Bleomycin-induced DNA cleavage: studies in vitro and in intact cells

Bleomycin, an important chemotherapeutic agent useful in the treatment of testicular carcinoma, can cause lung parenchymal injury. Prior studies showing that different cell types have different susceptibilities to bleomycin suggest that intracellular conditions for bleomycin-DNA interactions vary. The purpose of the present studies was to test the effects of therapeutic concentrations of bleomycin on DNA in vitro, and then to study the effects of these levels of bleomycin on intracellular DNA. In the absence of ferrous ion (Fe+2), bleomycin caused no DNA strand scission at any concentration tested. The addition of as little as 100 nmol/L Fe+2 resulted in DNA strand scission at concentrations of bleomycin greater than or equal to 10 nmol/L. Glutathione dramatically augmented the DNA strand breakage. With use of a viral minichromosome replicating in cultured cells, bleomycin caused DNA strand scission at levels equivalent to therapeutic serum concentrations. These studies reveal that bleomycin causes dose-dependent DNA damage at therapeutic serum concentrations in vitro and in intact cells. The observation that in vitro DNA damage is dependent on the availability of ferrous ion and is augmented by glutathione suggests that different cells, which may differ in their intracellular levels of Fe+2 and reducing capacity, may vary in their sensitivity to bleomycin-induced DNA strand scission.