Membrane transport of folates

Membrane transporters in a human genome-scale metabolic knowledgebase and their implications for disease

Membrane transporters enable efficient cellular metabolism, aid in nutrient sensing, and have been associated with various diseases, such as obesity and cancer. Genome-scale metabolic network reconstructions capture genomic, physiological, and biochemical knowledge of a target organism, along with a detailed representation of the cellular metabolite transport mechanisms. Since the first reconstruction of human metabolism, Recon 1, published in 2007, progress has been made in the field of metabolite transport. Recently, we published an updated reconstruction, Recon 2, which significantly improved the metabolic coverage and functionality. Human metabolic reconstructions have been used to investigate the role of metabolism in disease and to predict biomarkers and drug targets. Given the importance of cellular transport systems in understanding human metabolism in health and disease, we analyzed the coverage of transport systems for various metabolite classes in Recon 2. We will review the current knowledge on transporters (i.e., their preferred substrates, transport mechanisms, metabolic relevance, and disease association for each metabolite class). We will assess missing coverage and propose modifications and additions through a transport module that is functional when combined with Recon 2. This information will be valuable for further refinements. These data will also provide starting points for further experiments by highlighting areas of incomplete knowledge. This review represents the first comprehensive overview of the transporters involved in central metabolism and their transport mechanisms, thus serving as a compendium of metabolite transporters specific for human metabolic reconstructions.

Nutrient transport in the mammary gland: calcium, trace minerals and water soluble vitamins

Milk nutrients are secreted by epithelial cells in the alveoli of the mammary gland by several complex and highly coordinated systems. Many of these nutrients are transported from the blood to the milk via transcellular pathways that involve the concerted activity of transport proteins on the apical and basolateral membranes of mammary epithelial cells. In this review, we focus on transport mechanisms that contribute to the secretion of calcium, trace minerals and water soluble vitamins into milk with particular focus on the role of transporters of the SLC series as well as calcium transport proteins (ion channels and pumps). Numerous members of the SLC family are involved in the regulation of essential nutrients in the milk, such as the divalent metal transporter-1 (SLC11A2), ferroportin-1 (SLC40A1) and the copper transporter CTR1 (SLC31A1). A deeper understanding of the physiology and pathophysiology of these transporters will be of great value for drug discovery and treatment of breast diseases.

Macrophage uptake and accumulation of folates are polarization-dependent in vitro and in vivo and are regulated by activin A

Vitamin B9, commonly known as folate, is an essential cofactor for one-carbon metabolism that enters cells through three major specialized transporter molecules (RFC, FR, and PCFT), which differ in expression pattern, affinity for substrate, and ligand-binding pH dependency. We now report that the expression of the folate transporters differs between macrophage subtypes and explains the higher accumulation of 5-MTHF-the major folate form found in serum-in M2 macrophages in vitro and in vivo. M1 macrophages display a higher expression of RFC, whereas FRβ and PCFT are preferentially expressed by anti-inflammatory and homeostatic M2 macrophages. These differences are also seen in macrophages from normal tissues involved in folate transit (placenta, liver, colon) and inflamed tissues (ulcerative colitis, RA), as M2-like macrophages from normal tissues express FRβ and PCFT, whereas TNF-α-expressing M1 macrophages from inflamed tissues are RFC+. Besides, we provide evidences that activin A is a critical factor controlling the set of folate transporters in macrophages, as it down-regulates FRβ, up-regulates RFC expression, and modulates 5-MTHF uptake. All of these experiments support the notion that folate handling is dependent on the stage of macrophage polarization.

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.

Appropriate nutrient supplementation in celiac disease

Reduced levels of iron, folate, vitamin B12, vitamin D, zinc, and magnesium are common in untreated celiac disease (CD) patients probably due to loss of brush border proteins and enzymes needed for the absorption of these nutrients. In the majority of patients, removal of gluten from the diet leads to histological recovery and normalization of iron, vitamin, and mineral levels. Iron deficiency anemia is the most common extra-intestinal sign of CD and usually resolves with adherence to a gluten-free diet. However, deficiencies of both folate and vitamin B12 may persist in some patients on a gluten-free diet, thus requiring vitamin supplementation to improve subjective health status. Similarly, exclusion of gluten from the diet does not always normalize bone mineral density; in these cases, supplementation of vitamin D and calcium is recommended. Resolution of mucosal inflammation may not be sufficient to abrogate magnesium deficiency. Since gluten-free cereal products have a lower magnesium content as compared with gluten-containing counterparts, a magnesium-enriched diet should be encouraged in CD patients. In this article we discuss the frequency and clinical relevance of nutrient deficiency in CD and whether and when nutrient supplementation is needed.

Preparation and biological evaluation of (99m) TcN-labeled pteroyl-lys derivative as a potential folate receptor imaging agent

In order to develop a novel (99m) Tc-labeled folate receptor (FR) imaging agent, a dithiocarbamate derivative, pteroyl-lys-DTC, was synthesized and radiolabeled with (99m) Tc through the [(99m) TcN](2+) intermediate. The radiochemical purity of the corresponding (99m) Tc-complex, (99m) TcN-pteroyl-lys-DTC, was over 95% as measured by reversed-phase HPLC. The (99m) TcN complex was stable under physiological conditions. (99m) TcN-pteroyl-lys-DTC exhibited specific FR binding in FR-positive KB cells in vitro. The biodistribution in tumor-bearing mice showed that the (99m) TcN-labeled radiotracer had good uptake (3.56 ± 0.09%ID/g at 2 h postinjection) in FR-positive KB tumors, as well as in the kidneys (30.34 ± 3.53%ID/g at 2 h postinjection). After coinjection with excess folic acid, the uptake in tumor and kidneys was significantly blocked. The results indicated that (99m) TcN-pteroyl-lys-DTC was able to target the FR-positive tumor cells and tissues specifically both in vitro and in vivo.

