Cloning and functional expression of a human liver organic cation transporter

Pharmacological and physiological functions of the polyspecific organic cation transporters: OCT1, 2, and 3 (SLC22A1-3)

For the elimination of environmental toxins and metabolic waste products, the body is equipped with a range of broad-specificity transporters that are generally present in the liver, kidney, and intestine. The polyspecific organic cation transporters OCT1, 2, and 3 (SLC22A1-3) mediate the facilitated transport of a variety of structurally diverse organic cations, including many drugs, toxins, and endogenous compounds. OCT1 and OCT2 are found in the basolateral membrane of hepatocytes, enterocytes, and renal proximal tubular cells. OCT3 has a more widespread tissue distribution and is considered to be the major component of the extraneuronal monoamine transport system (or uptake-2), which is responsible for the peripheral elimination of monoamine neurotransmitters. Studies with knockout mouse models have directly demonstrated that these transporters can have a major impact on the pharmacological behavior of various substrate organic cations. The recent identification of polymorphic genetic variants of human OCT1 and OCT2 that severely affect transport activity thus suggests that some of the interpatient differences in response and sensitivity to cationic drugs may be caused by variable activity of these transporters.

Impact of drug transporter studies on drug discovery and development

Drug transporters are expressed in many tissues such as the intestine, liver, kidney, and brain, and play key roles in drug absorption, distribution, and excretion. The information on the functional characteristics of drug transporters provides important information to allow improvements in drug delivery or drug design by targeting specific transporter proteins. In this article we summarize the significant role played by drug transporters in drug disposition, focusing particularly on their potential use during the drug discovery and development process. The use of transporter function offers the possibility of delivering a drug to the target organ, avoiding distribution to other organs (thereby reducing the chance of toxic side effects), controlling the elimination process, and/or improving oral bioavailability. It is useful to select a lead compound that may or may not interact with transporters, depending on whether such an interaction is desirable. The expression system of transporters is an efficient tool for screening the activity of individual transport processes. The changes in pharmacokinetics due to genetic polymorphisms and drug-drug interactions involving transporters can often have a direct and adverse effect on the therapeutic safety and efficacy of many important drugs. To obtain detailed information about these interindividual differences, the contribution made by transporters to drug absorption, distribution, and excretion needs to be taken into account throughout the drug discovery and development process.

Evolutionary conservation predicts function of variants of the human organic cation transporter, OCT1

The organic cation transporter, OCT1, is a major hepatic transporter that mediates the uptake of many organic cations from the blood into the liver where the compounds may be metabolized or secreted into the bile. Because OCT1 interacts with a variety of structurally diverse organic cations, including clinically used drugs as well as toxic substances (e.g., N-methylpyridinium, MPP(+)), it is an important determinant of systemic exposure to many xenobiotics. To understand the genetic basis of extensive interindividual differences in xenobiotic disposition, we functionally characterized 15 protein-altering variants of the human liver organic cation transporter, OCT1, in Xenopus oocytes. All variants that reduced or eliminated function (OCT1-R61C, OCT1-P341L, OCT1-G220V, OCT1-G401S, and OCT1-G465R) altered evolutionarily conserved amino acid residues. In general, variants with decreased function had amino acid substitutions that resulted in more radical chemical changes (higher Grantham values) and were less evolutionarily favorable (lower blosum62 values) than variants that maintained function. A variant with increased function (OCT1-S14F) changed an amino acid residue such that the human protein matched the consensus of the OCT1 mammalian orthologs. Our results indicate that changes at evolutionarily conserved positions of OCT1 are strong predictors of decreased function and suggest that a combination of evolutionary conservation and chemical change might be a stronger predictor of function.

Increased synaptosomal dopamine content and brain concentration of paraquat produced by selective dithiocarbamates

Exposure to pesticides may be a risk factor for Parkinson's disease based on epidemiologic data in humans, animal models and in vitro studies. Different dithiocarbamate pesticides potentiate the toxicity of both 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and paraquat in mouse models of Parkinsonism by an unknown mechanism. This study examined the effects of commercially used dithiocarbamates on [3H]dopamine transport in striatal synaptosomal vesicles and on the concentration of [14C]paraquat in vivo in mice. Different ethylenebis-dithiocarbamates and diethyl-dithiocarbamate increased dopamine accumulation in synaptosomes, whereas dimethyl-dithiocarbamate and methyl-dithiocarbamate did not. Increased dopamine accumulation in synaptosomes was dose dependent and was related to the carbon backbone of these molecules. The dithiocarbamates that increased accumulation of dopamine did not alter the influx of dopamine, but rather delayed the efflux out of synaptosomes. These same dithiocarbamates also increased the tissue content of [14C]paraquat in vivo by a mechanism that appeared to be distinct from the dopamine transporter. There was a consistent relationship between the dithiocarbamates that increased synaptosomal accumulation of dopamine and tissue content of paraquat, with those previously demonstrated to enhance paraquat toxicity in vivo. These results suggest that selective dithiocarbamates may alter the kinetics of different endogenous and exogenous compounds to enhance their neurotoxicity.

Xenobiotic transporter expression and function in the human mammary gland

Xenobiotic transport in the mammary gland has tremendous clinical, toxicological and nutritional implications. Mechanisms such as passive diffusion, carrier-mediated transport, and transcytosis mediate xenobiotic transfer into milk. In vivo animal and human studies suggest the functional expression of both xenobiotic and nutrient transporters in the lactating mammary gland and the potential involvement of such systems in the significant accumulation of certain compounds in milk. In vitro cell culture systems provide further evidence for carrier-mediated transport across the lactating mammary epithelium. Additionally, molecular characterization studies indicate the expression of various members of the organic cation transporter, organic anion transporter, organic anion polypeptide transporter, oligopeptide transporter, nucleoside and nucleobase transporter, multidrug resistant transporter, and multidrug resistant-like protein transporter families at the lactating mammary epithelium. The in vivo relevance of the expression of such xenobiotic and nutrient transporters and their involvement in drug disposition at the mammary gland requires investigation.

Transport of cimetidine by flounder and human renal organic anion transporter 1

The H(2)-receptor antagonist cimetidine is efficiently excreted by the kidneys. In vivo studies indicated an interaction of cimetidine not only with transporters for basolateral uptake of organic cations but also with those involved in excretion of organic anions. We therefore tested cimetidine as a possible substrate of the organic anion transporters cloned from winter flounder (fROAT) and from human kidney (hOAT1). Uptake of [(3)H]cimetidine into fROAT-expressing Xenopus laevis oocytes exceeded uptake into control oocytes. At -60-mV clamp potential, 1 mM cimetidine induced an inward current, which was smaller than that elicited by 0.1 mM PAH. Cimetidine concentrations exceeding 0.1 mM decreased PAH-induced inward currents, indicating interaction with the same transporter. At pH 6.6, no current was seen with 0.1 mM cimetidine, whereas at pH 8.6 a current was readily detectable, suggesting preferential translocation of uncharged cimetidine by fROAT. Oocytes expressing hOAT1 also showed [(3)H]cimetidine uptake. These data reveal cimetidine as a substrate for fROAT/hOAT1 and suggest that organic anion transporters contribute to cimetidine excretion in proximal tubules.

