Ligand-dependent and independent modulation of aryl hydrocarbon receptor localization, degradation, and gene regulation

Examining the Aryl Hydrocarbon Receptor Network in the Placental Tissues of Pregnancies Complicated by Pre-Eclampsia: An Explorative Case-Control Analysis

Severe maternal and newborn morbidity and mortality associated with pre-eclampsia, which are caused partly by premature delivery, affect a factual proportion of pregnancies. Despite its prevalence, the underlying causes of pre-eclampsia remain elusive, with emerging evidence implicating the aryl hydrocarbon receptor (AhR) in its pathogenesis. This study sought to elucidate the involvement of the AhR and its associated pathway in pre-eclampsia by comparing placental components of the AhR pathway in pregnant individuals with and without pre-eclampsia. This case-control investigation was conducted at the University Hospital of Udine from May 2021 to February 2023. The AhR was assessed using immunohistochemistry and immunofluorescence, and its mRNA was evaluated using a Real-Time Quantitative Reverse Transcription PCR. Levels of mRNA expression were also estimated for other components of the AhR pathway (CYP1B1, IDO1, ARNT, TIPARP, S100A4, and AHRR). Our findings show decreased levels of expression of AhR, IDO1, ARNT, TiPARP, and S100A4 in the placental tissues of individuals with pre-eclampsia compared to controls (p < 0.05). The AhR exhibited a distinct localization within the syncytiotrophoblast (nuclei and cytoplasm) and CD45-positive cells (nuclei and cytoplasm). Furthermore, a significant positive correlation between the AhR and S100A4 (rho = 0.81) was observed in normal placentas, while CYP1B1 displayed a significant negative correlation with the AhR (rho = -0.72), within addition to its negative correlation with TiPARP (rho = -0.83). This study illuminates pre-eclampsia's molecular aberrations, suggesting new diagnostic, therapeutic, and mechanistic approaches. This study emphasizes the need for more research to validate and broaden these findings to improve the management of this complex pregnancy condition.

Activation of Chaperone-Mediated Autophagy Inhibits the Aryl Hydrocarbon Receptor Function by Degrading This Receptor in Human Lung Epithelial Carcinoma A549 Cells

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor and a substrate protein of a Cullin 4B E3 ligase complex responsible for diverse cellular processes. In the lung, this receptor is responsible for the bioactivation of benzo[a]pyrene during tumorigenesis. Realizing that the AHR function is affected by its expression level, we are interested in the degradation mechanism of AHR in the lung. Here, we have investigated the mechanism responsible for AHR degradation using human lung epithelial A549 cells. We have observed that the AHR protein levels increase in the presence of chloroquine (CQ), an autophagy inhibitor, in a dose-dependent manner. Treatment with 6-aminonicotinamide (6-AN), a chaperone-mediated autophagy (CMA) activator, decreases AHR protein levels in a concentration-dependent and time-dependent manner. This decrease suppresses the ligand-dependent activation of the AHR target gene transcription, and can be reversed by CQ but not MG132. Knockdown of lysosome-associated membrane protein 2 (LAMP2), but not autophagy-related 5 (ATG5), suppresses the chloroquine-mediated increase in the AHR protein. AHR is resistant to CMA when its CMA motif is mutated. Suppression of the epithelial-to-mesenchymal transition in A549 cells is observed when the AHR gene is knocked out or the AHR protein level is reduced by 6-AN. Collectively, we have provided evidence supporting that AHR is continuously undergoing CMA and activation of CMA suppresses the AHR function in A549 cells.

A new insight into the aryl hydrocarbon receptor/cytochrome 450 signaling pathway in MG63, HOS, SAOS2, and U2OS cell lines

Osteosarcoma is the most common malignant tumor of bone, with rapid progressive growth, early distant metastases, and frequent recurrence after surgical treatment. Osteosarcoma is characterized by changes in the ratio and expression of different cytochrome P450 (CYP) isoforms that can affect the effectiveness of anticancer therapies. The inducible expression of CYP1 genes depends on the ligand-dependent functionality of the aryl hydrocarbon receptor (AHR). In this study, we examined the AHR/CYP1 signaling pathway in four osteosarcoma cell lines (MG63, HOS, SAOS2, and U2OS) induced by the known AHR ligands: indirubin, indole-3-carbinol, and beta-naphthoflavone. Using qPCR and Western blot analysis, we explored the effects of these ligands on the expression of the CYP1 genes and studied the correlation between these responses and the changes in the mRNA and protein levels of AHR and the AHR nuclear translocator (ARNT) in these osteosarcoma cell lines. The results show that the AHR/CYP1 signaling pathway retains its function only in MG63 and HOS cells, and is impaired in SAOS2 and U2OS cells. Our data should be taken into account when recommending new strategies for the treatment of osteosarcoma and when evaluating new drugs against osteosarcoma in vitro.

Metabolomics and proteomics study reveals the effects of benzo[a]pyrene on the viability and migration of KYSE-150 esophageal cells

A representative polycyclic aromatic hydrocarbon, benzo[a]pyrene (B[a]P), has been widely detected in environmental compartments and is highly carcinogenic to humans. Oral ingestion of B[a]P is the dominant exposure pathway. The esophagus acts as the first contact point when B[a]P enters the human body. However, its role in the development of human esophageal cancer is rarely discussed. Herein, we employed untargeted metabolomics in combination with proteomics to explore B[a]P-related intracellular responses in human esophageal cell lines. Our results demonstrated that B[a]P exposure induced significant metabolic disorders, further leading to overproduction of reactive oxygen species (ROS) and disturbance of the cellular viability process and migration ability of esophageal cells. In response, glutathione (GSH) was consumed to meet the demand for cellular detoxification, and thioredoxin (TXN) was upregulated to balance the cellular redox. These alterations caused the reregulation of some specific protein families, including S100A proteins, ribosomal proteins, and histone H1 proteins. Such changes impeded the viability and migration of esophageal cells, which could adversely affect wound healing of the epithelium. These cellular responses indicate that B[a]P will cause serious cellular damage to esophageal cells and increase the carcinogenic risk even as a result of short-term exposure. SYNOPSIS: Our omics study demonstrated how benzo[a]pyrene hampered the migration of esophageal cells and proposed a plausible mechanism underlying its carcinogenicity, which may contribute to our understanding of environmental pollutants.

Roles of the ubiquitin ligase CUL4B and ADP-ribosyltransferase TiPARP in TCDD-induced nuclear export and proteasomal degradation of the transcription factor AHR

The aryl hydrocarbon receptor (AHR) is a transcription factor activated by exogenous halogenated polycyclic aromatic hydrocarbon compounds, including the environmental toxin TCDD, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and naturally occurring dietary and endogenous compounds. The activated AHR enhances transcription of specific genes including phase I and phase II metabolism enzymes and other targets genes such as the TCDD-inducible poly(ADP-ribose) polymerase (TiPARP). The regulation of AHR activation is a dynamic process: immediately after transcriptional activation of the AHR by TCDD, the AHR is exported from the nucleus to the cytoplasm where it is subjected to proteasomal degradation. However, the mechanisms regulating AHR degradation are not well understood. Here, we studied the role of two enzymes reported to enhance AHR breakdown: the cullin 4B (CUL4B)^AHR complex, an E3 ubiquitin ligase that targets the AHR and other proteins for ubiquitination, and TiPARP, which targets proteins for ADP-ribosylation, a posttranslational modification that can increase susceptibility to degradation. Using a WT mouse embryonic fibroblast (MEF) cell line and an MEF cell line in which CUL4B has been deleted (MEF^Cul4b-null), we discovered that loss of CUL4B partially prevented AHR degradation after TCDD exposure, while knocking down TiPARP in MEF^Cul4b-null cells completely abolished AHR degradation upon TCDD treatment. Increased TCDD-activated AHR protein levels in MEF^Cul4b-null and MEF^Cul4b-null cells in which TiPARP was knocked down led to enhanced AHR transcriptional activity, indicating that CUL4B and TiPARP restrain AHR action. This study reveals a novel function of TiPARP in controlling TCDD-activated AHR nuclear export and subsequent proteasomal degradation.

