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

Structural basis for recognition and transport of folic acid in mammalian cells

Structural studies on mammalian vitamin transport lag behind other metabolites. Folates, also known as B9 vitamins, are essential cofactors in one-carbon transfer reactions in biology. Three different systems control folate uptake in the human body; folate receptors function to capture and internalise extracellular folates via endocytosis, whereas two major facilitator superfamily transporters, the reduced folate carrier (RFC; SLC19A1) and proton-coupled folate transporter (PCFT; SLC46A1) control the transport of folates across cellular membranes. Targeting specific folate transporters is being pursued as a route to developing new antifolates with improved pharmacology. Recent structures of the proton-coupled folate transporter, PCFT, revealed key insights into antifolate recognition and the mechanism of proton-coupled transport. Combined with previously determined structures of folate receptors and new predictions for the structure of the RFC, we are now able to develop a structure-based understanding of folate and antifolate recognition to accelerate efforts in antifolate drug development.

Effect of rare coding variants of charged amino acid residues on the function of human organic anion transporting polypeptide 1B3 (SLCO1B3)

Human organic anion transporting polypeptide 1B3 (OATP1B3, gene symbol SLCO1B3) is a liver-specific uptake transporter. Its function was reported to be largely affected by some positively charged amino acid residues. However, so far the effect of naturally occurring genetic variants of charged residues on OATP1B3's function has not been explored yet. Therefore, in the present study nonsynonymous single nucleotide variants that led to the replacement of charged residues of OATP1B3 were investigated. Our results demonstrated that rare coding variants c.542G > A (p.R181H) and c.592G > A (p.D198N) had a great effect on the function of OATP1B3 mainly due to their influence on protein's surface expression. Further mutation studies showed that a negatively charged residue at position 198 was indispensable to the proper expression of OATP1B3 on the plasma membrane, while a positively charged reside at position 181 was not a must. Structural modeling indicated that R181 is located at the center of putative transmembrane domain 4 (TM4) and its side chain faces towards TM2 instead of towards the substrate translocation pathway, whereas D198 is located at the border of TM4 and intracellular loop 2 and may electrostatically repulse negatively charged phospholipid head groups. In conclusion, our results indicated that rare coding variants that cause changes of charged amino acid residues might have large influence on the function and expression of OATP1B3.

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.

Biology of the major facilitative folate transporters SLC19A1 and SLC46A1

This chapter focuses on the biology of the major facilitative membrane folate transporters, the reduced folate carrier (RFC), and the proton-coupled folate transporter (PCFT). Folates are essential vitamins, and folate deficiency contributes to a variety of heath disorders. RFC is ubiquitously expressed and is the major folate transporter in mammalian cells and tissues. PCFT mediates intestinal absorption of dietary folates. Clinically relevant antifolates such as methotrexate (MTX) are transported by RFC, and the loss of RFC transport is an important mechanism of MTX resistance. PCFT is abundantly expressed in human tumors and is active under pH conditions associated with the tumor microenvironment. Pemetrexed (PMX) is an excellent substrate for PCFT as well as for RFC. Novel tumor-targeted antifolates related to PMX with selective membrane transport by PCFT over RFC are being developed. The molecular picture of RFC and PCFT continues to evolve relating to membrane topology, N-glycosylation, energetics, and identification of structurally and functionally important domains and amino acids. The molecular bases for MTX resistance associated with loss of RFC function, and for the rare autosomal recessive condition, hereditary folate malabsorption (HFM), attributable to mutant PCFT, have been established. From structural homologies to the bacterial transporters GlpT and LacY, homology models were developed for RFC and PCFT, enabling new mechanistic insights and experimentally testable hypotheses. RFC and PCFT exist as homo-oligomers, and evidence suggests that homo-oligomerization of RFC and PCFT monomeric proteins may be important for intracellular trafficking and/or transport function. Better understanding of the structure and function of RFC and PCFT should facilitate the rational development of new therapeutic strategies for cancer as well as for HFM.

