Characterization of a human alternatively spliced truncated reduced folate carrier increasing folate accumulation in parental leukemia cells

The effects of drug transporters on the efficacy of methotrexate in the treatment of rheumatoid arthritis

The ATP-binding cassette (ABC) and solute carrier (SLC) transporter families consist of common drug transporters that mediate the efflux and uptake of drugs, respectively, and play an important role in the absorption, distribution, metabolism and excretion of drugs in vivo. Rheumatoid arthritis (RA) is an autoimmune disease characterized by erosive arthritis, and there are many RA patients worldwide. Methotrexate (MTX), the first-choice treatment for RA, can reduce the level of inflammation, prevent joint erosion and functional damage, and greatly reduce pain in RA patients. However, many patients show resistance to MTX, greatly affecting the efficacy of MTX. Many factors, such as irrational drug use and heredity, are associated with drug resistance. Considering the effect of drug transporters on drugs, many studies have compared the expression of drug transporters in drug-resistant and drug-sensitive patients, and abnormal transporter expression and transport activity have been found in patients with MTX resistance. Thus, drug transporters are involved in drug resistance. This article reviews the effects of transporters on the efficacy of MTX in the treatment of RA.

The major facilitative folate transporters solute carrier 19A1 and solute carrier 46A1: biology and role in antifolate chemotherapy of cancer

This review summarizes the biology of the major facilitative membrane transporters, the reduced folate carrier (RFC) (Solute Carrier 19A1) and the proton-coupled folate transporter (PCFT) (Solute Carrier 46A1). Folates are essential vitamins, and folate deficiency contributes to a variety of health disorders. RFC is ubiquitously expressed and is the major folate transporter in mammalian cells and tissues. PCFT mediates the intestinal absorption of dietary folates and appears to be important for transport of folates into the central nervous system. Clinically relevant antifolates for cancer, such as methotrexate and pralatrexate, are transported by RFC, and loss of RFC transport is an important mechanism of methotrexate resistance in cancer cell lines and in patients. PCFT is expressed in human tumors, and is active at pH conditions associated with the tumor microenvironment. Pemetrexed is an excellent substrate for both RFC and PCFT. Novel tumor-targeted antifolates related to pemetrexed with selective membrane transport by PCFT over RFC are being developed. In recent years, there have been major advances in understanding the structural and functional properties and the regulation of RFC and PCFT. The molecular bases for methotrexate resistance associated with loss of RFC transport and for hereditary folate malabsorption, attributable to mutant PCFT, were determined. Future studies should continue to translate molecular insights from basic studies of RFC and PCFT biology into new therapeutic strategies for cancer and other diseases.

The SLC19A1 80G>A polymorphism is not associated with male infertility

Previous studies have revealed that genetic factors may be involved in regulating folate turnover, e.g. methylenetetrahydrofolate reductase polymorphism in the development of male infertility. Folate transporter, encoded by the SLC19A1 gene, commonly referred to as reduced folate carrier (RFC) is a transmembrane protein, which transfers hydrophilic folates across the cell membrane. It was hypothesized that common polymorphism within the SLC19A1 gene (rs1051266:G>A, 80G>A) may alter RFC function. The aim of this study was to investigate a potential association between the SLC19A1 80G>A polymorphism and male infertility in a case-control study. The SLC19A1 80G>A polymorphism was determined by means of a polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) assay in 213 infertile Caucasian men and 226 ethnically matched controls. The distribution of SLC19A1 genotypes in the infertile men was as follows: GG 26.8%, GA 51.2%, AA 22.1% and in fertile men: GG 24.8%, GA 50.4%, AA 24.8%, and was comparable in the both the evaluated groups. Odds ratios (95% confidence interval, CI): 0.90 (0.59-1.38) and 0.88 (0.56-1.36) for dominant and recessive models remained non-significant, also after adjustment for age: 0.89 (0.57-1.37) and 0.80 (0.51-1.25), respectively. Our study demonstrated that polymorphism 80G>A of the SLC19A1 gene is not associated with male infertility.

