TiO2 nanoparticles cause mitochondrial dysfunction, activate inflammatory responses, and attenuate phagocytosis in macrophages: A proteomic and metabolomic insight

Titanium dioxide nanoparticles (TiO2 NPs) are widely used in food and cosmetics but the health impact of human exposure remains poorly defined. Emerging evidence suggests that TiO2 NPs may elicit immune responses by acting on macrophages. Our proteomic study showed that treatment of macrophages with TiO2 NPs led to significant re-organization of cell membrane and activation of inflammation. These observations were further corroborated with transmission electron microscopy (TEM) experiments, which demonstrated that TiO2 NPs were trapped inside of multi-vesicular bodies (MVB) through endocytotic pathways. TiO2 NP caused significant mitochondrial dysfunction by increasing levels of mitochondrial reactive oxygen species (ROS), decreasing ATP generation, and decreasing metabolic flux in tricarboxylic acid (TCA) cycle from C-13-labelled glutamine using GC-MS-based metabolic flux analysis. Further lipidomic analysis showed that TiO2 NPs significantly decreased levels of cardiolipins, an important class of mitochondria] phospholipids for maintaining proper function of electron transport chains. Furthermore, TiO2 NP exposure activates inflammatory responses by increasing mRNA levels of TNF-alpha, iNOS, and COX-2. Consistently, our targeted metabolomic analysis showed significantly increased production of COX-2 metabolites including PGD(2), PGE(2), and 15d-PGJ(2). In addition, TiO2 NP also caused significant attenuation of phagocytotic function of macrophages. In summary, our studies utilizing multiple powerful omit techniques suggest that human exposure of TiO2 NPs may have profound impact on macrophage function through activating inflammatory responses and causing mitochondrial dysfunction without physical presence in mitochondria.