Journal Title (Medline/Pubmed accepted abbreviation): J. Nutr.
Page numbers: 2145-2152
doi (if applicable): 10.3945/jn.110.128421
Amino acids serve as precursors for protein synthesis and also as cellular signaling molecules to regulate protein synthesis. Leucine (Leu), a branched chain amino acid, consistently stimulates protein synthesis. There is evidence that Leu works through the mTOR signaling pathway via translation initiation factors, but this mechanism is not yet completely understood.
Translation is the process of producing protein from mRNA. The process of translation is necessary in order to produce muscle protein as well as enzymes needed for other biochemical functions. Translation initiation factors are involved early in the translation process. Regulating these factors will in turn regulate translation. mTOR (mammalian target of rapamycin) is a protein that stimulates protein synthesis and cell growth. It is responsible for communicating with other enzymes including S6K, initiation factors, and others in order to amplify the downstream signal.
Research Objective:To understand the mechanism by which leucine regulates protein synthesis through the mTOR signaling pathway and translation initiation signaling in both fast-twitch and oxidative muscle fibers.
Subjects:Twenty-one 5-day old piglets
Treatment: The piglets were divided into 3 groups: low protein diet (LP, 1.3 g/kg body weight), high protein diet (HP, 6.4 g/kg body weight), or Leu (LP+L, 1.3 g protein/kg body weight + 5.1 g additional leucine/kg body weight to equal 6.4 g protein/kg body weight). The “protein” was a combination of whey and skim milk.
Experimental design:Piglets were gavage fed at time=0 and t=60 min with the protein solutions. Solutions also contained equal amounts of lactose, fat, vitamins, and other ingredients. Blood samples were collected at t=0, 30, 60, and 90 min and analyzed for plasma amino acids, insulin, and glucose. At t=60 min, piglets were injected with l-[4-3H] phenylalanine and euthanized 30 min later. Samples of muscle and other organs were harvested for analysis of rates of protein synthesis.
Both LP+L and HP diets were significantly more effective at activating protein synthesis than the low protein diet. Phosphorylation of mTOR, S6K1, and 4E-BP1 was enhanced in fast-twitch, glycolytic muscle with the HP and LP+L diet, indicating the protein translation was higher in these muscles than those fed with LP. To corroborate this observation, the quantity of 4E-BP1•eIF4E complex was lower in the LP+L and HP groups (eIF4E is released from 4E-BP1 to participate in translation.) Protein synthesis and protein synthetic efficiency were greater in muscles of oxidative fiber types in piglets fed the LP+L or HP diet compared to the LP diet. Protein synthesis rates were shown to be greater in some non-muscle cells as well (kidney, pancreas, et al.) but not the liver.