Lipopolysaccharide and platelet-activating factor stimulate expression of platelet-activating factor acetylhydrolase via distinct signaling pathways

Structure and hepatoprotective activity of Usp10/NF-κB/Nrf2 pathway-related Morchella esculenta polysaccharide

The water-soluble Morchella esculenta polysaccharide 2 (MEP2) was purified and isolated from an aqueous extract of the Morchella esculenta fruiting bodies. MEP2, having a molecular weight of 959 kDa, has a →4)-α-D-Glcp-(1→ glucan backbone, and this branch was substituted at the H-6 position by an α-D-Glcp-(1 → 4)-α-D-Glcp-(1→ residue and an α-D-Glcp-(1→ residue. The hepatoprotective activity and potential mechanism of action of MEP2 were also investigated. MEP2 ameliorated severe liver damage and regulated the liver function indicators and the alcohol-related enzyme levels in chronic alcohol-induced mice. Combined with biochemical detection, the gut microbiota, metabolites, and proteomics results revealed that MEP2 regulates the levels of hepatic cytokines related to inflammatory response and oxidative stress, as well as those of intestinal Bacteroides, Oscillospira, Parabacteroides, Alistipes, and Prevotella, through the ubiquitin-specific peptidase 10 (Usp10)/nuclear factor κB (NF-κB)/nuclear factor erythroid-2 related factor 2 (Nrf2) signaling pathway in the liver of mice induced by long-term alcohol intake. These data provide experimental evidence for the application of MEP2 in chronic alcohol-induced liver injury.

Urocortin participates in LPS-induced apoptosis of THP-1 macrophages via S1P-cPLA2 signaling pathway

There is little literature showing the effect of urocortin (UCN) on macrophage apoptosis. The underlying mechanism is also unclear. This work was to investigate the involvement of UCN in the regulation of LPS-induced macrophage apoptosis and hence in the prevention from the atherosclerotic lesion development through targeting PLA2. Flow cytometry analysis showed that cell apoptosis was increased by more than 50% after LPS treatment in human THP-1 macrophage. Lp-PLA2 and cPLA2 were found to mediate LPS-induced macrophage apoptosis and NF-κB differentially influenced the expression of Lp-PLA2 and cPLA2. However, the reverse regulation of the expression of Lp-PLA2 and cPLA2 by NF-κB suggested that NF-κB may not be a key target for regulating macrophage apoptosis. Interestingly, we found that the approximate three folds upregulation of cPLA2 was in line with the induction of S1P formation and cell apoptosis by LPS. Inversely, LPS obviously decreased UCN expression by about 50% and secretion by about 25%. Both the enzyme inhibitor and knockdown expression of cPLA2 could completely abolish LPS-induced cell apoptosis. In addition, suppression of S1P synthesis by Sphk1 inhibitor PF-543 reduced the expression of cPLA2 and cell apoptosis but at the same time restored the normal level of UCN in cell culture supernatant. Furthermore, addition of exogenous UCN also reversed LPS-induced expression of cPLA2 and apoptosis. Taken together, UCN may be the reverse regulator of LPS-S1P-cPLA2-apoptosis pathway, thereby contributing to the prevention from the formation of unstable plaques.

Lipoprotein-associated phospholipase A2: The story continues

Inflammation is thought to play an important role in the pathogenesis of vascular diseases. Lipoprotein-associated phospholipase A2 (Lp-PLA2) mediates vascular inflammation through the regulation of lipid metabolism in blood, thus, it has been extensively investigated to identify its role in vascular inflammation-related diseases, mainly atherosclerosis. Although darapladib, the most advanced Lp-PLA2 inhibitor, failed to meet the primary endpoints of two large phase III trials in atherosclerosis patients cotreated with standard medical care, the research on Lp-PLA2 has not been terminated. Novel pathogenic, epidemiologic, genetic, and crystallographic studies regarding Lp-PLA2 have been reported recently, while novel inhibitors were identified through a fragment-based lead discovery strategy. More strikingly, recent clinical and preclinical studies revealed that Lp-PLA2 inhibition showed promising therapeutic effects in diabetic macular edema and Alzheimer's disease. In this review, we not only summarized the knowledge of Lp-PLA2 established in the past decades but also emphasized new findings in recent years. We hope this review could be valuable for helping researchers acquire a much deeper insight into the nature of Lp-PLA2, identify more potent and selective Lp-PLA2 inhibitors, and discover the potential indications of Lp-PLA2 inhibitors.

