A human-mouse somatic hybrid line selected for human deoxycytidine deaminase.

A new selective medium has been developed for cells containing the enzyme deoxycytidine deaminase. This medium contains hypoxanthine, aminopterin, and 5-methyldeoxycytidine (HAM medium). To survive in the presence of the aminopterin, the cells must utilize deoxycytidine deaminase to convert the 5-methyldeoxycytidine to thymidine. The cells must also have thymidine kinase and hypoxanthine phosphoribosyltransferase. A mouse cell line deficient in deoxycytidine deaminase has been isolated from a deoxycytidine kinase-deficient line, using 5-bromodeoxycytidine as the selective agent. A hybrid line between this double mutant and a human diploid fibroblast was isolated in HAM medium. The hybrid line contains the chromosomes expected of a human-mouse hybrid. The deoxycytidine deaminase isozyme patterns on cellogel show that the human-mouse hybrid cell line produces an enzyme with an electrophoretic mobility intermediate between that of the human and that of the mouse.     

Mechanisms of cross-resistance between nucleoside analogues and vincristine or daunorubicin in leukemic cells

The aim of this study was to clarify the biochemical and molecular mechanisms behind the cross-resistance to nucleoside analogues (NAs) in four erythroleukemic cell lines with acquired resistance to the anthracycline daunorubicin and to the vinca alkaloid vincristine, expressing high levels of p-glycoprotein (P-gp, MDR1). All resistant strains exhibited cross-resistance to NA (cladribine and cytosine arabinoside)-induced apoptosis, assessed by caspase-3-like activation and were less sensitive to NA cytotoxicity in MTT assay. Real-time PCR and enzyme activity analysis showed reduced amounts of deoxycytidine kinase (35-80%) and elevated levels of 5'-nucleotidases (50-100%). The ratio 5'-nucleotidase to deoxycytidine kinase increased between 2.5- and 7.5-folds in resistant cells. This is in agreement with the observation that 5'-nucleotidase/deoxycytidine kinase ratio might be an important factor in predicting resistance to NAs. Implications of this finding for combining anthracyclines or vinca alkaloids with NAs toward leukemic cells are discussed.     

Heat- and alkali-induced deamination of 5-methylcytosine and cytosine residues in DNA

5-Methylcytosine residues in DNA underwent deamination at high temperatures. Furthemore, their rate of deamination at neutral or alkaline pH was greater than that of cytosine residues in DNA. As sources of [¹⁴C]5-methylcytosine-containing DNA, we used bacteriophage XP-12 DNA, in which 5-methylcytosine residues completely replace C residues, and calf thymus DNA experimentally substituted with [¹⁴C]5-methylcytosine residues. Upon incubation at 95°C in a physiological buffer or at 60°C in 1 M NaOH, the respective rates of deamination of 5-methylcytosine residues were about 3- and 1.5-times those of cytosine residues. Under the same conditions, the free 5-methyldeoxycytidine was converted to thymidine more rapidly than deoxycytidine was converted to deoxyuridine. The reactions at physiological pH and elevated temperature suggest that deamination of 5-methylcytosine residues may yield a significant portion of spontaneous mutations in vivo, especially in view of the lack of thymine-specific mismatch repair systems with specificity and efficiency comparable to that of uracil excision repair systems.     

Human T-lymphoblast deoxycytidine kinase: purification and properties

Previous observations present tremendous variations in the properties of deoxycytidine kinase. To clarify the properties and physiologic role of deoxycytidine kinase, we have undertaken its purification. Deoxycytidine kinase was purified from cultured human T-lymphoblasts (MOLT-4) to 90% purity with an estimated specific activity of 8 mumol min-1 (mg of protein)-1. The purification procedure included ammonium sulfate precipitation, Superose-12 HPLC gel filtration chromatography, DE-52 ion-exchange chromatography, AMP-Sepharose 4B affinity chromatography, and dCTP-Sepharose-4B affinity chromatography. Deoxyguanosine, deoxyadenosine, and cytidine phosphorylating activities copurified with deoxycytidine kinase to final specific activities of 7.2, 13.5, and 4 mumol min-1 (mg of protein)-1, respectively. The enzyme is very unstable at low protein concentration and is stabilized by storage at -85 degrees C with 1 mg/mL bovine serum albumin, 20% glycerol (v/v), 200 mM potassium chloride, and 25 mM dithiothreitol. The molecular weight was 60,000, and the Stokes radius was 32 A by gel filtration chromatography. The subunit molecular weight was 30,500. This enzyme had apparent Km values of 1.5, 430, 500, 450, and 40 microM for deoxycytidine, deoxyguanosine, deoxyadenosine, cytidine, and cytosine arabinoside, respectively. The pH optimum ranged from 6.5 to 9.0. Mg2+ and Mn2+ were the preferred divalent cations. ATP, GTP, dGTP, ITP, dITP, TTP, and XTP were substrates for the enzymes. Our study indicates that deoxycytidine kinase is a dimer with two subunits and has phosphorylating activity for deoxyguanosine, deoxyadenosine, cytidine, and cytosine arabinoside. This highly purified enzyme will facilitate the study of its regulation and phosphorylation of anticancer or antiviral nucleoside analogues.     

