Thymidine transport in cultured mammalian cells. Kinetic analysis, temperature dependence and specificity of the transport system

Hydrogen peroxide-induced thymidine incorporation into cultured rat astrocytes

We characterized [methyl-(3)H]thymidine ([(3)H]thymidine) and [5-(3)H]uridine ([(3)H]uridine) incorporation into cultured astrocytes and neurons in the presence and absence of hydrogen peroxide (H2O2) in order to define the response to oxidative stress in the central nervous system. [(3)H]Thymidine incorporation into cultured astrocytes was remarkably decreased by N(6),2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP), a permeable analogue of cAMP, which induced a morphological change from the polygonal form (undifferentiated astrocytes) to the process-bearing one (differentiated astrocytes). H2O2 induced [(3)H]thymidine, but not [(3)H]uridine, incorporation into cultured astrocytes at only an early time from 24 h after DBcAMP treatment, although the absolute quantities of [(3)H]thymidine incorporation into astrocytes pretreated with DBcAMP were less than those into astrocytes pretreated without DBcAMP. Hydroxyurea, a replicative DNA synthesis inhibitor, suppressed dose-dependently and completely [(3)H]thymidine incorporation into astrocytes pretreated without DBcAMP, but not astrocytes pretreated with DBcAMP. H2O2 did not stimulate [(3)H]thymidine or [(3)H]uridine incorporation into astrocytes pretreated without DBcAMP and neurons. These findings indicate that only astrocytes pretreated with DBcAMP are able to increase thymidine incorporation specifically in the presence of H2O2 for a purpose other than proliferation, including the repair of H2O2-induced DNA injury, for example.

PET imaging of cellular proliferation

PET cellular proliferation imaging has its roots in a long history of in vitro cellular proliferation studies to characterize cancer and in the understanding of the biology of thymidine incorporation into DNA gained from these studies. PET imaging represents the logical translation of the in vitro work to measure in vivo tumor proliferation. Preclinical studies of [11C]-thymidine and other PET-labeled thymidine analogues set the stage for early clinical studies that provided very promising results. Recent progress in the application of [18F]-FLT, a clinically practical PET thymidine analogue, to patient studies sets the next stage for clinical PET cellular proliferation imaging. Further mechanistic studies of the imaging agents and well-designed clinical trials will be important in moving PET proliferation imaging into what is likely to be a significant role in the care of cancer patients by providing a quantitative measure of tumor response to cytotoxic or cytostatic therapy.

Homeostatic control of uridine and the role of uridine phosphorylase: a biological and clinical update

Uridine, a pyrimidine nucleoside essential for the synthesis of RNA and bio-membranes, is a crucial element in the regulation of normal physiological processes as well as pathological states. The biological effects of uridine have been associated with the regulation of the cardio-circulatory system, at the reproduction level, with both peripheral and central nervous system modulation and with the functionality of the respiratory system. Furthermore, uridine plays a role at the clinical level in modulating the cytotoxic effects of fluoropyrimidines in both normal and neoplastic tissues. The concentration of uridine in plasma and tissues is tightly regulated by cellular transport mechanisms and by the activity of uridine phosphorylase (UPase), responsible for the reversible phosphorolysis of uridine to uracil. We have recently completed several studies designed to define the mechanisms regulating UPase expression and better characterize the multiple biological effects of uridine. Immunohistochemical analysis and co-purification studies have revealed the association of UPase with the cytoskeleton and the cellular membrane. The characterization of the promoter region of UPase has indicated a direct regulation of its expression by the tumor suppressor gene p53. The evaluation of human surgical specimens has shown elevated UPase activity in tumor tissue compared to paired normal tissue.

Expression of thymidine phosphorylase in epithelial ovarian cancer: correlation with angiogenesis, apoptosis, and ultrasound-derived peak systolic velocity

OBJECTIVE

The objective of this study was to determine whether the expression of thymidine phosphorylase (TP) by epithelial ovarian cancer cells correlates with the density of microvessels within the tumor, apoptotic index (AI) within the tumor cells, and ultrasound-derived blood flow.

METHODS

Color Doppler imaging and pulsed Doppler spectral analysis ultrasonography were used to scan 44 patients with an overt ovarian mass immediately before laparotomy. Sections of malignant tumors were analyzed for the cellular expression of TP and the intratumoral density of microvessels by immunohistochemistry using antibodies to TP and factor-VIII-related antigen, respectively. Moreover, AI was evaluated by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling methods.

RESULTS

Forty-four epithelial ovarian cancers were studied (6 low malignant potential, 15 serous cystadenocarcinoma, 9 mucinous cystadenocarcinoma, 8 endometrioid adenocarcinoma, 4 clear cell carcinoma, and 2 malignant Brenner tumors); 19 were Stage I, 6 Stage II, 15 Stage III, and 4 Stage IV. Fourteen tumors (32%) were classified as being TP positive. The proportion of Stage I tumors that was TP positive (16%) was significantly lower (P = 0.022) that the corresponding value for Stages II-IV (44%), although the values for microvessel count, AI, and peak systolic velocity (PSV) were similar. AI was significantly lower in TP-positive tumors than in TP-negative tumors (P = 0.015). The PSV was significantly higher in TP-positive tumors (P = 0.02). There was a significant correlation between the microvessel count and the PSV (r = 0.34, P = 0.024). Moreover, AI was significantly inversely related to the PSV (r = 0. 35, P = 0.023). The PSV in a subgroup with a high microvessel count and low AI was significantly higher than that in a subgroup with a low microvessel count and high AI (P = 0.0006). These findings significantly associated with TP expression (P = 0.024).

CONCLUSIONS

The intratumoral PSV, as determined by color Doppler imaging and spectral analysis, might reflect the coordination of angiogenesis and apoptosis associated with TP expression in epithelial ovarian cancer.

