Potentiation of sulphur mustard or cisplatin-induced toxicity by caffeine in Chinese hamster cells correlates with formation of DNA double-strand breaks during replication on a damaged template.

Caffeine inhibited the elongation of nascent DNA and induced breaks in the template DNA of sulphur mustard-treated Chinese hamster cells. The sizes of template and nascent DNAs, as indicated by alkaline sucrose gradient sedimentation, were similar suggesting that incision of template DNA occurred opposite gaps formed in nascent DNA by the action of caffeine, forming, effectively, double-strand breaks in DNA. Double-strand break formation was demonstrated, by means of elution of labelled DNA through polycarbonate filters at neutral pH, in both sulphur mustard- and cisplatin-treated cells when they were incubated in the presence of caffeine for 24 h. Double-strand breaks were only formed in that DNA which had been replicated in the presence of caffeine after treatment with sulphur mustard or cisplatin. Non-toxic concentrations of cycloheximide abolished the potentiation by caffeine of sulphur mustard-induced toxicity to Chinese hamster cells and at the same time abolished the formation of the low molecular weight nascent DNA, and as a consequence of its inhibitory effect on DNA synthesis, and the formation of double-strand breaks in DNA. Potentiation of the lethal and clastogenic effects of genotoxic agents by caffeine is therefore due to effects on the rate and mode of DNA synthesis which lead finally to double-strand breaks in DNA.