Amino acid-induced stimulation of translation initiation in rat skeletal muscle

Mammalian Target of Rapamycin (mTOR) Signaling at the Crossroad of Muscle Fiber Fate in Sarcopenia

The mammalian target of rapamycin (mTOR) is a major regulator of skeletal myocyte viability. The signaling pathways triggered by mTOR vary according to the type of endogenous and exogenous factors (e.g., redox balance, nutrient availability, physical activity) as well as organismal age. Here, we provide an overview of mTOR signaling in skeletal muscle, with a special focus on the role played by mTOR in the development of sarcopenia. Intervention strategies targeting mTOR in sarcopenia (e.g., supplementation of plant extracts, hormones, inorganic ions, calorie restriction, and exercise) have also been discussed.

Effect of using amino acids in the freeze-drying of siRNA lipoplexes on gene knockdown in cells after reverse transfection

Recently, small interfering RNA (siRNA)/cationic liposome complexes (siRNA lipoplexes) have become a crucial research tool for studying gene function. Easy and reliable siRNA transfection with a large set of siRNAs is required for the successful screening of gene function. Reverse (Rev)-transfection with freeze-dried siRNA lipoplexes is validated for siRNA transfection with a large set of siRNAs in a multi-well plate. In our previous study, it was shown that Rev-transfection with siRNA lipoplexes freeze-dried in disaccharides or trisaccharides resulted in long-term stability with a high level of gene-knockdown activity. In the present study, the effects of amino acids used as cryoprotectants in the freeze-drying of siRNA lipoplexes on gene knockdown via Rev-transfection were assessed. A total of 15 types of amino acids were used to prepare freeze-dried siRNA lipoplexes, and it was found that the freeze-drying of siRNA lipoplexes with amino acid concentrations >100 mM strongly suppressed targeted gene expression regardless of the amino acid type; however, some amino acids strongly upregulated or downregulated gene expression in the cells transfected with negative control siRNA. Amongst the amino acids tested, the presence of asparagine showed specific gene-knockdown activity, forming large cakes after freeze-drying and retaining a favorable siRNA lipoplex size after rehydration. These findings provide valuable information regarding amino acids as cryoprotectants for Rev-transfection using freeze-dried siRNA lipoplexes for the efficient delivery of siRNA into cells.

The distribution of eukaryotic initiation factor 4E after bouts of resistance exercise is altered by shortening of recovery periods

Insufficient duration of recovery between resistance exercise bouts reduces the effects of exercise training, but the influence on muscle anabolic responses is not fully understood. Here, we investigated the changes in the distribution of eukaryotic initiation factor (eIF) 4E, a key regulator of translation initiation, and related factors in mouse skeletal muscle after three successive bouts of resistance exercise with three durations of recovery periods (72 h: conventional, 24 h: shorter, and 8 h: excessively shorter). Bouts of resistance exercise dissociated eIF4E from eIF4E binding protein 1, with the magnitude increasing with shorter recovery. Whereas bouts of resistance exercise with 72 h recovery increased the association of eIF4E and eIF4G, those with shorter recovery did not. Similar results were observed in muscle protein synthesis. These results suggest that insufficient recovery inhibited the association of eIF4E and eIF4G, which might cause attenuation of protein synthesis activation after bouts of resistance exercise.

Effects of long-term exposures to low iron and branched-chain amino acid containing diets on aging skeletal muscle of Fisher 344 × Brown Norway rats

Aging skeletal muscle displays an altered iron status that may promote oxidative stress and sarcopenia. A diet containing low iron (LI) could reduce muscle iron status and attenuate age-related muscle atrophy. Supplemental branched-chain amino acids (BCAA) may also alleviate sarcopenia by promoting muscle protein synthesis and iron status improvement. This study examined individual and combined effects of LI and BCAA diets on anabolic signaling and iron status in skeletal muscle of aging rats. Twenty-nine-month-old male Fisher 344 × Brown Norway rats consumed the following control-base diets: control + regular iron (35 mg iron/kg) (CR; n = 11); control + LI (∼6 mg iron/kg) (CL; n = 11); 2×BCAA + regular iron (BR; n = 10); and 2×BCAA + LI (BL; n = 12) for 12 weeks. Although LI and/or 2×BCAA did not affect plantaris muscle mass, 2×BCAA groups showed lower muscle iron content than did CR and CL groups (P < 0.05). p70 ribosomal protein S6 kinase phosphorylation was greater in 2×BCAA and LI animals compared with CR animals (P < 0.05). Interactions between IRON and BCAA were observed for proteins indicative of mitochondrial biogenesis (peroxisome proliferator-activated receptor gamma coactivator 1 alpha) and oxidative capacity (cytochrome c oxidase subunit 2 and citrate synthase) (P < 0.05) wherein the combined diet (BL) negated potential benefits of individual diets. Antioxidant capacity, superoxide dismutase activity, and oxidative injury (3-nitrotyrosine, protein carbonyls, and 4-hydroxynonenal) were similar between groups. In conclusion, 12 weeks of LI and 2×BCAA diets showed significant impacts on increasing anabolic signaling as well as ameliorating iron status; however, these interventions did not affect muscle mass.

Prediction and analysis of essential genes using the enrichments of gene ontology and KEGG pathways

Identifying essential genes in a given organism is important for research on their fundamental roles in organism survival. Furthermore, if possible, uncovering the links between core functions or pathways with these essential genes will further help us obtain deep insight into the key roles of these genes. In this study, we investigated the essential and non-essential genes reported in a previous study and extracted gene ontology (GO) terms and biological pathways that are important for the determination of essential genes. Through the enrichment theory of GO and KEGG pathways, we encoded each essential/non-essential gene into a vector in which each component represented the relationship between the gene and one GO term or KEGG pathway. To analyze these relationships, the maximum relevance minimum redundancy (mRMR) was adopted. Then, the incremental feature selection (IFS) and support vector machine (SVM) were employed to extract important GO terms and KEGG pathways. A prediction model was built simultaneously using the extracted GO terms and KEGG pathways, which yielded nearly perfect performance, with a Matthews correlation coefficient of 0.951, for distinguishing essential and non-essential genes. To fully investigate the key factors influencing the fundamental roles of essential genes, the 21 most important GO terms and three KEGG pathways were analyzed in detail. In addition, several genes was provided in this study, which were predicted to be essential genes by our prediction model. We suggest that this study provides more functional and pathway information on the essential genes and provides a new way to investigate related problems.

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.

Disruption of REDD1 gene ameliorates sepsis-induced decrease in mTORC1 signaling but has divergent effects on proteolytic signaling in skeletal muscle

Sepsis-induced skeletal muscle atrophy and weakness are due in part to decreased mTORC1-mediated protein synthesis and increased proteolysis via the autophagy-lysosomal system and ubiquitin-proteasome pathway. The REDD1 (regulated in development and DNA damage-1) protein is increased in sepsis and can negatively regulate mTORC1 activity. However, the contribution of REDD1 to the sepsis-induced change in muscle protein synthesis and degradation has not been determined. Sepsis was produced by cecal ligation and puncture in female REDD1(-/-) or wild-type (WT) mice, and end points were assessed 24 h later in gastrocnemius; time-matched, pair-fed controls of each genotype were included. Sepsis increased REDD1 protein 300% in WT mice, whereas REDD1 was absent in REDD1(-/-) muscle. Sepsis decreased protein synthesis and phosphorylation of downstream targets of mTORC1 (S6K1 Thr(389), rpS6 Ser(240/244), 4E-BP1 Ser(65)) in WT but not REDD1(-/-) mice. However, Akt and PRAS40 phosphorylation was suppressed in both sham and septic muscle from REDD1(-/-) mice despite unaltered PDK1, PP2A, or TSC2 expression. Sepsis increased autophagy as indicated by decreased ULK1 Ser(757) phosphorylation and p62 abundance and increased LC3B-II/I in WT mice, whereas these changes were absent in septic REDD1(-/-) mice. Conversely, REDD1 deletion did not prevent the sepsis-induced decrease in IGF-I mRNA or the concomitant increase in IL-6, TNFα, MuRF1, and atrogin1 mRNA expression. Lastly, 5-day survival in a separate set of septic mice did not differ between WT and REDD1(-/-) mice. These data highlight the central role of REDD1 in regulating both protein synthesis and autophagy in skeletal muscle during sepsis.

Alcohol impairs skeletal muscle protein synthesis and mTOR signaling in a time-dependent manner following electrically stimulated muscle contraction

Alcohol (EtOH) decreases protein synthesis and mammalian target of rapamycin (mTOR)-mediated signaling and blunts the anabolic response to growth factors in skeletal muscle. The purpose of the current investigation was to determine whether acute EtOH intoxication antagonizes the contraction-induced increase in protein synthesis and mTOR signaling in skeletal muscle. Fasted male mice were injected intraperitoneally with 3 g/kg EtOH or saline (control), and the right hindlimb was electrically stimulated (10 sets of 6 contractions). The gastrocnemius muscle complex was collected 30 min, 4 h, or 12 h after stimulation. EtOH decreased in vivo basal protein synthesis (PS) in the nonstimulated muscle compared with time-matched Controls at 30 min, 4 h, and 12 h. In Control, but not EtOH, PS was decreased 15% after 30 min. In contrast, PS was increased in Control 4 h poststimulation but remained unchanged in EtOH. Last, stimulation increased PS 10% in Control and EtOH at 12 h, even though the absolute rate remained reduced by EtOH. The stimulation-induced increase in the phosphorylation of S6K1 Thr(421)/Ser(424) (20-52%), S6K1 Thr(389) (45-57%), and its substrate rpS6 Ser(240/244) (37-72%) was blunted by EtOH at 30 min, 4 h, and 12 h. Phosphorylation of 4E-BP1 Ser(65) was also attenuated by EtOH (61%) at 4 h. Conversely, phosphorylation of extracellular signal-regulated kinase Thr(202)/Tyr(204) was increased by stimulation in Control and EtOH mice at 30 min but only in Control at 4 h. Our data indicate that acute EtOH intoxication suppresses muscle protein synthesis for at least 12 h and greatly impairs contraction-induced changes in synthesis and mTOR signaling.

