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

Spatial expression patterns of peptide transporters in the human and rat gastrointestinal tracts, Caco-2 in vitro cell culture model, and multiple human tissues

This study sought to identify the spatial patterns of expression of peptide transporter 1 (PepT1), peptide transporter 3 (PTR3), peptide/histidine transporter 1 (PHT1), and the human peptide transporter 1 (HPT-1) mRNA in complementary DNA (cDNA) libraries of the human and rat gastrointestinal tracts (GIT), Caco-2 in vitro cell culture model, and in a human multiple tissue panel. Human PTR3 and PHT1 are putative peptide transporters recently discovered. Using sequence-specific primers designed to amplify regions of PepT1, PTR3, PHT1, and HPT-1, we were able to identify the expression of mRNA for each of these transporters in human cDNA panels (Clontech, Palo Alto, CA), the rat GIT, and in Caco-2 cDNA libraries by the polymerase chain reaction (PCR) and Southern Blot analysis. These studies suggest that in the human GIT, PepT1 appears to be localized predominantly in the duodenum, with decreasing expression in the jejunum and ileum. In contrast, PTR3 and HPT-1 were widely expressed in the human GIT, with predominant expression in the different regions of the colon. PHT1 appeared to be expressed in low levels throughout the human GI tract. Interestingly, the mRNAs for all 4 peptide transporters were expressed in Caco-2 cells throughout 30 days of culture. PepT1, PTR3, PHT1, and HPT-1 were also widely expressed in the rat GIT. Human tissue cDNA panel screening suggests that PTR3 and PHT1 are more uniformly expressed, whereas PepT1 and HPT-1 demonstrated site-specific expression. These results suggest that PepT1, PTR3, PHT1, and HPT-1 all may act to facilitate the diffusion of peptides and peptide-based pharmaceuticals in the GIT. PTR3, PHT1, and HPT-1 expressions in Caco-2 cell monolayers strongly suggest that their function needs to be further elucidated and their contribution to peptide transport not ignored. Taken together, these results demonstrate the potential for molecular biological characterization in localizing active transporter systems that can potentially be targeted for enhancing the absorption of peptide-based pharmaceuticals.

Prodrug based optimal drug delivery via membrane transporter/receptor

The carrier-mediated absorption of drugs and prodrugs across epithelial and endothelial barriers is emerging as a novel trend in biotherapeutics. This review examines the important advances in this field in the past decade. The feasibility of drug absorption of the parent drug or the appropriately modified prodrug via these transporters is discussed in detail. Several successful examples of synthesis of prodrugs recognised by the targeted transporters are described. The applicability of this approach in translocating drugs across the almost impenetrable blood-brain barrier (BBB) has also been examined.

PEPT1-mediated uptake of dipeptides enhances the intestinal absorption of amino acids via transport system b(0,+)

Free amino acids and short chain peptides are the main digestion products of dietary proteins in the small intestine. Whether there is a direct interference in transport of both groups of degradation products is not known. We used human intestinal Caco-2 cells to investigate whether the absorption of dipeptides by the peptide transporter PEPT1 alters the apical uptake of free cationic and neutral amino acids. Influx of L-[3H]Arg into Caco-2 cells was Na+-independent and mediated mainly by the b(0,+) system recognizing both cationic and neutral amino acids. Preincubation of cells with 10 mM of selected neutral, mono- or dicationic dipeptides increased the influx of L-Arg up to fourfold. Preloading with equivalent concentrations of the corresponding free amino acids also increased L-Arg influx but dipeptides always proved to be more efficient. The observed trans-stimulation was found to be specific for cationic amino acids since transport of L-[3H]Ala remained unaffected. We here demonstrate for the first time a direct interplay in amino acid and peptide transport in intestinal cells that may selectively alter the kinetics of amino acid absorption.

Localization of the peptide transporter PEPT2 in the lung: implications for pulmonary oligopeptide uptake

Pulmonary delivery of peptidomimetic antibiotics is frequently used for local drug therapy in pulmonary infections. Identification of transport pathways into airway epithelia can lead to new strategies of therapy. Here we describe the distribution of the beta-lactam-transporting high-affinity proton-coupled peptide transporter PEPT2 in mammalian lungs. Using reverse transcriptase-polymerase chain reaction and Northern blot analysis, PEPT2-mRNA was detected in lung extracts. The expression of PEPT2-mRNA and protein was localized to alveolar type II pneumocytes, bronchial epithelium, and endothelium of small arteries of rat lung by nonisotopic in situ hybridization and immunohistochemistry. In addition, transport studies using murine whole-organ preparations revealed transporter-mediated uptake of a fluorophore-conjugated dipeptide derivative into bronchial epithelial cells and type II pneumocytes. This transport was competitively inhibited by cephalosporins and dipeptides that are reported as PEPT2-carried substrates. Cell specificity of the PEPT2-mediated uptake pattern was confirmed by double labeling with Lycopersicon esculentum lectin. Together these data suggest that PEPT2 is the molecular basis for the transport of peptides and peptidomimetics in pulmonary epithelial cells. In conclusion PEPT2 may be an interesting target for pulmonary delivery of peptides and peptidomimetics.

Drug inhibition of Gly-Sar uptake and hPepT1 localization using hPepT1-GFP fusion protein

An hPepT1-GFP fusion construct was made to study drug inhibition of dipeptide uptake and apical, basolateral, or subcellular hPepT1 localization. The hPepT1 stop codon was mutated by polymerase chain reaction and was subsequently cloned into the pEGFP-N1 vector. The hPepT1-GFP fusion construct was then transfected into Caco-2 and HeLa cells, and drug inhibition was studied by inhibiting 3H-Gly-Sar uptake. Western blot analysis was used to determine hPepT1-GFP expression levels and confocal microscopy was used to examine the localization. Both anti-hPepT1 antibody and anti-GFP antibody recognized a 120-kd hPepT1-GFP fusion protein in the transfected cells. The 3H-Gly-Sar uptake in transfected HeLa cells was enhanced more than 20 times compared with the control. Valacyclovir (5 mmol/L) was able to completely inhibit 3H-Gly-Sar uptake in these transfected cells. Confocal microscopy showed that the hPepT1-GFP mainly localized in the Caco-2 cell apical membrane, but was present throughout the entire HeLa cell membranes. The hPepT1-GFP fusion protein was not found in either early endosome or lysosome of Caco-2 cells under normal conditions; however, it was detected in some subsets of lysosomes and early endosomes in phorbol 12-myristate 13-acetate (PMA)-treated Caco-2 cells.

