UMLS:C0023418 - FACTA Search

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Query: UMLS:C0023418 (leukemia)
93,477 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Methotrexate (MTX) inhibition of the growth of mouse or human leukemia cells in culture was partially prevented by either thymidine (dThd) or hypoxanthine. 5-Fluoro-2'-deoxyuridine (FdUrd) also decreased the growth-inhibitory potency of MTX in the presence of small concentrations of 5-formyltetrahydrofolate (citrovorum factor) and sufficient exogenous dThd to support the synthesis of thymidylate nucleotides by salvage mechanisms. In addition, citrovorum factor-induced reversal of MTX was several orders of magnitude more efficient in the presence of both FdUrd and dThd than in the presence of dThd alone or in the absence of both nucleosides. Likewise, the presence of FdUrd (3 microM) and dThd (5.6 microM) completely prevented the lethality of 0.3 mM MTX to L1210 cells in culture medium supplemented with micromolar concentrations of citrovorum factor. We propose that this protection against the cytotoxic effects of MTX by dThd, hypoxanthine, and FdUrd have a common biochemical mechanism--namely, inhibition of the de novo synthesis of thymidylate by either a direct [FdUrd; inhibition of thymidylate synthetase (thymidylate synthase; 5,10-methylenetetrahydrofolate:dUMP C-methyl-transferase, EC 2.1.1.45)] or indirect (dThd and hypoxanthine; feedback inhibition by anabolites on ribonucleotide reductase and deoxycytidylate deaminase) effect. The resultant decreased rate of loss of reduced folates due to de novo thymidylate synthesis would allow a higher degree of inhibition of dihydrofolate reductase to be endured without damage to the cell.
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PMID:Role of thymidylate synthetase activity in development of methotrexate cytotoxicity. 16 May 58

Net accumulation of methotrexate by carrier-mediated transport in different murine tumor cells in vitro exhibits a positive correlation with the relative drug pharmacokinetics and therapeutic responsiveness in these tumors in vivo. The transport of methotrexate by Sarcoma 180, Ehrlich carcinoma, P388, P288, and L1210 leukemia cells is qualitatively similar. Influx of drug exhibits saturation kinetics and is highly temperature dependent (Q10, 6.1 to 9.4). Efflux of exchangeable methotrexate from all of the different tumor cells exhibited first-order kinetics and the same high temperature dependence seen for influx (Q10, 6.1 to 8.0). The major kinetic determinant of responsiveness is the Km for influx. Values vary from 3.1 to 11.2 X 10(-6) M and are highest in cells from a nonresponsive Sarcoma 180 tumor, somewhat lower in the poorly responsive Ehrlich tumor, lower in moderately responsive P388 and P288 leukemias, and lowest in the highly responsive L1210 leukemia. Values for the influx Vmax differ to some extent, but in a manner not correlatable with responsiveness. The level of responsiveness of the P388 leukemia in vivo can also be partially attributed to an efflux rate that is lower than that measured for the other tumor cells. Steady-state levels of drug accumulation in vitro reflected influx and efflux rates and were consistently correlatable with therapeutic responsiveness. There was no significant difference in the extent to which folate and reduced 5-substituted folate derivatives compete with methotrexate for uptake in cells from all five tumors. The average value for Ki measured with folate for each tumor cell type was 50- and 80-fold higher than for 5-formyltetrahydrofolate and 5-methyltetrahydrofolate.
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PMID:Kinetic correlates of methotrexate transport and therapeutic responsiveness in murine tumors. 94 37

