The uptake of nucleosides by cells in culture. I. Inhibition by heterologous nucleosides

Effects of some antiviral isatinisothiosemicarbazones on cellular and viral ribonucleic acid synthesis in Mengovirus-infected FL cells

Three antiviral isatinisothiosemicarbazones strongly inhibited the incorporation of [(3)H]uridine into the ribonucleic acid (RNA) of FL cells as a consequence of the inhibition of uridine transport. After prelabeling of cells at a low temperature (1 h at 16 C) with uptake of [(3)H]uridine into the acid-soluble nucleotide pool, the later addition of the test compounds revealed only a small or negligible influence on host-directed RNA synthesis. The pulse-labeled soluble nucleotide pool of FL cells was sufficient to give a gradual increase in incorporation into RNA over a period of 7 h. With the same method of prelabeling at the beginning of the experiment, it was also possible to detect virus-induced RNA synthesis in the presence of actinomycin D. In this way the specific inhibitory action of the three isatinisothiosemicarbazones on viral RNA synthesis could be demonstrated.

Is adenosine deaminase involved in adenosine transport?

The hypothesis that adenosine metabolizing enzymes may have a key role in the transport of adenosine is discussed. The enhancement of adenosine transport by inhibitors of adenosine deaminase (the enzyme which deaminates adenosine to inosine) and the ecto-localization of adenosine deaminase suggest a contribution of the enzyme in taking up nucleosides. Two possible mechanisms are suggested: 1) transport and deamination of adenosine as a coupled process, or 2) uptake of inosine after cleavage of adenosine by ecto-adenosine deaminase. In both cases, the so-called adenosine deaminase binding protein which is a membrane protein could be the real nucleoside transporter. This behaviour of adenosine deaminase as an ectoenzyme anchored to a membrane protein remembers the behaviour of periplasmic binding proteins of bacteria. Thus, adenosine deaminase as well as, for instance, adenosine kinase would be a kind of 'periplasmic proteins' of eukaryotic cells. The function of adenosine deaminase and adenosine kinase would then be to take adenosine and give it to the true transporters.

Differential effects of hyperoxia and hydrogen peroxide on thymidine kinase and adenosine kinase activities of cultured endothelial cells

To compare the respective sensitivity of two nucleoside kinases, adenosine kinase and thymidine kinase, to oxidative stress, we measured these enzyme activities in cultured aortic endothelial cells exposed for 48 h to various O2 concentrations, and in cell extracts treated with H2O2 or the enzyme system hypoxanthine-xanthine oxidase. Adenosine kinase activity was not significantly influenced by the exposure to hyperoxia, nor by treatment with the enzyme system hypoxanthine-xanthine oxidase or with H2O2. On the other hand, there was a dose-dependent inhibitory effect on thymidine kinase activity resulting from exposure to various O2 concentrations or from treatment with various amounts of xanthine oxidase. Incubation of cell extracts in the presence of H2O2 also resulted in a significant reduction of thymidine kinase activity. These results indicate that thymidine kinase exhibits a selective sensitivity to the toxic effect of O2 concentrations and of O2 intermediates such as H2O2.

The effects of 5-bromodeoxyuridine on fusion of the cranial neural folds in the mouse embryo