Long-Term Exposure of Early-Transformed Human Mammary Cells to Low Doses of Benzo[a]pyrene and/or Bisphenol A Enhances Their Cancerous Phenotype via an AhR/GPR30 Interplay

It is of utmost importance to decipher the role of chronic exposure to low doses of environmental carcinogens on breast cancer progression. The early-transformed triple-negative human mammary MCF10AT1 cells were chronically (60 days) exposed to low doses (10⁻¹⁰ M) of Benzo[a]pyrene (B[a]P), a genotoxic agent, and/or Bisphenol A (BPA), an endocrine disruptor. Our study revealed that exposed MCF10AT1 cells developed, in a time-dependent manner, an acquired phenotype characterized by an increase in cancerous properties (anchorage independent growth and stem-like phenotype). Co-exposure of MCF10AT1 cells to B[a]P and BPA led to a significantly greater aggressive phenotype compared to B[a]P or BPA alone. This study provided new insights into the existence of a functional interplay between the aryl hydrocarbon receptor (AhR) and the G protein-coupled receptor 30 (GPR30) by which chronic and low-dose exposure of B[a]P and/or BPA fosters the progression of MCF10AT1 cells into a more aggressive substage. Experiments using AhR or GPR30 antagonists, siRNA strategies, and RNAseq analysis led us to propose a model in which AhR signaling plays a “driver role” in the AhR/GPR30 cross-talk in mediating long-term and low-dose exposure of B[a]P and/or BPA. Retrospective analysis of two independent breast cancer cohorts revealed that the AhR/GPR30 mRNA expression signature resulted in poor breast cancer prognosis, in particular in the ER-negative and the triple-negative subtypes. Finally, the study identified targeting AhR and/or GPR30 with specific antagonists as a strategy capable of inhibiting carcinogenesis associated with chronic exposure to low doses of B[a]P and BPA in MCF10AT1 cells. Altogether, our results indicate that the engagement of both AhR and GPR30 functions, in particular in an ER-negative/triple-negative context of breast cells, favors tumor progression and leads to poor prognosis.

The Action of Di-(2-Ethylhexyl) Phthalate (DEHP) in Mouse Cerebral Cells Involves an Impairment in Aryl Hydrocarbon Receptor (AhR) Signaling

Di-(2-ethylhexyl) phthalate (DEHP) is used as a plasticizer in various plastic compounds, such as polyvinyl chloride (PVC), and products including baby toys, packaging films and sheets, medical tubing, and blood storage bags. Epidemiological data suggest that phthalates increase the risk of the nervous system disorders; however, the impact of DEHP on the brain cells and the mechanisms of its action have not been clarified. The aim of the present study was to investigate the effects of DEHP on production of reactive oxygen species (ROS) and aryl hydrocarbon receptor (AhR), as well as Cyp1a1 and Cyp1b1 mRNA and protein expression in primary mouse cortical neurons and glial cells in the in vitro mono-cultures. Our experiments showed that DEHP stimulated ROS production in both types of mouse neocortical cells. Moreover, the results strongly support involvement of the AhR/Cyp1A1 signaling pathway in the action of DEHP in neurons and glial cells. However, the effects of DEHP acting on the AhR signaling pathways in these two types of neocortical cells were different. In neurons, AhR mRNA expression did not change, but AhR protein expression decreased in response to DEHP. A similar trend was observed for Cyp1a1 and Cyp1b1 mRNA and protein expression. Failure to induce Cyp1a1 in neurons was confirmed by EROD assay. In primary glial cells, a decrease in AhR protein level was accompanied by a decrease in AhR mRNA expression. In glial cells, mRNA and protein expression of Cyp1a1 as well as Cyp1a1-related EROD activity were significantly increased. As for Cyp1b1, both in neurons and glial cells Cyp1b1 mRNA expression did not significantly change, whereas Cyp1b1 protein level were decreased. We postulate that developmental exposure to DEHP which dysregulates AhR/Cyp1a1 may disrupt defense processes in brain neocortical cells that could increase their susceptibility to environmental toxins.

Alpha-naphthoflavone induces apoptosis through endoplasmic reticulum stress via c-Src-, ROS-, MAPKs-, and arylhydrocarbon receptor-dependent pathways in HT22 hippocampal neuronal cells

α-Naphthoflavone (αNF) is a prototype flavone, also known as a modulator of aryl hydrocarbon receptor (AhR). In the present study, we investigated the molecular mechanisms of αNF-induced cytotoxic effects in HT22 mouse hippocampal neuronal cells. αNF induced apoptotic cell death via activation of caspase-12 and -3 and increased expression of endoplasmic reticulum (ER) stress-associated proteins, including C/EBP homologous protein (CHOP). Inhibition of ER stress by treatment with the ER stress inhibitor, salubrinal, or by CHOP siRNA transfection reduced αNF-induced cell death. αNF activated mitogen-activated protein kinases (MAPKs), such as p38, JNK, and ERK, and inhibition of MAPKs reduced αNF-induced CHOP expression and cell death. αNF also induced accumulation of reactive oxygen species (ROS) and an antioxidant, N-acetylcysteine, reduced αNF-induced MAPK phosphorylation, CHOP expression, and cell death. Furthermore, αNF activated c-Src kinase, and inhibition of c-Src by a kinase inhibitor, SU6656, or siRNA transfection reduced αNF-induced ROS accumulation, MAPK activation, CHOP expression, and cell death. Inhibition of AhR by an AhR antagonist, CH223191, and siRNA transfection of AhR and AhR nuclear translocator reduced αNF-induced AhR-responsive luciferase activity, CHOP expression, and cell death. Finally, we found that inhibition of c-Src and MAPKs reduced αNF-induced transcriptional activity of AhR. Taken together, these findings suggest that αNF induces apoptosis through ER stress via c-Src-, ROS-, MAPKs-, and AhR-dependent pathways in HT22 cells.

Expression, Localization, and Activity of the Aryl Hydrocarbon Receptor in the Human Placenta

The human placenta is an organ between the blood of the mother and the fetus, which is essential for fetal development. It also plays a role as a selective barrier against environmental pollutants that may bypass epithelial barriers and reach the placenta, with implications for the outcome of pregnancy. The aryl hydrocarbon receptor (AhR) is one of the most important environmental-sensor transcription factors and mediates the metabolism of a wide variety of xenobiotics. Nevertheless, the identification of dietary and endogenous ligands of AhR suggest that it may also fulfil physiological functions with which pollutants may interfere. Placental AhR expression and activity is largely unknown. We established the cartography of AhR expression at transcript and protein levels, its cellular distribution, and its transcriptional activity toward the expression of its main target genes. We studied the profile of AhR expression and activity during different pregnancy periods, during trophoblasts differentiation in vitro, and in a trophoblast cell line. Using diverse methods, such as cell fractionation and immunofluorescence microscopy, we found a constitutive nuclear localization of AhR in every placental model, in the absence of any voluntarily-added exogenous activator. Our data suggest an intrinsic activation of AhR due to the presence of endogenous placental ligands.

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.

Aryl hydrocarbon receptor inhibits inflammation in DSS‑induced colitis via the MK2/p‑MK2/TTP pathway

The pathogenesis of inflammatory bowel disease (IBD) is believed to be associated with the abnormal expression of inflammatory factors. The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor, which can suppress the inflammatory response and attenuate experimental colitis. However, the detailed mechanism underlying the effects of AhR remains unclear. The present study investigated the role of AhR in the pathogenesis of IBD. Colitis was induced in mice by administration of 3% dextran sulphate sodium (DSS) for 7 days. The mice were also administered injections of the AhR agonist, 6-formylindolo(3,2-b)carbazole (FICZ), starting 2 days after the first administration of DSS. Furthermore, LoVo cells were treated with lipopolysaccharide (LPS) in the presence or absence of FICZ for 8 h. The protein expression levels of AhR, cytochrome P450 1A1 (CYP1A1) and tristetraprolin (TTP) were assessed by western blotting and immunofluorescence, whereas mRNA expression levels were assessed by reverse transcription-quantitative polymerase chain reaction. The results indicated that injection of mice with FICZ significantly attenuated DSS-induced colitis; in addition, the expression levels of inflammatory cytokines were markedly downregulated. Conversely, the expression levels of AhR and TTP were upregulated. In addition, mice in the AhR-knockout + DSS group exhibited elevated inflammatory cytokine production and developed more severe colitis. In LoVo cells, incubation with FICZ decreased the expression levels of inflammatory cytokines, whereas AhR and TTP expression was increased. In addition, the levels of phosphorylated-mitogen-activated protein kinase-activated protein kinase 2 (p-MK2) were decreased. These results suggested that AhR deficiency resulted in increased susceptibility to colitis, whereas activation of AhR by FICZ could ameliorate DSS-induced colitis via the MK2/p-MK2/TTP pathway.

Retinoid-xenobiotic interactions: the Ying and the Yang

The literature provides compelling evidence pointing to tight metabolic interactions between retinoids and xenobiotics. These are extensive and important for understanding xenobiotic actions in the body. Within the body, retinoids affect xenobiotic metabolism and actions and conversely, xenobiotics affect retinoid metabolism and actions. This article summarizes data that establish the importance of retinoid-dependent metabolic pathways for sustaining the body's responses to xenobiotic exposure, including the roles of all-trans- and 9-cis-retinoic acid for protecting mammals from harmful xenobiotic effects and for ensuring xenobiotic elimination from the body. This review will also consider molecular mechanisms underlying xenobiotic toxicity focusing on how this may contribute to retinoid deficiency and disruption of normal retinoid homeostasis. Special attention is paid to xenobiotic molecular targets (nuclear receptors, regulatory proteins, enzymes, and transporters) which affect retinoid metabolism and signaling.

