Quantitative study of deoxycytidine incorporation in large and small lymphocytes of the mouse

Using radioautographic smear preparations of thymocytes and mesenteric lymph node (MLN) cells labelled with three different tritiated pyrimidine deoxyribonucleosides, the incorporation of DNA precursors was studied separately on large lymphocytes and small lymphocytes. Radioautographic reaction due to generally tritiated deoxycytidine ( [G-3H]CdR) labelling in vivo in large lymphocytes was more intense than that in small lymphocytes. When mice were sacrificed 6 hr after the administration of tritiated thymidine ( [3H]TdR), small lymphocytes were labelled more heavily than large lymphocytes. However, labelling intensity with [3H]TdR in large lymphocytes was greatly enhanced by the administration of 5-fluoro-deoxyuridine, whereas in small lymphocytes labelling intensity was only fairly enhanced by the same treatment. When cells were incubated in vitro with 5-tritium labelled deoxycytidine [( 5-3H]CdR) for 10 min, there was no significant difference in labelling intensities between large and small lymphocytes. In the case of [G-3H]CdR incorporation, the labelling intensity in large lymphocytes was found to be significantly stronger than that in small lymphocytes. Large as well as small lymphocytes incorporated [3H]TdR very well in vitro. However, addition of 5 X 0 X 10(-5) M of non-radioactive CdR to the medium greatly decreased the incorporation of [3H]TdR by large lymphocytes, whereas the effect of non-radioactive CdR in small lymphocytes was not so marked as that in large lymphocytes. Furthermore, the [3H]TdR-labelling percentages were decreased at the same rate by the addition of non-radioactive CdR in both large and small lymphocytes. These results indicate that large lymphocytes and a proportion of small lymphocytes have a strong tendency to convert CdR to thymidine mono-phosphate, which is utilized for DNA synthesis, whereas this ability is relatively weak in the rest of small lymphocytes. Thus, it is probably that this metabolic ability changes during the transition of the large lymphocyte to the small lymphocyte.     

Mitochondrial and herpesvirus-specific deoxypyrimidine kinases.

To characterize and compare the thymidine (TdR) and deoxycytidine (CdR) kinase isozymes of uninfected and herpesvirus-infected cells: (i) the subcellular distribution of the isozymes has been studied; (ii) a specific assay for CdR kinase has been devised; (iii) the TdR kinase isozymes have been partially purified; and (iv) the purified enzymes have been analyzed by disc polyacrylamide gel electrophoresis, isoelectric focusing, and glycerol gradient centrifugation and by substrate competition and dCTP inhibition studies. The results indicate that there are interesting individual differences with respect to nucleoside acceptor specificity between the cytosol and mitochondrial pyrimidine deoxyribonucleoside kinases of uninfected cells and between the enzymes induced by different herpesviruses. In the cytosol of uninfected mouse, chicken, and owl monkey kidney cells, two different proteins, TdR kinase F and CdR kinase 2, catalyze the phosphorylations of TdR and CdR, respectively. TdR kinase F does not phosphorylate CdR, nor does CdR kinase 2 phosphorylate TdR. A second TdR kinase isozyme present in HeLa(BU25) mitochondria (TdR kinase B) also lacks CdR phosphorylating activity. In contrast, a genetically distinctive deoxypyrimidine kinase (TdR kinase A) of mouse, human, and chick mitochondria catalyzes the phosphorylation of both TdR and CdT. Three herpesviruses, marmoset herpesvirus and herpes simplex virus types 1 and 2, induce in the cytosol fraction of LM(TK-) mouse cells isozymes which share common properties with mitochondrial TdR kinase A, including the ability to catalyze the phosphorylation of both TdR and CdR. However, the herpesvirus-induced deoxypyrimidine kinases differ from mitochondrial TdR kinase A with respect to sedimentation coefficient, sensitivity to dCTP inhibition, and antigenic determinants. The herpesvirus-specific and the mitochondrial deoxypyrimidine kinases exhibit a preference for TdR over CdR as nucleoside acceptor. Pseudorabies virus and herpesvirus of turkeys induce cytosol TdR kinases resembling the other herpesvirus-induced TdR kinases in several properties, but like cellular TdR kinase F, the pseudorabies virus and herpesvirus of turkeys TdR kinases lack detectable CdR phosphorylating activities. Finally, a marmoset herpesvirus nutant resistant to bromodeoxyuridine, equine herpesvirus type 1, and Herpesvirus aotus induces neither TdR nor CdR phosphorylating enzymes during productive infections.     

