Novel membrane transporter OCTN1 mediates multispecific, bidirectional, and pH-dependent transport of organic cations

In the present study, functional characteristics of organic cation transporter (OCTN)1, which was cloned as the pH-dependent tetraethylammonium (TEA) transporter when expressed in mammalian human embryonic kidney (HEK)293 cells, were further investigated using Xenopus oocytes as well as HEK293 cells as gene expression systems. When OCTN1-derived complementary RNA was injected into Xenopus oocytes, pH-dependent transport of [14C]TEA was observed as the same in HEK293 cells. In contrast, a replacement of sodium ions with potassium ions in the surrounding medium did not cause any change in [14C]TEA uptake in Xenopus oocytes expressed with OCTN1. In addition, when OCTN1 was expressed in HEK293 cells, efflux of TEA from the cells was pH dependent, with an accelerated rate at acidic external medium pH. Accordingly, membrane potential or sodium ions are suggested to have no influence on [14C]TEA transport and the transport activity of OCTN1 is directly affected by pH itself. Furthermore, addition of the unlabeled TEA in external medium enhanced the efflux of preloaded [14C]TEA. These observations suggest that OCTN1 is a pH-dependent and bidirectional TEA transporter. OCTN1-mediated [14C]TEA uptake was inhibited by various organic cations such as cimetidine, procainamide, pyrilamine, quinidine, quinine, and verapamil. In addition, uptakes of cationic compounds such as [3H]pyrilamine, [3H]quinidine, and [3H]verapamil and zwitterionic L-[3H]carnitine were increased by expression of OCTN1 in Xenopus oocytes. Accordingly, OCTN1 was functionally demonstrated to be a multispecific and pH-dependent organic cation transporter, which presumably functions as a proton/organic cation antiporter at the renal apical membrane and other tissues.

Decreased tissue distribution of L-carnitine in juvenile visceral steatosis mice

We kinetically analyzed the disposition of L-carnitine of juvenile visceral steatosis (JVS) mice compared with that of normal mice to elucidate the mechanism of the systemic L-carnitine deficiency of JVS mice. There were significant differences in the plasma concentration-time course of total radioactive carnitine (L-[3H]carnitine, [acetyl-3H]carnitine, and other [acyl-3H]carnitines) between normal and JVS mice after a single i.v. or p.o. administration of L-[3H]carnitine (250 ng/kg). The oral bioavailability of L-[3H]carnitine in JVS mice (0.341) was about half of that in normal mice (0.675). The cumulative urinary excretion of total radioactive carnitine in JVS mice was about 10-fold more than that in normal mice, and the total clearance of unchanged L-[3H]carnitine for JVS mice (6.70 ml/min) was significantly higher than that for normal mice (2.45 ml/min). The distribution volume at the steady state of unchanged L-[3H]carnitine in JVS mice (1.10 liters/kg) was significantly smaller than that in normal mice (8.16 liters/kg). At 4 h after an i.v. administration, the apparent tissue-to-plasma concentration ratios of unchanged L-[3H]carnitine for various tissues of JVS mice, except for brain, were about one half to one 20th of those in normal mice. In conclusion, this in vivo disposition kinetic study of L-carnitine supports the previous in vitro finding that the L-carnitine transporter is absent or functionally deficient in JVS mice because the renal reabsorption, the intestinal absorption, and the apparent tissue-to-plasma concentration ratios in JVS mice are significantly lower than those in normal mice. The JVS mouse should be a useful experimental model for studying carnitine deficiency diseases.

