Acidosis-sensing glutamine pump SNAT2 determines amino acid levels and mammalian target of rapamycin signalling to protein synthesis in L6 muscle cells

SNAT2-mediated regulation of estrogen and progesterone in the proliferation of goat mammary epithelial cells

The development of the goat mammary gland is mainly under the control of ovarian hormones particularly estrogen and progesterone (P4). Amino acids play an essential role in mammary gland development and milk production, and sodium-coupled neutral amino acid transporter 2 (SNAT2) was reported to be expressed in the mammary gland of rats and bovine mammary epithelial cells, which may affect the synthesis of milk proteins or mammary cell proliferation by mediating prolactin, 17β-estradiol (E2) or methionine function. However, whether SNAT2 mediates the regulatory effects of E2 and P4 on the development of the ruminant mammary gland is still unclear. In this study, we show that E2 and P4 could increase the proliferation of goat mammary epithelial cells (GMECs) and regulate SNAT2 mRNA and protein expression in a dose-dependent manner. Further investigation revealed that SNAT2 is abundantly expressed in the mammary gland during late pregnancy and early lactation, while knockdown and overexpression of SNAT2 in GMECs could inhibit or enhance E2- and P4-induced cell proliferation as well as mammalian target of rapamycin (mTOR) signaling. We also found that the accelerated proliferation induced by SNAT2 overexpression in GMECs was suppressed by the mTOR signaling pathway inhibitor rapamycin. This indicates that the regulation of GMECs proliferation mediated by SNAT2 in response to E2 and P4 is dependent on the mTOR signaling pathway. Finally, we found that the total content of the amino acids in GMECs changed after knocking-down and overexpressing SNAT2. In summary, the results demonstrate that the regulatory effects of E2 and P4 on GMECs proliferation may be mediated by the SNAT2-transported amino acid pathway. These results may offer a novel nutritional target for improving the development of the ruminant mammary gland and milk production.

SLC38A10 Deficiency in Mice Affects Plasma Levels of Threonine and Histidine in Males but Not in Females: A Preliminary Characterization Study of SLC38A10−/− Mice

Solute carriers belong to the biggest group of transporters in the human genome, but more knowledge is needed to fully understand their function and possible role as therapeutic targets. SLC38A10, a poorly characterized solute carrier, is preliminary characterized here. By using a knockout mouse model, we studied the biological effects of SLC38A10 deficiency in vivo. We performed a transcriptomic analysis of the whole brain and found seven differentially expressed genes in SLC38A10-deficient mice (Gm48159, Nr4a1, Tuba1c, Lrrc56, mt-Tp, Hbb-bt and Snord116/9). By measuring amino acids in plasma, we found lower levels of threonine and histidine in knockout males, whereas no amino acid levels were affected in females, suggesting that SLC38A10−/− might affect sexes differently. Using RT-qPCR, we investigated the effect of SLC38A10 deficiency on mRNA expression of other SLC38 members, Mtor and Rps6kb1 in the brain, liver, lung, muscle, and kidney, but no differences were found. Relative telomere length measurement was also taken, as a marker for cellular age, but no differences were found between the genotypes. We conclude that SLC38A10 might be important for keeping amino acid homeostasis in plasma, at least in males, but no major effects were seen on transcriptomic expression or telomere length in the whole brain.

Comparative analysis of amino acid content and protein synthesis-related genes expression levels in breast muscle among different duck breeds/strains

Evidences have found important effects of breeds/strains on the content of amino acids (AAs) which is an important substrate for protein synthesis and contributes greatly to meat quality. Therefore, the objective of the present study was to compare the AAs content and protein synthesis-related genes expression levels in breast muscle of native breed (Jianchang duck (J)), hybrid strains (BH1, BH2, and MC♂ × (BGF2♂ × GF2♀)♀ (MC)), and commercial breed (Cherry Verry duck). Results showed that a total of 17 AAs (TAAs) was detected from breast muscle among 5 duck breeds/strains including 11 essential AAs (EAAs). Among these AAs, the contents of Proline, Threonine, Glutamine, Serine, Methionine, Phenylalanine, Histidine, and Cysteine were significant difference among 5 duck breeds/strains. The contents of EAAs, TAAs, and flavor AAs were higher in breast muscle of J and BH2 than those in other duck breeds/strains, and the ratio of EAAs/TAAs was higher in breast muscle of BH2. Furthermore, the expression levels of eukaryotic translation initiation factor 4E-binding protein 1, mammalian target of rapamycin, and proton-coupled amino acid transporter 1 were the highest in breast muscle of BH2, and that of solute carrier family 38 member 2 was the highest in breast muscle of J. Meanwhile, principal component analysis results showed that principal component 1 of BH1, principal component 3 of BH2, and principal component 2 of MC were positively corelated with EAAs/TAAs, and principal component 1 was positively correlated with flavor AAs and EAAs. In conclusion, compared to BH1, MC, and Cherry Verry duck, AA content was higher in breast muscle of BH2 and J, which might be associated with the higher expression levels of mammalian target of rapamycin, eukaryotic translation initiation factor 4E-binding protein 1, and proton-coupled amino acid transporter 1 in breast muscle of BH2 and solute carrier family 38 member 2 in breast muscle of J. The comparative analysis of AA content in breast muscle among different duck breeds/strains could provide an important basis for improving the nutritional value of duck meat in the breeding process.

Clinical Consequences of Metabolic Acidosis-Muscle

Metabolic acidosis is common in people with chronic kidney disease and can contribute to functional decline, morbidity, and mortality. One avenue through which metabolic acidosis can result in these adverse clinical outcomes is by negatively impacting skeletal muscle; this can occur through several pathways. First, metabolic acidosis promotes protein degradation and impairs protein synthesis, which lead to muscle breakdown. Second, metabolic acidosis hinders mitochondrial function, which decreases oxidative phosphorylation and reduces energy production. Third, metabolic acidosis directly limits muscle contraction. The purpose of this review is to examine the specific mechanisms of each pathway through which metabolic acidosis affects muscle, the impact of metabolic acidosis on physical function, and the effect of treating metabolic acidosis on functional outcomes.

SLC38A10 Knockout Mice Display a Decreased Body Weight and an Increased Risk-Taking Behavior in the Open Field Test

The solute carrier 38 family (SLC38) is a family of 11 members. The most common substrate among these are alanine and glutamine, and members are present in a wide range of tissues with important functions for several biological processes, such as liver and brain function. Some of these transporters are better characterized than others and, in this paper, a behavioral characterization of SLC38A10^−/− mice was carried out. A battery of tests for general activity, emotionality, motor function, and spatial memory was used. Among these tests, the elevated plus maze, Y-maze, marble burying and challenging beam walk have not been tested on the SLC38A10^−/− mice previously, while the open field and the rotarod tests have been performed by the International Mouse Phenotyping Consortium (IMPC). Unlike the results from IMPC, the results from this study showed that SLC38A10^−/− mice spend less time in the wall zone in the open field test than WT mice, implying that SLC38A10-deficient mice have an increased explorative behavior, which suggests an important function of SLC38A10 in brain. The present study also confirmed IMPC's data regarding rotarod performance and weight, showing that SLC38A10^−/− mice do not have an affected motor coordination impairment and have a lower body weight than both SLC38A10^+/− and SLC38A10^+/+ mice. These results imply that a complete deficiency of the SLC38A10 protein might affect body weight homeostasis, but the underlying mechanisms needs to be studied further.

