Genetic variant Arg57His in human H+/peptide cotransporter 2 causes a complete loss of transport function

Molecular Insights to the Structure-Interaction Relationships of Human Proton-Coupled Oligopeptide Transporters (PepTs)

Human proton-coupled oligopeptide transporters (PepTs) are important membrane influx transporters that facilitate the cellular uptake of many drugs including ACE inhibitors and antibiotics. PepTs mediate the absorption of di- and tri-peptides from dietary proteins or gastrointestinal secretions, facilitate the reabsorption of peptide-bound amino acids in the kidney, and regulate neuropeptide homeostasis in extracellular fluids. PepT1 and PepT2 have been the most intensively investigated of all PepT isoforms. Modulating the interactions of PepTs and their drug substrates could influence treatment outcomes and adverse effects with certain therapies. In recent studies, topology models and protein structures of PepTs have been developed. The aim of this review was to summarise the current knowledge regarding structure-interaction relationships (SIRs) of PepTs and their substrates as well as the potential applications of this information in therapeutic optimisation and drug development. Such information may provide insights into the efficacy of PepT drug substrates in patients, mechanisms of drug-drug/food interactions and the potential role of PepTs targeting in drug design and development strategies.

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.

Mirogabalin, a novel α2δ ligand, is not a substrate of LAT1, but of PEPT1, PEPT2, OAT1, OAT3, OCT2, MATE1 and MATE2-K

Mirogabalin is a α₂δ ligand as well as pregabalin. The aim of this study was to clarify whether mirogabalin is a substrate of human LAT1, which involved in absorption and disposition of pregabalin, and to investigate transporters involved in renal secretion and absorption of mirogabalin using transporter-expressing cells and fresh human kidney slices.We employed uptake assay of [³H]mirogabalin by HEK293T or HEK293 cells transiently overexpress human OAT1, OAT3, OCT2, LAT1/4F2hc, LAT2/4F2hc, PEPT1, and PEPT2 proteins. Transport assay of MDCKII cells transiently overexpress OCT2/MATE1, and OCT2/MATE2-K proteins was conducted. Contribution of transporters to renal secretion was investigated by uptake assay using human kidney slices.Uptake clearances of [³H]mirogabalin by OAT1-, OAT3-, OCT2-, PEPT1-, and PEPT2-expressing cells were higher than that by vector cells, but by LAT1/4F2hc and LAT2/4F2hc-expressing cells were not. In transport assay using OCT2/MATE1 and OCT2/MATE2-K cells, [³H]mirogabalin showed directional transport from basolateral to apical side. Contribution of OAT1, OAT3, and OCT2 was observed by uptake of [³H]mirogabalin into the kidney slices.These results indicate that mirogabalin is not a substrate of LAT1, but of PEPT1 and PEPT2 involved in absorption and of OAT1, OAT3, OCT2, MATE1 and/or MATE2-K involved in its urinary secretion.

Extracellular domain of PepT1 interacts with TM1 to facilitate substrate transport

Mammalian peptide transporters, PepT1 and PepT2, mediate uptake of small peptides and are essential for their absorption. PepT also mediates absorption of many drugs and prodrugs to enhance their bioavailability. PepT has twelve transmembrane (TM) helices that fold into an N-terminal domain (NTD, TM1-6) and a C-terminal domain (CTD, TM7-12) and has a large extracellular domain (ECD) between TM9-10. It is well recognized that peptide transport requires movements of the NTD and CTD, but the role of the ECD in PepT1 remains unclear. Here we report the structure of horse PepT1 encircled in lipid nanodiscs and captured in the inward-open apo conformation. The structure shows that the ECD bridges the NTD and CTD by interacting with TM1. Deletion of ECD or mutations to the ECD-TM1 interface impairs the transport activity. These results demonstrate an important role of ECD in PepT1 and enhance our understanding of the transport mechanism in PepT1.

Oligopeptide transporter Slc15A modulates macropinocytosis in Dictyostelium by maintaining intracellular nutrient status

Macropinocytosis mediates non-selective bulk uptake of extracellular fluid. It is the major route by which axenic Dictyostelium cells obtain nutrients and has emerged as a nutrient-scavenging pathway for mammalian cells. How environmental and cellular nutrient status modulates macropinocytic activity is not well understood. By developing a high-content imaging-based genetic screen in Dictyostelium, we identified Slc15A, an oligopeptide transporter localized at the plasma membrane and early macropinosome, as a novel macropinocytosis regulator. We show that deletion of slc15A, but not two other related slc15 genes, leads to reduced macropinocytosis, slower cell growth, and aberrantly increased autophagy in cells grown in nutrient-rich medium. Expression of Slc15A or supplying cells with free amino acids rescues these defects. In contrast, expression of transport-defective Slc15A or supplying cells with amino acids in their di-peptide forms fails to rescue these defects. Therefore, Slc15A modulates the level of macropinocytosis by maintaining the intracellular availability of key amino acids via oligopeptide extraction from the early macropinocytic pathway. We propose that Slc15A constitutes part of a positive feedback mechanism coupling cellular nutrient status and macropinocytosis.