Characterizing tumors using metabolic imaging: PET imaging of cellular proliferation and steroid receptors

Treatment decisions in oncology are increasingly guided by information on the biologic characteristics of tumors. Currently, patient-specific information on tumor biology is obtained from the analysis of biopsy material. Positron emission tomography (PET) provides quantitative estimates of regional biochemistry and receptor status and can overcome the sampling error and difficulty in performing serial studies inherent with biopsy. Imaging using the glucose metabolism tracer, 2 -deoxy-2- fluoro-D-glucose (FDG), has demonstrated PET's ability to guide therapy in clinical oncology. In this review, we highlight PET approaches to imaging two other aspects of tumor biology: cellular proliferation and tumor steroid receptors. We review the biochemical and biologic processes underlying the imaging, positron-emitting radiopharmaceuticals that have been developed, quantitative image-analysis considerations, and clinical studies to date. This provides a basis for evaluating future developments in these promising applications of PET metabolic imaging.

Thymidine phosphorylase, 2-deoxy-D-ribose and angiogenesis

Angiogenesis is the term used to describe the formation of new blood vessels from the existing vasculature. In order to attract new vessels, a tissue must release an endothelial-cell chemoattractant. 2-Deoxy-D-ribose is produced in vivo by the catalytic action of thymidine phosphorylase (TP) on thymidine and has recently been identified as an endothelial-cell chemoattractant and angiogenesis-inducing factor. TP, previously known only for its role in nucleotide salvage, is now known to be angiogenic. TP expression is elevated in many solid tumours and in chronically inflamed tissues, both known areas of active angiogenesis. There is evidence that TP is also involved in physiological angiogenesis such as endometrial angiogenesis during the menstrual cycle. The majority of known endothelial-cell chemoattractants are polypeptides that bind to endothelial-cell-surface receptors. In contrast, 2-deoxy-D-ribose appears to lack a cell-surface receptor. Glucose is another sugar that acts as an endothelial-cell chemoattractant. The migratory activity of glucose is blocked by ouabain. It is possible that 2-deoxy-D-ribose and glucose stimulate endothelial-cell migration via a similar mechanistic pathway.

Azidothymidine in combination with 5-fluorouracil in human colorectal cell lines: in vitro synergistic cytotoxicity and DNA-induced strand-breaks

The in vitro cytotoxicity of the combination of azidothymidine (AZT) and 5-fluorouracil (5-FU) against the human colorectal cancer cells SW-480, SW-620 and COLO-320DM was evaluated. The cytotoxic effects of 5-FU and AZT were determined by the assay using 2,3-bis(2-methoxy-4-nitro-5-sulfophenil)-2H-tetrazolium-5-carbo xanilide inner salt (XXT), while drug-induced DNA strand-breaks were measured using a fluorometric analysis of DNA unwinding. After an exposure of 72 h, 5-FU and AZT induced a dose-dependent cytotoxicity against each cell line. The addition of 3, 10 and 30 microM AZT to various concentrations of 5-FU, as well as the addition of 0.5, 1 and 3 microns 5-FU to various concentrations of AZT, resulted in an enhanced cytotoxic effect. Isobologram analysis and the combination index (CI) method demonstrated that the interaction between 5-FU and AZT was clearly synergistic in each cell line, except for the 30% level of effect in SW-620, where borderline synergism was observed. The evaluation of DNA strand-breaks after an exposure of 16 h to 5-FU, AZT or 5-FU + AZT demonstrated that the 5-FU + AZT combination produced the greatest DNA damage, and that this interaction was synergistic in each cell line. In conclusion, our study supports the evidence that the potential antitumour activity of AZT can be modulated by combining it with agents which inhibit thymidylate (dTMP) formation, such as 5-FU, and that the increased cytotoxicity is related to enhanced DNA damage. These findings should encourage further experimental and clinical studies of the potential use of AZT in combination with inhibitors of de novo dTMP synthesis.

Identification of a novel membrane transporter associated with intracellular membranes by phenotypic complementation in the yeast Saccharomyces cerevisiae

A partial mouse cDNA was isolated by its ability to functionally complement a thymidine transport deficiency in plasma membranes of the yeast, Saccharomyces cerevisiae. The full-length cDNA encoded a previously unidentified 27-kDa protein (mouse transporter protein (MTP)) with four predicted transmembrane-spanning domains. MTP mRNA was detected in cells of several mammalian species, and its predicted protein sequence exhibited near identity (98%) with that of a human cDNA (HUMORF13). MTP and its homologs evidently reside in an intracellular membrane compartment because a protein (about 24 kDa) that was recognized by MTP-specific antibodies was observed in a subcellular fraction of rat hepatocytes enriched for Golgi membranes. Deletion of the hydrophilic C terminus of MTP, which encompassed two putative signal motifs for intracellular localization (Tyr-X-X-hydrophobic amino acid), allowed expression of recombinant protein (MTP deltaC) in plasma membranes of Xenopus laevis oocytes. MTP deltaC-expressing oocytes exhibited greater fragility than nonexpressing oocytes, and those that survived the experimental manipulations were capable of mediated uptake of thymidine, uridine, and adenosine. Thymidine uptake by MTP deltaC-expressing oocytes was inhibited by thymine and dTMP. MTP may function in the transport of nucleosides and/or nucleoside derivatives between the cytosol and the lumen of an intracellular membrane-bound compartment.

Mechanisms of nitrosothiol-induced antimitogenesis in aortic smooth muscle cells

The purpose of this work was to investigate the effect of a nitrosothiol vasodilator, S-nitroso-N-acetylpenicillamine (SNAP), on serum-induced cell proliferation, thymidine uptake, RNA synthesis, protein synthesis and thymidine kinase activity in a cultured rat aortic smooth muscle cell line. SNAP decreased the rate of serum-stimulated cell proliferation, thymidine uptake and incorporation, uridine and leucine incorporation and thymidine kinase activity in concentration-dependent fashion. The threshold concentration of SNAP for inhibition of cell proliferation and thymidine uptake was similar and in the range of 1-3 microM. Uridine incorporation, indicative of RNA synthesis, was inhibited beginning at 10 microM SNAP, whereas leucine incorporation, indicative of protein synthesis, and thymidine kinase activity, an enzyme of importance to DNA synthesis, were inhibited beginning at 100 microM SNAP. These results indicate that inhibition of cell proliferation induced by relatively low concentrations of SNAP (1-10 microM) is independent of inhibition of protein synthesis or thymidine kinase activity, whereas higher concentrations of SNAP may inhibit cell proliferation by decreasing protein synthesis.