Acetoacetate protects macrophages from lactic acidosis-induced mitochondrial dysfunction by metabolic reprograming

Lactic acidosis, the extracellular accumulation of lactate and protons, is a consequence of increased glycolysis triggered by insufficient oxygen supply to tissues. Macrophages are able to differentiate from monocytes under such acidotic conditions, and remain active in order to resolve the underlying injury. Here we show that, in lactic acidosis, human monocytes differentiating into macrophages are characterized by depolarized mitochondria, transient reduction of mitochondrial mass due to mitophagy, and a significant decrease in nutrient absorption. These metabolic changes, resembling pseudostarvation, result from the low extracellular pH rather than from the lactosis component, and render these cells dependent on autophagy for survival. Meanwhile, acetoacetate, a natural metabolite produced by the liver, is utilized by monocytes/macrophages as an alternative fuel to mitigate lactic acidosis-induced pseudostarvation, as evidenced by retained mitochondrial integrity and function, retained nutrient uptake, and survival without the need of autophagy. Our results thus show that acetoacetate may increase tissue tolerance to sustained lactic acidosis.

Lactic acidosis is a metabolic state that occurs in injured tissues. Here the authors show that macrophages, in order to remain functional in acidosis, reduce their mitochondrial mass by mitophagy and rely on autophagy for survival, with mitochondrial integrity retained using acetoacetate as alternative fuel.

Low pH up‐regulates interleukin‐6 mRNA in L6‐G8C5 rat skeletal muscle cells independent of pH sensing by SNAT2(SLC38A2) transporters

Exercise is known to create a transient, but potent increase in skeletal muscle expression of potentially anti‐inflammatory myokine interleukin‐6 (IL‐6). This effect may be clinically important in managing chronic inflammatory states. It has previously been proposed that lactic acidosis following exercise promotes this IL‐6 up‐regulation, but the mechanism of this acidosis effect is unknown. Rat skeletal muscle cell line L6‐G8C5 has been used previously to model metabolic effects of acidosis, sensing low pH through the resulting inhibition of amino acid transporter SNAT2(SLC38A2). Use of ionophore ionomycin to model the rise in intracellular Ca²⁺ concentration occurring in contracting muscle strongly up‐regulates IL‐6 mRNA in L6‐G8C5 myotubes. This study used this model to test the hypothesis that low extracellular pH (7.1) enhances ionomycin‐induced IL‐6 mRNA up‐regulation by inhibiting SNAT2. Incubation of L6‐G8C5 myotubes for 6 h with 0.5 µM ionomycin at control pH (7.4) resulted in a 15‐fold increase in IL‐6 mRNA which was further enhanced (1.74‐fold) at pH 7.1. In contrast low pH had no significant effect on IL‐6 mRNA without ionomycin, nor on the IL‐6 mRNA increase that was induced by cyclic stretch. Even though pH 7.1 halved the transport activity of SNAT2, alternative methods of SNAT2 inhibition (JNK inhibitor SP600125; SNAT2 antagonist MeAIB; or SNAT2 silencing with siRNA) did not mimic the enhancing effect of low pH on IL‐6 mRNA. On the contrary, JNK inhibition blunted the effect of pH 7.1 with ionomycin, but had no effect at pH 7.4. It is concluded that low pH promotes Ca²⁺/ionomycin–induced up‐regulation of IL‐6 mRNA through a novel SNAT2‐independent JNK‐dependent pH‐sensing pathway not previously described in this skeletal muscle model.

Amino Acid Homeostasis in Mammalian Cells with a Focus on Amino Acid Transport

Amino acid homeostasis is maintained by import, export, oxidation, and synthesis of nonessential amino acids, and by the synthesis and breakdown of protein. These processes work in conjunction with regulatory elements that sense amino acids or their metabolites. During and after nutrient intake, amino acid homeostasis is dominated by autoregulatory processes such as transport and oxidation of excess amino acids. Amino acid deprivation triggers processes such as autophagy and the execution of broader transcriptional programs to maintain plasma amino acid concentrations. Amino acid transport plays a crucial role in the absorption of amino acids in the intestine, the distribution of amino acids across cells and organs, the recycling of amino acids in the kidney, and the recycling of amino acids after protein breakdown.

A GC-MS/Single-Cell Method to Evaluate Membrane Transporter Substrate Specificity and Signaling

Amino acid transporters play a vital role in metabolism and nutrient signaling pathways. Typically, transport activity is investigated using single substrates and competing amounts of other amino acids. We used GC-MS and LC-MS for metabolic screening of Xenopus laevis oocytes expressing various human amino acid transporters incubated in complex media to establish their comprehensive substrate profiles. For most transporters, amino acid selectivity matched reported substrate profiles. However, we could not detect substantial accumulation of cationic amino acids by SNAT4 and ATB0,+ in contrast to previous reports. In addition, comparative substrate profiles of two related sodium neutral amino acid transporters known as SNAT1 and SNAT2, revealed the latter as a significant leucine accumulator. As a consequence, SNAT2, but not SNAT1, was shown to be an effective activator of the eukaryotic cellular growth regulator mTORC1. We propose, that metabolomic profiling of membrane transporters in Xe nopus laevis oocytes can be used to test their substrate specificity and role in intracellular signaling pathways.

The role of the glutamine transporter ASCT2 in antineoplastic therapy

Cancer cells are metabolically reprogrammed to support their high rates of proliferation, continuous growth, survival, invasion, metastasis, and resistance to cancer treatments. Among changes in cancer cell bioenergetics, the role of glutamine metabolism has been receiving increasing attention. Increased glutaminolysis in cancer cells is associated with increased expression of membrane transporters that mediate the cellular uptake of glutamine. ASCT2 (Alanine, Serine, Cysteine Transporter 2) is a Na⁺-dependent transmembrane transporter overexpressed in cancer cells and considered to be the primary transporter for glutamine in these cells. The possibility of inhibiting ASCT2 for antineoplastic therapy is currently under investigation. In this article, we will present the pharmacological agents currently known to act on ASCT2, which have been attracting attention in antineoplastic therapy research. We will also address the impact of ASCT2 inhibition on the prognosis of some cancers. We conclude that ASCT2 inhibition and combination of ASCT2 inhibitors with other anti-tumor therapies may be a promising antineoplastic strategy. However, more research is needed in this area.

Dexamethasone acutely suppresses the anabolic SNAT2/SLC38A2 amino acid transporter protein in L6-G8C5 rat skeletal muscle cells

Chronic metabolic acidosis plays a role in cachexia by enhancing total proteolysis in skeletal muscle. Glucocorticoid also triggers proteolysis and plays a permissive role in the effect of acidosis. The System A amino acid transporter SNAT2/SLC38A2 is ubiquitously expressed in mammalian cells including muscle, performing Na⁺‐dependent active import of neutral amino acids, and is strongly inhibited by low pH. Exposure of rat skeletal muscle cell line L6‐G8C5 to low pH rapidly inhibits SNAT2 transport activity and enhances total proteolysis rate. Pharmacological inhibition or silencing of SNAT2 also enhances proteolysis. This study tests the hypothesis that the glucocorticoid dexamethasone (DEX), like low pH, inhibits SNAT2 activity in L6‐G8C5 myotubes, thus contributing to total proteolysis. Incubation with 500 nM DEX for 4 h reduced the System A amino acid transport rate to half the rate in control cultures. This inhibition depended on glucocorticoid receptor‐mediated gene transcription, but SNAT2 mRNA levels were unaffected by DEX. In contrast, the SNAT2 protein assessed by immunoblotting was significantly depleted. The co‐inhibitory effects of DEX and low pH on System A transport activity were additive in stimulating total proteolysis. In keeping with this mechanism, DEX’s inhibitory effect on SNAT2 transport activity was significantly blunted by the proteasome inhibitor MG132. Proof of principle was achieved in similar experiments using recombinant expression of a GFP‐tagged SNAT2 fusion protein in HEK293A cells. It is concluded that DEX acutely depletes the SNAT2 transporter protein, at least partly through proteasome‐dependent degradation of this functionally important transporter.

