Amino acids critical for substrate affinity of rat organic cation transporter 1 line the substrate binding region in a model derived from the tertiary structure of lactose permease

The large extracellular loop of organic cation transporter 1 influences substrate affinity and is pivotal for oligomerization

Polyspecific organic anion transporters (OATs) and organic cation transporters (OCTs) of the SLC22 transporter family play a pivotal role in absorption, distribution, and excretion of drugs. Polymorphisms in these transporters influence therapeutic effects. On the basis of functional characterizations, homology modeling, and mutagenesis, hypotheses for how OCTs bind and translocate structurally different cations were raised, assuming functionally competent monomers. However, homo-oligomerization has been described for OATs and OCTs. In the present study, evidence is provided that the large extracellular loops (EL) of rat Oct1 (rOct1) and rat Oat1 (rOat1) mediate homo- but not hetero-oligomerization. Replacement of the cysteine residues in the EL of rOct1 by serine residues (rOct1(6ΔC-l)) or breaking disulfide bonds with dithiothreitol prevented oligomerization. rOct1 chimera containing the EL of rOat1 (rOct1(rOat1-l)) showed oligomerization but reduced transporter amount in the plasma membrane. For rOct1(6ΔC-l) and rOct1(rOat1-l), similar K(m) values for 1-methyl-4-phenylpyridinium(+) (MPP(+)) and tetraethylammonium(+) (TEA(+)) were obtained that were higher compared with rOct1 wild type. The increased K(m) of rOct1(rOat1-l) indicates an allosteric effect of EL on the cation binding region. The similar substrate affinity of the oligomerizing and non-oligomerizing loop mutants suggests that oligomerization does not influence transport function. Independent transport function of rOct1 monomers was also demonstrated by showing that K(m) values for MPP(+) and TEA(+) were not changed after treatment with dithiothreitol and that a tandem protein with two rOct1 monomers showed about 50% activity with unchanged K(m) values for MPP(+) and TEA(+) when one monomer was blocked. The data help to understand how OCTs work and how mutations in patients may affect their functions.

Substrate recognition and translocation by polyspecific organic cation transporters

Organic cation transporters (OCTs) of the SLC22 family play a pivotal role in distribution and excretion of cationic drugs. They mediate electrogenic translocation of cations in both directions. OCTs are polyspecific transporters. During substrate translocation they perform a series of conformational changes involving an outward-facing conformation, an occluded state and an inward-facing conformation. Mutagenesis of OCT1 in combination with homology modeling showed that identical amino acids form the innermost parts of the outward-open and inward-open binding clefts. In addition to low affinity substrate binding sites, OCT1 contains high affinity substrate binding sites that can mediate inhibition via non-transported compounds.

Differences in the substrate binding regions of renal organic anion transporters 1 (OAT1) and 3 (OAT3)

This study examined the selectivity of organic anion transporters OAT1 and OAT3 for structural congeners of the heavy metal chelator 2,3-dimercapto-1-propanesulfonic acid (DMPS). Thiol-reactive reagents were also used to test structural predictions based on a homology model of OAT1 structure. DMPS was near equipotent in its ability to inhibit OAT1 (IC(50) = 83 μM) and OAT3 (IC(50) = 40 μM) expressed in Chinese hamster ovary cells. However, removal of a thiol group (3-mercapto-1-propanesulfonic acid) resulted in a 2.5-fold increase in IC(50) toward OAT1 vs. a ∼55-fold increase in IC(50) toward OAT3. The data suggested that compound volume/size is important for binding to OAT1/OAT3. The sensitivity to HgCl(2) of OAT1 and OAT3 was also dramatically different, with IC(50) values of 104 and 659 μM, respectively. Consistent with cysteines of OAT1 being more accessible from the external medium than those of OAT3, thiol-reactive reagents reacted preferentially with OAT1 in cell surface biotinylation assays. OAT1 was less sensitive to HgCl(2) inhibition and less reactive toward membrane-impermeant thiol reactive reagents following mutation of cysteine 440 (C440) to an alanine. These data indicate that C440 in transmembrane helix 10 of OAT1 is accessible from the extracellular space. Indeed, C440 was exposed to the aqueous phase of the presumptive substrate translocation pathway in a homology model of OAT1 structure. The limited thiol reactivity in OAT3 suggests that the homologous cysteine residue (C428) is less accessible. Consistent with their homolog-specific selectivities, these data highlight structural differences in the substrate binding regions of OAT1 and OAT3.

Plant sucrose transporters from a biophysical point of view

The majority of higher plants use sucrose as their main mobile carbohydrate. Proton-driven sucrose transporters play a crucial role in cell-to-cell and long-distance distribution of sucrose throughout the plant. A very negative plant membrane potential and the ability of sucrose transporters to accumulate sucrose concentrations of more than 1 M indicate that plants evolved transporters with unique structural and functional features. The knowledge about the transport mechanism and structural/functional domains of these nano-machines is, however, still fragmentary. In this review, the current knowledge about the biophysical properties of plant sucrose transporters is summarized and discussed.

Interactions of tyrosine kinase inhibitors with organic cation transporters and multidrug and toxic compound extrusion proteins

The drug-drug interaction (DDI) potential of tyrosine kinase inhibitors (TKI) as interacting drugs via transporter inhibition has not been fully assessed. Here, we estimated the half maximal inhibitory concentration (IC(50)) values for 8 small-molecule TKIs (imatinib, dasatinib, nilotinib, gefitinib, erlotinib, sunitinib, lapatinib, and sorafenib) on [(14)C]metformin transport by human organic cation transporters (OCT), OCT1, OCT2, and OCT3, and multidrug and toxic compound extrusion (MATE) proteins, MATE1 and MATE2-K, using human embryonic kidney cells stably expressing these transporters. We then compared the estimated IC(50) values to the maximum clinical concentrations of unbound TKIs in plasma (unbound C(max,sys,p)). Results showed that imatinib, nilotinib, gefitinib, and erlotinib exerted selectively potent inhibitory effects, with unbound C(max,sys,p)/IC(50) values ≥0.1, on MATE1, OCT3, MATE2-K, and OCT1, respectively. In comparison to the common form of OCT1, the OCT1 polymorphism, M420del, was more sensitive to drug inhibition by erlotinib. Major metabolites of several TKIs showed IC(50) values similar to those for unchanged TKIs. Taken together, these findings suggest the potential of clinical transporter-mediated DDIs between specific TKIs and OCTs and MATEs, which may affect the disposition, efficacy, and toxicity of metformin and other drugs that are substrates of these transporters. The study provides the basis for further clinical DDI studies with TKIs.

Organic cation transporters (OCTs, MATEs), in vitro and in vivo evidence for the importance in drug therapy

Organic cation transporters (OCTs) of the solute carrier family (SLC) 22 and multidrug and toxin extrusion (MATE) transporters of the SLC47 family have been identified as uptake and efflux transporters, respectively, for xenobiotics including several clinically used drugs such as the antidiabetic agent metformin, the antiviral agent lamivudine, and the anticancer drug oxaliplatin. Expression of human OCT1 (SLC22A1) and OCT2 (SLC22A2) is highly restricted to the liver and kidney, respectively. By contrast, OCT3 (SLC22A3) is more widely distributed. MATEs (SLC47A1, SLC47A2) are predominantly expressed in human kidney. Data on in vitro studies reporting a large number of substrates and inhibitors of OCTs and MATEs are systematically summarized. Several genetic variants of human OCTs and in part of MATE1 have been reported, and some of them result in reduced in vitro transport activity corroborating data from studies with knockout mice. A comprehensive overview is given on currently known genotype-phenotype correlations for variants in OCTs and MATE1 related to protein expression, pharmacokinetics/-dynamics of transporter substrates, treatment outcome, and disease susceptibility.

Role of organic cation transporters in drug-induced toxicity

INTRODUCTION

Membrane transporters are important determinants of in vivo drug disposition, therapeutic efficacy and adverse drug reactions. Many commonly used drugs are organic cations and substrates of organic cation transporters (OCTs). These transporters have a large binding site containing partially overlapping interaction domains for different substrates and are specifically distributed around the body. Consequently, drug interactions with these transporters can result in specific toxicity.

