Abstract PR02: Bacteroides fragilis toxin induces epithelial-to-mesenchymal transition and stem-like phenotype in breast epithelial cells and concomitantly activates Notch1 and βcatenin axes

The last decade established significant contributions of the microbiome to many organ-specific cancers. 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. The present study examines the contribution of procarcinogenic bacteria in breast cancer initiation, growth, and progression. Extensive data mining and metagenomic analyses of existing datasets revealed the presence of toxin-producing Bacteroides fragilis in malignant breast. B. fragilis is a procarcinogenic bacteria known for its potential to initiate and promote colon cancer; its pathogenicity has been attributed to its unique toxin “BFT.” B. fragilis-infected mice exhibited a significant increase in circulating BFT and distinct morphologic alterations in mammary gland. In vitro, upon treatment with BFT, no changes were observed in cell growth and clonogenicity. However, significant increase in migration and invasion potential and decreased adhesion of MCF10A and MCF7 cells were observed. BFT-treated cells displayed acquisition of fibroblast-like appearance and increased formation of pseudopodia/microtentacles emanating from the cell membrane along with molecular markers of epithelial-to-mesenchymal transition. Decreased expression of epithelial marker, E-cadherin, along with elevated levels of mesenchymal markers, N-cadherin and vimentin, were observed. BFT also increased the expression of EMT-related transcription factors, Snail, Slug and Twist. BFT-treated cells attained stem cell-like phenotype, exhibiting an increased ability to form secondary and tertiary mammospheres and elevated expression of pluripotency-factors (Oct4, Nanog, and Sox2). Mechanistic studies showed that BFT induced expression and nuclear translocation of NICD (cleaved NOTCH) and β-catenin resulting in activation of downstream targets. Inhibition of Notch1 and β-catenin using γ-secretase and β-catenin inhibitors successfully inhibited functional effects of BFT. Mammary gland implantation and in vivo limiting dilution assays were utilized to corroborate the in vitro findings. BFT-pretreated MCF7 cells exhibit increased tumor growth and form multifocal tumors in mice. MCF10A-KRas cells, pretreated with BFT, also showed increased tumor progression and multifocal tumors in mice. In vivo limiting dilution assay using breast tumors from BFT-pretreated MCF7 cells exhibited a striking increase in tumor-initiating cells. Follow-up analyses of these tumors demonstrated increased migratory, invasive, and mammospheres-forming behavior, confirming that brief BFT exposure elicits long-term molecular changes. Collectively, these findings present the first in vitro and in vivo evidence to show that Bacteroides fragilis toxin induces EMT, invasion/migration and stem cell-like phenotype and leads to concomitant activation of Notch and β-catenin axes. This abstract is also being presented as Poster A66. Citation Format: Sheetal Parida, Shaoguang Wu, Nethaji Muniraj, Sumit Siddharth, Arumugam Nagaligam, Cynthia L. Sears, Dipali Sharma. Bacteroides fragilis toxin induces epithelial-to-mesenchymal transition and stem-like phenotype in breast epithelial cells and concomitantly activates Notch1 and βcatenin axes [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr PR02.