No neoplastic alteration of metabolically competent rat liver cells in vitro by chemical carcinogens: 3-methylcholanthrene, dimethylnitrosamine and Natulan

Epithelial rat liver cell line RL-19 was checked for aryl hydrocarbon hydroxylase and dimethylnitrosamine demethylase activity. Aryl hydrocarbon hydroxylase activity was found at the rate of about 14.5 pmoles 3-hydroxy-benzopyrene per min per mg protein. This activity was not inducible by 3-methylcholanthrene or by phenobarbital and was independent of the subculture level. From the 45th up to the 59th subculture the mean demethylase activity was about 1.08 nmoles HCHO per min per mg protein, but was decreased to 0.64 nmoles HCHO per min per mg protein at the 131st subculture. RL-19 cells were treated with 3-methylcholanthrene (0.5-1.0 microgram/ml), dimethylnitrosamine (100-400 micrograms/ml), or Natulan (50 micrograms/ml), respectively, for 7 to 10 days. During a 6 months subsequent cultivation no neoplastic changes were observed as revealed by morphological investigation, soft agar assay, and transplantation. It is suggested that metabolic competence for carcinogen activation is only one prerequisite for neoplastic alteration in vitro, and that RL-19 cells are refractory to the action of carcinogens in spite of their metabolic capacity.     

Cytostatic and cytotoxic effects of (E)-2'-deoxy-2'-(fluoromethylene)-cytidine on a solid tumor and a leukemia cell line

(E)-2'-deoxy-2'-(fluoromethylene)-cytidine (FMdC), a deoxycytidine analog displaying a very high toxicity toward a variety of solid tumor cell lines and xenografts, is activated intracellularly by deoxycytidine kinase (dCK). We have compared cytotoxicity of FMdC towards a human promyeolocytic leukemia line HL-60 and a human colorectal carcinoma line COLO-205. Despite dCK activity being by far the highest in cells of lymphoid origin, the effects of FMdC were detectable at the lowest drug concentration only in a solid tumor cell line, and at higher concentrations they were qualitatively similar in the two tumor lines (increased cell protein content, cell cycle block and apoptosis). Apparently, low dCK activity in solid tumor cells sufficiently activates FMdC to yield cytotoxic effects, while high dCK activity in leukemia cells does not increase its cytotoxicity.     

In vitro antiproliferative effects on human tumor cell lines of extracts from the Bangladeshi medicinal plant Aegle marmelos Correa.

In the present paper we show that extracts from Aegle marmelos Correa are able to inhibit the in vitro proliferation of human tumor cell lines, including the leukemic K562, T-lymphoid Jurkat, B-lymphoid Raji, erythroleukemic HEL, melanoma Colo38, and breast cancer MCF7 and MDA-MB-231 cell lines. Molecules present within the studied Aegle marmelos C. extracts were identified by gas-chromatography/mass-spectrometry analysis; three derivatives (butyl p-tolyl sulfide, 6-methyl-4-chromanone and butylated hydroxyanisole) were found to exhibit strong activity in inhibiting in vitro cell growth of human K562 cells. The antiproliferative activity of these compounds was found to be comparable to that of known antitumor agents, including cisplatin, chromomycin, cytosine arabinoside and 5-fluorouracil. In addition, the antiproliferative activity of butyl-p-tolyl sulfide, 6-methyl-4-chromanone and 5-methoxypsolaren was associated to activation of the differentiation pattern of K562 cells.     

DNA cytosine methylation and heat-induced deamination

The heat-induced conversion of 5-methylcytosine (m⁵C) residues to thymine residues and of cytosine to uracil residues in single-stranded DNA was studied. The calculated rates for deamination at 37°C and pH 7.4 were 9.5×10^–10 and 2.1×10^–10 sec^–1, respectively. N⁴-Methyldeoxycytidine, which is in the DNA of certain thermophilic bacteria, was more heat-resistant than was deoxycytidine and much more than was 5-methyldeoxycytidine. Thermophilic bacteria which contain N⁴-methylcytosine rather than m⁵C in their genomes may thereby largely avoid heat-induced mutation due to deamination, which is incurred by the many organisms that contain m⁵C in their DNA.     

