Lipid peroxidation modification of protein generates Nepsilon-(4-oxononanoyl)lysine as a pro-inflammatory ligand

The unique reactivity of EKODE lipid peroxidation products allows in vivo detection of inflammation

Lipid peroxidation is a complex biochemical process associated with oxidative stress, and its products play crucial roles in cellular signaling and the pathophysiology of many diseases. Among the diverse array of lipid peroxidation (LPO) products, epoxyketooctadecenoic acids (EKODEs) have emerged as intriguing molecules with potential impacts on inflammatory diseases. EKODEs arise from linoleic acid reacting with reactive oxygen and nitrogen species present during inflammation. A hallmark of many LPO products is an electrophilic chemical functionality that can react with different biological nucleophiles to form adducts that impact a broad swath of physiologic processes. Here, we present the identification of reactivity patterns exhibited by the EKODE class of LPO products that arise due to the unique chemistry of the EKODE electrophiles, namely α, β-unsaturated epoxyketones of variable regiochemistry. Our initial investigations with models of the EKODE reactive core showed that surrogates of lysine did not react, and histidine nucleophiles formed reversible Michael adducts. However, when models of cysteine nucleophiles were tested, a unique reactivity profile emerged where rapid Michael addition was followed by slow rearrangement and epoxide opening at an unpredicted electrophilic site, affording what we postulated to be an advanced lipoxidation end product (ALE). After confirming the EKODE reactivity in model systems, we produced polyclonal antibodies of a stable epitope of the EKODE-based ALE and used these antibodies to investigate an approach for in vivo monitoring of inflammatory disease progression.

Role of Lectin-Like Oxidized Low-Density Lipoprotein Receptor-1 in Inflammation and Pathogen-Associated Interactions

BACKGROUND

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is known as a major receptor for oxidized low-density lipoproteins (oxLDL) and plays a significant role in the genesis of atherosclerosis. Recent research has shown its involvement in cancer, ischemic stroke, and diabetes. LOX-1 is a C-type lectin receptor and is involved in the activation of immune cells and inflammatory processes. It may further interact with pathogens, suggesting a role in infections or the host's response.

SUMMARY

This review compiles the current knowledge of potential implications of LOX-1 in inflammatory processes and in host-pathogen interactions with a particular emphasis on its regulatory role in immune responses. Also discussed are genomic and structural variations found in LOX-1 homologs across different species as well as potential involvements of LOX-1 in inflammatory processes from the angle of different cell types and organ-specific interactions.

KEY MESSAGES

The results presented reveal both similar and different structures in human and murine LOX-1 and provide clues as to the possible origins of different modes of interaction. These descriptions raise concerns about the suitability, particularly of mouse models, that are often used in the analysis of its functionality in humans. Further research should also aim to better understand the mostly unknown binding and interaction mechanisms between LOX-1 and different pathogens. This pursuit will not only enhance our understanding of LOX-1 involvement in inflammatory processes but also identify potential targets for immunomodulatory approaches.

Binding properties of recombinant LDL receptor and LOX-1 receptor to LDL measured using bio-layer interferometry and atomic force microscopy

Oxidation of low-density lipoproteins (LDLs) triggers a recognition by scavenger receptors such as lectin-like oxidized LDL receptor-1 (LOX-1) and is related to inflammation and cardiovascular diseases. Although LDLs that are recognized by LOX-1 can be risk-related LDLs, conventional LDL detection methods using commercially available recombinant receptors remain undeveloped. Using a bio-layer interferometry (BLI), we investigated the binding of recombinant LOX-1 (reLOX-1) and LDL receptors to the oxidized LDLs. The recombinant LDL receptor preferably bound minimally modified LDLs, while the reLOX-1 recognized extensively oxidized LDLs. An inversed response of the BLI was observed during the binding in the case of reLOX-1. AFM study showed that the extensively oxidized LDLs and aggregates of LDLs were observed on the surface, supporting the results. Altogether, a combined use of these recombinant receptors and the BLI method is useful in detecting high-risk LDLs such as oxidized LDLs and modified LDLs.

Chemically Tuned, Reversible Fluorogenic Electrophile for Live Cell Nanoscopy

We report a chemically tuned fluorogenic electrophile designed to conduct live-cell super-resolution imaging by exploiting its stochastic reversible alkylation reaction with cellular nucleophiles. Consisting of a lipophilic BODIPY fluorophore tethered to an electrophilic cyanoacrylate warhead, the new probe cyanoAcroB remains nonemissive due to internal conversion along the cyanoacrylate moiety. Intermittent fluorescence occurs following thiolate Michael addition to the probe, followed by retro-Michael reaction, tuned by the cyano moiety in the acrylate warhead and BODIPY decoration. This design enables long-term super-resolved imaging of live cells by preventing fluorescent product accumulation and background increase, while preserving the pool of the probe. We demonstrate the imaging capabilities of cyanoAcroB via two methods: (i) single-molecule localization microscopy imaging with nanometer accuracy by stochastic chemical activation and (ii) super-resolution radial fluctuation. The latter tolerates higher probe concentrations and low imaging powers, as it exploits the stochastic adduct dissociation. Super-resolved imaging with cyanoAcroB reveals that electrophile alkylation is prevalent in mitochondria and endoplasmic reticulum. The 2D dynamics of these organelles within a single cell are unraveled with tens of nanometers spatial and sub-second temporal resolution through continuous imaging of cyanoAcroB extending for tens of minutes. Our work underscores the opportunities that reversible fluorogenic probes with bioinspired warheads bring toward illuminating chemical reactions with super-resolved features in live cells.

