Leucine regulates translation initiation in rat skeletal muscle via enhanced eIF4G phosphorylation

Evaluation of Feeding Beta-Hydroxy-Beta-Methylbutyrate (HMB) to Mouse Dams during Gestation on Birth Weight and Growth Variation of Offspring

This study was designed to determine if feeding β-hydroxy-β-methylbutyrate (HMB) to pregnant mice would improve birth weight uniformity and growth performance of offspring. Dams (Agouti Avy) were assigned to one of four treatments: control (CON; n = 13), low-level HMB (LL; 3.5 mg/g; n = 14), high-level HMB (HL; 35 mg/g; n = 15), and low-level pulse dose fed from gestational days 6 to 10 (PUL; 3.5 mg/g; n = 14). Randomly selected dams (n = 27) were euthanized on gestational day 18 to collect placentae and pup weights. The remaining dams gave birth and lactated for 28 days. Dams only received HMB during gestation. Dietary HMB did not influence the performance of dams. Dietary treatment during gestation did not affect litter size or birth weight of pups. Variation was not different among treatments in terms of birth weight of offspring. Placental weights were not affected by treatments. Overall, growth performance of offspring after weaning was similar among all treatments. Body composition of offspring at 5 and 8 weeks of age was similar regardless of HMB treatment during gestation. In conclusion, dietary HMB supplementation in pregnant mice did not affect birth weight, variations in birth weight, or growth performance of offspring.

Physicochemical, technofunctional, in vitro antioxidant, and in situ muscle protein synthesis properties of a sprat (Sprattus sprattus) protein hydrolysate

Introduction

Sprat (Sprattus sprattus) is an underutilized fish species that may act as an economic and sustainable alternative source of protein due to its good amino acid (AA) profile along with its potential to act as a source of multiple bioactive peptide sequences.

Method and results

This study characterized the physicochemical, technofunctional, and in vitro antioxidant properties along with the AA profile and score of a sprat protein enzymatic hydrolysate (SPH). Furthermore, the impact of the SPH on the growth, proliferation, and muscle protein synthesis (MPS) in skeletal muscle (C2C12) myotubes was examined. The SPH displayed good solubility and emulsion stabilization properties containing all essential and non-essential AAs. Limited additional hydrolysis was observed following in vitro-simulated gastrointestinal digestion (SGID) of the SPH. The SGID-treated SPH (SPH-SGID) displayed in vitro oxygen radical antioxidant capacity (ORAC) activity (549.42 μmol TE/g sample) and the ability to reduce (68%) reactive oxygen species (ROS) production in C2C12 myotubes. Muscle growth and myotube thickness were analyzed using an xCELLigence™ platform in C2C12 myotubes treated with 1 mg protein equivalent.mL^-1 of SPH-SGID for 4 h. Anabolic signaling (phosphorylation of mTOR, rpS6, and 4E-BP1) and MPS (measured by puromycin incorporation) were assessed using immunoblotting. SPH-SGID significantly increased myotube thickness (p < 0.0001) compared to the negative control (cells grown in AA and serum-free medium). MPS was also significantly higher after incubation with SPH-SGID compared with the negative control (p < 0.05).

Conclusions

These preliminary in situ results indicate that SPH may have the ability to promote muscle enhancement. In vivo human studies are required to verify these findings.

High Protein Intake at Lunch Is Negatively Associated with Blood Pressure in Community-Dwelling Older Adults: A Cross-Sectional Study

BACKGROUND

The present study was conducted to explore the association between protein intake across the main meals and hypertension (HTN)-related parameters in community-dwelling Brazilian older adults.

METHODS

Brazilian community-dwelling older adults were recruited in a senior center. Dietary habits were assessed through 24 h recall. Protein intake was classified as high and low according to median and recommended dietary allowance values. Absolute and body weight (BW)-adjusted protein consumption levels were quantified and analyzed according to ingestion across the main meals. Systolic (SBP) and diastolic blood pressure (DBP) were measured using an oscilometric monitor. Participants were categorized as hypertensive according to physician diagnosis or the detection of high SBP and/or DBP values.

RESULTS

One hundred ninety-seven older adults were enrolled in the present study. Protein intake at lunch was independently and negatively associated with SBP. Furthermore, a lower prevalence of HTN (diagnosed by a physician) was observed in participants with higher intakes of protein. These results remained significant after adjustment for many covariates. However, significance was lost when kilocalories and micronutrients were included in the model.

CONCLUSIONS

Findings of the present study indicate that protein intake at lunch was independently and negatively associated with systolic BP in community-dwelling older adults.

Whey protein and vitamin D supplementation in institutionalized older adults: A randomized trial

Background: The increase in life expectancy and in the number of individuals over 60 years old brings new demands to health professionals and services based on the physiological changes that occur in this population. The aging process results in changes in body composition, increasing body fat and reducing muscle mass, in addition to a reduction in bone mass. Aim: The aim of this study was to examine the effect of whey protein and vitamin D supplementation on body composition and skeletal muscle in older adults living in long-term care facilities. Methods: This study is a double-blind randomized controlled trial. Thirty older adults (>60 years old) were randomized and allocated in three groups: group receiving resistance training and supplementation receiving resistance training, whey protein and vitamin D; group received resistance and placebo training receiving resistance training and placebo, and control group without any intervention. Body composition was measured by dual-energy X-ray absorptiometry at baseline, 12 weeks, and 24 weeks. Results: The mean age was 74.87 (± 8.14) years. A significant difference (p = 0.042) was observed between the group receiving resistance training and supplementation and control groups in relation to lean mass increase (kg) at 24 weeks. After 24 weeks of intervention, there was a significant increase in Relative index of muscle mass for the two groups that underwent resistance training, group received resistance and placebo training (p = 0.042) and group receiving resistance training and supplementation (p = 0.045), in relation to the control. Conclusion: Combined supplementation of whey protein and vitamin D with resistance training can significantly improve lean mass, total mass, and relative index of muscle mass in institutionalized older adults.

A Cell-Based Assessment of the Muscle Anabolic Potential of Blue Whiting (Micromesistius poutassou) Protein Hydrolysates

Blue whiting (BW) represents an underutilised fish species containing a high-quality protein and amino acid (AA) profile with numerous potentially bioactive peptide sequences, making BW an economic and sustainable alternative source of protein. This study investigated the impact of three different BW protein hydrolysates (BWPH-X, Y and Z) on growth, proliferation and muscle protein synthesis (MPS) in skeletal muscle (C2C12) myotubes. BWPHs were hydrolysed using different enzymatic and heat exposures and underwent simulated gastrointestinal digestion (SGID), each resulting in a high degree of hydrolysis (33.41-37.29%) and high quantities of low molecular mass peptides (86.17-97.12% <1 kDa). C2C12 myotubes were treated with 1 mg protein equivalent/mL of SGID-BWPHs for 4 h. Muscle growth and myotube thickness were analysed using an xCelligence™ platform. Anabolic signalling (phosphorylation of mTOR, rpS6 and 4E-BP1) and MPS measured by puromycin incorporation were assessed using immunoblotting. BWPH-X significantly increased muscle growth (p < 0.01) and myotube thickness (p < 0.0001) compared to the negative control (amino acid and serum free media). Muscle protein synthesis (MPS), as measured by puromycin incorporation, was significantly higher after incubation with BWPH-X compared with the negative control, but did not significantly change in response to BWPH-Y and Z treatments. Taken together, these preliminary findings demonstrate the anabolic potential of some but not all BWPHs on muscle enhancement, thus providing justification for human dietary intervention studies to confirm and translate the results of such investigations to dietary recommendations and practices.

A Review of Rehabilitation Benefits of Exercise Training Combined with Nutrition Supplement for Improving Protein Synthesis and Skeletal Muscle Strength in Patients with Cerebral Stroke

Cerebral vascular accident (CVA) is one of the main causes of chronic disability, and it affects the function of daily life, so it is increasingly important to actively rehabilitate patients' physical functions. The research confirmed that the nutrition supplement strategy is helpful to improve the effect of sports rehabilitation adaptation and sports performance. The patients with chronic strokes (whose strokes occur for more than 6 months) have special nutritional needs while actively carrying out rehabilitation exercises, but there are still few studies to discuss at present. Therefore, this paper will take exercise rehabilitation to promote muscle strength and improve muscle protein synthesis as the main axis and, through integrating existing scientific evidence, discuss the special needs of chronic stroke patients in rehabilitation exercise intervention and nutrition supplement one by one. At the same time, we further evaluated the physiological mechanism of nutrition intervention to promote training adaptation and compared the effects of various nutrition supplement strategies on stroke rehabilitation. Literature review pointed out that immediately supplementing protein nutrition (such as whey protein or soybean protein) after resistance exercise or endurance exercise can promote the efficiency of muscle protein synthesis and produce additive benefits, thereby improving the quality of muscle tissue. Recent animal research results show that probiotics can prevent the risk factors of neural function degradation and promote the benefits of sports rehabilitation. At the same time, natural polyphenols (such as catechin or resveratrol) or vitamins can also reduce the oxidative stress injury caused by animal stroke and promote the proliferation of neural tissue. In view of the fact that animal research results still make up the majority of issues related to the role of nutrition supplements in promoting nerve repair and protection, and the true benefits still need to be confirmed by subsequent human studies. This paper suggests that the future research direction should be the supplement of natural antioxidants, probiotics, compound nutritional supplements, and integrated human clinical research.