The effects of 500 and 300 mg/kg bromodeoxyuridine (BUdR) on the process of fusion of the neural folds were tested after injection into pregnant mice on day 8 of gestation (192 hours postcoitum). Various doses of the natural nucleoside, thymidine (TdR), were also tested. Both doses of BUdR retarded growth to the same extent, but only the larger dose caused neural tube defects in 28.8% of embryos. Treatment with the larger dose also caused extensive cell necrosis to appear in the neuroepithelium of the neural folds between 12 and 15 hours after treatment. No changes were detectable with the light microscope up to this time. Measurement of the cell generation time in treated and control embryos indicated that the BUdR prolonged the cycle by about 2 hours and that the dying cells were in the second DNA synthetic phase following incorporation of the analog. Treatment with the smaller dose of BUdR caused minimal cell necrosis. This was taken as evidence for the importance of cell necrosis in the pathogenesis of BUdR-induced neural tube defects. Treatment with excess TdR did not cause either neural tube defects or cell necrosis, and a dose of TdR equimolar with the large dose of BUdR (400 mg/kg TdR) did not retard growth. Doses of 800 and 1,200 mg/kg TdR retarded growth to the same extent as BUdR. The administration of an equimolar amount of TdR, along with the teratogenic dose of BUdR, prevented the occurrence of cell necrosis and neural tube defects. When treatments were given on day 9 of gestation, 500 mg/kg BUdR caused cell necrosis in the neuroepithelium about 15 hours after treatment but no neural tube defects were produced by day 9 after treatment. It is suggested that in this case cell necrosis occurred too late to interfere with neural fold fusion. It was concluded that the ability of BUdR to cause exencephaly in mouse embryos was due to cell necrosis in the neuroepithelium.

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

Transport of the nucleoside analog cytosine-arabinoside (CAR) in transformed hamster cells in culture has been studied in conditions of minimal metabolic conversion. Uptake (zero-trans in) properties at 20 degrees C over a limited range of CAR concentrations were characterized by a Km of 350 micrometer and a maximal velocity (V) of 780 micrometer.min-1 (V/Km = 2.28 min-1). Equilibrium exhcange at 20 degrees C over a wider range of concentrations was best described by a saturable component with a Km of 500 micrometer and a v of 1230 micrometer.min-1 (V/Km = 2.26 min-1) and either a saturable component of high Km or a nonsaturable component of k = 0.3 min-1. For the saturable component, the v/Km values were similar in both procedures. CAR transport was inhibited by various metabolizable nucleosides. Uptake of some of these nucleosides was inhibited by CAR. CAR transport and uridine uptake were inhibited in a reversible but partially competitive fashion by high affinity probes like S-(p-nitrobenzyl-6-mercaptoinosine (NBMI) (Ki less than 0.5 nM) and in an irreversible fashion by SH reagents such as N-ethylmaleiimide (NEM). The organomercurial p-hydroxymercuribenzene sulfonate (pMBS) markedly stimulated transport of these nucleosides, but also markedly potentiated the inhibitory effects of either NBMI or NEM. The effects are interpreted either in terms of models which invoke allosteric properties or in terms of two transport systems which display distinct chemical susceptibilities to externally added probes.

Further studies of the action of methionyl adenylate on chick embryo fibroblasts

Methionyl adenylate (Met-AMP) inhibits protein synthesis by interacting with methionyl-tRNA synthetase. Addition of 1--3 mM inhibitor to chick embryo fibroblasts rapidly stops protein synthesis and DNA synthesis but not RNA synthesis. These effects can be reversed by renewal of the medium. The extent and reversibility of protein and DNA syntheses depend on the concentration of MetAMP in the cultures, the length of exposure and the cellular density. MetAMP is recognised by several enzymes as substrate and/or as inhibitor. MetAmp is degraded to methionol plus 5'-adenylic acid by 5'-phosphodiesterase. Adenosine deaminase, adenylic acid deaminase and 3':5'-phosphodiesterase cannot use MetAMP as substrate but the last enzyme is inhibited. The presence of MetAMP in cultures provokes a small but reproducible increase in the level of methionyl-tRNA synthetase and 5'-phosphodiesterase.

