Evidence for the involvement of substrate cycles in the regulation of deoxyribonucleoside triphosphate pools in 3T6 cells

Deoxycytidine Release from Pancreatic Stellate Cells Promotes Gemcitabine Resistance

Pancreatic ductal adenocarcinoma (PDAC) is a leading cause of cancer deaths in the United States. The deoxynucleoside analogue gemcitabine is among the most effective therapies to treat PDAC, however, nearly all patients treated with gemcitabine either fail to respond or rapidly develop resistance. One hallmark of PDAC is a striking accumulation of stromal tissue surrounding the tumor, and this accumulation of stroma can contribute to therapy resistance. To better understand how stroma limits response to therapy, we investigated cell-extrinsic mechanisms of resistance to gemcitabine. Conditioned media from pancreatic stellate cells (PSC), as well as from other fibroblasts, protected PDAC cells from gemcitabine toxicity. The protective effect of PSC-conditioned media was mediated by secretion of deoxycytidine, but not other deoxynucleosides, through equilibrative nucleoside transporters. Deoxycytidine inhibited the processing of gemcitabine in PDAC cells, thus reducing the effect of gemcitabine and other nucleoside analogues on cancer cells. These results suggest that reducing deoxycytidine production in PSCs may increase the efficacy of nucleoside analog therapies. SIGNIFICANCE: This study provides important new insight into mechanisms that contribute to gemcitabine resistance in PDAC and suggests new avenues for improving gemcitabine efficacy.

Mechanism of the allosteric regulation of Streptococcus mutans 2'-deoxycytidylate deaminase

In cells, dUMP is the intermediate precursor of dTTP in its synthesis during deoxynucleotide metabolism. In Gram-positive bacteria and eukaryotes, zinc-dependent deoxycytidylate deaminases (dCDs) catalyze the conversion of dCMP to dUMP. The activity of dCD is allosterically activated by dCTP and inhibited by dTTP. Here, the crystal structure of Streptococcus mutans dCD (SmdCD) complexed with dTTP is presented at 2.35 Å resolution, thereby solving the first pair of activator-bound and inhibitor-bound structures from the same species to provide a more definitive description of the allosteric mechanism. In contrast to the dTTP-bound dCD from the bacteriophage S-TIM5 (S-TIM5-dCD), dTTP-bound SmdCD adopts an inactive conformation similar to the apo form. A structural comparison suggests that the distinct orientations of the triphosphate group in S-TIM5-dCD and SmdCD are a result of the varying protein binding environment. In addition, calorimetric data establish that the modulators bound to dCD can be mutually competitively replaced. The results reveal the mechanism underlying its regulator-specific activity and might greatly enhance the understanding of the allosteric regulation of other dCDs.

Requirement for deoxycytidine kinase in T and B lymphocyte development

Deoxycytidine kinase (dCK) is a rate-limiting enzyme in deoxyribonucleoside salvage, a metabolic pathway that recycles products of DNA degradation. dCK phosphorylates and therefore activates nucleoside analog prodrugs frequently used in cancer, autoimmunity, and viral infections. In contrast to its well established therapeutic relevance, the biological function of dCK remains enigmatic. Highest levels of dCK expression are found in thymus and bone marrow, indicating a possible role in lymphopoiesis. To test this hypothesis we generated and analyzed dCK knockout (KO) mice. dCK inactivation selectively and profoundly affected T and B cell development. A 90-fold decrease in thymic cellularity was observed in the dCK KO mice relative to wild-type littermates. Lymphocyte numbers in the dCK KO mice were 5- to 13-fold below normal values. The severe impact of dCK inactivation on lymphopoiesis was unexpected given that nucleoside salvage has been thought to play a limited, "fine-tuning" role in regulating deoxyribonucleotide triphosphate pools produced by the de novo pathway. The dCK KO phenotype challenges this view and indicates that, in contrast to the great majority of other somatic cells, normal lymphocyte development critically requires the deoxyribonucleoside salvage pathway.

Two dNTP triphosphohydrolases from Pseudomonas aeruginosa possess diverse substrate specificities

Nucleotide hydrolases are known to hydrolyze not only noncanonical dNTPs to reduce the risk of mutation, but also canonical dNTPs to maintain the dNTP concentrations in the cell. dGTP triphosphohydrolase from Escherichia coli is known as an enzyme that hydrolyzes dGTP. Recently, we identified a triphosphohydrolase from Thermus thermophilus HB8 that hydrolyzes all canonical dNTPs through a complex activation mechanism. These dNTP triphosphohydrolases are widely distributed in eubacteria, but it is difficult to predict whether they possess hydrolytic activity for dGTP or dNTP. To obtain information concerning the structure-function relationships of this protein family, we characterized two dNTP triphosphohydrolases, PA1124 and PA3043, from Pseudomonas aeruginosa. Molecular phylogenic analysis showed that dNTP triphosphohydrolases can be classified into three groups. Experimentally, PA1124 had a preference for dGTP, similar to the E. coli enzyme, whereas PA3043 displayed a broad substrate specificity. Both enzymes hydrolyzed substrates in the absence of additional dNTP as an activating effector. These kinetic data suggest that PA3043 is a novel type distinct from both the E. coli and T. thermophilus enzymes. On the basis of these results, we propose that the dNTP triphosphohydrolase family should be classified into at least three subfamilies.

