Inhibition of protein synthesis in cortical neurons during exposure to hydrogen peroxide

Dietary Enteromorpha polysaccharide-Zn supplementation regulates amino acid and fatty acid metabolism by improving the antioxidant activity in chicken

Background

Enteromorpha prolifera (E. prolifera) polysaccharide has become a promising feed additive with a variety of physiological activities, such as anti-oxidant, anti-cancer, anti-diabetic, immunomodulatory, hypolipidemic, and cation chelating ability. However, whether Enteromorpha polysaccharide-trace element complex supplementation regulates amino acid and fatty acid metabolism in chicken is largely unknown. This study was conducted to investigate the effects of E. prolifera polysaccharide (EP)-Zn supplementation on growth performance, amino acid, and fatty acid metabolism in chicken.

Methods

A total of 184 one-day-old Ross-308 broiler chickens were randomly divided into two treatment groups with 8 replicates, 12 chickens per replicate, and fed either the basal diet (control group) or basal diet plus E. prolifera polysaccharide-Zinc (400 mg EP-Zn/kg diet).

Results

Dietary EP-Zn supplementation significantly increased (P < 0.05) the body weight, average daily gain, muscle antioxidant activity, serum HDL level, and reduced serum TG and LDL concentration. In addition, dietary EP-Zn supplementation could modulate ileal amino acid digestibility and upregulate the mRNA expression of amino acid transporter genes in the jejunum, ileum, breast muscle, and liver tissues (P < 0.05). Compared with the control group, breast meat from chickens fed EP-Zn had higher (P < 0.05) Pro and Asp content, and lower (P < 0.05) Val, Phe, Gly, and Cys free amino acid content. Furthermore, EP-Zn supplementation upregulated (P < 0.05) the mRNA expressions of mTOR and anti-oxidant related genes, while down-regulated protein degradation related genes in the breast muscle. Breast meat from EP-Zn supplemented group had significantly lower (P < 0.05) proportions of Σn-3 PUFA, and a higher percentage of Σn-6 PUFA and the ratio of n-6/n-3 PUFA. Besides, EP-Zn supplementation regulated lipid metabolism by inhibiting the gene expression of key enzymes involved in the fatty acid synthesis and activating genes that participated in fatty acid oxidation in the liver tissue.

Conclusions

It is concluded that EP-Zn complex supplementation regulates apparent ileal amino acid digestibility, enhances amino acid metabolism, and decreases oxidative stress-associated protein breakdown, thereby improving the growth performance. Furthermore, it promotes fatty acid oxidation and restrains fat synthesis through modulating lipid metabolism-related gene expression.

Graphical abstract

Mitochondrial Dysfunction Is a Common Denominator Linking Skeletal Muscle Wasting Due to Disease, Aging, and Prolonged Inactivity

Skeletal muscle is the most abundant tissue in the body and is required for numerous vital functions, including breathing and locomotion. Notably, deterioration of skeletal muscle mass is also highly correlated to mortality in patients suffering from chronic diseases (e.g., cancer). Numerous conditions can promote skeletal muscle wasting, including several chronic diseases, cancer chemotherapy, aging, and prolonged inactivity. Although the mechanisms responsible for this loss of muscle mass is multifactorial, mitochondrial dysfunction is predicted to be a major contributor to muscle wasting in various conditions. This systematic review will highlight the biochemical pathways that have been shown to link mitochondrial dysfunction to skeletal muscle wasting. Importantly, we will discuss the experimental evidence that connects mitochondrial dysfunction to muscle wasting in specific diseases (i.e., cancer and sepsis), aging, cancer chemotherapy, and prolonged muscle inactivity (e.g., limb immobilization). Finally, in hopes of stimulating future research, we conclude with a discussion of important future directions for research in the field of muscle wasting.

Effect of hydrogen peroxide concentration on the maintenance and differentiation of cultured skeletal muscle cells

We have studied the effects of hydrogen peroxide (H₂O₂) on the differentiation and maintenance of C2C12 myoblasts. The effects of H₂O₂ were evaluated by cell viability, total protein concentration, the relative amount of muscle-related proteins, sarcomere structure, and active tension generation. Oxidative stress is one of the major causes of myopathy after exercise and thus establishing the method to evaluate the effects on muscle function is essential. The primary function of striated muscle is to generate force, thus, the measurement of active tension is important in assessing the effect of chemicals on muscle. Among the indices we tested, the sarcomere structure was the most sensitive to the H₂O₂ exposure while the cell viability was less sensitive. The effects of H₂O₂ on active tension correlated with a decrease in the amount of muscle proteins. In this study, our results showed that the effect of chemicals on muscle should be measured in multiple ways, including active tension generation, for a better understanding of its physiological impact.

Effect of Eukarion-134 on Akt-mTOR signalling in the rat soleus during 7 days of mechanical unloading

NEW FINDINGS

What is the central question of this study? Translocation of nNOSμ initiates catabolic signalling via FoxO3a and skeletal muscle atrophy during mechanical unloading. Recent evidence suggests that unloading-induced muscle atrophy and FoxO3a activation are redox sensitive. Will a mimetic of superoxide dismutase and catalase (i.e. Eukarion-134) also mitigate suppression of the Akt-mTOR pathway? What is the main finding and its importance? Eukarion-134 rescued Akt-mTOR signalling and sarcolemmal nNOSμ, which were linked to protection against the unloading phenotype, muscle fibre atrophy and partial fibre-type shift from slow to fast twitch. The loss of nNOSμ from the sarcolemma appears crucial to Akt phosphorylation and is redox sensitive, although the mechanisms remain unresolved.

