Effects of UR-12633, a new antagonist of platelet-activating factor, in rodent models of endotoxic shock

Platelet-activating Factor Mediates Endotoxin Tolerance by Regulating Indoleamine 2,3-Dioxygenase-dependent Expression of the Suppressor of Cytokine Signaling 3

Indoleamine 2,3-dioxygenase (IDO) mediates immune tolerance, and suppressor of cytokine signaling 3 (SOCS3) negatively regulates the JAK/STAT signal transduction pathway. We determined previously that platelet-activating factor (PAF) protects mice against LPS-induced endotoxic shock, but its detailed mechanism of action was unknown. We performed survival experiments in IDO^+/+ and IDO^-/- mice using an LPS-induced endotoxemia model and rated organ injury (neutrophil infiltration and liver function). Using ELISA and Western blotting, we also investigated the mechanism of PAF-mediated endotoxin tolerance during endotoxemia. PAF-mediated endotoxin tolerance was dependent on IDO in vivo and in vitro and was not observed in IDO^-/- mice. JAK/STAT signaling, crucial for SOCS3 expression, was also impaired in the absence of IDO. In an IDO- and STAT-dependent manner, PAF mediated a decrease in IL-12 and a dramatic increase in IL-10 and reduced mouse mortality. In addition, PAF attenuated LPS-mediated neutrophil infiltration into the lungs and interactions between neutrophil-like (THP-1) and endothelial cells (human umbilical vein endothelial cells). These results indicate that PAF-mediated endotoxin tolerance is initiated via IDO- and JAK/STAT-dependent expression of SOCS3. Our study has revealed a novel tolerogenic mechanism of IDO action and an important association between IDO and SOCS3 with respect to endotoxin tolerance.

Lipopolysaccharide Cross-Tolerance Delays Platelet-Activating Factor-Induced Sudden Death in Swiss Albino Mice: Involvement of Cyclooxygenase in Cross-Tolerance

Lipopolysaccharide (LPS) signaling through Toll-like receptor-4 (TLR-4) has been implicated in the pathogenesis of many infectious diseases. Some believe that TLR-mediated pathogenicity is due, in part, to the lipid pro-inflammatory mediator platelet-activating factor (PAF), but this has been questioned. To test the direct contribution of PAF in endotoxemia in murine models, we injected PAF intraperitoneally into Swiss albino mice in the presence and absence of LPS. PAF alone (5 μg/mouse) caused death within 15-20 min, but this could be prevented by pretreating mice with PAF-receptor (PAF-R) antagonists or PAF-acetylhydrolase (PAF-AH). A low dose of LPS (5 mg/kg body wt) did not impair PAF-induced death, whereas higher doses (10 or 20 mg/kg body wt) delayed death, probably via LPS cross-tolerance. Cross-tolerance occurred only when PAF was injected simultaneously with LPS or within 30 min of LPS injection. Tolerance does not appear to be due to an abundant soluble mediator. Histologic examination of lungs and liver and measurement of circulating TNF-α and IL-10 levels suggested that the inflammatory response is not diminished during cross-tolerance. Interestingly, aspirin, a non-specific cyclooxygenase (COX) inhibitor, partially blocked PAF-induced sudden death, whereas NS-398, a specific COX-2 inhibitor, completely protected mice from the lethal effects of PAF. Both COX inhibitors (at 20 mg/kg body wt) independently amplified the cross-tolerance exerted by higher dose of LPS, suggesting that COX-derived eicosanoids may be involved in these events. Thus, PAF does not seem to have a protective role in endotoxemia, but its effects are delayed by LPS in a COX-sensitive way. These findings are likely to shed light on basic aspects of the endotoxin cross-tolerance occurring in many disease conditions and may offer new opportunities for clinical intervention.

Strategies to improve drug development for sepsis

Sepsis, a common and potentially fatal systemic illness, is triggered by microbial infection and often leads to impaired function of the lungs, kidneys or other vital organs. Since the early 1980s, a large number of therapeutic agents for the treatment of sepsis have been evaluated in randomized controlled clinical trials. With few exceptions, the results from these trials have been disappointing, and no specific therapeutic agent is currently approved for the treatment of sepsis. To improve upon this dismal record, investigators will need to identify more suitable therapeutic targets, improve their approaches for selecting candidate compounds for clinical development and adopt better designs for clinical trials.