Transporters in the Mammary Gland-Contribution to Presence of Nutrients and Drugs into Milk

A large number of nutrients and bioactive ingredients found in milk play an important role in the nourishment of breast-fed infants and dairy consumers. Some of these ingredients include physiologically relevant compounds such as vitamins, peptides, neuroactive compounds and hormones. Conversely, milk may contain substances-drugs, pesticides, carcinogens, environmental pollutants-which have undesirable effects on health. The transfer of these compounds into milk is unavoidably linked to the function of transport proteins. Expression of transporters belonging to the ATP-binding cassette (ABC-) and Solute Carrier (SLC-) superfamilies varies with the lactation stages of the mammary gland. In particular, Organic Anion Transporting Polypeptides 1A2 (OATP1A2) and 2B1 (OATP2B1), Organic Cation Transporter 1 (OCT1), Novel Organic Cation Transporter 1 (OCTN1), Concentrative Nucleoside Transporters 1, 2 and 3 (CNT1, CNT2 and CNT3), Peptide Transporter 2 (PEPT2), Sodium-dependent Vitamin C Transporter 2 (SVCT2), Multidrug Resistance-associated Protein 5 (ABCC5) and Breast Cancer Resistance Protein (ABCG2) are highly induced during lactation. This review will focus on these transporters overexpressed during lactation and their role in the transfer of products into the milk, including both beneficial and harmful compounds. Furthermore, additional factors, such as regulation, polymorphisms or drug-drug interactions will be described.

Identification of two novel genes SLC15A2 and SLCO1B3 associated with maintenance dose variability of warfarin in a Chinese population

Warfarin is a commonly prescribed and effective oral anticoagulant. Genetic polymorphisms associated with warfarin metabolism and sensitivity have been implicated in the wide inter-individual dose variation that is observed. Several algorithms integrating patients’ clinical characteristics and genetic polymorphism information have been explored to predict warfarin dose. However, most of these algorithms could explain only over half of the variation in a warfarin maintenance dose, suggesting that additional genetic factors may exist and need to be identified. Here, a drug absorption, distribution, metabolism and excretion (ADME) Core Panel Kit-based pharmacogenetic study was performed to screen for warfarin dose-associated SNP sites in Han-Chinese population patients taking warfarin therapy, and the screen was followed by pyrosequencing-based validation. Finally, we confirmed that the common variant rs9923231 in VKORC1 and two novel genes, SLC15A2 (rs1143671 and rs1143672) and SLCO1B3 (rs4149117 and rs7311358), are associated with the warfarin maintenance dose. As has been shown for those carriers with the variant rs9923231 in VKORC1, it was suggested that those subjects with homozygous minor alleles in those four SNPs should take a lower warfarin dose than those carrying the wild type alleles. Together with the established predictor rs9923231 in VKORC1, those four novel variants on SLC15A2 and SLCO1B3 should be considered as useful biomarkers for warfarin dose adjustment in clinical practice in Han-Chinese populations.

Species Differences in Human and Rodent PEPT2-Mediated Transport of Glycylsarcosine and Cefadroxil in Pichia Pastoris Transformants

The proton-coupled oligopeptide transporter PEPT2 (SLC15A2) plays an important role in the disposition of di/tripeptides and peptide-like drugs in kidney and brain. However, unlike PEPT1 (SLC15A1), there is little information about species differences in the transport of PEPT2-mediated substrates. The purpose of this study was to determine whether PEPT2 exhibited a species-dependent uptake of glycylsarcosine (GlySar) and cefadroxil using yeast Pichia pastoris cells expressing cDNA from human, mouse, and rat. In such a system, the functional activity of PEPT2 was evaluated with [³H]GlySar as a function of time, pH, substrate concentration, and specificity, and with [³H]cefadroxil as a function of concentration. We observed that the uptake of GlySar was pH-dependent with an optimal uptake at pH 6.5 for all three species. Moreover, GlySar showed saturable uptake kinetics, with K_m values in human (150.6 µM) > mouse (42.8 µM) ≈ rat (36.0 µM). The PEPT2-mediated uptake of GlySar in yeast transformants was specific, being inhibited by di/tripeptides and peptide-like drugs, but not by amino acids and nonsubstrate compounds. Cefadroxil also showed a saturable uptake profile in all three species, with K_m values in human (150.8 μM) > mouse (15.6 μM) ≈ rat (11.9 μM). These findings demonstrated that the PEPT2-mediated uptake of GlySar and cefadroxil was specific, species dependent, and saturable. Furthermore, based on the K_m values, mice appeared similar to rats but both were less than optimal as animal models in evaluating the renal reabsorption and pharmacokinetics of peptides and peptide-like drugs in humans.

