Purification and properties of human cytosolic folylpoly-gamma-glutamate synthetase and organization, localization, and differential splicing of its gene

Folylpolyglutamate synthetase inactivation in relapsed ALL induces a druggable folate metabolic vulnerability

AIMS

The antifolate methotrexate (MTX) is an anchor drug used in acute lymphoblastic leukemia (ALL) with poorly understood chemoresistance mechanisms in relapse. Herein we find decreased folate polyglutamylation network activities and inactivating FPGS mutations, both of which could induce MTX resistance and folate metabolic vulnerability in relapsed ALL.

METHODS

We utilized integrated systems biology analysis of transcriptomic and genomic data from relapse ALL cohorts to infer hidden ALL relapse drivers and related genetic alternations during clonal evolution. The drug sensitivity assay was used to determine the impact of relapse-specific FPGS mutations on sensitivity to different antifolates and chemotherapeutics in ALL cells. We used liquid chromatography-mass spectrometry (LC-MS) to quantify MTX and folate polyglutamate levels in folylpoly-γ-glutamate synthetase (FPGS) mutant ALL cells. Enzymatic activity and protein degradation assays were also conducted to characterize the catalytic properties and protein stabilities of FPGS mutants. An ALL cell line-derived mouse leukemia xenograft model was used to evaluate the in vivo impact of FPGS inactivation on leukemogenesis and sensitivity to the polyglutamatable antifolate MTX as well as non-polyglutamatble lipophilic antifolate trimetrexate (TMQ).

RESULTS

We found a significant decrease in folate polyglutamylation network activities during ALL relapse using RNA-seq data. Supported by functional evidence, we identified multifactorial mechanisms of FPGS inactivation in relapsed ALL, including its decreased network activity and gene expression, focal gene deletion, impaired catalytic activity, and increased protein degradation. These deleterious FPGS alterations induce MTX resistance and inevitably cause marked intracellular folate shrinkage, which could be efficiently targeted by a polyglutamylation-independent lipophilic antifolate TMQ in vitro and in vivo.

CONCLUSIONS

MTX resistance in relapsed ALL relies on FPGS inactivation, which inevitably induces a folate metabolic vulnerability, allowing for an efficacious antifolate ALL treatment strategy that is based upon TMQ, thereby surmounting chemoresistance in relapsed ALL.

Spina Bifida Prevention: A Narrative Review of Folic Acid Supplements for Childbearing Age Women

Neural tube defects (NTDs) are malformations that occur during embryonic development, and they account for most central nervous system birth anomalies. Genetic and environmental factors have been shown to play a role in the etiology of NTDs. The different types of NTDs are classified according to anatomic location and severity of the defect, with most of the neural axis anomalies occurring in the caudal spinal or cranial areas. Spina bifida is a type of NTD that is characterized by an opening in the vertebral arch, and the level of severity is determined by the extent to which the neural tissue protrudes through the opened arch(es). Prevention of NTDs by administration of folic acid has been studied and described in the literature, yet there are approximately 300,000 cases of NTDs that occur annually, with 88,000 deaths occurring per year worldwide. A daily intake of at least 400 μg of folic acid is recommended especially for women of childbearing age. To provide the benefits of folic acid, prenatal vitamins are recommended in pregnancy, and many countries have been fortifying foods such as cereal grain products with folic acid; however, not all countries have instituted folic acid fortification programs. The present investigation includes a description of the pharmacology of folic acid, neural tube formation, defects such as spina bifida, and the relevance of folic acid to developing spina bifida. Women's knowledge and awareness of folic acid regarding its importance in the prevention of spina bifida is a major factor in reducing incidence worldwide.

Increasing Dosage of Leucovorin Results in Pharmacokinetic and Gene Expression Differences When Administered as Two-Hour Infusion or Bolus Injection to Patients with Colon Cancer

The combination of 5-fluorouracil (5-FU) and leucovorin (LV) forms the chemotherapy backbone for patients with colorectal cancer. However, the LV administration is often standardized and not based on robust scientific data. To address these issues, a randomized pharmacokinetics study was performed in patients with colon cancer. Thirty patients were enrolled, receiving 60, 200 or 500 mg/m² LV as a single two-hour infusion. Blood, tumor, mucosa, and resection margin biopsies were collected. Folate concentrations were analyzed with LC-MS/MS and gene expression with qPCR. Data from a previous study where patients received LV as bolus injections were used as comparison. Saturation of methylenetetrahydrofolate (MeTHF) and tetrahydrofolate (THF) levels was seen after two-hour infusion and polyglutamated MeTHF + THF levels in tumors decreased with increasing LV dosage. The decrease was associated with decreased FPGS and increased GGH expression, which was not observed after LV bolus injection. In the bolus group, results indicate activation of a metabolic switch possibly promoting TYMS inhibition in response to 5-FU. Different metabolic mechanisms appear to be induced when LV is administered as infusion and bolus injection. Since maximal inhibition of TYMS by the 5-FU metabolite 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) requires excess polyglutamated MeTHF, the results point in favor of the bolus regimen.

Folylpoly-ɣ-glutamate synthetase association to the cytoskeleton: Implications to folate metabolon compartmentalization

Folates are essential for nucleotide biosynthesis, amino acid metabolism and cellular proliferation. Following carrier-mediated uptake, folates are polyglutamylated by folylpoly-ɣ-glutamate synthetase (FPGS), resulting in their intracellular retention. FPGS appears as a long isoform, directed to mitochondria via a leader sequence, and a short isoform reported as a soluble cytosolic protein (cFPGS). However, since folates are labile and folate metabolism is compartmentalized, we herein hypothesized that cFPGS is associated with the cytoskeleton, to couple folate uptake and polyglutamylation and channel folate polyglutamates to metabolon compartments. We show that cFPGS is a cytoskeleton-microtubule associated protein: Western blot analysis revealed that endogenous cFPGS is associated with the insoluble cellular fraction, i.e., cytoskeleton and membranes, but not with the cytosol. Mass spectrometry analysis identified the putative cFPGS interactome primarily consisting of microtubule subunits and cytoskeletal motor proteins. Consistently, immunofluorescence microscopy with cytosol-depleted cells demonstrated the association of cFPGS with the cytoskeleton and unconventional myosin-1c. Furthermore, since anti-microtubule, anti-actin cytoskeleton, and coatomer dissociation-inducing agents yielded perinuclear pausing of cFPGS, we propose an actin- and microtubule-dependent transport of cFPGS between the ER-Golgi and the plasma membrane. These novel findings support the coupling of folate transport with polyglutamylation and folate channeling to intracellular metabolon compartments. SIGNIFICANCE: FPGS, an essential enzyme catalyzing intracellular folate polyglutamylation and efficient retention, was described as a soluble cytosolic enzyme in the past 40 years. However, based on the lability of folates and the compartmentalization of folate metabolism and nucleotide biosynthesis, we herein hypothesized that cytoplasmic FPGS is associated with the cytoskeleton, to couple folate transport and polyglutamylation as well as channel folate polyglutamates to biosynthetic metabolon compartments. Indeed, using complementary techniques including Mass-spectrometry proteomics and fluorescence microscopy, we show that cytoplasmic FPGS is associated with the cytoskeleton and unconventional myosin-1c. This novel cytoskeletal localization of cytoplasmic FPGS supports the dynamic channeling of polyglutamylated folates to metabolon compartments to avoid oxidation and intracellular dilution of folates, while enhancing folate-dependent de novo biosynthesis of nucleotides and DNA/protein methylation.

Elimination of human folypolyglutamate synthetase alters programming and plasticity of somatic cells

Folates are vital cofactors for the regeneration of S-adenosyl methionine, which is the methyl source for DNA methylation, protein methylation, and other aspects of one-carbon (C1) metabolism. Thus, folates are critical for establishing and preserving epigenetic programming. Folypolyglutamate synthetase (FPGS) is known to play a crucial role in the maintenance of intracellular folate levels. Therefore, any modulation in FPGS is expected to alter DNA methylation and numerous other metabolic pathways. To explore the role of polyglutamylation of folate, we eliminated both isoforms of FPGS in human cells (293T), producing FPGS knockout (FPGS^ko) cells. The elimination of FPGS significantly decreased cell proliferation, with a major effect on oxidative phosphorylation and a lesser effect on glycolysis. We found a substantial reduction in global DNA methylation and noteworthy changes in gene expression related to C1 metabolism, cell division, DNA methylation, pluripotency, Glu metabolism, neurogenesis, and cardiogenesis. The expression levels of NANOG, octamer-binding transcription factor 4, and sex-determining region Y-box 2 levels were increased in the mutant, consistent with the transition to a stem cell-like state. Gene expression and metabolite data also indicate a major change in Glu and GABA metabolism. In the appropriate medium, FPGS^ko cells can differentiate to produce mainly cells with characteristics of either neural stem cells or cardiomyocytes.-Srivastava, A. C., Thompson, Y. G., Singhal, J., Stellern, J., Srivastava, A., Du, J., O'Connor, T. R., Riggs, A. D. Elimination of human folypolyglutamate synthetase alters programming and plasticity of somatic cells.

