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Bejarano E, Rodríguez-Navarro JA. Autophagy and amino acid metabolism in the brain: implications for epilepsy. Amino Acids 2014; 47:2113-26. [DOI: 10.1007/s00726-014-1822-z] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2014] [Accepted: 07/31/2014] [Indexed: 12/31/2022]
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2
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Yang X, Mei S, Wang X, Li X, Liu R, Ma Y, Hao L, Yao P, Liu L, Sun X, Gu H, Liu Z, Cao W. Leucine facilitates insulin signaling through a Gαi protein-dependent signaling pathway in hepatocytes. J Biol Chem 2013; 288:9313-20. [PMID: 23404499 DOI: 10.1074/jbc.m112.409409] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
In this study, we addressed the direct effect of leucine on insulin signaling. In investigating the associated mechanisms, we found that leucine itself does not activate the classical Akt- or ERK1/2 MAP kinase-dependent signaling pathways but can facilitate the insulin-induced phosphorylations of Akt(473) and ERK1/2 in a time- and dose-dependent manner in cultured hepatocytes. The leucine-facilitated insulin-induced phosphorylation of Akt at residue 473 was not affected by knocking down the key component of mTORC1 or -2 complexes but was blocked by inhibition of c-Src (PP2), PI3K (LY294002), Gαi protein (pertussis toxin or siRNA against Gαi1 gene, or β-arrestin 2 (siRNA)). Similarly, the leucine-facilitated insulin activation of ERK1/2 was also blunted by pertussis toxin. We further show that leucine facilitated the insulin-mediated suppression of glucose production and expression of key gluconeogenic genes in a Gαi1 protein-dependent manner in cultured primary hepatocytes. Together, these results show that leucine can directly facilitate insulin signaling through a Gαi protein-dependent intracellular signaling pathway. This is the first evidence showing that macronutrients like amino acid leucine can facilitate insulin signaling through G proteins directly.
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Affiliation(s)
- Xuefeng Yang
- Department of Nutrition and Food Hygiene and MOE Key Laboratory of Environment and Health, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, Hubei, China.
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3
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Zanchi NE, Nicastro H, Lancha AH. Potential antiproteolytic effects of L-leucine: observations of in vitro and in vivo studies. Nutr Metab (Lond) 2008; 5:20. [PMID: 18637185 PMCID: PMC2488337 DOI: 10.1186/1743-7075-5-20] [Citation(s) in RCA: 51] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/20/2008] [Accepted: 07/17/2008] [Indexed: 11/25/2022] Open
Abstract
The purpose of present review is to describe the effect of leucine supplementation on skeletal muscle proteolysis suppression in both in vivo and in vitro studies. Most studies, using in vitro methodology, incubated skeletal muscles with leucine with different doses and the results suggests that there is a dose-dependent effect. The same responses can be observed in in vivo studies. Importantly, the leucine effects on skeletal muscle protein synthesis are not always connected to the inhibition of skeletal muscle proteolysis. As a matter of fact, high doses of leucine incubation can promote suppression of muscle proteolysis without additional effects on protein synthesis, and low leucine doses improve skeletal muscle protein ynthesis but have no effect on skeletal muscle proteolysis. These research findings may have an important clinical relevancy, because muscle loss in atrophic states would be reversed by specific leucine supplementation doses. Additionally, it has been clearly demonstrated that leucine administration suppresses skeletal muscle proteolysis in various catabolic states. Thus, if protein metabolism changes during different atrophic conditions, it is not surprising that the leucine dose-effect relationship must also change, according to atrophy or pathological state and catabolism magnitude. In conclusion, leucine has a potential role on attenuate skeletal muscle proteolysis. Future studies will help to sharpen the leucine efficacy on skeletal muscle protein degradation during several atrophic states.
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Affiliation(s)
- Nelo E Zanchi
- Laboratory of Applied Nutrition and Metabolism, Physical Education and School of Sports, University of São Paulo, São Paulo, Brazil.
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4
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Kadowaki M, Karim MR, Carpi A, Miotto G. Nutrient control of macroautophagy in mammalian cells. Mol Aspects Med 2006; 27:426-43. [PMID: 16999992 DOI: 10.1016/j.mam.2006.08.010] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
A growing number of evidences indicate a strict causality between the reduction of autophagic functionality and aging. In this context the preservation of a proper autophagic response is of paramount importance to preserve the cellular processes in aging cell. Nutrients availability, especially for amino acids, is the most physiological key regulator of macroautophagy. In mammalian cells the knowledge of the mechanism and the underlying regulation of macroautophagy has been greatly improved in recent years and we focus on the role of nutrients, in particular on their involvement in preventing cellular aging through the modulation of autophagy. This review covers the main features of macroautophagy regulation by nutrients, in particular amino acids as well as glucose and vitamins, and its mechanisms, focusing primarily on the mammalian hepatocyte, which has been extensively utilized to dissect signaling pathways underlying the regulation of macroautophagy.
