Detection and biological activities of carboxyethylpyrrole ethanolamine phospholipids (CEP-EPs)

Glycative stress as a cause of macular degeneration

People are living longer and rates of age-related diseases such as age-related macular degeneration (AMD) are accelerating, placing enormous burdens on patients and health care systems. The quality of carbohydrate foods consumed by an individual impacts health. The glycemic index (GI) is a kinetic measure of the rate at which glucose arrives in the blood stream after consuming various carbohydrates. Consuming diets that favor slowly digested carbohydrates releases sugar into the bloodstream gradually after consuming a meal (low glycemic index). This is associated with reduced risk for major age-related diseases including AMD, cardiovascular disease, and diabetes. In comparison, consuming the same amounts of different carbohydrates in higher GI diets, releases glucose into the blood rapidly, causing glycative stress as well as accumulation of advanced glycation end products (AGEs). Such AGEs are cytotoxic by virtue of their forming abnormal proteins and protein aggregates, as well as inhibiting proteolytic and other protective pathways that might otherwise selectively recognize and remove toxic species. Using in vitro and animal models of glycative stress, we observed that consuming higher GI diets perturbs metabolism and the microbiome, resulting in a shift to more lipid-rich metabolomic profiles. Interactions between aging, diet, eye phenotypes and physiology were observed. A large body of laboratory animal and human clinical epidemiologic data indicates that consuming lower GI diets, or lower glycemia diets, is protective against features of early AMD (AMDf) in mice and AMD prevalence or AMD progression in humans. Drugs may be optimized to diminish the ravages of higher glycemic diets. Human trials are indicated to determine if AMD progression can be retarded using lower GI diets. Here we summarized the current knowledge regarding the pathological role of glycative stress in retinal dysfunction and how dietary strategies might diminish retinal disease.

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.

Ocular surface toll like receptors in ageing

Background

To evaluate changes in Toll Like Receptors (TLRs) expression at the ocular surface of healthy volunteers within different age groups.

Methods

Fifty-one healthy volunteers were enrolled in a pilot observational study. Clinical function tests (OSDI questionnaire, Schirmer test type I and Break Up time) were assessed in all subjects. Temporal Conjunctival imprints were performed for molecular and immunohistochemical analysis to measure TLRs expression (TLR2, 4, 3, 5, 7, 8, 9 and MyD88).

Results

Immunofluorescence data showed an increased TLR2 and decreased TLR7 and TLR8 immunoreactivity in old conjunctival imprints. Up-regulation of TLR2 and down-regulation of TLR7, TLR8 and MyD88 transcripts expression corroborated the data. A direct correlation was showed between increasing ICAM-1 and increasing TLR2 changes with age. Within the age OSDI score increases, T-BUT values decrease, and goblet cells showed a decreasing trend.

Conclusion

Changes in TLRs expression are associated with ageing, suggesting physiological role of TLRs in modulating ocular surface immunity. TLRs age related changes may participate to the changes of ocular surface homeostatic mechanisms which lead to inflammAging.

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.

Retinal Pigment Epithelium Expressed Toll-like Receptors and Their Potential Role in Age-Related Macular Degeneration

(1) Background: Inflammation is a major pathomechanism in the development and progression of age-related macular degeneration (AMD). The retinal pigment epithelium (RPE) may contribute to retinal inflammation via activation of its Toll-like receptors (TLR). TLR are pattern recognition receptors that detect the pathogen- or danger-associated molecular pattern. The involvement of TLR activation in AMD is so far not understood. (2) Methods: We performed a systematic literature research, consulting the National Library of Medicine (PubMed). (3) Results: We identified 106 studies, of which 54 were included in this review. Based on these studies, the current status of TLR in AMD, the effects of TLR in RPE activation and of the interaction of TLR activated RPE with monocytic cells are given, and the potential of TLR activation in RPE as part of the AMD development is discussed. (4) Conclusion: The activation of TLR2, -3, and -4 induces a profound pro-inflammatory response in the RPE that may contribute to (long-term) inflammation by induction of pro-inflammatory cytokines, reducing RPE function and causing RPE cell degeneration, thereby potentially constantly providing new TLR ligands, which could perpetuate and, in the long run, exacerbate the inflammatory response, which may contribute to AMD development. Furthermore, the combined activation of RPE and microglia may exacerbate neurotoxic effects.