Aryl hydrocarbon receptor knockout rats are insensitive to the pathological effects of repeated oral exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin

Sustained activation of the aryl hydrocarbon receptor (AHR) is believed to be the initial key event in AHR receptor-mediated tumorigenesis in the rat liver. The role of AHR in mediating pathological changes in the liver prior to tumor formation was investigated in a 4-week, repeated-dose study using adult female wild-type (WT) and AHR knockout (AHR-KO) rats treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Beginning at 8 weeks of age, AHR-KO and WT rats were dosed by oral gavage with varying concentrations of TCDD (0, 3, 22, 100, 300 and 1000 ng kg(-1)  day(-1) ). Lung, liver and thymus histopathology, hematology, serum chemistry and the distribution of TCDD in liver and adipose tissue were examined. Treatment-related increases in the severity of liver and thymus pathology were observed in WT, but not AHR-KO rats. In the liver, these included hepatocellular hypertrophy, bile duct hyperplasia, multinucleated hepatocytes and inflammatory cell foci. A loss of cellularity in the thymic cortex and thymic atrophy was observed. Treatment-related changes in serum chemistry parameters were also observed in WT, but not AHR-KO rats. Finally, dose-dependent accumulation of TCDD was observed primarily in the liver of WT rats and primarily in the adipose tissue of AHR-KO rats. The results suggest that AHR activation is the initial key event underlying the progression of histological effects leading to liver tumorigenesis following TCDD treatment. Copyright © 2015 John Wiley & Sons, Ltd.

Proteasomal interaction as a critical activity modulator of the human constitutive androstane receptor

The CAR (constitutive androstane receptor; NR1I3) is a critical xenobiotic sensor that regulates xenobiotic metabolism, drug clearance, energy and lipid homoeostasis, cell proliferation and development. Although constitutively active, in hepatocytes CAR is normally held quiescent through a tethering mechanism in the cytosol, anchored to a protein complex that includes several components, including heat-shock protein 90. Release and subsequent nuclear translocation of CAR is triggered through either direct binding to ligand activators such as CITCO {6-(4-chlorophenyl)imidazo[2,1-b][1,3]thiazole-5-carbaldehyde O-(3,4-dichlorobenzyl)oxime} or through indirect chemical activation, such as with PB (phenobarbital). In the present study, we demonstrate that proteasomal inhibition markedly disrupts CAR function, repressing CAR nuclear trafficking, disrupting CAR's interaction with nuclear co-activators and inhibiting induction of CAR target gene responses in human primary hepatocytes following treatment with either PB or CITCO. Paradoxically, these effects occur following accumulation of ubiquitinated hCAR (human CAR). Furthermore, a non-proteolytic function was indicated by its interaction with a SUG1 (suppressor for Gal1), a subunit of the 26S proteasome. Taken together, these data demonstrate that the proteasome complex functions at multiple levels to regulate the functional biology of hCAR activity.

Methylated pentavalent arsenic metabolites are bifunctional inducers, as they induce cytochrome P450 1A1 and NAD(P)H:quinone oxidoreductase through AhR- and Nrf2-dependent mechanisms

Activation of the aryl hydrocarbon receptor (AhR) ultimately leads to the induction of the carcinogen-activating enzyme cytochrome P450 1A1 (CYP1A1), and activation of the nuclear factor-erythroid 2 p45-related factor 2 (Nrf2) in addition to the AhR pathway induces the expression of the NADP(H):quinone oxidoreductase (NQO1). Therefore, the aim of this study was to examine the effect of As(III) pentavalent metabolites, MMA(V), DMA(V), and TMA(V), on AhR and Nrf2 activation and on the expression of their prototypical downstream targets CYP1A1 and NQO1, respectively. Our results showed that treatment of HepG2 cells with MMA(V), DMA(V), or TMA(V) in the absence and presence of 2,3,7,8-tetrachlorodibenzo-p-dioxin or sulforaphane significantly induced both CYP1A1 and NQO1 at the mRNA, protein, and catalytic activity levels. Furthermore, these metabolites increased the AhR-dependent XRE-driven and the Nrf2-dependent ARE-driven luciferase reporter activities, which coincided with increased nuclear accumulation of both transcription factors. However, none of these metabolites were shown to be AhR ligands. The induction of CYP1A1 by these metabolites seems to be ligand-independent, possibly through a decrease in HSP90 protein expression levels. The metabolites also increased ROS production, which was significantly higher than that produced by As(III). Upon knockdown of AhR and Nrf2 the MMA(V)-, DMA(V)-, and TMA(V)-mediated induction of both CYP1A1 and NQO1 proteins was significantly decreased. In conclusion, this study demonstrates for the first time that methylated pentavalent arsenic metabolites are bifunctional inducers, as they increase CYP1A1 by activating the AhR/XRE signaling pathway and they increase NQO1 by activating the Nrf2/ARE signaling pathway in addition to the AhR/XRE pathway.

Dexamethasone accelerates degradation of aryl hydrocarbon receptor (AHR) and suppresses CYP1A1 induction in placental JEG-3 cell line

The JEG-3 choriocarcinoma cell line has been proposed as a model cell line of human placental trophoblast for induction studies via aryl hydrocarbon receptor (AHR). We examined whether glucocorticoid dexamethasone influences AHR-mediated induction of CYP1A1 enzyme in the JEG-3 cell line. We found that dexamethasone dose- and time-dependently suppresses CYP1A1 transactivation in gene reporter assays, CYP1A1 mRNA induction, and upregulation of 7-ethoxyresorufin-O-deethylase (EROD) activity by 3-methylcholanthrene (MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in JEG-3 cells. Co-transfection of JEG-3 cells with glucocorticoid receptor (GR) expression construct and treatment with dexamethasone abolished the effect of MC on CYP1A1 promoter construct in transient transfection gene reporter assays. RU486, a GR antagonist, suppressed the effect of dexamethasone on MC-induced transactivation of AHR responsive reporter constructs. We also found that dexamethasone stimulates both ligand-dependent and ligand-independent degradation of AHR but not of aryl hydrocarbon receptor nuclear translocator (ARNT) protein in JEG-3 cells. In experiments with proteasome inhibitors MG132 and bortezomib, we found that the degradation is not sensitive to proteasome inhibition in JEG-3. We can conclude that dexamethasone suppresses AHR-mediated CYP1A1 induction in JEG-3 cells through the unique mechanism of AHR-GR crosstalk, which involves accelerated degradation of AHR.

Induction of a chloracne phenotype in an epidermal equivalent model by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is dependent on aryl hydrocarbon receptor activation and is not reproduced by aryl hydrocarbon receptor knock down

BACKGROUND

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent activator of the aryl hydrocarbon receptor (AhR) and causes chloracne in humans. The pathogenesis and role of AhR in chloracne remains incompletely understood.

OBJECTIVE

To elucidate the mechanisms contributing to the development of the chloracne-like phenotype in a human epidermal equivalent model and identify potential biomarkers.

METHODS

Using primary normal human epidermal keratinocytes (NHEK), we studied AhR activation by XRE-luciferase, AhR degradation and CYP1A1 induction. We treated epidermal equivalents with high affinity TCDD or two non-chloracnegens: β-naphthoflavone (β-NF) and 2-(1'H-indole-3'-carbonyl)-thiazole-4-carboxylic acid methyl ester (ITE). Using Western blotting and immunochemistry for filaggrin (FLG), involucrin (INV) and transglutaminase-1 (TGM-1), we compared the effects of the ligands on keratinocyte differentiation and development of the chloracne-like phenotype by H&E.

RESULTS

In NHEKs, activation of an XRE-luciferase and CYP1A1 protein induction correlated with ligand binding affinity: TCDD>β-NF>ITE. AhR degradation was induced by all ligands. In epidermal equivalents, TCDD induced a chloracne-like phenotype, whereas β-NF or ITE did not. All three ligands induced involucrin and TGM-1 protein expression in epidermal equivalents whereas FLG protein expression decreased following treatment with TCDD and β-NF. Inhibition of AhR by α-NF blocked TCDD-induced AhR activation in NHEKs and blocked phenotypic changes in epidermal equivalents; however, AhR knock down did not reproduce the phenotype.

CONCLUSION

Ligand-induced CYP1A1 and AhR degradation did not correlate with their chloracnegenic potential, indicating that neither CYP1A1 nor AhR are suitable biomarkers. Mechanistic studies showed that the TCDD-induced chloracne-like phenotype depends on AhR activation whereas AhR knock down did not appear sufficient to induce the phenotype.

