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

Monoclonal Anti-AMP Antibodies Are Sensitive and Valuable Tools for Detecting Patterns of AMPylation

AMPylation is a post-translational modification that modifies amino acid side chains with adenosine monophosphate (AMP). Recently, a role of AMPylation as a universal regulatory mechanism in infection and cellular homeostasis has emerged, driving the demand for universal tools to study this modification. Here, we describe three monoclonal anti-AMP antibodies (mAbs) from mouse that are capable of protein backbone-independent recognition of AMPylation, in denatured (western blot) as well as native (ELISA, IP) applications, thereby outperforming previously reported tools. These antibodies are highly sensitive and specific for AMP modifications, highlighting their potential as tools for new target identification, as well as for validation of known targets. Interestingly, applying the anti-AMP mAbs to various cancer cell lines reveals a previously undescribed broad and diverse AMPylation pattern. In conclusion, these anti-AMP mABs will further advance the current understanding of AMPylation and the spectrum of modified targets.

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Generation of murine monoclonal anti-AMP antibodies via synthetic AMPylated peptide

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Characterization in the applications western blot, ELISA, and immunoprecipitation

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Sensitive and specific recognition of AMPylation independent of protein backbone

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Expansion of toolbox for the detection and enrichment of AMPylation

Chemical suppressors of mlo-mediated powdery mildew resistance

Loss-of-function of barley mildew locus o (Mlo) confers durable broad-spectrum penetration resistance to the barley powdery mildew pathogen, Blumeria graminis f. sp. hordei (Bgh). Given the importance of mlo mutants in agriculture, surprisingly few molecular components have been identified to be required for this type of resistance in barley. With the aim to identify novel cellular factors contributing to mlo-based resistance, we devised a pharmacological inhibitor screen. Of the 41 rationally chosen compounds tested, five caused a partial suppression of mlo resistance in barley, indicated by increased levels of Bgh host cell entry. These chemicals comprise brefeldin A (BFA), 2',3'-dideoxyadenosine (DDA), 2-deoxy-d-glucose, spermidine, and 1-aminobenzotriazole. Further inhibitor analysis corroborated a key role for both anterograde and retrograde endomembrane trafficking in mlo resistance. In addition, all four ribonucleosides, some ribonucleoside derivatives, two of the five nucleobases (guanine and uracil), some guanine derivatives as well as various polyamines partially suppress mlo resistance in barley via yet unknown mechanisms. Most of the chemicals identified to be effective in partially relieving mlo resistance in barley also to some extent compromised powdery mildew resistance in an Arabidopsis mlo2 mlo6 double mutant. In summary, our study identified novel suppressors of mlo resistance that may serve as valuable probes to unravel further the molecular processes underlying this unusual type of disease resistance.

Dipyridamole Potentiates the Activity of Various Acyclic Nucleoside Phosphonates against Varicella-Zoster Virus, Herpes Simplex Virus and Human Cytomegalovirus

Dypiridamole (DPM) is widely used in the treatment of cardiovascular diseases as a coronary vasodilator and inhibitor of platelet aggregation. Phosphonylmethoxyethyl (PME) and 3-hydroxy-2-phosphonylmethoxypropyl (HPMP) derivatives of purines and pyrimidines are potent and selective inhibitors of varicella-zoster virus (VZV), herpes simplex virus (HSV) and human cytomegalovirus (HCMV). We have found that DPM markedly potentiates the antiviral effects of the PME derivatives of adenine (PMEA) and 2,6-diaminopurine (PMEDAP), and of the HPMP derivatives of adenine (HPMPA), 3-deazaadenine (HPMPc3A) and cyclic HPMPA (cHPMPA). This was reflected by a significant decrease in the 50% inhibitory concentration of the acyclic nucleoside phosphonates for VZV-, HSV- and HCMV-induced cytopathic effect or plaque formation. DPM did not enhance the activity of vidarabine, acyclovir or ganciclovir. These results were confirmed by virus yield assays (for HSV and HCMV) and flow cytometry (for VZV).

Alkyl-fluorinated thymidine derivatives for imaging cell proliferation

Derivatives of 2'-deoxyuridine that contain fluoroalkyl groups at the C5 position and derivatives of thymidine that contain fluoroalkyl groups at the N3 position were synthesized and examined in three in vitro assays designed to evaluate their potential as radiopharmaceuticals for imaging cellular proliferation. Three of the former nucleosides and five of the latter were synthesized. The three assays were as follows: (a) phosphoryl transfer assay, which showed that all three of the former nucleosides and four of the latter ones were phosphorylated by recombinant human thymidine kinase 1 (TK1) and that N(3)-(2-fluoroethyl)-thymidine (NFT202) was the most potent substrate of the eight nucleosides studied; (b) transport assay, which indicated that all eight nucleosides had good affinity for an 6-[(4-nitrobenzyl)thio]-9-beta-d-ribofuranosylpurine-sensitive mouse erythrocyte nucleoside transporter, with inhibition constants in the range of 0.02-0.55 mM; and (c) degradation assay, which showed that all but one of the former nucleosides and none of the latter were degraded by recombinant Escherichia coli thymidine phosphorylase (an enzyme that catalyzes the glycosidic bond of thymidine and 2'-deoxyuridine derivatives). From these in vitro screening assays, we selected NFT202 as a candidate for subsequent in vivo evaluation because this compound met the three minimum requirements of the in vitro screening assays and had the most potent phosphorylation activity as a substrate for recombinant human TK1.

Mechanisms of adenosine-induced cytotoxicity and their clinical and physiological implications

Extracellular ATP (ATPo) and adenosine are cytotoxic to several cancer cell lines, suggesting their potential use for anticancer therapy. Adenosine causes cytotoxicity, either when added exogenously or when generated from ATPo hydrolysis, via mechanisms which are not mutually exclusive and which involve, adenosine receptor activation, pyrimidine starvation and/or increases in intracellular S-adenosylhomocysteine: S-adenosylmethionine ratio. Given that adenosine also appears to protect against cytotoxicity via mechanisms including immunity against damage by oxygen free radicals, an understanding of the contribution of adenosine to ATPo-induced cytotoxicity is thus crucial, when considering any potential therapeutic use for these compounds. However, such an understanding has been largely hindered by the fact that many studies have not focused enough on the possibility that both ATPo and adenosine may mediate cytotoxicity in the same system. Such studies can benefit from use a range of ATPo concentrations when assessing the contribution of adenosine to ATPo-induced cytotoxicity. Whilst future molecular and pharmacological studies are needed to establish the nature of the cytotoxic adenosine receptor, it is possible that more than just one adenosine receptor type is involved and that the cytotoxic receptor(s) type is more likely to have a low affinity for adenosine. Activation of the adenosine receptor(s) would thus lead to cytotoxicity only at relatively high adenosine concentrations, while lower adenosine concentrations mediate non-cytotoxic physiological effects.