Facilitation by pyrimidine deoxyribonucleosides and hypoxanthine of mutagenic and cytotoxic effects of monofunctional alkylating agents in Chinese hamster cells.

Hypoxanthine (Hx), thymidine (TdR) and deoxycytidine (CdR), at concentrations of 10(-5) M increased the yield of 8-azaguanine-resistant (AzGr) mutants induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in cultured Chinese hamster V79 cells. The cytotoxicity of MNNG was increased 2-fold in the presence of Hx, and more than 10-fold in the presence of TdR. This cytotoxic effect of TdR was abolished by equal concentrations of CdR, which by itself did not increase the cytotoxicity of MNNG. However, the yield of MNNG-induced AzGr colonies was increased 2--10-fold in the presence of both CdR and TdR. The AzGr colonies displayed phenotypes characteristic of hypoxanthine: guaninephosphoribosyltransferase-deficient (HGPRT-) mutants, or indicative of mutant HGPRT with altered substrate affinities. The nucleosides did not affect the growth or expression time of the HGPRT- mutants; the same extent of alkali-labile DNA damage occurred in cells treated with alkylating agents in the presence and absence of TdR and CdR; and the increase in mutation frequency in the presence of these nucleosides occurred not only with MNNG, but also with ethylating agents. Nucleosides treated with MNNG were not mutagenic, and treatment of the cells with TdR and CdR only prior to treatment with MNNG or only during selection with AzG did not increase the induced mutation frequency. Therefore, the interpretation is proposed that CdR, TdR and Hx produce nucleotide-pool imbalances that increase lethal and mutagenic errors of replication of alkylated DNA.     

Increased Activity of [gamma]-Glutamylcysteine Synthetase in Tomato Cells Selected for Cadmium Tolerance

Two cell lines of tomato (Lycopersicon esculentum Mill cv VFNT-Cherry) were systematically compared for their capacity to tolerate cadmium. Unselected CdS cells died in the presence of 0.3 mM CdCl2. CdR6-0 cells, which were selected from CdS, survived and grew in medium supplemented with 0.3 mM CdCl2. Growth of CdR6-0 cells under this condition was accompanied by synthesis of cadmium-binding phytochelatins and maintenance of cellular glutathione (GSH) levels. CdR6-0 cells also exhibited increased tolerance to buthionine sulfoximine, in both the presence and absence of 0.1 mM CdCl2. The specific activity of [gamma]-glutamylcysteine synthetase (EC 6.3.2.2) was approximately 2-fold higher in CdR6-0 cells than in CdS cells, whereas there was no difference between cell lines in specific activity of GSH synthetase (EC 6.3.2.3). Increased activity of the first enzyme of GSH biosynthesis in CdR6-0 cells, presumably a result of selection for increased cadmium tolerance, provides an enhanced capacity to synthesize GSH and to maintain the production of phytochelatins in response to cadmium. This adaptation may contribute to the enhanced cadmium tolerance of CdR6-0 cells.     

Chinese hamster ovary cells cultured in low concentrations of fetal bovine serum: cloning efficiency, growth in suspension, and selection of drug-resistant mutant phenotypes

