Enteral Leucine Supplementation Increases Protein Synthesis in Skeletal and Cardiac Muscles and Visceral Tissues of Neonatal Pigs through mTORC1-dependent Pathways

Leucine (Leu) activates mammalian target of rapamycin (mTOR) to upregulate protein synthesis (PS). To examine enteral Leu effects on PS and signaling activation, 5-d-old piglets were fed for 24 h diets containing: 1) low protein (LP), 2) LP supplemented with Leu (LP+L), or 3) high protein (HP). PS in skeletal muscles, heart, liver, pancreas, and jejunum, but not kidney, were greater in LP+L than LP, but lower than 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 versus 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. Our results suggest that enteral Leu supplementation of a low protein diet enhances PS in most tissues through mTOR complex 1 pathways.