The Impact of Titanium Dioxide Nanoparticles on the Lysosome-Autophagy System and Cellular Clearance

Titanium dioxide (TiO₂) is “generally regarded as safe” by the US Food and Drug administration and is found in a wide variety of consumer products, from cosmetics and pharmaceuticals to food additives. Recent studies suggest that the cellular uptake of nano-sized TiO₂ may cause oxidative stress, lysosomal impairment, and inflammation. However, the impact of nano-sized TiO₂ on cell physiology remains unclear. To address this knowledge gap, we evaluated the cellular response to the uptake of anatase TiO₂ nanoparticles (15, 50, and 100 nm in diameter), with particular emphasis on the lysosome-autophagy system, which is the cell’s first response to the uptake of a material perceived as foreign and potentially harmful. Following characterization of the TiO₂ nanoparticles, cytotoxicity analyses were conducted and each of the three TiO₂ nanoparticles were found to induce minimal toxicity at concentrations in the cell culture medium up to 1000 µg/mL. Induction of the lysosome-autophagy system was then tested by monitoring the subcellular location of the transcription factor EB, a master regulator of lysosome- and autophagy-related genes. We found TiO₂ nanoparticles to induce activation of the transcription factor EB at concentrations as low as 10 µg/mL and to induce autophagic vesicle formation. However, uptake of high concentrations of TiO₂ nanoparticles (100 µg/mL and above) was also found to prevent vesicle turnover. We hypothesized that this observed effect on vesicle turnover was due to lysosomal impairment. Therefore, we analyzed lysosomal hydrolase function and membrane stability following TiO₂ nanoparticle uptake and observed a correlation between lysosomal function and integrity and vesicle turnover. In conclusion, we established that cellular uptake of TiO₂ nanoparticles induces activation of the lysosome-autophagy system, and thus upregulation of cellular clearance, but results in a blockage of autophagic flux and degradation via impairment of the lysosome.