Purpose: We have previously reported on the isolation and characterization of a multiprotein DNA replication complex (MRC) from HeLa cells that fully supports in vitro DNA replication. Based upon its ability to replicate DNA in a cell-free environment (devoid of other cellular processes) the MRC may serve as a unique model system for investigating the mechanisms of action of anticancer drugs that directly affect DNA synthesis. The experiments described in this report were performed to establish whether the MRC could serve as a model system to examine in detail the mechanism of action of camptothecin, a DNA topoisomerase I inhibitor.
Methods: We examined the effects of increasing concentrations of camptothecin on HeLa cell survival, intact HeLa cell DNA synthesis and MRC-mediated in vitro DNA replication. We also performed topoisomerase I assays in the presence of increasing concentrations of camptothecin to study the direct effects of the agent on MRC-associated topoisomerase I activity. Furthermore, we employed an SDS precipitation assay to measure the formation of MRC-associated topoisomerase I-cleavable complexes in the presence of increasing concentrations of camptothecin.
Results: We found a close correlation between the IC50 values for intact HeLa cell DNA synthesis (0.15 microM) and MRC-mediated in vitro DNA synthesis (0.05 microM). Similarly, we found that 0.05 microM camptothecin inhibited MRC-associated topoisomerase I activity by approximately 50%. In addition, we found that the formation of MRC-associated topoisomerase I-cleavable complexes increased linearly with increasing concentrations of camptothecin.
Conclusions: The data presented in this report support the use of the MRC as a model system to study the mechanism of action of camptothecin. We anticipate that future studies with the MRC will help elucidate the cellular consequences of camptothecin-cleavable complex formation.