Enhancement of retention and cytotoxicity of 2-chlorodeoxyadenosine in cultured human leukemic lymphoblasts by nitrobenzylthioinosine, an inhibitor of equilibrative nucleoside transport

In leukemic cells exposed to 2-chlorodeoxyadenosine (2-CdA), levels of the nucleoside drug and its phosphate metabolites decay with time in the absence of external 2-CdA; an intrinsic part of this process is the efflux of 2-CdA. The effects of nitrobenzylthioinosine (NBMPR) and of dipyridamole (DPM), both potent inhibitors of es (e, equilibrative; s, sensitive to NBMPR) nucleoside transport processes, were studied in four lines of cultured leukemic lymphoblasts. Suspensions of 2-CdA-loaded cells were diluted 10-fold with 2-CdA-free medium to initiate the cellular 2-CdA decay processes, which followed a biexponential time course. When diluting media contained NBMPR or DPM, intracellular levels of 2-CdA and its metabolites were substantially increased (P < 0.001) compared with cells in media lacking the transport inhibitors, and 2-CdA loss followed a monoexponential time course. As a consequence, the AUCs (area under time-course plots of intracellular 2-CdA and its metabolites) were significantly (P < 0.001) lower in untreated control cells compared to inhibitor-treated cells. These results suggest that nucleoside transport processes contribute to the efflux of 2-CdA from the cultured lymphoblasts. The cytotoxicity of 1-h exposure to 2-CdA of Reh-A2 and CCRF-CEM cells was enhanced three-fold by subsequent exposure to 0.5 microM NBMPR relative to that of control cells subjected to the same manipulations without NBMPR exposure. However, before such a strategy may be considered to have a therapeutic application, careful examination of effects in normal lymphocytes and ex vivo leukemic lymphoblasts must first be undertaken. Leukemia (2000) 14, 52-60.

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.

Sensitivity of acute leukemia cells to cytarabine is a correlate of cellular es nucleoside transporter site content measured by flow cytometry with SAENTA-fluorescein

Cytarabine (araC) is converted to araC 5'-triphosphate after entering leukemia cells as a substrate for nucleoside transport processes. This study tested the relationship between araC cytotoxicity, measured in an in vitro tetrazolium dye reduction assay of cell viability, and the cellular abundance of es nucleoside transport elements, assayed by a flow cytometric method that used the es-specific stain, 5-(SAENTA-x8)-fluorescein (5-(Sx8)-F), in cultured leukemia cells and in myeloblasts and lymphoblasts (blasts) from leukemia patients. Cellular es site abundance (B(max) value for 5-(Sx8)-F binding) varied sixfold among nine leukemic myeloblast samples from patients. In cultured OCI/AML-2 myeloblasts and CCRF-CEM T-lymphoblasts, and in fresh leukemic blasts, es sites were fractionally blocked by treatment with graded concentrations of nitrobenzylthioinosine (NBMPR), an inhibitory es site ligand, to simulate the variation in es expression found in leukemic blasts from patients with acute myeloid leukemia. When the cytotoxicity of a single concentration of araC was determined in NBMPR-treated leukemia cells, cell kill correlated closely with the intensity of 5-(Sx8)-F fluorescence (r = .92 to .99), a measure of the cell surface abundance of functional es nucleoside transporter sites. Concentrations of NBMPR that achieved half-maximal reduction (4.3 to 12 nmol/L) of cellular 5-(Sx8)-F fluorescence (measured by flow cytometry) approximated IC50 values (1 to 10 nmol/L) previously found for inhibition by NBMPR of es-mediated nucleoside fluxes in several cell types, supporting the view that 5-(Sx8)-F interacted with the estransporter. The correlation of araC cytotoxicity and the B(max) for 5-(Sx8)-F binding to es sites in cultured leukemia cells and in leukemic blasts from acute leukemia patients (r = .95) suggests that the flow cytometry assay of es capacity may be useful in predicting clinical response to araC.

Measurement of nitrobenzylthioinosine in plasma and erythrocytes: a pharmacokinetic study in mice

PURPOSE

Nitrobenzylthioinosine (NBMPR), a potent inhibitor of nucleoside transport in many cell types, modulates the in vivo disposition of several cytotoxic nucleoside analogs. In this study, a radioligand binding assay was developed for measurement of the NBMPR content of plasma and erythrocytes.

METHODS

The assay was based on the competition between NBMPR and [3H]NBMPR for high-affinity sites on human erythrocytes membranes. With this assay, we followed in mice changes in the NBMPR content of blood plasma and erythrocytes, following the intraperitoneal injection of the disodium salt of NBMPR 5'-monophosphate (NBMPR-P), a prodrug form of NBMPR.

RESULTS

The radioligand binding assay was able to measure precisely as little as 2.5 pmol of NBMPR, allowing the direct determination of NBMPR concentrations in plasma as low as 16 nM. As few as 8 x 10(3) molecules of NBMPR per cell could be determined in erythrocytes. The NBMPR content of plasma from mice injected with NBMPR-P was maximal at about 20 min after injection and declined to < 0.2% of the peak value by 10 h. Erythrocyte-associated NBMPR was also maximal at 20 min, and declined to 11% of the peak value by 10 h after injection. Time courses for the disappearance of NBMPR from plasma and erythrocytes were monoexponential and yielded half-life values of 0.39 h and 0.68 h, respectively, an apparent volume of distribution of 0.61 l/kg, and a clearance of 1.1 l/h per kg.

CONCLUSIONS

The radioligand binding assay is a sensitive and facile method for monitoring NBMPR concentrations in mammalian plasma and tissue extracts.

Intracellular pharmacokinetics of 2-chlorodeoxyadenosine in leukemia cells from patients with chronic lymphocytic leukemia

2-Chlorodeoxyadenosine (2-CdA) is an important agent in the treatment of hairy cell leukemia and chronic lymphocytic leukemia (CLL). Others have reported that levels of 2-CdA phosphates present in human leukemia cells decline rapidly when the cells are in 2-CdA-free medium (Santana et al. J Clin Oncol 1991; 9: 416-422). In the present study, time-courses of 2-CdA loss from CLL cells were biexponential: the mean half-life of the initial phase was 0.30 +/- 0.18 h; the presence of 0.5 microM nitrobenzylthioinosine (NBMPR, a classical inhibitor of nucleoside transport) in the suspending medium, significantly decreased the initial rate of 2-CdA efflux (mean half-life, 0.43 +/- 0.22 h). As a consequence, AUCs (areas under time-course plots) were significantly higher in the NBMPR-treated cells (4.56 +/- 2.01 pmol.h/10(6) cells, n = 19) than in untreated control cells (3.83 +/- 1.74 pmol.h/10(6) cells; n = 19). 2-CdA was the principal efflux product released into the medium from 2-CdA-loaded CLL cells. We conclude that nucleoside transport processes contribute to the efflux of 2-CdA from CLL cells and that NBMPR may be useful as a retentive agent.

Cloning and functional expression of a complementary DNA encoding a mammalian nucleoside transport protein

Expression screening in Xenopus oocytes was used to isolate a cDNA from rat jejunal epithelium encoding a Na(+)-dependent nucleoside transport protein (named cNT1). The cDNA sequence of cNT1 predicts a protein of 648 amino acids (relative molecular mass 71,000) with 14 potential transmembrane domains. Data base searches indicate significant sequence similarity to the NUPC proton/nucleoside symporter of Escherichia coli. There is no sequence similarity between cNT1 and proteins of mammalian origin. Functionally, cNT1 exhibited the transport characteristics of the nucleoside transport system cit (selective for pyrimidine nucleosides and adenosine) and accepted both 3'-azido-3'-deoxythymidine (AZT) and 2',3'-dideoxycytidine (ddC) as permeants (Km = 0.49 and 0.51 mM, respectively). The demonstration of transport of AZT by cNT1 expressed in Xenopus oocytes provides the first direct evidence that AZT enters cells by transporter-mediated processes, as well as by passive diffusion. Consistent with the tissue distribution of system cit transport activity, transcripts for cNT1 were detected in kidney as well as jejunum. cNT1 therefore belongs to a potential new gene family and may be involved in the intestinal absorption and renal handling of pyrimidine nucleoside analogs used to treat acquired immunodeficiency syndrome (AIDS).

