Deoxyguanosine toxicity in a mouse T lymphoma: relationship to purine nucleoside phosphorylase-associated immune dysfunction

Targeting host deoxycytidine kinase mitigates Staphylococcus aureus abscess formation

Host-directed therapy (HDT) is an emerging approach to overcome antimicrobial resistance in pathogenic microorganisms. Specifically, HDT targets host-encoded factors required for pathogen replication and survival without interfering with microbial growth or metabolism, thereby eliminating the risk of resistance development. By applying HDT and a drug repurposing approach, we demonstrate that (R)-DI-87, a clinical-stage anticancer drug and potent inhibitor of mammalian deoxycytidine kinase (dCK), mitigates Staphylococcus aureus abscess formation in organ tissues upon invasive bloodstream infection. Mechanistically, (R)-DI-87 shields phagocytes from staphylococcal death-effector deoxyribonucleosides that target dCK and the mammalian purine salvage pathway-apoptosis axis. In this manner, (R)-DI-87-mediated protection of immune cells amplifies macrophage infiltration into deep-seated abscesses, a phenomenon coupled with enhanced pathogen control, ameliorated immunopathology, and reduced disease severity. Thus, pharmaceutical blockade of dCK represents an advanced anti-infective intervention strategy against which staphylococci cannot develop resistance and may help to fight fatal infectious diseases in hospitalized patients.

Targeting host deoxycytidine kinase mitigates Staphylococcus aureus abscess formation

Host-directed therapy (HDT) is an emerging approach to overcome antimicrobial resistance in pathogenic microorganisms. Specifically, HDT targets host-encoded factors required for pathogen replication and survival without interfering with microbial growth or metabolism, thereby eliminating the risk of resistance development. By applying HDT and a drug repurposing approach, we demonstrate that (R)-DI-87, a clinical-stage anti-cancer drug and potent inhibitor of mammalian deoxycytidine kinase (dCK), mitigates Staphylococcus aureus abscess formation in organ tissues upon invasive bloodstream infection. Mechanistically, (R)-DI-87 shields phagocytes from staphylococcal death-effector deoxyribonucleosides that target dCK and the mammalian purine salvage pathway-apoptosis axis. In this manner, (R)-DI-87-mediated protection of immune cells amplifies macrophage infiltration into deep-seated abscesses, a phenomenon coupled with enhanced pathogen control, ameliorated immunopathology, and reduced disease severity. Thus, pharmaceutical blockade of dCK represents an advanced anti-infective intervention strategy against which staphylococci cannot develop resistance and may help to fight fatal infectious diseases in hospitalized patients.

Nucleotide imbalance decouples cell growth from cell proliferation

Nucleotide metabolism supports RNA synthesis and DNA replication to enable cell growth and division. Nucleotide depletion can inhibit cell growth and proliferation, but how cells sense and respond to changes in the relative levels of individual nucleotides is unclear. Moreover, the nucleotide requirement for biomass production changes over the course of the cell cycle, and how cells coordinate differential nucleotide demands with cell cycle progression is not well understood. Here we find that excess levels of individual nucleotides can inhibit proliferation by disrupting the relative levels of nucleotide bases needed for DNA replication and impeding DNA replication. The resulting purine and pyrimidine imbalances are not sensed by canonical growth regulatory pathways like mTORC1, Akt and AMPK signalling cascades, causing excessive cell growth despite inhibited proliferation. Instead, cells rely on replication stress signalling to survive during, and recover from, nucleotide imbalance during S phase. We find that ATR-dependent replication stress signalling is activated during unperturbed S phases and promotes nucleotide availability to support DNA replication. Together, these data reveal that imbalanced nucleotide levels are not detected until S phase, rendering cells reliant on replication stress signalling to cope with this metabolic problem and disrupting the coordination of cell growth and division.

SAMHD1 Limits the Efficacy of Forodesine in Leukemia by Protecting Cells against the Cytotoxicity of dGTP

The anti-leukemia agent forodesine causes cytotoxic overload of intracellular deoxyguanosine triphosphate (dGTP) but is efficacious only in a subset of patients. We report that SAMHD1, a phosphohydrolase degrading deoxyribonucleoside triphosphate (dNTP), protects cells against the effects of dNTP imbalances. SAMHD1-deficient cells induce intrinsic apoptosis upon provision of deoxyribonucleosides, particularly deoxyguanosine (dG). Moreover, dG and forodesine act synergistically to kill cells lacking SAMHD1. Using mass cytometry, we find that these compounds kill SAMHD1-deficient malignant cells in patients with chronic lymphocytic leukemia (CLL). Normal cells and CLL cells from patients without SAMHD1 mutation are unaffected. We therefore propose to use forodesine as a precision medicine for leukemia, stratifying patients by SAMHD1 genotype or expression.

