Levuglandins and isolevuglandins: stealthy toxins of oxidative injury

Lipofuscin, Its Origin, Properties, and Contribution to Retinal Fluorescence as a Potential Biomarker of Oxidative Damage to the Retina

Lipofuscin accumulates with age as intracellular fluorescent granules originating from incomplete lysosomal digestion of phagocytosed and autophagocytosed material. The purpose of this review is to provide an update on the current understanding of the role of oxidative stress and/or lysosomal dysfunction in lipofuscin accumulation and its consequences, particularly for retinal pigment epithelium (RPE). Next, the fluorescence of lipofuscin, spectral changes induced by oxidation, and its contribution to retinal fluorescence are discussed. This is followed by reviewing recent developments in fluorescence imaging of the retina and the current evidence on the prognostic value of retinal fluorescence for the progression of age-related macular degeneration (AMD), the major blinding disease affecting elderly people in developed countries. The evidence of lipofuscin oxidation in vivo and the evidence of increased oxidative damage in AMD retina ex vivo lead to the conclusion that imaging of spectral characteristics of lipofuscin fluorescence may serve as a useful biomarker of oxidative damage, which can be helpful in assessing the efficacy of potential antioxidant therapies in retinal degenerations associated with accumulation of lipofuscin and increased oxidative stress. Finally, amendments to currently used fluorescence imaging instruments are suggested to be more sensitive and specific for imaging spectral characteristics of lipofuscin fluorescence.

Protein Lipoxidation: Basic Concepts and Emerging Roles

Protein lipoxidation is a non-enzymatic post-translational modification that consists of the covalent addition of reactive lipid species to proteins. This occurs under basal conditions but increases in situations associated with oxidative stress. Protein targets for lipoxidation include metabolic and signalling enzymes, cytoskeletal proteins, and transcription factors, among others. There is strong evidence for the involvement of protein lipoxidation in disease, including atherosclerosis, neurodegeneration, and cancer. Nevertheless, the involvement of lipoxidation in cellular regulatory mechanisms is less understood. Here we review basic aspects of protein lipoxidation and discuss several features that could support its role in cell signalling, including its selectivity, reversibility, and possibilities for regulation at the levels of the generation and/or detoxification of reactive lipids. Moreover, given the great structural variety of electrophilic lipid species, protein lipoxidation can contribute to the generation of multiple structurally and functionally diverse protein species. Finally, the nature of the lipoxidised proteins and residues provides a frameshift for a complex interplay with other post-translational modifications, including redox and redox-regulated modifications, such as oxidative modifications and phosphorylation, thus strengthening the importance of detailed knowledge of this process.

Contribution of Oxidative Stress and Impaired Biogenesis of Pancreatic β-Cells to Type 2 Diabetes

Significance: Type 2 diabetes development involves multiple changes in β-cells, related to the oxidative stress and impaired redox signaling, beginning frequently by sustained overfeeding due to the resulting lipotoxicity and glucotoxicity. Uncovering relationships among the dysregulated metabolism, impaired β-cell "well-being," biogenesis, or cross talk with peripheral insulin resistance is required for elucidation of type 2 diabetes etiology. Recent Advances: It has been recognized that the oxidative stress, lipotoxicity, and glucotoxicity cannot be separated from numerous other cell pathology events, such as the attempted compensation of β-cell for the increased insulin demand and dynamics of β-cell biogenesis and its "reversal" at dedifferentiation, that is, from the concomitantly decreasing islet β-cell mass (also due to transdifferentiation) and low-grade islet or systemic inflammation. Critical Issues: At prediabetes, the compensation responses of β-cells, attempting to delay the pathology progression-when exaggerated-set a new state, in which a self-checking redox signaling related to the expression of Ins gene expression is impaired. The resulting altered redox signaling, diminished insulin secretion responses to various secretagogues including glucose, may lead to excretion of cytokines or chemokines by β-cells or excretion of endosomes. They could substantiate putative stress signals to the periphery. Subsequent changes and lasting glucolipotoxicity promote islet inflammatory responses and further pathology spiral. Future Directions: Should bring an understanding of the β-cell self-checking and related redox signaling, including the putative stress signal to periphery. Strategies to cure or prevent type 2 diabetes could be based on the substitution of the "wrong" signal by the "correct" self-checking signal.