Influence of molecular structure on substrate binding to the human organic cation transporter, hOCT1

Organic cation transporters play a critical role in the elimination of therapeutic compounds in the liver and the kidney. We used computational quantitative structure activity approaches to predict molecular features that influence interaction with the human ortholog of the organic cation transporter (hOCT1). [(3)H]tetraethylammonium uptake in HeLa cells stably expressing hOCT1 was inhibited to varying extents by a diverse set of 30 molecules. A subset of 22 of these was used to produce, using Catalyst, a pharmacophore that consisted of three hydrophobic features and a positive ionizable feature. The correlation coefficient of observed versus predicted IC(50) was 0.86 for this training set, which was superior to calculated logP alone (r = 0.73) as a predictor of hOCT1 inhibition. A descriptor-based quantitative structure-activity relationship study using Cerius(2) resulted in an equation relating five molecular descriptors to log IC(50) with a correlation coefficient of 0.95. Furthermore, a group of phenylpyridinium and quinolinium compounds were used to investigate the spatial limitations of the hOCT1 binding site. The affinity for hOCT was higher for 4-phenylpyridiniums > 3-phenylpyridiniums > quinolinium, indicating that substrate affinity was influenced by the distribution of hydrophobic mass. In addition, supraplanar hydrophobic mass was found to increase the affinity for binding hOCT1. These results indicate how a combination of computational and in vitro approaches may yield insight into the binding affinity of transporters and may be applicable to predicting these properties for new therapeutics.

Regulation of human organic cation transporter hOCT2 by PKA, PI3K, and calmodulin-dependent kinases

Properties and regulation of the human organic cation (OC) transporter type 2 (hOCT2) expressed in HEK-293 cells were extensively characterized using the fluorescent OC 4-[4-(dimethylamino)styryl]-N-methylpyridinium (ASP(+)). ASP(+) uptake was electrogenic and inhibited by TPA(+) (EC(50) = 2.7 microM), tetraethylammonium (EC(50) = 35 microM), cimetidine (EC(50) = 36 microM), or quinine (EC(50) = 6.7 microM). Stimulation with carbachol or ATP decreased initial uptake by 44 +/- 3 (n = 14) and 34 +/- 4% (n = 21), respectively, independently of PKC but dependent on phosphatidylinositol 3-kinase (PI3K). PKA stimulation decreased uptake by 18 +/- 4% (n = 40). Inhibition of calmodulin (CaM), Ca(2+)/CaM-dependent kinase II, or myosin light chain kinase decreased uptake by 63 +/- 2 (n = 15), 40 +/- 4 (n = 30), and 31 +/- 4% (n = 16), respectively. Inhibition of CaM resulted in a significant change in the EC(50) value for the inhibition of ASP(+) uptake by tetraethylammonium. In conclusion, we demonstrate that the hOCT2 is inhibited by PI3K and PKA and activated by a CaM-dependent signaling pathway, probably via a change in substrate affinity.

Drug transport proteins in the liver

Together with drug metabolising enzymes, transmembrane transporters are important determinants of drug metabolism and drug clearance by the liver. Hepatic uptake of organic anions, cations, prostaglandins and bile salts is supported by dedicated transporter proteins in the basolateral (sinusoidal) membrane of hepatocytes: OATPs, OATs, OCTs, PGTs and NTCP, respectively. ATP-binding cassette (ABC) transporter proteins in the canalicular membrane of hepatocytes mediate the hepatic efflux of drugs, bile salts and metabolites against a steep concentration gradient from liver to bile. This transport is driven by ATP hydrolysis. Drugs, endogenous metabolites, bile salts and cytokines affect the expression levels of these transporters. They act through a family of ligand-activated transcription factors, the nuclear hormone receptors. Consequently, the levels of the various transporter proteins are subject to genetic polymorphism in the encoding genes as well as in these transcription factors. Adverse drug reactions may be caused by genetic or disease-induced variations of transporter expression or drug-drug interactions at the level of these transporters.

Organic anion and cation transporters occur in pairs of similar and similarly expressed genes

Organic anion and cation transporters (OATs, OCTs, OCTNs, and ORCTLs), transmembrane proteins essential to renal xenobiotic excretion, are encoded by a group of related genes. As yet there have been no studies of the transcriptional regulation of this important gene family. While such studies have traditionally been labor-intensive, comparative genomics approaches are now available that have proven reliable guides to critical regulatory elements. We report here the genomic sequencing of murine OAT1 (the cDNA of which was originally cloned by us as NKT) and OAT3 (Roct), and derivation of phylogenetic footprints (evolutionarily conserved non-coding sequences) by comparison to the human genome. We find binding sites within these footprints for several transcription factors implicated in kidney development, including PAX1, PBX, WT1, and HNF1. Additionally, we note that OATs and OCTs occur in the human and mouse genomes as tightly linked pairs (OAT1 and OAT3, UST3 and OAT5, OAT4 and URAT1/RST, OCT1 and 2, OCTN1 and 2, ORCTL3 and 4) that are also close phylogenetic relations, with Flipt1 and 2, and OAT2 the only unpaired family members. Finally, we find that pair-members have similar tissue distributions, suggesting that the pairing might exist to facilitate the co-regulation of the genes within each pair.

Human organic anion transporting polypeptide-C (SLC21A6) is a major determinant of rifampin-mediated pregnane X receptor activation

Rifampin, a member of the rifamycin class of antibiotics, is well known for its ability to induce drug-metabolizing enzymes and transporters, through activation of the pregnane X receptor. Available data suggest rifampin entry into hepatocytes may be transporter-mediated. Accordingly, it is therefore plausible that modulation of the achievable intracellular concentration of rifampin by drug uptake transporters would influence the degree of induction. In this study, we expressed an array of known hepatic uptake transporters to show the key hepatic rifampin uptake transporters are liver-specific members of the organic anion transporting polypeptide family (OATP). Indeed, both OATP-C and OATP8 seemed capable of mediating rifampin uptake into HeLa cells. OATP-C, however, seemed to have far greater affinity and capacity for rifampin transport. In addition, several allelic variants of OATP-C known to be present among European and African Americans were found to have markedly decreased rifampin transport activity. In cell-based, transactivation assays, OATP-C expression was associated with increased cellular rifampin retention as well as potentiation of PXR reporter gene activity. This is the first demonstration of an uptake transporter such as OATP-C, in modulating PXR function, and sheds important new insight into our understanding of the molecular determinants of PXR-mediated inductive processes.

Identification of genetic variations of the human organic cation transporter hOCT1 and their functional consequences

By systematic mutation screening of the polyspecific organic cation transporter hOCT1 (SLC22A1) in 57 Caucasians, 25 genetic variations were identified and further analysed for population frequency. Five mutations resulting in the amino acid changes Arg61Cys, Cys88Arg, Phe160Leu, Gly401Ser, and Met420del, with respective allele frequencies of 9.1, 0.6, 22, 3.2, and 16%, were functionally characterized upon expression in Xenopus oocytes. Phe160Leu and Met420del exhibited substrate affinities and selectivites identical to hOCT1 wild-type. In contrast, uptake of 0.1 microm [3H]1-methyl-4-phenylpyridinium ([3H]MPP) by Arg61Cys, Cys88Arg and Gly401Ser were reduced to 30, 1.4 and 0.9% compared to wild-type, respectively. Since transport of 1 microm [3H]serotonin by Cys88Arg and Gly401Ser was reduced to only 13 and 12% of wild-type, these mutants exhibit a changed substrate selectivity. The data show that the mutants Arg61Cys, Cys88Arg and Gly401Ser could affect the disposition of OCT1 substrates and as a consequence may alter the duration and intensity of effects of drugs and neurotransmitters which are substrates for hOCT1.

