Regulation of protein turnover by L-glutamine in porcine intestinal epithelial cells.

L-Glutamine (Gln) plays an important role in sustaining the intestinal mucosal mass of humans and animals. However, the underlying mechanisms are largely unknown. This study tested the hypothesis that Gln regulates protein turnover in intestinal epithelial cells. Intestinal porcine epithelial cells (IPEC-1) were cultured for 3 h (short-term study) or 96 h (long-term study) in Gln-free Dulbecco's modified Eagle-F12 Ham medium containing 0, 0.5 or 2.0 mM Gln. To determine effects of ammonia (a metabolite of Gln, i.e., 0.18 mM ammonia produced from 2 mM Gln in 3 h) on protein turnover, additional experiments were conducted in which medium contained 0.5 mM Gln and 0, 0.2, 0.5 or 2.0 mM NH(4)Cl. Variables of analysis included cell growth, protein synthesis, proteolysis and mammalian target of rapamycin (mTOR) signaling. IPEC-1 cell growth increased with extracellular Gln concentrations. Compared with 0 mM Gln, the addition of 0.5 and 2 mM Gln to medium stimulated protein synthesis and inhibited protein degradation in those cells in both the short- and long-term studies. Ammonia (0.05 to 2.0 mM) did not affect protein synthesis, although higher levels of ammonia (0.5 and 2.0 mM) reduced protein degradation in IPEC-1 cells. Consistent with the data on protein turnover, 0.5 and 2 mM Gln increased abundance of phosphorylated eIF4E-binding protein-1 and phosphorylated S6 kinase-1 proteins. Collectively, these results demonstrate that physiological levels of Gln regulate protein turnover independent of ammonia production in intestinal cells through the mTOR signaling pathway.