Permeation of Nucleosides, Nucleic Acid Bases, and Nucleotides in Animal Cells

Enhancement of 5-fluorouracil's anticancer activity by dipyridamole

Although the interaction between FUra and DP in HCT 116 cells is fairly complex, data from other investigators indicate that in cell lines in which inhibition of TS is growth limiting at relatively low concentrations of fluoropyrimidines, DP appears to augment the cytotoxicity of FUra and FdUrd by blocking the salvage of dThd (Miller et al., 1987; Schwartz et al., 1987). The previous in vitro data regarding the ability of DP to modulate the toxicity of fluoropyrimidines was obtained in exponentially growing cells. An additional observation that warrants consideration is a report that the inhibition of nucleoside incorporation by DP changed as a function of time in culture (Zhen et al., 1986). Hepatoma 3924A cells in lag and log phase were highly sensitive to DP with IC50 values for dThd incorporation of 0.2 and 0.32 microM, respectively. In contrast, stationary phase cells were insensitive to DP (IC50 = 38.9 microM). Amphotericin B, an antifungal agent which perturbs cell membranes, restored the sensitivity to DP in stationary cells. Several investigators have presented information on the effect of DP on fluoropyrimidines in normal tissues. Lee and Park (1987) examined the effect of DP on FUra and MTX toxicity in a soft-agar cloning assay against two human cancer cell lines and on pooled normal human bone marrow (CFU-C). DP (1 microM) potentiated the action of both MTX (0.1 microM) and FUra (5 microM) on Hep-2 (epidermoid carcinoma), MCF-7 (breast carcinoma) and CFU-C in medium supplemented with either non-dialyzed or dialyzed serum. Woodcock et al. (1987) incubated gallbladder mucosa, obtained from patients undergoing elective surgery for cholelithiasis, with control medium or varying concentrations of DP for 1 hr, and then exposed the mucosal cells to 2.5 microCi [3H]-FdUrd (2.5 microM). After 1 hr, the uptake of FdUrd into the tissue was inhibited to 49% and 42% of control by 0.1 microM and 1 microM, respectively.

Regulation of lipid metabolism by dipyridamole and adenosine antagonists in rat adipocytes

1. Acute effects of dipyridamole, an inhibitor of adenosine transport, direct activators of adenylate cyclase and thirteen adenosine antagonist analogs on fatty acid synthesis have been examined in terms of the control of [1-14C]acetate incorporation into labeled fatty acids in the presence of glucose. 2. This monosaccharide acts as a stimulator of lipogenesis by generating NADPH for the lipid synthesis. 3. The relationship between lipogenesis and lipolysis was compared with a variety of adenylate cyclase stimulators. 4. The data obtained reveals that dipyridamole potentiated the inhibitory or stimulatory effects of isoproterenol and forskolin on lipogenesis and on lipolysis, respectively. 5. In these cases the data show that it exists an inverse relationship between lipogenesis and lipolysis. 6. Dipyridamole and methylxanthine analogs only moderately affect the rate of lipolysis whereas its effects are more potent on lipogenesis and lend further support to the hypothesis that dipyridamole antagonize adenosine actions as well as methyl xanthines. 7. These results suggest that dipyridamole and adenosine antagonists alter lipogenesis independently of the lipolytic process and that it exists an inverse relationship between lipogenesis and lipolysis under some conditions whereas there are not under others.

Adenosine induction of rapid catabolism of adenine ribonucleotides and independent elevation of the ATP content in quiescent mouse fibroblasts

The influence of adenosine on the ribonucleotide metabolism in quiescent BALB/c 3T3 cells was studied. The cellular adenine ribonucleotides were labelled by pretreating the cells with [2-3H]-adenine. After addition of adenosine to the cell cultures, the amount and radioactivity of the cellular purine ribonucleotides and the radioactivity of the purine compounds in the medium were determined. It appeared that adenosine gave rise both to rapid catabolism of adenine ribonucleotides with inosine 5'-monophosphate (IMP) as an intermediate and to expansion of the cellular adenosine 5'-triphosphate (ATP) pool. The maximal rates and the apparent activation constants for the two processes have been determined. Experiments with varying concentrations of coformycin (an inhibitor of adenosine 5'-monophosphate [AMP] deaminase and adenosine deaminase) and of 5'-amino-5'-deoxyadenosine (an inhibitor of adenosine kinase), respectively, showed that each compound may almost completely inhibit the adenosine-induced catabolism. This effect can be obtained under conditions where there was little or no effect by the two inhibitors on the rate of expansion of the cellular ATP pool. These results may best be explained by assuming that the process of expansion of the ATP pool is independent of the induced catabolism of adenine ribonucleotides, even though both processes seem to depend on the phosphorylation of adenosine to AMP. The total increase in the pool size of ATP and of guanosine 5'-triphosphate (GTP), both caused by adenosine, seems not to have regulatory effect on adenine ribonucleotide catabolism.

