Receptor-Mediated Mechanism Controlling Tissue Levels of Bioactive Lipid Oxidation Products

The OSE complotype and its clinical potential

Cellular death, aging, and tissue damage trigger inflammation that leads to enzymatic and non-enzymatic lipid peroxidation of polyunsaturated fatty acids present on cellular membranes and lipoproteins. This results in the generation of highly reactive degradation products, such as malondialdehyde (MDA) and 4-hydroxynonenal (4-HNE), that covalently modify free amino groups of proteins and lipids in their vicinity. These newly generated neoepitopes represent a unique set of damage-associated molecular patterns (DAMPs) associated with oxidative stress termed oxidation-specific epitopes (OSEs). OSEs are enriched on oxidized lipoproteins, microvesicles, and dying cells, and can trigger sterile inflammation. Therefore, prompt recognition and removal of OSEs is required to maintain the homeostatic balance. This is partially achieved by various humoral components of the innate immune system, such as natural IgM antibodies, pentraxins and complement components that not only bind OSEs but in some cases modulate their pro-inflammatory potential. Natural IgM antibodies are potent complement activators, and 30% of them recognize OSEs such as oxidized phosphocholine (OxPC-), 4-HNE-, and MDA-epitopes. Furthermore, OxPC-epitopes can bind the complement-activating pentraxin C-reactive protein, while MDA-epitopes are bound by C1q, C3a, complement factor H (CFH), and complement factor H-related proteins 1, 3, 5 (FHR-1, FHR-3, FHR-5). In addition, CFH and FHR-3 are recruited to 2-(ω-carboxyethyl)pyrrole (CEP), and full-length CFH also possesses the ability to attenuate 4-HNE-induced oxidative stress. Consequently, alterations in the innate humoral defense against OSEs predispose to the development of diseases associated with oxidative stress, as shown for the prototypical OSE, MDA-epitopes. In this mini-review, we focus on the mechanisms of the accumulation of OSEs, the pathophysiological consequences, and the interactions between different OSEs and complement components. Additionally, we will discuss the clinical potential of genetic variants in OSE-recognizing complement proteins – the OSE complotype - in the risk estimation of diseases associated with oxidative stress.

Investigational drugs in clinical trials for macular degeneration

INTRODUCTION

Intravitreal anti-vascular endothelial growth factor (VEGF) injections for exudative age-related macular degeneration (eAMD) are effective and safe but require frequent injections and have nonresponding patients. Geographic atrophy/dry AMD (gaAMD) remains an unmet medical need . New therapies are needed to address this leading cause of blindness in the increasing aged population.

AREAS COVERED

This paper reviews the pathogenesis of macular degeneration, current and failed therapeutics, therapies undergoing clinical trials and a rationale for why certain AMD therapies may succeed or fail .

EXPERT OPINION

VEGF- inhibitors reduce both vascular leakage and neovascularization. Experimental therapies that only address neovascularization or leakage will unlikely supplant anti-VEGF therapies. The most promising future therapies for eAMD, are those that target, more potently inhibit and have a more sustained effect on the VEGF pathway such as KSI-301, RGX-314, CLS-AX, EYEP-1901, OTX-TKI.

GaAMD is a phenotype of phagocytic retinal cell loss. Inhibiting phagocytic activity of retinal microglial/macrophages at the border of GA and reducing complement derived activators of microglial/macrophage is the most promising strategy. Complement inhibitors (Pegcetacoplan and Avacincaptad pegol) will likely obtain FDA approval but will serve to pave the way for combined complement and direct phagocytic inhibitors such as AVD-104.

