New Pharmacogenomics Research Network: An Open Community Catalyzing Research and Translation in Precision Medicine

The goal of pharmacogenomics research is to discover genetic polymorphisms that underlie variation in drug response. Increasingly, pharmacogenomics research involves large numbers of patients and the application of new technologies and methodologies to enable discovery. The Pharmacogenomics Research Network (PGRN) has become a community-driven network of investigators spanning scientific and clinical disciplines. Here, we highlight the activities and types of resources that enable PGRN members to enhance and drive basic and translational research in pharmacogenomics.

Variants in Pharmacokinetic Transporters and Glycemic Response to Metformin: A Metgen Meta-Analysis

Therapeutic response to metformin, a first-line drug for type 2 diabetes (T2D), is highly variable, in part likely due to genetic factors. To date, metformin pharmacogenetic studies have mainly focused on the impact of variants in metformin transporter genes, with inconsistent results. To clarify the significance of these variants in glycemic response to metformin in T2D, we performed a large-scale meta-analysis across the cohorts of the Metformin Genetics Consortium (MetGen). Nine candidate polymorphisms in five transporter genes (organic cation transporter [OCT]1, OCT2, multidrug and toxin extrusion transporter [MATE]1, MATE2-K, and OCTN1) were analyzed in up to 7,968 individuals. None of the variants showed a significant effect on metformin response in the primary analysis, or in the exploratory secondary analyses, when patients were stratified according to possible confounding genotypes or prescribed a daily dose of metformin. Our results suggest that candidate transporter gene variants have little contribution to variability in glycemic response to metformin in T2D.

Metabolomic and Genome-wide Association Studies Reveal Potential Endogenous Biomarkers for OATP1B1

Transporter-mediated drug-drug interactions (DDIs) are a major cause of drug toxicities. Using published genome-wide association studies (GWAS) of the human metabolome, we identified 20 metabolites associated with genetic variants in organic anion transporter, OATP1B1 (P < 5 × 10-8 ). Of these, 12 metabolites were significantly higher in plasma samples from volunteers dosed with the OATP1B1 inhibitor, cyclosporine (CSA) vs. placebo (q-value < 0.2). Conjugated bile acids and fatty acid dicarboxylates were among the metabolites discovered using both GWAS and CSA administration. In vitro studies confirmed tetradecanedioate (TDA) and hexadecanedioate (HDA) were novel substrates of OATP1B1 as well as OAT1 and OAT3. This study highlights the use of multiple datasets for the discovery of endogenous metabolites that represent potential in vivo biomarkers for transporter-mediated DDIs. Future studies are needed to determine whether these metabolites can serve as qualified biomarkers for organic anion transporters. Quantitative relationships between metabolite levels and modulation of transporters should be established.

A Longitudinal HbA1c Model Elucidates Genes Linked to Disease Progression on Metformin

One-third of type-2 diabetic patients respond poorly to metformin. Despite extensive research, the impact of genetic and nongenetic factors on long-term outcome is unknown. In this study we combine nonlinear mixed effect modeling with computational genetic methodologies to identify predictors of long-term response. In all, 1,056 patients contributed their genetic, demographic, and long-term HbA1c data. The top nine variants (of 12,000 variants in 267 candidate genes) accounted for approximately one-third of the variability in the disease progression parameter. Average serum creatinine level, age, and weight were determinants of symptomatic response; however, explaining negligible variability. Two single nucleotide polymorphisms (SNPs) in CSMD1 gene (rs2617102, rs2954625) and one SNP in a pharmacologically relevant SLC22A2 gene (rs316009) influenced disease progression, with minor alleles leading to less and more favorable outcomes, respectively. Overall, our study highlights the influence of genetic factors on long-term HbA1c response and provides a computational model, which when validated, may be used to individualize treatment.

A Molecular Basis for Innovation in Drug Excipients

Excipients are ubiquitous in drug formulation, ensuring that active ingredient drugs are properly released on dosing, retain their properties over time, and are palatable, among other roles. Despite their crucial roles, surprisingly little is known about their systemic availability and activities on molecular targets. Here we review key excipient properties, introduce a public-accessible database that enumerates and categorizes them, and sketch a strategy for exploring their possible direct actions on molecular targets.

Genomewide Association Studies in Pharmacogenomics: Meeting Report of the NIH Pharmacogenomics Research Network-RIKEN (PGRN-RIKEN) Collaboration

Genomewide association studies (GWAS) have resulted in the identification of many heritable genetic factors that underlie risk for human disease or variation in physiologic traits. In contrast, there are fewer GWAS of drug response phenotypes, despite extensive unexplained interindividual variability. To address this urgent need, the NIH Pharmacogenomics Research Network (PGRN) and the Center for Integrative Medical Sciences (IMS) at RIKEN support a collaboration, PGRN-RIKEN, with the goal of accelerating GWAS of drug response phenotypes.

Genome-wide association study identifies ABCG2 (BCRP) as an allopurinol transporter and a determinant of drug response

The first-line treatment of hyperuricemia, which causes gout, is allopurinol. The allopurinol response is highly variable, with many users failing to achieve target serum uric acid (SUA) levels. No genome-wide association study (GWAS) has examined the genetic factors affecting allopurinol effectiveness. Using 2,027 subjects in Kaiser Permanente's Genetic Epidemiology Research on Adult Health and Aging (GERA) Cohort, we conducted a GWAS of allopurinol-related SUA reduction, first in the largest ethnic group, non-Hispanic white (NHW) subjects, and then in a stratified transethnic meta-analysis. ABCG2, encoding the efflux pump BCRP, was associated with SUA reduction in NHW subjects (P = 2 × 10(-8) ), and a missense allele (rs2231142) was associated with a reduced response (P = 3 × 10(-7) ) in the meta-analysis. Isotopic uptake studies in cells demonstrated that BCRP transports allopurinol and genetic variants in ABCG2 affect this transport. Collectively, this first GWAS of allopurinol response demonstrates that ABCG2 is a key determinant of response to the drug.

