A novel anticancer ribonucleoside, TAS106, switches necrosis to apoptosis in x-irradiated MKN45 cells through survivin down-regulation and G2/M checkpoint abrogation

1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl)cytosine (ECyd, TAS106) is a newly developed anti-tumor agent targeting RNA synthesis. In this study, we examined whether the exposure of gastric tumor MKN45 cells to X-rays in the presence of ECyd at the low concentrations, which induces no apoptosis itself, induced apototic cell death. We report here that a low dose of ECyd converted X-ray-induced necrosis to caspase-dependent apoptosis in gastric tumor cell line MKN45. This conversion from necrosis to apoptosis was linked to the abrogation of the X-ray-induced G2/M checkpoint. This apoptosis was significantly reduced by the treatment with N-tosyl-L-phenylalanyl-chloromethyl ketone (TPCK) or benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk). These results suggested that chymotrypsin-like and caspase-like proteases were responsible for apoptosis induced by co-treatment of X-rays with ECyd. Western blot analysis showed that X-rays increased the expression of cyclin B1, phospho-Cdc2 and Wee1, whereas co-treatment with X-rays and TAS106 decreased the expression of these signaling molecules associated with G2/M arrest. Furthermore, TAS106 was shown to decrease the expression of survivin but not Bcl-2 and Bcl-XL. Overexpression of wild-type survivin inhibited the induction of apoptosis in co-treatment of X-rays with TAS106. These results indicated that TAS106 sensitized X-ray-induced apoptosis through the down-regulation of survivin and abrogation of the cell cycle machinery. It is well known that apoptotic cells, relative to necrotic cells, are easily eliminated from normal tissue by phagocytosis without severe inflammatory responses. Therefore, manipulation of the survivin/caspase pathway by using the combination of a low concentration of TAS106 and radiation may facilitate the elimination of cancer cells in mitosis