AREAS COVERED

This review describes the general properties of OCT and illustrates their importance for the development of important drug toxicities using the examples of metformin and cisplatin. Additionally, this review discusses the role of OCT polymorphisms in the modulation of these toxic effects.

EXPERT OPINION

Understanding how drugs interact with membrane transporters is pivotally important in explaining the mechanisms of specific toxicities and also in designing new drugs or new therapeutic protective protocols by specific competition at the transporter. Defining the pharmacogenomics of these transporters will be essential to personalized medicine, enabling physicians to choose drugs for patients based on efficacy, availability, cost, safety, tolerability and convenience.

Tyrosine 112 is essential for organic cation transport by the plasma membrane monoamine transporter

Plasma membrane monoamine transporter (PMAT) is a polyspecific organic cation transporter in the solute carrier 29 (SLC29) family. Previous studies suggested that the major substrate recognition sites are located within transmembrane domains (TM) 1-6, and interaction of PMAT with organic cations may involve aromatic residues. In this study, we analyzed the roles of tyrosine and tryptophan residues located within TM1-6 with a goal of identifying potential residues involved in substrate recognition and translocation. The six tyrosines and one tryptophan in this region were each mutated to alanine followed by analysis of the mutant's membrane localization and transport activity toward 1-methyl-4-phenylpyridinium (MPP(+)), serotonin (5-HT), and dopamine. Two mutants, Y85A and Y112A, exhibited normal cell surface expressions but lost their transport activities toward organic cations. At position Y85, aromatic substitution with phenylalanine or tryptophan fully restored organic cation transport activity. Interestingly, at position Y112, phenylalanine substitution was not allowed. Tryptophan substitution at Y112 partially restored transport activity toward 5-HT and dopamine but severely impaired MPP(+) transport. Detailed kinetic analyses revealed that tryptophan substitution at Y85 and Y112 affected the apparent binding affinity (K(m)) and maximal transport velocity (V(max)) in a substrate-dependent manner. Together, our data suggest that Y85 and Y112 are important molecular determinants for PMAT function, and Y112 is indispensable for optimal interaction with organic cation substrates. Our analyses also suggest the involvement of transmembrane domains 1 and 2 in forming the substrate permeation pathway of PMAT.

Role of organic cation transporter 3 (SLC22A3) and its missense variants in the pharmacologic action of metformin

OBJECTIVES

The goals of this study were to determine the role of organic cation transporter 3 (OCT3) in the pharmacological action of metformin and to identify and functionally characterize genetic variants of OCT3 with respect to the uptake of metformin and monoamines.

METHODS

For pharmacological studies, we evaluated metformin-induced activation of AMP-activated protein kinase, a molecular target of metformin. We used quantitative PCR and immunostaining to localize the transporter and isotopic uptake studies in cells transfected with OCT3 and its nonsynonymous genetic variants for functional analyses.

RESULTS

Quantitative PCR and immunostaining showed that OCT3 was expressed high on the plasma membrane of skeletal muscle and liver, target tissues for metformin action. Both the OCT inhibitor, cimetidine, and OCT3-specific short hairpin RNA significantly reduced the activating effect of metformin on AMP-activated protein kinase. To identify genetic variants in OCT3, we used recent data from the 1000 Genomes and the Pharmacogenomics of Membrane Transporters projects. Six novel missense variants were identified. In functional assays, using various monoamines and metformin, three variants, T44M (c.131C>T), T400I (c.1199C>T) and V423F (c.1267G>T) showed altered substrate specificity. Notably, in cells expressing T400I and V423F, the uptakes of metformin and catecholamines were significantly reduced, but the uptakes of metformin, 1-methyl-4-phenylpyridinium and histamine by T44M were significantly increased more than 50%. Structural modeling suggested that these two variants may be located in the pore lining (T400) or proximal (V423) membrane-spanning helixes.

CONCLUSION

Our study suggests that OCT3 plays a role in the therapeutic action of metformin and that genetic variants of OCT3 may modulate metformin and catecholamine action.

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.

Protein kinase inhibition differentially regulates organic cation transport

Previous studies showed that amantadine transport increased while tetraethylammonium (TEA) transport decreased in kidney tissue from diabetic rats. Changes in transport activity were reversed by exogenous insulin. We hypothesized that this difference in transport regulation is due to differential regulation of different transport systems. Native human embryonic kidney cortex cells (HEK293 cell line) and rat organic cation transporter (rOCT)-transfected cells were used to test the hypothesis. In support of differential regulation, short-term glucose starvation stimulated amantadine transport and inhibited TEA transport, but the effect was bicarbonate-modulated only for amantadine. cAMP analogues inhibited TEA transport while stimulating amantadine transport. This effect was additive to the effect of insulin, and the presence of bicarbonate affected the extent of the change. Our findings indicated that regulation of rOCT 1 and 2 was mediated by transmembrane adenylyl cyclase, and regulation of amantadine transport was mediated by soluble adenylyl cyclase, suggesting that intracellular microdomains of cAMP may be important in determining overall cellular transport for organic cations. Soluble adenylyl cyclase activity is known to be modulated by bicarbonate and lactate. These observations support our hypothesis and reconcile our previous studies demonstrating increased transport affinity for amantadine in the presence of bicarbonate and decreased transport affinity in the presence of lactate.

Drug uptake systems in liver and kidney: a historic perspective

Drugs and their metabolites are eliminated mainly by excretion into urine and bile. Studies in whole animals, isolated organs, cells, and membrane vesicles led to the conclusion that different transport systems are responsible for the transport of different classes of organic compounds (small, large, anionic, and cationic). In the early 1990s, functional expression cloning resulted in the identification of the first transporters for organic anions and cations. Eventually, all the major transport systems involved in the uptake of these organic compounds were cloned and characterized, and we now know that they belong to the organic anion transporters (OATs) and organic cation transporters (OCTs) of the SLC22A superfamily and the organic anion-transporting polypeptides (OATPs) of the SLCO superfamily of polyspecific drug transporters. Today we can explain, at the molecular level, why small and hydrophilic organic compounds are excreted predominantly through urine whereas large and amphipathic compounds are excreted mainly through bile, and we can start to predict drug-drug interactions in the case of new compounds.

Putative transmembrane domain 12 of the human organic anion transporter hOAT1 determines transporter stability and maturation efficiency

Human organic anion transporter hOAT1 plays a critical role in the body disposition of clinically important drugs. In transmembrane segment (TM) 12, residues Tyr-490 and dileucine Leu-503/Leu-504 were identified to be critical for hOAT1 function. Substitution of Tyr-490 with alanine led to a dramatic reduction in protein expression of hOAT1 and its transport activity. The contribution of the side chain of Tyr-490 to transport activity was then evaluated by replacing this residue with Trp or Phe. Substitution of Tyr-490 with Trp or Phe partially or fully recovered the protein expression of hOAT1 and its transport activity, respectively, that were lost by substitution of Tyr-490 with alanine, suggesting that the aromatic ring and the size of the side chain of Tyr-490 are critical for hOAT1 expression and function. Studies with protease inhibitors and pulse-chase labeling further showed that the loss of expression of hOAT1 and its transport activity by replacing Tyr-490 with alanine resulted from accelerated degradation of the transporter, whereas its maturation efficiency was not affected. In contrast to Tyr-490, substitution of Leu-503/Leu-504 with alanine also resulted in complete loss of protein expression of hOAT1 and its transport activity. However, such loss of protein expression could not be prevented by treating mutant-expressing cells with protease inhibitors. Pulse-chase experiments showed that the mutant transporter (L503/L504A) was trapped in the endoplasmic reticulum without conversion into mature form of the transporter. Our results are the first to highlight the central role of TM 12 in maintaining the stability and in promoting the maturation efficiency of hOAT1.

