The Aryl Hydrocarbon Receptor Complex and the Control of Gene Expression (original) (raw)
Related papers
Molecular Cancer Research, 2009
In this study, we examined the role of estrogen receptors (ER) in aryl hydrocarbon receptor (AHR)-dependent transactivation. Chromatin immunoprecipitation assays showed that AHR agonists differentially induced recruitment of ERα to the AHR target genes CYP1A1 and CYP1B1. Cotreatment with 17β-estradiol significantly increased β-naphthoflavone (BNF)-and 2,3,7,8-tetrachlorodibenzop-dioxin-induced recruitment of ERα to CYP1A1, whereas 3,3′-diindolylmethane induced promoter occupancy of ERα at CYP1A1 that was unaffected by cotreatment with 17β-estradiol. Cyclical recruitment of AHR and ERα to CYP1A1 was only observed in cells treated with BNF. Stable and subtype-specific knockdown of ERα or ERβ using shRNA showed that suppression of ERα significantly reduced, whereas knockdown of ERβ significantly enhanced, AHR agonist-induced Cyp1a1 expression in HC11 mouse mammary epithelial cells. AHR agonist-induced Cyp1b1 expression was reduced by ERβ knockdown but unaffected by ERα knockdown. The siRNA-mediated knockdown of ERα in MCF-7 human breast cancer cells did not affect 2,3,7,8-tetrachlorodibenzo-p-dioxin-dependent regulation of CYP1A1 and CYP1B1 mRNA expression. In agreement with our in vitro findings in the HC11 cells, ERα knockout mice exhibit reduced BNF-dependent induction of Cyp1a1 mRNA. These results establish ligand-and promoter-specific influences on the cyclical recruitment patterns for AHR and show ER species-, subtype-, and promoter-specific modulation of AHR-dependent transcription. (Mol Cancer Res
Archives of Biochemistry and Biophysics, 2002
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) activates the aryl hydrocarbon receptor (AhR) to mediate transcriptional activity of dioxin-responsive genes. The transactivation domain (TAD) of human AhR (hAhR) has potentially distinct acidic, glutaminerich, and proline/serine/threonine-rich subdomains. Cotransfection of exogenous hAhR into BP8 cells with isolated subdomains of hAhR TAD fused to glutathione S-transferase exhibited squelching of TCDD-dependent dioxin-response element (DRE)-driven luciferase reporter-gene activity with each subdomain. To study the potential cross talk between AhR-and estrogen receptor (ER)mediated activities, BP8 cells were cotransfected with hAhR TAD subdomain constructs and ERa. The three hAhR TAD subdomains inhibited the 17b-estradiol-induced estrogen-response element-mediated reporter-gene transactivation. Cotransfection of hAhR with the ligand-binding domain (LBD) of ERa also squelched TCDD-dependent DRE-driven reporter-gene activity in the presence of 17b-estradiol. Similar results were observed in T47D cells that express functional AhR and ERa. These results indicate that the isolated subdomains of hAhRÕs TAD and LBD of ERa are capable of squelching ligand-dependent transactivation of either the AhR or the ER, by titrating crucial proteins from an existing common pool of cofactors.
Journal of Biological Chemistry, 2005
The aryl hydrocarbon receptor (AHR) and the aryl hydrocarbon receptor nuclear translocator (ARNT) form a heterodimeric transcription factor upon binding a wide variety of environmental pollutants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). AHR target gene activation can be repressed by estrogen and estrogen-like compounds. In this study, we demonstrate that a significant component of TCDD-inducible Cyp1a1 transcription is the result of recruitment of estrogen receptor (ER)-␣ by AHR/ARNT as a transcriptional corepressor. Both AHR and ARNT were capable of interacting directly with ER␣, as ascertained by glutathione S-transferase pull-down. 17-estradiol repressed TCDDactivated Cyp1a1 and Cyp1b1 gene transcription in MCF-7 cells in the presence of cycloheximide, as determined by reverse transcription/real-time PCR. Furthermore, chromatin immunoprecipitation (ChIP) assays have shown that ER␣ is present at the Cyp1a1 enhancer only after co-treatment with E2 and TCDD, in MCF-7 cells. Sequential two-step ChIP assays were performed which demonstrate that AHR and ER␣ are present together at the same time on the Cyp1a1 enhancer during transrepression. Taken together these data support a role for ER-mediated transrepression of AHR-dependent gene regulation.
