Methotrexate recognition by the human reduced folate carrier SLC19A1

Folates are essential nutrients with important roles as cofactors in one-carbon transfer reactions, being heavily utilized in the synthesis of nucleic acids and the metabolism of amino acids during cell division^1,2. Mammals lack de novo folate synthesis pathways and thus rely on folate uptake from the extracellular milieu³. The human reduced folate carrier (hRFC, also known as SLC19A1) is the major importer of folates into the cell^1,3, as well as chemotherapeutic agents such as methotrexate^4-6. As an anion exchanger, RFC couples the import of folates and antifolates to anion export across the cell membrane and it is a major determinant in methotrexate (antifolate) sensitivity, as genetic variants and its depletion result in drug resistance^4-8. Despite its importance, the molecular basis of substrate specificity by hRFC remains unclear. Here we present cryo-electron microscopy structures of hRFC in the apo state and captured in complex with methotrexate. Combined with molecular dynamics simulations and functional experiments, our study uncovers key determinants of hRFC transport selectivity among folates and antifolate drugs while shedding light on important features of anion recognition by hRFC.

Characterization of newly established Pralatrexate-resistant cell lines and the mechanisms of resistance

Background

Pralatrexate (PDX) is a novel antifolate approved for the treatment of patients with relapsed/refractory peripheral T-cell lymphoma, but some patients exhibit intrinsic resistance or develop acquired resistance. Here, we evaluated the mechanisms underlying acquired resistance to PDX and explored potential therapeutic strategies to overcome PDX resistance.

Methods

To investigate PDX resistance, we established two PDX-resistant T-lymphoblastic leukemia cell lines (CEM and MOLT4) through continuous exposure to increasing doses of PDX. The resistance mechanisms were evaluated by measuring PDX uptake, apoptosis induction and folate metabolism-related protein expression. We also applied gene expression analysis and methylation profiling to identify the mechanisms of resistance. We then explored rational drug combinations using a spheroid (3D)-culture assay.

Results

Compared with their parental cells, PDX-resistant cells exhibited a 30-fold increase in half-maximal inhibitory concentration values. Induction of apoptosis by PDX was significantly decreased in both PDX-resistant cell lines. Intracellular uptake of [¹⁴C]-PDX decreased in PDX-resistant CEM cells but not in PDX-resistant MOLT4 cells. There was no significant change in expression of dihydrofolate reductase (DHFR) or folylpolyglutamate synthetase (FPGS). Gene expression array analysis revealed that DNA-methyltransferase 3β (DNMT3B) expression was significantly elevated in both cell lines. Gene set enrichment analysis revealed that adipogenesis and mTORC1 signaling pathways were commonly upregulated in both resistant cell lines. Moreover, CpG island hypermethylation was observed in both PDX resistant cells lines. In the 3D-culture assay, decitabine (DAC) plus PDX showed synergistic effects in PDX-resistant cell lines compared with parental lines.

Conclusions

The resistance mechanisms of PDX were associated with reduced cellular uptake of PDX and/or overexpression of DNMT3B. Epigenetic alterations were also considered to play a role in the resistance mechanism. The combination of DAC and PDX exhibited synergistic activity, and thus, this approach might improve the clinical efficacy of PDX.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-08607-9.

Pharmacogenomic Markers of Methotrexate Response in the Consolidation Phase of Pediatric Acute Lymphoblastic Leukemia Treatment

Methotrexate (MTX) is one of the staples of pediatric acute lymphoblastic leukemia (ALL) treatment. MTX targets the folate metabolic pathway (FMP). Abnormal function of the enzymes in FMP, due to genetic aberrations, leads to adverse drug reactions. The aim of this study was to investigate variants in pharmacogenes involved in FMP and their association with MTX pharmacokinetics (MTX elimination profile) and toxicity in the consolidation therapy phase of pediatric ALL patients. Eleven variants in the thymidylate synthetase (TYMS), methylenetetrahydrofolate reductase (MTHFR), dihydrofolate reductase (DHFR), SLC19A1 and SLCO1B genes were analyzed in 148 patients, using PCR- and sequencing-based methodology. For the Serbian and European control groups, data on allele frequency distribution were extracted from in-house and public databases. Our results show that the A allele of SLC19A1 c.80 variant contributes to slow MTX elimination. Additionally, the AA genotype of the same variant is a predictor of MTX-related hepatotoxicity. Patients homozygous for TYMS 6bp deletion were more likely to experience gastrointestinal toxicity. No allele frequency dissimilarity was found for the analyzed variants between Serbian and European populations. Statistical modelling did not show a joint effect of analyzed variants. Our results indicate that SLC19A1 c.80 variant and TYMS 6bp deletion are the most promising pharmacogenomic markers of MTX response in pediatric ALL patients.

The Promise of Pharmacogenomics in Reducing Toxicity During Acute Lymphoblastic Leukemia Maintenance Treatment

Pediatric acute lymphoblastic leukemia (ALL) affects a substantial number of children every year and requires a long and rigorous course of chemotherapy treatments in three stages, with the longest phase, the maintenance phase, lasting 2–3 years. While the primary drugs used in the maintenance phase, 6-mercaptopurine (6-MP) and methotrexate (MTX), are necessary for decreasing risk of relapse, they also have potentially serious toxicities, including myelosuppression, which may be life-threatening, and gastrointestinal toxicity. For both drugs, pharmacogenomic factors have been identified that could explain a large amount of the variance in toxicity between patients, and may serve as effective predictors of toxicity during the maintenance phase of ALL treatment. 6-MP toxicity is associated with polymorphisms in the genes encoding thiopurine methyltransferase (TPMT), nudix hydrolase 15 (NUDT15), and potentially inosine triphosphatase (ITPA), which vary between ethnic groups. Moreover, MTX toxicity is associated with polymorphisms in genes encoding solute carrier organic anion transporter family member 1B1 (SLCO1B1) and dihydrofolate reductase (DHFR). Additional polymorphisms potentially associated with toxicities for MTX have also been identified, including those in the genes encoding solute carrier family 19 member 1 (SLC19A1) and thymidylate synthetase (TYMS), but their contributions have not yet been well quantified. It is clear that pharmacogenomics should be incorporated as a dosage-calibrating tool in pediatric ALL treatment in order to predict and minimize the occurrence of serious toxicities for these patients.

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.

