Folylpolyglutamate Synthetase Activity Research Articles

Erythrocyte Folyl Polyglutamate Synthetase Activity Profiling as a Potential Tool for the Prediction of Methotrexate Efficacy and Toxicity in Rheumatoid Arthritis

Methotrexate (MTX) is the cornerstone of therapy in the treatment of rheumatoid arthritis (RA). However, its efficacy and toxicity are variable and remain unpredictable. Interindividual variation in the metabolism of MTX by the enzyme folyl polyglutamate synthetase (FPGS) has been associated with response variability in RA. In this work, we propose the development of a FPGS phenotyping assay that can be evaluated as a tool for the prediction of efficacy and toxicity in patients with RA prior to initiating MTX therapy. FPGS activity was measured in erythrocyte lysate by monitoring methotrexate polyglutamate (MTX + Glun) formation using ultra-performance liquid chromatography tandem–mass spectrometry (UPLC/MS/MS). Erythrocyte FPGS activity was measured in newly diagnosed RA (n = 35) and osteoarthritis (n = 7) patients. The enzymatic assay was optimized for measuring FPGS activity in 25 µL of packed erythrocytes over two hours. The coefficient of variation for intra- and inter-day analysis was found to be 5% and 12%, respectively. The method was used to measure FPGS enzyme kinetics, resulting in a mean (SD) Km of 30.3 (4.8) µM and a Vmax of 612 (193) pmol MTX + Glu2/h/mL of packed erythrocytes. Mean (SD) erythrocyte FPGS activity in patients with RA was found to be 445.93 (344.50) pmol MTX + Glu2/h/mL and with a 26-fold difference in the range (range: 83–2179 pmol MTX + Glu2/h/mL) whereas for patients with OA, it was found to be 409.80 (157.66) pmol MTX + Glu2/h/mL with a 3.5-fold difference in the range (range: 200.95–683.93 pmol MTX + Glu2/h/mL). Monitoring erythrocyte FPGS activity may be a feasible strategy of phenotyping for methotrexate efficacy and toxicity in patients with RA.

Methotrexate Provokes Disparate Folate Metabolism Gene Expression and Alternative Splicing in Ex Vivo Monocytes and GM-CSF- and M-CSF-Polarized Macrophages.

Macrophages constitute important immune cell targets of the antifolate methotrexate (MTX) in autoimmune diseases, including rheumatoid arthritis. Regulation of folate/MTX metabolism remains poorly understood upon pro-inflammatory (M1-type/GM-CSF-polarized) and anti-inflammatory (M2-type/M-CSF-polarized) macrophages. MTX activity strictly relies on the folylpolyglutamate synthetase (FPGS) dependent intracellular conversion and hence retention to MTX-polyglutamate (MTX-PG) forms. Here, we determined FPGS pre-mRNA splicing, FPGS enzyme activity and MTX-polyglutamylation in human monocyte-derived M1- and M2-macrophages exposed to 50 nmol/L MTX ex vivo. Moreover, RNA-sequencing analysis was used to investigate global splicing profiles and differential gene expression in monocytic and MTX-exposed macrophages. Monocytes displayed six-eight-fold higher ratios of alternatively-spliced/wild type FPGS transcripts than M1- and M2-macrophages. These ratios were inversely associated with a six-ten-fold increase in FPGS activity in M1- and M2-macrophages versus monocytes. Total MTX-PG accumulation was four-fold higher in M1- versus M2-macrophages. Differential splicing after MTX-exposure was particularly apparent in M2-macrophages for histone methylation/modification genes. MTX predominantly induced differential gene expression in M1-macrophages, involving folate metabolic pathway genes, signaling pathways, chemokines/cytokines and energy metabolism. Collectively, macrophage polarization-related differences in folate/MTX metabolism and downstream pathways at the level of pre-mRNA splicing and gene expression may account for variable accumulation of MTX-PGs, hence possibly impacting MTX treatment efficacy.

Development and Validation of a Sensitive UHPLC-MS/MS-Based Method for the Analysis of Folylpolyglutamate Synthetase Enzymatic Activity in Peripheral Blood Mononuclear Cells: Application in Rheumatoid Arthritis and Leukemia Patients.

