Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P42345 (mTOR)
 26,049 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Regulation of translation of mRNAs coding for specific proteins plays an important role in controlling cell growth, differentiation, and transformation. Two proteins have been implicated in the regulation of specific mRNA translation: eukaryotic initiation factor eIF4E and ribosomal protein S6. Increased phosphorylation of eIF4E as well as its overexpression are associated with stimulation of translation of mRNAs with highly structured 5'-untranslated regions. Similarly, phosphorylation of S6 results in preferential translation of mRNAs containing an oligopyrimidine tract at the 5'-end of the message. In the present study, leucine stimulated phosphorylation of the eIF4E-binding protein, 4E-BP1, in L6 myoblasts, resulting in dissociation of eIF4E from the inactive eIF4E.4E-BP1 complex. The increased availability of eIF4E was associated with a 1.6-fold elevation in ornithine decarboxylase relative to global protein synthesis. Leucine also stimulated phosphorylation of the ribosomal protein S6 kinase, p70(S6k), resulting in increased phosphorylation of S6. Hyperphosphorylation of S6 was associated with a 4-fold increase in synthesis of elongation factor eEF1A. Rapamycin, an inhibitor of the protein kinase mTOR, prevented all of the leucine-induced effects. Thus, leucine acting through an mTOR-dependent pathway stimulates the translation of specific mRNAs both by increasing the availability of eIF4E and by stimulating phosphorylation of S6.
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PMID:Leucine regulates translation of specific mRNAs in L6 myoblasts through mTOR-mediated changes in availability of eIF4E and phosphorylation of ribosomal protein S6. 1020 76

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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PMID:Leucine, glutamine, and tyrosine reciprocally modulate the translation initiation factors eIF4F and eIF2B in perfused rat liver. 1059 1

Loss of muscle mass usually characterizes different pathologies (sepsis, cancer, trauma) and also occurs during normal aging. One reason for muscle wasting relates to a decrease in food intake. This study addressed the role of leucine as a regulator of protein breakdown in mouse C2C12 myotubes and aimed to determine which cellular responses regulate the process. Determination of the rate of protein breakdown indicated that leucine is one key regulator of this process in myotubes because starvation for this amino acid is responsible for 30-40% of the total increase generated by total amino acid starvation. Leucine restriction rapidly accelerates the rate of protein breakdown (+11 to 15% (p < 0.001) after 1 h of starvation) in a dose-dependent manner. By using various inhibitors, evidence is provided that acceleration of protein catabolism results mainly from an induction of autophagy, activation of lysosome-dependent proteolysis, without modification of mRNA levels encoding the lysosomal cathepsins B, L, or D. Those results suggest that autophagy is an essential cellular response for increasing protein breakdown in muscle following food deprivation. Induction of autophagy precedes a decrease in global protein synthesis (-20% to -30% (p < 0.001)) that occurs after 3 h of leucine starvation. Inhibition of the mammalian target of rapamycin (mTOR) activity does not abolish the effect of leucine starvation and the level of phosphorylated ribosomal S6 protein is not affected by leucine withdrawal. These latter data provide clear evidence that the mTOR signaling pathway is not involved in the mediation of leucine effects on both protein synthesis and degradation in C2C12 myotubes.
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PMID:Leucine limitation induces autophagy and activation of lysosome-dependent proteolysis in C2C12 myotubes through a mammalian target of rapamycin-independent signaling pathway. 1089 13

Amino acid availability is known to regulate diverse cell processes including the activation of p70 S6 kinase, initiation factors involved in mRNA translation, gene expression and cellular amino acid uptake. Essential amino acids, in particular the branched-chain amino acids (e.g. leucine), have been shown to be the dominant players in mediating these effects, although the precise nature by which they regulate these processes remain poorly understood. In this study we have investigated the mechanisms involved in the leucine-induced modulation of p70 S6 kinase and addressed whether this kinase participates in the up-regulation of the System A amino acid transporter in L6 muscle cells. Incubation of muscle cells that had been amino acid-deprived for 1 h with L-leucine (2 mM) led to a rapid (>2-fold) activation of p70 S6 kinase, which was suppressed by both wortmannin and rapamycin. Consistent with this finding, addition of leucine caused a rapid ( approximately 5-fold) but transient stimulation of phosphatidylinositol 3-kinase (PI3K). PI3K activation was inhibited by wortmannin and was not dependent upon insulin receptor substrate-1 activation. Unlike stimulation by insulin, activation of neither protein kinase B nor p42/p44 mitogen-activated protein kinase accompanied the leucine-induced stimulation of PI3K. However, the leucine-induced activation of PI3K and p70 S6 kinase did result in the concomitant inactivation of glycogen synthase kinase-3 (GSK-3). Leucine enhanced System A transport by approximately 50%. We have shown previously that this stimulation is protein-synthesis-dependent and in the current study we show that it was blocked by both wortmannin and rapamycin. Our findings indicate that PI3K and the mammalian target of rapamycin are components of a nutrient signalling pathway that regulates the activation of p70 S6 kinase and induction of System A in L6 cells. The activation of this pathway by leucine is also responsible for the inactivation of GSK-3, and this is likely to have important regulatory implications for translation initiation.
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PMID:L-leucine availability regulates phosphatidylinositol 3-kinase, p70 S6 kinase and glycogen synthase kinase-3 activity in L6 muscle cells: evidence for the involvement of the mammalian target of rapamycin (mTOR) pathway in the L-leucine-induced up-regulation of system A amino acid transport. 1094 49