Transmembrane domains 4, 5, 7, 8, and 10 of the human reduced folate carrier are important structural or functional components of the transmembrane channel for folate substrates

The human reduced folate carrier (hRFC) facilitates membrane transport of folates and antifolates. hRFC is characterized by 12 transmembrane domains (TMDs). To identify residues or domains involved in folate binding, we used substituted cysteine (Cys) accessibility methods (SCAM) with sodium (2-sulfonatoethyl)methanethiosulfonate (MTSES). We previously showed that residues in TMD11 of hRFC were involved in substrate binding, whereas those in TMD12 were not (Hou, Z., Stapels, S. E., Haska, C. L., and Matherly, L. H. (2005) J. Biol. Chem. 280, 36206-36213). In this study, 232 Cys-substituted mutants spanning TMDs 1-10 and conserved stretches within the TMD6-7 (residues 204-217) and TMD10-11 connecting loop domains were transiently expressed in hRFC-null HeLa cells. All Cys-substituted mutants showed moderate to high levels of expression on Western blots, and only nine mutants including R133C, I134C, A135C, Y136C, S138C, G163C, Y281C, R373C, and S313C were inactive for methotrexate transport. MTSES did not inhibit transport by any of the mutants in TMDs 1, 3, 6, and 9 or for positions 204-217. Whereas most of the mutants in TMDs 2, 4, 5, 7, 8, and 10, and in the TMD10-11 connecting loop were insensitive to MTSES, this reagent inhibited methotrexate transport (25-75%) by 26 mutants in these TMDs. For 13 of these (Y126C, S137C, V160C, S168C, W274C, S278C, V284C, V288C, A311C, T314C, Y376C, Q377C, and V380C), inhibition was prevented by leucovorin, another hRFC substrate. Combined with our previous findings, these results implicate amino acids in TMDs 4, 5, 7, 8, 10, and 11, but not in TMDs 1, 2, 3, 6, 9, or 12, as important structural or functional components of the putative hydrophilic cavity for binding of anionic folate substrates.

Proteomic analysis for the identification of proteins related to methotrexate resistance

Methotrexate(MTX) is one of the most important and frequently used drugs in cancer therapy, but the efficacy of this drug is often compromised by the development of resistance in cancer cells. To seek and identify differentially expressed proteins related to MTX resistance and provide clues for the mechanism of MTX resistance, proteins from cell line MTX300 (resistant to 300 micromol/L MTX) and its control cell line 3T3R500 were separated by two-dimensional electrophoresis (2-DE). The colloidal Coomassie brilliant blue-stained 2-DE gels were subjected to image analysis, which revealed several spots with high levels of differential expression between MTX300 and 3T3R500. The protein spot with highest differential expression was submitted for tryptic peptide mass fingerprinting(PMF) for identification by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). MS analysis and database searches revealed it to be dihydrofolate reductase (DHFR), which was subsequently confirmed by Western blot. The result suggested that DHFR might play an important role in the MTX resistance.

The inverse relationship between reduced folate carrier function and pemetrexed activity in a human colon cancer cell line

Pemetrexed, a new generation antifolate recently approved for the treatment of mesothelioma and non-small cell lung cancer, is an excellent substrate for the reduced folate carrier (RFC). To explore the carrier's effect on pemetrexed activity, RFC was inactivated in HCT-15 colon cancer cells by mutagenesis and PT632 selective pressure. A clone (PT1) was obtained with a glycine to arginine substitution at amino acid 401, resulting in the loss of RFC function. PT1 cells were resistant to PT632 (178-fold), methotrexate (4-fold), and ZD1694 (Tomudex, raltitrexed; 20-fold), but were 3-fold collaterally sensitive to pemetrexed when grown in 25 nmol/L of 5-formyltetrahydrofolate. PT1 cells transfected with wild-type RFC had antifolate sensitivities comparable to that of wild-type HCT-15 cells, indicating that the RFC mutation was the sole basis for resistance. Folate pools were contracted in PT1 cells by 32% or 60%, as measured by radiolabeling intracellular folates or by an enzyme binding assay, respectively. This was reflected in marked (6.5-fold) collateral sensitivity to trimetrexate. The initial uptake of pemetrexed in PT1 cells was markedly reduced ( approximately 85%) but intracellular pemetrexed levels increased to approximately 60% and approximately 70% to that of wild-type cells after 2 hours and 6 days, respectively. There was increased pemetrexed inhibition of glycinamide ribonucleotide transformylase and, to a lesser extent, thymidylate synthase in PT1 cells growing in 5-formyltetrahydrofolate based on nucleoside protection analyses. Hence, loss of RFC function leads to collateral sensitivity to pemetrexed in HCT-15 cells, likely due to cellular folate pool contraction resulting in partial preservation of pemetrexed polyglutamylation and increased target enzyme inhibition. micro

Comparison of folic acid uptake characteristics by human placental choriocarcinoma cells at acidic and physiological pH

The aim of this work was to characterize the placental uptake of folic acid from the maternal circulation. Using 2 human trophoblast cell lines (BeWo and JAR), we verified that uptake of3H-folic acid was pH-dependent, increasing significantly with decreasing extracellular pH. In BeWo cells, uptake of3H-folic acid at pH 5.5 was (i) Na+-independent; (ii) inhibited by folic acid, 5-methyltetrahydrofolate (5-MTHF), and methotrexate (MTX); (iii) inhibited by the anion transport inhibitors 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (DIDS) and 4-acetamido-4′-isothiocyano-2,2′-disulfonic acid stilbene (SITS); (iv) inhibited by the proton ionophore carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP); (v) not inhibited by blockers of receptor-mediated endocytosis (cytochalasin D and monensin); (vi) trans-inhibited by MTX and folic acid; and (vii) not affected by an anti-reduced folate transporter-1 (RFC) antibody. At pH 7.5, uptake of3H-folic acid was (i) Na+-independent; (ii) inhibited by folic acid and MTX, but not by 5-MTHF; (iii) inhibited by SITS, but not by DIDS; (iv) not affected by FCCP; (v) inhibited by monensin (but not by cytochalasin D); (vi) trans-inhibited by folic acid (but not by MTX); and (vii) inhibited by an anti-RFC antibody. In conclusion, in BeWo cells, both RFC and receptor-mediated endocytosis seem to be involved in3H-folic acid uptake at pH 7.5, whereas at pH 5.5, RFC and (or) a low pH-operating transporter distinct from RFC are involved.

