Topological and functional analysis of the human reduced folate carrier by hemagglutinin epitope insertion

Immunodeficiency associated with a novel functionally defective variant of SLC19A1 benefits from folinic acid treatment

Insufficient dietary folate intake, hereditary malabsorption, or defects in folate transport may lead to combined immunodeficiency (CID). Although loss of function mutations in the major intestinal folate transporter PCFT/SLC46A1 was shown to be associated with CID, the evidence for pathogenic variants of RFC/SLC19A1 resulting in immunodeficiency was lacking. We report two cousins carrying a homozygous pathogenic variant c.1042 G > A, resulting in p.G348R substitution who showed symptoms of immunodeficiency associated with defects of folate transport. SLC19A1 expression by peripheral blood mononuclear cells (PBMC) was quantified by real-time qPCR and immunostaining. T cell proliferation, methotrexate resistance, NK cell cytotoxicity, Treg cells and cytokine production by T cells were examined by flow cytometric assays. Patients were treated with and benefited from folinic acid. Studies revealed normal NK cell cytotoxicity, Treg cell counts, and naive-memory T cell percentages. Although SLC19A1 mRNA and protein expression were unaltered, remarkably, mitogen induced-T cell proliferation was significantly reduced at suboptimal folic acid and supraoptimal folinic acid concentrations. In addition, patients' PBMCs were resistant to methotrexate-induced apoptosis supporting a functionally defective SLC19A1. This study presents the second pathogenic SLC19A1 variant in the literature, providing the first experimental evidence that functionally defective variants of SLC19A1 may present with symptoms of immunodeficiency.

A guide to plasma membrane solute carrier proteins

This review aims to serve as an introduction to the solute carrier proteins (SLC) superfamily of transporter proteins and their roles in human cells. The SLC superfamily currently includes 458 transport proteins in 65 families that carry a wide variety of substances across cellular membranes. While members of this superfamily are found throughout cellular organelles, this review focuses on transporters expressed at the plasma membrane. At the cell surface, SLC proteins may be viewed as gatekeepers of the cellular milieu, dynamically responding to different metabolic states. With altered metabolism being one of the hallmarks of cancer, we also briefly review the roles that surface SLC proteins play in the development and progression of cancer through their influence on regulating metabolism and environmental conditions.

Identification of the Receptor Used by the Ecotropic Mouse GLN Endogenous Retrovirus

Approximately 10% of the mouse genome is composed of endogenous retroviruses belonging to different families. In contrast to the situation in the human genome, several of these families correspond to recent, still-infectious elements capable of encoding complete viral particles. The mouse GLN endogenous retrovirus is one of these active families. We previously identified one fully functional provirus from the sequenced genome of the C57BL/6 mouse strain. The GLN envelope protein gives the infectious viral particles an ecotropic host range, and we had demonstrated that the receptor was neither CAT1 nor SMIT1, the two previously identified receptors for mouse ecotropic retroviral envelope proteins. In this study, we have identified SLC19A1, the reduced folate carrier, as the cellular protein used as a receptor by the GLN retrovirus. The ecotropic tropism exhibited by this envelope is due to the presence or absence of an N-linked glycosylation site in the first extracellular loop as well as the specific amino acid sequence of the extracellular domains of the receptor. Like all the other retroviral envelope proteins from the gammaretrovirus genus whose receptors have been identified, the GLN envelope protein uses a member of the solute carrier superfamily as a receptor.IMPORTANCE Endogenous retroviruses are genomic traces of past infections present in all vertebrates. Most of these elements degenerate over time and become nonfunctional, but the mouse genome still contains several families with full infection abilities. The GLN retrovirus is one of them, and its members encode particles that are able to infect only mouse cells. Here, we identified the cellular protein used as a receptor by GLN for cell entry. It is SLC19A1, the reduced folate carrier. We show that GLN infection is limited to mouse cells due to both a mutation in the mouse gene preventing the glycosylation of SLC19A1 and also other residues conserved within the rat but not in the hamster and human proteins. Like all other gammaretroviruses whose receptors have been identified, GLN uses a member of the solute carrier superfamily for cell entry, highlighting the role of these proteins for retroviral infection in mammals.

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.

Folate and thiamine transporters mediated by facilitative carriers (SLC19A1-3 and SLC46A1) and folate receptors

The reduced folate carrier (RFC, SLC19A1), thiamine transporter-1 (ThTr1, SLC19A2) and thiamine transporter-2 (ThTr2, SLC19A3) evolved from the same family of solute carriers. SLC19A1 transports folates but not thiamine. SLC19A2 and SLC19A3 transport thiamine but not folates. SLC19A1 and SLC19A2 deliver their substrates to systemic tissues; SLC19A3 mediates intestinal thiamine absorption. The proton-coupled folate transporter (PCFT, SLC46A1) is the mechanism by which folates are absorbed across the apical-brush-border membrane of the proximal small intestine. Two folate receptors (FOLR1 and FOLR2) mediate folate transport across epithelia by an endocytic process. Folate transporters are routes of delivery of drugs for the treatment of cancer and inflammatory diseases. There are autosomal recessive disorders associated with mutations in genes encoded for SLC46A1 (hereditary folate malabsorption), FOLR1 (cerebral folate deficiency), SLC19A2 (thiamine-responsive megaloblastic anemia), and SLC19A3 (biotin-responsive basal ganglia disease).

