Lipoxidation-derived reactive carbonyl species as potential drug targets in preventing protein carbonylation and related cellular dysfunction

Isolevuglandins Promote Mitochondrial Dysfunction and Electrophysiologic Abnormalities in Atrial Cardiomyocytes

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia, yet the cellular and molecular mechanisms underlying the AF substrate remain unclear. Isolevuglandins (IsoLGs) are highly reactive lipid dicarbonyl products that mediate oxidative stress-related injury. In murine hypertension, the lipid dicarbonyl scavenger 2-hydroxybenzylamine (2-HOBA) reduced IsoLGs and AF susceptibility. We hypothesized that IsoLGs mediate detrimental pathophysiologic effects in atrial cardiomyocytes that promote the AF substrate. Using Seahorse XFp extracellular flux analysis and a luminescence assay, IsoLG exposure suppressed intracellular ATP production in atrial HL-1 cardiomyocytes. IsoLGs caused mitochondrial dysfunction, with reduced mitochondrial membrane potential, increased mitochondrial reactive oxygen species (ROS) with protein carbonylation, and mitochondrial DNA damage. Moreover, they generated cytosolic preamyloid oligomers previously shown to cause similar detrimental effects in atrial cells. In mouse atrial and HL-1 cells, patch clamp experiments demonstrated that IsoLGs rapidly altered action potentials (AP), implying a direct effect independent of oligomer formation by reducing the maximum Phase 0 upstroke slope and shortening AP duration due to ionic current modifications. IsoLG-mediated mitochondrial and electrophysiologic abnormalities were blunted or totally prevented by 2-HOBA. These findings identify IsoLGs as novel mediators of oxidative stress-dependent atrial pathophysiology and support the investigation of dicarbonyl scavengers as a novel therapeutic approach to prevent AF.

Reactive X (where X = O, N, S, C, Cl, Br, and I) species nanomedicine

Reactive oxygen, nitrogen, sulfur, carbonyl, chlorine, bromine, and iodine species (RXS, where X = O, N, S, C, Cl, Br, and I) have important roles in various normal physiological processes and act as essential regulators of cell metabolism; their inherent biological activities govern cell signaling, immune balance, and tissue homeostasis. However, an imbalance between RXS production and consumption will induce the occurrence and development of various diseases. Due to the considerable progress of nanomedicine, a variety of nanosystems that can regulate RXS has been rationally designed and engineered for restoring RXS balance to halt the pathological processes of different diseases. The invention of radical-regulating nanomaterials creates the possibility of intriguing projects for disease treatment and promotes advances in nanomedicine. In this comprehensive review, we summarize, discuss, and highlight very-recent advances in RXS-based nanomedicine for versatile disease treatments. This review particularly focuses on the types and pathological effects of these reactive species and explores the biological effects of RXS-based nanomaterials, accompanied by a discussion and the outlook of the challenges faced and future clinical translations of RXS nanomedicines.

Lipid peroxidation derived reactive carbonyl species in free and conjugated forms as an index of lipid peroxidation: limits and perspectives

Reactive carbonyl species (RCS) formed by lipidperoxidation as free forms or as enzymatic and non-enzymatic conjugates are widely used as an index of oxidative stress. Besides general measurements based on derivatizing reactions, more selective and sensitive MS based analyses have been proposed in the last decade. Untargeted and targeted methods for the measurement of free RCS and adducts have been described and their applications to in vitro and ex vivo samples have permitted the identification of many biological targets, reaction mechanisms and adducted moieties with a particular relevance to RCS protein adducts. The growing interest in protein carbonylation can be explained by considering that protein adducts are now recognized as being involved in the damaging action of oxidative stress so that their measurement is performed not only to obtain an index of lipid peroxidation but also to gain a deeper insight into the molecular mechanisms of oxidative stress. The aim of the review is to discuss the most novel analytical approaches and their application for profiling reactive carbonyl species and their enzymatic and non-enzymatic metabolites as an index of lipid-oxidation and oxidative stress. Limits and perspectives will be discussed.

L-carnosine and its Derivatives as New Therapeutic Agents for the Prevention and Treatment of Vascular Complications of Diabetes

Vascular complications are among the most serious manifestations of diabetes. Atherosclerosis is the main cause of reduced life quality and expectancy in diabetics, whereas diabetic nephropathy and retinopathy are the most common causes of end-stage renal disease and blindness. An effective therapeutic approach to prevent vascular complications should counteract the mechanisms of injury. Among them, the toxic effects of Advanced Glycation (AGEs) and Lipoxidation (ALEs) end-products are well-recognized contributors to these sequelae. L-carnosine (β-alanyl-Lhistidine) acts as a quencher of the AGE/ALE precursors Reactive Carbonyl Species (RCS), which are highly reactive aldehydes derived from oxidative and non-oxidative modifications of sugars and lipids. Consistently, L-carnosine was found to be effective in several disease models in which glyco/lipoxidation plays a central pathogenic role. Unfortunately, in humans, L-carnosine is rapidly inactivated by serum carnosinase. Therefore, the search for carnosinase-resistant derivatives of Lcarnosine represents a suitable strategy against carbonyl stress-dependent disorders, particularly diabetic vascular complications. In this review, we present and discuss available data on the efficacy of L-carnosine and its derivatives in preventing vascular complications in rodent models of diabetes and metabolic syndrome. We also discuss genetic findings providing evidence for the involvement of the carnosinase/L-carnosine system in the risk of developing diabetic nephropathy and for preferring the use of carnosinase-resistant compounds in human disease. The availability of therapeutic strategies capable to prevent both long-term glucose toxicity, resulting from insufficient glucoselowering therapy, and lipotoxicity may help reduce the clinical and economic burden of vascular complications of diabetes and related metabolic disorders.

