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

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

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

Engineering of Dual-Function Vitreoscilla Hemoglobin: A One-Pot Strategy for the Synthesis of Unnatural α-Amino Acids

Despite a well-developed and growing body of work on the directed evolution of hemoproteins, the potential of hemoproteins to catalyze non-natural reactions remains underexplored. This paper reports a new biocatalytic strategy for the one-pot synthesis of unnatural α-amino acids. Engineered variants of dual-function Vitreoscilla hemoglobin were found to efficiently catalyze N-H insertion and C-H sp3 alkylation, providing moderate to excellent yields (57%-95%) of unnatural α-amino acid derivatives and turnover numbers (1425-2375).

SLC15A4 Serves as a Novel Prognostic Biomarker and Target for Lung Adenocarcinoma

BACKGROUND

SLC15A family members are known as electrogenic transporters that take up peptides into cells through the proton-motive force. Accumulating evidence indicates that aberrant expression of SLC15A family members may play crucial roles in tumorigenesis and tumor progression in various cancers, as they participate in tumor metabolism. However, the exact prognostic role of each member of the SLC15A family in human lung cancer has not yet been elucidated.

MATERIALS AND METHODS

We investigated the SLC15A family members in lung cancer through accumulated data from TCGA and other available online databases by integrated bioinformatics analysis to reveal the prognostic value, potential clinical application and underlying molecular mechanisms of SLC15A family members in lung cancer.

RESULTS

Although all family members exhibited an association with the clinical outcomes of patients with NSCLC, we found that none of them could be used for squamous cell carcinoma of the lung and that SLC15A2 and SLC15A4 could serve as biomarkers for lung adenocarcinoma. In addition, we further investigated SLC15A4-related genes and regulatory networks, revealing its core molecular pathways in lung adenocarcinoma. Moreover, the IHC staining pattern of SLC15A4 in lung adenocarcinoma may help clinicians predict clinical outcomes.

CONCLUSION

SLC15A4 could be used as a survival prediction biomarker for lung adenocarcinoma due to its potential role in cell division regulation. However, more studies including large patient cohorts are required to validate the clinical utility of SLC15A4 in lung adenocarcinoma.

Impairment of Intestinal Monocarboxylate Transporter 6 Function and Expression in Diabetic Rats Induced by Combination of High-Fat Diet and Low Dose of Streptozocin: Involvement of Butyrate-Peroxisome Proliferator-Activated Receptor-γ Activation

Generally, diabetes remarkably alters the expression and function of intestinal drug transporters. Nateglinide and bumetanide are substrates of monocarboxylate transporter 6 (MCT6). We investigated whether diabetes down-regulated the function and expression of intestinal MCT6 and the possible mechanism in diabetic rats induced by a combination of high-fat diet and low-dose streptozocin. Our results indicated that diabetes significantly decreased the oral plasma exposure of nateglinide. The plasma peak concentration and area under curve in diabetic rats were 16.9% and 28.2% of control rats, respectively. Diabetes significantly decreased the protein and mRNA expressions of intestinal MCT6 and oligopeptide transporter 1 (PEPT1) but up-regulated peroxisome proliferator-activated receptor γ (PPARγ) protein level. Single-pass intestinal perfusion demonstrated that diabetes prominently decreased the absorption of nateglinide and bumetanide. The MCT6 inhibitor bumetanide, but not PEPT1 inhibitor glycylsarcosine, significantly inhibited intestinal absorption of nateglinide in rats. Coadministration with bumetanide remarkably decreased the oral plasma exposure of nateglinide in rats. High concentrations of butyrate were detected in the intestine of diabetic rats. In Caco-2 cells (a human colorectal adenocarcinoma cell line), bumetanide and MCT6 knockdown remarkably inhibited the uptake of nateglinide. Butyrate down-regulated the function and expression of MCT6 in a concentration-dependent manner but increased PPARγ expression. The decreased expressions of MCT6 by PPARγ agonist troglitazone or butyrate were reversed by both PPARγ knockdown and PPARγ antagonist 2-chloro-5-nitro-N-phenylbenzamide (GW9662). Four weeks of butyrate treatment significantly decreased the oral plasma concentrations of nateglinide in rats, accompanied by significantly higher intestinal PPARγ and lower MCT6 protein levels. In conclusion, diabetes impaired the expression and function of intestinal MCT6 partly via butyrate-mediated PPARγ activation, decreasing the oral plasma exposure of nateglinide.