Folylpolyglutamate synthetase (FPGS) is a crucial enzyme in both cellular folate homeostasis and the intracellular retention of folate analogue drugs such as methotrexate (MTX), which is commonly used for the treatment of (pediatric) leukemia and the anchor drug in rheumatoid arthritis (RA) treatment. To date, assessment of FPGS catalytic activity relies on assays using radioactive substrates that are labor-intensive and require relatively large numbers of cells. Here, we describe a nonradioactive, ultra-high-performance liquid chromatography-tandem mass spectrometer (UHPLC-MS/MS)-based method allowing for sensitive and accurate measurements of FPGS activity in low cell numbers (ie, 1-2 × 10) of biological specimens, including leukemic blast cells of acute lymphoblastic leukemia patients and peripheral blood mononuclear cells of patients with RA. The UHPLC-MS/MS assay was validated with 2 CCRF-CEM human leukemia cells, one proficient and one deficient in FPGS activity. Linearity of time and protein input were tested by measuring FPGS activity at 30-180 minutes of incubation time and 10-300 mcg protein extract. In addition, FPGS enzyme kinetic parameters were assessed. The FPGS enzymatic assay showed a linear relation between FPGS activity and protein input (R ≥ 0.989) as well as incubation time (R ≥ 0.996). Moreover, the UHPLC-MS/MS method also allowed for evaluation of FPGS enzyme kinetic parameters revealing Km values for the substrates MTX and L-glutamic acid of 64 µmol/L and 2.2 mmol/L, respectively. The mean FPGS activity of acute lymphoblastic leukemia blast cells (n = 4) was 3-fold higher than that of CCRF-CEM cells and 44-fold and 88-fold higher than that of peripheral blood mononuclear cells from MTX-naive (n = 9) and MTX-treated RA patients (n = 6), respectively. Collectively, given its sensitivity with low cell numbers and avoidance of radioactive substrates, UHPLC-MS/MS-based analysis of FPGS activity may be eligible for routine therapeutic drug monitoring of MTX in RA and leukemia for therapy (non)response evaluations.

Structural Analyses of Helicobacter Pylori FolC Conducting Glutamation in Folate Metabolism

FolC plays important roles in the folate metabolism of cells by attaching l-Glu to dihydropteroate (DHP) and folate, which are known activities of dihydrofolate synthetase (DHFS) and folylpolyglutamate synthetase (FPGS), respectively. Here, we determined the crystal structure of Helicobacter pylori FolC (HpFolC) at 1.95 Å resolution using the single-wavelength anomalous diffraction method. HpFolC has globular N- and C-terminal domains connected by a single loop, and a binding site for ATP is located between the two domains. Apo-HpFolC was crystallized in the presence of citrate in a crystallization solution, which was held in the ATP-binding site. Structural motifs such as the P-loop and Ω-loop of HpFolC for binding of ATP and two magnesium ions are well conserved in spite of the low overall sequence similarity to other FolC/FPGSs. The Ω-loop would also recognize a folate molecule, and the DHP-binding loop of HpFolC is expected to exhibit a unique recognition mode on DHP, compared with other FolCs. Because human FolC is known to only exhibit FPGS activity, the DHFS activity of bacterial FolC is an attractive target for the eradication of pathogenic bacteria. Consequently, our structural analyses of HpFolC provide a valuable foundation for a universal antibacterial strategy against H. pylori as well as other pathogenic bacteria.

Folylpolyglutamate synthetase splicing alterations in acute lymphoblastic leukemia are provoked by methotrexate and other chemotherapeutics and mediate chemoresistance

Methotrexate (MTX), a folate antagonist which blocks de novo nucleotide biosynthesis and DNA replication, is an anchor drug in acute lymphoblastic leukemia (ALL) treatment. However, drug resistance is a primary hindrance to curative chemotherapy in leukemia and its molecular mechanisms remain poorly understood. We have recently shown that impaired folylpolyglutamate synthetase (FPGS) splicing possibly contributes to the loss of FPGS activity in MTX-resistant leukemia cell line models and adult leukemia patients. However, no information is available on the possible splicing alterations in FPGS in pediatric ALL. Here, using a comprehensive PCR-based screen we discovered and characterized a spectrum of FPGS splicing alterations including exon skipping and intron retention, all of which proved to frequently emerge in both pediatric and adult leukemia patient specimens. Furthermore, an FPGS activity assay revealed that these splicing alterations resulted in loss of FPGS function. Strikingly, pulse-exposure of leukemia cells to antifolates and other chemotherapeutics markedly enhanced the prevalence of several FPGS splicing alterations in antifolate-resistant cells, but not in their parental antifolate-sensitive counterparts. These novel findings suggest that an assortment of deleterious FPGS splicing alterations may constitute a mechanism of antifolate resistance in childhood ALL. Our findings have important implications for the rational overcoming of drug resistance in individual leukemia patients.

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

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). 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. 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). 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.