Na+- and K+-dependent uridine transport in rat renal brush-border membrane vesicles

The transport of uridine into rat renal brush-border membrane vesicles was investigated using an inhibitor-stop filtration method. Uridine was not metabolized under these conditions. The rapid efflux of intravesicular uridine was prevented by adding 1 mM phloridzin to the ice-cold stop solution. In the presence of inwardly directed gradients of either Na+ or K+, zero-trans uridine uptake exhibited a transient overshoot phenomenon indicating active transport. The overshoot was much more pronounced with Na+ than K+ and it was not observed when either Na+ or K+ was at equilibrium across the membrane. The K+-induced overshoot was not due to the presence of a membrane potential alone, as an inwardly directed gradient of choline chloride failed to produce it. The amplitude of the overshoot was increased by raising either the Na+ or K+ concentration outside the membrane or by using more lipophilic anions (reactive order was NO3- greater than SCN- greater than Cl- greater than SO4(2-). Zero-trans efflux studies showed that the uridine transport is bidirectional. Li+ could substitute poorly for Na+ but not at all for K+. Stoichiometries of 1:1 and greater than 1:1 were observed for Na+: uridine and K+: uridine coupling, respectively. A preliminary analysis of the interactions between Na+ and K+ for uridine uptake showed complex interactions which can best be explained by the involvement of two different systems for nucleoside transport in the rat renal brush-border membrane, one requiring Na+ and the other K+ as transport coupler.

The role of blood platelets in nucleoside metabolism: assay, cellular location and significance of thymidine phosphorylase in human blood

The enzyme thymidine phosphorylase (thymidine: orthophosphate deoxyribosyltransferase, EC 2.4.2.4), which plays a crucial role in nucleic acid metabolism in both prokaryotic and eukaryotic cells by regulating the availability of thymidine, is present in mammalian blood. Here we describe a simple, rapid HPLC-based micromethod for the assay of blood thymidine phosphorylase. We have arbitrarily defined 1 unit of blood thymidine phosphorylase activity as the activity required to produce a 1-nM increment in the plasma concentration of thymine after incubation for 1 h at 37 degrees C with a saturating concentration of exogenous thymidine. In normal adults, whole (peripheral venous) blood thymidine phosphorylase activity with blood cells intact was 64 +/- 11 units (mean +/- S.D., n = 20, range 45-89). The apparent Michaelis constant for thymidine was of the order of 10(-4) M but varied nearly 5-fold between different individuals. Activity increased when blood cells were permeabilised or lysed with non-ionic detergents, implying that thymidine phosphorylase is an intracellular enzyme which may be influenced by exogenous as well as intracellular factors. When blood from normal donors was fractionated, thymidine phosphorylase activity consistently co-isolated with platelets. Whole-blood thymidine phosphorylase activity correlated well with platelet parameters. Although thymidine phosphorylase activity was also detected in plasma and serum, the small size and notorious fragility of platelets suggest its platelet origin. Blood from leukaemic donors showed significantly increased thymidine phosphorylase activity compared to normal controls (mean activity +/- S.D. was 96 +/- 27 units; range 58-140, n = 8). Thymine formation from thymidine was temperature- and pH-dependent in whole blood. 2'-Deoxyuridine and 3 of its 5-halogenated analogues (but not 3'-azido-3'-deoxythymidine (AZT), were catabolised by blood thymidine phosphorylase, even during blood clotting at room temperature. Assumptions about in vivo concentrations of these compounds should therefore be interpreted cautiously. In the presence of high concentrations of thymine and suitable deoxyribose donors, small amounts of thymidine were formed in some blood samples, so it is conceivable that thymidine catabolism may be reversible in vivo under some circumstances.

Pharmacological characterization of rapidly accumulated adenosine by dissociated brain cells from adult rat

Mechanically dissociated brain cells from adult rats were used to study biochemically and pharmacologically their capacity to accumulate rapidly [3H]adenosine. The assay, which used an inhibitor-stop method to prevent further uptake into cells, was characterized with respect to protein and optimal substrate concentrations, and incubation times that ranged from 5 to 180 s. The accumulation of [3H]adenosine using 15-s incubation periods, conditions under which less than 10% of accumulated [3H]adenosine was metabolized, was best described kinetically by a two-component system with Km and Vmax values for the high-affinity component of 0.8 microM and 6.2 pmol/mg protein/15 s and for the low-affinity component 259 microM and 2,217 pmol/mg protein/15 s, respectively. The potencies with which nucleosides, adenosine deaminase resistant adenosine receptor agonists, and nucleoside uptake inhibitors competed for these uptake components were determined. Of the nucleosides examined, adenosine was the "preferred" substrate for the uptake site. The Ki value of adenosine for the high-affinity component was 10.7 microM. Inosine and uridine competed for a single lower affinity uptake system: Ki values were 142 and 696 microM, respectively. Nucleoside uptake inhibitors--nitrobenzylthioinosine, dipyridamole, and dilazep--were the most potent inhibitors of [3H]adenosine accumulation tested: the Ki values for the high-affinity system were 0.11, 1.3, and 570 nM, respectively. The adenosine analogs S-phenylisopropyladenosine, R-phenylisopropyladenosine, and cyclohexyladenosine inhibited the high-affinity component with Ki values of 2.3, 9.3, and 14.5 microM, respectively. N-Ethylcarboxamidoadenosine competed for a single lower affinity uptake system: Ki, 292 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Species differences in sensitivity of nucleoside transport in erythrocytes and cultured cells to inhibition by nitrobenzylthioinosine, dipyridamole, dilazep and lidoflazine

