Hypoxanthine transport in mammalian cells: cell type-specific differences in sensitivity to inhibition by dipyridamole and uridine

Physiology of Nucleoside Transporters: Back to the Future

Nucleoside transporters (NTs) are integral membrane proteins responsible for mediating and facilitating the flux of nucleosides and nucleobases across cellular membranes. NTs are also responsible for the uptake of nucleoside analog drugs used in the treatment of cancer and viral infections, and they are the target of certain compounds used in the treatment of some types of cardiovascular disease. The important role of NTs as drug transporters and therapeutic targets has necessarily led to intense interest into their structure and function and the relationship between these proteins and drug efficacy. In contrast, we still know relatively little about the fundamental physiology of NTs. In this review, we discuss various aspects of the physiology of NTs in mammalian systems, particularly noting tissues and cells where there has been little recent research. Our central thesis is reference back to some of the older literature, combined with current findings, will provide direction for future research into NT physiology that will lead to a fuller understanding of the role of these intriguing proteins in the everyday lives of cells, tissues, organs, and whole animals.

Hypoxanthine transport in human tumour cell lines: relationship to the inhibition of hypoxanthine rescue by dipyridamole

Hypoxanthine (HPX) uptake was investigated in four human tumour cell lines previously characterised as being sensitive (ds: A549 and MCF7) or insensitive (di: COR-L23 and T-47D) to dipyridamole (DP)-induced inhibition of HPX rescue from antipurine antifolate-induced growth inhibition. The aim of the study was to determine the mechanism underlying the differential sensitivity of HPX rescue to DP. The time-course of HPX uptake in the two ds cell lines was different in comparison to the two di cell lines. The initial rate of HPX uptake in the di cell lines was more rapid than in the ds cell lines such that at 60 sec the amount of HPX taken up by the former was 2-6 times higher than that taken up by the later. The K(t) and T(max) for HPX transport in di COR-L23 cells were 870 microM and 4.75 microM/10(6) cells/min and 1390 microM and 1.78 microM/10(6) cells/min in ds A549 cells. HPX transport was not sodium-dependent in these cells. Equilibrative nucleoside transporter 2 (ENT2)-mediated thymidine transport was also higher in di cells. DP inhibited HPX uptake into ds cell lines by > or =48% and by < or =20% in the di cell lines. Competition studies with HPX and thymidine transport via ENT2 indicated an overlap between nucleoside and nucleobase transport transporters in the breast cancer cell lines (MCF7 and T-47D). These studies showed that more rapid and extensive HPX uptake, as well as reduced sensitivity to DP inhibition, is associated with the inability of DP to prevent HPX rescue from antipurine antifolate-induced growth inhibition in certain human tumour cell lines.

Nucleoside transport inhibition in ischemic myocardium results in enhanced taurine efflux

We measured with the microdialysis technique energy-related metabolites in ischemic myocardium over time in an experimental pig model. Emphasis was put on the dipyridamole effect when administered in the microdialysis probe inserted in ischemic myocardium. Not only adenosine but also taurine and pyruvate concentrations were significantly higher in the microdialysate during the periods of ischemia and extracorporeal circulation with cardioplegia. The enhanced efflux of taurine in ischemic myocardium induced by dipyridamole is a new finding. A mechanistic role of taurine in the prevention of Ca(2+) overload in ischemic myocytes is discussed. Also, taurine may have stimulatory effects on glycolysis in ischemic heart.

Nucleosides and nucleotides: natural bioactive substances in milk and colostrum

Nucleotides, nucleosides and nucleobases belong to the non-protein-nitrogen (NPN) fraction of milk. The largest amounts of ribonucleosides and ribonucleotides--ribose forms only were considered in this review--were measured directly after parturition in bovine milk and other ruminants as well as in the milk of humans. Generally, concentrations of most of the nucleos(t)ides tend to decrease gradually with advancing lactation period or nursing time. The species-specific pattern of these minor constituents in milk from different mammals is a remarkable property and confirms, at least, the specific physiological impact of these minor compounds in early life. The physiological capacity of these compounds in milk is given by the total potentially available nucleosides. The main dietary sources of nucleos(t)ides are nucleoproteins and nucleic acids which are converted in the course of intestinal digestion into nucleosides and nucleobases the preferred forms for absorption in the intestine. Thus, nucleosides and nucleobases are suggested to be the acting components of dietary and/or supplemented nucleic acid-related compounds in the gut. They are used by the body as exogenous trophochemical sources and can be important for optimal metabolic functions. Up to 15 % of the total daily need for a breast-fed infant was calculated to come from this dietary source. Concerning their biological role they not only act as metabolites but are also involved as bioactive substances in the regulation of body functions. Dietary nucleotides affect immune modulation, e.g. they enhance antibody responses of infants as shown by a study with more than 300 full-term healthy infants. Dietary nucleos(t)ides are found to contribute to iron absorption in the gut and to influence desaturation and elongation rates in fatty acid synthesis, in particular long-chain polyunsaturated fatty acids in early stages of life. The in vitro modulation of cell proliferation and apoptosis has been described by ribonucleosides, in particular by modified components using human cell culture models. Due to the bio- and trophochemical properties of dietary nucleos(t)ides, the European Commission has allowed the use of supplementation with specific ribonucleotides in the manufacture of infant and follow-on formula. From the technochemical point of view, the ribonucleoside pattern is influenced by thermal treatment of milk. In addition ribonucleosides are useful indicators for quantifying adulterations of milk and milk products.