Genetic variants in transcription factors are associated with the pharmacokinetics and pharmacodynamics of metformin

One-third of type 2 diabetes patients do not respond to metformin. Genetic variants in metformin transporters have been extensively studied as a likely contributor to this high failure rate. Here, we investigate, for the first time, the effect of genetic variants in transcription factors on metformin pharmacokinetics (PK) and response. Overall, 546 patients and healthy volunteers contributed their genome-wide, pharmacokinetic (235 subjects), and HbA1c data (440 patients) for this analysis. Five variants in specificity protein 1 (SP1), a transcription factor that modulates the expression of metformin transporters, were associated with changes in treatment HbA1c (P < 0.01) and metformin secretory clearance (P < 0.05). Population pharmacokinetic modeling further confirmed a 24% reduction in apparent clearance in homozygous carriers of one such variant, rs784888. Genetic variants in other transcription factors, peroxisome proliferator-activated receptor-α and hepatocyte nuclear factor 4-α, were significantly associated with HbA1c change only. Overall, our study highlights the importance of genetic variants in transcription factors as modulators of metformin PK and response.

Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines for ivacaftor therapy in the context of CFTR genotype

Cystic fibrosis (CF) is a life-shortening disease arising as a consequence of mutations within the CFTR gene. Novel therapeutics for CF are emerging that target CF transmembrane conductance regulator protein (CFTR) defects resulting from specific CFTR variants. Ivacaftor is a drug that potentiates CFTR gating function and is specifically indicated for CF patients with a particular CFTR variant, G551D-CFTR (rs75527207). Here, we provide therapeutic recommendations for ivacaftor based on preemptive CFTR genotype results.

Correction for Volume Shift during Equilibrium Dialysis by Measurement of Protein Concentration

Volume shift during equilibrium dialysis produces errors in estimating the fraction of drug unbound. This study describes a method in which protein concentration in the plasma is used to correct binding data for volume shifts. Data are presented for phenytoin, a drug that does not bind to the dialysis system, and for verapamil, a drug that does. The conventional method of not correcting for volume shift, the method described previously by one of us (TNT) for a drug that does not bind to the dialysis system, and the proposed method of determining fraction unbound are compared and discussed. It is concluded that the second method is simple and can be used to determine the unbound fraction for a drug, such as phenytoin, which does not bind to the dialysis system. If a drug binds to the dialysis system, as does verapamil, the proposed method of measuring protein concentration before and after dialysis can be reliably used to correct for volume shift.

Transporters in drug development and clinical pharmacology

More than 400 membrane transporters in two major superfamilies-ATP-binding cassette (ABC) and solute carrier (SLC)-are annotated in the human genome. Preclinical and clinical studies indicate that transport is an important determinant of drug disposition, as well as therapeutic and adverse drug effects. Importantly, transporters may represent the rate-determining step of drug absorption, distribution, and elimination in the intestine, liver, kidney, and blood-brain barrier (BBB), and they are often the sites of drug-drug interactions.

SLC classification: an update

The 386 human SLC superfamily members are diverse in sequence, structure, and function. Using sequence similarity, we previously classified the SLC superfamily members and identified relationships among families. With the recent determination of new SLC structures and identification of previously unknown human SLC families, an update of our previous classification is timely. Here, we comprehensively compare the SLC sequences and structures and discuss the applicability of structure-based ligand discovery to key SLC members.

The effect of novel promoter variants in MATE1 and MATE2 on the pharmacokinetics and pharmacodynamics of metformin

Interindividual variation in response to metformin, first-line therapy for type 2 diabetes, is substantial. Given that transporters are determinants of metformin pharmacokinetics, we examined the effects of promoter variants in both multidrug and toxin extrusion protein 1 (MATE1) (g.-66T → C, rs2252281) and MATE2 (g.-130G → A, rs12943590) on variation in metformin disposition and response. The pharmacokinetics and glucose-lowering effects of metformin were assessed in healthy volunteers (n = 57) receiving metformin. The renal and secretory clearances of metformin were higher (22% and 26%, respectively) in carriers of variant MATE2 who were also MATE1 reference (P < 0.05). Both MATE genotypes were associated with altered post-metformin glucose tolerance, with variant carriers of MATE1 and MATE2 having an enhanced (P < 0.01) and reduced (P < 0.05) response, respectively. Consistent with these results, patients with diabetes (n = 145) carrying the MATE1 variant showed enhanced metformin response. These findings suggest that promoter variants of MATE1 and MATE2 are important determinants of metformin disposition and response in healthy volunteers and diabetic patients.

Genetic and epigenetic regulation of the organic cation transporter 3, SLC22A3

Human organic cation transporter 3 (OCT3 and SLC22A3) mediates the uptake of many important endogenous amines and basic drugs in a variety of tissues. OCT3 is identified as one of the important risk loci for prostate cancer, and is markedly underexpressed in aggressive prostate cancers. The goal of this study was to identify genetic and epigenetic factors in the promoter region that influence the expression level of OCT3. Haplotypes that contained the common variants, g.-81G>delGA (rs60515630) (minor allele frequency 11.5% in African American) and g.-2G>A (rs555754) (minor allele frequency>30% in all ethnic groups) showed significant increases in luciferase reporter activities and exhibited stronger transcription factor-binding affinity than the haplotypes that contained the major alleles. Consistent with the reporter assays, OCT3 messenger RNA expression levels were significantly higher in Asian (P<0.001) and Caucasian (P<0.05) liver samples from individuals who were homozygous for g.-2A/A in comparison with those homozygous for the g.-2G/G allele. Studies revealed that the methylation level in the basal promoter region of OCT3 was associated with OCT3 expression level and tumorigenesis capability in various prostate cancer cell lines. The methylation level of the OCT3 promoter was higher in 62% of prostate tumor samples compared with matched normal samples. Our studies demonstrate that genetic polymorphisms in the proximal promoter region of OCT3 alter the transcription rate of the gene and may be associated with altered expression levels of OCT3 in human liver. Aberrant methylation contributes to the reduced expression of OCT3 in prostate cancer.

Discovery of regulatory elements in human ATP-binding cassette transporters through expression quantitative trait mapping

ATP-Binding Cassette (ABC) membrane transporters determine the disposition of many drugs, metabolites and endogenous compounds. Coding region variation in ABC transporters is the cause of many genetic disorders, but much less is known about the genetic basis and functional outcome of ABC transporter expression level variation. We used genotype and mRNA transcript level data from human lymphoblastoid cell lines to assess population and gender differences in ABC transporter expression, and to guide the discovery of genomic regions involved in transcriptional regulation. Nineteen of 49 ABC genes were differentially expressed between individuals of African, Asian and European descent suggesting an important influence of race on expression level of ABC transporters. Twenty-four significant associations were found between transporter transcript levels and proximally located genetic variants. Several of the associations were experimentally validated in reporter assays. Through influencing ABC expression levels, these SNPs may affect disease susceptibility and response to drugs.

