Repair of potentially lethal radiation damage in human squamous carcinoma cells after chronic hypoxia

Purpose : To examine the repair of radiation induced potentially lethal damage in A431 and CaSki cells after chronic hypoxia. Methods and Materials : Cells in exponential phase are subjected to hypoxia (< 10 ppm oxygen) for up to 12 h and then are allowed to reoxygenate in air for up to 4 h. Cells are then irradiated with γ rays. Cell survivals are measured by clonogenic assay immediately and at different times after irradiation. Results : Compared to aerobic controls, an increase in the level of potentially lethal damage repair (PLDR) is demonstrated in A431 cells reoxygenated for 10 min after > 4 h of hypoxia. The repair returned to aerobic control level by 3 h of reoxygenation. PLDR of A431 cells reached maximum at about 9 h after irradiation in cells reoxygenated for 10 min after hypoxia. However, the repair is maximum at 6 h in cells reoxygenated for 3 h after hypoxia and in aerobic cells not previously exposed to hypoxia. Reoxygenation after chronic hypoxia did not affect the PLDR capacity and repair kinetics of CaSki cells. Conclusion : The results suggest that radiosensitization by reoxygenation after chronic hypoxia is not related to inhibition of PLDR.