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With ulcerative colitis (UC)-associated tumorigenesis, p53 gene alteration is considered to be a key event. To clarify whether the p53-checkpoint is operating in foci of inflammation and that its disruption is a feature of UC-associated neoplasms, the present immunohistochemical study was conducted. Since accumulation of butyric acid with active UC is associated with apoptosis, effects ofin vitroexposure of newly established UC-cancer derived cell lines to organic acids were also assessed. The regulatory subunit of ribonucleotide reductase, p53R2, was found to be localized with p53in situ, and levels of p53, phospho-p53, p53R2 and inducible nitric oxide synthase were significantly intercorrelated. However, p53R2 expression was clearly reduced with progression through UC-associated dysplasia to carcinoma, demonstrating an inverse relation with p53 overexpression.In vitrotreatment with butyrate or propionic acid, but not succinic acid, elicited a positive response in the p53–p53R2 system. Moreover, p53-dependent DNA repair, investigated by radioactive nucleotide incorporation, was induced by butyric acid and inhibited by short-interferingp53andp53R2RNAs. Therefore, it was concluded that the p53–p53R2-dependent DNA repair system is constitutively stimulated by butyric acid, which accumulates in UC inflammatory lesions. Since failure of the p53-G1 checkpoint may cause dysfunction of repair under the influence of butyrate, gene alterations may increase and spread through the genome, leading to tumorigenesis.