Characterisation of a gene cluster involved in bacterial degradation of the cyanobacterial toxin microcystin LR

A novel pathway for degradation of the cyanobacterial heptapeptide hepatotoxin microcystin LR was identified in a newly isolated Sphingomonas sp. (Bourne et al. 1996 Appl. Environ. Microbiol. 62: 4086-4094). We now report the cloning and molecular characterisation of four genes from this Sphingomonas sp. that exist on a 5.8-kb genomic fragment and encode the three hydrolytic enzymes involved in this pathway together with a putative oligopeptide transporter. The heterologously expressed degradation pathway proteins are enzymatically active. Microcystinase (MlrA), the first enzyme in the degradative pathway, is a 336-residue endopeptidase, which displays only low sequence identity with a hypothetical protein from Methanobacterium thermoautotrophicum. Inhibition of microcystinase by EDTA and 1,10-phenanthroline suggests that it is a metalloenzyme. The most likely residues that could potentially chelate an active-site transition metal ion are in the sequence HXXHXE, which would be unique for a metalloproteinase. Situated immediately downstream of mlrA with the same direction of transcription is a gene mlrD, whose conceptual translation (MlrD, 442 residues) shows significant sequence identity and similar potential transmembrane spanning regions to the PTR2 family of oligopeptide transporters. A gene mlrB is situated downstream of the mlrA and mlrD genes, but transcribed in the opposite direction. The gene encodes the enzyme MlrB (402 residues) which cleaves linear microcystin LR to a tetrapeptide degradation product. This enzyme belongs to the "penicillin-binding enzyme" family of active site serine hydrolases. The final gene in the cluster mlrC, is located upstream of the mlrA gene and is transcribed in the opposite direction. It codes for MlrC (507 residues) which mediates further peptidolytic degradation of the tetrapeptide. This protein shows significant sequence identity to a hypothetical protein from Streptomyces coelicolor. It is suspected to be a metallopeptidase based on inhibition by metal chelators. It is postulated on the basis of comparison with other microorganisms that the genes in this cluster may all be involved in cell wall peptidoglycan cycling and subsequently act fortuitously in hydrolysis of microcystin LR.

Characterisation of the H(+)/peptide cotransporter of eel intestinal brush-border membranes

H(+)/peptide cotransport in brush-border membrane vesicles (BBMVs) from eel (Anguilla anguilla) intestine was studied by measuring d-[(3)H]-phenylalanyl-l-alanine uptake and by monitoring peptide-dependent intravesicular acidification using the pH-sensitive dye Acridine Orange. d-[(3)H]-phenylalanyl-l-alanine influx was greatly stimulated by an inside-negative membrane potential and enhanced by an inwardly directed H(+) gradient. In parallel, vesicular H(+) influx was significantly increased in the presence of extravesicular d-phenylalanyl-l-alanine or a series of glycyl and l-prolyl peptides. H(+)/peptide cotransport displayed saturable kinetics involving a single carrier system with apparent substrate affinities of 0.9-2.6 mmol l(−1) depending on the particular peptide. All substrates tested competed with this system. Pre-incubation of BBMVs with dipeptides prevented diethylpyrocarbonate inhibition of transport activity, suggesting that the substrates mask histidine residues involved in the catalytic function of the transporter. Using human PepT1-specific primers, a reverse transcription-polymerase chain reaction (RT-PCR) signal was detected in eel intestine. Our results suggest that, in eel intestine, a brush-border membrane ‘low-affinity’-type H(+)/peptide cotransport system is present that shares kinetic features with the mammalian intestinal PepT1-type transporters.

Membrane transporters

Carrier-mediated drug transport is relatively unexplored in comparison with passive transcellular and paracellular drug transport. Yet, there is a host of transporter proteins that can be targeted for improving epithelial drug absorption. Generally, these are transport mechanisms for amino acids, dipeptides, monosaccharides, monocarboxylic acids, organic cations, phosphates, nucleosides, and water-soluble vitamins. Among them, the dipeptide transporter mechanism has received the most attention. Dipeptide transporters are H(+)-coupled, energy-dependent transporters that are known to play an essential role in the oral absorption of beta-lactam antibiotics, angiotensin-converting enzyme (ACE) inhibitors, renin inhibitors, and an anti-tumor drug, bestatin. Moreover, several investigators have demonstrated the utility of the dipeptide transporter as a platform for improving the oral bioavailability of drugs such as zidovudine and acyclovir through dipeptide prodrug derivatization. Thus far, at least four proton-coupled peptide transporters have been cloned. The first one cloned was PepT1 from the rabbit small intestine. The focus of this presentation will be structure-function, intracellular trafficking, and regulation of PepT1. Disease, dietary, and possible excipient influences on PepT1 function will also be discussed.

Cellular and molecular aspects of drug transport in the kidney

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 (OAT), organic cation transporter (OCT), and organic anion-transporting polypeptide (oatp) gene families have been found to mediate the transport of diverse organic anions and cations. It has also been suggested that ATP-dependent primary active transporters such as MDR1/P-glycoprotein and the multidrug resistance-associated protein (MRP) gene 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 is the consequence of interaction and/or transport via 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 cloned 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.

Molecular pharmacology of renal organic anion transporters

Renal organic anion transport systems play an important role in the elimination of drugs, toxic compounds, and their metabolites, many of which are potentially harmful to the body. The renal proximal tubule is the primary site of carrier-mediated transport from blood to urine of a wide variety of anionic substrates. Recent studies have shown that organic anion secretion in renal proximal tubule is mediated by distinct sodium-dependent and sodium-independent transport systems. Knowledge of the molecular identity of these transporters and their substrate specificity has increased considerably in the past few years by cloning of various carrier proteins. However, a number of fundamental questions still have to be answered to elucidate the participation of the cloned transporters in the overall tubular secretion of anionic xenobiotics. This review summarizes the latest knowledge on molecular and pharmacological properties of renal organic anion transporters and homologs, with special reference to their nephron and plasma membrane localization, transport characteristics, and substrate and inhibitor specificity. A number of the recently cloned transporters, such as the p-aminohippurate/dicarboxylate exchanger OAT1, the anion/sulfate exchanger SAT1, the peptide transporters PEPT1 and PEPT2, and the nucleoside transporters CNT1 and CNT2, are key proteins in organic anion handling that possess the same characteristics as has been predicted from previous physiological studies. The role of other cloned transporters, such as MRP1, MRP2, OATP1, OAT-K1, and OAT-K2, is still poorly characterized, whereas the only information that is available on the homologs OAT2, OAT3, OATP3, and MRP3-6 is that they are expressed in the kidney, but their localization, not to mention their function, remains to be elucidated.