Function, Regulation, and Pathophysiological Relevance of the POT Superfamily, Specifically PepT1 in Inflammatory Bowel Disease

Mammalian members of the proton-coupled oligopeptide transporter family are integral membrane proteins that mediate the cellular uptake of di/tripeptides and peptide-like drugs and couple substrate translocation to the movement of H⁺ , with the transmembrane electrochemical proton gradient providing the driving force. Peptide transporters are responsible for the (re)absorption of dietary and/or bacterial di- and tripeptides in the intestine and kidney and maintaining homeostasis of neuropeptides in the brain. These proteins additionally contribute to absorption of a number of pharmacologically important compounds. In this overview article, we have provided updated information on the structure, function, expression, localization, and activities of PepT1 (SLC15A1), PepT2 (SLC15A2), PhT1 (SLC15A4), and PhT2 (SLC15A3). Peptide transporters, in particular, PepT1 are discussed as drug-delivery systems in addition to their implications in health and disease. Particular emphasis has been placed on the involvement of PepT1 in the physiopathology of the gastrointestinal tract, specifically, its role in inflammatory bowel diseases. © 2018 American Physiological Society. Compr Physiol 8:731-760, 2018.

Substrates of the human oligopeptide transporter hPEPT2

Oligopeptide transporters serve important functions in nutrition and pharmacology. In particular, these transporters help maintain the homeostasis of peptides. The peptide-transporter PEPT2 is a high-affinity and low-capacity type oligopeptide transporter from the proton-coupled oligopeptide transporter family. PEPT2 has recently received attention because of its potential application in targeted drug delivery. PEPT2 is widely distributed in kidney, central nervous system, and lung of organisms. In general, all dipeptides, tripeptides, and peptide-like drugs such as β-lactam antibiotics and angiotensin-converting enzyme inhibitors could be mediated and transported as a substrate of PEPT2. The design of many extant drugs and prodrugs is based on the substrate structure of PEPT2 to accelerate absorption via peptide transporters. Thus, this paper summarizes the substrate features of PEPT2 to promote the rational design of drugs and prodrugs that target peptide transporters.

Sertraline inhibits the transport of PAT1 substrates in vivo and in vitro

BACKGROUND AND PURPOSE

Intestinal nutrient transporters may mediate the uptake of drugs. The aim of this study was to investigate whether sertraline interacts with the intestinal proton-coupled amino acid transporter 1 PAT1 (SLC36A1).

EXPERIMENTAL APPROACH

In vitro investigations of interactions between sertraline and human (h)PAT1, hSGLT1 (sodium-glucose linked transporter 1) and hPepT1 (proton-coupled di-/tri-peptide transporter 1) were conducted in Caco-2 cells using radiolabelled substrates. In vivo pharmacokinetic investigations were conducted in male Sprague-Dawley rats using gaboxadol (10 mg·kg(-1), p.o.) as a PAT1 substrate and sertraline (0-30.6 mg·kg(-1)). Gaboxadol was quantified by hydrophilic interaction chromatography followed by MS/MS detection.

KEY RESULTS

Sertraline inhibited hPAT1-mediated L-[(3)H]-Pro uptake in Caco-2 cells. This interaction between sertraline and PAT1 appeared to be non-competitive. The uptake of the hSGLT1 substrate [(14)C]-α-methyl-D-glycopyranoside and the hPepT1 substrate [(14)C]-Gly-Sar in Caco-2 cells was also decreased in the presence of 0.3 mM sertraline. In rats, the administration of sertraline (0.1-10 mM, corresponding to 0.3-30.6 mg·kg(-1), p.o.) significantly reduced the maximal gaboxadol plasma concentration and AUC after its administration p.o.

