Response of methylenetetrahydrofolate levels to methotrexate in Krebs ascites cells

Reversal of Methotrexate-Induced Folate Pool Depletion by Thymidine in a Human Leukemia Cell Line in Vitro

In an In vitro study using a human monocytic leukemia cell line, U-937, the effects of interferon-γ (IFN-γ) in combination with the antifolate methotrexate and the role of thymidine introduced as a biochemical modulator were investigated. Methotrexate alone or in combination with INF-γ was found to enhance the induction of morphologic and functional monocytic differentiation in the U-937 cell line. Various cellular effects with the addition of thymidine to the medium with methotrexate and IFN-γ were studied. Enhanced inhibition of cell growth and perturbation of the cell cycle were noted when methotrexate and IFN-γ were used in combination, but not when methotrexate was used alone. The reduction of cellular folate by methotrexate was also enhanced in combination with IFN-γ. Cell cycle delay, resulting in cell growth inhibition of folate depletion, caused the induction of differentiation in U-937 cells, which was found to be greater with methotrexate + IFN-γ than with methotrexate alone. Cellular differentiation, as assessed by nitroblue tetrazolium reduction assay, indirect immunofluorescence and morphology, showed better effects towards the differentiation of U-937 cells when the agents were used in combination. However, addition of thymidine to the medium was found to cancel all the aforementioned effects. The addition of thymidine to the medium also caused reversal of the inhibitory effect of methotrexate and IFN-γ on cell growth and repletion of the endogenous folate level. Repletion of the folate level by exogenous thymidine is a new possibility for the role of the thymidine in cellular growth.

Low-dose methotrexate enhances cycling of highly anaplastic cancer cells

We previously showed that cellular RedOx state governs the G₁-S transition of AH130 hepatoma, a tumor spontaneously reprogrammed to the embryonic stem cell stage. This transition is impaired when the mithocondrial electron transport system is blocked by specific inhibitors (antimycin A) or the respiratory chain is saturated by adding to the cells high concentrations of pyruvate. The antimycin A or pyruvate block is removed by the addition of adequate concentrations of folate (F). This suggests that the G₁-S transition of AH130 cells depends on a respiration-linked step of DNA synthesis related to folate metabolism. In the study reported here, we characterized the effects of methotrexate (MTX), an inhibitor of dihydofolate-reductase, on the G₁-S transition of hepatoma cells, in the absence or the presence of exogenously added F, dihydrofolate (FH₂) or tetrahydrofolate (FH₄). MTX, at 1 μM or higher concentrations, inhibited G₁-S transition. This inhibition was completely removed by exogenous folates. Surprisingly, 10 nM MTX stimulated G₁-S transition. The addition of F, but not FH₂ or FH₄, significantly increased this effect. Furthermore, 10 nM MTX removed the block of the G₁-S transition operated by antimycin A or pyruvate, an effect which was enhanced in the presence of F. Finally, the stimulatory effect of 10 nM MTX was inhibited in the presence of serine. Our findings indicated that, under certain conditions, MTX may stimulate, rather than inhibiting, the cycling of cancer cells exhibiting a stem cell-like phenotype, such as AH130 cells. This may impact the therapeutic use of MTX and of folates as supportive care.

Antitumour effects of pure diastereoisomers of 5-formyltetrahydrofolate in hepatic transplants of a rodent colon carcinoma model