Folate pathway genes linked to mitochondrial biogenesis and respiration are associated with outcome of patients with stage III colorectal cancer

5-fluorouracil in combination with the folate leucovorin is the cornerstone in treatment of colorectal cancer. Transport of leucovorin into cells, and subsequent metabolic action, require expression of several genes. The aim was to analyze if tumoral expression of genes putatively involved in leucovorin transport, polyglutamation, or metabolism was associated with outcome of patients with stage III colorectal cancer treated with adjuvant chemotherapy. A total of 363 stage III colorectal cancer patients who received adjuvant bolus 5-fluorouracil + leucovorin alone, or in combination with oxaliplatin according to Nordic bolus regimes were included. Expression of 11 folate pathway genes was determined in tumors using quantitative real-time polymerase chain reaction and related to disease-free survival. The median follow-up time was 5 years. During follow-up, 114 (31%) patients suffered from recurrent disease. A high tumoral expression of the genes SLC46A1/PCFT, SLC19A1/RFC-1, ABCC3/MRP3, GGH, and MTHFD1L, which are involved in folate transport, polyglutamation, or metabolism, was associated with longer disease-free survival of the patients. Each of these genes either encodes mitochondrial enzymes or is being regulated by mitochondrial transcription factors. Expression of the SLC46A1/PCFT gene was most strongly associated with disease-free survival, regardless of treatment regimen. In conclusion, the results show that expression of folate pathway genes are associated with outcome of colorectal cancer patients treated with adjuvant 5-fluorouracil in combination with leucovorin. A prospective study needs to be conducted to determine if expression of these genes can be used to predict response to leucovorin and other folates that are now being tested in clinical studies. Moreover, there seems to be a link between folate metabolism and mitochondrial biogenesis and respiration that deserves further exploration.

Characterization and Interrelations of One-Carbon Metabolites in Tissues, Erythrocytes, and Plasma in Mice with Dietary Induced Folate Deficiency

Studies on one-carbon metabolism for the assessment of folate deficiency have focused on either metabolites of folate metabolism or methionine cycle. To bridge the gap between deficiency markers in these pathways we designed a dietary induced folate deficiency study using male C57BL/6N mice. After weaning (3 weeks) mice were fed a defined control diet (1 week) before being fed a folate deficient diet (n = 6 mice) and the control diet (n = 6 mice) for 12 additional weeks. Thereafter, we determined total homocysteine in plasma and folate in erythrocytes as well as S-adenosylmethionine, S-adenosylhomocysteine, and six folate vitamers in tissues including 5-methyltetrahydrofolate, 5-formyltetrahydrofolate, 5,10-methenyltetrahydrofolate, tetrahydrofolate, 10-formylfolic acid, and folic acid by means of stable isotope dilution assays coupled with liquid chromatography tandem mass spectrometry. In all organs, except heart (mainly 5-mehtyltetrahydrofolate), tetrahydrofolate constitutes the main vitamer. Moreover, in liver tetrahydrofolate was most abundant followed by 5-methyltetrahydrofolate (heart: tetrahydrofolate), 5-formyltetrahydrofolate, and 5,10-methenyltetrahydrofolate. Because of the significant decrease (p < 0.05) of folate status and S-adenosylmethionine/S-adenosylhomocysteine ratio accompanied with increasing S-adenosylhomocysteine (p < 0.05), hepatocytes are most susceptible to folate deficiency. To the best of our knowledge, we herein present the first method for simultaneous quantitation of eight metabolites for both folate and methionine cycle in one tissue sample, tHcy in plasma, and erythrocyte folate to shed light on physiological interrelations of one-carbon metabolism.

Maternal folic acid consumption during gestation and its long-term effects on offspring's liver: a systematic review

Abstract Objectives: describing the effects of maternal supplementation with folic acid (FA) exclusively during gestation on offspring's liver at later stages in life. Supplementation with FA during gestation has been recommended by the medical society worldwide. The liver has a central role on the substances of metabolism and homeostasis and some studies have shown that a high intake of FA at other periods in life may cause hepatic damage. Methods: a systematic review through which the following databases were consulted: Medline, through platforms of Pubmed, Lilacs and Scielo. The research was performed by keywords such as: "Folic acid", "Gestation", "Rat", "Offspring" and "Liver". Articles which evaluate the effect of FA consumption during both gestation and lactation were excluded. Results: FA consumption avoids disorders on expression of peroxisome proliferator-activated receptor alpha (PPARα) and glucocorticoid receptor (GccR), its lack did not change enzyme activity of the male offspring's liver in adulthood. Supplementation with FA during gestation did not change iron hepatic levels or lipid composition, but had an antioxidant effect on it. Conclusions: supplementation with FA at recommended doses did not cause toxic effects and is very likely to avoid deleterious effects in the liver of the offspring regarding the epigenetic level.

Folylpoly-γ-glutamate synthetase: A key determinant of folate homeostasis and antifolate resistance in cancer

Mammalians are devoid of autonomous biosynthesis of folates and hence must obtain them from the diet. Reduced folate cofactors are B9-vitamins which play a key role as donors of one-carbon units in the biosynthesis of purine nucleotides, thymidylate and amino acids as well as in a multitude of methylation reactions including DNA, RNA, histone and non-histone proteins, phospholipids, as well as intermediate metabolites. The products of these S-adenosylmethionine (SAM)-dependent methylations are involved in the regulation of key biological processes including transcription, translation and intracellular signaling. Folate-dependent one-carbon metabolism occurs in several subcellular compartments including the cytoplasm, mitochondria, and nucleus. Since folates are essential for DNA replication, intracellular folate cofactors play a central role in cancer biology and inflammatory autoimmune disorders. In this respect, various folate-dependent enzymes catalyzing nucleotide biosynthesis have been targeted by specific folate antagonists known as antifolates. Currently, antifolates are used in drug treatment of multiple human cancers, non-malignant chronic inflammatory disorders as well as bacterial and parasitic infections. An obligatory key component of intracellular folate retention and intracellular homeostasis is (anti)folate polyglutamylation, mediated by the unique enzyme folylpoly-γ-glutamate synthetase (FPGS), which resides in both the cytoplasm and mitochondria. Consistently, knockout of the FPGS gene in mice results in embryonic lethality. FPGS catalyzes the addition of a long polyglutamate chain to folates and antifolates, hence rendering them polyanions which are efficiently retained in the cell and are now bound with enhanced affinity by various folate-dependent enzymes. The current review highlights the crucial role that FPGS plays in maintenance of folate homeostasis under physiological conditions and delineates the plethora of the molecular mechanisms underlying loss of FPGS function and consequent antifolate resistance in cancer.

Mutation Screening and Association Study of the Folylpolyglutamate Synthetase (FPGS) Gene with Susceptibility to Childhood Acute Lymphoblastic Leukemia

BACKGROUND

Folylpolyglutamate synthetase (FPGS), an important enzyme in the folate metabolic pathway, plays a central role in intracellular accumulation of folate and antifolate in several mammalian cell types. Loss of FPGS activity results in decreased cellular levels of antifolates and consequently to polyglutamatable antifolates in acute lymphoblastic leukemia (ALL).

MATERIALS AND METHODS

During May 1997 and December 2003, 134 children diagnosed with ALL were recruited from one hospital in Thailand. We performed a mutation analysis in the coding regions of the FPGS gene and the association between single nucleotide polymorphisms (SNPs) within FPGS in a case-control sample of childhood ALL patients. Mutation screening was conducted by polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) and subsequently with direct sequencing (n=72). Association analysis between common FPGS variants and ALL risk was done in 98 childhood ALL cases and 95 healthy volunteers recruited as controls.

RESULTS

Seven SNPs in the FPGS coding region were identified by mutation analysis, 3 of which (IVS13+55C>T, g.1297T>G, and g.1508C>T) were recognized as novel SNPs. Association analysis revealed 3 of 6 SNPs to confer significant increase in ALL risk these being rs7039798 (p= 0.014, OR=2.14), rs1544105 (p=0.010, OR= 2.24), and rs10106 (p=0.026, OR= 1.99).

CONCLUSIONS

These findings suggested that common genetic polymorphisms in the FPGS coding region including rs7039789, rs1544105, and rs10106 are significantly associated with increased ALL risk in Thai children.

