Lipopolysaccharide induces ICAM‐1 expression in human pulmonary alveolar epithelial cells through a TLR4/MyD88/TRAF6/NADPH oxidase‐dependent pathway

Abstract

Lipopolysaccharide (LPS), a key component of the outer membranes of Gram‐negative bacteria, plays an important role in the induction of adhesion molecules expression and reactive oxygen species (ROS) generation in airway inflammatory diseases. Here, we investigated the mechanisms of LPS‐induced intercellular adhesion molecule‐1 (ICAM‐1) expression in human pulmonary alveolar epithelial cells (HPAEpiCs). We found that LPS induced ICAM‐1 protein and mRNA expression, promoter activity, and the adhesion of THP‐1 cells, as well as the phosphorylation of c‐Src, PDGFR, EGFR, Akt, and p65, which were attenuated by pretreatment with an anti‐TLR4 Ab or the inhibitor of c‐Src (PP1), PDGFR (AG1296), EGFR (AG1478), PI3K (LY294002), NADPH oxidase [diphenylene iodonium chloride (DPI) and apocynin (APO), ROS (edaravone), or NF‐κB (Bay11–7082) and transfection with siRNA of TLR4, MyD88, CD14, or TNFR‐associated factor (TRAF)6. LPS‐induced TLR4, MyD88, TRAF6, c‐Src, p47phox, and Rac‐1 complex formation was revealed by immunoprecipitation using an anti‐TLR4 Ab, followed by Western blot against an anti‐TLR4, anti‐MyD88, anti‐TRAF6, anti‐c‐Src, anti‐p47phox, or anti‐Rac‐1 Ab. These results demonstrated that LPS‐induced ICAM‐1 expression was mediated through TLR4/MyD88/TRAF6/p47phox/Rac‐1/c‐Src/EGFR, PDGFR/PI3K, in turn initiates NF‐κB activation, and ultimately induces ICAM‐1 expression and the adhesion of THP‐1 cells in HPAEpiCs.