Peptide transporters in the intestine and the kidney

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.

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

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

Biochemical and morphological developments are partially impaired in intestinal mucosa from growing pigs fed reduced-protein diets supplemented with crystalline amino acids1

The objective of this study was to determine if a reduction in dietary CP, with partial replacement of the intact protein with crystalline AA (CAA), would alter growth, morphology, and free or peptide-bound AA concentrations of intestinal mucosa in growing pigs. Twenty-four barrows (37.0 +/- 1.5 kg of BW) were fed 1 of 4 diets for 24 d: 16.1% CP with no CAA, or 12.8, 10.1, or 7.8% CP (analyzed values, as-fed) containing CAA. As CP decreased, CAA were gradually increased to meet requirements on a true ileal digestible basis. Pigs were euthanized 2 h postmeal on d 24, and mucosal samples from duodenum, jejunum, and ileum were collected. Reducing dietary CP decreased ADG, G:F, and final weight (linear, P < 0.05). With reduced dietary CP, mucosal protein concentration decreased in the jejunum (quadratic, P < 0.05) and tended to decrease in the ileum (linear, P = 0.062). Reduction of the dietary CP concentration from 16.1 to 7.8% tended to decrease the crypt depth (linear, P < 0.10) and decreased villus width (linear, P < 0.05) in duodenum and jejunum mucosa but did not reduce villus height or villus surface area in any regions of the small intestine. In the duodenum, a reduction in dietary CP increased free Lys, Met, and Thr (linear, P < 0.05) and peptide-bound Lys and Thr (quadratic, P < 0.10). In the jejunum, reducing CP decreased free Cys (linear P < 0.05) and tended to decrease free Asn and His (linear, P < 0.10) and peptide-bound His (quadratic, P = 0.061) and Ile, Leu, and Val (linear, P < 0.10). In the ileum, reducing CP decreased free Asn, Ser, Tyr, Arg, His, Phe (linear, P < 0.05), and Leu (linear, P = 0.054) and peptide-bound Gly and Ser (linear, P < 0.05) and tended to decrease peptide-bound Ile, Leu, Phe, Val (linear, P < 0.10), and Lys (linear P < 0.05). In conclusion, reduced-CP diets supplemented with CAA lead to a reduction in growth performance, associated with biochemical and morphological modifications of the intestinal mucosa.

Renal Tubular Drug Transporters

The kidney plays an important role in the elimination of numerous hydrophilic xenobiotics, including drugs, toxins, and endogenous compounds. It has developed high-capacity transport systems to prevent urinary loss of filtered nutrients, as well as electrolytes, and simultaneously to facilitate tubular secretion of a wide range of organic ions. Transport systems for organic anions and cations are primarily involved in the secretion of drugs in renal tubules. The identification and characterization of organic anion and cation transporters have been progressing at the molecular level. To date, many members of the organic anion transporter, organic cation transporter, and organic anion-transporting polypeptide families have been found to mediate the transport of diverse organic ions. It has also been suggested that ATP-dependent primary active transporters such as MDR1/P-glycoprotein and the multidrug resistance-associated protein family function as efflux pumps of renal tubular cells for more hydrophobic molecules and anionic conjugates. Tubular reabsorption of peptide-like drugs such as beta-lactam antibiotics across the brush-border membranes appears to be mediated by two distinct H+/peptide cotransporters: PEPT1 and PEPT2. Renal disposition of drugs occurs through interaction with these diverse secretory and absorptive transporters in renal tubules. Studies of the functional characteristics, such as substrate specificity and transport mechanisms, and of the localization of drug transporters could provide information regarding the cellular network involved in renal handling of drugs. Detailed information concerning molecular and cellular aspects of drug transporters expressed in the kidney has facilitated studies of the mechanisms underlying renal disposition as well as transporter-mediated drug interactions.

The V-type H+ ATPase: molecular structure and function, physiological roles and regulation

It was nearly 30 years before the V-type H+ ATPase was admitted to the small circle of bona fide transport ATPases alongside F-type and P-type ATPases. The V-type H+ ATPase is an ATP-driven enzyme that transforms the energy of ATP hydrolysis to electrochemical potential differences of protons across diverse biological membranes via the primary active transport of H+. In turn, the transmembrane electrochemical potential of H+ is used to drive a variety of (i) secondary active transport systems via H+-dependent symporters and antiporters and (ii) channel-mediated transport systems. For example, expression of Cl- channels or transporters next to the V-type H+ ATPase in vacuoles of plants and fungi and in lysosomes of animals brings about the acidification of the endosomal compartment, and the expression of the H+/neurotransmitter antiporter next to the V-type H+ ATPase concentrates neurotransmitters in synaptic vesicles. First found in association with endosomal membranes, the V-type H+ ATPase is now also found in increasing examples of plasma membranes where the proton pump energizes transport across cell membranes and entire epithelia. The molecular details reveal up to 14 protein subunits arranged in (i) a cytoplasmic V1 complex, which mediates the hydrolysis of ATP, and (ii) a membrane-embedded V0 complex, which translocates H+ across the membrane. Clever experiments have revealed the V-type H+ ATPase as a molecular motor akin to F-type ATPases. The hydrolysis of ATP turns a rotor consisting largely of one copy of subunits D and F of the V1 complex and a ring of six or more copies of subunit c of the V0 complex. The rotation of the ring is thought to deliver H+ from the cytoplasmic to the endosomal or extracellular side of the membrane, probably via channels formed by subunit a. The reversible dissociation of V1 and V0 complexes is one mechanism of physiological regulation that appears to be widely conserved from yeast to animal cells. Other mechanisms, such as subunit-subunit interactions or interactions of the V-type H+ ATPase with other proteins that serve physiological regulation, remain to be explored. Some diseases can now be attributed to genetic alterations of specific subunits of the V-type H+ ATPase.

Transport systems for opioid peptides in mammalian tissues

Transmembrane transport of endogenous as well as synthetic opioid peptides is a critical determinant of pharmacokinetics and biologic efficacy of these peptides. This transport process influences the distribution of opioid peptides across the blood-brain barrier and their elimination from the body. A multitude of transport systems that recognize opioid peptides as substrates have been characterized at the functional level, and these transport systems are expressed differentially at different sites in the body. Many of these transport systems have been identified at the molecular level. These include the H(+)-coupled peptide transporters PEPT1 and PEPT2, the adenosine triphosphate-dependent efflux transporters P-glycoprotein and multidrug resistance-related protein 2, and several members of the organic anion-transporting polypeptide gene family. There are however many additional transport systems that are known to transport opioid peptides but their molecular identities still remain unknown.

Characterization of mouse organic anion transporter 5 as a renal steroid sulfate transporter

A family of organic anion transporters (OAT) recently identified has important roles for the excretion or reabsorption of endogenous and exogenous compounds, and several new isoforms have been reported in this decade. Although the transepithelial transport properties of organic anions are gradually being understood, many portions of their functional characteristics in functions remain to be elucidated. A recently reported new cDNA encoding a mouse OAT5 (mOAT5) was constructed, using 3'-RACE PCR, with the total RNA isolated from a mouse kidney. When mOAT5 was expressed in Xenopus oocytes, mOAT5 transported estrone sulfate, dehydroepiandrosterone sulfate and ochratoxin A. Estrone sulfate uptake by mOAT5 displayed a time-dependent and sodium-independent manner. The Km values of estrone sulfate and dehydroepiandrosterone sulfate were 2.2 and 3.8 microM, respectively. mOAT5 interacted with chemically heterogeneous steroid or organic sulfates, such as nitrophenyl sulfate, methylumbelliferyl sulfate and estradiol sulfates. In contrast to the sulfate conjugates, mOAT5-mediated estrone sulfate uptake was not inhibited by the steroid or organic glucuronides. The mOAT5 protein having about 85 kDa molecular weight was shown to be mainly localized in the apical membrane of the proximal tubules of the outer medulla. These results suggest an important role of mOAT5 for the excretion or reabsorption of steroid sulfates in the kidney.

Prediction of glycylsarcosine transport in Caco-2 cell lines expressing PEPT1 at different levels

H(+)-coupled peptide transporter 1 (PEPT1) and the basolateral peptide transporter mediate the absorption of small peptides and peptide-like drugs in the small intestine. Recently, we constructed a mathematical model to simulate glycylsarcosine (Gly-Sar) transport in Caco-2 cells. In this study, we attempted to adjust our model to a change in the expression level of PEPT1. To obtain cell lines expressing PEPT1 at different levels, recloning of Caco-2 cells was performed, and nine clones were isolated. Compared with parental cells, clones 1 and 9 exhibited the lowest and the highest levels of [(14)C]Gly-Sar uptake from the apical side, respectively, whereas activities of the basolateral peptide transporter were comparable. Kinetic analysis demonstrated that the difference in the activity of PEPT1 was accounted by variations in V (max). Moreover, PEPT1 mRNA level was positively related to the activity of [(14)C]Gly-Sar uptake (r=0.55). Based on these findings, the V (max) value of PEPT1 was defined as a variable using the amount of PEPT1 mRNA as an index of the expression level. With this improved model, Gly-Sar transport in clones 1 and 9 was well-predicted, suggesting that our model can simulate Gly-Sar transport in cells expressing PEPT1 at different levels.