Alterations in differentiation and pyrimidine pathway enzymes in 5-azacytidine resistant variants of a myoblast line

5-azacytidine at concentrations higher than 5 muM inhibited the differentiation of a rat myoblast line in vitro. It was also somewhat cytotoxic at this level. Variants resistant to the cytotoxic effect of 5-azacytidine were obtained which were simultaneously unable to differentiate into myotubes and exhibited altered morphology. These characteristics were retained by the variants when subcultured in the absence of the drug for over 700 generations. Several of the azacytidine resistant cells were more susceptible than the parental line to the lethal action of 5-bromodeoxyuridine and adenosine, but not that of cytosine arabinoside, ouabain or 8-azaguanine. The variants were capable of transporting uridine, thymidine and 5-azacytidine. The uridine kinase activity was one-half to one-third of than in the parental cells but it was not missing completely in any of the variants. Two independently isolated variants selected for detailed study showed a 2- to 3-fold increase in the activity of orotidylic acid decarboxylase. This enzyme in the variants in contrast to that of the parental cells was completely insensitive to the inhibitory effect of a nucleotide generated from ATP and 5-azacytidine in cell extracts (probably 5-azacytidine monophosphate). These observations point to the possibility the 5-azacytidine resistance arises in myoblasts due to an alteration of the components of two target pathways of this drug, viz., the de novo pyrimidine pathway and an undefined sequence leading to the synthesis of membrane components.

Defective transport of thymidine by cultured cells resistant to 5-bromodeoxyuridine

A line of HeLa cells resistant to 5-bromo-2'-deoxyuridine (BUdR) was established by continuous culture in growth medium containing BUdR; during the selection period, BUdR concentrations, initially 15 micrometer, were gradually increased to 100 micrometer. Cells of a clone (HeLa/B5) established from this line were also resistant to 5-fluoro-2'-deoxyuridine (FUdR), but not to the free base, 5-fluorouracil. Although extracts of HeLa/B5 cells exhibited levels of thymidine kinase activity comparable to those of parental cells, rates of uptake of BUdR, FUdR, and thymidine into intact cells were much reduced. The kinetics of uptake of uridine and adenosine, nucleosides which appear to be transported independently of thymidine in HeLa cells, were similar for HeLa/B5 and the parental line (HeLa/O). Relative to thymidine uptake by HeLa/O cells, that by HeLa/B5 cells was distinctly less sensitive to nitrobenzylthioinosine (NBMPR), a specific inhibitor of nucleoside transport in various types of animal cells. Despite this difference in NBMPR sensitivity, both cell lines possessed the same number of high affinity NBMPR binding sites per mg cell protein. The altered kinetics of thymidine uptake and the NBMPR insensitivity of that function in HeLA/B5 cells suggest that resistance to BUdR is due to an altered thymidine transport mechanism.

Utilization of uridine for RNA synthesis in the insect cell line CP-1268 derived from the codling moth, Laspeyresia pomonella

The utilization of [3H]-5-uridine by CP-1268 cells was studied. Uridine was rapidly transported into these cells by a concentration dependent, saturable process. Exogenous uridine rapidly equilibrated with cellular nucleotide pools and virtually all of the uridine transported into the cells was phosphorylated. Uridine incorporation into RNA was studied by continuous and pulse-labeling techniques in the prescence or absence of actinomycin D and cordycepin. These studies have shown that the pattern of unstable RNA precursor and relatively stable RNA product relationship known to exist in mammalian cells similarly exists in insect cells in vitro. This pattern varied markedly with pulse-labeling time and required the addition of RNA inhibitors to block reincorporation of intracellular labeled metabolites during the chase.

Inhibition of protein and nucleic acid synthesis of animal cells in vitro mediated by high molecular weight inhibitors in human liver extract