Differential usage of the transport systems for folic acid and methotrexate in normal human T-lymphocytes and leukemic cells

Methotrexate (MTX) has been used as an effective anti-cancer drug for a long time. Conceptually, it is accepted that MTX and folic acid are transported by folate receptors (FRs) in cancerous cells, but the exact mechanism of MTX uptake in human leukemia is unknown. The objective of this study was to investigate different transport systems for FA and MTX, and to delineate their uptake mechanism in MOLT4, K562, Hut78 leukemia cells and normal human T cells. In MOLT4, uptake of MTX was higher than FA, similar to that of K562, Hut78 and normal T cells. In MOLT4 cells, MTX uptake was maximum at pH 7.4 whereas FA uptake was maximum at pH 4.5. Uptake of FA and MTX was significantly inhibited by anions, suggesting anion-dependent transport system. FA uptake was found to be energy dependent whereas MTX uptake was energy independent. RT-PCR and immunofluorescence results demonstrated the presence of reduced folate carrier as well as proton coupled folate transporter and absence of FR in MOLT4 and normal T cells. These data suggest the existence of two separate and independent carrier-mediated transport systems for the uptake of FA and MTX in normal and leukemic human T cells.

Oligomeric structure of the human reduced folate carrier: identification of homo-oligomers and dominant-negative effects on carrier expression and function

The ubiquitously expressed reduced folate carrier (RFC) is the major transport system for folate cofactors in mammalian cells and tissues. Previous considerations of RFC structure and mechanism were based on the notion that RFC monomers were sufficient to mediate transport of folate and antifolate substrates. The present study examines the possibility that human RFC (hRFC) exists as higher order homo-oligomers. By chemical cross-linking, transiently expressed hRFC in hRFC-null HeLa (R5) cells with the homobifunctional cross-linker 1,3-propanediyl bis-methanethiosulfonate and Western blotting, hRFC species with molecular masses of hRFC homo-oligomers were identified. Hemagglutinin- and Myc epitope-tagged hRFC proteins expressed in R5 cells were co-immunoprecipitated from both membrane particulate and surface-enriched membrane fractions, indicating that oligomeric hRFC is expressed at the cell surface. By co-expression of wild type and inactive mutant S138C hRFCs, combined with surface biotinylation and confocal microscopy, a dominant-negative phenotype was demonstrated involving greatly decreased cell surface expression of both mutant and wild type carrier caused by impaired intracellular trafficking. For another hRFC mutant (R373A), expression of oligomeric wild type-mutant hRFC was accompanied by a significant and disproportionate loss of wild type activity unrelated to the level of surface carrier. Collectively, our results demonstrate the existence of hRFC homo-oligomers. They also establish the likely importance of these higher order hRFC structures to intracellular trafficking and carrier function.

Role of lysine 411 in substrate carboxyl group binding to the human reduced folate carrier, as determined by site-directed mutagenesis and affinity inhibition

Reduced folate carrier (RFC) is the major membrane transporter for folates and antifolates in mammalian tissues. Recent studies used radioaffinity labeling with N-hydroxysuccinimide (NHS)-[(3)H]methotrexate (MTX) to localize substrate binding to residues in transmembrane domain (TMD) 11 of human RFC. To identify the modified residue(s), seven nucleophilic residues in TMD11 were mutated to Val or Ala and mutant constructs expressed in RFC-null HeLa cells. Only K411A RFC was not inhibited by NHS-MTX. By radioaffinity labeling with NHS-[(3)H]MTX, wild-type (wt) RFC was labeled; for K411A RFC, radiolabeling was abolished. When Lys411 was replaced with Ala, Arg, Gln, Glu, Leu, and Met, only K411E RFC showed substantially decreased transport. Nine classic diamino furo[2,3-d]pyrimidine antifolates with unsubstituted alpha- and gamma-carboxylates (1), hydrogen- or methyl-substituted alpha-(2,3) or gamma-(4,5) carboxylates, or substitutions of both alpha- and gamma-carboxylates (6-9) were used to inhibit [(3)H]MTX transport with RFC-null K562 cells expressing wt and K411A RFCs. For wt and K411A RFCs, inhibitory potencies were in the order 4 > 5 > 1 > 3 > 2; 6 to 9 were poor inhibitors. Inhibitions decreased in the presence of physiologic anions. When NHS esters of 1, 2, and 4 were used to covalently modify wt RFC, inhibitory potencies were in the order 2 > 1 > 4; inhibition was abolished for K411A RFC. These results establish that Lys411 participates in substrate binding via an ionic association with the substrate gamma-carboxylate; however, this is not essential for transport. An unmodified alpha-carboxylate is required for high-affinity substrate binding to RFC, whereas the gamma-carboxyl is not essential.