Reply to Grant, W.B. Comment on "Coelho-Junior et al. Protein Intake and Frailty in Older Adults: A Systematic Review and Meta-Analysis of Observational Studies. Nutrients 2022, 14, 2767"

We recently conducted a systematic review and meta-analysis to assess the association between protein consumption and frailty in older adults [...].

Branched-Chain Amino Acids and Mitochondrial Biogenesis: An Overview and Mechanistic Summary

Branched-chain amino acids (BCAA) are essential in the diet and promote several vital cell responses which may have benefits for health and athletic performance, as well as disease prevention. While BCAA are well-known for their ability to stimulate muscle protein synthesis, their effects on cell energetics are also becoming well-documented, but these receive less attention. In this review, much of the current evidence demonstrating BCAA ability (as individual amino acids or as part of dietary mixtures) to alter regulators of cellular energetics with an emphasis on mitochondrial biogenesis and related signaling is highlighted. Several studies have shown, both in vitro and in vivo, that BCAA (either individual or as a mixture) may promote signaling associated with increased mitochondrial biogenesis including the upregulation of master regulator of mitochondrial biogenesis peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), as well as numerous downstream targets and related function. However, sparse data in humans and the difficulty of controlling variables associated with feeding studies leave the physiological relevance of these findings unclear. Future well-controlled diet studies will be needed to assess if BCAA consumption is associated with increased mitochondrial biogenesis and improved metabolic outcomes in healthy and/or diseased human populations.

Protein Intake and Sarcopenia in Older Adults: A Systematic Review and Meta-Analysis

Background: The present systematic review and meta-analysis investigated the cross-sectional and longitudinal associations between protein intake and sarcopenia in older adults. Methods: Observational studies that investigated the association between protein intake and sarcopenia as the primary or secondary outcome in people aged 60 years and older were included. Studies published in languages other than English, Italian, Portuguese, and Spanish were excluded. Studies were retrieved from MEDLINE, SCOPUS, EMBASE, CINAHL, AgeLine, and Food Science Source databases through January 31, 2022. A pooled effect size was calculated based on standard mean differences. Results: Five cross-sectional studies, one longitudinal study, and one case-control study that investigated 3353 community-dwelling older adults with a mean age of approximately 73 years were included. The meta-analysis of four studies indicated that older adults with sarcopenia consumed significantly less protein than their peers with no sarcopenia. Conclusions: Results of the present study suggest that an inadequate protein intake might be associated with sarcopenia in older adults.

Protein Intake and Frailty in Older Adults: A Systematic Review and Meta-Analysis of Observational Studies

BACKGROUND

The present systematic review and meta-analysis investigated the cross-sectional and longitudinal associations between protein intake and frailty in older adults.

METHODS

We conducted a systematic review and meta-analysis of cross-sectional and longitudinal studies that investigated the association between protein intake and frailty in older adults. Cross-sectional, case-control, and longitudinal cohort studies that investigated the association between protein intake and frailty as a primary or secondary outcome in people aged 60+ years were included. Studies published in languages other than English, Italian, Portuguese, or Spanish were excluded. Studies were retrieved on 31 January 2022.

RESULTS

Twelve cross-sectional and five longitudinal studies that investigated 46,469 community-dwelling older adults were included. The meta-analysis indicated that absolute, bodyweight-adjusted, and percentage of protein relative to total energy consumption were not cross-sectionally associated with frailty. However, frail older adults consumed significantly less animal-derived protein than robust people. Finally, high protein consumption was associated with a significantly lower risk of frailty.

CONCLUSIONS

Our pooled analysis indicates that protein intake, whether absolute, adjusted, or relative to total energy intake, is not significantly associated with frailty in older adults. However, we observed that frail older adults consumed significantly less animal protein than their robust counterparts.

Blood Flow Restriction Training for the Intervention of Sarcopenia: Current Stage and Future Perspective

Sarcopenia is a geriatric syndrome that is characterized by a progressive and generalized skeletal muscle disorder and can be associated with many comorbidities, including obesity, diabetes, and fracture. Its definitions, given by the AWGS and EWGSOP, are widely used. Sarcopenia is measured by muscle strength, muscle quantity or mass and physical performance. Currently, the importance and urgency of sarcopenia have grown. The application of blood flow restriction (BFR) training has received increased attention in managing sarcopenia. BFR is accomplished using a pneumatic cuff on the proximal aspect of the exercising limb. Two main methods of exercise, aerobic exercise and resistance exercise, have been applied with BFR in treating sarcopenia. Both methods can increase muscle mass and muscle strength to a certain extent. Intricate mechanisms are involved during BFRT. Currently, the presented mechanisms mainly include responses in the blood vessels and related hormones, such as growth factors, tissue hypoxia-related factors and recruitment of muscle fiber as well as muscle satellite cells. These mechanisms contribute to the positive balance of skeletal muscle synthesis, which in turn mitigates sarcopenia. As a more suited and more effective way of treating sarcopenia and its comorbidities, BFRT can serve as an alternative to traditional exercise for people who have marked physical limitations or even show superior outcomes under low loads. However, the possibility of causing stress or muscle damage must be considered. Cuff size, pressure, training load and other variables can affect the outcome of sarcopenia, which must also be considered. Thoroughly studying these factors can help to better determine an ideal BFRT scheme and better manage sarcopenia and its associated comorbidities. As a well-tolerated and novel form of exercise, BFRT offers more potential in treating sarcopenia and involves deeper insights into the function and regulation of skeletal muscle.

Phosphorylation of Eukaryotic Initiation Factor 4G1 (eIF4G1) at Ser1147 Is Specific for eIF4G1 Bound to eIF4E in Delayed Neuronal Death after Ischemia

Ischemic strokes are caused by a reduction in cerebral blood flow and both the ischemic period and subsequent reperfusion induce brain injury, with different tissue damage depending on the severity of the ischemic insult, its duration, and the particular areas of the brain affected. In those areas vulnerable to cerebral ischemia, the inhibition of protein translation is an essential process of the cellular response leading to delayed neuronal death. In particular, translation initiation is rate-limiting for protein synthesis and the eukaryotic initiation factor (eIF) 4F complex is indispensable for cap-dependent protein translation. In the eIF4F complex, eIF4G is a scaffolding protein that provides docking sites for the assembly of eIF4A and eIF4E, binding to the cap structure of the mRNA and stabilizing all proteins of the complex. The eIF4F complex constituents, eIF4A, eIF4E, and eIF4G, participate in translation regulation by their phosphorylation at specific sites under cellular stress conditions, modulating the activity of the cap-binding complex and protein translation. This work investigates the phosphorylation of eIF4G1 involved in the eIF4E/eIF4G1 association complex, and their regulation in ischemia-reperfusion (IR) as a stress-inducing condition. IR was induced in an animal model of transient cerebral ischemia and the results were studied in the resistant cortical region and in the vulnerable hippocampal CA1 region. The presented data demonstrate the phosphorylation of eIF4G1 at Ser¹¹⁴⁷, Ser¹¹⁸⁵, and Ser¹²³¹ in both brain regions and in control and ischemic conditions, being the phosphorylation of eIF4G1 at Ser¹¹⁴⁷ the only one found in the eIF4E/eIF4G association complex from the cap-containing matrix (m⁷GTP-Sepharose). In addition, our work reveals the specific modulation of the phosphorylation of eIF4G1 at Ser¹¹⁴⁷ in the vulnerable region, with increased levels and colocalization with eIF4E in response to IR. These findings contribute to elucidate the molecular mechanism of protein translation regulation that underlies in the balance of cell survival/death during pathophysiological stress, such as cerebral ischemia.