High-dose intravenous zidovudine with 5-fluorouracil and leucovorin. A phase I trial

BACKGROUND

The inhibition of pyrimidine metabolism by 5-fluorouracil (5-FU) enhances the anti-cancer effects of zidovudine (formerly called AZT) in in vitro and in vivo model systems without additive toxicity. Zidovudine-induced DNA damage correlates with cytotoxicity.

METHODS

A Phase I trial of high-dose continuous-infusion intravenous zidovudine therapy in combination with 5-FU and leucovorin therapy was performed. Eighteen patients with advanced malignant tumors were treated with 43 courses of oral leucovorin (50 mg every 4 hours); continuous-infusion 5-FU (800 mg/M2/day) for 72 hours (3 days); and zidovudine, begun 24 hours after the start of 5-FU and leucovorin, for 48 hours, and terminating with the end of the 5-FU infusion. Zidovudine plasma levels and zidovudine-induced DNA damage were assessed.

RESULTS

Zidovudine administered in doses of 2-20 g/M2/day, added no obvious toxicity to the basic chemotherapeutic treatment with 5-FU and leucovorin but resulted in a dose-dependent biologic effect manifested by an increase in DNA strand breaks in peripheral blood cells. At doses greater than 15 g/M2/day, altered plasma kinetics of zidovudine were observed; plasma zidovudine levels increased dramatically in relation to the dose of zidovudine. Limitations in drug administration restricted administration of higher intravenous doses without achieving a maximally tolerated dose. No responses were seen in this heavily pretreated population.

CONCLUSIONS

Based on the results of preclinical studies, plasma zidovudine levels greater than those achieved at the maximal dose (133 microns) are required for increased anti-cancer activity with 5-FU. Additional studies using a bolus or rapid infusion as a method of achieving higher peak levels are indicated.

Adenine and hypoxanthine transport in human erythrocytes: distinct substrate effects on carrier mobility

Transport of adenine and hypoxanthine in human erythrocytes proceeds via two mechanisms: (1) a common carrier for both nucleobases and (2) unsaturable permeation 4-5-fold faster for adenine for hypoxanthine. The latter process was resistant to inactivation by diazotized sulfanilic acid. Carrier mediated transport of both substrates was investigated using zero-trans and equilibrium exchange protocols. Adenine displayed a much higher affinity for the carrier (Km approximately 5-8 microM) than hypoxanthine (Km approximately 90-120 microM) but maximum fluxes at 25 degrees C were generally 5-10-fold lower for adenine (Vmax approximately 0.6-1.4 pmol/microliters per s) than for hypoxanthine (Vmax approximately 9-11 pmol/microliters per s). The carrier behaved symmetrically with respect to influx and efflux for both substrates. Adenine, but not hypoxanthine reduced carrier mobility more than 10-fold. The mobility of the unloaded carrier, calculated from the kinetic data of either hypoxanthine or adenine transport, was the same thus providing further evidence that these substrates share a common transporter and that their membrane transport is adequately described by the alternating conformation model of carrier-mediated transport.

Tissue-specific expansion of uridine pools in mice. Effects of benzylacyclouridine, dipyridamole and exogenous uridine

The concentration of uridine (Urd) in murine tissues appears to be controlled by Urd catabolism, concentrative Urd transport, and the non-concentrative, facilitated diffusion of Urd. Previous reports document the tissue-specific disruption of these processes, and subsequently intracellular pools of free Urd in mice, by the administration of exogenous Urd (250 mg/kg) or the Urd phosphorylase (EC 2.4.2.3; uracil:ribose-1-phosphate phosphotransferase) inhibitor 5-benzylacyclouridine (BAU) (240 mg/kg). We now report the effect of combinations of BAU (120 mg/kg, p.o.), the nucleoside transport inhibitor dipyridamole (DP) (25 mg/kg, i.p.), and exogenous Urd (250 mg/kg, i.v.) on Urd pools in mice. This dose of BAU increased Urd pools 2- to 6-fold, in a tissue-specific manner, for up to 5 hr. DP increased Urd pools 3-fold in spleen, over a 4-hr period, but did not affect other tissues. Administration of BAU 1 hr prior to exogenous Urd resulted in a 50- to 100-fold expansion of tissue normal after 6 hr. Administration of DP 1 hr prior to exogenous Urd caused a tissue-specific 40- to 100-fold increase in Urd pools which, except in spleen, returned to normal within 2 hr. The marked additive effects of these combinations were in contrast to those obtained following the administration of BAU 1 hr prior to DP. This regimen increased Urd pools from 4- to 9-fold, in a tissue-specific manner. In addition, Urd pools remained elevated for up to 9 hr, except in spleen where the Urd concentration was elevated for up to 15 hr. Analysis of enzyme activities indicated that DP does not enhance the inhibitory effect of BAU against murine liver Urd phosphorylase. However, DP did inhibit plasma clearance of BAU, and this effect may partially explain the apparent synergistic effect of this combination. In spite of the prolonged and dramatic expansion of tissue Urd pools produced by BAU + DP, the total Ura nucleotide content in spleen, gut and colon tumor 38 (CT38) increased by less than 70% over a 12-hr period following administration of this combination. These findings are discussed in light of their biochemical and therapeutic implications.

