Determination of CTP synthetase activity in crude cell homogenates by a fast and sensitive non-radiochemical assay using anion-exchange high-performance liquid chromatography

CTP synthetase activity assay by liquid chromatography tandem mass spectrometry in the multiple reaction monitoring mode

Cytidine 5'-triphosphate synthetase (CTPS) is known to be a central enzyme in the de novo synthesis of CTP. We have recently demonstrated that a deficiency in CTPS1 is associated with an impaired capacity of activated lymphocytes to proliferate leading to a combined immunodeficiency disease. In order to better document its role in immunomodulation, we developed a method for measuring CTPS activity in human lymphocytes. Using liquid chromatography-mass spectrometry, we quantified CTPS activity by measuring CTP in cell lysates. A stable isotope analog of CTP served as internal standard. We characterized the kinetic parameters V_max and K_m of CTPS and verified that an inhibition of the enzyme activity was induced after 3-deazauridine (3DAU) treatment, a known inhibitor of CTPS. We then determined CTPS activity in healthy volunteers, in a family whose child displayed a homozygous mutation in CTPS1 gene and in patients who had developed or not a chronic lung allograft dysfunction (CLAD) after lung transplantation. Linearity of the CTP determination was observed up to 451 μmol/L, with accuracy in the 15% tolerance range. Michaelis-Menten kinetics for lysates of resting cells were K_m =280±310 μmol/L for UTP, V_max =83±20 pmol/min and, for lysates of activated PBMCs, K_m =230±280 μmol/L for UTP, V_max =379±90 pmol/min. Treatment by 3DAU and homozygous mutation in CTPS1 gene abolished the induction of CTPS activity associated with cell stimulation, and CTPS activity was significantly reduced in the patients who developed CLAD. We conclude that this test is suitable to reveal the involvement of CTPS alteration in immunodeficiency.

Retinoic acid reduces the cytotoxicity of cyclopentenyl cytosine in neuroblastoma cells

In this paper, it is demonstrated that all-trans, 9-cis and 13-cis retinoic acid (RA) decreased the sensitivity of SK-N-BE(2)c neuroblastoma cells towards the chemotherapeutic agent cyclopentenyl cytosine (CPEC), a potent inhibitor of cytosine-5'-triphosphate synthetase. Retinoic acid attenuated CPEC-induced apoptosis as reflected by a decreased caspase-3 induction. Retinoic acid decreased the accumulation of CPEC, whereas the salvage of cytidine was strongly increased. Metabolic labeling studies using [(3)H]uridine showed a strongly decreased biosynthesis of CTP via CTP synthetase. Retinoic acid likely confers resistance of neuroblastoma cells to CPEC in part by slowing down proliferation, and in part by shifting the synthesis of CTP towards the salvage of cytidine, thereby bypassing CTP synthetase.

The cytostatic- and differentiation-inducing effects of cyclopentenyl cytosine on neuroblastoma cell lines

This paper describes the effects of cyclopentenyl cytosine (CPEC) on the proliferation and cell-cycle distribution of the SK-N-BE(2)c and SK-N-SH neuroblastoma cell lines, as well as their ability to recover from treatment with CPEC. The IC50 value of SK-N-BE(2)c for CPEC, determined after 48 hr was 80 nM. SK-N-BE(2)c cells showed a time- and concentration-dependent accumulation in the S-phase of the cell cycle after 2 and 3 days of incubation with 50-250 nM CPEC, followed by a G0/G1-phase arrest after 4 days. After incubation with 50 nM CPEC for 2 days, SK-N-BE(2)c cells fully recovered and resumed logarithmic proliferation. In contrast, a complete and persistent growth arrest occurred when SK-N-BE(2)c cells were incubated for 2 days with 100 or 250 nM CPEC. The IC50 value of SK-N-SH, determined after 48 hr, for CPEC was > or =1 microM. SK-N-SH cells incubated with 250 nM or 1 microM CPEC showed a time-dependent accumulation in the S-phase of the cell cycle, followed by an accumulation in the G0/G1-phase, which reached a maximum of 84.1% after 7 days of incubation with 1 microM CPEC. SK-N-SH cells did not resume proliferation after removal of the drug. In addition, CPEC strongly induced differentiation in SK-N-SH cells. After 48 hr incubation with 250 nM CPEC, 90% of the cell population was differentiated. Both neuronal type and Schwannian type cells were observed. We conclude that at very low concentrations, CPEC has profound cytostatic- and differentiation-inducing effects on the neuroblastoma cells studied.

