Background: The last decade established significant contributions of microbiome to many organ specific cancers. Existence of distinct breast microbiota has been recently established but their biological impact in breast cancer progression remains elusive. A few recent studies suggested the existence of distinct breast microbiota and a shift in microbial community composition in diseased breast compared to normal breast however, their functional impact and underlying mechanisms are unknown. Present study was designed to examine the contribution of pro-carcinogenic bacteria in breast cancer initiation, progression and metastasis. Utilizing extensive data mining and metagenomic analyses, we discovered the presence of toxin producing enterotoxigenic Bacteroides fragilis (ETBF) in malignant breast. ETBF is a pro-carcinogenic bacteria known for its potential to initiate and/or promote colon cancer and its pathogenicity has been attributed to its unique toxin B. fragilis toxin (BFT)’.

Results: Using mammary intraductal model we discovered that ETBF can successfully colonize the breast confirmed by qPCR and Fluorescent in situ hybridization where it induces local inflammation, fibrosis and hyperproliferation of breast epithelial cells. Mice bearing gut ETBF infection exhibit significant circulating BFT confirmed by qPCR and ELISA and distinct morphological alterations in mammary gland as observed from whole breast mounting and histological evaluation. Gut colonization with ETBF rapidly induces hyperplasia in mammary glands with systemic and local breast inflammation validated by flow cytometry, immunohistochemistry and cytokine profiling. While no changes are observed in cell growth and clonogenicity upon BFT treatment, significant increase in migration and invasion potential and decreased adhesion of MCF10A and MCF7 cells are observed. BFT leads to prominent cytoskeletal reorganization, and increase in migration, invasion and stemness potential of breast cancer cells. Our results indicate that breast cancer cells exposed to BFT ensue to exhibit increase tumor growth, form multifocal tumors and show a striking increase in tumor-initiating cells upon in vivo limiting dilution in immunocompromised mice exhibiting retention of ‘BFT memory’ from the initial exposure. Mechanistically, RNA-sequencing shows enrichment of βcatenin and Notch pathway in secondary tumors derived from BFT-exposed breast cancer cells. Inhibitors of βcatenin and Notch axis abrogates BFT-induced migration and invasion potential indicating the functional importance of this axis. Intriguingly, gut colonization with ETBF at a physiologically relevant level strongly induces growth and metastatic progression of 4T1 tumor cells implanted in mammary ducts monitored by whole animal bioluminescent imaging. In vivo and ex vivo analyses of tumors and distant organs reveal a significant induction of lung and liver metastasis of breast cancer by ETBF while gut colonization with non-toxigenic Bacteroides fragilis (NTBF) does not exhibit any tumor-augmenting impact. We mechanistically evaluate the oncogenic impact of alpha bug ETBF on breast cancer progression and its role in promoting liver and lung metastasis using multiple mice models and multiple techniques including multi-color flow cytometry, immunohistochemistry, quantitative PCR, multiplexed ELISA, ex vivo functional assays and western blotting.

Conclusion: Collectively, these findings present the first evidence to show that gut colonization with Bacteroides fragilis rapidly induces inflammation, fibrosis and hyperplasia in the breast. In syngeneic breast cancer model, gut colonization with ETBF aggravates breast cancer progression and induces enhanced lung and liver metastasis via systemic immune modulation, cytokine synthesis and activation of pro-oncogenic pathways.

Citation Format: Sheetal Parida, Sumit Siddharth, Guannan Wang, Himavanth Gatla, Shaoguang Wu, Brian Ladle, Kathleen Gabrielson, Cynthia L Sears, Dipali Sharma. Gut pathogen, Bacteroides fragilis promotes breast cancer liver and lung metastasis [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS19-02.