Secretory transport of ranitidine and famotidine across Caco-2 cell monolayers

The secretory transport of the H(2)-antagonists, ranitidine and famotidine, across Caco-2 cell monolayers was found to be a saturable process. Both drugs exhibited greater permeability in the basolateral (BL) to apical (AP) direction than in the AP to BL direction, indicating apically directed secretion; BL to AP transport was inhibited by P-glycoprotein (P-gp) inhibitors verapamil and cyclosporin A. The cellular uptake of ranitidine across the BL membrane was saturable and temperature dependent, indicative of carrier-mediated transport. The K(m) and V(max) for the uptake process were estimated to be 66.9 mM and 20.9 nmol/mg of protein/min, respectively. The uptake of [(14)C]ranitidine across the BL membrane was inhibited by unlabeled ranitidine and structurally diverse organic cations. The tetraethylammonium (TEA)-sensitive organic cation transporters are not involved in the uptake of ranitidine and famotidine across the BL membrane of Caco-2. This conclusion was based on the evidence that functionally active TEA-sensitive organic cation transporters did not exist in the BL membranes of the Caco-2 cells, whereas the functionally active TEA-sensitive organic cation transporter(s) in LLC-PK(1) cells did not contribute to the transport of ranitidine or famotidine across the cell monolayers. Thus, we conclude that the secretory transport of ranitidine and famotidine across Caco-2 cell monolayers is mediated by 1) a carrier in the BL membrane that is distinct from the TEA-sensitive organic cation transporter(s) and 2) P-gp in the apical membrane.

Molecular identification of a novel carnitine transporter specific to human testis. Insights into the mechanism of carnitine recognition

l-Carnitine is an essential component of mitochondrial fatty acid beta-oxidation and plays a pivotal role in the maturation of spermatozoa within the male reproductive tract. Epididymal plasma contains the highest levels of l-carnitine found in the human body, and initiation of sperm motility occurs in parallel to l-carnitine increase in the epididymal lumen. Using a specific carrier, epididymal epithelium secretes l-carnitine into the lumen by an active transport mechanism; however, the structure-activity relationship comprising the carnitine-permeation pathway is poorly understood. We discovered a novel carnitine transporter (CT2) specifically located in human testis. Analyzing the primary structure of CT2 revealed that it is phylogenetically located between the organic cation transporter (OCT/OCTN) and anion transporter (OAT) families. Hence, CT2 represents a novel transporter family. When expressed in Xenopus oocytes, CT2 mediates the high affinity transport of l-carnitine but does not accept mainstream OCT/OCTN cationic or OAT anionic substrates. We synthesized and tested various carnitine-related compounds and investigated the physicochemical properties of substrate recognition by semi-empirical computational chemistry. The data suggest that the quaternary ammonium cation bulkiness and relative hydrophobicity be the most important factors that trigger CT2-substrate interactions. Immunohistochemistry showed that the CT2 protein is located in the luminal membrane of epididymal epithelium and within the Sertoli cells of the testis. The identification of CT2 represents an interesting evolutionary link between OCT/OCTNs and OATs, as well as provides us with an important insight into the maturation of human spermatozoa.

Functional characterization of mouse cation transporter mOCT2 compared with mOCT1

We characterized the function of mouse organic cation transporter OCT2 (TC 2.A.19.1.5) in comparison with that of OCT1 (TC 2.A.19.1.1). Uptake of [(3)H]1-methyl-4-phenylpyridinium ([(3)H]MPP(+)) by Xenopus laevis oocytes injected with mOCT1 (Slc22a1) or mOCT2 (Slc22a2) cRNA was attenuated by an increase of extracellular K(+) concentration and under acidic extracellular conditions. The uptakes of [(3)H]MPP(+) via mOCT1 and mOCT2 were saturable, with similar Michaelis constants (K(t)) of 10 and 24 microM, respectively. mOCT2 also mediated the uptake of [(14)C]tetraethylammonium with a K(t) value of 36 microM, which is similar to that of mOCT1. Quinine, tetraethylammonium, cimetidine, procainamide, choline, and N(')-methylnicotinamide inhibited the uptake of [(3)H]MPP(+) via mOCT1, as well as via mOCT2, and the inhibitory potencies for mOCT1 were comparable to but slightly higher than those for mOCT2. Thus, although the transport properties of mOCT2 are similar to those of mOCT1 in respect to the membrane-potential dependency, pH-sensitivity, and affinities for MPP(+) and tetraethylammonium, several organic cations had weaker inhibitory effects on [(3)H]MPP(+) uptake by mOCT2 than by mOCT1.

Guanidine transport across the apical and basolateral membranes of human intestinal Caco-2 cells is mediated by two different mechanisms

The functional characteristics of the intestinal absorption and secretion of guanidine as a model of a nutritionally and metabolically essential organic cation were examined in the Caco-2 human intestinal cell line. Both apical and basolateral transport of [14C]-guanidine were studied using Caco-2 cells grown on polycarbonate permeable membranes. The basolateral-to-apical flux of [14C]-guanidine (i.e., its secretion) was quantitatively higher than the apical-to-basolateral transport (i.e., its absorption). When Na+ was replaced by K+ or Li+, both apical and basolateral accumulation were significantly inhibited. Studies using the cell monolayers and apical membrane vesicles obtained from Caco-2 cells showed a potential-independent mechanism of guanidine apical uptake and efflux. Conversely, basolateral uptake and efflux were membrane potential dependent. Kinetic analysis revealed that both saturable and nonsaturable mechanisms accounted for the apical and basolateral accumulations. The [14C]-guanidine efflux from cells through the apical and basolateral membranes was significantly reduced at 4 degrees C, suggesting carrier-mediated mechanisms. Moreover, the apical efflux was stimulated by an inwardly directed H+ gradient. Influx and efflux of [14C]-guanidine were unaffected by the presence of tetraethylammonium, cimetidine or decynium-22 in the donor compartment. Only quinine significantly reduced [14C]-guanidine entrance through apical and basolateral membranes and its exit through the basolateral membrane. In conclusion, our results suggest that the influx and the efflux through the apical membrane is mediated by different transporters, whereas transport across the basolateral membrane is mediated by a member of the organic cation transporter family with high affinity for guanidine.

