Molecular cloning of the human placental folate transporter

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Functional role of serine 318 of the proton-coupled folate transporter in methotrexate transport

This study revealed the importance of serine 318 (S318) residue for proton-coupled folate transporter (PCFT, SLC46A1) functioning. Substitution of S318 with arginine or lysine impaired transport of methotrexate (MTX), but substitution with alanine (has a simple side chain structure), or cysteine (structurally similar to serine), had no significant effect on MTX transport. The initial uptake rate of MTX by S318A and S318C mutant at pH 5.0, followed by Michaelis-Menten kinetics with a K_m value of approximately 2.3 μM (for S318A) and 2.9 μM (for S318C), was similar to that of the wild-type. The normalized V_max value of the S318A mutant, calculated by dividing the V_max value by the Western blot protein band's relative intensity, was approximately 2-fold greater than that of the wild-type. The normalized V_max value of the S318C mutant was approximately 0.8-fold smaller than the wild-type. Results obtained showed that the substitution of S318 with basic amino acid residues results in the loss of transport activity, even though PCFT mutants are expressed at the cell membrane. Alternatively, the substitution of S318 with neutral amino acids did not significantly affect the transport function of PCFT.

Involvement of mTOR, JNK and PI3K in the negative effect of ethanol and metformin on the human first-trimester extravillous trophoblast HTR-8/SVneo cell line

Our aim was to investigate the effect of two xenobiotics to which pregnant woman may be exposed, the drug of abuse ethanol (EtOH) (and its metabolite acetaldehyde (ACA)) and the therapeutic agent metformin (METF), on placentation-related processes in an extravillous trophoblastic (EVTs) cell line (HTR-8/SVneo cells). EtOH, ACA and METF (24 h) significantly reduced cell proliferation rates, culture growth, viability and migratory capacity of HTR-8/SVneo cells. Moreover, both EtOH (100 μM) and METF (1 mM) increased the apoptosis index and inhibited ³H-deoxy-D-glucose (³H-DG) and ³H-folic acid (³H-FA) uptake. mTOR, JNK and PI3K intracellular signaling pathways were involved in the effect of EtOH upon ³H-FA uptake and in the effect of METF upon cell viability, and mTOR and JNK in the effect of EtOH upon cell viability and ³H-DG uptake. We show that EtOH and METF have a detrimental effect in placentation-related processes of HTR-8/SVneo cells. Moreover, mTOR, JNK and PI3K appear to mediate some of these negative effects.

FASD: folic acid and formic acid — an unholy alliance in the alcohol abusing mother

Alcohol consumption during pregnancy remains a significant cause of preventable birth defects and developmental disabilities; however, the mechanism of toxicity remains unclear. Methanol is present as a congener in many alcoholic beverages and is formed endogenously. Because ethanol is preferentially metabolized over methanol, it has been found in the sera and cerebro-spinal fluid of alcoholics. Toxicity resulting from methanol has been attributed to formic acid. Formic acid is present in significantly higher quantities in the biofluids of alcoholics. These higher levels can be cytotoxic and cause neuronal cell death. However, the adverse effects can be mitigated by adequate levels of hepatic folic acid, because formic acid elimination depends on folic acid. During pregnancy, folate concentrations are at least 2-fold higher in cord blood then in maternal blood, owing to increased folate requirements. The reverse has been demonstrated in pregnancies with alcohol abuse, suggesting downregulation of folate transporters and low fetal folate levels. Moreover, formic acid can cross the placenta and its adverse effects can be mitigated by folic acid. Thus, the combination of low fetal folate levels and presence of formic acid form a potent cytotoxic combination that may play a significant role in the etiology of fetal alcohol spectrum disorder.

Influx transporter variants as predictors of cancer chemotherapy-induced toxicity: systematic review and meta-analysis

AIM

Chemotherapeutic agents have been shown to increase lung patient survival, however their use may be limited by their serious adverse effects. We aimed to assess int impact of pharmacogenetic variation of influx transporters on inter-individual patient variation in adverse drug reactions.

PATIENTS & METHODS

We conducted a meta-analysis and systemic review and identified 16 publications, totaling 1510 patients, to be eligible for review.

RESULTS

Meta-analysis showed east-Asian patients expressing SLCO1B1 521T>C or 1118G>A to have a two- to fourfold increased risk of irinotecan-induced neutropenia but not diarrhea. American patients, expressing SLC19A1 IVS2(4935) G>A, were further associated with pemetrexed/gemcitabine-induced grade 3+ leukopenia.

CONCLUSION

Future studies should look to robust validation of SLCO1B1 and SLC19A1 as prognostic markers in the management of lung cancer patients.