ABSTRACT

Mechanical unloading stimulates rapid changes in skeletal muscle morphology, characterized by atrophy of muscle fibre cross-sectional area and a partial fibre-type shift from slow to fast twitch. Recent studies revealed that oxidative stress contributes to activation of forkhead box O3a (FoxO3a), proteolytic signalling and unloading-induced muscle atrophy via translocation of the μ-splice variant of neuronal nitric oxide synthase (nNOSμ) and activation of FoxO3a. There is limited understanding of the role of reactive oxygen species in the Akt-mammalian target of rapamycin (mTOR) pathway signalling during unloading. We hypothesized that Eukarion-134 (EUK-134), a mimetic of the antioxidant enzymes superoxide dismutase and catalase, would protect Akt-mTOR signalling in the unloaded rat soleus. Male Fischer 344 rats were separated into the following three study groups: ambulatory control (n = 11); 7 days of hindlimb unloading + saline injections (HU, n = 11); or 7 days of HU + EUK-134; (HU + EUK-134, n = 9). EUK-134 mitigated unloading-induced dephosphorylation of Akt, as well as FoxO3a, in the soleus. Phosphorylation of mTOR in the EUK-treated HU rats was not different from that in control animals. However, EUK-134 did not significantly rescue p70S6K phosphorylation. EUK-134 attenuated translocation of nNOSμ from the membrane to the cytosol, reduced nitration of tyrosine residues and suppressed upregulation of caveolin-3 and dysferlin. EUK-134 ameliorated HU-induced remodelling, atrophy of muscle fibres and the 12% increase in type II myosin heavy chain-positive fibres. Attenuation of the unloaded muscle phenotype was associated with decreased reactive oxygen species, as assessed by ethidium-positive nuclei. We conclude that oxidative stress affects Akt-mTOR signalling in unloaded skeletal muscle. Direct linkage of abrogation of nNOSμ translocation with Akt-mTOR signalling during unloading is the subject of future investigation.

Partial Support Ventilation and Mitochondrial-Targeted Antioxidants Protect against Ventilator-Induced Decreases in Diaphragm Muscle Protein Synthesis

Mechanical ventilation (MV) is a life-saving intervention in patients in respiratory failure. Unfortunately, prolonged MV results in the rapid development of diaphragm atrophy and weakness. MV-induced diaphragmatic weakness is significant because inspiratory muscle dysfunction is a risk factor for problematic weaning from MV. Therefore, developing a clinical intervention to prevent MV-induced diaphragm atrophy is important. In this regard, MV-induced diaphragmatic atrophy occurs due to both increased proteolysis and decreased protein synthesis. While efforts to impede MV-induced increased proteolysis in the diaphragm are well-documented, only one study has investigated methods of preserving diaphragmatic protein synthesis during prolonged MV. Therefore, we evaluated the efficacy of two therapeutic interventions that, conceptually, have the potential to sustain protein synthesis in the rat diaphragm during prolonged MV. Specifically, these experiments were designed to: 1) determine if partial-support MV will protect against the decrease in diaphragmatic protein synthesis that occurs during prolonged full-support MV; and 2) establish if treatment with a mitochondrial-targeted antioxidant will maintain diaphragm protein synthesis during full-support MV. Compared to spontaneously breathing animals, full support MV resulted in a significant decline in diaphragmatic protein synthesis during 12 hours of MV. In contrast, diaphragm protein synthesis rates were maintained during partial support MV at levels comparable to spontaneous breathing animals. Further, treatment of animals with a mitochondrial-targeted antioxidant prevented oxidative stress during full support MV and maintained diaphragm protein synthesis at the level of spontaneous breathing animals. We conclude that treatment with mitochondrial-targeted antioxidants or the use of partial-support MV are potential strategies to preserve diaphragm protein synthesis during prolonged MV.

Mitochondrial signaling contributes to disuse muscle atrophy

It is well established that long durations of bed rest, limb immobilization, or reduced activity in respiratory muscles during mechanical ventilation results in skeletal muscle atrophy in humans and other animals. The idea that mitochondrial damage/dysfunction contributes to disuse muscle atrophy originated over 40 years ago. These early studies were largely descriptive and did not provide unequivocal evidence that mitochondria play a primary role in disuse muscle atrophy. However, recent experiments have provided direct evidence connecting mitochondrial dysfunction to muscle atrophy. Numerous studies have described changes in mitochondria shape, number, and function in skeletal muscles exposed to prolonged periods of inactivity. Furthermore, recent evidence indicates that increased mitochondrial ROS production plays a key signaling role in both immobilization-induced limb muscle atrophy and diaphragmatic atrophy occurring during prolonged mechanical ventilation. Moreover, new evidence reveals that, during denervation-induced muscle atrophy, increased mitochondrial fragmentation due to fission is a required signaling event that activates the AMPK-FoxO3 signaling axis, which induces the expression of atrophy genes, protein breakdown, and ultimately muscle atrophy. Collectively, these findings highlight the importance of future research to better understand the mitochondrial signaling mechanisms that contribute to disuse muscle atrophy and to develop novel therapeutic interventions for prevention of inactivity-induced skeletal muscle atrophy.