Oxidized Lipids: Common Immunogenic Drivers of Non-Alcoholic Fatty Liver Disease and Atherosclerosis

The prevalence of non-alcoholic fatty liver disease (NAFLD), ranging from simple steatosis to inflammatory steatohepatitis (NASH) and cirrhosis, continues to rise, making it one of the major chronic liver diseases and indications for liver transplantation worldwide. The pathological processes underlying NAFLD not only affect the liver but are also likely to have systemic effects. In fact, growing evidence indicates that patients with NAFLD are at increased risk for developing atherosclerosis. Indeed, cardiovascular complications are the leading cause of mortality in NAFLD patients. Here, we aim to address common pathophysiological molecular pathways involved in chronic fatty liver disease and atherosclerosis. In particular, we focus on the role of oxidized lipids and the formation of oxidation-specific epitopes, which are important targets of host immunity. Acting as metabolic danger signals, they drive pro-inflammatory processes and thus contribute to disease progression. Finally, we summarize encouraging studies indicating that oxidized lipids are promising immunological targets to improve intervention strategies for NAFLD and potentially limit the risk of developing atherosclerosis.

Inflammation-dependent oxidative stress metabolites as a hallmark of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease, with poor prognosis and no cure. Substantial evidence implicates inflammation and associated oxidative stress as a potential mechanism for ALS, especially in patients carrying the SOD1 mutation and, therefore, lacking anti-oxidant defense. The brain is particularly vulnerable to oxidation due to the abundance of polyunsaturated fatty acids, such as docosahexaenoic acid (DHA), which can give rise to several oxidized metabolites. Accumulation of a DHA peroxidation product, CarboxyEthylPyrrole (CEP) is dependent on activated inflammatory cells and myeloperoxidase (MPO), and thus marks areas of inflammation-associated oxidative stress. At the same time, generation of an alternative inactive DHA peroxidation product, ethylpyrrole, does not require cell activation and MPO activity. While absent in normal brain tissues, CEP is accumulated in the central nervous system (CNS) of ALS patients, reaching particularly high levels in individuals carrying a SOD1 mutation. ALS brains are characterized by high levels of MPO and lowered anti-oxidant activity (due to the SOD1 mutation), thereby aiding CEP generation and accumulation. Due to DHA oxidation within the membranes, CEP marks cells with the highest oxidative damage. In all ALS cases CEP is present in nearly all astrocytes and microglia, however, only in individuals carrying a SOD1 mutation CEP marks >90% of neurons, thereby emphasizing an importance of CEP accumulation as a potential hallmark of oxidative damage in neurodegenerative diseases.

Modification of Extracellular Matrix by the Product of DHA Oxidation Switches Macrophage Adhesion Patterns and Promotes Retention of Macrophages During Chronic Inflammation

Oxidation of polyunsaturated fatty acids contributes to different aspects of the inflammatory response due to the variety of products generated. Specifically, the oxidation of DHA produces the end-product, carboxyethylpyrrole (CEP), which forms a covalent adduct with proteins via an ϵ-amino group of lysines. Previously, we found that CEP formation is dramatically increased in inflamed tissue and CEP-modified albumin and fibrinogen became ligands for α_Dβ₂ (CD11d/CD18) and α_Mβ₂ (CD11b/CD18) integrins. In this study, we evaluated the effect of extracellular matrix (ECM) modification with CEP on the adhesive properties of M1-polarized macrophages, particularly during chronic inflammation. Using digested atherosclerotic lesions and in vitro oxidation assays, we demonstrated the ability of ECM proteins to form adducts with CEP, particularly, DHA oxidation leads to the formation of CEP adducts with collagen IV and laminin, but not with collagen I. Using integrin α_Dβ₂-transfected HEK293 cells, WT and αD-/- mouse M1-polarized macrophages, we revealed that CEP-modified proteins support stronger cell adhesion and spreading when compared with natural ECM ligands such as collagen IV, laminin, and fibrinogen. Integrin α_Dβ₂ is critical for M1 macrophage adhesion to CEP. Based on biolayer interferometry results, the isolated α_D I-domain demonstrates markedly higher binding affinity to CEP compared to the "natural" α_Dβ₂ ligand fibrinogen. Finally, the presence of CEP-modified proteins in a 3D fibrin matrix significantly increased M1 macrophage retention. Therefore, CEP modification converts ECM proteins to α_Dβ₂-recognition ligands by changing a positively charged lysine to negatively charged CEP, which increases M1 macrophage adhesion to ECM and promotes macrophage retention during detrimental inflammation, autoimmunity, and chronic inflammation.

