Reactivity of Peroxynitrite and Nitric Oxide with LDL

Metabolism Regulation and Redox State: Insight into the Role of Superoxide Dismutase 1

Energy metabolism and redox state are strictly linked; energy metabolism is a source of reactive oxygen species (ROS) that, in turn, regulate the flux of metabolic pathways. Moreover, to assure redox homeostasis, metabolic pathways and antioxidant systems are often coordinately regulated. Several findings show that superoxide dismutase 1 (SOD1) enzyme has effects that go beyond its superoxide dismutase activity and that its functions are not limited to the intracellular compartment. Indeed, SOD1 is secreted through unconventional secretory pathways, carries out paracrine functions and circulates in the blood bound to lipoproteins. Striking experimental evidence links SOD1 to the redox regulation of metabolism. Important clues are provided by the systemic effects on energy metabolism observed in mutant SOD1-mediated amyotrophic lateral sclerosis (ALS). The purpose of this review is to analyze in detail the involvement of SOD1 in redox regulation of metabolism, nutrient sensing, cholesterol metabolism and regulation of mitochondrial respiration. The scientific literature on the relationship between ALS, mutated SOD1 and metabolism will also be explored, in order to highlight the metabolic functions of SOD1 whose biological role still presents numerous unexplored aspects that deserve further investigation.

Role of the eNOS Uncoupling and the Nitric Oxide Metabolic Pathway in the Pathogenesis of Autoimmune Rheumatic Diseases

Atherosclerosis and its clinical complications constitute the major healthcare problems of the world population. Due to the central role of endothelium throughout the atherosclerotic disease process, endothelial dysfunction is regarded as a common mechanism for various cardiovascular (CV) disorders. It is well established that patients with rheumatic autoimmune diseases are characterized by significantly increased prevalence of cardiovascular morbidity and mortality compared with the general population. The current European guidelines on cardiovascular disease (CVD) prevention in clinical practice recommend to use a 1,5-factor multiplier for CV risk in rheumatoid arthritis as well as in other autoimmune inflammatory diseases. However, mechanisms of accelerated atherosclerosis in these diseases, especially in the absence of traditional risk factors, still remain unclear. Oxidative stress plays the major role in the endothelial dysfunction and recently is strongly attributed to endothelial NO synthase dysfunction (eNOS uncoupling). Converted to a superoxide-producing enzyme, uncoupled eNOS not only leads to reduction of the nitric oxide (NO) generation but also potentiates the preexisting oxidative stress, which contributes significantly to atherogenesis. However, to date, there are no systemic analyses on the role of eNOS uncoupling in the excess CV mortality linked with autoimmune rheumatic diseases. The current review paper addresses this issue.

Fatty acid nitration in human low-density lipoprotein

Lipid nitration occurs during physiological and pathophysiological conditions, generating a variety of biomolecules capable to modulate inflammatory cell responses. Low-density lipoprotein (LDL) oxidation has been extensively related to atherosclerotic lesion development while oxidative modifications confer the particle pro-atherogenic features. Herein, we reviewed the oxidation versus nitration of human LDL protein and lipid fractions. We propose that unsaturated fatty acids present in LDL can be nitrated under mild nitration conditions, suggesting an anti-atherogenic role for LDL carrying nitro-fatty acids (NFA).

The Role of (Modified) Lipoproteins in Vascular Function: A Duet Between Monocytes and the Endothelium