Mouse RS21-C6 is a mammalian 2'-deoxycytidine 5'-triphosphate pyrophosphohydrolase that prefers 5-iodocytosine

Free nucleotides in living cells play important roles in a variety of biological reactions, and often undergo chemical modifications of their base moieties. As modified nucleotides may have deleterious effects on cells, they must be eliminated from intracellular nucleotide pools. We have performed a screen for ITP-binding proteins because ITP is a deaminated product of ATP, the most abundant nucleotide, and identified RS21-C6 protein, which bound not only ITP but also ATP. Purified, recombinant RS21-C6 hydrolyzed several canonical nucleoside triphosphates to the corresponding nucleoside monophosphates. The pyrophosphohydrolase activity of RS21-C6 showed a preference for deoxynucleoside triphosphates and cytosine bases. The k(cat)/K(m) (s(-1) m(-1)) values were 3.11 x 10(4), 4.49 x 10(3) and 1.87 x 10(3) for dCTP, dATP and dTTP, respectively, and RS21-C6 did not hydrolyze dGTP. Of the base-modified nucleotides analyzed, 5-I-dCTP showed an eightfold higher k(cat)/K(m) value compared with that of its corresponding unmodified nucleotide, dCTP. RS21-C6 is expressed in both proliferating and non-proliferating cells, and is localized to the cytoplasm. These results show that RS21-C6 produces dCMP, an upstream precursor for the de novo synthesis of dTTP, by hydrolyzing canonical dCTP. Moreover, RS21-C6 may also prevent inappropriate DNA methylation, DNA replication blocking or mutagenesis by hydrolyzing modified dCTP.

Synthesis of virus-specific high-mobility DNA after temperature upshift of SC-1 cells chronically infected with moloney murine leukemia virus mutant ts1

Premature termination products of reverse transcription that consist physically of viral minus-sense single-stranded DNA that is shorter than one long terminal repeat and partial DNA duplexes are dramatically increased in the central nervous system (CNS) of FVB/N mice that are infected by ts1, a temperature-sensitive mutant of Moloney murine leukemia virus. Due to their migration in agarose gels, these incomplete physical forms of DNA have been designated high-mobility (HM) DNA. In non-CNS tissues, the level of HM DNA is either low or not detectable. In order to determine the conditions that are necessary for the synthesis of HM DNA in vivo, we have characterized the physical forms of HM DNA that were synthesized in vitro in chronically infected SC-1 cells after temperature upshift. At the permissive temperature of 34 degrees C, the chronically infected SC-1 cells did not synthesize HM DNA. After temperature upshift of the cultured cells from 34 to 37 degrees C, the chronically infected SC-1 cells developed extremely high levels of HM DNA. Following temperature downshift of the cultured cells from 37 to 34 degrees C, a decrease in the level of HM DNA and an increase in the level of unintegrated linear proviral DNA occurred simultaneously. These results suggested that the accumulation of HM DNA both in vitro and in vivo may be the result of superinfection.

Intracellular compartmentation of organelles and gradients of low molecular weight species

Intracellular compartmentation of metabolites without intervening membranes is an important concept that has emerged from consideration of the metabolic inhomogeneities associated with a highly organized and structured cytoplasm within mammalian cells. This recognition is primarily due to the development of experimental approaches to measure metabolite or ion concentrations at specific subcellular sites, thereby providing a means to study concentration gradients within the aqueous cytoplasm in intact cells. The presence of mitochondrial clusters has been shown to create gradients of low molecular weight species, such as O2, ATP, and pH, with important implications for substrate supply for function and regulation of cellular processes. Moreover, the existence of kinetically distinct precursor pools has been shown to result in functional compartmentation of biochemical pathways, such as DNA replication and carbohydrate metabolism. The creation of these specialized microzones of metabolism in accordance with their association with cellular organelles or membranal structures may be integral to normal function and regulation of adult mammalian cells.

Differential effects of hydroxyurea upon deoxyribonucleoside triphosphate pools, analyzed with vaccinia virus ribonucleotide reductase

Hydroxyurea inhibits DNA synthesis by destroying the catalytically essential free radical of class I ribonucleoside diphosphate (rNDP) reductase, thereby blocking the de novo synthesis of deoxyribonucleotides. In mammalian cells, including those infected by vaccinia virus, hydroxyurea treatment causes a differential depletion of the four deoxyribonucleoside triphosphate pools, suggesting that the activities of rNDP reductase are differentially sensitive to hydroxyurea. In the presence of different substrates and allosteric modifiers, we measured rates of free radical destruction in the vaccinia virus-coded rNDP reductase, by following absorbance at 417 nm as a function of time after hydroxyurea addition. Also, we followed enzyme activity directly, by using a recently developed assay that allows simultaneous monitoring of the four activities, in the presence of substrates and effectors at concentrations that approximate the intracellular environment. We found the primary determinant of radical loss to be not the ensemble of allosteric ligands bound but the activity of the enzyme. Nucleoside triphosphate effectors accelerated radical decay, compared with rates seen with the free enzyme. Adding substrate to the holoenzyme, under conditions where the enzymatic reaction is proceeding, further accelerated radical decay. Alternative models are discussed, to account for selective depletion of purine nucleotide pools by hydroxyurea.

