Role of amino acids in the translational control of protein synthesis in mammals

Methionine Promotes Milk Protein Synthesis via the PI3K-mTOR Signaling Pathway in Human Mammary Epithelial Cells

Breast milk is widely considered to be the most natural, safe, and complete food for infants. However, current breastfeeding rates fall short of the recommendations established by the World Health Organization. Despite this, there are few studies that have focused on the promotion of human lactation through nutrient supplementation. Therefore, the aim of this study was to investigate the effect of methionine on milk synthesis in human mammary epithelial cells (MCF-10A cells) and to explore the underlying mechanisms. To achieve this, MCF-10A cells were cultured with varying concentrations of methionine, ranging from 0 to 1.2 mM. Our results indicated that 0.6 mM of methionine significantly promoted the synthesis of milk protein. An RNA-seq analysis revealed that methionine acted through the PI3K pathway. This finding was validated through real-time quantitative polymerase chain reaction (RT-qPCR) and Western blotting. In addition, PI3K inhibition assays confirmed that methionine upregulated the expression of both mTOR and p-mTOR through activation of PI3K. Taken together, these findings suggest that methionine positively regulates milk protein synthesis in MCF-10A cells through the PI3K-mTOR signaling pathway.

Effects of Individual Amino Acids on PPARα Transactivation, mTORC1 Activation, ApoA-I Transcription and pro-ApoA-I Secretion

A higher concentration of apolipoprotein A-I (ApoA-I) is associated with increased high density lipoprotein functionality and reverse cholesterol transport (RCT). A promising strategy to prevent cardiovascular diseases is therefore to improve RCT by increasing de novo ApoA-I production. Since experimental animal models have suggested effects of amino acids on hepatic lipoprotein metabolism, we here examined the effects of different amino acids on hepatic ApoA-I production. Human hepatocytes (HepG2) were exposed to six individual amino acids for 48 h. ApoA-I transcription and secreted pro-ApoA-I protein concentrations were analyzed using quantitative polymerase chain reaction (qPCR) and enzyme-linked immunosorbent assays (ELISA), respectively. Additionally, CPT1 and KEAP1 mRNA expression, peroxisome proliferator-activated receptor alpha (PPARα) transactivation, and mechanistic target of rapamycin complex 1 (mTORC1) phosphorylation were determined. Leucine, glutamic acid, and tryptophan increased ApoA-I and CPT1 mRNA expression. Tryptophan also strongly increased PPARα transactivation. Glutamine, proline, and histidine increased pro-ApoA-I protein concentrations but mTORC1 phosphorylation remained unchanged regardless of the amino acid provided. In conclusion, individual amino acids have different effects on ApoA-I mRNA expression and pro-ApoA-I production which can partially be explained by specific effects on PPARα transactivation, while mTORC1 phosphorylation remained unaffected.

Free Amino Acid Alterations in Patients with Gynecological and Breast Cancer: A Review

Gynecological and breast cancers still remain a significant health problem worldwide. Diagnostic methods are not sensitive and specific enough to detect the disease at an early stage. During carcinogenesis and tumor progression, the cellular need for DNA and protein synthesis increases leading to changes in the levels of amino acids. An important role of amino acids in many biological pathways, including biosynthesis of proteins, nucleic acids, enzymes, etc., which serve as an energy source and maintain redox balance, has been highlighted in many research articles. The aim of this review is a detailed analysis of the literature on metabolomic studies of gynecology and breast cancers with particular emphasis on alterations in free amino acid profiles. The work includes a brief overview of the metabolomic methodology and types of biological samples used in the studies. Special attention was paid to the possible role of selected amino acids in the carcinogenesis, especially proline and amino acids related to its metabolism. There is a clear need for further research and multiple external validation studies to establish the role of amino acid profiling in diagnosing gynecological and breast cancers.

Protein metabolism and related body function: mechanistic approaches and health consequences

The development and maintenance of body composition and functions require an adequate protein intake with a continuous supply of amino acids (AA) to tissues. Body pool and AA cellular concentrations are tightly controlled and maintained through AA supply (dietary intake, recycled from proteolysis and de novo synthesis), AA disposal (protein synthesis and other AA-derived molecules) and AA losses (deamination and oxidation). Different molecular regulatory pathways are involved in the control of AA sufficiency including the mechanistic target of rapamycin complex 1, the general control non-derepressible 2/activating transcription factor 4 system or the fibroblast growth factor 21. There is a tight control of protein intake, and human subjects and animals appear capable of detecting and adapting food and protein intake and metabolism in face of foods or diets with different protein contents. A severely protein deficient diet induces lean body mass losses and ingestion of sufficient dietary energy and protein is a prerequisite for body protein synthesis and maintenance of muscle, bone and other lean tissues and functions. Maintaining adequate protein intake with age may help preserve muscle mass and strength but there is an ongoing debate as to the optimal protein intake in older adults. The protein synthesis response to protein intake can also be enhanced by prior completion of resistance exercise but this effect could be somewhat reduced in older compared to young individuals and gain in muscle mass and function due to exercise require regular training over an extended period.

Leucine Promotes Milk Synthesis in Bovine Mammary Epithelial Cells via the PI3K-DDX59 Signaling

Leucine is an essential amino acid in the milk production of bovine mammary glands, but the regulatory roles and molecular mechanisms of leucine are still not known well. This study investigated the roles of leucine on milk synthesis and explored the corresponding mechanism in bovine mammary epithelial cells (BMECs). Leucine (0, 0.25, 0.5, 0.75, 1.0, and 1.25 mM) was added to BMECs that were cultured in FBS-free OPTI-MEM medium. Leucine significantly promoted milk protein and milk fat synthesis and also increased phosphorylation of mTOR signaling protein and the protein expression levels of SREBP-1c, with the most significant effects at 0.75 mM concentration. Leucine increased the expression and nuclear localization of DDX59, and loss and gain of gene function experiments further reveal that DDX59 mediates the stimulation of leucine on the mRNA expression variation of mTOR and SREBP-1c genes. PI3K inhibition experiment further detected that leucine upregulated expression of DDX59 and its downstream signaling via PI3K activation. ChIP-qPCR analysis further proved the binding of DDX59 to the promoter regions of mTOR and SREBP-1c. In summary, these data prove that DDX59 positively regulates the mTOR and SREBP-1c signaling pathways leading to synthesis of milk, and leucine regulates these two signaling pathways through the PI3K-DDX59 signaling.

