The Q-rich subdomain of the human Ah receptor transactivation domain is required for dioxin-mediated transcriptional activity

The Impact of the Aryl Hydrocarbon Receptor on Antenatal Chemical Exposure-Induced Cardiovascular-Kidney-Metabolic Programming

Early life exposure lays the groundwork for the risk of developing cardiovascular-kidney-metabolic (CKM) syndrome in adulthood. Various environmental chemicals to which pregnant mothers are commonly exposed can disrupt fetal programming, leading to a wide range of CKM phenotypes. The aryl hydrocarbon receptor (AHR) has a key role as a ligand-activated transcription factor in sensing these environmental chemicals. Activating AHR through exposure to environmental chemicals has been documented for its adverse impacts on cardiovascular diseases, hypertension, diabetes, obesity, kidney disease, and non-alcoholic fatty liver disease, as evidenced by both epidemiological and animal studies. In this review, we compile current human evidence and findings from animal models that support the connection between antenatal chemical exposures and CKM programming, focusing particularly on AHR signaling. Additionally, we explore potential AHR modulators aimed at preventing CKM syndrome. As the pioneering review to present evidence advocating for the avoidance of toxic chemical exposure during pregnancy and deepening our understanding of AHR signaling, this has the potential to mitigate the global burden of CKM syndrome in the future.

The intrinsically disordered transcriptional activation domain of CIITA is functionally tuneable by single substitutions: An exception or a new paradigm?

During protein evolution, some amino acid substitutions modulate protein function ("tuneability"). In most proteins, the tuneable range is wide and can be sampled by a set of protein variants that each contains multiple amino acid substitutions. In other proteins, the full tuneable range can be accessed by a set of variants that each contains a single substitution. Indeed, in some globular proteins, the full tuneable range can be accessed by the set of site-saturating substitutions at an individual "rheostat" position. However, in proteins with intrinsically disordered regions (IDRs), most functional studies-which would also detect tuneability-used multiple substitutions or small deletions. In disordered transcriptional activation domains (ADs), studies with multiple substitutions led to the "acidic exposure" model, which does not anticipate the existence of rheostat positions. In the few studies that did assess effects of single substitutions on AD function, results were mixed: the ADs of two full-length transcription factors did not show tuneability, whereas a fragment of a third AD was tuneable by single substitutions. In this study, we tested tuneability in the AD of full-length human class II transactivator (CIITA). Sequence analyses and experiments showed that CIITA's AD is an IDR. Functional assays of singly-substituted AD variants showed that CIITA's function was highly tuneable, with outcomes not predicted by the acidic exposure model. Four tested positions showed rheostat behavior for transcriptional activation. Thus, tuneability of different IDRs can vary widely. Future studies are needed to illuminate the biophysical features that govern whether an IDR is tuneable by single substitutions.

Aryl Hydrocarbon Receptor as an Anticancer Target: An Overview of Ten Years Odyssey

Aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor belonging to the basic helix-loop-helix (bHLH)/per-Arnt-sim (PAS) superfamily, is traditionally known to mediate xenobiotic metabolism. It is activated by structurally diverse agonistic ligands and regulates complicated transcriptional processes through its canonical and non-canonical pathways in normal and malignant cells. Different classes of AhR ligands have been evaluated as anticancer agents in different cancer cells and exhibit efficiency, which has thrust AhR into the limelight as a promising molecular target. There is strong evidence demonstrating the anticancer potential of exogenous AhR agonists including synthetic, pharmaceutical, and natural compounds. In contrast, several reports have indicated inhibition of AhR activity by antagonistic ligands as a potential therapeutic strategy. Interestingly, similar AhR ligands exert variable anticancer or cancer-promoting potential in a cell- and tissue-specific mode of action. Recently, ligand-mediated modulation of AhR signaling pathways and the associated tumor microenvironment is emerging as a potential approach for developing cancer immunotherapeutic drugs. This article reviews advances of AhR in cancer research covering publication from 2012 to early 2023. It summarizes the therapeutic potential of various AhR ligands with an emphasis on exogenous ligands. It also sheds light on recent immunotherapeutic strategies involving AhR.

The Aryl Hydrocarbon Receptor (AHR): A Novel Therapeutic Target for Pulmonary Diseases?

The aryl hydrocarbon receptor (AHR) is a cytoplasmic transcription factor that is well-known for regulating xenobiotic metabolism. Studies in knockout and transgenic mice indicate that the AHR plays a vital role in the development of liver and regulation of reproductive, cardiovascular, hematopoietic, and immune homeostasis. In this focused review on lung diseases associated with acute injury and alveolar development, we reviewed and summarized the current literature on the mechanistic role(s) and therapeutic potential of the AHR in acute lung injury, chronic obstructive pulmonary disease, and bronchopulmonary dysplasia (BPD). Pre-clinical studies indicate that endogenous AHR activation is necessary to protect neonatal and adult lungs against hyperoxia- and cigarette smoke-induced injury. Our goal is to provide insight into the high translational potential of the AHR in the meaningful management of infants and adults with these lung disorders that lack curative therapies.

Role of Aryl Hydrocarbon Receptor Activation in Inflammatory Chronic Skin Diseases

Aryl Hydrocarbon Receptor (AhR) is an evolutionary transcription factor which acts as a crucial sensor of different exogenous and endogenous molecules Recent data indicate that AhR is implicated in several physiological processes such as cell physiology, host defense, proliferation and differentiation of immune cells, and detoxification. Moreover, AhR involvement has been reported in the development and maintenance of several pathological conditions. In recent years, an increasing number of studies have accumulated highlighting the regulatory role of AhR in the physiology of the skin. However, there is evidence of both beneficial and harmful effects of AHR signaling. At present, most of the evidence concerns inflammatory skin diseases, in particular atopic dermatitis, psoriasis, acne, and hidradenitis suppurativa. This review exam-ines the role of AhR in skin homeostasis and the therapeutic implication of its pharmacological modulation in these cutaneous inflammatory diseases.

Species-Specific Differences in Aryl Hydrocarbon Receptor Responses: How and Why?

The aryl hydrocarbon receptor (AhR) is a transcription factor that regulates a wide range of biological and toxicological effects by binding to specific ligands. AhR ligands exist in various internal and external ecological systems, such as in a wide variety of hydrophobic environmental contaminants and naturally occurring chemicals. Most of these ligands have shown differential responses among different species. Understanding the differences and their mechanisms helps in designing better experimental animal models, improves our understanding of the environmental toxicants related to AhR, and helps to screen and develop new drugs. This review systematically discusses the species differences in AhR activation effects and their modes of action. We focus on the species differences following AhR activation from two aspects: (1) the molecular configuration and activation of AhR and (2) the contrast of cis-acting elements corresponding to AhR. The variations in the responses seen in humans and other species following the activation of the AhR signaling pathway can be attributed to both factors.

