DNA-Protein Crosslinking in UV-Irradiated Human and ICR 2A Cell Lines

Abstract

The levels of DNA-protein crosslinks (DPC) were measured using the alkaline elution assay in normal human skin fibroblasts and ICR 2A frog cell lines following UV-irradiation. For experiments utilizing human cells, the cultures were exposed to 0-200 J/m² of 254 nm UV and incubated 0-24 hr. It was found for these cells, that the levels of DPC increased upon incubation. In addition, when the DPC were eliminated by treatment with proteinase K, large numbers of DNA strand breaks (SSB) were revealed. The yields of SSB also increased upon incubation of the UV-irradiated cells, with kinetics similar to that observed for DPC induction. These results are suggestive of a role for a topoisomerase in this crosslinking process. DPC induction was also investigated in ICR 2A cells and two cell lines, DRP 36 and DRP 153, derived from ICR 2A, based upon their sensitivity to the solar UV induction of non-dimer DNA damages. For these experiments, the cells were treated with 150 kJ/m² of sunlamp UV, which was filtered through 48A Mylar to eliminate wavelengths shorter than approximately 315 nm. In addition, the cells were exposed to photoreactivating light to eliminate most of the small yield of dimers produced by this treatment. Upon incubation of the ICR 2A cells, a rapid increase in the level of DPC was detected that reached a maximum within about 2-4 hr following irradiation. In contrast, the enhancement in DPC was much less pronounced, and decreased more rapidly in the two mutant cell lines. These results suggest that this enhancement in DPC may be indicative of a process that plays a role in cellular survival following UV-irradiation.