Nutrient signalling in the regulation of human muscle protein synthesis

Abstract

The mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) are important nutrient- and energy-sensing and signalling proteins in skeletal muscle. AMPK activation decreases muscle protein synthesis by inhibiting mTOR signalling to regulatory proteins associated with translation initiation and elongation. On the other hand, essential amino acids (leucine in particular) and insulin stimulate mTOR signalling and protein synthesis. We hypothesized that anabolic nutrients would be sensed by both AMPK and mTOR, resulting in an acute and potent stimulation of human skeletal muscle protein synthesis via enhanced translation initiation and elongation. We measured muscle protein synthesis and mTOR-associated upstream and downstream signalling proteins in young male subjects (n=14) using stable isotopic and immunoblotting techniques. Following a first muscle biopsy, subjects in the 'Nutrition' group ingested a leucine-enriched essential amino acid-carbohydrate mixture (EAC). Subjects in the Control group did not consume nutrients. A second biopsy was obtained 1 h later. Ingestion of EAC significantly increased muscle protein synthesis, modestly reduced AMPK phosphorylation, and increased Akt/PKB (protein kinase B) and mTOR phosphorylation (P<0.05). mTOR signalling to its downstream effectors (S6 kinase 1 (S6K1) and 4E-binding protein 1 (4E-BP1) phosphorylation status) was also increased (P<0.05). In addition, eukaryotic elongation factor 2 (eEF2) phosphorylation was significantly reduced (P<0.05). Protein synthesis and cell signalling (phosphorylation status) was unchanged in the control group (P>0.05). We conclude that anabolic nutrients alter the phosphorylation status of both AMPK- and mTOR-associated signalling proteins in human muscle, in association with an increase in protein synthesis not only via enhanced translation initiation but also through signalling promoting translation elongation.

Figures

Figure 1. Substrate and hormone kinetics following nutrient intake

Control data are from fasting subjects. Nutrition data are from subjects following an ingestion of essential amino acids and carbohydrate. Following nutrient ingestion: A, arterial glucose and insulin concentrations; B, glucose uptake across the leg; C, arterial phenylalanine concentration; D, muscle free phenylalanine and leucine concentrations. *P < 0.05 versus Control.

Figure 2. Phosphorylation status of upstream regulators of mTOR

Data in A–C are presented as Pre and Post. Pre data are from the first muscle biopsy in both the Control and Nutrition groups and indicate fasting phosphorylation status. Post data are from the second muscle biopsy obtained 1 h later. In the Control group no nutrients were ingested and the second biopsy was able to assess if there was any effect of the biopsy procedure on phosphorylation status. The Post data in the Nutrition group reflect the change in phosphorylation status 1 h after ingestion of essential amino acids and carbohydrate. A, AMPKα phosphorylation at Thr 172; B, TSC2 phosphorylation at Thr 1462; C, Akt/PKB phosphorylation at Ser 473; D, IRS-1 Ser 312 phosphorylation in the Nutrition group at baseline (basal) and 1 h after nutrient ingestion (Nutrition). *P < 0.05 versus Pre.

Figure 3. Phosphorylation status of downstream components of the mTOR signalling pathway

Data are presented as Pre and Post. Pre data are from the first muscle biopsy in both the Control and Nutrition groups and indicate fasting phosphorylation status. Post data are from the second muscle biopsy obtained 1 h later. In the Control group no nutrients were ingested and the second biopsy was able to assess if there was any effect of the biopsy procedure on phosphorylation status. The Post data in the Nutrition group reflect the change in phosphorylation status 1 h after ingestion of essential amino acids and carbohydrate. A, mTOR phosphorylation at Ser 2448; B, 4E-BP1 phosphorylation at Thr 37/46; C, S6K1 phosphorylation at Thr 389; D, eEF2 phosphorylation at Thr 56. *P < 0.05 versus Pre.

Figure 4. Phenylalanine enrichmnts (tracer/tracee) in the femoral artery and vein during the hour following nutrient ingestion

Blood enrichments were stable throughout the experiment and were not different between time points (P > 0.05). Enrichments were also stable in the Control group (data not shown) and were not different from the Nutrition group (P > 0.05). Intracellular phenylalanine enrichment in the muscle free pool is shown for the biopsy collected immediately before and 1 h following nutrient ingestion. Muscle free pool enrichment was not different between the two time points (P > 0.05).

Figure 5. Muscle protein synthesis (FSR)

The basal FSR in the Control group is the synthesis rate in the absence of nutrients. Nutrition data reflect the large increase in muscle protein synthesis following an ingestion of essential amino acids and carbohydrate. *P < 0.05 versus Control.