Nitrotyrosine and protein carbonyls are equally distributed in HT22 cells after nitrosative stress

Small molecule probes for peroxynitrite detection

Peroxynitrite (ONOO‒/ONOOH) is a short-lived but highly reactive species that is formed in the diffusion-controlled reaction between nitric oxide and the superoxide radical anion. It can oxidize certain biomolecules and has been considered as a key cellular oxidant formed under various pathophysiological conditions. It is crucial to selectively detect and quantify ONOO- to determine its role in biological processes. In this review, we discuss various approaches used to detect ONOO‒ in cell-free and cellular systems with the major emphasis on small-molecule chemical probes. We review the chemical principles and mechanisms responsible for the formation of the detectable products, and plausible limitations of the probes. We recommend the use of boronate-based chemical probes for ONOO‒, as they react directly and rapidly with ONOO-, they produce minor but ONOO‒‒specific products, and the reaction kinetics and mechanism have been rigorously characterized. Specific experimental approaches and protocols for the detection of ONOO- in cell-free, cellular, and in vivo systems using boronate-based molecular probes are provided (as shown in Boxes 1-6).

Automated determination of 8-OHdG in cells and tissue via immunofluorescence using a specially created antibody

Despite powerful DNA repair systems, oxidative damage/modification to DNA is an inevitable side effect of metabolism, ionizing radiation, lifestyle habits, inflammatory pathologies such as type-2 diabetes or metabolic syndrome, cancer and natural aging. One of the most common oxidative DNA modifications is 8-OHdG (8‑hydroxy-2'-deoxyguanosine), which is the most widely used marker in research and clinical diagnostics. 8-OHdG is easily and specifically detectable in various samples such as urine, plasma, cells and tissues via a large variety of methods like ELISA, HPLC, chromatographic methods, and immunochemistry. Formed by oxidation of guanine and being representative for the degree of DNA damage, 8-OHdG can be also used as biomarker for risk assessment of various cancers as well as degenerative diseases. Here, we present a highly specific, self-developed 8-OHdG antibody in successful comparison to a commercially one, tested in cells (FF95, HCT116, and HT22) and intestinal tissue, focusing on automatized evaluation via fluorescence/confocal microscopy.

Prevention of colitis-induced liver oxidative stress and inflammation in a transgenic mouse model with increased omega-3 polyunsaturated fatty acids

Inflammatory bowel disease (IBD) is an immune-mediated gut dysfunction, which might also be associated with an inflammatory phenotype in the liver. It is known that the nutritional intake of omega-3 polyunsaturated fatty acids (n-3 PUFA) is inversely correlated to the severity and occurrence of IBD. In order to investigate whether n-3 PUFA can also reduce liver inflammation and oxidative liver damage due to colon inflammation, we explored the dextran sulfate sodium (DSS)-induced colitis model in wild-type and fat-1 mice with endogenously increased n-3 PUFA tissue content. Besides confirming previous data of alleviated DSS-induced colitis in the fat-1 mouse model, the increase of n-3 PUFA also resulted in a significant reduction of liver inflammation and oxidative damage in colitis-affected fat-1 mice as compared to wild-type littermates. This was accompanied by a remarkable increase of established inflammation-dampening n-3 PUFA oxylipins, namely docosahexaenoic acid-derived 19,20-epoxydocosapentaenoic acid and eicosapentaenoic acid-derived 15-hydroxyeicosapentaenoic acid and 17,18-epoxyeicosatetraenoic acid. Taken together, these observations demonstrate a strong inverse correlation between the anti-inflammatory lipidome derived from n-3 PUFA and the colitis-triggered inflammatory changes in the liver by reducing oxidative liver stress.

Analytical Toolbox to Unlock the Diversity of Oxidized Lipids

ConspectusLipids are diverse class of small biomolecules represented by a large variety of chemical structures. In addition to the classical biosynthetic routes, lipids can undergo numerous modifications via introduction of small chemical moieties forming hydroxyl, phospho, and nitro derivatives, among others. Such modifications change the physicochemical properties of a parent lipid and usually result in new functionalities either by mediating signaling events or by changing the biophysical properties of lipid membranes. Over the last decades, a large body of evidence indicated the involvement of lipid modifications in a variety of physiological and pathological events. For instance, lipid (per)oxidation for a long time was considered as a hallmark of oxidative stress and related proinflammatory signaling. Recently, however, with the burst in the development of the redox biology field, oxidative modifications of lipids are also recognized as a part of regulatory and adaptive events that are highly specific for particular cell types, tissues, and conditions.The initial diversity of lipid species and the variety of possible lipid modifications result in an extremely large chemical space of the epilipidome, the subset of the natural lipidome formed by enzymatic and non-enzymatic lipid modifications occurring in biological systems. Together with their low natural abundance, structural annotation of modified lipids represents a major analytical challenge limiting the discovery of their natural variety and functions. Furthermore, the number of available chemically characterized standards representing various modified lipid species remains limited, making analytical and functional studies very challenging. Over the past decade we have developed and implemented numerous analytical methods to study lipid modifications and applied them in the context of different biological conditions. In this Account, we outline the development and evolution of modern mass-spectrometry-based techniques for the structural elucidation of modified/oxidized lipids and corresponding applications. Research of our group is mostly focused on redox biology, and thus, our primary interest was always the analysis of lipid modifications introduced by redox disbalance, including lipid peroxidation (LPO), oxygenation, nitration, and glycation. To this end, we developed an array of analytical solutions to measure carbonyls derived from LPO, oxidized and nitrated fatty acid derivatives, and oxidized and glycated complex lipids. We will briefly describe the main analytical challenges along with corresponding solutions developed by our group toward deciphering the complexity of natural epilipdomes, starting from in vitro-oxidized lipid mixtures, artificial membranes, and lipid droplets, to illustrate the diversity of lipid modifications in the context of metabolic diseases and ferroptotic cell death.