Protein and Essential Amino Acids to Protect Musculoskeletal Health during Spaceflight: Evidence of a Paradox?

Long-duration spaceflight results in muscle atrophy and a loss of bone mineral density. In skeletal muscle tissue, acute exercise and protein (e.g., essential amino acids) stimulate anabolic pathways (e.g., muscle protein synthesis) both independently and synergistically to maintain neutral or positive net muscle protein balance. Protein intake in space is recommended to be 12%-15% of total energy intake (≤1.4 g∙kg-1∙day-1) and spaceflight is associated with reduced energy intake (~20%), which enhances muscle catabolism. Increasing protein intake to 1.5-2.0 g∙kg-1∙day-1 may be beneficial for skeletal muscle tissue and could be accomplished with essential amino acid supplementation. However, increased consumption of sulfur-containing amino acids is associated with increased bone resorption, which creates a dilemma for musculoskeletal countermeasures, whereby optimizing skeletal muscle parameters via essential amino acid supplementation may worsen bone outcomes. To protect both muscle and bone health, future unloading studies should evaluate increased protein intake via non-sulfur containing essential amino acids or leucine in combination with exercise countermeasures and the concomitant influence of reduced energy intake.

Leucine-rich diet supplementation modulates foetal muscle protein metabolism impaired by Walker-256 tumour

BACKGROUND

Cancer-cachexia induces a variety of metabolic disorders of protein turnover and is more pronounced when associated with pregnancy. Tumour-bearing pregnant rats have impaired protein balance, which decreases protein synthesis and increases muscle breakdown. Because branched-chain amino acids, especially leucine, stimulate protein synthesis, we investigated the effect of a leucine-rich diet on protein metabolism in the foetal gastrocnemius muscles of tumour-bearing pregnant rats.

METHODS

Foetuses of pregnant rats with or without Walker 256 tumours were divided into six groups. During the 20 days of the experiment, the pregnant groups were fed with either a control diet (C, control rats; W, tumour-bearing rats; Cp, rats pair-fed the same normoprotein-diet as the W group) or with a leucine-rich diet (L, leucine rats; LW, leucine tumour-bearing rats; and Lp, rats pair-fed the same leucine-rich diet as the LW group). After the mothers were sacrificed, the foetal gastrocnemius muscle samples were resected, and the protein synthesis and degradation and tissue chymotrypsin-like, cathepsin and calpain enzyme activities were assayed. The muscle oxidative enzymes (catalase, glutathione-S-transferase and superoxide dismutase), alkaline phosphatase enzyme activities and lipid peroxidation (malondialdehyde) were also measured.

RESULTS

Tumour growth led to a reduction in foetal weight associated with decreased serum protein, albumin and glucose levels and low haematocrit in the foetuses of the W group, whereas in the LW foetuses, these changes were less pronounced. Muscle protein synthesis (measured by L-[3H]-phenylalanine incorporation) was reduced in the W foetuses but was restored in the LW group. Protein breakdown (as assessed by tyrosine release) was enhanced in the L and W groups, but chymotrypsin-like activity increased only in group W and tended toward an increase in the LW foetuses. The activity of cathepsin H was significantly higher in the W group foetuses, but the proteolytic calcium-dependent pathway showed similar enzyme activity. In parallel, an intense oxidative stress process was observed only in the group W foetuses.

CONCLUSIONS

These data suggested that the proteasomal and lysosomal proteolytic pathways and oxidative stress are likely to participate in the process of foetal muscle catabolism of Walker's tumour-bearing pregnant rats. The present work shows that foetal muscle can be protected by supplementation with a leucine-rich diet.

Upregulation of amino acid transporter expression induced by L-leucine availability in L6 myotubes is associated with ATF4 signaling through mTORC1-dependent mechanism

OBJECTIVE

Essential amino acids, especially l-leucine, initiate the signaling of the mammalian target of rapamycin complex-1 (mTORC1) and protein synthesis in skeletal muscle. Current information on the relation between amino acid transporter mechanisms and mTORC1 signaling is sparse. The objectives of this study were to determine whether an increase in leucine availability upregulates the gene transcription and translation of amino acid transporters and other amino acid members in an mTORC1-dependent pathway that control amino acid use (general control non-repressed-2 and activating transcription factor-4) and to measure the factors related to protein synthesis and proteolysis.

METHODS

L6 skeletal muscle cells that had been treated with l-leucine (0.105 g/L) were incubated for 30 min to stimulate the transcription of L-type amino acid transporter-1, CD98, and sodium-coupled neutral amino acid transporter-2 and increase activating transcription factor-4 protein, which is dependent on the mTORC1 signaling pathway.

RESULTS

A rapid, high level of p70 S6 kinase-1 phosphorylation was detected but was suppressed by rapamycin (P < 0.05). The addition of leucine decreased the atrogin-1 transcription abundance in an insulin-involved manner (P < 0.05), which could not be completely blocked by rapamycin (P = 0.055).

CONCLUSIONS

Our findings indicate that the mTOR is a component of the nutrient signaling pathway, which regulates system A and L amino acid transporters, the initiation factors involved in mRNA translation, and is downstream of forkhead box-O in L6 myotubes.

Omega-3 polyunsaturated fatty acids augment the muscle protein anabolic response to hyperinsulinaemia-hyperaminoacidaemia in healthy young and middle-aged men and women

Increased dietary LCn-3PUFA (long-chain n-3 polyunsaturated fatty acid) intake stimulates muscle protein anabolism in individuals who experience muscle loss due to aging or cancer cachexia. However, it is not known whether LCn-3PUFAs elicit similar anabolic effects in healthy individuals. To answer this question, we evaluated the effect of 8 weeks of LCn-3PUFA supplementation (4 g of Lovaza®/day) in nine 25-45-year-old healthy subjects on the rate of muscle protein synthesis (by using stable isotope-labelled tracer techniques) and the activation (phosphorylation) of elements of the mTOR (mammalian target of rapamycin)/p70S6K (p70 S6 kinase) signalling pathway during basal post-absorptive conditions and during a hyperinsulinaemic-hyperaminoacidaemic clamp. We also measured the concentrations of protein, RNA and DNA in muscle to obtain indices of the protein synthetic capacity, translational efficiency and cell size. Neither the basal muscle protein fractional synthesis rate nor basal signalling element phosphorylation changed in response to LCn-3PUFA supplementation, but the anabolic response to insulin and amino acid infusion was greater after LCn-3PUFA [i.e. the muscle protein fractional synthesis rate during insulin and amino acid infusion increased from 0.062±0.004 to 0.083±0.007%/h and the phospho-mTOR (Ser2448) and phospho-p70S6K (Thr389) levels increased by ∼50%; all P<0.05]. In addition, the muscle protein concentration and the protein/DNA ratio (i.e. muscle cell size) were both greater (P<0.05) after LCn-3PUFA supplementation. We conclude that LCn-3PUFAs have anabolic properties in healthy young and middle-aged adults.

Control of translation initiation through integration of signals generated by hormones, nutrients, and exercise

Control of translation initiation in a tissue of an intact mammalian organism is a highly complex process requiring the continuous integration of multiple positive and negative stimuli. For a tissue such as skeletal muscle, which has the capacity to undergo dramatic changes in size and protein content, translation initiation contributes importantly to the regulation of global rates of protein synthesis and is controlled by numerous stimuli, including those arising from nutrients and hormones in the circulating blood, as well as from contraction-induced signaling within the tissue. Many of the pathways conveying signals generated by these stimuli converge on mTORC1, a serine-threonine protein kinase that has been termed the nutrient and energy sensor of the cell and that plays a prominent role in the regulation of cell growth. Control of translation initiation by mTORC1 is mediated through phosphorylation of downstream targets that modulate the binding of mRNA to the 43 S preinitiation complex. Control of translation initiation is also mediated through modulation of binding of initiator methionyl-tRNA to the 40 S ribosomal subunit. Together, modulation of these two regulatory steps in translation initiation accounts in large part for changes in protein synthesis in skeletal muscle produced by the integration of inputs from hormones, nutrients, and exercise.