The Adductomics of Isolevuglandins: Oxidation of IsoLG Pyrrole Intermediates Generates Pyrrole⁻Pyrrole Crosslinks and Lactams

Isoprostane endoperoxides generated by free radical-induced oxidation of arachidonates, and prostaglandin endoperoxides generated through enzymatic cyclooxygenation of arachidonate, rearrange nonenzymatically to isoprostanes and a family of stereo and structurally isomeric γ-ketoaldehyde seco-isoprostanes, collectively known as isolevuglandins (isoLGs). IsoLGs are stealthy toxins, and free isoLGs are not detected in vivo. Rather, covalent adducts are found to incorporate lysyl ε-amino residues of proteins or ethanolamino residues of phospholipids. In vitro studies have revealed that adduction occurs within seconds and is uniquely prone to cause protein–protein crosslinks. IsoLGs accelerate the formation of the type of amyloid beta oligomers that have been associated with neurotoxicity. Under air, isoLG-derived pyrroles generated initially are readily oxidized to lactams and undergo rapid oxidative coupling to pyrrole–pyrrole crosslinked dimers, and to more highly oxygenated derivatives of those dimers. We have now found that pure isoLG-derived pyrroles, which can be generated under anoxic conditions, do not readily undergo oxidative coupling. Rather, dimer formation only occurs after an induction period by an autocatalytic oxidative coupling. The stable free-radical TEMPO abolishes the induction period, catalyzing rapid oxidative coupling. The amine N-oxide TMAO is similarly effective in catalyzing the oxidative coupling of isoLG pyrroles. N-acetylcysteine abolishes the generation of pyrrole–pyrrole crosslinks. Instead pyrrole-cysteine adducts are produced. Two unified single-electron transfer mechanisms are proposed for crosslink and pyrrole-cysteine adduct formation from isoLG-pyrroles, as well as for their oxidation to lactams and hydroxylactams.

Covalent modification of biological targets with natural products through Paal-Knorr pyrrole formation

Covering: up to June 2017Natural products and endogenous metabolites engage specific targets within tissues and cells through complex mechanisms. This review examines the extent to which natural systems have adopted the Paal-Knorr reaction to engage nucleophilic amine groups within biological targets. Current understanding of this mode of reactivity is limited by only a few examples of this reaction in a biological context. This highlight is intended to stimulate the scientific community to identify potential research directions and applications of the Paal-Knorr reaction in native and engineered biological systems.

Isolevuglandins as a gauge of lipid peroxidation in human tumors

The cellular production of free radicals or reactive oxygen species (ROS) can lead to protein, lipid or DNA modifications and tumor formation. The cellular lipids undergo structural changes through the actions of enzymes (e.g. cyclooxygenases) or free radicals to form a class of compounds called Isolevuglandins (IsoLGs). The recruitment and continued exposure of tissue to ROS and IsoLGs causes increased cell proliferation, mutagenesis, loss of normal cell function and angiogenesis. The elevated concentration of ROS in cancerous tissues suggests that these mediators play an important role in cancer development. We hypothesized that tumors with elevated ROS levels would similarly possess an increased concentration of IsoLGs when compared with normal tissue. Using D11, an ScFv recombinant antibody specific for IsoLGs, we utilized immunohistochemistry to visualize the presence of IsoLG in human tumors compared to normal adjacent tissue (NAT) to the same tumor. We found that IsoLG concentrations were elevated in human breast, colon, kidney, liver, lung, pancreatic and tongue tumor cells when compared to NAT and believe that IsoLGs can be used as a gauge indicative of lipid peroxidation in tumors.

Total Synthesis Confirms the Molecular Structure Proposed for Oxidized Levuglandin D2

Levuglandins (LG)D₂ and LGE₂ are γ-ketoaldehyde levulinaldehyde derivatives with prostanoid side chains produced by spontaneous rearrangement of the endoperoxide intermediate PGH₂ in the biosynthesis of prostaglandins. Covalent adduction of LGs with the amyloid peptide Aβ_1-42 promotes formation of the type of oligomers that have been associated with neurotoxicity and are a pathologic hallmark of Alzheimer's disease. Within 1 min of their generation during the production of PGH₂ by cyclooxygenation of arachidonic acid, LGs are sequestered by covalent adduction to proteins. In view of this high proclivity for covalent adduction, it is understandable that free LGs have never been detected in vivo. Recently a catabolite, believed to be an oxidized derivative of LGD₂ (ox-LGD₂), a levulinic acid hydroxylactone with prostanoid side chains, was isolated from the red alga Gracilaria edulis and detected in mouse tissues and in the lysate of phorbol-12-myristate-13-acetate-treated THP-1 cells incubated with arachidonic acid. Such oxidative catabolism of LGD₂ is remarkable because it must be outstandingly efficient to prevail over adduction with proteins and because it requires a unique dehydrogenation. We now report a concise total synthesis that confirms the molecular structure proposed for ox-LGD₂. The synthesis also produces ox-LGE₂, which readily undergoes allylic rearrangement to Δ⁶-ox-LGE₂.

