In vivo protection against endotoxin by plasma high density lipoprotein

The inflammatory cytokines. New developments in the pathophysiology and treatment of septic shock

Bacterial products [lipopolysaccharide (LPS) with Gram-negative bacteria and toxins, superantigens or cell wall fragments with Gram-positive bacteria] are the main activators of the septic shock cascade. These molecules interact with monocytes, macrophages and endothelial cells to produce inflammatory cytokines [tumour necrosis factor (TNF) and interleukins 1 and 6], and may activate other harmful pathways such as the coagulation system, complement cascade and lipid mediators. As a therapeutic strategy, antibodies directed against LPS have been well studied, although, on the whole, the clinical results have been disappointing. Other possible interventions that have not yet been tested clinically include natural intracellular antibacterial proteins (e.g. bacterial permeability-increasing protein) and high density lipoprotein (responsible for detoxifying LPS in the body). The stimulation pathway of responsive cells by bacterial products is also another possible target for intervention. Compounds under investigation include soluble CD14 and antibodies directed against CD14 or LPS binding protein. Antibodies directed against the cytokines are another option. Anti-TNF antibodies are currently being investigated, but conclusive evidence of their activity is still lacking. Soluble receptors (e.g. interleukin-1 receptor antagonist, or soluble TNF receptor) are another possibility; one soluble TNF receptor is still undergoing clinical investigation.

High-density lipoproteins inhibit cytokine-induced expression of endothelial cell adhesion molecules

While an elevated plasma concentration of HDLs is protective against the development of atherosclerosis and ensuing coronary heart disease (CHD), the mechanism of this protection is unknown. One early cellular event in atherogenesis is the adhesion of mononuclear leukocytes to the endothelium. This event is mediated principally by vascular cell adhesion molecule-1 (VCAM-1) but also involves other molecules, such as intercellular adhesion molecule-1 (ICAM-1) and E-selectin. We have investigated the effect of isolated plasma HDLs and reconstituted HDLs on the expression of these molecules by endothelial cells. We show that physiological concentrations of HDLs inhibit tumor necrosis factor-alpha (TNF-alpha) or interleukin-1 (IL-1) induction of these leukocyte adhesion molecules in a concentration-dependent manner. Steady state mRNA levels of TNF-alpha-induced VCAM-1 and E-selectin are significantly reduced by physiological concentrations of HDLs. An an HDL concentration of 1 mg/mL apolipoprotein A-I, the protein expressions of VCAM-1, ICAM-1, and E-selectin were inhibited by 89.6 +/- 0.4% (mean +/-SD, n=4), 64.8 +/- 1.0%, and 79.2 +/- 0.4%, respectively. In contrast, HDLs have no effect on the expression of platelet endothelial cell adhesion molecule (PECAM) or on the expression of the p55 and p75 subunits of the TNF-alpha receptor. HDLs were effective when added from 16 hours before to 5 minutes after cytokine stimulation. HDLs had no effect on TNF-alpha-induced expression of ICAM-1 by human foreskin fibroblasts, suggesting that the effect is cell-type restricted.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of murine plasma phospholipid transfer protein activity and mRNA levels by lipopolysaccharide and high cholesterol diet

Plasma phospholipid transfer protein mediates the net movement of phospholipids between lipoproteins and between lipid bilayers and high density lipoprotein. In this study, the mouse phospholipid transfer protein cDNA was cloned by reverse transcription polymerase chain reactions based on the cDNA sequence of human phospholipid transfer protein. The predicted amino acid sequence of mouse phospholipid transfer protein shows the protein to be 476 amino acids long and to have a sequence identity of 83% with that of human phospholipid transfer protein. Mouse plasma phospholipid transfer protein activity is 1.5-2 times that of human plasma phospholipid transfer protein activity. As in humans, mouse peripheral tissues displayed a higher abundance of phospholipid transfer protein mRNA than observed in central organs. The order of phospholipid transfer protein mRNA expression was as follows: lung > adipose tissue, placenta, testis > brain > muscle, heart, liver. We examined the regulation of phospholipid transfer protein expression by dietary cholesterol and by bacterial lipopolysaccharide. A high fat, high cholesterol diet caused a significant increase (35%) in plasma phospholipid transfer protein activity and a significant increase (18%) in high density lipoprotein phospholipids. This increased activity was accompanied by approximately 100% increase in phospholipid transfer protein mRNA in lung. After lipopolysaccharide injection, plasma phospholipid transfer protein activity was decreased by approximately 66%. This decrease in activity was associated with a similar decrease in phospholipid transfer protein mRNA in lung, adipose tissue, and liver. The decrease in plasma phospholipid transfer protein activity was also associated with a significant increase (17%) in high density lipoprotein phospholipid concentration. The opposite changes in phospholipids levels with lipopolysaccharide treatment and dietary cholesterol despite similarly increased high density lipoprotein phospholipids levels indicate that high density lipoprotein phospholipids levels are likely determined both by phospholipid transfer protein levels and by gradients of phospholipids concentration between high density lipoprotein and other phospholipids sources.