Molecular characterization of the aryl hydrocarbon receptor 2 gene in black rockfish, Sebastes schlegelii, and its expression patterns upon exposure to benzo[a]pyrene, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and β-naphthoflavone

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the toxicity of halogenated and polycyclic aromatic hydrocarbons in vertebrates. Thus, increased knowledge of AhR-mediated responses to xenobiotics is imperative. Sebastes schlegelii is increasingly being used as a model for studying environmental toxicology; hence, in this study, the presence of AhR2 was evaluated in S. schlegelii. The results showed that the predicted AhR2 amino acid sequence contained regions characteristic of other vertebrate AhRs, including the basic helix-loop-helix and PER-ARNT-SIM domains in the N-terminal half, but it had minor similarity with other vertebrate AhRs across the C-terminal half; it did not contain the distinct glutamine-rich domains found in mammalian AhR2. Phylogenetic analysis demonstrated that S. schlegelii AhR2 was clustered within the teleost AhR2 branch. Additionally, AhR2 mRNA was detectable in all 11 tissues tested, with the highest mRNA levels in the heart, pyloric ceca, and liver. Furthermore, exposure to the AhR agonists showed that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, 1 μg/g body weight) induced a significantly higher increases in AhR2 expression in the gills, liver, kidneys, and spleen in 48 h than benzo[a]pyrene (2 μg/g body weight), and β-naphthoflavone (50-μg/g body weight); AhR2 mRNA levels upon TCDD exposure were up-regulated by 16- and 10-fold in the gills and liver, respectively. These findings indicated that AhR was a highly sensitive receptor against TCDD. Thus, investigating AhR2 expression in the presence of other xenobiotics might offer further information for the elucidation of its crucial role in mediating toxicant metabolism in S. schlegelii.

Plant Occurring Flavonoids as Modulators of the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a transcription factor deeply implicated in health and diseases. Historically identified as a sensor of xenobiotics and mainly toxic substances, AhR has recently become an emerging pharmacological target in cancer, immunology, inflammatory conditions, and aging. Multiple AhR ligands are recognized, with plant occurring flavonoids being the largest group of natural ligands of AhR in the human diet. The biological implications of the modulatory effects of flavonoids on AhR could be highlighted from a toxicological and environmental concern and for the possible pharmacological applicability. Overall, the possible AhR-mediated harmful and/or beneficial effects of flavonoids need to be further investigated, since in many cases they are contradictory. Similar to other AhR modulators, flavonoids commonly exhibit tissue, organ, and species-specific activities on AhR. Such cellular-context dependency could be probably beneficial in their pharmacotherapeutic use. Flavones, flavonols, flavanones, and isoflavones are the main subclasses of flavonoids reported as AhR modulators. Some of the structural features of these groups of flavonoids that could be influencing their AhR effects are herein summarized. However, limited generalizations, as well as few outright structure-activity relationships can be suggested on the AhR agonism and/or antagonism caused by flavonoids.

Potential functional variants in AHR signaling pathways are associated with age at puberty in swine

Puberty in female pigs is defined as age at first estrus and gilts that have an earlier age at puberty are more likely to have greater lifetime productivity. Because age at puberty is predictive for sow longevity and lifetime productivity, but not routinely measured in commercial herds, it would be beneficial to use genomic or marker-assisted selection to improve these traits. A GWAS at the US Meat Animal Research Center (USMARC) identified several loci associated with age at puberty in pigs. Candidate genes in these regions were scanned for potential functional variants using sequence information from the USMARC swine population founder animals and public databases. In total, 135 variants (SNP and insertion/deletions) in 39 genes were genotyped in 1284 phenotyped animals from a validation population sired by Landrace and Yorkshire industry semen using the Agena MassArray system. Twelve variants in eight genes were associated with age at puberty (P < 0.005) with estimated additive SNP effects ranging from 1.6 to 5.3 days. Nine of these variants were non-synonymous coding changes in AHR, CYP1A2, OR2M4, SDCCAG8, TBC1D1 and ZNF608, two variants were deletions of one and four codons in aryl hydrocarbon receptor, AHR, and the most significant SNP was near an acceptor splice site in the acetyl-CoA carboxylase alpha, ACACA. Several of the loci identified have a physiological and a genetic role in sexual maturation in humans and other animals and are involved in AHR-mediated pathways. Further functional validation of these variants could identify causative mutations that influence age at puberty in gilts and possibly sow lifetime productivity.

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.

Refinement of coding SNPs in the human aryl hydrocarbon receptor gene using ISNPranker: An integrative-SNP ranking web-tool

Different bioinformatic methods apply various approaches to predict how much the effect of a SNP could be deleterious and therefore their results may differ significantly. However, variation studies often need to consider an integrated prediction result to analyze the effect of SNPs. To address this problem, we used an algorithm to map ordinal predictions to a numeral space and averaging them, and based on it we developed the ISNPranker web-tool (http://isnpranker.semilab.ir/). It takes heterogonous outputs of different predictors and generates integrated numerical predictions and ranks SNPs based on them. Afterward, we used ISNPranker to identify the most deleterious coding SNPs (cSNPs) of the human aryl hydrocarbon receptor (AHR) gene. AHR is a ligand-activated transcription factor that governs many molecular and cellular mechanisms and cSNPs may affect its structure, interactions, and function. Forty validated cSNPs of AHR were initially analyzed using 16 publicly available SNP analyzers and the results were introduced to the ISNPranker and integrated predictions were obtained. The cSNPs were ranked in 34 levels of danger and rs200257782 in the ARNT dimerization domain (ADD_121-289) of AHR was identified as the most deleterious cSNP. The rs148360742, which affect ADD_40-79 and Hsp90 binding domain (HBD_27-79) was in the second rank and the third and fourth ranks were occupied by ADD_121-289-located variations rs571123681 and rs141667112 respectively. In conclusion, we introduced ISNPranker, which is a web-tool for integrative ranking of SNPs, and we showed that AHR structure and function may be highly sensitive to the cSNPs in the ARNT dimerization domain.

Targeting the aryl hydrocarbon receptor with a novel set of triarylmethanes

The aryl hydrocarbon receptor (AhR) is a chemical sensor upregulating the transcription of responsive genes associated with endocrine homeostasis, oxidative balance and diverse metabolic, immunological and inflammatory processes, which have raised the pharmacological interest on its modulation. Herein, a novel set of 32 unsymmetrical triarylmethane (TAM) class of structures has been synthesized, characterized and their AhR transcriptional activity evaluated using a cell-based assay. Eight of the assayed TAM compounds (14, 15, 18, 19, 21, 22, 25, 28) exhibited AhR agonism but none of them showed antagonist effects. TAMs bearing benzotrifluoride, naphthol or heteroaromatic (indole, quinoline or thiophene) rings seem to be prone to AhR activation unlike phenyl substituted or benzotriazole derivatives. A molecular docking analysis with the AhR ligand binding domain (LBD) showed similarities in the binding mode and in the interactions of the most potent TAM identified 4-(pyridin-2-yl (thiophen-2-yl)methyl)phenol (22) compared to the endogenous AhR agonist 5,11-dihydroindolo[3,2-b]carbazole-12-carbaldehyde (FICZ). Finally, in silico predictions of physicochemical and biopharmaceutical properties for the most potent agonistic compounds were performed and these exhibited acceptable druglikeness and good ADME profiles. To our knowledge, this is the first study assessing the AhR modulatory effects of unsymmetrical TAM class of compounds.

