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(1/215) Functional production and reconstitution of the human equilibrative nucleoside transporter (hENT1) in Saccharomyces cerevisiae. Interaction of inhibitors of nucleoside transport with recombinant hENT1 and a glycosylation-defective derivative (hENT1/N48Q).

We have produced recombinant human equilibrative nucleoside transporter (hENT1) in the yeast Saccharomyces cerevisiae and have compared the binding of inhibitors of equilibrative nucleoside transport with the wild-type transporter and a N-glycosylation-defective mutant transporter. Equilibrium binding of 3H-labelled nitrobenzylmercaptopurine ribonucleoside {6-[(4-nitrobenzyl)thio]-9-beta-d-ribofuranosyl purine; NBMPR} to hENT1-producing yeast revealed a single class of high-affinity sites that were shown to be in membrane fractions by (1) equilibrium binding (means+/-S.D.) of [3H]NBMPR to intact yeast (Kd 1.2+/-0.2 nM; Bmax 5.0+/-0.5 pmol/mg of protein) and membranes (Kd 0.7+/-0.2 nM; Bmax 6.5+/-1 pmol/mg of protein), and (2) reconstitution of hENT1-mediated [3H]thymidine transport into proteoliposomes that was potently inhibited by NBMPR. Dilazep and dipyridamole inhibited NBMPR binding to hENT1 with IC50 values of 130+/-10 and 380+/-20 nM respectively. The role of N-linked glycosylation in the interaction of NBMPR with hENT1 was examined by the quantification of binding of [3H]NBMPR to yeast producing either wild-type hENT1 or a glycosylation-defective mutant (hENT1/N48Q) in which Asn-48 was converted into Gln. The Kd for binding of NBMPR to hENT1/N48Q was 10. 5+/-1.6 nM, indicating that the replacement of an Asn residue with Gln decreased the affinity of hENT1 for NBMPR. The decreased affinity of hENT1/N48Q for NBMPR was due to an increased rate of dissociation (koff) and a decreased rate of association (kon) of specifically bound [3H]NBMPR because the values for hENT1-producing and hENT1/N48Q-producing yeast were respectively 0.14+/-0.02 and 0. 36+/-0.05 min-1 for koff, and (1.2+/-0.1)x10(8) and (0.40+/-0. 04)x10(8) M-1.min-1 for kon. These results indicated that the conservative conversion of an Asn residue into Gln at position 48 of hENT1 and/or the loss of N-linked glycosylation capability altered the binding characteristics of the transporter for NBMPR, dilazep and dipyridamole.  (+info)

(2/215) Quantification of extracellular and intracellular adenosine production: understanding the transmembranous concentration gradient.

BACKGROUND: Inhibitors of adenosine membrane transport cause vasodilation and enhance the plasma adenosine concentration. However, it is unclear why the plasma adenosine concentration rises rather than falls when membrane transport is inhibited. We tested the hypothesis that the cytosolic adenosine concentration exceeds the interstitial concentration under well-oxygenated conditions. METHODS AND RESULTS: In isolated, isovolumically working guinea pig hearts (n=50), the release rate of adenosine and accumulation of S-adenosylhomocysteine (after 20 minutes of 200 micromol/L homocysteine), a measure of the free cytosolic adenosine concentration, were determined in the absence and presence of specific and powerful blockers of adenosine membrane transport (nitrobenzylthioinosine 1 micromol/L), adenosine deaminase (erythro-9-hydroxy-nonyl-adenine 5 micromol/L), and adenosine kinase (iodotubericidine 10 micromol/L). Data analysis with a distributed multicompartment model revealed a total cardiac adenosine production rate of 2294 pmol. min-1. g-1, of which 8% was produced in the extracellular region. Because of a high rate of intracellular metabolism, however, 70.3% of extracellularly produced adenosine was taken up into cellular regions, an effect that was effectively eliminated by membrane transport block. The resulting approximately 2.8-fold increase of the interstitial adenosine concentration evoked near-maximal coronary dilation. CONCLUSIONS: We rejected the hypothesis that the cytosolic adenosine concentration exceeds the interstitial. Rather, there is significant extracellular production, and the parenchymal cell represents a sink, not a source, for adenosine under well-oxygenated conditions.  (+info)

(3/215) 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.  (+info)

(4/215) Effects of A1-adenosine receptor antagonists on purinergic transmission in the guinea-pig vas deferens in vitro.

1. Intracellularly recorded excitatory junction potentials (ej.ps) were used to study the effects of adenosine receptor antagonists on neurotransmitter release from postganglionic sympathetic nerve terminals in the guinea-pig vas deferens in vitro. 2. The A1 adenosine receptor antagonists, 8-phenyltheophylline (10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (0.1 microM), increased the amplitude of e.j.ps evoked during trains of 20 stimuli at 1 Hz in the presence, but not in the absence, of the alpha2-adrenoceptor antagonist, yohimbine (1 microM) or the non-selective alpha-adrenoceptor antagonist, phentolamine (1 microM). 3. Adenosine (100 microM) reduced the amplitude of e.j.ps, both in the presence and in the absence of phentolamine (1 microM). This inhibitory effect of adenosine is most likely caused by a reduction in transmitter release as there was no detectable change in spontaneous ej.p. amplitudes. 4. In the presence of phentolamine, application of the adenosine uptake inhibitor, S-(p-nitrobenzyl)-6-thioinosine (0.1 microM), had no effect on ej.p. amplitudes. 5. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (100 microM), significantly increased the amplitudes of all e.j.ps evoked during trains of 20 stimuli at 1 Hz, both in the presence and in the absence of phentolamine (1 microM). 6. These results suggest that endogenous adenosine modulates neurotransmitter release by an action at prejunctional A1 adenosine receptors only when alpha2-adrenoceptors are blocked.  (+info)

(5/215) Metabolism and selective toxicity of 6-nitrobenzylthioinosine in Toxoplasma gondii.