A lipooligosaccharide-binding site on HepG2 cells similar to the gonococcal opacity-associated surface protein Opa

The lacto-N-neotetraose-containing lipooligosaccharide (LOS) present on the surface of most Neisseria gonorrhoeae organisms may serve many important functions in gonococcal pathogenesis. This surface glycolipid contains the cross-reactive epitope to human paragloboside and can be sialylated by gonococci grown in the presence of CMP-N-acetylneuraminic acid. Another possible role for this glycolipid could be to mimic human asialocarbohydrates and act as a ligand for asialoglycoprotein receptors contained on numerous human cells. The most noted of this large family of receptors is that expressed on the surface of hepatic cells. In a model cell system, using the hepatoma tissue culture cell line HepG2, we wanted to investigate if the presence of this asialoglycoprotein receptor influenced the adherence and/or invasion of gonococci expressing the lacto-N-neotetraose structure. Piliated variants of the gonococcal wild-type strain 1291 and its isogeneic LOS mutant 1291E were used in adherence-invasion assays. This gonococcal strain is somewhat unusual in that it expresses large amounts of predominantly one species of LOS, thus reducing the complexity of interpreting the data. The data from these assays suggested that the Gal(beta 1-4)GlcNAc(beta 1-3)Gal(beta 1-4)Glc carbohydrate structure on the wild-type LOS affected the adherence-invasion of gonococci into the HepG2 cells. In studies to determine whether the major hepatic asialoglycoprotein receptor was involved in these interactions, we found that the HepG2 cells contained two receptors which bound gonococcal LOS. One of these was the asialoglycoprotein receptor, and the data concerning this receptor will be reported elsewhere. The data on the second receptor are reported here. Purified, 125I-labeled gonococcal LOS was used to identify specific high-affinity LOS-binding sites. These binding experiments revealed one major binding site corresponding to a protein with a molecular mass of 70 kDa (p70). Several lines of evidence in this study suggested that the oligosaccharide region of LOS played an important role in LOS binding to the p70 of HepG2 cells. In addition, we show that this human LOS receptor has some similarities to the gonococcal Opa proteins.

Soluble CD14 acts as a shuttle in the neutralization of lipopolysaccharide (LPS) by LPS-binding protein and reconstituted high density lipoprotein

We have recently shown that lipopolysaccharide (LPS)-binding protein (LBP) is a lipid transfer protein that catalyzes two distinct reactions: movement of bacterial LPS (endotoxin) from LPS micelles to soluble CD14 (sCD14) and movement of LPS from micelles to reconstituted high density lipoprotein (R-HDL) particles. Here we show that LBP facilitates a third lipid transfer reaction: movement of LPS from LPS-sCD14 complexes to R-HDL particles. This action of LBP is catalytic, with one molecule of LBP enabling the movement of multiple LPS molecules into R-HDL. LBP-catalyzed movement of LPS from LPS-sCD14 complexes to R-HDL neutralizes the capacity of LPS to stimulate polymorphonuclear leukocytes. Our findings show that LPS may be transferred to R-HDL either by the direct action of LBP or by a two-step reaction in which LPS is first transferred to sCD14 and subsequently to R-HDL. We have observed that the two-step pathway of LPS transfer to R-HDL is strongly favored over direct transfer. Neutralization of LPS by LBP and R-HDL was accelerated more than 30-fold by addition of sCD14. Several observations suggest that sCD14 accelerates this reaction by serving as a shuttle for LPS: addition of LBP and sCD14 to LPS micelles resulted in LPS-sCD14 complexes that could diffuse through a 100-kD cutoff filter; LPS-sCD14 complexes appeared transiently during movement of LPS to R-HDL facilitated by purified LBP; and sCD14 could facilitate transfer of LPS to R-HDL without becoming part of the final LPS-R-HDL complex. Complexes of LPS and sCD14 were formed transiently when LPS was incubated in plasma, suggesting that these complexes may play a role as intermediates in the neutralization of LPS under physiological conditions. These findings detail a new activity for sCD14 and suggest a novel mechanism for lipid transfer by LBP.