The AHR Signaling Attenuates Autoimmune Responses During the Development of Type 1 Diabetes

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcriptional factor widely expressed in immune cells. Its ligands range from xenobiotics and natural substances to metabolites, which renders it capable of sensing and responding to a variety of environmental cues. Although AHR signaling has long been recognized to be implicated in the pathogenesis of autoimmune disorders, such as rheumatoid arthritis (RA), colitis, and systemic lupus erythematosus (SLE), its effect on the pathogenesis of type 1 diabetes (T1D) remains less understood. In this review, we intend to summarize its potential implication in T1D pathogenesis and to sort out the related regulatory mechanisms in different types of immune cells. Emerging evidence supports that β cell destruction caused by autoimmune responses can be rectified by AHR signaling. Upon activation by its ligands, AHR not only modulates the development and functionality of immune cells, but also suppresses the expression of inflammatory cytokines, through which AHR attenuates autoimmune responses during the course of T1D development. Since AHR-initiated biological effects vary between different types of ligands, additional studies would be necessary to characterize or de novo synthesize effective and safe ligands aimed to replenish our arsenal in fighting autoimmune responses and β mass loss in a T1D setting.

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.

Homozygous stop mutation in AHR causes autosomal recessive foveal hypoplasia and infantile nystagmus

Herein we present a consanguineous family with three children affected by foveal hypoplasia with infantile nystagmus, following an autosomal recessive mode of inheritance. The patients showed normal electroretinography responses, no signs of albinism, and no anterior segment or brain abnormalities. Upon whole exome sequencing, we identified a homozygous mutation (c.1861C>T;p.Q621*) in the aryl hydrocarbon receptor (AHR) gene that perfectly co-segregated with the disease in the larger family. AHR is a ligand-activated transcription factor that has been intensively studied in xenobiotic-induced toxicity. Further, it has been shown to play a physiological role under normal cellular conditions, such as in immunity, inflammatory response and neurogenesis. Notably, knockout of the Ahr gene in mouse impairs optic nerve myelin sheath formation and results in oculomotor deficits sharing many features with our patients: the eye movement disorder in Ahr-/- mice appears early in development and presents as conjugate horizontal pendular nystagmus. We therefore propose AHR to be a novel disease gene for a new, recessively inherited disorder in humans, characterized by infantile nystagmus and foveal hypoplasia.

Targeting Aryl hydrocarbon receptor for next-generation immunotherapies: Selective modulators (SAhRMs) versus rapidly metabolized ligands (RMAhRLs)

Aryl Hydrocarbon Receptor (AhR) constitutes a major network hub of genomic and non-genomic signaling pathways, connecting host's immune cells to environmental factors. It shapes innate and adaptive immune processes to environmental stimuli with species-, cell- and tissue-type dependent specificity. Although an ever increasing number of studies has thrust AhR into the limelight as attractive target for the development of next-generation immunotherapies, concerns exist on potential safety issues associated with small molecule modulation of the receptor. Selective AhR modulators (SAhRMs) and rapidly metabolized AhR ligands (RMAhRLs) are two classes of receptor agonists that are emerging as interesting lead compounds to bypass AhR-related toxicity in favor of therapeutic effects. In this article, we discuss SAhRMs and RMAhRLs reported in literature, covering concepts underlying their definitions, specific binding modes, structure-activity relationships and AhR-mediated functions.

Transcription Factors AhR/ARNT Regulate the Expression of CYP6CY3 and CYP6CY4 Switch Conferring Nicotine Adaptation

Nicotine is one of the most toxic secondary plant metabolites in nature and it is highly toxic to herbivorous insects. The overexpression of CYP6CY3 and its homologous isozyme CYP6CY4 in Myzus persicae nicotianae is correlated with nicotine tolerance. The expanded (AC)n repeat in promoter is the cis element for CYP6CY3 transcription. These repeat sequences are conserved in the CYP6CY3 gene from Aphis gossypii and the homologous P450 genes in Acyrthosiphon pisum. The potential transcriptional factors that may regulate CYP6CY3 were isolated by DNA pulldown and sequenced in order to investigate the underlying transcriptional regulation mechanism of CYP6CY3. These identified transcriptional factors, AhR and ARNT, whose abundance was highly correlated with an abundance of the CYP6CY3 gene, were validated. RNAi and co-transfection results further confirm that AhR and ARNT play a major role in the transcriptional regulation of the CYP6CY3 gene. When the CYP6CY3 transcript is destabilized by AhR/ARNT RNAi, the transcription of the CYP6CY4 is dramatically up-regulated, indicating a compensatory mechanism between the CYP6CY3 and CYP6CY4 genes. Our present study sheds light on the CYP6CY3 and CYP6CY4 mediated nicotine adaption of M. persicae nicotianae to tobacco. The current studies shed light on the molecular mechanisms that underlie the genotypic and phenotypic changes that are involved in insect host shifts and we conclude that AhR/ARNT regulate the expression of CYP6CY3 and CYP6CY4 cooperatively, conferring the nicotine adaption of M. persicae nicotianae to tobacco.

A splicing mutation in aryl hydrocarbon receptor associated with retinitis pigmentosa

Retinitis pigmentosa (RP) refers to a group of retinal degenerative diseases, which often lead to vision loss. Although 70 genes have been identified in RP patients, the genetic cause of approximately 30% of RP cases remains unknown. We aimed to identify the cause of the disease in a cohort of RP families by whole exome sequencing. A rare homozygous splicing variant, c.1160 + 1G>A, which introduced skipping of exon 9 of the aryl hydrocarbon receptor (AHR), was identified in family RD-134. This variant is very rare in several exome databases and leads to skipping of exon 9 in the transcript. AHR is expressed in the human retina and is a ligand-activated transcription factor with multiple functions. Mutant AHR failed to promote 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD)-induced xenobiotic responsive element (XRE) luciferase activity. In parallel, mutation in AHR abolished activation of its downstream target gene, such as CYP1A1 and CYP1A2. To investigate the in vivo roles of Ahr in the retina, we generated a retina-specific conditional knockout mouse model of Ahr. Comparing with wild-type mouse, Ahr knockout mice exhibited reduced electroretinogram responses at 9 months of age. Retinal histology revealed retinal histology showed the degeneration of photoreceptors with a thinner outer nuclear layer. Thus, our data demonstrate that AHR is associated with RP.

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.

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.

Transcription factor aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator is involved in regulation of the xenobiotic tolerance-related cytochrome P450 CYP6DA2 in Aphis gossypii Glover

The cotton aphid, Aphis gossypii, is one of the most economically important agricultural pests worldwide as it is polyphagous and resistant to many classes of insecticides. Overexpression of the cytochrome P450 monooxygenase (P450) CYP6DA2 has previously been found to be associated with gossypol and spirotetramat tolerance in the cotton aphid. In the present study, the elements located in the promoter region (-357:-343; -250:-241; -113:-104) of CYP6DA2 were shown to control promoter activity, and gossypol induction was observed. We hypothesized that the expression of CYP6DA2 is subject to transcriptional regulation. To investigate the underlying mechanism, we assessed two transcription factors, aryl hydrocarbon receptor (AhR) and aryl hydrocarbon receptor nuclear translocator (ARNT), and found that the abundance of AhR was highly correlated with CYP6DA2 abundance. RNA interference of AhR or ARNT significantly decreased the levels of the target gene as well as those of its counterpart, and both dramatically repressed CYP6DA2 expression. Cotransfection of the ARNT, AhR, or AhR plus ARNT and CYP6DA2 promoter constructs elevated CYP6DA2 promoter activity, with the AhR plus ARNT cotransfection being the most effective. Thus, these elements located in the promoter were responsible for CYP6DA2 transcription, and CYP6DA2 expression was regulated by the transcription factors AhR and ARNT.

