Submitted to: American Journal of Physiology - Endocrinology and Metabolism
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2008
Publication Date: 8/5/2008
Citation: Suryawan, A., Jeyapalan, A.S., Orellana, R.A., Wilson, F.A., Nguyen, H.V., Davis, T.A. 2008. Leucine stimulates protein synthesis in skeletal muscle of neonatal pigs by enhancing mTORC1 activation. American Journal of Physiology: Endocrinology and Metabolism. 295(4):E868-E875.
Interpretive Summary: Skeletal muscle from new-born pigs grows rapidly, in part due to an enhanced sensitivity to the increase of amino acid level after eating. We conducted a study to determine the ability of leucine (an essential amino acid) to stimulate protein synthesis with help from mTORC1 (a group of signaling components). We found leucine increases mTORC1 activation, resulting in the activation of signaling components of protein synthesis apparatus (such as 4EBP1, S6K1, eIF-4EBP1, eIF-4G, etc). In conclusion, leucine stimulates protein synthesis by activating mTORC1 and protein synthesis apparatus.
Technical Abstract: Skeletal muscle in the neonate grows at a rapid rate due in part to an enhanced sensitivity to the postprandial rise in amino acids, particularly leucine. To elucidate the molecular mechanism by which leucine stimulates protein synthesis in neonatal muscle, overnight-fasted 7-day-old piglets were treated with rapamycin [an inhibitor of mammalian target of rapamycin (mTOR) complex (mTORC)1] for 1 h and then infused with leucine for 1 h. Fractional rates of protein synthesis and activation of signaling components that lead to mRNA translation were determined in skeletal muscle. Rapamycin completely blocked leucine-induced muscle protein synthesis. Rapamycin markedly reduced raptor-mTOR association, an indicator of mTORC1 activation. Rapamycin blocked the leucine-induced phosphorylation of mTOR, S6 kinase 1 (S6K1), and eukaryotic initiation factor (eIF)4E-binding protein-1 (4E-BP1) and formation of the eIF4E•eIF4G complex and increased eIF4E•4E-BP1 complex abundance. Rapamycin had no effect on the association of mTOR with rictor, a crucial component for mTORC2 activation, or G protein beta-subunit-like protein (GbetaL), a component of mTORC1 and mTORC2. Neither leucine nor rapamycin affected the phosphorylation of AMP-activated protein kinase (AMPK), PKB, or tuberous sclerosis complex (TSC)2, signaling components that reside upstream of mTOR. Eukaryotic elongation factor (eEF)2 phosphorylation was not affected by leucine or rapamycin, although current dogma indicates that eEF2 phosphorylation is mTOR dependent. Together, these in vivo data suggest that leucine stimulates muscle protein synthesis in neonates by enhancing mTORC1 activation and its downstream effectors.