Role of ribonucleotide reductase and deoxynucleotide pools in the oxygen-dependent control of DNA replication in Ehrlich ascites cells

Cultured Ehrlich ascites cells were exposed to different oxygen tensions (ranging from nearly complete anoxia to 95% O2 at 10(5) Pa) and to transient (5-10 h) hypoxia (0.02% O2 at 10(5) Pa). Treated cells were examined with respect to the intracellular concentration of the M2-specific tyrosyl free radical of ribonucleotide reductase by EPR spectroscopy, and with respect to the pool sizes of all four deoxynucleoside triphosphates by an enzymatic assay employing DNA polymerase I of Escherichia coli. From 2% to 0.02% O2, the free radical level decreased continually from a normal value to just above detectability by the EPR measurement employed, and quickly recovered when hypoxic cells were resupplied with atmospheric O2. Concurrently, analogous changes of the size of the dCTP pool occurred, whereas the pool sizes dATP and dGTP underwent no changes, and the size of the dTTP pool only moderate changes. The changes of the free radical concentration and of the dCTP pool correlated well with the suppression or reactivation of DNA replication under the respective O2 conditions. The results consistently support the hypothesis of a fast-acting regulatory pathway that controls the rate of DNA replication in proliferating cells according to sufficient availability of O2. Therefore, ribonucleotide reductase may serve, in addition to providing DNA building blocks, as a pO2 sensor, which transmits the signal in the form of an altered intracellular dCTP concentration, directly or indirectly, to the nuclear-replication machinery.

Substrate cycling between 5-amino-4-imidazolecarboxamide riboside and its monophosphate in isolated rat hepatocytes

AICA (5-amino-4-imidazolecarboxamide)-riboside is taken up by isolated rat hepatocytes and converted by adenosine kinase (ATP:adenosine 5'-phosphotransferase, EC 2.7.1.20) into AICAR (ZMP), an intermediate of the de novo synthesis of purine nucleotides. We investigated if, in these cells, a cycle analogous to the adenosine-AMP substrate cycle operates between AICAriboside and ZMP. When 50 microM ITu, an inhibitor of adenosine kinase, was added to hepatocytes that had metabolized AICAriboside for 30 min, the concentration of ZMP decreased immediately. This was mirrored by a reincrease of AICAriboside. Rates of the ITu-induced decrease of ZMP and the increase of AICAriboside, calculated at different concentrations of ZMP, were first order, up to the highest concentration of ZMP (approx. 5 mumol/g of cells). Dephosphorylation of ZMP added to crude cytosolic extracts of rat liver displayed hyperbolic kinetics, with a Vmax of 0.65 mumol/min per g protein and an apparent Km of 5 mM, and was markedly inhibited by Pi, an inhibitor of IMP-GMP 5'-nucleotidase (5'-ribonucleotide phosphohydrolase, EC 3.1.3.5). We conclude that hepatocyte ZMP is continuously dephosphorylated, most likely by IMP-GMP 5'-nucleotidase, into AICAriboside, which is rephosphorylated into ZMP by adenosine kinase. Substrate cycling was also shown to occur between other nucleoside analogs and their phosphorylated counterparts.

Mammalian deoxyribonucleoside kinases

The mammalian deoxyribonucleoside kinases are deoxycytidine kinase, thymidine kinase 1 and 2 and deoxyguanosine kinase. These enzymes phosphorylate deoxyribonucleosides and thereby provide an alternative to de novo synthesis of DNA precursors. Their activities are essential for the activation of several chemotherapeutically important nucleoside analogues. In recent years, these enzymes have been thoroughly characterised with regard to structure, substrate specificity and patterns of expression. In this review, these results are reviewed and furthermore, the physiologic metabolic role of the anabolic enzymes is discussed in relation to catabolic pathways. The significance of this information for the development of therapeutic protocols and choice of animal model systems is discussed. Finally, alternative pathways for nucleoside analogue phosphorylation are surveyed, such as the phosphotransfer capacity of 5'-nucleotidase.

Dynamics of the dATP pool in cultured mammalian cells

Conditions for labeling the dATP pool of V79 and 3T3 cells from [3H]deoxyadenosine (salvage) or [3H]adenine (via ribonucleotide reduction) were established. With deoxyadenosine the specific radioactivity of dATP reached a constant value after 60 min. In resting 3T3 cells this value was 30 times higher than in S-phase cells. Turnover of dATP and absolute rates of DNA synthesis and excretion of breakdown products of dATP were determined from the accumulation of isotope in various compartments and the specific activity of dATP. In S-phase cells the dATP pool had a half-life of 4 min, identical to that of dTTP determined earlier. Deoxyadenosine was the major breakdown product of dATP in the presence of an inhibitor of adenosine deaminase. The rate of deoxyadenosine excretion of V79 cells amounted to 4% of the rate of dATP incorporation into DNA. Inhibition of DNA replication increased deoxyadenosine excretion 5- to 10-fold, demonstrating a continued de novo synthesis of dATP, albeit at a slightly reduced rate. Our results fit a model involving a substrate cycle between dAMP and deoxyadenosine regulating the dATP pool, similar to the model of substrate cycles involved in the regulation of pyrimidine deoxyribonucleotide pools developed earlier.

Inhibition of RNA- and DNA-synthesis by citrinin and its effects on DNA precursor-metabolism in V79-E cells

1. The RNA synthesis of V79-E cells was inhibited by the mycotoxin citrinin time- and concentration-dependently. 2. Among the different RNA species mainly the rRNA synthesis was found to be inhibited by 200 microM citrinin. 3. At different precursor concentrations DNA synthesis was inhibited by citrinin after 30 min at least whereas labelling of the acid soluble fractions was found to be 3-fold higher than in untreated cells. 4. Remarkable perturbation of the DNA precursor metabolism, including release of precursor into the medium, was found to occur during citrinin treatment.