Defect of SLC38A3 promotes epithelial-mesenchymal transition and predicts poor prognosis in esophageal squamous cell carcinoma

Objective

Solute carrier family 38 (SLC38s) transporters play important roles in amino acid transportation and signaling transduction. However, their genetic alterations and biological roles in tumors are still largely unclear. This study aimed to elucidate the genetic signatures of SLC38s transporters and their implications in esophageal squamous cell carcinoma (ESCC).

Methods

Analyses on somatic mutation and copy number alterations (CNAs) of SLC38A3 were performed as described. Immunohistochemistry (IHC) assay and Western blot assay were used to detect the protein expression level. MTS assay, colony formation assay, transwell assay and wound healing assay were used to explore the malignant phenotypes of ESCC cells. Immunofluorescence assay was used to verify the colocalization of two indicated proteins and immunopreciptation assay was performed to confirm the interaction of proteins.

Results

Our findings revealed that SLC38s family was significantly disrupted in ESCC, with high frequent CNAs and few somatic mutations. SLC38A3 was the most frequent loss gene among them and was linked to poor survival and lymph node metastasis. The expression of SLC38A3 was lower in tumor tissues compared to that in normal tissues, which was also significantly associated with worse clinical outcome. Further experiments revealed that depletion of SLC38A3 could promote EMT in ESCC cell lines, and the interaction of SLC38A3 and SETDB1 might lead to the reduced transcription of Snail. Pharmacogenomic analyses demonstrated that fifteen inhibitors were showed significantly correlated with SLC38A3 expression.

Conclusions

Our investigations have provided insights that SLC38A3 could act as a suppressor in EMT pathway and serve as a prognostic factor and predictor of differential drug sensitivities in ESCC.

A selectively suppressing amino acid transporter: Sodium-coupled neutral amino acid transporter 2 inhibits cell growth and mammalian target of rapamycin complex 1 pathway in skeletal muscle cells

Sodium-coupled neutral amino acid transporter 2 (SNAT2), also known as solute carrier family 38 member 2 (SLC38A2), is expressed in the skeletal muscle. Our research previously indicated that SNAT2 mRNA expression level in the skeletal muscle was modulated by genotype and dietary protein. The aim of this study was to investigate the key role of the amino acid transporter SNAT2 in muscle cell growth, differentiation, and related signaling pathways via SNAT2 suppression using the inhibitor α-methylaminoisobutyric acid (MeAIB). The results showed that SNAT2 suppression down-regulated both the mRNA and protein expression levels of SNAT2 in C2C12 cells, inhibited cell viability and differentiation of the cell, and regulated the cell distribution in G0/G1 and S phases (P < 0.05). Meanwhile, most of the intercellular amino acid content of the cells after MeAIB co-culturing was significantly lower (P < 0.05). Furthermore, the mRNA expression levels of system L amino acid transporter 1 (LAT1), silent information regulator 1, and peroxisome proliferator-activated receptor-gamma co-activator 1 alpha, as well as the protein expression levels of amino acid transporters LAT1 and vacuolar protein sorting 34, were all down-regulated. The phosphorylated protein expression levels of mammalian target of rapamycin (mTOR), regulatory-associated protein of mTOR, 4E binding protein 1, and ribosomal protein S6 kinase 1 after MeAIB treatment were also significantly down-regulated (P < 0.05), which could contribute to the importance of SNAT2 in amino acid transportation and skeletal muscle cell sensing. In conclusion, SNAT2 suppression inhibited C2C12 cell growth and differentiation, as well as the availability of free amino acids. Although the mTOR complex 1 signaling pathway was found to be involved, its response to different nutrients requires further study.

Multifaceted regulation of the system A transporter Slc38a2 suggests nanoscale regulation of amino acid metabolism and cellular signaling

Amino acids are essential for cellular protein synthesis, growth, metabolism, signaling and in stress responses. Cell plasma membranes harbor specialized transporters accumulating amino acids to support a variety of cellular biochemical pathways. Several transporters for neutral amino acids have been characterized. However, Slc38a2 (also known as SA1, SAT2, ATA2, SNAT2) representing the classical transport system A activity stands in a unique position: Being a secondarily active transporter energized by the electrochemical gradient of Na⁺, it creates steep concentration gradients for amino acids such as glutamine: this may subsequently drive the accumulation of additional neutral amino acids through exchange via transport systems ASC and L. Slc38a2 is ubiquitously expressed, yet in a cell-specific manner. In this review, we show that Slc38a2 is regulated at the transcriptional and translational levels as well as by ions and proteins through direct interactions. We describe how Slc38a2 senses amino acid availability and passes this onto intracellular signaling pathways and how it regulates protein synthesis, cellular proliferation and apoptosis through the mechanistic (mammalian) target of rapamycin (mTOR) and general control nonderepressible 2 (GCN2) pathways. Furthermore, we review how this extensively regulated transporter contributes to cellular osmoadaptation and how it is regulated by endoplasmic reticulum stress and various hormonal stimuli to promote cellular metabolism, cellular signaling and cell survival. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Glutamine protects against LPS-induced inflammation via adjusted NODs signaling and enhanced immunoglobulins secretion in rainbow trout leukocytes

To evaluate effects of glutamine (GLN) on fish immune responses, leukocytes were isolated from head kidney of rainbow trout and cultured in GLN-free DMEM media supplemented with different combinations of lipopolysaccharide (LPS) and GLN. LPS significantly increased expression of pro-inflammatory cytokines, while GLN supplementation alleviated LPS-induced inflammation. Leukocytes in +GLN + LPS group showed more active GLN anabolism and catabolism, which signals could be sensed by O-GlcNAcylation, and then affected LPS binding to cell surface (LBP) and adjusted NODs signaling. The mRNA expression of immunoglobulins (Igs) and their receptor (pIgR) was also significantly increased after GLN supplementation. Further analysis showed that GLN increased the percentage of IgM⁺ B cells and IgT⁺ B cells, accompanied with the increased IgM and IgT secretion in culture media, which further increased complement C3 expression to perform effector functions. All these results illustrated the regulating mechanism of GLN against LPS-induced inflammation both via adjusted NODs signaling and increased Igs⁺ B cells to secrete Igs.

Physical decline and survival in the elderly are affected by the genetic variability of amino acid transporter genes

Amino acid (AA) availability is a rate-limiting factor in the regulation of muscle protein metabolism and, consequently, a risk factor for age-related decline in muscle performance. AA transporters are emerging as sensors of AA availability and activators of mTORC1 signalling, acting as transceptors. Here, we evaluated the association of 58 single nucleotide polymorphisms (SNPs) in 10 selected AA transporter genes with parameters of physical performance (Hand Grip, Activity of Daily Living, Walking time). By analysing a sample of 475 subjects aged 50-89 years, we found significant associations with SLC7A5/LAT1, SLC7A8/LAT2, SLC36A1/PAT1, SLC38A2/SNAT2, SLC3A2/CD98, SLC38A7/SNAT7 genes. Further investigation of the SNPs in a cross-sectional study including 290 subjects aged 90-107 years revealed associations of SLC3A2/CD98, SLC38A2/SNAT2, SLC38A3/SNAT3, SLC38A9/SNAT9 variability with longevity. Finally, a longitudinal study examining the survival rate over 10 years showed age-dependent complexity due to possible antagonistic pleiotropic effects for a SNP in SLC38A9/SNAT9, conferring a survival advantage before 90 years of age and a disadvantage later, probably due to the remodelling of AA metabolism. On the whole, our findings support the hypothesis that AA transporters may impact on the age-related physical decline and survival at old age in a complex way, likely through a mechanism involving mTORC1 signalling.

