Lethal effects of fluorodeoxyuridine on cultured mammalian cells at various stages of the cell cycle

The lethal damage induced by the exposure of synchronized Chinese hamster cells to various concentrations of 5-fluoro-2′deoxyuridine (FUdR) was not selectively restricted to cells exposed during the period of DNA synthesis S. The colony survival fraction observed after treatment for one hour with 5 × 10^?5 M FUdR was very low (0.0001–0.0003) whether the drug was administered during early G₁, late G₁, early S or in middle S. The survival of cells treated with the same concentration of FUdR during mitosis, however, was significantly higher (0.62) showing that mitotic cells were less sensitive to FUdR. Administration of 10^?7M thymidine or “conditioned” medium for one hour reversed the lethal effect of FUdR or improved the survival, depending on the time after removal of the FUdR at which these substances were given.     

INHIBITION OF MYOBLAST FUSION AFTER ONE ROUND OF DNA SYNTHESIS IN 5-BROMODEOXYURIDINE

The thymidine analogue 5-bromodeoxyuridine (BUdR) has a differential effect on the synthesis of tissue-specific products and molecules required for growth and division. Proliferating myogenic cells cultured in BUdR fail to fuse and fail to initiate the synthesis of contractile protein filaments. Conversely, BUdR has but a minor effect on cell viability and reproductive integrity. Low concentrations of BUdR result in an enhancement of cell number relative to the controls; higher concentrations are cytotoxic. Suppression of myogenesis is reversible after at least 10 cell generations of growth in the analogue. Cells that do not synthesize DNA, such as postmitotic myoblasts and myotubes, are not affected by BUdR. Incorporation of BUdR for one round of DNA synthesis was accomplished by first incubating myogenic cells, prior to fusion, in 5-fluorodeoxyuridine (FUdR) to block DNA synthesis and collect cells in the presynthetic phase. The cells were then allowed to synthesize either normal DNA or BU-DNA for one S period by circumventing the FUdR block with BUdR or BUdR plus thymidine (TdR). The cultures were continued in FUdR to prevent dilution of the incorporated analogue by further division. After 3 days, the cultures from the FUdR-BUdR series showed the typical BUdR effect; the cells were excessively flattened and few multinucleated myotubes formed. Cells in the control cultures were of normal morphology, and multinucleated myotubes were present. These results were confirmed in another experiment in which BUdR-³H was added to 2-day cultures in which myotubes were forming. Fusion of thymidine-³H-labeled cells begins at 8 hr after the preceding S phase. In contrast, cells which incorporate BUdR-³H for one S period do not fuse with normal myotubes.     

The vestigial locus of Drosophila melanogaster is involved in resistance to inhibitors of dTMP synthesis

The vestigal (vg) gene encodes a nuclear protein which plays a major role in the formation of the wing of Drosophila. Resistance or sensitivity to aminopterin, an inhibitor of the dihydrofolate reductase enzyme in D. melanogaster, seems to be associated with a specific alteration in vg gene function. Wild-type and vg mutant strains selected for growth on increasing concentrations of aminopterin display changes in physiological and biochemical parameters such as viability on normal and aminopterin-containing media, duration of development, wing phenotype, dihydrofolate reductase activity, and cross-resistance to fluorodeoxyuridine (FUdR) and to methotrexate. Our results indicate that the mechanisms of resistance differ in the wild-type and mutant strains. The vg ^83b27 mutant, in which the major part of intron 2 of the vg gene is deleted, is associated with a high rate of resistance to FUdR, an inhibitor of thymidylate synthetase. Moreover, vg ^83b27/vg ^BGheterozygotes, which are wild type when grown on normal medium, display a strong vg phenotype when grown on aminopterin. Our results indicate a role for the vestigial locus in mediating resistance to inhibitors of dTMP synthesis.     

The initiation of melanogenesis in the chick retinal pigment epithelium.

