Cell Cycle Specific Recovery from Fractionated Exposures of Ultraviolet Light

Previous studies, using caffeine, have indicated that a deoxyribonucleic acid (DNA) synthesis period (S phase)-dependent potentiation of ultraviolet light (UV) damage occurs in mouse L cells, but the question remains whether this potentiation is due to a peculiar interaction of caffeine with irradiated cells or whether it is due to interference with a recovery process which can be demonstrated in the absence of the drug. To answer this question synchronized populations of L cells were exposed to various first exposures of UV. As a function of time after this initial exposure survival curves of cell colony-forming ability were determined. Results indicate that cells surviving the initial exposure delivered in G₁ phase, which pass through S phase, show no effect of this exposure when irradiated in G₂ phase. That is, the survival curve for such cells in G₂ is the same as for cells irradiated in G₂ phase which have not been irradiated in G₁ phase. Conversely, cells which are exposed to UV in G₂ phase and are allowed to progress to G₁ phase are more sensitive to UV than cells in G₁ phase which are not preirradiated in G₂ phase. If these same cells are allowed to pass through S phase, they recover completely from the previous G₂ irradiation. These results are consistent with the hypothesis that UV-irradiated cells recover from damage during their first passage through S phase.