Abstract
Absorption of neutral amino acids across the luminal membrane of intestinal enterocytes is mediated by the broad neutral amino acid transporter B0AT1 (SLC6A19). Its intestinal expression depends on co-expression of the membrane-anchored peptidase angiotensin converting enzyme 2 (ACE2) and is additionally enhanced by aminopeptidase N (CD13). We investigated in this study the expression of B0AT1 and its auxiliary peptidases as well as its transport function along the rat small intestine. Additionally, we tested its possible short- and long-term regulation by dietary proteins and amino acids. We showed by immunofluorescence that B0AT1, ACE2 and CD13 co-localize on the luminal membrane of small intestinal villi and by Western blotting that their protein expression increases in distal direction. Furthermore, we observed an elevated transport activity of the neutral amino acid L-isoleucine during the nocturnal active phase compared to the inactive one. Gastric emptying was delayed by intragastric application of an amino acid cocktail but we observed no acute dietary regulation of B0AT1 protein expression and L-isoleucine transport. Investigation of the chronic dietary regulation of B0AT1, ACE2 and CD13 by different diets revealed an increased B0AT1 protein expression under amino acid-supplemented diet in the proximal section but not in the distal one and for ACE2 protein expression a reverse localization of the effect. Dietary regulation for CD13 protein expression was not as distinct as for the two other proteins. Ring uptake experiments showed a tendency for increased L-isoleucine uptake under amino acid-supplemented diet and in vivo L-isoleucine absorption was more efficient under high protein and amino acid-supplemented diet. Additionally, plasma levels of branched-chain amino acids were elevated under high protein and amino acid diet. Taken together, our experiments did not reveal an acute amino acid-induced regulation of B0AT1 but revealed a chronic dietary adaptation mainly restricted to the proximal segment of the small intestine.
Highlights
Proteins in ingested food are hydrolyzed to small oligopeptides and amino acids on their way from the oral cavity to the small intestine where they are absorbed across the mucosa
Three independent experiments have been performed in which rats were fed either a normal protein diet (NP), a high protein diet (HP) or a normal protein diet supplemented with amino acids (AA) and data were subsequently pooled, because no dietary effect on the expression of B0AT1 and/or angiotensin converting enzyme 2 (ACE2) mRNA was observed
The impact of nutrition on function and expression of the neutral amino acid transporter B0AT1 and its auxiliary peptidases in small intestine was investigated in this study
Summary
Proteins in ingested food are hydrolyzed to small oligopeptides and amino acids on their way from the oral cavity to the small intestine where they are absorbed across the mucosa. There is yet not much known about the dietary regulation of amino acid transporters in contrast to that of the diand tripeptide transporter PepT1 (SLC15A1) This proton/peptide co-transporter is transporting small oligopeptides and a range of peptide-like drugs through the brush-border membrane of intestinal epithelial cells [23, 24]. The few published studies about dietary regulation of amino acid transporters have revealed for instance that a high protein diet leads to an increased uptake of some amino acids and to an increased mRNA level of the apical Excitatory Amino Acid Transporter 3 (EAAT3, SLC1A1) [26, 29]. We tested the potential co-regulation of the associated peptidases ACE2 and CD13 along the small intestine taking a possible diurnal regulation into account
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