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

Short-term muscle disuse lowers myofibrillar protein synthesis rates and induces anabolic resistance to protein ingestion

Disuse leads to rapid loss of skeletal muscle mass and function. It has been hypothesized that short successive periods of muscle disuse throughout the lifespan play an important role in the development of sarcopenia. The physiological mechanisms underlying short-term muscle disuse atrophy remain to be elucidated. We assessed the impact of 5 days of muscle disuse on postabsorptive and postprandial myofibrillar protein synthesis rates in humans. Twelve healthy young (22 ± 1 yr) men underwent a 5-day period of one-legged knee immobilization (full leg cast). Quadriceps cross-sectional area (CSA) of both legs was assessed before and after immobilization. Continuous infusions of l-[ring-(2)H5]phenylalanine and l-[1-(13)C]leucine were combined with the ingestion of a 25-g bolus of intrinsically l-[1-(13)C]phenylalanine- and l-[1-(13)C]leucine-labeled dietary protein to assess myofibrillar muscle protein fractional synthetic rates in the immobilized and nonimmobilized control leg. Immobilization led to a 3.9 ± 0.6% decrease in quadriceps muscle CSA of the immobilized leg. Based on the l-[ring-(2)H5]phenylalanine tracer, immobilization reduced postabsorptive myofibrillar protein synthesis rates by 41 ± 13% (0.015 ± 0.002 vs. 0.032 ± 0.005%/h, P < 0.01) and postprandial myofibrillar protein synthesis rates by 53 ± 4% (0.020 ± 0.002 vs. 0.044 ± 0.003%/h, P < 0.01). Comparable results were found using the l-[1-(13)C]leucine tracer. Following protein ingestion, myofibrillar protein bound l-[1-(13)C]phenylalanine enrichments were 53 ± 18% lower in the immobilized compared with the control leg (0.007 ± 0.002 and 0.015 ± 0.002 mole% excess, respectively, P < 0.05). We conclude that 5 days of muscle disuse substantially lowers postabsorptive myofibrillar protein synthesis rates and induces anabolic resistance to protein ingestion.

Computational modeling to predict nitrogen balance during acute metabolic decompensation in patients with urea cycle disorders

Nutritional management of acute metabolic decompensation in amino acid inborn errors of metabolism (AA IEM) aims to restore nitrogen balance. While nutritional recommendations have been published, they have never been rigorously evaluated. Furthermore, despite these recommendations, there is a wide variation in the nutritional strategies employed amongst providers, particularly regarding the inclusion of parenteral lipids for protein-free caloric support. Since randomized clinical trials during acute metabolic decompensation are difficult and potentially dangerous, mathematical modeling of metabolism can serve as a surrogate for the preclinical evaluation of nutritional interventions aimed at restoring nitrogen balance during acute decompensation in AA IEM. A validated computational model of human macronutrient metabolism was adapted to predict nitrogen balance in response to various nutritional interventions in a simulated patient with a urea cycle disorder (UCD) during acute metabolic decompensation due to dietary non-adherence or infection. The nutritional interventions were constructed from published recommendations as well as clinical anecdotes. Overall, dextrose alone (DEX) was predicted to be better at restoring nitrogen balance and limiting nitrogen excretion during dietary non-adherence and infection scenarios, suggesting that the published recommended nutritional strategy involving dextrose and parenteral lipids (ISO) may be suboptimal. The implications for patients with AA IEM are that the medical course during acute metabolic decompensation may be influenced by the choice of protein-free caloric support. These results are also applicable to intensive care patients undergoing catabolism (postoperative phase or sepsis), where parenteral nutritional support aimed at restoring nitrogen balance may be more tailored regarding metabolic fuel selection.

Aryl hydrocarbon receptor-dependent up-regulation of the heterodimeric amino acid transporter LAT1 (SLC7A5)/CD98hc (SLC3A2) by diesel exhaust particle extract in human bronchial epithelial cells

The heterodimeric L-type amino acid transporter (LAT) 1/CD98hc is overexpressed in lung cancers with a poor prognosis factor. Factors that contribute to LAT1/CD98hc overexpression in lung cells remain however to be determined, but the implication of atmospheric pollution can be suspected. The present study was therefore designed to analyze the effects of diesel exhaust particle (DEP) extract (DEPe) on LAT1/CD98hc expression in bronchial epithelial BEAS-2B cells. Exposure to DEPe up-regulated LAT1 and CD98hc mRNA levels in a concentration-dependent manner, with DEPe EC50 values (around 0.2 μg/mL) relevant to environmental situations. DEPe concomitantly induced LAT1/CD98hc protein expression and LAT1-mediated leucine accumulation in BEAS-2B cells. Inhibition of the aryl hydrocarbon receptor (AhR) pathway through the use of a chemical AhR antagonist or the siRNA-mediated silencing of AhR expression was next found to prevent DEPe-mediated induction of LAT1/CD98hc, indicating that this regulation depends on AhR, known to be activated by major chemical DEP components like polycyclic aromatic hydrocarbons. DEPe exposure was finally shown to induce mRNA expression and activity of matrix metalloproteinase (MMP)-2 in BEAS-2B cells, in a CD98hc/focal adhesion kinase (FAK)/extracellular regulated kinase (ERK) manner, thus suggesting that DEPe-mediated induction of CD98hc triggers activation of the integrin/FAK/ERK signaling pathway known to be involved in MMP-2 regulation. Taken together, these data demonstrate that exposure to DEPe induces functional overexpression of the amino acid transporter LAT1/CD98hc in lung cells. Such a regulation may participate to pulmonary carcinogenic effects of DEPs, owing to the well-documented contribution of LAT1 and CD98hc to cancer development.

Transport of Iodothyronines by Human L-Type Amino Acid Transporters

Thyroid hormone (TH) transporters facilitate cellular TH influx and efflux, which is paramount for normal physiology. The L-type amino acid transporters LAT1 and LAT2 are known to facilitate TH transport. However, the role of LAT3, LAT4, and LAT5 is still unclear. Therefore, the aim of this study was to further characterize TH transport by LAT1 and LAT2 and to explore possible TH transport by LAT3, LAT4, and LAT5. FLAG-LAT1-5 constructs were transiently expressed in COS1 cells. LAT1 and LAT2 were cotransfected with the CD98 heavy chain. Cellular transport was measured using 10 nM (125)I-labeled T4, T3, rT3, 3,3'-T2, and 10 μM [(125)I]3'-iodotyrosine (MIT) as substrates. Intracellular metabolism of these substrates was determined in cells cotransfected with either of the LATs with type 1 or type 3 deiodinase. LAT1 facilitated cellular uptake of all substrates and LAT2 showed a net uptake of T3, 3,3'-T2, and MIT. Expression of LAT3 or LAT4 did not affect transport of T4 and T3 but resulted in the decreased cellular accumulation of 3,3'-T2 and MIT. LAT5 did not facilitate the transport of any substrate. Cotransfection with LAT3 or LAT4 strongly diminished the cellular accumulation of 3,3'-T2 and MIT by LAT1 and LAT2. These data were confirmed by metabolism studies. LAT1 and LAT2 show distinct preferences for the uptake of the different iodocompounds, whereas LAT3 and LAT4 specifically facilitate the 3,3'-T2 and MIT efflux. Together our findings suggest that different sets of transporters with specific influx or efflux capacities may cooperate to regulate the cellular thyroid state.

Age-related differences in lean mass, protein synthesis and skeletal muscle markers of proteolysis after bed rest and exercise rehabilitation

Bed rest-induced muscle loss and impaired muscle recovery may contribute to age-related sarcopenia. It is unknown if there are age-related differences in muscle mass and muscle anabolic and catabolic responses to bed rest. A secondary objective was to determine if rehabilitation could reverse bed rest responses. Nine older and fourteen young adults participated in a 5-day bed rest challenge (BED REST). This was followed by 8 weeks of high intensity resistance exercise (REHAB). Leg lean mass (via dual-energy X-ray absorptiometry; DXA) and strength were determined. Muscle biopsies were collected during a constant stable isotope infusion in the postabsorptive state and after essential amino acid (EAA) ingestion on three occasions: before (PRE), after bed rest and after rehabilitation. Samples were assessed for protein synthesis, mTORC1 signalling, REDD1/2 expression and molecular markers related to muscle proteolysis (MURF1, MAFBX, AMPKα, LC3II/I, Beclin1). We found that leg lean mass and strength decreased in older but not younger adults after bedrest (P < 0.05) and was restored after rehabilitation. EAA-induced mTORC1 signalling and protein synthesis increased before bed rest in both age groups (P < 0.05). Although both groups had blunted mTORC1 signalling, increased REDD2 and MURF1 mRNA after bedrest, only older adults had reduced EAA-induced protein synthesis rates and increased MAFBX mRNA, p-AMPKα and the LC3II/I ratio (P < 0.05). We conclude that older adults are more susceptible than young persons to muscle loss after short-term bed rest. This may be partially explained by a combined suppression of protein synthesis and a marginal increase in proteolytic markers. Finally, rehabilitation restored bed rest-induced deficits in lean mass and strength in older adults.

