Human intestinal H+/peptide cotransporter. Cloning, functional expression, and chromosomal localization

Slc15a1 is involved in the transport of synthetic F5-peptide into the seminiferous epithelium in adult rat testes

Spermiation and BTB restructuring, two critical cellular events that occur across seminiferous epithelium in mammalian testis during spermatogenesis, are tightly coordinated by biologically active peptides released from laminin chains. Our earlier study reported that F5-peptide, synthesized based on a stretch of 50 amino acids within laminin-γ3 domain IV, could reversibly induce the impairment of spermatogenesis, disruption of BTB integrity, and germ cell loss, and thus is a promising male contraceptive. However, how F5-peptide when administered intratesticularly enters seminiferous tubules and exerts effects beyond BTB is currently unknown. Here we demonstrated that Slc15a1, a peptide transporter also known as Pept1, was predominantly present in peritubular myoid cells, interstitial Leydig cells, vascular endothelial cells and germ cells, while absent in Sertoli cells or BTB site. The steady-state protein level of Slc15a1 in adult rat testis was not affected by F5-peptide treatment. Knockdown of Slc15a1 by in vivo RNAi in rat testis was shown to prevent F5-peptide induced disruptive effects on spermatogenesis. This study suggests that Slc15a1 is involved in the transport of synthetic F5-peptide into seminiferous epithelium, and thus Slc15a1 is a novel target in testis that could be genetically modified to improve the bioavailability of F5-peptide as a prospective male contraceptive.

Salt Bridge Swapping in the EXXERFXYY Motif of Proton-coupled Oligopeptide Transporters

Proton-coupled oligopeptide transporters (POTs) couple the inward transport of di- or tripeptides with an inwardly directed transport of protons. Evidence from several studies of different POTs has pointed toward involvement of a highly conserved sequence motif, E1XXE2RFXYY (from here on referred to as E1XXE2R), located on Helix I, in interactions with the proton. In this study, we investigated the intracellular substrate accumulation by motif variants with all possible combinations of glutamate residues changed to glutamine and arginine changed to a tyrosine, the latter being a natural variant found in the Escherichia coli POT YjdL. We found that YjdL motif variants with E1XXE2R, E1XXE2Y, E1XXQ2Y, or Q1XXE2Y were able to accumulate peptide, whereas those with E1XXQ2R, Q1XXE2R, or Q1XXQ2Y were unable to accumulate peptide, and Q1XXQ2R abolished uptake. These results suggest a mechanism that involves swapping of an intramotif salt bridge, i.e. R-E2 to R-E1, which is consistent with previous structural studies. Molecular dynamics simulations of the motif variants E1XXE2R and E1XXQ2R support this mechanism. The simulations showed that upon changing conformation arginine pushes Helix V, through interactions with the highly conserved FYING motif, further away from the central cavity in what could be a stabilization of an inward facing conformation. As E2 has been suggested to be the primary site for protonation, these novel findings show how protonation may drive conformational changes through interactions of two highly conserved motifs.

The physiological role of drug transporters

Transporters comprise the largest family of membrane proteins in human organism, including members of solute carrier transporter and ATP-binding cassette transporter families. They play pivotal roles in the absorption, distribution and excretion of xenobiotic and endogenous molecules. Transporters are widely expressed in various human tissues and are routinely evaluated during the process of drug development and approval. Over the past decade, increasing evidence shows that drug transporters are important in both normal physiology and disease. Currently, transporters are utilized as therapeutic targets to treat numerous diseases such as diabetes, major depression, hypertension and constipation. Despite the steady growth of the field of transporter biology, more than half of the members in transporter superfamily have little information available about their endogenous substrate(s) or physiological functions. This review outlines current research methods in transporter studies, and summarizes the drug-transporter interactions including drug-drug and drug-endogenous substrate interactions. In the end, we also discuss the therapeutic perspective of transporters based on their physiological and pathophysiological roles.

The role of drug transporters in the kidney: lessons from tenofovir

Tenofovir disoproxil fumarate, the prodrug of nucleotide reverse transcriptase inhibitor tenofovir, shows high efficacy and relatively low toxicity in HIV patients. However, long-term kidney toxicity is now acknowledged as a modest but significant risk for tenofovir-containing regimens, and continuous use of tenofovir in HIV therapy is currently under question by practitioners and researchers. Co-morbidities (hepatitis C, diabetes), low body weight, older age, concomitant administration of potentially nephrotoxic drugs, low CD4 count, and duration of therapy are all risk factors associated with tenofovir-associated tubular dysfunction. Tenofovir is predominantly eliminated via the proximal tubules of the kidney, therefore drug transporters expressed in renal proximal tubule cells are believed to influence tenofovir plasma concentration and toxicity in the kidney. We review here the current evidence that the actions, pharmacogenetics, and drug interactions of drug transporters are relevant factors for tenofovir-associated tubular dysfunction. The use of creatinine and novel biomarkers for kidney damage, and the role that drug transporters play in biomarker disposition, are discussed. The lessons learnt from investigating the role of transporters in tenofovir kidney elimination and toxicity can be utilized for future drug development and clinical management programs.

Regulation of drug transporters in the testis by environmental toxicant cadmium, steroids and cytokines

The blood-testis barrier (BTB) provides an efficient barrier to restrict paracellular and transcellular transport of substances, such as toxicants and drugs, limiting their entry to the testis to cause injury. This is achieved by the coordinated actions of efflux and influx transporters at the BTB, which are integral membrane proteins that interact with their substrates, such as drugs and toxicants. An efflux transporter (e.g., P-glycoprotein) can either restrict the entry of drugs/toxicants into the testis or actively pump drugs/toxicants out of Sertoli and/or germ cells if they have entered the seminiferous epithelium via influx pumps. This thus provides an effective mechanism to safeguard spermatogenesis. Using Sertoli cells cultured in vitro with an established tight junction (TJ)-permeability barrier which mimicked the BTB in vivo and treated with cadmium chloride (CdCl2), and also in adult rats (~300 g b.w.) treated with CdCl2 (3 mg/kg b.w., via i.p.) to induce testicular injury, cadmium was found to significantly downregulate the expression of efflux (e.g., P-glycoprotein, Mrp1, Abcg1) and influx (e.g., Oatp3, Slc15a1, Scl39a8) transporters. For instance, treatment of Sertoli cells with cadmium induced significant loss of P-glycoprotein and Oatp-3 at the cell-cell interface, which likely facilitated cadmium entry into the Sertoli cell. These findings illustrate that one of the mechanisms by which cadmium enters the testis is mediated by downregulating the expression of drug transporters at the BTB. Furthermore, cytokines and steroids were found to have differential effects in regulating the expression of drug transporters. Summary, the expression of drug transporters in the testis is regulated by toxicants, steroids and cytokines.