Therapeutic Benefit of Autophagy Modulation in Pompe Disease

The complexity of the pathogenic cascade in lysosomal storage disorders suggests that combination therapy will be needed to target various aspects of pathogenesis. The standard of care for Pompe disease (glycogen storage disease type II), a deficiency of lysosomal acid alpha glucosidase, is enzyme replacement therapy (ERT). Many patients have poor outcomes due to limited efficacy of the drug in clearing muscle glycogen stores. The resistance to therapy is linked to massive autophagic buildup in the diseased muscle. We have explored two strategies to address the problem. Genetic suppression of autophagy in muscle of knockout mice resulted in the removal of autophagic buildup, increase in muscle force, decrease in glycogen level, and near-complete clearance of lysosomal glycogen following ERT. However, this approach leads to accumulation of ubiquitinated proteins, oxidative stress, and exacerbation of muscle atrophy. Another approach involves AAV-mediated TSC knockdown in knockout muscle leading to upregulation of mTOR, inhibition of autophagy, reversal of atrophy, and efficient cellular clearance on ERT. Importantly, this approach reveals the possibility of reversing already established autophagic buildup, rather than preventing its development.

Voglibose-mediated alterations in neurometabolomic profiles in the hypothalamus of high-fat diet-fed mice

The alpha-glucosidase inhibitor voglibose (VO) was recently reported to have a protective effect against weight gain as well as affect various metabolic changes related to food intake and gut-brain signaling. We hypothesized that VO prevents weight gain by altering neurometabolome profiles in the hypothalamus to reduce food intake. To test this hypothesis, we assessed metabolite profiles in the hypothalamus of standard diet- or high-fat (HF) diet-fed mice in the absence or presence of VO. In total, 29 male C57BL/6 mice were divided into 3 groups: (1) lean control, (2) HF, and (3) HF + VO. Vehicle or VO was administered for 12 weeks. The results showed that there were alterations in levels of metabolites across several metabolic pathways in the hypothalamus. VO treatment increased levels of many amino acids including arginine, glutamine, histidine, isoleucine, leucine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine in the hypothalamus. In addition, levels of 2-hydroxy-2-methyl-butyric acid in the hypothalamus were significantly increased after VO administration in HF diet-fed mice. Among lipid metabolites, levels of fatty acids were higher in the hypothalamus of VO-treated mice than in that of HF diet-fed mice. In terms of the energy status, the ATP/ADP ratio was higher in the hypothalamus of VO-treated mice (P < 0.001), thereby indicating an energy surplus. In conclusion, VO supplementation altered metabolite profiles in the hypothalamus to enhance catabolism, which is possibly responsible for the hypophagic effect of VO in HF diet-fed mice.

Amino acid homeostasis and signalling in mammalian cells and organisms

Cells have a constant turnover of proteins that recycle most amino acids over time. Net loss is mainly due to amino acid oxidation. Homeostasis is achieved through exchange of essential amino acids with non-essential amino acids and the transfer of amino groups from oxidised amino acids to amino acid biosynthesis. This homeostatic condition is maintained through an active mTORC1 complex. Under amino acid depletion, mTORC1 is inactivated. This increases the breakdown of cellular proteins through autophagy and reduces protein biosynthesis. The general control non-derepressable 2/ATF4 pathway may be activated in addition, resulting in transcription of genes involved in amino acid transport and biosynthesis of non-essential amino acids. Metabolism is autoregulated to minimise oxidation of amino acids. Systemic amino acid levels are also tightly regulated. Food intake briefly increases plasma amino acid levels, which stimulates insulin release and mTOR-dependent protein synthesis in muscle. Excess amino acids are oxidised, resulting in increased urea production. Short-term fasting does not result in depletion of plasma amino acids due to reduced protein synthesis and the onset of autophagy. Owing to the fact that half of all amino acids are essential, reduction in protein synthesis and amino acid oxidation are the only two measures to reduce amino acid demand. Long-term malnutrition causes depletion of plasma amino acids. The CNS appears to generate a protein-specific response upon amino acid depletion, resulting in avoidance of an inadequate diet. High protein levels, in contrast, contribute together with other nutrients to a reduction in food intake.

The response of muscle protein synthesis following whole-body resistance exercise is greater following 40 g than 20 g of ingested whey protein

The currently accepted amount of protein required to achieve maximal stimulation of myofibrillar protein synthesis (MPS) following resistance exercise is 20–25 g. However, the influence of lean body mass (LBM) on the response of MPS to protein ingestion is unclear. Our aim was to assess the influence of LBM, both total and the amount activated during exercise, on the maximal response of MPS to ingestion of 20 or 40 g of whey protein following a bout of whole‐body resistance exercise. Resistance‐trained males were assigned to a group with lower LBM (≤65 kg; LLBM n = 15) or higher LBM (≥70 kg; HLBM n = 15) and participated in two trials in random order. MPS was measured with the infusion of ¹³C_6‐phenylalanine tracer and collection of muscle biopsies following ingestion of either 20 or 40 g protein during recovery from a single bout of whole‐body resistance exercise. A similar response of MPS during exercise recovery was observed between LBM groups following protein ingestion (20 g – LLBM: 0.048 ± 0.018%·h⁻¹; HLBM: 0.051 ± 0.014%·h⁻¹; 40 g – LLBM: 0.059 ± 0.021%·h⁻¹; HLBM: 0.059 ± 0.012%·h⁻¹). Overall (groups combined), MPS was stimulated to a greater extent following ingestion of 40 g (0.059 ± 0.020%·h⁻¹) compared with 20 g (0.049 ± 0.020%·h⁻¹; P = 0.005) of protein. Our data indicate that ingestion of 40 g whey protein following whole‐body resistance exercise stimulates a greater MPS response than 20 g in young resistance‐trained men. However, with the current doses, the total amount of LBM does not seem to influence the response.

Carbohydrate intake and resistance-based exercise: are current recommendations reflective of actual need?

Substantial research has been completed examining the impact of carbohydrate (CHO) intake on endurance exercise, whereas its role in resistance-based exercise performance, adaptation and cell signalling has yet to be fully characterised. This empirical shortcoming has precluded the ability to establish specific CHO recommendations for resistance exercise. This results in recommendations largely stemming from findings based on endurance exercise and/or anecdotal evidence despite the distinct energetic demands and molecular responses mediating adaptation from endurance- and resistance-based exercise. Moreover, the topic of CHO and exercise has become one of polarising nature with divergent views - some substantiated, others lacking evidence. Current literature suggests a moderately high daily CHO intake (3-7 g/kg per d) for resistance training, which is thought to prevent glycogen depletion and facilitate performance and adaptation. However, contemporary investigation, along with an emerging understanding of the molecular underpinnings of resistance exercise adaptation, may suggest that such an intake may not be necessary. In addition to the low likelihood of true glycogen depletion occurring in response to resistance exercise, a diet restrictive in CHO may not be detrimental to acute resistance exercise performance or the cellular signalling activity responsible for adaptation, even when muscle glycogen stores are reduced. Current evidence suggests that signalling of the mammalian target of rapamycin complex 1, the key regulatory kinase for gene translation (protein synthesis), is unaffected by CHO restriction or low muscular glycogen concentrations. Such findings may call into question the current view and subsequent recommendations of CHO intake with regard to resistance-based exercise.

