Proton Coupled Oligopeptide Transporter 1 (PepT1) Function, Regulation, and Influence on the Intestinal Homeostasis

Conformational Landscape of the Di- and Tripeptide Permease A Transport Cycle

Dipeptide and tripeptide permease A (DtpA) transporter is a bacterial homologue of the human PepT that is responsible for the uptake of di- and tripeptides from the small intestine and transports them across the cell membrane utilizing an inward-directed proton electrochemical gradient. Despite its importance, the structural dynamics governing the conformational transitions of DtpA remain poorly understood. In this study, we employed Adaptive Bandit enhanced sampling molecular dynamics simulations to investigate the five major conformational states of DtpA adopted during the transport cycle. We identified key metastable states and transitions underlying the transport cycle using Markov State Models (MSMs). Our findings reveal that intra- and interhelical interactions drive conformational changes by inducing bending and rotation of helices lining the pore, resulting in its opening and closure. This study explains the substrate transport mechanism in DtpA, enhancing our understanding of bacterial proton-dependent oligopeptide transporters (POTs) and opening new drug design and development opportunities.

Inhibitory Effect of Whey Protein-Derived Peptide Leu-Asp-Gln-Trp on Xanthine Oxidase

Hyperuricemia is associated with various diseases, and xanthine oxidase (XO) is the rate-limiting enzyme in uric acid (UA) production. A previous study reported that Leu-Asp-Gln-Trp (LDQW) in whey protein hydrolysate (WPH) suppressed lipid droplet accumulation in differentiated 3T3-L1 adipocyte-like cells. However, our understanding of LDQW remains limited, further, its efficacy against hyperuricemia has not been elucidated. This study evaluated the XO inhibitory activity of LDQW, one of the bioactive peptides in WPH. In this study, UA produced by the reaction between XO and xanthine was determined using two methods: monitoring the absorbance at 290 nm using absorptiometry and detection using liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis. Allopurinol was used as the positive control, whereas tryptophan and Ala-Leu-Pro-Met (ALPM) were used for comparison. Both absorptiometry and LC-MS/MS analyses demonstrated that LDQW significantly inhibited XO activity in a concentration-dependent manner. The LC-MS/MS analysis results indicated that LDQW, tryptophan, and ALPM inhibition ratios at 20 mM were 58.0% ± 2.8%, 4.4% ± 3.7%, and 45.0% ± 1.0%, respectively. Moreover, it was suggested that Asp-Gln-Trp, a potential digestive peptide predicted by the enzymatic digestion of LDQW in silico, also possessed XO inhibitory activity comparable to that of LDQW in LC-MS/MS analysis. These findings suggest that LDQW is a promising bioactive peptide with potential ameliorative effects against hyperuricemia, similar to those of other XO inhibitory peptides.

Collagen Hydrolysates as Nutritional Support in Canine Osteoarthritis: A Narrative Review

Osteoarthritis (OA) is a common disease in dogs with severe impact on their welfare. The multimodal management of OA includes feeding therapeutic diets and nutraceuticals to slow down OA progression. Collagen hydrolysates (CH) are a nutritional supplement that may exert anabolic effects on osteoarthritic joint cartilage as well as disease-modifying effects. After oral intake, CH is absorbed, mainly as amino acids, di- and tripeptides that are transported amongst others to the joint. In addition to reducing cartilage degradation, CH metabolites may reduce synovial inflammation and subchondral bone sclerosis during OA. Preliminary evidence in dogs suffering from the consequences of OA support the clinical efficacy of CH with reported reductions in lameness. However, effects on biomarker level of cartilage metabolism and inflammation are inconclusive. Additionally, current studies show a lack of standardised dosing regimens and the use of not validated outcomes. Future work should therefore elucidate further on the bioavailability of CH in dogs in order to establish adequate dosing recommendations. Furthermore, high-quality placebo-controlled randomised controlled trials are essential to dstudies have evaluated the cetermine the clinical efficacy of CH to reduce lameness, prevent OA progression and thereby improve the level of evidence.

Interaction of dietary replacements of fishmeal by protein blend and feeding frequency on growth performance and protein utilization of gibel carp (Carassius gibelio var. CAS V)

Feeding frequency represents a potential strategy to improve the utilization of protein sources by fish. This study investigated its impact on the utilization of protein blend in gibel carp. The dietary fishmeal was totally substituted with three protein blends consisting of Tenebrio molitor meal, Chlorella meal, Clostridium autoethanogenum protein, cottonseed protein concentrate, at ratios of 1:1:8:2, 1:1:6:4, and 1:1:4:6, respectively. During an 8-week feeding trial, a total of 960 healthy fish (18.10 g) were randomly assigned to eight groups, each with three replicates. Then they were fed either twice daily (two meals per day) or four times daily (four meals per day) with four different diets. Higher feeding frequency increased feed intake and intestinal trypsin activity (P < 0.05), and up-regulated the expression levels of genes related to amino acid or peptide transporter (pept1, y + lat2) and sensory receptors (casr, gprc6a, mglur4) in intestine (P < 0.05). Moreover, it accelerated muscle protein turnover by increasing free amino acid content, aspartate aminotransferase activity and akt1 transcript levels (P < 0.05), ultimately promoting growth. However, higher feeding frequency reduced protein apparent digestibility and feed efficiency (P < 0.05). Dietary blended proteins elevated trypsin and chymotrypsin activities (P < 0.01). Notably, the adverse effects observed with blended proteins (ratio at 1:1:8:2) on total essential amino acid digestibility and muscle protein metabolism-related gene expression were mitigated with increased feeding frequency, thus alleviating growth inhibition. Furthermore, the blended proteins at a ratio of 1:1:6:4 increased protein apparent digestibility (P < 0.05), down-regulated mstn expression level (P < 0.05), and up-regulated expression levels of genes related to protein synthesis (akt1, mtor, s6k1, eif4b, eif4e; P < 0.05); thereby promoting protein utilization and muscle growth at four meals per day. Overall, feeding frequency interacted synergistically with blended proteins to influence growth and protein utilization in gibel carp, and a protein blend with a ratio of 1:1:6:4 was a superior alternative to fishmeal at both feeding frequencies. Future strategies aimed at replacing dietary fishmeal should consider the role of feeding frequency as a critical factor.

Enhanced Gut-to-Liver Oral Drug Delivery via Ligand-Modified Nanoparticles by Attenuating Protein Corona Adsorption

The development of effective oral drug delivery systems for targeted gut-to-liver transport remains a significant challenge due to the multiple biological barriers including the harsh gastrointestinal tract (GIT) environment and the complex protein corona (PC) formation. In this study, we developed ligand-modified nanoparticles (NPs) that enable gut-to-liver drug delivery by crossing the GIT and attenuating PC formation. Specifically, mesoporous silica nanoparticles (MSNs) were functionalized with peptides targeting the neonatal Fc receptor (FcRn), capitalizing on FcRn expression in the small intestine and liver for targeted drug delivery. We showed that MSNs decorated with a small cyclic FcRn binding peptide (MSNs-FcBP) obtained enhanced diffusion in intestinal mucus and superior transportation across the intestine compared to unmodified MSNs and MSNs decorated with a large IgG Fc fragment (MSNs-Fc), which correlated with diminished protein adsorption and weaker interaction with mucin. After entering the blood circulation, reduced serum PC formation by MSNs-FcBP reduces the proteolytic and phagocytic propensity of the reticuloendothelial system, ultimately ameliorating accumulation in hepatocytes. Pharmacokinetic and pharmacodynamic studies in diabetic mice revealed that MSNs-FcBP effectively transported the therapeutic agent exenatide across the intestinal epithelium, leading to a significant hypoglycemic response and improved glucose tolerance. This study underscores the critical role of ligand selection in limiting protein corona formation, thereby significantly enhancing gut-to-liver drug delivery by increasing mucus permeation and minimizing serum-protein interactions. The effective delivery of exenatide in diabetic mice illustrates the potential of this strategy to optimize oral drug bioavailability and therapeutic efficacy.

