Single immunoglobulin IL-1 receptor-related protein attenuates the lipopolysaccharide-induced inflammatory response in A549 cells

The lipopolysaccharide (LPS)-Toll-like receptor 4 (TLR4) signaling pathway in alveolar epithelial cells plays an important role in many pathologic processes such as acute lung injury (ALI). The single immunoglobulin IL-1 receptor-related protein (SIGIRR) is an inhibitor of LPS-TLR4 signaling, but its expression and function in alveolar epithelial cells are still unknown. In this study, we examined the expression of SIGIRR in normal human lung tissue using immunohistochemistry, reverse transcription-PCR (RT-PCR) and Western blot and found that SIGIRR was expressed in alveolar epithelial cells. Treatment of an alveolar epithelial cell line, A549, with LPS and we observed a downregulation of SIGIRR mRNA, which returned to normal levels 24 h after LPS exposure. A549 cells were then transfected with a SIGIRR eukaryotic expression vector to over-express SIGIRR or, as a control, with an empty vector. Following LPS exposure, the transcriptional activity of NF-κB was measured using a dual-luciferase reporter assay system, and the concentration of IL-1β, TNF-α and IL-6 was determined by ELISA, and cell proliferation was measured by MTT. In A549 cells that over-expressed SIGIRR, LPS treatment resulted in a significant decrease in the transcriptional activity of NF-κB and cell growth inhibition ratio, as well as lower levels of secreted IL-1β, TNF-α and IL-6. In conclusion, SIGIRR in A549 cells inhibits the transcriptional activity of NF-κB and reduces the amount cytokines produced, protecting these cells from acute LPS-induced damage.