Effects of the lipophilic anticancer drug teniposide (VM-26) on membrane transport

The epipodophyllotoxin glucopyranosides have previously been shown to interact with membrane lipids and to alter the activity of several lipid-embedded membrane proteins. To determine if these agents are acting as general membrane perturbants, we have further examined their effects on membrane processes in Ehrlich ascites tumor cells. [3H]VM-26 and [3H]VP-16 were taken up rapidly and concentrated within the cells in proportion to their lipophilicity. Neither agent was found to have any significant effect on the influx of L-[3H]leucine or alpha-[3H]aminoisobutyric acid. Likewise, these drugs had no significant effects on the hexose transporter. The nucleoside transporter, which is structurally and functionally similar to the hexose transporter, was dramatically affected, however. VM-26 was a non-competitive inhibitor of equilibrium-exchange influx of cytosine arabinoside in Ehrlich cells with a Ki of 15 microM. Equilibrium-exchange influx increased with temperature in control cells (Q10 = 2) but not in VM-26-treated cells; thus, VM-26 was a more potent inhibitor at higher temperatures. VM-26 also significantly reduced zero-trans influx in Ehrlich, P388, L5178Y, and ML-1 cells, and these effects were immediate in onset. VM-26 inhibited high-affinity binding of the nucleoside transport inhibitor nitrobenzylmercaptopurine riboside (NBMPR), but VM-26 enhanced non-specific NBMPR binding to Ehrlich cells. The apparent specificity of the epipodophyllotoxins for the nucleoside transporter is discussed.

Dideoxycytidine permeation and salvage by mouse leukemia cells and human erythrocytes

Transmembrane equilibration of dideoxycytidine (ddCyd) in P388 mouse leukemia cells and human erythrocytes was only 1% as rapid as that of uridine and 2'-deoxycytidine which is mediated by the facilitated nucleoside transporter of these cells. ddCyd entry was nonsaturable up to a concentration of 1 mM but was partially inhibited by dipyridamole, nitrobenzylthioinosine and nucleosides, but not by nucleobases. Thus, entry was partly (70-80%) mediated, though very inefficiently, by the nucleoside carrier. Intracellular phosphorylation of ddCyd in P388 cells was also very inefficient compared to that of 2'-deoxycytidine and uridine and not rate limited by its slow entry into the cells.

Enhancement of fluorouracil therapy by the manipulation of tissue uridine pools

Evidence for transport systems that actively concentrate uridine in normal tissues provides a previously unexploited opportunity for manipulation to therapeutic advantage. The ability to expand these pools in a tissue-specific manner by administration of exogenous uridine, inhibition of uridine phosphorylase with BAU or blockade of the facilitated transport of nucleosides with dipyridamole is established. If the apparent defect in the active transport mechanism for uridine in neoplastic cells in culture as well as several model tumors reflect the properties of human neoplasms, a new exploitable therapeutic difference may exist. These approaches may, in the near future, increase the therapeutic effectiveness not only of fluorouracil and the other fluoropyrimidines but also of other agents which disrupt uridine metabolism such as PALA and pyrazofurin.

Stimulation by tumor necrosis factor of HL-60 thymidine salvage pathway metabolism dissociated from proliferation

The effect of human tumor necrosis factor (TNF) on early-passage HL-60 cells was studied. A transient phase of increased [3H]thymidine (TdR) incorporation was noted at 20-24 hr of exposure to TNF. This increase was disproportionate to the much slighter stimulation of the percentage of S-phase cells, which was measured by flow cytometry. Evidence for increased metabolic trapping of [3H]TdR following TNF treatment was apparent from whole cell uptake experiments. The salvage pathway enzyme TdR kinase was therefore measured and was found to be elevated comparably to [3H]TdR uptake. The mechanism of TNF regulation of TdR kinase was further investigated by a series of combination treatment experiments using other biologic factors and pharmacologic inhibitors of various intracellular steps. The response to TNF was not potentiated or reproduced by IL-1, IL-2, IL-3, IL-4, G-CSF, M-CSF, GM-CSF or alpha- or gamma-interferon. Blockers of early signal transduction steps, including H7, W7, sphingosine, and pertussis toxin, failed to inhibit TNF stimulation of [3H]TdR incorporation. mRNA synthesis inhibition with alpha-amanitin blocked this TNF effect, as did cAMP but not cGMP analogues. A sensitizing effect was noted with amiloride or cytochalasin B, characterized by greater relative increases of [3H]TdR incorporation and TdR kinase activity in response to TNF. In the presence of cytochalasin B, TNF treatment resulted in no change or slight decreases in the percentage of S-phase cells. Regulation of TdR kinase could thereby be dissociated from the usual cell cycle control. This study thus documents a unique example of stimulation of thymidine salvage pathway metabolism by a biologic factor, dissociable from overall cell cycle regulation.

Membrane transport and the antineoplastic action of nucleoside analogues

This article summarizes recent studies characterizing nucleoside transport in mammalian cells and discusses evidence for a role of membrane transport in the pharmacologic action of nucleoside analogues. Some of these studies have also addressed the controversy concerning the multiplicity in transport routes. It seems clear that erythrocytes and, perhaps, some other mammalian cells possess a single, broadly specific system for transporting nucleosides. However, substantial evidence from valid studies discriminating between transport and intracellular metabolism suggests that at least some mammalian cells, including some tumor cells, possess more than a single system. Evidence now exists for a determining role of membrane transport of nucleoside analogues in their cytotoxicity and, in the case of one pyrimidine nucleoside (AraC), in therapeutic responsiveness in leukemic patients. There are also numerous examples of transport-related resistance to nucleoside analogues. Included in this article are the results of studies from the authors' laboratory pertaining to the therapeutic activity of the purine nucleoside, FAraA, in murine tumor models. These studies provide evidence for a determining role of both membrane transport and intracellular phosphorylation in the selective antitumor action of this agent against murine leukemia. Substantially increased transport inward of FAraA occurs at pharmacologically achievable concentrations of this agent in tumor cells as compared to drug-limiting, normal proliferative epithelium of the small intestine. The basis for this differential appears to be the kinetic duality of FAraA and adenosine transport inward found in tumor cells, but not in proliferative intestinal epithelial cells. Tumor cells have highly saturable (low influx Km) and poorly saturable (high influx Km) systems for adenosine transport, both of which are shared by FAraA. In contrast, proliferative epithelial cells have only a poorly saturable system for these substrates. If a similar kinetic duality of nucleoside transport is found in other tumor cells certain implications arise concerning the significance of the duality to neoplastic transformation.

