DNA double-stranded breaks in mammalian cells after exposure to intercalating agents

Previous work has shown that exposing mouse L1210 cells to intercalating agents such as adriamycin, ellipticine and actinomycin D results in DNA single-stranded breaks and DNA-protein crosslinks. To characterize further the interaction between these drugs and intracellular DNA we have employed a modification of the alkaline elution technique which allows the detection of DNA double-stranded breaks. Ellipticine (1.25-5.0 microgram/ml) adriamycin (0.5-3.0 microgram/ml) and actinomycin D (1.5-3.0 microgram/ml) all caused double-stranded breaks in DNA from L1210 cells following a 1 h treatment. The number of double-stranded breaks found per single strand break was highest for ellipticine, despite the fact that this is least cytotoxic of the three drugs. By comparing the single and double strand break frequency caused by radiation to that caused by ellipticine, it appears that most if not all of the drug-induced single strand breaks observed actually represent double-strand breaks. We suggest that these double-strand breaks may result from the action of an intracellular enzyme, perhaps topoisomerase, which breaks both strans in concert to relieve the topological strain caused by drug intercalation.