Recognition of Oxidized Lipids by Macrophages and Its Role in Atherosclerosis Development

Atherosclerosis is a multifactorial chronic disease that has a prominent inflammatory component. Currently, atherosclerosis is regarded as an active autoimmune process that involves both innate and adaptive immune pathways. One of the drivers of this process is the presence of modified low-density lipoprotein (LDL). For instance, lipoprotein oxidation leads to the formation of oxidation-specific epitopes (OSE) that can be recognized by the immune cells. Macrophage response to OSEs is recognized as a key trigger for initiation and a stimulator of progression of the inflammatory process in the arteries. At the same time, the role of oxidized LDL components is not limited to pro-inflammatory stimulation, but includes immunoregulatory effects that can have protective functions. It is, therefore, important to better understand the complexity of oxidized LDL effects in atherosclerosis in order to develop new therapeutic approaches to correct the inflammatory and metabolic imbalance associated with this disorder. In this review, we discuss the process of oxidized LDL formation, mechanisms of OSE recognition by macrophages and the role of these processes in atherosclerosis.

Intermittent pressure imitating rolling manipulation ameliorates injury in skeletal muscle cells through oxidative stress and lipid metabolism signalling pathways

BACKGROUND

Traditional Chinese medicine manipulation (TCMM) is often used to treat human skeletal muscle injury, but its mechanism remains unclear due to difficulty standardizing and quantifying manipulation parameters.

METHODS

Here, dexamethasone sodium phosphate (DSP) was utilized to induce human skeletal muscle cell (HSkMC) impairments. Cells in a three-dimensional environment were divided into the control normal group (CNG), control injured group (CIG) and rolling manipulation group (RMG). The RMG was exposed to intermittent pressure imitating rolling manipulation (IPIRM) of TCMM via the FX‑5000™ compression system. Skeletal muscle damage was assessed via the cell proliferation rate, superoxide dismutase (SOD) activity, malondialdehyde (MDA) content and creatine kinase (CK) activity. Isobaric tagging for relative and absolute protein quantification (iTRAQ) and bioinformatic analysis were used to evaluate differentially expressed proteins (DEPs).

RESULTS

Higher-pressure IPIRM ameliorated the skeletal muscle cell injury induced by 1.2 mM DSP. Thirteen common DEPs after IPIRM were selected. Key biological processes, molecular functions, cellular components, and pathways were identified as mechanisms underlying the protective effect of TCMM against skeletal muscle damage. Some processes (response to oxidative stress, response to wounding, response to stress and lipid metabolism signalling pathways) were related to skeletal muscle cell injury. Western blotting for 4 DEPs confirmed the reliability of iTRAQ.

CONCLUSIONS

Higher-pressure IPIRM downregulated the CD36, Hsp27 and FABP4 proteins in oxidative stress and lipid metabolism pathways, alleviating excessive oxidative stress and lipid metabolism disorder in injured HSkMCs. The techniques used in this study might provide novel insights into the mechanism of TCMM.

Long-chain acyl-CoA synthetase-1 mediates the palmitic acid-induced inflammatory response in human aortic endothelial cells

Saturated fatty acid (SFA) induces proinflammatory response through a Toll-like receptor (TLR)-mediated mechanism, which is associated with cardiometabolic diseases such as obesity, insulin resistance, and endothelial dysfunction. Consistent with this notion, TLR2 or TLR4 knockout mice are protected from obesity-induced proinflammatory response and endothelial dysfunction. Although SFA causes endothelial dysfunction through TLR-mediated signaling pathways, the mechanisms underlying SFA-stimulated inflammatory response are not completely understood. To understand the proinflammatory response in vascular endothelial cells in high-lipid conditions, we compared the proinflammatory responses stimulated by palmitic acid (PA) and other canonical TLR agonists [lipopolysaccharide (LPS), Pam3-Cys-Ser-Lys4 (Pam₃CSK₄), or macrophage-activating lipopeptide-2)] in human aortic endothelial cells. The expression profiles of E-selectin and the signal transduction pathways stimulated by PA were distinct from those stimulated by canonical TLR agonists. Inhibition of long-chain acyl-CoA synthetases (ACSL) by a pharmacological inhibitor or knockdown of ACSL1 blunted the PA-stimulated, but not the LPS- or Pam₃CSK₄-stimulated proinflammatory responses. Furthermore, triacsin C restored the insulin-stimulated vasodilation, which was impaired by PA. From the results, we concluded that PA stimulates the proinflammatory response in the vascular endothelium through an ACSL1-mediated mechanism, which is distinct from LPS- or Pam₃CSK₄-stimulated responses. The results suggest that endothelial dysfunction caused by PA may require to undergo intracellular metabolism. This expands the understanding of the mechanisms by which TLRs mediate inflammatory responses in endothelial dysfunction and cardiovascular disease.

