New families of bioactive oxidized phospholipids generated by immune cells: identification and signaling actions

Mapping the Human Platelet Lipidome Reveals Cytosolic Phospholipase A2 as a Regulator of Mitochondrial Bioenergetics during Activation

Human platelets acutely increase mitochondrial energy generation following stimulation. Herein, a lipidomic circuit was uncovered whereby the substrates for this are exclusively provided by cPLA2, including multiple fatty acids and oxidized species that support energy generation via β-oxidation. This indicates that acute lipid membrane remodeling is required to support energetic demands during platelet activation. Phospholipase activity is linked to energy metabolism, revealing cPLA2 as a central regulator of both lipidomics and energy flux. Using a lipidomic approach (LipidArrays), we also estimated the total number of lipids in resting, thrombin-activated, and aspirinized platelets. Significant diversity between genetically unrelated individuals and a wealth of species was revealed. Resting platelets demonstrated ∼5,600 unique species, with only ∼50% being putatively identified. Thrombin elevated ∼900 lipids >2-fold with 86% newly appearing and 45% inhibited by aspirin supplementation, indicating COX-1 is required for major activation-dependent lipidomic fluxes. Many lipids were structurally identified. With ∼50% of the lipids being absent from databases, a major opportunity for mining lipids relevant to human health and disease is presented.

Nrf2 deficiency causes lipid oxidation, inflammation, and matrix-protease expression in DHA-supplemented and UVA-irradiated skin fibroblasts

Fish oil rich in docosahexaenoic acid (DHA) has beneficial effects on human health. Omega-3 polyunsaturated fatty acids are precursors of eicosanoids and docosanoids, signaling molecules that control inflammation and immunity, and their dietary uptake improves a range of disorders including cardiovascular diseases, ulcerative colitis, rheumatoid arthritis, and psoriasis. The unsaturated nature of these fatty acids, however, makes them prone to oxidation, especially when they are incorporated into (membrane) phospholipids. The skin is an organ strongly exposed to oxidative stress, mainly due to solar ultraviolet radiation. Thus, increased levels of PUFA in combination with oxidative stress could cause increased local generation of oxidized lipids, whose action spectrum reaches from signaling molecules to reactive carbonyl compounds that can crosslink biomolecules. Here, we investigated whether PUFA supplements to fibroblasts are incorporated into membrane phospholipids and whether an increase of PUFA within phospholipids affects the responses of the cells to UV exposure. The redox-sensitive transcription factor Nrf2 is the major regulator of the fibroblast stress response to ultraviolet radiation or exposure to oxidized lipids. Here we addressed how Nrf2 signaling would be affected in PUFA-supplemented human dermal fibroblasts and mouse dermal fibroblasts from Nrf2-deficient and wild type mice. We found, using HPLC-tandem MS, that DHA supplements to culture media of human and murine fibroblasts were readily incorporated into phospholipids and that subsequent irradiation of the supplemented cells with UVA resulted in an increase in 1-palmitoyl-2-(epoxyisoprostane-E2)-sn-glycero-3-phosphorylcholine and Oxo-DHA esterified to phospholipid, both of which are Nrf2 agonists. Also, induction of Nrf2 target genes was enhanced in the DHA-supplemented fibroblasts after UVA irradiation. In Nrf2-deficient murine fibroblasts, the expression of the target genes was, as expected, decreased, but surprisingly, expression of TNFα and MMP13 was strongly induced in DHA-supplemented, UVA-irradiated cells. Also, Nrf2-deficient cells had increased levels of oxidized phospholipids relative to the unoxidized precursors after UVA irradiation. Our data suggest that under ultraviolet stress a functioning Nrf2 system is required to prevent DHA-induced inflammation and matrix degradation in dermal fibroblasts.

Oxidative lipidomics coming of age: advances in analysis of oxidized phospholipids in physiology and pathology

SIGNIFICANCE

Oxidized phospholipids are now well recognized as markers of biological oxidative stress and bioactive molecules with both pro-inflammatory and anti-inflammatory effects. While analytical methods continue to be developed for studies of generic lipid oxidation, mass spectrometry (MS) has underpinned the advances in knowledge of specific oxidized phospholipids by allowing their identification and characterization, and it is responsible for the expansion of oxidative lipidomics.