Multiple myeloma: expression of nucleoside transporters on malignant plasma cells and their relationship to cellular proliferation

The expression of nucleoside transporters is a limiting factor in the pharmacology of the nucleoside analogue, cytosine arabinoside (AraC) and is associated with cellular proliferation. We investigated the expression of nucleoside transporters on plasma cells from the bone marrow of 51 patients with multiple myeloma by 2-colour immunofluorescence flow cytometry, utilising 5-(SAENTA-x8)-fluorescein, a fluorescent ligand for the nucleoside transporter and anti-CD38 conjugated to phycoerythrin, as CD38 expression has unique characteristics on plasma cells. Mean nucleoside transporter expression on bone marrow plasma cells from patients with myeloma (1777 +/- 2181 transporters/plasma cell) was not significantly different from expression on plasma cells from normal bone marrow (997 +/- 1096 transporters/plasma cell). However, analysis of disease subgroups revealed a significant trend towards increased transporter expression in patients with progressive disease compared to those with stable disease (chi 2 = 4.0, p < 0.05). Nucleoside transporter expression correlated significantly with the plasma cell labeling index (LI) (r = 0.45, p < 0.01) and serum thymidine kinase levels (r = 0.66, p < 0.01), both markers of cellular proliferation but not with c-myc oncoprotein expression. These findings suggest that flow cytometric measurement of nucleoside transporter expression on plasma cells provides a rapid and convenient measurement of disease activity or quiescence in myeloma.

Functional expression of the nitrobenzylthioinosine-sensitive nucleoside transporter of human choriocarcinoma (BeWo) cells in isolated oocytes of Xenopus laevis

Cultured human choriocarcinoma (BeWo) cells have previously been shown to exhibit, in comparison with other cultured cell types, elevated nitrobenzylthioinosine (NBMPR)-sensitive transport activity and large numbers (> 10(7)/cell) of high-affinity NBMPR-binding sites [Boumah, Hogue and Cass (1992) Biochem. J. 288, 987-996]. The present study investigates whether NBMPR-sensitive nucleoside transport activity could be induced in Xenopus laevis oocytes by microinjection of poly(A)+ RNA isolated from proliferating cultures of BeWo cells. Expression of uridine transport activity was assayed by comparing rates of uptake (22 degrees C) of 100 microM [3H]uridine by RNA-injected oocytes with uptake by water-injected or uninjected oocytes. A 4-fold stimulation of uridine uptake (2.0 versus 0.5 pmol/90 min per oocyte) was seen when oocytes were injected with 50 ng of BeWo poly(A)+ RNA, and this stimulation was abolished when the RNA-injected oocytes were assayed in the presence of 10 microM NBMPR. The expressed uridine transport activity in oocytes was highly sensitive to NBMPR, with a 50% reduction seen at 1.1 nM NBMPR (IC50 value). The IC50 value for NBMPR inhibition of uptake of 100 microM [3H]uridine by intact BeWo cells was 1.4 nM. Inward fluxes of [3H]uridine in the RNA-injected oocytes were greatly reduced in the presence of high concentrations (2 mM) of non-radioactive nucleosides (adenosine, thymidine, inosine) that are known permeants of NBMPR-sensitive nucleoside transport processes. These results establish that the abundance of NBMPR-sensitive nucleoside transporter mRNA in poly(A)+ RNA preparations from BeWo cells is sufficient to achieve production of functionally active transporter protein in Xenopus oocytes and that, when expressed in Xenopus oocytes, the transporters exhibit NBMPR sensitivity and permeant selectively similar to that of the native transporters.

Functional expression of Na(+)-dependent nucleoside transport systems of rat intestine in isolated oocytes of Xenopus laevis. Demonstration that rat jejunum expresses the purine-selective system N1 (cif) and a second, novel system N3 having broad specificity for purine and pyrimidine nucleosides

Isolated stage VI oocytes from Xenopus laevis expressed uridine transport activity after microinjection of mRNA from rat jejunum. Uridine uptake during 30 min (10 microM, 20 degrees C) by mRNA-injected oocytes reached 2.5 pmol/oocyte, compared with endogenous uptake by water-injected oocytes of about 0.05 pmol/oocyte. The expressed transport activity was 96% Na(+)-dependent, saturable (apparent Km = 15 microM) and inhibited by phloridzin (IC50 = 100 microM). Nucleoside inhibition studies resolved the expressed transport activity into two components: 1) a novel Na(+)-dependent system of broad purine and pyrimidine specificity that was inhibited by low concentrations of guanosine, inosine, adenosine, uridine, thymidine, and cytidine and 2) a Na(+)-dependent system of narrower specificity that was inhibited by low concentrations of guanosine, inosine, adenosine, and uridine and by high concentrations of thymidine and cytidine. The characteristics of the latter system are consistent with those of the Na(+)-dependent nucleoside transport system N1 (cif), previously identified in a number of cell types and tissues, including intestinal epithelia and cultured cells of intestinal origin. The broad specificity system, which was also detected in mRNA-injected oocytes using thymidine as permeant, has been given the provisional designation N3 to distinguish it from the previously described N1 (purine-selective) and N2 (pyrimidine-selective) Na(+)-linked nucleoside transporters. Rat jejunal transporters N1 and N3 were both expressed maximally by the same mRNA size fraction (1.6-3.0 kb, peak 2.3 kb).

Inhibitory effects of propentofylline on [3H]adenosine influx. A study of three nucleoside transport systems

The neuroprotective effects of adenosine are well-recognized. Recently, propentofylline, a xanthine derivative, has been shown to increase extracellular concentrations of adenosine in ischemic brain and to limit the extent of neuronal damage in experimental models of cerebral ischemia. Since the concentration of adenosine in brain is controlled, in part, by nucleoside transporter proteins, the action of propentofylline was proposed to be due to inhibition of mediated transfer of adenosine across cell membranes. To determine the likelihood of this mechanism, we examined the inhibitory effects of propentofylline on [3H]adenosine transport by the three best-characterized nucleoside transport processes, es, ei, and cif in cultured cell lines under conditions where only a single transporter type was operative. Propentofylline inhibited [3H]adenosine uptake by each of the three transport processes in a concentration-dependent manner. The greatest inhibitory potency was for es transporters (L1210/B23.1 cells), with an IC50 value of 9 microM, followed by ei transporters, with IC50 values of 170 microM (L1210/C2 cells) and 166 microM (Walker 256 cells). Propentofylline was a weak inhibitor of cif transporter, with an IC50 value of 6 mM. These results demonstrate that propentofylline is an inhibitor of adenosine transport processes and suggest that its neuroprotective effects may be due to an increase in extracellular concentrations of adenosine by virtue of inhibition of es transporter function.

Flow cytometric quantitation of nucleoside transporter sites on human leukemic cells

Quantitation of equilibrative, nitrobenzylthioinosine (NBMPR) sensitive (es) nucleoside transporters on blast cells isolated from patients with acute myeloblastic leukemia is useful in predicting intracellular accumulation of the antileukemic nucleoside drug, cytosine arabinoside. We previously reported the synthesis of a fluorescein-labeled ligand for the es nucleoside transporter, 5-(SAENTA-x2)-fluorescein. This paper reports the synthesis of 5-(SAENTA-x8)-fluorescein in which the linkage between fluorescein and nucleoside ligand has been increased from 2 atoms to 8 atoms. This new ligand had a sixfold increase in affinity (Kd 0.9 +/- 0.1 nM) as well as an 86% increase in the cell associated fluorescence output compared to its prototype 5-(SAENTA-x2)-fluorescein. The fluorescence signal arising from 5-(SAENTA-x8)-fluorescein specifically bound to freshly isolated and cultured leukemic myeloblasts was converted to molecules of equivalent soluble fluorescein (MESF) using standardized fluorescein microbeads and compared with the number of es nucleoside transporter sites assayed concurrently by [3H]NBMPR equilibrium binding analysis. A high correlation between the two assays was observed (r = 0.98), which enabled the cell-bound fluorescence output of 5-(SAENTA-x8)-fluorescein to be expressed in numbers of es nucleoside transporter sites per cell. The improved properties of 5-(SAENTA-x8)-fluorescein over those of its prototype molecule make it a suitable reagent for flow cytometric quantitation of nucleoside transporter expression on leukemic cells isolated from patient samples.

