Gene amplification and increased expression of the reduced folate carrier in transport elevated K562 cells

Functional loss of the reduced folate carrier enhances the antitumor activities of novel antifolates with selective uptake by the proton-coupled folate transporter

Uptake of 6-substituted pyrrolo[2,3-d]pyrimidine thienoyl antifolates with four or three bridge carbons [compound 1 (C1) and compound 2 (C2), respectively] into solid tumors by the proton-coupled folate transporter (PCFT) represents a novel therapeutic strategy that harnesses the acidic tumor microenvironment. Although these compounds are not substrates for the reduced folate carrier (RFC), the major facilitative folate transporter, RFC expression may alter drug efficacies by affecting cellular tetrahydrofolate (THF) cofactor pools that can compete for polyglutamylation and/or binding to intracellular enzyme targets. Human tumor cells including wild-type (WT) and R5 (RFC-null) HeLa cells express high levels of PCFT protein. C1 and C2 inhibited proliferation of R5 cells 3 to 4 times more potently than WT cells or R5 cells transfected with RFC. Transport of C1 and C2 was virtually identical between WT and R5 cells, establishing that differences in drug sensitivities between sublines were independent of PCFT transport. Steady-state intracellular [³H]THF cofactors derived from [³H]5-formyl-THF were depleted in R5 cells compared with those in WT cells, an effect exacerbated by C1 and C2. Whereas C1 and C2 polyglutamates accumulated to similar levels in WT and R5 cells, there were differences in polyglutamyl distributions in favor of the longest chain length forms. In severe combined immunodeficient mice, the antitumor efficacies of C1 and C2 were greater toward subcutaneous R5 tumors than toward WT tumors, confirming the collateral drug sensitivities observed in vitro. Thus, solid tumor-targeted antifolates with PCFT-selective cellular uptake should have enhanced activities toward tumors lacking RFC function, reflecting contraction of THF cofactor pools.

Molecular basis of antifolate resistance

Folates play a key role in one-carbon metabolism essential for the biosynthesis of purines, thymidylate and hence DNA replication. The antifolate methotrexate has been rationally-designed nearly 60 years ago to potently block the folate-dependent enzyme dihydrofolate reductase (DHFR) thereby achieving temporary remissions in childhood acute leukemia. Recently, the novel antifolates raltitrexed and pemetrexed that target thymidylate synthase (TS) and glycineamide ribonucleotide transformylase (GARTF) were introduced for the treatment of colorectal cancer and malignant pleural mesothelioma. (Anti)folates are divalent anions which predominantly use the reduced folate carrier (RFC) for their cellular uptake. (Anti)folates are retained intracellularly via polyglutamylation catalyzed by folylpoly-gamma-glutamate synthetase (FPGS). As the intracellular concentration of antifolates is critical for their pharmacologic activity, polyglutamylation is a key determinant of antifolate cytotoxicity. However, anticancer drug resistance phenomena pose major obstacles towards curative cancer chemotherapy. Pre-clinical and clinical studies have identified a plethora of mechanisms of antifolate-resistance; these are frequently associated with qualitative and/or quantitative alterations in influx and/or efflux transporters of (anti)folates as well as in folate-dependent enzymes. These include inactivating mutations and/or down-regulation of the RFC and various alterations in the target enzymes DHFR, TS and FPGS. Furthermore, it has been recently shown that members of the ATP-binding cassette (ABC) superfamily including multidrug resistance proteins (MRP/ABCC) and breast cancer resistance protein (BCRP/ABCG2) are low affinity, high capacity ATP-driven (anti)folate efflux transporters. This transport activity is in addition to their established facility to extrude multiple cytotoxic agents. Hence, by actively extruding antifolates, overexpressed MRPs and/or BCRP confer antifolate resistance. Moreover, down-regulation of MRPs and/or BCRP results in decreased folate efflux thereby leading to expansion of the intracellular folate pool and antifolate resistance. This chapter reviews and discusses the panoply of molecular modalities of antifolate-resistance in pre-clinical tumor cell systems in vitro and in vivo as well as in cancer patients. Currently emerging novel strategies for the overcoming of antifolate-resistance are presented. Finally, experimental evidence is provided that the identification and characterization of the molecular mechanisms of antifolate-resistance may prove instrumental in the future development of rationally-based novel antifolates and strategies that could conceivably overcome drug-resistance phenomena.