1. The addition of human liver extract to HeLa cells induces a reversible inhibition of the incorporation of [3H] thymidine into the DNA, [3H] uridine into the RNA, and 14C-labelled amino acids into the protein of HeLa cells. The inhibitory effects appear after treatment for 1 h and reach a maximum after 4-8 h. These effects do not depend on a defective precursor penetration, isotopic dilution or degradation of labelled precursor (thymidine-degrading enzymes were inactivated by the addition of unlabelled thymine), reduced activity of thymidine and uridine kinase, medium impairment, or an impairment of the cell-membrane function. 2. The nucleic acid synthesis-inhibiting activity of the extract seems to be dependent on cellular protein synthesis but independent of RNA synthesis which indicates that the inhibitors act in an indirect way. Furthermore, the inhibitors seem to lack the tissue-specific character of chalones. 3. The extract contains separate inhibitors of DNA, RNA and protein synthesis. These inhibitors were found to have different physical-chemical characteristics and to be macromolecules with a protein or conjugated protein character (mol. wt. approx. 90 000). 4. The possibility that the activity of the high molecular weight inhibitors resides in low molecular weight factors (bound to protein carriers) was tested: No true low molecular weight inhibitors could be liberated by extraction with trichloroacetic acid/organic solvents or by dialysis/enzymatic treatments. Nucleosides such as thymidine, uridine, and cytidine, however, were liberated and could be shown to interfere with the uptake of [3H] thymidine/[3H] uridine.

Decreased 3H-uridine incorporation and increased 3H-adenosine incorporation by HeLa cells exposed to autologous culture fluid

Actively growing HeLa monolayer cultures briefly exposed to the culture fluids (CF) from confluent HeLa cultures and labeled simultaneously or subsequently, incorporated less 3H-uridine (3H-UR) but more 3H-adenosine (3H-AR) than control cultures similarly exposed to fresh medium and labeled. Exposure to CF inhibited the uptake as well as the incorporation of 3H-UR by cultures. The inhibition of 3H-UR incorporation by CF-exposed cultures could be reduced by increasing the concentration of 3H-UR in the labeling medium. Both the inhibition of 3H-UR incorporation and the stimulation of 3H-AR incorporation were prevented by washing the CF-treated cultures with phosphate buffered saline before labeling. Similarly, both effects could be producted in HeLa cultures exposed to fresh medium containing 1 X 10(-5) M uridine instead of to CF. Therefore, the observed effects of CF on label incorporation were probably due to the presence of uridine or a related compound, and the inhibition of 3H-UR incorporation resulted from reduced uptake of 3H-UR rather than from reduced RNA synthesis by exposed cells. The active agent in the CF, formed only when cultures were incubated at physiological temperatures, was not a product of medium decay. It was a cellular product formed equally well by cultures incubated in medium containing dialysed or whole serum.

Nucleolar necklaces in chick embryo fibroblast cells. I. Formation of necklaces by dichlororibobenzimidazole and other adenosine analogues that decrease RNA synthesis and degrade preribosomes

A number of chemicals, mostly adenosine analogues, cause the nucleolus of the chick embryo fibroblast to lose material and unravel over a period of several hours into beaded strands termed nucleolar necklaces (NN). The results of analyses of the fibroblasts, treated with the NN-forming chemical dichlororibobenzimidazole (DRB), suggests that the following biochemical alterations occur: DRB almost completely prevents the increase in both messenger RNA (mRNA) and heterogeneous nuclear RNA. It interferes with ribosome synthesis by decreasing the rate of 45S ribosomal RNA (rRNA) accumulation by 50%, slowing the rate of 18S rRNA appearance by 50%, and causing an extensive degradation (80%) of the 32S and 28S rRNA-containing preribisomes. Most of this preribosome degration probably occurs at or before the 32S rRNA preribosome stage. The degradation of these preribosomes appears to be due to the formation of defective 45S rRNA preribosomes rather than to a direct DRB interference with preribosome processing enzyme action. DRB inhibits total cellular RNA synthesis in less than 15 min, suggesting a direct interference with RNA synthesis. DRB also inhibits the uptake of nucleosides into the cell. DRB in the concentrations used does not appear to directly interfere with the translation of mRNA (i.e., protein synthesis). Other NN-forming adenoside analogues and high concentrations of adenosine (2 mM) cause biochemical alterations similar to those produced by DRB. To explain the preribosome degradation, we propose the hypothesis that DRB inhibits the synthesis of mRNA; as a consequence, some of the preribosomal proteins that normally coat the 32S rRNA portion of the 45S precursor RNA become limiting, and this defective portion is then subject to degradation by nucleases.

