Metabolism, incorporation into DNA, and interactions with 1-beta-D-arabinofuranosylcytosine of 5-hydroxymethyl-2'-deoxyuridine in human promyelocytic leukemia cells (HL-60)

5-Hydroxymethyl-2'-deoxyuridine (5HmdUrd) and 1-beta-D-arabinofuranosylcytosine (Ara-C) had a dose-dependent synergistic or antagonistic action on growth of human promyelocytic leukemic (HL-60) cells in suspension culture. For instance, in 3-day cultures, the cell number was reduced from 100% (with either 100 nM Ara-C or 10 microM 5HmdUrd alone) to 65% (with 100 nM Ara-C plus 10 microM 5HmdUrd), or from 35% (with 1.0 microM Ara-C alone) to 10% (with 1.0 microM Ara-C plus 10 microM 5HmdUrd), compared to the control cultures without drugs. 1.0 and 10 microM 5HmdUrd potentiated the incorporation of radioactive Ara-C (1.0 microM) into HL-60 cell nucleic acids in 2-day cultures by 56 and 64%, respectively. 5HmdUrd-induced enhancement of Ara-C incorporation is one explanation for the synergism of these two drugs. On the other hand, 10 nM Ara-C partially inhibited the toxicity of 100 microM 5HmdUrd. Radioactive 5HmdUrd was incorporated into DNA, but not RNA, the rate being 5% of that observed with thymidine. [3H]5HmdUrd-derived radioactivity remained stable in DNA for at least 24 h, indicating that the compound was not excised to a significant extent from DNA in these conditions. The incorporation of Ara-C and 5HmdUrd into DNA appeared to take place via different pathways, which is a second explanation for their synergism. Ara-C is the most important drug in the clinical chemotherapy of acute nonlymphoblastic leukemia. Experience with 5HmdUrd in experimental antileukemia chemotherapy has been promising. This novel combination of antileukemic agents merits further evaluation.

The DNA-repair enzyme uracil-DNA glycosylase in the human hematopoietic system

The expression of the DNA base-excision-repair enzyme uracil-DNA glycosylase in the human hematopoietic system followed a tightly regulated pattern: high enzyme activities were recorded in proliferating bone marrow progenitor cells and in peripheral blood T- and B-cells, both groups of cells requiring the integrity of their genetic information for their proper function. The blood quiescent immunocompetent cells retained their DNA-uracil exclusion capacity, even in the oldest age groups. Peripheral blood mature end cells, granulocytes, platelets and red cells had little activity, consistent with the fact that these cells are anuclear or short-lived, so that no template-primer functions of their DNA are required. Uracil-DNA glycosylase expression is high in all types of human leukemia, providing a selective advantage for survival of leukemic cells. Overall results show that a deficiency of this DNA base-excision-repair pathway is not likely to be an etiopathogenetic factor in the formation of non-random or other chromosomal abnormalities or in the leukemogenesis itself.

Effect of cytosine arabinoside on the human immunosystem: metabolism and cytotoxicity studied with mitogen-stimulated normal blood lymphocytes in vitro

The toxicity, metabolic effects and metabolism of cytosine arabinoside (Ara-C) were studied with normal human peripheral blood PHA-stimulated mononuclear cells in vitro. Clinically relevant Ara-C concentrations were toxic against mitogen-stimulated blood lymphocytes. Dose-dependent effects included: (i) increased cell loss, (ii) decreased DNA synthesis assessed by 3H-thymidine incorporation, (iii) decreased blastic transformation, (iv) decreased protein synthesis assessed by 14C-leucine incorporation, (v) an inhibition of the production of new cells, (vi) a delay in the proceeding of the PHA-stimulated cells to the cell cycle, (vii) an arresting of the cells in the S-phase, and (viii), a dose-dependent decrease of the number of mitoses in Ara-C-treated cultures. The mode of cell death was of the delayed type. The toxicity of Ara-C was effectively reversed by an excess of deoxycytidine, but not by cytidine or other conventional nucleosides, which is highly suggestive that the molecular mechanism of Ara-C toxicity is based on its anti-metabolic role in the salvage pathway of biosynthesis of DNA deoxycytidine. In fact, we demonstrated that Ara-C is metabolized to Ara-CTP and to a lesser extent also incorporated into DNA in human PHA-stimulated lymphocytes. Ara-C significantly decreased its own uptake and DNA incorporation. On the other hand, uracil arabinoside, which was the major catabolic product of Ara-C, was not toxic to human PHA-stimulated T-cells. The antiproliferative effect of Ara-C against human T-cells resembled that previously demonstrated with various cancer cell types.(ABSTRACT TRUNCATED AT 250 WORDS)

