Minimal molecular determinants of substrates for recognition by the intestinal peptide transporter

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Biology of Peptide Transporter 2 in Mammals: New Insights into Its Function, Structure and Regulation

Peptide transporter 2 (PepT2) in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also of significant medical and pharmacological significance in the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. In this review, we provide a comprehensive overview of the structure, substrate preferences and localization of PepT2 in mammals. As PepT2 is expressed in various organs, its function in the liver, kidney, brain, heart, lung and mammary gland has also been addressed. Finally, the regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed.

Protonation State of a Histidine Residue in Human Oligopeptide Transporter 1 (hPEPT1) Regulates hPEPT1-Mediated Efflux Activity

To clarify the role of an amino acid residue in the pH-dependent efflux process in Chinese hamster ovary (CHO) cells expressing the human oligopeptide transporter hPEPT1 (CHO/hPEPT1), we determined the effect of extracellular pH on the hPEPT1-mediated efflux process. The efflux of glycylsarcosine (Gly-Sar), a typical substrate for hPEPT1, was determined using an infinite dilution method after cells were preloaded with [³H]-Gly-Sar. The efflux of [³H]-Gly-Sar was stimulated by 5 mM unlabeled hPEPT1 substrates in the medium. This trans-stimulation phenomenon showed that hPEPT1 mediated the efflux of [³H]-Gly-Sar from CHO/hPEPT1 and that hPEPT1 is a bi-directional transporter. We then determined the effect of extracellular pH (varying from 8.0 to 3.5) on the efflux activity. The efflux activity by hPEPT1 decreased with the decrease in extracellular pH. The Henderson-Hasselbälch-type equation, which fitted well to the pH-profile of efflux activity, indicated that a single amino acid residue with a pK_a value of approximately 5.7 regulates the efflux activity. The pH-profile of the efflux activity remained almost unchanged irrespective of the proton gradient across the plasma membrane. In addition, the chemical modification of the histidine residue with diethylpyrocarbonate completely abolished the efflux activity from cells, which could be prevented by the presence of 10 mM Gly-Sar. These data indicate that the efflux process of hPEPT1 is also regulated in a pH-dependent manner by the protonation state of a histidine residue located at or near the substrate recognition site facing the extracellular space.

The Antibiotic Negamycin Crosses the Bacterial Cytoplasmic Membrane by Multiple Routes

Negamycin is a natural pseudodipeptide antibiotic with promising activity against Gram-negative and Gram-positive bacteria, including Enterobacteriaceae, Pseudomonas aeruginosa, and Staphylococcus aureus, and good efficacy in infection models. It binds to ribosomes with a novel binding mode, stimulating miscoding and inhibiting ribosome translocation.

Negamycin is a natural pseudodipeptide antibiotic with promising activity against Gram-negative and Gram-positive bacteria, including Enterobacteriaceae, Pseudomonas aeruginosa, and Staphylococcus aureus, and good efficacy in infection models. It binds to ribosomes with a novel binding mode, stimulating miscoding and inhibiting ribosome translocation. We were particularly interested in studying how the small, positively charged natural product reaches its cytoplasmic target in Escherichia coli. Negamycin crosses the cytoplasmic membrane by multiple routes depending on environmental conditions. In a peptide-free medium, negamycin uses endogenous peptide transporters for active translocation, preferentially the dipeptide permease Dpp. However, in the absence of functional Dpp or in the presence of outcompeting nutrient peptides, negamycin can still enter the cytoplasm. We observed a contribution of the DppA homologs SapA and OppA, as well as of the proton-dependent oligopeptide transporter DtpD. Calcium strongly improves the activity of negamycin against both Gram-negative and Gram-positive bacteria, especially at concentrations around 2.5 mM, reflecting human blood levels. Calcium forms a complex with negamycin and facilitates its interaction with negatively charged phospholipids in bacterial membranes. Moreover, decreased activity at acidic pH and under anaerobic conditions points to a role of the membrane potential in negamycin uptake. Accordingly, improved activity at alkaline pH could be linked to increased uptake of [³H]negamycin. The diversity of options for membrane translocation is reflected by low resistance rates. The example of negamycin demonstrates that membrane passage of antibiotics can be multifaceted and that for cytoplasmic anti-Gram-negative drugs, understanding of permeation and target interaction are equally important.

PTR2/POT/NPF transporters: what makes them tick?

PTR2/POT/NPF are a family of primarily proton coupled transporters that belong to the major facilitator super family and are found across most kingdoms of life. They are involved in uptake of nutrients, hormones, ions and several orally administered drug molecules. A wealth of structural and functional data is available for this family; the similarity between the protein structural features have been discussed and investigated in detail on several occasions, however there are no reports on the unification of substrate information. In order to fill this gap, we have collected information about substrates across the entire PTR2/POT/NPF family in order to provide key insights into what makes a molecule a substrate and whether there are common features among confirmed substrates. This review will be of particular interest for researchers in the field trying to probe the mechanisms responsible for the different selectivity of these transporters at a molecular resolution, and to design novel substrates.

Xenopus oocytes as a heterologous expression system for analysis of tight junction proteins

Claudins (cldns) represent the largest family of transmembrane tight junction (TJ) proteins, determining organ-specific epithelial barrier properties. Because methods for the analysis of multiple cldn interaction are limited, we have established the heterologous Xenopus laevis oocyte expression system for TJ protein assembly and interaction analysis. Oocytes were injected with cRNA encoding human cldn-1, -2, or -3 or with a combination of these and were incubated in pairs for interaction analysis. Immunoblotting and immunohistochemistry were performed, and membrane contact areas were analyzed morphometrically and by freeze fracture electron microscopy. Cldns were specifically detected in membranes of expressing oocytes, and coincubation of oocytes resulted in adhesive contact areas that increased with incubation time. Adjacent membrane areas revealed specific cldn signals, including "kissing-point"-like structures representing homophilic trans-interactions of cldns. Contact areas of oocytes expressing a combination markedly exceeded those expressing single cldns, indicating effects on adhesion. Ultrastructural analysis revealed a self-assembly of TJ strands and a cldn-specific strand morphology.-Vitzthum, C., Stein, L., Brunner, N., Knittel, R., Fallier-Becker, P., Amasheh, S. Xenopus oocytes as a heterologous expression system for analysis of tight junction proteins.

Function, Regulation, and Pathophysiological Relevance of the POT Superfamily, Specifically PepT1 in Inflammatory Bowel Disease

Mammalian members of the proton-coupled oligopeptide transporter family are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs and couple substrate translocation to the movement of H⁺ , with the transmembrane electrochemical proton gradient providing the driving force. Peptide transporters are responsible for the (re)absorption of dietary and/or bacterial di- and tripeptides in the intestine and kidney and maintaining homeostasis of neuropeptides in the brain. These proteins additionally contribute to absorption of a number of pharmacologically important compounds. In this overview article, we have provided updated information on the structure, function, expression, localization, and activities of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4), and PhT2 (SLC15A3). Peptide transporters, in particular, PepT1 are discussed as drug-delivery systems in addition to their implications in health and disease. Particular emphasis has been placed on the involvement of PepT1 in the physiopathology of the gastrointestinal tract, specifically, its role in inflammatory bowel diseases. © 2018 American Physiological Society. Compr Physiol 8:731-760, 2018.