An increase in essential amino acid availability upregulates amino acid transporter expression in human skeletal muscle

Essential amino acids (EAA) stimulate skeletal muscle mammalian target of rapamycin complex 1 (mTORC1) signaling and protein synthesis. It has recently been reported that an increase in amino acid (AA) transporter expression during anabolic conditions is rapamycin-sensitive. The purpose of this study was to determine whether an increase in EAA availability increases AA transporter expression in human skeletal muscle. Muscle biopsies were obtained from the vastus lateralis of seven young adult subjects (3 male, 4 female) before and 1-3 h after EAA ingestion (10 g). Blood and muscle samples were analyzed for leucine kinetics using stable isotopic techniques. Quantitative RT-PCR, and immunoblotting were used to determine the mRNA and protein expression, respectively, of AA transporters and members of the general AA control pathway [general control nonrepressed (GCN2), activating transcription factor (ATF4), and eukaryotic initiation factor (eIF2) alpha-subunit (Ser(52))]. EAA ingestion increased blood leucine concentration, delivery of leucine to muscle, transport of leucine from blood into muscle, intracellular muscle leucine concentration, ribosomal protein S6 (Ser(240/244)) phosphorylation, and muscle protein synthesis. This was followed with increased L-type AA transporter (LAT1), CD98, sodium-coupled neutral AA transporter (SNAT2), and proton-coupled amino acid transporter (PAT1) mRNA expression at 1 h (P < 0.05) and modest increases in LAT1 protein expression (3 h post-EAA) and SNAT2 protein expression (2 and 3 h post-EAA, P < 0.05). Although there were no changes in GCN2 expression and eIF2 alpha phosphorylation, ATF4 protein expression reached significance by 2 h post-EAA (P < 0.05). We conclude that an increase in EAA availability upregulates human skeletal muscle AA transporter expression, perhaps in an mTORC1-dependent manner, which may be an adaptive response necessary for improved AA intracellular delivery.

Molecular mechanism of chemoresistance by astrocyte elevated gene-1

Our recent findings show that astrocyte elevated gene-1 (AEG-1) is overexpressed in >90% of human hepatocellular carcinoma (HCC) samples, and AEG-1 plays a central role in regulating development and progression of HCC. In the present study, we elucidate a molecular mechanism of AEG-1-induced chemoresistance, an important characteristic of aggressive cancers. AEG-1 increases the expression of multidrug resistance gene 1 (MDR1) protein, resulting in increased efflux and decreased accumulation of doxorubicin, promoting doxorubicin resistance. Suppression of MDR1 by small interfering RNA or chemical reagents, or inhibition of AEG-1 or a combination of both genes, significantly increases in vitro sensitivity to doxorubicin. In nude mice xenograft studies, a lentivirus expressing AEG-1 short hairpin RNA, in combination with doxorubicin, profoundly inhibited growth of aggressive human HCC cells compared with either agent alone. We document that although AEG-1 does not affect MDR1 gene transcription, it facilitates association of MDR1 mRNA to polysomes, resulting in increased translation, and AEG-1 also inhibits ubiquitination and subsequent proteasome-mediated degradation of MDR1 protein. This study is the first documentation of a unique aspect of AEG-1 function (i.e., translational and posttranslational regulation of proteins). Inhibition of AEG-1 might provide a means of more effectively using chemotherapy to treat HCC, which displays inherent chemoresistance with aggressive pathology.

The effect of new proteasome inhibitors, belactosin A and C, on protein metabolism in isolated rat skeletal muscle

The proteasome inhibitors are used as research tools to study of the ATP-dependent ubiquitin-proteasome system. Some of them are at present undergoing clinical trials to be used as therapeutic agents for cancer or inflammation. These diseases are often accompanied by muscle wasting. We herein demonstrate findings about new proteasome inhibitors, belactosin A and C, and their direct effect on protein metabolism in rat skeletal muscle. M. soleus (SOL) and m. extensor digitorum longus (EDL) were dissected from both legs of male rats (40-60 g) and incubated in a buffer containing belactosin A or C (30 microM) or no inhibitor. The release of amino acids into the medium was estimated using high performance liquid chromatography to calculate total and myofibrillar proteolysis. Chymotrypsin-like activity (CTLA) of proteasome and cathepsin B, L activity were determined by fluorometric assay. Protein synthesis and leucine oxidation were detected using specific activity of L-[1-14C] leucine added to medium. Inhibited and control muscles from the same rat were compared using paired t-test. The results indicate that after incubation with both belactosin A and C total proteolysis and CTLA of proteasome decreased while cathepsin B, L activity did not change in both SOL and EDL. Leucine oxidation was significantly enhanced in SOL, protein synthesis decreased in EDL. Myofibrillar proteolysis was reduced in both muscles in the presence of belactosin A only. In summary, belactosin A and C affected basic parameters of protein metabolism in rat skeletal muscle. The response was both muscle- and belactosin-type-dependent.

No major sex differences in muscle protein synthesis rates in the postabsorptive state and during hyperinsulinemia-hyperaminoacidemia in middle-aged adults

Men have more muscle than women, but most studies evaluating sex differences in muscle protein metabolism have been unable to discern sexual dimorphism in basal muscle protein turnover rates in young and middle-aged adults. We hypothesized that the anabolic response to nutritional stimuli (i.e., amino acids and insulin) would be greater in young/middle-aged men than women. We therefore measured the rates of muscle protein synthesis (MPS) in 16 healthy individuals [8 men and 8 women, matched for age (mean +/- SE: 37.7 +/- 1.5 yr) and body mass index (25.2 +/- 0.7 kg/m2)] after an overnight fast (plasma insulin approximately 5 microU/ml and plasma phenylalanine approximately 60 microM) and during a hyperinsulinemic-hyperaminoacidemic-euglycemic clamp (plasma insulin approximately 28 microU/ml; plasma phenylalanine approximately 110 microM; plasma glucose approximately 5.4 mM). The rates of MPS were not different between men and women (ANOVA main effect for sex; P = 0.49). During the clamp, the rate of MPS increased by approximately 50% (P = 0.003) with no difference in the increases from basal values between men and women (+0.019 +/- 0.004 vs. +0.018 +/- 0.010%/h, respectively; P = 0.93). There were also no differences between men and women in the basal concentrations of muscle phosphorylated Akt(Ser473), Akt(Thr308), mTOR(Ser2448), and p70s6k(Thr389) or in the hyperinsulinemia-hyperaminoacidemia-induced increases in phosphorylation of those signaling elements (P > or = 0.25). We conclude that there are no major differences in the rate of MPS and its intracellular control during basal conditions and during hyperinsulinemia-hyperaminoacidema between young and middle-aged adult men and women.

Regulation of muscle growth in neonates

PURPOSE OF REVIEW

This review reports recent findings on the multiple factors that regulate skeletal muscle growth in neonates.

RECENT FINDINGS

Skeletal muscle is the fastest growing protein mass in neonates. The high rate of neonatal muscle growth is due to accelerated rates of protein synthesis accompanied by the rapid accumulation of muscle nuclei. Feeding profoundly stimulates muscle protein synthesis in neonates and the response decreases with age. The feeding-induced stimulation of muscle protein synthesis is modulated by enhanced sensitivity to the postprandial rise in insulin and amino acids. Insulin and amino acid signaling components have been identified that are involved in the feeding-induced stimulation of protein synthesis in neonatal muscle. The enhanced activation of these signaling components in skeletal muscle of the neonate contributes to the high rate of muscle protein synthesis and rapid gain in muscle protein mass in neonates.

SUMMARY

Recent findings suggest that the immature muscle has a heightened capacity to activate signaling cascades that promote translation initiation in response to the postprandial rise in insulin and amino acids thereby enabling their efficient utilization for muscle growth. This capacity is further supported by enhanced satellite cell proliferation, but how these two processes are linked remains to be established.

Activation of AMP-activated protein kinase by 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside prevents leucine-stimulated protein synthesis in rat skeletal muscle

Several stress conditions are characterized by activation of 5'-AMP-activated protein kinase (AMPK) and the development of leucine resistance in skeletal muscle. In the present study, we determined whether direct activation of the AMPK by 5-aminoimidazole-4-carboxamide-1-beta-D-ribonucleoside (AICAR) prevents the characteristic leucine-induced increase in protein synthesis by altering mammalian target of rapamycin (mTOR) signal transduction. Rats were injected with AICAR or saline (Sal) and 1 h thereafter received an oral gavage of leucine (or Sal). Efficacy of AICAR was verified by increased AMPK phosphorylation. AICAR decreased basal in vivo muscle (gastrocnemius) protein synthesis and completely prevented the leucine-induced increase, independent of a change in muscle adenine nucleotide concentration. AICAR also prevented the hyperphosphorylation of eukaryotic initiation factor (eIF) 4E binding protein (4E-BP1), ribosomal protein S6 kinase (S6K1), S6, and eIF4G in response to leucine, suggesting a decrease in mTOR activity. Moreover, AICAR prevented the leucine-induced redistribution of eIF4E from the inactive eIF4E.4E-BP1 to the active eIF4E.eIF4G complex. This ability of AICAR to produce muscle leucine resistance could not be attributed to a change in phosphorylation of tuberous sclerosis complex (TSC)2, the formation of a TSC1.TSC2 complex, the binding of raptor with mTOR, or the phosphorylation of eukaryotic elongation factor-2. However, the inhibitory actions of AICAR were associated with reduced phosphorylation of proline-rich Akt substrate-40 and increased phosphorylation of raptor, which represent potential mechanisms by which AICAR might be expected to inhibit leucine-induced increases in mTOR activity and protein synthesis under in vivo conditions.

Functional impact of high protein intake on healthy elderly people

Decline in muscle mass, protein synthesis, and mitochondrial function occurs with age, and amino acids are reported to enhance both muscle protein synthesis and mitochondrial function. It is unclear whether increasing dietary protein intake corrects postabsorptive muscle changes in aging. We determined whether a 10-day diet of high [HP; 3.0 g protein x kg fat-free mass (FFM)(-1) x day(-1)] vs. usual protein intake (UP; 1.5 g protein x kg FFM(-1) x day(-1)) favorably affects mitochondrial function, protein metabolism, and nitrogen balance or adversely affects insulin sensitivity and glomerular filtration rate (GFR) in 10 healthy younger (24+/-1 yr) and 9 older (70+/-2 yr) participants in a randomized crossover study. Net daily nitrogen balance increased equally in young and older participants, but postabsorptive catabolic state also increased, as indicated by higher whole body protein turnover and leucine oxidation with no change in protein synthesis. Maximal muscle mitochondrial ATP production rate was lower in older people, with no change occurring in diet. GFR was lower in older people, and response to HP was significantly different between the two groups, with a significant increase occurring only in younger people, thus widening the differences in GFR between the young and older participants. In conclusion, a short-term high-protein diet increased net daily nitrogen balance but increased the postabsorptive use of protein as a fuel. HP did not enhance protein synthesis or muscle mitochondrial function in either young or older participants. Additionally, widening differences in GFR between young and older patients is a potential cause of concern in using HP diet in older people.