Expression and pharmacological profile of the human organic cation transporters hOCT1, hOCT2 and hOCT3

1. Organic cation transporters (OCTs) are involved in the elimination of monoamines and cationic xenobiotics. To examine whether some cell lines express several different OCTs, we investigated seven human cell lines for the mRNA expression pattern of the human (h) transporters hOCT1, hOCT2 and hOCT3. hOCT1 mRNA was found in all cell lines, six additionally expressed hOCT3 and only two cell lines contained all three hOCTs. 2. Among the three OCTs only for the OCT3 (also designated as 'uptake(2)' or 'extraneuronal monoamine transporter') 'selective' inhibitors are described in the literature. The affinities of the OCT3 inhibitors for the other two OCTs are largely unknown. Therefore, we compared the potencies of eight compounds as inhibitors of hOCT-mediated uptake of the organic cation [(3)H]-1-methyl-4-phenylpyridinium ([(3)H]-MPP(+)) in human embryonic kidney 293 (HEK293) cells stably expressing hOCT1, hOCT2 or hOCT3. Decynium-22 inhibited hOCT3 with 10 fold higher potency than hOCT1 and hOCT2. Corticosterone was about 100 fold more potent as inhibitor of hOCT3 than of hOCT1 or hOCT2, and O-methylisoprenaline (OMI) inhibited almost exclusively hOCT3. Progesterone and beta-Oestradiol preferentially inhibited hOCT3 and hOCT1, whereas prazosin was a potent inhibitor of hOCT1 and hOCT3. Phenoxybenzamine (PbA) inhibited with about equal apparent potency all three hOCTs, whereas the PbA derivative SKF550 ((9-fluorenyl)-N-methyl-beta-chloroethylamine) preferentially inhibited hOCT3 and hOCT2. 3. PbA reversibly inhibited hOCT1 and irreversibly hOCT2 and hOCT3; SKF550 also irreversibly inhibited hOCT3 but hOCT2 in a reversible manner. 4. These compounds enable a functional discrimination of the three hOCTs: hOCT1 is selectively inhibited by prazosin, reversibly inhibited by PbA and it is not sensitive to inhibition by SKF550 and OMI; hOCT2 is reversibly inhibited by SKF550, irreversibly by PbA and not by prazosin, beta-oestradiol and OMI, whereas hOCT3 is selectively inhibited by corticosterone, OMI and decynium22.

Molecular cloning of rabbit organic cation transporter rbOCT2 and functional comparisons with rbOCT1

Multiple organic cation transporters (OCTs) are present in rabbit kidney and may play different functional roles. We cloned rabbit OCT2 (rbOCT2) and compared its function with that of rabbit OCT1 (rbOCT1). In transiently transfected COS-7 cells, rbOCT1 and rbOCT2 mediated uptake of [3H]tetraethylammonium (TEA) with K(t) values of 188 and 125 microM, respectively. n-Tetraalkylammonium compounds showed similar affinities for the two homologs, with IC50 values for inhibition of OCT1- and OCT2-mediated [3H]TEA transport, respectively, of 4,538 and 1,395 microM for tetramethylammonium, 88.5 and 3.9 microM for tetrapropylammonium, 13.9 and 5.3 microM for tetrabutylammonium, and 8.8 and 7.6 microM for tetrapentylammonium. However, the transporters had very different affinities for cimetidine (CIM): IC50 of 916 and 5.7 microM for rbOCT1 and rbOCT2, respectively. CIM inhibition of TEA uptake into single S2 segments of rabbit proximal tubule was used to estimate the contributions of OCT1 and OCT2 to basolateral organic cation uptake. The median IC50 for CIM inhibition of TEA uptake was 12.3 microM, suggesting that OCT2 is the major contributor to basolateral organic cation transport in the S2 segment of proximal tubule in rabbit kidney.

The human organic cation transporter (hOCT2) recognizes the degree of substrate ionization

The organic cation transporter, OCT2, plays a role in renal secretion of a broad array of weak bases. To determine whether the degree of ionization of these compounds plays a role in their interaction with OCT2, we examined the influence of external pH values on the activity of the human ortholog of OCT2, as expressed in Chinese hamster ovary-K1 cells. Importantly, changing the pH value from 7.0 to 8.0 had no effect on the rate of transport of the fixed cations, tetraethylammonium and 1-methyl-4-phenylpyridinium, i.e. the pH value did not have an effect upon the transporter itself. Cimetidine (pK(a) 6.92), a competitive inhibitor of hOCT2, displayed a 3.5-fold increase in IC(50) as pH values increased from 7 to 8. hOCT2-mediated cimetidine transport decreased over this pH range, the consequence of an increase in K(t) and decrease in J(max) at the higher pH value. The weak bases trimethoprim and 4-phenylpyridine showed a similar pattern of pH-sensitive interaction with hOCT2. The non-ionizable sterol, corticosterone, also inhibited hOCT2 activity, although it was neither competitive in nature nor was it sensitive to pH in the manner observed with weak bases. We conclude that the degree of ionization plays a critical role in binding of substrate to organic cation transporters.

The impact of efflux transporters in the brain on the development of drugs for CNS disorders

The development of drugs to treat disorders of the CNS requires consideration of achievable brain concentrations. Factors that influence the brain concentrations of drugs include the rate of transport into the brain across the blood-brain barrier (BBB), metabolic stability of the drug, and active transport out of the brain by efflux mechanisms. To date, three classes of transporter have been implicated in the efflux of drugs from the brain: multidrug resistance transporters, monocarboxylic acid transporters, and organic ion transporters. Each of the three classes comprises multiple transporters, each of which has multiple substrates, and the combined substrate profile of these transporters includes a large number of commonly used drugs. This system of transporters may therefore provide a mechanism through which the penetration of CNS-targeted drugs into the brain is effectively minimised. The action of these efflux transporters at the BBB may be reflected in the clinic as the minimal effectiveness of drugs targeted at CNS disorders, including HIV dementia, epilepsy, CNS-based pain, meningitis and brain cancers. Therefore, modulation of these efflux transporters by design of inhibitors and/or design of compounds that have minimal affinity for these transporters may well enhance the treatment of intractable CNS disorders.

Carriers involved in targeting the cytostatic bile acid-cisplatin derivatives cis-diammine-chloro-cholylglycinate-platinum(II) and cis-diammine-bisursodeoxycholate-platinum(II) toward liver cells

Molecular bases for targeting bile acid-cisplatin derivatives Bamet-R2 [cis-diammine-chloro-cholylglycinate-platinum(II)] and Bamet-UD2 [cis-diammine-bisursodeoxycholate-platinum(II)] toward liver cells were investigated. Carriers for bile acids [human Na(+)-taurocholate cotransporting polypeptide (NTCP)], organic anions [organic anion transporting polypeptide (OATP)], and organic cations [organic cation transporter (OCT)] were expressed in Xenopus laevis oocytes (XO) and Chinese hamster ovary (CHO) cells. Drug uptake was measured by flameless atomic absorption of platinum. Rat Oatp1- or rat Ntcp-transfected CHO cells were able to take up Bamets, but not cisplatin, severalfold more efficiently than wild-type cells. This uptake was enhanced by butyrate-induced expression of both carriers. Uptake of both Bamets by Ntcp-transfected CHO cells was stimulated by extracellular sodium. The amount of Bamets, but not cisplatin, taken up by XO was enhanced when expressing OATP-A, OATP-C, NTCP, OCT1, or OCT2, a nonhepatic OCT isoform used for comparative purposes. Bamet uptake by XO was inhibited by known substrates of these carriers (glycocholate for NTCP and OATP-C, ouabain for OATP-A, and quinine for OCT1 and OCT2). Drug uptake versus substrate concentration revealed saturation kinetics (K(m) was in the 8-58 microM range), with the following order of efficiency of transport (V(max)/K(m)) for Bamet-R2: OATP-C > OCT2 > OATP-A > NTCP > OCT1; and the following order of efficiency of transport for Bamet-UD2: OATP-C > OCT2 > OATP-A > OCT1 > NTCP. Increasing the generation of cationic forms of Bamets by incubation in the absence of chloride increased drug uptake by OATP-A, OCT1, and OCT2 but reduced that achieved by NTCP and OATP-C. These results suggest a role for carriers of organic anions and cations in Bamet-R2 and Bamet-UD2 uptake, which may determine their ability to accumulate in liver tumor cells and/or be taken up and efficiently excreted by hepatocytes.