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.

Are intact peptides absorbed from the healthy gut in the adult human?

For over 100 years it was believed that dietary protein must be completely hydrolysed before its constituent amino acids could be absorbed via specific amino acid transport systems. It is now known that the uptake of di- and tripeptides into the enterocyte is considerable, being transported across the intestinal endothelium by the PepT1 H+/peptide co-transporter. There is also evidence that some di- and tripeptides may survive cytosolic hydrolysis and be transported intact across the basolateral membrane. However, other than antigen sampling, the transport of larger intact macromolecules across the intestinal endothelium of the healthy adult human remains a controversial issue as there is little unequivocal in vivo evidence to support this postulation. The aim of the present review was to critically evaluate the scientific evidence that peptides/proteins are absorbed by healthy intestinal epithelia and pass intact into the hepatic portal system. The question of the absorption of oliogopeptides is paramount to the emerging science of food-derived bioactive peptides, their mode of action and physiological effects. Overall, we conclude that there is little unequivocal evidence that dietary bioactive peptides, other than di- and tripeptides, can cross the gut wall intact and enter the hepatic portal system in physiologically relevant concentrations.

Population pharmacokinetic modeling of cefadroxil renal transport in wild-type and Pept2 knockout mice

1. Cefadroxil is a broad-spectrum β-lactam antibiotic that is widely used in the treatment of various infectious diseases. Currently, poor understanding of the drug's pharmacokinetic profiles and disposition mechanism(s) prevents determining optimal dosage regimens and achieving ideal antibacterial responses in patients. In the present retrospective study, we developed a population pharmacokinetic model of cefadroxil in wild-type and Pept2 knockout mice using the nonlinear mixed effect modeling (NONMEM) approach. 2. Cefadroxil pharmacokinetics were best described by a two-compartment model, with both saturable and nonsaturable elimination processes to/from the central compartment. Through this modeling approach, pharmacokinetic parameters in wild-type and Pept2 knockout mice were well estimated, respectively, as follows: volume of central compartment V1 (3.43 versus 4.23 mL), volume of peripheral compartment V2 (5.98 versus 8.61 mL), intercompartment clearance Q (0.599 versus 0.586 mL/min) and linear elimination rate constant K10 (0.111 versus 0.070 min(-1)). Moreover, the secretion kinetics (i.e. V(m1) = 17.6 nmoL/min and K(m1) = 37.1 µM) and reabsorption kinetics (i.e. V(m2) = 15.0 nmoL/min and K(m2) = 27.1 µM) of cefadroxil were quantified in kidney, for the first time, under in vivo conditions. 3. Our model provides a unique tool to quantitatively predict the dose-dependent nonlinear disposition of cefadroxil, as well as the potential for transporter-mediated drug interactions.

Proton-coupled oligopeptide transporter family SLC15: physiological, pharmacological and pathological implications

Mammalian members of the proton-coupled oligopeptide transporter family (SLC15) are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs. The driving force for uphill electrogenic symport is the chemical gradient and membrane potential which favors proton uptake into the cell along with the peptide/mimetic substrate. The peptide transporters are responsible for the absorption and conservation of dietary protein digestion products in the intestine and kidney, respectively, and in maintaining homeostasis of neuropeptides in the brain. They are also responsible for the absorption and disposition of a number of pharmacologically important compounds including some aminocephalosporins, angiotensin-converting enzyme inhibitors, antiviral prodrugs, and others. In this review, we provide updated information on the structure-function of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4) and PhT2 (SLC15A3), and their expression and localization in key tissues. Moreover, mammalian peptide transporters are discussed in regard to pharmacogenomic and regulatory implications on host pharmacology and disease, and as potential targets for drug delivery. Significant emphasis is placed on the evolving role of these peptide transporters as elucidated by studies using genetically modified animals. Whenever possible, the relevance of drug-drug interactions and regulatory mechanisms are evaluated using in vivo studies.

Importance of Peptide transporter 2 on the cerebrospinal fluid efflux kinetics of glycylsarcosine characterized by nonlinear mixed effects modeling

PURPOSE

To develop a population pharmacokinetic model to quantitate the distribution kinetics of glycylsarcosine (GlySar), a substrate of peptide transporter 2 (PEPT2), in blood, CSF and kidney in wild-type and PEPT2 knockout mice.