Endothelial adenosine transporter characterization in perfused guinea pig hearts

Adenosine (Ado), a smooth muscle vasodilator and modulator of cardiac function, is taken up by many cell types via a saturable transporter, blockable by dipyridamole. To quantitate the influences of endothelial cells in governing the blood-tissue exchange of Ado and its concentration in the interstitial fluid, one must define the permeability-surface area products (PS) for Ado via passive transport through interendothelial gaps [PS(g)(Ado)] and across the endothelial cell luminal membrane (PS(ecl)) in their normal in vivo setting. With the use of the multiple-indicator dilution (MID) technique in Krebs-Ringer perfused, isolated guinea pig hearts (preserving endothelial myocyte geometry) and by separating Ado metabolites by HPLC, we found permeability-surface area products for an extracellular solute, sucrose, via passive transport through interendothelial gaps [PS(g)(Suc)] to be 1.9 +/- 0.6 ml. g(-1). min(-1) (n = 16 MID curves in 4 hearts) and took PS(g)(Ado) to be 1. 2 times PS(g)(Suc). MID curves were obtained with background nontracer Ado concentrations up to 800 micrometer, partially saturating the transporter and reducing its effective PS(ecl) for Ado. The estimated maximum value for PS(ecl) in the absence of background adenosine was 1.1 +/- 0.1 ml. g(-1). min(-1) [maximum rate of transporter conformational change to move the substrate from one side of the membrane to the other (maximal velocity; V(max)) times surface area of 125 +/- 11 nmol. g(-1). min(-1)], and the Michaelis-Menten constant (K(m)) was 114 +/- 12 microM, where +/- indicates 95% confidence limits. Physiologically, only high Ado release with hypoxia or ischemia will partially saturate the transporter.

Involvement of multiple transporters in the oral absorption of nucleoside analogues

Many nucleoside analogues such as azt, ddI, ddC, d4T, 3TC, acv and vacv are currently being used in the treatment of patients infected with HIV, suffering from AIDS, or AIDS-related opportunistic infections. The transport of nucleoside analogues across the gastrointestinal tract is mediated by a number of transporters that fall into three broad categories, i.e., Na(+)-dependent concentrative transporters, Na(+)-independent equilibrative transporters and H(+)/peptide transporters. The first two transporter classes contain a large number of subtypes that are based on the substrate specificity. Recent studies have shown that most of the anti-HIV nucleoside analogues are transported by one or more of the nucleoside transporters. Furthermore, certain analogues, such as acv, appear to be absorbed by non-carrier-mediated diffusion, whereas vacv is apparently transported by non-nucleoside transporters (e.g., the oligopeptide transporter, PepT1 and possibly others). Thus, it is desirable to understand the precise nature of the absorption mechanism of these drugs to improve bioavailability and reduce the variability that is commonly observed in vivo in human patients. A complete understanding of the complex interactions of nucleoside analogues with the various transporters will help in designing better delivery systems and strategies to improve efficacy. In the current report, the mechanisms of nucleoside and nucleoside-analogue transport are reviewed. Also, methods of exploiting prodrugs to improve the bioavailability characteristics of drugs are highlighted.

Contribution of the nucleoside transport system to doxorubicin transport in HL60 cells but not in mononuclear cells

Previously, we reported that pirarubicin (THP), an anthracycline, was transported, at least in part, via a nucleoside transport system in human leukemic HL60 cells, but not in mononuclear cells (MNCs). In this study, the contribution of the nucleoside transport system to the transport of other anthracyclines, doxorubicin (DOX), daunorubicin (DNR) and idarubicin (IDA), in HL60 cells and MNCs was investigated. The experiments were performed after both types of cells had been pretreated with a metabolic inhibitor, 2,4-dinitrophenol, to deplete cellular ATP. The DOX uptake by HL60 cells was partially inhibited by inhibitors of equilibrative nucleoside transporters. In HL60 cells, moreover, the uptake of DOX depended on an inwardly directed Na(+)-gradient, and was inhibited by concentrative nucleoside transporters, but there was no change in the DNR or IDA uptake under any of these conditions. On the other hand, the uptake of the three drugs by MNCs was not affected by any inhibitors of the nucleoside transporters, and there was no dependence of the uptake on an Na(+)-gradient. These results suggested that DOX, but not DNR or IDA, was partially transported in HL60 cells via the nucleoside transport system, whereas in MNCs the system did not contribute to the uptake of any of these three drugs. Thus, nucleoside transport systems contributing to the transport of anthracyclines may be different among different derivatives and cell types.

Uptake and metabolism of adenosine mediate a meiosis-arresting action on mouse oocytes

This study was carried out to evaluate the possible role of adenosine uptake and metabolism in mediating the inhibitory actions of this nucleoside on spontaneous mouse oocyte maturation. Uridine blocked 3H-adenosine uptake by oocyte-cumulus cell complexes (OCCs) and cumulus cell-enclosed oocytes (CEOs) by 82-85%, whereas uptake by denuded oocytes (DOs) was suppressed by 97%. Uridine had no effect on germinal vesicle breakdown (GVB) in CEOs when meiotic arrest was maintained with hypoxanthine or hypoxanthine plus adenosine but reversed the combined inhibitory action of these purines in DOs. Five of six adenosine analogs that bind to purinoceptors demonstrated meiosis-arresting activity but not in relation to their relative affinities for inhibitory or stimulatory adenosine receptors and only at high concentrations. Moreover, in DOs, uridine reversed the inhibitory effect of 2-chloroadenosine and 5'-N-ethylcarboxamidoadenosine, two receptor agonists that are poor substrates for adenosine-metabolizing enzymes. Results of experiments with adenosine kinase inhibitors showed that methylmercaptopurine riboside (MMPR) and tubercidin, but not 5'-amino-5'-deoxyadenosine, reversed meiotic arrest maintained by hypoxanthine +/- adenosine, but this required an additional inhibitory action on de novo purine synthesis. Inhibition of de novo purine synthesis alone was not sufficient because azaserine failed to reverse meiotic arrest. MMPR was a very potent meiosis-inducing agent, completely reversing meiotic arrest in CEOs and DOs in the presence of a variety of meiotic inhibitors. The adenosine deaminase inhibitor deoxycoformycin had opposite effects on oocyte maturation depending on the presence or absence of adenosine: the inhibitory action of hypoxanthine alone was bolstered, but the meiosis-arresting action of adenosine was reversed. These data therefore indicate that at low adenosine concentrations phosphorylation predominates, but at higher adenosine concentrations deaminated products contribute to the meiotic inhibition. This idea was borne out by the ability of inosine to mimic the synergistic interaction of adenosine with hypoxanthine. The action of adenosine is not due to deamination to inosine and conversion to nucleotides through the hypoxanthine salvage pathway because adenosine-mediated inhibition was not compromised in oocytes from mutant mice unable to salvage hypoxanthine.

Adenosine and the nervous system: pharmacological data and therapeutic perspectives

1. Adenosine acts on a family of G-protein-coupled receptors called purinoreceptors. 2. Four subtypes have been cloned and pharmacologically characterized. 3. The principal pharmacological data and structure-function relations for agonist interactions with P1 receptors are presented. 4. We conclude that the potent role of adenosine in the nervous system may be interesting for the development of drugs targeted at purines and their receptors.