Evidence for the involvement of endogenous thymidine in the density-inhibition of tumorigenic Chinese hamster V79 cells

Two distinct low-molecular-weight growth inhibitory activities were isolated from supernatants of a density-inhibited, tumorigenic V79 Chinese hamster cell line. By chromatographic analyses, one of these was purified to homogeneity and eventually proved to be thymidine (dThd). In order to investigate the biological role of dThd in a density-inhibited culture of these cells, a dThd-kinase deficient (TK-) clone resistant to the excess of dThd was isolated from V79 cells and the effect of the supernatants on growth of these TK- or TK-proficient (TK+) cells was examined. As a result, the growth of TK- cells was not inhibited but enhanced by the supernatant at the concentrations which significantly inhibited the growth of TK+ cells. Such TK-dependent differential responses to supernatants suggest the presence of deoxyribonucleosides including a high level of dThd in the supernatants. Since it is unlikely that dThd might derive from denatured DNA of dead cells, an accumulation of endogenous dThd in confluent culture appears to be responsible for dThd triphosphates which are synthesized de novo, degraded and excreted into the medium rather than incorporated into DNA as a consequence of aberrant growth in the presence of certain growth inhibitors produced by density-inhibited V79 cells.

In vitro effects of bryostatin 1 on the metabolism and cytotoxicity of 1-beta-D-arabinofuranosylcytosine in human leukemia cells

Bryostatin 1 is a macrocyclic lactone protein kinase C (PK-C) activator which has demonstrated promising antileukemic activity in preclinical studies. We have examined the effect of this agent on the metabolism and cytotoxicity of 1-beta-D-arabinofuranosylcytosine (ara-C) in both log phase and high-density human promyelocytic leukemia cells (HL-60). Exposure of low-density cells to 12.5 nM bryostatin 1 for 24 hr prior to a 4-hr incubation with 1 or 10 microM ara-C resulted in nearly a 2-fold increase in ara-CTP formation. When cells were maintained under high-cell density conditions (e.g. 5 x 10(6) cells/mL) for 24 hr prior to ara-C exposure, a 90% reduction in ara-CTP formation and ara-C DNA incorporation was observed. However, coincubation of high-density cells with bryostatin 1 for 24 hr increased ara-CTP formation 6- to 8-fold, yielding levels essentially equivalent to those achieved in low-density cells. Smaller (but still significant) increases in ara-C DNA incorporation were also noted. Enhancement of ara-CTP formation by bryostatin 1 occurred over a broad ara-C concentration range (0.1 to 100 microM), involved a temperature-dependent process, could not be mimicked by addition of hematopoietic growth factors, and was not related to neutralization of toxic or inhibitory substances in high-density medium. Exposure of cells to bryostatin 1 did not lead to morphologic or functional evidence of HL-60 cell maturation or an increase in cell viability, but did produce a decline in cellular proliferative activity as determined by thymidine and bromodeoxyuridine incorporation and cytofluorometric analysis. Bryostatin 1 did not exert its effects in high-density cells by inhibiting ara-C deamination or by interfering with ara-CTP dephosphorylation, but instead appeared to act by enhancing ara-C phosphorylation. Although cell-free extracts obtained from high-density cells exposed to bryostatin 1 exhibited levels of deoxycytidine kinase activity compared to controls, treated cells did display a significant decline in intracellular dCTP levels (e.g. 0.7 vs 1.3 pmol/10(6)), and nearly a 2-fold increase in ATP and UTP concentrations. Ara-CTP formation was also increased substantially by other PK-C activators including phorbol dibutyrate and mezerein (10-100 nM); this process was inhibited more than 70% by the PK-C inhibitor H-7 (50 microM), but not by the PK-C inhibitors staurosporine, tamoxifen, and HA1004. Finally, coadministration of ara-C and bryostatin 1 resulted in greater than expected inhibitory effects toward HL-60 cell clonogenic growth.(ABSTRACT TRUNCATED AT 400 WORDS)

Tissue-specific expansion of uridine pools in mice. Effects of benzylacyclouridine, dipyridamole and exogenous uridine

The concentration of uridine (Urd) in murine tissues appears to be controlled by Urd catabolism, concentrative Urd transport, and the non-concentrative, facilitated diffusion of Urd. Previous reports document the tissue-specific disruption of these processes, and subsequently intracellular pools of free Urd in mice, by the administration of exogenous Urd (250 mg/kg) or the Urd phosphorylase (EC 2.4.2.3; uracil:ribose-1-phosphate phosphotransferase) inhibitor 5-benzylacyclouridine (BAU) (240 mg/kg). We now report the effect of combinations of BAU (120 mg/kg, p.o.), the nucleoside transport inhibitor dipyridamole (DP) (25 mg/kg, i.p.), and exogenous Urd (250 mg/kg, i.v.) on Urd pools in mice. This dose of BAU increased Urd pools 2- to 6-fold, in a tissue-specific manner, for up to 5 hr. DP increased Urd pools 3-fold in spleen, over a 4-hr period, but did not affect other tissues. Administration of BAU 1 hr prior to exogenous Urd resulted in a 50- to 100-fold expansion of tissue normal after 6 hr. Administration of DP 1 hr prior to exogenous Urd caused a tissue-specific 40- to 100-fold increase in Urd pools which, except in spleen, returned to normal within 2 hr. The marked additive effects of these combinations were in contrast to those obtained following the administration of BAU 1 hr prior to DP. This regimen increased Urd pools from 4- to 9-fold, in a tissue-specific manner. In addition, Urd pools remained elevated for up to 9 hr, except in spleen where the Urd concentration was elevated for up to 15 hr. Analysis of enzyme activities indicated that DP does not enhance the inhibitory effect of BAU against murine liver Urd phosphorylase. However, DP did inhibit plasma clearance of BAU, and this effect may partially explain the apparent synergistic effect of this combination. In spite of the prolonged and dramatic expansion of tissue Urd pools produced by BAU + DP, the total Ura nucleotide content in spleen, gut and colon tumor 38 (CT38) increased by less than 70% over a 12-hr period following administration of this combination. These findings are discussed in light of their biochemical and therapeutic implications.