Protein metabolism in slow- and fast-twitch skeletal muscle during turpentine-induced inflammation

The aim of our study was to evaluate the differences in protein and amino acid metabolism after subcutaneous turpentine administration in the soleus muscle (SOL), predominantly composed of red fibres, and the extensor digitorum longus muscle (EDL) composed of white fibres. Young rats (40-60 g) were injected subcutaneously with 0.2 ml of turpentine oil/100 g body weight (inflammation) or with the same volume of saline solution (control). Twenty-four hours later SOL and EDL were dissected and incubated in modified Krebs-Heinseleit buffer to estimate total and myofibrillar proteolysis, chymotrypsin-like activity of proteasome (CHTLA), leucine oxidation, protein synthesis and amino acid release into the medium. The data obtained demonstrate that in intact rats, all parameters measured except protein synthesis are significantly higher in SOL than in EDL. In turpentine treated animals, CHTLA increased and protein synthesis decreased significantly more in EDL. Release of leucine was inhibited significantly more in SOL. We conclude that turpentine-induced inflammation affects more CHTLA, protein synthesis and leucine release in EDL compared to SOL.

Differences in muscle protein synthesis and anabolic signaling in the postabsorptive state and in response to food in 65-80 year old men and women

Women have less muscle than men but lose it more slowly during aging. To discover potential underlying mechanism(s) for this we evaluated the muscle protein synthesis process in postabsorptive conditions and during feeding in twenty-nine 65-80 year old men (n = 13) and women (n = 16). We discovered that the basal concentration of phosphorylated eEF2(Thr56) was approximately 40% less (P<0.05) and the basal rate of MPS was approximately 30% greater (P = 0.02) in women than in men; the basal concentrations of muscle phosphorylated Akt(Thr308), p70s6k(Thr389), eIF4E(Ser209), and eIF4E-BP1(Thr37/46) were not different between the sexes. Feeding increased (P<0.05) Akt(Thr308) and p70s6k(Thr389) phosphorylation to the same extent in men and women but increased (P<0.05) the phosphorylation of eIF4E(Ser209) and eIF4E-BP1(Thr37/46) in men only. Accordingly, feeding increased MPS in men (P<0.01) but not in women. The postabsorptive muscle mRNA concentrations for myoD and myostatin were not different between sexes; feeding doubled myoD mRNA (P<0.05) and halved that of myostatin (P<0.05) in both sexes. Thus, there is sexual dimorphism in MPS and its control in older adults; a greater basal rate of MPS, operating over most of the day may partially explain the slower loss of muscle in older women.

Chronic alcohol feeding impairs mTOR(Ser 2448) phosphorylation in rat hearts

BACKGROUND

Chronic alcohol administration impairs protein synthesis ultimately causing a loss of proteins in cardiac muscle. Inhibition of protein synthesis resides in the process of mRNA translation. The present set of experiments were designed to examine the potential regulatory effect of chronic alcohol consumption on mammalian target of rapamycin (mTOR), a serine/threonine kinase important in controlling signaling cascades in the mRNA translation initiation pathway in rat hearts.

METHODS

Rats were fed a diet containing ethanol for 20 to 26 weeks. Pair-fed rats served as controls. Rates of protein synthesis were measured following intravenous infusion of [(3)H]-L-phenylalanine (150 mM, 30 microCi/ml; 1 ml/100 g body weight). The phosphorylation state of mTOR, eukaryotic initiation factor 4G (eIF4G), protein kinase B (PKB) and S6K1 in heart were measured using immunoblot techniques with phospho-specific antibodies.

RESULTS

Protein synthesis was reduced by 35% in animals consuming a diet containing ethanol. The fall in protein synthesis was accompanied by diminished S6K1(Thr(389)) and eIF4G (Ser(1108)) phosphorylation, both downstream effectors of mTOR signaling. These changes in phosphorylation of S6K1 and eIF4G were not associated with differences in the distribution of mTOR between TORC1 and TORC2. Instead, phosphorylation of mTOR on Ser(2448) but not on Ser(2481) was significantly reduced following feeding rats an ethanol containing diet. Decreased phosphorylation of mTOR(Ser(2448)) was not associated with a corresponding lessening of tumor suppressor complex 2 phosphorylation or expression of regulated in development and DNA damage 1, both upstream regulators of mTOR. Likewise, phosphorylation of PKB on either Ser(473) or Thr(308) was unaffected by long-term alcohol consumption.

CONCLUSIONS

Chronic ethanol consumption does not alter the distribution of mTOR between TORC1 and TORC2, but instead diminishes mTOR phosphorylation on Ser(2448) independent of changes in tumor suppressor complex 2 and PKB phosphorylation. Furthermore, the data suggest that protein synthesis in rats fed a diet containing ethanol is limited by mTOR-dependent reduction in phosphorylation of S6K1(Thr(389)) and eIF4G(Ser(1108)) secondary to reduced phosphorylation of mTOR(Ser(2448)).

Rapamycin limits formation of active eukaryotic initiation factor 4F complex following meal feeding in rat hearts

Feeding promotes protein synthesis in cardiac muscle through a stimulation of the messenger RNA translation initiation phase of protein synthesis by enhancing assembly of active eukaryotic initiation factor (eIF)4F complex. The experiments reported herein examined the potential role for a rapamycin-sensitive signaling pathway in increasing formation of active eIF4G-eIF4E complex during meal feeding. Hearts from male Sprague-Dawley rats fed a meal consisting of rat nonpurified diet were sampled prior to and 3 h following the meal in the presence or absence of treatment with rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR) complex 1. Rapamycin prevented the meal feeding-induced stimulation of myocardial protein synthesis. Inhibition of mTOR with rapamycin decreased the association of rapamycin-associated TOR protein with mTOR and prevented the feeding-induced assembly of eIF4G-eIF4E complex. In contrast, the abundance of eIF4E binding protein-1 (4E-BP1)-eIF4E complex was unaffected by either meal feeding or rapamycin. Pretreatment with rapamycin completely prevented the feeding-induced phosphorylation of eIF4G(Ser(1108)), whereas the inhibitor only partially attenuated meal feeding-induced 70-kDa ribosomal protein S6 kinase1(Thr(389)) phosphorylation and extent of 4E-BP1 in the gamma-form. Meal feeding-induced phosphorylation of protein kinase B on either Ser(473) or Thr(308) was unaffected by rapamycin. These findings suggest the extent of phosphorylation of eIF4G following meal feeding occurs by a rapamycin-sensitive mechanism in cardiac muscle. Furthermore, the rapamycin-sensitive reductions in phosphorylation of eIF4G may also lead to decreased formation of active eIF4G-eIF4E complex.

Glucose stimulates protein synthesis in skeletal muscle of neonatal pigs through an AMPK- and mTOR-independent process

Skeletal muscle protein synthesis is elevated in neonates in part due to an enhanced response to the rise in insulin and amino acids after eating. In vitro studies suggest that glucose plays a role in protein synthesis regulation. To determine whether glucose, independently of insulin and amino acids, is involved in the postprandial rise in skeletal muscle protein synthesis, pancreatic-substrate clamps were performed in neonatal pigs. Insulin secretion was inhibited with somatostatin and insulin was infused to reproduce fasting or fed levels, while glucose and amino acids were clamped at fasting or fed levels. Fractional protein synthesis rates and translational control mechanisms were examined. Raising glucose alone increased protein synthesis in fast-twitch glycolytic muscles but not in other tissues. The response in muscle was associated with increased phosphorylation of protein kinase B (PKB) and enhanced formation of the active eIF4E.eIF4G complex but no change in phosphorylation of AMP-activated protein kinase (AMPK), tuberous sclerosis complex 2 (TSC2), mammalian target of rapamycin (mTOR), 4E-binding protein-1 (4E-BP1), ribosomal protein S6 kinase (S6K1), or eukaryotic elongation factor 2 (eEF2). Raising glucose, insulin, and amino acids increased protein synthesis in most tissues. The response in muscle was associated with phosphorylation of PKB, mTOR, S6K1, and 4E-BP1 and enhanced eIF4E.eIF4G formation. The results suggest that the postprandial rise in glucose, independently of insulin and amino acids, stimulates protein synthesis in neonates, and this response is specific to fast-twitch glycolytic muscle and occurs by AMPK- and mTOR-independent pathways.