Nucleobase transporter-mediated permeation of 2',3'-dideoxyguanosine in human erythrocytes and human T-lymphoblastoid CCRF-CEM cells

Several 2',3'-dideoxynucleosides (ddNs), agents that inhibit the replication of human immunodeficiency virus and hepatitis B virus, enter mammalian cells by simple diffusion. In this report, we show that the membrane permeation of 2',3'-dideoxyguanosine (ddG) in human erythrocytes and CCRF-CEM cells, in contrast with that of other ddNs, is transporter-mediated. Inward fluxes of ddG in both cell types were inhibited by adenine, hypoxanthine, and acyclovir, but not by inhibitors of nucleoside transport (nitrobenzylthioinosine, dipyridamole, dilazep). Fluxes of ddG in human erythrocytes were attributable to a single, rate-saturable process (Km, 380 +/- 90 microM and Vmax, 7.9 +/- 0.8 pmol/s/microliter cell water) that was competitively inhibited by adenine (Ki, 16 microM). These results showed that ddG entered human erythrocytes and CCRF-CEM cells by a transporter-mediated process that was also the basis for entry of purine nucleobases. In contrast, inward fluxes of 2,6-diaminopurine-2',3'-dideoxyriboside (ddDAPR), a prodrug of ddG, were not affected by purine nucleobases or nucleoside transport inhibitors in either cell type. Thus, the permeation properties of ddDAPR resembled those of 2',3'-dideoxyadenosine, a diffusional permeant (cell uptake is transporter-independent), and contrasted with those of ddG, the deamination product of ddDAPR. This study demonstrated that the nucleobase moiety of ddNs is an important determinant of membrane permeation.

L1210/B23.1 cells express equilibrative, inhibitor-sensitive nucleoside transport activity and lack two parental nucleoside transport activities

Cultured mouse leukemia L1210 cells express the nucleoside-specific membrane transport processes designated es, ei, and cif. The es and ei processes are equilibrative, but may be distinguished by the high sensitivity of the former to 6-[(4-nitrobenzyl)thio]-9-beta-D-ribofuranosylpurine (NBMPR); the cif process is mediated by a Na+/nucleoside cotransporter of low sensitivity to NBMPR. Cells of an ei-deficient clonal line, L1210/MC5-1, were mutagenized, and clones were selected in soft agar medium that contained (i) NBMPR (an inhibitor of es processes), (ii) erythro-9-(2-hydorxy-3-nonyl)adenine (an inhibitor of adenosine deaminase), and (iii) arabinofuranosyladenine (a cytotoxic substrate for the three nucleotide transporters). The selection medium did not allow es activity and selected against cells that expressed the Na(+)-linked cif process. Cells of the L1210/B23.1 clonal isolate were deficient in cif transport activity, and inward fluxes of formycin B, a poorly metabolized analog of inosine, were virtually abolished by NBMPR in these cells. In the mutant cells, nonisotopic formycin B behaved as a countertransport substrate during influx of [3H]formycin B, and inward fluxes of the latter were competitively inhibited by purine and pyrimidine nucleosides. The transport behavior of L1210/B23.1 cells indicates that (i) the mutation/selection procedure impaired or deleted the Na(+)-linked cif process and (ii) es nucleoside transport activity is expressed in the mutant cells.

Transport function and subcellular distribution of purified human erythrocyte glucose transporter reconstituted into rat adipocytes

In order to delineate the insulin-independent (constitutive) and insulin-dependent regulations of the plasma membrane glucose transporter concentrations in rat adipocytes, we introduced purified human erythrocyte GLUT-1 (HEGT) into rat adipocytes by poly(ethylene glycol)-induced vesicle-cell fusion and its transport function and subcellular distribution in the host cell were measured. HEGT in adipocytes catalysed 3-O-methylglucose equilibrium exchange with a turnover number that is indistinguishable from that of the basal adipocyte transporters. However, insulin did not stimulate significantly the HEGT function in adipocytes where it stimulated the native transporter function by 7-8-fold. The steady state distribution and the transmembrane orientation assays revealed that more than 85% of the HEGT that were inserted in the physiological, cytoplasmic side-in orientation at the adipocytes plasma membrane were moved into low-density microsomes (LDM), while 90% of the HEGT that were inserted in the wrong, cytoplasmic side-out orientation were retained in the plasma membrane. Furthermore, more than 70% of the LDM-associated HEGT were found in a small subset of LDM that also contained 80% of the LDM-associated GLUT-4, the insulin-regulatable, native adipocyte glucose transporter. However, insulin did not cause redistribution of HEGT from LDM to the plasma membrane under the condition where it recruited GLUT-4 from LDM to increase the plasma membrane GLUT-4 content 4-5-fold. These results demonstrate that the erythrocyte GLUT-1 introduced in adipocytes transports glucose with an intrinsic activity similar to that of the adipocyte GLUT-1 and/or GLUT-4, and enters the constitutive GLUT-4 translocation pathway of the host cell provided it is in physiological transmembrane orientation, but fails to enter the insulin-dependent GLUT-4 recruitment pathway. We suggested that the adipocyte plasma membrane glucose transporter concentration is constitutively kept low by a mechanism where a cell-specific constituent interacts with a cytoplasmic domain common to GLUT-1 and GLUT-4, while the insulin-dependent recruitment requires a cytoplasmic domain specific to GLUT-4.

Protection against fludarabine neurotoxicity in leukemic mice by the nucleoside transport inhibitor nitrobenzylthioinosine

Fludarabine phosphate (F-ara-AMP, Fludara) is rapidly converted in the circulation to fludarabine (F-ara-A) and is among the most effective single agents in the treatment of chronic lymphocytic leukemia. Although current treatment protocols are well tolerated, severe neurotoxicity was a consequence of high-dose F-ara-AMP regimens used in early phase I trials against adult acute leukemia. The present study showed that in mice implanted with leukemia L1210, fatal neurotoxicity, which initially manifested as hind-limb paralysis, was a consequence of high-dose F-ara-AMP treatment. However, the incidence of neurotoxicity was reduced by the coadministration of NBMPR-P, the 5'-phosphate of nitrobenzylthioinosine, a potent inhibitor of the es equilibrative nucleoside transport (NT) system. NBTGR-P, the 5'-phosphate of nitrobenzylthioguanosine (also a potent NT inhibitor) similarly prevented F-ara-AMP neurotoxicity in this experimental system. Treatment with F-ara-AMP/NBMPR-P combinations was more effective with respect to the fractional yield of "cured" mice than were the same treatment regimens without NBMPR-P.

Sodium-dependent nucleoside transport in rabbit intestinal epithelium

Cellular uptake of formycin B, a poorly metabolized analog of inosine, by the isolated epithelium of rabbit jejunum is three times higher in the presence of Na+ than without this cation. The Na(+)-dependent nucleoside transport system is located in the apical membrane of the enterocytes and is capable of uphill transport, as shown for formycin B and adenosine with brush border membrane vesicles. According to present and earlier evidence, nucleoside transport across the basolateral membrane appears to have the properties of facilitated diffusion. Na(+)-dependent formycin B transport activity in intestinal epithelium decreases from jejunum to ileum and is absent in descending colon. As with Na(+)-coupled cotransport systems for other organic solutes, apical entry of formycin B is driven by the electrochemical Na+ gradient into the cell. In contrast to the facilitated diffusion system for nucleosides, Na(+)-dependent formycin B transport is not inhibited by nitrobenzylthioinosine, but both carrier systems are sensitive to inhibitors of D-glucose transport. Natural purine nucleosides and uridine are strong inhibitors of Na(+)-dependent formycin B transport. Transepithelial flux measurements substantiated that the Na(+)-dependent transport mechanism for formycin B functions as an absorptive system.