Knockout of the aryl hydrocarbon receptor results in distinct hepatic and renal phenotypes in rats and mice

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor which plays a role in the development of multiple tissues and is activated by a large number of ligands, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In order to examine the roles of the AHR in both normal biological development and response to environmental chemicals, an AHR knockout (AHR-KO) rat model was created and compared with an existing AHR-KO mouse. AHR-KO rats harboring either 2-bp or 29-bp deletion mutation in exon 2 of the AHR were created on the Sprague-Dawley genetic background using zinc-finger nuclease (ZFN) technology. Rats harboring either mutation type lacked expression of AHR protein in the liver. AHR-KO rats were also insensitive to thymic involution, increased hepatic weight and the induction of AHR-responsive genes (Cyp1a1, Cyp1a2, Cyp1b1, Ahrr) following acute exposure to 25 μg/kg TCDD. AHR-KO rats had lower basal expression of transcripts for these genes and also accumulated ~30-45-fold less TCDD in the liver at 7 days post-exposure. In untreated animals, AHR-KO mice, but not AHR-KO rats, had alterations in serum analytes indicative of compromised hepatic function, patent ductus venosus of the liver and persistent hyaloid arteries in the eye. AHR-KO rats, but not AHR-KO mice, displayed pathological alterations to the urinary tract: bilateral renal dilation (hydronephrosis), secondary medullary tubular and uroepithelial degenerative changes and bilateral ureter dilation (hydroureter). The present data indicate that the AHR may play significantly different roles in tissue development and homeostasis and toxicity across rodent species.

Combustion derived ultrafine particles induce cytochrome P-450 expression in specific lung compartments in the developing neonatal and adult rat

Vehicle exhaust is rich in polycyclic aromatic hydrocarbons (PAH) and can be a dominant contributor to ultrafine urban particulate matter (PM). Exposure to ultrafine PM is correlated with respiratory infections and asthmatic symptoms in young children. The lung undergoes substantial growth, alveolarization, and cellular maturation within the first years of life, which may be impacted by environmental pollutants such as PM. PAHs in PM can serve as ligands for the aryl hydrocarbon receptor (AhR) that induces expression of certain isozymes in the cytochrome P-450 superfamily, such as CYP1A1 and CYP1B1, localized in specific lung cell types. Although AhR activation and induction has been widely studied, its context within PM exposure and impact on the developing lung is poorly understood. In response, we have developed a replicable ultrafine premixed flame particle (PFP) generating system and used in vitro and in vivo models to define PM effects on AhR activation in the developing lung. We exposed 7-day neonatal and adult rats to a single 6-h PFP exposure and determined that PFPs cause significant parenchymal toxicity in neonates. PFPs contain weak AhR agonists that upregulate AhR-xenobiotic response element activity and expression and are capable inducers of CYP1A1 and CYP1B1 expression in both ages with different spatial and temporal patterns. Neonatal CYP1A1 expression was muted and delayed compared with adults, possibly because of differences in the enzyme maturation. We conclude that the inability of neonates to sufficiently adapt in response to PFP exposure may, in part, explain their susceptibility to PFP and urban ultrafine PM.

Aryl hydrocarbon receptor is a target of 17-Allylamino-17-demethoxygeldanamycin and enhances its anticancer activity in lung adenocarcinoma cells

We have demonstrated that aryl hydrocarbon receptor (AhR) is overexpressed in lung adenocarcinoma (AD). AhR is usually associated with heat shock protein 90 (Hsp90) in the cytoplasm. 17-Allylamino-17-demethoxygeldanamycin (17-AAG), an Hsp90 inhibitor, is currently under evaluation for its anticancer activity in clinical trials. Here we investigated whether AhR plays a role in 17-AAG-mediated anticancer activity by functioning as a downstream target or by modulating its anticancer efficacy. AhR expression in lung AD cells was modulated by siRNA interference or overexpression. Tumor growth was determined with colony formation in vitro or in vivo. Anticancer activity of 17-AAG was determined by measuring cell viability, cell cycle distribution, and expression of cell cycle regulators. Proteins and mRNA levels were examined by immunoblotting and the real-time reverse transcription-polymerase chain reaction, respectively. In this study, AhR overexpression positively modulated growth of lung AD cells, at least partially, via RelA-dependent mechanisms. Although treatment with 17-AAG reduced AhR levels and AhR-regulated gene expression in lung AD cells, AhR expression increased anticancer activity of 17-AAG. In addition, 17-AAG treatment reduced cell viability, CDK2, CDK4, cyclin E, cyclin D1, and phosphorylated Rb levels in AhR-expressing lung AD cells. NAD(P)H:quinone oxidoreductase (NQO1), which is regulated by AhR, was shown to increase anticancer activity of 17-AAG in cells. Knockdown of NQO1 expression attenuated the reduction of cell cycle regulators by 17-AAG treatment in AhR overexpressed cells. We demonstrated that AhR protein not only functions as a downstream target of 17-AAG, but also enhances anticancer activity of 17-AAG in lung AD cells.

p23 co-chaperone protects the aryl hydrocarbon receptor from degradation in mouse and human cell lines

The aryl hydrocarbon receptor (AhR) is a ligand-sensitive transcription factor which is responsible for most 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicities. Without ligand, the AhR complex is cytoplasmic and contains p23. Our objective was to investigate whether the wild type p23 levels are important for the AhR function. We generated eight p23-specific knockdown stable cell lines via either electroporation or lentiviral infection. Five of these stable cell lines were generated from a mouse hepatoma cell line (Hepa1c1c7) and three were from human hepatoma and cervical cell lines (Hep3B and HeLa). All of them expressed lower AhR protein levels, leading to reduced ligand-induced, DRE-driven downstream activity. The AhR protein levels in p23-specific knockdown stable cells were reversed back to wild type levels after exogenous p23 was introduced. Reduction of the AhR protein levels in these stable cells was caused by a decrease in the AhR message levels and an increase of the AhR protein degradation in the absence of ligand. This ligand-independent degradation of AhR was not reversed by MG132, suggesting that the 26S proteasome was not responsible for the degradation. In addition, MG132 could not protect AhR from the ligand-induced degradation in both mouse and human p23-knockdown stable cells.

TGF-β1 signaling plays a dominant role in the crosstalk between TGF-β1 and the aryl hydrocarbon receptor ligand in prostate epithelial cells

Crosstalk between the aryl hydrocarbon receptor (AhR) and transforming growth factor-β1 (TGF-β1) signaling has been observed in various experimental models. However, both molecular mechanism underlying this crosstalk and tissue-specific context of this interaction are still only partially understood. In a model of human non-tumorigenic prostate epithelial cells BPH-1, derived from the benign prostatic hyperplasia, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) persistently activates the AhR signaling pathway and induces expression of xenobiotic metabolizing enzymes, such as CYP1A1 or CYP1B1. Here we demonstrate that TGF-β1 suppresses the AhR-mediated gene expression through multiple mechanisms, involving inhibition of AhR expression and down-regulation of nuclear AhR, via a SMAD4-dependent pathway. In contrast, TCDD-induced AhR signaling does not affect either TGF-β1-regulated gene expression or epithelial-to-mesenchymal transition. These observations suggest that, in the context of prostate epithelium, TGF-β1 signaling plays a dominant role in the crosstalk with AhR signaling pathway. Given the importance of TGF-β1 signaling in regulation of prostate epithelial tissue homeostasis, as well as the recently revealed role of AhR in prostate development and tumorigenesis, the above findings contribute to our understanding of the mechanisms underlying the crosstalk between the two signaling pathways in the prostate-specific context.

A mutant Ahr allele protects the embryonic kidney from hydrocarbon-induced deficits in fetal programming

BACKGROUND

The use of experimental model systems has expedited the elucidation of pathogenetic mechanisms of renal developmental disease in humans and the identification of genes that orchestrate developmental programming during nephrogenesis.

OBJECTIVES

We conducted studies to evaluate the role of AHR polymorphisms in the disruption of renal developmental programming by benzo(a)pyrene (BaP).

METHODS

We used metanephric cultures of C57BL/6J (C57) mice expressing the Ahr(b-1) allele and B6.D2N-Ahr(d)/J (D2N) mice expressing a mutant allele deficient in ligand binding (Ahr(d)) to investigate molecular mechanisms of renal development. Deficits in fetal programming were evaluated in the offspring of pregnant mice treated with BaP during nephrogenesis.