A variant line (CEM-7A) "overproducing" the reduced folate/MTX carrier system was isolated from human CCRF-CEM leukemia cells grown under selective conditions in medium containing 0.25 nM 5-formyl-THF as the sole folate source. This line exhibits a 95-fold increased Vmax for [3H]-MTX influx as compared to parental cells. The values for [3H]-MTX influx Km, efflux t1/2 and structural specificity for other (anti)folate compounds were unchanged. The amount of carrier protein, estimated by NHS-[3H]-MTX affinity labeling, was approximately 30-fold higher in CEM-7A cells than in parental cells. Influx of [3H]-MTX in CEM-7A cells was found to be down-regulated 6-7-fold after preincubation of cells with adenosine, 5-formyl-THF or 5-methyl-THF, but could be prevented exclusively by inhibitors of dihydrofolate reductase. The underlying mechanism(s) of these effects have not as yet been elucidated. A radioiodinated photoaffinity analog of MTX was used to prove the molecular events in carrier-mediated MTX uptake in parental CCRF-CEM cells, CEM-7A cells, and a line exhibiting a MTX-transport defect (CEM-MTX). Specific labeling of an 80-85 kDa membrane protein was observed in parental cells, but not in CEM/MTX cells. Uptake of photoprobe and levels of the 80-85 kDa membrane protein were significantly increased in CEM-7A cells. Due to extensive glycosylation the MW of the carrier protein in human cells seems to be substantially higher than that of its counterpart in murine L1210 leukemia cells (46-48 kDa). Pulse-labeling experiments at 37 degrees C demonstrated that in CEM-7A cells photoprobe uptake proceeds via a specific pathway. The 80-85 kDa membrane protein is involved in the initial binding and translocation of photoprobe, after which a 38 kDa cytosolic protein is responsible for further intracellular distribution. At this time, the combination of photoaffinity labeling techniques and the availability of variant cell lines overexpressing the reduced folate/MTX carrier protein has provided new insights into the MTX transport process in human leukemia cell lines. In the near future this approach should also allow a further elucidation of the regulatory aspects of carrier function.
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PMID:Molecular events in the membrane transport of methotrexate in human CCRF-CEM leukemia cell lines. 132 3

We have previously described a subline of L1210 murine leukemia cells (LL1) selected in a low level of 5-formyltetrahydrofolate which overexpresses a membrane-bound folate-binding protein (FBP1) and exhibits a rearrangement at the locus encoding this protein. Genomic clones containing the entire FBP1-encoding DNA from both L1210 and LL1 were isolated and characterized. Sequence analysis indicates that, with exception of the 5'-region, the FBP1-encoding locus in both cell lines is identical. The rearrangement in LL1 results from the insertion of an intracisternal A particle (IAP) in the head-to-head (antisense) orientation 72 base pairs (bp) upstream of the FBP1 ATG start codon. The IAP likely provides an alternative promoter for FBP1 expression which may produce a novel transcript with enhanced stability. Presence of the IAP appears to inactivate or relocate normal cis-acting regulatory sequences as expression of the FBP1 transcript in LL1 is not regulated by the folate status of the cell.
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PMID:Insertion of an intracisternal A particle within the 5'-regulatory region of a gene encoding folate-binding protein in L1210 leukemia cells in response to low folate selection. Association with increased protein expression. 142 88

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.
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PMID:Compartmentation of intracellular folates. Failure to interconvert tetrahydrofolate cofactors to dihydrofolate in mitochondria of L1210 leukemia cells treated with trimetrexate. 183 61

Plasma homocysteine was determined in 12 children with acute lymphoblastic leukemia. The patients were investigated prior to chemotherapy (stage I), during seven weeks of induction chemotherapy (stage II), and thereafter during intermittent high-dose methotrexate (HD-MTX) therapy (stage III). The patients were followed for a period of three to 15 months, and the study included a total of 80 HD-MTX courses. Before start of chemotherapy (stage I), the average plasma homocysteine level in the children with leukemia was 13.18 +/- 6.23 (SD) mumol/liter, which is significantly (P less than 0.001) higher than the level in control children (6.52 +/- 1.21 mumol/liter). The plasma homocysteine level in the patients was positively correlated with the peripheral white blood cell count (P less than 0.01) and negatively correlated with serum folate (P less than 0.02). The serum folate was normal or subnormal in these patients. During induction therapy with cytotoxic drugs such as vincristine, asparaginase, and intrathecal MTX (stage II), there was a drastic change in plasma homocysteine as a function of time. A reciprocal alteration in serum folate was observed, suggesting fluctuating intracellular folate status at this stage of therapy. At the end of stage II (about seven weeks), there was a significant (P less than 0.01) reduction in total homocysteine (to 7.08 +/- 3.84 mumol/liter). HD-MTX (8 g/m2) therapy with 5-formyltetrahydrofolate "rescue" (stage III) was usually begun about seven weeks after start of chemotherapy, and the patients were followed for two to eight courses separated by three to eight weeks. Plasma homocysteine showed a transient increase (26-64%) following each MTX infusion. After three MTX infusions, basal total plasma homocysteine was reduced to 5.56 +/- 1.12 mumol/liter. During most MTX infusions, there was a variable reduction (17-56%) in plasma methionine followed by a rebound increase. It is concluded that plasma homocysteine in children with acute lymphoblastic leukemia is elevated prior to therapy, probably because of occasional folate deficiency and increased burden of proliferating cells. During induction therapy, monitoring plasma homocysteine and serum folate both suggest a labile folate homeostasis, usually a deficiency state. HD-MTX induced a temporary intracellular folate depletion before 5-formyl-tetrahydrofolate was administered, as judged by a transient homocysteinemia. The methionine depletion may interfere with the antileukemic effect of MTX.
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PMID:Plasma homocysteine in children with acute lymphoblastic leukemia: changes during a chemotherapeutic regimen including methotrexate. 198 22

