The lipopolysaccharide-recognition mechanism in cells expressing TLR4 and CD14 but lacking MD-2.
We analysed the lipopolysaccharide (LPS)-recognition mechanism in cells expressing TLR4 and CD14 but lacking MD-2. When TLR4 and CD14 were transiently expressed in HEK293 cells, cell-surface expression of TLR4 was observed, although the expression level was lower than that in cells coexpressing MD-2. We found that membrane CD14-TLR4 complexes were formed in these cells in response to LPS stimulation even in the absence of MD-2 expression, although NF-kappaB-dependent reporter activity was not induced. A strong activation of NF-kappaB was observed when these cells were stimulated with LPS followed by soluble MD-2 in this order, even when excess LPS was removed after formation of the CD14-TLR4 complex by washing cells prior to sMD-2 addition. From these results, we propose an additional LPS-recognition mechanism. In cells expressing TLR4 and CD14 but lacking MD-2, LPS is first transferred to membrane CD14 with the aid of LPS binding protein, which leads to the formation of the TLR4-CD14 complex. Then, the binding of soluble MD-2 to this complex triggers the transmembrane signal transduction. Cells expressing TLR4 and CD14 but lacking MD-2, such as airway epithelial cells, may be activated in response to LPS by this mechanism.
Ohnishi
Takahiro
T
Division of Microbiology, National Institute of Health Sciences, Tokyo, Japan.
Muroi
Masashi
M
Tanamoto
Ken-ichi
K
D000209:Acute-Phase Proteins / Q000502:physiology; D002352:Carrier Proteins / Q000502:physiology; D002460:Cell Line; D006801:Humans; D018950:Lipopolysaccharide Receptors / Q000502:physiology*; D008070:Lipopolysaccharides / Q000633:toxicity*; D051218:Lymphocyte Antigen 96 / Q000502:physiology*; D008562:Membrane Glycoproteins / Q000502:physiology; D015398:Signal Transduction; D051197:Toll-Like Receptor 4 / Q000502:physiology*; D000099231:Lipopolysaccharide-Binding Protein
D000209:Acute-Phase Proteins; D002352:Carrier Proteins; D018950:Lipopolysaccharide Receptors; D008070:Lipopolysaccharides; D051218:Lymphocyte Antigen 96; D008562:Membrane Glycoproteins; D051197:Toll-Like Receptor 4; D000099231:Lipopolysaccharide-Binding Protein; C495377:LY96 protein, human; C495345:TLR4 protein, human