Protein Source and Muscle Health in Older Adults: A Literature Review

Research shows that higher dietary protein of up to 1.2 g/kg_bodyweight/day may help prevent sarcopenia and maintain musculoskeletal health in older individuals. Achieving higher daily dietary protein levels is challenging, particularly for older adults with declining appetites and underlying health conditions. The negative impact of these limitations on aging muscle may be circumvented through the consumption of high-quality sources of protein and/or supplementation. Currently, there is a debate regarding whether source of protein differentially affects musculoskeletal health in older adults. Whey and soy protein have been used as the most common high-quality proteins in recent literature. However, there is growing consumer demand for additional plant-sourced dietary protein options. For example, pea protein is rapidly gaining popularity among consumers, despite little to no research regarding its long-term impact on muscle health. Therefore, the objectives of this review are to: (1) review current literature from the past decade evaluating whether specific source(s) of dietary protein provide maximum benefit to muscle health in older adults; and (2) highlight the need for future research specific to underrepresented plant protein sources, such as pea protein, to then provide clearer messaging surrounding plant-sourced versus animal-sourced protein and their effects on the aging musculoskeletal system.

Leucine Supplementation Decreases HDAC4 Expression and Nuclear Localization in Skeletal Muscle Fiber of Rats Submitted to Hindlimb Immobilization

In this study we surveyed a rat skeletal muscle RNA-Seq for genes that are induced by hindlimb immobilization and, in turn, become attenuated by leucine supplementation. This approach, in search of leucine-atrophy protection mediating genes, identified histone deacetylase 4 (HDAC4) as highly responsive to both hindlimb immobilization and leucine supplementation. We then examined the impact of leucine on HDAC4 expression, tissue localization, and target genes. A total of 76 male Wistar rats (~280 g) were submitted to hindlimb immobilization and/or leucine supplementation for 3, 7 and 12 days. These animals were euthanized, and soleus muscle was removed for further analysis. RNA-Seq analysis of hindlimb immobilized rats indicated a sharp induction (log2 = 3.4) of HDAC4 expression which was attenuated by leucine supplementation (~50%). Real-time PCR and protein expression analysis by Western blot confirmed increased HDAC4 mRNA after 7 days of hindlimb immobilization and mitigation of induction by leucine supplementation. Regarding the HDAC4 localization, the proportion of positive nuclei was higher in the immobilized group and decreased after leucine supplementation. Also, we found a marked decrease of myogenin and MAFbx-atrogin-1 mRNA levels upon leucine supplementation, while CAMKII and DACH2 mRNA levels were increased by leucine supplementation. Our data suggest that HDAC4 inhibition might be involved in the anti-atrophic effects of leucine.

Protein Intake and Frailty: A Matter of Quantity, Quality, and Timing

Frailty is a geriatric syndrome that refers to a state of reduced resiliency to stressful events that occurs in response to physiological and/or psychosocial detriments. Frailty is a predictor of poor prognosis, given that frail older adults are at higher risk of many adverse health-related events. Hence, the identification of potential strategies to prevent the development and progression of frailty is of extreme importance for avoiding its negative outcomes. An adequate protein consumption is advocated as a possible intervention for the management of frailty in older adults due to its effects on muscle mass and physical function. However, empirical evidence is still needed to support this proposition. On the other hand, substantial evidence from observational studies has provided important information on the association between frailty and dietary protein-related parameters. Here, we provide a narrative review of the current literature regarding the association between protein intake (amount (how much?), quality (what type?), and distribution across meals (when?)) and frailty-related parameters. The ultimate aim of this work is to offer practical, evidence-based indications to healthcare professionals responsible for the care of frail older adults.

Role of Akt signaling pathway regulation in the speckled mousebird (Colius striatus) during torpor displays tissue specific responses

Pronounced heterothermic responses are relatively rare among birds. Along with taxa such as hummingbirds and caprimulgids, the order Coliiformes (mousebirds) is known to possess the physiological capacity for torpor. During torpor, body temperature is greatly reduced and a bird becomes unresponsive to external stimuli until ambient temperatures return to more favorable conditions. Under such conditions, these birds are forced to rely only on their internal fuel storage for energy and show great reduction in metabolic rates by decreasing energy-expensive processes. This study investigated the role of the key insulin-Akt signaling kinase pathway involved in regulating energy metabolism and protein translation in the liver, kidney, heart, skeletal muscle, and brain of the speckled mousebird (Colius striatus). The degree of phosphorylation of well-conserved target residues with important regulatory function was examined in both the euthermic control and torpid birds. The results demonstrated marked differences in responses between the tissues with decreases in RPS6 S235/236 phosphorylation in the kidney (0.52 fold of euthermic) and muscle (0.29 fold of euthermic) as well as decreases in GS3K3β S9 in muscle (0.60 fold of euthermic) and GSK3α S21 (0.71 fold of euthermic) phosphorylation in kidney during torpor, suggesting a downregulation of this pathway. Interestingly, the liver demonstrated an increase in RPS6 S235/236 (2.89 fold increase) and P70S6K T412 (1.44 fold increase) phosphorylation in the torpor group suggesting that protein translation is maintained in this tissue. This study demonstrates that avian torpor is a complex phenomenon and alterations in this signaling pathway follow a tissue specific pattern.

Leucine-Enriched Protein Supplementation Increases Lean Body Mass in Healthy Korean Adults Aged 50 Years and Older: A Randomized, Double-Blind, Placebo-Controlled Trial

Early prevention of sarcopenia could be an important strategy for muscle retention, but most studies have focused on subjects aged 65 or older. Therefore, in this study we investigated the effects of leucine-enriched protein supplementation on muscle condition in a sample including late middle-aged adults. A 12-week intervention was performed for 120 healthy community-dwelling adults by providing either leucine-enriched protein supplement [leucine 3 g, protein mixture (casein 50% + whey 40% + soy 10%) 17 g, vitamin D 800IU (20 µg), calcium 300 mg, fat 1.1 g, carbohydrate 2.5 g] or isocaloric carbohydrate supplement twice per day. Appendicular skeletal muscle mass index (ASMI) and lean body mass (LBM) were measured by dual-energy X-ray absorptiometry. A total of 111 participants completed the study, with a dropout rate of 9.2%. LBM normalized by height and body weight (LBM/Wt) was significantly increased (p < 0.001) in the intervention group (0 wk: 633.9 ± 8.5 vs. 12 wk 636.9 ± 8.4 in the intervention group; 0 wk: 638.6 ± 8.3 vs. 12 wk: 632.9 ± 8.1 in the control group). In subgroup analyses, significant differences remained only in subjects between 50 and 64 years of age. We concluded that leucine-enriched protein supplementation can have beneficial effects by preventing muscle loss, mainly for late middle-aged adults.

Differential regulation of mTORC1 activation by leucine and β-hydroxy-β-methylbutyrate in skeletal muscle of neonatal pigs

Leucine (Leu) and its metabolite β-hydroxy-β-methylbutyrate (HMB) stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1)-dependent protein synthesis in the skeletal muscle of neonatal pigs. This study aimed to determine whether HMB and Leu utilize common nutrient-sensing mechanisms to activate mTORC1. In study 1, neonatal pigs were fed one of five diets for 24 h: low protein (LP), high protein (HP), or LP supplemented with 4 (LP+HMB4), 40 (LP+HMB40), or 80 (LP+HMB80) μmol HMB·kg body wt^-1·day^-1. In study 2, neonatal pigs were fed for 24 h: LP, LP supplemented with Leu (LP+Leu), or HP diets delivering 9, 18, and 18 mmol Leu·kg body wt^-1·day^-1, respectively. The upstream signaling molecules that regulate mTORC1 activity were analyzed. mTOR phosphorylation on Ser2448 and Ser2481 was greater in LP+HMB40, LP+HMB80, and LP+Leu than in LP and greater in HP than in HMB-supplemented groups (P < 0.05), whereas HP and LP+Leu were similar. Rheb-mTOR complex formation was lower in LP than in HP (P < 0.05), with no enhancement by HMB or Leu supplementation. The Sestrin2-GATOR2 complex was more abundant in LP than in HP and was reduced by Leu (P < 0.05) but not HMB supplementation. RagA-mTOR and RagC-mTOR complexes were higher in LP+Leu and HP than in LP and HMB groups (P < 0.05). There were no treatment differences in RagB-SH3BP4, Vps34-LRS, and RagD-LRS complex abundances. Phosphorylation of Erk1/2 and TSC2, but not AMPK, was lower in LP than HP (P < 0.05) and unaffected by HMB or Leu supplementation. Our results demonstrate that HMB stimulates mTORC1 activation in neonatal muscle independent of the leucine-sensing pathway mediated by Sestrin2 and the Rag proteins.NEW & NOTEWORTHY Dietary supplementation with either leucine or its metabolite β-hydroxy-β-methylbutyrate (HMB) stimulates protein synthesis in skeletal muscle of the neonatal pig. Our results demonstrate that both leucine and HMB stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) phosphorylation in neonatal muscle. This leucine-stimulated process involves dissociation of the Sestrin2-GATOR2 complex and increased binding of Rag A/C to mTOR. However, HMB's activation of mTORC1 is independent of this leucine-sensing pathway.