Animal models of sepsis

Sepsis remains a common, serious, and heterogeneous clinical entity that is difficult to define adequately. Despite its importance as a public health problem, efforts to develop and gain regulatory approval for a specific therapeutic agent for the adjuvant treatment of sepsis have been remarkably unsuccessful. One step in the critical pathway for the development of a new agent for adjuvant treatment of sepsis is evaluation in an appropriate animal model of the human condition. Unfortunately, the animal models that have been used for this purpose have often yielded misleading findings. It is likely that there are multiple reasons for the discrepancies between the results obtained in tests of pharmacological agents in animal models of sepsis and the outcomes of human clinical trials. One of important reason may be that the changes in gene expression, which are triggered by trauma or infection, are different in mice, a commonly used species for preclinical testing, and humans. Additionally, many species, including mice and baboons, are remarkably resistant to the toxic effects of bacterial lipopolysaccharide, whereas humans are exquisitely sensitive. New approaches toward the use of animals for sepsis research are being investigated. But, at present, results from preclinical studies of new therapeutic agents for sepsis must be viewed with a degree of skepticism.

New modalities in treating pneumococcal pneumonia

Despite the availability of effective antibiotics and intensive medical care, pneumococcal pneumonia is still associated with substantial mortality. Early diagnosis is becoming increasingly possible. This article reviews several adjunctive measures that might be instituted at or soon after admission in patients who are hospitalized for community-acquired pneumonia that is found to be due to Streptococcus pneumoniae. Available data favor the use of a macrolide together with a β-lactam antibiotic for treatment, based largely on immunomodulatory activity of macrolides. Two large subgroup analyses from a single major study suggest that activated protein C (eg, drotrecogin) should be considered for patients with severe sepsis, organ failure, and an Acute Physiology and Chronic Health Evaluation II score > 25 due to pneumococcal pneumonia. Statins exert an anti-inflammatory effect and several retrospective studies suggest that their use might ameliorate the adverse effects of pneumonia. Because inflammation elsewhere in the body is associated with inflammation in coronary arteries and because pneumococcal pneumonia has been shown to precipitate myocardial infarction, statins might be of further benefit by decreasing the likelihood of associated myocardial infarction. Aspirin, which inhibits platelet aggregation in inflamed coronary arteries, might also be considered for initial therapy. One reason that the association between myocardial infarction and pneumonia was not previously recognized is that aspirin was widely used in the past when people had acute febrile conditions. The literature on the benefits of corticosteroids in pneumonia is not convincing, and a particularly well-done, very recent study shows no benefit with corticosteroid use in patients with pneumococcal pneumonia, and perhaps even a worse outcome. No clinical data favor the use of platelet-activating factor antagonists.

The novel role of platelet-activating factor in protecting mice against lipopolysaccharide-induced endotoxic shock

Background

Platelet-activating factor (PAF) has been long believed to be associated with many pathophysiological processes during septic shock. Here we present novel activities for PAF in protecting mice against LPS-mediated endotoxic shock.

Principal Findings

In vivo PAF treatment immediately after LPS challenge markedly improved the survival rate against mortality from endotoxic shock. Administration of PAF prominently attenuated LPS-induced organ injury, including profound hypotension, excessive polymorphonuclear neutrophil infiltration, and severe multiple organ failure. In addition, PAF treatment protects against LPS-induced lymphocytes apoptosis. These protective effects of PAF was correlated with significantly decreases in the production of the inflammatory mediators such as TNF-α, IL-1β, IL-12, and IFN-γ, while increasing production of the anti-inflammatory cytokine IL-10 in vivo and in vitro.

Conclusions

Taken together, these results suggest that PAF may protect mice against endotoxic shock via a complex mechanism involving modulation of inflammatory and anti-inflammatory mediators.

Signaling via platelet-activating factor receptors accounts for the impairment of neutrophil migration in polymicrobial sepsis

Sepsis is a systemic inflammatory response that results from the inability of the immune system to limit bacterial spread during an ongoing infection. Recently, we have documented an impaired neutrophil migration toward the infectious focus in severe sepsis. This impairment seems to be mediated by circulating cytokines, chemokines, and NO. Platelet-activating factor (PAF) plays an important role in the orchestration of different inflammatory reactions, including the release of cytokines, chemokines, and free radicals. Using a PAFR antagonist, PCA-4248, and PAFR-deficient mice, we investigated whether signaling via PAFR was relevant for the failure of neutrophils to migrate to the site of infection after lethal sepsis caused by cecum ligation and puncture in mice. In PAFR-deficient mice or mice pretreated with PCA-4248 (5 mg/kg) and subjected to lethal sepsis, neutrophil migration failure was prevented, and bacterial clearance was more efficient. There was also reduced systemic inflammation (low serum cytokine levels), lower nitrate levels in plasma, and higher survival rate. Altogether, the results firmly establish a role for PAFR in mediating the early impairment of neutrophil migration toward the infectious focus. Blockade of PAFR may prevent the establishment of severe sepsis.