Tissue distribution and renal developmental changes in rat organic cation transporter mRNA levels

Organic cation transporters (OCTs) are responsible for excretion of cationic substances into urine. Tissue OCT expression may be important for the disposition and excretion of xenobiotics. Therefore, OCT1, OCT2, OCT3, OCTN1, and OCTN2 mRNA levels were measured in adult rat tissues and rat kidney tissue at various stages of development from day 0 to 45. OCT1 mRNA expression was highest in kidney and spleen, moderate in skin, and low in the gastrointestinal tract, brain, lung, thymus, muscle, and prostate. OCT2 mRNA levels were highest in kidney, with low expression in other tissues, and with renal OCT2 levels being approximately 4 times higher in males than that in females. In gonadectomized males, OCT2 mRNA levels were attenuated to female levels, suggesting a role for testosterone in OCT2 expression. OCT3 was moderately expressed in kidney and was highest in blood vessel, skin, and thymus. OCTN1 was expressed in most of the tissues examined, with relatively higher expression in kidney and ileum and lower levels in thymus. Lastly, OCTN2 was expressed abundantly in kidney and ileum, moderately in large intestine, dorsal prostate, bladder, duodenum, and cerebellum, and minimally in thymus, spleen, and cerebral cortex. Renal OCT1, OCTN1, and OCTN2 mRNA levels increased gradually from postnatal day 0 through day 45 in both genders. Renal OCT2 levels remained the same in males and females through day 25 and then dramatically increased only in male kidney after day 30. In summary, OCT mRNA was detected primarily in kidney, and the high level of renal OCT expression may explain why the kidney is a target organ for xenobiotics with cationic properties.

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.

Cell membrane transport of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in the liver and systemic bioavailability

Modulation of hepatic disposition of MPTP could influence susceptibility to its neurotoxicity. Therefore, we studied hepatocellular transport of MPTP in the perfused rat liver and isolated rat hepatocytes. The perfused liver extensively extracted MPTP. Amiloride and tubocurarine, inhibitors of OCT1, increased MPTP recovery (253 +/- 78 and 283 +/- 64%, respectively) and reduced PS(influx) (0.69 +/- 0.36 to 0.27 +/- 0.11, and 0.97 +/- 0.50 to 0.23 +/- 0.05 ml/s/g, respectively). P-glycoprotein inhibitor, daunomycin, and Oatp 1 & 2 inhibitor, rifamycin, had no effect. In isolated hepatocytes, amiloride and tubocurarine increased hepatic uptake of MPTP (23 +/- 12 and 6 +/- 2%, respectively). Daunomycin reduced MPTP uptake by 22 +/- 8% and rifamycin had no effect. Only a small proportion of MPTP is taken up into hepatocytes by transporters; however, modulation of these transport mechanisms will influence systemic bioavailability.

Ventricular choline transport: a role for organic cation transporter 2 expressed in choroid plexus

To determine whether organic cation transporter (OCT) family members might mediate choline transport in choroid plexus (CP), the handling of choline by cloned transporters and by intact CP isolated from the adult rat was investigated. Expression of OCT1 and OCT2 in Xenopus oocytes increased hemicholinium-3-sensitive choline uptake. In contrast, OCT3 did not mediate choline transport. Estimated K(m) values for choline in rOCT1-, rOCT2-, and hOCT2-expressing oocytes were 346 +/- 50, 441 +/- 67, and 102 +/- 80 microm, respectively. Membrane potential was the major driving force for choline uptake in rat and human OCT2-expressing oocytes and in intact CP in vitro. Lowering of medium pH (6 versus 7.4) was equally effective at inhibiting choline uptake in CP, suggesting that there might be a non-OCT component of choline uptake that is responsive to an H(+) gradient. However, choline efflux from CP was not stimulated by a trans-applied H(+) gradient. Choline uptake by CP was Na(+)-independent with an estimated K(m) of 183 microm. Reverse transcriptase-polymerase chain reaction detected OCT2 and OCT3, but not OCT1, mRNA expression in CP. Transfection of intact CP with a rOCT2/green fluorescent protein fusion construct resulted in strong apical membrane fluorescence with no detectable signal in the basal and lateral plasma membranes. These data indicate that OCT2 mediates choline transport across the ventricular membrane of CP.

Distinct characteristics of organic cation transporters, OCT1 and OCT2, in the basolateral membrane of renal tubules

PURPOSE

This study was performed to determine the detailed mRNA distribution of organic cation transporters, rOCT1 and rOCT2, along the rat nephron and to distinguish the substrate affinities of these transporters.

METHODS

The distributions of rOCT1 and rOCT2 mRNA were determined by reverse transcriptase polymerase chain reaction analysis of microdissected nephron segments. Using MDCK cells transfected with rOCT1 or rOCT2 cDNA, the inhibitory effects of various compounds on the uptake of [14C]tetraethylammonium were assessed.

RESULTS

rOCT1 mRNA was detected primarily in the superficial and juxtamedullary proximal convoluted tubules, whereas rOCT2 mRNA was detected widely in the superficial and juxtamedullary proximal straight and convoluted tubules, medullary thick ascending limbs, distal convoluted tubule, and cortical collecting duct. The IC50 values for cationic drugs and endogenous cations on [14C]tetraethylammonium uptake across the basolateral membranes in the transfectants indicated that rOCT1 and rOCT2 had similar inhibitor specificity for many compounds but showed moderate differences in the specificity for several compounds, such as 1-methyl-4-phenylpyridinium, dopamine, disopyramide, and chlorpheniramine.

CONCLUSIONS

rOCT1 and rOCT2 possess similar but not identical multispecificities for various compounds with distinct distributions along the nephron, indicating that the two transporters share physiologic and pharmacologic roles in the renal handling of cationic compounds.

The gut as a barrier to drug absorption: combined role of cytochrome P450 3A and P-glycoprotein

Intestinal phase I metabolism and active extrusion of absorbed drug have recently been recognised as major determinants of oral bioavailability. Cytochrome P450 (CYP) 3A, the major phase I drug metabolising enzyme in humans, and the multidrug efflux pump, P-glycoprotein, are present at high levels in the villus tip of enterocytes in the gastrointestinal tract, the primary site of absorption for orally administered drugs. The importance of CYP3A and P-glycoprotein in limiting oral drug delivery is suggested to us by their joint presence in small intestinal enterocytes, by the significant overlap in their substrate specificities, and by the poor oral bioavailability of joint substrates for these 2 proteins. These proteins are induced or inhibited by many of the same compounds. A growing number of preclinical and clinical studies have demonstrated that the oral bioavailability of many CYP3A and/or P-glycoprotein substrate drugs can be increased by concomitant administration of CYP3A inhibitors and/or P-glycoprotein inhibitors. We believe that further understanding the physiology and biochemistry of the interactive nature of intestinal CYP3A and P-glycoprotein will be important in defining, controlling, and improving oral bioavailability of CYP3A/P-glycoprotein substrates.