Leucine attenuates muscle atrophy and autophagosome formation by activating PI3K/AKT/mTOR signaling pathway in rotator cuff tears

Rotator cuff tears (RCTs), the most common tendon injury, are always accompanied by muscle atrophy, which is characterized by excessive protein degradation. Autophagy-lysosome systems are the crucial proteolytic pathways and are activated in atrophying muscle. Thus, inhibition of the autophagy-lysosome pathway might be an alternative way to minimize skeletal muscle atrophy. In this present study, combined with a tendon transection-induced rat model of massive rotator cuff tears, HE staining and transmission electron microscopy methods, we found leucine supplementation effectively prevented muscle atrophy, muscle injury and autophagosome formation. Utilizing immunoblotting, we discovered that leucine supplementation is able to inhibit the rise in autophagy-related protein expression (including LC3, Atrogin-1, MuRF1, Bnip3 and FoxO3) driven by tendon transection. The PI3K/AKT/mTOR pathway that was essential in autophagosome formation and autophagy was blocked in atrophying muscle and reactivated in the presence of leucine. Once administrated with LY294002 (PI3K inhibitor) and Rapamycin (mTOR inhibitor), leucine mediated by the anti-atrophic effects was nearly blunted. These results suggest that leucine potentially attenuates tendon transection-induced muscle atrophy through autophagy inhibition via activating the PI3K/AKT/mTOR pathway.

BCAA Metabolism and Insulin Sensitivity - Dysregulated by Metabolic Status?

Branched-chain amino acids (BCAAs) appear to influence several synthetic and catabolic cellular signaling cascades leading to altered phenotypes in mammals. BCAAs are most notably known to increase protein synthesis through modulating protein translation, explaining their appeal to resistance and endurance athletes for muscle hypertrophy, expedited recovery, and preservation of lean body mass. In addition to anabolic effects, BCAAs may increase mitochondrial content in skeletal muscle and adipocytes, possibly enhancing oxidative capacity. However, elevated circulating BCAA levels have been correlated with severity of insulin resistance. It is hypothesized that elevated circulating BCAAs observed in insulin resistance may result from dysregulated BCAA degradation. This review summarizes original reports that investigated the ability of BCAAs to alter glucose uptake in consequential cell types and experimental models. The review also discusses the interplay of BCAAs with other metabolic factors, and the role of excess lipid (and possibly energy excess) in the dysregulation of BCAA catabolism. Lastly, this article provides a working hypothesis of the mechanism(s) by which lipids may contribute to altered BCAA catabolism, which often accompanies metabolic disease.

Leucine Differentially Regulates Gene-Specific Translation in Mouse Skeletal Muscle

Background: Amino acids, especially leucine, are particularly effective in promoting protein synthesis. Leucine is known to increase the rate of protein synthesis in skeletal muscle through the mechanistic target of rapamycin complex 1-dependent, as well as -independent, signaling pathways. However, the overall translation program is poorly defined, and it is unknown how the activation of these pathways differentially controls the translation of specific mRNAs.Objective: Ribosome profiling and RNA sequencing were used to precisely define the translational program activated by an acute oral dose of leucine.Methods: Adult male C57BL/6 mice were deprived of food overnight before the delivery of an acute dose of l-leucine (9.4 mg) (n = 6) or vehicle (n = 5) and tissues collected 30 min later. Ribosome footprints and total RNA were isolated and subjected to deep sequencing. Changes in gene-specific mRNA abundance and ribosome occupancy were determined between the leucine-treated and control groups by aligning sequence reads to Reference Sequence database mRNAs and applying statistical features of the Bioconductor package edgeR.Results: Our data revealed mRNA features that confer translational control of skeletal muscle mRNAs in response to an acute dose of leucine. The subset of skeletal muscle mRNAs that are activated consists largely of terminal oligopyrimidine mRNAs (false discovery rate: <0.05), whereas those with reduced translation had 5' untranslated regions with increased length. Only the small nuclear RNAs, which are required for ribosome biogenesis, were significantly altered in RNA abundance. The inferred functional translational program activated by dietary leucine includes increased protein synthesis capacity and energy metabolism, upregulation of sarcomere-binding proteins, modulation of circadian rhythm, and suppression of select immune components.Conclusions: These results clarify the translation program acutely stimulated by leucine in mouse skeletal muscle and establish new methodologies for use in future studies of skeletal muscle disease or aging and further examination of downstream effects of leucine on gene expression.

Acute myotube protein synthesis regulation by IL-6-related cytokines

IL-6 and leukemia inhibitory factor (LIF), members of the IL-6 family of cytokines, play recognized paradoxical roles in skeletal muscle mass regulation, being associated with both growth and atrophy. Overload or muscle contractions can induce a transient increase in muscle IL-6 and LIF expression, which has a regulatory role in muscle hypertrophy. However, the cellular mechanisms involved in this regulation have not been completely identified. The induction of mammalian target of rapamycin complex 1 (mTORC1)-dependent myofiber protein synthesis is an established regulator of muscle hypertrophy, but the involvement of the IL-6 family of cytokines in this process is poorly understood. Therefore, we investigated the acute effects of IL-6 and LIF administration on mTORC1 signaling and protein synthesis in C2C12 myotubes. The role of glycoprotein 130 (gp130) receptor and downstream signaling pathways, including phosphoinositide 3-kinase (PI3K)-Akt-mTORC1 and signal transducer and activator of transcription 3 (STAT3)-suppressor of cytokine signaling 3 (SOCS3), was investigated by administration of specific siRNA or pharmaceutical inhibitors. Acute administration of IL-6 and LIF induced protein synthesis, which was accompanied by STAT3 activation, Akt-mTORC1 activation, and increased SOCS3 expression. This induction of protein synthesis was blocked by both gp130 siRNA knockdown and Akt inhibition. Interestingly, STAT3 inhibition or Akt downstream mTORC1 signaling inhibition did not fully block the IL-6 or LIF induction of protein synthesis. SOCS3 siRNA knockdown increased basal protein synthesis and extended the duration of the protein synthesis induction by IL-6 and LIF. These results demonstrate that either IL-6 or LIF can activate gp130-Akt signaling axis, which induces protein synthesis via mTORC1-independent mechanisms in cultured myotubes. However, IL-6- or LIF-induced SOCS3 negatively regulates the activation of myotube protein synthesis.

Muscle Protein Signaling in C2C12 Cells Is Stimulated to Similar Degrees by Diverse Commercial Food Protein Sources and Experimental Soy Protein Hydrolysates

Dietary protein stimulates muscle protein synthesis and is essential for muscle health. We developed a screening assay using C2C12 mouse muscle cells to assess the relative abilities of diverse commercial protein sources and experimental soy protein hydrolysates (ESH), after simulated gut digestion (SGD), to activate the mechanistic target of rapamycin complex I (mTORC1) muscle protein synthesis signaling pathway (p70S6K^(Thr389) phosphorylation). Activation of mTORC1 was expressed as a percentage of a maximal insulin response. The bioactivities of proteins grouped by source including fish (81.3 ± 10.6%), soy (66.2 ± 4.7%), dairy (61.8 ± 4.3%), beef (53.7 ± 8.6%), egg (52.3 ± 10.6%), soy whey (43.4 ± 8.6%), and pea (31.4 ± 10.6%) were not significantly different from each other. Bioactivity for ESH ranged from 28.0 ± 7.5 to 98.2 ± 6.6%. The results indicate that both the protein source and processing conditions are key determinants for mTORC1 activation. Regression analyses demonstrated that neither leucine nor total branched-chain amino acid content of proteins is the sole predictor of mTORC1 activity and that additional factors are necessary.