Dietary protein considerations to support active aging

Given our rapidly aging world-wide population, the loss of skeletal muscle mass with healthy aging (sarcopenia) represents an important societal and public health concern. Maintaining or adopting an active lifestyle alleviates age-related muscle loss to a certain extent. Over time, even small losses of muscle tissue can hinder the ability to maintain an active lifestyle and, as such, contribute to the development of frailty and metabolic disease. Considerable research focus has addressed the application of dietary protein supplementation to support exercise-induced gains in muscle mass in younger individuals. In contrast, the role of dietary protein in supporting the maintenance (or gain) of skeletal muscle mass in active older persons has received less attention. Older individuals display a blunted muscle protein synthetic response to dietary protein ingestion. However, this reduced anabolic response can largely be overcome when physical activity is performed in close temporal proximity to protein consumption. Moreover, recent evidence has helped elucidate the optimal type and amount of dietary protein that should be ingested by the older adult throughout the day in order to maximize the skeletal muscle adaptive response to physical activity. Evidence demonstrates that when these principles are adhered to, muscle maintenance or hypertrophy over prolonged periods can be further augmented in active older persons. The present review outlines the current understanding of the role that dietary protein occupies in the lifestyle of active older adults as a means to increase skeletal muscle mass, strength and function, and thus support healthier aging.

Muscle protein turnover in the elderly and its potential contribution to the development of sarcopenia

The underlying aetiology of sarcopenia appears multifaceted and not yet fully defined, but ultimately involves the gradual loss of muscle protein content over time. The present evidence suggests that the loss of lean tissue in the elderly is exacerbated by low dietary protein intake. Moreover, acute stable-isotope-based methodologies have demonstrated that the muscle anabolic response to a given amount of protein may decline with age, a phenomenon that has been termed anabolic resistance. Although the mechanism responsible for the inability of muscle to mount a satisfactory anabolic response to protein provision with increasing age is presently unknown, it does not appear due to impaired digestion or absorption of dietary protein. Rather, the issue could reside with any combination of: a diminished delivery of amino acids to peripheral tissues, impaired uptake of amino acids into muscle cells, or an inability of amino acids to elicit intracellular events pivotal for anabolism to occur. Despite the presence of anabolic resistance to dietary protein, present evidence suggests that protein supplementation may be able to overcome these issues, particularly when combined with resistance exercise programmes. As such, protein supplementation may prove to be an effective approach to delay the loss of muscle mass with age and has led to calls for the recommended daily intake of protein to be increased for the elderly population.

Implications of exercise training and distribution of protein intake on molecular processes regulating skeletal muscle plasticity

To optimize its function, skeletal muscle exhibits exceptional plasticity and possesses the fundamental capacity to adapt its metabolic and contractile properties in response to various external stimuli (e.g., external loading, nutrient availability, and humoral factors). The adaptability of skeletal muscle, along with its relatively large mass and high metabolic rate, makes this tissue an important contributor to whole body health and mobility. This adaptational process includes changes in the number, size, and structural/functional properties of the myofibers. The adaptations of skeletal muscle to exercise are highly interrelated with dietary intake, particularly dietary protein, which has been shown to further potentiate exercise training-induced adaptations. Understanding the molecular adaptation of skeletal muscle to exercise and protein consumption is vital to elicit maximum benefit from exercise training to improve human performance and health. In this review, we will provide an overview of the molecular pathways regulating skeletal muscle adaptation to exercise and protein, and discuss the role of subsequent timing of nutrient intake following exercise.

Anti-1-amino-3-18F-fluorocyclobutane-1-carboxylic acid: physiologic uptake patterns, incidental findings, and variants that may simulate disease

Anti-1-amino-3-(18)F-fluorocyclobutane-1-carboxylic acid ((18)F-FACBC) is a synthetic amino acid analog PET radiotracer undergoing clinical trials for the evaluation of prostate and other cancers. We aimed to describe common physiologic uptake patterns, incidental findings, and variants in patients who had undergone (18)F-FACBC PET.

METHODS

Sixteen clinical trials involving 611 (18)F-FACBC studies from 6 centers, which included dosimetry studies on 12 healthy volunteers, were reviewed. Qualitative observations of common physiologic patterns, incidental uptake, and variants that could simulate disease were recorded and compared with similar observations in studies of the healthy volunteers. Quantitative analysis of select data and review of prior published reports and observations were also made.

RESULTS

The liver and pancreas demonstrated the most intense uptake. Moderate salivary and pituitary uptake and variable mild to moderate bowel activity were commonly visualized. Moderate bone marrow and mild muscle activity were present on early images, with marrow activity decreasing and muscle activity increasing with time. Brain and lungs demonstrated activity less than blood pool. Though (18)F-FACBC exhibited little renal excretion or bladder uptake during the clinically useful early imaging time window, mild to moderate activity might accumulate in the bladder and interfere with evaluation of adjacent prostate bed and seminal vesicles in 5%-10% of patients. Uptake might also occur from benign processes such as infection, inflammation, prostatic hyperplasia, and metabolically active benign bone lesions such as osteoid osteoma.

CONCLUSION

Common physiologic uptake patterns were similar to those noted in healthy volunteers. The activity in organs followed the presence of amino acid transport and metabolism described with other amino acid-based PET radiotracers. As with other PET radiotracers such as (18)F-FDG, focal nonphysiologic uptake may represent incidental malignancy. Uptake due to benign etiologies distinct from physiologic background also occurred and could lead to misinterpretations if the reader is unaware of them.

Leucine-enriched amino acid ingestion after resistance exercise prolongs myofibrillar protein synthesis and amino acid transporter expression in older men

BACKGROUND

Postexercise protein or amino acid ingestion restores muscle protein synthesis in older adults and represents an important therapeutic strategy for aging muscle. However, the precise nutritional factors involved are unknown.

OBJECTIVE

The purpose of this study was to determine the role of increased postexercise Leu ingestion on skeletal muscle myofibrillar protein synthesis (MyoPS), mammalian/mechanistic target of rapamycin complex 1 signaling, and amino acid transporter (AAT) mRNA expression in older men over a 24-h post-resistance exercise (RE) time course.

METHODS

During a stable isotope infusion trial (l-[ring-(13)C6]Phe; l-[1-(13)C]Leu), older men performed RE and, at 1 h after exercise, ingested 10 g of essential amino acids (EAAs) containing either a Leu content similar to quality protein (control, 1.85 g of Leu, n = 7) or enriched Leu (LEU; 3.5 g of Leu, n = 8). Muscle biopsies (vastus lateralis) were obtained at rest and 2, 5, and 24 h after exercise.

RESULTS

p70 S6 kinase 1 phosphorylation was increased in each group at 2 h (P < 0.05), whereas 4E binding protein 1 phosphorylation increased only in the LEU group (P < 0.05). MyoPS was similarly increased (∼90%) above basal in each group at 5 h (P < 0.05) and remained elevated (∼90%) at 24 h only in the LEU group (P < 0.05). The mRNA expression of select AATs was increased at 2 and 5 h in each group (P < 0.05), but AAT expression was increased at 24 h only in the LEU group (P < 0.05).

CONCLUSIONS

Leu-enriched EAA ingestion after RE may prolong the anabolic response and sensitivity of skeletal muscle to amino acids in older adults. These data emphasize the potential importance of adequate postexercise Leu ingestion to enhance the response of aging muscle to preventive or therapeutic exercise-based rehabilitation programs. This trial was registered at clinicaltrials.gov as NCT00891696.

