Regulation of rat myosin light-chain synthesis in heterokaryons between 5-bromodeoxyuridine-blocked rat myoblasts and differentiated chick myocytes

Terminal cell differentiation in a variety of model systems is inhibited by the thymidine analogue 5-bromodeoxyuridine (BUdR). We investigated the mode of action of BUdR by forming heterokaryons between undifferentiated BUdR-blocked rat myoblasts and differentiated chick skeletal myocytes. We analyzed newly synthesized proteins on two- dimensional polyacrylamide gels. The induction of rat skeletal myosin light-chain synthesis was reduced fivefold, as compared with controls, when chick myocytes were fused to BUdR-blocked rat myoblasts. This indicates that plasma membrane effects cannot be the proximate cause for the inhibition of myogenesis by BUdR, since BUdR is able to block the effect of chick inducing factors even when a differentiated chick myocyte is in direct cytoplasmic continuity with the BUdR-blocked rat nucleus. The observation that chick cells required an 80% substitution of BUdR for thymidine to block myogenesis, whereas L6 rat myoblasts required only a 20% substitution led to a hypothesis involving a DNA- mediated action of BUdR. This model yielded three testable predictions: (a) putative chick inducing molecules should be present in limiting quantities, (b) exploiting gene-dosage effects to increase the quantity of putative chick inducing factors might overcome the inhibition produced in the rat myoblasts by a 35% BUdR for thymidine substitution, and (c) these gene-dosage effects should be abolished by increasing the level of BUdR substitution in the rat myoblast to 60-80%. All three of these predictions have been verified, providing strong indirect evidence that the inhibition of myogenesis produced by BUdR is a direct result of its incorporation into cellular DNA.     

Hemoglobin synthesis in murine virus-induced leukemic cells in vitro. 3. Effects of 5-bromo-2'-deoxyuridine, dimethylformamide and dimethylsulfoxide

Erythroid differentiation of Friend leukemia cells is enhanced when the cells are grown for four days in the presence of dimethylsulfoxide (DMSO). Dimethylformamide (DMF) has a similar though less marked effect. 5-Bromo-2′-deoxyuridine (BUdR) (10^?5M) inhibits both DMF- and DMSO-stimulated differentiation. For maximum inhibition, BUdR must be present during the first two days of growth, during which time DNA synthesis is maximal. The addition of BUdR after the third day has no effect. Since BUdR is incorporated into DNA and thymidine prevents BUdR inhibition of DMSO-stimulated differentiation, it is likely that BUdR acts by virtue of its incorporation into DNA. Although BUdR alone had little effect upon cell multiplication, in combination with DMSO, cell growth was inhibited up to 40%. Since the BUdR-inhibition of the DMSO effect was approximately 70%, it is unlikely that its effect on differentiation is due to selective killing of those cells which are stimulated to differentiate.     

THE EFFECTS OF 5-BROMODEOXYURIDINE ON YOLK SAC ERYTHROPOIESIS IN THE CHICK EMBRYO

The effects of the thymidine analog, 5-bromodeoxyuridine (BUdR), on the formation of red cells in the yolk sac of the chick embryo were examined. The prospective area opaca vasculosa from a definitive primitive streak embryo was excised, disaggregated, and deposited into a cell clump, and the cell clump was placed in organ culture. Hemoglobin synthesis is detectable after about 16 hr in culture. The formation of erythropoietic foci and incorporation of ⁵⁵Fe into heme were used to measure the extent of erythropoiesis. Exposure to 40 µg/ml of BUdR within 6 hr after explantation almost completely eliminated red cell formation; subsequent transfer to thymidine medium showed that the inhibition was reversible, and there was no histological evidence of analog toxicity. Between 6 and 12 hr after initiation of organ culture, the tissue became completely refractory to BUdR. DNA synthesis, as monitored by thymidine-³H and BUdR-³H pulses, was extensive both during and after the period of BUdR sensitivity. Hence, during both BUdR sensitive and insensitive periods the analog was incorporated into DNA of cells which had not yet synthesized hemoglobin. It is proposed that between 6 and 12 hr a crucial regulatory event for terminal differentiation is perturbed by the presence of BUdR in the chromosomes.     

