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Sahebnasagh A, Hashemi J, Khoshi A, Saghafi F, Avan R, Faramarzi F, Azimi S, Habtemariam S, Sureda A, Khayatkashani M, Safdari M, Rezai Ghaleno H, Soltani H, Khayat Kashani HR. Aromatic hydrocarbon receptors in mitochondrial biogenesis and function. Mitochondrion 2021; 61:85-101. [PMID: 34600156 DOI: 10.1016/j.mito.2021.09.012] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2021] [Revised: 09/17/2021] [Accepted: 09/27/2021] [Indexed: 12/11/2022]
Abstract
Mitochondria are ubiquitous membrane-bound organelles that not only play a key role in maintaining cellular energy homeostasis and metabolism but also in signaling and apoptosis. Aryl hydrocarbons receptors (AhRs) are ligand-activated transcription factors that recognize a wide variety of xenobiotics, including polyaromatic hydrocarbons and dioxins, and activate diverse detoxification pathways. These receptors are also activated by natural dietary compounds and endogenous metabolites. In addition, AhRs can modulate the expression of a diverse array of genes related to mitochondrial biogenesis and function. The aim of the present review is to analyze scientific data available on the AhR signaling pathway and its interaction with the intracellular signaling pathways involved in mitochondrial functions, especially those related to cell cycle progression and apoptosis. Various evidence have reported the crosstalk between the AhR signaling pathway and the nuclear factor κB (NF-κB), tyrosine kinase receptor signaling and mitogen-activated protein kinases (MAPKs). The AhR signaling pathway seems to promote cell cycle progression in the absence of exogenous ligands, whereas the presence of exogenous ligands induces cell cycle arrest. However, its effects on apoptosis are controversial since activation or overexpression of AhR has been observed to induce or inhibit apoptosis depending on the cell type. Regarding the mitochondria, although activation by endogenous ligands is related to mitochondrial dysfunction, the effects of endogenous ligands are not well understood but point towards antiapoptotic effects and inducers of mitochondrial biogenesis.
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Affiliation(s)
- Adeleh Sahebnasagh
- Clinical Research Center, Department of Internal Medicine, North Khorasan University of Medical Sciences, Bojnurd, Iran
| | - Javad Hashemi
- Department of Pathobiology and Laboratory Sciences, School of Medicine, North Khorasan University of Medical Sciences, Bojnurd, Iran
| | - Amirhosein Khoshi
- Department of Clinical Biochemistry, School of Medicine, North Khorasan University of Medical Sciences, Bojnurd, Iran
| | - Fatemeh Saghafi
- Department of Clinical Pharmacy, Faculty of Pharmacy and Pharmaceutical Sciences Research Center, Shahid Sadoughi University of Medical Sciences, Yazd, Iran
| | - Razieh Avan
- Assistant Professor of Clinical Pharmacy, Department of Clinical Pharmacy, Medical Toxicology and Drug Abuse Research Center (MTDRC), Faculty of Pharmacy, Birjand University of Medical Sciences, Birjand, Iran
| | - Fatemeh Faramarzi
- Clinical Pharmacy Research Center, Iran University of Medical Sciences, Tehran, Iran
| | - Saeed Azimi
- Student Research Committee, Department of Clinical Pharmacy, Faculty of Pharmacy, Shahid Beheshti University of Medical Sciences, Tehran, Iran
| | - Solomon Habtemariam
- Pharmacognosy Research Laboratories and Herbal Analysis Services, School of Science, University of Greenwich, Central Avenue, Chatham-Maritime, Kent ME4 4TB, United Kingdom
| | - Antoni Sureda
- Research Group in Community Nutrition and Oxidative Stress, University of the Balearic Islands and Health Research Institute of Balearic Islands (IdISBa), Palma de Mallorca, Spain; CIBER Fisiopatología de la Obesidad y Nutrición (CIBEROBN), Instituto de Salud Carlos III (ISCIII), Madrid, Spain
| | - Maryam Khayatkashani
- School of Iranian Traditional Medicine, Tehran University of Medical Sciences, 14155-6559 Tehran, Iran
| | - Mohammadreza Safdari
- Department of Orthopedic Surgery, Faculty of Medicine, North Khorasan University of Medical Sciences, Bojnurd, Iran
| | - Hassan Rezai Ghaleno
- Department of Surgery, Faculty of Medicine, Lorestan University of Medical Sciences, Khorramabad, Iran
| | - Hosseinali Soltani
- Department of General Surgery, Imam Ali Hospital, North Khorasan University of Medical Sciences, Bojnurd, Iran
| | - Hamid Reza Khayat Kashani
- Department of Neurosurgery, Imam Hossein Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran.
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2
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Inamoto I, Chen G, Shin JA. The DNA target determines the dimerization partner selected by bHLHZ-like hybrid proteins AhRJun and ArntFos. MOLECULAR BIOSYSTEMS 2017; 13:476-488. [DOI: 10.1039/c6mb00795c] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
The molecular basis of protein–partner selection and DNA binding of the basic helix–loop–helix (bHLH) and basic region-leucine zipper (bZIP) superfamilies of dimeric transcription factors is fundamental toward understanding gene regulation.
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Affiliation(s)
- Ichiro Inamoto
- Department of Chemistry
- University of Toronto
- Mississauga
- Canada L5L 1C6
| | - Gang Chen
- Department of Chemistry
- University of Toronto
- Mississauga
- Canada L5L 1C6
| | - Jumi A. Shin
- Department of Chemistry
- University of Toronto
- Mississauga
- Canada L5L 1C6
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3
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Regulations and Advisories. Toxicol Ind Health 2016. [DOI: 10.1177/074823370001600312] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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4
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Suzuki T, Toyohara T, Akiyama Y, Takeuchi Y, Mishima E, Suzuki C, Ito S, Soga T, Abe T. Transcriptional Regulation of Organic Anion Transporting Polypeptide SLCO4C1 as a New Therapeutic Modality to Prevent Chronic Kidney Disease. J Pharm Sci 2011; 100:3696-707. [DOI: 10.1002/jps.22641] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/01/2011] [Revised: 04/26/2011] [Accepted: 05/10/2011] [Indexed: 12/12/2022]
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5
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Kinehara M, Fukuda I, Yoshida KI, Ashida H. Aryl hydrocarbon receptor-mediated induction of the cytosolic phospholipase A(2)alpha gene by 2,3,7,8-tetrachlorodibenzo-p-dioxin in mouse hepatoma Hepa-1c1c7 cells. J Biosci Bioeng 2010; 108:277-81. [PMID: 19716514 DOI: 10.1016/j.jbiosc.2009.04.015] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/28/2009] [Revised: 04/10/2009] [Accepted: 04/14/2009] [Indexed: 01/22/2023]
Abstract
Upon binding to ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AhR) is activated to form a heterodimer with an aryl hydrocarbon receptor nuclear translocator (Arnt). This complex binds to DNA. It has been shown that the AhR bonds to a DNA sequence called the dioxin response element (DRE), which controls the expression of battery genes. It is reported that TCDD releases arachidonic acid from membrane phospholipids via activation of phospholipase A(2)s (PLA(2)s) in various cell types. Recently, we demonstrated that the TCDD-activated AhR binds to the second intron of the Pla2g4a gene, which encodes cytosolic phospholipase A(2)alpha (cPLA(2)alpha), in mouse hepatoma Hepa-1c1c7 cells. This result suggests that Pla2g4a appears to be a target gene of the AhR. In the present study, we investigated whether the transcriptional regulation of Pla2g4a is dependent on the AhR in Hepa-1c1c7 cells. Treatment of the cells with TCDD increased mRNA expression of Pla2g4a and enzymatic activity of PLA(2,) while this increased expression was not observed in AhR-defective c12 cells. After transient transfection of an Ahr gene-expressing plasmid into the c12 cells, expression of Pla2g4a was increased by TCDD. These results indicate that Pla2g4a may be a novel target gene of the AhR, and its transcriptional induction is mediated through binding of the AhR to the second intron of Pla2g4a, although this target site does not have a typical DRE sequence.