RECENT ADVANCES

Studies of oxidized phospholipids in biological samples, from both animal models and clinical samples, have been facilitated by the recent improvements in MS, especially targeted routines that depend on the fragmentation pattern of the parent molecular ion and improved resolution and mass accuracy. MS can be used to identify selectively individual compounds or groups of compounds with common features, which greatly improves the sensitivity and specificity of detection. Application of these methods has enabled important advances in understanding the mechanisms of inflammatory diseases such as atherosclerosis, steatohepatitis, leprosy, and cystic fibrosis, and it offers potential for developing biomarkers of molecular aspects of the diseases.

CRITICAL ISSUES AND FUTURE DIRECTIONS

The future in this field will depend on development of improved MS technologies, such as ion mobility, novel enrichment methods and databases, and software for data analysis, owing to the very large amount of data generated in these experiments. Imaging of oxidized phospholipids in tissue MS is an additional exciting direction emerging that can be expected to advance understanding of physiology and disease.

Targeted lipidomic strategies for oxygenated metabolites of polyunsaturated fatty acids

Oxidation of polyunsaturated fatty acids (PUFA) through enzymatic or non-enzymatic free radical-mediated reactions can yield an array of lipid metabolites including eicosanoids, octadecanoids, docosanoids and related species. In mammals, these oxygenated PUFA mediators play prominent roles in the physiological and pathological regulation of many key biological processes in the cardiovascular, renal, reproductive and other systems including their pivotal contribution to inflammation. Mass spectrometry-based technology platforms have revolutionized our ability to analyze the complex mixture of lipid mediators found in biological samples, with increased numbers of metabolites that can be simultaneously quantified from a single sample in few analytical steps. The recent development of high-sensitivity and high-throughput analytical tools for lipid mediators affords a broader view of these oxygenated PUFA species, and facilitates research into their role in health and disease. In this review, we illustrate current analytical approaches for a high-throughput lipidomic analysis of eicosanoids and related mediators in biological samples. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance."

Urinary metabolomics for noninvasive detection of borderline and acute T cell-mediated rejection in children after kidney transplantation

The goal of this study was to evaluate the utility of urinary metabolomics for noninvasive diagnosis of T cell-mediated rejection (TCMR) in pediatric kidney transplant recipients. Urine samples (n = 277) from 57 patients with surveillance or indication kidney biopsies were assayed for 134 unique metabolites by quantitative mass spectrometry. Samples without TCMR (n = 183) were compared to borderline tubulitis (n = 54) and TCMR (n = 30). Partial least squares discriminant analysis identified distinct classifiers for TCMR (area under receiver operating characteristic curve [AUC] = 0.892; 95% confidence interval [CI] 0.827-0.957) and borderline tubulitis (AUC = 0.836; 95% CI 0.781-0.892), respectively. Application of the TCMR classifier to borderline tubulitis samples yielded a discriminant score (-0.47 ± 0.33) mid-way between TCMR (-0.20 ± 0.34) and No TCMR (-0.80 ± 0.32) (p < 0.001 for all comparisons). Discriminant scoring for combined borderline/TCMR versus No TCMR (AUC = 0.900; 95% CI 0.859-0.940) applied to a validation cohort robustly distinguished between samples with (-0.08 ± 0.52) and without (-0.65 ± 0.54, p < 0.001) borderline/TCMR (p < 0.001). The TCMR discriminant score was driven by histological t-score, ct-score, donor-specific antibody and biopsy indication, and was unaffected by renal function, interstitial or microcirculatory inflammation, interstitial fibrosis or pyuria. These preliminary findings suggest that urinary metabolomics is a sensitive, specific and noninvasive tool for TCMR identification that is superior to serum creatinine, with minimal confounding by other allograft injury processes.

