Increased expression and characterization of two distinct folate binding proteins in murine erythroleukemia cells

The Concept of Folic Acid in Health and Disease

Folates have a pterine core structure and high metabolic activity due to their ability to accept electrons and react with O-, S-, N-, C-bounds. Folates play a role as cofactors in essential one-carbon pathways donating methyl-groups to choline phospholipids, creatine, epinephrine, DNA. Compounds similar to folates are ubiquitous and have been found in different animals, plants, and microorganisms. Folates enter the body from the diet and are also synthesized by intestinal bacteria with consequent adsorption from the colon. Three types of folate and antifolate cellular transporters have been found, differing in tissue localization, substrate affinity, type of transferring, and optimal pH for function. Laboratory criteria of folate deficiency are accepted by WHO. Severe folate deficiencies, manifesting in early life, are seen in hereditary folate malabsorption and cerebral folate deficiency. Acquired folate deficiency is quite common and is associated with poor diet and malabsorption, alcohol consumption, obesity, and kidney failure. Given the observational data that folates have a protective effect against neural tube defects, ischemic events, and cancer, food folic acid fortification was introduced in many countries. However, high physiological folate concentrations and folate overload may increase the risk of impaired brain development in embryogenesis and possess a growth advantage for precancerous altered cells.

Free fatty acids induce the demethylation of the fructose 1,6-biphosphatase 2 gene promoter and potentiate its expression in hepatocytes

Obesity is a serious health issue as it is a social burden and the main risk factor for other metabolic diseases. Increasing evidence indicates that a high-fat diet (HFD) is the key factor for the development of obesity, but the key genes and their associated molecular mechanisms are still not fully understood. In this study, we performed integrated bioinformatic analysis and identified that fructose-1,6 biphosphatase 2 (FBP2) was involved in free fatty acids (FFAs)-induced lipid droplet accumulation in hepatocytes and HFD-induced obesity in mice. Our data showed that palmitate (PA) and oleic acid (OA) induced the expression of FBP2 in time- and dose-dependent manners, and accelerated the development of lipid droplets in LO2 human normal liver cells. In HFD-fed C57BL/6 mice, accompanied by insulin resistance and lipid droplet accumulation, the mRNA and protein levels of FBP2 in the livers also increased significantly. The results from the methylation sequencing PCR (MSP) and bisulfite specific PCR (BSP) indicated that PA/OA induced the demethylation of the FBP2 gene promoter in LO2 cells. Moreover, betaine, a methyl donor, attenuated the expression of the FBP2 gene, the accumulation of lipid droplets, and the expression of perilipin-2, a biomarker of lipid droplets, in LO2 cells. All these findings revealed that FBP2 might be involved in HFD-induced obesity, and it is of interest to investigate the role of FBP2 in the treatment and prevention of obesity and its associated complications.

HATRIC-based identification of receptors for orphan ligands

Cellular responses depend on the interactions of extracellular ligands, such as nutrients, growth factors, or drugs, with specific cell-surface receptors. The sensitivity of these interactions to non-physiological conditions, however, makes them challenging to study using in vitro assays. Here we present HATRIC-based ligand receptor capture (HATRIC-LRC), a chemoproteomic technology that successfully identifies target receptors for orphan ligands on living cells ranging from small molecules to intact viruses. HATRIC-LRC combines a click chemistry-based, protein-centric workflow with a water-soluble catalyst to capture ligand-receptor interactions at physiological pH from as few as 1 million cells. We show HATRIC-LRC utility for general antibody target validation within the native nanoscale organization of the surfaceome, as well as receptor identification for a small molecule ligand. HATRIC-LRC further enables the identification of complex extracellular interactomes, such as the host receptor panel for influenza A virus (IAV), the causative agent of the common flu.

Technologies for identifying receptor-ligand pairs on living cells at physiological conditions remain scarce. Here, the authors develop a mass spectrometry-based ligand receptor capture technology that can identify receptors for a diverse range of ligands at physiological pH with as few as a million cells.

Maternally Contributed Folate Receptor 1 Is Expressed in Ovarian Follicles and Contributes to Preimplantation Development

Folates have been shown to play a crucial role for proper development of the embryo as folate deficiency has been associated with reduced developmental capacity such as increased risk of fetal neural tube defects and spontanous abortion. Transcripts encoding the reduced folate carrier RFC1 (SLC19A1 protein) and the high-affinity folate receptor FOLR1 are expressed in oocytes and preimplantation embryos, respectively. In this study, we observed maternally contributed FOLR1 protein during mouse and human ovarian follicle development, and 2-cell mouse embryos. In mice, FOLR1 was highly enriched in oocytes from primary, secondary and tertiary follicles, and in the surrounding granulosa cells. Interestingly, during human follicle development, we noted a high and specific presence of FOLR1 in oocytes from primary and intermediate follicles, but not in the granulosa cells. The distribution of FOLR1 in follicles was noted as membrane-enriched but also seen in the cytoplasm in oocytes and granulosa cells. In 2-cell embryos, FOLR1-eGFP fusion protein was detected as cytoplasmic and membrane-associated dense structures, resembling the distribution pattern observed in ovarian follicle development. Knock-down of Folr1 mRNA function was accomplished by microinjection of short interference (si)RNA targeting Folr1, into mouse pronuclear zygotes. This revealed a reduced capacity of Folr1 siRNA-treated embryos to develop to blastocyst compared to the siRNA-scrambled control group, indicating that maternally contributed protein and zygotic transcripts sustain embryonic development combined. In summary, maternally contributed FOLR1 protein appears to maintain ovarian functions, and contribute to preimplantation development combined with embryonically synthesized FOLR1.

Enhanced receptor-mediated endocytosis and cytotoxicity of a folic acid-desacetylvinblastine monohydrazide conjugate in a pemetrexed-resistant cell line lacking folate-specific facilitative carriers but with increased folate receptor expression

The reduced folate carrier (RFC), proton-coupled folate transporter (PCFT), and folate receptors (FR) are folate-specific transporters. Antifolates currently in the clinic, such as pemetrexed, methotrexate, and pralatrexate, are transported into tumor cells primarily via RFC. Folic acid conjugated to cytotoxics, a new class of antineoplastics, are transported into cells via FR-mediated endocytosis. To better define the role of PCFT in antifolate resistance, a methotrexate-resistant cell line, M160-8, was selected from a HeLa subline in which the RFC gene was deleted and PCFT was highly overexpressed. These cells were cross-resistant to pemetrexed. PCFT function and the PCFT mRNA level in M160-8 cells were barely detectable, and FR-α function and mRNA level were increased as compared with the parent cells. While pemetrexed rapidly associated with FR and was internalized within endosomes in M160-8 cells, consistent with FR-mediated transport, subsequent pemetrexed and (6S)-5-formyltetrahydrofolate export into the cytosol was markedly impaired. In contrast, M160-8 cells were collaterally sensitive to EC0905, a folic acid-desacetylvinblastine monohydrazide conjugate also transported by FR-mediated endocytosis. However, in this case a sulfhydryl bond is cleaved to release the lipophilic cytotoxic moiety into the endosome, which passively diffuses out of the endosome into the cytosol. Hence, resistance to pemetrexed in M160-8 cells was due to entrapment of the drug within the endosome due to the absence of PCFT under conditions in which the FR cycling function was intact.

Novel folate binding protein-1 interactions in embryonic orofacial tissue

AIM

To identify proteins with which FolBp1 may interact within lipid rafts in tissue derived from embryonic orofacial tissue.