Platelet-activating factor: a previously unrecognized mediator of fever

Lipopolysaccharide (LPS)-induced systemic inflammation is accompanied by either hypothermia (prevails when the ambient temperature (Ta) is subneutral) or fever (prevails when Ta is neutral or higher). Because platelet-activating factor (PAF) is a proximal mediator of LPS inflammation, it should mediate both thermoregulatory responses to LPS. That PAF possesses hypothermic activity and mediates LPS-induced hypothermia is known. We asked whether PAF possesses pyrogenic activity (Expt 1) and mediates LPS fever (Expt 2). The study was conducted in Long-Evans rats implanted with jugular catheters. A complex with bovine serum albumin (BSA) was infused as a physiologically relevant form of PAF; free (aggregated) PAF was used as a control. In Expt 1, either form of PAF caused hypothermia when infused (83 pmol kg-1 min-1, 60 min, i.v.) at a subneutral Ta of 20 degrees C, but the response to the PAF-BSA complex (-4.5 +/- 0.5 degrees C, nadir) was ~4 times larger than that to free PAF. At a neutral Ta of 30 degrees C, both forms caused fever preceded by tail skin vasoconstriction, but the febrile response to PAF-BSA (1.0 +/- 0.1 degrees C, peak) was > 2 times higher than that to free PAF. Both the hypothermic (at 20 degrees C) and febrile (at 30 degrees C) responses to PAF-BSA started when the total amount of PAF infused was extremely small, < 830 pmol kg-1. In Expt 2 (conducted at 30 degrees C), the PAF receptor antagonist BN 52021 (29 micromol kg-1, i.v.) had no thermal effect of itself. However, it strongly (~2 times) attenuated the febrile response to PAF (5 nmol kg-1, i.v.), implying that this response involves the PAF receptor and is not due to a detergent-like effect of PAF on cell membranes. BN 52021 (but not its vehicle) was similarly effective in attenuating LPS (10 microg kg-1, i.v.) fever. It is concluded that PAF is a highly potent endogenous pyrogenic substance and a mediator of LPS fever.

Cardiovascular properties of yangambin, a lignan isolated from Brazilian plants

Yangambin was initially selected from a number of lignans isolated from Brazilian plants for its ability to antagonize Platelet-Activating Factor (PAF, 1-O-hexadecyl-2-acetyl- sn-glyceryl-3-phosphorylcholine)-induced biological effects. Subsequently it was shown that, besides its antagonistic properties at PAF receptors, yangambin also prevents the cardiovascular collapse observed during anaphylactic and endotoxic/septic shocks, as well as the vascular and cardiac hyporesponsiveness to catecholamines in endotoxic shock. It is suggested that this naturally occurring compound could be of potential interest in the adjunctive management of the above mentioned pathologies. In the present article, we review the main studies investigating the pharmacological properties of yangambin related to the cardiovascular function.

Novel agents in the therapy of endotoxic shock

Endotoxic shock, or Gram-negative septic shock, can occur as a component of Gram-negative sepsis and is characterised by hypotension, poor tissue perfusion and multi-organ dysfunction. Despite antibiotic therapy and intensive care management, the morbidity and mortality rates of Gram-negative septic shock remain high. Endotoxin mediates its effects through interaction with receptors on the surface of a variety of host cells. These interactions result in the production and release of numerous biochemical mediators including nitric oxide, cytokines, prostaglandins and leukotrienes and toxic oxygen radicals. It is these biochemical mediators that exert toxic effects during endotoxic shock and which are often the target of novel treatment strategies. Several of these pharmacological agents are currently being investigated for use in Gram-negative septic shock and include inhibitors of the enzyme responsible for nitric oxide production, scavengers of the nitric oxide molecule and cytokine modulators. Although many agents have been studied for potential use as modulators of cytokine levels, this study will focus on pentoxifylline and the 21-aminosteroids, or lazaroids. Examination of the literature regarding pharmacological agents used to treat endotoxic shock often yields confusing and contradictory results. The reasons for these mixed results include differences in models, drug dosages, dosing methods and intervals and timing of administration relative to disease duration and severity. However, despite mixed results, several of the drugs discussed in this paper offer promise in the therapy of an often frustrating and lethal condition.

