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

Regulation of adenosine transport by D-glucose in human fetal endothelial cells: involvement of nitric oxide, protein kinase C and mitogen-activated protein kinase

The effects of elevated D-glucose on adenosine transport were investigated in human cultured umbilical vein endothelial cells isolated from normal pregnancies. Elevated D-glucose resulted in a time- (8-12 h) and concentration-dependent (half-maximal at 10+/-2 mM) inhibition of adenosine transport, which was associated with a reduction in the Vmax for nitrobenzylthioinosine (NBMPR)-sensitive (es) saturable nucleoside with no significant change in Km. d-Fructose (25 mM), 2-deoxy-D-glucose (25 mM) or D-mannitol (20 mM) had no effect on adenosine transport. Adenosine transport was inhibited following incubation of cells with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA; 100 nM, 30 min to 24 h). D-Glucose-induced inhibition of transport was abolished by calphostin C (100 nM, an inhibitor of PKC), and was not further reduced by PMA. Increased PKC activity in the membrane (particulate) fraction of endothelial cells exposed to D-glucose or PMA was blocked by calphostin C but was unaffected by NG-nitro-L-arginine methyl ester (L-NAME; 100 microM, an inhibitor of nitric oxide synthase (NOS)) or PD-98059 (10 microM, an inhibitor of mitogen-activated protein kinase kinase 1). D-Glucose and PMA increased endothelial NOS (eNOS) activity, which was prevented by calphostin C or omission of extracellular Ca2+ and unaffected by PD-98059. Adenosine transport was inhibited by S-nitroso-N-acetyl-l, d-penicillamine (SNAP; 100 microM, an NO donor) but was increased in cells incubated with L-NAME. The effect of SNAP on adenosine transport was abolished by PD-98059. Phosphorylation of mitogen-activated protein kinases p44mapk (ERK1) and p42mapk (ERK2) was increased in endothelial cells exposed to elevated D-glucose (25 mM for 30 min to 24 h) and the NO donor SNAP (100 microM, 30 min). The effect of D-glucose was blocked by PD-98059 or L-NAME, which also prevented the inhibition of adenosine transport mediated by elevated D-glucose. Our findings provide evidence that D-glucose inhibits adenosine transport in human fetal endothelial cells by a mechanism that involves activation of PKC, leading to increased NO levels and p42-p44mapk phosphorylation. Thus, the biological actions of adenosine appear to be altered under conditions of sustained hyperglycaemia.

Transporter-mediated permeation of drugs across the blood-brain barrier

Drug distribution into the brain is strictly regulated by the presence of the blood-brain barrier (BBB) that is formed by brain capillary endothelial cells. Since the endothelial cells are connected to each other by tight junctions and lack pores and/or fenestrations, compounds must cross the membranes of the cells to enter the brain from the bloodstream. Therefore, hydrophilic compounds cannot cross the barrier in the absence of specific mechanisms such as membrane transporters or endocytosis. So, for efficient supply of hydrophilic nutrients, the BBB is equipped with membrane transport systems and some of those transporter proteins have been shown to accept drug molecules and transport them into brain. In the present review, we describe mainly the transporters that are involved in drug transfer across the BBB and have been molecularly identified. The transport systems described include transporters for amino acids, monocarboxylic acids, organic cations, hexoses, nucleosides, and peptides. Most of these transporters function in the direction of influx from blood to brain; the presence of efflux transporters from brain to blood has also been demonstrated, including P-glycoprotein, MRPs, and other unknown transporters. These efflux transporters seem to be functional for detoxication and/or prevention of nonessential compounds from entering the brain. Various drugs are transported out of the brain via such efflux transporters, resulting in the decrease of CNS side effects for drugs that have pharmacological targets in peripheral tissues or in the reduction of efficacy in CNS because of the lower delivery by efflux transport. To identify the transporters functional at the BBB and to examine the possible involvement of them in drug transports by molecular and physiological approaches will provide a rational basis for controlling drug distribution to the brain.

Adenosine- and adenine-nucleotide-mediated inhibition of normal and transformed keratinocyte proliferation is dependent upon dipyridamole-sensitive adenosine transport

Extracellular adenosine and its related nucleotides have been referred to as retaliatory metabolites that can be released into the extracellular environment during inflammation, wounding, and other pathologic states. We have previously reported that these compounds reversibly inhibit the proliferation of normal keratinocyte cultures and we now demonstrate that these compounds also arrest the proliferation of transformed keratinocytes. Although our study shows that keratinocytes express mRNA corresponding to the A2B purinoreceptors and that adenosine or AMP treatment elevates intracellular cAMP in these cells, our study also demonstrates that dipyridamole-inhibitable transport of adenosine into the keratinocyte is central to the mechanism by which adenosine and adenine nucleotides arrest proliferation in these cells. In support of this mechanism, our results demonstrate that human keratinocytes express mRNA corresponding to the recently cloned dipyridamole-sensitive human equilibrative nucleoside transporter. Interestingly, coincubation with adenosine deaminase reverses the antiproliferative action of adenosine and exerts no effect on the antiproliferative activity of the adenine nucleotides, thus supporting a model in which adenine nucleotides are enzymatically converted to adenosine and transported into the keratinocyte in a tightly coupled and adenosine-deaminase-resistant manner. Analysis of adenosine- and adenosine-monophosphate-treated keratinocytes demonstrated that quiescence is induced within 12-24 h, and fluorescence-activated cell sorter analysis suggests that treatment with these compounds may result in the inhibition of keratinocyte proliferation at both G1 and S phases of the cell cycle. In addition to their documented antiproliferative action on other cell types, adenosine, adenine nucleotides, and related analogs may also represent a potential new class of pharmacologic regulators of keratinocyte proliferation in vivo.

Retroviral vectors containing a variant dihydrofolate reductase gene for drug protection and in vivo selection of hematopoietic cells

Transfer of drug resistance genes to hematopoietic cells is being studied as a means to protect against the myelosuppression associated with cancer chemotherapy and as a strategy for the in vivo selection and amplification of genetically modified cells. The goal of this study was to test if retroviral-mediated gene transfer of a dihydrofolate reductase (DHFR) variant (L22Y) could be used for in vivo selection of transduced myeloid cells and to determine what proportion of transduced cells was required for protection from myelosuppression. Based on previous work suggesting that selection with antifolates may also require inhibition of nucleoside transport mechanisms, mice transplanted with DHFR-transduced bone marrow cells were treated with trimetrexate and the nucleoside transport inhibitor prodrug nitrobenzylmercaptopurine riboside phosphate. In vivo selection of transduced myeloid progenitors was seen in the bone marrow and in circulating mature peripheral blood cells following drug treatment. These results show that the novel combination of the L22Y-DHFR cDNA, trimetrexate and nitrobenzylmercaptopurine riboside phosphate can be used to select for transduced myeloid cells, and that this approach warrants further study in large animal models. A bicistronic vector containing a human CD24 reporter gene was used to determine the number of modified cells needed for chemoprotection. Partial protection from neutropenia was seen when greater than 10% of myeloid cells expressed the vector, and high levels of protection were obtained when the proportion exceeded 30%. These results suggest that gene transfer may be useful for myeloprotection in certain pediatric cancers, but that more efficient gene transfer will be required to apply this approach to adult cancer patients.

Genomic organization and expression of the mouse equilibrative, nitrobenzylthioinosine-sensitive nucleoside transporter 1 (ENT1) gene

We have cloned and characterized the genomic structure of the mouse gene for the NBMPR-sensitive equilibrative nucleoside transporter (mENT1), which is located on chromosome 17C. About 12-kb of genomic DNA was sequenced including the promoter region, 12 exons, 11 introns, and the 3'-untranslated region. All exon-intron junction sequences conform to the GT/AG rule. Primer extension analysis demonstrated a transcription initiation site located 252 bp upstream of the translation start site. Analysis of the 2.5-kb 5'-flanking sequence shows putative binding sites for several transcription factors, including GATA-1, IRF-2, Pit-1, myogenin, CREB, Sp-1, Ap-2, MAZ, and GR. We demonstrated that mouse ENT1 mRNA was highly expressed in heart, spleen, lung, liver, and testis. Lower levels of expression were detected in brain and kidney. Functional analysis of the 5'-flanking region showed that the nucleotide sequence from -652 to -111 contains cis-regulatory elements that promote gene expression. We found two Sp-1 binding sites (-296/-303, -307/-313) and two MAZ binding sites (-353/-359, -522/-528) in this region. Luciferase assay results suggest that MAZ and Sp-1 transcription factors are important positive regulators of transcription for the ENT1 gene in NG108-15 cells.

