Journal Article

Estrogen regulates Ah responsiveness in MCF-7 breast cancer cells

David C. Spink, Barbara H. Katz, Mirza M. Hussain, Brian T. Pentecost, Zhimin Cao and Barbara C. Spink

in Carcinogenesis

Volume 24, issue 12, pages 1941-1950
Published in print December 2003 | ISSN: 0143-3334
Published online December 2003 | e-ISSN: 1460-2180 | DOI:
Estrogen regulates Ah responsiveness in MCF-7 breast cancer cells

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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 17β-estradiol (E2) at the C-2 and C-4 positions, respectively. There is evidence of cross-talk between estrogen receptor α (ERα)- 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 ERα 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; ERα 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.

Keywords: Ah, aryl hydrocarbon; AhR, aryl hydrocarbon receptor; ARNT, aryl hydrocarbon receptor nuclear translocator; CYP, cytochrome P450; DMEM, Dulbecco's modified Eagle's medium; E2, 17β-estradiol; ERα, estrogen receptor α; EROD, ethoxyresorufin O-deethylase; MeOE2, methoxyestradiol; PBS, phosphate-buffered saline; PAH, polycyclic aromatic hydrocarbon; TCDD, 2,3,7,8-tetrachlorodibenzo-p-dioxin

Journal Article.  7989 words.  Illustrated.

Subjects: Clinical Cytogenetics and Molecular Genetics

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