Nutritional Support for Exercise-Induced Injuries

Nutrition is one method to counter the negative impact of an exercise-induced injury. Deficiencies of energy, protein and other nutrients should be avoided. Claims for the effectiveness of many other nutrients following injuries are rampant, but the evidence is equivocal. The results of an exercise-induced injury may vary widely depending on the nature of the injury and severity. Injuries typically result in cessation, or at least a reduction, in participation in sport and decreased physical activity. Limb immobility may be necessary with some injuries, contributing to reduced activity and training. Following an injury, an inflammatory response is initiated and while excess inflammation may be harmful, given the importance of the inflammatory process for wound healing, attempting to drastically reduce inflammation may not be ideal for optimal recovery. Injuries severe enough for immobilization of a limb result in loss of muscle mass and reduced muscle strength and function. Loss of muscle results from reductions in basal muscle protein synthesis and the resistance of muscle to anabolic stimulation. Energy balance is critical. Higher protein intakes (2-2.5 g/kg/day) seem to be warranted during immobilization. At the very least, care should be taken not to reduce the absolute amount of protein intake when energy intake is reduced. There is promising, albeit preliminary, evidence for the use of omega-3 fatty acids and creatine to counter muscle loss and enhance hypertrophy, respectively. The overriding nutritional recommendation for injured exercisers should be to consume a well-balanced diet based on whole, minimally processed foods or ingredients made from whole foods. The diet composition should be carefully assessed and changes considered as the injury heals and activity patterns change.

Protein Consumption and the Elderly: What Is the Optimal Level of Intake?

Maintaining independence, quality of life, and health is crucial for elderly adults. One of the major threats to living independently is the loss of muscle mass, strength, and function that progressively occurs with aging, known as sarcopenia. Several studies have identified protein (especially the essential amino acids) as a key nutrient for muscle health in elderly adults. Elderly adults are less responsive to the anabolic stimulus of low doses of amino acid intake compared to younger individuals. However, this lack of responsiveness in elderly adults can be overcome with higher levels of protein (or essential amino acid) consumption. The requirement for a larger dose of protein to generate responses in elderly adults similar to the responses in younger adults provides the support for a beneficial effect of increased protein in older populations. The purpose of this review is to present the current evidence related to dietary protein intake and muscle health in elderly adults.

The Link between Dietary Protein Intake, Skeletal Muscle Function and Health in Older Adults

Skeletal muscle mass and function are progressively lost with age, a condition referred to as sarcopenia. By the age of 60, many older adults begin to be affected by muscle loss. There is a link between decreased muscle mass and strength and adverse health outcomes such as obesity, diabetes and cardiovascular disease. Data suggest that increasing dietary protein intake at meals may counterbalance muscle loss in older individuals due to the increased availability of amino acids, which stimulate muscle protein synthesis by activating the mammalian target of rapamycin (mTORC1). Increased muscle protein synthesis can lead to increased muscle mass, strength and function over time. This review aims to address the current recommended dietary allowance (RDA) for protein and whether or not this value meets the needs for older adults based upon current scientific evidence. The current RDA for protein is 0.8 g/kg body weight/day. However, literature suggests that consuming protein in amounts greater than the RDA can improve muscle mass, strength and function in older adults.

Translational control during endoplasmic reticulum stress beyond phosphorylation of the translation initiation factor eIF2α

The accumulation of unfolded/misfolded proteins in the endoplasmic reticulum (ER) causes stress to which an unfolded protein response is activated to render cell survival or apoptosis (chronic stress). Transcriptional and translational reprogramming is tightly regulated during the unfolded protein response to ensure specific gene expression. The master regulator of this response is the PERK/eIF2α/ATF4 signaling where eIF2α is phosphorylated (eIF2α-P) by the kinase PERK. This signal leads to global translational shutdown, but it also enables translation of the transcription factor ATF4 mRNA. We showed recently that ATF4 induces an anabolic program through the up-regulation of selected amino acid transporters and aminoacyl-tRNA synthetases. Paradoxically, this anabolic program led cells to apoptosis during chronic ER stress in a manner that involved recovery from stress-induced protein synthesis inhibition. By using eIF2α-P-deficient cells as an experimental system, we identified a communicating network of signaling pathways that contribute to the inhibition of protein synthesis during chronic ER stress. This eIF2α-P-independent network includes (i) inhibition of mammalian target of rapamycin kinase protein complex 1 (mTORC1)-targeted protein phosphorylation, (ii) inhibited translation of a selective group of 5'-terminal oligopyrimidine mRNAs (encoding proteins involved in the translation machinery and translationally controlled by mTORC1 signaling), and (iii) inhibited translation of non-5'-terminal oligopyrimidine ribosomal protein mRNAs and ribosomal RNA biogenesis. We propose that the PERK/eIF2α-P/ATF4 signaling acts as a brake in the decline of protein synthesis during chronic ER stress by positively regulating signaling downstream of the mTORC1 activity. These studies advance our knowledge on the complexity of the communicating signaling pathways in controlling protein synthesis rates during chronic stress.

The SLC38 family of sodium-amino acid co-transporters

Transporters of the SLC38 family are found in all cell types of the body. They mediate Na(+)-dependent net uptake and efflux of small neutral amino acids. As a result they are particularly expressed in cells that grow actively, or in cells that carry out significant amino acid metabolism, such as liver, kidney and brain. SLC38 transporters occur in membranes that face intercellular space or blood vessels, but do not occur in the apical membrane of absorptive epithelia. In the placenta, they play a significant role in the transfer of amino acids to the foetus. Members of the SLC38 family are highly regulated in response to amino acid depletion, hypertonicity and hormonal stimuli. SLC38 transporters play an important role in amino acid signalling and have been proposed to act as transceptors independent of their transport function. The structure of SLC38 transporters is characterised by the 5 + 5 inverted repeat fold, which is observed in a wide variety of transport proteins.

GCN2 kinase is a key regulator of fibrogenesis and acute and chronic liver injury induced by carbon tetrachloride in mice

General control nonderepresible 2 (GCN2) is a highly conserved cytosolic kinase that modulates a complex response for coping with the stress owing to lack of amino acids. GCN2 has been recently shown to be involved in the regulation of metabolic balance and lipid degradation rate in the liver. We hypothesized that GCN2 could have a role in in hepatic fibrogenesis and in the response to acute or chronic liver injury. Activation of GCN2 in primary or immortalized human hepatic stellate cells by incubation with medium lacking the essential amino acid histidine correlated with decreased levels of collagen type I protein and mRNA, suggesting an antifibrogenic effect of GCN2. In vivo studies with Gcn2 knock-out mice (Gcn2(-/-)) showed increased susceptibility to both acute or chronic liver damage induced by CCl(4), as shown by higher alanine aminotransferase and aspartate aminotransferase activities, increased necrosis and higher inflammatory infiltrates compared with wild-type mice (WT). Chronic CCl(4) treatment increased deposition of interstitial collagen type I more in Gcn2(-/-) mice than in WT mice. Col1a1 and col1a2 mRNA levels also increased in CCl(4)-treated Gcn2(-/-) mice compared with WT mice. These results suggest that GCN2 is a key regulator of the fibrogenic response to liver injury.