The MyoPulser field stimulator, a do it yourself programmable electronic pacemaker for contracting cells and tissues

After providing the free software MYOCYTER that analyzes a large amount of data from videos of contracting cells, tissues or organs, we now present an "Arduino"-based programmable, customizable and cost-effective electronic pacemaker ("MyoPulser") that triggers contraction by electric stimulation of the sample at arbitrary frequencies. In this work, construction, functions and application of the MyoPulser are explained in detail, the electronic pacemaker is also tested on isolated cardiomyocytes and HT22-cells to quantify biological effects of pacing. The device enables the user to select between different pulse types (monophasic, alternating, bi- and polyphasic) adjust the length of an applied pulse (1-200 ms), the gap between two consecutive pulses (20-2000 ms), application of irregular pulses with random length and gaps (simulation of arrhythmia) in a user-defined range, as well as manual pulsing, while extensive data are recorded for every single pulse during the experiment. Electrostimulation of isolated B6 cardiomyocytes showed very little deviation of the observed cellular contraction from the applied pulse settings of the device, while the carbon electrodes used proved to be biologically inert in long-term experiments. Due to the open source code and the expandable setup, the MyoPulser can be easily adapted to even highly specific requirements and together with the software MYOCYTER it represents a complete cardiomyophysiological measuring station.

High glucose leads to redistribution of the proteasomal system

The impact of high glucose on the cellular redox state, causing both induction of antioxidative systems and also enhanced protein oxidation is discussed for a long time. It is established that elevated glucose levels are disrupting the cellular proteostasis and influencing the proteasomal system. However, it is still unresolved whether this is due to a reaction of the cellular proteasomal system towards the high glucose or whether this is a secondary reaction to inflammatory stimuli. Therefore, we used a dermal fibroblast cell line exposed to high glucose in order to reveal whether a response of the proteasomal system takes place. We investigated the α4 and the inducible iβ5 subunits of the 20S proteasome, as well as the Rpn1-subunit of the 19S proteasomal regulator complex, measured activity of the 20S, 20S1, and 26S proteasome and detected as well changes in expression as a redistribution into the nucleus. Interestingly, while the activity of the proteasomal forms rather decreased under high glucose treatment; higher expression levels of components of the proteasomal system and higher concentrations of protein-bound 3-nitrotyrosine and Nrf2 (nuclear factor [erythroid-derived 2]-like 2) were detected. However, no change in the cytosol-nucleus distribution could be detected for most of the quantified parameters. We concluded that high glucose alone, without additional inflammatory stimuli, provokes a regulatory response on the ubiquitin-proteasomal system.

Angiotensin receptor blocker use is associated with upregulation of the memory-protective angiotensin type 4 receptor (AT4R) in the postmortem brains of individuals without cognitive impairment

The reported primary dementia-protective benefits of angiotensin II type 1 receptor (AT1R) blockers (ARB) are believed, at least in part, to arise from systemic effects on blood pressure. However, there is a specific and independently regulated brain renin-angiotensin system (RAS). Brain RAS acts mainly through three receptor subtypes; AT1R, AT2R, and AT4R. The AT1R promotes inflammation and mitochondrial reactive oxygen species generation. AT2R increases nitric oxide. AT4R is essential for dopamine and acetylcholine release. It is unknown whether ARB use is associated with changes in the brain RAS. Here, we compared the impact of treatment with ARB on not cognitively impaired individuals and individuals with Alzheimer's dementia using postmortem frontal-cortex samples of age- and sex-matched participants (70-90 years old, n = 30 in each group). We show that ARB use is associated with higher brain AT4R, lower oxidative stress, and amyloid-β burden in NCI participants. In AD, ARB use was associated with lower brain AT1R but had no impact on inflammation, oxidative stress, or amyloid-β burden. Our results may suggest a potential role for AT4R in the salutary effects for ARB on the brains of not cognitively impaired older adults.