CONCLUSIONS AND IMPLICATIONS

Sertraline is an apparent non-competitive inhibitor of hPAT1-mediated transport in vitro. This inhibitory effect of sertraline is not specific to hPAT1 as substrate transport via hPepT1 and hSGLT1 was also reduced in the presence of sertraline. In vivo, sertraline reduced the amount of gaboxadol absorbed, suggesting that the inhibitory effect of sertraline on PAT1 occurs both in vitro and in vivo. Hence, sertraline could alter the bioavailability of drugs absorbed via PAT1.

Physiological and pharmacological mechanisms through which the DPP-4 inhibitor sitagliptin regulates glycemia in mice

Inhibition of dipeptidyl peptidase-4 (DPP-4) activity improves glucose homeostasis through a mode of action related to the stabilization of the active forms of DPP-4-sensitive hormones such as the incretins that enhance glucose-induced insulin secretion. However, the DPP-4 enzyme is highly expressed on the surface of intestinal epithelial cells; hence, the role of intestinal vs. systemic DPP-4 remains unclear. To analyze mechanisms through which the DPP-4 inhibitor sitagliptin regulates glycemia in mice, we administered low oral doses of the DPP-4 inhibitor sitagliptin that selectively reduced DPP-4 activity in the intestine. Glp1r(-/-) and Gipr(-/-) mice were studied and glucagon-like peptide (GLP)-1 receptor (GLP-1R) signaling was blocked by an i.v. infusion of the corresponding receptor antagonist exendin (9-39). The role of the dipeptides His-Ala and Tyr-Ala as DPP-4-generated GLP-1 and glucose-dependent insulinotropic peptide (GIP) degradation products was studied in vivo and in vitro on isolated islets. We demonstrate that very low doses of oral sitagliptin improve glucose tolerance and plasma insulin levels with selective reduction of intestinal but not systemic DPP-4 activity. The glucoregulatory action of sitagliptin was associated with increased vagus nerve activity and was diminished in wild-type mice treated with the GLP-1R antagonist exendin (9-39) and in Glp1r(-/-) and Gipr(-/-) mice. Furthermore, the dipeptides liberated from GLP-1 (His-Ala) and GIP (Tyr-Ala) deteriorated glucose tolerance, reduced insulin, and increased portal glucagon levels. The predominant mechanism through which DPP-4 inhibitors regulate glycemia involves local inhibition of intestinal DPP-4 activity, activation of incretin receptors, reduced liberation of bioactive dipeptides, and activation of the gut-to-pancreas neural axis.

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

Pharmaceutical and pharmacological importance of peptide transporters

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many β-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

[Intestinal absorption and secretion mechanism of carboxylate drugs]

Oral drug delivery is generally the most desirable means of administration, mainly because of patient acceptance, convenience in administration. Intestinal absorption mechanisms of anionic drugs have been mainly explained by the passive diffusion of nonionized compounds. However, several studies have suggested the involvement of specific transporters in intestinal absorption of weak acids including monocarboxylates. (-)-N-(trans-4-Isopropylcyclohexanecarbonyl)-D-phenylalanine (nateglinide) is a oral hypoglycemic agent possessing a carboxyl group and a peptide-type bond in its structure. Although nateglinide quickly reaches the maximal serum concentration after oral administration, nateglinide itself is not transported by PepT1 or MCT1. We demonstrated that nateglinide transport occurs via a single system that is H(+) dependent but is distinct from PepT1 or MCT1. In clinical, patients usually take many kinds of drugs at the same time. Thus, drug-drug interactions involving transporters can often directly affect the therapeutic safety and efficacy of many drugs. However, there have been few studies on food-drug interactions involving transporters. Dietary polyphenols have been widely assumed to be beneficial to human health. Polyphenols are commercially prepared and used as functional foods. We reported that ferulic acid, which is widely used as a functional food, affects the transport of clinical agents. The major dose-limiting toxicity after administration of irinotecan hydrochloride, 7-ethyl-10-(4-[1-piperidino]-1-piperidino)-carbonyloxycamptothecin (CPT-11) is severe diarrhea. We have found that a specific transport system mediates the uptake of active metabolite 7-ethyl-10-hydroxycamptothecin (SN-38) across the apical membrane in Caco-2 cells. Baicalin and sulfobromophthatlein inhibit this transporter. Inhibition of this transporter would be a useful means for reducing late-onset diarrhea.