Thymidine secretion by cultured chicken embryo fibroblasts and NIH/3T3 cells: quantification and time course

Cultured chicken embryo fibroblasts (CEF) as well as mouse NIH/3T3 fibroblasts secrete substantial amounts of thymidine into the medium. The rate of secretion is maximal when the cells become confluent. In the medium of confluent primary cultures of CEF up to 10 microM thymidine was measured. Lower concentrations (1 to 2 microM) were found in media of NIH/3T3 cultures. In media conditioned by NIH/3T3 cells, the degradation product thymine was also found, which was not detectable in the sera and the media of CEF. Moderate inhibitory effects on the growth of CEF and NIH/3T3 cells (at least 10% with a relatively large range of variation) indicated that thymidine (2 to 5 microM) might contribute to cellular homeostasis. The secretion of high amounts of endogenous thymidine into cell culture media may falsify the assessment of DNA synthesis with labeled thymidine; thus a change of the medium is recommended when performing a thymidine pulse.

Deoxyadenosine reverses hydroxyurea inhibition of vaccinia virus growth

Hydroxyurea, an inhibitor of ribonucleotide reductase, blocks replication of vaccinia virus. However, when medium containing hydroxyurea and dialyzed serum was supplemented with deoxyadenosine, the block to viral reproduction was circumvented, provided that an inhibitor of adenosine deaminase was also present. Deoxyguanosine, deoxycytidine, and deoxythymidine were ineffective alone and did not augment the deoxyadenosine effect. In fact, increasing concentrations of deoxyguanosine and deoxythymidine, but not deoxycytidine, eliminated the deoxyadenosine rescue, an effect that was reversed by the addition of low concentrations of deoxycytidine. These results suggested that the inhibition of viral replication by hydroxyurea was primarily due to a deficiency of dATP. Deoxyribonucleoside triphosphate pools in vaccinia virus-infected cells were measured at the height of viral DNA synthesis after a synchronous infection. With 0.5 mM hydroxyurea, the dATP pool was greater than 90% depleted, the dCTP and dGTP pools were 40 to 50% reduced, and the dTTP pool was increased. Assay of ribonucleotide reductase activity in intact virus-infected cells suggested that hydroxyurea may differentially affect reduction of the various substrates of the enzyme.

Deoxyribonucleotide metabolism in hydroxyurea-resistant V79 hamster cells

V79 hamster cells were made resistant against hydroxyurea by continuous culture at stepwise increasing drug concentrations. Two cell lines were cloned, resistant to 0.4 mM (V79/H0.4) and 4 mM (V79/H4) hydroxyurea, with a fivefold and a 20-fold increase in soluble ribonucleotide reductase activity. We investigated how the increased amount of enzyme affected the in situ activity of ribonucleotide reductase and deoxyribonucleotide metabolism, in particular substrate cycles between pyrimidine deoxyribonucleosides and their 5'-phosphates. The in situ activity of the reductase was only moderately elevated (1.3-fold in V79/H4 cells). In the fully resistant line, the steady-state level of dATP was increased fourfold, and that of dTTP twofold. These nucleotides are negative allosteric effectors of the reductase and we propose that the increased pools inhibit the enzyme and thereby maintain the in situ activity of the reductase at only a slightly increased level. The surplus deoxyribonucleotides was excreted from the cells as thymidine and deoxycytidine via substrate cycles. The data support and extend our previous model for the regulation of deoxyribonucleotide synthesis via the allosteric properties of ribonucleotide reductase and substrate cycles that link salvage and degradation of deoxyribonucleotides.

Lack of correlation between thymidine kinase activity and changes of DNA synthesis with tumour age: an in vivo study in Ehrlich ascites tumour

Thymidine kinase (TK) and its isoenzymes were studied in relation to age of Ehrlich ascites tumour cells growing in vivo. Various steps of the pathway of thymidine through deoxynucleotide metabolism were studied: [3H]-thymidine cellular uptake and incorporation into DNA; the cellular nucleotide pools; and the concentration of thymidine in ascites. In addition, the proportion of cells in the various parts of the cell cycle and the bromodeoxyuridine labelling index were determined. Four isoenzymes at pI 4.1, 5.3, 6.9 and 8.3 were identified using isoelectric focusing. The TK activity declined with age of the tumour by about 90%, mostly due to a decrease of the isoenzyme at pI 8.3. However, this decline was neither related to the changes in DNA synthesis rate of the cells with tumour age, nor to the proportion of cells in S-phase or the bromodeoxyuridine (BrdU) labelling index. In contrast, the contribution of DNA synthesis via the thymidine salvage pathway relative to the total DNA synthesis increased from less than 1% at exponential growth to about 15% at plateau phase of growth. Blocking of DNA synthesis by aphidicolin did not change the TK activity. We therefore conclude that changes in TK activity and changes in cell growth are epiphenomena rather than causally related to each other. All nucleotide pools decreased with tumour age. The inhibition of TK by an increase in the deoxythymidine triphosphate pool could therefore be excluded. With a decrease of the TK activity during tumour growth, increasing amounts of TdR were excreted by the cells and accumulated in the ascites fluid. To explain our results on TK activity we propose a substrate cycle in which thymidine monophosphate supplied by de novo synthesis is dephosphorylated and is then either phosphorylated by TK to thymidine monophosphate or excreted by the cell.