Intracellular imaging of metmyoglobin and oxygen using new dual purpose probe EYFP-Myoglobin-mCherry

The biological relevance of nitric oxide (NO) and reactive oxygen species (ROS) in signaling, metabolic regulation, and disease treatment has become abundantly clear. The dramatic change in NO/ROS processing that accompanies a changing oxygen landscape calls for new imaging tools that can provide cellular details about both [O₂ ] and the production of reactive species. Myoglobin oxidation to the met state by NO/ROS is a known sensor with absorbance changes in the visible range. We previously employed Förster resonance energy transfer to read out the deoxygenation/oxygenation of myoglobin, creating the subcellular [O₂ ] sensor Myoglobin-mCherry. We now add the fluorescent protein EYFP to this sensor to create a novel probe that senses both met formation, a proxy for ROS/NO exposure, and [O₂ ]. Since both proteins are present in the construct, it can also relieve users from the need to measure fluorescence lifetime, making [O₂ ] sensing available to a wider group of laboratories.

Ageing Causes Ultrastructural Modification to Calcium Release Units and Mitochondria in Cardiomyocytes

Ageing is associated with an increase in the incidence of heart failure, even if the existence of a real age-related cardiomyopathy remains controversial. Effective contraction and relaxation of cardiomyocytes depend on efficient production of ATP (handled by mitochondria) and on proper Ca2+ supply to myofibrils during excitation-contraction (EC) coupling (handled by Ca2+ release units, CRUs). Here, we analyzed mitochondria and CRUs in hearts of adult (4 months old) and aged (≥24 months old) mice. Analysis by confocal and electron microscopy (CM and EM, respectively) revealed an age-related loss of proper organization and disposition of both mitochondria and EC coupling units: (a) mitochondria are improperly disposed and often damaged (percentage of severely damaged mitochondria: adults 3.5 ± 1.1%; aged 16.5 ± 3.5%); (b) CRUs that are often misoriented (longitudinal) and/or misplaced from the correct position at the Z line. Immunolabeling with antibodies that mark either the SR or T-tubules indicates that in aged cardiomyocytes the sarcotubular system displays an extensive disarray. This disarray could be in part caused by the decreased expression of Cav-3 and JP-2 detected by western blot (WB), two proteins involved in formation of T-tubules and in docking SR to T-tubules in dyads. By WB analysis, we also detected increased levels of 3-NT in whole hearts homogenates of aged mice, a product of nitration of protein tyrosine residues, recognized as marker of oxidative stress. Finally, a detailed EM analysis of CRUs (formed by association of SR with T-tubules) points to ultrastructural modifications, i.e., a decrease in their frequency (adult: 5.1 ± 0.5; aged: 3.9 ± 0.4 n./50 μm2) and size (adult: 362 ± 40 nm; aged: 254 ± 60 nm). The changes in morphology and disposition of mitochondria and CRUs highlighted by our results may underlie an inefficient supply of Ca2+ ions and ATP to the contractile elements, and possibly contribute to cardiac dysfunction in ageing.

Muscle activity prevents the uncoupling of mitochondria from Ca2+ Release Units induced by ageing and disuse

In fast-twitch fibers from adult mice Ca²⁺ release units (CRUs, i.e. intracellular junctions of excitation-contraction coupling), and mitochondria are structurally linked to each other by small strands, named tethers. We recently showed that aging causes separation of a fraction of mitochondria from CRUs and a consequent impairment of the Ca²⁺ signaling between the two organelles. However, whether the uncoupling of mitochondria from CRUs is the result of aging per-se or the consequence of reduced muscle activity remains still unclear. Here we studied the association between mitochondria and CRUs: in a) extensor digitorum longus (EDL) muscles from 2 years old mice, either sedentary or trained for 1 year in wheel cages; and b) denervated EDL muscles from adult mice and rats. We analyzed muscle samples using a combination of structural (confocal and electron microscopy), biochemical (assessment of oxidative stress via western blot), and functional (ex-vivo contractile properties, and mitochondrial Ca²⁺ uptake) experimental procedures. The results collected in structural studies indicate that: a) ageing and denervation result in partial uncoupling between mitochondria and CRUs; b) exercise either maintains (in old mice) or restores (in transiently denervated rats) the association between the two organelles. Functional studies supported the hypothesis that CRU-mitochondria coupling is important for mitochondrial Ca²⁺ uptake, optimal force generation, and muscle performance. Taken together our results indicate that muscle activity maintains/improves proper association between CRUs and mitochondria.