The ABCs of membrane transporters in health and disease (SLC series): introduction

The field of transport biology has steadily grown over the past decade and is now recognized as playing an important role in manifestation and treatment of disease. The SLC (solute carrier) gene series has grown to now include 52 families and 395 transporter genes in the human genome. A list of these genes can be found at the HUGO Gene Nomenclature Committee (HGNC) website (see www.genenames.org/genefamilies/SLC). This special issue features mini-reviews for each of these SLC families written by the experts in each field. The existing online resource for solute carriers, the Bioparadigms SLC Tables (www.bioparadigms.org), has been updated and significantly extended with additional information and cross-links to other relevant databases, and the nomenclature used in this database has been validated and approved by the HGNC. In addition, the Bioparadigms SLC Tables functionality has been improved to allow easier access by the scientific community. This introduction includes: an overview of all known SLC and “non-SLC” transporter genes; a list of transporters of water soluble vitamins; a summary of recent progress in the structure determination of transporters (including GLUT1/SLC2A1); roles of transporters in human diseases and roles in drug approval and pharmaceutical perspectives.

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).

Structures of human folate receptors reveal biological trafficking states and diversity in folate and antifolate recognition

Antifolates, folate analogs that inhibit vitamin B9 (folic acid)-using cellular enzymes, have been used over several decades for the treatment of cancer and inflammatory diseases. Cellular uptake of the antifolates in clinical use occurs primarily via widely expressed facilitative membrane transporters. More recently, human folate receptors (FRs), high affinity receptors that transport folate via endocytosis, have been proposed as targets for the specific delivery of new classes of antifolates or folate conjugates to tumors or sites of inflammation. The development of specific, FR-targeted antifolates would be accelerated if additional biophysical data, particularly structural models of the receptors, were available. Here we describe six distinct crystallographic models that provide insight into biological trafficking of FRs and distinct binding modes of folate and antifolates to these receptors. From comparison of the structures, we delineate discrete structural conformations representative of key stages in the endocytic trafficking of FRs and propose models for pH-dependent conformational changes. Additionally, we describe the molecular details of human FR in complex with three clinically prevalent antifolates, pemetrexed (also Alimta), aminopterin, and methotrexate. On the whole, our data form the basis for rapid design and implementation of unique, FR-targeted, folate-based drugs for the treatment of cancer and inflammatory diseases.

Biological and structural evaluation of 10R- and 10S-methylthio-DDACTHF reveals a new role for sulfur in inhibition of glycinamide ribonucleotide transformylase

Glycinamide ribonucleotide transformylase (GAR Tfase) is a folate-dependent enzyme in the de novo purine biosynthesis pathway, which has long been considered a potential target for development of anti-neoplastic therapeutics. Here we report the biological and X-ray crystallographic evaluations of both independent C10 diastereomers, 10S- and 10R-methylthio-DDACTHF, bound to human GAR Tfase, including the highest-resolution apo GAR Tfase structure to date (1.52 Å). Both diastereomers are potent inhibitors (Ki = 210 nM for 10R, and Ki = 180 nM for 10S) of GAR Tfase and exhibit effective inhibition of human leukemia cell growth (IC₅₀ = 80 and 50 nM, respectively). Their inhibitory activity was surprisingly high, and these lipophilic C10-substituted analogues show distinct advantages over their hydrophilic counterparts, most strikingly in retaining potency in mutant human leukemia cell lines that lack reduced folate carrier protein activity (IC₅₀ = 70 and 60 nM, respectively). Structural characterization reveals a new binding mode for these diastereoisomers, in which the lipophilic thiomethyl groups penetrate deeper into a hydrophobic pocket within the folate-binding site. In silico docking simulations of three other sulfur-containing folate analogues also indicate that this hydrophobic cleft represents a favorable region for binding lipophilic substituents. Overall, these results suggest sulfur and its substitutions play an important role in not only the binding of anti-folates to GAR Tfase but also the selectivity and cellular activity (growth inhibition), thereby presenting new possibilities for the future design of potent and selective anti-folate drugs that target GAR Tfase.

Targeting CCL21-folic acid-upconversion nanoparticles conjugates to folate receptor-α expressing tumor cells in an endothelial-tumor cell bilayer model

The ability of some malignant cells to evade immunosurveillance has been a major contribution to the inability of the host's immune system to eradicate the neoplastic cells. This has led to the development of various immunological strategies to augment the host immune response as part of cancer treatment. In this study, we developed folic acid (FA)/secondary lymphoid tissue chemokine (CCL21)/upconversion fluorescent nanoparticles (UCNs) conjugates as a targeting and delivery system to attract immune cells to folate receptor (FR) expressing tumor cells. Our data show that FA-conjugated UCNs@mesoporous silica specifically target FR expressing ovarian carcinoma cell line, OVCAR-3, compared to the unconjugated mesoporous silica coated UCNs. Furthermore, the FA-UCNs@mesoporous silica can efficiently cross the endothelial cell monolayer and accumulate in the clusters of OVCAR-3 cells in our endothelial-tumor cell bilayer model. Our migration assay data suggest that the CCL21 loaded into the mesoporous layer is biologically active and can efficiently induce T cells migration in-vitro. No significant cytotoxic effect was observed throughout the study indicating good biocompatibility of the nanoconjugates. As proof-of-concept, we have shown that it is feasible to load biologically active chemokines onto UCNs to modulate T cell migration.

Identification and characterization of a 66-68-kDa protein as a methotrexate-binding protein in murine leukemia L1210 cells

We previously observed an unidentified, tyrosine-phosphorylated, membrane-associated, 66-68-kDa protein which was present in the L1210 murine leukemia cells but not present, at least in the tyrosine-phosphorylated form, in cisplatin-methotrexate (CDDP-MTX) cross-resistant L1210/DDP cells. We purified and characterized this 66-68-kDa protein by affinity chromatography purification using its two identified properties, tyrosine phosphorylation and MTX-binding, and yielded a single band of 66-68 kDa. The purified protein was subjected to trypsin digestion and the isolated peptide fragments were sequenced and yielded two partial peptide sequences: VEIIANDQ and VTNAVVTVPAYFNDSQRQA. The two peptide sequences were used to search for the mouse genome at the national center for biotechnology information (NCBI) database for Open Reading Frame Sequence (ORFs) containing these peptides using the TBLASTN function. A single gene was identified containing both sequences, the HSPa8 gene, which codes for the heat shock family protein, HSC70. We further demonstrated that HSC70 is a MTX-binding protein using a binding assay with MTX-agarose beads followed by Western blotting. The HSC70 also existed in various cancer cell lines and showed binding to MTX. Additionally, the HSC70 protein, cloned from the L1210 murine leukemia cells, was expressed and purified from E. coli cells using a polyhistidine-tag purification system and it also showed the binding properties with MTX. DnaK, the HSC70 homologue in E. coli, also binds to MTX. By using the purified truncated HSC70 domains, we identified the adenosine triphosphatase (ATPase) domain of HSC70 that can bind to MTX. Thus, we have tentatively characterized a new, novel property of HSC70 as a MTX-binding protein.