Effect of ionization on the variable uptake of valacyclovir via the human intestinal peptide transporter (hPepT1) in CHO cells

Carrier-mediated transport of valacyclovir (vacv), the L-valyl ester prodrug of acyclovir (acv), via the human peptide transporter (hPepT1) has been shown in Xenopus laevis oocytes and in cell lines such as Chinese hamster ovary (CHO) and Caco-2 transfected with the hPepT1 gene. However, significant differences in vacv uptake were observed in those models as extracellular pH varied. The purpose of this work was to characterize the interactions of various ionic species of vacv with the peptide transporter by overexpressing the transporter gene, hPepT1, in CHO cells. Based on the pK(a) values of vacv, it was determined that vacv exists as four different ionic species (di-cationic, cationic, neutral and anionic) with a predominance of cationic and neutral species at physiologically relevant pH conditions. Vacv uptake was shown to increase with increasing pH of the extracellular medium from 5.5 to 7.2. The uptake value was maximal at around pH 7.2 and did not vary for studies done at higher pH. Vacv uptake was concentration dependent and saturable at all pH conditions (5.5, 6.2, 6.8, 7.5 and 7.9) with apparent Michaelis-Menten constants, mean (S.D.), of 7.42(0.32), 6.64(1.20), 5.38(0.88), 2.69(0.23) and 2.23(0.33) mM, respectively. The current results demonstrate that the estimated affinities of the cationic and the neutral species of vacv with hPepT1 are significantly different (7.4 versus 1.2 mM, respectively). Given the axial and radial (microclimate) pH gradients known to exist in the intestine, the greater than six-fold difference in affinity constants suggests that intestinal pH fluctuations may significantly impact upon the variability of vacv uptake.

cDNA structure, genomic organization, and promoter analysis of the mouse intestinal peptide transporter PEPT1

We describe in this report the cDNA structure, functional characteristics, genomic organization, and promoter analysis of the mouse H(+)-coupled low-affinity peptide transporter PEPT1. The mouse PEPT1 cDNA cloned from a kidney cDNA library is approximately 3.1 kb long and encodes a protein of 709 amino acids. When expressed heterologously in mammalian cells and in Xenopus laevis oocytes, mouse PEPT1 mediates H(+)-coupled electrogenic transport of the dipeptide glycylsarcosine. The mouse pept1 gene, cloned from a genomic DNA library in bacterial artificial chromosome, is approximately 38 kb long and consists of 23 exons and 22 introns. 5'-Rapid amplification of cDNA ends with poly(A)(+) RNA from mouse intestine has identified the transcription start site that lies 31 bp upstream of the translation start site. The promoter region upstream of the transcription start site does not contain the TATA box but possesses three GC boxes which are the binding sites for the transcription activator SP1. Functional analysis of the promoter region using the luciferase reporter assay in Caco-2 cells (a human intestinal cell line that express PEPT1 constitutively) and five different 5'-deletion fragments of the promoter has shown that essential promoter/enhancer elements are present within 1140 bp upstream of the transcription start site.

Drug transport and targeting. Intestinal transport

A wide variety of transporters are found in the intestine, and are involved in the membrane transport of daily nutrients as well as drugs. These intestinal transporters are located in the brush border membrane as well as basolateral membrane. Each transporter exhibits its own substrate specificity, and some have broader specificities than others. In addition, the distribution and characteristics of the intestinal transporters exhibit regional differences along the intestine, implying diverse physiologic functions and in some cases pathologic responses. Indeed several genetic disorders have been shown to result from deficient intestinal transporters. The development of prodrugs that target to intestinal transporters has been successful in improving oral absorption. For example, the intestinal peptide transporter is utilized in order to increase the bioavailability of several classes of peptidomimetic drugs, especially ACE inhibitors and beta-lactam antibiotics. The bioavailability of poorly absorbed drugs can be improved by utilization of the transporters responsible for the intestinal absorption of various solutes and/or by inhibiting the transporter involved in the efflux system. Recent advances in gene cloning and molecular biology techniques make it possible to study the characteristics and distribution of transporters at the molecular level. Based on molecular characterizations of membrane transporters and accumulated biochemical data on their specificities and kinetics, structural modification and targeting of a specific transporter is a promising strategy for the design of drugs that improve bioavailability and tissue distribution.

Human proton/oligopeptide transporter (POT) genes: identification of putative human genes using bioinformatics

The proton-dependent oligopeptide transporters (POT) gene family currently consists of approximately 70 cloned cDNAs derived from diverse organisms. In mammals, two genes encoding peptide transporters, PepT1 and PepT2 have been cloned in several species including humans, in addition to a rat histidine/peptide transporter (rPHT1). Because the Candida elegans genome contains five putative POT genes, we searched the available protein and nucleic acid databases for additional mammalian/human POT genes, using iterative BLAST runs and the human expressed sequence tags (EST) database. The apparent human orthologue of rPHT1 (expression largely confined to rat brain and retina) was represented by numerous ESTs originating from many tissues. Assembly of these ESTs resulted in a contiguous sequence covering approximately 95% of the suspected coding region. The contig sequences and analyses revealed the presence of several possible splice variants of hPHT1. A second closely related human EST-contig displayed high identity to a recently cloned mouse cDNA encoding cyclic adenosine monophosphate (cAMP)-inducible 1 protein (gi:4580995). This contig served to identify a PAC clone containing deduced exons and introns of the likely human orthologue (termed hPHT2). Northern analyses with EST clones indicated that hPHT1 is primarily expressed in skeletal muscle and spleen, whereas hPHT2 is found in spleen, placenta, lung, leukocytes, and heart. These results suggest considerable complexity of the human POT gene family, with relevance to the absorption and distribution of cephalosporins and other peptoid drugs.

Transport of valganciclovir, a ganciclovir prodrug, via peptide transporters PEPT1 and PEPT2

In clinical trials, valganciclovir, the valyl ester of ganciclovir, has been shown to enhance the bioavailability of ganciclovir when taken orally by patients with cytomegalovirus infection. We investigated the role of the intestinal peptide transporter PEPT1 in this process by comparing the interaction of ganciclovir and valganciclovir with the transporter in different experimental systems. We also studied the interaction of these two compounds with the renal peptide transporter PEPT2. In cell culture model systems using Caco-2 cells for PEPT1 and SKPT cells for PEPT2, valganciclovir inhibited glycylsarcosine transport mediated by PEPT1 and PEPT2 with K(i) values (inhibition constant) of 1.68+/-0.30 and 0.043+/- 0.005 mM, respectively. The inhibition by valganciclovir was competitive in both cases. Ganciclovir did not interact with either transporter. Similar studies done with cloned PEPT1 and PEPT2 in heterologous expression systems yielded comparable results. The transport of valganciclovir via PEPT1 was investigated directly in PEPT1-expressing Xenopus laevis oocytes with an electrophysiological approach. Valganciclovir, but not ganciclovir, induced inward currents in PEPT1-expressing oocytes. These results demonstrate that the increased bioavailability of valganciclovir is related to its recognition as a substrate by the intestinal peptide transporter PEPT1. This prodrug is also recognized by the renal peptide transporter PEPT2 with high affinity.