Sepsis-induced changes in amino acid transporters and leucine signaling via mTOR in skeletal muscle

The present study tested the hypothesis that sepsis-induced leucine (Leu) resistance in skeletal muscle is associated with a down-regulation of amino acid transporters important in regulating Leu flux or an impairment in the formation of the Leu-sensitive mTOR-Ragulator complex. Sepsis in adult male rats decreased basal protein synthesis in gastrocnemius, associated with a reduction in mTOR activation as indicated by decreased 4E-BP1 and S6K1 phosphorylation. The ability of oral Leu to increase protein synthesis and mTOR kinase after 1 h was largely prevented in sepsis. Sepsis increased CAT1, LAT2 and SNAT2 mRNA content two- to fourfold, but only the protein content for CAT1 (20 % decrease) differed significantly. Conversely, sepsis decreased the proton-assisted amino acid transporter (PAT)-2 mRNA by 60 %, but without a coordinate change in PAT2 protein. There was no sepsis or Leu effect on the protein content for RagA-D, LAMTOR-1 and -2, raptor, Rheb or mTOR in muscle. The binding of mTOR, PRAS40 and RagC to raptor did not differ for control and septic muscle in the basal condition; however, the Leu-induced decrease in PRAS40·raptor and increase in RagC·raptor seen in control muscle was absent in sepsis. The intracellular Leu concentration was increased in septic muscle, compared to basal control conditions, and oral Leu further increased the intracellular Leu concentration similarly in both control and septic rats. Hence, while alterations in select amino acid transporters are not associated with development of sepsis-induced Leu resistance, the Leu-stimulated binding of raptor with RagC and the recruitment of mTOR/raptor to the endosome-lysosomal compartment may partially explain the inability of Leu to fully activate mTOR and muscle protein synthesis.

Skeletal muscle Ras-related GTP binding B mRNA and protein expression is increased after essential amino acid ingestion in healthy humans

Essential amino acids (EAAs) are potent stimulators of mechanistic target of rapamycin complex 1 (mTORC1) signaling and muscle protein synthesis. However, regulators upstream of mTORC1 that are responsive to EAA availability are not well described, especially in human skeletal muscle. The purpose of this study was to determine changes in leucyl-tRNA synthetase (LARS/LARS) and Ras-related GTP binding B (RAGB/RAGB) mRNA and protein expression in healthy human skeletal muscle after acute EAA ingestion. Muscle biopsies sampled from the vastus lateralis were obtained from 13 young adults (7 males, 6 females; aged 22.9 ± 0.9 y; body mass index 21.7 ± 0.9 kg/m(2)) in the fasting state (baseline) and 1 and 3 h after EAA (13 g; 2.4 g of Leu) ingestion. Real-time quantitative polymerase chain reaction and Western blotting were used to determine changes in LARS/LARS and RAGB/RAGB mRNA and protein expression, respectively. Stable isotope tracers and gas chromatography mass spectrometry were used to determine Leu intracellular concentrations and muscle protein synthesis. EAA ingestion increased RAGB/RAGB mRNA (∼60%) and protein (∼100%) abundance in adult skeletal muscle (P ≤ 0.05). EAAs also increased muscle Leu concentrations (∼130%), mTOR phosphorylation (∼30%), and muscle protein synthesis (∼50%; P ≤ 0.05) but did not alter muscle LARS/LARS abundance (P > 0.05). We conclude that acute EAA ingestion is capable of increasing RAGB expression in human skeletal muscle. Future work is needed to determine whether this adaptive response is important to promote muscle protein anabolism in humans. This trial was registered at clinicaltrials.gov as NCT01669590.

Keeping older muscle “young” through dietary protein and physical activity

Sarcopenia is characterized by decreases in both muscle mass and muscle function. The loss of muscle mass, which can precede decrements in muscle function, is ultimately rooted in an imbalance between the rates of muscle protein synthesis and breakdown that favors a net negative balance (i.e., synthesis < breakdown). A preponderance of evidence highlights a blunted muscle protein synthetic response to dietary protein, commonly referred to as “anabolic resistance,” as a major underlying cause of the insipid loss of muscle with age. Dietary strategies to overcome this decreased dietary amino acid sensitivity include the ingestion of leucine-enriched, rapidly digested proteins and/or greater protein ingestion in each main meal to maximally stimulate muscle anabolism. Anabolic resistance is also a hallmark of a sedentary lifestyle at any age. Given that older adults may be more likely to experience periods of reduced activity (either voluntarily or through acute illness), it is proposed that inactivity is the precipitating factor in the development of anabolic resistance and the subsequent progression from healthy aging to frailty. However, even acute bouts of activity can restore the sensitivity of older muscle to dietary protein. Provided physical activity is incorporated into the daily routine, muscle in older adults should retain its capacity for a robust anabolic response to dietary protein comparable to that in their younger peers. Therefore, through its ability to “make nutrition better,” physical activity should be viewed as a vital component to maintaining muscle mass and function with age.

Membrane transporters for the special amino acid glutamine: structure/function relationships and relevance to human health

Glutamine together with glucose is essential for body's homeostasis. It is the most abundant amino acid and is involved in many biosynthetic, regulatory and energy production processes. Several membrane transporters which differ in transport modes, ensure glutamine homeostasis by coordinating its absorption, reabsorption and delivery to tissues. These transporters belong to different protein families, are redundant and ubiquitous. Their classification, originally based on functional properties, has recently been associated with the SLC nomenclature. Function of glutamine transporters is studied in cells over-expressing the transporters or, more recently in proteoliposomes harboring the proteins extracted from animal tissues or over-expressed in microorganisms. The role of the glutamine transporters is linked to their transport modes and coupling with Na⁺ and H⁺. Most transporters share specificity for other neutral or cationic amino acids. Na⁺-dependent co-transporters efficiently accumulate glutamine while antiporters regulate the pools of glutamine and other amino acids. The most acknowledged glutamine transporters belong to the SLC1, 6, 7, and 38 families. The members involved in the homeostasis are the co-transporters B0AT1 and the SNAT members 1, 2, 3, 5, and 7; the antiporters ASCT2, LAT1 and 2. The last two are associated to the ancillary CD98 protein. Some information on regulation of the glutamine transporters exist, which, however, need to be deepened. No information at all is available on structures, besides some homology models obtained using similar bacterial transporters as templates. Some models of rat and human glutamine transporters highlight very similar structures between the orthologs. Moreover the presence of glycosylation and/or phosphorylation sites located at the extracellular or intracellular faces has been predicted. ASCT2 and LAT1 are over-expressed in several cancers, thus representing potential targets for pharmacological intervention.

Dose-dependent increases in p70S6K phosphorylation and intramuscular branched-chain amino acids in older men following resistance exercise and protein intake

Resistance exercise and whey protein supplementation are effective strategies to activate muscle cell anabolic signaling and ultimately promote increases in muscle mass and strength. In the current study, 46 healthy older men aged 60–75 (69.0 ± 0.55 years, 85.9 ± 1.8 kg, 176.8 ± 1.0 cm) performed a single bout of unaccustomed lower body resistance exercise immediately followed by ingestion of a noncaloric placebo beverage or supplement containing 10, 20, 30, or 40 g of whey protein concentrate (WPC). Intramuscular amino acid levels in muscle biopsy samples were measured by Gas Chromatography–Mass Spectrometry (GC‐MS) at baseline (before exercise and WPC supplementation) plus at 2 h and 4 h post exercise. Additionally, the extent of p70S6K phosphorylation at Thr389 in muscle biopsy homogenates was assessed by western blot. Resistance exercise alone reduced intramuscular branch chain amino acid (BCAA; leucine, isoleucine, and valine) content. Supplementation with increasing doses of whey protein prevented this fall in muscle BCAAs during postexercise recovery and larger doses (30 g and 40 g) significantly augmented postexercise muscle BCAA content above that observed following placebo ingestion. Additionally, the fold change in the phosphorylation of p70S6K (Thr389) at 2 h post exercise was correlated with the dose of whey protein consumed (r = 0.51, P < 001) and was found to be significantly correlated with intramuscular leucine content (r = 0.32, P = 0.026). Intramuscular BCAAs, and leucine in particular, appear to be important regulators of anabolic signaling in aged human muscle during postexercise recovery via reversal of exercise‐induced declines in intramuscular BCAAs.

In older men, p70S6K phosphorylation is increased in a dose‐dependent manner with increasing doses of whey protein intake after resistance exercise. Resistance exercise reduces intramuscular concentrations of branched‐chain amino acids but whey protein intake post exercise maintains these concentrations.

Nutrient regulation of insulin secretion and action

Pancreatic β-cell function is of critical importance in the regulation of fuel homoeostasis, and metabolic dysregulation is a hallmark of diabetes mellitus (DM). The β-cell is an intricately designed cell type that couples metabolism of dietary sources of carbohydrates, amino acids and lipids to insulin secretory mechanisms, such that insulin release occurs at appropriate times to ensure efficient nutrient uptake and storage by target tissues. However, chronic exposure to high nutrient concentrations results in altered metabolism that impacts negatively on insulin exocytosis, insulin action and may ultimately lead to development of DM. Reduced action of insulin in target tissues is associated with impairment of insulin signalling and contributes to insulin resistance (IR), a condition often associated with obesity and a major risk factor for DM. The altered metabolism of nutrients by insulin-sensitive target tissues (muscle, adipose tissue and liver) can result in high circulating levels of glucose and various lipids, which further impact on pancreatic β-cell function, IR and progression of the metabolic syndrome. Here, we have considered the role played by the major nutrient groups, carbohydrates, amino acids and lipids, in mediating β-cell insulin secretion, while also exploring the interplay between amino acids and insulin action in muscle. We also focus on the effects of altered lipid metabolism in adipose tissue and liver resulting from activation of inflammatory processes commonly observed in DM pathophysiology. The aim of this review is to describe commonalities and differences in metabolism related to insulin secretion and action, pertinent to the development of DM.

