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

Clinical Implications of Folate Transport in the Central Nervous System

Folates are essential for key biosynthetic processes in mammalian cells and play a crucial role in the maintenance of central nervous system homeostasis. Mammals lack the metabolic capacity for folate biosynthesis; hence, folate requirements are largely met through dietary sources. To date, three major folate transport pathways have been characterized: the folate receptors (FRs), reduced folate carrier (RFC), and proton-coupled folate transporter (PCFT). This article reviews current knowledge on the role of folate transport systems in mediating folate delivery to vital tissues, particularly the brain, and how these pathways are modulated by various regulatory mechanisms. We will also briefly highlight the clinical significance of cerebral folate transport in relation to neurodevelopmental disorders associated with folate deficiency.

Folic acid deficiency and vision: a review

Folic acid (FA), also termed folate, is an essential vitamin for health at all ages since it participates in the biosynthesis of nucleotides, amino acids, neurotransmitters, and certain vitamins. It is therefore crucial for rapidly growing tissues such as those of the fetus. It is becoming clear that FA deficiency and impaired folate pathways are implicated in many diseases of both early life and old age. FA can be transported into the cell by the folate receptor, the reduced folate transporter, and proton-coupled folate transporter. Folate transport proteins are present in certain eye tissues, which explains why FA plays an important role in eye development. The purpose of this literature review is to investigate the evidence relating FA deficiency to eye diseases.

Targeted drug delivery via folate receptors in recurrent ovarian cancer: a review

Ovarian cancer is the most common cause of gynecological cancer-related mortality, with the majority of women presenting with advanced disease; although chemotherapeutic advances have improved progression-free survival, conventional treatments offer limited results in terms of long-term responses and survival. Research has recently focused on targeted therapies, which represent a new, promising therapeutic approach, aimed to maximize tumor kill and minimize toxicity. Besides antiangiogenetic agents and poly (ADP-ribose) polymerase inhibitors, the folate, with its membrane-bound receptor, is currently one of the most investigated alternatives. In particular, folate receptor (FR) has been shown to be frequently overexpressed on the surface of almost all epithelial ovarian cancers, making this receptor an excellent tumor-associated antigen. There are two basic strategies to targeting FRs with therapeutic intent: the first is based on anti-FR antibody (ie, farletuzumab) and the second is based on folate–chemotherapy conjugates (ie, vintafolide/etarfolatide). Both strategies have been investigated in Phase III clinical trials. The aim of this review is to analyze the research regarding the activity of these promising anti-FR agents in patients affected by ovarian cancer, including anti-FR antibodies and folate–chemotherapy conjugates.

Folate receptor-targeted aminopterin therapy is highly effective and specific in experimental models of autoimmune uveitis and autoimmune encephalomyelitis

EC0746 is a rationally designed anti-inflammatory drug conjugate consisting of a modified folic acid-based ligand linked to a γ-hydrazide analog of aminopterin. In this report, EC0746's effectiveness was evaluated against experimental retinal S-antigen (PDSAg) induced autoimmune uveitis (EAU) and myelin-basic-protein induced autoimmune encephalomyelitis (EAE). In both models, functional FR-β was detected on activated macrophages in local (retinal or central-nervous-system, respectively) and systemic (peritoneal cavity) sites of inflammation. In myelin-rich regions of EAE rats, an increased uptake of (99m)Tc-EC20 (etarfolatide; a FR-specific radioimaging agent) was also observed. EC0746 treatment at disease onset suppressed the clinical severity of both EAU and EAE, and it strongly attenuated progressive histopathological changes in the affected organs. In all parameters assessed, EC0746 activity was completely blocked by a benign folate competitor, suggesting that these therapeutic outcomes were specifically FR-β mediated. EC0746 may emerge as a useful macrophage-modulating agent for treating inflammatory episodes of organ-specific autoimmunity.

Recent advances in ligand targeted therapy

BACKGROUND

Ligand targeted therapy (LTT) is a powerful pharmaceutical strategy to achieve selective drug delivery to pathological cells, for both therapeutic and diagnostic purposes, with the advantage of limited side effects and toxicity. This active drug targeting approach is based on the discovery that there are receptors overexpressed on pathological cells, compared to their expression in normal tissues.

PURPOSE

The purpose of this article is to review recently published data on LTT with applications, both in the field of cancer therapy and other diseases. Moreover, data on LTT exploiting receptors overexpressed at cytoplasmatic level are also reviewed.

METHODS

Data were deduced from Medline (PubMed) and SciFinder and their selections were made with preference to papers where the most relevant receptors were involved.

