2002 Volume 48 Issue 3 Pages 296-301
Some DNA topoisomerase inhibitors induce cleavage of DNA strands and lead to cell death. Therefore the inhibitors are utilized for chemotherapy against cancer. The treatment, however, can cause severe side effects such as genetic modification of normal cells. Camptothecin (CPT) and etoposide (VP16) belong to this class of anticancer agents that inhibit DNA topoisomerase I and II, respectively. In the present study, we investigated whether treatment with CPT and VP16 induces DNA breaks in nuclei reconstituted in Xenopus egg extract by observing focus formation of replication protein A (RPA), which is thought to accumulate on loci of DNA lesions prior to DNA repair. Treatment with CPT caused formation of discrete RPA foci at a later stage of incubation of demembranated Xenopus sperm nuclei with egg extracts. The RPA foci increased with the amount of CPT. However, at 30 min, before or just starting the initiation of DNA replication, the discrete RPA foci in CPT-containing extract were not obvious, suggesting that DNA replication is necessary for the focus formation induced by CPT. On the other hand, discrete RPA foci were induced by VP16 even at 30 min.