Association of PepT1 with lipid rafts differently modulates its transport activity in polarized and nonpolarized cells

The transporter PepT1, apically expressed in intestinal epithelial cells, is responsible for the uptake of di/tripeptides. PepT1 is also expressed in nonpolarized immune cells. Here we investigated the localization of PepT1 in lipid rafts in small intestinal brush border membranes (BBMs) and polarized and nonpolarized cells, as well as functional consequences of the association of PepT1 with lipid rafts. Immunoblot analysis showed the presence of PepT1 in low-density fractions isolated from mouse intestinal BBMs, polarized intestinal Caco2-BBE cells, and nonpolarized Jurkat cells by solubilization in ice-cold 0.5% Triton X-100 and sucrose gradient fractionation. PepT1 colocalized with lipid raft markers GM1 and N-aminopeptidase in intestinal BBMs and Caco2-BBE cell membranes. Disruption of lipid rafts with methyl-beta-cyclodextrin (MbetaCD) shifted PepT1 from low- to high-density fractions. Remarkably, we found that MbetaCD treatment increased PepT1 transport activity in polarized intestinal epithelia but decreased that in intestinal BBM vesicles and nonpolarized immune cells. Mutational analysis showed that phenylalanine 293, phenylalanine 297, and threonine 281 in transmembrane segment 7 of the human di/tripeptide transporter, hPepT1, are important for the targeting to lipid rafts and transport activity of hPepT1. In conclusion, the association of PepT1 with lipid rafts differently modulates its transport activity in polarized and nonpolarized cells.

Development and validation of a stereoselective HPLC method for the determination of thein vitro transport of nateglinide enantiomers in rat intestine

A simple stereoselective high performance liquid chromatographic method was developed for the determination of the in vitro transport of the enantiomers of nateglinide (N-(trans-4-isopropylcyclohexyl-carbonyl)-phenylalanine) in the rat intestine using a Chiralcel OJ-RH column (150 x 4.0 mm, 5 microm). The effects of the mobile phase composition, pH, the flow rate, and the temperature on the chromatographic separation were investigated. The enantioseparation was achieved at 33 degrees C using a mobile phase containing 100 mM potassium dihydrogen phosphate, pH 2.5, and ACN (32:68 v/v) delivered at a flow rate of 1 mL/min. The analytes were monitored at 210 nm and linearity (r >0.99) was obtained for a concentration range of 0.5-50 microg/mL. The LOD and LOQ were 0.2 and 0.5 microg/mL for the R-enantiomer and 0.2 and 0.8 microg/mL for the S-enantiomer, respectively. Both, the intra- and interday accuracy and precision of the calibration curves were determined. The method was successfully applied to estimate the in vitro passage of the enantiomers and the racemate of nateglinide in duodenum, jejunum, and ileum of rats. Generally, higher concentrations of nateglinide and the S-enantiomer were observed when the racemate was administered compared to administration of the individual enantiomers of nateglinide.