Radioimmunoassay of 5-hydroxymethyl-2'-deoxyuridine

5-Hydroxymethyl-2'-deoxyuridine is an antileukemic thymidine analogue. It is also a well known thymidine-derivative in DNA exposed to ionizing irradiation. We report the production and characterization of specific rabbit anti-5HmdUrd antisera. The antisera were used for the radioimmunological measurement of 5HmdUrd. The radioimmunoassay was capable of quantitating 2 pmol of 5 HmdUrd per tube corresponding to 0.2 mumol/l in a 10 microliter plasma sample. A good correlation between the results obtained with the radioimmunoassay and HPLC was demonstrated when the methods were applied to the measurement of plasma levels of 5HmdUrd in mice receiving experimental chemotherapy.

Hematopoietic and gastric uracil-DNA glycosylase activity in megaloblastic anemia and in atrophic gastritis with special reference to pernicious anemia

The activity of the DNA excision repair enzyme uracil-DNA glycosylase was measured in peripheral blood mononuclear cells and in bone marrow aspiration samples obtained from patients with pernicious anemia (PA) or other types of megaloblastic anemia (one case of tapeworm anemia and three cases of myelodysplastic syndromes). In addition, the expression of uracil-DNA glycosylase was investigated in biopsies from the antrum and body of the stomach obtained from nine PA patients, from five patients having atrophic gastritis (AG) not associated with PA, and from six control patients having transient upper abdominal complaints without AG. Our results revealed that there was a considerable interindividual variation in gastric uracil-DNA glycosylase activity. No clear correlation between the enzyme level and the level of gastric atrophy was noted, although AG is generally regarded as a risk factor of gastric cancer. Furthermore, uracil-DNA glycosylase activities in peripheral blood mononuclear cells and in bone marrow cells in PA and in myelodysplastic syndromes were similar to the activities observed previously in non-hematological patients and healthy persons. Transient uracil incorporation into DNA may have a role in the cellular abnormalities associated with megaloblastic hematopoiesis. The present findings demonstrated that the enzymatic activity required for rapid removal of uracil from DNA is also expressed in the megaloblastic state.

Antileukemic activity against L1210 leukemia, pharmacokinetics and hematological side effects of 5-hydroxymethyl-2'-deoxyuridine

The chemotherapeutic potential of 5-hydroxymethyl-2'-deoxyuridine (5HmdUrd) was examined in vitro and in vivo. The compound was toxic in 2-day cultures; 7, 66 and 88% inhibition in the growth of L1210 cells was achieved with 1, 10 and 100 microM 5HmdUrd, respectively. The maximal plasma concentration of 5HmdUrd at 15 min after a single i.p. injection (100 mg/kg) in DBA/2 mice was 193-244 mumol./l and the compound had a logarithmic disappearance curve with a half-life of 20 min. Chemotherapy given as two daily i.p. injections of 5HmdUrd (100 mg/kg) for five successive days resulted in a 239% increase in median lifespan and 2/6 long-term survivals among DBA/2 mice bearing leukemia L1210. This treatment resulted in temporary neutropenia and thrombocytopenia, which were followed by rebound thrombocytosis and neutrophilia of short duration. Our data indicate that 5HmdUrd can successfully be used in experimental cancer chemotherapy in vivo.