Spliceosome Inhibitor Meayamycin B As a Novel Potential Chemotherapeutic Agent in ALL and AML

IntroductionThe process of pre-mRNA splicing is gaining attention as a contributor to anticancer drug resistance and as a promising novel therapeutic target. Splicing of many genes involved in regulation of apoptosis was found to be altered in tumor cells leading to chemoresistance. Similarly, aberrant splicing of genes engaged in drug metabolism was reported to mediate resistance to several anchor drugs of chemotherapeutic protocols in the treatment of leukemia including daunorubicin, cytarabine and methotrexate. Therefore, targeting the spliceosome holds potential to directly activate apoptosis by inducing pro-apoptotic splicing profiles as well as to sensitize cells displaying drug resistance related to altered splicing of genes involved in drug metabolism. In this respect, meayamycin B (MAMB) is a novel compound, which potently inhibits the SF3B1 subunit, which is one of the core components of the spliceosome complex. MAMB was previously shown to induce shifts in splicing of one of the essential apoptosis regulators Mcl-1 in non-small cell lung cancer cell lines A549 and H1299, hence promoting expression of its pro-apoptotic isoform Mcl-1S. The resulting dominance of Mcl-1S was able to sensitize these tumor cells to Bcl-XL inhibitor leading to induction of cell death. Here we evaluated the in vitro impact of MAMB as a single drug in acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).MethodsTo achieve this goal, we first assessed the impact of MAMB in short-term exposure on splicing of selected apoptosis-related genes, in particular Mcl-1, and concomitant induction of apoptosis in 3 human ALL and 3 AML cell lines. In addition, MAMB sensitivity was assessed using a 72h MTT assay in a panel of ALL and AML cell lines including sublines displaying resistance to several conventional chemotherapeutics including methotrexate, bortezomib or imatinib. Finally, we assessed MAMB sensitivity in primary ALL and AML samples, as compared to healthy bone marrow specimens.ResultsAs previously shown in solid tumors, MAMB (0.5-1nM) was able to shift splicing of Mcl-1 but not Bcl-X in leukemic cell lines upon 24h exposure. The observed changes in splicing coincided with enhanced apoptosis as determined by flow cytometry. The fraction of apoptotic cells reached approximately 40% in the parental CCRF-CEM cell line (T-ALL) and 10% in the methotrexate-resistant subline which has lost folylpolyglutamate synthetase (FPGS) activity. Intriguingly, we previously found impaired splicing of FPGS in this methotrexate-resistant cell line as compared to intact splicing in parental cells. It should be emphasized that intact FPGS splicing is a key component of intracellular retention and activity of MTX. Induction of apoptosis in AML cell lines ranged between 17% in OCI-AML3 and 53% in KG1a. When MAMB sensitivity was assessed in the 72h MTT assay, the growth of both ALL and AML cell lines was efficiently inhibited with remarkable IC50 values varying between 0.07 and 0.16 nM. Intriguingly, even tumor cell lines which are resistant to conventional chemotherapeutics with different mechanisms of action, such as methotrexate, imatinib and bortezomib, were highly sensitive to MAMB. In line with our results with leukemic cell lines, both ALL and AML primary samples showed a remarkable sensitivity to MAMB (mean LC50 value 0.42 and 0.43 nM, respectively, Figure 1). The normal bone marrow cells obtained from healthy children showed a slightly higher resistance (mean LC50 value 0.57, p=0.03, Figure).ConclusionsOur results show that MAMB itself may constitute a therapeutic option for patients displaying drug resistance to conventional chemotherapy, especially in AML, which in general tends to be more resistant to current treatment options compared to ALL. In addition, as a splicing modulating agent it may also be used to reverse aberrant splicing profiles of genes involved in drug metabolism, thereby restoring sensitivity to standard chemotherapeutics. This novel paradigm warrants further exploration using drug combination studies. Further investigation in a larger cohort of leukemia patient samples is warranted. Moreover, mice studies will be performed to reveal possible toxicities and in vivo efficacy. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Thymidylate synthase inhibitors for thoracic tumors