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Affiliation(s)
- Motoni Kadowaki
- Department of Applied Biological Chemistry, Faculty of Agriculture, Niigata University, Ikarashi, Niigata 950-2181, Japan
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5
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Dann SG, Thomas G. The amino acid sensitive TOR pathway from yeast to mammals. FEBS Lett 2006; 580:2821-9. [PMID: 16684541 DOI: 10.1016/j.febslet.2006.04.068] [Citation(s) in RCA: 148] [Impact Index Per Article: 8.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2006] [Accepted: 04/24/2006] [Indexed: 10/24/2022]
Abstract
The target of rapamycin (TOR) is an ancient effector of cell growth that integrates signals from growth factors and nutrients. Two downstream effectors of mammalian TOR, the translational components S6K1 and 4EBP1, are commonly used as reporters of mTOR activity. The conical signaling cascade initiated by growth factors is mediated by PI3K, PKB, TSC1/2 and Rheb. However, the process through which nutrients, i.e., amino acids, activate mTOR remains largely unknown. Evidence exists for both an intracellular and/or a membrane bound sensor for amino acid mediated mTOR activation. Research in eukaryotic models, has implicated amino acid transporters as nutrient sensors. This review describes recent advances in nutrient signaling that impinge on mTOR and its targets including hVps34, class III PI3K, a transducer of nutrient availability to mTOR.
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Affiliation(s)
- Stephen G Dann
- University of Cincinnati Genome Research Institute, 2180 East Galbraith Road, Cincinnati, OH 45237, USA.
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6
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Harris RA, Joshi M, Jeoung NH. Mechanisms responsible for regulation of branched-chain amino acid catabolism. Biochem Biophys Res Commun 2004; 313:391-6. [PMID: 14684174 DOI: 10.1016/j.bbrc.2003.11.007] [Citation(s) in RCA: 121] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
Abstract
The branched-chain amino acids (BCAAs) are essential amino acids and therefore must be continuously available for protein synthesis. However, BCAAs are toxic at high concentrations as evidenced by maple syrup urine disease (MSUD), which explains why animals have such an efficient oxidative mechanism for their disposal. Nevertheless, it is clear that leucine is special among the BCAAs. Leucine promotes global protein synthesis by signaling an increase in translation, promotes insulin release, and inhibits autophagic protein degradation. However, leucine's effects are self-limiting because leucine promotes its own disposal by an oxidative pathway, thereby terminating its positive effects on body protein accretion. A strong case can therefore be made that the proper leucine concentration in the various compartments of the body is critically important for maintaining body protein levels beyond simply the need of this essential amino acid for protein synthesis. The goal of the work of this laboratory is to establish the importance of regulation of the branched chain alpha-ketoacid dehydrogenase complex (BCKDC) to growth and maintenance of body protein. We hypothesize that proper regulation of the activity state of BCKDC by way of its kinase (BDK) and its phosphatase (BDP) is critically important for body growth, tissue repair, and maintenance of body protein. We believe that growth and protection of body protein during illness and stress will be improved by therapeutic control of BCKDC activity. We also believe that it is possible that the negative effects of some drugs (PPAR alpha ligands) and dietary supplements (medium chain fatty acids) on growth and body protein maintenance can be countered by therapeutic control of BCDKC activity.
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Affiliation(s)
- Robert A Harris
- Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN 46202-5122, USA.
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7
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Kanazawa T, Taneike I, Akaishi R, Yoshizawa F, Furuya N, Fujimura S, Kadowaki M. Amino Acids and Insulin Control Autophagic Proteolysis through Different Signaling Pathways in Relation to mTOR in Isolated Rat Hepatocytes. J Biol Chem 2004; 279:8452-9. [PMID: 14610086 DOI: 10.1074/jbc.m306337200] [Citation(s) in RCA: 152] [Impact Index Per Article: 7.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Autophagy, a major bulk proteolytic pathway, contributes to intracellular protein turnover, together with protein synthesis. Both are subject to dynamic control by amino acids and insulin. The mechanisms of signaling and cross-talk of their physiological anabolic effects remain elusive. Recent studies established that amino acids and insulin induce p70 S6 kinase (p70(S6k)) phosphorylation by mTOR, involved in translational control of protein synthesis. Here, the signaling mechanisms of amino acids and insulin in macroautophagy in relation to mTOR were investigated. In isolated rat hepatocytes, both regulatory amino acids (RegAA) and insulin coordinately activated p70(S6k) phosphorylation, which was completely blocked by rapamycin, an mTOR inhibitor. However, rapamycin blocked proteolytic suppression by insulin, but did not block inhibition by RegAA. These contrasting results suggest that insulin controls autophagy through the mTOR pathway, but amino acids do not. Furthermore, micropermeabilization with Saccharomyces aureus alpha-toxin completely deprived hepatocytes of proteolytic responsiveness to RegAA and insulin, but still maintained p70(S6k) phosphorylation by RegAA. In contrast, Leu(8)-MAP, a non-transportable leucine analogue, did not mimic the effect of leucine on p70(S6k) phosphorylation, but maintained the activity on proteolysis. Finally, BCH, a System L-specific amino acid, did not affect proteolytic suppression or mTOR activation by leucine. All the results indicate that mTOR is not common to the signaling mechanisms of amino acids and insulin in autophagy, and that the amino acid signaling starts extracellularly with their "receptor(s)," probably other than transporters, and is mediated through a novel route distinct from the mTOR pathway employed by insulin.