Purine uptake and release in rat C6 glioma cells: nucleoside transport and purine metabolism under ATP-depleting conditions

Adenosine, through activation of membrane-bound receptors, has been reported to have neuroprotective properties during strokes or seizures. The role of astrocytes in regulating brain interstitial adenosine levels has not been clearly defined. We have determined the nucleoside transporters present in rat C6 glioma cells. RT-PCR analysis, (3)H-nucleoside uptake experiments, and [(3)H]nitrobenzylthioinosine ([(3)H]NBMPR) binding assays indicated that the primary functional nucleoside transporter in C6 cells was rENT2, an equilibrative nucleoside transporter (ENT) that is relatively insensitive to inhibition by NBMPR. [(3)H]Formycin B, a poorly metabolized nucleoside analogue, was used to investigate nucleoside release processes, and rENT2 transporters mediated [(3)H]formycin B release from these cells. Adenosine release was investigated by first loading cells with [(3)H]adenine to label adenine nucleotide pools. Tritium release was initiated by inhibiting glycolytic and oxidative ATP generation and thus depleting ATP levels. Our results indicate that during ATP-depleting conditions, AMP catabolism progressed via the reactions AMP --> IMP --> inosine --> hypoxanthine, which accounted for >90% of the evoked tritium release. It was surprising that adenosine was not released during ATP-depleting conditions unless AMP deaminase and adenosine deaminase were inhibited. Inosine release was enhanced by inhibition of purine nucleoside phosphorylase; ENT2 transporters mediated the release of adenosine or inosine. However, inhibition of AMP deaminase/adenosine deaminase or purine nucleoside phosphorylase during ATP depletion produced release of adenosine or inosine, respectively, via the rENT2 transporter. This indicates that C6 glioma cells possess primarily rENT2 nucleoside transporters that function in adenosine uptake but that intracellular metabolism prevents the release of adenosine from these cells even during ATP-depleting conditions.

Nucleoside transporter proteins of Saccharomyces cerevisiae. Demonstration of a transporter (FUI1) with high uridine selectivity in plasma membranes and a transporter (FUN26) with broad nucleoside selectivity in intracellular membranes

FUI1 and function unknown now 26 (FUN26) are proteins of uncertain function with sequence similarities to members of the uracil/allantoin permease and equilibrative nucleoside transporter families of transporter proteins, respectively. [(3)H]Uridine influx was eliminated by disruption of the gene encoding FUI1 (fui1) and restored by expression of FUI1 cDNA, whereas influx in transport-competent and fui1-negative yeast were unaffected, respectively, by disruption of the FUN26 gene or overexpression of FUN26 cDNA. FUI1 transported uridine with high affinity (K(m), 22 +/- 3 micrometer) and was unaffected or inhibited only partially by high concentrations (1 mm) of a variety of ribo- and deoxyribonucleosides or nucleobases. When FUN26 cDNA was expressed in oocytes of Xenopus laevis, inward fluxes of [(3)H]uridine, [(3)H]adenosine, and [(3)H]cytidine were stimulated, and uridine influx was independent of pH and not inhibited by dilazep, dipyridamole, or nitrobenzylmercaptopurine ribonucleoside. Fractionation of yeast membranes containing immunotagged recombinant FUN26 (shown to be functional in oocytes) demonstrated that the protein was primarily in intracellular membranes. These results indicated that FUI1 has high selectivity for uracil-containing ribonucleosides and imports uridine across cell-surface membranes, whereas FUN26 has broad nucleoside selectivity and most likely functions to transport nucleosides across intracellular membranes.

Cloning and in vitro expression of the cDNA encoding a putative nucleoside transporter from Arabidopsis thaliana

Nucleoside transporters are integral membrane proteins involved in the uptake or release of nucleosides. Their function constitutes an essential step of the salvage pathway of nucleotide synthesis. In order to study the function of these proteins in higher plants, we cloned the cDNA corresponding to the AtENT1 gene that encodes a putative nucleoside transporter in Arabidopsis thaliana by RT-PCR. The amino acid sequence of the AtENT1 protein deduced from the cloned cDNA shared similarity to those of eukaryotic equilibrative nucleoside transporters. Structure prediction indicated that the deduced AtENT1 protein might possess eleven putative transmembrane domains. Southern hybridization revealed that AtENT1 had one homologue in the Arabidopsis genome. Northern blot analysis showed that AtENT1 might be constitutively expressed in most Arabidopsis organs and in plants at different developmental stages. Two AtENT1 fusion genes, AtENT1-His-tag and GFP-AtENT1-His-tag, were expressed in insect cells. Confocal microscopy demonstrated that the GFP-AtENT1-His-tag fusion protein was targeted specifically to the plasma membrane of insect cells.

Genetics and biochemistry of Leishmania membrane transporters

The ability to clone and functionally express genes encoding membrane transporters in Leishmania and related parasitic protozoa has illuminated the processes whereby these parasites acquire nutrients from their hosts. It is now possible to probe the physiological functions of these permeases and investigate their role in drug delivery and resistance.

Selective loss of nucleoside carrier expression in rat hepatocarcinomas

Evidence that hepatoma cell lines show differential expression of concentrative nucleoside transporters (CNT1 and CNT2) prompted us to study the transporter proteins in 2 models of hepatocarcinogenesis, the chemically induced Solt and Farber model and the albumin-SV40 large T antigen (Alb-SV40) transgenic rat. CNT1 expression was lower in tumor biopsy specimens from Alb-SV40 rat livers than in normal tissue. Immunocytochemistry revealed that the CNT1 protein was indeed absent in the tumor lesions. CNT1 was also absent in a cell line, L25, derived from the Alb-SV40 transgenic rat liver tumors, whereas another cell line, L37, derived from the normal-appearing parenchyma, retained the expression of both carrier isoforms. The protein expression correlated with the nucleoside transport properties of these cell lines. Moreover, although CNT2 expression was highly dependent on the growth characteristics of the 2 cell lines, as was CNT1 (albeit to a lower extent) in L37 cells, it was not expressed in L25 cells at any stage of cell growth. In contrast to the transgenic model of hepatocarcinogenesis, in the chemically induced tumors the expression of CNT2 was lower, although still detectable. In summary, these data indicate that hepatocarcinogenesis leads to a selective loss or diminished expression of nucleoside carrier isoforms, a feature that may be relevant to our understanding of the molecular basis of the bioavailability of those drugs that are nucleoside derivatives and may be substrates of these carriers. The transport properties and isoform-expression profile of the L25 and L37 cell lines make them suitable hepatocyte culture models with which to study nucleoside transport processes and drug sensitivity.

Cloning of a novel inosine-guanosine transporter gene from Leishmania donovani by functional rescue of a transport-deficient mutant

Purine transport is an indispensable nutritional function for protozoan parasites, since they are incapable of purine biosynthesis and must, therefore, acquire purines from the host milieu. Exploiting a mutant cell line (FBD5) of Leishmania donovani deficient in inosine and guanosine transport activity, the gene encoding this transporter (LdNT2) has been cloned by functional rescue of the mutant phenotype. LdNT2 encodes a polypeptide of 499 amino acids that shows substantial homology to other members of the equilibrative nucleoside transporter family. Molecular analysis revealed that LdNT2 is present as a single gene copy within the leishmanial genome and encodes a single transcript of 3 kilobase pairs. Transfection of FBD5 parasites with LdNT2 re-established their ability to take up inosine and guanosine with a concurrent restoration of sensitivity to the inosine analog formycin B. Kinetic analyses reveal that LdNT2 is highly specific for inosine (K(m) = 0.3 micrometer) and guanosine (K(m) = 1.7 micrometer) and does not recognize other naturally occurring nucleosides. Expression of LdNT2 cRNA in Xenopus oocytes significantly augmented their ability to take up inosine and guanosine, establishing that LdNT2 by itself suffices to mediate nucleoside transport. These results authenticate genetically and biochemically that LdNT2 is a novel nucleoside transporter with an unusual and strict specificity for inosine and guanosine.

