2'-deoxyguanosine toxicity for B and mature T lymphoid cell lines is mediated by guanine ribonucleotide accumulation

Pathogen-driven nucleotide overload triggers mitochondria-centered cell death in phagocytes

Staphylococcus aureus is a dangerous pathogen that evolved refined immuno-evasive strategies to antagonize host immune responses. This involves the biogenesis of death-effector deoxyribonucleosides, which kill infectious foci-penetrating macrophages. However, the exact mechanisms whereby staphylococcal death-effector deoxyribonucleosides and coupled imbalances of intracellular deoxyribonucleotide species provoke immune cell death remain elusive. Here, we report that S. aureus systematically promotes an overload of deoxyribonucleotides to trigger mitochondrial rupture in macrophages, a fatal event that induces assembly of the caspase-9-processing apoptosome and subsequent activation of the intrinsic pathway of apoptosis. Remarkably, genetic disruption of this cascade not only helps macrophages coping with death-effector deoxyribonucleoside-mediated cytotoxicity but also enhances their infiltration into abscesses thereby ameliorating pathogen control and infectious disease outcomes in laboratory animals. Combined with the discovery of protective alleles in human CASP9, these data highlight the role of mitochondria-centered apoptosis during S. aureus infection and suggest that gene polymorphisms may shape human susceptibility toward a predominant pathogen.

N6-methyl-2'-deoxyadenosine promotes self-renewal of BFU-E progenitor in erythropoiesis

Red blood cells supply the oxygen required for all human cells and are in demand for emerging blood-loss therapy. Here we identified N6-methyl-2'-deoxyadenosine (6mdA) as an agonist that promotes the hyperproliferation of burst-forming unit erythroid (BFU-E) progenitor cells. In addition, 6mdA represses the apoptosis of erythroid progenitor cells (EPCs). Combined use of with SCF and EPO enabled cultures of isolated BFU-E to be expanded up to 5,000-fold. Transcriptome analysis showed that 6mdA upregulates the expression of the EPC-associated factors c-Kit, Myb, and Gata2 and downregulates that of the erythroid maturation-related transcription factors Gata1, Spi1, and Klf1. Mechanistic studies suggested that 6mdA enhances and prolongs the activation of erythropoiesis-associated master gene c-Kit and its downstream signaling, leading to expansion and accumulation of EPCs. Collectively, we demonstrate that 6mdA can efficiently stimulate the EPC hyperproliferation and provide a new regenerative medicine recipe to improve ex vivo generation of red blood cells.

Staphylococcus aureus Multiplexes Death-Effector Deoxyribonucleosides to Neutralize Phagocytes

Adenosine synthase A (AdsA) is a key virulence factor of Staphylococcus aureus, a dangerous microbe that causes fatal diseases in humans. Together with staphylococcal nuclease, AdsA generates deoxyadenosine (dAdo) from neutrophil extracellular DNA traps thereby igniting caspase-3-dependent cell death in host immune cells that aim at penetrating infectious foci. Powered by a multi-technological approach, we here illustrate that the enzymatic activity of AdsA in abscess-mimicking microenvironments is not restricted to the biogenesis of dAdo but rather comprises excessive biosynthesis of deoxyguanosine (dGuo), a cytotoxic deoxyribonucleoside generated by S. aureus to eradicate macrophages of human and animal origin. Based on a genome-wide CRISPR-Cas9 knock-out screen, we further demonstrate that dGuo-induced cytotoxicity in phagocytes involves targeting of the mammalian purine salvage pathway-apoptosis axis, a signaling cascade that is concomitantly stimulated by staphylococcal dAdo. Strikingly, synchronous targeting of this route by AdsA-derived dGuo and dAdo boosts macrophage cell death, indicating that S. aureus multiplexes death-effector deoxyribonucleosides to maximize intra-host survival. Overall, these data provide unique insights into the cunning lifestyle of a deadly pathogen and may help to design therapeutic intervention strategies to combat multidrug-resistant staphylococci.

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.

Lupus-Associated Functional Polymorphism in PNP Causes Cell Cycle Abnormalities and Interferon Pathway Activation in Human Immune Cells

OBJECTIVE

Systemic lupus erythematosus (SLE) is frequently characterized by activation of the type I interferon (IFN) pathway. We previously observed that a missense single-nucleotide polymorphism (rs1049564) in the purine nucleoside phosphorylase (PNP) gene was associated with high levels of IFN in SLE. PNP is a key enzyme involved in purine metabolism. In this study, we performed functional follow-up of this polymorphism in human cells.

METHODS

Type I IFN was measured in patient sera, using a reporter cell assay. Structural modeling of the PNP variant was performed using PyMOL software. PNP messenger RNA (mRNA) and protein levels and type I IFN-induced gene expression were measured in lymphoblastoid cell lines with known PNP rs1049564 genotypes. The cell cycle was assayed using flow cytometry.

RESULTS

Structural modeling indicated no major disruption in folding related to rs1049564. We observed that homozygous rs1049564 TT lymphoblastoid cells had decreased PNP mRNA expression and protein levels, and that cells with the TT genotype had reduced PNP enzymatic activity even when the amount of PNP was controlled. Cells with the TT genotype had a 2-fold increase in S-phase block as compared with cells with the homozygous CC phenotype. The S-phase block could be pharmacologically reversed with hypoxanthine and adenosine, supporting the notion that relative PNP deficiency is the cause of the S-phase block. Type I IFN-induced transcripts were increased in a dose-response manner related to the rs1049564 T allele, at both baseline and after type I IFN stimulation.