Products of Docosahexaenoate Oxidation as Contributors to Photosensitising Properties of Retinal Lipofuscin

Retinal lipofuscin which accumulates with age in the retinal pigment epithelium (RPE) is subjected to daily exposures to high fluxes of visible light and exhibits potent photosensitising properties; however, the molecules responsible for its photoreactivity remain unknown. Here, we demonstrate that autooxidation of docosahexaenoate (DHE) leads to the formation of products absorbing, in addition to UVB and UVA light, also visible light. The products of DHE oxidation exhibit potent photosensitising properties similar to photosensitising properties of lipofuscin, including generation of an excited triplet state with similar characteristics as the lipofuscin triplet state, and photosensitised formation of singlet oxygen and superoxide. The quantum yields of singlet oxygen and superoxide generation by oxidised DHE photoexcited with visible light are 2.4- and 3.6-fold higher, respectively, than for lipofuscin, which is consistent with the fact that lipofuscin contains some chromophores which do contribute to the absorption of light but not so much to its photosensitising properties. Importantly, the wavelength dependence of photooxidation induced by DHE oxidation products normalised to equal numbers of incident photons is also similar to that of lipofuscin—it steeply increases with decreasing wavelength. Altogether, our results demonstrate that products of DHE oxidation include potent photosensitiser(s) which are likely to contribute to lipofuscin photoreactivity.

Toll-like Receptor 2 Facilitates Oxidative Damage-Induced Retinal Degeneration

Retinal degeneration is a form of neurodegenerative disease and is the leading cause of vision loss globally. The Toll-like receptors (TLRs) are primary components of the innate immune system involved in signal transduction. Here we show that TLR2 induces complement factors C3 and CFB, the common and rate-limiting factors of the alternative pathway in both retinal pigment epithelial (RPE) cells and mononuclear phagocytes. Neutralization of TLR2 reduces opsonizing fragments of C3 in the outer retina and protects photoreceptor neurons from oxidative stress-induced degeneration. TLR2 deficiency also preserves tight junction expression and promotes RPE resistance to fragmentation. Finally, oxidative stress-induced formation of the terminal complement membrane attack complex and Iba1+ cell infiltration are strikingly inhibited in the TLR2-deficient retina. Our data directly implicate TLR2 as a mediator of retinal degeneration in response to oxidative stress and present TLR2 as a bridge between oxidative damage and complement-mediated retinal pathology.

Oxidative stress and complement deposition are common to many retinal degenerative diseases. Mulfaul et al. demonstrate that TLR2 blockade protects against photoreceptor neuronal cell death and RPE fragmentation in experimental models of oxidative stress-induced retinal degeneration and present TLR2 as a bridge between oxidative damage and complement-mediated retinal pathology.

4-Hydroxy-7-oxo-5-heptenoic acid (HOHA) lactone induces apoptosis in retinal pigment epithelial cells

Retinal pigment epithelial (RPE) cell dysfunction and death play vital roles in age-related macular degeneration (AMD) pathogenesis. Previously we showed that oxidative cleavage of docosahexenoate (DHA) phospholipids generates an α,β-unsaturated aldehyde, 4-hydroxy-7-oxohept-4-enoic acid (HOHA) lactone, that forms ω-carboxyethylpyrrole (CEP) derivatives through adduction to proteins and ethanolamine phospholipids. CEP derivatives and autoantibodies accumulate in the retinas and blood plasma of individuals with AMD and are a biomarker of AMD. They promote the choroidal neovascularization of "wet AMD". Immunization of mice with CEP-modified mouse serum albumin induces "dry AMD"-like lesions in their retinas as well as interferon-gamma and interleukin-17 production by CEP-specific T cells that promote inflammatory M1 polarization of macrophages. The present study confirms that oxidative stress or inflammatory stimulus produces CEP in both the primary human ARPE-19 cell line and hRPE cells. Exposure of these cells to HOHA lactone fosters production of reactive oxygen species. Thus, HOHA lactone participates in a vicious cycle, promoting intracellular oxidative stress leading to oxidative cleavage of DHA to produce more HOHA lactone. We now show that HOHA lactone is cytotoxic, inducing apoptotic cell death through activation of the intrinsic pathway. This suggests that therapeutic interventions targeting HOHA lactone-induced apoptosis may prevent the loss of RPE cells during the early phase of AMD. We also discovered that ARPE-19 cells are more susceptible than hRPE cells to HOHA lactone cytotoxicity. This is consistent with the view that, compared to normal RPE cells, ARPE-19 cells exhibit a diseased RPE phenotype that also includes elevated expression of the mesenchymal indicator vimentin, elevated integrin a5 promotor strength and deficient secretion of the anti-VEGF molecule pigment-epithelium-derived factor fostering weaker tight junctions.