METHODS

A yeast two-hybrid screen of a cDNA library, derived from orofacial tissue from gestational day 11 to 13 mouse embryos, was conducted.

KEY FINDINGS

Using the full-length FolBp1 protein as bait, two proteins that bind FolBp1 were identified, Bat2d, and a fibronectin type III-containing domain protein. Results were confirmed by glutathione S-transferase pull-down assays.

SIGNIFICANCE

As a component of membrane lipid raft protein complexes, these binding proteins may represent "helper" or chaperone proteins that associate with FolBp1 in order to facilitate the transport of folate across the plasma membrane. The protein-protein interactions detected, while limited in number, may be critical in mediating the role of FolBp1 in folate transport, particularly in the developing embryo.

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

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

Valproic acid-induced deregulation in vitro of genes associated in vivo with neural tube defects

The utility of an in vitro system to search for molecular targets and markers of developmental toxicity was explored, using microarrays to detect genes susceptible to deregulation by the teratogen valproic acid (VPA) in the pluripotent mouse embryonal carcinoma cell line P19. Total RNA extracted from P19 cells cultured in the absence or presence of 1, 2.5, or 10mM VPA for 1.5, 6, or 24 h was subjected to replicated microarray analysis, using CodeLink UniSet I Mouse 20K Expression Bioarrays. A moderated F-test revealed a significant VPA response for 2972 (p < 10(-3)) array probes (19.4% of the filtered gene list), 421 of which were significant across all time points. In a core subset of VPA target genes whose expression was downregulated (68 genes) or upregulated (125 genes) with high probability (p < 10(-7)) after both 1.5 and 6 h of VPA exposure, there was a significant enrichment of the biological process Gene Ontology term transcriptional regulation among downregulated genes, and apoptosis among upregulated, and two of the downregulated genes (Folr1 and Gtf2i) have a knockout phenotype comprising exencephaly, the major malformation induced by VPA in mice. The VPA-induced gene expression response in P19 cells indicated that approximately 30% of the approximately 200 genes known from genetic mouse models to be associated with neural tube defects may be potential VPA targets, suggestive of a combined deregulation of multiple genes as a possible mechanism of VPA teratogenicity. Gene expression responses related to other known effects of VPA (histone deacetylase inhibition, G(1)-phase cell cycle arrest, induction of apoptosis) were also identified. This study indicates that toxicogenomic responses to a teratogenic compound in vitro may correlate with known in vitro and in vivo effects, and that short-time (< or =6 h) exposures in such an in vitro system could provide a useful component in mechanistic studies and screening tests in developmental toxicology.

The reduced folate carrier (RFC) is cytotoxic to cells under conditions of severe folate deprivation. RFC as a double edged sword in folate homeostasis

The reduced folate carrier (RFC), a bidirectional anion transporter, is the major uptake route of reduced folates essential for a spectrum of biochemical reactions and thus cellular proliferation. However, here we show that ectopic overexpression of the RFC, but not of folate receptor alpha, a high affinity unidirectional folate uptake route serving here as a negative control, resulted in an approximately 15-fold decline in cellular viability in medium lacking folates but not in folate-containing medium. Moreover to explore possible mechanisms of adaptation to folate deficiency in various cell lines that express the endogenous RFC, we first determined the gene expression status of the following genes: (a) RFC, (b) ATP-driven folate exporters (i.e. MRP1, MRP5, and breast cancer resistance protein), and (c) folylpoly-gamma-glutamate synthetase and gamma-glutamate hydrolase (GGH), enzymes catalyzing folate polyglutamylation and hydrolysis, respectively. Upon 3-7 days of folate deprivation, semiquantitative reverse transcription-PCR analysis revealed a specific approximately 2.5-fold decrease in RFC mRNA levels in both breast cancer and T-cell leukemia cell lines that was accompanied by a consistent fall in methotrexate influx, serving here as an RFC transport activity assay. Likewise a 2.4-fold decrease in GGH mRNA levels and approximately 19% decreased GGH activity was documented for folate-deprived breast cancer cells. These results along with those of a novel mathematical biomodeling devised here suggest that upon severe short term (i.e. up to 7 days) folate deprivation RFC transport activity becomes detrimental as RFC, but not ATP-driven folate exporters, efficiently extrudes folate monoglutamates out of cells. Hence down-regulation of RFC and GGH may serve as a novel adaptive response to severe folate deficiency.

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.

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.

Folate-mediated one-carbon metabolism and neural tube defects: balancing genome synthesis and gene expression

Neural tube defects (NTDs) refer to a cluster of neurodevelopmental conditions associated with failure of neural tube closure during embryonic development. Worldwide prevalence of NTDs ranges from approximately 0.5 to 60 per 10,000 births, with regional and population-specific variation in prevalence. Numerous environmental and genetic influences contribute to NTD etiology; accumulating evidence from population-based studies has demonstrated that folate status is a significant determinant of NTD risk. Folate-mediated one-carbon metabolism (OCM) is essential for de novo nucleotide biosynthesis, methionine biosynthesis, and cellular methylation reactions. Periconceptional maternal supplementation with folic acid can prevent occurrence of NTDs in the general population by up to 70%; currently several countries fortify their food supply with folic acid for the prevention of NTDs. Despite the unambiguous impact of folate status on NTD risk, the mechanism by which folic acid protects against NTDs remains unknown. Identification of the mechanism by which folate status affects neural tube closure will assist in developing more efficacious and better targeted preventative measures. In this review, we summarize current research on the relationship between folate status and NTDs, with an emphasis on linking genetic variation, folate nutriture, and specific metabolic and/or genomic pathways that intersect to determine NTD outcomes.

Folate deficiency induces cell-specific changes in the steady-state transcript levels of genes involved in folate metabolism and 1-carbon transfer reactions in human colonic epithelial cells

Intracellular folate homeostasis is essential for the 1-carbon transfer reactions necessary for DNA synthesis and biological methylation reactions in colonic epithelial cells. Perturbed 1-carbon transfer reactions resulting from folate depletion predispose normal colonic epithelial cells to neoplastic transformation while inhibiting the growth of colon cancer cells. Using an in vitro model of folate deficiency, we determined the effects of folate deficiency on the steady-state transcript levels of genes involved in intracellular folate metabolism and 1-carbon transfer reactions in HCT116 and Caco2 human colon adenocarcinoma cells. In HCT116 cells, folate depletion was associated with changes in transcript levels of genes favoring increased folate uptake and intracellular folate retention, the provision of metabolically more effective substrates for folate-dependent enzymes, and reduced folate hydrolysis and efflux. In HCT116 cells, folate depletion was associated with changes in transcript levels of genes favoring the preferential shuttling of the flux of 1-carbon units to the methionine cycle over the nucleotide synthesis pathway. In Caco2 cells, some adaptive responses in response to folate depletion were not as apparent as in HCT116 cells, and in some cases, the direction of change was counterintuitive. In Caco2 cells, the metabolic priority in response to folate depletion was to shuttle the available folate pools to the nucleotide biosynthesis pathway at the expense of biological methylation reactions. In both HCT116 and Caco2 cells, folate depletion was associated with the conservation of the existing pattern and extent of DNA methylation.

