l-Tryptophan Activates Mammalian Target of Rapamycin and Enhances Expression of Tight Junction Proteins in Intestinal Porcine Epithelial Cells

J Nutr. 2015 Jun;145(6):1156-62. doi: 10.3945/jn.114.209817. Epub 2015 Apr 15.

Abstract

Background: Besides serving as a substrate for protein synthesis, L-tryptophan (L-Trp) is used via serotonin-, kynurenine-, and niacin-synthetic pathways to produce bioactive compounds crucial for whole-body homeostasis. It is unknown whether L-Trp itself can regulate metabolic pathways in animal cells.

Objective: This study tested the hypothesis that L-Trp may activate mammalian target of rapamycin (mTOR) complex 1 and enhance expression of tight junction (TJ) proteins in intestinal porcine epithelial cells.

Methods: Jejunal enterocytes, intestinal porcine epithelial cell line 1 (IPEC-1) isolated from newborn pigs, were cultured in customized Dulbecco's modified Eagle medium (DMEM) supplemented with or without L-Trp for the indicated time periods. Cell proliferation, L-Trp metabolism, protein turnover, mRNA abundance for L-Trp transporters [solute carrier family 3 member 1 (SLC3A1), solute carrier family 6 member 14 (SLC6A14), solute carrier family 6 member 19 (SLC6A19), and Na(+)/K(+) ATPase subunit-α1 (ATP1A1)], abundance of proteins involved in mTOR signaling, and TJ proteins were determined.

Results: L-Trp was not degraded in IPEC-1 cells. Compared with basal medium containing 0.04 mmol/L L-Trp, 0.4 and 0.8 mmol/L L-Trp enhanced (P < 0.05) protein synthesis by 45-52% and cell growth by 17% and 25% on day 1 and 72% and 51% on day 2, respectively, while reducing (P < 0.05) protein degradation by 12% and 22%, respectively. These effects of L-Trp were associated with mTOR activation and increased (P < 0.05) mRNA abundance for L-Trp transporters (SLC6A19, SLC6A14, and SLC3A1) by 1.5-2.7 fold and ATP1A1 by 3 fold. L-Trp also upregulated (P < 0.05) the abundance of occludin, claudin-4, zonula occludens (ZO) 1 and 2 by 0.5-2 fold but did not affect expression of claudin-1 or ZO-3 in IPEC-1 cells.

Conclusion: L-Trp is not catabolized by pig small intestinal epithelial cells but can regulate intracellular protein turnover and expression of TJ proteins in these cells.

Keywords: cell proliferation; intestinal epithelial cells; mammalian target of rapamycin; pig; tight junction; tryptophan.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Animals
  • Cell Proliferation / drug effects
  • Cells, Cultured
  • Claudin-1 / genetics
  • Claudin-1 / metabolism
  • Claudin-4 / genetics
  • Claudin-4 / metabolism
  • Enterocytes / drug effects*
  • Enterocytes / metabolism
  • Intestinal Mucosa / metabolism
  • Intestine, Small / drug effects
  • Intestine, Small / metabolism
  • Intestines / cytology
  • Intestines / drug effects*
  • Mechanistic Target of Rapamycin Complex 1
  • Multiprotein Complexes / genetics
  • Multiprotein Complexes / metabolism*
  • Occludin / genetics
  • Occludin / metabolism
  • RNA, Messenger / genetics
  • RNA, Messenger / metabolism
  • Signal Transduction
  • Swine
  • TOR Serine-Threonine Kinases / genetics
  • TOR Serine-Threonine Kinases / metabolism*
  • Tight Junction Proteins / genetics
  • Tight Junction Proteins / metabolism*
  • Tight Junctions / drug effects
  • Tight Junctions / metabolism
  • Tryptophan / pharmacology*
  • Up-Regulation

Substances

  • Claudin-1
  • Claudin-4
  • Multiprotein Complexes
  • Occludin
  • RNA, Messenger
  • Tight Junction Proteins
  • Tryptophan
  • Mechanistic Target of Rapamycin Complex 1
  • TOR Serine-Threonine Kinases