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Affiliation(s)
- Masaki Kinehara
- Department of Agrobioscience, Kobe University, Rokkodai, Nada-ku, Hyogo, Japan
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6
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Toyohara T, Suzuki T, Morimoto R, Akiyama Y, Souma T, Shiwaku HO, Takeuchi Y, Mishima E, Abe M, Tanemoto M, Masuda S, Kawano H, Maemura K, Nakayama M, Sato H, Mikkaichi T, Yamaguchi H, Fukui S, Fukumoto Y, Shimokawa H, Inui KI, Terasaki T, Goto J, Ito S, Hishinuma T, Rubera I, Tauc M, Fujii-Kuriyama Y, Yabuuchi H, Moriyama Y, Soga T, Abe T. SLCO4C1 transporter eliminates uremic toxins and attenuates hypertension and renal inflammation. J Am Soc Nephrol 2009; 20:2546-55. [PMID: 19875811 DOI: 10.1681/asn.2009070696] [Citation(s) in RCA: 96] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/03/2022] Open
Abstract
Hypertension in patients with chronic kidney disease (CKD) strongly associates with cardiovascular events. Among patients with CKD, reducing the accumulation of uremic toxins may protect against the development of hypertension and progression of renal damage, but there are no established therapies to accomplish this. Here, overexpression of human kidney-specific organic anion transporter SLCO4C1 in rat kidney reduced hypertension, cardiomegaly, and inflammation in the setting of renal failure. In addition, SLCO4C1 overexpression decreased plasma levels of the uremic toxins guanidino succinate, asymmetric dimethylarginine, and the newly identified trans-aconitate. We found that xenobiotic responsive element core motifs regulate SLCO4C1 transcription, and various statins, which act as inducers of nuclear aryl hydrocarbon receptors, upregulate SLCO4C1 transcription. Pravastatin, which is cardioprotective, increased the clearance of asymmetric dimethylarginine and trans-aconitate in renal failure. These data suggest that drugs that upregulate SLCO4C1 may have therapeutic potential for patients with CKD.
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Affiliation(s)
- Takafumi Toyohara
- Division of Nephrology, Endocrinology, and Vascular Medicine, Tohoku University Graduate School of Biomedical Engineering, Sendai, Japan
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7
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Kinehara M, Fukuda I, Yoshida KI, Ashida H. High-throughput evaluation of aryl hydrocarbon receptor-binding sites selected via chromatin immunoprecipitation-based screening in Hepa-1c1c7 cells stimulated with 2,3,7,8-tetrachlorodibenzo-p-dioxin. Genes Genet Syst 2009; 83:455-68. [PMID: 19282623 DOI: 10.1266/ggs.83.455] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022] Open
Abstract
Upon binding to ligands such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an aryl hydrocarbon receptor (AhR) is activated to form a heterodimer with an aryl hydrocarbon receptor nuclear translocator (Arnt) and binds to DNA. It has been shown that the binding of AhR to DNA depends on the dioxin response element (DRE) and controls xenobiotic-response genes. AhR-binding DNA fragments from mouse hepatoma Hepa-1c1c7 cells stimulated with TCDD were once enriched in a chromatin immunoprecipitation (ChIP) DNA library and screened through a high-throughput southwestern chemistry-based enzyme-linked immunosorbent assay (SW-ELISA). After screening 1700 fragments, the ChIP-SW-ELISA screening strategy allowed us to isolate 77 fragments tightly interacting with AhR in the presence of TCDD. Only 39 of the 77 fragments appeared to contain a typical DRE, indicating that in some cases the DRE was dispensable for AhR-binding, while 75 fragments were located within promoter-distal regions. Genomic mapping of the 77 fragments enabled us to estimate 121 potential AhR targets including known targets such as Cyp1A1 and Cyp1B1, but only a limited number exhibited an altered expression dependent on TCDD. This study revealed the fact that TCDD-activated AhR frequently binds to promoter-distal regions even without a DRE and is not always involved in transcriptional regulation, suggesting that within the genome DNA-binding of AhR could take place often in many regions without cis-regulatory elements and might not be a key determinant to establish its regulatory function.
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Affiliation(s)
- Masaki Kinehara
- Department of Agrobioscience, Graduate School of Agricultural Science, Kobe University, Japan
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8
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Johnson IT, Williamson G, Musk SRR. Anticarcinogenic Factors in Plant Foods: A New Class of Nutrients? Nutr Res Rev 2007; 7:175-204. [DOI: 10.1079/nrr19940011] [Citation(s) in RCA: 128] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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9
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Okino ST, Quattrochi LC, Pookot D, Iwahashi M, Dahiya R. A Dioxin-Responsive Enhancer 3′ of the HumanCYP1A2Gene. Mol Pharmacol 2007; 72:1457-65. [PMID: 17785579 DOI: 10.1124/mol.107.039826] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The human CYP1A genes CYP1A1 and CYP1A2 are in a head-to-head orientation on chromosome 15. Both CYP1A genes and CYP1B1 are transcriptionally induced by the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor that binds 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin). Although the TCDD-responsive enhancers for CYP1A1 and CYP1B1 are well characterized, a similar CYP1A2 enhancer has not been identified. In the human prostate cell line RWPE-1, CYP1A2 mRNA expression is dramatically induced by TCDD. Therefore, analysis of the native CYP1A2 gene in these cells can provide insight into its induction mechanism. To identify sites that may bind AhR on the CYP1A locus, we scanned 75 kilobases of chromosome 15 sequence for high-affinity AhR binding sites. We then analyzed most of the sites for TCDD-inducible AhR interaction by chromatin immunoprecipitation. As expected, the CYP1A1 and CYP1B1 enhancers bind AhR in TCDD-treated cells. It is noteworthy that we identify a region 3' of CYP1A2 that also binds AhR in response to TCDD. We cannot detect AhR binding at other sites on the CYP1A locus. In vivo footprinting demonstrates that two AhR binding sites in the CYP1A2 3' region are occupied in TCDD-treated cells. Reporter-gene studies show that these sites confer TCDD-responsiveness to a heterologous promoter. AhR also binds to the CYP1A2 3' region in TCDD-treated LS180 cells but not in HepG2 and ND-1 cells. In the latter cell lines, the CYP1A2 3' region is extensively methylated. In summary, we identify a novel TCDD-responsive enhancer for CYP1A2. We were surprised to find that this enhancer is not conserved across species and is primarily human-specific.
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Affiliation(s)
- Steven T Okino
- Department of Urology, San Francisco Veterans Affairs Medical Center and UCSF, 4150 Clement Street, San Francisco, CA 94121, USA
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10
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Chan IS, Fedorova AV, Shin JA. The GCN4 bZIP targets noncognate gene regulatory sequences: quantitative investigation of binding at full and half sites. Biochemistry 2007; 46:1663-71. [PMID: 17279629 PMCID: PMC2435288 DOI: 10.1021/bi0617613] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
We previously reported that a basic region/leucine zipper (bZIP) protein, a hybrid of the GCN4 basic region and C/EBP leucine zipper, not only recognizes cognate target sites AP-1 (5'-TGACTCA-3') and cAMP-response element (CRE) (5'-TGACGTCA-3') but also binds selectively to noncognate DNA sites: C/EBP (CCAAT/enhancer binding protein, 5'-TTGCGCAA), XRE1 (xenobiotic response element, 5'-TTGCGTGA), HRE (HIF response element, 5'-GCACGTAG), and E-box (5'-CACGTG). In this work, we used electrophoretic mobility shift assay (EMSA) and circular dichroism (CD) for more extensive characterization of the binding of wt bZIP dimer to noncognate sites as well as full- and half-site derivatives, and we examined changes in flanking sequences. Quantitative EMSA titrations were used to measure dissociation constants of this hybrid, wt bZIP, to DNA duplexes: Full-site binding affinities gradually decrease from cognate sites AP-1 and CRE with Kd values of 13 and 12 nM, respectively, to noncognate sites with Kd values of 120 nM to low microM. DNA-binding selectivity at half sites is maintained; however, half-site binding affinities sharply decrease from the cognate half site (Kd = 84 nM) to noncognate half sites (all Kd values > 2 microM). CD shows that comparable levels of alpha-helical structure are induced in wt bZIP upon binding to cognate AP-1 or noncognate sites. Thus, noncognate sites may contribute to preorganization of stable protein structure before binding target DNA sites. This work demonstrates that the bZIP scaffold may be a powerful tool in the design of small, alpha-helical proteins with desired DNA recognition properties.