The Mechanism of the Selective Antiproliferation Effect of Guanine-Based Biomolecules and Its Compensation

As endogenous biomolecules, guanine, guanine-based nucleosides, and nucleotides are essential for cellular DNA/RNA synthesis, energy metabolism, and signal transduction. However, these biomolecules have been found to have a cell-specific antiproliferation effect at higher concentrations, and the mechanism is unclear. In this study, we demonstrate that guanine deaminase (GDA) is a major factor in determining the cell-type selectivity to the antiproliferation effect of guanine-based biomolecules. GDA catalyzes the deamination of guanine to xanthine, which is an essential part of the guanine degradation pathway. GDA deficient cells could not efficiently remove the excess guanine-based biomolecules. These excess molecules disturb the metabolism of adenine-, cytosine-, and thymine-based nucleotides; subsequently inhibit the DNA synthesis and cell growth; and eventually result in the apoptosis/death of GDA deficient cells. The inhibition of DNA synthesis could be relieved by simultaneous addition of adenine- and cytosine-based nucleosides, and the inhibited DNA synthesis could be restarted by post addition of them, which subsequently reduces the antiproliferation effect of guanine-based biomolecules or even totally restores the cell proliferation. These results provide important information for the development of guanine-based drugs or guanine-rich oligonucleotide drugs, as well as for the safety evaluation of food with a high level of guanine-based compounds.

Effects of silver nanoparticles and ions on a co-culture model for the gastrointestinal epithelium

Background

The increased incorporation of silver nanoparticles (Ag NPs) into consumer products makes the characterization of potential risk for humans and other organisms essential. The oral route is an important uptake route for NPs, therefore the study of the gastrointestinal tract in respect to NP uptake and toxicity is very timely. The aim of the present study was to evaluate the effects of Ag NPs and ions on a Caco-2/TC7:HT29-MTX intestinal co-culture model with mucus secretion, which constitutes an important protective barrier to exogenous agents in vivo and may strongly influence particle uptake.

Methods

The presence of the mucus layer was confirmed with staining techniques (alcian blue and toluidine blue). Mono and co-cultures of Caco-2/TC7 and HT29-MTX cells were exposed to Ag NPs (Ag 20 and 200 nm) and AgNO₃ and viability (alamar blue), ROS induction (DCFH-DA assay) and IL-8 release (ELISA) were measured. The particle agglomeration in the media was evaluated with DLS and the ion release with ultrafiltration and ICP-MS. The effects of the Ag NPs and AgNO₃ on cells in co-culture were studied at a proteome level with two-dimensional difference in gel electrophoresis (2D-DIGE) followed by Matrix Assisted Laser Desorption Ionization - Time Of Flight/ Time Of Flight (MALDI-TOF/TOF) mass spectrometry (MS). Intracellular localization was assessed with NanoSIMS and TEM.

Results

The presence of mucus layer led to protection against ROS and decrease in IL-8 release. Both Ag 20 and 200 nm NPs were taken up by the cells and Ag NPs 20 nm were mainly localized in organelles with high sulfur content. A dose- and size-dependent increase in IL-8 release was observed with a lack of cytotoxicity and oxidative stress. Sixty one differentially abundant proteins were identified involved in cytoskeleton arrangement and cell cycle, oxidative stress, apoptosis, metabolism/detoxification and stress.

Conclusions

The presence of mucus layer had an impact on modulating the induced toxicity of NPs. NP-specific effects were observed for uptake, pro-inflammatory response and changes at the proteome level. The low level of overlap between differentially abundant proteins observed in both Ag NPs and AgNO₃ treated co-culture suggests size-dependent responses that cannot only be attributed to soluble Ag.

Electronic supplementary material

The online version of this article (doi:10.1186/s12989-016-0117-9) contains supplementary material, which is available to authorized users.

A carbon ion beam irradiated MWCNT/AuNPs composite sensor for a sensitive assay of purine-nucleosides of DNA

Sensor for purine nucleosides has been developed using irradiation with high energy carbon ion beam.

The discovery of first-in-class drugs: origins and evolution

Analysis of the origins of new drugs approved by the US Food and Drug Administration (FDA) from 1999 to 2008 suggested that phenotypic screening strategies had been more productive than target-based approaches in the discovery of first-in-class small-molecule drugs. However, given the relatively recent introduction of target-based approaches in the context of the long time frames of drug development, their full impact might not yet have become apparent. Here, we present an analysis of the origins of all 113 first-in-class drugs approved by the FDA from 1999 to 2013, which shows that the majority (78) were discovered through target-based approaches (45 small-molecule drugs and 33 biologics). In addition, of 33 drugs identified in the absence of a target hypothesis, 25 were found through a chemocentric approach in which compounds with known pharmacology served as the starting point, with only eight coming from what we define here as phenotypic screening: testing a large number of compounds in a target-agnostic assay that monitors phenotypic changes. We also discuss the implications for drug discovery strategies, including viewing phenotypic screening as a novel discipline rather than as a neoclassical approach.