Role for circulating lipoproteins in protection from endotoxin toxicity

Previous studies have shown that endotoxin (lipopolysaccharide [LPS])-induced death can be prevented by preincubating LPS with lipoproteins in vitro or by infusing large quantities of lipids into animals prior to LPS administration. In the present study we determined whether physiological levels of lipids also provide protection. Serum lipid levels were decreased by two different mechanisms: administration of 4-aminopyrolo-(3,4-D)pyrimide, which prevents the hepatic secretion of lipoproteins, and administration of pharmacological doses of estradiol, which increases the number of hepatic low-density lipoprotein receptors, leading to increased lipoprotein clearance. In both hypolipidemic models, LPS-induced mortality is markedly increased compared with that of controls with normal serum lipid levels. In both hypolipidemic models, administration of exogenous lipoproteins, which increase levels of serum lipids into the physiological range, reduces the increased mortality to levels similar to that seen in normal animals. In normal lipidemic animals, 63% of 125I-LPS in plasma is associated with lipoproteins, where it would not be capable of stimulating cytokine production. In contrast, in hypolipidemic animals, very little LPS (12 to 17%) is associated with lipoproteins. Rather, more LPS is in the lipoprotein-free plasma compartment, where it could exert biological effects. In both hypolipidemic models, LPS produces a greater increase in serum tumor necrosis factor levels than it does in controls (three- to fivefold increase), and administration of exogenous lipoproteins prevents this increase. Cytokines, in particular tumor necrosis factor, are responsible for most of the toxic effects of LPS. These data provide evidence that physiological levels of serum lipids protect animals from LPS toxicity. Thus, lipoproteins, in addition to playing a role in lipid transport, may have protective functions. Moreover, as part of the immune response, cytokine-induced increases in serum lipid levels may play a role in host defense by decreasing the toxicities of biological and chemical agents.

Decreased cholesteryl ester transfer protein (CETP) mRNA and protein and increased high density lipoprotein following lipopolysaccharide administration in human CETP transgenic mice

The plasma cholesteryl ester transfer protein (CETP) mediates the exchange of HDL cholesteryl esters (CE) and VLDL triglycerides leading to catabolism of HDL. There is some evidence that HDL ameliorates the toxicity of LPS, and LPS is known to influence several enzymes affecting HDL metabolism. Therefore, the effects of LPS on CETP and plasma lipoproteins were examined in human CETP transgenic mice. Administration of LPS to mice expressing a CETP transgene linked to its natural flanking sequences (NFR-CETP Tg) resulted in a rapid marked decrease in hepatic CETP mRNA and plasma CETP concentration. Corticosteroid injection produced a similar decrease in hepatic CETP mRNA and adrenalectomy abolished this response to LPS. LPS caused disproportionate reductions in plasma CETP activity compared to mass, and was found to be a potent inhibitor of CETP activity when added directly to plasma. LPS was injected into mice expressing (A) a human apoA-I transgene, (B) apoA-I and NFR-CETP transgenes, or (C) apoA-I and LPS-inducible metallothionein promoter-driven CETP transgenes, producing (A) minimal changes in HDL cholesterol, (B) decreased plasma CETP and increased HDL cholesterol, and (C) increased plasma CETP and decreased HDL cholesterol. Thus, LPS administration produces a profound decrease in hepatic CETP mRNA, primarily as a result of adrenal corticosteroid release. The decrease in plasma CETP activity after LPS administration may reflect both this effect as well as a direct interaction between CETP and LPS. The decrease of CETP in response to LPS has major effects on HDL levels, and may represent an adaptive response to preserve or increase HDL and thereby modify the response to LPS.