Mouse models of neural tube defects: investigating preventive mechanisms

Neural tube defects (NTD), including anencephaly and spina bifida, are a group of severe congenital abnormalities in which the future brain and/or spinal cord fail to close. In mice, NTD may result from genetic mutations or knockouts, or from exposure to teratogenic agents, several of which are known risk factors in humans. Among the many mouse NTD models that have been identified to date, a number have been tested for possible primary prevention of NTD by exogenous agents, such as folic acid. In genetic NTD models such as Cart1, splotch, Cited2, and crooked tail, and NTD induced by teratogens including valproic acid and fumonisins, the incidence of defects is reduced by maternal folic acid supplementation. These folate-responsive models provide an opportunity to investigate the possible mechanisms underlying prevention of NTD by folic acid in humans. In another group of mouse models, that includes curly tail, axial defects, and the Ephrin-A5 knockout, NTD are not preventable by folic acid, reflecting the situation in humans in which a subset of NTD appear resistant to folic acid therapy. In this group of mutants alternative preventive agents, including inositol and methionine, have been shown to be effective. Overall, the data from mouse models suggests that a broad-based in utero therapy may offer scope for prevention of a greater proportion of NTD than is currently possible.

A Mutation Inactivating the Mitochondrial Inner Membrane Folate Transporter Creates a Glycine Requirement for Survival of Chinese Hamster Cells

A mutant Chinese hamster ovary cell line, glyB, that required exogenous glycine for survival and growth was reported previously (Kao, F., Chasin, L., and Puck, T. T. (1969) Proc. Natl. Acad. Sci. U. S. A. 64, 1284-1291). We now report that the defect in glyB cells causative of this phenotype is a point mutation in an inner mitochondrial membrane protein required for transport of folates into mitochondria. The CHO mitochondrial folate transporter (mft) was sequenced and compared with that from glyB cells. The hamster sequence was nearly identical to that of the recently reported human mitochondrial folate transporter. The corresponding cDNA from glyB cells contained a single nucleotide change that introduced a glutamate in place of the glycine in wild-type hamster MFT at codon 192 in a predicted transmembrane domain. Transfection of the wild-type hamster cDNA into glyB cells allowed cell survival in the absence of glycine and the accumulation of folates in mitochondria, whereas transfection of the Glu-192 cDNA did not. Genomic sequence analysis and fluorescence in situ hybridization demonstrated a single mutated allele of the mft gene in glyB cells, whereas there were two alleles in CHO cells. We conclude that we have defined the cause of the glyB auxotrophy and that the glyB mft mutation identified a region of this mitochondrial folate carrier vital to its transport function.

Resistance to antifolates

The antifolates were the first class of antimetabolites to enter the clinics more than 50 years ago. Over the following decades, a full understanding of their mechanisms of action and chemotherapeutic potential evolved along with the mechanisms by which cells develop resistance to these drugs. These principals served as a basis for the subsequent exploration and understanding of the mechanisms of resistance to a variety of diverse antineoplastics with different cellular targets. This section describes the bases for intrinsic and acquired antifolate resistance within the context of the current understanding of the mechanisms of actions and cytotoxic determinants of these agents. This encompasses impaired drug transport into cells, augmented drug export, impaired activation of antifolates through polyglutamylation, augmented hydrolysis of antifolate polyglutamates, increased expression and mutation of target enzymes, and the augmentation of cellular tetrahydrofolate-cofactor pools in cells. This chapter also describes how these insights are being utilized to develop gene therapy approaches to protect normal bone marrow progenitor cells as a strategy to improve the efficacy of bone marrow transplantation. Finally, clinical studies are reviewed that correlate the cellular pharmacology of methotrexate with the clinical outcome in children with neoplastic diseases treated with this antifolate.

The folate receptor as a molecular target for tumor-selective radionuclide delivery

The cell-membrane folate receptor is a potential molecular target for tumor-selective drug delivery, including radiolabeled folate-chelate conjugates for diagnostic imaging. We review here some background on the folate receptor as tumor-associated molecular target for drug delivery, and briefly survey the literature on tumor-targeting with radiolabeled folate-chelate conjugates.

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.

Dietary interactions influence the effects of bovine folate-binding protein on the bioavailability of tetrahydrofolates in rats

The newborns of mammals have a high folate demand, yet obtain adequate folate nutrition solely from their mothers' milk despite its low folate content. Milk folate is entirely bound by an excess of folate-binding protein (FBP), prompting speculation that FBP may affect the bioavailability of the limited folate supply. Previous research has shown that FBP-bound folic acid is more gradually absorbed, thereby reducing the peak plasma folate concentration and preventing loss into the urine. Natural folates are reduced derivatives of folic acid, with milk predominantly containing 5-methyltetrahydrofolate, yet little research has been carried out to determine the role of FBP in the bioavailability of reduced folates. We studied the effect of FBP on folate nutrition of rats in both single-dose and 4-wk feeding experiments. The effect of FBP was influenced by the presence of other milk components. FBP increased bioavailability of dietary folate when it was consumed with other whey proteins or with soluble casein. However, in the presence of acid-precipitated casein or a whey preparation enriched in lipids, bioavailability was decreased. These results highlight the difficulties of extrapolating from experimental results obtained using purified diets alone and of studying interactions among dietary components. They suggest that the addition of FBP-rich foods to folate-rich foods could enhance the bioavailability of natural folates, but that the outcome of such a combination would depend on interactions with other components of the diet.

Severe folate restriction results in depletion of and alteration in the composition of the intracellular folate pool, moderate sensitization to methotrexate and trimetrexate, upregulation of endogenous DHFR activity, and overexpression of metallothionein II and folate receptor alpha that, upon folate repletion, confer drug resistance to CHL cells

DC-3F/FA3 cells (FA3) were derived from antifolate-sensitive CHL cells by selection for growth in folate-free media containing 15 pM [6S]-5CHOFH4. These cells undergo a 30-fold decrease in intracellular folates, overexpress folate receptor alpha (FR alpha) and metallothionein II, and display increased sensitivity to the dihydrofolate reductase (DHFR) targeted anti-folates methotrexate (MTX) and trimetrexate (TMTX), which can be attributed primarily to the folate pool status. Upon folate repletion by growth in 15 nM [6S]-5CHOFH4, they display a 5- and 10-fold increase in resistance to both drugs, respectively, even though folate pools are restored by only 43%. Enforced overexpression of FR alpha in transfectants cultured in nanomolar folate did not confer resistance to MTX but did support a modest 2-fold increase in resistance to TMTX. Enforced overexpression of MTII had a similar effect, but when both were overexpressed together no increase in resistance beyond that conferred by each one separately was noted, suggesting that both confer resistance to TMTX through a common downstream mechanism. Analysis of three independent low folate selected clones, FA3, FA7, and FA14, showed that each had a 5- to 6-fold increase in DHFR activity accompanied by a similar increase in DHFR protein level. However, no differences were detected in the DHFR gene copy number or in the steady-state amount of DHFR mRNA, suggesting that a posttranscriptional mechanism was responsible for the increase in DHFR expression.

The half-life of the transcript encoding the folate receptor alpha in KB cells is reduced by cytosolic proteins expressed in folate-replete and not in folate-depleted cells

The KB cell, a transformed human cell line, constitutively expresses a high level of the glycosylphosphatidylinositol (GPI) anchored folate receptor protein alpha (FR alpha) and thereby can grow in medium containing <1 nM folate. When transferred from a folate-replete (FR) medium to one folate-deficient (FD), intracellular folate diminishes about 50-fold and expression of the FR alpha increases 6-fold. This up-regulation is mediated by a 4.5-fold increase in the initial transcription rate and a 2.4-fold prolongation of the mRNA half-life that together provide a higher level of the steady-state mRNA abundance. An RNA gel -shift assay of a 5' region of the mRNA that includes all of the non-coding and 24 nt of coding sequence, and a 3' region comprised only of coding sequence, identified unique complexes with cytosolic proteins from the FR-KB cells that were not observed with the cytosol from FD-KB cells. It appears, therefore, that expression of these folate-dependent cytosolic trans-active proteins function to maintain a shorter half-life of the mRNA in the FR-KB cells by binding to 5' and 3' cis elements, reducing the stability of this transcript.