Nutrient signaling components controlling protein synthesis in striated muscle

Accretion of muscle mass is dependent upon faster rates of protein synthesis than degradation. When an animal is deprived of dietary protein, loss of body weight and negative nitrogen balance ensue. Likewise, refeeding accelerates protein synthesis and results in resumption of positive nitrogen balance. Amino acids and anabolic hormones both interact to maximally enhance rates of protein synthesis acutely during refeeding through an acceleration of the messenger RNA (mRNA) translation initiation. The review will illuminate the molecular mechanisms responsible for increasing mRNA translation initiation in striated muscle. The hastening of mRNA translation initiation most likely results from a stimulation of mammalian target of rapamycin (mTOR) acting through its downstream effector proteins eukaryotic initiation factors (eIF)4E binding protein1 and possibly eIF4G to enhance assembly of eIF4G with eIF4E and 70-kDa ribosomal S6 kinase1. Amino acids and leucine in particular are as effective as a complete meal in stimulating mRNA translation initiation by targeting these specific signal transduction systems. The physiologic importance lies in the potential ability of amino acids as specific nutrients designed to counteract the accelerated host protein wasting associated with a number of disease entities, including cancer, HIV infection, sepsis, and diabetes, and to improve nutrition to maintain muscle mass in aging populations and ensure muscle growth in neonatal populations.

Rapamycin blunts nutrient stimulation of eIF4G, but not PKCepsilon phosphorylation, in skeletal muscle

Phosphorylation of eukaryotic initiation factor 4G (eIF4G) is hypothesized to be an important contributor to the stimulation of protein synthesis in skeletal muscle following meal feeding. The experiments reported herein examined the potential role for a rapamycin-sensitive signaling pathway in mediating the meal feeding-induced elevations in phosphorylation of eIF4G. Gastrocnemius from male Sprague-Dawley rats trained to consume a meal consisting of rat chow was sampled prior to and following 3 h of having the meal provided in the presence or absence of treatment with rapamycin, an inhibitor of the mammalian target of rapamycin (mTOR) complex 1 (TORC1). Pretreatment with rapamycin prevented the feeding-induced phosphorylation of mTOR, eIF4G, and S6K1 but only partially attenuated the shift in 4E-BP1 into the gamma-form. In contrast, the feeding-induced increase in phosphorylation of PKCepsilon was not reduced by rapamycin. Rapamycin also prevented the augmented association of eIF4G with eIF4E and the decreased association of eIF4E with 4E-BP1. Similar findings were observed in gastrocnemius from animals after oral administration of leucine. Perfusion of gastrocnemius with medium containing rapamycin partially prevented the leucine-induced increase in phosphorylation of eIF4G. Thus, rapamycin attenuated a feeding- or leucine-induced phosphorylation of eIF4G in skeletal muscle both in vivo and in situ. The latter observation implies that the effects observed with rapamycin were the result of modulation of skeletal muscle signaling mechanisms responsible for eIF4G phosphorylation.

Glucose phosphorylation is required for insulin-dependent mTOR signalling in the heart

OBJECTIVE

Insulin regulates both glucose uptake and postnatal cardiac growth. The anabolic effects of insulin are mediated by the mammalian target of rapamycin (mTOR), an evolutionarily conserved kinase which is also a convergence point between nutrient sensing and cell growth. We postulated that mTOR signalling in the heart requires the metabolism of glucose.

METHODS

We interrogated the insulin-mediated mTOR signalling pathway in response to different metabolic interventions regulating substrate metabolism in the isolated working rat heart and in isolated cardiomyocytes.

RESULTS

Although insulin enhanced Akt activity, phosphorylation of mTOR and its downstream targets (p70S6K and 4EBP1) required the addition of glucose. Glucose-dependent p70S6K phosphorylation was independent of the hexosamine biosynthetic pathway, the AMP kinase pathway, and the pentose phosphate pathway. However, inhibition of glycolysis downstream of hexokinase markedly enhanced p70S6K phosphorylation. Furthermore, 2-deoxyglucose activated p70S6K suggesting that phosphorylation of glucose is required for carbohydrate-mediated mTOR signalling in the heart. Lastly, we also found enhanced p70S6K phosphorylation in the hearts of diabetic rats.

CONCLUSION

Phosphorylation of glucose is necessary for insulin-dependent mTOR activity in the heart, suggesting a link between intermediary metabolism and cardiac growth.

Postprandial leucine deficiency failed to alter muscle protein synthesis in growing and adult rats

OBJECTIVE

This study examined the effect of a specific acute postprandial leucine deficiency on skeletal muscle protein synthesis in growing and adult rats. Because the anabolic action of dietary leucine supplementation is controversial, except during aging, we hypothesized that the maximum leucine effect might be already achieved for a normal postprandial rise of leucine. Preventing this rise during the 1- to 3-h period after feeding may reveal the leucine regulation.

METHODS

On the day of the experiment, rats were fasted (postabsorptive, PA group) or fed for 1 h a control meal (postprandial, control, PP group) or a leucine-poor meal (postprandial, PP-Leu group). Muscle protein synthesis was assessed in vivo, over the 1- to 3-h period after meal distribution, using the flooding dose method (L-1-(13)C phenylalanine).

RESULTS

As expected, the postprandial increase in plasma free leucine was specifically abolished after feeding the leucine-poor meal, whereas all the other plasma free amino acids were roughly at normal postprandial levels. Plasma insulin increased after feeding in young rats but was constant in adult rats. Plasma insulin was similar whatever dietary leucine levels. Rates of muscle protein synthesis were stimulated by feeding in gastrocnemius and soleus muscles from young rats but only in gastrocnemius muscles from adult rats. The PP-Leu group did not differ from the control PP group regarding muscle protein synthesis.

CONCLUSION

The rise in plasma free leucine is not required for the stimulation of muscle protein synthesis during the 1- to 3-h period after feeding young and adult rats, as previously observed in old rats.

Elevated plasma free fatty acids decrease basal protein synthesis, but not the anabolic effect of leucine, in skeletal muscle

Elevations in free fatty acids (FFAs) impair glucose uptake in skeletal muscle. However, there is no information pertaining to the effect of elevated circulating lipids on either basal protein synthesis or the anabolic effects of leucine and insulin-like growth factor I (IGF-I). In chronically catheterized conscious rats, the short-term elevation of plasma FFAs by the 5-h infusion of heparin plus Intralipid decreased muscle protein synthesis by approximately 25% under basal conditions. Lipid infusion was associated with a redistribution of eukaryotic initiation factor (eIF)4E from the active eIF4E.eIF4G complex to the inactive eIF4E.4E-BP1 complex. This shift was associated with a decreased phosphorylation of eIF4G but not 4E-BP1. Lipid infusion did not significantly alter either the total amount or phosphorylation state of mTOR, TSC2, S6K1, or the ribosomal protein S6 under basal conditions. In control rats, oral leucine increased muscle protein synthesis. This anabolic response was not impaired by lipid infusion, and no defects in signal transduction pathways regulating translation initiation were detected. In separate rats that received a bolus injection of IGF-I, lipid infusion attenuated the normal redistribution of eIF4E from the active to inactive complex and largely prevented the increased phosphorylation of 4E-BP1, eIF4G, S6K1, and S6. This IGF-I resistance was associated with enhanced Ser(307) phosphorylation of insulin receptor substrate-1 (IRS-1). These data indicate that the short-term elevation of plasma FFAs impairs basal protein synthesis in muscle by altering eIF4E availability, and this defect may be related to impaired phosphorylation of eIF4G, not 4E-BP1. Moreover, hyperlipidemia impairs IGF-I action but does not produce leucine resistance in skeletal muscle.

Nutrient regulation of PKCepsilon is mediated by leucine, not insulin, in skeletal muscle

Nutrients enhance signaling pathways involved in skeletal muscle growth through an increased rate of protein synthesis. These studies have led to an understanding of the potential role of the mammalian target of rapamycin (mTOR) in this process. However, activation of mTOR cannot account for all the stimulatory effects of nutrients. The purpose of these experiments was to examine the effect of nutrients on the cellular distribution and activation state of novel PKC isoforms (PKCepsilon and PKCdelta) in the gastrocnemius of rats by use of modification state-dependent phosphopeptide-specific antibodies. The phosphorylation of PKCepsilon on the catalytic domain autophosphorylation site (Ser(729)) was elevated during feeding and then returned to basal levels when the feeding period ended. Meal feeding augmented the phosphorylation of the downstream effectors of mTOR, namely S6K1 and 4E-BP1. In contrast, the phosphorylation of PKCdelta on either the catalytic domain autophosphorylation site (Ser(643)) or activation loop site (Thr(505)) was unaffected. Similar results were obtained when animals were given leucine either acutely via gavage or chronically by dietary supplementations. The effect of leucine was not mimicked by injecting animals with insulin but could be induced by gavage with norleucine, a structural analog of leucine that does not increase plasma insulin concentration. Thus rises in insulin secondary to meal intake or leucine gavage are probably not responsible for increased phosphorylation of PKCepsilon in response to meal feeding. Elevating the leucine concentration stimulated the phosphorylation of PKCepsilon in gastrocnemius from perfused hindlimb and caused a shift in the distribution of PKCepsilon from the membrane fraction to the cytosolic fraction. The results indicate that leucine leads to an activation (autophosphorylation) and subcellular redistribution of PKCepsilon, but not PKCdelta, in gastrocnemius both in vivo and in vitro. Furthermore, activation of the mTOR signaling pathway above basal conditions does not appear to be necessary to induce phosphorylation or translocation of PKCepsilon, suggesting that multiple signaling pathways become activated with leucine.

