Although daunomycin and adriamycin are considered effective antitumor drugs and have been used in the clinic for over 40 years, their mechanism of action is still a matter of debate. We investigated the influence of daunomycin on interaction between linker or core histones and DNA in live HeLa cells in vitro, using image and flow cytometry. Exposure to daunomycin at clinically relevant concentrations (25-250 nM) caused dissociation of wild-type H1.1 as well as 4 H1 point mutants from DNA, followed by their accumulation in nucleoli and aggregation of chromatin. A detectable dissociation of H2B core histones occurred only at much higher concentrations of the drug (500 nM). Replication of DNA and synthesis of RNA were not halted by daunomycin (up to 2500 nM); however the characteristic subnuclear distribution of sites of transcription and replication was lost. Dissociation of the H1.1 linker histones and subsequent loss of higher order chromatin structures may constitute an important component of the mechanism of cytotoxicity of daunomycin.
Keywords: DNA; chromatin aggregation; daunomycin; higher order chromatin structure; histone H1; intercalation.