Amino acid transport across mammalian intestinal and renal epithelia

Circadian Regulation of Macronutrient Absorption

Various intestinal functions exhibit circadian rhythmicity. Disruptions in these rhythms as in shift workers and transcontinental travelers are associated with intestinal discomfort. Circadian rhythms are controlled at the molecular level by core clock and clock-controlled genes. These clock genes are expressed in intestinal cells, suggesting that they might participate in the circadian regulation of intestinal functions. A major function of the intestine is nutrient absorption. Here, we will review absorption of proteins, carbohydrates, and lipids and circadian regulation of various transporters involved in their absorption. A better understanding of circadian regulation of intestinal absorption might help control several metabolic disorders and attenuate intestinal discomfort associated with disruptions in sleep-wake cycles.

LAT1 is the transport competent unit of the LAT1/CD98 heterodimeric amino acid transporter

LAT1 (SLC7A5) and CD98 (SLC3A2) constitute a heterodimeric transmembrane protein complex that catalyzes amino acid transport. Whether one or both subunits are competent for transport is still unclear. The present work aims to solve this question using different experimental strategies. Firstly, LAT1 and CD98 were immuno-detected in protein extracts from SiHa cells. Under oxidizing conditions, i.e., without addition of SH (thiol) reducing agent DTE, both proteins were revealed as a 120kDa major band. Upon DTE treatment separated bands, corresponding to LAT1(35kDa) or CD98(80kDa), were detected. LAT1 function was evaluated in intact cells as BCH sensitive [(3)H]His transport inhibited by hydrophobic amino acids. Antiport of [(3)H]His was measured in proteoliposomes reconstituted with SiHa cell extract in presence of internal His. Transport was increased by DTE. Hydrophobic amino acids were best inhibitors in addition to hydrophilic Tyr, Gln, Asn and Lys. Cys, Tyr and Gln, included in the intraliposomal space, were transported in antiport with external [(3)H]His. Similar experiments were performed in proteoliposomes reconstituted with the recombinant purified hLAT1. Results overlapping those obtained with native protein were achieved. Lower transport of [(3)H]Leu and [(3)H]Gln with respect to [(3)H]His was detected. Kinetic asymmetry was found with external Km for His lower than internal one. No transport was detected in proteoliposomes reconstituted with recombinant hCD98. The experimental data demonstrate that LAT1 is the sole transport competent subunit of the heterodimer. This conclusion has important outcome for following studies on functional characterization and identification of specific inhibitors with potential application in human therapy.

Growth performance, serum biochemical profile, jejunal morphology, and the expression of nutrients transporter genes in deoxynivalenol (DON)- challenged growing pigs

BACKGROUND

Fusarium infection with concurrent production of deoxynivalenol (DON) causes an increasing safety concern with feed worldwide. This study was conducted to determine the effects of varying levels of DON in diets on growth performance, serum biochemical profile, jejunal morphology, and the differential expression of nutrients transporter genes in growing pigs.

RESULTS

A total of twenty-four 60-day-old healthy growing pigs (initial body weight = 16.3 ± 1.5 kg SE) were individually housed and randomly assigned to receive one of four diets containing 0, 3, 6 or 12 mg DON/kg feed for 21 days. Differences were observed between control and the 12 mg/kg DON treatment group with regards to average daily gain (ADG), although the value for average daily feed intake (ADFI) in the 3 mg/kg DON treatment group was slightly higher than that in control (P<0.01). The relative liver weight in the 12 mg/kg DON treatment group was significantly greater than that in the control (P<0.01), but there were no significant differences in other organs. With regard to serum biochemistry, the values of blood urea nitrogen (BUN), alkaline phosphatase (ALP), alanine aminotransferase (ALT) and aspartate amino transferase (AST) in the 3 treatment groups were higher than those in the control, and the serum concentrations of L-valine, glycine, L-serine, and L-glutamine were significantly reduced in the 3 treatment groups, especially in the 12 mg/kg DON group (P<0.01). Serum total superoxide dismutase (T-SOD), glutathione peroxidase (GSH-Px) were markedly decreased after exposure to DON contaminated feeds (P<0.01). The villi height was markedly decreased and the lymphocyte cell numbers markedly increased in the 3 DON contaminated feeds (P<0.01). The mRNA expression levels of excitatory amino acid transporter-3 (EAAC-3), sodium-glucose transporter-1 (SGLT-1), dipeptide transporter-1 (PepT-1), cationic amino acid transporter-1 (CAT-1) and y(+)L-type amino acid transporter-1 (LAT-1) in control were slightly or markedly higher than those in the 3 DON treatment groups.

CONCLUSIONS

These results showed that feeds containing DON cause a wide range of effects in a dose-dependent manner. Such effects includes weight loss, live injury and oxidation stress, and malabsorption of nutrients as a result of selective regulation of nutrient transporter genes such as EAAC-3, SGLT-1, PepT-1, CAT-1 and LAT-1.

Effect of supplementing a fibrous diet with a xylanase and β-glucanase blend on growth performance, intestinal glucose uptake, and transport-associated gene expression in growing pigs

The present study evaluated supplemental carbohydrase effect on performance, intestinal nutrient uptake, and transporter mRNA expressions in growing pigs offered a high-fiber diet manufactured with distillers dried grains with solubles (DDGS). Twenty-four pigs (22.4 ± 0.7 kg BW) were randomly assigned to 1of 3 nutritionally adequate diets (8 pigs per diet) based on corn and soybean meal (SBM) with either 0 (control) or 30% DDGS (high fiber [HF]). The third diet was supplemented with a xylanase and β-glucanase blend (XB) in addition to the 30% DDGS (HF+XB). Parameters determined were ADFI, ADG, G:F, plasma glucose and plasma urea nitrogen (PUN) concentrations, jejunal tissue electrophysiological properties, and mRNA expressions of the sodium-dependent glucose transport 1 (SGLT1) and cationic AA transporter, bo,+AT, in the jejunal and ileal tissues. In addition, mRNA expressions of the short-chain fatty acid transporters, monocarboxylate transporter 1 (MCT1) and sodium-coupled monocarboxylate transporter, and mucin genes were quantified in the ileum. Feed intake, plasma glucose, and jejunal tissue electrophysiological properties were not affected (P > 0.05) by diet. However, control-fed pigs had superior growth rate and feed efficiency and higher PUN (P < 0.05) than HF- and HF+XB-fed pigs. The HF diet increased (P < 0.05) SGLT1 mRNA expression in the jejunum and decreased (P < 0.05) bo,+ mRNA expression in the ileum. The XB supplementation also increased bo,+ mRNA expression in the ileum relative to HF-fed pigs. Additionally, MCT1 mRNA expression was greater (P < 0.05) in the ileum of the HF- and HF+XB-fed pigs. In the present study, XB supplementation influenced nutrient transporter mRNA expression, although it was not accompanied by improved pig performance.