Protein adductomics: A comprehensive analysis of protein modifications by electrophiles

Human individuals are continually exposed to various exogenous and endogenous reactive electrophiles, which readily react with nucleophilic biomacromolecules, such as protein, and form a variety of covalent adducts. The covalent modifications of protein are thought to be involved in various physiological and pathological processes. Recently, the "adductome", a new concept that represents the totality of covalent adducts bound to nucleophilic biomolecules, has been offered as a useful technique for characterizing essentially all reactive electrophilic compounds in biological samples. The primary advantage of this approach is that non-targeted comprehensive analysis can readily be extended to investigate covalent adduct pattern of different situation of exposure and thereby makes it possible to detect/identify not only known but also unknown adducts. In this review, we provide a summary of the concept and methodology of protein adductomics, especially focusing on redox protein adductomics.

Modification of endothelial nitric oxide synthase by 4-oxo-2(E)-nonenal(ONE) in preeclamptic placentas

Preeclampsia (PE) is a leading cause of pregnancy complications, affecting 3-7% of pregnant women worldwide. The pathophysiology of preeclampsia involves a redox imbalance, oxidative stress and a reduced nitric oxide (NO) bioavailability. The molecular and cellular mechanisms leading to the dysfunction of the placental endothelial NO synthase (eNOS) are not clarified. This study was designed to investigate whether aldehydes generated by lipid peroxidation products (LPP), may contribute to placental eNOS dysfunction in PE. The analysis of placentas from PE-affected patients and normal pregnancies, showed a significant increase in protein carbonyl content, indicative of oxidative stress-induced protein modification, as shown by the accumulation of acrolein, 4-hydroxynonenal (HNE), and 4-oxo-2(E)-nonenal (ONE) adducts in PE placentas. In contrast, the levels of these LPP-adducts were low in placentas from normal pregnancies. Immunofluorescence and confocal experiments pointed out a colocalization of eNOS with ONE-Lys adducts, whereas eNOS was not modified in normal placentas. LC-MS/MS analysis of recombinant eNOS preincubated with ONE, allowed to identify several ONE-modified Lys-containing peptides, confirming that eNOS may undergo post-translational modification by LPP. The preincubation of HTR-8/SVneo human trophoblasts (HTR8) with ONE, resulted in ONE-Lys modification of eNOS and a reduced generation of NO. ONE inhibited the migration of HTR8 trophoblasts in the wound closure model, and this was partly restored by the NO donor, NOC-18, which confirmed the important role of NO in the invasive potential of trophoblasts. In conclusion, placental eNOS is modified by ONE in PE placentas, which emphasizes the sensitivity of this protein to oxidative stress in the disturbed redox environment of preeclamptic pregnancies.

A Dual Perspective of the Action of Lysine on Soybean Oil Oxidation Process Obtained by Combining 1H NMR and LC-MS: Antioxidant Effect and Generation of Lysine-Aldehyde Adducts

Little is still known about both the effect of amino acids on the oxidation course of edible oils and the modifications that the former may undergo during this process. Bearing this in mind, the objective of this work was to study the evolution of a system consisting of soybean oil with 2% of l-lysine under heating at 70 °C and stirring conditions, analyzing how the co-oxidation of the oil and of the amino acid affects their respective evolutions, and trying to obtain information about the action mechanism of lysine on soybean oil oxidation. The study of the oil progress by 1H Nuclear Magnetic Resonance (1H NMR) showed that the presence of lysine noticeably delays oil degradation and oxidation products generation in comparison with a reference oil without lysine. Regarding lysine evolution, the analysis by 1H NMR and Liquid Chromatography-Mass Spectrometry of a series of aqueous extracts obtained from the oil containing lysine over time revealed the formation of lysine adducts, most of them at the position, with n-alkanals, malondialdehyde, (E)-2-alkenals, and toxic oxygenated α β-unsaturated aldehydes. However, this latter finding does not seem enough to explain the antioxidant action of lysine.

