Methionine sulfoxide reductase A protects neuronal cells against brief hypoxia/reoxygenation

Protective effects of taurine against oxidative stress in the heart of MsrA knockout mice

Taurine has been shown to have potent anti-oxidant properties under various pathophysiological conditions. We reported previously a cellular dysfunction and mitochondrial damage in cardiac myocytes of methionine sulfoxide reductase A (MsrA) gene knockout mice (MsrA(-/-)). In the present study, we have explored the protective effects of taurine against oxidative stress in the heart of MsrA(-/-) mice with or without taurine treatment. Cardiac cell contractility and Ca(2+) dynamics were measured using cell-based assays and in vivo cardiac function was monitored using high-resolution echocardiography in the tested animals. Our data have shown that MsrA(-/-) mice exhibited a progressive cardiac dysfunction with a significant decrease of ejection fraction (EF) and fraction shortening (FS) at age of 8 months compared to the wild type controls at the same age. However, the dysfunction was corrected in MsrA(-/-) mice treated with taurine supplement in the diet for 5 months. We further investigated the cellular mechanism underlying the protective effect of taurine in the heart. Our data indicated that cardiac myocytes from MsrA(-/-) mice treated with taurine exhibited an improved cell contraction and could tolerate oxidative stress better. Furthermore, taurine treatment reduced significantly the protein oxidation levels in mitochondria of MsrA(-/-) hearts, suggesting an anti-oxidant effect of taurine in cardiac mitochondria. Our study demonstrates that long-term treatment of taurine as a diet supplement is beneficial to a heart that is vulnerable to environmental oxidative stresses.

Protein damage, repair and proteolysis

Proteins are continuously affected by various intrinsic and extrinsic factors. Damaged proteins influence several intracellular pathways and result in different disorders and diseases. Aggregation of damaged proteins depends on the balance between their generation and their reversal or elimination by protein repair systems and degradation, respectively. With regard to protein repair, only few repair mechanisms have been evidenced including the reduction of methionine sulfoxide residues by the methionine sulfoxide reductases, the conversion of isoaspartyl residues to L-aspartate by L-isoaspartate methyl transferase and deglycation by phosphorylation of protein-bound fructosamine by fructosamine-3-kinase. Protein degradation is orchestrated by two major proteolytic systems, namely the lysosome and the proteasome. Alteration of the function for both systems has been involved in all aspects of cellular metabolic networks linked to either normal or pathological processes. Given the importance of protein repair and degradation, great effort has recently been made regarding the modulation of these systems in various physiological conditions such as aging, as well as in diseases. Genetic modulation has produced promising results in the area of protein repair enzymes but there are not yet any identified potent inhibitors, and, to our knowledge, only one activating compound has been reported so far. In contrast, different drugs as well as natural compounds that interfere with proteolysis have been identified and/or developed resulting in homeostatic maintenance and/or the delay of disease progression.

Resveratrol preconditioning increases methionine sulfoxide reductases A expression and enhances resistance of human neuroblastoma cells to neurotoxins

Methionine sulfoxide reductases A (MsrA) has been postulated to act as a catalytic antioxidant system involved in the protection of oxidative stress-induced cell injury. Recently, attention has turned to MsrA in coupling with the pathology of Parkinson's disease, which is closely related to neurotoxins that cause dopaminergic neuron degeneration. Here, we firstly provided evidence that pretreatment with a natural polyphenol resveratrol (RSV) up-regulated the expression of MsrA in human neuroblastoma SH-SY5Y cells. It was also observed that the expression and nuclear translocation of forkhead box group O 3a (FOXO3a), a transcription factor that activates the human MsrA promoter, increased after RSV pretreatment. Nicotinamide , an inhibitor of silent information regulator 1 (SIRT1), prevented RSV-induced elevation of FOXO3a and MsrA expression, indicating that the effect of RSV was mediated by a SIRT1-dependent pathway. RSV preconditioning increased methionine sulfoxide(MetO)-reducing activity in SH-SY5Y cells and enhanced their resistance to neurotoxins, including chloramine-T and 1-methyl-4-phenyl-pyridinium. In addition, the enhancement of cell resistance to neurotoxins caused by RSV preconditioning can be largely prevented by MsrA inhibitor dimethyl sulfoxide. Our findings suggest that treatment with polyphenols such as RSV can be used as a potential regulatory strategy for MsrA expression and function.

Wanted and wanting: antibody against methionine sulfoxide

Methionine residues in protein can be oxidized by reactive oxygen or nitrogen species to generate methionine sulfoxide. This covalent modification has been implicated in processes ranging from normal cell signaling to neurodegenerative diseases. A general method for detecting methionine sulfoxide in proteins would be of great value in studying these processes, but development of a chemical or immunochemical technique has been elusive. Recently, an antiserum raised against an oxidized corn protein, DZS18, was reported to be specific for methionine sulfoxide in proteins (Arch. Biochem. Biophys. 485:35-40; 2009). However, data included in that report indicate that the antiserum is not specific. Utilizing well-characterized native and methionine-oxidized glutamine synthetase and aprotinin, we confirm that the antiserum does not possess specificity for methionine sulfoxide.

A low pKa cysteine at the active site of mouse methionine sulfoxide reductase A

Methionine sulfoxide reductase A is an essential enzyme in the antioxidant system which scavenges reactive oxygen species through cyclic oxidation and reduction of methionine and methionine sulfoxide. Recently it has also been shown to catalyze the reverse reaction, oxidizing methionine residues to methionine sulfoxide. A cysteine at the active site of the enzyme is essential for both reductase and oxidase activities. This cysteine has been reported to have a pK(a) of 9.5 in the absence of substrate, decreasing to 5.7 upon binding of substrate. Using three independent methods, we show that the pK(a) of the active site cysteine of mouse methionine sulfoxide reductase is 7.2 even in the absence of substrate. The primary mechanism by which the pK(a) is lowered is hydrogen bonding of the active site Cys-72 to protonated Glu-115. The low pK(a) renders the active site cysteine susceptible to oxidation to sulfenic acid by micromolar concentrations of hydrogen peroxide. This characteristic supports a role for methionine sulfoxide reductase in redox signaling.

Methionine sulfoxide reductase contributes to meeting dietary methionine requirements

Methionine sulfoxide reductases are present in all aerobic organisms. They contribute to antioxidant defenses by reducing methionine sulfoxide in proteins back to methionine. However, the actual in vivo roles of these reductases are not well defined. Since methionine is an essential amino acid in mammals, we hypothesized that methionine sulfoxide reductases may provide a portion of the dietary methionine requirement by recycling methionine sulfoxide. We used a classical bioassay, the growth of weanling mice fed diets varying in methionine, and applied it to mice genetically engineered to alter the levels of methionine sulfoxide reductase A or B1. Mice of all genotypes were growth retarded when raised on chow containing 0.10% methionine instead of the standard 0.45% methionine. Retardation was significantly greater in knockout mice lacking both reductases. We conclude that the methionine sulfoxide reductases can provide methionine for growth in mice with limited intake of methionine, such as may occur in the wild.

