Induction of human high K(M) 5'-nucleotidase in cultured 293 cells

Enhanced migration of breast and lung cancer cells deficient for cN-II and CD73 via COX-2/PGE2/AKT axis regulation

PURPOSE

Purine metabolism involves various intracellular and extracellular enzymes, including cN-II and CD73 that dephosphorylate intracellular and extracellular nucleoside monophosphates into their corresponding nucleosides. We conducted a study to better understand the biological roles of these enzymes in breast and lung cancer cells.

METHODS

We modified cN-II and/or CD73 expression in human breast cancer cells (MDA-MB-231), human lung cancer cells (NCI-H292) and murine breast cancer cells (4T1) using the CRISPR/Cas9 technique, and evaluated their impact on various cellular parameters such as proliferation, migration, invasion, intracellular nucleotide pools and nucleotide metabolism-related gene expression under extracellular nucleotide stress conditions.

RESULTS

Intracellular nucleotide contents were found to be altered in the modified cancer cell models both at their basal levels and after exposure to adenosine or AMP. Altered cN-II and CD73 levels were also found to be associated with cell migration and invasion alterations, involving TIMP-2, MMP-2 and MMP-9 expression, as well as alterations in the COX-2/PGE2/AKT pathway.

CONCLUSION

Our results highlight new cell-specific roles of cN-II and CD73 in cancer cell biology and provide insight into their interactions with different intracellular pathways.

Cytosolic 5'-Nucleotidase II Silencing in a Human Lung Carcinoma Cell Line Opposes Cancer Phenotype with a Concomitant Increase in p53 Phosphorylation

Purine homeostasis is maintained by a purine cycle in which the regulated member is a cytosolic 5'-nucleotidase II (cN-II) hydrolyzing IMP and GMP. Its expression is particularly high in proliferating cells, indeed high cN-II activity or expression in hematological malignancy has been associated to poor prognosis and chemoresistance. Therefore, a strong interest has grown in developing cN-II inhibitors, as potential drugs alone or in combination with other compounds. As a model to study the effect of cN-II inhibition we utilized a lung carcinoma cell line (A549) in which the enzyme was partially silenced and its low activity conformation was stabilized through incubation with 2-deoxyglucose. We measured nucleotide content, reduced glutathione, activities of enzymes involved in glycolysis and Krebs cycle, protein synthesis, mitochondrial function, cellular proliferation, migration and viability. Our results demonstrate that high cN-II expression is associated with a glycolytic, highly proliferating phenotype, while silencing causes a reduction of proliferation, protein synthesis and migration ability, and an increase of oxidative performances. Similar results were obtained in a human astrocytoma cell line. Moreover, we demonstrate that cN-II silencing is concomitant with p53 phosphorylation, suggesting a possible involvement of this pathway in mediating some of cN-II roles in cancer cell biology.

Stably transfected adherent cancer cell models with decreased expression of 5'-nucleotidase cN-II

The 5'-nucleotidase cN-II has been shown to be associated with the sensitivity to nucleoside analogues, the survival of cytarabine treated leukemia patients and to cell proliferation. Due to the lack of relevant cell models for solid tumors, we developed four cell lines with low cN-II expression and characterized them concerning their in vitro sensitivity to cancer drugs and their intracellular nucleotide pools. All four cell models had an important decrease of cN-II expression but did not show modified sensitivity, cell proliferation or nucleotide pools. Our cell models will be important for the study of the role of cN-II in human cancer cells.

The druggability of intracellular nucleotide-degrading enzymes

Nucleotide metabolism is the target of a large number of anticancer drugs including antimetabolites and specific enzyme inhibitors. We review scientific findings that over the last 10-15 years have allowed the identification of several intracellular nucleotide-degrading enzymes as cancer drug targets, and discuss further potential therapeutic applications for Rcl, SAMHD1, MTH1 and cN-II. We believe that enzymes involved in nucleotide metabolism represent potent alternatives to conventional cancer chemotherapy targets.