Oxidation specific epitopes in asthma: New possibilities for treatment

Oxidative stress is an important feature of asthma pathophysiology that is not currently targeted by any of our frontline treatments. Reactive oxygen species, generated during times of heightened oxidative stress, can damage cellular lipids causing the production of oxidation specific epitopes (OSE). OSEs are elevated in chronic inflammatory diseases and promoting their clearance by the body, through pattern recognition receptors and IgM antibodies, prevents and resolves inflammation and tissue damage in animal models. Current research on OSEs in asthma is limited. Although they are present in the lungs of people with asthma during periods of exacerbation or allergen exposure, we do not know if they are linked with disease pathobiology. This article reviews our current understanding of OSEs in asthma and explores whether targeting OSE clearance mechanisms may be a novel therapeutic intervention for asthma.

Amount of Mononuclear Phagocyte Infiltrate Does Not Predict Area of Experimental Choroidal Neovascularization (CNV)

Purpose: Mononuclear phagocytes (MNPs) are present in neovascular age-related macular degeneration (nv AMD) which is also called choroidal neovascularization (CNV). The number and phenotype of the MNPs depend upon the local environment in the CNV and effect of nv AMD therapy. We investigated ocular cell infiltration and conditions that modulate angiogenesis in a laser-induced mouse CNV model.

Methods: We developed assays to quantify MNPs in our established mouse CNV model. One MNP assay quantified the number of subretinal cells peripheral to the CNV lesions. A second assay semiquantitatively assesses the number of MNPs localized to the CNV lesion. We used these assays to measure the effect of toll-like receptor-2 (TLR-2) activation, anti-vascular endothelial growth factor (VEGF) therapy, and chemokine (C-C motif) ligand 2 (Ccl2) genetic deletion on MNP infiltration after laser injury.

Results: Laser injury induced blood vessel growth and infiltration of MNPs. Systemic administration of a TLR-2 activating peptide increased laser-induced CNV area, MNP cell numbers, and MNP density over the CNV lesions. Systemic administration of a VEGF antibody reduced CNV area, while Ccl2 genetic deletion increased CNV area. Despite the change in amount of angiogenesis, MNP infiltration was, surprisingly, unchanged in these 2 conditions.

Conclusions: MNP quantification provides biological insights for candidate AMD therapies. The number of infiltrating MNP cells does not correlate with the amount of laser-induced CNV area.

Inhibition of integrin αDβ2-mediated macrophage adhesion to end product of docosahexaenoic acid (DHA) oxidation prevents macrophage accumulation during inflammation

A critical step in the development of chronic inflammatory diseases is the accumulation of proinflammatory macrophages in the extracellular matrix (ECM) of peripheral tissues. The adhesion receptor integrin α_Dβ₂ promotes the development of atherosclerosis and diabetes by supporting macrophage retention in inflamed tissue. We recently found that the end product of docosahexaenoic acid (DHA) oxidation, 2-(ω-carboxyethyl)pyrrole (CEP), serves as a ligand for α_Dβ₂ CEP adduct with ECM is generated during inflammation-mediated lipid peroxidation. The goal of this project was to identify a specific inhibitor for α_Dβ₂-CEP interaction that can prevent macrophage accumulation. Using a specially designed peptide library, Biacore-detected protein-protein interaction, and adhesion of integrin-transfected HEK 293 cells, we identified a sequence (called P5 peptide) that significantly and specifically inhibited α_D-CEP binding. In the model of thioglycollate-induced peritoneal inflammation, the injection of cyclic P5 peptide reduced 3-fold the macrophage accumulation in WT mice but had no effect in α_D-deficient mice. The tracking of adoptively transferred, fluorescently labeled WT and α_D ^-/- monocytes in the model of peritoneal inflammation and in vitro two-dimensional and three-dimensional migration assays demonstrated that P5 peptide does not affect monocyte transendothelial migration or macrophage efflux from the peritoneal cavity but regulates macrophage migration through the ECM. Moreover, the injection of P5 peptide into WT mice on a high-fat diet prevents macrophage accumulation in adipose tissue in an α_Dβ₂-dependent manner. Taken together, these results demonstrate the importance of α_Dβ₂-mediated macrophage adhesion for the accumulation of infiltrating macrophages in the inflamed ECM and propose P5 peptide as a potential inhibitor of atherogenesis and diabetes.

