A genetic analysis of processes regulating cytochrome P4501A1 expression

A CRISPR/Cas9 Whole-Genome Screen Identifies Genes Required for Aryl Hydrocarbon Receptor-Dependent Induction of Functional CYP1A1

Environmental pollutants including halogenated aromatic hydrocarbons and polycyclic aromatic hydrocarbons, including benzo[a]pyrene, exert their deleterious effects through the activation of the aryl hydrocarbon receptor (AHR) and by the resulting transcription of genes not yet fully identified. Ligand-bound AHR translocates from cytoplasm to nucleus, where it dimerizes with the aryl hydrocarbon receptor nuclear translocator (ARNT) protein. The AHR/ARNT dimer binds to enhancer regions of responsive genes to activate transcription. AHR also mediates carcinogenesis caused by PAHs, likely via CYP1A1, CYP1A2, and CYP1B1, which are massively induced by activated AHR in many tissues and generate carcinogenic electrophilic derivatives of PAHs. In the current study, we have used the mouse GeCKOv2 genome-wide CRISPR/Cas9 library to identify novel genes in the AHR pathway by taking advantage of a B[a]P selection assay that we previously used to identify core AHR pathway genes in Hepa-1c1c7 murine hepatoma cells. Besides Ahr, Arnt, and Cyp1a1, we report the identification of multiple additional putative AHR pathway genes including several that we validated. These include cytochrome P450 reductase (Por), which mediates redox regeneration of cytochromes P450, and 5 genes of the heme biosynthesis pathway: delta-aminolevulinate synthase 1 (Alas1), porphobilinogen deaminase (Hmbs), uroporphyrinogen decarboxylase (Urod), coproporphyrinogen oxidase (Cpox), and ferrochelatase (Fech): heme being an essential prosthetic group of cytochrome P450 proteins. Notably, several of these genes were identified by GeCKO screening, despite not being identifiable by reverse genetics approaches. This indicates the power of high-sensitivity genome-wide genetic screening for identifying genes in the AHR pathway.

Aryl hydrocarbon receptor nuclear translocator and upstream stimulatory factor regulate Cytochrome P450 2a5 transcription through a common E-box site

The aryl hydrocarbon receptor nuclear translocator (ARNT) belongs to the basic-helix-loop-helix (bHLH) transcription factors and regulates several genes as heterodimers with other bHLH proteins. ARNT is also able to homodimerize, but no mammalian target genes for the homodimer have been shown. We identified a palindromic E-box element in the 5' regulatory region of the murine cytochrome P450 (Cyp) 2a5 gene that was found to be important for Cyp2a5 transcription in primary hepatocytes, and was found by chromatin immunoprecipitation assays to interact with ARNT. Electrophoretic mobility-shift assay experiments with in vitro translated ARNT showed binding without heterodimerization partner, indicating binding as a homodimer. Transfection studies in wild-type and ARNT-deficient Hepa-1 cells revealed that ARNT expression is necessary for full activity of the Cyp2a5 promoter. In the liver-specific Arnt-null mouse line, the level of hepatic CYP2A5 mRNA was decreased significantly. Co-transfection studies with an ARNT expression vector lacking the transactivation domain (TAD) demonstrated that the ARNT TAD is needed for Cyp2a5 activation, which suggests that ARNT transactivates Cyp2a5 as a homodimer. In primary hepatocytes, the mRNA levels of both CYP2A5 and ARNT splice variant 1 were increased during cultivation. Upstream stimulatory factors 1 and 2a were also able to bind to the same E-box as ARNT, indicating that there may be competition for DNA binding between these factors. Indeed, the upstream stimulatory factors activated the Cyp2a5 promoter through the E-box only in the presence of hepatocyte nuclear factor-4alpha, while ARNT transactivation was independent of hepatocyte nuclear factor-4alpha. In conclusion, these results indicate that ARNT controls Cyp2a5 transcription and thus, for the first time, suggest active involvement of the ARNT homodimer in mammalian gene regulation.

Role of coactivators in transcriptional activation by the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) mediates the carcinogenic and other toxic effects of a variety of environmental pollutants, including 2,37,8-tetrachlorodibenzo-p-dioxin (TCDD), and some polycyclic aromatic hydrocarbons (PAHs). In most if not all cases, these deleterious effects depend upon modulation of gene transcription effected by the ligand-bound AHR. The responsive genes required for toxicity of TCDD have yet to be defined. However, induction of Cyp1a1 is known to represent a significant event in the toxicity of PAHs. Furthermore, the Cyp1a1 gene provides a model system for studying the mechanism of gene transcription by AHR. This review discusses the roles of transcriptional coactivator proteins in induction of Cyp1a1 by AHR ligands. Coactivators physically associate with the gene upon induction, and provide a bridge between AHR molecules, located at 5'enhancer elements, and general transcription factors, located at the promoter of the gene. Studies on the endogenous Cyp1a1 gene in its natural chromosomal setting are emphasized. The recent development of several new experimental techniques including the chromatin immunoprecipitation (ChIP) assay, RNA interference, and real-time PCR has provided a major boost to such studies. Future directions for research are also discussed. Since variations in coactivator expression or activity may result in inter-individual differences in response to AHR ligands, and may also underlie tissue-specific differences in sensitivity to such ligands during development, and in adulthood, the role of coactivators in transcriptional activation by AHR constitutes a very important area of research.