The application of stable-isotope tracers to study human musculoskeletal protein turnover: a tale of bag filling and bag enlargement

The nutritional regulation of protein and amino acid balance in human skeletal muscle carried out by the authors with Mike Rennie is reviewed in the context of a simple physiological model for the regulation of the maintenance and growth of skeletal muscle, the "Bag Theory". Beginning in London in the late 1970s the work has involved the use of stable isotopes to probe muscle protein synthesis and breakdown with two basic experimental models, primed-dose continuous tracer infusions combined with muscle biopsies and arterio-venous (A-V) studies across a limb, most often the leg, allowing both protein synthesis and breakdown as well as net balance to be measured. In this way, over a 30 year period, the way in which amino acids and insulin mediate the anabolic effect of a meal has been elaborated in great detail confirming the original concepts of bag filling within the muscle endomysial "bag", which is limited by the "bag" size unless bag enlargement occurs requiring new collagen synthesis. Finally we briefly review some new developments involving ² H₂ O labelling of muscle proteins.

Leucine alone or in combination with glutamic acid, but not with arginine, increases biceps femoris muscle and alters muscle AA transport and concentrations in fattening pigs

Forty-eight Duroc × Large White × Landrace pigs with an average initial body weight of 77.09 ± 1.37 kg were used to investigate the effects of combination of leucine (Leu) with arginine (Arg) or glutamic acid (Glu) on muscle growth, free amino acid profiles, expression levels of amino acid transporters and growth-related genes in skeletal muscle. The animals were randomly assigned to one of the four treatment groups (12 pigs/group, castrated male:female = 1:1). The pigs in the control group were fed a basal diet (13% Crude Protein), and those in the experimental groups were fed the basal diet supplemented with 1.00% Leu (L group), 1.00% Leu + 1.00% Arg (LA group) or 1.00% Leu + 1.00% Glu (LG group). The experiment lasted for 60 days. Results showed an increase (p < 0.05) in biceps femoris (BF) muscle weight in the L group and LG group relative to the basal diet group. In longissimus dorsi (LD) muscle, Lys, taurine and total essential amino acid concentration increased in the LG group relative to the basal diet group (p < 0.05). In LG group, Glu and carnosine concentrations increased (p < 0.05) in the BF muscle, when compared to the basal diet group. The Leu and Lys concentrations of BF muscle were lower in the LA group than that in the L group (p < 0.05). A positive association was found between BF muscle weight and Leu concentration (p < 0.05). The LG group presented higher (p < 0.05) mRNA levels of ASCT2, LAT1, PAT2, SANT2 and TAT1 in LD muscle than those in the basal diet group. The mRNA levels of PAT2 and MyoD in BF muscle were upregulated (p < 0.05) in the LG group, compared with those in the basal diet group. In conclusion, Leu alone or in combination with Glu is benefit for biceps femoris muscle growth in fattening pig.

Transcriptional programming of lipid and amino acid metabolism by the skeletal muscle circadian clock

Circadian clocks are fundamental physiological regulators of energy homeostasis, but direct transcriptional targets of the muscle clock machinery are unknown. To understand how the muscle clock directs rhythmic metabolism, we determined genome-wide binding of the master clock regulators brain and muscle ARNT-like protein 1 (BMAL1) and REV-ERBα in murine muscles. Integrating occupancy with 24-hr gene expression and metabolomics after muscle-specific loss of BMAL1 and REV-ERBα, here we unravel novel molecular mechanisms connecting muscle clock function to daily cycles of lipid and protein metabolism. Validating BMAL1 and REV-ERBα targets using luciferase assays and in vivo rescue, we demonstrate how a major role of the muscle clock is to promote diurnal cycles of neutral lipid storage while coordinately inhibiting lipid and protein catabolism prior to awakening. This occurs by BMAL1-dependent activation of Dgat2 and REV-ERBα-dependent repression of major targets involved in lipid metabolism and protein turnover (MuRF-1, Atrogin-1). Accordingly, muscle-specific loss of BMAL1 is associated with metabolic inefficiency, impaired muscle triglyceride biosynthesis, and accumulation of bioactive lipids and amino acids. Taken together, our data provide a comprehensive overview of how genomic binding of BMAL1 and REV-ERBα is related to temporal changes in gene expression and metabolite fluctuations.

Membrane topology of rat sodium-coupled neutral amino acid transporter 2 (SNAT2)

Sodium-coupled neutral amino acid transporter 2 (SNAT2) is a subtype of the amino acid transport system A that is widely expressed in mammalian tissues. It plays critical roles in glutamic acid-glutamine circulation, liver gluconeogenesis and other biological pathway. However, the topology of the SNAT2 amino acid transporter is unknown. Here we identified the topological structure of SNAT2 using bioinformatics analysis, Methoxy-polyethylene glycol maleimide (mPEG-Mal) chemical modification, protease cleavage assays, immunofluorescence and examination of glycosylation. Our results show that SNAT2 contains 11 transmembrane domains (TMDs) with an intracellular N terminus and an extracellular C terminus. Three N-glycosylation sites were verified at the largest extracellular loop. This model is consistent with the previous model of SNAT2 with the exception of a difference in number of glycosylation sites. This is the first time to confirm the SNAT2 membrane topology using experimental methods. Our study on SNAT2 topology provides valuable structural information of one of the solute carrier family 38 (SLC38) members.

The Regulatory Role of MeAIB in Protein Metabolism and the mTOR Signaling Pathway in Porcine Enterocytes

Amino acid transporters play an important role in cell growth and metabolism. MeAIB, a transporter-selective substrate, often represses the adaptive regulation of sodium-coupled neutral amino acid transporter 2 (SNAT2), which may act as a receptor and regulate cellular amino acid contents, therefore modulating cellular downstream signaling. The aim of this study was to investigate the effects of MeAIB to SNAT2 on cell proliferation, protein turnover, and the mammalian target of rapamycin (mTOR) signaling pathway in porcine enterocytes. Intestinal porcine epithelial cells (IPEC)-J2 cells were cultured in a high-glucose Dulbecco’s modified Eagle’s (DMEM-H) medium with 0 or 5 mmoL/L System A amino acid analogue (MeAIB) for 48 h. Cells were collected for analysis of proliferation, cell cycle, protein synthesis and degradation, intracellular free amino acids, and the expression of key genes involved in the mTOR signaling pathway. The results showed that SNAT2 inhibition by MeAIB depleted intracellular concentrations of not only SNAT2 amino acid substrates but also of indispensable amino acids (methionine and leucine), and suppressed cell proliferation and impaired protein synthesis. MeAIB inhibited mTOR phosphorylation, which might be involved in three translation regulators, EIF4EBP1, IGFBP3, and DDIT4 from PCR array analysis of the 84 genes related to the mTOR signaling pathway. These results suggest that SNAT2 inhibition treated with MeAIB plays an important role in regulating protein synthesis and mTOR signaling, and provide some information to further clarify its roles in the absorption of amino acids and signal transduction in the porcine small intestine.