Mechanistic insights of intestinal absorption and renal conservation of folate in chronic alcoholism

Folate mediated one-carbon metabolism is of fundamental importance for various cellular processes, including DNA synthesis and methylation of biological molecules. Due to the exogenous requirement of folate in mammals, there exists a well developed epithelial folate transport system for regulation of normal folate homeostasis. The intestinal and renal folate uptake is tightly and diversely regulated and disturbances in folate homeostasis like in alcoholism have pathological consequences. The study was sought to delineate the regulatory mechanism of folate uptake in intestine and reabsorption in renal tubular cells that could evaluate insights of malabsorption during alcoholism. The folate transporters PCFT and RFC were found to be associated with lipid rafts of membrane surfaces in intestine and kidney. Importantly, the observed lower intestinal and renal folate uptake was associated with decreased levels of folate transporter viz. PCFT and RFC in lipid rafts of intestinal and renal membrane surfaces. The decreased association of folate transporters in lipid rafts was associated with decreased protein and mRNA levels. In addition, immunohistochemical studies showed that alcoholic conditions deranged that localization of PCFT and RFC. These findings could explain the possible mechanistic insights that may result in folate malabsorption during alcoholism.

Pharmacogenomics in pediatric rheumatology

PURPOSE OF REVIEW

Despite major advancements in therapeutics, variability in drug response remains a challenge in both adults and children diagnosed with rheumatic disease. The genetic contribution to interindividual variability has emerged as a promising avenue of exploration; however, challenges remain in making this knowledge relevant in the clinical realm.

RECENT FINDINGS

New genetic associations in patients with rheumatic disease have been reported for disease modifying antirheumatic drugs, antimetabolites and biologic drugs. However, many of these findings are in need of replication, and few have taken into account the concept of ontogeny, specific to pediatrics.

SUMMARY

In the current era in which we practice, genetic variation will undoubtedly contribute to variability in therapeutic response and may be a factor that will ultimately impact individualized care. However, preliminary studies have shown that there are many hurdles that need to be overcome as we explore pharmacogenomic associations specifically in the field of pediatric rheumatology.

The effect of oxidative stress upon the intestinal uptake of folic acid: in vitro studies with Caco-2 cells

Folic acid (FA) is a vitamin essential for normal cellular functions, growth, and development. Because humans cannot synthesize this micronutrient, it must be obtained from dietary sources through intestinal absorption. The intestinal tract is a major target for oxidative stress. Our aim was to investigate the effect of oxidative stress upon the uptake of FA by Caco-2 cells. Oxidative stress was induced by exposure of the cells to tert-butyl hydroperoxide (TBH) for 1 h. TBH (3,000 μM) induced an increase in biomarkers of oxidative stress, while maintaining cell viability and proliferation. In relation to the apical uptake of (3)H-FA, TBH (3,000 μM) reduced the cellular accumulation of (3)H-FA (10 nM), although the characteristics (kinetics, pH dependence, and inhibitory profile) of (3)H-FA uptake were not changed. This effect was associated with a decrease in the mRNA steady-state levels of proton-coupled folate transporter and folate receptor alpha and of the efflux transporter multidrug resistance protein 2. Moreover, TBH (3,000 μM) did not affect the noncarrier-mediated apical uptake of (3)H-FA. Finally, the effect of TBH upon (3)H-FA apical uptake was not dependent on protein kinase A, protein kinase C, mitogen-activated protein kinases, phosphoinositide 3-kinase, nuclear factor kappa B, and protein tyrosine kinases, but was completely prevented by dietary polyphenols (resveratrol, quercetin, and EGCG). These results suggest that oxidative stress at the intestinal level may result in a reduction in the intestinal absorption of dietary FA and that polyphenolic dietary components may offer protection against oxidative stress-induced inhibition of intestinal FA absorption.

Paradoxical impact of two folate receptors, FRα and RFC, in ovarian cancer: effect on cell proliferation, invasion and clinical outcome

Despite being an essential vitamin, folate has been implicated to enhance tumor growth, as evidenced by reports on overexpression of folate receptor alpha (FRα) in carcinomas. The role of another folate transporter, reduced folate carrier (RFC), is largely unknown. This study investigated the roles of folate, FRα and RFC in ovarian cancers. We demonstrated FRα mRNA and protein overexpression and reduced RFC expression in association with FRα gene amplification and RFC promoter hypermethylation, respectively. FRα overexpression was associated with tumor progression while RFC expression incurred a favorable clinical outcome. Such reciprocal expression pattern was also observed in ovarian cancer cell lines. Folate was shown to promote cancer cell proliferation, migration and invasion in vitro, and down-regulate E-cadherin expression. This effect was blocked after either stable knockdown of FRα or ectopic overexpression of RFC. This hitherto unreported phenomenon suggests that, RFC can serve as a balancing partner of FRα and confer a protective effect in patients with high FRα-expressing ovarian carcinomas, as evidenced by their prolonged overall and disease-free survivals. In conclusion, we report on the paradoxical impact of FRα (putative oncogenic) and RFC (putative tumor suppressive) in human malignancies. FRα and RFC may potentially be explored as therapeutic target or prognostic marker respectively. We recommend caution and additional research on folate supplements in cancer patients.

