Nucleoside transport in mammalian cell membranes. III. Kinetic and chemical modification studies of cytosine-arabinoside and uridine transport in hamster cells in culture

Sensitivity to inhibition by N-ethylmaleimide: a property of nitrobenzylthioinosine-sensitive equilibrative nucleoside transporter of murine myeloma cells

Murine myeloma SP2/0-Ag14 cells possess both nitrobenzylthioinosine (NBMPR)-sensitive and NBMPR-insensitive equilibrative uridine transport systems. No Na(+)-dependent uridine transport system was detected. The NBMPR-insensitive transport system is similarly insensitive to inhibition by dilazep and dipyridamole. Dose-response curve for the inhibition of equilibrative uridine transport by N-ethylmaleimide (NEM), a sulfhydryl reagent, in these cells was biphasic. About 30-40% of the uridine transport was inhibited by NEM at IC50 value of 0.15 mM. The other 60-70% of the transport activity remained insensitive to NEM at concentration as high as 3 mM. The decrease in NBMPR-sensitive uridine transport in the presence of 0.3 mM NEM was due to a 3-fold decrease in transport affinity. Apparent Km values of 500 and 1600 microM and Vmax values of 13 and 12 microM/s were obtained for untreated and NEM-treated cells, respectively. NEM (0.3 mM) has little effect on the Km of NBMPR-insensitive transporter, with apparent Km values of 100 and 110 microM and Vmax values of 3.0 and 2.5 microM/s for untreated and NEM-treated cells, respectively. High sensitivity of NBMPR-sensitive transporter to NEM inhibition was also observed in HL-60 and MCF-7 cells. Decrease in specific 3H-NBMPR equilibrium binding affinity in myeloma cells was observed after treatment with 0.3 mM NEM. Apparent Kd values of 0.32 and 2.3 nM with Bmax values of 48,000 and 44,000 sites/cell were obtained for untreated and NEM-treated cells, respectively. NBMPR, dilazep and dipyridamole at 30 microM, and uridine at 10 mM failed to protect the NBMPR-sensitive transporter against NEM inhibition. It is possible that a critical sulfhydryl residue is closed to substrate binding/transporting site of the NBMPR-sensitive transporter. NEM, a sulfhydryl reagent containing an activated double bond, hinders the affinity of this transporter by forming a stable thiol ether bond with the reactive residue.

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.

Nucleoside transport in rat erythrocytes: two components with differences in sensitivity to inhibition by nitrobenzylthioinosine and p-chloromercuriphenyl sulfonate

The sensitivity of nucleoside transport by rat erythrocytes to inhibition by nitrobenzylthioinosine (NBMPR) and the slowly permeating organomercurial, p-chloromercuriphenyl sulfonate (pCMBS), was investigated. The dose response curve for the inhibition of uridine transport (100 microM) by NBMPR was biphasic--35% of the transport activity was inhibited with an IC50 value of 0.25 nM, but 65% of the activity remained insensitive to concentrations as high as 1 microM. These two components of uridine transport are defined as NBMPR-sensitive and NBMPR-insensitive, respectively. Uridine influx by both components was saturable and conformed to simple Michaelis-Menten kinetics, and was inhibited by other nucleosides. The uridine affinity of the NBMPR-sensitive transport component was threefold higher than for the NBMPR-insensitive transport mechanism (apparent Km for uridine 50 +/- 18 and 163 +/- 28 microM, respectively). The two transport systems also differed in their sensitivity to pCMBS. NBMPR-insensitive uridine transport was inhibited by pCMBS with an IC50 of approximately 25 microM, while 1 mM pCMBS had little effect on NBMPR-sensitive transport by intact cells. pCMBS inhibition was reduced in the presence of uridine and adenosine and reversed by the addition by beta-mercaptoethanol, suggesting that the pCMBS-sensitive thiol group is located on the exterior surface of the erythrocyte membrane within the nucleoside binding site of the transport system. Inhibition of uridine transport by NBMPR was associated with high-affinity [3H]NBMPR binding to the cell membrane (apparent Kd 46 +/- 25 pM). Binding of inhibitor to these sites was competitively blocked by uridine and inhibited by adenosine, thymidine, dipyridamole, dilazep and nitrobenzylthioguanosine. Assuming that each NBMPR-sensitive transport site binds a single molecule of NBMPR, the calculated translocation capacity of each site is 25 +/- 6 molecules/site per sec at 22 degrees C. pCMBS had no effect on [3H]NBMPR binding to intact cells but markedly inhibited binding to disrupted membranes indicating that the NBMPR-sensitive nucleoside transporter probably has a thiol group located on the inner surface of the membrane. Exposure of rat erythrocyte membranes to UV light in the presence of [3H]NBMPR resulted in covalent radiolabeling of a membrane protein(s) (apparent Mr on SDS gel electropherograms of 62,000). Labeling of this protein was abolished in the presence of nitrobenzylthioguanosine. We conclude that nucleoside transport by rat erythrocytes occurs by two facilitated-diffusion systems which differ in their sensitivity to inhibition by both NBMPR and pCMBS.