Deoxyribonucleoside toxicity in adenosine deaminase and purine nucleoside phosphorylase deficiency: implications for the development of new immunosuppressive agents

The immunodeficient state associated with adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) deficiency may result from the selective phosphorylation by thymus-derived lymphocytes of the ADA substrate deoxyadenosine and the PNP substrate deoxyguanosine, leading to the intracellular trapping of toxic deoxyribonucleoside triphosphates. Agents such as deoxycytidine might be able to favourably modify the immunodeficient state by inhibiting deoxyribonucleoside phosphorylation. Deficiencies of other nucleotide catabolic enzymes, if selectively expressed by lymphocytes, might also lead to immunodeficiency via nucleoside trapping in lymphoid tissues. Purine deoxyribonucleoside analogues, either alone or in combination with ADA inhibitors, may have value as lymphospecific antimetabolites.

Clofarabine and nelarabine: two new purine nucleoside analogs

PURPOSE OF REVIEW

Both clofarabine and nelarabine recently received an accelerated approval by the US Food and Drug Administration for use in refractory or relapsed pediatric acute lymphoblastic leukemia and in refractory-relapsed T-cell acute lymphoblastic leukemia or T-cell lymphoblastic lymphoma. Both drugs have been studied for their metabolism and mechanisms of action in preclinical investigations and for their efficacy in clinical trials. This review will summarize these investigations and will suggest future plans with these nucleoside analogs.

RECENT FINDINGS

Clofarabine and nelarabine were designed based on preclinical and clinical findings with other nucleoside analogs or normal deoxynucleotides such as dGTP. Studies in cell lines have demonstrated that triphosphate is the active metabolite for both these purine nucleoside analogs. Pharmacokinetic and pharmacodynamic investigations during clinical trials have verified the importance of triphosphate levels in achieving clinical responses. Several phase I and II clinical explorations have suggested the utility of clofarabine in acute leukemias and nelarabine in T-cell diseases. Dose-limiting toxicities were nonhematologic: hepatotoxicity for clofarabine and neurotoxicity for nelarabine.

SUMMARY

Clofarabine is the first deoxyadenosine analog that shows promise in adult and pediatric acute leukemias without untoward toxicity. Nelarabine, as expected from its design, is a drug that may be directed to T-cell diseases.

Mechanisms for T-cell selective cytotoxicity of arabinosylguanine

Nelarabine, prodrug of arabinosylguanine (ara-G), has demonstrated T-lymphoblastic antileukemic activity in cell lines and in the clinic. To investigate the mechanism for lineage-specific toxicity, the effects of ara-G were compared in CEM (T-lymphoblast), Raji (B-lymphoblast), and ML-1 (myeloid) cell lines. CEM cells were the most sensitive to ara-G-induced apoptosis and accumulated the highest levels of ara-G triphosphate (ara-GTP). However, compared with myeloid and B-lineage cell lines, CEM cells incorporated fewer ara-G molecules-which were at internucleotide positions in all 3 cell lines- into DNA. Ara-G induced an S-phase arrest in both Raji and ML-1, while in CEM the S-phase cells decreased with a concomitant increase in the sub-G1 population. Within 3 hours of ara-G treatment, the levels of soluble Fas ligand (sFasL) in the medium increased significantly in CEM cultures. In parallel, an induction of FasL gene expression was observed by real-time reverse transcriptase-polymerase chain reaction (RT-PCR). Pretreatment of CEM cells with a Fas antagonistic antibody inhibited ara-G-mediated cell death. These results demonstrate that high ara-GTP accumulation in T cells results in an S phase-dependent apoptosis induced by ara-G incorporation into DNA, which may lead to a T cell-specific signal for the induction and liberation of sFasL. Subsequently, the sFasL induces an apoptotic response in neighboring non-S-phase cells. In contrast, myeloid and B cells accumulated lower levels of ara-GTP and arrested in S phase, blocking any apoptotic signaling.

Mitochondrial basis for immune deficiency. Evidence from purine nucleoside phosphorylase-deficient mice

We generated purine nucleoside phosphorylase (PNP)-deficient mice to gain insight into the mechanism of immune deficiency disease associated with PNP deficiency in humans. Similar to the human disease, PNP deficiency in mice causes an immunodeficiency that affects T lymphocytes more severely than B lymphocytes. PNP knockout mice exhibit impaired thymocyte differentiation, reduced mitogenic and allogeneic responses, and decreased numbers of maturing thymocytes and peripheral T cells. T lymphocytes of PNP-deficient mice exhibit increased apoptosis in vivo and higher sensitivity to gamma irradiation in vitro. We propose that the immune deficiency in PNP deficiency is a result of inhibition of mitochondrial DNA repair due to the accumulation of dGTP in the mitochondria. The end result is increased sensitivity of T cells to spontaneous mitochondrial DNA damage, leading to T cell depletion by apoptosis.

