Equilibrative nucleoside transporters: mapping regions of interaction for the substrate analogue nitrobenzylthioinosine (NBMPR) using rat chimeric proteins

The Role of Solute Carrier Transporters in Efficient Anticancer Drug Delivery and Therapy

Transporter-mediated drug resistance is a major obstacle in anticancer drug delivery and a key reason for cancer drug therapy failure. Membrane solute carrier (SLC) transporters play a crucial role in the cellular uptake of drugs. The expression and function of the SLC transporters can be down-regulated in cancer cells, which limits the uptake of drugs into the tumor cells, resulting in the inefficiency of the drug therapy. In this review, we summarize the current understanding of low-SLC-transporter-expression-mediated drug resistance in different types of cancers. Recent advances in SLC-transporter-targeting strategies include the development of transporter-utilizing prodrugs and nanocarriers and the modulation of SLC transporter expression in cancer cells. These strategies will play an important role in the future development of anticancer drug therapies by enabling the efficient delivery of drugs into cancer cells.

Structure-Activity Relationship Studies of 4-((4-(2-fluorophenyl)piperazin-1-yl)methyl)-6-imino-N-(naphthalen-2-yl)-1,3,5-triazin-2-amine (FPMINT) Analogues as Inhibitors of Human Equilibrative Nucleoside Transporters

Equilibrative nucleoside transporters (ENTs) play a vital role in nucleotide synthesis, regulation of adenosine function and chemotherapy. Current inhibitors of ENTs are mostly ENT1-selective. Our previous study has demonstrated that 4-((4-(2-fluorophenyl)piperazin-1-yl)methyl)-6-imino-N-(naphthalen-2-yl)-1,3,5-triazin-2-amine (FPMINT) is a novel inhibitor of ENTs, which is more selective to ENT2 than to ENT1. The present study aimed to screen a series of FPMINT analogues and study their structure-activity relationship. Nucleoside transporter-deficient cells transfected with cloned human ENT1 and ENT2 were used as in vitro models. The results of the [³H]uridine uptake study showed that the replacement of the naphthalene moiety with the benzene moiety could abolish the inhibitory effects on ENT1 and ENT2. The addition of chloride to the meta position of this benzene moiety could restore only the inhibitory effect on ENT1 but had no effect on ENT2. However, the addition of the methyl group to the meta position or the ethyl or oxymethyl group to the para position of this benzene moiety could regain the inhibitory activity on both ENT1 and ENT2. The presence of a halogen substitute, regardless of the position, in the fluorophenyl moiety next to the piperazine ring was essential for the inhibitory effects on ENT1 and ENT2. Among the analogues tested, compound 3c was the most potent inhibitor. Compound 3c reduced V _max of [³H]uridine uptake in ENT1 and ENT2 without affecting K _m. The inhibitory effect of compound 3c could not be washed out. Compound 3c did not affect cell viability, protein expression and internalization of ENT1 and ENT2. Therefore, similar to FPMINT, compound 3c was an irreversible and non-competitive inhibitor. Molecular docking analysis also showed that the binding site of compound 3c in ENT1 may be different from that of other conventional inhibitors. It is expected that structural modification may further improve its potency and selectivity and lead to the development of useful pharmacological agents.

Testicular disposition of clofarabine in rats is dependent on equilibrative nucleoside transporters

Acute lymphoblastic leukemia (ALL) is the most common cancer in children and adolescents. Although the 5-year survival rate is high, some patients respond poorly to chemotherapy or have recurrence in locations such as the testis. The blood-testis barrier (BTB) can prevent complete eradication by limiting chemotherapeutic access and lead to testicular relapse unless a chemotherapeutic is a substrate of drug transporters present at this barrier. Equilibrative nucleoside transporter (ENT) 1 and ENT2 facilitate the movement of substrates across the BTB. Clofarabine is a nucleoside analog used to treat relapsed or refractory ALL. This study investigated the role of ENTs in the testicular disposition of clofarabine. Pharmacological inhibition of the ENTs by 6-nitrobenzylthioinosine (NBMPR) was used to determine ENT contribution to clofarabine transport in primary rat Sertoli cells, in human Sertoli cells, and across the rat BTB. The presence of NBMPR decreased clofarabine uptake by 40% in primary rat Sertoli cells (p = .0329) and by 53% in a human Sertoli cell line (p = .0899). Rats treated with 10 mg/kg intraperitoneal (IP) injection of the NBMPR prodrug, 6-nitrobenzylthioinosine 5'-monophosphate (NBMPR-P), or vehicle, followed by an intravenous (IV) bolus 10 mg/kg dose of clofarabine, showed a trend toward a lower testis concentration of clofarabine than vehicle (1.81 ± 0.59 vs. 2.65 ± 0.92 ng/mg tissue; p = .1160). This suggests that ENTs could be important for clofarabine disposition. Clofarabine may be capable of crossing the human BTB, and its potential use as a first-line treatment to avoid testicular relapse should be considered.

Toward a Molecular Basis of Cellular Nucleoside Transport in Humans

Nucleosides play central roles in all facets of life, from metabolism to cellular signaling. Because of their physiochemical properties, nucleosides are lipid bilayer impermeable and thus rely on dedicated transport systems to cross biological membranes. In humans, two unrelated protein families mediate nucleoside membrane transport: the concentrative and equilibrative nucleoside transporter families. The objective of this review is to provide a broad outlook on the current status of nucleoside transport research. We will discuss the role played by nucleoside transporters in human health and disease, with emphasis placed on recent structural advancements that have revealed detailed molecular principles of these important cellular transport systems and exploitable pharmacological features.

Current Progress on Equilibrative Nucleoside Transporter Function and Inhibitor Design

Physiological nucleosides are used for the synthesis of DNA, RNA, and ATP in the cell and serve as universal mammalian signaling molecules that regulate physiological processes such as vasodilation and platelet aggregation by engaging with cell surface receptors. The same pathways that allow uptake of physiological nucleosides mediate the cellular import of synthetic nucleoside analogs used against cancer, HIV, and other viral diseases. Physiological nucleosides and nucleoside drugs are imported by two families of nucleoside transporters: the SLC28 concentrative nucleoside transporters (CNTs) and SLC29 equilibrative nucleoside transporters (ENTs). The four human ENT paralogs are expressed in distinct tissues, localize to different subcellular sites, and transport a variety of different molecules. Here we provide an overview of the known structure-function relationships of the ENT family with a focus on ligand binding and transport in the context of a new hENT1 homology model. We provide a generic residue numbering system for the different ENTs to facilitate the interpretation of mutational data produced using different ENT homologs. The discovery of paralog-selective small-molecule modulators is highly relevant for the design of new therapies and for uncovering the functions of poorly characterized ENT family members. Here, we discuss recent developments in the discovery of new paralog-selective small-molecule ENT inhibitors, including new natural product-inspired compounds. Recent progress in the ability to heterologously produce functional ENTs will allow us to gain insight into the structure and functions of different ENT family members as well as the rational discovery of highly selective inhibitors.

