Studies concerning the specificity of the effect of leucine on the turnover of proteins in muscles of control and diabetic rats

Buse, Maria G.; Weigand, Dorothy A.

doi:10.1016/0005-2787(77)90341-0

Cited by 90 publications

(32 citation statements)

References 14 publications

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“…However, there was a trend toward a dose-dependent decrease in phenylalanine and tyrosine. This trend is consistent with an inhibitory effect of leucine on protein breakdown (8,9), which increases with food deprivation. Figure 9 shows that, at the lowest plasma leucine concentrations (i.e., 179, 267, and 548 M), the ratio of pS293 to total E1␣ immunoreactivity was not significantly different.…”

Section: Characterization Of Ps293 E1␣ Antibodysupporting

confidence: 86%

Potential role of leucine metabolism in the leucine-signaling pathway involving mTOR

Lynch

Halle²,

Fujii³

et al. 2003

American Journal of Physiology-Endocrinology and Metabolism

103

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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 α-keto acid dehydrogenase (BCKD) complex. The activity of this complex is regulated acutely by phosphorylation of the E1α-subunit at Ser293 (S293), which inactivates the complex. Because the α-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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Section: Characterization Of Ps293 E1␣ Antibodysupporting

confidence: 86%

Potential role of leucine metabolism in the leucine-signaling pathway involving mTOR

Lynch

Halle²,

Fujii³

et al. 2003

American Journal of Physiology-Endocrinology and Metabolism

103

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“…Nakano and Hara (32) refed streptozotocin diabetic rats a diet containing 25% casein and found that the incorporation rate of labeled phenylalanine into isolated gastrocnemius preparations was increased compared with fooddeprived diabetic controls. Furthermore, the addition of leucine to the incubation medium stimulates the incorporation of labeled precursors into muscle proteins in hemidiaphragms isolated from diabetic rats compared with hemidiaphragms incubated in the absence of the amino acid (33). In the current study, oral administration of leucine to diabetic rats enhanced muscle protein synthesis compared with diabetic controls.…”

Section: Discussionsupporting

confidence: 49%

“…Few studies have examined whether amino acids regulate muscle protein synthesis in diabetic rats (32,33). Nakano and Hara (32) refed streptozotocin diabetic rats a diet containing 25% casein and found that the incorporation rate of labeled phenylalanine into isolated gastrocnemius preparations was increased compared with fooddeprived diabetic controls.…”

Section: Discussionmentioning

confidence: 99%

Orally Administered Leucine Enhances Protein Synthesis in Skeletal Muscle of Diabetic Rats in the Absence of Increases in 4E-BP1 or S6K1 Phosphorylation

et al. 2002

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In this study, food-deprived (18 h) control rats and rats with alloxan-induced diabetes were orally administered saline or the amino acid leucine to assess whether it regulates protein synthesis independently of a change in serum insulin concentrations. Immediately after leucine administration, diabetic rats were infused with insulin (0.0, 4.0, or 20 pmol ⅐ min -1 ⅐ kg -1 ) for 1 h to examine the role of the hormone in the protein synthetic response to leucine. In control rats, leucine stimulated protein synthesis by 58% and increased phosphorylation of the translational repressor, eukaryotic initiation factor (eIF) 4E-binding protein (BP)-1, 4E-BP1, fivefold. Consequently, association of the mRNA cap-binding protein eukaryotic initiation factor (eIF)4E with 4E-BP1 was reduced to 50% of control values, and eIF4G•eIF4E complex assembly was increased 80%. Furthermore, leucine increased the phosphorylation of the 70-kDa ribosomal protein S6 (rp S6) and the ribosomal protein S6 kinase (S6K1). Diabetes attenuated protein synthesis compared with control rats. Nonetheless, in diabetic rats, leucine increased protein synthesis by 53% without concomitant changes in the phosphorylation of 4E-BP1 or S6K1. Skeletal muscle protein synthesis was stimulated in diabetic rats infused with insulin, but rates of synthesis remained less than values in nondiabetic controls that were administered leucine. Phosphorylation of 4E-BP1 and S6K1 was increased in diabetic rats infused with insulin in a dose-dependent manner, and the response was enhanced by leucine. The results suggest that leucine enhances protein synthesis in skeletal muscle through both insulin-dependent and -independent mechanisms. The insulin-dependent mechanism is associated with increased phosphorylation of 4E-BP1 and S6K1. In contrast, the insulin-independent effect on protein synthesis is mediated by an unknown mechanism. Diabetes 51: -936, 2002A fter consumption of a protein-containing meal, the fractional rate of protein synthesis of total mixed proteins in skeletal muscle of growing animals is upregulated. Two vital components of this response are elevations in circulating concentrations of the hormone insulin and an increase in amino acid supply. The relative importance of each of these components in regulating protein synthesis continues to be a topic of investigation and controversy. Several reports indicate that physiological increases in circulating insulin concentrations are not sufficient to stimulate protein synthesis in food-deprived rats (1-4). When postabsorptive rats are administered an oral bolus of carbohydrate alone, no change in protein synthesis is observed. In contrast, when food-deprived rats are administered an isocaloric macronutrient-mixed meal, rates of protein synthesis are stimulated (1). Plasma insulin concentrations in rats administered either meal are similar; hence, the enhanced rate of recovery in rats administered a mixed meal cannot be attributed to a differential insulin response between the groups. Likewise, when fasted rats are...