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Affiliation(s)
- Takumi Kanazawa
- Department of Applied Biological Chemistry, Faculty of Agriculture, Niigata University, Ikarashi, Niigata 950-2181, Japan
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8
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Abstract
The mammalian-target-of-rapamycin (mTOR) is a multidomain protein that is important in regulating several components of the translational machinery. mTOR signalling is stimulated by hormones (e.g., insulin) and by amino acids. Our recent data suggest that TOR signalling responds to intracellular amino acids rather than to external amino acid levels. The translational repressor eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) is regulated through mTOR and undergoes phosphorylation at multiple sites, which affects its function. It contains two regulatory motifs: the C-terminal TOS motif interacts with the mTOR-binding partner, raptor, and mediates phosphorylation of specific sites in 4E-BP1. However, the N-terminal RAIP motif affects a larger range of mTOR-regulated sites. Since this motif does not bind raptor, mTOR must signal to 4E-BP1 via additional mechanisms that are independent of raptor. The kinase that phosphorylates and inhibits elongation factor 2 (eEF2 kinase) is inactivated by insulin via mTOR. Insulin decreases the ability of eEF2 kinase to bind calmodulin, its essential activator, and this effect requires mTOR signalling and a novel phosphorylation site in eEF2 kinase, Ser78. Ser78 is not phosphorylated by known components of the mTOR pathway implying the existence of novel mTOR-regulated kinases that control eEF2 kinase.
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Affiliation(s)
- Christopher G Proud
- Division of Molecular Physiology, Faculty of Life Sciences, University of Dundee, Dundee DD1 5EH, UK.
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9
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Proud CG. Role of mTOR Signalling in the Control of Translation Initiation and Elongation by Nutrients. Curr Top Microbiol Immunol 2004; 279:215-44. [PMID: 14560960 DOI: 10.1007/978-3-642-18930-2_13] [Citation(s) in RCA: 65] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
Protein synthesis requires nutrients both as precursors (amino acids) and as a source of energy, since this process consumes a high proportion of cellular metabolic energy. Recent work has shown that both types of nutrients directly influence the activities of components of the translational machinery in mammalian cells. Amino acids positively regulate signalling through the mammalian target of the rapamycin (mTOR) pathway, although the degree of dependency on external amino acids varies between cell types. mTOR signalling modulates several key components involved in mRNA translation, in particular (via repressor proteins) the cap-binding initiation factor eIF4E, the ribosomal protein S6 kinases, and elongation factor eEF2. The branched-chain amino acid leucine is the most effective one in most cell types. It is currently unclear how mammalian cells sense prevailing amino acid levels, although this may involve intracellular amino acids. Cellular ATP levels can also influence mTOR activity. The activities of some translation factors are modulated by mTOR-independent mechanisms. Examples include the regulation of eEF2 by cellular energy levels, which may be controlled via the AMP-activated protein kinase, and the activity of the guanine nucleotide-exchange factor eIF2B, which is modulated by amino acids and metabolic fuels.
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Affiliation(s)
- C G Proud
- Division of Molecular Physiology, School of Life Sciences, University of Dundee, MSI/WTB Complex, Dundee, DD1 5EH, UK.
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Bohé J, Low A, Wolfe RR, Rennie MJ. Human muscle protein synthesis is modulated by extracellular, not intramuscular amino acid availability: a dose-response study. J Physiol 2003; 552:315-24. [PMID: 12909668 PMCID: PMC2343318 DOI: 10.1113/jphysiol.2003.050674] [Citation(s) in RCA: 325] [Impact Index Per Article: 15.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/02/2023] Open
Abstract
To test the hypothesis that muscle protein synthesis (MPS) is regulated by the concentration of extracellular amino acids, we investigated the dose-response relationship between the rate of human MPS and the concentrations of blood and intramuscular amino acids. We increased blood mixed amino acid concentrations by up to 240 % above basal levels by infusion of mixed amino acids (Aminosyn 15, 44-261 mg kg-1 h-1) in 21 healthy subjects, (11 men 10 women, aged 29 +/- 2 years) and measured the rate of incorporation of D5-phenylalanine or D3-leucine into muscle protein and blood and intramuscular amino acid concentrations. The relationship between the fold increase in MPS and blood essential amino acid concentration ([EAA], mM) was hyperbolic and fitted the equation MPS = (2.68 x [EAA])/(1.51 + [EAA]) (P < 0.01). The pattern of stimulation of myofibrillar, sarcoplasmic and mitochondrial protein was similar. There was no clear relationship between the rate of MPS and the concentration of intramuscular EAAs; indeed, when MPS was increasing most rapidly, the concentration of intramuscular EAAs was below basal levels. We conclude that the rates of synthesis of all classes of muscle proteins are acutely regulated by the blood [EAA] over their normal diurnal range, but become saturated at high concentrations. We propose that the stimulation of protein synthesis depends on the sensing of the concentration of extracellular, rather than intramuscular EAAs.