Reducing serum concentrations in media provides a potential cost advantage. To determine whether such media could be used for applied mutagenesis assays, we measured cloning efficiency and growth parameters in suspension of Chinese hamster ovary cells cultured in reduced serum with or without additives (1 microgram/ml insulin, 3 X 10(-7) M linoleic acid, 1 X 10(-8) M H2SeO3) or bovine serum albumin (BSA, 1% wt/vol). With the additives and less than or equal to 0.5% fetal bovine serum (FBS), Ham's F12 medium (without hypoxanthine and thymidine) was more optimal than alpha Eagle's minimum essential medium (MEM) (without ribosides and deoxyribosides) for low density cloning and high density suspension growth. Acceptable cloning efficiencies were obtained with 2% FBS plus BSA without additives in either medium; the addition of BSA resulted in improved colony size and more compact colony morphology. In alpha MEM, satisfactory growth rates and maximum saturation densities in suspension culture were obtained only with 5% FBS; in Ham's F12, 1% FBS + deoxycytidine + BSA yielded satisfactory suspension growth. Spontaneous mutant frequencies were compared for each medium containing 10% dialyzed FBS (DFBS), 1% FBS plus BSA, or 2% FBS plus BSA. The spontaneous frequency of azaadenine-resistant phenotypes (mutant at the aprt locus) in 1% FBS plus BSA was significantly lower than the frequency observed in 2% FBS plus BSA or 10% DFBS. Frequencies of spontaneous mutants resistant to thioguanine (hgprt locus) or fluorodeoxyuridine (tk locus) were similar with 10% DFBS, 1% FBS plus BSA, or 2% FBS plus BSA. Compared to alpha MEM with 10% DFBS, frequencies of drug-resistant mutants induced by ethyl methanesulfonate or mitomycin C (MMC) were not significantly lower in alpha MEM with 2% FBS plus BSA; observed mutant frequencies induced by dimethylnitrosamine or benzo(a)pyrene seemed to be decreased at lower survival levels.     

Cell cycle progression delay in conditioned medium does not play a role in the repair of X-ray damage in Chinese hamster V79 cells

We tested our hypothesis that the lower survival of X-irradiated cells in growth medium (GM) relative to that in conditioned medium (CM) is due to differences in nutrient concentration levels rather than to differential effects on cell progression and growth. Chinese hamster V79 cells in log and unfed plateau phase, grown in Eagle's minimal essential medium (MEM) with 15% serum (100% GM), were irradiated. Before plating, cells were incubated in situ in various concentrations of MEM with serum (GM, normal cell progression) or MEM without serum or in CM (no cell progression). Cell survival was the lowest in 100% MEM with or without serum and increased with the decrease in MEM and serum concentrations, reaching a plateau in 40% MEM or 40% growth medium (40% MEM with 6% serum), similar to that in conditioned medium. Growth kinetics was the same in 40 and 100% growth medium, but the D0 of cells in 40% growth medium was higher than that of cells in 100% GM. Similarly, the D0 of cells in 40% MEM was higher than that of cells in 100% MEM, although cell progression was absent in both media. The radiation sensitivity of cells was the same in 40% GM with progression and in 40% MEM and CM with no progression. Cells in low-nutrient media were flatter than those in 100% MEM or GM. There was a correlation between the nutrient concentration in the medium postirradiation and the D0. This correlation was independent of the presence or absence of serum and thus independent of cell cycle progression. The cell morphology which is dependent on the nutrient concentration appears to influence the ability of a fraction of cells to repair their radiation damage.     

Survival of cultured cells in the cold : A kinetic study with special reference to the effect of serum in culture media

The survival at 4 °C of mouse fibroblasts (strain L-929) and rat liver cells (strain JTC-25·P5) was kinetically analysed after they had been pre-incubated at 37 °C in medium with or without supplement of serum. Both the composition of medium used for preincubation at 37 °C and that employed for storage at 4 °C had influence on the survival period.When the cells had been grown at 37 °C in Eagle minimal essential medium (MEM) alone, they rapidly lost their viability at 4 °C from the beginning. However, when grown at 37 °C in MEM supplemented with calf serum, they maintained viability at 4 °C for about 16 days and 8 days for L cells and JTC-25·P5 cells respectively, before the initiation of rapid loss of viability. The presence of macromolecular fraction of calf serum in the medium during preincubation was found to be responsible for the prolongation of survival at 4 °C.     