Structured multicellular intestinal spheroids (SMIS) as a standardized model for infection biology

BACKGROUND

3D cell culture models have recently garnered increasing attention for replicating organ microarchitecture and eliciting in vivo-like responses, holding significant promise across various biological disciplines. Broadly, 3D cell culture encompasses organoids as well as single- and multicellular spheroids. While the latter have found successful applications in tumor research, there is a notable scarcity of standardized intestinal models for infection biology that mimic the microarchitecture of the intestine. Hence, this study aimed to develop structured multicellular intestinal spheroids (SMIS) specifically tailored for studying molecular basis of infection by intestinal pathogens.

RESULTS

We have successfully engineered human SMIS comprising four relevant cell types, featuring a fibroblast core enveloped by an outer monolayer of enterocytes and goblet cells along with monocytic cells. These SMIS effectively emulate the in vivo architecture of the intestinal mucosal surface and manifest differentiated morphological characteristics, including the presence of microvilli, within a mere two days of culture. Through analysis of various differentiation factors, we have illustrated that these spheroids attain heightened levels of differentiation compared to 2D monolayers. Moreover, SMIS serve as an optimized intestinal infection model, surpassing the capabilities of traditional 2D cultures, and exhibit a regulatory pattern of immunological markers similar to in vivo infections after Campylobacter jejuni infection. Notably, our protocol extends beyond human spheroids, demonstrating adaptability to other species such as mice and pigs.

CONCLUSION

Based on the rapid attainment of enhanced differentiation states, coupled with the emergence of functional brush border features, increased cellular complexity, and replication of the intestinal mucosal microarchitecture, which allows for exposure studies via the medium, we are confident that our innovative SMIS model surpasses conventional cell culture methods as a superior model. Moreover, it offers advantages over stem cell-derived organoids due to scalability and standardization capabilities of the protocol. By showcasing differentiated morphological attributes, our model provides an optimal platform for diverse applications. Furthermore, the investigated differences of several immunological factors compared to monotypic monolayers after Campylobacter jejuni infection underline the refinement of our spheroid model, which closely mimics important features of in vivo infections.

Extracellular Vesicles of the Probiotic Escherichia coli Nissle 1917 Reduce PepT1 Levels in IL-1β-Treated Caco-2 Cells via Upregulation of miR-193a-3p

PepT1, a proton-coupled oligopeptide transporter, is crucial for intestinal homeostasis. It is mainly expressed in small intestine enterocytes, facilitating the absorption of di/tri-peptides from dietary proteins. In the colon, PepT1 expression is minimal to prevent excessive responses to proinflammatory peptides from the gut microbiota. However, increased colonic PepT1 is linked to chronic inflammatory diseases and colitis-associated cancer. Despite promising results from animal studies on the benefits of extracellular vesicles (EVs) from beneficial gut commensals in treating IBD, applying probiotic EVs as a postbiotic strategy in humans requires a thorough understanding of their mechanisms. Here, we investigate the potential of EVs of the probiotic Nissle 1917 (EcN) and the commensal EcoR12 in preventing altered PepT1 expression under inflammatory conditions, using an interleukin (IL)-1-induced inflammation model in Caco-2 cells. The effects are evaluated by analyzing the expression of PepT1 (mRNA and protein) and miR-193a-3p and miR-92b, which regulate, respectively, PepT1 mRNA translation and degradation. The influence of microbiota EVs on PepT1 expression is also analyzed in the presence of bacterial peptides that are natural substrates of colonic PepT1 to clarify how the regulatory mechanisms function under both physiological and pathological conditions. The main finding is that EcN EVs significantly decreases PepT1 protein via upregulation of miR-193a-3p. Importantly, this regulatory effect is strain-specific and only activates in cells exposed to IL-1β, suggesting that EcN EVs does not control PepT1 expression under basal conditions but can play a pivotal role in response to inflammation as a stressor. By this mechanism, EcN EVs may reduce inflammation in response to microbiota in chronic intestinal disorders by limiting the uptake of bacterial proinflammatory peptides.

Absorption of bioactive peptides following collagen hydrolysate intake: a randomized, double-blind crossover study in healthy individuals

Background

Collagen hydrolysates (CH) in functional foods and supplements are dietary sources of amino acids (AAs) and di-and tripeptides linked to various health benefits. This study aimed to investigate the single-dose bioavailability of skin- and hide-derived CH from fish, porcine and bovine origin with different molecular weights (bovine 2,000 and 5,000 Da).

Methods

A randomized, double-blind crossover clinical study was performed with healthy volunteers assessing the plasma concentration of free and peptide-bound hydroxyproline (Hyp) as well as selected peptides reported to be abundantly present in collagen.

Results

The pharmacokinetic endpoints demonstrated comparable uptake of free Hyp from all CH. A higher amount of total compared to free Hyp indicated the uptake of substantial amounts of Hyp-containing di- or tripeptides.

Conclusion

Independently of source and molecular weight, all CH yielded relevant plasma concentrations of the investigated metabolites. Larger studies are needed to estimate an ideal level of selected circulating metabolites needed to trigger distinct physiological reactions in target tissues.

In Vitro Fermentation of Animal and Plant Protein Isolates by the Human Gut Microbiota Under High and Low Carbohydrate Conditions

SCOPE

There is a lack of research comparing how different protein isolates influence the microbiome, especially when carbohydrate (CHO) availability is varied. The objective is to determine changes in gut microbiota composition and function during fermentation of digested protein isolates under high and low CHO conditions.

METHODS AND RESULTS

Protein isolates from beef, egg white, milk, pea, and soy are subjected to in vitro digestion and fermentation with human fecal microbiota. Under low CHO conditions, the microbiota is primarily proteolytic with decreased concentrations of peptides and increased variance among microbial taxa and production of ammonia and branched chain fatty acids by the microbiota. Milk protein not only results in the highest production of butyrate and p-hydroxyphenylacetate but also has high concentrations of deleterious fermentation metabolites. Amino acid composition of the protein isolates is significantly correlated with abundances of many microbial taxa and metabolites, but the correlations are stronger in the low CHO medium.

CONCLUSION

This study shows that low CHO conditions increase proteolytic fermentation and result in increased differences in microbiota response to protein isolates. It also showed that amino acid composition is highly associated with microbiota composition and function especially under low CHO conditions.

Peptide Transporter 1-Mediated Dipeptide Transport Promotes Hepatocellular Carcinoma Metastasis by Activating MAP4K4/G3BP2 Signaling Axis

Cancer metastasis is the leading cause of mortality in patients with hepatocellular carcinoma (HCC). To meet the rapid malignant growth and transformation, tumor cells dramatically increase the consumption of nutrients, such as amino acids. Peptide transporter 1 (PEPT1), a key transporter for small peptides, has been found to be an effective and energy-saving intracellular source of amino acids that are required for the growth of tumor cells. Here, the role of PEPT1 in HCC metastasis and its underlying mechanisms is explored. PEPT1 is upregulated in HCC cells and tissues, and high PEPT1 expression is associated with poor prognosis in patients with HCC. PEPT1 overexpression dramatically promoted HCC cell migration, invasion, and lung metastasis, whereas its knockdown abolished these effects both in vitro and in vivo. Mechanistic analysis revealed that high PEPT1 expression increased cellular dipeptides in HCC cells that are responsible for activating the MAP4K4/G3BP2 signaling pathway, ultimately facilitating the phosphorylation of G3BP2 at Thr227 and enhancing HCC metastasis. Taken together, these findings suggest that PEPT1 acts as an oncogene in promoting HCC metastasis through dipeptide-induced MAP4K4/G3BP2 signaling and that the PEPT1/MAP4K4/G3BP2 axis can serve as a promising therapeutic target for metastatic HCC.