Temporal and spatial changes of peroxiredoxin 2 levels in aortic media at very early stages of atherosclerotic lesion formation in apoE-knockout mice

The events that trigger early onset of atherosclerotic lesion formation are poorly understood. Initially, microscopic atherosclerotic lesions appear in the aortic root in 10-week-old apoE-knockout mice that are fed normal chow. Using proteome and immunohistochemical analyses, we investigated proteins in aortic media whose expression changes in athero-prone regions at the beginning of lesion formation. Protein profiles of the root/arch and thoracic/abdominal regions of aortas in 10-week-old apoE-knockout mice were analyzed using 2D-gel electrophoresis. Proteins in 81 spots with different abundance were identified. Among them, we focused on proteins related to oxidative stress and smooth muscle cells (SMCs). The level of peroxiredoxin 2 (Prx2), a major cellular antioxidant enzyme that reduces hydrogen peroxide, was lower in aortic root/arch compared with thoracic/abdominal aorta. Immunohistochemical staining demonstrated that Prx2 expression in SMCs in the aortic root was high at 4 weeks and decreased at 10 weeks in apoE-knockout mice, while Prx2 expression in the aorta was unchanged in wild-type mice. The level of Prx2 expression correlated positively with the SMC differentiation markers, α-smooth muscle actin and transgelin, suggesting that a decline in Prx2 expression accompanies SMC dedifferentiation. Accumulated acrolein-modified proteins and the infiltration of macrophages in aortic media were observed in areas with low Prx2 expression. These results showed that Prx2 expression declines in athero-prone aortic root before lesion formation, and this reduction in Prx2 expression correlates with lipid peroxidation, SMC dedifferentiation, and macrophage recruitment.

Investigation of aspects of the interaction of the indicators of lipid exchange, trombocitarian block of hemostasis, total body reactivity and their influence on the psychosomatic harmonization of the man in chronic inflammatory processes

For the first time on the basis of clinical and laboratory researches were revealed the correlation of indicators of lipid metabolism, platelet level of hemostasis and general reactivity in patients with chronic inflammatory processes of the genital system. The new pathogenetic aspects of the course of inflammatory diseases have been established. It has been shown that chronic inflammatory process is accompanied by disorders of lipid metabolism. The degree of severity of these changes depends on the severity of the inflammatory process, the reduction of phagocytic activity of polymorphonuclear leukocytes and the activation of macrophage and specific cellular mechanisms, accompanied by auto-intoxication. It is established that the frequency of atherogenic dyslipoproteinemias, as well as the intensity of lipid peroxidation increases in accordance with the increase age of the examined patients. Most atherogenic dyslipoproteinemias are found in patients with viral and chlamydial infections. The summarized data of the correlation analysis allow us to establish that the growth of atherogenic properties of blood plasma of patients with chronic inflammatory processes of the reproductive system occurs in accordance with the increase of adhesive-aggregation properties of platelets and damage of the vascular endothelium. These shifts occur in parallel with the predominance of the monocyte-macrophage linkage of phagocytosis against the background of reduced activity of neutrophils and cellular mechanisms of development of immunopathological reactions.

Lipoproteins as targets and markers of lipoxidation

Lipoproteins are essential systemic lipid transport particles, composed of apolipoproteins embedded in a phospholipid and cholesterol monolayer surrounding a cargo of diverse lipid species. Many of the lipids present are susceptible to oxidative damage by lipid peroxidation, giving rise to the formation of reactive lipid peroxidation products (rLPPs). In view of the close proximity of the protein and lipid moieties within lipoproteins, the probability of adduct formation between rLPPs and amino acid residues of the proteins, a process called lipoxidation, is high. There has been interest for many years in the biological effects of such modifications, but the field has been limited to some extent by the availability of methods to determine the sites and exact nature of such modification. More recently, the availability of a wide range of antibodies to lipoxidation products, as well as advances in analytical techniques such as liquid chromatography tandem mass spectrometry (LC-MSMS), have increased our knowledge substantially. While most work has focused on LDL, oxidation of which has long been associated with pro-inflammatory responses and atherosclerosis, some studies on HDL, VLDL and Lipoprotein(a) have also been reported. As the broader topic of LDL oxidation has been reviewed previously, this review focuses on lipoxidative modifications of lipoproteins, from the historical background through to recent advances in the field. We consider the main methods of analysis for detecting rLPP adducts on apolipoproteins, including their advantages and disadvantages, as well as the biological effects of lipoxidized lipoproteins and their potential roles in diseases.

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Lipoproteins can be modified by reactive Lipid Peroxidation Products (rLPPs).

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Lipoprotein lipoxidation is known to occur in several inflammatory diseases.

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Biochemical, immunochemical and mass spectrometry methods can detect rLPP adducts.

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Due to higher information output, MS can facilitate localization of modifications.

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Antibodies against some rLPPs have been used to identify lipoxidation in vivo.