Structure and function of the reduced folate carrier a paradigm of a major facilitator superfamily mammalian nutrient transporter

Folates are essential for life and folate deficiency contributes to a host of health problems including cardiovascular disease, fetal abnormalities, neurological disorders, and cancer. Antifolates, represented by methotrexate, continue to occupy a unique niche among the modern day pharmacopoeia for cancer along with other pathological conditions. This article focuses on the biology of the membrane transport system termed the "reduced folate carrier" or RFC with a particular emphasis on RFC structure and function. The ubiquitously expressed RFC is the major transporter for folates in mammalian cells and tissues. Loss of RFC expression or function portends potentially profound physiological or developmental consequences. For chemotherapeutic antifolates used for cancer, loss of RFC expression or synthesis of mutant RFC protein with impaired function results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and low levels of substrate for polyglutamate synthesis. The functional properties for RFC were first documented nearly 40 years ago in murine leukemia cells. Since 1994, when RFC was first cloned, tremendous advances in the molecular biology of RFC and biochemical approaches for studying the structure of polytopic membrane proteins have led to an increasingly detailed picture of the molecular structure of the carrier, including its membrane topology, its N-glycosylation, identification of functionally and structurally important domains and amino acids, and helix packing associations. Although no crystal structure for RFC is yet available, biochemical and molecular studies, combined with homology modeling, based on homologous bacterial major facilitator superfamily transporters such as LacY, now permit the development of experimentally testable hypotheses designed to establish RFC structure and mechanism.

Human reduced folate carrier: translation of basic biology to cancer etiology and therapy

This review attempts to provide a comprehensive overview of the biology of the physiologically and pharmacologically important transport system termed the "reduced folate carrier" (RFC). The ubiquitously expressed RFC has unequivocally established itself as the major transport system in mammalian cells and tissues for a group of compounds including folate cofactors and classical antifolate therapeutics. Loss of RFC expression or function may have potentially profound pathophysiologic consequences including cancer. For chemotherapeutic antifolates used for cancer such as methotrexate or pemetrexed, synthesis of mutant RFCs or loss of RFC transcripts and proteins results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and insufficient substrate for polyglutamate synthesis. Since RFC was first cloned in 1994, tremendous advances have been made in understanding the complex transcriptional and posttranscriptional regulation of RFC, in identifying structurally and functionally important domains and amino acids in the RFC molecule as a prelude to establishing the mechanism of transport, and in characterizing the molecular defects in RFC associated with loss of transport in antifolate resistant cell line models. Many of the insights gained from laboratory models of RFC portend opportunities for modulating carrier expression in drug resistant tumors, and for designing a new generation of agents with improved transport by RFC or substantially enhanced transport by other folate transporters over RFC. Many of the advances in the basic biology of RFC in cell line models are now being directly applied to human cancers in the clinical setting, most notably pediatric acute lymphoblastic leukemia and osteogenic sarcoma.

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.

