Cloning of a human nucleoside transporter implicated in the cellular uptake of adenosine and chemotherapeutic drugs

Characterization of adenosine transport in H9c2 cardiomyoblasts

Adenosine plays a significant role in various physiological processes including cardioprotection. Nucleoside transporters modulate adenosine levels in the vicinity of adenosine receptors, which in turn modulate adenosine functional efficacy. In the current study, adenosine transport in the rat heart myoblast cell line H9c2 was characterized. Kinetic analysis of adenosine transport in H9c2 cells revealed a Km of 8.9+/-0.001 microM and a Vmax of 32.1+/-0.65 pmol/mg protein/min. Adenosine transport in H9c2 cells was Na+-independent. About 6% of the total adenosine uptake was sensitive to nitrobenzylmercaptopurine riboside (NBMPR); however, 94% was insensitive, suggesting that adenosine uptake by H9c2 cells was predominantly mediated by the equilibrative nucleoside transporter (ENT)-2 and only mildly by ENT-1. Results of RT-PCR demonstrated the presence of mRNA for ENT-1, ENT-2 and ENT-3. Upon culture in a cell differentiation medium containing fetal bovine serum (1%) and retinoic acid (10 nM), both the activity and mRNA expression of ENT-1 increased 3-fold, however, ENT-2 was unaffected. Pharmacological studies revealed that ENT-1 activity was stimulated by PKA and PKC-delta/epsilon, however, ENT-2 activity was unaffected. Taken together, the exceptionally high expression level of ENT-2 in H9c2 cells raises questions regarding the use of H9c2 cells as a model for physiological adenosine activity in the heart. Furthermore, this study may form the basis for further investigation into the effect of cell differentiation and protein kinases on the regulation of nucleoside transporters.

Nucleoside transporters: from scavengers to novel therapeutic targets

Hydrophilic purine and pyrimidine nucleosides rely on specialized carrier proteins for their membrane translocation. The recent identification of two gene families encoding equilibrative and concentrative nucleoside transporters in mammals and other organisms has provided the essential breakthrough to a more complete understanding of the biological significance of nucleoside transport. Although nucleoside salvage is a primary function of these proteins, recent data indicate functions beyond metabolic recycling. In brain and spinal cord, for example, nucleoside transporters have the potential to regulate synaptic levels of neuroactive purines such as adenosine and, thereby, indirectly modulate physiological processes through G-protein-coupled purine P1 receptors. As described in this review, recent research indicates novel putative functions for CNS nucleoside transporters in sleep, arousal, drug and alcohol addiction, nociception and analgesia. The therapeutic use of nucleoside analogue drugs and nucleoside transporter inhibitors in viral, neoplastic, cardiovascular and infectious disease is also described.

The role of human nucleoside transporters in cellular uptake of 4'-thio-beta-D-arabinofuranosylcytosine and beta-D-arabinosylcytosine

4'-Thio-beta-D-arabinofuranosyl cytosine (TaraC) is in phase I development for treatment of cancer. In human equilibrative nucleoside transporter (hENT) 1-containing CEM cells, initial rates of uptake (10 microM; picomoles per microliter of cell water per second) of [3H]TaraC and [3H]1-beta-D-arabinofuranosyl cytosine (araC) were low (0.007 +/- 003 and 0.034 +/- 0.003, respectively) compared with that of [3H]uridine (0.317 +/- 0.048), a highactivity hENT1 permeant. In hENT1- and hENT2-containing HeLa cells, initial rates of uptake (10 microM; picomoles per cell per second) of [3H]TaraC, [3H]araC, and [3H]deoxycytidine were low (0.30 +/- 0.003, 0.42 +/- 0.03, and 0.51 +/- 0.11, respectively) and mediated primarily by hENT1 (approximately 74, approximately 65, and approximately 61%, respectively). In HeLa cells with recombinant human concentrative nucleoside transporter (hCNT) 1 or hCNT3 and pharmacologically blocked hENT1 and hENT2, transport of 10 microM[3H]TaraC and [3H]araC was not detected. The apparent affinities of recombinant transporters (produced in yeast) for a panel of cytosine-containing nucleosides yielded results that were consistent with the observed low-permeant activities of TaraC and araC for hENT1/2 and negligible permeant activities for hCNT1/2/3. During prolonged drug exposures of CEM cells with hENT1 activity, araC was more cytotoxic than TaraC, whereas coexposures with nitrobenzylthioinosine (to pharmacologically block hENT1) yielded identical cytotoxicities for araC and TaraC. The introduction by gene transfer of hENT2 and hCNT1 activities, respectively, into nucleoside transport-defective CEM cells increased sensitivity to both drugs moderately and slightly. These results demonstrated that nucleoside transport capacity (primarily via hENT1, to a lesser extent by hENT2 and possibly by hCNT1) is a determinant of pharmacological activity of both drugs.

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.

Identification of AtENT3 as the main transporter for uridine uptake in Arabidopsis roots

Previous studies have shown that Arabidopsis equilibrative nucleoside transporters (AtENTs) possess transport activities when produced in yeast cells and are differentially expressed in Arabidopsis organs. Herein, we report further analysis on the nucleoside transport activities and transcriptional patterns of AtENT members. The recombinant proteins of AtENTs 3, 6, and 7, but not those of AtENTs 1, 2, 4, and 8, were found to transport thymidine with high affinity. Contrary to previous suggestion that AtENT1 may not transport uridine, this work showed that recombinant AtENT1 was a pH-dependent and high-affinity transporter of uridine. When grown on MS plates, the AtENT3 knockout plants were more tolerant to the cytotoxic uridine analog 5-fluorouridine than wild-type plants and the knockout plants of AtENT1 or AtENT6. Consistent with this observation, the AtENT3 knockout line exhibited a significantly decreased ability to take up [(3)H]uridine via the roots when compared with wild-type plants and the plants with mutated AtENT1 or AtENT6. This indicates that AtENT3, but not AtENTs 1 and 6, is the main transporter for uridine uptake in Arabidopsis roots. The transcription of AtENTs 1, 3, 4, 6, 7, and 8 was regulated in a complex manner during leaf development and senescence. In contrast, the six AtENT members were coordinately induced during seed germination. This work provides new information on the transport properties of recombinant AtENT proteins and new clues for future studies of the in vivo transport activities and physiological functions of the different ENT proteins in Arabidopsis plants.

Uneven distribution of nucleoside transporters and intracellular enzymatic degradation prevent transport of intact [14C] adenosine across the sheep choroid plexus epithelium as a monolayer in primary culture

Background

Efflux transport of adenosine across the choroid plexus (CP) epithelium might contribute to the homeostasis of this neuromodulator in the extracellular fluids of the brain. The aim of this study was to explore adenosine transport across sheep CP epithelial cell monolayers in primary culture.

Methods

To explore transport of adenosine across the CP epithelium, we have developed a method for primary culture of the sheep choroid plexus epithelial cells (CPEC) on plastic permeable supports and analysed [¹⁴C] adenosine transport across this cellular layer, [¹⁴C] adenosine metabolism inside the cells, and cellular uptake of [¹⁴C] adenosine from either of the chambers. The primary cell culture consisted of an enriched epithelial cell fraction from the sheep fourth ventricle CP and was grown on laminin-precoated filter inserts.