Mitochondrial thiols in antioxidant protection and redox signaling: distinct roles for glutathionylation and other thiol modifications

SIGNIFICANCE

The mitochondrial matrix contains much of the machinery at the heart of metabolism. This compartment is also exposed to a high and continual flux of superoxide, hydrogen peroxide, and related reactive species. To protect mitochondria from these sources of oxidative damage, there is an integrated set of thiol systems within the matrix comprising the thioredoxin/peroxiredoxin/methionine sulfoxide reductase pathways and the glutathione/glutathione peroxidase/glutathione-S-transferase/glutaredoxin pathways that in conjunction with protein thiols prevent much of this oxidative damage. In addition, the changes in the redox state of many components of these mitochondrial thiol systems may transduce and relay redox signals within and through the mitochondrial matrix to modulate the activity of biochemical processes.

RECENT ADVANCES

Here, mitochondrial thiol systems are reviewed, and areas of uncertainty are pointed out, focusing on recent developments in our understanding of their roles.

CRITICAL ISSUES

The areas of particular focus are on the multiple, overlapping roles of mitochondrial thiols and on understanding how these thiols contribute to both antioxidant defenses and redox signaling.

FUTURE DIRECTIONS

Recent technical progress in the identification and quantification of thiol modifications by redox proteomics means that many of the questions raised about the multiple roles of mitochondrial thiols can now be addressed.

Methionine sulfoxide reductase A: Structure, function and role in ocular pathology

Methionine is a highly susceptible amino acid that can be oxidized to S and R diastereomeric forms of methionine sulfoxide by many of the reactive oxygen species generated in biological systems. Methionine sulfoxide reductases (Msrs) are thioredoxin-linked enzymes involved in the enzymatic conversion of methionine sulfoxide to methionine. Although MsrA and MsrB have the same function of methionine reduction, they differ in substrate specificity, active site composition, subcellular localization, and evolution. MsrA has been localized in different ocular regions and is abundantly expressed in the retina and in retinal pigment epithelial (RPE) cells. MsrA protects cells from oxidative stress. Overexpression of MsrA increases resistance to cell death, while silencing or knocking down MsrA decreases cell survival; events that are mediated by mitochondria. MsrA participates in protein-protein interaction with several other cellular proteins. The interaction of MsrA with α-crystallins is of utmost importance given the known functions of the latter in protein folding, neuroprotection, and cell survival. Oxidation of methionine residues in α-crystallins results in loss of chaperone function and possibly its antiapoptotic properties. Recent work from our laboratory has shown that MsrA is co-localized with αA and αB crystallins in the retinal samples of patients with age-related macular degeneration. We have also found that chemically induced hypoxia regulates the expression of MsrA and MsrB2 in human RPE cells. Thus, MsrA is a critical enzyme that participates in cell and tissue protection, and its interaction with other proteins/growth factors may provide a target for therapeutic strategies to prevent degenerative diseases.

Myristoylated methionine sulfoxide reductase A protects the heart from ischemia-reperfusion injury

Methionine sulfoxide reductase A (MsrA) catalytically scavenges reactive oxygen species and also repairs oxidized methionines in proteins. Increasing MsrA protects cells and organs from a variety of oxidative stresses while decreasing MsrA enhances damage, but the mechanisms of action have not been elucidated. A single gene encodes MsrA of which ∼25% is targeted to the mitochondria, a major site of reactive oxygen species production. The other ∼75% is targeted to the cytosol and is posttranslationally modified by myristoylation. To determine the relative importance of MsrA in each compartment in protecting against ischemia-reperfusion damage, we created a series of transgenic mice overexpressing MsrA targeted to the mitochondria or the cytosol. We used a Langendorff model of ischemia-reperfusion and assayed both the rate pressure product and infarct size following ischemia and reperfusion as measures of injury. While the mitochondrially targeted MsrA was expected to be protective, it was not. Notably, the cytosolic form was protective but only if myristoylated. The nonmyristoylated, cytosolic form offered no protection against injury. We conclude that cytosolic MsrA protects the heart from ischemia-reperfusion damage. The requirement for myristoylation suggests that MsrA must interact with a hydrophobic domain to provide protection.

Methionine sulfoxide reductase A is a stereospecific methionine oxidase

Methionine sulfoxide reductase A (MsrA) catalyzes the reduction of methionine sulfoxide to methionine and is specific for the S epimer of methionine sulfoxide. The enzyme participates in defense against oxidative stresses by reducing methionine sulfoxide residues in proteins back to methionine. Because oxidation of methionine residues is reversible, this covalent modification could also function as a mechanism for cellular regulation, provided there exists a stereospecific methionine oxidase. We show that MsrA itself is a stereospecific methionine oxidase, producing S-methionine sulfoxide as its product. MsrA catalyzes its own autooxidation as well as oxidation of free methionine and methionine residues in peptides and proteins. When functioning as a reductase, MsrA fully reverses the oxidations which it catalyzes.

Methionine sulfoxide reductase A (MsrA) protects cultured mouse embryonic stem cells from H2O2-mediated oxidative stress

Methionine sulfoxide reductase A (MsrA), a member of the Msr gene family, can reduce methionine sulfoxide residues in proteins formed by oxidation of methionine by reactive oxygen species (ROS). Msr is an important protein repair system which can also function to scavenge ROS. Our studies have confirmed the expression of MsrA in mouse embryonic stem cells (ESCs) in culture conditions. A cytosol-located and mitochondria-enriched expression pattern has been observed in these cells. To confirm the protective function of MsrA in ESCs against oxidative stress, a siRNA approach has been used to knockdown MsrA expression in ES cells which showed less resistance than control cells to hydrogen peroxide treatment. Overexpression of MsrA gene products in ES cells showed improved survivability of these cells to hydrogen peroxide treatment. Our results indicate that MsrA plays an important role in cellular defenses against oxidative stress in ESCs. Msr genes may provide a new target in stem cells to increase their survivability during the therapeutic applications.

Compartmentalization and regulation of mitochondrial function by methionine sulfoxide reductases in yeast

Elevated levels of reactive oxygen species can damage proteins. Sulfur-containing amino acid residues, cysteine and methionine, are particularly susceptible to such damage. Various enzymes evolved to protect proteins or repair oxidized residues, including methionine sulfoxide reductases MsrA and MsrB, which reduce methionine (S)-sulfoxide (Met-SO) and methionine (R)-sulfoxide (Met-RO) residues, respectively, back to methionine. Here, we show that MsrA and MsrB are involved in the regulation of mitochondrial function. Saccharomyces cerevisiae mutant cells lacking MsrA, MsrB, or both proteins had normal levels of mitochondria but lower levels of cytochrome c and fewer respiration-competent mitochondria. The growth of single MsrA or MsrB mutants on respiratory carbon sources was inhibited, and that of the double mutant was severely compromised, indicating impairment of mitochondrial function. Although MsrA and MsrB are thought to have similar roles in oxidative protein repair each targeting a diastereomer of methionine sulfoxide, their deletion resulted in different phenotypes. GFP fusions of MsrA and MsrB showed different localization patterns and primarily localized to cytoplasm and mitochondria, respectively. This finding agreed with compartment-specific enrichment of MsrA and MsrB activities. These results show that oxidative stress contributes to mitochondrial dysfunction through oxidation of methionine residues in proteins located in different cellular compartments.