Mechanistic links between oxidative stress and disuse muscle atrophy

Long periods of skeletal muscle inactivity promote a loss of muscle protein resulting in fiber atrophy. This disuse-induced muscle atrophy results from decreased protein synthesis and increased protein degradation. Recent studies have increased our insight into this complicated process, and evidence indicates that disturbed redox signaling is an important regulator of cell signaling pathways that control both protein synthesis and proteolysis in skeletal muscle. The objective of this review is to outline the role that reactive oxygen species play in the regulation of inactivity-induced skeletal muscle atrophy. Specifically, this report will provide an overview of experimental models used to investigate disuse muscle atrophy and will also highlight the intracellular sources of reactive oxygen species and reactive nitrogen species in inactive skeletal muscle. We then will provide a detailed discussion of the evidence that links oxidants to the cell signaling pathways that control both protein synthesis and degradation. Finally, by presenting unresolved issues related to oxidative stress and muscle atrophy, we hope that this review will serve as a stimulus for new research in this exciting field.

Oxidative stress alters neuronal RNA- and protein-synthesis: Implications for neural viability

Recent studies have demonstrated that impaired protein synthesis occurs in several neurodegenerative conditions associated with oxidative stress. Studies have also demonstrated that administration of oxidative stressors is sufficient to impair different and discrete regulatory aspects of protein synthesis in neural cells, with the majority of these studies focused on the effects of oxidative stressors towards initiation factors. Currently, little is known with regards to oxidative stress effects on the rates of RNA- and protein-synthesis, or the relationship between oxidant-induced impairments in RNA-/protein-synthesis to subsequent neuron death. In the present study, we demonstrate that administration of an oxidative stressor (hydrogen peroxide) induces a significant and time-dependent decrease in both RNA- and protein-synthesis in primary neurons and neural SH-SY5Y cells. Increases in RNA oxidation and disruption of ribosome complexes were selectively observed following the longer durations of oxidant exposure. Significant correlations between the loss of RNA- and protein-synthesis and the amount of oxidant-induced neuron death were also observed. Interestingly, the addition of a protein synthesis inhibitor (cycloheximide) did not significantly alter the amount of neuron death induced by the oxidative stressor. These data demonstrate that oxidant exposure promotes a time-dependent decrease in both RNA- and protein-synthesis in neurons, and demonstrate a role for elevations in RNA oxidation and ribosome dysfunction as potential mediators of impaired protein synthesis. These data also suggest that there is a complex relationship between the ability of oxidative stressors to modulate RNA- and protein-synthesis, and the ability of oxidative stressors to ultimately induce neuron death.

Roles of mitogen-activated protein kinase signal-integrating kinases 1 and 2 in oxidant-mediated eIF4E phosphorylation

Oxidative stress alters cellular metabolic processes including protein synthesis. The eukaryotic initiation factor, eIF4E, acts in the rate-limiting steps of initiation and promotes nuclear export. Phosphorylation of eIF4E by mitogen activated protein kinase signal-integrating kinases 1 and 2 (Mnk) influences the affinity of eIF4E for the 5'-mRNA cap and fosters nuclear export activity. Although phosphorylation of eIF4E on Ser209 is observed following oxidant exposure, the contribution of Mnk isoforms and the significance of phosphorylation remain elusive. Using a Mnk inhibitor and fibroblasts derived from Mnk knockout mice, we demonstrate that that H2O2 enhances eIF4E phosphorylation in cells containing Mnk1. In contrast, cells containing only Mnk2 show little change or a decrease in eIF4E phosphorylation in response to H2O2. H2O2 also shifted eIF4GI protein from the nucleus to the cytoplasm suggesting that the increases in eIF4E phosphorylation may reflect enhanced substrate availability to cytoplasmic Mnk1. In Mnk1(+/+) cells, H2O2 also enhanced eIF4E phosphorylation in the nucleus to a greater degree than in the cytoplasm, an effect not observed in cells containing Mnk2. In response to H2O2, all MEFs showed increased eIF4E:4E-BP1 and 4E-BP2:eIF4E binding and reduced eIF4E:eIF4GI binding. We also observed a dramatic increase in the amount of Mnk1 associated with eIF4E following affinity chromatography. These changes coincided with a smaller reduction in global protein synthesis in response to H2O2 in the DKO cells. These findings suggest that changes in eIF4GI distribution may enhance eIF4E phosphorylation and that the presence of either Mnk1 or 2 or any degree of eIF4E phosphorylation negatively regulates global protein synthesis in response to oxidant stress.

Evidence for non-chemical, non-electrical intercellular signaling in intestinal epithelial cells

Synchrony between mechanically separated biological systems is well known. We posed the question: can cells induce synchronous behavior in neighboring cells which are mechanically separated and which cannot communicate via chemical or electrical mechanisms. Caco-2 cell cultures were divided into three groups. "Inducer" cells were exposed to H(2)O(2). "Detector" cells were placed in separate containers near the inducer cells but were not exposed to H(2)O(2). Control cells were exposed to fresh media and were kept in a distant laboratory area. Samples were measured for total protein concentration, NFkappaB activation and structural changes, 10, 30 and 60 min after exposure respectively. Exposing inducer cells to H(2)O(2) resulted in a significant reduction in total protein content (-50%), an increase in nuclear NFkappaB activation (+38%), and structural damage (56%) compared to controls. There was a similar reduction in total protein content (-48%), increase in the nuclear fraction of NFkappaB (+35%) and structural damage (25%) in detector cells. These findings provide evidence in support of a non-chemical, non-electrical communication. This signaling system possibly plays a role in synchronous, stimulus-appropriate cell responses to noxious stimuli and may explain a number of cellular behaviors that are hard to explain based only on conventional cell signaling systems.