IGF-I stimulates protein synthesis in skeletal muscle through multiple signaling pathways during sepsis

Chronic septic abscess formation causes an inhibition of protein synthesis in gastrocnemius not observed in rats with a sterile abscess. Inhibition is associated with an impaired mRNA translation initiation that can be ameliorated by elevating IGF-I but not insulin. The present study investigated the ability of IGF-I signaling to stimulate protein synthesis in gastrocnemius by accelerating mRNA translation initiation. Experiments were performed in perfused hindlimb preparations from rats 5 days after induction of a septic abscess. Protein synthesis in gastrocnemius from septic rats was accelerated twofold by the addition of IGF-I (10 nM) to perfusate. IGF-I increased the phosphorylation of translation repressor 4E-binding protein-1 (4E-BP1). Hyperphosphorylation of 4E-BP1 in response to IGF-I resulted in its dissociation from the inactive eukaryotic initiation factor (eIF) 4E.4E-BP1 complex. Assembly of the active eIF4F complex (as assessed by the association eIF4G with eIF4E) was increased twofold by IGF-I in the perfusate. In addition, phosphorylation of eIF4G and ribosomal protein S6 kinase-1 (S6K1) was also enhanced by IGF-I. Activation of mammalian target of rapamycin, an upstream kinase implicated in phosphorylating both 4E-BP1 and S6K1, was also observed. Thus the ability of IGF-I to accelerate protein synthesis during sepsis may be related to a stimulation of signaling to multiple steps in translation initiation with an ensuing increased phosphorylation of eIF4G, eIF4E availability, and S6K1 phosphorylation.

Reevaluation of amino acid stimulation of protein synthesis in murine- and human-derived skeletal muscle cells assessed by independent techniques

Murine L6 and human rhabdomyosarcoma cells were cultured standardized in low (0.28 mM) and normal (9 mM) amino acid (AA) concentrations to reevaluate by independent methods to what extent AA activate initiation of protein synthesis. Methods used were incorporation of radioactive AA into proteins, distribution analysis of RNA in density gradient, and Western blots on initiation factors of translation of proteins in cultured cells as well as in vivo (gastrocnemius, C57Bl mice) during starvation/refeeding. Incorporation rate of AA gave incorrect results in a variety of conditions, where phenylalanine stimulated the incorporation rate of phenylalanine into proteins, but not of tyrosine, and tyrosine stimulated incorporation of tyrosine but not of phenylalanine. Similar problems were observed when [35S]methionine was used for labeling of fractionated cellular proteins. However, the methods entirely independent of labeled AA incorporation indicated that essential AA activate initiation of translation, whereas nonessential AA did not. Branched-chain AA and glutamine, in combination with some other AA, also stimulated initiation of translation. Starvation/refeeding in vitro agreed qualitatively with results in vivo evaluated by initiation factors. Insulin at physiological concentrations (100 microM/ml) did not stimulate global protein synthesis at low or normal AA concentrations but did so at supraphysiological levels (3 mU/ml), confirmed by independent methods. Our results reemphasize that labeled AA should be used with caution for quantification of protein synthesis, since the precursor pool(s) for protein synthesis is not in complete equilibrium with surrounding AA. "Flooding" tracee experiments did not overcome this problem.

Endotoxin disrupts the leucine-signaling pathway involving phosphorylation of mTOR, 4E-BP1, and S6K1 in skeletal muscle

Endotoxin (i.e., lipopolysaccharide, LPS) impairs skeletal muscle protein synthesis. Although this impairment is not acutely associated with a decreased plasma concentration of total amino acids, LPS may blunt the anabolic response to amino acids. To examine this hypothesis, rats were injected intraperitoneally with LPS or saline (Sal) and 4 h thereafter were orally administered either leucine (Leu) or Sal. The gastrocnemius was removed 20 min later to assess signaling components important in the translational control of protein synthesis. In the Sal-Leu group phosphorylation of 4E-BP1 in muscle was markedly increased, compared to values from time-matched saline-treated control rats. This change was associated with a redistribution of eukaryotic initiation factor (eIF) 4E from the inactive eIF4E x 4E-BP1 complex to the active eIF4E x eIF4G complex. In LPS-treated rats, the Leu-induced phosphorylation of 4E-BP1 and changes in eIF4E distribution were partially or completely abrogated. LPS also antagonized the Leu-induced increase in phosphorylation of S6K1, ribosomal protein S6 and mTOR. Neither LPS nor leu altered the total amount or phosphorylation of TSC2 in muscle. The ability of LPS to blunt the anabolic effects of Leu could not be attributed to differences in the plasma concentrations of insulin or Leu between groups. Furthermore, the replacement of plasma insulin-like growth factor (IGF)-I in LPS-treated rats to basal levels also did not ameliorate the defect in leucine-induced phosphorylation of S6K1 or S6, although it did reverse the LPS-induced decrease in the constitutive phosphorylation of mTOR, S6 and 4E-BP1. Pretreatment with the glucocorticoid receptor antagonist RU486 was unable to prevent the LPS-induced leucine resistance. In contrast, to the abovementioned results with leucine, LPS did not prevent the ability of pharmacological levels of IGF-I to phosphorylate 4E-BP1, S6K1, mTOR or alter the availability of eIF4E. Hence, LPS working via a glucocorticoid-independent mechanism produces a leucine resistance in skeletal muscle that might be expected to impair the ability of this amino acid to stimulate translation initiation and protein synthesis.

Human soleus and vastus lateralis muscle protein metabolism with an amino acid infusion

The calf muscles, compared with the thigh, are less responsive to resistance exercise in ambulatory and bed-rested individuals, apparently due to muscle-specific differences in protein metabolism. We chose to evaluate the efficacy of using amino acids to elevate protein synthesis in the soleus, because amino acids have been shown to have a potent anabolic effect in the vastus lateralis. Mixed muscle protein synthesis in the soleus and vastus lateralis was measured before and after infusion of mixed amino acids in 10 individuals (28 +/- 1 yr). Phosphorylation of ribosomal protein p70 S6 kinase (p70S6K; Thr389) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1; Thr37/46) was also evaluated at rest and after 3 h of amino acid infusion. Basal protein synthesis was similar (P = 0.126), and amino acids stimulated protein synthesis to a similar extent (P = 0.004) in the vastus lateralis (0.043 +/- 0.011%/h) and soleus (0.032 +/- 0.017%/h). Phosphorylation of p70S6K (P = 0.443) and 4E-BP1 (P = 0.192) was not increased in either muscle; however, the soleus contained more total (P = 0.002) and phosphorylated (P = 0.013) 4E-BP1 than the vastus lateralis. These data support the need for further study of amino acid supplementation as a means to compensate for the reduced effectiveness of calf resistance exercise in ambulatory individuals and those exposed to extended periods of unloading. The greater 4E-BP1 in the soleus suggests that there is a muscle-specific distribution of general translational initiation machinery in human skeletal muscle.

Differential effect of sepsis on ability of leucine and IGF-I to stimulate muscle translation initiation

Polymicrobial sepsis impairs skeletal muscle protein synthesis, which results from impairment in translation initiation under basal conditions. The purpose of the present study was to test the hypothesis that sepsis also impairs the anabolic response to amino acids, specifically leucine (Leu). Sepsis was induced by cecal ligation and puncture, and 24 h later, Leu or saline (Sal) was orally administered to septic and time-matched nonseptic rats. The gastrocnemius was removed 20 min later for assessment of protein synthesis and signaling components important in peptide-chain initiation. Oral Leu increased muscle protein synthesis in nonseptic rats. Leu was unable to increase protein synthesis in muscle from septic rats, and synthetic rates remained below those observed in nonseptic + Sal rats. In nonseptic + Leu rats, phosphorylation of eukaryotic initiation factor (eIF)4E-binding protein 1 (4E-BP1) in muscle was markedly increased compared with values from time-matched Sal-treated nonseptic rats. This change was associated with redistribution of eIF4E from the inactive eIF4E.4E-BP1 to the active eIF4E.eIF4G complex. In septic rats, Leu-induced phosphorylation of 4E-BP1 and changes in eIF4E distribution were completely abrogated. Sepsis also antagonized the Leu-induced increase in phosphorylation of S6 kinase 1 and ribosomal protein S6. Sepsis attenuated Leu-induced phosphorylation of mammalian target of rapamycin and eIF4G. The ability of sepsis to inhibit anabolic effects of Leu could not be attributed to differences in plasma concentrations of insulin, insulin-like growth factor I, or Leu between groups. In contrast, the ability of exogenous insulin-like growth factor I to stimulate the same signaling components pertaining to translation initiation was not impaired by sepsis. Hence, sepsis produces a relatively specific Leu resistance in skeletal muscle that impairs the ability of this amino acid to stimulate translation initiation and protein synthesis.

Impaired anabolic response of muscle protein synthesis is associated with S6K1 dysregulation in elderly humans

Age-related loss of muscle protein may involve a decreased response to anabolic factors of muscle protein synthesis through dysregulation of translation factors. To verify this hypothesis, we simultaneously investigated muscle protein synthesis and expression of some factors implicated in insulin signal transduction during hyperinsulinemia and hyperaminoacidemia in 6 young (25+/-1 year; mean+/-sem) and 8 elderly subjects (72+/-2 year). Incorporation of L-[1-13C] leucine in muscle proteins (fractional synthesis rate, FSR) was measured in vastus lateralis, before and during a euglycemic hyperinsulinemic hyperaminoacidemic clamp, together with Western blot analysis of protein kinase B (PKB), mTOR, 4E-BP1, and S6K1 phosphorylation. In basal state, muscle protein FSR was reduced in elderly in comparison with young subjects (0.061+/-0.004% per hour) vs 0.082+/-0.010% per hour, elderly vs. young, P<0.05). During clamp, muscle protein FSR was stimulated in young (0.119+/-0.006% per hour; P<0.05), but this response was significantly lower in elderly subjects (0.084+/-0.005% per hour, P<0.05 vs young subjects). Phosphorylation of PKB, mTOR, and 4E-BP1 were similarly increased by insulin and amino acid in both groups, except for S6K1 phosphorylation, which was not stimulated in elderly subjects. In conclusion, 1) response of muscle protein synthesis to insulin and amino acid is impaired in elderly humans; 2) a defect in S6K1 pathway activation may be responsible for this alteration. This modification is a mechanistic basis of sarcopenia development during aging.