Glycans in the intestinal peptide transporter PEPT1 contribute to function and protect from proteolysis

Despite the fact that many membrane proteins carry extracellular glycans, little is known about whether the glycan chains also affect protein function. We recently demonstrated that the proton-coupled oligopeptide transporter 1 (PEPT1) in the intestine is glycosylated at six asparagine residues (N50, N406, N439, N510, N515, and N532). Mutagenesis-induced disruption of the individual N-glycosylation site N50, which is highly conserved among mammals, was detected to significantly enhance the PEPT1-mediated inward transport of peptides. Here, we show that for the murine protein the inhibition of glycosylation at sequon N50 by substituting N50 with glutamine, lysine, or cysteine or by replacing S52 with alanine equally altered PEPT1 transport kinetics in oocytes. Furthermore, we provide evidence that the uptake of [¹⁴C]glycyl-sarcosine in immortalized murine small intestinal (MODE-K) or colonic epithelial (PTK-6) cells stably expressing the PEPT1 transporter N50Q is also significantly increased relative to the wild-type protein. By using electrophysiological recordings and tracer flux studies, we further demonstrate that the rise in transport velocity observed for PEPT1 N50Q is bidirectional. In line with these findings, we show that attachment of biotin derivatives, comparable in weight with two to four monosaccharides, to the PEPT1 N50C transporter slows down the transport velocity. In addition, our experiments provide strong evidence that glycosylation of PEPT1 confers resistance against proteolytic cleavage by proteinase K, whereas a remarkable intrinsic stability against trypsin, even in the absence of N-linked glycans, was detected.NEW & NOTEWORTHY This study highlights the role of N50-linked glycans in modulating the bidirectional transport activity of the murine peptide transporter PEPT1. Electrophysiological and tracer flux measurements in Xenopus oocytes have shown that removal of the N50 glycans increases the maximal peptide transport rate in the inward and outward directions. This effect could be largely reversed by replacement of N50 glycans with structurally dissimilar biotin derivatives. In addition, N-glycans were detected to stabilize PEPT1 against proteolytic cleavage.

Substrates of the human oligopeptide transporter hPEPT2

Oligopeptide transporters serve important functions in nutrition and pharmacology. In particular, these transporters help maintain the homeostasis of peptides. The peptide-transporter PEPT2 is a high-affinity and low-capacity type oligopeptide transporter from the proton-coupled oligopeptide transporter family. PEPT2 has recently received attention because of its potential application in targeted drug delivery. PEPT2 is widely distributed in kidney, central nervous system, and lung of organisms. In general, all dipeptides, tripeptides, and peptide-like drugs such as β-lactam antibiotics and angiotensin-converting enzyme inhibitors could be mediated and transported as a substrate of PEPT2. The design of many extant drugs and prodrugs is based on the substrate structure of PEPT2 to accelerate absorption via peptide transporters. Thus, this paper summarizes the substrate features of PEPT2 to promote the rational design of drugs and prodrugs that target peptide transporters.

Possible interaction of quinolone antibiotics with peptide transporter 1 in oral absorption of peptide-mimetic drugs

The study investigated whether quinolone antibiotics inhibit the PEPT1-mediated uptake of its substrates. Among the quinolones examined, lomefloxacin, moxifloxacin (MFLX) and purlifloxacin significantly inhibited the uptake of PEPT1 substrate phenylalanine-Ψ(CN-S)-alanine (Phe-Ψ-Ala) in HeLa/PEPT1 cells to 31.6 ± 1.3%, 27.6 ± 2.9%, 36.8 ± 2.2% and 32.6 ± 1.4%, respectively. Further examination showed that MFLX was an uncompetitive inhibitor, with an IC50 value of 4.29 ± 1.29 mm. In addition, MFLX significantly decreased the cephalexin and valacyclovir uptake in HeLa/PEPT1 cells. In an in vivo study in rats, the maximum plasma concentration (C(max)) of orally administered Phe-Ψ-Ala was significantly decreased in the presence of MFLX (171 ± 1 ng/ml) compared with that in its absence (244 ± 9 ng/ml). The area under the concentration-time curve (AUC) of orally administered Phe-Ψ-Ala in the presence of MFLX (338 ± 50 ng/ml · h) tended to decrease compared with that in its absence (399 ± 75 ng/ml · h). The oral bioavailability of Phe-Ψ-Ala in the presence and absence of MFLX was 41.7 ± 6.2% and 49.2 ± 9.2%, respectively. The results indicate that administration of quinolone antibiotics concomitantly with PEPT1 substrate drugs may potentially result in drug-drug interaction.

Human oligopeptide transporter 2 (PEPT2) mediates cellular uptake of polymyxins

OBJECTIVES

Polymyxins are a last-line therapy to treat MDR Gram-negative bacterial infections. Nephrotoxicity is the dose-limiting factor for polymyxins and recent studies demonstrated significant accumulation of polymyxins in renal tubular cells. However, little is known about the mechanism of polymyxin uptake into these cells. Oligopeptide transporter 2 (PEPT2) is a solute carrier transporter (SLC) expressed at the apical membrane of renal proximal tubular cells and facilitates drug reabsorption in the kidney. In this study, we examined the role of PEPT2 in polymyxin uptake into renal tubular cells.

METHODS

We investigated the inhibitory effects of colistin and polymyxin B on the substrate uptake mediated through 15 essential SLCs in overexpressing HEK293 cells. The inhibitory potency of both polymyxins on PEPT2-mediated substrate uptake was measured. Fluorescence imaging was employed to investigate PEPT2-mediated uptake of the polymyxin fluorescent probe MIPS-9541 and a transport assay was conducted with MIPS-9541 and [(3)H]polymyxin B1.

RESULTS

Colistin and polymyxin B potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 11.4 ± 3.1 and 18.3 ± 4.2 μM, respectively). In contrast, they had no or only mild inhibitory effects on the transport activity of the other 14 SLCs evaluated. MIPS-9541 potently inhibited PEPT2-mediated [(3)H]glycyl-sarcosine uptake (IC50 15.9 μM) and is also a substrate of PEPT2 (Km 74.9 μM). [(3)H]polymyxin B1 was also significantly taken up by PEPT2-expressing cells (Km 87.3 μM).

CONCLUSIONS

Our study provides the first evidence of PEPT2-mediated uptake of polymyxins and contributes to a better understanding of the accumulation of polymyxins in renal tubular cells.

Nrf2 regulates the expression of the peptide transporter PEPT1 in the human colon carcinoma cell line Caco-2

BACKGROUND

PEPT1 is a rheogenic transport protein in the apical membrane of intestinal epithelial cells capable of transporting essentially all possible di- and tripeptides that are generated from the luminal protein breakdown. In addition, several anticancer, antimicrobial and antiviral drugs are taken up from the intestinal lumen via PEPT1 and therefore PEPT1 is a target for efficient drug delivery via prodrug approaches. Thus, understanding PEPT1 gene regulation is not only of importance for dietary adaptation but also for drug treatment.