Role of cysteine 416 in N-ethylmaleimide sensitivity of human equilibrative nucleoside transporter 1 (hENT1)

Human equilibrative nucleoside transporter 1 (hENT1), the first identified member of the ENT family of integral membrane proteins, is the primary mechanism for cellular uptake of physiologic nucleosides and many antineoplastic and antiviral nucleoside drugs. hENT1, which is potently inhibited by nitrobenzylthioinosine (NBMPR), possesses 11 transmembrane helical domains with an intracellular N-terminus and an extracellular C-terminus. As a protein with 10 endogenous cysteine residues, it is sensitive to inhibition by the membrane permeable sulfhydryl-reactive reagent N-ethylmaleimide (NEM) but is unaffected by the membrane impermeable sulfhydryl-reactive reagent p-chloromercuriphenyl sulfonate. To identify the residue(s) involved in NEM inhibition, we created a cysteine-less version of hENT1 (hENT1C-), with all 10 endogenous cysteine residues mutated to serine, and showed that it displays wild-type uridine transport and NBMPR-binding characteristics when produced in the Xenopus oocyte heterologous expression system, indicating that endogenous cysteine residues are not essential for hENT1 function. We then tested NEM sensitivity of recombinant wild-type hENT1, hENT1 mutants C1S to C10S (single cysteine residues replaced by serine), hENT1C- (all cysteine residues replaced by serine), and hENT1C- mutants S1C to S10C (single serine residues converted back to cysteine). Mutants C9S (C416S/hENT1) and S9C (S416C/hENT1C-) were insensitive and sensitive, respectively, to inhibition by NEM, identifying Cys⁴¹⁶ as the endofacial cysteine residue in hENT1 responsible for NEM inhibition. Kinetic experiments suggested that NEM modification of Cys⁴¹⁶, which is located at the inner extremity of TM10, results in the inhibition of hENT1 uridine transport and NBMPR binding by constraining the protein in its inward-facing conformation.

Glycodendrimer Nanocarriers for Direct Delivery of Fludarabine Triphosphate to Leukemic Cells: Improved Pharmacokinetics and Pharmacodynamics of Fludarabine

Fludarabine, a nucleoside analogue antimetabolite, has complicated pharmacokinetics requiring facilitated transmembrane transport and intracellular conversion to triphosphate nucleotide form (Ara-FATP), causing it to be susceptible to emergence of drug resistance. We are testing a promising strategy to improve its clinical efficacy by direct delivery of Ara-FATP utilizing a biocompatible glycodendrimer nanocarrier system. Here, we present results of a proof-of-concept experiment in several in vitro-cultured leukemic cell lines (CCRF, THP-1, U937) using noncovalent complexes of maltose-modified poly(propyleneimine) dendrimer and fludarabine triphosphate. We show that Ara-FATP has limited cytotoxic activity toward investigated cells relative to free nucleoside (Ara-FA), but complexation with the glycodendrimer (which does not otherwise influence cellular metabolism) drastically increases its toxicity. Moreover, we show that transport via hENT1 is a limiting step in Ara-FA toxicity, while complexation with dendrimer allows Ara-FATP to kill cells even in the presence of a hENT1 inhibitor. Thus, the use of glycodendrimers for drug delivery would allow us to circumvent naturally occurring drug resistance due to decreased transporter activity. Finally, we demonstrate that complex formation does not change the advantageous multifactorial intracellular pharmacodynamics of Ara-FATP, preserving its high capability to inhibit DNA and RNA synthesis and induce apoptosis via the intrinsic pathway. In comparison to other nucleoside analogue drugs, fludarabine is hereby demonstrated to be an optimal candidate for maltose glycodendrimer-mediated drug delivery in antileukemic therapy.

The SLC28 (CNT) and SLC29 (ENT) nucleoside transporter families: a 30-year collaborative odyssey

Specialized nucleoside transporter (NT) proteins are required for passage of nucleosides and hydrophilic nucleoside analogues across biological membranes. Physiologic nucleosides serve as central salvage metabolites in nucleotide biosynthesis, and nucleoside analogues are used as chemotherapeutic agents in the treatment of cancer and antiviral diseases. The nucleoside adenosine modulates numerous cellular events via purino-receptor cell signalling pathways. Human NTs are divided into two structurally unrelated protein families: the SLC28 concentrative nucleoside transporter (CNT) family and the SLC29 equilibrative nucleoside transporter (ENT) family. Human CNTs are inwardly directed Na(+)-dependent nucleoside transporters found predominantly in intestinal and renal epithelial and other specialized cell types. Human ENTs mediate bidirectional fluxes of purine and pyrimidine nucleosides down their concentration gradients and are ubiquitously found in most, possibly all, cell types. Both protein families are evolutionarily old: CNTs are present in both eukaryotes and prokaryotes; ENTs are widely distributed in mammalian, lower vertebrate and other eukaryote species. This mini-review describes a 30-year collaboration with Professor Stephen Baldwin to identify and understand the structures and functions of these physiologically and clinically important transport proteins.

Equilibrative nucleoside transporters-A review

Equilibrative nucleoside transporters (ENTs) are polytopic integral membrane proteins that mediate the transport of nucleosides, nucleobases, and therapeutic analogs. The best-characterized ENTs are the human transporters hENT1 and hENT2. However, non-mammalian eukaryotic ENTs have also been studied (e.g., yeast, parasitic protozoa). ENTs are major pharmaceutical targets responsible for modulating the efficacy of more than 30 approved drugs. However, the molecular mechanisms and chemical determinants of ENT-mediated substrate recognition, binding, inhibition, and transport are poorly understood. This review highlights findings on the characterization of ENTs by surveying studies on genetics, permeant and inhibitor interactions, mutagenesis, and structural models of ENT function.

Purine import into malaria parasites as a target for antimalarial drug development

Infection with Plasmodium species parasites causes malaria. Plasmodium parasites are purine auxotrophs. In all life cycle stages, they require purines for RNA and DNA synthesis and other cellular metabolic processes. Purines are imported from the host erythrocyte by equilibrative nucleoside transporters (ENTs). They are processed via purine salvage pathway enzymes to form the required purine nucleotides. The Plasmodium falciparum genome encodes four putative ENTs (PfENT1-4). Genetic, biochemical, and physiologic evidence suggest that PfENT1 is the primary purine transporter supplying the purine salvage pathway. Protein mass spectrometry shows that PfENT1 is expressed in all parasite stages. PfENT1 knockout parasites are not viable in culture at purine concentrations found in human blood (<10 μM). Thus, PfENT1 is a potential target for novel antimalarial drugs, but no PfENT1 inhibitors have been identified to test the hypothesis. Identifying inhibitors of PfENT1 is an essential step to validate PfENT1 as a potential antimalarial drug target.