RESULTS

Hydrocarbon challenge of metanephri from C57 mice altered Wilms' tumor suppressor gene (Wt1) mRNA splice variant ratios and reduced mRNAs of the Wt1 transcriptional targets syndecan-1 (Sdc1) paired box gene 2 (Pax2), epidermal growth factor receptor (Egfr), and retinoic acid receptor, alpha (Rarα). These changes correlated with down-regulation of effectors of differentiation [secreted frizzled-related sequence protein 1 (Sfrp1), insulin-like growth factor 1 receptor (Igf1r), wingless-related MMTV-integration site 4 (Wnt4), Lim homeobox protein 1 (Lhx1), E-cadherin]. In contrast, metanephri from D2N mice were spared hydrocarbon-induced changes in Wt1 splice variant ratios and deficits of differentiation. We observed similar patterns of dysmorphogenesis and progressive loss of renal function at postnatal weeks 7 and 52 in the offspring of pregnant C57 but not D2N mice gavaged with 0.1 or 0.5 mg/kg BaP on gestation days 10-13.

CONCLUSIONS

These findings support a functional link between AHR and WT1 in the regulation of renal morphogenesis and raise important questions about the contribution of human AHR polymorphisms to the fetal origins of adult-onset kidney disease.

Integration of genome-wide computation DRE search, AhR ChIP-chip and gene expression analyses of TCDD-elicited responses in the mouse liver

Background

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor (TF) that mediates responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Integration of TCDD-induced genome-wide AhR enrichment, differential gene expression and computational dioxin response element (DRE) analyses further elucidate the hepatic AhR regulatory network.

Results

Global ChIP-chip and gene expression analyses were performed on hepatic tissue from immature ovariectomized mice orally gavaged with 30 μg/kg TCDD. ChIP-chip analysis identified 14,446 and 974 AhR enriched regions (1% false discovery rate) at 2 and 24 hrs, respectively. Enrichment density was greatest in the proximal promoter, and more specifically, within ± 1.5 kb of a transcriptional start site (TSS). AhR enrichment also occurred distal to a TSS (e.g. intergenic DNA and 3' UTR), extending the potential gene expression regulatory roles of the AhR. Although TF binding site analyses identified over-represented DRE sequences within enriched regions, approximately 50% of all AhR enriched regions lacked a DRE core (5'-GCGTG-3'). Microarray analysis identified 1,896 number of TCDD-responsive genes (|fold change| ≥ 1.5, P1(t) > 0.999). Integrating this gene expression data with our ChIP-chip and DRE analyses only identified 625 differentially expressed genes that involved an AhR interaction at a DRE. Functional annotation analysis of differentially regulated genes associated with AhR enrichment identified overrepresented processes related to fatty acid and lipid metabolism and transport, and xenobiotic metabolism, which are consistent with TCDD-elicited steatosis in the mouse liver.

Conclusions

Details of the AhR regulatory network have been expanded to include AhR-DNA interactions within intragenic and intergenic genomic regions. Moreover, the AhR can interact with DNA independent of a DRE core suggesting there are alternative mechanisms of AhR-mediated gene regulation.

Are assumptions about the model type necessary in reaction-diffusion modeling? A FRAP application

At present, fluorescence recovery after photobleaching (FRAP) data are interpreted using various types of reaction-diffusion (RD) models: the model type is usually fixed first, and corresponding model parameters are inferred subsequently. In this article, we describe what we believe to be a novel approach for RD modeling without using any assumptions of model type or parameters. To the best of our knowledge, this is the first attempt to address both model-type and parameter uncertainties in inverting FRAP data. We start from the most general RD model, which accounts for a flexible number of molecular fractions, all mobile, with different diffusion coefficients. The maximal number of possible binding partners is identified and optimal parameter sets for these models are determined in a global search of the parameter-space using the Simulated Annealing strategy. The numerical performance of the described techniques was assessed using artificial and experimental FRAP data. Our general RD model outperformed the standard RD models used previously in modeling FRAP measurements and showed that intracellular molecular mobility can only be described adequately by allowing for multiple RD processes. Therefore, it is important to search not only for the optimal parameter set but also for the optimal model type.

Molecular mechanisms of cold-induced CYP1A activation in rat liver microsomes

Cytochrome P4501A (the CYP1A1 and CYP1A2 enzymes) is known to metabolize anthropogenic xenobiotics to carcinogenic and mutagenic compounds. CYP1A1 transcriptional activation is regulated via the aryl hydrocarbon receptor (AhR)-dependent signal transduction pathway. CYP1A2 activation may occur through the AhR-dependent or AhR-independent signal transduction pathways. We used male Wistar rats to explore possible mechanisms of CYP1A activation induced by exposure to cold and the effects of the protein-tyrosine kinase inhibitors genistein, herbimycin A, and geldanamycin on the properties of hepatic CYP1A1 and CYP1A2 proteins following exposure to cold and to classic CYP1A inducers. The molecular mechanisms of cold-induced CYP1A1 and CYP1A2 activation are different. The CYP1A2 activation apparently occurs at the post-transcriptional level. The CYP1A1 activation, whether caused by exposure to cold or by classic CYP1A inducers, is AhR-dependent and occurs at the transcriptional level. Protein tyrosine kinase inhibitors have no effect on benzo(a)pyrene-induced CYP1A expression but alter cold-induced CYP1A1 activity and the CYP1A1 mRNA level. Thus, treatment with herbimycin A or geldanamycin leads to an increase in CYP1A1 activity, while treatment with genistein increases CYP1A1 mRNA expression and decreases CYP1A2 activity. These data elucidate the molecular mechanisms of cold-induced CYP1A activation and the role of protein kinases in the regulation of CYP1A during exposure to cold. Our results can also help identify the differences between the molecular mechanisms underlying the effects of the classic CYP1A inducers and the effects of cooling.

Reactivation of L1 retrotransposon by benzo(a)pyrene involves complex genetic and epigenetic regulation

Benzo(a)pyrene (BaP), is an environmental pollutant present in tobacco smoke and a byproduct of fossil fuel combustion which likely contributes to the tumorigenic processes in human cancers including lung and esophageal. Long Interspersed Nuclear Element-1 (LINE-1) or L1 is a mobile element within the mammalian genome that propagates via a "copy-and-paste" mechanism using reverse transcriptase and RNA intermediates. L1 is strongly expressed during early embryogenesis and then silenced as cells initiate differentiation programming. Although the complex transcriptional control mechanisms of L1 are not well understood, L1 reactivation has been described in several human cancers and following exposure of mouse or human cells to BaP. In this study we investigated the molecular mechanisms and epigenetic events that regulate L1 reactivation following BaP exposure. We show that challenge of HeLa cells with BaP induces early enrichment of the transcriptionally-active chromatin markers histone H3 trimethylated at lysine 4 (H3K4Me3) and histone H3 acetylated at lysine 9 (H3K9Ac), and reduces association of DNA methyltransferase-1 (DNMT1) with the L1 promoter. These changes are followed by proteasome-dependent decreases in cellular DNMT1 expression and sustained reduction of cytosine methylation within the L1 promoter CpG island. Pharmacological inhibition of the proteasome signaling pathway with the inhibitor MG132 blocks degradation of DNMT1 and alters BaP-mediated histone epigenetic modifications. We conclude that genetic reactivation of L1 by BaP involves an ordered cascade of epigenetic events that begin with nucleosomal histone modifications and is completed with alterations in DNMT1 recruitment to the L1 promoter and reduced DNA methylation of CpG islands.

Camel urine inhibits the cytochrome P450 1a1 gene expression through an AhR-dependent mechanism in Hepa 1c1c7 cell line

AIM OF THE STUDY

Drinking camel urine has been used traditionally to treat numerous cases of cancer yet, the exact mechanism was not investigated. Therefore, we examined the ability of three different camel urines (virgin, lactating, and pregnant source) to modulate a well-known cancer-activating enzyme, the cytochrome P450 1a1 (Cyp1a1) in murine hepatoma Hepa 1c1c7 cell line.

MATERIALS AND METHODS

The effect of different camel urines, compared to bovine urines, on Cyp1a1 mRNA was determined using real-time polymerase chain reaction. Cyp1a1 protein and catalytic activity levels were determined using Western blot analysis and 7-ethoxyresorufin as a substrate, respectively. The role of aryl hydrocarbon receptor (AhR)-dependent mechanism was determined using electrophoretic mobility shift assay (EMSA) and the AhR-dependent luciferase reporter gene.

RESULTS

All types of camel, but not bovine, urines differentially inhibited the induction of Cyp1a1 gene expression by TCDD, the most potent Cyp1a1 inducer and known carcinogenic chemical. Importantly, virgin camel urine showed the highest degree of inhibition at the activity level, followed by lactating and pregnant camel urines. Furthermore, we have shown that virgin camel urine significantly inhibited the TCDD-mediated induction of Cyp1a1 at the mRNA and protein expression levels. Mechanistically, the ability of virgin camel urine to inhibit Cyp1a1 was strongly correlated with its ability to inhibit AhR-dependent luciferase activity and DNA binding as determined by EMSA, suggesting that AhR-dependent mechanism is involved.

