Estrogen regulates Ah responsiveness in MCF-7 breast cancer cells

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 (E-2) 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 E-2 exposure on Ah responsiveness in MCF-7 human breast cancer cells. Short-term exposure to 1 nM E-2 elevated the ratio of the 4- to 2-hydroxylation pathways of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced E-2 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-E-2 medium progressively lost Ah responsiveness, as indicated by diminished rates of TCDD-induced E-2 metabolism and ethoxyresorufin O-deethylase activity, and the reduced expression of the CYP1A1 and CYP1B1 mRNAs and proteins levels. These E-2-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 E-2-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 E-2-deprived cells was reversed by culture for four passages in medium supplemented with 1 nM E-2; ERalpha and AhR mRNAs returned to near-normal levels and the inducibility of the CYP1A1 and CYP1B1 mRNAs, proteins, and E-2 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.