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

Gene expression in the human intestine and correlation with oral valacyclovir pharmacokinetic parameters

The transport of valacyclovir, the l-valyl ester of acyclovir, has been suggested to be mediated by several carrier-mediated pathways in cell culture and animal models. The role and importance of these transporters in modulating valacyclovir absorption in humans has not been determined, however. Recent advances in genomic technology have facilitated the rapid and simultaneous determination of global mRNA expression profiles for thousands of genes in tissue biopsies directly associated with the absorption process, thereby dramatically increasing the value of studies in humans. In this article, we describe correlations of pharmacokinetic parameters following oral valacyclovir or acyclovir administration with expression levels of intestinal genes in humans. Highly positive and significant correlations were observed with 4F2hc, an activator of cation-preferring amino acid transport systems, and human oligopeptide transporter (HPT1), an oligopeptide transporter expressed at higher levels in the human intestine compared with oligopeptide transporter (PEPT1). The validation of HPT1 microarray data with reverse transcription-polymerase chain reaction and the enhanced valacyclovir uptake in HeLa/HPT1 cells suggest that the role of HPT1 in transport of peptides and peptidomimetics drugs needs to be examined in more detail. The interrelation of 4F2hc and HPT1 in transport may be of interest. No significant correlations of valacyclovir pharmacokinetic parameters with PEPT1 and with organic cation or anion transporter expression levels were observed. The highly negative correlations observed with known efflux pumps such as MDR1 (P-glycoprotein) and MRP2 (cMOAT), as well as with the CYP450 IIIA subfamily may indicate that these proteins may regulate the cellular accumulation and metabolism of acyclovir.

Transmembrane segment 5 of the dipeptide transporter hPepT1 forms a part of the substrate translocation pathway

This study is the first systematic attempt to investigate the role of transmembrane segment 5 of hPepT1, the most conserved segment across different species, in forming a part of the aqueous substrate translocation pathway. We used cysteine-scanning mutagenesis in conjunction with the sulfhydryl-specific reagents, MTSEA and MTSET. Neither of these reagents reduced wild-type-hPepT1 transport activity in HEK293 cells and Xenopus oocytes. Twenty-one single cysteine mutations in hPepT1 were created by replacing each residue within TMS5 with a cysteine. HEK293 cells were then transfected with each mutated protein and the steady-state protein level, [3H]Gly-Sar uptake activity, and sensitivity to the MTS reagents were measured. S164C-, L168C-, G173C-, and I179C-hPepT1 were not expressed on the plasma membrane. Y167C-, N171C-, and S174C-hPepT1 showed </=25% Gly-Sar uptake when compared with WT-hPepT1. P182C-hPepT1 showed approximately 40% specific activity whereas all the remaining transporters, although still sensitive to single cysteine mutations, exhibited more than 50% specific activity when compared to WT-hPepT1. The activity of F166C-, L176C-, S177C-, T178C-, I180C-, T181C-, and P182C-hPepT1 was partially inhibited, while the activity of F163C- and I170C-hPepT1 was completely inhibited by 2.5mM MTSEA. F163C, I165C, F166C, A169C, I170C, S177C, T181C, and P182C were clearly accessible to 1mM MTSET. Overall, these results suggest that TMS5 lines the putative aqueous channel and is slightly tilted from the vertical axis of the channel, with the exofacial half forming a classical amphipathic alpha-helix and the cytoplasmic half being highly solvent accessible.

Intestinal absorption of cephalexin in diabetes mellitus model rats

We investigated the intestinal absorption and pharmacokinetics of cephalexin, as well as the intestinal H+/oligopeptide transporter PEPT1 mRNA and protein levels in type 1 and type 2 diabetic rats. Cephalexin disappearance from the duodenum loop was significantly lower in streptozotocin-induced type 1 diabetic rats and higher in hyperinsulinemic type 2 diabetic GK and Zucker-fa/fa (Zucker) rats, than in control rats. These results were speculated to be due to the enhancement of intestinal absorption of cephalexin in GK and Zucker rats. Intestinal PEPT1 mRNA levels were not significantly different between control and diabetic rats; however, the brush-border membrane vesicle PEPT1 protein levels were increased in GK and Zucker rats. After oral administration of cephalexin, plasma cephalexin concentrations and pharmacokinetic parameters, area under the concentration versus time curve from 0 to infinity, AUC(0-->infinity), and maximum plasma concentration, Cmax, in GK and Zucker rats were markedly higher than in control rats. From these findings, it is considered that intestinal absorption of drugs mediated by PEPT1 may be enhanced in hyperinsulinemic type 2 diabetes mellitus rats.

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.

Postnatal development of nutrient transport in the intestine of dogs

OBJECTIVE

To measure nutrient absorption by the intestine during postnatal development of dogs.

ANIMAL

110 Beagles ranging from neonatal to adult dogs.

PROCEDURE

Rates of absorption for sugars (glucose, galactose, and fructose), amino acids (aspartate, leucine, lysine, methionine, and proline), a dipeptide (glycyl-sarcosine), and linoleic acid by the proximal, mid, and distal regions of the small intestine were measured as functions of age and concentration (kinetics) by use of intact tissues and brush-border membrane vesicles. Absorption of octanoic acid by the proximal portion of the colon was measured in intact tissues.

RESULTS

Rates of carrier-mediated transport by intact tissues decreased from birth to adulthood for aldohexoses and most amino acids but not for fructose and aspartate. Kinetics and characteristics of absorption suggest that there were changes in the densities, types, and proportions of various carriers for sugars and amino acids. Saturable absorption of linoleic acid in the small intestine and octanoic acid in the proximal portion of the colon increased after weaning.

CONCLUSIONS AND CLINICAL RELEVANCE

Rates of absorption decreased between birth and adulthood for most nutrients. However, because of intestinal growth, absorption capacities of the entire small intestine remained constant for leucine and proline and increased for glucose, galactose, fructose, aspartate, and proline but were less than predicted from the increase in body weight. Although postnatal ontogeny of nutrient absorption was consistent with changes in the composition of the natural and commercial diets of growing dogs, rates of amino acid and peptide absorption were lower than expected.

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.

Delivery of peptide drugs to the brain by adenovirus-mediated heterologous expression of human oligopeptide transporter at the blood-brain barrier

The feasibility of using adenovirus-mediated human oligopeptide transporter (hPEPT1) gene transfer to achieve peptide drug delivery to the brain across the blood-brain barrier was tested by examining the accumulation of model peptides in a rat brain endothelial cell line (RBEC1) and rat brain after transduction with a recombinant adenovirus encoding hPEPT1-enhanced yellow fluorescent protein fusion gene (AdhPEPT1-EYFP). In vitro uptake of [(3)H]GlySar was determined in RBEC1 transduced with AdhPEPT1-EYFP. In vivo, the accumulation of cefadroxil in rat brain was evaluated after transduction of AdhPEPT1-EYFP. At pH 6.0, the uptake of [(3)H]GlySar by RBEC1 transduced with AdhPEPT1-EYFP was increased 4-fold compared with that of nontransduced cells. At pH 7.4, uptake of [(3)H]GlySar in AdhPEPT1-EYFP transduced RBEC1 was 1.5 times higher than that of nontransduced cells. Unlabeled glycylsarcosine (10 mM) reduced the uptake of [(3)H]GlySar to a level comparable with that of nontransduced cells. At 30 min after intravenous administration of cefadroxil to rats transduced with AdhPEPT1-EYFP at 3.2 x 10(9) plaque-forming units/rat by an in situ brain perfusion method, the brain-to-plasma concentration ratio (Kp) of cefadroxil was increased about 2 times compared with that of nontransduced or AdGFP (control vector)-transduced rats, although this was not statistically significant. In contrast, Kp of [(14)C]inulin, a marker for extracellular fluid space, remained unchanged after adenoviral transduction. In conclusion, our results suggest that adenovirus-mediated heterologous expression of hPEPT1 in vivo could be a useful approach to deliver oligopeptides to the brain.

