Analysis of the murine AhR gene promoter

Activation of aryl hydrocarbon receptor (AhR) in Alzheimer's disease: role of tryptophan metabolites generated by gut host-microbiota

Gut microbiota in interaction with intestinal host tissues influences many brain functions and microbial dysbiosis has been linked with brain disorders, such as neuropsychiatric conditions and Alzheimer's disease (AD). L-tryptophan metabolites and short-chained fatty acids (SCFA) are major messengers in the microbiota-brain axis. Aryl hydrocarbon receptors (AhR) are main targets of tryptophan metabolites in brain microvessels which possess an enriched expression of AhR protein. The Ah receptor is an evolutionarily conserved, ligand-activated transcription factor which is not only a sensor of xenobiotic toxins but also a pleiotropic regulator of both developmental processes and age-related tissue degeneration. Major microbiota-produced tryptophan metabolites involve indole derivatives, e.g., indole 3-pyruvic acid, indole 3-acetaldehyde, and indoxyl sulfate, whereas indoleamine and tryptophan 2,3-dioxygenases (IDO/TDO) of intestine host cells activate the kynurenine (KYN) pathway generating KYN metabolites, many of which are activators of AhR signaling. Chronic kidney disease (CKD) increases the serum level of indoxyl sulfate which promotes AD pathogenesis, e.g., it disrupts integrity of blood-brain barrier (BBB) and impairs cognitive functions. Activation of AhR signaling disturbs vascular homeostasis in brain; (i) it controls blood flow via the renin-angiotensin system, (ii) it inactivates endothelial nitric oxide synthase (eNOS), thus impairing NO production and vasodilatation, and (iii) it induces oxidative stress, stimulates inflammation, promotes cellular senescence, and enhances calcification of vascular walls. All these alterations are evident in cerebral amyloid angiopathy (CAA) in AD pathology. Moreover, AhR signaling can disturb circadian regulation and probably affect glymphatic flow. It seems plausible that dysbiosis of gut microbiota impairs the integrity of BBB via the activation of AhR signaling and thus aggravates AD pathology. KEY MESSAGES: Dysbiosis of gut microbiota is associated with dementia and Alzheimer's disease. Tryptophan metabolites are major messengers from the gut host-microbiota to brain. Tryptophan metabolites activate aryl hydrocarbon receptor (AhR) signaling in brain. The expression of AhR protein is enriched in brain microvessels and blood-brain barrier. Tryptophan metabolites disturb brain vascular integrity via AhR signaling. Dysbiosis of gut microbiota promotes inflammation and AD pathology via AhR signaling.

Aryl hydrocarbon receptor affects circadian-regulated lipolysis through an E-Box-dependent mechanism

An internal circadian clock regulates timing of systemic energy homeostasis. The central clock in the hypothalamic suprachiasmatic nucleus (SCN) directs local clocks in peripheral tissues such as liver, muscle, and adipose tissue to synchronize metabolism with food intake and rest/activity cycles. Aryl hydrocarbon receptor (AhR) interacts with the molecular circadian clockworks. Activation of AhR dampens rhythmic expression of core clock genes, which may lead to metabolic dysfunction. Given the importance of appropriately-timed adipose tissue function to regulation of energy homeostasis, this study focused on mechanisms by which AhR may influence clock-controlled adipose tissue activity. We hypothesized that AhR activation in adipose tissue would impair lipolysis by dampening adipose rhythms, leading to a decreased lipolysis rate during fasting, and subsequently, altered serum glucose concentrations. Levels of clock gene and lipolysis gene transcripts in mouse mesenchymal stem cells (BMSCs) differentiated into mature adipocytes were suppressed by the AhR agonist β-napthoflavone (BNF), in an AhR dependent manner. BNF altered rhythms of core clock gene and lipolysis gene transcripts in C57bl6/J mice. BNF reduced serum free fatty acids, glycerol and liver glycogen. Chromatin immunoprecipitation indicated that BNF increased binding of AhR to E-Box elements in clock gene and lipolysis gene promoters. These data establish a link between AhR activation and impaired lipolysis, specifically by altering adipose tissue rhythmicity. In response to the decreased available energy from impaired lipolysis, the body increases glycogenolysis, thereby degrading more glycogen to provide necessary energy.

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.

Daytime Restricted Feeding Modifies the Temporal Expression of CYP1A1 and Attenuated Damage Induced by Benzo[a]pyrene in Rat Liver When Administered before CYP1A1 Acrophase

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that heterodimerizes with the AhR nuclear translocator (ARNT) to modulate CYP1A1 expression, a gene involved in the biotransformation of benzo[a]pyrene (BaP). The AhR pathway shows daily variations under the control of the circadian timing system. Daytime restricted feeding (DRF) entrains the expression of genes involved in the processing of nutrients and xenobiotics to food availability. Therefore, we evaluate if temporal AhR, ARNT, and CYP1A1 hepatic expression in rats are due to light/dark cycles or fasting/feeding cycles promoted by DRF. Our results show that AhR oscillates throughout the 24 h period in DRF and ad libitum feeding rats (ALF), showing maximum expression at the same time points. DRF modified the peak of ARNT expression at ZT5; meanwhile, ALF animals showed a peak of maximum expression at ZT17. An increased expression of CYP1A1 was linked to the meal time in both groups of animals. Although a high CYP1A1 expression has been previously associated with BaP genotoxicity, our results show that, compared with the ALF group, DRF attenuated the BaP-CYP1A1 induction potency, the liver DNA-BaP adducts, the liver concentration of unmetabolized BaP, and the blood aspartate aminotransferase and alanine aminotransferase activities when BaP is administered prior to the acrophase of CYP1A1 expression. These results demonstrate that DRF modifies the ARNT and CYP1A1 expression and protects from BaP toxicity.

