Although a large proportion of primary human breast cancers express the androgen receptor, and treatment with androgens exerts beneficial effects in women with breast cancer, the role and especially the mechanism of action of androgens in breast cancer development and growth are not well understood. The potential effect of androgens on bcl-2 protooncogene expression was investigated in a human breast cancer cell line whose proliferation is known to be inhibited by androgens. The estrogen-responsive ZR-75-1 cells were grown in the presence or absence of 5alpha-dihydrotestosterone (DHT), alone or in combination with 17beta-estradiol. DHT caused a marked down-regulation of Bcl-2 protein and messenger RNA levels in both the presence and absence of 17beta-estradiol. The inhibitory effect of DHT was completely prevented by coincubation with the pure antiandrogen hydroxyflutamide. The present data indicate that androgens can down-regulate bcl-2 protooncogene levels via an androgen receptor-mediated mechanism, thus providing a novel mechanism for their known inhibitory effect on breast cancer cell growth.

Although a large proportion of primary human breast cancers express the androgen receptor, and treatment with androgens exerts beneficial effects in women with breast cancer, the role and especially the mechanism of action of androgens in breast cancer development and growth are not well understood. The potential effect of androgens on bcl-2 protooncogene expression was investigated in a human breast cancer cell line whose proliferation is known to be inhibited by androgens. The estrogen-responsive ZR-75-1 cells were grown in the presence or absence of 5a-dihydrotestosterone (DHT), alone or in combination with 17b-estradiol. DHT caused a marked down-regulation of Bcl-2 protein and messenger RNA levels in both the presence and absence of 17b-estradiol. The inhibitory effect of DHT was completely prevented by coincubation with the pure antiandrogen hydroxyflutamide. The present data indicate that androgens can downregulate bcl-2 protooncogene levels via an androgen receptormediated mechanism, thus providing a novel mechanism for their known inhibitory effect on breast cancer cell growth. (Endocrinology 140: 416–421, 1999) T ANDROGEN receptor is expressed in 50–90% of breast tumors (1–3). Androgens or androgenic compounds, such as testosterone propionate (4–6), fluoxymesterone (7–9), and calusterone (10), have been shown to inhibit breast cancer growth in a proportion of women comparable to that achieved with other endocrine therapies. In fact, a direct growth inhibitory effect of androgens has been demonstrated in estrogen-responsive ZR-75-1 human breast cancer cells under both basal and estrogen-induced incubation conditions in vitro (11–19) as well as in vivo in nude mice (20). Other in vitro studies have shown that androgens can regulate the secretion of glycoproteins such as GCDFP-15 (12) and apolipoprotein D (18) as well as the expression of the estrogen receptor (13), but little is known regarding the mechanism of action of androgens on breast cancer cell growth. The control of cell number is determined by a balance between cell proliferation and cell death. In multicellular organisms, apoptosis or programmed cell death represents a mechanism for the removal of unnecessary, aged, or damaged cells (for review, see Ref. 21). Cells from a variety of human cancers have a decreased ability to undergo apoptosis. Bcl-2 is an oncoprotein that acts by inhibiting programmed cell death and binds to several proteins that can participate in cell death regulation (for review, see Refs. 22 and 23). To better understand the mechanism of androgen-induced growth inhibition in breast cancer, we examined the effect of 5a-dihydrotestosterone (DHT) on Bcl-2 protein levels in ZR75-1 human breast cancer cells. ZR-75-1 cells are growth inhibited by DHT, and they are also sensitive to the mitogenic effects of 17b-estradiol (E2), making them a unique model in which to study the antagonistic effects of androgens and estrogens in breast cancer (11, 12). The present data show that androgens down-regulate Bcl-2 expression in ZR-75-1 human breast cancer cells, thus offering a potential explanation for their inhibitory effect on cancer cell growth. Materials and Methods