Disruption of thiamine uptake and growth of cells by feline leukemia virus subgroup A

Feline leukemia virus (FeLV) is still a major cause of morbidity and mortality in domestic cats and some wild cats despite the availability of relatively effective vaccines against the virus. FeLV subgroup A (FeLV-A) is transmitted in natural infections, and FeLV subgroups B, C, and T can evolve directly from FeLV-A by mutation and/or recombination with endogenous retroviruses in domestic cats, resulting in a variety of pathogenic outcomes. The cell surface entry receptor for FeLV-A is a putative thiamine transporter (THTR1). Here, we have addressed whether FeLV-A infection might disrupt thiamine uptake into cells and, because thiamine is an essential nutrient, whether this disruption might have pathological consequences. First, we cloned the cat ortholog of the other of the two known thiamine transporters in mammals, THTR2, and we show that feline THTR1 (feTHTR1) and feTHTR2 both mediate thiamine uptake, but feTHTR2 does not function as a receptor for FeLV-A. We found that feTHTR1 is widely expressed in cat tissues and in cell lines, while expression of feTHTR2 is restricted. Thiamine uptake mediated by feTHTR1 was indeed blocked by FeLV-A infection, and in feline fibroblasts that naturally express feTHTR1 and not feTHTR2, this blockade resulted in a growth arrest at physiological concentrations of extracellular thiamine. The growth arrest was reversed at high extracellular concentrations of thiamine. Our results show that FeLV-A infection can indeed disrupt thiamine uptake with pathological consequences. A prediction of these experiments is that raising the plasma levels of thiamine in FeLV-infected cats may ameliorate the pathogenic effects of infection.

Mechanisms of membrane transport of folates into cells and across epithelia

Until recently, the transport of folates into cells and across epithelia has been interpreted primarily within the context of two transporters with high affinity and specificity for folates, the reduced folate carrier and the folate receptors. However, there were discrepancies between the properties of these transporters and characteristics of folate transport in many tissues, most notably the intestinal absorption of folates, in terms of pH dependency and substrate specificity. With the recent cloning of the proton-coupled folate transporter (PCFT) and the demonstration that this transporter is mutated in hereditary folate malabsorption, an autosomal recessive disorder, the molecular basis for this low-pH transport activity is now understood. This review focuses on the properties of PCFT and briefly addresses the two other folate-specific transporters along with other facilitative and ATP-binding cassette (ABC) transporters with folate transport activities. The role of these transporters in the vectorial transport of folates across epithelia is considered.

Subcellular localization and distribution of the reduced folate carrier in normal rat tissues

The reduced folate carrier (Rfc1; Slc19a1) mediated transport of reduced folates and antifolate drugs such as methotrexate (MTX) play an essential role in physiological folate homeostasis and MTX cancer chemotherapy. As no systematic reports are as yet available correlating Rfc1 gene expression and protein levels in all tissues crucial for folate and antifolate uptake, storage or elimination, we investigated gene and protein expression of rat Rfc1 (rRfc1) in selected tissues. This included the generation of a specific anti-rRfc1 antibody. Rabbits were immunised with isolated rRfc1 peptides producing specific anti-rRfc1 antiserum targeted to the intracellular C-terminus of the carrier. Using RT-PCR analysis, high rRfc1 transcript levels were detected in colon, kidney, brain, thymus, and spleen. Moderate rRfc1 gene expression was observed in small intestine, liver, bone marrow, lung, and testes whereas transcript levels were negligible in heart, skeletal muscle or leukocytes. Immunohistochemical analyses revealed strong carrier expression in the apical membrane of tunica mucosa epithelial cells of small intestine and colon, in the brush-border membrane of choroid plexus epithelial cells or in endothelial cells of small vessels in brain and heart. Additionally, high rRfc1 protein levels were localized in the basolateral membrane of renal tubular epithelial cells, in the plasma membrane of periportal hepatocytes, and sertoli cells of the testes. Taken together, our results demonstrated that rRfc1 is expressed almost ubiquitously but to very different levels. The predominant tissue distribution supports the essential role of Rfc1 in physiological folate homeostasis. Moreover, our results may contribute to understand antifolate pharmacokinetics and selected organ toxicity associated with MTX chemotherapy.

