Etoposide Induces the Dispersal of DNA Ligase I from Replication Factories

In eukaryotic cells DNA replication occurs in specific nuclear compartments, called replication factories, that undergo complex rearrangements during S-phase. The molecular mechanisms underlying the dynamics of replication factories are still poorly defined. Here we show that etoposide, an anticancer drug that induces double-strand breaks, triggers the redistribution of DNA ligase I and proliferating cell nuclear antigen from replicative patterns and the ensuing dephosphorylation of DNA ligase I. Moreover, etoposide triggers the formation of RPA foci, distinct from replication factories. The effect of etoposide on DNA ligase I localization is prevented by aphidicolin, an inhibitor of DNA replication, and by staurosporine, a protein kinase inhibitor and checkpoints' abrogator. We suggest that dispersal of DNA ligase I is triggered by an intra-S-phase checkpoint activated when replicative forks meet topoisomerase II-DNA–cleavable complexes. However, etoposide treatment of ataxia telangiectasia cells demonstrated that ataxia-telangiectasia-mutated activity is not required for the disassembly of replication factories and the formation of replication protein A foci.