Protein and cell wall polysaccharide carbonyl determination by a neutral pH 2,4-dinitrophenylhydrazine-based photometric assay

A new 2,4-dinitrophenylhydrazine (DNPH)-based photometric assay is developed for the quantification of carbonyls in protein samples from any biological source by protein carbonyl-DNPH hydrazone formation at acidic pH in the presence of denaturing urea, and subsequent hydrazone solubilization in the presence of SDS and stabilization from acid hydrolysis at pH 7.0. At this neutral (ntr) pH, interfering unreacted DNPH is uncharged and its thus increased hydrophobicity permits its 100% effective removal from the solubilizate with ethyl acetate/hexane wash. The ntrDNPH assay is more reliable and sensitive than the standard (std) DNPH photometric assay because it eliminates its main limitations: (i) interfering unreacted DNPH (pKa 1.55) that is nonspecifically bound to the TCA (pKa 0.7)-protein pellet is not effectively removed after wash with EtOH: ethyl acetate because it is positively charged, (ii) acid (TCA-induced) hydrolysis of the protein carbonyl-DNPH hydrazone, (iii) sample protein concentration re-determination, (iv) loss of sample acid (TCA)-soluble proteins, (v) DNA interference, and (vi) requires high protein quantity samples (≥ 1 mg). Considering ntrDNPH assay's very low protein limit (1 µg), its cumulative and functional sensitivities are 2600- and 2000-fold higher than those of the stdDNPH assay, respectively. The present study elucidates the DNA interference mechanism on the stdDNPH assay, and also develops a standardized protocol for sample protein treatment and fractionation (into cytoplasmic/aqueous, membrane/lipid-bound, and histone/DNA-bound proteins; see Supplement section V) in order to ensure reproducible carbonyl determination on defined cell protein fractions, and to eliminate assay interference from protein samples containing (i) Cys sulfenic acid groups (via their neutralization with dithiothreitol), and (ii) DNA (via its removal by streptomycin sulfate precipitation). Lastly, the ntrDNPH assay determines carbonyl groups on cell wall polysaccharides, thus paving the way on studies to investigate cell walls acting as antioxidant defense in plants, fungi, bacteria and lichens.

"Cyt/Nuc," a Customizable and Documenting ImageJ Macro for Evaluation of Protein Distributions Between Cytosol and Nucleus

Large amounts of data from multi-channel, high resolution, fluorescence microscopic images require tools that provide easy, customizable, and reproducible high-throughput analysis. The freeware "ImageJ" has become one of the standard tools for scientific image analysis. Since ImageJ offers recording of "macros," even a complex multi-step process can be easily applied fully automated to large numbers of images, saving both time and reducing human subjective evaluation. In this work, we present "Cyt/Nuc," an ImageJ macro, able to recognize and to compare the nuclear and cytosolic areas of tissue samples, in order to investigate distributions of immunostained proteins between both compartments, while it documents in detail the whole process of evaluation and pattern recognition. As practical example, the redistribution of the 20S proteasome, the main intracellular protease in mammalian cells, is investigated in NZO-mouse liver after feeding the animals different diets. A significant shift in proteasomal distribution between cytosol and nucleus in response to metabolic stress was revealed using "Cyt/Nuc" via automatized quantification of thousands of nuclei within minutes. "Cyt/Nuc" is easy to use and highly customizable, matches the precision of careful manual evaluation and bears the potential for quick detection of any shift in intracellular protein distribution.

Cross-talk between lipid and protein carbonylation in a dynamic cardiomyocyte model of mild nitroxidative stress

Reactive oxygen and nitrogen species (ROS/RNS) play an important role in the regulation of cardiac function. Increase in ROS/RNS concentration results in lipid and protein oxidation and is often associated with onset and/or progression of many cardiovascular disorders. However, interplay between lipid and protein modifications has not been simultaneously studied in detail so far. Biomolecule carbonylation is one of the most common biomarkers of oxidative stress. Using a dynamic model of nitroxidative stress we demonstrated rapid changes in biomolecule carbonylation in rat cardiomyocytes. Levels of carbonylated species increased as early as 15min upon treatment with the peroxynitrite donor, 3-morpholinosydnonimine (SIN-1), and decreased to values close to control after 16h. Total (lipids+proteins) vs. protein-specific carbonylation showed different dynamics, with a significant increase in protein-bound carbonyls at later time points. Treatment with SIN-1 in combination with inhibitors of proteasomal and autophagy/lysosomal degradation pathways allowed confirmation of a significant role of the proteasome in the degradation of carbonylated proteins, whereas lipid carbonylation increased in the presence of autophagy/lysosomal inhibitors. Electrophilic aldehydes and ketones formed by lipid peroxidation were identified and relatively quantified using LC-MS/MS. Molecular identity of reactive species was used for data-driven analysis of their protein targets. Combination of different enrichment strategies with LC-MS/MS analysis allowed identification of more than 167 unique proteins with 332 sites modified by electrophilic lipid peroxidation products. Gene ontology analysis of modified proteins demonstrated enrichment of several functional categories including proteins involved in cytoskeleton, extracellular matrix, ion channels and their regulation. Using calcium mobilization assays, the effect of nitroxidative stress on the activity of several ion channels was further confirmed.