Genotype-dependent effects of inhibitors of the organic cation transporter, OCT1: predictions of metformin interactions

Common genetic variants of the liver-specific human organic cation transporter 1 (OCT1; SLC22A1) have reduced transport capacity for substrates such as the antidiabetic drug metformin. The effect of the reduced OCT1 function on drug interactions associated with OCT1 has not been investigated and was, therefore, the focus of the study presented here. HEK293 cells expressing human OCT1-reference or the variants R61C, V408M, M420del and G465R were first used to study the kinetics and inhibition pattern of the OCT1 substrate 4-(4-(dimethylamino)styryl)-N-methylpyridinium (ASP(+)). In the second part OCT1-mediated (14)C-metformin uptake was studied in the presence of drugs administered concomitantly with metformin. Transport studies using ASP(+) showed that the function of the variants decreased in the following order: OCT1-reference=V408M=M420del >R61C >>G465R. Variants M420del and R61C were more sensitive to drug inhibition, with IC(50) values up to 23 times lower than those of the OCT1-reference. Uptake studies using (14)C-metformin were in qualitative agreement with those using ASP(+), with the exception that a larger reduction in transport capacity was observed for M420del. Concomitantly administered drugs, such as verapamil and amitriptyline, revealed potential drug-drug interactions at clinical plasma concentrations of metformin for OCT1-M420del.

The role of organic anion-transporting polypeptides and their common genetic variants in mycophenolic acid pharmacokinetics

The goal of this study was to determine the roles of the organic anion-transporting polypeptides (OATPs) OATP1A2, OATP1B1, and OATP1B3 and their genetic variants in the pharmacokinetics of the immunosuppressive drug mycophenolate mofetil (MMF). Using OATP-transfected human embryonic kidney (HEK) cells, we measured the uptake of mycophenolic acid (MPA) and its glucuronide (MPAG). MPAG, but not MPA, significantly accumulated in cells expressing OATP1B3 or OATP1B1 (P < 0.05). The pharmacokinetics of both MPA and MPAG were significantly influenced by the OATP1B3 polymorphism 334T>G/699G>A in 70 renal transplant patients receiving combination treatment of MMF with either tacrolimus or sirolimus, but not in 115 patients receiving MMF and cyclosporine. The decrease in dose-normalized (dn) MPA exposure and the concomitant increase in the MPAG/MPA metabolic ratio are consistent with reduced enterohepatic cycling in patients carrying the OATP1B3 334G-699A haplotype. Further studies demonstrated that this variant of OATP1B3 exhibited a reduced maximal velocity (V(max)) in transfected HEK cells, thereby providing functional evidence to support our clinical findings.

Effects of genetic variation in the novel organic cation transporter, OCTN1, on the renal clearance of gabapentin

Gabapentin is an anticonvulsant that is widely prescribed for epilepsy and other neuropathic disorders. The pharmacokinetics, particularly the absorption and renal elimination, of gabapentin appear to involve membrane transporters. In this study, we tested the hypothesis that organic cation transporter 1 (OCTN1), a multispecific transporter expressed at the apical membrane in intestine and kidney, plays a role in gabapentin pharmacokinetics and that the common variant of OCTN1, OCTN1-L503F, contributes to variation in the pharmacokinetics of the drug. We observed that OCTN1 facilitates the Na+-independent transport of gabapentin, and that the OCTN1-L503F variant is deficient in gabapentin transport activity in stably transfected HEK-293 cells (fourfold enhanced uptake of gabapentin by OCTN1-503L vs twofold enhanced uptake by OCTN1-L503F, compared to mock-transfected cells). In clinical studies, we found that in subjects homozygous for the L503F variant, gabapentin renal clearance (CL(R)) approximates the glomerular filtration rate (mean+/-SE: 110+/-12 ml/min, n=9), whereas in subjects homozygous for the reference allele, gabapentin undergoes net secretion in the kidney (141+/-7.8 ml/min, n=11, P<0.05). Creatinine clearance and OCTN1 genotype accounted for 56% of the variation in CL(R) and were the only significant predictors of CL(R) (P<0.05). Importantly, OCTN1 genotype was the only significant predictor of net secretion of gabapentin (P<0.008). Oral bioavailability of gabapentin was not affected by OCTN1 genotype. We conclude that OCTN1 contributes to active tubular secretion of gabapentin, and that this effect may be diminished or absent in individuals carrying the OCTN1-L503F polymorphism. These results provide clinical evidence of the role of genetic variation in renal drug transporters in active drug secretion in vivo.

Effect of genetic variation in the organic cation transporter 1, OCT1, on metformin pharmacokinetics

The goal of this study was to determine the effects of genetic variation in the organic cation transporter 1, OCT1, on the pharmacokinetics of the antidiabetic drug, metformin. Twenty healthy volunteers with known OCT1 genotype agreed to participate in the study. Each subject received two oral doses of metformin followed by collection of blood and urine samples. OCT1 genotypes had a significant (P<0.05) effect on metformin pharmacokinetics, with a higher area under the plasma concentration-time curve (AUC), higher maximal plasma concentration (Cmax), and lower oral volume of distribution (V/F) in the individuals carrying a reduced function OCT1 allele (R61C, G401S, 420del, or G465R). The effect of OCT1 on metformin pharmacokinetics in mice was less than in humans possibly reflecting species differences in hepatic expression level of the transporter. Our studies suggest that OCT1 genotype is a determinant of metformin pharmacokinetics.

The pharmacogenetics research network: from SNP discovery to clinical drug response

The NIH Pharmacogenetics Research Network (PGRN) is a collaborative group of investigators with a wide range of research interests, but all attempting to correlate drug response with genetic variation. Several research groups concentrate on drugs used to treat specific medical disorders (asthma, depression, cardiovascular disease, addiction of nicotine, and cancer), whereas others are focused on specific groups of proteins that interact with drugs (membrane transporters and phase II drug-metabolizing enzymes). The diverse scientific information is stored and annotated in a publicly accessible knowledge base, the Pharmacogenetics and Pharmacogenomics Knowledge base (PharmGKB). This report highlights selected achievements and scientific approaches as well as hypotheses about future directions of each of the groups within the PGRN. Seven major topics are included: informatics (PharmGKB), cardiovascular, pulmonary, addiction, cancer, transport, and metabolism.

