UVA-induced oxidative stress and DNA damage in human skin cells and photoprotection by antioxidant compounds

Solar ultraviolet radiation (UVR), especially UVA (320-400nm), is a potent inducer of oxidative DNA damage in human skin cells. Such damage has mutagenic and carcinogenic potential. It is therefore important to understand the mechanisms that control its extent, and develop means of protecting against such damage. The main purpose of this work was to develop reliable quantitative methods to measure UVR-induced oxidative stress, assess its biological consequences and its inhibition, in human skin cells in vitro. Most of this work studied the effects of UVA radiation, as this is the major (>95%) component of solar UVR and readily generates reactive oxygen species (ROS). Particular focuses of this work were the measurement of UVA-induced ROS, as well as the formation of two of the most intensively studied DNA photolesions, the guanine-derived lesion 8-oxo-7,8-dihydroguanine (8-oxoGua), and the cyclobutane pyrimidine dimer (CPD). These were assessed by an enzyme specific comet assay. Vitamin E was used as a model antioxidant and treatment of HaCaT keratinocytes pre-but also post-irradiation was found to reduce ROS levels, as well as UVA-induced 8-oxoGua and CPD formation. The post-UVR protection offered by vitamin E eliminated any possible sunscreen effects. Real-time PCR data of UVA-induced genes showed that vitamin E inhibited expression of heme oxygenase 1 (HOI) but not matrix metalloproteinase 12 (MMP12). Of three other antioxidants tested, only one (lanosterol) had an inhibitory effect on ROS and 8-oxoGua, but not on CPD formation. Studies with agents that modified ROS levels showed a clear link between ROS and 8-oxoGua but not CPD. Overall, the data showed that antioxidants have the potential to protect against potentially mutagenic epidermal DNA photodamage, an endpoint which, along with gene expression, can be readily applied to the in vivo human situation