The Disposal of Reactive Carbonyl Species through Carnosine Conjugation: What We Know Now

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Reactive Carbonyl Species are electrophiles generated by the oxidative cleavage of lipids and sugars. Such compounds have been described as important molecules for cellular signaling, whilst their accumulation has been found to be cytotoxic as they may trigger aberrant modifications of proteins (a process often referred to as carbonylation).

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A correlation between carbonylation of proteins and human disease progression has been shown in ageing, diabetes, obesity, chronic renal failure, neurodegeneration and cardiovascular disease. However, the fate of reactive carbonyl species is still far from being understood, especially concerning the mechanisms responsible for their disposal as well as the importance of this in disease progression.

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In this context, some data have been published on phase I and phase II deactivation of reactive carbonyl species. In the case of phase II mechanisms, the route involving glutathione conjugation and subsequent disposal of the adducts has been extensively studied both in vitro and in vivo for some of the more representative compounds, e.g. 4-hydroxynonenal.

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There is also emerging evidence of an involvement of carnosine as an endogenous alternative to glutathione for phase II conjugation. However, the fate of carnosine conjugates is still poorly investigated and, unlike glutathione, there is little evidence of the formation of carnosine adducts in vivo. The acquisition of such data could be of importance for the development of new drugs, since carnosine and its derivatives have been proposed as potential therapeutic agents for the mitigation of carbonylation associated with disease progression.

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Herein, we wish to review our current knowledge of the binding of reactive carbonyl species with carnosine together with the disposal of carnosine conjugates, emphasizing those aspects still requiring investigation such as conjugation reversibility and enzyme assisted catalysis of the reactions.

Exploring aggregation-induced emission through tuning of ligand structure for picomolar detection of pyrene

Tuning of ligand structures through controlled variation of ring number in fused-ring aromatic moiety appended to antipyrine allows detection of 7.8 × 10^-12  M pyrene via aggregation-induced emission (AIE) associated with 101-fold fluorescence enhancement. In one case, antipyrine unit is replaced by pyridine to derive bis-methylanthracenyl picolyl amine. The structures of four molecules have been confirmed by single crystal X-ray diffraction analysis. Among them, pyrene-antipyrine conjugate (L) undergoes pyrene triggered inhibition of photo-induced electron transfer (PET) leading to water-assisted AIE.

Isotopic labelling for the characterisation of HNE-sequestering agents in plant-based extracts and its application for the identification of anthocyanidins in black rice with giant embryo

Reactive carbonyl species (RCS) are cytotoxic molecules that originate from lipid peroxidation and sugar oxidation. Natural derivatives can be an attractive source of potential RCS scavenger. However, the lack of analytical methods to screen and identify bioactive compounds contained in complex matrices has hindered their identification. The sequestering actions of various rice extracts on RCS have been determined using ubiquitin and 4-hydroxy-2-nonenal (HNE) as a protein and RCS model, respectively. Black rice with giant embryo extract was found to be the most effective among various rice varieties. The identification of bioactive compounds was then carried out by an isotopic signature profile method using the characteristic isotopic ion cluster generated by the mixture of HNE: ²H₅-HNE mixed at a 1:1 stoichiometric ratio. An in-house database was used to obtain the structures of the possible bioactive components. The identified compounds were further confirmed as HNE sequestering agents through HPLC-UV analysis.

A mitochondria-targeted fluorescent probe for imaging endogenous malondialdehyde in HeLa cells and onion tissues

The mitochondria-targeted turn-on fluorescent probe (Mito-FMP) based on a benzoxadiazole platform was developed for detection of malondialdehyde (MDA). Mito-FMP performed with large enhancement of the optical signal (774-fold) in response to MDA in an aqueous system and has the capability of monitoring endogenous MDA in HeLa cells and onion tissues.

Evaluation of Stability and In Vitro Security of Nanoemulsions Containing Eucalyptus globulus Oil

Essential oil of Eucalyptus globulus presents several pharmacological properties. However, their therapeutic efficacy may be affected by limitations due to several conditions, rendering it difficult to obtain stable and effective pharmaceutical formulations. The use of nanotechnology is an alternative to improve their characteristics aiming to ensure their stability and effectiveness. Furthermore, studies about the possible toxic effects of nanostructures are necessary to evaluate safety when the formulation comes into contact with human cells. Hence, in this paper, we evaluate for the first time the stability and in vitro cytogenotoxicity of nanoemulsions containing Eucalyptus globulus in peripheral blood mononuclear cells. As a result, the stability study found that the best condition for storage up to 90 days was refrigeration (4°C); it was the condition that best preserved the nanometric features. The content of the major compounds of oil was maintained after nanoencapsulation and preserved over time. In tests to evaluate the safety of this formulation, we can conclude that, at a low concentration (approximately 0.1%), Eucalyptus globulus nanoemulsion did not cause toxicity in peripheral blood mononuclear cells and also showed a protective effect in cells against possible damage when compared to oil in free form.

Fluorescent probes for imaging formaldehyde in biological systems

Formaldehyde (FA) is a common environmental toxin but is also endogenously produced through a diverse array of essential biological processes, including mitochondrial one-carbon metabolism, metabolite oxidation, and nuclear epigenetic modifications. Its high electrophilicity enables reactivity with a wide variety of biological nucleophiles, which can be beneficial or detrimental to cellular function depending on the context. New methods that enable detection of FA in living systems can help disentangle the signal/stress dichotomy of this simplest reactive carbonyl species (RCS), and fluorescent probes for FA with high selectivity and sensitivity have emerged as promising chemical tools in this regard.

