Radiosensitisation of bladder cancer cells via short-chain fatty acids and/or other metabolites produced by the gut microbiota

Abstract

New non-toxic radiosensitisers are needed in the treatment of muscle-invasive bladder cancer because elderly patients are very vulnerable to chemotherapy-related toxicity of currently available radiosensitisers. Our previous study showed that high fibre diets sensitised RT112 xenografts to irradiation by modifying the gut microbiome and this phenotype was positively correlated with B. acidifaciens abundance. Short chain fatty acids (SCFAs) are major products of fibre fermentation by the gut microbiota. Therefore, we hypothesise that B. acidifaciens may radiosensitise tumours via secretion of SCFAs and/or other metabolites. We treated the RT112 human bladder cancer cell line with SCFAs to determine histone acetylation levels by Western Blot and radiosensitivity by clonogenic assay. We also used a cell viability assay to validate the cytotoxic effects of SCFAs and bacterial supernatants on cancer cells. We showed that all three SCFAs increased histone acetylation (10 mM acetate p=0.014, 10 mM propionate p=0.004, 10 mM butyrate p<0.001) of RT112 bladder cancer cells, in a dose-dependent manner. All SCFAs tended to increase radiosensitivity in the clonogenic assay, with butyrate having a statistically significant effect (p=0.002 for butyrate at 8Gy). In a cell viability assay, we saw that the combination of SCFAs in a physiological ratio conferred a stronger phenotype than single SCFAs, in a time-dependent manner. To validate the anti-tumoural effects of B. acidifaciens, we treated the bladder tumour cells with bacterial supernatants of B. acidifaciens and its cross-feeding with F. prausnitzii, and compared the effects to Bifidobacterium (acetate-producer) and F. prausnitzii (butyrate-producer). Bacterial supernatants of B. acidifaciens and its cross-feeding with F. prausnitzii significantly increased the cytotoxic response of bladder tumour cells compared to the other supernatants. In conclusion, our in vitro experiments in RT112 human bladder cancer cells suggest a role for short-chain fatty acids and/or other metabolites generated by the microbiota in radiosensitisation of tumour cells.