Carnosine and Homocarnosine, the Forgotten, Enigmatic Peptides of the Brain

Carnosine (beta-alanyl-histidine) and homocarnosine (gamma-aminobutyryl-histidine) are major constituents of excitable tissues, brain and skeletal muscles, but their physiological functions are yet unknown. Using primary cell culture systems, synthesis and uptake of carnosine exclusively by glial cells could be demonstrated. Uptake of carnosine was found to be mediated by a high affinity, energy-dependent dipeptide transport system, subsequently identified as the peptide transporter PepT2. With the synthesis of beta-Ala-Lys-Nepsilon-AMCA as a fluorescent reporter molecule, accumulation of this dipeptide derivative could be monitored under viable conditions not only in astroglia cells but also in folliculostellate cells of the anterior pituitary and in gonadal resident macrophages. This reporter dipeptide provided a most valuable tool to identify an intrapituitary communication system by tracing folliculostellate cells in acute slice preparation. Moreover, this substance could also be used to prepare pituitary cell cultures enriched with or depleted of folliculostellate cells that are needed for further studies.

Luminal kidney and intestine SLC6 amino acid transporters of B0AT-cluster and their tissue distribution in Mus musculus

The B degrees transport system mediates the Na(+)-driven uptake of a broad range of neutral amino acids into epithelial cells of small intestine and kidney proximal tubule. A corresponding transporter was identified in 2004 (A. Broer, K. Klingel, S. Kowalczuk, J. E. Rasko, J. Cavanaugh, and S. Broer. J Biol Chem 279: 24467-24476, 2004) within the SLC6 family and named B degrees AT1 (SLC6A19). A phylogenetically related transporter known as XT3 in human (SLC6A20) and XT3s1 in mouse was shown to function as an imino acid transporter, to localize also to kidney and small intestine and renamed SIT1 or Imino(B). Besides these two transporters with known functions, there are two other gene products belonging to the same phylogenetic B degrees AT-cluster, XT2 (SLC6A18) and rodent XT3 that are still "orphans." Quantitative real-time RT-PCR showed that the mRNAs of the four B degrees AT-cluster members are abundant in kidney, whereas only those of B degrees AT1 and XT3s1/SIT1 are elevated in small intestine. In brain, the XT3s1/SIT1 mRNA is more abundant than the other B degrees AT-cluster mRNAs. We show here by immunofluorescence that all four mouse B degrees AT-cluster transporters localize, with differential axial gradients, to the brush-border membrane of proximal kidney tubule and, with the possible exception of XT3, also of intestine. Deglycosylation and Western blotting of brush-border proteins demonstrated the glycosylation and confirmed the luminal localization of B degrees AT1, XT2, and XT3. In summary, this study shows the luminal brush-border localization of the Na(+)-dependent amino and imino acid transporters B degrees AT1 and XT3s1/SIT1 in kidney and intestine. It also shows that the structurally highly similar orphan transporters XT2 and XT3 have the same luminal but a slightly differing axial localization along the kidney proximal tubule.

Role and relevance of peptide transporter 2 (PEPT2) in the kidney and choroid plexus: in vivo studies with glycylsarcosine in wild-type and PEPT2 knockout mice

The strategic localization of peptide transporter 2 (PEPT2), a proton-coupled oligopeptide transporter, to the apical membrane of epithelial cells in the kidney and choroid plexus suggests that it plays an important role in the disposition of peptides/mimetics in the body. Therefore, the in vivo significance of PEPT2 was investigated in wild-type and PEPT2 null mice following an i.v. bolus dose (0.05 micromol/g body weight) of [14C]glycylsarcosine (GlySar). In PEPT2 null mice, the clearance (total and renal) of GlySar was markedly increased (2-fold), resulting in concomitantly lower systemic concentrations. In addition, renal reabsorption was almost abolished, and GlySar was eliminated by glomerular filtration. Of the 46% of GlySar reabsorbed in wild-type mice, PEPT2 accounted for 86% and PEPT1 accounted for 14% of reabsorbed substrate. Analysis of GlySar uptake in kidney sections revealed that PEPT2 was primarily localized in the outer medullary region. Wild-type mice also had greater choroid plexus concentrations of GlySar and a 5-fold greater choroid plexus/cerebrospinal fluid (CSF) ratio as compared with null mice at 60 min. Null mice exhibited a greater CSF/blood ratio at 60 min (0.9 versus 0.2) and area under the curve (AUC)(CSF)/AUC(blood) ratio over 60 min (0.45 versus 0.12), indicating that PEPT2 significantly reduces the exposure of GlySar in CSF. Our in vivo results demonstrate that PEPT2 is the predominant peptide transporter in kidney and that it acts as an efflux transporter in choroid plexus. Thus, PEPT2 may have profound effects on the sensitivity and/or toxicity of peptides and peptide-like drugs.

Delineation of Human Peptide Transporter 1 (hPepT1)-Mediated Uptake and Transport of Substrates with Varying Transporter Affinities Utilizing Stably Transfected hPepT1/Madin-Darby Canine Kidney Clones and Caco-2 Cells

In the present investigation, the uptake and transport kinetics of valacyclovir (VACV), 5-aminolevulinic acid (5-ALA), and benzylpenicillin (BENZ) were studied in stably transfected Madin-Darby canine kidney (MDCK)/human peptide transporter 1 (hPepT1)-V5&His clonal cell lines expressing varying levels of epitope-tagged hPepT1 protein (low, medium, and high expression) and in Caco-2 cells to delineate hPepT1-mediated transport kinetics. These compounds were selected due to the fact that they are known PepT1 substrates, yet also have affinity for other transporters. Caco-2 cells, traditionally used for studying peptide-based drug transport, were included for comparison purposes. The time, pH, sodium, and concentration dependence of cellular uptake and permeability were measured using mock, clonal hPepT1-MDCK, and Caco-2 cells. A pH-dependent effect was observed in the hPepT1-expressing clones and Caco-2 cells, with an increase of 1.96-, 1.84-, and 2.05-fold for VACV, 5-ALA, and BENZ uptake, respectively, at pH 6 versus 7.4 in the high-expressing hPepT1 cells. BENZ uptake was significantly decreased in Caco-2 and MDCK cells in Na(+)-depleted buffer, whereas VACV uptake only decreased in Caco-2 cells. Concentration-dependent uptake studies in the mock-corrected hPepT1-MDCK and Caco-2 cells demonstrated hPepT1 affinity ranking of VACV > 5-ALA > BENZ. The apical-to-basal apparent permeability coefficient (P(app)) values of VACV, 5-ALA, and BENZ in mock-corrected hPepT1-MDCK cells showed solely hPepT1-mediated transport in contrast to Caco-2 cells. Lower K(m) values and higher P(app) in Caco-2 cells compared with hPepT1-MDCK cells suggested the involvement of multiple transporters in Caco-2 cells. Thus, hPepT1-MDCK cells corrected for endogenous transporter expression may be a more appropriate model for screening compounds for their affinity to hPepT1.

Functional linkage of H+/peptide transporter PEPT2 and Na+/H+ exchanger in primary cultures of astrocytes from mouse cerebral cortex

In our previous studies, we demonstrated that the high-affinity type peptide transporter PEPT2 is expressed in rat cerebral cortex using synaptosomal membrane study and that the uptake of dipeptide [14C]glycylsarcosine into synaptosomes was stimulated by an inwardly directed H+ gradient (Fujita et al., Brain Res. 972, 52-61, 2004). However, there is no information available for the driving force of PEPT2 function in the nervous system. In the present study, we investigated functional characteristics of PEPT2 mediated transport of Gly-Sar in primary cultured astrocytes from mouse cerebral cortex and examined the effects of Na+/H+ exchanger (NHE) inhibitor on Gly-Sar uptake in mouse astrocytes. In mouse astrocytes, extracellular H+ influenced only the maximal velocity (Vmax) of Gly-Sar uptake without affecting the apparent affinity (Kt). Interestingly, removal of Na+ from uptake buffer significantly reduced Gly-Sar uptake and Gly-Sar uptake was modulated by NHE inhibitors. The treatment of EIPA, an NHE inhibitor, altered the Vmax value of Gly-Sar uptake but had no effect on its Kt value. RT-PCR revealed that NHE1 and NHE2 mRNA are expressed in mouse cerebrocortical astrocytes. These results demonstrated that NHE activity is required to allow optimal uptake of dipeptides mediated by PEPT2 into the astrocytes. This study represents the first description of the functional co-operation of PEPT2 and NHE1 and/or NHE2 in cerebrocortical astrocytes.