Polysome profiling in liver identifies dynamic regulation of endoplasmic reticulum translatome by obesity and fasting

Obesity-associated metabolic complications are generally considered to emerge from abnormalities in carbohydrate and lipid metabolism, whereas the status of protein metabolism is not well studied. Here, we performed comparative polysome and associated transcriptional profiling analyses to study the dynamics and functional implications of endoplasmic reticulum (ER)–associated protein synthesis in the mouse liver under conditions of obesity and nutrient deprivation. We discovered that ER from livers of obese mice exhibits a general reduction in protein synthesis, and comprehensive analysis of polysome-bound transcripts revealed extensive down-regulation of protein synthesis machinery, mitochondrial components, and bile acid metabolism in the obese translatome. Nutrient availability also plays an important but distinct role in remodeling the hepatic ER translatome in lean and obese mice. Fasting in obese mice partially reversed the overall translatomic differences between lean and obese nonfasted controls, whereas fasting of the lean mice mimicked many of the translatomic changes induced by the development of obesity. The strongest examples of such regulations were the reduction in Cyp7b1 and Slco1a1, molecules involved in bile acid metabolism. Exogenous expression of either gene significantly lowered plasma glucose levels, improved hepatic steatosis, but also caused cholestasis, indicating the fine balance bile acids play in regulating metabolism and health. Together, our work defines dynamic regulation of the liver translatome by obesity and nutrient availability, and it identifies a novel role for bile acid metabolism in the pathogenesis of metabolic abnormalities associated with obesity.

Chronic diseases including obesity and associated metabolic abnormalities have become the greatest threat to human health worldwide. How metabolic organs and organelles adapt to nutritional fluctuations, or fail to do so, remains incompletely understood. To explore these issues, we developed a new platform to explore translational responses in the liver, a critical organ for metabolic homeostasis. In this translatomic platform, we integrated polysome profiling and global analysis of polysome-associated mRNAs to systematically quantify protein synthesis on each transcript in obesity and during fasting. Our analysis demonstrated for the first time that protein synthesis is progressively suppressed in the obese liver and that the overall translatome profile of obese liver markedly resembles that of fasting lean mice, particularly in mitochondrial function and bile metabolism. We also examined the physiological impact of some of these alterations and concluded that aberrant bile acid metabolism in the obese liver represents a novel mechanism contributing to hyperglycemia and continuous weight gain. Together, our work reveals abnormal translational regulation as a novel aspect of obesity that could impact future directions in metabolic disease treatment, and we believe translatome profiling represents a new approach to unravel complex mechanisms regulating cellular function and disease pathology.

The transcription factor network associated with the amino acid response in mammalian cells

Mammals exhibit multiple adaptive mechanisms that sense and respond to fluctuations in dietary nutrients. Consumption of reduced total dietary protein or a protein diet that is deficient in 1 or more of the essential amino acids triggers wide-ranging changes in feeding behavior and gene expression. At the level of individual cells, dietary protein deficiency is manifested as amino acid (AA) deprivation, which activates the AA response (AAR). The AAR is composed of a collection of signal transduction pathways that terminate in specific transcriptional programs designed to catalyze adaptation to the nutrient stress or, ultimately, undergo apoptosis. Independently of the AAR, endoplasmic reticulum stress activates 3 signaling pathways, collectively referred to as the unfolded protein response. The transcription factor activating transcription factor 4 is one of the terminal transcriptional mediators for both the AAR and the unfolded protein response, leading to a significant degree of overlap with regard to the target genes for these stress pathways. Over the past 5 y, research has revealed that the basic leucine zipper superfamily of transcription factors plays the central role in the AAR. Formation of both homo- and heterodimers among the activating transcription factor, CCAAT enhancer-binding protein, and FOS/JUN families of basic leucine zipper proteins forms the nucleus of a highly integrated transcription factor network that determines the initiation, magnitude, and duration of the cellular response to dietary protein or AA limitation.

Regulators of protein metabolism are affected by cyclical nutritional treatments with diets varying in protein and energy content

There is evidence that the E3 ubiquitin ligases muscle ring finger-1 (MuRF1) and atrogin-1, which mediate the ubiquitination of certain proteins and thereby their proteolysis, are regulated by cyclical nutritional treatments varying in lysine content. In order to explore further the regulatory mechanisms involved in metabolic adaptation to dietary changes, we investigated the effects of daily variations in energy [2800 (E-) followed by 3200 kcal/kg (E+)], protein [230 (P+) followed by 150g/kg (P-)] or both [E-P+ followed by E+P-] on muscle protein metabolism in 2-week-old male broiler chickens. Growth performance was similar for all treatments. Expression of atrogin-1 and MuRF1 was changed by alternation of diets varying in protein (higher expression with P- vs. P+) and energy content (higher expression with E- vs. E+). The expression of atrogin-1 was regulated with mixed diets (increase in E+P- vs. E-P+) but not that of MuRF1. Such regulation may involve the mammalian target of rapamycin (mTOR), which was more phosphorylated with P+ than with P-. Eukaryotic initiation factor 4E binding protein, p70S6 kinase and ribosomal protein S6, which are mTOR targets known to control protein synthesis, were highly activated by increased protein content (P+ vs. P-). The mechanisms coordinating the protein synthesis/proteolysis balance remain to be characterized.

An inhibitor of eIF2 activity in the sRNA pool of eukaryotic cells

Eukaryotic protein synthesis is a multi-step and highly controlled process that includes an early initiation complex containing eukaryotic initiation factor 2 (eIF2), GTP, and methionine-charged initiator methionyl-tRNA (met-tRNAi). During studies to reconstruct formation of the ternary complex containing these molecules, we detected a potent inhibitor in low molecular mass RNA (sRNA) preparations of eukaryotic tRNA. The ternary complex inhibitor (TCI) was retained in the total sRNA pool after met-tRNAi was charged by aminoacyl tRNA synthetase, co-eluted with sRNA by size exclusion chromatography, but resolved from met-tRNAi by ion exchange chromatography. The adverse effect of TCI was not overcome by high GTP or magnesium omission and was independent of GTP regeneration. Rather, TCI suppressed the rate of ternary complex formation, and disrupted protein synthesis and the accumulation of heavy polymeric ribosomes in reticulocyte lysates in vitro. Lastly, a component or components in ribosome depleted cell lysate significantly reversed TCI activity. Since assembly of the met-tRNAi/eIF2/GTP ternary complex is integral to protein synthesis, awareness of TCI is important to avoid confusion in studies of translation initiation. A clear definition of TCI may also allow a better appreciation of physiologic or pathologic situations, factors, and events that control protein synthesis in vivo.