Cell proliferation and drug sensitivity of human glioblastoma cells are altered by the stable modulation of cytosolic 5'-nucleotidase II

Cytosolic 5'-nucleotidase II (cN-II) has been reported to be involved in cell survival, nucleotide metabolism and in the cellular response to anticancer drugs. With the aim to further evaluate the role of this enzyme in cell biology, we stably modulated its expression the human glioblastoma cell ADF in which the transient inhibition of cN-II has been shown to induce cell death. Stable cell lines were obtained both with inhibition, obtained with plasmids coding cN-II-targeting short hairpin RNA, and stimulation, obtained with plasmids coding Green Fluorescence Protein (GFP)-fused wild type cN-II or a GFP-fused hyperactive mutant (GFP-cN-II-R367Q), of cN-II expression. Silenced cells displayed a decreased proliferation rate while the over expressing cell lines displayed an increased proliferation rate as evidenced by impedance measurement using the xCELLigence device. The expression of nucleotide metabolism relevant genes was only slightly different between cell lines, suggesting a compensatory mechanism in transfected cells. Cells with decreased cN-II expression were resistant to the nucleoside analog fludarabine confirming the involvement of cN-II in the metabolism of this drug. Finally, we observed sensitivity to cisplatin in cN-II silenced cells and resistance to this same drug in cN-II over-expressing cells indicating an involvement of cN-II in the mechanism of action of platinum derivatives, and most probably in DNA repair. In summary, our findings confirm some previous data on the role of cN-II in the sensitivity of cancer cells to cancer drugs, and suggest its involvement in other cellular phenomenon such as cell proliferation.

Determination of the enzymatic activity of cytosolic 5'-nucleotidase cN-II in cancer cells: development of a simple analytical method and related cell line models

The cytosolic 5'-nucleotidase (cN-II) has been shown to be involved in the response of cancer cells to cytotoxic agents, and the quantification of its activity in biological samples is of great interest. In this context, we developed and validated an analytical method for determination of cN-II activity in cultured cancer cells. This non-radioactive method, using a Hypercarb column as stationary phase, was validated with a lower limit of quantification of 0.1 μM inosine. We used it to characterize cell line models with modified cN-II expression obtained with stable transfections. We show that the short hairpin RNA (shRNA)-mediated inhibition of cN-II expression in various malignant blood cells is associated with decreased protein expression and enzymatic activity (1.7-6.2-fold) as well as an increased sensitivity to cytotoxic agents (up to 14-fold). On the other hand, expression of green fluorescent protein (GFP)-fused wild type or hyperactive mutant (R367Q) cN-II increased the activity and also decreased the sensitivity to nucleoside analogues. Our results confirm the biological relevance of modulating cN-II in cancer cells, and we present a straightforward validated method for the determination of cN-II activity in cellular samples.

IMP-GMP specific cytosolic 5'-nucleotidase regulates nucleotide pool and prodrug metabolism

BACKGROUND

Type II cytosolic 5'-nucleotidase (cN-II) catalyzes the hydrolysis of purine and, to some extent, of pyrimidine monophosphates. Recently, a number of papers demonstrated the involvement of cN-II in the mechanisms of resistance to antitumor drugs such as cytarabine, gemcitabine and fludarabine. Furthermore, cN-II is involved in drug resistance in patients affected by hematological malignancies influencing the clinical outcome. Although the implication of cN-II expression and/or activity appears to be correlated with drug resistance and poor prognosis, the molecular mechanism by which cN-II mediates drug resistance is still unknown.

METHODS

HEK 293 cells carrying an expression vector coding for cN-II linked to green fluorescent protein (GFP) and a control vector without cN-II were utilized. A highly sensitive capillary electrophoresis method was applied for nucleotide pool determination and cytotoxicity exerted by drugs was determined with 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay.

RESULTS

Over-expression of cN-II causes a drop of nucleoside triphosphate concentration and a general disturbance of nucleotide pool. Over-expressing cells were resistant to fludarabine, gemcitabine and cytarabine independently of cN-II ability to hydrolyze their monophosphates.

CONCLUSIONS

An increase of cN-II expression is sufficient to cause both a general disturbance of nucleotide pool and an increase of half maximal inhibitory concentration (IC50) of the drugs. Since the monophosphates of cytarabine and gemcitabine are not substrates of cN-II, the protection observed cannot be directly ascribed to drug inactivation.

GENERAL SIGNIFICANCE

Our results indicate that cN-II exerts a relevant role in nucleotide and drug metabolism through not only enzyme activity but also a mechanism involving a protein-protein interaction, thus playing a general regulatory role in cell survival.

SENTENCE

Resistance to fludarabine, gemcitabine and cytarabine can be determined by an increase of cN-II both through dephosphorylation of active drugs and perturbation of nucleotide pool.