Expression of resistance markers to methotrexate predicts clinical improvement in patients with rheumatoid arthritis

BACKGROUND

Methotrexate is transported into the cell by the reduced folate carrier (RFC) and out of the cell by members of the multidrug resistance protein family (MRP). Transport proteins may affect the therapeutic efficacy of this drug in patients with rheumatoid arthritis.

OBJECTIVE

To investigate the potential benefit of the presence of RFC and the absence of functional MRP for the efficacy of methotrexate treatment.

METHODS

The study involved 163 patients (116 female, 47 male; mean age 59.5 years) on methotrexate (mean weekly dose 12.2 mg). RFC was determined using reverse transcriptase polymerase chain reaction, and MRP function by flow cytometry, using a calcein acetoxymethylesther/probenecid assay. Clinical response to methotrexate was evaluated by the EULAR response criteria and the ACR 20% improvement criteria. The clinical data were obtained at the beginning of methotrexate treatment and at the time of blood sampling during ongoing therapy. Patients were divided into four groups according to the presence (+) or absence (-) of RFC and functional (f) MRP.

RESULTS

fMRP+/RFC+ and fMRP-/RFC- patients more often had good EULAR response rates (60%, p = 0.014, and 53%, p = 0.035, respectively) in comparison with the fMRP-/RFC+ group (29%); fMRP+/RFC- patients had a low frequency of good disease activity responses.

CONCLUSIONS

Absence of fMRP plus presence of RFC did not prove to be related to beneficial effects of methotrexate, but the lack or the presence of both fMRP and RFC led to a significantly better therapeutic outcome. Determination of these markers may predict responsiveness to methotrexate.

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.

Relationship of p15 and p16 gene alterations to elevated dihydrofolate reductase in childhood acute lymphoblastic leukaemia

The downstream effects of p15 and p16 gene deletions and loss of transcripts on dihydrofolate reductase (DHFR) were examined in 63 B-precursor (BP) acute lymphoblastic leukaemia (ALL) samples. p15 and/or p16 gene deletions were seen in 6% and 8%, respectively, of BP-ALL samples; however, losses of p15 and/or p16 transcripts were seen in 26 out of 63 (41%) samples. Loss of p15 transcripts (36.5%) exceeded that for p16 (17.5%). For the 26 BP-ALLs that lacked p15 and/or p16 transcripts, only six (23%) exhibited low levels of DHFR by flow cytometry assay with Pt430, a fluorescent anti-folate. Conversely, 18 out of 37 (49%) BP-ALL samples with intact p15 and/or p16 genes and transcripts showed low levels of DHFR (P = 0.04). In p15- and p16-null K562 cells transfected with a tetracycline-inducible p15 cDNA construct, induction of p15 transcripts and protein was accompanied by decreased growth rates, decreased S-phase fraction, decreased retinoblastoma protein phosphorylation, and markedly reduced levels of DHFR transcripts and protein. Collectively, our results suggest that losses of p15 and/or p16 gene expression result in elevated levels of DHFR in BP-ALL in children. However, additional downstream factors undoubtedly also contribute to elevated levels of this enzyme target.

Methylenetetrahydrofolate reductase (MTHFR) polymorphisms and risk of molecularly defined subtypes of childhood acute leukemia