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Affiliation(s)
- I-San Chan
- Department of Chemistry, University of Toronto, Mississauga, Ontario, Canada L5L 1C6
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11
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Temchura VV, Frericks M, Nacken W, Esser C. Role of the aryl hydrocarbon receptor in thymocyte emigration in vivo. Eur J Immunol 2005; 35:2738-47. [PMID: 16114106 DOI: 10.1002/eji.200425641] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The aryl hydrocarbon receptor (AHR) is a ligand-dependent member of the PAS-bHLH-family of nuclear receptors. Anthropogenic ligands include environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Over-activation of the AHR causes thymus atrophy and immunosuppression. Signaling via the AHR changes the thymocyte differentiation program at several checkpoints, in particular within the CD4-CD8- double-negative (DN) thymocyte subset. Here, we show that AHR over-activation led to the preferential emigration of DN thymocytes to the periphery and accumulation in the spleen. Some of these recent thymic emigrants (RTE) had a novel "activated immature" phenotype (CD3-TCRbeta-CD25+/intCD44-CD45RB+/intCD62L+CD69- cells). Gene expression profiling of DN RTE revealed 15 genes that were up-regulated more than threefold by TCDD, including the S100A9 gene. Exposure of S100A9 null mice to TCDD showed a role for this protein in AHR-mediated thymic egress.
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Affiliation(s)
- Vladimir V Temchura
- Institute for Environmental Medical Research (IUF), Heinrich-Heine University of Düsseldorf, Düsseldorf, Germany
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12
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Role of coactivators in transcriptional activation by the aryl hydrocarbon receptor. Arch Biochem Biophys 2005; 433:379-86. [PMID: 15581594 DOI: 10.1016/j.abb.2004.09.031] [Citation(s) in RCA: 248] [Impact Index Per Article: 13.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2004] [Revised: 09/24/2004] [Indexed: 11/29/2022]
Abstract
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.
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13
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Riddick DS, Lee C, Bhathena A, Timsit YE. The 2001 Veylien Henderson Award of the Society of Toxicology of Canada. Positive and negative transcriptional regulation of cytochromes P450 by polycyclic aromatic hydrocarbons. Can J Physiol Pharmacol 2003; 81:59-77. [PMID: 12665258 DOI: 10.1139/y03-003] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Most responses to aromatic hydrocarbons such as 3-methylcholanthrene (MC) and 2,3,7,8-tetrachlorodibenzo-p-dioxin are mediated by the aromatic hydrocarbon receptor (AHR). The AHR regulates induction of drug-metabolizing enzymes such as cytochrome P450 1A1. However, the expression of several genes of biological significance is decreased by these chemicals. We are examining the mechanisms by which aromatic hydrocarbons suppress constitutive hepatic cytochromes P450, especially the male-specific rat liver cytochrome P450 2C11 (CYP2C11), which is regulated by pulsatile growth hormone (GH) secretion. Aromatic hydrocarbons suppress CYP2C11 via a transcriptional mechanism both in vivo and in cultured hepatocytes, and the AHR appears to be involved; however, studies of protein-DNA interactions and reporter genes driven by the CYP2C11 5'-flanking region have not provided a definitive mechanism for this response. MC attenuates the ability of GH to stimulate hepatic CYP2C11 expression in hypophysectomized (hypx) male rats, and this prompted studies of effects of aromatic hydrocarbons on hepatic GH signaling pathways as a novel aspect of endocrine disruption. Our studies with hypx rats also suggest that the hepatic AHR protein is regulated by a pituitary factor(s). The goal of these molecular mechanistic studies is to improve our understanding of how environmental contaminants modulate the expression of genes coding for xenobiotic- and hormone-metabolizing enzymes.
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Affiliation(s)
- David S Riddick
- Department of Pharmacology, Medical Sciences Building, University of Toronto, Toronto, ON M5S 1A8, Canada.
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Abstract
Cytochrome P4501A1 is a substrate-inducible microsomal enzyme that oxygenates polycyclic aromatic hydrocarbons, such as the carcinogen benzo(a)pyrene, as the initial step in their metabolic processing to water-soluble derivatives. Enzyme induction reflects increased transcription of the cognate CYP1A1 gene. The environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin is the most potent known cytochrome P4501A1 inducer. Two regulatory proteins, the aromatic (aryl) hydrocarbon receptor (AhR) and the AhR nuclear translocator (Arnt), mediate induction. AhR and Arnt are prototypical members of the basic helix-loop-helix/Per-Arnt-Sim class of transcription factors. Mechanistic analyses of cytochrome P4501A1 induction provide insights into ligand-dependent mammalian gene expression, basic helix-loop-helix/Per-Arnt-Sim protein function, and dioxin action; such studies also impact public health issues concerned with molecular epidemiology, carcinogenesis, and risk assessment.
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Affiliation(s)
- J P Whitlock
- Department of Molecular Pharmacology, Stanford University School of Medicine, California 94305-5332, USA.
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15
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Okino ST, Whitlock JP. The aromatic hydrocarbon receptor, transcription, and endocrine aspects of dioxin action. VITAMINS AND HORMONES 2000; 59:241-64. [PMID: 10714242 DOI: 10.1016/s0083-6729(00)59009-8] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The widespread and persistent environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin elicits adaptive and adverse biological responses by inducing changes in gene transcription. Some of dioxin's effects reflect disruption of endocrine homeostasis. The aromatic hydrocarbon receptor protein, together with its heterodimerization partner, the aromatic hydrocarbon receptor nuclear translocator protein, mediates dioxin action. There are notable similarities between the mechanism of dioxin action and the mechanisms of steroid/retinoid/thyroid hormone action. Studies of dioxin action may provide insights into the regulation of hormone-responsive genes and endocrine physiology.
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Affiliation(s)
- S T Okino
- Department of Molecular Pharmacology, Stanford University School of Medicine, California 94305, USA
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16
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Ruh MF, Tian S, Cox LK, Ruh TS. The effects of histone acetylation on estrogen responsiveness in MCF-7 cells. Endocrine 1999; 11:157-64. [PMID: 10709763 DOI: 10.1385/endo:11:2:157] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/10/1999] [Revised: 06/29/1999] [Accepted: 07/26/1999] [Indexed: 11/11/2022]
Abstract
Because histone acetylation is implicated in the facilitation of specific gene transcription, the effect of increasing histone acetylation on the expression of an endogenous gene was investigated. The ability of trichostatin A (TSA), a histone deacetylase inhibitor, to potentiate the estradiol (E2) induction of progesterone receptor (PR) levels in MCF-7 cells was studied. Although TSA alone had no effect on PR synthesis, measured by a whole-cell binding assay with [3H]R5020, TSA potentiated the effect of 10(-11) ME2 such that 10 ng of TSA/mL approximately doubled the hormone response. When TSA was removed from the cells after various incubation times (24 and 48 h) by successive washings with TSA-free medium, it was determined that TSA was required throughout the 96-h incubation period in order to achieve maximal potentiation for the E2 response. In addition, TSA potentiated E2 induction of pS2 mRNA. These results suggested that the estrogen receptor (ER) complex might alter histone acetylation as part of the gene activation process. To test this directly, MCF-7 cells were incubated for 48 h with E2 followed by incubation with sodium [3H]acetate for 1 h. From two-dimensional polyacrylamide gel electrophoresis, an increase in total acetate incorporation into histones in estrogen- treated cells compared to control was observed as well as a preferential increase in the mono- and diacetylated histone H4. Experiments with lysine-specific antiacetylated H4 antibodies suggest a preferential increase in acetylation at lysine 16, but not 5, 8, or 12. The results of this study support an important role for histone acetylation in the mechanism of action of the ER.
