PM2.5 induces Nrf2-mediated defense mechanisms against oxidative stress by activating PIK3/AKT signaling pathway in human lung alveolar epithelial A549 cells

It has been well documented in in vitro studies that ambient airborne particulate matter (PM) with an aerodynamic diameter less than 2.5 mu m (PM2.5) is capable of inducing oxidative stress, which plays a key role in PM2.5-mediated cytotoxicity. Although nuclear factor erythroid-2-related factor 2 (Nrf2) has been shown to regulate the intracellular defense mechanisms against oxidative stress, a potential of the Nrf2-mediated cellular defense against oxidative stress induced by PM2.5 remains to be determined. This study was aimed to explore the potential signaling pathway of Nrf2-mediated defense mechanisms against PM2.5-induced oxidative stress in human type II alveolar epithelial A549 cells. We exposed A549 cells to PM2.5 particles collected from Beijing at a concentration of 16 mu g/cm(2). We observed that PM2.5 triggered an increase of intracellular reactive oxygen species (ROS) in a time-dependent manner during a period of 2 h exposure. We also found that Nrf2 overexpression suppressed and Nrf2 knockdown increased PM2.5-induced ROS generation. Using Western blot and confocal microscopy, we found that PM2.5 exposure triggered significant translocation of Nrf2 into nucleus, resulting in AKT phosphorylation and significant transcription of ARE-driven phases II enzyme genes, such as NAD(P)H:quinone oxidoreductase (NQO-1), heme oxygenase-1 (HO-1), and glutamate-cysteine ligase catalytic subunit (GCLC) in A549 cells. Evaluation of signaling pathways showed that a phosphatidylinositol 3-kinase (PI3K) inhibitor (LY294002), but not an ERK 1/2 inhibitor (PD98059) or a p38 MAPK (SB203580), significantly down-regulated PM2.5-induced Nrf2 nuclear translocation and HO-1 mRNA expression, indicating PI3K/AKT is involved in the signaling pathway leads to the PM2.5-induced nuclear translocation of Nrf2 and subsequent Nrf2-mediated HO-1 transcription. Taken together, our results suggest that PM2.5-induced ROS may function as signaling molecules to activate Nrf2-mediated defenses, such as HO-1 expression, against oxidative stress induced by PM2.5 through the PI3K/AKT signaling pathway.