Understanding the role of 3-O-Acetyl-11-keto-β-boswellic acid in conditions of oxidative-stress mediated hepatic dysfunction during benzo(a)pyrene induced toxicity

The present study was planned to see whether 3-O-Acetyl-11- keto-β-boswellic acid has any protective effects against benzo(a)pyrene (BaP) induced toxicity or not. In vitro studies show concentration dependent linear association of radical scavenging activity of AK which is comparable to ascorbic acid taken as reference compound. For in vivo studies, the animals were divided randomly into five groups which included a) normal control, b) vehicle treated (olive oil), c) BaP treated, d) AK treated and e) AK + BaP (combined treated). BaP was administered at a dose of 50mg/kg in olive oil twice a week orally for 4 weeks and AK (50mg/kg) was given in olive oil thrice a week for 4 weeks before and after BaP exposure. BaP treated animals showed a significant increase (p < 0.001) in lipid peroxidation (LPO) and protein carbonyl contents (PCC) in hepatic tissue. Further, a significant increase (p < 0.001) in the liver marker enzymes as well as citrulline and nitric oxide levels in the hepatic tissue was also observed. Interestingly, AK when supplemented to BaP treated animals ameliorated the above said biochemical indices appreciately. The histopathological observations also showed appreciable improvement when BaP treated animals were supplemented with AK, thus emphasing the protective potential of AK.

HPLC-Based Mass Spectrometry Characterizes the Phospholipid Alterations in Ether-Linked Lipid Deficiency Models Following Oxidative Stress

Despite the fact that the discovery of ether-linked phospholipids occurred nearly a century ago, many unanswered questions remain concerning these unique lipids. Here, we characterize the ether-linked lipids of the nematode with HPLC-MS/MS and find that more than half of the phosphoethanolamine-containing lipids are ether-linked, a distribution similar to that found in mammalian membranes. To explore the biological role of ether lipids in vivo, we target fatty acyl-CoA reductase (fard-1), an essential enzyme in ether lipid synthesis, with two distinct RNAi strategies. First, when fard-1 RNAi is initiated at the start of development, the treated animals have severely reduced ether lipid abundance, resulting in a shift in the phosphatidylethanolamine lipid population to include more saturated fatty acid chains. Thus, the absence of ether lipids during development drives a significant remodeling of the membrane landscape. A later initiation of fard-1 RNAi in adulthood results in a dramatic reduction of new ether lipid synthesis as quantified with ¹⁵N-tracers; however, there is only a slight decrease in total ether lipid abundance with this adult-only fard-1 RNAi. The two RNAi strategies permit the examination of synthesis and ether lipid abundance to reveal a relationship between the amount of ether lipids and stress survival. We tested whether these species function as sacrificial antioxidants by directly examining the phospholipid population with HPLC-MS/MS after oxidative stress treatment. While there are significant changes in other phospholipids, including polyunsaturated fatty acid-containing species, we did not find any change in ether-linked lipids, suggesting that the role of ether lipids in stress resistance is not through their general consumption as free radical sinks. Our work shows that the nematode will be a useful model for future interrogation of ether lipid biosynthesis and the characterization of phospholipid changes in various stress conditions.

A carnosine intervention study in overweight human volunteers: bioavailability and reactive carbonyl species sequestering effect

Carnosine is a natural dipeptide able to react with reactive carbonyl species, which have been recently associated with the onset and progression of several human diseases. Herein, we report an intervention study in overweight individuals. Carnosine (2 g/day) was orally administered for twelve weeks in order to evaluate its bioavailability and metabolic fate. Two carnosine adducts were detected in the urine samples of all subjects. Such adducts are generated from a reaction with acrolein, which is one of the most toxic and reactive compounds among reactive carbonyl species. However, neither carnosine nor adducts have been detected in plasma. Urinary excretion of adducts and carnosine showed a positive correlation although a high variability of individual response to carnosine supplementation was observed. Interestingly, treated subjects showed a significant decrease in the percentage of excreted adducts in reduced form, accompanied by a significant increase of the urinary excretion of both carnosine and carnosine-acrolein adducts. Altogether, data suggest that acrolein is entrapped in vivo by carnosine although the response to its supplementation is possibly influenced by individual diversities in terms of carnosine dietary intake, metabolism and basal production of reactive carbonyl species.

Albumin Cys34 adducted by acrolein as a marker of oxidative stress in ischemia-reperfusion injury during hepatectomy

The aim of this study was to measure and identify the reactive carbonyl species (RCSs) released in the blood of humans subjected to hepatic resection. Pre-anesthesia malondialdehyde (MDA) plasma content (0.36 ± 0.11 nmol/mg protein) remained almost unchanged immediately after anaesthesia, before clamping and at the 10th min after ischemia, while markedly increased (to 0.59 ± 0.07 nmol/mg; p < 0.01, Tukey's post test) at the 10th min of reperfusion. A similar trend was observed for the protein carbonyls (PCs), whose pre-anesthesia levels (0.17 ± 0.13 nmol/mg) did not significantly change during ischemia, while increased more than fourfold at the 10th min of reperfusion (0.75 ± 0.17 nmol/mg; p < 0.01, Tukey's post test). RCSs were then identified as covalent adducts to the albumin Cys34, which we previously found as the most reactive protein nucleophilic site in plasma. By using a mass spectrometry (MS) approach based on precursor ion scanning, we found that acrolein (ACR) is the main RCS adducted to albumin Cys34. In basal conditions, the adducted albumin was 0.6 ± 0.4% of the native form but it increased by almost fourfold at the 10th min of reperfusion (2.3 ± 0.7%; p < 0.01, t-test analysis). Since RCSs are damaging molecules, we propose that RCSs, and ACR in particular, are new targets for novel molecular treatments aimed at reducing the ischemia/reperfusion damage by the use of RCS sequestering agents.

