Abstract 2181: Endocrine-targeting therapies modify the breast microbiome to shift metabolism and reduce cancer risk

Abstract Studies indicate that breast tissue has a distinct modifiable microbiome population. However, whether endocrine-targeting therapies modify the non-cancerous breast microbiome to impact tumorigenesis is unknown. To examine this question, we performed 16S bacterial sequencing on DNA isolated from female C57BL/6 mouse mammary glands (MG) administered tamoxifen citrate for 16 weeks and breast tissue from ovariectomized non-human primates administered tamoxifen citrate (TAM) for 2. 5 years. In both models, we show that TAM significantly shifted β-diversity and increased Firmicutes proportional abundance. TAM elevated the proportional abundance of Lactobacillus spp. , Streptococcus luticea, and Staphylococcus sciuri. Female BALB/c mice received intra-nipple injections of Lactobacillus and MG were harvested at 1-, 3-, 7-, and 14-days post-injection to show bacteria colonization and modulation of metabolism-associated gene and protein expression. B6. MMTV-PyMT mice were injected with Lactobacillus bacteria into the MG. Elevating MG Lactobacillus reduced tumorigenesis with an associated decrease in tumor proliferation and shifts in non-cancerous MG tissue metabolism. To elucidate the effects of combining TAM and probiotic bacteria on breast cancer outcomes, 4T1. 2ER+ tumor-bearing female BALB/c mice, fed either a control or Western diet were treated with TAM, Lactobacillus probiotic, or a combination of TAM + Lactobacillus. Results indicate that oral probiotic administration in combination with TAM reduced ER+ tumor growth. Plasma metabolomics revealed combination treatment elevated L-carnitine regardless of diet, suggesting drug-bug interactions modifying metabolite profiles. Cell viability assays on human MCF7 BC cells and non-cancerous mammary HMT-3522S1 cells treated with 1% probiotic-conditioned media (Pro-CM) revealed a significant decrease in cell viability in BC cells but not non-cancerous epithelial cells. Mitochondrial fuel oxidation Seahorse assay on murine 4T1. 2ER+ cells treated with 1% Pro-CM, 1μm 4-hydroxytamoxifen (4-OHT) or combination revealed changes in metabolic substrate dependency, supporting role of microbial metabolites impacting metabolism. Tumor sections from patients with ER+ BC treated with aromatase inhibitors and/or Faslodex in the neoadjuvant setting were stained for Gram-positive bacteria, Gram-negative bacteria, and Ki67. Gram-positive bacteria negatively correlated with Ki67 immunoreactivity, suggesting endocrine-targeting therapies promote tumor Gram-positive bacteria associated with a decrease in proliferation. Taken together, our data indicates that endocrine-targeting treatments modify the breast microbiome associated with a shift in tissue metabolism to decrease proliferation and reduce ER+ BC risk. Citation Format: Alana Arnone, Yu-Ting Tsai, J Mark Cline, Adam S. Wilson, Brian Westwood, Meghan E. Seger, Akiko Chiba, Marissa Howard-McNatt, Edward A. Levine, Alexandra Thomas, David R. Soto-Pantoja, Katherine L. Cook. Endocrine-targeting therapies modify the breast microbiome to shift metabolism and reduce cancer risk abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 2181.