The objectives of the present study were twofold: 1) to determine whether leucine is unique among the branched-chain amino acids (BCAA) in its ability to stimulate protein synthesis in skeletal muscle of food-deprived rats; and 2) to investigate whether changes in muscle protein synthesis after leucine administration involve a signaling pathway that includes the protein kinase mammalian target of rapamycin (mTOR). In the first set of experiments, food-deprived (18 h) male rats (200 g) were orally administered saline or 270 mg valine, isoleucine or leucine. In the second set of experiments, food-deprived rats were injected intravenously with rapamycin (0.75 mg/kg), a specific inhibitor of mTOR, before leucine administration. Only leucine stimulated protein synthesis in skeletal muscle above saline-treated controls (P: < 0.05). Furthermore, leucine was most effective among the BCAA at enhancing phosphorylation of eukaryotic initiation factor (eIF), 4E binding protein 1 (4E-BP1) and the 70-kDa ribosomal protein S6 kinase (S6K1). Leucine-dependent hyperphosphorylation of 4E-BP1 increased the availability of eIF4E to form the active eIF4G.eIF4E complex. To a lesser extent, isoleucine also enhanced phosphorylation of 4E-BP1 and S6K1. Rapamycin inhibited protein synthesis in both leucine-treated and food-deprived rats. Additionally, rapamycin prevented the stimulatory effects of leucine on eIF4E availability for binding eIF4G and inhibited leucine-dependent phosphorylation of S6K1. The data demonstrate that leucine is unique among the BCAA in its ability to stimulate protein synthesis in muscle of food-deprived rats. We show for the first time that leucine-dependent stimulation of translation initiation in vivo occurs via a rapamycin-sensitive pathway.
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PMID:Leucine stimulates translation initiation in skeletal muscle of postabsorptive rats via a rapamycin-sensitive pathway. 1101 66

Recent findings have demonstrated that the branched-chain amino acid leucine can activate the translational regulators, phosphorylated heat- and acid-stable protein regulated by insulin (PHAS-I) and p70 S6 kinase (p70S6k), in an insulin-independent and rapamycin-sensitive manner through mammalian target of rapamycin (mTOR), although the mechanism for this activation is undefined. It has been previously established that leucine-induced insulin secretion by beta-cells involves increased mitochondrial metabolism by oxidative decarboxylation and allosteric activation of glutamate dehydrogenase (GDH). We now show that these same intramitochondrial events that generate signals for leucine-induced insulin exocytosis are required to activate the mTOR mitogenic signaling pathway by beta-cells. Thus, a minimal model consisting of leucine and glutamine as substrates for oxidative decarboxylation and an activator of GDH, respectively, confirmed the requirement for these two metabolic components and mimicked closely the synergistic interactions achieved by a complete complement of amino acids to activate p70s6k in a rapamycin-sensitive manner. Studies using various leucine analogs also confirmed the close association of mitochondrial metabolism and the ability of leucine analogs to activate p70s6k. Furthermore, selective inhibitors of mitochondrial function blocked this activation in a reversible manner, which was not associated with a global reduction in ATP levels. These findings indicate that leucine at physiological concentrations stimulates p70s6k phosphorylation via the mTOR pathway, in part, by serving both as a mitochondrial fuel and an allosteric activator of GDH. Leucine-mediated activation of protein translation through mTOR may contribute to enhanced beta-cell function by stimulating growth-related protein synthesis and proliferation associated with the maintenance of beta-cell mass.
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PMID:Metabolic regulation by leucine of translation initiation through the mTOR-signaling pathway by pancreatic beta-cells. 1127 47