Influence of nutrient ingestion on amino acid transporters and protein synthesis in human skeletal muscle after sprint exercise

Nutrient ingestion is known to increase the exercise-induced stimulation of muscle protein synthesis following resistance exercise. Less is known about the effect of nutrients on muscle protein synthesis following sprint exercise. At two occasions separated by 1 mo, 12 healthy subjects performed three 30-s sprints with 20-min rest between bouts. In randomized order, they consumed a drink with essential amino acids and maltodextrin (nutrient) or flavored water (placebo). Muscle biopsies were obtained 80 and 200 min after the last sprint, and blood samples were taken repeatedly during the experiment. Fractional synthetic rate (FSR) was measured by continuous infusion of l-[2H5]phenylalanine up to 200 min postexercise. The mRNA expression and protein expression of SNAT2 were both 1.4-fold higher ( P < 0.05) after nutrient intake compared with placebo at 200 min postexercise. Phosphorylated Akt, mammalian target of rapamycin (mTOR), and p70S6k were 1.7- to 3.6-fold higher ( P < 0.01) 80 min after the last sprint with nutrient ingestion as compared with placebo. In addition, FSR was higher ( P < 0.05) with nutrients when plasma phenylalanine (FSRplasma) was used as a precursor but not when intracellular phenylalanine (FSRmuscle) was used. Significant correlations were also found between FSRplasma on the one hand and plasma leucine and serum insulin on the other hand in the nutrient condition. The results show that nutrient ingestion induces the expression of the amino acid transporter SNAT2 stimulates Akt/mTOR signaling and most likely the rate of muscle protein synthesis following sprint exercise.

NEW & NOTEWORTHY There is limited knowledge regarding the effect of nutrients on muscle protein synthesis following sprint as compared with resistance exercise. The results demonstrate that nutrient ingestion during repeated 30-s bouts of sprint exercise induces expression of the amino acid transporter SNAT2 and stimulates Akt/mTOR signaling and most likely the rate of muscle protein synthesis. Future studies to explore the chronic effects of nutritional ingestion during sprint exercise sessions on muscle mass accretion are warranted.

Glutamine up-regulates pancreatic sodium-dependent neutral aminoacid transporter-2 and mitigates islets apoptosis in diabetic rats

BACKGROUND

Glutamine aminoacid regulates insulin exocytosis from pancreatic β-cells. Liraglutide is a glucagon-like peptide-1 (GLP-1) analogue that has fascinated function in inhibiting β-cell apoptosis and preserving pancreatic β-cell mass. The present study investigated the benefit of adding glutamine to a regimen of liraglutide in diabetic rats focusing on their role in increasing insulin production and upregulation of the expression of sodium-dependent neutral aminoacid transporter-2 (SNAT2).

METHODS

In the present study, diabetes mellitus was induced in rats using streptozotocin (STZ, 50mg/kg, ip). Male rats were allocated into 5 groups, (i) vehicle group, (ii) STZ-diabetic rats, (iii) STZ-diabetic rats treated with liraglutide (150μg/kg, sc), (iv) STZ-diabetic rats treated with glutamine (po) and (v) STZ-diabetic rats treated with a combination of liraglutide and glutamine for four weeks. After finishing the therapeutic courses, the fasting blood glucose value was determined and rats were sacrificed. Pancreases were used for quantification of mRNA expression for SNAT2. Paraffin fixed samples were used for histologic staining and immunohistochemistry for insulin and apoptosis markers (activated caspase-3, BCL2 and BAX).

RESULTS

Treatment with liraglutide and/or glutamine enhanced insulin production and hence glycemic control in diabetic male rats with favorable effects on apoptosis markers. Treatment with glutamine and its combination with liraglutide significantly increased pancreatic expression of SNAT2 by approximately 30-35 folds.

CONCLUSION

Addition of glutamine to liraglutide regimen enhances the glycemic control and may have utility in clinical settings.

Fasting Enhances the Contrast of Bone Metastatic Lesions in 18F-Fluciclovine-PET: Preclinical Study Using a Rat Model of Mixed Osteolytic/Osteoblastic Bone Metastases

¹⁸F-fluciclovine (trans-1-amino-3-¹⁸F-fluorocyclobutanecarboxylic acid) is an amino acid positron emission tomography (PET) tracer used for cancer staging (e.g., prostate and breast). Patients scheduled to undergo amino acid-PET are usually required to fast before PET tracer administration. However, there have been no reports addressing whether fasting improves fluciclovine-PET imaging. In this study, the authors investigated the influence of fasting on fluciclovine-PET using triple-tracer autoradiography with ¹⁴C-fluciclovine, [5,6-³H]-2-fluoro-2-deoxy-d-glucose (³H-FDG), and ^99mTc-hydroxymethylene diphosphonate (^99mTc-HMDP) in a rat breast cancer model of mixed osteolytic/osteoblastic bone metastases in which the animals fasted overnight. Lesion accumulation of each tracer was evaluated using the target-to-background (muscle) ratio. The mean ratios of ¹⁴C-fluciclovine in osteolytic lesions were 4.6 ± 0.8 and 2.8 ± 0.6, respectively, with and without fasting, while those for ³H-FDG were 6.9 ± 2.5 and 5.1 ± 2.0, respectively. In the peri-tumor bone formation regions (osteoblastic), where ^99mTc-HMDP accumulated, the ratios of ¹⁴C-fluciclovine were 4.3 ± 1.4 and 2.4 ± 0.7, respectively, and those of ³H-FDG were 6.2 ± 3.8 and 3.3 ± 2.2, respectively, with and without fasting. These results suggest that fasting before ¹⁸F-fluciclovine-PET improves the contrast between osteolytic and osteoblastic bone metastatic lesions and background, as well as ¹⁸F-FDG-PET.

Dietary proteins and amino acids in the control of the muscle mass during immobilization and aging: role of the MPS response

Dietary proteins/essential amino acids (EAAs) are nutrients with anabolic properties that may increase muscle mass or attenuate muscle loss during immobilization and aging via the stimulation of muscle protein synthesis (MPS). An EAA's anabolic threshold, capable to maximize the stimulation of MPS has been hypothesized, but during certain conditions associated with muscle loss, this anabolic threshold seems to increase which reduces the efficacy of dietary EAAs to stimulate MPS. Preliminary studies have demonstrated that acute ingestion of dietary proteins/EAA (with a sufficient amount of leucine) was capable to restore the postprandial MPS during bed rest, immobilization or aging; however, whether these improvements translate into chronic increases (or attenuates loss) of muscle mass is equivocal. For example, although free leucine supplementation acutely increases MPS and muscle mass in some chronic studies, other studies have reported no increases in muscle mass following chronic leucine supplementation. In contrast, chronically increasing leucine intake via the consumption of an overall increase in dietary protein appears to be the most effective dietary intervention toward increasing or attenuating lean mass during aging; however, more research investigating the optimal dose and timing of protein ingestion is necessary. Several studies have demonstrated that decreases in postprandial MPS as a result of increased circulating oxidative and inflammatory are more responsible than muscle protein breakdown for the decreases in muscle mass during disuse and health aging. Therefore, nutritional interventions that reduce oxidation or inflammation in conjunction with higher protein intakes that overcome the anabolic resistance may enhance the MPS response to feeding and either increase muscle mass or attenuate loss. In preliminary studies, antioxidant vitamins and amino acids with antioxidant or anti-inflammatory properties show potential to restore the anabolic response associated with protein ingestion. More research, however, is required to investigate if these nutrients translate to increases in MPS and, ultimately, increased lean mass in aging humans. The purpose of the present review is to discuss the role of protein/EAA intake to enhance postprandial MPS during conditions associated with muscle loss, and bring new perspectives and challenges associated nutritional interventions aimed to optimize the anabolic effects of dietary protein/EAAs ingestion.