Over the last century, many studies have demonstrated that low-density lipoprotein (LDL) is a key risk factor of cardiovascular diseases (CVD) related to atherosclerosis. Thus, for these CVD patients, LDL lowering agents are commonly used in the clinic to reduce the risk for CVD. LDL, upon modification, will develop distinct inflammatory and proatherogenic potential, leading to impaired endothelial integrity, influx of immune cells and subsequent increased foam cell formation. LDL can also directly affect peripheral monocyte composition, rendering them in a more favorable position to migrate and accumulate in the subendothelial space. It has become apparent that other lipoprotein particles, such as triglyceride- rich lipoproteins or remnants (TRL) and lipoprotein(a) [Lp(a)] may also impact on atherogenic pathways. Evidence is accumulating that Lp(a) can promote peripheral monocyte activation, eventually leading to increased transmigration through the endothelium. Similarly, remnant cholesterol has been identified to play a key role in endothelial dysfunction and monocyte behavior. In this review, we will discuss recent developments in understanding the role of different lipoproteins in the context of inflammation at both the level of the monocyte and the endothelium.

Interplay between oxidant species and energy metabolism

It has long been recognized that energy metabolism is linked to the production of reactive oxygen species (ROS) and critical enzymes allied to metabolic pathways can be affected by redox reactions. This interplay between energy metabolism and ROS becomes most apparent during the aging process and in the onset and progression of many age-related diseases (i.e. diabetes, metabolic syndrome, atherosclerosis, neurodegenerative diseases). As such, the capacity to identify metabolic pathways involved in ROS formation, as well as specific targets and oxidative modifications is crucial to our understanding of the molecular basis of age-related diseases and for the design of novel therapeutic strategies.

Herein we review oxidant formation associated with the cell's energetic metabolism, key antioxidants involved in ROS detoxification, and the principal targets of oxidant species in metabolic routes and discuss their relevance in cell signaling and age-related diseases.

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Energy metabolism is both a source and target of oxidant species.

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Reactive oxygen species are formed in redox reactions in catabolic pathways.

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Sensitive targets of oxidant species regulate the flux of metabolic pathways.

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Metabolic pathways and antioxidant systems are regulated coordinately.

Myeloperoxidase scavenges peroxynitrite: A novel anti-inflammatory action of the heme enzyme

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Ferric myeloperoxidase accelerates the decomposition of peroxynitrite.

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Compound I of myeloperoxidase does not promote this removal of peroxynitrite.

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Peroxynitrite removal is enhanced by substances reacting well with myeloperoxidase Compound II.

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In the presence of H₂O₂, peroxynitrite diminishes the chlorinating activity of myeloperoxidase.

Peroxynitrite, a potent pro-inflammatory and cytotoxic species, interacts with a variety of heme containing proteins. We addressed the question whether (i) the interaction of myeloperoxidase (MPO, an enzyme generating hypochlorous acid from hydrogen peroxide and chloride ions) with peroxynitrite affects the clearance of peroxynitrite, and (ii) if peroxynitrite could modulate the chlorinating activity of MPO. Our results show that this interaction promotes the decomposition of the highly reactive pro-inflammatory oxidant, whereby MPO Compound II (but not Compound I) is formed. The efficiency of MPO to remove peroxynitrite was enhanced by l-tyrosine, nitrite and (−)-epicatechin, substances known to reduce Compound II with high reaction rate. Next, peroxynitrite (added as reagent) diminished the chlorinating activity of MPO in the presence of hydrogen peroxide. Alternatively, SIN-1, a peroxynitrite donor, reduced hypochlorous acid formation by MPO, as measured by aminophenyl fluorescein oxidation (time kinetics) and taurine chloramine formation (end point measurement). At inflammatory loci, scavenging of peroxynitrite by MPO may overcome the uncontrolled peroxynitrite decomposition and formation of reactive species, which lead to cell/tissue damage.