Nutrition for acute exercise-induced injuries

BACKGROUND/AIMS

Injuries are an unavoidable aspect of participation in physical activity. Little information about nutritional support for injuries exists.

REVIEW

Immediately following injury, wound healing begins with an inflammatory response. Excessive anti-inflammatory measures may impair recovery. Many injuries result in limb immobilization. Immobilization results in muscle loss due to increased periods of negative muscle protein balance. Oxidative capacity of muscle is also decreased. Nutrient and energy deficiencies should be avoided. Energy expenditure may be reduced during immobilization, but inflammation, wound healing and the energy cost of ambulation limit the reduction of energy expenditure. There is little rationale for increasing protein intake during immobilization. There is a theoretical rationale for leucine and omega-3 fatty acid supplementation to help reduce muscle atrophy. During rehabilitation and recovery from immobilization, increased activity, in particular resistance exercise will increase muscle protein synthesis and restore sensitivity to anabolic stimuli. Ample, but not excessive, protein and energy must be consumed to support muscle growth. During rehabilitation and recovery, nutritional needs are very much like those for any athlete desiring muscle growth.

CONCLUSION

Nutrition is important for optimal wound healing. The most important consideration is to avoid malnutrition and to apply a risk/benefit approach.

Double-stranded RNA-dependent protein kinase activation modulates endotoxin-induced diaphragm weakness

Diaphragm caspase-8 activation plays a key role in modulating sepsis-induced respiratory muscle dysfunction. It is also known that double-stranded RNA-dependent protein kinase (PKR) is a regulator of caspase-8 activation in neural tissue. We tested the hypothesis that the PKR pathway modulates sepsis-induced diaphragmatic caspase-8 activation. We first evaluated the time course of diaphragm PKR activation following endotoxin administration in mice. We then determined whether administration of a PKR inhibitor (2-aminopurine) prevents endotoxin-induced diaphragm caspase-8 activation and contractile dysfunction in mice. Finally, we investigated if inhibition of PKR (using either 2-aminopurine or transfection with dominant-negative PKR) blocks caspase-8 activation in cytokine treated C₂C₁₂ cells. Endotoxin markedly activated diaphragm PKR (with increases in both active phospho-PKR protein levels, P < 0.03, and directly measured PKR activity, P < 0.01) and increased active caspase-8 levels (P < 0.01). Inhibition of PKR with 2-aminopurine prevented endotoxin-induced diaphragm caspase-8 activation (P < 0.01) and diaphragm weakness (P < 0.001). Inhibition of PKR with either 2-aminopurine or transfection with dominant-negative PKR blocked caspase-8 activation in isolated cytokine-treated C₂C₁₂ cells. These data implicate PKR activation as a major factor mediating cytokine-induced skeletal muscle caspase-8 activation and weakness.

Control of immune response by amino acid metabolism

The interaction between pathogenic microorganisms and their hosts is regulated by reciprocal survival strategies, including competition for essential nutrients. Though paradoxical, mammalian hosts have learned to take advantage of amino acid catabolism for controlling pathogen invasion and, at the same time, regulating their own immune responses. In this way, ancient catabolic enzymes have acquired novel functions and evolved into new structures with highly specialized functions, which go beyond the struggle for survival. In this review, we analyze the evidence supporting a critical role for the metabolism of various amino acids in regulating different steps of both innate and adaptive immunity.

Stimulation of muscle protein synthesis by prolonged parenteral infusion of leucine is dependent on amino acid availability in neonatal pigs

The postprandial rise in amino acids, particularly leucine, stimulates muscle protein synthesis in neonates. Previously, we showed that a 1-h infusion of leucine increased protein synthesis, but this response was not sustained for 2 h unless the leucine-induced decrease in amino acids was prevented. To determine whether a parenteral leucine infusion can stimulate protein synthesis for a more prolonged, clinically relevant period if baseline amino acid concentrations are maintained, overnight food-deprived neonatal pigs were infused for 24 h with saline, leucine (400 mumol.kg(-1). h(-1)), or leucine with replacement amino acids. Amino acid replacement prevented the leucine-induced decrease in amino acids. Muscle protein synthesis was increased by leucine but only when other amino acids were supplied to maintain euaminoacidemia. Leucine did not affect activators of mammalian target of rapamycin (mTOR), i.e. protein kinase B, AMP-activated protein kinase, tuberous sclerosis complex 2, or eukaryotic elongation factor 2. There was no effect of treatment on the association of mTOR with regulatory associated protein of mammalian target of rapamycin (raptor), G-protein beta subunit-like protein, or rictor or the phosphorylation of raptor or proline-rich Akt substrate of 40 kDa. Phosphorylation of mTOR and its downstream targets, eukaryotic initiation factor (eIF) 4E binding protein and ribosomal protein S6 kinase, and the eIF4E . eIF4G association were increased and eIF2alpha phosphorylation was reduced by leucine and was not further altered by correcting for the leucine-induced hypoaminoacidemia. Thus, prolonged parenteral infusion of leucine activates mTOR and its downstream targets in neonatal skeletal muscle, but the stimulation of protein synthesis also is dependent upon amino acid availability.

Quantitative nuclear proteomics identifies mTOR regulation of DNA damage response

Cellular nutritional and energy status regulates a wide range of nuclear processes important for cell growth, survival, and metabolic homeostasis. Mammalian target of rapamycin (mTOR) plays a key role in the cellular responses to nutrients. However, the nuclear processes governed by mTOR have not been clearly defined. Using isobaric peptide tagging coupled with linear ion trap mass spectrometry, we performed quantitative proteomics analysis to identify nuclear processes in human cells under control of mTOR. Within 3 h of inhibiting mTOR with rapamycin in HeLa cells, we observed down-regulation of nuclear abundance of many proteins involved in translation and RNA modification. Unexpectedly, mTOR inhibition also down-regulated several proteins functioning in chromosomal integrity and up-regulated those involved in DNA damage responses (DDRs) such as 53BP1. Consistent with these proteomic changes and DDR activation, mTOR inhibition enhanced interaction between 53BP1 and p53 and increased phosphorylation of ataxia telangiectasia mutated (ATM) kinase substrates. ATM substrate phosphorylation was also induced by inhibiting protein synthesis and suppressed by inhibiting proteasomal activity, suggesting that mTOR inhibition reduces steady-state (abundance) levels of proteins that function in cellular pathways of DDR activation. Finally, rapamycin-induced changes led to increased survival after radiation exposure in HeLa cells. These findings reveal a novel functional link between mTOR and DDR pathways in the nucleus potentially operating as a survival mechanism against unfavorable growth conditions.