Circadian regulation of transporter expression and implications for drug disposition

Introduction: Solute Carrier (SLC) and ATP-binding cassette (ABC) transporters expressed in the intestine, liver, and kidney determine the absorption, distribution, and excretion of drugs. In addition, most molecular and cellular processes show circadian rhythmicity controlled by circadian clocks that leads to diurnal variations in the pharmacokinetics and pharmacodynamics of many drugs and affects their therapeutic efficacy and toxicity.Area covered: This review provides an overview of the current knowledge on the circadian rhythmicity of drug transporters and the molecular mechanisms of their circadian control. Evidence for coupling drug transporters to circadian oscillators and the plausible candidates conveying circadian clock signals to target drug transporters, particularly transcription factors operating as the output of clock genes, is discussed.Expert opinion: The circadian machinery has been demonstrated to interact with the uptake and efflux of various drug transporters. The evidence supports the concept that diurnal changes that affect drug transporters may influence the pharmacokinetics of the drugs. However, more systematic studies are required to better define the timing of pharmacologically important drug transporter regulation and determine tissue- and sex-dependent differences. Finally, the transfer of knowledge based on the results and conclusions obtained primarily from animal models will require careful validation before it is applied to humans.

FSH/LH-Dependent Upregulation of Ahr in Murine Granulosa Cells Is Controlled by PKA Signaling and Involves Epigenetic Regulation

The aryl hydrocarbon receptor (Ahr) is a ligand-activated transcription factor primarily known for its toxicological functions. Recent studies have established its importance in many physiological processes including female reproduction, although there is limited data about the precise mechanisms how Ahr itself is regulated during ovarian follicle maturation. This study describes the expression of Ahr in ovarian granulosa cells (GCs) of immature mice in a gonadotropin-dependent manner. We show that Ahr upregulation in vivo requires both follicle stimulating hormone (FSH) and luteinizing hormone (LH) activities. FSH alone increased Ahr mRNA, but had no effect on Ahr protein level, implicating a possible LH-dependent post-transcriptional regulation. Also, the increase in Ahr protein is specific to large antral follicles in induced follicle maturation. We show that Ahr expression in GCs of mid-phase follicular maturation is downregulated by protein kinase A (PKA) signaling and activation of Ahr promoter is regulated by chromatin remodeling.

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.

Multigenerational and Transgenerational Effects of Dioxins

Dioxins are ubiquitous and persistent environmental contaminants whose background levels are still reason for concern. There is mounting evidence from both epidemiological and experimental studies that paternal exposure to the most potent congener of dioxins, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), can lower the male/female ratio of offspring. Moreover, in laboratory rodents and zebrafish, TCDD exposure of parent animals has been reported to result in reduced reproductive performance along with other adverse effects in subsequent generations, foremost through the paternal but also via the maternal germline. These impacts have been accompanied by epigenetic alterations in placenta and/or sperm cells, including changes in methylation patterns of imprinted genes. Here, we review recent key studies in this field with an attempt to provide an up-to-date picture of the present state of knowledge to the reader. These studies provide biological plausibility for the potential of dioxin exposure at a critical time-window to induce epigenetic alterations across multiple generations and the significance of aryl hydrocarbon receptor (AHR) in mediating these effects. Currently available data do not allow to accurately estimate the human health implications of these findings, although epidemiological evidence on lowered male/female ratio suggests that this effect may take place at realistic human exposure levels.

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.

The aryl hydrocarbon receptor in the crossroad of signalling networks with therapeutic value

The aryl hydrocarbon receptor (AhR) is well-known for its major contributions to the cellular responses against environmental toxins and carcinogens. Notably, AhR has also emerged as a key transcription factor controlling many physiological processes including cell proliferation and apoptosis, differentiation, adhesion and migration, pluripotency and stemness. These novel functions have broadened our understanding of the signalling pathways and molecular intermediates interacting with AhR under both homeostatic and pathological conditions. Recent discoveries link AhR with the function of essential organs such as liver, skin and gonads, and with complex organismal structures including the immune and cardiovascular systems. The identification of potential endogenous ligands able to regulate AhR activity, opens the possibility of designing ad hoc molecules with pharmacological and/or therapeutic value to treat human diseases in which AhR may have a causal role. Integration of experimental data from in vitro and in vivo studies with "omic" analyses of human patients affected with cancer, immune diseases, inflammation or neurological disorders will likely contribute to validate the clinical relevance of AhR and the possible benefits of modulating its activity by pharmacologically-driven strategies. In this review, we will highlight signalling pathways involved in human diseases that could be targetable by AhR modulators and discuss the feasibility of using such molecules in therapy. The pros and cons of AhR-aimed approaches will be also mentioned.

