DNA‐PROTEIN CROSSLINKING IN NORMAL HUMAN SKIN FIBROBLASTS EXPOSED TO SOLAR ULTRAVIOLET WAVELENGTHS

Abstract Three normal human skin fibroblast cell lines were exposed to the simulated solar UV radiation produced by a fluorescent sunlamp under conditions in which the wavelength components shorter than either 295, 305 or 315 nm were excluded. The level of DNA‐protein crosslinks (DPC) was then measured in those cells using the alkaline elution technique either immediately after irradiation or following a 24 h incubation. In each case, cells were exposed to fluences that induce similar levels of DPC. For cells exposed to 10 kJ m² of sunlamp UV > 295 nm, the level of DPC exhibited a 2‐5‐fold increase following incubation. In contrast, 40‐100% of the DPC were removed upon incubation of cells irradiated with either 100 kJ m⁻² of sunlamp UV > 305 nm or 150 kJ m⁻² of sunlamp UV > 315 nm. A major difference between the effects induced by these wavelength regions is that, in addition to DPC, a very high level of pyrimidine dimers is also produced by sunlamp UV > 295 nm, whereas much lower dimer yields result from treatment with either sunlamp UV > 305 nm or sunlamp UV > 315 nm. 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 is discussed.