TNF-α-induced up-regulation of intercellular adhesion molecule-1 is regulated by a Rac-ROS-dependent cascade in human airway epithelial cells

Up-regulation of intercellular adhesion molecule-1 (ICAM-1) in the lung airway epithelium is associated with the epithelium-leukocyte interaction, critical for the pathogenesis of various lung airway inflammatory diseases such as asthma. However, little is known about how ICAM-1 is up-regulated in human airway epithelial cells. In this study, we show that tumor TNF-α induces monocyte adhesion to A549 human lung airway epithelium and also up-regulation of ICAM-1 expression. These effects were significantly diminished by pre-treatment with diphenyliodonium (DPI), an inhibitor of NADPH oxidase-like flavoenzyme. In addition, the level of reactive oxygen species (ROS) was increased in response to TNF-α in A549 cells, suggesting a potential role of ROS in the TNF-α-induced signaling to ICAM-1 expression and monocyte adhesion to airway epithelium. Further, we found out that expression of Rac^N17, a dominant negative mutant of Rac1, suppressed TNF-α-induced ROS generation, ICAM-1 expression, and monocyte adhesion to airway epithelium. These findings suggest that Rac1 lies upstream of ROS generation in the TNF-α-induced signaling to ICAM-1 expression in airway epithelium. Finally, pretreatment with pyrrolidine dithiocarbamate (PDTC), an inhibitor of NF-κB, reduced TNF-α-induced ICAM-1 expression and both DPI and Rac^N17 significantly diminished NF-κB activation in response to TNF-α. Together, we propose that Rac1-ROS-linked cascade mediate TNF-α-induced ICAM-1 up-regulation in the airway epithelium via NF-κB-dependent manner.