Cell killing, radiosensitization and cell cycle redistribution induced by chronic hypoxia

Some of the biological changes associated with extreme hypoxia at 37 degrees C (less than 10 ppm pO2) were examined in Chinese hamster V79 cells. Specifically, extreme hypoxia caused an initial decrease in plating efficiency to 55% in 4 hr after the onset of hypoxia. Beyond this time, the decline in plating efficiency was more gradual reaching 35% of control at 20 hr. Flow microfluorimetry (FMF) studies, in which cells are sorted on the basis of DNA content and then assayed for viability, demonstrated that mid S phase cells were most sensitive to chronic hypoxia, with surviving fraction equal to 2.5% at 20 hr. Furthermore, the viability of G1 and G2/M cells, after 20 hr of hypoxic storage, was also reduced to 20 and 7.6%, respectively. Hypoxia also caused alterations in the cell cycle distribution of initially asynchronous cells, as determined by dual parameter FMF measurements of both cellular DNA content and incorporated BudR. In particular, G2/M cells completed mitosis, while G1 cells showed little or no movement. Lastly, cells stored in chronic hypoxia displayed an enhanced radiosensitivity when compared to acutely hypoxic cells. Possible reasons for these observations are discussed.