Smaller Oxygen Nanobubbles More Effectively Suppress Colitis and Colon Carcinogenesis than Larger Oxygen Nanobubbles

Oxygen nanobubble (NBO2) water is reportedly a promising therapeutic and radiosensitizing agent against solid cancers. However, the significance of nanobubble size in inflammation-associated colorectal carcinogenesis in vivo remains elusive. We investigated whether small NBO2 water exerts stronger preventive effects against colitis and colorectal carcinogenesis in an azoxymethane/dextran sulfate sodium–induced mouse model of colitis-associated cancer. Differences in particle size between the small and large NBO2 water samples were confirmed by atomic force microscopy. The mice received drinking water containing either small or large sized NBO2 throughout the experiment. Small NBO2 water significantly reduced disease activity index scores, histopathological colitis scores, colonic shortening, CD68-positive inflammatory macrophage density, and tumor numbers. However, body weight, water intake, food intake, and spleen weight were unaffected. Immunohistochemistry revealed that small NBO2 water reduced the percentage of Ki-67-positive tumor cells and the proportions of hypoxia-inducible factor-1α–positive epithelial and stromal cells, whereas no significant differences were observed in CD8- or forkhead box P3-positive cells. We conclude that nanometer-sized oxygen bubbles prevent inflammation-associated colorectal carcinogenesis, and that particle size is a critical determinant of biological effects. Small amounts of NBO2 water may help control colitis and tumor development by alleviating hypoxia in the tumor microenvironment.