Nucleoside 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.

Membrane transport influences the rate of accumulation of cytosine arabinoside in human leukemia cells

The role of membrane transport in the cellular accumulation of 1-beta-D-arabinofuranosylcytosine (ara-C) was studied in freshly isolated human acute leukemia cells. Patient cells had low rates for ara-C transport as compared with human and murine experimental cells and correspondingly low binding capacities for the nucleoside transport inhibitor, nitrobenzylmercaptopurine riboside (NBMPR). At 1 microM ara-C, the rate of net cellular accumulation was close to the membrane transport rate, and NBMPR inhibited transport and accumulation to the same extent. The rate of ara-C accumulation was half maximal at only 3-5 microM, a level much lower than that required for murine cells (67-85 microM). At concentrations below 1 microM the rate of ara-C accumulation was determined primarily by the transport rate, but at higher concentrations above 10 microM, phosphorylation capacity was the principal determinant of the net uptake rate. This difference in the role of transport at high and low ara-C concentrations may explain, in part, the efficacy of high-dose ara-C in patients refractory to standard dose protocols.

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.

Effects of tri-n-butyltin chloride on energy metabolism, macromolecular synthesis, precursor uptake and cyclic AMP production in isolated rat thymocytes

The inhibitor of oxidative phosphorylation tri-n-butyltin chloride (TBTC) causes membrane damage and disintegration of isolated rat thymocytes at concentrations higher than 1 microM. From a concentration of 0.1 microM, TBTC disturbs energy metabolism as indicated by an increase in methylglucose uptake, glucose consumption and lactate production and by a decrease in cellular ATP levels. Over the same TBTC concentration range, the incorporation of DNA, RNA and protein precursors are markedly reduced. Moreover the production of cyclic AMP upon stimulation of the cells with prostaglandin E1 is effectively inhibited. These effects cannot be explained by an inhibition of nucleoside kinase activity, amino acid uptake or adenylate cyclase activity. The effects of TBTC on macromolecular synthesis and cyclic AMP production are possibly due to a disturbance of the cellular energy state.

Effects of various inhibitors of oxidative phosphorylation on energy metabolism, macromolecular synthesis and cyclic AMP production in isolated rat thymocytes. A regulating role for the cellular energy state in macromolecular synthesis and cyclic AMP production

Inhibitors of oxidative phosphorylation such as several triorganotin compounds, oligomycin, 2,4-dinitrophenol and carbonylcyanide p-trifluoromethoxyphenylhydrazone suppress energy metabolism of isolated rat thymocytes as indicated by a reduction of ATP levels, an increase in glucose consumption and by a marked accumulation of lactate. Also these compounds effectively inhibit the incorporation of DNA, RNA and protein precursors into acid-precipitable material of thymocytes. Moreover, the prostaglandin E1-induced elevation of cAMP is markedly reduced by these inhibitors. A correlation is observed between the effects on energy metabolism, macromolecular synthesis and cAMP production, since from a series of trialkyltin chlorides, tri-n-propyltin, tri-n-butyltin and tri-n-hexyltin are very effective inhibitors of these functions, while trimethyltin and tri-n-octyltin affect neither of them; other inhibitors of oxidative phosphorylation, each of them with quite different mechanisms of action, also inhibit macromolecular synthesis and cAMP production. The finding that a rise in intracellular ATP concentrations leads to a reversion of the tri-n-butyltin-induced inhibition of cAMP production and uridine incorporation, indicates a regulating role for the cellular energy state in these aspects of cellular function.

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

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

Obligatory relationship between the sterol biosynthetic pathway and DNA synthesis and cellular proliferation in glial primary cultures

Primary cultures of newborn rat brain, which are composed predominantly of astroglia, were used to examine the relationship between the sterol biosynthetic pathway and DNA synthesis and cellular proliferation. Reduction of the fetal calf serum content of the culture medium from 10 to 0.1% (vol/vol) for an interval of 48 h between days 4 and 6 in culture resulted in a quiescent state characterized by inhibition of DNA synthesis and cellular proliferation. When 10% fetal calf serum was returned to the medium for these quiescent cells, within 24 h DNA synthesis increased markedly. Preceding the rise in DNA synthesis was an increase in sterol synthesis, which occurred within 12 h of the return of the quiescent cells to the 10% fetal calf serum. Exposure of the quiescent cells to mevinolin, a specific inhibitor of sterol synthesis at the 3-hydroxy-3-methylglutaryl-CoA reductase step, completely inhibited the increase in DNA synthesis that followed serum repletion. The increase in total protein synthesis that followed serum repletion was not similarly inhibited by mevinolin. When mevinolin was removed after causing the 24-h inhibition of DNA synthesis, the cultured cells underwent active DNA synthesis and proliferation. Thus, inhibition of the sterol biosynthetic pathway resulted in a specific and reversible inhibition of DNA synthesis and glial proliferation in developing glial cells. These findings establish a valuable system for the examination of glial proliferation, i.e., primary glial cultures subjected to serum depletion and subsequent repletion. Moreover, the data establish an obligatory relationship between the sterol biosynthetic pathway and DNA synthesis and cellular proliferation in developing glia.

