Nanotoxicology on the right track : focus on metal and metal oxide nanoparticles

Abstract

The last decade has seen a rapid increase in the manufacture and use of nanomaterials, a development which should be met with appropriate safety assessment strategies in order to ensure the sustainable development of nanotechnology. With decreasing size, the percentage of atoms found at the surface of a given material increases substantially, leading to an increase in surface phenomena and acquisition of novel properties. These new traits can be appealing for industrial purposes, however, they can also enhance the intrinsic toxicity of the materials as compared to their bulk counterparts. Currently, nanotoxicology faces several challenges related to the multitude of materials that need to be tested, the possible interactions of the nanomaterials with the conventional toxicology assays and the potential emergence of novel nano-specific properties. Despite numerous research efforts being made in the last decade to evaluate the toxicity of nanomaterials, most of these studies fall short of several aspects, such as appropriate particle characterization, cellular uptake, relevant doses and exposure duration.

The aim of this thesis was to use in vitro models to address some of the challenges in nanotoxicology in order to improve our understanding of the interactions between nanomaterials and biological systems.

In Paper I we demonstrated that we can use the ToxTracker assay, which consists of reporter stem cells, to screen and predict the genotoxicity of metal oxide nanoparticles and at the same time obtain information about their mechanism of toxicity. In Paper II we used a panel of thoroughly characterized silver nanoparticles to address the issue of size-dependent toxicity in human lung cells. Our results showed that small (10 nm) particles were more cytotoxic than larger particles (˃40 nm) after acute exposure (24 hours), and that could be related to a ‘Trojan horse’ effect by which the particulate form facilitates the cellular uptake of metal, with subsequent release of toxic metal ions. In Paper III we selected two of the silver nanoparticles tested in Paper II and evaluated the effects following low-dose, long-term (6 week) exposure to human lung cells. By using both conventional assays and systems toxicology approaches (RNA-sequencing, genome wide DNA-methylation) we identified that chronic exposure to low doses of silver nanoparticles induced a cancer-like phenotype and had immunosuppressive effects in human lung cells. In Paper IV we explored the effects of antioxidant cerium oxide nanoparticles, which allegedly have promising therapeutic potential, in neural stem cells. On one hand, we showed that pretreatment with cerium oxide nanoparticles provided a temporary neuroprotective effect when cells were challenged with an oxidative stress inducer. On the other hand, by using both immunofluorescence and RNA-sequencing we revealed that the same antioxidant properties can have detrimental effects by suppressing neuronal differentiation, in which reactive oxygen species play an important role as signaling molecules.

In all, our studies show that by using well-characterized nanomaterials together with appropriate experimental setups, and a combination of traditional toxicological assays with novel tools such as ‘omics’, we can improve our understanding of the toxicity of nanomaterials and by these means contribute to the sustainable development of nanotechnology.