Abstract Thymidylate synthase (TS) is a valid target for treatment of non-small cell lung cancer (NSCLC) and mesothelioma (MPM). The TS inhibitor pemetrexed (PMX) is now commonly used in combination with cisplatin (CDDP) or carboplatin, in the first-line setting for both of these lethal diseases. A combination of another TS inhibitor, Tomudex with CDDP, demonstrated similar activity in MPM patients. However, the efficacy of TS inhibitors is limited by uptake, metabolism and target affinity. While uptake of most antifolates is mediated by the reduced folate carrier, PMX is also a good substrate for the proton-coupled folate transporter (PCFT), which displays optimal activity at the acidic pH of the tumor microenvironment. NSCLC and MPM have a variable expression of PCFT, which might be caused by the differential methylation status of the PCFT promoter, resulting in decreased anticancer activity. PMX and TDX activity also depends on polyglutamylation, catalyzed by folylpolyglutamate synthetase (FPGS), which results in intracellular retention and thus enhanced cytotoxicity. We demonstrated that resistance to the classical antifolate methotrexate in human leukemia cells with a marked loss of FPGS activity was associated with impaired splicing of FPGS pre-mRNA. Moreover, we recently showed that the sensitivity of MPM patients to PMX-carboplatin is related to tumor TS expression. Patients with low TS mRNA levels had a significantly longer overall survival (20 vs. 7 months) compared with patients with high expression. Intriguingly, recent trials demonstrated that histology plays an important role in the sensitivity of NSCLC patients, and PMX-CDDP is now approved only for adenocarcinoma patients. These results might be partly explained by the higher TS expression detected in lung tumors of squamous histology. In summary, the clinical success of TS inhibitors may depend on biomarker-driven patient selection, and further studies should evaluate the genetic/epigenetic factors involved in the modulation of these biomarkers in the preclinical and clinical setting.

Abstract 4628: Alterations in FPGS splicing as a plausible underlying mechanism of MTX resistance in ALL.

Abstract The antifolate methotrexate (MTX) is used in acute lymphoblastic leukemia (ALL) treatment. Cellular retention and consequent efficacy of MTX is enhanced by polyglutamylation, a reaction catalyzed by folylpolyglutamate synthetase (FPGS). It has been shown that loss of FPGS activity can confer antifolate resistance. However, the mechanism underlying this phenomenon remains elusive. We have recently shown that alterations in FPGS mRNA splicing may be a plausible determinant of MTX resistance but a comprehensive screen of the spectrum of splicing aberrations throughout the entire FPGS gene is lacking. The aim of the current study was to evaluate the pattern of FPGS mRNA splicing alterations as a predictor of MTX resistance and treatment failure in ALL. A PCR-based assay was devised that allowed for a screen of the entire FPGS mRNA sequence for splicing alterations. This method was subsequently used to screen antifolate-resistant human leukemia cell lines and ALL patient samples. Our results identified multiple FPGS mRNA splicing alterations (Table 1) in MTX resistant cell lines with marked loss of FPGS activity. Moreover, the level of partial retention of intron 8 was markedly increased upon a 24 hour exposure to MTX in the antifolate-resistant leukemia cell lines. Moreover, most of the detected FPGS splicing alterations either introduced a frame-shift or a premature stop codon, hence resulting in either transcripts targeted for the nonsense-mediated decay or truncated proteins devoid of FPGS activity. These in vitro FPGS splicing alterations where corroborated in vivo as aberrations detected in leukemic cell lines were also identified in a panel of 30 ALL patient specimens at diagnosis. In summary, impaired FPGS mRNA splicing could be responsible for loss of FPGS activity and might offer an opportunity for the design of reliable molecular tools allowing for identification of antifolate resistance in leukemia cells. Table 1. FPGS mRNA splicing alterations in cell lines and ALL patient samples. Alteration Frame shift (yes/no) Early stop codon position (nucleotide) Frequency in primary ALL samples (%) Exon 2 skipping − − 71 Exon 5 skipping + 477 50 Exon 6 skipping − − 93 Exon 12 skipping + 1173 57 Exon 14 skipping + 1356 50 Intron 5 retention + 792 43 Intron 6 retention + 660 35 Intron 5 + Intron 6 retention − 879 50 Intron 8 partial retention − 870 93 Intron 6 retention + Intron 8 partial retention + 660 50 Citation Format: Anna Wojtuszkiewicz, Yehuda Assaraf, Shachar Raz, Michal Stark, Edwin Sonneveld, Godefridus Peters, Gerrit Jansen, Gertjan Kaspers, Jacqueline Cloos. Alterations in FPGS splicing as a plausible underlying mechanism of MTX resistance in ALL. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4628. doi:10.1158/1538-7445.AM2013-4628

Alcohol-associated folate disturbances result in altered methylation of folate-regulating genes