Kinetics and substrate specificity of membrane-reconstituted peptide transporter DtpT of Lactococcus lactis

The peptide transport protein DtpT of Lactococcus lactis was purified and reconstituted into detergent-destabilized liposomes. The kinetics and substrate specificity of the transporter in the proteoliposomal system were determined, using Pro-[(14)C]Ala as a reporter peptide in the presence of various peptides or peptide mimetics. The DtpT protein appears to be specific for di- and tripeptides, with the highest affinities for peptides with at least one hydrophobic residue. The effect of the hydrophobicity, size, or charge of the amino acid was different for the amino- and carboxyl-terminal positions of dipeptides. Free amino acids, omega-amino fatty acid compounds, or peptides with more than three amino acid residues do not interact with DtpT. For high-affinity interaction with DtpT, the peptides need to have free amino and carboxyl termini, amino acids in the L configuration, and trans-peptide bonds. Comparison of the specificity of DtpT with that of the eukaryotic homologues PepT(1) and PepT(2) shows that the bacterial transporter is more restrictive in its substrate recognition.

Differential recognition of ACE inhibitors in Xenopus laevis oocytes expressing rat PEPT1 and PEPT2

PURPOSE

To examine the mechanism of inhibition of glycylsarcosine (GlySar) transport by quinapril and enalapril, and whether or not angiotensin converting enzyme (ACE) inhibitors are transported by PEPT2 as well as by PEPT1.

METHODS

Xenopus laevis oocytes were cRNA-injected with rat PEPT1 or PEPT2 and the transport kinetics of radiolabeled GlySar were studied in the absence and presence of quinapril and enalapril. The two-microelectrode voltage-clamp technique was also performed to probe the electrogenic uptake of captopril, quinapril and enalapril.

RESULTS

Kinetic analyses demonstrated that quinapril inhibited the uptake of GlySar in a noncompetitive manner in Xenopus oocytes injected with PEPT1 or PEPT2 (Ki = 0.8 or 0.4 mM, respectively). In contrast, a competitive interaction was observed between GlySar and enalapril (Ki = 10.8 mM for PEPT1 or 4.3 mM for PEPT2). Most significantly, captopril and enalapril, but not quinapril, induced inwardly-directed currents in both PEPT1- and PEPT2-expressed oocytes.

CONCLUSIONS

These results are unique in providing direct evidence for the substrate recognition and transport of some ACE inhibitors by the high- and low-affinity oligopeptide transporters. Our findings point to differences between PEPT1 and PEPT2 in their affinity to, rather than in their specificity for, ACE inhibitors.

Regulation of PepT1 peptide transporter expression in the rat small intestine under malnourished conditions

BACKGROUND AND AIMS

Many investigations suggested that peptide nutrition had a clinical advantage for nitrogen absorption. Recently, the cDNA encoding the H(+)/peptide cotransporter PepT1 was cloned. However, the regulatory mechanism of PepT1 expression under malnourished conditions has not been elucidated. The aim of this study was to clarify regulatory mechanisms of PepT1 expression.

METHODS

Sprague-Dawley rats were starved for 4 days, semistarved (50% amount of control) for 10 days, or given total parenteral nutrition (TPN) for 10 days. Rats with free feeding were used as control. Among those groups, the changes of PepT1 mRNA level in the jejunal mucosa and PepT1 protein density at the brush-border membranes were examined by Northern blot and by Western blot analysis, respectively.

RESULTS

Both starvation and TPN treatment caused a significant decrease in mucosal weight by 41 and 50% respectively. PepT1 mRNA level increased to 179% in the starved group and also to 161 and 164% in the TPN and semistarved groups, respectively. In contrast, sodium-dependent glucose transporter 1 mRNA expression showed no significant change. PepT1 protein density showed similar changes with the mRNA.

CONCLUSIONS

PepT1 gene expression was significantly enhanced under the malnourished conditions in spite of atrophic changes of intestinal mucosa.

A novel H(+)-coupled oligopeptide transporter (OPT3) from Caenorhabditis elegans with a predominant function as a H(+) channel and an exclusive expression in neurons

We have cloned and functionally characterized a novel, neuron-specific, H(+)-coupled oligopeptide transporter (OPT3) from Caenorhabditis elegans that functions predominantly as a H(+) channel. The opt3 gene is approximately 4.4 kilobases long and consists of 13 exons. The cDNA codes for a protein of 701 amino acids with 11 putative transmembrane domains. When expressed in mammalian cells and in Xenopus laevis oocytes, OPT3 cDNA induces H(+)-coupled transport of the dipeptide glycylsarcosine. Electrophysiological studies of the transport function of OPT3 in Xenopus oocytes show that this transporter, although capable of mediating H(+)-coupled peptide transport, functions predominantly as a H(+) channel. The H(+) channel activity of OPT3 is approximately 3-4-fold greater than the H(+)/peptide cotransport activity as determined by measurements of H(+) gradient-induced inward currents in the absence and presence of the dipeptide using the two-microelectrode voltage clamp technique. A downhill influx of H(+) was accompanied by a large intracellular acidification as evidenced from the changes in intracellular pH using an ion-selective microelectrode. The H(+) channel activity exhibits a K(0.5)(H) of 1.0 microM at a membrane potential of -50 mV. At the level of primary structure, OPT3 has moderate homology with OPT1 and OPT2, two other H(+)-coupled oligopeptide transporters previously cloned from C. elegans. Expression studies using the opt3::gfp fusion constructs in transgenic C. elegans demonstrate that opt3 gene is exclusively expressed in neurons. OPT3 may play an important physiological role as a pH balancer in the maintenance of H(+) homeostasis in C. elegans.

Inhibitory effect of novel oral hypoglycemic agent nateglinide (AY4166) on peptide transporters PEPT1 and PEPT2

The novel oral hypoglycemic agent nateglinide (AY4166) is a nonsulfonylurea insulin secretagogue, and its pharmacokinetic features include rapid absorption and elimination. As nateglinide is a dipeptide-like drug, we investigated the interaction of nateglinide with peptide transporters PEPT1 and PEPT2, which mediate the absorption of various peptide-like drugs. Nateglinide exhibited a potent inhibitory effect on [14C]glycylsarcosine uptake by the human colon adenocarcinoma cell line Caco-2 and rat PEPT-transfectants. Kinetic analysis revealed that these inhibitory effects were noncompetitive. Na(+)-coupled alanine or threonine uptake by Caco-2 cells was not inhibited by nateglinide, suggesting that the inhibitory effect of nateglinide on peptide transporters was not due to nonspecific interaction. There was little uptake of [14C]nateglinide by peptide transporters. Various sulfonylureas, such as glibenclamide, also inhibited [14C]glycylsarcosine uptake by rat PEPT-transfectants. In conclusion, nateglinide as well as sulfonylureas inhibit the transport activity of PEPT1 and PEPT2, although nateglinide itself is not transported by these transporters.

