The Role of Apoptosis Pathway in the Cytotoxicity Induced by Fresh and Aged Zinc Oxide Nanoparticles

Zinc oxide nanoparticles (ZnO NPs) are used in a wide range of applications including industry, commercial products and medicine field. Numerous mechanistic studies for ZnO NPs' toxicity were performed on pristine (fresh) NPs. However, the cytotoxicity induced by the transformed (aged) ZnO NPs and the underlying mechanisms remain unclear. Here, we observed the physicochemical transformation of ZnO NPs underwent over time, followed by evaluating the cytotoxicity of fresh and aged NPs. We found that fresh ZnO NPs induced higher apoptosis level than their aged counterparts. Accordingly, RNA sequencing data from aged ZnO NP-treated human-hamster hybrid (A_L) cells showed that p53, PI3k-Akt, FoXO, Glutathione, ErbB, HIF-1, Oxytocin and Jak-STAT signaling pathways were enriched but no apoptosis pathway. Quantitative PCR results revealed the significantly higher mRNA level of IL1B and CD69 in fresh NP-treated groups compared to that of aged ZnO NP- and zinc chloride-treated groups. The above results indicated that the lower cytotoxicity of aged ZnO NPs is partially attributed to their reduced potency in inducing apoptosis. The transcriptional regulation of multiple signal pathways activated by aged NPs may help to build the cellular homeostasis. Taken together, our findings highlight the influence of aging (environmental transformation) process of ZnO NPs on their toxicities and biological consequences.