Carrier-mediated lung distribution of HSR-903, a new quinolone antibacterial agent

HSR-903 [(S)-(-)-5-amino-7-(7-amino-5-azaspiro[2. 4]hept-5-yl)-1-cyclopropyl-6-fluoro-1, 4-dihydro-8-methyl-4-oxoquinoline-3-carboxylic acid methanesulfonate] is a newly synthesized quinolone with a potent antibacterial activity and a low toxicity. The lung concentration of unchanged HSR-903 was about nine times higher than that in plasma after oral administration (5 mg/kg) in rats. In comparative studies, HSR-903 was accumulated more efficiently than levofloxacin, ciprofloxacin, and lomefloxacin in rat lung. To clarify the mechanism of the specific distribution of HSR-903 into the lung, the uptake of [14C]HSR-903 was studied using isolated rat lung cells and an isolated rat lung perfusion technique. Initial uptake of HSR-903 by isolated lung cells was temperature dependent, saturable, stereospecific, and Na+ and Cl- dependent. The Hill coefficients (1. 90 for Na+ and 1.13 for Cl-) suggest that two Na+ and one Cl- are associated with the transport of one HSR-903 molecule. The uptake of HSR-903 was inhibited by other quinolone antibacterial agents, grepafloxacin, and sparfloxacin. The extraction ratio of HSR-903 in isolated lung perfusion was temperature dependent and saturable. These findings suggest that HSR-903 is taken up by the lung cells via a carrier-mediated transport mechanism, resulting in a concentrative distribution into the lung.

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

BACKGROUND & AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Primary systemic carnitine deficiency is caused by mutations in a gene encoding sodium ion-dependent carnitine transporter

Primary systemic carnitine deficiency (SCD; OMIM 212140) is an autosomal recessive disorder characterized by progressive cardiomyopathy, skeletal myopathy, hypoglycaemia and hyperammonaemia. SCD has also been linked to sudden infant death syndrome. Membrane-physiological studies have suggested a defect of the carnitine transport system in the plasma membrane in SCD patients and in the mouse model, juvenile visceral steatosis. Although the responsible loci have been mapped in both human and mouse, the underlying gene has not yet been identified. Recently, we cloned and analysed the function of a novel transporter protein termed OCTN2. Our observation that OCTN2 has the ability to transport carnitine in a sodium-dependent manner prompted us to search for mutations in the gene encoding OCTN2, SLC22A5. Initially, we analysed the mouse gene and found a missense mutation in Slc22a5 in jvs mice. Biochemical analysis revealed that this mutation abrogates carnitine transport. Subsequent analysis of the human gene identified four mutations in three SCD pedigrees. Affected individuals in one family were homozygous for the deletion of a 113-bp region containing the start codon. In the second pedigree, the affected individual was shown to be a compound heterozygote for two mutations that cause a frameshift and a premature stop codon, respectively. In an affected individual belonging to a third family, we found a homozygous splice-site mutation also resulting in a premature stop codon. These mutations provide the first evidence that loss of OCTN2 function causes SCD.

Improvement of L-dopa absorption by dipeptidyl derivation, utilizing peptide transporter PepT1

In the present study, possible enhancement of intestinal absorption of L-dopa by utilizing intestinal peptide transporter was examined using Caco-2 cells and Xenopus oocytes expressing human peptide transporter (hPepT1). To see whether this peptide transporter could be utilized for the improvement of L-dopa absorption, we employed a dipeptide-mimetic derivative of L-dopa, L-dopa-L-Phe. L-Dopa-L-Phe inhibited the uptake of [14C]Gly-Sar, but not that of L-[3H]-dopa by Caco-2 cells. Uptake of L-dopa-L-Phe was increased by expression of hPepT1 in Xenopus oocytes. The appearance of L-dopa and its metabolite, dopamine, on the basolateral side of Caco-2 cells was significantly higher after addition of L-dopa-L-Phe than after that of L-dopa and was reduced by the presence of Gly-Sar on the apical side. These results indicate that the L-dopa-L-Phe is absorbed more efficiently than L-dopa and is taken up via the peptide transporter, but not via the amino acid transporter, demonstrating the possibility of targeting the peptide transporter as a means for improving intestinal absorption of peptide-like drugs.