MATE1 has an external COOH terminus, consistent with a 13-helix topology

The mammalian members of the Multidrug And Toxin Extruder family, i.e., MATE1 and MATE2-K, are suspected of mediating the luminal step in renal secretion of organic cations. The 1,000+ prokaryotic/fungal/plant MATE family members are predicted to have 12 transmembrane helices (TMHs), whereas MATE1/2-K appear to have an additional (13th) COOH-terminal helix. Here, we determined whether rabbit MATE1 has an external COOH terminus, consistent with the presence of 13 TMHs. A V5 epitope tag at the COOH terminus of MATE1 was freely accessible to external V5 antibody, whereas tags at the NH(2) terminus, or at sites of truncation within the long cytoplasmic loop between predicted TMHs 12 and 13, were only accessible to the V5 antibody following permeabilization of the membrane. The truncated mutants that lacked TMH13 still retained transport activity, indicating that the terminal helix was not necessary for transport function. Cells that expressed a mutant lacking only TMH13 displayed similar K(t) and J(max) values to those of the full-length protein, although when normalized to protein expressed at the plasma membrane, the transport rate of the mutant was <10% that of full-length MATE1. An effectively cysteine-less MATE1 mutant (Delta13Cys) was functional and refractory to reaction with the impermeant marker of accessible cysteine residues, maleimide-PEO(2)-biotin. Delta13Cys mutants with an added cysteine residue at the truncation sites within the terminal cytoplasmic loop reacted with maleimide biotin only after permeabilization of the membrane, whereas a mutant with a cysteine residue at the COOH terminus was freely accessible to maleimide biotin. These data are consistent with a mammalian MATE topology that includes 13 TMHs and indicate that the terminal TMH, although not necessary for transport function, may influence the turnover characteristics of the transporter.

Organic cation transporters OCT1, 2, and 3 mediate high-affinity transport of the mutagenic vital dye ethidium in the kidney proximal tubule

The positively charged fluorescent dyes ethidium (Et+) and propidium (Pr2+) are widely used as DNA and necrosis markers. Et+is cytotoxic and mutagenic. The polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), and OCT3 (SLC22A3) mediate electrogenic facilitated diffusion of small (≤500 Da) organic cations with broad specificities. In humans, OCT2 mediates basolateral uptake by kidney proximal tubules (PT), whereas in rodents OCT1/2 are involved. In mouse kidney, perfused Et+accumulated predominantly in the S2/S3 segments of the PT, but not Pr2+. In cells stably overexpressing human OCTs (hOCTs), Et+uptake was observed with Kmvalues of 0.8 ± 0.2 μM (hOCT1), 1.7 ± 0.5 μM (hOCT2), and 2.0 ± 0.5 μM (hOCT3), whereas Pr2+was not transported. Accumulation of Et+was inhibited by OCT substrates quinine, 3-methyl-4-phenylpyridinium (MPP+), cimetidine, and tetraethylammonium (TEA+). For hOCT1 and hOCT2, the IC50values for MPP+, TEA+, and cimetidine were higher than for inhibition of previously tested transported substrates. For hOCT2, the inhibition of Et+uptake by MPP+and cimetidine was shown to be competitive. Et+also inhibited transport of 0.1 μM [3H]MPP+by all hOCT isoforms with IC50values between 0.4 and 1.3 μM, and the inhibition of hOCT1-mediated uptake of MPP+by Et+was competitive. In Oct1/2−/−mice, Et+uptake in the PT was almost abolished. The data demonstrate that Et+is taken up avidly by the PT, which is mediated by OCT1 and/or OCT2. Considering the high affinity of OCTs for Et+and their strong expression in various organs, strict safety guidelines for Et+handling should be reinforced.

Five amino acids in the innermost cavity of the substrate binding cleft of organic cation transporter 1 interact with extracellular and intracellular corticosterone

We have shown previously that Leu447 and Gln448 in the transmembrane helix (TMH) 10 of rat organic cation transporter rOCT1 are critical for inhibition of cation uptake by corticosterone. Here, we tested whether the affinity of corticosterone is different when applied from the extracellular or intracellular side. The affinity of corticosterone was determined by measuring the inhibition of currents induced by tetraethylammonium(+) (TEA(+)) in Xenopus laevis oocytes expressing rOCT1. Either corticosterone and TEA(+) were added to the bath simultaneously or the oocytes were preincubated with corticosterone, washed, and TEA(+)-induced currents were determined subsequently. In mutant L447Y, K(i) values for extracellular and intracellular corticosterone were decreased, whereas in mutant Q448E, only the K(i) for intracellular corticosterone was changed. Modeling of the interaction of corticosterone with rOCT1 in the inward- or outward-facing conformation predicted direct binding to Leu447, Phe160 (TMH2), Trp218 (TMH4), Arg440 (TMH10), and Asp475 (TM11) from both sides. In mutant F160A, affinities for extracellular and intracellular corticosterone were increased, whereas maximal inhibition was reduced in W218F and R440K. In stably transfected epithelial cells, the affinities for inhibition of 1-methyl-4-phenyl-pyridinium(+) (MPP(+)) uptake by extracellular and intracellular corticosterone were decreased when Asp475 was replaced by glutamate. In mutants F160A, W218Y, R440K, and L447F, the affinities for MPP(+) uptake were changed, and in mutant D475E, the affinity for TEA(+) uptake was changed. The data suggest that Phe160, Trp218, Arg440, Leu447, and Asp475 are located within an innermost cavity of the binding cleft that is alternatingly exposed to the extracellular or intracellular side during substrate transport.

Analysis of a large cluster of SLC22 transporter genes, including novel USTs, reveals species-specific amplification of subsets of family members

When the organic anion transporter Oat1 was first identified as NKT (Lopez-Nieto CE, You G, Bush KT, Barros EJ, Beier DR, Nigam SK. J Biol Chem 272: 6471-6478, 1997), it was argued that it, together with Oct1, may be part of a larger subfamily (now known as SLC22) involved in organic ion and xenobiotic transport. The least studied among SLC22 transporters are the so-called unknown substrate transporters (USTs). Here, five novel genes located in a cluster on mouse chromosome 19, immediately between Slc22a8 (Oat3)/Slc22a6 (Oat1) and Slc22a19 (Oat5), were identified as homologs of human USTs. These genes display preferential expression in liver and kidney, and one gene, AB056422, has several splicing variants with differential tissue expression and embryonic expression. Along with Slc22a6, Slc22a8, and Slc22a19, these Usts define the largest known cluster of mammalian Slc22 genes. Given the established functions of Oats, these genes may also be involved in organic anion transport. Usts have characteristic motifs and share a signature residue in the possible active site of transmembrane domain 7, a conserved, positively charged, amino acid, Arg356, possibly a site for interaction with organic anions. In certain species, Oat1 and Oat3 appeared to be highly conserved, whereas the Ust part of this cluster appeared to undergo repeated species-specific amplification, suggesting strong environmental selection pressure, and perhaps providing an explanation for copy number variation in the human locus. One Ust amplification in mouse appears to be recent. This cluster may be coordinately regulated and under selective pressure in a species-specific manner.

Charge-to-substrate ratio during organic cation uptake by rat OCT2 is voltage dependent and altered by exchange of glutamate 448 with glutamine

Uptake of substrate and electric charge was measured simultaneously in voltage-clamped Xenopus laevis oocytes expressing rat organic cation transporter 2 (rOCT2). At 0 mV, saturating substrate concentrations induced uptake of more positive elementary charges than monovalent organic cations, with charge-to-substrate ratios of 1.5 for guanidinium+, 3.5 for tetraethylammonium+, and 4.0 for 1-methyl-4-phenylpyridinium+. At negative holding potentials, the charge-to-substrate ratios decreased toward unity. At 0 mV, charge-to-substrate ratios higher than unity were observed at different extracellular pH and after replacement of extracellular Na+, K+, Ca2+, Mg2+, and/or Cl−. Charge-to-substrate ratios were not influenced by intracellular succinate2− or glutarate2−. The effects of membrane potential and ion substitution strongly suggest that the surplus of transported positive charge is not generated by passive ion permeabilities. Rather, we hypothetize that small cations are taken up together with organic cation substrates whereas the outward reorientation of the empty transporter is electroneutral. Nonselective cotransport of small cations was supported by the three-dimensional structures of rOCT2 in its inward-facing and outward-facing conformations, which we determined by homology modeling based on known corresponding structures of H+-lactose permease of E. coli, and by functional analysis of OCT mutants. In our model, the innermost cavity of the outward-open binding cleft is negatively charged by Glu448 and Asp475, whereas the inward-open innermost cavity is electroneutral, containing Asp379, Asp475, Lys215, and Arg440. Substitution of Glu448 by glutamine reduced the charge-to-TEA+ ratio at 0 mV to unity. The observed charge excess associated with organic cation uptake into depolarized cells may contribute to tubular damage in renal failure.