Oxidized mitochondrial protein degradation and repair in aging and oxidative stress

Proteins are main targets for oxidative damage that occurs during aging and in oxidative stress situations. Since the mitochondria is a major source of reactive oxygen species, mitochondrial proteins are especially exposed to oxidative modification, and elimination of oxidized proteins is crucial for maintaining the integrity of this organelle. Hence, enzymatic reversal of protein oxidation and protein degradation is critical for protein homeostasis while protein maintenance failure has been implicated in the age-related accumulation of oxidized proteins. Within the mitochondrial matrix, the ATP-stimulated mitochondrial Lon protease is believed to play an important role in the degradation of oxidized protein, and age-associated impairment of Lon-like protease activity has been suggested to contribute to oxidized protein buildup in the mitochondria. Oxidized protein repair is limited to certain oxidation products of the sulfur-containing amino acids cysteine and methionine. Oxidized protein repair systems, thioredoxin/thioredoxin reductase or glutaredoxin/glutathione/glutathione reductase that catalytically reduce disulfide bridges or sulfenic acids, and methionine sulfoxide reductase that reverses methionine sulfoxide back to methionine within proteins, are present in the mitochondrial matrix. Thus, the role of the mitochondrial Lon protease and the oxidized protein repair system methionine sulfoxide reductase is further addressed in the context of oxidative stress and aging.

Transgenic mice overexpressing methionine sulfoxide reductase A: characterization of embryonic fibroblasts

Methionine residues in protein can be oxidized by reactive oxygen species to generate methionine sulfoxide. Aerobic organisms have methionine sulfoxide reductases capable of reducing methionine sulfoxide back to methionine. Methionine sulfoxide reductase A acts on the S-epimer of methionine sulfoxide, and it is known that altering its cellular level by genetic ablation or overexpression has notable effects on resistance to oxidative stress and on life span in species from microorganisms to animals. In mammals, the enzyme is present in both the cytosol and the mitochondria, and this study was undertaken to assess the contribution of each subcellular compartment's reductase activity to resistance against oxidative stresses. Nontransgenic mouse embryonic fibroblasts lack methionine sulfoxide reductase A activity, providing a convenient cell type to determine the effects of expression of the enzyme in each compartment. We created transgenic mice with methionine sulfoxide reductase A targeted to the cytosol, mitochondria, or both and studied embryonic fibroblasts derived from each line. Unexpectedly, none of the transgenic cells gained resistance to a variety of oxidative stresses even though the expressed enzymes were catalytically active when assayed in vitro. Noting that activity in vivo requires thioredoxin and thioredoxin reductase, we determined the levels of these proteins in the fibroblasts and found that they were very low in both the nontransgenic and the transgenic cells. We conclude that overexpression of methionine sulfoxide reductase A did not confer resistance to oxidative stress because the cells lacked other proteins required to constitute a functional methionine sulfoxide reduction system.

Dual sites of protein initiation control the localization and myristoylation of methionine sulfoxide reductase A

Methionine sulfoxide reductase A is an essential enzyme in the antioxidant system, which scavenges reactive oxygen species through cyclic oxidation and reduction of methionine and methionine sulfoxide. In mammals, one gene encodes two forms of the reductase, one targeted to the cytosol and the other to mitochondria. The cytosolic form displays faster mobility than the mitochondrial form, suggesting a lower molecular weight for the former. The apparent size difference and targeting to two cellular compartments had been proposed to result from differential splicing of mRNA. We now show that differential targeting is effected by use of two initiation sites, one of which includes a mitochondrial targeting sequence, whereas the other does not. We also demonstrate that the mass of the cytosolic form is not less than that of the mitochondrial form; the faster mobility of cytosolic form is due to its myristoylation. Lipidation of methionine sulfoxide reductase A occurs in the mouse, in transfected tissue culture cells, and even in a cell-free protein synthesis system. The physiologic role of myristoylation of MsrA remains to be elucidated.

A high-throughput screening compatible assay for activators and inhibitors of methionine sulfoxide reductase A

The methionine sulfoxide reductase (Msr) system has been shown to play an important role in protecting cells against oxidative damage. This family of enzymes can repair damage to proteins resulting from the oxidation of methionine residues to methionine sulfoxide, caused by reactive oxygen species. Previous genetic studies in animals have shown that increased levels of methionine sulfoxide reductase enzyme A (MsrA), an important member of the Msr family, can protect cells against oxidative damage and increase life span. A high-throughput screening (HTS) compatible assay has been developed to search for both activators and inhibitors of MsrA. The assay involves a coupled reaction in which the oxidation of NADPH is measured by either spectrophotometric or fluorometric analysis. Previous studies had shown that MsrA has a broad substrate specificity and can reduce a variety of methyl sulfoxide compounds, including dimethylsulfoxide (DMSO). Since the chemicals in the screening library are dissolved in DMSO, which would compete with any of the standard substrates used for the determination of MsrA activity, an assay has been developed that uses the DMSO that is the solvent for the compounds in the library as the substrate for the MsrA enzyme. A specific activator of MsrA could have important therapeutic value for diseases that involve oxidative damage, especially age-related diseases, whereas a specific inhibitor of MsrA would have value for a variety of research studies.

Oxidation of methionine 35 reduces toxicity of the amyloid beta-peptide(1-42) in neuroblastoma cells (IMR-32) via enzyme methionine sulfoxide reductase A expression and function

The beta amyloid peptide (Abeta), the major protein component of brain senile plaques in Alzheimer's disease, is known to be directly responsible for the production of free radicals that may lead to neurodegeneration. Our recent evidence suggest that the redox state of methionine residue in position 35 (Met-35) of Abeta has the ability to deeply modify peptide's neurotoxic actions. Reversible oxidation of methionine in proteins involving the enzyme methionine sulfoxide reductase type A (MsrA) is postulated to serve a general antioxidant role and a decrease in MsrA has been implicated in Alzheimer's disease. In rat neuroblastoma cells (IMR-32), we used Abeta(1-42), in which the Met-35 is present in the reduced state, with a modified peptide with oxidized Met-35 (Abeta(1-42)Met35(OX)), as well as an Abeta-derivative in which Met-35 is substituted with norleucine (Abeta(1-42)Nle35) to investigate the relationship between Met-35 redox state, expression and function of MsrA and reactive oxygen species (ROS) generation. The obtained results shown that MsrA activity, as well as mRNA levels, increase in IMR-32 cells treated with Abeta(1-42)Met35(OX), differently to that shown by the reduced derivative. The increase in MsrA function and expression was associated with a decline of ROS levels. None of these effects were observed when cells were exposed to Abeta containing oxidized Met35 (Abeta1-42)Met35(OX). Taken together, the results of the present study indicate that the differential toxicity of Abeta peptides containing reduced or oxidised Met-35 depends on the ability of the latter form to reduce ROS generation by enhancing MsrA gene expression and function and suggests the therapeutic potential of MsrA in Alzheimer's disease.

