Salmonella-type heptaacylated lipid A is inactive and acts as an antagonist of lipopolysaccharide action on human line cells

The polysaccharide portion plays an indispensable role in Salmonella lipopolysaccharide-induced activation of NF-kappaB through human toll-like receptor 4

The lipid A portion has been identified as the active center responsible for lipopolysaccharide (LPS)-induced macrophage activation. However, we found that Salmonella (Salmonella enterica serovars Abortusequi, Minnesota, and Typhimurium) lipid A is inactive in human macrophages, despite its LPS being highly active. Thus we investigated the critical role of polysaccharide in Salmonella LPS-induced activation of NF-kappaB. In human monocytic cell line THP-1, Salmonella lipid A and synthetic Salmonella-type lipid A (516) did not induce NF-kappaB-dependent reporter activity up to 1 micro g/ml, whereas strong activation was observed in response to Salmonella LPS. The difference in activity between this lipid A and LPS was further examined by using 293 cells expressing human CD14/Toll-like receptor 4 (TLR4)/MD-2, and similar results were obtained in these cells as well. A polysaccharide preparation obtained from Salmonella LPS was inactive in 293 cells expressing human CD14/TLR4/MD-2 even in combination with 516. Salmonella enterica serovar Minnesota Re LPS, whose structure consists of lipid A and two molecules of 2-keto-3-deoxyoctonic acid, but not its lipid A exhibited strong activity in THP-1 cells and 293 cells expressing human CD14/TLR4/MD-2. These results indicate that the polysaccharide portion covalently bound to lipid A plays the principal role in Salmonella LPS-induced activation of NF-kappaB through human CD14/TLR4/MD-2.

Regions of the mouse CD14 molecule required for toll-like receptor 2- and 4-mediated activation of NF-kappa B

Regions of mouse CD14 required for Toll-like receptor 2 (TLR2)- and TLR4-mediated activation of NF-kappaB were studied in transiently transfected 293 cells. Wild-type CD14 enhanced lipopolysaccharide (LPS)-induced NF-kappaB-dependent reporter activity in cells expressing TLR4/MD-2, and deletion of amino acid regions 35-44, 144-153, 235-243, and 270-275 impaired the TLR4-mediated activation. Unlike human CD14, mouse CD14 truncated at amino acid 151 lost the activity. Deletion of amino acids 35-44 or 235-243 also abrogated TLR2-mediated activation of NF-kappaB, whereas mutants lacking 144-153 and 270-275 retained the activity. Deletion and alanine substitution experiments revealed that amino acids 151-153 and 273-275 were required for the TLR4-mediated activation. Both deletion mutants lacking amino acids 35-44 and 235-243 and alanine substitution mutants in regions 151-153 and 273-275 were expressed on the cell surface and retained the ability to associate with TLR4. A cross-linking study with photoreactive LPS showed that the labeling intensities to CD14 mutants/TLR4/MD-2 were paralleled by the ability of CD14 mutants to increase TLR4-mediated activation. These results indicate that different regions of mouse CD14 are required for TLR4- and TLR2-mediated activation of NF-kappaB and suggest that amino acids 35-44, 151-153, 235-243, and 273-275 of mouse CD14 play an important role in LPS binding and its transfer to TLR4/MD-2.

Solution structure and dynamics of the outer membrane enzyme PagP by NMR

The bacterial outer membrane enzyme PagP transfers a palmitate chain from a phospholipid to lipid A. In a number of pathogenic Gram-negative bacteria, PagP confers resistance to certain cationic antimicrobial peptides produced during the host innate immune response. The global fold of Escherichia coli PagP was determined in both dodecylphosphocholine and n-octyl-beta-d-glucoside detergent micelles using solution NMR spectroscopy. PagP consists of an eight-stranded anti-parallel beta-barrel preceded by an amphipathic alpha helix. The beta-barrel is well defined, whereas NMR relaxation measurements reveal considerable mobility in the loops connecting individual beta-strands. Three amino acid residues critical for enzymatic activity localize to extracellular loops near the membrane interface, positioning them optimally to interact with the polar headgroups of lipid A. Hence, the active site of PagP is situated on the outer surface of the outer membrane. Because the phospholipids that donate palmitate in the enzymatic reaction are normally found only in the inner leaflet of the outer membrane, PagP activity may depend on the aberrant migration of phospholipids into the outer leaflet. This finding is consistent with an emerging paradigm for outer membrane enzymes in providing an adaptive response toward disturbances in the outer membrane.