Essential oils improve nursery pigs' performance and appetite via modulation of intestinal health and microbiota

Optimal intestinal health and functionality are essential for animal health and performance, and simultaneously intestinal nutrient transporters and intestinal peptides are also involved in appetite and feed intake control mechanisms. Given the potential of essential oil (EO) in improving animal performance and improving feed palatability, we hypothesized that dietary supplementation of cinnamaldehyde and carvacrol could improve performance and appetite of nursery pigs by modulating intestinal health and microbiota. Cinnamaldehyde (100 mg/kg), carvacrol (100 mg/kg), and their mixtures (including 50 mg/kg cinnamaldehyde and 50 mg/kg carvacrol) were supplemented into the diets of 240 nursery pigs for 42 d, and data related to performance were measured. Thereafter, the influence of EO on intestinal health, appetite and gut microbiota and their correlations were explored. EO supplementation increased (P < 0.05) the body weight, average daily gain (ADG) and average daily feed intake (ADFI) of piglets, and reduced (P < 0.05) diarrhea rates in nursery pigs. Furthermore, EO increased (P < 0.05) the intestinal absorption area and the abundance of tight junction proteins, and decreased (P < 0.05) intestinal permeability and local inflammation. In terms of intestinal development and the mucus barrier, EO promoted intestinal development and increased (P < 0.05) the number of goblet cells. Additionally, we found that piglets in the EO-supplemented group had upregulated (P < 0.05) levels of transporters and digestive enzymes in the intestine, which were significantly associated with daily gain and feed utilization. In addition, EO supplementation somewhat improved appetite in nursery pigs, increased the diversity of the gut microbiome and the abundance of beneficial bacteria, and there was a correlation between altered bacterial structure and appetite-related hormones. These findings indicate that EO is effective in promoting growth performance and nutrient absorption as well as in regulating appetite by improving intestinal health and bacterial structure.

Cell-based homologous expression system for in-vitro characterization of environmental effects on transmembrane peptide transport in fish

All organisms encounter environmental changes that lead to physiological adjustments that could drive evolutionary adaptations. The ability to adjust performance in order to cope with environmental changes depends on the organism's physiological plasticity. These adjustments can be reflected in behavioral, physiological, and molecular changes, which interact and affect each other. Deciphering the role of molecular adjustments in physiological changes will help to understand how multiple levels of biological organization are synchronized during adaptations. Transmembrane transporters, which facilitate a cell's interaction with its surroundings, are prime targets for molecular studies of the environmental effects on an organism's physiology. Fish are subjected to environmental fluctuations and exhibit different coping mechanisms. To study the molecular adjustments of fish transporters to their external surrounding, suitable experimental systems must be established. The Mozambique tilapia (Oreochromis mossambicus) is an excellent model for environmental stress studies, due to its extreme salinity tolerance. We established a homologous cellular-based expression system and uptake assay that allowed us to study the effects of environmental conditions on transmembrane transport. We applied our expression system to investigate the effects of environmental conditions on the activity of PepT2, a transmembrane transporter critical in the absorption of dietary peptides and drugs. We created a stable, modified fish cell-line, in which we exogenously expressed the tilapia PepT2, and tested the effects of water temperature and salinity on the uptake of a fluorescent di-peptide, β-Ala-Lys-AMCA. While temperature affected only Vmax, medium salinity had a bi-directional effect, with significantly reduced Vmax in hyposaline conditions and significantly increased Km in hypersaline conditions. These assays demonstrate the importance of suitable experimental systems for fish ecophysiology studies. Furthermore, our in-vitro results show how the effect of hypersaline conditions on the transporter activity can explain expression shifts seen in the intestine of saltwater-acclimated fish, emphasizing the importance of complimentary studies in better understanding environmental physiology. This research highlights the advantages of using homologous expression systems to study environmental effects encountered by fish, in a relevant cellular context. The presented tools and methods can be adapted to study other transporters in-vitro.

Molecular basis of TASL recruitment by the peptide/histidine transporter 1, PHT1

PHT1 is a histidine /oligopeptide transporter with an essential role in Toll-like receptor innate immune responses. It can act as a receptor by recruiting the adaptor protein TASL which leads to type I interferon production via IRF5. Persistent stimulation of this signalling pathway is known to be involved in the pathogenesis of systemic lupus erythematosus (SLE). Understanding how PHT1 recruits TASL at the molecular level, is therefore clinically important for the development of therapeutics against SLE and other autoimmune diseases. Here we present the Cryo-EM structure of PHT1 stabilized in the outward-open conformation. By combining biochemical and structural modeling techniques we propose a model of the PHT1-TASL complex, in which the first 16 N-terminal TASL residues fold into a helical structure that bind in the central cavity of the inward-open conformation of PHT1. This work provides critical insights into the molecular basis of PHT1/TASL mediated type I interferon production.

Intestinal Amino Acid Transport and Metabolic Health

Amino acids derived from protein digestion are important nutrients for the growth and maintenance of organisms. Approximately half of the 20 proteinogenic amino acids can be synthesized by mammalian organisms, while the other half are essential and must be acquired from the nutrition. Absorption of amino acids is mediated by a set of amino acid transporters together with transport of di- and tripeptides. They provide amino acids for systemic needs and for enterocyte metabolism. Absorption is largely complete at the end of the small intestine. The large intestine mediates the uptake of amino acids derived from bacterial metabolism and endogenous sources. Lack of amino acid transporters and peptide transporter delays the absorption of amino acids and changes sensing and usage of amino acids by the intestine. This can affect metabolic health through amino acid restriction, sensing of amino acids, and production of antimicrobial peptides.

Development, characterization and in vivo zinc absorption capacity of a novel soy meal hydrolysate-zinc complexes

Background

Zinc is an essential trace element for the human body. Recently, a novel Zn-binding peptide, Lys-Tyr-Lys-Arg-Gln-Arg-Trp (PP), was purified and identified from soy protein hydrolysates with high Zn-binding capacity (83.21 ± 2.65%) by our previous study. The preparation of soy meal hydrolysates (SMHs)-Zn complexes is convenient and low-cost, while PP (Lys-Tyr-Lys-Arg-Gln-Arg-Trp)-Zn complexes have a higher coordination rate but a relatively high cost. The aim of this study was to investigate the effect of soy meal hydrolysates (SMHs)-Zn complexes on zinc absorption in mice model, and synthetic soy peptide (PP)-Zn complexes with high Zn-binding capacity were used as control. Firstly, SMHs were prepared by enzymolysis, and the PP (Lys-Tyr-Lys-Arg-Gln-Arg-Trp) were synthesized based on previous studies. The binding mechanism of soy hydrolysates and zinc was analyzed by spectral analysis. Furthermore, the cytotoxicity of the SMHs-Zn complexes was also studied using the CCK-8 method. The effect of zinc absorption was evaluated based on Zn content, total protein and albumin content, relevant enzyme system, and the PeT1 and ZnT1 mRNA expression levels.

Result

The result showed that zinc was bound with carboxyl oxygen and amino nitrogen atoms on SMHs, with hydrophobic and electrostatic interactions as auxiliary stabilizing forces. SMHs-Zn were proved to have great solubility and a small particle size at different pH values, and it showed a beneficial effect on Caco-2 cells growth. Moreover, it was proved that SMHs-Zn and PP-Zn could increase the levels of zinc and the activity of Zn-related enzymes in mice. SMHs-Zn possessed higher PepT1 and ZnT1 mRNA expression levels than PP-Zn in the small intestine.

Conclusion

SMHs-Zn with a lower Zn-binding capacity had similar effects on zinc absorption in mice as PP-Zn, suggesting that the bioavailability of peptide-zinc complexes in mice was not completely dependent on their Zn-binding capacity, but may also be related to the amino acid composition.