Differences between diploid and transformed human cells in the incorporation of 5-bromodeoxyuridine into DNA

In diploid human cells, the DNA precursor pool equilibration times for exogenous thymidine are about twice those for the thymidine analogue 5-bromodeoxyuridine (BUdR); in cells that were either transformed chemically or derived from malignant tumours, the pool equilibration times are the same for thymidine and 5-bromodeoxyuridine and are closer in value to the shorter (bromodeoxyuridine) times of the diploid cells. Thymidine, if present in the culture medium with BUdR, is incorporated into DNA preferentially in diploid cells (by 2 or 3 to 1). Discrimination against bromodeoxyuridine is evident within 2 h of incubation of the two precursors with diploid cells, but is not observed even after 24 h in any of the transformed cell lines tested. Experiments were performed to test the effect of inhibitors of the mammalian DNA polymerases alpha (N-ethylmaleimide) and beta (incubation of cells at 45 °C) upon the ability of cells to synthesise DNA and to incorporate thymidine preferentially when present with equimolar BUdR. In diploid cells, overall in vivo DNA synthesis is more sensitive to N-ethylmaleimide and more resistant to 45 °C treatment than is DNA synthesis in the transformed cell lines. N-Ethylmaleimide decreases the capacity of diploid cells to discriminate against BUdR, whereas heating increases it. Transformed cells treated with N-ethylmaleimide remain unable to discriminate against BUdR; some transformed lines, when heated at 45 °C, become less incapable of such discrimination.     

THE EFFECT OF 5-BROMODEOXYURIDINE ON DNA REPLICATION AND CELL DIVISION IN TETRAHYMENA PYRIFORMIS

Populations of Tetrahymena pyriformis were grown in a chemically defined medium containing the thymidine analogue 5-bromodeoxyuridine (BUdR). About 65% of the thymidine sites in DNA were substituted by BUdR. During the first generation in the presence of BUdR, all DNA became hybrid. After the following cell division, in about 80% of the cells the second DNA replication round was initiated but no further cell division took place. The cells could be rescued by removing BUdR and adding thymidine. New replication took place before the first cell division. However, although the cells contained double heavy as well as hybrid DNA, only the hybrid DNA was replicated. After a full replication of the hybrid DNA, normal growth was restored. Melting profiles of normal, hybrid, and double heavy DNA indicated a structural change of the double heavy DNA.     

Incorporation of 5-bromo-2′deoxyuridine (BUdR) in Dictyostelium discoideum DNA

The modality of incorporation of 5-bromo-2′deoxyuridine (BUdR) in the DNA of Dictyostelium discoideum was studied after one generation of growth of the amoebae in the presence of different concentrations of the drug. The analog was incorporated following the semiconservative pattern of DNA replication. BUdR incorporation in monosubstituted DNA has been measured both by CsCl isopycnic centrifugation or by base analysis chromatography; substitution of thymidine by its analog reaches a maximal value of 30% (60% in the substituted strand). Up to 20% substitution it is proportional to the drug concentration in the growth medium. In these conditions, thymidine substitution is higher in repetitive sequences of the DNA than in unique sequences; the percent of increase of thymidine substitution in repetitive fractions versus total DNA is inversely proportional to thymidine substitution in total DNA.     

