Structural basis for endotoxic and antagonistic activities: investigation with novel synthetic lipid A analogs

Abstract

Our early work using homogeneous synthetic preparations demonstrated the presence of a lipid A analog which antagonizes endotoxic activities of LPS and lipid A. The first example was a tetraacylated biosynthetic precursor, now known as precursor Ia or lipid IVa, that contains four 3-hydroxytetradecanoyl moieties linked to the bisphosphorylated disaccharide backbone common to the endotoxic hexa-acyl Escherichia coli lipid A. Various compounds with both endotoxic and antagonistic activities have subsequently been reported from either natural or synthetic sources, but little is known about the factors determining the type of the activities of the respective compounds. To approach this issue, we have synthesized a series of lipid A analogs with various numbers and chain lengths of acyl groups on the backbone. Some were prepared by the aid of a novel affinity separation procedure. The phosphate moieties were also synthetically replaced. Biological tests showed that at least three acyl groups are required for antagonistic activity but one or even both of the phosphates can be replaced with other acidic moieties without losing the activity. The effect of Kdo residues linked to lipid A is also briefly discussed. Molecular dynamics calculations reasonably explain possible conformations required for the biological activity.

Differential Induction of the Toll-Like Receptor 4-MyD88-Dependent and -Independent Signaling Pathways by Endotoxins

“…Lipid A structure and configuration of endotoxin are known to determine the agonist or antagonist activity (24,27,37). For example, the KDO 2 -lipid A structure of meningococcal lipooligosaccharide (LOS) is necessary for the maximal agonist activation of macrophages via the TLR4 complex (51).…”

Neisseria meningitidisLipooligosaccharide Structure-Dependent Activation of the Macrophage CD14/Toll-Like Receptor 4 Pathway

“…Schromm et al (34) found that the number, nature, and location of negatively charged molecules modulate the molecular conformation of E. coli LPS and lipid A and are linked to IL-6 inducing capacity. Recently, synthetic lipid A with two KDO molecules was found to have enhanced agonist activity compared to one KDO molecule or none (18,50). A decreased number or lack of KDO and hydroxymyristic acid is proposed as a contributor to low endotoxic activity of Leptospira interrogans (46), Francisella tularensis (47), Legionella pneumophila, and different Rhizobium species LPSs (32,51).…”

Heterologous Expression of 3-O-Deacylase in <b><i>Acinetobacter baumannii </i></b>Modulates the Endotoxicity of Lipopolysaccharide

“…Lipid A moieties that deviate from this pattern often demonstrate a significant low endotoxic activity [Alexander and Rietschel, 2001]. For example, lipid A molecules with penta-or hepta-acyl chains are ∼ 100 times less active, while lipid A with tetra-acyl chains lacks agonistic activity altogether and acts as an antagonist instead [Hajjar et al, 2002;Kusumoto et al, 2003]. In this study, our data revealed that the whole cells, OMVs and purified LPS under PagL expression lessened toxicity.…”

“…The structure-function analysis of lipid A signaling indicates that the length and number of acyl side chains are critical for TLR4 signaling [Hajjar et al, 2002;Kusumoto et al, 2003]. Optimal lipid A potency is achieved by a biphosphorylated, hexa-acylated lipid A species that has acyl chains 12-14 carbons in length [Raetz and Whitfield, 2002].…”