Characterization and Regulation of the Amino Acid Transporter SNAT2 in the Small Intestine of Piglets

The sodium-dependent neutral amino acid transporter 2 (SNAT2), which has dual transport/receptor functions, is well documented in eukaryotes and some mammalian systems, but has not yet been verified in piglets. The objective of this study was to investigate the characteristics and regulation of SNAT2 in the small intestine of piglets. The 1,521-bp porcine full cDNA sequence of SNAT2 (KC769999) from the small intestine of piglets was cloned. The open reading frame of cDNA encodes 506 deduced amino acid residues with a calculated molecular mass of 56.08 kDa and an isoelectric point (pI) of 7.16. Sequence alignment and phylogenetic analysis revealed that SNAT2 is highly evolutionarily conserved in mammals. SNAT2 mRNA can be detected in the duodenum, jejunum and ileum by real-time quantitative PCR. During the suckling period from days 1 to 21, the duodenum had the highest abundance of SNAT2 mRNA among the three segments of the small intestine. There was a significant decrease in the expression of SNAT2 mRNA in the duodenal and jejunal mucosa and in the expression of SNAT2 protein in the jejunal and ileal mucosa on day 1 after weaning (P < 0.05). Studies with enterocytes in vitro showed that amino acid starvation and supplementation with glutamate, arginine or leucine enhanced, while supplementation with glutamine reduced, SNAT2 mRNA expression (P < 0.05). These results regarding the characteristics and regulation of SNAT2 should help to provide some information to further clarify its roles in the absorption of amino acids and signal transduction in the porcine small intestine.

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

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.

Intestinal B(0)AT1 (SLC6A19) and PEPT1 (SLC15A1) mRNA levels in European sea bass (Dicentrarchus labrax) reared in fresh water and fed fish and plant protein sources

The objective of the present study was to examine the effect of diets with descending fish meal (FM) inclusion levels and the addition of salt to the diet containing the lowest FM level on growth performances, feed conversion ratio, and intestinal solute carrier family 6 member 19 (SLC6A19) and oligopeptide transporter 1 (PEPT1) transcript levels, in freshwater-adapted European sea bass (Dicentrarchus labrax). We first isolated by molecular cloning and sequenced a full-length cDNA representing the neutral amino acid transporter SLC6A19 in sea bass. The cDNA sequence was deposited in GenBank database (accession no. KC812315). The twelve transmembrane domains and the ‘de novo’ prediction of the three-dimensional structure of SLC6A19 protein (634 amino acids) are presented. We then analysed diet-induced changes in the mRNA copies of SLC6A19 and PEPT1 genes in different portions of sea bass intestine using real-time RT-PCR. Sea bass were fed for 6 weeks on different diets, with ascending levels of fat or descending levels of FM, which was replaced with vegetable meal. The salt-enriched diet was prepared by adding 3 % NaCl to the diet containing 10 % FM. SLC6A19 mRNA in the anterior and posterior intestine of sea bass were not modulated by dietary protein sources and salt supplementation. Conversely, including salt in a diet containing a low FM percentage up-regulated the mRNA copies of PEPT1 in the hindgut. Fish growth correlated positively with the content of FM in the diets. Interestingly, the addition of salt to the diet containing 10 % FM improved feed intake, as well as specific growth rate and feed conversion ratio.

The in ovo administration of L-trans pyrrolidine-2,4-dicarboxylic acid regulates small intestinal growth in chicks

Glutamate, which is one of the most important contributors to oxidative metabolism in the intestinal mucosa, is mainly transported by the excitatory amino acids transporters (EAATs) that are expressed in enterocytes. The objective of this study was to evaluate the effects of in ovo administration of l-trans pyrrolidine-2,4-dicarboxylic acid (l-trans-PDC), a potent competitive inhibitor of glutamate uptake by EAATs, on the growth of the small intestine in chicks. Two series of experiments were conducted with hatching eggs; 100 μl of various l-trans-PDC solutions (0, 0.075 or 0.225 mg/egg for the Control group, low-dose l-trans pyrrolidine 2,4-dicarboxylic acid group (L-PDC) or high-dose l-trans pyrrolidine 2,4-dicarboxylic acid group (H-PDC), respectively) was injected into the albumen sac of these hatching eggs before incubation. Hatchlings were sacrificed by cervical dislocation to determine the embryonic development in Experiment I, whereas the birds in Experiment II were raised or sampled at hatching, days 7 and 14 (D7 and D14) for further study. Gene expression in the small intestines was determined by real-time RT-PCR; and serum concentration of free amino acids was determined by an amino acid analyzer. The results showed that the hatchability was decreased by in ovo administration of l-trans-PDC. The small intestinal weights of the H-PDC group were decreased (P<0.05) at hatching and increased (P<0.05) on D7 and D14 compared with those in the Control group. In addition, the gene expression of EAAT2 in the completed or segmental small intestines was not changed (P>0.05); EAAT3 gene expression in the duodenum (P<0.05), jejunum (P=0.084) and ileum (P=0.060) on D14 was lower in the H-PDC group than in the Control group. Furthermore, the serum concentrations of free proline, threonine and phenylalanine but not glutamate or aspartate were increased (P<0.06) in H-PDC group. In conclusion, this paper is the first to report that in ovo administration of l-trans-PDC induces small intestinal growth retardation during the embryonic period and catch-up growth after hatching.

Mice lacking neutral amino acid transporter B(0)AT1 (Slc6a19) have elevated levels of FGF21 and GLP-1 and improved glycaemic control

OBJECTIVE

Type 2 diabetes arises from insulin resistance of peripheral tissues followed by dysfunction of β-cells in the pancreas due to metabolic stress. Both depletion and supplementation of neutral amino acids have been discussed as strategies to improve insulin sensitivity. Here we characterise mice lacking the intestinal and renal neutral amino acid transporter B(0)AT1 (Slc6a19) as a model to study the consequences of selective depletion of neutral amino acids.

METHODS

Metabolic tests, analysis of metabolite levels and signalling pathways were used to characterise mice lacking the intestinal and renal neutral amino acid transporter B(0)AT1 (Slc6a19).

RESULTS

Reduced uptake of neutral amino acids in the intestine and loss of neutral amino acids in the urine causes an overload of amino acids in the lumen of the intestine and reduced systemic amino acid availability. As a result, higher levels of glucagon-like peptide 1 (GLP-1) are produced by the intestine after a meal, while the liver releases the starvation hormone fibroblast growth factor 21 (FGF21). The combination of these hormones generates a metabolic phenotype that is characterised by efficient removal of glucose, particularly by the heart, reduced adipose tissue mass, browning of subcutaneous white adipose tissue, enhanced production of ketone bodies and reduced hepatic glucose output.

CONCLUSIONS

Reduced neutral amino acid availability improves glycaemic control. The epithelial neutral amino acid transporter B(0)AT1 could be a suitable target to treat type 2 diabetes.