Spatio-temporal monitoring of lipid peroxyl radicals in live cell studies combining fluorogenic antioxidants and fluorescence microscopy methods

Lipid peroxidation of polyunsaturated fatty acids in cells may occur via their catalytic autoxidation through peroxyl radicals under oxidative stress conditions. Lipid peroxidation is related to a number of pathologies, and may be invoked in new forms of regulated cell death, yet it may also have beneficial roles in cell signaling cascades. Antioxidants are a natural line of defense against lipid peroxidation, and may accordingly impact the biological outcome associated with the redox chemistry of lipid peroxidation. Critical to unraveling the physiological and pathological role of lipid peroxidation is the development of novel probes with the partition, chemical sensitivity and more importantly, molecular specificity, enabling the spatial and temporal imaging of peroxyl radicals in the lipid membranes of live cells, reporting on the redox status of the cell membrane. This review describes our recent progress to visualize lipid peroxidation in model membrane systems and in live cell studies. Our work portrays the mechanistic insight leading to the development of a highly sensitive probe to monitor lipid peroxyl radicals (LOO^•). It also describes technical aspects including reagents and fluorescence microscopy methodologies to consider in order to achieve the much sought after monitoring of rates of lipid peroxyl radical production in live cell studies, be it under oxidative stress but also under cell homeostasis. This review seeks to bring attention to the study of lipid redox reactions and to lay the groundwork for the adoption of fluorogenic antioxidant probeshancement and maximum intensity recorded in turn provide a benchmark to estimate, when compared to the control BODIPY dye lacking the intramolecular PeT based switch, the overall exte and related fluorescence microscopy methods toward gaining rich spatiotemporal information on lipid peroxidation in live cells.

Redox Signaling by Reactive Electrophiles and Oxidants

The concept of cell signaling in the context of nonenzyme-assisted protein modifications by reactive electrophilic and oxidative species, broadly known as redox signaling, is a uniquely complex topic that has been approached from numerous different and multidisciplinary angles. Our Review reflects on five aspects critical for understanding how nature harnesses these noncanonical post-translational modifications to coordinate distinct cellular activities: (1) specific players and their generation, (2) physicochemical properties, (3) mechanisms of action, (4) methods of interrogation, and (5) functional roles in health and disease. Emphasis is primarily placed on the latest progress in the field, but several aspects of classical work likely forgotten/lost are also recollected. For researchers with interests in getting into the field, our Review is anticipated to function as a primer. For the expert, we aim to stimulate thought and discussion about fundamentals of redox signaling mechanisms and nuances of specificity/selectivity and timing in this sophisticated yet fascinating arena at the crossroads of chemistry and biology.

Targeting Sirtuins: Substrate Specificity and Inhibitor Design

Lysine residues across the proteome are modified by posttranslational modifications (PTMs) that significantly enhance the structural and functional diversity of proteins. For lysine, the most abundant PTM is ɛ-N-acetyllysine (Kac), which plays numerous roles in regulation of important cellular functions, such as gene expression (epigenetic effects) and metabolism. A family of enzymes, namely histone deacetylases (HDACs), removes these PTMs. A subset of these enzymes, the sirtuins (SIRTs), represent class III HDAC and, unlike the rest of the family, these hydrolases are NAD⁺-dependent. Although initially described as deacetylases, alternative deacylase functions for sirtuins have been reported, which expands the potential cellular roles of this class of enzymes. Currently, sirtuins are investigated as therapeutic targets for the treatment of diseases that span from cancers to neurodegenerative disorders. In the present book chapter, we review and discuss the current literature on novel ɛ-N-acyllysine PTMs, targeted by sirtuins, as well as mechanism-based sirtuin inhibitors inspired by their substrates.

Mitochondria Alkylation and Cellular Trafficking Mapped with a Lipophilic BODIPY-Acrolein Fluorogenic Probe

Protein and DNA alkylation by endogenously produced electrophiles is associated with the pathogenesis of neurodegenerative diseases, to epigenetic alterations and to cell signaling and redox regulation. With the goal of visualizing, in real-time, the spatiotemporal response of the cell milieu to electrophiles, we have designed a fluorogenic BODIPY-acrolein probe, AcroB, that undergoes a >350-fold fluorescence intensity enhancement concomitant with protein adduct formation. AcroB enables a direct quantification of single post-translational modifications occurring on cellular proteins via recording fluorescence bursts in live-cell imaging studies. In combination with super-resolution imaging, protein alkylation events may be registered and individually counted, yielding a map of protein-electrophile reactions within the cell lipid milieu. Alkylation is predominantly observed within mitochondria, a source, yet not a sink, of AcroB adducts, illustrating that a mitochondrial constitutive excretion mechanism ensures rapid disposal of compromised proteins. Sorting within the Golgi apparatus and trafficking along microtubules and through the cell endo- and exocytic pathways can be next visualized via live-cell imaging. Our results offer a direct visualization of cellular response to a noncanonical acrolein warhead. We envision AcroB will enable new approaches for diagnosis of pathologies associated with defective cellular trafficking. AcroB may help elucidate key aspects of mitochondria electrophile adduct excretion and cell endocytic and exocytic pathways. Conceptually, AcroB provides a new paradigm on fluorescence microscopy studies where chemical perturbation is achieved and simultaneously reported by the probe.