Rescue of embryonic lethality in reduced folate carrier-deficient mice by maternal folic acid supplementation reveals early neonatal failure of hematopoietic organs

The reduced folate carrier (RFC1) is an important route by which the major blood folate, 5-methyltetrahydrofolate, is transported into mammalian cells. In this study we determined the consequences of inactivation of RFC1 in mice by homologous recombination. While RFC1-null embryos died in utero before embryonic day 9.5 (E9.5), near-normal development could be sustained in RFC1(-)/- embryos examined at E18.5 by supplementation of pregnant RFC1(+/-) dams with 1-mg daily subcutaneous doses of folic acid. About 10% of these animals went on to live birth but died within 12 days. These RFC1(-)/- mice showed a marked absence of erythropoiesis in bone marrow, spleen, and liver along with lymphoid depletion in the splenic white pulp and thymus. In addition, there was some impairment of renal and seminiferous tubule development. These data indicate that in the absence of RFC1 function, neonatal animals die due to failure of hematopoietic organs.

The rate of folate receptor alpha (FR alpha) synthesis in folate depleted CHL cells is regulated by a translational mechanism sensitive to media folate levels, while stable overexpression of its mRNA is mediated by gene amplification and an increase in transcript half-life

DC-3F/FA3 cells (FA3) were obtained by selection of Chinese hamster lung fibroblasts for growth in folic acid free media, supplemented with 15 pM [6S]-5-formyltetrahydrofolic acid. These cells, as a result of low level gene amplification and RNA stabilization, were found to overexpress folate receptor alpha (FR alpha) mRNA by more than five hundred fold. The expression level of the receptor, a 43 kDa GPI-linked plasma membrane glycoprotein, was found to be inversely related to changes in media folate concentrations while its steady state mRNA level remained unaffected. In low folate, the rate of receptor synthesis was found to increase by more than three fold, while its half-life stabilized as compared to that observed in high folate media. Although DC-3F cells were found to contain low amounts of FR alpha mRNA, receptor expression was undetectable, and changing media folate concentrations had no effect on the expression of either. Hence, while selection for growth in low folate leads to stable overexpression of FR alpha mRNA, receptor expression is regulated at the level of protein synthesis by a mechanism sensitive to media folate levels.

Synthesis and evaluation of pteroic acid-conjugated nitroheterocyclic phosphoramidates as folate receptor-targeted alkylating agents

A novel nitroheterocyclic bis(haloethyl)phosphoramidate prodrug linked through lysine to a pteroic acid has been prepared and evaluated as a potential alkylating agent to target tumor cells that overexpress the folate receptor. The prodrug exhibited IC(50) values in the micromolar range and was 10-400-fold less cytotoxic in vitro than the phosphoramidate that lacks the lysine-pteroyl moiety. The data does not support a contribution of the folate receptor to cytotoxicity. In an attempt to determine the basis for the decreased cytotoxicity in the pteroyl-lysyl analogue, compounds were prepared in which the lysine-pteroyl moiety was replaced with lysine alone or with an n-propyl group. The n-propyl and the lysyl analogues were on average 3.8- and 21-fold less potent than the unsubstituted bis(haloethyl)phosphoramidate, respectively. Chemical reduction of the prodrugs followed by (31)P NMR kinetics demonstrated that all of the phosphoramidate anions cyclized to the aziridinium ion at similar rates and gave comparable product distributions, suggesting that changes in chemical activation did not account for the differences in cytotoxicity. It is likely that folate receptor-mediated transport is not sufficient to deliver adequate intracellular concentrations of the cytotoxic phosphoramide mustard.

Identification of two putative novel folate receptor genes in humans and mouse

Utilizing a 'database mining' strategy to detect novel folate receptors (FR), we identified two potential novel members in the mouse and human. The mouse gene (Folbp3) was sequenced and found to predict a 28.2 kDa protein that consists of 244 amino acids that is highly expressed in both the thymus and spleen, suggesting a potential role in the immune system. The human gene (FR-delta) is mapped to chromosome 11q14, and predicts a 27.7 kDa protein that is comprised of 241 amino acids. However, expression of the human gene was not detected in 59 samples from both adult and embryonic tissue sources, suggesting a highly restricted spatial/temporal expression pattern, an alternatively spliced variant or an additional FR pseudogene. Using T31 mouse radiation hybrid mapping, Folbp3 was mapped to a region on mouse chromosome 9 that is syntenic to human chromosome 19p13. As the chromosomal locations of Folbp1 murine and Folbp2 genes were previously unknown, we utilized the same approach and mapped both genes to a region of mouse chromosome 7 that is syntenic to the human FR loci on chromosome 11q13.

Retrovirally mediated complementation of the glyB phenotype. Cloning of a human gene encoding the carrier for entry of folates into mitochondria

The transduction of a human placental cDNA retroviral library into glyB cells, a Chinese hamster ovary K1 subline that is deficient in the transport of folates into mitochondria, resulted in the complementation of glycine auxotrophy of these cells. A 2.6-kilobase pair cDNA insert flanked by retroviral sequences had integrated into genomic DNA in rescued cells. An open reading frame in this cDNA encoded a 35-kDa protein homologous to several inner mitochondrial wall transporters for intermediate metabolites. The subcloned cDNA complemented the glycine auxotrophy of glyB cells and reinstated folate accumulation in the mitochondria of transfected cells. The human origin, chromosomal location, and intron-exon organization of the isolated mitochondrial folate transporter gene were deduced from the expressed sequence tag database and human genome project data.

Clustering of mutations in the first transmembrane domain of the human reduced folate carrier in GW1843U89-resistant leukemia cells with impaired antifolate transport and augmented folate uptake

We have studied the molecular basis for the resistance of human CEM leukemia cells to GW1843, a thymidylate synthase inhibitor. GW1843-resistant cells displayed a approximately 100-fold resistance to GW1843 and methotrexate but were collaterally sensitive to the lipophilic antifolates trimetrexate and AG337, which enter cells by diffusion. These cells exhibited a 12-fold decreased methotrexate influx but surprisingly had a 2-fold decreased folic acid growth requirement. This was associated with a 4-fold increased influx of folic acid, a 3.5-fold increased steady-state level of folic acid, and a 2.3-fold expansion of the cellular folate pool. Characterization of the transport kinetic properties revealed that GW1843-resistant cells had the following alterations: (a) 11-fold decreased transport K(m) for folic acid; (b) 6-fold increased transport K(m) for GW1843; and (c) a slightly increased transport V(max) for folic acid. Sequence analysis showed that GW1843-resistant cells contained the mutations Val-29 --> Leu, Glu-45 --> Lys, and Ser-46 --> Ile in the first transmembrane domain of the reduced folate carrier. Transfection of the mutant-reduced folate carrier cDNA into methotrexate transport null cells conferred resistance to GW1843. This is the first demonstration of multiple mutations in a confined region of the human reduced folate carrier in an antifolate-resistant mutant. We conclude that certain amino acid residues in the first transmembrane domain play a key role in (anti)folate binding and in the conferring of drug resistance.