Sodium-dependent nucleoside transport in mouse leukemia L1210 cells

Nucleoside permeation in L1210/AM cells is mediated by (a) equilibrative (facilitated diffusion) transporters of two types and by (b) a concentrative Na(+)-dependent transport system of low sensitivity to nitrobenzylthioinosine and dipyridamole, classical inhibitors of equilibrative nucleoside transport. In medium containing 10 microM dipyridamole and 20 microM adenosine, the equilibrative nucleoside transport systems of L1210/AM cells were substantially inhibited and the unimpaired activity of the Na(+)-dependent nucleoside transport system resulted in the cellular accumulation of free adenosine to 86 microM in 5 min, a concentration three times greater than the steady-state levels of adenosine achieved without dipyridamole. Uphill adenosine transport was not observed when extracellular Na+ was replaced by Li+, K+, Cs+, or N-methyl-D-glucammonium ions, or after treatment of the cells with nystatin, a Na+ ionophore. These findings show that concentrative nucleoside transport activity in L1210/AM cells required an inward transmembrane Na+ gradient. Treatment of cells in sodium medium with 2 mM furosemide in the absence or presence of 2 mM ouabain inhibited Na(+)-dependent adenosine transport by 50 and 75%, respectively. However, because treatment of cells with either agent in Na(+)-free medium decreased adenosine transport by only 25%, part of this inhibition may be secondary to the effects of furosemide and ouabain on the ionic content of the cells. Substitution of extracellular Cl- by SO4(-2) or SCN- had no effect on the concentrative influx of adenosine.

Substrate specificity, kinetics, and stoichiometry of sodium-dependent adenosine transport in L1210/AM mouse leukemia cells

Two equilibrative (facilitated diffusion) nucleoside transport processes and a concentrative Na(+)-dependent co-transport process contribute to zero-trans inward fluxes of nucleosides in L1210 mouse leukemia cells. Na(+)-linked inward adenosine fluxes in L1210/AM cells (a clone deficient in adenosine, deoxyadenosine, and deoxycytidine kinase activities) were measured as initial rates of [3H]adenosine influx in medium containing Na+ salts and 10 microM dipyridamole. The Na(+)-linked transporter distinguished between the D- and L-enantiomers of adenosine, the latter being a virtual nonpermeant in the initial-rate assay. Adenine arabinoside, inosine, 2'-deoxyadenosine and 2'-deoxyadenosine derivatives with halogen atoms at the purine C-2 position were recognized as substrates of the Na(+)-linked system because of their inhibition of adenosine (10 microM) fluxes under the condition of Na(+)-dependence with IC50 values ranging between 25 and 183 microM; uridine, deoxycytidine, and cytosine arabinoside (each at 400 microM) inhibited adenosine fluxes by 10-40%. Inward Na(+)-linked adenosine fluxes were saturable with respect to extracellular adenosine and Na+ concentrations [( Na+]o); Km and Vmax values for adenosine influx were 9.4 +/- 2.6 microM and 1.67 +/- 0.2 pmol/microliter cell water/s when [Na+]o was 100 mM. The stoichiometry of Na+:adenosine co-transport, determined by Hill analysis of the dependence of adenosine fluxes on [Na+]o, was 1:1. The thiol-reactive agents, N-ethylmaleimide (NEM), showdomycin and p-chloromercuriphenylsulphonate (pCMPS), inhibited Na(+)-linked adenosine fluxes with IC50 values of 40, 10, and 2 microM, respectively. This inhibition was partially reversed by the presence of adenosine in incubation media containing pCMPS, but not NEM. Thiol groups accessible to pCMPS may be involved in substrate recognition by the transporter and in the permeation step.

Glucose and nucleoside transporters of human erythrocytes: effects of detergents on immunoadsorption of a membrane protein to its monoclonal antibody

Immunoadsorption of membrane proteins solubilized in detergents has been used widely for identification, purification and quantitation of transporters and receptors. In an effort to separate the glucose and nucleoside nucleoside transporters of human erythrocytes (GT and NT, respectively) that copurify in a membrane protein fraction band 4.5, we examined in the present study the effects of seven different detergents on the immunoadsorption of GT to its monoclonal antibody, 65D4 (Craik, et al. (1988) Biochem. Cell Biol. 66, 839-852). The following results were obtained. (1) The maximum extent of the immunoadsorption of GT by 65D4 varied between 52 to 98% in different detergents. For non-ionic detergents, there was an apparent inverse correlation between the maximum immunoreactivity of GT and the aggregation number or micellar size of detergents. (2) The immunoprecipitate of GT by 65D4 was contaminated with nucleoside transporter to an extent that varied from 2 to 35 mol% in different detergents. There is an inverse correlation between the extent of the contamination and the detergent aggregation number. However, this contamination was quantitatively accounted for by a time-dependent, non-specific aggregation of NT with GT in detergents. (3) A high degree of purification of NT in band 4.5 by immunoadsorptive removal of GT with 65D4 was achieved in C12E8 as predicted by the observed low NT-GT aggregation and the relatively high epitope-accessibility of GT in this detergent. Based on these findings, we conclude that certain detergents can reduce the immunoreactivity of membrane proteins significantly by modulating epitope accessibility, and may also produce a false immuno-cross-reactivity by inducing nonspecific protein aggregation.

Expression of sodium-linked nucleoside transport activity in monolayer cultures of IEC-6 intestinal epithelial cells

Mature, confluent monolayer cultures of IEC-6 rat intestinal epithelial cells in conventional growth media express both Na(+)-linked, concentrative nucleoside transport (NT) activity and equilibrative, inhibitor-sensitive NT activity, but do not show morphologic differentiation. Na(+)-dependent fluxes of Ado and formycin B were minor in early subconfluent IEC-6 monolayers, but increased severalfold to become the major component of influx of these agents in confluent monolayers grown in medium containing Nu-Serum, a commercial medium supplement with a low serum content. In monolayers cultured in medium with fetal bovine serum, cell proliferation rates were similar to those in medium supplemented with Nu-Serum, but expression of Na(+)-linked NT activity was 6-8-fold lower than in monolayers grown in the latter medium. Inclusion of hydrocortisone in growth medium with Nu-Serum caused a 2-fold increase in the expression of Na(+)-linked NT activity. Experiments in which components of medium supplementation were withheld showed that insulin and epidermal growth factor were important in expression of the Na(+)-linked NT activity. Because the Na(+)-linked NT system has a brush border location in fresh intestinal epithelium, it is concluded that the regulated expression of this activity in the IEC-6 monolayers is a differentiative change.

Parasite-induced processes for adenosine permeation in mouse erythrocytes infected with the malarial parasite Plasmodium yoelii

In mouse erythrocytes harbouring the malarial parasite Plasmodium yoelii, three processes contributed to inward fluxes of adenosine, one of which is attributed to the native nucleoside transporter, because of the inhibitory effects of nitrobenzylthioinosine (NBMPR). New (parasite-induced) permeation processes of low NBMPR-sensitivity were (i) saturable fluxes with preference for the D enantiomer (D-Ado) and (ii) apparently unsaturable fluxes that proceeded by a channel-like route without enantiomeric selectivity. Parasite-induced fluxes of L- and D-Ado were similarly inhibited by furosemide [IC50 (concn. causing half-maximal inhibition) 15-17 microM], whereas D-Ado fluxes in uninfected erythrocytes were 10-fold less sensitive.

Sodium-dependent and equilibrative nucleoside transport systems in L1210 mouse leukemia cells: effect of inhibitors of equilibrative systems on the content and retention of nucleosides

The presence of 10 microM dipyridamole in incubation media of L1210/C2 cells decreased initial rates of zero-trans influx of formycin B (FB, 50 microM), a poorly metabolized inosine analogue, from 4.84 pmol/microliters cell water/s to 0.87 pmol/microliter cell water/s. However, after a 5-min interval of uptake, free FB levels in dipyridamole-treated cells were 165 pmol/microliters cell water, 2.3-fold greater than in dipyridamole-free cultures. This indicated the presence of a concentrative, dipyridamole-insensitive nucleoside transport (NT) system in L1210 cells, in addition to the equilibrative NT systems known to be expressed in these cells. The concentrative system was demonstrable only in the presence of NT inhibitors and required extracellular Na+. The presence of 8 microM 6-[(4-nitrobenzyl)thio]-9-beta-D- ribofuranosylpurine or 15 microM dilazep also induced an accumulation of free FB above steady-state levels, although of a lesser magnitude than that observed with dipyridamole. It appears that NT inhibitors induced nucleoside accumulation by inhibiting bidirectional nucleoside movements mediated by the equilibrative component of nucleoside transport in L1210/C2 cells without interfering with inward FB fluxes mediated by the Na(+)-dependent transporter. The presence of NT inhibitors also enhanced the cellular accumulation and retention of arabinosyladenine and its 5'-triphosphate in these cells. The increased cellular accumulation of 9-beta-D-arabinofuranosyladenine and 9-beta-D-arabinofuranosyladenine triphosphate by dipyridamole was associated with enhanced antiproliferative activity of 9-beta-D-arabinofuranosyladenine towards the leukemia cells.