Aryl hydrocarbon receptor gene transitions (c.-742C>T; c.1661G>A) and idiopathic male infertility: a case-control study with in silico and meta-analysis

Aryl hydrocarbon receptor (AHR) is responsible for crucial events in male reproductive biology. Here, the association of the AHR transitions c.-742C>T and c.1661G>A with idiopathic male infertility was investigated in a case-control study, which is followed by a meta-analysis and a bioinformatic investigation. Blood and semen samples were obtained from a total of 135 idiopathic infertile men and 130 healthy controls. Participants were genotyped for the transitions using a PCR-RFLP method. A meta-analysis of five sets of data evaluated the association of c.1661G>A with male infertility, and using an in silico analysis, the possible molecular effects of the transitions predicted. Genotypes and alleles of AHR-c.-742C>T and c.1661G>A polymorphisms were not associated with the risk of male infertility significantly. However, the frequency of C/A haplotype was significantly associated with the increased risk of male infertility, and T/A haplotype was higher among controls significantly. Also, the frequencies of combined genotypes CT/GG, CT/GA and TT/GG were significantly associated with decreased risk of infertility. And, the meta-analysis showed that the AA versus GA/GG recessive model is associated with decreased risk of male infertility among the Iranian population. In silico analysis predicted that c.-742C>T does not alter the binding sites of the proposed transcription factors, but c.1661G>A poses a tolerable structural disturbance in AHR protein. In conclusion, these results showed that AHR c.-742C>T and c.1661G>A transitions separately could not be nominated as a risk or protective factor for male infertility. However, some combined models could affect infertility risk, especially among Iranian men.

The Q-rich/PST domain of the AHR regulates both ligand-induced nuclear transport and nucleocytoplasmic shuttling

The aryl hydrocarbon receptor (AHR) shuttles continuously between cytoplasm and nucleus, unless ligand-binding triggers association with the AHR nuclear translocator (ARNT) and subsequent binding to cognate DNA motifs. We have now identified Val 647 as mandatory residue for export from the nucleus and AHR-function. This residue prevents inactivation of the receptor as a consequence of nuclear sequestration via constitutive import. Concomitantly mutants lacking this residue are exclusively localised in the nucleus. Although ligands accelerate nuclear import transiently, stable nuclear transition depends on a motif adjacent to Val 647 that comprises residues 650-661. Together, this defined region within the Q-rich domain regulates intracellular trafficking of the AHR in context of both nucleocytoplasmic shuttling and receptor activation. Nuclear export therefore depends on the previously characterised N-terminal NES and the newly identified motif that includes V647. Nucleocytoplasmic distribution of full-length human AHR is further affected by a section of the PST domain that shows sequence similarities with nuclear export signals. In concert, these motifs maintain a predominant cytoplasmic compartmentalisation, receptive for ligand binding.

Mucosal Interactions between Genetics, Diet, and Microbiome in Inflammatory Bowel Disease

Numerous reviews have discussed gut microbiota composition changes during inflammatory bowel diseases (IBD), particularly Crohn’s disease (CD). However, most studies address the observed effects by focusing on studying the univariate connection between disease and dietary-induced alterations to gut microbiota composition. The possibility that these effects may reflect a number of other interconnected (i.e., pantropic) mechanisms, activated in parallel, particularly concerning various bacterial metabolites, is in the process of being elucidated. Progress seems, however, hampered by various difficult-to-study factors interacting at the mucosal level. Here, we highlight some of such factors that merit consideration, namely: (1) the contribution of host genetics and diet in altering gut microbiome, and in turn, the crosstalk among secondary metabolic pathways; (2) the interdependence between the amount of dietary fat, the fatty acid composition, the effects of timing and route of administration on gut microbiota community, and the impact of microbiota-derived fatty acids; (3) the effect of diet on bile acid composition, and the modulator role of bile acids on the gut microbiota; (4) the impact of endogenous and exogenous intestinal micronutrients and metabolites; and (5) the need to consider food associated toxins and chemicals, which can introduce confounding immune modulating elements (e.g., antioxidant and phytochemicals in oils and proteins). These concepts, which are not mutually exclusive, are herein illustrated paying special emphasis on physiologically inter-related processes.

Regulation of aryl hydrocarbon receptor-mediated transcription in human retinal pigmented epithelial cells

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor with pleiotropic effects in normal physiology or vascular development, xenobiotic metabolism, and cancer. A previous study has reported that BRG1, a component of the SWI/SNF complex, is a coactivator for AHR and is recruited to the promoter region of the CYP1A1 gene in mouse hepatocytes. Recent data suggest that AHR is also expressed in human retinal pigment epithelial cells (ARPE-19), which play a crucial role in retinal physiology and the visual cycle. Multiple studies have shown that the AHR plays an important role in the pathogenesis of retinal diseases including age-related macular degeneration. However, the mechanism of AHR transcriptional activation in retinal pigment cells has not been reported. Here, we demonstrate that the AHR signaling pathway is active in ARPE-19 cells, as in hepatocytes, but with different target gene specificity. We also found that chromatin remodeling by the BRG1-containing SWI/SNF complex is required for the AHR-mediated expression of target genes in ARPE-19 cells. We identified a novel enhancer region (-12 kb) of the CYP1A1 gene in ARPE-19 cells, to which both AHR and BRG1 are recruited in a ligand-dependent manner. BRG1 is associated with the AHR in ARPE-19 cells, and the C-terminal activation domain of the AHR directly interacts with BRG1. Furthermore, depletion of BRG1 caused a reduction in chromatin accessibility at the CYP1A1 enhancer. These results suggest that ARPE-19 cells possess an AHR-mediated transcription pathway with different target gene specificity, and that BRG1 is required for AHR-mediated transcription in ARPE-19 cells.

Are aryl hydrocarbon receptor and G-protein-coupled receptor 30 involved in the regulation of seasonal testis activity in photosensitive rodent-the bank vole (Myodes glareolus)?