Amino Acid (Leucine) Chromatography: A Study of Branched-Chain Aminoaciduria in Type 2 Diabetes

INTRODUCTION

Diabetes is a disease characterized by insulin deficiency resulting in glucose intolerance and in abnormalities of other metabolic fuels including protein. Recently, a number of studies have revealed that branched-chain amino acids (BCAAs) (leucine, isoleucine, and valine) play an important role in the regulation of protein synthesis by activating mammalian target of rapamycin (mTOR) in pancreatic β cells. BCAAs have positive effects on the regulation of glucose homeostasis. Leucine is an important nutrient signal as evidenced by recent observations, which showed increased fasting concentrations of circulating BCAAs being associated with an increased risk of type 2 diabetes (T2D) and insulin resistance in humans. Leucine seems to have direct effects on hypothalamic and brainstem functioning involved in satiety, which can potentially contribute to obesity and T2D. A number of observational studies indicate that elevated activity of BCAAs could be associated with poor metabolic health and T2D complications. Although these associations were consistently observed in humans, the mechanisms underlying this relationship remain to be completely understood. In this study, we have attempted to evaluate urinary excretion of leucine among patients of T2D and compared them with healthy controls by using a low-cost and non-invasive amino acid chromatography technique.

METHODS

The study was carried out in the Department of Biochemistry, Central Research Unit, Prathima Institute of Medical Sciences (PIMS), Karimnagar, Telangana, India, during the period between July and September 2016. A group of 55 normal healthy subjects (control group A), and 55 patients suffering from T2D on treatment (test group B), were enrolled in the study. The urine samples were collected from normal and T2D subjects. Thin-layer chromatography (TLC) for leucine was performed on all the urine samples.

RESULTS

A strong correlation (p=0.0004) was found between the urinary excretion of leucine among the control (Rf=0.174 ±0.089) and T2D (Rf=0.247 ±0.030) patients.

CONCLUSION

Excretion of BCAAs (leucine) in detectable and increased quantities reflect the presence of an altered metabolic state attributable to T2D, which in turn could lead to early diabetic complications. This method (TLC), being non-invasive and cost-effective, could be recommended for assessing the progression and management of type 2 diabetes patients.

Novel metabolic and physiological functions of branched chain amino acids: a review

It is widely known that branched chain amino acids (BCAA) are not only elementary components for building muscle tissue but also participate in increasing protein synthesis in animals and humans. BCAA (isoleucine, leucine and valine) regulate many key signaling pathways, the most classic of which is the activation of the mTOR signaling pathway. This signaling pathway connects many diverse physiological and metabolic roles. Recent years have witnessed many striking developments in determining the novel functions of BCAA including: (1) Insufficient or excessive levels of BCAA in the diet enhances lipolysis. (2) BCAA, especially isoleucine, play a major role in enhancing glucose consumption and utilization by up-regulating intestinal and muscular glucose transporters. (3) Supplementation of leucine in the diet enhances meat quality in finishing pigs. (4) BCAA are beneficial for mammary health, milk quality and embryo growth. (5) BCAA enhance intestinal development, intestinal amino acid transportation and mucin production. (6) BCAA participate in up-regulating innate and adaptive immune responses. In addition, abnormally elevated BCAA levels in the blood (decreased BCAA catabolism) are a good biomarker for the early detection of obesity, diabetes and other metabolic diseases. This review will provide some insights into these novel metabolic and physiological functions of BCAA.

l-Leucine Increases Skeletal Muscle IGF-1 but Does Not Differentially Increase Akt/mTORC1 Signaling and Serum IGF-1 Compared to Ursolic Acid in Response to Resistance Exercise in Resistance-Trained Men

OBJECTIVE

Ursolic acid administration following resistance exercise increases mammalian target of rapamycin complex 1 (mTORC1) activity and skeletal muscle IGF-1 concentration in murines in a manner similar to l-leucine yet remains unexamined in humans. This study examined serum and skeletal muscle insulin-like growth factor-1 (IGF-1) and Akt/mTORC1 signaling activity following ingestion of either ursolic acid or l-leucine immediately after resistance exercise.

METHODS

Nine resistance-trained men performed 3 lower-body resistance exercise sessions involving 4 sets of 8-10 repetitions at 75%-80% one repetition maximum (1-RM) on the angled leg press and knee extension exercises. Immediately following each session, participants orally ingested 3 g cellulose placebo (PLC), l-leucine (LEU), or ursolic acid (UA). Blood samples were obtained pre-exercise and at 0.5, 2, and 6 hours postexercise. Muscle biopsies were obtained pre-exercise and at 2 and 6 hours postexercise.

RESULTS

Plasma leucine increased in LEU at 2 hours postexercise compared to PLC (p = 0.04). Plasma ursolic acid increased in UA at 2 h and 6 hours postexercise compared to PLC and LEU (p < 0.003). No significant differences were observed for serum insulin (p = 0.98) and IGF-1 (p = 0.99) or skeletal muscle IGF-1 receptor (IGF-1R; p = 0.84), Akt (p = 0.55), mTOR (p = 0.09), and p70S6K (p = 0.98). Skeletal muscle IGF-1 was significantly increased in LEU at 2 hours postexercise (p = 0.03) and 6 hours postexercise (p = 0.04) compared to PLC and UA.

CONCLUSION

Three grams of l-leucine and ursolic acid had no effect on Akt/mTORC1 signaling or serum insulin or IGF-1; however, l-leucine increased skeletal muscle IGF-1 concentration in resistance-trained men.

Human Skeletal Muscle Protein Metabolism Responses to Amino Acid Nutrition

Healthy individuals maintain remarkably constant skeletal muscle mass across much of adult life, suggesting the existence of robust homeostatic mechanisms. Muscle exists in dynamic equilibrium whereby the influx of amino acids (AAs) and the resulting increases in muscle protein synthesis (MPS) associated with the intake of dietary proteins cancel out the efflux of AAs from muscle protein breakdown that occurs between meals. Dysregulated proteostasis is evident with aging, especially beyond the sixth decade of life. Women and men aged 75 y lose muscle mass at a rate of ∼0.7% and 1%/y, respectively (sarcopenia), and lose strength 2- to 5-fold faster (dynapenia) as muscle "quality" decreases. Factors contributing to the disruption of an otherwise robust proteostatic system represent targets for potential therapies that promote healthy aging. Understanding age-related impairments in anabolic responses to AAs and identifying strategies to mitigate these factors constitute major areas of interest. Numerous studies have aimed to identify 1) the influence of distinct protein sources on absorption kinetics and muscle anabolism, 2) the latency and time course of MPS responses to protein/AAs, 3) the impacts of protein/AA intake on muscle microvascular recruitment, and 4) the role of certain AAs (e.g., leucine) as signaling molecules, which are able to trigger anabolic pathways in tissues. This review aims to discuss these 4 issues listed, to provide historical and modern perspectives of AAs as modulators of human skeletal muscle protein metabolism, to describe how advances in stable isotope/mass spectrometric approaches and instrumentation have underpinned these advances, and to highlight relevant differences between young adults and older individuals. Whenever possible, observations are based on human studies, with additional consideration of relevant nonhuman studies.

Global deletion of BCATm increases expression of skeletal muscle genes associated with protein turnover

Consumption of a protein-containing meal by a fasted animal promotes protein accretion in skeletal muscle, in part through leucine stimulation of protein synthesis and indirectly through repression of protein degradation mediated by its metabolite, α-ketoisocaproate. Mice lacking the mitochondrial branched-chain aminotransferase (BCATm/Bcat2), which interconverts leucine and α-ketoisocaproate, exhibit elevated protein turnover. Here, the transcriptomes of gastrocnemius muscle from BCATm knockout (KO) and wild-type mice were compared by next-generation RNA sequencing (RNA-Seq) to identify potential adaptations associated with their persistently altered nutrient signaling. Statistically significant changes in the abundance of 1,486/∼39,010 genes were identified. Bioinformatics analysis of the RNA-Seq data indicated that pathways involved in protein synthesis [eukaryotic initiation factor (eIF)-2, mammalian target of rapamycin, eIF4, and p70S6K pathways including 40S and 60S ribosomal proteins], protein breakdown (e.g., ubiquitin mediated), and muscle degeneration (apoptosis, atrophy, myopathy, and cell death) were upregulated. Also in agreement with our previous observations, the abundance of mRNAs associated with reduced body size, glycemia, plasma insulin, and lipid signaling pathways was altered in BCATm KO mice. Consistently, genes encoding anaerobic and/or oxidative metabolism of carbohydrate, fatty acids, and branched chain amino acids were modestly but systematically reduced. Although there was no indication that muscle fiber type was different between KO and wild-type mice, a difference in the abundance of mRNAs associated with a muscular dystrophy phenotype was observed, consistent with the published exercise intolerance of these mice. The results suggest transcriptional adaptations occur in BCATm KO mice that along with altered nutrient signaling may contribute to their previously reported protein turnover, metabolic and exercise phenotypes.