Nucleoside transporters in the purinome

The purinome is a rich complex of proteins and cofactors that are involved in fundamental aspects of cellular homeostasis and cellular responses. The purinome is evolutionarily ancient and is made up of thousands of members. Our understanding of the mechanisms linking some parts of this complex network and the physiological relevance of the various connections is well advanced. However, our understanding of other parts of the purinome is less well developed. Our research focuses on the adenosine or nucleoside transporters (NTs), which are members of the membrane purinome. Nucleoside transporters are integral membrane proteins that are responsible for the flux of nucleosides, such as adenosine, and nucleoside analog drugs, used in a variety of anti-cancer, anti-viral and anti-parasite therapies, across cell membranes. Nucleoside transporters form the SLC28 and SLC29 families of solute carriers and the protein members of these families are widely distributed in human tissues including the central nervous system (CNS). NTs modulate purinergic signaling in the CNS primarily through their effects on modulating prevailing adenosine levels inside and outside the cell. By clearing the extracellular milieu of adenosine, NTs can terminate adenosine receptor-dependent signaling and this raises the possibility of regulatory feedback loops that tie together receptor signaling with transporter function. Despite the important role of NTs as modulators of purinergic signaling in the human body, very little is known about the nature or underlying mechanisms of regulation of either the SLC28 or SLC29 families, particularly within the context of the CNS purinome. Here we provide a brief overview of our current understanding of the regulation of members of the SLC29 family and highlight some interesting avenues for future research.

Cysteine cross-linking defines the extracellular gate for the Leishmania donovani nucleoside transporter 1.1 (LdNT1.1)

Equilibrative nucleoside transporters are a unique family of proteins that enable uptake of nucleosides/nucleobases into a wide range of eukaryotes and internalize a myriad of drugs used in the treatment of cancer, heart disease, AIDs, and parasitic infections. In previous work we generated a structural model for such a transporter, the LdNT1.1 nucleoside permease from the parasitic protozoan Leishmania donovani, using ab initio computation. The model suggested that aromatic residues present in transmembrane helices 1, 2, and 7 interact to form an extracellular gate that closes the permeation pathway in the inward-open conformation. Mutation of residues Phe-48(TM1) and Trp-75(TM2) abrogated transport activity, consistent with such prediction. In this study cysteine mutagenesis and oxidative cross-linking were combined to analyze proximity relationships of helices 1, 2, and 7 in LdNT1.1. Disulfide bond formation between introduced paired cysteines at the interface of such helices (A61C(TM1)/F74C(TM2), A61C(TM1)/G350C(TM7), and F74C(TM2)/G350C(TM7)) was analyzed by transport measurement and gel mobility shifts upon oxidation with Cu (II)-(1,10-phenanthroline)(3). In all cases cross-linking inhibited transport. However, if LdNT1.1 ligands were included during cross-linking, inhibition of transport was reduced, suggesting that ligands moved the three gating helices apart. Moreover, all paired cysteine mutants exhibited a mobility shift upon oxidation, corroborating the formation of a disulfide bond. These data support the notion that helices 1, 2, and 7 constitute the extracellular gate of LdNT1.1, thus further validating the computational model and the previously demonstrated importance of F48(TM1) and Trp-75(TM2) in tethering together helices that are part of the gate.

Identification of cysteines involved in the effects of methanethiosulfonate reagents on human equilibrative nucleoside transporter 1

Inhibitor and substrate interactions with equilibrative nucleoside transporter 1 (ENT1; SLC29A1) are known to be affected by cysteine-modifying reagents. Given that selective ENT1 inhibitors, such as nitrobenzylmercaptopurine riboside (NBMPR), bind to the N-terminal half of the ENT1 protein, we hypothesized that one or more of the four cysteine residues in this region were contributing to the effects of the sulfhydryl modifiers. Recombinant human ENT1 (hENT1), and the four cysteine-serine ENT1 mutants, were expressed in nucleoside transport-deficient PK15 cells and probed with a series of methanethiosulfonate (MTS) sulfhydryl-modifying reagents. Transporter function was assessed by the binding of [(3)H]NBMPR and the cellular uptake of [(3)H]2-chloroadenosine. The membrane-permeable reagent methyl methanethiosulfonate (MMTS) enhanced [(3)H]NBMPR binding in a pH-dependent manner, but decreased [(3)H]2-chloroadenosine uptake. [2-(Trimethylammonium)ethyl] methane-thiosulfonate (MTSET) (positively charged, membrane-impermeable), but not sodium (2-sulfonatoethyl)-methanethiosulfonate (MTSES) (negatively charged), inhibited [(3)H]NBMPR binding and enhanced [(3)H]2-chloroadenosine uptake. Mutation of Cys222 in transmembrane (TM) 6 eliminated the effect of MMTS on NBMPR binding. Mutation of Cys193 in TM5 enhanced the ability of MMTS to increase [(3)H]NBMPR binding and attenuated the effects of MMTS and MTSET on [(3)H]2-chloroadenosine uptake. Taken together, these data suggest that Cys222 contributes to the effects of MTS reagents on [(3)H]NBMPR binding, and Cys193 is involved in the effects of these reagents on [(3)H]2-chloroadenosine transport. The results of this study also indicate that the hENT1-C193S mutant may be useful as a MTSET/MTSES-insensitive transporter for future cysteine substitution studies to define the extracellular domains contributing to the binding of substrates and inhibitors to this critical membrane transporter.

Characterization of mammalian equilibrative nucleoside transporters (ENTs) by mass spectrometry

Equilibrative nucleoside transporters (ENTs) are integral membrane proteins that facilitate the movement of nucleosides and hydrophilic nucleoside analog (NA) drugs across cell membranes. ENTs are also targets for cardioprotectant drugs, which block re-uptake of the purine nucleoside adenosine, thereby enhancing purinergic receptor signaling pathways. ENTs are therefore important contributors to drug bioavailability and efficacy. Despite this important clinical role, very little is known about the structure and regulation of ENTs. Biochemical and structural studies on ENT proteins have been limited by their low endogenous expression levels, hydrophobicity and labile nature. To address these issues, we developed an approach whereby tagged mammalian ENT1 protein was over-expressed in mammalian cell lines, confirmed to be functional and isolated by affinity purification to sufficient levels to be analyzed using MALDI-TOF and tandem MS mass spectrometry. This proteomic approach will allow for a more detailed analysis of the structure, function and regulation of ENTs in the future.

An ab Initio structural model of a nucleoside permease predicts functionally important residues

Permeases belonging to the equilibrative nucleoside transporter family promote uptake of nucleosides and/or nucleobases into a wide range of eukaryotes and mediate the uptake of a variety of drugs used in the treatment of cancer, heart disease, AIDS, and parasitic infections. No experimental three-dimensional structure exists for any of these permeases, and they are not present in prokaryotes, the source of many membrane proteins used in crystal structure determination. To generate a structural model for such a transporter, the LdNT1.1 nucleoside permease from the parasitic protozoan Leishmania donovani was modeled using ab initio computation. Site-directed mutations that strongly impair transport or that alter substrate specificity map to the central pore of the ab initio model, whereas mutations that have less pronounced phenotypes map to peripheral positions. The model suggests that aromatic residues present in transmembrane helices 1, 2, and 7 may interact to form an extracellular gate that closes the permeation pathway in the inward oriented conformation. Mutation of two of these three residues abrogated transport activity, consistent with the prediction of the model. The ab initio model is similar to one derived previously using threading analysis, a distinct computational approach, supporting the overall accuracy of both models. However, significant differences in helix orientation and residue position between the two models are apparent, and the mutagenesis data suggest that the ab initio model represents an improvement regarding structural details over the threading model. The putative gating interaction may also help explain differences in substrate specificity between members of this family.