Dipyridamole potentiates the in vitro activity of MTA (LY231514) by inhibition of thymidine transport

The novel pyrrolopyrimidine-based antifolate LY231514 (MTA), inhibits multiple folate-requiring enzymes including thymidylate synthase, glycinamide ribonucleotide formyltransferase and dihydrofolate reductase. Both thymidine and hypoxanthine are required to reverse MTA growth inhibition in leukaemia and colon cancer cells. Prevention of MTA growth inhibition by thymidine and/or hypoxanthine was investigated in two human lung (A549, COR L23) and two breast (MCF7, T47D) tumour cell lines, and the effect of the nucleoside/base transport inhibitor dipyridamole (DP) on thymidine and hypoxanthine rescue defined. MTA IC50 values (continuous exposure three population doublings) were: A549-640 nM, COR L23-28 nM, MCF7-52 nM and T47D-46 nM. Thymidine (1 microM) completely prevented growth inhibition at the MTA IC50 in all cell lines. At 10 x IC50, growth inhibition was only partially reversed by thymidine (< or = 10 microM); both thymidine and hypoxanthine (30 microM) being required for complete reversal, reflecting the multi-targeted nature of MTA. Growth inhibition by MTA was not affected by hypoxanthine alone. A non-toxic concentration (1 microM) of DP prevented thymidine/hypoxanthine rescue of MTA indicating that DP may potentiate MTA activity by preventing nucleoside and/or base salvage. Thymidine transport was inhibited by > or = 89% by 1 microM DP in all cell lines, whereas hypoxanthine transport was inhibited only in A549 and MCF7 cells. Therefore, prevention of end-product reversal of MTA-induced growth inhibition by DP can be explained by inhibition of thymidine transport alone.

Hypoxanthine uptake at the fetal side of human placenta proceeds through a nucleobase-preferring carrier and a non-saturable process

Uptake and metabolism of hypoxanthine by human placenta were studied using the single-circulation paired-tracer technique. In isolated cotyledons perfused through the fetal (basal) circulation, at mean pressures of 31.7 +/- 4.0 mmHg and mean flow rates maintained at 5.5 +/- 0.15 ml/min, the [3H]hypoxanthine uptake was 36 +/- 2.4 per cent (16.5 +/- 1.1 pmol/g wet weight). Hypoxanthine uptake was significantly inhibited by unlabelled (mM) hypoxanthine (0.5), adenine (0.5), guanine (0.5) and papaverine (15.0), but was unaffected by nitrobenzylthioinosine (0.01). Adenosine failed to inhibit hypoxanthine uptake. The kinetic analysis of hypoxanthine uptake showed it to be partially mediated by a saturable (apparent K(m) = 12.1 +/- 1.85 microns; Jmax = 7.1 +/- 0.52 nmol/min) and Na(+)-dependent mechanism. A greater fraction of hypoxanthine influx proceeded through a non-saturable process. Thin layer chromatographic analysis of venous perfusate after the intra-arterial injection of [3H]hypoxanthine showed a negligible degradation of nucleobase. These overall results show that hypoxanthine uptake at the fetal side of human placenta occurs by a saturable plus a non-saturable process. The carrier showed specificity for nucleobases and high affinity-low capacity for hypoxanthine. Since the fetal blood concentration of hypoxanthine is normally low, its uptake would be mediated by the high affinity transport system. Because the non-saturable mechanism can be operative at high concentrations of hypoxanthine, it may have primary importance to clear the nucleobase coming from the fetus during intrauterine hypoxia.

