Role of autophagy in response to titanium dioxide nanoparticles - 04/04/15

The development and the increasing production of manufactured nanoparticles (NP) are raising safety concerns because of the potential effects of NP on human health, particularly at the respiratory level. Several studies have shown that exposure to manufactured NP can induce pathogenic biological effects, with lung remodelling (fibrosis, emphysema…), depending on the physicochemical characteristics of the NP. Currently, oxidative stress and inflammation are the most widely accepted paradigms of NP toxicity; however, the exact underlying mechanisms in the biological effects of NP still remain unknown. Autophagy is a physiological process that allows the autodigestion of the subcellar components and which is also involved in the elimination of intracellular pathogens. It has been shown that this process can negatively regulate inflammation and oxidative stress. Thus, the hypothesis of this study is that a defective autophagy could be a new mechanism explaining, at least in part, NP effects.

We choose to focus on TiO₂ NP since it is one of the most abundantly produced and widely used NP.

We used seven TiO₂ NPs presenting different physicochemicals properties (size, crystal phase, surface embedding) and compared their effects to those of micron-size TiO₂ and carbon black (CB - for chemical composition effects). NPs were characterized by electron microscopy, dynamic light scattering and X-ray diffraction. Murine macrophages (RAW cell line) were exposed to 50μg/mL TiO₂ and CB NP for 6hours. Effects of NP on the autophagy process were analysed by looking at the expression of autophagy markers (LC3-II and p62) and lysosomal proteins (LAMPs and cathepsins). We also analysed the cytoskeleton network by fluorescence microscopy. Moreover, expression of inflammatory cytokines was determined in macrophages exposed to NP.

All particles, except the micrometric one, induced an increase of LC3-II protein expression, indicating an accumulation of autophagosomes. These particles also increased p62 protein level, suggesting an autophagy blockade. This perturbation of the autophagy process by NP does not seem to result from a disruption of the cytoskeleton but from a defect in the lysosome function as suggested by a decrease of mature cathepsins expression. Moreover, preliminary results showed that exposure to TiO₂ NP induce inflammation, at different levels depending on their physicochemicals characteristics.

These results suggest that some TiO₂ NP, depending on their physicochemical properties, can block the autophagy process. The future work will be to understand the mechanisms explaining this autophagy dysfunction and to determine its consequence on the toxicity induced by these NP.