Characterization of a Chinese hamster cell with a temperature-sensitive mutation in folate metabolism

Folylpolyglutamate synthetase gene transcription is regulated by a multiprotein complex that binds the TEL-AML1 fusion in acute lymphoblastic leukemia

Acute Lymphoblastic Leukemia (ALL) non-random fusions influence clinical outcome and alter the accumulation of MTX-PGs in vivo. Analysis of primary ALL samples uncovered subtype-specific patterns of folate gene expression. Using an FPGS-luciferase reporter gene assay, we determined that E2A-PBX1 and TEL-AML1 expression decreased FPGS transcription. ChIP assays uncovered HDAC1, AML1, mSin3A, E2F, and Rb interactions with the FPGS promoter region. We demonstrate that FPGS expression is epigenetically regulated through binding of selected ALL fusions to a multiprotein complex, which also controls the cell cycle dependence of FPGS expression. This study provides insights into the pharmacogenomics of MTX in ALL subtypes.

A mouse gene that coordinates epigenetic controls and transcriptional interference to achieve tissue-specific expression

The mouse fpgs gene uses two distantly placed promoters to produce functionally distinct isozymes in a tissue-specific pattern. We queried how the P1 and P2 promoters were differentially controlled. DNA methylation of the CpG-sparse P1 promoter occurred only in tissues not initiating transcription at this site. The P2 promoter, which was embedded in a CpG island, appeared open to transcription in all tissues by several criteria, including lack of DNA methylation, yet was used only in dividing tissues. The patterns of histone modifications over the two promoters were very different: over P1, histone activation marks (acetylated histones H3 and H4 and H3 trimethylated at K4) reflected transcriptional activity and apparently reinforced the effects of hypomethylated CpGs; over P2, these marks were present in tissues whether P2 was active, inactive, or engaged in assembly of futile initiation complexes. Since P1 transcriptional activity coexisted with silencing of P2, we sought the mechanism of this transcriptional interference. We found RNA polymerase II, phosphorylated in a pattern consistent with transcriptional elongation, and only minimal levels of initiation factors over P2 in liver. We concluded that mouse fpgs uses DNA methylation to control tissue-specific expression from a CpG-sparse promoter, which is dominant over a downstream promoter masked by promoter occlusion.

Molecular basis for decreased folylpoly-γ-glutamate synthetase expression in a methotrexate resistant CCRF-CEM mutant cell line

A CCRF-CEM mutant, CEM-p, has been shown to exhibit resistance to methotrexate due to decreased methotrexate polyglutamate accumulation. To ascertain the mechanism(s) responsible for this phenotype, we analyzed FPGS and SLC19A1 mRNA expression, examined FPGS promoter activity, and determined nucleotide sequence of the FPGS promoter and full length cDNA from CCRF-CEM and CEM-p cells. We identified in FPGS from CEM-p cells three amino acid substitutions that altered the ATP binding P-loop, glutamate/folate binding, and a conserved domain located at the carboxyl-terminal. Our data demonstrated for the first time the importance of the highly conserved domain (VTGSLHLVGGV) located at the carboxyl-terminal for FPGS activity.

Folic acid

Folic acid is an essential nutrient from the B complex group of vitamins. Folate, as a cofactor, is involved in numerous intracellular reactions, and this is reflected in the various derivatives that have been isolated from biological sources. Folic acid is involved in single carbon transfer reactions and serves as a source of single carbon units in different oxidative states. The processes involved in the absorption, transport, and intracellular metabolism of this cofactor are complex. Much of folate is bound tightly to enzymes, indicating that there is not excess of this cofactor and that its cellular availability is protected as well as being strictly regulated. In animals, the liver controls the supply of folate through first pass metabolism, biliary secretion, enterohepatic recirculation, as well as through senescent erythrocyte recycling. Deficiencies of folate can occur for many reasons, including reduced intake, increased metabolism, and/or increased requirements as well as through genetic defects. The effects of folate deficiency include hyperhomocysteinemia, megaloblastic anemia, and mood disorders. Folate deficiency has also been implicated in disorders associated with neural tube defects. Supplementation of grain products such as cereals has been undertaken in several countries as a cost-effective means of reducing the prevelance of neural tube defects. Recently, common polymorphisms have been discovered in several genes associated with folate pathways that may play a role in diseases associated with folate deficiency, particularly mild folate deficiency.