Oxidative stress and disuse muscle atrophy

Skeletal muscle inactivity is associated with a loss of muscle protein and reduced force-generating capacity. This disuse-induced muscle atrophy results from both increased proteolysis and decreased protein synthesis. Investigations of the cell signaling pathways that regulate disuse muscle atrophy have increased our understanding of this complex process. Emerging evidence implicates oxidative stress as a key regulator of cell signaling pathways, leading to increased proteolysis and muscle atrophy during periods of prolonged disuse. This review will discuss the role of reactive oxygen species in the regulation of inactivity-induced skeletal muscle atrophy. The specific objectives of this article are to provide an overview of muscle proteases, outline intracellular sources of reactive oxygen species, and summarize the evidence that connects oxidative stress to signaling pathways contributing to disuse muscle atrophy. Moreover, this review will also discuss the specific role that oxidative stress plays in signaling pathways responsible for muscle proteolysis and myonuclear apoptosis and highlight gaps in our knowledge of disuse muscle atrophy. By presenting unresolved issues and suggesting topics for future research, it is hoped that this review will serve as a stimulus for the expansion of knowledge in this exciting field.

Hydrogen peroxide impairs GRK2 translation via a calpain-dependent and cdk1-mediated pathway

Oxidative mechanisms of injury are involved in many neurodegenerative diseases such as stroke, ischemia-reperfusion injury and multiple sclerosis. G protein-coupled receptor kinase 2 (GRK2) plays a key role in G protein-coupled receptor (GPCR) signaling modulation, and its expression levels are decreased after brain hypoxia/ischemia and reperfusion as well as in several inflammatory conditions. We report here that hydrogen peroxide downregulates GRK2 expression in C6 rat glioma cells. The hydrogen peroxide-induced decrease in GRK2 is prevented by a calpain protease inhibitor, but does not involve increased GRK2 degradation or changes in GRK2 mRNA level. Instead we show that hydrogen peroxide treatment impairs GRK2 translation in a process that requires Cdk1 activation and involves the mTOR pathway. This novel mechanism for the control of GRK2 expression in glial cells upon oxidative stress challenge may contribute to the modulation of GPCR signaling in different pathological conditions.

N-acetyl-cysteine abolishes hydrogen peroxide-induced modification of eukaryotic initiation factor 4F activity via distinct signalling pathways

During the oxidative stress generated by hydrogen peroxide (H2O2) in nerve growth factor (NGF)-differentiated PC12 cells, eIF4E binding protein (4E-BP1) and initiation factor 4E (eIF4E) phosphorylated levels decrease significantly, and an enhancement of the association of 4E-BP1 to eIF4E, which in turn decreases eIF4F formation is observed. The treatment with N-acetyl-cysteine (NAC) completely abolishes the H2O2-induced decrease in eIF4E phosphorylated levels, whereas the decrease in 4E-BP1 phosphorylated levels and eIF4F activity inhibition are significantly but not fully reversed. Rapamycin, the mammalian target of rapamycin (FRAP/mTOR) inhibitor, prevents the effect of NAC on H2O2-induced eIF4F complex formation inhibition. Besides the inhibitor induces a similar decrease in 4E-BP1 phosphorylated levels to that promote by H2O2. However, rapamycin has no effect on the NAC-induced recovery in phosphorylated eIF4E levels. Neither the MAP kinase inhibitors, PD98056 and SB203580, or the protein phosphatase 2A inhibitor, okadaic acid, mimic NAC effect on the H2O2-induced eIF4E dephosphorylation. Altogether our findings suggest that the effects caused by oxidative stress on eIF4s factors depends on two MAP kinase-independent signal transduction pathways, being at least one of them rapamycin-dependent.

Depletion of intracellular Ca2+ store itself may be a major factor in thapsigargin-induced ER stress and apoptosis in PC12 cells

The mechanisms of intracellular calcium store depletion and store-related Ca(2+) dysregulation in relation to apoptotic cell death in PC12 cells were investigated at physiological temperatures with a leak-resistant fluorescent indicator dye Fura-PE3/AM by a cooled CCD imaging analysis system. Electron microscopic observations have shown thapsigargin (TG; 100 nM)-induced apoptosis in PC12 cells. Thorough starvation of stored Ca(2+) by BAPTA/AM (50 microM), or La(3+) (100 microM) enhanced while dantrolene (100 microM) attenuated the TG-induced apoptosis by preventing a calcium release from internal stores. An immunoblotting analysis revealed an enhanced expression of GRP78, the hallmark of endoplasmic reticulum (ER) stress when cells were treated by TG along with BAPTA/AM. These results indicate that the depletion of the intracellular Ca(2+) stores itself induces the ER stress and apoptosis in PC12 cells without any involvement of the capacitative calcium entry (CCE) or a sustained elevation of intracellular Ca(2+) concentrations ([Ca(2+)](i)).