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Affiliation(s)
- M F Ruh
- Department of Pharmacological and Physiological Science, St. Louis University School of Medicine, MO 63104, USA.
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Davarinos NA, Pollenz RS. Aryl hydrocarbon receptor imported into the nucleus following ligand binding is rapidly degraded via the cytosplasmic proteasome following nuclear export. J Biol Chem 1999; 274:28708-15. [PMID: 10497241 DOI: 10.1074/jbc.274.40.28708] [Citation(s) in RCA: 215] [Impact Index Per Article: 8.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that dimerizes with the AHR nuclear translocator protein to mediate gene regulation. However, the AHR protein is rapidly depleted in vitro and in vivo following exposure to ligands. The purpose of the studies in this report was to characterize the mechanism of AHR degradation and determine the consequence of blocking the degradation process. Western blot and immunological analysis of rat smooth muscle (A7), murine Hepa-1, and human HepG2 cells show that ligand-induced degradation of AHR is blocked when the proteasome is inhibited by MG-132. AHR degradation is also blocked in Hepa-1 and HepG2 cells when nuclear export is inhibited with leptomycin B. Mutation of a putative nuclear export signal present in the AHR results in the accumulation of AHR in the nucleus and reduced levels of degradation following ligand exposure. In addition, inhibition of AHR degradation results in an increase in the concentration of AHR.AHR nuclear translocator complexes associated with DNA and extends the duration that the complex resides in the nucleus. These findings show that nuclear export and degradation of the AHR protein are two additional steps in the AHR-mediated signal transduction pathway and suggest novel areas for regulatory control.
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Affiliation(s)
- N A Davarinos
- Department of Biochemistry, Medical University of South Carolina, Charleston, South Carolina 29403, USA
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18
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Wilson CL, Safe S. Mechanisms of ligand-induced aryl hydrocarbon receptor-mediated biochemical and toxic responses. Toxicol Pathol 1998; 26:657-71. [PMID: 9789953 DOI: 10.1177/019262339802600510] [Citation(s) in RCA: 101] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
The ubiquitous environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) is a member of a broad group of halogenated aromatic hydrocarbons (HAHs) that is known to induce a wide range of toxic and biochemical responses in laboratory animals and humans. The effects of HAH exposure are mediated by binding to the cytosolic aryl hydrocarbon receptor (AhR), which is expressed in a tissue- and cell type-specific manner. The AhR is a ligand-activated transcription factor belonging to the basic helix-loop-helix/Per-AhR-Arnt-Sim (bHLH/PAS) superfamily of proteins. The mechanism of induction of gene transcription by TCDD involves ligand recognition and binding by the AhR, nuclear translocation, and dimerization with the AhR cofactor, AhR nuclear translocator (Arnt). The nuclear heterodimer interacts with cognate xenobiotic responsive elements (XREs) in promoter/enhancer regions of multiple Ah-responsive genes. Subsequent changes in chromatin structure and/or interaction of the AhR complex with the basal transcriptional machinery play a significant role in AhR-mediated gene expression. Although Arnt is a necessary component of a functional nuclear AhR complex, this protein also forms transcriptionally active heterodimers with other bHLH/PAS factors, including those involved in the transcriptional response to hypoxia. Arnt is ubiquitously expressed in mammalian systems, and results from transgenic mouse studies suggest that this protein plays a vital role in early mammalian embryonic development. Similar experiments suggest that the AhR may be involved in development of various organ systems. Thus, molecular mechanistic studies of TCDD action have contributed significantly to an improved understanding of the role of at least 2 bHLH/PAS proteins, as well as organ- and tissue-specific biochemical and toxic responses to this class of environmental toxins.
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Affiliation(s)
- C L Wilson
- Department of Veterinary Physiology and Pharmacology, Texas A&M University, College Station 77843-4466, USA
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19
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Gao L, Dong L, Whitlock JP. A novel response to dioxin. Induction of ecto-ATPase gene expression. J Biol Chem 1998; 273:15358-65. [PMID: 9624117 DOI: 10.1074/jbc.273.25.15358] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We used differential display to discover a new gene that the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) regulates in mouse hepatoma cells. Its predicted amino acid sequence suggests that the gene encodes an ecto-ATPase that contains multiple glycosylation sites, conserved cysteine residues, and apyrase conserved regions. cDNA expression experiments in mouse hepatoma cells confirm that the new gene encodes an ecto-ATPase. Wild-type mouse hepatoma cells contain both constitutive and TCDD-inducible ecto-ATPase activity. Induction of ecto-ATPase gene expression by TCDD is direct and occurs at the transcriptional level. Studies in mutant hepatoma cells indicate that induction requires both the aromatic hydrocarbon receptor (AhR) and the AhR nuclear translocator (Arnt). Furthermore, induction requires AhR's transactivation domain, but not that of Arnt. Our findings reveal new aspects of dioxin's biological effects and TCDD-dependent gene regulation.
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Affiliation(s)
- L Gao
- Department of Molecular Pharmacology, Stanford University School of Medicine, Stanford, California 94305-5332, USA
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20
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Whitlock JP, Chichester CH, Bedgood RM, Okino ST, Ko HP, Ma Q, Dong L, Li H, Clarke-Katzenberg R. Induction of drug-metabolizing enzymes by dioxin. Drug Metab Rev 1997; 29:1107-27. [PMID: 9421687 DOI: 10.3109/03602539709002245] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Affiliation(s)
- J P Whitlock
- Department of Molecular Pharmacology, Stanford University School of Medicine, California 94305-5332, USA
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21
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Xu L, Ruh TS, Ruh MF. Effect of the histone deacetylase inhibitor trichostatin A on the responsiveness of rat hepatocytes to dioxin. Biochem Pharmacol 1997; 53:951-7. [PMID: 9174108 DOI: 10.1016/s0006-2952(97)00113-5] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Since histone acetylation has been implicated in the facilitation of specific gene transcription, we investigated the effect of increasing histone acetylation through inhibition of histone deacetylase on 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induction of P4501A activity in cultured rat hepatocytes. Inhibition of histone deacetylation was accomplished with addition of trichostatin A (TSA) to the incubation medium, and P4501A activity was measured spectrofluorometrically by determination of the rate of resorufin formation by ethoxyresorufin-O-deethylase (EROD). While TSA alone (5-200 ng/mL) had no effect on EROD activity, TSA potentiated the effect of various concentrations (10(-12) to 10(-10) M) of TCDD. Addition of 200 ng TSA/mL with TCDD resulted in an increased EROD activity of approximately 200% compared with TCDD alone. When TSA was removed from the cells after various incubation times (2, 6, 24 hr) by successive washings with TSA-free medium, it was determined that TSA was required for 24 hr in order to potentiate the effects of a 48-hr incubation with TCDD. In addition to measurement of EROD activity, P4501A1 and 1A2 microsomal protein were determined by western immunoblotting analysis. While neither P4501A1 nor 1A2 was detectable in the presence of TSA alone, P4501A1 was present after incubation of cells with TCDD in the presence or absence of TSA. TCDD plus TSA also resulted in the formation of P4501A2. The results of this study suggest an important role for histone acetylation in the action of TCDD on induction of P4501A enzymes.
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Affiliation(s)
- L Xu
- Department of Biology, St. Louis University School of Medicine, MO 63104, U.S.A
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22
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Abstract
The aryl hydrocarbon (or dioxin) receptor (AhR) is a ligand-activated basic helix-loop-helix (bHLH) protein that heterodimerizes with the bHLH protein ARNT (aryl hydrocarbon nuclear translocator) forming a complex that binds to xenobiotic regulatory elements in target gene enhancers. Genetic, biochemical, and molecular biology studies have revealed that the AhR mediates the toxic and biological effects of environmentally persistent dioxins and related compounds. Cloning of the receptor and its DNA-binding partner, ARNT, has facilitated detailed efforts to understand the mechanisms of AhR-mediated signal transduction. These studies have determined that this unique receptor consists of several functional domains and belongs to a subfamily of bHLH proteins that share a conserved motif termed the PAS domain. In addition, recent genetic studies have revealed that expression of the AhR is a requirement for proper embryonal development, which appears to be a common function shared by many other bHLH proteins. This review is a summary of recent molecular studies of AhR-mediated gene regulation.