ATF4-dependent transcription mediates signaling of amino acid limitation

Mammals respond to dietary nutrient fluctuations; for example, deficiency of dietary protein or an imbalance of essential amino acids activates an amino acid response (AAR) signal transduction pathway, consisting of detection of uncharged tRNA by the GCN2 kinase, eIF2alpha phosphorylation and ATF4 expression. In concert with heterodimerization partners, ATF4 activates specific genes via a CCAAT-enhancer binding protein-activating transcription factor response element (CARE). This review outlines the ATF4-dependent transcriptional mechanisms associated with the AAR, focusing on progress during the past 5 years. Recent evidence suggests that maternal nutrient deprivation not only has immediate metabolic effects on the fetus, but also triggers gene expression changes in adulthood, possibly through epigenetic mechanisms. Therefore, understanding the transcriptional programs initiated by amino acid limitation is crucial and timely.

Ribosomal protein S6 kinase from TOP mRNAs to cell size

Ribosomal protein S6 kinase (S6K) has been implicated in the phosphorylation of multiple substrates and is subject to activation by a wide variety of signals that converge at mammalian target of rapamycin (mTOR). In the course of the search for its physiological role, it was proposed that S6K activation and ribosomal protein S6 (rpS6) phosphorylation account for the translational activation of a subgroup of transcripts, the TOP mRNAs. The structural hallmark of these mRNAs is an oligopyrimidine tract at their 5'-terminus, known as the 5'-TOP motif. TOP mRNAs consists of about 90 members that encode multiple components of the translational machinery, such as ribosomal proteins and translation factors. The translation efficiency of TOP mRNAs indeed correlates with S6K activation and rpS6 phosphorylation, yet recent biochemical and genetic studies have established that, although S6K and TOP mRNAs respond to similar signals and are regulated by mTOR, they maintain no cause and effect relationship. Instead, S6K is primarily involved in regulation of cell size, and affects glucose homeostasis, but is dispensable for global protein synthesis, whereas translational efficiency of TOP mRNAs is a determinant of the cellular protein synthesis capacity. Despite extensive studies of their function and mode of regulation, the mechanism underlying the effect of S6K on the cell size, as well as the trans-acting factor that mediates the translational control of TOP mRNAs, still await their identification.

Insulin regulates milk protein synthesis at multiple levels in the bovine mammary gland

The role of insulin in milk protein synthesis is unresolved in the bovine mammary gland. This study examined the potential role of insulin in the presence of two lactogenic hormones, hydrocortisone and prolactin, in milk protein synthesis. Insulin was shown to stimulate milk protein gene expression, casein synthesis and (14)C-lysine uptake in mammary explants from late pregnant cows. A global assessment of changes in gene expression in mammary explants in response to insulin was undertaken using Affymetrix microarray. The resulting data provided insight into the molecular mechanisms stimulated by insulin and showed that the hormone stimulated the expression of 28 genes directly involved in protein synthesis. These genes included the milk protein transcription factor, ELF5, translation factors, the folate metabolism genes, FOLR1 and MTHFR, as well as several genes encoding enzymes involved in catabolism of essential amino acids and biosynthesis of non-essential amino acids. These data show that insulin is not only essential for milk protein gene expression, but stimulates milk protein synthesis at multiple levels within bovine mammary epithelial cells.

Role of sulfur amino acids in controlling nutrient metabolism and cell functions: implications for nutrition

Protein synthesis is affected when an insufficient level of sulfur amino acids is available. This defect may originate from dietary amino acid deficiency and/or excessive amino acid utilisation for other purposes such as the synthesis of glutathione and acute-phase proteins during catabolic stress. Sulfur amino acids are recognised to exert other significant functions since they are precursors of essential molecules, are involved in the methylation process, participate in the control of oxidative status, and may act as mediators affecting metabolism and cell functions. Despite this increased understanding of the role of sulfur amino acids, many questions still remain unanswered due to the complexity of the mechanisms involved. Moreover, surprising effects of dietary sulfur amino acids have been reported, with the development of disorders in cases of both deficiency and excess. These findings indicate the importance of defining adequate levels of intake and providing a rationale for nutritional advice. The aim of the present review is to provide an overview on the roles of sulfur amino acids as regulators of nutrient metabolism and cell functions, with emphasis placed on the implications for nutrition.

Specificity of amino acid regulated gene expression: analysis of genes subjected to either complete or single amino acid deprivation

Amino acid deprivation activates the amino acid response (AAR) pathway that enhances transcription of genes containing an amino acid response element (AARE). The present data reveal a quantitative difference in the response to deprivation of individual amino acids. The AAR leads to increased eukaryotic initiation factor 2alpha (eIF2alpha) phosphorylation and ATF4 translation. When HepG2 cells were deprived of an individual essential amino acid, p-eIF2alpha and activating transcription factor 4 were increased, but the correlation was relatively weak. Complete amino acid starvation in either Earle's balanced salt solution or Krebs-Ringer bicarbonate buffer (KRB) resulted in activation of transcription driven by a SNAT2 genomic fragment that contained an AARE. However, for the KRB, a proportion of the transcription was AARE-independent suggesting that amino acid-independent mechanisms were responsible. Therefore, activation of AARE-driven transcription is triggered by a deficiency in any one of the essential amino acids, but the response is not uniform. Furthermore, caution must be exercised when using a medium completely devoid of amino acids.

Leucine stimulates protein synthesis in skeletal muscle of neonatal pigs by enhancing mTORC1 activation

Skeletal muscle in the neonate grows at a rapid rate due in part to an enhanced sensitivity to the postprandial rise in amino acids, particularly leucine. To elucidate the molecular mechanism by which leucine stimulates protein synthesis in neonatal muscle, overnight-fasted 7-day-old piglets were treated with rapamycin [an inhibitor of mammalian target of rapamycin (mTOR) complex (mTORC)1] for 1 h and then infused with leucine for 1 h. Fractional rates of protein synthesis and activation of signaling components that lead to mRNA translation were determined in skeletal muscle. Rapamycin completely blocked leucine-induced muscle protein synthesis. Rapamycin markedly reduced raptor-mTOR association, an indicator of mTORC1 activation. Rapamycin blocked the leucine-induced phosphorylation of mTOR, S6 kinase 1 (S6K1), and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1) and formation of the eIF4E.eIF4G complex and increased eIF4E.4E-BP1 complex abundance. Rapamycin had no effect on the association of mTOR with rictor, a crucial component for mTORC2 activation, or G protein beta-subunit-like protein (GbetaL), a component of mTORC1 and mTORC2. Neither leucine nor rapamycin affected the phosphorylation of AMP-activated protein kinase (AMPK), PKB, or tuberous sclerosis complex (TSC)2, signaling components that reside upstream of mTOR. Eukaryotic elongation factor (eEF)2 phosphorylation was not affected by leucine or rapamycin, although current dogma indicates that eEF2 phosphorylation is mTOR dependent. Together, these in vivo data suggest that leucine stimulates muscle protein synthesis in neonates by enhancing mTORC1 activation and its downstream effectors.