In vitro inhibition of cytidine triphosphate synthetase activity by cyclopentenyl cytosine in paediatric acute lymphocytic leukaemia

Cytidine triphosphate (CTP) synthetase is a key enzyme for the synthesis of cytosine (deoxy)ribonucleotides, catalysing the conversion of uridine triphosphate (UTP) into CTP, and has a high activity in several malignancies. In this preclinical study, the enzyme activity and mRNA expression of the enzyme and (deoxy)ribonucleotide concentrations were analysed in leukaemic cells of 57 children suffering from acute lymphocytic leukaemia (ALL). In addition, in vitro experiments were performed with the CTP synthetase inhibitor cyclopentenyl cytosine (CPEC). A significantly higher activity of CTP synthetase (6.5 +/- 3.9 nmol CTP/mg/h) was detected in ALL cells than in lymphocytes of healthy controls (1.8 +/- 0.9 nmol CTP/mg/h, P < 0.001) that was independent of white blood cell (WBC) count, blast percentage, age, gender or type of ALL. The enzyme activity was not correlated with the CTP synthetase mRNA expression. The activity of CTP synthetase in ALL cells compared with non-malignant CD34+ bone marrow controls (5.6 +/- 2.4 nmol CTP/mg/h) was not statistically different. In vitro treatment of ALL cells with CPEC induced a dose-dependent decrease of the CTP concentration. The lowest concentration of CPEC (0.63 microM) induced a depletion of CTP of 41 +/- 20% and a depletion of dCTP of 27 +/- 21%. The degree of CTP depletion of ALL cells after treatment with CPEC was positively correlated with the activity of CTP synthetase. The inhibition of CTP synthetase in situ was confirmed by flux studies using radiolabelled uridine. From these results, it can be expected that CPEC has a cytostatic effect on lymphoblasts of children with ALL.

Cyclopentenyl cytosine inhibits cytidine triphosphate synthetase in paediatric acute non-lymphocytic leukaemia: a promising target for chemotherapy

Cytidine triphosphate (CTP) synthetase is a key enzyme in the anabolic pathways of cytosine and uracil ribonucleotide metabolism. The enzyme catalyses the conversion of uridine triphosphate (UTP) into CTP, and has a high activity in various malignancies, which has led to the development of inhibitors of CTP synthetase for therapeutic purposes. We studied both CTP synthetase activity and ribonucleotide concentrations in leukaemic cells of 12 children suffering from acute non-lymphocytic leukaemia (ANLL), and performed incubation experiments with cyclopentenyl cytosine (CPEC), a nucleoside analogue that is capable of inhibiting CTP synthetase. The CTP synthetase activity in ANLL cells (5.1+/-2.3 nmol CTP/mg/h) was significantly higher compared with granulocytes of healthy controls (0.6+/-0.4 nmol CTP/mg/h, P=0.0002), but was not different from the CTP synthetase activity in non-malignant CD34+ bone marrow cells (5. 6+/-2.4 nmol CTP/mg/h). Major shifts were observed in the various ribonucleotide concentrations in ANLL cells compared with granulocytes: the absolute amount of ribonucleotides was increased with a substantial rise of the CTP (2.4 versus 0.4 pmol/microg protein, P=0.0007) and UTP (8.7 versus 1.6 pmol/microg protein, P=0. 0007) concentrations in ANLL cells compared with granulocytes. Treatment of ANLL cells in vitro with CPEC induced a major depletion (77% with 2.5 microM of CPEC) in the concentration of CTP, whilst the concentrations of the other ribonucleotides remained unchanged. Therefore, the high activity of CTP synthetase in acute non-lymphocytic leukaemic cells can be inhibited by CPEC, which provides a key to a new approach for the treatment of ANLL.

Cytidine triphosphate synthase activity and mRNA expression in normal human blood cells

Cytidine triphosphate (CTP) synthase is one of the key enzymes in pyrimidine nucleotide anabolic pathways. The activity of this enzyme is elevated in various malignancies including acute lymphocytic leukemia (ALL). In this study we investigated the activity of CTP synthase in various human blood cells isolated from healthy volunteers by density centrifugation and elutriation centrifugation. We also investigated the mRNA expression of CTP synthase in lymphocytes and monocytes. The highest activity of CTP synthase was found in thrombocytes (6.48 nmol CTP x mg(-1) x h(-1)), followed by that of monocytes (2.23), lymphocytes (1.69), granulocytes (0.52) and erythrocytes (0.42). The activity of CTP synthase in whole blood samples was at an intermediate level (1.27). The mRNA expression of CTP synthase in monocytes was comparable to that observed in lymphocytes.