Isolation and functional characterization of the PfNT1 nucleoside transporter gene from Plasmodium falciparum

Plasmodium falciparum, the causative agent of the most lethal form of human malaria, is incapable of de novo purine synthesis, and thus, purine acquisition from the host is an indispensable nutritional requirement. This purine salvage process is initiated by the transport of preformed purines into the parasite. We have identified a gene encoding a nucleoside transporter from P. falciparum, PfNT1, and analyzed its function and expression during intraerythrocytic parasite development. PfNT1 predicts a polypeptide of 422 amino acids with 11 transmembrane domains that is homologous to other members of the equilibrative nucleoside transporter family. Southern analysis and BLAST searching of The Institute for Genomic Research (TIGR) malaria data base indicate that PfNT1 is a single copy gene located on chromosome 14. Northern analysis of RNA from intraerythrocytic stages of the parasite demonstrates that PfNT1 is expressed throughout the asexual life cycle but is significantly elevated during the early trophozoite stage. Functional expression of PfNT1 in Xenopus laevis oocytes significantly increases their ability to take up naturally occurring D-adenosine (K(m) = 13.2 microM) and D-inosine (K(m) = 253 microM). Significantly, PfNT1, unlike the mammalian nucleoside transporters, also has the capacity to transport the stereoisomer L-adenosine (K(m) > 500 microM). Inhibition studies with a battery of purine and pyrimidine nucleosides and bases as well as their analogs indicate that PfNT1 exhibits a broad substrate specificity for purine and pyrimidine nucleosides. These data provide compelling evidence that PfNT1 encodes a functional purine/pyrimidine nucleoside transporter whose expression is strongly developmentally regulated in the asexual stages of the P. falciparum life cycle. Moreover, the unusual ability to transport L-adenosine and the vital contribution of purine transport to parasite survival makes PfNT1 an attractive target for therapeutic evaluation.

Kinetic and pharmacological properties of cloned human equilibrative nucleoside transporters, ENT1 and ENT2, stably expressed in nucleoside transporter-deficient PK15 cells. Ent2 exhibits a low affinity for guanosine and cytidine but a high affinity for inosine

We stably transfected the cloned human equilibrative nucleoside transporters 1 and 2 (hENT1 and hENT2) into nucleoside transporter-deficient PK15NTD cells. Although hENT1 and hENT2 are predicted to be 50-kDa proteins, hENT1 runs as 40 kDa and hENT2 migrates as 50 and 47 kDa on SDS-polyacrylamide gel electrophoresis. Peptide N-glycosidase F and endoglycosidase H deglycosylate hENT1 to 37 kDa and hENT2 to 45 kDa. With hENT1 being more sensitive, there is a 7000-fold and 71-fold difference in sensitivity to nitrobenzylthioinosine (NBMPR) (IC(50), 0.4 +/- 0.1 nM versus 2.8 +/- 0.3 microM) and dipyridamole (IC(50), 5.0 +/- 0.9 nM versus 356 +/- 13 nM), respectively. [(3)H]NBMPR binds to ENT1 cells with a high affinity K(d) of 0.377 +/- 0.098 nM, and each ENT1 cell has 34,000 transporters with a turnover number of 46 molecules/s for uridine. Although both transporters are broadly selective, hENT2 is a generally low affinity nucleoside transporter with 2.6-, 2.8-, 7. 7-, and 19.3-fold lower affinity than hENT1 for thymidine, adenosine, cytidine, and guanosine, respectively. In contrast, the affinity of hENT2 for inosine is 4-fold higher than hENT1. The nucleobase hypoxanthine inhibits [(3)H]uridine uptake by hENT2 but has minimal effect on hENT1. Taken together, these results suggest that hENT2 might be important in transporting adenosine and its metabolites (inosine and hypoxanthine) in tissues such as skeletal muscle where ENT2 is predominantly expressed.

Cellular pharmacology of the D- and L-enantiomers of beta-5-o-carboranyl-2'-deoxyuridine

The cellular pharmacology of the D- and L-enantiomers of beta-5-o-carboranyl-2'-deoxyuridine (CDU), compounds designed for boron neutron capture therapy (BNCT), were studied using human CEM lymphoblast and U-251 glioblastoma cells, at a physiologically achievable concentration (1 microM). Accumulation of the enantiomers was rapid and indistinguishable, reaching cellular concentrations > 40-fold higher than extracellular levels, with approximately 5% persisting in cells after incubation in fresh medium for more than 2 hr. Uptake was not affected by nucleoside uptake inhibitors, but was inhibited by the purine base uptake inhibitor papaverine.

Identification and characterization of a novel hepatic canalicular ATP diphosphohydrolase

We have identified and characterized a novel ATP diphosphohydrolase (ATPDase) with features of E-type ATPases from porcine liver. Immunoblotting with a specific monoclonal antibody to this ectoenzyme revealed high expression in liver with lesser amounts in kidney and duodenum. This ATPDase was localized by immunohistochemistry to the bile canalicular domain of hepatocytes and to the luminal side of the renal ductular epithelium. In contrast, ATPDase/cd39 was detected in vascular endothelium and smooth muscle in these organs. We purified the putative ATPDase from liver by immunoaffinity techniques and obtained a heavily glycosylated protein with a molecular mass estimated at 75 kDa. This enzyme hydrolyzed all tri- and diphosphonucleosides but not AMP or diadenosine polyphosphates. There was an absolute requirement for divalent cations (Ca(2+) > Mg(2+)). Biochemical activity was unaffected by sodium azide or other inhibitors of ATPases. Kinetic parameters derived from purified preparations of hepatic ATPDase indicated V(max) of 8.5 units/mg of protein with apparent K(m) of 100 microM for both ATP or ADP as substrates. NH(2)-terminal amino acid sequencing revealed near 50% identity with rat liver lysosomal (Ca(2+)-Mg(2+))-ATPase. The different biochemical properties and localization of the hepatic ATPDase suggest pathophysiological functions that are distinct from the vascular ATPDase/cd39.

Erythrocyte membrane transport

Erythrocytes are endowed with functional entities that support either cellular functions or the systemic delivery of O2 from lung to tissue and removal of CO2 from tissue to lung. The latter depend largely on the blood's circulatory capacity. They are associated, respectively, with cytosolic haemoglobin and the major membrane polypeptide band 3 (anion exchanger 1, AE1). As a membrane transporter, AE1 mediates Cl-/HCO3- exchange, thus enhancing the blood capacity for carrying CO2 and for acid-base homeostasis. By interacting with lipids and proteins, the multifunctional AE1 tethers the membrane cytoskeleton multiprotein complex to the membrane and confers upon erythrocytes both mechanical and viscoelastic properties. Those in turn allow cells to withstand the shear forces of circulation and squeeze through capillaries. Most other major membrane transporters are apparently essential for maintaining a stable erythrocyte cell shape and flexibility via a functional membrane cytoskeleton. These include the membrane transporters of glucose, nucleoside and purine for fueling the Na/K and Ca pumps via ATP production, and of amino acid and oxidized glutathione transport for maintaining the cell redox status. All membrane transporters detected in mature erythrocytes are synthesized early in erythrocyte differentiation. Their contribution to erythrocyte and to systemic physiology is presently being re-assessed by targeted gene disruption and replacement. For example, organisms with reduced or disrupted AE1 gene expression showed major erythrocyte instabilities and defective anion exchange capacity and acidosis, but remain alive.

Chronic hypoxia enhances adenosine release in rat PC12 cells by altering adenosine metabolism and membrane transport

Acute exposure to hypoxia causes a release of adenosine (ADO) that is inversely related to the O2 levels in oxygen-sensitive pheochromocytoma (PC12) cells. In the current study, chronic exposure (48 h) of PC12 cells to moderate hypoxia (5% O2) significantly enhanced the release of ADO during severe, acute hypoxia (1% O2). Investigation into the intra- and extracellular mechanisms underpinning the secretion of ADO in PC12 cells chronically exposed to hypoxia revealed changes in gene expression and activities of several key enzymes associated with ADO production and metabolism, as well as the down-regulation of a nucleoside transporter. Decreases in the enzymatic activities of ADO kinase and ADO deaminase accompanied by an increase in those of cytoplasmic and ecto-5'-nucleotidases bring about an increased capacity to produce intra- and extracellular ADO. This increased potential to generate ADO and decreased capacity to metabolize ADO indicate that PC12 cells shift toward an ADO producer phenotype during hypoxia. The reduced function of the rat equilibrative nucleoside transporter rENT1 also plays a role in controlling extracellular ADO levels. The hypoxia-induced alterations in the ADO metabolic enzymes and the rENT1 transporter seem to increase the extracellular concentration of ADO. The biological significance of this regulation is unclear but is likely to be associated with modulating cellular activity during hypoxia.