CONCLUSION

The PNP rs1049564 T allele is a loss-of-function variant that induces S-phase block and IFN pathway activation in lymphocytes. The S-phase block could be rescued in our in vitro experiments, suggesting the potential for personalized treatment.

Antiproliferation effect of guanosine on HCT 116 cells involves MAPK and AMPK pathways

This study aims to investigate the mechanisms associated with the antiproliferation effect of guanosine on human colon carcinoma HCT 116 cells. In this study, guanosine induced more drastic cell cycle arrest effect than cell death effect on HCT 116 cells. The cell cycle arrest effect of guanosine on HCT 116 cells appeared to be associated with the increased activation of mitogen-activated protein kinases (MAPK) such as ERK1/2, p38 and JNK. The decrease of AMP-activated protein kinase (AMPK) activation and cyclin D1 expression was also involved. Thus, the antiproliferation of colon cancer cells of guanosine could be mediated by the disruption of MAPK and AMPK pathways.

Inhibitory effect of extracellular purine nucleotide and nucleoside concentrations on T cell proliferation

The release of nucleic acids and derivatives after tissue-injury may affect cellular immune-response. We studied the impact of extracellular ribo-, desoxyribonucleotides and nucleosides on T-cell immunity. Peripheral-blood-mononuclear-cells (PBMCs) or isolated CD3⁺T-cells obtained from 6 healthy donors were stimulated via CD3/CD28 Dynabeads or dendritic cells (DCs) in the presence or absence of pyrimidine-, purine-nucleotides and -nucleosides (range 2-200µM). Addition of deoxy-, guanosine-triphosphate (dGTP, GTP) and guanosine resulted concentration dependent in a complete, adenosine-triphosphate (ATP) in a partial inhibition of the induced T-cell-proliferation. Deoxyadenosine-triphosphate (dATP), adenosine and the pyrimidine-ribo- and -deoxyribonucleotides displayed no inhibitory capacity. Inhibitory effects of dGTP and GTP, but not of guanosine and ATP were culture-media-dependent and could be almost abrogated by use of the serum-free lymphocyte-culture-media X-Vivo15 instead of RPMI1640 with standard-supplementation. In contrast to RPMI1640, X-Vivo15 resulted in a significant down-regulation of the cell-surface-located ectonucleotidases CD39 (Ecto-Apyrase) and CD73 (Ecto-5'-Nucleotidase), critical for the extracellular nucleotides-hydrolysis to nucleosides, explaining the loss of inhibition mediated by dGTP and GTP, but not Guanosine. In line with previous findings ATP was found to exert immunosuppressive effects on T-cell-proliferation. Purine-nucleotides, dGTP and GTP displayed a higher inhibitory capacity, but seem to be strictly dependent on the microenvironmental conditions modulating the responsiveness of the respective T-lymphocytes. Further evaluation of experimental and respective clinical settings should anticipate these findings.

Cytotoxicity of guanine-based degradation products contributes to the antiproliferative activity of guanine-rich oligonucleotides

Guanine-rich oligonucleotides with lower nuclease resistance exhibited higher antiproliferative activity; guanine-based compounds showed highly concentration-dependent cytotoxicity.

Guanine-rich oligonucleotides (GROs) have attracted considerable attention as anticancer agents, because they exhibit cancer-selective antiproliferative activity and can form G-quadruplex structures with higher nuclease resistance and cellular uptake. Recently, a GRO, AS1411 has reached phase II clinical trials for acute myeloid leukemia and renal cell carcinoma. The antiproliferative activity of GROs has been associated with various protein targets; however the real mechanisms of action remain unclear. In this study, we showed evidence that antiproliferative activity of GROs (including AS1411) is mainly contributed by the cytotoxicity of their guanine-based degradation products, such as monophosphate deoxyguanosine (dGMP), deoxyguanosine (dG) and guanine. The GROs with lower nuclease resistance exhibited higher antiproliferative activity. Among nucleotides, nucleosides and nucleobases, only guanine-based compounds showed highly concentration-dependent cytotoxicity. Our results suggest that it is necessary to reconsider the cancer-selective antiproliferative activity of GROs. Since guanine-based compounds are endogenous substances in living organisms, systematic studies of the cytotoxicity of these compounds will provide new information for the understanding of certain diseases and offer useful information for drug design.

Guanosine acts intracellularly to initiate apoptosis in NB4 cells: A role for nucleoside transport

Guanosine initiated apoptosis in NB4 cells in a transport-dependent manner. Apoptosis was partially attributed to an imbalance in nucleosides with some protection upon the addition of pyrimidines. The effect of guanosine on cell proliferation and viability was biphasic whereby cells were able to recover from an initial cell cycle arrest and re-enter the cell cycle upon removal of guanosine in a time-dependent fashion. However, exposure to guanosine beyond 24 h prevented recovery and ultimately led to death. Death occurred with a decrease in bcl-2 protein expression, thus suggesting that the pathway to apoptosis involved change(s) in the intracellular environment that were ultimately sensed by the mitochondria. Expression of the unique guanosine-specific nucleoside transporter csg in NB4 cells may provide an opportunity to harness guanosine-mediated cell death in the treatment of APL and related malignancies while sparing normal cells.