Uridine uptake in a unicellular eukaryote during the interdivision period and after growth arrest

The rate of uridine uptake was measured in Tetrahymena after shiftdown to non-nutrient physiological salt solution. Uptake follows Michaelis-Menten kinetics and an apparent Km of transport of 2 X 10-minus 6 M has been estimated. This value is in good agreement with those reported for tissue-derived cells in culture. Incorporation of uridine into RNA follows similar kinetics suggesting that uptake is rate limiting for incorporation. Within three hours after shiftdown the rate of uptake is decreased by an order of magnitude. Also at three hours after shiftdown pairing occurs between cells of complementary mating types. It seems likely that the change in uptake is a reflection of a surface change associated with differentiation. The rate of uptake was also measured during the interdivision period using cells synchronized by a physical selection procedure. A change in rate occurs at the time the cells begin replication of DNA and is essentially stable thereafter. These results indicate that there exists in Tetrahymena a relationship between surface properties as assayed by uridine uptake and properties of growth and differentiation.

Inhibition of mitogen mediated lymphocyte blastogenesis by adenosine

The effect of adenosine on the proliferative response of human peripheral circulating lymphocytes to stimulation by concanavalin A, phytohemagglutinin and pokeweed mitogen was evaluated. Increasing concentrations of adenosine substantially inhibited mitogen mediated lymphocyte blastogenesis. Erythro-9(2-hydroxyl-3-nonyl) adenine. HCl enhanced the inhibitory effect of adenosine. Inosine, the deamination product of adenosine, had an inhibitory effect which was less than that of adenosine. Inhibition by adenosine may be relevant to the normal regulation of immune function and may account in part for the pathophysiological relationship between severe combined immunodeficiency disease and adenosine deaminase deficiency.

Tissue-specific suppression of differentiation by 5-bromodeoxyuridine in vitro

Embryonic rat tooth germs were treated with 5-bromodeoxyuridine in vitro. Morphodifferentiation and histodifferentiation were arrested. Epithelial and mesenchymal deoxyribonucleic acid and protein synthesis were unaffected, whereas ribonucleic acid and collagen synthesis were retarded in the mesenchyme only. Inhibition of odontogenesis by this agent appears to be tissue-specific.

Uptake of orotate and thymidine by normal and regenerating rat livers

1. At 1h after operation livers from partially hepatectomized rats showed a 60-100% increase in the capacity to concentrate (3)H radioactivity from orotate, thymidine or uridine with respect to the radioactivity in plasma. Uptake of [(3)H]cytidine into liver was unaffected, as was entry of any precursor studied into any tissue other than liver. 2. This increase in intracellular radioactivity was detectable 10min after operation with both orotate and thymidine. With orotate the augmentation had disappeared by 3 days, but with thymidine it was still evident 8 days after partial hepatectomy, when [(3)H]thymidine incorporation into DNA was no longer increased. Competition studies established that orotate was not entering the liver by the same mechanism as thymidine. 3. In the soluble fraction of the liver all the (3)H radioactivity from orotate was present as uridine nucleotides. Thymidine was not phosphorylated, and was believed to be catabolized.

