Molecular interactions of the combined effects of bleomycin and x-rays on mammalian cell survival.

The interactions between bleomycin and X-ray damage and repair have been examined in rat and human tumor cells. Bleomycin itself indices extensive DNA single-strand breaks but does not appear to inhibit the repair of X-ray-induced DNA single-strand breaks. Quantitative analysis of these interactions is complicated by the retention of active bleomycin within cells that remains capable of further DNA degradation even under the conditions of alkaline sucrose gradient cell lysis. DNA double-strand breaks and/or disruptions of DNA-lipid complexes also occur following bleomycin exposure. X-ray-induced excision repair replication is only minimally influenced by even high concentrations of bleomycin. A small amount of excision repair is demonstrable in nonirradiated cells treated with high concentrations of bleomycin consistent with repair of bleomycin-induced nucleotide damage in cellular DNA by a "cut and patch" repair mechanism. Repair of bleomycin-induced DNA single-strand breaks also occurs. The data indicate that bleomycin and X-ray damage are quite similar both in their induction and repair, but that lesions occur and are repaired independently. The enzymatic mechanisms appear similar in the two cell types despite substantial differences in their sensitivity to bleomycin.