Role of tissue transglutaminase in GTP depletion-induced apoptosis of insulin-secreting (HIT-T15) cells

The role of tissue transglutaminase (tTG), a calcium-dependent and GTP-modulated enzyme, in apoptotic death induced by GTP depletion in islet beta-cells was investigated. GTP depletion and apoptosis were induced by mycophenolic acid (MPA) in insulin-secreting HIT-T15 cells. MPA treatment increased in situ tTG activity (but not protein levels) in a dose- and time-dependent manner in parallel with the induction of apoptosis. MPA-induced increases of both tTG activity and apoptosis were entirely blocked by co-provision of guanosine but not adenosine. MPA-enhanced tTG activity could be substantially reduced by co-exposure to monodansylcadaverine or putrescine (tTG inhibitors), and largely blocked by lowering free Ca(2+) concentrations in the culture medium. However, MPA-induced cell death was either not changed or was only slightly reduced under these conditions. By contrast, a pan-caspase inhibitor (Z-VAD-FMK) entirely prevented apoptosis induced by MPA, but did not block the enhanced tTG activity, indicating that GTP depletion can induce apoptosis and activate tTG either independently or as part of a cascade of events involving caspases. Importantly, the morphological changes accompanying apoptosis could be markedly prevented by tTG inhibitors. These findings suggest that the effect of the marked increase in tTG activity in GTP depletion-induced apoptosis of insulin-secreting cells may be restricted to some terminal morphological changes.

The mononucleotide-dependent, nonantisense mechanism of action of phosphodiester and phosphorothioate oligonucleotides depends upon the activity of an ecto-5'-nucleotidase

Many reports indicate different nonantisense yet sequence-specific effects of antisense phosphorothioate oligonucleotides. Products of enzymatic degradation of the oligonucleotides can also influence cell proliferation. The cytotoxic effects of deoxyribonucleoside-5'-phosphates (dNMPs) and their 5'-phosphorothioate analogs, deoxyribonucleoside-5'-monophosphorothioates (dNMPSs) on 4 human cell types (HeLa, HL-60, K-562, and endothelial cells) were examined, and the effects were correlated with the catabolism of these compounds. The results indicate that differences in cytotoxicity of dNMPs or dNMPSs in these cells depend upon different activity of an ecto-5'-nucleotidase. It has also been found that dNMPSs stimulate proliferation of human umbilical vein endothelial cells and HL-60 cells in a concentration-dependent manner. This stimulation might be caused by the binding of deoxynucleoside-5'-phosphorothioates to as-yet unidentified nucleotide receptor(s) at the cell surface. (Blood. 2001;98:995-1002)

VX-497: a novel, selective IMPDH inhibitor and immunosuppressive agent

Inosine monophosphate dehydrogenase (IMPDH) is an essential rate-limiting enzyme in the purine metabolic pathway, catalyzing the de novo synthesis of guanine nucleotides required for lymphocyte proliferation. IMPDH has therefore been an attractive target for developing immunosuppressive drugs (e.g., CellCept and mizoribine). Here we describe the immunosuppressive activity of VX-497, a novel noncompetitive inhibitor of IMPDH. VX-497 (MW 452.5) is orally bioavailable and inhibits the proliferation of primary human, mouse, rat, and dog lymphocytes at concentrations of approximately 100 nM. The inhibitory effect of VX-497 on lymphocytes is reversed in the presence of exogenous guanosine, but not in the presence of adenosine or uridine, confirming that the antilymphocytic activity of VX-497 is specifically due to inhibition of IMPDH. The antiproliferative effect of VX-497 in cells is also reversed within 48 h of its removal. Based on evaluation of VX-497 in several lymphoid and nonlymphoid cells, the antiproliferative effect of VX-497 is observed to be most pronounced on lymphoid and keratinocyte cells as compared with fibroblasts. In vivo, oral administration of VX-497 inhibits the primary IgM antibody response in a dose-dependent manner, with an ED(50) value of approximately 30-35 mg/kg in mice. Single daily dosing of VX-497 is observed to be as effective as twice-daily dosing in this model of immune activation. These studies demonstrate that VX-497 is a potent, specific, and reversible IMPDH inhibitor that selectively inhibits lymphocyte proliferation.

Selective inhibition of human Molt-4 leukemia type II inosine 5'-monophosphate dehydrogenase by the 1,5-diazabicyclo[3.1.0]hexane-2,4-diones