Protective role of phospholipid oxidation products in endotoxin-induced tissue damage

Lipopolysaccharide (LPS), an outer-membrane component of Gram-negative bacteria, interacts with LPS-binding protein and CD14, which present LPS to toll-like receptor 4 (refs 1, 2), which activates inflammatory gene expression through nuclear factor kappa B (NF kappa B) and mitogen-activated protein-kinase signalling. Antibacterial defence involves activation of neutrophils that generate reactive oxygen species capable of killing bacteria; therefore host lipid peroxidation occurs, initiated by enzymes such as NADPH oxidase and myeloperoxidase. Oxidized phospholipids are pro-inflammatory agonists promoting chronic inflammation in atherosclerosis; however, recent data suggest that they can inhibit expression of inflammatory adhesion molecules. Here we show that oxidized phospholipids inhibit LPS-induced but not tumour-necrosis factor-alpha-induced or interleukin-1 beta-induced NF kappa B-mediated upregulation of inflammatory genes, by blocking the interaction of LPS with LPS-binding protein and CD14. Moreover, in LPS-injected mice, oxidized phospholipids inhibited inflammation and protected mice from lethal endotoxin shock. Thus, in severe Gram-negative bacterial infection, endogenously formed oxidized phospholipids may function as a negative feedback to blunt innate immune responses. Furthermore, identified chemical structures capable of inhibiting the effects of endotoxins such as LPS could be used for the development of new drugs for treatment of sepsis.

Structural and functional analyses of bacterial lipopolysaccharides

Bacterial lipopolysaccharides (LPSs) are powerful immunomodulators in infected hosts, and may cause endotoxic shock. Most of them share a common architecture but vary considerably in structural motifs from one genus, species, and strain to another. Cells of the innate immune response recognize evolutionarily conserved LPS molecular patterns of endotoxins and structural details thereby greatly influencing their response.

MD-2, a novel accessory molecule, is involved in species-specific actions of Salmonella lipid A

Salmonella lipid A is inactive in human macrophages despite being potently active in murine macrophages. We investigated the molecular basis for this species-specific action of Salmonella lipid A. When murine CD14 (mCD14), mTLR4, and mMD-2 were all expressed in human monocytic THP-1 cells, these cells were capable of responding to Salmonella lipid A. Expressing each of these proteins separately did not impart such responsiveness. Expression of mTLR4 plus mMD-2, but not mCD14 plus mTLR4 or mCD14 plus mMD-2, conferred this responsiveness. In THP-1 cells expressing mCD14, mTLR4, and mMD-2, replacing mCD14 with human CD14 had no effect on responsiveness to Salmonella lipid A or synthetic Salmonella-type lipid A (compound 516). When mTLR4 was replaced with human TLR4, the responses to these lipid A preparations were decreased to half, and the replacement of mMD-2 decreased responsiveness to one-third, although the responses to Escherichia coli lipid A or synthetic E. coli-type lipid A (compound 506) were not affected. These results indicate that both TLR4 and MD-2 participate in the species-specific action of Salmonella lipid A.