Selective and ATP-dependent translocation of peptides by the homodimeric ATP binding cassette transporter TAP-like (ABCB9)

The transporter associated with antigen processing (TAP)-like (TAPL, ABCB9) belongs to the ATP-binding cassette transporter family, which translocates a vast variety of solutes across membranes. The function of this half-size transporter has not yet been determined. Here, we show that TAPL forms a homodimeric complex, which translocates peptides across the membrane. Peptide transport strictly requires ATP hydrolysis. The transport follows Michaelis-Menten kinetics with low affinity and high capacity. Different nucleotides bind and energize the transport with a slight predilection for purine bases. The peptide specificity is very broad, ranging from 6-mer up to at least 59-mer peptides with a preference for 23-mers. Peptides are recognized via their backbone, including the free N and C termini as well as side chain interactions. Although related to TAP, TAPL is unique as far as its interaction partners, transport properties, and substrate specificities are concerned, thus excluding that TAPL is part of the peptide-loading complex in the classic route of antigen processing via major histocompatibility complex class I molecules.

In vitro and pharmacophore-based discovery of novel hPEPT1 inhibitors

PURPOSE

The human proton-coupled small peptide carrier (hPEPT1) is a low-affinity, high-capacity transporter with broad substrate specificity. We have taken an iterative in vitro and in silico approach to the discovery of molecules with hPEPT1 affinity.

METHODS

A pharmacophore-based approach was taken to identifying hPEPT1 inhibitors. The well-characterized and relatively high affinity ligands Gly-Sar, bestatin, and enalapril were used to generate a common features (HIPHOP) pharmacophore. This consisted of two hydrophobic features, a hydrogen bond donor, acceptor, and a negative ionizable feature.

RESULTS

The pharmacophore was used to search the Comprehensive Medicinal Chemistry (CMC) database of more than 8000 drug-like molecules and retrieved 145 virtual hits mapping to the pharmacophore features. The highest scoring compounds within this set were selected and tested in a stably transfected CHO-hPepT1 cell model. The antidiabetic repaglinide and HMG CoA reductase inhibitor fluvastatin were found to inhibit hPEPT1 with sub-millimolar potency (IC(50) 178 +/- 1.0 and 337 +/- 4 microM, respectively). The pharmacophore was also able to identify known hPEPT1 substrates and inhibitors in further database mining of more than 500 commonly prescribed drugs.

CONCLUSIONS

This study demonstrates the potential of combining computational and in vitro approaches to determine the affinity of compounds for hPEPT1 and, in turn, provides insights into key molecular interactions with this transporter.

Increased protein level of PEPT1 intestinal H+-peptide cotransporter upregulates absorption of glycylsarcosine and ceftibuten in 5/6 nephrectomized rats

In chronic renal failure (CRF), dietary protein is one of the factors that deteriorates residual renal functions. Numerous studies have indicated that the products of protein digestion are mainly absorbed as small peptides. However, how small peptides are absorbed in CRF remains poorly understood. H(+)-coupled peptide transporter (PEPT1/SLC15A1) plays an important role in the absorption of small peptides and peptide-like drugs in the small intestine. Because dietary protein intake is one of the risk factors for renal failure, the alteration of intestinal PEPT1 might have implications in the progression of renal disease as well as the pharmacokinetics of peptide-like drugs. In this study, we examined the alteration of intestinal PEPT1 in 5/6 nephrectomized (5/6 NR) rats, extensively used as a model of chronic renal failure. Absorption of [(14)C]glycylsarcosine and ceftibuten was significantly increased in 5/6 NR rats compared with sham-operated rats, without a change in intestinal protease activity. Western blot analysis indicated that the amount of intestinal PEPT1 protein in 5/6 NR rats was increased mainly at the upper region. On the other hand, the amount of intestinal PEPT1 mRNA was not significantly different from that of sham-operated rats. These findings indicate that the increase in absorption of small peptides and peptide-like drugs, caused by the upregulation of intestinal PEPT1 protein, might contribute to the progression of renal failure as well as the alteration of drug pharmacokinetics.

Computational modelling of H+-coupled peptide transport via human PEPT1

H+-coupled peptide transporter 1 (PEPT1) mediates the transport of small peptides and peptide-like drugs in a pH- and voltage-dependent manner. Here, we investigated the transport mechanisms of PEPT1 for neutral and charged substrates by experimental studies and computational simulation. Uptake studies revealed that the Michaelis-Menten constant (Km) of glycylsarcosine (Gly-Sar), a neutral substrate, decreased with a fall in pH from 7.4 to 5.5, but at pH 5.0, the Km increased again. In contrast, the Km value of an anionic substrate, ceftibuten, declined steadily with decreasing pH. Based on these findings and information from the literature, we hypothesized the transport mechanisms in which (1) H+ binds to not only the H+-binding site, but also the substrate-binding site; and (2) H+ at the substrate-binding site inhibits the interaction of neutral and cationic substrates, but is necessary for that of anionic substrates. To validate these hypotheses, a computational model was constructed and various properties of substrate transport by PEPT1 were simulated. Our model reproduced the voltage dependence, hyperbolic saturation and bell-shaped pH-profile of Gly-Sar transport. Moreover, the various transport properties of negatively and positively charged substrates were also reconstructed. These findings indicated that the inferred mechanisms are able to sufficiently interpret the transport of both neutral and charged substrates by PEPT1.

Intestinal absorption of drugs mediated by drug transporters: mechanisms and regulation

The absorption of drugs from the gastrointestinal tract is one of the important determinants for oral bioavailability. Development of in vitro experimental techniques such as isolated membrane vesicles and cell culture systems has allowed us to elucidate the transport mechanisms of various drugs across the plasma membrane. Recent introduction of molecular biological techniques resulted in the successful identification of drug transporters responsible for the intestinal absorption of a wide variety of drugs. Each transporter exhibits its own substrate specificity, though it usually shows broad substrate specificity. In this review, we first summarize the recent advances in the characterization of drug transporters in the small intestine, classified into peptide transporters, organic cation transporters and organic anion transporters. In particular, peptide transporter (PEPT1) is the best-characterized drug transporter in the small intestine, and therefore its utilization to improve the oral absorption of poorly absorbed drugs is briefly described. In addition, regulation of the activity and expression levels of drug transporters seems to be an important aspect, because alterations in the functional characteristics and/or expression levels of drug transporters in the small intestine could be responsible for the intra- and interindividual variability of oral bioavailability of drugs. As an example, regulation of the activity and expression of PEPT1 is summarized.

Dietary protein level and stage of development affect expression of an intestinal peptide transporter (cPepT1) in chickens

The objective of this study was to evaluate the effect of dietary protein intake and stage of development on chicken intestinal peptide transporter (cPepT1) mRNA abundance. Chicks were sampled at embryonic d 16 (e 16) until the day of hatch (d 0). After hatch, mixed sex Cobb chicks were randomly assigned to diets containing 12, 18, or 24% crude protein (CP). Intakes of the 18 and 24% CP-fed birds were restricted to that consumed by the 12% CP-fed birds. Chickens were sampled on d 0, 1, 3, 5, 7, 10, 14, 21, 28, and 35. The experiment was repeated with the addition of a 4th group with free access to the 24% CP diet. PepT1 mRNA abundance in the duodenum, jejunum, and ileum was quantified by Northern blot. PepT1 mRNA was barely detectable between e 16 and 20 but increased 14- to 50-fold by d 0 (P < 0.001). In chickens fed 12% CP, cPepT1 mRNA abundance decreased throughout the experiment, whereas those fed the restricted 18 or 24% CP diets showed an increase in cPepT1 mRNA abundance (protein x time interaction, P < 0.01). In chicks with free access to the 24% CP diet, cPepT1 mRNA decreased until d 14 but returned to an intermediate level at d 35 (protein x time interaction, P < 0.01). The relative abundance of cPepT1 mRNA varied among intestinal segments with greater abundance occurring in the duodenal or jejunal sections (P < 0.05). Our results indicate that expression of cPepT1 mRNA is regulated by both dietary protein intake and developmental stage.

