Does photocatalytic activity of TiO₂ nanoparticles correspond to photo-cytotoxicity? Cellular uptake of TiO₂ nanoparticles is important in their photo-cytotoxicity

Abstract

Titanium dioxide (TiO₂) nanoparticles are important industrial nano-objects with wide applications, including as photocatalysts and sunscreen components. Recently, the phototoxicity of TiO₂ nanoparticles has been a concern. However, phototoxicity caused by photocatalytic activity may differ between anatase and rutile nanoparticles. In the present study, we compared the phototoxicity of anatase and rutile nanoparticles. Human keratinocyte HaCaT cells were treated with stable TiO₂ nanoparticle suspensions. Without UVA irradiation, TiO₂ nanoparticles did not affect mitochondrial activity or cell membranes. However, exposure to rutile nanoparticle suspensions inhibited cell growth and induced HO-1 gene expression without UVA irradiation. These effects may be explained by the hydrophobic surface of rutile nanoparticles. Next, TiO₂-exposed cells were irradiated with UVA for 4 h and effects of TiO₂ nanoparticles on cells were examined. The rutile nanoparticles did not show any cellular effects after UVA irradiation. However, the anatase nanoparticles caused strong phototoxicity. Decreased mitochondrial activity, cell membrane damage and the induction of oxidative stress were observed in the cells exposed to anatase nanoparticles with UVA irradiation. Cellular uptake of the nanoparticles was observed in both anatase- and rutile-exposed cells. These results suggest that internalized anatase nanoparticles are important for phototoxicity. Additionally, the exposure of a 3D skin model to TiO₂ nanoparticles did not result in significant toxicity. In conclusion, rutile nanoparticles used in sunscreen did not exhibit phototoxic activity. Despite the strong phototoxic activity of anatase nanoparticles in cell cultures, they demonstrated no phototoxicity using a 3D skin model.

Keywords:

• Nanoparticle
• oxidative stress
• phototoxicity
• sunscreen
• titanium dioxide

Acknowledgements

We thank Mr. Yoshimasa Kamata (Tayca Corporation) for excellent advice for application of TiO₂ nanoparticles.

Disclosure statement

The authors report no conflicts of interest.