Topology of transmembrane channel-like gene 1 protein

Mutations of transmembrane channel-like gene 1 (TMC1) cause hearing loss in humans and mice. TMC1 is the founding member of a family of genes encoding proteins of unknown function that are predicted to contain multiple transmembrane domains. The goal of our study was to define the topology of mouse TMC1 expressed heterologously in tissue culture cells. TMC1 was retained in the endoplasmic reticulum (ER) membrane of five tissue culture cell lines that we tested. We used anti-TMC1 and anti-HA antibodies to probe the topologic orientation of three native epitopes and seven HA epitope tags along full-length TMC1 after selective or complete permeabilization of transfected cells with digitonin or Triton X-100, respectively. TMC1 was present within the ER as an integral membrane protein containing six transmembrane domains and cytosolic N- and C-termini. There is a large cytoplasmic loop, between the fourth and fifth transmembrane domains, with two highly conserved hydrophobic regions that might associate with or penetrate, but do not span, the plasma membrane. Our study is the first to demonstrate that TMC1 is a transmembrane protein. The topologic organization revealed by this study shares some features with that of the shaker-TRP superfamily of ion channels.

Membrane topological analysis of the proton-coupled folate transporter (PCFT-SLC46A1) by the substituted cysteine accessibility method

The proton-coupled folate transporter (PCFT) mediates intestinal folate absorption. Loss-of-function mutations in this gene are the molecular basis for the autosomal recessive disorder, hereditary folate malabsorption. In this study, the substituted cysteine accessibility method was utilized to localize extra- or intracellular loops connecting predicted PCFT transmembrane domains. Cysteine-less PCFT was generated by replacement of all seven cysteine residues with serine and was shown to be functional, following which cysteine residues were introduced into predicted loops. HeLa cells, transiently transfected with these PCFT mutants, were then labeled with an impermeant, cysteine-specific biotinylation reagent (MTSEA-biotin) with or without permeabilization of cells. The biotinylated proteins were precipitated by streptavidin beads and assessed by Western blotting analysis. The biotinylation of PCFT was further confirmed by blocking cysteine residues with impermeant 2-sulfonatoethyl methanethiosulfonate. Two extracellular cysteine residues (66, 298) present in WT-PCFT were not biotinylated; however, in the absence of either one, biotinylation occurred. Likewise, biotinylation occurred after treatment with beta-mercaptoethanol. Taken together, these analyses establish a PCFT secondary structure of 12 transmembrane domains with the N- and C- termini directed to the cytoplasm. The data indicate further that there is a disulfide bridge, which is not required for function, between the native C66 and C298 residues in the first and fourth transmembrane domains, respectively.

Identification of dynein light chain road block-1 as a novel interaction partner with the human reduced folate carrier

The reduced folate carrier (RFC) is a major folate transport system in mammalian cells. RFC is highly expressed in the intestine and believed to play a role in folate absorption. Studies from our laboratory and others have characterized different aspects of the intestinal folate absorption process, but little is known about possible existence of accessory protein(s) that interacts with RFC and influences its physiology and/or cell biology. We investigated this issue by employing a bacterial two-hybrid system to screen a BacterioMatch II human intestinal cDNA library using the large intracellular loop between transmembrane domains 6 and 7 of the human RFC (hRFC) as bait. Our screening has resulted in the identification of dynein light chain road block-1 (DYNLRB1) as an interacting partner with hRFC. Existence of a direct protein-protein interaction between hRFC and DYNLRB1 was confirmed by in vitro pull-down assay and in vivo mammalian two-hybrid luciferase assay and coimmunoprecipitation analysis. Furthermore, confocal imaging of live human intestinal epithelial HuTu-80 cells demonstrated colocalization of DYNLRB1 with hRFC. Coexpression of DYNLRB1 with hRFC led to a significant (P < 0.05) increase in folate uptake. On the other hand, inhibiting the endogenous DYNLRB1 with gene-specific small interfering RNA or pharmacologically with a specific inhibitor (vanadate) led to a significant (P < 0.05) decrease in folate uptake. This study demonstrates for the first time the identification of DYNLRB1 as an interacting protein partner with hRFC. Furthermore, DYNLRB1 appears to influence the function and cell biology of hRFC.

Membrane transporters and folate homeostasis: intestinal absorption and transport into systemic compartments and tissues

Members of the family of B9 vitamins are commonly known as folates. They are derived entirely from dietary sources and are key one-carbon donors required for de novo nucleotide and methionine synthesis. These highly hydrophilic molecules use several genetically distinct and functionally diverse transport systems to enter cells: the reduced folate carrier, the proton-coupled folate transporter and the folate receptors. Each plays a unique role in mediating folate transport across epithelia and into systemic tissues. The mechanism of intestinal folate absorption was recently uncovered, revealing the genetic basis for the autosomal recessive disorder hereditary folate malabsorption, which results from loss-of-function mutations in the proton-coupled folate transporter gene. It is therefore now possible to piece together how these folate transporters contribute, both individually and collectively, to folate homeostasis in humans. This review focuses on the physiological roles of the major folate transporters, with a brief consideration of their impact on the pharmacological activities of antifolates.