A combined evaluation of biochemical and morphological changes during human neuroblastoma cell differentiation

1. The effects of retinoic acid, gamma-interferon, cytosine arabinoside, nerve growth factor, tumor necrosis factor, and 12-O-tetradecanoylphorbol 13-acetate on the human neuroblastoma cell line, LAN-5, were studied. Intracellular levels of acetylcholinesterase, neuron-specific enolase, catecholamines and related neurotransmitters, vasointestinal peptide, and substance P were evaluated after induction. 2. Cell morphology was strongly affected by retinoic acid, gamma-interferon, cytosine arabinoside, and 12-O-tetradecanoylphorbol 13-acetate. The main effects of retinoic acid and gamma-interferon were the loosening of cell clusters and the extension of long neurites; cytosine arabinoside induced cell body swelling and marked neuritogenesis. Following 12-O-tetradecanoylphorbol 13-acetate treatment, the cells became small, round, and neuritic. Conversely, modifications induced by nerve growth factor and tumor necrosis factor were mild. Cell proliferation rate was reduced by retinoic acid, gamma-interferon, cytosine arabinoside, and 12-O-tetradecanoylphorbol 13-acetate, while nerve growth factor and tumor necrosis factor were devoid of effects. 3. Acetylcholinesterase activity was significantly stimulated by retinoic acid and by gamma-interferon. Neuron-specific enolase activity was unaffected by all treatments except 12-O-tetradecanoylphorbol 13-acetate, which enhanced it by 1.6-fold. 4. The cellular catecholamine and related metabolite content was lowered by retinoic acid and gamma-interferon, while cytosine arabinoside and, even more, 12-O-tetradecanoylphorbol 13-acetate showed a stimulatory activity on their intracellular accumulation. 5. Finally, the cell-associated vasointestinal peptide level was strikingly increased by gamma-interferon and, to a lesser extent, by retinoic acid, cytosine arabinoside, and 12-O-tetradecanoylphorbol 13-acetate. 6. It is concluded that the most relevant biochemical changes associated with LAN-5 cells differentiation involve the repertoire of neurotransmitters and neuropeptides. These events vary in quality and in quantity, likely due to the pattern complexity of gene expression triggered by each inducer in determining the diversity of neuronal phenotypes.     

A simple quantitative method for the estimation of free ecto-sulfhydryl groups of spermatozoa

A simple rapid quantitative method has been developed for the estimation of sperm ecto-SH groups on the basis of their high affinity binding to the mercurial: [203Hg]p-chloromercuriphenylsulfonic acid (PCMPS) used as a surface probe. The thiol reagent did not penetrate the sperm plasma membrane, as evidenced by the extremely rapid time course of the binding reaction and undetectable uptake of [203Hg]PCMPS by intact goat spermatozoa. The binding reaction was not due to contaminating broken or damaged cells, if any. The method consists of incubating of highly motile goat spermatozoa with PCMPS in a modified Ringer solution at 37 degrees C for 5 min, agglutination of the labelled cells with polyethyleneimine (100 micrograms/ml) and filtration and washing of the cell suspension through Whatman No. 1 filter discs under mild vacuum. The binding interaction is proportional to cell concentration, specific and saturable at 50 microM PCMPS. The method is capable of estimating free ecto-SH as low as 25 pmoles. Spermatozoa possess 286 +/- 61 pmoles of free ecto-SH groups/10(6) cells. Scatchard analysis showed the presence in goat spermatozoa of multiple classes of ecto-SH groups differing in their affinity for PCMPS.     

Isolation and cell cycle analysis of temperature-sensitive mutants from chinese hamster cells

HeLa cell mitochondria were allowed to incorporate ³H-thymidine in a cell free system and the effect of ethidium bromide, cytosine arabinoside and cytosine arabinoside triphosphate on the labeling of mitochondrial DNA was studied. The labeled products, isolated by sedimentation velocity in CsCl-ethidium bromide two-step gradients, showed similar sedimentation profiles as in vivo labeled mtDNA. Cytosine arabinoside triphosphate and ethidium bromide strongly inhibited the labeling of mitochondrial DNA, whereas cytosine arabinoside appeared to be much less effective. Tritiated deoxycytidine was found to be incorporated by isolated mitochondria, whereas cytosine arabinoside was shown to enter the mitochondrial acid-soluble pool but not to be incorporated in acid-insoluble form. These results are in agreement with the previously reported findings of in vivo experiments.     