Physiology, structure, and regulation of the cloned organic anion transporters

1. The transport of negatively charged drugs, xenobiotics, and metabolites by epithelial tissues, particularly the kidney, plays critical roles in controlling their distribution, concentration, and retention in the body. Thus, organic anion transporters (OATs) impact both their therapeutic efficacy and potential toxicity. 2. This review summarizes current knowledge of the properties and functional roles of the cloned OATs, the relationships between transporter structure and function, and those factors that determine the efficacy of transport. Such factors include plasma protein binding of substrates, genetic polymorphisms among the transporters, and regulation of transporter expression. 3. Clearly, much progress has been made in the decade since the first OAT was cloned. However, unresolved questions remain. Several of these issues--drug-drug interactions, functional characterization of newly cloned OATs, tissue differences in expression and function, and details of the nature and consequences of transporter regulation at genomic and intracellular sites--are discussed in the concluding Perspectives section.

Organic cation transporters

1. Organic cation transporters (OCTs) translocate endogenous (e.g. dopamine) and exogenous (e.g. drugs) substances of cationic nature and, therefore, play an important role in the detoxification of exogenous compounds. This review aims to furnish essential information on OCTs, with an emphasis on pharmacological aspects. 2. Analysis of the literature on OCTs makes clear that there is a species- and organ-specific distribution of the different isoforms, which can also be differentially regulated. OCTs are responsible for the excretion and/or distribution of many drugs and also for serious tissue-specific side-effects such as cisplatin-induced nephrotoxicity. The presence of single nucleotide polymorphisms in these transporters significantly influences the response of patients to medication, as demonstrated for the antidiabetic drug metformin. 3. A substantial amount of research has to be undertaken to clarify further the OCT structure-function relationships specifically to define the role of oligomerization on their activity and regulation, to identify intracellular interaction partners of OCTs, and to characterize their pharmacogenetic aspects.

Implications of the alternating access model for organic anion transporter kinetics

Many transport proteins, including the clinically important organic anion transporters (OATs), appear to function via an "alternating access" mechanism. In analyzing the kinetics of these transporters, the terms K(m) and V(max) are often treated in the field as denoting, respectively, the affinity of the substrate for the transporter and the turnover (conformational switch) rate of the substrate-transporter complex. In fact, the expressions for both these parameters have very complex forms comprising multiple rate constants from conformational switch as well as association/dissociation steps in the cycling of the transporter and, therefore, do not have straightforward physical meanings. However, if the rapid equilibrium assumption is made (namely, that the association/dissociation steps occur far more rapidly than the conformational switch steps), these expressions become greatly simplified and their physical meaning clear, though still distinct from the conventional interpretations. V(max) will be a function of not just the rate of substrate-transporter complex turnover but also the rate of the "return" conformational switch and will vary largely with the slower of these two steps (the rate-limiting step). K(m) will be seen to be related to substrate affinity by a term that varies inversely with the substrate-transporter complex turnover rate, essentially because the greater this rate, the greater the extent to which transporters will be distributed in a conformation inaccessible to substrate. Here, an intuitive approach is presented to demonstrate these conclusions. The phenomena of trans-stimulation and trans-inhibition are discussed in the context of this analysis.

Xenobiotic transporters of the human organic anion transporting polypeptides (OATP) family

1. The organic anion transporting polypeptides (humans OATP; other species Oatp) belong to the SLCO gene superfamily of transporters and are twelve transmembrane domain glycoproteins expressed in various epithelial cells. Some OATPs/Oatps are expressed in a single organ, while others are expressed ubiquitously. 2. The functionally characterized members mediate sodium-independent transport of a variety of structurally independent, mainly amphipathic organic compounds, including bile salts, hormones and their conjugates, toxins, and various drugs. 3. This review summarizes the general features and the substrates of the eleven human OATPs. Furthermore, it reviews what is known about the mechanism of their multispecificity, their predicted structure, their role in drug-food interactions, and their role in cancer. 4. Finally, some open questions are raised that need to be addressed to advance OATP research in the near future.

Structural requirements for drug inhibition of the liver specific human organic cation transport protein 1

The liver-specific organic cation transport protein (OCT1; SLC22A1) transports several cationic drugs including the antidiabetic drug metformin and the anticancer agents oxaliplatin and imatinib. In this study, we explored the chemical space of registered oral drugs with the aim of studying the inhibition pattern of OCT1 and of developing predictive computational models of OCT1 inhibition. In total, 191 structurally diverse compounds were examined in HEK293-OCT1 cells. The assay identified 47 novel inhibitors and confirmed 15 previously known inhibitors. The enrichment of OCT1 inhibitors was seen in several drug classes including antidepressants. High lipophilicity and a positive net charge were found to be the key physicochemical properties for OCT1 inhibition, whereas a high molecular dipole moment and many hydrogen bonds were negatively correlated to OCT1 inhibition. The data were used to generate OPLS-DA models for OCT1 inhibitors; the final model correctly predicted 82% of the inhibitors and 88% of the noninhibitors of the test set.

Genetic variants of the organic cation transporter 2 influence the disposition of metformin

Genetic variants of the organic cation transporter 2 (protein, OCT2; gene, SLC22A2) were evaluated for their contribution to the variations in the pharmacokinetics of metformin, especially to its renal elimination. Genetic variants of SLC22A2 (c.596C>T, c.602C>T, and c.808G>T) showed significant differences in metformin pharmacokinetics when compared with the reference genotype, with higher peak plasma concentration (C(max)) and area under the curve (AUC) and lower renal clearance (Cl(renal)), thereby suggesting that a decrease in transport function associated with the SLC22A2 variants results in reduced Cl(renal) of metformin and consequently leads to increased plasma concentrations.

Cell free expression and functional reconstitution of eukaryotic drug transporters

Polyspecific organic cation and anion transporters of the SLC22 protein family are critically involved in absorption and excretion of drugs. To elucidate transport mechanisms, functional and biophysical characterization of purified transporters is required and tertiary structures must be determined. Here, we synthesized rat organic cation transporters OCT1 and OCT2 and rat organic anion transporter OAT1 in a cell free system in the absence of detergent. We solubilized the precipitates with 2% 1-myristoyl-2-hydroxy- sn-glycero-3-[phospho- rac-(1-glycerol)] (LMPG), purified the transporters in the presence of 1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) or octyl glucoside, and reconstituted them into proteoliposomes. From 1 mL reaction vessels 0.13-0.36 mg of transporter proteins was purified. Thus, from five to ten 1 mL reaction vessels sufficient protein for crystallization was obtained. In the presence of 1% LMPG and 0.5% CHAPS, OCT1 and OAT1 formed homo-oligomers but no hetero-oligomers. After reconstitution of OCT1, OCT2, and OAT1 into proteoliposomes, similar Michaelis-Menten K m values were measured for uptake of 1-methyl-4-phenylpyridinium and p-aminohippurate (PAH (-)) by the organic cation and anion transporters, respectively, as after expression of the transporters in cells. Using the reconstituted system, evidence was obtained that OAT1 operates as obligatory and electroneutral PAH (-)/dicarboxylate antiporter and contains a low-affinity chloride binding site that stimulates turnover. PAH (-) uptake was observed only with alpha-ketoglutarate (KG (2-)) on the trans side, and trans-KG (2-) increased the PAH (-) concentration in voltage-clamped proteoliposomes transiently above equilibrium. The V max of PAH (-)/KG (2-) antiport was increased by Cl (-) in a manner independent of gradients, and PAH (-)/KG (2-) antiport was independent of membrane potential in the absence or presence of Cl (-).

