Joakim Huber

Protective role of phospholipid oxidation products in endotoxin-induced tissue damage

Lipopolysaccharide (LPS), an outer-membrane component of Gram-negative bacteria, interacts with LPS-binding protein and CD14, which present LPS to toll-like receptor 4 (refs 1, 2), which activates inflammatory gene expression through nuclear factor κB (NFκB) and mitogen-activated protein-kinase signalling. Antibacterial defence involves activation of neutrophils that generate reactive oxygen species capable of killing bacteria; therefore host lipid peroxidation occurs, initiated by enzymes such as NADPH oxidase and myeloperoxidase. Oxidized phospholipids are pro-inflammatory agonists promoting chronic inflammation in atherosclerosis; however, recent data suggest that they can inhibit expression of inflammatory adhesion molecules. Here we show that oxidized phospholipids inhibit LPS-induced but not tumour-necrosis factor-α-induced or interleukin-1β-induced NFκB-mediated upregulation of inflammatory genes, by blocking the interaction of LPS with LPS-binding protein and CD14. Moreover, in LPS-injected mice, oxidized phospholipids inhibited inflammation and protected mice from lethal endotoxin shock. Thus, in severe Gram-negative bacterial infection, endogenously formed oxidized phospholipids may function as a negative feedback to blunt innate immune responses. Furthermore, identified chemical structures capable of inhibiting the effects of endotoxins such as LPS could be used for the development of new drugs for treatment of sepsis.

Oxidized phospholipids induce expression of human heme oxygenase-1 involving activation of cAMP-responsive element-binding protein.

Heme oxygenase-1 (HO-1) catalyzes the rate-limiting step in heme degradation, protects against oxidative stress, and shows potent anti-inflammatory effects. Oxidized phospholipids, which are generated during inflammation and apoptosis, modulate the inflammatory response by inducing the expression of several genes including HO-1. Here we investigated the signaling pathways and transcriptional events involved in the induction of HO-1 gene expression by oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC) in human umbilical vein endothelial cells. OxPAPC up-regulated HO-1 mRNA and protein in a time- and concentration-dependent manner, whereas pro-inflammatory agents like TNF-α and lipopolysaccharide did not significantly induce HO-1 expression in human umbilical vein endothelial cells. Signaling pathways involved in the OxPAPC-mediated HO-1 induction included protein kinases A and C, as well as the mitogen-activated protein kinases p38 and ERK. The cAMP-responsive element-binding protein (CREB) was phosphorylated via these pathways in response to OxPAPC treatment and expression of a dominant-negative mutant of CREB inhibited OxPAPC-induced activity of a human heme oxygenase-1 promoter-driven luciferase reporter construct. We identified a cAMP-responsive element and a Maf recognition element to be involved in the transcriptional activation of the HO-1 promoter by OxPAPC. In gel shift assays we observed binding of CREB to the cAMP-responsive element after OxPAPC treatment. Induction of HO-1 expression by lipid oxidation products via CREB may represent a feedback mechanism to limit inflammation and associated tissue damage.

Role of group II secretory phospholipase A2 in atherosclerosis. 2. Potential involvement of biologically active oxidized phospholipids

Secretory nonpancreatic phospholipase A2 (group II sPLA2) is induced in inflammation and present in atherosclerotic lesions. In an accompanying publication we demonstrate that transgenic mice expressing group II sPLA2 developed severe atherosclerosis. The current study was undertaken to determine whether 1 mechanism by which group II sPLA2 might contribute to the progression of inflammation and atherosclerosis is by increasing the formation of biologically active oxidized phospholipids. In vivo measurements of bioactive lipids were performed, and in vitro studies tested the hypothesis that sPLA2 can increase the accumulation of bioactive phospholipids. We have shown previously that 3 oxidized phospholipids derived from the oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (PAPC) stimulated endothelial cells to bind monocytes, a process that is known to be an important step in atherogenesis. We now show that these 3 biologically active phospholipids are significantly increased in livers of sPLA2 transgenic mice fed a high-fat diet as compared with nontransgenic littermates. We present in vitro evidence for several mechanisms by which these phospholipids may be increased in sPLA2 transgenics. These studies demonstrated that polyunsaturated free fatty acids, which are liberated by sPLA2, increased the formation of bioactive phospholipids in LDL, resulting in increased ability to stimulate monocyte-endothelial interactions. Moreover, sPLA2-treated LDL was oxidized by cocultures of human aortic endothelial cells and smooth muscle cells more efficiently than untreated LDL. Analysis by electrospray ionization-mass spectrometry revealed that the bioactive phospholipids, compared with unoxidized PAPC, were less susceptible to hydrolysis by human recombinant group II sPLA2. In addition, HDL from the transgenic mice and human HDL treated with recombinant sPLA2 in vitro failed, in the coculture system, to protect against the formation of biologically active phospholipids in LDL. This lack of protection may in part relate to the decreased levels of paraoxonase seen in the HDL isolated from the transgenic animals. Taken together, these studies show that levels of biologically active oxidized phospholipids are increased in sPLA2 transgenic mice; they also suggest that this increase may be mediated by effects of sPLA2 on both LDL and HDL.