Results and conclusion

CPEC grew as monolayers forming typical polygonal islands, reaching optical confluence on the third day after the seeding. Transepithelial electrical resistance increased over the time after seeding up to 85 ± 9 Ω cm² at day 8, while permeability towards [¹⁴C] sucrose, a marker of paracellular diffusion, simultaneously decreased. These cells expressed some features typical of the CPEC in situ, including three nucleoside transporters at the transcript level that normally mediate adenosine transport across cellular membranes. The estimated permeability of these monolayers towards [¹⁴C] adenosine was low and the same order of magnitude as for the markers of paracellular diffusion.

However, inhibition of the intracellular enzymes, adenosine kinase and adenosine deaminase, led to a significant increase in transcellular permeability, indicating that intracellular phosphorylation into nucleotides might be a reason for the low transcellular permeability. HPLC analysis with simultaneous detection of radioactivity revealed that [¹⁴C] radioactivity which appeared in the acceptor chamber after the incubation of CPEC monolayers with [¹⁴C] adenosine in the donor chamber was mostly present as [¹⁴C] hypoxanthine, a product of adenosine metabolic degradation. Therefore, it appears that CPEC in primary cultures act as an enzymatic barrier towards adenosine. Cellular uptake studies revealed that concentrative uptake of [¹⁴C] adenosine was confined only to the side of these cells facing the upper or apical chamber, indicating uneven distribution of nucleoside transporters.

Pyrimidine and purine analogues, effects on cell cycle regulation and the role of cell cycle inhibitors to enhance their cytotoxicity

In anti-cancer treatment, deoxynucleoside analogues are widely used in combination chemotherapy. Improvement can be achieved by rational design of novel combinations with cell cycle inhibitors. These compounds inhibit protein kinases, preventing the cell cycle from continuing when affected by deoxynucleoside analogs. The efficacy is dependent on the site of cell cycle inhibition, whether multiple cyclin-dependent kinases are inhibited and whether the inhibitors should be given before or after the deoxynucleoside analogs. The action of cell cycle inhibition in vivo may be limited by unfavorable pharmacokinetics. Preclinical and clinical studies will be discussed, aiming to design improved future strategies.

HIF-1-dependent repression of equilibrative nucleoside transporter (ENT) in hypoxia

Extracellular adenosine (Ado) has been implicated as central signaling molecule during conditions of limited oxygen availability (hypoxia), regulating physiologic outcomes as diverse as vascular leak, leukocyte activation, and accumulation. Presently, the molecular mechanisms that elevate extracellular Ado during hypoxia are unclear. In the present study, we pursued the hypothesis that diminished uptake of Ado effectively enhances extracellular Ado signaling. Initial studies indicated that the half-life of Ado was increased by as much as fivefold after exposure of endothelia to hypoxia. Examination of expressional levels of the equilibrative nucleoside transporter (ENT)1 and ENT2 revealed a transcriptionally dependent decrease in mRNA, protein, and function in endothelia and epithelia. Examination of the ENT1 promoter identified a hypoxia inducible factor 1 (HIF-1)–dependent repression of ENT1 during hypoxia. Using in vitro and in vivo models of Ado signaling, we revealed that decreased Ado uptake promotes vascular barrier and dampens neutrophil tissue accumulation during hypoxia. Moreover, epithelial Hif1α mutant animals displayed increased epithelial ENT1 expression. Together, these results identify transcriptional repression of ENT as an innate mechanism to elevate extracellular Ado during hypoxia.

Molecular Imaging in the Development of Cancer Therapeutics

Researchers have made great progress in defining genetic and molecular alterations that contribute to cancer. New therapeutic targets have been identified and targeted therapeutic agents have been developed, but our ability to evaluate potential drugs has not kept pace. Molecular imaging technologies that monitor biological processes and/or measure levels of targeted macromolecules can contribute significantly to preclinical and clinical drug evaluation. This article describes the drug discovery process, economic problems facing drug discovery and development, and successes and failures in this realm. We briefly describe the available molecular imaging tools, with emphasis on positron emission tomography. We discuss biological processes that are altered in tumors and can be measured by molecular imaging; examples include gene expression, signal transduction, tumor cell metabolism, proliferation, apoptosis, hypoxia, and angiogenesis. We conclude with a proposal to integrate molecular imaging into the drug development process.

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.

The ribonucleotide reductase subunit M2B subcellular localization and functional importance for DNA replication in physiological growth of KB cells

Ribonucleoside diphosphate reductase (EC 1.17.4.1) (RR) is a potential target for antineoplastic agents due to its crucial role in DNA replication and repair. The expression and activity of RR subunits are highly regulated to maintain an optimal dNTP pool, which is required to maintain genetic fidelity. The human RR small subunit M2B (p53R2) is thought to contribute to DNA repair in response to DNA damage. However, it is not clear whether M2B is involved in providing dNTPs for DNA replication under physiological growth conditions. Serum starvation synchronized studies showed that a rapid increase of M2B was associated with cyclin E, which is responsible for regulation of G(1)/S-phase transition. A living cell sorting study that used KB cells in normal growth, further confirmed that M2B increased to maximum levels at the G(1)/S-phase transition, and decreased with DNA synthesis. Confocal studies revealed that M2B redistributed from the cytoplasm to the nucleus earlier than hRRM2 in response to DNA replication. Nuclear accumulation of M2B is associated with dynamic changes in dNTP at early periods of serum addition. By using M2B-shRNA expression vectors, inhibition of M2B may result in growth retardation in KB cells. We conclude that M2B may translocate from the cytoplasm into the nucleus and allow dNTPs to initiate DNA synthesis in KB cells under physiological conditions. Thus, our findings suggested that M2B might play an important role for initiating DNA replication of KB cells in normal growth.

Localization of the NBMPR-sensitive equilibrative nucleoside transporter, ENT1, in the rat dorsal root ganglion and lumbar spinal cord

ENT1 is an equilibrative nucleoside transporter that enables trans-membrane bi-directional diffusion of biologically active purines such as adenosine. In spinal cord dorsal horn and in sensory afferent neurons, adenosine acts as a neuromodulator with complex pro- and anti-nociceptive actions. Although uptake and release mechanisms for adenosine are believed to exist in both the dorsal horn and sensory afferent neurons, the expression profile of specific nucleoside transporter subtypes such as ENT1 is not established. In this study, immunoblot analysis with specific ENT1 antibodies (anti-rENT1(227-290) or anti-hENT1(227-290)) was used to reveal the expression of ENT1 protein in tissue homogenates of either adult rat dorsal horn or dorsal root ganglia (DRG). Immunoperoxidase labeling with ENT1 antibodies produced specific staining in dorsal horn which was concentrated over superficial laminae, especially the substantia gelatinosa (lamina II). Immunofluorescence double-labeling revealed a punctate pattern for ENT1 closely associated, in some instances, with cell bodies of either neurons (confirmed with NeuN) or glia (confirmed with CNPase). Electron microscopy analysis of ENT1 expression in lamina II indicated its presence within pre- and post-synaptic elements, although a number of other structures, including myelinated and unmyelinated, axons were also labeled. In sensory ganglia, ENT1 was localized to a high proportion of cell bodies of all sizes that co-expressed substance P, IB4 or NF, although ENT1 was most highly expressed in the peptidergic population. These data provide the first detailed account of the expression and cellular distribution of ENT1 in rat dorsal horn and sensory ganglia. The functional significance of ENT1 expression with regard to the homeostatic regulation of adenosine at synapses remains to be established.