METHODS

In silico analysis of the Pept1 promoter was performed using MatInspector. Pept1 promoter constructs were generated and cotransfected with an Nrf2 expression plasmid. Caco-2 cells were stimulated with Nrf2 inducers followed by electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP). Biological relevance was investigated using western blot analysis and transport activity assays.

RESULTS

Reporter gene assays showed transcriptional activation of the Pept1 promoter in response to Nrf2 overexpression. EMSA as well as ChIP analysis validated Nrf2 binding to the ARE located closest to the start codon (Pept1-ARE1). Induction of the Nrf2 pathway resulted in increased endogenous PEPT1 protein abundance as well as transport activity. Moreover, we demonstrate that also the induction of autophagy by MG132 resulted in elevated Nrf2 binding to Pept1-ARE1 and increased PEPT1 protein expression.

CONCLUSION

In summary, we identified a biologically active Nrf2 binding site within the Pept1 promoter which links Pept1 to the cellular defense program activated by Nrf2.

GENERAL SIGNIFICANCE

This study identifies Pept1 as an inducible target gene of the Nrf2 pathway.

Comparative digestive physiology

In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of carbohydrates, proteins, fats, and material(s) refractory to rapid digestion (e.g., cellulose). The expression of digestive enzymes and nutrient transporters approximately matches the dietary load of their respective substrates, with relatively modest excess capacity. Mechanisms explaining differences in hydrolase activity between populations and species include gene copy number variations and single-nucleotide polymorphisms. Transcriptional and posttranscriptional adjustments mediate phenotypic changes in the expression of hydrolases and transporters in response to dietary signals. Many species respond to higher food intake by flexibly increasing digestive compartment size. Fermentative processes by symbiotic microorganisms are important for cellulose degradation but are relatively slow, so animals that rely on those processes typically possess special enlarged compartment(s) to maintain a microbiota and other GI structures that slow digesta flow. The taxon richness of the gut microbiota, usually identified by 16S rRNA gene sequencing, is typically an order of magnitude greater in vertebrates than invertebrates, and the interspecific variation in microbial composition is strongly influenced by diet. Many of the nutrient transporters are orthologous across different animal phyla, though functional details may vary (e.g., glucose and amino acid transport with K+ rather than Na+ as a counter ion). Paracellular absorption is important in many birds. Natural toxins are ubiquitous in foods and may influence key features such as digesta transit, enzymatic breakdown, microbial fermentation, and absorption.

A randomized, double-blind clinical study on the safety and tolerability of an iron multi-amino acid chelate preparation in premenopausal women

ABSTRACT Considerable risk of iron deficiency has been identified in premenopausal women because of the adverse effects associated with commercial iron preparations. This study examined the safety and tolerability of a novel iron multi-amino acid chelate (IMAAC) preparation in premenopausal women. A single-centre, randomized, double-blind, three-arm placebo-controlled (n = 60) study was conducted where subjects received one of three test materials: IMAAC (600 mg) or ferrous sulfate (600 mg) each containing 25 mg of elemental iron, or placebo as a single daily dose for 7 days. After testing, there were no significant differences found in any of the hematological outcomes between the different test groups. The safety analyses showed that a significantly (p = .044) higher number of patients reported adverse events when taking the ferrous sulfate supplement compared to IMAAC. A significantly lower number of adverse effects (p = .008) were reported by subjects on IMAAC. The current study demonstrated the superiority of the IMAAC preparation over ferrous sulfate with regards to tolerability and adverse effects.

Characterization of oligopeptide transporter (PepT1) in grass carp (Ctenopharyngodon idella)

The oligopeptide transporter (PepT1) is located on the brush-border membrane of the intestinal epithelium, and plays an important role in dipeptide and tripeptide absorptions from protein digestion. In this study, we cloned the PepT1 cDNA from grass carp and characterized its expression profile in response to dietary protein and feed additives (sodium butyrate) treatments. The PepT1 gene encodes a protein of 714 amino acids with high sequence similarity with other vertebrate homologues. Expression analysis revealed highest levels of PepT1 mRNA expression in the foregut of grass carp. In addition, PepT1 mRNA expression exhibited diurnal variation in all three bowel segments of intestine with lower levels of expression in daytime than nighttime. During embryonic development, PepT1 showed a dynamic pattern of expression reaching maximal levels of expression in the gastrula stage and minimal levels in the organ stage. The PepT1 expression showed constant levels from 14 to 34 day post-hatch. To determine whether fish diet of different protein contents may have any effect on PepT1 expression, we extended our research to dietary regulation of PepT1 expression. We found that dietary protein levels had a significant effect on PepT1 gene expression. In addition, PepT1 mRNA levels were higher after feeding with fish meal than with soybean meal. Moreover, in vitro and in vivo sodium butyrate treatments increased PepT1 expression in the intestine of grass carp. The results demonstrate for the first time that PepT1 mRNA expression is regulated in a temporal and spatial pattern during development, and dietary protein and feed additives had a significant effects on PepT1 gene expression in grass carp.

Sodium caprate as an enhancer of macromolecule permeation across tricellular tight junctions of intestinal cells

Sodium caprate is a promising candidate for inducing drug absorption enhancement. The mechanism of that uptake-enhancing effect is not fully understood so far. We investigated how caprate acts in an established human intestinal cell line, HT-29/B6, on the transient opening of transcellular (across the cell membranes) and paracellular (across the tight junction) pathways. Sodium caprate (10 mm) caused a rapid and reversible decrease of transepithelial resistance which is based, as measured by two-path impedance spectroscopy, exclusively on resistance changes of the paracellular pathway. Measurements of paracellular marker fluxes revealed an increased permeability for fluorescein (330 Da) and FITC-dextran (4 and 10 kDa), indicating an opening of the paracellular barrier. Confocal microscopy revealed a marked reduction of tricellulin in tricellular tight junctions and of claudin-5 in bicellular tight junctions. This was not due to altered protein expression, as occludin, claudins or tricellulin were not significantly changed in Western blots. Visualization of the translocation site of the cell membrane-impermeable marker molecule sulpho-NHS-SS-biotin (607 Da) indicated the tricellular tight junction to be the predominant pathway. We suggest that caprate's known enhancing effect on intestinal drug uptake is based on increased permeability in tricellular cell contacts, mediated by reversible removal of tricellulin from the tricellular tight junction.

