The involvement of topoisomerase I in the induction of DNA-protein crosslinks and DNA single-strand breaks in cells of ultraviolet-irradiated human and frog cell lines.

Exposure of GM 4390 human skin fibroblasts and ICR 2A frog cells to 10 kJ m(-2) of ultraviolet B (UVB) radiation resulted in the formation of DNA-protein crosslinks (DPCs) and DNA single-strand breaks (SSBs). However, upon incubation, there were rapid increases in the yields of both DPCs and SSBs. An enhancement in these DNA alterations was detected within 12 min after irradiation and their levels continued to rise by 5-8-fold within 15 h after exposure to UV radiation. Using an antibody-based assay that measures covalent complex formation between topoisomerase (topo) I and genomic DNA, it was found that topo I is one of the proteins involved in these DPCs induced by UV radiation. The levels and rate of increase of topo I-DNA covalent complexes were similar to the UV-radiation-dependent formation of DPCs and SSBs. A UV-radiation-sensitive mutant frog cell line, DRP 153, was also examined and was found to be deficient in this induction of DPCs and SSBs by UV radiation. When these cells were transfected with the human SUVCC3 gene, the resulting transformant displayed kinetics for the induction of DPCs and SSBs similar to the human and parental frog cells. However, human topo I was not defected in the transformed frog cells, indicating that SUVCC3 does not encode topo I. It is likely that SUVCC3 encodes an associated enzymatic activity which permits normal stimulation of topo I-DNA covalent complexes in UV-irradiated cells.