ANTIESTROGENIC EFFECT OF 2,3,7,8-TETRACHLORODIBENZO-P-DIOXIN ON 17-BETA-ESTRADIOL-INDUCED PS2 EXPRESSION

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) exhibits a broad spectrum o f antiestrogenic activities in rodents and mammalian cells in culture. The effects of TCDD on 17 beta-estradiol (E2)-induction of pS2, a pro gnostic marker for breast cancer, were investigated in MCF-7, ZR-75, H eLa, and Hepa 1c1c7 wild-type and mutant cells. These effects were com pared to the suppressive activities of the congener, 2,8-dichlorodiben zo-p-dioxin, and the established antiestrogens, ICI 164,384 and tamoxi fen, in order to determine the relative potency of TCDD and to disting uish the mechanism of action of Ah receptor-mediated antiestrogens. Tr eatment of MCF-7 cells with 10 nM TCDD decreased E2-induced secreted p S2 protein levels by 50% and the induction of the transiently transfec ted -1100 to -86 pS2 promoter-regulated reporter gene (pS2-LUC) by 57% . Comparable effects on PS2-LUC activity were observed in HeLa and ZR- 75 cells. In contrast, TCDD had minimal effects on pS2ERE(-405 to -393 )-LUC induction, whereas treatment with 10 nM ICI 164,384 caused a 60% decrease in luciferase activity. In Hepa 1c1c7 wild-type and clone 1 (C1) mutant cells, TCDD also reduced E2 induction of pS2-LUC activity but had little effect in clone 4 (C4) or clone 12 (C12) mutant cells. However, suppression was reestablished following transfection of the h uman Ah receptor nuclear translocator (ARNT) complementary DNA express ion vector into C4 cells and the mouse Ah receptor (AhR) complementary DNA expression vector into C12 cells. Induction of pS2-LUC activity b y the ligand-dependent and -independent chimeric estrogen receptors (H E15, HE19, ERcVP16, and ERGR) were also used to examine the role of E2 metabolism and the mechanism of TCDD-mediated antiestrogenic activity . Induction by HE15 and ERcVP16 was suppressed by 57 and 74%, respecti vely, following treatment with TCDD, whereas ICI 164,384 was significa ntly less effective (38 and 20%, respectively). These results demonstr ate a role for the Ah receptor in TCDD-mediated suppression of E2-indu ced pS2 expression. Data is presented demonstrating that the effect re quires sequences within the pS2 promoter other than the estrogen respo nse element and is independent of E2 oxidative metabolism.