Differences in sensitivity of uridine transport in erythrocytes and cultured cells to inhibition by dipyridamole, dilazep and lidoflazine were largely species-specific; uridine transport in human cells, and probably in pig and rabbit cells, was 2-3- and 10-times more sensitive to inhibition by dipyridamole (IC50 approx. 50 nM) and about 10- and 20-times more sensitive to dilazep inhibition (IC50 approx. 5 nM) than transport in mouse and rat cells, respectively. Uridine transport in human erythrocytes and HeLa cells was strongly inhibited by lidoflazine (IC50 10-140 nM), whereas that in both mouse and rat cells was highly resistant (IC50 greater than 10 microM). Superimposed on species-specific differences were some cell type specific differences in sensitivity of nucleoside transport to these inhibitors. Uridine transport in Walker 256 rat carcinoma cells was more resistant to dipyridamole and dilazep than that of other rat cells. Transport in human Hep-2 cells was more resistant to lidoflazine (IC50 2000 nM) than that of human erythrocytes and HeLa cells, whereas it showed similar sensitivity to dilazep and dipyridamole. Uridine transport in Chinese hamster cells was also more resistant to dilazep than that of baby hamster kidney cells. In addition HeLa cells and clones thereof expressed uridine transporters (about 50% each) with difference of about 1000-fold in sensitivity to inhibition by dilazep (IC50 approx. 5 nM and 5 microM, respectively).

Adenosine uptake and [3H]nitrobenzylthioinosine binding in developing rat brain

The ontogenesis of adenosine transport sites as labelled with [3H]nitrobenzylthioinosine ([3H]NBI) was examined using radioligand binding and membrane preparations from whole brain and 4 brain regions of rats between the postnatal ages of one day through to adulthood. In whole brain, cerebral cortex and cerebellum, [3H]NBI binding was two-fold higher in 6-day-old than in 50-day-old rats. In contrast, [3H]NBI binding was higher in adults than in one-day-old rats by 4-fold in hypothalamus and 8-fold in superior colliculus. In cortex and hypothalamus, the levels of [3H]NBI binding in newborn and adult rats were reflected by changes in Bmax and not Kd values. As a measure of the utility of [3H]NBI as a probe for identifying functional adenosine transport sites, we examined [3H]NBI binding to and [3H]adenosine accumulation by intact brain cells prepared from adult and newborn rats. For [3H]NBI binding to brain cells from adult rats, the values of Kd were 0.092 nM and of Bmax were 274 fmol/mg protein. For newborns, slightly higher Kd and Bmax values were observed; 0.2 nM and 395 fmol/mg protein, respectively. [3H]Adenosine accumulation was higher in brain cells from one-day-old than from adult rat brains. Kinetically this uptake was best described by a two-component model: the Vmax values for the high- and low-affinity uptake, and the Km value for the high-affinity component in one-day-old rats were greater than in adults.(ABSTRACT TRUNCATED AT 250 WORDS)

Purine nucleosides as cell-specific modulators of 5-fluorouracil metabolism and cytotoxicity

Purine nucleosides and ribose-5-phosphate (Rib-5-P) were used to modulate the metabolism and cytotoxicity of 5-fluorouracil (5-FU) in order to get a better understanding of the mechanism of action of 5-FU. In extracts from five different cell lines both Rib-5-P and inosine were relatively good precursors for Rib-1-P, but deoxyinosine was a moderate to poor precursor for deoxyRib-1-P. In the human colon carcinoma WiDr and the human epithelial intestinal Intestine 407 inosine enhanced Rib-1-P concentrations 3-6-fold. Incubation with deoxyinosine resulted in the appearance of deoxyRib-1-P in both cell lines in levels comparable to those of Rib-1-P. dIMP had the same effect as deoxyinosine in Intestine 407 cells, but not in WiDr cells. Both inosine and deoxyinosine caused a depletion of phosphoribosyl-pyrophosphate. In WiDr cells deoxyinosine (0.1-1.0 mM) clearly potentiated the growth inhibition by 0.1-0.5 microM 5-FU after 24 h of culture, but growth between 24 and 48 h was normal. In Intestine 407 cells the potentiation of 5-FU cytotoxicity by deoxyinosine was even more pronounced at 48 h than at 24 h. In WiDr cells dIMP did not potentiate 5-FU cytotoxicity, but in Intestine 407 cells the effect was comparable to that of deoxyinosine. The lack of potentiation in WiDr was accompanied by a low metabolism of dIMP. Growth inhibition by 5-FU and deoxyinosine could be reversed by thymidine in Intestine 407 cells but not completely in WiDr cells. Since the predominant target of the deoxyinosine-5-FU combination was thymidylate synthase, we analyzed the inhibition of this enzyme by FdUMP and the retention of the inhibition in cell culture. In both cell lines FdUMP was a potent competitive inhibitor of thymidylate synthase with a Ki of between 0.5 and 2 nM. Culture of cells in the presence of 5-FU and deoxyinosine resulted in an almost complete inhibition of thymidylate synthase activity after 24 h but after 48 h the activity was partly recovered. In Intestine 407 cells replenishment of the culture medium at 24 h even enhanced the recovery. Analysis of 5-FU anabolism into nucleic acids demonstrated that deoxyinosine inhibited the incorporation of 5-FU into RNA. It is concluded that in Intestine 407 cells addition of deoxyinosine enhanced the effects of 5-FU on growth inhibition due to increased formation of FdUMP leading to enhanced inhibition of thymidylate synthase.(ABSTRACT TRUNCATED AT 400 WORDS)

