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.