A Study of Single Nucleotide Polymorphisms of the SLC19A1/RFC1 Gene in Subjects with Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a group of neurodevelopmental disorders with complex genetic etiology. Recent studies have indicated that children with ASD may have altered folate or methionine metabolism, suggesting that the folate–methionine cycle may play a key role in the etiology of ASD. SLC19A1, also referred to as reduced folate carrier 1 (RFC1), is a member of the solute carrier group of transporters and is one of the key enzymes in the folate metabolism pathway. Findings from multiple genomic screens suggest the presence of an autism susceptibility locus on chromosome 21q22.3, which includes SLC19A1. Therefore, we performed a case-control study in a Japanese population. In this study, DNA samples obtained from 147 ASD patients at the Kanazawa University Hospital in Japan and 150 unrelated healthy Japanese volunteers were examined by the sequence-specific primer-polymerase chain reaction method pooled with fluorescence correlation spectroscopy. p < 0.05 was considered to represent a statistically significant outcome. Of 13 single nucleotide polymorphisms (SNPs) examined, a significant p-value was obtained for AA genotype of one SNP (rs1023159, OR = 0.39, 95% CI = 0.16–0.91, p = 0.0394; Fisher’s exact test). Despite some conflicting results, our findings supported a role for the polymorphism rs1023159 of the SLC19A1 gene, alone or in combination, as a risk factor for ASD. However, the findings were not consistent after multiple testing corrections. In conclusion, although our results supported a role of the SLC19A1 gene in the etiology of ASD, it was not a significant risk factor for the ASD samples analyzed in this study.

Inside the biochemical pathways of thymidylate synthase perturbed by anticancer drugs: Novel strategies to overcome cancer chemoresistance

Our current understanding of the mechanisms of action of antitumor agents and the precise mechanisms underlying drug resistance is that these two processes are directly linked. Moreover, it is often possible to delineate chemoresistance mechanisms based on the specific mechanism of action of a given anticancer drug. A more holistic approach to the chemoresistance problem suggests that entire metabolic pathways, rather than single enzyme targets may better explain and educate us about the complexity of the cellular responses upon cytotoxic drug administration. Drugs, which target thymidylate synthase and folate-dependent enzymes, represent an important therapeutic arm in the treatment of various human malignancies. However, prolonged patient treatment often provokes drug resistance phenomena that render the chemotherapeutic treatment highly ineffective. Hence, strategies to overcome drug resistance are primarily designed to achieve either enhanced intracellular drug accumulation, to avoid the upregulation of folate-dependent enzymes, and to circumvent the impairment of DNA repair enzymes which are also responsible for cross-resistance to various anticancer drugs. The current clinical practice based on drug combination therapeutic regimens represents the most effective approach to counteract drug resistance. In the current paper, we review the molecular aspects of the activity of TS-targeting drugs and describe how such mechanisms are related to the emergence of clinical drug resistance. We also discuss the current possibilities to overcome drug resistance by using a molecular mechanistic approach based on medicinal chemistry methods focusing on rational structural modifications of novel antitumor agents. This paper also focuses on the importance of the modulation of metabolic pathways upon drug administration, their analysis and the assessment of their putative roles in the networks involved using a meta-analysis approach. The present review describes the main pathways that are modulated by TS-targeting anticancer drugs starting from the description of the normal functioning of the folate metabolic pathway, through the protein modulation occurring upon drug delivery to cultured tumor cells as well as cancer patients, finally describing how the pathways are modulated by drug resistance development. The data collected are then analyzed using network/netwire connecting methods in order to provide a wider view of the pathways involved and of the importance of such information in identifying additional proteins that could serve as novel druggable targets for efficacious cancer therapy.

Stereoselective recognition of amethopterin enantiomers by the rat proton-coupled folate transporter

The stereoselective transport of methotrexate (L-amethopterin, L-MTX) and its enantiomer (D-amethopterin, D-MTX) by the rat proton-coupled folate transporter (rPCFT) were examined using rPCFT-expressing HEK293 cells. The initial rate of uptake of [3H]-L-MTX by the rPCFT followed Michaelis-Menten kinetics, with a Km value of 2.1 µM. Dixon plots revealed that the uptake of L-MTX by the rPCFT was inhibited in a competitive manner by unlabeled L-MTX and D-MTX, with Ki values of approximately 1.3 and 150 µM, respectively. The initial rate of uptake of D-MTX by the rPCFT also followed Michaelis-Menten kinetics, with a Km value of 190 µM. The results of the current study demonstrate that the different enantiomers of MTX are transported in a highly stereoselective manner by the rPCFT, with the uptake clearance of L-MTX being approximately 46-fold greater than that of D-MTX. The observed stereoselectivity of the rPCFT was found to be comparable with that of the human PCFT.

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.

Effect of epigallocatechin gallate on drug transport mediated by the proton-coupled folate transporter

Folic acid (FA) is a water-soluble vitamin, and orally ingested FA is absorbed from the small intestine by the proton-coupled folate transporter (PCFT). In the present study, we investigated whether epigallocatechin gallate (EGCG), one of the tea catechins, affects the transport of FA by PCFT. EGCG inhibited the uptake of FA into Caco-2 cells and human PCFT-expressing HEK293 cells (PCFT-HEK293 cells). The initial rate of uptake of FA into PCFT-HEK293 cells followed Michaelis-Menten kinetics (K(m) = 1.9 µM). Dixon plots revealed that PCFT-mediated FA uptake was competitively inhibited by EGCG (K(i) ≒ 9 µM). The uptake of the PCFT substrate methotrexate (MTX) was competitively inhibited by EGCG as well (K(i) ≒ 15 µM). In conclusion, it is suggested that when FA or MTX is ingested with tea, it is likely that the intestinal absorption of these compounds by PCFT is inhibited, which could result in insufficient efficacy.