Light- and clock-control of genes involved in detoxification

Circadian regulation of hepatic detoxification seems to be amongst the key roles of the biological clock. The liver is the major site for biotransformation, and in mammals, it contains several clock-controlled transcription factors such as proline and acidic amino acid-rich basic leucine zipper proteins (PAR bZIP) and basic-helix-loop-helix Per-Arnt-Sim (bHLH-PAS) family that act as circadian regulators of detoxification genes. This investigation explored the existence of daily and circadian expression of transcription factors involved in detoxification, as well as the temporal profile of a set of their target genes in zebrafish liver. In our study, zebrafish were able to synchronize to a light-dark (LD) cycle and displayed a diurnal pattern of activity. In addition, the expression of clock genes presented daily and circadian rhythmicity in liver. Apart from hlfa, the expression of PAR bZIP transcription factors also displayed daily rhythms, which appeared to be both light-dependent and clock-controlled, as circadian rhythms free-ran under constant conditions (continuous darkness, DD). Under LD, tefb, dbpa and dbpb expression peaked at the end of the darkness period whereas tefa showed peak levels of expression at the onset of the photophase. In addition, these four genes exhibited circadian expression under DD, with higher expression levels at the end of the subjective night. The expression of the bHLH-PAS transcription factor arh2 also showed circadian rhythmicity in zebrafish liver, peaking in the middle of the subjective night and approximately 3-4 h before peak expression of the PAR bZIP genes. Regarding the detoxification genes, the major target gene of AhR, cyp1a, showed daily and circadian expression with an acrophase 2 h after ahr2. Under LD, abcb4 also showed daily rhythmicity, with an acrophase 1-2 h after that of PAR bZIP factors during the transition between darkness and light phases, when zebrafish become active. However, the expression of six detoxification genes showed circadian rhythmicity under DD, including cyp1a and abcb4 as well as gstr1, mgst3a, abcg2 and sult2_st2. In all cases, the acrophases of these genes were found during the second half of the subjective night, in phase with the PAR bZIP transcription factors. This suggested that their expression is clock-controlled, either directly by core clock genes or through transcription factors. This study presents new data demonstrating that the process of detoxification is under circadian control in fish. Results showed that time of day should be considered when designing toxicological studies or administering drugs to fish.

Aryl Hydrocarbon Receptor Deficiency Alters Circadian and Metabolic Rhythmicity

PAS domain-containing proteins can act as environmental sensors that capture external stimuli to allow coordination of organismal physiology with the outside world. These proteins permit diverse ligand binding and heterodimeric partnership, allowing for varied combinations of PAS-dependent protein-protein interactions and promoting crosstalk among signaling pathways. Previous studies report crosstalk between circadian clock proteins and the aryl hydrocarbon receptor (AhR). Activated AhR forms a heterodimer with the circadian clock protein Bmal1 and thereby functionally inhibits CLOCK/Bmal1 activity. If physiological activation of AhR through naturally occurring, endogenous ligands inhibits clock function, it seems plausible to hypothesize that decreased AhR expression releases AhR-induced inhibition of circadian rhythms. Because both AhR and the clock are important regulators of glucose metabolism, it follows that decreased AhR will also alter metabolic function. To test this hypothesis, rhythms of behavior, metabolic outputs, and circadian and metabolic gene expression were measured in AhR-deficient mice. Genetic depletion of AhR enhanced behavioral responses to changes in the light-dark cycle, increased rhythmic amplitude of circadian clock genes in the liver, and altered rhythms of glucose and insulin. This study provides evidence of AhR-induced inhibition that influences circadian rhythm amplitude.

Role of Aryl Hydrocarbon Receptor in Circadian Clock Disruption and Metabolic Dysfunction

The prevalence of metabolic syndrome, a clustering of three or more risk factors that include abdominal obesity, increased blood pressure, and high levels of glucose, triglycerides, and high-density lipoproteins, has reached dangerous and costly levels worldwide. Increases in morbidity and mortality result from a combination of factors that promote altered glucose metabolism, insulin resistance, and metabolic dysfunction. Although diet and exercise are commonly touted as important determinants in the development of metabolic dysfunction, other environmental factors, including circadian clock disruption and activation of the aryl hydrocarbon receptor (AhR) by dietary or other environmental sources, must also be considered. AhR binds a range of ligands, which prompts protein-protein interactions with other Per-Arnt-Sim (PAS)-domain-containing proteins and subsequent transcriptional activity. This review focuses on the reciprocal crosstalk between the activated AhR and the molecular circadian clock. AhR exhibits a rhythmic expression and time-dependent sensitivity to activation by AhR agonists. Conversely, AhR activation influences the amplitude and phase of expression of circadian clock genes, hormones, and the behavioral responses of the clock system to changes in environmental illumination. Both the clock and AhR status and activation play significant and underappreciated roles in metabolic homeostasis. This review highlights the state of knowledge regarding how AhR may act together with the circadian clock to influence energy metabolism. Understanding the variety of AhR-dependent mechanisms, including its interactions with the circadian timing system that promote metabolic dysfunction, reveals new targets of interest for maintenance of healthy metabolism.