Properties and regulation of organic cation transport in freshly isolated human proximal tubules

The kidney, and more specifically the proximal tubule, is the main site of elimination of cationic endogenous metabolites and xenobiotics. Although numerous studies exist on renal organic cation transport of rat and rabbit, no information is available from humans. Therefore, we examined organic cation transport and its regulation across the basolateral membrane of isolated human proximal tubules. mRNA for the cation transporters hOCT1 and hOCT2 as well as hOCTN1 and hOCTN2 was detected in these tubules. Organic cation transport across the basolateral membrane of isolated collapsed proximal tubules was recorded with the fluorescent dye 4-(4-dimethylamino)styryl-N-methylpyridinium (ASP(+)). Depolarization of the cells by rising extracellular K(+) concentration to 145 mm reduced ASP(+) uptake by 20 +/- 5% (n = 15), indicating its electrogeneity. The substrates of organic cation transport tetraethylammonium (K(i) = 63 microm) and cimetidine (K(i) = 11 microm) as well as the inhibitor quinine (K(i) = 2.9 microm) reduced ASP(+) uptake concentration dependently. Maximal inhibition reached with these substances was approximately 60%. Stimulation of protein kinase C with 1,2-dioctanoyl-sn-glycerol (DOG, 1 microm) or ATP (100 microm) inhibited ASP(+) uptake by 30 +/- 3 (n = 16) and 38 +/- 13% (n = 6), respectively. The effect of DOG could be reduced with calphostin C (0.1 microm, n = 7). Activation of adenylate cyclase by forskolin (1 microm) decreased ASP(+) uptake by 29 +/- 3% (n = 10). hANP (10 nm) or 8-bromo-cGMP (100 microm) also decreased ASP(+) uptake by 17 +/- 3 (n = 9) or 32 +/- 5% (n = 10), respectively. We show for the first time that organic cation transport across the basolateral membrane of isolated human proximal tubules, most likely mediated via hOCT2, is electrogenic and regulated by protein kinase C, the cAMP- and the cGMP-dependent protein kinases.

Mechanism of thiamine uptake by human jejunal brush-border membrane vesicles

Thiamine, a water-soluble vitamin, is essential for normal cellular functions, growth and development. Thiamine deficiency leads to significant clinical problems and occurs under a variety of conditions. To date, however, little is known about the mechanism of thiamine absorption in the native human small intestine. The objective of this study was, therefore, to characterize the mechanism of thiamine transport across the brush-border membrane (BBM) of human small intestine. With the use of purified BBM vesicles (BBMV) isolated from the jejunum of organ donors, thiamine uptake was found to be 1) independent of Na(+) but markedly stimulated by an outwardly directed H(+) gradient (pH 5.5(in)/pH 7.5(out)); 2) competitively inhibited by the cation transport inhibitor amiloride (inhibitor constant of 0.12 mM); 3) sensitive to temperature and osmolarity of the incubation medium; 4) significantly inhibited by thiamine structural analogs (amprolium, oxythiamine, and pyrithiamine), but not by unrelated organic cations (tetraethylammonium, N-methylnicotinamide, or choline); 5) not affected by the addition of ATP to the inside and outside of the BBMV; 6) potential insensitive; and 7) saturable as a function of thiamine concentration with an apparent Michaelis-Menten constant of 0.61 +/- 0.08 microM and a maximal velocity of 1.00 +/- 0.47 pmol. mg protein(-1). 10 s(-1). Carrier-mediated thiamine uptake was also found in BBMV of human ileum. These data demonstrate the existence of a Na(+)-independent, pH-dependent, amiloride-sensitive, electroneutral carrier-mediated mechanism for thiamine absorption in native human small intestinal BBMV.

Interaction of cations, anions, and weak base quinine with rat renal cation transporter rOCT2 compared with rOCT1

The rat organic cation transporter (rOCT)-2 was characterized by electrical and tracer flux measurements compared with rOCT1. By applying choline gradients to voltage-clamped Xenopus oocytes expressing rOCT2, potential-dependent currents could be induced in both directions. Tracer flux measurements with seven organic cations revealed similar Michaelis-Menten constant values for both transporters, with the exception of guanidine. In parallel experiments with rOCT2 and rOCT1, inhibition of tetraethylammonium transport by 12 cations, 2 weak bases, corticosterone, and the anions para-amminohippurate, alpha-ketoglutarate, and probenecid was characterized. The IC(50) values of many inhibitors were similar for both transporters, whereas others were significantly different. Mepiperphenidol and O-methylisoprenaline showed an approximately 70-fold lower and corticosterone a 38-fold higher affinity for rOCT2. With the use of these inhibitors together with previous information on cation transporters, experimental protocols are proposed to dissect out the individual contributions of rOCT2 and rOCT1 in intact proximal tubule preparations. Inhibition experiments at different pH levels strongly suggest that the weak base quinine passively permeates the plasma membrane at physiological pH and inhibits rOCT2 from the intracellular side.

Maintenance of serotonin in the intestinal mucosa and ganglia of mice that lack the high-affinity serotonin transporter: Abnormal intestinal motility and the expression of cation transporters

The enteric serotonin reuptake transporter (SERT) has been proposed to play a critical role in serotonergic neurotransmission and in the initiation of peristaltic and secretory reflexes. We analyzed potential compensatory mechanisms and enteric function in the bowels of mice with a targeted deletion of SERT. The guts of these animals were found to lack mRNA encoding SERT; moreover, high-affinity uptake of 5-HT into epithelial cells, mast cells, and enteric neurons was present in the SERT +/+ bowel but absent in the SERT -/- bowel. However, both the SERT +/+ gut and the -/- gut expressed molecules capable of transporting 5-HT, but with affinities and selectivity much lower than those of SERT. These included the dopamine transporter (DAT) and polyspecific organic cation transporters OCT-1 and OCT-3. DAT and OCT immunoreactivities were present in both the submucosal and myenteric plexuses, and the OCTs were also located in the mucosal epithelium. 5-HT was found in all of its normal sites in the SERT -/- bowel, which contained mRNA encoding tryptophan hydroxylase, but no 5-HT was present in the blood of SERT -/- animals. Stool water and colon motility were increased in most SERT -/- animals; however, the increase in motility (diarrhea) occasionally alternated irregularly with decreased motility (constipation). The watery diarrhea is probably attributable to the potentiation of serotonergic signaling in SERT -/- mice, whereas the transient constipation may be caused by episodes of enhanced 5-HT release leading to 5-HT receptor desensitization.

