Link Between Cellular Senescence and Changes in The Number and Size of Phosphorylated Histone H2ax Foci in Irradiated Human Fibroblasts

Purpose: To analyze the relationship between cellular aging and changes in the number and size of phosphorylated histone H2AX (γH2AX) foci in human fibroblasts and their descendants (up to 15 passages) after exposure to low and high doses of X-ray radiation. Material and methods: The work was performed on a culture of human skin fibroblasts. Cells were irradiated in the exponential growth phase on an X-ray biological unit RUB RUST-M1 (Russia), equipped with two X-ray emitters, at a dose rate of 40 mGy/min (dose 100 mGy) or 850 mGy/min (doses 2000 and 5000 mGy) and temperature 4˚C. Immunocytochemical staining was used to assess the number and size of γH2AX foci and the proportion of proliferating cells using antibodies to γH2AX and Ki67 (a cell proliferation marker protein), respectively. To assess cellular senescence, the proportion of cells positive for senescence-associated β-galactosidase (CA-β-gal(+)) was analyzed. Statistical and mathematical analysis of the obtained data was carried out using the statistical software package Statistica 8.0 (StatSoft). Results: The studies showed that irradiation of cultured human fibroblasts at a low dose (100 mGy) does not lead to statistically significant changes in the number and size of γH2AX foci, as well as the proportion of non-proliferating and senescent cells in the progenies of irradiated cells up to the 15th passage after irradiation. The phenomenon of aging-associated persistence of an increased number and size of γH2AX foci in passages of cells irradiated at a dose of 5000 mGy was discovered. Mathematical analysis of the relationship between changes in the proportion of CA-β-gal(+) cells, the number and size of γH2AX foci in populations of irradiated cells indicates that radiation-induced cellular aging is more associated with the size, rather than the number, of γH2AX foci.