Abstract 6101: Exopolysaccharide secreted by Bacillus subtilis alters breast cancer cell growth and stemness

Recently, multiple studies have shown that a unique microbiome is associated with the breast, with distinct differences in microbial compositions between healthy and patients with benign versus malignant breast cancer tissue. It is well known that bacteria can produce metabolites and other signaling molecules to alter the host9s response. However, little is known about whether bacteria in the breast microenvironment interact with breast cancer cells and if that interaction changes cancer progression. We aim to investigate if, and how commensal bacteria and/or their bacterial products alter growth and metastasis of breast cancer cells. Upon initial analysis of sequencing data deposited by Heiken et al, we identified the presence of the gut commensal bacterium Bacillus subtilis in some breast tissues of patients with breast cancer. Previous studies have shown that exopolysaccharide (EPS) secreted by B. subtilis can induce tolerance in the myeloid compartment to protect the host from inflammatory diseases such as diarrhea, sepsis, and graft-versus-host-disease. We first examined the effect of EPS on breast cancer proliferation by performing in vitro proliferation assays on a diverse panel of breast cancer cell lines, including hormone receptor positive and negative lines. Results showed that EPS suppressed proliferation of multiple breast cancer cell lines (T47D, MDA-MB-468, MDA-MB-453, HCC1428) in a concentration dependent manner while other cell lines were unaffected (BT549, MDA-MB-231, ZR-75-30, MCF7). Annexin V apoptosis assay and cell cycle staining indicated that EPS acted via distinct mechanisms depending on the cell line, increasing apoptosis in MDA-MB-468 cells while inducing G0/G1 cell cycle arrest in T47D cells. In addition to proliferation by bulk tumor cells, the survival of breast cancer stem cells emerges as a significant challenge to breast cancer therapies as these stem-like cells are thought to repopulate the tumor and lead to recurrence. We tested if EPS treatment alters cancer stem cell survival using the mammosphere forming assay, a 3D culture system which specifically selects for cells with stem cell characteristics. Our preliminary mammosphere results show that pretreatment with EPS increased cancer stem cell survival in T47D and MCF7 cells. Overall, we found that EPS secreted by B. subtilis can directly suppress the proliferation of some breast cancer cell lines but enhance cancer stem cell survival. Thus, effects of B. subtilis on breast cancer cells are potentially multifaceted. Future work will focus on assessing the effect of EPS on breast tumor growth in vivo and elucidating the signaling mechanism by which EPS inhibits bulk cancer cells but stimulates cancer stem cells. Elucidating this mechanism would further our understanding of how local breast microbes may influence breast cancer growth and outcome. Citation Format: Mai Rachel Nguyen, Clodia Osipo, Katherine L. Knight. Exopolysaccharide secreted by Bacillus subtilis alters breast cancer cell growth and stemness [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6101.