Human and rodent cell lines showing no differences in the induction but differing in the repair kinetics of radiation‐induced DNA base damage

Abstract

Purpose: To compare the induction and repair of radiation‐induced base damage in human and rodent cell lines.Material and methods: Experiments were performed with two human (normal fibroblasts HSF1 and tumour HeLa cells) and two rodent (mouse L929 and hamster CHO‐K1) cell lines. Base damage was determined with the alkaline comet assay combined with the repair enzyme formamidopyrimidine‐glycosylase (Fpg). Proteins were detected by Western blot.Results: The induction of Fpg‐sensitive sites was measured in human and rodent cell lines for doses up to 8 or 5 Gy, respectively. Comets were analysed in terms of tail moments, which were transformed into Gy‐equivalents. The amount of Fpg‐sensitive sites increased linearly with doses up to 4 Gy, whereby the ratio of single‐strand breaks (ssb) to Fpg‐sensitive sites was nearly identical for human and rodent cells with ssb:Fpg‐sensitive sites=1:0.41±0.07 and 1:0.45±0.05, respectively. For doses exceeding 4 Gy, the amount of Fpg‐sensitive sites did not increase further, indicating a dose limit up to which the comet assay can be used to detect Fpg‐sensitive sites. Repair of Fpg‐sensitive sites was studied for an X‐ray dose of 4 Gy. For all four cell lines, the repair was measured to be completed 24 h after irradiation, but with pronounced differences in the kinetics. In both rodent cell lines, 50% of Fpg‐sensitive sites were removed after t½=25±10 min in contrast to t½=80±20 min in the two human cell lines. The two species also differed in the level of polymerase ß with, on average, a three‐ to fivefold higher level in rodent cells compared with human cells.Conclusions: Repair of radiation‐induced Fpg‐sensitive sites was much faster in rodent than in human cells, which might result from the higher level of polymerase ß found in rodent cells.