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.

The Concise Guide to PHARMACOLOGY 2013/14: transporters

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Transporters are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, catalytic receptors, nuclear hormone receptors and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Folic acid transport to the human fetus is decreased in pregnancies with chronic alcohol exposure

BACKGROUND

During pregnancy, the demand for folic acid increases since the fetus requires this nutrient for its rapid growth and cell proliferation. The placenta concentrates folic acid into the fetal circulation; as a result the fetal levels are 2 to 4 times higher than the maternal level. Animal and in vitro studies have suggested that alcohol may impair transport of folic acid across the placenta by decreasing expression of transport proteins. We aim to determine if folate transfer to the fetus is altered in human pregnancies with chronic alcohol consumption.

METHODOLOGY/PRINCIPAL FINDINGS

Serum folate was measured in maternal blood and umbilical cord blood at the time of delivery in pregnancies with chronic and heavy alcohol exposure (n = 23) and in non-drinking controls (n = 24). In the alcohol-exposed pairs, the fetal:maternal serum folate ratio was ≤ 1.0 in over half (n = 14), whereas all but one of the controls were >1.0. Mean folate in cord samples was lower in the alcohol-exposed group than in the controls (33.15 ± 19.89 vs 45.91 ± 20.73, p = 0.04).

CONCLUSIONS/SIGNIFICANCE

Our results demonstrate that chronic and heavy alcohol use in pregnancy impairs folate transport to the fetus. Altered folate concentrations within the placenta and in the fetus may in part contribute to the deficits observed in the fetal alcohol spectrum disorders.

Stereoselective transport of amethopterin enantiomers by the proton-coupled folate transporter

Stereoselective transport of methotrexate (L-amethopterin, L-MTX) and its antipode (D-amethopterin, D-MTX) by the proton-coupled folate transporter (PCFT) was examined using PCFT-expressing HEK293 cells (PCFT-HEK293 cells). Uptake of both L-MTX and D-MTX was pH-dependent and decreased with an increase in the extracellular pH from 5.0 to 7.4. The initial uptake rate of L-MTX into PCFT-HEK293 cells followed Michaelis-Menten kinetics with a K(m) value of approximately 5.0 microM. Dixon plots revealed that L-MTX uptake was inhibited competitively by unlabeled L-MTX, D-MTX, and folic acid (FA), with K(i) values of approximately 3.6, 180, and 2.1 microM, respectively. The initial uptake rate of D-MTX into PCFT-HEK293 cells also followed Michaelis-Menten kinetics with a K(m) value of 211 microM. The V(max) value of D-MTX was similar to that of L-MTX. The present study revealed that the transport of MTX enantiomers by PCFT is highly stereoselective with the uptake clearance of L-MTX being approximately 40-fold greater than that of D-MTX. It was also revealed that this high stereoselectivity results from the difference in K(m) values, and not V(max) values, between the enantiomers. The observed stereoselectivity was consistent with the differences in the intestinal absorption of MTX enantiomers in humans.

Sodium-coupled and electrogenic transport of B-complex vitamin nicotinic acid by slc5a8, a member of the Na/glucose co-transporter gene family

SMCT (sodium-coupled monocarboxylate transporter; slc5a8) is a Na+-coupled transporter for lactate, pyruvate and short-chain fatty acids. Similar to these already known substrates of SMCT, the water-soluble B-complex vitamin nicotinic acid also exists as a monocarboxylate anion (nicotinate) under physiological conditions. Therefore we evaluated the ability of SMCT to mediate the uptake of nicotinate. In mammalian cells, the cloned mouse SMCT (slc5a8) induced the uptake of nicotinate. The SMCT-induced uptake was Na+-dependent. The Michaelis constant for the uptake process was 296+/-88 microM. The Na+-activation kinetics indicated that at least two Na+ ions are involved in the process. Among the various structural analogues tested, nicotinate was the most effective substrate. Nicotinamide and methylnicotinate were not recognized by the transporter. 2-pyrazine carboxylate and isonicotinate interacted with the transporter to a moderate extent. SMCT-mediated uptake of nicotinate was inhibited by lactate and pyruvate. In the Xenopus laevis oocyte expression system, SMCT-mediated nicotinate transport was electrogenic, as evident from the nicotinate-induced inward currents under voltage-clamp conditions. Substrate-induced currents in this expression system corroborated the substrate specificity determined in the mammalian cell expression system. The kinetic parameters with regard to the affinity of the transporter for nicotinate and the Hill coefficient for Na+ activation, determined by using the oocyte expression system, were also similar to those obtained from the mammalian cell expression system. We conclude that SMCT functions not only as a Na+-coupled transporter for short-chain fatty acids and lactate but also as a Na+-coupled transporter for the water-soluble vitamin nicotinic acid.