Control of protein synthesis by amino acid availability is an active and centrally important area of research that has produced several recent advances in our understanding of how these substrates serve not only as precursors but also as signaling molecules. One particularly noteworthy advance is the identification of the unique specificity of leucine in signaling to stimulate protein synthesis in skeletal muscle. Leucine mediated signaling results in a stimulation of initiation of mRNA translation and involves increases in the phosphorylation status of the translational repression 4E-BP1 and the ribosomal protein S6 kinase S6K1. It requires sustained activation of the mammalian target of rapamycin protein kinase. Leucine, however, also signals to stimulate protein synthesis in skeletal muscle by a mammalian target of rapamycin protein kinase independent (i.e. rapamycin insensitive) pathway, suggesting that the amino acid may signal through multiple pathways. Furthermore, leucine signaling in skeletal muscle differs from that in liver, suggesting that various responses may be tissue specific. Finally, there continues to be active research on the beneficial effects of glutamine as a unique supplement in catabolic circumstances. In this case, however, the signaling properties and mechanism of action of glutamine remain as an unsolved mystery.
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PMID:Control of protein synthesis by amino acid availability. 1179 Sep 48

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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PMID:Leucine is a direct-acting nutrient signal that regulates protein synthesis in adipose tissue. 1216 44

Acute administration of leucine and norleucine activates the mammalian target of rapamycin (mTOR) cell-signaling pathway and increases rates of protein synthesis in a number of tissues in fasted rats. Although persistent stimulation of mTOR signaling is thought to increase protein synthetic capacity, little information is available concerning the effects of chronic administration of these agonists on protein synthesis, mTOR signal transduction, or leucine metabolism. Hence, we developed a model of chronic leucine/norleucine supplementation via drinking water and examined the effects of chronic (12 days) supplementation on protein synthesis in adipose tissue, kidney, heart, liver, and skeletal muscle from ad libitum-fed rats. The relative concentration of proteins involved in mTOR signaling and the two initial steps in leucine oxidation were also examined. Leucine or norleucine supplementation was accompanied by increased rates of protein synthesis in adipose tissue, liver, and skeletal muscle, but not in heart or kidney. Supplementation was not associated with increases in the anabolic hormones insulin or insulin-like growth factor I. Chronic supplementation did not cause apparent adaptation in either components of the mTOR cell-signaling pathway that respond to leucine (mTOR, ribosomal protein S6 kinase, and eukaryotic initiation factor 4E-binding protein-1) or the first two steps in leucine metabolism (the mitochondrial isoform of branched-chain amino acid transaminase, branched-chain keto acid dehydrogenase, and branched-chain keto acid dehydrogenase kinase), which may be involved in terminating the signal from leucine. These results suggest that provision of leucine or norleucine supplementation via the drinking water results in stimulation of postprandial protein synthesis in adipose tissue, skeletal muscle, and liver without notable adaptive changes in signaling proteins or metabolic enzymes.
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PMID:Tissue-specific effects of chronic dietary leucine and norleucine supplementation on protein synthesis in rats. 1221 1

Leucine has been shown to stimulate adipose tissue protein synthesis in vivo as well as leptin secretion, protein synthesis, hyper-plastic growth, and tissue morphogenesis in in vitro experiments using freshly isolated adipocytes. Recently, others have proposed that leucine oxidation in the mitochondria may be required to activate the mammalian target of rapamycin (mTOR), the cytosolic Ser/Thr protein kinase that appears to mediate some of these effects. The first irreversible and rate-limiting step in leucine oxidation is catalyzed by the branched-chain alpha-keto acid dehydrogenase (BCKD) complex. The activity of this complex is regulated acutely by phosphorylation of the E1alpha-subunit at Ser293 (S293), which inactivates the complex. Because the alpha-keto acid of leucine regulates the activity of BCKD kinase, it has been suggested as a potential target for leucine regulation of mTOR. To study the regulation of BCKD phosphorylation and its potential link to mTOR activation, a phosphopeptide-specific antibody recognizing this site was developed and characterized. Phospho-S293 (pS293) immunoreactivity in liver corresponded closely to diet-induced changes in BCKD activity state. Immunoreactivity was also increased in TREMK-4 cells after the induction of BCKD kinase by a drug-inducible promoter. BCKD S293 phosphorylations in adipose tissue and gastrocnemius (which is mostly inactive in vivo) were similar. This suggests that BCKD complex in epididymal adipose tissue from food-deprived rats is mostly inactive (unable to oxidize leucine), as is the case in muscle. To begin to test the leucine oxidation hypothesis of mTOR activation, the dose-dependent effects of orally administered leucine on acute activation of S6K1 (an mTOR substrate) and BCKD were compared using the pS293 antibodies. Increasing doses of leucine directly correlated with increases in plasma leucine concentration. Phosphorylation of S6K1 (Thr389, the phosphorylation site leading to activation) in adipose tissue was maximal at a dose of leucine that increased plasma leucine approximately threefold. Changes in BCKD phosphorylation state required higher plasma leucine concentrations. The results seem more consistent with a role for BCKD and BCKD kinase in the activation of leucine metabolism/oxidation than in the activation of the leucine signal to mTOR.
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PMID:Potential role of leucine metabolism in the leucine-signaling pathway involving mTOR. 1281 18


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