The purine nucleoside analogue NBMPR (nitrobenzylthioinosine or 6-[(4-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine) was selectively phosphorylated to its nucleoside 5'-monophosphate by Toxoplasma gondii but not mammalian adenosine kinase (EC 2.7.1.20). NBMPR was also cleaved in toxoplasma to its nucleobase, nitrobenzylmercaptopurine. However, nitrobenzylmercaptopurine was not a substrate for either adenosine kinase or hypoxanthine-guanine-xanthine phosphoribosyltransferase (EC 2.4.2.8). Because of this unique and previously unknown metabolism of NBMPR by the parasite, the effect of NBMPR as an antitoxoplasmic agent was tested. NBMPR killed T. gondii grown in human fibroblasts in a dose-dependent manner, with a 50% inhibitory concentration of approximately 10 microM and without apparent toxicity to host cells. Doses of up to 100 microM had no significant toxic effect on uninfected host cells. The promising antitoxoplasmic effect of NBMPR led to the testing of other 6-substituted 9-beta-D-ribofuranosylpurines, which were shown to be good ligands of the parasite adenosine kinase (M. H. Iltzsch, S. S. Uber, K. O. Tankersley, and M. H. el Kouni, Biochem. Pharmacol. 49:1501-1512, 1995), as antitoxoplasmic agents. Among the analogues tested, 6-benzylthioinosine, p-nitrobenzyl-6-selenopurine riboside, N(6)-(p-azidobenzyl)adenosine, and N(6)-(p-nitrobenzyl)adenosine, like NBMPR, were selectively toxic to parasite-infected cells. Thus, it appears that the unique characteristics of purine metabolism in T. gondii render certain 6-substituted 9-beta-D-ribofuranosylpurines promising antitoxoplasmic drugs.  (+info)

(6/215) 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.  (+info)

(7/215) Retroviral transfer of the hENT2 nucleoside transporter cDNA confers broad-spectrum antifolate resistance in murine bone marrow cells.

Antifolate drugs such as methotrexate are commonly used in cancer chemotherapy. It may be possible to increase the antitumor activity of antifolates by the coadministration of drugs that inhibit nucleoside transport, thereby blocking the capacity of tumor cells to salvage nucleotide precursors. An important limitation of this approach is severe myelosuppression caused by many of these drug combinations. For this reason, we have developed a gene therapy strategy to protect bone marrow cells against combined treatment with antifolates and nitrobenzylmercaptopurine riboside (NBMPR), a potent inhibitor of the es nucleoside transporter. A retroviral vector (MeiIRG) was constructed that expressed the NBMPR-insensitive ei transporter, hypothesizing that transduced bone marrow cells would survive drug treatment because of the preservation of nucleoside salvage pathways. In vitro clonogenic assays confirmed that the MeiIRG vector did protect myeloid progenitors against the toxic effects of 3 different antifolates when each was combined with NBMPR. On testing this system in vivo, decreased myelosuppression was observed in mice transplanted with MeiIRG-transduced bone marrow cells and subsequently treated with trimetrexate and NBMPR-P. In these mice, significant increases were noted in absolute neutrophil count nadirs, reticulocyte indices, and the numbers of myeloid progenitors in the bone marrow. Furthermore, a survival advantage was associated with transfer of the MeiIRG vector, indicating that significant dose intensification was possible with this approach. In summary, the MeiIRG vector can decrease the toxicity associated with the combined use of antifolates and NBMPR-P and thereby may provide a strategy for simultaneously sensitizing tumor cells while protecting hematopoietic cells.  (+info)

(8/215) Antiangiogenic scheduling of chemotherapy improves efficacy against experimental drug-resistant cancer.

To reveal the antiangiogenic capability of cancer chemotherapy, we developed an alternative antiangiogenic schedule for administration of cyclophosphamide. We show here that this antiangiogenic schedule avoided drug resistance and eradicated Lewis lung carcinoma and L1210 leukemia, an outcome not possible with the conventional schedule. When Lewis lung carcinoma and EMT-6 breast cancer were made drug resistant before therapy, the antiangiogenic schedule suppressed tumor growth 3-fold more effectively than the conventional schedule. When another angiogenesis inhibitor, TNP-470, was added to the antiangiogenic schedule of cyclophosphamide, drug-resistant Lewis lung carcinomas were eradicated. Each dose of the antiangiogenic schedule of cyclophosphamide induced the apoptosis of endothelial cells within tumors, and endothelial cell apoptosis preceded the apoptosis of drug-resistant tumor cells. This antiangiogenic effect was more pronounced in p53-null mice in which the apoptosis of p53-null endothelial cells induced by cyclophosphamide was so vigorous that drug-resistant tumors comprising 4.5% of body weight were eradicated. Thus, by using a dosing schedule of cyclophosphamide that provided more sustained apoptosis of endothelial cells within the vascular bed of a tumor, we show that a chemotherapeutic agent can more effectively control tumor growth in mice, regardless of whether the tumor cells are drug resistant.  (+info)