The Contribution of Singlet Oxygen to Insulin Resistance

Insulin resistance contributes to the development of diabetes and cardiovascular dysfunctions. Recent studies showed that elevated singlet oxygen-mediated lipid peroxidation precedes and predicts diet-induced insulin resistance (IR), and neutrophils were suggested to be responsible for such singlet oxygen production. This review highlights literature suggesting that insulin-responsive cells such as endothelial cells, hepatocytes, adipocytes, and myocytes also produce singlet oxygen, which contributes to insulin resistance, for example, by generating bioactive aldehydes, inducing endoplasmic reticulum (ER) stress, and modifying mitochondrial DNA. In these cells, nutrient overload leads to the activation of Toll-like receptor 4 and other receptors, leading to the production of both peroxynitrite and hydrogen peroxide, which react to produce singlet oxygen. Cytochrome P450 2E1 and cytochrome c also contribute to singlet oxygen formation in the ER and mitochondria, respectively. Endothelial cell-derived singlet oxygen is suggested to mediate the formation of oxidized low-density lipoprotein which perpetuates IR, partly through neutrophil recruitment to adipose tissue. New singlet oxygen-involving pathways for the formation of IR-inducing bioactive aldehydes such as 4-hydroperoxy-(or hydroxy or oxo)-2-nonenal, malondialdehyde, and cholesterol secosterol A are proposed. Strategies against IR should target the singlet oxygen-producing pathways, singlet oxygen quenching, and singlet oxygen-induced cellular responses.

Heterogeneity of peptide adducts with carbonylated lipid peroxidation products

Highly reactive lipid peroxidation-derived carbonyls (oxoLPP) modify protein nucleophiles via Michael addition or Schiff base formation. Once formed, Michael adducts can be further stabilized via cyclic hemiacetals with or without loss of water. Depending on the mechanism of their formation, peptide-oxoLPP can carry aldehyde or keto groups and thus be a part of the total protein carbonylation level. If a carbonyl function is lost during consecutive reactions, the oxoLPP-peptide adducts will not be detected using the common carbonyl labeling protocols. Because of the differences in adduct stabilities, it is possible to address the heterogeneity of peptide/protein-oxoLPP adducts by careful evaluation of tandem mass spectra of modified peptides. Here, we used hydrophilic interaction liquid chromatography-tandem mass spectrometry analysis of lysine, cysteine and histidine containing model peptides co-incubated with oxidized 1-palmitoyl-2-linoleoyl-sn-glycerophosphatidylcholine to characterize the collision-induced dissociation behavior of peptide-carbonyl adducts. Numerous modifications were detected based on the analysis of tandem mass spectra, including Schiff bases on lysine (two), Michael adducts on lysine (six), cysteine (eleven) and histidine (two), as well as 4-hydroxy-2-aldehydes derived dehydrated cyclic hemiacetals on cysteine (five) and histidine (one). Additionally, cysteine and histidine side chains were modified by lipid-bound aldehydes as Michael adducts and dehydrated hemiacetals. The tandem mass spectra revealed collision-induced dissociation characteristics specific for each class of oxoLPP-peptide adducts.

Oxidative and reductive metabolism of lipid-peroxidation derived carbonyls

Extensive research has shown that increased production of reactive oxygen species (ROS) results in tissue injury under a variety of pathological conditions and chronic degenerative diseases. While ROS are highly reactive and can incite significant injury, polyunsaturated lipids in membranes and lipoproteins are their main targets. ROS-triggered lipid-peroxidation reactions generate a range of reactive carbonyl species (RCS), and these RCS spread and amplify ROS-related injury. Several RCS generated in oxidizing lipids, such as 4-hydroxy trans-2-nonenal (HNE), 4-oxo-2-(E)-nonenal (ONE), acrolein, malondialdehyde (MDA) and phospholipid aldehydes have been shown to be produced under conditions of oxidative stress and contribute to tissue injury and dysfunction by depleting glutathione and other reductants leading to the modification of proteins, lipids, and DNA. To prevent tissue injury, these RCS are metabolized by several oxidoreductases, including members of the aldo-keto reductase (AKR) superfamily, aldehyde dehydrogenases (ALDHs), and alcohol dehydrogenases (ADHs). Metabolism via these enzymes results in RCS inactivation and detoxification, although under some conditions, it can also lead to the generation of signaling molecules that trigger adaptive responses. Metabolic transformation and detoxification of RCS by oxidoreductases prevent indiscriminate ROS toxicity, while at the same time, preserving ROS signaling. A better understanding of RCS metabolism by oxidoreductases could lead to the development of novel therapeutic interventions to decrease oxidative injury in several disease states and to enhance resistance to ROS-induced toxicity.