Effects of serine and glycine on proliferation of an Ishikawa cell variant

The proliferation of cells on an Ishikawa human endometrial adenocarcinoma line variant (Ishikawa-Var I) is markedly influenced by the medium used to culture them, viz. MEM vs BM (basal medium; DMEM/Ham's F12, 1/1, with additional glutamine and HETES), under serum-free conditions. Components of BM which are not present in MEM were systematically tested in order to identify those that might account for these differences. Cells were cultured for various periods of time, up to 8 days, in serum-free MEM to which the components to be tested were added. Cell population densities were evaluated using a fluorometric DNA assay when the cells were grown in multiwell plates, or by cell counting when the cells were cultured in plastic dishes. It was found that addition to MEM of a mixture of the amino acids that this medium lacks, significantly increased cell density. By testing individual amino acids at the concentrations present in BM, it could be demonstrated that addition of serine alone was sufficient to obtain the densities achieved with BM. Glycine, a metabolic precursor of serine, had a similar but smaller effect. None of the other missing compounds of BM was effective. Effects of serine on DNA synthesis were also estimated by measuring incorporation of [3H]thymidine for 1 h after a 24 h culture period in MEM. The effect of serine was similar and additive to that of 1% charcoal-treated fetal bovine serum. A serine concentration dependence studied either with this method or measuring DNA/well after 8 days in culture showed detectable effects at 0.005 mM concentration and maximal responses at about 0.025 mM. These findings are of potential importance in studies on regulatory mechanisms of cell proliferation. A possibility to be explored, for instance, is that serine added to the medium increases intracellular phosphatidylserine concentrations leading to increases in the activity of protein kinase C, a stimulator of cell proliferation in some systems.     

Interferon effects upon fluorouracil metabolism by HL-60 cells

In order to better understand the synergistic antiproliferative effects of interferon in combination with fluorouracil (FUra), we studied effects of alpha 2-interferon upon FUra induced inhibition of thymidylate synthase of HL-60 cells. The 50% inhibitory dose for FUra decreased from approximately 75 microM to 10 microM following interferon treatment, as measured by whole cell activity assays. Enhanced FUra inhibition of cytosolic [3H] - FdUMP binding of interferon treated cells was also noted. FdUMP accumulation following FUra treatment increased over 10 fold in interferon treated cells, but dUMP did not increase. These results suggest that interferon can sensitize cells to FUra inhibition of thymidylate synthase by enhancing accumulation of FdUMP.     

DIFFERENT LABELLING PATTERNS IN MOUSE LYMPHOID TISSUES WITH [3H]DEOXYCYTIDINE AND [3H]THYMIDINE

The percentages of labelled lymphocytes in smear preparations of mouse thymus were higher than those in similar preparations of mesenteric lymph nodes with either generally labelled tritiated deoxycytidine, [³H]CdR, or tritiated thymidine, [³H]TdR. Lymphocytes in the thymus cortex and in germinal centres of mesenteric lymph nodes were intensely labelled with [³H]CdR, whereas with [³H]TdR lymphocytes in the peripheral region of thymus and medullary cords of mesenteric lymph nodes were heavily labelled. The majority of lymphocytes in thymic cortex and germinal centres of mesenteric lymph nodes were labelled weakly with [³H]TdR. Thus, labelling patterns with [³H]CdR differed from those with [³H]TdR in lymphoid tissues of the mouse. Mouse lymphocytes can utilize [³H]CdR as a precursor molecule for cytosine and thymine in DNA. The ratio of radioactivity of thymine to that of cytosine was measured biochemically in DNA extracted from lymphocytes labelled with [³H]CdR. This radioactivity ratio in thymus was higher than that in mesenteric lymph nodes. These results suggest that the metabolic activities of utilizing CdR for DNA synthesis differ within lymphocyte populations in various lymphoid tissues in the mouse.     

Cytotoxicity of commonly used nitroxide radical spin probes

Since nitroxide radical spin probes are used frequently to test biophysical properties of cells, their use should be restricted to conditions that do not perturb normal cell growth and viability. Eight commonly used nitroxide radical spin probes have been tested for their effects on the survival of CHO cells. These include water-soluble spin probes Tempol, Tempamine, CTPO, CTPC and 4-maleimido-Tempo, and lipid soluble spin probes 5-Doxyl-, 12-Doxyl-, and 16-Doxylstearates. With the exception of 4-maleimido-Tempo, none of the water soluble spin labels inhibited cell survival at concentrations as high as 1 mM. At concentrations of 75 microM and higher, 4-maleimido-Tempo inhibited cell survival in a dose dependent manner. At concentrations commonly used for spin labeling of cells (30-50 microM) none of the lipid soluble spin probes tested was cytotoxic. At 100 microM only 5-Doxylstearate inhibited cell survival, whereas 12-Doxylstearate and 16-Doxylstearate had no effect.     