Postprandial blood cell transcriptomics in response to the ingestion of dairy products by healthy individuals

The aim of this intervention study was to measure genome-wide postprandial gene expression in human blood cells after the ingestion of a single serving of milk, to identify the downstream physiological processes regulated by the differentially expressed genes, and to use this gene expression signature as a reference to compare it with the response following the ingestion of a transformed dairy product, namely, yogurt. We conducted a randomized, controlled, single-blinded, crossover study on six healthy male individuals. After an overnight fast, 540 g of milk or yogurt was ingested by the subjects. Blood samples were collected before (0 h) and after (2 h/4 h/6 h) ingestion, and the blood cell transcriptome was analyzed using a linear kinetic analysis that increases the statistical power of the study. The differentially expressed transcripts identified after the ingestion of milk (575 transcripts) and yogurt (625 transcripts) modulated similar biological processes. In particular, genes involved in protein biosynthesis and mitochondrial activities followed biphasic kinetics being down-regulated at 2 h and more pronouncedly up-regulated at 6 h. The opposite kinetics were observed for inflammatory and apoptotic processes during the same time frame. The human blood cell transcriptome appeared to be specifically modulated by specific nutrients present in bovine milk, a property that was further modified when milk was fermented to yogurt. The coordinated changes in postprandial expression of genes involved in basic biological processes suggest that postprandial blood cell transcriptomics may allow insight into the nutritional effects of selected foods in the prevention or development of chronic metabolic and inflammatory disorders.

Pivotal roles of peptide transporter PEPT1 and ATP-binding cassette (ABC) transporter ABCG2 in 5-aminolevulinic acid (ALA)-based photocytotoxicity of gastric cancer cells in vitro

BACKGROUND

Recently, 5-aminolevulinic acid-based photodynamic therapy (ALA-PDT) is being widely used in cancer therapy owing to the tumor-specific accumulation of photosensitizing protoporphyrin IX (PpIX) after the administration of ALA. In the present study, by focusing on genes involved in the porphyrin biosynthesis pathway, we aimed to explore biomarkers that are predictive for the efficacy of ALA-PDT.

METHODS

We used five lines of human gastric cancer cells to measure the ALA-based photocytotoxicity. ALA-induced production of PpIX in cancer cells was quantified by fluorescence spectrophotometry. To examine the potential involvement of PEPT1 and ABCG2 in the ALA-PDT sensitivity, stable cell lines overexpressing PEPT1 were established and ABCG2-specific siRNA used.

RESULTS

We observed that three cell lines were photosensitive, whereas the other two cell lines were resistant to ALA-based photocytotoxicity. The ALA-based photocytotoxicity was found to be well correlated with intracellular PpIX levels, which suggests that certain enzymes and/or transporters involved in ALA-induced PpIX production are critical determinants. We found that high expression of the peptide transporter PEPT1 (ALA influx transporter) and low expression of the ATP-binding cassette transporter ABCG2 (porphyrin efflux transporter) determined ALA-induced PpIX production and cellular photosensitivity in vitro.

CONCLUSION

PEPT1 and ABCG2 are key players in regulating intracellular PpIX levels and determining the efficacy of ALA-based photocytotoxicity against gastric cancer cells in vitro. Evaluation of the expression levels of PEPT1 and ABCG2 genes could be useful to predict the efficacy of ALA-PDT. Primers specific to those target genes are practical and useful biomarkers for predicting the photo-sensitivity to ALA-PDT.

Bioavailability through PepT1: the role of computer modelling in intelligent drug design

In addition to being responsible for the majority of absorption of dietary nitrogen, the mammalian proton-coupled di- and tri-peptide transporter PepT1 is also recognised as a major route of drug delivery for several important classes of compound, including beta-lactam antibiotics and angiotensin-converting enzyme inhibitors. Thus there is considerable interest in the PepT1 protein and especially its substrate binding site. In the absence of a crystal structure, computer modelling has been used to try to understand the relationship between PepT1 3D structure and function. Two basic approaches have been taken: modelling the transporter protein, and modelling the substrate. For the former, computer modelling has evolved from early interpretations of the twelve transmembrane domain structure to more recent homology modelling based on recently crystallised bacterial members of the major facilitator superfamily (MFS). Substrate modelling has involved the proposal of a substrate binding template, to which all substrates must conform and from which the affinity of a substrate can be estimated relatively accurately, and identification of points of potential interaction of the substrate with the protein by developing a pharmacophore model of the substrates. Most recently, these two approaches have moved closer together, with the attempted docking of a substrate library onto a homology model of the human PepT1 protein. This article will review these two approaches in which computers have been applied to peptide transport and suggest how such computer modelling could affect drug design and delivery through PepT1.

Pharmaceutical and pharmacological importance of peptide transporters

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many β-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

Delta-aminolevulinic acid is a substrate for the amino acid transporter SLC36A1 (hPAT1)

BACKGROUND AND PURPOSE

delta-Aminolevulinic acid (ALA) is used in cancer patients for photodynamic diagnosis or therapy. Oral administration of ALA has been used in patients with prostate and bladder cancer. The present aim was to investigate the mechanism of intestinal absorption of ALA and its transport via the amino acid transporter SLC36A1.

EXPERIMENTAL APPROACH

In vitro investigations of ALA affinity for and uptake via SLC36A1 and SLC15A1 were performed in Caco-2 cell monolayers. Interaction of ALA with SLC15A1 was investigated in MDCK/SLC15A1 cells, whereas interactions with SLC36A1 were investigated in COS-7 cells transiently expressing SLC36A1.

KEY RESULTS

ALA inhibited SLC36A1-mediated L-[(3)H]Pro and SLC15A1-mediated [(14)C]Gly-Sar uptake in Caco-2 cell monolayers with IC(50) values of 11.3 and 2.1 mM respectively. In SLC36A1-expressing COS-7 cells, the uptake of [(14)C]ALA was saturable with a K(m) value of 6.8 +/- 3.0 mM and a V(max) of 96 +/- 13 pmol x cm(-2) x min(-1). Uptake of [(14)C]ALA was pH and concentration dependent, and could be inhibited by glycine, proline and GABA. In a membrane potential assay, translocation of ALA via SLC36A1 was concentration dependent, with a K(m) value of 3.8 +/- 1.0 mM. ALA is thus a substrate for SLC36A1. In Caco-2 cells, apical [(14)C]ALA uptake was pH dependent, but Na(+) independent, and completely inhibited by 5-hydroxy-L-tryptophan and L-4,4'-biphenylalanyl-l-proline. CONCLUSIONS AND IMPLICATIONS. ALA was a substrate for SLC36A1, and the apical absorption in Caco-2 cell was only mediated by SLC36A1 and SLC15A1. This advances our understanding of intestinal absorption mechanisms of ALA, as well as its potential for drug interactions.

Projection structure of DtpD (YbgH), a prokaryotic member of the peptide transporter family

Cellular uptake of di- and tripeptides has been characterized in numerous organisms, and various transporters have been identified. In contrast, structural information on peptide transporters is very sparse. Here, we have cloned, overexpressed, purified, and biochemically characterized DtpD (YbgH) from Escherichia coli, a prokaryotic member of the peptide transporter family. Its homologues in mammals, PEPT1 (SLC15A1) and PEPT2 (SLC15A2), not only transport peptides but also are of relevance for uptake of drugs as they accept a large spectrum of peptidomimetics such as beta-lactam antibiotics, antivirals, peptidase inhibitors, and others as substrates. Uptake experiments indicated that DtpD functions as a canonical peptide transporter and is, therefore, a valid model for structural studies of this family of proteins. Blue native polyacrylamide gel electrophoresis, gel filtration, and transmission electron microscopy of single-DtpD particles suggest that the transporter exists in a monomeric form when solubilized in detergent. Two-dimensional crystallization of DtpD yielded first tubular crystals that allowed the determination of a projection structure at better than 19 A resolution. This structure of DtpD represents the first structural view of a member of the peptide transporter family.

