Isotope-dilution analysis of rate-limiting steps and pools affecting the incorporation of thymidine and deoxycytidine into cultured thymus cells

Metabolic optimization of adoptive T cell transfer cancer immunotherapy: A historical overview

After prolonged extracorporeal multiplication in physiological culture media, there can be curative infusions of a cancer patient's own cytotoxic T cells (adoptive T cell transfer; ACT), which must achieve efficient activation in potentially adverse tumour microenvironments. With spectacular, yet irregular, success, improvements are needed. Developing lymphoid cells are biologically selected, not only for 'near-self' reactivity (positive selection), but also to avoid self-reactivity (negative selection). Thus, success requires harnessing near-self cells while avoiding extreme autoimmune phenomena. Abrupt metabolic changes accompanying T cell activation to leave the G₀ stage and enter the G₁ stage of the cell cycle (eg enhanced glycolysis) are accompanied by increased transcription of the G0S9 gene that mediates salvage synthesis of NAD⁺ from nicotinamide; the latter has recently been shown to increase the efficiency of ACT. Despite theoretical and experimental advances, there has not been parallel progress in simulating in vivo conditions with culture media that were initially formulated for their positive benefits for tumour cell lines (cell survival and proliferation). Yet for lymphoid cells, inhibition or death (ie immunological tolerance) is as important as their activation and proliferation (immunological response). Thus, use of media optimized for the latter may mask the former. The resilience of established culture protocols may have been partly politically driven. However, unphysiological conditions have sometimes yielded fortuitous insights. Optimization of culture media for specific tissues must consider the nature of problems addressed in research settings and the need to avoid mishaps in clinical settings.

Further Verification of the Isotope Dilution Approach for Estimating the Degree of Participation of [H]thymidine in DNA Synthesis in Studies of Aquatic Bacterial Production

The optimal concentration of [H]thymidine (i.e., the maximal degree of participation in DNA synthesis) as determined by adding increasing amounts of labeled thymidine at the same specific activity was similar to the concentration of thymidine inhibiting the de novo pathway as determined by isotope dilution plots. These experiments provide further verification of the isotope dilution approach for determining the degree of participation of [H]thymidine in DNA synthesis.

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.

Substrate specificity of Epstein-Barr virus thymidine kinase

Purified recombinant protein encoded by the BXLF-I open reading frame of the Epstein-Barr virus genome has thymidine kinase activity. The substrate behaviors of various nucleosides toward this enzyme were tested. Halogenated deoxyuridines, zidovudine, and bromovinyldeoxyuridine are efficient substrates, while acyclovir and dihydroxypropylmethylguanine are relatively poor substrates for the Epstein-Barr virus thymidine kinase.

Mechanism(s) regulating inhibition of thymidylate synthase and growth by gamma-L-glutaminyl-4-hydroxy-3-iodobenzene, a novel melanin precursor, in melanogenic melanoma cells

A proposed mechanism for the melanoma specific activity of phenolic amines is based upon the ability of the enzyme tyrosinase to oxidize these prodrugs to toxic intermediates. In this study, we synthesized an iodinated analog of gamma-L-glutaminyl-4-hydroxybenzene (GHB) with increased antimelanoma activity in both human and murine melanoma cell lines. GHB and gamma-L-glutaminyl-4-hydroxy-3-iodobenzene (I-GHB) were shown to be substrates for both mammalian and mushroom tyrosinase. Glutathione, a cellular antioxidant, inhibited tyrosinase mediated formation of gamma-L-glutaminyl-3,4-benzoquinone (GBQ) from GHB, inhibited melanin production, and blocked the inhibition of the enzyme thymidylate synthase by oxidized GHB. Buthionine sulfoximine (BSO) depletion of cellular glutathione enhanced the growth inhibitory activity and the inhibition of in situ thymidylate synthase by phenolic amines in melanoma cells. GHB and I-GHB were shown to be approximately 5- and 10-fold more cytotoxic, respectively, in highly metastatic B16-BL6 cells than in weakly metastatic B16-F1 cells with approximately equal tyrosinase activity. B16-BL6 cells had approximately 20-fold higher gamma-glutamyltranspeptidase (gamma-GTPase) activity than B16-F1 cells which suggested the possible involvement of this enzyme in the activation of the cytotoxicity of the phenolic amines. 4-Aminophenol, a product of gamma-GTPase reaction with GHB, was a substrate for tyrosinase and a potent inhibitor of in situ thymidylate synthase activity in melanogenic cells. In pigmented melanoma cells containing the enzyme tyrosinase, the quinone mediated mechanism of phenolic amine cytotoxicity may be uniquely important and the cellular antioxidant glutathione essential in the detoxification of these quinone-generated intermediates.