Quercetin Alleviates Inflammation and Energy Deficiency Induced by Lipopolysaccharide in Chicken Embryos

Energy deficiency causes multiple organ dysfunctions after LPS induction. Quercetin is a phenolic compound found in herbal medicines. However, the effects of quercetin in alleviating LPS-induced energy deficiency remain unclear. In the present study, an in vivo LPS-induced inflammation model was established in chicken embryos. Specific pathogen-free chicken embryos (n = 120) were allocated to control, PBS with or without ethanol, quercetin (10, 20, or 40 nmol, respectively), and LPS (125 ng/egg) with or without quercetin groups. Fifteen day old embryonated eggs were injected with the abovementioned solutions via the allantoic cavity. On embryonic day 19, the tissues of the embryos were collected for histopathological examination using frozen oil red O staining, RNA extraction, real-time quantitative polymerase chain reaction, and immunohistochemical investigations. The glycogen and lipid contents in the liver increased after LPS stimulation as compared with the PBS group, whereas quercetin decreased the accumulation as compared with the LPS group. The mRNA expressions of AMPKα1 and AMPKα2 in the duodena, ceca, and livers were upregulated after LPS induction as compared with the PBS group, while quercetin could downregulate these expressions as compared with the LPS group. The immunopositivity of AMPKα2 in the villus, crypt, lamina propria, tunica muscularis, and myenteric plexus in the duodena and in the cytoplasms of hepatocytes significantly increased after LPS induction when compared with the PBS group (p < 0.01), whereas the immunopositivity to AMPKα2 in the quercetin treatment group significantly decreased when compared with the LPS group (p < 0.01 or p < 0.05). The LPS-induced high expressions of transcription factor PPARα and glucose transporter (SGLT1) were blocked by quercetin in the duodena, ceca, and livers. Quercetin treatment improved the LPS-induced decrease in APOA4 in the duodena, ceca, and livers. The mRNA expression of PEPT1 in the duodena and ceca increased after LPS challenge, whereas quercetin could downregulate PEPT1 gene expression. These data demonstrate that quercetin improved the energy deficiency induced by LPS in chicken embryos. The LPS-induced inflammation model was established to avoid the effect of LPS exposure from the environment and intestinal flora. The results form the basis the administration of quercetin pretreatment (in ovo infection) to improve the energy state of chicken embryos and improve the inflammation response.

Urine metabolomics reveals biomarkers and the underlying pathogenesis of diabetic kidney disease

PURPOSE

Diabetic kidney disease (DKD) is the most common complication of type 2 diabetes mellitus (T2DM), and its pathogenesis is not yet fully understood and lacks noninvasive and effective diagnostic biomarkers. In this study, we performed urine metabolomics to identify biomarkers for DKD and to clarify the potential mechanisms associated with disease progression.

METHODS

We applied a liquid chromatography-mass spectrometry-based metabolomics method combined with bioinformatics analysis to investigate the urine metabolism characteristics of 79 participants, including healthy subjects (n = 20), T2DM patients (n = 20), 39 DKD patients that included 19 DKD with microalbuminuria (DKD + micro) and 20 DKD with macroalbuminuria (DKD + macro).

RESULTS

Seventeen metabolites were identified between T2DM and DKD that were involved in amino acid, purine, nucleotide and primarily bile acid metabolism. Ultimately, a combined model consisting of 2 metabolites (tyramine and phenylalanylproline) was established, which had optimal diagnostic performance (area under the curve (AUC) = 0.94). We also identified 19 metabolites that were co-expressed within the DKD groups and 41 metabolites specifically expressed in the DKD + macro group. Ingenuity pathway analysis revealed three interaction networks of these 60 metabolites, involving the sirtuin signaling pathway and ferroptosis signaling pathway, as well as the downregulation of organic anion transporter 1, which may be important mechanisms that mediate the progression of DKD.

CONCLUSIONS

This work reveals the metabolic alterations in T2DM and DKD, constructs a combined model to distinguish them and delivers a novel strategy for studying the underlying mechanism and treatment of DKD.

The Role of Solute Carrier Transporters in Efficient Anticancer Drug Delivery and Therapy

Transporter-mediated drug resistance is a major obstacle in anticancer drug delivery and a key reason for cancer drug therapy failure. Membrane solute carrier (SLC) transporters play a crucial role in the cellular uptake of drugs. The expression and function of the SLC transporters can be down-regulated in cancer cells, which limits the uptake of drugs into the tumor cells, resulting in the inefficiency of the drug therapy. In this review, we summarize the current understanding of low-SLC-transporter-expression-mediated drug resistance in different types of cancers. Recent advances in SLC-transporter-targeting strategies include the development of transporter-utilizing prodrugs and nanocarriers and the modulation of SLC transporter expression in cancer cells. These strategies will play an important role in the future development of anticancer drug therapies by enabling the efficient delivery of drugs into cancer cells.

Effect of dietary soybean saponin Bb on the growth performance, intestinal nutrient absorption, morphology, microbiota, and immune response in juvenile Chinese soft-shelled turtle (Pelodiscus sinensis)

Soybean meal is widely applied in the aquafeeds due to the limitation of fish meal resources. Numerous studies have manifested that dietary soybean saponin, an anti-nutrient factor in soybean meal, may slow growth and induce intestinal inflammation in aquatic animals, but the possible causes are unclear. The juvenile Pelodiscus sinensis (mean initial body weight: 6.92 ± 0.03 g) were fed basal diet (CON group) and 2.46% soybean saponin Bb-supplemented diet (SAP group) for 35 days to further explore the effects of dietary soybean saponin Bb on the growth performance, apparent digestibility coefficients, intestinal morphology, the gut microbiota, intestinal transporters/channels, and immune-related gene expression. The results indicated that dietary soybean saponin Bb significantly decreased final body weight, specific growth rate, protein deposition ratio, and apparent digestibility coefficients (dry matter, crude protein, and crude lipid) of nutrients in Pelodiscus sinensis, which may be closely correlated with markedly atrophic villus height and increased lamina propria width in the small intestine. In addition, plasma contents of cholesterol, calcium, phosphorus, potassium, lysozyme, and C3 were significantly decreased in the SAP group compared with the control group. Soybean saponin Bb significantly downregulated the mRNA levels of glucose transporter 2, fatty acid binding protein 1 and fatty acid binding protein 2, amino acid transporter 2, b^0,+-type amino acid transporter 1, and sodium-dependent phosphate transport protein 2b in the small intestine. At the same time, the expressions of key transcription factors (STAT1, TBX21, FOS), chemokines (CCL3), cytokines (TNF-α, IL-8), and aquaporins (AQP3, AQP6) in the inflammatory response were increased by soybean saponin Bb in the large intestine of a turtle. Additionally, dietary supplementation of SAP significantly reduced the generic abundance of beneficial bacteria (Lactobacillus, Bifidobacterium, and Bacillus) and harmful bacteria (Helicobacter and Bacteroides). In a nutshell, dietary supplementation of 2.46% soybean saponin not only hindered the growth performance by negatively affecting the macronutrients absorption in the small intestine but also induced an inflammatory response in the large intestine possibly by damaging the intestinal morphology, disturbing the intestinal microbiota and decreasing intestinal epithelial cell membrane permeability.

Advances in Pharmacokinetic Mechanisms of Transporter-Mediated Herb-Drug Interactions

As the use of herbs has become more popular worldwide, there are increasing reports of herb-drug interactions (HDIs) following the combination of herbs and drugs. The active components of herbs are complex and have a variety of pharmacological activities, which inevitably affect changes in the pharmacokinetics of chemical drugs in vivo. The absorption, distribution, metabolism, and excretion of drugs in vivo are closely related to the expression of drug transporters. When the active components of herbs inhibit or induce the expression of transporters, this can cause changes in substrate pharmacokinetics, resulting in changes in the efficacy and toxicity of drugs. In this article, the tissue distribution and physiological functions of drug transporters are summarized through literature retrieval, and the effects of herbs on drug transporters and the possible mechanism of HDIs are analyzed and discussed in order to provide ideas and a reference for further guiding of safe clinical drug use.