Coding sequence, genomic organization and expression of a folate binding protein gene in the rat

The complementary DNA (cDNA) and the gene encoding the folate binding protein alpha isoform (FBPalpha) have been reported for the human and mouse protein. However, there is no information about this gene in the rat, an animal that could be a model to study expression of this protein in vivo when folate metabolism is modified. Accordingly, the cloning and characterization of this gene in the rat have been the subject of this research. The gene has seven exons and six introns and is approximately 10kb in size. The organization and nucleotide sequence of the coding exons are similar to those of the corresponding human and mouse genes, which are the only other mammalian FBP genes cloned. However, the amino acid sequence of the rat FBPalpha is less homologous, having 48% identity with the published sequences for all the mammalian FBP isoforms. A finding not previously reported is the expression of two FBPalpha transcripts in the kidney that differ in the length of the 5' untranslated sequences, as determined by rapid amplification of cDNA end-polymerase chain reaction amplification (RACE-PCR). The brain expresses a single transcript intermediate in size between the two transcripts expressed in the kidney. The kidney transcripts are encoded by the same gene and appear to be regulated either from two independent promoters or from a single promoter in association with alternative RNA splicing.

Folate deficiency reduces the GPI-anchored folate-binding protein in rat renal tubules

A folate-binding protein (FBP) anchored to cell membranes by a glycosyl phosphatidylinositol (GPI) adduct is constitutively expressed in some transformed and cultured cell lines. Its expression is upregulated when these cells are grown in medium containing low folate, but whether this occurs in vivo with nutritional folate deficiency is unknown. To address this question, the GPI-FBP in the liver, kidney, and brain of rats on control and folate-deficient (FD) diets was measured. The GPI-FBP in the kidney of FD rats decreased significantly in contrast to the upregulation of this protein in cultured cells. Northern blot analysis and nuclear run-on assays indicated that transcription of the GPI-FBP gene in the kidney was not reduced by folate deficiency. This decrease of the GPI-FBP appears to result from its proteolysis, similar to the enzymatic degradation of the apoprotein that occurs in vitro. Because the GPI-FBP is on the brush borders of the proximal renal tubules and provides for the reabsorption of folate, this function diminishes when the protein decreases in folate deficiency.

Characterization of a human alternatively spliced truncated reduced folate carrier increasing folate accumulation in parental leukemia cells

Human CEM-7A cells established by gradual deprivation of leucovorin from the growth medium, display 100-fold overexpression of methotrexate transport activity. We found that this was associated with 10-fold reduced folate carrier gene amplification and 50-fold overexpression of both the principal 3 kb reduced folate carrier transcript and, surprisingly, a novel truncated 2 kb reduced folate carrier mRNA poorly expressed in parental CEM cells. The molecular basis for the generation of this truncated reduced folate carrier transcript and its potential functional role in folate accumulation were studied. Reduced folate carrier genomic and cDNA sequencing revealed that the truncated transcript had an internal deletion of 987 nucleotides which was a result of an alternative splicing utilizing a cryptic acceptor splice site within exon 6. This deletion consisted of the 3'-most 480 nucleotides of the reduced folate carrier ORF and the following 507 nucleotides of the 3'-UTR. These resulted in a truncated reduced folate carrier protein, which lacks the C-terminal 160 amino acids, but instead contains 58 new C-terminal amino acids obtained from reading through the 3'-UTR. Consequently, a truncated reduced folate carrier protein is generated that lacks the 12th transmembrane domain and contains a new and much shorter C-terminus predicted to reside at the extracellular face. Western analysis with plasma-membrane fraction from CEM-7A cells revealed marked overexpression of both a broadly migrating approximately 65-90 kDa native reduced folate carrier and a approximately 40-45 kDa truncated reduced folate carrier, the core molecular masses of which were confirmed by in vitro translation. However, unlike the native reduced folate carrier, the truncated reduced folate carrier protein failed to bind the affinity labels NHS-[3H]MTX and NHS-[3H]folic acid. Stable transfection of the truncated reduced folate carrier cDNA into mouse L1210 leukemia cells: increased folate accumulation, decreased their leucovorin and folic acid growth requirements, and increased their sensitivity to methotrexate. This constitutes the first documentation of an expressed alternatively spliced truncated reduced folate carrier that, when coexpressed along with the native carrier, augments folate accumulation and consequently decreases the cellular folate growth requirement. The possible mechanisms by which the truncated reduced folate carrier may increase folate accumulation and/or metabolism in cells coexpressing the truncated and native reduced folate carrier are discussed.

Retinoic acid-dependent upregulation of mouse folate receptor-alpha expression in embryonic stem cells, and conservation of alternative splicing patterns

The action of retinoic acid (RA) in normal development and differentiation is mediated by changes in the expression of RA-responsive target genes. We used differential display reverse transcriptase polymerase chain reaction (RT-PCR) to identify RA-responsive genes expressed in embryonic stem (ES) cells and found that murine folate receptor-alpha (FR-alpha) expression is rapidly induced by RA treatment. The observed increase in FR-alpha expression occurs within 3h, is independent of protein synthesis and does not occur when ES cells are differentiated by removal of leukaemia inhibitory factor (LIF), evidence that the response to RA is both direct and specific. Two messenger RNA (mRNA) isoforms of FR-alpha featuring novel sequence in the 5' untranslated region (UTR) were cloned, and both were found to be upregulated by RA. Other splice variants in both the 5' UTR and 3' UTR of FR-alpha mRNA were also identified. There is a striking similarity between these splicing patterns and those reported for human FR-alpha, which also generates multiple isoforms by alternative splicing in the 5' and 3' UTR. The conservation of these splicing patterns in the non-coding regions of the FR-alpha gene between mouse and human suggests that these regions, and in particular the 5' UTR, play an important role in regulating expression of this gene.

Complete mapping of divergent amino acids responsible for differential ligand binding of folate receptors alpha and beta

The folate receptor (FR) type alpha may be distinguished from FR-beta by its higher affinity for the circulating folate coenzyme, (6S)-5-methyltetrahydrofolate (5-CH3H4folate), and its opposite stereospecificity for reduced folate coenzymes. Previous studies showed that a single leucine to alanine substitution at position 49 of the mature protein sequence is responsible for the functional divergence of FR-beta (Shen, F., Zheng, X., Wang, H., and Ratnam, M. (1997) Biochemistry 36, 6157-6163); however, the results also indicated that the minimum requirement for conversion of FR-beta to the functional equivalent of FR-alpha should include amino acid substitution(s) downstream of residue 92 in addition to mutation of L49A. To pinpoint those residues, chimeric FR-betaL49A/FR-alpha constructs including progressively shorter segments of FR-alpha downstream of position 92 as well as selected point mutants were studied. Simultaneous substitution of Leu-49, Phe-104, and Gly-166 in FR-beta with the corresponding FR-alpha residues Ala, Val, and Glu, respectively, reconstituted the ligand binding characteristics of FR-alpha. The results also exclude a role for other residues in FR-alpha in determining its functional divergence. A homology model of FR-alpha based on the three-dimensional structure of the chicken riboflavin-binding protein is used to show the position of residues 49, 104, and 166 in relation to the hydrophobic cleft corresponding to the riboflavin-binding pocket.

Folate receptor type beta is a neutrophilic lineage marker and is differentially expressed in myeloid leukemia

BACKGROUND

The membrane-associated folate receptor (FR) type beta is elevated in the spleen in patients with chronic myeloid leukemia (CML) and acute myeloid leukemia (AML). In this study, the authors investigated possible cell type and differentiation stage specificity of expression of FR-beta in normal and leukemic hematopoietic cells.