Selective potentiation of lometrexol growth inhibition by dipyridamole through cell-specific inhibition of hypoxanthine salvage

The novel antifolate lometrexol (5,10-dideazatetrahydrofolate) inhibits de novo purine biosynthesis, and co-incubation with hypoxanthine abolishes its cytotoxicity. The prevention of hypoxanthine rescue from an antipurine antifolate by the nucleoside transport inhibitor dipyridamole was investigated for the first time in nine human and rodent cell lines from seven different tissues of origin. In A549, HeLa and CHO cells, dipyridamole prevented hypoxanthine rescue and so growth was inhibited by the combination of lometrexol, dipyridamole and hypoxanthine, but in HT29, HCT116, KK47, MDA231, CCRF CEM and L1210 cells dipyridamole had no effect and the combination did not inhibit growth. Dipyridamole inhibited hypoxanthine uptake in A549 but not in CCRF CEM cells. Dipyridamole prevented the hypoxanthine-induced repletion of dGTP pools, depleted by lometrexol, in A549 but not in CCRF CEM cells. Thus, the selective growth-inhibitory effect of the combination of lometrexol, dipyridamole and hypoxanthine is apparently due to the dipyridamole sensitivity (ds) or insensitivity (di) of hypoxanthine transport. Both the human and murine leukaemic cells are of the di phenotype. If this reflects the transport phenotype of normal bone marrow it would suggest that the combination of lometrexol, dipyridamole and hypoxanthine might be selectively toxic to certain tumour types and have reduced toxicity to the bone marrow.

Hypoxanthine enters human vascular endothelial cells (ECV 304) via the nitrobenzylthioinosine-insensitive equilibrative nucleoside transporter

The transport properties of the nucleobase hypoxanthine were examined in the human umbilical vein endothelial cell line ECV 304. Initial rates of hypoxanthine influx were independent of extracellular cations: replacement of Na+ with Li+, Rb+, N-methyl-D-glucamine or choline had no significant effect on hypoxanthine uptake by ECV 304 cells. Kinetic analysis demonstrated the presence of a single saturable system for the transport of hypoxanthine in ECV 304 cells with an apparent K(m) of 320 +/- 10 microM and a Vmax of 5.6 +/- 0.9 pmol/10(6) cells per s. Hypoxanthine uptake was inhibited by the nucleosides adenosine, uridine and thymidine (apparent Ki 41 +/- 6, 240 +/- 27 and 59 +/- 8 microM respectively) and the nucleoside transport inhibitors nitrobenzylthioinosine (NBMPR), dilazep and dipyridamole (apparent Ki 2.5 +/- 0.3, 11 +/- 3 and 0.16 +/- 0.006 microM respectively), whereas the nucleobases adenine, guanine and thymine had little effect (50% inhibition at > 1 mM). ECV 304 cells were also shown to transport adenosine via both the NBMPR-sensitive and -insensitive nucleoside carriers. Hypoxanthine specifically inhibited adenosine transport via the NBMPR-insensitive system in a competitive manner (apparent Ki 290 +/- 14 microM). These results indicate that hypoxanthine entry into ECV 304 endothelial cells is mediated by the NBMPR-insensitive nucleoside carrier present in these cells.

Vesnarinone inhibits nucleoside and nucleobase transport

Vesnarinone is a novel synthetic oral inotropic agent that has been successfully used for treatment of patients with congestive heart failure. In addition to its cardiotonic activity, the drug has been proposed to have mild cytostatic and anti-HIV-1 effects. We have observed that vesnarinone profoundly inhibits radiolabeled thymidine and uridine incorporation into cells despite its modest inhibitory effect on DNA synthesis, RNA synthesis or cell proliferation. Here we demonstrate that vesnarinone inhibits both nucleoside and nucleobase transport in mammalian cells. This pharmacological action may be involved in some of its multiple biological effects.

Inhibition of purine nucleobase transport in human erythrocytes and cell lines by papaverine. Investigation of structure-activity relationship

Papaverine was found to be an effective inhibitor of hypoxanthine transport not only in human erythrocytes, but also in the human cell lines HL60 (myeloic) and U937 (monocytic). IC50 values for inhibition of hypoxanthine influx ranged from 6 to 20 microM. In erythrocytes papaverine was found to be a non-competitive inhibitor of hypoxanthine equilibrium-exchange transport with a Ki value of approximately 13 microM, which is in close agreement with the respective IC50 values estimated for zero-trans influx of hypoxanthine. In addition papaverine also had a slight inhibitory effect on unmediated nucleobase transport, most likely due to a perturbation of the membrane lipid environment. Several papaverine analogs were tested for their inhibitory effect on nucleobase transport. Only ethaverine was as effective as papaverine. Drotaverine and berberine were moderately inhibitory while laudanosine had no inhibitory effect at all. Isoquinoline acted as a very weak inhibitor.

Purine base transport in wild-type and mycophenolic acid-resistant Tritrichomonas foetus

The purine base transport systems of wild-type and mycophenolic acid-resistant (MPAR) Tritrichomonas foetus have been characterized. Wild-type T. foetus has two carriers, one for hypoxanthine (Km = 0.7 +/- 0.3 mM, Vm = 80 +/- 20 pmol microliters-1min-1) and guanine (Km = 0.09 +/- 0.02 mM, Vm = 17 +/- 3 pmol microliters-1min-1), and a second for xanthine (Km = 0.6 +/- 0.2 mM, Vm = 25 +/- 5 pmol microliters-1min-1). Adenine transport was not saturable (k = 0.16 +/- 0.01 min-1) and therefore appears to enter the parasite by passive diffusion through the membrane. T. foetus MPAR has lost the hypoxanthine/guanine transporter. Xanthine and adenine transport are similar in wild-type and MPAR T. foetus. No purine nucleoside transporter could be identified.