A method to produce fully characterized ubiquitin covalently modified by 4-hydroxy-nonenal, glyoxal, methylglyoxal, and malondialdehyde

Reactive carbonyl species (RCS) and the corresponding protein adducts (advanced glycoxidation or lipoxidation end products, i.e. AGEs and ALEs) are now widely studied from different points of view, since they can be considered as biomarkers, pathogenic factors, toxic mediators and drug targets. One of the main limits of the research in this field is the lack of standardized and fully characterized AGEs and ALEs to be used for biological, toxicological, and analytical studies. In this work, we set up a procedure to prepare and fully characterize a set of AGEs and ALEs by incubating ubiquitin - a model protein selected as target for carbonylation - with four different RCS: 4-hydroxy-trans-2-nonenal (HNE), methylglyoxal (MGO), glyoxal (GO), and malondialdehyde (MDA). After 24 h of incubation, the extent of protein carbonylation was estimated using a recently developed quantitative strategy based on high-resolution mass spectrometry. The resulting AGEs and ALEs were fully characterized by both intact protein and bottom-up analyses in terms of: stoichiometry of the total amount of modified protein, elucidation of the structure of the RCS-deriving adducts, and localization of the RCS-modified amino acids. Each RCS exhibited different reactivity toward ubiquitin, as detected by quantifying the extent of protein modification. The order of reactivity was MGO > GO > HNE > MDA. A variety of reaction products was identified and mapped on lysine, arginine, and histidine residues of the protein. In summary, a highly standardized and reproducible method to prepare fully characterized AGEs/ALEs is here presented.

Sulforaphane protects the heart from doxorubicin-induced toxicity

Cardiotoxicity is one of the major side effects encountered during cancer chemotherapy with doxorubicin (DOX) and other anthracyclines. Previous studies have shown that oxidative stress caused by DOX is one of the primary mechanisms for its toxic effects on the heart. Since the redox-sensitive transcription factor, Nrf2, plays a major role in protecting cells from the toxic metabolites generated during oxidative stress, we examined the effects of the phytochemical sulforaphane (SFN), a potent Nrf2-activating agent, on DOX-induced cardiotoxicity. These studies were carried out both in vitro and in vivo using rat H9c2 cardiomyoblast cells and wild type 129/sv mice, and involved SFN pretreatment followed by SFN administration during DOX exposure. SFN treatment protected H9c2 cells from DOX cytotoxicity and also resulted in restored cardiac function and a significant reduction in DOX-induced cardiomyopathy and mortality in mice. Specificity of SFN induction of Nrf2 and protection of H9c2 cells was demonstrated in Nrf2 knockdown experiments. Cardiac accumulation of 4-hydroxynonenal (4-HNE) protein adducts, due to lipid peroxidation following DOX-induced oxidative stress, was significantly attenuated by SFN treatment. The respiratory function of cardiac mitochondria isolated from mice exposed to DOX alone was repressed, while SFN treatment with DOX significantly elevated mitochondrial respiratory complex activities. Co-administration of SFN reversed the DOX-associated reduction in nuclear Nrf2 binding activity and restored cardiac expression of Nrf2-regulated genes at both the RNA and protein levels. Together, our results demonstrate for the first time that the Nrf2 inducer, SFN, has the potential to provide protection against DOX-mediated cardiotoxicity.

Reactive species and oxidative stress in optic nerve vulnerable to secondary degeneration

Secondary degeneration contributes substantially to structural and functional deficits following traumatic injury to the CNS. While it has been proposed that oxidative stress is a feature of secondary degeneration, contributing reactive species and resultant oxidized products have not been clearly identified in vivo. The study is designed to identify contributors to, and consequences of, oxidative stress in a white matter tract vulnerable to secondary degeneration. Partial dorsal transection of the optic nerve (ON) was used to model secondary degeneration in ventral nerve unaffected by the primary injury. Reactive species were assessed using fluorescent labelling and liquid chromatography/tandem mass spectroscopy (LC/MS/MS). Antioxidant enzymes and oxidized products were semi-quantified immunohistochemically. Mitophagy was assessed by electron microscopy. Fluorescent indicators of reactive oxygen and/or nitrogen species increased at 1, 3 and 7days after injury, in ventral ON. LC/MS/MS confirmed increases in reactive species linked to infiltrating microglia/macrophages in dorsal ON. Similarly, immunoreactivity for glutathione peroxidase and haem oxygenase-1 increased in ventral ON at 3 and 7days after injury, respectively. Despite increased antioxidant immunoreactivity, DNA oxidation was evident from 1day, lipid oxidation at 3days, and protein nitration at 7days after injury. Nitrosative and oxidative damage was particularly evident in CC1-positive oligodendrocytes, at times after injury at which structural abnormalities of the Node of Ranvier/paranode complex have been reported. The incidence of mitochondrial autophagic profiles was also significantly increased from 3days. Despite modest increases in antioxidant enzymes, increased reactive species are accompanied by oxidative and nitrosative damage to DNA, lipid and protein, associated with increasing abnormal mitochondria, which together may contribute to the deficits of secondary degeneration.

A novel high resolution MS approach for the screening of 4-hydroxy-trans-2-nonenal sequestering agents

An in vitro high resolution mass spectrometry (MS) method was set-up to test the ability of compounds, mixtures and extracts to inhibit protein carbonylation induced by reactive carbonyl species (RCS). The method consists of incubating the protein target (ubiquitin) with 4-hydroxy-trans-2-nonenal (HNE) in the presence and absence of the tested compound. After 24h of incubation, the reaction is stopped and the protein is analyzed by high-resolution MS. The extent of protein carbonylation is determined by measuring the area of the +11 multicharged peak of the HNE adduct in respect to the native form. The method was validated by measuring the effect of well-known RCS sequestering agents, namely aminoguanidine, pyridoxamine, hydralazine and carnosine, yielding a good reproducibility and the possibility to be automatable. All the compounds were found to dose-dependently inhibit the protein carbonylation with the following order of potency carnosine≈hydralazine≫aminoguanidine>pyridoxamine, as determined by calculating the UC50 values, that is the concentration required to inhibit ubiquitin carbonylation by 50%. A good correlation was found with the results obtained by measuring HNE consumption using an HPLC method optimized by a mobile phase set at pH 7.4, in order to stabilize the eluted adducts. The MS approach was then applied to test the effect of two selected natural extracts on protein carbonylation, i.e. green coffee bean extract and procyanidins from Vitis vinifera. In summary, this paper reports a validated and highly reproducible MS method to test the ability of pure compounds as well as natural extracts to act as protein carbonylation inhibitors.

