The Aryl Hydrocarbon Receptor Directly Regulates Expression of the Potent Mitogen Epiregulin

Abstract

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to cause a large number of adverse effects, mediated largely by its binding to the aryl-hydrocarbon receptor (AhR) and subsequent modulation of gene expression. It is thought that AhR mediates these effects through the untimely and disproportionate expression of specific genes. However, the exact mechanism, or the genes involved, through which TCDD leads to these effects is still unknown. This study reports the discovery of a novel target gene, epiregulin, which is regulated by TCDD-activated AhR. Epiregulin is a growth regulator which belongs to the epidermal growth factor (EGF) family. Using real time quantitative PCR (qPCR), it was established that TCDD upregulates epiregulin gene expression. The promoter region of epiregulin has a dioxin responsive element (DRE) 56 nucleotides upstream of the transcription start site, along with three potential Sp1 binding sites. Chromatin immunoprecipitation (ChIP) assays with an anti-AhR antibody showed promoter occupancy upon TCDD treatment. Luciferase reporter assays using a vector harboring the first 125 base pairs of the epiregulin rat promoter revealed an increase in signal on TCDD treatment, which was lost upon mutation of the DRE. Epiregulin and TCDD treatment mediated a dose-dependent increase in primary mouse keratinocyte growth. These results demonstrate that AhR directly increases epiregulin expression, which could play an important role in TCDD mediated tumor promotion observed in rodent models.

Green Tea Polyphenols Reverse Cooperation between c-Rel and CK2 that Induces the Aryl Hydrocarbon Receptor, Slug, and an Invasive Phenotype

“…Although the mechanism through which the AhR regulates Slug is still under investigation, the possibility that AhR and c-Rel coregulate the Slug promoter is appealing, given past work showing cooperation between AhR and the p65 NF-nB subunit (34). Lastly, our findings are consistent with results of other investigators suggesting a role for exogenous ligand-activated AhR in regulating factors involved in branching and invasive growth such as Slug (18), epiregulin (35), and Cox II (36) in human mammary tumor lines and in other cell types.…”

Expression and role of EGFR ligands induced in airway cells by PM2.5and its components

“…a-naphthoflavone, an AhR antagonist, diminished PMinduced secretion of amphiregulin and TGF-a ( fig. 3), suggesting AhR-dependent transcriptional regulation of amphiregulin and TGF-a, which has already been described for another EGFR ligand, epiregulin, in 2,3,7,8-tetrachlorodibenzo-pdioxin-treated mouse cells [25]. This hypothesis is reinforced by the identification of consensus xenobiotic responsive element sequences upstream of the transcriptional start site in human amphiregulin and TGF-a promoters, to which a transcriptional complex including PAH and AhR binds to transactivate genes.…”

NF‐κB and epithelial to mesenchymal transition of cancer

“…Historically, the AhR has been studied for its responsiveness to a variety of environmental chemicals (e.g., polycyclic aromatic hydrocarbons, PCBs, and dioxins) and its ability to transactivate genes encoding phase I cytochrome P-450 enzymes that metabolize endogenous substrates (e.g., 17b-estradiol) and some environmental chemicals into mutagenic intermediates [Safe and Krishnan, 1995;Spink et al, 2003]. However, more recent studies have indicated roles for the AhR, even in the absence of environmental ligands, in the transcriptional regulation of genes critical for growth [Ma and Whitlock, 1996;Elizondo et al, 2000;Tohkin et al, 2000;Abdelrahim et al, 2003;Patel et al, 2006], apoptosis Patel et al, 2006;Wu et al, 2007], and in EMT and tumor progression [Mulero-Navarro et al, 2005]. For example, immortalized mouse mammary fibroblasts lacking AhR have impaired tumorigenicity in a subcutaneous mouse xenograph model [Mulero-Navarro et al, 2005], and DMBA treatment induced a more highly invasive phenotype in c-Rel-driven mammary tumor cell line [Shin et al, 2006].…”