Reduced oxygen tension results in reduced human T cell proliferation and increased intracellular oxidative damage and susceptibility to apoptosis upon activation

Cell culture and in vitro models are the basis for much biological research, especially in human immunology. Ex vivo studies of T cell physiology employ conditions attempting to mimic the in vivo situation as closely as possible. Despite improvements in controlling the cellular milieu in vitro, most of what is known about T cell behavior in vitro is derived from experiments on T cells exposed to much higher oxygen levels than are normal in vivo. In this study, we report a reduced proliferative response and increased apoptosis susceptibility after T cell activation at 2% oxygen compared to in air. To explain this observation, we tested the hypothesis of an impaired efficacy of intracellular protective mechanisms including antioxidant levels, oxidized protein repair (methionine sulfoxide reductases), and degradation (proteasome) activities. Indeed, after activation, there was a significant accumulation of intracellular oxidized proteins at more physiological oxygen levels concomitant with a reduced GSH:GSSG ratio. Proteasome and methionine sulfoxide reductase activities were also reduced. These data may explain the increased apoptotic rate observed at more physiological oxygen levels. Altogether, this study highlights the importance of controlling oxygen levels in culture when investigating oxygen-dependent phenomena such as oxidative stress.

Design, synthesis, and examination of neuron protective properties of alkenylated and amidated dehydro-silybin derivatives

A series of C7-O- and C20-O-amidated 2,3-dehydrosilybin (DHS) derivatives ((+/-)-1a-f and (+/-)-2), as well as a set of alkenylated DHS analogues ((+/-)-4a-f), were designed and de novo synthesized. A diesteric derivative of DHS ((+/-)-3) and two C23 esterified DHS analogues ((+/-)-5a and (+/-)-5b) were also prepared for comparison. The cell viability of PC12 cells, Fe(2+) chelation, lipid peroxidation (LPO), free radical scavenging, and xanthine oxidase inhibition models were utilized to evaluate their antioxidative and neuron protective properties. The study revealed that the diether at C7-OH and C20-OH as well as the monoether at C7-OH, which possess aliphatic substituted acetamides, demonstrated more potent LPO inhibition and Fe(2+) chelation compared to DHS and quercetin. Conversely, the diallyl ether at C7-OH and C20-OH was more potent in protection of PC12 cells against H(2)O(2)-induced injury than DHS and quercetin. Overall, the more lipophilic alkenylated DHS analogues were better performing neuroprotective agents than the acetamidated derivatives. The results in this study would be beneficial for optimizing the therapeutic potential of lignoflavonoids, especially in neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

Methionine sulfoxide reductase B2 is highly expressed in the retina and protects retinal pigmented epithelium cells from oxidative damage

Methionine sulfoxide reductase B2 (MSRB2) is a mitochondrial enzyme that converts methionine sulfoxide (R) enantiomer back to methionine. This enzyme is suspected of functioning to protect mitochondrial proteins from oxidative damage. In this study we report that the retina is one of the human tissues with highest levels of MSRB2 mRNA expression. Other tissues with high expression were heart, kidney and skeletal muscle. Overexpression of an MSRB2-GFP fusion protein increased the MSR enzymatic activity three-fold in stably transfected cultured RPE cells. This overexpression augmented the resistance of these cells to the toxicity induced by 7-ketocholesterol, tert-butyl hydroperoxide and all-trans retinoic acid. By contrast, knockdown of MSRB2 by a miRNA in stably transfected cells did not convey increased sensitivity to the oxidative stress. In the monkey retina MSRB2 localized to the ganglion cell layer (GLC), the outer plexiform layer (OPL) and the retinal pigment epithelium (RPE). MSRB2 expression is most pronounced in the OPL of the macula and foveal regions suggesting an association with the cone synaptic mitochondria. Our data suggests that MSRB2 plays an important function in protecting cones from multiple type of oxidative stress and may be critical in preserving central vision.

Methionine sulfoxide reductase A expression is regulated by the DAF-16/FOXO pathway in Caenorhabditis elegans

The methionine sulfoxide reductase system has been implicated in aging and protection against oxidative stress. This conserved system reverses the oxidation of methionine residues within proteins. We analyzed one of the components of this system, the methionine sulfoxide reductase A gene, in Caenorhabditis elegans. We found that the msra-1 gene is expressed in most tissues, particularly in the intestine and the nervous system. Worms carrying a deletion of the msra-1 gene are more sensitive to oxidative stress, show chemotaxis and locomotory defects, and a 30% decrease in median survival. We established that msra-1 expression decreases during aging and is regulated by the DAF-16/FOXO3a transcription factor. The absence of this enzyme decreases median survival and affects oxidative stress resistance of long lived daf-2 worms. A similar effect of MSRA-1 absence in wild-type and daf-2 (where most antioxidant enzymes are activated) backgrounds, suggests that the lack of this member of the methionine repair system cannot be compensated by the general antioxidant response. Moreover, FOXO3a directly activates the human MsrA promoter in a cell culture system, implying that this could be a conserved mechanism of MsrA regulation. Our results suggest that repair of oxidative damage in proteins influences the rate at which tissues age. This repair mechanism, rather than the general decreased of radical oxygen species levels, could be one of the main determinants of organisms' lifespan.

Autoantibody against one of the antioxidant repair enzymes, methionine sulfoxide reductase A, in systemic sclerosis: association with pulmonary fibrosis and vascular damage

Systemic sclerosis (SSc) is a connective tissue disease characterized by fibrosis and vascular changes in the skin and internal organs with autoimmune background. It has been suggested that oxidative stress plays an important role in the development of SSc. To determine the prevalence and clinical correlation of autoantibody to methionine sulfoxide reductase A (MSRA), one of the antioxidant repair enzymes, in SSc, serum anti-MSRA autoantibody levels were examined in patients with SSc by enzyme-linked immunosorbent assay using recombinant MSRA. The presence of anti-MSRA antibody was evaluated by immunoblotting. To determine the functional relevance of anti-MSRA antibody in vivo, we assessed whether anti-MSRA antibody was able to inhibit MSRA enzymatic activity. Serum anti-MSRA antibody levels in SSc patients were significantly higher compared to controls and this autoantibody was detected in 33% of SSc patients. Serum anti-MSRA levels were significantly elevated in SSc patients with pulmonary fibrosis, cardiac involvement, or decreased total antioxidant power compared with those without them. Anti-MSRA antibodies also correlated positively with renal vascular damage determined as pulsatility index by color-flow Doppler ultrasonography of the renal interlobar arteries and negatively with pulmonary function tests. Furthermore, anti-MSRA antibody levels correlated positively with serum levels of 8-isoprostane and heat shock protein 70 that are markers of oxidative and cellular stresses. Remarkably, MSRA activity was inhibited by IgG isolated from SSc sera containing IgG anti-MSRA antibody. These results suggest that elevated anti-MSRA autoantibody is associated with the disease severity of SSc and may enhance the oxidative stress by inhibiting MSRA enzymatic activity.

