Nonsteroidal Estrogens and Estrogen Antagonists: Mechanism of Action and Health Implications

Abstract

Endogenous estrogens comprise a family of compounds that have a polycyclic structure and contain a steroid nucleus. These compounds exhibit a wide diversity of biochemical and biological activities. Estrogens both induce and stimulate the cellular proliferation and differentiation of specific cell types in target tissues, a prime example being the critical role of these hormones in sexual differentiation and reproduction. Estrogens also exert a critical role in body fat deposition and in preventing bone resorption in the maintenance of bone structure. Given that these compounds exert such a wide range of activities without requiring absolute structural specificity and that there are in existence a tremendous number of related naturally occurring and synthetic polycyclic compounds, it is highly likely that compounds other than endogenous steroidal estrogens will possess at least some estrogenic properties. This, in fact, is the case. Many of these compounds have been found to inhibit the action of endogenous mammalian steroidal estrogens. Compounds exhibiting this inhibitory activity have been called antiestrogens. Although the terms antiestrogen and antiestrogenic are now widely used in the literature, they do not accurately reflect or describe the properties of these compounds. Antiestrogens, in the strict sense of the word, should be substances that in and of themselves exhibit biological properties opposite to those of estrogens. Typically, what is actually measured is the ability of these substances to inhibit estrogen activity in the presence of estrogens. Therefore the terms estrogen antagonist or estrogen inhibitor would more correctly describe this biological activity. The inhibitory effect is analogous to that which is observed for enzyme inhibitors that, as a result of some structural similarity to natural substrates, are bound and acted upon by the relevant enzymes, and even if the degree of catalysis is low, the net result is that the activity of the enzyme on the natural substrate(s) is inhibited. In this issue of the Journal, Liu et al. (7) report on the estrogenic and estrogen inhibitory properties of indolo[3,2b]carbazole (ICZ), an acid-derived condensation product of indole-3-carbinol (I3C). I3C is a naturally occurring compound found in vegetables such as cabbage, broccoli, Brussels sprouts, and cauliflower. ICZ was found to induce CYP1A1, a member of the cytochrome P450 family, in the MCF-7 mammary tumor cell line. These findings are similar to those reported from the same laboratory for 3-methylcholanthrene (MC) and 2,3,7,8-tetrachlorodibenzo-/?-dioxin (TCDD) (2,3). In separate assays, ICZ was also found to stimulate the proliferation of MCF-7 cells and the incorporation of [ 3H]thymidine into DNA, similar to that observed for estradiol (E2) but to lesser extents and with higher molar concentrations required to mediate the effects. Similar findings were reported for the induction of procathepsin D and for chloramphenicol acetyl transferase (CAT) activity. Both E2 and ICZ separately increased the secretion of procathepsin D in MCF-7 cells and the CAT reporter gene activity in MCF-7 cells transiently transfected with an estrogen-resp onsive vit-CAT plasmid. When cells were exposed to both compounds at the same time, the levels of activity measured in both of the above assays were reduced as compared with those obtained in parallel from cells treated with E2 alone, thus demonstrating by independent assays the estrogen inhibitory effects of ICZ. ICZ was also shown to decrease the number of occupied estrogen receptors (ERs) and to compete with E2 for binding to ER, although this binding was very weak; the IC50 (concentration that causes 50% inhibition of growth) was 23 \iM, compared with 0.063 [iM for tamoxifen. It has been previously shown that I3C, ICZ, MC, and TCDD bind to the aryl hydrocarbon (Ah) receptor and are considered Ah receptor agonists (4-6). Liu et al. conclude that the induction of CYP1A1 gene expression is not the primary factor in the inhibitory estrogenic properties of ICZ, since the ICZ inhibition is seen at lower concentrations and after a shorter amount of time than is required for CYP1A1 induction. The suggestion that ICZ can inhibit estrogen activity via induction of the cytochrome P450 monoxygenase system comes from studies showing that TCDD and I3C increase CYPlA2-catalyzed 2hydroxylation of estradiol and, as a consequence, decrease the estrogen response (7). Liu et al. (7) propose that Ah receptor agonists act antagonistically toward estrogens by an as-yet-unknown cellular signal transduction interaction between the Ah