Eimeria tenella: identification of secretory and surface proteins from expressed sequence tags

To identify new vaccine candidates, Eimeria tenella expressed sequence tags (ESTs) from public databases were analysed for secretory molecules with an especially developed automated in silico strategy termed DNAsignalP. A total of 12,187 ESTs were clustered into 2881 contigs followed by a blastx search, which resulted in a significant number of E. tenella contigs with homologies to entries in public databases. Amino acid sequences of appropriate homologous proteins were analysed for the occurrence of an N-terminal signal sequence using the algorithm signalP. The resulting list of 84 entries comprised 51 contigs whose deduced proteins showed homologies to proteins of apicomplexan parasites. Based on function or localisation, we selected candidate proteins classified as (i) secreted proteins of Apicomplexa parasites, (ii) secreted enzymes, and (iii) transport and signalling proteins. To verify our strategy experimentally, we used a functional complementation system in yeast. For five selected candidate proteins we found that these were indeed secreted. Our approach thus represents an efficient method to identify secretory and surface proteins out of EST databases.

Gene expression and thioguanine nucleotide disposition in acute lymphoblastic leukemia after in vivo mercaptopurine treatment

To elucidate interpatient variability in thioguanine nucleotide (TGN) concentrations in acute lymphoblastic leukemia (ALL) cells, we determined the TGN concentrations in leukemic blasts from 82 children with newly diagnosed ALL after intravenous administration of mercaptopurine (MP). Patients treated with MP alone achieved higher TGN concentrations than those treated with the combination of methotrexate plus mercaptopurine (MTX + MP). Analysis of the expression of approximately 9600 genes in ALL cells obtained at diagnosis identified 60 gene probes significantly associated with TGN accumulation in patients treated with MP alone and 75 gene probes in patients treated with MTX + MP, with no overlap between the 2 sets of genes. Genes significantly associated with intracellular TGN accumulation after MP alone included those encoding MP metabolic enzymes and transporters (eg, SLC29A1). Inhibition of SLC29A1 by nitrobenzylmercaptopurine ribonucleoside (NBMPR) caused a 33% to 45% reduction of TGN in ALL cells in vitro (P < .006), consistent with the gene expression findings. Genes associated with TGN concentration after combination therapy included those involved in protein and adenosine triphosphate (ATP)-biosynthesis. Together, these in vivo and in vitro data provide new insight into the genomic basis of interpatient differences in intracellular TGN accumulation and reveal significant differences between treatment with MP alone and treatment with MP and MTX.

Analysis of human equilibrative nucleoside transporter 1 (hENT1) protein in non-Hodgkin's lymphoma by immunohistochemistry

The human equilibrative nucleoside transporter 1 (hENT1) is a member of the equilibrative nucleoside transporter family that mediates cellular entry of gemcitabine, cytarabine, and fludarabine. Deficiency in hENT1 confers resistance to toxicity of these drugs in a variety of model systems. Since some nucleoside analogs have a role in treating patients with non-Hodgkin's lymphoma (NHL), this study was undertaken to assess hENT1 abundance in NHL. A total of 115 cases of NHL of various subtypes and 15 reactive lymph nodes were evaluated for the presence of hENT1 protein using immunohistochemistry applied to frozen tissues. Samples were considered positive when >or=50% of neoplastic cells showed immunostaining. In reactive lymph nodes, hENT1 was confined to the germinal centers, whereas mantle zone B-cells and interfollicular T-cells were negative. In NHL, a relatively high frequency of hENT1 positivity was found in Burkitt lymphoma/leukemia (63%), diffuse large B-cell lymphoma (DLCL; 45%), and follicular lymphoma (40%). In DLCL, 26% of cases were positive for CD10, and CD10-positive DLCL cases were more likely to be hENT1 positive than CD10-negative cases (P=0.025). A lower frequency of hENT1 positivity was found in mantle cell lymphoma (13%) and peripheral T-cell lymphomas (37%). All marginal zone lymphomas (n=5), chronic lymphocytic leukemia small lymphocytic lymphomas (n=10), plasmacytoma (n=3), acute lymphoblastic lymphoma/leukemia, and anaplastic large-cell lymphomas (n=5) were negative. In conclusion, hENT1 was most frequently found in benign and malignant follicular center cells. Prospective studies to assess the value of hENT1 immunostaining in predicting resistance to nucleoside chemotherapy for NHL are warranted.

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.

Identification and Mutational Analysis of Amino Acid Residues Involved in Dipyridamole Interactions with Human and Caenorhabditis elegans Equilibrative Nucleoside Transporters

The equilibrative nucleoside transporters, hENT1 and CeENT1 from humans and Caenorhabditis elegans, respectively, are inhibited by nanomolar concentrations of dipyridamole and share a common 11-transmembrane helix (TM) topology. Random mutagenesis and screening by functional complementation in yeast for clones with reduced sensitivities to dipyridamole yielded mutations at Ile429 in TM 11 of CeENT1 and Met33 in TM 1 of hENT1. Mutational analysis of the corresponding residues of both proteins suggested important roles for these residues in competitive inhibition of hENT1 and CeENT1 by dipyridamole. To verify the roles of these residues in dipyridamole interactions, hENT2, which naturally exhibits low dipyridamole sensitivity, was mutated to contain side chains favorable for high affinity dipyridamole binding (i.e. a Met at the TM 1 and/or an Ile at the TM 11 positions). The single mutants exhibited increased hENT2 sensitivity to inhibition by dipyridamole, and the double mutant was the most sensitive, with an IC50 value that was only 2% of that of wild type. Functional analysis of the TM 1 and 11 mutants of hENT1 and CeENT1 revealed that Ala and Thr in the TM 1 and 11 positions, respectively, impaired uridine and adenosine transport and that Leu442 of hENT1 was involved in permeant selectivity. Mechanistic and structural models of dipyridamole interactions with the TM 1 and 11 residues are proposed. This study demonstrated that the corresponding residues in TMs 1 and 11 of hENT1, hENT2, and CeENT1 are important for dipyridamole interactions and nucleoside transport.

Subtype-specific regulation of equilibrative nucleoside transporters by protein kinase CK2

Two subtypes of equilibrative transporters, es (equilibrative inhibitor-sensitive) and ei (equilibrative inhibitor-insensitive), are responsible for the majority of nucleoside flux across mammalian cell membranes. Sequence analyses of the representative genes, ENT1 {equilibrative nucleoside transporter 1; also known as SLC29A1 [solute carrier family 29 (nucleoside transporters), member 1]} and ENT2 (SLC29A2), suggest that protein kinase CK2-mediated phosphorylation may be involved in the regulation of es- and ei-mediated nucleoside transport. We used human osteosarcoma cells transfected with catalytically active or inactive alpha' and alpha subunits of CK2 to assess the effects of CK2 manipulation on nucleoside transport activity. Expression of inactive CK2alpha' (decreased CK2alpha' activity) increased the number of binding sites (approximately 1.5-fold) for the es-specific probe [3H]NBMPR ([3H]nitrobenzylthioinosine), and increased (approximately 1.8-fold) the V(max) for 2-chloro[3H]adenosine of the NBMPR-sensitive (es) nucleoside transporter. There was a concomitant decrease in the V(max) of the NBMPR-resistant (ei-mediated) uptake of 2-chloro[3H]adenosine. This inhibition of CK2alpha' activity had no effect, however, on either the K(D) of [3H]NBMPR binding or the K(m) of 2-chloro[3H]adenosine uptake. Quantitative PCR showed a transient decrease in the expression of both hENT1 (human ENT1) and hENT2 mRNAs within 4-12 h of induction of the inactive CK2alpha' subunit, but both transcripts had returned to control levels by 24 h. These data suggest that inhibition of CK2alpha' reduced ei activity by attenuation of hENT2 transcription, while the increase in es/hENT1 activity was mediated by post-translational action of CK2. The observed modification in es activity was probably due to a CK2alpha'-mediated change in the phosphorylation state of the ENT1 protein, or an interacting protein, effecting an increase in the plasma membrane lifetime of the transport proteins.