Observations have been made on chick pigment retinal epithelium between 2 and 5 days of development. 2-Thiouracil has been demonstrated to be an effective agent for measuring the rate of melanin synthesis.Using [³H]thymidine and colcimid, we have found that the cells undergo a marked withdrawal from the cell cycle between 3 and 3.5 days of incubation in ovo, indicating that a majority of the population is synchronized. This withdrawal is followed, approximately 24 hr later, by a rapid rise in melanin synthesis from the basal level which first appears at approximately 3 days.5-Bromodeoxyuridine (BUdR) has been used to determine the time at which melanin synthesis is initiated. When BUdR is administered as early as 2 days in ovo, it is incapable of blocking the appearance of basal levels of melanin even though the cells divide at least three times in the presence of this thymidine analog. However, BUdR is capable of delaying the rapid rise in the rate of melanin synthesis first observed at 4.5 days. This delay has been found to correlate, using [³H]BUdR, with a delay in the withdrawal of the cells from the division cycle.In pursuing the idea of a correlation between withdrawal and the rapid increase of melanin formation, 5-fluorodeoxyuridine (FUdR) was used. Histological and biochemical evidence suggests that those cells which have been prevented from dividing by FUdR increase their rate of melanin synthesis to the high level of the postmitotic control cells described above.Therefore, it seems that (1), in light of work done by others, the initial decision to make melanin is made prior to 2 days in ovo, and (2) the mechanism by which cells shift their synthetic capabilities to high levels of melanin production is withdrawal mediated.     

Massive synthesis of ribonucleic Acid and cellulase in the pea epicotyl in response to indoleacetic Acid, with and without concurrent cell division

Measurements were made over a 4-day period of the effect of added indoleacetic acid (IAA), puromycin, actinomycin D and 5-fluorodeoxyuridine (FUdR) on growth and the levels of total DNA, RNA, protein and cellulase in segments of tissue at the apex of decapitated etiolated epicotyls of Pisum sativum, L. var. Alaska.

The hormone induced swelling of parenchyma cells and cell division. By 3 days after IAA application, the amounts of DNA and protein were approximately double, RNA triple and cellulase 12 to 16 times the levels in controls. All of these changes were prevented by both puromycin and actinomycin D. FUdR prevented DNA synthesis and cell division but not swelling or synthesis of RNA, protein and cellulase.

It is concluded that IAA-induced RNA synthesis is required for cellulase synthesis and lateral cell expansion, whether or not cell division takes place.

     

Changing rates of DNA and RNA synthesis in Drosophila embryos

Rates of DNA and RNA synthesis during Drosophila embryogenesis were measured by labeling octane-treated embryos with [¹⁴C]thymidine and [³H]uridine. Radioactivity incorporated per hour was converted to rates of synthesis using measurements of the pool-specific activity during the labeling periods. The rate of DNA synthesis during early embryogenesis increases to a maximum at 6 hr after oviposition and then decreases sharply. Measured rates of DNA synthesis were used to calculate that the total amount of DNA per embryo doubles every 18 min at blastoderm, every 70–80 min during gastrulation, and less than once every 7 hr at later stages. The rate of RNA accumulation per embryo increases continuously during the first 14 hr of embryogenesis. The rate of nuclear RNA synthesis per diploid amount of DNA, however, decreases fivefold between blastoderm and primary organogenesis. The cytoplasmic poly(A)⁺ RNA synthesized by blastoderm embryos associates rapidly with polysomes. The relatively high rate of synthesis of polysomal poly(A)⁺ RNA per nucleus at blastoderm allows the small number of nuclei present at blastoderm to make a significant quantitative contribution to the informational RNA active in the early embryo. At the end of blastoderm, approximately 14% of the mRNA being translated in the embryo has been synthesized after fertilization.     

Protein and nucleic acid synthesis during imbibition of dormant and after-ripened Vaccaria pyramidata seeds.