Light-induced generation and toxicity of docosahexaenoate-derived oxidation products in retinal pigmented epithelial cells

Oxidative cleavage of docosahexaenoate (DHA) in retinal pigmented epithelial (RPE) cells produces 4-hydroxy-7-oxohept-5-enoic acid (HOHA) esters of 2-lysophosphatidylcholine (PC). HOHA-PC spontaneously releases a membrane-permeant HOHA lactone that modifies primary amino groups of proteins and ethanolamine phospholipids to produce 2-(ω-carboxyethyl)pyrrole (CEP) derivatives. CEPs have significant pathological relevance to age-related macular degeneration (AMD) including activation of CEP-specific T-cells leading to inflammatory M1 polarization of macrophages in the retina involved in "dry AMD" and TLR2-dependent induction of angiogenesis that characterizes "wet AMD". RPE cells accumulate DHA from shed rod photoreceptor outer segments through phagocytosis and from plasma lipoproteins secreted by the liver through active uptake from the choriocapillaris. As a cell model of light-induced oxidative damage of DHA phospholipids in RPE cells, ARPE-19 cells were supplemented with DHA, with or without the lipofuscin fluorophore A2E. In this model, light exposure, in the absence of A2E, promoted the generation HOHA lactone-glutathione (GSH) adducts, depletion of intracellular GSH and a competing generation of CEPs. While DHA-rich RPE cells exhibit an inherent proclivity toward light-induced oxidative damage, photosensitization by A2E nearly doubled the amount of lipid oxidation and expanded the spectral range of photosensitivity to longer wavelengths. Exposure of ARPE-19 cells to 1 μM HOHA lactone for 24 h induced massive (50%) loss of lysosomal membrane integrity and caused loss of mitochondrial membrane potential. Using senescence-associated β-galactosidase (SA β-gal) staining that detects lysosomal β-galactosidase, we determined that exposure to HOHA lactone induces senescence in ARPE-19 cells. The present study shows that products of light-induced oxidative damage of DHA phospholipids in the absence of A2E can lead to RPE cell dysfunction. Therefore, their toxicity may be especially important in the early stages of AMD before RPE cells accumulate lipofuscin fluorophores.

Involvement of a gut-retina axis in protection against dietary glycemia-induced age-related macular degeneration

Age-related macular degeneration (AMD) is the major cause of blindness in developed nations. AMD is characterized by retinal pigmented epithelial (RPE) cell dysfunction and loss of photoreceptor cells. Epidemiologic studies indicate important contributions of dietary patterns to the risk for AMD, but the mechanisms relating diet to disease remain unclear. Here we investigate the effect on AMD of isocaloric diets that differ only in the type of dietary carbohydrate in a wild-type aged-mouse model. The consumption of a high-glycemia (HG) diet resulted in many AMD features (AMDf), including RPE hypopigmentation and atrophy, lipofuscin accumulation, and photoreceptor degeneration, whereas consumption of the lower-glycemia (LG) diet did not. Critically, switching from the HG to the LG diet late in life arrested or reversed AMDf. LG diets limited the accumulation of advanced glycation end products, long-chain polyunsaturated lipids, and their peroxidation end-products and increased C3-carnitine in retina, plasma, or urine. Untargeted metabolomics revealed microbial cometabolites, particularly serotonin, as protective against AMDf. Gut microbiota were responsive to diet, and we identified microbiota in the Clostridiales order as being associated with AMDf and the HG diet, whereas protection from AMDf was associated with the Bacteroidales order and the LG diet. Network analysis revealed a nexus of metabolites and microbiota that appear to act within a gut-retina axis to protect against diet- and age-induced AMDf. The findings indicate a functional interaction between dietary carbohydrates, the metabolome, including microbial cometabolites, and AMDf. Our studies suggest a simple dietary intervention that may be useful in patients to arrest AMD.

Systemic and ocular fluid compounds as potential biomarkers in age-related macular degeneration

Biomarkers can help unravel mechanisms of disease and identify new targets for therapy. They can also be useful in clinical practice for monitoring disease progression, evaluation of treatment efficacy, and risk assessment in multifactorial diseases, such as age-related macular degeneration (AMD). AMD is a highly prevalent progressive retinal disorder for which multiple genetic and environmental risk factors have been described, but the exact etiology is not yet fully understood. Many compounds have been evaluated for their association with AMD. We performed an extensive literature review of all compounds measured in serum, plasma, vitreous, aqueous humor, and urine of AMD patients. Over 3600 articles were screened, resulting in more than 100 different compounds analyzed in AMD studies, involved in neovascularization, immunity, lipid metabolism, extracellular matrix, oxidative stress, diet, hormones, and comorbidities (such as kidney disease). For each compound, we provide a short description of its function and discuss the results of the studies in relation to its usefulness as AMD biomarker. In addition, biomarkers identified by hypothesis-free techniques, including metabolomics, proteomics, and epigenomics, are covered. In summary, compounds belonging to the oxidative stress pathway, the complement system, and lipid metabolism are the most promising biomarker candidates for AMD. We hope that this comprehensive survey of the literature on systemic and ocular fluid compounds as potential biomarkers in AMD will provide a stepping stone for future research and possible implementation in clinical practice.