Hepatobiliary transport kinetics of HSR-903, a new quinolone antibacterial agent

HSR-903 is a newly synthesized quinolone antibacterial agent with low toxicity. The biliary and urinary excretion of unchanged HSR-903, its R-isomer, and their glucuronides was determined after iv bolus administration (5 mg/kg) to normal Sprague-Dawley rats (SDR) and Eisai hyperbilirubinemic mutant rats (EHBR). The values for the biliary excretion clearance of HSR-903 and its glucuronide in EHBR were decreased to approximately 40 and 2% of those in SDR, respectively, whereas the values for the urinary excretion clearance of HSR-903 and its glucuronide were comparable in SDR and EHBR. The biliary excretion clearance values for the R-isomer and its glucuronide were approximately 3 times greater than those for HSR-903. These results demonstrated that the enantiomers of HSR-903 and their conjugates were excreted into bile in a stereospecific manner. The hepatic uptake of [14C]HSR-903 in vivo was evaluated by means of integration plot analysis. The results indicated that the hepatic uptake of [14C]HSR-903 was very fast and was blood flow-limited. To clarify the mechanism of excretion of HSR-903 into bile, the uptake and efflux of [14C]HSR-903 were studied using isolated hepatocytes from SDR and EHBR. The initial uptake of HSR-903 by hepatocytes was temperature-dependent, saturable, and stereospecific. Unlabeled HSR-903 (S-isomer), the R-isomer, grepafloxacin, and sparfloxacin significantly inhibited the uptake of [14C]HSR-903. The efflux of [14C]HSR-903 from hepatocytes from EHBR was significantly slower than that from hepatocytes from SDR. The addition of sodium azide or bromosulfophthalein reduced the efflux of [14C]HSR-903. These results demonstrate that HSR-903 is actively excreted into bile via the canalicular multispecific organic anion transporter, which is deficient in EHBR.

Direct evidence for peptide transporter (PepT1)-mediated uptake of a nonpeptide prodrug, valacyclovir

Xenopus laevis oocytes were used as a gene expression system to characterize the carrier-mediated transport of valacyclovir (vacv), the L-valine ester prodrug of the acyclic nucleoside acyclovir (acv). A significant increase in the uptake of [3H]vacv by Xenopus laevis oocytes injected with human intestinal peptide transporter (hPepT1) cRNA compared to the uptake by water injected oocytes indicated that vacv was translocated by hPepT1. Vacv uptake was found to be concentration dependent, saturable (K(m) = 5.94 +/- 1.91 mM and Jmax = 1.68 +/- 0.25 nmoles/hr/oocyte), pH dependent, and inhibited by various known substrates of hPepT1 but not by acv, valine or pentaglycine. Vacv also inhibited the uptake of 14C-glycylsarcosine, a known substrate of hPepT1, in a concentration-dependent manner (Ki = 4.08 +/- 1.02 mM). These results demonstrate that human intestinal peptide transporter hPepT1 has broad specificity since it recognizes vacv as a substrate even though it lacks a typical peptide bond.

Adsorptive-mediated endocytosis of a basic peptide in enterocyte-like Caco-2 cells

The internalization of a basic peptide, 001-C8 [H-MeTyr-Arg-MeArg-D-Leu-NH(CH2)8NH2], into enterocyte-like Caco-2 cells was evaluated. Internalization of 125I-labeled 001-C8 (125I-001-C8) increased time dependently and reached steady state at 60 min. The steady-state internalization of 125I-001-C8 (7.24 +/- 0. 41 microl/mg protein) was temperature and concentration dependent and was significantly decreased by dansylcadaverine (500 microM), protamine (1 mM), poly-L-lysine (1 mM), E-2078 (1 mM), and ebiratide (1 mM), whereas poly-L-glutamic acid (1 mM), tyrosine (1 mM), and glycylglycine (25 mM) were not inhibitory. Predigestion of acid mucopolysaccharides by heparinase I, heparitinase, and chondroitinase ABC also decreased the internalization. The maximal internalization, the half-saturation constant, and the nonsaturable internalization of 125I-001-C8 were 1.13 +/- 0.23 pmol/mg protein, 0. 47 +/- 0.43 microM, and 3.13 +/- 0.19 microl/mg protein, respectively. Confocal microscopy also indicated the internalization of fluorescence-derived 001-C8 [001-C8-4-nitrobenz-2-oxa-1,3-diazole (001-C8-NBD)]. Granular staining seen within the cell, excluding nuclei, indicated the sequestration of 001-C8-NBD within endocytotic vesicles. Dansylcadaverine and protamine strongly decreased the granular distribution of 001-C8-NBD within the cell. These results demonstrate that 001-C8 is taken up by Caco-2 cells via adsorptive-mediated endocytosis.

