Titanium Dioxide (TiO₂) Nanoparticles Induce JB6 Cell Apoptosis Through Activation of the Caspase-8/Bid and Mitochondrial Pathways

Abstract

Titanium dioxide (TiO₂), a commercially important material, is used in a wide variety of products. Although TiO₂ is generally regarded as nontoxic, the cytotoxicity, pathogenicity, and carcinogenicity of TiO₂ nanoparticles have been recently recognized. The present study investigated TiO₂ nanoparticle-induced cell apoptosis and molecular mechanisms involved in this process in a mouse epidermal (JB6) cell line. Using the 3-(4,5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide (MTT) assay, TiO₂ nanoparticles were found to exhibit higher cytotoxicity than fine particles. YO-PRO-1 iodide (YP) staining demonstrated that both TiO₂ nanoparticles and fine particles induced cell death through apoptosis. The signaling pathways involved in TiO₂ particle-induced apoptosis were investigated. Western-blot analysis showed an activation of caspase-8, Bid, BAX, and caspase-3 and a decrease of Bcl-2 in JB6 cells treated with TiO₂ particles. Time-dependent poly(ADP)ribose polymerase (PARP) cleavage induced by TiO₂ nanoparticles was observed. TiO₂ particles also induced cytochrome c release from mitochondria to cytosol. Further studies demonstrated that TiO₂ nanoparticles induced significant changes in mitochondrial membrane permeability, suggesting the involvement of mitochondria in the apoptotic process. In conclusion, evidence indicated that TiO₂ nanoparticles exhibit higher cytotoxicity and apoptotic induction compared to fine particles in JB6 cells. Caspase-8/Bid and mitochondrial signaling may play a major role in TiO₂ nanoparticle-induced apoptosis involving the intrinsic mitochondrial pathway. Unraveling the complex mechanisms associated with these events may provide further insights into TiO₂ nanoparticle-induced pathogenicity and potential to induce carcinogenicity.

Acknowledgements

The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the National Institute for Occupational Safety and Health. We thank Dr. Shengqiao Li for statistical analysis, Diane Schwegler-Berry and Sherri Friend for image analysis, and Donna Pack for surface area analysis.