2-(ω-Carboxyethyl)pyrrole Antibody as a New Inhibitor of Tumor Angiogenesis and Growth

BACKGROUND

Angiogenesis is a fundamental process in the progression, invasion, and metastasis of tumors. Therapeutic drugs such as bevacizumab and ranibuzumab have thus been developed to inhibit vascular endothelial growth factor (VEFG)-promoted angiogenesis. While these anti-angiogenic drugs have been commonly used in the treatment of cancer, patients often develop significant resistance that limits the efficacy of anti-VEGF therapies to a short period of time. This is in part due to the fact that an independent pathway of angiogenesis exists, which is mediated by 2-(ω-carboxyethyl)pyrrole (CEP) in a TLR2 receptor-dependent manner that can compensate for inhibition of the VEGF-mediated pathway.

AIMS

In this work, we evaluated a CEP antibody as a new tumor growth inhibitor that blocks CEP-induced angiogenesis.

METHOD

We first evaluated the effectiveness of a CEP antibody as a monotherapy to impede tumor growth in two human tumor xenograft models. We then determined the synergistic effects of bevacizumab and CEP antibody in a combination therapy, which demonstrated that blocking of the CEP-mediated pathway significantly enhanced the anti-angiogenic efficacy of bevacizumab in tumor growth inhibition indicating that CEP antibody is a promising chemotherapeutic drug. To facilitate potential translational studies of CEP-antibody, we also conducted longitudinal imaging studies and identified that FMISO-PET is a non-invasive imaging tool that can be used to quantitatively monitor the anti-angiogenic effects of CEP-antibody in the clinical setting.

RESULTS

That treatment with CEP antibody induces hypoxia in tumor tissue WHICH was indicated by 43% higher uptake of [18F]FMISO in CEP antibody-treated tumor xenografs than in the control PBS-treated littermates.

Retinal pigment epithelium and microglia express the CD5 antigen-like protein, a novel autoantigen in age-related macular degeneration

We report on a novel autoantigen expressed in human macular tissues, identified following an initial Western blot (WB)-based screening of sera from subjects with age-related macular degeneration (AMD) for circulating auto-antibodies (AAbs) recognizing macular antigens. Immunoprecipitation, 2D-gel electrophoresis (2D-GE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS), direct enzyme-linked immunosorbent assays (ELISA), WBs, immunohistochemistry (IHC), human primary and ARPE-19 immortalized cell cultures were used to characterize this novel antigen. An approximately 40-kDa autoantigen in AMD was identified as the scavenger receptor CD5 antigen-like protein (CD5L), also known as apoptosis inhibitor of macrophage (AIM). CD5L/AIM was localized to human RPE by IHC and WB methods and to retinal microglial cells by IHC. ELISAs with recombinant CD5L/AIM on a subset of AMD sera showed a nearly 2-fold higher anti-CD5L/AIM reactivity in AMD vs. Control sera (p = 0.000007). Reactivity ≥0.4 was associated with 18-fold higher odds of having AMD (χ² = 21.42, p = 0.00063). Circulating CD5L/AIM levels were also nearly 2-fold higher in AMD sera compared to controls (p = 0.0052). The discovery of CD5L/AIM expression in the RPE and in retinal microglial cells adds to the known immunomodulatory roles of these cells in the retina. The discovery of AAbs recognizing CD5L/AIM identifies a possible novel disease biomarker and suggest a potential role for CD5L/AIM in the pathogenesis of AMD in situ. The possible mechanisms via which anti-CD5L/AIM AAbs may contribute to AMD pathogenesis are discussed. In particular, since CD5L is known to stimulate autophagy and to participate in oxidized LDL uptake in macrophages, we propose that anti-CD5L/AIM auto-antibodies may play a role in drusen biogenesis and inflammatory RPE damage in AMD.

4-Hydroxy-7-oxo-5-heptenoic Acid Lactone Induces Angiogenesis through Several Different Molecular Pathways

