High-level L-Gln compromises intestinal amino acid utilization efficiency and inhibits protein synthesis by GCN2/eIF2α/ATF4 signaling pathway in piglets fed low-crude protein diets

Gln, one of the most abundant amino acids (AA) in the body, performs a diverse range of fundamental physiological functions. However, information about the role of dietary Gln on AA levels, transporters, protein synthesis, and underlying mechanisms in vivo is scarce. The present study aimed to explore the effects of low-crude protein diet inclusion with differential doses of L-Gln on intestinal AA levels, transporters, protein synthesis, and potential mechanisms in weaned piglets. A total of 128 healthy weaned piglets (Landrace × Yorkshire) were randomly allocated into four treatments with four replicates. Pigs in the four groups were fed a low-crude protein diet containing 0%, 1%, 2%, or 3% L-Gln for 28 d. L-Gln administration markedly (linear, <i>P</i> < 0.05) increased Ala, Arg, Asn, Asp, Glu, Gln, His, Ile, Lys, Met, Orn, Phe, Ser, Thr, Tyr, and Val levels and promoted trypsin activity in the jejunal content of piglets. Moreover, L-Gln treatment significantly enhanced concentrations of colonic Gln and Trp, and serum Thr (linear, <i>P</i> < 0.01), and quadratically increased serum Lys and Phe levels (<i>P</i> < 0.05), and decreased plasma Glu, Ile, and Leu levels (linear, <i>P</i> < 0.05). Further investigation revealed that L-Gln administration significantly upregulated <i>Atp1a1</i>, <i>Slc1a5</i>, <i>Slc3a2</i>, <i>Slc6a14</i>, <i>Slc7a5</i>, <i>Slc7a7</i>, and <i>Slc38a1</i> relative expressions in the jejunum (linear, <i>P</i> < 0.05). Additionally, dietary supplementation with L-Gln enhanced protein abundance of general control nonderepressible 2 (GCN2, <i>P</i> = 0.010), phosphorylated eukaryotic initiation factor 2 subunit alpha (eIF2α, <i>P</i> < 0.001), and activating transcription factor 4 (ATF4) in the jejunum of piglets (<i>P</i> = 0.008). These results demonstrated for the first time that a low crude protein diet with high-level L-Gln inclusion exhibited side effects on piglets. Specifically, 2% and 3% L-Gln administration exceeded the intestinal utilization capacity and compromised the jejunal AA utilization efficiency, which is independent of digestive enzyme activities. A high level of L-Gln supplementation would inhibit protein synthesis by GCN2/eIF2α/ATF4 signaling in piglets fed low-protein diets, which, in turn, upregulates certain AA transporters to maintain AA homeostasis.