Auto-induction mechanism of aryl hydrocarbon receptor 2 (AHR2) gene by TCDD-activated AHR1 and AHR2 in the red seabream (Pagrus major)

The toxic effects of dioxins and related compounds (DRCs) are mediated by the aryl hydrocarbon receptor (AHR). Our previous study identified AHR1 and AHR2 genes from the red seabream (Pagrus major). Moreover, we found that AHR2 mRNA levels were notably elevated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure in the early life stage of red seabream embryos, while AHR1 mRNA level was not altered. In this study, to investigate the regulatory mechanism of these AHR transcripts, we cloned and characterized 5'-flanking regions of AHR1 and AHR2 genes. Both of the 5'-flanking regions in these AHR genes contained three potential xenobiotic-responsive elements (XREs). To assess whether the 5'-flanking region is transactivated by rsAHR1 and rsAHR2 proteins, we measured the transactivation potency of the luciferase reporter plasmids containing the 5'-flanking regions by AHR1 and AHR2 proteins that were transiently co-expressed in COS-7. Only reporter plasmid (pGL4-rsAHR2-3XREs) that contained three putative XRE sites in the 5'-flanking region of AHR2 gene showed a clear TCDD dose-dependent transactivation by AHR1 and AHR2 proteins. TCDD-EC₅₀ values for the rsAHR2-derived XRE transactivation were 1.3 and 1.4 nM for AHR1 and AHR2, respectively. These results suggest that the putative XREs of AHR2 gene have a function for AHR1- and AHR2-mediated transactivation, supporting our in ovo observation of an induction of AHR2 mRNA levels by TCDD exposure. Mutations in XREs of AHR2 gene led to a decrease in luciferase induction. Electrophoretic mobility shift assay showed that XRE1, the closest XRE from the start codon in AHR2 gene, is mainly responsible for the binding with TCDD-activated AHR. This suggests that TCDD-activated AHR1 and AHR2 up-regulate the AHR2 mRNA levels and this auto-induced AHR2 may amplify the signal transduction of its downstream targets including CYP1A in the red seabream.

Interplay between Dioxin-mediated signaling and circadian clock: a possible determinant in metabolic homeostasis

The rotation of the earth on its axis creates the environment of a 24 h solar day, which organisms on earth have used to their evolutionary advantage by integrating this timing information into their genetic make-up in the form of a circadian clock. This intrinsic molecular clock is pivotal for maintenance of synchronized homeostasis between the individual organism and the external environment to allow coordinated rhythmic physiological and behavioral function. Aryl hydrocarbon receptor (AhR) is a master regulator of dioxin-mediated toxic effects, and is, therefore, critical in maintaining adaptive responses through regulating the expression of phase I/II drug metabolism enzymes. AhR expression is robustly rhythmic, and physiological cross-talk between AhR signaling and circadian rhythms has been established. Increasing evidence raises a compelling argument that disruption of endogenous circadian rhythms contributes to the development of disease, including sleep disorders, metabolic disorders and cancers. Similarly, exposure to environmental pollutants through air, water and food, is increasingly cited as contributory to these same problems. Thus, a better understanding of interactions between AhR signaling and the circadian clock regulatory network can provide critical new insights into environmentally regulated disease processes. This review highlights recent advances in the understanding of the reciprocal interactions between dioxin-mediated AhR signaling and the circadian clock including how these pathways relate to health and disease, with emphasis on the control of metabolic function.

Transcriptional repression of the Ahr gene by LHCGR signaling in preovulatory granulosa cells is controlled by chromatin accessibility

Recent advances in establishing the role of the aryl hydrocarbon receptor (Ahr) in normophysiology have discovered its fundamental role, amongst others, in female reproduction. Considering previous studies suggesting the hormonal modulation of Ahr, we aimed to investigate whether in murine granulosa cells (GCs) the gonadotropins regulate Ahr expression and how this is mechanistically implemented. We found that the FSH-like substance--pregnant mare serum gonadotropin--led to stimulation of Ahr expression. More importantly hCG produced relatively rapid reduction of Ahr mRNA in GCs of preovulatory follicles. We show for the first time that LHCGR signaling in regulating the Ahr message involves protein kinase A pathway and is attributable to decreased transcription rate. Finally, we found that Ahr promoter accessibility was decreased by hCG, implicating chromatin remodeling in Ahr gene regulation by LH.

