Titanium dioxide induces apoptotic cell death through reactive oxygen species-mediated Fas upregulation and Bax activation

Ki-Chun Yoo¹, Chang-Hwan Yoon¹, Dongwook Kwon², Kyung-Hwan Hyun¹, Soo Jung Woo¹, Rae-Kwon Kim¹, Eun-Jung Lim¹, Yongjoon Suh¹, Min-Jung Kim¹, Tae Hyun Yoon², Su-Jae Lee<sup>1

1</sup>Laboratory of Molecular Biochemistry, ²Laboratory of Nanoscale Characterization and Environmental Chemistry, Department of Chemistry, Hanyang University, Seoul, Republic of Korea

Background: Titanium dioxide (TiO₂) has been widely used in many areas, including biomedicine, cosmetics, and environmental engineering. Recently, it has become evident that some TiO₂ particles have a considerable cytotoxic effect in normal human cells. However, the molecular basis for the cytotoxicity of TiO₂ has yet to be defined.
Methods and results: In this study, we demonstrated that combined treatment with TiO₂ nanoparticles sized less than 100 nm and ultraviolet A irradiation induces apoptotic cell death through reactive oxygen species-dependent upregulation of Fas and conformational activation of Bax in normal human cells. Treatment with P25 TiO₂ nanoparticles with a hydrodynamic size distribution centered around 70 nm (TiO₂^P25–70) together with ultraviolet A irradiation-induced caspase-dependent apoptotic cell death, accompanied by transcriptional upregulation of the death receptor, Fas, and conformational activation of Bax. In line with these results, knockdown of either Fas or Bax with specific siRNA significantly inhibited TiO₂-induced apoptotic cell death. Moreover, inhibition of reactive oxygen species with an antioxidant, N-acetyl-L-cysteine, clearly suppressed upregulation of Fas, conformational activation of Bax, and subsequent apoptotic cell death in response to combination treatment using TiO₂^P25–70 and ultraviolet A irradiation.
Conclusion: These results indicate that sub-100 nm sized TiO₂ treatment under ultraviolet A irradiation induces apoptotic cell death through reactive oxygen species-mediated upregulation of the death receptor, Fas, and activation of the preapoptotic protein, Bax. Elucidating the molecular mechanisms by which nanosized particles induce activation of cell death signaling pathways would be critical for the development of prevention strategies to minimize the cytotoxicity of nanomaterials.

Keywords: TiO₂, reactive oxygen species, apoptotic cell death, Fas upregulation, Bax activation, mitochondrial membrane potential loss, caspase activation