loss on AHR activity. Our findings reveal specific expression patterns of AHR interactors following exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and in ccRCC. We identified several AHR interactors significantly associated with poor survival rates in ccRCC patients. Notably, the upregulation of the androgen receptor (AR) and retinoblastoma-associated protein (RB1) by TCDD, coupled with their respective downregulation in ccRCC and association with poor survival rates, suggests novel therapeutic targets. The strategic activation of the AHR via selective AHR modulators (SAhRMs) could stimulate its anticancer activity, specifically targeting RB1 and AR to reduce cell cycle progression and metastasis formation in ccRCC. Our study provides comprehensive insights into the complex interplay between the AHR and HIF pathways in ccRCC pathogenesis, offering novel strategies for targeted therapeutic interventions.

The Ah Receptor: Adaptive Metabolism, Ligand Diversity, and the Xenokine Model

The Ah receptor (AHR) has been studied for almost five decades. Yet, we still have many important questions about its role in normal physiology and development. Moreover, we still do not fully understand how this protein mediates the adverse effects of a variety of environmental pollutants, such as the polycyclic aromatic hydrocarbons (PAHs), the chlorinated dibenzo-p-dioxins ("dioxins"), and many polyhalogenated biphenyls. To provide a platform for future research, we provide the historical underpinnings of our current state of knowledge about AHR signal transduction, identify a few areas of needed research, and then develop concepts such as adaptive metabolism, ligand structural diversity, and the importance of proligands in receptor activation. We finish with a discussion of the cognate physiological role of the AHR, our perspective on why this receptor is so highly conserved, and how we might think about its cognate ligands in the future.

The emerging role of aryl hydrocarbon receptor in the activation and differentiation of Th17 cells

The aryl hydrocarbon receptor (AHR) is a cytoplasmic transcription factor, which plays an essential role in the xenobiotic metabolism in a wide variety of cells. The AHR gene is evolutionarily conserved and it has a central role not only in the differentiation and maturation of many tissues, but also in the toxicological metabolism of the cell by the activation of metabolizing enzymes. Several lines of evidence support that both AHR agonists and antagonists have profound immunological effects; and recently, the AHR has been implicated in antibacterial host defense. According to recent studies, the AHR is essential for the differentiation and activation of T helper 17 (Th17) cells. It is well known that Th17 cells have a central role in the development of inflammation, which is crucial in the defense against pathogens. In addition, Th17 cells play a major role in the pathogenesis of several autoimmune diseases such as rheumatoid arthritis. Therefore, the AHR may provide connection between the environmental chemicals, the immune regulation, and autoimmunity. In the present review, we summarize the role of the AHR in the Th17 cell functions.

Quantitative analysis of the Ah receptor/cytochrome P450 CYP1B1/CYP1A1 signalling pathway

Cytochrome P450 (CYP) drug metabolising enzymes CYP1A1 and CYP1B1 are regulated through the ligand-activated aryl hydrocarbon (Ah) receptor. Differential expression of CYP1A1 and CYP1B1 mRNA and protein has previously been reported in human tissues with the presence of the message often extrapolated to indicate the presence of protein. The aim of this study was to clarify these potentially misleading findings, by analysing components of the Ah receptor pathway (CYP1B1, CYP1A1, Ah receptor and ARNT) using a combination of quantitative real-time RT-PCR and immunoblotting. Three human cell lines (MOG-G-CCM, MCF7 and HEPG2) known to differentially express CYP1A1 and CYP1B1 mRNA and protein were exposed to the Ah receptor agonist 3-MC, and basal and inducible levels of CYP1A1, CYP1B1, Ah receptor and ARNT were determined. The key finding of this study was the demonstration of equivalent levels of CYP1B1 mRNA in both the treated and untreated MOG-G-CCM cell lines, with expression of the corresponding CYP1B1 protein only after exposure to an Ah receptor agonist. This finding suggests that a post-transcriptional mechanism is involved in the regulation of CYP1B1. In addition, the expression pattern of CYP1B1 mRNA and protein in the MOG-G-CCM cells highlights this cell line as a potential model for studying CYP1B1 expression in human tissue.