Inosine 5'-monophosphate dehydrogenase (IMPDH) is the rate-limiting enzyme in de novo purine biosynthesis. IMPDH activity results from expression of two isoforms. Type I is constitutively expressed and predominates in normal resting cells, while Type II is selectively up-regulated in neoplastic and replicating cells. Inhibitors of IMPDH activity selectively targeting the Type II isoform have great potential as cancer chemotherapeutic agents. For this study, an expression system was developed which yields 35-50 mg of soluble, purified recombinant Type I and II protein from 1 L of bacteria. In addition, three 1,5-diazabicyclo[3.1.0]hexane-2,4-diones were synthesized and shown to act as specific inhibitors of human recombinant Type II IMPDH. The agents are competitive inhibitors with respect to the endogenous substrate IMP and K(i) values range from 5 to 44 microM but were inactive as inhibitors of the Type I isoform at concentrations ranging from 0.5 to 500 microM. IC(50) values for recombinant Type II inhibition were determined and compared to IC(50) values obtained from Molt-4 cell extracts of IMPDH. Cytotoxicity assays revealed that the compounds inhibited Molt-4 leukemia growth with ED(50) values of 3.2-7.6 microM. Computational docking studies predict that the compounds bind to IMPDH in the IMP-binding site, although interactions with residues differ from those previously determined to interact with bound IMP. While all residues predicted to interact directly with the bound compounds are conserved in the Type I and Type II isoforms, sequence divergence within a helix adjacent to the active site may contribute to the observed selectivity for the human Type II isoform. These compounds represent the first class of selective IMPDH Type II inhibitors which may serve as lead compounds for the development of isoform-selective cancer chemotherapy.

ATP depletion, purine riboside triphosphate accumulation and rat thymocyte death induced by purine riboside

Purine riboside (purine-1-D-ribofuranoside, nebularine), an adenosine analog, exerts cytotoxic effect both in vivo and in vitro. However, exact biochemical mechanism for its toxicity and sensitivity of lymphoid cells remains unknown. The present experiments have examined the sequential metabolic changes leading to cell death, induced in cultured rat thymocytes during incubation with purine riboside. Among 22 analogs tested, purine-riboside and tubercidin were most toxic as determined by trypan blue exclusion and lactate dehydrogenase leakage from the cells. 2-Chloroadenosine and 2'-deoxyadenosine were only moderately toxic, whereas other analogs tested were without effect on cell viability. In the presence of purine riboside, more than 90% of ATP was lost after 2 h of incubation. Hypoxanthine accumulated in the medium and the formation of purine-riboside triphosphate exceeded 4-fold the physiological concentration of ATP in the cell. Inhibition of adenosine kinase by 5-iodotubercidin reversed the cytotoxic effect of purine riboside. Interestingly, cells virtually deprived of ATP after 2 h of incubation with purine riboside maintained high nucleotide energy charge value and high viability. Purine riboside triphosphate was capable to replace ATP in stimulation of glycolysis in cell-free thymus extract. We conclude that for a short time (a few hours) purine riboside triphosphate formed in the cell may serve in the absence of ATP as an intermediate of cellular energy metabolism in rat thymocytes. However, possibly due to toxic effects of purine-riboside triphosphate, cells were finally dying. Thus, ATP depletion and adenosine kinase mediated purine riboside phosphates formation are the principle causes of rat thymocytes death exposed to purine riboside.

A purine nucleoside phosphorylase (PNP) inhibitor induces apoptosis via caspase-3-like protease activity in MOLT-4 T cells

Children with congenital homozygous deficiency of purine nucleoside phosphorylase (PNP) have abnormalities in purine metabolism that result in T-cell selective immune deficiency. The mechanism of action for cell death has been attributed to intracellular accumulation of dGTP, a potent inhibitor of ribonucleotide reductase and subsequently DNA synthesis, in thymocytes and T-cells but not B-cells. However, the mode of cell death has not been determined to be either necrosis or apoptosis. To examine the involvement of apoptosis in T-cells following PNP inhibition, MOLT-4 cells, a human T cell leukemia cell line, were co-treated with the PNP inhibitor, CI-1000 (2-amino 3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2-d]-pyrimidin-4-one HCl), and 2'-deoxyguanosine (dGuo) which resulted in a concentration-dependent loss of cell viability (trypan blue) and inhibition of tritiated thymidine ([3H]-TdR) uptake. Staining of cells with the DNA dye Hoechst 33,258 showed nuclear morphology characteristic of apoptosis. Western blots (24 h lysates) were probed with antibodies against several proteins implicated in apoptosis. Anti-PARP revealed the presence of an 85 kD PARP breakdown product while, anti-alpha-spectrin revealed the accumulation a 120 kD breakdown product, both suggestive of CPP32 cleavage (caspase-3; an ICE-like cysteine protease). Western blots also detected the loss of the intact 32 kD caspase-3 isoform, a biochemical event associated with caspase-3 activation. Corresponding fluorometric activity assays detected a marked increase in caspase-3-like activity using the substrate Ac-DEVD-MCA. Lastly, a pan caspase inhibitor (Z-D-DCB) and 2'-deoxycytidine (dCyd), which is known to prevent dGTP accumulation following PNP inhibition, were able to prevent cell death and all indicators of caspase-3-like activity in MOLT-4 cells co-treated with dGuo and CI-1000. In summary, we provided several lines of evidence for the role of apoptosis and the contribution of caspase-3-like proteases in T-cell death following PNP inhibition.