Identification and characterization of purine nucleoside transporters from Crithidia fasciculata

To initiate a molecular dissection into the mechanism by which purine transport is up-regulated in Crithidia, genes encoding nucleoside transporters from Crithidia fasciculata were cloned and functionally characterized. Sequence analysis revealed CfNT1 and CfNT2 to be members of the equilibrative nucleoside transporter family, and the genes isolated encompassed polypeptides of 497 and 502 amino acids, respectively, each with 11 predicted membrane-spanning domains. Heterologous expression of CfNT1 cRNA in Xenopus laevis oocytes or CfNT2 in nucleoside transport-deficient Leishmania donovani demonstrated that CfNT1 is a novel high affinity adenosine transporter that also recognizes inosine, hypoxanthine, and pyrimidine nucleosides, while CfNT2 is a high affinity permease specific for inosine and guanosine. Southern blot analysis revealed that CfNT2 is present as a single copy within the C. fasciculata genome. Starvation of parasites for purines increased CfNT2 transport activity by an order of magnitude, although Northern blot analysis indicated CfNT2 transcript levels increased by <2-fold. These data imply that this metabolic adaptation can mainly be ascribed to post-transcriptional events. Conversely, Southern analysis of CfNT1 suggests that it is a member of a highly homologous multi-copy gene family, indicating that adenosine transport by C. fasciculata is more complex than previously thought.

Functional characterization of novel human and mouse equilibrative nucleoside transporters (hENT3 and mENT3) located in intracellular membranes

The first mammalian examples of the equilibrative nucleoside transporter family to be characterized, hENT1 and hENT2, were passive transporters located predominantly in the plasma membranes of human cells. We now report the functional characterization of members of a third subgroup of the family, from human and mouse, which differ profoundly in their properties from previously characterized mammalian nucleoside transporters. The 475-residue human and mouse proteins, designated hENT3 and mENT3, respectively, are 73% identical in amino acid sequence and possess long N-terminal hydrophilic domains that bear typical (DE)XXXL(LI) endosomal/lysosomal targeting motifs. ENT3 transcripts and proteins are widely distributed in human and rodent tissues, with a particular abundance in placenta. However, in contrast to ENT1 and ENT2, the endogenous and green fluorescent protein-tagged forms of the full-length hENT3 protein were found to be predominantly intracellular proteins that co-localized, in part, with lysosomal markers in cultured human cells. Truncation of the hydrophilic N-terminal region or mutation of its dileucine motif to alanine caused the protein to be relocated to the cell surface both in human cells and in Xenopus oocytes, allowing characterization of its transport activity in the latter. The protein proved to be a broad selectivity, low affinity nucleoside transporter that could also transport adenine. Transport activity was relatively insensitive to the classical nucleoside transport inhibitors nitrobenzylthioinosine, dipyridamole, and dilazep and was sodium ion-independent. However, it was strongly dependent upon pH, and the optimum pH value of 5.5 probably reflected the location of the transporter in acidic, intracellular compartments.

Nucleoside transport at the blood-testis barrier studied with primary-cultured sertoli cells

Nucleosides are essential for nucleotide synthesis in testicular spermatogenesis. In the present study, the mechanism of the supply of nucleosides to the testicular system across the blood-testis barrier was studied using primary-cultured Sertoli cells from rats and TM4 cells from mice. Uptake of uridine by these cells was time- and concentration-dependent. Uridine uptake was decreased under Na(+)-free conditions, and the system was presumed to be high affinity, indicating an Na(+)-dependent concentrative nucleoside transporter (CNT) is involved. On the other hand, nitrobenzylthioinosine, a potent inhibitor of Na(+)-independent equilibrative nucleoside transporters (ENTs), inhibited uridine uptake by the Sertoli cells in a concentration-dependent manner. Expression of nucleoside transporters ENT1, ENT2, ENT3, CNT1, CNT2, and CNT3 was detected in Sertoli cells by reverse transcriptase-polymerase chain reaction analysis. Inhibition studies of the uptake of uridine by various nucleosides both in the presence and absence of Na(+) indicated that the most of those expressed nucleoside transporters, ENTs and CNTs, are involved functionally. These results demonstrated that Sertoli cells are equipped with multiple nucleoside transport systems, including ENT1, ENT2, and CNTs, to provide nucleosides for spermatogenesis.

Functional characterization of adenosine transport across the BBB in mice

We investigated transport characteristics of adenosine across the blood-brain barrier (BBB) in mice. Uptake clearance across the BBB was measured by using an in situ mouse brain perfusion technique and cultured mouse brain capillary endothelial cell line (MBEC4 cells). Nucleoside transporter was cloned by RT-PCR and expressed on Xenopus laevis oocyte. Both in situ and in vitro studies revealed that the adenosine uptake is concentration-dependent, Na(+)-independent and S-(p-nitrobenzyl)-6-thioinosine (NBMPR)-sensitive. The K(t) values of in situ and in vitro studies were 31.7 +/- 13.8 microM and 11.9 +/- 2.84 microM, respectively. A good correlation was found for the inhibitory effects of nucleoside analogs to adenosine uptake between in situ and in vitro studies. RT-PCR revealed the expression of RNA of mouse equilibrative nucleoside transporter (mENT1) in mouse brain capillary and MBEC4 cells. In mENT1 expressed on X. laevis oocyte, K(t) value of adenosine transport was 6.9 +/- 2.7 microM (and comparable to those in situ and in vitro studies). In conclusion, we characterized the adenosine transport across the BBB in mice by using in situ brain perfusion technique and MBEC4 cells and found that these transports share common characteristics with mENT1-mediated transport. Transport of adenosine across the BBB in mice may be attributable to mENT1.

In vivo labeling of fission yeast DNA with thymidine and thymidine analogs

In vivo labeling of DNA with thymidine and thymidine analogs has long been a cornerstone of replication studies. Unfortunately, yeast lack a thymidine salvage pathway and thus do not incorporate exogenous thymidine. Specifically, yeast neither efficiently take up exogenous thymidine from their growth media nor phosphorylate it to thymidylate, the precursor of dTTP. We have overcome these problems in fission yeast by expressing the human equilibrative nucleoside transporter 1 (hENT1) along with herpes simplex virus thymidine kinase (tk). hENT1 tk cells are healthy and efficiently incorporate exogenous thymidine and thymidine analogs. We present protocols for labeling DNA with tritiated thymidine, for in situ detection of incorporated BrdU by immunofluorescence, for double labeling with CldU and IdU, for CsCl gradient separation of IdU-labeled DNA, and for using hENT1 and tk as both positive and negative selection markers.