Genome-wide RNAi high-throughput screen identifies proteins necessary for the AHR-dependent induction of CYP1A1 by 2,3,7,8-tetrachlorodibenzo-p-dioxin

The aryl hydrocarbon receptor (AHR) has a plethora of physiological roles, and upon dysregulation, carcinogenesis can occur. One target gene of AHR encodes the xenobiotic and drug-metabolizing enzyme CYP1A1, which is inducible by the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via the AHR. An siRNA library targeted against over 5600 gene candidates in the druggable genome was used to transfect mouse Hepa-1 cells, which were then treated with TCDD, and subsequently assayed for CYP1A1-dependent ethoxyresorufin-o-deethylase (EROD) activity. Following redundant siRNA activity (RSA) statistical analysis, we identified 93 hits that reduced EROD activity with a p value ≤ .005 and substantiated 39 of these as positive hits in a secondary screening using endoribonuclease-prepared siRNAs (esiRNAs). Twelve of the corresponding gene products were subsequently confirmed to be necessary for the induction of CYP1A1 messenger RNA by TCDD. None of the candidates were deficient in aryl hydrocarbon nuclear translocator expression. However 6 gene products including UBE2i, RAB40C, CRYGD, DCTN4, RBM5, and RAD50 are required for the expression of AHR as well as for induction of CYP1A1. We also found 2 gene products, ARMC8 and TCF20, to be required for the induction of CYP1A1, but our data are ambiguous as to whether they are required for the expression of AHR. In contrast, SIN3A, PDC, TMEM5, and CD9 are not required for AHR expression but are required for the induction of CYP1A1, implicating a direct role in Cyp1a1 transcription. Our methods, although applied to Cyp1a1, could be modified for identifying proteins that regulate other inducible genes.

Genetic and epigenetic regulation of AHR gene expression in MCF-7 breast cancer cells: role of the proximal promoter GC-rich region

The aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, contributes to carcinogenesis through its role in the regulation of cytochrome P450 1 (CYP1)-catalyzed metabolism of carcinogens. Here, we investigated genetic and epigenetic mechanisms that affect AhR expression. Analyses of the human AHR proximal promoter in MCF-7 human breast cancer cells using luciferase assays and electrophoretic mobility shift assays revealed multiple specificity protein (Sp) 1 binding sequences that are transcriptional activators in vitro. The regulation of AhR expression was evaluated in long-term estrogen exposed (LTEE) MCF-7 cells, which showed increased AhR expression, enhanced CYP1 inducibility, and increased capacity to form DNA adducts when exposed to the dietary carcinogen, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. The increased AhR expression in LTEE cells was found not to result from increased mRNA stability, differential RNA processing, or decreased DNA methylation. Analysis of the AHR proximal promoter region using chromatin immunoprecipitation confirmed that enhanced expression of AhR in LTEE cells involves changes in histone modifications, notably decreased trimethylation of histone 3, lysine 27. Upon further examination of the GC-rich Sp1-binding region, we confirmed that it contains a polymorphic (GGGGC)(n) repeat. In a population of newborns from New York State, the allele frequency of (GGGGC)(n) was n = 4 > 5 ≫ 6, 2. Circular dichroism spectroscopy revealed the ability of sequences of this GC-rich region to form guanine-quadruplex structures in vitro. These studies revealed multiple levels at which AhR expression may be controlled, and offer additional insights into mechanisms regulating AhR expression that can ultimately impact carcinogenesis.

Aryl hydrocarbon receptor-mediated Cyp1a1 expression is modulated in a CLOCK-dependent circadian manner

The expression of genes involved in xenobiotic detoxification is under the control of the circadian clock. The aryl hydrocarbon receptor (AhR) is one of the transcription factors responsible for the induction of detoxification enzymes in response to xenobiotic toxins, and the expression of AhR has been suggested to be regulated by a circadian oscillator. In this study, we investigated whether toxin-mediated activation of the AhR signaling pathway was modulated by CLOCK protein, a key component of the mammalian circadian clock. The expression of AhR and its DNA binding ability in the lungs of wild-type mice showed significant 24-h oscillation. Clock mutant (Clk/Clk) mice, producing CLOCK protein deficient in transcriptional activity, failed to show significant oscillation in the expression of AhR. The mRNA levels of AhR in the lungs of Clk/Clk mice were significantly lower than in wild-type mice. A single intraperitoneal injection of benzo[α]pyrene, a ligand of AhR, induced the expression of Cyp1a1 in the lungs of wild-type mice, but the induction varied depending on the benzo[α]pyrene injection time. The dosing time-dependency of benzo[α]pyrene-induced Cyp1a1 expression was also modulated by Clock gene mutation. These findings suggest that CLOCK protein affects the toxin-induced expression of detoxification enzymes through modulating the activity of AhR. Our present findings provide a molecular link between the circadian clock and xenobiotic detoxification.

Nrf2-dependent induction of NQO1 in mouse aortic endothelial cells overexpressing catalase