Expression of Folate Pathway Genes in Stage III Colorectal Cancer Correlates with Recurrence Status Following Adjuvant Bolus 5-FU-Based Chemotherapy

Colorectal cancer is commonly treated with 5-fluorouracil and 5-formyltetrahydrofolate (leucovorin). Metabolic action of leucovorin requires several enzymatic steps that are dependent on expression of corresponding coding genes. To identify folate pathway genes with possible impact on leucovorin metabolism, a retrospective study was performed on 193 patients with stage III colorectal cancer. Relative expression of 22 genes putatively involved in leucovorin transport, polyglutamation and metabolism was determined in tumor and mucosa samples using quantitative real-time polymerase chain reaction. After surgery, patients received adjuvant 5-fluorouracil-based bolus chemotherapy with leucovorin during six months, and were followed for 3 to 5 years. Cox regression analysis showed that high tumoral expression of the genes SLC46A1/PCFT (proton-coupled folate transporter) and SLC19A1/RFC-1 (reduced folate carrier 1) correlated significantly (p < 0.001 and p < 0.01, respectively) with a decreased risk of recurrent disease, measured as disease-free survival (DFS). These two genes are involved in the transport of folates into the cells and each functions optimally at a different pH. We conclude that SLC46A1/PCFT and SLC19A1/RFC-1 are associated with DFS of patients with colorectal cancer and hypothesize that poor response to 5-fluorouracil plus leucovorin therapy in some patients may be linked to low expression of these genes. Such patients might need a more intensified therapeutic approach than those with high gene expression. Future prospective studies will determine if the expression of any of these genes can be used to predict response to leucovorin.

γ-Glutamyl hydrolase modulation significantly influences global and gene-specific DNA methylation and gene expression in human colon and breast cancer cells

γ-Glutamyl hydrolase (GGH) plays an important role in folate homeostasis by catalyzing hydrolysis of polyglutamylated folate into monoglutamates. Polyglutamylated folates are better substrates for several enzymes involved in the generation of S-adenosylmethionine, the primary methyl group donor, and hence, GGH modulation may affect DNA methylation. DNA methylation is an important epigenetic determinant in gene expression, in the maintenance of DNA integrity and stability, and in chromatin modifications, and aberrant or dysregulation of DNA methylation has been mechanistically linked to the development of human diseases including cancer. Using a recently developed in vitro model of GGH modulation in HCT116 colon and MDA-MB-435 breast cancer cells, we investigated whether GGH modulation would affect global and gene-specific DNA methylation and whether these alterations were associated with significant gene expression changes. In both cell lines, GGH overexpression decreased global DNA methylation and DNA methyltransferase (DNMT) activity, while GGH inhibition increased global DNA methylation and DNMT activity. Epigenomic and gene expression analyses revealed that GGH modulation influenced CpG promoter DNA methylation and gene expression involved in important biological pathways including cell cycle, cellular development, and cellular growth and proliferation. Some of the observed altered gene expression appeared to be regulated by changes in CpG promoter DNA methylation. Our data suggest that the GGH modulation-induced changes in total intracellular folate concentrations and content of long-chain folylpolyglutamates are associated with functionally significant DNA methylation alterations in several important biological pathways.

Humanizing mouse folate metabolism: conversion of the dual-promoter mouse folylpolyglutamate synthetase gene to the human single-promoter structure

The mouse is extensively used to model human folate metabolism and therapeutic outcomes with antifolates. However, the folylpoly-γ-glutamate synthetase (fpgs) gene, whose product determines folate/antifolate intracellular retention and antifolate antitumor activity, displays a pronounced species difference. The human gene uses only a single promoter, whereas the mouse uses two: P2, akin to the human promoter, at low levels in most tissues; and P1, an upstream promoter used extensively in liver and kidney. We deleted the mouse P1 promoter through homologous recombination to study the dual-promoter mouse system and to create a mouse with a humanized fpgs gene structure. Despite the loss of the predominant fpgs mRNA species in liver and kidney (representing 95 and 75% of fpgs transcripts in these tissues, respectively), P1-knockout mice developed and reproduced normally. The survival of these mice was explained by increased P2 transcription due to relief of transcriptional interference, by a 3-fold more efficient translation of P2-derived than P1-derived transcripts, and by 2-fold higher stability of P2-derived FPGS. In combination, all 3 effects reinstated FPGS function, even in liver. By eliminating mouse P1, we created a mouse model that mimicked the human housekeeping pattern of fpgs gene expression.

The molecular chaperone Hsp70 activates protein phosphatase 5 (PP5) by binding the tetratricopeptide repeat (TPR) domain

Protein phosphatase 5 (PP5) is auto-inhibited by intramolecular interactions with its tetratricopeptide repeat (TPR) domain. Hsp90 has been shown to bind PP5 to activate its phosphatase activity. However, the functional implications of binding Hsp70 to PP5 are not yet clear. In this study, we find that both Hsp90 and Hsp70 bind to PP5 using a luciferase fragment complementation assay. A fluorescence polarization assay shows that Hsp90 (MEEVD motif) binds to the TPR domain of PP5 almost 3-fold higher affinity than Hsp70 (IEEVD motif). However, Hsp70 binding to PP5 stimulates higher phosphatase activity of PP5 than the binding of Hsp90. We find that PP5 forms a stable 1:1 complex with Hsp70, but the interaction appears asymmetric with Hsp90, with one PP5 binding the dimer. Solution NMR studies reveal that Hsc70 and PP5 proteins are dynamically independent in complex, tethered by a disordered region that connects the Hsc70 core and the IEEVD-TPR contact area. This tethered binding is expected to allow PP5 to carry out multi-site dephosphorylation of Hsp70-bound clients with a range of sizes and shapes. Together, these results demonstrate that Hsp70 recruits PP5 and activates its phosphatase activity which suggests dual roles for PP5 that might link chaperone systems with signaling pathways in cancer and development.

Folate and B12 in prostate cancer

Mechanisms postulated to link folate and B12 metabolism with cancer, including genome-wide hypomethylation, gene-specific promoter hypermethylation, and DNA uracil misincorporation, have been observed in prostate tumor cells. However, epidemiological studies of prostate cancer risk, based on dietary intakes and blood levels of folate and vitamin B12 and on folate-pathway gene variants, have generated contradictory findings. In a meta-analysis, circulating concentrations of B12 (seven studies, OR = 1.10; 95% CI 1.01, 1.19; P = 0.002) and (in cohort studies) folate (five studies, OR = 1.18; 95% CI 1.00, 1.40; P = 0.02) were positively associated with an increased risk of prostate cancer. Homocysteine was not associated with risk of prostate cancer (four studies, OR = 0.91; 95% CI 0.69, 1.19; P = 0.5). In a meta-analysis of folate-pathway polymorphisms, MTR 2756A > G (eight studies, OR = 1.06; 95% CI 1.00, 1.12; P = 0.06) and SHMT1 1420C > T (two studies, OR = 1.11; 95% CI 1.00, 1.22; P = 0.05) were positively associated with prostate cancer risk. There were no effects due to any other polymorphisms, including MTHFR 677C > T (12 studies, OR = 1.04; 95% CI 0.97, 1.12; P = 0.3). The positive association of circulating B12 with an increased risk of prostate cancer could be explained by reverse causality. However, given current controversies over mandatory B12 fortification, further research to eliminate a causal role of B12 in prostate cancer initiation and/or progression is required. Meta-analysis does not entirely rule out a positive association of circulating folate with increased prostate cancer risk. As with B12, even a weak positive association would be a significant public health issue, given the high prevalence of prostate cancer and concerns about the potential harms versus benefits of mandatory folic acid fortification.

Comprehensive quantitative measurement of folate polyglutamates in human erythrocytes by ion pairing ultra-performance liquid chromatography/tandem mass spectrometry

RATIONALE

The erythrocyte folate pool is reflective of an individual's long-term folate status; however, comprehensive quantitative determination of the various folate isoforms including polyglutamation (Glu(n)) status has posed an analytical problem. Factors complicating such analysis are the absence of authentic (isotope-labeled) standards and the large number of potential analytes. The present work presents high-throughput analytical methodology for the indirect comprehensive quantitation of the erythrocyte folate pool with commercially available standards.

METHODS

The erythrocyte folate pool was determined comprehensively by utilizing a cascade of three complementary ultra-performance liquid chromatography (UPLC) tandem mass spectrometry (MS/MS) assays. In a first assay utilizing ion-pairing UPLC/MS/MS the relative (%) polyglutamation distribution (Glu(3-10)) of 5-methyltetrahydrofolate, tetrahydrofolate and 5-formyltetrahydrofolate is determined in a thermal extract obtained from packed erythrocytes, not requiring analytical standards. Quantitation of the erythrocyte folate pool was accomplished by performing two additional stable isotope dilution UPLC/MS/MS assays to determine whole blood and plasma folate content, utilizing commercially available [(13)C(5)]-labeled analogs of the Glu(1) analytes. Based on the values provided by each individual assay the comprehensive erythrocyte folate content could be calculated.