Targeted prodrug design to optimize drug delivery

Classical prodrug design often represents a nonspecific chemical approach to mask undesirable drug properties such as limited bioavailability, lack of site specificity, and chemical instability. On the other hand, targeted prodrug design represents a new strategy for directed and efficient drug delivery. Particularly, targeting the prodrugs to a specific enzyme or a specific membrane transporter, or both, has potential as a selective drug delivery system in cancer chemotherapy or as an efficient oral drug delivery system. Site-selective targeting with prodrugs can be further enhanced by the simultaneous use of gene delivery to express the requisite enzymes or transporters. This review highlights evolving strategies in targeted prodrug design, including antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and peptide transporter-associated prodrug therapy.

Genomic structure of proton-coupled oligopeptide transporter hPEPT1 and pH-sensing regulatory splice variant

Proton-coupled oligopeptide transporter PEPT1 facilitates the transport of dipeptides and peptoid drugs (including antibiotics) across the cell membranes of endothelial and epithelial cells. Substrate transport by the proton symport is driven by pH gradients, while the profile of pH sensitivity is regulated by a closely related protein, hPEPT1-RF. We investigated the genomic structure of hPEPT1 and hPEPT1-RF. Analysis of the high-throughput genomic sequence (HTGS) database revealed that hPEPT1 and hPEPT1-RF are splice variants encoded by the same gene located in chromosome 13, consisting of 24 exons. hPEPT1 is encoded by 23 exons and hPEPT1-RF by 6 exons. Coding sequences of hPEPT1-RF share 3 exons completely and 2 exons partially with hPEPT1. The genomic organization of hPEPT1 shows high similarity with its mouse orthologue. Exon-intron boundaries occur mostly in the loops connecting transmembrane segments (TMSs), suggesting a modular gene structure reflecting the TMS-loop repeat units in hPEPT1. The putative promoter region of hPEPT1 contains TATA boxes and GC-rich regions and a potential insulin responsive element.

beta-lactam antibiotics as substrates for OCTN2, an organic cation/carnitine transporter

Therapeutic use of cephaloridine, a beta-lactam antibiotic, in humans is associated with carnitine deficiency. A potential mechanism for the development of carnitine deficiency is competition between cephaloridine and carnitine for the renal reabsorptive process. OCTN2 is an organic cation/carnitine transporter that is responsible for Na(+)-coupled transport of carnitine in the kidney and other tissues. We investigated the interaction of several beta-lactam antibiotics with OCTN2 using human cell lines that express the transporter constitutively as well as using cloned human and rat OCTN2s expressed heterologously in human cell lines. The beta-lactam antibiotics cephaloridine, cefoselis, cefepime, and cefluprenam were found to inhibit OCTN2-mediated carnitine transport. These antibiotics possess a quaternary nitrogen as does carnitine. Several other beta-lactam antibiotics that do not possess this structural feature did not interact with OCTN2. The interaction of cephaloridine with OCTN2 is competitive with respect to carnitine. Interestingly, many of the beta-lactam antibiotics that were not recognized by OCTN2 were good substrates for the H(+)-coupled peptide transporters PEPT1 and PEPT2. In contrast, cephaloridine, cefoselis, cefepime, and cefluprenam, which were recognized by OCTN2, did not interact with PEPT1 and PEPT2. The interaction of cephaloridine with OCTN2 was Na(+)-dependent, whereas the interaction of cefoselis and cefepime with OCTN2 was largely Na(+)-independent. Furthermore, the Na(+)-dependent, OCTN2-mediated cellular uptake of cephaloridine could be demonstrated by direct uptake measurements. These studies show that OCTN2 plays a crucial role in the pharmacokinetics and therapeutic efficacy of certain beta-lactam antibiotics such as cephaloridine and that cephaloridine-induced carnitine deficiency is likely to be due to inhibition of carnitine reabsorption in the kidney.

Molecular recognition templates of peptides: driving force for molecular evolution of peptide transporters

Small peptides derived from protein hydrolysis occur ubiquitously. To utilize these structurally diverse compounds, organisms possess generic peptide transporters for di- (Dpp), tri- (Tpp), and oligopeptides (Opp). Using conformational analysis, we describe the predominant conformers of di-, tri-, and oligopeptides in water; dipeptides occur as nine main groups, defined by specific combinations of torsional angles. The molecular recognition templates (MRTs) of substrates for Dpp and Tpp comprise distinct groups of dipeptide conformers plus folded tripeptide conformers with matching spatial distribution of recognition features; for Opp, the MRT involves specific oligopeptide conformers with extended backbones. For any peptide, the proportion of its conformers in a particular MRT correlates with its relative binding and transport by each transporter. Thus, peptide transporters have evolved complementary specificities to optimize utilization of the universal peptide pool. The general applicability of MRTs should facilitate rational design and targeting of peptide-based prodrugs.

Tissue selective drug delivery utilizing carrier-mediated transport systems

This paper describes some successful examples of a tissue selective drug delivery by utilizing specialized transporter(s) expressed in the targeted tissue cells. These are as follows: (1) oral delivery via H(+)/oligopeptide transporter, rat or human Pept1, in the intestine for beta-lactam antibiotics and a newly synthesized dipeptide, L-dopa-L-phenylalanine; (2) tumor cell specific delivery via the newly discovered H(+)/oligopeptide transporter(s) expressed in human fibrosarcoma cell line HT-1080 for model oligopeptides, glycylsarcosine and carnosine; (3) oral and hepatic delivery via an H(+)/monocarboxylate transporter in the intestine and an organic anion transporter in the liver for HMG-CoA reductase inhibitor, pravastatin; and (4) lung selective delivery via some type of transporter and avoidance of transfer into the brain via P-glycoprotein at the blood-brain barrier for a new quinolone antibacterial, HSR-903.

Biopharmaceutics of transmucosal peptide and protein drug administration: role of transport mechanisms with a focus on the involvement of PepT1

Non-invasive delivery of peptide and protein drugs will soon become a reality. This is due partly to a better understanding of the endogenous transport mechanisms, including paracellular transport, endocytosis, and carrier-mediated transport of mucosal routes of peptide and protein drug administration. This paper focuses on work related to the elucidation of structure-function, intracellular trafficking, and regulation of the intestinal dipeptide transporter, PepT1.

Function and immunolocalization of overexpressed human intestinal H+/peptide cotransporter in adenovirus-transduced Caco-2 cells

PURPOSE

To determine the localization of the human intestinal H+/peptide cotransporter (hPepT1) and its function in intestinal epithelial cells after adenoviral transduction.

METHODS

Caco-2 cells grown on Transwell membrane filters were transduced with a recombinant replication-deficient adenovirus carrying the hPepT1 gene. The transport of Gly-Sar across both apical and basolateral membranes was measured after adenoviral transduction as a function of pH, temperature, inhibitors, and substrate concentration. The localization of hPepT1 was examined by immunocytochemistry using confocal laser scanning microscopy.