Redesigning channel-forming peptides: amino acid substitutions that enhance rates of supramolecular self-assembly and raise ion transport activity

Three series of 22-residue peptides derived from the transmembrane M2 segment of the glycine receptor alpha1-subunit (M2GlyR) have been designed, synthesized, and tested to determine the plasticity of a channel-forming sequence and to define whether channel pores with enhanced conductive properties could be created. Sixteen sequences were examined for aqueous solubility, solution-association tendency, secondary structure, and half-maximal concentration for supramolecular assembly, channel activity, and ion transport properties across epithelial monolayers. All peptides interact strongly with membranes: associating with, inserting across, and assembling to form homooligomeric bundles when in micromolar concentrations. Single and double amino acid replacements involving arginine and/or aromatic amino acids within the final five C-terminal residues of the peptide cause dramatic effects on the concentration dependence, yielding a range of K1/2 values from 36 +/- 5 to 390 +/- 220 microM for transport activity. New water/lipid interfacial boundaries were established for the transmembrane segment using charged or aromatic amino acids, thus limiting the peptides' ability to move perpendicularly to the plane of the bilayer. Formation of discrete water/lipid interfacial boundaries appears to be necessary for efficient supramolecular assembly and high anion transport activity. A peptide sequence is identified that may show efficacy in channel replacement therapy for channelopathies such as cystic fibrosis.

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.

Restoration of transport activity by co-expression of human reduced folate carrier half-molecules in transport-impaired K562 cells: localization of a substrate binding domain to transmembrane domains 7-12

Reduced folates such as 5-methyl tetrahydrofolate and classical antifolates such as methotrexate are actively transported into mammalian cells by the reduced folate carrier (RFC). RFC is characterized by 12 stretches of mostly hydrophobic, alpha-helix-promoting amino acids, internally oriented N and C termini, and a large central linker connecting transmembrane domains (TMDs) 1-6 and 7-12. Previous studies showed that deletion of the majority of the central loop domain between TMDs 6 and 7 abolished transport, but this segment could be replaced with mostly non-homologous sequence from the SLC19A2 thiamine transporter to restore transport function. In this report, we expressed RFC from separate TMD1-6 and TMD7-12 RFC half-molecule constructs, each with a unique epitope tag, in RFC-null K562 cells to restore transport activity. Restored transport exhibited characteristic transport kinetics for methotrexate, a capacity for trans-stimulation by pretreatment with leucovorin, and inhibition by N-hydroxysuccinimide methotrexate, a documented affinity inhibitor of RFC. The TMD1-6 half-molecule migrated on SDS gels as a 38-58 kDa glycosylated species and was converted to 27 kDa by N-glycosidase F or tunicamycin treatments. The 40 kDa TMD7-12 half-molecule was unaffected by these treatments. Using transfected cells expressing both TMDs 1-6 and TMDs 7-12 as separate polypeptides, the TMD7-12 half-molecule was covalently radiolabeled with N-hydroxysuccinimide [(3)H]methotrexate. No radioactivity was incorporated into the TMD1-6 half-molecule. Digestion with endoproteinase GluC decreased the size of the radiolabeled 40 kDa TMD7-12 polypeptide to approximately 20 kDa. Our results demonstrate that a functional RFC can be reconstituted with RFC half-molecules and localize a critical substrate binding domain to within TMDs 7-12.

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.

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.

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.

Characterization of a cysteine-less human reduced folate carrier: localization of a substrate-binding domain by cysteine-scanning mutagenesis and cysteine accessibility methods