Purine and pyrimidine transport and permeation in human erythrocytes

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

Treatment of schistosomiasis by purine nucleoside analogues in combination with nucleoside transport inhibitors

In contrast to their effects on mammalian cells, the nucleoside transport inhibitors nitrobenzylthioinosine 5'-monophosphate (NBMPR-P) dilazep, benzylacyclouridine (BAU), and to a lesser extent, dipyridamole have no significant effect on the in vitro uptake of adenosine analogues by Schistosoma mansoni [el Kouni and Cha, Biochem. Pharmac. 36, 1099 (1987)]. Coadministration of either NMBPR-P or dilazep with potentially lethal doses of tubercidin (7-deazaadenosine), nebularine or 9-deazaadenosine protected mice from the toxicity of these adenosine analogues. Dipyridamole caused partial protection, whereas BAU did not protect the animals from this toxicity. Toyocamycin caused delayed mortality (after 16 weeks) which could not be prevented by coadministration of NBMPR-P. In S. mansoni infected mice, treated with the combination of NBMPR-P and 9-deazaadenosine was not effective against the parasite. On the other hand, the combinations of NBMPR-P or dilazep with either tubercidin or nebularine were highly toxic to the parasite but not the host. Combination therapy caused a marked reduction in the number of pairing of worms. Effectiveness of combination therapy could also be noted by a drastic decrease in the number of eggs in the liver and small intestine. All eggs found were dead, indicating a direct effect on ovigenesis. Although dipyridamole was less effective than NBMPR-P or dilazep in protecting the host from the toxicity of tubercidin or nebularine, the combinations with dipyridamole produced similar significant therapeutic effects in animals that survived. Mice receiving the combination of tubercidin (or nebularine) plus NBMPR-P or dilazep, as well as those that survived the combination with dipyridamole, appeared healthy and were found to have normal size livers and spleens. These results suggest that highly selective toxicity against schistosomes can be achieved by coadministration of various nucleoside transport inhibitors with adenosine analogues.

Purification and reconstitution studies of the nucleoside transporter from pig erythrocytes

The pig erythrocyte nucleoside transporter has been identified as a band 4.5 polypeptide (Mr 64,000) on the basis of photoaffinity labelling experiments with the nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR). This protein was purified 140-fold by treatment of haemoglobin-free erythrocytes 'ghosts' with EDTA (pH 11.2) to remove extrinsic proteins, extraction of the protein-depleted membranes with n-octyl-glucoside and subsequent gradient-elution ion-exchange chromatography on DEAE-cellulose. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the purified material revealed the presence of only two detectable protein bands, one which co-migrated with the radiolabelled NBMPR-binding protein, and a lower molecular weight species with an Mr of 43,000. The latter protein may be a degradation product of the band 3 anion-exchange transporter. The overall purification of the NBMPR-binding protein with respect to the Mr 64,000 band was 350-fold. Reversible NBMPR-binding to the partially-purified band 4.5 preparation was saturable (apparent Kd 7.2 nM). Adjustment of the chromatography conditions to allow elution of the NBMPR-binding protein along with the majority of solubilised membrane phospholipid reduced the apparent Kd value to 3.0 nM. Purification of reversible NBMPR-binding activity during ion-exchange chromatography was paralleled by an increase in the specific activity of nitrobenzylthioguanosine (NBTGR) -sensitive uridine transport as assayed in proteoliposomes reconstituted by a freeze-thaw-sonication procedure.