Genetic variations and gene expression of transporters in drug disposition and response

BACKGROUND

The importance of transporters in drug disposition and response has led to increasing interest in genetic variations and expression differences of their genes.

OBJECTIVE

This review summarizes: i) genetic variations in transporters and associated drug response; and ii) a pharmacogenomic approach to correlate transporter expression and drug response.

METHODS

Several transporters in ATP-binding cassette family and solute carrier family are discussed.

CONCLUSION

The field of transporter pharmacogenomics is in its early stage. Transporter expression at mRNA levels could be more directly related to their functions and more practical to be assayed in high throughput. Correlating microarray expression of transporters with anticancer drug activity in the NCI-60 panel has provided an approach for identifying drug-transporter relationships and predicting drug response.

N-linked glycosylation and its impact on the electrophoretic mobility and function of the human proton-coupled folate transporter (HsPCFT)

The human proton-coupled folate transporter (HsPCFT, SLC46A1) mediates intestinal absorption of folates and transport of folates into the liver, brain and other tissues. On Western blot, HsPCFT migrates as a broad band (~55 kDa), higher than predicted (~50 kDa) in cell lines. Western blot analysis required that membrane preparations not be incubated in the loading buffer above 50 degrees C to avoid aggregation of the protein. Treatment of membrane fractions from HsPCFT-transfected HeLa cells with peptidyl N-glycanase F, or cells with tunicamycin, resulted in conversion to a ~35 kDa species. Substitution of asparagine residues of two canonical glycosylation sites to glutamine, individually, yielded a ~47 kDa protein; substitution of both sites gave a smaller (~35 kDa) protein. Single mutants retained full transport activity; the double mutant retained a majority of activity. Transport function and molecular size were unchanged when the double mutant was hemagglutinin (HA) tagged at either the NH(2) or COOH terminus and probed with an anti-HA antibody excluding degradation of the deglycosylated protein. Wild-type or deglycosylated HsPCFT HA, tagged at amino or carboxyl termini, could only be visualized on the plasma membrane when HeLa cells were first permeabilized, consistent with the intracellular location of these domains.

Measures of adiposity and body fat distribution in relation to serum folate levels in postmenopausal women in a feeding study

OBJECTIVE

To assess the associations between serum folate concentration and measures of adiposity in postmenopausal women.

DESIGN

This study was conducted as a cross-sectional analysis within the control segment of a randomized, crossover trial in which postmenopausal women (n=51) consumed 0 g (control), 15 g (one drink) and 30 g (two drinks) alcohol (ethanol)/day for 8 weeks as part of a controlled diet. Subjects in one treatment arm were crossed-over to another arm after a 2- to 5-week washout period. Body mass index (BMI) was measured, and dual energy X-ray absorptiometry (DEXA) scan administered to the women during the control (0 g alcohol) treatment, and a blood sample from this group was collected at baseline and week 8 of each diet period and analyzed for folate, B12, homocysteine and methylmalonic acid.

SETTING

This study was conducted at the Beltsville Human Nutrition Research Center, MD, USA.

RESULTS

In multivariate analysis, women who were overweight had a 12% lower, and obese women had a 22% lower serum folate concentrations compared to normal weight women (P-trend=0.02). Vitamin B12 also decreased with increasing BMI (P-trend=0.08). Increased BMI, percent body fat, and absolute amounts of central and peripheral fat were all significantly associated with decreased serum folate, but were unrelated to serum B12, homocysteine or methylmalonic acid.

CONCLUSIONS

Our data show that adiposity is associated with lower serum folate levels in postmenopausal women. With obesity at epidemic proportions, these data, if confirmed by prospective or randomized controlled studies, have important public health implications.

Mechanisms of antifolate resistance and methotrexate efficacy in leukemia cells

Antifolates are the first class of antimetabolites introduced to clinic about 6 decades ago. Now, after several years of administration of antifolates against malignancies and particularly leukemia, we are still trying to achieve a full understanding of the mechanisms of action and resistance to these agents. The present article covers different factors able to influence efficacy of antifolates on leukemic cells, the known mechanisms of resistance to methotrexate (MTX) and strategies to overcome these mechanisms. The dominant factors that are contributed to tolerance to cytocidal effects of MTX including pharmacokinetic factors, impaired transmembrane uptake as the most frequent rote of provoking resistance to MTX, augmented drug efflux, impaired intracellular polyglutamation as a determining process of drug efficacy, alterations in expression or activity of target enzymes and increased intracellular folate pools; and finally role of 7-hydroxymethotrexate on response or resistance to MTX will be discussed in more detail. Finally, strategies to overcome resistance to antifolates are discussed.