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.

Mechanism of corneal permeation of L-valyl ester of acyclovir: targeting the oligopeptide transporter on the rabbit cornea

PURPOSE

To delineate mechanisms associated with the corneal transport of a L-valine prodrug of an antiviral agent, acyclovir.

METHOD

The permeability and enzymatic hydrolysis of L-Val-ACV were evaluated using freshly excised rabbit cornea. Transport mechanism across rabbit cornea was investigated through a competitive inhibition study of L-Val-ACV with other substrates of human peptide transporter (hPepT1).

RESULTS

L-Valyl ester of Acyclovir (L-Val-ACV) was approximately threefold more permeable across the intact rabbit cornea than acyclovir (ACV). Dipeptides, beta-lactam antibiotics, and angiotensin converting enzyme (ACE) inhibitors, strongly inhibited the transport of L-Val-ACV indicating that a carrier mediated transport system specific for peptides is primarily responsible for the corneal permeation of L-Val-ACV. L-Val-ACV transport was found to be saturable (Km = 2.26 +/- 0.34 mM, Jmax = 1.087 +/- 0.05 nmoles cm(-2) min(-1)), energy and pH dependent. CONCLUSIONS Functional evidence of an oligopeptide transport system present on the rabbit cornea has been established. The peptide transporter on the corneal epithelium may be targeted to improve the ocular bioavailability of poorly absorbed drugs.

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.

Inhibition of intestinal dipeptide transport by the neuropeptide VIP is an anti-absorptive effect via the VPAC1 receptor in a human enterocyte-like cell line (Caco-2)

1. Optimal dipeptide and peptidomimetic drug transport across the intestinal mucosal surface is dependent upon the co-operative functional activity of the di/tripeptide transporter hPepT1 and the Na(+)/H(+) exchanger NHE3. The ability of the anti-absorptive enteric neuropeptide VIP (vasoactive intestinal peptide) to modulate dipeptide uptake was determined using human intestinal (Caco-2) epithelial cell monolayers. 2. Uptake of glycylsarcosine (Gly-Sar) across the apical membrane of Caco-2 cell monolayers is inhibited by basolateral exposure to either VIP, pituitary adenylate cyclase-activating polypeptide (PACAP), or the VPAC(1) receptor agonist [(11,22,28)Ala]-VIP. Inhibition of Gly-Sar uptake is observed only in the presence of extracellular Na(+). Reverse-transcription polymerase chain reaction (RT-PCR) demonstrates that VPAC(1) mRNA is expressed in Caco-2 cells whereas VPAC(2) mRNA is not detected. 3. The VIP-induced inhibition of Gly-Sar uptake is abolished in the presence of the protein kinase A (PKA) inhibitor H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide.2HCl). 4. (22)Na(+) uptake across the apical membrane is inhibited by the selective NHE3 inhibitor S1611. Experiments with BCECF [2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein]-loaded Caco-2 cells demonstrate that VIP reduces the NHE3-dependent recovery of intracellular pH (pH(i)) after dipeptide-induced acidification. Western blot of Caco-2 cell protein demonstrates expression of the NHE regulatory factor NHERF1 (expression of which is thought to be required for PKA-mediated inhibition of NHE3). 5. VIP has no effect on Gly-Sar uptake in the presence of S1611 suggesting that VIP and S1611 both modulate dipeptide uptake via the same mechanism. 6. These observations demonstrate that VIP (and PACAP) modulate activity of the H(+)/dipeptide transporter hPepT1 in a Na(+)-dependent manner consistent with the modulation being indirect through inhibition of NHE3.

Characterisation of intestinal peptide transporter of the Antarctic haemoglobinless teleost Chionodraco hamatus

H(+)/peptide cotransport was studied in brush-border membrane vesicles (BBMV) from the intestine of the haemoglobinless Antarctic teleost Chionodraco hamatus by monitoring peptide-dependent intravesicular acidification with the pH-sensitive dye Acridine Orange. Diethylpyrocarbonate-inhibited intravesicular acidification was specifically achieved in the presence of extravesicular glycyl-L-proline (Gly-L-Pro) as well as of glycyl-L-alanine (Gly-L-Ala) and D-phenylalanyl-L-alanine (D-Phe-L-Ala). H(+)/Gly-L-Pro cotransport displayed saturable kinetics, involving a single carrier system with an apparent substrate affinity (K(m,app)) of 0.806+/-0.161 mmol l(-1). Using degenerated primers from eel and human (PepT1) transporter sequence, a reverse transcription-polymerase chain reaction (RT-PCR) signal was detected in C. hamatus intestine. RT-PCR paralleled kinetic analysis, confirming the hypothesis of the existence of a PepT1-type transport system in the brush-border membranes of icefish intestine. Functional expression of H(+)/peptide cotransport was successfully performed in Xenopus laevis oocytes after injection of poly(A)(+) RNA (mRNA) isolated from icefish intestinal mucosa. Injection of mRNA stimulated D-Phe-L-Ala uptake in a dose-dependent manner and an excess of glycyl-L-glutamine inhibited this transport. H(+)/peptide cotransport in the Antarctic teleost BBMV exhibited a marked difference in temperature optimum with respect to the temperate teleost Anguilla anguilla, the maximal activity rate occurring at approximately 0 degrees C for the former and 25 degrees C for the latter. Temperature dependence of icefish and eel intestinal mRNA-stimulated uptake in the heterologous system (oocytes) was comparable.

Enhanced Permeability of Phenylalanyl-glycine (Phe-Gly) Across the Intestinal Membranes by Chemical Modification with Various Fatty Acids

We synthesized four novel lipophilic derivatives of phenylalanyl-glycine (Phe-Gly), C4-Phe-Gly, Phe-Gly-C4, C6-Phe-Gly and C8-Phe-Gly by chemical modification with butyric acid (C4), caproic acid (C6) and octanoic acid (C8). The effect of the acylation on the stability, permeability and accumulation of Phe-Gly in the intestine was investigated by in vitro studies. The stability of Phe-Gly in homogenates of duodenal and colonic membranes was low, but was significantly improved by the acylation except for Phe-Gly-C4. In the transport studies, a modified Ussing chamber was used for the intestinal permeability experiments with Phe-Gly and its acyl derivatives. The permeability of native Phe-Gly and Phe-Gly-C4 across the intestinal membrane was not observed during the transport studies. However, the permeability of Phe-Gly was much improved by chemical modification with various fatty acids to its N-terminal portion. The permeability of acyl-Phe-Gly derivatives across the intestinal membrane decreased with increasing the chain length of fatty acids. In addition, the intestinal tissue accumulation of acyl-Phe-Gly derivatives at the end of the transport studies was much higher than that of native Phe-Gly. The intestinal tissue accumulation of acyl-Phe-Gly in the duodenum increased as the chain length of fatty acids increased. Furthermore, intestinal permeability of C4-Phe-Gly was slightly inhibited in the presence of 5 mM ceftibuten and was significantly reduced under low temperature condition. We observed a directional difference in the transport of C4-Phe-Gly (the mucosal to serosal transport of C4-Phe-Gly was higher than its serosal to mucosal transport) suggesting that C4-Phe-Gly might be transported by a carrier-mediated process as well as other dipeptides. These findings indicate that acylation might be useful approach to enhance the transport of Phe-Gly, a model dipeptide, transported by a carrier-mediated process.

