Microbe-associated molecular patterns differentially mediate carcinogenic alterations of the breast tissue in the context of obesity.

Obesity is a risk factor for breast cancer. Obesity alters the microbiome and microbiome perturbations are reported in breast cancer patients. Yet, the impact of obesity-mediated microbial shifts on breast cancer risk remains unclear. Here, we investigate the effect of microbial-associated molecular pattern (MAMP) signaling on genome instability and inflammation. We show in human samples that obesity chronically elevates breast tissue levels of two MAMPs: lipopolysaccharide (LPS) and flagellin. In contrast, obesity was not associated with chronic elevations in lipoteichoic acid (LTA). Injections of LPS and flagellin in mouse mammary glands (MG) were sufficient to induce DNA damage and inflammation. Moreover, DNA damage was reduced in MG of animals on a high-fat diet by knockdown of toll-like receptors for LPS and flagellin (TLR4 and TLR5), but not TLR2 (LTA receptor). Experiments with breast acini cultures demonstrated LPS and flagellin (but not LTA) induce DNA double-strand breaks via TLR and reactive oxygen species (ROS) generation. Similarly, LPS and flagellin mediated nuclear factor-kappa B (NF-κB) pathway activation and increased expression of inflammatory cytokines. Analyses of non-cancerous breast tissue microbiome revealed an enrichment of Proteobacteria in obese women. Proteobacteria often contain LPS and many of these bacteria are flagellated. Tissue-resident Proteobacteria abundance correlated with breast tissue DNA damage. Our findings show that LPS and flagellin are systemic and local mediators of obesity-induced microbiome alterations, predisposing the breast to pre-malignant changes. These results underscore the importance of considering the tissue-resident microbiome as a biomarker of risk to improve primary prevention of breast cancer. Significance: Obesity differentially modulates non-cancerous breast tissue microbial-associated molecular pattern signaling, enriching LPS and flagellin, to promote oxidative stress and DNA damage.