Determination of absorption characteristics of AG337, a novel thymidylate synthase inhibitor, using a perfused rat intestinal model

The purpose of this study was to determine the intestinal absorption characteristics of AG337, a mechanism-based inhibitor of thymidylate synthase, using a perfused rat intestinal model. Effects of site, pH, temperature, concentration, Na+, and inhibitors on the absorption of AG337 were determined, after the compound was shown to be stable in buffers of various pH, blank perfusate, and intestinal homogenate. The results indicated that absorption of AG337 was temperature-, pH-, Na+-, concentration-, and site-dependent. The best site of absorption is duodenum, where the absorption was 3-10 times (p < 0. 05) higher than absorption at jejunum, ileum, and colon. Among the four pH's studied, the best was at pH 6.5 (p < 0.05). Absorption was 80% lower in the absence of Na+, and 75% lower when the temperature of the perfusate was decreased to 4 degreesC. Permeability of AG337 also decreased about 75% when the concentration was raised to 100 microM. These results suggest that a nutrient carrier may be involved in the transport of AG337. To determine the carrier responsible for the absorption of AG337, its absorption was determined in the presence of various inhibitors at different concentrations. The results indicated that transport of AG337 was inhibited significantly (p < 0.01) by 100 microM of adenine, hypoxanthine, and xanthine. The transport was also inhibited significantly (p < 0.01) by a mixture of 100 microM each of adenine, hypoxanthine, and xanthine, but not by a mixture of 100 microM each of thymine and uracil. A higher concentration of hypoxanthine resulted in increased inhibition. In contrast, prototypical inhibitors of nucleoside transporter, dipyridamole and nitrobenzylthioinosine (NBMPR), did not significantly decrease the transport of AG337. The results also showed that absorption of AG337 had a significant nonsaturable component, with a nonsaturable Pw of 0.8. In conclusion, absorption of AG337 in the rat intestine has been shown to be mainly via a purine base carrier with a significant nonsaturable component.

Possibility of contribution of nucleoside transport systems to pirarubicin uptake by HL60 cells but not mononuclear cells

Previously, we reported that pirarubicin (THP), an anthracycline, was taken up, at least in part, by both human leukemic HL60 cells and mononuclear cells (MNCs) via a carrier-mediated system. In this study, the possibility of a contribution of nucleoside transport systems to the uptake of THP by HL60 cells and MNCs was investigated. The experiments were performed after both types of cells had been pretreated with a metabolic inhibitor, 2,4-dinitrophenol, to deplete cellular ATP. In HL60 cells, THP uptake was increased and decreased significantly by treatment with equilibrative nucleoside transport inhibitors, nitrobenzylthioinosine (NBMPR), nitrobenzylthioguanosine and dilazep, in the presence and absence, respectively, of an inwardly directed Na+-gradient. THP uptake by HL60 cells showed an overshoot in the presence of the gradient, and was decreased by treatment of the cells with monensin, indicating that the uptake partially depended on the Na+-gradient. In HL60 cells in which equilibrative nucleoside transport was inhibited by NBMPR, THP uptake in the presence of the gradient was inhibited by Na+-dependent concentrative nucleoside transport inhibitors, but no inhibition was observed in the absence of the gradient. In MNCs, conversely, there was no effect of any equilibrative nucleoside transport inhibitor or the Na+-gradient on THP uptake. These results suggested that THP was taken up, at least in part, via both equilibrative and concentrative nucleoside transport systems in HL60 cells, but not in MNCs.

Characterization and regulation of adenosine transport in T84 intestinal epithelial cells

Adenosine release from mucosal sources during inflammation and ischemia activates intestinal epithelial Cl- secretion. Previous data suggest that A2b receptor-mediated Cl- secretory responses may be dampened by epithelial cell nucleoside scavenging. The present study utilizes isotopic flux analysis and nucleoside analog binding assays to directly characterize the nucleoside transport system of cultured T84 human intestinal epithelial cells and to explore whether adenosine transport is regulated by secretory agonists, metabolic inhibition, or phorbol ester. Uptake of adenosine across the apical membrane displayed characteristics of simple diffusion. Kinetic analysis of basolateral uptake revealed a Na(+)-independent, nitrobenzylthioinosine (NBTI)-sensitive facilitated-diffusion system with low affinity but high capacity for adenosine. NBTI binding studies indicated a single population of high-affinity binding sites basolaterally. Neither forskolin, 5'-(N-ethylcarboxamido)-adenosine, nor metabolic inhibition significantly altered adenosine transport. However, phorbol 12-myristate 13-acetate significantly reduced both adenosine transport and the number of specific NBTI binding sites, suggesting that transporter number may be decreased through activation of protein kinase C. This basolateral facilitated adenosine transporter may serve a conventional function in nucleoside salvage and a novel function as a regulator of adenosine-dependent Cl- secretory responses and hence diarrheal disorders.

Characterization of a novel Na+-dependent, guanosine-specific, nitrobenzylthioinosine-sensitive transporter in acute promyelocytic leukemia cells

NB4 cells are the only bona fide in vitro model of human acute promyelocytic leukemia. We have examined cytidine and guanosine transport in this cell line and characterized a novel guanosine-specific transporter. Cytidine transport occurred predominately by equilibrative nitrobenzylthioinosine (NBMPR)-sensitive (es) transport. In the presence of Na+, guanosine at various concentrations accumulated at least 6-fold above equilibrium. The initial rate of guanosine transport in Na+ buffer decreased by 75% with the addition of 1 microM NBMPR and the IC50 for NBMPR inhibition was 0.7 +/- 0.1 nM. Replacement of Na+ with choline also resulted in a 75% decrease in total guanosine transport. The potent inhibition of guanosine transport by NBMPR and the loss of transport in choline suggested that a Na+-dependent NBMPR-sensitive transporter was responsible for the majority of guanosine uptake. This concentrative, sensitive transporter is Na+ dependent with a stoichiometric coupling ratio of 1:1. This novel transporter, referred to as csg, is guanosine-specific with total guanosine transport inhibited by only 50% in the presence of 1 mM competing nucleosides. HL-60, acute myelocytic leukemia cells, do not exhibit csg activity while L1210, murine acute lymphocytic leukemia cells, exhibit csg transport. The presence of the csg transporter suggests an important role for guanosine in particular forms of leukemia and may provide a new target for cytotoxic therapy.

Nucleoside transport in erythrocytes from bottle-nosed dolphin (Tursiops truncatus)

Entry of adenosine, and thymidine, into erythrocytes from adult dolphins was rapid, showed saturation at higher substrate concentrations, and was strongly inhibited by low concentrations of nitrobenzylthioinosine (NBMPR). Kinetic parameters were estimated from the concentration dependence of initial rates of tracer entry at 21 degrees C, as K(m) 0.14 +/- 0.05 mM and Vmax 24.4 +/- 1.9 mumol/litre cell water/sec for zero trans entry of adenosine, and K(m) 0.96 +/- 0.21 mM and Vmax 25.4 +/- 1.7 mumol/litre cell water/sec for thymidine. Adenosine, and thymidine, entry were inhibited by both purine and pyrimidine nucleosides. Mass law analysis of a saturable component of nitrobenzylthioinosine binding to dolphin red cell membranes gave values of Bmax 65.4 +/- 1.2 pmol/mg protein, and K(d) of 1.53 +/- 0.08 nM for a single class of sites. Photo-irradiation of dolphin red cell membranes in the presence of tritiated nitrobenzylthioinosine led to radioactive labeling of polypeptides M(r) 52, 500-58,000, on SDS-PAGE.