Nucleotidase activities in soluble and membrane fractions of three different mammalian cell lines

Soluble cytoplasmic and membrane fractions were prepared from three cultured mammalian cell lines: 3T3 mouse fibroblasts, V79 hamster lung cells, and human "Cherry" B-lymphoblastoid cells. By using relatively specific nucleotidase assays, together with a phosphotransferase assay, the activities of three different enzymes (low-Km nucleotidase, high-Km nucleotidase, and 5'(3')-nucleotidase) capable of dephosphorylating deoxyribonucleoside 5'-monophosphates were determined in these fractions. The three nucleotidases exist simultaneously in all cell lines, but their relative amounts showed large variations. The 5'(3')-nucleotidase dominated Cherry and 3T3 cells, while in V79 cells equal amounts of this enzyme and the high-Km nucleotidase were recovered. In the membrane fractions, the low-Km nucleotidase was the predominant enzyme. We found no evidence for cell-cycle control of any nucleotidase. We postulated earlier that substrate cycles, involving 5'-nucleotidases and deoxyribonucleoside kinases, provide a mechanism for the regulation of deoxyribonucleotide pools. We suggest that both the low-Km nucleotidase and the 5'(3)-nucleotidase are candidate enzymes for such cycles.

The effects of folate deficiency on thymidylate synthetase activity, deoxyuridine suppression, cell size and doubling time in a cultured human myeloid cell line

To help understand the pathogenesis of megaloblastic anaemia, we have studied folate-deprived HL60 cells. Cells grown with no added folic acid developed macrocytosis, a prolonged doubling time, a grossly increased deoxyuridine-suppressed value, and markedly reduced thymidylate synthetase activity. Cells in medium containing 50 nmol/l added folic acid became macrocytic with similar biochemical changes, but their doubling time was only marginally prolonged. In 100 nmol/l added folic acid, there was slight macrocytosis and a normal doubling time, but marked biochemical alterations were still present. The findings demonstrate that folate deficiency causes diminished thymidylate synthetase activity and macrocytosis in human myeloid cells, but that such cells may nevertheless demonstrate no prolongation of their doubling time, indicating either that their supply of thymidine triphosphate (TTP) is sufficient, or that misincorporation of deoxyuridine triphosphate (dUTP) is occurring. This suggests that the ineffectiveness of haemopoiesis in folate deficiency may result from damage to bone marrow cells in some way other than arrest in S-phase by a reduced delivery of TTP to the DNA replication fork.

Effects of cytosine arabinoside and hydroxyurea on the synthesis of deoxyribonucleotides and DNA replication in L1210 cells

Experiments were carried out in L1210 cells to examine the importance of 'substrate cycles' in regulating the intracellular levels of deoxyribonucleoside 5'-triphosphate. L1210 cells were incubated with [14C]cytidine or [14C]adenosine in the presence and absence of hydroxyurea or cytosine arabinoside (araC). These incubations were carried out for either 30 or 120 min. Inhibition of ribonucleotide reductase by hydroxyurea resulted in the blockage of the flux of ribonucleotides to deoxyribonucleotides (greater than 90%) as expected. When DNA synthesis was inhibited with araC, there was a marked decrease in the incorporation of [14C]cytidine or [14C]adenosine into DNA as deoxyribonucleotides. However, there was not a corresponding increase in the deoxyribonucleotide levels in the acid-soluble fraction or deoxyribonucleosides in the culture medium. AraC treatment decreased the total formation of deoxyribonucleotides. These data indicate that L1210 cells do not regulate the intracellular pools of dNTPs via 'substrate cycles' which involve activation of phosphatases when DNA synthesis is blocked or activation of kinases when ribonucleotide reductase is inhibited.

Hydroxyurea increases the phosphorylation of 3'-fluorothymidine and 3'-azidothymidine in CEM cells

The triphosphates of the nucleoside analogues 3'-azidothymidine and 3'-fluorothymidine inhibit reverse transcriptase and are of therapeutic interest for the treatment of retrovirus infections. At equimolar concentrations 3'-fluorothymidine was more effectively transformed to the triphosphate by human CEM cells than azidothymidine which mainly accumulates as the monophosphate. Hydroxyurea, a drug that inhibits de novo synthesis of deoxyribonucleotides, considerably increased the ability of cells to phosphorylate both analogues. Addition of as little as 50 microM hydroxyurea decreased the amount of dideoxynucleoside required to attain a given intracellular concentration of its triphosphate by an order of magnitude. Hydroxyurea is known to shift the balance of substrate cycles between natural deoxynucleosides and their 5'-phosphates in the direction of synthesis and thereby to increase the import and intracellular phosphorylation of the nucleoside. The present results demonstrate a similar effect for the two analogues and raise the possibility of using this effect in therapy.

Deoxyribonucleoside triphosphate pools in regenerating rat liver: effect of hydroxyurea and exogenous deoxypyrimidines

Hydroxyurea (HU) causes inhibition of DNA synthesis in regenerating rat liver due to an inhibition of the ribonucleotide reductase. We studied the consequences of a continuous HU infusion for deoxyribonucleoside triphosphate (dNTP) pools in the liver after partial hepatectomy and tried to modify imbalances by application of deoxyribonucleosides in vivo. In normal liver, an intracellular concentration of 0.16, 0.84, 0.33 and 0.27 pmol/micrograms DNA was observed for dATP, dCTP, dGTP and dTTP, respectively. In regenerating liver the dNTP pools show minor changes until 18 h after partial hepatectomy. During and after a continuous HU infusion 14--24 h after partial hepatectomy, the intracellular dNTP pools change considerably. At 19.5 h after partial hepatectomy, 5.5 h after the start of HU infusion, and at 25 h after partial hepatectomy, 1 h after termination of HU infusion, the dTTP pool was more than 10-times, and the dGTP pool about 2-times higher than in controls, while the dATP and dCTP pools remain relatively unchanged. Simultaneous infusion of HU and deoxythymidine (dThd) 14--25 h after partial hepatectomy results in a further increase of the dTTP pool during and after HU infusion. Administration of deoxycytidine (dCyd) leads to a moderate increase of the dCTP pool and a weak decrease of the dTTP pool during HU infusion. The combined application of dCyd and dThd after HU infusion had similar effects on dNTP pools as observed with dThd alone. These results show that intracellular pools of dNTPs in hepatocytes can be altered by exogenous factors in a controlled pattern. This system can be used as a model for studying the implications of induced dNTP pool dysbalances for the initiation of liver carcinogenesis by mutagenic chemicals.

