Endotoxin-binding Proteins Modulate the Susceptibility of Bacterial Endotoxin to Deacylation by Acyloxyacyl Hydrolase

Abstract

Acyloxyacyl hydrolase (AOAH) is an eukaryotic lipase that partially deacylates and detoxifies Gram-negative bacterial lipopolysaccharides and lipooligosaccharides (LPSs orLOSsTissue invasion by even minute quantities of many Gramnegative bacteria (GNB) 2 initiates rapid mobilization of the innate immune responses of the host. In these circumstances, both GNB recognition and many responses depend upon activation of the exquisitely sensitive Toll-like receptor 4 (TLR4) by endotoxins, structurally unique and abundant glycolipids that occupy much of the outer leaflet of the GNB outer membrane (3). Maximal TLR4-dependent host responses to endotoxin are orchestrated through a sequential set of interactions of endotoxin with lipopolysaccharide-binding protein (LBP), membrane or soluble CD14, and soluble or TLR4-associated MD-2 (3-5). Although timely mobilization of host responses is essential, equally important is the regulation of the duration and intensity of host responses to endotoxin to prevent over-exuberant and sustained responses that can result in severe pathological consequences (6).One mechanism of dampening host responses to endotoxin is for the host to modify endotoxin itself, converting it from a potent TLR4 agonist to a much weaker agonist with antagonistic properties. To date, the best described host endotoxin-detoxifying enzyme is acyloxyacyl hydrolase (AOAH) (6). AOAH reduces endotoxin activity by catalyzing the release of secondary fatty acyl chains that are attached to primary 3-hydroxy fatty acyl chains within the bioactive lipid A region (7,8). AOAH thus converts hexaacylated endotoxin species that are potent TLR4 agonists to pentaacylated or tetraacylated forms that have reduced or no agonist properties (9 -13) and, at least in vitro, possess the ability to inhibit TLR4 activation by intact, hexaacylated species (10,11,13, 14). Recent studies indicate that hexaacylated and partially deacylated or underacylated endotoxin react similarly with LBP, CD14, and MD-2 to form monomeric complexes with MD-2 (13). However, only hexaacylated endotoxin-human MD-2 is a potent TLR4 agonist (13, 14).AOAH-dependent deacylation of endotoxin has been demonstrated in vivo (1,15,16) and in complex in vitro settings that roughly simulate the extracellular and intracellular conditions of inflammatory fluids (1, 2, 15,

The bactericidal/permeability-increasing protein (BPI) in infection and inflammatory disease

“…Acyloxyacyl hydrolase converts hexa-acylated endotoxins that are potent TLR4 agonists to tetra-acylated species that are TLR4 antagonists [3,105,106]. Endotoxin associated with intact bacteria and bacterial remnants is also partially deacylated in the extracellular inflammatory fluid [88] but at a much slower rate.…”

Lipopolysaccharide binding protein promoter variants influence the risk for Gram-negative bacteremia and mortality after allogeneic hematopoietic cell transplantation

“…41 Finally, Gioannini et al recently found that LBP binds to LPS to expose fatty acyl chains within lipid A, which allows acyloxyacyl hydrolase and eukaryotic lipase to bind and detoxify LPS. 42 Although these data suggest that high levels of circulating LBP may be beneficial for curtailing the inflammatory response to high concentrations of LPS, the resultant attenuation of cellular response to GN bacteria, especially when leukocyte counts are already low and innate immunity is even more important in host defense (as commonly found early after transplantation), may permit unchecked GN bacterial growth and ultimately result in death.The association of SNP 1683 with a 5-fold increase in risk of death after transplantation among patients with GN bacteremia, and a borderline effect among patients without GN bacteremia, suggests several possible mechanisms by which LBP variants might influence mortality risk. This finding is consistent with the known biology of LBP and the major role it plays in modulating the host immune response to GN bacteria and LPS.…”

“…41 Finally, Gioannini et al recently found that LBP binds to LPS to expose fatty acyl chains within lipid A, which allows acyloxyacyl hydrolase and eukaryotic lipase to bind and detoxify LPS. 42 Although these data suggest that high levels of circulating LBP may be beneficial for curtailing the inflammatory response to high concentrations of LPS, the resultant attenuation of cellular response to GN bacteria, especially when leukocyte counts are already low and innate immunity is even more important in host defense (as commonly found early after transplantation), may permit unchecked GN bacterial growth and ultimately result in death.…”

Transfer of Monomeric Endotoxin from MD-2 to CD14

“…On the basis of these and many other studies, we and others have proposed that potent TLR4-dependent cell activation by endotoxin depends on sequential protein-endotoxin and protein-protein interactions between endotoxin, LBP, CD14, MD-2, and TLR4 (1-4, 9 -11). Thus, LBP binds to endotoxin as presented natively in the bacterial outer membrane or as aggregates after extraction and purification (4,(12)(13)(14)(15). In so doing, LBP catalyzes delivery and transfer of individual molecules of endotoxin to CD14 (either as a soluble extracellular protein (sCD14) or GPI-linked membrane protein (mCD14)) (4,7,(13)(14)(15)(16).…”