Differential gene regulation by the human and mouse aryl hydrocarbon receptor
2010
The human aryl hydrocarbon receptor (hAHR) and mouse aryl hydrocarbon receptor (mAHR b) share limited (58%) transactivation domain (TAD) sequence identity. Compared to the mAHR b allele, the hAHR displays 10-fold lower relative affinity for prototypical ligands, such as 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD). However, in previous studies, we have demonstrated that the hAHR can display a higher relative ligand-binding affinity than the mAHR b for specific AHR ligands, such as indirubin. Each receptor has also been shown to differentially recruit LXXLL coactivator motif proteins and to utilize different TAD subdomains in gene transactivation. Using hepatocytes isolated from C57BL/6J mice (Ahr b/b) and AHR Ttr transgenic mice, which express hAHR protein specifically in hepatocytes, we investigated whether the hAHR and mAHR b differentially regulate genes. DNA microarray and quantitative PCR analysis of Ahr b/b and AHR Ttr primary mouse hepatocytes treated with 10nM TCDD revealed that a number of established AHR target genes such as Cyp1a1 and Cyp1b1 are significantly induced by both receptors. Remarkably, of the 1752 genes induced by mAHR b and 1186 genes induced by hAHR, only 265 genes (~18%) were significantly activated by both receptors in response to TCDD. Conversely, of the 1100 and 779 genes significantly repressed in mAHR b and hAHR hepatocytes, respectively, only 462 (~49%) genes were significantly repressed by both receptors in response to TCDD treatment. Genes identified as differentially expressed are known to be involved in a number of biological pathways, including cell proliferation and inflammatory response, which suggest that compared to the mAHR b , the hAHR may play contrasting roles in TCDD-induced toxicity and endogenous AHR-mediated gene regulation.
Molecular endocrinology (Baltimore, Md.), 2008
The biological effects of dioxins are mediated by the aryl hydrocarbon receptor (AhR) and its dimerization partner, the AhR nuclear translocator (ARNT), and include interference with hormonal signaling pathways like the response to estrogens. The effects of estrogens are mediated by two estrogen receptor (ER) isoforms, ERalpha and ERbeta, which belong to the family of nuclear receptors. We have previously shown that ARNT can act as coactivator of the ERs. In this study, we show that recruitment of ARNT to AhR or hypoxia-inducible factor-1alpha signaling pathways as well as small interfering RNA-mediated down-regulation of ARNT levels lead to a reduction in ER transcriptional activity. Using chromatin immunoprecipitation assays, we demonstrate that this decrease coincides with reduced recruitment of ARNT to estradiol-regulated promoters. We show further that coactivation by ARNT as well as inhibition by dioxin acts stronger on ERbeta than on ERalpha activity. Additionally, we demonst...
PLoS ONE, 2012
The activated AHR/ARNT complex (AHRC) regulates the expression of target genes upon exposure to environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Importantly, evidence has shown that TCDD represses estrogen receptor (ER) target gene activation through the AHRC. Our data indicates that AHR and ARNT act independently from each other at non-dioxin response element sites. Therefore, we sought to determine the specific functions of AHR and ARNT in estrogen-dependent signaling in human MCF7 breast cancer and human ECC-1 endometrial carcinoma cells. Knockdown of AHR with siRNA abrogates dioxin-inducible repression of estrogen-dependent gene transcription. Intriguingly, knockdown of ARNT does not effect TCDD-mediated repression of estrogen-regulated transcription, suggesting that AHR represses ER function independently of ARNT. This theory is supported by the ability of the selective AHR modulator 39,49-dimethoxy-a-naphthoflavone (DiMNF) to repress estrogen-inducible transcription. Furthermore, basal and estrogen-activated transcription of the genes encoding cathepsin-D and pS2 are down-regulated in MCF7 cells but upregulated in ECC-1 cells in response to loss of ARNT. These responses are mirrored at the protein level with cathepsin-D. Furthermore, knock-down of ARNT led to opposite but corresponding changes in estrogen-stimulated proliferation in both MCF7 and ECC-1 cells. We have obtained experimental evidence demonstrating a dioxin-dependent repressor function for AHR and a dioxin-independent co-activator/co-repressor function for ARNT in estrogen signalling. These results provide us with further insight into the mechanisms of transcription factor crosstalk and putative therapeutic targets in estrogenpositive cancers.