Cardiac myocyte guanosine transport and metabolism

Guanosine transport and metabolism were examined in adult rat cardiac myocytes. Myocytes transported guanosine via saturable [Km = 18 microM, maximum velocity (Vmax) = 3.61 pmol.mg-1.s-1] and nonsaturable (rate constant = 1.47 X 10(-2] processes. The saturable process was inhibited by nitrobenzyl-thioinosine, inosine [inhibition constant (Ki) = 180 microM], and adenosine (Ki = 112 microM). Extracellular guanosine taken up by myocytes was slowly phosphorylated to guanine nucleotides. The majority of guanosine (98%) existed as free intracellular guanosine after 60 s. Countertransport of nucleosides could not be demonstrated in these cells at physiological concentrations in the presence of up to a 10-fold gradient of nucleoside. These studies indicate that adult rat cardiac myocytes can be used to assess myocardial guanosine transport separate from its metabolism. Comparable inhibition of guanosine and adenosine transport by each other and by inosine support the hypothesis that guanosine and adenosine are transported by a common carrier.

Biochemistry of an olfactory purinergic system: dephosphorylation of excitatory nucleotides and uptake of adenosine

The olfactory organ of the spiny lobster, Panulirus argus, is composed of chemosensory sensilla containing the dendrites of primary chemosensory neurons. Receptors on these dendrites are activated by the nucleotides AMP, ADP, and ATP but not by the nucleoside adenosine. It is shown here that the lobster chemosensory sensilla contain enzymes that dephosphorylate excitatory nucleotides and an uptake system that internalizes the nonexcitatory dephosphorylated product adenosine. The uptake of [3H]-adenosine is saturable with increasing concentration, linear with time for up to 3 h, sodium dependent, insensitive to moderate pH changes and has a Km of 7.1 microM and a Vmax of 5.2 fmol/sensillum/min (573 fmol/micrograms of protein/min). Double-label experiments show that sensilla dephosphorylate nucleotides extracellularly; 3H from adenine-labeled AMP or ATP is internalized, whereas 32P from phosphate-labeled nucleotides is not. The dephosphorylation of AMP is very rapid; 3H from AMP is internalized at the same rate as 3H from adenosine. Sensillar 5'-ectonucleotidase activity is inhibited by ADP and the ADP analog alpha, beta-methylene ADP. Collectively, these results indicate that the enzymes and the uptake system whereby chemosensory sensilla of the lobster inactivate excitatory nucleotides and clear adenosine from extracellular spaces are very similar to those present in the internal tissues of vertebrates, where nucleotides have many neuroactive effects.

Effects of dipyridamole on adenosine concentration, insulin sensitivity and glucose utilisation in soleus muscle of the rat

Adenosine has been shown to modulate the sensitivity of skeletal muscle to insulin (Budohoski et al. 1984). In an attempt to further characterize the modulatory action of adenosine on insulin sensitivity in skeletal muscle we have investigated the effect of the nucleoside transport inhibitor dipyridamole in isolated incubated soleus muscle strips. At a concentration of 50 microM, dipyridamole increased the concentration of adenosine in the soleus muscle by 36% and in the incubation medium by 32%. At this concentration of dipyridamole, the basal rates (in the presence of 1 microunit of insulin/ml) of lactate formation, 2-deoxy [2,6-3H]glucose phosphorylation and glucose oxidation were decreased by 48%, 43% and 47% respectively, whilst the rate of glycogen synthesis was unaffected. Insulin-stimulated rates (in the presence of 10,000 microunits of insulin/ml) of lactate formation, 2-deoxy [2,6-3H] glucose phosphorylation, glycogen synthesis and glucose oxidation were decreased by 70%, 30%, 26% and 20% respectively in the presence of 50 microM dipyridamole. Although 50 microM dipyridamole was required to exert a significant effect on medium and soleus muscle adenosine concentrations, statistically significant effects on glycolytic rate were observed at concentrations as low as 2 microM dipyridamole. It is concluded that the results are not consistent with dipyridamole exerting an effect on skeletal muscle carbohydrate metabolism solely through elevation of the intracellular or interstitial adenosine concentration, but strongly suggest that dipyridamole inhibits glucose transport and/or phosphorylation in skeletal muscle.

Na+/adenosine co-transport in Vibrio parahaemolyticus

Adenosine transport in Vibrio parahaemolyticus was studied. Na+ greatly stimulated adenosine uptake. Addition of adenosine to a cell suspension under anaerobic conditions elicited Na+ uptake, and the Na+ uptake was inhibited by monensin, an Na+ ionophore. Imposition of an electrochemical potential of Na+ or a membrane potential in energy-depleted cells elicited adenosine uptake. Therefore, adenosine transport in this organism was concluded to proceed by an Na+/adenosine co-transport mechanism. The Na+/adenosine co-transport system was induced when cells were grown in the presence of adenosine, and repressed by glucose. Although Na+ uptake elicited by adenosine was reduced by glucose, it was enhanced by methyl alpha-glucoside, which reduced the intracellular ATP level. Thus, the effects of glucose and the glucoside on the Na+/adenosine co-transport system did not seem to be due to inducer exclusion, but to be related to the intracellular ATP level.