Chronic dipyridamole administration downregulates [3H]nitrobenzylthioinosine binding site affinity in guinea pig kidney but not heart and brain

Specific binding of the nucleoside transporter probe, [3H]nitrobenzylthioinosine, ([3H]NBMPR) was measured in washed guinea pig cardiac, renal and forebrain membranes after 14 days of treatment with dipyridamole (37.5 mg/kg, s.c., b.i.d.) or vehicle. When compared to values in vehicle-treated animals, a 100 percent increase in equilibrium dissociation constant (Kd) was observed in the kidney of dipyridamole-treated animals (0.51 +/- 0.04 to 1.0 +/- 0.06, p < 0.01). The maximal binding capacity (Bmax) was unaltered. No changes were observed in the heart or forebrain. The increase in Kd suggests that chronic dipyridamole treatment decreases the apparent binding affinity of NBMPR for kidney nucleoside transporters. Cardiac and brain nucleoside transporters may be either less susceptible to chronic dipyridamole administration or have a different adaptive mechanism.

Influence of both salvage and DNA damage response pathways on resistance to chemotherapeutic antimetabolites

The resistance of 3 human embryo fibroblast cell lines to the antimetabolites methotrexate (MTX), N-phosphonacetyl-L-aspartate (PALA) and 5-fluorouracil (5-FU) has been studied. The cell lines were of common genetic origin, all originating from the normal KMS parental cell line, which was irradiated with 60Co to produce the immortalised derivative KMST which, in turn, was transfected with an activated N-ras oncogene to produce the tumourigenic KN-NM cell line. Previous work from this group, using dialysed versus nondialysed serum, has provided evidence for the involvement of salvage pathways of purine and pyrimidine biosynthesis in the increased resistance to antimetabolites of those cell lines (KMST and KN-NM) tending towards increased tumourigenicity. The present study has extended this work by using the nucleoside and nucleobase transport inhibitor dipyridamole, to further assess the contribution of the salvage pathways to the increased cellular resistance to the three antimetabolites. The salvage pathways were found to contribute to the resistance of cell lines to PALA and MTX, but had no effect on the resistance to 5-FU. The addition of excess uridine in the case of PALA, and hypoxanthine plus thymidine in the case of MTX, could be used to "rescue" cells from the effects of dipyridamole-induced salvage pathway inhibition. The data will be discussed in relation to 1. the effect of limited substrate availability, 2. the induction of DNA damage and DNA damage-response pathways, and 3. DNA-damage protection by the salvage pathways of purine and pyrimidine biosynthesis.

Salvage pathway of pyrimidine synthesis: divergence of substrate specificity in two related species of teleostean fish

For nucleotide synthesis, cells use purine and pyrimidine nucleosides generated either through de novo synthesis or through utilization of salvage pathways. In the pyrimidine salvage pathway, thymidine is taken up by transport proteins and phosphorylated by the enzyme thymidine kinase to thymidine monophosphate. So far, all vertebrates analyzed are able to use radioactively labeled thymidine for the biosynthesis of nucleotides in brain tissue. However, when standard autoradiographic, immunohistochemical and biochemical procedures were applied for the detection of the incorporation of tritiated thymidine and the thymidine analogue 5-bromo-2'-deoxyuridine into DNA to two species of gymnotiform fish, a divergence in substrate specificity has been revealed. Although brain cells of the two species, Apteronotus leptorhynchus and Eigenmannia sp., can utilize 5-bromo-2'-deoxyuridine for pyrimidine synthesis, only Eigenmannia sp. is able to incorporate tritiated thymidine into DNA during the S phase of the cell cycle. We hypothesize that this inability to use thymidine for nucleotide synthesis is caused either by a defect in the transport system mediating the uptake of thymidine or by a deficiency in the thymidine kinase of A. leptorhynchus.

Oral absorption of anti-acquired immune deficiency syndrome nucleoside analogues. 2. Carrier-mediated intestinal transport of stavudine in rat and rabbit preparations

The intestinal transport and metabolism of stavudine (d4T), a nucleoside analogue of thymidine used in the treatment of AIDS, was studied using single-pass intestinal perfusion (SPIP), intestinal brush-border membrane vesicles (BBMV), and mucosal homogenates in rats and rabbits. In the SPIP, d4T demonstrated concentration-dependent mean wall permeability (P+/-w) at perfusate concentrations ranging from 0.001 to 25 mM. In coperfusion studies using 0.1 mM thymidine, 1 mM formycin B, or 5 microM NBTI as putative inhibitors of d4T transport, the P+/-w of 5 microM d4T was reduced to 48%, 62%, and 70% of the control value, respectively, suggesting the involvement of multiple nucleoside carriers in the intestinal uptake of d4T. d4T uptake in rat BBMV was significantly greater in the presence of a sodium ion gradient compared with a sodium-free (choline) gradient. The permeability of d4T, in the presence of a sodium gradient, was concentration-dependent and inhibited by 10 mM thymidine but not significantly reduced by 10 mM formycin B. In the presence of 10 microM NBTI, the permeability of d4T was not inhibited; however; the binding of d4T to rat and rabbit BBMV was significantly reduced. Formycin B did not significantly reduce the d4T uptake in rat or rabbit BBMV suggesting that d4T does not interact with the purine-selective N1 nucleoside carrier. However, because formycin B inhibited d4T uptake in the SPIP and since d4T inhibited formycin B uptake in rat but not rabbit BBMV, it appears to interact with the N3 carrier which has been demonstrated in rat but not rabbit intestine. Also, an interaction with the sodium-independent facilitative transporter at the basolateral membrane cannot be ruled out. The low hybrid K(m) and high passive permeability of d4T likely account for the lack of saturable absorption behavior observed in humans, whereas the brush-border and intracellular stability of d4T preserve the high bioavailability observed after oral dosing.

Characterization of equilibrative and concentrative Na+-dependent (cif) nucleoside transport in acute promyelocytic leukemia NB4 cells

Nucleoside transport processes can be classified by the transport mechanism, e=equilibrative and c=concentrative, by the sensitivity to inhibition by nitrobenzylthioinosine (NBMPR), s=sensitive and i=insensitive, and also by permeant selectivity. To characterize nucleoside transport in acute promyelocytic NB4 cells, nucleoside transport was resolved into different components by selective elimination of transport processes with NBMPR and with Na+-deficient media. Initial transport rates were estimated from time course experiments. For adenosine, uridine, and formycin B, equilibrative transport accounted for approximately 60% of their uptake, with ei and es transport contributing almost equally, and Na+-dependent transport accounting for the remaining 40% of the total uptake. Thymidine uptake was mediated exclusively by equilibrative systems with ei and es systems each contributing 50% to total uptake. Adenosine accumulated above equilibrative concentrations, suggesting that a concentrative transport process was active and/or that metabolism led to adenosine's accumulation. Formycin B, a nonmetabolizable analog, also accumulated in the cells, supporting the concentrative potential of the Na+-dependent transporter. Kinetic analyses also provided evidence for three distinct high affinity transport mechanisms. NBMPR binding assays indicated the presence of two high affinity (Km 0.10 and 0.35 nM) binding sites. In conclusion, NB4 cells express ei and es transport, as well as a large ci transport component, which appears to correspond to cif (f=formycin B or purine selective) nucleoside transport, not previously described in human cells.