Oxidative modifications of extracellular matrix promote the second wave of inflammation via β2 integrins

Early stages of inflammation are characterized by extensive oxidative insult by recruited and activated neutrophils. Secretion of peroxidases, including the main enzyme, myeloperoxidase, leads to the generation of reactive oxygen species. We show that this oxidative insult leads to polyunsaturated fatty acid (eg, docosahexaenoate), oxidation, and accumulation of its product 2-(ω-carboxyethyl)pyrrole (CEP), which, in turn, is capable of protein modifications. In vivo CEP is generated predominantly at the inflammatory sites in macrophage-rich areas. During thioglycollate-induced inflammation, neutralization of CEP adducts dramatically reduced macrophage accumulation in the inflamed peritoneal cavity while exhibiting no effect on the early recruitment of neutrophils, suggesting a role in the second wave of inflammation. CEP modifications were abundantly deposited along the path of neutrophils migrating through the 3-dimensional fibrin matrix in vitro. Neutrophil-mediated CEP formation was markedly inhibited by the myeloperoxidase inhibitor, 4-ABH, and significantly reduced in myeloperoxidase-deficient mice. On macrophages, CEP adducts were recognized by cell adhesion receptors, integrin αMβ2 and αDβ2 Macrophage migration through CEP-fibrin gel was dramatically augmented when compared with fibrin alone, and was reduced by β2-integrin deficiency. Thus, neutrophil-mediated oxidation of abundant polyunsaturated fatty acids leads to the transformation of existing proteins into stronger adhesive ligands for αMβ2- and αDβ2-dependent macrophage migration. The presence of a carboxyl group rather than a pyrrole moiety on these adducts, resembling characteristics of bacterial and/or immobilized ligands, is critical for recognition by macrophages. Therefore, specific oxidation-dependent modification of extracellular matrix, aided by neutrophils, promotes subsequent αMβ2- and αDβ2-mediated migration/retention of macrophages during inflammation.

Characterization of covalent modifications of HDL apoproteins by endogenous oxidized phospholipids

High density lipoprotein (HDL) is cardioprotective, unless it is pathologically modified under oxidative stress. Covalent modifications of lipid-free apoA-I, the most abundant apoprotein in HDL, compromise its atheroprotective functions. HDL is enriched in oxidized phospholipids (oxPL) in vivo in oxidative stress. Furthermore, oxidized phospholipids can covalently modify HDL apoproteins. We have now carried out a systematic analysis of modifications of HDL apoproteins by endogenous oxPL. Human HDL or plasma were oxidized using a physiologically relevant MPO-H₂O₂-NO₂^- system or AIPH, or were exposed to synthetic oxPL. Protein adduction by oxPL was assessed using LC-MS/MS and MALDI-TOF MS. The pattern of HDL apoprotein modification by oxPL was independent of the oxidation systems used. ApoA-I and apoA-II were the major modification targets. OxPL with a γ-hydroxy (or oxo)-alkenal were mostly responsible for modifications, and the Michael adduct was the most abundant adduct. Histidines and lysines in helices 5-8 of apoA-I were highly susceptible to oxPL modifications, while lysines in helices 1, 2, 4 and 10 were resistant to modification by oxPL. In plasma exposed to oxidation or synthetic oxPL, oxPL modification was highly selective, and four histidines (H155, H162, H193 and H199) in helices 6-8 of apoA-I were the main modification target. H710 and H3613 in apoB-100 of LDL and K190 of human serum albumin were also modified by oxPL but to a lesser extent. Comparison of oxPL with short chain aldehyde HNE using MALDI-TOF MS demonstrated high selectivity and efficiency of oxPL in the modification of HDL apoproteins. These findings provide a novel insight into a potential mechanism of the loss of atheroprotective function of HDL in conditions of oxidative stress.