Vitality measurement using spectrum shift in Hoechst 33342 stained cells.

A bivariate flow cytometric technique has been developed in which Hoechst 33342 stained vital cells are discriminated from early damaged cells by differences in their fluorescence emission spectra. A discrete population of cells with intact cell membranes passing from vital to dead could be identified by using propidium iodide exclusion in combination with a concentration-independent spectrum shift of Hoechst fluorescence to longer wavelengths. The appearance after injury of two subsets of cells with intact membranes representing vital and early damaged cells, respectively, indicates two types of binding of Hoechst 33342 to these cells, presumably resulting from differences in chromatin structure. The method was tested in murine and human hemopoietic cell lines by cytotoxic treatment with cytosine arabinoside and interleukin 3 deprivation in factor-dependent lines. The power of the method was demonstrated by a cell cycle analysis of the early damaged cells after both cytotoxic treatment and growth factor deprivation.     

Ectoprotein kinase activity of the isolated rat adipocyte.

Intact adipocytes exhibit ectoprotein kinase activity as reflected by their ability to catalyze the transfer of the terminal phosphate of (γ-³²P) ATP to histone added to a cell suspension. This activity is substrate, time and cell number dependent. Lineweaver-Burk plots gave Km and Vmax values for ATP of 5 × 10^?5 M and 7.14 pmoles/min/1.5 × 10⁵ cells. Cyclic AMP but not cyclic GMP in μM concentrations stimulates ectoprotein kinase activity. The controlled tryptic digestion of intact cells results in reduction of ectoprotein kinase activity. This activity is not due to leakage of intracellular protein kinases during the preparative procedure nor to penetration of histone into the cells. Additional phosphoproteins not accessible to endogenous protein kinase activity are also localized on the external surface of the intact fat cell.     

Potential of ribozymes against deoxycytidine kinase to confer drug resistance to cytosine nucleoside analogs

Hematopoietic toxicity is the dose-limiting side effect produced in cancer chemotherapy with deoxycytidine nucleoside analogs. Deletion of the deoxycytidine kinase (dCK), results in a drug resistance phenotype to these analogs. An interesting gene therapy strategy to confer drug resistance to cytosine nucleoside analogs would be to specifically inactivate the dCK in normal hematopoietic stem cell. In this study, we designed hammerhead ribozymes that can specifically cut and downregulate the murine dCK mRNA. Three different ribozymes were identified and shown to cleave in vitro the dCK RNA. After introduction of ribozyme cDNA into murine L1210 leukemic cells by retroviral transfer, two of the ribozymes showed some capacity in reducing dCK activity. However, analysis of transduced L1210 clones showed that the significant reduction in the dCK mRNA was not sufficient to confer drug resistance to cytosine arabinoside. Nevertheless, these results provide a new avenue of modulating the dCK enzyme activity and with improved modifications may have the potential for use in gene therapy to confer drug resistance to deoxycytidine analogs.     

Determination of the deoxycytidine kinase activity in cell homogenates with a non-radiochemical assay using reversed-phase high performance liquid chromatography; Identification of a novel metabolite of 2-chlorodeoxyadenosine

A non-radioactive procedure to measure the deoxycytidine kinase (dCK) activity in crude cell free homogenates was developed. 2-Chlorodeoxyadenosine (CdA) was used as the substrate for dCK and was separated from its product 2-chlorodeoxyadenosine-5'-monophosphate (CdAMP) by reversed-phase HPLC. A complete separation of CdA and its metabolites was achieved in 30 min. The minimum amount of CdAMP that could be detected was 1 pmol. The assay was linear with reaction times up to at least 3h. With respect to the protein concentration, the reaction was linear with protein concentrations up to 760 microg/ml in the assay. An amount of 8 x 10(3) cells was already sufficient to determine the specific dCK activity in SK-N-BE(2)c cells. CdA was not only converted to CdAMP but also to 2-chloroadenine and, surprisingly, also to 2-chlorodeoxyinosine, in MOLT-3 cells. The deamination of CdA was completely inhibited by deoxycoformycin, which clearly demonstrates that CdA is a substrate for adenosine deaminase.     

Antiproliferative activity and mechanism of action of fatty acid derivatives of gemcitabine in leukemia and solid tumor cell lines and in human xenografts

Gemcitabine is a deoxycytidine analog, which can be inactivated by deamination catalyzed by deoxycytidine deaminase (dCDA). Altered transport over the cell membrane is a mechanism of resistance to gemcitabine. To facilitate accumulation, the fatty acid derivative CP-4125 was synthesized. Since, the fatty acid is acylated at the site of action of dCDA, a decreased deamination was expected. CP-4125 was equally active as gemcitabine in a panel of rodent and human cell lines and in human melanoma xenografts bearing mice. In contrast to gemcitabine, CP-4125 was not deaminated but inhibited deamination of deoxycytidine and gemcitabine. Pools of the active triphosphate of gemcitabine increased for over 20 hr after CP-4125 exposure, while these pools decreased directly after removal of gemcitabine. In conclusion: CP-4125 is an interesting new gemcitabine derivative.     