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.

Genetic and epigenetic alterations of the reduced folate carrier in untreated diffuse large B-cell lymphoma

The reduced folate carrier (RFC) is a transmembrane protein that mediates cellular uptake of reduced folates and antifolate drugs, including methotrexate (MTX). Acquired alterations of the RFC gene have been associated with resistance to MTX in cancer cell lines and primary osteosarcomas. Here, we examined RFC for mutations and promoter hypermethylation in (i) the inherently MTX-resistant lymphoma cell line (RL); (ii) 30 paired cases of acute lymphoblastic leukemia (ALL) obtained at diagnosis and at relapse after treatment with MTX; and (iii) 25 cases of diffuse large B-cell lymphoma (DLBCL) at diagnosis, none of which had been previously exposed to MTX. Aberrant hypermethylation of the RFC promoter occurred in RL cells and two of the primary DLBCLs. In one additional DLBCL, a single-base substitution in RFC was identified, leading to the introduction of a premature termination codon (c.1396C>T; p.Q466X). A missense mutation affecting the 11th transmembrane domain of RFC (c.1250T>C; p.I417T) was found in one case of ALL at diagnosis. In ALL, RFC promoter hypermethylation was found neither at diagnosis nor at relapse and thus is not a common cause of low levels of RFC expression associated with adverse outcome. In DLBCL, genetic and epigenetic alterations of RFC were detected at diagnosis in the absence of a selective MTX pressure, suggesting that these alterations may possibly contribute to the development of lymphoma.

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.

The molecular identity and characterization of a Proton-coupled Folate Transporter--PCFT; biological ramifications and impact on the activity of pemetrexed

Membrane transport of folates is essential for the survival of all mammalian cells and transport of antifolates is an important determinant of antifolate activity. While a major focus of attention has been on transport mediated by the reduced folate carrier and folate receptors, a very prominent carrier-mediated folate transport activity has been recognized for decades with a low-pH optimum and substrate specificity distinct from that of the reduced folate carrier which operates most efficiently at neutral pH. This low-pH transporter represents the mechanism by which folates are absorbed in the small intestine and it is also widely expressed in other human tissues and solid tumors. Recently, this laboratory discovered the molecular identity of this transporter which is genetically unrelated to the reduced folate carrier. This transporter is proton-coupled, electrogenic, and manifests a substrate specificity that is similar to that of the low-pH transport activity previously described in mammalian cells. The key role this transporter plays in intestinal folate absorption has been confirmed by the demonstration of a mutation in this gene in the rare autosomal recessive disorder, hereditary folate malabsorption. This article reviews (1) the characteristics and prevalence of the low-pH folate transport activity, (2) its relationship to, and properties of, the recently identified Proton-Coupled Folate Transporter (PCFT), (3) the physiological and pharmacological roles of this transporter, particularly with respect to pemetrexed, and (4) the historical controversy, now resolved, on the mechanism of intestinal folate absorption.

Cloning and functional characterization of a folate transporter from the nematode Caenorhabditis elegans

Two putative orthologs to the human reduced folate carrier (hRFC), folt-1 and folt-2, which share a 40 and 31% identity, respectively, with the hRFC sequence, have been identified in the Caenorhabditis elegans genome. Functional characterization of the open reading frame of the putative folt-1 and folt-2 showed folt-1 to be a specific folate transporter. Transport of folate by folt-1 expressed in a heterologous expression system showed an acidic pH dependence, saturability (apparent K(m) of 1.23 +/- 0.18 microM), a similar degree of inhibition by reduced and substituted folate derivatives, sensitivity to the anti-inflammatory drug sulfasalazine (apparent K(i) of 0.13 mM), and inhibition by anion transport inhibitors, e.g., DIDS. Knocking down (silencing) or knocking out the folt-1 gene led to a significant inhibition of folate uptake by intact living C. elegans. We also cloned the 5'-regulatory region of the folt-1 gene and confirmed promoter activity of the construct in vivo in living C. elegans. With the use of the transcriptional fusion construct (i.e., folt-1::GFP), the expression pattern of folt-1 in different tissues of living animal was found to be highest in the pharynx and intestine. Furthermore, folt-1::GFP expression was developmentally and adaptively regulated in vivo. These studies demonstrate for the first time the existence of a specialized folate uptake system in C. elegans that has similar characteristics to the folate uptake process of the human intestine. Thus C. elegans provides a genetically tractable model that can be used to study integrative aspects of the folate uptake process in the context of the whole animal level.

