Osmotic stress inhibits thymidine incorporation into soybean protoplast DNA

DNA synthesis in protoplasts isolated from soybean cell suspension cultures has been investigated by [³H] thymidine uptake and incorporation kinetics. Initial rates of incorporation in exponential and 5-fluorodeoxyuridine synchronized protoplasts are inhibited by increased osmolarities of the medium. The inhibition was not readily reversible during 3 h culture in low osmotic medium. Velocity sedimentation analyses of replicating DNA from such protoplasts shows a complex pattern of inhibition. The inhibition probably effects replicon initiation as well as strand elongation and ligation of replication intermediates.     

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

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

Synchrony between DNA synthesis and thymidine incorporation into DNA in regenerating rat liver

DNA synthesis in regenerating liver was studied to determine whether the onset of stimulated DNA synthesis preceded the onset of increased incorporation of thymidine into DNA. Thymidine incorporation into hepatic DNA was not stimulated 15 h after operation, but was stimulated after 18 h; peak stimulation occurred 30 h after operation. Thymidine kinase activity was stimulated 24 h after operation; highest kinase activity was observed at 36 h. The onset of stimulated DNA synthesis was estimated by following the incorporation of labeled aspartic acid, sodium formate, adenine or orotic acid into appropriate DNA bases, viz., thymine, adenine, adenine or cytosine, respectively. Incorporation of adenine and orotic acid was stimulated between 15 h and 18 h after operation; incorporation of aspartic acid and sodium formate was stimulated between 18 h and 21 h after operation.The incorporation of thymidine into DNA was accelerated by stress stimulus and was inhibited by hydrocortisone. Changes in thymidine kinase activity also were correspondingly accelerated or delayed. Incorporation of labeled thymidine, adenine, formate, orotic acid or thymine into appropriate DNA bases, viz., thymine, adenine, adenine, cytosine or thymine, respectively, was stimulated by stress stimulus or was inhibited by hydrocortisone.It was concluded from these data that stimulation of DNA synthesis and of thymidine incorporation into DNA was essentially synchronized in regenerating rat liver. Results from this study were compared with results from similar studies in 2 other tissues, and the limitations, attendant with using thymidine incorporation into DNA as an indicator of stimulated DNA synthesis, were discussed.     

Measurement of bacterial growth rates in subsurface sediments using the incorporation of tritiated thymidine into DNA

Microbial growth rates in subsurface sediment from three sites were measured using incorporation of tritiated thymidine into DNA. Sampling sites included Lula, Oklahoma, Traverse City, Michigan, and Summit Lake, Wisconsin. Application of the thymidine method to subsurface sediments required (1) thymidine concentrations greater than 125 nM, (2) incubation periods of less than 4 hours, (3) addition of SDS and EDTA for optimum macromolecular extraction, and (4) DNA purification, in order to accurately measure the rate of thymidine incorporation into DNA. Macromolecule extraction recoveries, as well as the percentage of tritium label incorporated into the DNA fraction, were variable and largely dependent upon sediment composition. In general, sandy sediments yielded higher extraction recoveries and demonstrated a larger percentage of label incorporated into DNA than sediments that contained a high silt-clay component. Reported results also indicate that the acid-base hydrolysis procedure routinely used for macromolecular fractionation in water samples may not be routinely applicable to the modified sediment procedure where addition of SDS and EDTA are required for macromolecule extraction. Growth rates exhibited by subsurface communities are relatively slow, ranging from 5.1 to 10.2×10⁵ cells g^–1 day^–1. These rates are 2–1,000-fold lower than growth rates measured in surface sediments. These data lend support to the supposition that subsurface microbial communities are nutritionally stressed.     

Design and synthesis of boronic-acid-labeled thymidine triphosphate for incorporation into DNA

The boronic acid moiety is a versatile functional group useful in carbohydrate recognition, glycoprotein pull-down, inhibition of hydrolytic enzymes and boron neutron capture therapy. The incorporation of the boronic-acid group into DNA could lead to molecules of various biological functions. We have successfully synthesized a boronic acid-labeled thymidine triphosphate (B-TTP) linked through a 14-atom tether and effectively incorporated it into DNA by enzymatic polymerization. The synthesis was achieved using the Huisgen cycloaddition as the key reaction. We have demonstrated that DNA polymerase can effectively recognize the boronic acid-labeled DNA as the template for DNA polymerization, that allows PCR amplification of boronic acid-labeled DNA. DNA polymerase recognitions of the B-TTP as a substrate and the boronic acid-labeled DNA as a template are critical issues for the development of DNA-based lectin mimics via in vitro selection.     

1-beta-D-arabinofuranosylcytosine stimulates thymidine incorporation into the DNA of contact-inhibited cells

In rapidly proliferating cells l-β-d-arabinofuranosylcytosine (ara-C) is a potent inhibitor of DNA synthesis whose effect can be irreversible and consequently cytocidal. Whereas thymidine incorporation is greatly reduced in rapidly proliferating cells in the presence of ara-C, contact-inhibited cells, similarly treated, show increased thymidine incorporation by as much as 7-fold. This ara-C-induced stimulation appears to result from an influence on thymidine utilization rather than increased DNA synthesis.     