Thymidylate synthase as a target enzyme for the melanoma-specific toxicity of 4-S-cysteaminylphenol and N-acetyl-4-S-cysteaminylphenol

The rationale for melanoma-specific antitumor agents containing phenolic amines is based in part on the ability of the enzyme tyrosinase to oxidize these prodrugs to toxic intermediates. The phenolic amine compounds 4-S-cysteaminylphenol (4-S-CAP) and N-acetyl-4-S-cysteaminylphenol (N-Ac-4-S-CAP) inhibited in situ thymidylate synthase activity in pigmented melanoma cell lines but had little or no effect on nonpigmented and nonmelanoma cell lines. Theophylline, a cyclic adenosine monophosphate (cAMP) phosphodiesterase inhibitor, increased tyrosinase activity and potentiated the inhibition of in situ thymidylate synthase by N-Ac-4-S-CAP. The inhibition of in situ thymidylate synthase by both drugs in pigmented melanoma cells correlated with the inhibition of DNA synthesis and cell growth and was not due to an indirect effect caused by inhibition of the enzyme dihydrofolate reductase. 4-S-CAP inhibition of thymidylate synthase activity in cell free extracts required oxidation of the drug. In the presence of tyrosinase, the concentration causing a 50% inhibition of thymidylate synthase activity (IC50) in cell-free extracts was less than 10 microM, but no inhibition was observed in its absence, even at a drug concentration of 500 microM. Two reducing agents, dithioerythritol and glutathione, effectively blocked the inhibition of thymidylate synthase by oxidized 4-S-CAP. In pigmented melanoma cells containing the enzyme tyrosinase, the quinone-mediated mechanism of inhibition of DNA synthesis via inhibition of thymidylate synthase may be uniquely important in the expression of phenolic amine cytotoxicity.

Programmed activation of T-lymphocytes. A theoretical basis for short term treatment of AIDS with azidothymidine

When its T-lymphocyte host cell is activated, the latent (DNA) form of human immunodeficiency virus (HIV) is activated to produce RNA copies which are liberated as virus particles from the cell. In this process the cell is destroyed together with the latent virus. If administered at this time, 3'-azidothymidine (AZT) would specifically prevent the liberated RNA copies replicating and establishing latency in new host cells. The RNA copies would then be degraded by viral or host ribonucleases. Thus, one DNA copy of HIV and its RNA progeny would be eliminated from the body. However, many DNA copies of HIV would remain in other cells. The main problem of therapy with AZT is that activation of host cells to become permissive for production of virus is random in time. Activation depends on chance encounters of an infected person with the particular foreign antigens to which individual T-cells bearing latent HIV can specifically respond. It is primarily for this reason that AZT must be administered continuously. If all T-cell could be polyclonally stimulated at one time, all HIV-bearing T-cells would be destroyed and concomitant administration of AZT for a short term would prevent the replication of all liberated viruses. Unlike most renewable 'end' cells in the body, the maturation of T cells involves processes of positive and negative selection. To preserve the 'educated' T-cell population, T-cell renewal occurs at the end cell, rather than at the stem cell level. It is possible that normal physiological signals concerned with this homeostatic regulation of T-lymphocyte population size could be harnessed to produce synchronous activation of all T-lymphocytes. Tumor necrosis factor-alpha has some of the properties expected of a postulated polyclonal activator needed for this programmed activation of T-lymphocytes.

Assay of mitogen-induced effects on cellular incorporation of precursors for scavenger, de novo, and net DNA synthesis

In summary we have presented data on [3H]dThd incorporation into DNA for a positive and negative growth modulator. These data clearly do not correspond to those based on net DNA synthesis (ortho[32P]phosphate incorporation into purified DNA) and previous knowledge of the effects of these hormones on cell number and cellular DNA accumulation. The paradox was resolved by directly analyzing effects of hormones on de novo pyrimidine biosynthesis. E2 stimulated both de novo and scavenger pathways in the first wave of DNA synthesis and only de novo in the second (and subsequent) waves. In contrast, tam progressively inhibited de novo pyrimidine biosynthesis. These hormonal effects on intracellulr pyrimidine pool sizes rendered [3H]dThd incorporation data by itself uninterpretable. [3H]dThd is a useful measure of DNA synthesis only if verified by independent measures of net DNA synthesis for the same time course and treatment conditions. In addition it may prove beneficial in various experimental systems to develop direct assays of de novo pyrimidine biosynthesis to assess mitogen effects. The experiments presented may also prove to be useful in evaluating the effects of mitogens and antimitogens on cells synchronized in the cell cycle by any of a variety of means.