Effect of the arrangement of tandem repeating units of class A amphipathic alpha-helixes on lipid interaction

Exchangeable apolipoproteins possess tandem repeating units of class A amphipathic helical segments and many of them are linked together by proline residues. To understand the optimal arrangement of the amphipathic helixes for lipid association, we have studied the interactions of three model class A amphipathic helical peptides with lipids. The three peptides are: 37pA, a dimer of 18A (DWLKAFYDKVAEKLKEAF) linked together by a Pro (18A-Pro-18A); 37aA, a dimer of 18A linked together by an Ala (18A-Ala-18A); and 36A, a dimer of 18A without any linker residue (18A-18A). Circular dichroism (CD) spectra showed that the peptides are predominantly alpha-helical in aqueous and lipid environments. Temperature dependent CD studies indicated that in buffer helix stability decreases in the order 36A > 37aA > 37pA; however, in the presence of dimyristoyl phosphatidylcholine (DMPC), the above order is reversed. The retention times of the peptides on a C18 reversed-phase high performance liquid chromatography column decreased in the order 36A > 37aA > 37pA, consistent with the lengths of the nonpolar faces of the alpha-helixes being in the same order; the retention time of the parent 18A was shorter than 37pA. While 37pA adsorbed to egg phosphatidylcholine monolayers most strongly, the degree and rate of association of 36A were significantly lower. Differential scanning calorimetry indicated that, while 37pA was most effective in reducing the enthalpy of the gel to liquid-crystalline phase transition of DMPC multilamellar vesicles, 36A was least effective; 36A was even less effective than 18A. Fluorescence quenching experiments with iodide and acrylamide indicated that, in the presence of DMPC, Trp residues in 36A are most exposed to the quenchers while in 37pA they are least exposed. In the presence of DMPC, shielding of Trp in 18A from the quenchers was more than that observed with Trp residues in 36A. The results of this study suggest that the arrangement of tandem repeating amphipathic helical units which results in the formation of a class A amphipathic helix with a nonpolar face longer than five or six turns reduces the ability of the helix to associate with phospholipid.

Mechanisms for the modulation of membrane bilayer properties by amphipathic helical peptides

The amphipathic helix, in which hydrophobic and hydrophilic residues are grouped on opposing faces, is a structural motif found in many peptides and proteins that bind to membranes. One of the physical properties of membranes that can be altered by the binding of amphipathic helices is membrane monolayer curvature strain. Class A amphipathic helices, which are present in exchangeable plasma lipoproteins, can stabilize membranes by reducing negative monolayer curvature strain; proline-punctuated class A amphipathic helical segments are particularly effective in this regard. This property is suggested to be associated with some of the beneficial biological effects of this protein. On the other hand, lytic amphipathic helical peptides can act by increasing negative curvature strain or by forming pores composed of helical clusters. Thus, different amphipathic helical peptides can be membrane stabilizing or be lytic to membranes, depending on the structural motif of the helix, which in turn determines the nature of its association with membranes. Features of these peptides that are responsible for their specific properties are discussed.

Selective modulation of lipopolysaccharide-induced death and cytokine production by various muramyl peptides

Pretreatment of animals with the adjuvant muramyl dipeptide enhances both the production of circulating tumor necrosis factor and the sensitivity to the lethal effect of a lipopolysaccharide (LPS) challenge. The present study examined the capacity of various adjuvant muramyl dipeptide derivatives to potentiate responsiveness to LPS administration. Cytokine levels in serum were determined at various time intervals after LPS administration by bioassays and immunoassays; the cytokines examined were tumor necrosis factor, interleukin-1, interleukin-6, and gamma interferon. The time course of cytokine response was not modified by the pretreatment, but most of the levels were strongly enhanced. However, of the four compounds which were found to be potent priming agents, only two caused an increased sensitivity to LPS lethality, showing that elevated titers of cytokines in serum were not correlated with host sensitization. Interestingly, previous studies have shown that these two compounds also display neurobiological properties, implying a possible role of the central nervous system in LPS lethality. However, two hydrophilic derivatives with low activity as priming agents were capable of decreasing the toxicity of LPS when given after the challenge in galactosamine-sensitized mice. These results illustrate the diversity of responses elicited by immunological priming. They raise unanswered questions on the importance of endogenous mediators in the pathophysiological alterations during toxic shock.