Intramuscular fat and inflammation differ in older adults: the impact of frailty and inactivity

OBJECTIVES

Intramuscular adipose tissue (IMAT) is recognized as a negative predictor of both muscle and mobility function in older adults, however the mechanism by which IMAT may negatively influence muscle and mobility function is currently unknown. The release of pro-inflammatory cytokines from IMAT provides a potential reason for these negative associations. To explore this hypothesis we compared IMAT and muscular inflammation in age-and BMI-matched older non-obese frail and non-frail adults. We also sought to examine the relationship between IMAT and inflammation, and muscle and mobility function in this group of older adults.

DESIGN

A case-control sampling was used for this study. Age-and BMI-matched non-obese frail and non-frail individuals (<65 years) were recruited.

MEASUREMENTS

MRI was used to quantify thigh IMAT and lean tissue. Unilateral muscle biopsies were used to quantify muscular inflammation as represented by interleukin-6 (IL-6) and tumor-necrosis factor alpha (TNF-α). Muscle and mobility function was also measured using a maximal voluntary isometric contraction, six-minute walk, and self-selected gait speed.

PARTICIPANTS

26 older (80.7 +/- 5.4 years) individuals (8 frail and 18 non-frail) were enrolled.

RESULTS

The frail-group had increased IMAT (p<0.01) and decreased lean tissue (p<0.01), and elevated IL-6 muscle mRNA (p=0.02) and IL-6 protein content (p=0.02) compared to the non-frail group. IMAT was significantly associated with IL-6 mRNA (r=0.43, p=.04) and protein expression within the muscle (r=0.41, p= 0.045). IL-6 mRNA was significantly associated with six-minute walk (r=-0.63, p<0.01), and gait speed (r=-0.60, p <0.01) and IL-6 protein was significantly associated with muscle force (r=-0.54, p=0.01), six-minute walk (r=-0.66, p<0.01), and gait speed (r=-0.76, p<0.01). No significant relationships were found for any variables with TNF-a.

CONCLUSION

Non-obese, older, frail individuals have increased IMAT and muscular inflammation when compared to their non-frail, age- and BMI-matched peers. A significant relationship exists between IMAT and muscle IL-6 expression as well as between IL-6 and muscle and mobility function of these older adults. This IMAT-inflammatory pathway provides a potential link between IMATs and decreased muscle and mobility function.

Soy-dairy protein blend and whey protein ingestion after resistance exercise increases amino acid transport and transporter expression in human skeletal muscle

Increasing amino acid availability (via infusion or ingestion) at rest or postexercise enhances amino acid transport into human skeletal muscle. It is unknown whether alterations in amino acid availability, from ingesting different dietary proteins, can enhance amino acid transport rates and amino acid transporter (AAT) mRNA expression. We hypothesized that the prolonged hyperaminoacidemia from ingesting a blend of proteins with different digestion rates postexercise would enhance amino acid transport into muscle and AAT expression compared with the ingestion of a rapidly digested protein. In a double-blind, randomized clinical trial, we studied 16 young adults at rest and after acute resistance exercise coupled with postexercise (1 h) ingestion of either a (soy-dairy) protein blend or whey protein. Phenylalanine net balance and transport rate into skeletal muscle were measured using stable isotopic methods in combination with femoral arteriovenous blood sampling and muscle biopsies obtained at rest and 3 and 5 h postexercise. Phenylalanine transport into muscle and mRNA expression of select AATs [system L amino acid transporter 1/solute-linked carrier (SLC) 7A5, CD98/SLC3A2, system A amino acid transporter 2/SLC38A2, proton-assisted amino acid transporter 1/SLC36A1, cationic amino acid transporter 1/SLC7A1] increased to a similar extent in both groups (P < 0.05). However, the ingestion of the protein blend resulted in a prolonged and positive net phenylalanine balance during postexercise recovery compared with whey protein (P < 0.05). Postexercise myofibrillar protein synthesis increased similarly between groups. We conclude that, while both protein sources enhanced postexercise AAT expression, transport into muscle, and myofibrillar protein synthesis, postexercise ingestion of a protein blend results in a slightly prolonged net amino acid balance across the leg compared with whey protein.

Insulin increases mRNA abundance of the amino acid transporter SLC7A5/LAT1 via an mTORC1-dependent mechanism in skeletal muscle cells

Abstract Amino acid transporters (AATs) provide a link between amino acid availability and mammalian/mechanistic target of rapamycin complex 1 (mTORC1) activation although the direct relationship remains unclear. Previous studies in various cell types have used high insulin concentrations to determine the role of insulin on mTORC1 signaling and AAT mRNA abundance. However, this approach may limit applicability to human physiology. Therefore, we sought to determine the effect of insulin on mTORC1 signaling and whether lower insulin concentrations stimulate AAT mRNA abundance in muscle cells. We hypothesized that lower insulin concentrations would increase mRNA abundance of select AAT via an mTORC1-dependent mechanism in C2C12 myotubes. Insulin (0.5 nmol/L) significantly increased phosphorylation of the mTORC1 downstream effectors p70 ribosomal protein S6 kinase 1 (S6K1) and ribosomal protein S6 (S6). A low rapamycin dose (2.5 nmol/L) significantly reduced the insulin-(0.5 nmol/L) stimulated S6K1 and S6 phosphorylation. A high rapamycin dose (50 nmol/L) further reduced the insulin-(0.5 nmol/L) stimulated phosphorylation of S6K1 and S6. Insulin (0.5 nmol/L) increased mRNA abundance of SLC38A2/SNAT2 (P ≤ 0.043) and SLC7A5/LAT1 (P ≤ 0.021) at 240 min and SLC36A1/PAT1 (P = 0.039) at 30 min. High rapamycin prevented an increase in SLC38A2/SNAT2 (P = 0.075) and SLC36A1/PAT1 (P ≥ 0.06) mRNA abundance whereas both rapamycin doses prevented an increase in SLC7A5/LAT1 (P ≥ 0.902) mRNA abundance. We conclude that a low insulin concentration increases SLC7A5/LAT1 mRNA abundance in an mTORC1-dependent manner in skeletal muscle cells.

The catalytic subunit of the system L1 amino acid transporter (slc7a5) facilitates nutrient signalling in mouse skeletal muscle

The System L1-type amino acid transporter mediates transport of large neutral amino acids (LNAA) in many mammalian cell-types. LNAA such as leucine are required for full activation of the mTOR-S6K signalling pathway promoting protein synthesis and cell growth. The SLC7A5 (LAT1) catalytic subunit of high-affinity System L1 functions as a glycoprotein-associated heterodimer with the multifunctional protein SLC3A2 (CD98). We generated a floxed Slc7a5 mouse strain which, when crossed with mice expressing Cre driven by a global promoter, produced Slc7a5 heterozygous knockout (Slc7a5+/−) animals with no overt phenotype, although homozygous global knockout of Slc7a5 was embryonically lethal. Muscle-specific (MCK Cre-mediated) Slc7a5 knockout (MS-Slc7a5-KO) mice were used to study the role of intracellular LNAA delivery by the SLC7A5 transporter for mTOR-S6K pathway activation in skeletal muscle. Activation of muscle mTOR-S6K (Thr389 phosphorylation) in vivo by intraperitoneal leucine injection was blunted in homozygous MS-Slc7a5-KO mice relative to wild-type animals. Dietary intake and growth rate were similar for MS-Slc7a5-KO mice and wild-type littermates fed for 10 weeks (to age 120 days) with diets containing 10%, 20% or 30% of protein. In MS-Slc7a5-KO mice, Leu and Ile concentrations in gastrocnemius muscle were reduced by ∼40% as dietary protein content was reduced from 30 to 10%. These changes were associated with >50% decrease in S6K Thr389 phosphorylation in muscles from MS-Slc7a5-KO mice, indicating reduced mTOR-S6K pathway activation, despite no significant differences in lean tissue mass between groups on the same diet. MS-Slc7a5-KO mice on 30% protein diet exhibited mild insulin resistance (e.g. reduced glucose clearance, larger gonadal adipose depots) relative to control animals. Thus, SLC7A5 modulates LNAA-dependent muscle mTOR-S6K signalling in mice, although it appears non-essential (or is sufficiently compensated by e.g. SLC7A8 (LAT2)) for maintenance of normal muscle mass.