The proteasome is an integral part of solar ultraviolet a radiation-induced gene expression

Solar ultraviolet (UV) A radiation is a well known trigger of signaling responses in human skin fibroblasts. One important consequence of this stress response is the increased expression of matrix metalloproteinase-1 (MMP-1), which causes extracellular protein degradation and thereby contributes to photoaging of human skin. In the present study we identify the proteasome as an integral part of the UVA-induced, intracellular signaling cascade in human dermal fibroblasts. UVA-induced singlet oxygen formation was accompanied by protein oxidation, the cross-linking of oxidized proteins, and an inhibition of the proteasomal system. This proteasomal inhibition subsequently led to an accumulation of c-Jun and phosphorylated c-Jun and activation of activator protein-1, i.e. transcription factors known to control MMP-1 expression. Increased transcription factor activation was also observed if the proteasome was inhibited by cross-linked proteins or lactacystin, indicating a general mechanism. Most importantly, inhibition of the proteasome was of functional relevance for UVA-induced MMP-1 expression, because overexpression of the proteasome or the protein repair enzyme methionine sulfoxide reductase prevented the UVA-induced induction of MMP-1. These studies show that an environmentally relevant stimulus can trigger a signaling pathway, which links intracellular and extracellular protein degradation. They also identify the proteasome as an integral part of the UVA stress response.

Nutrition intervention: a strategy against systemic inflammatory syndrome

BACKGROUND

Sepsis and septic shock syndrome are the leading causes of death in critically ill patients. Lipopolysaccharide (LPS) released by the colonic microorganisms may translocate across a compromised lumen, leading to upregulated reactive oxidative stress, inflammation, and sepsis. The authors examined an enteral formula high in cysteine (antioxidant precursor), omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), and prebiotic fructooligosaccharides (FOS) against systemic inflammatory syndrome.

METHODS

Rats were allocated to (1) standard soy-based diet high in cysteine and crude fiber and devoid of EPA-DHA (CHOW); (2) whey-peptide-based liquid diet high in cysteine, EPA-DHA, and FOS (CYSPUFA); or (3) casein-based liquid isonitrogenous diet, low in cysteine and devoid of EPA-DHA-FOS (CASN). Liquid diets provided 25% and CHOW, 23% of calories as protein. After 6 days on diets, rats received an intraperitoneal injection of LPS or saline. Animals gained weight on their respective diets and lost weight after LPS administration. The CYSPUFA group lost considerably less weight (vs CASN or CHOW, P < .05). Inflammatory cytokines significantly increased by 4 hours and subsided 18 hours after assault. The CASN group showed elevated liver enzyme alanine aminotransferase release from damaged hepatocytes and developed severe hepatic pathology with low hematocrit. The CHOW group developed more severe hepatic lesions compared with those on liquid diets. Concentration of liver enzyme and pathology were improved in rats receiving CYSPUFA.

CONCLUSIONS

Data indicate that CYSPUFA, a diet rich in EPA-DHA-FOS, protects against LPS-induced systemic inflammatory responses and warrants clinical studies in critically ill patients.

Sulindac enhances the killing of cancer cells exposed to oxidative stress

Background

Sulindac is an FDA-approved non-steroidal anti-inflammatory drug (NSAID) that affects prostaglandin production by inhibiting cyclooxygenases (COX) 1 and 2. Sulindac has also been of interest for more than decade as a chemopreventive for adenomatous colorectal polyps and colon cancer.

Principal Findings

Pretreatment of human colon and lung cancer cells with sulindac enhances killing by an oxidizing agent such as tert-butyl hydroperoxide (TBHP) or hydrogen peroxide. This effect does not involve cyclooxygenase (COX) inhibition. However, under the conditions used, there is a significant increase in reactive oxygen species (ROS) within the cancer cells and a loss of mitochondrial membrane potential, suggesting that cell death is due to apoptosis, which was confirmed by Tunel assay. In contrast, this enhanced killing was not observed with normal lung or colon cells.

Significance

These results indicate that normal and cancer cells handle oxidative stress in different ways and sulindac can enhance this difference. The combination of sulindac and an oxidizing agent could have therapeutic value.

Deletion of mouse MsrA results in HBO-induced cataract: MsrA repairs mitochondrial cytochrome c

PURPOSE

Considerable evidence indicates a role for methionine sulfoxide reductase A (MsrA) in lens cell resistance to oxidative stress through its maintenance of mitochondrial function. Correspondingly, increased protein methionine sulfoxide (PMSO) is associated with lens aging and human cataract formation, suggesting that loss of MsrA activity is associated with this disease. Here we tested the hypothesis that loss of MsrA protein repair is associated with cataract formation. To test this hypothesis we examined the effect of MsrA deletion on lens opacity in mice treated with hyperbaric oxygen, identified lens mitochondrial proteins oxidized upon deletion of MsrA and determined the ability of MsrA to repair the identified proteins.

METHODS

Wild-type and MsrA knockout mice were treated or not treated with 100 treatments of hyperbaric oxygen (HBO) over an 8 month period and lenses were examined by in vivo light scattering measurements documented by slit-lamp imaging. Co-immunoprecipitation of MsrA was conducted against five specific protein representatives of the five complexes of the electron transport chain in addition to cytochrome c (cyt c). Cyt c in lens protein from the knockout and wild-type lenses was subjected to cyanogen bromide (CNBr) cleavage to identify oxidized methionines. Methionine-specific CNBr cleavage was used to differentiate oxidized and un-oxidized methionines in cyt c in vitro and the ability of MsrA to restore the activity of oxidized cyt c was evaluated. Mass spectrometry analysis of cyt c was used to confirm oxidation and repair by MsrA in vitro.

RESULTS

HBO treatment of MsrA knockout mice led to increased light scattering in the lens relative to wild-type mice. MsrA interacted with four of the five complexes of the mitochondrial electron transport chain as well as with cyt c. Cyt c was found to be aggregated and degraded in the knockout lenses consistent with its oxidation. In vitro analysis of oxidized cyt c revealed the presence of two oxidized methionines (met 65 and met 80) that were repairable by MsrA. Repair of the oxidized methionines in cyt c restored the activity of cytochrome c oxidase and reduced cytochrome c peroxidase activity.

CONCLUSIONS

These results establish that MsrA deletion causes increased light scattering in mice exposed to HBO and they identify cyt c as oxidized in the knockout lenses. They also establish that MsrA can restore the in vitro activity of cyt c through its repair of PMSO. These results support the hypothesis that MsrA is important for the maintenance of lens transparency and provide evidence that repair of mitochondrial cyt c by MsrA could play an important role in defense of the lens against cataract formation.

Quinacrine protects neuronal cells against heat-induced injury

The effects of quinacrine (QA) on heat-induced neuronal injury have been explored, with the intention of understanding the mechanisms of QA protection. Primary cultivated striatum neurons from newborn rats were treated with QA 1h before heat treatment at 43 degrees C which lasted for another 1h, and necrosis and apoptosis were detected by Annexin-V-FITC and propidium iodide (PI) double staining. Neuronal apoptosis was determined using terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) techniques. Cell membrane fluidity, activity of cytosolic phospholipase A(2) (cPLA(2)) and the level of arachidonic acid (AA) were also examined. Membrane surface ultrastructure of striatum neurons was investigated by atomic force microscopy (AFM). Results showed that heat treatment induced great striatum neurons death, with many dying neurons undergoing necrosis rather than apoptosis. QA alone had little effect on the survival of striatum neurons, while QA pretreatment before heat treatment decreased necrosis. Heat treatment also resulted in decreased membrane fluidity and increased cPLA(2) activity as well as arachidonic acid level; these effects were reversed by QA pretreatment. QA pretreatment also significantly prevented damage to the membrane surface ultrastructure of heat-treated neurons. These results suggest that QA protects striatum neurons against heat-induced neuronal necrosis, and also demonstrate that inhibition of cPLA(2) activity and stabilization of membranes may contribute to protective effect of quinacrine.

