INDOLO[3,2-B]CARBAZOLE - A DIETARY-DERIVED FACTOR THAT EXHIBITS BOTH ANTIESTROGENIC AND ESTROGENIC ACTIVITY

Background: Indole-3-carbinol (I3C) and related compounds have been id entified in vegetables of the Brassica genus. I3C and its acid-derived condensation product, indolo[3,2-b]carbazole (ICZ), bind to the aryl hydrocarbon (Ah) receptor and induce CYP1A1/1A2 gene expression in bot h in vivo and in vitro models. I3C also inhibits mammary tumor develop ment in rodent models. Purpose: The major focus of this study was to i nvestigate the induction of CYP1A1-dependent activity and antiestrogen ic effects of ICZ in the MCF-7 human breast cancer cell line and deter mine if induction of CYP1A1 is required for observed antiestrogenic re sponses. Methods: The induction of CYP1A1 in MCF-7 cells was determine d by measuring time- and concentration-dependent changes in ethoxyreso rufin O-deethylase (EROD) activity in response to ICZ treatment. The e ffects of ICZ on occupied nuclear estrogen receptor (ER) levels and in hibition of estrogen (17 beta-estradiol [E(2)])-induced cell prolifera tion, [H-3]thymidine uptake, secretion of the 52-kd protein, and nucle ar progesterone receptor (PR) levels were also measured. Chloramphenic ol acetyl transferase (CAT) activity was assayed in MCF-7 cells transi ently transfected with an estrogen-responsive vit-CAT plasmid. Competi tive binding to rat cytosolic ER was also examined. Results: ICZ (grea ter than or equal to 10 nM) induced CYP1A1 in MCF-7 human breast cance r cells. This compound also elicited a diverse spectrum of antiestroge nic responses, including inhibition of E(2)-induced cell proliferation , [H-3]thymidine uptake, occupied nuclear PR binding, and CAT activity in cells transfected with the estrogen-responsive vit-CAT plasmid. In nuclear extracts from ICZ-treated cells, there was a decrease in ER l evels and binding to an estrogen-responsive element in a gel shift ass ay. I3C also decreased nuclear ER binding in MCF-7 cells. ICZ bound wi th low affinity to the ER and exhibited weak estrogen-like activity. C onclusions: Like other Ah receptor agonists, ICZ is antiestrogenic in human breast cancer cells, and this activity is consistent with the in hibitory activity of I3C on mammary tumor formation in rodents. ICZ-in duced antiestrogenic responses can be observed at times or concentrati ons in which EROD activity is unchanged, indicating an interaction bet ween the Ah receptor and ER-mediated endocrine pathways that is indepe ndent of P450-induced hormone metabolism. ICZ also is a weak estrogen in MCF-7 cells and binds to the ER. Implications: The current focus on the role of dietary and environmental estrogens in human disease shou ld take into account the possible contra-active effects of Ah receptor agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), ICZ, I3C , and related compounds that exhibit antiestrogenic activity.