Novel SLC22A16 polymorphisms and influence on doxorubicin pharmacokinetics in Asian breast cancer patients

OBJECTIVE

To identify novel polymorphisms in the solute carrier SLC22A16 gene and determine their influence on the pharmacokinetics of doxorubicin and doxorubicinol in Asian breast cancer patients.

METHODS

SLC22A16 coding regions were screened in a total of 400 healthy subjects belonging to three distinct Asian ethnic groups (Chinese [n = 100], Malays [n = 100] and Indians [n = 100]) and in the Caucasian population (n = 100). Pharmacokinetic parameters of doxorubicin and doxorubicinol were estimated in Asian breast cancer patients undergoing adjuvant chemotherapy to investigate genotype-phenotype correlations.

RESULTS

Four novel polymorphisms (c.146A>G [exon 2], c.312T>C, c.755T>C [exon 4] and c.1226T>C [exon 5]) were identified. The genotypic frequency of the homozygous c.146GG polymorphism was approximately twofold higher in the healthy Chinese (13%) & Malay (18%) populations compared with the Indian (7%) and Caucasian (9%) populations. The genotypic frequency of the c.1226T>C polymorphism was observed to be significantly higher among the Caucasian (11%) and Indian (8%) study subjects compared with the Chinese (1%) and Malay (1%) ethnic groups (p < 0.005 in each case). Breast cancer patients harboring the 146GG genotype showed a trend towards higher exposure levels to doxorubicin (AUC(0 negative infinity)/dose/body surface area [BSA] [hm(-5)]: 21.6; range: 18.8-27.7) compared with patients with either the reference genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 17.4; range: 8.2-26.3, p = 0.066) or heterozygotes (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 15.4; range: 6.2-38.0, p = 0.055). The exposure levels of doxorubicinol were also higher in patients harboring the variant 146GG genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 13.3; range: 8.8-21.7) when compared with patients harboring the reference genotype (AUC(0 negative infinity)/dose/BSA[hm(-5)]): 9.8; range: 6.1-24.3, p = 0.137) or heterozygotes (AUC(0 negative infinity)/dose/BSA[hm(-5)]: 8.98; range: 3.7-20.6, p = 0.047).

CONCLUSION

Among the four novel SLC22A16 polymorphisms identified, the c.146A>G and c.1226T>C polymorphisms exhibited interethnic variations in allele and genotype frequencies. This exploratory study suggests that the c.146A>G variation could contribute to the variations in the pharmacokinetics of doxorubicin and doxorubicinol in Asian cancer patients. Further in vitro studies are required to determine the functional impact of these novel polymorphisms on doxorubicin pharmacokinetics in cancer patients.

Structure and function of the reduced folate carrier a paradigm of a major facilitator superfamily mammalian nutrient transporter

Folates are essential for life and folate deficiency contributes to a host of health problems including cardiovascular disease, fetal abnormalities, neurological disorders, and cancer. Antifolates, represented by methotrexate, continue to occupy a unique niche among the modern day pharmacopoeia for cancer along with other pathological conditions. This article focuses on the biology of the membrane transport system termed the "reduced folate carrier" or RFC with a particular emphasis on RFC structure and function. The ubiquitously expressed RFC is the major transporter for folates in mammalian cells and tissues. Loss of RFC expression or function portends potentially profound physiological or developmental consequences. For chemotherapeutic antifolates used for cancer, loss of RFC expression or synthesis of mutant RFC protein with impaired function results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and low levels of substrate for polyglutamate synthesis. The functional properties for RFC were first documented nearly 40 years ago in murine leukemia cells. Since 1994, when RFC was first cloned, tremendous advances in the molecular biology of RFC and biochemical approaches for studying the structure of polytopic membrane proteins have led to an increasingly detailed picture of the molecular structure of the carrier, including its membrane topology, its N-glycosylation, identification of functionally and structurally important domains and amino acids, and helix packing associations. Although no crystal structure for RFC is yet available, biochemical and molecular studies, combined with homology modeling, based on homologous bacterial major facilitator superfamily transporters such as LacY, now permit the development of experimentally testable hypotheses designed to establish RFC structure and mechanism.

Eukaryotic major facilitator superfamily transporter modeling based on the prokaryotic GlpT crystal structure

The major facilitator superfamily (MFS) of transporters represents the largest family of secondary active transporters and has a diverse range of substrates. With structural information for four MFS transporters, we can see a strong structural commonality suggesting, as predicted, a common architecture for MFS transporters. The rate for crystal structure determination of MFS transporters is slow, making modeling of both prokaryotic and eukaryotic transporters more enticing. In this review, models of eukaryotic transporters Glut1, G6PT, OCT1, OCT2 and Pho84, based on the crystal structures of the prokaryotic GlpT, based on the crystal structure of LacY are discussed. The techniques used to generate the different models are compared. In addition, the validity of these models and the strategy of using prokaryotic crystal structures to model eukaryotic proteins are discussed. For comparison, E. coli GlpT was modeled based on the E. coli LacY structure and compared to the crystal structure of GlpT demonstrating that experimental evidence is essential for accurate modeling of membrane proteins.

Identification of cysteines in rat organic cation transporters rOCT1 (C322, C451) and rOCT2 (C451) critical for transport activity and substrate affinity

Effects of the sulfhydryl reagent methylmethanethiosulfonate (MMTS) on functions of organic cation transporters (OCTs) were investigated. Currents induced by 10 mM choline [ Imax(choline)] in Xenopus laevis oocytes expressing rat OCT1 (rOCT1) were increased four- to ninefold after 30-s incubation with 5 mM MMTS whereas Imax(choline) by rat OCT2 was 70% decreased. MMTS activated the rOCT1 transporter within the plasma membrane without changing stoichiometry between translocated charge and cation. After modification of oocytes expressing rOCT1 or rOCT2 with MMTS, I0.5(choline) values for choline-induced currents were increased. For rOCT1 it was shown that MMTS increased I0.5 values for different cations by different degrees. Mutagenesis of individual cysteine residues in rOCT1 revealed that modification of cysteine 322 in the large intracellular loop, and of cysteine 451 at the transition of the transmembrane α-helix (TMH) 10 to the short intracellular loop between the TMH 10 and 11 is responsible for the observed effects of MMTS. After replacement of cysteine 451 by methionine, the IC50(choline) for choline to inhibit MPP uptake by rOCT1 was increased whereas the I0.5(choline) value for choline-induced current remained unchanged. At variance, in double mutant Cys322Ser, Cys451Met, I0.5(choline) was increased compared with rOCT1 wild-type whereas in the single mutant Cys322Ser I0.5(choline) was not changed. The data suggest that modification of rOCT1 at cysteines 322 and 451 leads to an increase in turnover. They indicate that cysteine 451 in rOCT1 interacts with the large intracellular loop and that cysteine 451 in both rOCT1 and rOCT2 is critical for the affinity of choline.

Pharmacophore modelling of stereoselective binding to the human organic cation transporter (hOCT1)

BACKGROUND AND PURPOSE

The human organic cation transporter-1 (hOCT1) is a polyspecific transporter that plays a role in drug distribution, metabolism and excretion. Previous studies have demonstrated that hOCT1 binding can be stereoselective, but the mechanism for stereochemical recognition has not been described. The purpose of this study was to develop a pharmacophore model to describe stereoselective binding to hOCT1.

EXPERIMENTAL APPROACH

A set of 22 compounds including 8 pairs of enantiomers and five pairs of diastereomers was used to develop a pharmacophore model. The pharmacophore modeling was carried out using Catalyst version 4.11 and HypoGen and was based upon the correlation of the structures and activities (K(i) values) of the compounds used in the study.

KEY RESULTS

The resulting model contained a positive ion, hydrophobic and two hydrogen-bond acceptor interaction sites. The relative enantioselectivity of 8/8 enantiomeric pairs and diastereoselectivity of 5/5 diastereomers was described by mapping to a combination of at least 3 of the 4 functional feature sites of the model.