Oxidative stress and angiogenesis have been implicated not only in normal phenomena such as tissue healing and remodeling but also in many pathological processes. However, the relationships between oxidative stress and angiogenesis still remain unclear, although oxidative stress has been convincingly demonstrated to influence the progression of angiogenesis under physiological and pathological conditions. The retina is particularly susceptible to oxidative stress because of its intensive oxygenation and high abundance of polyunsaturated fatty acyls. In particular, it has high levels of docosahexanoates, whose oxidative fragmentation produces 4-hydroxy-7-oxo-5-heptenoic acid lactone (HOHA-lactone). Previously, we found that HOHA-lactone is a major precursor of 2-(ω-carboxyethyl)pyrrole (CEP) derivatives, which are tightly linked to age-related macular degeneration (AMD). CEPs promote the pathological angiogenesis of late-stage AMD. We now report additional mechanisms by which HOHA-lactone promotes angiogenesis. Using cultured ARPE-19 cells, we observed that HOHA-lactone induces secretion of vascular endothelial growth factor (VEGF), which is correlated to increases in reactive oxygen species and decreases in intracellular glutathione (GSH). Wound healing and tube formation assays provided, for the first time, in vitro evidence that HOHA-lactone induces the release of VEGF from ARPE-19 cells, which promotes angiogenesis by human umbilical vein endothelial cells (HUVEC) in culture. Thus, HOHA-lactone can stimulate vascular growth through a VEGF-dependent pathway. In addition, results from MTT and wound healing assays as well as tube formation experiments showed that GSH-conjugated metabolites of HOHA-lactone stimulate HUVEC proliferation and promote angiogenesis in vitro. Previous studies demonstrated that HOHA-lactone, through its CEP derivatives, promotes angiogenesis in a novel Toll-like receptor 2-dependent manner that is independent of the VEGF receptor or VEGF expression. The new studies show that HOHA-lactone also participates in other angiogenic signaling pathways that include promoting the secretion of VEGF from retinal pigmented epithelial cells.

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.

Bioactive 4-Oxoheptanedioic Monoamide Derivatives of Proteins and Ethanolaminephospholipids: Products of Docosahexaenoate Oxidation

Oxidative stress causes lipid-derived oxidative modification of biomolecules that has been implicated in many pathological states. Phospholipids containing polyunsaturated fatty acids are major targets of free radical-initiated oxidation. Phospholipids that incorporate docosahexaenoate (DHA) are highly enriched in important neural structures including the brain and retina, where DHA comprises 40% and 60% of total fatty acids, respectively. Oxidative fragmentation of 2-docosahexaenoyl-1-palmityl-sn-glycerophosphocholine generates esters of 4-hydroxy-7-oxohept-5-enoic acid (HOHA) and 4-keto-7-oxohept-5-enoic acid (KOHA) with 2-lysophosphatidylcholine, HOHA-PC, and KOHA-PC. Covalent HOHA adducts that incorporate the primary amino groups of proteins and ethanolamine phospholipids in carboxyethylpyrrole (CEP) derivatives were detected immunologically with anti-CEP antibodies in human tumors, retina, and blood. Now, we generated an anti-OHdiA antibody to test the hypothesis that KOHA adducts, which incorporate the primary amino groups of proteins or ethanolamine phospholipids in 4-oxo-heptanedioic (OHdiA) monoamide derivatives, are present in vivo. However, whereas the anti-CEP antibody is highly specific and does not cross-react with the OHdiA monoamide epitope, the anti-OHdiA monoamide antibody cross-reacted with CEP epitopes making it of little value as an analytical tool for OHdiA monoamides but suggesting the possibility that OHdiA monoamides would exhibit receptor-mediated biological activity similar to that of CEP. An LC-MS/MS method was developed that allows quantification of OHdiA derivatives in biological samples. We now find that KOHA-PC forms OHdiA monoamide adducts of proteins and ethanolamine phospholipids and that OHdiA-protein levels are significantly higher than OHdiA-ethanloamine phospholipid levels in blood from healthy human subjects, 0.45 μM and 0.18 μM, respectively (n = 3, and p = 0.027). OHdiA monoamide epitopes are angiogenic, causing TLR2-dependent adhesion and tube formation by human umbilical vein endothelial cells. OHdiA monoamide epitopes are only slightly less potent than CEP epitopes that contribute to the pathological angiogenesis of age-related macular degeneration and tumor growth.

Molecular Structures of Isolevuglandin-Protein Cross-Links

Isolevuglandins (isoLGs) are stereo and structurally isomeric γ-ketoaldehydes produced through free radical-induced oxidation of arachidonates. Some isoLG isomers are also generated through enzymatic cyclooxygenation. Post-translational modification of proteins by isoLGs is associated with loss-of-function, cross-linking and aggregation. We now report that a low level of modification by one or two molecules of isoLG has a profound effect on the activity of a multi subunit protease, calpain-1. Modification of one or two key lysyl residues apparently suffices to abolish catalytic activity. Covalent modification of calpain-1 led to intersubunit cross-linking. Hetero- and homo-oligomers of the catalytic and regulatory subunits of calpain-1 were detected by SDS-PAGE with Western blotting. N-Acetyl-glycyl-lysine methyl ester and β-amyloid(11-17) peptide EVHHQKL were used as models for characterizing the cross-linking of protein lysyl residues resulting from adduction of iso[4]LGE₂. Aminal, bispyrrole, and trispyrrole cross-links of these two peptides were identified and fully characterized by mass spectrometry. Aminal and bispyrrole dimers were both detected. Furthermore, a complex mixture of derivatives of the bispyrrole cross-link containing one or more additional atoms of oxygen was found. Interesting differences are evident in the predominant cross-link type generated in the reaction of iso[4]LGE₂ with these peptides. More aminal cross-links versus bispyrrole are formed during the reaction of the dipeptide with iso[4]LGE₂. In contrast, more bispyrrole versus aminal cross-links are formed during the reaction of EVHHQKL with iso[4]LGE₂. It is tempting to speculate that the EVHHQKL peptide-pyrrole modification forms noncovalent aggregates that favor the production of covalent bispyrrole cross-links because β-amyloid(11-17) tends to spontaneously oligomerize.

