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

Methotrexate Polyglutamates Analysis by Chromatography Methods in Biological Matrices: A Review

Methotrexate (MTX) is used as an immunosuppressant and antineoplastic drug in clinical practice. MTX is a parent drug and converts to MTX polyglutamates (MTXPGs) to exhibit its biological activity. Clinical studies found that MTXPG levels were associated with MTX response and toxicities, especially at low doses. Due to huge variance of MTX response and toxicities between individuals, therapeutic drug monitoring is necessary for its use in individualized therapy. Various chromatography methods coupled with ultraviolet-visible detector, fluorescence detector and mass spectrometry have been reported for MTXPG analysis in various biological matrices. The aim of this paper is to review the chromatographic based methods for the measurement of total and/or individual MTXPGs. We searched Embase, Science Direct and PubMed databases using "methotrexate polyglutamate" and "chromatography" as search terms, and found 745 articles. Of those, 14 articles were extracted for this study. The key steps for method development (sample pretreatment, parameter optimization of liquid chromatography and mass spectrometry, selection of internal standard) and validation (lower limit of quantitation, accuracy, precision, recovery, matrix effect and stability) were analyzed and summarized, which might be helpful for researchers to develop their own methods.

Inhibition of the NEDD8 conjugation pathway induces calcium-dependent compensatory activation of the pro-survival MEK/ERK pathway in acute lymphoblastic leukemia

De novo and acquired drug resistance and subsequent relapse remain major challenges in acute lymphoblastic leukemia (ALL). We previously identified that pevonedistat (TAK-924, MLN4924), a first-in-class inhibitor of NEDD8 activating enzyme (NAE), elicits ER stress and has potent in vitro and in vivo efficacy against ALL. However, in pevonedistat-treated ALL cell lines, we found consistent activation of the pro-survival MEK/ERK pathway, which has been associated with relapse and poor outcome in ALL. We uncovered that inhibition of the MEK/ERK pathway in vitro and in vivo sensitized ALL cells to pevonedistat. The observed synergistic apoptotic effect appears to be mediated by inhibition of the MEK/ERK pro-survival cascade leading to de-repression of the pro-apoptotic BIM protein. Mechanistically, Ca²⁺ influx via the Ca²⁺-release-activated Ca²⁺ (CRAC) channel induced protein kinase C β2 (PKC-β2) was responsible for activation of the MEK/ERK pathway in pevonedistat-treated ALL cells. Sequestration of Ca²⁺ using BAPTA-AM or blockage of store-operated Ca²⁺ entry (SOCE) using BTP-2 both attenuated the compensatory activation of MEK/ERK signaling in pevonedistat-treated ALL cells. Pevonedistat significantly altered the expression of Orai1 and stromal interaction molecule 1 (STIM1), resulting in significantly decreased STIM1 protein levels relative to Orai1. Further, we identified eIF2α as an important post-transcriptional regulator of STIM1, suggesting that pevonedistat-induced eIF2α de-phosphorylation selectively down-regulates translation of STIM1 mRNA. Consequently, our data suggest that pevonedistat potentially activates SOCE and promotes Ca²⁺ influx leading to activation of the MEK/ERK pathway by altering the stoichiometric Orai1:STIM1 ratio and inducing ER stress in ALL cells.

Effect of the Polymorphism of Folylpolyglutamate Synthetase on Treatment of High-Dose Methotrexate in Pediatric Patients with Acute Lymphocytic Leukemia

BACKGROUND The aim of this study was to investigate the association of the polymorphism of folylpolyglutamate synthetase (FPGS) with the dynamic plasma concentration of methotrexate (MTX) in pediatric patients with acute lymphocytic leukemia (ALL), as well as the prognosis. MATERIAL AND METHODS 57 ALL patients and 31 age and sex-matched children (control) were included in this study. Polymerase chain reaction-restriction fragment length polymorphism was performed for the analysis of the genotype of FPGS rs1544105 and high-performance liquid chromatography for measurement of MTX plasma concentration after 24-h and 44-h treatment. Overall survival was analyzed by Kaplan-Meier method. RESULTS No differences were observed between patients and controls regarding the distribution frequency of genotype and alleles of rs1544105. Patients carrying AA genotype had a significantly higher plasma concentration of MTX after 24 h than those carrying GG or GA (P<0.05) and no differences were found after 44 h. Kaplan-Meier survival analysis showed a longer median survival time in patients with AA than other genotypes with significant difference in overall survival. CONCLUSIONS Polymorphism of FPGS rs1544105 might be used as an effective approach for prediction of the treatment outcome of MTX.

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.

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.

Binding of a Smad4/Ets-1 complex to a novel intragenic regulatory element in exon12 of FPGS underlies decreased gene expression and antifolate resistance in leukemia

Polyglutamylation of antifolates catalyzed by folylpoly-γ-glutamate synthetase (FPGS) is essential for their intracellular retention and cytotoxic activity. Hence, loss of FPGS expression and/or function results in lack of antifolate polyglutamylation and drug resistance. Members of the TGF-β/Smad signaling pathway are negative regulators of hematopoiesis and deregulation of this pathway is considered a major contributor to leukemogenesis. Here we show that FPGS gene expression is inversely correlated with the binding of a Smad4/Ets-1 complex to exon12 of FPGS in both acute lymphoblastic leukemia cells and acute myeloid leukemia blast specimens. We demonstrate that antifolate resistant leukemia cells harbor a heterozygous point mutation in exon12 of FPGS which disrupts FPGS activity by abolishing ATP binding, and alters the binding pattern of transcription factors to the genomic region of exon12. This in turn results in the near complete silencing of the wild type allele leading to a 97% loss of FPGS activity. We show that exon12 is a novel intragenic transcriptional regulator, endowed with the ability to drive transcription in vitro, and is occupied by transcription factors and chromatin remodeling agents (e.g. Smad4/Ets-1, HP-1 and Brg1) in vivo. These findings bear important implications for the rational overcoming of antifolate resistance in leukemia.