Metabolic Syndrome, Redox State, and the Proteasomal System

SIGNIFICANCE

Since the metabolic syndrome (MS) and pathologies associated with/resulting from metabolic dysregulations became a worldwide spreading and growing problem, the mechanisms mediating the according cellular changes got into a focus of interest. The ubiquitin-proteasomal system (UPS) is the main regulator of both the functional and dysfunctional protein pool of (not only) mammalian cells-thus, it is obvious that an impact on this system may also affect cellular functionality that directly depends on permanent regulation/adaption of the cell's proteostasis. However, the according research is still at the beginning. Recent Advances: It was also recently shown that maintaining a highly functional UPS positively correlates with increased health or even life span, thus modulation or restoration of UPS function may be an effective approach alleviating or even preventing MS detrimental consequences.

CRITICAL ISSUES

Even if many consequences of metabolic dysregulation such as a slight but chronic redox shift to a more oxidative state (i.e., a low-grade systemic inflammation that increases reactive oxygen species formation, lipid peroxidation, protein oxidation, formation of advanced glycation end products, glycosylation, S-glutathionylation, redox shifts, endoplasmic reticulum stress, unfolded protein response, expression of transcription factors, and release of cytokines) are already known to affect the highly redox-regulated UPS, experimental data about UPS changes that are directly mediated by glucotoxic and/or lipotoxic stress are still rarely published.

FUTURE DIRECTIONS

It may be taken into account that many MS-related pathologic changes result from UPS dysfunction or dysregulation. In this review, the main interface between MS effects and their impact on the UPS are highlighted since they may direct to new therapeutic approaches. Antioxid. Redox Signal. 25, 902-917.

Fluorescence labeling of carbonylated lipids and proteins in cells using coumarin-hydrazide

Carbonylation is a generic term which refers to reactive carbonyl groups present in biomolecules due to oxidative reactions induced by reactive oxygen species. Carbonylated proteins, lipids and nucleic acids have been intensively studied and often associated with onset or progression of oxidative stress related disorders. In order to reveal underlying carbonylation pathways and biological relevance, it is crucial to study their intracellular formation and spatial distribution. Carbonylated species are usually identified and quantified in cell lysates and body fluids after derivatization using specific chemical probes. However, spatial cellular and tissue distribution have been less often investigated. Here, we report coumarin-hydrazide, a fluorescent chemical probe for time- and cost-efficient labeling of cellular carbonyls followed by fluorescence microscopy to evaluate their intracellular formation both in time and space. The specificity of coumarin-hydrazide was confirmed in time- and dose-dependent experiments using human primary fibroblasts stressed with paraquat and compared with conventional DNPH-based immunocytochemistry. Both techniques stained carbonylated species accumulated in cytoplasm with strong perinuclear clustering. Using a complimentary array of analytical methods specificity of coumarin-hydrazide probe towards both protein- and lipid-bound carbonyls has been shown. Additionally, co-distribution of carbonylated species and oxidized phospholipids was demonstrated.

•

Coumarin-hydrazide (CHH) chemical probe was used to label cellular carbonyls.

•

CHH fluorescence microscopy allowed to monitor protein and lipid carbonyl distribution.

•

CHH specificity towards protein- and lipid-bound carbonyls was demonstrated.

•

CHH labeling and DNPH immunocytochemistry for microscopy imaging were compared.

Oxidative stress in aging human skin

Oxidative stress in skin plays a major role in the aging process. This is true for intrinsic aging and even more for extrinsic aging. Although the results are quite different in dermis and epidermis, extrinsic aging is driven to a large extent by oxidative stress caused by UV irradiation. In this review the overall effects of oxidative stress are discussed as well as the sources of ROS including the mitochondrial ETC, peroxisomal and ER localized proteins, the Fenton reaction, and such enzymes as cyclooxygenases, lipoxygenases, xanthine oxidases, and NADPH oxidases. Furthermore, the defense mechanisms against oxidative stress ranging from enzymes like superoxide dismutases, catalases, peroxiredoxins, and GSH peroxidases to organic compounds such as L-ascorbate, α-tocopherol, beta-carotene, uric acid, CoQ10, and glutathione are described in more detail. In addition the oxidative stress induced modifications caused to proteins, lipids and DNA are discussed. Finally age-related changes of the skin are also a topic of this review. They include a disruption of the epidermal calcium gradient in old skin with an accompanying change in the composition of the cornified envelope. This modified cornified envelope also leads to an altered anti-oxidative capacity and a reduced barrier function of the epidermis.