Influence of Methylenetetrahydrofolate Reductase C677T, A1298C, and G80A Polymorphisms on the Survival of Pediatric Patients with Acute Lymphoblastic Leukemia

The influence of genic polymorphisms involved in metabolism of chemotherapeutic agents as the methotrexate (MTX) has been studied mainly in acute lymphoblastic leukemia (ALL) of childhood. Advances in treatment may be attributed to identification of prognostic factors added to chemotherapy protocol. The aim of this study was to analyze the association of the C677T, A1298C, and G80A polymorphisms on MTHFR gene and on the overall survival of pediatric patients (n = 126) with lymphoblastic leukemia treated with MTX according to the Brazilian protocol in 187 months. The C677T and G80A polymorphisms were genotyped by PCR-RFLP and A1298C polymorphism by allele-specific PCR. We observed that ALL patients presented rate (dead/alive) of 0.36 for the 677CC genotype, corresponding also to lower overall survival (P = 0.0013); on the other hand, the 677TT genotype showed a better survival (98%). Thus, we believe that patients with 80AA genotype presented a small reduction in MTX plasma level, suggesting that ALL children, carrying the 80AA genotype, showed a high toxicity to MTX (P < 0.0001).

A second target of benzamide riboside: dihydrofolate reductase

Dihydrofolate reductase (DHFR) is an essential enzyme involved in de novo purine and thymidine biosynthesis. For several decades, selective inhibition of DHFR has proven to be a potent therapeutic approach in the treatment of various cancers including acute lymphoblastic leukemia, non-Hodgkin's lymphoma, osteogenic sarcoma, carcinoma of the breast, and head and neck cancer. Therapeutic success with DHFR inhibitor methotrexate (MTX) has been compromised in the clinic, which limits the success of MTX treatment by both acquired and intrinsic resistance mechanisms. We report that benzamide riboside (BR), via anabolism to benzamide adenine dinucleotide (BAD) known to potently inhibit inosine monophosphate dehydrogenase (IMPDH), also inhibits cell growth through a mechanism involving downregulation of DHFR protein. Evidence to support this second site of action of BR includes the finding that CCRF-CEM/R human T-cell lymphoblasic leukemia cells, resistant to MTX as a consequence of gene amplification and overexpression of DHFR, are more resistant to BR than are parental cells. Studies of the mechanism by which BR lowers DHFR showed that BR, through its metabolite BAD, reduced NADP and NADPH cellular levels by inhibiting nicotinamide adenine dinucleotide kinase (NADK). As consequence of the lack of NADPH, DHFR was shown to be destabilized. We suggest that, inhibition of NADK is a new approach to downregulate DHFR and to inhibit cell growth.

Roles of one-carbon metabolism in preimplantation period--effects on short-term development and long-term programming--

One-carbon metabolism (OCM) can be seen as integrated metabolic pathways centered on the metabolism of two nutritional substances, folate and methionine. Mammalian oocytes and preimplantation embryos express almost all enzymes that participate in OCM, suggesting that they can independently metabolize OCM nutrients. A deficiency or excess of OCM nutrients and their metabolites during in vitro culture affects preimplantation development of mammalian embryos. Recent in vivo studies have demonstrated that specific OCM dietary interventions during the periconceptional (mainly oocyte growth and preimplantation) period can cause epigenetic alterations in DNA of offspring and program the long-term consequences in their health in adulthood. The epigenetic processes are likely to be implicated in the effects of OCM nutrients; however, understanding their effects at the level of specific genes and their implications in assisted reproductive technology will require further investigations.

GCPII imaging and cancer

Glutamate carboxypeptidase II (GCPII) in the central nervous system is referred to as the prostate-specific membrane antigen (PSMA) in the periphery. PSMA serves as a target for imaging and treatment of prostate cancer and because of its expression in solid tumor neovasculature has the potential to be used in this regard for other malignancies as well. An overview of GCPII/PSMA in cancer, as well as a discussion of imaging and therapy of prostate cancer using a wide variety of PSMA-targeting agents is provided.

Characterization of the human folate receptor alpha via novel antibody-based probes

Folate receptor alpha (FRA) is a cell surface protein whose aberrant expression in malignant cells has resulted in its pursuit as a therapeutic target and marker for diagnosis of cancer. The development of immune-based reagents that can reproducibly detect FRA from patient tissue processed by varying methods has been difficult due to the complex post-translational structure of the protein whereby most reagents developed to date are highly structure-sensitive and have resulted in equivocal expression results across independent studies. The aim of the present study was to generate novel monoclonal antibodies (mAbs) using modified full length FRA protein as immunogen in order to develop a panel of mAbs to various, non-overlapping epitopes that may serve as diagnostic reagents able to robustly detect FRA-positive disease. Here we report the development of a panel of FRA-specific mAbs that are able to specifically detect FRA using an array of diagnostic platforms and methods. In addition, the methods used to develop these mAbs and their diverse binding properties provide additional information on the three dimensional structure of FRA in its native cell surface configuration.

Interleukin-6 regulates anti-arthritic effect of methotrexate via reduction of SLC19A1 expression in a mouse arthritis model

Introduction

Methotrexate (MTX) enters cells via the reduced folate carrier SLC19A1, suggesting that SLC19A1 is associated with the efficacy of MTX. We here examined the relationship between the efficacy of MTX and the expression of SLC19A1 in glucose 6-phosphate isomerase (GPI)-induced arthritis. We found that interleukin-6 (IL-6) regulated the expression of SLC19A1, so we studied the effect of a combination of MTX and anti-mouse IL-6 receptor antibody (MR16-1).

Methods

GPI-induced arthritis was induced by intradermal immunization with recombinant GPI. MTX was given from the first day of immunization. Mice were injected once with MR16-1 10 days after immunization. The levels of SLC19A1 mRNA in whole hind limbs and immune cells were measured. Synovial cells from arthritic mice were cultured with cytokines, and cell proliferation and gene expressions were measured.

