Purine nucleotide synthesis in normal and leukemic blood cells

ATP depletion triggers acute myeloid leukemia differentiation through an ATR/Chk1 protein-dependent and p53 protein-independent pathway

Despite advances in oncology drug development, most commonly used cancer therapeutics exhibit serious adverse effects. Often the toxicities of chemotherapeutics are due to the induction of significant DNA damage that is necessary for their ability to kill cancer cells. In some clinical situations, the direct induction of significant cytotoxicity is not a requirement to meet clinical goals. For example, differentiation, growth arrest, and/or senescence is a valuable outcome in some cases. In fact, in the case of acute myeloid leukemia (AML), the use of the differentiation agent all-trans-retinoic acid (ATRA) has revolutionized the therapy for a subset of leukemia patients and led to a dramatic survival improvement. Remarkably, this therapeutic approach is possible even in many elderly patients, who would not be able to tolerate therapy with traditional cytotoxic chemotherapy. Because of the success of ATRA, there is widespread interest in identifying differentiation strategies that may be effective for the 90-95% of AML patients who do not clinically respond to ATRA. Utilizing an AML differentiation agent that is in development, we found that AML differentiation can be induced through ATP depletion and the subsequent activation of DNA damage signaling through an ATR/Chk1-dependent and p53-independent pathway. This study not only reveals mechanisms of AML differentiation but also suggests that further investigation is warranted to investigate the potential clinical use of low dose chemotherapeutics to induce differentiation instead of cytotoxicity. This therapeutic approach may be of particular benefit to patients, such as elderly AML patients, who often cannot tolerate traditional AML chemotherapy.

Role of thiamin (vitamin B-1) and transketolase in tumor cell proliferation

Metabolic control analysis predicts that stimulators of transketolase enzyme synthesis such as thiamin (vitamin B-1) support a high rate of nucleic acid ribose synthesis necessary for tumor cell survival, chemotherapy resistance, and proliferation. Metabolic control analysis also predicts that transketolase inhibitor drugs will have the opposite effect on tumor cells. This may have important implications in the nutrition and future treatment of patients with cancer.

In vivo antitumor activity and metabolism of a series of 5-deazaacyclotetrahydrofolate (5-DACTHF) analogues

This study compares the antitumor activity and metabolism of the purine de novo biosynthesis inhibitor 5-deazaacyclotetrahydrofolate and a series of analogues. All compounds have similar IC50 values for inhibition of MCF-7 cell growth, activity of glycineamide ribonucleotide transformylase, and methotrexate uptake by MOLT-4 cells, the latter a measure of cellular uptake potential. Only 5-deazaacyclotetrahydrofolate and the 2'-fluoro and 3'-fluoro analogues demonstrated significant inhibition of colon 38 adenocarcinoma or HCT-116 colon carcinoma growth in vivo. This correlated with the Km of these compounds for folylpolyglutamate synthetase. 5-Deazaacyclotetrahydrofolate and 2'-fluoro-5-deazaacyclotetrahydrofolate which displayed the strongest antitumor activity were detectable in colon 38 tumor tissue 24 hr after dosing and were present nearly exclusively as the polyglutamated species. These results indicate that polyglutamation represents a critical step in the in vivo antitumor activity of these compounds.

Purine de novo synthesis as the basis of synergism of methotrexate and 6-mercaptopurine in human malignant lymphoblasts of different lineages