Decreased methionine sulphoxide reductase A expression renders melanocytes more sensitive to oxidative stress: a possible cause for melanocyte loss in vitiligo

BACKGROUND

Methionine is one of the major targets of reactive oxygen species (ROS). It is readily oxidized to methionine-S-sulphoxide and methionine-R-sulphoxide, which can be reduced by methionine sulphoxide reductase (MSR) A and B, respectively. MSR represents a unique repair mechanism in the skin antioxidant network. It functions both as a protein repairer and as a ROS scavenger. However, the expression and activity of MSR are significantly reduced in vitiligo.

OBJECTIVES

To investigate whether the decreased expression of MSRA is one of the reasons why melanocytes are especially vulnerable to oxidative stress in vitiligo. Methods We downregulated MSRA expression in immortalized human epidermal melanocyte cell line PIG1 by using the short interfering RNA (siRNA)-targeted gene silencing method. We checked the changes in MSRA transcript and protein level by using reverse transcriptase-polymerase chain reaction and Western blot, respectively. Then we monitored the viability of MSRA-silenced melanocytes under oxidative stress. All statistical analysis was performed by unpaired two-tailed Student's t-test.

RESULTS

The siRNA specific for MSRA successfully suppressed MSRA expression in melanocytes. The lower MSRA expression in melanocytes led to an increased sensitivity to oxidative stress, resulting in more cell death. Furthermore, a remarkable loss of viable cells was found in MSRA-silenced melanocytes even in the absence of exogenously added oxidative stress.

CONCLUSIONS

MSRA is crucial for melanocytes to fight against oxidative stress in vitiligo. In addition, it is also important for normal cell survival. Any means to enhance MSRA appears to have therapeutic potential for the treatment of vitiligo.

Expression, subcellular localization, and antioxidant role of mammalian methionine sulfoxide reductases in Saccharomyces cerevisiae

Despite the growing body of evidence suggesting a role for MsrA in antioxidant defense, little is currently known regarding the function of MsrB in cellular protection against oxidative stress. In this study, we overexpressed the mammalian MsrB and MsrA genes in Saccharomyces cerevisiae and assessed their subcellular localization and antioxidant functions. We found that the mitochondrial MsrB3 protein (MsrB3B) was localized to the cytosol, but not to the mitochondria, of the yeast cells. The mitochondrial MsrB2 protein was detected in the mitochondria and, to a lesser extent, the cytosol of the yeast cells. In this study, we report the first evidence that MsrB3 overexpression in yeast cells protected them against H(2)O(2)-mediated cell death. Additionally, MsrB2 overexpression also provided yeast cells with resistance to oxidative stress, as did MsrA overexpression. Our results show that mammalian MsrB and MsrA proteins perform crucial functions in protection against oxidative stress in lower eukaryotic yeast cells.

Overexpression of methionine sulfoxide reductases A and B2 protects MOLT-4 cells against zinc-induced oxidative stress

Among the amino acids, methionine is the most susceptible to oxidation, and methionine sulfoxide can be catalytically reduced within proteins by methionine sulfoxide reductase A (MsrA) and B (MsrB). As one of the very few repair systems for oxidized proteins, MsrA and MsrB enzymes play a major role in protein homeostasis during aging and have also been involved in cellular defenses against oxidative stress, by scavenging reactive oxygen species. To elucidate the role of zinc on the Msr system, the effects of zinc treatment on control and stably overexpressing MsrA and MsrB2 MOLT-4 leukemia cells have been analyzed. Here we show that zinc treatment has a pro-antioxidant effect in MOLT-4 cells by inducing the transcription of metallothioneins and positively modulating the activity of the Msr enzymes. In contrast, due to its pro-oxidant effect, zinc also led to increased cell death, reactive oxygen species production, and protein damage. Our results indicate that overexpression of the Msr enzymes, due to their antioxidant properties, counteracts the pro-oxidant effects of zinc treatment, which lead to a cellular protection against protein oxidative damage and cell death, by reducing the production of reactive oxygen species.

Origin and evolution of the protein-repairing enzymes methionine sulphoxide reductases

The majority of extant life forms thrive in an O2-rich environment, which unavoidably induces the production of reactive oxygen species (ROS) during cellular activities. ROS readily oxidize methionine (Met) residues in proteins/peptides to form methionine sulphoxide [Met(O)] that can lead to impaired protein function. Two methionine sulphoxide reductases, MsrA and MsrB, catalyse the reduction of the S and R epimers, respectively, of Met(O) in proteins to Met. The Msr system has two known functions in protecting cells against oxidative damage. The first is to repair proteins that have lost activity due to Met oxidation and the second is to function as part of a scavenger system to remove ROS through the reversible oxidation/reduction of Met residues in proteins. Bacterial, plant and animal cells lacking MsrA are known to be more sensitive to oxidative stress. The Msr system is considered an important cellular defence mechanism to protect against oxidative stress and may be involved in ageing/senescence. MsrA is present in all known eukaryotes and eubacteria and a majority of archaea, reflecting its essential role in cellular life. MsrB is found in all eukaryotes and the majority of eubacteria and archaea but is absent in some eubacteria and archaea, which may imply a less important role of MsrB compared to MsrA. MsrA and MsrB share no sequence or structure homology, and therefore probably emerged as a result of independent evolutionary events. The fact that some archaea lack msr genes raises the question of how these archaea cope with oxidative damage to proteins and consequently of the significance of msr evolution in oxic eukaryotes dealing with oxidative stress. Our best hypothesis is that the presence of ROS-destroying enzymes such as peroxiredoxins and a lower dissolved O2 concentration in those msr-lacking organisms grown at high temperatures might account for the successful survival of these organisms under oxidative stress.

Multiple signal transduction pathways in okadaic acid induced apoptosis in HeLa cells

Okadaic acid (OA) is the major component of diarrhetic shell fish poisoning toxins and a potent inhibitor of protein phosphatase 1 and 2A. We investigated the signal transduction pathways involved in OA induced cell death in HeLa cells. OA induced cytotoxicity and apoptosis at IC50 of 100nM. OA treatment resulted in time dependent increase in reactive oxygen species and depleted intracellular glutathione levels. Loss of mitochondrial membrane permeability led to translocation of bax, cytochrome-c and AIF from mitochondria to cytosol. The cells under fluorescence microscope showed typical apoptotic morphology with condensed chromatin, and nuclear fragmentation. We investigated the mitochondrial-mediated caspase cascade. The time dependent activation and cleavage of of bax, caspases-8, 10, 9, 3 and 7 was observed in Western blot analysis. In addition to caspase-dependent pathway AIF mediated caspase-independent pathway was involved in OA mediated cell death. OA also caused time dependent inhibition of protein phosphatase 2A activity and phosphorylation of p38 and p42/44 MAP kinases. Inhibitor studies with Ac-DEVO-CHO and Z-VAD-FMK could not prevent the phosphorylation of p38 and p42/44 MAP kinases. Our experiments with caspase inhibitors Ac-DEVD-CHO, Z-IETD-FMK and Z-VAD-FMK inhibited capsase-3, 8 cleavages but did not prevent OA-induced apoptosis and DNA fragmentation. Similarly, pretreatment with cyclosporin-A and N-acetylcysteine could not prevent the DNA fragmentation. In summary, the results of our study show that OA induces multiple signal transduction pathways acting either independently or simultaneously leading to apoptosis.

