Lack of correlation between G1 arrest and radiation age-response in three synchronized human tumour cell lines.

PURPOSE: To determine radiosensitivity as a function of cell age (the age-response) in three human tumour cell lines, and investigate the dependence of the age-response on G1 arrest and on cell-age heterogeneity in synchronized cell populations. MATERIALS AND METHODS: Variation in radiosensitivity throughout the cell cycle and G1 arrest was measured in mitotically selected populations of synchronized human tumour cells. In order to examine the effects of desynchronization and cell age heterogeneity on the measured age-response, a mathematical model was developed based on an existing kinetic model of the cell cycle. The model was used to describe the age-response for mitotically selected populations of cells, which was then compared with experimentally measured age responses. RESULTS: Three different human tumour cell lines had qualitatively similar age-responses, with periods of radiosensitivity in mitosis and in late G1 phase/early S phase, and periods of radioresistance in early/mid G1 phase and late S/G2 phase. Radiosensitivity appeared to increase in G1 phase before the onset of DNA synthesis. One of the cell lines displayed a prolonged G1 arrest after irradiation in G1 phase. Model results demonstrated that the measured age-responses were consistent with a simple model in which the cell cycle was divided into four regions. Radiosensitivity was assumed to be constant within each region, and changed abruptly at the borders between regions. CONCLUSIONS: Human tumour cell lines can exhibit qualitatively similar age-responses despite having markedly different G1 checkpoint responses. This suggests that modulation of the G1 arrest response may not prove to be a useful clinical strategy because it may not lead to significant cell age specific changes in radiosensitivity. The mathematical model of the radiation response of mitotically selected synchronized cells was a useful way to quantitatively describe cell age heterogeneity in these populations, and demonstrated the important impact of this heterogeneity on measured age-responses.