Recent Advances in Understanding Amino Acid Sensing Mechanisms that Regulate mTORC1

The mammalian target of rapamycin (mTOR) is the central regulator of mammalian cell growth, and is essential for the formation of two structurally and functionally distinct complexes: mTORC1 and mTORC2. mTORC1 can sense multiple cues such as nutrients, energy status, growth factors and hormones to control cell growth and proliferation, angiogenesis, autophagy, and metabolism. As one of the key environmental stimuli, amino acids (AAs), especially leucine, glutamine and arginine, play a crucial role in mTORC1 activation, but where and how AAs are sensed and signal to mTORC1 are not fully understood. Classically, AAs activate mTORC1 by Rag GTPases which recruit mTORC1 to lysosomes, where AA signaling initiates. Plasma membrane transceptor L amino acid transporter 1 (LAT1)-4F2hc has dual transporter-receptor function that can sense extracellular AA availability upstream of mTORC1. The lysosomal AA sensors (PAT1 and SLC38A9) and cytoplasmic AA sensors (LRS, Sestrin2 and CASTOR1) also participate in regulating mTORC1 activation. Importantly, AAs can be sensed by plasma membrane receptors, like G protein-coupled receptor (GPCR) T1R1/T1R3, and regulate mTORC1 without being transported into the cells. Furthermore, AA-dependent mTORC1 activation also initiates within Golgi, which is regulated by Golgi-localized AA transporter PAT4. This review provides an overview of the research progress of the AA sensing mechanisms that regulate mTORC1 activity.

Identification of a Disulfide Bridge in Sodium-Coupled Neutral Amino Acid Transporter 2(SNAT2) by Chemical Modification

Sodium-coupled neutral amino acid transporter 2 (SNAT2) belongs to solute carrier 38 (SLC38) family of transporters, which is ubiquitously expressed in mammalian tissues and mediates transport of small, neutral amino acids, exemplified by alanine(Ala, A). Yet structural data on SNAT2, including the relevance of intrinsic cysteine residues on structure and function, is scarce, in spite of its essential roles in many tissues. To better define the potential of intrinsic cysteines to form disulfide bonds in SNAT2, mutagenesis experiments and thiol-specific chemical modifications by N-ethylmaleimide (NEM) and methoxy-polyethylene glycol maleimide (mPEG-Mal, MW 5000) were performed, with or without the reducing regent dithiothreitol (DTT) treatment. Seven single mutant transporters with various cysteine (Cys, C) to alanine (Ala, A) substitutions, and a C245,279A double mutant were introduced to SNAT2 with a hemagglutinin (HA) tag at the C-terminus. The results showed that the cells expressing C245A or C279A were labeled by one equivalent of mPEG-Mal in the presence of DTT, while wild-type or all the other single Cys to Ala mutants were modified by two equivalents of mPEG-Mal. Furthermore, the molecular weight of C245,279A was not changed in the presence or absence of DTT treatment. The results suggest a disulfide bond between Cys245 and Cys279 in SNAT2 which has no effect on cell surface trafficking, as well as transporter function. The proposed disulfide bond may be important to delineate proximity in the extracellular domain of SNAT2 and related proteins.

Enteral β-hydroxy-β-methylbutyrate supplementation increases protein synthesis in skeletal muscle of neonatal pigs

Many low-birth weight infants are at risk for poor growth due to an inability to achieve adequate protein intake. Administration of the amino acid leucine stimulates protein synthesis in skeletal muscle of neonates. To determine the effects of enteral supplementation of the leucine metabolite β-hydroxy-β-methylbutyrate (HMB) on protein synthesis and the regulation of translation initiation and degradation pathways, overnight-fasted neonatal pigs were studied immediately (F) or fed one of five diets for 24 h: low-protein (LP), high-protein (HP), or LP diet supplemented with 4 (HMB4), 40 (HMB40), or 80 (HMB80) μmol HMB·kg body wt(-1)·day(-1) Cell replication was assessed from nuclear incorporation of BrdU in the longissimus dorsi (LD) muscle and jejunum crypt cells. Protein synthesis rates in LD, gastrocnemius, rhomboideus, and diaphragm muscles, lung, and brain were greater in HMB80 and HP and in brain were greater in HMB40 compared with LP and F groups. Formation of the eIF4E·eIF4G complex and S6K1 and 4E-BP1 phosphorylation in LD, gastrocnemius, and rhomboideus muscles were greater in HMB80 and HP than in LP and F groups. Phosphorylation of eIF2α and eEF2 and expression of SNAT2, LAT1, MuRF1, atrogin-1, and LC3-II were unchanged. Numbers of BrdU-positive myonuclei in the LD were greater in HMB80 and HP than in the LP and F groups; there were no differences in jejunum. The results suggest that enteral supplementation with HMB increases skeletal muscle protein anabolism in neonates by stimulation of protein synthesis and satellite cell proliferation.

System-L amino acid transporters play a key role in pancreatic β-cell signalling and function

The branched-chain amino acids (BCAA) leucine, isoleucine and valine, are essential amino acids that play a critical role in cellular signalling and metabolism. They acutely stimulate insulin secretion and activate the regulatory serine/threonine kinase mammalian target of rapamycin complex 1 (mTORC1), a kinase that promotes increased β-cell mass and function. The effects of BCAA on cellular function are dependent on their active transport into the mammalian cells via amino acid transporters and thus the expression and activity of these transporters likely influence β-cell signalling and function. In this report, we show that the System-L transporters are required for BCAA uptake into clonal β-cell lines and pancreatic islets, and that these are essential for signalling to mTORC1. Further investigation revealed that the System-L amino acid transporter 1 (LAT1) is abundantly expressed in the islets, and that knockdown of LAT1 using siRNA inhibits mTORC1 signalling, leucine-stimulated insulin secretion and islet cell proliferation. In summary, we show that the LAT1 is required for regulating β-cell signalling and function in islets and thus may be a novel pharmacological/nutritional target for the treatment and prevention of type 2 diabetes.

Supplementation of branched-chain amino acids in protein-restricted diets modulates the expression levels of amino acid transporters and energy metabolism associated regulators in the adipose tissue of growing pigs

This experiment was conducted to investigate the effects of branched-chain amino acids (BCAA) supplemented in protein-restricted diets on the growth performance and the expression profile of amino acid transporters and energy metabolism related regulators in the white adipose tissue (WAT) of different regional depots including dorsal subcutaneous adipose (DSA) and abdominal subcutaneous adipose (ASA). A total of 24 crossbred barrows (7.40 ± 0.70 kg) were randomly divided into 4 groups and were fed the following isocaloric diets for 33 days: 1) a recommended adequate protein diet (AP, 20% CP, as a positive control); 2) a low protein diet (LP, 17% CP); 3) the LP diet supplemented with BCAA (LP + B, 17% CP) to reach the same level of the AP diet group; 4) the LP diet supplemented with 2 times the amount of BCAA (LP + 2B, 17% CP). The daily gain and daily feed intake of the LP diet group were the lowest among all the treatments (P < 0.01). The feed conversion was improved markedly in the group of LP + B compared with the LP diet group (P < 0.05). No significant difference was noted for the serum biochemical parameter concentrations of glucose, triglyceride, nonesterified fatty acid and insulin among the groups (P > 0.05). Moreover, BCAA supplementation down-regulated the expression levels of amino acid transporters including L-type amino acid transporter 1 and sodium-coupled neutral amino acid transporter 2 in DSA, but up-regulated the expression level of L-type amino acid transporter 4 in ASA (P < 0.05). Meanwhile, the energy sensor AMP-activated protein kinase α was activated in the DSA of pigs fed LP diet and in the ASA of the pigs fed AP or LP + 2B diets (P < 0.05). The mRNA expression profile of the selected mitochondrial component and mitochondrial biogenesis associated regulators in DSA and ASA also responded differently to dietary BCAA supplementation. These results suggested that the growth performance of growing pigs fed protein restricted diets supplemented with BCAA could catch up to that of the pigs fed AP diets. The results also partly demonstrated that the regulation mechanisms of BCAA are different in the adipose tissues of different depots.