The human reduced folate carrier (hRFC) mediates the transport of reduced folates and classical anti-folates into mammalian cells. Whereas the functionally important domains in hRFC are poorly characterized, previous studies with anti-folate-resistant cells suggest critical roles for transmembrane domain (TMD) 1 and residues (Gly44, Glu45, Ser46 and Ile48) in or flanking this region. An hRFC mutant devoid of cysteine residues was prepared by deleting the C-terminal 56 amino acids, including four cysteine residues, and mutagenizing the remaining cysteine residues to serine residues. A fully functional cysteine-less hRFC protein was expressed in transport-impaired MtxRIIOuaR2-4 Chinese-hamster ovary cells. To explore the role of residues in or flanking TMD1 in transport, all 24 amino acids from Trp25 to Ile48 of hRFC were mutated individually to cysteine residues, and the mutant hRFCs were transfected into MtxRIIOuaR2-4 cells. All of the 24 cysteine mutants were expressed and, with the exception of R42C (Arg42-->Cys), were capable of mediating methotrexate uptake above the low level in MtxRIIOuaR2-4 cells. We found that by treating the transfected cells with the small, water-soluble, thiol-reactive anionic reagent, sodium (2-sulphonatoethyl) methanethiosulphonate, methotrexate transport by several of the cysteine-substituted hRFC mutants was significantly inhibited, including Q40C, G44C, E45C and I48C. Sodium (2-sulphonatoethyl) methanethiosulphonate transport inhibition of the Q40C, G44C and I48C mutants was protected by leucovorin [(6R, S)-5-formyltetrahydrofolate], indicating that these residues lie at or near a substrate-binding site. Using surface-labelling reagents [N-biotinylaminoethyl methanethiosulphonate and 3-(N-maleimidylpropionyl)biocytin, combined with 4-acetamido-4'-maleimidylstilbene-2,2'-disulphonic acid] with cysteine mutants from positions 37-48, the extracellular TMD1 boundary was found to lie between residues 39 and 40, and amino acids 44-46 and 48 were localized to the TMD1 exofacial loop. Collectively, our results imply that amino acids 40, 44, 48 and, possibly, 42 serve important roles in hRFC transport, albeit not as structural components of the putative transmembrane channel for folate substrates.

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.

Uptake of 5-methyltetrahydrofolate into PC-3 human prostate cancer cells is carrier-mediated

Uptake of 5-methyltetrahydrofolate into the PC-3 human prostate cancer cells was linear for the first 60 min. There was no difference in the initial rate of uptake in cells incubated in folate-free medium for 24 or 48 hr compared to control cells grown in folate-containing medium. The initial rate of 5-methyltetrahydrofolate uptake showed little dependence on extracellular pH and it was independent of extracellular sodium ions. Transport of 5-methyltetrahydrofolate into PC-3 cells was saturable - K(m) = 0.74 micro M and V(max) = 7.78 nmol/10(9)cells/min and these kinetic constants were not different in cells incubated for 24 hr in folate-free medium (K(m) = 0.80 +/- 0.22, V(max) = 8.52 +/- 0.50; P = 0.09, N = 3). Uptake of 5-methyltetrahydrofolate was inhibited by structural analogs with the K(i) values being 0.50, 1.79, and 31.8 micro M for 5-formyltetrahydrofolate, methotrexate, and folic acid, respectively. Uptake of 5-methyltetrahydrofolate was inhibited by the energy poisons, sodium cyanide, sodium arsenate, p-chloromercuriphenylsulfonate, and sodium azide. Uptake was inhibited by increasing concentrations of sulfate and phosphate ions, suggesting that 5-methyltetrahydrofolate may be transported by an anion-exchange mechanism. These results show that 5-methyltetrahydrofolate is transported into PC-3 prostate cancer cells by a carrier-mediated process.

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.

Large diversity in transport-mediated methotrexate resistance in human leukemia cell line CCRF-CEM established in a high concentration of leucovorin

To elucidate the mechanism(s) of methotrexate (MTX) resistance as a possible reason underlying treatment failure in high-dose MTX regimens combined with leucovorin (LV) rescue, we established MTX-resistant human T-cell leukemia cell line CCRF-CEM cells in the presence of excess LV, and characterized their properties. Continuous exposure of the cells to escalating concentrations of MTX up to 20 microM in the presence of 1000 nM LV resulted in establishment of three MTX-resistant sublines with a wide disparity of resistance degree over a 4 logarithmic range (approximately 40-, 900- and 44,000-fold, respectively). Transmembrane transport of MTX in these sublines was diminished to 52%, 35% and 12%, respectively. Intracellular retention of MTX in these sublines was not different from that of the parent cells. A cell growth study in various concentrations of LV showed that cells with higher resistance to MTX required more LV for optimal growth. In parallel with the resistance levels, there was an increase in mRNA expression of dihydrofolate reductase gene and a decrease in that of thymidylate synthase gene, but no change in that of reduced folate carrier (RFC1) gene, as assessed by northern blot analysis. Sequencing of the RFC1 gene in all 3 sublines revealed a point mutation in codon 47 (TCC-->TTC) resulting in substitution of Phe for Ser residue, and additional deletion of CTG of codon 112 in the subline with the highest resistance. In summary, MTX exposure to CCRF-CEM cells in the presence of 1000 nM LV resulted in the establishment of heterogeneous cell populations with a wide range of transport-mediated MTX resistance, which was associated with differential alterations of RFC gene. These cell lines may serve as models for investigation of the molecular mechanism(s) underlying refractory tumors in high-dose MTX regimens with LV rescue.