Protection of capsaicin against hypoxia–reoxygenation-induced apoptosis of rat hippocampal neurons

The aim of this study was to investigate the effect of capsaicin on hypoxia–reoxygenation (H/R)-induced apoptosis in primary rat hippocampal neurons. Three hours of hypoxia (1% O2) and subsequent reoxygenation for 24 h significantly increased the apoptotic death of hippocampal neurons, as evidenced by increases in both TUNEL-positive cell number and caspase-3 activity. Pretreatment with capsaicin (3–30 µmol/L) or the caspase-3-specific inhibitor acetyl-DEVD-CHO (100 µmol/L) markedly attenuated H/R-induced apoptosis in hippocampal neurons. Capsaicin also markedly induced the phosphorylation of Akt. The phosphoinositide 3-kinase (PI3K) inhibitor LY294002 (10 µmol/L) prevented any capsaicin-induced survival effect in hippocampal neurons. Intracellular levels of reactive oxygen species (ROS), which were greatly increased after H/R, were significantly inhibited by capsaicin, pyrrolidine dithiocarbamate (PDTC) (50 µmol/L), and LY294002. Taken together, these data suggest that capsaicin protects against H/R-induced apoptosis of hippocampal neurons via the PI3K/Akt-mediated signaling pathway, which is related to the inhibition of oxidative stress and caspase-3 activation.

Antioxidant therapy in Alzheimer's disease: theory and practice

Alzheimer disease treatment has yet to yield a successful therapy that addresses the source of the damage found in brains. Of the varied proposed theories of AD etiology, reactive oxygen species (ROS) generation is cited as a common factor. Efforts to reduce the pathology associated with ROS via antioxidants therefore offer new hope to patients suffering from this devastative disease.

Retinoic acid regulates the human methionine sulfoxide reductase A (MSRA) gene via two distinct promoters

MSRAs (methionine sulfoxide reductases A) are enzymes that reverse the effects of oxidative damage by reducing methionine sulfoxide back to methionine and recovering protein function. In this study we demonstrate that the transcriptional regulation of the human MSRA gene is complex and driven by two distinct promoters. Both promoters demonstrate high expression in human brain and kidney tissues. The upstream (promoter 1) regulates the msrA1 transcript that codes for the mitochondrial form of MSRA and is highly active in a broad range of cell lines. The downstream promoter (promoter 2) regulates the msrA2/3 transcripts that code for the cytosolic/nuclear forms of MSRA and is generally less active. Promoter 2 contains a 65 bp putative enhancer region that is very active in the retinal pigment epithelium-derived D407 cell line. Both promoters are partially regulated by all-trans retinoic acid via RARA and other RARs.

Methionine in proteins defends against oxidative stress

A variety of reactive oxygen species react readily with methionine residues in proteins to form methionine sulfoxide, thus scavenging the reactive species. Most cells contain methionine sulfoxide reductases, which catalyze a thioredoxin-dependent reduction of methionine sulfoxide back to methionine. Thus, methionine residues may act as catalytic antioxidants, protecting both the protein where they are located and other macromolecules. To test this hypothesis directly, we replaced 40% of the methionine residues in Escherichia coli with norleucine, the carbon-containing analog, in which the sulfur of methionine is substituted by a methylene group (-CH2-). The intracellular free methionine and S-adenosylmethionine pools were not altered, nor was the specific activity of the key enzyme, glutamine synthetase. When unstressed, both control and norleucine-substituted cells survived equally well at stationary phase for at least 32 h. However, oxidative stress was more damaging to the norleucine-substituted cells. They died more rapidly than control cells when challenged by hypochlorite, hydrogen peroxide, or ionizing radiation. One of the most abundant proteins in the cell, elongation factor Tu, was found to be more oxidatively modified in norleucine-substituted cells, consistent with loss of the antioxidant defense provided by methionine residues. The results of these studies support the hypothesis that methionine in protein acts as an endogenous antioxidant in cells.

Methionine sulfoxide reductase A protects dopaminergic cells from Parkinson's disease-related insults

Parkinson's disease (PD) is a neurologic disorder characterized by dopaminergic cell death in the substantia nigra. PD pathogenesis involves mitochondrial dysfunction, proteasome impairment, and alpha-synuclein aggregation, insults that may be especially toxic to oxidatively stressed cells including dopaminergic neurons. The enzyme methionine sulfoxide reductase A (MsrA) plays a critical role in the antioxidant response by repairing methionine-oxidized proteins and by participating in cycles of methionine oxidation and reduction that have the net effect of consuming reactive oxygen species. Here, we show that MsrA suppresses dopaminergic cell death and protein aggregation induced by the complex I inhibitor rotenone or mutant alpha-synuclein, but not by the proteasome inhibitor MG132. By comparing the effects of MsrA and the small-molecule antioxidants N-acetylcysteine and vitamin E, we provide evidence that MsrA protects against PD-related stresses primarily via methionine sulfoxide repair rather than by scavenging reactive oxygen species. We also demonstrate that MsrA efficiently reduces oxidized methionine residues in recombinant alpha-synuclein. These findings suggest that enhancing MsrA function may be a reasonable therapeutic strategy in PD.

Mitochondrial function and redox control in the aging eye: role of MsrA and other repair systems in cataract and macular degenerations

Oxidative stress occurs when the level of prooxidants exceeds the level of antioxidants in cells resulting in oxidation of cellular components and consequent loss of cellular function. Oxidative stress is implicated in wide range of age-related disorders including Alzheimer's disease, Parkinson's disease amyotrophic lateral sclerosis (ALS), Huntington's disease and the aging process itself. In the anterior segment of the eye, oxidative stress has been linked to lens cataract and glaucoma while in the posterior segment of the eye oxidative stress has been associated with macular degeneration. Key to many oxidative stress conditions are alterations in the efficiency of mitochondrial respiration resulting in superoxide (O(2)(-)) production. Superoxide production precedes subsequent reactions that form potentially more dangerous reactive oxygen species (ROS) species such as the hydroxyl radical (OH), hydrogen peroxide (H(2)O(2)) and peroxynitrite (OONO(-)). The major source of ROS in the mitochondria, and in the cell overall, is leakage of electrons from complexes I and III of the electron transport chain. It is estimated that 0.2-2% of oxygen taken up by cells is converted to ROS, through mitochondrial superoxide generation, by the mitochondria. Generation of superoxide at complexes I and III has been shown to occur at both the matrix side of the inner mitochondrial membrane and the cytosolic side of the membrane. While exogenous sources of ROS such as UV light, visible light, ionizing radiation, chemotherapeutics, and environmental toxins may contribute to the oxidative milieu, mitochondria are perhaps the most significant contribution to ROS production affecting the aging process. In addition to producing ROS, mitochondria are also a target for ROS which in turn reduces mitochondrial efficiency and leads to the generation of more ROS in a vicious self-destructive cycle. Consequently, the mitochondria have evolved a number of antioxidant and key repair systems to limit the damaging potential of free oxygen radicals and to repair damaged proteins (Fig. 1). The aging eye appears to be at considerable risk from oxidative stress. This review will outline the potential role of mitochondrial function and redox balance in age-related eye diseases, and detail how the methionine sulfoxide reductase (Msr) protein repair system and other redox systems play key roles in the function and maintenance of the aging eye.