Downregulation of E3 ubiquitin ligases and mitophagy-related genes in skeletal muscle of physically inactive, frail older women: a cross-sectional comparison

BACKGROUND AND OBJECTIVES

Reduced lean mass and physical function is a characteristic of frailty. However, it is currently unknown if proteolysis through the E3 ubiquitin ligases and the autophagic lysosomal pathway is dysregulated in inactive frail older women. The purpose of this study was to determine the expression of key markers of ubiquitin-mediated and autophagic lysosomal proteolysis in inactive (N = 7) compared with active (N = 7) older women.

METHODS

Strength, mobility, leg lean mass, and physical activity assessment were used to characterize activity levels and frailty in older women. Vastus lateralis biopsies were collected after an overnight fast and were assessed for gene and protein targets related to E3 ubiquitin ligases and the autophagic lysosomal system.

RESULTS

We found that AMP-activated protein kinase alpha (Thr172) was increased (p = .045), and forkhead box O3A (FOXO3A) gene expression (p = .047) was lower in inactive frail older women. Foxo3a (Ser253), Beclin1 (Ser93/96), and class III phosphatidylinositol-3-kinase (VPS34) protein expression were not different between the groups (p > .05). Neural precursor cell-expressed developmentally downregulated protein 4, muscle ring finger 1, muscle atrophy F-box, and the autophagy/mitophagy gene expression markers, Beclin1, autophagy-related-7, BCL2/adenovirus E1B 19 kDa interacting protein 3 (BNIP3), dynamin-related protein 1, and Parkinson protein 2 (PARKIN) were lower in inactive frail older women (p < .05). Autophagy/mitophagy markers were positively correlated with the 6-minute walk and leg lean mass (p < .05).

CONCLUSIONS

We conclude that physical inactivity in frail older women is associated with a downregulation of ubiquitin-mediated and autophagic lysosomal skeletal muscle gene expression, perhaps related to low muscle mass and poor physical function.

Bolus ingestion of individual branched-chain amino acids alters plasma amino acid profiles in young healthy men

Physiological conditions in humans affect plasma amino acid profiles that might have potential for medical use. Because the branched-chain amino acids (BCAAs) leucine, isoleucine and valine are used as medicines and supplements, we investigated the acute effects of individual BCAAs (10-90 mg/kg body weight) or mixed BCAAs ingested as a bolus on plasma amino acid profiles in young healthy men. Plasma leucine levels rapidly increased and peaked around 30 min after leucine ingestion. Concentrations of plasma isoleucine, valine and phenylalanine subsequently decreased after ingestion, and those of methionine and tyrosine tended to decrease. The effects of ingested leucine on other plasma amino acids were biphasic, being higher at lower doses (10-20 mg/kg body weight). Isoleucine or valine intake also caused corresponding plasma amino acid concentrations to rapidly elevate, and peaks at 30-40 min after ingestion were much higher than that of plasma leucine after leucine ingestion. However, the increase in plasma isoleucine and valine concentrations essentially did not affect those of other plasma amino acids. The rate of decline among peak plasma BCAA concentrations was the highest for leucine, followed by isoleucine and valine. Oral mixed BCAAs promoted the decline in plasma isoleucine and valine concentrations. These results suggest that plasma leucine is a regulator of the plasma concentrations of BCAAs, methionine and aromatic amino acids.

Whey protein intake after resistance exercise activates mTOR signaling in a dose-dependent manner in human skeletal muscle

PURPOSE

Protein ingestion after resistance exercise increases muscle protein synthesis (MPS) in a dose-dependent manner. However, the molecular mechanism(s) for the dose-dependency of MPS remains unclear. This study aimed to determine the dose response of mammalian target of rapamycin (mTOR) signaling in muscle with ingestion of protein after resistance exercise.

METHODS

Fifteen male subjects performed four sets of six unilateral isokinetic concentric knee extensions. Immediately after exercise, eight subjects consumed water only. The other seven subjects, in a randomized-order crossover design, took either a 10 [3.6 g essential amino acids (EAA)] or 20 g (7.1 g EAA) solution of whey protein. Muscle biopsies from the vastus lateralis muscle were taken 30 min before and 1 h after resistance exercise. Phosphorylation of Akt (Ser473), mTOR (Ser2448), 4E-BP1 (Thr37/46), and S6K1 (Thr389) was measured by western blotting.

RESULTS

Concentric knee extension exercise alone did not increase phosphorylation of Akt and mTOR 1 h after exercise, but ingesting protein after exercise significantly increased the phosphorylation of Akt and mTOR in a dose-dependent manner (P < 0.05). 4E-BP1 phosphorylation significantly decreased after resistance exercise (P < 0.05), but subjects who took 10 or 20 g of protein after exercise showed increased 4E-BP1 from post-exercise dephosphorylation (P < 0.05). S6K1 phosphorylation significantly increased after resistance exercise (P < 0.05), and 20 g of protein further increased S6K1 phosphorylation compared with ingestion of 10 g (P < 0.05).

CONCLUSIONS

These findings suggest that whey protein intake after resistance exercise activates mTOR signaling in a dose-dependent manner in untrained men.

PAT1 (SLC36A1) shows nuclear localization and affects growth of smooth muscle cells from rats

The proton-coupled amino acid transporter 1 (PAT1) is a transporter of amino acids in small intestinal enterocytes. PAT1 is, however, also capable of regulating cell growth and sensing the availability of amino acids in other cell types. The aim of the present study was to investigate the localization and function of PAT1 in smooth muscle cells (SMCs). The PAT1 protein was found in smooth muscles from rat intestine and in the embryonic rat aorta cell line A7r5. Immunolocalization and cellular fractionation studies revealed that the majority of the PAT1 protein located within the cell nucleus of A7r5 cells. These results were confirmed in primary SMCs derived from rat aorta and colon. A 3'-untranslated region of the PAT1 transcript directed the nuclear localization. Neither cellular starvation nor cell division altered the nuclear localization. In agreement, uptake studies of l-proline, a PAT1 substrate, in A7r5 cells suggested an alternative role for PAT1 in SMCs than in transport. To shed light on the function of PAT1 in A7r5 cells, experiments with downregulation of the PAT1 level by use of a siRNA approach were conducted. The growth rates of the cells were evaluated, and knockdown of PAT1 led to induced cellular growth, suggesting a role for PAT1 in regulating cellular proliferation of SMCs.

Role of amino acid transporters in amino acid sensing

Amino acid (AA) transporters may act as sensors, as well as carriers, of tissue nutrient supplies. This review considers recent advances in our understanding of the AA-sensing functions of AA transporters in both epithelial and nonepithelial cells. These transporters mediate AA exchanges between extracellular and intracellular fluid compartments, delivering substrates to intracellular AA sensors. AA transporters on endosomal (eg, lysosomal) membranes may themselves function as intracellular AA sensors. AA transporters at the cell surface, particularly those for large neutral AAs such as leucine, interact functionally with intracellular nutrient-signaling pathways that regulate metabolism: for example, the mammalian target of rapamycin complex 1 (mTORC1) pathway, which promotes cell growth, and the general control non-derepressible (GCN) pathway, which is activated by AA starvation. Under some circumstances, upregulation of AA transporter expression [notably a leucine transporter, solute carrier 7A5 (SLC7A5)] is required to initiate AA-dependent activation of the mTORC1 pathway. Certain AA transporters may have dual receptor-transporter functions, operating as "transceptors" to sense extracellular (or intracellular) AA availability upstream of intracellular signaling pathways. New opportunities for nutritional therapy may include targeting of AA transporters (or mechanisms that upregulate their expression) to promote protein-anabolic signals for retention or recovery of lean tissue mass.

The SLC38 family of sodium-amino acid co-transporters

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

Amino acid transporters in the regulation of human skeletal muscle protein metabolism

PURPOSE OF REVIEW

To highlight recent research on amino acid sensing and signaling and the role of amino acid transporters in the regulation of human skeletal muscle protein metabolism.

RECENT FINDINGS

The mechanisms that sense amino acid availability and activate mechanistic target of rapamycin complex 1 signaling and protein synthesis are emerging, with multiple new proteins and intracellular amino acid sensors recently identified. Amino acid transporters have a role in the delivery of amino acids to these intracellular sensors and new findings provide further support for amino acid transporters as possible extracellular amino acid sensors. There is growing evidence in human skeletal muscle that amino acid transporter expression is dynamic and responsive to various stimuli, indicating amino acid transporters may have a unique role in the regulation of human skeletal muscle adaptation.

SUMMARY

There is a clear need to further examine the role of amino acid transporters in human skeletal muscle and their link to cellular amino acid sensing and signaling in the control of protein metabolism. A better understanding of amino acid transport and transporters will allow us to optimize nutritional strategies to accelerate muscle health and improve outcomes for clinical populations.