Localization of a substrate binding domain of the human reduced folate carrier to transmembrane domain 11 by radioaffinity labeling and cysteine-substituted accessibility methods

The human reduced folate carrier (hRFC) mediates the membrane transport of reduced folates and classical anti-folates into mammalian cells. RFC is characterized by 12 transmembrane domains (TMDs), internally oriented N and C termini, and a large central linker connecting TMDs 1-6 and 7-12. By co-expression and N-hydroxysuccinimide methotrexate (Mtx) radioaffinity labeling of hRFC TMD 1-6 and TMD 7-12 half-molecules, combined with endoproteinase GluC digestion, a substrate binding domain was previously localized to within TMDs 8-12 (Witt, T. L., Stapels, S. E., and Matherly, L. H. (2004) J. Biol. Chem. 279, 46755-46763). In this report, this region was further refined to TMDs 11-12 by digestion with 2-nitro-5-thiocyanatobenzoic acid. A transportcompetent cysteine-less hRFC was used as a template to prepare single cysteine-replacement mutant constructs in which each residue from Glu-394 to Asp-420 of TMD 11 and Tyr-435 to His-457 of TMD 12 was replaced individually by a cysteine. The mutant constructs were transfected into hRFC-null HeLa cells. Most of the 50 single cysteine-substituted constructs were expressed at high levels on Western blots. With the exception of G401C hRFC, all mutants were active for Mtx transport. Treatment with sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES) had no effect on hRFC activity for all of the cysteine mutants within TMD 12 and for the majority of the cysteine mutants within TMD 11. However, MTSES inhibited Mtx uptake by the T404C, A407C, T408C, T412C, F416C, I417C, V418C, and S419C mutants by 25-65%. Losses of activity by MTSES treatment for T404C, A407C, T412C, and I417C hRFCs were appreciably reversed in the presence of excess leucovorin, a hRFC substrate. Our results strongly suggest that residues within TMD 11 are likely critical structural and/or functional components of the putative hRFC transmembrane channel for anionic folate and anti-folate substrates.

Modulation of IMPDH2, survivin, topoisomerase I and vimentin increases sensitivity to methotrexate in HT29 human colon cancer cells

We determined differentially expressed genes in HT29 human colon cancer cells, both after short treatment with methotrexate (MTX) and after the resistance to MTX had been established. Screening was performed using Atlas Human Cancer 1.2K cDNA arrays. The analysis was carried out using Atlas image 2.01 and genespring 6.1 software. Among the differentially expressed genes we chose for further validation were inosine monophosphate dehydrogenase type II (IMPDH2), inosine monophosphate cyclohydrolase and survivin as up-regulated genes, and topoisomerase I (TOP1) and vimentin as down-regulated genes. Changes in mRNA levels were validated by quantitative RT-PCR. Additionally, functional analyses were performed inhibiting the products of the selected genes or altering their expression to test if these genes could serve as targets to modify MTX cytotoxicity. Inhibition of IMPDH or TOP1 activity, antisense treatment against survivin, or overexpression of vimentin, sensitized resistant HT29 cells to MTX. Therefore, these proteins could constitute targets to develop modulators in MTX chemotherapy.

A prominent low-pH methotrexate transport activity in human solid tumors: contribution to the preservation of methotrexate pharmacologic activity in HeLa cells lacking the reduced folate carrier

Whereas the major folate transporter, the reduced folate carrier (RFC), has a physiological pH optimum, transport activities for folates and antifolates have been detected with low pH optima. Because the interstitial pH in solid tumors is generally acidic, the mechanisms by which antifolates are transported at low pH could be an important determinant of drug activity under these conditions. The current study quantitated the low pH methotrexate (MTX) transport activity in human solid tumor cell lines from the National Cancer Institute tumor panel and other sources. MTX influx at pH 5.5 was equal to, or greater than, influx at pH 7.4 in 29 of 32 cell lines. To assess the role of RFC in transport at low pH in one of these cell lines, a HeLa clonal line (R5) was selected for MTX resistance due to a genomic deletion of the carrier gene. MTX influx was depressed by 70% in R5 versus wild-type HeLa cells at pH 7.4. At pH 6.5, influx in these two lines was similar; as the pH was decreased to 5.5 influx increased in both cell lines. Similarly, whereas net MTX uptake over 1 h was markedly decreased in R5 cells at pH 7.4, net uptake in HeLa and R5 cells was comparable at pH 6.5. Also, as compared with MCF7 breast cancer cells, MTX uptake was markedly decreased at pH 7.4, but only minimally at pH 6.5, in the MDA-MB-231 human breast cancer cell line that lacks RFC expression. When grown with folic acid (2 micro M) at pH 7.4, the IC(50) for MTX was 14-fold higher in R5 as compared with wild-type HeLa cells; the difference was only 4-fold when cells when grown at pH 6.9; the IC(50)s were identical at this pH when the medium folate was 25 nM 5-formyltetrahydrofolate. These data demonstrate that transport activity at low pH is prevalent in human solid tumors, is RFC-independent in R5 cells and MDA-MB-231 breast cancer cells, and can preserve MTX activity in the absence of RFC at an acidic pH relevant to solid tumors in vivo.

Preservation of folate transport activity with a low-pH optimum in rat IEC-6 intestinal epithelial cell lines that lack reduced folate carrier function

Intestinal folate transport has been well characterized, and rat small intestinal epithelial (IEC-6) cells have been used as a model system for the study of this process on the cellular level. The major intestinal folate transport activity has a low-pH optimum, and the current paradigm is that this process is mediated by the reduced folate carrier (RFC), despite the fact that this carrier has a neutral pH optimum in leukemia cells. The current study addressed the question of whether constitutive low-pH folate transport activity in IEC-6 cells is mediated by RFC. Two independent IEC-6 sublines, IEC-6/A4 and IEC-6/PT1, were generated by chemical mutagenesis followed by selective pressure with antifolates. In IEC-6/A4 cells, a premature stop resulted in truncation of RFC at Gln(420). A green fluorescent protein (GFP) fusion with the truncated protein was not stable. In IEC-6/PT1 cells, Ser(135) was deleted, and this alteration resulted in the failure of localization of the GFP fusion protein in the plasma membrane. In both cell lines, methotrexate (MTX) influx at neutral pH was markedly decreased compared with wild-type IEC-6 cells, but MTX influx at pH 5.5 was not depressed. Transient transfection of the GFP-mutated RFC constructs into RFC-null HeLa cells confirmed their lack of transport function. These results indicate that in IEC-6 cells, folate transport at neutral pH is mediated predominantly by RFC; however, the folate transport activity at pH 5.5 is RFC independent. Hence, constitutive folate transport activity with a low-pH optimum in this intestinal cell model is mediated by a process entirely distinct from that of RFC.

The H(+)-dependent reduced folate carrier 1 of humans and the sodium-dependent methotrexate carrier-1 of the rat are orthologs

Previously, two different carrier systems for uptake of reduced folates and the antifolate methotrexate (Mtx) were described: the pH-dependent folate sensitive reduced folate carrier 1 (RFC1) from human, hamster and mouse and a sodium-dependent and folate insensitive Mtx carrier-1 (MTX-1) from rat. It was found that all critical residues of the homologous amino acid sequence were identical. RFC1- as well as MTX-1-mediated uptake of a marker substrate into suitable human and rat cell lines increased with proton concentration, was sodium-dependent at neutral pH, and inhibited by folate at acidic pH. It is concluded that RFC1 and MTX-1 are orthologs.