Reactive gamma-ketoaldehydes as novel activators of hepatic stellate cells in vitro

AIMS

Products of lipid oxidation, such as 4-hydroxynonenal (4-HNE), are key activators of hepatic stellate cells (HSC) to a pro-fibrogenic phenotype. Isolevuglandins (IsoLG) are a family of acyclic γ-ketoaldehydes formed through oxidation of arachidonic acid or as by-products of the cyclooxygenase pathway. IsoLGs are highly reactive aldehydes which are efficient at forming protein adducts and cross-links at concentrations 100-fold lower than 4-hydroxynonenal. Since the contribution of IsoLGs to liver injury has not been studied, we synthesized 15-E₂-IsoLG and used it to investigate whether IsoLG could induce activation of HSC.

RESULTS

Primary human HSC were exposed to 15-E₂-IsoLG for up to 48h. Exposure to 5μM 15-E₂-IsoLG in HSCs promoted cytotoxicity and apoptosis. At non-cytotoxic doses (50 pM-500nM) 15-E₂-IsoLG promoted HSC activation, indicated by increased expression of α-SMA, sustained activation of ERK and JNK signaling pathways, and increased mRNA and/or protein expression of cytokines and chemokines, which was blocked by inhibitors of JNK and NF-kB. In addition, IsoLG promoted formation of reactive oxygen species, and induced an early activation of ER stress, followed by autophagy. Inhibition of autophagy partially reduced the pro-inflammatory effects of IsoLG, suggesting that it might serve as a cytoprotective response.

INNOVATION

This study is the first to describe the biological effects of IsoLG in primary HSC, the main drivers of hepatic fibrosis.

CONCLUSIONS

IsoLGs represent a newly identified class of activators of HSC in vitro, which are biologically active at concentrations as low as 500 pM, and are particularly effective at promoting a pro-inflammatory response and autophagy.

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.

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.

Cyclooxygenase inhibition targets neurons to prevent early behavioural decline in Alzheimer's disease model mice

Identifying preventive targets for Alzheimer's disease is a central challenge of modern medicine. Non-steroidal anti-inflammatory drugs, which inhibit the cyclooxygenase enzymes COX-1 and COX-2, reduce the risk of developing Alzheimer's disease in normal ageing populations. This preventive effect coincides with an extended preclinical phase that spans years to decades before onset of cognitive decline. In the brain, COX-2 is induced in neurons in response to excitatory synaptic activity and in glial cells in response to inflammation. To identify mechanisms underlying prevention of cognitive decline by anti-inflammatory drugs, we first identified an early object memory deficit in APPSwe-PS1ΔE9 mice that preceded previously identified spatial memory deficits in this model. We modelled prevention of this memory deficit with ibuprofen, and found that ibuprofen prevented memory impairment without producing any measurable changes in amyloid-β accumulation or glial inflammation. Instead, ibuprofen modulated hippocampal gene expression in pathways involved in neuronal plasticity and increased levels of norepinephrine and dopamine. The gene most highly downregulated by ibuprofen was neuronal tryptophan 2,3-dioxygenase (Tdo2), which encodes an enzyme that metabolizes tryptophan to kynurenine. TDO2 expression was increased by neuronal COX-2 activity, and overexpression of hippocampal TDO2 produced behavioural deficits. Moreover, pharmacological TDO2 inhibition prevented behavioural deficits in APPSwe-PS1ΔE9 mice. Taken together, these data demonstrate broad effects of cyclooxygenase inhibition on multiple neuronal pathways that counteract the neurotoxic effects of early accumulating amyloid-β oligomers.