5'-S-(2-aminoethyl)-N6-(4-nitrobenzyl)-5'-thioadenosine (SAENTA), a novel ligand with high affinity for polypeptides associated with nucleoside transport. Partial purification of the nitrobenzylthioinosine-binding protein of pig erythrocytes by affinity chromatography

Derivatives of N6-(4-aminobenzyl)adenosine (substituted at the aminobenzyl group) and 5'-linked derivatives of N6-(4-nitrobenzyl)adenosine (NBAdo) were evaluated as inhibitors of site-specific binding of [3H]nitrobenzylthioinosine (NBMPR) to pig erythrocyte membranes. Potent inhibitors were SAENTA [5'-S-(2-aminoethyl)-N6-(4-nitrobenzyl)-5'-thioadenosine] and acetyl-SAENTA (the 2-acetamidoethyl derivative of SAENTA). SAENTA was coupled to derivatized agarose-gel beads (Affi-Gel 10) to form an affinity matrix for chromatographic purification of NBMPR-binding polypeptides, which in pig erythrocytes are part of, or are associated with, the equilibrative nucleoside transporter. When pig erythrocyte membranes were solubilized with octyl glucoside (n-octyl beta-D-glucopyranoside) and applied to SAENTA-Affi-Gel 10 (SAENTA-AG10), polypeptides that migrated as a broad band on SDS/PAGE with an apparent molecular mass of 58-60 kDa were selectively retained by the affinity gel. These polypeptides were identified as components of the nucleoside transporter of pig erythrocytes by reactivity with a monoclonal antibody (mAb 11C4) that recognizes the NBMPR-binding protein of pig erythrocytes. Retention of the immunoreactive polypeptides by SAENTA-AG10 was blocked by NBAdo. The immunoreactive polypeptides were released from SAENTA-AG10 by elution under denaturing conditions with 1% SDS or by elution with detergent solutions containing competitive ligands (NBAdo or NBMPR). A 72-fold enrichment of the immunoreactive polypeptides was achieved by a single passage of solubilized, protein-depleted membranes through a column of SAENTA-AG10, followed by elution with detergent solutions containing NBAdo. These results demonstrate that polypeptide components of NBMPR-sensitive nucleoside-transport systems may be partly purified by affinity chromatography using gel media bearing SAENTA groups.

Potencies of mioflazine and its derivatives as inhibitors of adenosine transport in isolated erythrocytes from different species

The potency of mioflazine and related drugs (Janssen Pharmaceutica, Belgium) as inhibitors of adenosine transport in isolated erythrocytes from several species were measured and compared with those of dilazep and 6-(4-nitrobenzylmercapto)purine ribonucleoside (NBMPR). [8-3H]Adenosine was used as the permeant at 1 microM and incubation times were 10 s, and assays were conducted in the presence and absence of varying doses of potential transport inhibitors. The species investigated included mouse, hamster, rabbit, baboon and man. Dilazep was the most potent compound throughout with an IC50 of about 2 nM. In the mouse and hamster mioflazine and its derivatives were considerably less potent (IC50 values greater than 200 nM) with the exception of R57974 with IC50 values of about 150 and 60 nM in mouse and hamster, respectively. In the man and baboon the derivatives had IC50 values in the same order of magnitude as NBMPR (less than 100 nM), and in the rabbit they had potencies close to that of NBMPR, ranging between 10-60 nM. Nucleoside transport inhibitors are of potential importance as host protectors during treatment of parasitic infections with cytotoxic nucleosides. Present data indicate that mioflazine and its derivatives are not very potent in some of the preferred animal models for parasitic infections (mouse, hamster) but are more effective in primates such as man and baboon.

Enantiomeric selectivity of adenosine transport systems in mouse erythrocytes and L1210 cells

In mediating the entry of adenosine into mouse erythrocytes and mouse leukaemia L1210 cells, nucleoside transport systems were stereoselective, showing a marked preference for the D-enantiomer of adenosine (D-Ado). Inward zero-trans fluxes of the mirror-image isomer, L-adenosine (L-Ado), in those cells were slow relative to those of D-Ado. Contributing to L-Ado fluxes in both cell types were (i) a transporter-mediated process of high nitrobenzylthioinosine-sensitivity and (ii) simple diffusion.

Interaction of [3H]dilazep at nucleoside transporter-associated binding sites on S49 mouse lymphoma cells

Dilazep, a tertiary amine that is greater than 96% protonated at pH 7.4, is a potent inhibitor of facilitated diffusion (equilibrative) nucleoside transport (NT) in animal cells. In this study, saturable reversible binding of [3H]dilazep was demonstrated at sites on S49 mouse lymphoma cells but not in AE1 cells, an NT-deficient mutant of S49 cells. Mass law analysis of dilazep binding under equilibrium conditions revealed two saturable components, representing binding sites that differed about 50-fold in affinity for dilazep (Kd values of 0.21 and 10 nM). At pH 7.4, the low affinity sites were more abundant (Bmax, 3.5 X 10(5) sites/cell) than the high affinity site (Bmax, 3.0 X 10(4) sites/cell). Binding of dilazep was pH dependent; at pH 9.0, binding at the high affinity sites predominated, whereas, at pH 5.0, the low affinity component predominated, suggesting that these components represented binding of nonprotonated and protonated dilazep molecules, respectively. Nitrobenzylthioinosine (NBMPR) and physostigmine selectively blocked binding of nonprotonated and protonated species of dilazep, respectively, at pH 7.4, yielding Scatchard plots that were similar to control plots obtained at pH 5.0 and 9.0. First-order plots of the dissociation of [3H]dilazep-binding site complexes in the presence of excess nonradioactive dilazep at pH 7.4 were nonlinear and were resolved into rapid (rate constant, 3.4-4.7 min-1) and slow (rate constant, 0.13-0.15 min-1) components. In the presence of site-saturating concentrations of NBMPR or high concentrations of nucleoside permeants, dissociation of site-bound [3H]dilazep was incomplete and only the slow component of dissociation was apparent (rate constant, 0.11-0.19 min-1). The combined presence of nonradioactive dilazep and NBMPR yielded time courses of [3H]dilazep-site dissociation equivalent to those obtained in the presence of nonradioactive dilazep alone. These results are consistent with a model in which protonated and nonprotonated species of dilazep bind at separate sites on S49 cells. The absence of both high and low affinity sites on AE1 cells suggests that, in S49 cells, both populations of sites are associated with NT polypeptides. The high affinity sites that bind nonprotonated species of dilazep appear to overlap with NBMPR binding sites on these cells.