Synthesis of a novel C-nucleoside, 2-amino-7-(2-deoxy-beta-D-erythro- pentofuranosyl)-3H,5H-pyrrolo-[3,2-d]pyrimidin-4-one (2'-deoxy-9-deazaguanosine)

A synthesis of the C-nucleoside, 2-amino-7-(2-deoxy-beta-D-erythro- pentofuranosyl)-3H,5H-pyrrolo[3,2-d]pyrimidin-4-one (9-deaza-2'-deoxyguanosine) was achieved starting from 2-amino-6-methyl-3H-pyrimidin-4-one (5) and methyl 2-deoxy-3,5-di-O-(p-nitrobenzoyl)-D-erythro-pento-furanoside (11). The anomeric configuration of the C-nucleoside was established by 1H NMR, NOEDS and ROESY. This C-nucleoside did not inhibit the growth of T-cell lymphoma cells.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Deoxyribonucleoside triphosphate levels: a critical factor in the maintenance of genetic stability

DNA precursor pool imbalances can elicit a variety of genetic effects and modulate the genotoxicity of certain DNA-damaging agents. These and other observations indicate that the control of DNA precursor concentrations is essential for the maintenance of genetic stability, and suggest that factors which offset this control may contribute to environmental mutagenesis and carcinogenesis. In this article, we review the biochemical and genetic mechanisms responsible for regulating the production and relative amounts of intracellular DNA precursors, describe the many outcomes of perturbations in DNA precursor levels, and discuss implications of such imbalances for sensitivity to DNA-damaging agents, population monitoring, and human diseases.

Regulation of IMP dehydrogenase gene expression by its end products, guanine nucleotides

To study the regulation of IMP dehydrogenase (IMPDH), the rate-limiting enzyme of guanine nucleotide biosynthesis, we examined the effects of nucleosides, nucleotides, nucleotide analogs, or the IMPDH inhibitor mycophenolic acid (MPA) on the steady-state levels of IMPDH mRNA. The results indicated that IMPDH gene expression is regulated inversely by the intracellular level of guanine ribonucleotides. We have shown that treatment with guanosine increased the level of cellular guanine ribonucleotides and subsequently reduced IMPDH steady-state mRNA levels in a time- and dose-dependent manner. Conversely, MPA treatment diminished the level of guanine ribonucleotides and increased IMPDH mRNA levels. Both of these effects on the steady-state level of IMPDH mRNA could be negated by cotreatment with guanosine and MPA. The down regulation of IMPDH gene expression by guanosine or its up regulation by MPA was not due to major changes in transcriptional initiation and elongation or mRNA stability in the cytoplasm but rather was due to alterations in the levels of the IMPDH mRNA in the nucleus. These results suggest that IMPDH gene expression is regulated by a posttranscriptional, nuclear event in response to fluctuations in the intracellular level of guanine ribonucleotides.

Regulation of IMP dehydrogenase gene expression by its end products, guanine nucleotides

To study the regulation of IMP dehydrogenase (IMPDH), the rate-limiting enzyme of guanine nucleotide biosynthesis, we examined the effects of nucleosides, nucleotides, nucleotide analogs, or the IMPDH inhibitor mycophenolic acid (MPA) on the steady-state levels of IMPDH mRNA. The results indicated that IMPDH gene expression is regulated inversely by the intracellular level of guanine ribonucleotides. We have shown that treatment with guanosine increased the level of cellular guanine ribonucleotides and subsequently reduced IMPDH steady-state mRNA levels in a time- and dose-dependent manner. Conversely, MPA treatment diminished the level of guanine ribonucleotides and increased IMPDH mRNA levels. Both of these effects on the steady-state level of IMPDH mRNA could be negated by cotreatment with guanosine and MPA. The down regulation of IMPDH gene expression by guanosine or its up regulation by MPA was not due to major changes in transcriptional initiation and elongation or mRNA stability in the cytoplasm but rather was due to alterations in the levels of the IMPDH mRNA in the nucleus. These results suggest that IMPDH gene expression is regulated by a posttranscriptional, nuclear event in response to fluctuations in the intracellular level of guanine ribonucleotides.

Characterization of deoxyguanosine-resistant hypoxanthine-guanine phosphoribosyltransferase(-)metastatic variants altered in soybean-agglutinin-binding properties and cell-surface glycoproteins

The isolation of deoxyguanosine-resistant 10T1/2 mouse cell lines following stepwise selection in the presence of increasing concentrations of drug led to the identification of a highly metastatic line, as measured by the ability to form secondary tumors in syngenic mice after intravenous injection. This metastatic deoxyguanosine-resistant mutant was determined to be deficient in hypoxanthine-guanine phosphoribosyltransferase activity, accounting for the resistance to deoxyguanosine. Lectin-binding studies determined that the metastatic potential of high- and low-metastatic revertant clones of this deoxyguanosine-resistant mutant was negatively correlated to soybean agglutinin binding, but not to concanavalin A or wheat germ agglutinin binding. Examination of labelled cell-surface glycoproteins led to the identification of two glycoproteins, gp80 and gp48, which were present on the low-metastatic wild-type cell line but absent from the highly metastatic drug-resistant cells. Our studies suggest that these cell-surface glycoprotein alterations play a role in determining the malignant properties of the cells, and indicate that metastatic variants with the properties described in this report would be useful biological tools for investigations into the roles played by specific cell-surface structures in mechanisms of tumor progression.

