Effects of several dioxin-like compounds on estrogen metabolism in the malignant MCF-7 and nontumorigenic MCF-10A human mammary epithelial cell lines

Abstract

In human breast tissue, estrone (E 1) and estradiol (E 2) are mainly hydroxylated by cytochrome P450 1A1 (CYP1A1) and 1B1 (CYP1B1) to 2-hydroxyestrogens (2-OHE 1/2) and 4-hydroxyestrogens (4-OHE 1/2), respectively. Several studies show that 4-OHE 1/2, but not 2-OHE 1/2, may act as a carcinogen and a high estrogen 4-/2-hydroxylation ratio appears to be a marker for the presence of neoplasms. In this study, we investigated the effects of several dioxin-like compounds on estrogen 2- and 4-hydroxylation in a malignant (MCF-7) and a nontumorigenic (MCF-10A) human mammary epithelial cell line. 2- and 4-methoxyestrogen (MeOE 1/2) formations were used as measures of the 2- and 4-hydroxylation pathways, respectively. 2,3,7,8-Tetrachlorodibenzo- p-dioxin (TCDD), 2,3,4,7,8-pentachlorodibenzofuran (PCDF), 3,3′,4,4′,5-pentachlorobiphenyl (PCB 126), and 3,3′4,4′,5,5′-hexachlorobiphenyl (PCB 169) concentration dependently induced 2-MeOE 1/2 formation and ethoxyresorufin- O-deethylation (EROD) activity through induced CYP1A1 expression in MCF-7 and MCF-10A cells. 2,3′,4,4′,5-pentachlorobiphenyl (PCB 118) had no such effect. Effects on CYP1B1 expression and 4-MeOE 1/2 formation were less pronounced; only TCDD caused an induction, whereas PCB 169 was a potent and selective inhibitor of 4-MeOE 1/2 formation (IC 50 0.7 and 2.2 nM PCB 169 in MCF-7 and MCF-10A cells, respectively). MCF-10A cells were less responsive toward dioxin-like compounds and the apparent EC 50 values for CYP1A1 and CYP1B1 induction in this study were 10–100 fold higher than in MCF-7 cells. The constitutive 4-/2-MeOE 1/2 ratios were 2.99 ± 0.78 and 0.93 ± 0.40 in MCF-7 and MCF-10A, respectively. Incubation with dioxin-like compounds resulted in a concentration-dependent decrease in the 4-/2-MeOE 1/2 ratio, but an increase in potentially carcinogenic estrogen metabolites in both MCF-7 and MCF-10A cells. This indicates that even though the 4-/2-OHE 1/2 ratio may be used as indicator for the presence of neoplasms, it is readily lowered by dioxin-like compounds and its value as a prognostic parameter for cancer risk should be further examined.