Estrogen regulates Ah responsiveness in MCF-7 breast cancer cells

Abstract

Cytochrome P450 (CYP)1A1 and CYP1B1, which are under the regulatory control of the aryl hydrocarbon (Ah) receptor (AhR), catalyze the metabolic activation of numerous procarcinogens and the hydroxylation of 17beta-estradiol (E2) at the C-2 and C-4 positions, respectively. There is evidence of cross-talk between estrogen receptor alpha (ERalpha)- and AhR-mediated signaling in breast and endometrial cells. To further examine these interactions, we investigated the short- and long-term effects of E2 exposure on Ah responsiveness in MCF-7 human breast cancer cells. Short-term exposure to 1 nM E2 elevated the ratio of the 4- to 2-hydroxylation pathways of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced E2 metabolism and the ratio of the induced CYP1B1 to CYP1A1 mRNA levels, as determined by real-time PCR. Cells maintained long-term (9-12 months) in low-E2 medium progressively lost Ah responsiveness, as indicated by diminished rates of TCDD-induced E2 metabolism and ethoxyresorufin O-deethylase activity, and the reduced expression of the CYP1A1 and CYP1B1 mRNAs and proteins levels. These E2-deprived cells showed elevated levels of ERalpha mRNA, depressed levels of AhR mRNA, and unchanged levels of the AhR nuclear translocator mRNA. Transient transfection studies using a CYP1B1-promoter-luciferase reporter construct showed that reduced CYP1B1 promoter activity in E2-deprived cells could be restored by co-transfection with an AhR expression construct, indicating that AhR expression was limiting in these cells. The reduced Ah responsiveness of E2-deprived cells was reversed by culture for four passages in medium supplemented with 1 nM E2; ERalpha and AhR mRNAs returned to near-normal levels and the inducibility of the CYP1A1 and CYP1B1 mRNAs, proteins, and E2 metabolic activities by TCDD was restored. These studies indicate that the continued presence of estrogen is required to maintain high levels of AhR expression and inducibility of the procarcinogen-bioactivating enzymes, CYP1A1 and CYP1B1, in MCF-7 cells.

Aryl Hydrocarbon Receptor-Mediated Transcription: Ligand-Dependent Recruitment of Estrogen Receptor α to 2,3,7,8-Tetrachlorodibenzo- p-Dioxin-Responsive Promoters

“…Similar results were seen in MCF-7 cells (data not shown). These data suggest and support a previously proposed role for ER␣ in AhR signaling (43,46). New mechanism of cross talk between ER␣ and AhR.…”

“…The impact of ER␣ on AhR-mediated transcription is less clear and has been shown to exhibit cell-, species-, sex-, and age-related differences (39). However, the TCDD-dependent activation of AhR target genes in breast cancer cell lines and AhR expression and inducibility have been shown to correlate with ER␣ expression levels (43,46). These data provide evidence of a role for ER␣ in AhR activity.…”

“…The introduction of exogenous ER␣ into an ER-negative breast cancer cell line restores TCDD-dependent gene expression (46). The continued presence of E2 is required to maintain high levels of AhR expression and inducibility (43). The incidence of liver hyperplasia is significantly higher in female rats exposed to TCDD (22), and cotreatment of ovariectomized rats with E2 and TCDD resulted in a potentiation of CYP1A1 expression compared to rats given TCDD alone (40).…”

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

“…This raises the possibility that the AhR may contribute directly to promoting a more invasive phenotype via regulation of genes such as Slug. Furthermore, AhR directly regulates cytochrome P450 enzymes (e.g., CYP1B1) that metabolize PAH into mutagenic intermediates in mammary epithelial cells (39,40). Thus, the increased expression of AhR in tumors may also lead to heightened susceptibility to environmental PAH during the later stages of tumor progression.…”

Human CYP1B1 Is Regulated by Estradiol via Estrogen Receptor

“…Many polycyclic aromatic hydrocarbons are known to induce CYP1B1 and their own metabolism through binding to and activation of the AhR. A recent report (23) demonstrated that AhR might be responsible for E2-induced CYP1B1 expression in ER-positive cells. In that study, AhR expression was significantly increased in response to E2 stimulation.…”