Metabolism, excretion, and pharmacokinetics of S-allyl-L-cysteine in rats and dogs

The metabolism, excretion, and pharmacokinetics of S-allyl-l-cysteine (SAC), an active key component of garlic supplements, were examined in rats and dogs. A single dose of SAC was administered orally or i.v. to rats (5 mg/kg) and dogs (2 mg/kg). SAC was well absorbed (bioavailability >90%) and its four metabolites-N-acetyl-S-allyl-l-cysteine (NAc-SAC), N-acetyl-S-allyl-l-cysteine sulfoxide (NAc-SACS), S-allyl-l-cysteine sulfoxide (SACS), and l-γ-glutamyl-S-allyl-l-cysteine-were identified in the plasma and/or urine. Renal clearance values (<0.01 l/h/kg) of SAC indicated its extensive renal reabsorption, which contributed to the long elimination half-life of SAC, especially in dogs (12 hours). The metabolism of SAC to NAc-SAC, principal metabolite of SAC, was studied in vitro and in vivo. Liver and kidney S9 fractions of rats and dogs catalyzed both N-acetylation of SAC and deacetylation of NAc-SAC. After i.v. administration of NAc-SAC, SAC appeared in the plasma and its concentration declined in parallel with that of NAc-SAC. These results suggest that the rate and extent of the formation of NAc-SAC are determined by the N-acetylation and deacetylation activities of liver and kidney. Also, NAc-SACS was detected in the plasma after i.v. administration of either NAc-SAC or SACS, suggesting that NAc-SACS could be formed via both N-acetylation of SACS and S-oxidation of NAc-SAC. In conclusion, this study demonstrated that the pharmacokinetics of SAC in rats and dogs is characterized by its high oral bioavailability, N-acetylation and S-oxidation metabolism, and extensive renal reabsorption, indicating the critical roles of liver and kidney in the elimination of SAC.

Low-protein amino acid-supplemented diets for growing pigs: effect on expression of amino acid transporters, serum concentration, performance, and carcass composition

Pigs fed protein-bound AA appear to have a higher abundance of AA transporters for their absorption in the jejunum compared with the duodenum. However, there is limited data about the effect of dietary free AA, readily available in the duodenum, on the duodenal abundance of AA transporters and its impact on pig performance. Forty-eight pigs (24.3 kg initial BW) distributed in 4 treatments were used to evaluate the effect of the CP level and form (free vs. protein bound) in which AA are added to diets on the expression of AA transporters in the 3 small intestine segments, serum concentration of AA, and performance. Dietary treatments based on wheat and soybean meal (SBM) were 1) low-CP (14%) diet supplemented with L-Lys, L-Thr, DL-Met, L-Leu, L-Ile, L-Val, L-His, L-Trp, and L-Phe (LPAA); 2) as in the LPAA but with added L-Gly as a N source (LPAA+N); 3) intermediate CP content (16%) supplemented with L-Lys HCl, L-Thr, and DL-Met (MPAA); and 4) high-CP (22%) diet (HP) without free AA. At the end of the experiment, 8 pigs from LPAA and HP were sacrificed to collect intestinal mucosa and blood samples and to dissect the carcasses. There were no differences in ADG, ADFI, G:F, and weights of carcass components and some visceral organs between treatments. Weights of the large intestine and kidney were higher in HP pigs (P < 0.01). Expression of b(0,+) in the duodenum was higher in pigs fed the LPAA compared with the HP diet (P= 0.036) but there was no difference in the jejunum and ileum. In the ileum, y+ L expression tended to be higher in pigs fed the LPAA diet (P = 0.098). Expression of b(0,+) in LPAA pigs did not differ between the duodenum and the jejunum, but in HP pigs, the expression of all AA transporters was higher in the jejunum than in the duodenum or ileum (P < 0.05). The serum concentration of Arg, His, Ile, Leu, Phe, and Val was higher but serum Lys and Met were lower in pigs fed the HP diet (P < 0.05). These results indicate that LPAA can substitute up to 8 percentage units of protein in HP wheat-SBM diets without affecting pig performance; nonessential N does not seem to be limiting in very low-protein wheat-SBM diets for growing pigs. Also, the inclusion of free AA in the diet appears to affect their serum concentration and the expression of the AA transporter b0,+ in the duodenum of pigs.

Differentiation of malignant tumours from granulomas by using dynamic [(18)F]-fluoro-L-α-methyltyrosine positron emission tomography

Background

Previous clinical studies have revealed the potential of [¹⁸F]-fluoro-L-α-methyltyrosine (¹⁸F-FAMT) for the differential diagnosis of malignant tumours from sarcoidosis. However, one concern regarding the differential diagnosis with ¹⁸F-FAMT is the possibility of false negatives given the small absolute uptake of ¹⁸F-FAMT that has been observed in some malignant tumours. The aim of this study was to evaluate a usefulness of dynamic ¹⁸F-FAMT positron emission tomography (PET) for differentiating malignant tumours from granulomas.

Methods

Rats bearing both granulomas (Mycobacterium bovis bacillus Calmette-Guérin (BCG)-induced) and tumours (C6 glioma cell-induced) underwent dynamic 2-deoxy-2-[¹⁸F]-fluoro-D-glucose (¹⁸F-FDG) PET and ¹⁸F-FAMT PET for 120 min on consecutive days. Time-activity curves, static images, mean standardized uptake values (SUVs) and the SUV ratios (SUVRs; calculated by dividing SUV at each time point by that of 2 min after injection) were assessed.

Results

In tumours, ¹⁸F-FAMT showed a shoulder peak immediately after the initial distribution followed by gradual clearance compared with granulomas. Although the mean SUV in the tumours (1.00 ± 0.10) was significantly higher than that in the granulomas (0.88 ± 0.12), a large overlap was observed. In contrast, the SUVR was markedly higher in tumours than in granulomas (50 min/2 min, 0.72 ± 0.06 and 0.56 ± 0.05, respectively) with no overlap. The dynamic patterns, SUVR, and mean SUV of ¹⁸F-FDG in the granulomas were comparable to those in the tumours.

Conclusions

Dynamic ¹⁸F-FAMT and SUVR analysis might compensate for the current limitations and help in improving the diagnostic accuracy of ¹⁸F-FAMT.

Gene-based and pathway-based genome-wide association study of alcohol dependence

BACKGROUND

The organization of risk genes within signaling pathways may provide clues about the converging neurobiological effects of risk genes for alcohol dependence.

AIM

Identify risk genes and risk gene pathways for alcohol dependence.

METHODS

We conducted a pathway-based genome-wide association study (GWAS) of alcohol dependence using a gene-set-rich analytic approach. Approximately one million genetic markers were tested in the discovery sample which included 1409 European-American (EA) alcohol dependent individuals and 1518 EA healthy comparison subjects. An additional 681 African-American (AA) cases and 508 AA healthy subjects served as the replication sample.

RESULTS

We identified several genome-wide replicable risk genes and risk pathways that were significantly associated with alcohol dependence. After applying the Bonferroni correction for multiple testing, the 'cellextracellular matrix interactions' pathway (p<2.0E-4 in EAs) and the PXN gene (which encodes paxillin) (p=3.9E-7 in EAs) within this pathway were the most promising risk factors for alcohol dependence. There were also two nominally replicable pathways enriched in alcohol dependence-related genes in both EAs (0.015≤p≤0.035) and AAs (0.025≤p≤0.050): the 'Na+/Cl- dependent neurotransmitter transporters' pathway and the 'other glycan degradation' pathway.

CONCLUSION

These findings provide new evidence highlighting several genes and biological signaling processes that may be related to the risk for alcohol dependence.

