Differential regulation of mTORC1 activation by leucine and beta-hydroxy-beta-methylbutyrate in skeletal muscle of neonatal pigs

Authors: SURYAWAN, AGUS - Children'S Nutrition Research Center (CNRC); RUDAR, MARKO - Children'S Nutrition Research Center (CNRC); FIOROTTO, MARTA - Children'S Nutrition Research Center (CNRC); DAVIS, TERESA - Children'S Nutrition Research Center (CNRC)

Submitted to: Journal of Applied Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2020
Publication Date: 1/16/2020
Citation: Suryawan, A.A., Rudar, M., Fiorotto, M.L., Davis, T.A. 2020. Differential regulation of mTORC1 activation by leucine and beta-hydroxy-beta-methylbutyrate in skeletal muscle of neonatal pigs. Journal of Applied Physiology. 128(2):286-295. https://doi.org/10.1152/japplphysiol.00332.2019.
DOI: https://doi.org/10.1152/japplphysiol.00332.2019

Interpretive Summary: Infant health and development depend on optimal nutrition during the newborn period. In this study we used newborn pigs as animal model for human infants to study how nutritional supplements (leucine and its metabolite, hydroxy methylbutyrate can support muscle growth of newborn babies. While both leucine and hydroxy methylbutyrate promote protein synthesis, a major determinant of muscle growth in infants, the underlying mechanisms are not clear. Here we show that leucine and HMB differ in how they affect several key signal transduction processes that regulate protein synthesis. Our findings provide important information that may be beneficial for optimizing the nutritional management of newborn infants and promoting their lean growth.

Technical Abstract: Leucine (Leu) and its Beta-hydroxy-Beta-methylbutyrate (HMB) stimulate mechanistic target of rapamycin (mTOR) complex 1 (mTORC1)-dependent protein synthesis in the skeletal muscle of neonatal pigs. This study aimed to determine whether HMB and Leu utilize common nutrient-sensing mechanisms to activate mTORC1. In study 1, neonatal pigs were fed one of five diets for 24 h: low protein (LP), high protein (HP), or LP supplemented with 4 (LP+HMB4), 40 (LP+HMB40), or 80 (LP+HMB80) µmol HMB·kg body wt-1·day-1. In study 2, neonatal pigs were fed for 24 h: LP, LP supplemented with Leu (LP+Leu), or HP diets delivering 9, 18, and 18 mmol Leu·kg body wt-1·day-1, respectively. The upstream signaling molecules that regulate mTORC1 activity were analyzed. mTOR phosphorylation on Ser2448 and Ser2481 was greater in LP+HMB40, LP+HMB80, and LP+Leu than in LP and greater in HP than in HMB-supplemented groups (P < 0.05), whereas HP and LP+Leu were similar. Rheb-mTOR complex formation was lower in LP than in HP (P < 0.05), with no enhancement by HMB or Leu supplementation. The Sestrin2-GATOR2 complex was more abundant in LP than in HP and was reduced by Leu (P < 0.05) but not HMB supplementation. RagA-mTOR and RagC-mTOR complexes were higher in LP+Leu and HP than in LP and HMB groups (P < 0.05). There were no treatment differences in RagB-SH3BP4, Vps34-LRS, and RagD-LRS complex abundances. Phosphorylation of Erk1/2 and TSC2, but not AMPK, was lower in LP than HP (P < 0.05) and unaffected by HMB or Leu supplementation. Our results demonstrate that HMB stimulates mTORC1 activation in neonatal muscle independent of the leucine-sensing pathway mediated by Sestrin2 and the Rag proteins.