TLR2 Plays a Key Role in Platelet Hyperreactivity and Accelerated Thrombosis Associated With Hyperlipidemia

RATIONALE

Platelet hyperreactivity, which is common in many pathological conditions, is associated with increased atherothrombotic risk. The mechanisms leading to platelet hyperreactivity are complex and not yet fully understood.

OBJECTIVE

Platelet hyperreactivity and accelerated thrombosis, specifically in dyslipidemia, have been mechanistically linked to the accumulation in the circulation of a specific group of oxidized phospholipids (oxPC_CD36) that are ligands for the platelet pattern recognition receptor CD36. In the current article, we tested whether the platelet innate immune system contributes to responses to oxPC_CD36 and accelerated thrombosis observed in hyperlipidemia.

METHODS AND RESULTS

Using in vitro approaches, as well as platelets from mice with genetic deletion of MyD88 (myeloid differentiation factor 88) or TLRs (Toll-like receptors), we demonstrate that TLR2 and TLR6 are required for the activation of human and murine platelets by oxPC_CD36. oxPC_CD36 induce formation of CD36/TLR2/TLR6 complex in platelets and activate downstream signaling via TIRAP (Toll-interleukin 1 receptor domain containing adaptor protein)-MyD88-IRAK (interleukin-1 receptor-associated kinase)1/4-TRAF6 (TNF receptor-associated factor 6), leading to integrin activation via the SFK (Src family kinase)-Syk (spleen tyrosine kinase)-PLCγ2 (phospholipase Cγ2) pathway. Intravital thrombosis studies using ApoE^-/- mice with genetic deficiency of TLR2 or TLR6 have demonstrated that oxPC_CD36 contribute to accelerated thrombosis specifically in the setting of hyperlipidemia.

CONCLUSIONS

Our studies reveal that TLR2 plays a key role in platelet hyperreactivity and the prothrombotic state in the setting of hyperlipidemia by sensing a wide range of endogenous lipid peroxidation ligands and activating innate immune signaling cascade in platelets.

Context-Dependent Role of Oxidized Lipids and Lipoproteins in Inflammation

Oxidized low-density lipoprotein (OxLDL), which contains hundreds of different oxidized lipid molecules, is a hallmark of hyperlipidemia and atherosclerosis. The same oxidized lipids found in OxLDL are also formed in apoptotic cells, and are present in tissues as well as in the circulation under pathological conditions. In many disease contexts, oxidized lipids constitute damage signals, or patterns, that activate pattern-recognition receptors (PRRs) and significantly contribute to inflammation. Here, we review recent discoveries and emerging trends in the field of oxidized lipids and the regulation of inflammation, focusing on oxidation products of polyunsaturated fatty acids esterified into cholesteryl esters (CEs) and phospholipids (PLs). We also highlight context-dependent activation and biased agonism of Toll-like receptor-4 (TLR4) and the NLRP3 inflammasome, among other signaling pathways activated by oxidized lipids.

