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

Cell-free DNA in plasma as an essential immune system regulator

The cell-free DNA (cfDNA) is always present in plasma, and it is biomarker of growing interest in prenatal diagnostics as well as in oncology and transplantology for therapy efficiency monitoring. But does this cfDNA have a physiological role? Here we show that cfDNA presence and clearance in plasma of healthy individuals plays an indispensable role in immune system regulation. We exposed THP1 cells to healthy individuals' plasma with (NP) and without (TP) cfDNA. In cells treated with NP, we found elevated expression of genes whose products maintain immune system homeostasis. Exposure of cells to TP triggered an innate immune response (IIR), documented particularly by elevated expression of pro-inflammatory interleukin 8. The results of mass spectrometry showed a higher abundance of proteins associated with IIR activation due to the regulation of complement cascade in cells cultivated with TP. These expression profiles provide evidence that the presence of cfDNA and its clearance in plasma of healthy individuals regulate fundamental mechanisms of the inflammation process and tissue homeostasis. The detailed understanding how neutrophil extracellular traps and their naturally occurring degradation products affect the performance of immune system is of crucial interest for future medical applications.

Inhibition of Duck Hepatitis B Virus Replication in vivo by the Nucleoside Analogue Ganciclovir (9-[2-hydroxy-1-(hydroxymethyl) ethoxymethyl] Guanine)

Treatment of ducks congenitally infected with the duck hepatitis B virus (DHBV) using the guanosine analogue ganciclovir resulted in prompt and profound inhibition of viral DNA replication in serum and liver. By the end of the treatment period all the replicative intermediates, except the supercoiled DNA form, could not be detected. Within 2 weeks of cessation of treatment viral replication returned and, in some cases, rebound occurred. Sequential treatment with prednisolone followed by ganciclovir also resulted in inhibition of viral replication and, even though relapse was observed after therapy was discontinued, the rebound phenomenon was reduced. Ganciclovir significantly and selectively inhibited DHBV DNA replication but may be more efficacious if used in combination with compounds targeted to the viral supercoiled DNA form.

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.

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

Human 293 cells were stably transfected with a plasmid introducing a receptor for the ecdysone analog muristerone. The cells were further stably transfected with muristerone-inducible expression vectors carrying either the cDNA for the human high K(M) 5'-nucleotidase or the coding sequence of the nucleotidase linked to the 5'-end of the sequence for the green fluorescent protein. Upon induction, both types of transfectants overproduced nucleotidase activity in a time- and dose-dependent manner. Western blots gave values close to the expected subunit molecular masses of 65 and 92 kDa, respectively, excluding processing of the induced proteins. Cells induced to overexpress the nucleotidase showed a decreased growth rate and contained smaller pools of each of the four common ribonucleoside triphosphates. They showed no increased resistance to the toxicity of 2-chlorodeoxyadenosine.

Human high-Km 5'-nucleotidase effects of overexpression of the cloned cDNA in cultured human cells

5'-Nucleotidases participate, together with nucleoside kinases, in substrate cycles involved in the regulation of deoxyribonucleotide metabolism. Three major classes of nucleotidases are known, one on the plasma membrane and two in the cytosol. The two cytosolic classes have been named high-Km nucleotidases and 5'(3')-nucleotidases. Starting from two plasmids with partial sequences (Oka, J., Matsumoto, A., Hosokawa, Y. & Inoue, S. (1994) Biochem. Biophys. Res. Commun. 205, 917-922) we cloned the complete cDNA of the human high-Km nucleotidase into vectors suitable for transfection of Escherichia coli or mammalian cells. After transfection, E. coli overproduced large amounts of the enzyme. Most of the enzyme was present in inclusion bodies that also contained many partially degraded products of the protein. Part of the enzyme, corresponding to approximately 2% of the soluble proteins, was in a soluble active form. Stably transfected human 293 cells were obtained with a vector where the 3'-end of the nucleotidase coding sequence is linked to the 5'-end of the green fluorescent protein coding sequence. Several green clones overproduced both mRNA and fusion protein. Two clones with 10-fold higher enzyme activity were analyzed further. The nucleotidase activity of cell extracts showed the same substrate specificity and allosteric regulation as the high-Km enzyme. The growth rate of the two clones did not differ from the controls. The cells were not resistant to deoxyguanosine or deoxyadenosine, and did not show an increased ability to phosphorylate dideoxyinosine. Both ribonucleoside and deoxyribonucleoside triphosphate pools were decreased slightly, suggesting participation of the enzyme in their regulation.