CONCLUSIONS

The present work provides the first evidence that camel urine but not that of bovine inhibits the TCDD-mediated toxic effect by inhibiting the expression of Cyp1a1, at both transcriptional and post-transcriptional levels through an AhR-dependent mechanism.

Regulation of human carbonyl reductase 3 (CBR3; SDR21C2) expression by Nrf2 in cultured cancer cells

Carbonyl reduction is a central metabolic process that controls the level of key regulatory molecules as well as xenobiotics. Carbonyl reductase 3 (CBR3; SDR21C2), a member of the short-chain dehydrogenase/reductase (SDR) superfamily, has been poorly characterized so far, and the regulation of its expression is a complete mystery. Here, we show that CBR3 expression is regulated via Nrf2, a key regulator in response to oxidative stress. In human cancer cell lines, CBR3 mRNA was expressed differentially, ranging from very high (A549, lung) to very low (HT-29, colon; HepG2, liver) levels. CBR3 protein was highly expressed in SW-480 (colon) cells but was absent in HCT116 (colon) and HepG2 cells. CBR3 mRNA could be induced in HT-29 cells by Nrf2 agonists [sulforaphane (SUL, 7-fold) and diethyl maleate (DEM, 4-fold)] or hormone receptor ligand Z-guggulsterone (5-fold). Aryl hydrocarbon receptor agonist B[k]F failed to induce CBR3 mRNA after incubation for 8 h but elevated CBR3 levels after 24 h, most likely mediated by B[k]F metabolites that can activate Nrf2 signaling. Inhibition of Nrf2-activating upstream kinase MEK/ERK by PD98059 weakened DEM-mediated induction of CBR3 mRNA. Proteasome inhibitors MG-132 (5 μM) and bortezomib (50 nM) dramatically increased the level of CBR3 mRNA, obviously because of the increase in the level of Nrf2 protein. While siRNA-mediated knockdown of Nrf2 led to a decrease in the level of CBR3 mRNA in A549 cells (30% of control), Keap1 knockdown increased the level of CBR3 mRNA expression in HepG2 (9.3-fold) and HT-29 (2.7-fold) cells. Here, we provide for the first time evidence that human CBR3 is a new member of the Nrf2 gene battery.

Neural precursor cell proliferation is disrupted through activation of the aryl hydrocarbon receptor by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Neurogenesis involves the proliferation of multipotent neuroepithelial stem cells followed by differentiation into lineage-restricted neural precursor cells (NPCs) during the embryonic period. Interestingly, these progenitor cells express robust levels of the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that regulates expression of genes important for growth regulation, and xenobiotic metabolism. Upon binding 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a pervasive environmental contaminant and potent AhR ligand, AhR, is activated and disrupts gene expression patterns to produce cellular toxicity. Because of its widespread distribution in the brain during critical proliferative phases of neurogenesis, it is conceivable that AhR participates in NPC expansion. Therefore, this study tested the hypothesis that AhR activation by TCDD disrupts signaling events that regulate NPC proliferation. The C17.2 NPC line served as a model system to (1) assess whether NPCs are targets for TCDD-induced neurotoxicity and (2) characterize the effects of TCDD on NPC proliferation. We demonstrated that C17.2 NPCs express an intact AhR signaling pathway that becomes transcriptionally active after TCDD exposure. (3)H-thymidine and alamar blue reduction assays indicated that TCDD suppresses NPC proliferation in a concentration-dependent manner without the loss of cell viability. Cell cycle distribution analysis by flow cytometry revealed that TCDD-induced growth arrest results from an impaired G1 to S cell cycle transition. Moreover, TCDD exposure altered p27( kip1) and cyclin D1 cell cycle regulatory protein expression levels consistent with a G1 phase arrest. Initial studies in primary NPCs isolated from the ventral forebrain of embryonic mice demonstrated that TCDD reduced cell proliferation through a G1 phase arrest, corroborating our findings in the C17.2 cell line. Together, these observations suggest that the inappropriate or sustained activation of AhR by TCDD during neurogenesis can interfere with signaling pathways that regulate neuroepithelial stem cell/NPC proliferation, which could adversely impact final cell number in the brain and lead to functional impairments.

Characterization of the recalcitrant CYP1 phenotype found in Atlantic killifish (Fundulus heteroclitus) inhabiting a Superfund site on the Elizabeth River, VA

Fundulus heteroclitus (Atlantic killifish) found at the Atlantic Wood Industries Superfund site on the Elizabeth River (ER) in Portsmouth, VA (USA), have been shown to be resistant to the teratogenic effects of creosote-contaminated sediments found at this highly contaminated site. Many of the polycyclic aromatic hydrocarbons (PAHs) found at the ER are known to activate the aryl hydrocarbon receptor (AHR), and are thought to mediate their toxic effects through this pathway. Activation of the AHR results in the induction of several Phase I and II metabolic enzymes. It has been previously shown that the AHR of killifish from the ER are refractory to induction by AHR agonists. To more fully characterize this altered AHR response, we exposed embryos from the ER and from a reference site on King's Creek, VA (KC) to two PAHs, benzo[alpha]pyrene (BaP) and benzo[k]fluoranthene (BkF), and to the dioxin-like compound (DLC), 3,3',4,4',5-pentachlorobiphenyl (PCB126). We compared their developmental and molecular responses by screening the embryos for CYP1A enzyme activity, cardiac deformities, and mRNA expression of CYP1A, CYP1B1, CYP1C1, and AHR2. Basal gene expression of both CYP1A and CYP1B1 was 40% higher in the KC control embryos compared to those from the ER, while AHR2 and CYP1C1 were not significantly different between the populations. Exposure of KC embryos to BaP, BkF, and PCB126 induced CYP1A activity and cardiac deformities. In contrast, CYP1A activity was induced in ER embryos only in response to BkF exposure, although this induction in ER embryos was significantly lower than that observed in KC fish at comparable concentrations. ER embryos did not develop cardiac deformities in response to any of the chemicals tested. CYP1A, CYP1B1 and CYP1C1 mRNA were all significantly induced in the KC embryos after exposure to BaP, BkF and PCB126. Exposure to BaP and BkF in ER embryos resulted in a significant induction of CYP1A mRNA, albeit significantly lower than observed in KC fish. Interestingly, BaP exposure resulted in induction of CYP1B1 at comparable levels in embryos from both populations. CYP1s were not induced in ER embryos in response to PCB126, nor was CYP1C1 for any treatment examined. Additionally, AHR2 was not significantly induced for any of the treatment groups. This study further characterizes the AHR response in killifish, and provides greater insight into the adapted ER phenotype. The ER adaptation involves the suppression of normal AHR-inducible gene expression for all three CYP1 genes, and therefore is likely an alteration in AHR signaling or control.

3-methylcholanthrene induces differential recruitment of aryl hydrocarbon receptor to human promoters

The aryl hydrocarbon receptor (AHR) is a ligand-activated protein that mediates the toxic actions of polycyclic aromatic and halogenated compounds. Identifying genes directly regulated by AHR is important in understanding the pathways regulated by this receptor. Here we used the techniques of chromatin immunoprecipitation and DNA microarrays (ChIP-chip) to detect AHR-bound genomic regions after 3-methylcholanthrene (3MC) treatment of T-47D human breast cancer cells. We identified 241 AHR-3MC-bound regions, and transcription factor-binding site analysis revealed a strong overrepresentation of the AHR-responsive element. Conventional ChIP confirmed recruitment of AHR to 26 regions with target gene responses to 3MC varying from activation to inhibition to having no effect. A comparison of identified AHR-3MC-bound regions with AHR-2,3,7,8-tetrchlorodibenzo-p-dioxin (TCDD)-bound regions from our previous study (Ahmed, S., Valen, E., Sandelin, A., and Matthews, J. (2009). Toxicol. Sci. 111, 254-266) revealed that 127 regions were common between the data sets. Time course ChIPs for six of the regions showed that 3MC induced gene-specific changes in histone H3 acetylation and methylation and induced differential oscillatory binding of AHR, with a periodicity between 1.5 and 2 h. Re-treatment of cells with 3MC failed to alter the oscillatory binding profiles of AHR or aryl hydrocarbon receptor nuclear translocator. Cells became responsive to 3MC but not TCDD after 24 h of exposure to 3MC, highlighting important differences in AHR responsiveness between the two ligands. Our results reveal a number of novel AHR-bound promoter regions and target genes that exhibit differential kinetic binding profiles and regulation by AHR.

