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.

Background: Tumor development and progression is a long and complex process influenced by a combination of intrinsic (eg, gene mutation) and extrinsic (eg, environmental pollution) factors. As a detoxification organ, the liver plays an important role in human exposure and response to various environmental pollutants including nanomaterials (NMs). Hepatocellular carcinoma (HCC) is one of the most common malignant tumors and remains a serious threat to human health. Whether NMs promote liver cancer progression remains elusive and assessing long-term exposure to subtoxic doses of nanoparticles (NPs) remains a challenge. In this study, we focused on the promotional effects of nano zinc oxide (nZnO) on the malignant progression of human HCC cells HepG2, especially aged nZnO that has undergone physicochemical transformation. Methods: In in vitro experiments, we performed colony forming efficiency, soft agar colony formation, and cell migration/invasion assays on HepG2 cells that had been exposed to a low dose of nZnO (1.5 μg/mL) for 3 or 4 months. In in vivo experiments, we subcutaneously inoculated HepG2 cells that had undergone long-term exposure to nZnO for 4 months into BALB/c athymic nude mice and observed tumor formation. ZnCl₂ was administered to determine the role of zinc ions. Results: Chronic low-dose exposure to nZnO significantly intensified the malignant progression of HCC cells, whereas aged nZnO may exacerbate the severity of malignant progression. Furthermore, through transcriptome sequencing analysis and in vitro cellular rescue experiments, we demonstrated that the mechanism of nZnO-induced malignant progression of HCC could be linked to the activation of Claudin-2 (CLDN2), one of the components of cellular tight junctions, and the dysregulation of its downstream signaling pathways. Conclusion: Long-term exposure of fresh and aged nZnO promotes hepatocellular carcinoma malignancy by up-regulating CLDN2. The implications of this work can be profound for cancer patients, as the use of various nanoproducts and unintentional exposure to environmentally transformed NMs may unknowingly hasten the progression of their cancers.