Within the reproductive system both aryl hydrocarbon receptor (AHR) and G-protein-coupled receptor 30 (GPR30) contribute to estrogen signaling and controlling of reproductive physiology. The specific question is whether and how AHR and GPR30 are involved in regulation of testis function in seasonally breeding rodents. Bank vole testes were obtained from animals reared under 18 hours light:6 hours dark (LD) and 6 hours light:18 hours dark (SD) conditions. Aryl hydrocarbon receptor and GPR30 expression were analyzed by quantitative reverse transcriptase-polymerase chain reaction, Western blot, and immunohistochemistry and/or immunofluorescent staining. In addition, the activity of enzymes involved in the intracellular signal transduction; extracellular signal-regulated kinase (ERK), protein kinase (PKA), matrix metalloproteinase 9 (MMP 9) and the concentrations of cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), and calcium (Ca(2+)) were examined by immunohistochemical, immunoenzymatic, and colorimetric assays, respectively. Aryl hydrocarbon receptor and GPR30 were expressed in testes of actively reproducing voles and regressed ones although their expression at the messenger RNA and AHR also at protein level appeared to be photoperiod-dependent. A specific cellular localization and expression of AHR and GPR30 correlated with the expression of ERK, PKA, and MMP 9. Moreover, we found robust differences in the levels of cAMP, cGMP, and Ca(2+) in testicular homogenates between LD and SD voles. In the testes of LD voles, the levels of second messengers were always higher compared to SD. In vole testis, AHR and GPR30 can induce signaling pathways that involve ERK, PKA, MMP 9 and cAMP, cGMP, Ca(2+). In addition, in AHR, signaling the engagement of both photoperiod and estrogens, whereas in GPR30, signaling only estrogens is reported. It is likely that in vole, because of a differential activity of signaling molecules, signal transduction via AHR rather than through GPR30 plays a role in regulation of seasonal changes of testis physiology.

An in silico approach to investigate the source of the controversial interpretations about the phenotypic results of the human AhR-gene G1661A polymorphism

Aryl hydrocarbon receptor (AhR) acts as an enhancer binding ligand-activated intracellular receptor. Chromatin remodeling components and general transcription factors such as TATA-binding protein (TBP) are evoked on AhR-target genes by interaction with its flexible transactivation domain (TAD). AhR-G1661A single nucleotide polymorphism (SNP: rs2066853) causes an arginine to lysine substitution in the acidic sub-domain of TAD at position 554 (R554K). Although, numerous studies associate the SNP with some abnormalities such as cancer, other reliable investigations refuse the associations. Consequently, the interpretation of the phenotypic results of G1661A-transition has been controversial. In this study, an in silico analysis were performed to investigate the possible effects of the transition on AhR-mRNA, protein structure, interaction properties and modifications. The analysis revealed that the R554K substitution affects secondary structure and solvent accessibility of adjacent residues. Also, it causes to decreasing of the AhR stability; altering the hydropathy features of the local sequence and changing the pattern of the residues at the binding site of the TAD-acidic sub-domain. Generating of new sites for ubiquitination and acetylation for AhR-K554 variant respectively at positions 544 and 560 was predicted. Our findings intensify the idea that the AhR-G1661A transition may affects AhR-TAD interactions, especially with the TBP, which influence AhR-target genes expression. However, the previously reported flexibility of the modular TAD could act as an intervening factor, moderate the SNP effects and causes distinct outcomes in different individuals and tissues.

The adverse outcome pathway for rodent liver tumor promotion by sustained activation of the aryl hydrocarbon receptor

An Adverse Outcome Pathway (AOP) represents the existing knowledge of a biological pathway leading from initial molecular interactions of a toxicant and progressing through a series of key events (KEs), culminating with an apical adverse outcome (AO) that has to be of regulatory relevance. An AOP based on the mode of action (MOA) of rodent liver tumor promotion by dioxin-like compounds (DLCs) has been developed and the weight of evidence (WoE) of key event relationships (KERs) evaluated using evolved Bradford Hill considerations. Dioxins and DLCs are potent aryl hydrocarbon receptor (AHR) ligands that cause a range of species-specific adverse outcomes. The occurrence of KEs is necessary for inducing downstream biological responses and KEs may occur at the molecular, cellular, tissue and organ levels. The common convention is that an AOP begins with the toxicant interaction with a biological response element; for this AOP, this initial event is binding of a DLC ligand to the AHR. Data from mechanistic studies, lifetime bioassays and approximately thirty initiation-promotion studies have established dioxin and DLCs as rat liver tumor promoters. Such studies clearly show that sustained AHR activation, weeks or months in duration, is necessary to induce rodent liver tumor promotion--hence, sustained AHR activation is deemed the molecular initiating event (MIE). After this MIE, subsequent KEs are 1) changes in cellular growth homeostasis likely associated with expression changes in a number of genes and observed as development of hepatic foci and decreases in apoptosis within foci; 2) extensive liver toxicity observed as the constellation of effects called toxic hepatopathy; 3) cellular proliferation and hyperplasia in several hepatic cell types. This progression of KEs culminates in the AO, the development of hepatocellular adenomas and carcinomas and cholangiolar carcinomas. A rich data set provides both qualitative and quantitative knowledge of the progression of this AOP through KEs and the KERs. Thus, the WoE for this AOP is judged to be strong. Species-specific effects of dioxins and DLCs are well known--humans are less responsive than rodents and rodent species differ in sensitivity between strains. Consequently, application of this AOP to evaluate potential human health risks must take these differences into account.

The evolving role of the aryl hydrocarbon receptor (AHR) in the normophysiology of hematopoiesis

In addition to its role as a toxicological signal mediator, the Aryl Hydrocarbon Receptor (AHR) is also a transcription factor known to regulate cellular responses to oxidative stress and inflammation through transcriptional regulation of molecules involved in the signaling of nucear factor-erythroid 2-related factor-2 (Nrf2), p53 (TRP53), retinoblastoma (RB1), and NFκB. Recent research suggests that AHR activation of these signaling pathways may provide the molecular basis for understanding AHR's evolving role in endogenous developmental functions during hematopoietic stem-cell maintenance and differentiation. Recent developments into the hematopoietic roles for AHR are reviewed, aiming to reconcile divergent findings as to the endogenous function of AHR in hematopoiesis. Potential mechanistic explanations for AHR's involvement in hematopoietic differentiation are discussed, focusing on its known role as a cell cycle mediator and its interactions with Hypoxia-inducible transcription factor-1 alpha (HIF1-α). Understanding the physiological mechanisms of AHR activation and signaling have far reaching implications ranging from explaining the action of various toxicological agents to providing novel ways to expand stem cell populations ex vivo for use in transplant therapies.

Use of natural AhR ligands as potential therapeutic modalities against inflammatory disorders

The aim of this review is to discuss research involving ligands for the aryl hydrocarbon receptor (AhR) and their role in immunomodulation. While activation of the AhR is well known for its ability to regulate the biochemical and toxic effects of environmental chemicals, more recently an exciting discovery has been made indicating that AhR ligation can also regulate T-cell differentiation, specifically through activation of Foxp3(+) regulatory T cells (Tregs) and downregulation of the proinflammatory Th17 cells. Such findings have opened new avenues of research on the possibility of targeting the AhR to treat inflammatory and autoimmune diseases. Specifically, this review will discuss the current research involving natural and dietary AhR ligands. In addition, evidence indicating the potential use of these ligands in regulating inflammation in various diseases will be highlighted. The importance of the AhR in immunological processes can be illustrated by expression of this receptor on a majority of immune cell types. In addition, AhR signaling pathways have been reported to influence a number of genes responsible for mediating inflammation and other immune responses. As interest in the AhR and its ligands increases, it seems prudent to consolidate current research on the contributions of these ligands to immune regulation during the course of inflammatory diseases.