RESULTS

The various assays have been validated for intra- and inter-run precision, accuracy, linearity and are robust. The method was sensitive enough to measure the comprehensive erythrocyte folate distribution in a Down's syndrome patient with extremely low folate, bearing the C677T mutation in the gene encoding for methylenetetrahydrofolate reductase.

CONCLUSIONS

The erythrocyte folate pool can be comprehensively quantitated by running three complementary UPLC/MS/MS assays. The present assays are robust and allow for high-throughput analysis. The method can be utilized to support larger investigations that investigate the relationship between folate isoform and polyglutamation distribution and disease pathogenesis.

Compartmentalization of Mammalian folate-mediated one-carbon metabolism

The recognition that mitochondria participate in folate-mediated one-carbon metabolism grew out of pioneering work beginning in the 1950s from the laboratories of D.M. Greenberg, C.G. Mackenzie, and G. Kikuchi. These studies revealed mitochondria as the site of oxidation of one-carbon donors such as serine, glycine, sarcosine, and dimethylglycine. Subsequent work from these laboratories and others demonstrated the participation of folate coenzymes and folate-dependent enzymes in these mitochondrial processes. Biochemical and molecular genetic approaches in the 1980s and 1990s identified many of the enzymes involved and revealed an interdependence of cytoplasmic and mitochondrial one-carbon metabolism. These studies led to the development of a model of eukaryotic one-carbon metabolism that comprises parallel cytosolic and mitochondrial pathways, connected by one-carbon donors such as serine, glycine, and formate. Sequencing of the human and other mammalian genomes has facilitated identification of the enzymes that participate in this intercompartmental one-carbon metabolism, and animal models are beginning to clarify the roles of the cytoplasmic and mitochondrial isozymes of these enzymes. Identifying the mitochondrial transporters for the one-carbon donors and elucidating how flux through these pathways is controlled are two areas ripe for exploration.

Functional analysis of folate polyglutamylation and its essential role in plant metabolism and development

Cellular folates function as co-enzymes in one-carbon metabolism and are predominantly decorated with a polyglutamate tail that enhances co-enzyme affinity, subcellular compartmentation and stability. Polyglutamylation is catalysed by folylpolyglutamate synthetases (FPGSs) that are specified by three genes in Arabidopsis, FPGS1, 2 and 3, which reportedly encode plastidic, mitochondrial and cytosolic isoforms, respectively. A mutational approach was used to probe the functional importance of folate polyglutamylation in one-carbon metabolism and development. Biochemical analysis of single FPGS loss-of-function mutants established that folate polyglutamylation is essential for organellar and whole-plant folate homeostasis. However, polyglutamylated folates were still detectable, albeit at lower levels, in organelles isolated from the corresponding isozyme knockout lines, e.g. in plastids and mitochondria of the fpgs1 (plastidial) and fpgs2 (mitochondrial) mutants. This result is surprising given the purported single-compartment targeting of each FPGS isozyme. These results indicate redundancy in compartmentalised FPGS activity, which in turn explains the lack of anticipated phenotypic defects for the single FPGS mutants. In agreement with this hypothesis, fpgs1 fpgs2 double mutants were embryo-lethal, fpgs2 fpgs3 mutants exhibited seedling lethality, and fpgs1 fpgs3 mutants were dwarfed with reduced fertility. These phenotypic, metabolic and genetic observations are consistent with targeting of one or more FPGS isozymes to multiple organelles. These data confirm the importance of polyglutamylation in folate compartmentation, folate homeostasis and folate-dependent metabolic processes, including photorespiration, methionine and pantothenate biosynthesis.

A central role for gamma-glutamyl hydrolases in plant folate homeostasis

Most cellular folates carry a short poly-γ-glutamate tail, and this tail is believed to affect their efficacy and stability. The tail can be removed by γ-glutamyl hydrolase (GGH; EC 3.4.19.9), a vacuolar enzyme whose role in folate homeostasis remains unclear. In order to probe the function of GGH, we modulated its level of expression and subcellular location in Arabidopsis plants and tomato fruit. Three-fold overexpression of GGH in vacuoles caused extensive deglutamylation of folate polyglutamates and lowered the total folate content by approximately 40% in Arabidopsis and tomato. No such effects were seen when GGH was overexpressed to a similar extent in the cytosol. Ablation of either of the major Arabidopsis GGH genes (AtGGH1 and AtGGH2) alone did not significantly affect folate status. However, a combination of ablation of one gene plus RNA interference (RNAi)-mediated suppression of the other (which lowered total GGH activity by 99%) increased total folate content by 34%. The excess folate accumulated as polyglutamate derivatives in the vacuole. Taken together, these results suggest a model in which: (i) folates continuously enter the vacuole as polyglutamates, accumulate there, are hydrolyzed by GGH, and exit as monoglutamates; and (ii) GGH consequently has an important influence on polyglutamyl tail length and hence on folate stability and cellular folate content.

Characterisation of the bifunctional dihydrofolate synthase-folylpolyglutamate synthase from Plasmodium falciparum; a potential novel target for antimalarial antifolate inhibition

Unusually for a eukaryote, the malaria parasite Plasmodium falciparum expresses dihydrofolate synthase (DHFS) and folylpolyglutamate synthase (FPGS) as a single bifunctional protein. The two activities contribute to the essential pathway of folate biosynthesis and modification. The DHFS activity of recombinant PfDHFS–FPGS exhibited non-standard kinetics at high co-substrate (glutamate and ATP) concentrations, being partially inhibited by increasing concentrations of its principal substrate, dihydropteroate (DHP). Binding of DHP to the catalytic and inhibitory sites exhibited dissociation constants of 0.50 μM and 1.25 μM, respectively. DHFS activity measured under lower co-substrate concentrations, where data fitted the Michaelis–Menten equation, yielded apparent K_m values of 0.88 μM for DHP, 22.8 μM for ATP and 5.97 μM for glutamate. Of the substrates tested in FPGS assays, only tetrahydrofolate (THF) was efficiently converted to polyglutamylated forms, exhibiting standard kinetics with an apparent K_m of 0.96 μM; dihydrofolate, folate and the folate analogue methotrexate (MTX) were negligibly processed, emphasising the importance of the oxidation state of the pterin moiety. Moreover, MTX inhibited neither DHFS nor FPGS, even at high concentrations. Conversely, two phosphinate analogues of 7,8-dihydrofolate that mimic tetrahedral intermediates formed during DHFS- and FPGS-catalysed glutamylation were powerfully inhibitory. The K_i value of an aryl phosphinate analogue against DHFS was 0.14 μM and for an alkyl phosphinate against FPGS 0.091 μM, with each inhibitor showing a high degree of specificity. This, combined with the absence of DHFS activity in humans, suggests PfDHFS–FPGS might represent a potential new drug target in the previously validated folate pathway of P. falciparum.

Inhibition of human folylpolyglutamate synthetase by diastereomeric phosphinic acid mimics of the tetrahedral intermediate

Phosphorus-containing pseudopeptides, racemic at the C-terminal alpha-carbon, are potent mechanism-based inhibitors of folylpolyglutamate synthetase (FPGS). They are mimics of the tetrahedral intermediate postulated to form during FPGS-catalyzed biosynthesis of poly(gamma-l-glutamates). In the present paper, the FPGS inhibitory activity of each diastereomer coupled to three heterocycles is reported. The high R(f) pseudopeptide containing the 5,10-dideazatetrahydropteroyl (DDAH(4)Pte) heterocycle is most potent (K(is) = 1.7 nM). While the heterocyclic portion affects absolute FPGS inhibitory potency, the high R(f) species is more potent in each pair containing the same heterocycle. This species presumably has the same stereochemistry as the natural folate polyglutamate, i.e., (l-Glu-gamma-l-Glu). Unexpectedly, the low R(f) (presumed l-Glu-gamma-d-Glu) species are only slightly less potent (<30-fold) than their diastereomers. Further study of this phenomenon comparing l-Glu-gamma-l-Glu and l-Glu-gamma-d-Glu dipeptide-containing FPGS substrates shows that <1% contamination of commercial d-Glu precursors by l-Glu may give misleading information if l-Glu-gamma-l-Glu substrates have low K(m) values.