Differential effects of 2'-deoxyguanosine on peripheral blood mononuclear cell proliferation in healthy donors and Hashimoto's thyroiditis patients

The aim of this study was to determine possible differences in peripheral blood mononuclear cells (PBMC) proliferation of healthy donors and Hashimoto's thyroiditis patients and whether a statistical approach to cell proliferation analysis might be used to discern the differences. The effect of a wide range of 2'-deoxyguanosine (dGuo) concentrations (0-1250 microM) on the mitogen-induced proliferation of PBMC was studied in healthy donors and Hashimoto's thyroiditis patients. Activity levels of purine nucleoside phosphorylase (PNP) and adenosine deaminase (ADA) in PBMC were also measured. For the first time in a study of these models of dGuo toxicity in vitro, the analysis of polynomial trends of orders from 1 to 7 was applied to evaluate cell proliferation. A dose-dependent inhibition of mitogen-induced PBMC proliferation was observed in both groups. Data for linear trend established that PBMC from Hashimoto's thyroiditis patients were more sensitive to dGuo toxicity than PBMC from healthy donors. A positive quadratic trend at low dGuo doses was found in the cell proliferation of Hashimoto's thyroiditis patients. A decrease in PNP activity (P < 0.025) and an increase in ADA activity (P < 0.005) was observed in PBMC of Hashimoto's thyroiditis group. The differences in PBMC proliferation subjected to dGuo toxicity between the two groups could be related with the distinct pattern of purine salvage enzymes observed.

Evidence for a pathway independent from 2'-deoxyguanosine and reversible by IL-2 by which purine nucleoside phosphorylase inhibitors block T-cell proliferation

Patients with homozygous deficiency of purine nucleoside phosphorylase (PNP) present with a T-cell selective immune deficiency. To elucidate the potential use of PNP inhibitors in the therapy of cutaneous T-cell lymphomas (CTCLs) the authors studied the effects of CI-1000 (formerly PD141955-2) and CI-972 on a T-cell line MyLa established from a patient with mycosis fungoides. Both PNP inhibitors had significant, dose-dependent, inhibitory effects on the proliferation of the T-cell line. CI-1000 (ED50: 3.7 microM) was approximately six-fold more potent in blocking 3H-thymidine uptake than CI-972 (ED50: 22.5 microM). The inhibitory effect of either substance could not be increased by addition of deoxyguanosine. Flow cytometric analysis revealed that both PNP inhibitors caused a block in the S-phase of the cell cycle. The inhibitory effect on proliferation was reversible partially by addition of IL-2. When testing proliferation inhibition of both substances on an IL-2-dependent T-cell line (SeAx), their inhibitory effects were reduced significantly. These data document a mechanism of action of the PNP inhibitors independent of deoxyguanosine and partially reversible by IL-2. The authors' observations suggest the potential use of PNP inhibitors in the therapy of cutaneous T-cell lymphomas and provide evidence for a pathway independent from deoxyguanosine by which PNP inhibitors might function in T cells.

IMP dehydrogenase inhibitors as immunomodulators

IMP dehydrogenase is a key enzyme in the de novo pathway of purine biosynthesis and is responsible for catalyzing the first step in the formation of guanine ribonucleotides from inosine monophosphate. Mizoribine, an immunosuppressive agent in wide-spread clinical use in Japan, has been demonstrated to inhibit this enzyme. We have investigated the effects of mizoribine on human T cell activation. Stimulation of purified human peripheral blood T lymphocytes with phorbol ester and ionomycin leads to a five-fold increase in guanine ribonucleotide levels over 72 hours. The addition of mizoribine to these cultures at concentrations that are achieved in vivo leads to a dose-dependent inhibition of proliferation and concomitant 50% decrease in guanine ribonucleotide levels, an effect that is reversible with the addition of guanosine, which repletes the GTP pool. Similar effects are seen with direct stimulation via the CD3/T cell receptor complex. Inhibition of proliferation occurs at the G1/S interface of the cell cycle and is additive to that produced by cyclosporine. In order to determine whether inhibition of IMP dehydrogenase is a common mechanism of immunosuppression for drugs such as azathioprine and 6-mercaptopurine that interfere with purine biosynthesis, we compared the effects of these agents on the metabolism of purified T lymphocytes. The results of these studies demonstrate that mizoribine and mycophenolic acid, a highly specific inhibitor of IMP dehydrogenase, inhibit proliferation directly by the depletion of guanine ribonucleotides; 6-mercaptopurine, on the other hand, has a mixed effect on adenine and guanine ribonucleotide pools, whereas azathioprine inhibits proliferation by a mechanism completely independent of its effects on the purine metabolic pathway. We conclude from these studies that inhibitors of IMP dehydrogenase have potential as specific immunosuppressive agents.

Fibroblast growth factor-dependent metabolism of hypoxanthine via the salvage pathway for purine synthesis in porcine aortic endothelial cells

In this study we examined the metabolism of hypoxanthine in fibroblast growth factor (FGF)-stimulated porcine aortic endothelial cells (PAEC). Our previous report indicated that hypoxanthine in fetal bovine serum (FBS) was an essential component for both basal and FGF-dependent growth of PAEC (Hayashi et al., Exp Cell Res 185: 217-228, 1989). Besides hypoxanthine, the addition of various purine bases and purine nucleosides, but not xanthine, xanthosine or any pyrimidine metabolites, restored the limited growth of PAEC cultured in medium containing 10% dialyzed FBS in the presence or absence of FGF. The metabolism of [14C]hypoxanthine was compared in PAEC treated with and without FGF. Treatment of PAEC with FGF for 24 hr enhanced the radioactivity incorporation from [14C]hypoxanthine into both the acid-soluble and -insoluble fractions approximately 2-fold. Upon chromatographic analyses of hypoxanthine metabolites in the acid-soluble nucleotide fraction, it was found that in control PAEC hypoxanthine was largely metabolized to IMP, adenine nucleotides and uric acid, whereas in FGF-treated cells it was converted to ATP, ADP, GTP, xanthine and uric acid. The radioactivity of IMP was lowered in FGF-stimulated cells. The addition of FGF to PAEC increased phosphoribosyl pyrophosphate (PRPP) synthetase activity by approximately 8-fold and the PRPP content by approximately 2-fold, but it did not increase hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity or hypoxanthine transport. On the other hand, methotrexate, an inhibitor of de novo synthesis of purine, did not affect the growth of PAEC. Analyses of the rate of [14C]formate incorporation into total purine compounds showed that PAEC had a low capacity to synthesize purines de novo, which was not stimulated by FGF. These data indicate that FGF stimulates the synthesis of PRPP necessary for the salvage synthesis of purine nucleotides in conjunction with purine bases, e.g. hypoxanthine.

