MTOR Activation occurs Independent of Changes in Skeletal Muscle LAT1 Protein Content after Protein Ingestion

Abstract

Activation of the mammalian target of rapamycin complex 1 (mTORC1) after protein ingestion has been shown to be obligatory to augment the postprandial muscle protein synthetic response. The delivery of dietary protein derived amino acids into skeletal muscle tissue is facilitated by amino acid transporters. In particular, the system L amino acid transporter (LAT1/solute-linked carrier (SLC)7A5) transports large neutral amino acids such as phenylalanine, tyrosine, and leucine across the muscle cell membrane and may also serve as an intracellular sensor for mTORC1 signaling. PURPOSE: We aimed to examine variations in skeletal muscle LAT1 (55 kDa) protein content and phosphorylation of mTORC1 (Ser2448) in relation to dietary protein digestion and absorption kinetics after ingestion of single meal containing ample amounts of protein. METHODS: Seven healthy, young men (24±1 y; BMI= 24.8±1.2 kg/m2) received a primed continuous infusion of L-[ring-2H5]phenylalanine, L-[ring-2H2]tyrosine, and L-[1-13C]leucine and ingested 38 g of intrinsically L-[1-13C]phenylalanine and L-[1-13C]leucine labeled milk protein. Blood samples were drawn every 0.5-1 h during the infusion protocol. Biopsies from the vastus lateralis were collected before and after protein ingestion at 1, 2, 3, and 5 h of the postprandial period. RESULTS: Postprandial release of dietary protein derived phenylalanine rapidly increased in circulation after protein ingestion and remained elevated throughout the 5 h postprandial period (Time effect: P<0.001). mTORC1 phosphorylation was increased (18±6%) throughout the postprandial phase (time effect: P<0.05). However, protein ingestion did not modulate LAT1 protein content during the postprandial period (P=0.53). CONCLUSIONS: We conclude that the increase in mTORC1 phosphorylation after protein ingestion occurs without modulations in overall skeletal muscle LAT1 protein content.