2-Deoxyglucose and NMDA inhibit protein synthesis in neurons and regulate phosphorylation of elongation factor-2 by distinct mechanisms

Cerebral ischaemia is associated with brain damage and inhibition of neuronal protein synthesis. A deficit in neuronal metabolism and altered excitatory amino acid release may both contribute to those phenomena. In the present study, we demonstrate that both NMDA and metabolic impairment by 2-deoxyglucose or inhibitors of mitochondrial respiration inhibit protein synthesis in cortical neurons through the phosphorylation of eukaryotic elongation factor (eEF-2), without any change in phosphorylation of initiation factor eIF-2alpha. eEF-2 kinase may be activated both by Ca(2+)-independent AMP kinase or by an increase in cytosolic Ca2+. Although NMDA decreases ATP levels in neurons, only the effects of 2-deoxyglucose on protein synthesis and phosphorylation of elongation factor eEF-2 were reversed by Na(+) pyruvate. Protein synthesis inhibition by 2-deoxyglucose was not as a result of a secondary release of glutamate from cortical neurons as it was not prevented by the NMDA receptor antagonist 5-methyl-10,11-dihydro-5H-dibenzo-(a,d)-cyclohepten-5,10-imine hydrogen maleate (MK 801), nor to an increase in cytosolic-free Ca2+. Conversely, 2-deoxyglucose likely activates eEF-2 kinase through a process involving phosphorylation by AMP kinase. In conclusion, we provide evidence that protein synthesis can be inhibited by NMDA and metabolic deprivation by two distinct mechanisms involving, respectively, Ca(2+)-dependent and Ca(2+)-independent eEF-2 phosphorylation.

Glucosamine-induced phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 is mediated by the protein kinase R-like endoplasmic-reticulum associated kinase

In diabetic animals, enhanced production of vascular endothelial growth factor is thought to be a major contributor to the development of diabetic retinopathy. In the present study, glucosamine-treated R28 retinal neuronal cells were used as an experimental model system to explore the possible involvement of the hexosamine biosynthetic pathway in the diabetes-induced changes in mRNA translation. Glucosamine treatment enhanced vascular endothelial growth factor production subsequent to changes in phosphorylation of the alpha-subunit of eukaryotic initiation factor 2, with no change in vascular endothelial growth factor mRNA content. Possible mechanisms through which glucosamine might act to increase eukaryotic initiation factor 2alpha phosphorylation include enhanced O-linked glycosylation of protein kinase or phosphatase regulatory proteins and/or induction of oxidative stress. However, increasing global protein O-glycosylation through inhibition of O-beta-N-acetylglucosaminidase did not mimic the effect of glucosamine on eukaryotic initiation factor 2alpha phosphorylation. Likewise, attenuating glucosamine-induced oxidative stress with two different antioxidants did not reduce glucosamine-induced eukaryotic initiation factor 2alpha phosphorylation. Glucosamine treatment was also found to promote eukaryotic initiation factor 2alpha phosphorylation in wild-type mouse embryonic fibroblasts, but not in mouse embryonic fibroblasts lacking the eukaryotic initiation factor 2alpha kinase referred to as RNA-dependent protein kinase-like endoplasmic-reticulum associated kinase, implicating the kinase in the glucosamine-induced increase in eukaryotic initiation factor 2alpha phosphorylation. Overall, the results are consistent with glucosamine causing activation of RNA-dependent protein kinase-like endoplasmic-reticulum associated kinase, which phosphorylates eukaryotic initiation factor 2alpha and consequently upregulates translation of mRNAs encoding specific proteins, such as vascular endothelial growth factor.

Mechanisms of disuse muscle atrophy: role of oxidative stress

Prolonged periods of skeletal muscle inactivity lead to a loss of muscle protein and strength. Advances in cell biology have progressed our understanding of those factors that contribute to muscle atrophy. To this end, abundant evidence implicates oxidative stress as a potential regulator of proteolytic pathways leading to muscle atrophy during periods of prolonged disuse. This review will address the role of reactive oxygen species and oxidative stress as potential contributors to the process of disuse-mediated muscle atrophy. The first section of this article will discuss our current understanding of muscle proteases, sources of reactive oxygen in muscle fibers, and the evidence linking oxidative stress to disuse muscle atrophy. The second section of this review will highlight gaps in our knowledge relative to the specific role of oxidative stress in the regulation of disuse muscle atrophy. By discussing unresolved issues and suggesting topics for future research, it is hoped that this review will serve as a stimulus for the expansion of knowledge in this exciting field.

Hyperoxia alters the expression and phosphorylation of multiple factors regulating translation initiation

Hyperoxia is cytotoxic and depresses many cellular metabolic functions including protein synthesis. Translational control is exerted primarily during initiation by two mechanisms: 1) through inhibition of translation initiation complex formation via sequestration of the cap-binding protein, eukaryotic initiation factor (eIF) 4E, with inhibitory 4E-binding proteins (4E-BP); and 2) by prevention of eIF2-GTP-tRNA(i)(Met) formation and eIF2B activity by phosphorylated eIF2alpha. In this report, exposure of human lung fibroblasts to 95% O2 decreased the incorporation of thymidine into DNA at 6 h and the incorporation of leucine into protein beginning at 12 h. The reductions in DNA and protein synthesis were accompanied by increased phosphorylation of eIF4E protein and reduced phosphorylation of 4E-BP1. At 24 h, hyperoxia shifted 4E-BP1 phosphorylation to lesser-phosphorylated isoforms, increased eIF4E expression, and increased the association of eIF4E with 4E-BP1. Although hyperoxia did not change eIF2alpha expression, it increased its phosphorylation at Ser51, but not until 48 h. In addition, the activation of eIF2alpha was not accompanied by the formation of stress granules. These findings suggest that hyperoxia diminishes protein synthesis by increasing eIF4E phosphorylation and enhancing the affinity of 4E-BP1 for eIF4E.

Does phosphorylation of eukaryotic elongation factor eEF2 regulate protein synthesis in ischemic preconditioning?