Clinical Relevance of Biomarkers of Oxidative Stress

SIGNIFICANCE

Oxidative stress is considered to be an important component of various diseases. A vast number of methods have been developed and used in virtually all diseases to measure the extent and nature of oxidative stress, ranging from oxidation of DNA to proteins, lipids, and free amino acids.

RECENT ADVANCES

An increased understanding of the biology behind diseases and redox biology has led to more specific and sensitive tools to measure oxidative stress markers, which are very diverse and sometimes very low in abundance.

CRITICAL ISSUES

The literature is very heterogeneous. It is often difficult to draw general conclusions on the significance of oxidative stress biomarkers, as only in a limited proportion of diseases have a range of different biomarkers been used, and different biomarkers have been used to study different diseases. In addition, biomarkers are often measured using nonspecific methods, while specific methodologies are often too sophisticated or laborious for routine clinical use.

FUTURE DIRECTIONS

Several markers of oxidative stress still represent a viable biomarker opportunity for clinical use. However, positive findings with currently used biomarkers still need to be validated in larger sample sizes and compared with current clinical standards to establish them as clinical diagnostics. It is important to realize that oxidative stress is a nuanced phenomenon that is difficult to characterize, and one biomarker is not necessarily better than others. The vast diversity in oxidative stress between diseases and conditions has to be taken into account when selecting the most appropriate biomarker.

Mass spectrometry detection of isolevuglandin adduction to specific protein residues

The aging process seems to be associated with oxidative stress and hence increased production of lipid peroxidation products, including isolevuglandins (isoLGs). The latter are highly reactive γ-ketoaldehydes which can form covalent adducts with primary amino groups of enzymes and proteins and alter the properties of these biomolecules. Yet little is currently known about amino acid-containing compounds affected by isoLG modification in different age-related pathological processes. To facilitate the detection of these biomolecules, we developed a strategy in which the purified enzyme (or protein) of interest is first treated with authentic isoLG in vitro to evaluate whether it contains reactive lysine residues prone to modification with isoLGs. The data obtained serve as a basis for making the "GO/NO GO" decision as to whether to pursue a further search of this isoLG modification in a biological sample. In this chapter, we describe the conditions for the in vitro isoLG modification assay and how to use mass spectrometry to identify the isoLG-modified peptides and amino acid residues. Our studies were carried out on cytochrome P450 27A1, an important metabolic enzyme, and utilized iso[4]levuglandin E2 as a prototypical isoLG. The isoLG-treated cytochrome P450 was subjected to proteolysis followed by liquid chromatography-tandem mass spectrometry for peptide separation and analysis by Mascot, a proteomics search engine, for the presence of modified peptides. The developed protocol could be applied to characterization of other enzymes/proteins and other types of unconventional posttranslational protein modification.

Isolevuglandin adducts in disease

SIGNIFICANCE

A diverse family of lipid-derived levulinaldehydes, isolevuglandins (isoLGs), is produced by rearrangement of endoperoxide intermediates generated through both cyclooxygenase (COX) and free radical-induced cyclooxygenation of polyunsaturated fatty acids and their phospholipid esters. The formation and reactions of isoLGs with other biomolecules has been linked to alcoholic liver disease, Alzheimer's disease, age-related macular degeneration, atherosclerosis, cardiac arythmias, cancer, end-stage renal disease, glaucoma, inflammation of allergies and infection, mitochondrial dysfunction, multiple sclerosis, and thrombosis. This review chronicles progress in understanding the chemistry of isoLGs, detecting their production in vivo and understanding their biological consequences.

CRITICAL ISSUES

IsoLGs have never been isolated from biological sources, because they form adducts with primary amino groups of other biomolecules within seconds. Chemical synthesis enabled investigation of isoLG chemistry and detection of isoLG adducts present in vivo.

RECENT ADVANCES

The first peptide mapping and sequencing of an isoLG-modified protein present in human retina identified the modification of a specific lysyl residue of the sterol C27-hydroxylase Cyp27A1. This residue is preferentially modified by iso[4]LGE2 in vitro, causing loss of function. Adduction of less than one equivalent of isoLG can induce COX-associated oligomerization of the amyloid peptide Aβ1-42. Adduction of isoLGE2 to phosphatidylethanolamines causes gain of function, converting them into proinflammatory isoLGE2-PE agonists that foster monocyte adhesion to endothelial cells.