Peptide transporters and their roles in physiological processes and drug disposition

1. The peptide transporters belong to the peptide transporter (PTR) family and serve as integral membrane proteins for the cellular uptake of di- and tripeptides in the organism. By their ability also to transport peptidomimetics and other substrates with therapeutic activities or precursors of pharmacologically active agents, they are of considerable importance in pharmacology. 2. PEPT1 is the low-affinity, high-capacity transporter and is mainly expressed in the small intestine, whereas PEPT2 is the high-affinity, low-capacity transporter and has a broader distribution in the organism. 3. Targeted mouse models have revealed PEPT2 to be the dominant transporter for the reabsorption of di- and tripeptides and its pharmacological substrates in the organism, and for the removal of these substrates from the cerebrospinal fluid. Moreover, the peptide transporters undergo physiological and pharmacological regulation and, of great interest, are present in disease states where PEPT1 exhibits ectopic expression in colonic inflammation. 4. The paper reviews the structural characteristics of the peptide transporters, the structural requirements for substrates, the distribution of the peptide transporters in the organism, and finally their regulation in the organism in healthy and pathological situations.

Cysteine scanning of transmembrane domain three of the human dipeptide transporter: Implications for substrate transport

The human intestinal dipeptide transporter (hPepT1) transports dipeptides and pharmacologically active drugs from the intestine to the blood. The role of transmembrane domain 3 (TMD3) of hPepT1 was studied using cysteine-scanning mutagenesis and methane thiosulfonate (MTS) cysteine modification. Each amino acid in TMD3 was individually mutated to a cysteine and Gly-Sar uptake by each mutated and modified transporter was determined relative to wild-type hPepT1. Uptake data for mutated transporters modified with the lipid-insoluble cysteine-modifying reagent MTSET suggested tilting of TMD3 relative to the substrate translocation pathway; the extracellular region of TMD3 showed little MTSET reactivity, indicative of solvent inaccessibility, whereas the intracellular part of TMD3 was relatively solvent accessible. Modification at 10 positions of TMD3 with MTSEA, a lipid-soluble cysteine-modifying reagent, gave unusual and statistically significant increases in Gly-Sar uptake relative to untreated mutants. We interpret these data in terms of the spatial properties of the hPepT1 substrate translocation channel and possible interactions of TMD3 with other transmembrane domains.

Substrate-induced changes in the density of peptide transporter PEPT1 expressed in Xenopus oocytes

The adaptation of the capacity of the intestinal peptide transporter PEPT1 to varying substrate concentrations may be important with respect to its role in providing bulk quantities of amino acids for growth, development, and other nutritional needs. In the present study, we describe a novel phenomenon of the regulation of PEPT1 in the Xenopus oocyte system. Using electrophysiological and immunofluorescence methods, we demonstrate that a prolonged substrate exposure of rabbit PEPT1 (rPEPT1) caused a retrieval of transporters from the membrane. Capacitance as a measure of membrane surface area was increased in parallel with the increase in rPEPT1-mediated transport currents with a slope of approximately 5% of basal surface per 100 nA. Exposure of oocytes to the model peptide Gly-l-Gln for 2 h resulted in a decrease in maximal transport currents with no change of membrane capacitance. However, exposure to substrate for 5 h decreased transport currents but also, in parallel, surface area by endocytotic removal of transporter proteins from the surface. The reduction of the surface expression of rPEPT1 was confirmed by presteady-state current measurements and immunofluorescent labeling of rPEPT1. A similar simultaneous decrease of current and surface area was also observed when endocytosis was stimulated by the activation of PKC. Cytochalasin D inhibited all changes evoked by either dipeptide or PKC stimulation, whereas the PKC-selective inhibitor bisindolylmaleimide only affected PKC-stimulated endocytotic processes but not substrate-dependent retrieval of rPEPT1. Coexpression experiments with human Na(+)-glucose transporter 1 (hSGLT1) revealed that substrate exposure selectively affected PEPT1 but not the activity of hSGLT1.

Enhanced cancer cell growth inhibition by dipeptide prodrugs of floxuridine: increased transporter affinity and metabolic stability

Dipeptide monoester prodrugs of floxuridine were synthesized, and their chemical stability in buffers, resistance to glycosidic bond metabolism, affinity for PEPT1, enzymatic activation and permeability in cancer cells were determined and compared to those of mono amino acid monoester floxuridine prodrugs. Prodrugs containing glycyl moieties were the least stable in pH 7.4 buffer ( t 1/2 < 100 min). The activation of all floxuridine prodrugs was 2- to 30-fold faster in cell homogenates than their hydrolysis in buffer, suggesting enzymatic action. The enzymatic activation of dipeptide monoester prodrugs containing aromatic promoieties in cell homogenates was 5- to 20-fold slower than that of other dipeptide and most mono amino acid monoester prodrugs ( t 1/2 approximately 40 to 100 min). All prodrugs exhibited enhanced resistance to glycosidic bond metabolism by thymidine phosphorylase compared to parent floxuridine. In general, the 5'-O-dipeptide monoester floxuridine prodrugs exhibited higher affinity for PEPT1 than the corresponding 5'-O-mono amino acid ester prodrugs. The permeability of dipeptide monoester prodrugs across Caco-2 and Capan-2 monolayers was 2- to 4-fold higher than the corresponding mono amino acid ester prodrug. Cell proliferation assays in AsPC-1 and Capan-2 pancreatic ductal cell lines indicated that the dipeptide monoester prodrugs were equally as potent as mono amino acid prodrugs. The transport and enzymatic profiles of 5'- l-phenylalanyl- l-tyrosyl-floxuridine, 5'- l-phenylalanyl- l-glycyl-floxuridine, and 5'- l-isoleucyl- l-glycyl-floxuridine suggest their potential for increased oral uptake, delayed enzymatic bioconversion and enhanced resistance to metabolism to 5-fluorouracil, as well as enhanced uptake and cytotoxic activity in cancer cells, attributes that would facilitate prolonged systemic circulation for enhanced therapeutic action.