The oligopeptide transporter hPepT1: gateway to the innate immune response

Bacterial products that are normally present in the lumen of the colon, such as N-formylated peptides and muramyl-dipeptide, are important for inducing the development of mucosal inflammation. The intestinal dipeptide transporter, hPepT1, which is expressed in inflamed but not in noninflamed colonic epithelial cells, mediates the transport of these bacterial products into the cytosol of colonic epithelial cells. The small bacterial peptides subsequently induce an inflammatory response, including the induction of MHC class I molecules expression and cytokines secretion, via the activation of nucleotide-binding site and leucine-rich repeat (NBS-LRR) proteins, for example NOD2, and activation of NF-kappaB. Subsequent secretion of chemoattractants by colonic epithelial cells induces the movement of neutrophils through the underlying matrix, as well as across the epithelium. These bacterial products can also reach the lamina propria through the paracellular pathway and across the basolateral membrane of epithelial cells. As a consequence, small formylated peptides can interact directly with immune cells through specific membrane receptors. Since immune cells, including macrophages, also express hPepT1, they can transport small bacterial peptides into the cytosol where these may interact with the NBS-LRR family of intracellular receptors. As in intestinal epithelial cells, the presence of these small bacterial peptides in immune cells may trigger immune response activation.

Coadministration of gemfibrozil and itraconazole has only a minor effect on the pharmacokinetics of the CYP2C9 and CYP3A4 substrate nateglinide

BACKGROUND AND AIMS

Gemfibrozil, and particularly its combination with itraconazole, greatly increases the area under the plasma concentration-time curve [AUC(0, infinity)] and response to the cytochrome P450 (CYP) 2C8 and 3A4 substrate repaglinide. In vitro, gemfibrozil is a more potent inhibitor of CYP2C9 than of CYP2C8. Our aim was to investigate the effects of the gemfibrozil-itraconazole combination on the pharmacokinetics and pharmacodynamics of another meglitinide analogue, nateglinide, which is metabolized by CYP2C9 and CYP3A4.

METHODS

In a randomized crossover study with two phases, nine healthy subjects took 600 mg gemfibrozil and 100 mg itraconazole (first dose 200 mg) twice daily or placebo for 3 days. On day 3, they ingested a single 30-mg dose of nateglinide. Plasma nateglinide and blood glucose concentrations were measured for up to 12 h.

RESULTS

During the gemfibrozil-itraconazole phase, the AUC(0, infinity) and C(max) of nateglinide were 47% (range 23-74%; P < 0.0001) and 30% (range - 8% to 104%; P = 0.0146) higher than during the placebo phase, respectively, but the t(max) and t1/2 of nateglinide remained unchanged. The combination of gemfibrozil and itraconazole had no effect on the formation of the M7 metabolite of nateglinide but impaired its elimination. The blood glucose response to nateglinide was not significantly changed by coadministration of gemfibrozil and itraconazole.

CONCLUSIONS

The combination of gemfibrozil and itraconazole has only a limited influence on the pharmacokinetics of nateglinide. This is in marked contrast to the substantial effect of this combination on the pharmacokinetics of repaglinide. The findings suggest that in vivo gemfibrozil, probably due to its metabolites, is a much more potent inhibitor of CYP2C8 than of CYP2C9.

Purification by p-aminobenzoic acid (PABA)-affinity chromatography and the functional reconstitution of the nateglinide/H+ cotransport system in the rat intestinal brush-border membrane

(-)-N-(trans-4-isopropylcyclohexanecarbonyl)-D-phenylalanine (nateglinide) is a novel oral hypoglycemic agent possessing a peptide-type bond and a carboxyl group in its structure. Recently, we have shown that nateglinide transport occurs via the ceftibuten/H+ cotransport system, which is distinct from PepT1, and that the fluorescein/H+ cotransport system is involved in the uptake of nateglinide. The aim of this study was to characterize the functional properties of the intestinal nateglinide transporter. In the first part of this study, we demonstrated that the ceftibuten/H+ cotransport system is identical to the fluorescein/H+ cotransport system. We succeeded in purification of the nateglinide transporter from brush-border membranes of the rat small intestine using p-aminobenzoic acid (PABA)-affinity chromatography. We then investigated the functional properties of the nateglinide transporter using proteoliposomes prepared from the PABA-affinity chromatography elute. We demonstrated that nateglinide, ceftibuten, and fluorescein are transported by the same transporter in the intestine.