RESULTS

The apical-to-basolateral and basolateral-to-apical transport of Gly-Sar in Caco-2 cells after viral transduction was increased 3.3 and 3.5-fold, respectively. The similar magnitude of Gly-Sar permeability from either direction indicates involvement of identical transport pathways in both membranes. This was further confirmed by immunocytochemistry showing that hPepT1 was localized in the apical and basolateral membrane of Caco-2 cells after adenoviral transduction. In both directions, Gly-Sar transport was enhanced in the presence of a pH gradient. In addition, the basolateral-to-apical Gly-Sar transport was dependent on temperature, multiplicity of infection (MOI), and Gly-Sar concentration. It was inhibited in the presence of excess Gly-Pro and cephalexin.

CONCLUSIONS

Caco-2 cell monolayers represent an appropriate model to study gene expression in intestinal epithelial cells. Transport characteristics of Gly-Sar from the basolateral to the apical side in adenovirus-transduced Caco-2 cells are in agreement with those from the apical to the basolateral side, indicating that hPepT1 is also expressed in the basolateral membrane and displays a similar level of transport enhancement after adenovirus mediated hPepT1 gene expression.

Manipulation of activity and orientation of membrane-reconstituted di-tripeptide transport protein DtpT of Lactococcus lactis

The di-tripeptide transport system (DtpT) of Lactococcus lactis was purified to apparent homogeneity by pre-extraction of crude membrane vesicles with octaethylene glycol monodecyl ether (C10E8), followed by solubilization with n-dodecyl-beta-D-maltoside (DDM) and chromatography on a Ni-NTA resin. The DtpT protein was reconstituted into detergent-destabilized preformed liposomes prepared from E. coli phospholipid/phosphatidylcholine. A variety of detergents were tested for their ability to mediate the membrane reconstitution of DtpT and their effectiveness to yield proteoliposomes with a high transport activity. The highest activities were obtained with TX100, C12E8 and DM, whereas DDM yielded relatively poor activities, in particular when this detergent was used at concentrations beyond the onset of solubilization of the preformed liposomes. Parallel with the low activity, significant losses of lipid were observed when the reconstitution was performed at high DDM concentrations. This explained at least part of the reduced transport activity as the DtpT protein was highly dependent on the final lipid-to-protein ratios in the proteoliposomes. Consistent with the difference in mechanism of DDM- and TX100-mediated membrane protein reconstitution, the orientation of the DtpT protein in the membrane was random with DDM and inside-in when TX100 was used. The methodology to determine the orientation of membrane-reconstituted proteins from the accessibility of cysteines for thiol-specific reagents is critically evaluated.

Peptide transporter in the rat small intestine: ultrastructural localization and the effect of starvation and administration of amino acids

Peptide transporter-1 is a H+/peptide cotransporter responsible for the uptake of small peptides and peptide-like drugs, and is present in the absorptive epithelial cells of the villi in the small intestine (duodenum, jejunum, and ileum). It has been localized to the apical microvillous plasma membrane of the absorptive epithelial cells of the rat small intestine using the immunogold electron microscopic technique. Digital image analysis of the jejunum revealed that the transporter protein was abundant at the tip of the villus and that the amount decreased from the tip of the villus to its base. The effect of dietary administration of amino acids and starvation on the expression of PepT1 in the jejunum was examined by immunoblotting and image analysis of immunofluorescence. Starvation markedly increased the amount of peptide transporter present, whereas dietary administration of amino acids reduced it. The gradient of the transporter protein along the crypt-villus axis was maintained under either condition. These observations show that it is specific to the microvillous plasma membrane and that its expression is regulated by the nutritional condition.

Intestinal peptide transport systems and oral drug availability

The intestinal peptide transport system has broad substrate specificities. In addition to its physiological function of absorbing di- and tripeptides resulting from the digestion of dietary proteins, this transport system also absorbs some orally administered peptidomimetic drugs, including beta-lactam antibiotics, angiotensin converting enzyme inhibitors, renin inhibitors, bestatin, thrombin inhibitors, and thyrotropin-releasing hormone and its analogues. There have been several studies on the mechanism and substrate structure-affinity relationship for this transport system. Rapid progress has been made recently in studies on the molecular basis of the intestinal peptide transport system. A protein apparently involved in peptide transport has been isolated from rabbit small intestines, and genes for human intestinal peptide transporters have been cloned, sequenced and functionally expressed. This review summarizes these studies and addresses the pharmaceutical potential of the intestinal peptide transport system.

sigma Receptor ligand-induced up-regulation of the H(+)/peptide transporter PEPT1 in the human intestinal cell line Caco-2

We determined the effects of (+)pentazocine, a selective sigma(1) ligand, on the uptake of glycylsarcosine (Gly-Sar) in the human intestinal cell line Caco-2 which expresses the low affinity/high capacity peptide transporter PEPT1. Confluent Caco-2 cells were treated with various concentrations of (+)pentazocine for desired time (mostly 24 hr). The activity of PEPT1 was assessed by measuring the uptake of [(14)C]Gly-Sar in the presence of a H(+) gradient. (+)Pentazocine increased the uptake of [(14)C]Gly-Sar mediated by PEPT1 in a concentration- and time-dependent manner. Kinetic analyses have indicated that (+)pentazocine increased the maximal velocity (V(max)) for Gly-Sar uptake in Caco-2 cells without affecting the Michaelis-Menten constant (K(t)). In addition, semi-quantitative RT-PCR revealed that treatment of (+)pentazocine increased PEPT1 mRNA in Caco-2 cells in a concentration-dependent manner. These data suggest that sigma(1) receptor ligand (+)pentazocine up-regulates PEPT1 in Caco-2 cells at the level of increased mRNA, causing an increase in the density of the transporter protein in the cell membrane.

Functional and molecular expression of intestinal oligopeptide transporter (Pept-1) after a brief fast

The intestinal oligopeptide transporter, cloned as Pept-1, has major roles in the assimilation of dietary proteins and absorption of peptidomimetic medications. The initial aim of the present experiment was to investigate whether the functional expression of this transporter is affected by dietary intake. Functional expression was determined as the rate of uptake of glycylglutamine (Gly-Gln) by brush-border membrane vesicles (BBMVs) prepared from the jejunum of fed and fasted rats. Surprisingly, the rate of dipeptide uptake was greatly increased after 1 day of fasting. The subsequent aim of the experiment became the investigation of the mechanism of this alteration in transport, which showed that 1 day of fasting increased (1) the maximal Gly-Gln uptake (Vmax) by twofold without changing the Km of Gly-Gln uptake by BBMVs, (2) the amount of intestinal oligopeptide transporter (Pept-1) protein by threefold in the brush-border membrane, and (3) the abundance of Pept-1 mRNA by threefold in the intestinal mucosa. We conclude that 1 day of fasting increases dipeptide transport in rat intestine by increasing the population of Pept-1 in the brush-border membrane. The mechanism appears to be an increase in Pept-1 gene expression.

