Altered Cleavage of DNA Sequences by Bleomycin and Its Deglycosylated Derivative in the Presence of Actinomycin

Abstract

The antitumour antibiotics bleomycin and actinomycin are commonly used therapeutically in combination. One causes metal ion- and oxygen-dependent oxidative damage to DNA, while the other acts at the level of DNA via intercalation of its phenoxazone chromophore and probable inhibition of topoisomerases. Both drugs bind and/or cleave DNA primarily at guanine-containing sequences, which could lead to mutual interference. Using three different restriction fragments we show that binding of actinomycin to DNA causes major alterations in the sequence specificity of bleomycin.Fe-mediated cleavage, including the appearance of new cleavage sites and the suppression of others. The subtle sequence-dependence of the interference is illustrated by the different effects of actinomycin on DNA cleavage by the deglycobleomycin.Fe complex. Actinomycin sharply decreases the extent of cleavage at GpC sites by both bleomycin and deglycobleomycin whereas cleavage at GpT sites is much less affected, while novel cleavage sites are generated at GpA, ApT and, to a lesser extent, TpT steps. A dramatic increase in bleomycin.Fe cutting at GpA is barely detectable with deglycobleomycin.Fe, confirming that the carbohydrate moiety of bleomycin is important for DNA recognition. The results contribute to a better understanding of how two individually well-characterized small molecules interact simultaneoulsy with specific sequences in DNA and as such assist clarification of the principles governing drug-DNA recognition.