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Affiliation(s)
- J C Rowlands
- Department of Bioscience, Karolinska Institute, NOVUM, Huddinge, Sweden
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23
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Dioxin induces transcription of fos and jun genes by ah receptor-dependent and -independent pathways. Toxicol Appl Pharmacol 1996. [DOI: 10.1016/s0041-008x(96)80029-9] [Citation(s) in RCA: 92] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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24
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Chen YH, Tukey RH. Protein kinase C modulates regulation of the CYP1A1 gene by the aryl hydrocarbon receptor. J Biol Chem 1996; 271:26261-6. [PMID: 8824276 DOI: 10.1074/jbc.271.42.26261] [Citation(s) in RCA: 113] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
Transcriptional activation of the human CYP1A1 gene by halogenated and polycyclic aromatic hydrocarbons is mediated by the aryl hydrocarbon receptor (AhR) complex, a ligand-dependent transcription factor. A competent AhR comprises at least two components following nuclear translocation and DNA binding, the AhR and the AhR nuclear translocator (Arnt) protein, whose combined action on human CYP1A1 gene transcription is shown to be dependent upon functional protein kinase C (PKC). In the present study, we examined the effects of phorbol 12-myristate 13-acetate, a potent PKC activator, on the ligand-induced transcriptional activation of the CYP1A1 gene and cellular function of the AhR in human HepG2 101L cells. The 101L cells carry a stable transgene consisting of 1800 bases of 5'-flanking DNA and the promoter of the human CYP1A1 gene linked to the firefly luciferase structural gene (Postlind, H., Vu, T. P., Tukey, R. H. & Quattrochi, L. C. (1993) Toxicol. Appl. Pharmacol. 118, 255-262). Pretreatment of cells with 12-myristate 13-acetate enhanced ligand-induced CYP1A1 gene expression 2-3-fold. Inhibition of PKC activity blocked directly the transcriptional activation and the transactivation of the CYP1A1 gene, indicating a role for PKC in the AhR-mediated transcriptional activation process. However, the DNA binding activities of the in vitro activated and the induced nuclear AhR as measured by electrophoretic mobility shift analysis were not affected when CYP1A1 transcription was inhibited, indicating the actions of PKC to be a nuclear event that works in concert with or precedes AhR binding to the gene. These results illustrate that PKC is absolutely essential for the cellular and molecular events that control induction of CYP1A1 gene transcription.
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Affiliation(s)
- Y H Chen
- Department of Pharmacology and UCSD Cancer Center, University of California, San Diego, La Jolla, California 92093-0636, USA
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25
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Corton JC. Overlapping but unique DNA binding specificities of the Ah receptor and constitutive dioxin-responsive element binding proteins from human keratinocytes. Toxicol Lett 1996; 85:67-75. [PMID: 8650695 DOI: 10.1016/0378-4274(96)03636-3] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
To understand the relationships between the protein architecture assembled on dioxin-responsive elements (DRE) and transcriptional regulation by dioxin in human keratinocytes, the nuclear DRE-binding proteins from human keratinocytes were identified and characterized. In addition to the aryl hydrocarbon receptor (AHR) complex inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), nuclear extracts from 3 human keratinocyte cell lines also contained one or more proteins that bound specifically to the DRE but whose levels were unaffected by TCDD or by anti-AHR antibody pretreatment. Alteration of a conserved T, within the core DRE sequence needed for transcriptional activation by the AHR complex, did not affect AHR binding but severely affected the ability of the constitutive proteins to bind. These data suggest that the nonidentical interplay of the AHR and constitutive DRE-binding proteins on the DRE is important in the regulation of genes whose expression is controlled by DRE.
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Affiliation(s)
- J C Corton
- Chemical Industry Institute of Toxicology, Research Triangle Park, NC 27709, USA
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26
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Dunn RT, Ruh TS, Burroughs LK, Ruh MF. Purification and characterization of an Ah receptor binding factor in chromatin. Biochem Pharmacol 1996; 51:437-45. [PMID: 8619888 DOI: 10.1016/0006-2952(95)02199-x] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Dioxin induces biological responses through interaction with a specific intracellular receptor, the Ah receptor, and the subsequent interaction of the Ah receptor with chromatin. We previously reported the binding of the Ah receptor, partially purified form rabbit liver, to receptor binding factors (termed AhRBFs) in chromatin. Rabbit liver chromatin proteins (CP) were isolated by absorption of chromatin to hydroxylapatite followed by sequential extraction with 3 M NaCl and 1-8 M guanidine hydrochloride (GdnHCl). In the present study, we continued the purification of the CP5 fraction, which exhibited AhRBF activity. The proteins in CP5 were separated by CL-Sepharose 6B column chromatography resolving lower molecular weight fractions. To assay for receptor binding, a portion of each Cl-Sepharose 6B fraction was reconstituted to rabbit double-stranded DNA (dsDNA) using a reverse gradient dialysis of 7.5 to 0.0 M GdnHCl. These reconstituted chromatins were then examined for binding to [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin ([3H]TCDD)-receptor complexes by the streptomycin filter binding assay. Two protein fractions with a molecular weight in the range of 10,000-14,000 demonstrated high affinity binding to the Ah receptor. The binding of AhRBFs reconstituted to dsDNA was shown, by competition experiments with Ah receptor bound by unlabeled TCDD (TCDD-R), to be > 90% specific for [3H]TCDD-R. Further purification was achieved by preparative ADS-PAGE, and AhRBF activity was attributed to two fractions with molecular weights between 12,000 and 10,000. A kDa protein with AhRBF activity was found to have an isoelectric point (pI) of > or = 10. The 12 kDa AhRBF was sequenced by Edman degradation after cyanogen bromide cleavage and identified as histone H4. Although histone H4 has been postulated to interact with transcription factors in a variety of systems, this is the first report of a specific interaction of AhR with histone H4.
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Affiliation(s)
- R T Dunn
- St. Louis University Health Sciences Center, Department of Pharmacological and Physiological Science, MO 63104, USA
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27
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Whysner J, Williams GM. 2,3,7,8-Tetrachlorodibenzo-p-dioxin mechanistic data and risk assessment: gene regulation, cytotoxicity, enhanced cell proliferation, and tumor promotion. Pharmacol Ther 1996; 71:193-223. [PMID: 8910955 DOI: 10.1016/0163-7258(96)00068-x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been found to cause several tumor types in rodents, but TCDD has not been proven to cause cancer in humans, although there have been reported associations. TCDD does not bind to DNA, and indirect tests for DNA damage have been mostly negative. Tumorigenicity by TCDD in rodents has been linked to cellular necrosis, enhanced cell proliferation and tumor promotion. TCDD binds to the Ah receptor, which induces CYP1A1. This binding may be involved in tumorigenicity in rodents; however, additional TCDD-induced toxic changes appear to be required. Biopersistence and organ distribution may play an important role in TCDD dosage extrapolation to humans, but these have not been adequately determined.
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Affiliation(s)
- J Whysner
- Toxicology and Risk Assessment Program, American Health Foundation, Valhalla, NY 10595-1599, USA
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28
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Abstract
The potent developmental toxicity of dioxin in multiple species has been known for a number of years. However, recent studies have indicated that dioxin also induces functional developmental defects, many of which are delayed. Subtle structural deficits, not detectable at birth, have also been described in multiple species and in both sexes. Certain defects have been reported not only in animals but also in children prenatally exposed to complex mixtures containing dioxinlike compounds. None of the effects can be attributed to modulation of any one endocrine system. For example, dioxin does not bind to the estrogen receptor, but it can cause effects that are both estrogenic and antiestrogenic. However, viewing dioxin and related compounds as endocrine disruptors that may alter multiple pathways sheds some light on the complexities of this potent class of growth dysregulators.