Selective Preservation of Pemetrexed Pharmacological Activity in HeLa Cells Lacking the Reduced Folate Carrier

A methotrexate (MTX)-resistant HeLa subline (R5), developed in this laboratory, with impaired transport due to a genomic deletion of the reduced folate carrier (RFC) was only 2-fold resistant to pemetrexed (PMX), but 200- and 400-fold resistant to raltitrexed (ZD1694) and N(alpha)-(-4-amino-4-deoxypteroyl)-N(delta)-hemiphthaloyl-1-ornithine (PT523), respectively, compared with parental HeLa cells when grown with 2 microM folic acid. When folic acid was replaced with the more physiological 25 nM 5-formyltetrahydrofolate, R5 cells were 2-fold collaterally sensitive to PMX but still 40- and 200-fold resistant to ZD1694 and PT523, respectively. Sensitivity to PT523 and PMX could be completely restored, and sensitivity to ZD1694 nearly restored, by transfection of RFC cDNA into R5 cells, indicating that the defect in drug transport was the only, or major, factor in resistance. The preserved PMX activity in R5 cells could not be related to the very low expression of folate receptors. Rather, retained PMX activity in R5 cells was associated with residual transport by another process that exhibits good affinity for PMX (Kt = 12 microM) with much lower affinities for ZD1694, MTX, and PT523 (Kis of approximately 90, 100, and 250 microM, respectively). PMX transported by this route was rapidly converted to higher polyglutamates and, when grown with 25 nM 5-formyl-tetrahydrofolate, the rate of formation of these derivatives and their net accumulation in R5 cells was comparable to that of wild-type cells. These data suggest that selective preservation of PMX pharmacological activity in RFC-null R5 cells is due, in part, to partial preservation of transport by secondary process with a higher affinity for PMX than the other antifolates evaluated.

Sodium-dependent methotrexate carrier-1 is expressed in rat kidney: cloning and functional characterization

Previous Northern blot studies suggested strong expression of a homolog to the sodium-dependent hepatocellular methotrexate transporter in the kidneys. Here, we report on the cloning of the cDNA for the renal methotrexate carrier isoform-1 (RK-MTX-1) and its functional characterization. Sequencing revealed 97% homology to the rat liver methotrexate carrier with an identical open reading frame. Differences were located in the 5′-untranslated region and resulted in the absence of putative regulatory elements (Barbie box, Ah/ARNT receptor) identified in the cDNA for the hepatocellular carrier. For functional characterization, MTX-1 cDNA was stably expressed in Madin-Darby canine kidney (MDCK) cells. A sodium-dependent transport of methotrexate with a Kmof 41 μM and a Vmaxof 337 pmol·mg protein-1·min-1was observed. This uptake was blocked by the reduced folates dihydro- and tetrahydrofolate as well as by methotrexate itself. Folate was inhibiting only weakly, whereas 5-methyltetrahydrofolate was a strong inhibitor. Further inhibitors of the methotrexate transport included the bile acids cholate and taurocholate and xenobiotics like bumetanide and BSP. PAH, ouabain, bumetanide, cholate, taurocholate, and acetyl salicylic acid were tested as potential substrates. However, none of these substances was transported by MTX-1. Furthermore, expression of RK-MTX-1 in MDCK cells enhanced methotrexate toxicity in these cells fivefold. Analysis of a fusion protein of RK-MTX-1 and the influenza virus hemagglutinin epitope by immunoblotting revealed a major band at 72 kDa within the cell membrane but not in the soluble fraction of transfected MDCK. Indirect immunofluorescence staining revealed an exclusive localization of the carrier in the plasma membrane, and by confocal laser-scanning microscopy we were able to demonstrate that the protein is expressed in the serosal region of MDCK tubules grown in a morphogenic collagen gel model.

Reduced folate carrier mutations are not the mechanism underlying methotrexate resistance in childhood acute lymphoblastic leukemia

BACKGROUND

Although the majority of children with acute lymphoblastic leukemia (ALL) are cured with combination chemotherapy containing methotrexate (MTX), drug resistance contributes to treatment failure for a substantial fraction of patients. The primary transporter for folates and MTX is the reduced folate carrier (RFC). Impaired drug transport is a documented mechanism of MTX resistance in patients with ALL; however, to the authors' knowledge it is not known whether inactivating RFC mutations are a contributing factor.

METHODS

The authors devised a genomic polymerase chain reaction-single strand conformational polymorphism assay followed by sequencing and screened the entire RFC coding region for sequence alterations in DNA from 246 leukemia specimens from patients with diverse ethnic variation, 24 at the time of recurrence and the rest at the time of diagnosis. This cohort was comprised of 203 B-precursor ALL specimens (82.5%), 32 T-lineage ALL specimens (13%), and 11 acute myeloblastic leukemia specimens (4.5%).

RESULTS

Of 246 DNA samples, only 3 diagnosis B-precursor ALL specimens (1.2%) were found to harbor alterations in the RFC gene, including heterozygous single nucleotide changes resulting in D56H and D522N substitutions in the first extracellular loop and the C-terminus of this transporter, respectively. The third sample had a sequence alteration in exon 3 that could not be identified because of the lack of availability of DNA.

CONCLUSIONS

Whereas inactivating RFC mutations are a frequent mechanism of MTX resistance in human leukemia cell lines and in patients with osteosarcoma, they are not common and do not appear to play any significant role in intrinsic or acquired resistance to MTX in childhood leukemia. This is the first study of RFC mutations in multiple pediatric leukemia specimens.

Resistance to antifolates

The antifolates were the first class of antimetabolites to enter the clinics more than 50 years ago. Over the following decades, a full understanding of their mechanisms of action and chemotherapeutic potential evolved along with the mechanisms by which cells develop resistance to these drugs. These principals served as a basis for the subsequent exploration and understanding of the mechanisms of resistance to a variety of diverse antineoplastics with different cellular targets. This section describes the bases for intrinsic and acquired antifolate resistance within the context of the current understanding of the mechanisms of actions and cytotoxic determinants of these agents. This encompasses impaired drug transport into cells, augmented drug export, impaired activation of antifolates through polyglutamylation, augmented hydrolysis of antifolate polyglutamates, increased expression and mutation of target enzymes, and the augmentation of cellular tetrahydrofolate-cofactor pools in cells. This chapter also describes how these insights are being utilized to develop gene therapy approaches to protect normal bone marrow progenitor cells as a strategy to improve the efficacy of bone marrow transplantation. Finally, clinical studies are reviewed that correlate the cellular pharmacology of methotrexate with the clinical outcome in children with neoplastic diseases treated with this antifolate.