Glucocorticoids modulate amino acid-induced translation initiation in human skeletal muscle

Amino acids are unique anabolic agents in that they nutritively signal to mRNA translation initiation and serve as substrates for protein synthesis in skeletal muscle. Glucocorticoid excess antagonizes the anabolic action of amino acids on protein synthesis in laboratory animals. To examine whether excessive glucocorticoids modulate mixed amino acid-signaled translation initiation in human skeletal muscle, we infused an amino acid mixture (10% Travasol) systemically to 16 young healthy male volunteers for 6 h in the absence (n = 8) or presence (n = 8) of glucocorticoid excess (dexamethasone 2 mg orally every 6 h for 3 days). Vastus lateralis muscles were biopsied before and after amino acid infusion, and the phosphorylation of eukaryotic initiation factor (eIF) 4E-binding protein 1 (4E-BP1), ribosomal protein S6 kinase (p70(S6K)), and eIF2alpha and the guanine nucleotide exchange activity of eIF2B were measured. Systemic infusion of mixed amino acids significantly stimulated the phosphorylation of 4E-BP1 (P < 0.04) and p70(S6K) (P < 0.001) and the dephosphorylation of eIF2alpha (P < 0.003) in the control group. Dexamethasone treatment did not alter the basal phosphorylation state of 4E-BP1, p70(S6K), or eIF2alpha; however, it abrogated the stimulatory effect of amino acid infusion on the phosphorylation of 4E-BP1 (P = 0.31) without affecting amino acid-induced phosphorylation of p70(S6K) (P = 0.002) or dephosphorylation of eIF2alpha (P = 0.003). Neither amino acid nor dexamethasone treatment altered the guanine nucleotide exchange activity of eIF2B. We conclude that changes of amino acid concentrations within the physiological range stimulate mRNA translation by enhancing the binding of mRNA to the 43S preinitiation complex, and the activity of p70(S6K) and glucocorticoid excess blocks the former action in vivo in human skeletal muscle.

Impact of Src homology 2-containing inositol 5'-phosphatase 2 on the regulation of insulin signaling leading to protein synthesis in 3T3-L1 adipocytes cultured with excess amino acids

Src homology 2-containing inositol 5'-phosphatase 2 (SHIP2) possesses 5'-phosphatase activity to specifically hydrolyze the phosphatidylinositol 3-kinase product PI(3,4,5)P3 in the regulation of insulin signaling. In the present study, we examined the impact of SHIP2 on the regulation of insulin signaling leading to protein synthesis in 3T3-L1 adipocytes cultured with standard and excess concentrations of amino acids. Insulin-induced translocation of PDK1 to the plasma membrane, phosphorylation of Akt and p70S6-kinase and ribosomal protein S6, increase in the amount of 4E-BP1 gamma-form, association of eIF4E with eIF4G, and protein synthesis were decreased by overexpression of wild-type SHIP2 by adenovirus-mediated gene transfer. The effect of SHIP2 overexpression on the regulation of insulin-induced phosphorylation of Akt and p70S6-kinase was somewhat augmented by the incubation with 5-fold excess concentrations of amino acids for 30 min. In contrast, the impact of SHIP2 expression was diminished in insulin-induced phosphorylation of p70S6-kinase and S6, but not of Akt, after the incubation for 16 h. Interestingly, incubation with the excess concentrations of amino acids for 30 min induced activation of phosphatidylinositol 3-kinase and phosphorylation of Akt, whereas phosphorylation of p70S6-kinase and S6 was decreased. Furthermore, although the exposure for longer time periods up to 24 h did not elicit phosphorylation of Akt, it markedly induced phosphorylation of p70S6-kinase and S6. These results indicate that SHIP2 plays an important role in the negative regulation of insulin signaling for the protein synthesis and that the impact of SHIP2 is altered, dependent on the acute or chronic exposure of excess concentrations of amino acids in culture.

Proteins and amino acids in enteral nutrition

PURPOSE OF REVIEW

This review reports recent findings on the effect of enterally fed protein and amino acids on metabolism, function, and clinical outcome, particularly during the neonatal period.

RECENT FINDINGS

Splanchnic tissues metabolize significant proportions of some enteral amino acids and this likely contributes to the higher requirement for these amino acids when they are provided enterally versus parenterally. Splanchnic tissues are particularly key in the provision of nutrition to preterm infants, who possess an exceedingly high protein anabolic drive, but limited tolerance to aggressive enteral feeding. The protein anabolic response to specific proteins is influenced by the rate of digestion and the pattern of feeding, as well as the amino acid composition of the proteins. The post-prandial rise in amino acids and insulin stimulates neonatal tissue protein synthesis by modulation of the nutrient and insulin signaling pathways that lead to translation initiation. A flurry of investigations into the metabolic response and clinical impact of individual amino acids suggests that leucine, glutamine, and arginine, in particular, have specific roles in regulating protein synthesis and immune function.

SUMMARY

Recent findings suggest that enteral nutrition support that provides an optimum combination of proteins and amino acids can have a beneficial impact on the clinical outcome of patients.

Role of mTOR Signalling in the Control of Translation Initiation and Elongation by Nutrients

Protein synthesis requires nutrients both as precursors (amino acids) and as a source of energy, since this process consumes a high proportion of cellular metabolic energy. Recent work has shown that both types of nutrients directly influence the activities of components of the translational machinery in mammalian cells. Amino acids positively regulate signalling through the mammalian target of the rapamycin (mTOR) pathway, although the degree of dependency on external amino acids varies between cell types. mTOR signalling modulates several key components involved in mRNA translation, in particular (via repressor proteins) the cap-binding initiation factor eIF4E, the ribosomal protein S6 kinases, and elongation factor eEF2. The branched-chain amino acid leucine is the most effective one in most cell types. It is currently unclear how mammalian cells sense prevailing amino acid levels, although this may involve intracellular amino acids. Cellular ATP levels can also influence mTOR activity. The activities of some translation factors are modulated by mTOR-independent mechanisms. Examples include the regulation of eEF2 by cellular energy levels, which may be controlled via the AMP-activated protein kinase, and the activity of the guanine nucleotide-exchange factor eIF2B, which is modulated by amino acids and metabolic fuels.

Alcohol impairs leucine-mediated phosphorylation of 4E-BP1, S6K1, eIF4G, and mTOR in skeletal muscle

Acute alcohol (EtOH) intoxication impairs skeletal muscle protein synthesis. Although this impairment is not associated with a decrease in the total plasma amino acid concentration, EtOH may blunt the anabolic response to amino acids. To examine this hypothesis, rats were administered EtOH or saline (Sal) and 2.5 h thereafter were orally administered either leucine (Leu) or Sal. The gastrocnemius was removed 20 min later to assess protein synthesis and signaling components important in translational control of protein synthesis. Oral Leu increased muscle protein synthesis by the same magnitude in Sal- and EtOH-treated rats. However, the increase in the latter group was insufficient to overcome the suppressive effect of EtOH, and the rate of synthesis remained lower than that observed in rats from the Sal-Sal group. Leu markedly increased phosphorylation of Thr residues 36, 47, and 70 on 4E-binding protein (BP)1 in muscle from rats not receiving EtOH, and this response was associated with a redistribution of eukaryotic initiation factor (eIF) 4E from the inactive eIF4E. 4E-BP1 to the active eIF4E. eIF4G complex. In EtOH-treated rats, the Leu-induced phosphorylation of 4E-BP1 and changes in eIF4E availability were partially abrogated. EtOH also prevented the Leu-induced increase in phosphorylation of eIF4G, the serine/threonine protein kinase S6K1, and the ribosomal protein S6. Moreover, EtOH attenuated the Leu-induced phosphorylation of the mammalian target of rapamycin (mTOR). The ability of EtOH to blunt the anabolic effects of Leu could not be attributed to differences in the plasma concentrations of insulin, insulin-like growth factor I, or Leu. Finally, although EtOH increased the plasma corticosterone concentration, inhibition of glucocorticoid action by RU-486 was unable to prevent EtOH-induced defects in the ability of Leu to stimulate 4E-BP1, S6K1, and mTOR phosphorylation. Hence, ethanol produces a leucine resistance in skeletal muscle, as evidenced by the impaired phosphorylation of 4E-BP1, eIF4G, S6K1, and mTOR, that is independent of elevations in endogenous glucocorticoids.

Regulation of translation initiation by insulin and amino acids in skeletal muscle of neonatal pigs

Previous studies have shown that intravenous infusion of insulin and/or amino acids reproduces the feeding-induced stimulation of muscle protein synthesis in neonates and that insulin and amino acids act independently to produce this effect. The goal of the present study was to delineate the regulatory roles of insulin and amino acids on muscle protein synthesis in neonates by examining translational control mechanisms, specifically the eukaryotic translation initiation factors (eIFs), which enable coupling of initiator methionyl-tRNAi and mRNA to the 40S ribosomal subunit. Insulin secretion was blocked by somatostatin in fasted 7-day-old pigs (n = 8-12/group), insulin was infused to achieve plasma levels of approximately 0, 2, 6, and 30 microU/ml, and amino acids were clamped at fasting or fed levels or, at the high insulin dose, below fasting. Both insulin and amino acids increased the phosphorylation of ribosomal protein S6 kinase (S6K1) and the eIF4E-binding protein (4E-BP1), decreased the binding of 4E-BP1 to eIF4E, increased eIF4E binding to eIF4G, and increased fractional protein synthesis rates but did not affect eIF2B activity. In the absence of insulin, amino acids had no effect on these translation initiation factors but increased the protein synthesis rates. Raising insulin from below fasting to fasting levels generally did not alter translation initiation factor activity but raised protein synthesis rates. The phosphorylation of S6K1 and 4E-BP1 and the amount of 4E-BP1 bound to eIF4E and eIF4E bound to eIF4G were correlated with insulin level, amino acid level, and protein synthesis rate. Thus insulin and amino acids regulate muscle protein synthesis in skeletal muscle of neonates by modulating the availability of eIF4E for 48S ribosomal complex assembly, although other processes also must be involved.