Expression of the peptide transporter hPepT1 in human colon: a potential route for colonic protein nitrogen and drug absorption

Substrates of the proton-coupled peptide transporter, hPepT1, include dietary di- and tripeptides plus therapeutically important drugs such as the beta-lactam antibiotics and angiotensin-converting enzyme inhibitors. Expression and function of hPepT1 in the small bowel is well established. We have compared levels of hPepT1 mRNA expression in regions of human gut by RT-PCR methods and examined the expression of hPepT1 in normal human colon using an anti-hPepT1 antipeptide antibody. hPepT1 mRNA was expressed in the large intestine, although at lower levels than in the small intestine. Quantitatively, expression in ileum was 4.6-fold greater than in sigmoid colon. Immunoreactive hPepT1 was detected in human colon at lower levels than in ileum. The pattern of expression differed between the two tissues: whilst expression in the ileum was localised to the apical enterocyte membrane along the length of the crypt-villus axis, expression in the colonocyte was detected at the apical membrane towards the luminal surface but predominantly at the basal membrane towards the base of the crypt. We conclude that distal regions of the bowel express hPepT1, which may provide a mechanism for colonic protein-nitrogen absorption and for absorption of therapeutically important peptidomimetic drugs.

Transporters and xenobiotic disposition

Metabolism alone does not adequately account for the observed intersubject variability in drug disposition or response. Carrier-mediated processes, or transporters, are increasingly recognized to be importantly involved in drug absorption, distribution, and excretion. Thus for many drugs, transport and metabolism must be considered together to better predict drug disposition in vivo. Accordingly, this review will outline relevant background information regarding drug transporters and the role of such transporters in the drug disposition process.

[Biopharmaceutical studies on molecular mechanisms of membrane transport]

By incorporating the transporter-mediated or receptor-mediated transport process in physiologically based pharmacokinetic models, we succeeded in the quantitative prediction of plasma and tissue concentrations of beta-lactam antibiotics, insulin, pentazocine, quinolone antibacterial agents, and inaperizone and digoxin. The author's research on transporter-mediated pharmacokinetics focuses on the molecular and functional characteristics of drug transporters such as oligopeptide transporter, monocarboxylic acid transporter, anion antiporter, organic anion transporters, organic cation/carnitine transporters (OCTNs), and the ATP-binding cassette transporters P-glycoprotein and MRP2. We have successfully demonstrated that these transporters play important roles in the influxes and/or effluxes of drugs in intestinal and renal epithelial cells, hepatocytes, and brain capillary endothelial cells that form the blood-brain barrier. In the systemic carnitine deficiency (SCD) phenotype mouse model, juvenile visceral steatosis (jvs) mouse, a mutation in the OCTN2 gene was found. Furthermore, several types of mutation in human SCD patients were found, demonstrating that OCTN2 is a physiologically important carnitine transporter. Interestingly, OCTNs transport carnitine in a sodium-dependent manner and various cationic drugs transport it in a sodium-independent manner. OCTNs are thought to be multifunctional transporters for the uptake of carnitine into tissue cells and for the elimination of intracellular organic cationic drugs.

PepT1-mediated fMLP transport induces intestinal inflammation in vivo

In the present study, the effect of H(+)/peptide transporter (PepT1)-mediated N-formylmethionyl-leucyl-phenylalanine (fMLP) transport on inflammation in vivo in the rat small intestine, which expresses high PepT1 levels, and in the rat colon, which does not express PepT1, were investigated using myeloperoxidase (MPO) activity and histological analysis. We found that 10 microM fMLP perfusion in the jejunum for 4 h significantly increased MPO activity and altered the architecture of jejunal villi. In contrast, 10 microM fMLP perfusion in the colon for 4 h did not induce any inflammation. In addition, we have shown that 50 mM Gly-Gly alone did not affect basal MPO activity but completely inhibited the MPO activity induced by 10 microM fMLP in the jejunum. Together, these experiments demonstrate that 1) the differential expression of PepT1 between the small intestine and the colon plays an important role in epithelial-neutrophil interactions and 2) the inhibition of fMLP uptake by jejunal epithelial cells (expressing PepT1) reduces the neutrophil ability to move across the epithelium, in agreement with our previously published in vitro study. This report constitutes the first in vivo study showing the implication of a membrane transporter (PepT1) in intestinal inflammation.

Mutation screening of two candidate genes from 13q32 in families affected with Bipolar disorder: human peptide transporter (SLC15A1) and human glypican5 (GPC5)

BACKGROUND

Multiple candidate regions as sites for Schizophrenia and Bipolar susceptibility genes have been reported, suggesting heterogeneity of susceptibility genes or oligogenic inheritance. Linkage analysis has suggested chromosome 13q32 as one of the regions with evidence of linkage to Schizophrenia and, separately, to Bipolar disorder (BP). SLC15A1 and GPC5 are two of the candidate genes within an approximately 10-cM region of linkage on chromosome 13q32. In order to identify a possible role for these candidates as susceptibility genes, we performed mutation screening on the coding regions of these two genes in 7 families (n-20) affected with Bipolar disorder showing linkage to 13q32.

RESULTS

Genomic organization revealed 23 exons in SLC15A1 and 8 exons in GPC5 gene respectively. Sequencing of the exons did not reveal mutations in the GPC5 gene in the 7 families affected with BP. Two polymorphic variants were discovered in the SLC15A1 gene. One was T to C substitution in the third position of codon encoding alanine at 1403 position of mRNA in exon 17, and the other was A to G substitution in the untranslated region at position 2242 of mRNA in exon 23.

CONCLUSIONS

Mutation analysis of 2 candidate genes for Bipolar disorder on chromosome 13q32 did not identify any potentially causative mutations within the coding regions or splice junctions of the SLC15A1 or GPC5 genes in 7 families showing linkage to 13q32. Further studies of the regulatory regions are needed to completely exclude these genes as causative for Bipolar disorder.

Theoretical predictions of drug absorption in drug discovery and development

The clinical development of new drugs is often terminated because of unfavourable pharmacokinetic properties such as poor intestinal absorption after oral administration. Intestinal permeability and solubility are two of the most important factors that determine the absorption properties of a compound. Efficient and reliable computational models that predict these properties as early as possible in drug discovery and development are therefore desirable. In this review, we first discuss the implementation of predictive models of intestinal drug permeability and solubility in drug discovery and development. Secondly, we discuss the mechanisms of intestinal drug permeability and computational methods that can be used to predict it. We then discuss factors influencing drug solubility and models for predicting it. We finally speculate that once these and other predictive computational models are implemented in drug discovery and development, these processes will become much more effective. Further, an increased fraction of drug candidates that are less likely to fail during clinical development will be selected.

Monitoring intracellular pH changes in response to osmotic stress and membrane transport activity using 5-chloromethylfluorescein

Intracellular free H+ concentration (pHi) responds to numerous extracellular stimuli. The use of fluorescent indicator dyes to measure pHi is strongly influenced by the ability of target cells to retain activated dye within the cytoplasmic compartment. Here, 3 pH-sensitive indicator dye - acetoxymethyl (AM) esters of SNARF-1 and BCECF, and the thiol-reactive 5-chloromethyfluorescein (CMFDA) - were examined for monitoring pHi. The stability of pH measurements was strongly affected by temperature, cell type, indicator dye, and use of transport inhibitors to prevent dye export. Cellular retention of CMFDA, which forms covalent complexes, was sufficient to permit monitoring of transient pHi changes over extended time periods in a multi-well plate assay format. In human embryonic kidney (HEK293) and Chinese hamster ovary (CHO) cells, increasing osmotic pressure caused a significant rise in pHi. In contrast, activation of native or transfected beta-adrenergic, cholinergic, and d and m opioid receptors did not measurably affect pHi in HEK293 cells. Decreases in pHi were observed in CHO cells expressing the human H+/peptide transporter PEPT1 upon addition of dipeptide substrates. The use of CMFDA in multi-well formats should facilitate study of osmotic and transport activity and screening for drugs that affect pHi.