Cloning and expression of human deoxycytidine kinase cDNA

Deoxycytidine (dCyd) kinase is required for the phosphorylation of several deoxyribonucleosides and certain nucleoside analogs widely employed as antiviral and chemotherapeutic agents. Detailed analysis of this enzyme has been limited, however, by its low abundance and instability. Using oligonucleotides based on primary amino acid sequence derived from purified dCyd kinase, we have screened T-lymphoblast cDNA libraries and identified a cDNA sequence that encodes a 30.5-kDa protein corresponding to the subunit molecular mass of the purified protein. Expression of the cDNA in Escherichia coli results in a 40-fold increase in dCyd kinase activity over control levels. In dCyd kinase-deficient murine L cells, transfection with dCyd kinase cDNA in a mammalian expression vector produces a 400-fold increase over control in dCyd phosphorylating activity. The expressed enzyme has an apparent Km of 1.0 microM for dCyd and is also capable of phosphorylating dAdo and dGuo. Northern blot analysis reveals a single 2.8-kilobase mRNA expressed in T lymphoblasts at 5- to 10-fold higher levels than in B lymphoblasts, and decreased dCyd kinase mRNA levels are present in T-lymphoblast cell lines resistant to arabinofuranosylcytosine and dideoxycytidine. These findings document that this cDNA encodes the T-lymphoblast dCyd kinase responsible for the phosphorylation of dAdo and dGuo as well as dCyd and arabinofuranosylcytosine.

Molecular cloning of the cDNA for a mutant mouse ribonucleotide reductase M1 that produces a dominant mutator phenotype in mammalian cells

Mammalian ribonucleotide reductase is regulated by the binding of dATP and other nucleotide effectors to allosteric sites on subunit M1. Using mRNA from a mutant mouse T-lymphoma (S49) cell line, we have isolated a cDNA which encodes an altered, dATP feedback-resistant subunit M1. The mutant cDNA contains a single point mutation (a G-to-A transition) at codon 57, converting aspartic acid to asparagine. Proof that this mutation is responsible for the phenotype of dATP feedback resistance is provided by the following evidence. (i) The mutation was detected only in mutant S49 cells containing dATP feedback-resistant ribonucleotide reductase and not in wild-type or other mutant S49 cells. (ii) Transfection of Chinese hamster ovary cells with an expression plasmid containing the mutant M1 cDNA resulted in the production of dATP feedback-resistant ribonucleotide reductase. Transfected CHO cells expressing the mutant M1 cDNA exhibited a 15- to 25-fold increase in the frequency of spontaneous mutation to 6-thioguanine resistance, confirming that dATP feedback-resistant ribonucleotide reductase produces a mutator phenotype in mammalian cells. The availability of a cDNA which encodes dATP feedback-resistant subunit M1 thus provides a means of manipulating by transfection the frequency of spontaneous mutation in mammalian cells.

Molecular cloning of the cDNA for a mutant mouse ribonucleotide reductase M1 that produces a dominant mutator phenotype in mammalian cells

Mammalian ribonucleotide reductase is regulated by the binding of dATP and other nucleotide effectors to allosteric sites on subunit M1. Using mRNA from a mutant mouse T-lymphoma (S49) cell line, we have isolated a cDNA which encodes an altered, dATP feedback-resistant subunit M1. The mutant cDNA contains a single point mutation (a G-to-A transition) at codon 57, converting aspartic acid to asparagine. Proof that this mutation is responsible for the phenotype of dATP feedback resistance is provided by the following evidence. (i) The mutation was detected only in mutant S49 cells containing dATP feedback-resistant ribonucleotide reductase and not in wild-type or other mutant S49 cells. (ii) Transfection of Chinese hamster ovary cells with an expression plasmid containing the mutant M1 cDNA resulted in the production of dATP feedback-resistant ribonucleotide reductase. Transfected CHO cells expressing the mutant M1 cDNA exhibited a 15- to 25-fold increase in the frequency of spontaneous mutation to 6-thioguanine resistance, confirming that dATP feedback-resistant ribonucleotide reductase produces a mutator phenotype in mammalian cells. The availability of a cDNA which encodes dATP feedback-resistant subunit M1 thus provides a means of manipulating by transfection the frequency of spontaneous mutation in mammalian cells.

