To examine the dependency of p53 status and the effect of wortmannin on the repair of radiation-induced damage, referring to the response of intratumor quiescent (Q) cells. Human head and neck squamous cell carcinoma cells transfected with mutant TP53 (SAS/mp53) or with neo vector (SAS/neo) were injected subcutaneously into left hind legs of nude mice. Tumor-bearing mice received 5-bromo-2'-deoxyuridine (BrdU) continuously to label all intratumor proliferating (P) cells. They received high dose-rate γ-ray irradiation (HDRI) immediately followed by caffeine or wortmannin administration, or reduced dose-rate γ-ray irradiation simultaneously with caffeine or wortmannin administration. Nine hours after the start of irradiation, the tumor cells were isolated and incubated with a cytokinesis blocker, and the micronucleus (MN) frequency in cells without BrdU labeling (Q cells) was determined using immunofluorescence staining for BrdU. SAS/neo tumor cells, especially intratumor Q cell populations, showed a marked reduction in sensitivity due to the repair of radiation-induced damage, compared with the total or Q cell populations within SAS/mp53 tumors that showed little repair capacity. In both total and Q cell populations within SAS/neo tumors, wortmannin efficiently suppressed the reduction in sensitivity caused by leaving an interval between HDRI and the assay and decreasing the irradiation dose rate. The repair of radiation-induced damage was thought to be a p53-dependent event. From the viewpoint of tumor control, including intratumor Q-cell control, wortmannin treatment in combination with γ-ray irradiation is thought to be useful for suppressing the repair of radiation-induced damage.

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