Folate plays a critical role in maintaining normal metabolic, energy, differentiation and growth status of all mammalian cells. The steady-state accumulation of folate seems to depend on the activity of two enzymes: folylpolyglutamate synthetase (FPGS), which adds glutamate residues, and gamma-glutamyl hydrolase (GGH), which removes them, enabling it to be transported across the biological membranes. Overexpression of GGH and downregulation of FPGS would be expected to decrease intracellular folate in its polyglutamylated form, thereby increasing efflux of folate and its related molecules, which might lead to resistance to drugs or folate deficiency. The study was sought to delineate the activity of GGH and expression FPGS in tissues involved in folate homeostasis during alcoholism and the epigenetic regulation of these enzymes and transporters regulating intracellular folate levels. We determined the activity of GGH and expression of FPGS in tissues after 3 months of ethanol feeding to rats at 1 g/kg body weight/day. The results showed that there was not any significant change in the activity of folate hydrolyzing enzyme GGH in ethanol-fed rats while there was significant down regulation in the expression of FPGS. Ethanol feeding decreased the total as well as polyglutamated folate levels. There was tissue-specific hyper/hypo methylation of folate transporter genes viz. PCFT and RFC by chronic ethanol feeding. Moreover, hypermethylation of FPGS gene was observed in intestine and kidney without any change in methylation levels of GGH in the ethanol-fed rats. In conclusion, the initial deconjugation of polyglutamylated folate by GGH was not impaired in ethanol-fed rats while the conversion of monoglutamylated folate to polyglutamylated form might be impaired. There was tissue-specific altered methylation of folate transporter genes by chronic ethanol feeding.

Modeling Mechanisms of In Vivo Variability in Methotrexate Accumulation and Folate Pathway Inhibition in Acute Lymphoblastic Leukemia Cells

Methotrexate (MTX) is widely used for the treatment of childhood acute lymphoblastic leukemia (ALL). The accumulation of MTX and its active metabolites, methotrexate polyglutamates (MTXPG), in ALL cells is an important determinant of its antileukemic effects. We studied 194 of 356 patients enrolled on St. Jude Total XV protocol for newly diagnosed ALL with the goal of characterizing the intracellular pharmacokinetics of MTXPG in leukemia cells; relating these pharmacokinetics to ALL lineage, ploidy and molecular subtype; and using a folate pathway model to simulate optimal treatment strategies. Serial MTX concentrations were measured in plasma and intracellular MTXPG concentrations were measured in circulating leukemia cells. A pharmacokinetic model was developed which accounted for the plasma disposition of MTX along with the transport and metabolism of MTXPG. In addition, a folate pathway model was adapted to simulate the effects of treatment strategies on the inhibition of de novo purine synthesis (DNPS). The intracellular MTXPG pharmacokinetic model parameters differed significantly by lineage, ploidy, and molecular subtypes of ALL. Folylpolyglutamate synthetase (FPGS) activity was higher in B vs T lineage ALL (p<0.005), MTX influx and FPGS activity were higher in hyperdiploid vs non-hyperdiploid ALL (p<0.03), MTX influx and FPGS activity were lower in the t(12;21) (ETV6-RUNX1) subtype (p<0.05), and the ratio of FPGS to γ-glutamyl hydrolase (GGH) activity was lower in the t(1;19) (TCF3-PBX1) subtype (p<0.03) than other genetic subtypes. In addition, the folate pathway model showed differential inhibition of DNPS relative to MTXPG accumulation, MTX dose, and schedule. This study has provided new insights into the intracellular disposition of MTX in leukemia cells and how it affects treatment efficacy.

Genoproteomic Mining of Urothelial Cancer Suggests γ-Glutamyl Hydrolase and Diazepam-Binding Inhibitor as Putative Urinary Markers of Outcome after Chemotherapy

Urinary biomarkers for the detection of bladder cancer have been developed, but no similar markers exist for prediction of clinical outcomes after receiving chemotherapy. Here we evaluate an approach that combines genomic, proteomic, and therapeutic outcome datasets to identify novel putative urinary biomarkers of clinical outcome after neoadjuvant methotrexate, vinblastine, doxorubicin, and cisplatin (MVAC). Using this method, we identified gamma-glutamyl hydrolase (GGH), emmprin, survivin, and diazepam-binding inhibitor (DBI). Interestingly, GGH is a protein associated with methotrexate resistance, whereas emmprin, survivin, and DBI had been previously identified as predictors of outcome after platinum-containing chemotherapeutic regimens when assessed on tumor tissue. Using disease-free survival as a marker for clinical outcome, we evaluated the ability of GGH, emmprin, survivin, and DBI expression in tumor tissue to stratify 27 patients treated with neoadjuvant MVAC. DBI (P = 0.046) but not GGH (P = 0.190), emmprin (P = 0.066), or survivin (P = 0.393) successfully stratified patients. When GGH was used with DBI the significance of stratification improved (P = 0.024), whereas the addition of survivin or emmprin to this latter two-gene model reduced its significance (P = 0.036 and P = 0.040, respectively). Although these predictive results were obtained on tumor tissues, the presence of GGH and DBI in urine serves as a rationale for developing them as urinary markers of clinical outcomes for patients treated with neoadjuvant MVAC.