Renal conservation of folates role of folate transport proteins

Folates play vital roles in one-carbon metabolism that produces the early substrates necessary for nucleotide synthesis and salvage. Folates are essential vitamins in that humans cannot synthesize them and are totally dependent on the diet to obtain them. As water-soluble vitamins, they would be easily filtered by the kidney and lost to the tubular fluid but for a highly efficient renal conservation mechanism. This renal "folate trap" is made up of alpha-folate receptors and reduced folate carriers. The locations of these transporters are such that they direct folate transport from the apical/luminal sides of kidney cells to the basolateral/plasma sides. In addition, other transporters such as organic anion transporters and multidrug resistance proteins are also found in kidney cells and play a role in renal elimination of folate analogues such as antifolate cancer chemotherapy drugs. This chapter discusses how these transporter activities manifest themselves in folate and antifolate pharmacokinetics. It also discusses effects of alcohol on renal reabsorption of folates.

Polymorphisms and methylation of the reduced folate carrier in osteosarcoma

High-dose methotrexate is a standard component in the treatment of osteogenic sarcoma. Impaired methotrexate uptake associated with decreased reduced folate carrier expression is a common mechanism of methotrexate resistance in osteogenic sarcoma samples. We investigated whether promoter methylation and polymorphisms in the 3' untranslated region are involved in regulating reduced folate carrier expression. In a cohort of 66 osteogenic sarcoma specimens, quantitative methylation-specific polymerase chain reaction and single-strand conformation polymorphism were performed. We found detectable levels of promoter methylation in 84.3% of samples. When related to the reduced folate carrier mRNA levels, a trend was observed that reduced folate carrier expression is lower in samples (median, 0.7) with greater than 10% DNA methylation as compared with those (median, 2.3) with less than 10% DNA methylation. The heterozygous polymorphisms of 2582 T/G and 2617C/T in the 3' untranslated region showed reduced folate carrier expression (median, 0.9) as compared with the wild-type 2582T and 2617C (median, 4.2). The data suggest promoter methylation and polymorphisms in the 3' untranslated region of the reduced folate carrier may be involved in its transcriptional regulation in osteogenic sarcoma. Further study is required to confirm this finding.

Quantitative correlation between promoter methylation and messenger RNA levels of the reduced folate carrier

Background

Methotrexate (MTX) uptake is mediated by the reduced folate carrier (RFC). Defective drug uptake in association with decreased RFC expression is a common mechanism of MTX resistance in many tumor types. Heavy promoter methylation was previously identified as a basis for the complete silencing of RFC in MDA-MB-231 breast cancer cells, its role and prevalence in RFC transcription regulation are, however, not widely studied.

Methods

In the current study, RFC promoter methylation was assessed using methylation specific PCR in a panel of malignant cell lines (n = 8), including MDA-MB-231, and M805, a MTX resistant cell line directly established from the specimen of a patient with malignant fibrohistocytoma, whom received multiple doses of MTX. A quantitative approach of real-time PCR for measuring the extent of RFC promoter methylation was developed, and was validated by direct bisulfite genomic sequencing. RFC mRNA levels were determined by quantitative real-time RT-PCR and were related to the extent of promoter methylation in these cell lines.

Results

A partial promoter methylation and RFC mRNA down-regulation were observed in M805. Using the quantitative approach, a reverse correlation (correlation coefficient = -0.59, p < 0.05) was identified between the promoter methylation and RFC mRNA levels in this a panel of malignant cell lines.

Conclusion

This study further suggests that promoter methylation is a potential basis for MTX resistance. The quantitative correlation identified in this study implies that promoter methylation is possibly a mechanism involved in the fine regulation of RFC transcription.

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.

Expression status of folate receptor alpha is significantly correlated with prognosis in non-small-cell lung cancers

BACKGROUND

To evaluate the prognostic value of folate receptor alpha (FOLR1) and/or reduced folate carrier (RFC1) expression, which are well-characterized folate transporters, in completely resected non-small-cell lung cancer (NSCLC).

METHODS

We quantitatively examined gene expression of FOLR1 and RFC1 in surgical specimens resected from NSCLC patients. A total of 119 consecutive patients from January 2003 to June 2004 were included.