Nateglinide uptake by a ceftibuten transporter in the rat kidney brush-border membrane

Nateglinide, a novel oral hypoglycemic agent, possesses a carbonyl group and a peptide-type bond in its structure. We previously reported that nateglinide transport occurs via a single system that may be identical to the ceftibuten/H(+) cotransport system by the rat small intestine. We speculated that the absorption system present on the intestinal epithelium may be similar to that found on the renal tubular epithelium. The aim of this study was to characterize the transporters on the apical side of the kidney that may contribute to the reabsorption of ceftibuten and nateglinide. The uptake of nateglinide by rat renal brush-border membranes is associated with an H(+)-coupled transport system. Ceftibuten competitively inhibited H(+)-dependent nateglinide uptake. In contrast, Gly-Sar, cephradine and cephalexin had no effect on nateglinide uptake. Nateglinide competitively inhibited H(+)-driven transporter-mediated ceftibuten uptake. We conclude that nateglinide transport occurs via a single system that is H(+)-dependent and may be identical to the ceftibuten/H(+) cotransport system.

Intestinal uptake of nateglinide by an intestinal fluorescein transporter

Nateglinide, a novel oral hypoglycemic agent, rapidly reaches its maximum serum concentration after oral administration, suggesting that it is rapidly absorbed in the intestine. However, nateglinide itself is not transported by MCT1 or PEPT1. The aim of this study was to characterize the transporters on the apical side of the small intestine that are responsible for the rapid absorption of nateglinide. It has been reported that the uptake of fluorescein by Caco-2 cells occurs via an H+-driven transporter and that the intestinal fluorescein transporter is probably not MCT1. We examined the contribution of the fluorescein transporter to the uptake of nateglinide by Caco-2 cells. Fluorescein competitively inhibited H+-dependent nateglinide uptake. All of fluorescein transporter inhibitors examined reduced the uptake of nateglinide. Furthermore, nateglinide inhibited fluorescein uptake. We conclude that the intestinal nateglinide/H+ cotransport system is identical to the intestinal fluorescein/H+ cotransport system.

H+-dependent transport mechanism of nateglinide in the brush-border membrane of the rat intestine

(-)-N-(trans-4-Isopropylcyclohexanecarbonyl)-D-phenylalanine (nateglinide) is a novel oral hypoglycemic agent possessing a carboxyl group and a peptide-type bond in its structure. Although nateglinide quickly reaches the maximal serum concentration after oral administration, nateglinide itself is not transported by PepT1 or MCT1. The aim of this study was to characterize the transporters on the apical side of the small intestine that are responsible for the rapid absorption of nateglinide. The uptake of nateglinide by rat intestinal brush-border membrane vesicles is associated with a proton-coupled transport system. Ceftibuten competitively inhibited H(+)-dependent nateglinide uptake. Glycylsarcosine (Gly-Sar), cephradine, and cephalexin did not significantly inhibit the uptake of nateglinide. The combination of Gly-Sar and nateglinide greatly reduced the uptake of ceftibuten. The effect of the combined treatment was significantly greater than that of Gly-Sar alone. Furthermore, nateglinide competitively inhibited H(+)-driven ceftibuten transporter-mediated ceftibuten uptake. Ceftibuten transport occurs via at least two H(+)-dependent transport systems: one is PepT1, and the other is the ceftibuten/H(+) cotransport system. On the other hand, we demonstrated that nateglinide transport occurs via a single system that is H(+) dependent but is distinct from PepT1 and may be identical to the ceftibuten/H(+) cotransport system.