Molecular analysis of murine leukemia cell lines resistant to 5, 10-dideazatetrahydrofolate identifies several amino acids critical to the function of folylpolyglutamate synthetase

Four L1210 murine leukemia cell lines resistant to 5, 10-dideazatetrahydrofolate (DDATHF) and other folate analogs, but sensitive to continuous exposure to methotrexate, were developed by chemical mutagenesis followed by DDATHF selective pressure. Endogenous folate pools were modestly reduced but polyglutamate derivatives of DDATHF and ALIMTA (LY231514, MTA) were markedly decreased in these mutant cell lines. Membrane transport was not a factor in drug resistance; rather, folypolyglutamate synthetase (FPGS) activity was decreased by >98%. In each cell line, FPGS mRNA expression was unchanged but both alleles of the FPGS gene bore a point mutation in highly conserved domains of the coding region. Four mutations were in the predicted ATP-, folate-, and/or glutamate-binding sites of FPGS, and two others were clustered in a peptide predicted to be beta sheet 5, based on the crystal structure of the Lactobacillus casei enzyme. Transfection of cDNAs for three mutant enzymes into FPGS-null Chinese hamster ovary cells restored a reduced level of clonal growth, whereas a T339I mutant supported growth at a level comparable to that of the wild-type enzyme. The two mutations predicted to be in beta sheet 5, and one in the loop between NH(2)- and COOH-terminal domains did not support cell growth. When sets of mutated cDNAs were co-transfected into FPGS-null cells to mimic the genotype of drug-selected resistant cells, clonal growth was restored. These results demonstrate for the first time that single amino acid substitutions in several critical regions of FPGS can cause marked resistance to tetrahydrofolate antimetabolites, while still allowing cell survival.

Effect of polyglutamylation of 5,10-methylenetetrahydrofolate on the binding of 5-fluoro-2'-deoxyuridylate to thymidylate synthase purified from a human colon adenocarcinoma xenograft

CH2-H4PteGlu and H4PteGlu exist in human colon adenocarcinoma xenografts predominantly in the form of polyglutamate species at concentrations of less than 3 microM. The interaction of polyglutamates of [6R]CH2-H4PteGlu in the formation and stability of [6-3H]FdUMP-thymidylate synthase-CH2-H4PteGlun ternary complexes has therefore been examined using enzyme purified from a human colon adenocarcinoma xenograft. Dissociation of these complexes was first-order and was dependent upon the concentration of folate. [6R]CH2-H4PteGlu3-6 (0.9 to 1.6 microM) were greater than 200-fold and [6R]CH2-H4PteGlu2 (18.2 microM) was 18-fold more effective than [6R]CH2-H4PteGlu1 (335 microM) at stabilizing ternary complexes for a T1/2 for dissociation of 100 min. Polyglutamylation of CH2-H4PteGlu also increased the affinity of binding of [6-3H]FdUMP to thymidylate synthase as determined by Scatchard analysis at folate concentrations of 10 microM, where the Kd in the presence of [6R]CH2-H4PteGlu1 was in the order of 4.0 x 10(-8) M, and for [6R]CH2-H4PteGlu3-5 was between 3.7 and 5.5 x 10(-9) M. To examine whether this effect was due to differences in the rates at which [6-3H]FdUMP was bound (kon) or dissociated (koff) from the enzyme, the apparent rate of [6-3H]FdUMP binding was determined in the presence of [6R]CH2H4PteGlu1, [6R]CH2-H4PteGlu3 and [6R]CH2-H4PteGlu4. The kon values were similar and were in the range of 1.7 to 2.3 x 10(6) M-1 min-1 for 10 or 20 microM folate concentrations. Differences in binding affinity determined for [6R]CH2-H4PteGlu1 and longer polyglutamate forms of [6R]CH2-H4PteGlu were thus due to differences in koff. The Vmax for the initial velocity of [6-3H]FdUMP binding was achieved at 10 microM folate. Consequently, at concentrations of CH2-H4PteGlu polyglutamates present in tumors, inhibition of thymidylate synthase by FdUMP in vivo would be expected to be transient, based upon the concentration of [6R]CH2-H4PteGlun required for maximal formation and stability of the covalent ternary complex. It would be advantageous for modulation of CH2-H4PteGlun pools to increase the concentrations of the longer polyglutamate species (n greater than or equal to 3) to maximize the interaction between FdUMP, thymidylate synthase and CH2-H4PteGlu.

