Combined effects of gamma-radiation and ethylene oxide in human diploid fibroblasts

Human diploid VH-10 fibroblasts were pre-exposed to gamma-rays and then treated with ethylene oxide (EtO). In the reverse experiment, the cells were pretreated with EtO and then exposed to gamma-rays. Two different dose rates of gamma-rays were used: a low dose rate (LDR, 0.66 Gy/min) and a high dose rate (HDR, 10 Gy/min). Cell killing, mutagenicity and DNA double-strand breakage were studied in both types of experiment. The induction of mutations in the HPRT locus was studied by selection in medium containing 6-thioguanine. DNA double-strand breakage, measured as fraction of activity released (FAR), was investigated using pulsed field gel electrophoresis. Concerning mutagenesis, it was found that pre-exposure of the cells to gamma-radiation (1 Gy) followed by treatment with EtO (2.5 mMh) led to an additive co-interaction, irrespective of dose rate. On the other hand, the reverse experimental procedure (pretreatment with EtO followed by gamma-ray exposure) resulted in an antagonistic effect, which was most pronounced when the HDR was applied. In the latter case, the resultant mutant frequency was two times lower than the sum of the mutant frequencies after the individual treatments. However, the effect of the combined treatment on FAR was different: FAR increased with both combinations of agents used compared with the separate and hypothetically expected effects. Moreover, the HDR exposure led to an additional increase in FAR compared with the LDR one.