Regulation of theCyp2a5Gene Involves an Aryl Hydrocarbon Receptor-Dependent Pathway

We have investigated the role of the aryl hydrocarbon receptor (AHR) in the regulation of the Cyp2a5 gene. The C57BL/6 and DBA/2 mouse strains with a genetically determined difference in AHR function were used to study the CYP2A5 induction by typical AHR ligands, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3-methylcholanthrene. The CYP2A5 mRNA up-regulation in these mouse strains showed a difference in response, typical for AHR-regulated genes, both by TCDD in cultured primary hepatocytes and by 3-methylcholanthrene in vivo. In primary hepatocytes, TCDD caused a 3-fold elevation of the CYP2A5 protein level and a similar induction of the CYP2A5-catalyzed coumarin 7-hydroxylation activity. In reporter gene assays, the Cyp2a5 promoter region -3033 to +10 mediated a 2- to 5-fold induction of luciferase activity by TCDD treatment in primary hepatocytes and in Hepa-1 hepatoma cells with an intact AHR/AHR nuclear translocator (ARNT) complex. In Hepa-1 variant cell lines with deficiencies in the AHR/ARNT complex, the absence of ARNT abolished the induction. A putative AHR response element (XRE) was identified in the Cyp2a5 promoter at the position -2514 to -2492 and found to interact with the AHR/ARNT heterodimer. Transfection experiments combined with mutation of the XRE site indicated that the site partly mediates the TCDD induction of Cyp2a5. An additional AHR-dependent mechanism also regulates the proximal promoter of the Cyp2a5 gene. In conclusion, our studies showed that AHR ligands up-regulate Cyp2a5 transcriptionally by an AHR/ARNT-dependent mechanism and established Cyp2a5 as a novel AHR-regulated gene.

Different structural requirements of the ligand binding domain of the aryl hydrocarbon receptor for high- and low-affinity ligand binding and receptor activation

The aryl hydrocarbon receptor (AhR) functions as a ligand-activated transcription factor that is responsible for the regulation of several response genes, of which the best characterized is the CYP1A1 gene. The present study was undertaken to elucidate the mechanism of activation of the AhR by omeprazole (OME), 2-mercapto-5-methoxybenzimidazole (MMB), and primaquine (PRQ), compounds that have previously been reported to induce CYP1A1 expression but that are not typical AhR ligands. All compounds caused a significant increase in luciferase activity in rat H4IIE and human HepG2 hepatoma cells transfected with a Gal4-AhR construct and the corresponding Gal4-Luc reporter gene. Furthermore, MMB and PRQ, but not OME, were capable of transforming cytosolic AhR to a DNA-binding form and displacing AhR-bound [3H]TCDD in rat hepatic cytosol in vitro. By performing site-directed mutagenesis of residues in the ligand-binding domain of the Gal4-AhR, a construct containing a Y320F substitution was found to be resistant to activation by OME, MMB, and PRQ, but not by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Comparable affinities of [3H]TCDD-binding to the wild-type and the Y320F mutant Gal4-proteins, expressed in human embryonic kidney 293 cells, were obtained in the ligand-binding assay. In contrast, the competition of receptor-bound [3H]TCDD by PRQ was absent from Gal4-Y320F but not from Gal4-AhR cell extracts. The results of this study confirm that MMB and PRQ are low-affinity ligands for the AhR and suggest that high- and low-affinity ligands interact with different residues of the AhR ligand-binding pocket. In addition, the data presented here indicate that Tyr320 plays an important role in AhR activation.

Pharmacological doses of some synthetic retinoids can modulate both the aryl hydrocarbon receptor and retinoid receptor pathways

Retinoids have been demonstrated to have pharmacological application in the areas of dermatology and oncology. In addition to the natural retinoids such as all-trans-retinoic acid and 9-cis-retinoic acid, many new potential retinoid drugs have been synthesized, including retinoic acid receptor (RAR)-subtype selective agonists, retinoid X receptor (RXR)-selective agonists, RAR-selective antagonists, anti-AP1-specific retinoids and retinoids that induce apoptosis. Recent studies demonstrate that some retinoids, in addition to modulating the RAR/RXR pathway, are also capable at pharmacological concentrations of binding to aryl hydrocarbon receptor (AhR) and activating the AhR/AhR nuclear translocator pathway. Future studies are necessary to ascertain the consequences, if any, of activation of the AhR signaling pathway by pharmacological doses of specific retinoids.