Nucleoside transport in Walker 256 rat carcinosarcoma and S49 mouse lymphoma cells. Differences in sensitivity to nitrobenzylthioinosine and thiol reagents

The characteristics of nucleoside transport were examined in Walker 256 rat carcinosarcoma and S49 mouse lymphoma cells. In Walker 256 cells the initial rates of uridine, thymidine and adenosine uptake were insensitive to the nucleoside transport inhibitor nitrobenzylthioinosine (NBMPR) (1 microM), but were partially inhibited by dipyridamole (10 microM), another inhibitor of nucleoside transport. In contrast, the transport of these nucleosides in S49 cells was completely blocked by both inhibitors. Nucleoside transport in Walker 256 and S49 cells also differed in its sensitivity to the thiol reagent p-chloromercuribenzenesulphonate (pCMBS). Uridine transport in Walker 256 cells was inhibited by pCMBS with an IC50 (concentration producing 50% inhibition) of less than 25 microM, and inhibition was readily reversed by beta-mercaptoethanol. In S49 cells uridine transport was only inhibited at much higher concentrations of pCMBS (IC50 approximately equal to 300 microM). In other respects nucleoside transport in Walker 256 and S49 cells were quite similar. The Km and Vmax. values for uridine transport were nearly identical, and the transporters of both cell lines appeared to accept a broad range of nucleosides as substrates. Uridine transport in Walker 256 cells was non-concentrative and did not require an energy source. These studies demonstrate that nucleoside uptake in Walker 256 cells is mediated by a facilitated-diffusion mechanism which differs markedly from that of S49 cells in its sensitivity to the transport inhibitor NBMPR and the thiol reagent pCMBS.

The human erythrocyte ghost: a new experimental model for studying adenosine transport

Previous work on adenosine transport has always had problems with the interference of adenosine metabolism, due to its high metabolic rate and because the enzymes involved are consistently present in most tissues. A new experimental model for studying adenosine transport in human erythrocyte ghosts is presented in this work: Human erythrocyte ghosts were sealed in the presence of erythro-3(2-hydroxynonyl)adenine and P1-P5-di(adenosine)5'-pentaphosphate, inhibitors of adenosine deaminase and adenosine kinase, respectively. These ghosts proved to lack adenosine metabolism when incubated in [U-14C]adenosine at 10 microM concentration at lack 37 degrees C for 60 min. Ghosts were 99.4% sealed in the correct orientation and had constant intracellular water volume. With these characteristics, the erythrocyte ghost preparation has many advantages for studying adenosine transport without adenosine metabolism interference. Adenosine transport was studied following the technique of W. R. Lieb and W. D. Stein [(1974) Biochim. Biophys. Acta 373, 165-177, 178-196.] Experiments to study Zero-trans influx and efflux, equilibrium exchange, and infinite-trans influx and efflux are presented. Adenosine transport did not behave linearly in any of these experimental procedures. Adenosine basic kinetic constants, calculated according to the procedure of Lieb and Stein, were R1----2 = 4.1 X 10(-4), R2----1 = 3.97 X 10(-4), Ree = 1.94 X 10(-4), Roo = 6.08 X 10(-4), K1----2 = 125.67 microM, and K2----1 = 84.36 microM. Lieb and Stein rejection criteria were used to distinguish a simple pore from a simple carrier. The data accumulated indicate that adenosine transport is carried out by a system that satisfies the criteria used for the simple carrier model. Asymmetric behavior was observed indicating lower affinity of the carrier for adenosine influx, although Vmax values for influx and efflux were similar.

Solute-excluded volumes near the Novikoff cell surface

A differential centrifugation technique, in which all extracellular water except that intimately associated with the cell (pericellular domain) is removed, has been applied to isolated Novikoff hepatoma cells. The pericellular volumes accessible to albumin, inulin, raffinose, and sucrose were inversely related to the molecular weights of the test solutes. This phenomenon was not detectable in erythrocytes or in fat cells. Selective removal of cell surface components by enzymatic treatment produced proportional changes in the relative volumes of distribution accessible to the solutes. This discrimination in the volume accessible to each of the solutes is analogous to that obtained in gel chromatographic separation and represents, in effect, excluded volumes which are inversely related to solute size. This exclusion is associated with components of the Novikoff cell surface, including the surface coat and the microvilli that cover the Novikoff cell. These structures provide an additional level of discrimination for the Novikoff cell not seen in certain other cell types.

Interferon inhibition of thymidine incorporation into DNA through effects on thymidine transport and uptake

Replenishment of medium after 72 hr of growth of HeLa-S3 cells in dense suspension cultures increased [3H]-thymidine uptake into cells and incorporation into DNA, with the levels reaching a peak approximately 12 hr following medium change; beta interferon inhibits the enhanced uptake of [3H]-thymidine and labeling of DNA in a dose-dependent manner. Some reduction in these processes is observed at a concentration as low as 1 u/ml, and approximately 75% inhibition at 640 u/ml. Kinetic analysis has revealed that the rate of labeling of the acid-soluble pool with [3H]-thymidine, measured either at 22 degrees C or 37 degrees C, is reduced in interferon-treated (640 u/ml, 24 hr) HeLa-S3 cells. At 22 degrees C, the initial rate of thymidine transport at a high (500 microM) thymidine concentration, determined within the first 30 sec of [3H]-thymidine addition was depressed by 44% in interferon-treated HeLa cells. At 37 degrees C, labeled precursors accumulate in acid-soluble material for approximately 8 min after the addition of [3H]-thymidine, after which an apparent equilibrium level is attained. At this temperature, the rate of thymidine uptake and the apparent equilibrium level attained were depressed by 70% in interferon-treated HeLa cells. The reduced incorporation of [3H]-thymidine into DNA in interferon-treated HeLa-S3 cells can be largely explained by interferon inhibition of thymidine transport and phosphorylation.

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

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

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

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

Kinetics of transport and metabolism of 1-beta-D-arabinofuranosylcytosine and structural analogs by everted perfused rat jejunum

Few reports have dealt with the kinetics and metabolism of AraC and analogs by rat intestine. Using everted rat jejunum with continuous perfusion, it was possible to demonstrate that AraC and Cyd cross the intestinal barrier(s) by a carrier mediated process which was saturable and exhibited fairly good fitting of the flux rate by Michaelis-Menten equation. The transport rate of different analogs was not consistent with the pH-partition theory of membrane transport of drugs being rather dependent on the chemical structure of the nucleoside. A free amino group of cytosine increased the rate of transport within the present series of AraC analogs. There was a detectable deaminase as well as esterase activity towards AraC and its analogs in rat jejunum.

