In this study, we aimed at evaluating the possible enhancing effect exerted by the combined use of sodium butyrate (SB) and X-rays on eradicating the human colorectal cancer cell line HCT 116 containing wild-type p53. We assessed the effect of this combination on the molecular pathways leading to cell death. HCT 116 cells were subjected to SB (1 mM) treatment followed by X-irradiation (5 Gy), and the effects on cell death, cell proliferation and cell cycle were examined. We also analyzed the apoptosis-indicating protein expression, mitochondrial membrane potential and intracellular superoxide formation. Treatment with SB alone significantly induced cell cycle arrest and apoptosis, whereas X-irradiation showed no effect on cell death despite its ability to block cell proliferation. Growth arrest and cell death were enhanced in the combined treatment groups. A marked reduction in the growth rate of the combined-treatment group was observed compared to that of the single-treatment groups. The apoptotic mitochondrial pathway was significantly enhanced with the combined use of the two agents. It was observed to be involved in the increased expression levels of p53 and p21, as well as in the release of cytochrome c and the alteration of the balance of anti- and pro-apoptotic Bcl-2 family proteins. Enhanced superoxide formation was also observed. However, the death receptor pathway was found to play no role in this phenomenon. These results suggest that X-irradiation promotes cell killing in synergy with SB treatment. Thus, the combined treatment led to a mutual potentiation of the killing effects of each agent.

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