Cell Cycle Age Dependence for Radiation-Induced G₂ Arrest: Evidence for Time-Dependent Repair

Exponentially growing eucaryotic cells, irradiated in interphase, are delayed in progression to mitosis chiefly by arrest in G₂. The sensitivity of Chinese hamster ovary cells to|${\rm G}_{2}\text{-arrest}$| induction by X rays increases through the cell cycle, up to the X-ray transition point (TP) in G₂. This age response can be explained by cell cycle age-dependent changes in susceptibility of the target(s) for G₂ arrest and/or by changes in capability for postirradiation recovery from|${\rm G}_{2}\text{-arrest}$| damage. Discrimination between sensitivity changes and repair phenomena is possible only if the level of|${\rm G}_{2}\text{-arrest-causing}$| damage sustained by a cell at the time of irradiation and the level ultimately expressed as arrest can be determined. The ability of caffeine to ameliorate radiation-induced G₂ arrest, while inhibiting repair of|${\rm G}_{2}\text{-arrest-causing}$| damage makes such an analysis possible. CHO cell monolayers were irradiated (1.5 Gy), then exposed to 5 mM caffeine for periods of 0-10 hr. Cell progression was monitored by the mitotic cell selection procedure. In the presence of caffeine, progression of irradiated cells was relatively unperturbed, but on caffeine removal, G₂ arrest was expressed. The duration of G₂ arrest was independent of the length of the prior caffeine exposure and, since cells of all ages were ultimately examined, the duration of arrest was also independent of cell cycle age at the time of irradiation. This finding indicates that the target for|${\rm G}_{2}\text{-arrest}$| induction is present throughout the cell cycle and that the level of|${\rm G}_{2}\text{-arrest}$| damage incurred is initially constant for all cell cycle phases. The data are consistent with the existence of a time-dependent recovery mechanism to explain the age dependence for radiation induction of G₂ arrest.