Identification of a novel dioxin-inducible cytochrome P450

Representational difference analysis was used to isolate cDNAs corresponding to 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin)-inducible genes from mouse Hepa-1 cells. One cDNA encoded a novel cytochrome P450. The human homolog was also isolated and later proved to be human CYP2S1. The induction of mouse CYP2S1 mRNA by dioxin represents a primary response and required the aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator proteins. The induction of CYP2S1 also occurred in mouse liver and lung, with the highest expression found in lung. CYP2S1 was also inducible in a human lung epithelial cell line. The dioxin-inducibility of CY2S1 is exceptional, because all previously well-characterized cases of the induction of cytochromes P450 by dioxin involve members of the CYP1 family.

The Human Cytochrome P450 1A1 mRNA Is Rapidly Degraded In HepG2 Cells

The cytochromes P450 are a superfamily of enzymes that can carry out a wide range of oxidative reactions. While the transcriptional control of the cytochrome P450 genes has been relatively well-studied, posttranscriptional regulatory mechanisms that contribute to the regulation of P450s are much less well understood. We followed the decay of CYP1A1, CYP1A2, and CYP1B1 mRNAs after induction by the AH receptor ligand 2,3,7,8,-tetrachlorodibenzo-p-dioxin. CYP1A2 and CYP1B1 mRNAs were long-lived in this cell line (to > 24 h). In contrast, the CYP1A1 mRNA decays remarkably quickly. To determine if this rapid decay was unique to CYP1A1, we assessed the decay of selected human P450 and liver-specific mRNAs in HepG2 cells as a comparison. We analyzed albumin, phosphofructokinase, and GAPDH mRNAs and found that they were long-lived, with half-lives >24 h. We show that CYP2E1 mRNA can be detected in HepG2 cells by RT-PCR and that this mRNA also has a basal half-life of >24 h. Thus the CYP1A1 mRNA with its half-life of 2.4 h was one of the shortest-lived mRNA studied and is the most unstable of the cytochrome P450 mRNAs we have tested. The rapid decay of CYP1A1 mRNA is associated with a rapid loss in poly(A) tail length, suggesting that deadenylation is the first step in the decay pathway. The short half-life appears to be conserved across species, which suggests that this characteristic of the CYP1A1 mRNA is important for its function.

Expression of functional aromatic hydrocarbon receptor and aromatic hydrocarbon nuclear translocator proteins in murine bone marrow stromal cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) acting through the aromatic hydrocarbon receptor (AhR) and its dimerization partner, the AhR nuclear translocator protein (arnt), elicits numerous toxicological effects including immunosuppression and thymic atrophy. Previous work has shown that TCDD alters bone marrow prothymocyte populations. These effects could be mediated at the lymphocyte level directly and/or through effects on bone marrow stromal cells, a population important in the support of lymphopoiesis. The purpose of this study was to characterize AhR and arnt expression in three murine bone marrow stromal cell lines (S17, M2-10B4, and BMS2) and in primary stromal cell cultures. Immunoblot analysis detected AhR protein in M2-10B4 and BMS2 cells. AhR protein was also detected in the primary cultures. Arnt protein could be detected in all cell cultures. Electrophoretic mobility shift assays detected TCDD-dependent dioxin-responsive element (DRE) binding in all three cell lines. DNA binding was sequence-specific and dependent on AhR, as demonstrated by the addition of unlabeled DRE DNA or of anti-AhR antibody. Results obtained with the primary cultures paralleled those seen with the stromal cell lines. The ED50 for induction of TCDD-dependent DRE binding in M2-10B4 cells was 0.21 nM. TCDD treatment did not induce stromal P4501A1 mRNA expression but did increase P4501B1 mRNA levels in all three cell lines and in the primary cultures. These results indicate that murine bone marrow stromal cells express AhR and arnt proteins. Furthermore, these proteins are functional in terms of their DRE-binding ability and potential to regulate mRNA levels in a gene-specific fashion.