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

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

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

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

Effects of Ca2+-channel antagonists on nucleoside and nucleobase transport in human erythrocytes and cultured mammalian cells

Lidoflazine strongly inhibited the equilibrium exchange of uridine in human erythrocytes (Ki approximately 16 nM). Uridine zero-trans influx was similarly inhibited by lidoflazine in cultured HeLa cells (IC50 approximately to 80 nM), whereas P388 mouse leukemia and Novikoff rat hepatoma cells were three orders of magnitude more resistant (IC50 greater than 50 microM). Uridine transport was also inhibited by nifedipine, verapamil, diltiazem, prenylamine and trifluoperazine, but only at similarly high concentrations in both human erythrocytes and the cell lines. IC50 values ranged from about 10 microM for nifedipine and about 20 microM for verapamil to more than 100 microM for diltiazem, prenylamine and trifluoperazine. The concentrations required for inhibition of nucleoside transport are several orders higher than those blocking Ca2+ channels. Lidoflazine competitively inhibited the binding of nitrobenzylthioinosine to high-affinity sites in human erythrocytes, but did not inhibit the dissociation of nitrobenzylthioinosine from these sites on the transporter as is observed with dipyridamole and dilazep.

Ribonucleoside uptake and phosphorylation during fertilization and early development of the sea-urchin, Strongylocentrotus purpuratus

Uptake of the four ribonucleosides normally present in RNA increases nearly 50-fold shortly after fertilization in eggs of the sea-urchin, Strongylocentrotus purpuratus. Uridine, adenosine and cytidine are phosphorylated (greater than 95%) to their mono-, di- and triphosphates immediately after transport into the fertilized egg. Although guanosine is transported to an extent equal to the other three ribonucleosides, less than 12% of its phosphorylated after transport. In vitro nucleoside and nucleotide kinase assays of unfertilized egg homogenates indicate that the uridine, adenosine and cytidine kinases as well as the uridylate, adenylate, cytidylate and guanylate kinases are present in the egg prior to fertilization. Substrate competition measurements indicate that adenosine phosphorylation is catalyzed by a monospecific enzyme, while uridine and cytidine phosphorylations are catalyzed by a common kinase. Guanosine kinase activity was not detectable in unfertilized egg homogenates. Between 3 h and 5 h after fertilization the phosphorylation of transported guanosine begins to increase as it enters the embryo. By 7 h after fertilization, more than 95% of the guanosine entering the embryo is phosphorylated to the mono-, di- and triphosphates. More than 80% is phosphorylated to guanosine triphosphate. The timing of increased guanosine phosphorylation correlates with a decrease in the acid-soluble GTP pools in the embryo, suggesting that increased guanosine kinase activity is a response to increased GTP demand. These results, in view of the importance of GTP in many cellular processes, imply a crucial role for guanosine kinase activation in GTP pool maintenance and cellular metabolism during early sea-urchin development.

Metabolism of adenosine analogues by Schistosoma mansoni and the effect of nucleoside transport inhibitors

Schistosoma mansoni incorporated tubercidin, nebularine, 9-deazaadenosine, 5'-deoxy-5'-iodo-2-fluoroadenosine, 7,9-dideaza-7-thiaadenosine and toyocamycin but not sangivamycin, 3'-deoxy-sangivamycin, or 1-methylformycin into their nucleotide pool after a 4-hr incubation in vitro. In contrast to mammalian systems, addition of nucleoside transport inhibitors nitrobenzylthioinosine 5'-monophosphate (NBMPR-P), dilazep, or benzylacyclouridine had no significant effect on the pattern of incorporation of these adenosine analogues. Dipyridamole, on the other hand, reduced, but did not prevent, the incorporation of these analogues into the nucleoside 5'-triphosphate pool. These results suggest that the transport of purine nucleosides in schistosomes is different from that of their mammalian hosts. Therefore, coadministration of a specific nucleoside transport inhibitor with tubercidin, nebularine, 9-deazaadenosine, 5'-deoxy-5'-iodo-2-fluoroadenosine, toyocamycin, or 7,9-dideaza-7-thiaadenosine may result in high selective toxicity against schistosomes, as was the case with the combination of tubercidin plus NBMPR-P [el Kouni et al., Proc. natn. Acad. Sci. U.S.A. 80, 6667 (1983); el Kouni et al., Biochem. Pharmac. 34, 3921 (1985)], by protecting the host but not the parasite from the toxicity of these analogues.

Augmentation of methotrexate cytotoxicity in human colon cancer cells achieved through inhibition of thymidine salvage by dipyridamole