Development and characterization of a novel mouse line humanized for the intestinal peptide transporter PEPT1

The proton-coupled oligopeptide transporter PEPT1 (SLC15A1) is abundantly expressed in the small intestine, but not colon, of mammals and found to mediate the uptake of di/tripeptides and peptide-like drugs from the intestinal lumen. However, species differences have been observed in both the expression (and localization) of PEPT1 and its substrate affinity. With this in mind, the objectives of this study were to develop a humanized PEPT1 mouse model (huPEPT1) and to characterize hPEPT1 expression and functional activity in the intestines. Thus, after generating huPEPT1 mice in animals previously nulled for mouse Pept1, phenotypic, PCR, and immunoblot analyses were performed, along with in situ single-pass intestinal perfusion and in vivo oral pharmacokinetic studies with a model dipeptide, glycylsarcosine (GlySar). Overall, the huPEPT1 mice had normal survival rates, fertility, litter size, gender distribution, and body weight. There was no obvious behavioral or pathological phenotype. The mRNA and protein profiles indicated that huPEPT1 mice had substantial PEPT1 expression in all regions of the small intestine (i.e., duodenum, jejunum, and ileum) along with low but measurable expression in both proximal and distal segments of the colon. In agreement with PEPT1 expression, the in situ permeability of GlySar in huPEPT1 mice was similar to but lower than wildtype animals in small intestine, and greater than wildtype mice in colon. However, a species difference existed in the in situ transport kinetics of jejunal PEPT1, in which the maximal flux and Michaelis constant of GlySar were reduced 7-fold and 2- to 4-fold, respectively, in huPEPT1 compared to wildtype mice. Still, the in vivo function of intestinal PEPT1 appeared fully restored (compared to Pept1 knockout mice) as indicated by the nearly identical pharmacokinetics and plasma concentration–time profiles following a 5.0 nmol/g oral dose of GlySar to huPEPT1 and wildtype mice. This study reports, for the first time, the development and characterization of mice humanized for PEPT1. This novel transgenic huPEPT1 mouse model should prove useful in examining the role, relevance, and regulation of PEPT1 in diet and disease, and in the drug discovery process.

Molecular insights into proton coupled peptide transport in the PTR family of oligopeptide transporters

Background

Cellular uptake of small peptides is an important physiological process mediated by the PTR family of proton-coupled peptide transporters. In bacteria peptides can be used as a source of amino acids and nitrogen. Similarly in humans peptide transport is the principle route for the uptake and retention of dietary protein in the form of short di- and tri-peptides for cellular metabolism.

Scope of the review

Recent crystal structures of bacterial PTR family transporters, combined with biochemical studies of transport have revealed key molecular details underpinning ligand promiscuity and the mechanism of proton-coupled transport within the family.

Major conclusions

Pairs of salt bridge interactions between transmembrane helices work in tandem to orchestrate alternating access transport within the PTR family. Key roles for residues conserved between bacterial and eukaryotic homologues suggest a conserved mechanism of peptide recognition and transport that in some cases has been subtly modified in individual species.

General significance

Physiological studies on PepT1 and PepT2, the mammalian members of this family, have identified these transporters as being responsible for the uptake of many pharmaceutically important drug molecules, including antibiotics and antiviral medications and demonstrated their promiscuity can be used for improving the oral bioavailability of poorly absorbed compounds. The insights gained from recent structural studies combined with previous physiological and biochemical analyses are rapidly advancing our understanding of this medically important transporter superfamily. This article is part of a Special Issue entitled Structural biochemistry and biophysics of membrane proteins.

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Crystal structures of PTR family transporters

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Identification of mechanistically important salt bridge interactions.

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Conservation of key functional residues between bacterial and mammalian homologues.

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High resolution structural information on peptide binding.

Analysing the substrate multispecificity of a proton-coupled oligopeptide transporter using a dipeptide library

Peptide uptake systems that involve members of the proton-coupled oligopeptide transporter (POT) family are conserved across all organisms. POT proteins have characteristic substrate multispecificity, with which one transporter can recognize as many as 8,400 types of di/tripeptides and certain peptide-like drugs. Here we characterize the substrate multispecificity of Ptr2p, a major peptide transporter of Saccharomyces cerevisiae, using a dipeptide library. The affinities (K_i) of di/tripeptides toward Ptr2p show a wide distribution range from 48 mM to 0.020 mM. This substrate multispecificity indicates that POT family members have an important role in the preferential uptake of vital amino acids. In addition, we successfully establish high performance ligand affinity prediction models (97% accuracy) using our comprehensive dipeptide screening data in conjunction with simple property indices for describing ligand molecules. Our results provide an important clue to the development of highly absorbable peptides and their derivatives including peptide-like drugs.

Proton-coupled oligopeptide transporters (POTs) can recognize and mediate the uptake of up to 8,400 di/tripeptides or peptide-like drugs. Ito et al. comprehensively map the substrate specificity of the yeast POT Ptr2p, and use this information to construct models for the prediction of ligand affinity.

Colonic expression of the peptide transporter PEPT1 is downregulated during intestinal inflammation and is not required for NOD2-dependent immune activation

BACKGROUND

PEPT1 was proposed to be expressed only in inflamed colonic tissues in which it could contribute to inflammatory bowel disease (IBD) development by transporting bacterial peptides, such as muramyl dipeptide (MDP), that activate intracellular pattern recognition receptors, such as the nucleotide-binding and oligomerization domain 2. To better define the pathological relevance of this transporter, we analyzed PEPT1 expression during intestinal inflammation and studied the susceptibility of Pept1-deficient (Pept1) mice to experimental colitis.

METHODS

Wild-type and Pept1 mice were treated with dextran sulfate sodium and 2,4,6-trinitrobenzene sulfonic acid to induce colitis, and MDP-induced cytokine expression was studied in colonic tissue cultures. PEPT1 expression was characterized in mouse models of Crohn's disease-like ileitis (Tnf) or colitis (Il-10, Il-10XTlr2) and endoscopic tissue samples from descending colon of patients with IBD (n = 11) and controls (n = 17). Moreover, the prevalence of the PEPT1 single-nucleotide polymorphism rs2297322 was tested in German patients with IBD (n = 458) and controls (n = 452).

RESULTS

PEPT1 expression was consistently reduced under condition of acute or chronic experimental inflammation. Wild-type and Pept1 mice revealed comparable susceptibility to dextran sulfate sodium-induced and 2,4,6-trinitrobenzene sulfonic acid-induced colitis, and MDP-induced cytokine expression was PEPT1-independent. PEPT1 expression levels were also decreased in descending colon of patients with IBD during acute inflammation, but the rs2297322 single-nucleotide polymorphism was not associated with IBD susceptibility in the German cohort.

CONCLUSIONS

PEPT1 expression is reduced during intestinal inflammation and PEPT1 is neither required for MDP-induced immune response nor is the PEPT1 rs2297322 single-nucleotide polymorphism associated with IBD susceptibility in our German cohort. These data strongly argue against a primary role of PEPT1 in the initiation or progression of IBD.

Antibiotrophs: The complexity of antibiotic-subsisting and antibiotic-resistant microorganisms

Widespread overuse of antibiotics has led to the emergence of numerous antibiotic-resistant bacteria; among these are antibiotic-subsisting strains capable of surviving in environments with antibiotics as the sole carbon source. This unparalleled expansion of antibiotic resistance reveals the potent and diversified resistance abilities of certain bacterial strains. Moreover, these strains often possess hypermutator phenotypes and virulence transmissibility competent for genomic and proteomic propagation and pathogenicity. Pragmatic and prospicient approaches will be necessary to develop efficient therapeutic methods against such bacteria and to understand the extent of their genomic adaptability. This review aims to reveal the niches of these antibiotic-catabolizing microbes and assesses the underlying factors linking natural microbial antibiotic production, multidrug resistance, and antibiotic-subsistence.

Peptide transporter isoforms are discriminated by the fluorophore-conjugated dipeptides β-Ala- and d-Ala-Lys-N-7-amino-4-methylcoumarin-3-acetic acid

Peptide transporters of the SLC15 family are classified by structure and function into PEPT1 (low‐affinity/high‐capacity) and PEPT2 (high‐affinity/low‐capacity) isoforms. Despite the differences in kinetics, both transporter isoforms are reckoned to transport essentially all possible di‐ and tripeptides. We here report that the fluorophore‐conjugated dipeptide derivatives β‐Ala‐Lys‐N‐7‐amino‐4‐methylcoumarin‐3‐acetic acid (β‐AK‐AMCA) and d‐Ala‐Lys‐N‐7‐amino‐4‐methylcoumarin‐3‐acetic acid (d‐AK‐AMCA) are transported by distinct PEPT isoforms in a species‐specific manner. Transport of the fluorophore peptides was studied (1) in vitro after heterologous expression in Xenopus oocytes of PEPT1 and PEPT2 isoforms from different vertebrate species and of PEPT1 and PEPT2 transporters from Caenorhabditis elegans by using electrophysiological and fluorescence methods and (2) in vivo in C. elegans by using fluorescence methods. Our results indicate that both substrates are transported by the vertebrate “renal‐type” and the C. elegans “intestinal‐type” peptide transporter only. A systematic analysis among species finds four predicted amino acid residues along the sequence that may account for the substrate uptake differences observed between the vertebrate PEPT1/nematode PEPT2 and the vertebrate PEPT2/nematode PEPT1 subtype. This selectivity on basis of isoforms and species may be helpful in better defining the structure–function determinants of the proteins of the SLC15 family.