RESULTS

In adenocarcinoma, the FOLR1 gene expression was downregulated in smokers and male patients (P = 0.006 and P < 0.001, respectively). In addition, FOLR1 expression values in patients with well-differentiated, early p-stage, pT1, pN0, EGFR mutant, and p53 wild-type cancers were significantly higher than those for poorly differentiated, advanced p-stage, pT2-4, pN1-3, EGFR wild-type, and p53 mutant (P < 0.001, P < 0.001, P < 0.001, P = 0.002, P < 0.001 and P = 0.023, respectively). In squamous cell carcinoma, FOLR1 expression values in patients with pN1-3 was significantly higher than those with pN0 (P = 0.037). Moreover, the 3-year survival rate and disease-free survival rate of high-FOLR1-expressing patients (94.7% and 75.4%) were significantly higher than those of low-FOLR1-expressing patients (80.9% and 60.8%) (P = 0.008 and P = 0.038). A multivariate analysis confirmed that high FOLR1 expression was an independent and significant factor predicting a favorable prognosis (P = 0.043).

CONCLUSIONS

Higher levels of FOLR1 appear to be associated with better prognoses for patients with lung adenocarcinomas.

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.

Folic acid transport via high affinity carrier-mediated system in human retinoblastoma cells

The primary objective of this study was to investigate the expression of a specialized carrier-mediated system for folic acid and to delineate its uptake mechanism and intracellular trafficking in a human derived retinoblastoma cell line (Y-79). Uptake of [3H]Folic acid was determined at various concentrations, pH, temperatures, in the absence of sodium and chloride ions and in the presence of structural analogs, methyltetrahydro folate (MTF) and methotrexate (MTX), vitamins, membrane transport and metabolic inhibitors to delineate the mechanism of uptake. Kinetics of uptake was studied in the presence of various intracellular regulatory pathways; protein kinases A and C (PKA and PKC), protein tyrosine kinase (PTK) and calcium-calmodulin modulators. Reverse transcription polymerase chain reaction (RT-PCR) was performed to confirm the molecular identity of folate carrier systems. The uptake was found to be linear up to 30min. The rate of uptake followed saturation kinetics with apparent Km of 8.29+/-0.74nM, 17.03+/-1.98nM and 563.23+/-115.2nM and Vmax of 393.47+/-9.33, 757.58+/-26.21 and 653.17+/-31.7fmol/(minmg) protein for folic acid, MTF and MTX, respectively. The process was chloride, temperature and energy dependent but sodium and pH independent; inhibited by the structural analogs MTF and MTX but not by structurally unrelated vitamins. Membrane transport inhibitors did not affect the uptake of [3H]Folic acid, however endocytic inhibitor, colchicine, significantly inhibited the [3H]Folic acid uptake indicating the involvement of receptor mediated endocytosis process. PKC, PTK and Ca2+/calmodulin pathways appeared to play important roles in the regulation of folic acid uptake. Molecular evidence of the presence of folate receptor (FR) precursor was identified by RT-PCR analysis. This research work demonstrated, for the first time, the functional and molecular existence of a specialized high affinity carrier-mediated system for folic acid uptake, in human retinoblastoma cells.

H+-coupled nutrient, micronutrient and drug transporters in the mammalian small intestine

The H(+)-electrochemical gradient was originally considered as a driving force for solute transport only across cellular membranes of bacteria, plants and yeast. However, in the mammalian small intestine, a H(+)-electrochemical gradient is present at the epithelial brush-border membrane in the form of an acid microclimate. Over recent years, a large number of H(+)-coupled cotransport mechanisms have been identified at the luminal membrane of the mammalian small intestine. These transporters are responsible for the initial stage in absorption of a remarkable variety of essential and non-essential nutrients and micronutrients, including protein digestion products (di/tripeptides and amino acids), vitamins, short-chain fatty acids and divalent metal ions. Proton-coupled cotransporters expressed at the mammalian small intestinal brush-border membrane include: the di/tripeptide transporter PepT1 (SLC15A1); the proton-coupled amino-acid transporter PAT1 (SLC36A1); the divalent metal transporter DMT1 (SLC11A2); the organic anion transporting polypeptide OATP2B1 (SLC02B1); the monocarboxylate transporter MCT1 (SLC16A1); the proton-coupled folate transporter PCFT (SLC46A1); the sodium-glucose linked cotransporter SGLT1 (SLC5A1); and the excitatory amino acid carrier EAAC1 (SLC1A1). Emerging research demonstrates that the optimal intestinal absorptive capacity of certain H(+)-coupled cotransporters (PepT1 and PAT1) is dependent upon function of the brush-border Na(+)-H(+) exchanger NHE3 (SLC9A3). The high oral bioavailability of a large number of pharmaceutical compounds results, in part, from absorptive transport via the same H(+)-coupled cotransporters. Drugs undergoing H(+)-coupled cotransport across the intestinal brush-border membrane include those used to treat bacterial infections, hypercholesterolaemia, hypertension, hyperglycaemia, viral infections, allergies, epilepsy, schizophrenia, rheumatoid arthritis and cancer.

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.