Results

MTX inhibited the development of GPI-induced arthritis; however, the efficacy of MTX gradually diminished. SLC19A1 expression in immunized mice with arthritis was lower than in intact mice; moreover, SLC19A1 expression in arthritic mice was further decreased when they were treated with MTX. IL-6 was highly expressed in whole hind limbs of arthritic mice. In an in vitro study using synovial cells from arthritic mice, IL-6 + soluble IL-6 receptor (sIL-6R) weakened the anti-proliferative effect of MTX and reduced SLC19A1 expression. Finally, although MR16-1 did not improve arthritis at all when administered on day 10, MTX in combination with MR16-1 more potently reduced the development of arthritis than did MTX alone. When used in combination with MTX, MR16-1 apparently reversed the decrease in SLC19A1 induced by MTX alone.

Conclusions

In the present study, we demonstrated for the first time that IL-6 reduced the efficacy of MTX by decreasing the expression of SLC19A1, which is important for MTX uptake into cells.

Genetic aspects of folate metabolism

The vitamin folate functions within the cell as a carrier of one-carbon units. The requirement for one-carbon transfers is ubiquitous and all mammalian cells carry out folate dependent reactions. In recent years, low folate status has been linked to risk of numerous adverse health conditions throughout life from birth defects and complications of pregnancy to cardiovascular disease, cancer and cognitive dysfunction in the elderly. In many instances inadequate intake of folate seems to be the primary contributor but there is also evidence that an underlying genetic susceptibility can play a modest role by causing subtle alterations in the availability, metabolism or distribution of intermediates in folate related pathways. Folate linked one-carbon units are essential for DNA synthesis and repair and as a source of methyl groups for biological methylation reactions. The notion of common genetic variants being linked to risk of disease was relatively novel in 1995 when the first functional folate-related polymorphism was discovered. Numerous polymorphisms have now been identified in folate related genes and have been tested for functionality either as a modifier of folate status or as being associated with risk of disease. Moreover, there is increasing research into the importance of folate-derived one-carbon units for DNA and histone methylation reactions, which exert crucial epigenetic control over cellular protein synthesis. It is thus becoming clear that genetic aspects of folate metabolism are wide-ranging and may touch on events as disparate as prenatal imprinting to cancer susceptibility. This chapter will review the current knowledge in this area.

Alcohol-associated folate disturbances result in altered methylation of folate-regulating genes

Folate plays a critical role in maintaining normal metabolic, energy, differentiation and growth status of all mammalian cells. The steady-state accumulation of folate seems to depend on the activity of two enzymes: folylpolyglutamate synthetase (FPGS), which adds glutamate residues, and gamma-glutamyl hydrolase (GGH), which removes them, enabling it to be transported across the biological membranes. Overexpression of GGH and downregulation of FPGS would be expected to decrease intracellular folate in its polyglutamylated form, thereby increasing efflux of folate and its related molecules, which might lead to resistance to drugs or folate deficiency. The study was sought to delineate the activity of GGH and expression FPGS in tissues involved in folate homeostasis during alcoholism and the epigenetic regulation of these enzymes and transporters regulating intracellular folate levels. We determined the activity of GGH and expression of FPGS in tissues after 3 months of ethanol feeding to rats at 1 g/kg body weight/day. The results showed that there was not any significant change in the activity of folate hydrolyzing enzyme GGH in ethanol-fed rats while there was significant down regulation in the expression of FPGS. Ethanol feeding decreased the total as well as polyglutamated folate levels. There was tissue-specific hyper/hypo methylation of folate transporter genes viz. PCFT and RFC by chronic ethanol feeding. Moreover, hypermethylation of FPGS gene was observed in intestine and kidney without any change in methylation levels of GGH in the ethanol-fed rats. In conclusion, the initial deconjugation of polyglutamylated folate by GGH was not impaired in ethanol-fed rats while the conversion of monoglutamylated folate to polyglutamylated form might be impaired. There was tissue-specific altered methylation of folate transporter genes by chronic ethanol feeding.

Synthesis and evaluation of folate-based chlorambucil delivery systems for tumor-targeted chemotherapy

The development of tumor-targeting drug delivery systems, able to selectively transport cytotoxic agents into the tumor site by exploiting subtle morphological and physiological differences between healthy and malignant cells, currently stands as one of the most attractive anticancer strategies used to overcome the selectivity problems of conventional chemotherapy. Owing to frequent overexpression of folate receptors (FRs) on the surface of malignant cells, conjugation of cytotoxic agents to folic acid (FA) via suitable linkers have demonstrated to enhance selective drug delivery to the tumor site. Herein, the chemical synthesis and biological evaluation of two novel folate-conjugates bearing the anticancer agent chlorambucil (CLB) tethered to either an aminoether (4,7,10-trioxa-1,13-tridecanediamine) or a pseudo-β-dipeptide (β-Ala-ED-β-Ala) linker is reported. The two drug delivery systems have been prepared in high overall yields (54% and 34%) through straightforward and versatile synthetic routes. Evaluation of cell specificity was examined using three leukemic cell lines, undifferentiated U937 (not overexpressing FRs, FR(-)), TPA-differentiated U937 (overexpressing FRs, FR(+)), and TK6 (FR(+)) cells. Both conjugates exhibited high specificity only to FR(+) cells (particularly TK6), demonstrating comparable antitumor activity to CLB in its free form. These data confirm the reliability of folate-based drug delivery systems for targeted antitumor therapy; likewise, they lay the foundations for the development of other folate-conjugates with antitumor potential.

A low molecular weight folate receptor targeted contrast agent for magnetic resonance tumor imaging

PURPOSE

This study aims to develop a low molecular weight folate receptor (FR) contrast agent for MR tumor imaging.

PROCEDURES

Gadolinium-tetraazacyclododecane tetraacetic acid (Gd.DOTA) was conjugated to folic acid to create Gd.DOTA.Folate. The efficacy of Gd.DOTA.Folate to bind FR was evaluated in vitro by inductively coupled mass spectrometry (ICP-MS) and in vivo by magnetic resonance imaging (MRI) tumor enhancement over 14 h, utilizing an overexpressing α-FR cell line (IGROV-1), compared to an α-FR-negative cell line (OVCAR-3). Gd.DOTA.Folate localization ex vivo was verified by laser ablation ICP-MS.