METHODS

A stepwise compartment modeling approach was performed to describe the concentration profiles of GlySar in blood, CSF, and kidney simultaneously using nonlinear mixed effects modeling (NONMEM). The final model was selected based on the likelihood ratio test and graphical goodness-of-fit.

RESULTS

The profiles of GlySar in blood, CSF, and kidney were best described by a four-compartment model. The estimated systemic elimination clearance, volume of distribution in the central and peripheral compartments were 0.236 vs 0.449 ml/min, 3.79 vs 4.75 ml, and 5.75 vs 9.18 ml for wild-type versus knockout mice. Total CSF efflux clearance was 4.3 fold higher for wild-type compared to knockout mice. NONMEM parameter estimates indicated that 77% of CSF efflux clearance was mediated by PEPT2 and the remaining 23% was mediated by the diffusional and bulk clearances.

CONCLUSIONS

Due to the availability of PEPT2 knockout mice, we were able to quantitatively determine the significance of PEPT2 in the efflux kinetics of GlySar at the blood-cerebrospinal fluid barrier.

Crystal structure of a prokaryotic homologue of the mammalian oligopeptide-proton symporters, PepT1 and PepT2

PepT1 and PepT2 are major facilitator superfamily (MFS) transporters that utilize a proton gradient to drive the uptake of di- and tri-peptides in the small intestine and kidney, respectively. They are the major routes by which we absorb dietary nitrogen and many orally administered drugs. Here, we present the crystal structure of PepT(So), a functionally similar prokaryotic homologue of the mammalian peptide transporters from Shewanella oneidensis. This structure, refined using data up to 3.6 Å resolution, reveals a ligand-bound occluded state for the MFS and provides new insights into a general transport mechanism. We have located the peptide-binding site in a central hydrophilic cavity, which occludes a bound ligand from both sides of the membrane. Residues thought to be involved in proton coupling have also been identified near the extracellular gate of the cavity. Based on these findings and associated kinetic data, we propose that PepT(So) represents a sound model system for understanding mammalian peptide transport as catalysed by PepT1 and PepT2.

Distribution of glycylsarcosine and cefadroxil among cerebrospinal fluid, choroid plexus, and brain parenchyma after intracerebroventricular injection is markedly different between wild-type and Pept2 null mice

The purpose of this study was to define the cerebrospinal fluid (CSF) clearance kinetics, choroid plexus uptake, and parenchymal penetration of PEPT2 substrates in different regions of the brain after intracerebroventricular administration. To accomplish these objectives, we performed biodistribution studies using [(14)C]glycylsarcosine (GlySar) and [(3)H]cefadroxil, along with quantitative autoradiography of [(14)C]GlySar, in wild-type and Pept2 null mice. We found that PEPT2 deletion markedly reduced the uptake of GlySar and cefadroxil in choroid plexuses at 60 mins by 94% and 82% (P<0.001), respectively, and lowered their CSF clearances by about fourfold. Autoradiography showed that GlySar concentrations in the lateral, third, and fourth ventricle choroid plexuses were higher in wild-type as compared with Pept2 null mice (P<0.01). Uptake of GlySar by the ependymal-subependymal layer and septal region was higher in wild-type than in null mice, but the half-distance of penetration into parenchyma was significantly less in wild-type mice. The latter is probably because of the clearance of GlySar from interstitial fluid by brain cells expressing PEPT2, which stops further penetration. These studies show that PEPT2 knockout can significantly modify the spatial distribution of GlySar and cefadroxil (and presumably other peptides/mimetics and peptide-like drugs) in brain.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

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.

Nature and consequences of mammalian brain and CSF efflux transporters: four decades of progress

In the last 40 years, especially with the application of new neurochemical and molecular biological techniques, there has been explosive progress in understanding how certain ligands and drugs are transported across the blood-brain barrier and choroid plexus out of brain and CSF. In the CNS, there are several separate efflux transporters with very broad specificity that are responsible for much of the efflux transport. This review focuses on three such transporters: organic acid transporter-3, peptide transporter-2 and P-glycoprotein for which there is substantial new information including 'knockout' models in mice and, in one case, dogs. Moreover, the structural biology and transport mechanism of P-glycoprotein at 3.8 angstroms is described. The overall objective is to show how this new knowledge provides a more thorough understanding (e.g., of molecular mechanisms) of efflux transport and in several cases leads to clinically relevant information that allows better treatment of certain CNS disorders (e.g., meningitis and brain cancer).