Nutritional epigenetics with a focus on amino acids: implications for the development and treatment of metabolic syndrome

Recent findings from human and animal studies indicate that maternal undernutrition or overnutrition affects covalent modifications of the fetal genome and its associated histones that can be carried forward to subsequent generations. An adverse outcome of maternal malnutrition is the development of metabolic syndrome, which is defined as a cluster of disorders including obesity, hyperglycemia, hyperinsulinemia, hyperlipidemia, hypertension and insulin resistance. The transgenerational impacts of maternal nutrition are known as fetal programming, which is mediated by stable and heritable alterations of gene expression through covalent modifications of DNA and histones without changes in DNA sequences (namely, epigenetics). The underlying mechanisms include chromatin remodeling, DNA methylation (occurring at the 5'-position of cytosine residues within CpG dinucleotides), histone modifications (acetylation, methylation, phosphorylation, ubiquitination and sumoylation) and expression and activity of small noncoding RNAs. The enzymes catalyzing these reactions include S-adenosylmethionine-dependent DNA and protein methyltransferases, DNA demethylases, histone acetylase (lysine acetyltransferase), general control nonderepressible 5 (GCN5)-related N-acetyltransferase (a superfamily of acetyltransferase) and histone deacetylase. Amino acids (e.g., glycine, histidine, methionine and serine) and vitamins (B6, B12 and folate) play key roles in provision of methyl donors for DNA and protein methylation. Therefore, these nutrients and related metabolic pathways are of interest in dietary treatment of metabolic syndrome. Intervention strategies include targeting epigenetically disturbed metabolic pathways through dietary supplementation with nutrients (particularly functional amino acids and vitamins) to regulate one-carbon-unit metabolism, antioxidative reactions and gene expression, as well as protein methylation and acetylation. These mechanism-based approaches may effectively improve health and well-being of affected offspring.

Effect of leucine uptake on hepatic and skeletal muscle gene expression in rats: a microarray analysis

[Purpose]

This study was performed to explore the physiological functions of leucine by exploring genes with leucine-dependent variability using DNA microarray.

[Methods]

Sprague-Dawley rats (n = 20) were separated into a HPD (30% High Protein Diet, n = 10) group and a NPD (0% Non Protein Diet, n = 10) group and fed a protein diet for 2 weeks. At the end of the 2-week period, the rats were fasted for 12-16 hours, further separated into subgroups within the HPD (Saline, n = 5, Leucine, n = 5) and NPD (Saline, n = 5, Leucine, n = 5) groups and administered with a leucine solution. The liver and muscles were harvested after 2 hours for RNA extraction. RNA purification from the isolated muscles and target gene identification using DNA chip were performed. The target gene was determined based on the results of the DNA chip experiment, and mRNA expression of the target gene was analyzed using Real-Time PCR.

[Results]

In the skeletal muscle, 27 genes were upregulated while 52 genes were down regulated after leucine administration in the NPD group. In the liver, 160 genes were up-regulated while 126 were down-regulated. The per2 gene was one of the genes with leucine-dependent induction in muscles and liver.

[Conclusion]

This study was performed to explore the physiological functions of leucine, however, a large number of genes showed variability. Therefore, it was difficult to definitively identify the genes linked with a particular physiological function. Various nutritional effects of leucine were observed. High variability in cytokines, receptors, and various membrane proteins were observed, which suggests that leucine functions as more than a nutrient. The interpretation may depend on investigators’ perspectives, therefore, discussion with relevant experts and the BCAA (Branched-Chain Amino Acids) society may be needed for effective utilization of this data.

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 supplementation improves skeletal muscle regeneration after cryolesion in rats

This study was undertaken in order to provide further insight into the role of leucine supplementation in the skeletal muscle regeneration process, focusing on myofiber size and strength recovery. Young (2-month-old) rats were subjected or not to leucine supplementation (1.35 g/kg per day) started 3 days prior to cryolesion. Then, soleus muscles were cryolesioned and continued receiving leucine supplementation until 1, 3 and 10 days later. Soleus muscles from leucine-supplemented animals displayed an increase in myofiber size and a reduction in collagen type III expression on post-cryolesion day 10. Leucine was also effective in reducing FOXO3a activation and ubiquitinated protein accumulation in muscles at post-cryolesion days 3 and 10. In addition, leucine supplementation minimized the cryolesion-induced decrease in tetanic strength and increase in fatigue in regenerating muscles at post-cryolesion day 10. These beneficial effects of leucine were not accompanied by activation of any elements of the phosphoinositide 3-kinase/Akt/mechanistic target of rapamycin signalling pathway in the regenerating muscles. Our results show that leucine improves myofiber size gain and strength recovery in regenerating soleus muscles through attenuation of protein ubiquitination. In addition, leucine might have therapeutic effects for muscle recovery following injury and in some muscle diseases.

Nutritional strategies to counteract muscle atrophy caused by disuse and to improve recovery

Periods of immobilisation are often associated with pathologies and/or ageing. These periods of muscle disuse induce muscle atrophy which could worsen the pathology or elderly frailty. If muscle mass loss has positive effects in the short term, a sustained/uncontrolled muscle mass loss is deleterious for health. Muscle mass recovery following immobilisation-induced atrophy could be critical, particularly when it is uncompleted as observed during ageing. Exercise, the best way to recover muscle mass, is not always applicable. So, other approaches such as nutritional strategies are needed to limit muscle wasting and to improve muscle mass recovery in such situations. The present review discusses mechanisms involved in muscle atrophy following disuse and during recovery and emphasises the effect of age in these mechanisms. In addition, the efficiency of nutritional strategies proposed to limit muscle mass loss during disuse and to improve protein gain during recovery (leucine supplementation, whey proteins, antioxidants and anti-inflammatory compounds, energy intake) is also discussed.

Branched-chain amino acid supplementation: impact on signaling and relevance to critical illness

The changes that occur in mammalian systems following trauma and sepsis, termed systemic inflammatory response syndrome, elicit major changes in carbohydrate, protein, and energy metabolism. When these events persist for too long they result in a severe depletion of lean body mass, multiple organ dysfunction, and eventually death. Nutritional supplementation has been investigated to offset the severe loss of protein, and recent evidence suggests that diets enriched in branched-chain amino acids (BCAAs) may be especially beneficial. BCAAs are metabolized in two major steps that are differentially expressed in muscle and liver. In muscle, BCAAs are reversibly transaminated to the corresponding α-keto acids. For the complete degradation of BCAAs, the α-keto acids must travel to the liver to undergo oxidation. The liver, in contrast to muscle, does not significantly express the branched-chain aminotransferase. Thus, BCAA degradation is under the joint control of both liver and muscle. Recent evidence suggests that in liver, BCAAs may perform signaling functions, more specifically via activation of mTOR (mammalian target of rapamycin) signaling pathway, influencing a wide variety of metabolic and synthetic functions, including protein translation, insulin signaling, and oxidative stress following severe injury and infection. However, understanding of the system-wide effects of BCAAs that integrate both metabolic and signaling aspects is currently lacking. Further investigation in this respect will help rationalize the design and optimization of nutritional supplements containing BCAAs for critically ill patients.

Role of p70S6K1-mediated phosphorylation of eIF4B and PDCD4 proteins in the regulation of protein synthesis

Modulation of mRNA binding to the 40 S ribosomal subunit during translation initiation controls not only global rates of protein synthesis but also regulates the pattern of protein expression by allowing for selective inclusion, or exclusion, of mRNAs encoding particular proteins from polysomes. The mRNA binding step is modulated by signaling through a protein kinase known as the mechanistic target of rapamycin complex 1 (mTORC1). mTORC1 directly phosphorylates the translational repressors eIF4E binding proteins (4E-BP) 1 and 2, releasing them from the mRNA cap binding protein eIF4E, thereby promoting assembly of the eIF4E·eIF4G complex. mTORC1 also phosphorylates the 70-kDa ribosomal protein S6 kinase 1 (p70S6K1), which subsequently phosphorylates eIF4B, and programmed cell death 4 (PDCD4), which sequesters eIF4A from the eIF4E·eIF4G complex, resulting in repressed translation of mRNAs with highly structured 5'-untranslated regions. In the present study, we compared the role of the 4E-BPs in the regulation of global rates of protein synthesis to that of eIF4B and PDCD4. We found that maintenance of eIF4E interaction with eIF4G was not by itself sufficient to sustain global rates of protein synthesis in the absence of mTORC1 signaling to p70S6K1; phosphorylation of both eIF4B and PDCD4 was additionally required. We also found that the interaction of eIF4E with eIF4G was maintained in the liver of fasted rats as well as in serum-deprived mouse embryo fibroblasts lacking both 4E-BP1 and 4E-BP2, suggesting that the interaction of eIF4G with eIF4E is controlled primarily through the 4E-BPs.