Low folate intake as well as alterations in folate metabolism as a result of polymorphisms in the enzyme methylenetetrahydrofolate reductase (MTHFR) have been associated with an increased incidence of neural tube defects, vascular disease, and some cancers. Polymorphic variants of MTHFR lead to enhanced thymidine pools and better quality DNA synthesis that could afford some protection from the development of leukemias, particularly those with translocations. We now report associations of MTHFR polymorphisms in three subgroups of pediatric leukemias: infant lymphoblastic or myeloblastic leukemias with MLL rearrangements and childhood lymphoblastic leukemias with either TEL-AML1 fusions or hyperdiploid karyotypes. Pediatric leukemia patients (n = 253 total) and healthy newborn controls (n = 200) were genotyped for MTHFR polymorphisms at nucleotides 677 (C-->T) and 1,298 (A-->C). A significant association for carriers of C677T was demonstrated for leukemias with MLL translocations (MLL+, n = 37) when compared with controls [adjusted odd ratios (OR) = 0.36 with a 95% confidence interval (CI) of 0.15-0.85; P = 0.017]. This protective effect was not evident for A1298C alleles (OR = 1.14). In contrast, associations for A1298C homozygotes (CC; OR = 0.26 with a 95% CI of 0.07--0.81) and C677T homozygotes (TT; OR = 0.49 with a 95% CI of 0.20--1.17) were observed for hyperdiploid leukemias (n = 138). No significant associations were evident for either polymorphism with TEL-AML1+ leukemias (n = 78). These differences in allelic associations may point to discrete attributes of the two alleles in their ability to alter folate and one-carbon metabolite pools and impact after DNA synthesis and methylation pathways, but should be viewed cautiously pending larger follow-up studies. The data provide evidence that molecularly defined subgroups of pediatric leukemias have different etiologies and also suggest a role of folate in the development of childhood leukemia.

Carrier-mediated membrane transport of folates in mammalian cells

Mediated internalization of folates is required for cellular macromolecular biosynthesis. Multiple carrier-mediated mechanisms have been identified that can fulfill this role in a variety of mammalian cell types, including neoplastic cells, with and without proliferative potential. The absorption of dietary folates also relies on the function of a carrier-mediated system in mature luminal epithelium of small intestine. The various carrier-mediated systems can be distinguished by their preferences for various folate compounds as permeants as well as by differences in temperature and pH dependence. The widely studied one-carbon, reduced-folate transport system is mediated by a transporter encoded by the newly discovered RFC-1 (reduced-folate carrier) gene. The characteristics of this gene in rodent and human cells are similar, consistent with the close similarity between these species of folate transport mediated by this transporter. However, differences occur in the form of tissue-specific expression, alternate splicing, and 5' end mRNA heterogeneity, as well as in promoter utilization regulating transcription. RFC-1 gene expression also appears to regulate luminal epithelial cell folate absorption in small intestine. However, the properties of RFC-1-mediated folate transport in these cells is anomalous when compared with that seen in nonabsorptive cell types. Detailed mechanisms as to the regulation of RFC-1 transcription are now emerging along with other information on structure and function of the transporter and its alteration following mutation.

Impaired membrane transport in methotrexate-resistant CCRF-CEM cells involves early translation termination and increased turnover of a mutant reduced folate carrier

The basis for impaired reduced folate carrier (RFC) activity in methotrexate-resistant CCRF-CEM (CEM/Mtx-1) cells was examined. Parental and CEM/Mtx-1 cells expressed identical levels of the 3. 1-kilobase RFC transcript. A approximately 85-kDa RFC protein was detected in parental cells by photoaffinity labeling and on Western blots with RFC-specific antiserum. In CEM/Mtx-1 cells, RFC protein was undetectable. By reverse transcriptase-polymerase chain reaction and sequence analysis, G to A point mutations were identified in CEM/Mtx-1 transcripts at positions 130 (P1; changes glycine 44 --> arginine) and 380 (P2; changes serine 127 --> asparagine). A 4-base pair (CATG) insertion detected at position 191 (in 19-30% of cDNA clones) resulted in a frameshift and early translation termination. Wild-type RFC was also detected (0-9% of clones). Wild-type RFC and double-mutated RFC (RFCP1+P2) cDNAs were transfected into transport-impaired K562 and Chinese hamster ovary cells. Although RFC transcripts paralleled wild-type protein, for the RFCP1+P2 transfectants, disproportionately low RFCP1+P2 protein was detected. This reflected an increased turnover of RFCP1+P2 over wild-type RFC. RFCP1+P2 did not restore methotrexate transport; however, uptake was partially restored by constructs with single mutations at the P1 or P2 loci. Cumulatively, our results show that loss of transport function in CEM/Mtx-1 cells results from complete loss of RFC protein due to early translation termination and increased turnover of a mutant RFC protein.