Amino acids as regulators of gene expression at the level of mRNA translation

Amino acids act through a number of signaling pathways and mechanisms to mediate control of gene expression at the level of mRNA translation. This report reviews recent findings that illustrate the manner through which amino acids act to regulate the initiation phase of mRNA translation. The report focuses on signaling pathways that involve the eukaryotic initiation factor-2 (eIF2) protein kinase, general control non-derepressing kinase-2 and the mammalian target of rapamycin (mTOR) protein kinase. It also describes the mechanisms through which amino acid-induced modulation of eIF2 phosphorylation and mTOR-mediated signaling cause derepression of translation of specific mRNAs and result in an overall change in the pattern of gene expression. Finally, it provides examples of mRNAs whose translation is modulated through these mechanisms.

Endotoxemia reduces skeletal muscle protein synthesis in neonates

Protein synthesis in skeletal muscle is reduced by as much as 50% as early as 4 h after a septic challenge in adults. However, the effect of sepsis on muscle protein synthesis has not been determined in neonates, a highly anabolic population whose muscle protein synthesis rates are elevated and uniquely sensitive to insulin and amino acid stimulation. Neonatal piglets (n = 10/group) were infused for 8 h with endotoxin [lipopolysaccharide (LPS), 0 and 10 microg. kg(-1). h(-1)]. Plasma amino acid and glucose concentrations were kept at the fed level by infusion of dextrose and a balanced amino acid mixture. Fractional protein synthesis rates were determined by use of a flooding dose of [(3)H]phenylalanine. LPS infusion produced a septic-like state, as indicated by an early and sustained elevation in body temperature, heart rate, and plasma tumor necrosis factor-alpha, interleukin-1, cortisol, and lactate concentrations. Plasma levels of insulin increased, whereas glucose and amino acids decreased, suggesting the absence of insulin resistance. LPS significantly reduced protein synthesis in longissimus dorsi muscle by only 11% and in gastrocnemius by only 15%, but it had no significant effect in masseter and cardiac muscles. LPS increased protein synthesis in the liver (22%), spleen (28%), kidney (53%), jejunum (19%), diaphragm (21%), lung (50%), and skin (13%), but not in the stomach, pancreas, or brain. These findings suggest that, when substrate supply is maintained, skeletal muscle protein synthesis in neonates compared with adults is relatively resistant to the catabolic effects of sepsis.

Regulation of mammalian translation factors by nutrients

Protein synthesis requires both amino acids, as precursors, and a substantial amount of metabolic energy. It is well established that starvation or lack of nutrients impairs protein synthesis in mammalian cells and tissues. Branched chain amino acids are particularly effective in promoting protein synthesis. Recent work has revealed important new information about the mechanisms involved in these effects. A number of components of the translational machinery are regulated through signalling events that require the mammalian target of rapamycin, mTOR. These include translational repressor proteins (eukaryotic initiation factor 4E-binding proteins, 4E-BPs) and protein kinases that act upon the small ribosomal subunit (S6 kinases). Amino acids, especially leucine, positively regulate mTOR signalling thereby relieving inhibition of translation by 4E-BPs and activating the S6 kinases, which can also regulate translation elongation. However, the molecular mechanisms by which amino acids modulate mTOR signalling remain unclear. Protein synthesis requires a high proportion of the cell's metabolic energy, and recent work has revealed that metabolic energy, or fuels such as glucose, also regulate targets of the mTOR pathway. Amino acids and glucose modulate a further important regulatory step in translation initiation, the activity of the guanine nucleotide-exchange factor eIF2B. eIF2B controls the recruitment of the initiator methionyl-tRNA to the ribosome and is activated by insulin. However, in the absence of glucose or amino acids, insulin no longer activates eIF2B. Since control of eIF2B is independent of mTOR, these data indicate the operation of additional, and so far unknown, regulatory mechanisms that control eIF2B activity.

The GCN2 eIF2alpha kinase is required for adaptation to amino acid deprivation in mice

The GCN2 eIF2alpha kinase is essential for activation of the general amino acid control pathway in yeast when one or more amino acids become limiting for growth. GCN2's function in mammals is unknown, but must differ, since mammals, unlike yeast, can synthesize only half of the standard 20 amino acids. To investigate the function of mammalian GCN2, we have generated a Gcn2(-/-) knockout strain of mice. Gcn2(-/-) mice are viable, fertile, and exhibit no phenotypic abnormalities under standard growth conditions. However, prenatal and neonatal mortalities are significantly increased in Gcn2(-/-) mice whose mothers were reared on leucine-, tryptophan-, or glycine-deficient diets during gestation. Leucine deprivation produced the most pronounced effect, with a 63% reduction in the expected number of viable neonatal mice. Cultured embryonic stem cells derived from Gcn2(-/-) mice failed to show the normal induction of eIF2alpha phosphorylation in cells deprived of leucine. To assess the biochemical effects of the loss of GCN2 in the whole animal, liver perfusion experiments were conducted. Histidine limitation in the presence of histidinol induced a twofold increase in the phosphorylation of eIF2alpha and a concomitant reduction in eIF2B activity in perfused livers from wild-type mice, but no changes in livers from Gcn2(-/-) mice.

Acute IGF-I infusion stimulates protein synthesis in skeletal muscle and other tissues of neonatal pigs

Studies have shown that protein synthesis in skeletal muscle of neonatal pigs is uniquely sensitive to a physiological rise in both insulin and amino acids. Protein synthesis in cardiac muscle, skin, and spleen is responsive to insulin but not amino acid stimulation, whereas in the liver, protein synthesis responds to amino acids but not insulin. To determine the response of protein synthesis to insulin-like growth factor I (IGF-I) in this model, overnight-fasted 7- and 26-day-old pigs were infused with IGF-I (0, 20, or 50 microg. kg(-1). h(-1)) to achieve levels within the physiological range, while amino acids and glucose were clamped at fasting levels. Because IGF-I infusion lowers circulating insulin levels, an additional group of high-dose IGF-I-infused pigs was also provided replacement insulin (10 ng. kg(-0.66). min(-1)). Tissue protein synthesis was measured using a flooding dose of L-[4-(3)H]phenylalanine. In 7-day-old pigs, low-dose IGF-I increased protein synthesis by 25-60% in various skeletal muscles as well as in cardiac muscle (+38%), skin (+24%), and spleen (+32%). The higher dose of IGF-I elicited no further increase in protein synthesis above that found with the low IGF-I dose. Insulin replacement did not alter the response of protein synthesis to IGF-I in any tissue. The IGF-I-induced increases in tissue protein synthesis decreased with development. IGF-I infusion, with or without insulin replacement, had no effect on protein synthesis in liver, jejunum, pancreas, or kidney. Thus the magnitude, tissue specificity, and developmental change in the response of protein synthesis to acute physiological increases in plasma IGF-I are similar to those previously observed for insulin. This study provides in vivo data indicating that circulating IGF-I and insulin act on the same signaling components to stimulate protein synthesis and that this response is highly sensitive to stimulation in skeletal muscle of the neonate.

IGF-I and insulin regulate eIF4F formation by different mechanisms in muscle and liver in the ovine fetus

The mechanisms by which insulin-like growth factor I (IGF-I) and insulin regulate eukaryotic initiation factor (eIF)4F formation were examined in the ovine fetus. Insulin infusion increased phosphorylation of eIF4E-binding protein (4E-BP1) in muscle and liver. IGF-I infusion did not alter 4E-BP1 phosphorylation in liver. In muscle, IGF-I increased 4E-BP1 phosphorylation by 27%; the percentage in the gamma-form in the IGF-I group was significantly lower than that in the insulin group. In liver, only IGF-I increased eIF4G. Both IGF-I and insulin increased eIF4E. eIF4G binding in muscle, but only insulin decreased the amount of 4E-BP1 associated with eIF4E. In liver, only IGF-I increased eIF4E. eIF4G binding. Insulin increased the phosphorylation of p70 S6 kinase (p70(S6k)) in both muscle and liver and protein kinase B (PKB/Akt) in muscle, two indicative signal proteins in the phosphatidylinositol (PI) 3-kinase pathway. IGF-I increased PKB/Akt phosphorylation in muscle but had no effect on p70(S6k) phosphorylation in muscle or liver. We conclude that insulin and IGF-I modulate eIF4F formation; however, the two hormones have different regulatory mechanisms. Insulin increases phosphorylation of 4E-BP1 and eIF4E. eIF4G binding in muscle, whereas IGF-I regulates eIF4F formation by increasing total eIF4G. Insulin, but not IGF-I, decreased 4E-BP1 content associated with eIF4E. Insulin regulates translation initiation via the PI 3-kinase-p70(S6k) pathway, whereas IGF-I does so mainly via mechanisms independent of the PI 3-kinase-p70(S6k) pathway.