Peptide transporters of the SLC15 family can be classified by structure and function into the PEPT1 (low‐affinity/high‐capacity) and PEPT2 (high‐affinity/low‐capacity) phenotype. We found that the fluorophore‐conjugated dipeptide derivatives β‐Ala‐Lys‐N‐7‐amino‐4‐methylcoumarin‐3‐acetic acid (β‐AK‐AMCA) and d‐Ala‐Lys‐N‐7‐amino‐4‐methylcoumarin‐3‐acetic acid (d‐AK‐AMCA) are transported only by distinct PEPT isoforms in a species‐specific manner. This selectivity on basis of isoforms and species should be helpful in further defining the substrate‐binding domain of peptide transporters.

Glucose-dependent insulinotropic polypeptide regulates dipeptide absorption in mouse jejunum

Glucose-dependent insulinotropic polypeptide (GIP) secreted from jejunal mucosal K cells augments insulin secretion and plays a critical role in the pathogenesis of obesity and Type 2 diabetes mellitus. In recent studies, we have shown GIP directly activates Na-glucose cotransporter-1 (SGLT1) and enhances glucose absorption in mouse jejunum. It is not known whether GIP would also regulate other intestinal nutrient absorptive processes. The present study investigated the effect of GIP on proton-peptide cotransporter-1 (PepT1) that mediates di- and tripeptide absorption as well as peptidomimetic drugs. Immunohistochemistry studies localized both GIP receptor (GIPR) and PepT1 proteins on the basolateral and apical membranes of normal mouse jejunum, respectively. Anti-GIPR antibody detected 50-, 55-, 65-, and 70-kDa proteins, whereas anti-PepT1 detected a 70-kDa proteins in mucosal homogenates of mouse jejunum. RT-PCR analyses established the expression of GIPR- and PepT1-specific mRNA in mucosal cells of mouse jejunum. Absorption of Gly-Sar (a nondigestible dipeptide) measured under voltage-clamp conditions revealed that the imposed mucosal H(+) gradient-enhanced Gly-Sar absorption as an evidence for the presence of PepT1-mediated H(+):Gly-Sar cotransport on the apical membranes of mouse jejunum. H(+):Gly-Sar absorption was completely inhibited by cephalexin (a competitive inhibitor of PepT1) and was activated by GIP. The GIP-activated Gly-Sar absorption was completely inhibited by RP-cAMP (a cAMP antagonist). In contrast to GIP, the ileal L cell secreting glucagon-like peptide-1 (GLP-1) did not affect the H(+):Gly-Sar absorption in mouse jejunum. We conclude from these observations that GIP, but not GLP-1, directly activates PepT1 activity by a cAMP-dependent signaling pathway in jejunum.

Transcriptional and functional regulation of the intestinal peptide transporter PEPT1

Dietary proteins are cleaved within the intestinal lumen to oligopeptides which are further processed to small peptides (di- and tripeptides) and free amino acids. Although the transport of amino acids is mediated by several specific amino acid transporters, the proton-coupled uptake of the more than 8000 different di- and tripeptides is performed by the high-capacity/low-affinity peptide transporter isoform PEPT1 (SLC15A1). Its wide substrate tolerance also allows the transport of a repertoire of structurally closely related compounds and drugs, which explains their high oral bioavailability and brings PEPT1 into focus for medical and pharmaceutical approaches. Although the first evidence for the interplay of nutrient supply and PEPT1 expression and function was described over 20 years ago, many aspects of the molecular processes controlling its transcription and translation and modifying its transporter properties are still awaiting discovery. The present review summarizes the recent knowledge on the factors modulating PEPT1 expression and function in Caenorhabditis elegans, Danio rerio, Mus musculus and Homo sapiens, with focus on dietary ingredients, transcription factors and functional modulators, such as the sodium-proton exchanger NHE3 and selected scaffold proteins.

PepT1 expressed in immune cells has an important role in promoting the immune response during experimentally induced colitis

We and others have shown that the dipeptide cotransporter PepT1 is expressed in immune cells, including macrophages that are in close contact with the lamina propria of the small and large intestines. In the present study, we used PepT1-knockout (KO) mice to explore the role played by PepT1 in immune cells during dextran sodium sulfate (DSS)-induced colitis. DSS treatment caused less severe body weight loss, diminished rectal bleeding, and less diarrhea in PepT1-KO mice than in wild-type (WT) animals. A histological examination of colonic sections revealed that the colonic architecture was less disrupted and the extent of immune cell infiltration into the mucosa and submucosa following DSS treatment was reduced in PepT1-KO mice compared with WT animals. Consistent with these results, the DSS-induced colitis increase in colonic myeloperoxidase activity was significantly less in PepT1-KO mice than in WT littermates. The colonic levels of mRNAs encoding the inflammatory cytokines CXCL1, interleukin (IL)-6, monocyte chemotactic protein-1, IL-12, and interferon-γ were significantly lower in DSS-treated PepT1-KO mice than in DSS-treated WT animals. Colonic immune cells from WT had significantly higher level of proinflammatory cytokines then PepT1 KO. In addition, we observed that knocking down the PepT1 expression decreases chemotaxis of immune cells recruited during intestinal inflammation. Antibiotic treatment before DSS-induced colitis eliminated the differential expression of inflammatory cytokines between WT and PepT1-KO mice. In conclusion, PepT1 in immune cells regulates the secretion of proinflammatory cytokines triggered by bacteria and/or bacterial products, and thus has an important role in the induction of colitis. PepT1 may transport small bacterial products, such as muramyl dipeptide and the tripeptide L-Ala-gamma-D-Glu-meso-DAP, into macrophages. These materials may be sensed by members of the nucleotide-binding site-leucine-rich repeat family of intracellular receptors, ultimately resulting in altered homeostasis of the intestinal microbiota.

IL-16 induces intestinal inflammation via PepT1 upregulation in a pufferfish model: new insights into the molecular mechanism of inflammatory bowel disease

Inflammatory bowel disease (IBD) has long been a worldwide health care problem with a persistently increasing incidence. Although its clinical features have been well described, its etiology and pathogenesis remain unclear. IL-16 is a chemoattractant cytokine with various effects on cellular activities and diseases. However, the involvement of IL-16 in IBD remains poorly understood. In this study, to our knowledge we report for the first time the mechanism by which IL-16 induces intestinal inflammation by upregulating the expression of oligopeptide transporter member 1 (PepT1) in a Tetraodon nigroviridis fish model. The dextran sodium sulfate-induced colitis model in this species revealed that IL-16 levels significantly increase accompanied by elevations in PepT1 in the colon. Moreover, the signs of colitis were dramatically attenuated by IL-16 depletion using anti-IL-16 Abs. In vivo IL-16 administration induced remarkable intestinal inflammation with typical ulcerative colitis-like features, including histologic damage, inflammatory cell infiltration, increased myeloperoxidase activity, and proinflammatory cytokines expression, which corresponded with significant PepT1 upregulation in the colon. The IL-16-induced PepT1 expression and its upregulated fMLF transport were also demonstrated in vitro. To our knowledge, our study provides the first evidence of the connection between IL-16 and PepT1, which provides new insights into the molecular mechanism underlying IBD development. Additionally, this study suggests that fish species are an attractive model for studying IBD. By providing a better understanding of IL-16 biology from fish to mammals, this study should aid the development of IL-16-based therapies for IBD.