Oxidized lipids and lysophosphatidylcholine induce the chemotaxis, up-regulate the expression of CCR9 and CXCR4 and abrogate the release of IL-6 in human monocytes

Lipids through regulation of chronic inflammation play key roles in the development of various diseases. Here, we report that a mixed population of human primary monocytes migrated towards LPC, as well as oxidized linoleic acid isoforms 9-S-HODE, 9-R-HODE and 13-R-HODE. Incubation with 9-R-HODE, 13-R-HODE and LPC resulted in increased expression of CXCR4, the receptor for SDF-1α/CXCL12, correlated with increased monocyte migration towards SDF-1α/CXCL12. Further, we report increased expression of CCR9, the receptor for TECK/CCL25, after stimulation with these lipids. Upon examining the migratory response towards TECK/CCL25, it was observed that an increase in CCR9 expression upon pre-treatment with 9-S-HODE, 9-R-HODE, 13-R-HODE and LPC resulted in increased migration of monocytes expressing CCR9. Only LPC but not any other lipid examined increased the influx of intracellular Ca²⁺ in monocytes. Finally, 9-S-HODE, 9-R-HODE, 13-R-HODE, or LPC inhibited the release of IL-6 from monocytes suggesting that these lipids may play important role in controlling inflammatory responses.

Oxidized PLs and vascular inflammation

Oxidized PLs (OxPLs) generated in health and disease are now recognized as important mediators of cellular signalling. There is an increasing body of evidence showing that PL peroxidation is not only increased in vascular disorders, but is also a physiological event of relevance to coagulation, innate immunity, and self-tolerance. Nonenzymatically formed OxPLs generated during chronic inflammation is an uncontrolled event, generating hundreds of diverse structures, and prone to more deleterious bioactivities. In contrast, enzymatic formation of OxPLs is tightly regulated, involving receptors and intracellular signaling, acting as part of the normal physiological response to injury in order to restore homeostasis. In the present review, the major nonenzymatic OxPLs structures found during vascular inflammation are summarized, along with a brief description of their known biological activities. Also, we review what is currently known about enzymatic formation of OxPLs by acutely activated immune cells and their signaling actions under homeostatic and pathological conditions.

Emerging importance of omega-3 fatty acids in the innate immune response: molecular mechanisms and lipidomic strategies for their analysis

The beneficial health properties of dietary omega-3 polyunsaturated fatty acids, particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) have long been known and their metabolic dysfunction has been linked to a range of diseases including various inflammatory disorders, cardiovascular diseases, and cancer. However, the molecular mechanisms underlying their health benefits have remained unclear. Recent technological advances in lipidomic analytical strategies have resulted in the discovery of a range of bioactive mediators derived from EPA and DHA that possess potent anti-inflammatory and pro-resolving properties and that may be responsible, at least in part, for the beneficial effects observed. These mediators include resolvins, protectins and maresins, as well as EPA derivatives of classical arachidonic acid derived eicosanoids, such as prostaglandin E3 . The aim of this review is to provide an overview of the biosynthetic pathways and biological properties of these omega-3 mediators, with a particular focus on the emerging importance of the counter-regulatory role of omega-3 and -6 fatty acids in the spatial and temporal regulation of the inflammatory response. It will also provide an insight into a range of lipidomic approaches, which are currently available to analyse these fatty acids and their metabolites in biological matrices.

Human platelets generate phospholipid-esterified prostaglandins via cyclooxygenase-1 that are inhibited by low dose aspirin supplementation

Oxidized phospholipids (oxPLs) generated nonenzymatically display pleiotropic biological actions in inflammation. Their generation by cellular cyclooxygenases (COXs) is currently unknown. To determine whether platelets generate prostaglandin (PG)-containing oxPLs, then characterize their structures and mechanisms of formation, we applied precursor scanning-tandem mass spectrometry to lipid extracts of agonist-activated human platelets. Thrombin, collagen, or ionophore activation stimulated generation of families of PGs comprising PGE₂ and D₂ attached to four phosphatidylethanolamine (PE) phospholipids (16:0p/, 18:1p/, 18:0p/, and 18:0a/). They formed within 2 to 5 min of activation in a calcium, phospholipase C, p38 MAP kinases, MEK1, cPLA₂, and src tyrosine kinase-dependent manner (28.1 ± 2.3 pg/2 × 10⁸ platelets). Unlike free PGs, they remained cell associated, suggesting an autocrine mode of action. Their generation was inhibited by in vivo aspirin supplementation (75 mg/day) or in vitro COX-1 blockade. Inhibitors of fatty acyl reesterification blocked generation significantly, while purified COX-1 was unable to directly oxidize PE in vitro. This indicates that they form in platelets via rapid esterification of COX-1 derived PGE₂/D₂ into PE. In summary, COX-1 in human platelets acutely mediates membrane phospholipid oxidation via formation of PG-esterified PLs in response to pathophysiological agonists.