Metabolism of 4-Hydroxy-7-oxo-5-heptenoic Acid (HOHA) Lactone by Retinal Pigmented Epithelial Cells

4-Hydroxy-7-oxo-5-heptenic acid (HOHA)-lactone is a biologically active oxidative truncation product released (t1/2 = 30 min at 37 °C) by nonenzymatic transesterification/deacylation from docosahexaenoate lipids. We now report that HOHA-lactone readily diffuses into retinal pigmented epithelial (RPE) cells where it is metabolized. A reduced glutathione (GSH) Michael adduct of HOHA-lactone is the most prominent metabolite detected by LC-MS in both the extracellular medium and cell lysates. This molecule appeared inside of ARPE-19 cells within seconds after exposure to HOHA-lactone. The intracellular level reached a maximum concentration at 30 min and then decreased with concomitant increases in its level in the extracellular medium, thus revealing a unidirectional export of the reduced GSH-HOHA-lactone adduct from the cytosol to extracellular medium. This metabolism is likely to modulate the involvement of HOHA-lactone in the pathogenesis of human diseases. HOHA-lactone is biologically active, e.g., low concentrations (0.1-1 μM) induce secretion of vascular endothelial growth factor (VEGF) from ARPE-19 cells. HOHA-lactone is also a precursor of 2-(ω-carboxyethyl)pyrrole (CEP) derivatives of primary amino groups in proteins and ethanolamine phospholipids that have significant pathological and physiological relevance to age-related macular degeneration (AMD), cancer, and wound healing. Both HOHA-lactone and the derived CEP can contribute to the angiogenesis that defines the neovascular "wet" form of AMD and that promotes the growth of tumors. While GSH depletion can increase the lethality of radiotherapy, because it will impair the metabolism of HOHA-lactone, the present study suggests that GSH depletion will also increase levels of HOHA-lactone and CEP that may promote recurrence of tumor growth.

Efficient Quantitative Analysis of Carboxyalkylpyrrole Ethanolamine Phospholipids: Elevated Levels in Sickle Cell Disease Blood

γ-Hydroxy-α,β-unsaturated aldehydes, generated by oxidative damage of polyunsaturated phospholipids, form pyrrole derivatives that incorporate the ethanolamine phospholipid (EP) amino group such as 2-pentylpyrrole (PP)-EP and 2-(ω-carboxyalkyl)pyrrole (CAP)-EP derivatives: 2-(ω-carboxyethyl)pyrrole (CEP)-EP, 2-(ω-carboxypropyl)pyrrole (CPP)-EP, and 2-(ω-carboxyheptyl)pyrrole (CHP)-EP. Because EPs occur in vivo in various forms, a complex mixture of pyrrole-modified EPs with different molecular weights is expected to be generated. To provide a sensitive index of oxidative stress, all of the differences in mass related to the glycerophospholipid moieties were removed by releasing a single CAP-ethanolamine (ETN) or PP-ETN from each mixture by treatment with phospholipase D. Accurate quantization was achieved using the corresponding ethanolamine-d4 pyrroles as internal standards. The product mixture obtained by phospholipolysis of total blood phospholipids from sickle cell disease (SCD) patients was analyzed by LC-MS/MS. The method was applied to measure CAP-EP and PP-EP levels in blood plasma from clinical monitoring of SCD patients. We found uniformly elevated blood levels of CEP-EP (63.9 ± 9.7 nM) similar to mean levels in blood from age-related macular degeneration (AMD) patients (56.3 ± 37.1 nM), and 2-fold lower levels (27.6 ± 3.6 nM, n = 5) were detected in plasma from SCD patients hospitalized to treat a sickle cell crisis, although mean levels remain higher than those (12.1 ± 10.5 nM) detected in blood from healthy controls. Plasma levels of CPP-EPs from SCD clinic patients were 4-fold higher than those of SCD patients hospitalized to treat a sickle cell crisis (45.1 ± 10.9 nM, n = 5 versus 10.9 ± 3.4 nM, n = 6; p < 0.002). PP-EP concentration in plasma from SCD clinic patients is nearly 4.8-fold higher than its level in plasma samples from SCD patients hospitalized to treat a sickle cell crisis (7.06 ± 4.05 vs 1.48 ± 0.92 nM; p < 0.05). Because CAP-EPs promote angiogenesis and platelet activation, the elevated levels present in SCD blood can contribute to the hypercoaguability and vaso-occlusive events that are critical pathophysiologic features of SCD.

