Camptothecin induces protein-linked DNA breaks via mammalian DNA topoisomerase I.

TLDR

Camptothecin is a cytotoxic agent that strongly inhibits nucleic acid synthesis and induces chromosomal DNA strand breaks in mammalian cells. The authors propose that camptothecin blocks the rejoining step of the breakage‑reunion reaction of mammalian DNA topoisomerase I. This blockage causes accumulation of a cleavable complex resembling the transient intermediate of eukaryotic DNA topoisomerase I. Camptothecin does not bind DNA directly but induces immediate, reversible single‑strand breaks by forming a covalent cleavable complex with topoisomerase I, which also inhibits its catalytic activity and likely underlies its cytotoxic effects.

Abstract

Camptothecin, a cytotoxic drug, is a strong inhibitor of nucleic acid synthesis in mammalian cells and a potent inducer of strand breaks in chromosomal DNA. Neither the equilibrium dialysis nor the unwinding measurement indicates any interaction between camptothecin and purified DNA. However, camptothecin induces extensive single strand DNA breaks in reactions containing purified mammalian DNA topoisomerase I. DNA breakage in vitro is immediate and reversible. Analyses of camptothecin-induced DNA breaks show that topoisomerase I is covalently linked to the 3' end of the broken DNA. In addition, camptothecin inhibits the catalytic activity of mammalian DNA topoisomerase I. We propose that camptothecin blocks the rejoining step of the breakage-reunion reaction of mammalian DNA topoisomerase I. This blockage results in the accumulation of a cleavable complex which resembles the transient intermediate proposed for eukaryotic DNA topoisomerase I. The inhibition of nucleic acid synthesis and the induction of DNA strand breaks observed in vivo may be related to the formation of this drug-induced cleavable complex.