Overexpression of catalase has been shown to accelerate benzo(a)pyrene (BaP) detoxification in mouse aortic endothelial cells (MAECs). NAD(P)H:quinone oxidoreductase-1 (NQO1) is an enzyme that catalyzes BaP-quinone detoxification. Aryl hydrocarbon receptor (AhR) and nuclear factor erythroid 2-related factor-2 (Nrf2) are transcription factors that control NQO1 expression. Here, we investigated the effects of catalase overexpression on NQO1, Nrf2, and AhR expression. The levels of NQO1 mRNA and protein were comparable in MAECs isolated from wild-type and transgenic mice that overexpress human catalase (hCatTg). BaP treatment increased NQO1 mRNA and protein levels in both groups, with a significantly greater induction in hCatTg MAECs than in wild-type cells. BaP-induced NQO1 promoter activity was dramatically higher in hCatTg MAECs than in wild-type cells. Our data also showed that the basal level of AhR and the BaP-induced level of Nrf2 were significantly higher in hCatTg MAECs than in wild-type cells. Inhibition of specificity protein-1 (Sp1) binding to the AhR promoter region by mithramycin A reversed the enhancing effect of catalase overexpression on AhR expression. Knockdown of AhR by RNA interference diminished BaP-induced expression of Nrf2 and NQO1. Knockdown of Nrf2 significantly decreased NQO1 mRNA and protein levels in cells with or without BaP treatment. NQO1 promoter activity was abrogated by mutation of the Nrf2-binding site in this promoter. In contrast, mutation of the AhR-binding site in the NQO1 promoter did not affect the promoter activity. These results suggest that catalase overexpression upregulates BaP-induced NQO1 expression by enhancing the Sp1-AhR-Nrf2 signaling cascade.

An introduction to the molecular basics of aryl hydrocarbon receptor biology

Depending on their chemical structure and properties, environmental chemicals and other xenobiotics that enter the cell can affect cellular function by either nonselective binding to cellular macromolecules or by interference with cellular receptors, which would initiate a more defined cell biological response. One of these intracellular chemosensor molecules is the aryl hydrocarbon receptor (AhR), a transcription factor of the bHLH/PAS family that is known to mediate the biochemical and toxic effects of dioxins, polyaromatic hydrocarbons and related compounds. Numerous investigations have revealed that the AhR is not only a master regulator of drug metabolism activated by anthropogenic chemicals, but is also triggered by natural and endogenous ligands and can influence cell biological endpoints such as growth and differentiation. Cutting-edge research has identified new intriguing functions of the AhR, such as during proteasomal degradation of steroid hormone receptors, the cellular UVB stress response and the differentiation of certain T-cell subsets. In this review we provide both a survey of the fundamental basics of AhR biology and an insight into new functional aspects of AhR signaling to further stimulate research on this intriguing transcription factor at the interface between toxicology, cell biology and immunology.

Overexpression of antioxidant enzymes upregulates aryl hydrocarbon receptor expression via increased Sp1 DNA-binding activity

We previously reported upregulation of aryl hydrocarbon receptor (AhR) expression as a mechanism by which overexpression of Cu/Zn-superoxide dismutase (SOD) and/or catalase accelerates benzo(a)pyrene (BaP) detoxification in mouse aorta endothelial cells (MAECs). The objective of this study was to investigate the regulatory role of specificity protein-1 (Sp1) in AhR expression in MAECs that overexpress Cu/Zn-SOD and/or catalase. Our data demonstrated comparable levels of nuclear Sp1 protein in the transgenic and wild-type MAECs; however, binding of Sp1 protein to the AhR promoter region was more than 2-fold higher in MAECs overexpressing Cu/Zn-SOD and/or catalase than in wild-type cells. Inhibition of Sp1 binding to the AhR promoter by mithramycin A reduced AhR expression and eliminated the differences between wild-type MAECs and three lines of transgenic cells. Functional promoter analysis indicated that AhR promoter activity was significantly higher in MAECs overexpressing catalase than in wild-type cells. Mutation of an AhR promoter Sp1-binding site or addition of hydrogen peroxide to the culture medium reduced AhR promoter activity, and decreased the differences between wild-type MAECs and transgenic cells overexpressing catalase. These results suggest that increased Sp1 binding to the AhR promoter region is an underlying mechanism for upregulation of AhR expression in MAECs that overexpress Cu/Zn-SOD and/or catalase.

Regulation of aryl hydrocarbon receptor expression and function by glucocorticoids in mouse hepatoma cells

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates most biological responses to 2,3, 7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related aromatic hydrocarbons. Although the role of the AHR in control of drug metabolism and endocrine disruption is partly understood, we know little about the regulation of the AHR itself by endocrine factors. Our work with hypophysectomized rats suggested that hepatic AHR protein level is positively regulated by pituitary-dependent factors. A current hypothesis is that adrenal glucocorticoids elevate AHR expression and enhance responsiveness to AHR agonists. Dexamethasone (DEX) at concentrations that activate the glucocorticoid receptor (GR) increased AHR mRNA, protein, and TCDD-binding by approximately 50% in Hepa-1 mouse hepatoma cells. This response was blocked by the GR antagonist 17beta-hydroxy-11beta-[4-dimethylamino phenyl]-17alpha-[1-propynyl]estra-4,9-dien-3-one (RU486), suggesting GR involvement. This small magnitude increase in AHR levels was functionally significant; pretreatment of Hepa-1 cells with DEX caused a 75% increase in the maximum induction of an AHR-activated luciferase reporter plasmid by TCDD. A luciferase reporter under control of the proximal 2.5 kilobases of the mouse Ahr 5'-flanking region and promoter was induced approximately 2.5-fold by DEX when cotransfected with a mouse GR expression plasmid. This is the first demonstration that glucocorticoids increase AHR levels in hepatoma cells via a GR-dependent transcriptional mechanism, suggesting a novel aspect of cross-talk between the AHR and the GR.