Ischemia/reperfusion-associated translation inhibition in the hippocampus is attenuated significantly at reinitiation and elongation steps by ischemic preconditioning (Burda et al. [2003] Neurochem. Res. 28:1237-1243). To address potential regulation of the elongation step by changes in eukaryotic elongation factor 2 (eEF2) phosphorylation with and without acquired ischemic tolerance (IT), Wistar rats were preconditioned by 5-min sublethal ischemia and 2 days later, 30-min lethal ischemia was induced. Given the important role that oxidative stress plays in the ischemic process, eEF2 phosphorylation was also studied in a model of oxidative stress in vitro. Three blocks of our results support a lack of correlation between eEF2 phosphorylation status and protein synthesis rate. First, eEF2 was dephosphorylated significantly (activated) after transient cerebral ischemia in rats with and without IT or H2O2-treated cells; however, protein synthesis was significantly inhibited under these three conditions. Second, after 30-min reperfusion, the protein synthesis rate was maintained below control levels in cortex and hippocampus of rats without IT. Eukaryotic EF2 phosphorylated levels were notably low only in the cortex, whereas levels in the hippocampus were close to that of sham controls. In rats with IT, protein synthesis was virtually restored in both brain regions, but phosphorylated eEF2 levels were even higher than in rats without IT. Third, after 4-hr reperfusion, the protein synthesis rate in cortex and hippocampus was observed to be below sham control values in rats with and without IT. Conversely, phosphorylated eEF2 levels were below sham control in rats with IT and reached sham control values in rats without IT.

Myocardial ischemia and increased heart work modulate the phosphorylation state of eukaryotic elongation factor-2

Protein synthesis, in particular peptide chain elongation, is an energy-consuming biosynthetic process. AMP-activated protein kinase (AMPK) is a key regulatory enzyme involved in cellular energy homeostasis. Therefore, we tested the hypothesis that, as in liver, it could mediate the inhibition of protein synthesis by oxygen deprivation in heart by modulating the phosphorylation of eukaryotic elongation factor-2 (eEF2), which becomes inactive in its phosphorylated form. In anoxic cardiomyocytes, AMPK activation was associated with an inhibition of protein synthesis and an increase in phosphorylation of eEF2. Rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR), did not mimic the effect of oxygen deprivation to inhibit protein synthesis in cardiomyocytes or lead to eEF2 phosphorylation in perfused hearts, suggesting that AMPK activation did not inhibit mTOR/p70 ribosomal protein S6 kinase (p70S6K) signaling. Human recombinant eEF2 kinase (eEF2K) was phosphorylated by AMPK in a time- and AMP-dependent fashion, and phosphorylation led to eEF2K activation, similar to that observed in extracts from ischemic hearts. In contrast, increasing the workload resulted in a dephosphorylation of eEF2, which was rapamycin-insensitive, thus excluding a role for mTOR in this effect. eEF2K activity was unchanged by increasing the workload, suggesting that the decrease in eEF2 phosphorylation could result from the activation of an eEF2 phosphatase.

Reversible inhibition of the protein phosphatase 1 by hydrogen peroxide. Potential regulation of eIF2 alpha phosphorylation in differentiated PC12 cells

Oxidative inactivation of protein tyrosine phosphatases and calcineurin is a well established mechanism; however, little information with regard to the effect of oxidants on PP1 and PP2A activity is available. Herein, we show that PP1 activity is inhibited by H(2)O(2) treatment in differentiated PC12 cells both in vitro and in vivo experiments. Thiol-antioxidant N-acetyl-cysteine (NAC) and reduced glutathione (GSH), when added in vitro to lysates from H(2)O(2)-treated cells, reversed PP1 inhibition. H(2)O(2) treatment increased eIF2 alpha phosphorylated levels (eIF2 alpha P) in a time- and dose-dependent fashion and promoted protein synthesis inhibition. Interestingly, NAC pretreatment protected cells from H(2)O(2)-induced PP1 inactivation and, consequently, it abolished increased H(2)O(2)-induced eIF2 alpha phosphorylation and protein synthesis inhibition. In addition, PP1 inhibitor tautomycin prevented both NAC-induced PP1 reactivation and eIF2 alpha P dephosphorylation in H(2)O(2)-treated cells. Taken together, our findings support a role for PP1 in eIF2 alpha phosphorylation and oxidative stress-triggered translation down regulation.

Transforming growth factor-alpha attenuates N-methyl-D-aspartic acid toxicity in cortical cultures by preventing protein synthesis inhibition through an Erk1/2-dependent mechanism

Transforming growth factor-alpha (TGF-alpha), a ligand of the epidermal growth factor receptor, reduces the infarct size after focal cerebral ischemia in rat, but the molecular basis underlying the protection is unknown. Excitotoxicity and global inhibition of translation are acknowledged to contribute significantly to the ischemic damage. Here we studied whether TGF-alpha can rescue neurons from excitotoxicity in vitro and how it affects calcium homeostasis, protein synthesis, and the associated Akt and extracellular signal-regulated kinase 1/2 (Erk1/2) intracellular signaling pathways in mixed neuron-glia cortical cultures. We found that 100 ng/ml TGF-alpha attenuated neuronal cell death induced by a 30-min exposure to 35 microM N-methyl-D-aspartic acid (NMDA) (as it reduced lactate dehydrogenase release, propidium iodide staining, and caspase-3 activation) and decreased the elevation of intracellular Ca2+ elicited by NMDA. TGF-alpha induced a prompt and sustained phosphorylation of Erk1/2 and prevented the loss of Akt-P induced by NMDA 3 h after exposure. The protective effect of TGF-alpha was completely prevented by PD 98059, an inhibitor of the Erk1/2 pathway. Studies of incorporation of [3H]leucine into proteins showed that NMDA decreased the rate of protein synthesis, and TGF-alpha attenuated this effect. TGF-alpha stimulated the phosphorylation of the eukaryotic initiation factor 4E (eIF4E) but did not affect eIF2 alpha, two proteins involved in translation regulation. PD 98059 abrogated the TGF-alpha effect on eIF4E. Our data demonstrate that TGF-alpha exerts a neuroprotective action against NMDA toxicity, in which Erk1/2 activation plays a key role, and suggest that the underlying mechanisms involve recovery of translation inhibition, mediated at least in part by eIF4E phosphorylation.

