Toll-like receptor 2-mediated sequential activation of MyD88 and MAPKs contributes to lipopolysaccharide-induced sp-a gene expression in human alveolar epithelial cells

Surfactant proteins (SPs) produced by pulmonary epithelial cells participate in the regulation of sepsisinduced acute lung injury. Our previous study has shown that lipopolysaccharide (LPS), a Gram-negative bacterial outer membrane component, can regulate sp-a gene expression in human lung carcinoma type II epithelial A549 cells. This study was further designed to evaluate the signal-transducing mechanisms of LPS-induced sp-a gene expression. Exposure of A549 cells to LPS induced SP-A mRNA and protein production in time-dependent manners. Application of toll-like receptor 2 (TLR2) siRNA into A549 cells decreased the levels of this receptor and simultaneously inhibited LPS-induced SP-A mRNA expression. Sequentially, LPS enhanced phosphorylation of mitogen-activated protein kinase (MEK) 4 and c-Jun NH2 terminal kinase 1 (JNK1) in time-dependent manners. Application of TLR2 siRNA decreased LPS-enhanced phosphorylation of MEK4 and JNK1. After knocking-down the translation of MyD88 by RNA interference, the LPS-triggered MEK4 phosphorylation was attenuated. Consequently, LPS augmented the translocation of c-Jun from the cytoplasm to nuclei without affecting c-Fos. Pretreatment of A549 cells with SP600125, an inhibitor of JNK1, significantly lowered LPS-induced SP-A mRNA production. Analyses of an electrophoretic mobility shift assay and a reporter gene further showed that LPS increased the transactivation activity of AP-1 in A549 cells. Therefore, the present study demonstrates that LPS can induce sp-a gene expression in human type II epithelial A549 cells through TLR2-mediated sequential activation of MyD88-MEK4-JNK1-AP-1. (C) 2010 Elsevier GmbH. All rights reserved.