Functional analysis of genetic variants in the human concentrative nucleoside transporter 3 (CNT3; SLC28A3)

The human concentrative nucleoside transporter, CNT3 (SLC28A3), plays an important role in mediating the cellular entry of a broad array of physiological nucleosides and synthetic anticancer nucleoside analog drugs. As a first step toward understanding the genetic basis for interindividual differences in the disposition and response to antileukemic nucleoside analogs, we examined the genetic and functional diversity of CNT3. In all, 56 variable sites in the exons and flanking intronic region of SLC28A3 were identified in a collection of 270 DNA samples from US populations (80 African-Americans, 80 European-Americans, 60 Asian-Americans, and 50 Mexican-Americans). Of the 16 coding region variants, 12 had not been previously reported. Also, 10 resulted in amino-acid changes and three of these had total allele frequencies of >/=1%. Nucleotide diversity (pi) at nonsynonymous and synonymous sites was estimated to be 1.81 x 10(4) and 18.13 x 10(4), respectively, suggesting that SLC28A3 is under negative selection. All nonsynonymous variants, constructed by site-directed mutagenesis and expressed in Xenopus laevis oocytes, transported purine and pyrimidine model substrates, except for c. 1099G>A (p. Gly367Arg). This rare variant alters an evolutionarily conserved site in the putative substrate recognition domain of CNT3. The presence of three additional evolutionarily conserved glycine residues in the vicinity of p. Gly367Arg that are also conserved in human paralogs suggest that these glycine residues are critical in the function of the concentrative nucleoside transporter family. The genetic analysis and functional characterization of CNT3 variants suggest that this transporter does not tolerate nonsynonymous changes and is important for human fitness.

Functional characteristics and steroid hormone-mediated regulation of an organic cation transporter in Madin-Darby canine kidney cells

Organic cation transporters (OCT1-3) play an important role in renal elimination of many drugs. The goals of this study were to 1) identify a cell culture model which constitutively expressed OCT2 that could be used to study the characteristics and regulation of this transporter, and 2) to study the mechanisms by which xenobiotics and hormones regulate the activity of OCT2. We characterized the endogenous organic cation transporter (OCT) activity in Madin-Darby canine kidney (MDCK) cells. The activity was localized to the basolateral membrane and was pH and membrane potential-dependent. The uptake of the model organic cation, tetraethylammonium, was saturable (Km, 19.5 +/- 4.6 microM; Vmax, 350 +/- 19.4 pmol/mg of protein/10 min) and was inhibited by known OCT inhibitors (e.g., cimetidine and quinidine). A cDNA fragment (711 base pairs) isolated by reverse transcriptase-polymerase chain reaction (RT-PCR) was greater than 83% identical to OCT2 cDNAs from mammalian species; no OCT1 or OCT3 was detected by RT-PCR, suggesting that OCT2 may be the primary basolateral OCT in MDCK. OCT2 mRNA levels were increased significantly following exposure of MDCK to the steroid hormones, dexamethasone (2.0-fold), hydrocortisone (2.4-fold), and testosterone (1.8-fold) with comparable increases in activity. Other compounds tested, including the cytochrome P450 inducers, rifampicin, phenobarbital, and phenytoin, and the OCT substrates, verapamil and metformin, had no inducing effects. Collectively, these data indicate that MDCK can serve as a useful and convenient tool in screening candidate drugs for interaction with OCT2 and for studying the regulation of this transporter. Furthermore, our data demonstrate that steroid hormones induce the transcription of OCT2 in the kidney.

Arginine 454 and lysine 370 are essential for the anion specificity of the organic anion transporter, rOAT3

Organic anion transporters (OATs) and organic cation transporters (OCTs) mediate the flux of xenobiotics across the plasma membranes of epithelia. Substrates of OATs generally carry negative charge(s) whereas substrates of OCTs are cations. The goal of this study was to determine the domains and amino acid residues essential for recognition and transport of organic anions by the rat organic anion transporter, rOAT3. An rOAT3/rOCT1 chimera containing transmembrane domains 1-5 of rOAT3 and 6-12 of rOCT1 retained the specificity of rOCT1, suggesting that residues involved in substrate recognition reside within the carboxyl-terminal half of these transporters. Mutagenesis of a conserved basic amino acid residue, arginine 454 to aspartic acid (R454D), revealed that this amino acid is required for organic anion transport. The uptakes of p-aminohippurate (PAH), estrone sulfate, and ochratoxin A were approximately 10-, approximately 48-, and approximately 32-fold enhanced in oocytes expressing rOAT3 and were only approximately 2-, approximately 6-, and approximately 5-fold enhanced for R454D. Similarly, mutagenesis of the conserved lysine 370 to alanine (K370A) suggested that K370 is important for organic anion transport. Interestingly, the charge specificity of the double mutant, R454DK370A, was reversed in comparison to rOAT3-R454DK370A preferentially transported the organic cation, MPP(+), in comparison to PAH (MPP(+) uptake/PAH uptake = 3.21 for the double mutant vs 0.037 for rOAT3). These data indicate that arginine 454 and lysine 370 are essential for the anion specificity of rOAT3. The studies provide the first insights into the molecular determinants that are critical for recognition and translocation of organic anions by a member of the organic anion transporter family.

Localization of GFP-tagged concentrative nucleoside transporters in a renal polarized epithelial cell line

Many nucleosides undergo active reabsorption within the kidney, probably via nucleoside transporters. To date, two concentrative nucleoside transporters have been cloned, the sodium-dependent purine-selective nucleoside transporter (SPNT) and concentrative nucleoside transporter 1 (CNT1). We report the stable expression of green fluorescence protein (GFP)-tagged SPNT and CNT1 in Madin-Darby canine kidney (MDCK) cells, a polarized renal epithelial line. We demonstrate that the GFP tag does not alter the substrate selectivity and only modestly affects the kinetic activity of the transporters. By using confocal microscopy and functional studies, both SPNT and CNT1 are localized primarily to the apical membrane of MDCK and LLC-PK(1) cells. Apical localization of these transporters suggests a role in renal nucleoside reabsorption and regulation of tubular function via the adenosine pathway.