Acute and chronic effects of some dietary bioactive compounds on folic acid uptake and on the expression of folic acid transporters by the human trophoblast cell line BeWo

Folic acid (FA) is a vitamin that acts as a coenzyme in the biosynthesis of purine and pyrimidine precursors of nucleic acids, which are critically important during pregnancy. Our group has previously shown that both reduced folate carrier (RFC1) and folate receptor alpha (FRalpha) seem to be involved in the uptake of [3H]folic acid ([3H]FA) by a human trophoblast cell line (BeWo) and by human primary cultured cytotrophoblasts. Our aim was to study the interaction between FA and some nutrients/bioactive substances. For this, we tested the acute and chronic effects of some dietary compounds on [3H]FA apical uptake and on the expression of both RFC1 and FRalpha mRNA in BeWo cells. Our results show that [3H]FA uptake was significantly reduced by acute exposure to epicatechin, isoxanthohumol (1-400 microM) or theophylline (0.1-100 microM); isoxanthohumol seemed to act as a competitive inhibitor, whereas epicatechin and theophylline caused an increase in both Km and Vmax. On the other hand, [3H]FA uptake was significantly increased by chronic exposure to xanthohumol, quercetin or isoxanthohumol (0.1-10 microM), and this increase does not seem to result from changes in the level of RFC1 or FRalpha gene expression. Moreover, [3H]FA uptake was significantly reduced by chronic exposure to ethanol (0.01%). This reduction seems to be, at least in part, due to a reduction in FRalpha expression. These results are compatible with an association between a deficient FA supply to the placenta/fetus and ethanol toxicity in pregnancy.

Chiral recognition of amethopterin enantiomers by the reduced folate carrier in Caco-2 cells

Stereoselectivity of the human reduced folate carrier (RFC1) in Caco-2 cells was examined using methotrexate (L-amethopterin, L-MTX) and its antipode (D-amethopterin, D-MTX) as model substrates. The initial uptake rate of L-MTX into Caco-2 cells followed Michaelis-Menten kinetics with a Km value of approximately 1 microM. The Eadie-Hofstee plot of the RFC1-mediated L-MTX uptake showed that it was mediated by a single transport system, RFC1. Dixon plots revealed that L-MTX uptake was inhibited competitively by folic acid (FA), L-MTX and D-MTX, with Ki values of approximately 0.8, 1.5 and 180 microM, respectively. The results showed that the affinities of FA and L-MTX to RFC1 were approximately 120-fold greater than that of D-MTX. The uptake of L- and D-MTX into Caco-2 cells was also measured using LC-MS/MS analysis, which revealed that the L-MTX uptake was at least 7-fold greater than that of D-MTX. The present study revealed significant stereoselectivity of RFC1 toward amethopterin enantiomers with the L-isomer being much more favored.

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.

G80A reduced folate carrier SNP influences the absorption and cellular translocation of dietary folate and its association with blood pressure in an elderly population

The functional consequences of the G80A RFC SNP on the expressed reduced folate carrier protein were evaluated by looking at the relationship between intake of folate, plasma folate and cellular stores of the vitamin. The effect on homocysteine was also examined. Homocysteine is a thiol that is known to be inversely associated with folate, and which is considered to be both thrombo- and athrogenic. At high levels, homocysteine may also interfere with nitric oxide mediated vasodilation, cause oxidative injury to, and proliferation of the vascular endothelium, and alter the elastic properties of the vascular wall, contributing to increased blood pressure. Participants (119; 52 male, 67 female) from a NSW retirement village were assessed. Independent of gender, the assimilation of folate from dietary sources into red cells showed a significant association for GG (r=0.399; p=0.022) and GA (r=0.564; p<0.0001) subjects, but not homozygous recessive (AA) individuals (r=0.223; p=0.236). The same genotype based pattern of significance was shown for the association between dietary folate and plasma folate (GG: r=0.524; p=0.002, GA: r=0.408; p=0.002). No genotype-related pattern of significance was shown for the association between dietary folate and homocysteine. When examined by gender, some differences were apparent; one-way ANOVA showed that genotype influenced diastolic blood pressure in males (p=0.019), while only females showed a significant correlation between dietary folate and blood pressure within specific genotypes (Systolic pressure GA: r=-0.372; p=0.025, carriage of A: r=0.-0.357; p=0.011. Diastolic pressure GA: r=-0.355; p=0.034, carriage of A: r=0.-0.310; p=0.029). The G80A RFC SNP had an impact on the absorption and cellular translocation of dietary folate and its association with blood pressure in an elderly population.

A putative thiamine transport protein is a receptor for feline leukemia virus subgroup A

Feline leukemia virus (FeLV) is a horizontally transmitted virus that causes a variety of proliferative and immunosuppressive diseases in cats. There are four subgroups of FeLV, A, B, C, and T, each of which has a distinct receptor requirement. The receptors for all but the FeLV-A subgroup have been defined previously. Here, we report the identification of the cellular receptor for FeLV-A, which is the most transmissible form of FeLV. The receptor cDNA was isolated using a gene transfer approach, which involved introducing sequences from a feline cell line permissive to FeLV-A into a murine cell line that was not permissive. The feline cDNA identified by this method was approximately 3.5 kb, and included an open reading frame predicted to encode a protein of 490 amino acids. This feline cDNA conferred susceptibility to FeLV-A when reintroduced into nonpermissive cells, but it did not render these cells permissive to any other FeLV subgroup. Moreover, these cells specifically bound FeLV-A-pseudotyped virus particles, indicating that the cDNA encodes a binding receptor for FeLV-A. The feline cDNA shares approximately 93% amino acid sequence identity with the human thiamine transport protein 1 (THTR1). The human THTR1 receptor was also functional as a receptor for FeLV-A, albeit with reduced efficiency compared to the feline orthologue. On the basis of these data, which strongly suggest the feline protein is the orthologue of human THTR1, we have named the feline receptor feTHTR1. Identification of this receptor will allow more detailed studies of the early events in FeLV transmission and may provide insights into FeLV pathogenesis.