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Affiliation(s)
- L S Birnbaum
- Health Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711, USA
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29
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Okino ST, Whitlock JP. Dioxin induces localized, graded changes in chromatin structure: implications for Cyp1A1 gene transcription. Mol Cell Biol 1995; 15:3714-21. [PMID: 7791778 PMCID: PMC230609 DOI: 10.1128/mcb.15.7.3714] [Citation(s) in RCA: 69] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
In mouse hepatoma cells, the environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, or dioxin) induces Cyp1A1 gene transcription, a process that requires two basic helix-loop-helix regulatory proteins, the aromatic hydrocarbon receptor (AhR) and the aromatic hydrocarbon receptor nuclear translocator (Arnt). We have used a ligation-mediated PCR technique to analyze dioxin-induced changes in protein-DNA interactions and chromatin structure of the Cyp1A1 enhancer-promoter in its native chromosomal setting. Dioxin-induced binding of the AhR/Arnt heteromer to enhancer chromatin is associated with a localized (about 200 bp) alteration in chromatin structure that is manifested by increased accessibility of the DNA; these changes probably reflect direct disruption of a nucleosome by AhR/Arnt. Dioxin induces analogous AhR/Arnt-dependent changes in chromatin structure and accessibility at the Cyp1A1 promoter. However, the changes at the promoter must occur by a different, more indirect mechanism, because they are induced from a distance and do not reflect a local effect of AhR/Arnt binding. Dose-response experiments indicate that the changes in chromatin structure at the enhancer and promoter are graded and mirror the graded induction of Cyp1A1 transcription by dioxin. We discuss these results in terms of a TCDD-induced shift in an equilibrium between nucleosomal and nonnucleosomal chromatin configurations.
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Affiliation(s)
- S T Okino
- Department of Molecular Pharmacology, Stanford University School of Medicine, California 94305-5332, USA
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30
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Jørgensen EC, Autrup H. Effect of a negative regulatory element (NRE) on the human CYP1A1 gene expression in breast carcinoma MCF-7 and hepatoma HepG2 cells. FEBS Lett 1995; 365:101-7. [PMID: 7781760 DOI: 10.1016/0014-5793(95)00456-j] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
The expression of the cytochrome P4501A1 gene, CYP1A1, is induced by e.g. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) mainly by transcriptional mechanisms. The inducers mediate their effect upon binding and activation of the aryl hydrocarbon receptor (AHR) transcription-factor complex. Utilizing chimeric CYP1A1/CAT constructs transient gene expression experiments indicate that the putative negative regulatory element (NRE) of CYP1A1 influence the relative TCDD induced CAT activity in HepG2 cells, whereas this effect was not observed in MCF-7 cells. Differences in the formation of cell-specific protein-DNA complexes were demonstrated by gel retardation assays suggesting a functional difference of NRE in these two cell lines.
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Affiliation(s)
- E C Jørgensen
- Department of Environmental and Occupational Medicine, University of Aarhus, Denmark
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31
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Harper N, Steinberg M, Thomsen J, Safe S. Halogenated aromatic hydrocarbon-induced suppression of the plaque-forming cell response in B6C3F1 splenocytes cultured with allogenic mouse serum: Ah receptor structure activity relationships. Toxicology 1995; 99:199-206. [PMID: 7610466 DOI: 10.1016/0300-483x(95)03064-m] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The immunosuppressive effects of halogenated aromatic hydrocarbons (HAHs) were investigated in B6C3F1 female mice and in mouse splenocytes cultured with allogenic mouse serum using the Mishell-Dutton model for in vitro immunization to trinitrophenyl-lipopolysaccharide (TNP-LPS). Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), 1,2,3,7,8-PeCDF, 1,3,6,8-tetrachlorodibenzofuran (TCDF), 3,3',4,4',5-pentachlorobiphenyl (pentaCB), or 3,3',4,4',5,5'-hexaCB resulted in a dose-dependent suppression of the splenic plaque-forming cell (PFC) response both in vivo and in vitro. The effective dose required to decrease 50% (ED50) of the response to 2,3,7,8-TCDD, 2,3,4,7,8-PeCDF, 1,2,3,7,8-PeCDF, 1,3,6,8-TCDF, 3,3',4,4',5-pentaCB, or 3,3',4,4',5,5'-hexaCB in vivo was 14.1, 5.5, 1695, 34,800, 21, and 19 nmol/kg, respectively, and in vitro was 7.0, 10.6, 149, 2325, 9.1 and 9.1 nM, respectively. There was an excellent rank order and linear correlation between the in vivo versus in vitro activities for these HAHs (r < 0.99) and the relative immunosuppressive potencies of these compounds paralleled their binding affinities for the aryl hydrocarbon (Ah) receptor. These results show that splenocytes cultured with allogenic mouse serum is an Ah-responsive in vitro assay which can be used for quantitating the immunosuppressive effects of HAHs.
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Affiliation(s)
- N Harper
- Texas A & M University, College Station 77843-4466, USA
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32
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Unkila M, Pohjanvirta R, Tuomisto J. Biochemical effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds on the central nervous system. Int J Biochem Cell Biol 1995; 27:443-55. [PMID: 7641074 DOI: 10.1016/1357-2725(95)00023-i] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds are an important class of environmental contaminants which induce several types of biochemical alterations. Their effects have been most thoroughly characterized in the liver, especially regarding the Ah receptor-mediated induction of xenobiotic metabolizing enzymes. The behavioral signs exhibited by animals exposed to TCDD (progressive anorexia and body weight loss) suggest a role for the central nervous system (CNS) in TCDD toxicity. At lethal doses, TCDD affects the metabolism of serotonin, a neurotransmitter able to modulate food intake in the brain. This effect is associated with an elevated concentration of free tryptophan in the plasma. There does not appear to be any major changes in catecholaminergic neurotransmitter systems in TCDD-treated rats. Cytochrome P-450 related enzyme activities are induced by TCDD in the brain. As is the case in the liver, this induction does not correlate with susceptibility to TCDD lethality in rats. The involvement of the CNS in TCDD toxicity is still obscure. Elucidation of this role as well as the mechanism of TCDD-induced wasting may well advance our understanding of the regulation of food intake and body weight.
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Affiliation(s)
- M Unkila
- National Public Health Institute, Department of Toxicology, Kuopio, Finland
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33
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Yang JH, Rhim JS. 2,3,7,8-Tetrachlorodibenzo-p-dioxin: molecular mechanism of carcinogenesis and its implication in human in vitro model. Crit Rev Oncol Hematol 1995; 18:111-27. [PMID: 7695826 DOI: 10.1016/1040-8428(94)00125-d] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Affiliation(s)
- J H Yang
- Department of Preventive Medicine, Taegu Catholic University, School of Medicine, Republic of Korea
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34
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Safe SH. Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds. Pharmacol Ther 1995; 67:247-81. [PMID: 7494865 DOI: 10.1016/0163-7258(95)00017-b] [Citation(s) in RCA: 221] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
The aryl hydrocarbon (Ah) receptor binds several different structural classes of chemicals, including halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic and heteropolynuclear aromatic hydrocarbons. TCDD induces expression of several genes including CYP1A1, and molecular biology studies show that the Ah receptor acts as a nuclear ligand-induced transcription factor that interacts with xenobiotic or dioxin responsive elements located in 5'-flanking regions of responsive genes. TCDD also elicits diverse toxic effects, modulates endocrine pathways and inhibits a broad spectrum of estrogen (17 beta-estradiol)-induced responses in rodents and human breast cancer cell lines. Molecular biology studies show that TCDD inhibited 17 beta-estradiol-induced cathepsin D gene expression by targeted interaction of the nuclear Ah receptor with imperfect dioxin responsive elements strategically located within the estrogen receptor-Sp1 enhancer sequence of this gene.