RESULTS

ICP-MS confirmed Gd.DOTA.Folate uptake by IGROV-1 cells and competitive binding with free folic acid inhibited binding. IGROV-1 tumors showed an increase in R (1) at 2 h, which increased significantly over 14 h post-Gd.DOTA.Folate with clear enhancement on MR images. This was not observed in controls.

CONCLUSION

These data support the use of FR-targeted small molecular weight MRI contrast agents for tumor imaging in vivo.

Substrate- and pH-specific antifolate transport mediated by organic anion-transporting polypeptide 2B1 (OATP2B1-SLCO2B1)

Human organic anion-transporting polypeptide (OATP) 2B1 (OATP-B; SLCO2B1) is expressed in the apical membrane of the small intestine and the hepatocyte basolateral membrane and transports structurally diverse organic anions with a wide spectrum of pH sensitivities. This article describes highly pH-dependent OATP2B1-mediated antifolate transport and compares this property with that of sulfobromophthalein (BSP), a preferred OATP2B1 substrate. At pH 5.5 and low substrate concentrations (~2.5 μM), only [(3)H]pemetrexed influx [in contrast to methotrexate (MTX), folic acid, and reduced folates] could be detected in OATP2B1-transfected HeLa R1-11 cells that lack endogenous folate-specific transporters. Influx was optimal at pH 4.5 to 5.5, falling precipitously with an increase in pH >6.0; BSP influx was independent of pH. Influx of both substrates at low pH was markedly inhibited by the proton ionophore 4-(trifluoromethoxy)phenylhydrazone; BSP influx was also suppressed at pH 7.4. At 300 μM MTX, influx was one-third that of pemetrexed; influx of folic acid, (6S)5-methyltetrahydrofolate, or (6S)5-formyltetrahydrofolate was not detected. There were similar findings in OATP2B1-expressing Xenopus laevis oocytes. The pemetrexed influx K(m) was ~300 μM; the raltitrexed influx K(i) was ~70 μM at pH 5.5. Stable expression of OAPT2B1 in HeLa R1-11 cells resulted in substantial raltitrexed, but modest pemetrexed, growth inhibition consistent with their affinities for this carrier. Hence, OATP2B1 represents a low-affinity transport route for antifolates (relative affinities: raltitrexed > pemetrexed > MTX) at low pH. In contrast, the high affinity of this transporter for BSP relative to antifolates seems to be intrinsic to its binding site and independent of the proton concentration.

Thymidylate synthase inhibitors for non-small cell lung cancer

INTRODUCTION

The folate-dependent enzyme thymidylate synthase (TS) plays a pivotal role in DNA replication/repair and cancer cell proliferation, and represents a valid target for the treatment of several tumor types, including NSCLC. NSCLC is the leading cause of cancer-related mortality, and several TS inhibitors have gone into preclinical and clinical testing, with pemetrexed emerging for its approval and widespread use as first-/second-line and maintenance therapy for this disease.

AREAS COVERED

This review summarizes the therapeutic options in NSCLC, as well as the background and rationale for targeting TS. The authors also review recent pharmacogenetic studies and data from clinical trials evaluating novel TS inhibitors, hoping that the reader will gain a comprehensive overview of the field of TS inhibition, specifically relating to drugs used or being developed for lung cancer patients.

EXPERT OPINION

TS is a validated target in NSCLC. However, benefits from conventional chemotherapy in NSCLC have plateaued, and more cost-effective results should be obtained with individualized treatment. Accordingly, the clinical success for TS inhibitors may depend on our ability to correctly administer these agents following biomarker-driven patient selection, including TS genotype and expression, and using the right combination therapy.

Expression of RFC/SLC19A1 is associated with tumor type in bladder cancer patients

Urinary bladder cancer (UBC) ranks ninth in worldwide cancer. In Egypt, the pattern of bladder cancer is unique in that both the transitional and squamous cell types prevail. Despite much research on the topic, it is still difficult to predict tumor progression, optimal therapy and clinical outcome. The reduced folate carrier (RFC/SLC19A1) is the major transport system for folates in mammalian cells and tissues. RFC is also the primary means of cellular uptake for antifolate cancer chemotherapeutic drugs, however, membrane transport of antifolates by RFC is considered as limiting to antitumor activity. The purpose of this study was to compare the mRNA expression level of RFC/SLC19A1 in urothelial and non-urothelial variants of bladder carcinomas. Quantification of RFC mRNA in the mucosa of 41 untreated bladder cancer patients was performed using RT-qPCR. RFC mRNA steady-state levels were ∼9-fold higher (N = 39; P<0.0001) in bladder tumor specimens relative to normal bladder mRNA. RFC upregulation was strongly correlated with tumor type (urothelial vs. non-urothelial; p<0.05) where median RFC mRNA expression was significantly (p<0.05) higher in the urothelial (∼14-fold) compared to the non-urothelial (∼4-fold) variant. This may account for the variation in response to antifolate-containing regimens used in the treatment of either type. RFC mRNA levels were not associated with tumor grade (I, II and III) or stage (muscle-invasive vs. non-muscle invasive) implying that RFC cannot be used for prognostic purposes in bladder carcinomas and its increased expression is an early event in human bladder tumors pathogenesis. Further, RFC can be considered as a potential marker for predicting response to antifolate chemotherapy in urothelial carcinomas.

Functional characterization of folate transport proteins in Staten's Seruminstitut rabbit corneal epithelial cell line

PURPOSE

The overall objective of this study was to investigate and characterize the expression of folate transport proteins in Staten's Seruminstitut rabbit corneal (SIRC) epithelial cell line.

METHODS

[(3)H]Folic acid uptake was studied with respect to time, pH, temperature, sodium, and chloride ion dependency. Inhibition studies were conducted with structural analogs, vitamins, and metabolic inhibitors. [(3)H]Folic acid uptake was also determined with varying concentrations of cold folic acid. Uptake kinetics was studied in the presence of various modulators of intracellular regulatory pathways, protein kinases A and C (PKA and PKC), protein tyrosine kinase (PTK), and calcium-calmodulin modulators. Ex vivo corneal permeability studies were carried out with [(3)H]folic acid in the presence and absence of 1 mM cold folic acid.