Variegated clonality and rapid emergence of new molecular lesions in xenografts of acute lymphoblastic leukemia are associated with drug resistance

The use of genome-wide copy-number analysis and massive parallel sequencing has revolutionized the understanding of the clonal architecture of pediatric acute lymphoblastic leukemia (ALL) by demonstrating that this disease is composed of highly variable clonal ancestries following the rules of Darwinian selection. The current study aimed to analyze the molecular composition of childhood ALL biopsies and patient-derived xenografts with particular emphasis on mechanisms associated with acquired chemoresistance. Genomic DNA from seven primary pediatric ALL patient samples, 29 serially passaged xenografts, and six in vivo selected chemoresistant xenografts were analyzed with 250K single-nucleotide polymorphism arrays. Copy-number analysis of non-drug-selected xenografts confirmed a highly variable molecular pattern of variegated subclones. Whereas primary patient samples from initial diagnosis displayed a mean of 5.7 copy-number alterations per sample, serially passaged xenografts contained a mean of 8.2 and chemoresistant xenografts a mean of 10.5 copy-number alterations per sample, respectively. Resistance to cytarabine was explained by a new homozygous deletion of the DCK gene, whereas methotrexate resistance was associated with monoallelic deletion of FPGS and mutation of the remaining allele. This study demonstrates that selecting for chemoresistance in xenografted human ALL cells can reveal novel mechanisms associated with drug resistance.

[The mbr-FPGS efficient expression plasmid enhances the sensitivity of Jurkat cells to methotrexate]

Folylpolyglutamate synthetase (FPGS) is the key enzyme that converts chemotherapy drug Methotrexate (MTX) into MTXPG. The expression level of FPGS directly influences MTX-sensitivity of tumor cells. Compared with B-cell acute lymphocytic leukemia (B-ALL), T-cell acute lymphocytic leukemia (T-ALL) cells express a lower level of FPGS, which results in insensitivity of the cells to MTX. Our previous work has demonstrated that 279 bp mbr element located within the 3'-UTR of the BCL2 gene possesses enhancer function. In this study, FPGS expression plasmid containing mbr element at the 5' upstream of the gene was constructed and transfected into Jurkat cells to sensitize the cells to MTX. Western blotting and MTT assay were applied to detect the FPGS expression level and suppression rate of the cells treated by MTX, respectively. We found that the mbr enhanced the expression of FPGS significantly and increased sensitivity of Jurkat cells to MTX efficiently, while FPGS expression plasmid without mbr element had less effect. Our data provides a new clue for the clinical application of mbr regulatory element and may contribute to improvement of MTX treatment in T-ALL.

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

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

Distinct mechanistic activity profile of pralatrexate in comparison to other antifolates in in vitro and in vivo models of human cancers

PURPOSE

This study evaluated mechanistic differences of pralatrexate, methotrexate, and pemetrexed.

METHODS

Inhibition of dihydrofolate reductase (DHFR) was quantified using recombinant human DHFR. Cellular uptake and folylpolyglutamate synthetase (FPGS) activity were determined using radiolabeled pralatrexate, methotrexate, and pemetrexed in NCI-H460 non-small cell lung cancer (NSCLC) cells. The tumor growth inhibition (TGI) was assessed using MV522 and NCI-H460 human NSCLC xenografts.

RESULTS

Apparent K ( i ) values for DHFR inhibition were 45, 26, and >200 nM for pralatrexate, methotrexate, and pemetrexed, respectively. A significantly greater percentage of radiolabeled pralatrexate entered the cells and was polyglutamylatated relative to methotrexate or pemetrexed. In vivo, pralatrexate showed superior anti-tumor activity in both NSCLC models, with more effective dose-dependent TGI in the more rapidly growing NCI-H460 xenografts.

CONCLUSIONS

Pralatrexate demonstrated a distinct mechanistic and anti-tumor activity profile relative to methotrexate and pemetrexed. Pralatrexate exhibited enhanced cellular uptake and increased polyglutamylation, which correlated with increased TGI in NSCLC xenograft models.

Aberrant splicing of folylpolyglutamate synthetase as a novel mechanism of antifolate resistance in leukemia

Folylpoly-γ-gluatamate synthetase (FPGS) catalyzes the polyglutamylation and thus intracellular retention of folates and antifolates (eg, methotrexate; MTX) through the addition of multiple glutamate equivalents to their γ-carboxyl residue. Since polyglutamylation of antifolates is crucial for their pharmacological activity in leukemia, loss of FPGS function results in decreased cellular levels of polyglutamylation-dependent antifolates and consequent drug resistance. Whereas resistance to pulse exposure to antifolates is frequently associated with loss of FPGS activity, the underlying molecular mechanism remains elusive. Here we explored the molecular basis of antifolate resistance in human MTX-resistant leukemia cell lines displaying marked loss of FPGS activity. We demonstrate that these MTX-resistant cells exhibit impaired splicing of FPGS mRNA based on intron retention and/or exon skipping, thereby resulting in loss of FPGS function due to premature translation termination. Furthermore, analysis of FPGS transcripts in blood or bone marrow specimens from patients with acute lymphoblastic leukemia revealed exon 12 skipping, both at diagnosis and at relapse, the latter of which occurs after high-dose MTX-containing chemotherapy. These results constitute the first demonstration of the loss of FPGS function via aberrant mRNA splicing, thereby resulting in loss of antifolate retention and drug resistance. The clinical ramifications of these novel findings are discussed.