Human DNA glycosylase enzyme TDG repairs thymine mispaired with exocyclic etheno-DNA adducts

Lipid peroxidation directly reacts with DNA and produces various exocyclic etheno-base DNA adducts, some of which are considered to contribute to carcinogenesis. However, the system for repairing them in humans is largely unknown. We hypothesized that etheno-DNA adducts are repaired by base excision repair initiated by DNA glycosylase. To test this hypothesis, we examined the activities of the DNA glycosylase proteins OGG1, SMUG1, TDG, NEIL1, MUTYH, NTH1, MPG, and UNG2 against double-stranded oligonucleotides containing 1,N(6)-ethenoadenine (εA), 3,N(4)-ethenocytosine (εC), butanone-ethenocytosine (BεC), butanone-ethenoguanine (BεG), heptanone-ethenocytosine (HεC), or heptanone-ethenoguanine (HεG) using a DNA cleavage assay. We found that TDG is capable of removing thymine that has mispaired with εC, BεC, BεG, HεC, or HεG in vitro. We next examined the effect of TDG against etheno-DNA adducts in human cells. TDG-knockdown cells exhibited the following characteristics: (a) higher resistance to cell death caused by the induction of etheno-DNA adducts; (b) lower repair activity for εC; and (c) a modest acceleration of mutations caused by εC, compared with the rate in control cells. All these characteristics suggest that TDG exerts a repair activity against etheno-DNA adducts in human cells. These results suggest that TDG has novel repair activities toward etheno-DNA adducts.

Activated human mast cells induce LOX-1-specific scavenger receptor expression in human monocyte-derived macrophages

Objective

Activated mast cells in atherosclerotic lesions degranulate and release bioactive compounds capable of regulating atherogenesis. Here we examined the ability of activated human primary mast cells to regulate the expression of the major scavenger receptors in cultured human primary monocyte-derived macrophages (HMDMs).

Results

Components released by immunologically activated human primary mast cells induced a transient expression of lectin-like oxidized LDL receptor (LOX-1) mRNA in HMDMs, while the expression of two other scavenger receptors, MSR1 and CD36, remained unaffected. The LOX-1-inducing secretory components were identified as histamine, tumor necrosis factor alpha (TNF-α), and transforming growth factor beta (TGF-β1), which exhibited a synergistic effect on LOX-1 mRNA expression. Histamine induced a transient expression of LOX-1 protein. Mast cell –induced increase in LOX-1 expression was not associated with increased uptake of oxidized LDL by the macrophages.

Conclusions

Mast cell-derived histamine, TNF-α, and TGF-β1 act in concert to induce a transient increase in LOX-1 expression in human primary monocyte-derived macrophages. The LOX-1-inducing activity potentially endows mast cells a hitherto unrecognized role in the regulation of innate immune reactions in atherogenesis.

Stable histone adduction by 4-oxo-2-nonenal: a potential link between oxidative stress and epigenetics

Lipid electrophiles modify cellular targets, altering their function. Here, we describe histones as major targets for modification by 4-oxo-2-nonenal, resulting in a stable Lys modification structurally analogous to other histone Lys acylations. Seven adducts were identified in chromatin isolated from intact cells: four 4-ketoamides to Lys and three Michael adducts to His. A 4-ketoamide adduct residing at H3K27 was identified in stimulated macrophages. Modification of histones H3 and H4 prevented nucleosome assembly.

Natural antibodies as a sensor of electronegative damage-associated molecular patterns (DAMPs)

Natural antibodies (Abs), predominantly IgMs, play an important function in the host response to the recognition of endogenous danger signals called damage-associated molecular patterns (DAMPs). Many of the natural IgM Abs also show several different antigenic cross-reactivities toward covalently modified proteins, such as oxidized low-density lipoproteins and advanced glycation end products. Of interest, a recent study has shown that these DAMPs have several physicochemical characteristics that differ from native proteins, such as an increased negative charge due to modification of the lysine residues. This finding may provide a mechanistic insight into the multispecificity of the natural Abs.

The formation of lipid hydroperoxide-derived amide-type lysine adducts on proteins: a review of current knowledge

Lipid peroxidation is an important biological reaction. In particular, polyunsaturated fatty acid (PUFA) can be oxidized easily. Peroxidized lipids often react with other amines accompanied by the formation of various covalent adducts. Novel amide-type lipid-lysine adducts have been identified from an in vitro reaction mixture of lipid hydroperoxide with a protein, biological tissues exposed to conditions of oxidative stress and human urine from a healthy person. In this chapter, the current knowledge of amide type adducts is reviewed with a focus on the evaluation of functional foods and diseases with a history of discovery of hexanoyl-lysine (HEL). Although there is extensive research on HEL and other amide-type adducts, the mechanism of generation of the amide bond remains unclear. We have found that the decomposed aldehyde plus peroxide combined with a lysine moiety does not fully explain the formation of the amide-type lipid-lysine adduct that is generated by lipid hydroperoxide. Singlet oxygen or an excited state of the ketone generated from the lipid hydroperoxide may also contribute to the formation of the amide linkage. The amide-adducts may prove useful not only for the detection of oxidative stress induced by disease but also for the estimation of damage caused by an excess intake of PUFA.