Maternal Gestational Diabetes Mellitus increases placental and foetal lipoprotein-associated Phospholipase A2 which might exert protective functions against oxidative stress

Increased Lipoprotein associated phospholipase A2 (LpPLA2) has been associated with inflammatory pathologies, including Type 2 Diabetes. Studies on LpPLA2 and Gestational Diabetes Mellitus (GDM) are rare, and have focused mostly on maternal outcome. In the present study, we investigated whether LpPLA2 activity on foetal lipoproteins is altered by maternal GDM and/or obesity (a major risk factor for GDM), thereby contributing to changes in lipoprotein functionality. We identified HDL as the major carrier of LpPLA2 activity in the foetus, which is in contrast to adults. We observed marked expression of LpPLA2 in placental macrophages (Hofbauer cells; HBCs) and found that LpPLA2 activity in these cells was increased by insulin, leptin, and pro-inflammatory cytokines. These regulators were also increased in plasma of children born from GDM pregnancies. Our results suggest that insulin, leptin, and pro-inflammatory cytokines are positive regulators of LpPLA2 activity in the foeto-placental unit. Of particular interest, functional assays using a specific LpPLA2 inhibitor suggest that high-density lipoprotein (HDL)-associated LpPLA2 exerts anti-oxidative, athero-protective functions on placental endothelium and foetus. Our results therefore raise the possibility that foetal HDL-associated LpPLA2 might act as an anti-inflammatory enzyme improving vascular barrier function.

Plasma PAF-AH (PLA2G7): Biochemical Properties, Association with LDLs and HDLs, and Regulation of Expression

This chapter is focused on the plasma form of PAF-acetylhydrolase (PAF-AH), a lipoprotein-bound, calcium-independent phospholipase A2 activity also referred to as lipoprotein-associated phospholipase A2 and PLA2G7. PAF-AH catalyzes the removal of the acyl group at the sn-2 position of PAF and truncated phospholipids generated in settings of inflammation and oxidant stress. Here, I discuss current knowledge related to the structural features of this enzyme, including the molecular basis for association with lipoproteins and susceptibility to oxidative inactivation. The circulating form of PAF-AH is constitutively active and its expression is upregulated by mediators of inflammation at the transcriptional level. Several new mechanisms of regulation have been identified in recent years, including effects mediated by PPARs, VEGFR, and the state of cellular differentiation. Moreover, I discuss recent studies describing significant variations in the structure and regulation of PAF-AH from diverse species, which is likely to have important implications for the function of this enzyme in vivo.

Platelet Activating Factor (PAF) biosynthesis is inhibited by phenolic compounds in U-937 cells under inflammatory conditions

Interleukin 1 beta (IL-1β) induced platelet activating factor (PAF) synthesis in U-937 cells through stimulation of acetyl-CoA:lysoPAF-acetyltransferase (lyso PAF-AT) at 3 h and DTT-independentCDP-choline-1-alkyl-2-acetyl-sn-glycerol cholinophosphotransferase (PAF-CPT) at 0.5 h. The aim of this study was to investigate the effect of tyrosol (T), resveratrol (R) and their acetylated derivatives(AcDs) which exhibit enhanced bioavailability, on PAF synthesis in U-937 after IL-1β stimulation. The specific activity of PAF enzymes and intracellular levels were measured in cell homogenates. T and R concentration capable of inducing 50% inhibition in IL-1β effect on lyso PAF-AT was 48 μΜ ± 11 and 157 μΜ ± 77, for PAF-CPT 246 μΜ ± 61 and 294 μΜ ± 102, respectively. The same order of concentration was also observed on inhibiting PAF levels produced by IL-1β. T was more potent inhibitor than R (p<0.05). AcDs of T retain parent compound inhibitory activity, while in the case of R only two AcDs retain the activity. The observed inhibitory effect by T,R and their AcDs, may partly explain their already reported beneficial role.

Platelet-activating factor: a role in preterm delivery and an essential interaction with Toll-like receptor signaling in mice

Platelet-activating factor (PAF), a potent phospholipid activator of inflammation that signals through its cognate receptor (platelet-activating factor receptor, PTAFR), has been shown to induce preterm delivery in mice. Toll-like receptors (TLRs) are transmembrane receptors that mediate innate immunity. We have shown previously that Escherichia coli-induced preterm delivery in mice requires TLR signaling via the adaptor protein myeloid differentiation primary response gene 88 (MyD88), but not an alternative adaptor, Toll/IL-1 receptor domain-containing adapter protein-inducing interferon-beta (TRIF). In the present work, we analyzed the role of endogenously produced PAF in labor using mice lacking (knockout [KO]) PAF acetylhydrolase (PAF-AH; the key degrading enzyme for PAF). PAF-AH KO mice are more susceptible to E. coli-induced preterm delivery and inflammation than controls. In peritoneal macrophages, the PTAFR agonist carbamyl PAF induces production of inflammatory markers previously demonstrated to be upregulated during bacterially induced labor, including: inducible nitric oxide synthase (Nos2), the chemokine Ccl5 (RANTES), tumor necrosis factor (Tnf), and level of their end-products (NO, CCL5, TNF) in a process dependent upon both IkappaB kinase and calcium/calmodulin-dependent protein kinase II. Interestingly, this induced expression was completely eliminated not only in macrophages deficient in PTAFR, but also in those lacking either TLR4, MyD88, or TRIF. The dependence of PAF effects upon TLR pathways appears to be related to production of PTAFR itself: PAF-induced expression of Ptafr mRNA was eliminated completely in TLR4 KO and partially in MyD88 and TRIF KO macrophages. We conclude that PAF signaling plays an important role in bacterially induced preterm delivery. Furthermore, in addition to its cognate receptor, PAF signaling in peritoneal macrophages requires TLR4, MyD88, and TRIF.