Signaling properties of 4-hydroxyalkenals formed by lipid peroxidation in diabetes

Peroxidation of polyunsaturated fatty acids is intensified in cells subjected to oxidative stress and results in the generation of various bioactive compounds, of which 4-hydroxyalkenals are prominent. During the progression of type 2 diabetes mellitus, the ensuing hyperglycemia promotes the generation of reactive oxygen species (ROS) that contribute to the development of diabetic complications. It has been suggested that ROS-induced lipid peroxidation and the resulting 4-hydroxyalkenals markedly contribute to the development and progression of these pathologies. Recent findings, however, also suggest that noncytotoxic levels of 4-hydroxyalkenals play important signaling functions in the early phase of diabetes and act as hormetic factors to induce adaptive and protective responses in cells, enabling them to function in the hyperglycemic milieu. Our studies and others' have proposed such regulatory functions for 4-hydroxynonenal and 4-hydroxydodecadienal in insulin secreting β-cells and vascular endothelial cells, respectively. This review presents and discusses the mechanisms regulating the generation of 4-hydroxyalkenals under high glucose conditions and the molecular interactions underlying the reciprocal transition from hormetic to cytotoxic agents.

Lipoxidation adducts with peptides and proteins: deleterious modifications or signaling mechanisms?

Protein lipoxidation refers to the modification by electrophilic lipid oxidation products to form covalent adducts, which for many years has been considered as a deleterious consequence of oxidative stress. Oxidized lipids or phospholipids containing carbonyl moieties react readily with lysine to form Schiff bases; alternatively, oxidation products containing α,β-unsaturated moieties are susceptible to nucleophilic attack by cysteine, histidine or lysine residues to yield Michael adducts, overall corresponding to a large number of possible protein adducts. The most common detection methods for lipoxidized proteins take advantage of the presence of reactive carbonyl groups to add labels, or use antibodies. These methods have limitations in terms of specificity and identification of the modification site. The latter question is satisfactorily addressed by mass spectrometry, which enables the characterization of the adduct structure. This has allowed the identification of lipoxidized proteins in physiological and pathological situations. While in many cases lipoxidation interferes with protein function, causing inhibition of enzymatic activity and increased immunogenicity, there are a small number of cases where lipoxidation results in gain of function or activity. For certain proteins lipoxidation may represent a form of redox signaling, although more work is required to confirm the physiological relevance and mechanisms of such processes. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Exploring the space of histidine containing dipeptides in search of novel efficient RCS sequestering agents

The study reports a set of forty proteinogenic histidine-containing dipeptides as potential carbonyl quenchers. The peptides were chosen to cover as exhaustively as possible the accessible chemical space, and their quenching activities toward 4-hydroxy-2-nonenal (HNE) and pyridoxal were evaluated by HPLC analyses. The peptides were capped at the C-terminus as methyl esters or amides to favor their resistance to proteolysis and diastereoisomeric pairs were considered to reveal the influence of configuration on quenching. On average, the examined dipeptides are less active than the parent compound carnosine (βAla + His) thus emphasizing the unfavorable effect of the shortening of the βAla residue as confirmed by the control dipeptide Gly-His. Nevertheless, some peptides show promising activities toward HNE combined with a remarkable selectivity. The results emphasize the beneficial role of aromatic and positively charged residues, while negatively charged and H-bonding side chains show a detrimental effect on quenching. As a trend, ester derivatives are slightly more active than amides while heterochiral peptides are more active than their homochiral diastereoisomer. Overall, the results reveal that quenching activity strongly depends on conformational effects and vicinal residues (as evidenced by the reported QSAR analysis), offering insightful clues for the design of improved carbonyl quenchers and to rationalize the specific reactivity of histidine residues within proteins.

The lipid peroxidation product 4-hydroxy-2-nonenal: Advances in chemistry and analysis

4-Hydroxy-2-nonenal (HNE) is one of the most studied products of phospholipid peroxidation, owing to its reactivity and cytotoxicity. It can be formed by several radical-dependent oxidative routes involving the formation of hydroperoxides, alkoxyl radicals, epoxides, and fatty acyl cross-linking reactions. Cleavage of the oxidized fatty acyl chain results in formation of HNE from the methyl end, and 9-oxo-nonanoic acid from the carboxylate or esterified end of the chain, although many other products are also possible. HNE can be metabolized in tissues by a variety of pathways, leading to detoxification and excretion. HNE-adducts to proteins have been detected in inflammatory situations such as atherosclerotic lesions using polyclonal and monoclonal antibodies, which have also been applied in ELISAs and western blotting. However, in order to identify the proteins modified and the exact sites and nature of the modifications, mass spectrometry approaches are required. Combinations of enrichment strategies with targetted mass spectrometry routines such as neutral loss scanning are now facilitating detection of HNE-modified proteins in complex biological samples. This is important for characterizing the interactions of HNE with redox sensitive cell signalling proteins and understanding how it may modulate their activities either physiologically or in disease.

D-Carnosine octylester attenuates atherosclerosis and renal disease in ApoE null mice fed a Western diet through reduction of carbonyl stress and inflammation

BACKGROUND AND PURPOSE

Lipoxidation-derived reactive carbonyl species (RCS) such as 4-hydroxy-2-nonenal (HNE) react with proteins to form advanced lipoxidation end products (ALEs), which have been implicated in both atherosclerosis and renal disease. L-carnosine acts as an endogenous HNE scavenger, but it is rapidly inactivated by carnosinase. This study aimed at assessing the effect of the carnosinase-resistant, D-carnosine, on HNE-induced cellular injury and of its bioavailable prodrug D-carnosine octylester on experimental atherosclerosis and renal disease.