Role of human nucleoside transporters in the cellular uptake of two inhibitors of IMP dehydrogenase, tiazofurin and benzamide riboside

Benzamide riboside (BR) and tiazofurin (TR) are converted to analogs of NAD that inhibit IMP dehydrogenase (IMPDH), resulting in cellular depletion of GTP and dGTP and inhibition of proliferation. The current work was undertaken to identify the human nucleoside transporters involved in cellular uptake of BR and TR and to evaluate their role in cytotoxicity. Transportability was examined in Xenopus laevis oocytes and Saccharomyces cerevisiae that produced individual recombinant human concentrative nucleoside transporter (CNT) and equilibrative nucleoside transporter (ENT) types (hENT1, hENT2, hCNT1, hCNT2, or hCNT3). TR was a better permeant than BR with a rank order of transportability in oocytes of hCNT3 >> hENT1 > hENT2 > hCNT2 >> hCNT1. The concentration dependence of inhibition of [(3)H]uridine transport in S. cerevisiae by TR exhibited lower K(i) values than BR: hCNT3 (5.4 versus 226 microM), hENT2 (16 versus 271 microM), hENT1 (57 versus 168 microM), and hCNT1 (221 versus 220 microM). In cytotoxicity experiments, BR was more cytotoxic than TR to cells that were either nucleoside transport-defective or -competent, and transport-competent cells were more sensitive to both drugs. Exposure to nitrobenzylmercaptopurine ribonucleoside conferred resistance to BR and TR cytotoxicity to hENT1-containing CEM cells, thereby demonstrating the importance of transport capacity for manifestation of cytoxicity. A breast cancer cell line with mutant p53 exhibited 9-fold higher sensitivity to BR than the otherwise similar cell line with wild-type p53, suggesting that cells with mutant p53 may be potential targets for IMPDH inhibitors. Further studies are warranted to determine whether this finding can be generalized to other cell types.

Equilibrative nucleoside transporter 2 is expressed in human umbilical vein endothelium, but is not involved in the inhibition of adenosine transport induced by hyperglycaemia

Human equilibrative, Na(+)-independent nucleoside transport is mediated by membrane proteins sensitive (system es, hENT1) or insensitive (system ei, hENT2) to nitrobenzylthioinosine (NBMPR). Gestational diabetes and elevated extracellular concentrations of D-glucose reduce adenosine transport in human umbilical vein endothelium (HUVEC). We studied hENT2 and hENT1 expression in HUVEC, and the effect of D-glucose on their activity and expression in HUVEC preincubated with 25 mM D-glucose (24 h). hENT2 and hENT1 mRNA were quantified by real-time reverse transcription polymerase chain reaction, and their proteins were detected by Western blotting. hENT2 and hENT1 proteins are co-expressed in HUVEC and are located at the plasma membrane, however, hENT2 was mainly cytoplasmatic and perinuclear in location. D-Glucose reduced hENT1 and hENT2 mRNA expression, but only hENT1 protein abundance at the plasma membrane. Adenosine transport was inhibited by D-glucose and NMBPR (1 microM) in intact cells and membrane vesicles. Hypoxanthine inhibited adenosine transport in the absence or in the presence of 1 microM NBMPR. D-Glucose reduced NBMPR maximal binding in intact cells, membrane vesicles, and plasma membrane fractions. In conclusion, the present study demonstrates that hENT2 and hENT1 are co-expressed in HUVEC, and even when adenosine transport is also mediated by hENT2, the hENT2-mediated transport activity is not involved in the d-glucose-induced down-regulation of total adenosine transport.

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.

Uridine recognition motifs of human equilibrative nucleoside transporters 1 and 2 produced in Saccharomyces cerevisiae

The sugar moiety of nucleosides has been shown to play a major role in permeant-transporter interaction with human equilibrative nucleoside transporters 1 and 2 (hENT1 and hENT2). To better understand the structural requirements for interactions with hENT1 and hENT2, a series of uridine analogs with sugar modifications were subjected to an assay that tested their abilities to inhibit [3H]uridine transport mediated by recombinant hENT1 and hENT2 produced in Saccharomyces cerevisiae. hENT1 displayed higher affinity for uridine than hENT2. Both transporters barely tolerated modifications or inversion of configuration at C(3'). The C(2')-OH at uridine was a structural determinant for uridine-hENT1, but not for uridine-hENT2, interactions. Both transporters were sensitive to modifications at C(5') and hENT2 displayed more tolerance to removal of C(5')-OH than hENT1; addition of an O-methyl group at C(5') greatly reduced interaction with either hENT1 or hENT2. The changes in binding energies between transporter proteins and the different uridine analogs suggested that hENT1 formed strong interactions with C(3')-OH and moderate interactions with C(2')-OH and C(5')-OH of uridine, whereas hENT2 formed strong interactions with C(3')-OH, weak interactions with C(5')-OH, and no interaction with C(2')-OH.

Identification and characterization of a novel monoamine transporter in the human brain

Precise control of monoamine neurotransmitter levels in the extracellular fluids of the brain is critical in maintaining efficient and robust neurotransmission. High affinity transporters in the solute carrier SLC6A family function in removing monoamines from the neurosynaptic cleft. Emerging evidence suggests that these transporters are only one part of a system of transporters that work in concert to maintain brain homeostasis of monoamines. Here we report the cloning and characterization of a new human plasma membrane monoamine transporter, PMAT. The PMAT cDNA encodes a protein of 530 amino acid residues with 10-12 transmembrane segments. PMAT is not homologous to known neurotransmitter transporters but exhibits low homology to members of the equilibrative nucleoside transporter family. When expressed in Madin-Darby canine kidney cells and Xenopus laevis oocytes, PMAT efficiently transports serotonin (K(m) = 114 mum), dopamine (K(m) = 329 mum), and the neurotoxin 1-methyl-4-phenylpyridinium (K(m) = 33 mum). In contrast, there is no significant interaction of PMAT with nucleosides or nucleobases. PMAT-mediated monoamine transport does not require Na(+) or Cl(-) but appears to be sensitive to changes in membrane potential. Northern blot analysis showed that PMAT is predominantly expressed in the human brain and widely distributed in the central nervous system. These studies demonstrate that PMAT may be a novel low affinity transporter for biogenic amines, which, under certain conditions, might supplement the role of the high affinity transporters in the brain.

Intestinal solute carriers: an overview of trends and strategies for improving oral drug absorption

A large amount of absorptive intestinal membrane transporters play an important part in absorption and distribution of several nutrients, drugs and prodrugs. The present paper gives a general overview on intestinal solute carriers as well as on trends and strategies for targeting drugs and/or prodrugs to these carriers in order to increasing oral bioavailability and distribution. A number of absorptive intestinal transporters are described in terms of gene and protein classification, driving forces, substrate specificities and cellular localization. When targeting absorptive large capacity membrane transporters in the small intestine in order to increase oral bioavailabilities of drug or prodrug, the major influence on in vivo pharmacokinetics is suggested to be dose-dependent increase in bioavailability as well as prolonged blood circulation due to large capacity facilitated absorption, and renal re-absorption, respectively. In contrast, when targeting low-capacity transporters such as vitamin transporters, dose independent saturable absorption kinetics are suggested. We thus believe that targeting drug substrates for absorptive intestinal membrane transporters could be a feasible strategy for optimizing drug bioavailability and distribution.

