EFFECTS OF 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN, 12-O-TETRADECANOYLPHORBOL-13-ACETATE AND 17-BETA-ESTRADIOL ON ESTROGEN-RECEPTOR REGULATION IN MCF-7 HUMAN BREAST-CANCER CELLS

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exhibits remarkably potent antiestrogenic activity. To further elucidate the role of estrogen rec eptor (ER) regulation in this response, we examined the effects of exp osure to TCDD in MCF-7 human breast cancer cells on ER mRNA levels by using an RNase protection assay, on ER accumulation by using an ER imm unocytochemical essay (ER-ICA), and on ER function by competitive bind ing assays under conditions of saturating 17 beta-estradiol (E(2)). Co mparative studies were conducted with E(2) and 12-O-tetradecanoylphorb ol-13-acetate (TPA), as both compounds are known to suppress ER expres sion. Our results indicate that 1 nM E(2) and 100 nM TPA both suppress ER mRNA levels as early as 4 h after exposure and to 33.6% and 16.5% of control levels, respectively, after 72 h. In contrast, no significa nt effect on ER mRNA levels was attributed to exposure to 10 nM TCDD. A greater than 50% reduction in positive staining was observed by ER-I CA after 72 h exposure to 1 nM E(2) and to 100 nM TPA, while only an 1 1% reduction in positive staining was observed with 10 nM TCDD. Specif ic binding of [H-3]E(2) under saturating conditions (10 nM E(2)) in wh ole cells was reduced by 50% in cultures exposed to 100 nM TPA, althou gh no effect on binding was observed with exposure to 10 nM TCDD. In c ontrast, specific binding using subsaturating 1 nM [H-3]E(2) was depre ssed by 49% in MCF-7 cells exposed to 10 nM TCDD for 72 h. This depres sion was inhibited by a 1-h treatment with 5 mu M alpha-naphthoflavone , which inhibits TCDD-induced, P450-mediated, E(2) metabolism, and sub sequent E(2) depletion. In conclusion, while TPA and E(2) effectively down-regulate ER expression, TCDD, under antiestrogenic conditions, ha s little if any effect on total ER levels in MCF-7 cells, and thus ER modulation is probably not necessary for the suppression of estrogenic activity in MCF-7 cells by TCDD. (C) 1996 Wiley-Liss, Inc.