Effect of cytosine arabinoside on the human immunosystem: toxicity against quiescent human peripheral blood mononuclear cells in vitro

The toxic and metabolic effects of cytosine arabinoside (Ara-C) were studied in vitro with normal human peripheral blood mononuclear cells. The majority of target cells were T-lymphocytes. Dose-dependent toxicity of clinically relevant Ara-C concentrations was manifested by increased cell loss, inhibition of spontaneous blastic transformation, inhibition of DNA synthesis assessed by 3H-thymidine incorporation, inhibition of protein synthesis assessed by 14C-leucine incorporation, and inhibition of the mitogenic response of T-lymphocytes when challenged with phytohemagglutinin after Ara-C treatment. Cell death among the resting cells was delayed and it was mainly, if not entirely, cell-cycle-independent, since most of the cells stayed in the G0- or G1-phase. The toxicity of Ara-C was effectively reversed by an excess of deoxycytidine. This suggests that the molecular mechanism of Ara-C toxicity against quiescent peripheral blood mononuclear cells is based on its anti-metabolic role in the salvage pathway of biosynthesis of DNA deoxycytidine.

Uracil-DNA glycosylase activity in human acute leukemia

The expression of the DNA excision repair enzyme uracil-DNA glycosylase was investigated in bone marrow and peripheral samples from seven patients with acute lymphoblastic leukemia (ALL), from 17 patients with acute non-lymphocytic leukemia (ANLL), and from one patient with chronic granulocytic leukemia (CGL) in blast crisis. In addition, uracil-DNA glycosylase activities were determined in nine human leukemia/lymphoma cell lines. There was a clear correlation between the percentage of blast cells and the enzyme activity when mononuclear cell fractions from patient samples were analysed. The following uracil-DNA glycosylase activities were recorded (mean +/- S.D., number of samples): ALL = 45.6 +/- 14.8 U/mg of protein, N = 10; ANLL = 41.1 +/- 13.8 U/mg of protein, N = 22; CGL (blast crisis) = 44.7 U/mg of protein. The uracil-DNA glycosylase activity in nine human leukemia/lymphoma cell lines ranged from 35.2 to 66.0 U/mg of protein, and no striking differences were observed between the T-ALL, B-ALL, null cell ALL or myeloid lines. Similarly, the various biological features, such as the common ALL surface antigen, the terminal deoxynucleotidyl transferase enzyme, the sub-type of leukemia, chromosomal aberrations, or previous chemotherapy, did not apparently affect the expression of uracil-DNA glycosylase. We propose that the integrity of the genetic information is well protected by uracil-DNA glycosylase in different forms of leukemia, including cases with a low proportion of S-phase blasts, as assessed by flow cytometry in the present work. When compared to the activities in benign hematopoietic progenitor cells, studied previously in this laboratory, no big differences between the benign and malignant hematopoiesis were demonstrated. Hence, it is unlikely that selectivity of chemotherapy towards malignant vs benign hematopoietic growth could be based on the enzyme uracil-DNA glycosylase.

In vitro and in vivo studies of a promising antileukemic thymidine analogue, 5-hydroxymethyl-2' deoxyuridine

The toxicity and metabolism of a thymidine analogue, 5-hydroxymethyl-2'-deoxyuridine (5HmdUrd) were studied with human leukemia cells (HL-60) and with human platelets. 3 X 10(-5) M 5HmdUrd caused a 50% inhibition in the proliferation of HL-60 cells. The compound was hydrolyzed to 5-hydroxymethyluracil (5HmUra) by the enzyme thymidine phosphorylase (EC 2.4.2.4) present in leukemia cells; this catabolic product was non-toxic. The catabolism of 5HmdUrd by human platelet thymidine phosphorylase could be inhibited by 6-aminothymine. The toxicity of 5HmdUrd was effectively reversed by deoxycytidine and 5HmdUrd increased the incorporation of deoxycytidine into dCTP and DNA several fold. The two latter phenomena are explicable in terms of a feedback action to ribonucleotide reductase, resulting in deoxycytidylate starvation, which is a known effect of excess thymidine. We report here also our preliminary observations that 5HmdUrd is active against mouse leukemia in vivo.

Uracil-DNA glycosylase in benign and malignant maturing human hematopoietic cells

The expression of uracil-DNA glycosylase was studied in human normal hematopoietic bone marrow cells and in malignant counterparts obtained from patients with chronic granulocytic leukemia. We observed that the expression of the enzyme was highest in the proliferating granulocytic compartment (myeloblasts through myelocytes) and that it was diminished in more mature cells. Furthermore, we demonstrated that uracil-DNA glycosylase activity was higher in immature red blood cells or reticulocytes than in more mature red cells. The same tendency was also demonstrated in human malignant monoblasts, which were induced to terminal maturation by phorbol ester. It can be concluded from these results that uracil-DNA glycosylase expression is equal in benign and malignant hematopoietic progenitor cells; no selectivity towards malignant vs. benign progenitors can be expected in possible chemotherapeutic approaches relying on uracil-DNA glycosylase.