Purine deoxynucleosides and adenosine dialdehyde decrease 5-amino-4-imidazolecarboxamide (Z-base)-dependent purine nucleotide synthesis in cultured T and B lymphoblasts

Deoxyadenosine (dAdo) and deoxyguanosine (dGuo) decrease methionine synthesis from homocysteine in cultured lymphoblasts; because of the possible trapping of 5-methyltetrahydrofolate this could lead to decreased purine nucleotide synthesis. Since purine deoxynucleosides could also inhibit purine synthesis de novo at an early step not involving folate metabolism, we measured in azaserine-treated cells 5-amino-4-imidazolecarboxamide (Z-base)-dependent purine nucleotide synthesis using [14C]formate. In the T lymphoblasts, Z-base-dependent purine nucleotide synthesis was decreased 26% by 0.3 microM-dAdo, 21% by 1 microM-dGuo and 28% by 1 microM-adenosine dialdehyde, a potent S-adenosylhomocysteine hydrolase inhibitor; homocysteine fully reversed the inhibitions. The B lymphoblasts were considerably less sensitive to the deoxynucleoside-induced decrease in Z-base-dependent purine nucleotide synthesis, with 100 microM-dAdo required for significant inhibition and no inhibition by dGuo at this concentration; homocysteine partly reversed the inhibition by dAdo. The observed decrease in Z-base-dependent purine nucleotide synthesis could not be attributed either to dUMP depletion changing the folate pools or to decreased ATP availability because dUrd was without effect and during the experimental period the intracellular ATP concentration did not change significantly. Cells with 5,10-methylenetetrahydrofolate reductase deficiency were relatively resistant to inhibition of Z-base-dependent purine nucleotide synthesis by dAdo and adenosine dialdehyde. Our results suggest that deoxynucleosides decrease purine nucleotide synthesis by trapping 5-methyltetrahydrofolate.

Deoxyadenosine triphosphate as a mediator of deoxyguanosine toxicity in cultured T lymphoblasts

The mechanism by which 2'-deoxyguanosine is toxic for lymphoid cells is relevant both to the severe cellular immune defect of inherited purine nucleoside phosphorylase (PNP) deficiency and to attempts to exploit PNP inhibitors therapeutically. We have studied the cell cycle and biochemical effects of 2'-deoxyguanosine in human lymphoblasts using the PNP inhibitor 8-aminoguanosine. We show that cytostatic 2'-deoxyguanosine concentrations cause G1-phase arrest in PNP-inhibited T lymphoblasts, regardless of their hypoxanthine guanine phosphoribosyltransferase status. This effect is identical to that produced by 2'-deoxyadenosine in adenosine deaminase-inhibited T cells. 2'-Deoxyguanosine elevates both the 2'-deoxyguanosine-5'-triphosphate (dGTP) and 2'-deoxyadenosine-5'-triphosphate (dATP) pools; subsequently pyrimidine deoxyribonucleotide pools are depleted. The time course of these biochemical changes indicates that the onset of G1-phase arrest is related to increase of the dATP rather than the dGTP pool. When dGTP elevation is dissociated from dATP elevation by coincubation with 2'-deoxycytidine, dGTP does not by itself interrupt transit from the G1 to the S phase. It is proposed that dATP can mediate both 2'-deoxyguanosine and 2'-deoxyadenosine toxicity in T lymphoblasts.

Functional and mechanistic studies on the toxicity of deoxyguanosine for the in vitro proliferation and differentiation of human peripheral blood B lymphocytes

Deoxyguanosine (dGuo) has been implicated as the toxic metabolite causing a severe impairment of cellular immunity in children with a genetic deficiency of purine nucleoside phosphorylase (PNP). In peripheral blood T cells of normal donors both the pathway which leads to phosphorylation of dGuo (ultimately resulting in deoxyguanosine triphosphate, dGTP) and the salvage pathway which starts with degradation of dGuo by PNP (resulting in the formation of guanosine triphosphate, GTP) contribute to the inhibition of proliferation. In normal peripheral blood B cells, addition of dGuo leads to an inhibition of proliferation and differentiation. The concentrations of dGuo needed to cause a 50% inhibition are equivalent for peripheral blood T cells and B cells. Inhibition of B cell differentiation can be observed at the level of intracytoplasmic as well as secreted Ig and concerns all Ig isotypes. The early phase of B cell activation which takes place during a 24-h preculture with formalinized Cowan I Staphylococci is not affected by dGuo; it is not until proliferation and differentiation of B cells, brought about by culturing in the presence of crude concanavalin A supernatant, occurs that inhibitory effects of dGuo become evident. Addition of dGuo to B cell cultures results in an intracellular accumulation of GTP and dGTP. Addition of 8-aminoguanosine, a PNP inhibitor, next to dGuo, completely prevents the dGuo-mediated inhibition. Under these circumstances the dGuo-mediated increase in intracellular GTP is abrogated while dGTP accumulation still occurs. This indicates that the inhibitory effect of dGuo on the proliferation and differentiation of peripheral blood B lymphocytes of normal donors is independent of dGTP accumulation.