In HCT 116 cells, a human colon cancer cell line, the levels of thymidine (0.6 microM) and hypoxanthine (9 microM) contributed to the tissue culture medium by the fetal bovine serum significantly reduced the growth inhibition and lethality produced by 0.1 microM methotrexate. Dipyridamole, an inhibitor of nucleoside transport, potentiated the growth inhibitory effects of methotrexate when the cells were grown in medium that was changed daily. However, when the medium was supplemented with dialyzed serum, methotrexate cytotoxicity was not increased by dipyridamole. Similarly, in cloning experiments, dipyridamole increased the cell killing produced by methotrexate. The potentiation of methotrexate toxicity produced by dipyridamole was mediated through inhibition of thymidine uptake. The uptake of 1 microM thymidine was inhibited 50% by 0.12 microM dipyridamole but neither hypoxanthine nor guanine uptake was decreased by dipyridamole (5 microM). As a result, the decrease in dTTP pools produced by methotrexate was augmented by dipyridamole. In contrast, dipyridamole did not influence the effect of methotrexate on ribonucleoside triphosphate pools. HCT 116 cells avidly salvaged low concentrations of thymidine, and methotrexate increased this capacity. Conversion of 0.11 microM thymidine to thymidine triphosphate was increased by 55%, from 16.6 to 25.7 pmoles/10(6) cells, following exposure to 1.0 microM methotrexate. Dipyridamole blocked this pool expansion. This study suggests that the salvage of physiological levels of thymidine may diminish the cytotoxic effects of methotrexate on human colon cancer cells. Inhibition of thymidine uptake by dipyridamole may be an effective strategy to increase the cytotoxicity of methotrexate.

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

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

Interaction of [3H]dipyridamole with the nucleoside transporters of human erythrocytes and cultured animal cells

Equilibrium binding of [3H]dipyridamole identified high-affinity (Kd approximately 10 nM) binding sites on human erythrocytes (approximately 5 X 10(5) sites/cell) and on HeLa cells (approximately 5 X 10(6) sites/cell). The equilibration of dipyridamole with these sites on human erythrocytes was compatible with a second-order process which proceeded at 22 degrees C with a rate constant of about 6 X 10(6) M-1 sec-1. Binding of dipyridamole to these sites correlated kinetically with the inhibition of the equilibrium exchange of 500 microM uridine in these cells and was inhibited in a concentration-dependent manner by nucleosides and other inhibitors of nucleoside transport, such as nitrobenzylthioinosine, dilazep and lidoflazine, but not by hypoxanthine, which is not a substrate for the nucleoside transporter of human erythrocytes. The results indicate that the substrate binding site of the transporter is part of the high-affinity dipyridamole binding site. Bound [3H]dipyridamole became displaced from these sites on human erythrocytes by incubation with an excess of unlabeled dipyridamole or high concentrations of nucleosides and inhibitors of nucleoside transport, but neither by hypoxanthine nor sugars. Dissociation of [3H]dipyridamole behaved as a simple first-order process, but the rate constant was about one order of magnitude lower (about 3 X 10(-3) sec-1) than anticipated for typical ligand-protein binding on the basis of the measured association rate and equilibrium constants. The reason for this discrepancy has not been resolved. No high-affinity dipyridamole binding sites were detected on Novikoff rat hepatoma cells, P388, L1210 and S49 mouse leukemia cells or Chinese hamster ovary cells, and their absence correlated with a greater resistance of nucleoside transport in these cells to inhibition by dipyridamole. All cells expressed considerable low affinity (Kd greater than 0.5 microM) and nonspecific binding of dipyridamole.

Phase I trial of combination therapy of cancer with N-phosphonacetyl-L-aspartic acid and dipyridamole

While N-phosphonacetyl-L-aspartic acid (PALA), an inhibitor of de novo pyrimidine biosynthesis, demonstrated a unique spectrum of activity during preclinical drug evaluation, multiple clinical trials have shown it to possess minimal clinical activity. One explanation for the disappointing results is the possibility that tumor cells are able to utilize circulating uridine in the synthesis of pyrimidines (salvage pathway). Dipyridamole, an inhibitor of nucleoside transport, has been demonstrated experimentally to potentiate the cytotoxicity of PALA significantly. In addition, this agent has a long safety record when used clinically in man. A phase I trial of this two-drug combination was therefore conducted, with a fixed oral dose of dipyridamole (50 mg/m2 every 6 h) and an escalating i.v. dose of PALA administered every 3 weeks. The dose-limiting toxicity with this schedule was diarrhea and abdominal cramping pain at a PALA dose of 3900-4200 mg/m2. Among the 65 patients participating in this trial 4 objective responses (2 partial, 2 minimal) were observed. Because of the potential for unique clinical synergy between PALA and dipyridamole further investigation should be considered.

Two high affinity nucleoside transporters in Leishmania donovani

A rapid sampling kinetic technique has been used to evaluate the nucleoside transport functions of Leishmania donovani. The results indicated that L. donovani promastigotes possessed two independent purine nucleoside transporters with nonoverlapping substrate specificity. The first transported inosine, guanosine, and their analogs, while the second carried adenosine, analogs of adenosine, and the pyrimidine nucleosides, uridine, cytidine, and thymidine. The apparent Km values of the two nucleoside permeases for their purine nucleoside substrates were extraordinarily low, in the micromolar range. The organisms were capable of concentrating purine nucleosides from the medium and converting them to the nucleotide level with great efficiency and rapidity. Inosine and adenosine transport could be distinguished by different sensitivities to sulfhydryl reagents, suggesting structural differences between the two transporters. Finally, the two nucleoside transport systems of L. donovani were virtually refractory to inhibition by 4-nitrobenzylthioinosine and dipyridamole, two potent inhibitors of nucleoside entry into mammalian cells.