RESULTS

Linear increase in [(3)H]folic acid uptake was observed over 30 min. The process followed saturation kinetics with apparent K(m) of 14.2 ± 0.2 nM, V(max) of (1.5 ± 0.1)*10(-5) micro.moles/min/mg protein, and K(d) of (2.1 ± 0.2)*10(-6) min(-1). The uptake process was found to be dependent on pH, sodium ions, chloride ions, temperature, and energy. Uptake was inhibited in the presence of structural analogs (cold folic acid, methyltetrahydro folate, and methotrexate), but structurally unrelated vitamins did not show any effect. Membrane transport inhibitors SITS, DIDS, probenecid and endocytic inhibitor, colchicine significantly inhibited the [(3)H]folic acid uptake indicating the involvement of receptor/transporter mediated process. PKA, PTK, and Ca(2+)/calmodulin pathways significantly regulate the process. RT-PCR and Western blot analysis confirmed the presence of folate receptor-α (FR-alpha) and proton-coupled folate transporter (PCFT). Permeability of [(3)H]folic acid across the rabbit cornea was (1.48 ± 0.13)*10(-05) cm/sec, and in the presence of cold folic acid it was (1.08 ± 0.10)*10(-05) cm/sec.

CONCLUSIONS

This work demonstrated the functional and molecular presence of FR-alpha and PCFT in SIRC epithelial cell line.

The antiepileptic drugs phenobarbital and carbamazepine reduce transport of methotrexate in rat choroid plexus by down-regulation of the reduced folate carrier

Intrathecal methotrexate (MTX) has been associated with severe neurotoxicity. Because carrier-associated removal of MTX from the cerebrospinal fluid (CSF) into blood remains undefined, we determined the expression and function of MTX transporters in rat choroid plexus (CP). MTX neurotoxicity usually manifests as seizures requiring therapy with antiepileptic drugs (AEDs) such as phenobarbital (PB). Because we have demonstrated that PB reduces activity of MTX influx carrier reduced folate carrier (Rfc1) in liver, we investigated the influence of the AEDs PB, carbamazepine (CBZ), or gabapentin on Rfc1-mediated MTX transport in CP. Reverse transcriptase-polymerase chain reaction and Western blot analysis showed similar expression of the MTX influx carrier Rfc1 and organic anion transporter 3 or efflux transporter multidrug resistance-associated protein 1 (Mrp1) and breast cancer resistance protein (Bcrp) in rat CP tissue and choroidal epithelial Z310 cells. Confocal microscopy revealed subcellular localization of Rfc1 and Bcrp at the apical and of Mrp1 at the basolateral CP membrane. Uptake, efflux, and inhibition studies indicated MTX transport activity of Rfc1, Mrp1, and Bcrp. PB and CBZ but not gabapentin significantly inhibited Rfc1-mediated uptake of MTX in CP cells. Studies on the regulatory mechanism showed that PB significantly inhibited Rfc1 translation but did not alter carrier gene expression. Altogether, removal of intrathecal MTX across the blood-CSF barrier may be achieved through Rfc1-mediated uptake from the CSF followed by MTX extrusion into blood, particularly via Mrp1. Antiepileptic treatment with PB or CBZ causes post-transcriptional down-regulation of Rfc1 activity in CP. This mechanism may result in enhanced MTX toxicity in patients with cancer who are receiving intrathecal MTX chemotherapy by reduced CSF clearance of the drug.

Maternal folic acid supplement intake and semen quality in Danish sons: a follow-up study

OBJECTIVE

To examine whether maternal folic acid supplement intake during pregnancy is related to better semen quality in male offspring.

DESIGN

A follow-up study.

SETTING

Two major Danish municipalities, Aalborg and Odense.

PATIENT(S)

The study population included 347 singleton sons of mothers enrolled into the Healthy Habits for Two cohort when pregnant in 1984-87.

INTERVENTION(S)

Information on maternal folic acid supplement intake during pregnancy was provided by self-administered questionnaire in the 36th week of gestation.

MAIN OUTCOME MEASURE(S)

Semen characteristics and serum concentrations of sex hormones.

RESULT(S)

The distribution of semen characteristics among sons whose mothers took folic acid supplement during pregnancy (n = 88, 25%) did not differ from the distributions among those without (n = 75, 22%) or with unknown folic acid supplement intake (n = 84, 53%). On the contrary, serum levels of FSH and LH were significantly higher in the folic acid supplement group.

CONCLUSION(S)

The hypothesis that folic acid supplement intake during pregnancy will improve semen quality in male offspring was not corroborated by a follow-up study in young Danish men.

Effect of paternal folate deficiency on placental folate content and folate receptor α expression in rats

We investigated the effect of paternal folate status on folate content and expression of the folate transporter folate receptor α (FRα) in rat placental tissues. Rats were mated after males were fed a diet containing 0 mg of folic acid/kg of diet (paternal folate-deficient, PD) or 8 mg folic acid/kg of diet (paternal folate-supplemented, PS) for 4 weeks. At 20 days of gestation, the litter size, placental weight, and fetal weight were measured, and placental folate content (n = 8/group) and expression of FRα (n = 10/group) were analyzed by microbiological assay and Western blot analysis, respectively. Although there was no difference observed in litter size or fetal weight, but significant reduction (10%) in the weight of the placenta was observed in the PD group compared to that in the PS group. In the PD group, placental folate content was significantly lower (by 35%), whereas FRα expression was higher (by 130%) compared to the PS group. Our results suggest that paternal folate status plays a critical role in regulating placental folate metabolism and transport.