Kinetics of thymidine entry into tonsillar lymphocytes and its alteration in the presence of a lymphokine

Thymidine uptake into the acid soluble cell fraction of human tonsillar lymphocytes was studied in vitro. Uptake was linear for 15-20 minutes at low concentrations (less than 1.2 microM) of thymidine. The plot of uptake versus time could be extrapolated to the origin. Value for KM (0.5-0.6 microM) and values for Vmax were determined. In the presence of a lymphokine which inhibited thymidine incorporation into DNA the uptake of thymidine into the acid soluble cell fraction was also inhibited. The decreased uptake could be characterized by an increase in the apparent KM, without the alteration of Vmax. Lymphokines which inhibit thymidine incorporation may influence and regulate in vivo the entry of the exogenous thymidine into the cells.

Membrane damage by staphylococcal alpha-toxin to different types of cultured mammalian cell

Staphylococcal alpha-toxin was shown to be more membrane-damaging to epithelial-like cells than to neuroblasts or normal fibroblasts. Mouse adrenal cortex tumor (Y1Ac) epithelioid cells and human embryonal lung (MRC-5) fibroblasts were used for further comparison. Alpha-toxin was considerably more cytotoxic to adrenal cells than to fibroblasts. This difference did not depend on the presence fibronectin on the fibroblast surface, or on a general difference in the response to other membrane-damaging hemolytic toxins and detergents. Incubation of adrenal cells at 0 degree C with alpha-toxin induced some irreversible change, and membrane damage and a cytotoxic effect developed upon further incubation in toxin-free growth medium. In fibroblasts the membrane damage progressed slowly and only in the continued presence of the toxin. Toxin-induced damage to transport and synthetic functions in fibroblasts was reversible upon removal of the toxin after prolonged exposure. It is proposed that adrenal cells may carry a cell-surface receptor to which alpha-toxin binds specifically, thereby allowing the toxin to exert its cell damaging effect.

Kinetic and thermodynamic studies on nitrobenzylthioinosine binding to the nucleoside transporter of Chinese hamster ovary cells

The binding of [G-3H]nitrobenzylthioinosine to intact Chinese hamster ovary cells has been studied kinetically and thermodynamically. The association of nitrobenzylthioinosine with cells is a second-order process which proceeds at 24 degrees C with a rate constant of 2 X 10(7) M-1 X S-1. Dissociation of the complex was characterized as a simple first-order process with rate constant on the order of 7 X 10(-3)S-1. The quotient of these is comparable to the dissociation constant as measured in equilibrium binding studies, 2.2 X 10(-10) M. The temperature dependence of the rate of association indicated an Arrhenius activation energy of 8.4 kcal X mol-1, while that of the equilibrium constant for dissociation indicated a standard enthalpy change of 8.8 kcal X mol-1. The large increase in affinity of nitrobenzylthioinosine as compared to natural nucleosides is attributable to an entropy-driven interaction with the binding site. Thymidine, dipyridamole and papaverine each decrease the apparent dissociation constant for the nitrobenzylthioinosine-cell complex; the latter, inhibitors of nucleoside transport, decrease the rate of dissociation of the complex.

Nucleoside transport in human erythrocytes. A simple carrier with directional symmetry in fresh cells, but with directional asymmetry in cells from outdated blood

Kinetic characteristics of the transport of uridine, a non-metabolized permeant in human erythrocytes, have been compared in erythrocytes from fresh and outdated stored blood. Uridine transport kinetics in fresh cells conformed to the predictions of a simple carrier model operating with directional symmetry, but in erythrocytes from outdated blood the kinetic characteristics of uridine transport were those of an asymmetric system. The latter result agrees with earlier reports by others. The mobility of the loaded and empty carriers differed by about 6- and 12-fold in fresh and outdated blood, respectively.

Transport and metabolism of 5'-methylthioadenosine in human erythrocytes

The transport and metabolism of 5'-deoxy-5'-S-methylthioadenosine have been studied in intact human erythrocytes. The sulfur nucleoside is rapidly accumulated into red cells and the extent of uptake largely exceeds the theoretical equilibrium between inner and outer compartment owing to its conversion into a non-permeable compound, namely 5-methylthioribose 1-phosphate. To characterize the nucleoside transport, phosphate-depleted erythrocytes, in which the methylthioadenosine metabolism is negligible, have been employed. The results indicate that: (i) the transport occurs via a facilitated-diffusion mechanism; (ii) the process is not energy-dependent and (iii) no specific cation is required. The kinetic analyses of both the transport and the metabolism show that the uptake of methylthioadenosine is a result of the tandem action of a transport step of high capacity (Vmax = 604 +/- 51 pmol/10(6) cells per min) and low affinity (Km = 3270 +/- 321 microM) followed by a metabolic step of low capacity (Vmax = 6.6 pmol/10(6) cells per min) and high affinity (Km = 30 microM). Furthermore, a substrate inhibition exerted by methylthioadenosine at high concentration (over 200 microM) on its specific phosphorylase is reported for the first time. Experiments performed with several analogs of the thioether indicate that the adenine amino group and the hydrophobic substituent at the 5'-position are critical for the transport carrier recognition. Adenine is the most powerful inhibitor of methylthioadenosine transport.

On the functional symmetry of nucleoside transport in mammalian cells

The transport of uridine and thymidine has been examined in HeLa cells, in Novikoff rat hepatoma cells and in human erythrocytes, with the purpose of comparing influx, efflux and isotopic exchange at chemical equilibrium. The results support the following conclusions: (i) In all three cell types influx and efflux are comparable; (ii) HeLa and Novikoff cells show no trans-effect, while erythrocytes show a 5-fold trans-stimulation; (iii) a single kinetic entity accounts for nucleoside transport in HeLa and Novikoff cells - no parallel routes of permeation with Km less than 40 microM were detected. For the cultured cells, the flux data conform to the kinetic model of a single, carrier-mediated transport system symmetrical with respect to direction, and with equal mobilities of substrate-loaded and empty carrier.

