Radioresistance mechanisms of side population cells in mouse melanoma cell line B16

As we showed earlier, side population (SP) cells are more resistant to low-LET radiation than the rest of mouse melanoma B16 cells (Matchuk et al., 2012). The goal of this study was elucidation of some mechanisms of radioresistance; therefore, we analyzed the SP and non-SP cell-cycle distribution, spontaneous and radiation-induced DNA double-strand breaks (the number of γ-H2AX foci), and intracellular NO concentration. The obtained results indicate that SP cells have a significantly lower number of double-strand DNA breaks after irradiation at a dose of 3 Gy than do non-SP cells (24.4 vs. 40.3, respectively, p < 0.05, Mann-Whitney U-test). The SP cells are more quiescent than are non-SP ones (the G₁/G₀-fraction is 85 vs. 39%, respectively, p < 0.01). Most non-SP cells are in the S or G₂/M phases (61%), which are believed to be rather radiosensitive. Thus, the difference between the SP and non-SP cell radiosensitivity can be partly explained by peculiarities of the cell cycle distribution. The NO concentration is 1.5 times higher in the SP than in the non-SP cells (p < 0.05); since it is known that NO inhibits apoptosis, being one of the mechanisms of genetic stability maintenance, that there is a higher number of the spontaneous double-strand DNA breaks in the SP cells is not surprising (p < 0.05). The above-given results to a certain extent explain the higher resistance of the SP cells to low-LET radiation in comparison with the non-SP cells. Further study of this problem may become the basis for development of tools to target SP cells and, eventually, for more effective treatment of oncological diseases.