The Positive Effects of Exogenous Pancreatin on Growth Performance, Nutrient Digestion and Absorption, and Intestinal Microbiota in Piglets

Pancreatin secretion is dramatically decreased over time after weaning, thus affecting the utilization of nutrients in piglets. Therefore, exogenous pancreatin is expected to alleviate this situation. This experiment was conducted to investigate the effects of exogenous pancreatin on the growth performance, nutrient digestion and absorption, and intestinal microbiota of piglets. One hundred eighty piglets (Duroc × Landrace × Yorkshire, 40 days) were randomly allotted to three treatments (basal diets supplemented with 0, 250, or 500 mg/kg pancreatin) with three replicate pens per treatment and 20 piglets per pen. Compared with the control diet, dietary 500 mg/kg pancreatin significantly increased (p < 0.05) the average daily gain (ADG) and the apparent digestibility of crude protein and crude fat of piglets. Regarding endogenous enzymes, pancrelipase activity in the pancreas, duodenal mucosa, and small intestinal digesta as well as trypsin activity in the jejunal digesta were increased in piglets fed a diet supplemented with 500 mg/kg pancreatin (p < 0.05). Moreover, amylopsin activity was significantly strengthened in the pancreas, duodenal mucosa, and digesta in piglets fed a diet with 500 mg/kg pancreatin (p < 0.05). The mRNA expression of nutrient transporters, including oligopeptide transporter-1 (PepT1), excitatory amino acid transporter-1 (EAAC1), cationic amino acid transporter-1 (CAT1), sodium glucose cotransporter-1 (SGLT1), glucose transporter-2 (GLUT2), and fatty acid transporter-4 (FATP4), in the jejunum significantly increased after dietary supplementation with 500 mg/kg pancreatin (p < 0.05). An increased villus height-to-crypt depth ratio of the ileum was observed in the 500 mg/kg pancreatin-treated group (p < 0.05). The composition of the colonic microbiota modulated by the addition of 500 mg/kg pancreatin was characterized by an increased relative abundance of Lactobacillus (p < 0.05), and the predicted functions revealed that 500 mg/kg pancreatin supplementation enhanced the functional abundance of genetic information processing in colonic microorganisms and environmental information processing. Our findings suggested that the addition of 500 mg/kg pancreatin improved the growth performance of piglets, improved intestinal structure, and modulated the colon microbiota, thereby increasing nutrient digestibility.

Evaluation of PepT1 (SLC15A1) Substrate Characteristics of Therapeutic Cyclic Peptides

The human peptide transporter hPepT1 (SLC15A1), physiologically transporting dipeptides and tripeptides generated during food digestion, also plays a role in the uptake of small bioactive peptides and peptide-like drugs. Moreover, it might be addressed in prodrug strategies of poorly absorbed drugs. We hypothesised that the cyclic drug peptides octreotide and pasireotide could be substrates of this transporter because their diameter can resemble the size of dipeptides or tripeptides due to their strong structural curvature and because they reach the systemic circulation in Beagle dogs. For investigating possible hPepT1 substrate characteristics, we generated and characterised a CHO-K1 cell line overexpressing SLC15A1 by transfection and selection via magnetic beads. Possible inhibition of the uptake of the prototypical substrate Gly-Sar by octreotide and pasireotide was screened, followed by quantifying the uptake of the cyclic peptides in cells overexpressing SLC15A1 compared with the parental cell line. Although inhibition of Gly-Sar uptake was observed, uptake of octreotide and pasireotide was not increased in SLC15A1 overexpressing cells, indicating a lack of transport by hPepT1. Our data clearly indicate that octreotide and pasireotide are nonsubstrate inhibitors of hPepT1 and that their oral bioavailability cannot be explained by absorption via hPepT1.

Cryo-EM Structure of an Atypical Proton-Coupled Peptide Transporter: Di- and Tripeptide Permease C

Proton-coupled Oligopeptide Transporters (POTs) of the Major Facilitator Superfamily (MFS) mediate the uptake of short di- and tripeptides in all phyla of life. POTs are thought to constitute the most promiscuous class of MFS transporters, with the potential to transport more than 8400 unique substrates. Over the past two decades, transport assays and biophysical studies have shown that various orthologues and paralogues display differences in substrate selectivity. The E. coli genome codes for four different POTs, known as Di- and tripeptide permeases A-D (DtpA-D). DtpC was shown previously to favor positively charged peptides as substrates. In this study, we describe, how we determined the structure of the 53 kDa DtpC by cryogenic electron microscopy (cryo-EM), and provide structural insights into the ligand specificity of this atypical POT. We collected and analyzed data on the transporter fused to split superfolder GFP (split sfGFP), in complex with a 52 kDa Pro-macrobody and with a 13 kDa nanobody. The latter sample was more stable, rigid and a significant fraction dimeric, allowing us to reconstruct a 3D volume of DtpC at a resolution of 2.7 Å. This work provides a molecular explanation for the selectivity of DtpC, and highlights the value of small and rigid fiducial markers such as nanobodies for structure determination of low molecular weight integral membrane proteins lacking soluble domains.

DL-methionine and DL-methionyl-DL-methionine increase intestinal development and activate Wnt/β-catenin signaling activity in domestic pigeons (Columba livia)

This experiment was undertaken to investigate the effects of parental dietary DL-methionine (DL-Met) and DL-methionyl-DL-methionine (DL-Met-Met) supplementation on the intestinal development of young squabs. A total of 108 pairs of breeding pigeons and 432 one-day-old squabs were randomly divided into 3 groups: the control group (CON) was fed a basal diet (CP = 15%) and the experimental groups were fed a basal diet supplemented with 0.3% DL-Met or DL-Met-Met. Each pair of breeding pigeons nourished 4 young squabs, and 8 squabs from each treatment were randomly sampled at the end of the experiment. The results indicated that DL-Met and DL-Met-Met supplementation improved the intestinal morphology and structure in the squabs, as reflected by the increased relative intestinal weight of each small intestinal segment, villus height, and villus to crypt ratio. In addition, DL-Met and DL-Met-Met supplementation significantly increased the protein expression of cell proliferation markers (Ki67 and PCNA) and tight junction proteins (ZO-1 and Claudin-1) in the jejunum and strengthened the fluorescence signal intensity of Ki67, PCNA and Villin. Moreover, the expression of Wnt/β-catenin signaling pathway-related proteins (Frizzled 7 [FZD7], p-GSK-3β, Active β-catenin, β-catenin, TCF4, c-Myc, and Cyclin D1), and intestinal peptide transporter 1 (PepT1) in the jejunum was considerably higher in the treatment group than in the CON group (P < 0.05), with the DL-Met-Met group having the highest expression. Consistently, the molecular docking results predicted the possibility that DL-Met or DL-Met-Met binds to the membrane receptor FZD7, which mediates Wnt/β-catenin signaling. Collectively, the improvement of the intestinal development in squabs after parental dietary 0.3% DL-Met and DL-Met-Met supplementation could be through activation of Wnt/β-catenin signaling pathway, and DL-Met-Met is superior to DL-Met. Our findings may provide basic data for further optimizing the feeding formula of breeding pigeons and improving the growth and development of squabs.

Amino acid transporters as modulators of glucose homeostasis

Amino acids modulate glucose homeostasis. Cytosolic levels of amino acids are regulated by amino acid transporters, modulating insulin release, protein synthesis, cell proliferation, cell fate, and metabolism. In β-cells, amino acid transporters modulate incretin-stimulated insulin release. In the liver, amino acid transporters provide glutamine and alanine for gluconeogenesis. Intestinal amino acid transporters facilitate the intake of amino acids causing protein restriction when inactive. Adipocyte development is regulated by amino acid transporters through activation of mechanistic target of rapamycin (mTORC1) and amino acid-related metabolites. The accumulation and metabolism of branched-chain amino acids (BCAAs) in muscle depends on transporters. The integration between amino acid metabolism and transport is critical for the maintenance and function of tissues and cells involved in glucose homeostasis.