The mTORC1/4E-BP pathway coordinates hemoglobin production with L-leucine availability

In multicellular organisms, the mechanisms by which diverse cell types acquire distinct amino acids and how cellular function adapts to their availability are fundamental questions in biology. We found that increased neutral essential amino acid (NEAA) uptake was a critical component of erythropoiesis. As red blood cells matured, expression of the amino acid transporter gene Lat3 increased, which increased NEAA import. Inadequate NEAA uptake by pharmacologic inhibition or RNAi-mediated knockdown of LAT3 triggered a specific reduction in hemoglobin production in zebrafish embryos and murine erythroid cells through the mTORC1 (mammalian target of rapamycin complex 1)/4E-BP (eukaryotic translation initiation factor 4E-binding protein) pathway. CRISPR-mediated deletion of members of the 4E-BP family in murine erythroid cells rendered them resistant to mTORC1 and LAT3 inhibition and restored hemoglobin production. These results identify a developmental role for LAT3 in red blood cells and demonstrate that mTORC1 serves as a homeostatic sensor that couples hemoglobin production at the translational level to sufficient uptake of NEAAs, particularly L-leucine.

Comprehensive cytotoxic evaluation of morin, a bioflavonoid against verapamil on rat gastrointestinal epithelium for novel pharmaceutical application involving P-glycoprotein inhibition

OBJECTIVE

In this study, a comprehensive and comparative cytotoxic evaluation of morin against verapamil on rat intestinal epithelium as P-gp inhibitors through in-vitro gastrointestinal short-term toxicity assays involving permeability studies for safety evaluation was investigated.

METHODS

In this study, the effect of morin (1 mM or 10 mM) or verapamil (1 mM or 10 mM) or sodium deoxycholate (10 mM) was investigated on intestinal epithelium and isolated brush border membrane using biomarker assays. Cytotoxicity was determined using 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) assay. The nutrients transport was assessed using everted sacs studies. Paracellular permeability was measured using Lucifer yellow, followed by morphometric analysis of intestinal sacs.

KEY FINDINGS

Our results indicated that morin was effective in maintaining cell viability with no significant changes (P > 0.05) in the activity of intestinal brush border markers, membrane integrity and morphometric analysis as compared with control. On the contrary, dramatic (P < 0.01) changes were noticed in the release of membrane markers, cell viability and surface characteristics of intestinal segments when treated with verapamil or sodium deoxycholate as compared with control or morin.

CONCLUSIONS

Our findings confirm that morin is non-toxic to rat intestinal epithelium against verapamil demonstrating the potential use of bioflavonoid as safe and novel pharmaceutical adjuvant as P-gp inhibitor.

mRNA expression of amino acid transporters, aminopeptidase, and the di- and tri-peptide transporter PepT1 in the intestine and liver of posthatch broiler chicks

Amino acid (AA) transporter proteins are responsible for the movement of amino acids in and out of cells. Aminopeptidase cleaves AAs from the N-terminus of polypeptides making them available for transport, while PepT1 is a di- and tripeptide transporter. In the intestine, these proteins are present on the brush border and basolateral membranes of enterocytes, and are essential for the uptake of AAs into enterocytes and their release into circulation. The purpose of this study was to determine the level of transcription of these genes after hatch in 3 regions of the small intestine, the ceca, and liver. Heritage broiler chicks (n=5) were sampled at day after hatch and days 3, 5, 7, 10, 12, 14, 17, and 21 posthatch, and mRNA expression level was measured using absolute quantitation. The small intestine (duodenum, jejunum, and ileum) expressed the largest quantities of each gene tested. The expression in the ceca and liver was 1 to 3 orders of magnitude less than that of the small intestine. The expression of basolateral transporters in the small intestine was more constant over days posthatch than the expression of brush border transporters. In the ceca the expression of the brush border transporters decreased over the sampling period, while expression of basolateral genes was relatively constant. In the liver the expression of Na+ independent cationic and zwitterionic amino acid transporter (bo,+AT), Na+ independent cationic amino acid transporter 2 (CAT2), excitatory amino acid transporter 3 (EAAT3), and the heavy chain corresponding to the bo,+) system (rBAT) significantly decreased at 12 days posthatch; however, the expression of Na+ independent cationic and Na+ dependent neutral amino acid transporter 1 (y+LAT1), Na+ coupled neutral amino acid transporter 1; (SNAT1), and Na+ coupled neutral amino acid transporter 2 (SNAT2) significantly increased at day 5 posthatch compared to day 1 and these levels remained throughout the rest of the sampling period. The current results suggest that at 1 day posthatch chicks are capable of AA processing and transport in the intestine as well as the liver. Additionally the ability of the ceca in transporting AA from the lumen may decrease with age. The liver should be capable of amino acid transport, but its capabilities may be more specific since the expression of several transporters in this organ is either absent or very low.

Regulation of mammalian nucleotide metabolism and biosynthesis

Nucleotides are required for a wide variety of biological processes and are constantly synthesized de novo in all cells. When cells proliferate, increased nucleotide synthesis is necessary for DNA replication and for RNA production to support protein synthesis at different stages of the cell cycle, during which these events are regulated at multiple levels. Therefore the synthesis of the precursor nucleotides is also strongly regulated at multiple levels. Nucleotide synthesis is an energy intensive process that uses multiple metabolic pathways across different cell compartments and several sources of carbon and nitrogen. The processes are regulated at the transcription level by a set of master transcription factors but also at the enzyme level by allosteric regulation and feedback inhibition. Here we review the cellular demands of nucleotide biosynthesis, their metabolic pathways and mechanisms of regulation during the cell cycle. The use of stable isotope tracers for delineating the biosynthetic routes of the multiple intersecting pathways and how these are quantitatively controlled under different conditions is also highlighted. Moreover, the importance of nucleotide synthesis for cell viability is discussed and how this may lead to potential new approaches to drug development in diseases such as cancer.

Differences in plasma metabolomics between sows feddl-methionine and its hydroxy analogue reveal a strong association of milk composition and neonatal growth with maternal methionine nutrition

The aim of the present study was to determine whether increased consumption of methionine asdl-methionine (DLM) or its hydroxy analoguedl-2-hydroxy-4-methylthiobutanoic acid (HMTBA) could benefit milk synthesis and neonatal growth. For this purpose, eighteen cross-bred (Landrace × Yorkshire) primiparous sows were fed a control (CON), DLM or HMTBA diet (n6 per diet) from 0 to 14 d post-partum. At postnatal day 14, piglets in the HMTBA group had higher body weight (P= 0·02) than those in the CON group, tended (P= 0·07) to be higher than those in the DLM group, and had higher (P< 0·05) mRNA abundance of jejunal fatty acid-binding protein 2, intestinal than those in the CON and DLM groups. Compared with the CON diet-fed sows, milk protein, non-fat solid, and lysine, histidine and ornithine concentrations decreased in the DLM diet-fed sows (P< 0·05), and milk fat, lactose, and cysteine and taurine concentrations increased in the HMTBA diet-fed sows (P< 0·05). Plasma homocysteine and urea N concentrations that averaged across time were increased (P< 0·05) in sows fed the DLM diet compared with those fed the CON diet. Metabolomic results based on1H NMR spectroscopy revealed that consumption of the HMTBA and DLM diets increased (P< 0·05) both sow plasma methionine and valine levels; however, consumption of the DLM diet led to lower (P< 0·05) plasma levels of lysine, tyrosine, glucose and acetate and higher (P< 0·05) plasma levels of citrate, lactate, formate, glycerol,myo-inositol andN-acetyl glycoprotein in sows. Collectively, neonatal growth and milk synthesis were regulated by dietary methionine levels and sources, which resulted in marked alterations in amino acid, lipid and glycogen metabolism.