Bioactive metabolites of docosahexaenoic acid

Docosahexaenoic acid (DHA) is an essential fatty acid that is recognized as a beneficial dietary constituent and as a source of the anti-inflammatory specialized proresolving mediators (SPM): resolvins, protectins and maresins. Apart from SPMs, other metabolites of DHA also exert potent biological effects. This article summarizes current knowledge on the metabolic pathways involved in generation of DHA metabolites. Over 70 biologically active metabolites have been described, but are often discussed separately within specific research areas. This review follows DHA metabolism and attempts to integrate the diverse DHA metabolites emphasizing those with identified biological effects. DHA metabolites could be divided into DHA-derived SPMs, DHA epoxides, electrophilic oxo-derivatives (EFOX) of DHA, neuroprostanes, ethanolamines, acylglycerols, docosahexaenoyl amides of amino acids or neurotransmitters, and branched DHA esters of hydroxy fatty acids. These bioactive metabolites have pleiotropic effects that include augmenting energy expenditure, stimulating lipid catabolism, modulating the immune response, helping to resolve inflammation, and promoting wound healing and tissue regeneration. As a result they have been shown to exert many beneficial actions: neuroprotection, anti-hypertension, anti-hyperalgesia, anti-arrhythmia, anti-tumorigenesis etc. Given the chemical structure of DHA, the number and geometry of double bonds, and the panel of enzymes metabolizing DHA, it is also likely that novel bioactive derivatives will be identified in the future.

Carboxyethylpyrroles: From Hypothesis to the Discovery of Biologically Active Natural Products

Our research on the roles of lipid oxidation in human disease is guided by chemical intuition. For example, we postulated that 2-(ω-carboxyethyl)pyrrole (CEP) derivatives of primary amines would be produced through covalent adduction of a γ-hydroxyalkenal generated, in turn, through oxidative fragmentation of docosahexaenoates. Our studies confirmed the natural occurrence of this chemistry, and the biological activities of these natural products and their extensive involvements in human physiology (wound healing) and pathology (age-related macular degeneration, autism, atherosclerosis, sickle cell disease, and tumor growth) continue to emerge. This perspective recounts these discoveries and proposes new frontiers where further developments are likely. Perhaps more significantly, it depicts an effective chemistry-based approach to the discovery of novel biochemistry.

Biological and pathophysiological roles of end-products of DHA oxidation

BACKGROUND

Polyunsaturated fatty acids (PUFA) are known to be present and/or enriched in vegetable and fish oils. Among fatty acids, n-3 PUFA are generally considered to be protective in inflammation-related diseases. The guidelines for substituting saturated fatty acids for PUFAs have been highly publicized for decades by numerous health organizations. Recently, however, the beneficial properties of n-3 PUFA are questioned by detailed analyses of multiple randomized controlled clinical trials. The reported heterogeneity of results is likely due not only to differential effects of PUFAs on various pathological processes in humans, but also to the wide spectrum of PUFA's derived products generated in vivo.

SCOPE OF REVIEW

The goal of this review is to discuss the studies focused on well-defined end-products of PUFAs oxidation, their generation, presence in various pathological and physiological conditions, their biological activities and known receptors. Carboxyethylpyrrole (CEP), a DHA-derived oxidized product, is especially emphasized due to recent data demonstrating its pathophysiological significance in many inflammation-associated diseases, including atherosclerosis, hyperlipidemia, thrombosis, macular degeneration, and tumor progression.

MAJOR CONCLUSIONS

CEP is a product of radical-based oxidation of PUFA that forms adducts with proteins and lipids in blood and tissues, generating new powerful ligands for TLRs and scavenger receptors. The interaction of CEP with these receptors affects inflammatory response, angiogenesis, and wound healing.

GENERAL SIGNIFICANCE

The detailed understanding of CEP-mediated cellular responses may provide a basis for the development of novel therapeutic strategies and dietary recommendations.

Innate sensing of oxidation-specific epitopes in health and disease

Ageing, infections and inflammation result in oxidative stress that can irreversibly damage cellular structures. The oxidative damage of lipids in membranes or lipoproteins is one of these deleterious consequences that not only alters lipid function but also leads to the formation of neo-self epitopes - oxidation-specific epitopes (OSEs) - which are present on dying cells and damaged proteins. OSEs represent endogenous damage-associated molecular patterns that are recognized by pattern recognition receptors and the proteins of the innate immune system, and thereby enable the host to sense and remove dangerous biological waste and to maintain homeostasis. If this system is dysfunctional or overwhelmed, the accumulation of OSEs can trigger chronic inflammation and the development of diseases, such as atherosclerosis and age-related macular degeneration. Understanding the molecular components and mechanisms that are involved in this process will help to identify individuals with an increased risk of developing chronic inflammation, and will also help to indicate novel modes of therapeutic intervention.