Alanylglutamine dipeptide and growth hormone maintain PepT1-mediated transport in oxidatively stressed Caco-2 cells

Reactive oxygen species (ROS) produced by gut mucosal cells during conditions such as inflammatory bowel disease (IBD) may impair mucosal repair and nutrient transport/absorptive function. Absorption of di- and tripeptides in the small intestine and colon is mediated by the H(+)-dependent transporter PepT1, but effects of oxidative stress on di- and tripeptide transport are unknown. We assessed whether exposure to hydrogen peroxide (H(2)O(2)) influences dipeptide transport in human colonic epithelial (Caco-2) cells. Uptake of [(14)C]glycylsarcosine (Gly-Sar) was used to evaluate PepT1-mediated dipeptide transport. Exposure to 1-5 mmol/L H(2)O(2) for 24 h caused a dose-dependent decrease in Gly-Sar transport, which was associated with decreased PepT1 transport velocity (V(max)). Treatment with alanylglutamine (Ala-Gln) or growth hormone (GH) did not alter Caco-2 Gly-Sar transport in the absence of H(2)O(2). However, both Ala-Gln and GH prevented the decrease in dipeptide transport observed with 1 mmol/L H(2)O(2) treatment. Ala-Gln, but not GH, maintained cellular glutathione and prevented the decrease in PepT1 protein expression. Thus, these agents should be further investigated as potential therapies to improve absorption of small peptides in disorders associated with oxidative injury to the gut mucosa.

Effect of insulin on cephalexin uptake and transepithelial transport in the human intestinal cell line Caco-2

We investigated whether cephalexin transport in Caco-2 cells is regulated by insulin. After the insulin pretreatment, cephalexin uptake, and transport as well as PEPT1 mRNA and protein expression in the cells were measured. Cephalexin uptake was significantly increased by the insulin pretreatment. Insulin significantly increased cephalexin saturable uptake, but had no significant effect on the non-saturable one. PEPT1 protein expression on the apical membrane, but not PEPT1 mRNA expression, was increased by the insulin pretreatment. The enhancement of cephalexin uptake by the insulin pretreatment was inhibited by genistein, a tyrosine kinase inhibitor, and colchicine, an agent that disrupts protein translocation. Apical-to-basolateral transport of cephalexin has increased by the insulin pretreatment at the apical side and long-term insulin pretreatment at the basolateral side. It is considered that insulin mainly binds to its receptor on the apical and basolateral membranes, thereby promoting PEPT1 translocation from the intracellular pool to the apical membrane surface; consequently, PEPT1 protein expression on the apical membrane is increased.

Analysis of the transport properties of side chain modified dipeptides at the mammalian peptide transporter PEPT1

This study was initiated to examine systematically the effect of side chain modifications at dipeptides on their transport via PEPT1. We synthesized a series of Xaa(R)-Ala and Ala-Xaa(R) dipeptides with the functional groups of the side chains modified by structurally different blocking groups R. Recognition and transport of these derivatives by PEPT1 was measured in Caco-2 cells, in transgenic Pichia pastoris cells and in Xenopus laevis oocytes expressing PEPT1. The dipeptide derivatives displayed K(i) values between 0.002 and 4 mM. Electrophysiological analyses showed that the Ala-Xaa(R) derivatives were transported by PEPT1. In contrast, most Xaa(R)-Ala derivatives--although recognized--did not show significant transport rates. Substitution of a terminal phenyl residue in the side chain blocking group by a p-nitrophenyl residue enhanced the affinity of several dipeptide derivatives for interaction with PEPT1. However, none of these compounds showed electrogenic transport in oocytes. With a K(i) value of 0.002 mM, Lys[Z(NO(2))]-Val displayed the highest affinity to PEPT1 ever reported. We conclude that the transport of side chain modified dipeptides into enterocytes depends (a) on the position of the modified trifunctional amino acid in the dipeptide, (b) the distance between its alpha-carbon and the side chain blocking group and (c) the hydrophobic character of the side chain modification.

Efflux properties of basolateral peptide transporter in human intestinal cell line Caco-2

Small peptides and some pharmacologically active compounds are absorbed from the small intestine by the apical H(+)-coupled peptide transporter 1 (PEPT1) and the basolateral peptide transporter. Here we investigated the efflux properties of the basolateral peptide transporter in Caco-2 cells using two strategies, efflux measurements and a kinetic analysis of transepithelial transport of glycylsarcosine (Gly-Sar). [(14)C]Gly-Sar efflux through the basolateral membrane was not affected significantly by the external pH. Both approaches revealed that the basolateral peptide transporter was saturable in the efflux direction, and that the affinity was lower than that in the influx direction. For two peptide-like drugs, there was no difference in substrate recognition by the basolateral peptide transporter between the two sides of the membrane. Using the kinetic parameters of PEPT1 and the basolateral peptide transporter, a computational model of Gly-Sar transport in Caco-2 cells was constructed. The simulation fitted the experimental data well. Our findings suggested that substrate affinity of the basolateral peptide transporter is apparently asymmetric, but pH-dependence and substrate specificity are symmetric for the two directions of transport. The behaviour of Gly-Sar in Caco-2 cells could be predicted by a mathematical model describing the peptide transporters.

Preliminary investigation into the expression of proton-coupled oligopeptide transporters in neural retina and retinal pigment epithelium (RPE): lack of functional activity in RPE plasma membranes

PURPOSE

To determine the expression and functional activity of proton-coupled oligopeptide transporters (POT) in retinal pigment epithelial (RPE) cells.

METHODS

RT-PCR was used to probe the presence of POT mRNA in freshly isolated bovine RPE (BRPE) and human RPE (HRPE) cells, a human RPE cell line (ARPE-19), and human and bovine neural retina. [14C]GlySar uptake was used to characterize POT activity in cultured ARPE-19 cells and freshly isolated BRPE cell sheet suspensions.

RESULTS

PHT1 mRNA was expressed in BRPE, HRPE, ARPE-19, and bovine and human neural retina. In contrast, PEPT2 and PHT2 were expressed only in bovine and human retina, and PEPT1 could not be detected. GlySar exhibited a linear uptake over 6 h at pH values of 6.0 and 7.4, with greater uptake at pH 7.4 (p < 0.01). GlySar uptake did not exhibit saturability (5-2000 microM) and was unchanged when studied in the presence of 1 mM L-histidine. In contrast, GlySar uptake was significantly decreased when studied at 4 degrees C or in the presence of endocytic inhibitors at 37 degrees C (p < 0.01). Studies in BRPE cell sheet suspensions validated the results obtained in ARPE-19 cells and strongly suggested the absence of POT on the apical and basolateral membranes of RPE.

CONCLUSIONS

PHT1 mRNA is present in native bovine and human RPE and a human RPE cell line. However, the data argue against PHT1 being expressed on plasma membranes of RPE. Overall, GlySar appears to be taken up by RPE cells via a low-affinity, endocytic process.

Inhibitory effect of zinc on PEPT1-mediated transport of glycylsarcosine and beta-lactam antibiotics in human intestinal cell line Caco-2

PURPOSE

The aim of this study was to examine the effects of zinc on the intestinal peptide transporters (PEPT1 and basolateral peptide transporter) and to elucidate the mechanism of the interactions.

METHODS

Caco-2 cells were pretreated with zinc, and the uptake studies were carried out.

RESULTS

Zinc treatment resulted in the inhibition of [14C]glycylsarcosine (Gly-Sar) uptake via PEPT1 in a concentration-dependent manner, whereas it showed moderate inhibitory effect on the basolateral peptide transporter. Zinc also inhibited the uptake of oral beta-lactam antibiotics such as ceftibuten and cephradine by PEPT1. Kinetic analysis showed that zinc treatment increased Km values without affecting Vmax values of the [14C]Gly-Sar uptake. The inhibition of [14C]Gly-Sar uptake induced by zinc was observed in the presence of an H+ gradient but not in the absence of an H+ gradient.

CONCLUSIONS

These results indicate that zinc is a competitive inhibitor of PEPT1. Zinc inhibited the PEPT1 function, possibly by interacting with histidine residues of PEPT1 that are part of an H+-binding site. These findings would provide important information for clinical, physiologic, and biochemical aspects of peptide transporters.