Methotrexate (MTX) causes an inhibition of purine de novo synthesis (PDNS), resulting in increased intracellular availability of 5-phosphoribosyl-1-pyrophosphate (PRPP) in human malignant lymphoblasts with an active PDNS. Normal bone marrow cells and peripheral blood lymphocytes lack this capacity. The increased levels of PRPP can be used for enhanced incorporation of 6-mercaptopurine (6MP), indicating a potential time-, sequence- and dose-dependent synergism of both drugs. The effects of 0.02 microM and 0.2 microM MTX on the PDNS of MOLT-4 (T-), RAJI (B-) and KM-3 (non-B-non-T-) human malignant lymphoblasts were studied with respect to PRPP levels, aminoimidazolecarboxamide ribonucleosidemonophosphate (AICAR) levels and the incorporation of labeled glycine into purine metabolites. These results were correlated with the activity of the PDNS (labeled glycine incorporation) and the purine salvage pathway (labeled hypoxanthine incorporation) in untreated cells. Inhibition of PDNS by 0.02 microM MTX was complete in KM-3 cells with a moderately active PDNS and salvage pathway. RAJI cells, with a relatively low PDNS and high salvage pathway, demonstrated an incomplete, but increasing inhibition of PDNS, whereas inhibition of PDNS in MOLT-4 cells with both pathways active was minimal and recovered in time. Treatment with 0.2 microM MTX resulted in a complete inhibition of PDNS in all cell lines. After treatment with MTX an enhanced incorporation of labeled hypoxanthine and 6MP was noticed, confirming the potential rescue from MTX cytotoxicity by hypoxanthine and a potential synergism of MTX and 6MP on cytotoxicity. The enhanced incorporation of 6MP was more obvious in RAJI and KM-3 cells in comparison with MOLT-4 cells. These data demonstrate the important role of both the activities of the PDNS and the purine salvage pathway in malignant lymphoblasts of different subclasses with respect to the synergism of MTX and 6MP.

Effects of methotrexate on purine and pyrimidine metabolism and cell-kinetic parameters in human malignant lymphoblasts of different lineages

MOLT-4 (T-), RAJI (B-), and KM-3 (non-B-non-T-, common ALL) malignant lymphoblasts demonstrated significant differences in their activities of purine de novo synthesis (PDNS) and purine salvage pathway and in their cell-kinetic parameters. Incubations with concentrations of methotrexate (0.02 and 0.2 microM), which can be maintained during many hours in the oral maintenance therapy of acute lymphoblastic leukemia, indicated large differences between the three cell lines with respect to the inhibition of PDNS, depending on the concentration of methotrexate (MTX) and on the activities of the two pathways. These dose- and cell line-dependent differences corresponded to the perturbations of cell-kinetics and purine and pyrimidine (deoxy)ribonucleotide pools in the three cell lines. Exposure of MOLT-4 cells to 0.02 microM MTX resulted in an incomplete inhibition of DNA synthesis in early S phase, as shown by DNA-flow cytometry and increase of dCTP levels, which recovered spontaneously after 48 hr. Almost no impairment of RNA synthesis occurred (unbalanced growth). In RAJI cells, exposed to 0.02 microM MTX, DNA synthesis was delayed in the S phase, not arrested, and RNA synthesis was not impaired, also indicating an unbalanced growth pattern, which, however, did not recover in time. KM-3 cells were arrested in G1 phase and subsequently in early S phase after incubation with 0.02 microM MTX, and perturbations of ribonucleotides indicated a complete inhibition of RNA synthesis, resulting in a balanced growth pattern. Cytotoxicity was more pronounced in KM-3 cells. The reliability of the soft agar colony forming assay after low dose MTX treatment is discussed. Exposure of MOLT-4 and KM-3 cells to 0.2 microM MTX resulted in a complete inhibition of DNA synthesis, with cessation of cell progression through all parts of the cell cycle and arrest in G1 phase. RAJI cells showed an increasing accumulation of cells in G1 phase without complete cessation of cell cycle progression. Perturbations of ribonucleotide pools suggested an inhibition of RNA synthesis in all cell lines, indicating a balanced growth pattern in KM-3 cells and MOLT-4 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose-related effects of methotrexate on purine and pyrimidine nucleotides and on cell-kinetic parameters in MOLT-4 malignant human T-lymphoblasts