Inhibition of apoptosis in cultured porcine granulosa cells by inhibitors of caspase and serine protease activity

Protease inhibitors were used to test the hypothesis that caspases and other proteases were active during apoptosis in cultured porcine granulosa cells. Cells isolated from 3 to 6 mm follicles were cultured for 24 h in Dulbecco's modified Eagles medium: Hams F12 (1:11 containing 1% fetal bovine serum. Final inhibitor concentrations, added in 10 microL of dimethylsulfoxide, were 0, 1, 5, 25 and 125 microM. Cells with compromised plasma membrane integrity, identified by uptake ethidium homodimer, increased during culture in the absence of inhibitors from 37% to 43%. Apoptotic (A0) cells, identified by DNA fluorescence flow cytometry, increased (P < 0.05) from 1.7% to 29%. The serine protease inhibitor N-tosyl-L-phenylalanine chloromethyl ketone (TPCK) at 125 microM was lethal increasing (P < 0.05) cells with compromised membranes to 92%. In response to TPCK, A0 cells decreased from 55% to 1.2%; progesterone and estradiol production were decreased by 94% and 98%, respectively. The general caspase inhibitor, benzyloxycarbonyl-valinyl-alaninyl-aspartyl fluoro methylketone, decreased (P < 0.05) A0 cells linearly from 33% to 3 % between 0 and 125 microM without significant effect on steroidogenesis or on the percentage of cells with compromised plasma membranes. Other inhibitors only had a marginal effect on apoptosis; concentrations of > or = 1 microM decreased (P < 0.05) A0 cells from 29% to 18% to 21% and had no significant effect on membrane integrity or steroid production. We conclude that caspases are associated with apoptosis in cultured porcine granulosa cells. Death induced by TPCK was through a non-apoptotic mechanism.     

Metabolism of 5-fluorouracil in sensitive and resistant Novikoff hepatoma cells.

N1-S1/FdUrd Novikoff hepatoma cells, which lack thymidine kinase activity, are resistant to 5-fluorouracil (FUra) as well as 5-fluorodeoxyuridine (FdUrd), suggesting that the pathway, FUra leads to FdUrd leads to FdUMP, is utilized for the conversion of FUra to FdUMP. However, the inhibition of thymidylate synthetase activity, the presumed target of FdUMP, by 1 X 10(-4) M FUra in intact N1-S1 Novikoff hepatoma cells, which have significant levels of thymidine kinase activity, is completely eliminated by 5 X 10(-4) M hydroxyurea, which is a potent inhibitor of ribonucleotide reductase. These results imply that the formation of ribonucleotides and does not involve thymidine kinase. This apparent dichotomy can be explained by the fact that, in addition to the well known lack of thymidine kinase activity, [14C]FUra conversion to ribonucleotides is greatly depressed in the N1-S1/FdUrd cells. Hence, the formation of FdUMP from FUra in Novikoff hepatoma cells apparently proceeds primarily via the intermediate formation of ribonucleotides. The decreased conversion of FUra to ribonucleotides in N1-S1/FdUrd cells decreases not only the ability of the analog to inhibit DNA synthesis, but also its effect on RNA metabolism. FUra, at a concentration of 1 X 10(-5) M, inhibits rRNA maturation in N1-S1 cells, but not in N1-S1/FdUrd cells. Since N1-S1/FdUrd cells are completely resistant to 1 X 10(-5) M FUra, whereas N1-S1 cells are completely inhibited by 1 X 10(-5) M FUra, even in the presence of 1 X 10(-4) M thymidine, the effects of FUra on RNA metabolism appear to contribute significantly to the cytotoxicity of the analog at higher drug concentrations.     

Effects of sugars on melanogenesis in cultured melanoma cells.