Functional redundancy of two nucleoside transporters of the ENT family (CeENT1, CeENT2) required for development ofCaenorhabditis elegans

The genome of Caenorhabditis elegans encodes multiple homologues of the two major families of mammalian equilibrative and concentrative nucleoside transporters. As part of a programme aimed at understanding the biological rationale underlying the multiplicity of eukaryote nucleoside transporters, we have now demonstrated that the nematode genes ZK809.4 (ent-1) and K09A9.3 (ent-2) encode equilibrative transporters, which we designate CeENT1 and CeENT2 respectively. These transporters resemble their human counterparts hENT1 and hENT2 in exhibiting similar broad permeant specificities for nucleosides, while differing in their permeant selectivities for nucleobases. They are insensitive to the classic inhibitors of mammalian nucleoside transport, nitrobenzylthioinosine, dilazep and draflazine, but are inhibited by the vasoactive drug dipyridamole. Use of green fluorescent protein reporter constructs indicated that the transporters are present in a limited number of locations in the adult, including intestine and pharynx. Their potential roles in these tissues were explored by using RNA interference to disrupt gene expression. Although disruption of ent-1 or ent-2 expression alone had no effect, simultaneous disruption of both genes yielded pronounced developmental defects involving the intestine and vulva.

Human equilibrative nucleoside transporter (ENT) family of nucleoside and nucleobase transporter proteins

1. The human (h) SLC29 family of integral membrane proteins is represented by four members, designated equilibrative nucleoside transporters (ENTs) because of the properties of the first-characterized family member, hENT1. They belong to the widely distributed eukaryotic ENT family of equilibrative and concentrative nucleoside/nucleobase transporter proteins. 2. A predicted topology of eleven transmembrane helices has been experimentally confirmed for hENT1. The best-characterized members of the family, hENT1 and hENT2, possess similar broad permeant selectivities for purine and pyrimidine nucleosides, but hENT2 also efficiently transports nucleobases. hENT3 has a similar broad permeant selectivity for nucleosides and nucleobases and appears to function in intracellular membranes, including lysosomes. 3. hENT4 is uniquely selective for adenosine, and also transports a variety of organic cations. hENT3 and hENT4 are pH sensitive, and optimally active under acidic conditions. ENTs, including those in parasitic protozoa, function in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis and, in humans, are also responsible for the cellular uptake of nucleoside analogues used in the treatment of cancers and viral diseases. 4. By regulating the concentration of adenosine available to cell surface receptors, mammalian ENTs additionally influence physiological processes ranging from cardiovascular activity to neurotransmission.

Fine tuning of rabbit equilibrative nucleoside transporter activity by an alternatively spliced variant

The full-length cDNA encoding an equilibrative nucleoside transporter (rbENT2) and its novel C-terminal variant, rbENT2A, were isolated from rabbit trachea. Rabbit ENT2 protein consists of 456 amino acid residues; rbENT2A is shorter by 41 residues. Both rbENT2 and rbENT2A transcripts are found in rabbit tissues including intestine, kidney cortex, kidney, and trachea, at varying levels of expression. When transfected in a heterologous expression system-Madin Darby canine kidney (MDCK) epithelial cell line-both rbENT2 and rbENT2A were expressed. rbENT2 had a molecular mass of 49 kDa; rbENT2A had a molecular mass of 44 kDa. Clones of both transporters yielded functional proteins that were capable of mediating uridine uptake and efflux without the needing to be coupled to a secondary ion (e.g. Na(+)). Remarkably, rbENT2A displayed a higher affinity (K(m) = 41 microM) and a lower capacity (V(max) = 0.6 nmol/mg protein/5 min) towards substrates than rbENT2 (K(m) = 272.8 microM, V(max) = 1.26 nmol/mg protein/5 min). Pharmacological profiles showed that nitro-benzyl-mercapto-purine-ribose (NBMPR) potently inhibited (3)H-uridine uptake mediated by rbENT2A, but not uptake mediated by rbENT2. The constitutive splicing, broad expression, markedly different kinetics, and distinct pharmacological characteristics of rbENT2A appear to act in conjunction with the wild type, rbENT2, to fine-tune basolateral nucleoside transport function in rabbit trachea.

The role of nucleoside transporters in cancer chemotherapy with nucleoside drugs

Nucleoside analogs are important components of treatment regimens for various malignancies. Nucleoside-specific membrane transporters mediate plasma membrane permeation of physiologic nucleosides and most nucleoside analogs, for which the initial event is cellular conversion of nucleosides to active agents. Understanding of the roles of nucleoside transporters in nucleoside drug toxicity and resistance will provide opportunities for potentiating anticancer efficacy and avoiding resistance. Because transportability is a possible determinant of toxicity and resistance of many nucleoside analogs, nucleoside transporter abundance might be a prognostic marker to assess drug resistance. Elucidation of the structural determinants of nucleoside analogs for interaction with transporter proteins as well as the structural features of transporter proteins required for permeant interaction and translocation will lead to "transportability guidelines" for the rational design and therapeutic application of nucleoside analogs as anticancer drugs. It should eventually be possible to develop clinical assays that predict sensitivity and/or resistance to nucleoside anti-cancer drugs and thus to identify those patient populations that will most likely benefit from optimal nucleoside analog treatments. This review discusses recent results from structure/function studies of human nucleoside transporters, the role of nucleoside transport processes in the cytotoxicity and resistance of several anticancer nucleoside analogs and strategies to improve the nucleoside transporter-related anticancer effects of nucleoside analogs.

Physiological roles of vascular nucleoside transporters

Nucleoside transporters (NTs) comprise 2 widely expressed families, the equilibrative nucleoside transporters (diffusion-limited channels) and concentrative nucleoside transporters (sodium-dependent transporters). Because of their anatomic position at the blood-tissue interface, vascular NTs are in an ideal position to influence vascular nucleoside levels, particularly adenosine, which among others plays an important role in tissue protection during acute injury. For example, endothelial NTs contribute to preserving the vascular integrity during conditions of limited oxygen availability (hypoxia). Indeed, hypoxia-inducible factor-1-dependent repression of NTs results in enhanced extracellular adenosine signaling and thus attenuates hypoxia-associated increases in vascular leakage. In addition, vascular NTs also contribute to cardiac ischemic preconditioning, coronary vasodilation, and inhibition of platelet aggregation. Moreover, vascular nucleoside uptake via NTs is important for nucleoside recovery, particularly in cells lacking de novo nucleotide synthesis pathways (erythrocytes, leukocytes). Taken together, vascular NTs are critical in modulating adenosine-mediated responses during conditions such as inflammation or hypoxia.