Human red-blood-cell Ca2+-antagonist binding sites. Evidence for an unusual receptor coupled to the nucleoside transporter

The human red blood cell ghost Ca2+-antagonist binding sites were characterized with (+/-)-[3H]nimodipine. The labelled 1,4-dihydropyridine bound in a non-cooperative, reversible manner with a Kd of 52 nM at 25 degrees C to 9.65 pmol sites/mg ghost protein. The stereochemistry of the binding domain was evaluated with the optically pure enantiomers of chiral 1,4-dihydropyridines. In contrast to the 1,4-dihydropyridine-selective receptors on Ca2+ channels in electrically excitable tissues, the (+) enantiomer of nimodipine and the (-) enantiomer of the benzoxadiazol 1,4-dihydropyridine (PN 200-110) were bound with higher affinity than the respective optical antipodes. The human red blood cell ghost [3H]nimodipine-labelled sites also interacted with the inorganic Ca2+-antagonist La3+ (increase in the number of binding sites), and were allosterically regulated by the optical enantiomers of the phenylalkylamine-type Ca2+-antagonists (e.g. verapamil, desmethoxyverapamil, methoxyverapamil). The benzothiazepines d- or l-cis-diltiazem were without effect. Nucleosides (adenosine approximately equal to inosine greater than cytidine) were inhibitory at the nimodipine-labelled site, as were the nucleoside uptake inhibitors dipyridamole, hexobendine, dilazep, nitrobenzylthioinosine and nitrobenzylthioguanosine. The binding sites have essential sulfhydryl groups, show trypsin sensitivity, but are relatively heat stable. When nitrobenzylthioinosine was employed as a covalent probe to inactivate the red blood cell ghost nucleoside carrier, [3H]nimodipine binding was irreversibly lost. (+)-Nimodipine greater than (-)-nimodipine inhibited [14C]adenosine transport into human red blood cells. A good correlation between IC50 values for inhibition of [3H]nimodipine binding and IC50 values for inhibition of [14C]adenosine uptake was found for 18 compounds. Sheep red blood cells (which lack the nucleoside transporter) had no detectable [3H]nimodipine binding sites. It is concluded that the Ca2+-antagonist receptor sites of the human erythrocyte are coupled to the nucleoside transporter.