Concentration-dependent processivity of multiple glutamate ligations catalyzed by folylpoly-gamma-glutamate synthetase

Folylpoly-gamma-glutamate synthetase (FPGS, EC 6.3.2.17) is an ATP-dependent ligase that catalyzes formation of poly-gamma-glutamate derivatives of reduced folates and antifolates such as methotrexate and 5,10-dideaza-5,6,7,8-tetrahydrofolate (DDAH 4PteGlu 1). While the chemical mechanism of the reaction catalyzed by FPGS is known, it is unknown whether single or multiple glutamate residues are added following each folate binding event. A very sensitive high-performance liquid chromatography method has been used to analyze the multiple ligation reactions onto radiolabeled DDAH 4PteGlu 1 catalyzed by FPGS to distinguish between distributive or processive mechanisms of catalysis. Reaction time courses, substrate trapping, and pulse-chase experiments were used to assess folate release during multiple glutamate additions. Together, the results of these experiments indicate that hFPGS can catalyze the processive addition of approximately four glutamate residues to DDAH 4PteGlu 1. The degree of processivity was determined to be dependent on the concentration of the folate substrate, thus suggesting a mechanism for the regulation of folate polyglutamate synthesis in cells.

Folate and Vitamin B12 Metabolism: Overview and Interaction with Riboflavin, Vitamin B6, and Polymorphisms

This paper provides a general review on folate and vitamin B12 nutrition and metabolism and the metabolic interrelationship between these vitamins. The effects of some common polymorphisms in folate and vitamin B12 genes and the influence of vitamin B6 and riboflavin status on folate and vitamin B12 metabolism are also discussed.

Mechanisms of antifolate resistance and methotrexate efficacy in leukemia cells

Antifolates are the first class of antimetabolites introduced to clinic about 6 decades ago. Now, after several years of administration of antifolates against malignancies and particularly leukemia, we are still trying to achieve a full understanding of the mechanisms of action and resistance to these agents. The present article covers different factors able to influence efficacy of antifolates on leukemic cells, the known mechanisms of resistance to methotrexate (MTX) and strategies to overcome these mechanisms. The dominant factors that are contributed to tolerance to cytocidal effects of MTX including pharmacokinetic factors, impaired transmembrane uptake as the most frequent rote of provoking resistance to MTX, augmented drug efflux, impaired intracellular polyglutamation as a determining process of drug efficacy, alterations in expression or activity of target enzymes and increased intracellular folate pools; and finally role of 7-hydroxymethotrexate on response or resistance to MTX will be discussed in more detail. Finally, strategies to overcome resistance to antifolates are discussed.

The role of impaired transport in (pre)clinical resistance to methotrexate: insights on new antifolates

Impaired transport appears to be a common mechanism of resistance of neoplastic cells to the antifolate methotrexate. The extensive knowledge of the molecular, biochemical and functional properties of the membrane transport systems for folates, in particular the reduced folate carrier (RFC) and membrane folate receptors (MFR), has made an important contribution to the rational design of novel antifolates that are either more efficiently internalized via these transporters or can simply bypass these transport routes. The current status of exploiting the RFC and MFR for transport of novel antifolates active in preclinical model systems and a clinical setting will be reviewed, with a special reference to childhood acute lymphoblastic leukemia (ALL) and acute non-lymphoblastic leukemia (ANLL).

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.

The role of multidrug resistance efflux transporters in antifolate resistance and folate homeostasis

Members of the ATP-binding cassette (ABC) transporters including P-glycoprotein (Pgp/ABCB1), multidrug resistance proteins (MRPs/ABCC) as well as breast cancer resistance protein (BCRP/ABCG2) function as ATP-dependent drug efflux transporters, which form a unique defense network against multiple chemotherapeutic drugs and cellular toxins. Among antitumor agents is the important group of folic acid antimetabolites known as antifolates. Antifolates such as methotrexate (MTX), pemetrexed and raltitrexed exert their cytotoxic activity via potent inhibition of folate-dependent enzymes essential for purine and pyrimidine nucleotide biosynthesis and thereby block DNA replication. Overexpression of MRPs and BCRP confers resistance upon malignant cells to various hydrophilic and lipophilic antifolates. Apart from their central role in mediating resistance to antifolates and other anticancer drugs, MRPs and BCRP have been recently shown to transport naturally occurring reduced folates. This was inferred from various complementary systems as follows: (a) Cell-free systems including ATP-dependent uptake of radiolabeled folate/MTX into purified inside-out membrane vesicles from stable transfectants and/or cells overexpressing these transporters, (b) Decreased accumulation of radiolabeled folate/MTX in cultured tumor cells overexpressing these transporters, as well as (c) In vivo rodent models such as Eisi hyperbillirubinemic rats (EHBR) that hereditarily lack MRP2 in their canalicular membrane and thereby display a bile that is highly deficient in various reduced folate cofactors and MTX, when compared with wild type Sprague-Dawley (SD) rats. In all cases, these folate/antifolate transporters functioned as high capacity, low affinity ATP-driven exporters. While the mechanism of cellular retention of (anti)folates is mediated via (anti)folylpolyglutamylation, certain efflux transporters including MRP5 (ABCC5) and BCRP were shown to transport both mono-, di- as well as triglutamate derivatives of MTX and folic acid. Furthermore, overexpression of MRPs and BCRP has been shown to result in decreased cellular folate pools, whereas loss of ABC transporter expression brought about a significant expansion in the intracellular reduced folate pool. The latter finding has important implications to antifolate-based chemotherapy as an augmented cellular folate pool results in a significant level of resistance to certain antifolates. Hence, the aims of the present review are: (a) To summarize and discuss the cumulative evidence supporting a functional role for various multidrug resistance efflux transporters of the ABC superfamily which mediate resistance to hydrophilic and lipophilic antifolates, (b) To describe and evaluate the recent data suggesting a role for these efflux transporters in regulation of cellular folate homeostasis under folate replete and deplete conditions. Furthermore, novel developments and future perspectives regarding the identification of novel antifolate target proteins and mechanisms of action, as well as rationally designed emerging drug combinations containing antifolates along with receptor tyrosine kinase inhibitors are being discussed.

Methenyltetrahydrofolate synthetase is a high-affinity catecholamine-binding protein

Recombinant mouse 5,10-methenyltetrahydrofolate synthetase (MTHFS) was expressed in Escherichia coli and shown to co-purify with a chromophore that had a lambda(max) at 320nm. The chromophore remained bound to MTHFS during extensive dialysis, but dissociated from MTHFS when its substrate, 5-formyltetrahydrofolate, was bound. The chromophore was identified as an oxidized catecholamine by mass spectrometry and absorption spectroscopy. Purified recombinant mouse MTHFS and rabbit liver MTHFS proteins were shown to bind oxidized N-acetyldopamine (NADA) tightly. The addition of NADA to cell culture medium accelerated markedly folate turnover and decreased both folate accumulation and total cellular folate concentrations in MCF-7 cells. Expression of the MTHFS cDNA in MCF-7 cells increased the concentration of NADA required to deplete cellular folate. The results of this study are the first to identify a link between catecholamines and one-carbon metabolism and demonstrate that NADA accelerates folate turnover and impairs cellular folate accumulation in MCF-7 cells.

Analysis of folylpoly-gamma-glutamate synthetase gene expression in human B-precursor ALL and T-lineage ALL cells

BACKGROUND

Expression of folylpoly-gamma-glutamate synthetase (FPGS) gene is two- to three-fold higher in B-precursor ALL (Bp- ALL) than in T-lineage ALL (T-ALL) and correlates with intracellular accumulation of methotrexate (MTX) polyglutamates and lymphoblast sensitivity to MTX. In this report, we investigated the molecular regulatory mechanisms directing FPGS gene expression in Bp-ALL and T-ALL cells.

METHODS

To determine FPGS transcription rate in Bp-ALL and T-ALL we used nuclear run-on assays. 5'-RACE was used to uncover potential regulatory regions involved in the lineage differences. We developed a luciferase reporter gene assay to investigate FPGS promoter/enhancer activity. To further characterize the FPGS proximal promoter, we determined the role of the putative transcription binding sites NFY and E-box on FPGS expression using luciferase reporter gene assays with substitution mutants and EMSA.

RESULTS

FPGS transcription initiation rate was 1.6-fold higher in NALM6 vs. CCRF-CEM cells indicating that differences in transcription rate led to the observed lineage differences in FPGS expression between Bp-ALL and T-ALL blasts. Two major transcripts encoding the mitochondrial/cytosolic and cytosolic isoforms were detected in Bp-ALL (NALM6 and REH) whereas in T-ALL (CCRF-CEM) cells only the mitochondrial/cytosolic transcript was detected. In all DNA fragments examined for promoter/enhancer activity, we measured significantly lower luciferase activity in NALM6 vs. CCRF-CEM cells, suggesting the need for additional yet unidentified regulatory elements in Bp-ALL. Finally, we determined that the putative transcription factor binding site NFY, but not E-box, plays a role in FPGS transcription in both Bp- and T-lineage.