Properties of the superoxide-generating oxidase of B-lymphocyte cell lines. Determination of Michaelis parameters.

The capacity of three B-lymphocyte cell lines to generate superoxide (O2.-) was examined. The Burkitt lymphoma lines P.3HR-1 and Jijoye gave no response to phorbol 12-myristate 13-acetate (PMA) at 100 ng/ml but produced up to 0.35 nmol of O2.-/min per mg of protein when stimulated with 5 micrograms of PMA/ml; the cell line RPMI 1788 produced Nitro Blue Tetrazolium-positive responses to low PMA concentrations and approx. 0.4 nmol of O2.-/min per mg of protein at 5 micrograms of PMA/ml. Each cell line contained approx. 10 pmol of low-potential cytochrome b (cytochrome b-245)/mg of protein. Homogenates of PMA-activated cells gave 10-20-fold greater rates of O2.- produced per mg of protein. The Km for NADPH varied between approx. 250 microM for P3.HR-1 and RPMI 1788 cell lines and 30.5 +/- 6.5 microM for the Jijoye cell line; the Km values for NADH were higher. Determination of intracellular NADPH concentration showed that this might limit the rate of O2.- production since in each cell line it was at or below the Km concentration.     

Serotonin and β-phenylethylamine deamination by semicarbazide-sensitive amine oxidase from Callista chione hepatopancreas

Amine oxidase activity against 5-HT and PEA has been studied in hepatopancreas homogenates of Callista chione (Mollusca, Bivalvia). Specific activity values were 3 pmoles/mg protein/min and 0.05 nmoles/mg protein/min with 5-HT and PEA, respectively. Values of activities measured with the two substrates were differently affected by variations of pH (6.5–8) and temperature (17°C–67°C). Monoamine osidase inhibitors such as clorgyline and deprenyl used at a concentration of 10¹⁴ M did not significantly affect 5-HT and PEA deamination. Semicarbazide 10⁴ M completely inhibited deamination of both substrates indicating that, at least under the experimental conditions adopted, 5-HT and PEA were deaminated by a semicarbazide-sensitive amine oxidase.     

Intracellular adenosine triphosphate as a measure of human tumor cell viability and drug modulated growth

Summary Adenosine triphosphate is the primary energy unit for cells, and levels of this compound offer a potential marker for cell viability and growth. The availability of a bioluminescence assay allows for a rapid, sensitive, and reproducible measurement of ATP. A method is described for the quantification of intracellular ATP levels in human cancer cells. ATP levels were linearly related to the number of viable cells and increased with time in human cancer cell line cultures correlating with growth kinetics. The effect of 5-fluorouracil, doxorubicin, methotrexate, cytosine arabinoside, nitrogen mustard, melphalan, vinblastine, and cisplatin on the growth of human cancer cell lines was studied utilizing ATP levels. ATP levels and colony formation in agar of drug-exposed cells were compared. Overall there was a significant correlation between drug effects on colony formation and ATP levels. The ATP assay is rapid, simple, reproducible, and a relatively inexpensive method of quantifying drug effects on malignant cells. This makes it a potentially useful method for screening new anticancer drugs in human cancer cell lines.     

Antiproliferative Activity and Mechanism of Action of Fatty Acid Derivatives of Gemcitabine in Leukemia and Solid Tumor Cell Lines and in Human Xenografts

Gemcitabine is a deoxycytidine analog, which can be inactivated by deamination catalyzed by deoxycytidine deaminase (dCDA). Altered transport over the cell membrane is a mechanism of resistance to gemcitabine. To facilitate accumulation, the fatty acid derivative CP‐4125 was synthesized. Since, the fatty acid is acylated at the site of action of dCDA, a decreased deamination was expected. CP‐4125 was equally active as gemcitabine in a panel of rodent and human cell lines and in human melanoma xenografts bearing mice. In contrast to gemcitabine, CP‐4125 was not deaminated but inhibited deamination of deoxycytidine and gemcitabine. Pools of the active triphosphate of gemcitabine increased for over 20 hr after CP‐4125 exposure, while these pools decreased directly after removal of gemcitabine. In conclusion: CP‐4125 is an interesting new gemcitabine derivative.