Enteral leucine supplementation increases protein synthesis in skeletal and cardiac muscles and visceral tissues of neonatal pigs through mTORC1-dependent pathways

INTRODUCTION

Leucine (Leu) activates mammalian target of rapamycin (mTOR) to upregulate protein synthesis (PS).

RESULTS

PS in skeletal muscles, heart, liver, pancreas, and jejunum, but not kidney, were greater in low protein supplemented with Leu (LP+L) than LP, but lower than high protein (HP). In longissimus dorsi muscle, protein kinase B phosphorylation was similar in LP and LP+L, but lower than HP. Although less than HP, p70 ribosomal S6 kinase 1 (S6K1) and eukaryotic initiation factor (eIF) 4E binding protein 1 (4EBP1) association with regulatory associated protein of mammalian target of rapamycin was greater in LP+L than LP, resulting in higher S6K1 and 4EBP1 phosphorylation. Feeding LP+L vs. LP decreased 4EBP1·eIF4E and increased eIF4E·eIF4G formation, but not to HP. Similar results were obtained for S6K1 and 4EBP1 phosphorylation in gastrocnemius, masseter, heart, liver, pancreas, and jejunum, but not kidney. eIF2α and elongation factor 2 phosphorylation was unaffected by treatment. DICUSSION: Our results suggest that enteral Leu supplementation of a low protein diet enhances PS in most tissues through mTOR complex 1 pathways.

METHODS

To examine enteral Leu effects on PS and signaling activation, 5-d-old piglets were fed for 24 h diets containing: (i) LP, (ii) LP+L, or (iii) HP.

Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men

Leucine is a nutrient regulator of muscle protein synthesis by activating mTOR and possibly other proteins in this pathway. The purpose of this study was to examine the role of leucine in the regulation of human myofibrillar protein synthesis (MPS). Twenty-four males completed an acute bout of unilateral resistance exercise prior to consuming either: a dose (25 g) of whey protein (WHEY); 6.25 g whey protein with total leucine equivalent to WHEY (LEU); or 6.25 g whey protein with total essential amino acids (EAAs) equivalent to WHEY for all EAAs except leucine (EAA-LEU). Measures of MPS, signalling through mTOR, and amino acid transporter (AAT) mRNA abundance were made while fasted (FAST), and following feeding under rested (FED) and post-exercise (EX-FED) conditions. Leucinaemia was equivalent between WHEY and LEU and elevated compared to EAA-LEU (P=0.001). MPS was increased above FAST at 1–3 h post-exercise in both FED (P <0.001) and EX-FED (P <0.001) conditions with no treatment effect.At 3–5 h, only WHEY remained significantly elevated above FAST in EX-FED(WHEY 184% vs. LEU 55% and EAA-LEU 35%; P =0.036). AAT mRNA abundance was increased above FAST after feeding and exercise with no effect of leucinaemia. In summary, a low dose of whey protein supplemented with leucine or all other essential amino acids was as effective as a complete protein (WHEY) in stimulating postprandial MPS; however only WHEY was able to sustain increased rates of MPS post-exercise and may therefore be most suited to increase exercise-induced muscle protein accretion.

Contrarily to whey and high protein diets, dietary free leucine supplementation cannot reverse the lack of recovery of muscle mass after prolonged immobilization during ageing

During ageing, immobilization periods increase and are partially responsible of sarcopaenia by inducing a muscle atrophy which is hardly recovered from. Immobilization-induced atrophy is due to an increase of muscle apoptotic and proteolytic processes and decreased protein synthesis. Moreover, previous data suggested that the lack of muscle mass recovery might be due to a defect in protein synthesis response during rehabilitation. This study was conducted to explore protein synthesis during reloading and leucine supplementation effect as a nutritional strategy for muscle recovery. Old rats (22–24 months old) were subjected to unilateral hindlimb casting for 8 days (I8) and allowed to recover for 10–40 days (R10–R40). They were fed a casein (±leucine) diet during the recovery. Immobilized gastrocnemius muscles atrophied by 20%, and did not recover even at R40. Amount of polyubiquitinated conjugates and chymotrypsin- and trypsin-like activities of the 26S proteasome increased. These changes paralleled an ‘anabolic resistance' of the protein synthesis at the postprandial state (decrease of protein synthesis, P-S6 and P-4E-BP1). During the recovery, proteasome activities remained elevated until R10 before complete normalization and protein synthesis was slightly increased. With free leucine supplementation during recovery, if proteasome activities were normalized earlier and protein synthesis was higher during the whole recovery, it nevertheless failed in muscle mass gain. This discrepancy could be due to a ‘desynchronization' between the leucine signal and the availability of amino acids coming from casein digestion. Thus, when supplemented with leucine-rich proteins (i.e. whey) and high protein diets, animals partially recovered the muscle mass loss.

MNK2 inhibits eIF4G activation through a pathway involving serine-arginine-rich protein kinase in skeletal muscle

Skeletal muscle mass is regulated by activity, metabolism, and the availability of nutrients. During muscle atrophy, MNK2 expression increases. We found that MNK2 (mitogen-activated protein kinase-interacting kinase 2), but not MNK1, inhibited proteins involved in promoting protein synthesis, including eukaryotic translation initiation factor 4G (eIF4G) and mammalian target of rapamycin (mTOR). Phosphorylation at serine 1108 (Ser¹¹⁰⁸) of eIF4G, which is associated with enhanced protein translation, is promoted by insulin-like growth factor 1 and inhibited by rapamycin or starvation, suggesting that phosphorylation of this residue is regulated by mTOR. In cultured myotubes, small interfering RNA (siRNA) knockdown of MNK2 increased eIF4G Ser¹¹⁰⁸ phosphorylation and overcame rapamycin's inhibitory effect on this phosphorylation event. Phosphorylation of Ser¹¹⁰⁸ in eIF4G, in gastrocnemius muscle, was increased in mice lacking MNK2, but not those lacking MNK1, and this increased phosphorylation was maintained in MNK2-null animals under atrophy conditions and upon starvation. Conversely, overexpression of MNK2 decreased eIF4G Ser¹¹⁰⁸ phosphorylation. An siRNA screen revealed that serine-arginine-rich protein kinases linked increased MNK2 activity to decreased eIF4G phosphorylation. In addition, we found that MNK2 interacted with mTOR and inhibited phosphorylation of the mTOR target, the ribosomal kinase p70S6K (70-kD ribosomal protein S6 kinase), through a mechanism independent of the kinase activity of MNK2. These data indicate that MNK2 plays a unique role, not shared by its closest paralog MNK1, in limiting protein translation through its negative effect on eIF4G Ser¹¹⁰⁸ phosphorylation and p70S6K activation.

The ribosome-related protein, SBDS, is critical for normal erythropoiesis

Although anemia is common in Shwachman- Diamond syndrome (SDS), the underlying mechanism remains unclear. We asked whether SBDS, which is mutated in most SDS patients, is critical for erythroid development. We found that SBDS expression is high early during erythroid differentiation. Inhibition of SBDS in CD34(+) hematopoietic stem cells and early progenitors (HSC/Ps) and K562 cells led to slow cell expansion during erythroid differentiation. Induction of erythroid differentiation resulted in markedly accelerated apoptosis in the knockdown cells; however, proliferation was only mildly reduced. The percentage of cells entering differentiation was not reduced. Differentiation also increased the oxidative stress in SBDS-knockdown K562 cells, and antioxidants enhanced the expansion capability of differentiating SBDS-knockdown K562 cells and colony production of SDS patient HSC/Ps. Erythroid differentiation also resulted in reduction of all ribosomal subunits and global translation. Furthermore, stimulation of global translation with leucine improved the erythroid cell expansion of SBDS-knockdown cells and colony production of SDS patient HSC/Ps. Leucine did not reduce the oxidative stress in SBDS-deficient K562 cells. These results demonstrate that SBDS is critical for normal erythropoiesis. Erythropoietic failure caused by SBDS deficiency is at least in part related to elevated ROS levels and translation insufficiency because antioxidants and leucine improved cell expansion.