CONCLUSION

We demonstrated that the minimal FPGS promoter region previously described in CCRF-CEM is not sufficient to effectively drive FPGS transcription in NALM6 cells, suggesting that different regulatory elements are required for FPGS gene expression in Bp-cells. Our data indicate that the control of FPGS expression in human hematopoietic cells is complex and involves lineage-specific differences in regulatory elements, transcription initiation rates, and mRNA processing. Understanding the lineage-specific mechanisms of FPGS expression should lead to improved therapeutic strategies aimed at overcoming MTX resistance or inducing apoptosis in leukemic cells.

Synthesis of (6R)- and (6S)-5,10-dideazatetrahydrofolate oligo-gamma-glutamates: kinetics of multiple glutamate ligations catalyzed by folylpoly-gamma-glutamate synthetase

Folylpoly-gamma-glutamate synthetase (FPGS, EC 6.3.2.17) catalyzes the ATP-dependent ligation of glutamic acid to reduced folates including (6S)-5,6,7,8-tetrahydrofolate (H4PteGlu), as well as to anticancer drugs such as 5,10-dideaza-5,6,7,8-tetrahydrofolate ((6R)-DDAH4PteGlu1, (6R)-DDATHF, Lometrexol). Synthesis of unlabeled mono- and polyglutamates, DDAH4PteGlu(n) (6R, n = 1-6; 6S, n = 1-2), as well as (6R)-DDAH4Pte[14C]Glu1, was effected from (6R)- or (6S)-5,10-dideazatetrahydropteroyl azide and glutamic acid, H-Glu-gamma-Glu(n)-gamma-Glu-OH (n = 0-4), or [14C]glutamic acid, respectively. These compounds were evaluated as FPGS substrates to determine steady-state kinetic constants. Michaelis-Menten kinetics were observed for (6R)-DDAH4PteGlu1, the isomer corresponding to H(4)PteGlu, whereas marked substrate inhibition was observed for (6S)-DDAH4PteGlu(n) (n = 1-2) and (6R)-DDAH4PteGlu(n) (n = 2-5), but not (6R)-DDAH4PteGlu6. Multiple ligation of glutamate renders a quantitative analysis of these data difficult. However, approximate values of K(M) = 0.65-1.6 microM and K(I) = 144-417 microM for DDAH4PteGlu(n) were obtained using a simple kinetic model.

Primary acute lymphoblastic leukemia cells use a novel promoter and 5'noncoding exon for the human reduced folate carrier that encodes a modified carrier translated from an upstream translational start

The human reduced folate carrier (hRFC) is reported to be regulated by up to seven alternatively spliced noncoding exons (A1, A2, A, B, C, D, and E). Noncoding exon and promoter usage was analyzed in RNAs from 27 childhood acute lymphoblastic leukemia (ALL) specimens by real-time PCR and/or 5' rapid amplification of cDNA ends (5' RACE) assay. By real-time PCR, total hRFC transcripts in ALL spanned a 289-fold range. Over 90% of hRFC transcripts were transcribed with A1, A2, and B 5' untranslated regions (UTRs). Analysis of 5' RACE clones showed that the A1 + A2 5'UTRs contained A1 sequence alone or a fusion of A1 and A2, implying the existence of a single, alternatively spliced 1021-bp A1/A2 noncoding region. High frequency sequence polymorphisms (AGG deletion, C/T transition) identified in the A1/A2 region by 5'RACE were confirmed in normal DNAs. By reporter assays in HepG2 hepatoma and Jurkat leukemia cells, A1/A2 promoter activity was localized to a 134-bp minimal region. Translation from an upstream AUG in the A1/A2 noncoding region in-frame with the normal translation start resulted in synthesis of a larger ( approximately 7 kDa) hRFC protein with transport properties altered from those for wild-type hRFC. Although there was no effect on transcript or protein stabilities, in vitro translation from A1/A2 transcripts was decreased compared with those with the B 5'UTR. Our results document the importance of the hRFC A1/A2 upstream region in childhood ALL and an intricate transcriptional and posttranscriptional regulation of hRFC-A1/A2 mRNAs. Furthermore, they suggest that use of the A1/A2 5'UTR may confer a transport phenotype distinct from the other 5'UTRs due to altered translation efficiency and transport properties.

Transcriptional regulation of the human reduced folate carrier promoter C: synergistic transactivation by Sp1 and C/EBP beta and identification of a downstream repressor

The human reduced folate carrier (hRFC) is ubiquitously but differentially expressed in human tissues and its levels are regulated by up to six alternatively spliced non-coding regions (designated A1/A2, A, B, C, D, and E) and by at least four promoters. By transient transfections of HepG2 human hepatoma cells with 5' and 3' deletion constructs spanning 2883 bp of upstream sequence, a transcriptionally important region was localized to within 177 bp flanking the transcriptional start sites for exon C. By gel shift and chromatin immunoprecipitation assays, Sp1 and C/EBP beta transcription factors were found to bind consensus elements (GC-box, CCAAT-box) within this region. The functional importance of these elements was confirmed by transient tranfections of HepG2 cells with hRFC-C reporter constructs in which these elements were mutated, and by co-transfections of Drosophila SL-2 cells with wild-type hRFC-C promoter and expression constructs for Sp1 and C/EBP beta. Whereas both Sp1 and C/EBP beta transactivated hRFC-C promoter activity, C/EBP alpha and gamma were transcriptionally inert. Sp1 combined with C/EBP beta resulted in a synergistic transactivation. In HepG2 cells, transfections with Sp1 and C/EBP beta both increased endogenous levels of hRFC-C transcripts. By 3' deletion analysis, a repressor sequence was localized to within 71 bp flanking the minimal promoter. On gel shifts, a novel transcriptional repressor was localized to within 30 bp. Collectively, these results identify transcriptionally important regions in the hRFC-C minimal promoter that include a GC-box and CCAAT-box, and suggest that cooperative interactions between Sp1 and C/EBP beta are essential for hRFC-C transactivation. Another possible factor in the tissue-specific regulation of the hRFC-C region involves the downstream repressor flanking the minimal promoter.

Fusogenic domain and lysines in saposin C

Saposin C, a sphingolipid activator protein with fusogenic activity, interacts specifically with the membrane containing negatively charged, unsaturated phospholipids. The kinetics and mechanism of saposin C-induced membrane fusion were previously investigated using acidic phospholipid liposomes. A hypothetic clip-on model for such a fusion process was illustrated by the ionic binding between saposin C and lipids, as well as the inter-saposin C hydrophobic interaction. Here, we report the location of the fusogenic domain in a linear sequence at the amino-terminal half of saposin C. This domain consisted of the first and second helical sequences. Selected positively charged lysines in the fusogenic domain were mutated to study the roles of basic residues in the saposin C-induced vesicle fusion. Based on the results, Lys13 and Lys17 were critical for the fusogenic activity, but had no effect on the enzymatic activation of acid beta-glucosidase (GCase). These results clearly indicate the segregation of the fusion and activation function into two different regions of saposin C. Interestingly, all the Lys mutant saposin Cs anchored on the acidic phospholipid membrane. Our data suggest that saposin C's fusogenic and activation functions have different requirements for the orientation and insertion manners of helical peptides in membranes.

Membrane transport of folates

The chapter reviews the current understanding of the transport mechanisms for folates in mammalian cells--their molecular identities and organization, tissue expression, regulation, structures, and their kinetic and thermodynamic properties. This encompasses a variety of diverse processes. Best characterized is the reduced folate carrier, a member of the SLC19 family of facilitative carriers. But other facilitative organic anion carriers (SLC21), largely expressed in epithelial tissues, transport folates as well. In addition to these bi-directional carrier systems are the membrane-localized folate receptors alpha and beta, that mediate folate uptake unidirectionally into cells via an endocytotic process. There are also several transporters, typified by the family of multidrug resistance-associated proteins, that unidirectionally export folates from cells. There are transport activities for folates, that function optimally at low pH, related in part to the reduced folate carrier, with at least one activity that is independent of this carrier. The reduced folate carrier-associated low-pH route mediates intestinal folate transport. This review considers how these different transport processes contribute to the generation of transmembrane folate gradients and to vectorial flows of folates across epithelia. The role of folate transporters in mouse development, as assessed by homologous deletion of folate receptors and the reduced folate carrier, is described. Much of the focus is on antifolate cancer chemotherapeutic agents that are often model surrogates for natural folates in transport studies. In particular, antifolate transport mediated by the reduced folate carrier is a major determinant of the activity of, and resistance to, these agents. Finally, many of the key in vitro findings on the properties of antifolate transporters are now beginning to be extended to patient specimens, thus setting the stage for understanding response to these drugs in the clinical setting at the molecular level.