Mutation of Gly51 to serine in the P-loop ofLactobacillus caseifolylpolyglutamate synthetase abolishes activity by altering the conformation of two adjacent loops

Based upon the three-dimensional structure of Lactobacillus casei folylpolyglutamate synthetase (FPGS), site-directed mutagenesis studies were performed on three residues associated with the ATPase site: Gly51, Ser52 and Ser73. Gly51 and Ser52 are at the end of the P-loop, which is involved in triphosphate binding. A G51S mutant enzyme and a G51S/S52T double-mutant enzyme were made in order to alter the FPGS P-loop to more closely resemble the sequences found in other ATPase and GTPase enzymes. Ser73 is on a neighboring loop (the Omega-loop) and precedes a proline residue found to be in a cis conformation. The carbonyl O atom of Ser73 is one of the protein ligands for the essential Mg(2+) ion involved in ATP binding and hydrolysis and the Omega-loop is involved in binding the folate substrate 5,10-methylenetetrahydrofolate. The serine residue was mutated to alanine and this is the only one of the three mutants which retains some FPGS activity. The structures of the G51S, G51S/S52T and S73A mutant proteins have been solved to high resolution, along with the structure of the apo wild-type FPGS. The P-loop in both the G51S and G51S/S52T mutant proteins remains unaltered, yet both structures show a large conformational rearrangement of the Omega-loop in which a cis-Pro residue has switched conformation to a trans-peptide. The structure of the Omega-loop is severely disrupted and as a consequence structural rearrangements are observed in the peptide linker joining the two domains of the enzyme. Magnesium binding in the active site is also disrupted by the presence of the serine side chain at position 51 and by the repositioning of the carbonyl O atom of Ser73 and a water molecule is bound in place of the Mg(2+) ion. The S73A mutant protein retains the cis-Pro configuration in the Omega-loop and the Mg(2+) site remains intact. The cis-Pro is also observed in the structure of the substrate-free form of FPGS (apoFPGS), maintained in the absence of Mg(2+) by a hydrogen-bonding network involving water molecules in the active site. It is only in the complete absence of water or Mg(2+) in the binding site that the cis-Pro switches to the trans conformation.

Escherichia coli FolC Structure Reveals an Unexpected Dihydrofolate Binding Site Providing an Attractive Target for Anti-microbial Therapy

In some bacteria, such as Escherichia coli, the addition of L-glutamate to dihydropteroate (dihydrofolate synthetase activity) and the subsequent additions of L-glutamate to tetrahydrofolate (folylpolyglutamate synthetase (FPGS) activity) are catalyzed by the same enzyme, FolC. The crystal structure of E. coli FolC is described in this paper. It showed strong similarities to that of the FPGS enzyme of Lactobacillus casei within the ATP binding site and the catalytic site, as do all other members of the Mur synthethase superfamily. FolC structure revealed an unexpected dihydropteroate binding site very different from the folate site identified previously in the FPGS structure. The relevance of this site is exemplified by the presence of phosphorylated dihydropteroate, a reaction intermediate in the DHFS reaction. L. casei FPGS is considered a relevant model for human FPGS. As such, the presence of a folate binding site in E. coli FolC, which is different from the one seen in FPGS enzymes, provides avenues for the design of specific inhibitors of this enzyme in antimicrobial therapy.

Disparate mechanisms of antifolate resistance provoked by methotrexate and its metabolite 7-hydroxymethotrexate in leukemia cells: implications for efficacy of methotrexate therapy

AbstractMethotrexate (MTX) is one of the leading drugs in the treatment of leukemia, but extensive metabolism to 7-hydroxymethotrexate (7-OHMTX) can limit its therapeutic efficacy. In this study we investigated whether 7-OHMTX itself can provoke anti-folate resistance that may further disrupt MTX efficacy. For this purpose, we developed resistance to 7-OHMTX as well as MTX in 2 human leukemia cell lines (CCRF-CEM and MOLT-4) by stepwise exposure to increasing concentrations of 7-OHMTX and MTX. Consequently, both leukemia cell lines displayed marked levels of resistance to 7-OHMTX (> 10-fold) and MTX (> 75-fold). The underlying mechanism of resistance in the MTX-exposed cells was a marked decrease (> 10-fold) in reduced folate carrier (RFC)-mediated cellular uptake of MTX. This was associated with transcriptional silencing of the RFC gene in MTX-resistant CCRF-CEM cells. In contrast, the molecular basis for the resistance to 7-OHMTX was due solely to a marked decreased (> 95%) in folylpolyglutamate synthetase (FPGS) activity, which conferred more than 100-fold MTX resistance upon a short-term exposure to this drug. This is the first demonstration that 7-OHMTX can provoke distinct modalities of antifolate resistance compared with the parent drug MTX. The implications of this finding for MTX efficacy and strategies to circumvent MTX resistance are discussed.