The hypoxanthine transporter of Novikoff rat hepatoma cells exhibits directional symmetry and equal mobility when empty or substrate-loaded

The kinetics of hypoxanthine transport were measured in hypoxanthine phosphoribosyltransferase-deficient Novikoff cells by rapid kinetic techniques applying both zero-trans and equilibrium exchange protocols. The data indicate operation of a simple carrier with directional symmetry and equal mobility when substrate loaded and empty. Zero-trans influx and efflux were about equivalent and so were zero-trans influx and equilibrium exchange flux. The apparent Michaelis-Menten constant and maximum velocity were about 500 microM and 100 pmol/s per microliter cell H2O, respectively. The time courses of accumulation of radioactively labeled hypoxanthine at a concentration above the Michaelis-Menten constant differed noticeably in zero-trans and equilibrium exchange mode, but computer simulations showed that the difference is predicted by the symmetrical carrier model and does not reflect trans-stimulation.

VP16-213 and podophyllotoxin. A study on the relationship between chemical structure and biological activity

VP16-213, a semi-synthetic derivative of podophyllotoxin, is an effective antitumor agent in the treatment of a variety of leukemias and solid tumors. A comparison of the mechanism of action of VP16-213 and podophyllotoxin has revealed that although both drugs inhibit the uptake of nucleosides into HeLa cells, they exhibit other biological properties which are quite distinct. Podophyllotoxin is a potent inhibitor of microtubule assembly in vitro, while VP16-213 has no effect in this system. VP16-213 induces single stranded breaks in HeLa cells DNA, an effect which may be related to its antitumor activity. In contrast to VP16-213 treated cells, podophyllotoxin-treated cells maintain DNA integrity. Structure-activity relationship studies have identified some of chemical sites of VP16-213 and podophyllotoxin responsible for each of their biological properties. These studies illustrate that chemical modification of podophyllotoxin can generate derivatives which possess new and unique biological properties.

Erythrocyte nucleoside transport: asymmetrical binding of nitrobenzylthioinosine to nucleoside permeation sites

1. Nitrobenzylthioinosine is a potent and specific inhibitor of nucleoside translocation in animal cells. Kinetic and inhibitor binding studies were undertaken to clarify how this inhibitor interacts with the nucleoside transporter from human and nucleoside-permeable type sheep erythrocytes.2. [(3)H]nitrobenzylthioinosine inhibition of zero-trans [U-(14)C]uridine influx into nucleoside-permeable type sheep cells was consistent with simple competitive inhibition (apparent K(i) 1 nmol/l). Analysis of results using total inhibitor levels instead of cell-free inhibitor concentrations did not affect the inhibition pattern, but increased the apparent K(i) value by 5-fold.3. In contrast, [(3)H]nitrobenzylthioinosine was a non-competitive inhibitor of zero-trans [U-(14)C]uridine efflux (apparent Ki 1.5 nmol/l). Dipyridamole, another potent inhibitor of nucleoside translocation, also inhibited zero-trans [U-(14)C]uridine influx in a competitive manner (apparent K(i) 20-40 nmol/l).4. [(3)H]nitrobenzylthioinosine bound to high-affinity sites on cell membranes from human and nucleoside-permeable type sheep cells (apparent K(D) values approximately 1 nmol/l). Binding of inhibitor to these sites was competitively blocked by uridine, a well characterized substrate for the nucleoside transporter (apparent K(i) 1.25 and 0.9 mmol/l, respectively). These apparent K(i) values are close to the apparent K(m) for uridine equilibrium exchange in human erythrocytes.5. Similarly, deoxycytidine was found to be a competitive inhibitor of high-affinity [(3)H]nitrobenzylthioinosine binding activity (apparent K(i) 1.0 and 1.2 mmol/l for human and nucleoside-permeable type sheep cell membranes, respectively). This contrasts with a previous report that this nucleoside had no effect on inhibitor binding activity. Transport studies confirmed that deoxycytidine is a substrate for the erythrocyte nucleoside transporter. Apparent K(m) and V(max) values for [U-(14)C]-deoxycytidine zero-trans influx into human and nucleoside-permeable type sheep cells were comparable to those obtained for [U-(14)C]uridine.6. It is suggested from these results that nitrobenzylthioinosine competes directly with nucleosides for the permeation site of the nucleoside transporter, but that inhibitor binds preferentially to the external membrane surface.

Absence of binding sites for the transport inhibitor nitrobenzylthioinosine on nucleoside transport-deficient mouse lymphoma cells

Cells of an adenosine-resistant clone (AE1) of S49 mouse lymphoma cells were compared with cells of the parental line with respect to (a) characteristics of nucleoside transport, (b) high affinity binding of the inhibitor of nucleoside transport, nitrobenzylthioinosine (NBMPR), and (c) the antiproliferative effects of the nucleoside antibiotics, tubercidin, arabinosyladenine and showdomycin. Rates of inward transport of uridine, thymidine, adenosine, 2'-deoxyadenosine, tubercidin, showdomycin, and arabinosyladenine in AE1 cells were less than 1% of those in cells of the parental S49 line. The inhibitor of nucleoside transport, NBMPR, reduced rates of inward nucleoside transport in S49 cells to levels comparable to those seen in the transport-defective mutant. S49 cells possessed high affinity sites that bound NBMPR (6.6 X 10(4) sites/cell, Kd = 0.2 nM), whereas site-specific binding of NBMPR to AE1 cells was not demonstrable, indicating that loss of nucleoside transport activity in AE1 cells was accompanied by loss of the high affinity NBMPR binding sites. Relative to S49 cells, AE1 cells were resistant to the antiproliferative effects of tubercidin and showdomycin, but differences between the two cell lines in sensitivity toward arabinosyladenine were minor, suggesting that nucleoside transport activity was required for cytotoxicity of tubercidin and showdomycin, but not for that of arabinosyladenine.