Screening a large reference sample to identify very low frequency sequence variants: comparisons between two genes

Most human sequence variation is in the form of single-nucleotide polymorphisms (SNPs). It has been proposed that coding-region SNPs (cSNPs) be used for direct association studies to determine the genetic basis of complex traits. The success of such studies depends on the frequency of disease-associated alleles, and their distribution in different ethnic populations. If disease-associated alleles are frequent in most populations, then direct genotyping of candidate variants could show robust associations in manageable study samples. This approach is less feasible if the genetic risk from a given candidate gene is due to many infrequent alleles. Previous studies of several genes demonstrated that most variants are relatively infrequent (<0.05). These surveys genotyped small samples (n<75) and thus had limited ability to identify rare alleles. Here we evaluate the prevalence and distribution of such rare alleles by genotyping an ethnically diverse reference sample that is more than six times larger than those used in previous studies (n=450). We screened for variants in the complete coding sequence and intron-exon junctions of two candidate genes for neuropsychiatric phenotypes: SLC6A4, encoding the serotonin transporter; and SLC18A2, encoding the vesicular monoamine transporter. Both genes have unique roles in neuronal transmission, and variants in either gene might be associated with neurobehavioral phenotypes.

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.

A novel proton-dependent nucleoside transporter, CeCNT3, from Caenorhabditis elegans

In this study, we describe the cloning and characterization of a proton-dependent, broadly selective nucleoside transporter from Caenorhabditis elegans. Recently, we constructed a broadly selective nucleoside transporter which accepts both purine and pyrimidine nucleosides. Based on these studies, we hypothesized that CNTs with novel substrate selectivities exist in nature and that a CNT homolog in the C. elegans genomic database may function as a broadly selective nucleoside transporter. We cloned the cDNA for this transporter, termed CeCNT3 because of its broad selectivity, using polymerase chain reaction-based methods. CeCNT3 is predicted to have 575 amino acid residues (63.4 kDa) with 11 to 14 putative transmembrane domains and exhibits approximately 30% identity to members of the mammalian CNT family. This transporter exhibits a novel substrate selectivity, transporting a wide range of purine and pyrimidine nucleosides (inosine, guanosine, adenosine, uridine, and thymidine) but not cytidine. The apparent Km values for inosine and thymidine are 15.2 +/- 5.3 microM and 11.0 +/- 2.4 microM, respectively. Kinetic studies demonstrate that purine and pyrimidine nucleosides share a common recognition site in the transporter. In contrast to all known members of the mammalian CNT family, CeCNT3-mediated transport of nucleosides is proton-, but not sodium-, dependent. Mutation of tyrosine 332 in CeCNT3 decreased both the maximum uptake rate and apparent Km of thymidine, suggesting that this residue is in the domain of nucleoside recognition and translocation. The broad nucleoside specificity of CeCNT3 may be explained by this and other residues that restrict purine and pyrimidine nucleoside uptake and that discriminate among pyrimidine nucleosides.

Electrophysiological analysis of the substrate selectivity of a sodium-coupled nucleoside transporter (rCNT1) expressed in Xenopus laevis oocytes

Nucleoside transporters that mediate cellular uptake of therapeutic nucleoside analogs are major determinants of the pharmacokinetic properties of these compounds. Understanding the substrate selectivity of these transporters is critical in the development of therapeutic nucleoside analogs with optimal pharmacokinetic properties, including high oral bioavailability and tissue-specific distribution. In general, substrate selectivity of nucleoside transporters has been evaluated indirectly by inhibition studies. The purpose of this study was to directly measure the transport of nucleoside analogs by the sodium-coupled pyrimidine-selective transporter rCNT1 using electrophysiology methods. We used a two-electrode voltage clamp assay to investigate the substrate selectivity of rCNT1; 19 structurally diverse nucleosides and nucleoside analogs were studied. Uridine-induced currents in voltage-clamped oocytes expressing rCNT1 were sodium-, voltage-, and concentration-dependent (K(0.5) = 21 microM), and were blocked by adenosine. Uridine-induced currents increased approximately 5-fold upon hyperpolarization of membrane potential from -10 to -150 mV. Uridine, thymidine, and cytidine induced currents in rCNT1-expressing oocytes, whereas guanosine, inosine, and adenosine did not. Uridine, deoxyuridine, and cytidine analogs with modifications at the 3-, 4-, or 5-position were found to be substrates of rCNT1, whereas uridine and cytidine analogs modified at the 6-position were not. In addition, it was found that the 5'-hydroxyl group of the sugar is not required for transport by rCNT1. These results enhance our understanding of the structural basis for substrate selectivity of nucleoside transporters and should prove useful in the development of therapeutic nucleoside analogs.

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.

Interactions of HIV protease inhibitors with a human organic cation transporter in a mammalian expression system

Recently, we cloned a human organic cation transporter, hOCT1, which is expressed primarily in the liver. hOCT1 plays an important role in the cellular uptake and elimination of various xenobiotics including therapeutically important drugs. HIV protease inhibitors are a new class of therapeutic agents. The purpose of this study was to elucidate the interactions of HIV protease inhibitors with hOCT1 and to determine whether hOCT1 is involved in the elimination of these compounds. We studied the interactions of HIV protease inhibitors with hOCT1 in a transiently transfected human cell line, HeLa. Uptake studies were carried out 40 h post-transfection using the radiolabeled model organic cation, [(14)C]tetraethylammonium (TEA), under different experimental conditions. In cis-inhibition studies, all of the HIV protease inhibitors tested, i.e., indinavir (IC(50) of 62 microM), nelfinavir (IC(50) of 22 microM), ritonavir (IC(50) of 5.2 microM), and saquinavir (IC(50) of 8.3 microM) inhibited TEA uptake in HeLa cells expressing hOCT1. However, none of the HIV protease inhibitors trans-stimulated [(14)C]TEA uptake, suggesting that they are poorly translocated by hOCT1. Nelfinavir, ritonavir, and saquinavir demonstrated an apparent "trans-inhibition" effect. No enhanced uptake of [(14)C]saquinavir was observed in hOCT1 DNA-transfected cells versus empty vector-transfected cells. These data suggest that HIV protease inhibitors are potent inhibitors, but poor substrates, of hOCT1. Some HIV protease inhibitors may potently inhibit the uptake and elimination of cationic drugs that are substrates for hOCT1, leading to potential drug-drug interactions. Other transporters, e.g., MDR1 and MRP1, in HIV-targeted cells may control the intracellular concentrations of HIV protease inhibitors.