Cytosolic 5'-nucleotidase II interacts with the leucin rich repeat of NLR family member Ipaf

IMP/GMP preferring cytosolic 5'-nucleotidase II (cN-II) is a bifunctional enzyme whose activities and expression play crucial roles in nucleotide pool maintenance, nucleotide-dependent pathways and programmed cell death. Alignment of primary amino acid sequences of cN-II from human and other organisms show a strong conservation throughout the entire vertebrata taxon suggesting a fundamental role in eukaryotic cells. With the aim to investigate the potential role of this homology in protein-protein interactions, a two hybrid system screening of cN-II interactors was performed in S. cerevisiae. Among the X positive hits, the Leucin Rich Repeat (LRR) domain of Ipaf was found to interact with cN-II. Recombinant Ipaf isoform B (lacking the Nucleotide Binding Domain) was used in an in vitro affinity chromatography assay confirming the interaction obtained in the screening. Moreover, co-immunoprecipitation with proteins from wild type Human Embryonic Kidney 293 T cells demonstrated that endogenous cN-II co-immunoprecipitated both with wild type Ipaf and its LRR domain after transfection with corresponding expression vectors, but not with Ipaf lacking the LRR domain. These results suggest that the interaction takes place through the LRR domain of Ipaf. In addition, a proximity ligation assay was performed in A549 lung carcinoma cells and in MDA-MB-231 breast cancer cells and showed a positive cytosolic signal, confirming that this interaction occurs in human cells. This is the first report of a protein-protein interaction involving cN-II, suggesting either novel functions or an additional level of regulation of this complex enzyme.

Structure-based design of a bisphosphonate 5′(3′)-deoxyribonucleotidase inhibitor

Based on previously known inhibitor–enzyme complex structures, we developed a promising inhibitor by mimicking the phosphate ion and achieved 50- and 100-fold increases in the inhibitory potency towards cdN and mdN, respectively.

Expression of bovine cytosolic 5'-nucleotidase (cN-II) in yeast: nucleotide pools disturbance and its consequences on growth and homologous recombination

Cytosolic 5'-nucleotidase II is a widespread IMP hydrolyzing enzyme, essential for cell vitality, whose role in nucleotide metabolism and cell function is still to be exactly determined. Cytosolic 5'-nucleotidase overexpression and silencing have both been demonstrated to be toxic for mammalian cultured cells. In order to ascertain the effect of enzyme expression on a well-known eukaryote simple model, we expressed cytosolic 5'-nucleotidase II in Saccharomyces cerevisiae, which normally hydrolyzes IMP through the action of a nucleotidase with distinct functional and structural features. Heterologous expression was successful. The yeast cells harbouring cytosolic 5'-nucleotidase II displayed a shorter duplication time and a significant modification of purine and pyrimidine derivatives concentration as compared with the control strain. Furthermore the capacity of homologous recombination in the presence of mutagenic compounds of yeast expressing cytosolic 5'-nucleotidase II was markedly impaired.

The pyrimidine nucleotide carrier PNC1 and mitochondrial trafficking of thymidine phosphates in cultured human cells

In cycling cells cytosolic de novo synthesis of deoxynucleotides is the main source of precursors for mitochondrial (mt) DNA synthesis. The transfer of deoxynucleotides across the inner mt membrane requires protein carriers. PNC1, a SLC25 family member, exchanges pyrimidine nucleoside triphosphates in liposomes and its downregulation decreases mtUTP concentration in cultured cells. By an isotope-flow protocol we confirmed transport of uridine nucleotides by PNC1 in intact cultured cells and investigated PNC1 involvement in the mt trafficking of thymidine phosphates. Key features of our approach were the manipulation of PNC1 expression by RNA interference or inducible overexpression, the employment of cells proficient or deficient for cytosolic thymidine kinase (TK1) to distinguish the direction of flow of thymidine nucleotides across the mt membrane during short pulses with [(3)H]-thymidine, the determination of mtdTTP specific radioactivity to quantitate the rate of mtdTTP export to the cytoplasm. Downregulation of PNC1 in TK1(-) cells increased labeled dTTP in mitochondria due to a reduced rate of export. Overexpression of PNC1 in TK1(+) cells increased mtdTTP pool size and radioactivity, suggesting an involvement in the import of thymidine phosphates. Thus PNC1 is a component of the network regulating the mtdTTP pool in human cells.