Downregulation of the reduced folate carrier transport activity by phenobarbital-type cytochrome P450 inducers and protein kinase C activators

The sodium dependent reduced folate carrier (Rfc1; Slc19a1) provides the major route for cellular uptake of reduced folates and antifolate drugs such as methotrexate (MTX) into various tissues. Despite its essential role in folate homeostasis and cancer treatment, little is known about Rfc1 regulation. A barbiturate recognition box, which as yet has only been found in the promoter region of xenobiotic metabolizing enzymes, particularly those of the CYP450 enzyme family, was predicted in the 5' untranslated region of rat rfc1 cDNA. We have therefore investigated the regulation of Rfc1 by phenobarbital (PB)-type CYP450 inducers on the functional, transcriptional and translational level in a suitable in vitro model for rat liver. A decrease of >75% in substrate uptake was observed following treatment (48 h) with 1-10 times therapeutic plasma concentrations of PB-type CYP450 inducers like PB, carbamazepine, chlorpromazine, clotrimazole and with 0.1-1 ng/ml of the constitutive androstane receptor agonist TCPOBOP. This was not associated with reduced mRNA and protein levels. Further mechanistic investigations revealed that short-term treatment (2 h) of cells with protein phosphatase 1/2A inhibitor okadaic acid (80.5 ng/ml) and proteinkinase C inducer phorbol 12-myristate 13-acetate (PMA; 0.62 microg/ml) almost abolished Rfc1 mediated MTX uptake. Finally, the reduction in Rfc1 activity caused by PB, TCPOBOP and PMA was reversed by simultaneous incubation with the specific PKC inhibitor bisindolylmaleimide (BIM; 21 ng/ml). These results demonstrate that clinically relevant concentrations of PB-type CYP450 inducers cause a significant PKC-dependent reduction in Rfc1 uptake activity at the posttranscriptional level.

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.

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.

Infection-dependent nuclear localization of US17, a member of the US12 family of human cytomegalovirus-encoded seven-transmembrane proteins

The human cytomegalovirus (HCMV) US12 gene family is a group of predicted seven-transmembrane, G-protein-coupled receptor-related proteins, about which little is known. Specific rabbit polyclonal antibodies detected US17 and US18 beginning 54 and 36 h after infection, respectively, with expression of both proteins dependent on viral DNA synthesis. While US14 and US18 are expressed exclusively in the cytoplasm, we unexpectedly found abundant expression of US17 in both the cytoplasm and nucleoplasm. N- and C-terminally tagged versions of US17 were readily detected in the cytoplasm of transfected mammalian cells, but not in nuclei, suggesting that nuclear localization involves other viral proteins or an infection-triggered cellular process. There was no specific colocalization between US17 and other nuclear expressed HCMV-encoded proteins (IE-2, DNA polymerase processivity factor, and pp28/UL99). To determine whether the observed nuclear localization might be the product of a process by which a soluble C-terminal segment of the full-length protein is expressed, we constructed a recombinant virus that incorporates a synthetic epitope at its N terminus, which in conjunction with the antipeptide antibody that targets its predicted cytoplasmic C-terminal segment, enables simultaneous independent detection of both termini. In cells infected with the recombinant, the US17 N and C termini had limited colocalization, with the N-terminal segment not detected in nuclei, supporting the segmentation hypothesis. Consistent with this, a fragment with an apparent molecular size of 10 kDa was detected by immunoblotting. We have identified the first viral example of a seven-transmembrane protein that is either segmented or expressed in nuclei. Further study will be required to learn the mechanism by which this occurs and the function of the nuclear localizing segment. This likely represents yet another mechanism by which a virus has hijacked or modified cellular regulatory pathways for its benefit.

Membrane topology of mouse stearoyl-CoA desaturase 1

Stearoyl-CoA desaturase (SCD) is an integral membrane protein anchored in the endoplasmic reticulum. It catalyzes the biosynthesis of monounsaturated fatty acids that are required for the synthesis of triglycerides, cholesteryl esters, and phospholipids. Four mouse isoforms of SCD (SCD1-4) and two human isoforms have been characterized. In the current study, we characterize the topology of the mouse SCD1 isoform. Hydropathy analysis of the 355-amino acid mouse SCD1 protein predicts that the protein contains four transmembrane domains (TMDs) and three loops connecting the membrane-spanning domains. To define the topology of the protein, recombinant SCD1 constructs containing epitope tags were transiently expressed in HeLa cells and analyzed by indirect immunofluorescence and cysteine derivatization. Our data provide evidence that the N and C termini of SCD1 are oriented toward the cytosol with four transmembrane domains separated by two very short hydrophilic loops in the ER lumen and one large hydrophilic loop in the cytosol. In addition, based on the previous observation that SCD is a thiol enzyme, we sought to investigate whether the cysteine residues were essential for enzyme activity through mutagenesis studies, and our data suggest that the cysteines in SCD are not catalytically essential.

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.

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.