G80A reduced folate carrier SNP modulates cellular uptake of folate and affords protection against thrombosis via a non homocysteine related mechanism

Dietary folate is absorbed in the jejunum by the 'Reduced Folate Carrier' binding protein. This protein also sequesters extracellular folate for use by many cells in the body. As several biosynthetic pathways require folate for critical life processes, any change in the properties of this protein could lower folate bioavailability, cellular levels of the vitamin, and thus influence health. Since folate lowers thrombogenic homocysteine, we examined the prevalence of a common genetic polymorphism encoding the Reduced Folate Carrier (G80A RFC) to see if it acts as a risk factor for thrombotic vascular disease via an effect on homocysteine disposition in a cohort of 156 patients. The odds ratio indicates a significant protective effect of the mutant A allele against thrombosis: OR = 0.56(95% CI; 0.34-0.92). chi2; p = 0.022 (Yates corrected chi2; p = 0.031). The polymorphism had no impact on homocysteine, but did increase the level of extracellular to intracellular folate as might be predicted by the biological role of the expressed protein. This, and not homocysteine level, may be what affords protection against thrombosis.

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

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

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

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

Placental folate transport and binding are not impaired in pregnancies complicated by fetal growth restriction

Maternal folate deficiency is associated with fetal growth restriction, however, transfer of folate across placentae of pregnancies complicated by fetal growth restriction has never been investigated. We studied whether maternal to fetal 5-methyltetrahydrofolate (5MTF) transport in the ex vivo dually perfused isolated cotyledon, binding of [(3)H] folate (PteGlu) to the syncytial microvillous membrane, and protein expression of folate receptor alpha (FR-alpha) and reduced folate carrier (RFC) in these placentae are disturbed. Placental clearance of 5MTF from the maternal perfusate appeared to be non-saturable over a range of 50 to 500 nm, independent of albumin and flow-independent. No statistically significant differences between placentae complicated with fetal growth restriction and uncomplicated pregnancies were observed. Binding characteristics of [(3)H-]PteGlu to microvillous membranes of fetal growth restriction versus control placentae were similar: B(max)of 3.9+/-2.0 (mean+/-s.d.) versus 4.0+/-1.6 pmol/mg protein and a K(d)of 0.037+/-0.010 versus 0.040+/-0.018 nm. Expression of FR-alpha and RFC were not different in placentae of both groups studied. In conclusion, fetal growth restriction appears not to be associated with impaired maternal to fetal placental folate transport, placental receptor binding, or expression of FR-alpha and RFC.

Structure and regulation of the murine reduced folate carrier gene: identification of four noncoding exons and promoters and regulation by dietary folates

The upstream structure and regulation of the mouse reduced folate carrier (mRFC) gene was characterized. By 5'-rapid amplification of cDNA ends assay and DNA sequencing from mouse tissues and 7-15-day stage embryos, mRFC transcripts with four unique 5' noncoding exons, designated mRFC-a,-b,-c, and -d, were identified mapping over 6300 bp. The 5' noncoding exons were characterized by multiple transcription starts and, for form b, two alternate splice forms. mRFC transcript forms were measured by real-time reverse transcription-PCR in mouse tissues and embryos and in L1210 leukemia and BNL CL.2 liver cell lines. The highest mRFC levels were detected in kidney and brain. mRFC-b and -c were the major transcript forms, with low levels of mRFC-a and -d. The 5'-flanking regions for exons a-d each exhibited promoter activity in reporter gene assays. mRFC transcripts and individual noncoding exons were measured in small intestine and kidney from mice fed folate-deficient or -replete diets. Mice fed the folate-deficient diet exhibited a significant (13.8-fold) increase in total mRFC transcripts and protein in the small intestine, reflecting increases in each of the mRFC-b, -c, and -d forms. Only minor changes in mRFC transcript levels or distributions were detected for kidney. Levels of folate-binding protein alpha were also increased in both small intestine and kidney in folate-deficient mice (91- and 2-fold, respectively). Multidrug resistance-associated proteins 1 and 3 were, likewise, elevated in intestine from folate-deficient mice (53- and 168-fold, respectively); however, there were no significant changes in kidney. Our results document the existence of four unique noncoding exons and promoters for mRFC and demonstrate a facile induction of mRNAs for mRFC and multidrug resistance-associated proteins 1 and 3 in intestine in response to changes in dietary folate intake.