The regulation of nucleic acid and protein synthesis in dormant, thermodormant, and after-ripened embryos of Vaccaria pyramidata (Caryophyllaceae) has been studied. Germination of after-ripened V. pyramidata seeds is prevented by inhibitors of protein, RNA, and DNA synthesis. The synthesis of both protein and RNA is activated at the beginning of imbibition, whereas [³H]thymidine incorporation does not start until the second period of the imbibition phase. [³H]Thymidine incorporation is greatly reduced in embryos treated with cycloheximide or 6-methylpurine. There is no correlation between the level of [³H]uracil and l-[¹⁴C]leucine incorporation into macromolecules and the physiological state of the seeds: tRNA, ribosomal RNA, and poly(A)-containing RNA (probably mRNA) as well as proteins are synthesized at the same rate in both dormant and thermodormant embryos as in after-ripened embryos. The protein patterns of dormant and after-ripened embryos are similar, as shown by electrophoresis and electrofocusing of double-labeled proteins. The level of DNA synthesis, measured as [³H]thymidine incorporation, may, on the other hand, indicate the physiological activity of the seeds: [³H]Thymidine is incorporated at a high rate in after-ripened embryos only and remains at a low level in dormant or thermodormant embryos. This correlation is, however, observed only in the axes. DNA synthesis in the cotyledons does not show any relation to the developmental stage of the seeds. These results are discussed in relation to the regulation of dormancy and after-ripening of seeds.     

Relation between DNA synthesis and germination of vaccaria Pyramidata seeds

Dormant seeds of Vaccaria pyramidata are characterized by a high level of [¹⁴C]Ieucine incorporation into proteins, a high level of [³H]uracil incorporation into RNA and an undiminished synthesis of poly(A)⁺-RNA; however, DNA synthesis is very much reduced. Germination of dormant Vaccaria seeds can be induced either partially by cytokinins or more completely by cytokinins and ethylene. Cytokinins cause a drastically increased [³H]thymidine incorporation in cotyledons of imbibing seeds. This effect is enhanced by ethylene. The incorporation of [³H]thymidine in radicles is also stimulated, but to a lesser extent. Cytokinin-dependent stimulation of [³H]thymidine incorporation can be referred to as an activation of nuclear DNA synthesis.     

Mode of Action of Lomofungin

Lomofungin inhibited the growth of some yeasts and mycelial fungi at concentrations between 5 and 10 μg/ml. At such concentrations, there was no decrease in endogenous and exogenous oxygen consumption, and even 50 μg of antibiotic per ml caused only slight decreases. The permeation of the cell membrane was changed so that leakage of ninhydrin-positive substances was reduced, and the uptake of ¹⁴C-labeled glucose, amino acids, uracil, and thymidine was decreased at concentrations as low as 4 μg/ml. Protein synthesis in whole cells of Saccharomyces cerevisiae was reduced 35% at 10 μg/ml. However, the antibiotic did not reduce the incorporation of phenylalanine-U-¹⁴C into polypeptides with cell-free systems of Rhizoctonia solani and S. cerevisiae. The synthesis of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) was inhibited even at concentrations of lomofungin of 4 μg/ml. Since RNA synthesis was inhibited at lower concentrations and earlier than DNA synthesis, the primary site of action of the antibiotic appears to be the synthesis of RNA.     

Mode of Action of Lomofungin

Lomofungin inhibited the growth of some yeasts and mycelial fungi at concentrations between 5 and 10 μg/ml. At such concentrations, there was no decrease in endogenous and exogenous oxygen consumption, and even 50 μg of antibiotic per ml caused only slight decreases. The permeation of the cell membrane was changed so that leakage of ninhydrin-positive substances was reduced, and the uptake of ¹⁴C-labeled glucose, amino acids, uracil, and thymidine was decreased at concentrations as low as 4 μg/ml. Protein synthesis in whole cells of Saccharomyces cerevisiae was reduced 35% at 10 μg/ml. However, the antibiotic did not reduce the incorporation of phenylalanine-U-¹⁴C into polypeptides with cell-free systems of Rhizoctonia solani and S. cerevisiae. The synthesis of ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) was inhibited even at concentrations of lomofungin of 4 μg/ml. Since RNA synthesis was inhibited at lower concentrations and earlier than DNA synthesis, the primary site of action of the antibiotic appears to be the synthesis of RNA.     