SUMOylation of AhR modulates its activity and stability through inhibiting its ubiquitination

Aryl hydrocarbon receptor (AhR) is a transcription factor that belongs to the basic helix-loop-helix (bHLH) Per-Arnt-Sim homology domain (PAS) family. AhR can be activated by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (2, 3, 7, 8-TCDD) and once activated, it promotes the abnormal expression of cytochrome P450, leading to several diseases, including cancer. In this study, we showed that AhR is subjected to post-translational modification by SUMOylation and this modification could be reversed by SENP1. Two SUMOylation sites were identified, one in the bHLH domain (K63) and the other in the TAD domain (K510) of AhR. Substitution of either K63 or K510 with arginine resulted in reduced SUMOylation for AhR. Treatment of MCF-7 cells with TCDD led to a reduced level of SUMOylated AhR in a time-dependent manner, and this occurred mainly in the nucleus. SUMOylation of AhR enhanced its stability through inhibiting its ubiquitination. Moreover, SUMOylation also repressed the transactivation activity of AhR and this could be reversed by TCDD. These results suggested that SUMOylation of AhR might play an important role in the regulation of its function, and TCDD may activate the transcriptional activity of AhR through downregulating its SUMOylation.

A dominant negative zebrafish Ahr2 partially protects developing zebrafish from dioxin toxicity

The toxicity by 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) is thought to be caused by activation of the aryl hydrocarbon receptor (AHR). However, our understanding of how AHR activation by TCDD leads to toxic effects is poor. Ideally we would like to manipulate AHR activity in specific tissues and at specific times. One route to this is expressing dominant negative AHRs (dnAHRs). This work describes the construction and characterization of dominant negative forms of the zebrafish Ahr2 in which the C-terminal transactivation domain was either removed, or replaced with the inhibitory domain from the Drosophila engrailed repressor protein. One of these dnAhr2s was selected for expression from the ubiquitously active e2fα promoter in transgenic zebrafish. We found that these transgenic zebrafish expressing dnAhr2 had reduced TCDD induction of the Ahr2 target gene cyp1a, as measured by 7-ethoxyresorufin-O-deethylase activity. Furthermore, the cardiotoxicity produced by TCDD, pericardial edema, heart malformation, and reduced blood flow, were all mitigated in the zebrafish expressing the dnAhr2. These results provide in vivo proof-of-principle results demonstrating the effectiveness of dnAHRs in manipulating AHR activity in vivo, and demonstrating that this approach can be a means for blocking TCDD toxicity.

Aryl hydrocarbon receptor modulation of estrogen receptor α-mediated gene regulation by a multimeric chromatin complex involving the two receptors and the coregulator RIP140

Although crosstalk between aryl hydrocarbon receptor (AhR) and estrogen receptor α (ERα) is well established, the mechanistic basis and involvement of other proteins in this process are not known. Because we observed an enrichment of AhR-binding motifs in ERα-binding sites of many estradiol (E2)-regulated genes, we investigated how AhR might modulate ERα-mediated gene transcription in breast cancer cells. Gene regulations were categorized based on their pattern of stimulation by E2 and/or dioxin and were denoted E2-responsive, dioxin-responsive, or responsive to either ligand. ERα, AhR, aryl hydrocarbon receptor translocator, and receptor interacting protein 140 (RIP140) were recruited to gene regulatory regions in a gene-specific and E2/dioxin ligand-specific manner. Knockdown of AhR markedly increased the expression of ERα-mediated genes upon E2 treatment. This was not attributable to a change in ERα level, or recruitment of ERα, phosphoSer5-RNA Pol II, or several coregulators but rather was associated with greatly diminished recruitment of the coregulator RIP140 to gene regulatory sites. Changing the cellular level of RIP140 revealed coactivator or corepressor roles for this coregulator in E2- and dioxin-mediated gene regulation, the choice of which was determined by the presence or absence of ERα at gene regulatory sites. Coimmunoprecipitation and chromatin immunoprecipitation (ChIP)-reChIP studies documented that E2- or dioxin-promoted formation of a multimeric complex of ERα, AhR, and RIP140 at ERα-binding sites of genes regulated by either E2 or dioxin. Our findings highlight the importance of cross-regulation between AhR and ERα and a novel mechanism by which AhR controls, through modulating the recruitment of RIP140 to ERα-binding sites, the kinetics and magnitude of ERα-mediated gene stimulation.

Modulation of Signal Proteins: A Plausible Mechanism to Explain How a Potentized Drug Secale Cor 30C Diluted beyond Avogadro's Limit Combats Skin Papilloma in Mice

In homeopathy, ability of ultra-high diluted drugs at or above potency 12C (diluted beyond Avogadro's limit) in ameliorating/curing various diseases is often questioned, particularly because the mechanism of action is not precisely known. We tested the hypothesis if suitable modulations of signal proteins could be one of the possible pathways of action of a highly diluted homeopathic drug, Secale cornutum 30C (diluted 10(60) times; Sec cor 30). It could successfully combat DMBA + croton oil-induced skin papilloma in mice as evidenced by histological, cytogenetical, immunofluorescence, ELISA and immunoblot findings. Critical analysis of several signal proteins like AhR, PCNA, Akt, Bcl-2, Bcl-xL, NF-κB and IL-6 and of pro-apoptotic proteins like cytochrome c, Bax, Bad, Apaf, caspase-3 and -9 revealed that Sec cor 30 suitably modulated their expression levels along with amelioration of skin papilloma. FACS data also suggested an increase of cell population at S and G2 phases and decrease in sub-G1 and G1 phages in carcinogen-treated drug-unfed mice, but these were found to be near normal in the Sec cor 30-fed mice. There was reduction in genotoxic and DNA damages in bone marrow cells of Sec Cor 30-fed mice, as revealed from cytogenetic and Comet assays. Changes in histological features of skin papilloma were noted. Immunofluorescence studies of AhR and PCNA also suggested reduced expression of these proteins in Sec cor 30-fed mice, thereby showing its anti-cancer potentials against skin papilloma. Furthermore, this study also supports the hypothesis that potentized homeopathic drugs act at gene regulatory level.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

Polymorphisms and functional differences in aryl hydrocarbon receptors (AhR) in Japanese field mice, Apodemus speciosus

Dioxins, which are unintentionally generated toxic pollutants, exert a variety of adverse effects on organisms. The majority of these effects, which include teratogenesis, immunosuppression, tumor promotion, and endocrine disruption, are mediated through aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor. Genetic variations in AhR result in different survivability under exposure to dioxin contamination, which might affect the genetic structure of wildlife populations through differential susceptibility to dioxin exposure. The aim of this study was to clarify the polymorphisms of AhR in Japanese field mice, Apodemus speciosus, and their functional differences in order to develop a molecular indicator for dioxin sensitivity. Wild Japanese field mice had abundant polymorphisms in AhR coding region. Seventy-one single nucleotide polymorphisms, 27 of which occur amino acid substitutions, and consequently 49 alleles were identified in 63 individuals. In the functional analysis of AhR variants using transient reporter assays, a Gln to Arg mutation at amino acid 799 exhibited a significant decrease in the level of transactivational properties (p=0.015) which might modify the dioxin susceptibility of an individual.