The actions of exogenous leucine on mTOR signalling and amino acid transporters in human myotubes

Background

The branched-chain amino acid (BCAA) leucine has been identified to be a key regulator of skeletal muscle anabolism. Activation of anabolic signalling occurs via the mammalian target of rapamycin (mTOR) through an undefined mechanism. System A and L solute carriers transport essential amino acids across plasma membranes; however it remains unknown whether an exogenous supply of leucine regulates their gene expression. The aim of the present study was to investigate the effects of acute and chronic leucine stimulation of anabolic signalling and specific amino acid transporters, using cultured primary human skeletal muscle cells.

Results

Human myotubes were treated with leucine, insulin or co-treated with leucine and insulin for 30 min, 3 h or 24 h. Activation of mTOR signalling kinases were examined, together with putative nutrient sensor human vacuolar protein sorting 34 (hVps34) and gene expression of selected amino acid transporters. Phosphorylation of mTOR and p70S6K was transiently increased following leucine exposure, independently to insulin. hVps34 protein expression was also significantly increased. However, genes encoding amino acid transporters were differentially regulated by insulin and not leucine.

Conclusions

mTOR signalling is transiently activated by leucine within human myotubes independently of insulin stimulation. While this occurred in the absence of changes in gene expression of amino acid transporters, protein expression of hVps34 increased.

Visfatin exerts angiogenic effects on human umbilical vein endothelial cells through the mTOR signaling pathway

The biologically active factors known as adipocytokines are secreted primarily by adipose tissues and can act as modulators of angiogenesis. Visfatin, an adipocytokine that has recently been reported to have angiogenic properties, is upregulated in diabetes, cancer, and inflammatory diseases. Because maintenance of an angiogenic balance is critically important in the management of these diseases, understanding the molecular mechanism by which visfatin promotes angiogenesis is very important. In this report, we describe our findings demonstrating that visfatin stimulates the mammalian target of the rapamycin (mTOR) pathway, which plays important roles in angiogenesis. Visfatin induced the expression of hypoxia-inducible factor 1α (HIF1α) and vascular endothelial growth factor (VEGF) in human endothelial cells. Inhibition of the mTOR pathway by rapamycin eliminated the angiogenic and proliferative effects of visfatin. The visfatin-induced increase in VEGF expression was also eliminated by RNA interference-mediated knockdown of the 70-kDa ribosomal protein S6 kinase (p70S6K), a downstream target of mTOR. Visfatin inactivated glycogen synthase kinase 3β (GSK3β) by phosphorylating it at Ser-9, leading to the nuclear translocation of β-catenin. Both rapamycin co-treatment and p70S6K knockdown inhibited visfatin-induced GSK3β phosphorylation at Ser-9 and nuclear translocation of β-catenin. Taken together, these results indicate that mTOR signaling is involved in visfatin-induced angiogenesis, and that this signaling leads to visfatin-induced VEGF expression and nuclear translocation of β-catenin. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Maternal amino acid supplementation for intrauterine growth restriction

Maternal dietary protein supplementation to improve fetal growth has been considered as an option to prevent or treat intrauterine growth restriction. However, in contrast to balanced dietary supplementation, adverse perinatal outcomes in pregnant women who received high amounts of dietary protein supplementation have been observed. The responsible mechanisms for these adverse outcomes are unknown. This review will discuss relevant human and animal data to provide the background necessary for the development of explanatory hypotheses and ultimately for the development therapeutic interventions during pregnancy to improve fetal growth. Relevant aspects of fetal amino acid metabolism during normal pregnancy and those pregnancies affected by IUGR will be discussed. In addition, data from animal experiments which have attempted to determine mechanisms to explain the adverse responses identified in the human trials will be presented. Finally, we will suggest new avenues for investigation into how amino acid supplementation might be used safely to treat and/or prevent IUGR.

Prostaglandin F2&alpha; stimulates PI3K/ERK/mTOR signaling and skeletal myotube hypertrophy

Cyclooxygenase (COX) enzymes mediate the synthesis of proinflammatory prostaglandin (PG) species from cellular arachidonic acid. COX/PGs have been implicated in skeletal muscle growth/regeneration; however, the mechanisms by which PGs influence skeletal muscle adaptation are poorly understood. The present study aimed to investigate PGF(2α) signaling and its role in skeletal myotube hypertrophy. PGF(2α) or the FP receptor agonist fluprostenol increased C2C12 myotube diameter. This effect was abolished by the FP receptor antagonist AL8810 and mammalian target of rapamycin (mTOR) inhibition. PGF(2α) stimulated time- and dose-dependent increases in the phosphorylation of extracellular receptor kinase (ERK)1/2 (Thr202/Tyr204), p70S6 kinase (p70S6K) (Thr389 and Thr421/Ser424), and eukaryotic initiation factor 4G (eIF4G) (Ser1108) without influencing Akt (Ser473). Pretreatment with the phosphoinositide 3-kinase (PI3K) inhibitor LY294002 and the ERK inhibitor PD98059 blocked F prostanoid receptor signaling responses, whereas rapamycin blocked heightened p70S6K/eIF4G phosphorylation without influencing ERK1/2 phosphorylation. These data suggest that activation of the F prostanoid receptor is coupled to C2C12 myotube growth and intracellular signaling via a PI3K/ERK/mTOR-dependent pathway.

Leucine supplementation of a low-protein meal increases skeletal muscle and visceral tissue protein synthesis in neonatal pigs by stimulating mTOR-dependent translation initiation

Protein synthesis and eukaryotic initiation factor (eIF) activation are increased in skeletal muscle of neonatal pigs parenterally infused with amino acids. Leucine appears to be the most effective single amino acid to trigger these effects. To examine the response to enteral leucine supplementation, overnight food-deprived 5-d-old pigs were gavage fed at 0 and 60 min a: 1) low-protein diet (LP); 2) LP supplemented with leucine (LP+L) to equal leucine in the high-protein diet (HP); or 3) HP diet. Diets were isocaloric and equal in lactose. Fractional protein synthesis rates and translation initiation control mechanisms were examined in skeletal muscles and visceral tissues 90 min after feeding. Protein synthesis rates in longissimus dorsi, gastrocnemius, and masseter muscles, heart, jejunum, kidney, and pancreas, but not liver, were greater in the LP+L group compared with the LP group and did not differ from the HP group. Feeding LP+L and HP diets compared with the LP diet increased phosphorylation of mammalian target of rapamycin (mTOR), 4E-binding protein 1, ribosomal protein S6 kinase-1, and eIF4G and formation of the active eIF4E·eIF4G complex in longissimus dorsi muscle. In all tissues except liver, activation of mTOR effectors increased in pigs fed LP+L and HP vs. LP diets. Our results suggest that leucine supplementation of a low-protein meal stimulates protein synthesis in muscle and most visceral tissues to a rate similar to that achieved by feeding a high-protein meal and this stimulation involves activation of mTOR downstream effectors.

Muscle full effect after oral protein: time-dependent concordance and discordance between human muscle protein synthesis and mTORC1 signaling

BACKGROUND

We previously showed that human muscle protein synthesis (MPS) increased during infusion of amino acids (AAs) and peaked at ≈120 min before returning to baseline rates, despite elevated plasma AA concentrations.

OBJECTIVE

We tested whether a protein meal elicited a similar response and whether signaling responses that regulate messenger RNA translation matched MPS changes.

DESIGN

Eight postabsorptive healthy men (≈21 y of age) were studied during 8.5 h of primed continuous infusion of [1,2-¹³C₂]leucine with intermittent quadriceps biopsies for determination of MPS and anabolic signaling. After 2.5 h, subjects consumed 48 g whey protein.

RESULTS

At 45-90 min after oral protein bolus, mean (± SEM) myofibrillar protein synthesis increased from 0.03 ± 0.003% to 0.10 ± 0.01%/h; thereafter, myofibrillar protein synthesis returned to baseline rates even though plasma essential AA (EAA) concentrations remained elevated (+130% at 120 min, +80% at 180 min). The activity of protein kinase B (PKB) and phosphorylation of eukaryotic initiation factor 4G preceded the rise of MPS and increases in phosphorylation of ribosomal protein kinase S6 (S6K1), and 4E-binding protein 1 (4EBP1) was superimposable with MPS responses until 90 min. However, although MPS decreased thereafter, all signals, with the exception of PKB activity (which mirrored insulin responses), remained elevated, which echoed the slowly declining plasma EAA profile. The phosphorylation of eukaryotic initiation factor 2α increased only at 180 min. Thus, discordance existed between MPS and the mammalian target of rapamycin complex 1 (mTORC1) and signaling (ie, S6K1 and 4EBP1 phosphorylation).

CONCLUSIONS

We confirm our previous findings that MPS responses to AAs are transient, even with oral protein bolus. However, changes in MPS only reflect elevated mTORC1 signaling during the upswing in MPS.