Protein carbonylation as a major hallmark of oxidative damage: update of analytical strategies

Protein carbonylation, one of the most harmful irreversible oxidative protein modifications, is considered as a major hallmark of oxidative stress-related disorders. Protein carbonyl measurements are often performed to assess the extent of oxidative stress in the context of cellular damage, aging and several age-related disorders. A wide variety of analytical techniques are available to detect and quantify protein-bound carbonyls generated by metal-catalyzed oxidation, lipid peroxidation or glycation/glycoxidation. Here we review current analytical approaches for protein carbonyl detection with a special focus on mass spectrometry-based techniques. The utility of several carbonyl-derivatization reagents, enrichment protocols and especially advanced mass spectrometry techniques are compared and discussed in detail. Furthermore, the mechanisms and biology of protein carbonylation are summarized based on recent high-throughput proteomics data.

The proteasome and the degradation of oxidized proteins: part III-Redox regulation of the proteasomal system

Here, we review shortly the current knowledge on the regulation of the proteasomal system during and after oxidative stress. After addressing the components of the proteasomal system and the degradation of oxidatively damaged proteins in part I and II of this series, we address here which changes in activity undergo the proteasome and the ubiquitin-proteasomal system itself under oxidative conditions. While several components of the proteasomal system undergo direct oxidative modification, a number of redox-regulated events are modulating the proteasomal activity in a way it can address the major tasks in an oxidative stress situation: the removal of oxidized proteins and the adaptation of the cellular metabolism to the stress situation.

The proteasome and the degradation of oxidized proteins: Part II - protein oxidation and proteasomal degradation

Here, we review the role of oxidative protein modification as a signal for recognition and degradation of proteins. It was clearly demonstrated that the ATP- and ubiquitin-independent 20S proteasome is playing a key role in the selective removal of oxidized proteins. Furthermore, the current knowledge of the substrate susceptibility on the degradation of oxidized proteins and the role of the immunoproteasome will be highlighted.

Lipopolysaccharide challenge of the mammary gland in cows induces nitrosative stress that impairs milk oxidative stability

The aim of this work was to study the effects of mastitis induced by intramammary lipopolysaccharide (LPS) challenge on milk oxidative stability, as well as to understand the underlying biochemical processes that cause such changes. LPS challenge was associated with nitric oxide burst from the surrounding mammary epithelial cells and consequently induced nitrosative stress that was induced by the formation of NO2• from nitrite by lactoperoxidase. This response was associated with an ∼3-fold increased formation of hazardous compounds: nitrotyrosines, carbonyls and lipid peroxides. We sustained the involvement of xanthine oxidase as a major source of hydrogen peroxide. In consistent with previous findings, catalase has been shown to play a major role in modulating the nitrosative stress by oxidizing nitrite to nitrate. The current hygienic quality criteria cannot detect mixing of low-quality milk, such as milk with high somatic cells, and nitrite with high-quality milk. Thus, development of an improved quality control methodology may be important for the production of high-quality milk.

Regulation of pancreatic β-cell survival by nitric oxide: clinical relevance

The reduction of pancreatic β-cell mass is an important factor in the development of type 1 and type 2 diabetes. Understanding the mechanisms that regulate the maintenance of pancreatic β-cell mass as well as β-cell death is necessary for the establishment of therapeutic strategies. In this context, nitric oxide (NO) is a diatomic, gaseous, highly reactive molecule with biological activity that participates in the regulation of pancreatic β-cell mass. Two types of cellular responses can be distinguished depending on the level of NO production. First, pancreatic β-cells exposed to inflammatory cytokines, lipid stress or hyperglycaemia produce high concentrations of NO, mainly due to the activation of inducible NO synthase (iNOS), thus promoting cell death. Meanwhile, under homeostatic conditions, low concentrations of NO, constitutively produced by endothelial NO synthase (eNOS), promote cell survival. Here, we will discuss the current knowledge of the NO-dependent mechanisms activated during cellular responses, emphasizing those related to the regulation of cell survival.

Nitric oxide in paraquat-mediated toxicity: A review

Paraquat, a cationic herbicide, produces degenerative lesions in the lung and in the nervous system after systemic administration to man and animals. Many cases of acute poisoning and death have been reported over the past few decades. Although a definitive mechanism of toxicity of paraquat has not been delineated, a cyclic single electron reduction/oxidation is a critical mechanistic event. The redox cycling of paraquat has two potentially important consequences relevant to the development of toxicity: the generation of the superoxide anion, which can lead to the formation of more toxic reactive oxygen species which are highly reactive to cellular macromolecules; and the oxidation of reducing equivalents (e.g., NADPH, reduced glutathione), which results in the disruption of important NADPH-requiring biochemical processes necessary for normal cell function. Nitric oxide is an important signaling molecule that reacts with superoxide derived from the paraquat redox cycle, to form the potent oxidant peroxynitrite, which causes serious cell damage. Although nitric oxide has been involved in the mechanism of paraquat-mediated toxicity, the role of nitric oxide has been controversial as both protective and harmful effects have been described. The present review summarizes recent findings in the field and describes new knowledge on the role of nitric oxide in the paraquat-mediated toxicity.