The effects of methotrexate (MTX) on cytotoxicity (trypan blue exclusion and soft agar clonal growth), cell cycle perturbation, and purine and pyrimidine ribonucleotide and deoxyribonucleotide pools have been studied in MOLT-4 malignant T-lymphoblasts. Two concentrations of MTX, 0.02 microM and 0.2 microM have been utilized, which can be maintained in vivo during many hours in the maintenance therapy of acute lymphoblastic leukemia (ALL). The results are correlated with the effects of MTX on the inhibition of purine de novo synthesis. Treatment with 0.02 microM MTX results in an accumulation of cells in early S phase after 20 hr, as measured by DNA flow cytometry and by a significant increase of dCTP levels, followed by a slow progression of a cohort of cells through the cell cycle. Cytotoxicity also becomes evident starting from this point of time. The effects on deoxyribonucleotide pools are discussed in correlation with the inhibition of DNA synthesis. The changes in ribonucleotide pools are associated with the partial inhibition of purine de novo synthesis at 20-28 hr and suggest an inhibition of RNA synthesis. After 48 hr a reutilization of nucleotide precursors due to nucleic acid breakdown and a recovery of purine de novo synthesis is shown, associated with a recovery of RNA synthesis, whereas cytotoxicity increases. Treatment of MOLT-4 cells with 0.2 microM MTX results in a rapid complete cessation of cell progression through all parts of the cell cycle after 8 hr, associated with a depletion of all deoxyribonucleotide pools, complete inhibition of purine de novo synthesis, inhibition of RNA synthesis and a marked cytotoxicity. Ribonucleotide pools demonstrate a reutilization of nucleotide precursors after 12 hr of incubation without a recovery of purine de novo synthesis and RNA synthesis. These data show a close dose- and time-dependent correlation of the effects of MTX on purine de novo synthesis, UMP levels and other (deoxy)ribonucleotide pools, and on RNA and DNA synthesis in MOLT-4 cells having an active purine de novo synthesis. This correlation is absent in normal bone marrow cells and peripheral blood lymphocytes. These data can be used in order to elucidate the synergistic effects of sequential administration of MTX and 6-mercaptopurine.

Inhibition of salvage pathway enzymes by adenine arabinoside 5'-monophosphate (ara-AMP)

The effect of 9-beta-arabinofuranosyladenine 5'-monophosphate (ara-AMP) on the purine salvage pathway has been studied. On a dose-dependent basis ara-AMP inhibits the incorporation of adenine-8-14C into nucleotides in intact erythrocytes. The partially purified enzymes of the purine salvage pathway, the adenine phosphoribosyltransferase and the 5'-phosphoribosyl-1-pyrophosphate (PP-ribose-P) synthetase, but not the hypoxanthine-guanine phosphoribosyltransferase, are inhibited by ara-AMP in a non-competitive manner. The possible adverse drug interactions which might occur by the simultaneous use of ara-AMP and other antimetabolites are discussed.

Inosine 5'-phosphate dehydrogenase activity in normal and leukemic blood cells

The enzyme inosinic acid dehydrogenase (EC 1.2.1 [14]) was measured and partially purified (10- to 15-fold) from normal and leukemic leukocytes. From the normal blood cells, the highest activities could be detected in lymphocytes and bone marrow cells. Dependent on the blast cell count, the leukemic IMP dehydrogenase had a higher mean specific activity than the enzymes of fractionated, immature bone marrow cells, or normal granulocytes. The partially purified enzymes from the various blood cells were apparently identical; they exhibited hyperbolic substrate saturation kinetics and were inhibited by a number of purine nucleotides. For the leukemic blast cell enzyme, the Km values for the substrates, IMP and NAD+, were 28 +/- 11; 227 +/- 98 microM, and 34 +/- 10; 240 +/- 67 microM for the partially purified enzyme from normal, immature bone marrow cells. The hypoxanthine-guanine and adenine phosphoribosyltransferase activities increased in the leukemic cells when compared with mature granulocytes, but nearly always showed similar activities when compared with fractionated bone marrow cells. Only one of the 30 investigated leukemic patients exhibited a marked decrease in hypoxanthine phosphoribosyltransferase activity of 0.5 nmol/mg/h. The phosphoribosyltransferase-specific activities of the leukemic cells are more variable than for the normal ones and no correlation of enzyme activities and blast cell count was apparent.