GTP and guanosine synergistically enhance NGF-induced neurite outgrowth from PC12 cells

Six per cent of rat pheochromocytoma (PC12) cells extended neurites (processes greater than one cell diameter in length) in the presence of 300 microM extracellular GTP or 300 microM guanosine for 48 hr, compared to only 2.5% of cells in control cultures. In the presence of 40 ng/ml of 2.5S NGF, about 20-35% of PC12 cells had neurites after 48 hr, and the addition of 300 microM guanosine or GTP together with NGF synergistically increased the proportion of cells with neurites to 40-65%. GTP and guanosine also increased the average number of branches per neurite, from 0.6 in NGF-treated cultures to 1.2 (guanosine) or 1.5 (GTP). Neurites formed after exposure to NGF alone had axonal characteristics as determined by immunocytochemistry with antibody, SMI-31, against axonal-specific polyphosphorylated neurofilament epitopes. Neurites generated with the addition of both guanosine or GTP had the same characteristics. GTP probably did not exert its effects via the P2X or P2Y purinoceptors because the adenine nucleotides ATP, ATP gamma S, ADP beta S, and ADP, which are all agonists of these receptors, inhibited rather than enhanced, NGF-induced neurite outgrowth. UTP also enhanced the proportion of cells with neurites, although not to the same degree as did GTP. This may indicate activity through a P2U-like nucleotide receptor. However, the response profile obtained, GTP > UTP >> ATP, does not fit the profile of any known P2Y, P2X or P2U receptor. The poorly hydrolyzable GTP analogues, GTP gamma S and GDP beta s were also unable to enhance the proportion of cells with neurites. This implied that GTP may produce its effects through a GTP-specific ectoenzyme or kinase. This idea was supported by results showing that another poorly hydrolyzable analogue, GMP-PCP, competitively inhibited the effects of GTP on neurite outgrowth. GTP did not exert its effects after hydrolysis to guanosine since the metabolic intermediates GDP and GMP were also ineffective in enhancing the proportion of cells with neurites. Moreover, the effects of GTP and guanosine were mutually additive, implying that these two purines utilized different signal transduction mechanisms. The effects of guanosine were not affected by the nucleoside uptake inhibitors nitrobenzylthioinosine (NBTI) and dipyridamole, indicating that a transport mechanism was not involved. Guanosine also did not activate the purinergic P1 receptors, because the A2 receptor antagonists, 1,3-dipropyl-7-methylxanthine (DPMX) or CGS15943, and the A1 receptor antagonist, 1,3-dipropyl-8-(2-amino-4-chloro)xanthine (PACPX) did not inhibit its reaction. Therefore guanosine enhanced neurite outgrowth by a signal transduction mechanism that does not include the activation of the P1 purinoceptors. The enhancement of the neuritogenic effects of NGF by GTP and guanosine may have physiological implications in sprouting and functional recovery after neuronal injury in the CNS, due to the high levels of nucleosides and nucleotides released from dead or injured cells.

An assay for the determination of reduced methotrexate accumulation in cells displaying limited viability in vitro

Amongst the mechanisms known to mediate resistance to methotrexate (MTX), a major component in the treatment of childhood leukemia, reduced drug accumulation appears to have direct clinical relevance. However, due to the poor viability of patient-derived acute lymphoblastic leukemia cells in vitro, determination of this parameter in clinical samples is associated with a number of difficulties. We have therefore developed an assay for reduced MTX accumulation, which controls for the metabolic state of the cell population under study by utilizing accumulation of the nucleoside thymidine as an independent indicator of this parameter. To establish this assay, we have utilized pediatric leukemic cell populations maintained as xenografts in nude mice, which, despite displaying sensitivity to MTX, demonstrated reduced accumulation of MTX when assayed using standard methodology. When accumulation of MTX by such cell populations was expressed, however, relative to their accumulation of thymidine, MTX accumulation was shown to be equal to that of drug-sensitive CCRF-CEM cells maintained in long-term culture. In contrast, significantly less MTX was accumulated, in this assay, by xenografted cell populations with demonstrated resistance to MTX. Identical results were obtained using either fresh or cryopreserved cells. The data thus indicate that by controlling for variable metabolic status of leukemic cells, it is possible to accurately assess MTX accumulation in leukemic samples displaying limited viability 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.

A diamidine-resistant Trypanosoma equiperdum clone contains a P2 purine transporter with reduced substrate affinity

Following the demonstration that the transport of melaminophenyl arsenical drugs in Trypanosoma brucei is dependent upon an unusual adenosine nucleoside transporter (Carter and Fairlamb, Nature 361 (1993) 173-175) we have investigated adenosine transport in the related parasite Trypanosoma equiperdum (Botat1.1) and a cloned derivative resistant to the diamidine drug berenil (diminazene aceturate) with limited cross-resistance to the melaminophenyl arsenical cymelarsen. The parental strain possesses a bipartite adenosine transport system consisting of one component which is inhibited in a dose-dependent and saturable manner with increasing concentrations of inosine and a second component which is similarly inhibited by adenine. Uptake of adenosine on this second transporter is also inhibited in a dose-dependent fashion by berenil and cymelarsen. Both transporters have high affinity for adenosine (apparent Km values of 0.60 and 0.70 mM and Vmax values of 8.4 and 6.9 pmol (s (10(8) trypanosomes))-1 at 25 degrees C, respectively). Thus T. equiperdum shares with T. brucei a system comprising two adenosine transporters named P1 and P2, respectively. The P1 transporter is similar in the sensitive and resistant T. equiperdum clones, whereas the P2 transporter has reduced transport capacity at physiological adenosine concentration and decreased affinity for adenosine in the drug-resistant clone.

Parasite-induced permeation of nucleosides in Plasmodium falciparum malaria

A mechanism which mediates the transport of the nonphysiological nucleoside, L-adenosine, was demonstrated in Plasmodium falciparum infected erythrocytes and naturally released merozoites. L-Adenosine was not a substrate for influx in freed intraerythrocytic parasites or in normal human erythrocytes nor was L-adenosine transported in a variety of cell types including other parasitic protozoa such as Crithidia luciliae, Trichomonas vaginalis, Giardia intestinalis, or the mammalian cells, Buffalo Green Monkey and HeLa cells. L-Adenosine transport in P. falciparum infected cells was nonsaturable, with a rate of 0.13 +/- 0.01 pmol/microliter cell water per s per microM L-adenosine, yet the transport was inhibited by furosemide, phloridzin and piperine with IC50 values between 1-13 microM, distinguishing the transport pathway from simple diffusion. The channel-like permeation was selective as disaccharides were not permeable to parasitised cells. In addition, an unusual metabolic property of parasitic adenosine deaminase was found in that L-adenosine was metabolised to L-inosine by both P. falciparum infected erythrocytes and merozoites, an activity which was inhibited by 50 nM deoxycoformycin. No other cell type examined displayed this enzymic activity. The results further substantiate that nucleoside transport in P. falciparum infected cells was significantly altered compared to uninfected erythrocytes and that L-adenosine transport and metabolism was a biochemical property of Plasmodium infected cells and merozoites and not found in normal erythrocytes nor any of the other cell types investigated.

Mechanisms of adenosine-mediated actions on cellular and clinical cardiac electrophysiology

OBJECTIVE

To provide insights into the molecular mechanisms of adenosine-mediated cardiac cellular electrophysiology and how information about these mechanisms can be used to facilitate diagnostic and therapeutic approaches to various clinical arrhythmias.

