Comparison of manganese oxide nanoparticles and manganese sulfate with regard to oxidative stress, uptake and apoptosis in alveolar epithelial cells

Due to their physicochemical characteristics, metal oxide nanoparticles (NPs) interact differently with cells compared to larger particles or soluble metals. Oxidative stress and cellular metal uptake were quantified in rat type II alveolar epithelial cells in culture exposed to three different NPs: manganese(II,III) oxide nanoparticles (Mn3O4-NPs), the soluble manganese sulfate (Mn-salt) at corresponding equivalent doses, titanium dioxide (TiO2-NPs) and cerium dioxide nanoparticles (CeO2-NPs). In the presence of reactive oxygen species an increased apoptosis rate was hypothesized. Oxidative stress was assessed by detection of fluorescently labeled reactive oxygen species and by measuring intracellular oxidized glutathione. Catalytic activity was determined by measuring catalyst-dependent oxidation of thiols (DTT-assay) in a cell free environment. Inductively coupled plasma mass spectrometry was used to quantify cellular metal uptake. Apoptosis rate was determined assessing the activity of caspase-3 and by fluorescence microscopic quantification of apoptotic nuclei. Reactive oxygen species were mainly generated in cells treated with Mn3O4-NPs. Only Mn3O4-NPs oxidized intracellular glutathione. Catalytic activity could be exclusively shown for Mn3O4-NPs. Cellular metal uptake was similar for all particles, whereas Mn-salt could hardly be detected within the cell. Apoptosis was induced by both, Mn3O4-NPs and Mn-salt. The combination of catalytic activity and capability of passing the cell membrane contributes to the toxicity of Mn3O4-NPs. Apoptosis of samples treated with Mn-salt is triggered by different, potentially extracellular mechanisms.

► Type II alveolar epithelial cells were exposed to Mn3O4-NPs and Mn-salt. ► Mn3O4-NPs in contrast to Mn-salt led to intracellular oxidative stress. ► Intracellular manganese contents were higher upon exposure to Mn3O4-NPs. ► Mn3O4-NPs and Mn-salt induced a similar apoptosis pattern.