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.

Increasing oral absorption of polar neuraminidase inhibitors: a prodrug transporter approach applied to oseltamivir analogue

Poor oral absorption is one of the limiting factors in utilizing the full potential of polar antiviral agents. The neuraminidase target site requires a polar chemical structure for high affinity binding, thus limiting oral efficacy of many high affinity ligands. The aim of this study was to overcome this poor oral absorption barrier, utilizing prodrug to target the apical brush border peptide transporter 1 (PEPT1). Guanidine oseltamivir carboxylate (GOCarb) is a highly active polar antiviral agent with insufficient oral bioavailability (4%) to be an effective therapeutic agent. In this report we utilize a carrier-mediated targeted prodrug approach to improve the oral absorption of GOCarb. Acyloxy(alkyl) ester based amino acid linked prodrugs were synthesized and evaluated as potential substrates of mucosal transporters, e.g., PEPT1. Prodrugs were also evaluated for their chemical and enzymatic stability. PEPT1 transport studies included [(3)H]Gly-Sar uptake inhibition in Caco-2 cells and cellular uptake experiments using HeLa cells overexpressing PEPT1. The intestinal membrane permeabilities of the selected prodrugs and the parent drug were then evaluated for epithelial cell transport across Caco-2 monolayers, and in the in situ rat intestinal jejunal perfusion model. Prodrugs exhibited a pH dependent stability with higher stability at acidic pHs. Significant inhibition of uptake (IC(50) <1 mM) was observed for l-valyl and l-isoleucyl amino acid prodrugs in competition experiments with [(3)H]Gly-Sar, indicating a 3-6 times higher affinity for PEPT1 compared to valacyclovir, a well-known PEPT1 substrate and >30-fold increase in affinity compared to GOCarb. The l-valyl prodrug exhibited significant enhancement of uptake in PEPT1/HeLa cells and compared favorably with the well-absorbed valacyclovir. Transepithelial permeability across Caco-2 monolayers showed that these amino acid prodrugs have a 2-5-fold increase in permeability as compared to the parent drug and showed that the l-valyl prodrug (P(app) = 1.7 × 10(-6) cm/s) has the potential to be rapidly transported across the epithelial cell apical membrane. Significantly, only the parent drug (GOCarb) appeared in the basolateral compartment, indicating complete activation (hydrolysis) during transport. Intestinal rat jejunal permeability studies showed that l-valyl and l-isoleucyl prodrugs are highly permeable compared to the orally well absorbed metoprolol, while the parent drug had essentially zero permeability in the jejunum, consistent with its known poor low absorption. Prodrugs were rapidly converted to parent in cell homogenates, suggesting their ability to be activated endogenously in the epithelial cell, consistent with the transport studies. Additionally, l-valyl prodrug was found to be a substrate for valacyclovirase (K(m) = 2.37 mM), suggesting a potential cell activation mechanism. Finally we determined the oral bioavailability of our most promising candidate, GOC-l-Val, in mice to be 23% under fed conditions and 48% under fasted conditions. In conclusion, GOC-l-Val prodrug was found to be a very promising antiviral agent for oral delivery. These findings indicate that the carrier-mediated prodrug approach is an excellent strategy for improving oral absorption of polar neuraminidase inhibitors. These promising results demonstrate that the oral peptide transporter-mediated prodrug strategy has enormous promise for improving the oral mucosal cell membrane permeability of polar, poorly absorbed antiviral agents and treating influenza via the oral route of administration.

Emergence of antibiotic-resistant extremophiles (AREs)

Excessive use of antibiotics in recent years has produced bacteria that are resistant to a wide array of antibiotics. Several genetic and non-genetic elements allow microorganisms to adapt and thrive under harsh environmental conditions such as lethal doses of antibiotics. We attempt to classify these microorganisms as antibiotic-resistant extremophiles (AREs). AREs develop strategies to gain greater resistance to antibiotics via accumulation of multiple genes or plasmids that harbor genes for multiple drug resistance (MDR). In addition to their altered expression of multiple genes, AREs also survive by producing enzymes such as penicillinase that inactivate antibiotics. It is of interest to identify the underlying molecular mechanisms by which the AREs are able to survive in the presence of wide arrays of high-dosage antibiotics. Technologically, "omics"-based approaches such as genomics have revealed a wide array of genes differentially expressed in AREs. Proteomics studies with 2DE, MALDI-TOF, and MS/MS have identified specific proteins, enzymes, and pumps that function in the adaptation mechanisms of AREs. This article discusses the molecular mechanisms by which microorganisms develop into AREs and how "omics" approaches can identify the genetic elements of these adaptation mechanisms. These objectives will assist the development of strategies and potential therapeutics to treat outbreaks of pathogenic microorganisms in the future.

Interaction of the antiviral drug telaprevir with renal and hepatic drug transporters

Telaprevir is a new, direct-acting antiviral drug that has been approved for the treatment of chronic hepatitis C viral infection. First data on drug-drug interactions with co-medications such as cyclosporine, tacrolimus and atorvastatin have been reported recently. Drug transporting proteins have been shown to play an important role in clinically observed drug-drug interactions. The aim of this study was therefore to systematically investigate the potential of telaprevir to inhibit drug transporting proteins. The effect of telaprevir on substrate uptake mediated by drug transporters located in human kidney and liver was investigated on a functional level in HEK293 cell lines that over-express single transporter. Telaprevir was shown to exhibit significant inhibition of the human renal drug transporters OCT2 and MATE1 with IC(50) values of 6.4 μM and 23.0 μM, respectively, whereas no inhibitory effect on OAT1 and OAT3 mediated transport by telaprevir was demonstrated. Liver drug transporters were inhibited with an IC(50) of 2.2 μM for OATP1B1, 6.8 μM for OATP1B3 and 20.7 μM for OCT1. Our data show that telaprevir exhibited significant potential to inhibit human drug transporters. In view of the inhibitory potential of telaprevir, clinical co-administration of telaprevir together with drugs that are substrates of renal or hepatic transporters should be carefully monitored.

Alternating access mechanism in the POT family of oligopeptide transporters

Proton-dependent oligopeptide transporters are required for the uptake of diet-derived peptides in all kingdoms of life. The crystal structure of a bacterial transporter in the inward open conformation, together with a published structure in an occluded conformation, reveals the peptide transport mechanism.

Short chain peptides are actively transported across membranes as an efficient route for dietary protein absorption and for maintaining cellular homeostasis. In mammals, peptide transport occurs via PepT1 and PepT2, which belong to the proton-dependent oligopeptide transporter, or POT family. The recent crystal structure of a bacterial POT transporter confirmed that they belong to the major facilitator superfamily of secondary active transporters. Despite the functional characterization of POT family members in bacteria, fungi and mammals, a detailed model for peptide recognition and transport remains unavailable. In this study, we report the 3.3-Å resolution crystal structure and functional characterization of a POT family transporter from the bacterium Streptococcus thermophilus. Crystallized in an inward open conformation the structure identifies a hinge-like movement within the C-terminal half of the transporter that facilitates opening of an intracellular gate controlling access to a central peptide-binding site. Our associated functional data support a model for peptide transport that highlights the importance of salt bridge interactions in orchestrating alternating access within the POT family.