Na+-dependent and -independent transport of uridine and its phosphorylation in mouse spleen cells

Rapid kinetic techniques were used to study the transport and salvage of uridine and other nucleosides in mouse spleen cells. Spleen cells express two nucleoside transport systems: (1) the non-concentrative, symmetrical, Na+-independent transporter with broad substrate specificity, which has been found in all mammalian cells and is sensitive to inhibition by dipyridamole and nitrobenzylthioinosine; and (2) a Na+-dependent nucleoside transport, which is specific for uridine and purine nucleosides and resistant to inhibition by dipyridamole and nitrobenzylthioinosine. The kinetic properties of the two transporters were determined by measuring uridine influx in ATP-depleted cells and dipyridamole-treated cells, respectively. The Michaelis-Menten constants for Na+-independent and -dependent transport were about 40 and 200 microM, respectively, but the first-order rate constants were about the same for both transport systems. Nitrobenzylthioinosine-sensitivity of the facilitated nucleoside transporter correlated with the presence of about 10,000 high-affinity (Kd = 0.6 nM) nitrobenzylthioinosine-binding sites per cell. The turnover number of the nitrobenzylthioinosine-sensitive nucleoside transporter was comparable to that of mouse P388 leukemia cells. The activation energy of this transporter was 20 kcal/mol. Entry of uridine via either of the transport routes was rapidly followed by its phosphorylation and conversion to UTP. The Michaelis-Menten constant for the in situ phosphorylation of uridine was about 50 microM and the first-order rate constants for phosphorylation and transport were about the same. The spleen cells also efficiently salvaged adenosine, adenine, and hypoxanthine, but not thymidine.

Membrane transport and the antineoplastic action of nucleoside analogues

This article summarizes recent studies characterizing nucleoside transport in mammalian cells and discusses evidence for a role of membrane transport in the pharmacologic action of nucleoside analogues. Some of these studies have also addressed the controversy concerning the multiplicity in transport routes. It seems clear that erythrocytes and, perhaps, some other mammalian cells possess a single, broadly specific system for transporting nucleosides. However, substantial evidence from valid studies discriminating between transport and intracellular metabolism suggests that at least some mammalian cells, including some tumor cells, possess more than a single system. Evidence now exists for a determining role of membrane transport of nucleoside analogues in their cytotoxicity and, in the case of one pyrimidine nucleoside (AraC), in therapeutic responsiveness in leukemic patients. There are also numerous examples of transport-related resistance to nucleoside analogues. Included in this article are the results of studies from the authors' laboratory pertaining to the therapeutic activity of the purine nucleoside, FAraA, in murine tumor models. These studies provide evidence for a determining role of both membrane transport and intracellular phosphorylation in the selective antitumor action of this agent against murine leukemia. Substantially increased transport inward of FAraA occurs at pharmacologically achievable concentrations of this agent in tumor cells as compared to drug-limiting, normal proliferative epithelium of the small intestine. The basis for this differential appears to be the kinetic duality of FAraA and adenosine transport inward found in tumor cells, but not in proliferative intestinal epithelial cells. Tumor cells have highly saturable (low influx Km) and poorly saturable (high influx Km) systems for adenosine transport, both of which are shared by FAraA. In contrast, proliferative epithelial cells have only a poorly saturable system for these substrates. If a similar kinetic duality of nucleoside transport is found in other tumor cells certain implications arise concerning the significance of the duality to neoplastic transformation.

Purine and pyrimidine transport and permeation in human erythrocytes

Time courses of the uptake of radiolabeled hypoxanthine, adenine and uracil were measured by rapid kinetic techniques over substrate ranges from 0.02 to 5000 microM in suspensions of human erythrocytes at 25 or 30 degrees C. At concentrations above 25 microM, the rate of intracellular phosphoribosylation of hypoxanthine and adenine was insignificant relative to their rates of entry into the cell and time courses of transmembrane equilibration of the substrates could be measured and analyzed by integrated rate analysis. Hypoxanthine and uracil are transported by simple facilitated carriers with directional symmetry, high capacity and Michaelis-Menten constants of about 0.2 and 5 mM, respectively. Adenine is probably transported by a carrier with similar properties but no saturability was detectable up to a concentration of 5 mM. Cytosine entered the cells much more slowly than the other three nucleobases, and its entry seems not to be mediated by a carrier. The hypoxanthine transporter resembles that of one group of mammalian cell lines, which does not exhibit any overlap with the nucleoside transporter and is resistant to inhibitors of nucleoside transport. Results from studies on the effects of the nucleobases on the influx and countertransport of each other were complex and did not allow unequivocal conclusions as to the number of independent carriers involved. At concentrations below 5 microM, radiolabel from adenine and hypoxanthine accumulated intracellularly to higher than equilibrium levels. Part of this accumulation reflected metabolic trapping, especially when the medium contained 50 mM phosphate. But part was due to an apparent concentrative accumulation of free adenine and hypoxanthine up to 3-fold at medium concentrations much less than 1 microM and when cells were incubated in phosphate-free medium. This concentrative accumulation could be due to the functioning of additional high-affinity, low-capacity, active transport systems for adenine and hypoxanthine, but other factors could be responsible, such as saturable binding to intracellular components.