A structurally altered human reduced folate carrier with increased folic acid transport mediates a novel mechanism of antifolate resistance

CEM/MTX is a subline of human CCRF-CEM leukemia cells which displays >200-fold resistance to methotrexate (MTX) due to defective transport via the reduced folate carrier (RFC). CEM/MTX-low folate (LF) cells, derived by a gradual deprivation of folic acid from 2.3 microM to 2 nM (LF) in the cell culture medium of CEM/MTX cells, resulted in a >20-fold overexpression of a structurally altered RFC featuring; 1) a wild type Km value for MTX transport but a 31-fold and 9-fold lower Km values for folic acid and leucovorin, respectively, relative to wild type RFC; 2) a 10-fold RFC1 gene amplification along with a >20-fold increased expression of the main 3.1-kilobase RFC1 mRNA; 3) a marked stimulation of MTX transport by anions (i.e. chloride); and 4) a G --> A mutation at nucleotide 227 of the RFC cDNA in both CEM/MTX-LF and CEM/MTX, resulting in a lysine for glutamate substitution at amino acid residue 45 predicted to reside within the first transmembrane domain of the human RFC. Upon transfer of CEM/MTX-LF cells to folate-replete medium (2.3 microM folic acid), the more efficient folic acid uptake in CEM/MTX-LF cells resulted in a 7- and 24-fold elevated total folate pool compared with CEM and CEM/MTX cells, respectively (500 versus 69 and 21 pmol/mg of protein, respectively). This markedly elevated intracellular folate pool conferred a novel mechanism of resistance to polyglutamatable (e.g. ZD1694, DDATHF, and AG2034) and lipophilic antifolates (e.g. trimetrexate and pyrimethamine) by abolishing their polyglutamylation and circumventing target enzyme inhibition.

Structure and organization of the human reduced folate carrier gene

The human reduced folate carrier gene was found to contain 7 exons, including two alternative non-coding exons (exons 1 and 2), spanning approximately 29 kb. Two transcript variants involving exon 7 were detected in K562 cells by RT-PCR, distinguishable from the wild-type transcript by deletions of 625 bp (KS32) and 988 bp (KS1). The presence of consensus splice donor and acceptor elements in the deleted KS1 isoform suggested that this form was likely a splice variant; however, KS32 likely arose during reverse transcription rather than by alternative splicing.

Effects of the loss of capacity for N-glycosylation on the transport activity and cellular localization of the human reduced folate carrier

The role of N-glycosylation in reduced folate carrier (RFC) transport and membrane targeting was examined in transport-deficient K562 (K500E) cells transfected with human RFC cDNAs. Treatment of cells expressing wild-type RFC with tunicamycin (0-3 microg) resulted in a progressive shift of the approximately 85 kDa RFC on western blots to 65 kDa. At 3 microg/ml tunicamycin, the nearly complete loss of glycosylated RFC was accompanied by a approximately 25% decreased rate of methotrexate uptake. A deglycosylated RFC cDNA construct in which asparagine-58 was replaced by glutamine (Gln58-RFC) was expressed in K500E cells as a 65 kDa protein and restored transport capacity for methotrexate and (6S)5-formyl tetrahydrofolate. With both wild-type and Gln58-RFC constructs, expression of cDNA-encoded RFC protein far exceeded relative levels of RFC uptake. Wild-type and Gln58-RFCs containing a hemagglutinin (HA) epitope at the carboxyl terminus were similarly functional and, by immunofluorescence staining with rhodamine-conjugated anti-HA antibody, were localized to plasma membranes. Collectively, our results demonstrate that N-glycosylation of human RFC plays no significant role in either transport function or membrane targeting. The discrepancy between the stoichiometries of RFC expression and transport activity for both wild-type RFC and Gln58-RFC implies that identical regulatory controls and/or non-RFC transport components are necessary to completely restore transport function in the transfected cells.