Molecular evidence for a functional ecdysone signaling system in Brugia malayi

Background

Filarial nematodes, including Brugia malayi, the causative agent of lymphatic filariasis, undergo molting in both arthropod and mammalian hosts to complete their life cycles. An understanding of how these parasites cross developmental checkpoints may reveal potential targets for intervention. Pharmacological evidence suggests that ecdysteroids play a role in parasitic nematode molting and fertility although their specific function remains unknown. In insects, ecdysone triggers molting through the activation of the ecdysone receptor: a heterodimer of EcR (ecdysone receptor) and USP (Ultraspiracle).

Methods and Findings

We report the cloning and characterization of a B. malayi EcR homologue (Bma-EcR). Bma-EcR dimerizes with insect and nematode USP/RXRs and binds to DNA encoding a canonical ecdysone response element (EcRE). In support of the existence of an active ecdysone receptor in Brugia we also cloned a Brugia rxr (retinoid X receptor) homolog (Bma-RXR) and demonstrate that Bma-EcR and Bma-RXR interact to form an active heterodimer using a mammalian two-hybrid activation assay. The Bma-EcR ligand-binding domain (LBD) exhibits ligand-dependent transactivation via a GAL4 fusion protein combined with a chimeric RXR in mammalian cells treated with Ponasterone-A or a synthetic ecdysone agonist. Furthermore, we demonstrate specific up-regulation of reporter gene activity in transgenic B. malayi embryos transfected with a luciferase construct controlled by an EcRE engineered in a B. malayi promoter, in the presence of 20-hydroxy-ecdysone.

Conclusions

Our study identifies and characterizes the two components (Bma-EcR and Bma-RXR) necessary for constituting a functional ecdysteroid receptor in B. malayi. Importantly, the ligand binding domain of BmaEcR is shown to be capable of responding to ecdysteroid ligands, and conversely, ecdysteroids can activate transcription of genes downstream of an EcRE in live B. malayi embryos. These results together confirm that an ecdysone signaling system operates in B. malayi and strongly suggest that Bma-EcR plays a central role in it. Furthermore, our study proposes that existing compounds targeting the insect ecdysone signaling pathway should be considered as potential pharmacological agents against filarial parasites.

Filarial parasites such as Brugia malayi and Onchocerca volvulus are the causative agents of the tropical diseases lymphatic filariasis and onchocerciasis, which infect 150 million people, mainly in Africa and Southeast Asia. Filarial nematodes have a complex life cycle that involves transmission and development within both mammalian and insect hosts. The successful completion of the life cycle includes four molts, two of which are triggered upon transmission from one host to the other, human and mosquito, respectively. Elucidation of the molecular mechanisms involved in the molting processes in filarial nematodes may yield a new set of targets for drug intervention. In insects and other arthropods molting transitions are regulated by the steroid hormone ecdysone that interacts with a specialized hormone receptor composed of two different proteins belonging to the family of nuclear receptors. We have cloned from B. malayi two members of the nuclear receptor family that show many sequence and biochemical properties consistent with the ecdysone receptor of insects. This finding represents the first report of a functional ecdysone receptor homolog in nematodes. We have also established a transgenic hormone induction assay in B. malayi that can be used to discover ecdysone responsive genes and potentially lead to screening assays for active compounds for pharmaceutical development.

Dioxin and immune regulation: emerging role of aryl hydrocarbon receptor in the generation of regulatory T cells

The immune toxicity of the ubiquitous environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), commonly referred to as dioxin, has been studied for over 35 years but only recently has the profound immune suppression induced by TCDD exposure been linked to induction of regulatory T cells (Tregs). The effects of TCDD are mediated through its binding to the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor. The subsequent AHR-dependent effects on immune responses are determined by the cell types involved, their activation status, and the type of antigenic stimulus. Collectively, studies indicate that TCDD inhibits CD4+ T cell differentiation into T helper (Th)1, Th2, and Th17 effector cells, while inducing Foxp3-negative and/or preserving Foxp3+ Tregs. Although it is not yet clear how activation of AHR by TCDD induces Tregs, there is a potential therapeutic role for alternative AHR ligands in the treatment of immune-mediated disorders.

The mouse and human Ah receptor differ in recognition of LXXLL motifs

The aryl hydrocarbon receptor (AhR) is a ligand inducible transcription factor that exhibits interspecies differences, with the human and mouse AhR C-terminal transactivation domain sharing only 58% amino acid sequence identity. The AhR has a transactivation domain comprised of proline/serine/threonine-rich, glutamine-rich, and acidic amino acid subdomains. A truncated mAhR and hAhR containing only the acidic subdomain displayed widely differing transactivation potentials. Whether the glutamine-rich subdomain of the mouse AhR and the human AhR differentially recruit LXXLL-motif coactivators was investigated. Transiently expressed GAL4 DNA binding domain (GAL4DBD)-LXXLL-motif fusion proteins were used to map the critical LXXLL binding sequence of the hAhR to amino acid residues 663-688. Several LXXLL-motif GAL4DBD fusion proteins dramatically differed in their ability to influence the transactivation potential of the mAhR and hAhR. These findings suggest that the human and mouse AhR may display differential recruitment of coactivators and hence may exhibit divergent regulation of target genes.

Aryl hydrocarbon receptor splice variants in the dioxin-resistant rat: tissue expression and transactivational activity

The AHR locus encodes the aryl hydrocarbon receptor (AHR), a transcriptional regulator of multiple drug-metabolizing enzymes and mediator of toxicity of dioxin-like chemicals. The Han/Wistar (Kuopio) rat strain (H/W) is remarkably resistant to lethal effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) because of a point mutation in the exon/intron 10 boundary in AHR genomic structure that leads to use of 3 alternative cryptic splice sites, potentially creating 3 alternative transcripts and 2 protein products. The deletion variant (DV), which lacks 43 amino acids in the transactivation domain, has the highest intrinsic transactivation activity in vitro; amino acids 766 to 783 suppress transactivation function. However, DV expression levels in H/W rats in vivo are low in liver, lung, thymus, kidney, and testis; insertion variant mRNAs (IVs) are the dominant mRNA forms in H/W rats in which wild-type AHR mRNA is undetectable. In dioxin-sensitive rat strains and lines that are homozygous for wild-type AHR alleles, wild-type AHR mRNA is the most abundant transcript but some IV transcripts are detectable. TCDD treatment in vivo increases transcript levels for both the DV and IVs in H/W rats and increases wild-type transcript levels in dioxin-sensitive rats but does not alter which transcript forms are expressed. In silico modeling indicates that the DV mRNA has lost considerable secondary structure, whereas at the protein level, the transactivation domain of the IV in the dioxin-resistant H/W rat has greater alpha-helical content and a more hydrophobic terminus than wild-type AHR, which may produce a protein conformation that is less amenable to interaction with other regulatory proteins.