Comparative in vitro and in vivo activities of two 9-deazaguanine analog inhibitors of purine nucleoside phosphorylase, CI-972 and PD 141955

An in-parallel comparison is presented of the in vitro and in vivo properties of two 9-deazaguanine analog inhibitors of purine nucleoside phosphorylase (PNP), CI-972 [8-amino-9-deaza-9-(3-thienylmethyl)guanine] and PD 141955 [9-deaza-9-(3-thienylmethyl)guanine] (published Ki values of 0.83-8.0 and 0.08 microM, respectively). Despite structural similarities, PD 141955 was considerably more potent and active in all systems studied. The respective IC50 values for inhibition of MOLT-4 cell growth in the absence and presence of 10 microM 2'-deoxyguanosine (GdR) were greater than 50 and 5.06 microM for CI-972 and 15.4 and 0.061 microM for PD 141955. PD 141955 induced accumulation of dGTP in GdR-treated MOLT-4 and CEM cells at log-lower concentrations than were required of CI-972, and the magnitude of dGTP accumulation in PD 141955-treated T cell cultures was markedly greater (e.g. 366 vs 100 pmol/10(6) CEM cells at 10 microM). PD 141955 administered orally produced a dose-dependent elevation of plasma inosine and guanosine in rats over a broad concentration range. Mean plasma inosine concentrations following a 150 mg/kg p.o. dose peaked at 6.21 and 13.2 microM in CI-972 and PD 141955-treated rats, respectively. Low levels of inosine were detectable at 50 micrograms/kg following oral administration of PD 141955.

Reciprocal alterations of GMP reductase and IMP dehydrogenase activities during differentiation in HL-60 leukemia cells

The study was undertaken to elucidate the regulatory roles of GMP reductase (GMPR) and IMP dehydrogenase (IMPDH) on purine interconversion during differentiation. Treatment of HL-60 cells with retinoic acid (1 microM) induced granulocytic differentiation which was accompanied with a 2.4-fold increase in GMPR and 55% decrease in IMPDH activities. Maturation induced by 12-O-tetradecanoylphorbol 13-acetate or dimethylsulfoxide was also associated with similar reciprocal alterations. Incubation with guanosine (200 microM), which expands the guanine nucleotide pool, elevated GMPR (1.9-fold) and decreased IMPDH (73%) activities. The synchronous and opposing alterations in GMPR and IMPDH activities should amplify the metabolic response due to differentiation or guanylate pool expansion.

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.

Selective in vitro inhibition of human MOLT-4 T lymphoblasts by the novel purine nucleoside phosphorylase inhibitor, CI-972

CI-972 (2,6-diamino-3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3,2- d]pyrimidin-4-one monohydrochloride, monohydrate) is a competitive inhibitor of PNPase (E.C. 2.4.2.1., Ki = 0.83 microM) entering clinical trials as a T cell-selective immunosuppressive agent. Neither CI-972 (less than or equal to 50 microM) nor dGuo (less than or equal to 10 microM) inhibited [3H]Thd uptake by human MOLT-4 (T cell) or MGL-8 (B cell) lymphoblasts, but in the presence of 10 microM dGuo, the IC50 for CI-972 decreased to 3.0 microM for MOLT-4 but remained at greater than 50 microM for MGL-8. Inhibition of MOLT-4 growth was associated with an increase in dGTP that was dependent on CI-972 concentration and inhibited by 2'-deoxycytidine. Growth could not be restored by hypoxanthine or adenine. No alterations in GTP pools were noted in MOLT-4, and neither GTP nor dGTP were altered in MGL-8.

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.

Guanine ribonucleotide depletion inhibits T cell activation. Mechanism of action of the immunosuppressive drug mizoribine

The immunosuppressive drug, mizoribine, has been used to prevent rejection of organ allografts in humans and in animal models. Based on studies in cell lines, mizoribine has been postulated to be an inhibitor of inosine monophosphate (IMP) dehydrogenase (EC1.2.1.14), a pivotal enzyme in the formation of guanine ribonucleotides from IMP. To further characterize the mechanism of action of this drug, we studied the effect of mizoribine on human peripheral blood T cells stimulated with alloantigen, anti-CD3 MAb, or pharmacologic mitogens. Mizoribine (1-50 micrograms/ml) was able to inhibit T cell proliferation by 10-100% in a dose-dependent fashion to all stimuli tested. Measurements of purine ribonucleotide pools by HPLC showed that mizoribine led to a decrease in intracellular GTP levels, and that repletion of GTP reversed its antiproliferative effects. We also examined sequential events occurring after T cell stimulation. Early events in T cell activation, as assessed by steady-state mRNA levels of c-myc, IL-2, c-myb, histone, and cdc2 kinase, as well as surface IL-2 receptor expression, were unaffected. However, cell cycle analysis revealed decreased numbers of cells in S, G2, and M phases, and showed that the G1/S block was reversed with GTP repletion. These data indicate that mizoribine has an effect on T cell proliferation by a mechanism distinct from that of cyclosporine or corticosteroids, and therefore may be useful in combination immunosuppressive regimens.