Age-related loss of stress-induced nuclear proteasome activation is due to low PARP-1 activity

Changes in protein turnover are among the dominant metabolic changes during aging. Of special importance is the maintenance of nuclear protein homeostasis to ensure a coordinated cellular metabolism. Therefore, in the nucleus a special PARP-1-mediated mechanism of proteasomal activation exists to ensure a rapid degradation of oxidized nuclear proteins. It was already demonstrated earlier that the cytosolic proteasomal system declines dramatically with aging, whereas the nuclear proteasome remains less affected. We demonstrate here that the stress-mediated proteasomal activation in the nucleus declines during replicative senescence of human fibroblasts. Furthermore, we clearly show that this decline in the PARP-1-mediated proteasomal activation is due to a decline in the expression and activity of PARP-1 in senescent fibroblasts. In a final study we show that this process also happens in vivo, because the protein expression level of PARP-1 is significantly lower in the skin of aged donors compared to that of young ones. Therefore, we conclude that the rate-limiting factor in poly(ADP-ribose)-mediated proteasomal activation in oxidative stress is PARP-1 and not the nuclear proteasome itself.

Biomarkers of protein oxidation from a chemical, biological and medical point of view

In physiological conditions intracellular radical formation is mostly due to mitochondrial activity. This is in contrast to clinical and pathophysiological situations, where the oxidant formation is additionally driven by xenobiotics and inflammation. Oxidative damage accumulation in macromolecules especially in proteins has been considered as a cause of cellular damage and pathology impairing the clinical outcome of patients. However, up to now strategies to measure oxidative stress in clinical settings are limited. A lot of parameters and techniques are available for the determination of oxidized proteins in biological systems. Unfortunately, most of them are no reliable markers in clinical settings due to their unknown clinical relevance or the lack in clinical feasibility. Major problems are the sample availability, sample stability and cost-, time- and man-power intensive methods. The present review focuses on the measurement of protein oxidation products from a chemical, biological, and medical point of view.

Effects of water-filtered infrared A irradiation on human fibroblasts

Infrared radiation is a substantial part of the solar energy output reaching the earth surface. Therefore, exposure of humans to infrared radiation is common. However, whether and how infrared (IR) or infrared A acts on human skin cells is still under debate. Recently the generation of reactive oxygen species by water-filtered infrared A (wIRA) irradiation was postulated. wIRA shows a spectral distribution similar to that of solar irradiation at the earth's surface. Thus, the need for protection of human skin from both solar- and artificially generated infrared A irradiation was concluded. Here we demonstrate that in human dermal fibroblasts this reactive oxygen species generation is dependent on heat formation by infrared A and can be reproduced by thermal exposure. On the other hand wIRA irradiation had no detectable effect if the temperature in the cells was kept constant, even if irradiance exceeded the extraterrestrial solar irradiance in the IR range by a factor of about 4 and the maximum at noontime in the tropics by a factor up to about 6. This could be demonstrated by the measurement of oxidant formation using H(2)DCFDA and the determination of protein carbonyls. In additional experiments we could show that during thermal exposure the mitochondria contribute significantly to oxidant production. Further experiments revealed that the major absorbance of infrared is due to absorption of the energy by cellular water.

The proteasome and its role in the degradation of oxidized proteins

The generation of free radicals and the resulting oxidative modification of cell structures are omnipresent in mammalian cells. This includes the permanent oxidation of proteins leading to the disruption of the protein structure and an impaired functionality. In consequence, these oxidized proteins have to be removed in order to prevent serious metabolic disturbances. The most important cellular proteolytic system responsible for the removal of oxidized proteins is the proteasomal system. For normal functioning, the proteasomal system needs the coordinated interaction of numerous components. This review describes the fundamental functions of the 20S "core" proteasome, its regulators, and the roles of the proteasomal system beyond the removal of oxidized proteins in mammalian cells.

Age-related differences in oxidative protein-damage in young and senescent fibroblasts

Aging is accompanied by an accumulation of oxidized proteins and cross-linked modified protein material. The intracellular formation and accumulation of highly oxidized and cross-linked proteins, the so-called lipofuscin, is a typical sign of senescence. However, little is known whether the lipofuscin accumulation during aging is related to environmental conditions, as oxidative stress, and whether the accumulation of oxidized proteins and lipofuscin is preferentially taking place in the cytosol or the nucleus and finally, what is the role of lysosomes in this process. Therefore, we investigated human skin fibroblasts in an early stage of proliferation ("young cells") and in a late stage ("senescent cells"). Such cells were compared for the amount of protein carbonyls and lipofuscin and their distribution within the cytosol and the nucleus. Furthermore, cells were exposed to single and repeated doses of hydrogen peroxide and paraquat, measuring the same set of parameters. In addition to that the role of the proteasome to degrade oxidized proteins in young and senescent cells was tested. Furthermore, detailed microscopic analysis was performed testing the intracellular distribution of lipofuscin. The results clearly demonstrated that repeated/chronic oxidative stress induces a senescence-like phenotype of the distribution of oxidized proteins as well as of lipofuscin. It could be demonstrated that most of the lipofuscin is located in lysosomes and that senescent cells contain less lysosomes not lipofuscin-laden in comparison to young cells.