Effect of Aminopterin and Deoxyribonucleosides on the Cleavage and Embryogenesis of the Sea Urchin, Hemicentrotus pulcherrimus

In the embryos of the sea urchin, Hemicentrotus pulcherrimus , reared with 150 μM aminopterin from the time of fertilization, cessation of the development occurred at the blastula stage, at which the dTTP level became quite low. Another addition of thymidine to the embryo culture containing aminopterin resulted in an elevation of dTTP concentration in the embryos and allowed them to develop normally. Decrease in the dTTP level, resulting from the inhibition of thymidylate synthesis by aminopterin, probably causes a failure of egg cleavage and development. 5-Bromo-2'-deoxyuridine (BUdR) also released the aminopterin-inhibition of egg cleavage and allowed the treated embryos to develop to early gastrulae. Thereafter, the degeneration of archenteron occurred and these embryos became large permanent blastulae. Other deoxyribonucleosides failed to cancel the inhibition by aminopterin of egg cleavage. In the embryos kept with both BUdR and aminopterin, BUdR incorporation into DNA occurred at a similar rate as in thymidine incorporation in the embryos kept with thymidine and aminopterin, and was inhibited by another addition of thymidine. Without aminopterin treatment, BUdR incorporation hardly occurred and the embryos developed normally. BUdR incorporation into DNA in place of thymidine probably occurs in aminopterin-treated embryos, resulting in abnormal development.     

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.     

Inhibition of sporulation in Bacillus subtilis by bromodeoxyuridine and the effect on DNA replication

A 5-bromo-2'-deoxyuridine (BUdR)-tolerant derivative of a thymidine (TdR)-requiring strain of Bacillus subtilis was used to examine the effect of BUdR, an analogue of TdR, on sporulation. At a TdR:BUdR ratio which had little effect on growth, sporulation was inhibited if cells were exposed to BUdR during the period of DNA synthesis at the onset of the process. Cells recovered from BUdR inhibition of sporulation if the analogue was removed and DNA replication allowed to continue with TdR alone. BUdR prolonged the period of DNA synthesis during sporulation and experiments with chloramphenicol suggested that this was due in part to unscheduled initiation of new rounds of replication.     

Mutagenic mechanism of 5-bromodeoxyuridine in Chinese hamster cells

Resistance to 6-thioguanine was induced by 5-bromodeoxyuridine (BUdR) in synchronous Chinese hamster cells. The yield of mutant colonies was not proportional to the amount of BUdR incorporated into DNA; thus mutants were not due to mispairing of BUdR with guanine during replication. Few mutants were induced until BUdR concentrations exceeded that of the intracellular thymidine triphosphate pool and mutant yield was depressed by addition of thymidine to the medium. These data suggest that BUdR exerts an allosteric effect on the DNA synthesizing system which renders it more error prone.     

Effect of L-phenylalanine on I-methyladenine production and spontaneous oocyte maturation in starfish

5-Bromodeoxyuridine (BUdR)-resistant cells were obtained from N-methyl-N′-nitro-N-nitrosoguanidine (NTG)-treated soybean protoplasts and cultured in liquid nutrient medium containing BUdR (20 μg/ml) and uridine (100 μg/ml). Addition of uridine to the medium improved growth of the BUdR-resistant cells. The growth of BUdR-resistant cells was partly inhibited when hypoxanthine, aminopterine, glycine and thymidine were added to the medium. Both BUdR-resistant and BUdR-sensitive cells exhibited thymidine kinase activity. CsCl density gradient analyses showed that the DNA of BUdR-resistant cells, which were cultured in the presence of BUdR, had a buoyant density of 1.703 g/ml, while the DNA of the parental soybean cells grown without BUdR had a buoyant density of 1.692 g/ml. Uptake of ³H-thymidine or ¹⁴C-BUdR by the cells occurred in both BUdR-resistant and BUdR-sensitive cells. CsCl density gradient patterns of labelled DNA also demonstrated that ¹⁴C-BUdR and ³H-thymidine were incorporated into the DNA of BUdR-resistant cells, as well as into that of BUdR-sensitive cells.     