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Affiliation(s)
- Julien Bohé
- Division of Metabolism, Department of Surgery, University of Texas Medical Branch, Shriners Burns Hospital, Galveston, TX 77550, USA
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11
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Beugnet A, Tee AR, Taylor PM, Proud CG. Regulation of targets of mTOR (mammalian target of rapamycin) signalling by intracellular amino acid availability. Biochem J 2003; 372:555-66. [PMID: 12611592 PMCID: PMC1223408 DOI: 10.1042/bj20021266] [Citation(s) in RCA: 236] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/12/2002] [Revised: 02/07/2003] [Accepted: 02/28/2003] [Indexed: 11/17/2022]
Abstract
In mammalian cells, amino acids affect the phosphorylation state and function of several proteins involved in mRNA translation that are regulated via the rapamycin-sensitive mTOR (mammalian target of rapamycin) pathway. These include ribosomal protein S6 kinase, S6K1, and eukaryotic initiation factor 4E-binding protein, 4E-BP1. Amino acids, especially branched-chain amino acids, such as leucine, promote phosphorylation of 4E-BP1 and S6K1, and permit insulin to further increase their phosphorylation. However, it is not clear whether these effects are exerted by extracellular or intracellular amino acids. Inhibition of protein synthesis is expected to increase the intracellular level of amino acids, whereas inhibiting proteolysis has the opposite effect. We show in the present study that inhibition of protein synthesis by any of several protein synthesis inhibitors tested allows insulin to regulate 4E-BP1 or S6K1 in amino-acid-deprived cells, as does the addition of amino acids to the medium. In particular, insulin activates S6K1 and promotes initiation factor complex assembly in amino-acid-deprived cells treated with protein synthesis inhibitors, but cannot do so in the absence of these compounds. Their effects occur at concentrations commensurate with their inhibition of protein synthesis and are not due to activation of stress-activated kinase cascades. Inhibition of protein breakdown (autophagy) impairs the ability of insulin to regulate 4E-BP1 or S6K1 under such conditions. These and other data presented in the current study are consistent with the idea that it is intracellular amino acid levels that regulate mTOR signalling.
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Affiliation(s)
- Anne Beugnet
- School of Life Sciences, MSI/WTB Complex, University of Dundee, Dow Street, Scotland, UK
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12
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Abstract
Proteolysis, as well as protein synthesis, is a major process that contributes to the body protein turnover. Despite the huge variety of proteases in the body, there are very few proteolytic systems contributing to the complete hydrolysis of proteins to amino acids. The autophagic-lysosomal pathway is responsible for bulk proteolysis, whereas the ubiquitin-proteasome pathway plays a significant role in the fine control of the degradation of specific proteins. Both systems can produce free amino acids as a final product, but only the autophagy system is physiologically controlled by plasma amino acids. Recently, the study of amino acids as regulators of macromolecular turnover has been focused on for their signal transduction mechanism. In autophagic proteolysis, several amino acids have a direct regulatory potential: Leu, Gln, Tyr, Phe, Pro, Met, Trp and His in the liver, and Leu in the skeletal muscle. These amino acids are recognized at the plasma membrane, indicating the possible existence of an amino acid receptor/sensor for their recognition and subsequent intracellular signaling. Another line of evidence has emerged that protein kinase cascades such as mTOR, Erk, eIF2alpha etc. may be involved in the regulation of autophagy, and that amino acids, in combination with insulin, may exert their effects through these pathways. From the viewpoint of amino acid safety, the contribution of proteolysis to possible adverse effects caused by excessive amino acid intake is not clear. At present, there is one report that excess glutamine at 10-fold the plasma level has an abnormal inhibitory effect on hepatic proteolysis, due to a lysosomotropic toxicity of ammonia derived from glutamine degradation. Whether this may lead to an adverse effect in humans remains to be clarified.
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Affiliation(s)
- Motoni Kadowaki
- Department of Applied Biological Chemistry, Faculty of Agriculture, Niigata University, Ikarashi, Niigata 950-2181, Japan.
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Kobayashi H, Børsheim E, Anthony TG, Traber DL, Badalamenti J, Kimball SR, Jefferson LS, Wolfe RR. Reduced amino acid availability inhibits muscle protein synthesis and decreases activity of initiation factor eIF2B. Am J Physiol Endocrinol Metab 2003; 284:E488-98. [PMID: 12556349 DOI: 10.1152/ajpendo.00094.2002] [Citation(s) in RCA: 71] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We have examined the effect of a hemodialysis-induced 40% reduction in plasma amino acid concentrations on rates of muscle protein synthesis and breakdown in normal swine. Muscle protein kinetics were measured by tracer methodology using [(2)H(5)]phenylalanine and [1-(13)C]leucine and analysis of femoral arterial and venous samples and tissue biopsies. Net amino acid release by muscle was accelerated during dialysis. Phenylalanine utilization for muscle protein synthesis was reduced from the basal value of 45 +/- 8 to 25 +/- 6 nmol x min(-1) x 100 ml leg(-1) between 30 and 60 min after start of dialysis and was stimulated when amino acids were replaced while dialysis continued. Muscle protein breakdown was unchanged. The signal for changes in synthesis appeared to be changes in plasma amino acid concentrations, as intramuscular concentrations remained constant throughout. The changes in muscle protein synthesis were accompanied by a reduction or stimulation, respectively, in the guanine nucleotide exchange activity of eukaryotic initiation factor (eIF)2B following hypoaminoacidemia vs. amino acid replacement. We conclude that a reduction in plasma amino acid concentrations below the normal basal value signals an inhibition of muscle protein synthesis and that corresponding changes in eIF2B activity suggest a possible role in mediating the response.
