Ultrafine silicon dioxide nanoparticles cause lung epithelial cells apoptosis via oxidative stress-activated PI3K/Akt-mediated mitochondria- and endoplasmic reticulum stress-dependent signaling pathways

Silicon dioxide nanoparticles (SiO 2 NPs) are widely applied in industry, chemical, and cosmetics. SiO 2 NPs is known to induce pulmonary toxicity. In this study, we investigated the molecular mechanisms of SiO 2 NPs on pulmonary toxicity using a lung alveolar epithelial cell (L2) model. SiO 2 NPs, which primary particle size was 12 nm, caused the accumulation of intracellular Si, the decrease in cell viability, and the decrease in mRNAs expression of surfactant, including surfactant protein (SP)-A, SP-B, SP-C, and SP-D. SiO 2 NPs induced the L2 cell apoptosis. The increases in annexin V fluorescence, caspase-3 activity, and protein expression of cleaved-poly (ADP-ribose) polymerase (PARP), cleaved-caspase-9, and cleaved-caspase-7 were observed. The SiO 2 NPs induced caspase-3 activity was reversed by pretreatment of caspase-3 inhibitor Z-DEVD-FMK. SiO 2 NPs exposure increased reactive oxygen species (ROS) production, decreased mitochondrial transmembrane potential, and decreased protein and mRNA expression of Bcl-2 in L2 cells. SiO 2 NPs increased protein expression of cytosolic cytochrome c and Bax, and mRNAs expression of Bid, Bak, and Bax. SiO 2 NPs could induce the endoplasmic reticulum (ER) stress-related signals, including the increase in CHOP, XBP-1, and phospho-eIF2α protein expressions, and the decrease in pro-caspase-12 protein expression. SiO 2 NPs increased phosphoinositide 3-kinase (PI3K) activity and AKT phosphorylation. Both ROS inhibitor N -acetyl-l-cysteine (NAC) and PI3K inhibitor LY294002 reversed SiO 2 NPs-induced signals described above. However, the LY294002 could not inhibit SiO 2 NPs-induced ROS generation. These findings demonstrated first time that SiO 2 NPs induced L2 cell apoptosis through ROS-regulated PI3K/AKT signaling and its downstream mitochondria- and ER stress-dependent signaling pathways.