Hepatic sinusoidal membrane transport of anionic drugs mediated by anion transporter Npt1

The purpose of our study was to establish the localization of the anion transporter Npt1 in liver and the relevance of Npt1 to carrier-mediated hepatic transport of beta-lactam antibiotics. Immunocytochemical examination of mouse liver with antiserum for Npt1 showed basolateral (sinusoidal) membrane localization. Function of Npt1 was characterized in Xenopus laevis oocytes. Injection of in vitro-transcribed cRNA into oocytes resulted in an increased uptake of [14C]benzylpenicillin (PCG). The Npt1-mediated uptake was saturable with a Michaelis constant (Km) of 0.46 +/- 0.18 mM and a maximum rate (Vmax) of 46.6 +/- 8.5 pmol/60 min/oocyte, and the uptake of [14C]PCG was independent of Na+ and pH, but dependent on chloride ion. Npt1-mediated [14C]PCG uptake was inhibited by several beta-lactam antibiotics and probenecid. Oocytes injected with Npt1-cRNA demonstrated significantly enhanced transport activity for other anionic compounds such as [14C]faropenem, [14C]foscarnet and [3H]mevalonic acid, as well as [14C]PCG, compared with water-injected oocytes. In conclusion, Npt1 is suggested to participate in hepatic sinusoidal membrane transport of organic anions such as beta-lactam antibiotics as well as inorganic anions for the efflux from hepatocyte-to-blood direction.

Molecular and functional identification of sodium ion-dependent, high affinity human carnitine transporter OCTN2

Primary carnitine deficiency, because of a defect of the tissue plasma membrane carnitine transporters, causes critical symptoms. However, the transporter has not been molecularly identified. In this study, we screened a human kidney cDNA library and assembled a cDNA-encoding OCTN2 as a homologue of the organic cation transporter OCTN1, and then we examined the function of OCTN2 as a carnitine transporter. OCTN2-cDNA encodes a polypeptide of 557 amino acids with 75.8% similarity to OCTN1. Northern blot analysis showed that OCTN2 is strongly expressed in kidney, skeletal muscle, heart, and placenta in adult humans. When OCTN2 was expressed in HEK293 cells, uptake of L-[3H]carnitine was strongly enhanced in a sodium-dependent manner with Km value of 4.34 microM, whereas typical substrates for previously known organic cation transporters, tetraethylammonium and guanidine, were not good substitutes. OCTN2-mediated L-[3H]carnitine transport was inhibited by the D-isomer, acetyl-D,L-carnitine, and gamma-butyrobetaine with high affinity and by glycinebetaine with lower affinity, whereas choline, beta-hydroxybutyric acid, gamma-aminobutyric acid, lysine, and taurine were not inhibitory. Because the observed tissue distribution of OCTN2 is consistent with the reported distribution of carnitine transport activity and the functional characteristics of OCTN2 coincide with those reported for plasma membrane carnitine transport, we conclude that OCTN2 is a physiologically important, high affinity sodium-carnitine cotransporter in humans.