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

We have measured by rapid kinetic techniques the zero-trans influx of hypoxanthine in various cell lines and its sensitivity to inhibition by uridine, dipyridamole, nitrobenzylthioinosine and nitrobenzylthiopurine. The results and those reported earlier divided the cells into two distinct groups. In mouse P388, L1210 and L929 cells uridine and hypoxanthine had little effect on the transport of each other, supporting the view that nucleosides and hypoxanthine are transported by different carriers. In these cells, hypoxanthine transport was also uniquely resistant to inhibition by dipyridamole (IC50 (50% inhibition dose) greater than 30 microM). In Novikoff and HTC rat hepatoma, Chinese hamster ovary and Ehrlich ascites tumor cells, on the other hand, hypoxanthine and uridine inhibited the transport of each other about 50% at a concentration corresponding to the Michaelis-Menten constant of their transport, and hypoxanthine transport was strongly inhibited by dipyridamole (IC50 = 100 to 400 nM). Although these results are compatible with the view that nucleosides and hypoxanthine are transported by a common carrier in these cells, this conclusion is not supported by the finding that uridine transport is strongly inhibited in some of these cell lines, as in the first group of cells, by nitrobenzylthioinosine, whereas hypoxanthine transport is highly resistant in all cell lines tested. In contrast, the transport of both substrates is highly resistant to inhibition by nitrobenzylthiopurine. The Michaelis-Menten constants for uridine transport are about the same in all cell lines. The Michaelis-Menten constants for hypoxanthine transport are similar to those for uridine transport in some cell lines, but are much higher in others.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleoside transport in cultured mammalian cells. Multiple forms with different sensitivity to inhibition by nitrobenzylthioinosine or hypoxanthine

The zero-trans influx of 500 microM uridine by CHO, P388, L1210 and L929 cells was inhibited by nitrobenzylthioinosine ( NBTI ) in a biphasic manner; 60-70% of total uridine influx by CHO cells and about 90% of that in P388, L1210 and L929 cells was inhibited by nmolar concentrations of NBTI (ID50 = 3-10 nM) and is designated NBTI -sensitive transport. The residual transport activity, designated NBTI -resistant transport, was inhibited by NBTI only at concentrations above 1 microM (ID50 = 10-50 microM). S49 cells exhibited only NBTI -sensitive uridine transport, whereas Novikoff cells exhibited only NBTI -resistant uridine transport. In all instances NBTI -sensitive transport correlated with the presence of between 7 7 X 10(4) and 7 X 10(5) high-affinity NBTI binding sites/cell (Kd = 0.3-1 nM). Novikoff cells lacked such sites. The two types of nucleoside transport, NBTI -resistant and NBTI -sensitive, were indistinguishable in substrate affinity, temperature dependence, substrate specificity, inhibition by structurally unrelated substances, such as dipyridamole or papaverine, and inhibition by sulfhydryl reagents or hypoxanthine. We suggest, therefore, that a single nucleoside transporter can exist in an NBTI -sensitive and an NBTI -resistant form depending on its disposition in the plasma membrane. The sensitive form expresses a high-affinity NBTI binding site(s) which is probably made up of the substrate binding site plus a hydrophobic region which interacts with the lipophilic nitrobenzyl group of NBTI . The latter site seems to be unavailable in NBTI -resistant transporters. The proportion of NBTI -resistant and sensitive uridine transport was constant during proportion of NBTI -resistant and sensitive uridine transport was constant during progression of P388 cells through the cell cycle and independent of the growth stage of the cells in culture. There were additional differences in uridine transport between cell lines which, however, did not correlate with NBTI sensitivity and might be related to the species origin of the cells. Uridine transport in Novikoff cells was more sensitive to inhibition by dipyridamole and papaverine than that in all other cell lines tested, whereas uridine transport in CHO cells was the most sensitive to inactivation by sulfhydryl reagents.

Cytosine arabinoside transport by human leukaemic cells

The membrane transport of cytosine arabinoside (araC) has been studied in blasts freshly isolated from a variety of acute leukaemias. The major fraction of araC influx was facilitated and this fraction was 80-87% at l microM araC and 68-80% at 200 microM araC. Competitive kinetics were observed between araC and deoxycytidine for entry into leukaemic blasts and, moreover, araC influx was blocked by phloretin, a broad-spectrum inhibitor of facilitated transport systems. Kinetic analysis of facilitated araC influx gave KmS which varied over a 10-fold range between patients and which were positively correlated to the Vmax. Nucleoside influx Vmax also varied over an 80-fold range between individuals, although the mean araC transport was 4-fold greater in myeloblasts than in lymphoblasts. Larger transport of araC may explain the greater sensitivity of acute myeloid leukaemia to this drug.

