NADPH oxidase and ERK1/2 are involved in cadmium induced-STAT3 activation in HepG2 cells

The molecular mechanism of Cd-induced signal transduction is not well understood. The aims of this study were to determine the system that generates reactive oxygen species in response to Cd that contribute to intracellular signaling on the activation of the STAT3 pathway in HepG2 cells and to address the participation of STAT3 in the production of Hsp70. Cadmium induced a significant increase in STAT3 DNA-binding after 1h treatment. Serine phosphorylation of STAT3 was observed as a result of cadmium treatment while no tyrosine phosphorylation was detected. Cells were pretreated with inhibitors for several ROS generating systems, only diphenylen iodonium, an inhibitor of NADPH oxidase, decreased STAT3 activation. Cd induced 2.6-fold NADPH oxidase activity. Antioxidant treatment with pegylated-catalase reduced STAT3 activation. Cells were pretreated with different MAPK's inhibitors. ERK contributes in approximately 60%, and JNK in a small proportion, while p38 does not contribute in STAT3 activation. Cells were pretreated with a specific STAT3 peptide inhibitor that decreased the Cd-induced Hsp70 expression. Data suggest that STAT3 is phosphorylated at serine 727 by a Cd stress-activated signaling pathway inducing NADPH oxidase activity which produced ROS, leading ERK activation. MAPK promotes STAT3 phosphorylation that could induce a protective mechanism against Cd toxicity.