Intracellular Generation of Superoxide by TiO2 Nanoparticles Decreases Histone Deacetylase 9 (HDAC9), an Epigenetic Modifier.

Titanium dioxide (TiO2) nanoparticles are used on a massive scale in commercial and industrial products. Of specific concern is how the inhalation of these nanoparticles in a manufacturing setting may affect human health. We examine the cellular response to TiO2 nanoparticles using a combination of cell-free spectroscopic assays, fluorescence microscopy, Western blotting, and TiO2 nanoparticle surface modifications. These experiments show that TiO2 nanoparticles generate superoxide, both in solution and in cells, and this intracellular superoxide decreases expression of histone deacetylase 9 (HDAC9), an epigenetic modifier. We use protein coronas formed from superoxide dismutase (SOD) and catalase, enzymes that scavenge reactive oxygen species (ROS), to probe the relationship between TiO2 nanoparticles, ROS, and the subsequent cellular response. These protein coronas provide nanoparticle-localized scavengers that demonstrate that the nanoparticles are the source of the intracellular superoxide. Importantly, the use of a SOD corona or surface passivated TiO2 nanoparticles prevents the decrease of HDAC9. These experiments elucidate the underlying mechanism of TiO2 nanoparticle-mediated cellular responses including oxidative stress and changes in gene expression. They also provide the first demonstration of a protein corona as a tool for probing cellular responses to nanoparticles. Overall, this research shows that low, nontoxic concentrations of TiO2 nanoparticles alter an enzyme responsible for epigenetic modifications, which points to concerns regarding long-term exposures in manufacturing settings.