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

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

Functional analysis of altered reduced folate carrier sequence changes identified in osteosarcomas

Osteosarcomas are common primary malignant bone tumors that do not respond to conventional low-dose treatments of methotrexate (Mtx), suggesting an intrinsic resistance to this drug. Previous work has shown that cDNAs generated from osteosarcoma mRNA from a fraction of patients contain sequence changes in the reduced folate carrier (RFC), the membrane protein transporter for Mtx. In this study, the functionality of the altered RFC proteins was assessed by fusing the green fluorescent protein (GFP) to the C-terminal, and examining the ability of the transfected constructs to complement a hamster cell line null for the carrier. Confocal microscopy and cell surface biotinylation indicated that all altered proteins were properly localized at the cell membrane. Only one of those examined, Leu291Pro, was unable to complement the null carrier line, but did bind Mtx at the cell surface. Thus, this alteration confers drug resistance since the carrier is unable to translocate the substrate across the cell membrane. Three alterations, Ser46Asn, Ser4Pro and Gly259Trp, while able to complement the carrier null line, conferred some degree of resistance to Mtx via a decreased rate of transport (Vmax). Another set of alterations, Glu21Lys, Ala7Val, and the combined changes Thr222Ile, Met254Thr, complemented the carrier null line and did not confer resistance to Mtx. Thus, some, but not all of these identified alterations in the RFC may contribute to the lack of responsiveness of osteosarcomas to Mtx treatment.

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

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

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.

Analysis of the membrane topology for transmembrane domains 7-12 of the human reduced folate carrier by scanning cysteine accessibility methods

The hRFC (human reduced folate carrier) is the major membrane transporter for both reduced folates and antifolates in human tissues and tumours. The primary amino acid sequence of hRFC predicts a membrane topology involving 12 TMDs (transmembrane domains) with cytosolic oriented N- and C-termini, and a large internal loop connecting TMDs 6 and 7. Previous studies using haemagglutinin epitope insertion and scanning glycosylation mutagenesis methods verified portions of the predicted topology model, including TMDs 1-8 and the N- and C-termini of hRFC. However, the topology structure for TMDs 9-12 remains controversial. To further determine the membrane topology of the hRFC protein, single cysteine residues were introduced into the predicted extracellular or cytoplasmic loops of a fully functional cysteine-less hRFC expressed in transport impaired MtxRIIOua(R)2-4 Chinese hamster ovary cells. The membrane orientations of the substituted cysteines were determined by treatments with the thiol reagents 3-(N-maleimidylpropionyl)-biocytin (biotin maleimide) and 4-acetamido-4'maleimidylstilbene-2,2'-disulphonic acid (stilbenedisulphonate maleimide; SM) or N-ethylmaleimide, combined with the cell-permeabilizing reagent SLO (streptolysin O). We found that cysteine residues placed in the predicted extracellular loops between TMDs 7 and 8 (position 301), 9 and 10 (360), and 11 and 12 (429) could be biotinylated with 200 microM biotin maleimide, and labelling could be blocked with SM. However, biotinylation of cysteines placed in the predicted intracellular loops between TMDs 8 and 9 (position 332) and TMDs 10 and 11 (position 388) was only detected after cell permeabilization with SLO and was abolished by pre-treatment with N -ethylmaleimide. These results strongly support a 12-TMD topology structure for the hRFC protein.

Disruption of transport activity in a D93H mutant thiamine transporter 1, from a Rogers Syndrome family

Rogers syndrome is an autosomal recessive disorder resulting in megaloblastic anemia, diabetes mellitus, and sensorineural deafness. The gene associated with this disease encodes for thiamine transporter 1 (THTR1), a member of the SLC19 solute carrier family including THTR2 and the reduced folate carrier (RFC). Using transient transfections into NIH3T3 cells of a D93H mutant THTR1derived from a Rogers syndrome family, we determined the expression, post-translational modification, plasma membrane targeting and thiamine transport activity. We also explored the impact on methotrexate (MTX) transport activity of a homologous missense D88H mutation in the human RFC, a close homologue of THTR1. Western blot analysis revealed that the D93H mutant THTR1 was normally expressed and underwent a complete N-glycosylation. However, while this mutant THTR1 was targeted to the plasma membrane, it was completely devoid of thiamine transport activity. Consistently, introduction into MTX transport null cells of a homologous D88H mutation in the hRFC did not result in restoration of MTX transport activity, thereby suggesting that D88 is an essential residue for MTX transport activity. These results suggest that the D93H mutation does not interfere with transporter expression, glycosylation and plasma membrane targeting. However, the substitution of this negatively charged amino acid (Asp93) by a positively charged residue (His) in an extremely conserved region (the border of transmembrane domain 2/intracellular loop 2) in the SLC19 family, presumably inflicts deleterious structural alterations that abolish thiamine binding and/or translocation. Hence, this functional characterization of the D93H mutation provides a molecular basis for Rogers syndrome.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Polarized expression of members of the solute carrier SLC19A gene family of water-soluble multivitamin transporters: implications for physiological function

Humans lack biochemical pathways for the synthesis of the micro-nutrients thiamine and folate. Cellular requirements are met through membrane transport activity, which is mediated by proteins of the SLC19A gene family. By using live-cell confocal imaging methods to resolve the localization of all SLC19A family members, we show that the two human thiamine transporters are differentially targeted in polarized cells, establishing a vectorial transport system. Such polarization decreases functional redundancy between transporter isoforms and allows for independent regulation of thiamine import and export pathways in cells.