Insulin signaling and the general amino acid control response. Two distinct pathways to amino acid synthesis and uptake

ATF4 is a transcription factor that induces a genetic program for amino acid synthesis and amino acid uptake. Previous work demonstrated that ATF4 expression is increased either by insulin or by the general amino acid control (GAAC) response, an evolutionarily ancient pathway that is activated when eukaryotic cells are deprived of amino acids. It is not known whether insulin and the GAAC pathway increase ATF4 expression by the same or different mechanisms. In these studies, we demonstrate that insulin-mediated ATF4 expression occurs as part of a coordinated anabolic program that does not require an essential component of the GAAC pathway, the protein kinase GCN2. Moreover, insulin and the GAAC pathway have an additive effect on expression of ATF4 and downstream mRNAs for amino acid synthesis and uptake. These data suggest that the GAAC pathway may facilitate insulin-mediated anabolism when exogenous amino acids are limiting. We conclude that insulin signaling and the GAAC response comprise two distinct yet complimentary pathways to ATF4 expression, allowing anabolism to be finely tuned to amino acid availability.

Mechanisms involved in the nutritional regulation of mRNA translation: features of the avian model

Abstract:Insulin and amino acids are key factors in regulating protein synthesis. The mechanisms of their action have been widely studied for several years. The insulin signal is mediated by the activation of intracellular kinases such as phosphatidylinositol–3'kinase and the mammalian target of rapamycin (mTOR), affecting the phosphorylation of some major effectors involved in the regulation of translation initiation, i.e. p70 S6 kinase (p70S6K) and the translational repressor eukaryotic initiation factor 4E binding protein (4E-BP1). The amino acid–induced signalling cascade also originates from mTOR and promotes p70S6K and 4E–BP1 activation. However, the mechanisms of regulation are complex and little understood, especiallyin vivo. Elucidating these mechanisms is important for both fundamental physiology and nutritional applications, i.e. better control of the use of nutrients and optimisation of dietary amino acid supplies in various physiological and physiopathological situations. In comparative physiology, the chicken is an interesting model to gain better understanding of the nutritional regulation of mRNA translation because of the very high rates of muscle growth and protein synthesis, and the unusual features compared with mammals. In the present review we provide an overview of the roles of insulin and amino acids as regulators of protein synthesis in both mammals and avian species.

Metabolic mechanisms of cancer-induced inhibition of immune responses

During progression, tumors become refractory to the offensive weapons of the immune system. It has been known for a long time that the tumor microenvironment presents a profound modification in the metabolism of arachidonic acid and amino acids such as l-triptophan and l-arginine. However, only in the last decade we have started to appreciate how these changes might cause dysfunctions in cells of both adaptive and innate immune system. The knowledge of these complex and partially interconnected metabolic pathways is offering new targets for an integrated pharmacological approach aiming at freeing tumor-specific T lymphocytes from the latches of cancer influence.

Retrograde nuclear accumulation of cytoplasmic tRNA in rat hepatoma cells in response to amino acid deprivation

Until recently, transport of tRNA was presumed to be unidirectional, from the nucleus to the cytoplasm. Our published findings, however, revealed that cytoplasmic tRNAs move retrograde to the nucleus in Saccharomyces cerevisiae and that nuclear accumulation of cytoplasmic tRNAs occurs when cells are nutrient deprived. The findings led us to examine whether retrograde nuclear accumulation of cytoplasmic tRNAs occurs in higher eukaryotes. Using RNA FISH and Northern and Western analyses we show that tRNAs accumulate in nuclei of a hepatoma cell line in response to amino acid deprivation. To discern whether tRNA nuclear accumulation results from nuclear import of cytoplasmic tRNAs, transcription of new RNAs was inhibited, and the location of "old" tRNAs in response to nutrient stress was determined. Even in the absence of new RNA synthesis, there were significant tRNA nuclear pools after amino acid depletion, providing strong evidence that retrograde traffic is responsible for the tRNA nuclear pools. Further analyses showed that retrograde tRNA nuclear accumulation in hepatoma cells is a reversible and energy-dependent process. The data provide evidence for retrograde tRNA nuclear accumulation in intact mammalian cells and support the hypothesis that nuclear accumulation of cytoplasmic tRNA and tRNA re-export to the cytoplasm may constitute a universal mechanism for posttranscriptional regulation of global gene expression in response to nutrient availability.

Role of the transcription factor ATF4 in the anabolic actions of insulin and the anti-anabolic actions of glucocorticoids

In most mammalian cells, insulin and glucocorticoids promote anabolism and catabolism, respectively. Whereas the opposing effects of insulin and glucocorticoids on catabolic gene expression have been explained at the molecular level, comparatively little is known about how these hormones alter anabolic gene expression. These studies identify ATF4 as an anabolic transcription factor that is repressed by glucocorticoids and induced by insulin. Insulin-mediated induction of ATF4 required the mammalian target of rapamycin complex 1, was required for the activation of a genetic program for the cellular uptake of essential amino acids and the synthesis of nonessential amino acids and aminoacyl-tRNAs, and was coupled to the repression of Foxo-dependent genes needed for protein and lipid catabolism. These results suggest that ATF4 plays a central role in hormonal regulation of amino acid and protein anabolism by coupling amino acid uptake and synthesis, as well as the generation of charged tRNAs, to mammalian target of rapamycin complex 1-mediated mRNA translation.

Of bears, frogs, meat, mice and men: complexity of factors affecting skeletal muscle mass and fat

Extreme loss of skeletal muscle mass (atrophy) occurs in human muscles that are not used. In striking contrast, skeletal muscles do not rapidly waste away in hibernating mammals such as bears, or aestivating frogs, subjected to many months of inactivity and starvation. What factors regulate skeletal muscle mass and what mechanisms protect against muscle atrophy in some species? Severe atrophy also occurs with ageing and there is much clinical interest in reducing such loss of muscle mass and strength (sarcopenia). In the meat industry, a key aim is optimizing the control of skeletal muscle growth and meat quality. The impaired response of muscle to insulin resulting in diabetes, that is a consequence of the metabolic impact of increasing obesity and fat deposition in humans, is also of increasing clinical concern. Intensive research in these fields, combined with mouse models, is reviewed with respect to the molecular control of muscle growth (myogenesis) and atrophy/hypertrophy and fat deposition (adipogenesis) in skeletal muscle, with a focus on IGF-1/insulin signaling.