PepT1 mRNA expression is induced by starvation and its level correlates with absorptive transport of cefadroxil longitudinally in the rat intestine

PURPOSE

To establish how closely intestinal transport activity for beta-lactam antibiotics is correlated with PepT1 expression, absolute expression level of PepT1 mRNA and transport activity were determined longitudinally in the small intestine of fed and starved rats.

METHODS

For evaluation of absolute expression levels of PepTl mRNA, quantitative RT-PCR by LightCycler was used. The transport function was determined by quantifying the absorptive transport of cefadroxil across intestinal tissue sheets in a Ussing chamber.

RESULTS

PepT1 mRNA expression was highest at the lower region and lowest at the upper region in the fed rats. The value of PepT1 was about 1/5-1/6 of that of GAPDH. The expression level in the starved rats was increased in all segments, but more profoundly in the upper region. Cefadroxil transport across intestinal tissue was higher in the lower region and lower in the upper region in fed rats, and increased in the upper region in starved rats. An excellent correlation was observed between expression levels and the permeability coefficients (r2 = 0.859, p < 0.05).

CONCLUSIONS

The intestinal transport of cefadroxil is directly proportional to PepT1 expression, suggesting that the PepT1 expression level in the rat small intestine is the major determinant of the absorption of peptide-like compounds.

Transport and uptake of nateglinide in Caco-2 cells and its inhibitory effect on human monocarboxylate transporter MCT1

1 Nateglinide, a novel oral hypoglycemic agent, rapidly reaches the maximum serum concentration after oral administration, suggesting that it is rapidly absorbed in the gastrointestinal tract. The aim of this work is to clarify the intestinal absorption mechanism of nateglinide by means of in vitro studies. 2 We examined the transcellular transport and the apical uptake of [(14)C]nateglinide in a human colon carcinoma cell line (Caco-2). We also examined whether nateglinide is transported via monocarboxylate transport-1 (MCT1) by means of an uptake study using MCT1-expressing Xenopus laevis oocytes. 3 In Caco-2 cells, the transcellular transport of [(14)C]nateglinide from the apical to basolateral side was greater than that in the opposite direction. The uptake of [(14)C]nateglinide from the apical side was concentration-dependent, H(+)-dependent, and Na(+)-independent. Kinetic analysis revealed that the Kt and Jmax values of the initial uptake rate of [(14)C]nateglinide were 448 micro M and 43.2 nmol mg protein(-1) 5 min(-1), respectively. Various monocarboxylates, including salicylic acid and valproic acid, and glibenclamide significantly inhibited the uptake of [(14)C]nateglinide. 4 The uptake study using MCT1-expressing oocytes showed that nateglinide inhibits the MCT1-mediated uptake of [(14)C]L-lactic acid, though nateglinide itself is not transported by MCT1. 5 Taken together, these results suggest that the uptake of nateglinide from the apical membranes of Caco-2 cells is, at least in part, mediated by a proton-dependent transport system(s) distinct from MCT1.

Mammalian peptide transporters as targets for drug delivery

Peptide transporters are integral plasma membrane proteins that mediate the cellular uptake of dipeptides and tripeptides in addition to a variety of peptidomimetics. The carriers, which occur predominantly in the brush-border membranes of epithelial cells of the small intestine, lung, choroid plexus and kidney, contribute to absorption, distribution and elimination of their substrates. The cellular uptake of peptides and peptidomimetics involves the cotransport of protons down an inwardly directed, electrochemical proton gradient that provides the driving force and causes the electrogenicity of the translocation step. Peptide transporters represent excellent targets for the delivery of pharmacologically active compounds because their substrate-binding site can accommodate a wide range of molecules of differing size, hydrophobicity and charge.

Mucosal tissue injury in cancer therapy. More than muscositis and mouthwash

PURPOSE

The purpose of this article is as follows: 1) to describe the characteristics and scope of mucosal tissue injury associated with cancer treatment; 2) to discuss recent advances in related basic and clinical science; and 3) to articulate research needs and opportunities to be addressed through collaborative interdisciplinary research.

OVERVIEW

Mucosal tissue injury is both a direct and indirect consequence of cancer therapy, with manifestations that include damage and a number of other potentially serious sequelae. Current research in mucosal tissue injury is focused on the biology, immunology, and genetics of mucosal injury; clinical problems; assessment and management; and processes and outcomes of care.

CLINICAL IMPLICATIONS

Results from these various areas of research enhance the understanding of the mechanisms of mucosal tissue injury, provide direction for the development of policy and for clinical practice, and help to define research needs and opportunities. Future research on the complex process of mucosal tissue injury will be interdisciplinary and will cross the boundaries among basic, translational, and clinical science.

Enhanced delivery of drugs to the liver by adenovirus-mediated heterologous expression of the human oligopeptide transporter PEPT1

To explore the feasibility of drug delivery to the liver by the use of adenovirus-mediated human oligopeptide transporter (hPEPT1) gene transfer, we examined the accumulation of L-[(3)H]carnosine in the hepatoma cell line (HepG2 and WIFB9) and mouse liver. We constructed a recombinant adenovirus encoding hPEPT1-enhanced yellow fluorescent protein (EYFP) fusion gene (AdhPEPT1-EYFP). In vitro uptake of L-[(3)H]carnosine was determined in HepG2 and WIFB9 cells transduced with AdhPEPT1-EYFP. In vivo, the accumulation of L-[(3)H]carnosine in mouse liver was evaluated after transduction of AdhPEPT1-EYFP. At pH 6.0, the uptake of L-[(3)H]carnosine by HepG2 and WIFB9 cells transduced with AdhPEPT1-EYFP was increased 15- and 2-fold, respectively, compared with the cells without transduction. At pH 7.4, uptake of L-[(3)H]carnosine in AdhPEPT1-EYFP transduced HepG2 cells was 3 times greater than that of nontransduced cells. In the presence of carnosine or glycylsarcosine as an inhibitor at 20 mM, the uptake of L-[(3)H]carnosine was reduced to a level comparable to that of nontransduced cells. At 30 min after intravenous administration of L-[(3)H]carnosine to mice transduced with AdhPEPT1-EYFP at 1 x 10(10) plaque-forming units/mouse, the tissue-to-plasma concentration ratio (K(p)) of L-[(3)H]carnosine in the liver was significantly increased to 7 times that of nontransduced mice. In contrast, the K(p) value of [(14)C]inulin, a marker for extracellular fluid space, remained unchanged after adenoviral transduction suggesting minimal pathological damage of tissues. hPEPT1-EYFP was localized at both the basolateral and apical membranes in HepG2 cells, WIFB9 cells, and mouse liver. In conclusion, our results suggest that delivery of oligopeptide to the liver by adenovirus-mediated heterologous expression of hPEPT1 in vivo is feasible.

Transport of peptidomimetic drugs by the intestinal Di/tri-peptide transporter, PepT1

The apical membrane of small intestinal enterocytes possess an uptake system for di- and tripeptides. The physiological function of the system is to transport small peptides resulting from digestion of dietary protein. Moreover, due to the broad substrate specificity of the system, it is also capable of transporting a number of orally administered peptidomimetic drugs. Absorbed peptides may be hydrolysed in the cells due to the high peptidase activity present in the cytosol. Peptidomimetic drugs may, if resistant to the cellular enzyme activity, pass the basolateral membrane via a basolateral peptide transport mechanism and enter the systemic circulation. As the number of new peptide and peptidomimetic drugs are rapidly increasing, the peptide transport system has gained increasing attention as a possible drug delivery system for small peptides and peptide-like compounds. In this paper we give an updated introduction to the transport system and discuss the substrate characteristics of the di/tri-peptide transporter system with special emphasis on chemically modified substrates and prodrugs.