Stimulation of Na+/Cl--coupled opioid peptide transport system in SK-N-SH cells by L-kyotorphin, an endogenous substrate for H+-coupled peptide transporter PEPT2

We have recently identified a Na+/Cl--coupled transport system in mammalian cells for endogenous and synthetic opioid peptides. This transport system does not transport dipeptides/tripeptides, but is stimulated by these small peptides. Here we investigated the influence of L-kyotorphin (L-Tyr-L-Arg), an endogenous dipeptide with opioid activity, on this transport system. The activity of the transport system, measured in SK-N-SH cells (a human neuronal cell line) with deltorphin II as a model substrate, was stimulated approximately 2.5-fold by L-kyotorphin, with half-maximal stimulation occurring at approximately 100 microM. The stimulation was associated primarily with an increase in the affinity for deltorphin II. The stimulation caused by L-kyotorphin was stereospecific; L-Tyr-D-Arg (D-kyotorphin) had minimal effect. The influence of L-kyotorphin was observed also in a different cell line which expressed the opioid peptide transport system. While L-kyotorphin is a stimulator of opioid peptide transport, it is a transportable substrate for the H+-coupled peptide transporter PEPT2, which is expressed widely in the brain. Since the activity of the opioid peptide transport system is modulated by extracellular L-kyotorphin and since PEPT2 is an important determinant of extracellular L-kyotorphin in the brain, the expression/activity of PEPT2 may be a critical factor in the modulation of opioidergic neurotransmission in vivo.

Role and relevance of PEPT2 in drug disposition, dynamics, and toxicity

Pept2 knockout mice are an important tool to evaluate the evolving role and relevance of this proton-coupled oligopeptide transporter beyond drug disposition, where the transporter also modulates the pharmacodynamic and toxicodynamic effects of drug substrates. Our in vivo studies with glycylsarcosine in Pept2 knockout mice have established "proof of concept" that PEPT2 can have a significant effect on dipeptide disposition. Subsequent studies with the aminocephalosporin antibiotic cefadroxil have shown relevance to pharmacology and infectious disease. Finally, studies with the endogenous peptidomimetic 5-aminolevulinic acid have demonstrated relevance to toxicology in the framework of porphyria- and lead-induced neurotoxicity. These studies have consistently demonstrated the dual action of PEPT2 with respect to its apical localization in choroid plexus epithelium and kidney in: 1) effluxing substrates from CSF into choroid plexus, thereby affecting regional pharmacokinetics in brain; and 2) reabsorbing substrates from renal tubular fluid into proximal tubules, thereby affecting systemic pharmacokinetics and exposure. Moreover, these studies have shown that the regional effect of PEPT2 in limiting substrate concentrations in the CSF is more dramatic than its effect in increasing systemic exposure. In the case of 5-aminolevulinic acid, such regional modulation of drug disposition translates directly into significant changes in neurotoxicity.

Peptide transporter 2 (PEPT2) expression in brain protects against 5-aminolevulinic acid neurotoxicity

The proton-coupled oligopeptide transporter PEPT2 (or SLC15A2) is the major protein involved in the reclamation of peptide-bound amino acids and peptide-like drugs in kidney. PEPT2 is also important in effluxing peptides and peptidomimetics from CSF at the choroid plexus, thereby limiting their exposure in brain. In this study, we report a neuroprotective role for PEPT2 in modulating the toxicity of a heme precursor, 5-aminolevulinic acid (5-ALA). Our findings demonstrate that in PEPT2-deficient mice, 5-ALA administration results in reduced survivability, a worsening of neuromuscular dysfunction, and CSF concentrations of substrate that are 8-30 times higher than that in wild-type control animals. The ability of PEPT2 to limit 5-ALA exposure in CSF suggests that it may also have relevance as a secondary genetic modifier of conditions (such as acute hepatic porphyrias and lead poisoning) in which 5-ALA metabolism is altered and in which 5-ALA toxicity is important.

Impact of genetic knockout of PEPT2 on cefadroxil pharmacokinetics, renal tubular reabsorption, and brain penetration in mice