Identification of lysine-411 in the human reduced folate carrier as an important determinant of substrate selectivity and carrier function by systematic site-directed mutagenesis

Site-directed mutagenesis was used to characterize the functional role of lysine-411, a conserved amino acid located in putative transmembrane domain (TMD) 11 of the human reduced folate carrier (hRFC). Lysine-411 was mutagenized to arginine, glutamate, and leucine, and the mutant constructs (K411R-, K411E-, and K411L-hRFC, respectively) were transfected into hRFC-deficient K562 cells. The mutant hRFC constructs were all expressed at high levels and restored 22-36% of the methotrexate (MTX) transport level in wild-type (K43-6) hRFC transfectants. Although 5-formyl tetrahydrofolate (5-CHO-H(4)PteGlu) uptake levels for both the K411E- and K411L-hRFCs were also impaired (approximately 33% and 28%, respectively), a complete restoration of the wild-type level was observed for K411R-hRFC. While loss of MTX transport activity for the K411R-hRFC transfectant was associated with an incomplete restoration of MTX sensitivity compared to K43-6 cells, these cells were similarly sensitive to Tomudex. The K411R-hRFC transfectants showed an approximately threefold decreased growth requirement for 5-CHO-H(4)PteGlu compared to K43-6 cells. The 5-CHO-H(4)PteGlu transport stimulation observed for the wild-type carrier in chloride-free buffer was also observed for K411R-hRFC, however, this response was decreased for the K411E- and K411L-hRFCs. The preservation of low levels of transport for the K411E- and K411L-hRFCs suggest that the amino acid at position 411 does not directly participate in the binding of anionic hRFC substrates. However, a functionally important role for a basic amino acid at position 411 was, nonetheless, implied by the increased MTX transport for wild-type hRFC over the K411 mutant hRFCs, and the highly selective uptake of 5-CHO-H(4)PteGlu over MTX for K411R-hRFC.

Functional role of arginine 373 in substrate translocation by the reduced folate carrier

The reduced folate carrier (RFC) plays a critical role in the cellular uptake of folates. However, little is known regarding the mechanism used to transport substrates or the tertiary structure of the protein. Through the analysis of a Chinese hamster ovary cell line deficient in folate uptake, we have identified a single residue in TM10 (Arg-373) of RFC that appears to play a critical role in the translocation of substrate. Replacement of this position with various amino acids (KHQNA) diminished the rate of translocation by 16-50-fold, although substrate binding, protein stability, and localization were unaffected. Furthermore, the translocation capabilities of an R373C mutant in a cysteine-less form of the reduced folate carrier were enhanced 2.5-fold by the positively charged methanethiosulfonate reagent, confirming the essential role of a positive charge at this position. When considering the membrane-impermeable nature of this reagent, the data further suggest that the Arg-373 residue is located within the substrate translocation pathway of the RFC protein. Moreover, cross-linking analysis of the Arg-373 residue demonstrates that it is within 6 A of residue Glu-394 (TM11), providing the first definitive tertiary structural information for this protein.

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.