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.

Mechanism and regulation of folate uptake by human pancreatic epithelial MIA PaCa-2 cells

After the liver, the pancreas contains the second highest level of folate among human tissues, and folate deficiency adversely affects its physiological function. Despite that, nothing is currently known about the cellular mechanisms involved in folate uptake by cells of this important exocrine organ or about folate uptake regulation. We have begun to address these issues, and in this report we present the results of our findings on the mechanism of folate uptake by the human-derived pancreatic MIA PaCa-2 cells. Our results show folic acid uptake to be 1). temperature and energy dependent; 2). pH dependent, with a markedly higher uptake at acidic pH compared with neutral or alkaline pH; 3). Na(+) independent; 4). saturable as a function of substrate concentration (apparent K(m) = 0.762 +/- 0.10 microM); 5). inhibited (with similar affinity) by reduced, substituted, and oxidized folate derivatives; and 6). sensitive to the inhibitory effect of anion transport inhibitors. RT-PCR and Western blot analysis showed expression of the human reduced folate carrier (hRFC) at the RNA and protein levels, respectively. The functional contribution of hRFC in carrier-mediated folate uptake was confirmed by gene silencing using gene-specific small interfering RNA. Evidence also was found suggesting that the folate uptake process by MIA PaCa-2 cells is regulated by cAMP- and protein tyrosine kinase (PTK)-mediated pathways. These studies demonstrate for the first time the involvement of a specialized, acidic pH-dependent, carrier-mediated mechanism for folate uptake by human pancreatic MIA PaCa-2 cells. The results also show the involvement of hRFC in the uptake process and suggest the possible involvement of intracellular cAMP- and PTK-mediated pathways in the regulation of folate uptake.

Regulation of reduced-folate transporter-1 (RFT-1) by homocysteine and identity of transport systems for homocysteine uptake in retinal pigment epithelial (RPE) cells

Reduced-folate transporter-1 (RFT-1) transports reduced-folates, such as N5-methyltetrahydrofolate (MTF), the predominant circulating form of folate. In RPE, RFT-1 is localized to the apical membrane and is thought to transport folate from RPE to photoreceptor cells. Folate is required for DNA, RNA, protein synthesis and the conversion of homocysteine (Hcy) to methionine. Decreased folate levels are associated with increased Hcy levels. In the present study, we asked whether RFT-1 activity in RPE is altered under high Hcy conditions and examined the transport mechanism for Hcy in RPE. Treatment of ARPE-19 cells, a human RPE cell line, with Hcy at concentrations higher than 50 microM led to a significant decrease in RFT-1 activity. This effect increased as the treatment time increased. The inhibitory effect of Hcy on RFT-1 activity was not non-specific, as the activities of several other nutrient transporters were not affected under identical conditions. The effect of Hcy on RFT-1 was associated primarily with a decrease in the maximal velocity with no detectable change in substrate affinity. The decrease in RFT-1 activity was accompanied by parallel changes in RFT-1 mRNA and protein. Uptake of Hcy in ARPE-19 cells occurred via several transport systems, including Na+-independent systems L and b(0,+) and the Na+-dependent systems B0, ATB(0,+) and A. Studies of the interaction of Hcy with one of the cloned transporters (ATB(0,+)) provided direct evidence for the translocation of Hcy across the membrane via the transporter. We conclude that several transport systems operate in ARPE-19 cells for the entry of Hcy and that high levels of Hcy have deleterious effects on the expression and activity of RFT-1 in these 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.

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.

Effects of the glutamate carboxypeptidase II (GCP2 1561C>T) and reduced folate carrier (RFC1 80G>A) allelic variants on folate and total homocysteine levels in kidney transplant patients

BACKGROUND

The effect of the glutamate carboxypeptidase II GCP2 1561C>T and the reduced folate carrier 1 RFC1 80G>A polymorphisms on folate and total homocysteine (tHcy) plasma levels of kidney transplant patients are unknown.

METHODS

In a cross-sectional study of 730 kidney allograft recipients, GCP2 1561C>T, RFC1 80G>A, folate, and tHcy plasma levels were analyzed using linear regression models that allowed dependent covariates to follow a gamma distribution for univariate and multivariate analyses.

RESULTS

The allele frequency for GCP2 1561C>T was 0.05, and 0.43 for RFC1 80G>A. Heterozygosity or homozygosity for GCP2 1561C>T was associated with higher folate plasma levels compared to patients without mutation (P < 0.0001), while RFC1 80G>A showed no influence. Multiple testing, also including MTHFR 677C>T and MTHFR 1298A>C, revealed no interaction between the different genotypes and the folate plasma concentration. Neither GCP2 1561C>T nor RFC1 80G>A showed an association with tHcy plasma levels.

CONCLUSION

We conclude that GCP2 1561C>T is associated with elevated folate levels. GCP2 1561C>T and RFC1 80G>A are no major determinants of tHcy plasma levels in kidney transplant patients.

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.