Modification of radiation-induced genetic damage in Drosophila melanogaster male germ cells with nucleic acid precursors. 3. Effects on the premeiotic cells

Using a 2-day brood pattern, the effect of 5-bromodeoxyuridine (BUdR) or 5-bromodeoxycytidine (BCdR) pre-treatment on the radiation-induced yield of sex-linked recessive lethals and translocations was studied in the spermatocytes and late gonial cells (p.i. DNA synthesis cells) of D. melanogaster. The p.i. DNA synthesis cells were irradiated (I.2 kR γ-radiation) in the pre-meiotic or post-meiotic stage. Irradiation of p.i. DNA synthesis cells in the pre-meiotic stage resulted in enhanced lethal frequency with BUdR (3.0%) and BCdR (2.9%) over the other pre-treatment conditions: saline (S), thymidine (TdR) and deoxycitydine (CdR) in the spermatocytes but not in the late gonial cells. The radiosensitizing property was evident with BCdR even when the p.i. DNA synthesis cells were irradiated in the post-meiotic stage; but not with BUdR pre-treatment. Probable reasons for the contradicting results reported in the literature were discussed.     

FLOW CYTOMETRIC CELL CYCLE ANALYSIS USING THE QUENCHING OF 33258 HOECHST FLUORESCENCE BY BROMODEOXYURIDINE INCORPORATION

Cultures of L cells were grown in medium containing 2.0 mg/l bromodeoxyuridine (BUdR) and stained with the fluorescent dye 33258 Hoechst for flow cytometric analysis. During exposure to BUdR, the cells replace thymidine by BUdR in the newly synthesized DNA. The new DNA is not stainable with 33258 Hoechst, which is highly specific for thymidine. The temporal development of the fluorescence distributions after addition of BUdR to the growth medium has been investigated in the flow cytometer, and the data were used to calculate the mean durations of the phases G₁, S and G₂+ M in exponentially growing cultures as well as the cycle transit times in synchronized cultures. The percentage of non-cycling cells was determined in each experiment.     

Interruption of Somatic Embryogenesis in Daucus carota L. by 5-Bromodeoxyuridine

Embryogenic Daucus carota L. cells grown in 9 micromolar 2.4-dichlorophenoxyacetic acid are resistant to greater than 5 micromolar 5-bromodeoxyuridine (BrdU). In contrast, 5 micromolar BrdU strongly inhibits somatic embryogenesis within 24 hours after transfer of cells to an auxin-free medium. DNA synthesis rates in control and BrdU-treated cultures are rapid and similar; however, the DNA content does not reach levels as great in the presence of BrdU as in control cultures. BrdU substitutes for thymidine in the DNA in 28% of the available sites 48 hours after auxin removal. Following DNA repair, somatic embryogenesis resumes. BrdU DNA incorporation leads to somatic embryogenesis inhibition and provides an alternative to auxin treatment for the interruption of carrot cell culture differentiation.     

Effects of thymidine analogues on murine and human cells.

Three mouse tumour cell lines grew continuously in 3 micro M 5-bromodeoxyuridine (BUdR). One line (MC-2) produced a retrovirus and altered in morphology in the presence of BUdR or 5-iododeoxyuridine (IUdR). These effects, which could be reversed by growth in normal medium were similar to those reported for the B-16 mouse melanoma line. The B-16 line used in this study, however, as well as a variety of human cells (six melanoma lines and three fibroblast strains), were much more sensitive to BUdR, 0.03-0.1 micro M being the maximum tolerated levels for continuous growth. No virus production or changes in morphology were induced in these cells by BUdR, deoxyuridine (UdR), 5-fluorodeoxyuridine (FUdR) or thymidine (TdR). The results of cell labelling and growth studies showed a correlation of incorporation of BUdR into DNA with toxicity. Compared on a competitive basis with 1 micro M TdR, the order of incorporation of 1 micro M nucleosides by two human cell lines was TdR = BUdR = IUdR greater than UdR greater than FUdR. In contrast to previous reports that FUdR is incorporated into RNA but not into DNA, half of the FUdR label was found in alkalistable, DNase-sensitive material. Over 90% of the other compounds was incorporated into DNA. All of the UdR and 60% of the IUdR label was incorporated as thymidine; this conversion could be inhibited by labelling in the presence of FUdR.     