The inhibitory mechanisms of the tyrosine kinase inhibitors herbimycin a, genistein, and tyrphostin B48 with regard to the function of the aryl hydrocarbon receptor in Caco-2 cells

The aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by dioxin and related xenobiotics. Although the activation of AhR is inhibited by tyrosine kinase inhibitors, the molecular mechanism has not been clarified. In the current study, the inhibitory mechanisms of several inhibitors of tyrosine kinase, herbimycin A, genistein, and tyrphostin B48, on AhR activation was analyzed in human Caco-2 cells. All the inhibitors suppressed the transcriptional activation of AhR induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Herbimycin A induced down-regulation of the AhR protein by inhibiting its molecular chaperone heat shock protein 90 (HSP90). In contrast, genistein and tyrphostin B48 inhibited the nuclear localization of AhR induced by TCDD, although the amount of AhR protein was not altered. The inhibitory effects of genistein and tyrphostin B48 on endogenous tyrosine kinase activity were evaluated by detection of alterations in the tyrosine phosphorylation states of cellular proteins.

Antagonism of aryl hydrocarbon receptor signaling by 6,2',4'-trimethoxyflavone

The aryl hydrocarbon receptor (AHR) is regarded as an important homeostatic transcriptional regulator within physiological and pathophysiological processes, including xenobiotic metabolism, endocrine function, immunity, and cancer. Agonist activation of the AHR is considered deleterious based on toxicological evidence obtained with environmental pollutants, which mediate toxic effects through AHR. However, a multitude of plant-derived constituents, e.g., polyphenols that exhibit beneficial properties, have also been described as ligands for the AHR. It is conceivable that some of the positive aspects of such compounds can be attributed to suppression of AHR activity through antagonism. Therefore, we conducted a dioxin response element reporter-based screen to assess the AHR activity associated with a range of flavonoid compounds. Our screen identified two flavonoids (5-methoxyflavone and 7,4'-dimethoxyisoflavone) with previously unidentified AHR agonist potential. In addition, we have identified and characterized 6,2',4'-trimethoxyflavone (TMF) as an AHR ligand that possesses the characteristics of an antagonist having the capacity to compete with agonists, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene, thus effectively inhibiting AHR-mediated transactivation of a heterologous reporter and endogenous targets, e.g., CYP1A1, independent of cell lineage or species. Furthermore, TMF displays superior action by virtue of having no partial agonist activity, in contrast to other documented antagonists, e.g., alpha-napthoflavone, which are partial weak agonists. TMF also exhibits no species or promoter dependence with regard to AHR antagonism. TMF therefore represents an improved tool allowing for more precise dissection of AHR function in the absence of any conflicting agonist activity.

Genomewide analysis of aryl hydrocarbon receptor binding targets reveals an extensive array of gene clusters that control morphogenetic and developmental programs

BACKGROUND

The vertebrate aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that regulates cellular responses to environmental polycyclic and halogenated compounds. The naive receptor is believed to reside in an inactive cytosolic complex that translocates to the nucleus and induces transcription of xenobiotic detoxification genes after activation by ligand.

OBJECTIVES

We conducted an integrative genomewide analysis of AHR gene targets in mouse hepatoma cells and determined whether AHR regulatory functions may take place in the absence of an exogenous ligand.

METHODS

The network of AHR-binding targets in the mouse genome was mapped through a multipronged approach involving chromatin immunoprecipitation/chip and global gene expression signatures. The findings were integrated into a prior functional knowledge base from Gene Ontology, interaction networks, Kyoto Encyclopedia of Genes and Genomes pathways, sequence motif analysis, and literature molecular concepts.

RESULTS

We found the naive receptor in unstimulated cells bound to an extensive array of gene clusters with functions in regulation of gene expression, differentiation, and pattern specification, connecting multiple morphogenetic and developmental programs. Activation by the ligand displaced the receptor from some of these targets toward sites in the promoters of xenobiotic metabolism genes.

CONCLUSIONS

The vertebrate AHR appears to possess unsuspected regulatory functions that may be potential targets of environmental injury.

Heat shock protein 90 inhibitors suppress aryl hydrocarbon receptor-mediated activation of CYP1A1 and CYP1B1 transcription and DNA adduct formation

The aryl hydrocarbon receptor (AhR), a client protein of heat shock protein 90 (HSP90), plays a significant role in polycyclic aromatic hydrocarbon (PAH)-induced carcinogenesis. Tobacco smoke, a source of PAHs, activates the AhR, leading to enhanced transcription of CYP1A1 and CYP1B1, which encode proteins that convert PAHs to genotoxic metabolites. The main objectives of this study were to determine whether HSP90 inhibitors suppress PAH-mediated induction of CYP1A1 and CYP1B1 or block benzo(a)pyrene [B(a)P]-induced formation of DNA adducts. Treatment of cell lines derived from oral leukoplakia (MSK-Leuk1) or esophageal squamous cell carcinoma (KYSE450) with a saline extract of tobacco smoke, B(a)P, or dioxin induced CYP1A1 and CYP1B1 transcription, resulting in enhanced levels of message and protein. Inhibitors of HSP90 [17-allylamino-17-demethoxygeldanamycin (17-AAG); celastrol] suppressed these inductive effects of PAHs. Treatment with 17-AAG and celastrol also caused a rapid and marked decrease in amounts of AhR protein without modulating levels of HSP90. The formation of B(a)P-induced DNA adducts in MSK-Leuk1 cells was inhibited by 17-AAG, celastrol, and alpha-naphthoflavone, a known AhR antagonist. The reduction in B(a)P-induced DNA adducts was due, at least in part, to reduced metabolic activation of B(a)P. Collectively, these results suggest that 17-AAG and celastrol, inhibitors of HSP90, suppress the activation of AhR-dependent gene expression, leading, in turn, to reduced formation of B(a)P-induced DNA adducts. Inhibitors of HSP90 may have a role in chemoprevention in addition to cancer therapy.

Role of cAMP in mediating AHR signaling

Regulation of the nuclear import of many transcription factors represents a step in gene regulation which is crucial for a number of cellular processes. The aryl hydrocarbon receptor (AHR), a basic helix-loop-helix protein of the PAS (PER-ARNT-SIM) family of transcriptional regulators is a cytosol-associated and ligand-activated receptor. The environmental toxin dioxin binds with high affinity to AHR rendering it nuclear and leading to the activation of AHR sensitive genes. However, the fact, that the AHR mediates a large variety of physiological events without the involvement of any known exogenous ligand, including liver and vascular system development, maturation of the immune system, regulation of genes involved in cellular growth, cell differentiation and circadian rhythm, speaks for an important role of AHR in cell biology independent of the presence of an exogenous ligand. Different approaches were applied to study mechanism(s) which render AHR nuclear and design its function in absence of exogenous ligands. We found that AHR is sensitive to cAMP signaling mediated by cAMP-dependent protein kinase (PKA) which fundamentally differs from AHR signaling mediated by the exogenous ligand dioxin. It has been shown that PKA mediated signaling can be confined by compartmentalization of signaling components in microdomains conferring specificity to signaling by the ubiquitous second messenger cAMP. Moreover, A-kinase-anchoring proteins (AKAPs) and newly discovered cAMP receptors, Epac (exchange protein directly activated by cAMP), may give us a further chance to enter into new dimensions of cAMP signal transmissions that potentially may bring us closer to AHR physiology.

Cross-talk between xenobiotic detoxication and other signalling pathways: clinical and toxicological consequences

1. Recent investigations on nuclear receptors and other transcription factors involved in the regulation of genes encoding xenobiotic metabolizing and transport systems reveal that xenobiotic-dependent signalling pathways are embedded in, and establish functional interactions with, a tangle of regulatory networks involving the glucocorticoid and oestrogen receptors, the hypoxia-inducible factor, the vitamin D receptor and other transcription factors/nuclear receptors controlling cholesterol/bile salt homeostasis and liver differentiation. 2. Such functional interferences provide new insight, first for understanding how xenobiotics might exert adverse effects, and second how physiopathological stimuli affect xenobiotic metabolism.

Phosphodiesterases link the aryl hydrocarbon receptor complex to cyclic nucleotide signaling

The aryl hydrocarbon receptor (AHR) is a major transcription factor regulated by different mechanisms. The classical view of AHR activation by xenobiotics needs to be amended by recent findings on the regulation of AHR by endogenous ligands and by crosstalk with other signaling pathways. In the cytosol the AHR recruits a large number of binding partners, including HSP90, p23, XAP2 and the ubiquitin ligases cullin 4B and CHIP. Furthermore, XAP2 binds the cyclic nucleotide phosphodiesterases PDE2A and PDE4A5. PDE2A inhibits nuclear translocation of AHR suggesting an important regulatory role of cyclic nucleotides in AHR trafficking. Signaling involving cAMP is organized in subcellular compartments and a distinct cAMP compartment might be required for proper AHR mobility and function. We conclude that the AHR complex integrates ligand binding and cyclic nucleotide signaling to generate an adequate transcriptional response.