Role of the intestinal peptide transporter PEPT1 in oseltamivir absorption: in vitro and in vivo studies

It was reported that oseltamivir (Tamiflu) absorption was mediated by human peptide transporter (hPEPT) 1. Understanding the exact mechanism(s) of absorption is important in the context of drug-drug and diet-drug interactions. Hence, we investigated the mechanism governing the intestinal absorption of oseltamivir and its active metabolite (oseltamivir carboxylate) in wild-type [Chinese hamster ovary (CHO)-K1] and hPEPT1-transfected cells (CHO-PEPT1), in pharmacokinetic studies in juvenile and adult rats, and in healthy volunteers. In vitro cell culture studies showed that the intracellular accumulation of oseltamivir and its carboxylate into CHO-PEPT1 and CHO-K1 was always similar under a variety of experimental conditions, demonstrating that these compounds are not substrates of hPEPT1. Furthermore, neither oseltamivir nor its active metabolite was capable of inhibiting Gly-Sar uptake in CHO-PEPT1 cells. In vivo pharmacokinetic studies in juvenile and adult rats showed that the disposition of oseltamivir and oseltamivir carboxylate, after oral administration of oseltamivir, was sensitive to the feed status but insensitive to the presence of milk and Gly-Sar. Moreover, oseltamivir and oseltamivir carboxylate exhibited significantly higher exposure in rats under fasted conditions than under fed conditions. In humans, oral dosing after a high-fat meal resulted in a statistically significant but moderate lower exposure than after an overnight fasting. This change has no clinical implications. Taken together, the results do not implicate either rat Pept1 or hPEPT1 in the oral absorption of oseltamivir.

Species-dependent uptake of glycylsarcosine but not oseltamivir in Pichia pastoris expressing the rat, mouse, and human intestinal peptide transporter PEPT1

The purpose of this study was to determine whether glycylsarcosine (a model dipeptide) and oseltamivir (an antiviral prodrug) exhibited a species-dependent uptake in yeast Pichia pastoris expressing the rat, mouse, and human homologs of PEPT1. Experiments were performed with [(3)H]glycylsarcosine (GlySar) in yeast P. pastoris expressing human, mouse, and rat peptide transporter 1 (PEPT1), in which uptake was examined as a function of time, concentration, potential inhibitors, and the dose-response inhibition of GlySar by oseltamivir. Studies with [(14)C]oseltamivir were also performed under identical experimental conditions. We found that GlySar exhibited saturable uptake in all three species, with K(m) values for human (0.86 mM) > mouse (0.30 mM) > rat (0.16 mM). GlySar uptake in the yeast transformants was specific for peptides (glycylproline) and peptide-like drugs (cefadroxil, cephradine, and valacyclovir), but was unaffected by glycine, l-histidine, cefazolin, cephalothin, cephapirin, acyclovir, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, tetraethylammonium, and elacridar. Although oseltamivir caused a dose-dependent inhibition of GlySar uptake [IC(50) values for human (27.4 mM) > rat (18.3 mM) > mouse (10.7 mM)], the clinical relevance of this interaction would be very low in humans. Of importance, oseltamivir was not a substrate for the intestinal PEPT1 transporter in yeast expressing the three mammalian species tested. Instead, the prodrug exhibited nonspecific binding to the yeast vector and PEPT1 transformants. Finally, the mouse appeared to be a better animal model than the rat for exploring the intestinal absorption and pharmacokinetics of peptides and peptide-like drugs in human.

The role and pathophysiological relevance of membrane transporter PepT1 in intestinal inflammation and inflammatory bowel disease

Intestinal inflammation is characterized by epithelial disruption, leading to loss of barrier function and the recruitment of immune cells, including neutrophils. Although the mechanisms are not yet completely understood, interactions between environmental and immunological factors are thought to be critical in the initiation and progression of intestinal inflammation. In recent years, it has become apparent that the di/tripeptide transporter PepT1 may play an important role in the pathogenesis of such inflammation. In healthy individuals, PepT1 is primarily expressed in the small intestine and transports di/tripeptides for metabolic purposes. However, during chronic inflammation such as that associated with inflammatory bowel disease, PepT1 expression is upregulated in the colon, wherein the protein is normally expressed either minimally or not at all. Several recent studies have shown that PepT1 binds to and transports various bacterial di/tripeptides into colon cells, leading to activation of downstream proinflammatory responses via peptide interactions with innate immune receptors. In the present review, we examine the relationship between colonic PepT1-mediated peptide transport in the colon and activation of innate immune responses during disease. It is important to understand the mechanisms of PepT1 action during chronic intestinal inflammation to develop future therapies addressing inappropriate immune activation in the colon.

Parameters for pyrethroid insecticide QSAR and PBPK/PD models for human risk assessment

In this review we have examined the status of parameters required by pyrethroid QSAR-PBPK/PD models for assessing health risks. In lieu of the chemical,biological, biochemical, and toxicological information developed on the pyrethroids since 1968, the finding of suitable parameters for QSAR and PBPK/PD model development was a monumental task. The most useful information obtained came from rat toxicokinetic studies (i.e., absorption, distribution, and excretion), metabolism studies with 14C-cyclopropane- and alcohol-labeled pyrethroids, the use of known chiral isomers in the metabolism studies and their relation to commercial products. In this review we identify the individual chiralisomers that have been used in published studies and the chiral HPLC columns available for separating them. Chiral HPLC columns are necessary for isomer identification and for developing kinetic values (Vm,, and Kin) for pyrethroid hydroxylation. Early investigators synthesized analytical standards for key pyrethroid metabolites, and these were used to confirm the identity of urinary etabolites, by using TLC. These analytical standards no longer exist, and muste resynthesized if further studies on the kinetics of the metabolism of pyrethroids are to be undertaken.In an attempt to circumvent the availability of analytical standards, several CYP450 studies were carried out using the substrate depletion method. This approach does not provide information on the products formed downstream, and may be of limited use in developing human environmental exposure PBPK/PD models that require extensive urinary metabolite data. Hydrolytic standards (i.e., alcohols and acids) were available to investigators who studied the carboxylesterase-catalyzed hydrolysis of several pyrethroid insecticides. The data generated in these studies are suitable for use in developing human exposure PBPK/PD models.Tissue:blood partition coefficients were developed for the parent pyrethroids and their metabolites, by using a published mechanistic model introduced by Poulin and Thiele (2002a; b) and log DpH 7.4 values. The estimated coefficients, especially those of adipose tissue, were too high and had to be corrected by using a procedure in which the proportion of parent or metabolite residues that are unbound to plasma albumin is considered, as described in the GastroPlus model (Simulations Plus, Inc.,Lancaster, CA). The literature suggested that Km values be adjusted by multiplying Km by the substrate (decimal amount) that is unbound to microsomal or CYPprotein. Mirfazaelian et al. (2006) used flow- and diffusion-limited compartments in their deltamethrin model. The addition of permeability areas (PA) having diffusion limits, such as the fat and slowly perfused compartments, enabled the investigators to bring model predictions in line with in vivo data.There appears to be large differences in the manner and rate of absorption of the pyrethroids from the gastrointestinal tract, implying that GI advanced compartmental transit models (ACAT) need to be included in PBPK models. This is especially true of the absorption of an oral dose of tefluthrin in male rats, in which 3.0-6.9%,41.3-46.3%, and 5.2-15.5% of the dose is eliminated in urine, feces, and bile,respectively (0-48 h after administration). Several percutaneous studies with the pyrethroids strongly support the belief that these insecticides are not readily absorbed, but remain on the surface of the skin until they are washed off. In one articular study (Sidon et al. 1988) the high levels of permethrin absorption through the forehead skin (24-28%) of the monkey was reported over a 7- to 14-days period.Wester et al. (1994) reported an absorption of 1.9% of pyrethrin that had been applied to the forearm of human volunteers over a 7-days period.SAR models capable of predicting the binding of the pyrethroids to plasma and hepatic proteins were developed by Yamazaki and Kanaoka (2004), Saiakhov et al. (2000), Colmenarejo et al. (2001), and Colmenarejo (2003). QikProp(Schrodinger, LLC) was used to obtain Fu values for calculating partition coefficients and for calculating permeation constants (Caco-2, MDCK, and logBBB). ADMET Predictor (Simulations Plus Inc.) provided Vm~,x and Km values for the hydroxylation of drugs/pyrethroids by human liver recombinant cytochrome P450 enzymes making the values available for possible use in PBPK/PD models.The Caco-2 permeability constants and CYP3A4 Vmax and Km values are needed in PBPK/PD models with GI ACAT sub models. Modeling work by Chang et al.(2009) produced rate constants (kcat) for the hydrolysis of pyrethroids by rat serumcarboxylesterases. The skin permeation model of Potts and Guy (1992) was used topredict K, values for the dermal absorption of the 15 pyrethroids.The electrophysiological studies by Narahashi (1971) and others (Breckenridgeet al. 2009; Shafer et al. 2005; Soderlund et al. 2002; Wolansky and Harrill 2008)demonstrated that the mode of action of pyrethroids on nerves is to interfere with the changes in sodium and potassium ion currents. The pyrethroids, being highly lipid soluble, are bound or distributed in lipid bilayers of the nerve cell membrane and exert their action on sodium channel proteins. The rising phase of the action potential is caused by sodium influx (sodium activation), while the falling phase is caused by sodium activation being turned off, and an increase in potassium efflux(potassium activation). The action of allethrin and other pyrethroids is caused by an inhibition or block of the normal currents. An equation by Tatebayashi and Narahashi (1994) that describes the action of pyrethroids on sodium channels was found in the literature. This equation, or some variation of it, may be suitable for use in the PD portion of pyrethroid PBPK models.