Phospholipid vesicle fusion induced by saposin C

Saposin C is a small Trp-free, multifunctional glycoprotein that enhances the hydrolytic activity of acid beta-glucosidase in lysosomes. Saposin C's functions have been shown to include neuritogenic/neuroprotection effects and membrane fusion induction. Here, the mechanism and kinetics of saposin C's fusogenic activity were evaluated by fluorescence spectroscopic methods including dequenching, fluorescence resonance energy transfer, and stopped-flow analyses. Trp or dansyl groups were introduced as fluorescence reporters into selected sites of saposin C to serve as topological probes for protein-protein and protein-membrane interactions. Saposin C induction of liposomal vesicle enlargement was dependent upon anionic phospholipids and acidic pH. The initial fusion burst was completed in the timeframe of a few seconds to minutes and was dependent upon the unsaturated anionic phospholipid content. Two events were associated with saposin C-membrane interaction: membrane insertion of the saposin C terminal helices and reorientation of its central helical region. The latter conformational change likely exposed a binding site for saposins anchored on vesicles. Addition of selected saposin C peptides prior to intact saposin C in reaction mixtures abolished the liposomal fusion. These results indicated that saposin-membrane and saposin-saposin interactions are needed for the fusion process.

Mammalian fibroblasts lacking mitochondrial NAD+-dependent methylenetetrahydrofolate dehydrogenase-cyclohydrolase are glycine auxotrophs

Primary fibroblasts established from embryos of NAD-dependent mitochondrial methylenetetrahydrofolate dehydrogenase-cyclohydrolase (NMDMC) knockout mice were spontaneously immortalized or transformed with SV40 Large T antigen. Mitotracker Red CMXRos staining of the cells indicates the presence of intact mitochondria with a membrane potential. The nmdmc(-/-) cells are auxotrophic for glycine, demonstrating that NMDMC is the only methylenetetrahydrofolate dehydrogenase normally expressed in the mitochondria of these cell lines. Growth of null mutant but not wild type cells on complete medium with dialyzed serum is stimulated about 2-fold by added formate or hypoxanthine. Radiolabeling experiments demonstrated a 3-10 x enhanced incorporation of radioactivity into DNA from formate relative to serine by nmdmc(-/-) cells. The generation of one-carbon units by mitochondria in nmdmc(-/-) cells is completely blocked, and the cytoplasmic folate pathways alone are insufficient for optimal purine synthesis. The results demonstrate a metabolic role for NMDMC in supporting purine biosynthesis. Despite the recognition of these metabolic defects in the mutant cell lines, the phenotype of nmdmc(-/-) embryos that begin to die at E13.5 is not improved when pregnant dams are given a glycine-rich diet or daily injections of sodium formate.

The role of multidrug resistance proteins MRP1, MRP2 and MRP3 in cellular folate homeostasis

Previously, we reported that the multidrug resistance proteins MRP1, MRP2 and MRP3 confer resistance to therapeutic antifolates by mediating their cellular extrusion. We now determined whether MRPs also play a role in controlling cellular homeostasis of natural folates. In MRP1, MRP2 and MRP3-transfected 2008 human ovarian carcinoma cells total cellular folate content was 32-38% lower than in 2008 cells (105+/-14pmolfolate/mgprotein) when grown in medium containing 2.3 microM folic acid (FA). Under these conditions cellular growth rates were not compromised. However, when cells were challenged under folate-depleted conditions with a short exposure (4 hr) to FA or leucovorin, MRP1 and MRP3 overexpressing cells were impaired in their growth. In contrast to wild-type cells, MRP1 transfected cells retained only 60% of the maximum growth when exposed to 500 nM leucovorin or 500 microM FA. For 2008/MRP1 and 2008/MRP3 cells FA growth stimulation capacity was dramatically decreased when, during a 4 hr exposure, metabolism into rapidly polyglutamatable and retainable dihydrofolate was blocked by the dihydrofolate reductase inhibitor trimetrexate. To retain growth under such conditions MRP1 overexpressing cells required much higher concentrations of FA (EC(50) > 500 microM) compared to 2008 cells (EC(50): 12 microM). These results suggest that down- and up-regulation of MRP1 (and MRP3) expression can influence cellular folate homeostasis, in particular when cellular retention by polyglutamylation of folates is attenuated.

Loss of folylpoly-gamma-glutamate synthetase activity is a dominant mechanism of resistance to polyglutamylation-dependent novel antifolates in multiple human leukemia sublines

We have studied the molecular basis of drug resistance in human CCRF-CEM leukemia cells exposed to high dose intermittent pulses of novel polyglutamatable antifolates that target various folate-dependent enzymes. These include the dihydrofolate reductase (DHFR) inhibitors edatrexate, methotrexate and aminopterin, the thymidylate synthase (TS) inhibitors ZD1694 and GW1843, the glycinamide ribonucleotide formyltransferase (GARTF) inhibitor DDATHF as well as the multitargeted antifolate LY231514 inhibiting both TS, DHFR and GARTF. Fourteen antifolate-resistant sublines were isolated, 11 of which displayed a drug resistance phenotype that was based on impaired folylpoly-gamma-glutamate synthetase (FPGS) activity as these cell lines: 1) typically lost 90-99% of parental FPGS activity; 2) expressed 1.4-3.3-fold less FPGS mRNA (only 4 cell lines); 3) displayed up to 10(5)-fold resistance to polyglutamylation-dependent antifolates including ZD1694 and MTA; 4) retained sensitivity to polyglutamylation-independent antifolates including ZD9331 and PT523; 5) were up to 19-fold hypersensitive to the lipid-soluble antifolates trimetrexate and AG377; 6) had a normal or a small decrease in [(3)H]MTX transport; and 7) had a 2.1-8.3-fold decreased cellular folate pools and a consequently increased folate growth requirement. The remaining 3 antifolate-resistant sublines lost 94-97% of parental [(3)H]MTX transport and thus displayed a high level resistance to all hydrophilic antifolates. To screen for mutations in the hFPGS gene, we devised an RT-PCR single strand conformational polymorphism (SSCP) assay. RT-PCR-SSCP analysis and DNA sequencing showed that only a single FPGS-deficient subline harbored an FPGS mutation (Cys346Phe). Three-dimensional modeling of the human FPGS based on the crystal structure of Lactobacillus casei FPGS suggested that this mutation maps to the active site and interferes with the catalytic activity of the enzyme due to a putative bulky clash between the mutant Phe346 and a native Phe350 within alpha-helix A10 in a highly conserved C-terminal hydrophobic core. This was consistent with a 23-fold decreased affinity of the mutant Cys346Phe FPGS for L-glutamate. We conclude that decreased FPGS activity is a dominant mechanism of resistance to polyglutamylation-dependent novel antifolates upon a high-dose intermittent exposure schedule. The finding that cells may exhibit 5 orders of magnitude of resistance to polyglutamylation-dependent antifolates but in the same time retain parental sensitivity or hypersensitivity to polyglutamylation-independent antifolates or lipophilic antifolates offers a potentially promising treatment strategy in the overcoming of FPGS-based anticancer drug resistance.

The human reduced folate carrier gene is regulated by the AP2 and sp1 transcription factor families and a functional 61-base pair polymorphism

Recently, our laboratory reported an intricate regulation of the human reduced folate carrier (hRFC) gene, involving multiple promoters and noncoding exons. We localized promoter activity to a 452-bp GC-rich region upstream of noncoding exon A, including a 47-bp basal promoter with a CRE/AP-1-like consensus element that bound the bZip family of DNA-binding proteins (e.g. CREB-1 and c-Jun). We now report that three nearly identical tandem repeats (49-61 bp) in the hRFC-A upstream region are involved in regulating promoter activity. By in vitro binding assays, multiple transcription factors (e.g. AP2 and Sp1/Sp3) bound this region. When AP2 was cotransfected with the hRFC-A reporter construct into HT1080 cells, promoter activity increased 3-fold. In Drosophila SL2 cells, Sp1 transactivated promoter A and showed synergism with CREB-1. However, c-Jun was antagonistic to the effects of Sp1. A sequence variant in the hRFC-A repeated region was identified, involving an exact duplication of a 61-bp sequence. This variant had an allelic frequency of 78% in 72 genomic DNAs and resulted in a 63% increase in promoter activity. These results identify important regions in the hRFC-A promoter and critical roles for AP2 and Sp1, in combination with the bZip transcription factors. Moreover, they document a functionally novel polymorphism that increases promoter activity and may contribute to interpatient variations in hRFC expression and effects on tissue folate homeostasis and antitumor response to antifolates.

