title: Structural regions of MD-2 that determine the agonist-antagonist activity of lipid IVa.
abstract: A cell surface receptor complex consisting of CD14, Toll-like receptor (TLR4), and MD-2 recognizes lipid A, the active moiety of lipopolysaccharide (LPS). Escherichia coli-type lipid A, a typical lipid A molecule, potently activates both human and mouse macrophage cells, whereas the lipid A precursor, lipid IVa, activates mouse macrophages but is inactive and acts as an LPS antagonist in human macrophages. This animal species-specific activity of lipid IVa involves the species differences in MD-2 structure. We explored the structural region of MD-2 that determines the agonistic and antagonistic activities of lipid IVa to induce nuclear factor-kappaB activation. By expressing human/mouse chimeric MD-2 together with mouse CD14 and TLR4 in human embryonic kidney 293 cells, we found that amino acid regions 57-79 and 108-135 of MD-2 determine the species-specific activity of lipid IVa. We also showed that the replacement of Thr(57), Val(61), and Glu(122) of mouse MD-2 with corresponding human MD-2 sequence or alanines impaired the agonistic activity of lipid IVa, and antagonistic activity became evident. These mutations did not affect the activation of nuclear factor-kappaB, TLR4 oligomerization, and inducible phosphorylation of IkappaBalpha in response to E. coli-type lipid A. These results indicate that amino acid residues 57, 61, and 122 of mouse MD-2 are critical to determine the agonist-antagonist activity of lipid IVa and suggest that these amino acid residues may be involved in the discrimination of lipid A structure.
authors.affiliation: Division of Microbiology, National Institute of Health Sciences, 1-18-1 Kamiyoga, Setagaya, Tokyo 158-8501, Japan.
authors.forename: Masashi
authors.initials: M
authors.lastname: Muroi
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Tanamoto
mesh_terms: D000818:Animals; D002460:Cell Line; D006017:Glycolipids / Q000378:metabolism*; D006801:Humans; D008050:Lipid A / Q000031:analogs & derivatives* / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018950:Lipopolysaccharide Receptors / Q000235:genetics / Q000378:metabolism; D051218:Lymphocyte Antigen 96 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D016328:NF-kappa B / Q000378:metabolism; D011498:Protein Precursors / Q000235:genetics / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology; D013045:Species Specificity; D051197:Toll-Like Receptor 4 / Q000235:genetics / Q000378:metabolism
chemical_list: D006017:Glycolipids; D008050:Lipid A; D018950:Lipopolysaccharide Receptors; C411728:Ly96 protein, mouse; D051218:Lymphocyte Antigen 96; D016328:NF-kappa B; D011498:Protein Precursors; D011993:Recombinant Fusion Proteins; C493487:Tlr4 protein, mouse; D051197:Toll-Like Receptor 4; C040142:lipid A precursors, bacterial