Identification of glucose and nucleoside transport proteins in neonatal pig erythrocytes using monoclonal antibodies against band 4.5 polypeptides of adult human and pig erythrocytes

Cytochalasin B and nitrobenzylthioinosine (NBMPR), which inhibit membrane transport of glucose and nucleosides, respectively, have served as photoaffinity ligands that become covalently linked at inhibitor binding sites on transporter-associated proteins. Thus, when membranes from erythrocytes of neonatal pigs with site-bound [3H]cytochalasin B or [3H]NBMPR were irradiated with uv light, two labeled membrane polypeptides (peak Mr values: 55,000 and 64,000, respectively) were identified. Treatment of the photolabeled membranes with endoglycosidase F increased the mobility of [3H]cytochalasin B- and [3H]NBMPR-labeled material (peak Mr values: 44,000 and 57,000, respectively) and limited digestion with trypsin yielded different polypeptide fragments (Mr values: 18,000-23,000 and 43,000, respectively). Identification of the photolabeled polypeptides as transporter components was established using monoclonal antibodies (MAbs) raised against partially purified preparations of band 4.5 from erythrocytes of adult pigs and humans. MAbs 65D4 and 64C7 (anti-human band 4.5), raised in this study, reacted with [3H]cytochalasin B-labeled material from membranes of human erythrocytes and bound to permeabilized erythrocytes but not to intact cells. MAb 65D4 also bound to erythrocytes of mice and neonatal pigs and to a variety of cultured cells (mouse, human, rat), including AE1 mouse lymphoma cells, which lack an NBMPR-sensitive nucleoside transporter. Also employed was MAb 11C4 (anti-pig band 4.5), which recognizes the NBMPR-binding protein of erythrocyte membranes from adult pigs. When membrane proteins from neonatal and adult pigs were subjected to electrophoretic analysis and blots were probed with different MAbs, MAb 65D4 (anti-human band 4.5) bound to material that comigrated with [3H]cytochalasin B-labeled polypeptides (band 4.5) from neonatal, but not adult, pig erythrocytes, whereas MAb 11C4 (anti-pig band 4.5) bound to material that comigrated with [3H]NBMPR-labeled band 4.5 polypeptides of erythrocytes from both neonatal and adult pigs. These results, which indicate structural differences in the cytochalasin B- and NBMPR-binding proteins of pig erythrocytes, establish the presence of both proteins in erythrocytes of neonatal pigs and suggest that only the NBMPR-binding protein is present in erythrocytes of adult pigs.

External location of sites on pig erythrocyte membranes that bind nitrobenzylthioinosine

Nucleoside transport in erythrocytes of various species is inhibited by the binding of nitrobenzylthioinosine (NBMPR) to high affinity sites associated with nucleoside transport elements of the plasma membrane. The present study examined binding of [3H]NBMPR to unsealed ghosts and to sealed right-side-out vesicles (ROVs) and inside-out vesicles (IOVs) prepared from pig erythrocytes. Kd values for NBMPR dissociation from the ligand-site complex in unsealed ghosts, ROVs and IOVs were similar (1.6-2.4 nM), and Bmax values (mean +/- SD) were, respectively, 22.2 +/- 5.5, 25.8 +/- 6.4, and 37.3 +/- 4.0 molecules/fg of protein, reflecting differences in the protein content of the membrane preparations. When temperatures were decreased from 22 degrees to 4 degrees, NBMPR binding to erythrocyte membrane preparations was reduced in IOVs relative to that in unsealed ghosts and ROVs. At 22 degrees, the association of NBMPR molecules with IOVs was slower than with ROVs and unsealed ghosts, differences that were virtually eliminated by permeabilization of the membrane preparations with saponin. Thus, the binding sites were more accessible to external NBMPR in sealed ROVs and unsealed ghosts than in sealed IOVs, indicating that the NBMPR sites are located on the extracellular aspect of the membrane.

Stage-specific alteration of nucleoside membrane permeability and nitrobenzylthioinosine insensitivity in Plasmodium falciparum infected erythrocytes

In human erythrocytes, the intracellular presence of malarial parasites (Plasmodium falciparum) markedly changed the permeation characteristics of the nucleosides, adenosine and tubercidin, an adenosine analogue. We report parasite-induced changes in the kinetics of cellular uptake of the nucleosides and in the appearance in infected cells of a nucleoside permeation route of low sensitivity to the classical inhibitor of erythrocytic nucleoside transport, nitrobenzylthioinosine (NBMPR). These changes and a diminution in NBMPR effectiveness during parasite maturation to the trophozoite or schizont stage, suggest the presence in the infected cells of an altered or new nucleoside permeation mechanism of low sensitivity to NBMPR. The incorporation of adenosine into polynucleotides was also of low sensitivity to 10 microM NBMPR. Binding studies of [3H]NBMPR with both normal erythrocytes and those harbouring parasites at each morphological stage indicated that fewer high affinity NBMPR binding sites were present on cells containing mature parasites than on the uninfected cells. The apparent low sensitivity to NBMPR of nucleoside permeation in erythrocytes containing P. falciparum forms may enable therapeutic measures with cytotoxic nucleosides to be directed with selectivity toward parasite-containing cells.

Nucleoside permeation in mouse erythrocytes infected with Plasmodium yoelii

In normal mouse erythrocytes, nucleoside permeation was almost completely blocked in the presence of binding site-saturating concentrations of nitrobenzylthioinosine, whereas permeation in erythrocytes infected with the malarial parasite, Plasmodium yoelii, was substantial under these conditions, suggesting the presence of a permeation mechanism of low sensitivity to nitrobenzylthioinosine in the infected cells. Binding sites for nitrobenzylthioinosine were more numerous on infected erythrocytes than on uninfected cells. When mice infected with P. yoelii were treated with combinations of tubercidin and nitrobenzylthioinosine 5'-monophosphate, progression of parasitemia was delayed and survival times were increased.

Characterization of monoclonal antibodies that recognize band 4.5 polypeptides associated with nucleoside transport in pig erythrocytes

Three monoclonal antibodies have been raised against partially purified band 4.5 polypeptides [Steck (1974) J. Cell Biol. 62, 1-19] from pig erythrocyte membranes. The antibodies were capable of binding to both intact pig erythrocytes and protein-depleted membrane preparations and recognized detergent-solubilized polypeptides from adult and neonatal pig erythrocytes that were photolabelled with [G-3H]nitrobenzylthioinosine (NBMPR), a potent specific inhibitor of nucleoside transport. The antibodies did not recognize polypeptides from neonatal pig erythrocytes that were photolabelled with the glucose-transport inhibitor [3H]cytochalasin B. Reactivity with polypeptides of apparent Mr 64,000 [10% (w/v) acrylamide gels] was demonstrated by Western-blot analysis. The antibodies recognized pig band 4.5 polypeptides after prolonged treatment with endoglycosidase F, a finding consistent with reactivity against polypeptide, rather than carbohydrate, determinants. Trypsin digestion of NBMPR-labelled protein-depleted pig erythrocyte membranes generated two labelled polypeptide fragments (Mr 43,000 and 26,000). Two of the antibodies recognized both fragments on Western blots, whereas the third bound to the larger, but not to the smaller, fragment. The antibodies had no significant effect on reversible binding of NBMPR to protein-depleted pig erythrocyte membranes and did not bind to NBMPR-labelled polypeptides in human, rabbit or mouse erythrocytes.

[125I]iodohydroxynitrobenzylthioinosine: a new high-affinity nucleoside transporter probe

[125I]iodohydroxynitrobenzylthioinosine ([125I]IH-NBMPR), a new gamma-labeled nucleoside transport inhibitor, has been prepared at a theoretical specific activity of 2000 Ci/mmol (1 Ci = 37 GBq). IH-NBMPR was more acidic than hydroxynitrobenzylthioinosine (H-NBMPR), having a pKa of 4.6. Site-specific binding of [125I]IH-NBMPR to membrane-enriched fractions (MEF) from S49 mouse lymphoma cells was pH dependent, increasing with the fraction of undissociated molecules present; it was maximal at pH 4.5 and negligible at pH 7.0. Scatchard analysis of specific binding to MEF from S49 cells under equilibrium conditions at pH 5.0 yielded a Kd of 15 nM (equivalent to 4.0 nM for the undissociated fraction of inhibitor molecules) and maximum number of binding sites (Bmax) of 4.9 pmol/mg protein. Specific binding of IH-NBMPR could not be demonstrated in MEF from AE1 cells, a nucleoside transport-deficient mutant of S49 cells. Influx of uridine into mouse erythrocytes at pH 5.0 in the presence of 5 microM IH-NBMPR (1.4 microM undissociated IH-NBMPR) was reduced to about 7% of the control value, indicating that this compound is an effective nucleoside transport inhibitor. Photoactivation of site-bound [125I]IH-NBMPR, following equilibration of the ligand with MEF from S49 cells at pH 5.0, resulted in specific covalent labeling of a polypeptide with a relative molecular mass of 52,000-63,000, identified on sodium dodecyl sulfate-polyacrylamide gels. These results indicate that the new, iodinated ligand is an inhibitor of nucleoside transport and that it binds specifically and with high affinity to nucleoside transporter polypeptides in mammalian cells.