Concentrative nucleoside transporters (CNTs) in epithelia: from absorption to cell signaling

Concentrative and Equilibrative Nucleoside Transporter proteins (CNT and ENT, respectively) are encoded by gene families SLC28 and SLC29. They mediate the uptake of natural nucleosides and a variety of nucleoside-derived drugs, mostly used in anticancer therapy. CNT and ENT proteins are mostly localized in the apical and basolateral sides, respectively, in (re)absorptive epithelia. This anatomic distribution determines nucleoside and nucleoside-derived vectorial flux. CNT expression (particularly CNT2) is associated with differentiation and is also nutritionally regulated in intestinal epithelia, whereas ENT protein amounts (mostly ENT1) are increased when cells are exposed to proliferative stimuli such as EGF, TGF-alpha or wounding. Although all these features suggest a role for NT proteins in nucleoside salvage and (re)absorption, recent data demonstrate that CNT2 might be under purinergic control, in a manner that is dependent on energy metabolism. A physiological link between CNT2 function and intracellular metabolism is also supported by the evidence that extracellular adenosine can activate the AMP-dependent kinase (AMPK), by a mechanism which relies upon adenosine transport and phosphorylation. Thus the complex pattern of NT isoform expression in mammalian cells can fulfill physiological roles other than salvage.

Comprehensive examination of charged intramembrane residues in a nucleoside transporter

Permeases of the equilibrative nucleoside transporter family mediate the uptake of nucleosides and/or nucleobases in a diverse array of eukaryotes and transport a host of drugs used for treatment of cancer, heart disease, AIDS, and parasitic infections. To identify residues that play central roles in transport function, we have systematically substituted by site-directed mutagenesis all the charged residues located within predicted transmembrane domains of the Leishmania donovani nucleoside transporter 1.1, LdNT1.1, which transports adenosine and the pyrimidine nucleosides. Substitution of three of these ten residues by uncharged amino acids resulted in loss of >95% transport activity, and we hence designated them "key" residues. These amino acids were Glu94, Lys153, and Arg404 located in transmembrane domains 2, 4, and 9, respectively. In addition, previous studies on the related LdNT2 inosine/guanosine transporter identified the highly conserved Asp389 and Arg393 (equivalent to Asp374 and Arg378 in LdNT1.1) in transmembrane domain 8 as key residues. Among these residues, the mutants in Arg393 (LdNT2) and Arg404 were strongly impaired in trafficking to the plasma membrane, but the other mutants were expressed with high to moderate efficiency at the cell surface, indicating that their mutation impaired transport activity per se. A conservative K153R substitution exhibited a change in substrate specificity, acquiring the ability to transport inosine, a nucleoside that is not a substrate for the wild-type LdNT1.1 permease. These results imply that the Glu94, Lys153, and Asp374 residues may play central roles in the mechanism of substrate translocation in LdNT1.1.

Thirty Novel Genetic Variations in the SLC29A1 Gene Encoding Human Equilibrative Nucleoside Transporter 1 (hENT1)

Thirty-nine genetic variations, including thirty novel ones, were found in the human SLC29A1 gene, which encodes equilibrative nucleoside transporter 1, from 256 Japanese cancer patients administered gemcitabine. The found novel variations included -8,166G>A, -81,10A>G, -7,947G>A, -7,789T>C, -5,595G>A, -3,803_-3,783delTCGGGGAGGTGGCAGTGGGCG, -3,548G>C, -3,414G>A, -1355T>C, -34C>G, IVS1+141G>A, IVS1+260C>T, IVS1-82C>T, 177C>G, IVS3-6C>T, 564C>T, IVS8+44T>C, IVS8+90T>C, IVS8+97T>C, IVS8+131C>T, IVS8+169G>A, 933T>C, 954C>T, IVS11-52G>C, IVS11-46G>A, 1,288G>A, 1,641C>G, 1,703_1,704delGT, 1812C>T, and 1861C>T. The frequencies were 0.051 for IVS8+169G>A, 0.012 for -7,947G>A, 0.006 for IVS1+141G>A and 1,703_1,704delGT, 0.004 for -8,166G>A, -8,110A>G, -3,548G>C, -1,355T>C, -34C>G, IVS8+44T>C, and 1,812C>T, and 0.002 for the other 19 variations. Among them, 177C>G and 1,288G>A resulted in amino acid substitutions Asp59Glu and Ala430Thr, respectively. Using the detected polymorphisms, linkage disequilibrium analysis was performed, and 28 haplotypes were identified or inferred. Our findings would provide fundamental and useful information for genotyping SLC29A1 in the Japanese and probably other Asian populations.

Residue 33 of human equilibrative nucleoside transporter 2 is a functionally important component of both the dipyridamole and nucleoside binding sites

Human equilibrative nucleoside transporters 1 and 2 (hENT1 and hENT2) differ functionally in that hENT2 generally displays lower affinity for its nucleoside permeants and is less sensitive to inhibition by the coronary vasodilators dilazep and dipyridamole. In previous work, we demonstrated that mutation of residues 33 (Met versus Ile) of hENT1 and hENT2 altered sensitivity to dilazep and dipyridamole and that the hENT2 mutant (I33M) displayed a K(m) value for uridine that was lower than that of hENT2 and similar to that of hENT1 (J Biol Chem 277:395-401, 2002). In this study, we report results of an in-depth investigation of the role of residue 33 in hENT2. We found that hENT2-I33M displayed decreased K(m) values for both pyrimidine and purine nucleosides and increased V(max) values for purine nucleosides. Cys or Ser at position 33 had similar effects on the kinetic parameters of hENT2 as Met, indicating that hydrophobic (Met and Cys) or hydrogen-bonding energy (Ser) contributed to permeant binding by these residues. hENT2-I33M and I33C displayed increased sensitivities to dipyridamole compared with wild-type hENT2, hENT2-I33A, and hENT2-I33S, suggesting interaction of the sulfur atom of Met and Cys with aromatic moieties on dipyridamole. hENT2-I33C was inhibited by the membrane-impermeant sulfhydryl reactive reagent p-chloromercuribenzyl sulfonate, and uridine, adenosine, and dipyridamole protected against inhibition. Our results indicated that residue 33 resides in an extracellular domain as predicted by the current hENT2 topology model and suggested that it is a functionally important component of both the permeant and dipyridamole binding sites.

