Histone deacetylase inhibitor sodium butyrate enhances cellular radiosensitivity by inhibiting both DNA nonhomologous end joining and homologous recombination.

HDAC inhibitors have been proposed as radiosensitizers in cancer therapy. Their application would permit the use of lower radiation doses and would reduce the adverse effects of the treatment. However, the molecular mechanisms of their action remain unclear. In the present article, we have studied the radiosensitizing effect of sodium butyrate on HeLa cells. FACS analysis showed that it did not abrogate the γ-radiation imposed G2 cell cycle arrest. The dynamics of γ-H2AX foci disappearance in the presence and in the absence of butyrate, however, demonstrated that butyrate inhibited DSB repair. In an attempt to clarify which one of the two major DSBs repair pathways was affected, we synchronized HeLa cells in G1 phase and after γ-irradiation followed the repair of the DSBs by agarose gel electrophoresis. Since HR is not operational during G1 phase, by this approach we determined the rates of NHEJ only. The results showed that NHEJ decreased in the presence of butyrate. In another set of experiments, we followed the dynamics of disappearance of RAD51 foci in the presence and in the absence of butyrate after γ-radiation of HeLa cells. Since RAD51 takes part in HR only, this experiment allows the effect of butyrate on DSB repair by homologous recombination to be assessed. It showed that HR was also obstructed by butyrate. These results were confirmed by host cell reactivation assays in which the repair of plasmids containing a single DSB by NHEJ or HR was monitored. We suggest that after a DSB is formed, HDACs deacetylated core histones in the vicinity of the breaks in order to compact the chromatin structure and prevent the broken DNA ends from moving apart from each other, thus ensuring effective repair.