Folylpolyglutamate synthesis and role in the regulation of one-carbon metabolism

The physiological importance of folylpolyglutamates is now well established. These derivatives are the intracellular substrates and regulators of one-carbon metabolism, and their synthesis is required for normal folate retention by tissues. Over the last few years, a considerable amount of information has been obtained on the mechanism by which these compounds are synthesized, on how this synthesis is regulated, and on the effects of the polyglutamate chain on the interaction of folate substrates and inhibitors with folate-dependent enzymes. Many regulatory implications have been suggested by these studies, but the physiological relevance of some of these observations remains to be explored. Folates in mammalian tissues are metabolized to polyglutamates of chain lengths considerably longer than that required for folate retention, but the metabolic advantages of this are not entirely clear. Several in vivo model systems have been developed to explore the functioning of specific folylpolyglutamate chain lengths in metabolic cycles of one-carbon metabolism, and these are likely to shed further light on this point. The role of folate-binding proteins in folate transport, the metabolic role of glutamylhydrolases, and the role of folylpolyglutamates in putative multifunctional protein complexes are also areas that are being actively pursued at present and are likely to produce new insights in the future. Recent studies on the retention of antifolates by cells and on their substrate efficacy for folylpolyglutamate synthetases have also suggested mechanisms for the differential cytotoxicity of these agents for different tissues.

Direct selection of Chinese hamster ovary strains deficient in CTP synthetase activity

Mutant CHO cell strains dependent upon cytidine for growth and survival were isolated by a direct selection protocol. The mutants obtained are deficient in CTP synthetase activity (less than 2% residual activity) and have low reversion frequencies (less than 10(-7)). Cytidine deprivation of these stains leads to rapid depletion of intracellular CTP pools, but not dCTP pools, and a surprisingly rapid loss of cell viability. These properties should make the cytidine auxotrophs useful for a number of biochemical and genetic studies.

Characteristics of thymidylate synthase purified from a human colon adenocarcinoma

Thymidylate synthase has been purified greater than 4000-fold from a human colon adenocarcinoma maintained as a xenograft in immune-deprived mice. In this disease, the enzyme is an important target for the cytotoxic action of 5-fluorouracil, which is influenced by the reduced folate substrate CH2-H4PteGlu. Due to the importance of this interaction, and the existence in cells of folate species as polyglutamyl forms, the interaction of folylpolyglutamates with thymidylate synthase was examined. Polyglutamates of PteGlu were used as inhibitors, and the interaction of CH2-H4PteGlu polyglutamates as substrates or in an inhibitory ternary complex were also examined. Using PteGlu1-7, Ki values were determined. A maximal 125-fold decrease in Ki was observed between PteGlu1 and PteGlu4; further addition of up to three glutamyl residues did not result in an additional decrease in Ki. Despite the increased binding affinity of folypolyglutamates for this enzyme, no change in the Km values for either dUMP (3.6 microM) or CH2-H4PteGlu (4.3 microM) were detected when polyglutamates of [6R]CH2-H4PteGlu were used as substrates. Product inhibition studies demonstrated competitive inhibition between dTMP and dUMP in the presence of CH2-H4PteGlu5. In addition, CH2-H4PteGlu4 stabilized an inhibitory ternary complex formed between FdUMP, thymidylate synthase, and CH2-H4PteGlu4. Thus the data do not support a change in the order of substrate binding and product release upon polyglutamylation of CH2-H4PteGlu reported for non-human mammalian enzyme. This is the first study to characterize kinetically thymidylate synthase from a human colon adenocarcinoma.