Biochemical and pharmacological properties of CI-972, a novel 9-deazaguanine analog purine nucleoside phosphorylase (PNP) inhibitor

CI-972 (2,6-diamino-3,5-dihydro-7-(3-thienylmethyl)-4H-pyrrolo[3, 2-d]pyrimidin-4-one monohydrochloride, monohydrate) is a novel inhibitor of PNP (Ki = 0.83 microM) under development as a T cell-selective immunosuppressive agent. CI-972 inhibited proliferation (3H-thymidine uptake) of human MOLT-4 (T cell) but not MGL-8 (B cell) lymphoblasts with respective IC50s of 3.0 and greater than 50 microM when tested with 10 microM 2'-deoxyguanosine. Without addition of exogenous 2'-deoxyguanosine, CI-972 was not inhibitory to any human T or B lymphoblastoid cell line tested. 2'-Deoxycytidine (10 microM), but not hypoxanthine or adenine, restored MOLT-4 cell growth. Inhibition of 3H-thymidine uptake in MOLT-4 cells correlated with accumulation of dGTP, while alterations in guanine nucleotides were not observed. 2'-Deoxycytidine (10 microM) also blocked dGTP accumulation in MOLT-4 cells. CI-972 showed activity in vivo over a broad dose range: At 5-150 mg/kg p.o., CI-972 produced dose-dependent elevation of plasma inosine one hr after administration to rats (mean maximum of 2.62 vs. 0.06 microM in controls). Guanosine was also significantly elevated in a concentration-dependent manner, although the effect was not as impressive. Plasma nucleosides remained statistically-significantly elevated for up to four hr following a single oral dose of CI-972.

Cell differentiation and altered IMP dehydrogenase expression induced in human T-lymphoblastoid leukemia cells by mycophenolic acid and tiazofurin

The IMP dehydrogenase inhibitors mycophenolic acid (MPA) and tiazofurin (TZ) induce a time- and dose-dependent inhibition of cell growth, as well as differentiation in T-lymphoid CEM-2 leukemia cells. The differentiated cells have acquired a suppressor/cytotoxic T-lymphocyte phenotype characterized by reactivity with maturation-specific monoclonal antibodies. Coadministration of guanosine and hypoxanthine reduces the growth inhibition and diminishes the induction of differentiation by either MPA or TZ. No such reduction was observed for differentiation induced by phorbol 12-myristate 13-acetate (PMA), another inducer of a suppressor/cytotoxic phenotype in CEM-2 cells. During the first 2 days of treatment with MPA or TZ, a pattern of stable IMPDH mRNA levels and increased amounts of cellular enzyme was observed, perhaps, because of compensation for the inhibitor-mediated decrease in cellular IMPDH activity or a MPA- or TZ-mediated decrease in proteolysis of IMPDH. PMA treatment decreased the levels of IMPDH mRNA, protein, and activity. In addition, treatment of CEM-2 cells with either IMPDH inhibitors or PMA caused different alterations of the ribonucleotide pools. The lack of a consistent pattern of IMPDH expression in CEM-2 cells treated with IMPDH inhibitors or PMA indicates that no general association exists between the induction of cell differentiation and the expression of IMPDH. Nevertheless, our results indicating that IMPDH inhibitors can induce differentiation in CEM-2 cells suggest that this treatment may provide a useful approach to circumvent the differentiation block in some tumor cells.

Expression of deoxyadenosine and deoxyguanosine toxicity at different stages of lymphocyte activation

We have previously shown that deoxyguanosine (dGuo) is toxic to normal T and B lymphocytes, an effect mediated by intracellular accumulation of guanine ribonucleotides. In order to define the cellular processes that are sensitive to guanosine triphosphate (GTP) we have performed studies in which the effects of dGuo on normal T cells are compared with those of deoxyadenosine (dAdo) on adenosine deaminase (ADA)-deficient T cells. Kinetic studies show that dAdo exerts its toxic effects on processes that precede the onset of DNA synthesis, like interleukin 2 receptor expression, whereas dGuo added as late as 24-48 h after initiation of the culture still inhibits mitogen-induced proliferation. It can thus be concluded that dGuo toxicity as mediated through guanine ribonucleotides is manifested relatively late during the process of T-cell activation, whereas dAdo acts early in T-cell activation by a mechanism that cannot be explained by inhibition of ribonucleotide reductase.