Disparate Mechanisms of Antifolate Resistance Provoked by Methotrexate and Its Metabolite 7-Hydroxymethotrexate in Leukemia Cells: Implications for Efficacy of Methotrexate Therapy.

Methotrexate (MTX) is one of the leading drugs in the treatment of leukemia, but extensive metabolism to 7-hydroxymethotrexate (7-OHMTX) can limit its therapeutic efficacy. In this study we investigated whether 7-OHMTX itself can provoke antifolate resistance that may further disrupt MTX efficacy. For this purpose we developed resistance to 7-OHMTX as well as MTX in two human leukemia cell lines (CCRF-CEM and MOLT-4) by stepwise exposure to increasing concentrations of 7-OHMTX and MTX. Consequently, both leukemia cell lines displayed marked levels of resistance to 7-OHMTX (>10 fold) and MTX (>75 fold), respectively. The underlying mechanism of resistance in the MTX-exposed cells was a marked decrease (>10-fold) in reduced folate carrier (RFC)-mediated cellular uptake of MTX. This was associated with transcriptional silencing of the RFC gene in MTX-resistant CCRF-CEM cells. In contrast, the molecular basis for the resistance to 7-OHMTX was solely due to a marked decreased (> 95%) in folylpolyglutamate synthetase (FPGS) activity which conferred >100-fold MTX resistance upon a short term exposure to this drug. This is the first demonstration that 7-OHMTX can provoke distinct modalities of antifolate resistance as compared to the parent drug MTX.

Inactivation of the Leishmania tarentolae Pterin Transporter (BT1) and Reductase (PTR1) Genes Leads to Viable Parasites with Changes in Folate Metabolism and Hypersensitivity to the Antifolate Methotrexate

The protozoan parasite Leishmania is a folate and pterin auxotroph. The main biopterin transporter (BT1) and pterin reductase (PTR1) have already been characterized in Leishmania. In this study, we have succeeded in generating a BT1 and PTR1 null mutant in the same Leishmania tarentolae strain. These cells are viable with growth properties indistinguishable from wildtype cells. However, in response to the inactivation of BT1 and PTR1, at least one of the folate transporter genes was deleted, and the level of the folylpolyglutamate synthetase activity was increased, leading to increased polyglutamylation of both folate and methotrexate (MTX). Secondary events following gene inactivation should be considered when analyzing a phenotype in Leishmania. The BT1/PTR1 null mutant is hypersensitive to MTX, but in a step-by-step fashion, we could induce resistance to MTX in these cells. Several resistance mechanisms were found to co-exist including a reduced folate and MTX accumulation, demonstrating that cells with no measurable biopterin uptake but also greatly reduced folate uptake are viable, despite their auxotrophy for each of these substrates. The resistant cells have also amplified the gene coding for the MTX target dihydrofolate reductase. Finally, we found a marked reduction in MTX polyglutamylation in resistant cells. These studies further highlight the formidable ability of Leishmania cells to bypass the blockage of key metabolic pathways.

Induction of resistance to the multitargeted antifolate Pemetrexed (ALIMTA) in WiDr human colon cancer cells is associated with thymidylate synthase overexpression

Pemetrexed (ALIMTA, MTA) is a novel thymidylate synthase (TS) inhibitor and has shown activity against colon cancer, mesothelioma and nonsmall-cell lung cancer. We induced resistance to Pemetrexed in the human colon cancer cell line WiDr by using a continuous exposure to stepwise increasing Pemetrexed concentrations (up to 20 μM) as well as a more clinically relevant schedule with intermittent exposure (up to 50 μM) for 4 hr every 7 days, resulting in WiDr variants WiDr-cPEM and WiDr-4PEM, respectively. However, using the same conditions, it was not possible to induce resistance in the WiDr/F cell line, a variant adapted to growth under low folate conditions. Mechanisms of resistance to Pemetrexed were determined at the level of TS, folylpolyglutamate synthetase (FPGS) and reduced folate carrier (RFC). WiDr-4PEM and WiDr-cPEM showed cross-resistance to the polyglutamatable TS inhibitor Raltitrexed (6- and 19-fold, respectively) and the nonpolyglutamatable TS-inhibitor Thymitaq (6- and 42-fold, respectively) but not to 5-fluorouracil. The ratios of TS mRNA:β actin mRNA in WiDr-4PEM and WiDr-cPEM were 5-fold ( P=0.01) and 18-fold ( P=0.04) higher, respectively, compared to WiDr (ratio: 0.012). In addition, TS protein expression in the resistant WiDr variants was elevated 3-fold compared to WiDr, while the catalytic activity of TS with 1 μM dUMP increased from 30 pmol/hr/10 6 cells in WiDr cells to 2201 and 7663 pmol/hr/10 6 cells in WiDr-4PEM and WiDr-cPEM, respectively. The activity of FPGS was moderately decreased, but not significantly different in all WiDr variants. Finally, no evidence was found that decreased catalytic activity of RFC was responsible for the obtained Pemetrexed resistance. Altogether, these results indicate that resistance to Pemetrexed in the colon cancer cell line WiDr was solely due to upregulation of TS of which all related parameters (mRNA and protein expression and TS activity) were increased, rather than alterations in FPGS or RFC activity.