Functional role of specific amino acid residues in human thiamine transporter SLC19A2: mutational analysis

SLC19A2 is a membrane thiamine transporter expressed in a variety of human tissues, including the gastrointestinal tract. Little is currently known about the structure/function relationship of SLC19A2. We examined the effect of introducing mutations in SLC19A2 identical to those found in thiamine-responsive megaloblastic anemia syndrome (TRMA), on functional activity and membrane expression of the transporter. We also examined the effect of mutating the only conserved anionic residue (E138) in the transmembrane (TM) domains of the SLC19A2 and that of the putative glycosylation sites (N63, N314). Northern blot analysis showed SLC19A2 mRNA was expressed at the same level in HeLa cells transfected with wild-type or mutated SLC19A2. Introducing the clinically relevant mutations (D93H, S143F, G172D) or mutation at the conserved anionic residue (E138A) of SLC19A2 led to a significant (P < 0.01) inhibition of thiamine uptake. Mutations of the two potential N-linked glycosylation sites (N63Q, N314Q) of SLC19A2 did not affect functional activity; they did, however, lead to a noticeable reduction in apparent molecular weight of protein. Western blot analysis showed all proteins (except D93H) were expressed in the membrane (not the cytoplasmic) fraction of HeLa cells. These results provide direct confirmation that clinically relevant mutations in SLC19A2 observed in TRMA cause malfunctioning of the transporter and/or a defect in its translation/stability. Results also show conserved TM anionic residue of the SLC19A2 protein is critical for its function. Furthermore, native SLC19A2 is glycosylated, but this is not important for its function.

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.

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.

Transport of methotrexate into LNCaP human prostate cancer cells

Uptake of methotrexate into the LNCaP human prostate cancer cells was linear for the first 60 min. The initial rate of methotrexate uptake was highest at extracellular pH 4.5 and decreased markedly until pH 7.0 to 8.0. Transport of methotrexate into LNCaP cells showed two components, one saturable -K(m) = 0.13 +/- 0.06 microM and V(max) = 1.20 +/- 0.16 pmol x 45 min(-1) x mg(-1) protein at low concentrations and the other apparently not saturable up to 10 microM. Uptake of methotrexate was inhibited by structural analogs with the K(i) values being 6.53, 12.4, and 85.6 microM for 5-formyltetrahydrofolate, 5-methyltetrahydrofolate, and folic acid, respectively. Uptake of methotrexate into LNCaP cells was not inhibited by the energy poisons in contrast to methotrexate uptake into PC-3 prostate cancer cells. Uptake was inhibited by increasing concentrations of sulfate and phosphate ions and by the organic anions probenecid and DIDS, suggesting that methotrexate may be transported by an anion-exchange mechanism. These results show that methotrexate is transported into LNCaP prostate cancer cells by a carrier-mediated process.

Transport of methotrexate into PC-3 human prostate cancer cells

Transport of methotrexate (MTX) into human prostatic PC-3 cells was studied. Uptake of MTX vs concentration was saturable at pH 7.4 in cells grown in normal medium and in cells incubated for 24 h in folate-free medium (Km = 3.24 and 4.84 microM, respectively (P > 0.05, n = 3) and Vmax = 0.64 and 0.92 nmol x min(-1) x 10(-9) cells, respectively (P < 0.05, n = 3)). In contrast, uptake at pH 4.5 showed both a saturable component (Km = 1.03 microM, Vmax = 0.42 nmol x min(-1) x 10(-9) cells) and a nonsaturable, linear component. Uptake was inhibited by the structural analogs 5-methyltetrahydrofolate, 5-formyltetrahydrofolate, and folic acid (K(i) = 6.8, 10.9, and 89.6 microM, respectively). Uptake was inhibited by increasing concentrations of chloride ion, suggesting that MTX transport in PC-3 cells may be via an anion-exchange mechanism. Uptake was significantly decreased by high concentrations of sodium cyanide and sodium arsenate but not by sodium azide. Uptake was inhibited by the sulfhydryl inhibitor p-chloromercuriphenylsulfonate and by the anions probenecid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. Uptake of MTX was independent of sodium ions in the medium. It is concluded that PC-3 human prostate cancer cells have a carrier-mediated system for the uptake of MTX and other folates.