Persistent eIF2alpha(P) is colocalized with cytoplasmic cytochrome c in vulnerable hippocampal neurons after 4 hours of reperfusion following 10-minute complete brain ischemia

Upon brain reperfusion following ischemia, there is widespread inhibition of neuronal protein synthesis that is due to phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha), which persists in selectively vulnerable neurons (SVNs) destined to die. Other investigators have shown that expression of mutant eIF2alpha (S51D) mimicking phosphorylated eIF2alpha induces apoptosis, and expression of non-phosphorylatable eIF2alpha (S51A) blocks induction of apoptosis. An early event in initiating apoptosis is the release of cytochrome c from mitochondria, and cytochrome c release corresponds to the selective vulnerability of hippocampal CA1 neurons in rats after transient global cerebral ischemia. At present the signaling pathways leading to this are not well defined. We hypothesized that persistent eIF2alpha(P) reflects injury mechanisms that are causally upstream of release of cytochrome c and induction of apoptosis. At 4 h of reperfusion following 10-min cardiac arrest, vulnerable neurons in the striatum, hippocampal hilus and CA1 showed colocalized intense immunostaining for both persistent eIF2alpha(P) and cytoplasmic cytochrome c, while resistant neurons in the dentate gyrus and elsewhere did not immunostain for either. A lower intensity of persistent eIF2alpha(P) immunostaining was present in cortical layer V pyramidal neurons without cytoplasmic cytochrome c, possibly reflecting the lesser vulnerability of this area to ischemia. We did not observe cytoplasmic cytochrome c in any neurons that did not also display persistent eIF2alpha(P) immunostaining. Because phosphorylation of eIF2alpha during early brain reperfusion is carried out by PERK, these findings suggest that there is prolonged activation of the unfolded protein response in the reperfused brain.

Temporal and spatial gene expression patterns after experimental stroke in a rat model and characterization of PC4, a potential regulator of transcription

We have used the middle cerebral artery occlusion model in the rat in combination with microarray transcript imaging to study changes in gene activity after ischemic stroke. We analyze transcriptional changes in three regions of the affected, ipsilateral brain sphere using contralateral tissues from the same animal as a control over several time points in 180 individual RNA samples. After 1 h transcription factors and signaling molecules are expressed in all tissues followed by the induction of tissue repair-related genes in the cortices which undergo regeneration. Some of these genes are turned on by PC4, which is upregulated in tissues surrounding the infarct core. Interestingly, PC4 is a nerve growth factor (NGF)-inducible gene and has been associated in earlier studies with neuronal growth processes. The expression mode of PC4, the cellular localization of the gene product, and the functional properties of downstream genes induced in vivo and in vitro using transgenic cell lines suggest that PC4 is a regulator of transcription involved in tissue regeneration after ischemic stroke. The novel experimental strategy applied here is suited to provide insight into the molecular mechanisms underlying stroke and tissue regeneration and may enable the discovery of preventive medicines.

N-Methyl-D-aspartate receptor activation inhibits protein synthesis in cortical neurons independently of its ionic permeability properties

Transient cerebral ischemia, which is accompanied by a sustained release of glutamate, strongly depresses protein synthesis. We have previously demonstrated in cortical neurons that a glutamate-induced increase in intracellular Ca(2+) is likely responsible for the blockade of the elongation step of protein synthesis. In this study, we provide evidence indicating that NMDA mobilizes a thapsigargin-sensitive pool of intracellular Ca(2+). Exposure of cortical neurons to NMDA, in the absence of external Ca(2+), produced a transient rise in intracellular Ca(2+) that was suppressed by pretreatment with thapsigargin. This rise in intracellular Ca(2+) did not result from an influx of Na(+) via reversal of the mitochondrial Na(+)/Ca(2+) exchanger since it persisted in a Na(+)-free medium or in the presence of CGP 37157, an inhibitor of the exchanger. Moreover, the NMDA-induced increase in intracellular Ca(2+) required the presence of D-serine, was blocked by D(-)-2-amino-5-phosphonopentanoic acid, but was not reduced in the presence of external Mg(2+). This unexpected non-ionotropic effect of NMDA was associated with an inhibition of protein synthesis that was also insensitive to the absence of external Ca(2+) or Na(+), or presence of Mg(2+). NMDA treatment resulted in an increase in the phosphorylation of eEF-2 in the absence or presence of external Ca(2+). The initiation step of protein synthesis was not blocked by NMDA since the phosphorylation of initiation factor eIF-2alpha subunit was not altered by NMDA treatment. In conclusion, we provide evidence indicating that NMDA can inhibit protein synthesis in cortical neurons through a process that involves the mobilization of intracellular Ca(2+) stores via a mechanism that is not linked to the ionic properties of NMDA receptors.