Novel phosphatidylethanolamine derivatives accumulate in circulation in hyperlipidemic ApoE-/- mice and activate platelets via TLR2

A prothrombotic state and increased platelet reactivity are common in dyslipidemia and oxidative stress. Lipid peroxidation, a major consequence of oxidative stress, generates highly reactive products, including hydroxy-ω-oxoalkenoic acids that modify autologous proteins generating biologically active derivatives. Phosphatidylethanolamine, the second most abundant eukaryotic phospholipid, can also be modified by hydroxy-ω-oxoalkenoic acids. However, the conditions leading to accumulation of such derivatives in circulation and their biological activities remain poorly understood. We now show that carboxyalkylpyrrole-phosphatidylethanolamine derivatives (CAP-PEs) are present in the plasma of hyperlipidemic ApoE(-/-) mice. CAP-PEs directly bind to TLR2 and induces platelet integrin αIIbβ3 activation and P-selectin expression in a Toll-like receptor 2 (TLR2)-dependent manner. Platelet activation by CAP-PEs includes assembly of TLR2/TLR1 receptor complex, induction of downstream signaling via MyD88/TIRAP, phosphorylation of IRAK4, and subsequent activation of tumor necrosis factor receptor-associated factor 6. This in turn activates the Src family kinases, spleen tyrosine kinase and PLCγ2, and platelet integrins. Murine intravital thrombosis studies demonstrated that CAP-PEs accelerate thrombosis in TLR2-dependent manner and that TLR2 contributes to accelerate thrombosis in mice in the settings of hyperlipidemia. Our study identified the novel end-products of lipid peroxidation, accumulating in circulation in hyperlipidemia and inducing platelet activation by promoting cross-talk between innate immunity and integrin activation signaling pathways.

Antiangiogenic Effect of Ethanol Extract of Vigna angularis via Inhibition of Phosphorylation of VEGFR2, Erk, and Akt

Though dietary azuki bean (Vigna angularis) seed containing antioxidant proanthocyanidins was known to have multibiological activities including antioxidant, hypotensive, anti-inflammatory, and immunomodulatory activities, the antiangiogenic activity of ethanol extract of Vigna angularis (EVA) was never reported so far. In the present study, the antiangiogenic mechanism of EVA was examined in human umbilical vein endothelial cells (HUVECs). EVA showed weak cytotoxicity in HUVECs, while it significantly suppressed the VEGF induced proliferation of HUVECs. Consistently, wound healing assay revealed that EVA inhibited the VEGF induced migration of HUVECs. Also, EVA abrogated the VEGF induced tube formation of HUVECs in a concentration dependent fashion. Furthermore, Matrigel plug assay showed that EVA significantly reduced the hemoglobin level of Matrigel plug in mice compared to untreated control. Of note, EVA effectively attenuated the phosphorylation of VEGFR2, Erk, and Akt in VEGF-treated HUVECs. Overall, our findings suggest that EVA inhibits angiogenesis in VEGF-treated HUVECs via inhibition of phosphorylation of VEGFR2, ERK, and Akt.

Isolevuglandin-type lipid aldehydes induce the inflammatory response of macrophages by modifying phosphatidylethanolamines and activating the receptor for advanced glycation endproducts

AIMS

Increased lipid peroxidation occurs in many conditions associated with inflammation. Because lipid peroxidation produces lipid aldehydes that can induce inflammatory responses through unknown mechanisms, elucidating these mechanisms may lead to development of better treatments for inflammatory diseases. We recently demonstrated that exposure of cultured cells to lipid aldehydes such as isolevuglandins (IsoLG) results in the modification of phosphatidylethanolamine (PE). We therefore sought to determine (i) whether PE modification by isolevuglandins (IsoLG-PE) occurred in vivo, (ii) whether IsoLG-PE stimulated the inflammatory responses of macrophages, and (iii) the identity of receptors mediating the inflammatory effects of IsoLG-PE.

RESULTS

IsoLG-PE levels were elevated in plasma of patients with familial hypercholesterolemia and in the livers of mice fed a high-fat diet to induce obesity and hepatosteatosis. IsoLG-PE potently stimulated nuclear factor kappa B (NFκB) activation and expression of inflammatory cytokines in macrophages. The effects of IsoLG-PE were blocked by the soluble form of the receptor for advanced glycation endproducts (sRAGE) and by RAGE antagonists. Furthermore, macrophages derived from the bone marrow of Ager null mice failed to express inflammatory cytokines in response to IsoLG-PE to the same extent as macrophages from wild-type mice.