A permanent cell line C2M of mouse melanoma B16 was highly melanized in a modified Eagle's MEM supplemented with 10% calf serum, when the medium contained 1 mM galactose and 10 mM pyruvate instead of 5.5 mM glucose. The activity of the key anzyme for melanogenesis, tyrosinase (EC 1.14.18.1), of living cells cultured in the galactose-pyruvate medium was consistently 27 times higher than that of cells in normal MEM. This high level of tyrosinase activity was maintained in the stationary phase, in contrast to the activity of cells in normal medium, which decreased sharply in the stationary phase. It seems likely that tyrosinase activity is suppressed by the presence of glucose rather than stimulated by galactose. This modified medium should be useful obtaining a high level of tyrosinase activity in living cells in culture and in cell-free extracts.     

Survival of hippocampal and cortical neurons in a mixture of MEM+ and B27-supplemented neurobasal medium

Serum-free B-27 supplemented neurobasal (NB) and a 10% fetal bovine serum-supplemented Eagle's minimum essential medium (MEM+) are used to culture rat embryonic hippocampal neurons for different purposes. Although NB medium leads to enhanced cell survival, it contains biological antioxidants and is not suitable for the study of free radical damage and oxidation in cultured neurons. MEM+ without additional antioxidants has been used widely in the study of free radical damage and oxidation, although it does not support optimum neuronal survival in culture. Serum in MEM+ leads to enhanced cell survival but also promotes glial cell proliferation. In this study, we used a new combination medium (NM-2) that consists of both NB and MEM+ for growing primary hippocampal and cortical neuronal cultures. NM-2 enhanced neuronal survival 78.9% for dissociated neurons at a density of 50 cells/mm(2) and 83.1% for 100 cells/mm(2), while decreasing glial cell proliferation to 2-3% and completely inhibiting oligodendrocytes. The NM-2 minimized the effectiveness of antioxidants in the medium to the neurotoxin 4-hydroxynonenal. It also decreased neuronal clumping and provided a more even distribution of neurons. Neurons survived for 4 weeks in NM-2 without changing the original medium. NM-2 provides a good environment for studies of free radical damage and oxidation of neurons. The combination incorporates the best of both NB and MEM+ that results in high neuron survival rate, low glial cell proliferation, reduced antioxidant level, and provides relatively pure cultures of hippocampal and cortical neurons.     

Lead Enters Bovine Adrenal Medullary Cells Through Calcium Channels

Agents that stimulate secretion also accelerate the rate of Pb uptake into adrenal medullary cells. For example, when cells are suspended in a medium containing 5 microM Pb2+, depolarization by 77 mM K increases the rate of Pb uptake from 12 +/- 1 to 47 +/- 5 mumol/(L cells X min). K-induced Pb uptake has an apparent Km for Pb2+ of 2.6 microM, and is antagonized by Ca2+ with a K0.5 of 1.4 mM. The Ca channel blocker D-600 inhibits Pb entry with a K0.5 of 0.4 microM. Pb uptake is also stimulated by the Ca channel agonist BAY K 8644. These observations suggest that Pb passes through Ca channels. The permeability of the channels to Pb appears to be at least 10 times the permeability to Ca.     

Isolation and characterization of two 5-fluorouracil-substituted Escherichia coli initiator methionine transfer ribonucleic acids

Escherichia coli initiator methionine tRNA labeled in vivo with 5-fluorouracil (FUra) has been isolated and characterized. The tRNA, with essentially all its uracil and uracil-derived minor bases replaced by FUra, was purified by sequential chromatography, first on diethylaminoethylcellulose (DEAE-cellulose), at pH 8.9, followed by chromatography on Sepharose 4B, using a reverse salt gradient, then on DEAE-Sephadex A-50, and finally on benzoylated DEAE-cellulose. The last step resolved two FUra-substituted tRNAfMet-iso-accepting species, each with a specific activity over 1500 pmol/A260. Kinetic analysis shows both are aminoacylated at the same rate; apparent KmS for the two are 0.92 and 0.94 microM, compared with 1.7 microM for normal tRNAfMet. Chromatographic differences between the two forms of fluorinated tRNAfMet persist after aminoacylation, and the two tRNAs are not interconverted by denaturation and renaturation. The isoacceptors have nearly identical nucleoside composition, and both contain 7-methylguanosine and 2'-O-methylcytidine as the only modified nucleosides. Analysis of complete RNase T1 digests of the two methionine tRNAs shows that they differ in only one oligonucleotide. The sequence 20FpApGp, derived from the dihydrouridine loop and stem region, which is found in one of the isoaccepting forms of the tRNA, is replaced by an oligonucleotide containing adenine and guanine, but no FUra in the other. A modified FUra, with the properties of a 5-fluoro-5,6-dihydrouracil derivative, is detected in this tRNA. 19F NMR spectra of the two species of FUra-substituted initiator tRNA show 9-10 resolved resonances for the 12 FUra residues incorporated. The spectra differ primarily in the shift of one peak in the form lacking the sequence 20FpApGp, from 4.8 ppm downfield from free FUra (= 0 ppm) to 14.9 ppm upfield from the standard.     