Genetic demonstration that the mutationally expressed nucleobase transporter of mouse S49 cells is nonconcentrative

Somatic cell genetic analysis of purine base transporters in mouse S49 cells has demonstrated the existence of a unique high-affinity purine base transporter, which is mutationally expressed and is not found in wild-type S49 cells or any other cells of the animal kingdom (B. Aronow, et al. (1986) Mol. Cell. Biol. 6, 2957). In order to determine whether this nucleobase transport system is active and concentrative, a secondary mutation in hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) was inserted into the cell line expressing this novel base transporter. The HGPRTase-deficient cells were capable of transporting hypoxanthine at increased rates but did not accumulate the base to concentrations in excess of that in the culture medium. Moreover, neither sodium azide nor ouabain had significant effects on hypoxanthine transport rates, indicating that energy metabolism and the maintenance of a sodium gradient were not required for transport function. These studies suggest that the novel mutationally expressed base transporter is independent of subsequent metabolism and does not require energy or a functioning Na+-K+-dependent ATPase activity.

Comparison of the equilibrium exchange of nucleosides and 3-O-methylglucose in human erythrocytes and of the effects of cytochalasin B, phloretin and dipyridamole on their transport

Because of similarities in the physical and molecular properties of the nucleoside and sugar transporters of human erythrocytes and the photoaffinity labeling of the sugar transporter by 8-azidoadenosine (Jarvis et al. (1986) J. Biol. Chem. 261, 11077-11085), we have directly compared the equilibrium exchange of uridine and 3-O-methylglucose in these cells as measured by rapid kinetic techniques under identical experimental conditions. Both the Michaelis-Menten constant and maximum velocity were about 100-fold higher for 3-O-methylglucose exchange than for uridine exchange so that the first order rate constants for both transporters were about the same. When calculated on the basis of the number of nucleoside and sugar carriers per red cell estimated by equilibrium binding of nitrobenzylthioinosine and cytochalasin B, respectively, the turnover numbers for the sugar and nucleoside carriers with 3-O-methylglucose and uridine, respectively, as substrates were quite similar. Various sugars up to concentrations of 108 mM had no effect on the exchange of 500 microM uridine or adenosine, and uridine up to a concentration of 50 mM had no effect on the exchange of 10 mM 3-O-methylglucose. Adenosine, on the other hand, inhibited 3-O-methylglucose exchange in a concentration dependent manner, though not very effectively (IC50 approximately equal to 3 mM). Both uridine and 3-O-methylglucose exchange were inhibited in a concentration dependent manner by cytochalasin B, phloretin and dipyridamole, but cytochalasin B and phloretin were 100-times more effective in inhibiting 3-O-methylglucose than uridine exchange, whereas the opposite was the case for the inhibition by dipyridamole.

Expression of the high-affinity purine nucleobase transporter in mutant mouse S49 cells does not require a functional wild-type nucleoside-nucleobase transporter

A novel type of somatic mutation that causes the expression of a high-affinity purine base permease (B. Aronow, D. Toll, J. Patrick, P. Hollingsworth, K. McCartan, and B. Ullmann, Mol. Cell Biol. 6:2957-2962, 1986) has been inserted into nucleoside transport-deficient S49 cells. Two classes of mutants expressing this nucleobase permease were generated. The first, as exemplified by the AE1HADPAB2 cell line, possessed an augmented capacity to transport low concentrations of the three purine bases, hypoxanthine, guanine, and adenine. The second class of mutants, as typified by the AE1HADPAB5 clone, possessed an augmented capability to translocate low levels of hypoxanthine and guanine, but not adenine. Neither the AE1HADPAB2 nor the AE1HADPAB5 cells could transport nucleosides, suggesting that the expression of the high-affinity base transporter did not reverse the mutation in the nucleoside transport system. The transport of purine bases by both AE1HADPAB2 and AE1HADPAB5 cells was much less sensitive than that by wild-type cells to inhibition by dipyridamole, 4-nitrobenzylthionosine, and N-ethylmaleimide, potent inhibitors of nucleoside and nucleobase transport in wild-type S49 cells. Fusion of the AE1HADPAB2 and AE1HADPAB5 cell lines with wild-type cells indicated that the expression of the high-affinity base transporter behaved in a dominant fashion, while the nucleoside transport deficiency was a recessive trait. These data suggest that the high-affinity purine base transporter of mutant cells and the nucleoside transport function of wild-type cells are products of different genes and that expression of the former probably requires the unmasking or alteration of a specific genetic locus that is silent or different in wild-type cells.