PAF and its metabolic enzymes in healthy volunteers: interrelations and correlations with basic characteristics

PAF (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), a potent inflammatory mediator, is synthesized via the remodeling and the de novo route, key enzymes of which are acetyl-CoA:lyso-PAF acetyltransferase (lyso-PAF-AT) and DTT-insensitive CDP-choline:1-alkyl-2-acetyl-sn-glycerol cholinephosphotransferase (PAF-CPT), respectively. PAF-acetylhydrolase (PAF-AH) and its extracellular isoform lipoprotein-associated phospholipase-A(2) (Lp-PLA(2)) catabolize PAF. This study evaluated PAF levels together with leukocyte PAF-CPT, lyso-PAF-AT, PAF-AH and Lp-PLA(2) activities in 106 healthy volunteers. Men had lower PAF levels and higher activity of both catabolic enzymes and lyso-PAF-AT than women (P-values <0.05). Age was inversely correlated with PAF levels in men (r=-0.279, P=0.06) and lyso-PAF-AT in women (r=-0.280, P=0.05). In contrast, Lp-PLA(2) was positively correlated with age (r=0.201, P=0.04). Moreover, PAF-CPT was positively correlated with glucose (r=0.430, P=0.002) in women. In addition, Principal Component Analysis revealed three PAF metabolic patterns: (i) increased activities of PAF-CPT and PAF-AH, (ii) increased activities of PAF-CPT and lyso-PAF-AT and (iii) increased activity of Lp-PLA(2). The present study underlines the complexity of PAF's metabolism determinants.

Pseudomonas aeruginosa toxin ExoU induces a PAF-dependent impairment of alveolar fibrin turnover secondary to enhanced activation of coagulation and increased expression of plasminogen activator inhibitor-1 in the course of mice pneumosepsis

Background

ExoU, a Pseudomonas aeruginosa cytotoxin with phospholipase A₂ activity, was shown to induce vascular hyperpermeability and thrombus formation in a murine model of pneumosepsis. In this study, we investigated the toxin ability to induce alterations in pulmonary fibrinolysis and the contribution of the platelet activating factor (PAF) in the ExoU-induced overexpression of plasminogen activator inhibitor-1 (PAI-1).

Methods

Mice were intratracheally instilled with the ExoU producing PA103 P. aeruginosa or its mutant with deletion of the exoU gene. After 24 h, animal bronchoalveolar lavage fluids (BALF) were analyzed and lung sections were submitted to fibrin and PAI-1 immunohistochemical localization. Supernatants from A549 airway epithelial cells and THP-1 macrophage cultures infected with both bacterial strains were also analyzed at 24 h post-infection.

Results

In PA103-infected mice, but not in control animals or in mice infected with the bacterial mutant, extensive fibrin deposition was detected in lung parenchyma and microvasculature whereas mice BALF exhibited elevated tissue factor-dependent procoagulant activity and PAI-1 concentration. ExoU-triggered PAI-1 overexpression was confirmed by immunohistochemistry. In in vitro assays, PA103-infected A549 cells exhibited overexpression of PAI-1 mRNA. Increased concentration of PAI-1 protein was detected in both A549 and THP-1 culture supernatants. Mice treatment with a PAF antagonist prior to PA103 infection reduced significantly PAI-1 concentrations in mice BALF. Similarly, A549 cell treatment with an antibody against PAF receptor significantly reduced PAI-1 mRNA expression and PAI-1 concentrations in cell supernatants, respectively.

Conclusion

ExoU was shown to induce disturbed fibrin turnover, secondary to enhanced procoagulant and antifibrinolytic activity during P. aeruginosa pneumosepsis, by a PAF-dependent mechanism. Besides its possible pathophysiological relevance, in vitro detection of exoU gene in bacterial clinical isolates warrants investigation as a predictor of outcome of patients with P. aeruginosa pneumonia/sepsis and as a marker to guide treatment strategies.