Glucocorticoid-Enhanced Expression of Dioxin Target Genes through Regulation of the Rat Aryl Hydrocarbon Receptor

The aryl hydrocarbon receptor (AhR) and glucocorticoid receptor (GR) are ligand-activated transcription factors and members of the basic helix-loop-helix Period-aryl hydrocarbon nuclear translocator-single minded and nuclear hormone receptor superfamilies, respectively. Besides their individual role as activators of specific gene transcription, also interplay between both transcription factors can be an important mechanism of regulation. In this study, we report that GR can strongly activate AhR-mediated transcription and consequent gene expression in rat H4IIe cells. Reporter gene assays showed an enhanced effect of dexamethasone on the dioxin response mediated by GR in rat H4IIe cells and mouse Hepa 1c1c7 cells, but not in human HepG2 cells and human T47D cells. These deviations between the rodent and human cell lines were confirmed by CYP1A1 enzyme activities. In addition, quantitative reverse transcription-PCR showed enhanced GR-mediated effects of dexamethasone on endogenous 2,3,7,8-tetrachlorodibenzo-[p]-dioxin target genes as well in rat H4IIe cells, but not in human HepG2 and human T47D cells. Surprisingly, AhR itself was upregulated by combined dioxin/glucocorticoid exposure in rat H4IIe cells but not in the human cells which could be explained by the presence of two putative glucocorticoid response elements in the rat AhR promoter, but not in the human AhR promoter. This GR-mediated expression of dioxin target genes through upregulation of the AhR in rat but not in human cells opens the possibility that dioxin responses in rodent-based models for toxicity differ from humans and provides new insight into the interactions of stress-related pathways, biological effects of dioxin-like compounds and may possibly have implications for risk assessment.

Regulating the regulator: Factors that control levels and activity of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) participates in a wide range of critical cellular events in response to endogenous signals or xenobiotic chemicals. Hence, it is important that AHR levels and activity themselves be well controlled in target tissues. The AHR is essentially ubiquitous in its distribution in mammalian tissues. However, levels of the receptor vary widely across different tissues and among different cell types. AHR levels and activity are modulated by exposure to the receptor's own ligands and are influenced by other xenobiotic chemicals. Many different factors impinge on AHR levels and AHR activity. These factors may alter responsiveness of downstream pathways, thereby affecting normal physiologic functions as well as responses to toxic environmental chemicals such as dioxins. Our commentary appraises the current literature on factors that regulate AHR levels/activity and attempts to identify fruitful strategies towards discovery of key pathways by which AHR levels are modulated in response to endogenous signals and in response to xenobiotic chemicals. An extraordinarily large number of agents alter the level or activity of the AHR. We have not yet entered an age of enlightenment sufficient to achieve true understanding of the interplay of mechanisms that regulate AHR expression in space and in time.

Aryl hydrocarbon receptor (AHR) and AHR nuclear translocator (ARNT) expression in Baikal seal (Pusa sibirica) and association with 2,3,7,8-TCDD toxic equivalents and CYP1 expression levels

Most toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related planar halogenated aromatic hydrocarbons (PHAHs) are mediated by ligand-activated aryl hydrocarbon receptor (AHR) signaling pathway. To understand the regulation mechanism of AHR and AHR nuclear translocator (ARNT) expression in wild Baikal seal (Pusa sibirica) population contaminated by PHAHs, the present study investigated hepatic mRNA expression levels of AHR and its heterodimer, ARNT genes, in association with biological index (age, gender and body weight), PHAH accumulation and expression levels of cytochrome P450 (CYP) 1A and 1B. While there was no gender difference, the AHR mRNA expression levels were increased with ages (p = 0.014) and body weights (p = 0.015), indicating that AHR expression might be affected by these biological factors. The AHR mRNA expression levels exhibited significant positive correlations with total TEQs and most of individual congener TEQs derived from polychorinated dibenzo-p-dioxins, dibenzofurans and non-ortho coplanar polychorinated biphenyls (PCBs), indicating the transcriptional up-regulation of AHR expression by these congeners. On the other hand, there was no significant correlation between individual TEQs from mono-ortho coplanar PCBs and AHR expression. These results imply the structure-related transcriptional activity of AHR among PHAHs congeners. AHR mRNA levels showed positive correlations with both CYP1A protein (p = 0.039) and CYP1A1 mRNA expression levels (p = 0.046). In contrast to AHR expression, neither the total nor individual congener TEQs influenced ARNT at the transcriptional level. ARNT mRNA showed significant negative correlations with CYP1A/1B protein (p = 0.027 and p = 0.006) and CYP1A1 mRNA expression levels (p = 0.039), implying the existence of different transcriptional regulation between AHR and ARNT genes and negative regulation by CYP1A/1B-mediated signaling pathways. The present findings may render significant insight on the basic mechanisms underlying regulation of AHR and ARNT expressions associated with biological factors and PHAH exposure in wild mammalian populations.

