Subcellular Targets of Zinc Oxide Nanoparticles During the Aging Process: Role of Cross-talk Between Mitochondrial Dysfunction and Endoplasmic Reticulum Stress in the Genotoxic Response

Zinc oxide nanoparticles (ZnO NPs) are being produced abundantly and applied increasingly in various fields. The special physicochemical characteristics of ZnO NPs make them incline to undergo physicochemical transformation over time (aging), which modify their bioavailability and toxicity. However, the subcellular targets and the underlying molecular mechanisms involved in the genotoxicity induced by ZnO NPs during aging process are still unknown. This study found that the acute cytotoxic effects of fresh ZnO NPs was largely regulated by mitochondria-dependent apoptosis, which the level of cleaved Caspase-3 and mitochondria damage were significantly higher than that of 60-day-aged ZnO NPs. In contrast, aged ZnO NPs induced more reactive oxygen species (ROS) production and endoplasmic reticulum (ER) stress marker protein (BIP/GRP78) expression and their genotoxicity could be dramatically suppressed by either ROS scavengers (dimethyl sulfoxide, catalase, and sodium azide) or ER stress inhibitor (4-phenylbutyrate). Using mitochondrial-DNA deficient (rho(0)) A(L) cells, we further found that ER stress induced by aged ZnO NPs was triggered by ROS generated from mitochondria, which eventually mediated the genotoxicity of aged NPs. Our data provided novel information on better understanding the contribution of subcellular targets to the genotoxic response of ZnO NPs during the aging process.