Role of amino acids in insulin signaling in adipocytes and their potential to decrease insulin resistance of adipose tissue

Recently, our knowledge concerning the role of amino acids in signal transduction in mammals has greatly improved. This significant advance is mainly due to the remarkable discovery that the mammalian target of rapamycin (mTOR) protein kinase, known to be activated in response to a large number of hormones, growth factors and cytokines, is also under the tight control of branched-chain amino acids. Actually, both inputs are necessary to fully activate the mTOR pathway, the main function of which is to increase cell size, via the regulation of translational processes. However, amino acids are able to modulate other biological effects and appear to have unexpected actions, as evidenced by our recent work in rat adipocytes. The aim of this review is to summarize novel findings on the role of mTOR and amino acids in insulin signaling in adipocytes. A possible beneficial impact of the use of amino acids in the treatment of insulin resistance is discussed, and hypotheses about the molecular mechanisms underlying their effect are proposed.

Intravenous administration of amino acids during anesthesia stimulates muscle protein synthesis and heat accumulation in the body

The present study was conducted to determine the contribution of muscle protein synthesis to the prevention of anesthesia-induced hypothermia by intravenous administration of an amino acid (AA) mixture. We examined the changes of intraperitoneal temperature (Tcore) and the rates of protein synthesis (K(s)) and the phosphorylation states of translation initiation regulators and their upstream signaling components in skeletal muscle in conscious (Nor) or propofol-anesthetized (Ane) rats after a 3-h intravenous administration of a balanced AA mixture or saline (Sal). Compared with Sal administration, the AA mixture administration markedly attenuated the decrease in Tcore in rats during anesthesia, whereas Tcore in the Nor-AA group became slightly elevated during treatment. Stimulation of muscle protein synthesis resulting from AA administration was observed in each case, although K(s) remained lower in the Ane-AA group than in the Nor-Sal group. AA administration during anesthesia significantly increased insulin concentrations to levels approximately 6-fold greater than in the Nor-AA group and enhanced phosphorylation of eukaryotic initiation factor 4E-binding protein-1 (4E-BP1) and ribosomal protein S6 protein kinase relative to all other groups and treatments. The alterations in the Ane-AA group were accompanied by hyperphosphorylation of protein kinase B and the mammalian target of rapamycin (mTOR). These results suggest that administration of an AA mixture during anesthesia stimulates muscle protein synthesis via insulin-mTOR-dependent activation of translation initiation regulators caused by markedly elevated insulin and, thereby, facilitates thermal accumulation in the body.

Amino acid starvation induces the SNAT2 neutral amino acid transporter by a mechanism that involves eukaryotic initiation factor 2alpha phosphorylation and cap-independent translation

Nutritional stress caused by amino acid starvation involves a coordinated cellular response that includes the global decrease of protein synthesis and the increased production of cell defense proteins. Part of this response is the induction of transport system A for neutral amino acids that leads to the recovery of cell volume and amino acid levels once extracellular amino acid availability is restored. Hypertonic stress also increases system A activity as a mechanism to promote a rapid recovery of cell volume. Both a starvation-dependent and a hypertonic increase of system A transport activity are due to the induction of SNAT2, the ubiquitous member of SLC38 family. The molecular mechanisms underlying SNAT2 induction were investigated in tissue culture cells. We show that the increase in system A transport activity and SNAT2 mRNA levels upon amino acid starvation were blunted in cells with a mutant eIF2alpha that cannot be phosphorylated. In contrast, the induction of system A activity and SNAT2 mRNA levels by hypertonic stress were independent of eIF2alpha phosphorylation. The translational control of the SNAT2 mRNA during amino acid starvation was also investigated. It is shown that the 5'-untranslated region contains an internal ribosome entry site that is constitutively active in amino acid-fed and -deficient cells and in a cell-free system. We also show that amino acid starvation caused a 2.5-fold increase in mRNA and protein expression from a reporter construct containing both the SNAT2 intronic amino acid response element and the SNAT2-untranslated region. We conclude that the adaptive response of system A activity to amino acid starvation requires eukaryotic initiation factor 2alpha phosphorylation, increased gene transcription, and internal ribosome entry site-mediated translation. In contrast, the response to hypertonic stress does not involve eukaryotic initiation factor 2alpha phosphorylation, suggesting that SNAT2 expression can be modulated by specific signaling pathways in response to different stresses.

Determinants of Disuse-Induced Skeletal Muscle Atrophy: Exercise and Nutrition Countermeasures to Prevent Protein Loss

Muscle atrophy results from a variety of conditions such as disease states, neuromuscular injuries, disuse, and aging. Absence of gravitational loading during spaceflight or long-term bed rest predisposes humans to undergo substantial loss of muscle mass and, consequently, become unfit and/or unhealthy. Disuse- or inactivity-induced skeletal muscle protein loss takes place by differential modulation of proteolytic and synthetic systems. Transcriptional, translational, and posttranslational events are involved in the regulation of protein synthesis and degradation in myofibers, and these regulatory events are known to be responsive to contractile activity. However, regardless of the numerous studies which have been performed, the intracellular signals that mediate skeletal muscle wasting due to muscular disuse are not completely comprehended. Understanding the triggers of atrophy and the mechanisms that regulate protein loss in unloaded muscles may lead to the development of effective countermeasures such as exercise and dietary intervention. The objective of the present review is to provide a window into the molecular processes that underlie skeletal muscle remodeling and to examine what we know about exercise and nutrition countermeasures designed to minimize muscle atrophy.

Molecular mechanisms responsible for alcohol-induced myopathy in skeletal muscle and heart

Chronic alcohol abuse has the potential to modulate striated muscle physiology and function. The skeletal muscle alcoholic myopathy is characterized by muscle weakness and difficulties in gait and locomotion, while chronic alcohol consumption ultimately leads to a decrease in cardiac contractility and output. In both tissues a loss of protein mass results in part from a decreased protein synthesis that initially manifests as a defect in translational efficiency. This review focuses on recent developments in understanding the cellular and molecular mechanisms by which alcohol impairs mRNA translation in skeletal and cardiac muscle, including identification of the signaling pathways and biochemical sites negatively impacted. Defective signaling potentially results from resistance to the normal stimulating effects of anabolic hormones (insulin and insulin-like growth factor-I) and nutrients (leucine) as well as increased production of several negative regulators of muscle mass. Overall, the biochemical mechanisms contributing to the pathogenesis of loss of skeletal and cardiac muscle are reviewed.