RESULTS

Several groups have reported improved delivery of targeted nanocarriers, as compared to nontargeted ones, to pathological cells. LTT offers several advantages, but there are also limitations in the development of this strategy. Moreover, LTT have shown encouraging results in in vitro and in animal models in vivo; hence their clinical potential awaits investigation.

CONCLUSION

Recent studies highlight that the ligand density plays an important role in targeting efficacy. Furthermore, LTT applications in diseases different from cancer and those exploiting receptors overexpressed at cytoplasmatic level are growing.

Enhanced antitumor efficacy of folate-linked liposomal doxorubicin with TGF-β type I receptor inhibitor

Tumor cell targeting of drug carriers is a promising strategy and uses the attachment of various ligands to enhance the therapeutic potential of chemotherapy agents. Folic acid is a high-affinity ligand for folate receptor, which is a functional tumor-specific receptor. The transforming growth factor (TGF)-β type I receptor (TβR-I) inhibitor A-83-01 was expected to enhance the accumulation of nanocarriers in tumors by changing the microvascular environment. To enhance the therapeutic effect of folate-linked liposomal doxorubicin (F-SL), we co-administrated F-SL with A-83-01. Intraperitoneally injected A-83-01-induced alterations in the cancer-associated neovasculature were examined by magnetic resonance imaging (MRI) and histological analysis. The targeting efficacy of single intravenous injections of F-SL combined with A-83-01 was evaluated by measurement of the biodistribution and the antitumor effect in mice bearing murine lung carcinoma M109. A-83-01 temporarily changed the tumor vasculature around 3 h post injection. A-83-01 induced 1.7-fold higher drug accumulation of F-SL in the tumor than liposome alone at 24 h post injection. Moreover F-SL co-administrated with A-83-01 showed significantly greater antitumor activity than F-SL alone. This study shows that co-administration of TβR-I inhibitor will open a new strategy for the use of FR-targeting nanocarriers for cancer treatment.

All-trans retinoic acid is capable of inducing folate receptor β expression in KG-1 cells

The high expression of folate receptor (FR) on cancer cells might be a potential target for cancer therapy. In this study, the FR-β expression and the modulation effect of all-trans retinoic acid (ATRA) in a number of cancer cell lines were analyzed. The gateway of ATRA activity on FR-β expression was further studied by a panel of retinoid activators and inhibitors. The results revealed that ATRA was capable of upregulating the expression of FR-β protein in KG-1 cells in a dosage-dependent manner, not in KG-1a, NB4, HL60, 293, L1210, JAR, and Hela cells. FR-β mRNA expression in KG-1 cells was higher when ATRA was present in culture medium at 10⁻⁶ mol/L for 5 days, and it went down to baseline when ATRA was removed from the medium, vice versa. The upregulation of FR-β expression in KG-1 cells by ATRA was not associated with cell proliferation and differentiation. In addition, activators of retinoid acid receptor (RAR)α and RARγ, CD336, and CD2781 also induced FR-β expression. The induction of FR-β expression by CD336 could be inhibited by RARγ antagonist CD2665; RARβ agonist CD-417 and CD-2314 as well as retinoid X receptor (RXR) agonist LG100364 could not induce FR-β expression. These results indicate that ATRA within a certain range of concentration could reversibly induce the expression of FR-β in a dosage- and cell type-dependent manner, and its action in KG-1 cells might be associated with the signal transduction of retinoid receptor RARα and RARγ, rather than RARβ and RXRs.

High milk consumers have an increased risk of folate receptor blocking autoantibody production but this does not affect folate status in Spanish men and women

Folate receptor (FR)-blocking autoantibodies (FR-autoantibodies) have been reported in women with neural tube defect-affected pregnancies and subfertility and in children with progressive neurodevelopment disorders. We investigated their prevalence and association with folate status and milk intake in adults unexposed to folic acid fortification. A cross-sectional study of a randomly selected representative sample of a Spanish population (aged 18-75 y) stratified by age and gender was performed. Plasma and red cell folate, plasma cobalamin, fasting plasma total homocysteine (tHcy) concentration, methylenetetrahydrofolate reductase C677T polymorphism, and FR-autoantibody titer were determined in blood samples from 787 fasting participants. Lifestyle data were collected and milk intake estimated from a 3-d dietary record. FR-autoantibody prevalence was 7.2% [0.30 +/- 0.27 nmol (mean +/- SD) FR blocked/L], equally affecting men and women of all ages. Plasma and red cell folate and tHcy did not differ between carriers and noncarriers of FR-autoantibodies. Milk intake was higher in carriers (225 +/- 199 g/d) than in noncarriers (199 +/- 147 g/d) (P < 0.01). The risk of having FR-autoantibodies increased progressively with increasing quintile of milk intake and was significant in the highest quintile (> or =307 g/d) compared with the lowest (< or =67 g/d) [odds ratio (OR), 2.41 [95% CI: 1.02, 5.69]; P < 0.05; linear trend, P = 0.02]. We concluded that FR-autoantibodies occur in men and women of all ages and do not affect indicators of folate status such as plasma and red cell folate and tHcy. Higher milk intake is associated with increased risk of having FR-autoantibodies.