DESIGN

A review of (1) adenosine metabolism and receptors in the cardiac system, (2) adenosine-mediated signal transduction pathways in the regulation of cellular electrophysiology in various cardiac cell types, and (3) the clinical usefulness of adenosine in cardiac electrophysiology is presented.

RESULTS

The effects of adenosine on cardiac electrophysiologic properties are consequences of complex interactions among the specific cardiac target structures, the density and type of adenosine receptors, and the effector systems. The easy application of adenosine and its short half-life, favorable side-effects profile, and electrophysiologic properties make it an excellent diagnostic and therapeutic tool for the initial assessment of various tachyarrhythmias.

CONCLUSION

The direct adenosine-activated KACh (potassium acetylcholine) channel signal transduction system explains the effects of adenosine on the sinus node, atrioventricular node, and atrial myocardium. The indirect adenosine-inhibited adenylate cyclase system accounts for its negative inotropic effects on the catecholamine-entrained contractility in atrial and ventricular myocardium. Because of the recent purification and cloning of adenosine receptors and subunits of G proteins, additional adenosine-mediated electrophysiologic mechanisms can be explored.

Zidovudine induces the expression of cellular resistance affecting its antiviral activity

We have previously shown that multidrug-resistant cells expressing the multidrug transporter P-glycoprotein are less sensitive to the antiviral activity of AZT. Subsequently, we addressed the question whether AZT itself is able to induce cellular resistance to the drug. Indeed, CEM cells propagated in the presence of increasing concentrations of AZT become resistant to the antigrowth and antiviral activity of AZT but do not express detectable level of P-glycoprotein. Sensitivity of these cells to other compounds, such as vinblastine, vincristine, ddI, and ddC remained unchanged, indicating that, in contrast to P-glycoprotein-positive cells, AZT-induced resistance is specific for AZT. Interestingly, in AZT-induced resistant cells the intracellular accumulation of AZT and exogenous deoxythymidine, as well as thymidine kinase activity, are significantly reduced when compared with the parental cell line. Our findings show that AZT itself may directly induce the expression of cellular mechanisms leading to the acquisition of specific cellular resistance that can affect its antiviral activity.

Nucleoside transport in Crithidia luciliae

Nucleoside transport was evaluated in the trypanosomatid Crithidia luciliae by a rapid sampling technique. C. luciliae was shown to possess two independent nucleoside transporters, one which transported adenosine, deoxyadenosine, tubercidin, sangivamycin and the pyrimidine nucleoside thymidine, while the second was specific for guanosine, inosine and deoxyguanosine. The rapid influx occurred by a process of facilitated transport. The apparent Km values for adenosine and guanosine were 9.34 +/- 1.30 and 10.6 +/- 2.60 microM, respectively. The pyrimidine nucleoside thymidine was transported at a rate approximately 50% lower than the purine nucleosides, whilst uridine, deoxyuridine and deoxycytidine were not transported. The optical isomer, L-adenosine entered the organism by simple diffusion rather than by facilitated transport. In contrast to mammalian cells, neither of the nucleoside transporters in C. luciliae were inhibited by nitrobenzylthioinosine, dilazep, or dipyridamole, potent inhibitors of nucleoside transport in mammalian cells, whilst p-chloromercuribenzoate sulphonate inhibited both nucleoside transporters in C. luciliae.

Effects of adenosine and its analogues on isolated intracerebral arterioles. Extraluminal and intraluminal application

We evaluated the responses of brain parenchymal arterioles to intraluminal and extraluminal application of adenosine and its analogues. Intracerebral arterioles (28.4- to 60.3-microns diameter) were isolated from Sprague-Dawley rats, cannulated with micropipettes, and perfused in vitro. Both extraluminal and intraluminal adenosine, 5'-(N-ethylcarboxamido)adenosine (NECA), R-N6-(phenylisopropyl)adenosine (R-PIA), and S-N6-(phenylisopropyl)adenosine (S-PIA) elicited concentration-dependent dilation of these arterioles, but intraluminal application was less potent and efficacious than extraluminal application. Inosine was not vasoactive. A common order of agonist potency (NECA > adenosine > R-PIA > or = S-PIA) was determined for both extraluminal and intraluminal application. Theophylline (10 microM) caused a rightward shift of the adenosine concentration-response curve and a 50-fold reduction in potency. Intraluminal theophylline was one sixth as effective as extraluminal theophylline in antagonizing the extraluminal adenosine response, whereas intraluminal 8-sulfophenyltheophylline, a polar theophylline derivative, was ineffective. Polyadenylic acid (PolyA, 1 microM), an adenosine polymer that does not penetrate the endothelium, induced a dilation of 44.2 +/- 5.3% when applied extraluminally but had no effect when infused intraluminally. The dilator effect of PolyA was antagonized by theophylline. We conclude that: (1) intraluminal adenosine and its analogues are effective dilators of intracerebral arterioles, (2) the dilator effects of both intraluminally and extraluminally applied adenosine are predominantly mediated by A2-type receptors, and (3) adenosine receptors mediating vasodilation are not present on the luminal surface of the endothelium.

Purine nucleoside and nucleobase cell membrane transport in Giardia lamblia

Giardia lamblia is dependent on the salvage of preformed purines and pyrimidines. This study investigated purine nucleoside and nucleobase transport utilizing rapid uptake determinations. Nucleoside substrate/velocity curves exhibited the hyperbolic kinetics of a saturable carrier-mediated system. Deoxynucleosides exhibited a much lower affinity for the transporter. Inhibition studies confirmed the relative carrier affinities of these ribonucleosides and deoxyribonucleosides. The nucleobase adenine did not exhibit saturation kinetics at a comparable substrate range, and did not inhibit nucleoside transport. Dipyridamole markedly inhibited nucleoside but not nucleobase transport, confirming the separate entry pathways. When cells were depleted of ATP, the velocity of nucleoside and nucleobase transport was unchanged, indicating that it is a non-energy-dependent process. Three nucleoside analogs, formycin A, adenine arabinoside and 7-deazaadenosine, were studied. Transport kinetics ranged widely among this group and could not completely account for their cytotoxic effect. When the apparent Km and Vmax of the nucleosides were compared, an approximately linear relationship (r2 = 0.95) was noted. This suggests that a high affinity of the nucleoside permease for the substrate retards disassociation of the substrate-carrier complex, slowing net influx.

Intracellular accumulation of cytosine arabinoside in murine normal and neoplastic lymphocytes following their exposure to sodium 2-mercaptoethanesulphonate

The effect of mesna on intracellular accumulation of cytosine arabinoside (Ara-C) in murine normal and neoplastic lymphocytes was studied. Simultaneous exposure of cells to mesna at concentrations ranging from 0.25 to 1.0 mM and 3H-Ara-C (40.0 nM) resulted in a strong inhibition of Ara-C uptake in normal lymphocytes. Under the same experimental conditions, mesna did not affect the Ara-C uptake in neoplastic cells (cultured L5178Y mouse leukaemia cells and neoplastically transformed thymus cells). It was found that the inhibitory effect of mesna was not cell cycle-dependent, since mesna reduced the Ara-C uptake in both normal quiescent and PHA-stimulated cells. We therefore concluded that mesna may selectively reduce Ara-C uptake by normal cells in vitro.