Inhibition of Growth and of Thymidine Incorporation into DNA in Tetrahymena by Chlorpromazine,Pimozide and Penfluridol1

The antipsychotic drugs chlorpromazine, pimozide, and penfluridol caused a 50% inhibition of growth of Tetrahymena at concentrations of 4.5, 5.5, and 1.5 μM, respectively. The degree of growth inhibition was dependent on the concentration of cells; higher drug concentrations were needed to produce inhibition of denser cell cultures. Binding studies with penfluridol showed that 50% growth inhibition resulted when approximately 50 μmoles of drug were bound per 10⁶ cells. A 20-min preincubation of cells with chlorpromazine (14.7 μM) inhibited DNA synthesis by 46%, and with penfluridol (4 μM) DNA synthesis was inhibited by 27%. The incorporation of labeled thymidine into the thymidine triphosphate pool was inhibited by chlorpromazine but not by penfluridol, indicating that the drugs produce their growth inhibitory effects by different mechanisms. TDP kinase activity was demonstrated in a particle-free fraction of the cells. Its enzymatic activity was not affected by added chlorpromazine, penfluridol, or calmodulin, suggesting that inhibition of DNA synthesis by these drugs may be a consequence of growth inhibition.     

Tissue-specific inhibition of [3H]thymidine incorporation into DNA by carcinogenic N-nitrosamines

The N-nitrosamines N-nitrosodimethylamine (DMN), N'-nitrosonornicotine (NNN) and 4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were injected intraperitoneally 24 h before sacrifice in F344 rats and C57BL mice in doses of 297 mumoles/kg b.w. and 148 mumoles/kg b.w., respectively. 2 h before sacrifice, the animals were given an intraperitoneal injection of [3H]thymidine. The results showed that the examined N-nitrosamines inhibited the incorporation of [3H]thymidine into DNA in a few tissues of the rats and the mice. The results indicated that the N-nitrosamines exerted a tissue-specific inhibition of the [3H]thymidine incorporation in the tissues reported to be involved in the biotransformation of these substances. The observed inhibitory effects on the incorporation of [3H]thymidine by DMN, NNN and NNK were also correlated to a considerable extent to the reported sites of carcinogenicity. The present study indicates that measurements of [3H]thymidine incorporation into DNA in various tissues of experimental animals is a useful short-term bioassay to evaluate the potential tissue-specific carcinogenicity of the N-nitrosamines. The method may also be useful as a complement to other short-term in vivo tests in the screening of potential genotoxicity of several other chemicals.     

Temperature dependent phosphorylation of [H]thymidine and its incorporation into DNA by KB cells

The incorporation of [³H]TdR into DNA by KB cells cooled to 4 °C falls rapidly to about 1–2% of that of controls held at 37 °C. The amounts of four enzymes involved in TdR metabolism: TdR kinase, thymidylate kinase, cytoplasmic DNA polymerase, and nuclear DNA polymerase, never fall below 50% of those in the control cells even after 12 h at 4 °C. The activities of these enzymes were measured in vitro at different temperatures and it was found that whereas the two kinases retained appreciable activity at low temperature, the DNA polymerase activities were severely inhibited. Cultures of cells rewarmed to 37 °C after 12 h at 4 °C immediately re-started incorporation of labelled TdR into DNA, showing that sufficient enzyme activity for normal functioning had been preserved during the cold period.     

Glucocorticoid effects on thymidine incorporation into the DNA of S49 lymphoma cells

The effects of glucocorticoids on lymphoid cell growth and thymidine incorporation into DNA were studied using the S49 mouse lymphoma cell line. Glucocorticoid-mediated lymphocytolysis in these cells is preceded by an arrest in the g₁ phase of the cell cycle [1]. However, this arrest is only partial, and is reversible by washing out the hormone. Thus, although the overall impression is that these cells are arrested in G₁ and then begin to die, they apparently can escape the arrest and proceed through the cell cycle, albeit at a relatively low level. The mode of DNA synthesis in these glucocorticoid-treated cells is replicative and not repair. The importance of the inhibition of thymidine incorporation to the cell death process in S49 cells is evident from experiments in which cells are treated with both the hormone and various DNA synthesis inhibitors; a synergistic killing of the cells is obtained. Thus, the inhibition of DNA synthesis and cell proliferation is a rapid, and perhaps primary, effect of glucocorticoids on the complex process of hormone-mediated lymphocytolysis in this mouse cell line.     