The 5'-nucleotidases as regulators of nucleotide and drug metabolism

The 5'-nucleotidases are a family of enzymes that catalyze the dephosphorylation of nucleoside monophosphates and regulate cellular nucleotide and nucleoside levels. While the nucleoside kinases responsible for the initial phosphorylation of salvaged nucleosides have been well studied, many of the catabolic nucleotidases have only recently been cloned and characterized. Aside from maintaining balanced ribo- and deoxyribonucleotide pools, substrate cycles that are formed with kinase and nucleotidase activities are also likely to regulate the activation of nucleoside analogues, a class of anticancer and antiviral agents that rely on the nucleoside kinases for phosphorylation to their active forms. Both clinical and in vitro studies suggest that an increase in nucleotidase activity can inhibit nucleoside analogue activation and result in drug resistance. The physiological role of the 5'-nucleotidases will be covered in this review, as will the evidence that these enzymes can mediate resistance to nucleoside analogues.

Substrate cycles and drug resistance to 1-beta-D-arabinofuranosylcytosine (araC)

Acute myelogenous leukemia (AML) is the most common form of acute leukemia in adults. After diagnosis, patients with AML are mainly treated with standard induction chemotherapy combining cytarabine (araC) and anthracyclines. The majority of them achieve complete remission (CR) (65-80%). However, prospects for long-term survival are poor for the majority of patients. Resistance to chemotherapy therefore remains a major obstacle in the effective treatment of patients with AML. In this review, we highlight the current knowledge of substrate cycles involved in normal deoxynucleoside triphosphate (dNTPs) metabolism and their possible role in drug resistance to araC.

Regulation and activity of cytosolic 5′-nucleotidase II

In many vertebrate tissues, cytosolic 5'-nucleotidase II (cN-II) either hydrolyses or phosphorylates a number of purine (monophosphorylated) nucleosides through a scheme common to the Haloacid Dehalogenase superfamily members. It possesses a pivotal role in purine cellular metabolism and it acts on anti-tumoural and antiviral nucleoside analogues, thus being of potential therapeutic importance. cN-II is Mg2+-dependent, regulated and stabilised by several factors such as allosteric effectors ATP and 2,3-DPG, although these are not directly involved in the reaction stoichiometry. We review herein the experimental knowledge currently available about this remarkable enzymatic activity.

Pharmacological basis for cladribine resistance

The inherent or acquired resistance of leukemic cells to cytostatic agents is a major clinical challenge. The purpose of this review was to elucidate and analyse the available data concerning mechanisms of resistance of cladribine with emphasis on recent advances in the characterization of activating and inactivating enzymes in the induction of resistance to cladribine. All available in vitro and clinical data on cladribine was undertaken. Cladribine, unlike many other drugs, is toxic to both dividing and indolent lymphoid malignancies. Cladribine is a prodrug and must be phosphorylated intracellularly to cladribine-monophosphate (MP) by the nuclear/cystosol enzyme deoxycytidine kinase (dCK) and the mitochondrial enzyme deoxyguanosine kinase. The cytotoxicity mainly depends on the accumulation of cladribine-triphosphates (TP) after phosphorylation of cladribine-MP by nucleoside monophosphate kinase and nucleoside diphosphate kinase. 5'-Nucleotidase (5'-NT) dephosphorylates cladribine-MP and the accumulation of cladribine-TP depends on the ratio of dCK and 5'-NT in the cells. The mechanisms underlying cladribine resistance are multifactorial, e.g. decreased nucleoside transport, decreased activity or deficiency of dCK, altered intracellular pools of competing nucleotides, altered regulation of ribonucleotide reductase and increased drug inactivation by 5'-NT. Finally, cladribine resistance may be a consequence of a defective induction of apoptosis. In spite of the fact that more than one mechanism can contribute to a cladribine resistance phenotype, a reduction in dCK activity is probably the major determinant of cladribine resistance. Insight into the mechanism of action and resistance to cladribine is crucial for its optimal use as well as for the development of newer analogues.