Complementation mapping in microcell hybrids: localization of Fpgs and Ak-1 on Mus musculus chromosome 2

The gene encoding folylpolyglutamyl synthetase (FPGS) was assigned to mouse chromosome 2 by complementation mapping. Chinese hamster ovary cells (AuxBl) deficient in FPGS, and consequently auxotrophic for glycine, adenosine, and thymidine (gat-), were employed as recipients in microcell-mediated chromosome transfer experiments. Mouse chromosomes derived from diploid embryo fibroblasts were introduced into hamster AuxBl cells, and gat+ microcell hybrids were selected in medium lacking adenosine and thymidine. Mouse chromosome 2 was the only donor chromosome whose presence correlated with expression of FPGS activity. Furthermore, every gat+ hybrid clone expressed murine AK-1, a marker previously assigned to chromosome 2. Eight of 20 clones analyzed retained deletion chromosomes derived from mouse chromosome 2. These clones were used to localize murine Fpgs and Ak-1 to a region of this chromosome, namely 2 (cen leads to Cl).

Folate pentaglutamate and folate hexaglutamate mediated one-carbon metabolism

The proposal that folate polyglutamate cofactors of different chain lengths function differently in metabolism was investigated. We identified the one-carbon units present in rat liver folates within each of the liver folate polyglutamate groups, the folate penta-, hexa-, and heptaglutamates. This identification revealed that at the pentaglutamate level, 5-methyl-H4folate was the major form, accounting for 18% of the total liver folates, with small amounts of H4folate. At the hexaglutamate level, 5-methyl-H4folate and H4folate were major forms, accounting for 17% and 22% of the liver folates, respectively. At the heptaglutamate level, 5-methyl-H4folate occurred in small amounts while H4folate predominated. The rats used here had been fed a diet low in methionine and were exposed to N2O gas. These results are qualitatively similar to those from rat brain [Brody, T., Shin, Y. S., & Stokstad, E. L. R. (1976) J. Neurochem. 27, 409--413] where 5-methyl-H4folate was a major fraction of the folate pentaglutamates but a minor, if detectable, part of the hexa- and heptaglutamates. The folates 5-methyl-H4PteGlu5-7 were metabolically active in the liver, as illustrated by the severe contraction in the amounts of these folates following an injection of methionine. This indicates that "folate-binding proteins" do not prevent 5-methyl-H4folates from entering into one-carbon metabolism.

Changes in the chain length of folylpolyglutamates during liver regeneration

Partial hepatectomy significantly alters the proportions of folylpolyglutamates of different chain lengths in rat liver. Hexa- and heptaglutamates increase at the expense of tetra- and pentaglutamates. Methyl hexaglutamates account for most of the increase in hexaglutamates, and folates with formyl oxidation-level one-carbon substituents for most of the increase in heptaglutamates. These changes are reversible and believed to represent an element of regulation of one-carbon metabolism.

DNA-mediated gene transfer of beta-aspartylhydroxamate resistance into Chinese hamster ovary cells

Cell lines that have high levels of resistance to beta-aspartylhydroxamate and elevated levels of asparagine synthetase activity were selected in two steps from Chinese hamster ovary cells. Resistance to beta-aspartylhydroxmate was transferred into sensitive cells by using total genomic DNA derived from the dominant two-step mutants. The surviving colonies were characterized as transferants on the basis of transfer frequency, degree of resistance to beta-aspartylhydroxamate, increased level of asparagine synthetase activity, expression of the donor form of asparagine synthetase, codominance in hybrids, and instability of the phenotype in the absence of selection.

Evidence for functional hemizygosity at the Emtr locus in CHO cells through segregation analysis

The hypothesis of functional hemizygostiy at the emetine-resistant (Emtr, a non-X-linked recessive marker) locus in Chinese hamster ovary (CHO) cells has been examined by segregation analysis. The frequencies and the rates of segregation of the Emtr and Thgr (thioguanine-resistant, an X-linked recessive mutation) markers were determined from hybrids constructed between an Emtr-Thgr CHO cell line and various other Chinese hamster lines (V79, M3-1, CHO, GM7S, CHW and CHL). Thgr segregants were obtained at similar frequencies (10(-2)-10(-3)) from all the hybrids. The frequency of segregation of the Emtr marker, however, was similar to that of Thgr only in the CHO x CHO hybrids and was much lower (10(-4)-10(-6)) than the CHO x other Chinese hamster hybrids. Similar results were obtained when the segregation rates for the two markers from various hybrids were determined. These results are consistent with the hypothesis that in CHO cells, the gene responsible for Emtr is present in a single (functional) copy, whereas two copies of this gene are present in other Chinese hamster lines examined.