Nucleoside transport by perfused human placenta

Nucleoside transport and metabolism by human placenta was studied using the dual perfusion technique. With [3H] thymidine added to the maternal perfusate and neither perfusate recirculated (steady-state studies) around 40 per cent of the thymidine in the maternal outflow and 50 per cent of the transferred thymidine was degraded. In similar studies with adenosine, over 95 per cent of the nucleoside was degraded. Even with the bolus technique which sharply limits the duration of contact with the placenta, degradation of adenosine was over 95 per cent. Uptake as calculated by the dual-tracer method ([3H] adenosine/[14C] L-glucose) was equally rapid from the maternal and fetal perfusates, was saturable and inhibited by nitrosobenzylthioinosine, consistent with the facilitated diffusion system for nucleosides. Thymidine was taken up at one-third the rate of adenosine. Thymidine in large excess (500 microM) reduced adenosine uptake suggesting a common transporter. Zidovudine, a thymidine analogue used for the treatment of AIDS in which the ribose is modified at the 2' 3' position, did not compete with adenosine for uptake consistent with previous reports that zidovudine is transferred across the placenta by simple diffusion.

Comparison of uptake characteristics of thymidine and zidovudine in a human intestinal epithelial model system

The uptake of a natural nucleoside, thymidine (THY), and one of its deoxynucleoside analogues, zidovudine (AZT), was studied in a newly developed intestinal epithelial model system (CaCO-2). The results of the study indicated that the uptake rate of THY was saturable with a Km of 44.6 microM and maximum rate of 1.73 +/- 0.17 pmol/cm2/s. The uptake of AZT, on the other hand, was not saturable. When various naturally occurring nucleosides were used as inhibitors of THY uptake, it was shown that the uptake was inhibited by various pyrimidine nucleosides (e.g., uridine and cytidine), but not by purine nucleosides (e.g., guanosine). Neither group of nucleosides had any effect on the uptake of AZT. Finally, the uptake of THY was also inhibited by the metabolic inhibitor NaCN and a couple of specific nucleoside transporter inhibitors [i.e., dipyridamole and S-(p-nitrobenzyl-6-thioinosine], whereas none of these compounds had a significant effect on the uptake of AZT. The results showed, for the first time, the existence of a nucleoside carrier in the CaCO-2 cell culture model system. Also the results indicated that the uptake of AZT by the CaCO-2 cell monolayers is mainly via a passive diffusion process.

Permeation and metabolism of anti-HIV and endogenous nucleosides in human immune effector cells

Numerous anti-HIV drugs are synthetic analogs of endogenous nucleosides. Therefore it is of interest to see if a facilitated nucleoside transport system exists to mediate their uptake into human immune effector cells that are known HIV targets. Nucleoside permeation and metabolism in lymphocytes, macrophages and bone marrow cells isolated from healthy human volunteers were studied, using uridine as the prototype endogenous nucleoside. There are saturable broad specificity nucleoside transport systems in all three cell types, all of which were inhibited by dipyridamole. The Vmax and Km values for uridine transport were 0.05 +/- 0.01 pmol/sec/10(6) cells and 18.4 +/- 4.2 microM, respectively, for lymphocytes, 0.04 +/- 0.01 pmol/sec/10(6) cells and 25.3 +/- 6.6 microM, respectively, for macrophages, and 0.03 +/- 0.01 pmol/sec/10(6) cells and 90.2 +/- 10.1 microM, respectively, for bone marrow mononuclear cells. Anti-HIV dideoxynucleosides such as azidothymidine (AZT), 2',3'-dideoxycytidine (DDC), 2',3'-dideoxyinosine (DDI), 2',3'-dideoxyadenosine (DDA), and 2',3'-dideoxythymidine (DDT) are not substrates of this nucleotide transport system; hence, little or no drug accumulated inside the cells after 60 sec. Equilibration of cells with uridine or dideoxynucleosides for 2 hr resulted in high levels of cellular uridine and DDA, low levels of cellular AZT, but undetectable levels of the other analogs in all three cell types. Active metabolite levels in lymphocytes as assayed by HPLC correlated with the drug permeation results. Our data demonstrated that DDC, DDI, and DDT are not substrates for the nucleoside transporter and cannot diffuse readily across the cell membrane of human immune effector cells. Future anti-HIV drug development efforts should consider drugs that are substrates of the nucleotide transporter to ensure rapid and complete uptake into target cells.

Thymidine phosphorylase activity of platelet-derived endothelial cell growth factor is responsible for endothelial cell mitogenicity

Recombinant human platelet-derived endothelial cell growth factor, expressed in the yeast Saccharomyces cerevisiae was purified to greater than 98% purity by anion-exchange and hydroxyapatite chromatography. It was shown to possess thymidine phosphorolytic activity in vitro (pH optimum, pH 5.3; Km, 0.11 mM; Vmax, 12.5 mmol min-1 mg-1; turnover number, 9.4 s-1). Covalent modification simultaneously inhibited the enzymatic and mitogenic properties of the protein, while interaction with a cell-surface receptor was not required to stimulate mitogenesis. Purified Escherichia coli thymidine phosphorylase was also mitogenic toward endothelial cells. It is proposed that platelet-derived endothelial cell growth factor is human thymidine phosphorylase which promotes endothelial cell proliferation by reducing thymidine levels that would otherwise be inhibitory to endothelial cell growth.

Nucleoside transport in normal and neoplastic cells

The permeation of nucleosides across the plasma membrane of mammalian cells is complex and mediated by at least five distinct transporters that differ in their sensitivity to inhibitors and in their specificity for nucleosides. The basic properties and permeant specificity of these transporters are summarized in Table 3. It appears that there may be differences in the distribution of these transporters in tumors and normal tissues that might be exploited for chemotherapeutic purposes. The human tumor cell lines examined express predominantly the NBMPR-sensitive equilibrative transporter es which can be blocked by low concentrations of NBMPR and dipyridamole. It is reasonable to expect that tumors with transport properties similar to the CCRF-CEM and Rh28 cell lines (Table 1) that have no detectable NBMPR-insensitive transport activity will be highly susceptible to the therapeutic approach of combining a transport inhibitor such as dipyridamole or NBMPR with an inhibitor of de novo pyrimidine biosynthesis. On the other hand, this approach to therapy is unlikely to succeed against tumors with transport phenotypes similar to the WI-L2 cell line that may permit the salvage nucleosides in the presence of these inhibitors. The majority of tumor cells examined, however, fall between these extremes, and it is not yet known what level of NBMPR-insensitive transport activity can be tolerated without seriously compromising this therapeutic approach. With respect to normal tissues, the mature absorptive cells of the intestine have predominantly Na(+)-dependent nucleoside transporters that are insensitive to NBMPR and dipyridamole. The proliferating crypt cells also appear to have Na(+)-dependent nucleoside transport, although they may also have an NBMPR-sensitive component of transport (Belt, unpublished data). Bone marrow granulocyte-macrophage progenitor cells also appear to have one or more concentrative nucleoside transporters. Thus these tissues, which are most vulnerable to the toxicity of antimetabolites, may be able to salvage nucleosides in the presence of inhibitors of equilibrative transport and be protected from the toxicity of de novo synthesis inhibitors. It is likely, however, that a successful application of this therapeutic approach will require the analysis of the nucleoside transport phenotype of individual tumors in order to identify those patients that may benefit from such therapy. Since the development of antibodies and cDNA probes for the various nucleoside transporters is currently underway in several laboratories, it is likely that analysis of the nucleoside transport phenotype of tumors from biopsy material will be feasible in the future.