Influence of cAMP on reporter bioassays for dioxin and dioxin-like compounds

In reporter assays for detection of dioxins, the dioxin-responsive element (DRE) is generally used as a sensor sequence. In several systems, the CYP1A1 promoter containing DREs (DRE(cyp)) is inserted into a part of the long terminal repeat of mouse mammary tumor virus (LTR(MMTV)) to improve sensitivity of assays. We found that DRE(cyp)-LTR(MMTV) responds not only to dioxins and dioxin-like compounds but also to forskolin, a cAMP-elevating agent. This effect was dose-dependent and reproduced by other cAMP-elevating agents including 8-bromo-cAMP and 3-isobutyl-methylxanthine. The cAMP response element (CRE) and CRE-like sequences were absent in DRE(cyp)-LTR(MMTV) and not involved in this process. In contrast to the effect of dioxin, the activation of DRE(cyp)-LTR(MMTV) by cAMP was independent of the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor for DRE. Furthermore, neither DRE(cyp), LTR(MMTV) nor the consensus sequence of DRE alone was activated in response to cAMP. These data elucidated for the first time that the combination of DRE(cyp) with LTR(MMTV) causes a peculiar response to cAMP and suggested that use of AhR antagonists is essential to exclude false-positive responses of DRE(cyp)-LTR(MMTV)-based bioassays for detection and quantification of dioxins and dioxin-like compounds.

Single nucleotide polymorphism analysis and functional characterization of the human Ah receptor (AhR) gene promoter

The aryl hydrocarbon receptor (AhR) mediates biological and toxicological actions of e.g., halogenated aromatic hydrocarbons such as 2,3,7,8-tetrachlorodibenzo-p-dioxin. Although much is known about the biochemical and molecular mechanisms of AhR action, little is known about the control of the expression of the AhR gene itself. Therefore, we aimed at the identification and characterization of regions important for constitutive AhR gene expression. First, we screened 2.6 kb of the 5(')-flanking region of the AhR gene in 91 healthy Caucasian volunteers for naturally occurring genetic variants. Seven variants were detected. However, they do not seem to influence AhR gene expression in lymphocytes. Using a 2.7 kb AhR promoter luciferase reporter gene construct and various deletion constructs, a putative regulatory region was identified and characterized further by electrophoretic mobility shift assays and site-directed mutagenesis. These investigations were confirmed by cotransfection experiments in Drosophila SL2 cells. The obtained results prove an involvement of Sp1 in AhR gene regulation.

Transcriptional regulation of the AhR gene during adipose differentiation

The aryl hydrocarbon receptor (AhR) mediates a spectrum of toxicological and biological effects of dioxins. Studies on tissue distribution of the AhR in developing and adult animals demonstrated that the AhR is expressed in a tissue-specific and developmentally specific manner. Also, the expression level of the AhR in culture cells varies more than 50-fold among cell lines. Although the mode of AhR action has been studied extensively, the events that control the expression of the AhR gene itself are still poorly understood. We previously showed that the AhR protein is depleted during adipose differentiation, resulting in the loss of functional response to xenobiotics. In this study, to understand the mechanism by which the AhR is depleted during adipogenesis, we analyzed the AhR promoter activity during adipose differentiation in 3T3-L1 cells. Nuclear run-on assay revealed that the downregulation of the AhR during adipogenesis is primarily at the transcriptional level. To identify the sequence of the AhR promoter region responsible for differentiation-dependent suppression of AhR transcription, a series of deletion constructs linked to the CAT reporter were transfected into 3T3-L1 cells. A comparison of CAT activity between preadipocytes and adipocytes revealed that the sequence -378/-359 is core contributor to differentiation-dependent downregulation of AhR promoter activity. EMSA and UV crosslinking studies showed the presence of the factor bound to the sequence -378/-359. The binding activity was apparently higher in preadipocytes than in adipocytes. Consequently, the downregulation of the trans-acting factor may result in the suppression of AhR gene transcription during adipose differentiation.

Cell-specific regulation of human aryl hydrocarbon receptor expression by transforming growth factor-beta(1)

Previous studies showed that TGF-beta down-regulates aryl hydrocarbon (AhR) expression in human lung carcinoma cells A549. Here we analyzed the molecular mechanisms by which TGF-beta modulates AhR expression. A 5799-nucleotide 5'-flanking region of human AhR gene was isolated. Transient transfection studies of full-length (hAhRP) and deletion promoter constructs indicate the requirement of a cis-regulatory element encompassing -1980 to -1892 for full constitutive activity. Basal hAhRP activity occurs in a cell-specific manner; human hepatoma HepG2 cells possess a 10-fold higher activity compared with A549 cells. TGF-beta exerts cell-specific effects on hAhRP activity. Treatment of cells with 100 pM TGF-beta leads to a 50% inhibition in A549 and a 3-fold induction in HepG2 cells. Deletion mutagenesis identified a TGF-beta-responsive sequence containing a functional conserved Smad-binding element. Transient overexpression of Smad 2, 3, and 4 indicates that these signal transducers modulate hAhRP activity. The down-regulation of AhR by TGF-beta is modulated by 5'-TG-3'-interacting factor (TGIF). Transient overexpression of TGIF in MDA-MB231 and HepG2 cells led to inhibition of hAhRP activity and a similar decrease of AhR mRNA expression. Our findings indicate that Smad proteins are involved in the cell-specific regulation of AhR expression by TGF-beta.