PEPT1 is essential for the growth of pancreatic cancer cells: a viable drug target

PEPT1 is a proton-coupled peptide transporter that is up-regulated in PDAC cell lines and PDXs, with little expression in the normal pancreas. However, the relevance of this up-regulation to cancer progression and the mechanism of up-regulation have not been investigated. Herein, we show that PEPT1 is not just up-regulated in a large panel of PDAC cell lines and PDXs but is also functional and transport-competent. PEPT2, another proton-coupled peptide transporter, is also overexpressed in PDAC cell lines and PDXs, but is not functional due to its intracellular localization. Using glibenclamide as a pharmacological inhibitor of PEPT1, we demonstrate in cell lines in vitro and mouse xenografts in vivo that inhibition of PEPT1 reduces the proliferation of the cancer cells. These findings are supported by genetic knockdown of PEPT1 with shRNA, wherein the absence of the transporter significantly attenuates the growth of cancer cells, both in vitro and in vivo, suggesting that PEPT1 is critical for the survival of cancer cells. We also establish that the tumor-derived lactic acid (Warburg effect) in the tumor microenvironment supports the transport function of PEPT1 in the maintenance of amino acid nutrition in cancer cells by inducing MMPs and DPPIV to generate peptide substrates for PEPT1 and by generating a H+ gradient across the plasma membrane to energize PEPT1. Taken collectively, these studies demonstrate a functional link between PEPT1 and extracellular protein breakdown in the tumor microenvironment as a key determinant of pancreatic cancer growth, thus identifying PEPT1 as a potential therapeutic target for PDAC.

Amino Acid Homeostasis in Mammalian Cells with a Focus on Amino Acid Transport

Amino acid homeostasis is maintained by import, export, oxidation, and synthesis of nonessential amino acids, and by the synthesis and breakdown of protein. These processes work in conjunction with regulatory elements that sense amino acids or their metabolites. During and after nutrient intake, amino acid homeostasis is dominated by autoregulatory processes such as transport and oxidation of excess amino acids. Amino acid deprivation triggers processes such as autophagy and the execution of broader transcriptional programs to maintain plasma amino acid concentrations. Amino acid transport plays a crucial role in the absorption of amino acids in the intestine, the distribution of amino acids across cells and organs, the recycling of amino acids in the kidney, and the recycling of amino acids after protein breakdown.

IAF, QGF, and QDF Peptides Exhibit Cholesterol-Lowering Activity through a Statin-like HMG-CoA Reductase Regulation Mechanism: In Silico and In Vitro Approach

In this study, in silico approaches are employed to investigate the binding mechanism of peptides derived from cowpea β-vignin and HMG-CoA reductase. With the obtained information, we designed synthetic peptides to evaluate their in vitro enzyme inhibitory activity. In vitro, the total protein extract and <3 kDa fraction, at 5000 µg, support this hypothesis (95% and 90% inhibition of HMG-CoA reductase, respectively). Ile-Ala-Phe, Gln-Gly-Phe, and Gln-Asp-Phe peptides were predicted to bind to the substrate binding site of HMGCR via HMG-CoAR. In silico, it was established that the mechanism of HMG-CoA reductase inhibition largely entailed mimicking the interactions of the decalin ring of simvastatin and via H-bonding; in vitro studies corroborated the predictions, whereby the HMG-CoA reductase activity was decreased by 69%, 77%, and 78%, respectively. Our results suggest that Ile-Ala-Phe, Gln-Gly-Phe, and Gln-Asp-Phe peptides derived from cowpea β-vignin have the potential to lower cholesterol synthesis through a statin-like regulation mechanism.

Growth Hormone Overexpression Induces Hyperphagia and Intestinal Morphophysiological Adaptations to Improve Nutrient Uptake in Zebrafish

The excess of circulating growth hormone (GH) in most transgenic animals implies mandatory growth resulting in higher metabolic demand. Considering that the intestine is the main organ responsible for the digestion, absorption, and direction of dietary nutrients to other tissues, this study aimed to investigate the mechanisms by which gh overexpression modulates the intestine to support higher growth. For this purpose, we designed an 8-weeks feeding trial to evaluate growth parameters, feed intake, and intestinal morphometric indices in the adult gh-transgenic zebrafish (Danio rerio) model. To access the sensitivity of the intestine to the excess of circulating GH, the messenger RNA (mRNA) expression of intestine GH receptors (GHRs) (ghra and ghrb) was analyzed. In addition, the expression of insulin-like growth factor 1a (igf1a) and genes encoding for di and tripeptide transporters (pept1a and pept1b) were assessed. Gh-transgenic zebrafish had better growth performance and higher feed intake compared to non-transgenic sibling controls. Chronic excess of GH upregulates the expression of its cognate receptor (ghrb) and the main growth factor related to trophic effects in the intestine (igf1a). Moreover, transgenic zebrafish showed an increased intestinal absorptive area and higher expression of crucial genes related to the absorption of products from meal protein degradation. These results reinforce the ability of GH to modulate intestinal morphology and the mechanisms of assimilation of nutrients to sustain the energy demand for the continuous growth induced by the excess of circulating GH.

A Novel UPLC-MS/MS Method Identifies Organ-Specific Dipeptide Profiles

BACKGROUND

Amino acids have a central role in cell metabolism, and intracellular changes contribute to the pathogenesis of various diseases, while the role and specific organ distribution of dipeptides is largely unknown.

METHOD

We established a sensitive, rapid and reliable UPLC-MS/MS method for quantification of 36 dipeptides. Dipeptide patterns were analyzed in brown and white adipose tissues, brain, eye, heart, kidney, liver, lung, muscle, sciatic nerve, pancreas, spleen and thymus, serum and urine of C57BL/6N wildtype mice and related to the corresponding amino acid profiles.

RESULTS

A total of 30 out of the 36 investigated dipeptides were detected with organ-specific distribution patterns. Carnosine and anserine were most abundant in all organs, with the highest concentrations in muscles. In liver, Asp-Gln and Ala-Gln concentrations were high, in the spleen and thymus, Glu-Ser and Gly-Asp. In serum, dipeptide concentrations were several magnitudes lower than in organ tissues. In all organs, dipeptides with C-terminal proline (Gly-Pro and Leu-Pro) were present at higher concentrations than dipeptides with N-terminal proline (Pro-Gly and Pro-Leu). Organ-specific amino acid profiles were related to the dipeptide profile with several amino acid concentrations being related to the isomeric form of the dipeptides. Aspartate, histidine, proline and serine tissue concentrations correlated with dipeptide concentrations, when the amino acids were present at the C- but not at the N-terminus.

CONCLUSION

Our multi-dipeptide quantification approach demonstrates organ-specific dipeptide distribution. This method allows us to understand more about the dipeptide metabolism in disease or in healthy state.

Rapid and Sensitive Quantification of Intracellular Glycyl-Sarcosine for Semi-High-Throughput Screening for Inhibitors of PEPT-1

The peptide transporter PEPT-1 (SLC15A1) plays a major role in nutritional supply with amino acids by mediating the intestinal influx of dipeptides and tripeptides generated during food digestion. Its role in the uptake of small bioactive peptides and various therapeutics makes it an important target for the investigation of the systemic absorption of small peptide-like active compounds and prodrug strategies of poorly absorbed therapeutics. The dipeptide glycyl-sarcosine (Gly-Sar), which comprises an N-methylated peptide bond that increases stability against enzymatic degradation, is widely utilized for studying PEPT-1-mediated transport. To support experiments on PEPT-1 inhibitor screening to identify potential substrates, we developed a highly sensitive Gly-Sar quantification assay for Caco-2 cell lysates with a dynamic range of 0.1 to 1000 ng/mL (lower limit of quantification 0.68 nM) in 50 µL of cell lysate. The assay was validated following the applicable recommendations for bioanalytic method validation of the FDA and EMA. Sample preparation and quantification were established in 96-well cell culture plates that were also used for the cellular uptake studies, resulting in a rapid and robust screening assay for PEPT-1 inhibitors. This sample preparation principle, combined with the high sensitivity of the UPLC-MS/MS quantification, is suitable for screening assays for PEPT-1 inhibitors and substrates in high-throughput formats and holds the potential for automation. Applicability was demonstrated by IC₅₀ determinations of the known PEPT-1 inhibitor losartan, the known substrates glycyl-proline (Gly-Pro), and valaciclovir, the prodrug of aciclovir, which itself is no substrate of PEPT-1 and consequently showed no inhibition in our assay.