Identification of the minimal functional unit of the homo-oligomeric human reduced folate carrier

The reduced folate carrier (RFC) is the major transport system for folates in mammals. We previously demonstrated the existence of human RFC (hRFC) homo-oligomers and established the importance of these higher order structures to intracellular trafficking and carrier function. In this report, we examined the operational significance of hRFC oligomerization and the minimal functional unit for transport. In negative dominance experiments, multimeric transporters composed of different ratios of active (either wild type (WT) or cysteine-less (CLFL)) and inactive (either inherently inactive (Y281L and R373A) due to mutation, or resulting from inactivation of the Y126C mutant by (2-sulfonatoethyl) methanethiosulfonate (MTSES)) hRFC monomers were expressed in hRFC-null HeLa (R5) cells, and residual WT or CLFL activity was measured. In either case, residual transport activity with increasing levels of inactive mutant correlated linearly with the fraction of WT or CLFL hRFC in plasma membranes. When active covalent hRFC dimers, generated by fusing CLFL and Y126C monomers, were expressed in R5 cells and treated with MTSES, transport activity of the CLFL-CLFL dimer was unaffected, whereas Y126C-Y126C was potently (64%) inhibited; heterodimeric CLFL-Y126C and Y126C-CLFL were only partly (27 and 23%, respectively) inhibited by MTSES. In contrast to Y126C-Y126C, trans-stimulation of methotrexate uptake by intracellular folates for Y126C-CLFL and CLFL-Y126C was nominally affected by MTSES. Collectively, these results strongly support the notion that each hRFC monomer comprises a single translocation pathway for anionic folate substrates and functions independently of other monomers (i.e. despite an oligomeric structure, hRFC functions as a monomer).

Alternative transcripts of rat slc19a1: Cloning, genomic organisation, tissue specific promoters and alternative splicing

Recently, the rat genome project revealed the genomic sequence of slc19a1, coding for the methotrexate carrier-1, identical to the reduced folate carrier-1 of humans, on rat chromosome 20. At the same time, we have cloned and analysed the complete or partial cDNAs of now at least six different transcripts from rat liver and kidneys. Alignment with the genomic sequence revealed seven exons. The first two non-coding exons, exon I and Ia were used alternatively in kidneys and liver, respectively, suggesting usage of alternative promoters. Three minor mRNA forms resulted from absent splicing of intron III, a shortened exon III (exon IIIa), and a shortened exon IV (exon IVa). The minor transcripts were predicted to result in translation products with 7 or 6 instead of 12 transmembrane domains (TMDs) and a peptide mass of 38, 39 and 40 kDa instead of 58 kDa.

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.

Reduced folate carrier-1 80G>A polymorphism affects methotrexate treatment outcome in rheumatoid arthritis

The folate antagonist methotrexate (MTX) is a drug currently used in the treatment of rheumatoid arthritis (RA). MTX enters the cells through the reduced folate carrier (RFC-1) and is activated to polyglutamates. Previous studies have shown that RFC-1 expression may influence the efficacy of therapy with MTX. The studies suggest that G80A polymorphism in RFC-1 is associated with altered folate/antifolate levels and the subjects carrying homozygous mutant 80AA genotype tend to have higher plasma folate and MTX concentrations and higher erythrocyte polyglutamate levels compared with those with the wild type or heterozygous genotype. It is possible that this polymorphism might influence MTX treatment outcome in patients with RA. In the present study, we examined the association between RFC-1 G80A polymorphism and treatment outcome in patients with RA administered MTX. The study was carried out on 174 patients diagnosed with RA treated with MTX (7.5-15.0 mg weekly) plus low doses of methylprednisone. The RFC-1 80G>A polymorphism (resulting in a histidine-to-arginine substitution at codon 27 of RFC-1) was detected using a polymerase chain reaction-restriction fragment length polymorphism method. The probability of remission of RA symptoms was 3.32-fold higher in carriers of 80AA genotype as compared with patients with 80GG genotype (P=0.021, OR=3.32, 95% CI: 1.26-8.79). The frequency of A allele among MTX responders was 62.1, compared to 47.8% in a group of poor MTX responders (P=0.013, OR=1.78, 95% CI: 1.13-2.81). Moreover, the increase of aminotransferase activity was noted more frequently in carriers of 80AA genotype. The present data suggest that evaluation of RFC-1 gene 80G>A polymorphism may be a useful tool to optimize MTX therapy in patients with RA.