The PepT1-NOD2 signaling pathway aggravates induced colitis in mice

BACKGROUND & AIMS

The human di/tripeptide transporter human intestinal H-coupled oligonucleotide transporter (hPepT1) is abnormally expressed in colons of patients with inflammatory bowel disease, although its exact role in pathogenesis is unclear. We investigated the contribution of PepT1 to intestinal inflammation in mouse models of colitis and the involvement of the nucleotide-binding oligomerization domain 2 (NOD2) signaling pathway in the pathogenic activity of colonic epithelial hPepT1.

METHODS

Transgenic mice were generated in which hPepT1 expression was regulated by the β-actin or villin promoters; colitis was induced using 2,4,6-trinitrobenzene sulfonic acid (TNBS) or dextran sodium sulfate (DSS) and the inflammatory responses were assessed. The effects of NOD2 deletion in the hPepT1 transgenic mice also was studied to determine the involvement of the PepT1-NOD2 signaling pathway.

RESULTS

TNBS and DSS induced more severe levels of inflammation in β-actin-hPepT1 transgenic mice than wild-type littermates. Intestinal epithelial cell-specific hPepT1 overexpression in villin-hPepT1 transgenic mice increased the severity of inflammation induced by DSS, but not TNBS. Bone marrow transplantation studies showed that hPepT1 expression in intestinal epithelial cells and immune cells has an important role in the proinflammatory response. Antibiotics abolished the effect of hPepT1 overexpression on the inflammatory response in DSS-induced colitis in β-actin-hPepT1 and villin-hPepT1 transgenic mice, indicating that commensal bacteria are required to aggravate intestinal inflammation. Nod2-/-, β-actin-hPepT1 transgenic/Nod2-/-, and villin-hPepT1 transgenic/Nod2-/- littermates had similar levels of susceptibility to DSS-induced colitis, indicating that hPepT1 overexpression increased intestinal inflammation in a NOD2-dependent manner.

CONCLUSIONS

The PepT1-NOD2 signaling pathway is involved in aggravation of DSS-induced colitis in mice.

Effect of dose escalation on the in vivo oral absorption and disposition of glycylsarcosine in wild-type and Pept1 knockout mice

This study evaluated the in vivo absorption and disposition of glycylsarcosine (GlySar), after escalating oral doses, in wild-type and peptide transporter 1 (Pept1) knockout mice. [(3)H]GlySar was administered to mice at doses of 1, 10, 100, 1000, and 5000 nmol/g b.wt. Serial blood samples were obtained over 480 min, the plasma was harvested, and the area under the plasma concentration-time curve (AUC) was determined. It was observed that the GlySar AUC was 60, 45, and 30% lower in knockout than wild-type mice when evaluated over 2, 4, and 8 h, respectively (p < 0.01). Plasma levels of GlySar reached a plateau at 90 min in knockout mice and then rose to a second plateau at 240 min. In wild-type mice, the plasma levels rose continuously to reach a single plateau at 90 min. When partial AUC (0-120 min) was used as an indicator for rate of absorption, there was a 60% reduction in GlySar absorption rate in knockout mice compared with wild-type animals. Tissue distribution studies were also performed after 10 nmol/g oral doses of [(3)H]GlySar. When sampled 1 h after dosing, GlySar tissue concentrations were significantly lower in knockout versus wild-type mice and, with the exception of intestines, reflected differences in the systemic exposure of dipeptide between these two genotypes. Overall, PEPT1 ablation in mice resulted in significant reductions, in vivo, in the rate and extent of GlySar absorption. The AUC of GlySar was proportional to dose in both genotypes over 1 to 100 nmol/g, with minor decrements at the two highest doses.

The tandem chain extension aldol reaction used for synthesis of ketomethylene tripeptidomimetics targeting hPEPT1

The rationale for targeting the human di-/tripeptide transporter hPEPT1 for oral drug delivery has been well established by several drug and prodrug cases. The aim of this study was to synthesize novel ketomethylene modified tripeptidomimetics and to investigate their binding affinity for hPEPT1. Three related tripeptidomimetics of the structure H-Phe-ψ[COCH(2)]-Ser(Bz)-X(aa)-OH were synthesized applying the tandem chain extension aldol reaction, where amino acid derived β-keto imides were stereoselectively converted to α-substituted γ-keto imides. In addition, three corresponding tripeptides, composed of amide bonds, were synthesized for comparison of binding affinities. The six investigated compounds were all defined as high affinity ligands (K(i)-values <0.5 mM) for hPEPT1 by measuring the concentration dependent inhibition of apical [(14)C]Gly-Sar uptake in Caco-2 cells. Consequently, the ketomethylene replacement for the natural amide bond and α-side chain modifications appears to offer a promising strategy to modify tripeptidic structures while maintaining a high affinity for hPEPT1.

Hijacking solute carriers for proton-coupled drug transport

The physiological role of mammalian solute carrier (SLC) proteins is to mediate transmembrane movement of electrolytes, nutrients, micronutrients, vitamins, and endogenous metabolites from one cellular compartment to another. Many transporters in the small intestine, kidney, and solid tumors are H(+)-coupled, driven by local H(+)-electrochemical gradients, and transport numerous drugs. These transporters include PepT1 and PepT2 (SLC15A1/2), PCFT (SLC46A1), PAT1 (SLC36A1), OAT10 (SLC22A13), OATP2B1 (SLCO2B1), MCT1 (SLC16A1), and MATE1 and MATE2-K (SLC47A1/2).

Ibuprofen is a non-competitive inhibitor of the peptide transporter hPEPT1 (SLC15A1): possible interactions between hPEPT1 substrates and ibuprofen

BACKGROUND AND PURPOSE

Recently, we identified etodolac as a possible ligand for the human intestinal proton-couple peptide transporter (hPEPT1). This raised the possibility that other non-steroidal anti-inflammatory drugs, and especially ibuprofen, could also interact with hPEPT1. Here, we have assessed the interactions of ibuprofen with hPEPT1.