Reconstituted high-density lipoprotein neutralizes gram-negative bacterial lipopolysaccharides in human whole blood

We have tested hypotheses relating lipoprotein structure to function as measured by the relative ability to neutralize endotoxin by comparing natural human lipoproteins, a chemically defined form of reconstituted high-density lipoprotein (R-HDL), and a lipid emulsion (Intralipid). The human whole-blood system was used as an in vitro model of lipopolysaccharide (LPS) binding protein and CD14-dependent activation of cytokine production. When lipoproteins were compared on the basis of protein content, R-HDL was most effective in reducing tumor necrosis factor alpha (TNF-alpha) production followed in order by very low density lipoprotein, low-density lipoprotein, Intralipid, and natural HDL. However, when these particles were compared by protein, phospholipid, cholesterol, or triglyceride content by stepwise linear regression analysis, only phospholipid was correlated to effectiveness (r2 = 0.873; P < 0.0001). Anti-CD14 monoclonal antibodies MY4 and 3C10 inhibited LPS binding protein and CD14-dependent activation of TNF-alpha production by LPS at LPS concentrations up to approximately 1.0 ng/ml. R-HDL (2 mg of protein per ml) blocked TNF-alpha production by LPS from both smooth- and rough-type gram-negative bacteria at concentrations up to 100 ng of LPS per ml but had little effect on heat-killed gram-positive Staphylococcus aureus and no effect on other LPS-independent stimuli tested. These results support our hypothesis that LPS is neutralized by binding to phospholipid on the surface of R-HDL and demonstrate that R-HDL is a potent inhibitor of the induction of TNF-alpha by LPS from both rough- and smooth-form gram-negative bacteria in whole human blood.

Lipopolysaccharide (LPS)-binding protein is carried on lipoproteins and acts as a cofactor in the neutralization of LPS

Lipoproteins isolated from normal human plasma can bind and neutralize bacterial lipopolysaccharide (LPS) and may represent an important mechanism in host defense against gram-negative septic shock. Recent studies have shown that experimentally elevating the levels of circulating high-density lipoproteins (HDL) provides protection against death in animal models of endotoxic shock. We sought to define the components of HDL that are required for neutralization of LPS. To accomplish this we have studied the functional neutralization of LPS by native and reconstituted HDL using a rapid assay that measures the CD14-dependent activation of leukocyte integrins on human neutrophils. We report here that reconstituted HDL particles (R-HDL), prepared from purified apolipoprotein A-I (apoA-I) combined with phospholipid and free cholesterol, are not sufficient to neutralize the biologic activity of LPS. However, addition of recombinant LPS binding protein (LBP), a protein known to transfer LPS to CD14 and enhance responses of cells to LPS, enabled prompt binding and neutralization of LPS by R-HDL. Thus, LBP appears capable of transferring LPS not only to CD14 but also to lipoprotein particles. In contrast with R-HDL, apoA-I containing lipoproteins (LpA-I) isolated from plasma by selected affinity immunosorption (SAIS) on an anti-apoA-I column, neutralized LPS without addition of exogenous LBP. Several lines of evidence demonstrated that LBP is a constituent of LpA-I in plasma. Passage of plasma over an anti-apoA-I column removed more than 99% of the LBP detectable by ELISA, whereas 31% of the LBP was recovered by elution of the column. Similarly, the ability of plasma to enable activation of neutrophils by LPS (LBP/Septin activity) was depleted and recovered by the same process. Furthermore, an immobilized anti-LBP monoclonal antibody coprecipitated apoA-I. The results described here suggest that in addition to its ability to transfer LPS to CD14, LBP may also transfer LPS to lipoproteins. Since LBP appears to be physically associated with lipoproteins in plasma, it is positioned to play an important role in the neutralization of LPS.