FUTURE DIRECTIONS

Among the remaining questions on the biochemistry of isoLGs are the dependence of biological activity on isoLG isomer structure, the structures and mechanism of isoLG-derived protein-protein and DNA-protein cross-link formation, and its biological consequences.

Cholesterol in the retina: the best is yet to come

Historically understudied, cholesterol in the retina is receiving more attention now because of genetic studies showing that several cholesterol-related genes are risk factors for age-related macular degeneration (AMD) and because of eye pathology studies showing high cholesterol content of drusen, aging Bruch's membrane, and newly found subretinal lesions. The challenge before us is determining how the cholesterol-AMD link is realized. Meeting this challenge will require an excellent understanding these genes' roles in retinal physiology and how chorioretinal cholesterol is maintained. In the first half of this review, we will succinctly summarize physico-chemical properties of cholesterol, its distribution in the human body, general principles of maintenance and metabolism, and differences in cholesterol handling in human and mouse that impact on experimental approaches. This information will provide a backdrop to the second part of the review focusing on unique aspects of chorioretinal cholesterol homeostasis, aging in Bruch's membrane, cholesterol in AMD lesions, a model for lesion biogenesis, a model for macular vulnerability based on vascular biology, and alignment of AMD-related genes and pathobiology using cholesterol and an atherosclerosis-like progression as unifying features. We conclude with recommendations for the most important research steps we can take towards delineating the cholesterol-AMD link.

Lipidomics of oxidized polyunsaturated fatty acids

Lipid mediators are produced from the oxidation of polyunsaturated fatty acids through enzymatic and free radical-mediated reactions. When subject to oxygenation via cyclooxygenases, lipoxygenases, and cytochrome P450 monooxygenases, polyunsaturated fatty acids give rise to an array of metabolites including eicosanoids, docosanoids, and octadecanoids. These potent bioactive lipids are involved in many biochemical and signaling pathways, with inflammation being of particular importance. Moreover, because they are produced by more than one pathway and substrate, and are present in a variety of biological milieus, their analysis is not always possible with conventional assays. Liquid chromatography coupled to electrospray mass spectrometry offers a versatile and sensitive approach for the analysis of bioactive lipids, allowing specific and accurate quantitation of multiple species present in the same sample. Here we explain the principles of this approach to mediator lipidomics and present detailed protocols for the assay of enzymatically produced oxygenated metabolites of polyunsaturated fatty acids that can be tailored to answer biological questions or facilitate assessment of nutritional and pharmacological interventions.

Posttranslational modification by an isolevuglandin diminishes activity of the mitochondrial cytochrome P450 27A1

Posttranslational modification by isolevuglandins (isoLGs), arachidonate oxidation products, is an important yet understudied process associated with altered protein properties. This type of modification is detected in cytochrome P450 27A1 (CYP27A1), a multifunction enzyme expressed in almost every cell and involved in the metabolism of cholesterol and other sterols. Previously, the CYP27A1 Lys(358)-isoLG adduct was found in human retina afflicted with age-related macular degeneration. Yet, the effect of Lys(358) modification on enzyme activity was not investigated. Herein, we characterized catalytic properties of Lys(358) as well as Lys(476) CYP27A1 mutants before and after isoLG treatment and quantified the extent of modification by multiple reaction monitoring. The K358R mutant was less susceptible to isoLG-induced loss of catalytic activity than the wild type (WT), whereas the K476R mutant was nearly as vulnerable as the WT. Both mutants showed less isoLG modification than WT. Thus, modification of Lys(358), a residue involved in redox partner interactions, is the major contributor to isoLG-associated loss of CYP27A1 activity. Our data show the specificity of isoLG modification, provide direct evidence that isoLG adduction impairs enzyme activity, and support our hypothesis that isoLG modification in the retina is detrimental to CYP27A1 enzyme activity, potentially disrupting cholesterol homeostasis.