Nitric oxide produces HLA-G nitration and induces metalloprotease-dependent shedding creating a tolerogenic milieu

Human leucocyte antigen G (HLA-G) is a tolerogenic molecule that protects the fetus from maternal immune attack, may favour tumoral immunoescape and is up-regulated in viral and inflammatory diseases. The aim of this work was to discover if nitric oxide (NO) could affect HLA-G expression or function because NO is an important modulator of innate and adaptive immunity. For this purpose HLA-G expression and function were analysed following treatment with a NO donor or a peroxynitrite donor in various cell lines expressing HLA-G either spontaneously or upon transfection. Results showed NO-dependent nitration of both cellular and soluble HLA-G protein, but not all HLA-G moieties underwent nitration. Endogenous biosynthesis of NO by both U-937-HLA-G1 and M8-HLA-G5 stable transfectants also caused HLA-G nitration. The NO decreased total HLA-G cellular protein content and expression on the cell surface, while increasing HLA-G shedding into the culture medium. This effect was post-transcriptional and the result of metalloprotease activity. By contrast, NO pretreatment did not affect HLA-G capability to suppress NK cytotoxicity and lymphocyte proliferation. Our studies show that NO regulates the availability of HLA-G molecules without modifying their biological activities.

Oxidative stress related changes in the brain of hypercholesterolemic rabbits

In recent years, there has been increasing evidence that cholesterol plays a role in the pathology of Alzheimer disease. Since hypercholesterolemia was reported to increase the levels of reactive oxygen species and Alzheimer disease has clearly involved an oxidative component, it is possible that hypercholesterolemia is via increased oxidant production facilitating the disease development of the neurodegenerative disease. Therefore, we tested in an established model of enhanced cholesterol feed in rabbits the effects of serum cholesterol increase on oxidative stress parameters as well in serum as in the brain. In addition to that we tested the effects of vitamin E on the cholesterol-induced oxidative stress. Since Alzheimer disease is largely connected with increased protein oxidation whereas cholesterol is rather connected with lipid peroxidation processes, we tested both protein carbonyl levels and the formation of malondialdehyde, a marker of lipid peroxidation. We could clearly demonstrate an increase in serum malondialdehyde due to high cholesterol feeding, which is accompanied by an increase in protein oxidation parameters in the brain, especially in the hippocampus. Therefore, we suggest that specific neuropathological changes occur during the feeding of hypercholesterolemic diet.

Detection and localization of oxidized proteins in muscle cells by fluorescence microscopy

In meat, no detailed studies on the intracellular distribution of oxidized proteins during oxidative stress have been performed, to our knowledge. Therefore, we used fluorescence microscopy to detect and locate protein carbonyls, oxidation products of basic amino acids, generated in bovine M. Rectus abdominis during either exposition to a chemical free radical generating system, or refrigerated storage, or cooking. The technique consisted of an immunohistochemical detection of carbonyls by reaction with the specific probe DNPH (2,4-dinitrophenylhydrazine) followed by the sequential addition of a first antibody against DNPH-carbonylated proteins and a CY3-labeled secondary antibody. The fluorescence of the CY3 probe increased regularly with level of free radical generating system and storage time. Moreover, an important heterogeneity of carbonyl distribution was observed, with a higher oxidation level at the periphery than inside the muscle cells. Cooking induced fluorescence increase only at the periphery of cells. Specific coloration of collagen by Sirius red showed that collagen was not involved in fluorescence. We can deduce that accumulation of oxidized proteins observed in the cell periphery was linked to membrane protein oxidation and not to connective tissue oxidation. Biochemical assays were performed in parallel on membrane and myofibrillar proteins to provide complementary quantitative data on level of oxidized proteins.

Oxidized proteins: intracellular distribution and recognition by the proteasome

The formation of oxidized proteins is one of the highlights of oxidative stress. In order not to accumulate such proteins have to be degraded. The major proteolytic system responsible for the removal of oxidized proteins is the proteasome. The proteasome is distributed throughout the cytosolic and nuclear compartment of mammalian cells, with high concentrations in the nucleus. On the other hand a major part of protein oxidation is taking place in the cytosol. The present review highlights the current knowledge on the intracellular distribution of oxidized proteins and put it into contrast with the concentration and distribution of the proteasome.