Enteral leucine supplementation increases protein synthesis in skeletal and cardiac muscles and visceral tissues of neonatal pigs through mTORC1-dependent pathways

Abstract

Introduction: Leucine (Leu) activates mammalian target of rapamycin (mTOR) to upregulate protein synthesis (PS).

Results: PS in skeletal muscles, heart, liver, pancreas, and jejunum, but not kidney, were greater in low protein supplemented with Leu (LP+L) than LP, but lower than high protein (HP). In longissimus dorsi muscle, protein kinase B phosphorylation was similar in LP and LP+L, but lower than HP. Although less than HP, p70 ribosomal S6 kinase 1 (S6K1) and eukaryotic initiation factor (eIF) 4E binding protein 1 (4EBP1) association with regulatory associated protein of mammalian target of rapamycin was greater in LP+L than LP, resulting in higher S6K1 and 4EBP1 phosphorylation. Feeding LP+L vs. LP decreased 4EBP1·eIF4E and increased eIF4E·eIF4G formation, but not to HP. Similar results were obtained for S6K1 and 4EBP1 phosphorylation in gastrocnemius, masseter, heart, liver, pancreas, and jejunum, but not kidney. eIF2α and elongation factor 2 phosphorylation was unaffected by treatment. DICUSSION: Our results suggest that enteral Leu supplementation of a low protein diet enhances PS in most tissues through mTOR complex 1 pathways.

Methods: To examine enteral Leu effects on PS and signaling activation, 5-d-old piglets were fed for 24 h diets containing: (i) LP, (ii) LP+L, or (iii) HP.

Figure 1

Circulating insulin (A), glucose (B), and BCAA (C) concentrations in piglets fed LP (●), LP+L (○), and HP (▲) diets every 4 h for 24 h. Values are means + SEM, n = 7–10 per treatment. Means without a common symbol differ, p < 0.05.

Figure 2

Plasma concentrations of essential AA (A) and nonessential AA (B) in piglets fed LP (□), LP+L (■), or HP ( ) diets for 1530 min. Values are means + SEM, n = 7–10 per treatment. Means without a common symbol differ, p < 0.05.

Figure 3

PS rates (A) and phosphorylation of PKB (B), S6K1 (C) and 4EBP1 (D) in longissimus dorsi muscle of piglets fed LP (□), LP+L (■), or HP ( ) diets. Values are means + SEM, n = 7–10 per treatment. Means without a common symbol differed, p < 0.05.

Figure 4

Abundance of raptor·mTOR (A), raptor·PRAS40 (B), raptor·S6K1 (C) and raptor·4EBP1 (D) complexes in longissimus dorsi muscle of piglets fed LP (□), LP+L (■), or HP ( ) diets. Values are means + SEM, n = 7–10 per treatment. Means without a common symbol differed, P < 0.05.

Figure 5

Abundance of eIF4E·4EBP1 (A) and eIF4E·eIF4G (B), and phosphorylation of eIF2α (C) and eEF2 (D) in longissimus dorsi muscle of piglets fed LP (□), LP+L (■), or HP ( ) diets. Values are means + SEM, n = 7–10 per treatment. Means without a common symbol differed, p < 0.05.

Figure 6

PS rates in gastrocnemius (A), masseter (B), and heart (C) muscles of piglets fed LP (□), LP+L (■), or HP ( ) diets. Values are means + SEM, n = 7–10 per treatment. Means without a common symbol differed, p < 0.05.

Figure 7

PS rates in liver (A), pancreas (B), jejunum (C), and kidney (D) of piglets fed LP (□), LP+L (■), or HP ( ) diets. Values are means + SEM, n = 7–10 per treatment. Means without a common symbol differed, p < 0.05.