Analysis of inflammatory gene induction by oxidized phospholipids in vivo by quantitative real-time RT-PCR in comparison with effects of LPS.

Oxidized phospholipids are thought to play a role in the development of atherosclerosis and other chronic inflammatory processes. In this study, we analyzed the expression of inflammatory genes induced by oxidized L-alpha-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholin (OxPAPC) in vitro and in vivo using quantitative real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Cultured human umbilical vein endothelial cells (HUVEC) and monocyte-like U937 cells were treated with OxPAPC or lipopolysaccharide (LPS) for 3 h. For in vivo studies, OxPAPC or LPS was injected intravenously into female C57Bl/6J mice and different tissues were isolated after 3 h. We found that both OxPAPC and LPS induced expression of early growth response factor 1 (EGR-1) and monocyte chemoattractant protein 1 (MCP-1) in HUVEC and of JE, the mouse homologue of MCP-1, in liver and heart. Interestingly, OxPAPC but not LPS increased expression of heme oxygenase 1 (HO-1) in U937 cells, HUVEC, aorta, heart, liver, and isolated blood cells. In contrast, E-selectin was selectively induced by LPS, but not by OxPAPC. Finally, OxPAPC-induced expression of HO-1 was blocked by a platelet-activating factor (PAF) receptor antagonist. We conclude that oxidized phospholipids are biologically active in vivo and exert a specific response inducing a pattern of genes that is different from that induced by LPS. In addition, we demonstrate that the quantitative real-time RT-PCR technology is a proper tool to investigate differential inflammatory gene induction in vivo.

The isoprostane 8-iso-PGF2α stimulates endothelial cells to bind monocytes: differences from thromboxane-mediated endothelial activation

Increased isoprostane levels were found in patients with vascular disease and in atherosclerotic lesions. Isoprostanes interact with vascular cells via thromboxane receptors (TPs) and block TP‐ reduced atherosclerosis in apoE‐deficient mice. This effect was, however, independent of thromboxane formation, suggesting involvement of isoprostanes. We investigated the stimulation of human endothelial cells (ECs) by the isoprostane 8‐iso‐PGF2α with respect to induced monocyte adhesion, a key initiating event in atherogenesis. The isoprostane 8‐iso‐PGF2α stimulated ECs to specifically bind monocytes. The TP agonist U46619 induced monocyte and neutrophil binding and expression of E‐selectin and vascular cell adhesion molecule‐1 (VCAM‐ 1). The monocyte adhesion induced by 8‐iso‐PGF2α was independent of VCAM‐1 and could be blocked by a PKA (H89) and a MEK‐1 inhibitor (PD98059), whereas U46619‐induced monocyte adhesion was mediated by VCAM‐1 and could be blocked by a PKC inhibitor (bisindolylmaleimide I). The effects of 8‐iso‐PGF2α and U46619 could be blocked by a TP antagonist and did not involve NF‐MB translocation but required activation of p38MAPK. Our data demonstrate that 8‐iso‐PGF2α induces monocyte adhesion in ECs by activating TPs or a TP‐ related receptor but via signaling mechanisms that are critically different from those activated by U46619. Therefore, generation of 8‐iso‐PGF2α may induce atherosclerotic lesion formation by activation of ECs to bind monocytes.