Families of transmembrane sugar transport proteins

We describe here 20 families of secondary (pmf-driven) carriers which, in addition to nine families within the ATP-dependent ABC superfamily, and seven families of Gram-negative bacterial outer membrane porins, largely account for the stereospecific transport of sugars and their derivatives into and out of all living cells on earth. Family characteristics as well as struc-tural and functional properties of the family constituents are described. By reference to our website (http://www-biology.ucsd.edu/ approximately msaier/transport/), phylogenetic relationships, detailed substrate specificity information and both primary and secondary references are also available. This review provides a comprehensive guide to the diversity of carriers that mediate the transport of sugar-containing molecules across cell and organellar membranes.

Maintenance of differential methotrexate toxicity between cells expressing drug-resistant and wild-type dihydrofolate reductase activities in the presence of nucleosides through nucleoside transport inhibition

Methotrexate (MTX), a potent inhibitor of dihydrofolate reductase (DHFR), has been used widely as a chemotherapeutic agent and as a selective agent for cells expressing drug-resistant DHFR activity. MTX deprives rapidly dividing cells of reduced folates that are necessary for thymidylate synthesis and de novo purine nucleotide synthesis. However, MTX toxicity can be circumvented by salvaging thymidine (TdR) and purine nucleosides. Here we have investigated conditions under which nucleoside transport inhibition can be used to maintain differential MTX toxicity between unmodified cells and cells expressing drug-resistant DHFR activity in the presence of exogenous nucleosides. PA317 cells (a 3T3 derivative cell line) were rescued from the toxicity of 0.1 microM MTX by 1.0 microM TdR in the presence of 100 microM inosine. The nucleoside transport inhibitor dipyridamole (DP) resensitized these cells to MTX, even in the presence of exogenous nucleosides. Furthermore, PA317 cells transduced with any of three retroviruses encoding drug-resistant DHFRs remained resistant to MTX over all concentrations tested (up to 10.0 microM) in the presence of DP. Similar results were obtained in transduced HuH7 and K562 cell lines, a human hepatoma and a human leukemia cell line, respectively. We conclude that nucleoside transport inhibition increases the toxicity and selectivity of MTX in cultured cells, and therefore is an effective way to maintain differential MTX toxicity between unmodified and DHFR-modified cells. Our results support the use of nucleoside transport inhibition in in vivo selection protocols involving the liver and hematopoietic systems.

Differential transport of cytosine-containing nucleosides by recombinant human concentrative nucleoside transporter protein hCNT1

The transportability of cytosine-containing nucleosides by recombinant hCNT1 was investigated in transfected mammalian cells. Apparent K(m) values for hCNT1-mediated transport of uridine, cytidine and deoxycytidine were, respectively, 59, 140 and 150 microM. Uridine transport was inhibited 89, 32 and 11%, respectively, by 500 microM gemcitabine, cytarabine and lamivudine, demonstrating that, unlike gemcitabine (a high-affinity hCNT1 permeant), cytarabine and lamivudine are poor hCNT1 permeants.

The adenosine transporter of Toxoplasma gondii. Identification by insertional mutagenesis, cloning, and recombinant expression

Purine transport into the protozoan parasite Toxoplasma gondii plays an indispensable nutritional function for this pathogen. To facilitate genetic and biochemical characterization of the adenosine transporter of the parasite, T. gondii tachyzoites were transfected with an insertional mutagenesis vector, and clonal mutants were selected for resistance to the cytotoxic adenosine analog adenine arabinoside (Ara-A). Whereas some Ara-A-resistant clones exhibited disruption of the adenosine kinase (AK) locus, others displayed normal AK activity, suggesting that a second locus had been tagged by the insertional mutagenesis plasmid. These Ara-A(r) AK+ mutants displayed reduced adenosine uptake capability, implying a defect in adenosine transport. Sequences flanking the transgene integration point in one mutant were rescued from a genomic library of Ara-A(r) AK+ DNA, and Southern blot analysis revealed that all Ara-A(r) AK+ mutants were disrupted at the same locus. Probes derived from this locus, designated TgAT, were employed to isolate genomic and cDNA clones from wild-type libraries. Conceptual translation of the TgAT cDNA open reading frame predicts a 462 amino acid protein containing 11 transmembrane domains, a primary structure and membrane topology similar to members of the mammalian equilibrative nucleoside transporter family. Expression of TgAT cRNA in Xenopus laevis oocytes increased adenosine uptake capacity in a saturable manner, with an apparent K(m) value of 114 microM. Uptake was inhibited by various nucleosides, nucleoside analogs, hypoxanthine, guanine, and dipyridamole. The combination of genetic and biochemical studies demonstrates that TgAT is the sole functional adenosine transporter in T. gondii and a rational target for therapeutic intervention.

Gemcitabine transport in xenopus oocytes expressing recombinant plasma membrane mammalian nucleoside transporters

BACKGROUND

Gemcitabine, a pyrimidine analogue of deoxycytidine, is an anticancer nucleoside drug that requires functional plasma membrane nucleoside transporter proteins to reach its intracellular targets and cause cytotoxicity. Because of technical difficulties inherent in studying nucleoside transport in human cells, we rigorously defined gemcitabine membrane transportability by producing each of the available human (h) and rat (r) recombinant nucleoside transporters (NTs) individually in Xenopus laevis oocytes.

METHODS

Oocytes were microinjected with in vitro-transcribed RNAs derived from complementary DNAs encoding (C = concentrative) rCNT1, rCNT2, hCNT1, hCNT2, (E = equilibrative) rENT1, rENT2, hENT1, and hENT2. Uptake of [(3)H]gemcitabine and [(14)C] uridine was measured 3 days after microinjection to determine kinetic constants. We also used the two-electrode, voltage-clamp technique to investigate the electrophysiology of hCNT1-mediated gemcitabine transport.

RESULTS

Gemcitabine was transported by most of the tested proteins (the exceptions being the purine-selective rCNT2 and hCNT2), with the greatest uptake occurring in oocytes producing recombinant rCNT1 and hCNT1. Influxes of gemcitabine mediated by hCNT1, hENT1, and hENT2 were saturable and conformed to Michaelis-Menten kinetics with apparent K(m) values of 24, 160, and 740 microM, respectively. Gemcitabine had a limited ability to cross the lipid bilayer of oocyte membranes by simple diffusion. External application of gemcitabine to oocytes producing recombinant hCNT1 induced an inward current, which demonstrated that hCNT1 functions as a Na(+)/nucleoside co-transport protein and confirmed the transporter's ability to transport gemcitabine.

CONCLUSIONS

Mammalian nucleoside transporters vary widely in their affinity and capacity to transport gemcitabine. Variation in the tumor and tissue distribution of plasma membrane nucleoside transporter proteins may contribute to the solid tumor activities and schedule-dependent toxic effects of gemcitabine.

Involvement of multiple transporters in the oral absorption of nucleoside analogues

Many nucleoside analogues such as azt, ddI, ddC, d4T, 3TC, acv and vacv are currently being used in the treatment of patients infected with HIV, suffering from AIDS, or AIDS-related opportunistic infections. The transport of nucleoside analogues across the gastrointestinal tract is mediated by a number of transporters that fall into three broad categories, i.e., Na(+)-dependent concentrative transporters, Na(+)-independent equilibrative transporters and H(+)/peptide transporters. The first two transporter classes contain a large number of subtypes that are based on the substrate specificity. Recent studies have shown that most of the anti-HIV nucleoside analogues are transported by one or more of the nucleoside transporters. Furthermore, certain analogues, such as acv, appear to be absorbed by non-carrier-mediated diffusion, whereas vacv is apparently transported by non-nucleoside transporters (e.g., the oligopeptide transporter, PepT1 and possibly others). Thus, it is desirable to understand the precise nature of the absorption mechanism of these drugs to improve bioavailability and reduce the variability that is commonly observed in vivo in human patients. A complete understanding of the complex interactions of nucleoside analogues with the various transporters will help in designing better delivery systems and strategies to improve efficacy. In the current report, the mechanisms of nucleoside and nucleoside-analogue transport are reviewed. Also, methods of exploiting prodrugs to improve the bioavailability characteristics of drugs are highlighted.