Effects of heat stress on the gene expression of nutrient transporters in the jejunum of broiler chickens (Gallus gallus domesticus)

In broiler chickens, heat stress disrupts nutrient digestion and absorption. However, the underlying molecular mechanism is not clearly understood. Hence, to investigate the effects of high ambient temperatures on the expression levels of nutrient transporters in the jejunum of broiler chickens, seventy-two 35-day-old male broiler chickens with similar body weights were randomly allocated into two groups: control (24 ± 1 °C) and heat-stressed (32 ± 1 °C). The chickens in the heat-stressed group were exposed to 10 h of heat daily from 08:00 to 18:00 and then raised at 24 ± 1 °C. The rectal temperature and feed intake of the chickens were recorded daily. After 7 days, nine chickens per group were sacrificed by exsanguination, and the jejunum was collected. The results show that heat exposure significantly decreased the feed intake and increased the rectal temperature of the broiler chickens. The plasma concentrations of uric acid and triglyceride significantly increased and decreased, respectively, in the heat-stressed group. No significant differences in the levels of plasma glucose, total amino acids, and very low-density lipoprotein were observed between the heat-stressed and control groups. However, the plasma concentration of glucose tended to be higher (P = 0.09) in the heat-stressed group than in the control group. Heat exposure did not significantly affect the mRNA levels of Na(+)-dependent glucose transporter 1 and amino acid transporters y + LAT1, CAT1, r-BAT, and PePT-1. However, the expression levels of GLUT-2, FABP1, and CD36 were significantly decreased by heat exposure. The results of this study provide new insights into the mechanisms by which heat stress affects nutrient absorption in broiler chickens. Our findings suggest that periodic heat exposure might alter the jejunal glucose and lipid transport rather than amino acid transport. However, intestinal epithelial damage and cell loss should be considered when interpreting the effects of heat stress on the expression of intestinal transporters.

A novel biomarker for acute kidney injury using TaqMan-based unmethylated DNA-specific polymerase chain reaction

There has been increasing interest in the use of circulating DNA as biomarkers for various tissue injuries, cancers, and fetal conditions. DNA methylation is a well-characterized mechanism underlying the epigenetic regulation of gene expression, and many diagnostic tests based on DNA methylation patterns have been developed. We developed a novel TaqMan-based assay for the detection of acute kidney injury using a hypomethylated promoter region of Slc22a12, a urate transporter specifically expressed in proximal tubular cells. Bisulfite sequencing analysis confirmed that the CpG islands in the promoter region of mouse Slc22a12 were preferentially hypomethylated in the kidney cortex. TaqMan minor groove binder (MGB) probes reliably discriminated the DNA fragments corresponding to the unmethylated and methylated promoter regions of Slc22a12. Plasma levels of unmethylated DNA corresponding to the Slc22a12 promoter region were undetectable at baseline and were significantly elevated after acute kidney cortex necrosis. This study showed the usefulness of the TaqMan system in discriminating methylated and unmethylated DNA fragments, and the similar strategy can be applied for establishing biomarkers for various cellular injuries or pathological conditions.

Lactobacillus rhamnosus CNCM I-3690 and the commensal bacterium Faecalibacterium prausnitzii A2-165 exhibit similar protective effects to induced barrier hyper-permeability in mice

Impaired gut barrier function has been reported in a wide range of diseases and syndromes and in some functional gastrointestinal disorders. In addition, there is increasing evidence that suggests the gut microbiota tightly regulates gut barrier function and recent studies demonstrate that probiotic bacteria can enhance barrier integrity. Here, we aimed to investigate the effects of Lactobacillus rhamnosus CNCM I-3690 on intestinal barrier function. In vitro results using a Caco-2 monolayer cells stimulated with TNF-α confirmed the anti-inflammatory nature of the strain CNCM I-3690 and pointed out a putative role for the protection of the epithelial function. Next, we tested the protective effects of L. rhamnosus CNCM I-3690 in a mouse model of increased colonic permeability. Most importantly, we compared its performance to that of the well-known beneficial human commensal bacterium Faecalibacterium prauznitzii A2-165. Increased colonic permeability was normalized by both strains to a similar degree. Modulation of apical tight junction proteins expression was then analyzed to decipher the mechanism underlying this effect. We showed that CNCM I-3690 partially restored the function of the intestinal barrier and increased the levels of tight junction proteins Occludin and E-cadherin. The results indicate L. rhamnosus CNCM I-3690 is as effective as the commensal anti-inflammatory bacterium F. prausnitzii to treat functional barrier abnormalities.

Metabolism. Differential regulation of mTORC1 by leucine and glutamine

The mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) integrates environmental and intracellular signals to regulate cell growth. Amino acids stimulate mTORC1 activation at the lysosome in a manner thought to be dependent on the Rag small guanosine triphosphatases (GTPases), the Ragulator complex, and the vacuolar H⁺-adenosine triphosphatase (v-ATPase). We report that leucine and glutamine stimulate mTORC1 by Rag GTPase-dependent and -independent mechanisms, respectively. Glutamine promoted mTORC1 translocation to the lysosome in RagA and RagB knockout cells and required the v-ATPase but not the Ragulator. Furthermore, we identified the adenosine diphosphate ribosylation factor-1 GTPase to be required for mTORC1 activation and lysosomal localization by glutamine. Our results uncover a signaling cascade to mTORC1 activation independent of the Rag GTPases and suggest that mTORC1 is differentially regulated by specific amino acids.

Participation of b0,+and B0,+systems in the transport of mercury bound to cysteine in intestinal cells

The main source of exposure to mercury (Hg) as divalent inorganic Hg [Hg(ii)] and methylmercury (CH₃Hg) is the diet, in which complexes with the amino acid cysteine (Hg–Cys) may be found.

In vivo regulation of intestinal absorption of amino acids by leptin

Leptin is secreted by the gastric mucosa and is able to reach the intestinal lumen and bind to its receptors located in the apical membranes of enterocytes. We have previously demonstrated that apical leptin inhibits uptake of amino acids in rat intestine in vitro and in Caco-2 cells. The aim of the present work was to investigate the effect of leptin on absorption of amino acids using in vivo techniques, which generate situations closer to physiological conditions. In vivo intestinal absorption of amino acids in rats was measured by isolating a jejunal loop and using the single-pass perfusion system. Disappearance of glutamine (Gln), proline (Pro), and β-alanine (β-Ala) from the perfusate, in the absence or presence of leptin, was measured using a radioactivity method. Luminal leptin (25 nM) inhibited the absorption of 2 mM Pro, 5 mM β-Ala, and 5 mM Gln by approximately 45% after 5-15 min; the effect remained constant until the end of the experiment (80 min) and was rapidly and completely reversed when leptin was removed from the perfusion medium. Moreover, leptin was able to regulate the absorption of galactose and Gln in the same animal, indicating a direct action of the hormone on the specific transporters implicated in the uptake of each nutrient. The results of the present work indicate that luminal leptin decreases absorption of amino acids in vivo in a short-term manner and in a reversible way. These results, together with our previous findings, make it evident that leptin can be considered as a hormone which provides the intestine with a control mechanism to handle absorption of nutrients.