Specific monocyte adhesion to endothelial cells induced by oxidized phospholipids involves activation of cPLA2 and lipoxygenase

Oxidized phospholipids stimulate endothelial cells to bind monocytes, but not neutrophils, an initiating event in atherogenesis. Here, we investigate intracellular signaling events induced by oxidized phospholipids in human umbilical vein endothelial cells (HUVECs) that lead to specific monocyte adhesion. In a static adhesion assay, oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine and one of its components, 1-palmitoyl-2-oxovaleroyl-sn-glycero-3-phosphorylcholine, stimulated HUVECs to bind U937 cells and human peripheral blood monocytes but not HL-60 cells or blood neutrophils. Monocyte adhesion was dependent on protein kinases A and C, extracellular signal-regulated kinase 1/2, p38 mitogen activated protein kinases (MAPKs), and cytosolic phospholipase A2 (cPLA2). Inhibition of 12-lipoxygenase (12-LOX), but not cyclooxygenases, blocked monocyte adhesion, and addition of 12-hydroxyeicosatetraenoic acid (12-HETE) mimicked the effects of oxidized phospholipids. Peroxisome proliferator-activated receptor α (PPARα) was excluded as a possible target for 12-HETE, because monocyte adhesion was still induced in endothelial cells from PPARα null mice. Together, our results suggest that oxidized phospholipids stimulate HUVECs to specifically bind monocytes involving MAPK pathways, which lead to the activation of cPLA2 and 12-LOX. Further analysis of signaling pathways induced by oxidized phospholipids that lead to specific monocyte adhesion should ultimately lead to the development of novel therapeutic approaches against chronic inflammatory diseases.

Apoptotic Cells as Sources for Biologically Active Oxidized Phospholipids

Acute inflammation is characterized by an accumulation of polymorphonuclear cells (PMNs), generation of reactive oxygen species, subsequent apoptosis of PMNs, and finally phagocytosis of apoptotic cells by macrophages. Recently, it has been demonstrated that during apoptosis oxidation of membrane phospholipids, especially phosphatidylserine, occurs. Moreover, we have shown that membrane vesicles released from apoptotic cells contain biologically active oxidized phospholipids. The involvement of oxidized phospholipids in the development of atherosclerosis, which is described as a chronic inflammatory disease, is increasingly recognized. These oxidized phospholipids were shown to induce several proinflammatory genes, such as monocyte chemoattractant protein 1 or interleukin-8, and it is hypothesized that lipid oxidation products also play a role in other chronic inflammatory disorders. On the other hand, oxidized phospholipids were shown to exert antiendotoxin effects by inhibiting lipopolysaccharide-induced signaling, representing a possible feedback loop during gram-negative infection. Additionally, it has been described that oxidized phospholipids are capable of inducing genes such as heme oxygenase-1 that are important for the resolution of acute inflammation. Moreover, oxidized phospholipids serve as recognition signals on apoptotic cells facilitating phagocytosis. In this review, we discuss the hypothesis that oxidized phospholipids generated in apoptotic cells (a) propagate chronic inflammation and (b) contribute to the resolution of acute inflammation.

Oxidized Cholesteryl Linoleates Stimulate Endothelial Cells to Bind Monocytes via the Extracellular Signal–Regulated Kinase 1/2 Pathway

Oxidation products of cholesteryl esters have been shown to be present in oxidized low density lipoprotein and in atherosclerotic lesions. Monocyte adhesion to the endothelium is an initiating crucial event in atherogenesis. Here, we show that in vitro oxidized cholesteryl linoleate (oxCL) stimulated human umbilical vein endothelial cells (HUVECs) to bind human peripheral blood mononuclear cells as well as monocyte-like U937 cells but not peripheral blood neutrophils or neutrophil-like HL-60 cells. Among the oxidation products contained in oxCLs, 9-oxononanoyl cholesterol (9-ONC) and cholesteryl linoleate hydroperoxides stimulated U937 cell adhesion. OxCL-induced U937 cell adhesion was inhibited by an antibody against the connecting segment-1 region of fibronectin. Neither oxCL nor 9-ONC induced activation of the classical nuclear factor-&kgr;B pathway. In contrast, stimulation of HUVECs with oxCL resulted in phosphorylation of the extracellular signal–regulated kinase 1/2. Moreover, U937 cell adhesion induced by 9-ONC and oxCL was blocked by a mitogen-activated protein kinase/extracellular signal–regulated kinase inhibitor and a protein kinase C inhibitor. Taken together, oxCLs stimulate HUVECs to specifically bind monocytes, involving endothelial connecting segment-1 and the activation of a protein kinase C– and mitogen-activated protein kinase–dependent pathway. Thus, oxidized cholesteryl esters may play an important role as novel mediators in the initiation and progression of atherosclerosis.