Cytosolic and mitochondrial deoxyribonucleotidases: activity with substrate analogs, inhibitors and implications for therapy

Nucleoside analogs act as prodrugs that must be converted to 5'-phosphates by intracellular kinases to become active in the treatment of viral and oncological diseases. Activation may be reversed by dephosphorylation if the 5'-phosphates are substrates for 5'-nucleotidases. Dephosphorylation by cytosolic enzymes decreases the efficacy of the analogs, whereas dephosphorylation by mitochondrial enzymes may decrease mitochondrial toxicity. Both effects may influence the outcome of therapy. We investigated the dephosphorylation of the 5'-phosphates of commonly used nucleoside analogs by two cytosolic (cN-II and dNT-1) and one mitochondrial (dNT-2) nucleotidase. Most uracil/thymine nucleotide analogs were dephosphorylated by all three human enzymes but cytosine-containing nucleotide analogs were inactive. Only cN-II showed some activity with the monophosphates of the two purine analogs 2-chloro-2'-deoxyadenosine and 9-beta-D-arabinosylguanine. We conclude that overproduction of any of the three 5'-nucleotidases cannot explain development of resistance against cytosine analogs but that overproduction of cN-II could lead to resistance against purine analogs. Of the tested analogs, only (E)-5-(2-bromovinyl)-2'-deoxyuridine was preferentially dephosphorylated by mitochondrial dNT-2. We propose that in future developments of analogs this aspect be considered in order to reduce mitochondrial toxicity. We tested inhibition of dNT-1 and dNT-2 by a large variety of synthetic metabolically stable nucleoside phosphonate analogs and found one (PMcP-U) that inhibited dNT-1 and dNT-2 competitively and a second (DPB-T) that inhibited dNT-2 by mixed inhibition. Both inhibitors are useful for specific 5'-nucleotidase assays and structural studies and may open up possibilities for therapy.

Human mitochondrial 5'-deoxyribonucleotidase. Overproduction in cultured cells and functional aspects

Deoxynucleoside triphosphates (dNTPs) used for mitochondrial DNA replication are mainly formed by phosphorylation of deoxynucleosides imported into mitochondria from the cytosol. We earlier obtained evidence for a mitochondrial 5'-nucleotidase (dNT2) with a pronounced specificity for dUMP and dTMP and suggested that the enzyme protects mitochondrial DNA replication from excess dTTP. In humans, accumulation of dTTP causes a mitochondrial genetic disease. We now establish that dNT2 in vivo indeed is located in mitochondria. The native enzyme shows the same substrate specificity and affinity for inhibitors as the recombinant dNT2. We constructed ponasterone-inducible cell lines overproducing dNT2 with and without the green fluorescent protein (GFP) linked to its C terminus. The fusion protein occurred in mitochondria mostly in an inactive truncated form, with only a short C-terminal fragment of dNT2 linked to GFP. No truncation occurred when dNT2 and GFP were not linked. The cell mitochondria then contained a large excess of active dNT2 with or without the mitochondrial presequence. After removal of ponasterone overproduced dNT2 disappeared only slowly from the cells, whereas dNT2-mRNA was lost rapidly. Overproduction of dNT2 did not lead to an increased excretion of pyrimidine deoxyribonucleosides, in contrast to overproduction of the corresponding cytosolic deoxynucleotidase, suggesting that the mitochondrial enzyme does not affect overall cellular deoxynucleotide turnover.

5'-Nucleotidases: specific assays for five different enzymes in cell extracts

Several mammalian 5'-nucleotidases (5-NTs), attached to membranes or present in the cytosol or in mitochondria, remove the phosphate from ribo- and deoxyribonucleotides with different specificities for the sugar and base moieties. Some enzymes probably participate in signaling functions by producing adenosine from AMP. A more general function may be to prevent overproduction of deoxyribonucleotides. 5-NTs may affect the pharmacological activity of nucleoside analogs and also be involved in their mitochondrial toxicity. Here we describe for five cloned 5-NT specific assays that largely rely on new inhibitors for some of the enzymes. The assays can be used to quantitate each enzyme in crude cell extracts. To ascertain their validity we applied each assay to extracts from genetically modified cells that overproduce separately each of the five enzymes. The methodology should be useful in further studies of the physiological function of 5-NTs and their influence on the clinical use of nucleoside analogs.