Inhibition of herpesvirus-induced thymidine kinase and DNA polymerase by beta-hydroxynorvaline

Treatment of HSV-infected cells with 5-10 mM beta-hydroxynorvaline (Hnv), a threonine analog, specifically affects herpesvirus DNA replication: both the rate of and total DNA synthesis are reduced, the former approximately 15-fold by Hnv (6 h post-infection) and the latter by 12-fold (between 3 and 12 h post-infection). The effect on DNA replication was due to inhibition of HSV-1 thymidine kinase (TK) and DNA polymerase (DP) activities; the former is reduced by 75% and whereas DP returns to baseline levels (when compared to untreated and/or uninfected cells). Host cell TK and DP activities are unaffected. It is suggested that beta-hydroxynorvaline is incorporated into these enzyme(s), either close to or at the active site thus perturbing viral DNA synthesis. beta-Hydroxynorvaline should have unique utility as a targeted antiviral compound, acting on both membrane-mediated phenomena (fusion, penetration and attachment) and DNA replication.     

Differences in the incorporation of thymidine into DNA of normal and cystic fibrosis fibroblasts in vitro.

Although similar fractions of cells were in the S phase of the cell cycle, normal human skin fibroblasts were shown to incorporate more than twice the 3HTdR into their DNA in vitro than did cells obtained from individuals with cystic fibrosis (CF). Obligate heterozygotes incorporated an intermediate amount of the DNA precursor. Studied were initiated to determine the basis of the differential incorporation of 3HTdR among the genotypes. An analog of thymidine, BUdR, produced varied effects on the growth kinetics of the three genotypes. The growth of cells in BUdR resulted in a 50% increase in the population doubling times of all three genotypes, and caused the cell morphology to change from a spindle shape to one in which the cells became broadened and flat, with numerous cytoplasmic projections extending for distances of several cell diameters. The activities of thymidine kinase and the participation of the exogenous and de novo pathways in the synthesis of TMP were found to be approximately the same in all three genotypes. The data suggest that an alteration in the transport of thymidine into the cells may account for the differences in TdR incorporation into DNA, and this may be associated with other changes in cystic fibrosis that are apparently membrane associated.     

Synthesis of a convenient thymidine glycol phosphoramidite monomer and its site-specific incorporation into DNA fragments

An original phosphoramidite building block of the thymidine glycol lesion has been prepared taking into account the additional diol function and the high lability of this oxidatively induced nucleobase damage. Then the modified nucleoside was site-specifically inserted into DNA fragments by solid support assembling followed by a "one-step" mild final deprotection treatment.     

Inhibition of macrophage DNA synthesis by immunomodulators. II. Characterization of the suppression by muramyl dipeptide or lipopolysaccharide [3H]thymidine incorporation into macrophages

Guinea pig peritoneal exudate macrophages actively incorporated [3H]thymidine into trichloroacetic acid-insoluble fraction in vitro. The incorporation of [3H]thymidine was almost completely inhibited by aphidicolin, an inhibitor of DNA polymerase alpha and an autoradiograph showed heavy labeling in nuclei of 15% of macrophage populations. These results indicate that the observed thymidine incorporation was due to a nuclear DNA synthesis. The [3H]thymidine incorporation was markedly suppressed when macrophages were activated by immunoadjuvants such as muramyl dipeptide (MDP) or bacterial lipopolysaccharide (LPS). The suppression of [3H]thymidine incorporation by MDP was neither due to the decrease in thymidine transport through the cell membrane, nor due to dilution by newly synthesized "cold" thymidine. An autoradiograph revealed that MDP markedly decreased the number of macrophages the nuclei of which were labeled by [3H]thymidine. These results suggest that the suppression of [3H]thymidine incorporation by the immunoadjuvants reflects a true inhibition of DNA synthesis. The inhibition of DNA synthesis by MDP was also observed in vivo. Further, it was strongly suggested that the inhibition was not caused by some mediators, such as prostaglandin E2, released from macrophages stimulated by the immunoadjuvants but caused by a direct triggering of the adjuvants at least at the early stage of activation. Cyclic AMP appears to be involved in the inhibitory reaction.     

Underestimation of DNA synthesis by [h]thymidine incorporation in marine bacteria

A direct comparison of [H]thymidine incorporation with DNA synthesis was made by using an exponentially growing estuarine bacterial isolate and the naturally occurring bacterial populations in a eutrophic subtropical estuary and in oligotrophic offshore waters. Simultaneous measurements of [H]thymidine incorporation into DNA, fluorometrically determined DNA content, and direct counts were made over time. DNA synthesis estimated from thymidine incorporation values was compared with fluorometrically determined changes in DNA content. Even after isotope dilution, nonspecific macromolecular labeling, and efficiency of DNA recovery were accounted for, [H]thymidine incorporation consistently underestimated DNA synthesized by six- to eightfold. These results indicate that although the relationship of [H]thymidine incorporation to DNA synthesis appears consistent, there are significant sources of thymine bases incorporated into DNA which cannot be accounted for by standard [H]thymidine incorporation and isotope dilution assays.