Characterization of the folylpolyglutamate synthetase gene and polyglutamylation of folates in the protozoan parasite Leishmania.

Folates are polyglutamylated in most organisms by the enzyme folylpolyglutamate synthetase (FPGS). The Leishmania tarentolae FPGS gene was isolated. Its predicted product contains 538 amino acids and shows 33 and 30% identity with the human and yeast FPGS proteins, respectively. The level of folate polygtutamylation was studied in L. tarentolae promastigotes and in Leishmania infantum promastigotes and axenic amastigotes. In all species examined, folates were found predominantly as pentaglutamates, although monoglutamates were found in higher proportion in L. infantum axenic amastigote cells. Leishmania cells transfected with a FPGS containing plasmid (FPGS transfectant) exhibited a 6-fold increase in FPGS activity (32.7 pmol mg(-1) h(-1)) compared with wild-type cells (4.7 pmol mg(-1) h(-1)). HPLC analysis of the polyglutamylated forms of folates indicated a 2-fold increase of hexaglutamates in the FPGS transfectant compared with wild-type cells, while cells with one FPGS allele interrupted showed a higher proportion of short chain glutamates. The long-term accumulation of folates was greatly increased in the FPGS transfectant. Overall, this work indicates that FPGS activity is expressed in all forms of the parasite, and modulates the retention of folate, thereby possibly playing an important role in physiology.

Multiple mechanisms of resistance to methotrexate and novel antifolates in human CCRF-CEM leukemia cells and their implications for folate homeostasis

We determined the mechanisms of resistance of human CCRF-CEM leukemia cells to methotrexate (MTX) vs. those to six novel antifolates: the polyglutamatable thymidylate synthase (TS) inhibitors ZD1694, multitargeted antifolate, pemetrexed, ALIMTA (MTA) and GW1843U89, the non-polyglutamatable inhibitors of TS, ZD9331, and dihydrofolate reductase, PT523, as well as DDATHF, a polyglutamatable glycinamide ribonucleotide transformylase inhibitor. CEM cells were made resistant to these drugs by clinically relevant intermittent 24 hr exposures to 5–10 μM of MTX, ZD1694, GW1843U89, MTA and DDATHF, by intermittent 72 hr exposures to 5 μM of ZD9331 and by continuous exposure to stepwise increasing concentrations of ZD9331, GW1843U89 and PT523. Development of resistance required only 3 cycles of intermittent drug exposure to ZD1694 and MTA, but 5 cycles for MTX, DDATHF and GW1843U89 and 8 cycles for ZD9331. The predominant mechanism of resistance to ZD1694, MTA, MTX and DDATHF was impaired polyglutamylation due to ∼10-fold decreased folylpolyglutamate synthetase activity. Resistance to intermittent exposures to GW1843U89 and ZD9331 was associated with a 2-fold decreased transport via the reduced folate carrier (RFC). The CEM cell lines resistant to intermittent exposures to MTX, ZD1694, MTA, DDATHF, GW1843U89 and ZD9331 displayed a depletion (up to 4-fold) of total intracellular reduced folate pools. Resistance to continuous exposure to ZD9331 was caused by a 14-fold increase in TS activity. CEM/GW70, selected by continuous exposure to GW1843U89 was 50-fold resistant to GW1843U89, whereas continuous exposure to PT523 generated CEM/PT523 cells that were highly resistant (1550-fold) to PT523. Both CEM/GW70 and CEM/PT523 displayed cross-resistance to several antifolates that depend on the RFC for cellular uptake, including MTX (95- and 530-fold). CEM/GW70 cells were characterized by a 12-fold decreased transport of [ 3 H ]MTX. Interestingly, however, CEM/GW70 cells displayed an enhanced transport of folic acid, consistent with the expression of a structurally altered RFC resulting in a 2.6-fold increase of intracellular folate pools. CEM/PT523 cells displayed a markedly impaired (100-fold) transport of [ 3 H ]MTX along with 12-fold decreased total folate pools. In conclusion, multifunctional mechanisms of resistance in CEM cells have a differential impact on cellular folate homeostasis: decreased polyglutamylation and transport defects lead to folate depletion, whereas a structurally altered RFC protein can provoke expanded intracellular folate pools.