Mechanisms of transport of nucleosides and nucleoside analogues in choroid plexus

Nucleoside and nucleobase analogues are being used to treat a number of viral infections of the central nervous system (CNS) including herpes-simplex encephalitis, cytomegalovirus retinitis, and AIDS-related dementia complex. Delivery of nucleoside analogues to the CNS is considered a key challenge in the treatment of these diseases. In this review, we focus on the mechanisms of transport of nucleosides and nucleoside analogues in the choroid plexus. First, the structure and function of the choroid plexus are reviewed. Then, we focus on the mechanisms of nucleoside transport in mammalian cells. Specific emphasis is placed on the molecular and functional characteristics of various nucleoside transporters. A discussion is then devoted to the mechanisms of nucleoside transport in choroid plexus. Current knowledge of nucleoside transport systems in choroid plexus in several animal species is summarized followed by a short section on the mechanisms of transport of nucleobases in the choroid plexus. Finally, a brief section on future directions is included.

Cloning and functional expression of a gene encoding a P1 type nucleoside transporter from Trypanosoma brucei

Nucleoside transporters are likely to play a central role in the biochemistry of the parasite Trypanosoma brucei, since these protozoa are unable to synthesize purines de novo and must salvage them from their hosts. Furthermore, nucleoside transporters have been implicated in the uptake of antiparasitic and experimental drugs in these and other parasites. We have cloned the gene for a T. brucei nucleoside transporter, TbNT2, and shown that this permease is related in sequence to mammalian equilibrative nucleoside transporters. Expression of the TbNT2 gene in Xenopus oocytes reveals that the permease transports adenosine, inosine, and guanosine and hence has the substrate specificity of the P1 type nucleoside transporters that have been previously characterized by uptake assays in intact parasites. TbNT2 mRNA is expressed in bloodstream form (mammalian host stage) parasites but not in procyclic form (insect stage) parasites, indicating that the gene is developmentally regulated during the parasite life cycle. Genomic Southern blots suggest that there are multiple genes related in sequence to TbNT2, implying the existence of a family of nucleoside transporter genes in these parasites.

Common resistance mechanisms to nucleoside analogues in variants of the human erythroleukemic line K562

Variants of the human K562 were developed against the nucleoside analogues cytosine arabinoside, 2 chlorodeoxyadenosine, fludarabine and gemcitabine. The resistant lines displayed a high degree of cross-resistance to all nucleoside analogues, with little or no cross resistance to other agents. There was a profound accumulation defect of the different nucleoside analogues in all of the variants. There was a strong overexpression of 5'nucleotidase, measured by rt-PCR and enzyme activity, in all resistant variants. There was a two fold increase of ribonucleotide reductase in the fludarabine resistant line and increased expression of purine nucleoside phosphorylase in the 2 chlorodeoxyadenosine selected line. Karyotypic analysis revealed the loss of a 6(q16;q22) deletion present in the parental line in all of the resistant lines. This portion of chromosome 6 has been shown to contain the gene for 5'nucleotidase. Early events in the transport and metabolism appear to be involved in the resistance mechanisms to nucleoside analogues and are responsible for broad cross resistance to this family of compounds.

Identification of amino acid residues responsible for the pyrimidine and purine nucleoside specificities of human concentrative Na(+) nucleoside cotransporters hCNT1 and hCNT2

hCNT1 and hCNT2 mediate concentrative (Na(+)-linked) cellular uptake of nucleosides and nucleoside drugs by human cells and tissues. The two proteins (650 and 658 residues, 71 kDa) are 72% identical in sequence and contain 13 putative transmembrane helices (TMs). When produced in Xenopus oocytes, recombinant hCNT1 is selective for pyrimidine nucleosides (system cit), whereas hCNT2 is selective for purine nucleosides (system cif). Both transport uridine. We have used (i) chimeric constructs between hCNT1 and hCNT2, (ii) sequence comparisons with a newly identified broad specificity concentrative nucleoside transporter (system cib) from Eptatretus stouti, the Pacific hagfish (hfCNT), and (iii) site-directed mutagenesis of hCNT1 to identify two sets of adjacent residues in TMs 7 and 8 of hCNT1 (Ser(319)/Gln(320) and Ser(353)/Leu(354)) that, when converted to the corresponding residues in hCNT2 (Gly(313)/Met(314) and Thr(347)/Val(348)), changed the specificity of the transporter from cit to cif. Mutation of Ser(319) in TM 7 of hCNT1 to Gly enabled transport of purine nucleosides, whereas concurrent mutation of Gln(320) to Met (which had no effect on its own) augmented this transport. The additional mutation of Ser(353) to Thr in TM 8 converted hCNT1/S319G/Q320M, from cib to cif, but with relatively low adenosine transport activity. Additional mutation of Leu(354) to Val (which had no effect on its own) increased the adenosine transport capability of hCNT1/S319G/Q320M/S353T, producing a full cif-type transporter phenotype. On its own, the S353T mutation converted hCNT1 into a transporter with novel uridine-selective transport properties. Helix modeling of hCNT1 placed Ser(319) (TM 7) and Ser(353) (TM 8) within the putative substrate translocation channel, whereas Gln(320) (TM 7) and Leu(354) (TM 8) may exert their effects through altered helix packing.

Distribution of equilibrative, nitrobenzylthioinosine-sensitive nucleoside transporters (ENT1) in brain

Nucleoside transport processes may play a role in regulating endogenous levels of the inhibitory neuromodulator adenosine in brain. The cDNAs encoding species homologues of one member of the equilibrative nucleoside transporter (ENT) gene family have recently been isolated from rat (rENT1) and human (hENT1) tissues. The current study used RT-PCR, northern blot, in situ hybridization, and [3H]nitrobenzylthioinosine autoradiography to determine the distribution of mRNA and protein for ENT1 in rat and human brain. Northern blot analysis indicated that hENT1 mRNA is widely distributed in adult human brain. 35S-labeled sense and antisense riboprobes, transcribed from a 153-bp segment of rENT1, were hybridized to fresh frozen coronal sections from adult rat brain and revealed widespread rENT1 mRNA in pyramidal neurons of the hippocampus, granule neurons of the dentate gyrus, Purkinje and granule neurons of the cerebellum, and cortical and striatal neurons. Regional localization in rat brain was confirmed by RT-PCR. Thus, ENT1 mRNA has a wide cellular and regional distribution in brain, indicating that this nucleoside transporter subtype may be important in regulating intra- and extracellular levels of adenosine in brain.

Potentiation of the cytotoxicity of thymidylate synthase (TS) inhibitors by dipyridamole analogues with reduced alpha1-acid glycoprotein binding

Dipyridamole has been shown to enhance the in vitro activity of antimetabolite anticancer drugs through the inhibition of nucleoside transport. However, the clinical potential of dipyridamole has not been realized because of the avid binding of the drug to the plasma protein alpha1-acid glycoprotein (AGP). Dipyridamole analogues that retain potent nucleoside transport inhibitory activity in the presence of AGP are described and their ability to enhance the growth inhibitory and cytotoxic effects of thymidylate synthase (TS) inhibitors has been evaluated. Three dipyridamole analogues (NU3026, NU3059 and NU3060) were shown to enhance the growth inhibitory activity of the TS inhibitor CB3717 and block thymidine rescue in L1210 cells. The extent of potentiation at a fixed analogue concentration (10 microM) was related to the potency of inhibition of thymidine uptake. A further analogue, NU3076, was identified, which was more potent than dipyridamole with a Ki value for inhibition of thymidine uptake of 0.1 microM compared to 0.28 microM for dipyridamole. In marked contrast to dipyridamole, inhibition of thymidine uptake by NU3076 was not significantly affected by the presence of AGP (5 mg ml(-1)). NU3076 and dipyridamole produced equivalent potentiation of the cytotoxicity of the non-classical antifolate TS inhibitor, nolatrexed, in L1210 cells with both compounds significantly reducing the LC90, by > threefold in the absence of salvageable thymidine. Thymidine rescue of L1210 cells from nolatrexed cytotoxicity was partially blocked by both 1 microM NU3076 and 1 microM dipyridamole. NU3076 also caused a significant potentiation of FU cytotoxicity in L1210 cells. These studies demonstrate that nucleoside transport inhibition can be maintained in the absence of AGP binding with the dipyridamole pharmacophore and that such analogues can enhance the cytotoxicity of TS inhibitors.