EXPERIMENTAL APPROACH

Vascular smooth muscle cells (VSMCs) were exposed to HNE or H₂O₂ plus D-carnosine. ApoE null mice fed a Western, pro-atherogenic diet were treated with D-carnosine octylester for 12 weeks.

KEY RESULTS

In vitro, D-carnosine attenuated the effect of HNE, but not of H₂O₂, on VSMCs. In vivo, D-carnosine octylester-treated mice showed reduced lesion area and a more stable plaque phenotype compared with untreated animals, with reduced foam cell accumulation, inflammation and apoptosis and increased clearance of apoptotic bodies and collagen deposition, resulting in decreased necrotic core formation. Likewise, renal lesions were attenuated in D-carnosine octylester-treated versus untreated mice, with lower inflammation, apoptosis and fibrosis. This was associated with increased urinary levels of HNE-carnosine adducts and reduced protein carbonylation, circulating and tissue ALEs, expression of receptors for these products, and systemic and tissue oxidative stress.

CONCLUSIONS AND IMPLICATIONS

These data indicate RCS quenching with a D-carnosine ester was highly effective in attenuating experimental atherosclerosis and renal disease by reducing carbonyl stress and inflammation and that this may represent a promising therapeutic strategy in humans.

Strategies to decrease ongoing oxidant burden in chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a leading cause of mortality and morbidity globally, and its development is mainly associated with tobacco/biomass smoke-induced oxidative stress. Hence, targeting systemic and local oxidative stress with agents that can balance the antioxidant/redox system can be expected to be useful in the treatment of COPD. Preclinical and clinical trials have revealed that antioxidants/redox modulators can detoxify free radicals and oxidants, control expression of redox and glutathione biosynthesis genes, chromatin remodeling and inflammatory gene expression; and are especially useful in preventing COPD exacerbations. In this review, various novel approaches and problems associated with these approaches in COPD are reviewed.

Protection by polyphenols of postprandial human plasma and low-density lipoprotein modification: the stomach as a bioreactor

Recent studies dramatically showed that the removal of circulating modified low-density lipoprotein (LDL) results in complete prevention of atherosclerosis. The gastrointestinal tract is constantly exposed to food, some of it containing oxidized compounds. Lipid oxidation in the stomach was demonstrated by ingesting heated red meat in rats. Red wine polyphenols added to the rats' meat diet prevented lipid peroxidation in the stomach and absorption of malondialdehyde (MDA) in rat plasma. In humans, postprandial plasma MDA levels rose by 3-fold after a meal of red meat cutlets. MDA derived from meat consumption caused postprandial plasma LDL modification in human. The levels of plasma MDA showed a 75% reduction by consumption of red wine polyphenols during the meat meal. Locating the main biological site of action of polyphenols in the stomach led to a revision in the understanding of how antioxidants work in vivo and may help to elucidate the mechanism involved in the protective effects of polyphenols in human health.

Transforming dietary peptides in promising lead compounds: the case of bioavailable carnosine analogs

The ability of carnosine to prevent advanced glycoxidation end products (AGEs) and advanced lipoxidation end products (ALEs) formation, on the one hand, and the convincing evidence that these compounds act as pathogenetic factors, on the other hand, strongly support carnosine as a promising therapeutic agent for oxidative-based diseases. The mechanism/s by which carnosine inhibits AGEs and ALEs is still under investigation but an emerging hypothesis is that carnosine acts by deactivating the AGEs and ALEs precursors and in particular the reactive carbonyl species (RCS) generated by both lipid and sugar oxidation. The ability of carnosine to inhibit AGEs and ALEs formation and the corresponding biological effects has been demonstrated in several in vitro studies and in some animal models. However, such effects are in line of principle, limited in humans, due to the effect of serum carnosinase (absent in rodents), which catalyzes the carnosine hydrolysis to its constitutive amino acids. Such a limitation has prompted a great interest in the design of carnosine derivatives, which maintaining (or improving) the reactivity with RCS, are more resistant to carnosinase. The present paper intends to critically review the most recent studies oriented to obtaining carnosine derivatives, optimized in terms of reactivity with RCS, selectivity (no reaction with physiological aldehydes) and the pharmacokinetic profile (mainly through an enhanced resistance to carnosinase hydrolysis). The review also includes a brief description of AGEs and ALEs as drug targets and the evidence so far reported regarding the ability of carnosine as inhibitor of AGEs and ALEs formation and the proposed reaction mechanisms.

Association between age-related decline of kidney function and plasma malondialdehyde

Oxidative stress is a key factor linked renal function decline with age. However, there is still no large cohort study exploring the potential role of oxidative stress in mild insufficiency of kidney function (MIKF) and chronic kidney disease (CKD) after adjusting for confounding factors. This study tested the hypothesis that oxidative stress, indicated by plasma malondialdehyde (MDA), is associated with the prevalence of MIKF and CKD after controlling the effects of confounding factors. Plasma levels of MDA and serum levels of fasting glucose, cholesterol, triglycerides, creatinine, alanine aminotransferase, and aspartate aminotransferase were analyzed from 2,169 Chinese Han adults. A questionnaire and physical examination were performed to identify and suspect risk factors of renal function decline with age. Kidney function, as indicated by estimated glomerular filtration rate, showed a significant decline with age in both male and female. Although the association between age and plasma MDA levels was nonlinear, MDA was negatively related to kidney function. The multivariate-adjusted odds ratios showed that plasma MDA had a significantly graded relation to the prevalence of MIKF and CKD with or without adjustment for covariates. By comparison with the lowest quartile, individuals with the highest quartile of MDA level had a 99% and 223% increased risk of developing MIKF and CKD, respectively. Further results from multiinteraction analysis demonstrated that plasma MDA may be the mediator linking different covariates with renal function decline. The most striking finding of this study was that oxidative stress, as indicated by plasma MDA levels, is associated with the prevalence of MIKF and/or CKD. Although imposing an increasing burden on the kidney and/or promoting a cyclical process of oxidative stress in the body, high levels of MDA in plasma may link the decline of kidney function with age.