In vitro oxidative footprinting provides insight into apolipoprotein B-100 structure in low-density lipoprotein

Low-density lipoprotein (LDL) is a major cholesterol carrier in human blood. Oxidations of apolipoprotein B-100 (apo B-100, LDL protein) could be proatherogenic and play critical roles in early stages of plaque formation in the arterial wall. The structure of apo B-100 is still poorly understood, partially due to its size (550 KDa, 4563 amino acids). To gain an insight into LDL structure, we mapped the regions of apo B-100 in human LDL that were prone to oxidation using peroxynitrite and hypochlorite as probes. In this study, LDL was incubated with various concentrations of peroxynitrite and sodium hypochlorite in bicarbonate buffer. The LDL protein apo B-100 was delipidated, denatured, alkylated, and subjected to tryptic digestion. Tryptic peptides were analyzed employing LC-MS/MS. Database search was performed against the apo B-100 database (SwissProt accession #P04114) using "SEQUEST" algorithm to identify peroxynitrite and hypochlorite-mediated oxidations markers nitrotyrosine, nitrotryptophan, hydroxy-tryptophan, and 3-chlorotyrosine. Several site-specific oxidations were identified in apo B-100 after treatment of intact LDL particles with the oxidants. We hypothesize that these regions could be accessible to oxidant and critical for early events in atherosclerotic plaque deposition.

Oxidative modification of lipoproteins

Lipoproteins consist of lipids and apolipoproteins that have functional roles in lipid metabolism. It has been suggested that oxidation of lipoproteins by reactive oxygen species (ROS) may be involved in the inception of various diseases. In particular, the relationship between low-density lipoprotein (LDL) oxidation and atherosclerosis has been studied in great detail. The main target molecules of lipoprotein oxidation are polyunsaturated fatty acid residues of lipids and apolipoproteins. Extensive investigations have characterized oxidative modifications of apolipoprotein B100 (apo B100) in LDL. Furthermore, modifications of apo B100 by oxidized lipids have been confirmed in oxidized LDL and atherosclerotic lesions using immunological techniques. In this chapter, characteristics and oxidation mechanisms of lipoproteins by ROS are described from in vitro and in vivo studies. Oxidative modifications of apo B100 by lipid hydroperoxides, major products of lipid peroxidation at the early stage, are principally reported.

Nitro-fatty acids: formation, redox signaling, and therapeutic potential

SIGNIFICANCE

Nitrated derivatives of unsaturated fatty acids (nitro-fatty acids) are being formed and detected in human plasma, cell membranes, and tissue, triggering signaling cascades via covalent and reversible post-translational modifications of susceptible nucleophilic amino acids in transcriptional regulatory proteins and enzymes.

RECENT ADVANCES

Nitro-fatty acids modulate metabolic as well as inflammatory signaling pathways, including the p65 subunit of nuclear factor κB and the transcription factor peroxisome proliferator-activated receptor-γ. Moreover, nitro-fatty acids can activate heat shock as well as phase II antioxidant responses. As electrophiles, they also activate the Nuclear factor erythroid 2-related factor 2 pathway.

CRITICAL ISSUES

We first discuss the mechanisms of nitro-fatty acid formation as well as their key chemical and biochemical properties, including their capacity to release nitric oxide and exert antioxidant actions. The electrophilic properties of nitro-fatty acids to activate anti-inflammatory signaling pathways are discussed in detail. A critical issue is the influence of nitroarachidonic acid on prostaglandin endoperoxide H synthases, modulating inflammatory processes through redirection of arachidonic acid metabolism and signaling.

FUTURE DIRECTIONS

Based on this information, we analyze in vivo data supporting nitro-fatty acids as promising pharmacological tools to prevent inflammatory diseases associated with oxidative and nitrative stress conditions. A key future issue is to evaluate whether nitro-fatty acid supplementation would be useful for human diseases linked to inflammation as well as their potential toxicity when administered by long periods of time.

Nitro-fatty acids: novel anti-inflammatory lipid mediators

Nitro-fatty acids are formed and detected in human plasma, cell membranes, and tissue, modulating metabolic as well as inflammatory signaling pathways. Here we discuss the mechanisms of nitro-fatty acid formation as well as their key chemical and biochemical properties. The electrophilic properties of nitro-fatty acids to activate anti-inflammatory signaling pathways are discussed in detail. A critical issue is the influence of nitroarachidonic acid on prostaglandin endoperoxide H synthases, redirecting arachidonic acid metabolism and signaling. We also analyze in vivo data supporting nitro-fatty acids as promising pharmacological tools to prevent inflammatory diseases.