Role of phospholipid oxidation products in atherosclerosis

There is increasing clinical evidence that phospholipid oxidation products (Ox-PL) play a role in atherosclerosis. This review focuses on the mechanisms by which Ox-PL interact with endothelial cells, monocyte/macrophages, platelets, smooth muscle cells, and HDL to promote atherogenesis. In the past few years major progress has been made in identifying these mechanisms. It has been recognized that Ox-PL promote phenotypic changes in these cell types that have long-term consequences for the vessel wall. Individual Ox-PL responsible for specific cellular effects have been identified. A model of the configuration of bioactive truncated Ox-PL within membranes has been developed that demonstrates that the oxidized fatty acid moiety protrudes into the aqueous phase, rendering it accessible for receptor recognition. Receptors and signaling pathways for individual Ox-PL species are now determined and receptor independent signaling pathways identified. The effects of Ox-PL are mediated both by gene regulation and transcription independent processes. It has now become apparent that Ox-PL affects multiple genes and pathways, some of which are proatherogenic and some are protective. However, at concentrations that are likely present in the vessel wall in atherosclerotic lesions, the effects promote atherogenesis. There have also been new insights on enzymes that metabolize Ox-PL and the significance of these enzymes for atherosclerosis. With the knowledge we now have of the regulation and effects of Ox-PL in different vascular cell types, it should be possible to design experiments to test the role of specific Ox-PL on the development of atherosclerosis.

Lipid mediators in plasma of autism spectrum disorders

Background

Inflammation is increasingly recognized as being of both physiological and pathological importance in the immature brain. Cerebellar pathology occurs in autism, as a neurodevelopmental disorder with genetic and environmental origins. The genesis of this disorder is still not understood but inflammation in utero or early in childhood is an environmental risk factor.

Methods

Prostaglandin E2 (PGE2), cysteinyl leukotriene as two important lipid mediators together with 8 isoprostane as marker of oxidative stress were measured using ELISA in plasma of 20 male autistic patients compared to 19 age and gender matching control participants.

Results

PGE2, leukotrienes and isoprostanes recorded significantly elevated levels in autistics compared to controls. Role of these measured parameters in inflammation and autoimmunity as two etiological factors in autism were discussed in details.

Conclusion

Receiver Operating Characteristic (ROC) curve analysis shows satisfactory values of area under the curve (AUC) which could reflect the high degree of specificity and sensitivity of the altered PGE2, leukotrienes and isoprostanes as predictive biomarkers in autistic patients from Saudi Arabia.

Role of mitochondria in programmed cell death mediated by arachidonic acid-derived eicosanoids

Arachidonic acid-derived eicosanoids from cyclooxygenases, lipoxygenases, and cytochrome P450 are important lipid mediators involved in numerous homeostatic and pathophysiological processes. Most eicosanoids act primarily on their respective cell surface G-protein coupled receptors to elicit downstream signaling in an autocrine and paracrine fashion. Emerging evidence indicates that these hormones are also critical in apoptosis in a cell/tissue specific manner. In this review, we summarize the formation of eicosanoids and their roles as mediators in apoptosis, specifically on the roles of mitochondria in mediating these events and the signaling pathways involved. The biological relevance of eicosanoid-mediated apoptosis is also discussed.

Lipidomics of polyunsaturated-fatty-acid-derived oxygenated metabolites

Nutritionally important PUFAs (polyunsaturated fatty acids) mediate some of their bioactivities through formation of oxygenated metabolites. These bioactive lipids are formed by COX (cyclo-oxygenase), LOX (lipoxygenase) and cytochrome-P450-catalysed reactions, as well as non-enzymatic lipid peroxidation. These reactions produce numerous species, some of which can be formed through more than one pathway. MS-based lipidomics offers the selectivity and sensitivity required for qualitative and quantitative analysis of multiple lipid species, in a variety of biological systems, and can facilitate the study of these mediators.

Isolevuglandins and mitochondrial enzymes in the retina: mass spectrometry detection of post-translational modification of sterol-metabolizing CYP27A1

