Involvement of TGF-β and ROS in G1 Cell Cycle Arrest Induced by Titanium Dioxide Nanoparticles Under UVA Irradiation in a 3D Spheroid Model

BACKGROUND: As one of the most widely produced engineered nanomaterials, titanium dioxide nanoparticles (nano-TiO(2)) are used in biomedicine and healthcare products, and as implant scaffolds; therefore, the toxic mechanism of nano-TiO(2) has been extensively investigated with a view to guiding application. Three-dimensional (3D) spheroid models can simplify the complex physiological environment and mimic the in vivo architecture of tissues, which is optimal for the assessment of nano-TiO(2) toxicity under ultraviolet A (UVA) irradiation. METHODS AND RESULTS: In the present study, the toxicity of nano-TiO(2) under UVA irradiation was investigated in 3D H22 spheroids cultured in fibrin gels. A significant reduction of approximately 25% in spheroid diameter was observed following treatment with 100 μg/mL nano-TiO(2) under UVA irradiation after seven days of culture. Nano-TiO(2) under UVA irradiation triggered the initiation of the TGF-β/Smad signaling pathway, increasing the expression levels of TGF-β1, Smad3, Cdkn1a, and Cdkn2b at both the mRNA and protein level, which resulted in cell cycle arrest in the G1 phase. In addition, nano-TiO(2) under UVA irradiation also triggered the production of reactive oxygen species (ROS), which were shown to be involved in cell cycle regulation and the induction of TGF-β1 expression. CONCLUSION: Nano-TiO(2) under UVA irradiation induced cell cycle arrest in the G1 phase and the formation of smaller spheroids, which were associated with TGF-β/Smad signaling pathway activation and ROS generation. These results reveal the toxic mechanism of nano-TiO(2) under UVA irradiation, providing the possibility for 3D spheroid models to be used in nanotoxicology studies.