Uridine-5'-phosphate synthase: evidence for substrate cycling involving this bifunctional protein

Uridine 5'-phosphate (UMP) synthase contains two sequential catalytic activities for the synthesis of orotidine 5'-phosphate (OMP) from orotate (EC 2.4.2.10, orotate phosphoribosyltransferase) and the decarboxylation of OMP to form UMP (EC 4.1.1.23, OMP decarboxylase). Previous kinetic studies had indicated that partial channeling of OMP might occur [T.W. Traut and M.E. Jones (1977) J. Biol. Chem. 252, 8374-8381]; in the presence of a nucleotidase, there was no measurable formation of orotidine from OMP under conditions where OMP was maintained at a steady-state concentration [T.W. Traut (1980) Arch. Biochem. Biophys. 200, 590-594]. Recently claims were made that (i) the steady-state activities of UMP synthase could be modeled by Michaelis-Menten kinetics, and (ii) the nucleotidase activity in Ehrlich ascites cells was insufficient to degrade any significant amount of OMP [R.W. McClard and K.M. Shokat (1987) Biochemistry 26, 3378-3384]. The present studies show that UMP synthase has cooperative kinetics toward OMP, and that a substrate cycle involving orotate phosphoribosyltransferase, cytoplasmic nucleotidase, and uridine phosphorylase maintains the cyclic interconversion: orotate----OMP----orotidine----orotate, etc. It is therefore the complex steady-state kinetics of UMP synthase in the presence of OMP, and the existence of a substrate cycle that account for the results which were interpreted as channeling in the earlier studies.

Involvement of DNA polymerase delta in DNA repair synthesis in human fibroblasts at late times after ultraviolet irradiation

DNA repair synthesis following UV irradiation of confluent human fibroblasts has a biphasic time course with an early phase of rapid nucleotide incorporation and a late phase of much slower nucleotide incorporation. The biphasic nature of this curve suggests that two distinct DNA repair systems may be operative. Previous studies have specifically implicated DNA polymerase delta as the enzyme involved in DNA repair synthesis occurring immediately after UV damage. In this paper, we describe studies of DNA polymerase involvement in DNA repair synthesis in confluent human fibroblasts at late times after UV irradiation. Late UV-induced DNA repair synthesis in both intact and permeable cells was found to be inhibited by aphidicolin, indicating the involvement of one of the aphidicolin-sensitive DNA polymerases, alpha or delta. In permeable cells, the process was further analyzed by using the nucleotide analogue (butylphenyl)-2'-deoxyguanosine 5'-triphosphate, which inhibits DNA polymerase alpha several hundred times more strongly than it inhibits DNA polymerase delta. The (butylphenyl)-2'-deoxyguanosine 5'-triphosphate inhibition curve for late UV-induced repair synthesis was very similar to that for polymerase delta. It appears that repair synthesis at late times after UV irradiation, like repair synthesis at early times, is mediated by DNA polymerase delta.

Effects of 3'-azido-3'-deoxythymidine on the deoxynucleotide triphosphate pools of cultured human cells

The effects of 3'-azido-3'-deoxythymidine (AZT) on the deoxynucleotide pools of three human cell lines, HL-60, H-9, and K-562, were determined. The corresponding ED50s for inhibition of cell growth were 670, 100, and 100 microM AZT. In all three lines, exposure to 200 microM AZT caused dTTP and dGTP initially to fall and then to return towards control levels. In contrast to a previous report [Furman et al., (1986) Proc. Nat. Acad. Sci. USA 83, 8333-8337], dCTP levels increased. Pools of dATP were relatively unchanged. Qualitatively similar changes occurred in 10 microM AZT, but recovery was faster than at 200 microM AZT. After 24 hrs incubation with 200 microM AZT, AZT-5'-MP reached 2.8, 4.7, and 15.7 mM in the HL-60, H-9, and K-562 cells, respectively. When HL-60 and K-562 cells incubated in AZT were resuspended in fresh medium, AZT-5'-MP pools declined with respective t1/2 values equal to 34 and 68 min. The concentration of thymidine, and to a lesser extent deoxyuridine, increased in the media of treated cells. AZT-5'-MP was found in the media of cells treated with AZT.

Hydroxyurea: effects on deoxyribonucleotide pool sizes correlated with effects on DNA repair in mammalian cells

We have measured deoxyribonucleotide pool sizes in different cell types: normal human, transformed human (HeLa), and the permanent hamster line CHO-K1. The range of sizes of the four DNA precursor pools in CHO cells is far greater than in human cells. It is a general rule that hydroxyurea causes rapid depletion of pools (except for dTTP) until the pool present in smallest amount is exhausted; this suggests a tight coupling of the pools to DNA replication (the presumed main cause of the depletion). The effect of hydroxyurea on DNA repair after ultraviolet irradiation (namely, a relatively small accumulation of incomplete repair sites blocked at the resynthesis stage) is probably accounted for by the reduced availability of DNA precursors. However, depletion of the dCTP pool is not an adequate explanation for the observed enhancement by hydroxyurea of the inhibitory effect of cytosine arabinoside; we suggest other possible modes of action. Ultraviolet irradiation has only small effects on the levels of deoxyribonucleotides.