Involvement of double-stranded RNA-dependent protein kinase and phosphorylation of eukaryotic initiation factor-2alpha in neuronal degeneration

Inhibition of protein translation plays an important role in apoptosis. While double-stranded RNA-dependent protein kinase (PKR) is named as it is activated by double-stranded RNA produced by virus, its activation induces an inhibition of protein translation and apoptosis via the phosphorylation of the eukaryotic initiation factor 2alpha (eIF2alpha). PKR is also a stress kinase and its levels increase during ageing. Here we show that PKR activation and eIF2alpha phosphorylation play a significant role in apoptosis of neuroblastoma cells and primary neuronal cultures induced by the beta-amyloid (Abeta) peptides, the calcium ionophore A23187 and flavonoids. The phosphorylation of eIF2alpha and the number of apoptotic cells were enhanced in over-expressed wild-type PKR neuroblastoma cells exposed to Abeta peptide, while dominant-negative PKR reduced eIF2alpha phosphorylation and apoptosis induced by Abeta peptide. Primary cultured neurons from PKR knockout mice were also less sensitive to Abeta peptide toxicity. Activation of PKR and eIF2alpha pathway by Abeta peptide are triggered by an increase in intracellular calcium because the intracellular calcium chelator BAPTA-AM significantly reduced PKR phosphorylation. Taken together, these results reveal that PKR and eIF2alpha phosphorylation could be involved in the molecular signalling events leading to neuronal apoptosis and death and could be a new target in neuroprotection.

Zinc-induced inhibition of protein synthesis and reduction of connexin-43 expression and intercellular communication in mouse cortical astrocytes

Zinc released from a subpopulation of glutamatergic synapses, mainly localized in the cerebral cortex and the hippocampus, facilitates or reduces glutamatergic transmission by acting on neuronal AMPA and NMDA receptors, respectively. However, neurons are not the only targets of zinc. In the present study, we provide evidence that zinc inhibits protein synthesis in cultured astrocytes from the cerebral cortex of embryonic mice. This inhibition, which reached 85% in the presence of 100 micro m zinc, was partially and slowly reversible and resulted from the successive inhibition of the elongation and the initiation steps of the protein translation process. This was assessed by measuring the phosphorylation level of the elongation factor eEF-2 and of the alpha subunit of the initiation factor eIF-2. Due to the rapid turnover of connexin-43 that forms junction channels in cultured astrocytes, the zinc-induced decrease of protein synthesis led to a partial disappearance of connexin-43, which was associated with an inhibition of the cellular coupling in the astrocytic syncitium. In conclusion, zinc not only inhibits protein synthesis in neurons, as previously demonstrated, but also in astrocytes. The resulting decrease in the intercellular communication between astrocytes should alter the function of surrounding neurons as well as their survival.

Oxidant-induced hypertrophy of A549 cells is accompanied by alterations in eukaryotic translation initiation factor 4E and 4E-binding protein-1

Control of protein synthesis resides at the level of eukaryotic translation initiation (eIF) complex formation. Complex formation is regulated by the mRNA cap-binding protein, eIF4E, whose activity is influenced by phosphorylation and binding to 4E-binding protein 1 (4E-BP1). To provide a link between alterations in protein synthesis and the pathogenesis of oxidant-mediated lung disease, we investigated the effect of hydrogen peroxide (H2O2) on actively growing A549 cells. Cells were exposed to 200 or 400 microM H2O2 for 4 h and then assessed for changes in proliferation, protein synthesis, and eIF4E and 4E-BP1 status over 72 h. We found that both concentrations of H2O2 inhibited [3H]thymidine incorporation and cell division while inducing a G2/M-predominant growth arrest within 24 h. In addition, H2O2 increased cell size, [3H]leucine incorporation/cell, and total cell protein. Although time had little effect on eIF4E and 4E-BP1 expression and phosphorylation state of control cells, H2O2 induced a 2- to 3-fold increase in eIF4E and 4E-BP1 expression, a 5-fold increase in eIF4E phosphorylation, and a shift in the distribution of 4E-BP1 phosphorylation favoring lesser phosphorylated forms. These findings suggest that oxidant-mediated alterations in protein synthesis and cell morphology occur in concert with changes in factors known to regulate translation kinetics.

HSV-mediated delivery of virally derived anti-apoptotic genes protects the rat hippocampus from damage following excitotoxicity, but not metabolic disruption

Studies utilizing gene delivery to the nervous system indicate that various strategies are protective following acute neurological insults such as seizure and stroke. We have found that inhibitors of apoptosis are protective against excitotoxicity and heat stress but not energetic impairment in vitro. Here we studied the neuroprotective efficacy in vivo of these mediators: viral genes (crmA, p35, gamma34.5 KsBcl-2) that have evolved to suppress suicidal host responses to infection, by inhibiting apoptosis. We investigated these effects by utilizing modified herpes vectors to deliver the anti-apoptotic agents intracerebrally and examined them in the face of excitotoxic and metabolic insults. We found that p35 and gamma34.5 reduced by 45% a hippocampal CA3 lesion caused by kainic acid, while crmA and KsBcl-2 did not. None of the inhibitors protected the dentate gyrus of the hippocampus following 3-acetylpyridine, a hypoglycemia model, but we found crmA to worsen the damage. These data are similar to our results in neuronal cultures where the inhibitors protected against the excitotoxin domoic acid, but not against the metabolic poison, cyanide. Together, the results suggest that inhibitors of various apoptotic elements are capable of protecting under acute insult conditions both in vitro and in vivo, suggesting possible future therapeutic applications.