Nucleoside and nucleobase transport and metabolism in wild type and nucleoside transport-deficient Aedes albopictus cells

Nucleoside and nucleobase transport and metabolism were measured in ATP-depleted and normal Aedes albopictus mosquito cells (line C-7-10) by rapid kinetic techniques. The cells possess a facilitated diffusion system for nucleosides, which in its broad substrate specificity and kinetic properties resembles that present in many types of mammalian cells. The Michaelis-Menten constant for uridine transport at 28 degrees C is about 180 microM. However, the nucleoside transporter of the mosquito cells is resistant to inhibition by nmolar concentrations of nitrobenzylthioinosine and the cells lack high affinity nitrobenzylthioinosine binding sites. The cells also possess an adenine transporter, which is distinct from the nucleoside transporter. They lack, however, a hypoxanthine transport system and are deficient in hypoxanthine phosphoribosyltransferase activity, which explains their failure to efficiently salvage hypoxanthine from the medium. The cells possess uridine and thymidine phosphorylase activities and, in contrast to cultured mammalian cells, efficiently convert uracil to nucleotides. An adenosine-resistant variant (CAE-3-6) of the C-7-10 cell line is devoid of significant nucleoside transport activity but transports adenine normally. Residual entry of various nucleosides into these cells and of hypoxanthine and cytosine into wild type and mutant cells is strictly non-mediated. The rate of permeation of various nucleosides and of hypoxanthine into the CAE-3-6 cells is related to their hydrophobicity. Uridine permeation into CAE-3-6 cells exhibits an activation energy of about 20 kcal/mol. At high uridine concentrations permeation is sufficiently rapid to partly overcome the limitation in nucleoside salvage imposed by the nucleoside transport defect in these cells.

Expression of a novel high-affinity purine nucleobase transport function in mutant mammalian T lymphoblasts

The single nucleoside transport function of mouse S49 lymphoblasts also transports purine bases (B. Aronow and B. Ullman, J. Biol. Chem. 261:2014-2019, 1986). This transport of purine bases by S49 cells is sensitive to inhibition by dipyridamole (DPA) and 4-nitrobenzylthioinosine, two potent inhibitors of nucleoside transport. Therefore, wild-type S49 cells cannot salvage low hypoxanthine concentrations in the presence of 10 microM DPA and 11 microM azaserine; the latter is a potent inhibitor of purine biosynthesis. Among a mutagenized wild-type population, a cell line, JPA2, was isolated which could proliferate in 50 microM hypoxanthine-11 microM azaserine-10 microM DPA. The basis for the survival of JPA2 cells under these selective conditions was expression of a unique, high-affinity purine nucleobase transport function not present in wild-type cells. JPA2 cells could transport 5 microM concentrations of hypoxanthine, guanine, and adenine 15- to 30-fold more efficiently than parental cells did. Kinetic analyses revealed that the affinity of the JPA2 transporter for all three purine bases was much greater than that of the wild-type nucleobase transport system. Moreover, nucleobase transport in JPA2 cells, unlike that in parental cells, was insensitive to inhibition by DPA, 4-nitrobenzylthioinosine, sulfhydryl reagents, and nucleosides. No alterations in nucleoside transport capability, phosphoribosylpyrophosphate levels, or purine phosphoribosyltransferase enzymes were detected in JPA2 cells. Thus, JPA2 cells express a novel nucleobase transport capability which can be distinguished from the nucleoside transport function by multiple biochemical parameters.

Incomplete nucleoside transport deficiency with increased hypoxanthine transport capability in mutant T-lymphoblastoid cells

From a mutagenized population of wild-type mouse (S49) T-lymphoma cells, a clone, 80-5D2, was isolated in a single step by virtue of its ability to survive in 80 nM 5-fluorouridine. Unlike previously isolated nucleoside transport-deficient cell lines (A. Cohen, B. Ullman, and D. W. Martin, Jr., J. Biol. Chem. 254:112-116, 1979), 80-5D2 cells were only slightly less sensitive to growth inhibition by a variety of cytotoxic nucleosides and were capable of proliferating in hypoxanthine-amethopterin-thymidine-containing medium. The molecular basis for the phenotype of 80-5D2 cells was incomplete deficiency in the ability of the mutant cells to translocate nucleosides across the plasma membrane. Interestingly, mutant cells were more capable than wild-type cells of transporting the nucleobase hypoxanthine. Residual transport of adenosine into 80-5D2 cells was just as sensitive to inhibition by nucleosides and more sensitive to inhibition by hypoxanthine than that in wild-type cells, indicating that the phenomena of ligand binding and translocation can be uncoupled genetically. The 80-5D2 cells lacked cell surface binding sites for the potent inhibitor of nucleoside transport p-nitrobenzylthioinosine (NBMPR) and, consequently, were largely resistant to the physiological effects of NBMPR. However, the altered transporter retained its sensitivity to dipyridamole, another inhibitor of nucleoside transport. The biochemical phenotype of the 80-5D2 cell line supports the hypothesis that the determinants that comprise the nucleoside carrier site, the hypoxanthine carrier site, the NBMPR binding site, and the dipyridamole binding site of the nucleoside transport function of mouse S49 cells are genetically distinguishable.