Relationship of electrophilic stress to aging

This review begins with the premise that an organism's life span is determined by the balance between two countervailing forces: (i) the sum of destabilizing effects and (ii) the sum of protective longevity-assurance processes. Against this backdrop, the role of electrophiles is discussed, both as destabilizing factors and as signals that induce protective responses. Because most biological macromolecules contain nucleophilic centers, electrophiles are particularly reactive and toxic in a biological context. The majority of cellular electrophiles are generated from polyunsaturated fatty acids by a peroxidation chain reaction that is readily triggered by oxygen-centered radicals, but propagates without further input of reactive oxygen species (ROS). Thus, the formation of lipid-derived electrophiles such as 4-hydroxynon-2-enal (4-HNE) is proposed to be relatively insensitive to the level of initiating ROS, but to depend mainly on the availability of peroxidation-susceptible fatty acids. This is consistent with numerous observations that life span is inversely correlated to membrane peroxidizability, and with the hypothesis that 4-HNE may constitute the mechanistic link between high susceptibility of membrane lipids to peroxidation and shortened life span. Experimental interventions that directly alter membrane composition (and thus their peroxidizability) or modulate 4-HNE levels have the expected effects on life span, establishing that the connection is not only correlative but causal. Specific molecular mechanisms are considered, by which 4-HNE could (i) destabilize biological systems via nontargeted reactions with cellular macromolecules and (ii) modulate signaling pathways that control longevity-assurance mechanisms.

Protein modification by acrolein: relevance to pathological conditions and inhibition by aldehyde sequestering agents

Acrolein (ACR) is a toxic and highly reactive α,β-unsaturated aldehyde widely distributed in the environment as a common pollutant and generated endogenously mainly by lipoxidation reactions. Its biological effects are due to its ability to react with the nucleophilic sites of proteins, to form covalently modified biomolecules which are thought to be involved as pathogenic factors in the onset and progression of many pathological conditions such as cardiovascular and neurodegenerative diseases. Functional impairment of structural proteins and enzymes by covalent modification (crosslinking) and triggering of key cell signalling systems are now well-recognized signs of cell and tissue damage induced by reactive carbonyl species (RCS). In this review, we mainly focus on the in vitro and in vivo evidence demonstrating the ability of ACR to covalently modify protein structures, in order to gain a deeper insight into the dysregulation of cellular and metabolic pathways caused by such modifications. In addition, by considering RCS and RCS-modified proteins as drug targets, this survey will provide an overview on the newly developed molecules specifically tested for direct or indirect ACR scavenging, and the more significant studies performed in the last years attesting the efficacy of compounds already recognized as promising aldehyde-sequestering agents.

Cigarette smoke and α,β-unsaturated aldehydes elicit VEGF release through the p38 MAPK pathway in human airway smooth muscle cells and lung fibroblasts

BACKGROUND AND PURPOSE

Vascular endothelial growth factor (VEGF) is an angiogenic factor known to be elevated in the sputum of asymptomatic smokers as well as smokers with bronchitis type of chronic obstructive pulmonary disease. The aim of this study was to investigate whether acute exposure to cigarette smoke extract altered VEGF production in lung parenchymal cells.

EXPERIMENTAL APPROACH

We exposed human airway smooth muscle cells (ASMC), normal human lung fibroblasts (NHLF) and small airways epithelial cells (SAEC) to aqueous cigarette smoke extract (CSE) in order to investigate the effect of cigarette smoke on VEGF expression and release.

KEY RESULTS

Vascular endothelial growth factor release was elevated by sub-toxic concentrations of CSE in both ASMC and NHLF, but not in SAEC. CSE-evoked VEGF release was mimicked by its component acrolein at concentrations (10-100 µM) found in CSE, and prevented by the antioxidant and α,β-unsaturated aldehyde scavenger, N-acetylcysteine (NAC). Both CSE and acrolein (30 µM) induced VEGF mRNA expression in ASMC cultures, suggesting an effect at transcriptional level. Crotonaldehyde and 4-hydroxy-2-nonenal, an endogenous α,β-unsaturated aldehyde, stimulated VEGF release, as did H(2)O(2). CSE-evoked VEGF release was accompanied by rapid and lasting phosphorylation of p38 MAPK (mitogen-activated protein kinase), which was abolished by NAC and mimicked by acrolein. Both CSE- and acrolein-evoked VEGF release were blocked by selective inhibition of p38 MAPK signalling.

CONCLUSIONS AND IMPLICATIONS

α,β-Unsaturated aldehydes and possibly reactive oxygen species contained in cigarette smoke stimulate VEGF expression and release from pulmonary cells through p38 MAPK signalling.

Plasma reactive carbonyl species levels and risk of non-alcoholic fatty liver disease

BACKGROUND AND AIM

Oxidative stress plays a critical role in the pathogenesis of non-alcoholic fatty liver disease (NAFLD). However, there is still no large cohort study to explore the direct risk role of oxidative stress for NAFLD. This study is to test the hypothesis that elevated oxidative stress is a direct risk factor for the pathogenesis of NAFLD under controlling the potential effects of covariates.

METHODS

The levels of serum cholesterol, serum triglyceride, fasting plasma glucose and plasma reactive carbonyl species (RCS) were measured from 1204 Chinese Han adults, and the questionnaire and physical examination were administered to those with known and suspected risk factors for NAFLD.

