Expression of the aryl hydrocarbon receptor/transcription factor (AhR) and AhR-regulated CYP1

Abstract

Exposure to ubiquitous environmental chemicals, such as polycyclic aromatic hydrocarbons (PAH), may contribute to human breast cancer. In animals, PAH induce tumors in part by activating the aryl hydrocarbon receptor (AhR)/transcription factor. Historically, investigations into AhR-regulated carcinogenesis have focused on AhR-dependent transcriptional regulation of cytochrome P450 (CYP) enzymes which oxidize PAH to mutagenic intermediates. However, recent studies suggest that the AhR directly regulates cell growth. Given the postulated role of the AhR in carcinogenesis, we predicted that: (1) tissue predisposed to PAH tumorigenesis would express the AhR and (2) aberrant AhR and/or AhR-regulated gene expression would accompany malignant transformation. To test these hypotheses, AhR and CYP1 protein and/or mRNA levels were evaluated in rat mammary tumors induced with 7, 12-dimethylbenz[a]anthracene (DMBA), a prototypic PAH and AhR ligand. Results indicate modest AhR expression in normal mammary myoepithelial and ductal epithelial cells. In contrast, high AhR levels were detected in DMBA-induced tumors. Nuclear AhR localization in tumors suggested constitutive AhR activation. In situ hybridization and quantitative RT-PCR assays indicated high AhR mRNA levels in neoplastic epithelial cells. While both AhR-regulated CYP1A1 and CYP1B1 mRNAs were induced in breast tissue within 6 h of DMBA gavage, only CYP1B1 mRNA remained elevated in tumors. These results: (1) help explain targeting of breast tissue by carcinogenic PAH, (2) imply that AhR and CYP1B1 hyper-expression represent molecular biomarkers for, at least, PAH-induced mammary cell transformation, and (3) suggest mechanisms through which the AhR may contribute to carcinogenesis well after exogenous AhR ligands have been eliminated.

The RelA NF-κB subunit and the aryl hydrocarbon receptor (AhR) cooperate to transactivate the c-myc promoter in mammary cells

“…Upon activation, the receptor translocates to the nucleus, binds speci®c response elements (XREs), and induces transcription of a number of genes, including those encoding the P450 enzymes CYP1A1, CYP1A2, and CYP1B1. As predicted from the working model, we have recently found high levels of constitutively active AhR in PAHinduced rat mammary tumors (Trombino et al, 2000), that coincide with constitutively active NF-kB (Sovak et al, 1997).…”

“…Recently, we demonstrated that rodent and human mammary tumors are typi®ed by aberrant activation of NF-kB/Rel (Sovak et al, 1997) and overexpression of AhR (Trombino et al, 2000). These tumors are often also characterized by overexpression of c-myc (Berns et al, 1992;Borg et al, 1992;Pavelic et al, 1991;DWK and GES, unpublished observations).…”

“…This hypothesis is strongly supported by constitutive nuclear AhR expression (Chang and Puga, 1998;Singh et al, 1996), and constitutive AhR-mediated transcriptional activity (Chang and Puga, 1998;Ma and Whitlock, 1996) in mouse hepatoma, monkey kidney, and human epithelial carcinoma cell lines. Furthermore, we have recently demonstrated constitutive nuclear AhR expression and high levels of an AhR-regulated gene, CYP1B1, in rat mammary tumors (Trombino et al, 2000). While these results support a role for constitutive AhR activation in tumorigenesis, organ defects observed in AhR 7/7 mice suggest a role for developmentally regulated AhR activation in organogenesis (Hushka et al, 1998;Fernandez-Salguero et al, 1995;Lahvis and Brad®eld, 1998;Abbott et al, 1999;Robles et al, 2000).…”

BCScreen: A gene panel to test for breast carcinogenesis in chemical safety screening

“…Such changes may serve as a link between cellular events relevant to breast cancer that are also responsive to environmental chemical exposures. For example, in vivo and in vitro studies have shown that environmental exposure to carcinogens and endocrine disruptors may exert influence via changes in gene expression [20,53,49,47]. These and other gene expression changes could be used to build a centralized list of environmentally susceptible genes that are also important in breast cancer.…”

Aryl hydrocarbon receptor (AHR)

“…Recently, a surprising discovery has been made by David H. Sherr and his colleagues at Boston University that the AhR is overexpressed in DMBA-induced tumors in vivo in rats [4] as well as in mice [5]. Furthermore, the same group discovered that several in vitro cultured mouse as well as human breast cancer cell lines show high levels of expression of AhR [6].…”