Biochemical genetic analysis of pyrimidine biosynthesis in mammalian cells: I. Isolation of a mutant defective in the early steps of de novo pyrimidine synthesis

The isolation and characterization of a new mutant of Chinese hamster ovary cells is described. This mutant, Urd-A, shows an absolute requirement for exogenously added pyrimidines for growth. Complementation analysis indicates that the lesion in this mutant is recessive. Revertants can be isolated at frequencies suggesting that it is a single gene alteration. Biochemical analysis of cell-free extracts of CHO-K1 (Urd+) and Urd-A revealed that Urd-A possesses no more than 10% of wild-type levels of carbamyl phosphate synthetase (EC 2.7.2.9) activity, no more than 1% of wild-type levels of aspartate transcarbamylase (EC 1.2.3.2) activity, and undetectable levels of dihydroorotase (EC 3.5.2.3) activity. Thus, this mutant appears simultaneously to possess marked or complete deficiencies in the activities of the first three enzymes of pyrimidine biosynthesis. Activities of the other enzymes of the pathway appear normal. The use of this mutant for biochemical-genetic studies of pyrimidine biosynthesis is discussed.

Mutations affecting the antigenic properties of hypoxanthine-guanine phosphoribosyl transferase in cultured Chinese hamster cells

Cells of the mutant Chinese hamster strain RJK10 do not contain either hypoxanthine-guanine phosphoribosyl transferase activity (HGPRT) or protein that cross-reacts immunologically with HGPRT. HGPRT+ revertants have been isolated from RJK10 and those strains produce HGPRT with altered antigenic properties. HGPRT from the revertant cells is less reactive with anti-HGPRT serum than enzyme from the wild-type cells, and enzymes from the two sources are immunoprecipitated independently from mixtures of cell extracts. Thus one or more of the antigenic determinants present on Chinese hamster HGPRT are either missing or present in an altered form on HGPRT from revertants of RJK10. This indicates that RJK10 carries a mutation in the structural gene for HGPRT and that secondary mutations in the gene give rise to the revertants that produce the antigenically altered enzymes.

Somatic mutation as the basis for malignant transformation of BHK cells by chemical carcinogens

The chemical induction of malignant transformation in BHK cells seems to result from a somatic mutation. Stable transformants, whose frequency is significantly increased by mutagenic carcinogens, can revert to normal and often display temperature-restricted phenotypes indicative of an altered gene product.

Isolation and characterization of temperature-sensitive mutants of Chinese hamster ovary cells after treatment with UV and x-irradiation

The isolation of ten conditionally lethal temperature-sensitive mutants of the Chinese hamster ovary cell (CHO-Kl, pro-) by the BUdR-visible light selection procedure described. Treatment with radiation at doses known to cause single gene mutation in mammalian cells increases the mutation frequency by a factor of at least 14. These mutants will grow with normal plating efficiency at 34.5 degrees but will not grow at 39.5 degrees. Complementation analysis by two independent methods indicates that all mutants are recessive and allows the assignment of the mutants to six genetically independent complementation groups. Reversion analysis indicates that the TS-mutants are stable, spontaneous revertants arising at a frequency of less than 10(-6). Preliminary chromosome analysis revealed no systematic chromasomal abnormality in the mutants. Mitotic accumulation is used to study the generation time of the parental cells and representative mutants at 34.5 degrees and 39.5 degrees. The uses of these mutants for genetic analysis of mammalian cells in culture is discussed.

Mutant chinese hamster cells with a thermosensitive hypoxanthine-guanine phosphoribosyltransferase

By selecting variants of Chinese hamster cells that were resistant to 6-thioguanine at 39 degrees C, but which would continue to grow in HAT medium at 33 degrees C, we have isolated cell lines with thermosensitive phenotypes. These clones form colonies in HAT medium and incorporate 14-C-hypoxanthine much more efficiently at 33 degrees C than at 39 degrees C. The specific activity of hypoxanthine-guanine phosphoribo-syltransferase is at least 10 times higher in variant cells grown at 33 degrees C than in those grown in 39 degrees C, and the enzymes from the variant clones are inactivated in vitro at 39 degrees C 7-9 times more rapidly than is the enzyme from wild-type cells. The results are consistent with the conclusion that the selected clones have missense mutations in the structural gene for the enzyme.