The aim of this study was to examine the role of PEPT2, a proton-coupled oligopeptide transporter of the SLC15 family, on the disposition of the antibiotic cefadroxil in the body, particularly the kidney and brain. Pharmacokinetic, tissue distribution, and renal clearance studies were performed in wild-type and PEPT2 null mice after intravenous bolus administration of [(3)H]cefadroxil at 1, 12.5, 50, and 100 nmol/g body weight. Studies were also performed in the absence and presence of probenecid and quinine. Cefadroxil disposition kinetics was clearly nonlinear over the dose range studied (1-100 nmol/g), which was attributed to both saturable renal tubular secretion and reabsorption of the antibiotic. After an intravenous bolus dose of 1 nmol/g cefadroxil, PEPT2 null mice exhibited a 3-fold greater total clearance and 3-fold lower systemic concentrations of drug compared with wild-type animals. Renal clearance studies further demonstrated that the renal reabsorption of cefadroxil was almost completely abolished in PEPT2 null versus wild-type mice (3% versus 70%, p < 0.001). Of the 70% of cefadroxil reabsorbed in wild-type mice, PEPT2 accounted for 95% and PEPT1 accounted for 5% of reabsorbed substrate. Tissue distribution studies indicated that PEPT2 had a dramatic effect on cefadroxil tissue exposure, especially in brain where the cerebrospinal fluid (CSF)-to-blood concentration ratio of cefadroxil was 6-fold greater in PEPT2 null mice compared with wild-type animals. These findings demonstrate that renal PEPT2 is almost entirely responsible for the reabsorption of cefadroxil in kidney and that choroid plexus PEPT2 limits the exposure of cefadroxil (and perhaps other aminocephalosporins) in CSF.

Molecular cloning, functional characterization and tissue distribution of rat H+/organic cation antiporter MATE1

PURPOSE

Transport characteristics and tissue distribution of the rat H+/organic cation antiporter MATE1 (multidrug and toxin extrusion 1) were examined.

METHODS

Rat MATE1 cDNA was isolated by polymerase chain reaction (PCR) cloning. Transport characteristics of rat MATE1 were assessed by HEK293 cells transiently expressing rat MATE1. The mRNA expression of rat MATE1 was examined by Northern blot and real-time PCR analyses.

RESULTS

The uptake of a prototypical organic cation tetraethylammonium (TEA) by MATEI-expressing cells was concentration-dependent, and showed the greatest value at pH 8.4 and the lowest at pH 6.0-6.5. Intracellular acidification induced by ammonium chloride resulted in a marked stimulation of TEA uptake. MATE1 transported not only organic cations such as cimetidine and metformin but also the zwitterionic compound cephalexin. MATE1 mRNA was expressed abundantly in the kidney and placenta, slightly in the spleen, but not expressed in the liver. Real-time PCR analysis of microdissected nephron segments showed that MATE1 was primarily expressed in the proximal convoluted and straight tubules.

CONCLUSIONS

These findings indicate that MATE1 is expressed in the renal proximal tubules and can mediate the transport of various organic cations and cephalexin using an oppositely directed H+ gradient.

Induction of intestinal peptide transporter 1 expression during fasting is mediated via peroxisome proliferator-activated receptor alpha

We previously demonstrated that starvation markedly increased the amount of mRNA and protein levels of the intestinal H+/peptide cotransporter (PEPT1) in rats, leading to altered pharmacokinetics of the PEPT1 substrates. In the present study, the mechanism underlying this augmentation was investigated. We focused on peroxisome proliferator-activated receptor alpha (PPARalpha), which plays a pivotal role in the adaptive response to fasting in the liver and other tissues. In 48-h fasted rats, the expression level of PPARalpha mRNA in the small intestine markedly increased, accompanied by the elevation of serum free fatty acids, which are endogenous PPARalpha ligands. Oral administration of the synthetic PPARalpha ligand WY-14643 to fed rats increased the mRNA level of intestinal PEPT1. Furthermore, treatment of the human intestinal model, Caco-2 cells, with WY-14643 resulted in enhanced PEPT1 mRNA expression and uptake activity of glycylsarcosine. In the small intestine of PPARalpha-null mice, augmentation of PEPT1 mRNA during fasting was completely abolished. In the kidney, fasting did not induce PEPT1 expression in either PPARalpha-null or wild-type mice. Together, these results indicate that PPARalpha plays critical roles in fasting-induced intestinal PEPT1 expression. In addition to the well-established roles of PPARalpha, we propose a novel function of PPARalpha in the small intestine, that is, the regulation of nitrogen absorption through PEPT1 during fasting.

Drug-drug interactions involving membrane transporters in the human kidney

The kidneys play a critical role in the elimination of xenobiotics. Factors affecting the ability of the kidney to eliminate drugs may result in marked changes in the pharmacokinetics of a given compound. Drug-drug interactions due to competitive inhibition of renal organic anion or cation secretion systems have been noticed clinically for a long time. However, our understanding of the physical sites of interactions, that is, the specific transport proteins that the interacting drugs act on, has just begun very recently. This review summarises the latest progress in molecular identification and functional characterisation of major drug transporters in the human kidney. In particular, the review focuses on relating cloned renal drug transporters to clinically observed drug-drug interactions. The authors' opinion on the current status and future directions of research in these areas is also offered.

