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First published online on May 3, 2007.
 Copyright © 2007 by The Physiological Society
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Received April 13, 2007
Revised May 2, 2007
Accepted after revision May 2, 2007

Nutrient Signalling in the Regulation of Human Muscle Protein Synthesis

Satoshi Fujita1, Hans C. Dreyer1, Micah J. Drummond1, Erin L. Glynn1, Jerson G. Cadenas1, Fumiaki Yoshizawa2, Elena Volpi1, and Blake B. Rasmussen1*

1 University of Texas Medical Branch
2 Utsunomiya University

* To whom correspondence should be addressed. E-mail: blrasmus{at}utmb.edu.

The mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) and mTOR are important nutrient and energy sensing and signalling proteins in skeletal muscle. AMPK activation decreases muscle protein synthesis by inhibiting mTOR signalling to regulatory proteins associated with translation initiation and elongation. On the other hand, essential amino acids (leucine in particular) and insulin stimulate mTOR signalling and protein synthesis. We hypothesized that anabolic nutrients would be sensed by both AMPK and mTOR resulting in an acute and potent stimulation of human skeletal muscle protein synthesis via enhanced translation initiation and elongation. We measured muscle protein synthesis and mTOR-associated upstream and downstream signalling proteins in young male subjects (N=14) using stable isotopic and immunoblotting techniques. Following a 1st muscle biopsy, subjects in the Nutrition group ingested a leucine enriched essential amino acid-carbohydrate mixture (EAC). Subjects in the Control group did not consume nutrients. A 2nd biopsy was obtained 1 hr later. Ingestion of EAC significantly increased MPS, modestly reduced AMPK phosphorylation, and increased Akt/PKB and mTOR phosphorylation (P<0.05). mTOR signalling to its downstream effectors (S6K1 and 4E-BP1 phosphorylation status) was also increased (P<0.05). In addition, eEF2 phosphorylation was significantly reduced (P<0.05). Protein synthesis and cell signalling (phosphorylation status) was unchanged in the Control group (P>0.05). We conclude that anabolic nutrients alter the phosphorylation status of both AMPK and mTOR-associated signalling proteins in human muscle in association with an increase in protein synthesis not only via enhanced translation initiation but also through signalling promoting translation elongation.


Key words: Nutrition • Protein metabolism • Skeletal muscle




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