Marked difference in efficiency of the digestive enzymes pepsin, trypsin, chymotrypsin, and pancreatic elastase to cleave tightly folded proteins

In order for the intestinal mucosa to absorb dietary proteins they have to be digested into single amino acids or very short peptides of a length of not more than four amino acids. In order to study the efficiency of the digestive endopeptidases to digest folded proteins we have analyzed several target proteins under different conditions, native proteins, heat denatured and acid treated. The three pancreatic serine proteases, trypsin, chymotrypsin, and pancreatic elastase, were found to be remarkable inefficient in cleaving native folded proteins whereas pepsin, which acts at a very low pH (pH 1.2) was much more efficient, possibly due to the denaturing conditions and thereby better accessibility to internal cleavage sites at the low pH. Heat treatment improved the cleavage considerably by all three pancreatic enzymes, but acid treatment followed by return to neutral pH did not have any major effect. Cleavage at the low pH when the protein is in a denatured state, is apparently very efficient. This indicates that pepsin is the prime enzyme cleaving the properly folded native proteins and that the pancreatic enzymes primarily are involved in generating single amino acids or very short peptides for efficient uptake by the intestinal mucosa.

Properties and regulation of Gly-Sar uptake and transport in bovine intestinal epithelial cells

Intestinal absorption of peptides is vital for the overall health and productivity of dairy cows. This study investigated the regulation, uptake and transport of dipeptides in bovine intestinal epithelial cells (BIECs). We also evaluated the effects of time, pH, concentration of the dipeptides, temperature, presence of diethylpyrocarbonate (DEPC)-an inhibitor of PepT1, and other dipeptides (Met-Met, Lys-Lys or Met-Lys), on the uptake and transport of Gly-Sar-FITC, which was a common fluorophore-labelled dipeptide. Under controlled experiments, BIECs were treated with 25 μM LY294002 (a phosphatidylinositol 3-kinase (PI3K) inhibitor) and 25 μM Perifosine (a protein kinase B (AKT) inhibitor). The subsequent expression of PepT1 in the BIECs was assessed by reverse transcriptase polymerase chain reaction (RT-PCR) and Western blotting. It was found that the uptake and transport of Gly-Sar-FITC were significant high at 37℃ than that at 4℃. The optimal pH for transport and uptake of Gly-Sar-FITC was 6.0-6.5, whereas the two properties decreased significantly in the presence of DEPC, Met-Met, Lys-Lys and Met-Lys (p < 0.05). The apical-to-basolateral transport was also found to be significantly higher than the reverse transport (p < 0.05). PI3K and AKT inhibitors were found to significantly suppress the expression of PepT1, thus impairing uptake and transport of Gly-Sar-FITC. Findings of this study thus suggest that the uptake and transport of Gly-Sar-FITC in BIECs are mediated by PepT1, and the PI3K/AKT signalling pathway regulates the absorption of small peptides.

JAK-STAT Pathway Inhibition Partially Restores Intestinal Homeostasis in Hdac1- and Hdac2-Intestinal Epithelial Cell-Deficient Mice

We have previously reported that histone deacetylase epigenetic regulator Hdac1 and Hdac2 deletion in intestinal epithelial cells (IEC) disrupts mucosal tissue architecture and barrier, causing chronic inflammation. In this study, proteome and transcriptome analysis revealed the importance of signaling pathways induced upon genetic IEC-Hdac1 and Hdac2 deletion. Indeed, Gene Ontology biological process analysis of enriched deficient IEC RNA and proteins identified common pathways, including lipid metabolic and oxidation-reduction process, cell adhesion, and antigen processing and presentation, related to immune responses, correlating with dysregulation of major histocompatibility complex (MHC) class II genes. Top upstream regulators included regulators associated with environmental sensing pathways to xenobiotics, microbial and diet-derived ligands, and endogenous metabolites. Proteome analysis revealed mTOR signaling IEC-specific defects. In addition to mTOR, the STAT and Notch pathways were dysregulated specifically in jejunal IEC. To determine the impact of pathway dysregulation on mutant jejunum alterations, we treated mutant mice with Tofacitinib, a JAK inhibitor. Treatment with the inhibitor partially corrected proliferation and tight junction defects, as well as niche stabilization by increasing Paneth cell numbers. Thus, IEC-specific histone deacetylases 1 (HDAC1) and 2 (HDAC2) support intestinal homeostasis by regulating survival and translation processes, as well as differentiation and metabolic pathways. HDAC1 and HDAC2 may play an important role in the regulation of IEC-specific inflammatory responses by controlling, directly or indirectly, the JAK/STAT pathway. IEC-specific JAK/STAT pathway deregulation may be, at least in part, responsible for intestinal homeostasis disruption in mutant mice.

The duration of food withdrawal affects the intestinal structure, nutrients absorption, and utilization in broiler chicken

Food withdrawal is usually used for accurate feed metabolizable energy (ME) assessment in poultry, but its effects on intestinal structure and the absorption of nutrients are unclear. In this study, broilers were fed ad libitum (CT) or withdrew food for 12 (FH12), 24 (FH24), 36 (FH36), or 48 hours (FH48). We showed that food withdrawal increased the energy assimilation when compared with the CT. Food withdrawal improved the digestibility of ether extract and the level of lipid substances and fatty acid-derived β-hydroxybutyrate in serum. Compared to the CT, food withdrawal did not influence the digestibility of starch. Due to 12 hours or longer food withdrawal duration increased glutamate oxidation and uric acid excretion, the analyzed digestibility of crude protein was underestimated, although the upregulated amino acid transporter genes. In addition, histological analysis showed that short-term food withdrawal (12 hours) increased intestinal villus height, crypt depth, and proliferative cell, whereas prolonged food withdrawal (more than 24 hours) impaired villus structure due to the decreased cell proliferation. Moreover, proteomics analysis revealed upregulated pathways in birds withdrawn food for 36 hours involved in nutrient absorption and amino acid oxidation. In conclusion, food withdrawal changes nutrient absorption and utilization, especially for amino acid and ether extract, and results in increased ME. Both glutamate oxidation and fatty acid incomplete oxidation are involved in energy supply after refeeding. In contrast to short-term food withdrawal, prolonged food withdrawal impairs the intestinal structure and villus renewal. Our findings deserve attention from nutritionists who are analyzing food digestibility.

Expression of the Tas1r3 and Pept1 genes in the digestive tract of wagyu cattle

Animals have precise recognition systems for amino acids and peptides that regulate their feeding behavior as well as metabolic responses. Because of their particular gastrointestinal structure, ruminants are expected to have unique mechanisms of amino acid regulation in the digestive tract. To better understand these mechanisms in the ruminant digestive tract, the expression of Tas1r3 and Pept1 was studied along the gastrointestinal tract of Japanese Black cattle through quantitative RT-PCR and immunohistochemistry. Tas1r3 mRNA was detected ubiquitously along the gastrointestinal tract, and the most predominant expression was observed in the reticulum. In addition, the presence of Tas1r3 receptor was confirmed in the rumen through immunohistochemistry. The expression level of Pept1 mRNA was higher in the forestomach (rumen, reticulum, and omasum) and small intestine (duodenum) than that in the tongue, and predominant expression was observed in the rumen. By contrast, a negligible amount of Pept1 mRNA was detected in the abomasum and large intestine. Further studies on the roles of Tas1r3 and Pept1 in the digestive tract, in particular, in the four components of the stomach, will help us to understand the mechanisms of amino acids regulation in ruminants and provide the basis for formulating cattle diets to improve the health and productivity of cattle.

