12-O-TETRADECANOYLPHORBOL-13-ACETATE UP-REGULATES THE AH RECEPTOR ANDDIFFERENTIALLY ALTERS CYP1B1 AND CYP1A1 EXPRESSION IN MCF-7 BREAST-CANCER CELLS

Elevated expression of cytochrome P450 1B1 (CYP1B1) and estradiol 4-hy droxylation have been reported to be biomarkers of tumorigenesis in hu mans. The aromatic hydrocarbon receptor (AhR) regulates expression of human cytochrome P450 1A1 (CYP1A1) and CYP1B1, 17 beta-estradiool (E-2 ) 2- and 4-hydroxylases, respectively. There is also evidence that exp ression of estrogen receptor alpha (ER alpha) potentiates CYP1A1 induc ibility in breast cancer cells. To characterize these relationships fu rther, we examined the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA), which downregulates ERa, and the high-affinity AhR ligand, 2,3 ,7,8-tetrachlorodibenzo-p-dioxin (TCDD), on the expression of AhR, ER alpha, CYP1A1, and CYP1B1 in MCF-7 human breast cancer cells. Treatmen t with TPA,which suppressed ERa mRNA levels, caused a greater than fou rfold elevation of AhR mRNA and protein levels, whereas treatment with TCDD caused a decrease in AhR protein but no change in ER alpha or Ah R mRNA levels. In MCF-7 cells treated with TPA prior to treatment with TCDD, the AhR mRNA level was elevated, the ER alpha mRNA level remain ed suppressed, and the ratio of CYP1B1 to CYP1A1 mRNA was increased co mpared with treatment with TCDD alone. A corresponding increase in the ratio of the rates of 4- to 2-hydroxylation pathways of E-2 metabolis m was also observed in response to pretreatment with TPA prior to the addition of TCDD. These results demonstrate differential regulation of the human CYP1A1 and CYP1B1 genes and provide a cellular model to inv estigate further the mechanisms that may be involved in the elevated e xpression of CYP1B1 in tumorigenesis. J. Cell. Biochem. 70.289-296, 19 98. (C) 1998 Wiley-Liss, Inc.