H/dipeptide absorption across the human intestinal epithelium is controlled indirectly via a functional Na/H exchanger

BACKGROUND & AIMS

For optimal nutrient absorption to occur, the enterocyte must express a range of specialist ion-driven carrier proteins that function cooperatively in a linked and mutually dependent fashion. Thus, absorption via the human intestinal H(+)-coupled di/tripeptide transporter (hPepT1) is dependent on maintenance of the trans-apical driving force (the H(+)-electrochemical gradient) established, in part, by brush-border Na(+)/H(+) exchanger (NHE3) activity. This study aimed to examine whether physiologic regulation of NHE3 activity can limit hPepT1 capacity and, therefore, protein absorption after a meal.

METHODS

hPepT1 and NHE3 activities were determined in intact human intestinal epithelial Caco-2 cell monolayers by measurements of [(14)C]glycylsarcosine transport and uptake, (22)Na(+)-influx, H(+)-influx, and H(+)-efflux. Expression of NHE regulatory factors was determined by reverse-transcriptase polymerase chain reaction.

RESULTS

Optimal dipeptide transport was observed in the presence of a transapical pH gradient and extracellular Na(+). At apical pH 6.5, and only in Na(+)-containing media, protein kinase A activation (by forskolin or vasoactive intestinal peptide) or selective NHE3 inhibition (by S1611) reduced transepithelial dipeptide transport and cellular accumulation by a reduction in the capacity (without effect on affinity) of dipeptide uptake.

CONCLUSIONS

Protein kinase A-mediated modulation of intestinal dipeptide absorption is indirect via effects on the apical Na(+)/H(+) exchanger.

Distribution of the H+/peptide transporter PepT1 in human intestine: up-regulated expression in the colonic mucosa of patients with short-bowel syndrome

BACKGROUND

Intestinal adaptation after massive bowel resection in animal models is characterized by increased gut-mucosal growth and expression of nutrient transporters. Few data about these indexes exist in humans with short-bowel syndrome (SBS).

OBJECTIVE

The objective was to compare small-bowel and colonic mucosal growth and expression of the peptide transporter PepT1 in adults with or without SBS.

DESIGN

Mucosal biopsy specimens were obtained from the small bowel and colon of 33 control subjects with intact intestine and from 13 SBS patients dependent on parenteral nutrition because of chronic malabsorption. Gut-mucosal crypt depth, villus height, and villus width were measured, and expression of PepT1 was determined by Northern blotting, in situ hybridization, and immunohistochemistry.

RESULTS

The indexes of small-bowel and colonic mucosal growth were not significantly different between the 2 groups. PepT1 expression was high in the apical region of duodenal, jejunal, and ileal villus epithelial cells; low in absorptive colonocytes; and not significantly different in the distal small intestine of the 2 groups. However, the abundance of PepT1 mRNA in the colon of SBS patients was more than 5-fold that in control subjects (P < 0.01).

CONCLUSIONS

Gut adaptation in SBS patients does not appear to involve an increase in gut-mucosal crypt depth or villus size. PepT1 is abundant along the small-bowel brush border in humans; expression in the colon indicates that the large intestine has a mechanism for luminal di- and tripeptide transport. Up-regulation of colonic PepT1 in SBS may adaptively improve accrual of malabsorbed di- and tripeptides, independent of changes in the mucosal surface area.

Peptide transport in the mammary gland: expression and distribution of PEPT2 mRNA and protein

The lactating mammary gland utilizes free plasma amino acids as well as those derived by hydrolysis from circulating short-chain peptides for protein synthesis. Apart from the major route of amino acid nitrogen delivery to the gland by the various transporters for free amino acids, it has been suggested that dipeptides may also be taken up in intact form to serve as a source of amino acids. The identification of peptide transporters in the mammary gland may therefore provide new insights into protein metabolism and secretion by the gland. The expression and distribution of the high-affinity type proton-coupled peptide transporter PEPT2 were investigated in rat lactating mammary gland as well as in human epithelial cells derived from breast milk. By use of RT-PCR, PEPT2 mRNA was detected in rat mammary gland extracts and human milk epithelial cells. The expression pattern of PEPT2 mRNA revealed a localization in epithelial cells of ducts and glands by nonisotopic high resolution in situ hybridization. In addition, immunohistochemistry was carried out and showed transporter immunoreactivity in the same epithelial cells of the glands and ducts. In addition, two-electrode voltage clamp recordings using PEPT2-expressing Xenopus laevis oocytes demonstrated positive inward currents induced by selected dipeptides that may play a role in aminonitrogen handling in mammalian mammary gland. Taken together, these data suggest that PEPT2 is expressed in mammary gland epithelia, in which it may contribute to the reuptake of short-chain peptides derived from hydrolysis of milk proteins secreted into the lumen. Whereas PEPT2 also transports a variety of drugs, such as selected beta-lactams, angiotensin-converting enzyme inhibitors, and antiviral and anticancer metabolites, their efficient reabsorption via PEPT2 may reduce the burden of xenobiotics in milk.

Thyroid hormone regulates the activity and expression of the peptide transporter PEPT1 in Caco-2 cells

An oligopeptide transporter (PEPT1) in the small intestine plays an important role in the absorption of small peptides and peptide-like drugs. We examined the effect of thyroid hormone 3,5,3'-L-triiodothyronine (T(3)) on the activity and expression of PEPT1 in human intestinal Caco-2 cells. Treatment of Caco-2 cells with T(3) inhibited [(14)C]glycylsarcosine uptake in a time- and dose-dependent manner. [(14)C]glycylsarcosine uptake was reduced by pretreatment of the cells with 100 nM T(3) for 4 days (67% of control value), whereas methyl-alpha-D-[U-(14)C]glucopyranoside and [(3)H]threonine uptake were not decreased. Kinetic analysis showed that T(3) treatment significantly decreased the maximum uptake (V(max)) value for [(14)C]glycylsarcosine uptake but had no effect on the K(m) value. Moreover, T(3) treatment caused a significant decrease in the amount of PEPT1 mRNA (25% of the control). Western blotting indicated that the amount of PEPT1 protein in the apical membrane was decreased (70% of the control). These findings indicate that T(3) treatment inhibits the uptake of [(14)C]glycylsarcosine by decreasing the transcription and/or stability of PEPT1 mRNA.

Molecular cloning and functional expression of a chicken intestinal peptide transporter (cPepT1) in Xenopus oocytes and Chinese hamster ovary cells

To study peptide absorption in chickens, an intestinal peptide transporter cDNA (cPepT1) was isolated from a chicken duodenal cDNA library. The cDNA was 2914 bp long and encoded a protein of 714 amino acid residues with an estimated molecular size of 79.3 kDa and an isoelectric point of 7.48. cPepT1 protein is similar60% identical to PepT1 from rabbits, humans, mice, rats and sheep. Sixteen dipeptides, three tripeptides and four tetrapeptides that contained the essential amino acids Met, Lys and(or) Trp were used for functional analysis of cPepT1 in Xenopus oocytes and Chinese hamster ovary cells. For most di- and tripeptides tested, the substrate affinities were in the micromolar range, indicating that cPepT1 has high affinity for these peptides. Lys-Lys and Lys-Trp-Lys were exceptions, with substrate affinities in the millimolar range. Neither free amino acids nor tetrapeptides were transported by cPepT1. Northern blot analysis using a full-length cPepT1 cDNA as the probe demonstrated that cPepT1 is expressed strongly in the duodenum, jejunum and ileum, and at lower levels in kidney and ceca. The present study demonstrated for the first time the presence and functional characteristics of a peptide transport system from an avian species.

