Daunomycin disrupts nuclear assembly and the coordinate initiation of DNA replication in Xenopus egg extracts

We have used Xenopus egg extracts to investigate the effects of the antitumor drug daunomycin on DNA replication in vitro. Xenopus sperm nuclei replicated nearly synchronously in our egg extracts, thereby allowing us to determine the effects of the drug on both replication initiation and elongation. Titration experiments demonstrated that daunomycin effectively inhibited replication in the extract, with 50% inhibition at a total drug concentration of 2.7 microM. However, a high concentration of daunomycin (50 microM) also inhibited nuclear envelope assembly, a prerequisite for the initiation of replication in this system. Therefore, to bypass the effects of daunomycin on nuclear envelope assembly, sperm nuclei were preassembled in extract prior to drug addition. Initiation of replication in preassembled nuclei was also inhibited by daunomycin, with 50% inhibition at a drug concentration of 3.6 microM. At low drug concentrations, where replication did occur, the synchrony of initiations within individual nuclei was lost. This drug-induced disruption of initiation events may provide important clues regarding the mechanism(s) by which these events are coordinated in eukaryotic cells. Daunomycin also inhibited replication elongation in preassembled, preinitiated nuclei. However, the concentration of drug required for 50% inhibition of elongation was nearly fourfold higher than the required for inhibition of initiation. Taken together, these data demonstrate that Xenopus egg extract can be used to investigate the effects of DNA-binding antitumor drugs on a number of interrelated cellular processes, many of which are less tractable in whole cell systems.