A nucleoside transporter from Trypanosoma brucei involved in drug resistance

Drug resistance of pathogens is an increasing problem whose underlying mechanisms are not fully understood. Cellular uptake of the major drugs against Trypanosoma brucei spp., the causative agents of sleeping sickness, is thought to occur through an unusual, so far unidentified adenosine transporter. Saccharomyces cerevisiae was used in a functional screen to clone a gene (TbAT1) from Trypanosoma brucei brucei that encodes a nucleoside transporter. When expressed in yeast, TbAT1 enabled adenosine uptake and conferred susceptibility to melaminophenyl arsenicals. Drug-resistant trypanosomes harbor a defective TbAT1 variant. The molecular identification of the entry route of trypanocides opens the way to approaches for diagnosis and treatment of drug-resistant sleeping sickness.

Distribution of mRNA encoding a nitrobenzylthioinosine-insensitive nucleoside transporter (ENT2) in rat brain

Nucleoside transporters may play a role in regulating levels of extracellular adenosine and adenosine receptor activity. Two members of the equilibrative nucleoside transporter family have recently been cloned. ENT1 is potently inhibited by nitrobenzylthioinosine (NBMPR) (K(i) approximately 1 nM) and was previously found to have a wide distribution in rat and human brain. ENT2 is insensitive to inhibition by NBMPR at low nanomolar concentrations and there is limited information describing its distribution in rat brain. The present study used RT-PCR, northern blot and in situ hybridization and detected rENT2 transcript in several brain regions including hippocampus, cortex, striatum and cerebellum. Our results indicate a wide cellular and regional distribution for ENT2 in rat brain, similar to ENT1, indicating that control of adenosine levels in brain is achieved by multiple transport processes.

A human nucleobase transporter-like cDNA (SLC23A1): member of a transporter family conserved from bacteria to mammals

A family of related polytopic membrane proteins that mediate the transport of nucleobases has been extended to Homo sapiens by the cloning of a full-length human cDNA that encodes a nucleobase transporter-like protein. The protein is predicted to contain 11-14 transmembrane-spanning regions, exhibits 20-28% overall sequence identity to fungal and bacterial transporters, and contains a conserved signature motif found in this family. Fluorescence in situ hybridization localized the gene (HGMW-approved symbol SLC23A1) to human chromosome 20p13. Human nucleobase transporter-like mRNA was present in all tissues examined, with lower levels found in heart, skeletal muscle, and ovary. Expression of the 60-kDa cDNA-encoded protein was demonstrated by an in vitro transcription-translation approach. The identification of this nucleobase transporter-like protein will allow the further elucidation of the interaction of human cells with physiological nucleobases and pharmacologically important drugs such as 5-F-uracil, dideoxynucleosides, and acyclic nucleosides.

Differential modulation of nucleoside transport types in neuroblastoma cells by protein kinase activation

Nucleoside transport regulation in undifferentiated Neuro-2A cells has been studied and found to include Na+-dependent adenosine transport and facilitated diffusion adenosine transport. The latter corresponded to nitrobenzylthioinosine-sensitive nucleoside transport. Short-term treatment of Neuro-2A cells with physiologically relevant signals only modulated the facilitated diffusion component. The stimulation of undifferentiated cells with forskolin or other activators of the protein kinase A pathway, decreased NBTI-sensitive adenosine transport. Treatment of cells with an inactive analogue of forskolin, 1,9-dideoxi-forskolin, had no effect on NBTI-sensitive nucleoside transport. Therefore, the inhibition of protein kinase A activity by pre-incubation with H-89 or the cAMP antagonist, Rp-8-Br-cAMPS, completely prevented the inhibitory effect of forskolin. Similarly, the activation of protein kinase C with phorbol 12,13-dibutyrate (PDBu) and the calcium ionophore A-23187 decreased NBTI-sensitive adenosine transport. The effect of PDBu was reversed by pre-incubation of cells with staurosporine. Maximal transport inhibition was obtained by the simultaneous stimulation of cells with a phorbol ester and A-23187 or a phorbol ester and forskolin. The modulation of NBTI-sensitive nucleoside transport corresponded to changes in specific [3H]NBTI binding to Neuro-2A cells. Maximal inhibition correlated well with a maximal enhancement of cAMP production. However, the Na+-dependent adenosine transport in Neuro-2A cells was not modulated by any of these signals.

Common resistance mechanisms to deoxynucleoside analogues in variants of the human erythroleukaemic line K562

Resistant variants of the human leukaemic line K562 were developed using selection with the deoxynucleoside analogues cytosine arabinoside, 2-chlorodeoxyadenosine, fludarabine and gemcitabine. The resistant lines displayed a high degree of cross resistance to all deoxynucleoside analogues, with little or no cross resistance to other agents. There was a profound accumulation defect of all nucleoside analogues in the resistant variants but no significant defect in nucleoside transport in any of the variants. 5' nucleotidase activity was strongly increased and deoxycytidine kinase activity was moderately reduced in all of the resistant variants, resulting in reduced accumulation of triphosphate analogues. In addition a deletion in one of the alleles of the deoxycytidine kinase was detected in the fludarabine-resistant line. Ribonucleotide reductase activity was found to be strongly increased in the gemcitabine-selected line and purine nucleoside phosphorylase was increased in the 2-chlorodeoxyadenosine-selected line. Free nucleotide pools were increased in the 2-chlorodeoxyadenosine-selected line. There was no expression of the mdr1 gene by the resistant lines. Karyotypic analysis and FISH experiments using a 6q21 specific probe showed alterations in the 6(q16-q22) region which contains the 5'-nucleotidase gene. Early events in the activation and degradation of deoxynucleoside analogues appear to constitute common mechanisms of resistance to these compounds.

Nucleoside transport in human colonic epithelial cell lines: evidence for two Na+-independent transport systems in T84 and Caco-2 cells

RT-PCR of RNA isolated from monolayers of the human colonic epithelial cell lines T84 and Caco-2 demonstrated the presence of mRNA for the two cloned Na+-independent equilibrative nucleoside transporters, ENT1 and ENT2, but not for the cloned Na+-dependent concentrative nucleoside transporters, CNT1 and CNT2. Uptake of [3H]uridine by cell monolayers in balanced Na+-containing and Na+-free media confirmed the presence of only Na+-independent nucleoside transport mechanisms. This uptake was decreased by 70-75% in the presence of 1 microM nitrobenzylthioinosine, a concentration that completely inhibits ENT1, and was completely blocked by the addition of 10 microM dipyridamole, a concentration that inhibits both ENT1 and ENT2. These findings indicate the presence in T84 and Caco-2 cells of two functional Na+-independent equilibrative nucleoside transporters, ENT1 and ENT2.

Nucleoside transporters: molecular biology and implications for therapeutic development

The uptake of nucleosides (or nucleobases) is essential for nucleic acid synthesis in many human cell types and in parasitic organisms that cannot synthesize nucleotides de novo. The transporters responsible are also the route of entry for many cytotoxic nucleoside analogues used in cancer and viral chemotherapy. Moreover, by regulating adenosine concentrations in the vicinity of its cell-surface receptors, nucleoside transporters profoundly affect neurotransmission, vascular tone and other processes. The recent molecular characterization of two families of human nucleoside transporters has provided new insights into the mechanisms of natural nucleoside and drug uptake and into future developments of improved therapies.

Functional aspects of red cell antigens

Over 250 blood group determinants are known and most of these are located on integral red cell proteins and glycoproteins. The functions of some of these structures are known: Diego (band 3) is the red cell anion exchanger; Kidd, a urea transporter; Colton (aquaporin 1), a water channel; Cromer (DAF) and Knops (CRI), complement regulators; Diego (band 3) and Gerbich (glycophorin C/D) link the red cell membrane and the membrane skeleton. The Duffy glycoprotein is a chemokine receptor that may act as a scavenger for inflammatory mediators in the peripheral blood, but is also exploited as a receptor by Plasmodium vivax merozoites. The functions of some blood group antigens can be speculated upon because of structural similarity to proteins and glycoproteins of known function. For example, the Lutheran, LW, and Ok glycoproteins are members of the immunoglobulin superfamily of receptors and signal transducers, the Rh proteins and related glycoproteins show homology to ammonium transporters, and the Kell glycoprotein resembles a family of endopeptidases. Yet most blood groups systems contain null phenotypes associated with no apparent pathology. If these blood group antigens have important functions, other structures must be able to carry out those functions in their absence. Almost nothing is known of the biological significance of blood group polymorphism.