Peroxynitrite-mediated lipid oxidation and nitration: mechanisms and consequences

Lipid oxidation and nitration represents a novel area of research of relevance in the understanding of inflammatory processes. Peroxynitrite, the product of the diffusion-limited reaction between nitric oxide and superoxide anion, mediates oxidative modifications in lipid systems including cell membranes and lipoproteins. In this review, we discuss the mechanisms of lipid oxidation and nitration by peroxynitrite as well as the influence of physiological molecules and cell targets to redirect peroxynitrite reactivity. We also provide evidence to support that oxidation/nitration of lipids results in the formation of novel signaling modulators of key lipid-metabolizing enzymes.

Endothelial dysfunction: from molecular mechanisms to measurement, clinical implications, and therapeutic opportunities

Endothelial dysfunction has been implicated as a key factor in the development of a wide range of cardiovascular diseases, but its definition and mechanisms vary greatly between different disease processes. This review combines evidence from cell-culture experiments, in vitro and in vivo animal models, and clinical studies to identify the variety of mechanisms involved in endothelial dysfunction in its broadest sense. Several prominent disease states, including hypertension, heart failure, and atherosclerosis, are used to illustrate the different manifestations of endothelial dysfunction and to establish its clinical implications in the context of the range of mechanisms involved in its development. The size of the literature relating to this subject precludes a comprehensive survey; this review aims to cover the key elements of endothelial dysfunction in cardiovascular disease and to highlight the importance of the process across many different conditions.

Nitrated fatty acids: mechanisms of formation, chemical characterization, and biological properties

Nitrated derivatives of unsaturated fatty acids are formed under oxidative and nitrative stress conditions, and are detected and structurally characterized in cell membranes, cardiac tissue, human plasma, and urine. Nitro-fatty acids display pleiotropic activities, including modulation of macrophage activation, prevention of leukocyte and platelet activation, and promotion of blood vessel relaxation. However, mechanisms of formation and levels reached in inflammatory milieu are poorly characterized. In this review, we discuss potential mechanisms of formation of nitro-fatty acids and their key chemical and biochemical properties. A major focus is to analyze nitrated lipids as novel signaling mediators leading to secondary changes in protein function via electrophilic-based modifications as well as inhibition of inflammatory cell function, thus representing the convergence of lipid and nitric oxide signaling pathways.

Peroxynitrite: biochemistry, pathophysiology and development of therapeutics

Peroxynitrite--the product of the diffusion-controlled reaction of nitric oxide with superoxide radical--is a short-lived oxidant species that is a potent inducer of cell death. Conditions in which the reaction products of peroxynitrite have been detected and in which pharmacological inhibition of its formation or its decomposition have been shown to be of benefit include vascular diseases, ischaemia-reperfusion injury, circulatory shock, inflammation, pain and neurodegeneration. In this Review, we first discuss the biochemistry and pathophysiology of peroxynitrite and then focus on pharmacological strategies to attenuate the toxic effects of peroxynitrite. These include its catalytic reduction to nitrite and its isomerization to nitrate by metalloporphyrins, which have led to potential candidates for drug development for cardiovascular, inflammatory and neurodegenerative diseases.

Proteomic identification of nitrated brain proteins in amnestic mild cognitive impairment: a regional study

Oxidative stress is an imbalance between the level of antioxidants and oxidants in a cell. Oxidative stress has been shown in brain of subjects with mild cognitive impairment (MCI) as well Alzheimer's disease (AD). MCI is considered as a transition phase between control and AD. The focus of the current study was to identify nitrated proteins in the hippocampus and inferior parietal lobule (IPL) brain regions of subjects with amnestic MCI using proteomics. The identified nitrated proteins in MCI brain were compared to those previously reported to be nitrated and oxidatively modified in AD brain, a comparison that might provide an invaluable insight into the progression of the disease.