Cyanopyrazoles as analogs of purine precursors--II. Effects on macromolecular synthesis and cell cycle progression

The cytotoxic and cytokinetic effects, and in vitro inhibition of macromolecular synthesis by cyanopyrazoles were studied using Friend leukemia and Ehrlich ascites tumor cells. At concentrations in the range of 2.5 mM to 50 microM analog 3(5)-amino-4-cyano-5(3)-trichloromethylpyrazole (I) was highly cytotoxic and completely inhibited thymidine, uridine and leucine incorporation into macromolecular material. 24 hr incubation of FL cells with cytostatic concentrations of compound I (in the range of 2 to 0.5 microM) resulted in an accumulation of cells in the G2 + M phase. Analogs N-hydroxyethyl-3(5)-amino-4-cyano-5(3)-trichloromethylpyrazole (II) and 3(5)-amino-4-cyanopyrazole (III) were not cytotoxic at concentrations up to 5 mM and did not substantially inhibit precursor incorporation into macromolecules but exhibited a cytostatic activity. These compounds caused a decrease of FL cells in the G2 + M phase and an accumulation in the S phase. Analogs I and II displayed a similar in vivo inhibitory effect on thymidine incorporation into DNA in EAT cells. The results indicate that the cytotoxicity of cyanopyrazoles correlates with their ability to inhibit precursor incorporation into macromolecular material. On the other hand, the cytostatic action of compound I is not coupled to a block of nucleic acid synthesis.     

The role of trehalose in dehydration resistance of Saccharomyces cerevisiae

Abstract High levels of intracellular trehalose in stationary-phase cells of Saccharomyces cerevisiae or cells incubated in the absence of a nitrogen source were found to increase the resistance of the cells to dehydration. Exponential-phase cells showed negligible dehydration resistance. When stationary-phase cells were inoculated into fresh medium, trehalose was rapidly broken down, and this was correlated with a rapid loss of dehydration resistance. It appeared that a minimum internal concentration of 120 mM trehalose was required before there was a significant increase in dehydration resistance. Exogenous trehalose increased the dehydration resistance of S. cerevisiae : this effect was most marked for stationary-phase cells, where almost 100% survival was obtained at trehalose concentrations of 500 mM and above while maximum survival for exponential cells was less than 10%, even at 1000 mM external trehalose.     

Modulation of DNA precursor pools, DNA synthesis, and ultraviolet sensitivity of a repair-dificient CHO cell line by deoxycytidine

The presence of 2 mM deoxycytidine (CdR) in growth medium substantially increased the deoxycytidine triphosphate (dCTP) and deoxuthymidine triphosphate (dTTP) pools in a Chinese hamster ovary cell line, CHO-K1, and in a radiation-sensitive mutant, xrs-5, derived from it (Jeggo et al., 1982). We observed significant differences in alkaline-sucrose gradient profiles of pulse-labeled DNA from unirradiated CHO-K1 and xrs-5 cells. For the latter cell line, a sizable fraction of the DNA synthesized during 5 or 10 min of growth subsequent to a 5-min radiolabeling period was found to co-sediment with large-chromosome DNA. This characteristics of xrs-5 was dramatically reduced by the presence of 2 mM CdR in the culture medium, and the UV resistance of the mutant increased to nearly that of the parent cell line under these culture conditions. These results show that the locus conferring UV-radiation sensitivity to xrs-5 affects DNA replication and that replicative activity and UV-radiation sensivity are jointly modulated by CdR, possibly through its impact on the size of deoxynucleoside triphosphate pools.