EXPERIMENTAL APPROACH

The uptake of [(14)C]Gly-Sar, [(3)H]Ibuprofen and other radio-labelled compounds were investigated in Madin-Darby canine kidney cells (MDCK)/hPEPT1, MDCK/Mock, LLC-PK(1) or Caco-2 cells. The transepithelial transport of ibuprofen and hPEPT1 substrates was investigated in Caco-2 cell monolayers.

KEY RESULTS

Ibuprofen concentration dependently inhibited hPEPT1-mediated uptake of Gly-Sar in MDCK/hPEPT1 cells (K(i)(app) = 0.4 mM) but uptake of ibuprofen in Caco-2 cells and MDCK/hPEPT1 cells was not inhibited by hPEPT1 substrates. The maximum uptake rate for Gly-Sar uptake was reduced from 522 pmol·min(-1)·cm(-2) to 181 pmol·min(-1)·cm(-2) and 78 pmol·min(-1)·cm(-2) in the presence of 0.5 mM and 1 mM ibuprofen, respectively. The interaction between ibuprofen and hPEPT1 was thus non-competitive. In LLC-PK1 cells, ibuprofen (1 mM) did not influence the transporter-mediated uptake of glycine or α-methyl-D-glycopyranoside. In Caco-2 cell monolayers the absorptive transport of δ-aminolevulinic acid was reduced by 23% and 48% by ibuprofen (1 and 10 mM), respectively. Likewise the transport of Gly-Sar was reduced by 23% in the presence of ibuprofen (1 mM).

CONCLUSIONS AND IMPLICATIONS

Ibuprofen is a non-competitive inhibitor of hPEPT1. As ibuprofen reduced the transepithelial transport of δ-aminolevulinic acid, drug-drug interactions between ibuprofen and hPEPT1 drug substrates at their site of absorption are possible if administered together.

Lactobacillus plantarum consumption increases PepT1-mediated amino acid absorption by enhancing protein kinase C activity in spontaneously colitic mice

Although probiotic consumption has generally been shown to have many beneficial effects for the prevention and treatment of inflammatory bowel disease, the effects of Lactobacillus plantarum (LP) on intestinal nutrient absorption, particularly oligopeptide transporter 1 (PepT1)-mediated absorption of dietary protein under inflammatory conditions, has not yet been characterized. In this study, we first investigated the effects of LP consumption on plasma amino acid concentrations and PepT1-mediated absorption of cephalexin in the small intestine of wild-type (WT) mice and interleukin-10 knockout (IL-10(-/-)) mice, a model of spontaneous colitis. We then analyzed expression and distribution of PepT1 and protein kinase C (PKC) activity in the jejunum of these mice. LP consumption (10(9) colony-forming units/0.5 mL) delivered by gavage once per day for 4 wk increased the total plasma amino acid concentration and the concentration of plasma cephalexin through enhancement of PepT1-mediated uptake in LP treated IL-10(-/-) mice compared with IL-10(-/-) mice. However, Western blotting and quantitative PCR analysis revealed no significant differences in PepT1 protein and mRNA expression between LP-treated and untreated mice. Additionally, immunofluorescence analysis showed that PepT1 did not appear to be mislocalized in IL-10(-/-) mice. Interestingly, IL-10(-/-) mice had significantly lower PKC activity and expression of phosphorylated PKC compared with WT mice, and these decreases could be prevented by LP treatment. These data suggest that consumption of LP enhances PepT1-mediated amino acid absorption, likely through alterations in PKC activity, as opposed to changes in expression or distribution of PepT1 in the small intestine of IL-10(-/-) mice.

Functional role of the intracellular loop linking transmembrane domains 6 and 7 of the human dipeptide transporter hPEPT1

The human intestinal di-/tripeptide transporter (hPEPT1) is a 12-transmembrane protein that facilitates transport of peptides from the intestine into the circulation. hPEPT1 is also an important target in oral delivery of drugs, but mechanistic and structural data for the protein are limited. In particular, there is little information on the function of the loops of the transporter. In this study, we show that mutation of several charged residues in the largest intracellular loop of hPEPT1 (loop 6-7, amino acids 224-278) significantly reduces hPEPT1 function. This loop has an asymmetric distribution of charged residues, with only positive charges in the N-terminal half and all five negative charges in the loop located in a small part of the C-terminal half. Point mutagenesis to alanine of three positive residues in the N-terminal half of loop 6-7 and four negative residues in the C-terminal half of the loop significantly reduced glycylsarcosine uptake. E267 was particularly sensitive to mutation, and kinetic analyses of E267A- and E267K-hPEPT1 gave V (max) values 10-fold lower than that for the wild-type protein. Secondary structure prediction suggested that loop 6-7 includes two amphipathic α-helices, with net positive and negative charges, respectively. We interpret the mutagenesis data in terms of interactions of the charged residues in loop 6-7 that may influence conformational changes of hPEPT1 during and after substrate transport.

Role of insulin on jejunal PepT1 expression and function regulation in diabetic male and female rats

The aim of this study was to determine whether the jejunal oligopeptide transporter PepT1 is regulated by insulin and whether this regulation is sex-dependent in type 1 diabetic rats. PepT1 expression, real-time polymerase chain reaction, and Western blots were performed using jejunal segments from 4 groups of male and female rats: normal (nondiabetic), insulin-treated nondiabetic, streptozotocin (STZ)-induced diabetic (type 1 diabetes), and insulin-treated diabetic models. Furthermore, the same segments from all groups underwent perfusion to assess uptake of the dipeptide glycylsarcosine through PepT1. Our results showed that insulin treatment of nondiabetic female rats decreased blood glucose level but did not affect nondiabetic male rats. In both male and female diabetic rats, insulin did not completely decrease blood glucose level. Insulin treatment decreased PepT1 mRNA level in nondiabetic male rats and increased mRNA level in nondiabetic female rats without affecting the PepT1 protein level in either sex. Inducing diabetes with STZ increased PepT1 mRNA and protein levels in female rats; however, in diabetic male rats, the increase in mRNA level was accompanied by a decrease in PepT1 protein level. Treatment of diabetic male rats with insulin partially reversed the effect of diabetes on PepT1 mRNA and protein levels, whereas the same treatment completely restored both PepT1 mRNA and protein to control levels in insulin-treated diabetic female rats. In both nondiabetic male and female rats, insulin treatment had no effect on PepT1 influx rate, and STZ treatment decreased the transporter influx rate. Treatment of diabetic male and female rats with insulin significantly increased PepT1 influx rate; however, complete recovery was found only in diabetic female rats. These results clearly show that insulin and diabetes affected blood glucose level as well as PepT1 activity, expression, and protein levels in a sex-dependent manner. These results suggest that a factor, probably estrogen, could be responsible for the sex-dependent effects of diabetes and insulin in PepT1 level and activity.

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.

Serine side chain-linked peptidomimetic conjugates of cyclic HPMPC and HPMPA: synthesis and interaction with hPEPT1

Cidofovir (HPMPC), a broad spectrum antiviral agent, cannot be administered orally due to ionization of its phosphonic acid group at physiological pH. One prodrug approach involves conversion to the cyclic form (cHPMPC, 1) and esterification by the side chain hydroxyl group of a peptidomimetic serine. Transport studies in a rat model have shown enhanced levels of total cidofovir species in the plasma after oral dosing with L-Val-L-Ser-OMe cHPMPC, 2a. To explore the possibility that 2a and its three L/D stereoisomers 2b-d undergo active transport mediated by the peptide-specific intestinal transporter PEPT1, we performed radiotracer uptake and electrophysiology experiments applying the two-electrode voltage clamp technique in Xenopus laevis oocytes overexpressing human PEPT1 (hPEPT1, SLC15A1). 2a-d did not induce inward currents, indicating that they are not transported, but the stereoisomers with an L-configuration at the N-terminal valine (2a and 2b) potently inhibited transport of the hPEPT1 substrate glycylsarcosine (Gly-Sar). A "reversed" dipeptide conjugate, L-Ser-L-Ala-OiPr cHPMPC (4), also did not exhibit detectable transport, but completely abolished the Gly-Sar signal, suggesting that affinity of the transporter for these prodrugs is not impaired by a proximate linkage to the drug in the N-terminal amino acid of the dipeptide. Single amino acid conjugates of cHPMPC (3a and 3b) or cHPMPA (5, 6a and 6b) were not transported and only weakly inhibited Gly-Sar transport. The known hPEPT1 prodrug substrate valacyclovir (7) and its L-Val-L-Val dipeptide analogue (8) were used to verify coupled transport by the oocyte model. The results indicate that the previously observed enhanced oral bioavailability of 2a relative to the parent drug is unlikely to be due to active transport by hPEPT1. Syntheses of the novel compounds 2b-d and 3-6 are described, including a convenient solid-phase method to prepare 5, 6a and 6b.