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Affiliation(s)
- S H Safe
- Texas A&M University, College Station 77843-4466, USA
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35
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Catteau A, Bechtel YC, Poisson N, Bechtel PR, Bonaïti-Pellie C. A population and family study of CYP1A2 using caffeine urinary metabolites. Eur J Clin Pharmacol 1995; 47:423-30. [PMID: 7720764 DOI: 10.1007/bf00196856] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
CYP1A2 is a cytochrome P450 which is inducible by polycyclic aromatic hydrocarbons. This induction could be mediated via the Ah locus, which encodes a cytosolic receptor responsible for the regulation of the CYP1A1 gene. Enzyme activity in vivo can be measured by the urinary caffeine metabolite ratio (AFMU + 1X + 1U)/17U. Our goal was to determine, using this ratio, the possible existence of a genetic polymorphism in CYP1A2 induction. For this purpose, a population and family study, including smokers, were undertaken. In a first step, we investigated factors influencing enzyme activity in a population of 245 unrelated individuals. The induction effect of smoking and inhibiting effect of oral contraceptive use were confirmed. None of the other factors examined (age, sex, level of cigarette consumption, nicotine or tar amounts, filter, inhalation) accounted for the interindividual variability in the metabolic ratio. Using the statistical SKUMIX method, a unimodal (one peak) distribution of the ratio was concluded in 164 unrelated smokers, since a second distribution did not significantly improve the fit to the data (chi 2(1) = 1.39, P > 0.2). Segregation analysis was performed on 68 nuclear families and no major gene effect could be shown. Furthermore, the polygenic model did not provide a higher likelihood than the sporadic one, which argues against the existence of any familial resemblance. Although we cannot rule out the possibility that some environmental factors could obscure the phenotypes and occult a genetic determinism, we conclude that genetic factors are probably negligible in the determination of CYP1A2 activity measured by this method.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- A Catteau
- Unité de Recherches d'Epidémiologie Génétique (U.155 INSERM), Château de Longchamp, Paris, France
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36
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Carrier F, Chang CY, Duh JL, Nebert DW, Puga A. Interaction of the regulatory domains of the murine Cyp1a1 gene with two DNA-binding proteins in addition to the Ah receptor and the Ah receptor nuclear translocator (ARNT). Biochem Pharmacol 1994; 48:1767-78. [PMID: 7980646 DOI: 10.1016/0006-2952(94)90463-4] [Citation(s) in RCA: 27] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
The aromatic hydrocarbon (Ah) receptor complex is a ligand-activated transcriptional activator consisting of at least two protein components. The ligand-binding component is the AhR protein, a cytosolic receptor encoded by the Ahr gene, which, upon ligand binding, translocates to the nucleus in a heterodimeric complex with the ARNT (Ah receptor nuclear translocator) component. The complex binds to several discrete DNA domains containing aromatic hydrocarbon responsive elements (AhRE) present in the regulatory region of the murine cytochrome P(1)450 Cyp1a1 gene and of the other genes in the [Ah] gene battery. As a consequence of binding, a transcriptional complex is formed that activates the expression of these genes by as yet unidentified mechanisms. We have analyzed DNA-protein interactions in four of these domains, specifically, the AhREs located between -1085 and -482 (sites A, C, E, and D) of the upstream regulatory region of the murine Cyp1a1 gene. We found that two DNA-binding proteins, present in cytosolic and nuclear extracts of mouse Hepa-1 cells, showed overlapping DNA-binding specificities to those of the Ah receptor. One of these proteins had an apparent molecular mass of 35-40 kDa, bound only to AhRE3 (site D), and has been identified tentatively as a member of the C/EBP family of transcription factors. The second protein, purified by DNA-affinity chromatography, had an apparent molecular mass of 95 kDa and bound to a larger DNA motif that included the AhRE sequence, in AhRE3 and AhRE5 (sites D and A), but not in AhRE1 or AhRE2 (sites C and E). This protein was not AhR nor was it ARNT, since it was found in receptorless (Ahr-) and in nuclear translocation-defective (Arnt-) cells, as well as in cells that had not been exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin), a potent inducer of Cyp1a1 expression. Evidence from in vivo methylation protection indicated that two G residues flanking AhRE3, one of which is required for binding of the 95-kDa protein, may be protected from methylation in uninduced cells and become exposed upon dioxin treatment, suggesting that the 95-kDa protein may be constitutively bound to AhRE3, and be displaced by binding of the Ah receptor complex. These results lend support to the concept that the transcriptional regulation of the [Ah] battery genes could be modulated by combinatorial interactions of the Ah receptor complex with other transcription factors.
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Affiliation(s)
- F Carrier
- Laboratory of Molecular Pharmacology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892
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37
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Thomas T, Gallo MA. Polyamine-induced changes in the sedimentation profile and DNA binding of aryl hydrocarbon receptor. Toxicol Lett 1994; 74:35-49. [PMID: 8085269 DOI: 10.1016/0378-4274(94)90072-8] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
Aryl hydrocarbon (Ah) receptor mediates the toxic action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We studied the effects of polyamines--putrescine, spermidine and spermine--on the physical chemical properties of Ah receptor from A431 cells. Spermidine and spermine caused the precipitation of 9S oligomeric receptor with a gradual decrease in the receptor peak during density gradient sedimentation. RNase A treatment transformed the 9S Ah receptor to a 6S form and DNA binding increased by 2-fold. Following partial purification of transformed Ah receptor by preparative sucrose gradient centrifugation, it lost the ability to bind to DNA, but addition of spermidine increased DNA binding in a concentration-dependent manner. These data show that polyamines modulate the structure and DNA binding of Ah receptor and suggest that cellular polyamine levels might be important in the tissue specific toxicity of TCDD.
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Affiliation(s)
- T Thomas
- Department of Environmental and Community Medicine, University of Medicine and Dentistry of New Jersey-Robert Wood Johnson Medical School, Piscataway 08854
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38
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Down-regulation of nuclear aryl hydrocarbon receptor DNA-binding and transactivation functions: requirement for a labile or inducible factor. Mol Cell Biol 1994. [PMID: 8065302 DOI: 10.1128/mcb.14.9.5653] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Aryl hydrocarbons (AHs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene activate the sequence-specific DNA-binding activity of the AH receptor. In the rat hepatocyte-derived cell line LCS7, DNA-binding activity peaked after 30 min and was then down-regulated, reaching negligible levels by 2 h. Down-regulation could be blocked, and DNA-binding activity maintained at maximum for many hours by inhibiting protein or RNA synthesis, implying that down-regulation is a mediated process requiring a labile or inducible protein. CYP1A1 transcription and in vivo DNA-protein interactions at xenobiotic response elements were down-regulated in parallel with DNA-binding activity in nuclear extracts, and these changes could also be blocked by inhibitors of protein synthesis. The correlation between AH receptor DNA-binding activity, intensity of in vivo footprints at xenobiotic response elements, and CYP1A1 transcription rate implies that down-regulation of AH receptor DNA-binding activity is important in regulating CYP1A1 transcription and that receptor is required continuously to maintain transcription. This correlation extends to the murine hepatoma cell line Hepa-1c1c7, in which slower kinetics of activation and down-regulation of CYP1A1 transcription paralleled slower activation and down-regulation of AH receptor DNA-binding activity. The difference in kinetics between cell lines also implies that AH receptor DNA-binding activity is modulated by a mechanism that may be influenced by cell-specific regulatory pathways. The above observations in conjunction with mixing experiments and comparisons of cytoplasmic and nuclear extracts indicate that down-regulation of AH receptor DNA-binding activity is probably due either to degradation or to conversion of the receptor to form that is inactive in both DNA binding and transactivation.