Thymidine transport by cultured Novikoff hepatoma cells and uptake by simple diffusion and relationship to incorporation into deoxyribonucleic acid

The initial rate of thymidine-(3)H incorporation into the acid-soluble pool by cultured Novikoff rat hepatoma cells was investigated as a function of the thymidine concentration in the medium. Below, but not above 2 microM, thymidine incorporation followed normal Michaelis-Menten kinetics at 22 degrees , 27 degrees , 32 degrees , and 37 degrees C with an apparent K(m) of 0.5 microM, and the V(max) values increased with an average Q(10) of 1.8 with an increase in temperature. The intracellular acid-soluble (3)H was associated solely with thymine nucleotides (mainly deoxythymidine triphosphate [dTTP]). Between 2 and 200 microM, on the other hand, the initial rate of thymidine incorporation increased linearly with an increase in thymidine concentration in the medium and was about the same at all four temperatures. Pretreatment of the cells with 40 or 100 microMp-chloromercuribenzoate for 15 min or heat-shock (49.5 degrees C, 5 min) markedly reduced the saturable component of uptake without affecting the unsaturable component or the phosphorylation of thymidine. The effect of p-chloromercuribenzoate was readily reversed by incubating the cells in the presence of dithiothreitol. Persantin and uridine competitively inhibited thymidine incorporation into the acid-soluble pool without inhibiting thymidine phosphorylation. At concentrations below 2 microM, thymidine incorporation into DNA also followed normal Michaelis-Menten kinetics and was inhibited in an apparently competitive manner by Persantin and uridine. The apparent K(m) and K(i) values were about the same as those for thymidine incorporation into the nucleotide pool. The over-all results indicate that uptake is the rate-limiting step in the incorporation of thymidine into the nucleotide pool as well as into DNA. The cells possess an excess of thymidine kinase, and thymidine is phosphorylated as rapidly as it enters the cells and is thereby trapped. At low concentrations, thymidine is taken up mainly by a transport reaction, whereas at concentrations above 2 microM simple diffusion becomes the principal mode of uptake. Evidence is presented that indicates that uridine and thymidine are transported by different systems. Upon inhibition of DNA synthesis, net thymidine incorporation into the acid-soluble pool ceased rapidly. Results from pulse-chase experiments indicate that a rapid turnover of dTTP to thymidine may be involved in limiting the level of thymine nucleotides in the cell.

Application of the isotope-dilution principle to the analysis of factors affecting the incorporation of (3H) uridine and (3H) cytidine into cultured lymphocytes. Evaluation of pools in serum and culture media

1. Rat lymph-node cells were incubated in serum and medium 199 with [5-(3)H]uridine or [5-(3)H]cytidine and acid-precipitable radioactivity was measured. Results were interpreted in terms of an isotope-dilution model. 2. Both serum and medium 199 contained pools that inhibited radioactive labelling in a competitive manner. The serum activity was diffusible and inhibited labelling with [(3)H]cytidine more than with [(3)H]uridine; in these respects the activity resembled cytidine (14mum). 3. The pools in serum and plasma were the same size; however, the rate of labelling was greater in plasma, owing to a diffusible factor. 4. Paradoxically, relatively simple media (Earle's salts and Eagle's minimum essential) appeared to have a larger pool than the more complex pyrimidine-containing medium 199; this suggests a contribution to the pool by cells in the simple media. 5. In the absence of pools the average cell was capable of incorporating 2000 radioactive nucleoside molecules/s.

Interferon action: inhibition of vesicular stomatitis virus RNA synthesis induced by virion-bound polymerase

The particle-bound RNA polymerase activity of vesicular stomatitis virus (VSV) can be demonstrated in vivo. Linear synthesis of viral RNA persists for 5 to 6 hours at 34 degrees C in infected monolayers of chick embryo cells treated with cycloheximide and actinomycin D to block synthesis of protein and cell-specific RNA. At least 55 percent of the RNA made under these conditions is complementary to virion RNA. RNA synthesis mediated by VSV polymerase activity is inhibited in cells first treated with chick-derived interferon or polyriboinosinate* polyribocytidylate, but not by mouse interferon. The RNA product of VSV polymerase activity is present throughout the cytoplasm, and its synthesis is inhibited by the interferon system, as judged by autoradiographs that show the physical distribution, in cells, of RNA produced by virion polymerase in the absence of translation-a demonstration of the transcription product of the viral genome.