Time-resolved fluoroimmunoassay of 5-methyl-2'-deoxycytidine

We describe time-resolved fluoroimmunoassay of 5-methyl-2'-deoxycytidine (5MedCyd). The assay is based on the use of a highly specific antiserum raised in rabbits against BSA-conjugated 5-methylcytidine (5MeCyd). The tracer in the solid-phase time-resolved fluoroimmunoassay (TR-FIA) was antigen-selected anti-5MedCyd labeled with Europium. Thyroglobulin-linked 5MeCyd served as the solid-phase antigen. The measuring range for the fluoroimmunoassay was from less than 1 to 5000 pmol per assay of 5MedCyd. A good correlation between the results obtained with the TR-FIA and HPLC was demonstrated when the methods were applied to the measurement of methylation in human leukemic cells and other DNA samples. TR-FIA has several advantages over the more laborious techniques available so far: (i) high sensitivity, (ii) large assay ranges, (iii) rapidity and large number of simultaneous assays, (iv) simplicity, and (v) low cost provided that the laboratory has equipment for time-resolved fluorometry.

An improved radioimmunoassay for the quantitation of DNA methylation

A novel radioimmunoassay of 5MedCyd is described. The assay, employing a highly specific antiserum raised in rabbits against BSA-conjugated 5MeCyd, used 5-125iodo-2'-deoxycytidine as the tracer. The measuring range for the assay was found to be 1-1000 pmol per assay of 5MedCyd. When the methods were applied to the measurement of methylation in DNA samples a good correlation between the results obtained with the radioimmunoassay and HPLC was demonstrated. The method has several advantages over the more laborious and sophisticated techniques previously available: high sensitivity, large assay range, rapidity and potential for large number of simultaneous assays, simplicity, and low cost provided that the laboratory has equipment for gamma counting.

Uracil-DNA glycosylase activity in chronic lymphoproliferative disorders

The activity of uracil-DNA glycosylase, a repair enzyme for the excision of uracil from DNA, was studied in patients with chronic lymphoproliferative disorders and with malignant plasma cell dyscrasias. The biochemical assay was performed on mononuclear cells, isolated by density gradient centrifugation from peripheral blood, from bone marrow or from both. The activity of the uracil-DNA glycosylase of peripheral blood cells in 8/8 cases of myeloma and in 3/3 cases of Waldenström's macroglobulinemia was in the same range as in 22 non-hematological control patients, i.e. 2.4-25.1 U/mg of protein. Higher activities were found in 9/12 cases of chronic lymphocytic leukemia (CLL), in 2/4 cases of hairy cell leukemia (HCL), in 2/2 cases of chronic T-cell lymphocytosis and in the only case of small cell lymphocytic lymphoma. Follow-up of some CLL and HCL patients revealed that uracil-DNA glycosylase activity was fairly stable during the course of the disease. We conclude that malignant cells in chronic lymphoproliferative disorders are characterized by a normal or even increased capability to repair DNA, as exemplified by uracil-DNA glycosylase in this study.

Uracil-DNA glycosylase activity in human blood cells

We studied uracil-DNA glycosylase activities systematically in all types of human peripheral blood cells. The highest amounts of uracil-DNA glycosylase activity were found in cells capable of using their genetic information either in DNA replicative or repair synthesis or in DNA transcription. These cells included cytotoxic/suppressor and inducer/helper T lymphocytes, B lymphocytes and monocytes. On the other hand, the peripheral blood mature end cells, erythrocytes, platelets and granulocytes, contained very little if any uracil-DNA glycosylase activity. In addition to this biological capacity, we show that the housekeeping excision repair capacity of uracil-DNA glycosylase is well maintained in human peripheral blood mononuclear cells throughout life from the neonatal period to old age.