Nucleoside transporters of mammalian cells

In this review, we have summarized recent advances in our understanding of the biology of nucleoside transport arising from new insights provided by the isolation and functional expression of cDNAs encoding the major nucleoside transporters of mammalian cells. Nucleoside transporters are required for permeation of nucleosides across biological membranes and are present in the plasma membranes of most cell types. There is growing evidence that functional nucleoside transporters are required for translocation of nucleosides between intracellular compartments and thus are also present in organellar membranes. Functional studies during the 1980s established that nucleoside transport in mammalian cells occurs by two mechanistically distinct processes, facilitated diffusion and Na(+)-nucleoside cotransport. The determination of the primary amino acid sequences of the equilibrative and concentrative transporters of human and rat cells has provided a structural basis for the functional differences among the different transporter subtypes. Although nucleoside transporter proteins were first purified from human erythrocytes a decade ago, the low abundance of nucleoside transporter proteins in membranes of mammalian cells has hindered analysis of relationships between transporter structure and function. The molecular cloning of cDNAs encoding nucleoside transporters and the development of heterologous expression systems for production of recombinant nucleoside transporters, when combined with recombinant DNA technologies, provide powerful tools for characterization of functional domains within transporter proteins that are involved in nucleoside recognition and translocation. As relationships between molecular structure and function are determined, it should be possible to develop new approaches for optimizing the transportability of nucleoside drugs into diseased tissues, for development of new transport inhibitors, including reagents that are targeted to the concentrative transporters, and, eventually, for manipulation of transporter function through an understanding of the regulation of transport activity.

Es nucleoside transporter content of acute leukemia cells: role in cell sensitivity to cytarabine (araC)

Nucleoside analogs are important components of treatment regimens for acute leukemia in adults. Plasma membrane permeation of the nucleoside analog molecules, the initial event in the cellular conversion of nucleosides to active agents, is mediated by nucleoside-specific membrane transporters. The widely-expressed es nucleoside transporter accepts as substrates diverse nucleoside analogs, including cytarabine (araC), 2-chlorodeoxyadenosine, and fludarabine. The cellular content of es transporter sites has been measured in blasts from patients with acute lymphoblastic leukemia and acute myelogenous leukemia, by a sensitive, quantitative flow cytometry assay that employs the tightly-bound es ligand, SAENTA fluorescein. Values for es transporter expression varied ten-fold among samples from patients with acute myelogenous leukemia. In this article, we review current findings that document, in confocal fluorescence microscopy images and in flow cytometry assays of SAENTA fluorescein-stained cells, the patient-to-patient variance of es transporter expression in leukemic blasts from patients. Our data show a correlation between the expression of es transporters and the in vitro sensitivity to nucleoside drugs of blasts from acute leukemia patients. These findings show that the flow cytometry assay of es expression provides a facile means of predicting resistance of leukemia cells to the cytotoxicity of araC and other nucleosides.

Ribavirin and interferon alfa-2b in chronic hepatitis C: assessment of possible pharmacokinetic and pharmacodynamic interactions

AIMS

The primary objective of this study was to determine whether pharmacokinetic interactions occurred between interferon alpha-2b (IFN) and ribavirin in patients with chronic hepatitis C infections. Additionally this study assessed the single and multiple-dose pharmacokinetics of ribavirin and IFN, and compared the safety, tolerability and antiviral pharmacodynamics of IFN plus ribavirin compared with either drug alone.

METHODS

In this open label parallel group study, patients with chronic hepatitis C were randomized to receive IFN 3 million IU thrice weekly s.c. alone, ribavirin 600 mg twice daily p.o. alone or both drugs in combination over 6 weeks. Single and multiple dose pharmacokinetics and indices of antiviral pharmacodynamics were assessed during weeks 1 and 6, along with safety assessments during the study.

RESULTS

The range of mean ribavirin terminal phase half-lives after single doses was 44-49 h. Comparison of week 1 and week 6 AUC(0,12h) values showed accumulation in plasma of approximately 6-fold. The range of mean washout half-lives after week 6 was 274-298 h, reflecting release of ribavirin from deep compartment stores. The range of single and multiple dose IFN terminal phase half-lives was 5-7 h. IFN demonstrated an increase in bioavailability (approximately 2-fold) upon multiple dose administration. Ribavirin and IFN pharmacokinetic parameters for combined ribavirin and IFN were similar to those during monotherapy with either compound, although the power of this study to detect differences was low. Serum HCV-RNA titers and ALT concentrations were reduced by IFN alone, ribavirin alone reduced ALT concentrations only, and combined IFN plus ribavirin produced numerically greater falls in both measurements than either treatment alone. Serum concentrations of neopterin and activity of 2',5'-oligoadenylate synthetase (2'5'-OAS) were increased by IFN alone and in combination with ribavirin, whereas serum 2'5'-OAS activity was decreased and neopterin concentrations unaltered by ribavirin monotherapy. IFN and ribavirin monotherapy produced characteristic changes in safety laboratory tests (IFN--reductions in white cells, neutrophils and platelets; ribavirin--reduced haemoglobin) and characteristic adverse event profiles (IFN--headache, flu-like symptoms, fatigue, anorexia, nausea, myalgia, and insomnia; ribavirin--headache, fatigue, myalgia, and pruritus). There was no additive effect of combination therapy on safety laboratory tests or reported adverse events. All changes were fully reversible upon treatment cessation.

CONCLUSIONS

There was no evidence of pharmacokinetic interactions between IFN and ribavirin in this study. There were numerical trends indicating that the combination of IFN and ribavirin reduced titers of HCV-RNA to a greater extent than did either treatment alone, and the safety profile of combination therapy was similar to those of both monotherapy treatments.

Differential expression and regulation of nucleoside transport systems in rat liver parenchymal and hepatoma cells

Primary cultures of rat-liver parenchymal cells show carrier-mediated nucleoside uptake by a mechanism that mainly involves concentrative, Na+-dependent transport activity. In contrast, the hepatoma cell line FAO shows high nucleoside transport activity, although it is mostly accounted for by Na+-independent transport processes. This is associated with a low amount of sodium purine nucleoside transporter (SPNT) mRNA. SPNT encodes a purine-preferring transporter expressed in liver parenchymal cells. To analyze whether SPNT expression is modulated during cell proliferation, SPNT mRNA levels were determined in the early phase of liver growth after partial hepatectomy and in synchronized FAO cells that had been induced to proliferate. SPNT mRNA amounts increased as early as 2 hours after partial hepatectomy. FAO cells induced to proliferate after serum refeeding show an increase in SPNT mRNA levels, which is followed by an increase in Na+-dependent nucleoside uptake and occurs before the peak of 3H-thymidine incorporation into DNA. FAO cells also express significant equilibrative nucleoside transport activity, which may be accounted for by the expression of the nitrobenzylthioinosine (NBTI)-sensitive and -insensitive isoforms, rat equilibrative nucleoside transporter 1 (rENT1) and rENT2, respectively. Interestingly, rENT2 mRNA levels follow a similar pattern to that described for SPNT when FAO cells are induced to proliferate, whereas rENT1 appears to be constitutively expressed. Liver parenchymal cells show low and negligible mRNA levels for rENT1 and rENT2 transporters, respectively, although most of the equilibrative transport activity found in hepatocytes is NBTI-resistant. It is concluded that: 1) SPNT expression is regulated both in vivo and in vitro in a way that appears to be dependent on cell cycle progression; 2) SPNT expression may be a feature of differentiated hepatocytes; and 3) equilibrative transporters are differentially regulated, rENT2 expression being cell cycle-dependent. This is consistent with its putative role as a growth factor-induced delayed early response gene.