CONCLUSIONS AND IMPLICATIONS

The pharmacophore model describes stereoselective interactions with hOCT1 at one of the binding sites on the molecule.

Polyspecific organic cation transporters: structure, function, physiological roles, and biopharmaceutical implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.

Polyspecific organic cation transporters: structure, function, physiological roles, and biopharmaceutical implications

The body is equipped with broad-specificity transporters for the excretion and distribution of endogeneous organic cations and for the uptake, elimination and distribution of cationic drugs, toxins and environmental waste products. This group of transporters consists of the electrogenic cation transporters OCT1-3 (SLC22A1-3), the cation and carnitine transporters OCTN1 (SLC22A4), OCTN2 (SLC22A5) and OCT6 (SLC22A16), and the proton/cation antiporters MATE1, MATE2-K and MATE2-B. The transporters show broadly overlapping sites of expression in many tissues such as small intestine, liver, kidney, heart, skeletal muscle, placenta, lung, brain, cells of the immune system, and tumors. In epithelial cells they may be located in the basolateral or luminal membranes. Transcellular cation movement in small intestine, kidney and liver is mediated by the combined action of electrogenic OCT-type uptake systems and MATE-type efflux transporters that operate as cation/proton antiporters. Recent data showed that OCT-type transporters participate in the regulation of extracellular concentrations of neurotransmitters in brain, mediate the release of acetylcholine in non-neuronal cholinergic reactions, and are critically involved in the regulation of histamine release from basophils. The recent identification of polymorphisms in human OCTs and OCTNs allows the identification of patients with an increased risk for adverse drug reactions. Transport studies with expressed OCTs will help to optimize pharmacokinetics during development of new drugs.

Inorganic mercury interacts with cysteine residues (C451 and C474) of hOCT2 to reduce its transport activity

Human organic cation transporter 2 (hOCT2) is essential for the renal tubular secretion of many toxic organic cations. Previously, of the cysteines (C437, C451, C470, and C474) that occur within transmembrane helices that comprise the hydrophilic cleft (proposed site of substrate binding), only C474 was accessible to maleimide-PEO2-biotin (hydrophilic thiol-reactive reagent), and covalent modification of this residue caused lower transport rates (Pelis RM, Zhang X, Dangprapai Y, Wright SH, J Biol Chem 281: 35272–35280, 2006). Thus it was hypothesized that the environmental contaminant Hg2+(as HgCl2) would interact with C474 to reduce hOCT2-mediated transport. Uptake of [3H]tetraethylammonium (TEA) into Chinese hamster ovary cells stably expressing hOCT2 was reduced in a concentration-dependent manner by HgCl2, with an IC50of 3.9 ± 0.11 μM. Treatment with 10 μM HgCl2caused a sixfold reduction in the maximal rate of TEA transport but did not alter the affinity of hOCT2 for TEA. To determine which cysteines interact with Hg2+, a mutant with all four cleft cysteines converted to alanines (quadruple mutant), and four variants of this mutant, each with an individual cysteine restored, were created. The quadruple mutant was less sensitive to HgCl2than wild-type, whereas the C451- and C474-containing mutants were more sensitive than the quadruple mutant. Consistent with the HgCl2effect on transport, MTSEA-biotin only interacted with C451 and C474. These data indicate that C451 and C474 of hOCT2 reside in the aqueous milieu of the cleft and that interaction of Hg2+with these residues causes reduced TEA transport activity.

3-Hydroxyglutaric acid is transported via the sodium-dependent dicarboxylate transporter NaDC3

Patients with glutaryl-CoA dehydrogenase (GCDH) deficiency accumulate glutaric acid (GA) and 3-hydroxyglutaric acid (3OH-GA) in their blood and urine. To identify the transporter mediating the translocation of 3OH-GA through membranes, kidney tissue of Gcdh-/- mice have been investigated because of its central role in urinary excretion of this metabolite. Using microarray analyses of kidney-expressed genes in Gcdh-/- mice, several differentially expressed genes encoding transporter proteins were identified. Real-time polymerase chain reaction analysis confirmed the upregulation of the sodium-dependent dicarboxylate cotransporter 3 (NaDC3) and the organic cation transporter 2 (OCT2). Expression analysis of NaDC3 in Xenopus laevis oocytes by the two-electrode-voltage-clamp technique demonstrated the sodium-dependent translocation of 3OH-GA with a K (M) value of 0.95 mM. Furthermore, tracer flux measurements in Chinese hamster ovary cells overexpressing OCT2 showed that 3OH-GA inhibited significantly the uptake of methyl-4-phenylpyridinium, whereas 3OH-GA is not transported by OCT2. The data demonstrate for the first time the membrane translocation of 3OH-GA mediated by NaDC3 and the cis-inhibitory effect on OCT2-mediated transport of cations.

The chloride dependence of the human organic anion transporter 1 (hOAT1) is blunted by mutation of a single amino acid

Organic anion transporter 1 (OAT1) is key for the secretion of organic anions in renal proximal tubules. These organic anions comprise endogenous as well as exogenous compounds including frequently used drugs of various chemical structures. The molecular basis for the polyspecificity of OAT1 is not known. Here we mutated a conserved positively charged arginine residue (Arg(466)) in the 11(th) transmembrane helix of human OAT1. The replacement by the positively charged lysine (R466K) did not impair expression of hOAT1 at the plasma membrane of Xenopus laevis oocytes but decreased the transport of p-aminohippurate (PAH) considerably. Extracellular glutarate inhibited and intracellular glutarate trans-stimulated wild type and mutated OAT1, suggesting for the mutant R466K an unimpaired interaction with dicarboxylates. However, when Arg(466) was replaced by the negatively charged aspartate (R466D), glutarate no longer interacted with the mutant. PAH uptake by wild type hOAT1 was stimulated in the presence of chloride, whereas the R466K mutant was chloride-insensitive. Likewise, the uptake of labeled glutarate or ochratoxin A was chloride-dependent in the wild type but not in R466K. Kinetic experiments revealed that chloride did not alter the apparent K(m) for PAH but influenced V(max) in wild type OAT1-expressing oocytes. In R466K mutants the apparent K(m) for PAH was similar to that of the wild type, but V(max) was not changed by chloride removal. We conclude that Arg(466) influences the binding of glutarate, but not interaction with PAH, and interacts with chloride, which is a major determinant in substrate translocation.

Identification of Essential Histidine and Cysteine Residues of the H+/Organic Cation Antiporter Multidrug and Toxin Extrusion (MATE)

Multidrug and toxin extrusion 1 (MATE1) has been isolated as an H(+)/organic cation antiporter located at the renal brush-border membranes. Previous studies using rat renal brush-border membrane vesicles indicated that cysteine and histidine residues played critical roles in H(+)/organic cation antiport activity. In the present study, essential histidine and cysteine residues of MATE1 family were elucidated. When 7 histidine and 12 cysteine residues of rat (r)MATE1 conserved among species were mutated, substitution of His-385, Cys-62, and Cys-126 led to a significant loss of tetraethylammonium (TEA) transport activity. Cell surface biotinylation and immunofluorescence analyses with confocal microscopy indicated that rMATE1 mutant proteins were localized at plasma membranes. Mutation of the corresponding residues in human (h)MATE1 and hMATE2-K also diminished the transport activity. The transport of TEA via rMATE1 was inhibited by the sulfhydryl reagent p-chloromercuribenzenesulfonate (PCMBS) and the histidine residue modifier diethyl pyrocarbonate (DEPC) in a concentration-dependent manner. The PCMBS-caused inhibition of the transport via rMATE1 was protected by an excess of various organic cations such as TEA, suggesting that cysteine residues act as substrate-binding sites. In the case of DEPC, no such protective effects were observed. These results suggest that histidine and cysteine residues are required for MATE1 to function and that cysteine residues may serve as substrate-recognition sites.

