This review summarizes the literature on mammalian toxicity of ZnO nanoparticles (NPs) published between 2009 and 2011. The toxic effects of ZnO NPs are due to the compound's solubility. Whether the increased intracellular [Zn(2+)] is due to the NPs being taken up by cells or to NP dissolution in medium is still unclear. In vivo airway exposure poses an important hazard. Inhalation or instillation of the NPs results in lung inflammation and systemic toxicity. Reactive oxygen species (ROS) generation likely plays an important role in the inflammatory response. The NPs do not, or only to a minimal extent, cross the skin; this also holds for sunburned skin. Intraperitoneal administration induces neurological effects. The NPs show systemic distribution; target organs are liver, spleen, lung, and kidney and, in some cases, the heart. In vitro exposure of BEAS-2B bronchial epithelial cells and A549 alveolar adenocarcinoma cells results in cytotoxicity, increased oxidative stress, increased intracellular [Ca(2+)], decreased mitochondrial membrane potential, and interleukin (IL)-8 production. Decreased contractility of airway smooth muscle cells poses an additional hazard. In contrast to the results for BEAS-2B and A549 cells, in RKO colon carcinoma cells ZnO NPs and not Zn(2+) induce cytotoxicity and mitochondrial dysfunction. Short-term exposure of skin cells results in apoptosis but not in an inflammatory response, while long-term exposure leads to increased ROS generation, decreased mitochondrial activity, and formation of tubular intercellular structures. Macrophages, monocytes, and dendritic cells are affected; exposure results in cytotoxicity, oxidative stress, intracellular Ca(2+) flux, decreased mitochondrial membrane potential, and production of IL-1β and chemokine CXCL9. The NPs are phagocytosed by macrophages and dissolved in lysosomes. In vitro the Comet assay and the cytokinesis-blocked micronucleus assay show genotoxicity, whereas the Ames test does not. This is, however, not confirmed by in vivo genotoxicity assays. Protein binding results in increased stability.
Keywords: inflammation; intracellular calcium; lysosomes; mitochondrial membrane potential; reactive oxygen species; solubility.