Intestinal absorption of fluorescence-derivatized cationic peptide 001-C8-NBD via adsorptive-mediated transcytosis

The intestinal absorption of an intact oligopeptide was investigated in rats using a synthetic cationic peptide, 001-C8 (H-MeTyr-Arg-MeArg-D-Leu-NH(CH2)8NH2). The peptide was coupled with 4-nitrobenzo-2-oxa-1,3-diazole (NBD) to prepare a fluorescence-labeled derivative 001-C8-NBD (H-MeTyr-Arg-MeArg-D-Leu-NH(CH2)8NH-NBD) for the purpose of quantification. The degradation half-life of 001-C8-NBD in jejunal homogenate (1 mg/mL) was 99.5 min, which was significantly longer than that of natural leucine enkephalin (1.14 min). The absorption of 001-C8-NBD was evaluated by the vascular-perfusion method. Intact 001-C8-NBD appeared in the blood time-dependently and the absorption volume at 30 min (2.75 +/- 0.14 microL/cm intestine) was significantly larger than that of [3H]PEG 900 (0.88 +/- 0.13 microL/cm intestine), of which membrane permeability is very low. The absorption of 001-C8-NBD was greatly reduced by an adsorptive-mediated endocytosis inhibitor, protamine (10 mM). No inhibition of the absorption of [3H]PEG 900 by protamine was observed. The intestinal absorption was also measured by an in vivo loop method. The absorption clearance of 001-C8-NBD measured by this method (0.083 +/- 0.008 microL/min/cm intestine) was comparable to that obtained by the vascular perfusion method (0.092 +/- 0.005 microL/min/cm intestine). All of these data suggested that 001-C8-NBD was absorbed as the intact oligopeptide in the intestine in vivo. Adsorptive-mediated transcytosis is suggested to have enormous potential as an oral delivery system for peptide and/or protein drugs.

Gene-dose effect on carnitine transport activity in embryonic fibroblasts of JVS mice as a model of human carnitine transporter deficiency

Recently, the marked decline in renal carnitine reabsorption has been thought to account fotr the systemic carnitine deficiency in juvenile visceral steatosis (JVS) mice. We have conducted a kinetic analysis using embryonic fibroblasts derived from normal, heterozygous, and homozygous jvs mice and found that the high-affinity carnitine transporter (Km = 5.5 microM), which shows Na+ and temperature dependency and stereospecificity, is defective in homozygous jvs mice. Moreover, a gene dose-dependent decrease of carnitine transport activity, which was due to a decrease in the number of the transporter molecules, was found in heterozygous jvs mice. Similar phenomena have been observed in human primary carnitine deficiency. Therefore, JVS mice may be useful for understanding this extremely rare human hereditary disorder.

Application of fractal kinetics for carrier-mediated transport of drugs across intestinal epithelial membrane

PURPOSE

Fractal kinetics was used for the analysis of the carrier-mediated transport for drugs across the intestinal epithelial cells.

METHODS

The transport was examined under various agitation rates using a monolayer of Caco-2 cells and rabbit ileum sheets.

RESULTS

The passive transport of antipyrine across Caco-2 cells was increased with the increasing rate of agitation and was supposed to be caused by a change in the thickness of the unstirred water layer. On the contrary, in the case of L-lactic acid transport, which follows a carrier-mediated transport mechanism, the more the agitation rate controlling the fractal dimension was increased, the more the permeability rate across the Caco-2 cells was decreased. Fractal kinetic analysis of L-lactic acid transport indicated that the permeability was caused by a single saturable process. Similar agitation effects with L-lactic acid transport were observed in the transport of phenylalanine and cephradine in Caco-2 cells. However, the permeability rates of benzoic acid and 3-O-methyl-D-glucose across Caco-2 cells and L-lactic acid transport across the rabbit ileum tissue indicated the maximum levels at a designated agitation rate. This phenomenon was likely to be caused by the agitation effects controlling not only the fractal environment but also the unstirred water layer.

CONCLUSIONS

Carrier-mediated transports are well defined by fractal kinetics rather than classical kinetic analysis. Fractal kinetics are one of the important areas for understanding and confirming the properties of a carrier-mediated transport process.

Possible role of anion exchanger AE2 as the intestinal monocarboxylic acid/anion antiporter

PURPOSE

The purpose of the present study was to investigate the transport of organic monocarboxylic acids mediated by the anion exchanger AE2, which has been already reported to be present at several tissue cell membranes, including intestinal brush border membrane in rabbit.

METHODS

Membrane transport of organic monocarboxylic acids by AE2 was investigated by transient AE2-gene expression in HEK 293 cells and subsequent uptake studies by the cells.