LOX-1 ligands containing apolipoprotein B and carotid intima-media thickness in middle-aged community-dwelling US Caucasian and Japanese men

OBJECTIVE

The serum level of LOX-1 ligand containing ApoB (LAB) may reflect atherogenicity better than LDL cholesterol (LDLC), total LDL particles and usual measurement of oxidized LDL. The association between LAB and intima-media thickness (IMT) of carotid artery was investigated by ultrasound in US and Japan men.

METHODS

Participants were 297 US Caucasian and 310 Japanese men, aged 40-49 years without past history of cardiovascular disease. Serum LAB levels were measured by ELISAs with recombinant LOX-1 and monoclonal anti-apolipoprotein B antibody.

RESULTS

Serum LAB levels [median (interquartile range), μg/L] were 1321 (936, 1730) in US Caucasians and 940 (688, 1259) in Japanese. For Caucasian men, average IMT was higher in higher LAB quartile, which was 0.653, 0.667, 0.688, and 0.702 mm, respectively (p for trend = 0.02). Linear regression analysis showed serum LAB was significantly associated with IMT after adjustment for LDLC or total LDL particles in addition to other traditional or novel risk factors for atherosclerosis such as C-reactive protein. However, there was no significant relationship between LAB and IMT in Japanese men.

CONCLUSION

Serum LAB, a new candidate biomarker for residual risk, was associated with an increased carotid IMT in US Caucasian men independently of various risk factors; however, ethnic difference should be clarified in the future.

Multispecificity of immunoglobulin M antibodies raised against advanced glycation end products: involvement of electronegative potential of antigens

BACKGROUND

Advanced glycation end products (AGEs) can act as neoantigens to trigger immune responses.

RESULTS

Natural IgM antibodies against AGEs recognize multiple molecules, including DNA and chemically modified proteins.

CONCLUSION

There is a close relationship between the formation of AGEs and innate immune responses.

SIGNIFICANCE

Our findings highlight AGEs and related modified proteins as a source of multispecific natural antibodies Advanced glycation end products (AGEs) are a heterogeneous and complex group of compounds that are formed when reducing sugars, such as dehydroascorbic acid, react in a nonenzymatic way with amino acids in proteins and other macromolecules. AGEs are prevalent in the diabetic vasculature and contribute to the development of atherosclerosis. The presence and accumulation of AGEs in many different cell types affect the extracellular and intracellular structure and function. In the present study, we studied the immune response to the dehydroascorbic acid-derived AGEs and provide multiple lines of evidence suggesting that the AGEs could be an endogenous source of innate epitopes recognized by natural IgM antibodies. Prominent IgM titers to the AGEs were detected in the sera of normal mice and were significantly accelerated by the immunization with the AGEs. Patients with systemic lupus erythematosus (SLE), a potentially fatal systemic autoimmune disease characterized by the increased production of autoantibodies, showed significantly higher serum levels of the IgM titer against the AGEs than healthy individuals. A progressive increase in the IgM response against the AGEs was also observed in the SLE-prone mice. Strikingly, a subset of monoclonal antibodies, showing a specificity toward the AGEs, prepared from normal mice immunized with the AGEs and from the SLE mice cross-reacted with the double-stranded DNA. Moreover, they also cross-reacted with several other modified proteins, including the acetylated proteins, suggesting that the multiple specificity of the antibodies might be ascribed, at least in part, to the increased electronegative potential of the proteins. These findings suggest that the protein modification by the endogenous carbonyl compounds, generating electronegative proteins, could be a source of multispecific natural antibodies.

Redox-derived damage-associated molecular patterns: Ligand function of lipid peroxidation adducts

Endogenous electrophiles, such as α,β-unsaturated aldehydes and ketones generated during lipid peroxidation, exhibit a facile reactivity with proteins, generating a variety of intra and intermolecular covalent adducts. It has been postulated that these host-derived, modified proteins with electrophiles, which constitute the products of diverse classes of oxidative reactions, represent damage-associated molecular patterns (DAMPs). The DAMPs, that occur in vivo, can be a ligand of multiple proteins, which in turn, may lead to the profound innate and adaptive immune responses and mediate homeostatic functions consequent to inflammation and cell death.

LOX-1: a multiligand receptor at the crossroads of response to danger signals

PURPOSE OF REVIEW

LOX-1 is a multiligand receptor implicated in endothelial dysfunction and atherosclerosis, although it was originally identified as an oxidized LDL receptor. In this review, the roles of various LOX-1 ligands and their interaction with LOX-1 are discussed to understand the pathophysiological significance of LOX-1.