Dynamics of extracellular DNA in the marine environment

The production and turnover of dissolved DNA in subtropical estuarine and oligotrophic oceanic environments were investigated. Actively growing heterotrophic bacterioplankton (i.e., those capable of [3H]thymidine incorporation) were found to produce dissolved DNA, presumably through the processes of death and lysis, grazing by bacteriovores, and excretion. Production of dissolved DNA as determined by [3H]thymidine incorporation was less than or equal to 4% of the ambient dissolved DNA concentration per day. In turnover studies, the addition of [3H]DNA (Escherichia coli chromosomal) to seawater resulted in rapid hydrolysis and uptake or radioactivity by microbial populations. DNA was hydrolyzed by both cell-associated and extracellular nucleases, in both estuarine and offshore environments. Kinetic analysis performed for a eutrophic estuary indicated a turnover time for dissolved DNA as short as 6.5 h. Microautoradiographic studies of bacterial populations in Tampa Bay indicated that filamentous and attached bacteria took up most of the radioactivity from [3H]DNA. Dissolved DNA is therefore a dynamic component of the dissolved organic matter in the marine environment, and bacterioplankton play a key role in the cycling of this material.

Comparison of the inhibitory effects of hydroxyurea, 5-fluorodeoxyuridine, 3,4-dihydroxybenzylamine, and methotrexate on human squamous cell carcinoma

A cell line (SCC-25) derived from a human squamous cell carcinoma was found to have a higher level of in situ thymidylate synthase activity when compared to a faster-growing cell line, L1210 leukemia, and approximately 5 times the level of activity of a cell line with the same growth rate, S91-A melanoma. These results led to an examination of the effects of both direct and indirect inhibitors of this enzyme using intact SCC-25 cells. It was found that drugs that have an indirect effect on this enzyme--methotrexate (MTX), hydroxyurea (HU), and 3,4-dihydroxybenzylamine (3,4-DHBA)--acted as noncompetitive inhibitors and the inhibition of thymidylate synthase by these drugs did not result in the accumulation of its substrate, dUMP. This suggested that these drugs are also inhibiting steps leading to the formation of dUMP by a mechanism that is coordinated with the inhibition of thymidylate synthase. 5-Fluorodeoxyuridine (FUDR), a competitive inhibitor of thymidylate synthase, did cause an increase in the dUMP pool size indicating that this drug did not affect the synthesis of this substrate. There was a good correlation for the inhibition of growth, DNA synthesis, and thymidylate synthase with HU, 3,4-DHBA, and FUDR. However, the results suggested fundamentally different mechanistic reasons for the close relationship among these three inhibitory effects. Finally the inhibition of growth by MTX did not appear to correlate with the inhibition of DNA synthesis. The implication of these results for the therapy of epidermally derived proliferative disorders, especially with combinations such as MTX and 5-FU, is discussed.

Activity of an Attached and Free-Living Vibrio sp. as Measured by Thymidine Incorporation, p -Iodonitrotetrazolium Reduction, and ATP/DNA Ratios

Three independent techniques, [ 3 H]thymidine incorporation, the reduction rate of p -iodonitrotetrazolium violet (INT) to INT formazan normalized to DNA, and the ratio of ATP to DNA, were adapted to measure the activity of attached and unattached estuarine bacteria. In experiments employing the estuarine isolate Vibrio proteolytica , nutrient concentrations were manipulated by varying the concentration of peptone-yeast extract. In the presence of exogenous nutrients, the activity of free-living cells was greater than that of attached cells as measured by [ 3 H]thymidine incorporation and ATP/DNA ratios. In the absence of peptone-yeast extract, however, the activity of attached cells surpassed that of free-living cells as determined by [ 3 H]thymidine incorporation and INT formazan normalized to DNA. Of the three techniques, [ 3 H]thymidine incorporation was deemed most sensitive for detecting changes in activity resulting from slight differences in nutrient concentration. By this technique, attached cells were much less sensitive to changing nutrient concentrations than were free-living cells. Below a threshold concentration, attached cell activity remained constant, while the activity of unattached cells decreased as a function of decreasing nutrient concentration. The results suggest that loss of cell surface area available for substrate uptake due to attachment may be an important factor in determining the relative activities of attached and free-living cells.

Validity of the tritiated thymidine method for estimating bacterial growth rates: measurement of isotope dilution during DNA synthesis

The rate of tritiated thymidine incorporation into DNA was used to estimate bacterial growth rates in aquatic environments. To be accurate, the calculation of growth rates has to include a factor for the dilution of isotope before incorporation. The validity of an isotope dilution analysis to determine this factor was verified in experiments reported here with cultures of a marine bacterium growing in a chemostat. Growth rates calculated from data on chemostat dilution rates and cell density agreed well with rates calculated by tritiated thymidine incorporation into DNA and isotope dilution analysis. With sufficiently high concentrations of exogenous thymidine, de novo synthesis of deoxythymidine monophosphate was inhibited, thereby preventing the endogenous dilution of isotope. The thymidine technique was also shown to be useful for measuring growth rates of mixed suspensions of bacteria growing anaerobically. Thymidine was incorporated into the DNA of a range of marine pseudomonads that were investigated. Three species did not take up thymidine. The common marine cyanobacterium Synechococcus species did not incorporate thymidine into DNA.