RESULTS

Statistically significant high levels of blood pressure, fasting plasma glucose, serum cholesterol and triglyceride, body mass index, serum alanine aminotransferase and aspartate aminotransferase, and plasma RCS were observed in NAFLD subjects compared to healthy subjects (P < 0.01). Multivariate-adjusted odds ratio illustrated that, compared with the lowest quartile of plasma RCS levels, the highest quartile subjects had a 132% increase in the risk of developing NAFLD. Further results from multi-interaction analysis demonstrated that the underlying mechanism of the risk of NAFLD by unhealthy physical conditions and lifestyles might be, at least in part, through the oxidative stress.

CONCLUSIONS

Our findings provide credible evidence from a large population that oxidative stress, as indicated by plasma RCS levels, may be a direct risk factor for developing NAFLD.

Direct reaction of taurine with malondialdehyde: evidence for taurine as a scavenger of reactive carbonyl species

Though taurine has been reported very useful in preventing oxidative stress and various age-related diseases, the detailed biochemical mechanism of its biological functions is not well understood. Direct reaction of malondialdehyde (MDA) with taurine was studied using spectrofluorometry, spectrophotometry and liquid chromatography online with mass spectrometry (LC/MS). The results indicated that taurine reacted readily with MDA at supraphysiological conditions to yield mainly two products: a fluorescent 1,4-dihydropyridine and non-fluorescent enaminal derivatives. Taurine also significantly inhibited the formation of lipofuscin-like fluorescence induced by MDA-modified bovine serum albumin. These findings suggested that taurine effectively reduces carbonyl stress due to the amino group in its molecular structure, and we propose that it should be the mechanism related with the pathophysiological functions of taurine in the biological system.

The Nε-(carboxymethyl)lysine-RAGE axis: putative implications for the pathogenesis of obesity-related complications

Obesity is an important contributor to the burden of insulin resistance, Type 2 diabetes and cardiovascular disease. An important mechanism by which excess adiposity causes obesity-associated complications is the dysregulated production and secretion of biologically active molecules derived from adipocytes. These adipokines affect the vascular wall and contribute to the development of insulin resistance and Type 2 diabetes. However, factors that cause an increased production of pro-inflammatory adipokines, while decreasing anti-inflammatory adipokines, have not been fully clarified. Owing to local conditions in adipose tissue, that is, increased fatty acids, hypoxia and oxidative stress, we speculate that an increased formation of the major advanced lipoxidation end product, N^ε-(carboxymethyl)lysine (CML), may play a role. CML-adducts in proteins are major ligands for the receptor for advanced glycation end products (RAGE). The consequence of RAGE activation by CML is the activation of important signaling inflammatory pathways. The putative role of CML-modified proteins in obesity is addressed in this article. The identification of this pathway may provide an important strategy for novel therapeutic approaches against obesity-associated complications.

The carbonyl scavenger carnosine ameliorates dyslipidaemia and renal function in Zucker obese rats

The metabolic syndrome is a risk factor that increases the risk for development of renal and vascular complications. This study addresses the effects of chronic administration of the endogenous dipeptide carnosine (β-alanyl-L-histidine, L-CAR) and of its enantiomer (β-alanyl-D-histidine, D-CAR) on hyperlipidaemia, hypertension, advanced glycation end products, advanced lipoxidation end products formation and development of nephropathy in the non-diabetic, Zucker obese rat. The Zucker rats received a daily dose of L-CAR or D-CAR (30 mg/kg in drinking water) for 24 weeks. Systolic blood pressure was recorded monthly. At the end of the treatment, plasma levels of triglycerides, total cholesterol, glucose, insulin, creatinine and urinary levels of total protein, albumin and creatinine were measured. Several indices of oxidative/carbonyl stress were also measured in plasma, urine and renal tissue. We found that both L- and D-CAR greatly reduced obese-related diseases in obese Zucker rat, by significantly restraining the development of dyslipidaemia, hypertension and renal injury, as demonstrated by both urinary parameters and electron microscopy examinations of renal tissue. Because the protective effect elicited by L- and D-CAR was almost superimposable, we conclude that the pharmacological action of L-CAR is not due to a pro-histaminic effect (D-CAR is not a precursor of histidine, since it is stable to peptidic hydrolysis), and prompted us to propose that some of the biological effects can be mediated by a direct carbonyl quenching mechanism.

Intermediate filament carbonylation during acute acrolein toxicity in A549 lung cells: functional consequences, chaperone redistribution, and protection by bisulfite

Extensive protein carbonylation accompanies cellular exposure to acrolein, a ubiquitous smoke constituent implicated in life-threatening pulmonary edema in fire victims, a condition involving rapid erosion of the "watertight" properties of respiratory epithelium. Since the identities of lung epithelial proteins that sustain carbonylation by acrolein are unknown, we sought to identify significant targets in subcellular fractions from A549 cells after 30 min exposure to either subtoxic or acutely toxic acrolein concentrations (60 or 360 fmol acrolein/cell). The lower concentration mainly modified cytosolic proteins while the higher concentration also damaged nuclear, membrane, and cytoskeletal proteins. The multifunctional intermediate filament proteins vimentin, keratin-18, keratin-7 and keratin-8, were conspicuous targets. Consistent with their mechanical functions, a loss of cellular adhesive strength accompanied adduction of the two most abundant intermediate filaments in A549 cells, keratins-8 and -18. Acrolein also elicited redistribution of several chaperones (Hsp40, -70, -90, and -110) to intermediate filament fractions, suggesting chaperone-mediated autophagy contributes to the triage of acrolein-adducted proteins. The carbonyl scavenger bisulfite suppressed acrolein toxicity, intermediate filament adduction, vimentin cross-linking, Hsp90 redistribution, and loss of cellular adhesive strength, while also suppressing vimentin hyperphosphorylation. These novel observations identify intermediate filaments as key targets for the reactive smoke constituent acrolein.