Leucine pulses enhance skeletal muscle protein synthesis during continuous feeding in neonatal pigs

Infants unable to maintain oral feeding can be nourished by orogastric tube. We have shown that orogastric continuous feeding restricts muscle protein synthesis compared with intermittent bolus feeding in neonatal pigs. To determine whether leucine infusion can be used to enhance protein synthesis during continuous feeding, neonatal piglets received the same amount of formula enterally by orogastric tube for 25.25 h continuously (CON) with or without LEU or intermittently by bolus every 4 h (BOL). For the CON+LEU group, leucine pulses were administered parenterally (800 μmol·kg(-1)·h(-1)) every 4 h. Insulin and glucose concentrations increased after the BOL meal and were unchanged in groups fed continuously. LEU infusion during CON feeding increased plasma leucine after the leucine pulse and decreased essential amino acids compared with CON feeding. Protein synthesis in longissimus dorsi (LD), gastrocnemius, and soleus muscles, but not liver or heart, were greater in CON+LEU and BOL than in the CON group. BOL feeding increased protein synthesis in the small intestine. Muscle S6K1 and 4E-BP1 phosphorylation and active eIF4E·eIF4G complex formation were higher in CON+LEU and BOL than in CON but AMPKα, eIF2α, and eEF2 phosphorylation were unchanged. LC3-II-to-total LC3 ratio was lower in CON+LEU and BOL than in CON, but there were no differences in atrogin-1 and MuRF-1 abundance and FoxO3 phosphorylation. In conclusion, administration of leucine pulses during continuous orogastric feeding in neonates increases muscle protein synthesis by stimulating translation initiation and may reduce protein degradation via the autophagy-lysosome, but not the ubiquitin-proteasome pathway.

Short-term bed rest increases TLR4 and IL-6 expression in skeletal muscle of older adults

Bed rest induces significant loss of leg lean mass in older adults. Systemic and tissue inflammation also accelerates skeletal muscle loss, but it is unknown whether inflammation is associated to inactivity-induced muscle atrophy in healthy older adults. We determined if short-term bed rest increases toll-like receptor 4 (TLR4) signaling and pro-inflammatory markers in older adult skeletal muscle biopsy samples. Six healthy, older adults underwent seven consecutive days of bed rest. Muscle biopsies (vastus lateralis) were taken after an overnight fast before and at the end of bed rest. Serum cytokine expression was measured before and during bed rest. TLR4 signaling and cytokine mRNAs associated with pro- and anti-inflammation and anabolism were measured in muscle biopsy samples using Western blot analysis and qPCR. Participants lost ∼4% leg lean mass with bed rest. We found that after bed rest, muscle levels of TLR4 protein expression and interleukin-6 (IL-6), nuclear factor-κB1, interleukin-10, and 15 mRNA expression were increased after bed rest (P < 0.05). Additionally, the cytokines interferon-γ, and macrophage inflammatory protein-1β, were elevated in serum samples following bed rest (P < 0.05). We conclude that short-term bed rest in older adults modestly increased some pro- and anti-inflammatory cytokines in muscle samples while systemic changes in pro-inflammatory cytokines were mostly absent. Upregulation of TLR4 protein content suggests that bed rest in older adults increases the capacity to mount an exaggerated, and perhaps unnecessary, inflammatory response in the presence of specific TLR4 ligands, e.g., during acute illness.

Increased plasma citrulline in mice marks diet-induced obesity and may predict the development of the metabolic syndrome

In humans, plasma amino acid concentrations of branched-chain amino acids (BCAA) and aromatic amino acids (AAA) increase in states of obesity, insulin resistance and diabetes. We here assessed whether these putative biomarkers can also be identified in two different obesity and diabetic mouse models. C57BL/6 mice with diet-induced obesity (DIO) mimic the metabolic impairments of obesity in humans characterized by hyperglycemia, hyperinsulinemia and hepatic triglyceride accumulation. Mice treated with streptozotocin (STZ) to induce insulin deficiency were used as a type 1 diabetes model. Plasma amino acid profiling of two high fat (HF) feeding trials revealed that citrulline and ornithine concentrations are elevated in obese mice, while systemic arginine bioavailability (ratio of plasma arginine to ornithine + citrulline) is reduced. In skeletal muscle, HF feeding induced a reduction of arginine levels while citrulline levels were elevated. However, arginine or citrulline remained unchanged in their key metabolic organs, intestine and kidney. Moreover, the intestinal conversion of labeled arginine to ornithine and citrulline in vitro remained unaffected by HF feeding excluding the intestine as prime site of these alterations. In liver, citrulline is mainly derived from ornithine in the urea cycle and DIO mice displayed reduced hepatic ornithine levels. Since both amino acids share an antiport mechanism for mitochondrial import and export, elevated plasma citrulline may indicate impaired hepatic amino acid handling in DIO mice. In the insulin deficient mice, plasma citrulline and ornithine levels also increased and additionally these animals displayed elevated BCAA and AAA levels like insulin resistant and diabetic patients. Therefore, type 1 diabetic mice but not DIO mice show the “diabetic fingerprint” of plasma amino acid changes observed in humans. Additionally, citrulline may serve as an early indicator of the obesity-dependent metabolic impairments.

Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis

Quantity and timing of protein ingestion are major factors regulating myofibrillar protein synthesis (MPS). However, the effect of specific ingestion patterns on MPS throughout a 12 h period is unknown. We determined how different distributions of protein feeding during 12 h recovery after resistance exercise affects anabolic responses in skeletal muscle. Twenty-four healthy trained males were assigned to three groups (n = 8/group) and undertook a bout of resistance exercise followed by ingestion of 80 g of whey protein throughout 12 h recovery in one of the following protocols: 8 × 10 g every 1.5 h (PULSE); 4 × 20 g every 3 h (intermediate: INT); or 2 × 40 g every 6 h (BOLUS). Muscle biopsies were obtained at rest and after 1, 4, 6, 7 and 12 h post exercise. Resting and post-exercise MPS (l-[ring-(13)C6] phenylalanine), and muscle mRNA abundance and cell signalling were assessed. All ingestion protocols increased MPS above rest throughout 1-12 h recovery (88-148%, P < 0.02), but INT elicited greater MPS than PULSE and BOLUS (31-48%, P < 0.02). In general signalling showed a BOLUS>INT>PULSE hierarchy in magnitude of phosphorylation. MuRF-1 and SLC38A2 mRNA were differentially expressed with BOLUS. In conclusion, 20 g of whey protein consumed every 3 h was superior to either PULSE or BOLUS feeding patterns for stimulating MPS throughout the day. This study provides novel information on the effect of modulating the distribution of protein intake on anabolic responses in skeletal muscle and has the potential to maximize outcomes of resistance training for attaining peak muscle mass.

Addition of carbohydrate or alanine to an essential amino acid mixture does not enhance human skeletal muscle protein anabolism

In humans, essential amino acids (EAAs) stimulate muscle protein synthesis (MPS) with no effect on muscle protein breakdown (MPB). Insulin can stimulate MPS, and carbohydrates (CHOs) and insulin decrease MPB. Net protein balance (NB; indicator of overall anabolism) is greatest when MPS is maximized and MPB is minimized. To determine whether adding CHO or a gluconeogenic amino acid to EAAs would improve NB compared with EAA alone, young men and women (n = 21) ingested 10 g EAA alone, with 30 g sucrose (EAA+CHO), or with 30 g alanine (EAA+ALA). The fractional synthetic rate and phenylalanine kinetics (MPS, MPB, NB) were assessed by stable isotopic methods on muscle biopsies at baseline and 60 and 180 min following nutrient ingestion. Insulin increased 30 min postingestion in all groups and remained elevated in the EAA+CHO and EAA+ALA groups for 60 and 120 min, respectively. The fractional synthetic rate increased from baseline at 60 min in all groups (P < 0.05; EAA = 0.053 ± 0.018 to 0.090 ± 0.039% · h(-1); EAA+ALA = 0.051 ± 0.005 to 0.087 ± 0.015% · h(-1); EAA+CHO = 0.049 ± 0.006 to 0.115 ± 0.024% · h(-1)). MPS and NB peaked at 30 min in the EAA and EAA+CHO groups but at 60 min in the EAA+ALA group and NB was elevated above baseline longer in the EAA+ALA group than in the EAA group (P < 0.05). Although responses were more robust in the EAA+CHO group and prolonged in the EAA+ALA group, AUCs were similar among all groups for fractional synthetic rate, MPS, MPB, and NB. Because the overall muscle protein anabolic response was not improved in either the EAA+ALA or EAA+CHO group compared with EAA, we conclude that protein nutritional interventions to enhance muscle protein anabolism do not require such additional energy.