Rats fed soy protein isolate (SPI) have impaired hepatic CYP1A1 induction by polycyclic aromatic hydrocarbons as a result of interference with aryl hydrocarbon receptor signaling

Consumption of soy diets has been found to reduce cancer incidence in animals and is associated with reduced cancer risk in humans. Previously, we have demonstrated that female Sprague-Dawley rats fed purified AIN-93G diets with soy protein isolate (SPI) as the sole protein source had reduced CYP1A1 induction and basal aryl hydrocarbon receptor (AhR) levels relative to those fed the same diet containing casein (CAS). In the present study, the molecular mechanisms underlying reduced AhR expression have been studied. The SPI-effect on AhR was not observed after feeding diets containing the purified soy isoflavones genistein or daidzein. Rat hepatoma FGC-4 cells were treated with the serum obtained from rats fed CAS- or SPI-containing diets. Reduced AhR levels (P<0.05) were observed after 24 h exposure to SPI-serum without any changes in the overall expression of chaperone proteins--HSP90 and XAP2. SPI-serum-stimulated AhR degradation was inhibited by treating the cells with the proteasome inhibitor, MG132, and was observed to be preceded by ubiquitination of the receptor. A reduced association of XAP2 with the immunoprecipitated AhR complex was observed. SPI-serum-mediated AhR degradation was preceded by nuclear translocation of the receptor. However, the translocated receptor was found to be unable to heterodimerize with ARNT or to bind to XRE elements on the CYP1A1 enhancer. These data suggest that feeding SPI-containing diets antagonizes AhR signaling by a novel mechanism which differs from those established for known AhR antagonists.

Aryl hydrocarbon receptor signaling in rainbow trout hepatocytes: role of hsp90 and the proteasome

The objective of this study was to investigate the role of heat shock protein 90 (hsp90) and the proteasome in regulating aryl hydrocarbon receptor (AhR) activation and cytochrome P450 1A (Cyp1A) protein expression in rainbow trout (Oncorhynchus mykiss). We exposed trout hepatocytes in primary culture to the AhR agonist beta-napthoflavone (betaNF; 10(-6) M) and examined AhR and Cyp1A expression. betaNF-induced a significant temporal accumulation of AhR and Cyp1A1 mRNA abundance in trout hepatocytes. This transcript response was followed by a significantly higher AhR and Cyp1A protein expression. Exposure to geldanamycin (GA; 1000 ng mL(-1)), a benzoquinone ansamycin antibiotic used to inhibit hsp90 function, significantly reduced ( approximately 70%) betaNF-induced Cyp1A protein expression. Also, exposure to the proteasomal inhibitor MG-132 (50 microM) completely abolished betaNF-induced Cyp1A protein expression in trout hepatocytes. In addition, MG-132 treatment further enhanced the GA-mediated suppression of the Cyp1A response. The effect of MG-132 on Cyp1A response corresponded with a significant inhibition of BNF-mediated AhR mRNA abundance, but not protein content. Altogether our results suggest a betaNF-mediated autoregulation of AhR content in trout hepatocytes. We propose a key role for hsp90 and the proteasome in this ligand-mediated AhR regulation and Cyp1A response.

Targeting cytochrome P450 enzymes: a new approach in anti-cancer drug development

Cytochrome P450s (CYPs) represent a large class of heme-containing enzymes that catalyze the metabolism of multitudes of substrates both endogenous and exogenous. Until recently, however, CYPs have been largely overlooked in cancer drug development, acknowledged only for their role in phase I metabolism of chemotherapeutics. The first successful strategy targeting CYP enzymes in cancer therapy was the development of potent inhibitors of CYP19 (aromatase) for the treatment of breast cancer. Aromatase inhibitors ushered in a new era in hormone ablation therapy for estrogen dependent cancers, and have paved the way for similar strategies (i.e., inhibition of CYP17) that combat androgen dependent prostate cancer. Identification of CYPs involved in the inactivation of anti-cancer metabolites of vitamin D(3) and vitamin A has triggered development of agents that target these enzymes as well. The discovery of the over-expression of exogenous metabolizing CYPs, such as CYP1B1, in cancer cells has roused interest in the development of inhibitors for chemoprevention and of prodrugs designed to be activated by CYPs only in cancer cells. Finally, the expression of CYPs within tumors has been utilized in the development of bioreductive molecules that are activated by CYPs only under hypoxic conditions. This review offers the first comprehensive analysis of strategies in drug development that either inhibit or exploit CYP enzymes for the treatment of cancer.

Persistent binding of ligands to the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates many of the biological and toxic effects of halogenated aromatic hydrocarbons (HAHs), polycyclic aromatic hydrocarbons (PAHs), and other structurally diverse ligands. While HAHs are several orders of magnitude more potent in producing AhR-dependent biochemical effects than PAHs or other AhR agonists, only the HAHs have been observed to produce AhR-dependent toxicity in vivo. Here we have characterized the dissociation of a prototypical HAH ligand ([(3)H] 2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD]) and PAH-like ligand ([(3)H] beta-naphthoflavone [betaNF]) from the guinea pig, hamster, mouse, and rat hepatic cytosolic AhR in order to elucidate the relationship between the apparent ligand-binding affinities and the divergent potency of these chemicals. Both compounds dissociated very slowly from the AhR with the amount of specific binding remaining at 96 h ranging from 53% to 70% for [(3)H]TCDD and 26% to 85% for [(3)H] betaNF, depending upon the species examined. The rate of ligand dissociation was unaffected by protein concentration or incubation temperature. Preincubation of cytosol with 2,3,7,8-tetrachlorodibenzofuran, carbaryl, or primaquine, prior to the addition of [(3)H]TCDD, shifted the apparent IC(50) of these compounds as competitive AhR ligands by approximately 10- to 50-fold. Our results support the need for reassessment of previous AhR ligand-binding affinity calculations and competitive binding analysis since these measurements are not carried out at equilibrium binding conditions. Our studies suggest that AhR binding affinity/occupancy has little effect on the observed differences in the persistence of gene expression by HAHs and PAHs.

Specific blockage of ligand-induced degradation of the Ah receptor by proteasome but not calpain inhibitors in cell culture lines from different species

To firmly establish the pathway involved in ligand-induced degradation of the AHR, cell lines derived from mouse rat or human tissues were exposed to inhibitors specific to the proteasome or calpain proteases and exposed to TCDD. The level of endogenous AHR and CYP1A1 protein was then evaluated by quantitative Western blotting. Treatment of cells with the calpain inhibitors: calpeptin, calpain inhibitor III, or PD150606 either individually or in combinations up to 75 microM did not reduce TCDD-induced degradation of the AHR, the induction of endogenous CYP1A1 or the nuclear accumulation of the AHR. The activity of the inhibitors was verified with an in vivo calpain assay. In contrast, exposure of cells to the specific proteasome inhibitors: epoxomicin (1-5 microM), proteasome inhibitor I (5-10 microM) or lactacystin (5-15 microM) completely inhibited TCDD-induced degradation of the AHR. Inhibition of AHR degradation with these compounds did not reduce the induction of endogenous CYP1A1. In addition, exposure of the Hepa-1 line to the various proteasome inhibitors caused an accumulation of the AHR in the nucleus in the absence of TCDD exposure. Finally, Western blot analysis of the DNA bound AHR showed that its molecular mass was unchanged in comparison to the unliganded (cytoplasmic) AHR. Thus, these studies conclusively implicate the proteasome and not calpain proteases in the ligand-induced degradation of the mouse, rat and human AHR and suggest that the pharmacological use of proteasome inhibitors may impact the time course and magnitude of gene regulatory events mediated through the AHR.

Regulation of Aryl Hydrocarbon Receptor Expression in Rat Granulosa Cells1

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that mediates most of the toxic and endocrine-disruptive actions of aromatic compounds in the ovary. Paradoxically, this receptor has been shown to play important roles in normal female reproductive function as well. Although knowledge of AHR expression regulation in the ovary is of crucial significance to understand the receptor biology and its function in reproductive physiology, there are only limited data in this area. The purpose of the present study was to establish the possible regulation that AHR might undergo in ovarian cells. Here we show that the hormones FSH and estradiol are able to reduce AHR protein and transcript levels in granulosa cells in a way that parallels the changes observed in ovarian tissue across the rat estrous cycle. These findings suggest that estradiol and FSH would be cycle-associated endogenous modulators of AHR expression. In addition, we show that in granulosa cells the receptor is rapidly downregulated via proteasomal degradation following treatment with AHR ligands. However, prolonged treatment with an agonist caused an increase in Ahr mRNA levels. These actions would constitute a regulatory mechanism that both attenuates AHR signal rapidly and replenishes the cellular receptor pool in the long term. In conclusion, our results indicate that AHR expression is regulated by classical hormones and by its own ligands in granulosa cells.