Pharmacological analysis and molecular cloning of the canine equilibrative nucleoside transporter 1

We studied the binding of [3H]nitrobenzylthioinosine (NBMPR) and the uptake of [3H]formycin B by the es (equilibrative inhibitor-sensitive) nucleoside transporter of Madin Darby Canine Kidney (MDCK) cells. NBMPR inhibited [3H]formycin B uptake with a Ki of 2.7+/-0.6 nM, and [3H]NBMPR had a KD of 1.3+/-0.3 nM for binding to these cells; these values are significantly higher than those obtained in human and mouse cell models. In contrast, other recognized es inhibitors, such as dipyridamole, were significantly more effective as inhibitors of [3H]NBMPR binding and [3H]formycin B uptake by MDCK cells relative to that seen for human cells. We isolated a cDNA encoding the canine es nucleoside transporter (designated cENT1), and assessed its function by stable expression in nucleoside transport deficient PK15NTD cells. The PK15-cENT1 cells displayed inhibitor sensitivities that were comparable to those obtained for the endogenous es nucleoside transporter in MDCK cells. These data indicate that the dog es/ENT1 transporter has distinctive inhibitor binding characteristics, and that these characteristics are a function of the protein structure as opposed to the environment in which it is expressed.

Mutation of leucine-92 selectively reduces the apparent affinity of inosine, guanosine, NBMPR [S6-(4-nitrobenzyl)-mercaptopurine riboside] and dilazep for the human equilibrative nucleoside transporter, hENT1

We developed a yeast-based assay for selection of hENT1 (human equilibrative nucleoside transporter 1) mutants that have altered affinity for hENT1 inhibitors and substrates. In this assay, expression of hENT1 in a yeast strain deficient in adenine biosynthesis (ade2) permits yeast growth on a plate lacking adenine but containing adenosine, a hENT1 substrate. This growth was prevented when inhibitors of hENT1 [e.g. NBMPR [S6-(4-nitrobenzyl)-mercaptopurine riboside], dilazep or dipyridamole] were included in the media. To identify hENT1 mutants resistant to inhibition by these compounds, hENT1 was randomly mutagenized and introduced into this strain. Mutation(s) that allowed growth of yeast cells in the presence of these inhibitors were then identified and characterized. Mutants harbouring amino acid changes at Leu92 exhibited resistance to NBMPR and dilazep but not dipyridamole. The IC50 values of NBMPR and dilazep for [3H]adenosine transport by one of these mutants L92Q (Leu92-->Gln) were approx. 200- and 4-fold greater when compared with the value for the wild-type hENT1, whereas that for dipyridamole remained unchanged. Additionally, when compared with the wild-type transporter, [3H]adenosine transport by L92Q transporter was significantly resistant to inhibition by inosine and guanosine but not by adenosine or pyrimidines. The Km value for inosine transport was approx. 4-fold greater for the L92Q mutant (260+/-16 mM) when compared with the wild-type transporter (65+/-7.8 mM). We have identified for the first time an amino acid residue (Leu92) of hENT1 that, when mutated, selectively alters the affinity of hENT1 to transport the nucleosides inosine and guanosine and its sensitivity to the inhibitors NBMPR and dilazep.

Conserved residues F316 and G476 in the concentrative nucleoside transporter 1 (hCNT1) affect guanosine sensitivity and membrane expression, respectively

The functional significance of two highly conserved amino acid residues, F316 [putative transmembrane domain (TM)7] and G476 (putative TM11), in the concentrative nucleoside transporter hCNT1 (SLC28A1) was examined by performing site-directed mutagenesis. Conservative mutations at these positions (F316A, F316Y, G476A, and G476L) were generated and expressed in Madin-Darby canine kidney (MDCK) cells as fusion polypeptides with green fluorescent protein (GFP). Unlike wild-type hCNT1, G476A-GFP and G476L-GFP were not expressed in the plasma membrane in undifferentiated or differentiated MDCK cells and had no functional activity. Like wild-type hCNT1, F316A-GFP and F316Y-GFP were expressed in the plasma membrane of undifferentiated MDCK cells and in the apical membrane of differentiated MDCK cells. Remarkably, transport of [(3)H]uridine by F316Y-GFP or F316A-GFP was highly sensitive to inhibition by guanosine. Furthermore, genotyping of exon 11 of hCNT1 (TM7) in a panel of 260 anonymous human DNA samples revealed a novel F316H variant (TT>CA; 1/260). When expressed in MDCK cells, [(3)H]uridine transport by F316H was also found to be sensitive to inhibition by guanosine (IC(50) = 148 microM). The effect of the F316H mutation resembles the N4 type nucleoside transporter phenotype previously reported to be present in human kidneys. We suggest that the N4 transport system is a naturally occurring variant of hCNT1, perhaps at the F316 position. Collectively, our data show that G476 is important for correct membrane targeting, folding, and/or intracellular processing of hCNT1. In addition, we have discovered that hCNT1 displays natural variation at position F316 and that the variant F316H confers on the transporter an unusual sensitivity to inhibition by guanosine.

Glycine 154 of the equilibrative nucleoside transporter, hENT1, is important for nucleoside transport and for conferring sensitivity to the inhibitors nitrobenzylthioinosine, dipyridamole, and dilazep

hENT1 and hENT2 are members of the human equilibrative nucleoside transporter family. hENT1 is ubiquitously expressed and plays an important role in the disposition and pharmacological activity of nucleoside drugs and nucleosides, such as adenosine. hENT2 is expressed in only a few tissues (e.g. muscle). hENT1 and hENT2 differ in their affinity for nucleoside substrates and in their sensitivity to inhibitors, such as nitrobenzylthioinosine (NBMPR). hENT1 has higher (or equal) affinity to hENT2 for all natural nucleosides except inosine. hENT1 is also more sensitive to NBMPR inhibition (IC50 approximately 0.4-8 nM) when compared with hENT2 (IC50 approximately 2.8 microM). This difference in inhibition potency is substantially dependent on the difference in amino acid at position 154 in hENT1 (glycine) and hENT2 (serine). Since NBMPR competitively inhibits nucleoside transporter activity, we hypothesized that G154 may also play a role in the transport of natural nucleosides and in the inhibition by other hENT1 inhibitors, dipyridamole (DP), and dilazep (DZ). Our results, using a yeast expression system, demonstrate that substituting glycine 154 of hENT1 with serine of hENT2 converts hENT1 to a transporter that exhibits partial characteristics of hENT2. For example, this conversion reduces sensitivity of hENT1 to the inhibitors NBMPR, DP, and DZ and reduces its transport affinity for the natural nucleosides cytidine and adenosine. However, this conversion renders hENT1 less sensitive to inhibition by anti-HIV drugs azidothymidine, dideoxyinosine, and the nucleobase, hypoxanthine. Collectively, these results suggest that glycine 154 plays an important role in the transport of nucleosides and in sensitivity to the inhibitors NBMPR, DP, and DZ.