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Affiliation(s)
- Hisamine Kobayashi
- Department of Surgery, Shriners Burns Hospital, University of Texas Medical Branch, Galveston, Texas 77550, USA
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14
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Hidayat S, Yoshino KI, Tokunaga C, Hara K, Matsuo M, Yonezawa K. Inhibition of amino acid-mTOR signaling by a leucine derivative induces G1 arrest in Jurkat cells. Biochem Biophys Res Commun 2003; 301:417-23. [PMID: 12565877 DOI: 10.1016/s0006-291x(02)03052-8] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
We have previously demonstrated that N-acetylleucine amide, a derivative of L-leucine, inhibits leucine-induced p70(S6k) activation in a rat hepatoma cell line. In the present study, we investigated whether N-acetylleucine amide is capable of inhibiting amino acid-mTOR signaling. N-Acetylleucine amide caused cell cycle arrest at G1 stage in Jurkat cells, a human leukemia T cell line, concomitant with the inhibition of serum-induced p70(S6k) activation and p27 degradation. Treatment of Jurkat cells with this compound also exhibited dephosphorylation of retinoblastoma protein. These effects are similar to the inhibitory effects of rapamycin on amino acid-mTOR signaling pathway and suggest that N-acetylleucine amide acts as a rapamycin-like reagent to inhibit cell cycle progression in Jurkat cells.
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Affiliation(s)
- Sujuti Hidayat
- Biosignal Research Center, Kobe University, 1-1 Rokkodai-cho, Nada-ku, Kobe 657-8501, Japan
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15
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Abstract
Protein synthesis requires both amino acids, as precursors, and a substantial amount of metabolic energy. It is well established that starvation or lack of nutrients impairs protein synthesis in mammalian cells and tissues. Branched chain amino acids are particularly effective in promoting protein synthesis. Recent work has revealed important new information about the mechanisms involved in these effects. A number of components of the translational machinery are regulated through signalling events that require the mammalian target of rapamycin, mTOR. These include translational repressor proteins (eukaryotic initiation factor 4E-binding proteins, 4E-BPs) and protein kinases that act upon the small ribosomal subunit (S6 kinases). Amino acids, especially leucine, positively regulate mTOR signalling thereby relieving inhibition of translation by 4E-BPs and activating the S6 kinases, which can also regulate translation elongation. However, the molecular mechanisms by which amino acids modulate mTOR signalling remain unclear. Protein synthesis requires a high proportion of the cell's metabolic energy, and recent work has revealed that metabolic energy, or fuels such as glucose, also regulate targets of the mTOR pathway. Amino acids and glucose modulate a further important regulatory step in translation initiation, the activity of the guanine nucleotide-exchange factor eIF2B. eIF2B controls the recruitment of the initiator methionyl-tRNA to the ribosome and is activated by insulin. However, in the absence of glucose or amino acids, insulin no longer activates eIF2B. Since control of eIF2B is independent of mTOR, these data indicate the operation of additional, and so far unknown, regulatory mechanisms that control eIF2B activity.
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Affiliation(s)
- Christopher G Proud
- Division of Molecular Physiology, School of Life Sciences, University of Dundee, MSI/WTB Complex, Dow Street, UK.
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16
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Abstract
Amino acid availability is a potent regulator of muscle protein synthesis (MPS). We have performed a series of studies using stable isotope methodology and the arteriovenous balance approach to quantify many aspects of the response of MPS, breakdown, and the balance between synthesis and breakdown to changes in the availability of amino acids. A constant intake of amino acids stimulates MPS in a dose-dependent manner until concentrations are approximately doubled, after which further increases in concentration are ineffective. MPS rises more rapidly after bolus ingestion to a peak rate of MPS higher than during constant intake, but the response is transient. A reduction in amino acid availability below basal levels inhibits MPS. Ingestion of nonessential amino acids is not needed to stimulate MPS. When carbohydrate alone is ingested there is minimal effect on MPS, but there is an interactive effect with amino acid ingestion, meaning the response to amino acids plus glucose is more than the sum of their individual effects. Finally, acute anabolic responses in net MPS correspond quantitatively to differences in 24-h net muscle balances.
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Affiliation(s)
- Robert R Wolfe
- University of Texas Medical Branch and Shriners Burns Hospital, Department of Surgery, Galveston 77550, USA.
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17
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Lynch CJ, Patson BJ, Anthony J, Vaval A, Jefferson LS, Vary TC. Leucine is a direct-acting nutrient signal that regulates protein synthesis in adipose tissue. Am J Physiol Endocrinol Metab 2002; 283:E503-13. [PMID: 12169444 DOI: 10.1152/ajpendo.00084.2002] [Citation(s) in RCA: 110] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
In freshly isolated rat adipocytes, leucine or its analog norleucine activates the mammalian target of rapamycin (mTOR)-signaling pathway. This results in phosphorylation of the ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E-binding protein-1 (4E-BP1), two proteins involved in the initiation phase of protein synthesis. The purpose of the studies reported herein was to address the question of whether or not these in vitro effects of leucine and norleucine on adipocytes could be extended to the intact animal and to other tissues. To accomplish this, food-deprived (18 h) male Sprague-Dawley rats were orally administered solutions (2.5 ml/100 g body wt) containing normal saline (0.9% NaCl), a carbohydrate mixture (26.2% D-glucose and 26.2% sucrose), leucine (5.4%), or norleucine (5.4%). The protein synthetic responses of adipose tissue were measured and compared with those of other tissues. In addition, S6K1 and 4E-BP1 phosphorylation was measured, as was the plasma concentration of insulin and tissue ATP concentrations. Leucine administration stimulated protein synthesis in adipose tissue, gastrocnemius, and kidney but not in liver and heart. Norleucine stimulated protein synthesis in all of the tissues tested but, in contrast to leucine, without affecting plasma insulin concentrations. The carbohydrate meal had no effect on protein synthesis in any tissue tested but elicited a robust increase in plasma insulin. These findings provide support for a role of leucine as a direct-acting nutrient signal for stimulation of protein synthesis in adipose tissue as well as other select tissues. In adipose tissue, the effects of the different treatment conditions on the acute regulation of protein synthesis closely correlated with changes in phosphorylation of S6K1 and 4E-BP1; however, this correlation did not exist in all tissues examined. This result implies that leucine or norleucine may acutely stimulate protein synthesis, at least in some tissues, by a mechanism that is independent of both S6K1 and 4E-BP1 phosphorylation.