From bacteria to man: archaic proton-dependent peptide transporters at work

Uptake of nutrients into cells is essential to life and occurs in all organisms at the expense of energy. Whereas in most prokaryotic and simple eukaryotic cells electrochemical transmembrane proton gradients provide the central driving force for nutrient uptake, in higher eukaryotes it is more frequently coupled to sodium movement along the transmembrane sodium gradient, occurs via uniport mechanisms driven by the substrate gradient only, or is linked to the countertransport of a similar organic solute. With the cloning of a large number of mammalian nutrient transport proteins, it became obvious that a few "archaic'' transporters that utilize a transmembrane proton gradient for nutrient transport into cells can still be found in mammals. The present review focuses on the electrogenic peptide transporters as the best studied examples of proton-dependent nutrient transporters in mammals and summarizes the most recent findings on their physiological importance. Taking peptide transport as a general phenomenon found in nature, we also include peptide transport mechanisms in bacteria, yeast, invertebrates, and lower vertebrates, which are not that often addressed in physiology journals.

Variability in renal clearance of substrates for renal transporters in chinese subjects

The authors evaluated the inter- and intraindividual variability in the renal clearance of substrates of organic anion transporters (OAT) or organic cation transporters (OCT) using repeated drug application procedures. Two OAT substrates (ampicillin and cephalexin) and 2 OCT substrates (famotidine and metformin) were selected. Each drug was administered orally twice to healthy subjects, with sample sizes ranging from 12 to 28 (using bioequivalent formulations of each drug). The inter-(delta(inter)) and intrasubject (delta(intra)) variances in renal clearance were estimated based on analysis of variance, and the genetic contribution (r(GC)) was calculated as (delta(inter - intra))/delta(inter). The renal clearances of ampicillin, cephalexin, famotidine, and metformin averaged 5.21 (range, 2.87-11.20), 3.01 (range, 1.50-3.82), 4.96 (range, 2.84-8.17), and 9.44 (range, 5.66-15.43) mL/min/kg, with mean intraindividual coefficients of variation of 17.7%, 7.3%, 13.5%, and 9.0% and r(GC) values of 0.75, 0.89, 0.81, and 0.93, respectively. These high r(GC) values suggest a potential significant genetic contribution by the renal OATs and OCTs in Chinese subjects. Further studies in a larger population are needed to confirm the importance of these results as well as to identify specific genetic variants in these transporters responsible for such variability.

The PDZ domain protein PDZK1 interacts with human peptide transporter PEPT2 and enhances its transport activity

The proton-coupled peptide transporter PEPT2 (SLC15A2) mediates the high-affinity low-capacity transport of small peptides as well as various oral peptide-like drugs in the kidney. In contrast to its well-characterized transport properties, there is less information available on its regulatory mechanism, although the interaction of PEPT2 to the PDZ (PSD-95, DglA, and ZO-1)-domain protein PDZK1 has been preliminarily reported. To examine whether PDZK1 is a physiological partner of PEPT2 in kidneys, we started from a yeast two-hybrid screen of a human kidney cDNA library with the C-terminus of PEPT2 (PEPT2 C-terminus (PEPT2-CT)) as bait. We could identify PDZK1 as one of the positive clones. This interaction requires the PDZ motif of PEPT2-CT detected by a yeast two-hybrid assay, in vitro binding assay and co-immunoprecipitation. The binding affinities of second and third PDZ domains of PDZK1 to PEPT2-CT were measured by surface plasmon resonance. Co-immunoprecipitation using human kidney membrane fraction and localization of PEPT2 in renal apical proximal tubules revealed the physiological meaning of this interaction in kidneys. Furthermore, we clarified the mechanism of enhanced glycylsarcosine (Gly-Sar) transport activity in PEPT2-expressing HEK293 cells after the PDZK1 coexpression. This augmentation was accompanied by a significant increase in the V(max) of Gly-Sar transport via PEPT2 and it was also associated with the increased surface expression level of PEPT2. These results indicate that the PEPT2-PDZK1 interaction thus plays a physiologically important role in both oligopeptide handling as well as peptide-like drug transport in the human kidney.