Effect of 5-Fluoro-2′-Deoxyuridine on [3H]Thymidine Incorporation by Bacterioplankton in the Waters of Southwest Florida

The effect of 5-fluoro-2′-deoxyuridine (FdUrd) on [methyl-³H] thymidine incorporation by bacterioplankton populations in subtropical freshwater, estuarine, and oceanic environments was examined. In estuarine waters, intracellular isotope dilution was inhibited by FdUrd, which enabled us to estimate both intracellular and extracellular isotope dilution. In 2 of 10 cases, extracellular isotope dilution was significant. At low concentrations of [methyl-³H]thymidine or [6-³H]thymidine, FdUrd completely inhibited incorporation of radioactivity into protein and RNA. At high concentrations of [³H]thymidine, however, FdUrd had little effect on labeling patterns. The dihydrofolate reductase inhibitors amethopterin and trimethoprim had no effect on macromolecular labeling patterns. These results suggest that thymidylate synthase is not involved in nonspecific labeling and that FdUrd inhibits nonspecific labeling by blocking some other enzyme involved in thymidine catabolism. In oligotrophic oceanic and freshwater samples, FdUrd did not inhibit intracellular isotope dilution or [³H]thymidine labeling of protein and RNA, but caused some inhibition of [³H]thymidine incorporation into DNA. The ability of FdUrd to inhibit nonspecific macromolecular labeling during [³H]thymidine incorporation was significantly correlated (r = 0.84) with total thymidine incorporation (in picomoles per liter per hour). The results are discussed in terms of applications of FdUrd to routine bacterial production measurements and the general assumptions of [³H]thymidine incorporation.     

The Nop60B gene of Drosophila encodes an essential nucleolar protein that functions in yeast

The Cbf5 protein of Saccharomyces cerevisiae was originally identified as a low-affinity centromeric DNA-binding protein, and cbf5 mutants have a defect in rRNA synthesis. A closely related protein from mammals, NAP57, is a nucleolar protein that coimmunoprecipitates with the nucleolar phosphoprotein Nopp140. To study the function of this protein family in a higher eukaryote that is amenable to genetic approaches, the gene encoding a Drosophilamelanogaster homolog, Nop60B, was identified. The predicted Drosophila protein shares a high degree of sequence identity over a 380-residue region with both the mammalian and yeast proteins, and shares several conserved motifs with the prokaryotic tRNA pseudouridine 55 synthases. Nop60B RNA is found at high levels in nurse cells and in the oocyte, and is present throughout development. Nop60B protein is localized primarily to the nucleolus of interphase cells, and is absent from the chromosomes during mitosis. Nop60B mutants were generated and shown to be homozygous lethal. The Drosophila gene can rescue the lethal phenotype of yeast cbf5 mutations, showing that the function of this protein has been conserved from yeast to Drosophila.     

Molecular specificity of 2-aminopurine in Saccharomyces cerevisiae

The rates of DNA, RNA and protein synthesis were investigated by incorporation of radioactive precursors into the excised root tips of V. faba. 2-h exposure to 0.1% caffeine resulted in inhibition of protein synthesis to about 60% of the control rate. RNA synthesis was reduced in the range of 20–30%. The same concentration of caffeine did not affect the rate of DNA synthesis even during 12-h incubation, but concentrations higher than 1% caused a significant decrease in [³H]thymidine incorporation.     

Reversible inhibition of somatic embryo differentiation by bromodeoxyuridine in cultured cells of Daucus carota L.

Somatic embryogenesis induced in suspension cultures of carrot (Daucus carota) was inhibited by BUdR at concentrations which do not influence the rate of RNA or protein synthesis, or the growth of the cells. At the time of embryo induction 2 days treatment with 6 μM BUdR was sufficient for the complete supression of differentiation. The degree of inhibition showed a dependence on the applied concentration of BUdR, as well as on the level of BUdR substitution in DNA. The BUdR inhibition proved to be reversible and the embryogenic capacity of the carrot cells was restored after removel of the analogue. Addition of thymidine prevented the inhibitory effect of BUdR by decreasing the incorporation of BUdR into DNA. The experiments reported here indicate that the effect of BUdR on the somatic embryo differentiation was probably associated with BUdR substitution of thymidine in the DNA.     

Studies on the utilization of dietary thymidine and thymine by Drosophila melanogaster larvae

Deoxycytidine improves tolerance of Drosophila melanogaster to thymidine block, suggesting the presence of deoxycytidine kinase. At appropriate concentrations, a mixture of thymidine and deoxycytidine allows larvae to tolerate a higher concentration of 5-fluoro-2′-deoxyuridine than is tolerated with either thymidine or deoxycytidine alone. Thus, at this high concentration, 5-fluoro-2′-deoxyuridine appears to act primarily upon thymidylate synthetase, as it does at lower concentrations, rather than upon RNA metabolism, as has been suggested previously. Larvae can also be rescued from 5-fluoro-2′-deoxyuridine-induced death by a high concentration of thymine. The effect is enhanced by the presence of deoxyadenosine. Since this compound is known to increase the intracellular concentration of deoxyribose-1-phosphate, the main effect of thymine is probably due to its salvage utilization as a thymidine source, via the anabolic functioning of thymidine phosophorylase.     