An unexpected increase of toxicity of amino acid-containing ionic liquids

The influence of the structure of cations and anions on the biological activity of ionic liquids is addressed.

Porcine circovirus type 2 affects the serum profile of amino acids and intestinal expression of amino acid transporters in mice

PCV2 is highly pathogenic, however, its effect on the serum amino acids profile is unknown.

Direct In Vivo Human Intestinal Permeability (Peff ) Determined with Different Clinical Perfusion and Intubation Methods

Regional in vivo human intestinal effective permeability (Peff ) is calculated by measuring the disappearance rate of substances during intestinal perfusion. Peff is the most relevant parameter in the prediction of rate and extent of drug absorption from all parts of the intestine. Today, human intestinal perfusions are not performed on a routine basis in drug development. Therefore, it would be beneficial to increase the accuracy of the in vitro and in silico tools used to evaluate the intestinal Peff of novel drugs. This review compiles historical Peff data from 273 individual measurements of 80 substances from 61 studies performed in all parts of the human intestinal tract. These substances include: drugs, monosaccharaides, amino acids, dipeptides, vitamins, steroids, bile acids, ions, fatty acids, and water. The review also discusses the determination and prediction of Peff using in vitro and in silico methods such as quantitative structure-activity relationship, Caco-2, Ussing chamber, animal intestinal perfusion, and physiologically based pharmacokinetic (PBPK) modeling. Finally, we briefly outline how to acquire accurate human intestinal Peff data by deconvolution of plasma concentration-time profiles following regional intestinal bolus dosing.

Computational modelling of amino acid exchange and facilitated transport in placental membrane vesicles

Placental amino acid transport is required for fetal development and impaired transport has been associated with poor fetal growth. It is well known that placental amino acid transport is mediated by a broad array of specific membrane transporters with overlapping substrate specificity. However, it is not fully understood how these transporters function, both individually and as an integrated system. We propose that mathematical modelling could help in further elucidating the underlying mechanisms of how these transporters mediate placental amino acid transport.

The aim of this work is to model the sodium independent transport of serine, which has been assumed to follow an obligatory exchange mechanism. However, previous amino acid uptake experiments in human placental microvillous plasma membrane vesicles have persistently produced results that are seemingly incompatible with such a mechanism; i.e. transport has been observed under zero-trans conditions, in the absence of internal substrates inside the vesicles to drive exchange. This observation raises two alternative hypotheses; (i) either exchange is not fully obligatory, or (ii) exchange is indeed obligatory, but an unforeseen initial concentration of amino acid substrate is present within the vesicle which could drive exchange.

To investigate these possibilities, a mathematical model for tracer uptake was developed based on carrier mediated transport, which can represent either facilitated diffusion or obligatory exchange (also referred to as uniport and antiport mechanisms, respectively). In vitro measurements of serine uptake by placental microvillous membrane vesicles were carried out and the model applied to interpret the results based on the measured apparent Michaelis–Menten parameters K_m and V_max. In addition, based on model predictions, a new time series experiment was implemented to distinguish the hypothesised transporter mechanisms. Analysis of the results indicated the presence of a facilitated transport component, while based on the model no evidence for substantial levels of endogenous amino acids within the vesicle was found.

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Initial rate and time course data for serine uptake in placental membrane vesicles.

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Integrated model analysisof facilitative diffusion vs obligatory exchange.

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Dependency apparent Michaelis–Menten constants on internal concentrations.

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Uptake in placental vesicles was consistent with a facilitative transport component.

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No effects of any internal endogenous substrate in vesicles were apparent.

System L amino acid transporter LAT1 accumulates O-(2-fluoroethyl)-L-tyrosine (FET)

O-(2-fluoroethyl)-L-tyrosine (FET) labeled with fluorine-18 is an important and specific tracer for diagnostics of glioblastoma via positron emission tomography (PET). However, the mechanism of its quite specific accumulation in tumor tissue has not been understood so far. In this work we demonstrate that [(3)H]L-tyrosine is primarily transported by the system L transporter LAT1 in human LN229 glioblastoma cells. FET reduced tyrosine transport, suggesting that it shares the same uptake pathway. More importantly, accumulation of FET was significantly reduced after siRNA-mediated downregulation of LAT1. Xenopus laevis oocytes expressing human LAT1 together with the glycoprotein 4F2hc (necessary to pull LAT-1 to the plasma membrane) exhibited a similar accumulation of FET as observed in glioblastoma cells. In contrast, no accumulation was observed in control oocytes, not overexpressing an exogenous transporter. Because LAT1 works exclusively as an exchanger of amino acids, substrates at one side of the membrane stimulate exchange against substrates at the other side. Extracellular FET stimulated the efflux of intracellular [(3)H]L-leucine, demonstrating that FET is indeed an influx substrate for LAT1. However, FET injected into oocytes was not able to stimulate uptake of extracellular [(3)H]L-leucine, indicating that FET is not a good efflux substrate. Our data, therefore, suggest that FET is trapped within cells due to the asymmetry of its intra- and extracellular recognition by LAT1. If also found for other transporters in tumor cells, asymmetric substrate recognition may be further exploited for tumor-specific accumulation of PET-tracers and/or other tumor-related drugs.

L-Glutamate supplementation improves small intestinal architecture and enhances the expressions of jejunal mucosa amino acid receptors and transporters in weaning piglets

L-Glutamate is a major oxidative fuel for the small intestine. However, few studies have demonstrated the effect of L-glutamate on the intestinal architecture and signaling of amino acids in the small intestine. The aim of this study was to investigate the effects of dietary L-glutamate supplementation on the intestinal architecture and expressions of jejunal mucosa amino acid receptors and transporters in weaning piglets. A total of 120 weaning piglets aged 35 ± 1 days with an average body weight at 8.91 ± 0.45 kg were randomly allocated to two treatments with six replicates of ten piglets each, fed with diets containing 1.21% alanine, or 2% L-glutamate. L-Glutamate supplementation increased the activity of glutamate oxaloacetate transaminase (GOT) in the jejunal mucosa. Also, the mRNA expression level of jejunal mucosa glutamine synthetase (GS) was increased by L-glutamate supplementation. The height of villi in duodenal and jejunal segments, and the relative mRNA expression of occludin and zonula occludens protein-1 (ZO-1) in jejunal mucosa were increased by dietary L-glutamate supplementation. L-Glutamate supplementation increased plasma concentrations of glutamate, arginine, histidine, isoleucine, leucine, methionine, phenylalanine and threonine. L-Glutamate supplementation also increased the relative mRNA expression of the jejunal mucosa Ca(2+)-sensing receptor (CaR), metabotropic glutamate receptor 1 (mGluR1) and metabotropic glutamate receptor 4 (mGluR4), and neutral amino acid transporter B(0)-like (SLC1A5) in the jejunal mucosa. These findings suggest that dietary addition of 2% L-glutamate improves the intestinal integrity and influences the expression of amino acid receptors and transporters in the jejunum of weaning, which is beneficial for the improvement of jejunal nutrients for digestion and absorption.

