Novel Strategies in Cancer Prevention and Fertility Preservation with Tamoxifen

Abstract

Women at high risk for breast cancer are often also at high risk for ovarian cancer, reflecting similar risk factors and suggesting intertwined disease pathways and common prevention targets. A novel strategy to overcome obstacles in preventing ovarian neoplasia (low incidence, lack of specific disease markers, and difficulties in tissue sampling), the deadliest gynecologic cancer, may be to develop a prevention strategy that targets breast and ovarian cancer simultaneously. Tamoxifen, a selective estrogen receptor modulator, reduces hormone responsive breast cancer risk by 50% but its effects on risk of ovarian cancer, also hormonal responsive, are unclear. The goals of this work were to 1) develop and characterize a preclinical model of concurrent breast and ovarian cancer and 2) use this dual cancer model to examine the efficacy of tamoxifen to prevent both breast and ovarian cancer. Mammary carcinogens [7,12-dimethylbenz[α]anthracene (DMBA), N-methyl-N-nitrosourea and estradiol (Ey2)] were tested separately in combination with local ovarian DMBA administration to determine the best combined treatment to induce mammary and ovarian cancer concurrently and effectively in the rat. Results showed that systemic Ey2 and ovarian DMBA promoted the highest incidence of dysplasia in the mammary gland and ovary and elevated levels of mammary Ki-67 and cyclooxygenase 2 (COX-2) mimicking the human disease. Next, the ability of tamoxifen to prevent mammary and ovarian cancer simultaneously was evaluated. Tamoxifen which inhibited mammary carcinogenesis and normalized levels of Ki-67 and COX-2, had no effect on (neither accelerated nor inhibited) ovarian cancer progression. In addition, carcinogen treatment increased levels of stem cell markers, Oct-4 and aldehyde dehydrogenase-1, in the mammary gland; interestingly, this expansion was not reversed by tamoxifen. Intriguingly, while examining ovaries from this study, we serendipitously discovered an apparent protective effect of tamoxifen against DMBA-induced follicular destruction and this effect was further investigated. Chemotherapy and environmental toxicants (e.g. DMBA) deplete ovarian follicles and often lead to accelerated ovarian aging and premature ovarian failure; however, there is no established treatment that can protect the ovary from these toxic insults. In vivo, rats were treated with tamoxifen and DMBA or cyclophosphamide (the most ovotoxic chemotherapy) and total numbers of follicles in the ovary were determined. In vitro, ovarian organ culture and oocyte culture were carried out to examine local effects of tamoxifen on DMBA-induced follicle loss and doxorubicin-induced oocyte fragmentation, respectively. We demonstrated for the first time that tamoxifen protects ovarian follicles against not only DMBA- but also chemotherapy (cyclophosphamide and doxorubicin)-induced ovarian damage. Clinically, tamoxifen has already been tested for safe use as an adjuvant therapy for several cancers; therefore, if translated into clinical use, these results may have immediate impact on options for fertility preservation and quality of life in young female cancer patients undergoing chemotherapy. The long term goals of this work are to 1) use the dual cancer model to screen for promising agents that decrease risks for both breast and ovarian cancer and 2) examine the mechanism by which tamoxifen inhibits toxicant-induced ovarian follicle loss.