Reduced folate carrier protein expression in osteosarcoma: implications for the prediction of tumor chemosensitivity

BACKGROUND

High-dose methotrexate (MTX) is an important component of current protocols for the treatment of osteosarcoma. Although MTX uptake proceeds primarily through the reduced folate carrier (RFC) protein and efflux occurs via multidrug resistance protein 1 (MRP1), RFC protein expression in osteosarcoma remains unexamined.

METHODS

RFC and MRP1 expression (normalized to beta-actin expression) was examined with Western blot analysis in 11 osteosarcoma specimens obtained at diagnosis and 9 osteosarcoma specimens obtained on recurrence.

RESULTS

The average RFC level in specimens obtained on recurrence was significantly higher than the level in specimens obtained at diagnosis (P = 0.0005). Furthermore, in all three matched pairs of diagnosis and recurrence specimens, RFC levels were higher in recurrence specimens than in the corresponding diagnosis specimens. Potential correlations between RFC and MRP1 expression and histologic response to preoperative chemotherapy were examined. Tumors with poor histologic responses (i.e., </= 90% necrosis) had significantly lower RFC levels than did those with favorable responses to chemotherapy (P = 0.0016). In contrast, there was no correlation between MRP1 levels at diagnosis and histologic response to chemotherapy (P = 0.8764). The elevated MRP1 levels in specimens obtained on recurrence relative to MRP1 levels in specimens obtained at diagnosis were not statistically significant (P = 0.2056).

CONCLUSIONS

The significant correlation between low RFC levels at diagnosis and poor histologic response to preoperative chemotherapy suggests that RFC levels at diagnosis may be a useful predictor of chemosensitivity and warrants large-scale studies. In addition, postchemotherapy progression to recurrence is associated with a significant increase in RFC expression. To our knowledge, the current study is the first to examine RFC protein levels in tumor specimens. Cancer 2003.

The region between transmembrane domains 1 and 2 of the reduced folate carrier forms part of the substrate-binding pocket

A functional cysteine-less form of the hamster reduced folate carrier protein was generated by alanine replacement of the 14 cysteine residues. The predicted 12-transmembrane topology was examined by replacing selected amino acids, predicted to be exposed to the extracellular or cytosolic environments, with cysteines. The location of these cysteines was defined by their accessibility to biotin maleimide in the presence or absence of specific blocking agents. Amino acids predicted to be exposed to the extracellular environment (S46C, S179C, L300C, Y355C, and K430C) could be labeled with biotin maleimide; this modification could be blocked by prior treatment with nonpermeable reagents. Amino acids predicted to be within the cytosol (S152C, Cys224, and L475C) could be labeled only after streptolysin O permeabilization. In addition, the cysteine-less reduced folate carrier was exploited to evaluate a potential substrate-binding domain as suggested by previous studies. Nineteen cysteine replacements were generated between residues 39 and 75, a region located between the first and second transmembrane segments. From the biotinylation of these sites and the ability of various reagents to block this labeling, it appears that L41C, E45C, S46C, T49C, I66C, and L70C are exposed to the extracellular environment, whereas Q54C, Q61C, and T63C are slightly less accessible. Cysteines 39, 42, 44, 47, 51, and 73 were inefficiently biotinylated, suggesting that these sites are located in the membrane or within a tightly folded domain of the protein. Furthermore, biotinylation of cysteines 41, 46, 49, 70, and 71 could be prevented by prior treatment with either methotrexate or folinic acid, indicating that these sites form part of a substrate-binding pocket.

Reduced folate carrier and dihydrofolate reductase expression in acute lymphocytic leukemia may predict outcome: a Children's Cancer Group Study

PURPOSE

Methotrexate is a major component of current treatment regimens for children with acute lymphocytic leukemia (ALL). Potential mechanisms of methotrexate resistance include impaired drug uptake, decreased drug retention, and dihydrofolate reductase (DHFR) amplification. The purpose of this study was to assess whether reduced folate carrier (RFC) and DHFR expression in untreated leukemic blasts correlated with outcome.

METHODS

Quantitative real-time RT-PCR was used to measure RFC and DHFR mRNA expression in leukemic blasts from 40 newly diagnosed patients with ALL obtained in a blinded fashion from Children's Cancer Group studies.

RESULTS

Low RFC expression at diagnosis correlated significantly with an unfavorable event free survival. Surprisingly, low, not high, DHFR expression correlated significantly with an unfavorable event-free survival. Proliferative cell nuclear antigen (PCNA) expression demonstrated a weak inverse relationship between sample PCNA and DHFR or RFC expression, suggesting that DHFR and RFC expression may be markers for factors other than drug resistance.

CONCLUSIONS

These results suggest that impaired transport may be an important mechanism of intrinsic methotrexate resistance in ALL, and DHFR expression also may be an important prognostic factor in ALL. Additional studies are necessary to clarify the mechanism for the correlation of low DHFR expression with poor outcome.

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.

Residues 45 and 404 in the murine reduced folate carrier may interact to alter carrier binding and mobility

The reduced folate carrier (RFC), a facilitative transporter, plays a major role in the delivery of reduced folates and antifolates into cells. Previous studies indicated that mutations of E45K in the first transmembrane domain (TMD), and K404L in the 11th TMD, produce selective and opposite alterations in binding of natural folate substrates to murine RFC. The former mutation is frequently associated with antifolate resistance. The current study was designed to determine whether there might be an interaction between these sites by comparing the transport properties of RFC-null cell lines stably transfected with K404E, E45K, or E45K/K404E carriers. These studies demonstrated that: (1) All mutant carriers were inserted into the plasma membrane. (2) In the K404E mutant, the influx K(t)'s for 5-formyltetrahydrofolate and 5-methyltetrahydrofolate were markedly increased, and to a much smaller extent folic acid, as compared to L1210 cells. However, with introduction of a second E45K mutation the influx K(t) for these folates reverted to those of the E45K cells which retained wild-type binding for 5-methyltetrahydrofolate and enhanced binding of 5-formyltetrahydrofolate and folic acid. (3) The influx V(max) of the E45K mutant was markedly reduced. Introduction of the second K404E mutation doubled this parameter and the ratio of V(max) to K(t) for 5-formytetrahydrofolate was restored to approximately 50% that of the wild-type carrier consistent with a substantial increase in function. (4) Chloride inhibits wild-type RFC but the E45K mutant requires chloride for activity. The K404E mutant is also suppressed by chloride but introduction of the K404E mutation decreased the chloride-dependence of E45K. The results suggest that there is an interaction between the E45 and K404 residues in the first and 11th TMDs, respectively, but that the E45 residue appears to be the more dominant determinant of binding and anion sensitivity.