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

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

Coexistence of multiple mechanisms of PT523 resistance in human leukemia cells harboring 3 reduced folate carrier alleles: transcriptional silencing, inactivating mutations, and allele loss

The reduced folate carrier (RFC) is the dominant route for the uptake of various antifolates including PT523, a potent dihydrofolate reductase inhibitor (Ki = 0.35 pM) and an excellent transport substrate of the RFC (Kt = 0.7 μM). Here, we describe the multiple mechanisms of RFC inactivation in human leukemia PT523-resistant cells originally harboring 3 RFC alleles. Cellular exposure to gradually increasing PT523 concentrations resulted in sublines displaying up to 3500-fold resistance to various hydrophilic antifolates that rely on RFC for their cellular uptake. Antifolate-resistant cells lost RFC gene expression (65%-99% loss) due to impaired promoter binding of various transcription factors that regulate RFC gene expression. Additionally, DNA sequencing revealed that PT523-resistant cells contained a cluster of 4 nearly consecutive mutations residing on a single RFC allele including L143P, A147V, R148G, and Q150Stop. Southern blot analysis established the loss of an RFC allele in PT523-resistant cells. These alterations resulted in markedly decreased RFC protein levels (∼80%-99% loss) and consequently impaired [3H]methotrexate transport (87%-99% loss). This study provides the first evidence that acquisition of PT523 resistance in human leukemia cells harboring 3 RFC alleles is due to multiple coexisting alterations including transcriptional silencing, inactivating mutations, and RFC allele loss.

The Reduced Folate Carrier Gene Is a Novel Selectable Marker for Recombinant Protein Overexpression

Folate cofactors are one-carbon donors essential for the biosynthesis of purines and thymidylate. Mammalian cells are devoid of folate biosynthesis and are therefore folate auxotrophs that take up folate vitamins primarily via the reduced folate carrier (RFC). In this study, we showed that the human RFC (hRFC) gene can serve as a novel selectable marker for the overproduction of recombinant proteins. Toward this end, a hemagglutinin (HA) epitope tagged hRFC (hRFC-HA) was introduced into a bicistronic vector (pIRES2-EGFP), upstream of an enhanced green fluorescent protein (EGFP) reporter gene. Chinese hamster ovary cells deficient in RFC activity were isolated and transfected with this construct, followed by gradual deprivation of leucovorin, the sole folate source in the growth medium. Only cells with hRFC-HA overexpression were able to take up leucovorin and thereby survive these selective conditions. Western blot and immunofluorescence analyses confirmed that the hRFC-HA was overexpressed at extremely high levels, properly glycosylated and sorted out to the plasma membrane. This resulted in a approximately 450-fold increase in [3H]methotrexate influx and approximately 100-fold increased sensitivity to methotrexate, relative to untransfected RFC-deficient cells. Flow cytometric analysis consistently revealed that EGFP was overexpressed approximately 100-fold above the autofluorescence level. Overproduction of hRFC-HA and EGFP was stably maintained for at least 2 months in a constant concentration of leucovorin. These results establish a novel RFC-based metabolic selection system for the efficient overexpression of recombinant proteins. Furthermore, the possible implications to subcellular transporter localization and restoration of MTX sensitivity in drug-resistant tumors by RFC-based gene therapy are discussed.

Sulfasalazine is a potent inhibitor of the reduced folate carrier: Implications for combination therapies with methotrexate in rheumatoid arthritis

OBJECTIVE

To investigate whether interactions of sulfasalazine (SSZ) with reduced folate carrier (RFC), the dominant cell membrane transporter for natural folates and methotrexate (MTX), may limit the efficacy of combination therapy with MTX and SSZ in patients with rheumatoid arthritis.