Effects of antioxidant-rich foods on vascular reactivity: review of the clinical evidence

The foods and nutrients discussed in this paper are components of dietary patterns that have been associated with lower cardiovascular disease risk. The focus of this review is on the effects of antioxidant foods on vascular health and discussion of their potential mechanisms of action. The foods reviewed include fruits and vegetables, red grapes and red wine, tea, cocoa/chocolate, and olive oil. The primary challenge in studying the cardioprotective components of a dietary pattern is in identifying mechanism(s) of action as well as the bioactive nutrients responsible. In selecting papers for this review, we focused on studies of whole foods and beverages that met the following criteria: 1) they are commonly consumed in typical diets, 2) they appear to have direct antioxidant effects, and 3) they have demonstrated effects on endothelial function in several human studies. The evidence presented herein suggests that dietary consumption of fruits and vegetables, red grapes and red wine, tea, chocolate, and olive oil may improve vascular reactivity, in part, by attenuating the adverse effects of oxidation on endothelial function. Additional research is needed to better understand the mechanism(s) by which antioxidant-rich foods and beverages favorably affect endothelial function and the extent to which this reflects direct antioxidant effects.

Peroxynitrite transforms nerve growth factor into an apoptotic factor for motor neurons

Nerve growth factor (NGF) overexpression and increased production of peroxynitrite occur in several neurodegenerative diseases. We investigated whether NGF could undergo posttranslational oxidative or nitrative modifications that would modulate its biological activity. Compared to native NGF, peroxynitrite-treated NGF showed an exceptional ability to induce p75(NTR)-dependent motor neuron apoptosis at physiologically relevant concentrations. Whereas native NGF requires an external source of nitric oxide (NO) to induce motor neuron death, peroxynitrite-treated NGF induced motor neuron apoptosis in the absence of exogenous NO. Nevertheless, NO potentiated the apoptotic activity of peroxynitrite-modified NGF. Blocking antibodies to p75(NTR) or downregulation of p75(NTR) expression by antisense treatment prevented motor neuron apoptosis induced by peroxynitrite-treated NGF. We investigated what oxidative modifications were responsible for inducing a toxic gain of function and found that peroxynitrite induced tyrosine nitration in a dose-dependent manner. Moreover, peroxynitrite triggered the formation of stable high-molecular-weight oligomers of NGF. Preventing tyrosine nitration by urate abolished the effect of peroxynitrite on NGF apoptotic activity. These results indicate that the oxidation of NGF by peroxynitrite enhances NGF apoptotic activity through p75(NTR) 10,000-fold. To our knowledge, this is the first known posttranslational modification that transforms a neurotrophin into an apoptotic agent.

The biology of nitric oxide and other reactive intermediates in systemic lupus erythematosus

Formation of reactive nitrogen and oxygen intermediates (RNI and ROI) is an essential part of the innate immune response. Markers of systemic RNI production are increased in the setting of systemic lupus erythematosus (SLE) activity. Several lines of evidence suggest mechanisms through which the activity of inducible nitric oxide synthase (iNOS) is pathogenic in SLE, including the ability of peroxynitrite (ONOO(-), a product of iNOS activity) to modify proteins, lipids, and DNA. These modifications can alter enzyme activity and may increase the immunogenicity of self antigens, leading to a break in immune tolerance. In humans, observational data suggest that overexpression of iNOS and increased production of ONOO(-) lead to glomerular and vascular pathology. Therapies designed to target iNOS activity or scavenge ROI and RNI are in development and may provide the means to reduce the pathogenic consequences of ROI and RNI in SLE.