Decreased expression of the reduced folate carrier and folypolyglutamate synthetase is the basis for acquired resistance to the pemetrexed antifolate (LY231514) in an L1210 murine leukemia cell line

Pemetrexed (LY231514) is a new-generation antifolate that, in its polyglutamyl forms, is a potent inhibitor of thymidylate synthase and glycinamide ribonucleotide formyltransferase (GAR transformylase). This study explored the mechanisms of resistance to pemetrexed in L1210 murine leukemia cells using chemical mutagenesis with 5-formyltetrahydrofolate (5-formylTHF) as the growth substrate. A cell line, MTA-13, was identified that was 8.5-fold resistant to pemetrexed with comparable cross-resistance to ZD1694 (Tomudex) and lesser cross-resistance (5-fold) to ZD9331 [(2S)-2-(O-fluoro-p-[N-(2,7-dimethyl-4-oxo-3,4-dihydro-quinazolin-6-ylmethyl)-N-(prop-2-ynyl)amino]benzamido)-4-(tetrazol-5-yl)-butyric acid], DDATHF (dideazatetrahydrofolate) (3.5-fold), and methotrexate (MTX) (2.7-fold) but comparable sensitivity to trimetrexate. Influx of pemetrexed, MTX, and 5-formylTHF into MTA-13 cells was decreased by 56, 47, and 38% compared to wild-type cells. Folate receptor expression was negligible in both cell lines. Net drug uptake declined within 15min to a slower, constant rate over the next 45min, reflecting the rate of accumulation of pemetrexed polyglutamate derivatives. This rate in the MTA-13 line was half that of the wild-type cells. Accumulation of 50nM [3H]pemetrexed, 25nM [3H]5-formylTHF, or 50nM [3H]DDATHF after 3 days was decreased to 35, 46, and 56% the level of L1210 cells. The reduced folate carrier (RFC) message and protein were decreased by 50%, and folypolyglutamate synthetase (FPGS) message was decreased by 65% in MTA-13 cells. No mutations were detected in either protein by DNA sequence analysis. There was a slight decrease (approximately 25%) in thymidylate synthase mRNA, without mutations in the protein, and there was no change in GAR transformylase message. The data indicate that resistance to pemetrexed in the MTA-13 cell line was due to changes in both RFC and FPGS expression, two proteins that act in tandem to regulate polyglutamation of folates and antifolates in cells, resulting in cellular depletion of these active pemetrexed congeners.

Alterations in the expression of transcription factors and the reduced folate carrier as a novel mechanism of antifolate resistance in human leukemia cells

The human reduced folate carrier (hRFC) is the dominant transporter mediating the uptake of reduced folate cofactors and antifolate anticancer drugs. Defective antifolate uptake due to inactivating mutations in the hRFC gene is an established mechanism of drug resistance in various tumor cells. However, while antifolate transport is frequently impaired, either no or only a single hRFC allele is inactivated, suggesting that additional mechanism(s) of resistance are operative. Here we studied the relationship between the expression and function of transcription factors and antifolate resistance in transport-defective leukemia cells that poorly express or completely lack RFC mRNA. Stable transfection with a hRFC expression construct resulted in restoration of normal RFC mRNA expression and nearly wild type drug sensitivity in these antifolate-resistant cells. The loss of RFC gene expression prompted us to explore transcription factor binding to the hRFC promoter. The hRFC promoter contains an upstream GC-box and a downstream cAMP-response element (CRE)/AP-1-like element. Electrophoretic mobility shift assays and oligonucleotide competition revealed a substantial loss of nuclear factor binding to CRE and GC-box in these drug-resistant cell lines. Consistently, antibody-mediated supershift analysis showed a marked decrease in the binding of CRE-binding protein 1 (CREB-1) and specificity protein 1 (Sp1) to CRE and GC-box, respectively. Western blot analysis revealed undetectable expression of CREB-1, decreased ATF-1 levels, parental Sp1 levels, and increased levels of the short Sp3 isoforms, recently shown to repress hRFC gene expression. Transient transfections into these antifolate-resistant cells demonstrated a marked loss of GC-box-dependent, and CRE-driven reporter gene activities and introduction of CREB-1 or Sp1 expression constructs resulted in restoration of hRFC mRNA expression. These results establish a novel mechanism of antifolate resistance that is based on altered expression and function of transcription factors resulting in transcriptional silencing of the hRFC promoter.

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.

Role of the E45K-reduced folate carrier gene mutation in methotrexate resistance in human leukemia cells

Resistance to the antifolate methotrexate (MTX) can cause treatment failure in childhood acute lymphoblastic leukemia (ALL). This may result from defective MTX accumulation due to alterations in the human reduced folate carrier (hRFC) gene. We have identified an hRFC gene point mutation in a transport-defective CCRF-CEM human T-ALL cell line resulting in a lysine to glutamic acid substitution at codon 45 (E45K), which has been identified in other antifolate-resistant sublines (JBC 273:30 189, 1998; JBC 275:30 855, 2000). To characterize the role of this mutation in MTX resistance, transfection experiments were performed using hRFC-null CCRF-CEM cells. E45K transfectants demonstrated an initial rate of MTX influx that was approximately 0.5-fold that of CCRF-CEM cells, despite marked protein overexpression. Cytotoxicity studies revealed partial reversal of MTX and raltitrexed resistance in E45K transfectants, while trimetrexate resistance was significantly increased. Kinetic analysis indicated only minor differences in MTX kinetics between wild-type and E45K hRFCs, however, K(i)s for folic acid and 5-formyltetrahydrofolate were markedly reduced for E45K hRFC. This was paralleled by increased folic acid transport and reduced synthesis of MTX polyglutamates. Collectively, the results demonstrate that expression of E45K hRFC leads to increased MTX resistance due to decreased membrane transport and, secondarily, from alterations in binding affinities and transport of folate substrates. However, despite these findings, we could find no evidence of this mutation in 121 childhood ALL samples, suggesting that it does not contribute to clinical MTX resistance in this disease.

The emergence of resistance to targeted cancer therapeutics

Drug resistance has always been a concern in cancer treatment, often blamed on the genetic complexity and instability of tumor cells. While studies of cancer cell lines have implicated an array of potential mechanisms, it has been difficult to translate these insights into clinically meaningful improvements in cancer treatment. The successful deployment of molecularly targeted therapeutics in some cancers has led to widespread optimism that this approach will become broadly applicable. Despite their early promise in the clinic, the novel therapeutics are often plagued with the age old problem of acquired drug resistance. Progress in understanding why certain patients respond and why some develop resistance can be made rapidly through studies of the drug target in tumor tissue from patient. One important lesson is that many cancers, even in the most advanced stages, continue to rely on a limited number of critical oncogenic signals for maintenance of the malignant phenotype. This article reviews the mechanisms of drug resistance to a variety of cancer therapeutics and provides an approach for how measures of drug target activity can be incorporated into clinical trial design.