Board-invited review: Peptide absorption and utilization: Implications for animal nutrition and health

Over the last 50 yr, the study of intestinal peptide transport has rapidly evolved into a field with exciting nutritional and biomedical applications. In this review, we describe from a historical and current perspective intestinal peptide transport, the importance of peptides to whole-body nutrition, and the cloning and characterization of the intestinal peptide transporter, PepT1. We focus on the nutritional significance of peptide transport and relate these findings to livestock and poultry. Amino acids are transported into the enterocyte as free AA by a variety of AA transporters that vary in substrate specificity or as di- and tripeptides by the peptide transporter, PepT1. Expression of PepT1 is largely restricted to the small intestine in most species; however, in ruminants, peptide transport and activity is observed in the rumen and omasum. The extent to which peptides are absorbed and utilized is still unclear. In ruminants, peptides make a contribution to the portal-drained visceral flux of total AA and are detected in circulating plasma. Peptides can be utilized by the mammary gland for milk protein synthesis and by a variety of other tissues. We discuss the factors known to regulate expression of PepT1 including development, diet, hormones, diurnal rhythm, and disease. Expression of PepT1 is detected during embryological stages in both birds and mammals and increases with age, a strategic event that allows for the immediate uptake of nutrients after hatch or birth. Both increasing levels of protein in the diet and dietary protein deficiencies are found to upregulate the peptide transporter. We also include in this review a discussion of the use of dietary peptides and potential alternate routes of nutrient delivery to the cell. Our goal is to impart to the reader the nutritional implications of peptide transport and dietary peptides and share discoveries that shed light on various biological processes, including rapid establishment of intestinal function in early neonates and maintenance of intestinal function during fasting, starvation, and disease states.

Recognition of 2-aminothiazole-4-acetic acid derivatives by the peptide transporters PEPT1 and PEPT2

The H(+)/peptide cotransporters PEPT1 and PEPT2 have gained considerable interest in pharmaceutical sciences as routes for drug delivery. It is, therefore, of interest to develop uncommon artificial substrates for the two carriers. This study was initiated to investigate the binding affinity of 2-aminothiazole-4-acetic acid (ATAA) conjugates with amino acids to PEPT1 and PEPT2. The 2-aminothiazole-4-acetic acid derivatives have been synthesised and tested for their affinity to PEPT1 and PEPT2. The K(i) values were compared with in silico predicted values from CoMSIA models. C-terminal ATAA-Xaa conjugates proved to be low to medium inhibitors of the [(14)C]Gly-Sar uptake at both carrier systems whereas N-terminal Xaa-ATAA conjugates exhibited medium to high affinity. A promising candidate for further functionalisation is Val-ATAA which shows extraordinary high affinity to PEPT1.

Transepithelial transport of alpha-lipoic acid across human intestinal Caco-2 cell monolayers

Alpha-lipoic acid (LA) is used in dietary supplements or food with antioxidative functions. The mechanism for the intestinal absorption of alpha-lipoic acid was investigated in this study by using human intestinal Caco-2 cell monolayers. LA was rapidly transported across the Caco-2 cell monolayers, this transport being energy-dependent, suggesting transporter-mediated transport to be the mechanism involved. The LA transport was strongly dependent on the pH value, being accelerated in the acidic pH range. Furthermore, such monocarboxylic acids as benzoic acid and medium-chain fatty acids significantly inhibited LA transport, suggesting that a proton-linked monocarboxylic acid transporter (MCT) was involved in the intestinal transport of LA. The conversion of LA to the more antioxidative dihydrolipoic acid was also apparent during the transport process.

Monocarboxylate transporter 1 mediates DL-2-Hydroxy-(4-methylthio)butanoic acid transport across the apical membrane of Caco-2 cell monolayers

The methionine hydroxy analogue DL-2-hydroxy-(4-methylthio)butanoic acid (DL-HMB) is a supplementary source of methionine commonly added to commercial animal diets to satisfy the total sulfur amino acid requirement. In this study, we characterized DL-HMB transport across the apical membrane of Caco-2 cells to identify the transport mechanism involved in the intestinal absorption of this methionine source. DL-HMB transport induced a significant decrease in intracellular pH (pH(i)) and was inhibited in the presence of the protonophore carbonyl cyanide 4-(trifluoromethoxy)-phenylhydrazone. Moreover, both Na(+) removal and 5-(N-ethyl-N-isopropyl)amiloride, an inhibitor of apical Na(+)/H(+) exchanger (NHE3), significantly reduced substrate uptake and pH(i) recovery, suggesting cooperation between H(+)-dependent DL-HMB transport and NHE3 activity. cis-Inhibition experiments with L-Ala, beta-Ala, D-Pro, betaine, or glycyl-sarcosine excluded the participation of systems proton amino acid transporter 1 and peptide transporter 1. In contrast, alpha-cyano-4-hydroxycinnamate, phloretin, L-lactate, beta-hydroxybutyrate, butyrate, and pyruvate, inhibitors and substrates of monocarboxylate transporter 1 (MCT1), significantly reduced DL-HMB uptake. Dixon plot analysis of L-lactate transport in the presence of DL-HMB revealed a competitive interaction (inhibition constant, 17.5 +/- 0.11 mmol/L), confirming the participation of system MCT1. The kinetics of DL-HMB uptake was described by a model involving passive diffusion and a single low-affinity, high-capacity transport mechanism (K(D), 1.9 nL/microg protein; K(m), 13.1 +/- 0.04 mmol/L; and V(max), 43.6 +/- 0.14 pmol/microg protein) compatible with MCT1 kinetic characteristics. In conclusion, the methionine hydroxy analogue is transported in Caco-2 cell apical membrane by a transport mechanism with functional characteristics similar to those of MCT1.

Functional and structural characterization of a prokaryotic peptide transporter with features similar to mammalian PEPT1

The ydgR gene of Escherichia coli encodes a protein of the proton-dependent oligopeptide transporter (POT) family. We cloned YdgR and overexpressed the His-tagged fusion protein in E. coli BL21 cells. Bacterial growth inhibition in the presence of the toxic phosphonopeptide alafosfalin established YgdR functionality. Transport was abolished in the presence of the proton ionophore carbonyl cyanide p-chlorophenylhydrazone, suggesting a proton-coupled transport mechanism. YdgR transports selectively only di- and tripeptides and structurally related peptidomimetics (such as aminocephalosporins) with a substrate recognition pattern almost identical to the mammalian peptide transporter PEPT1. The YdgR protein was purified to homogeneity from E. coli membranes. Blue native-polyacrylamide gel electrophoresis and transmission electron microscopy of detergent-solubilized YdgR suggest that it exists in monomeric form. Transmission electron microscopy revealed a crown-like structure with a diameter of approximately 8 nm and a central density. These are the first structural data obtained from a proton-dependent peptide transporter, and the YgdR protein seems an excellent model for studies on substrate and inhibitor interactions as well as on the molecular architecture of cell membrane peptide transporters.

