Abstract 11: Prophylactic use of tamoxifen could reduce the risk of breast cancer in women who do not breast feed postpartum

Abstract Background: Triple negative breast cancer (TNBC) is associated with poor survival, particularly affecting African American women (AAW). Epidemiological studies indicate prolonged breast feeding reduces breast cancer (BC) risk, including TNBC. AAW have significantly lower rates of breast feeding compared to Caucasian women. To understand this link we developed a mouse model mimicking abrupt (AI) and gradual involution (GI). AI led to increased estrogen signaling, cell proliferation and chronic inflammation, which was followed by hyperplasia and squamous metaplasia in mammary glands1. There was an increase in the luminal progenitor (LP) cell population, the cells of origin of TNBC, and a decrease in mature luminal (ML) cells in AI glands. In this study, we sought to determine if blocking estrogen signaling with tamoxifen (TAM) could revert the negative effects of AI, and if so, could be a prophylactic option to reduce BC risk in women who do not breast feed. Methods: Uniparous FVB/N mice (~8 weeks) were allowed to nurse six pups per dam at partum. To induce AI, all pups were removed on postpartum (PP) day 7 (d7). For TAM treatment, 5mg sustained release TAM citrate pellet or placebo was implanted in the subscapular region on PP d8. Mammary glands were harvested on PP d28 and d120. FFPE sections were used for histology and immunohistochemistry. Single cell suspensions were analyzed for mammary epithelial subpopulations using Fluorescence Activated Cell Sorting. Affymetrix and qPCR were used for gene expression analysis. Mass cytometry was performed on mammary glands harvested at PP d120. Results: TAM treatment for 21 days completely abrogated hyperplastic and metaplastic changes in AI glands harvested on d120. Treatment initiation on PP d8, d15 and d35 had the same effect. TAM treatment reduced the cell proliferation and collagen deposition in AI glands. De-enrichment of estrogen signaling pathways and decrease in Elf5 expression, a luminal progenitor marker, were observed upon TAM treatment in d28 glands. Mass cytometry revealed a marked reduction in LP population and a significant increase in ML population in TAM treated AI glands on d120, restoring to the levels in age matched virgin mice. Significant increases in progenitor-like markers TSPAN8, Ly6D, CD200 and decreases in CD49f and CD47 expression in LP cells were observed, indicating return to a normal uniparous LP state. Expression of Ly-6D in ML cells, a ML cell marker, was also rescued upon TAM treatment. Conclusion: Using our mouse model of AI and GI, we show that suppression of estrogen signaling after initiation of AI offered marked protection against precancerous changes. TAM restored the balance of epithelial lineages and normalized the LP and basal cells in AI glands to the post-involution phenotype. Our data provide a rationale for considering short-term TAM treatment for women who do not breastfeed to reduce risk of BC. 1. Basree et.al. PMID 31315645 Citation Format: Bhuvaneswari Ramaswamy, Neelam Shinde, Gary K. Gray, Resham Mawalkar, Allen Zhang, Mustafa Basree, Xiaoli Zhang, Ramesh Ganju, Gina M. Sizemore, Joan S. Brugge, Sarmila Majumder. Prophylactic use of tamoxifen could reduce the risk of breast cancer in women who do not breast feed postpartum [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 11.