METHODS

An affinity-purified rabbit polyclonal antibody specific for FR-beta was employed for immunostaining representative bone marrow smears and peripheral blood smears from normal individuals and from a limited number of patients with various leukemias. Multiple samples of normal bone marrow and peripheral blood were analyzed for the expression of FR-beta and selected CD antigens by two- or three-color flow cytometry.

RESULTS

Of the morphologically identifiable cells, only neutrophils were positive for FR-beta. The leukemic blasts in CML patients showed expression of FR-beta with no apparent relation to the occurrence of the Philadelphia chromosome. Among acute nonlymphocytic leukemias, FR-beta was expressed in promyelocytic leukemia, in the myeloblast populations of myelomonocytic and erythroleukemias, and variably in M1/M2 AML. Neither the blasts of acute lymphocytic leukemia nor the more mature cells of chronic lymphocytic and hairy cell leukemias expressed FR-beta. The less differentiated FR-beta positive AML samples also were positive for CD34 and HLA-DR. Flow cytometric analysis of normal bone marrow and peripheral blood revealed low or insignificant coexpression of FR-beta with CD34, CD19, and CD3, whereas significant coexpression was observed with high levels of CD33, CD13, and CD11b; coexpression of FR-beta with CD14 was high in the immature bone marrow cells, comparable to that in myeloid cells, but relatively low in peripheral blood.

CONCLUSIONS

The results of this study suggest a narrow expression pattern of FR-beta marking the neutrophilic lineage and the possibility of defining a subtype or subtypes of myeloid leukemia based on FR-beta expression. The identification of FR-beta positive leukemias and the absence of the receptor in normal CD34 positive cells may enable selective receptor-mediated targeting of leukemic cells.

Mutations in the reduced folate carrier gene which confer dominant resistance to 5,10-dideazatetrahydrofolate

L1210/D3 mouse leukemia cells are resistant to 5, 10-dideazatetrahydrofolate due to expansion of cellular folate pools which block polyglutamation of the drug (Tse, A., and Moran, R. G. (1998) J. Biol. Chem. 273, 25944-25952). These cells were found to have two point mutations in the reduced folate carrier (RFC), resulting in a replacement of isoleucine 48 by phenylalanine and of tryptophan 105 by glycine. Each mutation contributes to the resistance phenotype. Genomic DNA from resistant cells contained both the wild-type and mutant alleles, but wild-type message was not detected. Folic acid was a much better substrate, and 5-formyltetrahydrofolate was a poorer substrate for transport in L1210/D3 cells relative to L1210 cells. Enhanced transport of folic acid was due to a marked, approximately 20-fold, decrease in the influx Km. Influx of methotrexate and 5,10-dideazatetrahydrofolate were minimally altered. Transfection of mutated rfc cDNA into RFC-null L1210/A cells produced the substrate specificity and 5, 10-dideazatetrahydrofolate resistance observed in the L1210/D3 line. Transfection of the mutant cDNA into wild-type cells also conferred resistance to 5,10-dideazatetrahydrofolate. We conclude that the I48F and W105G mutations in RFC caused resistance to 5, 10-dideazatetrahydrofolate, that the region of the RFC protein near these two positions defines the substrate-binding site, that the wild-type allele was silenced during the multistep development of resistance, and that this mutant phenotype represents a genetically dominant trait.

Structure and regulation of a polymorphic gene encoding folate receptor type gamma/gamma'

The human folate receptor (hFR) type gamma and gamma' are constitutively secreted proteins that are expressed primarily in hematopoietic tissues and are potential serum markers for certain hematopoietic malignancies. hFR-gamma' is a variant of hFR-gamma with a two base deletion in its cDNA resulting in a truncated polypeptide. The gene encoding hFR-gamma' was isolated from a placental genomic library. The gene has five exons, four introns and a 5' flanking sequence which contains multiple putative regulatory elements. From RNase protection assay and RACE analysis, the major site of transcriptional initiation was identified at -56 nt. Systematically deleted fragments in the 5' region of the genomic DNA of FR-gamma' were ligated into the PGL3Basic plasmid and the reporter luciferase activity was assayed in cell lysates from transiently transfected NIH3T3 cells. From those results, putative positive and negative regulatory regions in the 5' flanking sequences were noted, and a TATA-less proximal promoter was located between -206 and -22 nt. Gel mobility shift and supershift analyses as well as mutagenesis experiments revealed that Sp1 and ets binding elements in the proximal promoter region confer transcriptional activity. From partial sequencing of genomic DNA, genomic Southern blots, RACE analysis and RNase protection assays, it appears that hFR-gamma shares the gene organization of hFR-gamma'. The results of the analysis of genomic DNA in spleen tissues from several individuals, were consistent with the interpretation that hFR-gamma and hFR-gamma' are encoded by a polymorphic gene.

Targeting tumor cells with bispecific antibodies and T cells

It has been known for some time that mammalian immune systems are capable of eliminating large tumor burdens. Redirecting the immune response of a patient to an established tumor has now become the focus of various therapeutic strategies. In this report, two projects toward this goal are described. The first project involves the development of a transgenic mouse model for T cell directed therapeutics. These mice express specific T cell receptor alpha and beta transgenes on a background in which the recombinational-activating-gene-1 (RAG) has been knocked out. The mice express cytotoxic T cells but not either T helper cells or B cells. Despite these deficiencies, the animals are capable of eliminating tumors that express the appropriate peptide/major histocompatibility complex ligand that is recognized by the alphabeta transgenic T cell receptor. Human tumors grow as transplants in these mice, thereby allowing various agents that redirect the endogenous T cells against human tumors to be tested. The second project involves a description of such agents: bispecific antibodies that simultaneously bind to an immune effector cell and a tumor cell. The bispecific antibody described here consists of folate attached to anti-T cell receptor antibodies, or their fragments. A single-chain Fv coupled with folate can redirect the lysis of human tumor cells that bear the high affinity folate receptor. Preliminary in vivo data showed that the folate/antibody conjugates were also capable of mediating rejection of the human tumor. This transgenic mouse model should now allow the evaluation and optimization of bispecific agents that can redirect a patient's own T cell response.

A reduced folate carrier mutation produces substrate-dependent alterations in carrier mobility in murine leukemia cells and methotrexate resistance with conservation of growth in 5-formyltetrahydrofolate

With 5-formyltetrahydrofolate (5-CHO-THF) as the folate source a methotrexate (MTX) transport-deficient murine leukemia cell line, L1210-G1a, was isolated after chemical mutagenesis and MTX selection. This cell line was 10-fold resistant to MTX in comparison to parental L1210 cells, yet the EC50 for 5-CHO-THF was increased by a factor of only 2. The initial uptake of MTX, at a concentration of 1 microM, was decreased by a factor of 40, whereas influx of 5-CHO-THF dropped by a factor of only 8. This difference in initial uptake rates was attributed solely to changes in influx Vmax without a significant change in Km. Whereas the RFC1 mRNA level in L1210-G1a cells was indistinguishable from that of parental L1210 cells, a serine to asparagine substitution was identified at amino acid 46 within the first predicted transmembrane domain. This was a result of a homozygous mutation of G-->A in the genome. Transfection of the mutated RFC1 cDNA into MTXrA cells, which lack functional endogenous carrier, resulted in a clonal derivative MTXrA-S46N. The increase in influx of 5-CHO-THF and 5-CH3THF was 5 and 13 times greater than that for MTX in the transfectant, consistent with the influx ratio in the L1210-G1a line. The functional expression of the mutated RFC1 reduced the growth requirement for 5-CHO-THF by a factor of 30, compared with only a 3-fold decrease in the MTX IC50. This represents the first reported RFC1 mutation that confers resistance to MTX due to a markedly impaired influx with relative conservation of reduced folate transport. The kinetic changes are consistent with a substrate-dependent alteration in carrier mobility that favors reduced folates over MTX. These changes may account for the development of MTX resistance due to impaired drug transport in vivo, allowing tumor cells to meet their folate requirement with 5-CH3THF, the predominant blood folate.