Alternative fuels for cancer cells

Tumor metabolism is significantly altered to support the various metabolic needs of tumor cells. The most prominent change is the increased tumor glycolysis that leads to increased glucose uptake and utilization. However, it has become obvious that many non-glucose nutrients, such as amino acids, lactate, acetate, and macromolecules, can serve as alternative fuels for cancer cells. This knowledge reveals an unexpected flexibility and evolutionarily conserved model in which cancer cells uptake nutrients from their external environment to fulfill their necessary energetic needs. Tumor cells may have evolved the ability to utilize different carbon sources because of the limited supply of nutrients in their microenvironment, which can be driven by oncogenic mutations or tumor microenvironmental stresses. In certain cases, these factors permanently alter the tumor cells' metabolism, causing certain nutrients to become indispensable and thus creating opportunities for therapeutic intervention to eradicate tumors by their metabolic vulnerabilities.

Loss of function mutation of the Slc38a3 glutamine transporter reveals its critical role for amino acid metabolism in the liver, brain, and kidney

Glutamine, the most abundant amino acid in mammals, is critical for cell and organ functions. Its metabolism depends on the ability of cells to take up or release glutamine by transporters located in the plasma membrane. Several solute carrier (SLC) families transport glutamine, but the SLC38 family has been thought to be mostly responsible for glutamine transport. We demonstrate that despite the large number of glutamine transporters, the loss of Snat3/Slc38a3 glutamine transporter has a major impact on the function of organs expressing it. Snat3 mutant mice were generated by N-ethyl-N-nitrosurea (ENU) mutagenesis and showed stunted growth, altered amino acid levels, hypoglycemia, and died around 20 days after birth. Hepatic concentrations of glutamine, glutamate, leucine, phenylalanine, and tryptophan were highly reduced paralleled by downregulation of the mTOR pathway possibly linking reduced amino acid availability to impaired growth and glucose homeostasis. Snat3-deficient mice had altered urea levels paralleled by dysregulation of the urea cycle, gluconeogenesis, and glutamine synthesis. Mice were ataxic with higher glutamine but reduced glutamate and gamma-aminobutyric acid (GABA) levels in brain consistent with a major role of Snat3 in the glutamine-glutamate cycle. Renal ammonium excretion was lower, and the expression of enzymes and amino acid transporters involved in ammoniagenesis were altered. Thus, SNAT3 is a glutamine transporter required for amino acid homeostasis and determines critical functions in various organs. Despite the large number of glutamine transporters, loss of Snat3 cannot be compensated, suggesting that this transporter is a major route of glutamine transport in the liver, brain, and kidney.

Characterization and Regulation of the Amino Acid Transporter SNAT2 in the Small Intestine of Piglets

The sodium-dependent neutral amino acid transporter 2 (SNAT2), which has dual transport/receptor functions, is well documented in eukaryotes and some mammalian systems, but has not yet been verified in piglets. The objective of this study was to investigate the characteristics and regulation of SNAT2 in the small intestine of piglets. The 1,521-bp porcine full cDNA sequence of SNAT2 (KC769999) from the small intestine of piglets was cloned. The open reading frame of cDNA encodes 506 deduced amino acid residues with a calculated molecular mass of 56.08 kDa and an isoelectric point (pI) of 7.16. Sequence alignment and phylogenetic analysis revealed that SNAT2 is highly evolutionarily conserved in mammals. SNAT2 mRNA can be detected in the duodenum, jejunum and ileum by real-time quantitative PCR. During the suckling period from days 1 to 21, the duodenum had the highest abundance of SNAT2 mRNA among the three segments of the small intestine. There was a significant decrease in the expression of SNAT2 mRNA in the duodenal and jejunal mucosa and in the expression of SNAT2 protein in the jejunal and ileal mucosa on day 1 after weaning (P < 0.05). Studies with enterocytes in vitro showed that amino acid starvation and supplementation with glutamate, arginine or leucine enhanced, while supplementation with glutamine reduced, SNAT2 mRNA expression (P < 0.05). These results regarding the characteristics and regulation of SNAT2 should help to provide some information to further clarify its roles in the absorption of amino acids and signal transduction in the porcine small intestine.

Size control: the developmental physiology of body and organ size regulation

The developmental regulation of final body and organ size is fundamental to generating a functional and correctly proportioned adult. Research over the last two decades has identified a long list of genes and signaling pathways that, when perturbed, influence final body size. However, body and organ size are ultimately a characteristic of the whole organism, and how these myriad genes and pathways function within a physiological context to control size remains largely unknown. In this review, we first describe the major size-regulatory signaling pathways: the Insulin/IGF-, RAS/RAF/MAPK-, TOR-, Hippo-, and JNK-signaling pathways. We then explore what is known of how these pathways regulate five major aspects of size regulation: growth rate, growth duration, target size, negative growth and growth coordination. While this review is by no means exhaustive, our goal is to provide a conceptual framework for integrating the mechanisms of size control at a molecular-genetic level with the mechanisms of size control at a physiological level.

Acute acidosis attenuates leucine stimulated signal transduction and protein synthesis in rat skeletal muscle

BACKGROUND

Critical illnesses are often complicated by acute metabolic acidosis, which if persistent, adversely affects outcome. Among the harmful effects that it might cause are impaired utilization of nutrients, increased proteolysis and depressed protein synthesis, leading to muscle wasting. As the amino acid leucine stimulates protein synthesis by activating mTOR signaling, we explored whether in acidosis, impaired leucine-stimulated signaling might be a contributor to the depressed protein synthesis.

METHODS

Male pair-fed rats were gavaged with NH4Cl (acidosis) or NaCl (control) for 2 days and then gavaged once with leucine and sacrificed 45 min later. Extensor digitorum longus muscles were isolated, incubated with or without leucine and protein synthesis measured. The anterior tibial muscle signaling was analysed by Western immunobloting.

RESULTS

Despite pair-feeding, acidotic rats lost body and muscle weight vs. controls. Moreover, leucine-induced protein synthesis in isolated muscle from acidotic rats was impaired. In-vivo, 45 min after an oral leucine load, anterior tibial muscle mTOR and 4E-BP1 phosphorylation increased significantly and comparably in control and acidotic rats. In contrast, leucine-stimulated phosphorylation of S6K1, a regulator of translation initiation and protein synthesis, was attenuated to approximately 56% of the control value (p < 0.05).

CONCLUSION

This study reveals that an acute metabolic acidosis impairs leucine-stimulated protein synthesis and activation of signaling downstream of mTOR at the level of S6K1. We propose that this S6K1 abnormality may account in part, for the resistance to leucine-stimulated muscle protein synthesis, and may thereby contribute to the impaired nutrient utilization and ultimately the muscle wasting that develops in acidosis.

Toxoplasma gondii is dependent on glutamine and alters migratory profile of infected host bone marrow derived immune cells through SNAT2 and CXCR4 pathways

The obligate intracellular parasite, Toxoplasma gondii, disseminates through its host inside infected immune cells. We hypothesize that parasite nutrient requirements lead to manipulation of migratory properties of the immune cell. We demonstrate that 1) T. gondii relies on glutamine for optimal infection, replication and viability, and 2) T. gondii-infected bone marrow-derived dendritic cells (DCs) display both “hypermotility” and “enhanced migration” to an elevated glutamine gradient in vitro. We show that glutamine uptake by the sodium-dependent neutral amino acid transporter 2 (SNAT2) is required for this enhanced migration. SNAT2 transport of glutamine is also a significant factor in the induction of migration by the small cytokine stromal cell-derived factor-1 (SDF-1) in uninfected DCs. Blocking both SNAT2 and C-X-C chemokine receptor 4 (CXCR4; the unique receptor for SDF-1) blocks hypermotility and the enhanced migration in T. gondii-infected DCs. Changes in host cell protein expression following T. gondii infection may explain the altered migratory phenotype; we observed an increase of CD80 and unchanged protein level of CXCR4 in both T. gondii-infected and lipopolysaccharide (LPS)-stimulated DCs. However, unlike activated DCs, SNAT2 expression in the cytosol of infected cells was also unchanged. Thus, our results suggest an important role of glutamine transport via SNAT2 in immune cell migration and a possible interaction between SNAT2 and CXCR4, by which T. gondii manipulates host cell motility.

