Influence of Cell Cycle Phase on Radiation-Induced Cytotoxicity and DNA Damage in Human Colon Cancer (HT29) and Chinese Hamster Ovary Cells

Abstract

We have previously shown that fluorodeoxyuridine (FdUrd) radiosensitizes HT29 human colon carcinoma cells. Since treatment with FdUrd arrests cells at the G1/S-phase interface, a condition associated with increased radiation sensitivity in some cells, it seemed possible that redistribution of cells in the phases of the cell cycle might account for FdUrd-mediated radiosensitization. To begin to test this, HT29 cells were separated by centrifugal elutriation according to cell cycle phase and assessed for radiosensitivity, using a clonogenic assay, and radiation-induced DNA damage, using pulsed-field gel electrophoresis. We found that all of the elutriated fractions (which contained cells enriched in G1, G1/early S, mid to late S or G2/M phase) had the same radiation sensitivity and expressed a similar extent of radiation-induced DNA damage. To determine if the techniques used in this study could detect differences between the radiation sensitivity of cells in different phases of the cell cycle, analogous experiments were carried out using Chinese hamster ovary (CHO) cells. In contrast with the results of experiments with HT29 cells, but in agreement with previous studies, CHO cells separated under the same conditions as were used for HT29 cells showed a marked dependence on cell age of both clonogenic survival and radiation-induced DNA damage. Thus, within the limitations of the purity of separation obtained using elutriation, the radiation sensitivity of HT29 cells does not vary substantially as a function of cell cycle phase. Therefore, it seems unlikely that cell cycle redistribution alone explains the radiation sensitivity produced by exposure to FdUrd.