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“…Leucine was chosen because a mixture of the branched-chain amino acids can support protein synthesis in muscle as well as the full complement of amino acids (16,19,28,33). Of the branched-chain amino acids, leucine appears to be the specific effector branched-chain amino acid on protein synthesis in muscle (7,8,10,28).…”

Section: Methodsmentioning

confidence: 99%

Amino acid-induced stimulation of translation initiation in rat skeletal muscle

Vary

Jefferson

Kimball

1999

American Journal of Physiology-Endocrinology and Metabolism

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Vary, Thomas C., Leonard S. Jefferson, and Scot R. Kimball. Amino acid-induced stimulation of translation initiation in rat skeletal muscle. Am. J. Physiol. 277 (Endocrinol. Metab. 40): E1077-E1086, 1999.-Amino acids stimulate protein synthesis in skeletal muscle by accelerating translation initiation. In the two studies described herein, we examined mechanisms by which amino acids regulate translation initiation in perfused skeletal muscle hindlimb preparation of rats. In the first study, the effects of supraphysiological amino acid concentrations on eukaryotic initiation factors (eIF) 2B and 4E were compared with physiological concentrations of amino acids. Amino acid supplementation stimulated protein synthesis twofold. No changes were observed in eIF2B activity, in the amount of eIF4E associated with the eIF4E-binding protein (4E-BP1), or in the phosphorylation of 4E-BP1. The abundance of eIF4E bound to eIF4G and the extent of phosphorylation of eIF4E were increased by 800 and 20%, respectively. In the second study, we examined the effect of removing leucine on translation initiation when all other amino acids were maintained at supraphysiological concentrations. Removal of leucine from the perfusate decreased the rate of protein synthesis by 40%. The inhibition of protein synthesis was associated with a 40% decrease in eIF2B activity and an 80% fall in the abundance of eIF4E · eIF4G complex. The fall in eIF4G binding to eIF4E was associated with increased 4E-BP1 bound to eIF4E and a reduced phosphorylation of 4E-BP1. In contrast, the extent of phosphorylation of eIF4E was unaffected. We conclude that formation of the active eIF4E · eIF4G complex controls protein synthesis in skeletal muscle when the amino acid concentration is above the physiological range, whereas removal of leucine reduces protein synthesis through changes in both eIF2B and eIF4E. leucine; eukaryotic initiation factors 4E, 4G, and 2B; 4E-binding protein 1 AMINO ACIDS play a regulatory role in modulating both protein synthesis and protein degradation in a wide range of tissues. Much evidence indicates that a major portion of the increase in skeletal muscle protein synthesis in vivo by refeeding after a brief starvation is independent of changes in anabolic hormones (insulin) and is attributable to amino acids themselves (44, 53). During in vitro perfusion or incubation of skeletal muscles, inclusion of amino acids at 5 or 10 times normal plasma concentrations stimulates protein synthesis (16,19,28,33). Likewise, infusion of amino acids to achieve a relative hyperaminoacidemia stimulates protein synthesis in muscle in vivo (1,2,15,30,(43)(44)(45). Besides amino acids functioning as precursors, they also appear to have a regulatory role for protein synthesis. Amino acids increase rates of protein synthesis by enhancing translation efficiency (19, 28), through stimulating peptide-chain initiation relative to elongation (28). Of all the amino acids, the branched-chain amino acids (leucine, isoleucine, and valine) have a unique role in this pro...

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Studies concerning the specificity of the effect of leucine on the turnover of proteins in muscles of control and diabetic rats

Cited by 90 publications

References 14 publications

Potential role of leucine metabolism in the leucine-signaling pathway involving mTOR

Potential role of leucine metabolism in the leucine-signaling pathway involving mTOR

Orally Administered Leucine Enhances Protein Synthesis in Skeletal Muscle of Diabetic Rats in the Absence of Increases in 4E-BP1 or S6K1 Phosphorylation

Amino acid-induced stimulation of translation initiation in rat skeletal muscle

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