The human reduced folate carrier gene is ubiquitously and differentially expressed in normal human tissues: identification of seven non-coding exons and characterization of a novel promoter

Our previous study identified two alternate non-coding upstream exons (A and B) in the human reduced folate carrier (hRFC) gene, each controlled by a separate promoter. Each minimal promoter was regulated by unique cis -elements and transcription factors, including stimulating protein (Sp) 1 and Sp3 and the basic leucine zipper family of proteins, suggesting opportunities for cell- and tissue-specific regulation. Studies were performed to explore the expression patterns of hRFC in human tissues and cell lines. Levels of hRFC transcripts were measured on a multi-tissue mRNA array from 76 human tissues and tumour cell lines and on a multi-tissue Northern blot of representative tissues, each probed with full-length hRFC cDNA. hRFC transcripts were ubiquitously expressed, with the highest level in placenta and the lowest level in skeletal muscle. By rapid amplification of cDNA 5'-ends assay from nine tissues and two cell lines, hRFC transcripts containing both A and B 5'-untranslated regions (UTRs) were identified. However, five additional 5'-UTRs (designated A1, A2, C, D and E) were detected, mapping over 35 kb upstream from the hRFC translation start site. The 5'-UTRs were characterized by multiple transcription start sites and/or alternative splice forms. At least 18 unique hRFC transcripts were detected. A novel promoter was localized to a 453 bp fragment, including 442 upstream of exon C and 11 bp of exon C. A 346 bp repressor flanked the 3'-end of this promoter. Our results suggest an intricate regulation of hRFC gene expression involving multiple promoters and non-coding exons. Moreover, they provide a transcriptional framework for understanding the role of hRFC in the pathophysiology of folate deficiency and antifolate drug selectivity.

New perspectives on folate catabolism

Folate catabolism has been assumed to result from the nonenzymatic oxidative degradation of labile folate cofactors. Increased rates of folate catabolism and simultaneous folate deficiency occur in several physiological states, including pregnancy, cancer, and when anticonvulsant drugs are used. These studies have introduced the possibility that folate catabolism may be a regulated cellular process that influences intracellular folate concentrations. Recent studies have demonstrated that the iron storage protein ferritin can catabolize folate in vitro and in vivo, and increased heavy-chain ferritin synthesis decreases intracellular folate concentrations independent of exogenous folate levels in cell culture models. Ferritin levels are elevated in most physiological states associated with increased folate catabolism. Therefore, folate catabolism is emerging as an important component in the regulation of intracellular folate concentrations and whole-body folate status.

Folate, homocysteine and neural tube defects: an overview

Folate administration substantially reduces the risk on neural tube detects (NTD). The interest for studying a disturbed homocysteine (Hcy) metabolism in relation to NTD was raised by the observation of elevated blood Hcy levels in mothers of a NTD child. This observation resulted in the examination of enzymes involved in the folate-dependent Hcy metabolism. Thus far, this has led to the identification of the first and likely a second genetic risk factor for NTD. The C677T and A1298C mutations in the methylenetetrahydrofolate reductase (MTHFR) gene are associated with an increased risk of NTD and cause elevated Hcy concentrations. These levels can be normalized by additional folate intake. Thus, a dysfunctional MTHFR partly explains the observed elevated Hcy levels in women with NTD pregnancies and also, in part, the protective effect of folate on NTD. Although the MTHFR polymorphisms are only moderate risk factors, population-wide they may account for an important part of the observed NTD prevalence.

Rescue of embryonic lethality in reduced folate carrier-deficient mice by maternal folic acid supplementation reveals early neonatal failure of hematopoietic organs

The reduced folate carrier (RFC1) is an important route by which the major blood folate, 5-methyltetrahydrofolate, is transported into mammalian cells. In this study we determined the consequences of inactivation of RFC1 in mice by homologous recombination. While RFC1-null embryos died in utero before embryonic day 9.5 (E9.5), near-normal development could be sustained in RFC1(-)/- embryos examined at E18.5 by supplementation of pregnant RFC1(+/-) dams with 1-mg daily subcutaneous doses of folic acid. About 10% of these animals went on to live birth but died within 12 days. These RFC1(-)/- mice showed a marked absence of erythropoiesis in bone marrow, spleen, and liver along with lymphoid depletion in the splenic white pulp and thymus. In addition, there was some impairment of renal and seminiferous tubule development. These data indicate that in the absence of RFC1 function, neonatal animals die due to failure of hematopoietic organs.

A possible role for methotrexate in the treatment of childhood acute myeloid leukaemia, in particular for acute monocytic leukaemia

Acute myeloid leukaemia (AML) is thought to be methotrexate (MTX)-resistant. However, a small study suggested that acute monocytic leukemia (AML-M5) is sensitive to MTX. We measured MTX accumulation/polyglutamylation in 20 AML-nonM5, 37 AML-M5 and 83 common/preB-acute lymphoblastic leukaemia (c/preB-ALL) samples. Membrane transport was determined in 11 childhood AMLs (including 3 AML-M5) and in 25 c/preB-ALL samples. MTX sensitivity was determined in 23 AML-nonM5, 15 AML-M5 and 63 common/preB-ALL samples using the thymidylate synthase (TS) inhibition assay. MTX transport was higher in AML samples compared with c/preB-ALL precluding a transport defect in AML. Accumulation of long-chain polyglutamates MTX-Glu(4-6) was 3-fold lower for AML-nonM5 compared with c/preB-ALL cells (median 268 versus 889 pmol MTX-Glu(4-6)/10(9) cells; P < or = 0.001); for AML-M5 samples, median accumulation of MTX-Glu(4-6) was 0 pmol/10(9) cells (P < or = 0.001). After short-term MTX exposure, AML-nonM5 was 6-fold more resistant to MTX compared with c/preB-ALL cells (2.16 versus 0.39 microM; P < 0.001), while AML-M5 was 2-fold more resistant (P = 0.02). In both AML-nonM5 and AML-M5 cells, MTX resistance was circumvented by continuous MTX exposure (median TSI(50) values: 0.052 and 0.041 microM, respectively) compared with a c/preB-ALL value of 0.066 microM. In conclusion, AML-M5 is relatively sensitive to MTX compared with other AML-subtypes even though polyglutamylation of MTX is poor. Using continuous exposure, AML-nonM5 and AML-M5 cells were at least as sensitive to MTX as c/preB-ALL cells. This report suggests that MTX might be an overlooked drug in the treatment of childhood AML.

Retrovirally mediated complementation of the glyB phenotype. Cloning of a human gene encoding the carrier for entry of folates into mitochondria

The transduction of a human placental cDNA retroviral library into glyB cells, a Chinese hamster ovary K1 subline that is deficient in the transport of folates into mitochondria, resulted in the complementation of glycine auxotrophy of these cells. A 2.6-kilobase pair cDNA insert flanked by retroviral sequences had integrated into genomic DNA in rescued cells. An open reading frame in this cDNA encoded a 35-kDa protein homologous to several inner mitochondrial wall transporters for intermediate metabolites. The subcloned cDNA complemented the glycine auxotrophy of glyB cells and reinstated folate accumulation in the mitochondria of transfected cells. The human origin, chromosomal location, and intron-exon organization of the isolated mitochondrial folate transporter gene were deduced from the expressed sequence tag database and human genome project data.

Expression patterns of the multiple transcripts from the folylpolyglutamate synthetase gene in human leukemias and normal differentiated tissues

Folylpoly-gamma-glutamate synthetase (FPGS) catalyzes the activation of folate antimetabolites in mammalian tissues and tumors. We have determined the sequence, abundance, and function of human FPGS transcripts and found some striking differences to transcription of the mouse gene that allow production of FPGS isoforms in mouse liver and dividing tissues. Multiple human transcripts were identified, including the homolog of the mouse transcripts that initiate at two upstream exons. However, the human FPGS upstream promoter is infrequently used, and transcripts from this promoter include sequences homologous with only one of the upstream exons found in the mouse. The downstream promoter generates an array of transcripts, some of which do not produce active enzyme, a phenomenon not seen in the mouse. Hence, the dual promoter mechanism directing expression of FPGS isozymes in mouse tissues is not conserved in humans, and, unlike the mouse downstream promoter, the human downstream promoter is active in both dividing and differentiated tissues. This study raises questions about the differences in function served by the two mouse FPGS isozymes and how, or if, human tissues fulfill these functions. How humans and mice produce FPGS in only a subset of tissues using such different promoter structures also becomes a central issue.