Cells of the TP53-deficient human leukemia cell line HL60 continue to progress throughout the cell cycle and arrest in the G2/ M phase during protracted exposure to exponentially decreasing low-dose-rate radiation. We have hypothesized that${\rm G}_{2}/{\rm M}\text{-phase}$ arrest contributes to the extent of radiation-induced cell death by apoptosis as well as to overall cell killing. To test this hypothesis, we used caffeine and nocodazole to alter the duration of${\rm G}_{2}/{\rm M}\text{-phase}$ arrest of HL60 cells exposed to exponentially decreasing low-dose-rate irradiation and measured the activity of${\rm G}_{2}/{\rm M}\text{-phase}$ checkpoint proteins, redistribution of cells in the phases of the cell cycle, cell death by apoptosis, and overall survival after irradiation. The results from these experiments demonstrate that concomitant exposure of HL60 cells to caffeine (2 mM) during irradiation inhibited radiation-induced tyrosine 15 phosphorylation of the${\rm G}_{2}/{\rm M}\text{-phase}$ transition checkpoint protein CDC2/p34 kinase and reduced${\rm G}_{2}/{\rm M}\text{-phase}$ arrest by 40-46% compared to cells irradiated without caffeine. Radiation-induced apoptosis also decreased by 36-50% in cells treated with caffeine and radiation compared to cells treated with radiation alone. Radiation survival was significantly increased by exposure to caffeine. In contrast, prolongation of${\rm G}_{2}/{\rm M}\text{-phase}$ arrest by preincubation with nocodazole enhanced radiation-induced apoptosis and overall radiation-induced cell killing. To further study the role of cell death by apoptosis in the response to exponentially decreasing low-dose-rate irradiation, HL60 cells were transfected with the BCL2 proto-oncogene. The extent of${\rm G}_{2}/{\rm M}\text{-phase}$ arrest was similar for parental, neomycin-transfected control and BCL2-transfected cells during and after exponentially decreasing low-dose-rate irradiation. However, there were significant differences (P < 0.01) in the extent of radiation-induced apoptosis of parental and neomycin- and BCL2-transfected cells after irradiation, with significantly less radiation-induced apoptosis and higher overall survival in BCL2-transfected cells than similarly irradiated control cells. These data demonstrate that radiation-induced${\rm G}_{2}/{\rm M}\text{-phase}$ arrest and subsequent induction of apoptosis play an important role in the response of HL60 cells to low-dose-rate irradiation and suggest that it may be possible to increase radiation-induced apoptosis by altering the extent of${\rm G}_{2}/{\rm M}\text{-phase}$ arrest. These findings are clinically relevant and suggest a novel therapeutic strategy for increasing the efficacy of brachytherapy and radioimmunotherapy.

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