Overexpression of mitochondrial methionine sulfoxide reductase B2 protects leukemia cells from oxidative stress-induced cell death and protein damage

According to the mitochondrial theory of aging, mitochondrial dysfunction increases intracellular reactive oxidative species production, leading to the oxidation of macromolecules and ultimately to cell death. In this study, we investigated the role of the mitochondrial methionine sulfoxide reductase B2 in the protection against oxidative stress. We report, for the first time, that overexpression of methionine sulfoxide reductase B2 in mitochondria of acute T-lymphoblastic leukemia MOLT-4 cell line, in which methionine sulfoxide reductase A is missing, markedly protects against hydrogen peroxide-induced oxidative stress by scavenging reactive oxygen species. The addition of hydrogen peroxide provoked a time-gradual increase of intracellular reactive oxygen species, leading to a loss in mitochondrial membrane potential and to protein carbonyl accumulation, whereas in methionine sulfoxide reductase B2-overexpressing cells, intracellular reactive oxygen species and protein oxidation remained low with the mitochondrial membrane potential highly maintained. Moreover, in these cells, delayed apoptosis was shown by a decrease in the cleavage of the apoptotic marker poly(ADP-ribose) polymerase-1 and by the lower percentage of Annexin-V-positive cells in the late and early apoptotic stages. We also provide evidence for the protective mechanism of methionine sulfoxide reductase B2 against protein oxidative damages. Our results emphasize that upon oxidative stress, the overexpression of methionine sulfoxide reductase B2 leads to the preservation of mitochondrial integrity by decreasing the intracellular reactive oxygen species build-up through its scavenging role, hence contributing to cell survival and protein maintenance.

Identification of a new functional splice variant of the enzyme methionine sulphoxide reductase A (MSRA) expressed in rat vascular smooth muscle cells

Reactive oxygen species contribute to ageing of the vascular system and development of cardiovascular disease. Methionine-S-sulphoxide, an oxidized form of methionine, is repaired by the enzyme methionine sulphoxide reductase A (MSRA). The enzyme, targeted to mitochondria or the cytosol by alternative splicing, is vital for oxidative stress resistance. This study was designed to examine the endogenous expression and intracellular localization of MSRA in rat aortic vascular smooth muscle cells (VSMCs). We detected robust MSRA immunoreactivity exclusively in mitochondria. Sequence analysis of msrA transcripts revealed the presence of a novel mitochondrial splice variant, msrA2a, in cultured rat VSMCs as well as in aortic tissue preparations. The enzymatic activity of a recombinant MSRA2a protein was confirmed by the reduction of methionine sulphoxide in a model substrate peptide. We conclude that multiple MSRA variants participate in the repair of oxidized proteins in VSMC mitochondria, but that other protective mechanisms may exist in the cytoplasmic compartment.

Methionine deficiency and hepatic injury in a dietary steatohepatitis model

Methionine (Meth) is an essential amino acid involved in DNA methylation and glutathione biosynthesis. We examined the effect of Meth on the development of steatohepatitis. Rats were fed (five weeks) amino acid-based Meth-choline-sufficient (A-MCS) or total deficient (MCD) diets and gavaged daily (two weeks) with vehicle (B-vehicle/MCD), or Meth replacement (C-Meth/MCD). To assess the effect of short-term deficiency, after three weeks one MCS group was fed a deficient diet (D-MCS/MCD). Animals fed the deficient diet for two weeks lost (29%) weight and after five weeks weighed one third as much as those on the sufficient diet, and also developed anemia (P < 0.01). Hepatic transaminases progressively increased from two to five weeks (P < 0.01), leading to severe hepatic pathology. Meth administration normalized hematocrit, improved weight (P < 0.05), and suppressed abnormal enzymes activities (P < 0.01). Meth administration improved blood and hepatic glutathione (GSH), S-adenosylmethionine (SAMe), and hepatic lesions (P < 0.01). The deficient diet significantly upregulated proinflammatory and fibrotic genes, which was ameliorated by Meth administration. These data support a pivotal role for methionine in the pathogenesis of the dietary model of Meth-choline-deficient (MCD) steatohepatitis (NASH).

Methionine sulfoxide reductase A and a dietary supplement S-methyl-L-cysteine prevent Parkinson's-like symptoms

Parkinson's disease (PD), a common neurodegenerative disease, is caused by loss of dopaminergic neurons in the substantia nigra. Although the underlying cause of the neuronal loss is unknown, oxidative stress is thought to play a major role in the pathogenesis of PD. The amino acid methionine is readily oxidized to methionine sulfoxide, and its reduction is catalyzed by a family of enzymes called methionine sulfoxide reductases (MSRs). The reversible oxidation-reduction cycle of methionine involving MSRs has been postulated to act as a catalytic antioxidant system protecting cells from oxidative damage. Here, we show that one member of the MSR family, MSRA, inhibits development of the locomotor and circadian rhythm defects caused by ectopic expression of human alpha-synuclein in the Drosophila nervous system. Furthermore, we demonstrate that one way to enhance the MSRA antioxidant system is dietary supplementation with S-methyl-L-cysteine (SMLC), found abundantly in garlic, cabbage, and turnips. SMLC, a substrate in the catalytic antioxidant system mediated by MSRA, prevents the alpha-synuclein-induced abnormalities. Therefore, interventions focusing on the enzymatic reduction of oxidized methionine catalyzed by MSRA represent a new prevention and therapeutic approach for PD and potentially for other neurodegenerative diseases involving oxidative stress.

MsrA protects cardiac myocytes against hypoxia/reoxygenation induced cell death

Reactive oxygen species (ROS) are critical in tissue responses to ischemia-reperfusion. The enzyme methionine sulfoxide reductase-A (MsrA) is capable of protecting cells against oxidative damage by reversing damage to proteins caused by methionine oxidation or by decreasing ROS through a scavenger mechanism. The current study employed adenovirus mediated over-expression of MsrA in primary neonatal rat cardiac myocytes to determine the effect of this enzyme in protecting against hypoxia/reoxygenation in this tissue. Cells were transduced with MsrA encoding adenovirus and subjected to hypoxia/reoxygenation. Apoptotic cell death was decreased by greater than 45% in cells over-expressing MsrA relative to cells transduced with a control virus. Likewise total cell death as determined by levels of LDH release was dramatically decreased by MsrA over-expression. These observations indicate that MsrA is protective against hypoxia/reoxygenation stress in cardiac myocytes and point to MsrA as an important therapeutic target for ischemic heart disease.