An in vitro investigation of the differential cytotoxic responses of human and rat lung epithelial cell lines using TiO2 nanoparticles
The aim of this study was to compare the cytotoxicity endpoints in two different lung epithelial cell lines following in vitro exposures to well characterised nano (or ultrafine) and fine-sized TiO2 particles and, subsequently, compare them to previously reported in vivo lung toxicity data. Cells in culture (human lung epithelial (A549) or rat lung epithelial cells (L2)) were incubated with doses of 0.8, 8.0, or 16.0 µg/cm² of the following rutile TiO2 particle-types: ultrafine-sized TiO2 particles with an alumina surface coating; ultrafine-sized TiO2 particles with an alumina and amorphous silica surface coating; fine-sized TiO2 particles with an alumina surface coating; or fine-sized TiO2 particles with an alumina and amorphous silica surface coating. Following incubation, the particle-exposed cells were evaluated for cytotoxicity endpoints in the culture media, including lactate dehydrogenase (LDH) release, 1-(4,5-dimethylthiazol-2-yl)-3,5-diphenylformazan (MTT) activity, and changes in micro-total protein at post-incubation time points of 1, 4, 24, and 48 h. Results showed that cytotoxicity responses of human lung epithelial cells were similar, but not identical, to responses of rat lung epithelial cells. Generally, the ultrafine TiO2 particles increased the levels of LDH in the culture media of cells more often than the fine TiO2 particle-types. These in vitro findings were not consistent with results measured in previous in vivo studies in the lungs of rats using the same particle-types. In the in vivo system, neither the fine, nor ultrafine, TiO2 particles produced sustained cytotoxic effects in the lung fluids of rats. Further, results indicate that the 48 h time point in the in vitro time course study does not accurately reflect the cytotoxicological response of these nanoparticle-types. In vitro techniques will need to be further developed and validated. Further, other particle-types will need to be tested in order to provide useful screening data for predicting in vivo pulmonary toxicity for a variety of nanoparticles.