Genetic and biochemical studies of actidione sensitivity in Drosophila

A new mutant, act, a recessive gene carried on chromosome III of Drosophila, results in sensitivity to actidione, an inhibitor of protein synthesis. The mutant gene has no detectable effect except that it acts as a conditional lethal in the presence of actidione. The possibility that this gene encodes a ribosomal component was tested and eliminated using an in vitro system. Conditions are described for in vitro protein synthesis utilizing Drosophila microsomes and transfer RNA derived from Escherichia coli.This study was supported by USPHS Grant 1 R01 GM 15055.     

Synthesis of poly(A) containing RNA induced by ecdysterone in fat body cells of Calliphora vicina

The insect-hormone ecdysterone causes a de novo synthesis of RNA in the fat body cells of blowfly larvae. Hybridization of the RNA induced by the hormone with ³H-Poly(U) demonstrates that it contains poly(A) sequences and therefore is probably informational. DNA-RNA-hybridization experiments further show that qualitatively new mRNA species are synthesized under the influence of ecdysterone. The inducibility of RNA synthesis is strongly correlated to specific developmental stages.     

Interaction between Subunits of Heterodimeric Splicing Factor U2AF Is Essential In Vivo

The heterodimeric pre-mRNA splicing factor, U2AF (U2 snRNP auxiliary factor), plays a critical role in 3′ splice site selection. Although the U2AF subunits associate in a tight complex, biochemical experiments designed to address the requirement for both subunits in splicing have yielded conflicting results. We have taken a genetic approach to assess the requirement for the Drosophila U2AF heterodimer in vivo. We developed a novel Escherichia coli copurification assay to map the domain on the Drosophila U2AF large subunit (dU2AF⁵⁰) that interacts with the Drosophila small subunit (dU2AF³⁸). A 28-amino-acid fragment on dU2AF⁵⁰ that is both necessary and sufficient for interaction with dU2AF³⁸ was identified. Using the copurification assay, we scanned this 28-amino-acid interaction domain for mutations that abrogate heterodimer formation. A collection of these dU2AF⁵⁰ point mutants was then tested in vivo for genetic complementation of a recessive lethal dU2AF⁵⁰ allele. A mutation that completely abolished interaction with dU2AF³⁸ was incapable of complementation, whereas dU2AF⁵⁰ mutations that did not effect heterodimer formation rescued the recessive lethal dU2AF⁵⁰ allele. Analysis of heterodimer formation in embryo extracts derived from these interaction mutant lines revealed a perfect correlation between the efficiency of subunit association and the ability to complement the dU2AF⁵⁰ recessive lethal allele. These data indicate that Drosophila U2AF heterodimer formation is essential for viability in vivo, consistent with a requirement for both subunits in splicing in vitro.     

The type and time of occurrence of aminopterin-induced chromosome aberrations in cultured Potorous cells

Many inhibitors of DNA synthesis have been found to induce chromosome aberrations. Our kinetic studies indicate that treatment of cellswith 10^?7M aminopterin in the presence of 10^?4M glycine, 10^?4M hypoxanthine, and 10^?4M thymidine allows continued normal cell growth. Omission of thymidine, a treatment which is known to inhibit DNA synthesis while allowing RNA and protein synthesis to continue, leads to cessation of cell growth. Treament of Potorous cell cultures with aminopterin in the presence of hypoxanthine and glycine without thymidine led to the following observations: (1) only non-exchange chromatid aberrations were formed after aminopterin treatment; (2) the aberrations were induced only in cells treated during S, and the breaks were associated with the replicating region of the chromosome; (3) breaks were observed at the first metaphase after the beginning of treatment; and (4) thymidine could reverse the chromosome-breaking action of aminopterin. A model for the molecular mechanism is suggested.