RECENT FINDINGS

LOX-1 knockout mice showed resistance of endothelium-dependent vasorelaxation against oxidized LDL and retardation of atherosclerosis progression. LOX-1 ligand reduction in mice also attenuated atherosclerosis progression. In a human cohort study, high concentration of apoB-containing LOX-1 ligands predicted the incidence of cardiovascular disease. Furthermore, modified HDL, which existed in high concentration in the plasma of coronary artery disease patients, was found to induce impairment of endothelial nitric oxide release via LOX-1. In addition to lipoproteins, LOX-1 was found to work as a C-reactive protein receptor providing a scaffold for the activation of the complement system.

SUMMARY

LOX-1 is a unique molecule among the sensors of danger signals. LOX-1 is not only sensing danger signals such as modified LDL and heat shock protein, but also scaffolding other danger sensors including C-reactive protein and C1q, and directly commanding responses to danger signals by working as a cell adhesion molecule. Via these functions, LOX-1 might work as a surveillance molecule of vascular homeostasis.

Cancer, inflammation, and insights from ayurveda

A recent, exciting discovery relates to the concept of "shared pathology" between cancer and metabolic syndrome. One major pathway common to cancer and metabolic syndrome is chronic inflammation, which is a major driving force in carcinogenesis. Indeed, chronic inflammation precedes most cancers and is considered a "hallmark" of the neoplastic process. We discuss molecular and biochemical evidence which links diet, obesity, abnormal lipid metabolism, and type 2 diabetes mellitus with chronic inflammation. We also explain how each of these factors is linked with biochemical aberrations of carcinogenesis and the prevalence and risk of cancer. While there are reliable biomarkers for chronic inflammation, there are few markers for a mechanistic link between early inflammation and digestive disorders. Discovery of such a marker could lead to identification of a new subtype of patients with digestive disorders that predispose them to cancer and/or metabolic syndrome. In this context, we discuss the ayurvedic concept of "Ama" which is thought to be a toxic, proinflammatory waste-product of improper digestion. We then develop hypotheses and outline preclinical and clinical experiments designed to prove whether "Ama" can serve as a novel and reliable biomarker that links abnormal digestive status, with the onset of chronic inflammation.

The discovery of LOX-1, its ligands and clinical significance

LOX-1 is an endothelial receptor for oxidized low-density lipoprotein (oxLDL), a key molecule in the pathogenesis of atherosclerosis.The basal expression of LOX-1 is low but highly induced under the influence of proinflammatory and prooxidative stimuli in vascular endothelial cells, smooth muscle cells, macrophages, platelets and cardiomyocytes. Multiple lines of in vitro and in vivo studies have provided compelling evidence that LOX-1 promotes endothelial dysfunction and atherogenesis induced by oxLDL. The roles of LOX-1 in the development of atherosclerosis, however, are not simple as it had been considered. Evidence has been accumulating that LOX-1 recognizes not only oxLDL but other atherogenic lipoproteins, platelets, leukocytes and CRP. As results, LOX-1 not only mediates endothelial dysfunction but contributes to atherosclerotic plaque formation, thrombogenesis, leukocyte infiltration and myocardial infarction, which determine mortality and morbidity from atherosclerosis. Moreover, our recent epidemiological study has highlighted the involvement of LOX-1 in human cardiovascular diseases. Further understandings of LOX-1 and its ligands as well as its versatile functions will direct us to ways to find novel diagnostic and therapeutic approaches to cardiovascular disease.

Characterization of oxidation products from 1-palmitoyl-2-linoleoyl-sn-glycerophosphatidylcholine in aqueous solutions and their reactions with cysteine, histidine and lysine residues

This report focuses on studies of lipid peroxidation products reactivity towards the side chains of cysteine, histidine, and lysine residues in structurally unordered peptides. Thus we have analyzed linoleic acid peroxidation products (LaPP) obtained by incubating 1-palmitoyl-2-linoleoyl-sn-glycerophosphatidylcholine (PLPC) overnight with or without H(2)O(2) in the presence or absence of CuCl. In total, 55 different LaPP were identified with 26 containing reactive carbonyl groups. The strongest oxidation conditions (H(2)O(2) and Cu(I), i.e. a Fenton-like reagent) yielded 51 LaPP, whereas air oxidation produced only 12 LaPP. Independent of the oxidation conditions, around half of all LaPP were short-chain (oxidative cleavage) and the others long-chain (oxygen addition) PLPC oxidation products. The stronger oxidation conditions increased the number of LaPP, but also oxidized the added peptide Ac-PAAPAAPAPAEXTPV-OH (X=Cys, His or Lys) very quickly, especially under Fenton conditions. Thus, PLPC was oxidized by milder conditions (air or Cu(I)), incubated with the peptide and the peptide modifications were then analyzed by nano-RPC-ESI-Orbitrap-MS. Ten LaPP-derived peptide modifications were identified at lysine, whereas nine products were identified for cysteine and only three for histidine. Three high molecular weight LaPP still esterified to the GPC backbone were detected on Lys-containing peptide. Furthermore, three LaPP-derived mass shifts were obtained at cysteine, which have not previously been reported.