Determination of the external loops and the cellular orientation of the N- and the C-termini of the human organic anion transporter hOAT1

The OAT (organic anion transporter) family mediates the absorption, distribution and excretion of a diverse array of environmental toxins and clinically important drugs. OAT dysfunction significantly contributes to renal, hepatic, neurological and fetal toxicity and disease. As a first step to establish the topological model of hOAT1 (human OAT1), we investigated the external loops and the cellular orientation of the N- and the C-termini of this transporter. Combined approaches of immunofluorescence studies and site-directed chemical labelling were used for such purpose. Immunofluorescence microscopy of Myc-tagged hOAT1 expressed in cultured cells identified that both the N- and the C-termini of the transporter were located in the cytoplasm. Replacement of Lys59 in the predicted extracellular loop I with arginine resulted in a mutant (K59R), which was largely inaccessible for labelling by membrane-impermeable NHS (N-hydroxysuccinimido)-SS (dithio)-biotin present in the extracellular medium. This result suggests that loop I faces outside of the cell membrane. A single lysine residue introduced into putative extracellular loops III, V and VI of mutant K59R, which is devoid of extracellular lysine, reacted readily with membrane-impermeable NHS-SS-biotin, suggesting that these putative extracellular loops are in the extracellular domains of the protein. These studies provided the first experimental evidence on the extracellular loops and the cellular orientation of the N- and the C-termini of hOAT1.

Identification and Functional Characterization of Genetic Variants of Human Organic Cation Transporters in a Korean Population

Genetic variants of three human organic cation transporter genes (hOCTs) were extensively explored in a Korean population. The functional changes of hOCT2 variants were evaluated in vitro, and those genetic polymorphisms of hOCTs were compared among different ethnic populations. From direct DNA sequencing, 7 of 13 coding variants were nonsynonymous single-nucleotide polymorphisms (SNPs), including four variants from hOCT1 (F160L, P283L, P341L, and M408V) and three from hOCT2 (T199I, T201M, and A270S), whereas 6 were synonymous SNPs. The linkage disequilibrium analysis presented for three independent LD blocks for each hOCT gene showed no significant linkage among all three hOCT genes. The transporter activities of MDCK cells that overexpress the hOCT2-T199I, -T201M, and -A270S variants showed significantly decreased uptake of [(3)H]methyl-4-phenylpyridinium acetate (MPP(+)) or [(14)C]tetraethylammonium compared with those cells that overexpress wild-type hOCT2, and the estimated kinetic parameters of these variants for [(3)H]MPP(+) uptake in oocytes showed a 2- to 5-fold increase in K(m) values and a 10- to 20-fold decrease in V(max) values. The allele frequencies of the five functional variants hOCT1-P283L, -P341L, and hOCT2-T199I, -T201M, and -A270S were 1.3, 17, 0.7, 0.7, and 11%, respectively, in a Korean population; the frequency distributions of these variants were not significantly different from those of Chinese and Vietnamese populations. These findings suggest that genetic variants of hOCTs are not linked among three genes in a Korean population, and several of the hOCT genetic variants cause decreased transport activity in vitro compared with the wild type, although the clinical relevance of these variants remains to be evaluated.

The putative transmembrane segment 7 of human organic anion transporter hOAT1 dictates transporter substrate binding and stability

Human organic anion transporter hOAT1 plays a critical role in the body disposition of clinically important drugs. We examined the role of the putative transmembrane segment (TM) 7 in the function of hOAT1. Each residue within putative TM7 was replaced by alanine, and the uptake of para-aminohippurate was studied in cells expressing the mutants. We discovered four critical amino acid residues: Trp-346, Thr-349, Tyr-353, and Tyr-354. Substitution of Tyr-353 and Tyr-354 with alanine led to the loss of transport activity without affecting the surface expression of the transporter, whereas substitution of Trp-346 and Thr-349 with alanine lead to the loss of the total expression of the transporter. The effect of side chains of Tyr-353 and Tyr-354 on transporter functions were further evaluated by replacing these residues with Phe or Trp. Among all the mutants studied (Y353W, Y353F, Y354W, and Y354F), only mutant Y353F regained 30% transport activity, which was lost from replacement of Tyr-353 with alanine, suggesting that both the -OH group and the size of the side chain at positions 353 and 354 are critical for maintaining the full transport activity. To investigate the mechanisms underlying the loss of total protein expression when Trp-346 and Thr-349 were replaced with alanine, mutant-expressing cells were treated with lysosomal or proteasomal inhibitors. Our results showed that only proteasomal inhibitors resulted in the accumulation of mutant proteins, indicating that proteasome is involved in the degradation of the mutant transporters. Therefore, Trp-346 and Thr-349 are critically involved in the stability of the transporter.

Molecular determinants of substrate selectivity of a novel organic cation transporter (PMAT) in the SLC29 family

Plasma membrane monoamine transporter (PMAT or ENT4) is a newly cloned transporter assigned to the equilibrative nucleoside transporter (ENT) family (SLC29). Unlike ENT1-3, PMAT mainly functions as a polyspecific organic cation transporter. In this study, we investigated the molecular mechanisms underlying the unique substrate selectivity of PMAT. By constructing chimeras between human PMAT and ENT1, we showed that a chimera consisting of transmembrane domains (TM) 1-6 of PMAT and TM7-11 of hENT1 behaved like PMAT, transporting 1-methyl-4-phenylpyridinium (MPP+, an organic cation) but not uridine (a nucleoside), suggesting that TM1-6 contains critical domains responsible for substrate recognition. To identify residues important for the cation selectivity of PMAT, 10 negatively charged residues were chosen and substituted with alanine. Five of the alanine mutants retained PMAT activity, and four were non-functional due to impaired targeting to the plasma membrane. However, alanine substitution at Glu(206) in TM5 abolished PMAT activity without affecting cell surface expression. Eliminating the charge at Glu(206) (E206Q) resulted in loss of organic cation transport activity, whereas conserving the negative charge (E206D) restored transporter function. Interestingly, mutant E206Q, which possesses the equivalent residue in ENT1, gained uridine transport activity. Thr(220), another residue in TM5, also showed an effect on PMAT activity. Helical wheel analysis of TM5 revealed a distinct amphipathic pattern with Glu(206) and Thr(220) clustered in the center of the hydrophilic face. In summary, our results suggest that Glu(206) functions as a critical charge sensor for cationic substrates and TM5 forms part of the substrate permeation pathway in PMAT.

Molecular insights into the structure-function relationship of organic anion transporters OATs

The organic anion transporter (OAT) family encoded by SLC22A mediates the absorption, distribution, and excretion of a diverse array of environmental toxins, and clinically important drugs, including anti-HIV therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories, and therefore is critical for the survival of mammalian species. Several OATs have been identified: OAT1 (SLC22A6), OAT2 (SLC22A7), OAT3 (SLC22A8), OAT4 (SLC22A11), OAT5 (SLC22A19) OAT6 (SLC22A20) and URAT1 (SLC22A12). The expressions of these OATs have been detected in key organs such as kidney, liver, brain and placenta. OAT dysfunction in these organs may contribute to the renal, hepatic, neurological and fetal toxicity and diseases. In this review, we summarize, according to the work done by our laboratory as well as by others, the most updated molecular studies on these OAT members, especially on the aspect of their structure-function relationships. The functional roles of N-glycosylation, transmembrane domains and individual amino acids, cell surface assembly, as well as associating proteins will be discussed. In addition, we will show the recent analyses of coding region polymorphisms of OATs, which give us information on the genetic variants of OATs and their potential effects on OAT functions.