Preferential utilisation of deoxycytidine by undifferentiated (peanut positive) tonsillar lymphocytes

Peanut agglutinin (PNA), a D-galactose specific lectin, agglutinated 10-18% of lymphocytes isolated from tonsils of 3- to 6-yr-old children. PNA+ cells were found to be mainly B lymphocytes showing a 9.6 times higher specific activity of DNA polymerase alpha compared to the PNA- cells. The specific activity of deoxycytidine kinase as well as the incorporation of [5-3H]deoxycytidine were also much higher in PNA+ cell fraction than in PNA- fraction (7.7-fold and 6-fold, respectively). On the other hand, thymidine kinase activity and [5-3H]deoxythymidine incorporation were only 3.6 and 3.9 times higher, respectively. The data presented here show a high degree of DNA synthesis and preferential utilisation of [5-3H]deoxycytidine for DNA synthesis in this undifferentiated B lymphocyte fraction.

Metabolism of pyrimidine bases and nucleosides by pyrimidine-nucleoside phosphorylases in cultured human lymphoid cells

The anabolism of pyrimidine ribo- and deoxyribonucleosides from uracil and thymine was investigated in phytohemagglutinin-stimulated human peripheral blood lymphocytes and in a Burkitt's lymphoma-derived cell line (Raji). We studied the ability of these cells to synthesize pyrimidine nucleosides by ribo- and deoxyribosyl transfer between pyrimidine bases or nucleosides and the purine nucleosides inosine and deoxyinosine as donors of ribose 1-phosphate and deoxyribose 1-phosphate, respectively: these reactions involve the activities of purine-nucleoside phosphorylase, and of the two pyrimidine-nucleoside phosphorylases (uridine phosphorylase and thymidine phosphorylase). The ability of the cells to synthesize uridine was estimated from their ability to grow on uridine precursors in the presence of an inhibitor of pyrimidine de novo synthesis (pyrazofurin). Their ability to synthesize thymidine and deoxyuridine was estimated from the inhibition of the incorporation of radiolabelled thymidine in cells cultured in the presence of unlabelled precursors. In addition to these studies on intact cells, we determined the activities of purine- and pyrimidine-nucleoside phosphorylases in cell extracts. Our results show that Raji cells efficiently metabolize preformed uridine, deoxyuridine and thymidine, are unable to salvage pyrimidine bases, and possess a low uridine phosphorylase activity and markedly decreased (about 1% of peripheral blood lymphocytes) thymidine phosphorylase activity. Lymphocytes have higher pyrimidine-nucleoside phosphorylases activities, they can synthesize deoxyuridine and thymidine from bases, but at high an non-physiological concentrations of precursors. Neither type of cell is able to salvage uracil into uridine. These results suggest that pyrimidine-nucleoside phosphorylases have a catabolic, rather than an anabolic, role in human lymphoid cells. The facts that, compared to peripheral blood lymphocytes, lymphoblasts possess decreased pyrimidine-nucleoside phosphorylases activities, and, on the other hand, more efficiently salvage pyrimidine nucleosides, are consistent with a greater need of these rapidly proliferating cells for pyrimidine nucleotides.

Unresolved issues in the study of mammalian ribonucleotide reductase

Although research on mammalian ribonucleotide reductase and its role in cell replication has been intensified in recent years, there remain several areas in which there is not uniform agreement with respect to several of its important properties. The major issues include: 1) whether there is one enzyme which catalyzes the reduction of all four substrates or several enzymes for the four substrates; 2) whether the two subunits are coordinately regulated as the cells pass from G1 to S during the cell cycle; if not which subunit represents the limiting component and what are the respective half-lives of the individual subunits; 3) whether the allosteric regulation which has been demonstrated in the test tube is the actual mechanism in the intact cells; and 4) is mammalian ribonucleotide reductase part of an enzyme complex which channels ribonucleoside diphosphates into DNA. The data which have appeared in the literature are discussed in the context of these unresolved questions.

Eukaryotic DNA metabolism. Are deoxyribonucleotides channeled to replication sites?

DNA precursor biosynthesis is closely coordinated with DNA replication itself. In prokaryotic systems, firm evidence supports the idea that this coordination is achieved through the action of multienzyme complexes that physically link the synthesis of deoxyribonucleotides with their utilization in DNA replication. Much evidence favors a similar channeling mechanism in eukaryotes. However, recent studies suggest strongly that in mammalian cells DNA precursors are synthesized in cytoplasm and are then transported into the nucleus. This article reviews the pertinent evidence, attempts to reconcile contradictory findings, and highlights areas that need further investigation.

Activity of deoxyribonucleoside-activated nucleotidase in cells from various lymphoid mouse tissues and in concanavalin A-stimulated lymphocytes

The specific activity in cells from lymph nodes, spleen and thymus was 32, 28 and 25 nmol/min per mg of cytosol protein, respectively, whereas that in bone marrow cells was significantly lower (10 units/mg of protein). No difference in specific DAN activity between isolated B- and T-lymphocytes was observed. Two types of lymphoid mouse cell lines (MOPC-31C plasmacytoma cells, S49 Cyc- lymphoma cells) showed specific activities similar to the normal lymphoid cells. In concanavalin A- stimulated spleen lymphocytes in culture there was a rapid increase in DAN activity shortly after maximum DNA synthesis, reaching a plateau 2-3 times the original level. The enzyme (DAN) of mouse tissues possessed the characteristic properties previously detected for the rat enzyme.