Abstract P4-04-15: Hormonal prevention of HER2 positive mammary cancers

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Abstract Introduction: Approximately 25-30% of breast cancers overexpress human epidermal growth factor receptor 2 (HER2). HER2-positive breast cancers are more likely to be diagnosed in younger women and are highly aggressive. Currently, there are few treatments options available specifically for HER2-positive breast cancer which includes Herceptin, Kadcyla, Perjeta and Tykerb. There is a higher risk of recurrence of HER2-positive breast cancers compared to HER2-negative breast cancers. Hence it is important to devise effective strategies to prevent/treat HER2-positive breast cancer. We had earlier shown that short-term treatment with pregnancy levels of estrogen plus or minus progesterone was highly effective in inhibiting mammary tumor growth in MMTV. Here we attempt to understand the mechanism behind this protection. Materials and Methods: MMTV-neu transgenic mice were treated for 3 weeks, starting at 7 weeks of age, with 100 μg of estradiol in Silastic capsules. This dose of estradiol used in the Silastic capsules results in pregnancy levels of estradiol in the circulation. The control animals received empty Silastic capsules for the same duration. Mice were palpated weekly for 9 months to monitor for mammary cancer development. A set of mice were sacrificed at 7, 10, 16, 22, 28 and 36 weeks of age. Mammary gland was surgically isolated. Inguinal mammary glands were prepared as wholemounts. A piece of the mammary gland was fixed in formalin for histopathology and the rest was snap frozen in liquid nitrogen and stored at -80oC for further molecular analysis. Pathway focused microarrays (Cancer Pathway Finder, and PI3K-AKT Signaling) were performed and the differential regulated genes were validated using real-time RTPCR and immunoblotting. All data obtained were statistically analyzed using Graphpad Prism version 5.03. The repeated measures analysis of variance was used to evaluate the dose and time response to hormone treatment. Multiple comparisons between groups with significant differences were analyzed using Dunnett post–hoc test. Paired t–test was done to analyze intergroup differences. The values of p <0.05 were considered as statistically significant. Results: Short-term treatment with pregnancy levels of estradiol significantly reduced the mammary cancer incidence and multiplicity, while it significantly increased mammary cancer latency. Pathway focused microarray analysis of mammary glands at different time points demonstrated that mice treated with short-term estradiol has persistent changes in gene expression involved in key signaling pathways involved in cancer. Pro-angiogenic genes (Angpt1, Fgf2, Vegfc), anti-apoptotic genes (Bcl2, Birc3), pro-proliferative/survival genes (Akt, Ccnd1, Igf1r, Mtor, Pi3k) were significantly down-regulated in the mammary gland of short-term estradiol treated mice. On contrary, pro-apoptotic genes (Bad,Casp, Cflar) and anti-proliferative/survival genes (Pten, p53) were up-regulated. Conclusion: Our data demonstrates that short-term treatment with pregnancy levels of estradiol is highly effective in inhibiting mammary carcinogenesis in MMTV-neu mice. This protection against HER2 positive mammary cancers is achieved by altering key signaling pathways involved in cancer growth and progression. Citation Format: Subramani R, Lakshmanaswamy R. Hormonal prevention of HER2 positive mammary cancers [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P4-04-15.