A folate receptor beta-specific human monoclonal antibody recognizes activated macrophage of rheumatoid patients and mediates antibody-dependent cell-mediated cytotoxicity

Introduction

Folate receptor beta (FRβ) is only detectable in placenta and limited to some hematopoietic cells of myeloid lineage in healthy people. Studies have indicated that FRβ is over-expressed in activated macrophages in autoimmune diseases and some cancer cells. In this study we aimed to develop an FRβ-specific human monoclonal antibody (mAb) that could be used as a therapeutic agent to treat rheumatoid arthritis and other autoimmune diseases, as well as FRβ positive cancers.

Methods

Functional recombinant FRβ protein was produced in insect cells and used as antigen to isolate a mAb, m909, from a human naïve Fab phage display library. Binding of Fab and IgG1 m909 to FRβ was measured by ELISA, surface plasmon resonance, immune fluorescence staining, and flow cytometry. Antibody-dependent cell-mediated cytotoxicity (ADCC) was evaluated with FRβ positive CHO cells as target cells and isolated peripheral blood monocytes as effector cells in an in vitro assay.

Results

Fab m909 bound with relatively high affinity (equilibrium dissociation constant 57 nM) to FRβ. The IgG1 m909 showed much higher (femtomolar) avidity as measured by ELISA, and it bound to FRβ positive cells in a dose-dependent manner, but not to parental FRβ negative cells. m909 did not compete with folate for the binding to FRβ on cells. m909 was not only able to select FRβ positive, activated macrophages from synovial fluid cells of arthritis patients as efficiently as folate, but also able to mediate ADCC in FRβ positive cells.

Conclusions

Unlike folate-drug conjugates, m909 selectively binds to FRβ, does not recognize FRα, and has at least one effector function. m909 alone has potential to eliminate FRβ positive cells. Because m909 does not compete with folate for receptor binding, it can be used with folate-drug conjugates in a combination therapy. m909 can also be a valuable research reagent.

Nutritional deficiencies and phospholipid metabolism

Phospholipids are important components of the cell membranes of all living species. They contribute to the physicochemical properties of the membrane and thus influence the conformation and function of membrane-bound proteins, such as receptors, ion channels, and transporters and also influence cell function by serving as precursors for prostaglandins and other signaling molecules and modulating gene expression through the transcription activation. The components of the diet are determinant for cell functionality. In this review, the effects of macro and micronutrients deficiency on the quality, quantity and metabolism of different phospholipids and their distribution in cells of different organs is presented. Alterations in the amount of both saturated and polyunsaturated fatty acids, vitamins A, E and folate, and other micronutrients, such as zinc and magnesium, are discussed. In all cases we observe alterations in the pattern of phospholipids, the more affected ones being phosphatidylcholine, phosphatidylethanolamine and sphingomyelin. The deficiency of certain nutrients, such as essential fatty acids, fat-soluble vitamins and some metals may contribute to a variety of diseases that can be irreversible even after replacement with normal amount of the nutrients. Usually, the sequelae are more important when the deficiency is present at an early age.

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.

Regulatory mechanisms of intestinal folate uptake in a rat model of folate oversupplementation

Folic acid is essential for numerous biological functions, ranging from nucleotide biosynthesis to the remethylation of homocysteine. Folic acid is unable to cross the biological membranes by simple diffusion, so there exists a well-developed epithelial folate transport system for the regulation of normal folate homeostasis in the intestine. Any perturbances in the folate uptake system might lead to a state of folate deficiency, which in turn is strongly associated with the risk of various cancers, birth defects and CVD. Countries with obligatory folate fortification of food (USA and Canada) have documented a significant decrease in neural tube defects in newborns. However, the effect of folate oversupplementation on the intestinal absorption of folic acid has not been studied. We studied the process of folate transport and the expression of folate transporters in the rat intestine after folate oversupplementation. Rats were oversupplemented with tenfold the normal requirement of folic acid for periods of 10 and 60 d. Folate uptake in intestinal brush-border membrane vesicles followed saturable kinetics with pH optimum at 5·5. Acute, but not chronic, folate oversupplementation led to a significant down-regulation in intestinal folate uptake at acidic pH optima and was associated with a decrease in Vmax without any significant change in the Km of the folate uptake process. The decrease in folate uptake was also associated with the down-regulation in the protein levels of major folate transporters, proton-coupled folate transporter (PCFT) and reduced folate carrier (RFC), without altering their mRNA levels. Hence, it was concluded that acute folate oversupplementation results in a significant decrease in intestinal folate uptake by down-regulating the expressions of RFC and PCFT, via some post-transcriptional or translational mechanisms.

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.

Functional analysis of folate polyglutamylation and its essential role in plant metabolism and development

Cellular folates function as co-enzymes in one-carbon metabolism and are predominantly decorated with a polyglutamate tail that enhances co-enzyme affinity, subcellular compartmentation and stability. Polyglutamylation is catalysed by folylpolyglutamate synthetases (FPGSs) that are specified by three genes in Arabidopsis, FPGS1, 2 and 3, which reportedly encode plastidic, mitochondrial and cytosolic isoforms, respectively. A mutational approach was used to probe the functional importance of folate polyglutamylation in one-carbon metabolism and development. Biochemical analysis of single FPGS loss-of-function mutants established that folate polyglutamylation is essential for organellar and whole-plant folate homeostasis. However, polyglutamylated folates were still detectable, albeit at lower levels, in organelles isolated from the corresponding isozyme knockout lines, e.g. in plastids and mitochondria of the fpgs1 (plastidial) and fpgs2 (mitochondrial) mutants. This result is surprising given the purported single-compartment targeting of each FPGS isozyme. These results indicate redundancy in compartmentalised FPGS activity, which in turn explains the lack of anticipated phenotypic defects for the single FPGS mutants. In agreement with this hypothesis, fpgs1 fpgs2 double mutants were embryo-lethal, fpgs2 fpgs3 mutants exhibited seedling lethality, and fpgs1 fpgs3 mutants were dwarfed with reduced fertility. These phenotypic, metabolic and genetic observations are consistent with targeting of one or more FPGS isozymes to multiple organelles. These data confirm the importance of polyglutamylation in folate compartmentation, folate homeostasis and folate-dependent metabolic processes, including photorespiration, methionine and pantothenate biosynthesis.