Including carrier-mediated transport in oral uptake prediction of nutrients and pharmaceuticals in humans

Most toxicokinetic models consider passive diffusion as the only mechanism when modeling the oral uptake of chemicals. However, the overall uptake of nutrients and xenobiotics, such as pharmaceuticals and environmental pollutants, can be increased by influx transport proteins. We incorporated carrier-mediated transport into a one-compartment toxicokinetic model originally developed for passive diffusion only. The predictions were compared with measured oral uptake efficiencies of nutrients and pharmaceuticals, i.e. the fraction of the chemical reaching systemic circulation. Including carrier-mediated uptake improved model predictions for hydrophilic nutrients (RMSE=10% vs. 56%, Coefficient of Efficiency CoE=0.5 vs. -9.6) and for pharmaceuticals (RMSE=21% vs. 28% and CoE=-0.4 vs. -1.1). However, the negative CoE for pharmaceuticals indicates that further improvements are needed. Most important in this respect is a more accurate estimation of vMAX and KM as well as the determination of the amount of expressed and functional transport proteins both in vivo and in vitro.

Effect of L-lysine on expression of selected genes, serum concentration of amino acids, muscle growth and performance of growing pigs

Lysine (Lys) is the first limiting amino acid (AA) in most feed formulations for pigs and most abundant, along with leucine, in muscle proteins. An experiment was conducted with 17 pigs (17.7 ± 0.05 kg initial BW) to identify a role of dietary Lys in the control of protein synthesis in pigs. Fourteen pigs were randomly assigned to one of the two wheat-based dietary treatments: Lys-deficient, 3.0 g/kg (DEF) and Lys-adequate, 10.8 g/kg (ADE). Samples from jejunum mucosa, liver, Longissumus and Semitendinosus muscles, and blood were collected. The other three pigs were sacrificed at the beginning of the trial to measure basal carcass composition. Weight gain, gain:feed ratio, Lys intake and loin eye area were greater in ADE than in DEF pigs (p < 0.01). Muscle-related carcass characteristics were better, and myosin heavy chain IIb expression (MyHC IIb) in Semitendinosus was higher in ADE than in DEF pigs. Expression of AA transporters CAT-1 was lower (p < 0.05), serum Lys was higher and serum Val was lower in pigs fed the ADE diet. The higher muscularity, MyHC IIb expression in Semitendinosus muscle and Lys serum of pigs fed the ADE diet suggest that Lys increases growth rate not only by functioning as protein construction unit but also as potential control of the protein synthesis process.

Protein kinase C-mediated phosphorylation of RKIP regulates inhibition of Na-alanine cotransport by leukotriene D(4) in intestinal epithelial cells

Leukotriene D4 (LTD4) is an important immune inflammatory mediator that is known to be elevated in the mucosa of chronically inflamed intestine and alter nutrient absorption. LTD4 inhibits Na-alanine cotransport in intestinal epithelial cells by decreasing the affinity of the cotransporter ASCT1. LTD4 is known to increase intracellular Ca(++) and cAMP concentrations. However, the intracellular signaling mechanism of LTD4-mediated ASCT1 inhibition is unknown. In the present study, pretreatment with calcium chelator BAPTA-AM or inhibition of Ca(++)-dependent protein kinase C (PKC), specifically PKCα, resulted in the reversal of LTD4-mediated inhibition of ASCT1, revealing the involvement of the Ca(++)-activated PKC pathway. PKCα is known to phosphorylate Raf kinase inhibitor protein (RKIP), thus activating its downstream signaling pathway. Immunoblotting with anti-RKIP-Ser(153) antibody showed an increase in phosphorylation levels of RKIP in LTD4-treated cells. Downregulation of endogenous RKIP showed no decrease in ASCT1 activity by LTD4, thus confirming its involvement in ASCT1 regulation. Phosphorylation of RKIP by PKC is known to activate different signaling pathways, and in this study it was found to activate cAMP-activated protein kinase A (PKA) pathway. Although protein abundance of ASCT1 was not altered in any of the experimental conditions, there was an increase in the levels of phosphothreonine in ASCT1 protein, thus showing that phosphorylation changes were responsible for the altered affinity of ASCT1 by LTD4. In conclusion, LTD4 inhibits ASCT1 through PKC-mediated phosphorylation of RKIP, leading to the subsequent activation of PKA pathway, possibly through β2-andrenergic receptor activation.

Changes in plasma amino acid profiles, growth performance and intestinal antioxidant capacity of piglets following increased consumption of methionine as its hydroxy analogue

The aim of the present study was to determine whether early weaning-induced growth retardation could be attenuated by increased consumption of methionine as dl-methionine (DLM) or dl-2-hydroxy-4-methylthiobutyrate (HMTBA) in both lactating sows and weaned piglets. Therefore, diets containing DLM and HMTBA at 25 % of the total sulphur-containing amino acids (AA) present in the control (CON) diet were fed to lactating sows and weaned piglets and their responses were evaluated. Compared with the CON diet-fed sows, the HMTBA diet-fed sows exhibited a tendency (P< 0·10) towards higher plasma taurine concentrations and the DLM diet-fed sows had higher (P< 0·05) plasma taurine concentrations, but lower (P< 0·05) isoleucine concentrations. Suckling piglets in the HMTBA treatment group had higher (P< 0·05) intestinal reduced glutathione (GSH) content, lower (P< 0·05) oxidised glutathione (GSSG):GSH ratio, and higher (P< 0·05) plasma cysteine and glutathione peroxidase (GPx) activity than those in the CON and DLM treatment groups. The feed intake (P< 0·05) and body weight of piglets averaged across post-weaning (PW) days were higher (P< 0·05) in the HMTBA treatment group than in the DLM treatment group and were higher (P< 0·05) and tended (P< 0·10) to be higher, respectively, in the HMTBA treatment group than in the CON treatment group. Increased (P< 0·05) GSSG content and GSSG:GSH ratio and down-regulated (P< 0·05) expression of nutrient transport genes were observed in the jejunum of piglets on PW day 7 than on PW day 0. On PW day 14, the HMTBA diet-fed piglets had higher (P< 0·05) intestinal GSH content than the CON diet-fed piglets and higher (P< 0·05) plasma GPx activity, villus height and goblet cell numbers than the CON diet- and DLM diet-fed piglets. In conclusion, early weaning-induced growth retardation appears to be attenuated through changes in plasma AA profiles and elevation of growth performance and intestinal antioxidant capacity in piglets following increased consumption of methionine as HMTBA.

System a amino acid transport-targeted brain and systemic tumor PET imaging agents 2-amino-3-[(18)F]fluoro-2-methylpropanoic acid and 3-[(18)F]fluoro-2-methyl-2-(methylamino)propanoic acid

INTRODUCTION

Amino acid based radiotracers target tumor cells through increased uptake by membrane-associated amino acid transport (AAT) systems. In the present study, four structurally related non-natural (18)F-labeled amino acids, (R)- and (S)-[(18)F]FAMP 1 and (R)- and (S)-[(18)F]MeFAMP 2 have been prepared and evaluated in vitro and in vivo for their potential utility in brain and systemic tumor imaging based upon primarily system A transport with positron emission tomography (PET).