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Affiliation(s)
- Christopher J Lynch
- Department of Cellular and Molecular Physiology, The Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA.
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18
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Abstract
Peptide dendrimers are radial or wedge-like branched macromolecules consisting of a peptidyl branching core and/or covalently attached surface functional units. The multimeric nature of these constructs, the unambiguous composition and ease of production make this type of dendrimer well suited to various biotechnological and biochemical applications. Applications include use as biomedical diagnostic reagents, protein mimetics, anticancer and antiviral agents, vaccines and drug and gene delivery vehicles. This review focuses on the different types of peptide dendrimers currently in use and the synthetic methods commonly employed to generate peptide dendrimers ranging from stepwise solid-phase synthesis to chemoselective and orthogonal ligation.
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Affiliation(s)
- Kristen Sadler
- Department of Microbiology and Immunology, Vanderbilt University, Nashville, TN 37232, USA
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19
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Kimball SR. Regulation of translation initiation by amino acids in eukaryotic cells. PROGRESS IN MOLECULAR AND SUBCELLULAR BIOLOGY 2002; 26:155-84. [PMID: 11575165 DOI: 10.1007/978-3-642-56688-2_6] [Citation(s) in RCA: 77] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/19/2022]
Abstract
The translation of mRNA in eukaryotic cells is regulated by amino acids through multiple mechanisms. One such mechanism involves activation of mTOR (Fig. 1). mTOR controls a myriad of downstream effectors, including RNA polymerase I, S6K1, 4E-BP1, and eEF2 kinase. In yeast, and probably in higher eukaryotes, mTOR signals through Tap42p/alpha 4 to regulate protein phosphatases. Through phosphorylation of Tap42p/alpha 4, mTOR abrogates dephosphorylation of the downstream effectors by PP2 A and/or PP6, resulting in their increased phosphorylation. Although at this time still speculative, in vitro results using mTOR immunoprecipitates suggest that mTOR, or an associated kinase, may also be directly involved in phosphorylating some effectors. Enhanced RNA polymerase I activity results in increased transcription of rDNA genes, whereas increased S6K1 activity promotes preferential translation of TOP mRNAs, such as those encoding ribosomal proteins. Together, stimulated RNA polymerase I and S6K1 activities enhance ribosome biogenesis, increasing the translational capacity of the cell. Phosphorylation of 4E-BP1 prohibits its association with eIF4E, allowing eIF4E to bind to eIF4G and form the active eIF4F complex. Increased eIF4F formation preferentially stimulates translation of mRNAs containing long, highly-structured 5' UTRs. Finally, amino acids cause inhibition of the eEF2 kinase, resulting in an increase in the proportion of eEF2 in the active, dephosphorylated form. By inhibiting eEF2 phosphorylation, amino acids may not only stimulate translation elongation, but may also prevent activation of GCN2 by enhancing the rate of removal of deacylated tRNA from the P-site on the ribosome; a potential activator of GCN2. GCN2 may also be regulated directly by the accumulation of deacylated-tRNA caused by treatment with inhibitors of tRNA synthetases or in cells incubated in the absence of essential amino acids. However, because the Km of the tRNA synthetases for amino acids is well above the amino acid concentrations found in plasma of fasted animals, such a mechanism may not be operative in mammals in vivo. Activation of GCN2 results in increased phosphorylation of the alpha-subunit of eIF2, which in turn causes inhibition of eIF2B. Thus, by preventing activation of GCN2, amino acids preserve eIF2B activity, which promotes translation of all mRNAs, i.e., global protein synthesis is enhanced.
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MESH Headings
- Adaptor Proteins, Signal Transducing
- Amino Acids, Essential/metabolism
- Animals
- Carrier Proteins/metabolism
- Cell Cycle Proteins
- DNA-Binding Proteins
- Eukaryotic Initiation Factor-2/metabolism
- Eukaryotic Initiation Factor-2B/metabolism
- Fungal Proteins/genetics
- Humans
- Models, Biological
- Peptide Chain Initiation, Translational/physiology
- Phosphoproteins/metabolism
- Phosphorylation
- Protein Kinases/genetics
- Protein Kinases/metabolism
- Protein Serine-Threonine Kinases
- RNA, Fungal/genetics
- RNA, Fungal/metabolism
- RNA, Messenger/genetics
- RNA, Messenger/metabolism
- RNA, Transfer, Met/metabolism
- Ribosomal Protein S6 Kinases/metabolism
- Ribosomes/metabolism
- Saccharomyces cerevisiae/metabolism
- Saccharomyces cerevisiae Proteins
- Signal Transduction
- eIF-2 Kinase/metabolism
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Affiliation(s)
- S R Kimball
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania 17033, USA
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van Sluijters DA, Dubbelhuis PF, Blommaart EF, Meijer AJ. Amino-acid-dependent signal transduction. Biochem J 2000; 351 Pt 3:545-50. [PMID: 11042107 PMCID: PMC1221392 DOI: 10.1042/0264-6021:3510545] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Recent research carried out in several laboratories has indicated that, in addition to their role as intermediates in many metabolic pathways, amino acids can interact with insulin-dependent signal transduction. In this short review, the current state of this rapidly expanding field is discussed.