Selective delivery of folate-PEG-linked, nanoemulsion-loaded aclacinomycin A to KB nasopharyngeal cells and xenograft: effect of chain length and amount of folate-PEG linker

To investigate the use of folate-targeted nanoemulsion-loaded aclacinomycin A (ACM) to folate receptor (FR)-positive cells, we attempted to optimize the targeting ability of nanoemulsions by modifying the chain length and amount of the folate-PEG linker. Folate-linked, nanoemulsion-loaded ACM were formulated with 0.24 mol% of folate-poly (ethylene glycol)(3400)- (folate-PEG(3400)-) and folate-PEG(5000)-distearoylphosphatidylethanolamine (DSPE), and 0.03 mol% of folate-PEG(5000)-DSPE in nanoemulsions. Selective FR-mediated uptake was achieved in a human nasopharyngeal tumor cell line, KB, which overexpresses FR, but not in a human hepatoblastoma cell line, (FR(-)) HepG2. At the same amount of folate modification, the association with KB cells was increased with increasing the PEG-chain length. The association of 0.03 and 0.24 mol% folate-PEG(5000)-linked nanoemulsions with cells was 5- and 3.3-fold higher than that of non-folate nanoemulsion, respectively, while their cytotoxicity was similar. Both 0.03 and 0.24 mol% folate-PEG(5000)-linked nanoemulsions and non-folate nanoemulsion following intravenous injection inhibited tumor growth more significantly than ACM solution on day 24 following tumor inoculation (p < 0.01). This study demonstrates that a folate-linked nanoemulsion is feasible for tumor-targeted ACM delivery, and that folate modification with a sufficiently long PEG-chain and a small amount of nanoemulsion is an effective way of targeting nanoemulsion to tumor cells.

Targeted drug delivery via folate receptors

Targeted delivery via selective cellular markers can potentially increase the efficacy and reduce the toxicity of therapeutic agents. The folate receptor (FR) has two glycosyl phosphatidylinositol (GPI)-anchored isoforms, alpha and beta. FR-alpha expression is frequently amplified in epithelial cancers, whereas FR-beta expression is found in myeloid leukemia and activated macrophages associated with chronic inflammatory diseases. Conjugates of folic acid and anti-FR antibodies can be taken up by cancer cells via receptor-mediated endocytosis, thus providing a mechanism for targeted delivery to FR+ cells. The aim of this article is to provide a brief overview of applications of FR targeting in drug delivery, with an emphasis on the strategy of using folate as a targeting ligand. In order to do this, recent literature is surveyed on targeted delivery via both FR sub-types, as well as new findings on selective receptor upregulation in the targeted cells. A wide variety of molecules and drug carriers, including imaging agents, chemotherapeutic agents, oligonucleotides, proteins, haptens, liposomes, nanoparticles and gene transfer vectors have been conjugated to folate and evaluated for FR-targeted delivery. Substantial targeting efficacy has been found both in vitro and in vivo. In addition, mechanisms and methods for selective FR upregulation have been uncovered, which might enhance the effectiveness of the FR-targeted delivery strategy. FR-alpha serves as a useful marker for cancer, whereas FR-beta serves as a marker for myeloid leukemia and chronic inflammatory diseases. FR-targeted agents have shown promising efficacy in preclinical models and significant potential for future clinical application in a wide range of diseases.

Insilco analysis of functionally important residues in folate receptors

Lack of crystal structure data of folate binding proteins has left so many questions unanswered (for example, important residues in active site, binding domain, important amino acid residues involved in interactions between ligand and receptor). With sequence alignment and PROSITE motif identification, we attempted to answer evolutionarily significant residues that are of functional importance for ligand binding and that form catalytic sites. We have analyzed 46 different FRs and FBP sequences of various organisms obtained from Genbank. Multiple sequence alignment identified 44 highly conserved identical amino acid residues with 10 cysteine residues and 12 motifs including ECSPNLGPW (which might help in the structural stability of FR).