Sodium-dependent, concentrative nucleoside transport in cultured intestinal epithelial cells

In a simple salts medium, monolayers of IEC-6 intestinal cells achieved concentrations of unmetabolized formycin B (an analog of inosine) about 6-fold higher than in the medium. Rates of formycin B influx were a saturable function of Na+ concentrations in the medium. Although IEC-6 cells possess sites with high affinity for nitrobenzylthioinosine, a potent inhibitor of equilibrative (facilitated diffusion) nucleoside transport systems in certain cell types, the inhibitor had only minor effects on formycin B uptake in IEC-6 cells, but reduced efflux of the analog from these cells. These findings indicate the joint presence in IEC-6 cells of nucleoside transporters of two types, one that is concentrative and Na+-dependent, and another that is sensitive to nitrobenzylthioinosine and apparently equilibrative.

Photoaffinity labeling of the human erythrocyte glucose transporter with 8-azidoadenosine

8-Azidoadenosine was employed as a possible covalent probe of the erythrocyte nucleoside transporter. 8-Azidoadenosine was shown to enter human erythrocytes by a saturable mechanism (apparent Km for influx 80 microM) that was inhibited by nitrobenzylthioinosine (NBMPR), a potent inhibitor of nucleoside transport, and competitively inhibit uridine influx and NBMPR binding. Irradiation with UV light of human erythrocyte membranes or a partially purified preparation of the nucleoside transporter in the presence of [3H]8-azidoadenosine and dithiothreitol (as a free radical scavenger) resulted in selective covalent incorporation into the band 4.5 region of sodium dodecyl sulfate-polyacrylamide gels (Mr 66,000-45,000). Covalent labeling of band 4.5 was inhibited by adenosine, uridine, and inosine, but NBMPR had no effect. Surprisingly, D-glucose and cytochalasin B, but not L-glucose and cytochalasin E, blocked covalent attachment of the ligand. No incorporation of radioactivity into membranes from rabbit and pig erythrocytes was observed, cells which transport nucleosides rapidly, but have little or no functional glucose carrier. Limited treatment with trypsin of unsealed human erythrocyte membranes photolabeled with [3H]8-azidoadenosine yielded a single radioactive fragment of Mr 19,000, a pattern identical to that obtained with [3H]cytochalasin B-labeled membranes. These results suggest that, despite 8-azidoadenosine being a permeant for the nucleoside transporter, under photoactivation 8-azidoadenosine preferentially labeled the glucose carrier.

Nitrobenzylthioinosine: an in vivo inhibitor of pig erythrocyte energy metabolism

The potential role of plasma nucleosides as metabolic energy substrates for pig erythrocytes, which are impermeable to glucose, was investigated in vivo by infusion of anesthetized pigs with nitrobenzylthioinosine phosphate (NBMPR-P), a soluble prodrug form of the specific nucleoside transport inhibitor, nitrobenzylthioinosine. NBMPR-P administration (1 or 10 mg X kg-1 X h-1) led to complete in vivo blockade of erythrocyte nucleoside transport activity and was associated with a dramatic decrease in the erythrocyte [ATP]-to-[ADP] ratio from 11.4 at time 0 to 2.9 after 4 h (mean results from 3 animals). Plasma inosine concentrations increased progressively from 2-4 microM at time 0 to 20-70 microM after 4 h of drug administration. In contrast, plasma adenosine concentrations remained less than 0.4 microM in all samples. These data suggest that pig erythrocytes utilize plasma inosine as their physiological energy substrate.

Photoaffinity labelling of a nitrobenzylthioinosine-binding polypeptide from cultured Novikoff hepatoma cells

Site-specific binding of nitrobenzylthioinosine (NBMPR) to plasma membranes of some animal cells results in the inhibition of the facilitated diffusion of nucleosides. The present study showed that nucleoside transport in Novikoff UA rat hepatoma cells is insensitive to site-saturating concentrations of NBMPR. Equilibrium binding experiments demonstrated the presence of high-affinity sites for NBMPR in a membrane-enriched fraction from these cells. In the presence of uridine or dipyridamole, specific binding of NBMPR at these sites was inhibited. When Novikoff UA membranes were covalently labelled with [3H]NBMPR by using photoaffinity techniques, specifically bound radioactivity was incorporated exclusively into a polypeptide(s) with an apparent Mr of 72,000-80,000, determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Covalent labelling of this polypeptide was abolished in the presence of excess nitrobenzylthioguanosine (NBTGR) and reduced in the presence of adenosine, uridine or dipyridamole. The apparent Mr of the NBMPR-binding polypeptide in Novikoff UA cells is significantly higher than that reported for corresponding polypeptides in other cell types (Mr 45,000-66,000). When membrane-enriched preparations from S49 mouse lymphoma cells were photolabelled and mixed with labelled NovikoffUA membrane-enriched preparations, gel electrophoresis resolved the NBMPR-binding polypeptides from the two preparations.

Nucleoside transport and metabolism in erythrocytes from the Yucatan miniature pig. Evidence that inosine functions as an in vivo energy substrate

Erythrocytes from the Yucatan miniature pig, like those from the normal domestic pig, lack functional glucose transporters and were unable to utilize plasma glucose as an energy source. In contrast, inosine and adenosine entered the cells rapidly. The nucleoside transporter responsible for this uptake was identified as a band 4.5 polypeptide (5000 copies per cell; apparent Mr 45 000-66 000). Inosine concentrations in the physiological plasma range (1.6-2.5 microM) were found to maintain normal erythrocyte ATP levels and ATP/ADP ratios during prolonged in vitro incubation of cells at 37 degrees C, an effect that was blocked by the specific nucleoside transport inhibitor, nitrobenzylthioguanosine. In the absence of extracellular nucleoside, cells 'protected' themselves against some of the consequences of deprivation of energy substrate by glycolyzing the ribose moiety of inosine produced during ATP catabolism. Although erythrocytes from the miniature pig were capable of utilizing extracellular adenosine as an energy substrate, plasma samples from these animals contained less than 0.4 microM adenosine. It is concluded that inosine is a major physiological energy source of pig erythrocytes.

Toxicologic and antitumor studies on 5-hydroxymethyldeoxyuridine

Toxic effects of 5-hydroxymethyldeoxyuridine (HMUdR) were studied in white Swiss mice. Pathological, hematological, and clinical chemistry parameters were examined. Systemic toxicity was not observed in mice after ip administration of HMUdR in single doses up to 2000 mg/kg. Mice administered HMUdR daily for 15 days at 200 mg/kg, ip, manifested loss of weight, rough hair coat, diarrhea, swollen abdomens, weakness, lethargy, and a 20% mortality rate. Hematological and clinical chemistry parameters of all mice receiving HMUdR were within normal limits. At necropsy, all organs were grossly normal but microscopic examination of tissues of treated mice revealed the presence of shortened, thickened villi in the small intestine, nuclear vacuolation and necrosis of intestinal crypt epithelial cells, and some cytoplasmic vacuolation causing nuclear margination in hepatocytes. All histological lesions were reversible with cessation of treatment. HMUdR was active against murine L1210 and L5178Y leukemias in cell culture. The concentrations required to inhibit cell growth 50% compared to untreated cells was 2 and 4 microM, respectively. When HMUdR was administered ip at 50, 100, or 200 mg/kg in five daily doses to mice implanted with L1210 cells, life spans increased 20, 30, or 33% over placebo-treated controls.

The effect of pH on interaction of nitrobenzylthioinosine and hydroxynitrobenzylthioinosine with the nucleoside transporter of human erythrocyte membranes

Site-specific binding to human erythrocyte membranes of nitrobenzylthioinosine (NBMPR), un-ionized at physiological pH, was compared with that of hydroxynitrobenzylthioinosine (HNBMPR), pKa 6.4, at graded pH values. Binding of [3H]NBMPR was measured directly, and that of HNBMPR was assayed by competitive inhibition by HNBMPR of [3H]NBMPR binding. Kd and Bmax values for binding of [3H]NBMPR to erythrocyte membranes were independent of pH. Kd values for the competing ligand were determined by mass law analysis of equilibrium binding data using either (a) apparent ligand concentration (dissociated plus undissociated forms of HNBMPR) or (b) the concentration of undissociated HNBMPR. Kd values for HNBMPR calculated with the apparent ligand concentration increased 10-fold as the fraction of HNBMPR molecules present in the dissociated form was increased (by pH changes) from 14 to 88%, whereas Kd values for the undissociated form of HNBMPR were independent of pH. The results presented here demonstrate that the undissociated form of HNBMPR binds more tightly to the transport-inhibitory sites of erythrocytes than NBMPR and suggest that ionization of S6-substituted thiopurine ribonucleosides eliminates or greatly decreases their ability to interact with the binding sites.

