Phototoxicity of zinc oxide nanoparticles in HaCaT keratinocytes-generation of oxidative DNA damage during UVA and visible light irradiation.

Zinc oxide nanoparticles (nano-ZnO) are one of the most commonly used nanomaterials in industrial products including paints, cosmetics, and medical materials. Since ZnO is a well-known photocatalyst, it is important to further study if nano-ZnO cause phototoxic effect on skin cells under UVA-irradiation and visible light illumination. Human-derived keratinocytes (HaCaT) were treated with 1-20 microg/mL of nano-ZnO (< 50 nm) and then exposed to UVA (0.5-2 J/cm2). Twenty four hours later, cell viability, membrane integrity, and oxidative DNA damage were determined by MTS assay, lactate dehydrogenase (LDH) release, and the formation of 8-hydroxy-2'-deoxyguanosine (8-OHdG) adduct, respectively. High concentration of nano-ZnO (10-20 microg/mL) significantly induced cytotoxicity, whereas 0.5-2 J/cm2 of UVA irradiation dose-dependently aggravated nano-ZnO-induced cell death via induction of LDH release and DNA damage. The level of photocytotoxicity is mainly dependent on the level of reactive oxygen species (ROS) production. UVA irradiation of nano-ZnO in methanol induced lipid peroxidation in a light dose and substrate dose response manner. Electron spin resonance (ESR) spin trapping studies confirmed that both hydroxyl radical and superoxide anion radical were formed during photoirradiation, while nano-ZnO-induced hydroxyl radical formation is not evolved from superoxide. In addition, nano-ZnO dose-dependently induced single strand DNA break in supercoiled phi x 174 plasmid DNA. Under visible light illumination, nano-ZnO induced the LDH leakage, hydroxyl radical generation, and 8-OHdG formation in a dose-dependent manner. Collectively, these results suggest the photocytotoxic and photogenotoxic effects of nano-ZnO on human skin keratinocytes.