The effects of the two diastereoisomers of 5-formyltetrahydrofolate on tumour growth, thymidylate synthase (TS, EC 2.1.1.45) levels, and potentiation of 5-fluorouracil cytotoxicity were studied in an in vivo rat colon carcinoma model, transplanted to liver. The animals were randomized into eight groups, treated with daily i.v. tail vein injections of racemic (d,l)-5-formyltetrahydrofolate (5-CHO-FH4), 15 mg/kg, (1)-5-CHO-FH4 7.5 mg/kg, and (d)-5-CHO-FH4 7.5 mg/kg, 5-fluorouracil (FUra) 30 mg/kg, (d,l)-5-CHO-FH4 15 mg/kg+FUra 30 mg/kg, (l) 5-CHO-FH4 7.5 mg/kg+FUra 30 mg/kg, and (d)-5-CHO-FH4 7.5 mg/kg+FUra 30 mg/kg, and a sham-treated control group. The average tumour size of the groups was equal at the start of treatment. After six days' treatment the average tumour sizes were at laparotomy 3.3 +/- 1.0 g in the (d/l)-5-CHO-FH4 treated group, compared to 2.0 +/- 0.1 g in the FUra treated group and 7.1 +/- 3.1 g in the controls. Natural (l)-5-CHO-FH4 promoted tumour growth (average tumour weight 10.8 +/- 4.0 g), whereas the unnatural (d)-5-CHO-FH4 alone retarded it (average tumour weight 1.2 +/- 0.40 g). (l)-5-CHO-FH4 induced a significant increase in tumour tissue TS levels by [3H]FdUMP radioligand assay (27.5 +/- 8.4 pmol/g tumour tissue) compared to controls (16.8 +/- 6.1 pmol/g tumour tissue). Increases in 5,10-methylenetetrahydrofolate and tetrahydrofolate occurred with FUra alone, with a further statistically significant increase in both folates with the addition of (d)-5-CHO-FH4 to FUra.

Compartmentation of intracellular folates. Failure to interconvert tetrahydrofolate cofactors to dihydrofolate in mitochondria of L1210 leukemia cells treated with trimetrexate

Following exposure of L1210 leukemia cells to antifolates, tetrahydrofolate-dependent purine and pyrimidine biosyntheses are blocked despite the presence of the major portion of tetrahydrofolate cofactors. Previous studies from this laboratory demonstrated that this cannot be due to direct inhibition of thymidylate synthase by dihydrofolate polyglutamates or other endogenous folates and suggested that this phenomenon is due to compartmentation of tetrahydrofolate cofactors unavailable for interconversion and/or oxidation when dihydrofolate reductase activity is abolished by antifolates. The present paper evaluates the possibility that tetrahydrofolate cofactors in subcellular organelles, in particular, mitochondria, are unavailable for oxidation by thymidylate synthase. Particulate and cytosolic fractions were obtained from L1210 cells following homogenization and differential centrifugation. The crude mitochondrial fraction contained 20.1% of the total folate pool and included 5-formyltetrahydrofolate, 10-formyltetrahydrofolate and tetrahydrofolate in proportions similar to intact cells. The cytosolic fraction had an increased proportion of tetrahydrofolate and decreased proportions of 5-formyl- and 10-formyltetrahydrofolate relative to intact cells or the particulate fraction. Exposure of cells to 10 microM trimetrexate for 30 min produced approximately 45% interconversion of tetrahydrofolate cofactors to dihydrofolate in the cytosolic fraction, a level much greater than that observed in whole cell extracts (25-30%), but had no effect on folate pools in the crude mitochondrial fraction. These data indicate that subcellular compartmentation accounts, in part, for the failure to oxidize tetrahydrofolate cofactors to dihydrofolate in the presence of antifolate levels that abolish dihydrofolate reductase activity.

Evidence for a localized conversion of endogenous tetrahydrofolate cofactors to dihydrofolate as an important element in antifolate action in murine leukemia cells

