Abstract 199: Short-term treatment with pregnancy levels of estradiol prevents breast cancer by delaying promotion and progression.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC A full term pregnancy early in life reduces the risk of breast cancer in women. A similar phenomenon is also observed in rats and mice. Recent findings indicate that estradiol alone replacement therapy in postmenopausal women also reduces the risk of breast cancer. We have earlier demonstrated that short-term treatment with pregnancy levels of estradiol (STET) is very effective in preventing mammary carcinogenesis. In the current study our objective was to determine whether STET conferred protection against mammary carcinogenesis by blocking initiation or promotion. Rats were injected with N-methyl-N-nitrosourea at 7 weeks of age and divided into 2 groups. One group (n=30) did not receive any further treatment and served as controls and the second group (n=30) of rats was treated with 1 microgram of estradiol per day for 3 weeks using a subcutaneous implant of Alzet osmotic mini pump. A subset (n=5) were terminated 3 and 6 months post carcinogen treatment. The other animals were terminated 9 months post carcinogen treatment. Mammary gland wholemounts were prepared at each termination timepoint and examined for the presence of latent microscopic mammary cancers (LMMC). In another experiment we surgically isolated LMMC and transplanted them back into a host from the same group (control to control or STET to STET) or to a host from the other group (control to STET or STET to control). Short-term treatment with pregnancy levels of estradiol drastically reduced the incidence of overt mammary cancers. Although a significant reduction of overt cancers was observed in the pregnancy levels of estradiol treated groups, there was no difference in the incidence of LMMC between the estradiol treated and the controls. LMMC were isolated from the mammary glands. Breast cancer and estrogen receptor signaling specific microarrays were performed to find differences in gene expression in LMMC between the controls and STET groups. We were able to identify clear differences in gene expression in LMMC between the two groups. Genes involved in angiogenesis (Thbs1, Vegfa), apoptosis (Bad, Bcl2) and cell cycle (Ccnd1, Cdk2) were differentially expressed. Further several transcription factors (Esr1, Esr2, Gata3, Rb1) were also differentially expressed in LMMC between the controls and STET groups. The transplantation experiment indicated that the progression of LMMC was dependent on the systemic environment it is exposed to. The LMMC derived from control were not able to progress further to make overt cancers when transplanted STET hosts, while the LMMC derived from STET were able to progress to overt cancers when transplanted to control hosts. These findings demonstrate that STET confers protection against mammary cancer development by blocking promotion and progression of transformed cells. Understanding this phenomenon shall assist in the design of novel prevention and therapeutic strategies against breast cancer. Citation Format: Arunkumar Arumugam, Maricarmen Stout, Cathy Tsin, Amar Bhat, Taylor Yong, Sushmita Nandy, Christina Gutierrez, Rebecca Lopez, Thiyagarajan Boopalan, Jacqueline Parada, Rajkumar Lakshmanaswamy. Short-term treatment with pregnancy levels of estradiol prevents breast cancer by delaying promotion and progression. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 199. doi:10.1158/1538-7445.AM2013-199