Absorptive transport of amino acids by the rat colon

The capacity of the colon to absorb microbially produced amino acids (AAs) and the underlying mechanisms of AA transport are incompletely defined. We measured the profile of 16 fecal AAs along the rat ceco-colonic axis and compared unidirectional absorptive AA fluxes across mucosal tissues isolated from the rat jejunum, cecum, and proximal colon using an Ussing chamber approach, in conjunction with ¹H-NMR and ultra-performance liquid chromatography-mass spectrometry chemical analyses. Passage of stool from cecum to midcolon was associated with segment-specific changes in fecal AA composition and a decrease in total AA content. Simultaneous measurement of up to 16 AA fluxes under native luminal conditions, with correction for endogenous AA release, demonstrated absorptive transfer of AAs across the cecum and proximal colon at rates comparable (30-80%) to those across the jejunum, with significant Na⁺-dependent and H⁺-stimulated components. Expression profiling of 30 major AA transporter genes by quantitative PCR revealed comparatively high levels of transcripts for 20 AA transporters in the cecum and/or colon, with the levels of 12 exceeding those in the small intestine. Our results suggest a more detailed model of major apical and basolateral AA transporters in rat colonocytes and provide evidence for a previously unappreciated transfer of AAs across the colonic epithelium that could link the prodigious metabolic capacities of the luminal microbiota, the colonocytes, and the body tissues.NEW & NOTEWORTHY This study provides evidence for a previously unappreciated transfer of microbially generated amino acids across the colonic epithelium under physiological conditions that could link the prodigious metabolic capacities of the luminal microbiota, the colonocytes, and the body tissues. The segment-specific expression of at least 20 amino acid transporter genes along the colon provides a detailed mechanistic basis for uniport, heteroexchange, Na⁺-cotransport, and H⁺-cotransport components of colonic amino acid absorption.

Calcium-sensing receptor regulates intestinal dipeptide absorption via Ca2+ signaling and IKCa activation

Although absorption of di- and tripeptides into intestinal epithelial cells occurs via the peptide transporter 1 (PEPT1, also called solute carrier family 15 member 1 (SLC15A1)), the detailed regulatory mechanisms are not fully understood. We examined: (a) whether dipeptide absorption in villous enterocytes is associated with a rise in cytosolic Ca2+ ([Ca2+ ]cyt ), (b) whether the calcium sensing receptor (CaSR) is involved in dipeptide-elicited [Ca2+ ]cyt signaling, and (c) what potential consequences of [Ca2+ ]cyt signaling may enhance enterocyte dipeptide absorption. Dipeptide Gly-Sar and CaSR agonist spermine markedly raised [Ca2+ ]cyt in villous enterocytes, which was abolished by NPS-2143, a selective CaSR antagonist and U73122, an phospholipase C (PLC) inhibitor. Apical application of Gly-Sar induced a jejunal short-circuit current (Isc), which was reduced by NPS-2143. CaSR expression was identified in the lamina propria and on the basal enterocyte membrane of mouse jejunal mucosa in both WT and Slc15a1-/- animals, but Gly-Sar-induced [Ca2+ ]cyt signaling was significantly decreased in Slc15a1-/- villi. Clotrimazole and TRM-34, two selective blockers of the intermediate conductance Ca2+ -activated K+ channel (IKCa ), but not iberiotoxin, a selective blocker of the large-conductance K+ channel (BKCa ) and apamin, a selective blocker of the small-conductance K+ channel (SKCa ), significantly inhibited Gly-Sar-induced Isc in native tissues. We reveal a novel CaSR-PLC-Ca2+ -IKCa pathway in the regulation of small intestinal dipeptide absorption, which may be exploited as a target for future drug development in human nutritional disorders.

Boron delivery for boron neutron capture therapy targeting a cancer-upregulated oligopeptide transporter

Boron neutron capture therapy (BNCT) is a radiotherapy utilizing the neutron capture and nuclear fission reaction of ¹⁰B taken up into tumor cells. The most commonly used boron agent in BNCT, p-borono-l-phenylalanine (BPA), is accumulated in tumors by amino acid transporters upregulated in tumor cells. Here, by using dipeptides of BPA and tyrosine (BPA-Tyr and Tyr-BPA), we propose a novel strategy of selective boron delivery into tumor cells via oligopeptide transporter PEPT1 upregulated in various cancers. Kinetic analyses indicated that BPA-Tyr and Tyr-BPA are transported by oligopeptide transporters, PEPT1 and PEPT2. The intrinsic oligopeptide transport activity in tumor cells clearly correlated with PEPT1 protein expression level but not with PEPT2, suggesting that PEPT1 is the predominant oligopeptide transporter at least in tumor cell lines. Furthermore, using BPA-Tyr and Tyr-BPA, boron was successfully delivered into PEPT1-expressing pancreatic cancer AsPC-1 cells via a PEPT1-mediated mechanism. Intravenous administration of BPA-Tyr into the mice bearing AsPC-1 xenograft tumors resulted in significant boron accumulation in the tumors. It is proposed that the oligopeptide transporters, especially PEPT1, are promising candidates for molecular targets of boron delivery in BNCT. The BPA-containing dipeptides would have a potential for the development of novel boron carriers targeting PEPT1.

Amino Acid Transport Across the Mammalian Intestine

The small intestine mediates the absorption of amino acids after ingestion of protein and sustains the supply of amino acids to all tissues. The small intestine is an important contributor to plasma amino acid homeostasis, while amino acid transport in the large intestine is more relevant for bacterial metabolites and fluid secretion. A number of rare inherited disorders have contributed to the identification of amino acid transporters in epithelial cells of the small intestine, in particular cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. These are most readily detected by analysis of urine amino acids, but typically also affect intestinal transport. The genes underlying these disorders have all been identified. The remaining transporters were identified through molecular cloning techniques to the extent that a comprehensive portrait of functional cooperation among transporters of intestinal epithelial cells is now available for both the basolateral and apical membranes. Mouse models of most intestinal transporters illustrate their contribution to amino acid homeostasis and systemic physiology. Intestinal amino acid transport activities can vary between species, but these can now be explained as differences of amino acid transporter distribution along the intestine. © 2019 American Physiological Society. Compr Physiol 9:343-373, 2019.

Apically targeted oral micelles exhibit highly efficient intestinal uptake and oral absorption

Introduction

Polymeric micelles (PMs) hold promise for improving solubility and oral absorption of poorly soluble drugs. Unfortunately, the oral absorption of PMs is also limited by intestinal epithelium. To improve the oral delivery efficiency of micelles, transporter-mediated micelles could enhance the transport efficiency across the epithelial barrier, and they have attracted more attention.

Methods

Peptide transporter 1 (PepT1)-mediated micelles (Val-PMs/Phe-PMs) were designed by grafting valine (or phenylalanine) onto the surface of curcumin (Cur)-loaded-D-α-tocopheryl polyethylene glycol 1000 succinate micelles (TP-PMs). The oral absorption mechanism and oral bioavailability were further investigated in vitro and in vivo.

Results

The cellular study showed that Val-PMs/Phe-PMs had a high PepT1 affinity, resulting in a higher drug uptake and transcellular transport than TP-PMs. In rats, Val-PMs/Phe-PMs exhibited higher intestinal accumulation in the apical side of the intestinal epithelium than TP-PMs, promoting drug diffusion across epithelial barrier. The oral bioavailability of Cur was significantly improved by Val-PMs/Phe-PMs, which was about 10.50- and 3.40-fold greater than that of Cur-Sol and TP-PMs, respectively.

Conclusion

PepT-1-mediated micelles, using PepT1 as a target on intestinal epithelium, have unique functions with intestine and prove promising for oral delivery of poorly water-soluble drugs.