We report the first peptide mapping and sequencing of an in vivo isolevuglandin-modified protein. Mitochondrial cytochrome P450 27A1 (CYP27A1) is a ubiquitous multifunctional sterol C27-hydroxylase that eliminates cholesterol and likely 7-ketocholesterol from the retina and many other tissues. We investigated the post-translational modification of this protein with isolevuglandins, arachidonate oxidation products. Treatment of purified recombinant CYP27A1 with authentic iso[4]levuglandin E(2) (iso[4]LGE(2)) in vitro diminished enzyme activity in a time- and phospholipid-dependent manner. A multiple reaction monitoring protocol was then developed to identify the sites and extent of iso[4]LGE(2) adduction. CYP27A1 exhibited only three Lys residues, Lys(134), Lys(358), and Lys(476), that readily interact with iso[4]LGE(2) in vitro. Such selective modification enabled the generation of an internal standard, (15)N-labeled CYP27A1 modified with iso[4]LGE(2), for the subsequent analysis of a human retinal sample. Two multiple reaction monitoring transitions arising from the peptide AVLK(358)(-C(20)H(26)O(3))ETLR in the retinal sample were observed that co-eluted with the corresponding two (15)N transitions from the supplemented standard. These data demonstrate that modified CYP27A1 is present in the retina. We suggest that such protein modification impairs sterol elimination and likely has other pathological sequelae. We also propose that the post-translational modifications identified in CYP27A1 exemplify a general mechanism whereby oxidative stress and inflammation deleteriously affect protein function, contributing, for example, to cholesterol-rich lesions associated with age-related macular degeneration and cardiovascular disease. The proteomic protocols developed in this study are generally applicable to characterization of lipid-derived oxidative protein modifications occurring in vivo, including proteins bound to membranes.

Generation and biological activities of oxidized phospholipids

Glycerophospholipids represent a common class of lipids critically important for integrity of cellular membranes. Oxidation of esterified unsaturated fatty acids dramatically changes biological activities of phospholipids. Apart from impairment of their structural function, oxidation makes oxidized phospholipids (OxPLs) markers of "modified-self" type that are recognized by soluble and cell-associated receptors of innate immunity, including scavenger receptors, natural (germ line-encoded) antibodies, and C-reactive protein, thus directing removal of senescent and apoptotic cells or oxidized lipoproteins. In addition, OxPLs acquire novel biological activities not characteristic of their unoxidized precursors, including the ability to regulate innate and adaptive immune responses. Effects of OxPLs described in vitro and in vivo suggest their potential relevance in different pathologies, including atherosclerosis, acute inflammation, lung injury, and many other conditions. This review summarizes current knowledge on the mechanisms of formation, structures, and biological activities of OxPLs. Furthermore, potential applications of OxPLs as disease biomarkers, as well as experimental therapies targeting OxPLs, are described, providing a broad overview of an emerging class of lipid mediators.

Isolevuglandins covalently modify phosphatidylethanolamines in vivo: detection and quantitative analysis of hydroxylactam adducts

Levuglandins (LGs) and isolevuglandins (isoLGs, also called "isoketals" or "isoKs") are extraordinarily reactive products of cyclooxygenase- and free radical-induced oxidation of arachidonates. We now report the detection in vivo and quantitative analysis of LG/isoLG adducts that incorporate the amino group of phosphatidylethanolamines (PEs) into LG/isoLG-hydroxylactams. Notably, LC-MS/MS detection of these hydroxylactams is achieved with samples that are an order of magnitude smaller and sample processing is much simpler and less time consuming than required for measuring protein-derived LG/isoLG-lysyl lactams. A key feature of our protocol is treatment of biological phospholipid extracts with phospholipase A(2) to generate mainly 1-palmitoyl-2-lysoPE-hydroxylactams from heterogeneous mixtures of phospholipids with a variety of acyl groups on the 2 position. Over 160% higher mean levels of LG/isoLG-PE-hydroxylactam (P<0.001) were detected in liver from chronic ethanol-fed mice (32.4+/-6.3 ng/g, n=6) compared to controls (12.1+/-1.5 ng/g, n=4), and mean levels in plasma from patients with age-related macular degeneration (5.2+/-0.4 ng/ml, n=15) were elevated approximately 53% (P<0.0001) compared to those of healthy volunteers (3.4+/-0.1 ng/ml, n=15). Just as LG/isoLG-protein adducts provide a dosimeter of oxidative injury, this study suggests that LG/isoLG-PE-hydroxylactams are potential biomarkers for assessing risk for oxidative stress-stimulated diseases.

Isoprostanes and related compounds: update 2006

Last year Antioxidants and Redox Signaling devoted a special Forum issue (Volume 7, 2005) to reviewing the current literature on isoprostanes (IsoPs) and related compounds. Awide variety of topics provided readers of that issue with important information on the chemistry and biology of these molecules. During the past year, more than 200 articles have been published in the field of IsoP research, and noteworthy advances in the area have been made. This editorial highlights selected aspects of these advances as they relate to the Forum issue. The main focus of this discussion is on the role of the IsoPs as biomarkers and mediators of oxidant stress in human disease, their mechanism of formation and metabolism, and their biologic activities.