METHODS

The transport of enantiomers of [(18)F]FAMP 1 and [(18)F]MeFAMP 2 was measured through in vitro uptake assays in human derived cancer cells including A549 (lung), DU145 (prostate), SKOV3 (ovary), MDA MB468 (breast) and U87 (brain) in the presence and absence of amino acid transporter inhibitors. The in vivo biodistribution of these tracers was evaluated using tumor mice xenografts at 15, 30, 60 and 120 min post injection.

RESULTS

All four tracers showed moderate to high levels of uptake (1-9%ID/5×10(5) cells) by the cancer cell lines tested in vitro. AAT cell inhibition assays demonstrated that (R)-[(18)F]1 and (S)-[(18)F]1 entered these tumor cells via mixed AATs, likely but not limited to system A and system L. In contrast, (R)-[(18)F]2 and (S)-[(18)F]2 showed high selectivity for system A AAT. Similar to the results of in vitro cell studies, the tumor uptake of all four tracers was good to high and persisted over the 2 hours time course of in vivo studies. The accumulation of these tracers was higher in tumor than most normal tissues including blood, brain, muscle, bone, heart, and lung, and the tracers with the highest in vitro selectivity for system A AAT generally demonstrated the best tumor imaging properties. Higher uptake of these tracers was observed in the pancreas, kidney and spleen compared to tumors.

CONCLUSIONS

These preclinical studies demonstrate good imaging properties in a wide range of tumors for all four amino acids evaluated with (R)-[(18)F]2 having the highest selectivity for system A AAT.

Glutamate as a neurotransmitter in the healthy brain

Glutamate is the most abundant free amino acid in the brain and is at the crossroad between multiple metabolic pathways. Considering this, it was a surprise to discover that glutamate has excitatory effects on nerve cells, and that it can excite cells to their death in a process now referred to as "excitotoxicity". This effect is due to glutamate receptors present on the surface of brain cells. Powerful uptake systems (glutamate transporters) prevent excessive activation of these receptors by continuously removing glutamate from the extracellular fluid in the brain. Further, the blood-brain barrier shields the brain from glutamate in the blood. The highest concentrations of glutamate are found in synaptic vesicles in nerve terminals from where it can be released by exocytosis. In fact, glutamate is the major excitatory neurotransmitter in the mammalian central nervous system. It took, however, a long time to realize that. The present review provides a brief historical description, gives a short overview of glutamate as a transmitter in the healthy brain, and comments on the so-called glutamate-glutamine cycle. The glutamate transporters responsible for the glutamate removal are described in some detail.

Leucine stimulates ASCT2 amino acid transporter expression in porcine jejunal epithelial cell line (IPEC-J2) through PI3K/Akt/mTOR and ERK signaling pathways

Leucine has been shown to influence intestinal protein metabolism, cell proliferation and migration. Furthermore, our previous study demonstrated that branched-chain amino acids could modulate the intestinal amino acid and peptide transporters in vivo. As the possible mechanisms are still largely unknown, in the present work, we studied the transcriptional and translational regulation of leucine on amino acid transporter production in IPEC-J2 cells and the signaling pathways involved. Treatment of IPEC-J2 cells with 7.5 mM leucine enhanced the mRNA expression of the Na(+)-neutral AA exchanger 2 (ASCT2) and 4F2 heavy chain (4F2hc) and caused an increase in ASCT2 protein expression. Leucine also activated phosphorylation of 4E-BP1 and eIF4E through the phosphorylation of mTOR, Akt and ERK signaling pathways in IPEC-J2 cells. Pre-treatment of IPEC-J2 cells with inhibitors of mTOR and Akt (rapamycin and wortmannin) or an inhibitor of ERK (PD098059) for 30 min before leucine treatment attenuated the positive effect of leucine in enhancing the protein abundance of ASCT2. These results demonstrate that leucine could up-regulate the expression of the amino acid transporters (ASCT2) through transcriptional and translational regulation by ERK and PI3K/Akt/mTOR activation.

Effects of dietary lysine levels on apparent nutrient digestibility and serum amino Acid absorption mode in growing pigs

Two experiments were conducted to determine the effects of different dietary lysine levels on the apparent nutrient digestibility, the serum amino acid (AA) concentration, and the biochemical parameters of the precaval and portal vein blood in growing pigs. In Experiment 1, 15 noncannulated pigs received diets with different lysine densities (0.65%, 0.95%, and 1.25% lysine) for 13 d. A total collection digestion test was performed, and blood samples were collected from the precaval vein at the end of the experiment. In Experiment 2, four cannulated pigs were fed the same diets of Experiment 1. The experiment used a self-control experimental design and was divided into three periods. On d 5 of each period, at 0.5 h before feeding and hourly up to 8 h after feeding, single blood samples were collected from catheters placed in the portal vein. In Experiment 1, some serum AAs (including lysine), serum urinary nitrogen (SUN), and total protein (TP) concentrations were significantly affected by the dietary lysine levels (p<0.05). Moreover, the 0.65% lysine treatment showed a significant lower apparent digestibility of gross energy, dry matter, crude protein, and phosphorus than the other treatments (p<0.05). In Experiment 2, serum lysine, histidine, phenylalanine, threonine, valine, isoleucine (p = 0.0588), triglyceride, and SUN (p = 0.0572) concentrations were significantly affected by the dietary lysine levels (p<0.05). Additionally, almost all of the determined serum AA and total AA concentrations reached their lowest values at 0.5 h before feeding and their highest values at 2 h after feeding (p<0.05). These findings indicate that the greatest absorption of AA occurred at 2 h after feeding and that the dynamic profile of serum AA is affected by the dietary lysine levels. Moreover, when the dietary lysine content was 0.95%, the growing pigs achieved a better nutrient digestibility and serum metabolites levels.

Intestinal amino acid availability via PEPT-1 affects TORC1/2 signaling and the unfolded protein response

The intestinal peptide transporter PEPT-1 plays an important role in development, growth, reproduction, and stress tolerance in Caenorhabditis elegans, as revealed by the severe phenotype of the pept-1-deficient strain. The reduced number of offspring and increased stress resistance were shown to result from changes in the insulin/IGF-signaling cascade. To further elucidate the regulatory network behind the phenotypic alterations in PEPT1-deficient animals, a quantitative proteome analysis combined with transcriptome profiling was applied. Various target genes of XBP-1, the major mediator of the unfolded protein response, were found to be downregulated at the mRNA and protein levels, accompanied by a reduction of spliced xbp-1 mRNA. Proteome analysis also revealed a markedly reduced content of numerous ribosomal proteins. This was associated with a reduction in the protein synthesis rate in pept-1 C. elegans, a process that is strictly regulated by the TOR (target of rapamycine) complex, the cellular sensor for free amino acids. These data argue for a central role of PEPT-1 in cellular amino acid homeostasis. In PEPT-1 deficiency, amino acid levels dropped systematically, leading to alterations in protein synthesis and in the IRE-1/XBP-1 pathway.