Analysis of membrane topology of the human reduced folate carrier protein by hemagglutinin epitope insertion and scanning glycosylation insertion mutagenesis

The human reduced folate carrier (RFC) is the major membrane transport system for both reduced folates and chemotherapeutic antifolate drugs, such as methotrexate (MTX). Although the RFC protein has been subjected to intensive study in order to identify critical structural and functional determinants of transport, it is impossible to assess the significance of these studies without characterizing the essential domain structure and membrane topology. The primary amino acid sequence from the cloned cDNAs predicts that the human RFC protein has 12 transmembrane domains (TMDs) with a large cytosolic loop between TMDs 6 and 7, and cytosolic-facing N- and C-termini. To establish the RFC membrane topology, a hemagglutinin (HA) epitope was inserted into the individual predicted intracellular and extracellular loops. HA insertions into putative TMD interconnecting loops 3/4, 6/7, 7/8, and 8/9, and the N- and C-termini all preserved MTX transport activity upon expression in transport-impaired K562 cells. Immunofluorescence detection with HA-specific antibody under both permeabilized and non-permeabilized conditions confirmed extracellular orientations for loops 3/4 and 7/8, and cytosolic orientations for loops 6/7 and 8/9, and the N- and C-termini. Insertion of a consensus N-glycosylation site [NX(S/T)] into putative loops 5/6, 8/9, and 9/10 of deglycosylated RFC-Gln(58) had minimal effects on MTX transport. Analysis of glycosylation status on Western blots suggested an extracellular orientation for loop 5/6, and intracellular orientations for loops 8/9 and 9/10. Our findings strongly support the predicted topology model for TMDs 1-8 and the C-terminus of human RFC. However, our results raise the possibility of an alternative membrane topology for TMDs 9-12.

Role of the C-terminus and the long cytoplasmic loop in reduced folate carrier expression and function

The reduced folate carrier (RFC1), a member of the major facilitative superfamily, generates uphill transport of folates into cells through an exchange mechanism with intracellular organic anions. RFC1 has twelve transmembrane domains with N- and C-termini, and the long loop connecting the 6th and 7th transmembrane domains, directed to the cytoplasm. To elucidate the role of the C-terminus and the long cytoplasmic loop in carrier function, mutants with deletion of the entire C-terminus or with progressive deletions of the loop region were constructed and stably transfected into the murine MTX(r)A cell line, which lacks functional RFC1. While expression of the C-terminus-deleted RFC1 protein could not be detected in the cell lysate, the RFC1 mutant lacking 57 of 66 amino acid residues of the long cytoplasmic loop appeared to be inserted into the cytoplasmic membrane but was not functional. In cell lines in which 17 or 31 amino acids were deleted from the carboxyl half of the loop, there was partial preservation of methotrexate, 5-formyltetrahydrofolate, and 5-methyltetrahydrofolate transport. The loss of 5-formyltetrahydrofolate transport activity in the delta31 and delta17 mutants was due primarily to a decrease in substrate binding to the carrier. Mutants with partially truncated internal loops demonstrated an anion responsiveness similar to that of wild-type RFC1, indicating that this region of the carrier does not contain a site(s) that plays a role in anion exchange. This is the first study to describe the important role of the long cytoplasmic loop in substrate binding and the crucial role of the C-terminus in maintaining stability of RFC1.

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 microM of MTX, ZD1694, GW1843U89, MTA and DDATHF, by intermittent 72 hr exposures to 5 microM 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 approximately 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 [3H]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 [3H]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.

Expression of the reduced folate carrier SLC19A1 in IEC-6 cells results in two distinct transport activities

Intestinal absorption of folates has been characterized as a facilitative process with a low pH optimum. Studies with intestinal epithelial cells have suggested that this activity is mediated by the reduced folate carrier (RFC1). In this paper, we report on folate transport characteristics in an immortalized rat IEC-6 cell line that was found to exhibit the predominant influx activity for methotrexate (MTX) at pH 5.5 with a low level of activity at pH 7.4. Transfection of this cell line with an RFC1 construct resulted in clones exhibiting increased MTX uptake at both the pHs and high folic acid uptake only at the low pH. For the two clones with the highest level of transport activity, relative MTX influx at the two pHs was reversed. Moreover, the low pH MTX influx activity ([MTX](e) = 0.5 microM) was markedly inhibited by 20 microM folic acid while influx at neutral pH was not. Furthermore, in the presence and absence of glucose at low pH, MTX and folic acid influx activity was inhibited by azide, while MTX influx at pH 7.4 was stimulated by azide in the absence of glucose but was unchanged in the presence of glucose and azide. This was contrasted with the results of transfection of the same RFC1 construct into an L1210 murine leukemia cell line bearing a nonfunctional endogenous carrier. In this case, the activity expressed was only at pH 7.4. These data indicate that RFC1 can exhibit two distinct types of folate transport activities in intestinal cells that must depend on tissue-specific modulators.

Methylation-dependent silencing of the reduced folate carrier gene in inherently methotrexate-resistant human breast cancer cells

The molecular basis of methotrexate resistance was studied in human MDA-MB-231 breast cancer cells, which are inherently defective in methotrexate uptake and lack expression of the reduced folate carrier (RFC). Transfection of MDA-MB-231 cells with RFC cDNA restored methotrexate uptake and increased methotrexate sensitivity by approximately 50-fold. A CpG island in the promoter region of RFC was found to be methylated in MDA-MB-231 cells, but was unmethylated in RFC expressing, methotrexate-sensitive MCF-7 breast cancer cells. Chromatin immunoprecipitation with antibodies against acetylated histones H3 and H4 showed that the RFC promoter was enriched for acetylated histones on expressed, unmethylated alleles only. Treatment of MDA-MB-231 cells with 5-aza-2'-deoxycytidine restored RFC expression but also led to increased methotrexate efflux and did not reverse methotrexate resistance. This suggests that 5-aza-2'-deoxycytidine up-regulates both methotrexate uptake and some methotrexate-resistance mechanism(s). Reverse transcription-polymerase chain reaction analysis showed increased expression levels of several ATP-dependent efflux pumps in response to 5-aza-2'-deoxycytidine treatment, including P-glycoprotein and members of the multidrug resistance-associated protein family. Up-regulation of P-glycoprotein in response to 5-aza-2'-deoxycytidine was associated with demethylation of a CpG island in the MDR1 promoter, whereas the mechanism(s) for 5-aza-2'-deoxycytidine-induced up-regulation of multidrug resistance-associated proteins is probably indirect. Dipyridamole inhibited methotrexate efflux and reversed methotrexate resistance in 5-aza-2'-deoxycytidine-treated MDA-MB-231 cells.

