Effect of inhibition of DNA synthesis on recovery of X-irradiated L5178Y-S cells. I. Aphidicolin.

Irradiated L51 78Y-S cells (LY-S) were characterized by an exponential survival curve and the potentiation effect of split--dose irradiation. Previously we showed [13] that in LY-S cells the reduction of DNA replicative synthesis rate affected the balance between the fixation and repair of sublethal damage (SLD) and of potentially lethal damage (PLD) in favor of repair. We report here that a block of DNA synthesis by aphidicolin (APC), an inhibitor of DNA polymerase alpha, was sufficient to protect LY-S cells from fixation of PLD and SLD induced by X-rays. Treatment with APC 0.5 micrograms/ml for 2 h, efficiently inhibited DNA replication (95%) with minimal effect on survival. Inhibition of DNA synthesis by combined irradiation and APC was not significantly different from APC treatment alone. The level of protection by APC was dependent on the length of time between irradiation and APC application. An opposite effect was observed when the drug treatment had proceeded irradiation: The killing effect of X-ray increased. The effect of aphidicolin treatment remained even after removal of APC and was dependent on the drug concentration and time between drug removal and irradiation. These results are interpreted as indicating that X-ray damage was fixed in LY-S cells, because of their lack of ability to maintain the nucleotide pool balance, and that fixation took place during progression through the cell cycle.