Platelet-activating factor (PAF) receptor and genetically engineered PAF receptor mutant mice

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) is a biologically active phospholipid mediator. Although PAF was initially recognized for its potential to induce platelet aggregation and secretion, intense investigations have elucidated potent biological actions of PAF in a broad range of cell types and tissues, many of which also produce the molecule. PAF acts by binding to a unique G-protein-coupled seven transmembrane receptor. PAF receptor is linked to intracellular signal transduction pathways, including turnover of phosphatidylinositol, elevation in intracellular calcium concentration, and activation of kinases, resulting in versatile bioactions. On the basis of numerous pharmacological reports, PAF is thought to have many pathophysiological and physiological functions. Recently advanced molecular technics enable us not only to clone PAF receptor cDNAs and genes, but also generate PAF receptor mutant animals, i.e., PAF receptor-overexpressing mouse and PAF receptor-deficient mouse. These mutant mice gave us a novel and specific approach for identifying the pathophysiological and physiological functions of PAF. This review also describes the phenotypes of these mutant mice and discusses them by referring to previously reported pharmacological and genetical data.

Molecular basis of the interaction between plasma platelet-activating factor acetylhydrolase and low density lipoprotein

The platelet-activating factor acetylhydrolases are enzymes that were initially characterized by their ability to hydrolyze platelet-activating factor (PAF). In human plasma, PAF acetylhydrolase (EC 3.1.1.47) circulates in a complex with low density lipoproteins (LDL) and high density lipoproteins (HDL). This association defines the physical state of PAF acetylhydrolase, confers a long half-life, and is a major determinant of its catalytic efficiency in vivo. The lipoprotein-associated enzyme accounts for all of the PAF hydrolysis in plasma but only two-thirds of the protein mass. To characterize the enzyme-lipoprotein interaction, we employed site-directed mutagenesis techniques. Two domains within the primary sequence of human PAF acetylhydrolase, tyrosine 205 and residues 115 and 116, were important for its binding to LDL. Mutation or deletion of those sequences prevented the association of the enzyme with lipoproteins. When residues 115 and 116 from human PAF acetylhydrolase were introduced into mouse PAF acetylhydrolase (which normally does not associate with LDL), the mutant mouse PAF acetylhydrolase associated with lipoproteins. To analyze the role of apolipoprotein (apo) B100 in the formation of the PAF acetylhydrolase-LDL complex, we tested the ability of PAF acetylhydrolase to bind to lipoproteins containing truncated forms of apoB. These studies indicated that the carboxyl terminus of apoB plays a key role in the association of PAF acetylhydrolase with LDL. These data on the molecular basis of the PAF acetylhydrolase-LDL association provide a new level of understanding regarding the pathway for the catabolism of PAF in human blood.

Impaired anaphylactic responses with intact sensitivity to endotoxin in mice lacking a platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator with diverse biological activities in addition to its well-known ability to stimulate platelet aggregation. Pharmacologic studies had suggested a role for PAF in pregnancy, neuronal cell migration, anaphylaxis, and endotoxic shock. Here we show that disruption of the PAF receptor gene in mice caused a marked reduction in systemic anaphylactic symptoms. Unexpectedly, however, the PAF receptor-deficient mice developed normally, were fertile, and remained sensitive to bacterial endotoxin. These mutant mice clearly show that PAF plays a dominant role in eliciting anaphylaxis, but that it is not essential for reproduction, brain development, or endotoxic shock.

Pathobiology of lipopolysaccharide

Lipopolysaccharide is a component of the gram-negative bacterial cell wall that is responsible for 25,000-50,000 deaths in the United States each year. The sequelae of gram-negative infection and septicemia leading to death include fever, hypotension with inadequate tissue perfusion, and disseminated intravascular coagulation. It is clear that different cell types respond differently to lipopolysaccharide. Furthermore, various autacoids and cytokines are released that can affect cellular function even in cell types that do not recognize lipopolysaccharide. Despite advances made in the etiology of septic shock and organ failure, therapy is still for the most part supportive and largely ineffective. The aim of this review is to summarize the current understanding of the role of lipopolysaccharide in the development of septicemia by examining signal transduction and therapeutic approaches.