In vivo ocular pharmacokinetics of acyclovir dipeptide ester prodrugs by microdialysis in rabbits

In vivo corneal absorption of the dipeptide prodrugs of acyclovir (ACV) was evaluated using microdialysis in rabbits. A corneal well was placed on the cornea of the anesthetized New Zealand White rabbits with implanted linear probes into the aqueous humor. Two hundred microliters of a 1% solution of L-valine-ACV (VACV), glycine-valine-ACV (GVACV), valine-valine-ACV (VVACV), and valine-tyrosine-ACV (VYACV) was placed in the corneal well and was allowed to diffuse for a period of 2 h, following which the drug solution was aspirated and well removed. Samples were collected every 20 min throughout the infusion and postinfusion phases and were analyzed by HPLC to obtain the aqueous humor concentrations. Absorption rate constants of all the compounds were found to be lower than the elimination rate constants. GVACV exhibited highest absorption rate (ka) compared with other prodrugs, but all the prodrugs showed similar terminal elimination rate (lambda(z)). The time of maximum absorption (Tmax) of ACV after administration of VACV and the dipeptide prodrugs did not vary significantly (p < 0.05). GVACV exhibited the highest concentration (Cmax) and area under curve (AUC) upon absorption (p < 0.05) compared to VACV, VVACV, and VYACV. Dipeptide prodrugs of ACV were absorbed through the cornea at similar rates but to varying extents. The dipeptide prodrug GVACV owing to its enhanced absorption of ACV seems to be a promising candidate for the treatment of ocular HSV infections.

Apical sodium dependent bile acid transporter (ASBT, SLC10A2): a potential prodrug target

A major hurdle impeding the successful clinical development of drug candidates can be poor intestinal permeability. Low intestinal permeability may be enhanced by a prodrug approach targeting membrane transporters in the small intestine. Transporter specificity, affinity, and capacity are three factors in targeted prodrug design. The human apical sodium dependent bile acid transporter (SLC10A2) belongs to the solute carrier family (SLC) of transporters and is an important carrier protein expressed in the small intestine. In spite of its appearing to be an excellent target for prodrug design, few studies have targeted human apical sodium dependent bile acid transporter (hASBT) to improve oral bioavailability. This review discusses bile acids including their chemistry and their absorptive disposition. Additionally, hASBT-mediated prodrug targeting is discussed, including QSAR, in vitro models for hASBT assay, and the current progress in utilizing hASBT as a drug delivery target.

The renal type H+/peptide symporter PEPT2: structure-affinity relationships

The H(+)/peptide cotransporter PEPT2 is expressed in a variety of organs including kidney, lung, brain, mammary gland, and eye. PEPT2 substrates are di- and tripeptides as well as peptidomimetics, such as beta-lactam antibiotics. Due to the presence of PEPT2 at the bronchial epithelium, the aerosolic administration of peptide-like drugs might play a major role in future treatment of various pulmonary and systemic diseases. Moreover, PEPT2 has a significant influence on the in vivo disposition and half-life time of peptide-like drugs within the body, particularly in kidney and brain. PEPT2 is known to have similar but not identical structural requirements for substrate recognition and transport compared to PEPT1, its intestinal counterpart. In this review we compiled available affinity constants of 352 compounds, measured at different mammalian tissues and expression systems and compare the data whenever possible with those of PEPT1.

Synthesis and cytotoxic activities of beta-carboline amino acid ester conjugates

Beta-carboline represents a class of compounds with potent anti-tumor activity by intercalating with DNA. To further enhance the cytotoxic potency and bioavailability of beta-carboline, a series of novel beta-carboline amino acid ester conjugates were designed and synthesized, and the cytotoxic activities of these compounds were tested using a panel of human tumor cell lines. In addition, the membrane permeability of these compounds was evaluated in vitro using a Caco-2 cell monolayer model. The beta-carboline amino acid ester conjugates demonstrated improved cytotoxic activity compared to the parental beta-carbolines. In particular, the Lys/Arg conjugates were the most potent analogs with an IC(50) value of 4 and 1 microM against human cervical carcinoma cells. The low interaction energy of Arg conjugate based on molecular modeling may contribute to its enhanced cytotoxicity. Taken together, this study provided new insights into structure-activity relationships in the beta-carboline amino acid ester conjugates and identified the beta-carboline Lys/Arg conjugates as promising lead compounds for further in vivo biological and molecular evaluation.

Human PEPT1 Pharmacophore Distinguishes between Dipeptide Transport and Binding

The human intestinal oligopeptide transporter (PEPT1) facilitates the absorption of dipeptides, tripeptides, and many peptidomimetic drugs. In this study, a large number of peptides were selected to investigate the structural features required for PEPT1 transport. Binding affinity was determined in a Gly-Sar uptake inhibition assay, whereas functional transport was ranked in a membrane depolarization assay. Although most of the peptides tested could bind to PEPT1, not all were substrates. As expected, single amino acids and tetrapeptides could not bind to or be transported by PEPT1. Dipeptide transport was influenced by charge, hydrophobicity, size, and side chain flexibility. The extent of transport was variable, and unexpectedly, some dipeptides were not substrates of PEPT1. These included dipeptides with two positive charges or extreme bulk in either position 1 or 2. Our results identify key features required for PEPT1 transport in contrast to most previously described pharmacophores, which are based on the inhibition of transport of a known substrate.

AtGAT1, a high affinity transporter for gamma-aminobutyric acid in Arabidopsis thaliana

Functional characterization of Arabidopsis thaliana GAT1 in heterologous expression systems, i.e. Saccharomyces cerevisiae and Xenopus laevis oocytes, revealed that AtGAT1 (At1g08230) codes for an H(+)-driven, high affinity gamma-aminobutyric acid (GABA) transporter. In addition to GABA, other omega-aminofatty acids and butylamine are recognized. In contrast to the most closely related proteins of the proline transporter family, proline and glycine betaine are not transported by AtGAT1. AtGAT1 does not share sequence similarity with any of the non-plant GABA transporters described so far, and analyses of substrate selectivity and kinetic properties showed that AtGAT1-mediated transport is similar but distinct from that of mammalian, bacterial, and S. cerevisiae GABA transporters. Consistent with a role in GABA uptake into cells, transient expression of AtGAT1/green fluorescent protein fusion proteins in tobacco protoplasts revealed localization at the plasma membrane. In planta, AtGAT1 expression was highest in flowers and under conditions of elevated GABA concentrations such as wounding or senescence.

Measurement of electric current evoked by substrate transport via bi-directional H+/oligopeptide transporter over-expressed in HeLa cells: Electrogenic efflux and existence of a newly observed channel-like state

In the present study, we measured an electric current induced by substrate transport in a HeLa cell over-expressing a human intestinal di/tri-peptide transporter using the whole-cell patch-clamp technique. Gly-Sar, a typical substrate, induced an inward current associated with its uptake, which showed concentration-dependency following Michaelis-Menten-type kinetics with an apparent K(0.5) of 1.3mM as well as voltage-dependency. An outward current accompanying the efflux of Gly-Sar was also observed after washing out the cell. This outward current was voltage-dependent and was reduced by the inward proton gradient. In the case of hydrophobic dipeptides such as Gly-Phe and Gly-Leu, a distinctive current was observed: after washing out the cells, no outward current was observed, but rather, an 'inward leak' current was sustained in spite of the absence of transportable substrate. This leaky current was abolished by the perfusion of Gly-Sar and subsequent washing. It is considered that the hydrophobic substrate sticks within the substrate-binding site and causes the newly observed state, or the 'inward leak' current.