INTERACTIONS OF AMOXICILLIN AND CEFACLOR WITH HUMAN RENAL ORGANIC ANION AND PEPTIDE TRANSPORTERS

Amoxicillin and cefaclor are two of the widely used beta-lactam antibiotics in the treatment of urinary tract infections. Both drugs are eliminated mainly by the kidney and rely on renal excretion to exert their antibacterial activities in the urinary tract. Previous studies have suggested the involvement of organic anion and oligopeptide transporters in membrane transport of beta-lactams. The objective of the current study was to examine the kinetics of amoxicillin and cefaclor interactions with human renal transporters human organic anion transporter 1 (hOAT1), human peptide transporter 1 (hPepT1), and human peptide transporter 2 (hPepT2) in detail, both as substrates and as inhibitors. Using fluorescence protein tagging and cell sorting, we established Madin-Darby canine kidney cell lines stably expressing highly functional hOAT1, hPepT1, and hPepT2. Amoxicillin and cefaclor inhibited hOAT1-mediated [(3)H]para-aminohippuric acid uptake (K(i) = 11.0 and 1.15 mM, respectively). However, our uptake study revealed that neither drug was transported by hOAT1. Amoxicillin and cefaclor competitively inhibited hPepT2-mediated [(3)H]glycylsarcosine uptake (K(i) = 733 and 65 muM, respectively), whereas much lower affinity for hPepT1 was observed with both antibiotics. Direct uptake studies demonstrated that amoxicillin and cefaclor were transported by hPepT1 and hPepT2. Kinetic analysis showed that hPepT2-mediated uptake of both drugs was saturable with K(m) of 1.04 mM for amoxicillin and 70.2 muM for cefaclor. hPepT2, and to a lesser extent hPepT1, may play an important role in apical transport of amoxicillin and cefaclor in the renal tubule. hOAT1, in contrast, is not involved in basolateral uptake of these antibiotics.

Integrated physiology of proximal tubular organic anion transport

PURPOSE OF REVIEW

Renal organic anion transport proteins play important roles in the reabsorption and the secretion of endogenous and exogenous compounds. This review focuses on the interpretation of the physiological integration of identified transport molecules in the renal proximal tubules.

RECENT FINDINGS

To date, molecular identification of organic anion transport proteins is still continuing: rodent organic anion transporter 5, organic anion-transporting polypeptide 4C1, voltage-driven organic anion transporter 1, multidrug resistance-associated protein 4, and sodium-coupled monocarboxylate transporter have yielded additional information in this field. In addition, particularly at the apical membrane of the proximal tubules, the importance of the PDZ (PSD-95, DglA, and ZO-1) binding domain proteins has emerged in the formation of the multimolecular complex as a functional unit of membrane transport. Finally, discovery of dicarboxylate receptors in the renal tubular cells raises the possibility that dicarboxylate anions function as intrarenal signaling molecules. This novel aspect of renal organic anion transport, the potential modulation of signaling via dicarboxylate receptors, may be of significant relevance to renovascular hypertension and other renal diseases.

SUMMARY

Comprehensive understanding of the multimolecular complex, which is composed of transporters and their related signaling elements and is supported by the scaffold proteins underneath the plasma membrane, may be useful in clarifying complex transport phenomena such as renal apical organic anion handling. In addition to the recent proteomics approaches and conventional molecular physiology, it is necessary to develop novel methods to analyze the overall function of the multimolecular complex for the post-genomic era.

Genetic variants of the human H+/dipeptide transporter PEPT2: analysis of haplotype functions

PEPT2 is a high-affinity H+/dipeptide transporter expressed in kidney, brain, lung, and mammary gland. The physiological role of PEPT2 in kidney is to reabsorb small peptides generated by luminal peptidases. PEPT2 is also a transporter for peptide-like drugs such as penicillins and cephalosporins. We have conducted a haplotype analysis of 27 single nucleotide polymorphisms located in or near exons of the human gene encoding hPEPT2 (SLC15A2), using genotyping data from 247 genomic DNA samples from the Coriell collection. Our analysis reveals that hPEPT2 has a >6-kilobase sequence block with at least 10 abundant polymorphisms in almost complete linkage disequilibrium. As a result, only two main hPEPT2 variants exist (hPEPT2*1 and *2) with several phased amino acid substitutions, present in substantial frequencies in all ethnic groups tested. When expressed in Chinese hamster ovary cells, hPEPT2*1 and *2 displayed similar Vmax values for glycyl-sarcosine (Gly-Sar), but they differed significantly in their Km values (83 +/- 16 and 233 +/- 38 microM, respectively). Moreover, hPEPT2*1 and *2 differed in their pH sensitivity for H+/Gly-Sar transport. In addition, hPEPT2*1 and *2 generated varying levels of mRNA in nine heterozygous kidney tissue samples, including one allele expressing no detectable mRNA, suggesting the presence of cis-acting polymorphisms affecting transcription or mRNA processing. The results indicate that polymorphisms in the gene encoding hPEPT2 can alter substrate transport and therefore could affect drug disposition in vivo.