Expression of the high-affinity purine nucleobase transporter in mutant mouse S49 cells does not require a functional wild-type nucleoside-nucleobase transporter

A novel type of somatic mutation that causes the expression of a high-affinity purine base permease (B. Aronow, D. Toll, J. Patrick, P. Hollingsworth, K. McCartan, and B. Ullmann, Mol. Cell Biol. 6:2957-2962, 1986) has been inserted into nucleoside transport-deficient S49 cells. Two classes of mutants expressing this nucleobase permease were generated. The first, as exemplified by the AE1HADPAB2 cell line, possessed an augmented capacity to transport low concentrations of the three purine bases, hypoxanthine, guanine, and adenine. The second class of mutants, as typified by the AE1HADPAB5 clone, possessed an augmented capability to translocate low levels of hypoxanthine and guanine, but not adenine. Neither the AE1HADPAB2 nor the AE1HADPAB5 cells could transport nucleosides, suggesting that the expression of the high-affinity base transporter did not reverse the mutation in the nucleoside transport system. The transport of purine bases by both AE1HADPAB2 and AE1HADPAB5 cells was much less sensitive than that by wild-type cells to inhibition by dipyridamole, 4-nitrobenzylthionosine, and N-ethylmaleimide, potent inhibitors of nucleoside and nucleobase transport in wild-type S49 cells. Fusion of the AE1HADPAB2 and AE1HADPAB5 cell lines with wild-type cells indicated that the expression of the high-affinity base transporter behaved in a dominant fashion, while the nucleoside transport deficiency was a recessive trait. These data suggest that the high-affinity purine base transporter of mutant cells and the nucleoside transport function of wild-type cells are products of different genes and that expression of the former probably requires the unmasking or alteration of a specific genetic locus that is silent or different in wild-type cells.

Expression of a novel high-affinity purine nucleobase transport function in mutant mammalian T lymphoblasts

The single nucleoside transport function of mouse S49 lymphoblasts also transports purine bases (B. Aronow and B. Ullman, J. Biol. Chem. 261:2014-2019, 1986). This transport of purine bases by S49 cells is sensitive to inhibition by dipyridamole (DPA) and 4-nitrobenzylthioinosine, two potent inhibitors of nucleoside transport. Therefore, wild-type S49 cells cannot salvage low hypoxanthine concentrations in the presence of 10 microM DPA and 11 microM azaserine; the latter is a potent inhibitor of purine biosynthesis. Among a mutagenized wild-type population, a cell line, JPA2, was isolated which could proliferate in 50 microM hypoxanthine-11 microM azaserine-10 microM DPA. The basis for the survival of JPA2 cells under these selective conditions was expression of a unique, high-affinity purine nucleobase transport function not present in wild-type cells. JPA2 cells could transport 5 microM concentrations of hypoxanthine, guanine, and adenine 15- to 30-fold more efficiently than parental cells did. Kinetic analyses revealed that the affinity of the JPA2 transporter for all three purine bases was much greater than that of the wild-type nucleobase transport system. Moreover, nucleobase transport in JPA2 cells, unlike that in parental cells, was insensitive to inhibition by DPA, 4-nitrobenzylthioinosine, sulfhydryl reagents, and nucleosides. No alterations in nucleoside transport capability, phosphoribosylpyrophosphate levels, or purine phosphoribosyltransferase enzymes were detected in JPA2 cells. Thus, JPA2 cells express a novel nucleobase transport capability which can be distinguished from the nucleoside transport function by multiple biochemical parameters.