5-Hydroxymethyl-2'-deoxyuridine. Cytotoxicity and DNA incorporation studied by using a novel [2-14C]-derivative with normal and leukemic human hematopoietic cells

5-Hydroxymethyl-2'-deoxyuridine is a biologically active thymidine analogue. This investigation was aimed at characterizing the cytotoxicity of 5-hydroxymethyl-2'-deoxyuridine and its incorporation into DNA. Fifty percent inhibition of cellular proliferation, assessed by incorporation of [U-14C]-L-leucine in vitro, was caused by 1.7-5.8 X 10(-5) incorporation of [U-14C]-L-leucine in vitro, was caused by 1.7-5.8 X 10(-5) M 5-hydroxymethyl-2'-deoxyuridine in seven human leukemia cell lines. Higher concentrations of 5-hydroxymethyl-2'-deoxyuridine, i.e. 6-8 X 10(-5) M, were required for a comparable inhibition in human PHA-stimulated peripheral blood lymphocytes. A new synthesis procedure for [2-14C]5-hydroxymethyl-2'-deoxyuridine was developed. The net incorporation of [2-14C]5-hydroxymethyl-2'-deoxyuridine into DNA of hematopoietic cells was low. The possibility of a repair mechanism for 5-hydroxymethyluracil bound to DNA is discussed.

5-Ethyl-2'-deoxyuridine. Cytotoxicity and DNA incorporation demonstrated with human leukemic cells and PHA-stimulated lymphocytes in vitro

5-Ethyl-2'-deoxyuridine (5EtdUrd) is a biologically active thymidine analogue. The cytotoxicity of 5EtdUrd was investigated with seven established human leukemia cell lines as well as with human peripheral blood PHA-stimulated lymphocytes. All types of leukemia cells were susceptible to the toxicity of 5EtdUrd as assayed with a [U-14C]-L-leucine incorporation system developed for this study. A 50% inhibition of leucine incorporation in 3-day cultures was induced by 1.3-3.8 microM 5EtdUrd with leukemic cells, but the concentration required to induce similar inhibition with PHA-stimulated lymphocytes was approximately was approximately 100-fold. The toxicity of 5EtdUrd seemed to require active DNA synthesis, since the inhibition of leucine incorporation became obvious only after the first 24 hours of culture. The DNA incorporation studies were based on a new isotopically labeled 5EtdUrd derivative, [2-14C]5EtdUrd, synthesized for this study in our laboratory. It was demonstrated for the first time that most of the radioactivity derived from [2-14C]5EtdUrd in DNA was in 5-ethyluracil. 5EtdUrd has a powerful antileukemic potency in vitro. Its effects against human leukemia in vivo remain to be tested.

Radioimmunoassay of 5-methyl-2'-deoxycytidine. A method for the quantitation of DNA methylation

5-Methyl-2'-deoxycytidine (5MedCyd) is a minor constituent of mammalian cell DNA. We report the production and characterization of highly specific rabbit anti-5MedCyd antiserum. The antiserum was suitable for the radioimmunological measurement of 5MedCyd. This simple radioimmunoassay was capable of quantitating calf thymus DNA methylation at nanomolar levels of total DNA.

Reversal of deamination-related cytotoxicity of 5-methyl-2'-deoxycytidine by tetrahydrouridine in human leukemia cells

The present experiments were conducted to test the effects of the potent cytidine deaminase inhibitor tetrahydrouridine (THU) on the metabolism and cytotoxicity of 5-methyl-2'-deoxycytidine (5-Med-Cyd) in several human leukemia cell lines in vitro. It was observed that 5-Med-Cyd exerts its effects via deamination to thymidine, which is particularly toxic to human promyelocytic (HL-60) and T-cell (JM) leukemia cell lines in vitro. The deamination and the cytotoxicity of 5-Med-Cyd were effectively hindered by 10(-3) M THU in 3-day cultures of HL-60 cells. Although the catabolism of [14C]5-Med-Cyd in the HL-60 cell cultures was blocked by THU, no radioactive 5-Med-Cyd was incorporated into DNA. The cytotoxicity and DNA incorporation of fluorodeoxycytidine are enhanced by THU. Unlike that compound 5-Med-Cyd resembled more bromodeoxycytidine and iododeoxycytidine; THU decreases the toxicity of both of these deoxycytidine analogues.