Mucosal drug delivery

This review focuses on epithelial drug transport mechanisms in mucosal drug delivery: the final step of a four-part process. Reference is made to the mucosae lining the oral cavity and the gastrointestinal tract, the two mucosae most often succumbing to the side effects of cytotoxic chemotherapeutic drugs. This review will be devoted to carrier-mediated transport, particularly as it relates to the intestinal dipeptide transporter PepT1. This transporter protein appears to be enriched in tumor epithelial cells, to be rather robust to the cytotoxic effects of chemotherapeutic drugs, and to lend itself to the molecular engineering of drugs that target this transporter in tumor epithelial cells. In contrast to the gastrointestinal tract, much less is known about the type and capacity of drug transport processes in the buccal epithelial cells and about how these processes may be altered in disease state (including cancer) and be manipulated pharmaceutically to optimize drug absorption.

Transporter-mediated Drug Interactions

Since 1994, researchers have isolated various genes encoding transporter proteins involved in drug uptake into and efflux from tissues that play key roles in the absorption, distribution and secretion of drugs in animals and humans. The pharmacokinetic characteristics of drugs that are substrates for these transporters are expected to be influenced by coadministered drugs that work as inhibitors or enhancers of the transporter function. This review deals with recent progress in molecular and functional research on drug transporters, and then with transporter-mediated drug interactions in absorption and secretion from the intestine, secretion from the kidney and liver, and transport across the blood-brain barrier in humans. Although the participation of the particular transporters in observed drug-drug interactions can be difficult to confirm in humans, this review focuses mainly on pharmacokinetic interactions of clinically important drugs.

Comparison of bidirectional cephalexin transport across MDCK and caco-2 cell monolayers: interactions with peptide transporters

PURPOSE

Bidirectional transport studies were conducted to determine whether Madin-Darby canine kidney (MDCK) cell monolayers could be used as an alternative to the traditional Caco-2 assay as a fast-growing in vitro model of peptide transport.

METHODS

Transport of cephalexin and glycylsarcosine across MDCK and Caco-2 cell monolayers was quantified using LC-LC/MS. Glycylsarcosine, p-aminohippuric acid (PAH), and tetraethylammonium chloride (TEA) were tested as inhibitors of cephalexin transport.

RESULTS

The ratio of apparent cephalexin permeabilities (apical to basolateral/basolateral to apical) obtained from MDCK monolayers was almost 5-fold greater than that obtained from Caco-2 monolayers. The opposite trend was observed for glycylsarcosine. When MDCK monolayers were used, glycylsarcosine reduced the cephalexin/apparent permeability ratio almost 90%. PAH and TEA did not inhibit cephalexin transport across MDCK or Caco-2 cell monolayers.

CONCLUSION

MDCK cell monolayers may be a promising, fast-growing alternative to Caco-2 cells for identifying peptide transporter substrates. However, differences in the apical-to-basolateral transport of cephalexin and glycylsarcosine suggest that the basolateral transport mechanisms for these compounds are different in the two cell lines. Additionally, because the activity of the peptide transporter in MDCK cells was low, scaling factors may be required when using this cell line to predict in vivo drug absorption.

Distribution and function of the peptide transporter PEPT2 in normal and cystic fibrosis human lung

BACKGROUND

Aerosol administration of peptide based drugs has an important role in the treatment of various pulmonary and systemic diseases. The characterisation of pulmonary peptide transport pathways can lead to new strategies in aerosol drug treatment.

METHODS

Immunohistochemistry and ex vivo uptake studies were established to assess the distribution and activity of the beta-lactam transporting high affinity proton coupled peptide transporter PEPT2 in normal and cystic fibrosis human airway tissue.

RESULTS

PEPT2 immunoreactivity in normal human airways was localised to cells of the tracheal and bronchial epithelium and the endothelium of small vessels. In peripheral lung immunoreactivity was restricted to type II pneumocytes. In sections of cystic fibrosis lung a similar pattern of distribution was obtained with signals localised to endothelial cells, airway epithelium, and type II pneumocytes. Functional ex vivo uptake studies with fresh lung specimens led to an uptake of the fluorophore conjugated dipeptide derivative D-Ala-L-Lys-AMCA into bronchial epithelial cells and type II pneumocytes. This uptake was competitively inhibited by dipeptides and cephalosporins but not ACE inhibitors, indicating a substrate specificity as described for PEPT2.

CONCLUSIONS

These findings provide evidence for the expression and function of the peptide transporter PEPT2 in the normal and cystic fibrosis human respiratory tract and suggest that PEPT2 is likely to play a role in the transport of pulmonary peptides and peptidomimetics.

Characterization and regulation of a cloned ovine gastrointestinal peptide transporter (oPepT1) expressed in a mammalian cell line

To investigate the kinetics of peptide transport by the peptide transporter, PepT1, Chinese hamster ovary cells were transfected with an expression vector containing our cloned ovine PepT1 cDNA. Transport was assessed by uptake studies using the radiolabeled dipeptide, [(3)H]-Gly-Sar. Expression of oPepT1 was detected at 8-24 h post-transfection with an optimal time of 16-24 h. Uptake of Gly-Sar by oPepT1 was pH-dependent with an optimal pH of 5.5-6.0, concentration-dependent and saturable with an apparent K(m) value of 1.0 +/- 0.1 mmol/L and a maximum velocity of 14.3 +/- 0.4 nmol/(mg protein x 40 min). Competition studies with nonradiolabeled peptides and [(3)H]-Gly-Sar showed that all di- and tripeptides inhibited uptake of [(3)H]-Gly-Sar. In addition, three tetrapeptides (Met-Gly-Met-Met, Pro-Phe-Gly-Lys, and Val-Gly-Ser-Glu) also inhibited [(3)H]-Gly-Sar uptake. There was no inhibition of [(3)H]-Gly-Sar uptake detected in the presence of nonradiolabeled free amino acids. Treatment of the cells with staurosporine, an inhibitor of protein kinase C (PKC) significantly increased the transport system. This increase was specific and could be blocked if treatment was done in the presence of phorbol 12-myristate-13-acetate (PMA), an activator of PKC. The staurosporine- and PMA-induced changes in peptide transport activity were not affected by cotreatment with cycloheximide. These data demonstrate that the transport of peptide substrates by oPepT1 in transfected mammalian cells is similar to that in microinjected Xenopus oocytes and that PKC phosphorylation plays a regulatory role in oPepT1 function.

Cloning and functional characterization of a new subtype of the amino acid transport system N

We have cloned a new subtype of the amino acid transport system N2 (SN2 or second subtype of system N) from rat brain. Rat SN2 consists of 471 amino acids and belongs to the recently identified glutamine transporter gene family that consists of system N and system A. Rat SN2 exhibits 63% identity with rat SN1. It also shows considerable sequence identity (50-56%) with the members of the amino acid transporter A subfamily. In the rat, SN2 mRNA is most abundant in the liver but is detectable in the brain, lung, stomach, kidney, testis, and spleen. When expressed in Xenopus laevis oocytes and in mammalian cells, rat SN2 mediates Na(+)-dependent transport of several neutral amino acids, including glycine, asparagine, alanine, serine, glutamine, and histidine. The transport process is electrogenic, Li(+) tolerant, and pH sensitive. The transport mechanism involves the influx of Na(+) and amino acids coupled to the efflux of H(+), resulting in intracellular alkalization. Proline, alpha-(methylamino)isobutyric acid, and anionic and cationic amino acids are not recognized by rat SN2.