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39
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Reick M, Robertson RW, Pasco DS, Fagan JB. Down-regulation of nuclear aryl hydrocarbon receptor DNA-binding and transactivation functions: requirement for a labile or inducible factor. Mol Cell Biol 1994; 14:5653-60. [PMID: 8065302 PMCID: PMC359090 DOI: 10.1128/mcb.14.9.5653-5660.1994] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
Aryl hydrocarbons (AHs) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo[a]pyrene activate the sequence-specific DNA-binding activity of the AH receptor. In the rat hepatocyte-derived cell line LCS7, DNA-binding activity peaked after 30 min and was then down-regulated, reaching negligible levels by 2 h. Down-regulation could be blocked, and DNA-binding activity maintained at maximum for many hours by inhibiting protein or RNA synthesis, implying that down-regulation is a mediated process requiring a labile or inducible protein. CYP1A1 transcription and in vivo DNA-protein interactions at xenobiotic response elements were down-regulated in parallel with DNA-binding activity in nuclear extracts, and these changes could also be blocked by inhibitors of protein synthesis. The correlation between AH receptor DNA-binding activity, intensity of in vivo footprints at xenobiotic response elements, and CYP1A1 transcription rate implies that down-regulation of AH receptor DNA-binding activity is important in regulating CYP1A1 transcription and that receptor is required continuously to maintain transcription. This correlation extends to the murine hepatoma cell line Hepa-1c1c7, in which slower kinetics of activation and down-regulation of CYP1A1 transcription paralleled slower activation and down-regulation of AH receptor DNA-binding activity. The difference in kinetics between cell lines also implies that AH receptor DNA-binding activity is modulated by a mechanism that may be influenced by cell-specific regulatory pathways. The above observations in conjunction with mixing experiments and comparisons of cytoplasmic and nuclear extracts indicate that down-regulation of AH receptor DNA-binding activity is probably due either to degradation or to conversion of the receptor to form that is inactive in both DNA binding and transactivation.
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Affiliation(s)
- M Reick
- Molecular Biology Laboratory, Maharishi International University, Fairfield, Iowa 52557-1078, USA
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40
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Nakai JS, Winhall MJ, Bunce NJ. Comparative kinetic study of the binding between 2,3,7,8-tetrachlorodibenzo-p-dioxin and related ligands with the hepatic Ah receptors from several rodent species. JOURNAL OF BIOCHEMICAL TOXICOLOGY 1994; 9:199-209. [PMID: 7853354 DOI: 10.1002/jbt.2570090405] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
Kinetic analysis of the time course of association of [3H]-2,3,7,8-tetrachlorodibenzo-p-dioxin with hepatic cytosol from five rodent species gave additional evidence for differences in the properties of the Ah receptor ligand binding subunit between species. A parallel study of the association of six tritiated polychlorinated dibenzo-p-dioxins and dibenzofurans with hepatic Ah receptor from Wistar rat and C57BL/6 mouse showed that their rank order for kinetic affinity did not correlate with the rank ordering of their toxic potency and may vary according to the source of the Ah receptor.
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Affiliation(s)
- J S Nakai
- Department of Chemistry and Biochemistry, University of Guelph, Ontario, Canada
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Abstract
The environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin acts through the aromatic hydrocarbon (AH) receptor to enhance CYP1A1 gene transcription. There are notable mechanistic analogies between dioxin action and lipophilic hormone action. In addition, some of dioxin's adverse effects are associated with disruptions in endocrine homeostasis. Thus, dioxin biology intersects several disciplines, including toxicology, endocrinology, molecular biology, and public health.
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Affiliation(s)
- J P Whitlock
- Department of Molecular Pharmacology, Stanford University School of Medicine, Stanford, CA 94305-5332, USA
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Robertson RW, Zhang L, Pasco DS, Fagan JB. Aryl hydrocarbon-induced interactions at multiple DNA elements of diverse sequence--a multicomponent mechanism for activation of cytochrome P4501A1 (CYP1A1) gene transcription. Nucleic Acids Res 1994; 22:1741-9. [PMID: 8202380 PMCID: PMC308058 DOI: 10.1093/nar/22.9.1741] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
In vivo footprinting experiments, augmented with gel shift and transfection analyses suggest that activation of the CYP1A1 gene by aryl hydrocarbons may be a multicomponent process. During the first 30 minutes of exposure to aryl hydrocarbon carcinogens and environmental contaminants, in vivo footprints appear at nine distinct sites within a 281 bp region centered 950 bp upstream of the CYP1A1 transcription start site. Six of these sites are unrelated in sequence to the three xenobiotic response elements (XREs) within this region, at which the aryl hydrocarbon (AH) receptor is known to bind. These six display a variety of footprint patterns, are diverse in sequence and range in G-C content from 60 to 75%. This diversity suggests that multiple nuclear factors may be responsible for these six in vivo footprints. These observations are consistent with competition gel shift experiments showing that the nuclear factors binding at two of these sites are different from each other, as well as from the AH receptor. Gel shifts also indicate that the sequence-specific factors binding at these sites are expressed constitutively. This is consistent with a model in which in vivo footprints are induced at these six sites, not through direct activation or de novo synthesis of DNA-binding factors, but through a two phase mechanism in which binding of the nuclear AH receptor complex to XREs facilitates the binding of constitutive factors at these sites. This facilitation could be mediated either through specific protein-protein interactions or through alterations in chromatin structure that make these sites accessible to constitutive nuclear factors. A function for the sequences at which aryl hydrocarbons induce in vivo footprints is suggested by transfection experiments showing that one of these sequences cooperates with a weak XRE to confer on a reporter gene responsiveness to aryl hydrocarbons.
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Affiliation(s)
- R W Robertson
- Molecular Biology Laboratory, Maharishi International University, Fairfield, IA 52557-1078
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Vogel C, Döhr O, Abel J. Transforming growth factor-beta 1 inhibits TCDD-induced cytochrome P450IA1 expression in human lung cancer A549 cells. Arch Toxicol 1994; 68:303-7. [PMID: 8085941 DOI: 10.1007/s002040050073] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
The effect of transforming growth factor-beta 1 (TGF-beta 1) on the expression of cytochrome P450IA1 (CYPIA1) was examined in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-treated human lung cancer A549 cells. Using the reverse transcription-polymerase chain reaction (RT-PCR) it was demonstrated that TGF-beta 1 inhibits CYPIA1 expression in a dose dependent manner. Based on the inhibitory concentration 50 (IC50) of about 5 pM it is suggested that TGF-beta 1 has a physiological function in downregulation of this cytochrome. In the presence of cycloheximide, the effect of TGF-beta 1 on CYPIA1 mRNA disappeared. This finding indicates that protein synthesis may be required for the TGF-beta 1 mediated response of CYPIA1. The possible mechanisms by which TGF-beta 1 interacts with TCDD-responsive drug metabolizing enzymes are discussed.
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Affiliation(s)
- C Vogel
- Medical Institute of Environmental Hygiene, Heinrich-Heine-University of Düsseldorf, Germany
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44
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Lucier GW, Portier CJ, Gallo MA. Receptor mechanisms and dose-response models for the effects of dioxins. ENVIRONMENTAL HEALTH PERSPECTIVES 1993; 101:36-44. [PMID: 8390353 PMCID: PMC1519667 DOI: 10.1289/ehp.9310136] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/20/2023]
Abstract
There is increasing evidence that receptor-mediated events impact one or more stages responsible for tumor development in experimental animals and humans. Although many chemicals and endogenous hormones require receptor interactions as a necessary event in their carcinogenic activity, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its structural analogs are the most visible examples of receptor-mediated carcinogens. TCDD, or dioxin as it is frequently called, interacts with the Ah receptor (AhR), which functions in a manner analogous to receptors for steroids. TCDD produces a wide spectrum of biochemical and toxic responses in in vitro and in vivo systems, and the Ah receptor is generally considered necessary for most if not all of these responses. Risk assessments for dioxin made by the United States and other countries throughout the world have been based on its carcinogenecity in experimental animals. Recently, epidemiology studies have indicated that TCDD is a human carcinogen at high doses. Because TCDD appears to be acting like a potent and persistent hormone agonist, it appears reasonable to incorporate mechanistic information on receptor-mediated events in risk assessments for TCDD. This information may be obtained from steroid receptor action and from molecular data on the Ah receptor. In this paper, we evaluate the scientific foundation on which mechanistic models for estimating dioxin's risks should be based. These models need to recognize the mechanisms possible for the diversity of biological responses that are initiated by a single receptor interacting with a single ligand. The U.S. EPA is currently reevaluating dioxin's risks by examining the possibility of developing biologically based models.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- G W Lucier
- National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709
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