Nutritional strategies to attenuate muscle disuse atrophy

Situations such as recovery from injury or illness require otherwise healthy humans to undergo periods of disuse, which lead to considerable losses of skeletal muscle mass and, subsequently, numerous negative health consequences. It has been established that prolonged disuse (>10 days) leads to a decline in basal and postprandial rates of muscle protein synthesis, without an apparent change in muscle protein breakdown. It also seems, however, that an early and transient (1-5 days) increase in basal muscle protein breakdown may also contribute to disuse atrophy. A period of disuse reduces energy requirements and appetite. Consequently, food intake generally declines, resulting in an inadequate dietary protein consumption to allow proper muscle mass maintenance. Evidence suggests that maintaining protein intake during a period of disuse attenuates disuse atrophy. Furthermore, supplementation with dietary protein and/or essential amino acids can be applied to further aid in muscle mass preservation during disuse. Such strategies are of particular relevance to the older patient at risk of developing sarcopenia. More work is required to elucidate the impact of disuse on basal and postprandial rates of muscle protein synthesis and breakdown. Such information will provide novel targets for nutritional interventions to further attenuate muscle disuse atrophy and, as such, support healthy aging.

The role of amino acid transporters in nutrition

PURPOSE OF REVIEW

We consider recent advances in epithelial amino acid transport physiology and our understanding of the functioning of amino acid transporters as sensors, as well as carriers, of tissue nutrient supplies.

RECENT FINDINGS

Gut hormones (e.g. leptin) may regulate intestinal amino acid transporter activity by a variety of mechanisms, although the overall functional significance of such regulation is not yet fully understood. Important functional interactions between amino acid transporters and nutrient-signalling pathways which regulate metabolism [e.g. the mammalian target of rapamycin (mTOR)C1 pathway which promotes cell growth] have been revealed in recent studies. Amino acid transporters on endosomal (e.g. lysosomal) membranes may be of unexpected significance as intracellular nutrient sensors. It is also now evident that certain amino acid transporters may have dual receptor-transporter functions and act as 'transceptors' to sense amino acid availability upstream of signal pathways.

SUMMARY

Increased knowledge on the timescale of the amino acid sensor-signal-effector process(es) should help in the optimization of protein-feeding regimes to gain maximum anabolic effect. New opportunities for nutritional therapy include targeting of amino acid transceptors to promote protein-anabolic signals and mechanisms up-regulating amino acid transporter expression to improve absorptive capacity for nutrients.

Differential effects of military training on fat-free mass and plasma amino acid adaptations in men and women

Fat-free mass (FFM) adaptations to physical training may differ between sexes based on disparities in fitness level, dietary intake, and levels of plasma amino acids (AA). This investigation aimed to determine FFM and plasma AA responses to military training, examine whether adaptations differ between male and female recruits, and explore potential associations between FFM and AA responses to training. Body composition and plasma AA levels were assessed in US Army recruits (n = 209, 118 males, 91 females) before (baseline) and every three weeks during basic combat training (BCT), a 10-week military training course. Body weight decreased in men but remained stable in women during BCT (sex-by-time interaction, P < 0.05). Fifty-eight percent of recruits gained FFM during BCT, with more (P < 0.05) females (88%) gaining FFM than males (36%). Total plasma AA increased (P < 0.05) during BCT, with greater (P < 0.05) increases observed in females (17%) then in males (4%). Essential amino acids (EAA) and branched-chain amino acids (BCAA) were increased (P < 0.05) in females but did not change in males (sex-by-time interaction, P < 0.05). Independent of sex, changes in EAA (r = 0.34) and BCAA (r = 0.27) from baseline were associated with changes in FFM (P < 0.05); greater (P < 0.05) increases in AA concentrations were observed for those who gained FFM. Increases in FFM and plasma AA suggest that BCT elicits a more pronounced anabolic response in women compared to men, which may reflect sex-specific differences in the relative intensity of the combined training and physiological stimulus associated with BCT.

Consilience in sarcopenia of cirrhosis

Cirrhosis is the consequence of progression of many forms of necro-inflammatory disorders of the liver with hepatic fibrosis, hepatocellular dysfunction, and vascular remodeling. Reversing the primary hepatic disorder, liver transplantation, and controlling the complications are the major management goals. Since the former options are not available to the majority of cirrhotics, treating complications remains the mainstay of therapy. Sarcopenia and/or cachexia is the most common complication and adversely affects survival, quality of life, development of other complications of cirrhosis, and outcome after liver transplantation. With the increase in number of cirrhotic patients with hepatitis C and nonalcoholic fatty liver disease, the number of patients waiting for a liver transplantation is likely to continue to increase above the currently estimated 72.3/100,000 population. One of the critical clinical questions is to determine if we can treat sarcopenia of cirrhosis without transplantation. No effective therapies exist to treat sarcopenia because the mechanism(s) of sarcopenia in cirrhosis is as yet unknown. The reasons for this include the predominantly descriptive studies to date and the advances in our understanding of skeletal muscle biology and molecular regulation of atrophy and hypertrophy not being translated into the clinical practice of hepatology. Satellite cell biology, muscle autophagy and apoptosis, and molecular signaling abnormalities in the skeletal muscle of cirrhotics are also not known. Aging of the cirrhotic and transplanted population, use of mTOR inhibitors, and the lack of definitive outcome measures to define sarcopenia and cachexia in this population add to the difficulty in increasing our understanding of hepatic sarcopenia/cachexia and developing treatment options. Recent data on the role of myostatin, AMP kinase, impaired mTOR signaling resulting in anabolic resistance in animal models, and the rapidly developing field of nutriceuticals as signaling molecules need to be evaluated in human cirrhotics. Finally, the benefits of exercise reported in other disease states with sarcopenia may not be safe in cirrhotics due to the risk of gastrointestinal variceal bleeding due to an increase in portal pressure. This article focuses on the problems facing both muscle biologists and hepatologists in developing a comprehensive approach to sarcopenia in cirrhosis.

Myocardial oxidative metabolism and protein synthesis during mechanical circulatory support by extracorporeal membrane oxygenation

Extracorporeal membrane oxygenation (ECMO) provides essential mechanical circulatory support necessary for survival in infants and children with acute cardiac decompensation. However, ECMO also causes metabolic disturbances, which contribute to total body wasting and protein loss. Cardiac stunning can also occur, which prevents ECMO weaning, and contributes to high mortality. The heart may specifically undergo metabolic impairments, which influence functional recovery. We tested the hypothesis that ECMO alters oxidative metabolism and protein synthesis. We focused on the amino acid leucine and integration with myocardial protein synthesis. We used a translational immature swine model in which we assessed in heart 1) the fractional contribution of leucine (FcLeucine) and pyruvate to mitochondrial acetyl-CoA formation by nuclear magnetic resonance and 2) global protein fractional synthesis (FSR) by gas chromatography-mass spectrometry. Immature mixed breed Yorkshire male piglets (n = 22) were divided into four groups based on loading status (8 h of normal circulation or ECMO) and intracoronary infusion [(13)C(6),(15)N]-L-leucine (3.7 mM) alone or with [2-(13)C]-pyruvate (7.4 mM). ECMO decreased pulse pressure and correspondingly lowered myocardial oxygen consumption (∼40%, n = 5), indicating decreased overall mitochondrial oxidative metabolism. However, FcLeucine was maintained and myocardial protein FSR was marginally increased. Pyruvate addition decreased tissue leucine enrichment, FcLeucine, and Fc for endogenous substrates as well as protein FSR. The heart under ECMO shows reduced oxidative metabolism of substrates, including amino acids, while maintaining 1) metabolic flexibility indicated by ability to respond to pyruvate and 2) a normal or increased capacity for global protein synthesis.

Myosin heavy chain 2A and α-actin expression in human and murine skeletal muscles at feeding; particularly amino acids

BACKGROUND

Protein dynamics during non-steady state conditions as feeding are complex. Such studies usually demand combinations of methods to give conclusive information, particularly on myofibrillar proteins with slow turnover. Therefore, time course transcript analyses were evaluated as possible means to monitor changes in myofibrillar biosynthesis in skeletal muscles in conditions with clinical nutrition; i.e. long term exposure of nutrients.

METHODS

Muscle tissue from overnight intravenously fed surgical patients were used as a model combined with muscle tissue from starved and refed mice as well as cultured L6 muscle cells. Transcripts of acta 1 (α-actin), mhc2A (myosin) and slc38 a2/Snat 2 (amino acid transporter) were quantified (qPCR) as markers of muscle protein dynamics.