Functional expression and characterization of a purine nucleobase transporter gene from Leishmania major

Leishmania major, like all the other kinetoplastid protozoa, are unable to synthesize purines and rely on purine nucleobase and nucleoside acquisition across the parasite plasma membrane by specific permeases. Although, several genes have been cloned that encode nucleoside transporters in Leishmania and Trypanosoma brucei, much less progress has been made on nucleobase transporters, especially at the molecular level. The studies reported here have cloned and expressed the first gene for a L. major nucleobase transporter, designated LmaNT3. The LmaNT3 permease shows 33% identity to L. donovani nucleoside transporter 1.1 (LdNT1.1) and is, thus, a member of the equilibrative nucleoside transporter (ENT) family. ENT family members identified to date are nucleoside transporters, some of which also transport one or several nucleobases. Functional expression studies in Xenopus laevis oocytes revealed that LmaNT3 mediates high levels of uptake of hypoxanthine, xanthine, adenine and guanine. Moreover, LmaNT3 is an high affinity transporter with K(m) values for hypoxanthine, xanthine, adenine and guanine of 16.5 +/- 1.5, 8.5 +/- 0.6, 8.5 +/- 1.1, and 8.8 +/- 4.0 microM, respectively. LmaNT3 is, thus, the first member of the ENT family identified in any organism that functions as a nucleobase rather than nucleoside or nucleoside/nucleobase transporter.

Membrane transporters and channels: role of the transportome in cancer chemosensitivity and chemoresistance

Membrane transporters and channels (collectively the transportome) govern cellular influx and efflux of ions, nutrients, and drugs. We used oligonucleotide arrays to analyze gene expression of the transportome in 60 human cancer cell lines used by the National Cancer Institute for drug screening. Correlating gene expression with the potencies of 119 standard anticancer drugs identified known drug-transporter interactions and suggested novel ones. Folate, nucleoside, and amino acid transporters positively correlated with chemosensitivity to their respective drug substrates. We validated the positive correlation between SLC29A1 (nucleoside transporter ENT1) expression and potency of nucleoside analogues, azacytidine and inosine-glycodialdehyde. Application of an inhibitor of SLC29A1, nitrobenzylmercaptopurine ribonucleoside, significantly reduced the potency of these two drugs, indicating that SLC29A1 plays a role in cellular uptake. Three ABC efflux transporters (ABCB1, ABCC3, and ABCB5) showed significant negative correlations with multiple drugs, suggesting a mechanism of drug resistance. ABCB1 expression correlated negatively with potencies of 19 known ABCB1 substrates and with Baker's antifol and geldanamycin. Use of RNA interference reduced ABCB1 mRNA levels and concomitantly increased sensitivity to these two drugs, as expected for ABCB1 substrates. Similarly, specific silencing of ABCB5 by small interfering RNA increased sensitivity to several drugs in melanoma cells, implicating ABCB5 as a novel chemoresistance factor. Ion exchangers, ion channels, and subunits of proton and sodium pumps variably correlated with drug potency. This study identifies numerous potential drug-transporter relationships and supports a prominent role for membrane transport in determining chemosensitivity. Measurement of transporter gene expression may prove useful in predicting anticancer drug response.

Nucleoside transporter subtype expression and function in rat skeletal muscle microvascular endothelial cells

1. Microvascular endothelial cells (MVECs) form a barrier between circulating metabolites, such as adenosine, and the surrounding tissue. We hypothesize that MVECs have a high capacity for the accumulation of nucleosides, such that inhibition of the endothelial nucleoside transporters (NT) would profoundly affect the actions of adenosine in the microvasculature. 2. We assessed the binding of [(3)H]nitrobenzylmercaptopurine riboside (NBMPR), a specific probe for the inhibitor-sensitive subtype of equilibrative NT (es), and the uptake of [(3)H]formycin B (FB), by MVECs isolated from rat skeletal muscle. The cellular expression of equilibrative (ENT1, ENT2, ENT3) and concentrative (CNT1, CNT2, CNT3) NT subtypes was also determined using both qualitative and quantitative polymerase chain reaction techniques. 3. In the absence of Na(+), MVECs accumulated [(3)H]FB with a V(max) of 21+/-1 pmol microl(-1) s(-1). This uptake was mediated equally by es (K(m) 260+/-70 microm) and ei (equilibrative inhibitor-insensitive; K(m) 130+/-20 microm) NTs. 4. A minor component of Na(+)-dependent cif (concentrative inhibitor-insensitive FB transporter)/CNT2-mediated [(3)H]FB uptake (V(i) 0.008+/-0.005 pmol microl(-1) s(-1) at 10 microm) was also observed at room temperature upon inhibition of ENTs with dipyridamole (2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido-[5,4-d]pyrimidine)/NBMPR. 5. MVECs had 122,000 high-affinity (K(d) 0.10 nm) [(3)H]NBMPR binding sites (representing es transporters) per cell. A lower-affinity [(3)H]NBMPR binding component (K(d) 4.8 nm) was also observed that may be related to intracellular es-like proteins. 6. Rat skeletal muscle MVECs express es/ENT1, ei/ENT2, and cif/CNT2 transporters with characteristics typical of rat tissues. This primary cell culture model will enable future studies on factors influencing NT subtype expression, and the consequent effect on adenosine bioactivity, in the microvasculature.

Adenosine down-regulates the surface expression of dipeptidyl peptidase IV on HT-29 human colorectal carcinoma cells: implications for cancer cell behavior

Dipeptidyl peptidase IV (DPPIV) is a multifunctional cell-surface protein that, as well as having dipeptidase activity, is the major binding protein for adenosine deaminase (ADA) and also binds extracellular matrix proteins such as fibronectin and collagen. It typically reduces the activity of chemokines and other peptide mediators as a result of its enzymatic activity. DPPIV is aberrantly expressed in many cancers, and decreased expression has been linked to increases in invasion and metastasis. We asked whether adenosine, a purine nucleoside that is present at increased levels in the hypoxic tumor microenvironment, might affect the expression of DPPIV at the cell surface. Treatment with a single dose of adenosine produced an initial transient (1 to 4 hours) modest (approximately 10%) increase in DPPIV, followed by a more profound (approximately 40%) depression of DPPIV protein expression at the surface of HT-29 human colon carcinoma cells, with a maximal decline being reached after 48 hours, and persisting for at least a week with daily exposure to adenosine. This down-regulation ofDPPIV occurred at adenosine concentrations comparable to those present within the extracellular fluid of colorectal tumors growing in vivo, and was not elicited by inosine or guanosine. Neither cellular uptake of adenosine nor its phosphorylation was necessary for the down-regulation of DPPIV. The decrease in DPPIV protein at the cell surface was paralleled by decreases in DPPIV enzyme activity, binding of ADA, and the ability of the cells to bind to and migrate on cellular fibronectin. Adenosine, at concentrations that exist within solid tumors, therefore acts at the surface of colorectal carcinoma cells to decrease levels and activities of DPPIV. This down-regulation of DPPIV may increase the sensitivity of cancer cells to the tumor-promoting effects of adenosine and their response to chemokines and the extracellular matrix, facilitating their expansion and metastasis.