An aryl hydrocarbon receptor (AHR) homologue from the soft-shell clam, Mya arenaria: evidence that invertebrate AHR homologues lack 2,3,7,8-tetrachlorodibenzo-p-dioxin and beta-naphthoflavone binding

The aryl hydrocarbon receptor (AHR) mediates numerous toxic effects following exposure of vertebrate animals to certain aromatic environmental contaminants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To investigate possible effects of TCDD on invertebrates, a cDNA encoding an AHR homologue was cloned from the soft-shell clam, Mya arenaria. The predicted amino acid sequence contains regions characteristic of vertebrate AHRs: basic helix-loop-helix (bHLH) and PER-ARNT-SIM (PAS) domains and a glutamine-rich region. Phylogenetic analysis shows that the clam AHR sequence groups within the AHR subfamily of the bHLH-PAS family, in a clade containing AHR homologues from Drosophila melanogaster and Caenorhabditis elegans. AHR mRNA expression was detected in all tissue types tested: adductor muscle, digestive gland, foot, gill, gonad, mantle, and siphon. The in vitro-expressed clam AHR exhibited sequence-specific interactions with a mammalian xenobiotic response element (XRE). Velocity sedimentation analysis using either in vitro-expressed clam AHR or clam cytosolic proteins showed that this AHR homologue binds neither [(3)H]TCDD nor [(3)H]beta-naphthoflavone (BNF). Similarly, in vitro-expressed D. melanogaster and C. elegans AHR homologues lacked specific binding of these compounds. Thus, the absence of specific, high-affinity binding of the prototypical AHR ligands TCDD and BNF, is a property shared by known invertebrate AHR homologues, distinguishing them from vertebrate AHRs. Comparative studies of phylogenetically diverse organisms may help identify an endogenous ligand(s) and the physiological role(s) for this protein.

Characterization of the aromatic hydrocarbon receptor gene and its expression in Atlantic tomcod

Cytochrome P4501A1 (CYP1A1) mRNA is not inducible in Atlantic tomcod from the Hudson River that are treated with halogenated aromatic hydrocarbons (HAHs). In contrast, CYP1A1 mRNA is inducible in Hudson River tomcod that are treated with polycyclic aromatic hydrocarbons (PAHs) and in tomcod that are collected from cleaner rivers and treated with HAHs or PAHs. We hypothesize that CYP1A1 transcription is inhibited in Hudson River tomcod because of down-regulation of the aromatic hydrocarbon receptor (AhR) pathway and that separate molecular pathways modulate CYP1A1 transcription in fish treated with HAHs and PAHs. We initially evaluated levels of hepatic nuclear protein binding at enhancer elements (DREs) in the regulatory region of tomcod CYP1A1. No difference in levels of protein binding was observed between tomcod from the Hudson and Miramichi (cleaner) rivers that were untreated or were treated with benzo[a]pyrene. In contrast, levels of protein binding were lower in tomcod from the Hudson River that were treated with TCB than in similarly treated fish from the Miramichi River, suggesting differences between the populations in the structure or expression of AhR pathway molecules. To address this possibility, AhR DNA sequences were characterized from tomcod cDNA and genomic DNA libraries. In tomcod and mammals, AhR is represented by 11 exons, overall peptide sizes are similar, and amino acid sequences at basic, helix-loop-helix, PAAS A, and PAAS B domains are highly conserved. In contrast, little similarity was observed between tomcod and mammals in the sizes or sequences of AhR exons 10 and 11, including the absence in tomcod of glutamine-rich domains. No differences in levels of hepatic AhR mRNA were observed between the two populations or treatment groups when tomcod were untreated or were treated with aromatic hydrocarbons. In contrast, variation in levels of AhR mRNA expression was observed among tomcod tissues; however, no relationship was observed between levels of AhR mRNAs and CYP1A1 mRNAs in tissues from chemically or vehicle control-treated fish. RFLP analysis revealed extensive variation in exons 10 and 11 of AhR cDNA among tomcod from different rivers. Our results suggest that variation between tomcod populations in CYP1A1 mRNA inducibility is reflected by differences in levels of inducible hepatic protein binding to DREs. However, levels of hepatic AhR mRNA are not down-regulated in the Hudson River population, are not affected by AH treatments, and levels of AhR mRNA expression are not responsible for the differential inducibility of CYP1A1 transcription.