Modulation of the cleavage of glycosylphosphatidylinositol-anchored proteins by specific bacterial phospholipases

Many enzymes are tethered to the extracellular face of the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor. These proteins can be released in soluble form by the action of GPI-specific phospholipase. Little is currently known about the factors modulating this release. We investigated the effects of several experimental variables on the cleavage of the GPI-anchored proteins 5'nucleotidase, acetylcholinesterase, and alkaline phosphatase by phospholipases from Bacillus thuringiensis and Staphylococcus aureus. Phospholipase activity was not inhibited by isotonic salt and was relatively unaffected by buffer type and concentration. In both cases, the optimum pH for cleavage was approximately 6.5. Over 80% of 5'-nucleotidase activity present in the lymphocyte plasma membrane was cleaved by the B. thuringiensis enzyme, and the initial rate of release was linear with phospholipase concentration. All three GPI-anchored proteins were released from lymphocyte plasma membrane at comparable phospholipase concentrations, suggesting that they have similar anchor structures. The catalytic activity of 5'-nucleotidase appeared to increase following conversion to the soluble form. The relative surface charge of the host plasma membrane modulated catalytic activity towards GPI-anchored proteins, depending on the net charge of the phospholipase. Studies on purified lymphocyte 5'-nucleotidase reconstituted into bilayers of dimyristoylphosphatidylcholine indicated that the efficiency of phospholipase cleavage was 12- to 50-fold lower when compared with the native plasma membrane. The ability of the phospholipase to cleave the GPI anchor was further reduced when the bilayer was in the gel phase.

Mammalian deoxyribonucleoside kinases

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

Deoxycytidine kinase and deoxycytidine deaminase activities in human tumour xenografts

Deoxycytidine kinase (dCK) and deaminase (dCDA) are both key enzymes in the activation and inactivation, respectively, of several deoxycytidine antimetabolites. We determined the total dCK and dCDA activities using standard assays, in 28 human solid tumours grown as xenografts in nude mice, and four corresponding cell lines. dCK activities in colon tumours varied from 11 to 12 nmol/h/mg protein, in ovarian tumours from 3 to 10 nmol/h/mg protein, in soft tissue sarcomas from 2 to 7 nmol/h/mg protein and in squamous cell carcinomas of the head and neck about 45-fold, between 0.4 and 18 nmol/h/mg protein. The dCDA activities showed a larger variation, from 243 to 483, 14 to 1231, 3 to 7 and 1 to 222 nmol/h/mg protein, respectively. The ratios of dCK vs. dCDA activities in these tumours varied from 0.025 to 0.046, 0.004 to 0.240, 0.581 to 1.123 and from 0.012 to 4.227, respectively. In four cell lines (A2780, OVCAR-3, WiDr and UM-SCC-14C), sources for some of the above mentioned tumours, a different pattern in dCK and dCDA was observed than in the corresponding tumours. The variation in dCDA activities was in a smaller range (20-fold) than in the tumours (40-fold). In all cell lines dCK activity was higher than dCDA activity, in contrast to the corresponding tumours, in which the reverse pattern was observed. Previously, some of the tumours were tested for sensitivity to the deoxycytidine analogues 5-aza-deoxycytidine and 2',2'-difluorodeoxycytidine. In the sensitive tumours, both the highest and lowest dCK activity was observed, indicating that dCK activity in solid tumours is high enough to activate deoxycytidine analogues.

Quantitative immunoassay of human deoxycytidine kinase in malignant cells

Deoxycytidine kinase (dCK) is necessary for the activity of several nucleosides used for the chemotherapy of cancer and AIDS. However, the measurement of dCK catalytic activity in crude cell extracts may be imprecise, due to the presence of phosphatases and nucleotidases that degrade the enzyme products. We describe a simple immunoassay for dCK that can measure accurately as little as 5 ng enzyme protein in crude tissue extracts. The assay enabled us to show (i) that mutant cells deficient in dCK activity lack immunoreactive dCK protein, (ii) that dCK catalytic activity and immunoreactivity correlate closely in human tumors, and (iii) that immunoreactive dCK is particularly high in lymphocytes and lymphoid malignancies, although certain solid tumors may also contain the enzyme. The immunoassay of dCK could prove useful in the selection and monitoring of patients who are being treated with nucleosides that are activated by this enzyme.

Deoxyribonucleotide metabolism in hydroxyurea-resistant V79 hamster cells

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