Demonstration of equilibrative nucleoside transporters (hENT1 and hENT2) in nuclear envelopes of cultured human choriocarcinoma (BeWo) cells by functional reconstitution in proteoliposomes

The equilibrative nucleoside transporters (ENTs) are a newly recognized family of membrane proteins of which hENT1 is the nitrobenzylmercaptopurine ribonucleoside (NBMPR)-sensitive (es) and hENT2 the NBMPR-insensitive (ei) transporter of human cells. BeWo cells exhibit large numbers (>10(7)/cell) of NBMPR-binding sites and high es and ei nucleoside transport activities relative to other cell types. In this work, we have demonstrated that proliferating BeWo cells possess (i) mRNA encoding hENT1 and hENT2 and (ii) hENT1-specific immunoepitopes. We examined NBMPR binding and its inhibition of uridine transport in various BeWo membrane fractions and proteoliposomes derived therefrom to determine if NBMPR binding to intracellular membranes represented interaction with functional es transporters. Unfractionated membranes and fractions enriched 5-fold in plasma membranes relative to postnuclear supernatants exhibited high NBMPR binding activity. Intact nuclei and nuclear envelopes also exhibited abundant quantities of NBMPR-binding sites with affinities similar to those of enriched plasma membranes (Kd = 0.4-0.9 nM). When proteoliposomes were made from octyl glucoside-solubilized membranes, high affinity NBMPR-binding sites were not only observed in crude membrane preparations and plasma membrane-enriched fractions but also in nuclear envelope fractions. Proteoliposomes prepared from either unfractionated membranes or nuclear envelopes exhibited both hENT1-mediated (82-85%) and hENT2-mediated (15-18%) transport of [3H]uridine. These results provided evidence for the presence of functional es and ei transporters in nuclear membranes and endoplasmic reticulum, suggesting that hENT1 and hENT2 may function in the translocation of nucleosides between the cytosol and the luminal compartments of one or both of these membrane types.

Molecular characterization of two novel transporters from human and mouse kidney and from LLC-PK1 cells reveals a novel conserved family that is homologous to bacterial and Aspergillus nucleobase transporters

Nucleobase transport is important for the metabolism of nucleic acids and antiviral and antineoplastic drugs. This transport has been functionally described in several mammalian cells but has not been well characterized molecularly. We report the cloning of two novel transporters. YSPL2 encodes a 650-residue protein and has an ubiquitous 8 kb transcript. The human and pig homologs are 95% similar. YSPL3 encodes a 598-residue protein with a 3 kb transcript that is expressed only in kidney and liver. Human YSPL2 and YSPL3 are 60% similar at the amino acid level and both show 31% similarity to the first nucleobase permease gene described in vertebrates, YSPL1. These proteins appear to be members of a new family of possible nucleobase transporters with significant sequence similarities with bacterial and Aspergillus nucleobase transporters. Further functional studies will be needed to unveil the role of these transporters in nucleic acid metabolism in normal and in disease states.

Regulation of nucleoside transport by lipopolysaccharide, phorbol esters, and tumor necrosis factor-alpha in human B-lymphocytes

Nucleoside transport systems and their regulation in human B-lymphocytes have been characterized using the cell lines Raji and Bare lymphoma syndrome-1 (BLS-1) as experimental models. These cells express at least three different nucleoside transport systems as follows: a nitrobenzylthioinosine-sensitive equilibrative transport system of the es-type, which appears to be associated with hENT1 expression, and two Na+-dependent transport systems that may correspond to N1 and to the recently characterized N5-type, which is nitrobenzylthioinosine-sensitive and guanosine-preferring. B cell activators such as phorbol 12-myristate 13-acetate and lipopolysaccharide (LPS) up-regulate both concentrative transport systems but down-regulate the equilibrative es-type transporter, which correlates with lower hENT1 mRNA levels. These effects are dependent on protein kinase C activity. Phorbol 12-myristate 13-acetate and LPS also induce an increase in tumor necrosis factor-alpha (TNF-alpha) mRNA levels, which suggest that this cytokine may mediate some of the effects triggered by these agents, since addition of TNF-alpha alone can increase N1 and N5 transport activities by a mechanism that also depends on protein kinase C activation. Interestingly, TNF-alpha down-regulates es activity, but this effect cannot be abolished by inhibiting protein kinase C. This study reveals differential regulation of nucleoside transport systems following activation of human B-lymphocyte cell lines by agents of physiological relevance such as TNF-alpha and LPS. Moreover, it indicates that the recently characterized N5 transport system can also be regulated following B cell activation, which may be relevant to lymphocyte physiology and to the treatment of lymphocyte malignancies.

Decreased presynaptic sensitivity to adenosine after cocaine withdrawal

The nucleus accumbens (NAc) is a site mediating the rewarding properties of drugs of abuse, such as cocaine, amphetamine, opiates, nicotine, and alcohol (Wise and Bozarth, 1987; Koob, 1992; Samson andHarris, 1992; Woolverton and Johnson, 1992; Self and Nestler, 1995; Pontieri et al., 1996). Acute cocaine has been shown to decrease excitatory synaptic transmission mediated by the cortical afferents to the NAc (Nicola et al., 1996), but the effects of long-term cocaine treatment and withdrawal have not been explored. Here, we report that long-term (1 week) withdrawal from chronic cocaine reduced the potency of adenosine to presynaptically inhibit glutamate (Glu) release by activating adenosine A1 receptors. Adenosine A1 receptors were not desensitized, because the potency of the metabolically stable adenosine analog N6-cyclopentyl-adenosine was unchanged after chronic cocaine withdrawal. When adenosine transporters were blocked, the potency of adenosine to inhibit Glu release from naive and cocaine-withdrawn NAc slices was similar. These results suggest that one of the long-term consequences of cocaine withdrawal is an augmented uptake of adenosine. This long-lasting change expressed at the presynaptic excitatory inputs to the medium spiny output neurons in the NAc may help identify new therapeutic targets for the treatment of drug abuse.

Crithidia luciliae: functional expression of nucleoside and nucleobase transporters in Xenopus laevis oocytes

The expression of purine-specific nucleoside and base transporters of Crithidia luciliae has been demonstrated in Xenopus laevis oocytes. Poly(A)+-mRNA from C. luciliae, cultured in either purine-replete or purine-starved conditions, was microinjected into X. laevis oocytes. For "purine-replete" mRNA, expression of adenosine and hypoxanthine uptake in microinjected X. laevis oocytes was increased on average 9- and 3-fold above water-injected controls, respectively. Expression of adenosine and hypoxanthine uptake in oocytes microinjected with "purine-starved" mRNA was 8 and 3-fold above water-injected controls, respectively. Substrate competition indicated an adenosine/deoxyadenosine transporter and a separate base transporter specific for hypoxanthine. In contrast to C. luciliae in vivo, where the level of activity of adenosine and hypoxanthine transport was regulated by the level of purines in the medium, the heterologous expression of these transporters (from both purine replete and deplete cultures) in X. laevis oocytes was independent of the extracellular purine concentration. These results may suggest that the presence of specific transporter message is independent of the extracellular purine content, indicating that the regulation of activation and expression of these transporters in C. luciliae may not be under transcriptional control.

Molecular cloning, functional expression and chromosomal localization of a cDNA encoding a human Na+/nucleoside cotransporter (hCNT2) selective for purine nucleosides and uridine

Two Na(+)-dependent nucleoside transporters implicated in adenosine and uridine transport in mammalian cells are distinguished functionally on the basis of substrate specificity: CNT1 is selective for pyrimidine nucleosides but also transports adenosine; CNT2 (also termed SPNT) is selective for purine nucleosides but also transports uridine. Both proteins belong to a gene family that includes the NupC proton/nucleoside symporter of E. coli. cDNAs encoding members of the CNT family have been isolated from rat tissues (jejunum, brain, liver; rCNT1 and rCNT2/SPNT) and, most recently, human kidney (hCNT1 and hSPNT1). Here, the molecular cloning and functional characterization of a CNT2/SPNT-type transporter from human small intestine are described. The encoded 658-residue protein (hCNT2 in the nomenclature) had the same predicted amino acid sequence as human kidney hSPNT1, except for a polymorphism at residue 75 (Arg substituted by Ser), and was 83 and 72% identical to rCNT2 and hCNT1, respectively. Sequence differences between hCNT2 and rCNT2 were greatest at the N-terminus. In Xenopus oocytes, recombinant hCNT2 exhibited the functional characteristics of a Na(+)-dependent nucleoside transporter with selectivity for adenosine, other purine nucleosides and uridine (adenosine and uridine K(m) app values 8 and 40 microM, respectively). hCNT2 transcripts were found in kidney and small intestine but, unlike rCNT2, were not detected in liver. Deoxyadenosine, which undergoes net renal secretion in humans, was less readily transported than adenosine. hCNT2 also mediated small, but significant, fluxes of the antiviral purine nucleoside analogue 2',3'-dideoxyinosine. hCNT2 is, therefore potentially involved in both the intestinal absorption and renal handling of purine nucleosides (including adenosine), uridine and purine nucleoside drugs. The gene encoding hCNT2 was mapped to chromosome 15q15.