Mechanisms and clinical implications of renal drug excretion

The body defends itself against potentially harmful compounds like drugs, toxic compounds, and their metabolites by elimination, in which the kidney plays an important role. Renal clearance is used to determine renal elimination mechanisms of a drug, which is the result of glomerular filtration, active tubular secretion and reabsorption. The renal proximal tubule is the primary site of carrier-mediated transport from blood to urine. Renal secretory mechanisms exists for, anionic compounds and organic cations. Both systems comprises several transport proteins, and knowledge of the molecular identity of these transporters and their substrate specificity has increased considerably in the past decade. Due to overlapping specificities of the transport proteins, drug interactions at the level of tubular secretion is an event that may occur in clinical situation. This review describes the different processes that determine renal drug handling, the techniques that have been developed to attain more insight in the various aspects of drug excretion, the functional characteristics of the individual transport proteins, and finally the implications of drug interactions in a clinical perspective.

The role of neuronal and extraneuronal plasma membrane transporters in the inactivation of peripheral catecholamines

Catecholamines are translocated across plasma membranes by transporters that belong to two large families with mainly neuronal or extraneuronal locations. In mammals, neuronal uptake of catecholamines involves the dopamine transporter (DAT) at dopaminergic neurons and the norepinephrine transporter (NET) at noradrenergic neurons. Extraneuronal uptake of catecholamines is mediated by organic cation transporters (OCTs), including the classic corticosterone-sensitive extraneuronal monoamine transporter. Catecholamine transporters function as part of uptake and metabolizing systems primarily responsible for inactivation of transmitter released by neurons. Additionally, the neuronal catecholamine transporters, recycle catecholamines for rerelease, thereby reducing requirements for transmitter synthesis. In a broader sense, catecholamine transporters function as part of integrated systems where catecholamine synthesis, release, uptake, and metabolism are regulated in a coordinated fashion in response to the demands placed on the system. Location is also important to function. Neuronal transporters are essential for rapid termination of the signal in neuronal-effector organ transmission, whereas non-neuronal transporters are more important for limiting the spread of the signal and for clearance of catecholamines from the bloodstream. Besides their presynaptic locations, NET and DAT are also present at several extraneuronal locations, including syncytiotrophoblasts of the placenta and endothelial cells of the lung (NET), stomach and pancreas (DAT). The extraneuronal monoamine transporter shows a broad tissue distribution, whereas the other two non-neuronal catecholamine transporters (OCT1 and OCT2) are mainly localized to the liver, kidney, and intestine. Altered function of peripheral catecholamine transporters may be involved in disturbances of the autonomic nervous system, such as occurs in congestive heart failure and hypernoradrenergic hypertension. Peripheral catecholamine transporters provide important targets for clinical imaging of sympathetic nerves and diagnostic localization and treatment of neuroendocrine tumors, such as neuroblastomas and pheochromocytomas.

Transporters involved in the elimination of drugs in the kidney: organic anion transporters and organic cation transporters

Transporters in the kidney mediate the secretion or reabsorption of many compounds and thereby influence the plasma levels of their substrates. Organic anion transporters and organic cation transporters are two major classes of secretory transporters in the mammalian kidney. During the past decade, significant progress has been made in the cloning, functional expression, and initial characterization of these transporters. To date, five organic cation transporters and nine organic anion transporters have been cloned. In this review, we summarize the available data on organic anion and organic cation transporters, focusing in particular on their molecular characteristics, tissue distribution, and inhibitor and substrate selectivities. Currently we have a good understanding of the inhibitor selectivities for most of these transporters, and with the development of more robust assays, we will soon have a better understanding of their substrate selectivities. Based on the available data, summarized in this review, it appears that many compounds interact with multiple transporters. Furthermore, there appears to be substantial overlap in the selectivities of organic cation transporters, and the same appears true for organic anion transporters. At the present time, it is unclear what the roles of these multiple transporters are in renal drug elimination. With the development of new assays, reagents, and experimental methods, we will soon have a better understanding of the roles of each transporter isoform in the renal elimination of drugs.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Molecular and functional characterization of organic cation/carnitine transporter family in mice

Carnitine is essential for beta-oxidation of fatty acids, and a defect of cell membrane transport of carnitine leads to fatal systemic carnitine deficiency. We have already shown that a defect of the organic cation/carnitine transporter OCTN2 is a primary cause of systemic carnitine deficiency. In the present study, we further isolated and characterized new members of the OCTN family, OCTN1 and -3, in mice. All three members were expressed commonly in kidney, and OCTN1 and -2 were also expressed in various tissues, whereas OCTN3 was characterized by predominant expression in testis. When their cDNAs were transfected into HEK293 cells, the cells exhibited transport activity for carnitine and/or the organic cation tetraethylammonium (TEA). Carnitine transport by OCTN1 and OCTN2 was Na(+)-dependent, whereas that by OCTN3 was Na(+)-independent. TEA was transported by OCTN1 and OCTN2 but not by OCTN3. The relative uptake activity ratios of carnitine to TEA were 1.78, 11.3, and 746 for OCTN1, -2, and -3, respectively, suggesting high specificity of OCTN3 for carnitine and significantly lower carnitine transport activity of OCTN1. Thus, OCTN3 is unique in its limited tissue distribution and Na(+)-independent carnitine transport, whereas OCTN1 efficiently transported TEA with minimal expression of carnitine transport activity and may have a different role from other members of the OCTN family.

Expression and regulation of hepatic drug and bile acid transporters

Transport across hepatocyte plasma membranes is a key parameter in hepatic clearance and usually occurs through different carrier-mediated systems. Sinusoidal uptake of compounds is thus mediated by distinct transporters, such as Na(+)-dependent or Na(+)-independent anionic transporters and by some cationic transporters. Similarly, several membrane proteins located at the apical pole of hepatocytes have been incriminated in the excretion of compounds into the bile. Indeed, biliary elimination of anionic compounds, including glutathione S-conjugates, is mediated by MRP2, whereas bile salts are excreted by a bile salt export pump (BSEP) and Class I-P-glycoprotein (P-gp) is involved in the secretion of amphiphilic cationic drugs, whereas class II-P-gp is a phospholipid transporter. The expression of hepatic transporters and their activity are regulated in various situations, such as ontogenesis, carcinogenesis, cholestasis, cellular stress and after treatment by hormones and xenobiotics. Moreover, a direct correlation between a defect and the absence of transporter with hepatic disease has been demonstrated for BSEP, MDR3-P-gp and MRP2.

Function and regulation of ATP-binding cassette transport proteins involved in hepatobiliary transport

Hepatobiliary transport of endogenous and exogenous compounds is mediated by the coordinated action of multiple transport systems present at the sinusoidal (basolateral) and canalicular (apical) membrane domains of hepatocytes. During the last few years many of these transporters have been cloned and functionally characterized. In addition, the molecular bases of several forms of cholestatic liver disease have been defined. Combined, this has greatly expanded our understanding of the normal physiology of bile formation, the pathophysiology of intrahepatic cholestasis, as well as of drug elimination and disposition processes. In this review recent advances, with respect to function and regulation of ATP binding cassette transport proteins expressed in liver, are summarized and discussed.