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.

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.

Protein synthesis in skeletal muscle of neonatal pigs is enhanced by administration of β-hydroxy-β-methylbutyrate

Many low-birth-weight infants experience failure to thrive. The amino acid leucine stimulates protein synthesis in skeletal muscle of the neonate, but less is known about the effects of the leucine metabolite β-hydroxy-β-methylbutyrate (HMB). To determine the effects of HMB on protein synthesis and the regulation of translation initiation and degradation pathways, overnight-fasted neonatal pigs were infused with HMB at 0, 20, 100, or 400 μmol·kg body wt(-1)·h(-1) for 1 h (HMB 0, HMB 20, HMB 100, or HMB 400). Plasma HMB concentrations increased with infusion and were 10, 98, 316, and 1,400 nmol/ml in the HMB 0, HMB 20, HMB 100, and HMB 400 pigs. Protein synthesis rates in the longissimus dorsi (LD), gastrocnemius, soleus, and diaphragm muscles, lung, and spleen were greater in HMB 20 than in HMB 0, and in the LD were greater in HMB 100 than in HMB 0. HMB 400 had no effect on protein synthesis. Eukaryotic initiation factor (eIF)4E·eIF4G complex formation and ribosomal protein S6 kinase-1 and 4E-binding protein-1 phosphorylation increased in LD, gastrocnemius, and soleus muscles with HMB 20 and HMB 100 and in diaphragm with HMB 20. Phosphorylation of eIF2α and elongation factor 2 and expression of system A transporter (SNAT2), system L transporter (LAT1), muscle RING finger 1 protein (MuRF1), muscle atrophy F-box (atrogin-1), and microtubule-associated protein light chain 3 (LC3-II) were unchanged. Results suggest that supplemental HMB enhances protein synthesis in skeletal muscle of neonates by stimulating translation initiation.

Purification and characterization of calcium-binding soybean protein hydrolysates by Ca2+/Fe3+ immobilized metal affinity chromatography (IMAC)

Soybean protein hydrolysates (SPHs) can bind calcium in order to form soluble peptide-calcium complexes. However, amino acid composition and structural characteristics of the calcium chelating SPHs are still unclear. This study separated SPHs with calcium and iron immobilized metal affinity chromatography (IMAC), and examined the effects of SPHs with different amino acid composition on calcium binding capacity. Three fractions (FFe-1, FFe-2 and FFe-3) isolated with IMAC-Fe(3+) were shown possessing increased Glu, Gln, Lys and Pro content from FFe-1 to FFe-3, and improved amount of bound calcium. Furthermore, the fractions adsorbed on IMAC-Ca(2+) (Fe(3+)) were separated and identified with reverse-phase (RP)-HPLC and MALDI-TOF MS/MS. The results showed that the sequence of peptides from FCa-2 and FFe-3 fractions was DEGEQPRPFPFP.

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.

Inhibiting glutamine uptake represents an attractive new strategy for treating acute myeloid leukemia

Cancer cells require nutrients and energy to adapt to increased biosynthetic activity, and protein synthesis inhibition downstream of mammalian target of rapamycin complex 1 (mTORC1) has shown promise as a possible therapy for acute myeloid leukemia (AML). Glutamine contributes to leucine import into cells, which controls the amino acid/Rag/mTORC1 signaling pathway. We show in our current study that glutamine removal inhibits mTORC1 and induces apoptosis in AML cells. The knockdown of the SLC1A5 high-affinity transporter for glutamine induces apoptosis and inhibits tumor formation in a mouse AML xenotransplantation model. l-asparaginase (l-ase) is an anticancer agent also harboring glutaminase activity. We show that l-ases from both Escherichia coli and Erwinia chrysanthemi profoundly inhibit mTORC1 and protein synthesis and that this inhibition correlates with their glutaminase activity levels and produces a strong apoptotic response in primary AML cells. We further show that l-ases upregulate glutamine synthase (GS) expression in leukemic cells and that a GS knockdown enhances l-ase-induced apoptosis in some AML cells. Finally, we observe a strong autophagic process upon l-ase treatment. These results suggest that l-ase anticancer activity and glutamine uptake inhibition are promising new therapeutic strategies for AML.

Acute uremia suppresses leucine-induced signal transduction in skeletal muscle

Adequate nutrient intake in acute uremia is a key part of patient management especially as food utilization is usually impaired. Leucine is important as it comprises about one-fifth of essential amino acid needs and, apart from serving as a substrate, it directly activates the mTOR signaling pathway stimulating protein synthesis and inhibiting autophagy. Here we tested whether leucine activation of the mTOR signaling pathway in muscle is severely disrupted in acute uremia. Several abnormalities were identified in bilateral ureteral ligated (model of acute uremia) compared to sham-operated pair-fed control rats. Levels of several signaling proteins increased significantly while leucine-induced phosphorylation of mTOR and downstream proteins, 4e-BP1 and S6K1, was completely suppressed. Levels of LC3B-II, a specific autophagosomal membrane-associated protein used as a marker of autophagy, increased threefold in uremia. Furthermore, while leucine suppressed LC3B-II levels in controls, it failed to do so in uremic rats. Muscle IL-6 mRNA levels increased, while IGF-1 mRNA levels decreased in uremia. These findings establish that, in acute uremia, severe resistance to leucine-induced activation of the mTOR anabolic signaling pathway develops. Thus, leucine resistance, together with the reduction in IGF-1 and increase in IL-6 expression, may explain why the anabolic effect of nutritional therapy is diminished in acute uremic patients.

Combined walking exercise and alkali therapy in patients with CKD4-5 regulates intramuscular free amino acid pools and ubiquitin E3 ligase expression

Muscle-wasting in chronic kidney disease (CKD) arises from several factors including sedentary behaviour and metabolic acidosis. Exercise is potentially beneficial but might worsen acidosis through exercise-induced lactic acidosis. We studied the chronic effects of exercise in CKD stage 4-5 patients (brisk walking, 30 min, 5 times/week), and non-exercising controls; each group receiving standard oral bicarbonate (STD), or additional bicarbonate (XS) (Total n = 26; Exercising + STD n = 9; Exercising +XS n = 6; Control + STD n = 8; Control + XS n = 3). Blood and vastus lateralis biopsies were drawn at baseline and 6 months. The rise in blood lactate in submaximal treadmill tests was suppressed in the Exercising + XS group. After 6 months, intramuscular free amino acids (including the branched chain amino acids) in the Exercising + STD group showed a striking chronic depletion. This did not occur in the Exercising + XS group. The effect in Exercising + XS patients was accompanied by reduced transcription of ubiquitin E3-ligase MuRF1 which activates proteolysis via the ubiquitin-proteasome pathway. Other anabolic indicators (Akt activation and suppression of the 14 kDa actin catabolic marker) were unaffected in Exercising + XS patients. Possibly because of this, overall suppression of myofibrillar proteolysis (3-methylhistidine output) was not observed. It is suggested that alkali effects in exercisers arose by countering exercise-induced acidosis. Whether further anabolic effects are attainable on combining alkali with enhanced exercise (e.g. resistance exercise) merits further investigation.

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.

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.

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.