Ultraviolet-B-Induced Oxidative DNA Base Damage in Primary Normal Human Epidermal Keratinocytes and Inhibition by a Hydroxyl Radical Scavenger

To evaluate the effects of ultraviolet-induced environmental trauma on human skin cells, primary normal human epidermal keratinocytes were exposed to ultraviolet-B radiation (290–320 nm). We found that relatively low doses of ultraviolet-B (62.5–500 mJ per cm2) caused dose-dependent increases in 8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxo-dG), a biomarker of oxidative DNA damage. Unirradiated normal human epidermal keratinocytes contained 1.49 (± 0.11) 8-oxo-dG per 106 2′-deoxyguanosine (dG) residues in cellular DNA, which increased linearly to as high as 6.24 (± 0.85) 8-oxo-dG per 106 dG after irradiation with 500 mJ per cm2. Further, this oxidative damage was reduced by 60.7% when the cells were pretreated with 1 mM mannitol. As hydrogen peroxide (H2O2) is known to be generated during oxidative stress, its accumulation in ultraviolet-B-irradiated normal human epidermal keratinocytes was also assessed and correlated to 8-oxo-dG formation. An ultraviolet-B-induced increase in H2O2 was observed in normal human epidermal keratinocytes and its production was inhibited by the addition of catalase. Based on the ability of a neutral molecule like H2O2 to permeate membranes, our data indicate that, after ultraviolet-B irradiation, H2O2 migrates from the cytosol to the nucleus where it participates in a Fenton-like reaction that results in the production of hydroxyl radicals (OH·), which may then cause 8-oxo-dG formation in cellular DNA. This conclusion is supported by our data showing that OH· scavengers, such as mannitol, are effective inhibitors of oxidative DNA base damage. Although increased levels of 8-oxo-dG were previously found in immortalized mouse keratinocytes exposed to ultraviolet-B radiation, we now report the induction of 8-oxo-dG in normal human skin keratinocytes at ultraviolet-B doses relevant to human skin exposure.