The inhibition of de novo nucleotide, serine, and methionine biosynthesis in mammalian cells treated with antifolates has been attributed generally to a reduction in the levels of tetrahydrofolate cofactors. In L1210 leukemia cells grown in tritiated folic acid (1 microM), most of the endogenous radiolabeled folates were present as formyl-substituted tetrahydrofolates (60-73%, including 10- and 5-formyl and 5,10-methenyl tetrahydrofolate), with lower levels of tetrahydrofolate (including 5,10-methylene tetrahydrofolate), 5-methyl tetrahydrofolate, and non-metabolized folic acid. Trimetrexate (1 microM) caused an elevation of dihydrofolate levels within 5 min following drug addition, from approximately 1 to 20% of the total folates. Whereas total reduced folates were preserved, losses in the levels of individual forms ranged from minor changes in the formyl tetrahydrofolates (approx. 10% decrease), to significant losses in the levels of tetrahydrofolate (approx. 60%) and 5-methyl tetrahydrofolate (95%). Under these conditions, the incorporations of [3H]deoxyuridine into TMP and [14C]glycine into purines or of [14C]formate into biosynthetic products were inhibited (69-95%). The majority (59-100%) of the endogenous radiolabeled folates in L1210 cells grown in various concentrations (0.2 to 3 microM) of [3H]folic acid was bound to soluble intracellular proteins when cell-free extracts were fractionated by rapid gel filtration or charcoal adsorption. Total intracellular folate levels increased in proportion to the changes in medium folic acid concentration; however, cofactor binding was saturable. At low concentrations, below that which supported maximal growth (less than 0.75 microM), all of the intracellular folates were protein-bound; only when maximal growth was achieved, could unbound folates be detected. Incubation with trimetrexate (1 or 10 microM), methotrexate (10 microM), or calcium leuvovorin (50 microM) did not alter significantly the levels of total and protein-bound [3H]folates in cells grown in 1 microM [3H]folic acid. Under all conditions, formyl tetrahydrofolates were the major intracellular derivatives; however, these forms were poorly represented in the bound fraction. Conversely, all of the other intracellular folate forms were completely bound. Tetrahydrofolate was the predominant protein-bound derivative in control cells; in antifolate-treated cells, both bound tetrahydrofolate and 5-methyl tetrahydrofolate were largely replaced by protein-bound dihydrofolate. This interconversion in drug-treated cells was independent of (i) sustained levels of [3H]formyl tetrahydrofolates, or (ii) high extracellular concentrations of unlabeled calcium leucovorin (50 microM). Hence, protein-bound tetrahydrofolates must not only be substrates for enzyme mediated reactions (i.e. TMP synthesis) but also must slowly equilibrate with unbound cofactor. In this fashion, binding of endogenous folates to soluble proteins may function to "segregate' intracellular cofactor pools.(ABSTRACT TRUNCATED AT 400 WORDS)

Folinic acid modulation of fluorouracil: tissue kinetics of bolus administration

Thymidylate synthase (TS) is the enzyme target of 5-fluorouracil (FUra) that recent laboratory and clinical studies with folinic acid (calcium leucovorin) suggest may mediate important antitumor cytotoxicity. Measurement in carcinoma tissue of parameters related to TS inhibition by 5-fluorodeoxyuridylate (FdUMP), by analogy to hormone receptor analysis, should be useful to determine which patients should receive fluoropyrimidine drug therapy and to evaluate folinic acid requirements. Folinic acid is metabolized to 5,10-methylenetetrahydropteroylglutamine (CH2FH4), which must be present in large excess to effect desired levels of maximal inhibition of TS, by promoting formation and stabilization of TS-FdUMP-CH2FH4 ternary complexes. In patients with metastatic disease, serial biopsies of tumor and normal tissues for studies of pharmacodynamic responses to test-dose FUra or folinic acid are shown to be easily added to routine intraoperative management. A suitable methodologic approach is described and examples given of assays of free TS, FdUMP, dUMP, and CH2FH4 levels after FUra or folinic acid, that may be useful in future studies aimed at improving the cost-effectiveness of FUra-folinic acid combinations.

Effects of methotrexate on folates in Krebs ascites and L1210 murine leukemia cells

Changes in reduced folates upon exposure of Krebs ascites cells and L1210 murine leukemia cells to methotrexate (MTX) have been measured by stoichiometric entrapment of tissue methylenetetrahydrofolate into a stable ternary complex with thymidylate synthase and tritiated 5-fluoro-2'-deoxy-uridine-5'-monophosphate. Tetrahydrofolate and 5-methyltetrahydrofolate were determined after conversion to methylenetetrahydrofolate. In both tumor cell lines, treatment with methotrexate at levels which had little effect on methylenetetrahydrofolate and tetrahydrofolate concentrations resulted in nearly complete elimination of the methyltetrahydrofolate pool. Thus, an initial effect of methotrexate on folate metabolism appears to be on methyltetrahydrofolate.

Folate enzymes in Ehrlich ascites carcinoma-bearing mice

The activity of 4 enzymes involved in the formation and interconversion of folate coenzymes has been examined in liver and kidney of healthy and Ehrlich ascites carcinoma-bearing mice. In the liver, a 50% increase of methylenetetrahydrofolate reductase activity was shown soon after tumour cell inoculation, while the activity of formyltetrahydrofolate synthetase and methylenetetrahydrofolate dehydrogenase decrease by 20% at an advanced stage of tumour development. In kidneys of the host mouse the only change observed was a decrease of dihydrofolate reductase activity. The levels of activity of all assayed enzymes found in host organs were similar to that in Ehrlich carcinoma cells.