Inhibition of nucleoside and nucleobase transport and nitrobenzylthioinosine binding by dilazep and hexobendine

The transport of 500 microM uridine by human erythrocytes and S49, P388 and L1210 mouse leukemia cells, Chinese hamster ovary (CHO) cells and Novikoff rat hepatoma cells was inhibited strongly by dilazep and hexobendine with similar concentration dependence, but the sensitivity of transport in the various cell types varied greatly; IC50 values ranged from 5-30 nM for human erythrocytes and S49 and P388 cells to greater than 1 microM for CHO and Novikoff cells. The binding of nitrobenzylthioinosine (NBTI) to high-affinity sites on these cells (Kd approximately equal to 0.5 nM) was inhibited by hexobendine and dilazep in a similar pattern. Furthermore, these drugs, just as dipyridamole and papaverine, inhibited the dissociation of NBTI from high-affinity binding sites but only at concentrations 10-100 times higher than those inhibiting uridine transport. In contrast, high uridine concentrations (greater than 2 mM) accelerated the dissociation of NBTI. Dilazep also inhibited the transport of hypoxanthine, but only in those cell lines whose transporter is sensitive to inhibition by uridine and dipyridamole. Adenine transport was not inhibited significantly by dilazep in any of the cell lines tested, except for a slight inhibition in Novikoff cells. [14C]Hexobendine equilibrated across the plasma membrane in human erythrocytes within 2 sec of incubation at 25 degrees, but accumulated to 6-10 times the extracellular concentration in cells of the various cultured lines. Uptake was not affected by high concentrations of uridine, NBTI or dipyridamole. Hexobendine inhibited the growth of various cell lines to a lesser extent (IC50 = greater than or equal to 100 microM) than dipyridamole (IC50 = 15-40 microM). At 40 microM, however, it completely inhibited the growth of S49 cells that had been made nucleoside dependent by treatment with methotrexate or pyrazofurin.

Adenosine transporters in chromaffin cells. Quantification by dipyridamol monoacetate

Chromaffin cells from bovine adrenal medulla are a useful model to approach adenosine transport and metabolism in neural cells. Dipyridamol has been shown to be an adenosine transport inhibitor with high affinity. To quantify the adenosine transporters a labelled dipyridamol analogue, [14C]dipyridamol acetate, was synthesized. This compound had a Ki = 5.3 +/- 0.43 nM according to the Dixon method, and 4.58 +/- 0.46 nM when the receptor number molarity was taken into account showing, like dipyridamol, a non-competitive mechanism. The high-affinity receptors present in chromaffin cells showed a Kd = 6.8 +/- 0.8 nM and the receptor number was 630 000 +/- 40 000 per cell.

Incomplete nucleoside transport deficiency with increased hypoxanthine transport capability in mutant T-lymphoblastoid cells

From a mutagenized population of wild-type mouse (S49) T-lymphoma cells, a clone, 80-5D2, was isolated in a single step by virtue of its ability to survive in 80 nM 5-fluorouridine. Unlike previously isolated nucleoside transport-deficient cell lines (A. Cohen, B. Ullman, and D. W. Martin, Jr., J. Biol. Chem. 254:112-116, 1979), 80-5D2 cells were only slightly less sensitive to growth inhibition by a variety of cytotoxic nucleosides and were capable of proliferating in hypoxanthine-amethopterin-thymidine-containing medium. The molecular basis for the phenotype of 80-5D2 cells was incomplete deficiency in the ability of the mutant cells to translocate nucleosides across the plasma membrane. Interestingly, mutant cells were more capable than wild-type cells of transporting the nucleobase hypoxanthine. Residual transport of adenosine into 80-5D2 cells was just as sensitive to inhibition by nucleosides and more sensitive to inhibition by hypoxanthine than that in wild-type cells, indicating that the phenomena of ligand binding and translocation can be uncoupled genetically. The 80-5D2 cells lacked cell surface binding sites for the potent inhibitor of nucleoside transport p-nitrobenzylthioinosine (NBMPR) and, consequently, were largely resistant to the physiological effects of NBMPR. However, the altered transporter retained its sensitivity to dipyridamole, another inhibitor of nucleoside transport. The biochemical phenotype of the 80-5D2 cell line supports the hypothesis that the determinants that comprise the nucleoside carrier site, the hypoxanthine carrier site, the NBMPR binding site, and the dipyridamole binding site of the nucleoside transport function of mouse S49 cells are genetically distinguishable.

S49 mouse lymphoma cells are deficient in hypoxanthine transport

The rate of uptake of hypoxanthine in S49 cells was only about 2-5% of the rate of hypoxanthine transport observed in many other types of mammalian cells, and of the rate of uridine transport in this and other cell types. Part of the slow entry of hypoxanthine seems to be due to non-mediated permeation, but the remainder is saturable, strongly inhibited by uridine, nitrobenzylthioinosine and dipyridamole and not detectable in a nucleoside-transport-deficient mutant of S49 cells (AE1). The inhibition of hypoxanthine transport in S49 cells by nitrobenzylthioinosine resembles the inhibition of nucleoside transport in these and other mammalian cells, whereas it contrasts with the resistance of hypoxanthine transport to nitrobenzylthioinosine in all types of mammalian cells that have been investigated. We conclude that S49 cells lack the hypoxanthine transport system common to other types of cells and that hypoxanthine entry into these cells is mediated, although very inefficiently, by the nucleoside transporter. In contrast, adenine transport in S49 and AE1 cells was comparable to that in other types of cells.