Endotoxic properties of lipid A from Comamonas testosteroni

The lipid A from Comamonas testosteroni has been isolated and its complete chemical structure determined [Iida, T., Haishima, Y., Tanaka, A., Nishijima, K., Saito, S. & Tanamoto, K. (1996). Eur J Biochem 237, 468-475]. In this work, the relationship between its chemical structure and biological activity was studied. The lipid A was highly homogeneous chemically and was characterized by the relatively short chain length (C(10)) of the 3-hydroxy fatty acid components directly bound to the glucosamine disaccharide backbone by either amide or ester linkages. The lipid A exhibited endotoxic activity in all of the assay systems tested (mitogenicity in mouse spleen cells; induction of tumour necrosis factor alpha release from both mouse peritoneal macrophages and mouse macrophage-like cell line J774-1, as well as from the human monocytic cell line THP-1; induction of nitric oxide release from J774-1 cells; Limulus gelation activity and lethal toxicity in galactosamine-sensitized mice) to the same extent as did 'Salmonella minnesota' lipid A or Escherichia coli LPS used as controls. The strong endotoxic activity of the C. testosteroni lipid A indicates that the composition of 3-hydroxydecanoic acid is not responsible for the low endotoxicity of the lipid A observed in members of the genus Rhodopseudomonas, as has previously been suggested. Furthermore, both the lack of a second acylation of the 3-hydroxy fatty acid attached at the 3' position, and the substitution of the hydroxyl group of the 3-hydroxy fatty acid attached at position 2, do not affect the manifestation of endotoxic activity or species specificity.

Biological properties of lipid A isolated from Flavobacterium meningosepticum

The biological properties of the lipid A from Flavobacterium meningosepticum, which we recently isolated and whose complete chemical structure has been determined (H. Kato, T. Iida, Y. Haishima, A. Tanaka, and K. Tanamoto. J. Bacteriol. 180:3891--3899, 1998), were studied. The lipid A exhibited generally moderate activity compared to Salmonella enterica subsp. enterica serovar abortus equi lipopolysaccharide (LPS) used as a control in the assay systems tested; lethal toxicity in galactosamine-sensitized mice, mitogenicity in mouse spleen cells, induction of tumor necrosis factor alpha (TNF-alpha) release from mouse peritoneal macrophages and J774-1 mouse macrophage-like and human THP-1 line cells, nitric oxide induction activity from J774-1 cells, and Limulus gelation activity. The moderate activity of the F. meningosepticum lipid A may be explained by its unique fatty acid composition and the lack of a phosphate group in position 4'. It is noteworthy that the lipid A apparently induced TNF-alpha release from peritoneal macrophages in LPS-unresponsive C3H/HeJ mice and that the activation was suppressed by the LPS-specific antagonist, succinylated lipid A precursor. Significant splenocyte mitogenicity in C3H/HeJ mice was also observed with the lipid A. Taken together with the previous results concerning Porphyromonas gingivalis lipid A, which has a high level of structural similarity to the lipid A of F. meningosepticum, and the induction of TNF-alpha release in macrophages from C3H/HeJ mice, the lipid A of F. meningosepticum, which has novel fatty acids, may possibly play an role for the activation of C3H/HeJ macrophages.

Transfer of palmitate from phospholipids to lipid A in outer membranes of gram-negative bacteria

Regulated covalent modifications of lipid A are implicated in virulence of pathogenic Gram-negative bacteria. The Salmonella typhimurium PhoP/PhoQ-activated gene pagP is required both for biosynthesis of hepta-acylated lipid A species containing palmitate and for resistance to cationic anti-microbial peptides. Palmitoylated lipid A can also function as an endotoxin antagonist. We now show that pagP and its Escherichia coli homolog (crcA) encode an unusual enzyme of lipid A biosynthesis localized in the outer membrane. PagP transfers a palmitate residue from the sn-1 position of a phospholipid to the N-linked hydroxymyristate on the proximal unit of lipid A (or its precursors). PagP bearing a C-terminal His(6)-tag accumulated in outer membranes during overproduction, was purified with full activity and was shown by cross-linking to behave as a homodimer. PagP is the first example of an outer membrane enzyme involved in lipid A biosynthesis. Additional pagP homologs are encoded in the genomes of Yersinia and Bordetella species. PagP may provide an adaptive response toward both Mg(2+) limitation and host innate immune defenses.