Nucleoside transport-deficient mutants of PK-15 pig kidney cell line

Previous studies indicated that PK-15 pig kidney cells express solely a nitrobenzylthioinosine-sensitive, equilibrative nucleoside transporter. In the present study, PK-15 cells were mutagenized by treatment with ICR-170 and nucleoside transport-deficient mutants selected in a single step in growth medium containing tubercidin and cytosine arabinoside at a frequency of about 2 x 10(6). The mutants were simultaneously at least 100-times more resistant to tubercidin, cytosine arabinoside and 5-fluorodeoxyuridine than the wild-type parent cells. The mutants failed to transport thymidine and uridine and had lost all high affinity nitrobenzylthioinosine binding sites. Residual low level uptake of thymidine by the mutants was shown to be due to nonmediated permeation (passive diffusion), which explains the sensitivity of the mutants to growth inhibition by high concentrations of the nucleoside drugs. Passive diffusion of thymidine at a concentration of 16 microM was not rapid enough to support the growth of nucleoside transport-deficient mutant cells that had been made thymidine-dependent by treatment with methotrexate, whereas wild-type cells grew normally under these conditions. The nucleoside transport-deficient mutants exhibited about the same growth rate and plating efficiency (60-80%) as wild-type cells, but formed larger colonies than wild-type cells because of a more extensive spread of the cells on the surface of culture dishes. PK-15 cells adhere very strongly to the surface of culture dishes and have been transformed with high efficiency with plasmid DNA either via lipofection or electroporation.

Altered membrane permeability: a new approach to malaria chemotherapy

During its development: in the host erythrocyte, the malarial parasite causes profound alterations in the permeability of the host cell membrane. Nucleoside transport pathways, which are induced by the parasite in the host erythrocyte membrane, have properties significantly different from those of the host cell. Here, Annette Gero and Joanne Upston review the current knowledge o f the parasite-induced transporters and show that they can be used to selectively direct cytotoxic compounds into the parasite-infected cell, thereby indicating their chemotherapeutic potential.

High-affinity, equilibrative nucleoside transporter of pig kidney cell line (PK-15)

Nucleoside transport was determined in the cloned porcine kidney cell line PK-15 which exhibits properties of tubular cells. The cells did not express any Na(+)-dependent, concentrative nucleoside transport. They exhibited only nitrobenzylthioinosine-sensitive equilibrative nucleoside transport. Their transport activity, however, was only about 10% of that observed in many other mammalian cell lines. This low transport activity correlated with a relatively low number of high-affinity nitrobenzylthioinosine binding sites (5 x 10(3)/cell). Furthermore, although the equilibrative transporter of PK-15 cells exhibited a similar broad substrate specificity as the equilibrative nucleoside transporters of other mammalian cells, it exhibited a much higher affinity for certain nucleosides, especially cytidine and uridine, than the latter. The Michaelis-Menten constants for zero-trans transport and equilibrium exchange of uridine in ATP-depleted cells were about the same (about 40 microM), indicating equal mobility of the nucleoside-loaded and empty carrier. Concentrative nucleoside transport was detected in one set of PK-15 cultures, but was found to be due to mycoplasma contamination.

Thymidine kinase-deficient mutants of Aedes albopictus mosquito cells

Aedes albopictus mosquito cells resistant to the thymidine analog 5-bromodeoxyuridine (BrdU) were obtained using a single-step selection procedure. The resistant cells were characterized with respect to growth in the presence of BrdU, incorporation of [3H]uridine and [3H]thymidine, and thymidine kinase activity in crude extracts. The LC50 for TK-6 cells was 95 micrograms BrdU/ml, and for TK-8 cells was 45 micrograms/ml. Both clones incorporated [3H]uridine at levels corresponding to those in wild-type cells. TK-6 and TK-8 cells did not incorporate [3H]thymidine into acid-precipitable material, nor did they contain measurable thymidine kinase activity. Thymidine kinase activity in crude extracts from wild-type cells had a Km of 2 microM and a Vmax of 10 pmol.min-1.mg-1 protein.

Transport and metabolism of D-[3H]adenosine and L-[3H]adenosine in rat cerebral cortical synaptoneurosomes

The relationship between transport and metabolism in synaptoneurosomes was examined to determine the metabolic stability of rapidly accumulated D-[3H]adenosine and L-[3H]adenosine and the degree to which metabolism of the accumulated purines affected measurements of apparent KT and Vmax values for adenosine transport. For D-[3H]adenosine, high- and low-affinity accumulation processes were present. For the high-affinity system an inverse relationship was found between transport reaction times and KT and Vmax values. For incubations of 5, 15, and 600 s, which corresponded to 24, 32, and 76% phosphorylation of accumulated D-[3H]adenosine to nucleotides, apparent KT values were 9.4, 8.4, and 4.5 microM, respectively, and Vmax values were 850, 70, and 12 pmol/min/mg of protein, respectively. Pretreatment with 10 microM erythro-9-(2-hydroxy-3-nonyl)adenine, an adenosine deaminase inhibitor, and 5'-iodotubercidin, an adenosine kinase inhibitor, decreased the phosphorylation of accumulated D-[3H]adenosine to 6% with 5-s and 9% with 15-s incubations. This resulted in significantly higher KT values: 36 microM at 5 s and 44 microM at 15 s. At 10-min incubations in the presence of these inhibitors, metabolism of accumulated D-[3H]adenosine was 32%, and apparent KT and Vmax values at this time were not significantly different from those obtained without inhibitors. For L-[3H]adenosine, apparent KT and Vmax values for 20-s incubations were 38.7 microM and 330 pmol/min/mg of protein, respectively. Metabolism (mainly phosphorylation) of accumulated L-[3H]adenosine was observed only at incubations of greater than 30 s.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol metabolism and inhibition of nucleoside uptake lead to increased extracellular adenosine in hepatocytes

Recent evidence suggests that adenosine mediates many of the acute and chronic effects of ethanol in both cultured cells and whole animals. These adenosine-mediated effects of ethanol result from ethanol-induced increases in extracellular adenosine. Acute exposure of primary cultures of rat hepatocytes to 12.5-200 mM ethanol increased extracellular adenosine concentrations by 20-35%. Pretreatment of hepatocytes with 100 microM 4-methylpyrazole, an inhibitor of alcohol dehydrogenase, completely blocked ethanol-induced increases in extracellular adenosine at 12.5 and 25 mM ethanol. However, even in the presence of 4-methylpyrazole, ethanol at concentrations greater than 50 mM still increased extracellular adenosine concentrations. This increase appears to be due to ethanol inhibition of adenosine uptake via the nucleoside transporter (50% inhibitory concentration, 28 mM). After chronic treatment with 100 mM ethanol for 48 h, acute challenge with ethanol no longer inhibited adenosine uptake, i.e., the nucleoside transporter had become tolerant to ethanol. Moreover, in these chronically treated cells, ethanol-induced increases in extracellular adenosine were completely blocked by treatment with 4-methylpyrazole at all concentrations of ethanol. Taken together, these results suggest that increased extracellular adenosine in hepatocytes is dependent on both ethanol oxidation and inhibition of adenosine uptake via the nucleoside transporter.