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

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

Uptake and metabolism of nucleosides by embryos of the sea urchin Strongylocentrotus purpuratus

Uptake and metabolism of thymidine and adenosine have been studied in embryos of the sea urchin Strongylocentrotus purpuratus. Uptake of these nucleosides is found to be mutually competitive, with the Km for uptake of thymidine similar to its Ki for inhibition of adenosine uptake and vice versa. The metabolic studies show that adenosine is rapidly and completely phosphorylated upon entry, even at high exogenous concentrations which saturate the uptake mechanism. In contrast, at concentrations which saturate nucleoside uptake, thymidine becomes appreciably catabolized (up to 60%) to thymine and beta-amino-isobutyric acid in addition to its phosphorylation to thymine nucleotides. Negligible amounts of endogenous thymidine appear to remain unmetabolized following uptake in these embryos. The data provide strong in vivo evidence for separate metabolic pathways for thymidine and adenosine which have not previously been described in this organism. The observation of mutual competition during uptake, together with different routes of metabolism for these nucleosides, would suggest that the rate-limiting step in the uptake process is transport rather than metabolism. The specificity of this transport system for its nucleoside substrate has been examined in some detail in the present report. All naturally occurring nucleosides but only a limited number of nucleoside analogs are recognized by this membrane carrier. Neither purine nor pyrimidine bases are substrates for this transport system. Previous work by this laboratory has demonstrated the strict Na+-dependence of this carrier, its high affinity for nucleoside substrate, and its activation at fertilization. These observations and the substrate specificity studies of the present work together describe a unique transport system for nucleosides in sea urchin embryos which is quite different from those previously described 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.

Rapid and complete degradation of thymidine by human peripheral blood platelets: implications for genotoxicity assays

Small-scale washed cell preparations obtained by Percoll density-gradient fractionation of whole blood were used to study the metabolic fate of [3H]thymidine supplied to isolated human blood mononuclear cells and platelets incubated for up to 24 h in vitro. Two cell fractions were monitored: low molecular weight compounds which were soluble in Triton X-100 and TCA were investigated by thin-layer chromatography, and high molecular weight components, distinguished by their Triton and TCA insolubility, were examined by agarose-gel electrophoresis. Under the conditions used, greater than 99% of added [3H]thymidine was very rapidly degraded. Catabolites were recovered in the Triton-soluble (cytoplasmic) fraction and the extracellular medium. A negligible proportion of added label was associated with Triton- and TCA-insoluble cell fractions. These results confirm and clarify previous data and have important implications for genotoxicity tests which employ in vitro leukocyte cultures.

Transport and metabolism of adenosine in human erythrocytes: effect of transport inhibitors and regulation by phosphate

Rapid kinetic techniques were applied to determine the effect of transport inhibitors on the transport and metabolism of adenosine in human red cells. Dipyridamole inhibited the equilibrium exchange of 500 microM adenosine by deoxycoformycin-treated cells in a similar concentration dependent manner as the equilibrium exchange and zero-trans influx of uridine with 50% inhibition being observed at about 20 nM. Intracellular phosphorylation of adenosine at an extracellular concentration of 5 microM was inhibited only by dipyridamole concentrations greater than or equal to 100 nM, which inhibited transport about 95%. Lower concentrations of dipyridamole actually stimulated adenosine phosphorylation, because the reduced influx of adenosine lessened substrate inhibition of adenosine kinase. When the cells were not treated with deoxycoformycin, greater than 95% of the adenosine entering the cells at a concentration of 100 microM became deaminated. A 95-98% inhibition of adenosine transport by treatment with dipyridamole, dilazep, or nitrobenzylthioinosine inhibited its deamination practically completely, whereas adenosine phosphorylation was inhibited only 50-85%. Whether adenosine entering the cells is phosphorylated or deaminated is strictly based on the kinetic properties of the responsible enzymes, substrate inhibition of adenosine kinase, and the absolute intracellular steady state concentration of adenosine attained. The latter approaches the extracellular concentration of adenosine, since transport is not rate limiting, except when modulated by transport inhibitors. In spite of the extensive adenosine deamination in cells incubated with 100 microM adenosine, little IMP accumulated intracellularly when the medium phosphate concentration was 1 mM, but IMP formation increased progressively with increase in phosphate concentration to 80 mM. The intracellular phosphoribosylation of adenine and hypoxanthine were similarly dependent on phosphate concentration. The results indicate that adenosine is the main purine source for erythrocytes and is very efficiently taken up and converted to nucleotides under physiological conditions, whereas hypoxanthine and adenine are not significantly salvaged. Hypoxanthine resulting from nucleotide turnover in these cells is expected to be primarily released from the cells. Adenosine was also dephosphorylated in human red cells presumably by 5'-methylthioadenosine phosphorylase, but this reaction seems without physiological significance as it occurs only at high adenosine and phosphate concentrations and if deamination is inhibited.