Resistance of leukemic cells to 2-chlorodeoxyadenosine is due to a lack of calcium-dependent cytochrome c release

The purine nucleoside 2-chlorodeoxyadenosine (CdA) is often used in leukemia therapy. Its efficacy, however, is compromised by the emergence of resistant cells. In the present study, 3 CdA-resistant cell lines were generated and characterized. Their ability to accumulate 2-chloroadenosine triphosphate (CdATP) varied, reflecting differences in activities of deoxycytidine kinase (dCK) and deoxyguanosine kinase (dGK). Nonetheless, the selected lines were uniformly resistant to CdA-induced apoptosis, as assessed by caspase activation and DNA fragmentation. In contrast, cytosols from resistant cells were capable of robust caspase activation when incubated in the presence of cytochrome c and dATP. Moreover, replacement of dATP with CdATP also resulted in caspase activation in the parental and some of the resistant cell lines. Strikingly, CdA-induced decreases in mitochondrial transmembrane potential and release of cytochrome c from mitochondria were observed in the parental cells but not in any resistant lines. The lack of cytochrome c release correlated with an increased ability of mitochondria from resistant cells to sequester free Ca2+. Consistent with this enhanced Ca2+ buffering capacity, an early increase in cytosolic Ca2+ after CdA treatment of parental cells but not resistant cells was detected. Furthermore, CdA-resistant cells were selectively cross-resistant to thapsigargin but not to staurosporine- or Fas-induced apoptosis. In addition, CdA-induced caspase-3 activation and DNA fragmentation were inhibited by the Ca2+ chelator BAPTA-AM in sensitive cells. Taken together, the data indicate that the mechanism of resistance to CdA may be dictated by changes in Ca2+-sensitive mitochondrial events.

Pharmacological basis for cladribine resistance in a human acute T lymphoblastic leukaemia cell line selected for resistance to etoposide

Cross-resistance between different classes of anti-neoplastic agents can jeopardize successful combination cancer chemotherapy. In this study, we observed an unexpected cross-resistance between the podophyllotoxine derivative etoposide (VP) and the nucleoside analogue cladribine (CdA) in CCRF-CEM cells developed for resistance to VP. The resistant cells also displayed 14- and twofold resistance to cytarabine (ara-C) and gemcitabine respectively. Closer analysis of these cells showed that they contained lower amounts of topoisomerase (topo) IIalpha (P < 0.001) and beta protein (P < 0.026), formed substantially lower amounts of the topo II-DNA complex, and had a markedly decreased level of Fas (CD95/APO-1)-ligand mRNA expression. Interestingly, Fas expression in the resistant cells did not differ from that in the parental cell line. No differences were observed in the accumulation/efflux of daunorubicin or in the gene expressions of P-glycoprotein, multidrug resistance-associated protein and the lung resistance-related protein. The activity of deoxycytidine kinase (dCK), responsible for activation of CdA and ara-C, was the same for resistant and wild-type cells. However, there was an increase in the activity of the cytosolic 5'-nucleotidases (5'-NT), responsible for deactivation of nucleotides, amounting to 206% (P < 0.001) for the high Km and 134% (P < 0.331) for the low Km 5'-NT in resistant cells. The high Km 5'-NT is probably responsible for the decreased amount of the active metabolite CdA 5'-triphosphate [40% decreased (P < 0.045)], as well as for other purine ribonucleosides and deoxyribonucleosides triphosphates in the resistant cells. In contrast, a significantly higher deoxycytidine triphosphate (dCTP) level (167%, P < 0.001) was observed in the resistant cells. Thus, this study suggests that the major cause of resistance to the nucleoside analogues CdA and ara-C in cells selected for resistance to VP is a result of metabolic alterations producing increased activity of 5'-NT and higher dCTP levels. Furthermore, these results indicate that there is a common factor in the regulation of nucleotide-degrading enzymes and DNA topoisomerases, which may be altered in cross-resistant cells.

Cytosolic high K(m) 5'-nucleotidase and 5'(3')-deoxyribonucleotidase in substrate cycles involved in nucleotide metabolism

5'-Nucleotidases are the catabolic members of the substrate cycles postulated to be involved in the regulation of intracellular deoxyribonucleoside triphosphate pools. Here, we attempt to identify the nature of the nucleotidases. Earlier, we constructed various mammalian cell lines that can be induced to overproduce the high K(m) 5'-nucleotidase (hkm-NT) or the 5'(3')-deoxynucleotidase (dNT-1). Now we labeled control and induced human 293 cells and hamster V79 cells with radioactive hypoxanthine or uridine and during a chase measured quantitatively the metabolism of ribo- and deoxyribonucleotides, DNA replication, and excretion of nucleosides into the medium. Overproduction of hkm-NT greatly increased excretion of inosine and guanosine but did not affect adenosine or deoxyribonucleosides. dNT-1 overproduction increased excretion of deoxycytidine, thymidine, and in particular deoxyuridine but also uridine and cytidine. We conclude that the hkm-NT is not involved in the regulation of deoxyribonucleotide pools but affects IMP and GTP pools. dNT-1, instead, appears to be the catabolic arm of substrate cycles regulating pyrimidine nucleotide pools.