This study reports the isolation and characterization of a variant of the human CCRF-CEM leukemia cell line that overproduces the carrier protein responsible for the uptake of reduced folates and the folate analogue methotrexate. The variant was obtained by adapting CCRF-CEM cells for prolonged times to stepwise decreasing concentrations of 5-formyltetrahydrofolate as the sole folate source in the cell culture medium. From cells that were grown on less than 1 nM 5-formyl-tetrahydrofolate, a variant (CEM-7A) was isolated exhibiting a 95-fold increased Vmax for [3H]methotrexate influx compared to parental CCRF-CEM cells. The values for influx Km, efflux t0.5, and Ki for inhibition by other folate (analogue) compounds were unchanged. Affinity labeling of the carrier with an N-hydroxysuccinimide ester of [3H]methotrexate demonstrate an approximately 30-fold increased incorporation of [3H] methotrexate in CEM-7A cells. This suggests that the up-regulation of [3H]methotrexate influx is not only due to an increased amount of carrier protein, but also to an increased rate of carrier translocation or an improved cooperativity between carrier protein molecules. Incubation for 1 h at 37 degrees C of CEM-7A cells with a concentration of 5-formyltetrahydrofolate or 5-methyltetrahydrofolate in the physiological range (25 nM) resulted in a 7-fold decline in [3H]methotrexate influx. This down-regulation during incubations with 5-formyltetrahydrofolate or 5-methyltetrahydrofolate could be prevented by either the addition of 10-25 nM of the lipophilic antifolate trimetrexate or by preincubating CEM-7A cells with 25 nM methotrexate. The down-regulatory effect was specifically induced by reduced folates since incubation of CEM-7A cells with 25 nM of either methotrexate, 10-ethyl-10-deazaaminopterin, aminopterin, or folic acid, or a mixture of purines and thymidine, had no effect on [3H]methotrexate influx. Similarly, these down-regulatory effects on [3H]methotrexate transport by 5-formyltetrahydrofolate, and its reversal by trimetrexate or methotrexate, were also observed, though to a lower extent, for parental CCRF-CEM cells grown in folate-depleted medium rather than in standard medium containing high folate concentrations. These results indicate that mediation of reduced folate/methotrexate transport can occur at reduced folate concentrations in the physiological range, and suggest that the intracellular folate content may be a critical determinant in the regulation of methotrexate transport.
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PMID:Methotrexate transport in variant human CCRF-CEM leukemia cells with elevated levels of the reduced folate carrier. Selective effect on carrier-mediated transport of physiological concentrations of reduced folates. 221 1

The diasteromers of 5,10-dideaza-5,6,7,8-tetrahydrofolate (DDATHF) differing in chirality about carbon 6 were resolved and studied as inhibitors of folate-dependent processes in mouse leukemia cells. Both diastereomers of DDATHF were found to be potent inhibitors of leukemia cell growth due to effects on de novo purine synthesis. Cell growth inhibition by these compounds was prevented by 5-formyltetrahydrofolate in a dose-dependent manner. This indicated that the effects of the DDATHF diastereomers were due to inhibition of folate-dependent processes. Metabolite reversal experiments indicated that 5'-phosphoribosylglycinamide formyltransferase was the major site of action of these compounds in mouse cells. Another site in de novo purine synthesis was affected at higher concentrations of diastereomer B in L1210 cells. Low concentrations of both diastereomers were found to inhibit pure L1210 5'-phosphoribosylglycinamide formyltransferase competitively with the folate substrate. The two diastereomers were also efficient substrates for mouse liver folylpolyglutamate synthetase. We conclude that the 6R- and 6S-diastereomers of DDATHF are remarkably similar and equiactive antimetabolites inhibitory to de novo purine synthesis and that the biochemical processes involved in their cytotoxicity display little stereochemical specificity.
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PMID:The 6S- and 6R-diastereomers of 5, 10-dideaza-5, 6, 7, 8-tetrahydrofolate are equiactive inhibitors of de novo purine synthesis. 259 65