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Affiliation(s)
- D A van Sluijters
- Department of Biochemistry, Academic Medical Center, University of Amsterdam, Meibergdreef 15, 1105 AZ Amsterdam, The Netherlands
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Shah OJ, Antonetti DA, Kimball SR, Jefferson LS. Leucine, glutamine, and tyrosine reciprocally modulate the translation initiation factors eIF4F and eIF2B in perfused rat liver. J Biol Chem 1999; 274:36168-75. [PMID: 10593901 DOI: 10.1074/jbc.274.51.36168] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Leucine, glutamine, and tyrosine, three amino acids playing key modulatory roles in hepatic proteolysis, were evaluated for activation of signaling pathways involved in regulation of liver protein synthesis. Furthermore, because leucine signals to effectors that lie distal to the mammalian target of rapamycin, these downstream factors were selected for study as candidate mediators of amino acid signaling. Using the perfused rat liver as a model system, we observed a 25% stimulation of protein synthesis in response to balanced hyperaminoacidemia, whereas amino acid imbalance due to elevated concentrations of leucine, glutamine, and tyrosine resulted in a protein synthetic depression of roughly 50% compared with normoaminoacidemic controls. The reduction in protein synthesis accompanying amino acid imbalance became manifest at high physiologic concentrations and was dictated by the guanine nucleotide exchange activity of translation initiation factor eIF2B. Paradoxically, this phenomenon occurred concomitantly with assembly of the mRNA cap recognition complex, eIF4F as well as activation of the 70-kDa ribosomal S6 kinase, p70(S6k). Dual and reciprocal modulation of eIF4F and eIF2B was leucine-specific because isoleucine, a structural analog, was ineffective in these regards. Thus, we conclude that amino acid imbalance, heralded by leucine, initiates a liver-specific translational fail-safe mechanism that deters protein synthesis under unfavorable circumstances despite promotion of the eIF4F complex.
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Affiliation(s)
- O J Shah
- Department of Cellular and Molecular Physiology, The Pennsylvania State University, College of Medicine, Hershey, Pennsylvania 17033, USA
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Patti ME, Brambilla E, Luzi L, Landaker EJ, Kahn CR. Bidirectional modulation of insulin action by amino acids. J Clin Invest 1998; 101:1519-29. [PMID: 9525995 PMCID: PMC508730 DOI: 10.1172/jci1326] [Citation(s) in RCA: 372] [Impact Index Per Article: 14.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
Amino acids have been shown to stimulate protein synthesis, inhibit proteolysis, and decrease whole-body and forearm glucose disposal. Using cultured hepatoma and myotube cells, we demonstrate that amino acids act as novel signaling elements in insulin target tissues. Exposure of cells to high physiologic concentrations of amino acids activates intermediates important in the initiation of protein synthesis, including p70 S6 kinase and PHAS-I, in synergy with insulin. This stimulatory effect is largely due to branched chain amino acids, particularly leucine, and can be reproduced by its transamination product, ketoisocaproic acid. Concurrently, amino acids inhibit early steps in insulin action critical for glucose transport and inhibition of gluconeogenesis, including decreased insulin-stimulated tyrosine phosphorylation of IRS-1 and IRS-2, decreased binding of grb 2 and the p85 subunit of phosphatidylinositol 3-kinase to IRS-1 and IRS-2, and a marked inhibition of insulin-stimulated phosphatidylinositol 3-kinase. Taken together, these data support the hypothesis that amino acids act as specific positive signals for maintenance of protein stores, while inhibiting other actions of insulin at multiple levels. This bidirectional modulation of insulin action indicates crosstalk between hormonal and nutritional signals and demonstrates a novel mechanism by which nutritional factors contribute to insulin resistance.
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Affiliation(s)
- M E Patti
- Research Division, Joslin Diabetes Center, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02215, USA
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Fox HL, Kimball SR, Jefferson LS, Lynch CJ. Amino acids stimulate phosphorylation of p70S6k and organization of rat adipocytes into multicellular clusters. THE AMERICAN JOURNAL OF PHYSIOLOGY 1998; 274:C206-13. [PMID: 9458729 DOI: 10.1152/ajpcell.1998.274.1.c206] [Citation(s) in RCA: 113] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
In previous studies we have shown that rat adipocytes suspended in Matrigel and placed in primary culture migrate through the gel to form multicellular clusters over a 5- to 6-day period. In the present study, phosphorylation of the insulin-regulated 70-kDa ribosomal protein S6 kinase (p70S6k) was observed within 30 min of establishment of adipocytes in primary culture. Two inhibitors of the p70S6k signaling pathway, rapamycin and LY-294002, greatly reduced phosphorylation of p70S6k and organization of adipocytes into multicellular clusters. Of all the components of the cell culture medium, amino acids, and in particular a subset of neutral amino acids, were found to promote both phosphorylation of p70S6k and cluster formation. Lowering the concentrations of amino acids in the medium to levels approximating those in plasma of fasted rats decreased both phosphorylation of p70S6k and cluster formation. Furthermore, stimulation of p70S6k phosphorylation by amino acids was prevented by either rapamycin or LY-294002. These findings demonstrate that amino acids stimulate the p70S6k signaling pathway in adipocytes and imply a role for this pathway in multicellular clustering.