Systems biological approaches to measure and understand vaccine immunity in humans

Recent studies have demonstrated the utility of using systems approaches to identify molecular signatures that can be used to predict vaccine immunity in humans. Such approaches are now being used extensively in vaccinology, and are beginning to yield novel insights about the molecular networks driving vaccine immunity. In this review, we present a broad review of the methodologies involved in these studies, and discuss the promise and challenges involved in this emerging field of "systems vaccinology."

Immune cell dysfunction and inflammation in end-stage renal disease

Uraemia causes inflammation and reduces immune system function as evidenced by an increased risk of viral-associated cancers, increased susceptibility to infections and decreased vaccination responses in patients with end-stage renal disease (ESRD). The substantially increased risk of atherosclerosis in these patients is also probably related to uraemia-associated inflammation. Uraemia is associated with a reduction in the number and function of lymphoid cells, whereas numbers of myeloid cells in uraemic patients are normal or increased with increased production of inflammatory cytokines and reactive oxygen species. Similar to healthy elderly individuals, patients with ESRD have increased numbers of specific proinflammatory subsets of T cells and monocytes, suggesting the presence of premature immunological ageing in these patients. These cells might contribute to inflammation and destabilization of atherosclerotic plaques, and have, therefore, been identified as novel nonclassical cardiovascular risk factors. The cellular composition of the immune system does not normalize after successful kidney transplantation despite a rapid reduction in inflammation and oxidative stress. This finding suggests that premature ageing of the immune system in patients with ESRD might be related to a permanent skewing of the haematopoetic stem cell population towards myeloid-generating subsets, similar to that seen in healthy elderly individuals.

Phospholipid--driven gene regulation

Phospholipids (PLs), well known for their fundamental role in cellular structure, play critical signaling roles via their derivatives and cleavage products acting as second messengers in signaling cascades. Recent work has shown that intact PLs act as signaling molecules in their own right by modulating the activity of nuclear hormone transcription factors responsible for tuning genes involved in metabolism, lipid flux, steroid synthesis and inflammation. As such, PLs have been classified as novel hormones. This review highlights recent work in PL-driven gene regulation with a focus on the unique structural features of phospholipid-sensing transcription factors and what sets them apart from well known soluble phospholipid transporters.

Elevated level of pro-inflammatory eicosanoids and EPC dysfunction in diabetic patients with cardiac ischemia

BACKGROUND

Circulating endothelial progenitor cells (EPCs) are recruited from the blood system to sites of ischemia and endothelial damage, where they contribute to the repair and development of blood vessels. Since numerous eicosanoids including leukotrienes (LTs) and hydroxyeicosatetraenoic acids (HETEs) have been shown to exert potent pro-inflammatory activities, we examined their levels in chronic diabetic patients with severe cardiac ischemia in conjunction with the level and function of EPCs.

RESULTS

Lipidomic analysis revealed a diabetes-specific increase (p<0.05) in inflammatory and angiogenic eicosanoids including the 5-lipoxygenase-derived LTB (4.11±1.17 vs. 0.96±0.27 ng/ml), the lipoxygenase/CYP-derived 12-HETE (117.08±35.05 vs. 24.34±10.03 ng/ml), 12-HETrE (17.56±4.43 vs. 4.15±2.07 ng/ml), and the CYP-derived 20-HETE (0.32±0.04 vs. 0.06±0.05 ng/ml) the level of which correlated with BMI (p=0.0027). In contrast, levels of the CYP-derived EETs were not significantly (p=0.36) different between these two groups. EPC levels and their colony-forming units were lower (p<0.05) with a reduced viability in diabetic patients compared with non-diabetics. EPC function (colony-forming units (CFUs) and MTT assay) also negatively correlated with the circulating levels of HgA1C.

CONCLUSION

This study demonstrates a close association between elevated levels of highly pro-inflammatory eicosonoids, diabetes and EPC dysfunction in patients with cardiac ischemia, indicating that chronic inflammation impact negatively on EPC function and angiogenic capacity in diabetes.