Neural tube and craniofacial defects with special emphasis on folate pathway genes

Neural tube and orofacial defects are common congenital malformations in humans. While etiologically heterogeneous, they are for the most part multifactorial in their pathogenesis, having both genetic and environmental components in their development. In recent years, there has been a great deal of epidemiologic evidence demonstrating that women who received multivitamins containing folic acid periconceptionally had significantly reduced occurrence and recurrence risks for producing infants with such malformations. This risk reduction is not observed in all populations, further suggestive of a genetic regulation of this phenomenon. Unfortunately, the mechanisms underlying the beneficial effects of folic acid are not well-understood. In this article, we review the relevant epidemiologic data on both neural tube defects and orofacial malformations, the fundamental embryological processes involved in closing the neural tube, and the development of the craniofacies, and propose a working hypothesis for susceptibility to these malformations. This hypothesis is based on the interworkings of cellular folate transport, focusing on the key elements involved in potocytosis. We propose that infants with mutations in the folate receptor alpha gene might be at increased risk for congenital anomalies due to a reduced binding affinity for 5-methyltetrahydrofolate, the physiologic form of folic acid. Various experimental approaches to test the working hypothesis are considered.

Variable expression of RFC1 in human leukemia cell lines resistant to antifolates

The resistance to folate-based antifolates is associated with impaired function of the reduced folate carrier (RFC), one of the major routes of folate transport into cancer cells. To clarify the importance of RFC functions in the antifolate resistance, we have examined the expression of RFC1 and its phenotype as a folate transporter in human leukemia cell lines resistant to various antifolates. MOLT-3 cells resistant to ZD9331 (a thymidylate synthase (TS) inhibitor that utilizes the RFC for cell entry) (MOLT-3/ZD9331) showed decreased expression of RFC1 concomitant with diminished cellular uptake of [3H]methotrexate (MTX). K562 cells resistant to raltitrexed (ZD1694, another TS inhibitor that utilizes the RFC for cell entry) (K562/ ZD1694 x C) scarcely expressed RFC1, which is in accordance with the impaired uptake of folate analogs and the high degree of resistance to ZD1694 and MTX. On the other hand, no apparent decrease of RFCI1 expression was found in transport-deficient MTX-resistant MOLT-3 cells (MOLT-3/MTX10000) though its phenotype showed defective transport of MTX or ZD1694. In these cell lines with impaired RFC function, [3H]leucovorin (LV) uptake was only moderately decreased as compared to [3H]MTX or [3H]ZD1694 uptake. These cells grew with a minimal retardation in folate-free medium supplemented with 10 nM LV, suggesting that these cell lines with impaired RFC function had enough folate transporters to transport LV. In contrast to downregulation of RFC, the much greater uptake of [3H]MTX was observed in the MOLT-3/trimetrexate (TMQ)800-MTX10000 in parallel with increased RFC1 expression. These cell lines with the altered expression of RFC1 may serve as models useful for investigating the regulation of RFC1 expression and for understanding the molecular mechanism(s) behind the transport-mediated antifolate resistance.

PMA alters folate receptor distribution in the plasma membrane and increases the rate of 5-methyltetrahydrofolate delivery in mature MA104 cells

MA104 cells (a monkey kidney cell line) can internalize 5-methyltetrahydrofolate via a receptor mediated process termed potocytosis. Uptake is initiated by binding to an external folate receptor which cycles to an internal, but membrane bound compartment. These two pools can be measured by determining the amount of [3H]ligand removed by an acid-saline wash, i.e. acid labile and acid resistant pools. When assayed in confluent nonmitotic cells, 2/3 of the folate receptor pool is located in an internal (acid resistant) compartment, but phorbol 12-myristate 13-acetate (PMA) causes a shift such that 65-75% of the receptor pool resides on the surface of the plasma membrane. This new steady state is likely the result of an increased rate of receptor movement. In addition, PMA increases the rate of 5-methyl[3H]tetrahydrofolate delivery to the cytoplasm 1.8 fold. Using known inhibitors of potocytosis, we were able to show that the increased rate of delivery is receptor mediated. Comparison of the time courses of the PMA effects on folate receptor redistribution assessed by membrane binding of [3H]folic acid and 5-methyl[3H]tetrahydrofolate delivery to the cytoplasm suggests that PMA may be activating more than one protein kinase C independent signal transduction pathway. PMA is the first reported positive modulator of receptor mediated folate uptake.

Expression levels of functional folate receptors alpha and beta are related to the number of N-glycosylated sites

In a previous study with inhibitors of N-glycosylation, it was proposed that core glycosylation of the folate receptor (FR) is required for the proper folding of the protein [Luhrs (1991) Blood 77, 1171-1180]. The human FR isoforms type alpha and type beta have three and two candidate sites for N-glycosylation respectively, only one of which is conserved. The significance of N-glycosylation at each of these loci in the expression and function of FR was examined by eliminating the sites both individually and in combination by introducing Asn-->Gln substitutions. Translation experiments in vitro showed that the mutations did not alter the synthetic rates of the polypeptides. The recombinant proteins were expressed in human 293 fibroblasts. Treatment with N-glycanase and analysis by Western blotting of the wild-type and mutant proteins revealed that all of the candidate sites in both FR-alpha and FR-beta are glycosylated. When all of the N-glycosylation sites were abolished, 2% and 8% of FR-alpha and FR-beta respectively were expressed on the cell surface compared with the corresponding wild-type proteins; the residual FR polypeptides in the cell lysates were unable to bind [3H]folic acid. In both the proteins, the inclusion of each additional N-glycosylation site partly contributed to restoration of cell surface [3H]folic acid binding and receptor-mediated folate transport. Further, in FR-beta the introduction of an additional unnatural site of N-glycosylation resulted in the enhancement of the expression of the cell surface receptor compared with the wild-type protein. The results indicate that the total mass of N-glycosylation, not a specific locus of the modification, is critical for the efficient folding and optimal expression of functional FR-alpha and FR-beta.

Loss of folic acid exporter function with markedly augmented folate accumulation in lipophilic antifolate-resistant mammalian cells

We previously described a 1,000-fold pyrimethamine-resistant Chinese hamster ovary cell line (PyrR100) which retains parental dihydrofolate reductase activity and methotrexate (MTX) sensitivity. This study characterizes the basis for the 14-fold decrease in folic acid and leucovorin concentrations required for clonogenic growth of PyrR100 cells relative to parental AA8 cells. Under conditions in which folic acid reduction was blocked by trimetrexate, PyrR100 cells displayed relative to parental AA8 cells a: 1) 17- and 5-fold increase in the net transport of folic acid and MTX, respectively; 2) 23- and 5-fold decrease in the efflux rate constant for folic acid and MTX, respectively; and 3) 2-fold increase in folic acid influx with no significant change in MTX influx. The markedly increased net folic acid transport in PyrR100 cells could not be explained by cellular folic acid binding, mitochondrial sequestration, polyglutamylation, nor by a decreased membrane potential. The effect of energy deprivation on folic acid and MTX transport in both cell lines was quite different. Glucose and pyruvate deprivation nearly abolished the increase in net folic acid transport in PyrR100 cells. In contrast, energy deprivation increased net MTX transport in AA8 cells, whereas no change was seen with PyrR100 cells. Furthermore, while folic acid influx in PyrR100 and AA8 cells was markedly reduced with energy deprivation, MTX influx was not affected. Provision of glucose and pyruvate to energy-deprived cells resulted in a rapid onset of MTX efflux from parental AA8 cells but not from PyrR100 cells. Taken together these results indicate that the markedly enhanced net transport of folic acid and MTX in PyrR100 cells is largely due to the complete loss of exit pump activity. Furthermore, the energy source that sustains the augmented levels of folic acid appears linked to the influx process and is distinct from the energy source that sustains MTX gradients under these conditions. We conclude that the loss of folic acid efflux is an efficient means of augmenting cellular uptake of folate cofactors and subsequent survival on picomolar folate concentrations. This constitutes the first demonstration of the loss of folic acid exporter activity in mammalian cells as a response to lipophilic antifolate selective pressure.