Kinetic and selectivity differences between rodent, rabbit, and human organic cation transporters (OCT1)

Organic cation transporters play an important role in the absorption, distribution, and elimination of clinical agents, toxic substances, and endogenous compounds. In kidney preparations, significant differences in functional characteristics of organic cation transport between various species have been reported. However, the underlying molecular mechanisms responsible for these interspecies differences are not known. The goal of this study was to determine the kinetics and substrate selectivities of organic cation transporter (OCT1) homologs from mouse, rat, rabbit, and human that may contribute to interspecies differences in the renal and hepatic handling of organic cations. With a series of n-tetraalkylammonium (nTAA) compounds, a correlation between increasing alkyl chain length and affinity for the four OCT1 homologs was observed. However, the apparent affinity constants (K(i)) differed among the species homologs. For the mouse homolog mOCT1, apparent K(i) values ranged from 7 microM for tetrabutylammonium to 2000 microM for tetramethylammonium. In contrast, the human homolog hOCT1 exhibited weaker interactions with the nTAA compounds. Trans-stimulation studies and current measurements in voltage-clamped oocytes demonstrated that larger nTAA compounds were transported at greater rates in oocytes expressing hOCT1, whereas smaller nTAAs were transported at greater rates in oocytes expressing mOCT1 or rOCT1. The rabbit homolog rbOCT1 exhibited intermediate properties in its interactions with nTAAs compared with its rodent and human counterparts. This report demonstrates that the human OCT1 homolog has functional properties distinct from those of the rodent and rabbit OCT1 homologs. The study underscores potential difficulties in extrapolating data from preclinical studies in animal models to humans.

Functional characterization of a human purine-selective, Na+-dependent nucleoside transporter (hSPNT1) in a mammalian expression system

Nucleosides and nucleoside analogs are actively transported in the human kidney. With the recent cloning of a purine-selective, Na+-dependent, nucleoside transporter (hSPNT1, also termed hCNT2) from human kidney, it is now possible to study the interaction of nucleosides and nucleoside analogs with this transport protein and gain a more detailed knowledge of the underlying mechanisms of nucleoside transport in the human kidney. In this study we examined the substrate selectivity of hSPNT1 for nucleosides and nucleoside analogs. We determined that the naturally occurring nucleosides adenosine, inosine, and uridine are substrates for this carrier, whereas thymidine is not. The nucleoside analogs (0.5 mM) 2', 3'-dideoxyadenosine; 2',3'-dideoxyinosine; and 2-chloro-2'deoxyadenosine (2CdA), significantly inhibited the uptake of [3H]inosine in HeLa cells transiently transfected with hSPNT1. However, there was no significant Na+-dependent uptake of [3H]2', 3'-dideoxyinosine or [3H]2CdA in the transfected cells, suggesting that these nucleoside analogs are not permeants of hSPNT1. Interestingly, 2CdA was considerably less potent in inhibiting [3H]inosine uptake in HeLa cells expressing hSPNT1 than in cells expressing the rat homolog rSPNT (IC50 = 371 microM versus 13.8 microM), suggesting that there may be notable species differences in the kinetic interactions of some nucleoside analogs with purine- selective nucleoside transporters.

The interaction of n-tetraalkylammonium compounds with a human organic cation transporter, hOCT1

Polyspecific organic cation transporters in epithelia play an important role in the elimination of many endogenous bioactive amines and therapeutically important drugs. Recently, the first human organic cation transporter (hOCT1) was cloned from liver. The purpose of the current study was to determine the effect of molecular size and hydrophobicity on the transport of organic cations by hOCT1. We studied the interaction of a series of n-tetraalkylammonium (n-TAA) compounds (alkyl chain length, N, ranging from 1 to 6 carbons) with hOCT1 in a transiently transfected human cell line, HeLa. [14C]tetraethylammonium (TEA) uptake was measured under different experimental conditions. Both cis-inhibition and trans-stimulation studies were carried out. With the exception of tetramethylammonium, all of the n-TAAs significantly inhibited [14C]TEA uptake. A reversed correlation of IC50 values (range, 3.0-260 microM) with alkyl chain lengths or partition coefficients (LogP) was observed. trans-Stimulation studies revealed that TEA, tetrapropylammonium, tetrabutylammonium, as well as tributylmethylammonium trans-stimulated TEA uptake mediated by hOCT1. In contrast, tetramethylammonium and tetrapentylammonium did not trans-stimulate [14C]TEA uptake, and tetrahexylammonium demonstrated an apparent "trans-inhibition" effect. These data indicate that with increasing alkyl chain lengths (N >/= 2), n-TAA compounds are more poorly translocated by hOCT1 although their potency of inhibition increases. Similar findings were obtained with nonaliphatic hydrocarbons. These data suggest that a balance between hydrophobic and hydrophilic properties is necessary for binding and subsequent translocation by hOCT1.

Characterization of a bioengineered chimeric Na+-nucleoside transporter

Na+-dependent nucleoside transporters mediate the intracellular uptake of purine and pyrimidine nucleosides. The N1, N2, and N3 Na+-nucleoside transporters differ in substrate selectivity. N1 is purine-selective, N2 is pyrimidine-selective, and N3 is broadly selective. Recently, we created a chimeric transporter, T8, from the cloned rat N1 and N2 transporters. Whereas most chimeric proteins exhibit the characteristics of one of the two parent proteins, limited studies suggested that T8 possesses either a combined substrate selectivity of N1 and N2 or the selectivity of N3. The purpose of this study was to determine the substrate profile, transport mechanisms, and Na+-coupling stoichiometry of T8 and to compare these measurements with those of wild-type N1, N2, and N3. In Xenopus laevis oocytes expressing T8, Na+-dependent uptake of 3H-labeled purine (adenosine, inosine, and guanosine) and pyrimidine nucleosides (uridine, thymidine, and cytidine) was significantly enhanced (3.5-18.6-fold), which suggests that T8 accepts both purine and pyrimidine nucleosides as permeants. T8-mediated uptake of [3H]thymidine was competitively inhibited by inosine, and T8-mediated uptake of [3H]inosine was competitively inhibited by thymidine, which suggests that purine and pyrimidine nucleosides share a common binding site. Base-modified ribo- and 2'-deoxyribonucleosides were potent inhibitors of T8. In contrast, 2',3'-dideoxyinosine, 2',3'-dideoxycytidine, and 3'-azidothymidine, which are known inhibitors of N1 or N2, did not inhibit T8-mediated uptake. These data suggest that the substrate profile of T8 is not a combination of those of N1 and N2; rather, it is similar to that of N3. However, the Na+/nucleoside stoichiometric ratio of T8 was determined to be 1, consistent with both N1 and N2 but different from N3.