Nucleoside transporters in chronic lymphocytic leukaemia

Nucleoside derivatives have important therapeutic activity in chronic lymphocytic leukaemia (CLL). Experimental evidence indicates that in CLL cells most of these drugs induce apoptosis ex vivo, suggesting that programmed cell death is the mechanism of their therapeutic action, relying upon previous uptake and metabolic activation. Although defective apoptosis and poor metabolism often cause resistance to treatment, differential uptake and/or export of nucleosides and nucleotides may significantly modulate intracellular drug bioavailability and, consequently, responsiveness to therapy. Two gene families, SLC28 and SLC29, encode transporter proteins responsible for concentrative and equilibrative nucleoside uptake (CNT and ENT, respectively). Furthermore, selected members of the expanding ATP-binding cassette (ABC) protein family have recently been identified as putative efflux pumps for the phosphorylated forms of these nucleoside-derived drugs, ABCC11 (MRP8) being a good candidate to modulate cell sensitivity to fluoropyrimidines. Sensitivity of CLL cells to fludarabine has also been recently correlated with ENT-type transport function, suggesting that, besides the integrity of apoptotic pathways and appropriate intracellular metabolism, transport across the plasma membrane is also a relevant event during CLL treatment. As long as nucleoside transporter expression in leukaemia cells is not constitutive, the possibility of regulating nucleoside transporter function by pharmacological means may also contribute to improve therapy.

The equilibrative nucleoside transporter family, SLC29

The human SLC29 family of proteins contains four members, designated equilibrative nucleoside transporters (ENTs) because of the properties of the first-characterised family member, hENT1. They belong to the widely-distributed eukaryotic ENT family of equilibrative and concentrative nucleoside/nucleobase transporters and are distantly related to a lysosomal membrane protein, CLN3, mutations in which cause neuronal ceroid lipofuscinosis. A predicted topology of 11 transmembrane helices with a cytoplasmic N-terminus and an extracellular C-terminus has been experimentally confirmed for hENT1. The best-characterised members of the family, hENT1 and hENT2, possess similar broad substrate specificities for purine and pyrimidine nucleosides, but hENT2 in addition efficiently transports nucleobases. The ENT3 and ENT4 isoforms have more recently also been shown to be genuine nucleoside transporters. All four isoforms are widely distributed in mammalian tissues, although their relative abundance varies: ENT2 is particularly abundant in skeletal muscle. In polarised cells ENT1 and ENT2 are found in the basolateral membrane and, in tandem with concentrative transporters of the SLC28 family, may play a role in transepithelial nucleoside transport. The transporters play key roles in nucleoside and nucleobase uptake for salvage pathways of nucleotide synthesis, and are also responsible for the cellular uptake of nucleoside analogues used in the treatment of cancers and viral diseases. In addition, by regulating the concentration of adenosine available to cell surface receptors, they influence many physiological processes ranging from cardiovascular activity to neurotransmission.

Nucleoside anticancer drugs: the role of nucleoside transporters in resistance to cancer chemotherapy

The clinical efficacy of anticancer nucleoside drugs depends on a complex interplay of transporters mediating entry of nucleoside drugs into cells, efflux mechanisms that remove drugs from intracellular compartments and cellular metabolism to active metabolites. Nucleoside transporters (NTs) are important determinants for salvage of preformed nucleosides and mediated uptake of antimetabolite nucleoside drugs into target cells. The focus of this review is the two families of human nucleoside transporters (hENTs, hCNTs) and their role in transport of cytotoxic chemotherapeutic nucleoside drugs. Resistance to anticancer nucleoside drugs is a major clinical problem in which NTs have been implicated. Single nucleotide polymorphisms (SNPs) in drug transporters may contribute to interindividual variation in response to nucleoside drugs. In this review, we give an overview of the functional and molecular characteristics of human NTs and their potential role in resistance to nucleoside drugs and discuss the potential use of genetic polymorphism analyses for NTs to address drug resistance.

Nucleoside transporters in the disposition and targeting of nucleoside analogs in the kidney

Systemic disposition of nucleosides and nucleoside analogs is dependent on renal handling of these compounds. There are five known, functionally characterized nucleoside transporters with varying substrate specificities for nucleosides: concentrative nucleoside transporters (CNT1-CNT3; Solute Carrier (SLC) 28A1-28A3), which mediate the intracellular flux of nucleosides, and equilibrative nucleoside transporters (ENT1-ENT2; SLC29A1-SLC29A2), which mediate bi-directional facilitated diffusion of nucleosides. All five of these transporters are expressed in the kidney. Concentrative nucleoside transporters primarily localize to the apical membrane of renal epithelial cells while equilibrative nucleoside transporters primarily localize to the basolateral membrane. These transporters work in concert to mediate reabsorptive flux of naturally occurring nucleosides and nucleoside analogs. In addition, equilibrative transporters also participate in secretory flux of some nucleoside analogs. Nucleoside transporters also serve in the targeting of nucleoside analog therapies to renal tumors. This review examines the role that these transporters play in renal disposition of nucleosides and nucleoside analogs in both systemic and kidney-specific therapies.

Equilibrative nucleoside transporters of Arabidopsis thaliana. cDNA cloning, expression pattern, and analysis of transport activities

Equilibrative nucleoside transporters (ENTs) occur in diverse organisms. In the model plant Arabidopsis thaliana, eight potential ENTs (AtENTs) have been predicted by genome sequencing. We here report the cloning of the cDNAs for AtENTs 2, 3, 4, 6, 7, and 8. Conceptual translation of the cDNAs of AtENTs 2, 3, 4, 6, 7, and 8 yielded polypeptides possessing strong similarities to ENTs characterized previously. Eleven putative transmembrane domains were identified in each of the six AtENTs. In suspension cells, the transcription of AtENTs 1, 3, 4, 6, and 8 was increased by two treatments (nitrogen deprivation, application of 5-fluorouracil and methotrexate) that inhibited the de novo pathway of nucleotide synthesis, indicating that multiple members of the Arabidopsis ENT family may function in the salvage pathway of nucleotide synthesis. Except for AtENT1, the transcription of the remaining six AtENTs showed varying degrees of organ specificity. However, all seven AtENTs were expressed in the leaf and flower. In plant, insect, and yeast cells, ectopically expressed AtENT3 was targeted to the plasma membrane. AtENT3 expressed in yeast cells transported adenosine and uridine with high affinity. Furthermore, the activities of AtENT3 appear not to require a transmembrane proton gradient because protonophores did not abolish adenosine or uridine transport. In competition experiments, the transport of [3H]adenosine by AtENT3 was most significantly inhibited by a number of different purine and pyrimidine nucleosides and 2'-deoxynucleosides, although certain nucleobases and nucleotides were also found to have some inhibitory effect. This indicates that AtENT3 may possess broad substrate specificity. Adenosine and uridine transport by AtENT3, although partly sensitive to the vasodilator drugs dilazep and dipyridamole, was resistant to the nucleoside analogue nitrobenzylmercaptopurine ribonucleoside. We conclude that AtENT3 represents the first ei type ENT characterized from higher plants. The potential functions of ENTs in the biology of A. thaliana are discussed.

