High-fat diet reshapes microbial interactions to promote breast cancer stemness via Streptococcus mutans-induced leucine accumulation

BACKGROUND: Obesity and high-fat diet (HFD) are established risk factors for breast cancer, but the mechanisms by which dietary-induced gut microbiota alterations influence cancer progression are not fully understood. The interplay between microbial composition, metabolites, and cancer progression warrants further exploration. METHODS: We employed Mendelian randomization (MR) and Bayesian colocalization analyses based on genome-wide association studies (GWAS) to identify gut microbial genera causally linked to breast cancer risk. Multi-omics, including 16S rRNA sequencing, fecal metabolomics and RNA-seq, were performed to depict HFD-induced microbial and metabolic shifts in breast cancer-bearing mice. In vitro co-culture systems and in vivo murine models examined interactions between Roseburia intestinalis (R. intestinalis) and Streptococcus mutans (S. mutans) under HFD conditions. Functional assays, including immunofluorescence, qRT-PCR, probe-based assays and fluorescent in situ hybridization, and flow cytometry, evaluated cancer stemness, bacterial colonization, and the impact of leucine metabolism. RESULTS: MR analysis revealed Roseburia as a potential causal microbial risk factor for breast cancer, with colocalized genes enriched in fatty acid metabolism. HFD feeding promoted the co-occurrence of R. intestinalis and S. mutans, facilitating S. mutans intratumoral colonization and consequent leucine accumulation in the tumor microenvironment (TME). S. mutans-derived leucine robustly enhanced breast cancer cell proliferation and stemness, as evidenced by increased tumor sphere formation and upregulation of CD44, CD133, and SOX2. Functional blockade of leucine transport with BCH attenuated S. mutans-mediated tumor growth and limited tumor-associated macrophage activation in vivo. CONCLUSIONS: This study reveals that HFD-induced reshaping of microbial interactions, particularly a commensal-like interaction between S. mutans and R. intestinalis, is associated with leucine accumulation in the TME, thereby supporting breast cancer stemness and progression. Targeting S. mutans-mediated leucine accumulation represents a promising strategy for therapeutic intervention in obesity-related breast cancer. Our findings highlight the pivotal role of dietary-microbiota crosstalk in modulating the TME and cancer stemness.