Lipid nitration and formation of lipid-protein adducts: biological insights

Lipid-protein adducts are formed during oxidative and nitrative stress conditions associated with increasing lipid and protein oxidation and nitration. The focus of this review is the analysis of interactions between oxidative-modified lipids and proteins and how lipid nitration can modulate lipid-protein adducts formation. For this, two biologically-relevant models will be analysed: a) human low density lipoprotein, whose oxidation is involved in the early steps of atherogenesis, and b) alpha-synuclein/lipid membranes system, where lipid-protein adducts are being associated with the develop of Parkinson disease and other synucleinopathies.

Modified Low-Density Lipoproteins and High-Density Lipoproteins

It has long been known that the oxidative state of the various plasma lipoproteins modulates platelet aggregability, thereby contributing to atherogenesis. Low-density lipoprotein (LDL), occurring in vivo both in the native and oxidised forms, interacts directly with platelets, by binding to specific receptors. While the identity of the receptors for native LDL and some subfractions of high-density lipoproteins (HDL) remains disputed, apoE-containing HDL(2) binds to LRP8. The nature of these interactions as well as the distinction between candidate receptor proteins was elucidated using covalently modified apolipoproteins, which pointed to the participation of apolipoproteins in high affinity binding. However, the platelet effects initiated by binding of native lipoproteins remain controversial. Some of this ambiguity can be traced to the fact that native LDL inevitably undergoes substantial oxidisation upon modification, including by radiolabelling. The platelet-activating effects provoked by oxidised LDL are irrefutable, but many details remain unknown. The role of CD36 in platelet binding by oxidised LDL is well established, although additional receptors may exist. Much less is known about the interaction of oxidised HDL with platelets, since platelet activation was observed in some, but not all studies. Various frequently applied in vitro oxidation methods produce modified lipoprotein species that may not be relevant in vivo. Based on the reported modifications obtained by in vitro oxidation of LDL, early investigations focused mainly on the formation and the eventual effects of oxidised lipids. More recently, alterations to lipoproteins performed using hypochloric acid and myeloperoxidase redirected the attention to the role of modified apoproteins in triggering platelet responses.

Nitration of human plasminogen by RAW 264.7 macrophages reduces streptokinase-induced plasmin activity

Despite many years of study, clinical trials of new drugs to prevent thrombosis have often been disappointing. Part of the problem lies in our incomplete understanding of the regulation of plasminogen activation and/or inhibition in vivo. We have previously shown that in vitro nitration of plasminogen in plasma by peroxynitrite resulted in decreased plasmin activity. We hypothesized that macrophages may be agents of plasminogen nitration and designed this study to prove this hypothesis. We first better characterized our previous observations using purified plasminogen instead of whole plasma, studied the time and concentration dependence of these reactions, and co-incubated plasminogen with macrophages, as well as with non-inflammatory cells as controls, to assess nitration and impaired activity. When plasminogen (10μmol/L) is incubated in the presence of SIN-1 (0.01–2mmol/L), plasmin activity (generated by streptokinase) is reduced in a time- and concentration-dependent fashion. We performed experiments incubating human plasminogen in the presence of murine RAW264.7 macrophages, allowing for free diffusion of reactive oxygen species, while preventing the action of proteases. In this way we show that incubation of plasminogen with macrophages also decreases plasmin activity, while increasing nitration of the molecule, an effect that is already apparent after 2h and reaches a plateau of 60% inhibition after 24h of incubation. This effect appears specific for macrophages, since 31EG4 murine mammary cells used in parallel and under the same conditions failed to produce any deleterious changes in plasminogen. Our data on quick functional inactivation of plasminogen by nitration, mediated by macrophages, adds a new pathophysiological dimension to our previous work showing plasminogen as a target for peroxynitrite damage. Nitrosative stress may be implicated in impaired fibrinolysis. New therapeutic approaches for nitrosative stress in atherosclerosis and diabetes should limit the formation of superoxides and peroxynitrite.