Enteral delivery of proteins stimulates protein synthesis in human duodenal mucosa in the fed state through a mammalian target of rapamycin–independent pathway

Abstract

Background: Glutamine modulates duodenal protein metabolism in fasted healthy humans, but its effects in a fed state remain unknown. Objective: We aimed to assess the effects of either glutamine or an isonitrogenous protein mixture on duodenal protein metabolism in humans in the fed state. Design: Twenty-four healthy volunteers were randomly included in 2 groups. Each volunteer was studied on 2 occasions in a random order and received, during 5 h, either an enteral infusion of maltodextrins alone (0.25 g $ kg 21 $ h 21 ; both groups) that mimicked a carbohydrate fed state or maltodextrins with glutamine (group 1) or anSimultaneously, a continuous intravenous infusion of 13 C-leucine and 2 H 5 -phenylalanine (both 9 mmol $ kg 21 $ h 21 ) was performed.Endoscopic duodenal biopsies were taken. Leucine and phenylalanine enrichments were assessed by using gas chromatography-mass spectrometry in duodenal proteins and the intracellular free amino acids pool to calculate the mucosal fractional synthesis rate (FSR). Proteasome proteolytic activities and phosphokinase expression were assessed by using specific fluorogenic substrates and macroarrays, respectively. Results: The FSR and proteasome activity were not different after the glutamine supply compared with after maltodextrins alone. In contrast, the FSR increased (1.7-fold increase; P , 0.05) after protein-powder delivery without modification of total proteasome activity. The protein powder increased insulinemia, PI3 kinase, and erk phosphorylation but did not affect the mammalian target of rapamycin (mTOR) pathway and mitogen-activated protein kinase signal-integrating kinase 1 phosphorylation. A trend for an increase of eukaryotic translation initiation factor 4E phosphorylation was observed (P = 0.07). Conclusion: In the carbohydrate fed state, enteral proteins but not glutamine increased duodenal protein synthesis through an mTOR independent pathway in humans. This trial was registered at clinicaltrials.gov as NCT01254110. Am J Clin Nutr 2013;97:286-94.

Glutamine metabolism in advanced age

“…In contrast, in healthy subjects in the fed state, enteral proteins, but not glutamine, increased protein synthesis via an mTOR-independent pathway in humans. 66 Nakajo et al 38 have proposed a new concept for the biological role of phosphorylation of mTOR and intestinal cell growth: the signal induced by glutamine may stimulate cellular proliferation and increase cell number, whereas leucine or arginine induces the signal for cell growth (increase in cell size) in rat intestinal epithelial cells.…”

Does enteral protein administration stimulate duodenal mucosa protein synthesis through an mTORC1-independent signaling pathway?

“…Overall, the data reported by Coëffier et al (11) are interpreted to support a model in which amino acids act to stimulate protein synthesis in the duodenal mucosa through a mechanism distinct from the established one involving activation of mTORC1. Future studies should aim to confirm the initial conclusion and to identify the mTORC1-independent pathway involved in regulating protein synthesis in the duodenal mucosa.…”

“…It is important that Western blot analysis, rather than dot-blot analysis, be used to assess potential changes in p70S6K1 and mTOR phosphorylation, and changes in protein phosphorylation should be expressed relative to the respective target protein, rather than GAPDH, to account for potential changes in expression. Finally, possible mTORC1-independent pathways, such as those involving eIF2 and eIF2B, which were not assessed in the Coëffier et al study (11), should be evaluated in future studies. It is possible that such measurements were performed and not reported in the present study or are planned for a future report.…”

“…It is therefore imperative that future studies not only confirm the results of the present one but also test possible alternative pathways that might be involved in the effect. For example, in the Coëffier et al study (11), measurements were taken at a single time point, 5 h after the start of enteral delivery of the protein mixture and intravenous infusion of [ ]phenylalanine into protein may have occurred transiently at a time when mTORC1 signaling was activated, and that both may have returned to basal values by the time biopsies were taken, a time course analysis would be necessary. Moreover, a study that uses rapamycin to inhibit mTOR signaling could be performed, as has been used by others to assess the role of mTORC1 in the amino acid-and exerciseinduced stimulation of protein synthesis in humans (8,12).…”

Delayed gastric emptying and altered antrum protein metabolism during activity‐based anorexia

“…Immunoblots were performed as previously described; briefly, total proteins (25 μg proteins/lane) were separated on a 4%‐20% gradient polyacrylamide gel (Biorad, Marnes‐la‐Coquette, France). After transfer, nitrocellulose membrane were overnight incubated at 4°C with specific primary antibodies: goat polyclonal antibodies [anti‐HSPD1, #sc‐1052, SantaCruz Biotechnology; anti‐GAPDH, #SAB2500451, Sigma‐Aldrich], rabbit monoclonal antibody [anti‐HSPA8, #8444, Cell Signaling Technology], rabbit polyclonal antibody [anti VCL, #4650, Cell Signaling Technology], and mouse monoclonal antibody [anti‐ACTA2, #A5228, Sigma‐Aldrich].…”