Folate receptors as potential therapeutic targets in choroid plexus tumors of SV40 transgenic mice

A high affinity folate receptor is expressed in some human cancers, including choroid plexus tumors and ependymomas, and has been suggested as a target for therapeutics. In this report, the expression of folate receptors in an SV40 large T antigen transgenic mouse (SV11) was investigated. SV11 mice develop choroid plexus tumors, a property that may be related to the observation that SV40 has been isolated from human choroid plexus tumors and ependymomas. We report that SV11 choroid plexus tumors contain a high affinity folate receptor (KD of 1 nM), detectable by 125I-folate autoradiography and immunohistochemistry. Western blot analysis indicated an apparent molecular weight of 38 kDa. RT-PCR revealed the presence of transcripts for both alpha and beta isoforms of the folate receptor. Brain parenchyma has undetectable folate receptor, but normal choroid plexus has substantial levels (as does human choroid plexus). The folate receptors of the tumor are accessible from the bloodstream whereas those of the normal choroid plexus are not. Thus SV11 transgenic mice should be useful for evaluating therapeutic targeting of high affinity folate receptors, both for efficacy of specific agents and possible side effects.

pH dependence of methotrexate transport by the reduced folate carrier and the folate receptor in L1210 leukemia cells. Further evidence for a third route mediated at low pH

F2-MTX'A is an L1210 leukemia cell line with a functional detect in the reduced folate carrier and high level expression of folate receptor beta. The pH dependence of methotrexate (MTX) influx by folate receptor beta in F2-MTX'A cells was characterized and compared with that of the reduced folate carrier in parental L1210 cells. MTX influx by folate receptor beta had a pH optimum of 6.5, whereas influx mediated by the reduced folate carrier showed a pH optimum of 7.5. Increased folate receptor beta-mediated MTX influx at pH 6.5 relative to pH 7.5 was accompanied by a 5-fold increase in binding affinity of the receptor for MTX without a change in the number of binding sites. At pH 6.2, approximately 24% of MTX influx in F2-MTX'A cells proceeded by another mechanism. This transport route became active at pH < 7.5, operated optimally at pH 6.0 to 6.5, and, unlike folate receptor beta-mediated MTX influx, was insensitive to the presence of low levels of folic acid (100 nM). MTX influx by the low pH system showed saturability, with a Ki of 5.3 microM and a Vmax of 1.53 nmol/g dry wt/min, was energy dependent, was inhibited by sulfobromophthalein with a Ki of 148 microM, and had similar relative affinities for folic acid, leucovorin, and 5 methyltetrahydrofolate. Influx of 5-methyltetrahydrofolate was also mediated by this route. The data provide further confirmatory evidence for an MTX influx route in F2-MTX'A cells, optimal at low pH and distinct from the reduced folate carrier or the folate receptor.

Comparison of methotrexate polyglutamylation in L1210 leukemia cells when influx is mediated by the reduced folate carrier or the folate receptor. Lack of evidence for influx route-specific effects

We previously described a methotrexate-resistant L1210 cell line (MTXrA) that lacks a functional reduced folate carrier and does not appreciably express the folate receptor. In the present study, we utilized MTXrA cell lines stably transfected with cDNAs encoding either the folate receptor or the reduced folate carrier to investigate the influence of the route of folate influx on the rate and extent of methotrexate polyglutamylation. At an extracellular methotrexate concentration of 0.1 microM, influx in the folate receptor transfectant (MTXrA-TF1) and in the reduced folate carrier transfectant (MTXrA-R1) was equal and methotrexate polyglutamates accumulated at an identical rate, but the onset was delayed until dihydrofolate reductase was saturated with the monoglutamate (approxmately 3 hr). The onset of polyglutamate formation was immediate and identical among the lines in cells pretreated with the lipophilic dihydrofolate reductase inhibitor trimetrexate to block methotrexate binding to dihydrofolate reductase. The spectra of individual methotrexate polyglutamates that accumulated were similar, with the tetraglutamate present as the predominant form. A 100-fold higher methotrexate concentration was required to detect methotrexate uptake and polyglutamylation in the transport defective parent MTXrA line, demonstrating that diffusion or an unidentified low affinity route also supports polyglutamylation. Since the folate receptor and the reduced folate carrier achieve nearly identical rates of polyglutamylation despite very different mechanisms of methotrexate delivery, the data suggest that transport-mediated substrate channeling to folylpolyglutamate synthetase is unlikely to play a role in tetrahydrofolate metabolism. This study supports the notion that it is the intracellular concentration of methotrexate achieved within the cell that drives polyglutamylation irrespective of its route of entry.

Comparison of transport properties of the reduced folate carrier and folate receptor in murine L1210 leukemia cells

This laboratory previously described an L1210 murine leukemia cell line with a functional defect in the reduced folate carrier and increased expression of folate receptor-beta (F2-MTXrA). This cell line was used to characterize methotrexate (MTX) influx mediated by folate receptor-beta and to compare this with influx mediated by the reduced folate carrier in L1210 parental cells. Influx of 0.2 microM MTX in F2-MTXrA cells was one-third that of L1210 cells and was abolished by very low concentrations of folic acid. Kinetic analysis revealed that MTX transport mediated by folate receptor-beta exhibited an influx kappa t one-third, and an influx Vmax one-fourth, that of the reduced folate carrier. Metabolic inhibitors markedly suppressed influx in F2-MTXrA cells but had no effect on MTX influx in L1210 cells. MTX influx in both cell lines was inhibited by the organic anions probenecid, sulfobromophthalein, and CI-920, but to a lesser extent in F2-MTXrA cells. The inhibitory effects of these anions on transport in F2-MTXrA cells could be attributed to their inhibition of MTX binding to the folate receptor. Although MTX influx in both cell lines was not sodium dependent, removal of extracellular chloride increased influx 2-fold in L1210 cells while markedly inhibiting influx in F2-MTXrA cells. Substitution of Cl- with isethionate or NO3- partially restored influx in the latter cells, whereas SO4(2-) was inhibitory. Anions enhanced MTX binding to folate receptor-beta with isethionate > SO4(2-) > Cl-. Decreasing the buffer pH to 6.2 produced a 69% reduction, and a 260% increase, in MTX influx in L1210 cells and F2-MTXrA cells, respectively. The data indicate that folate receptor-beta-mediated MTX influx has properties fundamentally different from transport mediated by the reduced folate carrier in terms of energy, ion, and pH dependence. There was no evidence indicating that these processes are functionally linked.