Immunolocalization of the Proton-Coupled Oligopeptide Transporter PEPT2 in Developing Rat Brain

This study examined the tissue distribution, cellular localization, and developmental expression of the PEPT2 protein in rat brain. Immunoblot and immunocytochemistry analyses were performed with specific rat PEPT1 and PEPT2 antisera developed in our laboratory. Rats were examined from fetus (gestation for 17 days) to adult (day 75). On immunoblot analysis, the PEPT2 protein was detected in cerebral cortex, olfactory bulb, basal ganglia, cerebellum, and hindbrain sections of adult brain, with the strongest signals in cerebral cortex. No PEPT1 protein was found in brain. Expression levels of the PEPT2 protein in cerebral cortex were maximal in the fetus and declined rapidly with advancing age. Adult protein levels were approximately 14% of that observed in fetus. In immunofluorescence experiments, the strongest PEPT2 signals were observed in epithelial cells of the choroid plexus for both adult and neonate brains. The PEPT2 protein was exclusively expressed on the apical membrane (CSF-facing) of choroid plexus epithelia. In double labeling experiments, PEPT2 immunoreactivity in adult brain colocalized with NeuN, a neuronal marker, but not with GFAP, an astrocyte marker. In contrast, in neonatal brain, PEPT2 immunoreactivity colocalized with both GFAP and NeuN. These findings demonstrate that the PEPT2 protein is found throughout the brain. The apical expression of PEPT2 in choroid plexus suggests that it is involved in the export of neuropeptides, peptide fragments, and peptide-like drugs from cerebrospinal fluid. PEPT2 may also play a role in the regulation of neuropeptide concentrations in extracellular fluid, especially during early development.

A rapid in vitro screening for delivery of peptide-derived peptidase inhibitors as potential drug candidates via epithelial peptide transporters

Targeting drugs or prodrugs to a specific enzyme by simultaneously targeting cell membrane carriers for efficient transport should provide the highest bioavailability along with specificity at the site of action. The peptide transporters PEPT1 and PEPT2 are expressed in a variety of tissues, including the brush-border membranes of epithelial cells of the small intestine and kidney. The transporters accept a wide range of substrates and are therefore good targets for a transporter-mediated drug delivery. Here, we report a screening procedure for peptidomimetic drug candidates combining two independent expression systems: 1) a competition assay in transgenic Pichia pastoris yeast cells expressing either mammalian PEPT1 or PEPT2 for identifying substrate interaction with the transporter binding site; and 2) a Xenopus laevis-based oocyte expression of the peptide transporter for assessing electrogenic transport of drug candidates. Based on the known oral availability and in vivo efficacy of the dipeptidyl peptidase IV (DPIV) inhibitor isoleucine-thiazolidide and its peptide-like structure, we first tested whether this compound is a substrate of epithelial peptide transporters. Additionally, a series of structurally related inhibitors were analyzed for transport. We identified various compounds that serve as substrates of the intestinal peptide transporter PEPT1. In contrast, none of these DPIV inhibitors showed electrogenic transport by PEPT2, although a variety of the compounds displayed good affinities for competition in peptide uptake in PEPT2-expressing cells, suggesting that they may serve as efficient inhibitors. In conclusion, we have applied an in vitro screening system that predicts efficient intestinal absorption of peptide-derived peptidase inhibitors via PEPT1 in vivo.

Functional characterization of brain peptide transporter in rat cerebral cortex: identification of the high-affinity type H+/peptide transporter PEPT2

In this report, we studied the functional characteristics of a brain peptide transporter using synaptosomes prepared from rat cerebral cortex. Crude synaptosomes (P(2) fraction) were prepared from cerebral cortices in male Wistar rats. Uptake of [14C]glycylsarcosine (Gly-Sar), a substrate for H(+)/oligopeptide transporters PEPT1 and PEPT2, and [3H]histidine, a substrate for peptide/histidine transporters PHT1 and PHT2, was measured at 37 degrees C by a rapid filtration technique. The uptake of [14C]Gly-Sar into synaptosomes was stimulated by an inwardly directed H(+)-gradient. The uptake system exhibited a Michaelis-Menten constant (K(t)) of 110+/-20 microM for Gly-Sar. This value is comparable to the K(t) value for Gly-Sar uptake via the high-affinity H(+)/peptide transporter PEPT2. The H(+)-dependent uptake of [14C]Gly-Sar into synaptosomes was inhibited by di- and tripeptides and beta-lactam antibiotics, but was unaffected by amino acids glycine and histidine. In particular, kyotorphin (Tyr-Arg) completely inhibited Gly-Sar uptake with the K(i) value of 29+/-14 microM. These uptake properties of the brain peptide transporter (i.e., the K(t) value for Gly-Sar uptake and the K(i) value of kyotorphin for Gly-Sar uptake) are very similar to those of PEPT2. RT-PCR and Western blotting analyses revealed that PEPT2 is actually expressed in the cerebral cortex in rat. These results indicate that a H(+)-coupled high affinity peptide transport system is functionally expressed in the cerebral cortex and that this transport system is identical to PEPT2.

The intestinal H+/peptide symporter PEPT1: structure–affinity relationships

Peptide transporter 1, PEPT1, of the mammalian enterocyte is presently under intense investigation in many laboratories because of its nutritional importance in the absorption of protein hydrolysis products and because more recent studies have shown that many drugs and prodrugs gain entry into the systemic circulation via PEPT1. Until the exact structural features of the substrate binding site of PEPT1 become available, for example by X-ray crystallography, determination of affinities followed by proof of actual membrane translocation will have to suffice when testing for possible new substrates for PEPT1. Affinity constants reflect the strength of their interaction with the binding site of the transporter. A review of the literature shows a wide range of affinity constants between 2 microM and 30 mM. We consider affinity constants for substrates or inhibitors of PEPT1 lower than 0.5 mM as high affinity, between 0.5 and 5.0 mM as medium affinity and above 5 mM as low affinity. Values above 15 mM we consider with great caution. In this mini-review we discuss affinities and structural determinants which affect affinities of a variety of substrates for PEPT1.

Transport of amino acid-based prodrugs by the Na+- and Cl(-) -coupled amino acid transporter ATB0,+ and expression of the transporter in tissues amenable for drug delivery

We evaluated the potential of the Na(+)- and Cl(-)-coupled amino acid transporter ATB(0,+) as a delivery system for amino acid-based prodrugs. Immunofluorescence analysis indicated that ATB(0,+) is expressed abundantly on the luminal surface of cells lining the lumen of the large intestine and the airways of the lung and in various ocular tissues, including the conjunctival epithelium, the tissues easily amenable for drug delivery. We screened a variety of beta-carboxyl derivatives of aspartate and gamma-carboxyl derivatives of glutamate as potential substrates for this transporter using heterologous expression systems. In mammalian cells expressing the cloned ATB(0,+), several of the aspartate and glutamate derivatives inhibited glycine transport via ATB(0,+). Direct evidence for ATB(0,+)-mediated transport of these derivatives was obtained in Xenopus laevis oocytes using electrophysiological methods. Exposure of oocytes, which express ATB(0,+) heterologously, to aspartate beta-benzyl ester as a model derivative induced inward currents in a Na(+)- and Cl(-)-dependent manner with a Na(+)/Cl(-)/aspartate beta-benzyl ester stoichiometry of 2:1:1. ATB(0,+) transported not only the beta-carboxyl derivatives of aspartate and the gamma-carboxyl derivatives of glutamate but also valacyclovir, which is an alpha-carboxyl ester of acyclovir with valine. The transport of valacyclovir via ATB(0,+) was demonstrable in both heterologous expression systems. This process was dependent on Na(+) and Cl(-). The ability of ATB(0,+) to transport valacyclovir was comparable with that of the peptide transporter PEPT1. These findings suggest that ATB(0,+) has significant potential as a delivery system for amino acid-based drugs and prodrugs.

Mechanisms of Cefadroxil Uptake in the Choroid Plexus: Studies in Wild-Type and PEPT2 Knockout Mice

The choroid plexus uptake of [(3)H]cefadroxil was studied in peptide transporter 2 (PEPT2) wild-type and null mice as a function of temperature, transport inhibitors, pH, and saturability. At normal pH (7.4) and temperature (37 degrees C), the uptake of 1 microM cefadroxil was reduced by 83% in PEPT2(-/-) mice as compared with PEPT2(+/+) mice (p < 0.001). A further reduction was achieved in null animals by reducing the temperature to 4 degrees C, or by adding saturating concentrations of unlabeled cefadroxil or p-aminohippurate (p < 0.05). Glycylsarcosine coadministration could inhibit the uptake of cefadroxil in PEPT2(+/+) mice (p < 0.01) but not PEPT2(-/-) mice. Although a proton-stimulated uptake of cefadroxil was demonstrated in PEPT2(+/+) mice (pH 6.5 versus pH 7.4; p < 0.01), no pH dependence was observed in PEPT2(-/-) mice. Kinetic parameters for cefadroxil (without p-aminohippurate) in wild-type mice were: V(max) = 5.4 pmol/mg/min, K(m) = 34 microM, and K(d) = 0.0069 microl/mg/min; in the presence of p-aminohippurate, the parameters were: V(max) = 4.1 pmol/mg/min, K(m) = 27 microM, and K(d) = 0.0064 microl/mg/min. In null animals, the kinetic parameters of cefadroxil (without p-aminohippurate) were: V(max) = 2.7 pmol/mg/min, K(m) = 110 microM, and K(d) = 0.0084 microl/mg/min; in the presence of p-aminohippurate, only a K(d) = 0.010 microl/mg/min was observed. Based on kinetic and inhibitor analyses, it was determined that (under linear conditions), 80 to 85% of cefadroxil's uptake in choroid plexus is mediated by PEPT2, 10 to 15% by organic anion transporter(s), and 5% by nonspecific mechanisms. These findings demonstrate that PEPT2 is the primary transporter responsible for cefadroxil uptake in the choroid plexus. Moreover, the data suggest a role for PEPT2 in the clearance of peptidomimetics from cerebrospinal fluid.

