Abstract
Zinc oxide (ZnO) nanoparticles (NPs) are being widely utilized in industry due to their versatile properties. The in vitro cytotoxicity findings and the potential for exposures to ZnO NP from different sources via different routes of entry into the body have raised public health concerns. Although recent studies have shown the cytotoxic effects of these NPs, including oxidative stress, apoptosis and necrosis induction, genotoxicity, and others, irradiation-induced cytotoxicity has not been systematically studied. The goal of this study was to determine whether irradiation in the forms of visible light (approximately 400-600 nm), ultraviolet (UV) A (366 nm), and UVC (254 nm) would affect ZnO NPs-induced cytotoxicity. The results of this study demonstrated that the cytotoxicity of 60 to 80 nm ZnO NPs to A549 cells is dosage, time, and wavelength dependent. Nuclear decomposition by ZnO NPs, prior to membrane deformation, was found to be enhanced when exposed to irradiation. This study suggests that this phenomenon may be attributed to the irradiation-induced formation of positively charged sites on the ZnO NPs, which enhances nuclear affinity and generation of reactive oxygen species. Finally, the data demonstrated that while ZnO NPs act preferentially toward nuclear regions, destructions of cell membrane and the cytosol have also been observed. The photocatalytic properties of ZnO NPs play a critical role during the early stages of cell death, and their effects were reduced through the use of an antioxidant, N-acetylcysteine. In conclusion, both visible light and UV irradiations have been found to enhance the cytotoxic effect of ZnO NPs on the A549 cell line. This finding supports the need for further in vivo exposure studies relevant to actual conditions to confirm whether combined irradiation and ZnO NP exposure could enhance the nanotoxicity of ZnO NPs.