Abstract
Silver nanoparticles (AgNPs) and titanium dioxide nanoparticles (TiO2NPs) are two of the most commonly used nanomaterials in consumer products. However, less is known about their hazards especially effects on central nervous systems. ALT astrocyte-like, BV-2 microglia and differentiated N2a neuroblastoma cells were first direct exposed to 9 nm AgNPs, 6 nm TiO2NPs. We found that ALT take up more NPs than the other two cells, and had highest cytotoxicity. AgNPs did not easily induce cytokines from the three cells, but emitted ROS and NO from ALT and BV-2, respectively. TiO2NPs induced high levels of IL-1β in the three cell lines. LPS-activated BV-2 took up more NPs than normal BV-2, while causing cell proliferation and inhibition of ROS, MCP-1, IL-6 and TNF-α to AgNPs, or synergistic induction of ROS, IL-1β, IL-6 and MCP-1 to TiO2 NPs. Ca2+-regulated clathrin- and caveolae-independent endocytosis and phagocytosis were involved in the AgNP uptake in ALT, while clathrin- and caveolae-dependent endocytosis were involved in TiO2NP uptake. This led to more rapid translocation to ALT lysosome for AgNPs than TiO2NPs, which may explain the rapid ROS production in ALT when exposing to AgNPs. Though NPs did not directly lead to cytotoxicity of neuron-like cells after 24 h of exposure, by indirect NP exposure to bottom chamber of ALT/BV-2 in Transwell system, they caused late apoptosis of upper chamber neuron-like cells. The apoptosis of cells correlated to NP-mediated releasing of ROS from astrocytes and/or of RNS and TNF-α from microglia in normal and activated state. Finally, the Trojan-horse mechanism in the cellular uptake of AgNPs was verified. The ratio of AgNPs to total Ag (AgNPs+Ag (I)) in glia cells can be determined by rate of AgNP and Ag ion uptake and production of H2O2 which reacts with AgNPs to form more Ag (I). However, acid lysosomal mimic solutions did not promote AgNP dissolution, which suggested that AgNPs in acid environment of lysosome did not affect ratio of AgNP to total Ag in cells. Using X-ray absorption near edge structure (XANES), the major speciation of Ag (I) in cells was Ag(cysteine) and Ag(cysteine)2, indicating the possible binding of monomer cysteine or vital thiol proteins/peptides to Ag ions.