Radiosensitization and DNA repair inhibition by pentoxifylline in NIH3T3 p53 transfectants

Purpose: To examine the role of p53 mutations in the modulation of DNA repair and radiotoxicity by pentoxifylline. Materials and methods: NIH3T3 murine cells transfected with mutant p53 constructs were examined for the influence of pentoxifylline on radiotoxicity to Co-60 gamma-irradiation by colony assay. DNA repair (0-100 Gy) was measured by constant-field gel electrophoresis. Apoptosis was assessed by flow cytometry with the an nexin-V-binding assay. Results: In the two p53 hot-spot mutant cell lines p53-S269R and p53- + 15, the SF10 radiotoxicity enhancement factors induced by the pentoxifylline were 8.0 and 9.7, respectively. In the 1:63 deletion mutant p53-DeltaA cell line, the radiotoxicity enhancement factor was 2.6. No radiosensitization was obtained in the untransfected p53 wild-type cell line U-Wt and in the transfected p53 double-wild-type p53-Wt cell line. When pentoxifylline was added after irradiation at the time of maximum G2 block expression, no radiosensitization was observed in any of the five cell lines. Constant-field gel electrophoresis analyses after 20 h of repair showed that pentoxifylline suppresses DNA double-strand break repair in all p53 mutant cell lines, as indicated by repair inhibition factors of 2.0-2.3. No repair suppression was found in the p53 wild-type cell lines. Conclusions: p53 mutations are a general requirement for radiosensitization by pentoxifylline and the level of radiosensitization depends upon the location of the p53 mutation.