Increased Yield of Residual γH2AX Foci in p53-Deficient Human Lung Carcinoma Cells Exposed to Subpicosecond Beams of Accelerated Electrons.

We studied quantitative yield of residual (24 h post-irradiation) phosphorylated histone (γH2AX) foci as a marker of DNA double strand breaks in wild-type A549 and p53-deficient H1299 human lung carcinoma cells after exposure to subpicosecond (energy 4 MeV, pulse duration 400 fsec, peak dose rate during the pulse 16 GGy/s) and quasi-continuous (energy 3.6 MeV) beams of accelerated electrons in a dose range of 0.5-10.0 Gy. The efficiency of pulse irradiation in A549 and H1299 cells assessed by the yield of residual foci was higher than the efficiency of quasi-continuous exposure by 1.8 and 5.3 times, respectively. Significant differences in quantitative yield of residual γH2AX foci between wild-type and p53-deficient cell lines were observed only after exposure to subpicosecond, but not quasi-continuous beams of accelerated electrons.

Comparative Study of the γH2AX Foci Forming in Human Lung Fibroblasts Incubated in Media Containing Tritium-Labeled Thymidine or Amino Acids.

We compared the formation of γH2AX foci (marker of DNA double-strand breaks) in human lung fibroblasts (MRC-5 line) during their 24-h incubation in a medium containing 3H-labeled thymidine or amino acids (glycine, alanine, and proline) with specific radioactivity from 100 to 400 MBq/liter. A linear dependence of changes in the number of γH2AX foci on the specific radioactivity of the medium was revealed. The quantitative yield of DNA double-strand breaks under the influence of 3H-thymidine was more than 2-fold higher than under the influence of 3H-labeled amino acids. Comparative analysis of the yields of DNA double-strand breaks during cell incubation with 3H-labeled amino acids showed that 3H-alanine produced more pronounced effect that 3H-proline, which is consistent with the data on the content of their non-radioactive analogs in chromatin proteins.

Colony-Forming Ability and Residual Foci of DNA Repair Proteins in Human Lung Fibroblasts Irradiated with Subpicosecond Beams of Accelerated Electrons.

We performed a comparative study of the colony-forming ability and the number of residual foci of DNA repair proteins in cultured human lung fibroblasts (MRC-5 cell line) after exposure to subpicosecond beams of accelerated electrons with an energy of 3.6 MeV and quasi-continuous radiation (accelerated electrons with an energy of 4 MeV and X-rays). The yield of damages causing reproductive cell death after pulsed subpicosecond radiation exposure was higher by ~1.8 times than after quasi-continuous radiation exposure. The quantitative yield of residual γH2AX foci (phosphorylated H2AX histone, a protein marker of DNA double breaks) in cells irradiated with subpicosecond beams of accelerated electrons was shown to be ~2.0- 2.5-fold higher than in cells irradiated with quasi-continuous beams of accelerated electrons.

Ionizing radiation-induced genomic instability in CHO cells is followed by selection of radioresistant cell clones.

The DNA comet assay (neutral version) showed that irradiation of CHO cells in a dose of 1 Gy (gamma-radiation, (60)Co) is followed by an increase in the degree of DNA fragmentation. These changes were observed immediately after irradiation and on days 7-21. On days 2-4 and 23-28 after irradiation, the degree of DNA fragmentation in the descendants of irradiated cell did not differ from that in control samples. The increase in the degree of DNA fragmentation on days 7-21 probably results from induction of apoptosis. This assumption is confirmed by the study of cell death. The sensitivity of cells to repeated irradiation in a dose of 10 Gy significantly increased on days 9, 11, 16, and 18 after irradiation. However, these cells were resistant to repeated irradiation on days 21-28. Our results confirm the hypothesis that genomic instability is a selective mechanism, which mediates the formation of radioresistant cell clones.