5-Methyl-2'-deoxycytidine. Metabolism and effects on cell lethality studied with human leukemic cells in vitro

5-Methylcytosine ( 5MeCyt ) is a possible regulator of eukaryotic gene transcription. We investigated whether this compound could be introduced into DNA from exogenous deoxyribonucleoside 5-methyl-2'-deoxycytidine ( 5MedCyd ). High concentrations of 5MedCyd inhibited the growth of several types of human leukemic cell lines in vitro. However, the effect could be accounted for by dThd, a deamination product of 5MedCyd . We found that radioactivity from [methyl-14C] 5MedCyd and [2-14C] 5MedCyd was incorporated into DNA as thymidylate, and none was present as 5MeCyt . There are two conceivable metabolic pathways from 5MedCyd to thymidylate. The first consists of deoxycytidine or thymidine kinase and deoxycytidylate deaminase, and the second of sequential reactions catalyzed by deoxycytidine deaminase and thymidine kinase. No indication of the first pathway was demonstrable in human leukemic cells. We conclude that the DNA exclusion of 5MeCyt from exogenous 5MedCyd takes place because of powerful deoxycytidine deaminase activity in human malignant hematopoietic cells.

Uracil-DNA glycosylase and deoxyuridine triphosphatase: studies of activity and subcellular location in human normal and malignant lymphocytes

Crude extracts of human lymphocytic cells contain two enzymes which putatively exclude uracil from DNA: uracil-DNA glycosylase and deoxyuridine triphosphatase (dUTPase). Their activities were found in different types of benign and malignant cells: resting and mitogen-stimulated peripheral blood T- and B-lymphocytes, as well as in a lymphoblastic cell line with neither B- or T-differentiation. The proliferative stage of the cell determined the dUTPase activity rather than the phenotypic origin of the population. This was demonstrated with peripheral blood lymphocytes: the activity of dUTPase in resting cells was very low and a 15- to 27-fold increase took place during mitogenic stimulation. Mitogens slightly enhanced the uracil-DNA glycosylase activity. The physiological roles of these enzymes may be connected with their subcellular location. The subcellular distributions of the enzymes were different; uracil-DNA glycosylase was most abundantly present near to cellular DNA, i.e., in nuclei, but dUTPase was nearly exclusively a cytoplasmic enzyme.

Uracil in deoxyribonucleotide polymers reduces their template-primer activity for E. coli DNA polymerase I

The physical and biochemical properties of two pairs of synthetic DNA template-primers were investigated. The copolymer poly(dA-dU) . poly(dA-dU) and the homopolymer duplex poly(dA). poly(dU) were characterized by a lower Tm and by a higher buoyant density value than the respective thymine polynucleotides poly(dA-dT) . poly(dA-dT) and poly(dA) . poly(dT). The polymerizing and the primer terminus adding reactions of a homogenous E. coli DNA polymerase I preparation, as measured by incorporation of [3H]dAMP into the acid-insoluble fraction, were significantly poorer with uracil-containing template-primers than with thymine templates. Moreover, the uracil-containing polynucleotides inhibited the polymerizing activity of DNA polymerase I to a greater extent than the thymine polynucleotides, when the enzymatic activity was investigated with a dATP/dTTP/dUTP-free incorporation system making use of poly(dI-dC) . poly(dI-dC) as the template-primer.

Mitogen induction of deoxyuridine triphosphatase activity in human T and B lymphocytes

Deoxyuridine triphosphatase (dUTPase; deoxyuridine diphosphohydrolase; EC 3.6.1.23) activity during mitogen stimulation was investigated in human T-cell and B-cell enriched mononuclear leucocyte fractions as well as in a mixed lymphocyte population. The dUTPase activity was very low in the resting peripheral blood lymphocytes. A remarkable enhancement of enzymatic activity was observed when cells were stimulated with different mitogens; T-cells and non-separated lymphocytes with phytohaemagglutinin, and the B-cell enriched fraction with pokeweed mitogen. There was a positive correlation between dUTPase activity and the enhancement of macromolecule synthesis (protein and RNA). In particular, a highly significant correlation was observed between dUTPase activity and DNA synthesis in the three human lymphocyte populations studied. This supports the view that the enzyme dUTPase may have a significant role in cellular proliferation. The physiological role of the enzyme is discussed.