Molecular biology and regulation of nucleoside and nucleobase transporter proteins in eukaryotes and prokaryotes

The molecular cloning of cDNAs encoding nucleoside transporter proteins has greatly advanced understanding of how nucleoside permeants are translocated across cell membranes. The nucleoside transporter proteins identified thus far have been categorized into five distinct superfamilies. Two of these superfamilies, the equilibrative and concentrative nucleoside transporters, have human members and these will be examined in depth in this review. The human equilibrative nucleoside transporters translocate nucleosides and nucleobases bidirectionally down their concentration gradients and are important in the uptake of anticancer and antiviral nucleoside drugs. The human concentrative nucleoside transporters cotranslocate nucleosides and sodium unidirectionally against the nucleoside concentration gradients and play a vital role in certain tissues. The regulation of nucleoside and nucleobase transporters is being studied more intensely now that more tools are available. This review provides an overview of recent advances in the molecular biology and regulation of the nucleoside and nucleobase transporters.

Functional and molecular characterization of nucleobase transport by recombinant human and rat equilibrative nucleoside transporters 1 and 2. Chimeric constructs reveal a role for the ENT2 helix 5-6 region in nucleobase translocation

The human (h) and rat (r) equilibrative (Na(+)-independent) nucleoside transporters (ENTs) hENT1, rENT1, hENT2, and rENT2 belong to a family of integral membrane proteins with 11 transmembrane domains (TMs) and are distinguished functionally by differences in sensitivity to inhibition by nitrobenzylthioinosine and coronary vasoactive drugs. Structurally, the proteins have a large glycosylated loop between TMs 1 and 2 and a large cytoplasmic loop between TMs 6 and 7. In the present study, hENT1, rENT1, hENT2, and rENT2 were produced in Xenopus laevis oocytes and investigated for their ability to transport pyrimidine and purine nucleobases. hENT2 and rENT2 efficiently transported radiolabeled hypoxanthine, adenine, guanine, uracil, and thymine (apparent K(m) values 0.7-2.6 mm), and hENT2, but not rENT2, also transported cytosine. These findings were independently confirmed by hypoxanthine transport experiments with recombinant hENT2 produced in purine-cytosine permease (FCY2)-deficient Saccharomyces cerevisiae and provide the first direct demonstration that the ENT2 isoform is a dual mechanism for the cellular uptake of nucleosides and nucleobases, both of which are physiologically important salvage metabolites. In contrast, recombinant hENT1 and rENT1 mediated negligible oocyte fluxes of hypoxanthine relative to hENT2 and rENT2. Chimeric experiments between rENT1 and rENT2 using splice sites at rENT1 residues 99 (end of TM 2), 171 (between TMs 4 and 5), and 231 (end of TM 6) identified TMs 5-6 of rENT2 (amino acid residues 172-231) as a determinant of nucleobase transport activity, suggesting that this domain forms part(s) of the ENT2 substrate translocation channel.

Complex effects of sulfhydryl reagents on ligand interactions with nucleoside transporters: evidence for multiple populations of ENT1 transporters with differential sensitivities to N-ethylmaleimide

Functional studies have implicated cysteines in the interaction of ligands with the ENT1 nucleoside transporter. To better define these interactions, N-ethylmaleimide (NEM) and p-chloromercuribenzylsulfonate (pCMBS) were tested for their effects on ligand interactions with the [(3)H] nitrobenzylthioinosine (NBMPR) binding site of the ENT1 transporters of mouse Ehrlich ascites cells and human erythrocytes. NEM had biphasic, concentration-dependent effects on NBMPR binding to intact Ehrlich cells, plasma membranes, and detergent-solubilized membranes, with about 35% of the binding activity being relatively insensitive to NEM inhibition. NBMPR binding to human erythrocyte membranes also displayed heterogeneity in that about 33% of the NBMPR binding sites remained, albeit with lower affinity for NBMPR, even after treatment with NEM at concentrations in excess of 1 mM. However, unlike that seen for Ehrlich cells, no "reversal" in NBMPR binding to human erythrocyte membranes was observed at the higher concentrations of NEM. pCMBS inhibited 100% of the NBMPR binding to both Ehrlich cell and human erythrocyte membranes, but had no effect on the binding of NBMPR to intact cells. The effects of NEM on NBMPR binding could be prevented by coincubation of membranes with nonradiolabeled NBMPR, adenosine, or uridine. Treatment with NEM and pCMBS also decreased the affinity of other nucleoside transport inhibitors for the NBMPR binding site, but enhanced the affinities of nucleoside substrates. These data support the existence of at least two populations of ENT1 in both erythrocyte and Ehrlich cell membranes with differential sensitivities to NEM. The interaction of NEM with the mouse ENT1 protein may also involve additional sulphydryl groups not present in the human ENT1.

Mutation of residue 33 of human equilibrative nucleoside transporters 1 and 2 alters sensitivity to inhibition of transport by dilazep and dipyridamole

Human equilibrative nucleoside transporters (hENT) 1 and 2 differ in that hENT1 is inhibited by nanomolar concentrations of dipyridamole and dilazep, whereas hENT2 is 2 and 3 orders of magnitude less sensitive, respectively. When a yeast expression plasmid containing the hENT1 cDNA was randomly mutated and screened by phenotypic complementation in Saccharomyces cerevisiae to identify mutants with reduced sensitivity to dilazep, clones with a point mutation that converted Met33 to Ile (hENT1-M33I) were obtained. Characterization of the mutant protein in S. cerevisiae and Xenopus laevis oocytes revealed that the mutant had less than one-tenth the sensitivity to dilazep and dipyridamole than wild type hENT1, with no change in nitrobenzylmercaptopurine ribonucleoside (NBMPR) sensitivity or apparent uridine affinity. To determine whether the reciprocal mutation in hENT2 (Ile33 to Met) also altered sensitivity to dilazep and dipyridamole, hENT2-I33M was created by site-directed mutagenesis. Although the resulting mutant (hENT2-I33M) displayed >10-fold higher dilazep and dipyridamole sensitivity and >8-fold higher uridine affinity compared with wild type hENT2, it retained insensitivity to NBMPR. These data established that mutation of residue 33 (Met versus Ile) of hENT1 and hENT2 altered the dilazep and dipyridamole sensitivities in both proteins, suggesting that a common region of inhibitor interaction has been identified.

Topology of a human equilibrative, nitrobenzylthioinosine (NBMPR)-sensitive nucleoside transporter (hENT1) implicated in the cellular uptake of adenosine and anti-cancer drugs

The human equilibrative nucleoside transporter hENT1, the first identified member of the ENT family of integral membrane proteins, is the primary mechanism for the cellular uptake of physiologic nucleosides, including adenosine, and many anti-cancer nucleoside drugs. We have produced recombinant hENT1 in Xenopus oocytes and used native and engineered N-glycosylation sites in combination with immunological approaches to experimentally define the membrane architecture of this prototypic nucleoside transporter. hENT1 (456 amino acid residues) is shown to contain 11 transmembrane helical segments with an amino terminus that is intracellular and a carboxyl terminus that is extracellular. Transmembrane helices are linked by short hydrophilic regions, except for a large glycosylated extracellular loop between transmembrane helices 1 and 2 and a large central cytoplasmic loop between transmembrane helices 6 and 7. Sequence analyses suggest that this membrane topology is common to all mammalian, insect, nematode, protozoan, yeast, and plant members of the ENT protein family.