Functional characteristics of basolateral peptide transporter in the human intestinal cell line Caco-2

The apical H+-coupled peptide transporter (PEPT1) and basolateral peptide transporter in human intestinal Caco-2 cells were functionally compared by the characterization of [14C]glycylsarcosine transport. The glycylsarcosine uptake via the basolateral peptide transporter was less sensitive to medium pH than uptake via PEPT1 and was not transported against the concentration gradient. Kinetic analysis indicated that glycylsarcosine uptake across the basolateral membranes was apparently mediated by a single peptide transporter. Small peptides and beta-lactam antibiotics inhibited glycylsarcosine uptake by the basolateral peptide transporter, and these inhibitions were revealed to be competitive. Comparison of the inhibition constant values of various beta-lactam antibiotics between PEPT1 and the basolateral peptide transporter suggested that the former had a higher affinity than the latter. A histidine residue modifier, diethyl pyrocarbonate, inhibited glycylsarcosine uptake by both transporters, although the inhibitory effect was greater on PEPT1. These findings suggest that a single facilitative peptide transporter is expressed at the basolateral membranes of Caco-2 cells and that PEPT1 and the basolateral peptide transporter cooperate in the efficient transepithelial transport of small peptides and peptidelike drugs.

Glycylsarcosine uptake in rabbit renal brush border membrane vesicles isolated from outer cortex or outer medulla: evidence for heterogeneous distribution of oligopeptide transporters

Studies were initially performed in rabbit brush border membrane vesicles (BBMV) prepared from whole cortex plus outer medulla. In these studies using combined tissues, two distinct peptide/H+transport systems were found for glycylsarcosine (GlySar) uptake, with one representing a low-affinity/high-capacity system (Vm1 = 974 pmol/mg/10 sec and Km1 = 4819 microM) and the other a high-affinity/low-capacity system (Vm2 = 220 pmol/mg/10 sec and Km2 = 96 microM). Thus, under linear conditions, the high-affinity transporter accounted for about 92% of the total transport of dipeptide. To better define the regional heterogeneity of peptide transporter activity in kidney, subsequent studies were performed in vesicles prepared from separately harvested outer cortical and outer medullary tissue. In BBMV studies prepared from outer cortex, two saturable components were revealed for GlySar transport (low-affinity/high-capacity transport system: Vm1 = 1921 pmol/mg/10 sec and Km1 = 11714 microM; high-affinity/low-capacity transport system: Vm2 = 143 pmol/mg/10 sec and Km2= 138 microM). However, in BBMV studies prepared from outer medulla, only one saturable component was revealed for GlySar transport (high-affinity/low-capacity transport system:Vm2= 168 pmol/mg/10 sec and Km2 = 230 microM). Overall, these studies support the contention that peptides are handled sequentially in kidney (ie, first by low-affinity transporter PEPT1, and then by high-affinity transporter PEPT2) and that PEPT2 is primarily responsible for the renal reabsorption of peptides and peptidomimetics.

Localization of PEPT1 and PEPT2 proton-coupled oligopeptide transporter mRNA and protein in rat kidney

To determine the renal localization of oligopeptide transporters, Northern blot analyses were performed and polyclonal antisera were generated against PEPT1 and PEPT2, the two cloned rat H+/peptide transporters. Under high-stringency conditions, a 3.0-kb mRNA transcript of rat PEPT1 was expressed primarily in superficial cortex, whereas a 3.5-kb mRNA transcript of PEPT2 was expressed primarily in deep cortex/outer stripe of outer medulla. PEPT1 antisera detected a specific band on immunoblots of renal and intestinal brush-border membrane vesicles (BBMV) with an apparent mobility of approximately 90 kDa. PEPT2 antisera detected a specific broad band of approximately 85 kDa in renal but not in intestinal BBMV. PEPT1 immunolocalization experiments showed detection of a brush border antigen in S1 segments of the proximal tubule and in the brush border of villi from all segments of the small intestine. In contrast, PEPT2 immunolocalization was primarily confined to the brush border of S3 segments of the proximal tubule. All other nephron segments in rat were negative for PEPT1 and PEPT2 staining. Overall, our results conclusively demonstrate that although PEPT1 is expressed in early regions of the proximal tubule (pars convoluta), PEPT2 is specific for the latter regions of proximal tubule (pars recta).

Carrier-mediated or specialized transport of drugs across the blood-brain barrier

In the drug development process, it remains a difficult task to regulate the entry of the drugs. However, recent progress in studies of the transporter-mediated influx and efflux of endogenous and exogenous compounds, including synthetic drugs, across the blood-brain barrier (BBB) is beginning to provide a rational basis for controlling drug distribution to the brain. This paper describes mechanisms established in the last decade for carrier-mediated influx and efflux of drugs and endocytosis of biologically active peptides across the BBB. The transport systems at the BBB described here are the uptake transporters for nutrients, such as amino acids and hexoses, monocarboxylates, amines, carnitine and glutathione and efflux transporters, such as P-glycoprotein and multiple organic anion transporters. Delivery of cationized peptides across the BBB via adsorptive-mediated endocytosis is also described. By utilizing such highly specific transport mechanisms, it should be possible to establish strategies to regulate the entry of candidate drugs, including peptides, into the brain.

Hormonal regulation of oligopeptide transporter pept-1 in a human intestinal cell line

The intestinal oligopeptide transporter (cloned as Pept-1) has major roles in protein nutrition and drug therapy. A key unstudied question is whether expression of Pept-1 is hormonally regulated. In this experiment, we investigated whether insulin has such a role. We used a human intestinal cell monolayer (Caco-2) as the in vitro model of human small intestine and glycylglutamine (Gly-Gln) as the model substrate for Pept-1. Results showed that addition of insulin at a physiological concentration (5 nM) to incubation medium greatly stimulates Gly-Gln uptake by Caco-2 cells. This stimulation was blocked when genistein, an inhibitor of tyrosine kinase, was added to incubation medium. Studies of the mechanism of insulin stimulation showed the following. 1) Stimulation occurred promptly (30-60 min) after exposure to insulin. 2) There was no significant change in the Michaelis-Menten constant of Gly-Gln transport, but there was a nearly twofold increase in its maximal velocity. 3) Insulin effect persisted even when Golgi apparatus, which is involved in trafficking of newly synthesized Pept-1, was dismantled. 4) However, there was complete elimination of insulin effect by disruption of microtubules involved in trafficking of preformed Pept-1. 5) Finally, with insulin treatment, there was no change in Pept-1 gene expression, but the amount of Pept-1 protein in the apical membrane was increased. In conclusion, the results show that insulin, when it binds to its receptor, stimulates Gly-Gln uptake by Caco-2 cells by increasing the membrane population of Pept-1. The mechanism appears to be increased translocation of this transporter from a preformed cytoplasmic pool.