Adenosine uptake inhibitors

Adenosine is a purine nucleoside and modulates a variety of physiological functions by interacting with cell-surface adenosine receptors. Under several adverse conditions, including ischemia, trauma, stress, seizures and inflammation, extracellular levels of adenosine are increased due to increased energy demands and ATP metabolism. Increased adenosine could protect against excessive cellular damage and organ dysfunction. Indeed, several protective effects of adenosine have been widely reported (e.g., amelioration of ischemic heart and brain injury, seizures and inflammation). However, the effects of adenosine itself are insufficient because extracellular adenosine is rapidly taken up into adjacent cells and subsequently metabolized. Adenosine uptake inhibitors (nucleoside transport inhibitors) could retard the disappearance of adenosine from the extracellular space by blocking adenosine uptake into cells. Therefore, it is expected that adenosine uptake inhibitors will have protective effects in various diseases, by elevating extracellular adenosine levels. Protective or ameliorating effects of adenosine uptake inhibitors in ischemic cardiac and cerebral injury, organ transplantation, seizures, thrombosis, insomnia, pain, and inflammatory diseases have been reported. Preclinical and clinical results indicate the possibility of therapeutic application of adenosine uptake inhibitors.

Interaction of the novel adenosine uptake inhibitor 3-[1-(6,7-diethoxy-2-morpholinoquinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1H,3H)-quinazolinedione hydrochloride (KF24345) with the es and ei subtypes of equilibrative nucleoside transporters

Nucleosides such as adenosine, as well as many nucleoside-based drugs, permeate cell membranes via a family of equilibrative nucleoside transporters (ENTs). We assessed the effects of (3-[1-(6,7-diethoxy-2-morpholino-quinazolin-4-yl)piperidin-4-yl]-1,6-dimethyl-2,4(1H,3H)-quinazolinedione hydrochloride (KF24345), a novel anti-inflammatory agent that potentiates the actions of adenosine, on the es (inhibitor-sensitive) and ei (inhibitor-resistant) subtypes of ENTs in human, mouse, and rat cells. KF24345 was similar to the prototypical high-affinity inhibitor nitrobenzylthioinosine (NBMPR) for blocking the human es transporter (K(I) of approximately 0.4 nM), but was 50-fold more effective than NBMPR at blocking the human ei transporter (K(I) of approximately 100 nM). KF24345 displayed significantly less species heterogeneity in its affinity for the es transporter than did dipyridamole, a widely used inhibitor of nucleoside transport; KF24345 may thus prove useful as an inhibitor for studies of nucleoside metabolism in a range of animal models. Furthermore, KF24345 seemed to act as a noncompetitive inhibitor of both [(3)H]NBMPR binding and [(3)H]nucleoside uptake by human es transporters, and these kinetics were consistent with an observed slow dissociation of KF24345 from the inhibitor binding site. KF24345 also exhibited unusual biphasic profiles for inhibition of [(3)H]NBMPR binding to membranes prepared from a recombinant human es transporter model (PK15-hENT1), suggesting the presence of multiple populations of NBMPR binding proteins in these membranes. The atypical tight binding interaction of KF24345 with the es transporter may prove useful for the molecular delineation of inhibitor binding domains and will facilitate its use as an in vivo inhibitor of nucleoside transport in studies focused on the biological effects of adenosine.

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.

Allelic isoforms of the H+/nucleoside co-transporter (CaCNT) fromCandida albicans reveal separate high- and low-affinity transport systems for nucleosides

Contigs 19-10196 and 19-20196 of the Stanford Candida albicans genome sequence databank encode two putative allelic isoforms of C. albicans CaCNT, a recently characterized 608 amino acid residue H+-coupled fungal member of the CNT family of concentrative nucleoside transport proteins. The single Ser/Gly difference between CaCNT/19-20196 and CaCNT occurs at position 328 in putative TM 7, and corresponds to a Ser/Gly substitution previously shown to contribute to the contrasting pyrimidine and purine nucleoside selectivities of human (h) and rat (r) Na+-dependent CNT1 and CNT2. CaCNT/19-10196 differs from CaCNT by four amino acids, but has Gly at position 328. These new proteins were recreated by site-directed mutagenesis of CaCNT and characterized functionally by heterologous expression in Xenopus laevis oocytes. In marked contrast to h/rCNT1/2, both CaCNT/19-10196 and CaCNT/19-20196 exhibited permeant selectivities for purine nucleosides (adenosine, guanosine and inosine) and uridine similar to that of CaCNT. However, although H+-coupled, CaCNT/19-20196 exhibited a approximately 10-fold higher apparent Km for uridine than either CaCNT or CaCNT/19-10196. CaCNT/19-20196 also exhibited a low apparent affinity for inosine. We conclude that the three proteins correspond to high-affinity (CaCNT, CaCNT/19-10196) and low-affinity (CaCNT/19-20196) allelic isoforms of the C. albicans CNT nucleoside transporter. This is the first example of a single amino acid residue substitution altering a CNT protein's overall apparent affinity for nucleosides.

Efficient labeling of fission yeast Schizosaccharomyces pombe with thymidine and BUdR

In this paper we report the construction of a Schizosaccharomyces pombe strain that facilitates analysis of replicating DNA. The strain co-expresses the Herpes simplex virus thymidine kinase gene (hsv-tk) and a human equilibrative nucleoside transporter (hENT1). The double integrant efficiently incorporates 3H-thymidine into nuclear DNA as monitored by scintillation counting. These strains also incorporate the thymidine analog Bromodeoxy uridine (BUdR) into newly replicated DNA, which can be detected by immunofluorescence and flow cytometry. This strain provides a valuable tool for direct study of DNA replication in S.pombe.

Role of red blood cells in pharmacokinetics of chemotherapeutic agents

After oral or parenteral administration of chemotherapeutic agents, these drugs are transported to the tissues by the blood in different fractions: plasma water, plasma proteins or cells. Erythrocytes may play an important role in the storage, transport and metabolism of chemotherapeutic agents. Anthracyclines, ifosfamide and its metabolites, and topoisomerase I and I/II inhibitors are incorporated in red blood cells. They may be transported to the tumour tissue and mobilised from the erythrocyte by different active or passive transport mechanisms. Erythrocytes may also be used as carriers for drugs such as asparaginase. This leads to a decreased toxicity profile. Finally, it has been shown that red blood cells are important in the transport and metabolism of mercaptopurine. The erythrocyte concentration of mercaptopurine has a prognostic value in the treatment of childhood acute lymphoblastic leukemia. In this review, the role of red blood cells for various anticancer drugs is further discussed.

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.

Interferon-gamma regulates nucleoside transport systems in macrophages through signal transduction and activator of transduction factor 1 (STAT1)-dependent and -independent signalling pathways

The expressions of CNT and ENT (concentrative and equilibrative nucleoside transporters) in macrophages are differentially regulated by IFN-gamma (interferon-gamma). This cytokine controls gene expression through STAT1-dependent and/or -independent pathways (where STAT1 stands for signal transduction and activator of transcription 1). In the present study, the role of STAT1 in the response of nucleoside transporters to IFN-gamma was studied using macrophages from STAT1 knockout mice. IFN-gamma triggered an inhibition of ENT1-related nucleoside transport activity through STAT1-dependent mechanisms. Such inhibition of macrophage growth and ENT1 activity by IFN-gamma is required for DNA synthesis. Interestingly, IFN-gamma led to an induction of the CNT1- and CNT2-related nucleoside transport activities independent of STAT1, thus ensuring the supply of extracellular nucleosides for the STAT1-independent RNA synthesis. IFN-gamma up-regulated CNT2 mRNA and CNT1 protein levels and down-regulated ENT1 mRNA in both wild-type and STAT1 knockout macrophages. This is consistent with a STAT1-independent, long-term-mediated, probably transcription-dependent, regulation of nucleoside transporter genes. Moreover, STAT1-dependent post-transcriptional mechanisms are implicated in the regulation of ENT1 activity. Although nitric oxide is involved in the regulation of ENT1 activity in B-cells at a post-transcriptional level, our results show that STAT1-dependent induction of nitric oxide by IFN-gamma is not implicated in the regulation of ENT1 activity in macrophages. Our results indicate that both STAT1-dependent and -independent pathways are involved in the regulation of nucleoside transporters by IFN-gamma in macrophages.