Effect of polyethylene glycol linker chain length of folate-linked microemulsions loading aclacinomycin A on targeting ability and antitumor effect in vitro and in vivo

PURPOSE

To establish a novel formulation tumor-targeted drug carrier of lipophilic antitumor antibiotics, aclacinomycin A (ACM), folate-linked microemulsions were prepared and investigated both in vitro and in vivo.

EXPERIMENTAL DESIGN

Three kinds of folate-linked microemulsions with different polyethylene glycol (PEG) chain lengths loading ACM were formulated with 0.24 mol% folate-PEG(2000)-distearoylphosphatidylethanolamine (DSPE), folate-PEG(5000)-DSPE, and folate-lipid (without PEG linker) in microemulsions. In vitro studies were done in a human nasopharyngeal cell line, KB, which overexpresses the folate receptor (FR), and a human hepatoblastoma cell line, [FR(-)] HepG2. In vivo experiments were done in a KB xenograft by systemic administration of folate-linked microemulsions loading ACM.

RESULTS

The association of folate-linked microemulsions to KB cells could be blocked by 2 mmol/L free folic acid. Selective FR-mediated cytotoxicity of folate-linked microemulsions loading ACM was obtained in KB but not in HepG2 cells. The association of the folate-PEG(5000)-linked microemulsion and folate-PEG(2000)-linked microemulsion with the cells was 200- and 4-fold higher, whereas their cytotoxicity was 90- and 3.5-fold higher than those of nonfolate microemulsion, respectively. The folate-PEG(5000)-linked microemulsions showed 2.6-fold higher accumulation in solid tumors 24 hours after i.v. injection and greater tumor growth inhibition than free ACM.

CONCLUSION

These findings suggest that a folate-linked microemulsion is feasible for tumor-targeted ACM delivery. This study shows that folate modification with a sufficiently long PEG chain on emulsions is an effective way of targeting emulsion to tumor cells.

Homocysteine: exploring its potential as a pharmacodynamic biomarker of antifolate chemotherapy

For decades it has been well known that elevated levels of homocysteine are harmful to humans on the basis of clinical observations derived from classical model diseases, such as inherited metabolic disorders. This group of diseases includes classical homocystinuria and several other inherited diseases affecting the so-called 'transsulfuration pathways'. Homocysteine lies in a metabolic checkpoint that interconnects one-carbon-transferring reactions with metabolism of sulfur-containing amino acids, since every molecule of 5-methyltetrahydrofolate derived either from plasma or generated from other folate species must be demethylated to liberate the reduced tetrahydrofolate. This unidirectional mechanism operates in every cell and has no alternative in eukaryotic cells. Antifolates are a group of anticancer agents targeting various metabolic steps within folate metabolism. They exert an indirect influence on the rate of appearance/disappearance of homocysteine from cellular and plasma compartments. Recently, it has been postulated that homocysteine may be a marker of the 'pharmacodynamic effect' of methotrexate, but studies attesting to this role are only now emerging. Here, we explore the genetic disease of folate and homocysteine metabolism and discuss the links between these model disorders with pharmacology and pharmacogenetics of folate antagonists used in the clinic. We outline possible ways of how homocysteine may be used as a biomarker of antifolate therapy.

Enhanced in vitro DNA transfection efficiency by novel folate-linked nanoparticles in human prostate cancer and oral cancer

Novel folate-linked, cationic nanoparticles (NPs) were developed and evaluated for potential use for gene delivery to human oral cancer (KB cells) and human prostate cancer (LNCaP cells), which abundantly expressed folate binding proteins. Folate-polyethylenglycol-distearoylphosphatidylethanolamine conjugate (f-PEG-DSPE) was incorporated in NPs composed of 3([N-(N',N'-dimethylaminoethane)-carbamoyl] cholesterol (DC-Chol) and Tween 80. NP-0.3FT, -1FT and -1FLT, which contain 0.3 and 1 mol% f-PEG2000-DSPE, and 1 mol% f-PEG5000-DSPE, respectively, showed about 100-200 nm in size. The NP/plasmid DNA complex (nanoplex) remained in an injectable size (230-340 nm) and slightly increased its size in serum. The association of NP-1FT with KB cells was enhanced by f-PEG2000-DSPE and was blocked by co-incubation with free folic acid in medium. In transfection activity, the NP-1FT, but not NP-1FLT, showed high activity into KB and LNCaP cells in the presence of serum. The NP-0.3FT also showed high activity into LNCaP cells, but not KB cells. In RT-PCR analysis, KB cells strongly expressed folate receptors mRNA, but LNCaP cells did not. In contrast, LNCaP cells expressed mRNA of prostate-specific membrane antigen (PSMA), which interacts with the folate substrate. Uptake mechanism of folate-linked NPs in LNCaP cells may be different from that in KB cells. This is the first report that folate-linked NPs selectively deliver the DNA to LNCaP cells, suggesting that such NPs are potentially targeted vectors to prostate cancer for gene delivery.