Flow cytometric cell cycle analysis using the quenching of 33258 Hoechst fluorescence by bromodeoxyuridine incorporation.

Cultures of L cells were grown in medium containing 2.0 mg/l bromodeoxyuridine (BUdR) and stained with the fluorescent dye 33258 Hoechst for flow cytometric analysis. During exposure to BUdR, The cells replace thymidine by BUdR in the newly synthesized DNA. The new DNA is not stainable with 33258 Hoechst, which is highly specific for thymidine. The temporal development of the fluorescence distributions after addition of BUdR to the growth medium has been investigated in the flow cytometer, and the data were used to calculate the mean durations of the phases G1, S and G2 + M in exponentially growing cultures as well as the cycle transit times in synchronized cultures. The percentage of non-cycling cells was determined in each experiment.     

Tyrosine aminotransferase sensitivity to bromodeoxyuridine during restricted intervals of S phase in hepatoma cells

Synchronized hepatoma tissue culture (HTC) cells, accumulated at the G1/S boundary with aminopterin, were released into S phase with either thymidine or 5-bromodeoxyuridine (BUdR). Tyrosine aminotransferase (TAT) activity was found to be unaffected by BUdR over the initial 3 h of S phase, but then to rapidly decline to a new basal level of 40% of control by 9 h. There was no corresponding response in the activities of alcohol dehydrogenase, malate dehydrogenase, acid phosphatase, and alkaline phosphatase, or in the rate of protein and RNA synthesis. If BUdR incorporation was restricted to limited periods of S phase, TAT was found to be maximally suppressed by incorporation into the initial 40% of the DNA. Incorporation of the analogue into the latter 60% of DNA synthesized during S phase had no effect on TAT. This is the first report that the effect of BUdR on TAT in HTC cells is associated with incorporation of the analog into DNA synthesized during a specific interval of S phase.     

5-Bromodeoxyuridine: A specific inhibitor of cytokinin-habituation in tobacco cell culture

Summary Cytokinin-habituated cells of Nicotiana tabacum L. cv. Havana 425 are able to grow in culture without added cytokinin. The thymidine analogue, 5-bromodeoxyuridine (BUdR), which selectively inhibits differentiation of animal cells, blocks expression of the cytokinin-habituated phenotype in culture. This effect is prevented by thymidine and is reversible. These findings suggest that habituation and animal cell differentiation have a common mechanism. BUdR provides a useful tool for investigating the metabolism of cell division factors and its regulation in higher plant cells.Abbreviation BUdR 5-bromodeoxyuridine - BU 5-bromouracil     

Promotion of Flowering by DNA Base Analogues and Changes in Acid Phosphatase and Peroxidase Isozyme Composition in Dark-Grown Arabidopsis thaliana

A DNA base analogue, 5-bromodeoxyuridine (BUdR), promoted floweringof Arabidopsis thaliana in short and long photoperiods and evenin total darkness. The promotive effect of BUdR was nullifiedby thymidine which had a weak inhibitory effect by itself. AnotherDNA base analogue, 5-fluorodeoxyuridine (FUdR), inhibited theflowering at a low concentration (10^–8 M), but markedlyenhanced the promotive effect of BUdR if they were present togetherin the culture medium. In the flower-promoting medium containing both BUdR and FUdR,the number of acid phosphatase isozymes decreased temporarily,followed by an increase to the control level with a prolongedculture period. The number of peroxidase isozymes was greaterin plants grown in the medium with BUdR or BUdR $ FUdR thanin those without them. (Received October 22, 1987; Accepted March 25, 1988)