Induction of cytochrome P4501A1 by photooxidized tryptophan in Hepa lclc7 cells

Mouse hepatoma Hepa-lclc7 (Hepa-1) cells were cultivated in the presence of UV-irradiated amino acids. The results demonstrated that all of the amino acids tested, UV-oxidized tryptophan caused the highest induction of 7-ethoxyresorufin O-deethylase (EROD) activity compared with the controls (P < 0.01). The induction of EROD activity by oxidized tryptophan was dose dependent, and maximal induction was obtained at 12 hr after administration. Studies with various Hepa-1 mutants, which are defective in either the aryl hydrocarbon (Ah) receptor or Ah receptor nuclear translocator protein, indicated that the induction of EROD activity by oxidized tryptophan occurs through the Ah receptor. Gel mobility shift assays using nuclear extracts of Hepa-1 cells revealed that oxidized products of tryptophan can induce both Ah receptor transformation and binding of the liganded Ah receptor complex to its specific DNA recognition site. CYP1A1 mRNA, quantified by reverse transcription-polymerase chain reaction, and CYP1A1 protein were induced markedly in the oxidized tryptophan group compared with the controls. Injection of isolated oxidized tryptophan products into adult male rats caused significant induction of EROD activity in the pulmonary and hepatic microsomes compared with the controls (P < 0.01). These results demonstrated that oxidized tryptophan induces Ah receptor activation and binding of the liganded Ah receptor complex to its specific DNA recognition site, thereby initiating transcription and translation of the CYP1A1 gene with concomitant increase of EROD activity in Hepa-1 cells. Induction of EROD activity in the liver and lungs after injection of isolated oxidized tryptophan products into rats suggests that a similar mechanism may be operative in vivo.

Regulation of the CYP1A1 promoter in transgenic mice: an exquisitely sensitive on-off system for cell specific gene regulation

Mammalian cytochrome P-450s in the CYP1A gene family catalyse the oxidation of a wide range of drugs and foreign compounds resulting in their excretion. These enzymes are highly inducible by a range of compounds, including polycyclic aromatic hydrocarbons such as 3-methylcholanthrene (3-MC) and dioxins. Analysis of the CYP1A1 promoter has identified dioxin responsive enhancer elements which mediate the induction response. In order to evaluate this promoter as an in vivo regulatable expression system and to gain further insights into the tissue specific regulation of this gene, an 8.5 kb genomic fragment of the rat CYP1A1 promoter was cloned upstream of the lacZ reporter gene. This construct was used to generate transgenic mice and three independent lines were expanded for further study. The regulation of beta-galactosidase expression was determined in mock and 3-MC-treated mice in an extensive range of tissues. In untreated animals no transgene expression was detectable over non-transgenic controls. Treatment with 3-MC caused a profound increase in transgene expression (&gt; 1,000-fold) in many tissues including liver, adrenal, kidney and intestine. Inducible transgene expression was also detectable in many of the other tissues including the spleen, lung, pancreas and the reproductive organs. Although the absolute levels of induction varied, no significant differences in the pattern of transgene expression were observed between the three different transgenic mouse lines. In addition, the pattern of transgene expression correlated closely with the reported regulation of CYP1A1 protein. These results indicate that the CYP1A1 promoter can drive expression of heterologous genes in a truly on/off manner in a variety of tissues and cell types which will allow the expression of other proteins to be controlled in vivo. This reporter system also provides a model for establishing the environmental and hormonal factors regulating the expression of the CYP1A1 gene.

The role of the aryl hydrocarbon receptor nuclear translocator (ARNT) in hypoxic induction of gene expression. Studies in ARNT-deficient cells

Hypoxia-inducible factor-1 (HIF-1), a DNA-binding complex implicated in the regulation of gene expression by oxygen, has been shown to consist of a heterodimer of two basic helix-loop-helix Per-AHR-ARNT-Sim (PAS) proteins, HIF-1alpha, and HIF-1beta. One partner, HIF-1beta, had been recognized previously as the aryl hydrocarbon receptor nuclear translocator (ARNT), an essential component of the xenobiotic response. In the present work, ARNT-deficient mutant cells, originally derived from the mouse hepatoma line Hepa1c1c7, have been used to analyze the role of ARNT/HIF-1beta in oxygen-regulated gene expression. Two stimuli were examined: hypoxia itself and desferrioxamine, an iron-chelating agent that also activates HIF-1. Induction of the DNA binding and transcriptional activity of HIF-1 was absent in the mutant cells, indicating an essential role for ARNT/HIF-1beta. Analysis of deleted ARNT/HIF-1beta genes indicated that the basic, helix-loop-helix, and PAS domains, but not the amino or carboxyl termini, were necessary for function in the response to hypoxia. Comparison of gene expression in wild type and mutant cells demonstrated the critical importance of ARNT/HIF-1beta in the hypoxic induction of a wide variety of genes. Nevertheless, for some genes a reduced response to hypoxia and desferrioxamine persisted in these mutant cells, clearly distinguishing ARNT/HIF-1beta-dependent and ARNT/HIF-1beta-independent mechanisms of gene activation by both these stimuli.