PepT1-mediated epithelial transport of dipeptides and cephalexin is enhanced by luminal leptin in the small intestine

Dietary proteins are mostly absorbed as di- and tripeptides by the intestinal proton-dependent transporter PepT1. We have examined the effects of leptin on PepT1 function in rat jejunum and in monolayers of the human enterocyte-like 2 cell Caco-2. Leptin is produced by the stomach and secreted in the gut lumen. We show here that PepT1 and leptin receptors are expressed in Caco-2 and rat intestinal mucosal cells. Apical (but not basolateral) leptin increased Caco-2 cell transport of cephalexin (CFX) and glycylsarcosine (Gly-Sar), an effect that was associated with increased Gly-Sar uptake, increased membrane PepT1 protein, decreased intracellular PepT1 content, and no change in PepT1 mRNA levels. The maximal velocity (Vmax) for Gly-Sar transport was significantly increased by leptin, whereas the apparent Michaelis-Menten constant (Km) did not change. Furthermore, leptin-stimulated Gly-Sar transport was completely suppressed by colchicine, which disrupts cellular translocation of proteins to plasma membranes. Intrajejunal leptin also induced a rapid twofold increase in plasma CFX after jejunal perfusion with CFX in the rat, indicating enhanced intestinal absorption of CFX. These data revealed an unexpected action of gastric leptin in controlling ingestion of dietary proteins.

Pharmacogenomics of drug transporters: the next drug delivery challenge

Scientifically, the third millennium begins with a major triumph--the publishing of the human genomic map, which is destined to have a momentous impact on the quality of life in our time. Disease prevention, individualized medicine, and genotyped-based medicine will soon become a reality. Pharmacogenetics, the forerunner of pharmacogenomics, began in the 1950s with a series of observations relating drug response to various genetic factors. It took almost two more decades for scientists to discover that cytochrome p450 2D6 was responsible for the metabolism of many drugs. This landmark discovery helped focus attention on how gene expression could impact the response to drugs. The stage was set for a revolution in therapeutics some 30 years later as the Human Genome Project crossed the finishing line triumphantly. A parallel development in drug delivery that may also benefit from the fruits of the Human Genome Project is the growing acceptance/awareness of drug transporters as a gateway to epithelial drug transport. This presentation addresses an area in need of attention: the possible impact of genetic polymorphism of drug transporters in pharmacokinetics and the challenge it poses in drug delivery.

Ocular pharmacokinetics of cephalosporins using microdialysis

The purpose of this study was to delineate the ocular pharmacokinetics of cephalosporins and investigate the presence of peptide transporters in the retina. New Zealand albino rabbits were kept under anesthesia. A concentric microdialysis probe was implanted in the vitreous chamber and linear probe across the cornea in the aqueous humor. Isotonic phosphate buffer saline was perfused through the probes, and samples were collected every 20 min over a period of 10 hr. A 500 microg dose of cephalexin, cephazolin, and cephalothin was administered intravitreally. Inhibition experiments were carried out in vivo, using gly-pro and gly-sar. The vitreal half-lives of cephalexin, cefazolin, and cephalothin were 185.38 +/- 27.25 min, 111.40 +/- 17.17 min, and 146.68 +/- 47.52 min, respectively. Cephalexin generated higher aqueous humor concentrations compared to cefazolin. The pharmacokinetic parameters of cephalexin in the presence of gly-pro, i.e., AUC (44452.06 +/- 3326.55 microg x min/ml), clearance (0.0013 +/- 0.0004 ml/min) and vitreal half-life (825.12 +/- 499.95 min) were different from that of the control (14612.83 +/- 4036.47 microg x min/ml, 0.0036 +/- 0.0011 ml/min, and 187.96 +/- 65.12 min, respectively). Gly-pro did not inhibit cefazolin, and gly-sar showed no effect on the pharmacokinetics of both drugs. These studies indicate the involvement of a peptide carrier in the transport of cephalosporins across the retina. Although gly-pro inhibited the elimination of cephalexin from the vitreous, the effect of an alpha-amino group on peptide carriers was not clearly evident.

Intestinal peptide transport: ex vivo uptake studies and localization of peptide carrier PEPT1

The nature of protein breakdown products and peptidomimetic drugs such as beta-lactams is crucial for their transmembrane transport across apical enterocyte membranes, which is accomplished by the pH-dependent high-capacity oligopeptide transporter PEPT1. To visualize oligopeptide transporter-mediated uptake of oligopeptides, an ex vivo assay using the fluorophore-conjugated dipeptide derivative D-Ala-Lys-N(epsilon)-7-amino-4-methylcoumarin-3-acetic acid (D-Ala-Lys-AMCA) was established in the murine small intestine and compared with immunohistochemistry for PEPT1 in murine and human small intestine. D-Ala-Lys-AMCA was accumulated by enterocytes throughout all segments of the murine small intestine, with decreasing intensity from the top to the base of the villi. Goblet cells did not show specific uptake. Inhibition studies revealed competitive inhibition by the beta-lactam cefadroxil, the angiotensin-converting enzyme inhibitor captopril, and the dipeptide glycyl-glutamine. Controls were performed using either the inhibitor diethylpyrocarbonate or an incubation temperature of 4 degrees C to exclude unspecific uptake. Immunohistochemistry for PEPT1 localized immunoreactivity to the enterocytes, with the highest intensity at the apical membrane. This is the first study that visualizes dipeptide transport across the mammalian intestine and indicates that uptake assays using D-Ala-Lys-AMCA might be useful for characterizing PEPT1-specific substrates or inhibitors.

Expression of the myc/His-tagged human peptide transporter hPEPT1 in yeast for protein purification and functional analysis

The human intestinal peptide transporter hPEPT1 has been expressed in the yeast Pichia pastoris using the promoter of the glyceraldehyde-3-phosphate-dehydrogenase gene. A myc-epitope fused to a polyhistidine-tag was introduced at the C-terminus of hPEPT1 for ease of detection and purification. Yeast cells transformed with tagged hPEPT1 exhibited 30-fold increased dipeptide uptake compared to control cells with a substrate specificity and pH dependence similar to the native transporter. The tagged hPEPT1 protein was detected in crude membrane fractions of Pichia cells with an apparent molecular mass of 66 kDa and an expression level of approximately 64 pmol/mg membrane protein. These studies demonstrate that tagged hPEPT1 can be expressed functionally in P. pastoris with unaltered phenotypical characteristics allowing the yeast cells to be used for functional analysis such as screening for compounds utilizing the peptide transporter for absorption in the human intestine. Moreover, recombinant hPEPT1 can now easily be detected for further purification purposes using immobilized metal-affinity chromatography.

Identification of myelodysplastic syndrome-specific genes by DNA microarray analysis with purified hematopoietic stem cell fraction

Myelodysplastic syndrome (MDS) is a slowly progressing hematologic malignancy associated with a poor outcome. Despite the relatively high incidence of MDS in the elderly, differentiation of MDS from de novo acute myeloid leukemia (AML) still remains problematic. Identification of genes expressed in an MDS-specific manner would allow the molecular diagnosis of MDS. Toward this goal, AC133 surface marker-positive hematopoietic stem cell (HSC)-like fractions have been collected from a variety of leukemias in a large-scale and long-term genomics project, referred to as "Blast Bank," and transcriptome of these purified blasts from the patients with MDS were then compared with those from AML through the use of oligonucleotide microarrays. A number of genes were shown to be expressed in a disease-specific manner either to MDS or AML. Among the former found was the gene encoding the protein Delta-like (Dlk) that is distantly related to the Delta-Notch family of signaling proteins. Because overexpression of Dlk may play a role in the pathogenesis of MDS, the disease specificity of Dlk expression was tested by a quantitative "real-time" polymerase chain reaction analysis. Examination of the Blast Bank samples from 22 patients with MDS, 31 with AML, and 8 with chronic myeloid leukemia confirmed the highly selective expression of the Dlk gene in the individuals with MDS. Dlk could be the first candidate molecule to differentiate MDS from AML. The proposal is made that microarray analysis with the Blast Bank samples is an efficient approach to extract transcriptome data of clinical relevance for a wide range of hematologic disorders.