RESULTS

Functional transfection of AE2 was confirmed by the enhanced 36Cl- efflux from the cells. When preloaded with chloride anion. AE2-transfected cells demonstrated a significantly enhanced [14C]benzoic acid transport activity compared with mock-transfected cells. The AE2-mediated uptake was saturable with kinetic parameters of Km = 0.26 +/- 0.08 mM and Vmax = 6.14 +/- 0.52 nmol/mg protein/3 min and the uptake of [14C]benzoic acid was pH-dependent with a maximal uptake at pH 6.5. AE2-mediated [14C]benzoic acid uptake was inhibited by Cl-, HCO3-, and DIDS. AE2-transfected cells demonstrated significantly enhanced transport activity for nicotinic acid, propionic acid, butyric acid, and valproic acid as well as benzoic acid compared with mock-transfected cells.

CONCLUSIONS

AE2 is functionally involved in the anion antiport for organic monocarboxylic acids as well as inorganic anions and is supposed to play a partial role in the intestinal transport of organic acids.

Cloning and characterization of a novel human pH-dependent organic cation transporter, OCTN1

cDNA for a novel proton/organic cation transporter, OCTN1, was cloned from human fetal liver and its transport activity was investigated. OCTN1 encodes a 551-amino acid protein with 11 transmembrane domains and one nucleotide binding site motif. It is strongly expressed in kidney, trachea, bone marrow and fetal liver and in several human cancer cell lines, but not in adult liver. When expressed in HEK293 cells, OCTN1 exhibited saturable and pH-dependent [3H]tetraethyl ammonium uptake with higher activity at neutral and alkaline pH than at acidic pH. Furthermore, treatment with metabolic inhibitors reduced the uptake, which is consistent with the presence of the nucleotide binding site sequence motif. Although its subcellular localization and detailed functional characteristics are not clear at present, OCTN1 appears to be a novel proton antiporter that functions for active secretion of cationic compounds across the renal epithelial brush-border membrane. It may play a role in the renal excretion of xenobiotics and their metabolites.

Application of muscle microdialysis to evaluate the concentrations of the fluoroquinolones pazufloxacin and ofloxacin in the tissue interstitial fluids of rats

Muscle microdialysis has been used to determine the unbound concentrations of the fluoroquinolones, pazufloxacin and ofloxacin, in tissue interstitial fluids (Cisf,u) of rats under steady state conditions. Cisf,u was estimated from the concentration in dialysate and the in-vitro permeability rate constant by the extrapolation method based on the clearance concept. Paper-disks were inserted under the abdominal skin of rats, and the drug concentrations in the fluids penetrating into the disks (Cdisk) were measured and compared with Cisf,u. The Cisf,u of pazufloxacin and ofloxacin in muscle were close to their unbound concentrations in the venous plasma; these were 75.3% and 77.1%, respectively, of the total concentrations in plasma at the steady state. The Cdisk of pazufloxacin and ofloxacin were also close to their Cisf,u. These results indicate that the unbound concentrations of the fluoroquinolones in the tissue interstitial fluids were the same as those in the venous plasma. The disk insertion technique seems to be useful for evaluating drug concentrations in tissue interstitial fluid.

Nonlinear intestinal absorption of 5-hydroxytryptamine receptor antagonist caused by absorptive and secretory transporters