Mutational analysis of the functional role of conserved arginine and lysine residues in transmembrane domains of the murine reduced folate carrier

The reduced folate carrier (RFC1) plays a major role in the delivery of folates into mammalian cells. RFC1 is an anion exchanger with seven conserved positively charged amino acid residues within 12 predicted transmembrane domains. This article explores the role of these residues in transport function by the development of cell lines in which arginines and lysines in RFC1 were replaced with leucine by site-directed mutagenesis. Three cell lines transfected with R131L, R155L, or R366L all lacked activity, despite high levels of protein expression in the plasma membrane, suggesting the crucial role of these amino acid residues in RFC1 function. In several mutant carriers, R26L, R42L, and K332L, there was little or no change in the influx K(t) value for MTX or influx K(i) value for folic acid. However, the R26L, R42L, and K332L carriers had decreased affinity for reduced folates. This was most prominent for K404L, which had 11- and 4-fold increases in influx K(i) for 5-methyl-THF and 5-formyl-THF, respectively, compared with L1210 cells. The marked influx stimulation observed with wild-type carrier when extracellular chloride was decreased was significantly diminished when influx was mediated by the K404L carrier, but was only slightly decreased with the R26L, R42L, and K332L mutants. This suggested that the K404 residue may be a major site of inhibition by chloride in the wild-type carrier. These studies indicate the important role that some positively charged residues within transmembrane domains of RFC1 play in RFC1 function.

Impact of the reduced folate carrier on the accumulation of active thiamin metabolites in murine leukemia cells

The thiamin transporter encoded by SLC19A2 and the reduced folate carrier (RFC1) share 40% homology at the protein level, but the thiamin transporter does not mediate transport of folates. By using murine leukemia cell lines that express no, normal, or high levels of RFC1, we demonstrate that RFC1 does not mediate thiamin influx. However, high level RFC1 expression substantially reduced accumulation of the active thiamin coenzyme, thiamin pyrophosphate (TPP). This decreased level of TPP, synthesized intracellularly from imported thiamin, resulted from RFC1-mediated efflux of TPP. This conclusion was supported by the following observations. (i) Efflux of intracellular TPP was increased in cells with high expression of RFC1. (ii) Methotrexate inhibits TPP influx. (iii) TPP competitively inhibits methotrexate influx. (iv) Loading cells, which overexpress RFC1 to high levels of methotrexate to inhibit competitively RFC1-mediated TPP efflux, augment TPP accumulation. (v) There was an inverse correlation between thiamin accumulation and RFC1 activity in cells grown at a physiological concentration of thiamin. The modulation of thiamin accumulation by RFC1 in murine leukemia cells suggests that this carrier may play a role in thiamin homeostasis and could serve as a modifying factor in thiamin nutritional deficiency as well as when the high affinity thiamin transporter is mutated.

Clustering of mutations in the first transmembrane domain of the human reduced folate carrier in GW1843U89-resistant leukemia cells with impaired antifolate transport and augmented folate uptake

We have studied the molecular basis for the resistance of human CEM leukemia cells to GW1843, a thymidylate synthase inhibitor. GW1843-resistant cells displayed a approximately 100-fold resistance to GW1843 and methotrexate but were collaterally sensitive to the lipophilic antifolates trimetrexate and AG337, which enter cells by diffusion. These cells exhibited a 12-fold decreased methotrexate influx but surprisingly had a 2-fold decreased folic acid growth requirement. This was associated with a 4-fold increased influx of folic acid, a 3.5-fold increased steady-state level of folic acid, and a 2.3-fold expansion of the cellular folate pool. Characterization of the transport kinetic properties revealed that GW1843-resistant cells had the following alterations: (a) 11-fold decreased transport K(m) for folic acid; (b) 6-fold increased transport K(m) for GW1843; and (c) a slightly increased transport V(max) for folic acid. Sequence analysis showed that GW1843-resistant cells contained the mutations Val-29 --> Leu, Glu-45 --> Lys, and Ser-46 --> Ile in the first transmembrane domain of the reduced folate carrier. Transfection of the mutant-reduced folate carrier cDNA into methotrexate transport null cells conferred resistance to GW1843. This is the first demonstration of multiple mutations in a confined region of the human reduced folate carrier in an antifolate-resistant mutant. We conclude that certain amino acid residues in the first transmembrane domain play a key role in (anti)folate binding and in the conferring of drug resistance.

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

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

The reduced folate carrier in L1210 murine leukemia cells is a 58-kDa protein

The reduced folate carrier (RFC1) is a major transporter for both natural reduced folates and antifolate chemotherapeutics. Using polyclonal antibodies targeted to epitopes at the loop between the sixth and seventh predicted transmembrane domains or the distal C-terminus, we were able to demonstrate by Western blot analysis that the molecular size of RFC1 expressed in murine leukemia L1210 cells is 58 kDa as predicted by the open reading frame of its cDNA. 46- and 38-kDa proteins detected only in plasma membrane preparations were proteolytic degradation products that appeared during membrane preparation or treatment with the conventional SDS-PAGE loading buffer. These data resolve discrepancies reported previously for the molecular size of RFC1.

Discrimination among reduced folates and methotrexate as transport substrates by a phenylalanine substitution for serine within the predicted eighth transmembrane domain of the reduced folate carrier

A phenylalanine substitution for serine in the reduced folate carrier at residue 309 (RFC1-S309F) was identified in a methotrexate (MTX)-resistant cell line selected with 5-formyltetrahydrofolate (5-CHO-THF) as the sole folate source. The transport characteristics of the mutated carrier were studied by transfection into the MTXrA line, which lacks endogenous RFC1 function. The level of expression of carrier in the cell lines studied was determined by specific surface binding of 5-methyltetrahydrofolate (5-CH3-THF). Influx of 5-CH3-THF and 5-CHO-THF mediated by RFC1-S309F was 20- and 7-fold greater than that of MTX, respectively. Consistent with the influx difference between 5-CHO-THF and MTX, the growth requirement (EC50) for 5-CHO-THF in MTXrA-S309F cells was decreased by a factor of 9, while the MTX IC50 was reduced by a factor of only approximately 2 as compared with the recipient MTXrA cells. The decrease in 5-CH3-THF influx mediated by the mutated carrier was attributed to a decrease in the mobility of the 5-CH3-THF-carrier complex, since the influx Kt was essentially unchanged. However, the reduction in 5-CHO-THF and MTX influx was attributed to decreases in both carrier affinity and Vmax, although the decline in the MTX influx Vmax appeared to be much greater than for 5-CHO-THF. The inhibitory effect of chloride on 5-CHO-THF influx observed for L1210 cells was eliminated in the MTXrA-S309F line. This study represents another example of a single mutation in RFC1 that markedly impairs MTX influx but partially preserves transport of reduced folates when cells are selected with 5-CHO-THF as the available folate substrate. The data indicate that residues in the predicted eighth transmembrane domain of RFC1 can play an important role in the selectivity of folate binding and the mobility of the carrier-substrate complex.