Nitrobenzylthioinosine-insensitive uridine transport in human lymphoblastoid and murine leukemia cells

Both human lymphoblastoid (RPMI 6410) and murine leukemia (L1210) cells were found to have a component of uridine transport which is insensitive to the nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR). In both cell lines NBMPR-insensitive uridine transport is inhibited by other nucleosides and by the sulfhydryl reagent p-chloromercuribenzenesulfonate. In RPMI 6410 cells NBMPR-insensitive transport accounts for only 2% of the initial rate of uridine transport. In contrast, 20% of the initial rate of transport of L1210 cells is insensitive to NBMPR, and uridine uptake over longer periods (10 min) is completely insensitive to NBMPR.

Binding of nitrobenzylthioinosine to high-affinity sites on the nucleoside-transport mechanism of HeLa cells

Nitrobenzylthioinosine (NBMPR) binds reversibly, but with high affinity (Kd 0.1--1.2 nM), to inhibitory sites on nucleoside-transport elements of the plasma membrane in a variety of animal cells. The present study explored relationships in HeLa cells between NBMPR binding and inhibition of uridine transport. The Km value for inward transport of uridine by HeLa cells in both suspension and monolayer culture was about 0.1 mM. The affinity of the transport-inhibitory sites for uridine (Kd 1.7 mM), inosine (Kd 0.4 mM) and other nucleoside permeants was low relative to that for NBMPR. The pyrimidine homologue of NBMPR, nitrobenzylthiouridine, also exhibited low affinity for the NBMPR-binding sites. Pretreatment of HeLa cells with p-chloromercuribenzene sulphonate (p-CMBS) or N-ethylmaleimide (NEM) decreased binding of NBMPR to its high-affinity sites and inhibited uridine transport, indicating the presence of thiol groups essential to both processes. NEM, a more penetrable reagent than p-CMBS, inhibited binding and transport at much lower concentrations than the latter compound. Pretreatment of cells with concentrations of p-CMBS that alone had no effect on either NBMPR binding or uridine transport increased the sensitivity of transport to NBMPR inhibition and changed the shape of the NBMPR concentration-effect curve, suggesting synergistic inhibiton of uridine-transport activity by these two agents.

Nucleoside transport in mammalian cell membranes. IV. Organomercurials and organomercurial-mercaptonucleoside complexes as probes for nucleoside transport systems in hamster cells

Organomercurials form stable stoichiometric complexes with thiolated nucleosides. The complexes inhibited uptake of ribonucleosides and cytosine arabinoside (CAR) in various types of normal and transformed cells. The inhibition was competitive and reversible (Ki = 3--6 micrometer). The interaction between complexes and transport system displayed a 1:1 stoichiometry. Chemical factors which contributed to the inhibitory power were evaluated with a series of S-alkylated derivatives and S--Hg--R complexes of mercaptonucleosides. The inhibitory potency was not determined exclusively by the hydrophobic nature of either the S-alkylated or the S--Hg--R moieties. Chemical modification of cells with penetrating and nonpenetrating organomercurials lead to stimulation of nucleoside uptake and to an increase in its susceptibility to inhibition by S--Hg--R complexes or S-aklylated derivatives of mercaptopurine ribosides. The kinetic and chemical data obtained with nucleoside analogs and with chemical modifiers suggested complex features of nucleoside transport systems. Four distinct classes of sites were implied: (i) a substrate binding site susceptible directly to competitive inhibition by organomercurial-mercaptonucleoside complexes, (ii) an additional site susceptible either to S-arylalkylated or S-mercuriated derivatives of 6-mercaptopurine ribosides, (iii) SH-containing modifier sites which stimulate uridine uptake upon binding of organomercurials, and (iv) SH-containing modifier sites which inhibit the function upon binding of organomercurials. From the observation that only SH sites related to stimulation were susceptible to modification by macromolecular-SH modifier probes, some conclusions can be drawn regarding the disposition of the various sites in the cell membrane in general and among membrane components in particular.