Reactive Oxygen Species-Activated Akt/ASK1/p38 Signaling Pathway in Nickel Compound-Induced Apoptosis in BEAS 2B Cells

Nickel compounds are carcinogenic to humans, possibly through induction of reactive oxygen species (ROS) that damage macromolecules including DNA and proteins. The aim of the present study is to elucidate the role of the ROS-mediated Akt/apoptosis-regulating signal kinase (ASK) 1/p38 pathway in nickel-induced apoptosis. Exposure of human bronchial epithelial cells (BEAS-2B) to nickel compounds induced the generation of ROS and activation of Akt that is associated with the activation of ASK1 and p38 mitogen-activated protein kinase. Immunoblotting suggested a down-regulation of several antiapoptotic proteins, including Bcl-2 and Bcl-xL in the nickel compound-treated cells. Indeed, a notable cell apoptosis following nickel compound treatment is evident as revealed by flow cytometry analysis. N-Acetyl-L-cysteine (NAC, a general antioxidant) and vitamin E or catalase (a specific H2O2 inhibitor) all decreased nickel-induced ROS generation. Scavenging of nickel-induced ROS by NAC or catalase attenuated Akt, ASK1, and p38 MAPK activation and apoptosis, which implies involvement of ROS in the Akt/ASK1/p38 pathway. In addition, nickel-induced activation of p38 MAPK was attenuated by a small interference of RNA specific to ASK1 (siRNA ASK1), implying that p38 MAPK was downstream of ASK1, while ASK1 activation was not reversely regulated by the inhibition of p38 MAPK by SB203580, a widely used p38 MAPK inhibitor. Silencing Akt by siRNA reduced the activation of ASK I and p38 MAPK and cell apoptosis, whereas without nickel stimulation, siRNA Akt had no effect on the activation of ASK1 and p38 MAPK. Thus, these results suggest that the ROS-dependent Akt-ASK1-p38 axis is important for nickel-induced apoptosis.