Altered diurnal rhythm of intestinal peptide transporter by fasting and its effects on the pharmacokinetics of ceftibuten

We previously demonstrated that H+/peptide cotransporter PEPT1 shows a diurnal rhythm in the rat small intestine. In the present study, we examined the effect of food intake on the diurnal rhythm of intestinal PEPT1 using fed and fasted rats and also determined whether such variation affected the pharmacokinetics of peptide-like drugs. In fed rats, PEPT1 protein level was significantly higher at 8:00 PM than at 8:00 AM. However, during fasting for 2 to 4 days, the differences of PEPT1 protein levels between 8:00 AM and 8:00 PM gradually disappeared. Intestinal absorption of an oral antibiotic ceftibuten (CETB), a pharmacological substrate for PEPT1, was also greater at 8:00 PM than at 8:00 AM in fed rats, but not different in 4-day fasted rats. In contrast to PEPT1 protein levels, PEPT1 mRNA levels retained a diurnal rhythm after 4 days of fasting. Pharmacokinetic analyses of CETB after intraintestinal administration demonstrated that both Cmax and area under the plasma concentration-time curve from 0 to 3 h were greater at 8:00 PM than at 8:00 AM in fed rats. In contrast, pharmacokinetic parameters showed no significant difference between 8:00 AM and 8:00 PM for intraintestinal administration in 4-day fasted rats and for intravenous administration in fed and 4-day fasted rats. These findings suggested that the diurnal rhythm of intestinal PEPT1 transport activity was disrupted by fasting and that diurnal variation of intestinal PEPT1 functionality could influence the pharmacokinetics of peptide-like drugs such as CETB.

Identification of a novel Na+- and Cl--coupled transport system for endogenous opioid peptides in retinal pigment epithelium and induction of the transport system by HIV-1 Tat

The endogenous opioid peptides enkephalins, dynorphins and endorphins consist of five or more amino acids. These peptides participate in a multitude of biological functions in mammalian cells by interacting with different subtypes of opiate receptors located on the plasma membrane and in the nucleus. Here we report on the identification of a new peptide transport system in the human retinal pigment epithelial (RPE) cells that transports a variety of endogenous opioid peptides with high affinity. We identified this novel, hitherto unrecognized, transport system when we were analysing the differential effects of Tat, the transacting factor encoded by HIV-1, on various transport processes in RPE cells. This transport system is markedly induced by Tat. This opioid transport system is energized by transmembrane Na+ and Cl- gradients and is distinct from any of the previously identified transport systems for opioid peptides in mammalian cells. Free amino acids, dipeptides, tripeptides and non-peptide opiate receptor antagonists are excluded by this newly identified transport system. The affinities of endogenous opioid peptides for this system are in the range of 0.4-40 microM. The identification of the high-affinity Na+- and Cl--coupled transport system in mammalian cells that is specific for endogenous opioid peptides and is induced by HIV-1 Tat is of significance not only to the biology of opioid peptides but also to the pathology of HIV-1 infection in humans.

Modulatory effects of hormones, drugs, and toxic events on renal organic anion transport

The human body is exposed continuously to a wide variety of exogenous compounds, many of which are anionic compounds. In addition, products of phase II biotransformation reactions are negatively charged, viz. glucuronides, sulfate esters, or glutathiones. Renal transport of organic anions is an important defense mechanism of the organism against foreign substances. The combination of the rate of uptake and efflux and the intracellular disposition of organic anions in the proximal tubule determines the intracellular concentration and the nephrotoxic potential of a compound. Modulation of organic anion secretion is observed after exposure of proximal tubules to various hormones, and the subsequent receptor-mediated response is signaled by protein kinases. Transport of anionic compounds across the basolateral as well as the luminal membrane is modified by activation or inhibition of protein kinases. Protein kinase C activation reduces the uptake of organic anions mediated by the organic anion transporter 1 (OAT1/Oat1) and Oat3 and reduces Mrp2-mediated efflux. In addition, activation of protein kinase C has been shown to inhibit transport by the organic anion transporting polypeptide 1 (Oatp1) across the luminal membrane. Additional protein kinases have been implicated in the regulation of organic anion transport, and the role of nuclear factors in xenobiotic excretion is an emerging field. The physiological regulation of organic anion transporters may also be influenced by exogenous factors, such as exposure to xenobiotics and cellular stress. This commentary discusses the current knowledge of endogenous and exogenous influences on renal anionic xenobiotic excretion.

An update on renal peptide transporters

The brush-border membrane of renal epithelial cells contains PEPT1 and PEPT2 proteins that are rheogenic carriers for short-chain peptides. The carrier proteins display a distinct surface expression pattern along the proximal tubule, suggesting that initially di- and tripeptides, either filtered or released by surface-bound hydrolases from larger oligopeptides, are taken up by the low-affinity but high-capacity PEPT1 transporter and then by PEPT2, which possesses a higher affinity but lower transport capacity. Both carriers transport essentially all possible di- and tripeptides and numerous structurally related drugs. A unique feature of the mammalian peptide transporters is the capability of proton-dependent electrogenic cotransport of all substrates, regardless of their charge, that is achieved by variable coupling in proton movement along with the substrate down the transmembrane potential difference. This review focuses on the postcloning research efforts to understand the molecular physiology of peptide transport processes in renal tubules and summarizes available data on the underlying genes, protein structures, and transporter function as derived from studies in heterologous expression systems.

Current perspectives on established and putative mammalian oligopeptide transporters

Peptides and peptide-based drugs are increasingly being utilized as therapeutic agents for the treatment of numerous disorders. The increasing development of peptide-based therapeutic agents is largely due to technological advances including the advent of combinatorial peptide libraries, peptide synthesis strategies, and peptidomimetic design. Peptides and peptide-based agents have a broad range of potential clinical applications in the treatment of many disorders including AIDS, hypertension, and cancer. Peptides are generally hydrophilic and often exhibit poor passive transcellular diffusion across biological barriers. Insights into strategies for increasing their intestinal absorption have been derived from the numerous studies demonstrating that the absorption of protein digestion products occurs primarily in the form of small di- and tripeptides. The characterization of the pathways of intestinal, transepithelial transport of peptides and peptide-based drugs have demonstrated that a significant degree of absorption occurs through the role of proteins within the proton-coupled, oligopeptide transporter (POT) family. Considerable focus has been traditionally placed on Peptide Transporter 1 (PepT1) as the main mammalian POT member regulating intestinal peptide absorption. Recently, several new POT members, including Peptide/Histidine Transporter 1 (PHT1) and Peptide/Histidine Transporter 2 (PHT2) and their splice variants have been identified. This has led to an increased need for new experimental methods enabling better characterization of the biophysical and biochemical barriers and the role of these POT isoforms in mediating peptide-based drug transport.

Renal assimilation of short chain peptides: visualization of tubular peptide uptake

PURPOSE

Renal assimilation of short chain peptides plays an important role in systemic protein metabolism and amino acid homeostasis. The transepithelial peptide transport across the apical membrane of tubular cells is mediated almost exclusively by pH-dependent H(+)-peptide symport pathways. The current study was designed to identify by visualization functional peptide transport activity along the nephron structures.

METHODS

Visualization of peptide uptake was achieved by using the fluorescent dipeptide derivative D-Ala-Lys-AMCA and unlabelled cefadroxil and glycylglutamine as transport competitors to demonstrate specificity. To confirm these assays, rat specific cRNA probes were synthesized and non-isotopic high-resolution in-situ-hybridization and northern blot analysis were carried out to demonstrate the expression of the high-affinity peptide transporter PEPT2.