Identification of ligands and coligands for the ecdysone-regulated gene switch

The ecdysone-inducible gene switch is a useful tool for modulating gene expression in mammalian cells and transgenic animals. We have identified inducers derived from plants as well as certain classes of insecticides that increase the versatility of this gene regulation system. Phytoecdysteroids share the favorable kinetics of steroids, but are inert in mammals. The gene regulation properties of one of these ecdysteroids have been examined in cell culture and in newly developed strains of ecdysone-system transgenic mice. Ponasterone A is a potent regulator of gene expression in cells and transgenic animals, enabling reporter genes to be turned on and off rapidly. A number of nonsteroidal insecticides have been identified that also activate the ecdysone system. Because the gene-controlling properties of the ecdysone switch are based on a heterodimer composed of a modified ecdysone receptor (VgEcR) and the retinoid X receptor (RXR), we have tested the effect of RXR ligands on the VgEcR/RXR complex. Used alone, RXR ligands display no activity on the ecdysone switch. However, when used in combination with a VgEcR ligand, RXR ligands dramatically enhance the absolute levels of induction. This property of the heterodimer has allowed the development of superinducer combinations that increase the dynamic range of the system.

A deoxyribonucleotidase in mitochondria: involvement in regulation of dNTP pools and possible link to genetic disease

Three cytosolic and one plasma membrane-bound 5'-nucleotidases have been cloned and characterized. Their various substrate specificities suggest widely different functions in nucleotide metabolism. We now describe a 5'-nucleotidase in mitochondria. The enzyme, named dNT-2, dephosphorylates specifically the 5'- and 2'(3')-phosphates of uracil and thymine deoxyribonucleotides. The cDNA of human dNT-2 codes for a 25.9-kDa polypeptide with a typical mitochondrial leader peptide, providing the structural basis for two-step processing during import into the mitochondrial matrix. The deduced amino acid sequence is 52% identical to that of a recently described cytosolic deoxyribonucleotidase (dNT-1). The two enzymes share many catalytic properties, but dNT-2 shows a narrower substrate specificity. Mitochondrial localization of dNT-2 was demonstrated by the mitochondrial fluorescence of 293 cells expressing a dNT-2-green fluorescent protein (GFP) fusion protein. 293 cells expressing fusion proteins without leader peptide or with dNT-1 showed a cytosolic fluorescence. During in vitro import into mitochondria, the preprotein lost the leader peptide. We suggest that dNT-2 protects mitochondrial DNA replication from overproduction of dTTP, in particular in resting cells. Mitochondrial toxicity of dTTP can be inferred from a severe inborn error of metabolism in which the loss of thymidine phosphorylase led to dTTP accumulation and aberrant mitochondrial DNA replication. We localized the gene for dNT-2 on chromosome 17p11.2 in the Smith-Magenis syndrome-critical region, raising the possibility that dNT-2 is involved in the etiology of this genetic disease.

Mammalian 5'(3')-deoxyribonucleotidase, cDNA cloning, and overexpression of the enzyme in Escherichia coli and mammalian cells

5'(3')-Deoxyribonucleotidase is a ubiquitous enzyme in mammalian cells whose physiological function is not known. It was earlier purified to homogeneity from human placenta. We determined the amino acid sequences of several internal peptides and with their aid found an expressed sequence tag clone with the complete cDNA for a murine enzyme of 23.9 kDa. The DNA was cloned into appropriate plasmids and introduced into Escherichia coli and ecdyson-inducible 293 and V79 cells. The recombinant enzyme was purified to homogeneity from transformed E. coli and was found to be identical with the native enzyme. After induction with ponasterone, the transfected mammalian cells showed a gradual increase of enzyme activity. A human expressed sequence tag clone contained a large part of the cDNA of the human enzyme but lacked the 5'-end corresponding to 51 amino acids of the murine enzyme. Several polymerase chain reaction-based approaches to find this sequence met with no success. A mouse/human hybrid cDNA that had substituted the missing human 5'-end with the corresponding mouse sequence coded for a fully active enzyme.