Peptide transporter 1 is responsible for intestinal uptake of the dipeptide glycylsarcosine: studies in everted jejunal rings from wild-type and Pept1 null mice

The purpose of this study was to determine the relative importance of peptide transporter 1 (PEPT1) in the uptake of peptides/mimetics from mouse small intestine, using glycylsarcosine (GlySar). After isolating jejunal tissue from wild-type and Pept1 null mice, 2 cm intestinal segments were everted and mounted on glass rods for tissue uptake studies. [(14)C]GlySar (4 μM) was studied as a function of time, temperature, sodium and pH, concentration, and potential inhibitors. Compared with wild-type animals, Pept1 null mice exhibited a 78% reduction in GlySar uptake at pH 6.0 at 37°C. GlySar uptake showed pH dependence, with peak values between pH 6.0 and 6.5 in wild-type animals, whereas no such tendency was observed in Pept1 null mice. GlySar exhibited Michaelis-Menten uptake kinetics and a minor nonsaturable component in wild-type animals. In contrast, GlySar uptake occurred only by a nonsaturable process in Pept1 null mice. GlySar uptake was significantly inhibited by dipeptides, aminocephalosporins, angiotensin-converting enzyme inhibitors, and the antiviral prodrug valacyclovir; these inhibitors had little, if any, effect on the uptake of GlySar in Pept1 null mice. The findings demonstrate that PEPT1 plays a critical role in the uptake of GlySar in jejunum and suggest that PEPT1 is the major transporter responsible for the intestinal absorption of small peptides.

Significance and regional dependency of peptide transporter (PEPT) 1 in the intestinal permeability of glycylsarcosine: in situ single-pass perfusion studies in wild-type and Pept1 knockout mice

The purpose of this study was to evaluate the role, relevance, and regional dependence of peptide transporter (PEPT) 1 expression and function in mouse intestines using the model dipeptide glycylsarcosine (GlySar). After isolating specific intestinal segments, in situ single-pass perfusions were performed in wild-type and Pept1 knockout mice. The permeability of [(3)H]GlySar was measured as a function of perfusate pH, dipeptide concentration, potential inhibitors, and intestinal segment, along with PEPT1 mRNA and protein. We found the permeability of GlySar to be saturable (K(m) = 5.7 mM), pH-dependent (maximal value at pH 5.5), and specific for PEPT1; other peptide transporters, such as PHT1 and PHT2, were not involved, as judged by the lack of GlySar inhibition by excess concentrations of histidine. GlySar permeabilities were comparable in the duodenum and jejunum of wild-type mice but were much larger than that in ileum (approximately 2-fold). A PEPT1-mediated permeability was not observed for GlySar in the colon of wild-type mice (<10% residual uptake compared to proximal small intestine). Moreover, GlySar permeabilities were very low and not different in the duodenum, jejunum, ileum, and colon of Pept1 knockout mice. Functional activity of intestinal PEPT1 was confirmed by real-time polymerase chain reaction and immunoblot analyses. Our findings suggest that a loss of PEPT1 activity (e.g., due to polymorphisms, disease, or drug interactions) should have a major effect in reducing the intestinal absorption of di-/tripeptides, peptidomimetics, and peptide-like drugs.

Gene ablation for PEPT1 in mice abolishes the effects of dipeptides on small intestinal fluid absorption, short-circuit current, and intracellular pH

PEPT1 function in mouse intestine has not been assessed by means of electrophysiology and methods to assess its role in intracellular pH and fluid homeostasis. Therefore, the effects of the dipeptide glycilsarcosin (Gly-Sar) on jejunal fluid absorption and villous enterocyte intracellular pH (pH(i)) in vivo, as well as on enterocyte[(14)C]Gly-Sar uptake, short-circuit current (I(sc)) response, and enterocyte pH(i) in vitro were determined in wild-type and PEPT1-deficient mice and in mice lacking PEPT1. Immunohistochemistry for PEPT1 failed to detect any protein in enterocyte apical membranes in Slc15a1(-/-) animals. Saturable Gly-Sar uptake in Slc15a1(-/-) everted sac preparations was no longer detectable. Similarly, Gly-Sar-induced jejunal I(sc) response in vitro was abolished. The dipeptide-induced increase in fluid absorption in vivo was also abolished in animals lacking PEPT1. Since PEPT1 acts as an acid loader in enterocytes, enterocyte pH(i) was measured in vivo by two-photon microscopy in SNARF-4-loaded villous enterocytes of exteriorized jejuni in anesthetized mice, as well as in BCECF-loaded enterocytes of microdissected jejunal villi. Gly-Sar-induced pH(i) decrease was no longer observed in the absence of PEPT1. A reversal of the proton gradient across the luminal membrane did not significantly diminish Gly-Sar-induced I(sc) response, whereas a depolarization of the apical membrane potential by high K(+) or via Na(+)-K(+)-ATPase inhibition strongly diminished Gly-Sar-induced I(sc) responses. This study demonstrates for the first time that proton-coupled electrogenic dipeptide uptake in the native small intestine, mediated by PEPT1, relies on the negative apical membrane potential as the major driving force and contributes significantly to intestinal fluid transport.

The bioavailable octapeptide Gly-Ala-Hyp-Gly-Leu-Hyp-Gly-Pro stimulates nitric oxide synthesis in vascular endothelial cells

Gly-Ala-Hyp-Gly-Leu-Hyp-Gly-Pro (GAXGLXGP, X: Hyp), an octapeptide contained in chicken collagen hydrolysate, inhibits angiotensin I-converting enzyme activity in vitro. Intestinal Caco-2 and bovine aortic endothelial cells (BAECs) were used to investigate whether the transported GAXGLXGP improves vascular function. When GAXGLXGP was added to the apical side of Caco-2 monolayers, the intact form of GAXGLXGP was released to the basolateral side without incorporation into the cells. This transport was energy-independent but was associated with tight junction permeability. GAXGLXGP was then added to BAECs, and endothelial nitric oxide (NO) synthase (eNOS) activation was examined. GAXGLXGP at a concentration of 10 microM stimulated production of NO during a 1 h incubation. This event involved phosphorylation of eNOS at Ser(1179) without a change in the total eNOS protein level. These findings indicate that GAXGLXGP absorbed intact through the intestinal epithelium has direct effects on eNOS activity in vascular endothelial cells, leading to NO synthesis, thereby suggesting the potential for improvement in vascular function.

Computational tools for the interactive exploration of proteomic and structural data

Linking proteomics and structural data is critical to our understanding of cellular processes, and interactive exploration of these complementary data sets can be extremely valuable for developing or confirming hypotheses in silico. However, few computational tools facilitate linking these types of data interactively. In addition, the tools that do exist are neither well understood nor widely used by the proteomics or structural biology communities. We briefly describe several relevant tools, and then, using three scenarios, we present in depth two tools for the integrated exploration of proteomics and structural data.