DNA-protein crosslinking in normal human skin fibroblasts exposed to ultraviolet radiation

Cultured normal human skin fibroblasts were exposed to different fluences of 254 nm UV and the levels of DNA-protein crosslinks (DPC) measured with alkaline elution immediately after irradiation or following a 24-hour incubation (370C). For cells exposed to 10J/m/sup 2/ and then incubated, the level of DPC decreased to that of unexposed cells. When the fluences increased, the levels of DPC measured following a 24-hour incubation increased as compared with non-incubated cells. At fluences higher than 100J/m/sup 2/, the DPC levels of incubated cells exceeded the DPC levels of non-incubated cells. When the single strand breaks (SSB) and double strand breaks (DSB) were measured under a deproteinized condition with alkaline elution and neutral elution, respectively, the levels of SSB and DSB were higher for cells with than for cells without post-irradiation incubation. The simultaneous increase of DPC and proteinase-sensitive SSB and DSB for cells given post-irradiation incubation suggests that a significant part of the DPC observed during post-UV-irradiation incubation were the DNA strand breaks that were tightly associated with proteins. A potential role for type II DNA topoisomerase in the formation of these DPC resulting from either the change in conformational structure caused by the presence of a high level of dimers or an involvement of this enzyme in dimer excision repair will be discussed