INNOVATION

These studies are the first to identify IsoLG-PE as a mediator of macrophage activation and a specific receptor, RAGE, which mediates its biological effects.

CONCLUSION

PE modification by IsoLG forms RAGE ligands that activate macrophages, so that the increased IsoLG-PE generated by high circulating cholesterol levels or high-fat diet may play a role in the inflammation associated with these conditions.

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

RATIONALE

Oxidative stress is an important contributing factor in several human pathologies ranging from atherosclerosis to cancer progression; however, the mechanisms underlying tissue protection from oxidation products are poorly understood. Oxidation of membrane phospholipids, containing the polyunsaturated fatty acid docosahexaenoic acid, results in the accumulation of an end product, 2-(ω-carboxyethyl)pyrrole (CEP), which was shown to have proangiogenic and proinflammatory functions. Although CEP is continuously accumulated during chronic processes, such as tumor progression and atherosclerosis, its level during wound healing return to normal when the wound is healed, suggesting the existence of a specific clearance mechanism.

OBJECTIVE

To identify the cellular and molecular mechanism for CEP clearance.

METHODS AND RESULTS

Here, we show that macrophages are able to bind, scavenge, and metabolize carboxyethylpyrrole derivatives of proteins but not structurally similar ethylpyrrole derivatives, demonstrating the high specificity of the process. F4/80(hi) and M2-skewed macrophages are much more efficient at CEP binding and scavenging compared with F4/80(lo) and M1-skewed macrophages. Depletion of macrophages leads to increased CEP accumulation in vivo. CEP binding and clearance are dependent on 2 receptors expressed by macrophages, CD36 and toll-like receptor 2. Although knockout of each individual receptor results in diminished CEP clearance, the lack of both receptors almost completely abrogates macrophages' ability to scavenge CEP derivatives of proteins.

CONCLUSIONS

Our study demonstrates the mechanisms of recognition, scavenging, and clearance of pathophysiologically active products of lipid oxidation in vivo, thereby contributing to tissue protection against products of oxidative stress.

4-Hydroxy-7-oxo-5-heptenoic Acid (HOHA) Lactone is a Biologically Active Precursor for the Generation of 2-(ω-Carboxyethyl)pyrrole (CEP) Derivatives of Proteins and Ethanolamine Phospholipids

2-(ω-Carboxyethyl)pyrrole (CEP) derivatives of proteins were previously shown to have significant pathological and physiological relevance to age-related macular degeneration, cancer and wound healing. Previously, we showed that CEPs are generated in the reaction of ε-amino groups of protein lysyl residues with 1-palmityl-2-(4-hydroxy-7-oxo-5-heptenoyl)-sn-glycero-3-phosphatidylcholine (HOHA-PC), a lipid oxidation product uniquely generated by oxidative truncation of docosahexanenate-containing phosphatidylcholine. More recently, we found that HOHA-PC rapidly releases HOHA-lactone and 2-lyso-PC (t1/2 = 30 min at 37 °C) by nonenzymatic transesterification/deacylation. Now we report that HOHA-lactone reacts with Ac-Gly-Lys-OMe or human serum albumin to form CEP derivatives in vitro. Incubation of human red blood cell ghosts with HOHA-lactone generates CEP derivatives of membrane proteins and ethanolamine phospholipids. Quantitative analysis of the products generated in the reaction HOHA-PC with Ac-Gly-Lys-OMe showed that HOHA-PC mainly forms CEP-dipeptide that is not esterified to 2-lysophosphatidycholine. Thus, the HOHA-lactone pathway predominates over the direct reaction of HOHA-PC to produce the CEP-PC-dipeptide derivative. Myleoperoxidase/H2O2/NO2(-) promoted in vitro oxidation of either 1-palmityl-2-docosahexaneoyl-sn-glycero-3-phosphatidylcholine (DHA-PC) or docosahexaenoic acid (DHA) generates HOHA-lactone in yields of 0.45% and 0.78%, respectively. Lipid oxidation in human red blood cell ghosts also releases HOHA-lactone. Oxidative injury of ARPE-19 human retinal pigmented epithelial cells by exposure to H2O2 generated CEP derivatives. Treatment of ARPE-19 cells with HOHA-lactone generated CEP-modified proteins. Low (submicromolar), but not high, concentrations of HOHA-lactone promote increased vascular endothelial growth factor (VEGF) secretion by ARPE-19 cells. Therefore, HOHA-lactone not only serves as an intermediate for the generation of CEPs but also is a biologically active oxidative truncation product from docosahexaenoate lipids.