The Extracellular pH Dependency of Transport Activity by Human Oligopeptide Transporter 1 (hPEPT1) Expressed Stably in Chinese Hamster Ovary (CHO) Cells: A Reason for the Bell-Shaped Activity versus pH

Human oligopeptide transporter (hPEPT1) translocates di/tri-peptide by coupling to movement of proton down the electrochemical gradient. This transporter has the characteristics that the pH-profile of neutral dipeptide transport shows a bell-shaped curve with an optimal pH of 5.5. In the present study, we examined the reason for the decrease in the acidic region with hPEPT1-transfected CHO cells stably oeverexpressing hPEPT1 (CHO/hPEPT1). The pH profile of the transport activity vs. pH was measured in the presence of nigericin/monensin. Under this condition, the inwardly directed proton concentration gradient was dissipated while the membrane potential remained. As pH increased the activity increased, and the Henderson-Hasselbalch equation with a single pKa was fitted well to the activity curve. The pKa value was estimated to be 6.7+/-0.2. This value strongly suggests that there is a key amino acid residue, which is involved in pH regulation of transport activity. To identify the key amino acid residue, we examined the effects of various chemical modifications on pH-profile of the transport activity. Modification of carboxyl groups or hydroxyl groups had no significant influence on the pH-profile, whereas a chemical modification of histidine residue with diethylpyrocarbonate (DEPC) completely abolished the transport activity in CHO/hPEPT1 cells. On the other hand, this abolishment was almost prevented by the presence of 10 mM Gly-Sar. This protection was observed only in the presence of the substrate of hPEPT1, indicating that the histidine residue is located at the substrate recognition site. The pH-profile of the transport activity in CHO/hPEPT1 cells treated with DEPC in the presence of 10 mM Gly-Sar also showed a bell-shape similar to that in non-treated CHO/hPEPT1 cells. These data stressed that the histidine residue located at or near the substrate binding site is involved in the pH regulation of transport activity.

Affinity prediction for substrates of the peptide transporter PepT1

A quantitative method has been developed for determining the affinity of substrates for the peptide transporter PepT1, allowing oral availability of drugs via PepT1 to be estimated.

Substrate preference is altered by mutations in the fifth transmembrane domain of Ptr2p, the di/tri-peptide transporter of Saccharomyces cerevisiae

The integral membrane protein Ptr2p transports di/tri-peptides into the yeast Saccharomyces cerevisiae. The sequence FYXXINXG (FYING motif) in the 5th transmembrane domain (TM5) is invariably conserved among the members of the PTR (Peptide TRansport) family ranging from yeast to human. To test the role of TM5 in Ptr2p function, Ala-scanning mutagenesis of the 22 residues comprising TM5 was completed. All mutated transporters, with the exception of the Y248A mutant, were expressed as determined by immunoblots. In peptide-dependent growth assays, ten mutants of the non-FYING residues grew as well as wild-type Ptr2p on all twelve different peptides tested. All of the FYING motif mutants, except the non-expressed Y248A, plus seven other mutants in TM5 exhibited differential growth on peptides including Leu-Leu and Met-Met-Met indicating that these mutations conferred substrate preference. In assays measuring direct uptake of the radioactive peptides (3)H-Leu-Leu or (14)C-Met-Met-Met, the F, I and G mutants of the FYING motif did not demonstrate accumulation of these peptides over a ten minute interval. The mutation N252A of the FYING motif, along with L240A, M250A, and L258A, exhibited differential substrate preference for Met-Met-Met over Leu-Leu. Other mutations (T239A, Q241A, N242A, M245A, and A260) resulted in preference for Leu-Leu over Met-Met-Met. These data demonstrate that TM5, in particular its conserved FYING motif, is involved in substrate preference of Ptr2p.

Dipeptides as effective prodrugs of the unnatural amino acid (+)-2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (LY354740), a selective group II metabotropic glutamate receptor agonist

(+)-2-Aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid (1), also known as LY354740, is a highly potent and selective agonist for group II metabotropic glutamate receptors (mGlu receptors 2 and 3) tested in clinical trials. It has been shown to block anxiety in the fear-potentiated startle model. Its relatively low bioavailability in different animal species drove the need for an effective prodrug form that would produce a therapeutic response at lower doses for the treatment of anxiety disorders. We have investigated the increase of intestinal absorption of this compound by targeting the human peptide transporter hPepT1 for active transport of di- and tripeptides derived from 1. We have found that oral administration of an N dipeptide derivative of 1 (12a) in rats shows up to an 8-fold increase in drug absorption and a 300-fold increase in potency in the fear-potentiated startle model in rats when compared with the parent drug 1.

Effect of diabetes mellitus and insulin on the regulation of the PepT 1 symporter in rat jejunum

This investigation focused on studying the effects of insulin-dependent diabetes mellitus and insulin treatment on absorption of glycylsarcosine (Gly-Sar) across the Sprague-Dawley rat jejunum, using in situ perfusion in a physiologic acidic microenvironment at pH 6.0. Rats were divided into five groups: normal controls in group I, normal colchicine-treated rats in group II, normal cytochalasin-treated rats in group III, streptozotocin-induced diabetic rats in group IV, and insulin-treated diabetic rats in group V. Histologic studies of the five different groups showed morphologic changes upon induction of diabetes and treatments with colchicine and cytochalasin and several variations in post-1 month diabetic rats treated with insulin. The rate of uptake of Gly-Sar was significantly reduced in the diabetic state. The comparison of colchicine-treated and cytochalasin-treated rats to the diabetic group suggests that an intact cytoskeleton and tight junctions may play a role in jejunal dipeptide absorption. In the diabetic and insulin-treated group, the dipeptide influx rate was significantly increased compared to that of the nontreated controls. The regulation of the PepT 1 symporter was further assessed by immunostaining and Western blot analyses in the normal, diabetic, and diabetic and insulin-treated groups. Our results showed that a downregulation of PepT 1 in the diabetics seemed to be due in part to the low systemic insulin levels, and not necessarily to hyperglycemia. In addition, the results suggest a probable role of systemic insulin binding at the vascular site of the jejunal epithelium, and the role that this hormone may be playing in the regulation and probably cellular trafficking of PepT1.

Strategies to improve oral drug bioavailability

Efforts to improve oral drug bioavailability have grown in parallel with the pharmaceutical industry. As the number and chemical diversity of drugs has increased, new strategies have been required to develop orally active therapeutics. The past two decades have been characterised by an increased understanding of the causes of low bioavailability and a great deal of innovation in oral drug delivery technologies, marked by an unprecedented growth of the drug delivery industry. The advent of biotechnology and consequent proliferation of biopharmaceuticals have brought new challenges to the drug delivery field. In spite of the difficulties associated with developing oral forms of this type of therapeutics, significant progress has been made in the past few years, with some oral proteins, peptides and other macromolecules currently advancing through clinical trials. This article reviews the approaches that have been successfully applied to improve oral drug bioavailability, primarily, prodrug strategies, lead optimisation through medicinal chemistry and formulation design. Specific strategies to improve the oral bioavailability of biopharmaceuticals are also discussed.