Characterization of Drosophila and Caenorhabditis elegans NXF-like-factors, putative homologs of mammalian NXF

The basic helix-loop-helix/Per-Arnt-Sim (bHLH-PAS) homology protein family is an important class of transcriptional regulators that are involved in a wide variety of biological processes. In a previous study, we characterized a novel bHLH-PAS factor 'NXF' as a new member of the mammalian bHLH-PAS family. If model animals have ortholog genes for NXF, they might have advantages for characterization of its function and regulation in vivo. Here we document cDNA cloning and characterization of putative NXF homolog genes (for NXF-like-factors) in Drosophila and C. elegans. Some conservation of the exon-intron gene architecture was noted in the bHLH-PAS domains among NXF-like-factors and human NXF. The NXF-like-factors selectively interacted with an Arnt ortholog in vitro assay. Moreover, they selectively associated with the mammalian NXF:Arnt2 binding element in the presence of the Arnt ortholog, and synergistically activated a mammalian NXF:Arnt-responsive promoter. These data strongly suggest that the Drosophila and C. elegans NXF-like-factors reported here are homologs of mammalian NXF. Further elucidation of whether they are true orthologs with the same function in vivo is now necessary.

Basic helix-loop-helix transcription factor heterocomplex of Yas1p and Yas2p regulates cytochrome P450 expression in response to alkanes in the yeast Yarrowia lipolytica

The expression of the ALK1 gene, which encodes cytochrome P450, catalyzing the first step of alkane oxidation in the alkane-assimilating yeast Yarrowia lipolytica, is highly regulated and can be induced by alkanes. Previously, we identified a cis-acting element (alkane-responsive element 1 [ARE1]) in the ALK1 promoter. We showed that a basic helix-loop-helix (bHLH) protein, Yas1p, binds to ARE1 in vivo and mediates alkane-dependent transcription induction. Yas1p, however, does not bind to ARE1 by itself in vitro, suggesting that Yas1p requires another bHLH protein partner for its DNA binding, as many bHLH transcription factors function by forming heterodimers. To identify such a binding partner of Yas1p, here we screened open reading frames encoding proteins with the bHLH motif from the Y. lipolytica genome database and identified the YAS2 gene. The deletion of the YAS2 gene abolished the alkane-responsive induction of ALK1 transcription and the growth of the yeast on alkanes. We revealed that Yas2p has transactivation activity. Furthermore, Yas1p and Yas2p formed a protein complex that was required for the binding of these proteins to ARE1. These findings allow us to postulate a model in which bHLH transcription factors Yas1p and Yas2p form a heterocomplex and mediate the transcription induction in response to alkanes.

Calpain Mediates the Dioxin-Induced Activation and Down-Regulation of the Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated basic-helix-loop-helix transcription factor that binds polyaromatic hydrocarbons (PAH), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and mediates their toxicity. Binding of PAH to AhR in the cytoplasm triggers a poorly defined transformation step of the receptor into a nuclear transcription factor. In this study, we show that the calcium-dependent cysteine protease calpain plays a major role in the ligand-induced transformation and signaling of AhR. Fluorescence imaging measurements showed that TCDD treatment elevates intracellular calcium, providing the trigger for calpain activation, as measured toward t-butoxycarbonyl-Leu-Met-chloromethylaminocoumarin, a calpain-specific substrate. Inhibition of calpain activity by the N-benzyloxycarbonyl-Val-Phe-aldehyde (MDL28170) blocked the TCDD-induced nuclear translocation of AhR in Hepa1c1c7 mouse hepatoma cell line. Treatment of the human metastatic breast carcinoma cell line MT-2 with MDL28170 and 3-(4-iodophenyl)-2-mercapto-(Z)-2-propenoic acid (PD 150606), two calpain-selective inhibitors, completely abolished the TCDD-induced transactivation of AhR as assessed by transcription of CYP1A1 gene. Previous studies have established that after TCDD-induced transactivation, the AhR undergoes a massive depletion; we show here that selective calpain inhibitors can block this step, which suggests that the ligand-induced down-regulation of the AhR is calpain-dependent. The data presented support a major role for calpain in the AhR transformation, transactivation, and subsequent down-regulation, and provide a possible explanation for many of the reported phenomena of ligand-independent activation of AhR.

Molecular characterization of the aryl hydrocarbon receptors (AHR1 and AHR2) from red seabream (Pagrus major)

The aryl hydrocarbon receptor (AHR) mediates the toxic effects of planar halogenated aromatic hydrocarbons (PHAHs). Bony fishes exposed to PHAHs exhibit a wide range of developmental defects. However, functional roles of fish AHR are not yet fully understood, compared with those of mammalian AHRs. To investigate the potential sensitivity to PHAHs toxic effects, an AHR cDNA was initially cloned and sequenced from red seabream (Pagrus major), an important fishery resource in Japan. The present study succeeded in identifying two highly divergent red seabream AHR cDNA clones, which shared only 32% identity in full-length amino acid sequence. The phylogenetic analysis revealed that one belonged to AHR1 clade (rsAHR1) and another to AHR2 clade (rsAHR2). The rsAHR1 encoded a 846-residue protein with a predicted molecular mass of 93.2 kDa, and 990 amino acids and 108.9 kDa encoded rsAHR2. In the N-terminal half, both rsAHR genes included bHLH and PAS domains, which participate in ligand binding, AHR/ARNT dimerization and DNA binding. The C-terminal half, which is responsible for transactivation, was poorly conserved between rsAHRs. Quantitative analyses of both rsAHRs mRNAs revealed that their tissue expression profiles were isoform-specific; rsAHR1 mRNA expressed primarily in brain, heart, ovary and spleen, while rsAHR2 mRNA was observed in all tissues examined, indicating distinct roles of each rsAHR. Furthermore, there appeared to be species-differences in the tissue expression profiles of AHR isoforms between red seabream and other fish. These results suggest that there are isoform- and species-specific functions in piscine AHRs.

Human Pax-5 C-terminal Isoforms Possess Distinct Transactivation Properties and Are Differentially Modulated in Normal and Malignant B Cells

The transcription factor Pax-5 occupies a central role in B cell differentiation and has been implicated in the development of B cell lymphoma. The transcriptional activation function of Pax-5 requires an intact N-terminal DNA-binding domain and is strongly influenced by the C-terminal transactivation domain. We report the identification and characterization of five human Pax-5 isoforms, which occur through the alternative splicing of exons that encode for the C-terminal transactivation domain. These isoforms arise from the inclusion or exclusion of exon 7, exon 8, and/or exon 9. Three of the Pax-5 isoforms generate novel protein sequences rich in proline, serine, and threonine amino acids that are the hallmarks of transactivation domains. The Pax-5 isoforms are expressed in peripheral blood mononuclear cells, cancerous and non-cancerous B cell lines, as well as in primary B cell lymphoma tissue. Electrophoretic mobility shift assays demonstrate that the isoforms possess specific DNA binding activity and recognize the PAX-5 consensus binding sites. In reporter assays using the CD19 promoter, the transactivation properties of the various isoforms were significantly influenced by the changes in the C-terminal protein sequence. Finally, we demonstrate, for the first time, that human Pax-5 isoform expression is modulated by specific signaling pathways in B lymphocytes.