Peptide transporter in the rat small intestine: ultrastructural localization and the effect of starvation and administration of amino acids

Peptide transporter-1 is a H+/peptide cotransporter responsible for the uptake of small peptides and peptide-like drugs, and is present in the absorptive epithelial cells of the villi in the small intestine (duodenum, jejunum, and ileum). It has been localized to the apical microvillous plasma membrane of the absorptive epithelial cells of the rat small intestine using the immunogold electron microscopic technique. Digital image analysis of the jejunum revealed that the transporter protein was abundant at the tip of the villus and that the amount decreased from the tip of the villus to its base. The effect of dietary administration of amino acids and starvation on the expression of PepT1 in the jejunum was examined by immunoblotting and image analysis of immunofluorescence. Starvation markedly increased the amount of peptide transporter present, whereas dietary administration of amino acids reduced it. The gradient of the transporter protein along the crypt-villus axis was maintained under either condition. These observations show that it is specific to the microvillous plasma membrane and that its expression is regulated by the nutritional condition.

Regulation of the high-affinity H+/peptide cotransporter in renal LLC-PK1 cells

Di- and tripeptides and peptide mimetics such as beta-lactam antibiotics are efficiently reabsorbed from the tubular lumen by a high-affinity peptide transporter. We have recently identified and characterized this H+-coupled high-affinity peptide transport system in the porcine proximal tubular cell line LLC-PK1. Here we describe for the first time the regulation of the renal high-affinity peptide cotransporter at the cellular level. Uptake of 5 microM 3H-D-Phe-L-Ala into LLC-PK1 cells was significantly increased by lowering [Ca2+]in and decreased by increasing [Ca2+] in. Moreover, it was shown that the [Ca2+]in effects on peptide transport activity were dependent on Ca2+ entry from the extracellular site (e.g., via a store-regulated capacitative Ca2+ influx). Protein kinase C (PKC) was found to transmit the effects of [Ca2+]in on peptide transport. Although we demonstrate by pHin measurements that the PKC inhibitor staurosporine did decrease the transmembrane H+ gradient and consequently should have reduced the driving force for peptide uptake, the only effect on transport kinetics of 3H-D-Phe-L-Ala observed was a significant decrease in Km from 22.7+/-2.5 microM to 10.2+/-1.9 microM with no change in maximal velocity.

CHO/hPEPT1 cells overexpressing the human peptide transporter (hPEPT1) as an alternative in vitro model for peptidomimetic drugs

The present study characterized Chinese hamster ovary cells overexpressing a human intestinal peptide transporter, CHO/hPEPT1 cells, as an in vitro model for peptidomimetic drugs. The kinetic parameters of Gly-Sar uptake were determined in three different cell culture systems such as untransfected CHO cells (CHO-K1), transfected CHO cells (CHO/hPEPT1) and Caco-2 cells. Vmax in CHO/hPEPT1 cells was approximately 3-fold higher than those in Caco-2 cells and CHO-K1 cells, while Km values were similar in all cases. The uptake of beta-lactam antibiotics in CHO/hPEPT1 cells was three to twelve fold higher than that in CHO-K1 cells, indicating that CHO/hPEPT1 cells significantly enhanced the peptide transport activity. However, amino acid drugs also exhibited high cellular uptake in both CHO-K1 and CHO/hPEPT1 cells due to the high background level of amino acid transporters. Thus, cellular uptake study in CHO/hPEPT1 cells is not sensitive enough to distinguish the peptidyl drugs from amino acid drugs. The potential of CHO/hPEPT1 cells as an in vitro model for peptidomimetic drugs was also examined through the inhibition study on Gly-Sar uptake. Peptidomimetic drugs such as beta-lactam antibiotics and enalapril significantly inhibited Gly-Sar uptake whereas the nonpeptidyl compounds, L-dopa and alpha-methyldopa, did not compete with Gly-Sar for cellular uptake within the therapeutic concentrations. In conclusion, the present study demonstrates the further characterization of CHO/hPEPT1 cells as an uptake model as well as inhibition study and suggests their utility as an alternative in vitro model for drug candidates targeting the hPEPT1 transporter.

Transporters for ammonium, amino acids and peptides are expressed in pitchers of the carnivorous plant Nepenthes

Insect capture and digestion contribute substantially to the nitrogen budget of carnivorous plants. In Nepenthes, insect-derived nitrogenous compounds are imported from the pitcher fluid and transported throughout the plant via the vascular tissue to support growth. Import and distribution of nutrients may require transmembrane nitrogen transporters. Representatives of three classes of genes encoding transporters for the nitrogenous compounds ammonium, amino acids and peptides were identified in Nepenthes pitchers. The expression at the cellular level of an ammonium transporter gene, three amino acid transporter genes, and one peptide transporter gene were investigated in the insect trapping organs of Nepenthes. Expression of the ammonium transporter gene NaAMT1 was detected in the head cells of digestive glands in the lower part of the pitcher where NaAMT1 may function in ammonium uptake from the pitcher fluid. One amino acid transporter gene, NaAAP1, was expressed in bundle sheath cells surrounding the vascular tissue. To understand the locations where transmembrane transport could be required within the pitcher, symplasmic and apoplasmic continuity was probed using fluorescent dyes. Symplasmic connections were not found between cortical cells and vascular bundles. Therefore, the amino acid transporter encoded by NaAAP1 may be involved in transport of amino acids into the vascular tissue. In contrast, expression of the peptide transporter gene NaNTR1 was detected in phloem cells of the vascular tissue within pitchers. NaNTR1 may function in the export of nitrogen from the pitcher by loading peptides into the phloem.

Cellular and molecular mechanisms of dietary regulation on rat intestinal H+/Peptide transporter PepT1

BACKGROUND & AIMS

Dietary regulation is one of the most important factors of intestinal peptide transport. However, the cellular and molecular mechanisms of dietary regulation of the intestinal peptide transport system remain unknown. This study investigated the molecular mechanism of transcriptional activation of intestinal peptide transporter (PepT1) gene by the dietary protein. The promoter region of the rat PepT1 gene was isolated and characterized.

METHODS

PepT1 messenger RNA levels were determined by Northern blot analysis. In transient transfection experiments, effects of amino acid and dipeptide on luciferase activity were investigated.

RESULTS

The proximal promoter region of the rat PepT1 gene has a TATA-like box and a GC box sequence. The luciferase activities of the clone -351 RPT-LUC responded to particular amino acids (phenylalanine, arginine, and lysine) and dipeptides (Gly-Sar, Gly-Phe, Lys-Phe, and Asp-Lys). An AP-1 binding site and an amino acid-responsible element were present at -295 and -277 nucleotides relative to the transcription start site in this region.

CONCLUSIONS

These results suggest that the up-regulation of dipeptide transport activity by dietary protein is caused by transcriptional activation of the PepT1 gene by selective amino acids and dipeptides in the diet.