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

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.

Substrate-specific binding and conformational changes involving Ser313 and transmembrane domain 8 of the human reduced folate carrier, as determined by site-directed mutagenesis and protein cross-linking

RFC (reduced folate carrier) is the major transporter for reduced folates and antifolates [e.g. MTX (methotrexate)]. RFC is characterized by two halves, each with six TMD (transmembrane domain) α helices connected by a hydrophilic loop, and cytoplasmic N- and C-termini. We previously identified TMDs 4, 5, 7, 8, 10 and 11 as forming the hydrophilic cavity for translocation of (anti)folates. The proximal end of TMD8 (positions 311–314) was implicated in substrate binding from scanning-cysteine accessibility methods; cysteine replacement of Ser³¹³ resulted in loss of transport. In the present study, Ser³¹³ was mutated to alanine, cysteine, phenylalanine and threonine. Mutant RFCs were expressed in RFC-null R5 HeLa cells. Replacement of Ser³¹³ with cysteine or phenylalanine abolished MTX transport, whereas residual activity was preserved for the alanine and threonine mutants. In stable K562 transfectants, S313A and S313T RFCs showed substantially decreased V_max values without changes in K_t values for MTX compared with wild-type RFC. S313A and S313T RFCs differentially impacted binding of ten diverse (anti)folate substrates. Cross-linking between TMD8 and TMD5 was studied by expressing cysteine-less TMD1–6 (N₆) and TMD7–12 (C₆) half-molecules with cysteine insertions spanning these helices in R5 cells, followed by treatment with thiol-reactive homobifunctional cross-linkers. C₆–C₆ and N₆–N₆ cross-links were seen for all cysteine pairs. From the N₆ and C₆ cysteine pairs, Cys¹⁷⁵/Cys³¹¹ was cross-linked; cross-linking increased in the presence of transport substrates. The results of the present study indicate that the proximal end of TMD8 is juxtaposed to TMD5 and is conformationally active in the presence of transport substrates, and TMD8, including Ser³¹³, probably contributes to the RFC substrate-binding domain.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Marked suppression of the activity of some, but not all, antifolate compounds by augmentation of folate cofactor pools within tumor cells

Folates have been co-administered with some antifolates to diminish host toxicity; however, the extent to which this will reduce antitumor activity is not known. To further clarify this issue, studies were undertaken to characterize and quantitate the impact of alterations in intracellular folate levels on the activities of a variety of antifolates in L1210 murine leukemia cells. Intracellular folate cofactor levels increased almost in proportion to the increase in extracellular 5-formyltetrahydrofolate (5-CHO-THF) over a concentration range that encompassed physiological levels of 5-methyltetrahydrofolate. This resulted in a spectrum of increases in the ic50 values of antifolates upon continuous exposure to drugs [Lometrexol (DDATHF) (70x) > trimetrexate (TMQ) (30x), multitargeted antifolate, LY231514 (ALIMTA) (30x) > Raltitrexed, Tomudex (ZD1694) (10x), 6R-2',5'-thienyl-5,10-dideazatetrahydrofolic acid (LY309887) (10x) > methotrexate (MTX) (6x) > (2S)-2-[o-fluoro-p-[N-(2,7-dimethyl-4-oxo-3,4-dihydroquinazolin-6-ylmethyl)-N-(prop-2-ynyl)amino]benzamido]-4-(tetrazol-5-yl) butyric acid (ZD9331) (3x), N(alpha)-(4-amino-4-deoxypteroyl)-N(delta)-hemiphthaloyl-l-ornithine (PT523) (3x)]. Upon a 4-hr pulse exposure to drug, the ic50 values for DDATHF and ALIMTA were increased > 180- and 5-fold, respectively, with only a 2.5-fold increase in the extracellular 5-CHO-THF level within the physiological range. The reductions in drug sensitivities could be attributed to decreases in accumulation of polyglutamate derivatives of ALIMTA and DDATHF. Hence, in these studies, natural folates diminished the activity of agents that undergo polyglutamation by suppression of the formation of these active congeners at the level of folylpolyglutamate synthetase. For inhibitors of dihydrofolate reductase, the suppressive effect of endogenous folates appears to be due to competition between the antifolate and dihydrofolate at the level of the target enzyme. These data should be carefully considered in the design of regimens with antifolates, which incorporate co-administration of folates.