Different pathways for deoxyguanosine toxicity in T-lymphocytes of various developmental stages

The basis of the selective cellular immunodeficiency which occurs in patients with purine nucleoside phosphorylase (PNP) deficiency still is not completely understood. We studied the mechanism of deoxyguanosine (dGuo) toxicity in proliferating lymphoid T-cells of different maturation stage, i.e. in T-cells of adult peripheral blood and cord blood and in CD3+ and CD3- subfractions of thymocytes. The mitogen-induced proliferation of T-cells from peripheral blood and cord blood and of CD3+ and CD3- subfractions of thymocytes. The mitogen-induced proliferation of T-cells from peripheral blood and cord blood and of CD3+ thymocytes, as well as the spontaneous proliferation of CD3- thymocytes, are inhibited by dGuo. CD3+ and CD3- thymocytes are significantly more sensitive to dGuo than T-cells from peripheral blood or cord blood. Among the thymocyte subfractions CD3- thymocytes appeared to be extremely sensitive. In all cell types studied, inhibition of proliferation is accompanied by intracellular increases in both guanosine triphosphate (GTP) and deoxyguanosine triphosphate (dGTP) concentrations. By use of the PNP inhibitor 8-aminoguanosine, or the metabolites hypoxanthine or deoxycytidine, the metabolism of dGuo could be selectively directed to the formation of GTP or to dGTP. Based on the pattern of rescue from dGuo intoxication under these different metabolic conditions we conclude that in CD3- thymocytes dGuo toxicity is mediated by dGTP. In all other cell types studied GTP mediates dGuo intoxication. Altogether the results show that during the maturation from immature thymocytes to mature peripheral blood T-cells a shift occurs in the pattern of dGuo toxicity since dGuo toxicity in the former is primarily caused via the dCyd kinase pathway, and in the latter mainly the degradation route is involved. Since in PNP deficiency mature T-cells do occur in the peripheral blood, we must conclude that some cells escape the stage of T-cell maturation in the thymus which is extremely sensitive to dGuo. Furthermore, the results imply that as far as T-cell development in the normal thymus is concerned, survival and death of cells might be regulated by local (deoxy) nucleoside availability.

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.

Potential use of purine nucleosides and enzyme inhibitors for selective depletion of Thy-lymphoblasts from human bone marrow

The toxicity of the purine nucleoside, deoxyadenosine in the presence of the adenosine deaminase inhibitor, deoxycoformycin and of deoxyguanosine in the presence of the purine nucleoside phosphorylase inhibitor, 8-aminoguanosine was measured against two Thy-leukemic cell lines. Toxicity was assessed by the survival of clonogenic cells in a colony assay. The kill of clonogenic Thy-leukemic cells was 99.99% with both nucleoside enzyme inhibitor combinations following 4-h incubations when 50 microM concentration of nucleoside were used. With these nucleoside concentrations some reduction in toxicity was apparent when drug treated cells were cultured in the presence of deoxycytidine (50 microM), however, this reduction in toxicity was not apparent when higher nucleoside concentrations were used (100 microM). Survival of bone marrow myeloid progenitor cells (CFU.GM) was only slightly reduced by these nucleoside concentrations following 4 hour incubations. The presence of a twenty-fold excess of normal bone marrow cells reduced the cytotoxic effect but clonogenic cell incubation still ranged from 99.98 to 99.99% for deoxyguanosine and deoxyadenosine respectively. These combinations of nucleosides and enzyme inhibitors may have a therapeutic role in the elimination of malignant Thy cells from human bone marrow.

Human B lymphocytes and thymocytes but not peripheral blood mononuclear cells accumulate high dATP levels in conditions simulating ADA deficiency

Inherited adenosine deaminase (ADA) deficiency is associated with a lymphospecific cytotoxicity affecting both dividing and non-dividing cells. The metabolic basis for this was investigated using different cell types and the potentially toxic metabolite 2'-deoxyadenosine (dAR) in short-term experiments under physiological conditions simulating ADA deficiency (1 mM Pi 8.7 microM dAR). In the uncultured cells, [8-14C] dAR alone was metabolized almost completely only by thymocytes and tonsil-derived B-lymphocytes. The greater percentage of counts (greater than 75%) were in the medium (deoxyinosine, hypoxanthine). Cellular counts were predominantly in adenine nucleotides, and to a lesser extent guanine nucleotides. Interestingly, both thymocytes and tonsil-derived B-lymphocytes, and a partially ADA deficient B lymphoblast line, accumulated detectable amounts of dATP even in the absence of ADA inhibition. Peripheral blood lymphocytes (PBMs) did not, and showed little dAR metabolism. In experiments simulating ADA deficiency varying amounts of 2'-deoxycoformycin (2'dCF) were needed to completely inhibit ADA (20-60 microM), with thymocytes requiring the highest amount. ADA inhibited thymocytes and tonsillar B-lymphocytes accumulated very high dATP levels, which were sustained to an equal extent by both over a 60-min period; PBMs accumulated the lowest values. Results in cultured cells reflected findings in previous studies. Some counts were also found in ATP by a route excluding ADA or PNP. These results again question the hypothesis that B-cells are more resistant than T-cells to the toxic effects of dAR because of an inability to accumulate and sustain elevated dATP levels and underline the lack of comparability between enzyme activity in intact as distinct from lysed cells. They cast doubt on the validity of cultured cells as a model for ADA deficiency and suggest the observed toxicity in some instances might result from altered ATP or GTP pools through inadequate ADA inhibition. They indicate that combined immunodeficiency in ADA deficiency could relate to an equal sensitivity of B-cells and T-cell precursors to the toxic effects of dATP accumulation.