Reconstitution studies of the human erythrocyte nucleoside transporter

The human erythrocyte nucleoside transporter has been identified as a band 4.5 polypeptide (Mr 45,000-66,000) on the basis of reversible binding and photoaffinity labeling experiments with the nucleoside transport inhibitor, nitrobenzylthioinosine (NBMPR). In the present study, the NBMPR-binding protein was extracted from protein-depleted human erythrocyte "ghosts" with Triton X-100 and reconstituted into soybean phospholipid vesicles by a freeze-thaw-sonication procedure. The reconstituted proteoliposomes exhibited nitrobenzylthioguanosine (NBTGR)-sensitive [14C]uridine transport. A partially purified preparation of the NBMPR-binding protein, consisting largely of band 4.5 polypeptides, was also shown to have nucleoside transport activity. This band 4.5 preparation exhibited a 10-fold increase in uridine transport activity and a 7-fold increase in NBMPR-binding activity relative to the crude membrane extract. Uridine transport by the reconstituted band 4.5 preparation was saturable (apparent Km = 0.21 mM; Vmax = 9 nmol/mg of protein/5 s) and was inhibited by dipyridamole, dilazep, adenosine, and inosine. The vesicles reconstituted with the band 4.5 preparation also exhibited stereospecific glucose transport which was inhibited by cytochalasin B, but unaffected by NBTGR. In contrast, cytochalasin B was a poor inhibitor of NBTGR-sensitive uridine transport. These experiments implicate band 4.5 polypeptides in both nucleoside and sugar permeation.

Inward fluxes of adenosine in erythrocytes and cultured cells measured by a quenched-flow method

Dilazep, a vasodilator previously recognized as an inhibitor of adenosine permeation, very rapidly blocked the uptake of adenosine by cultured L5178Y cells, and accordingly was used as a quencher in a simple quenched-flow system for measuring cellular uptake of nucleosides during very short intervals. Time courses of cellular uptake of adenosine, assayed during intervals between 0.05 and 0.5s with the quenched-flow system, were linear and defined initial rates of adenosine uptake. The latter are rates of inward transport of adenosine. Kinetic constants for that process in cultured S49 cells determined with the quenched-flow procedure were similar to those determined with an assay dependent on manual timing. In studies of adenosine uptake kinetics in human erythrocytes at 22 degrees C and 37 degrees C in which the quenched-flow procedure was used, time courses of adenosine uptake were linear at both temperatures and defined initial uptake rates; kinetic constants (means +/- S.E.M.) at 22 degrees C (n = 8) were Km 25 +/- 14 microM and Vmax. 15 +/- 5 pmol/s per microliter of cell water and at 37 degrees C (n = 3) were Km 98 +/- 17 microM and Vmax. 80 +/- 9 pmol/s per microliter of cell water.

Separation of nitrobenzylthioinosine from high affinity binding sites on human erythrocytes by gel chromatography

The potent inhibitor of nucleoside transport, 6-[(4-nitrobenzyl) mercapto]-9-beta-D-ribofuranosylpurine (NBMPR), binds reversibly, but with high affinity (KD 0.3-1.4 nM) to plasma membrane sites on human erythrocytes, and occupancy of those sites by NBMPR correlates with inhibition of transport. An earlier study from this laboratory showed that, upon photoactivation of site-bound [3H]NBMPR on erythrocyte membranes by exposure to UV light, isotopic ligand molecules became covalently linked to membrane polypeptides, which migrated as a discrete band on gel electrophoretograms, thereby identifying nucleoside transport polypeptides. The present study showed that erythrocytes on which the high affinity sites were occupied by reversibly bound [3H]NBMPR were freed of the latter by slow passage at 37 degrees C through 9 X 300 mm columns of Sephadex G-200 gel. Photoactivation of site-bound [3H]NBMPR on the intact cells caused apparent covalent attachment of the ligand because cells so treated retained the isotopic label during passage through the gel columns. Apparent covalent binding to erythrocytes also resulted from photoactivation of site-bound [3H]nitrobenzylmercaptopurine arabinoside.

Dephosphorylation of nitrobenzylthioinosine 5'-monophosphate by ecto 5'-nucleotidase of HeLa cells

HeLa cells as well as human and mouse erythrocytes possess membrane sites which bind the inhibitor of nucleoside transport, nitrobenzylthioinosine (NBMPR), reversibly but tightly (KD, 10(-9)-10(-10) M). Site-specific binding of the ligand correlates with inhibition of nucleoside transport. The present study showed that the 5'-phosphate of NBMPR, NBMPR-P, was not transport inhibitory. Upon exposure to [35S]NBMPR-P or [G-3H]NBMPR-P, HeLa cells retained the isotopic labels virtually exclusively in the form of NBMPR. The dephosphorylation of [G-3H]NBMPR-P by HeLa cells, assayed by the production of extracellular [G-3H]NBMPR, was competitively inhibited by AMP, but was not affected by the presence of 5 microM NBMPR, a concentration sufficient to completely occupy the transport inhibitory sites. Thus, the sites at which dephosphorylation of NBMPR occurs in HeLa cells are separate from and function independently of the high affinity sites which bind NBMPR.

Photoaffinity labelling of the nucleoside transporter of cultured mouse lymphoma cells

Nitrobenzylthioniosine (NBMPR), a potent and specific inhibitor of nucleoside transport, is bound reversibly by high affinity sites on nucleoside transporter proteins of erythrocyte membranes and, upon photoactivation, NBMPR molecules become covalently bonded to the sites. This study showed that [3H]NBMPR molecules reversibly bound to intact S49 and L5178Y mouse lymphoma cells became covalently bound upon exposure to UV light. Electrophoretic analysis of plasma membrane fractions from the labelled cells showed that 3H was present in polypeptides which migrated as a major band with an apparent Mr of 45000-65000.

Identification of the nucleoside transporter in cultured mouse lymphoma cells. Photoaffinity labeling of plasma membrane-enriched fractions from nucleoside transport-competent (S49) and nucleoside transport-deficient (AE1) cells with [3H]nitrobenzylthioinosine

Plasma membrane-enriched fractions from disrupted S49 lymphoma cells contained high affinity sites for [3H]nitrobenzylthioinosine, a potent and specific inhibitor of nucleoside transport. These sites were absent from similar preparations from AE1 cells, a nucleoside-transport deficient clone derived from the S49 cell line. Reversible binding of [3H]nitrobenzylthioinosine to the S49 membrane preparations was inhibited by adenosine, nitrobenzylthioguanosine, and dipyridamole. Exposure of S49 membrane preparations to UV light in the presence of [3H]nitrobenzylthioinosine resulted in the covalent radiolabeling of a membrane protein(s) which migrated on sodium dodecyl sulfate-polyacrylamide gels with an apparent Mr of 45,000 to 66,000. Labeling of this protein was abolished in the presence of nitrobenzylthioguanosine and markedly reduced in the presence of adenosine and dipyridamole. AE1 membrane proteins were not covalently labeled under these conditions.

Nucleoside transport in human erythrocytes. Nitrobenzylthioinosine binding and uridine transport activities have similar radiation target sizes

Intact human erythrocytes were irradiated in the frozen state with a high-energy electron beam. Nitrobenzylthioinosine-sensitive uridine influx, equilibrium exchange uridine influx and high-affinity nitrobenzylthioinosine binding were inactivated as a simple exponential function of the radiation dose, indicating an in situ target size of 122 000. The results suggest that the nitrobenzylthioinosine-binding site(s) and the permeation site(s) of the transporter are present on the same transporter element.