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Affiliation(s)
- H L Fox
- Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey 17033, USA
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Blommaart EF, Luiken JJ, Meijer AJ. Autophagic proteolysis: control and specificity. THE HISTOCHEMICAL JOURNAL 1997; 29:365-85. [PMID: 9184851 DOI: 10.1023/a:1026486801018] [Citation(s) in RCA: 180] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
The rate of proteolysis is an important determinant of the intracellular protein content. Part of the degradation of intracellular proteins occurs in the lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very active and almost completely accounts for starvation-induced proteolysis. Factors inhibiting this process include amino acids, cell swelling and insulin. In the mechanisms controlling macroautophagy, protein phosphorylation plays an important role. Activation of a signal transduction pathway, ultimately leading to phosphorylation of ribosomal protein S6, accompanies inhibition of macroautophagy. Components of this pathway may include a heterotrimeric Gi3-protein, phosphatidylinositol 3-kinase and p70S6 kinase. Recent evidence indicates that lysosomal protein degradation can be selective and occurs via ubiquitin-dependent and -independent pathways.
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Affiliation(s)
- E F Blommaart
- Department of Biochemistry, University of Amsterdam, The Netherlands
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Abstract
Macroautophagy is a major cellular catabolic pathway involved in the regulation of cell homeostasis. It is initiated by the sequestration of intracellular material by a wrapping membrane and terminates with the fusion of autophagic vacuoles with the lysosomal compartment. Macroautophagy has been extensively studied at the morphological level and in terms of environmental responses (nutrient deprivation, hormones). Recently a burst of data has emerged concerning the intracellular molecular events involved in the control of macroautophagic sequestration. It is becoming clear that the initial sequestration step of macroautophagy is under the control of different signalling pathways.
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Affiliation(s)
- P Codogno
- INSERM U410 Neuroendocrinologie et Biologie Cellulaire Digestives, Faculté de Medecine Xavier Bichat, Paris, France
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27
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Abstract
The goals for the development of multiple antigen peptides (MAP) are to provide a rational and unambiguous system to multimerize different types of synthetic peptide antigens and to attach immunomodulating molecules for targeting and delivery. These goals have been largely realized and new designs of MAPs now permit a broad range of immune responses including CTLs and mucosal IgAs. Furthermore, significant advances by the inventiveness of many laboratories have led to applications of MAPs for serodiagnostic and other biochemical uses including those for drug discovery. An important aspect to accomplish various goals of MAPs is chemistry. New methodologies using unprotected peptides as building blocks have been developed to accommodate new and sophisticated design of MAPs. This review is written based on the personal perspective of my laboratory and will focus on the recent progress in MAPs, together with the chemistry to achieve their synthesis.
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Affiliation(s)
- J P Tam
- Vanderbilt University Medical Center, Department of Microbiology and Immunology, Nashville, TN 37232-2363, USA
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Affiliation(s)
- G E Mortimore
- Department of Cellular and Molecular Physiology, Hershey Medical Center, Pennsylvania State University, Hershey 17033, USA
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Blommaart EF, Luiken JJ, Blommaart PJ, van Woerkom GM, Meijer AJ. Phosphorylation of ribosomal protein S6 is inhibitory for autophagy in isolated rat hepatocytes. J Biol Chem 1995; 270:2320-6. [PMID: 7836465 DOI: 10.1074/jbc.270.5.2320] [Citation(s) in RCA: 520] [Impact Index Per Article: 17.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
In rat hepatocytes, autophagy is known to be inhibited by amino acids. Insulin and cell swelling promote inhibition by amino acids. Each of the conditions leading to inhibition of autophagic proteolysis was found to be associated with phosphorylation of a 31-kDa protein that we identified as ribosomal protein S6. A combination of leucine, tyrosine, and phenylalanine, which efficiently inhibits autophagic proteolysis, was particularly effective in stimulating S6 phosphorylation. The relationship between the percentage inhibition of proteolysis and the degree of S6 phosphorylation was linear. Thus, inhibition of autophagy and phosphorylation of S6 are under the control of the same signal transduction pathway. Stimulation of S6 phosphorylation by the presence of amino acids was due to activation of S6 kinase and not to inhibition of S6 phosphatase. The inhibition by amino acids of both autophagic proteolysis and autophagic sequestration of electro-injected cytosolic [14C]sucrose was partially prevented by rapamycin, a compound known to inhibit activation of p70 S6 kinase. In addition, rapamycin partially inhibited the rate of protein synthesis. We conclude that the fluxes through the autophagic and protein synthetic pathways are regulated in an opposite manner by the degree to which S6 is phosphorylated. Possible mechanisms by which S6 phosphorylation can cause inhibition of autophagy are discussed.
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Affiliation(s)
- E F Blommaart
- E.C. Slater Institute, University of Amsterdam, The Netherlands
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Inhibition of macroautophagy and proteolysis in the isolated rat hepatocyte by a nontransportable derivative of the multiple antigen peptide Leu8-Lys4-Lys2-Lys-beta Ala. J Biol Chem 1994. [DOI: 10.1016/s0021-9258(18)47254-7] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
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