Zinc Oxide Nanoparticles Interfere With Zinc Ion Homeostasis to Cause Cytotoxicity

Abstract

The toxicological effects of zinc oxide nanoparticles (ZnO-NPs) are attracting increasing concern as the field of nanotechnology progresses. Although the literature suggests that toxicity of ZnO-NPs may be related to their dissolution, the mechanism for ZnO-NP perturbation of cytosolic zinc concentration ([Zn²⁺]_c) homeostasis remains obscure. Using FluoZin-3 and RhodZin-3, this study investigated changes in both [Zn²⁺]_c and mitochondrial free Zn²⁺ concentration ([Zn²⁺]_m) under conditions of ZnO-NP treatment in vivo and in vitro. In human leukemia Jurkat cells and human lung carcinoma H1355 cells, ZnO-NP treatment resulted in an elevation of both [Zn²⁺]_c and [Zn²⁺]_m. In H1355 cells, ZnO-NP treatment induced depolarization of mitochondrial membrane potential, as well as caspase-3 activation and lactic dehydrogenase (LDH) release. In our in vivo experiments, when rats were exposed to ZnO-NPs, higher [Zn²⁺]_c and [Zn²⁺]_m were recorded in both broncho-alveolar lavage (BAL) cells and white blood cells isolated from ZnO-NP–exposed rats, compared with high efficiency particulate air-filter–protected controls LDH levels were also elevated in the BAL of ZnO-NP–exposed rats compared with controls. A mechanical toxicological pathway for ZnO-NP toxicity is suggested by these results: an elevation in [Zn²⁺]_c resulting from ZnO-NP dissolution in the intracellular endosome; cytosolic Zn²⁺ sequestration by mitochondria; and elevated [Zn²⁺]_m leading to mitochondrial dysfunction, caspase activation, and cell apoptosis. We conclude that exposure to ZnO-NPs interferes with the homeostasis of [Zn²⁺]_c, and that elevated [Zn²⁺]_c results in cell apoptosis.

zinc oxide nanoparticles, zinc ion, cytosolic zinc ion concentration, mitochondrial zinc ion concentration, mitochondria membrane potential, caspase-3