The aryl hydrocarbon receptor interacts with transcription factor IIB
Molecular pharmacology, 1998
The aryl hydrocarbon receptor (AHR) and its DNA binding partner, the AHR nuclear translocator (ARNT), are basic helix-loop-helix transcription factors that mediate many of the toxic and carcinogenic effects of polyhalogenated aromatic hydrocarbons. The basic regions of the AHR and ARNT contact the GCGTG recognition site, whereas both their helix-loop-helix domains and periodicity-ARNT-single-minded domains participate in heterodimerization. To delineate the transcription factors that may facilitate DNA binding and transcriptional activation of the AHR/ARNT heterodimer, we questioned whether transcription factor IIB (TFIIB) may interact with either the AHR or ARNT and whether this interaction may affect the ability of the AHR/ARNT complex to bind DNA. Coaffinity precipitation assays demonstrated that both the AHR and ARNT were capable of interacting with TFIIB. Domain mapping experiments revealed that TFIIB interacts with the periodicity-ARNT-single-minded and carboxyl-terminal regio...
Evidence that ligand binding is a key determinant of Ah receptor-mediated transcriptional activity
Archives of Biochemistry and Biophysics, 2005
The aryl hydrocarbon receptor (AhR) mediates the biological activity of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Whether the AhR can mediate enhanced transcriptional activity in the absence of ligand binding has not been established. Hepatocytes from AhR-null (AhR-KO) and wild-type (AhR-WT) neonatal mice were immortalized with Simian virus 40. Two point mutants of the AhR, A375I and A375F, were generated to test the hypothesis that the AhR requires ligand binding to exhibit signiWcant transcriptional activity, both mutants fail to bind ligand or exhibit enhanced activity in cells exposed to AhR ligands. Upon transient, co-expression of ARNT with AhR-A375I or AhR-A375F in AhR-KO cells, these mutants exhibited signiWcant ligand-independent transcriptional activity. However, in CV-1 cells, which others have previously shown to contain relatively high levels of AhR ligand(s), these AhR mutants exhibit essentially no constitutive activity. These results indicate that while the AhR can potentially exhibit activity in the absence of ligand binding, the high constitutive receptor activity observed in many cell lines appears to be due to the presence of endogenous AhR ligands.
Current Opinion in Toxicology, 2017
The aryl hydrocarbon receptor repressor (AhRR) was first described as a specific competitive repressor of aryl hydrocarbon receptor (AhR) activity based on its ability to dimerize with the AhR nuclear translocator (ARNT) and through direct competition of AhR/ARNT and AhRR/ ARNT complexes for binding to dioxin-responsive elements (DREs). Like AhR, AhRR belongs to the basic Helix-Loop-Helix/Per-ARNT-Sim (bHLH/PAS) protein family but lacks functional ligand-binding and transactivation domains. Transient transfection experiments with ARNT and AhRR mutants examining the inhibitory mechanism of AhRR suggested a more complex mechanism than the simple mechanism of negative feedback through sequestration of ARNT to regulate AhR signaling. Recently, AhRR has been shown to act as a tumor suppressor gene in several types of cancer cells. Furthermore, epidemiological studies have found epigenetic changes and silencing of AhRR associated with exposure to cigarette smoke and cancer development. Additional studies from our laboratories have demonstrated that AhRR represses other signaling pathways including NF-κB and is capable of regulating inflammatory responses. A better understanding of the regulatory mechanisms of AhRR in AhR signaling and adverse outcome pathways leading to deregulated inflammatory responses contributing to tumor promotion and other adverse health effects is expected from future studies. This review article summarizes the characteristics of AhRR as an inhibitor of AhR activity and highlights more recent findings pointing out the role of AhRR in inflammation and tumorigenesis.