Lipopolysaccharide Stimulates Surfactant Protein-A in Human Renal Epithelial HK-2 Cells through Upregulating Toll-like Receptor 4 Dependent MEK1/2-ERK1/2-NF-KB Pathway

Background： Surfactant protein-A （SP-A） contributes to the regulation of sepsis-induced acute kidney injury. In a previous study, we demonstrated that the expression of SP-A in the human renal tubular epithelial （HK-2） cells can be stimulated by lipopolysaccharide （LPS）. The present study evaluated the possible signal-transducing mechanisms of LPS-induced SP-A biosynthesis in the HK-2 cells. Methods： Tetrazolium salt colorimetry （MTT） assay was used to detect cell viability of HK-2 cells after LPS stimulation on different time points. HK-2 cells were stimulated with 100 ng/ml of LPS for different durations to determine the effects of LPS on SP-A and toll-like receptor 4 （TLR4） messenger RNA （mRNA） expression, as well as phosphorylation of mitogen-activated/ extracellular signal-regulated kinase （MEK） 1, extracellular signal-regulated kinase 1/2 （ERK1/2）, p38 mitogen-activated protein kinase （p38MAPK）, and nuclear factor-kappa B （NF-KB） inhibitor-alpha （IkB-a）. Then, HK-2 cells were pretreated with CLI-095, a TLR4 inhibitor, to analyze mRNA and protein levels of SP-A and TLR4 and expression of NF-KB ill the cytoplasm and nucleus of HK-2 before LPS exposure. Results： HK-2 cells exposed to 100 ng/ml of LPS for 1,6, and 24 h did not affect cell viability which showed no toxic effect of 100 ng/ml LPS on cells （P = 0.16）; however, the biosynthesis of SP-A mRNA and protein in HK-2 cells was significantly increased （P = 0.02）. As to the mechanism, LPS enhanced transmembrane receptor TLR4 protein expression. Sequentially, LPS time dependently augmented phosphorylation of MEKI, ERKI/2, and p38MAPK. In addition, levels of phospborylatedand nuclear NF-KB were augmented with LPS exposure for 2 h. LPS-induced SP-A and TLR4 mRNA as well as NF-KB expression were significantly inhibited by pretreatment with CLI-095. Conclusions： The present study exhibited that LPS can increase SP-A synthesis in human renal epithelial cells through sequentially activating the TLR4-related MEK1 -ERK 1/2-NF-kB-dependent pathway.