Rate of DNA synthesis in exponentially growing cell lines in the presence and absence of antimetabolites

Experimental tumor cell lines were used to show that in the presence of 5-fluorodeoxyuridine (FdUrd), the rate of DNA synthesis remains unaltered as long as a saturating concentration of thymidine is present. This unimpeded rate of DNA synthesis in combination with FdUrd-blocked de novo thymidylate synthesis makes it possible to accurately measure the total rate of increase of DNA using tritiated thymidine of known specific activity. The observed amount of incorporated tritiated thymidine is in excellent agreement with the calculated theoretical maximal incorporation in cultures with exponentially increasing DNA and cell number.

Isotope-dilution analysis of the effects of deoxyguanosine and deoxyadenosine on the incorporation of thymidine and deoxycytidine by hydroxyurea-treated thymus cells

It is presumed that the dGTP and dATP needed for replicative DNA synthesis can be formed by way of either ;salvage' pathways or biosynthesis de novo. This was examined by adding hydroxyurea to cultures of rat thymus cells to inhibit ribonucleoside diphosphate reductase, a key enzyme of the ;de novo' pathway. Most of the inhibition of the incorporation of [Me-(3)H]thymidine and deoxy[5-(3)H]cytidine by low concentrations of hydroxyurea (100-500mum) was prevented by substrates of the salvage pathway (400mum-deoxyguanosine and, to a lesser extent, 200mum-deoxyadenosine). However, isotope-dilution studies indicated that the purine deoxyribonucleosides prevented inhibition by decreasing pyrimidine deoxyribonucleotide competitor pools. Evidence was obtained that a hydroxyurea-induced increase in the thymidine-competitor pool (probably dTTP) was prevented to an equal extent by deoxyguanosine and by the inhibitor of thymidylate synthase, deoxy-5-fluorouridine. These compounds had almost identical effects on hydroxyurea dose-response curves and on thymidine isotope-dilution plots. The evidence suggests that exogenous purine deoxyribonucleosides cannot prevent the inhibition by hydroxyurea of thymus-cell DNA synthesis. This could mean that, with respect to the metabolism of purine deoxyribonucleotides, ribonucleoside diphosphate reductase is tightly coupled to DNA polymerase in a multienzyme complex. The complex would not permit entry of exogenous metabolic intermediates into the ;de novo' pathway, but would still be subject to the regulatory effects of these intermediates. Thus dGTP and dATP formed from exogenous purine deoxyribonucleosides by salvage pathways might deplete pyrimidine deoxyribonucleotide competitor pools by inhibiting relatively hydroxyurea-insensitive activities of ribonucleoside diphosphate reductase.

Channelling of exogenous phenylalanine to the sites of storage and the sites of alkaloid and protein biosynthesis in Penicillium cyclopium

Externally applied L-phenylalanine rapidly equilibrates with the cytosolic pool(s) in hyphae of emerged cultures of Penicillium cyclopium. If not incorporated into protein it is accumulated in the so called expandable pool, which is presumably localized in the vacuolar compartment. At high concentrations of exogenous L-phenylalanine practically all of the amino acid needed for protein synthesis comes from the extracellular source, contrary to alkaloid synthesis which under all conditions recruits more than 90% of the required L-phenylalanine from intracellular sources. Two pathways of alkaloid labelling can be distinguished, by which externally applied L-phenylalanine reaches the sites of alkaloid synthesis: (1) a direct way from cytosolic pool (primary labelling) and (2) an indirect way via the expandable pool (secondary labelling).

The rate of deoxyribonucleic acid synthesis by cultured Chinese-hamster ovary cells. An application of isotope-dilution analysis

The rate of DNA synthesis is exponentially growing cells was determined by isotopedilution analysis of the incorporation of [me-3H]thymidine. Thymidine concentrations greater than 7 micrometer were used so that the rate-limiting step governing incorporation would be at the level of DNA polymerase rather than at the level of thymidine kinase [Sjostrom & Forsdyke (1974) Biochem. J. 138, 253-262]. In early exponential phase the rate determined by isotope-dilution analysis closely correlated with the rates calculated either from growth curves or from known cell-cycle parameters. However, in late-exponential phase the rate calculated from the growth curve was less than that determined by isotope-dilution analysis. We conclude that, under certain conditions, the pool-corrected rate of incorporation of [me-3H]thymidine, as determined by isotope-dilution analysis, can accurately reflect the rate of DNA synthesis. Discrepancies between the observed rate of DNA synthesis and increase in cell number could reflect an exponential degeneration of post-S-phase cells.