RESULTS

The reporter molecule was accumulated by cells of the proximal tubulus but not in glomerular or endothelial cells. Inhibition studies revealed competitive inhibition of D-Ala-Lys-AMCA uptake by the betalactam cefadroxil and the dipeptide glycylglutamine. The control organs intestine and spleen did not show uptake of the systemically administered molecule. Non-isotopic mRNA in-situ-hybridization, using an antisense probe for rat PEPT2 confirmed up-take assays by identifying PEPT2 expression throughout segments of the straight proximal tubule at the inner cortex and outer stripe.

CONCLUSIONS

We demonstrate for the first time renal in vivo transport activity of a dipeptide that allows cells that participate in peptide reabsorption to be visualized. This functional assay may be used to investigate renal peptide transport mechanisms and test new compounds that are transported via proton-driven peptide transporters.

Switching between the two action modes of the dual-affinity nitrate transporter CHL1 by phosphorylation

To counteract fluctuating nutrient environments, plants have evolved high- and low-affinity uptake systems. These two systems were traditionally thought to be genetically distinct, but, recently, two Arabidopsis transporters, AtKUP1 and CHL1, were shown to have dual affinities. However, little is known about how a dual-affinity transporter works and the advantages of having a dual-affinity transporter. This study demonstrates that, in the case of CHL1, switching between the two modes of action is regulated by phosphorylation at threonine residue 101; when phosphorylated, CHL1 functions as a high-affinity nitrate transporter, whereas, when dephosphorylated, it functions as a low-affinity nitrate transporter. This regulatory mechanism allows plants to change rapidly between high- and low-affinity nitrate uptake, which may be critical when competing for limited nitrogen. These results demonstrate yet another regulatory role of phosphorylation in plant physiology.

Extracellular glucose concentration alters functional activity of the intestinal oligopeptide transporter (PepT-1) in Caco-2 cells

The objective of this study was to determine the effect of different cell culture media glucose concentrations on the functional activity of PepT-1 in Caco-2 cells. Uptake kinetics of Gly-Sar into Caco-2 cells that were maintained in iso-osmotic media containing 25 or 5.5 mM glucose were determined in the presence and absence of amino acid-selective chemical modifiers and dithiothreitol. Inhibition of Gly-Sar uptake into Caco-2 cells was measured in the presence of dipeptides and xenobiotics exhibiting various binding affinities for the PepT-1. The effect of extracellular glucose on PepT-1 gene expression was assessed using comparative RT-PCR. Long-term exposure of Caco-2 cells to 25 mM glucose reduced maximum transport capacity for Gly-Sar uptake without altering PepT-1 gene expression. In contrast, binding affinity of Gly-Sar and other dipeptides or xenobiotics was not significantly changed. Chemical modification of Lys and Tyr residues decreased V(max), while Cys modification increased the maximum transport capacity of the carrier. Preincubation of Caco-2 cells with dithiothreitol restored PepT-1 activity in cells maintained at 25 mM glucose. In conclusion, cell culture media containing 25 mM glucose decreases maximum transport capacity of PepT-1 in Caco-2 cells without affecting substrate recognition, at least in part, mediated via an oxidative pathway.

Targeted disruption of the PEPT2 gene markedly reduces dipeptide uptake in choroid plexus

The presence of multiple oligopeptide transporters in brain has generated considerable interest as to their physiological role in neuropeptide homeostasis, pharmacologic importance, and potential as a target for drug delivery through the blood-brain and blood-cerebrospinal fluid barriers. To understand further the purpose of specific peptide transporters in brain, we have generated PEPT2-deficient mice by targeted gene disruption. Homozygous PepT2 null mice lacked expression of PEPT2 mRNA and protein in choroid plexus and kidney, tissues in which PepT2 is normally expressed, whereas heterozygous mice displayed PepT2 expression levels that were intermediate between those of wild-type and homozygous null animals. Mutant PepT2 null mice were found to be viable, grew to normal size and weight, and were without obvious kidney or brain abnormalities. Notwithstanding the lack of apparent biological effects, the proton-stimulated uptake of 1.9 microm glycylsarcosine (a model, hydrolysis-resistant dipeptide) in isolated choroid plexus was essentially ablated (i.e. residual activity of 10.9 and 3.9% at 5 and 30 min, respectively). These novel findings provide strong evidence that, under the experimental conditions of this study, PEPT2 is the primary member of the peptide transporter family responsible for dipeptide uptake in choroid plexus tissue.

Effects of organic anion, organic cation, and dipeptide transport inhibitors on cefdinir in the isolated perfused rat kidney

Cefdinir (Omnicef; Abbott Laboratories) is a cephalosporin antibiotic primarily eliminated by the kidney. Nonlinear renal elimination of cefdinir has been previously reported. Cefdinir renal transport mechanisms were studied in the erythrocyte-free isolated perfused rat kidney. Studies were performed with drug-free perfusate and perfusate containing cefdinir alone to establish the baseline physiology and investigate cefdinir renal elimination characteristics. To investigate cefdinir renal transport mechanisms, inhibition studies were conducted by coperfusing cefdinir with inhibitors of the renal organic anion (probenecid), organic cation (tetraethylammonium), or dipeptide (glycylsarcosine) transport system. Cefdinir concentrations in biological samples were determined using reversed-phase high-performance liquid chromatography. Differences between treatments and controls were evaluated using analysis of variance and Dunnett's test. The excretion ratio (ER; the renal clearance corrected for the fraction unbound and glomerular filtration rate) for cefdinir was 5.94, a value indicating net renal tubular secretion. Anionic, cationic, and dipeptide transport inhibitors all significantly affected the cefdinir ER. With probenecid, the ER was reduced to 0.59, clearly demonstrating a significant reabsorptive component to cefdinir renal disposition. This finding was confirmed by glycylsarcosine studies, in which the ER was elevated to 7.95, indicating that reabsorption was mediated, at least in part, by the dipeptide transporter system. The effects of the organic cation tetraethylammonium, in which the ER was elevated to 7.53, were likely secondary in nature. The anionic secretory pathway was found to be the predominant mechanism for cefdinir renal excretion.

The presence of xenobiotic transporters in rat placenta

Understanding the role of transporters in placental handling of xenobiotics across the maternal-fetal interface is essential to evaluate the pharmacological and toxicological potential of therapeutic agents, drugs of abuse, and other xenobiotics to which the mother is exposed during pregnancy. Therefore, the purpose of this study was to assess mRNA levels of various transporters in placenta and to compare these to levels in maternal liver and kidney, predominant organs of excretion, to determine which transporters are likely to have a role in xenobiotic transfer within the placenta. During late stage pregnancy, relative amounts of mRNA levels of 40 genes representing 11 families/group of transporters were assessed in placenta with respect to relative maternal liver and kidney mRNA levels. Members of the following transporter families were assessed: three multidrug resistance (Mdr), six multidrug resistance-associated protein (Mrp), eight organic anion-transporting polypeptide (Oatp), three organic anion transporters (Oat), five organic cation transporters (Oct), two bile acid transporters (Na(+)/taurocholate-cotransporting polypeptide and bile salt export protein), four metal (ZnT1, divalent metal transporter 1, Menkes and Wilsons), a prostaglandin, two peptide, two sterolin, and four nucleoside transporters. Of the 40 genes evaluated, 16 [Mdr1a and 1b, Mrp1 and 5, Oct3 and Octn1, Oatp3 and 12, four metal, a prostaglandin, AbcG8, equilibrative nucleoside transporter 1 (ENT1), and ENT2] were expressed in placenta at concentrations similar to or higher than in maternal liver and kidney. The abundance of these mRNA transcripts in placenta suggests a role for these transporters in placental transport of xenobiotics and supports their role in the transport of endogenous substances.

Transcellular transport of a highly polar 3+ net charge opioid tetrapeptide

Oligopeptides are generally thought to have poor permeability across biological membranes. Recent studies, however, suggest significant distribution of [Dmt1]DALDA (Dmt-D-Arg-Phe-Lys-NH2; Dmt is 2',6'-dimethyltyrosine), a 3+ net charge opioid peptide, to the brain and spinal cord after subcutaneous administration. Peptide transporters (PEPT1 and PEPT2) play a major role in the uptake of di- and tripeptides across cell membranes, but their ability to transport tetrapeptides is not clear. The purpose of this study was to determine whether [Dmt1]DALDA can translocate across Caco-2 cell monolayers and whether PEPT1 plays a role in the uptake process. Our results show that [3H][Dmt1]DALDA can readily translocate across Caco-2 cells, with a permeability coefficient estimated to be 1.24 x 10(-5) cm/s. When incubated with Caco-2 cells, [3H][Dmt1]DALDA was detected in cell lysates by 5 min. The internalization of [Dmt1]DALDA was confirmed visually with a fluorescent [Dmt1]DALDA analog (H-Dmt-D-Arg-Phe-dnsDap-NH2; dnsDap is beta-dansyl-L-alpha,beta-diaminopropionic acid). The uptake of [3H][Dmt1]DALDA was concentration-dependent but temperature- and pH-independent. Treatment with diethylpyrocarbonate (DEPC) inhibited [14C]glycine-sarcosine uptake but increased [3H][Dmt1]DALDA uptake 34-fold. These findings suggest that PEPT1 is not involved in [Dmt1]DALDA internalization. [Dmt1]DALDA uptake was also observed in SH-SY5Y, human embryonic kidney 293, and CRFK cells, and was independent of whether the cells expressed opioid receptors. The efflux of [3H][Dmt1]DALDA from Caco-2 cells was temperature-dependent and was inhibited by DEPC, but was not affected by verapamil, an inhibitor of P-glycoprotein. These data show transcellular translocation of a highly polar 3+ charge tetrapeptide and suggest that [Dmt1]DALDA may not only distribute across the blood-brain barrier but also it may even have reasonable oral absorption.