Localization of organic cation transporters OCT1 and OCT2 in rat kidney

Renal excretion and reabsorption of organic cations are mediated by electrogenic and electroneutral organic cation transporters, which belong to a recently discovered family of polyspecific transporters. These transporters are electrogenic and exhibit differences in substrate specificity. In rat, the renal expression of the polyspecific cation transporters rOCT1 and rOCT2 was investigated. By in situ hybridization, significant amounts of both rOCT1 and rOCT2 mRNA were detected in S1, S2, and S3 segments of proximal tubules. By immunohistochemistry, expression of the rOCT1 protein was mainly observed in S1 and S2 segments of proximal tubules, with lower expression levels in the S3 segments. At variance, rOCT2 protein was mainly expressed in the S2 and S3 segments. Both transporters were localized to the basolateral cell membrane. Neither rOCT1 nor rOCT2 was detected in the vasculature, the glomeruli, and nephron segments other than proximal tubules. The data suggest that rOCT1 and rOCT2 are responsible for basolateral cation uptake in the proximal tubule, which represents the first step in cation secretion.

Structure, function, and regional distribution of the organic cation transporter OCT3 in the kidney

We examined in this study the expression of the potential-sensitive organic cation transporter OCT3 in the kidney. A functionally active OCT3 was cloned from a mouse kidney cDNA library. The cloned transporter was found to be capable of mediating potential-dependent transport of a variety of organic cations including tetraethylammonium. This function was confirmed in two different heterologous expression systems involving mammalian cells and Xenopus laevis oocytes. We have also isolated the mouse OCT3 gene and deduced its structure and organization. The OCT3 gene consists of 11 exons and 10 introns. In situ hybridization studies in the mouse kidney have shown that OCT3 mRNA is expressed primarily in the cortex. The expression is evident in the proximal and distal convoluted tubules. The expression of OCT3 in human kidney was confirmed by RT-PCR. We have also cloned OCT3 from human placenta and human kidney. Human OCT3 exhibits 86% identity with mouse OCT3 in amino acid sequence. Human OCT3 was found to transport tetraethylammonium and a variety of other organic cations. The transport process was electrogenic. We conclude that OCT3 is expressed in mammalian kidney and that it plays an important role in the renal clearance of cationic drugs.

Cellular and molecular aspects of drug transport in the kidney

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

Molecular cloning of a Na+-dependent nucleoside transporter from rabbit intestine

PURPOSE

Substantial species differences in the transport kinetics of nucleosides and therapeutic nucleoside analogs have been observed in various experimental systems. To explain these differences at a molecular level, it is necessary to clone the relevant transporters and examine their functional characteristics in heterologous expression systems. The goal of the present study was to clone the nucleoside transporters present in rabbit, an important preclinical animal model, and to functionally characterize the clone(s).

METHODS

A Polymerase Chain Reaction (PCR)-based homology cloning approach in conjunction with Rapid Amplification of cDNA Ends (RACE) was used to isolate a full-length cDNA. Characterization of this transporter was accomplished through heterologous expression in Xenopus laevis oocytes.

RESULTS

A full-length cDNA encoding a purine-selective, Na+-dependent nucleoside transporter, rbSPNT1, was isolated from rabbit small intestine. The encoded protein is 658 amino acid residues in length. Hydropathy analysis suggests that rbSPNT1 has 11 to 14 transmembrane domains. In Xenopus laevis oocytes expressing rbSPNT1, the uptake of uridine and inosine was enhanced significantly; uridine transport was inhibited by purine, but not pyrimidine nucleosides. mRNA transcripts for rbSPNT1 were detected primarily in intestine, lung, and kidney and at lower levels in liver, brain, and heart.

CONCLUSIONS

A full-length functional nucleoside transporter was cloned. Sequence analysis and functional characterization suggest that rbSPNT1 is the rabbit homolog of the purine-selective nucleoside transporter, N1. The cloned rbSPNT1 can be used to understand the molecular mechanisms responsible for the observed species differences in the transport of nucleosides and therapeutic nucleoside analogs.

The affinity of the organic cation transporter rOCT1 is increased by protein kinase C-dependent phosphorylation

Members of the organic cation transporter (OCT) family are mainly expressed in kidney, liver, intestine, and brain. The regulation of the OCT type 1 from rat (rOCT1) stably transfected in HEK293 cells was examined using a fluorimetric technique, 1-[(3)H]methyl-4-phenylpyridinium uptake studies, and fast-whole-cell patch-clamp recordings. For the fluorescence measurements, the cation 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP(+)) was used as substrate. Uptake of ASP(+) via rOCT1 was electrogenic, and its inhibition by other organic cations was consistent with previously reported radioactive tracer flux measurements. The inhibitor quinine was not translocated by the organic cation transporter in contrast to tetraethylammonium. Stimulation of diacyl glycerol-dependent protein kinase C (PKC) by sn-1,2-dioctanoyl glycerol (1 microM) resulted in an increase in initial ASP(+) uptake rate by 216 +/- 28% (n = 29). The effect was completely antagonized by the PKC inhibitor tamoxifen (20 microM, n = 22). Forskolin (1 microM), which activates adenylate cyclase and thereby protein kinase A (PKA), stimulated the initial rate of ASP(+) accumulation by 51 +/- 6% (n = 19). This effect was inhibited by the specific PKA inhibitor KT5720 (1 microM, n = 12). Inhibition of tyrosine kinases by aminogenestein (10 microM) reduced ASP(+) uptake by 63 +/- 7% (n = 7), while genestein or tyrphostin AG1295 (each 10 microM) were without significant effects. Incubation of the cells with sn-1, 2-dioctanoyl glycerol (1 microM) increased the affinities of the transporter to tetraethylammonium, tetrapenthylammonium, and quinine by a factor of 58, 14.5, and 2.4, respectively. Western blot analysis revealed that rOCT1 protein was phosphorylated at a serine residue upon stimulation of PKC. In conclusion, it has been demonstrated that the organic cation transport by rOCT1 is stimulated by PKC, PKA, and endogenous tyrosine kinase activation. The PKC phosphorylates rOCT1 and leads to a conformational change at the substrate binding site.

The X-linked lymphoproliferative syndrome gene product SH2D1A associates with p62dok (Dok1) and activates NF-kappa B

The X-linked lymphoproliferative syndrome (XLP) is a genetic disorder in which affected males have a morbid or fatal response to Epstein-Barr virus infection. The XLP deficiency has been mapped to a gene encoding a 128-residue protein, SH2D1A, which is comprised principally of a Src homology 2 (SH2) domain. We now report that SH2D1A associates with Dok1, a protein that interacts with Ras-GAP, Csk, and Nck. An SH2D1A SH2 domain mutant that has been identified in XLP does not associate with Dok1, in accord with the hypothesis that this interaction is linked to XLP. The association of SH2D1A with Dok1 also depends on phosphorylation of Dok1 Y(449) in the sequence ALYSQVQK. Further, overexpression of SH2D1A is found to activate NF-kappaB in 293T cells. NF-kappaB activation by SH2D1A does not depend on the wild-type SH2 domain and is inhibited by a dominant-negative IkappaB kinase beta. Thus, SH2D1A can affect multiple intracellular signaling pathways that are potentially important in the normal effective host response to Epstein-Barr virus infection.

Drug transport and targeting. Intestinal transport

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