METHODS

Human RFC-(over)expressing CEM cells of T cell origin were used to analyze the effect of SSZ on the RFC-mediated cellular uptake of radiolabeled MTX and the natural folate leucovorin. Moreover, both cells with and those without acquired resistance to SSZ were used to assess the antiproliferative effects of MTX in combination with SSZ.

RESULTS

Transport kinetic analyses revealed that SSZ was a potent noncompetitive inhibitor of RFC-mediated cellular uptake of MTX and leucovorin, with mean +/- SD K(i) (50% inhibitory concentration) values of 36 +/- 6 microM and 74 +/- 7 microM, respectively. Consistent with the inhibitory interaction of SSZ with RFC, a marked loss of MTX efficacy was observed when MTX was coadministered with SSZ: up to 3.5-fold for CEM cells in the presence of 0.25 mM of SSZ, and >400-fold for SSZ-resistant cells in the presence of 2.5 mM of SSZ. Importantly, along with diminished efficacy of MTX, evidence for cellular folate depletion was obtained by the demonstration of an SSZ dose-dependent decrease in leucovorin accumulation.

CONCLUSION

At clinically relevant plasma concentrations, interactions of SSZ with RFC provide a biochemical rationale for 2 important clinical observations: 1) the onset of (sub)clinical folate deficiency during SSZ treatment, and 2) the lack of additivity/synergism of the combination of SSZ and MTX when these disease-modifying antirheumatic drugs are administered simultaneously. Thus, when considering use of these drugs in combination therapies, the present results provide a rationale both for the use of folate supplementation and for spacing administration of these drugs over time.

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.

Loss of multidrug resistance protein 1 expression and folate efflux activity results in a highly concentrative folate transport in human leukemia cells

We studied the molecular basis of the up to 46-fold increased accumulation of folates and methotrexate (MTX) in human leukemia CEM-7A cells established by gradual deprivation of leucovorin (LCV). CEM-7A cells consequently exhibited 10- and 68-fold decreased LCV and folic acid growth requirements and 23-25-fold hypersensitivity to MTX and edatrexate. Although CEM-7A cells displayed a 74-86-fold increase in the reduced folate carrier (RFC)-mediated influx of LCV and MTX, RFC overexpression per se cannot induce a prominently increased folate/MTX accumulation because RFC functions as a nonconcentrative anion exchanger. We therefore explored the possibility that folate efflux activity mediated by members of the multidrug resistance protein (MRP) family was impaired in CEM-7A cells. Parental CEM cells expressed substantial levels of MRP1, MRP4, poor MRP5 levels, whereas MRP2, MRP3 and breast cancer resistance protein were undetectable. In contrast, CEM-7A cells lost 95% of MRP1 levels while retaining parental expression of MRP4 and MRP5. Consequently, CEM-7A cells displayed a 5-fold decrease in the [(3)H]folic acid efflux rate constant, which was identical to that obtained with parental CEM cells, when their folic acid efflux was blocked (78%) with probenecid. Furthermore, when compared with parental CEM, CEM-7A cells accumulated 2-fold more calcein fluorescence. Treatment of parental cells with the MRP1 efflux inhibitors MK571 and probenecid resulted in a 60-100% increase in calcein fluorescence. In contrast, these inhibitors failed to alter the calcein fluorescence in CEM-7A cells, which markedly lost MRP1 expression. Replenishment of LCV in the growth medium of CEM-7A cells resulted in resumption of normal MRP1 expression. These results establish for the first time that MRP1 is the primary folate efflux route in CEM leukemia cells and that the loss of folate efflux activity is an efficient means of markedly augmenting cellular folate pools. These findings suggest a functional role for MRP1 in the maintenance of cellular folate homeostasis.