Epidermal growth factor inhibits glycylsarcosine transport and hPepT1 expression in a human intestinal cell line

The human intestinal cell line Caco-2 was used as a model system to study the effects of epidermal growth factor (EGF) on peptide transport. EGF decreased apical-to-basolateral fluxes of [(14)C]glycylsarcosine ([(14)C]Gly-Sar) up to 50.2 +/- 3.6% (n = 6) of control values. Kinetic analysis of the fluxes showed that maximal flux (V(max)) of transepithelial transport decreased from 3.00 +/- 0.17 nmol x cm(-2) x min(-1) in control cells to 0.50 +/- 0.07 nmol x cm(-2) x min(-1) in cells treated with 5 ng/ml EGF (n = 6, P < 0.01). The apparent Michaelis-Menten constant (K(m)) was 2.71 +/- 0.31 mM (n = 6) in control cells and 1.89 +/- 0.28 mM (n = 6, not significantly different from control) in EGF-treated cells. Similarly, apical uptake of [(14)C]Gly-Sar decreased in cells treated with EGF, with an ED(50) value of 0.36 +/- 0.06 ng/ml (n = 6) EGF and a maximal inhibition of 80 +/- 0.02% (n = 6). V(max) decreased from 2.61 +/- 0.4 to 1.06 +/- 0.1 nmol x cm(-2) x min(-1) (n = 3, P < 0.05), whereas K(m) remained constant. Basolateral Gly-Sar uptake showed no changes in V(max) or K(m) after EGF treatment (n = 3). RT-PCR showed a decrease in hPepT1 mRNA (using glucose-6-phosphate dehydrogenase mRNA as control) in cells treated with EGF. Western blotting indicated a decrease in hPepT1 protein in cell lysates. We conclude that EGF treatment decreases Gly-Sar transport in Caco-2 cells by decreasing the number of peptide transporter molecules in the apical membrane.

Colonic epithelial hPepT1 expression occurs in inflammatory bowel disease: transport of bacterial peptides influences expression of MHC class 1 molecules

BACKGROUND & AIMS

hPepT1 is an intestinal epithelial apical membrane transporter responsible for uptake of di/tripeptides (including bacterial derived proinflammatory n-formyl peptides). hPepT1 expression normally has a strict axial gradient-highest in the proximal small intestine with no expression in the colon.

METHODS

Small intestinal-like cells (Caco2-BBE), and colonic-like cells (HT29-Cl.19A), and colonic mucosa from diseased and control patients were used in the present study.

RESULTS

hPepT1 expression occurs aberrantly in the colon with chronic ulcerative colitis (6 patients) and Crohn's disease (4 patients), but not in normal colon (4 patients) or colon with microscopic colitis (4 patients). To model expression of hPepT1 by colonic-like cells in inflamed states, we stably transfected HT29-Cl.19A cells with a modified hPepT1 tagged on the N-terminus with green fluorescence protein. Analysis of transfected cells revealed that: GFP-hPepT1 protein, like the natural protein, is targeted to the apical plasma membrane. In addition, the tagged protein retains the capability of di/tripeptide absorption, and the expression of the tagged protein by HT29-Cl.19A cells permits absorption of N-formyl-methionyl-leucyl-phenylalanine (fMLP), as occurs in hPepT1 expressing Caco2-BBE cells. fMLP uptake by colonic cells expressing GFP-hPepT1 specifically enhances major histocompatibility complex class I surface expression.

CONCLUSIONS

These data collectively indicate that, in some states of chronic inflammation, hPepT1 may be anomolously expressed in the colon. Further, transport of fMLP by hPepT1 potentially stimulates expression of key accessory immune molecule, MHC-1.

Developmental expression of PEPT1 and PEPT2 in rat small intestine, colon, and kidney

Mammalian peptide transporters (PEPT1 and PEPT2) play a pivotal role in the absorption of small peptides from the intestine and kidney, respectively, and in the disposition and targeting of peptide or mimetic drugs. However, there are few reports on the molecular basis of their regulation, especially in the young. The aim of this study was to determine the developmental expression of intestinal and renal oligopeptide transporters in rats from embryonic to adult ages. Intestinal segments were collected (i.e. duodenum, jejunum, ileum, and colon) along with whole kidney, and their mRNA and protein levels were measured. Expression levels of PEPT1 were maximal 3-5 d after birth in the duodenum, jejunum, and ileum, and then declined rapidly. Expression was increased transiently at d 24, most notably in the ileum. Adult protein levels were approximately 70% of that observed on d 3-5. Significant PEPT1 expression was observed in colon during the first week of life, but levels were undetectable shortly thereafter through adulthood. PEPT1 and PEPT2 expression is less regulated in rat kidney and more pronounced in older animals. Peptide transporters were also present as early as d 20 of fetal life for all tissues tested. These results are unique in providing the developmental expression of peptide transporter mRNA and protein in distinct regions of the small intestine, colon, and kidney in rat. Our findings suggest that intestinal expression of PEPT1 is induced postpartum, possibly by suckling, and again at the time of weaning, and that the colon may participate in peptide transport early in life.

Evolutionary relationships among G protein-coupled receptors using a clustered database approach

Guanine nucleotide-binding protein-coupled receptors (GPCRs) comprise large and diverse gene families in fungi, plants, and the animal kingdom. GPCRs appear to share a common structure with 7 transmembrane segments, but sequence similarity is minimal among the most distant GPCRs. To reevaluate the question of evolutionary relationships among the disparate GPCR families, this study takes advantage of the dramatically increased number of cloned GPCRs. Sequences were selected from the National Center for Biotechnology Information (NCBI) nonredundant peptide database using iterative BLAST (Basic Local Alignment Search Tool) searches to yield a database of approximately 1700 GPCRs and unrelated membrane proteins as controls, divided into 34 distinct clusters. For each cluster, separate position-specific matrices were established to optimize sequence comparisons among GPCRs. This approach resulted in significant alignments between distant GPCR families, including receptors for the biogenic amine/peptide, VIP/secretin, cAMP, STE3/MAP3 fungal pheromones, latrophilin, developmental receptors frizzled and smoothened, as well as the more distant metabotrobic glutamate receptors, the STE2/MAM2 fungal pheromone receptors, and GPR1, a fungal glucose receptor. On the other hand, alignment scores between these recognized GPCR clades with p40 (putative GPCR) and pm1 (putative GPCR), as well as bacteriorhodopsins, failed to support a finding of homology. This study provides a refined view of GPCR ancestry and serves as a reference database with hyperlinks to other sources. Moreover, it may facilitate database annotation and the assignment of orphan receptors to GPCR families.

Expression of a cloned ovine gastrointestinal peptide transporter (oPepT1) in Xenopus oocytes induces uptake of oligopeptides in vitro

We determined the primary structure, tissue distribution and in vitro functional characterization of a peptide transporter, oPepT1, from ovine intestine. Ovine PepT1 (oPepT1) cDNA was 2829-bp long, encoding a protein of 707 amino acid residues with an estimated molecular size of 78 kDa and an isoelectric point (pI) of 6.57. Transport function of oPepT1 was assessed by expressing oPepT1 in Xenopus oocytes using a two-electrode voltage-clamp technique. The transport process was electrogenic and pH dependent, but independent of Na+, Cl- and Ca2+. The oPepT1 displayed a broad substrate specificity for transport of neutral and charged dipeptides and tripeptides. All dipeptides and tripeptides examined evoked inward currents in a saturable manner, with an affinity constant (Kt) ranging from 27 micromol/L to 3.0 mmol/L. No responses were detected from tetrapeptides or free amino acids. Northern blot analysis demonstrated that oPepT1 was expressed in the small intestine, omasum and rumen, but was not expressed in liver and kidney. The presence of the peptide transporter in the forestomach at such levels could provide nutritionally important amino acid nitrogen to ruminants.

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.