Molecular and Integrative Physiology of Intestinal Peptide Transport

Intestinal protein digestion generates a huge variety and quantity of short chain peptides that are absorbed into intestinal epithelial cells by the PEPT1 transporter in the apical membrane of enterocytes. PEPT1 operates as an electrogenic proton/peptide symporter with the ability to transport essentially every possible di- and tripeptide. Transport is enantio-selective and involves a variable proton-to-substrate stoichiometry for uptake of neutral and mono- or polyvalently charged peptides. Neither free amino acids nor peptides containing four or more amino acids are accepted as substrates. The structural similarity of a variety of drugs with the basic structure of di- or tripeptides explains the transport of aminocephalosporins and aminopenicillins, selected angiotensin-converting inhibitors, and amino acid-conjugated nucleoside-based antiviral agents by PEPT1. The high transport capacity of PEPT1 allows fast and efficient intestinal uptake of the drugs but also of amino acid nitrogen even in states of impaired mucosal functions. Transcriptional and post-transcriptional regulation of PEPT1 occurs in response to alterations in the nutritional status and in disease states, suggesting a prime role of this transporter in amino acid absorption.

The proton oligopeptide cotransporter family SLC15 in physiology and pharmacology

Mammalian members of the SLC15 family are electrogenic transporters that utilize the proton-motive force for uphill transport of short chain peptides and peptido-mimetics into a variety of cells. The prototype transporters of this family are PEPT1 (SLC15A1) and PEPT2 (SLC15A2), which mediate the uptake of peptide substrates into intestinal and renal epithelial cells. More recently, other sites of functional expression of the two proteins have been identified such as bile duct epithelium (PEPT1), glia cells and epithelia of the choroid plexus, lung and mammary gland (PEPT2). Both proteins can transport essentially every possible di- and tripeptide regardless of the substrate's net charge, but operate stereoselectively. Based on peptide-like structures, various drugs and prodrugs are transported as well, allowing efficient intestinal absorption of the compounds via PEPT1. In kidney tubules both peptide transporters can mediate the renal reabsorption of the filtered compounds thus affecting their pharmacokinetics. Recently, two new peptide transporters, PHT1 (SLC15A4) and PHT2 (SLC15A3), were identified in mammals. They possess an overall amino acid identity with the PEPT-series of 20% to 25%. PHT1 and PHT2 were shown to transport free histidine and certain di- and tripeptides, but it is not yet clear whether they are located on the plasma membrane or represent lysosomal transporters for the proton-dependent export of histidine and dipeptides from lysosomal protein degradation into the cytosol.

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.

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.

Studies on intestinal absorption of sulpiride (1): carrier-mediated uptake of sulpiride in the human intestinal cell line Caco-2

We investigated whether the uptake of a specific antipsychotic agent, sulpiride, in Caco-2 cells is mediated by a carrier-mediated system. Caco-2 cell monolayers were cultured in plastic culture dishes and uptake and efflux studies were conducted. The determination of sulpiride was performed by HPLC. At 37 degrees C, sulpiride uptake in pH 6.0 was twice as much as in pH 7.4. At 4 degrees C, however, no significant difference was observed between pH 6.0 and 7.4. The uptake at 4 degrees C was markedly lower than that obtained at 37 degrees C. The subtraction of the uptake at 4 degrees C from the uptake at 37 degrees C indicated a saturable process, and the result of the Eadie-Hofstee plot analysis indicated that the uptake consists of two or more saturable components. The uptake was significantly inhibited by uncoupler, protonophore, amino acid modifying agent and proteinase. Sulpiride efflux was temperature-dependent and was significantly inhibited by uncoupler and amino acid modifying agent. These findings indicate that sulpiride uptake and efflux in Caco-2 cells are carrier-mediated. Furthermore, the uptake was significantly decreased by some substrates and inhibitors of peptide transporter, PEPT1, and organic cation transporters, OCTN1 and OCTN2, and was significantly increased by preloading with them. The uptake was also significantly increased by a typical substrate of P-glycoprotein. From these findings, we presumed that peptide transporter PEPT1 and organic cation transporters OCTN1 and OCTN2 are involved with this uptake. P-glycoprotein may also contribute to the efflux of sulpiride.

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.