Resistance to multiple novel antifolates is mediated via defective drug transport resulting from clustered mutations in the reduced folate carrier gene in human leukaemia cell lines

We have studied the molecular basis of resistance of multiple human leukaemia CCRF-CEM sublines to the novel antifolates ZD9331, GW1843, AG2034, PT523 and edatrexate, which use the reduced folate carrier (RFC) as their main cellular uptake route and that target different folate-dependent enzymes. Antifolate-resistant sublines established by stepwise and single-step selections displayed up to 2135-fold resistance to the selection drug, and up to 2323-fold cross-resistance to various hydrophilic antifolates. In contrast, these sublines were up to 17- and 20-fold hypersensitive to the lipophilic antifolates AG377 and trimetrexate, respectively. The total reduced folate pool of these antifolate-resistant sublines shrunk by 87-96%, resulting in up to 42-fold increased folic acid growth requirement. These sublines lost 92-97% of parental [(3)H]methotrexate influx rates. Genomic PCR single-strand conformational polymorphism analysis and sequencing revealed that most of these drug-resistant sublines harboured RFC mutations that surprisingly clustered in two confined regions in exons 2 and 3. The majority of these mutations resulted in frame-shift and/or premature translation termination and lack of RFC protein expression. The remaining mutations involved single amino acid substitutions predominantly residing in the first transmembrane domain (TMD1). Some RFC-inactivating mutations emerged during the early stages of antifolate selection and were stably retained during further drug selection. Furthermore, some sublines displayed a markedly decreased or abolished RFC mRNA and/or protein expression. This constitutes the first demonstration of clustering of multiple human RFC mutations in TMD1, thereby suggesting that it plays a functional role in folate/antifolate binding and/or translocation. This is the first molecular characterization of human RFC-associated modalities of resistance to various novel antifolates in multiple leukaemia sublines.

Cytoplasmic domains of the reduced folate carrier are essential for trafficking, but not function

The reduced folate carrier (RFC) protein has a secondary structure consistent with the predicted 12 transmembrane (TM) domains, intracellular N- and C-termini and a large cytoplasmic loop between TM6 and TM7. In the present study, the role of the cytoplasmic domains in substrate transport and protein biogenesis were examined using an array of hamster RFC deletion mutants fused to enhanced green fluorescent protein and expressed in Chinese hamster ovary cells. The N- and C-terminal tails were removed both individually and together, or the large cytoplasmic loop was modified such that the domain size and role of conserved sequences could be examined. The loss of the N- or C-terminal tails did not appear to significantly disrupt protein function, although both termini appeared to have a role in the efficiency with which molecules exited the endoplasmic reticulum to localize at the plasma membrane. There appeared to be both size and sequence requirements for the intracellular loop, which are able to drastically affect protein stability and function unless met. Furthermore, there might be an indirect role for the loop in substrate translocation, since even moderate changes significantly reduced the V(max) for methotrexate transport. Although these cytoplasmic domains do not appear to be absolutely essential for substrate transport, each one is important for biogenesis and localization.

Molecular and cellular biology of the human reduced folate carrier

The natural folates are water-soluble members of the B class of vitamins that are essential for cell proliferation and tissue regeneration. Since mammalian cells cannot synthesize folates de novo, tightly regulated and sophisticated cellular uptake processes have evolved to sustain sufficient levels of intracellular tetrahydrofolate cofactors to support the biosynthesis of purines, pyrimidines, serine, and methione. Membrane transport is also a critical determinant of the antitumor activity of antifolate therapeutics (methotrexate, Tomudex) used in cancer chemotherapy, and impaired uptake of antifolates is a frequent mode of drug resistance. The reduced folate carrier is the major transport system for folates and classical antifolates in mammalian cells and tissues. This review summarizes the remarkable advances in the cellular and molecular biology of the human reduced folate carrier over the past decade, relating to its molecular structure and transport function, mechanisms of transcriptional and posttranscriptional regulation, and its critical role in antifolate response and resistance. Many key in vitro findings have now begun to be extended to studies of reduced folate carrier levels and function in patient specimens, paving the way for translating basic laboratory studies in cultured cells to improvements in human health and treatment of disease. The results of research into the human reduced folate carrier should clarify the roles of changes in expression and function of this system that accompany nutritional folate deficiency and human disease, and may lead to improved therapeutic strategies for enhancing drug response and circumventing resistance in cancer patients undergoing chemotherapy with antifolates.

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.