A three-dimensional model of human organic anion transporter 1: aromatic amino acids required for substrate transport

Organic anion transporters (OATs) play a critical role in the handling of endogenous and exogenous organic anions by excretory and barrier tissues. Little is known about the OAT three-dimensional structure or substrate/protein interactions involved in transport. In this investigation, a theoretical three-dimensional model was generated for human OAT1 (hOAT1) based on fold recognition to the crystal structure of the glycerol 3-phosphate transporter (GlpT) from Escherichia coli. GlpT and hOAT1 share several sequence motifs as major facilitator superfamily members. The structural hOAT1 model shows that helices 5, 7, 8, 10, and 11 surround an electronegative putative active site ( approximately 830A(3)). The site opens to the cytoplasm and is surrounded by three residues not previously examined for function (Tyr(230) (domain 5) and Lys(431) and Phe(438) (domain 10)). Effects of these residues on p-aminohippurate (PAH) and cidofovir transport were assessed by point mutations in a Xenopus oocyte expression system. Membrane protein expression was severely limited for the Y230A mutant. For the K431A and F438A mutants, [(3)H]PAH uptake was less than 30% of wild-type hOAT1 uptake after protein expression correction. Reduced V(max) values for the F438A mutant confirmed lower protein expression. In addition, the F438A mutant exhibited an increased affinity for cidofovir but was not significantly different for PAH. Differences in handling of PAH and cidofovir were also observed for the Y230F mutant. Little uptake was determined for cidofovir, whereas PAH uptake was similar to wild-type hOAT1. Therefore, the hOAT1 structural model has identified two new residues, Tyr(230) and Phe(438), which are important for substrate/protein interactions.

Cysteine accessibility in the hydrophilic cleft of human organic cation transporter 2

Organic cation transporters (OCTs) are involved in the renal elimination of many cationic drugs and toxins. A hypothetical three-dimensional structure of OCT2 based on a homology model that used the Escherichia coli glycerol 3-phosphate transporter as a template has been described (Zhang, X., Shirahatti, N. V., Mahadevan, D., and Wright, S. H. (2005) J. Biol. Chem. 280, 34813-34822). To further define OCT structure, the accessibility to hydrophilic thiol-reactive reagents of the 13 cysteine residues contained in the human ortholog of OCT2 was examined. Maleimide-PEO2-biotin precipitated (surface biotinylation followed by Western blotting) and reduced tetraethylammonium transport by OCT2 expressed in Chinese hamster ovary cells, effects that were largely reversed by co-exposure to substrates and transport inhibitors, suggesting interaction with cysteines that are near to or part of a substrate-binding surface. Cysteines at amino acid position 437, 451, 470, and 474 were identified from the model as being located in transmembrane helices that participate in forming the hydrophilic cleft, the proposed region of substrate-protein interaction. To determine which residues are exposed to the solvent, a mutant with all four of these cysteines converted to alanine, along with four variants of this mutant each with an individual cysteine restored, were created. Maleimide-PEO2-biotin was only effective at precipitating and reducing transport by wild-type OCT2 and the mutant with cysteine 474 restored. Additionally, the smaller thiol-reactive reagent, methanethiosulfonate ethylsulfonate, reduced transport by wild-type OCT2 and the mutant with cysteine 474 restored. These data demonstrate that cysteine 474 of OCT2 is exposed to the aqueous milieu of the cleft and contributes to forming a pathway for organic cation transport.

Functional Role of the C Terminus of Human Organic Anion Transporter hOAT1

Human organic anion transporter hOAT1 plays critical roles in the body disposition of environmental toxins and clinically important drugs. In the present study, we examined the role of the C terminus of hOAT1 in its function. Combined approaches of cell surface biotinylation and transport analysis were employed for such purposes. It was found that deletion of the last 15 amino acids (residues 536-550) or the last 30 amino acids (residues 521-550) had no significant effect on transport activity. However, deletion of the entire C terminus (residues 506-550) completely abolished transport activity. Alanine scanning mutagenesis within the region of amino acids 506-520 led to the discovery of two critical amino acids: Glu-506 and Leu-512. Substitution of negatively charged Glu-506 with neutral amino acids alanine or glutamine resulted in complete loss of transport activity. However, such loss of transport activity could be rescued by substitution of Glu-506 with another negatively charged amino acid aspartic acid, suggesting the importance of negative charge at this position for maintaining the correct tertiary structure of the transporter, possibly by forming a salt bridge with a positively charged amino acid. Substitution of Leu-512 with amino acids carrying progressively smaller side chains including isoleucine, valine, and alanine resulted in mutants (L512I, L512V, and L512A) with increasingly impaired transport activity. However, the cell surface expression of these mutants was not affected. Kinetic analysis of mutant L512V revealed that the reduced transport activity of this mutant resulted mainly from a reduced maximum transport velocity Vmax without affecting the binding affinity (1/Km) of the transporter for its substrates, suggesting that the size of the side chain at position 512 critically affects transporter turnover number. Together, our results are the first to highlight the central role of the C terminus of hOAT1 in the function of this transporter.

Characterization of regulatory mechanisms and states of human organic cation transporter 2

Polyspecific organic cation transporters (OCTs) have a large substrate binding pocket with different interaction domains. To determine whether OCT regulation is substrate specific, suitable fluorescent organic cations were selected by comparing their uptake in wild-type (WT) human embryonic kidney (HEK)-293 cells and in HEK-293 cells stably transfected with hOCT2. N-amidino-3,5-diamino-6-chloropyrazine-carboxamide (amiloride) and 4-[4-(dimethylamino)-styryl]- N-methylpyridinium (ASP) showed concentration-dependent uptake in hOCT2 at 37°C. After subtraction of unspecific uptake determined in WT at 37°C or in hOCT2 at 8°C saturable specific uptake of both substrates was measured. Kmvalues of hOCT2-mediated uptake of 95 μM amiloride and 24 μM ASP were calculated. Inhibition of amiloride and ASP uptake by several organic cations was also measured [IC50(in μM) for amiloride and ASP, respectively, tetraethylammonium (TEA) 98 and 30, cimetidine 14 and 26, and tetrapentylammonium (TPA) 7 and 2]. Amiloride and ASP uptake were significantly reduced by inhibition of Ca2+/CaM complex (−55 ± 5%, n = 10 and −63 ± 2%, n = 15, for amiloride and ASP, respectively) and stimulation of PKC (−54 ± 5%, n = 14, and −31 ± 6%, n = 26) and PKA (−16 ± 5%, n = 16, and −18 ± 4%, n = 40), and they were increased by inhibition of phosphatidylinositol 3-kinase (+28 ± 6%, n = 8, and +55 ± 17%, n = 16). Inhibition of Ca2+/CaM complex resulted in a significant decrease of Vmax(160–99 photons/s) that can be explained in part by a reduction of the membrane-associated hOCT2 (−22 ± 6%, n = 9) as determined using FACScan flow cytometry. The data indicate that saturable transport by hOCT2 can be measured by the fluorescent substrates amiloride and ASP and that transport activity for both substrates is regulated similarly. Inhibition of the Ca2+/CaM complex causes changes in transport capacity via hOCT2 trafficking.

Functional influence ofN-glycosylation in OCT2-mediated tetraethylammonium transport

OCT2, an organic cation transporter critical for removal of many drugs and toxins from the body, contains consensus sites for N-glycosylation at amino acid position 71, 96, and 112. However, the extent to which these sites are glycosylated by the cell, and the influence glycosylation has on OCT2 function, remains unknown. To address these issues, the acquisition of N-glycosylation was disrupted by mutating the amino acid asparagine (N) to glutamine (Q) at these sites in the rabbit ortholog of OCT2, which was expressed in Chinese hamster ovary cells. Disruption of N-glycosylation followed by Western blotting indicated that each site is indeed glycosylated and that OCT2 contains no other sites of N-glycosylation. Plasma membrane expression (determined by surface biotinylation) of the N112Q mutant, but not N71Q or N96Q mutants, was fourfold lower than that of wild-type OCT2, and unglycosylated OCT2 (N71Q/N96Q/N112Q) was sequestered in an unidentified intracellular compartment. The N71Q, N96Q, and N112Q mutants had a higher affinity (∼2-fold) for tetraethylammonium (TEA). Maximum transport rate was reduced in the N96Q (3-fold) and N112Q (5-fold) mutants, but not the N71Q mutant, and unglycosylated OCT2 failed to transport TEA (associated with its absence in the plasma membrane). Whereas the reduction in maximum transport rate of the N112Q mutant is consistent with its reduced plasma membrane expression, the lower rate of the N96Q mutant, which appeared to traffic properly, suggests that glycosylation at N96 increases the transporter turnover number.