The mechanism of the nonlinear concentration dependence of intestinal absorption of the 5-hydroxytryptamine receptor antagonist azasetron was studied by use of rat in situ intestinal perfusion, as well as an in vitro Ussing-type chamber method mounted with rat intestinal tissue and cultured monolayers of human adenocarcinoma Caco-2 cells. The intestinal absorption rate constant of azasetron evaluated by the Doluisio method increased significantly with increasing concentration of azasetron up to 10 mM in a nonlinear fashion and tended to decrease at higher concentrations. Mucosal-to-serosal directed permeation of [14C]azasetron across rat ileal sheets evaluated by the in vitro Ussing-type chamber method also increased in a nonlinear fashion in a low concentration range, followed by a decrease as the concentration was further increased, whereas serosal-to-mucosal directed permeation decreased in a concentration-dependent manner. Vectorial transport of [14C]azasetron across a Caco-2 cell monolayer was observed, with higher transport in the basolateral-to-apical direction at a trace concentration of azasetron. When the initial uptake rate of azasetron by Caco-2 cells was measured, it was saturable with an apparent half-saturation concentration of 15 mM and was reduced in the presence of several cationic compounds. These observations suggest that azasetron is taken up by a carrier-mediated transport mechanism across the intestinal epithelial cells. When the steady-state uptake of [14C]azasetron was measured, it was increased in the presence of unlabeled azasetron and ondansetron. In addition, the steady-state uptake was enhanced in the presence of a P-glycoprotein inhibitor, cyclosporin A, and by ATP-depletion of the cells, although these treatments had no effect on the initial uptake of [14C]azasetron. Furthermore, the multidrug-resistant cancer cell line K562/ADM that overexpresses P-glycoprotein accumulated azasetron less extensively than did the parental drug-sensitive K562 cells. These results strongly suggest that azasetron is secreted into the intestinal lumen predominantly by P-glycoprotein. We conclude that intestinal transport of azasetron involves specialized transporters in both the absorptive and secretory directions, and the complex nonlinear intestinal absorption characteristics can be ascribed to the participation of multiple transport mechanisms.

Carrier-mediated transport of oligopeptides in the human fibrosarcoma cell line HT1080

To explore the feasibility of targeting human tumor cells via their transport systems, dipeptide uptake was studied in the human fibrosarcoma cell line HT1080 and the human fibroblast cell line IMR-90 by the use of hydrolysis-resistant glycylsarcosine (Gly-Sar). The uptake of [14C]Gly-Sar into HT1080 was time dependent. Kinetic analysis of the concentration dependence of the initial rate of [14C]Gly-Sar uptake showed that a carrier-mediated transport system with a K(m) of 11.4 +/- 3.3 mM and V(max) of 26.8 +/- 4.0 (nmol/15 min/mg protein) and a nonsaturable component (k(d) of 0.80 microl/15 min/mg protein) were responsible for the dipeptide uptake by HT1080 cells. The optimal pH for the maximal uptake was around 6.0. [14C]Gly-Sar uptake was inhibited by various di- and tripeptides and peptide-mimetic drugs, such as bestatin and cefadroxil. [14C]Gly-Sar uptake was not affected by the presence of amino acids or tetra- or pentapeptides. The uptake of cefadroxil was reduced significantly by unlabeled Gly-Sar. Moreover, Gly-Gly and Gly-Leu produced an increase in the apparent K(m) of the uptake of Gly-Sar without altering V(max). On the other hand, dipeptide uptake by IMR-90, which is a normal diploid cell line (not malignant), showed no saturable transport. These results suggest that HT1080 cells take up dipeptides via a pH-dependent transporter. This is the first report showing that a dipeptide transport system, which is similar but not identical to the well-characterized oligopeptide transporters PepT1 and PepT2, exists in fibroblast-derived tumor cells but not in normal fibroblasts. The present finding could be the basis of a novel strategy for the specific delivery of oligopeptide-mimetic anticancer drugs into tumor cells.

The predominant contribution of oligopeptide transporter PepT1 to intestinal absorption of beta-lactam antibiotics in the rat small intestine