Reconstitution of an efficient thymidine salvage pathway in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae is unable to incorporate exogenous nucleosides into DNA. We have made a number of improvements to existing strategies to reconstitute an efficient thymidine salvage pathway in yeast. We have constructed strains that express both a nucleoside kinase as well as an equilibrative nucleoside transporter. By also deleting the gene encoding thymidylate synthase (CDC21) we have constructed strains that are entirely dependent upon exogenous thymidine for viability and that can grow with normal kinetics at low thymidine concentrations. Using this novel approach, we show that depletion of a single deoxyribonucleoside causes reversible arrest of cells in S phase with concomitant phosphorylation and activation of the S phase checkpoint kinase, Rad53. We show that this strain also efficiently incorporates the thymidine analogue, BrdU, into DNA and can be used for pulse-chase labelling.

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.

Functional analysis of an inosine-guanosine transporter from Leishmania donovani. The role of conserved residues, aspartate 389 and arginine 393

Equilibrative nucleoside transporters encompass two conserved, charged residues that occur within predicted transmembrane domain 8. To assess the role of these "signature" residues in transporter function, the Asp389 and Arg393 residues within the LdNT2 nucleoside transporter from Leishmania donovani were mutated and the resultant phenotypes evaluated after transfection into Delta ldnt2 parasites. Whereas an R393K mutant retained transporter activity similar to that of wild type LdNT2, the R393L, D389E, and D389N mutations resulted in dramatic losses of transport capability. Tagging the wild type and mutant ldnt2 proteins with green fluorescent protein demonstrated that the D389N and D389E mutants targeted properly to the parasite cell surface and flagellum, whereas the expression of R393L at the cell surface was profoundly compromised. To test whether Asp389 and Arg393 interact, a series of mutants was generated, D389R/R393R, D389D/R393D, and D389R/R393D, within the green fluorescent protein-tagged LdNT2 construct. Although all of these ldnt2 mutants were transport-deficient, D389R/R393D localized properly to the plasma membrane, while neither D389R/R393R nor D389D/R393D could be detected. Moreover, a transport-incompetent D389N/R393N double ldnt2 mutant also localized to the parasite membrane, whereas a D389L/R393L ldnt2 mutant did not, suggesting that an interaction between residues 389 and 393 may be involved in LdNT2 membrane targeting. These studies establish genetically that Asp389 is critical for optimal transporter function and that a positively charged or polar residue at Arg393 is essential for proper expression of LdNT2 at the plasma membrane.

Increased resistance to nitrogen mustards and antifolates following in vitro selection of murine fibroblasts and primary hematopoietic cells transduced with a bicistronic retroviral vector expressing the rat glutathione S-transferase A3 and a mutant dihydrofolate reductase

We have constructed a retroviral bicistronic vector, MFG/GID, that transduces the expression of both the A3 isoform of the rat glutathione S-transferase (GST A3), and the tyr-22 variant of the human dihydrofolate reductase (DHFR(L22Y)). Transduction of murine 3T3 fibroblasts with this vector increased their in vitro resistance to chlorambucil (1.8-fold) and trimetrexate (TMTX) (748-fold). TMTX selection of a mixed population of 20% GID-transduced NIH 3T3 cells and 80% control cells resulted in a marked increase in the GST peroxidase activity associated with the GST A3 isoform (17.7-fold). MFG/GID-transduced primary clonogenic murine hematopoietic progenitor cells were likewise more resistant to TMTX and chlorambucil than control beta-gal-transduced cells. Selecting GID-transduced hematopoietic cells with a combination of TMTX and a nucleoside transport inhibitor resulted in a marked increase in resistance upon re-exposure to TMTX (99% survival). Similarly, GID-transduced hematopoietic cells selected with TMTX were more resistant to chlorambucil, with 40% survival at a drug concentration that killed practically all control cells. These results suggest that antifolate-mediated selection of MFG/GID-transduced hematopoietic cells could be used as a mean to enrich the population of transduced cells prior to or following transplantation, thus potentially conferring in vivo chemoprotection to nitrogen mustards and antifolates.

Functional expression and characterization of a sodium-dependent nucleoside transporter hCNT2 cloned from human duodenum

We have cloned and functionally expressed a sodium-dependent human nucleoside transporter, hCNT2, from a CNS cancer cell line U251. Our cDNA clone of hCNT2 had the same predicted amino acid sequence as the previously cloned hCNT2 transporter. Of the several cell lines studied, the best hCNT2 transport function was obtained when transiently expressed in U251 cells. Na(+)-dependent uptake of [3H]inosine in U251 cells transiently expressing hCNT2 was 50-fold greater than that in non-transfected cells, and uptake in Na(+)-containing medium was approximately 30-fold higher than that at Na(+)-free condition. The hCNT2 displayed saturable uptake of [3H]inosine with K(m) of 12.8 microM and V(max) of 6.66 pmol/mg protein/5 min. Uptake of [3H]inosine was significantly inhibited by the purine nucleoside drugs dideoxyinosine and cladribine, but not by acyclic nucleosides including acyclovir, ganciclovir, and their prodrugs valacyclovir and valganciclovir. This indicates that the closed ribose ring is important for binding of nucleoside drugs to hCNT2. Among several pyrimidine nucleosides, hCNT2 favorably interacted with the uridine analogue floxuridine. Interestingly, we found that benzimidazole analogues, including maribavir, 5,6-dichloro-2-bromo-1-beta-D-ribofuranosylbenzimidazole (BDCRB), and 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole (DRB), were strong inhibitors of inosine transport, even though they have a significantly different heterocycle structure compared to a typical purine ring. As measured by GeneChip arrays, mRNA expression of hCNT2 in human duodenum was 15-fold greater than that of hCNT1 or hENT2. Further, the rCNT2 expression in rat duodenum was 20-fold higher than rCNT1, rENT1 or rENT2. This suggests that hCNT2 (and rCNT2) may have a significant role in uptake of nucleoside drugs from the intestine and is a potential transporter target for the development of nucleoside and nucleoside-mimetic drugs.

Nitrobenzylthioinosine (NBT), a nucleoside transport inhibitor, protects against Shiga toxin cytotoxicity in human microvascular endothelial cells

Infections with Shiga toxin (Stx)-producing Escherichia coli (STEC) cause microvascular endothelial cell damage, resulting in hemorrhagic colitis and hemolytic uremic syndrome. The prevention of endothelial cell damage is therefore a crucial step in overcoming this disorder. Here, we report that nitrobenzylthioinosine (NBT), a nucleoside transport inhibitor, has a protective effect against the cytotoxicity of Stxs in human microvascular endothelial cells (HMVECs). The relative viability of cells treated with 1.5-15 pM of Stx1 was reduced to 10-20% of that without Stx1. However, the viability of cells treated with NBT (10-100 microM) remained higher than 80%, even in the presence of Stx1. NBT also protected against Stx1 cytotoxicity in sodium butyrate-treated hypersensitive HMVECs. The protective effect of NBT against Stx cytotoxicity may be due to the depletion of ATP in the cells, thereby inhibiting the entry of Stx1.