Compartmentation of guanine nucleotide precursors for DNA synthesis

We have studied the kinetics of guanine incorporation into DNA in mouse T-lymphoma (S-49) mutant cells [PNPase (purine-nucleoside phosphorylase)- and HGPRTase (hypoxanthine: guanine phosphoribosyltransferase)-deficient] that are incapable of converting dGuo (deoxyguanosine) to Gua (guanine) ribonucleotides. Of the two possible pathways for an exogenous guanine source to reach DNA, firstly: dGuo----dGMP----dGDP----dGTP and secondly: Gua----GMP----GDP----dGDP----dGTP only the second pathway was found to be functional in providing guanine for DNA replication, although deoxyguanosine readily produced toxic cellular dGTP levels via the first pathway. The functional guanine-nucleotide-precursor pools for DNA are rather small; further, the depletion of the small GMP pool, but not that of GDP, GTP and dGTP, correlated well with the inhibition of DNA synthesis by mycophenolic acid, an IMP dehydrogenase inhibitor. These results support the hypothesis that guanine-nucleotide incorporation into DNA is highly compartmentalized and that a small functional guanine-nucleotide pool, e.g., the GMP pool, may serve a crucial role in limiting the availability of DNA precursor substrate.

Human placental nucleoside kinase activities

Our studies have indicated that normal human placental cytosol contains a complex mixture of nucleoside kinase enzymes, some of which conform to previously characterized activities. Deoxyadenosine is phosphorylated by deoxycytidine kinase, adenosine kinase, and two as yet uncharacterized activities. Deoxyguanosine phosphorylation is associated with deoxycytidine kinase. More complete and detailed studies will be necessary to characterize these enzymes fully.

The effects of exogenous thymidine on endogenous deoxynucleotides and mutagenesis in mammalian cells

The intracellular deoxyribonucleoside triphosphate pools in mammalian cells affect diverse biological functions including the spontaneous or induced mutability. We have isolated from murine T-lymphosarcoma S49 cells, a mutant that is unable to convert dCMP to dUMP, contains deranged intracellular dNTP pools, and exhibits a mutator phenotype. The enzymatic defect in araC-6-1 cells is a deficiency of deoxycytidylate deaminase, which accounts for the high dCTP and low TTP intracellular pools. The addition of increasing concentrations of exogenous thymidine to araC-6-1 cells alters these dNTP pools in a predictable manner: increasing the TTP and diminishing the dCTP. Concomitant with this reversal of the dCTP:TTP ratio is a marked decrease in the mutation rate followed by an increase in the mutation rates at higher exogenous thymidine concentrations. This response of the mutation rate is in contrast to that seen in the control cell line containing normal deoxycytidylate deaminase. In the latter case, increasing thymidine concentration induces an enhanced mutation rate that parallels the later phase of the thymidine-induced mutation rate in araC-6-1 cells. The deficiency of deoxycytidylate deaminase, the endogeneous dNTP pool alterations, and the mutator phenotype of araC-6-1 cells are all recessive traits in cell-cell hybrids. These observations allow one to predict whether exogenous thymidine will be mutagenic, antimutagenic, or both for a given cell line and provide a basis for understanding conflicting reports in the literature concerning the effects of the thymidine on genomic stability.

On the mechanism of deoxyribonucleoside toxicity in human T-lymphoblastoid cells. Reversal of growth inhibition by addition of cytidine

High levels of deoxyadenosine and deoxyguanosine in patients with inherited deficiency of either adenosine deaminase or purine-nucleoside phosphorylase, respectively, are considered to be responsible for the associated immunological disorder. The mechanism involves phosphorylation to the corresponding deoxyribonucleoside triphosphates which subsequently inhibit the CDP-reducing activity of ribonucleotide reductase. Addition of deoxycytidine protects cells from the cytotoxic effects of deoxyadenosine and deoxyguanosine by competition for phosphorylation and by replenishing dCTP, the apparent limiting DNA precursor. Addition of cytidine, but not uridine, led to a reversal of deoxyguanosine and thymidine growth inhibition, comparable to that obtained with deoxycytidine. Analysis of the intracellular nucleotide pools showed that increased levels of cytidine ribonucleotides were sufficient to overcome the inhibitory effects of dGTP and dTTP on CDP reduction, thereby circumventing a depletion of the dCTP pool. A partial reversal of deoxyadenosine toxicity was also obtained with addition of cytidine. In this case little change in the dCTP level was observed, but a decreased dGTP pool appeared to be correlated with growth inhibition. High cytidine ribonucleotide levels partially prevented this effect. The present results may encourage the use of cytidine in combination with deoxycytidine as a pharmacological regime in treatment of immunodeficiency disease associated with increased deoxyribonucleotide levels.

