The actions of estrogen metabolites and phytoestrogens in breast carcinogenesis—Mechanistic insights from in vitro and in vivo studies

Abstract

Steroidal estrogens are suggested to induce breast cancer by way of a two-tiered mechanism, in which estrogens stimulate cell growth and proliferation and generate oxidative stress and DNA damage. 17β-estradiol (E2) is metabolized by the cytochrome P450 enzymes 1A1 (CYP1A1) and 1B1 (CYP1B1) to generate the catechol estrogens 2-hydroxyestradiol (2-OHE2) and 4-hydroxyestradiol (4-OHE2), respectively. Catechol estrogens are further metabolized by catechol-O-methyltransferase (COMT) to form 2- and 4-methoxyestradiol (2-MeOE2 and 4-McOE2). Formation of 2-OHE2 and 2-MeOE2 is considered a relatively benign pathway of estrogen metabolism, but 4-OHE2 is highly genotoxic and proposed to be a proximate cause of E2-induced carcinogenesis. The roles of estrogen metabolites and oxidative stress in breast cancer development remain unclear, and a greater understanding of the biologic actions of these genotoxic molecules would likely provide insight into cancer prevention strategies. Epidemiologic and experimental evidence indicate that phytoestrogens may alter estrogen metabolism and inhibit carcinogenesis. The phytoestrogens are a class of plant-derived compounds that are structurally similar to mammalian estrogens. The mechanisms of phytoestrogen action in the breast have yet to be elucidated and it is unknown whether these compounds exacerbate or prevent breast cancer. The goal of our studies was to investigate the potential mechanism(s) underlying phytoestrogen action in the mammary gland. In vitro and in vivo approaches were used to test the hypotheses that phytoestrogens exert chemoprotective effects against breast cancer by shifting estrogen metabolic pathways, reducing oxidative stress and modifying estrogen-mediated signaling pathways. The MCF10F human breast epithelial cell line and various breast cancer cell lines were used for in vitro experiments. In vivo experiments were carried out by using the ACI rat model of estrogen-induced breast cancer. The results of the experiments included in this thesis indicate that (a) estrogen metabolites and estrogen-associated oxidative stress play critical roles in breast carcinogenesis, (b) phytoestrogens alter mRNA and protein expression of estrogen-metabolizing enzymes CYP1A1, CYP1B1 and COMT, (c) quercetin significantly increases the severity of estrogen-induced breast tumors in ACI rats, and (d) genistein and quercetin induce expression of estrogen-sensitive genes and trigger activation of AKT and p44/42 MAPK in breast cancer cells.