Although recent evidence suggests that certain beta-lactam antibiotics are absorbed via a specific transport mechanism, its nature is unclear. To confirm whether peptide transport in the rat can be largely ascribed to the intestinal oligopeptide transporter PepT1, the transporter has been functionally characterized and its significance in the intestinal absorption of beta-lactam antibiotics was evaluated. For evaluation of transport activity complementary RNA (cRNA) of rat PepT1 was synthesized in-vitro and expressed in Xenopus laevis oocytes. cRNA induced uptake of several beta-lactam antibiotics and the dipeptide [14C]glycylsarcosine; this was specifically inhibited by various dipeptides and tripeptides but not by their constituent amino acids or by tetra- or pentapeptides. The transport activity of PepT1 for beta-lactam antibiotics correlated well with their in-vivo intestinal transport and absorption. Furthermore, mutual inhibitory effects on uptake were observed between glyclsarcosine and beta-lactam antibiotics. Hybrid depletion of the functional expression of rat PepT1 in oocytes injected with rat intestinal epithelial total mRNA was studied using an antisense oligonucleotide corresponding to the 5'-coding region of PepT1. In oocytes injected with rat mRNA pre-hybridized with the antisense oligonucleotide against rat PepT1, the uptake of [14C]glycylsarcosine was almost completely abolished, whereas its uptake was not influenced by a sense oligonucleotide for the same region of PepT1. Similarly, the uptake of beta-lactam antibiotics was also reduced by the antisense oligonucleotide against rat PepT1. These results demonstrate that the intestinal proton-coupled oligopeptide transporter PepT1 plays a predominant role in the carrier-mediated intestinal absorption of beta-lactam antibiotics and native oligopeptides in the rat.

[Molecular characterization of intestinal absorption of drugs by carrier-mediated transport mechanisms]

It has long been thought that intestinal absorption of most of the drugs proceeds by passive diffusion mechanism, in which lipid solubility of the drug molecule is a determinant factor. However, water-soluble natural compounds such as amino acids and sugars can move across cell membranes by the specialized carrier-mediated transport mechanisms. Although some drugs which are structurally analogous to natural compounds have been suggested to be absorbed by such transporters, no clear evidence for the involvement of carrier-mediated transport mechanisms has been obtained. In the present study, through the approach by means of the molecular cloning and functional expression of drug transporters as well as membrane physiological analysis for the drug transport across the intestinal epithelial cell membranes, participation of the carrier-mediated transport mechanisms for the drug absorption was clarified. They include peptide transporter, monocarboxylic acid transporter, anion antiporter, and P-glycoprotein. Most of them have a function for the uptake of drugs into epithelial cells, leading to the increased absorption of drugs, whereas P-glycoprotein excludes drugs into the lumen, thereby decreasing the apparent absorbability of drugs. A rat intestinal monocarboxylic acid-proton cotransporter, MCT1, and an anion antiporter, AE2, were suggested to contribute to the pH-dependent intestinal absorption of monocarboxylic acids such as benzoic acid, lactic acid, nicotinic acid, and valproic acid. An involvement of such pH-dependent transporters in the intestinal absorption of weak organic acids is important, because they may have an alternative mechanism against passive diffusion according to the pH-partition hypothesis. PepT1 cloned from rat intestinal epithelial cells as a peptide transporter was clarified to localize at the intestinal epithelia brush-border membrane and to function for the absorption of beta-lactam antibiotics by the proton-gradient energized mechanism. In contrast, P-glycoprotein functions for the secretion of drugs into the intestinal lumen, thereby decreasing intestinal absorption of an immunosuppressive, cyclosporin A and a 5-HT3 receptor antagonist, azasetron. These lines of studies on the clarification of carrier-mediated drug absorption mechanisms will provide new knowledge for the strategies to the enhancement of intestinal absorption of drugs.

[Tissue distribution of the multidrug-resistance gene product P-glycoprotein and its physiological function]

In normal tissues, P-glycoprotein(P-gp), which is expressed in various tumor cells, is found on the luminal surface of epithelia of the kidney proximal tubule, small intestine and colon and bile canalicular face of hepatocytes, as well as, in the adrenal and capillary endothelial cells in the brain and testis. The physiological function of P-gp remains unclear but growing amounts of information suggest that it can play a important role in the absorption from intestine, elimination from liver and kidney and distribution into brain across the blood-brain barrier for many cancer chemotherapeutic agents as well as other drugs which reverse multidrug resistance. The competition for the P-gp-mediated transports in tissues and organs, which express multidrug-transporter, might lead to unsuspected drug interactions among these drugs.