Immunodeficiencies associated with errors in purine metabolism

The genetic deficiencies of adenosine deaminase and purine nucleoside phosphorylase lead to blocks in the purine pathway. The intracellular accumulation of deoxynucleosides and deoxynucleotides is toxic to both dividing and nondividing lymphocytes via multiple mechanisms. T-lymphocytes are uniquely sensitive to purine-mediated cytotoxicity because of a functional imbalance of phosphorylating and dephosphorylating enzymatic activities. These inborn errors or purine metabolism are rare disorders. The study of these conditions, however, has uncovered unique enzymatic properties of lymphocytes and lymphocyte subclasses. A better understanding of the mechanisms of lymphocytotoxicity in these two purine enzyme defects may lead to better modes of therapeutic manipulation of the immune system.

The gene encoding the T-cell receptor alpha-chain maps close to the Np-2 locus on mouse chromosome 14

Serological and molecular genetic analyses of T-cell clones have shown that the T-cell antigen receptor apparently comprises two glycosylated, disulphide-linked polypeptide chains (alpha and beta), both of which span the cell membrane. Cloning of the genes encoding the two chains from mouse and human DNA has shown that the alpha- and beta-chains are composed of variable (V) and conserved (C) regions in agreement with peptide mapping data. Gene segments encoding variable and conserved domains of the beta-chain have been identified and undergo rearrangements during T-cell differentiation. The genes encoding the alpha-chain, so far described at the level of complementary DNA clones, also identify DNA rearrangements. Thus, the genes encoding the T-cell receptor show the same structure and dynamic behaviour as immunoglobulin genes, indicating that the two gene families belong to the same supergene family; this evolutionary relationship is supported by the fact that the genes encoding the beta-chain of the T-cell receptor are closely linked to immunoglobulin kappa light-chain genes on chromosome 6 in mouse. In man, however, the beta genes map to chromosome 7 (ref. 14) whereas the kappa-chain genes are located on chromosome 2, indicating that linkage between the two gene families is not needed for proper expression. Here we describe genomic clones encoding the constant portion of the T-cell receptor alpha-chain and map the gene to chromosome 14 in mouse, close to the gene for purine nucleoside phosphorylase (Np-2) which, in man, has been associated with T-cell immunodeficiencies.

Combination chemotherapy directed at the components of nucleoside diphosphate reductase

It would be expected that drugs directed at the rate-limiting step in a key metabolic pathway in tumor cell proliferation would provide a useful basis for therapy of neoplasms. Ribonucleotide reductase catalyzes the rate-limiting step in the de novo synthesis of dNTP's for DNA synthesis. Further, ribonucleotide reductase is composed of two non-identical protein subunits (non-heme iron and effector-binding subunits) which can be specifically and independently inhibited. As a result, combinations of drugs specifically directed at each of the subunits of ribonucleotide reductase have been shown to cause synergistic inhibition of L1210 cell growth in culture and synergistic cell kill. This approach offers a novel basis for the design of combination chemotherapy.

Changes in deoxynucleoside triphosphate pools induced by inhibitors and modulators of ribonucleotide reductase

Changes in dNTP pools have been studied by a number of investigators, in a wide range of cell types. The in vitro pertubations in dNTP pool levels induced, in particular, by deoxynucleosides which act as allosteric modulators, are not totally consistent with current 'in vitro models' of ribonucleotide reductase function. This problem has also been addressed by Henderson et al. (1980) who stress the profusion of such models. Possible explanations, apart from the technical problems of the range of different experimental conditions (e.g. concentration of modulator used, time of incubation, etc.) for the various cell lines include: Modulators presumably have unpredictable 'network' effects by inhibiting or stimulating many other enzymes involved in the de novo and salvage synthesis of purines and pyrimidines. It is possible there are two separate forms of ribonucleotide reductase, one specifically reducing CDP/UDP, the other ADP/GDP. This, in particular, would explain the lack of decrease in dCTP levels after elevation of the dATP pool. There may be variations in ribonucleotide function which in vivo are cell specific, e.g. in thymic-derived compared with non-T-cell types. Peculiarities of T-cells include: Their ability to elevate their dNTP pools on exposure to very low exogenous concentration of deoxynucleoside. This may reflect very low rates of dNTP catabolism. The biological response of T-cells to elevation of the dATP or dGTP pool is reflected by a G1 block compared to an S phase block in cell-cycle progression in non-T-cell lines. The possibility that, in thymic cells, ribonucleotide reduction is restricted to ADP/GDP while pyrimidine dNTPs are synthesized by salvage pathways. As well, possible variation in the pool localization of dNTPs depending on production by either de novo or salvage synthesis could produce dNTP pool changes not clearly in accord with in vitro models. Clearly, the solution to these problems (although not easy) requires systematic comparative study, using cells of various origin (particularly T vs non-T), of dNTP pool responses to deoxynucleoside modulators, with an attempt to explore the factors described above. However, in the detailed pursuit of such an analysis the concept, that these variations in the control of nucleotide metabolism in T and non-T-cell systems may reflect quite significant differences in growth control and cell-cycle progression, should not be lost.