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.

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.

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.

SLC19: the folate/thiamine transporter family

The SLC19 gene family of solute carriers is a family of three transporter proteins with significant structural similarity, transporting, however, substrates with different structure and ionic charge. The three members of this gene family are expressed ubiquitously and mediate the transport of two important water-soluble vitamins, folate and thiamine. The concentrative transport of substrates mediated by the members of this gene family is energized by transcellular H(+)/OH(-) gradient. SLC19A1 is expressed at highest levels in absorptive cells where it is located in a polarized manner either in the apical or basal membrane, depending on the cell type. It mediates the transport of reduced folate and its analogs, such as methotrexate, which are anionic at physiological pH. SLC19A2 is expressed ubiquitously and mediates the transport of thiamine, a cation at physiological pH. SLC19A3 is also widely expressed and is capable of transporting thiamine. This review summarizes the current knowledge on the structural, functional, molecular and physiological aspects of the SLC19 gene family.

Cell biology of the human thiamine transporter-1 (hTHTR1). Intracellular trafficking and membrane targeting mechanisms

The human thiamine transporter hTHTR1 is involved in the cellular accumulation of thiamine (vitamin B1) in many tissues. Thiamine deficiency disorders, such as thiamine-responsive megaloblastic anemia (TRMA), which is associated with specific mutations within hTHTR1, likely impairs the functionality and/or intracellular targeting of hTHTR1. Unfortunately, nothing is known about the mechanisms that control the intracellular trafficking or membrane targeting of hTHTR1. To identify molecular determinants involved in hTHTR1 targeting, we generated a series of hTHTR1 truncations fused with the green fluorescent protein and imaged the targeting and trafficking dynamics of each construct in living duodenal epithelial cells. Whereas the full-length fusion protein was functionally expressed at the plasma membrane, analysis of the truncated mutants demonstrated an essential role for both NH(2)-terminal sequence and the integrity of the backbone polypeptide for cell surface expression. Most notably, truncation of hTHTR1 within a region where several TRMA truncations are clustered resulted in intracellular retention of the mutant protein. Finally, confocal imaging of the dynamics of intracellular hTHTR1 vesicles revealed a critical role for microtubules, but not microfilaments, in hTHTR1 trafficking. Taken together, these results correlate hTHTR1 structure with cellular expression profile and reveal a critical dependence on hTHTR1 backbone integrity and microtubule-based trafficking processes for functional expression of hTHTR1.

Restoration of high-level transport activity by human reduced folate carrier/ThTr1 thiamine transporter chimaeras: role of the transmembrane domain 6/7 linker region in reduced folate carrier function

The reduced folate carrier (RFC; SLC19A1) is closely related to the thiamine transporter, SLC19A2 (ThTr1). Hydropathy models for these homologous transporters predict up to 12 transmembrane domains (TMDs), with internally oriented N- and C-termini and a large central loop between TMDs 6 and 7. The homologies are localized mostly in the TMDs. However, there is little similarity in their N- and C-terminal domains and the central peptide linkers connecting putative TMDs 1-6 and TMDs 7-12. To explore the functional role of the 61-amino acid central linker in the human RFC (hRFC), we introduced deletions of 49 and 60 amino acids into this region, differing by the presence of a stretch of 11 highly conserved amino acids between the human and rodent RFCs (positions 204-214). An additional hRFC construct was prepared in which only the 11 conserved amino acids were deleted. The resulting hRFC(D215-R263 Delta), hRFC(K204-R263 Delta) and hRFC(K204-R214 Delta) proteins were transfected into transport-impaired K562 cells. The deletion constructs were all expressed in plasma membranes; however, they were completely inactive for methotrexate and (6 S )5-formyl tetrahydrofolate transport. Insertion of non-homologous 73- and 84-amino acid fragments from the structurally analogous ThTr1 linker region into position 204 of hRFC(K204-R263 Delta) restored low levels of transport (16-21% of the wild type). Insertion of the ThTr1 linkers into hRFC(D215-R263 Delta) at position 215 restored 60-80% of wild-type levels of transport. Collectively, our results suggest that the role of the hRFC linker peptide is to provide the proper spatial orientation between the two halves of the hRFC protein for optimal function, and that this is largely independent of amino acid sequence. Our results also demonstrate a critical transport role for the stretch of 11 conserved amino acids starting at position 204 of hRFC.

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.

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

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