INTRODUCTION: Breast cancer is the most common carcinoma diagnosed in women today. It has been well documented that estrogen plays a critical role in breast cancer development and is a major target for treatment. However, anti-estrogen therapy for breast cancer produce side effects associated with deficiency of estrogen action to other estrogen-supportive tissues such as bone. On the other hand, hormone replacement therapy for post-menopause women may increase the risk of having breast cancer. It was reported that 4-estren-3alpha,17beta-diol (estren), a synthetic ligand of the estrogen (ER) or androgen (AR) receptor, is as potent as estradiol or 5α-dihydrotestosterone in protecting bone from the effects of sex steroid deficiency but does not increase the weight of reproductive tissues. Estren is also ineffective in promoting MCF-7 breast cancer cell growth in vitro. PURPOSE: To investigate the potency and mechanisms of estren in controlling breast cancer progression in vitro and its potential anti-cancer effect in vivo. METHODS: MCF-7 and T-47-D cell lines were pre-treated with either vehicle, ICI 182,780, an ER- specific inhibitor, or flutamide, an AR- specific inhibitor, for 1 hr, followed by the addition of estren, E2 or both. Cell growth was measured by 3H-Thymidine uptake. Apoptosis was measured by cell death ELISA and FACS with FITC annexin V. MCF-7 cells were injected in female severe combined immune deficient (SCID) mice and the tumor bearing mice were treated with placebo, estren or E2. RESULTS: Estren inhibited estrogen (E2)-enhanced MCF-7 cell proliferation. PD98059, the specific inhibitor of ERK phosphorylation, blocked E2 induced ERK phosphorylation, but had no effect on E2-enhanced cancer cell proliferation. Moreover, estren induced apoptosis in ER-positive MCF-7 and T-47-D but not the ER-negative MDA-MB-231 cells. ICI 182,780 completely abolished the pro-apoptotic effect of estren in both MCF-7 and T-47-D cells, suggesting that estren-mediated cancer cell apoptosis may be ER-dependent. In contrast, flutamide had no effect on estren-mediated cancer cell apoptosis. Further, estren rapidly induced ERK activation. Pre-treatment of MCF-7 cells with 20 μM of PD98059 blocked estren induced ERK phosphorylation and abrogated its pro-apoptotic effect as well. Consistent with the in vitro observations, preliminary results from an in vivo experiment performed in an established MCF-7 tumor model in female SCID mice indicate that estren does not stimulate MCF-7 tumor growth or uterine growth in vivo. CONCLUSION: Estren inhibits the proliferative effects of E2 and causes apoptosis of breast cancer cells. The pro-apoptotic actions of estren are ER-dependent. ERK phosphorylation may involve in the cancer-inhibition effect of estren. Our results provide evidence for estren to function as an antagonist of estrogen in controlling of human breast cancer progression. Acknowledgment: The U.S. Army DOD-BCRP supported this work.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]