RESULTS

Myosin heavy chain 2A transcripts decreased significantly in skeletal muscle tissue from overnight parenterally fed patients but did not change significantly in orally refed mice. Alpha-actin transcripts did not change significantly in muscle cells from fed patients, mice or cultured L6 cells during provision of AA. The AA transporter Snat 2 decreased in L6 cells refed by all AA and by various combinations of AA but did not change during feeding in muscle tissue from patients or mice.

CONCLUSION

Our results confirm that muscle cells are sensitive to alterations in extracellular concentrations of AA for induction of protein synthesis and anabolism. However, transcripts of myofibrillar proteins and amino acid transporters showed complex alterations in response to feeding with provision of amino acids. Therefore, muscle tissue transcript levels of actin and myosin do not reflect protein accretion in skeletal muscles at feeding.

A soy, whey and caseinate blend extends postprandial skeletal muscle protein synthesis in rats

BACKGROUND & AIMS

Blends of dairy and soy protein are used in commercial sports nutrition products; however, no studies have systematically compared blends to isolated protein sources and their effects on muscle protein synthesis (MPS). Dairy whey protein (WP), soy protein isolate (SP), and two blends (Blend 1 and Blend 2) consisting of ratios of 50:25:25 and 25:50:25 for whey:caseinate:soy, respectively, were evaluated for their ability to affect MPS.

METHODS

Male Sprague-Dawley rats were trained to eat 3 meals/day: a 4 g meal at 0700-0720 hours followed by ad lib feeding at 1300-1400 hours and 1800-1900 hours. After ~5 days of training, fasted rats were administered their respective 4 g meal at 0700-0720 hours and an intravenous flooding dose of (2)H5-phenylalanine 10 min prior to euthanasia. Individual rats were euthanized at designated postprandial time points. Blood and gastrocnemius samples were collected and the latter was used to measure mixed muscle protein fractional synthetic rates (FSR).

RESULTS

Plasma leucine concentrations peaked in all groups at 90 min and were still above baseline at 300 min post-meal. FSR tended to increase in all groups post-meal but initial peaks of FSR were different times (45, 90 and 135 min for WP or SP, Blend 1 and Blend 2, respectively). Blend 2 had a significantly higher FSR compared to WP alone at 135 min (P < 0.05).

CONCLUSIONS

Single source proteins and protein blends all enhance skeletal MPS after a meal, however, Blend 2 had a delayed FSR peak which was significantly higher than whey protein at 135 min.

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.

Survival by self-destruction: a role for autophagy in the placenta?

Autophagy is a burgeoning area of research from yeast to humans. Although previously described as a death pathway, autophagy is now considered an important survival phenomenon in response to environmental stressors to which most organs are exposed. Despite an ever expanding literature in non-placental cells, studies of autophagy in the placenta are lagging. We review the regulation of autophagy, summarize available placental studies of autophagy, and highlight potential areas for future research. We believe that such studies will yield novel insights into how placentas protect the survival of the species by "self-eating".

Aging differentially affects human skeletal muscle amino acid transporter expression when essential amino acids are ingested after exercise

BACKGROUND & AIMS

Amino acid transporters have been proposed as regulators of protein synthesis. The primary aim of this study was to determine whether amino acid transporter expression is increased in human muscle following resistance exercise (RE) coupled with essential amino acid (EAA) ingestion, and whether a differential response occurs with aging. Secondly, we aimed to compare this response to a previous study examining RE alone.

METHODS

Young (n = 7, 30 ± 2 yr) and older men (n = 6, 70 ± 2 yr) ingested EAA 1 h after RE. Muscle biopsies were obtained at rest and 3 and 6 h post exercise to examine amino acid transporter mRNA and protein expression.

RESULTS

In both age groups, RE + EAA increased mRNA of L-type amino acid transporter 1 (LAT1)/solute linked carrier (SLC)7A5, sodium-coupled neutral amino acid transporter 2 (SNAT2)/SLC38A2, and cationic amino acid transporter 1/SLC7A1 (p < 0.05). SNAT2 protein increased in young at 3 and 6 h (p < 0.05), whereas old maintained higher LAT1 protein (p < 0.05). Compared to RE alone, RE + EAA enhanced amino acid transporter expression only in young (p < 0.05).

CONCLUSIONS

RE increases muscle amino acid transporter expression in young and older adults, however, post exercise EAA ingestion enhances amino acid transporter expression only in young indicating that aging may influence the function of specific amino acid transporters.

Exercise and amino acid anabolic cell signaling and the regulation of skeletal muscle mass

A series of complex intracellular networks influence the regulation of skeletal muscle protein turnover. In recent years, studies have examined how cellular regulators of muscle protein turnover modulate metabolic mechanisms contributing to the loss, gain, or conservation of skeletal muscle mass. Exercise and amino acids both stimulate anabolic signaling potentially through several intracellular pathways including the mammalian target of rapamycin complex 1 and the mitogen activated protein kinase cell signaling cascades. As novel molecular regulators of muscle integrity continue to be explored, a contemporary analysis of the literature is required to understand the metabolic mechanisms by which contractile forces and amino acids affect cellular process that contribute to long-term adaptations and preservation of muscle mass. This article reviews the literature related to how exercise and amino acid availability affect cellular regulators of skeletal muscle mass, especially highlighting recent investigations that have identified mechanisms by which contractile forces and amino acids modulate muscle health. Furthermore, this review will explore integrated exercise and nutrition strategies that promote the maintenance of muscle health by optimizing exercise, and amino acid-induced cell signaling in aging adults susceptible to muscle loss.

Leucine and mTORC1: a complex relationship

Amino acid availability is a rate-limiting factor in the regulation of protein synthesis. When amino acid supplies become restricted, mammalian cells employ homeostatic mechanisms to rapidly inhibit processes such as protein synthesis, which demands high levels of amino acids. Muscle cells in particular are subject to high protein turnover rates to maintain amino acid homeostasis. Mammalian target of rapamycin complex 1 (mTORC1) is an evolutionary conserved multiprotein complex that coordinates a network of signaling cascades and functions as a key mediator of protein translation, gene transcription, and autophagy. Signal transduction through mTORC1, which is centrally involved in muscle growth through enhanced protein translation, is governed by intracellular amino acid supply. The branched-chain amino acid leucine is critical for muscle growth and acts in part through activation of mTORC1. Recent research has revealed that mTORC1 signaling is coordinated primarily at the lysosomal membranes. This discovery has sparked a wealth of research in this field, revealing several different signaling molecules involved in transducing the amino acid signal to mTORC1, including the Rag GTPases, MAP4K3, and Vps34/ULK1. This review evaluates the current knowledge regarding cellular mechanisms that control and sense the intracellular amino acid pool. We discuss the role of leucine and mTORC1 in the regulation of amino acid transport via the system L and system A transporters such as LAT1 and SNAT2, as well as protein degradation via autophagic and proteasomal pathways. We also describe the complexities of energy homeostasis via AMPK and cell receptor-mediated growth signals that also converge on mTORC1. Leucine is a particularly potent regulator of protein turnover, to the extent where leucine stimulation alone is sufficient to stimulate mTORC1 signal transduction. The significance of leucine in this context is not yet known; however, recent advancements in this area will also be covered within this review.

Abundance of amino acid transporters involved in mTORC1 activation in skeletal muscle of neonatal pigs is developmentally regulated

Previously we demonstrated that the insulin- and amino acid-induced activation of the mammalian target of rapamycin complex 1 (mTORC1) is developmentally regulated in neonatal pigs. Recent studies have indicated that members of the System A transporter (SNAT2), the System N transporter (SNAT3), the System L transporters (LAT1 and LAT2), and the proton-assisted amino acid transporters (PAT1 and PAT2) have crucial roles in the activation of mTORC1 and that the abundance of amino acid transporters is positively correlated with their activation. This study aimed to determine the effect of the post-prandial rise in insulin and amino acids on the abundance or activation of SNAT2, SNAT3, LAT1, LAT2, PAT1, and PAT2 and whether the response is modified by development. Overnight fasted 6- and 26-day-old pigs were infused for 2 h with saline (Control) or with insulin or amino acids to achieve fed levels while amino acids or insulin, respectively, as well as glucose were maintained at fasting levels. The abundance of SNAT2, SNAT3, LAT1, LAT2, PAT1, and PAT2 was higher in muscle of 6- compared with 26-day-old pigs. The abundance of the PAT2-mTOR complex was greater in 6- than in 26-day-old pigs, consistent with the higher activation of mTORC1. Neither insulin nor amino acids altered amino acid transporter or PAT2-mTOR complex abundance. In conclusion, the amino acid transporters, SNAT 2/3, LAT 1/2, and PAT1/2, likely have important roles in the enhanced amino acid-induced activation of mTORC1 in skeletal muscle of the neonate.