Structural biology and function of solute transporters: implications for identifying and designing substrates

Solute carrier (SLC) proteins have critical physiological roles in nutrient transport and may be utilized as a mechanism to increase drug absorption. However, we have little understanding of these proteins at the molecular level due to the absence of high-resolution crystal structures. Numerous efforts have been made in characterizing the peptide transporter (PepT1) and the apical sodium dependent bile acid transporter (ASBT) that are important for both their native transporter function as well as targets to increase absorption and act as therapeutic targets. In vitro and computational approaches have been applied to gain some insight into these transporters with some success. This represents an opportunity for optimizing molecules as substrates for the solute transporters and providing a further screening system for drug discovery. Clearly the future growth in knowledge of SLC function will be led by integrated in vitro and in silico approaches.

Structure, function, and regulation of renal organic anion transporters

Renal elimination of anionic drugs, xenobiotics, and toxins is necessary for the survival of mammalian species. This process is mediated by vectorial transport from blood to urine through the cooperative functions of specific transporters in the basolateral and apical membranes of the proximal tubule epithelium. The first step of this process is the extraction of organic anions from the peritubular blood plasma into proximal tubule cells largely through the organic anion transporter (OAT) pathway. Therefore, the OAT pathway is one of the major sites for body drug clearance/detoxification. As a result, it is also the site for drug-drug interaction and drug-induced nephrotoxicity. To maximize therapeutic efficacy and minimize toxicity, the structure-function relationships of OATs and their regulation must be defined. The recent cloning and identification of OATs have paved the way for such investigations. This review summarizes the available data on the general properties of OATs, focusing in particular on the recent progress made from the author's laboratory as well as from other's, on the molecular characterization of the structure-function relationships of OATs and their regulatory mechanisms.

Diurnal rhythm of H+-peptide cotransporter in rat small intestine

In mammals, most physiological, biochemical, and behavioral processes show a circadian rhythm. In the present study, we examined the diurnal rhythm of the H+-peptide cotransporter (PEPT1), which transports small peptides and peptide-like drugs in the small intestine and kidney, using rats maintained in a 12-h photoperiod with free access to chow. The transport of [14C]glycylsarcosine (Gly-Sar), a typical substrate for PEPT1 by in situ intestinal loop and everted intestine, was greater in the dark phase than the light phase. PEPT1 protein and mRNA levels varied significantly, with a maximum at 2000 and minimum at 800. Similar functional and expressional diurnal variations were observed in the intestinal Na+-glucose cotransporter (SGLT1). In contrast, renal PEPT1 and SGLT1 showed little diurnal rhythmicity in protein and mRNA expression. These findings indicate that the intestinal PEPT1 undergoes diurnal regulation in its activity and expression, and this could affect the intestinal absorption of dietary protein.

Effects of an alanyl-glutamine-based oral rehydration and nutrition therapy solution on electrolyte and water absorption in a rat model of secretory diarrhea induced by cholera toxin

OBJECTIVES

Recurring diarrhea and persistent diarrhea are commonly associated with malnutrition and long-term functional deficits. A beneficial approach would be to develop an alanyl-glutamine (AlaGln)-based oral rehydration and nutrition therapy (ORNT). We investigated the effect of an AlaGln-ORNT solution on electrolyte and water absorption in a rat model of secretory diarrhea induced by cholera toxin.

METHODS

Phenolsulfonphthalein (50 microg/mL) was used as a non-absorbable marker for calculation of net water and electrolyte transport. Solutions tested were Ringer's solution, a glutamine-based ORNT (Gln-ORNT) solution, and an AlaGln-ORNT solution. Cholera toxin (1 microg/mL) was injected into lumen of rat small intestinal segments and incubated for 18 h before the initiation of the perfusion.

RESULTS

Cholera toxin induced significant secretion of electrolyte and water in the control Ringer's solution. AlaGln-ORNT and Gln-ORNT solutions reduced the sodium secretory effect of cholera toxin by 128% and 36%, respectively. The net water secretion also was reduced by 95% and 60%, respectively, with the AlaGln-ORNT and Gln-ORTN solutions.

CONCLUSIONS

These results showed that AlaGln-ORNT solution can enhance water and electrolyte intestinal absorption even better than glutamine or glucose and thus provide a potential novel approach for ORNT to break the vicious cycle of diarrhea and malnutrition. Clinical trials are now needed in children and adults with diarrhea and malnutrition.

Role of PEPT2 in peptide/mimetic trafficking at the blood-cerebrospinal fluid barrier: studies in rat choroid plexus epithelial cells in primary culture

Recent studies have established the functional and molecular presence of a high-affinity peptide transporter, PEPT2, in whole tissue rat choroid plexus. However, the precise membrane location and directionality of PEPT2-mediated transport is uncertain at present. In this study, we examined the transport kinetics of a model dipeptide, glycylsarcosine (GlySar), along with the protein expression of PEPT2 using primary cell cultures of choroidal epithelium from neonatal rats. GlySar accumulation and transepithelial transport were 3 to 4 times higher when introduced from the apical as opposed to the basal side of the monolayers. GlySar apical uptake was also stimulated by an inwardly directed proton gradient. The uptake of GlySar was inhibited by di/tripeptides, carnosine, and alpha-amino cephalosporins but was unaffected by amino acids, cephalosporins lacking an alpha-amino group, and organic anions and cations. The Michaelis constant (K(m)) of GlySar was 59.6 microM for apical uptake and 1.4 mM for basal uptake; this is consistent with the high-affinity properties of PEPT2 at the apical membrane. Immunoblot analyses and immunofluorescent confocal microscopy demonstrated the presence of PEPT2, but not PEPT1, in rat choroid plexus epithelial cells. Moreover, PEPT2 was present in the apical and subapical regions of the cell but was absent in the basolateral membrane. These findings demonstrate, for the first time, that PEPT2 protein is present at the apical membrane of choroidal epithelial cells and that it is functionally active at this membrane surface. The results suggest that PEPT2 may have a role in the efflux of peptides and/or mimetics from cerebrospinal fluid to the blood.

Transport characteristics of peptides and peptidomimetics: II. Hydroxyethylamine bioisostere-containing peptidomimetics as substrates for the oligopeptide transporter and P-glycoprotein in the intestinal mucosa

Peptide bond bioisosteres, such as hydroxyethylamine (Hea), have frequently been used to stabilize metabolically labile peptide bonds in peptidomimetic drug design in an effort to increase the oral bioavailability of drug candidates. However, the impact of the peptide bond bioisosteres on the cell permeation characteristics of peptidomimetics is not well understood, particularly with respect to the effects on the substrate activity for proteins that can restrict (e.g. P-glycoprotein, P-gp) or facilitate (e.g. the oligopeptide transporter, OPT) intestinal mucosal permeation of peptidomimetics. In this study, terminally free and terminally modified (N-acetylated and C-amidated) peptidomimetics of H-Ala-Phe-OH and H-Ala-Phe-Ala-OH with the Ala-Phe peptide bonds replaced by Hea bioisosteres were synthesized. Transport characteristics of these peptidomimetics were investigated using Caco-2 cell monolayers as an in vitro model of the intestinal mucosa. The study showed that the Hea bioisostere stabilized the peptidomimetics to protease metabolism in Caco-2 cells. All terminally free peptidomimetics showed significant affinity and substrate activity for OPT. The affinity and substrate activity for OPT were stereoselective for peptidomimetics containing an S,S-configuration for the two adjacent chiral centers related to the Hea bioisostere. Three of the four terminally modified peptidomimetics showed significant substrate activity for P-gp and, interestingly, the substrate activity for P-gp was also stereoselective; however, it was in favor of an R,R-configuration for the two adjacent chiral centers related to the Hea bioisostere.