Serine 318 is essential for the pyrimidine selectivity of the N2 Na+-nucleoside transporter

Molecular cloning has isolated two subtypes of Na+-nucleoside transporters; one is pyrimidine-selective (N2), and the other is purine-selective (N1). Using chimeric rat N2/N1 transporters, we previously demonstrated that transmembrane domains (TM) 8 and 9 are the major sites for substrate binding and discrimination. Interestingly, when TM8 of N2 was replaced by that of N1, the resulting chimera, T8, lost the pyrimidine selectivity of N2 and accepted both purine and pyrimidine nucleosides. Five residues differ between rat N2 and N1 in TM8. To identify the critical residues responsible for transport selectivity, the five residues in N2 were systematically changed to their equivalents in N1. Replacing the serine residue at position 318 to its equivalent N1 residue, glycine, caused N2 to lose its selectivity for pyrimidine nucleosides and accept purine nucleosides as substrates. In contrast, replacing the other four residues did not change the pyrimidine selectivity of N2. Furthermore, when glycine 318 in chimera T8 was changed back to serine, the chimeric transporter regained pyrimidine selectivity. These observations suggest that serine 318 is located in the nucleoside permeation pathway and is responsible for the substrate selectivity of N2. An adjacent residue, glutamine 319, was found to be important in modulating the apparent affinity for nucleosides.

Functional characterization of an organic cation transporter (hOCT1) in a transiently transfected human cell line (HeLa)

Recently, a polyspecific organic cation transporter, hOCT1, was cloned from human liver. To date, limited studies examining the functional characteristics of the transporter have been performed. The purpose of the present study was to develop a mammalian expression system for hOCT1 and to characterize the interactions of various compounds with the cloned transporter. Lipofection was used to transiently transfect the hOCT1 plasmid DNA in a human cell line, HeLa. We tested the interaction of an array of organic cations and other compounds with hOCT1 by determining Ki values in inhibiting 14C-tetraethylammonium (TEA) transport in the transfected cells. Transient expression of hOCT1 activity was observed between 24 and 72 hr post-transfection, with maximal expression at approximately 40 hr. TEA transport was temperature dependent and saturable with Vmax and K(m) values of 2.89 +/- 0.448 nmol/mg protein/30 min and 229 +/- 78.4 microM, respectively. 14C-TEA uptake in hOCT1 plasmid DNA-transfected HeLa cells was trans-stimulated by unlabeled TEA and 1-methyl-4-phenyl-pyridinium. Organic cations, including clonidine, quinine, quinidine and verapamil (0.1 mM), significantly inhibited 14C-TEA uptake, whereas the organic anion, p-aminohippuric acid (5 mM), did not. The neutral compounds, corticosterone (Ki, 7.0 microM) and midazolam (Ki, 3.7 microM) potently inhibited 14C-TEA uptake. The Ki values of several compounds in interacting with hOCT1 differed substantially from the corresponding values for the rat organic cation transporter, rOCT1, and the human kidney-specific organic cation transporter, hOCT2, determined in previous studies. Transiently transfected HeLa cells represent a useful tool in studying the interactions and kinetics of organic cations and other xenobiotics with hOCT1 and in understanding the molecular events involved in organic cation transport.

Nicotine transport in a human choriocarcinoma cell line (JAR)

Smoking is a major health problem in pregnancy resulting in intrauterine growth retardation and birth complications. Nicotine, a toxic component of cigarette smoke, interferes with amino acid transport in the placenta and stimulates catecholamine release resulting in uteroplacental vasoconstriction. Transplacental transport of nicotine may be an important determinant of placental and fetal exposure. Our aim was to determine the mechanism of nicotine transport in the human choriocarcinoma cell line, JAR, as a model for the placenta. JAR cells were subcultured in 12-well plates following trypsinization at a seeding density of 0.5 x 10(6) cells/well (1.3 x 10(5) cells/cm2). Uptake studies of [3H]nicotine were carried out in JAR cell monolayers on day 2 after plating. [3H]Nicotine uptake was saturable (Km 156 microM), sensitive to temperature, and inhibited by unlabeled nicotine and various organic cations including mecamylamine and quinidine, but not by guanidine, tetraethylammonium (TEA), or neurotransmitters. Counterflux of [3H]nicotine uptake was produced by unlabeled nicotine and mecamylamine but not by cotinine or acetylcholine, consistent with a carrier-mediated transport process. The uptake could be driven by an inside-negative membrane potential or by an outwardly directed pH gradient. This is the first demonstration of a carrier-mediated transport mechanism for nicotine in a human cell line. This transport mechanism may have implications to the disposition of nicotine in the human placenta.

Role of organic cation transporters in drug absorption and elimination

Organic cation transporters are critical in drug absorption, targeting, and disposition. It has become increasingly clear that multiple mechanisms are involved in organic cation transport in the key tissues responsible for drug absorption and disposition: the kidney, liver, and intestine. In this review, we discuss current models of transepithelial flux of organic cations in these three tissues. Particular emphasis is placed on the more recent molecular studies that have paved the way for a more complete understanding of the physiological and pharmacological roles of the organic cation transporters. Such information is essential in predicting pharmacokinetics and pharmacodynamics and in the design and development of cationic drugs.

Molecular cloning and functional expression of a rabbit renal organic cation transporter

A cDNA encoding an organic cation transporter (rbOCT1) was isolated from rabbit kidney. The cDNA encodes a 554 amino acid protein that is highly homologous to other mammalian organic cation transporters. rbOCT1 mediated 3H-1-methyl-4-phenylpyridinium (3H-MPP+) transport in Xenopus laevis oocytes was saturable, sensitive to membrane potential, and inhibited by various organic cations. rbOCT1 mRNA transcripts are expressed in the kidney, liver, and intestine.