We report on membrane protein changes in an L1210 leukemia cell line with a highly specific defect in the function of the methotrexate (MTX)-tetrahydrofolate cofactor transport carrier. This clonal line, MTXrA, made 100-fold resistant to MTX, was derived in a single step and exhibited stable resistance over 120 generations in the absence of drug. The transport defect was associated with a 10-fold decrease in influx Vmax without a change in influx Km. There was no difference between the MTXrA and parent lines in the levels or affinities of specific cell surface binders for MTX nor in the labeling of the 44-kDa membrane protein upon treatment with the specific affinity label, N-hydroxysuccinimide ester of tritiated MTX. Consistent with impaired carrier function was the observation that trans-stimulation of MTX influx by intracellular 5-formyltetrahydrofolate observed in the parent line was not demonstrated in the MTXrA line. The transport defect was highly specific for the MTX-tetrahydrofolate cofactor transport carrier. Initial uptake rates for 5-fluoro-2'-deoxyuridine and 2-deoxyglucose were unchanged and influx and net transport of alpha-aminoisobutyric acid were, in fact, increased. There was no cross-resistance of this line to phenylalanine mustard or cytosine arabinoside, agents that utilize specific amino acid and nucleoside transport carriers, respectively. SDS-polyacrylamide gel electrophoresis of purified plasma membrane preparations stained with Coomassie Blue revealed several protein differences between the parental and MTXrA lines. Most prominent is a band at approximately 190 kDa which ran with slightly greater mobility than a lesser staining band in the parent line. [3H]Borohydride labeling of cells also identified a distinct protein peak in the MTXrA line at approximately 190 kDa eliminated by prior treatment of cells with neuraminidase. Absence of expression of protein or mRNA related to the multidrug resistance gene as well as lack of cross-resistance to daunorubicin or trimetrexate indicate that this mechanism of resistance to MTX is completely unrelated to the multidrug resistance phenomenon observed with high molecular weight heterocyclic compounds. These data represent the first demonstration of membrane protein differences in a highly resistant L1210 murine leukemia cell line with a marked unique defect in MTX transport which appears to be related to impaired mobility of the tetrahydrofolate-cofactor carrier. Further studies are now required to elucidate the possible role of one or more of these proteins in the transport defect.
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PMID:Membrane protein changes in an L1210 leukemia cell line with a translocation defect in the methotrexate-tetrahydrofolate cofactor transport carrier. 277 91

Properties of the methotrexate (MTX) transport carrier were examined in a stable single-step 16-fold MTX-resistant L1210 murine leukemia cell line with unchanged dihydrofolate reductase gene copy and thymidylate synthase and dihydrofolate reductase levels and activities. MTX influx was markedly depressed due to a decrease in Vmax without a change in Km. From this cell line a clonal variant with greater resistance to MTX was identified due solely to a further decrease in influx Vmax. Trans-stimulation of MTX influx by 5-formyltetrahydrofolate was induced in parental but not resistant cells. Analysis of specific MTX surface binding demonstrated a small increase in the number of carriers in the first- and second-step resistant lines. Affinity labeling of cells with an N-hydroxysuccinimide ester derivative of [3H]MTX demonstrated carriers with comparable molecular weights in the parent and second-step transport defective lines. In two partial revertants with increased MTX sensitivity isolated from the second-step resistant lines, MTX influx was increased but surface membrane-binding sites were unchanged suggesting that recovery of transport was due to normalization of carrier function rather than an increase in the number of carriers. These studies suggest that impaired MTX transport in these lines is not due to an alteration in the association of the transport carrier with its substrate at the cell surface. Rather, resistance may be due to an alteration in the mobility of the carrier possibly associated with a protein change in the carrier itself or the cell membrane that surrounds it.
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PMID:Evidence for a functional defect in the translocation of the methotrexate transport carrier in a methotrexate-resistant murine L1210 leukemia cell line. 283 83

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