The isoprostane 8-iso-PGE2 stimulates endothelial cells to bind monocytes via cyclic AMP- and p38 MAP kinase-dependent signaling pathways.

Increased levels of isoprostanes have been detected in human atherosclerotic lesions. To examine a possible role for 8-iso-prostaglandin E(2) (8-iso-PGE(2)) in atherogenesis, we tested the effect of 8-iso-PGE(2) on adhesion of leukocytes to human umbilical vein endothelial cells (EC). We demonstrate that 8-iso-PGE(2) stimulates EC to bind monocytes, but not neutrophils. This effect was inhibited by the thromboxane A(2) receptor antagonist SQ29548. Moreover, 8-iso-PGE(2) increased levels of cyclic AMP in EC, and monocyte adhesion induced by 8-iso-PGE(2) was blocked by a protein kinase A inhibitor, H89. In addition, 8-iso-PGE(2 )induced phosphorylation of p38 and extracellular signal-regulated kinase (ERK) 1/2 mitogen-activated protein (MAP) kinase and stimulated expression of EGR-1. A specific inhibitor of p38 MAP kinase (SB203580) abrogated monocyte binding, whereas an inhibitor of the ERK pathway (PD98059) did not block monocyte adhesion induced by 8-iso-PGE(2). Activation of nuclear factor-kappaB (NF-kappaB) and expression of NFkappaB-dependent genes intercellular adhesion molecule-1, vascular cell adhesion molecule-1, and E-selectin were not induced by 8-iso-PGE(2). Taken together, these results demonstrate that 8-iso-PGE(2) stimulates EC to specifically bind monocytes, but not neutrophils. This effect is mediated by cyclic AMP/protein kinase A- and p38 MAP kinase-dependent pathways and is independent of the classical inflammatory NFkappaB pathway. Thus, formation of 8-iso-PGE(2) may play an important role in chronic inflammatory diseases such as atherosclerosis by increasing adhesion and extravasation of monocytes.

Regulation of platelet adhesion by oxidized lipoproteins and oxidized phospholipids.

Activated platelets adhere to the endothelium and release vasoactive mediators which induce vasoconstriction and remodeling of the vessel wall. The influence of native and ex vivo oxidized lipoproteins enriched with oxidized 1-palmitoyl-2-arachidonoyl- sn -glycero-3-phosphorylcholine (ox-PAPC), the major lipid responsible for the biological activity of minimally oxidized LDL (mm-LDL), on platelet adhesion, membrane receptor expression and aggregation was studied. Influence of native and oxidized lipoproteins (5-100 w g protein/ml); ox-PAPC (0.5-50 w g/ml); ADP (1-10 w M) as well as the specific phosphatase 1 and 2A inhibitor okadaic acid (3-10 w M) on platelet adhesion, receptor expression and aggregation was measured. Platelets adhered to all the classes of lipoproteins immobilized in plastic microtiter wells (native lipoproteins: HDL<LDL<VLDL<oxidized lipoproteins<ox-PAPC-enriched lipoproteins). Flow cytometry revealed that lipoproteins increased CD41 expression. Preincubation of platelets with ox-PAPC alone, significantly up-regulated CD62p and CD41 receptors (higher dose) but potently inhibited anti-CD36 MoAb binding. Okadaic acid increased anti-CD41 and decreased anti-CD36 and anti-CD42b MoAbs binding. Neither ox-PAPC nor okadaic acid induced platelet aggregation. CD36 seems to be the main receptor responsible for binding of oxidized lipoproteins, particularly its ox-PAPC epitope. The effect of okadaic acid on CD36 and CD41 argue for the participation of phosphorylation-dependent reorganization of cellular trafficking and microtubule organization by ox-PAPC.