Membrane transporters in drug disposition

Many clinically used drugs and their metabolites as well as a variety of environmental toxins are organic cations at physiologic pH. Secretion in the renal proximal tubule constitutes a major pathway in the elimination of organic cations. In this report, the results of studies recently performed in this laboratory are presented. First, the molecular cloning of a novel splice variant of organic cation transporter from rat kidney (rOCT1A) is described. The functional characteristics of the transporter are discussed along with the implications of RNA splicing in enhancing transporter diversity. Second, the molecular cloning of the first human organic cation transporter (hOCT1) is described. Distinct interspecies differences in the tissue distribution and function of this transporter is presented. These studies have paved the way for elucidating molecular structure function relationships of organic cation transporters and for determining their physiologic role in drug absorption and elimination. The cloned transporters can be used in mammalian expression systems for screening candidate compounds identified during drug discovery and development and in the in vivo prediction of the pharmacokinetics of therapeutic agents.

Na(+)-dependent purine nucleoside transporter from human kidney: cloning and functional characterization

Many purine nucleosides and their analogs are actively transported in the kidney. Using homology cloning strategies and reverse transcriptase-polymerase chain reactions, we isolated a cDNA encoding a Na(+)-dependent nucleoside transporter, hSPNT1, from human kidney. Functional expression in Xenopus laevis oocytes identified hSPNT1 as a Na(+)-dependent nucleoside transporter that selectively transports purine nucleosides but also transports uridine. The Michaelis constant (K(m)) of uridine (80 microM) in interacting with hSPNT1 was substantially higher than that of inosine (4.5 microM). hSPNT1 (658 amino acids) is 81% identical to the previously cloned rat Na(+)-nucleoside transporter, SPNT, but differs markedly from SPNT in terms of its primary structure in the NH2 terminus. In addition, an Alu repetitive element (approximately 282 bp) is present in the 3'-untranslated region of the hSPNT1 cDNA. Northern analysis revealed that multiple transcripts of hSPNT1 are widely distributed in human tissues including human kidney. In contrast, rat SPNT transcripts are absent in kidney and highly localized to liver and intestine. The hSPNT1 gene was localized to chromosome 15. This is the first demonstration of a purine nucleoside transporter in human kidney.

Molecular determinants of substrate selectivity in Na+-dependent nucleoside transporters

In mammalian cells, the salvage of purine and pyrimidine nucleosides is mediated by both facilitated and Na+-dependent nucleoside transporters. These transporters also play important roles in the transmembrane flux of therapeutic nucleoside analogs, which are widely used in the treatment of cancer and viral infections. The N1, N2, and N3 Na+-dependent nucleoside transporters differ in terms of their transport selectivity for purine and pyrimidine nucleosides. N1 is purine-selective, N2 is pyrimidine-selective, and N3 is broadly selective. To identify structural domains involved in substrate binding and molecular determinants responsible for distinct transport selectivity, chimeric transporters were made from the cloned rat N1 and N2 transporters. Of the 14 transmembrane domains (TM) of N1 and N2, transplanting TM8-9 of N1 into N2 converted N2 from a pyrimidine- to a purine-selective transporter. Transplanting only TM8 generated a chimera with characteristics similar to the N3 transporter that has yet to be cloned. These data suggest that TM8-9 confer substrate selectivity and may form at least part of a substrate-binding site in Na+-dependent nucleoside transporters.

Na+-dependent gamma-aminobutyric acid (GABA) transport in the choroid plexus of rabbit

The goal of this study was to examine the mechanisms of transport of gamma-aminobutyric acid (GABA) in the choroid plexus. Choroid plexus slices from the rabbit were depleted of ATP with 2,4-dinitrophenol. GABA accumulated in the choroid plexus slices in a concentrative manner in the presence of an inwardly-directed Na+ gradient. Uptake occurred in the presence of Cl-; replacement of Cl- with gluconate abolished uptake. SCN-, NO3- or Br- were able to support uptake in the absence of Cl- to a significant extent (80, 68 and 61% of control, respectively). GABA uptake was saturable (Km of 37 +/- 8.5 microM, Vmax of 409 +/- 43 nmol/g/min). Na+-driven GABA uptake was inhibited by beta-alanine (IC50 = 22.9 microM) and hypotaurine (IC50 = 21.9 microM) but less potently by nipecotic acid (IC50 = 244 microM) and hydroxy-nipecotic acid (IC50 = 284 microM). Betaine, L-(2,4)-diaminobutyric acid, guvacine and 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridin-3-ol were weak inhibitors (IC50 > 500 microM). GABA inhibited Na+-driven uptake of taurine (IC50 = 230 microM); taurine, however, did not inhibit GABA uptake (IC50 > 1 mM). RT-PCR, using degenerate primers for cloned GABA transporters, did not result in the amplification of a band from rat choroid plexus RNA. The location of the choroid plexus in the ventricles of the brain, and its role in the secretion of the cerebrospinal fluid, suggest a role for the choroid plexus Na+-GABA transporter in the disposition of GABA in the brain.

Functional and molecular characteristics of Na(+)-dependent nucleoside transporters

Nucleoside transporters play a critical role in the absorption, disposition, and targeting of therapeutically used nucleosides and nucleoside analogs. This review is focused on the Na(+)-dependent, concentrative nucleoside transporters which are found in a variety of cells including renal, intestinal and hepatic epithelia. Five major Na(+)-dependent nucleoside transporter subtypes have been characterized in isolated tissue preparations: N1 is purine selective; N2 is pyrimidine selective and N3-N5 exhibit variable selectively for both purine and pyrimidine nucleosides. The recent cloning of N1 and N2 nucleoside transporters has provided the first information on the molecular function and structure of concentrative nucleoside transporters. In this manuscript we review the characteristics of the various subtypes of nucleoside transporters and the molecular structure, functional properties, and tissue distribution of the cloned Na(+)-dependent nucleoside transporters. In addition, the interactions of nucleosides and nucleoside analogs with the cloned transporters in mammalian and amphibian expression systems are presented. Mammalian expression systems may be particularly useful during drug development in screening potential compounds for improved bioavailability and tissue specific targeting. Finally, we present our view of future ares of study in the field of nucleoside transporters.