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.

Receptor induction and targeted drug delivery: a new antileukaemia strategy

Strategic modalities of drug delivery have the potential to greatly improve the therapeutic efficacy of available drugs in acute myelogenous leukaemia (AML). Folate receptor (FR) type beta is selectively expressed on the surface of approximately 70% of AMLs. Increased FR-beta expression in these cells can be induced by all-trans retinoic acid (ATRA) and other retinoid compounds in the absence of terminal differentiation or cell growth inhibition. An apparent post-transcriptional modification prevents FR-beta in normal haematopoietic cells from binding folate, in contrast to AML cells. FR-beta may, therefore, be used as a target for the selective delivery of chemotherapeutic drugs to AML cells; this treatment modality appears to be particularly efficacious when administered in conjunction with retinoid-induction of FR-beta. FR-targeted liposomal drug delivery can also bypass the P-glycoprotein (P-gp)-mediated drug efflux pump commonly associated with multiple drug resistance in AML. The rationale and merits of this novel experimental treatment for AML and the current status of this research are provided.

Modulation of the folate receptor type beta gene by coordinate actions of retinoic acid receptors at activator Sp1/ets and repressor AP-1 sites

Folate receptor (FR) type beta is a promising target for therapeutic intervention in acute myelogenous leukemia (AML) owing particularly to its specific up-regulation in AML cells by all-trans retinoic acid (ATRA). Here we identify functional elements in the FR-beta gene and examine the molecular mechanism of transcriptional induction of FR-beta by ATRA. The basal promoter activity of FR-beta resulted from synergistic interaction between Sp1 and ets binding sites (EBSs) and repression by upstream AP-1-like elements, whose action required EBSs. A minimal promoter containing the Sp1 and ets elements was ATRA-responsive. The repressor elements bound Fos family proteins; association of the proteins with the repressor elements correlated negatively with FR-beta expression in peripheral blood neutrophils and monocytes and also in KG-1 (AML) cells grown in the absence or in the presence of ATRA. Furthermore, down-regulation of FR-beta in KG-1 cells treated with O-tetradecanoylphorbol 13-acetate (TPA) was accompanied by increased AP-1 binding to the repressor elements. From chromatin immunoprecipitation (ChIP) assays, the nuclear retinoic acid receptor alpha (RARalpha) associated with the Sp1 region, and RARs beta and gamma associated with the AP-1 and Sp1 regions; treatment of KG-1 cells with ATRA did not alter Sp1 binding but increased the association of RARalpha and decreased the association of RARs beta and gamma. ATRA also decreased RAR expression levels. The results suggest that the FR-beta gene is a target for multiple coordinate actions of nuclear receptors for ATRA directly and indirectly acting on a transcriptional complex containing activating Sp1/ets and inhibitory AP-1 proteins. The multiple mechanisms favor the prediction that ATRA will induce FR-beta expression in a broad spectrum of AML cells. Further, optimal FR-beta induction may be expected when all 3 RAR subtypes bind agonist.

Folate receptor-targeted liposomes as vectors for therapeutic agents

The folate receptor is a cell surface protein that has recently been identified as a tumor marker, due to its differential overexpression in several malignancies. Current research indicates that folate can be covalently attached to the surface of liposomes to mediate their selective internalization by tumor cells through the folate receptor-mediated endocytic pathway. Optimized liposome formulations, characterized by improvements in drug loading, extended residence times in the circulation and improved drug release, have been developed to improve the biodistribution of therapeutic molecules. Theoretically, folate receptor-targeting can be combined with liposome encapsulation to synergistically affect disease outcome by enhancing the delivery of chemotherapeutic agents to neoplastic cells, while reducing systemic toxicities to normal tissues. The purpose of this chapter is to characterize the components of folate receptor-targeted liposomes, and summarize their applications in gene and drug delivery.

Dual regulation of ets-activated gene expression by SP1

The human folate receptor (hFR) type gamma gene is driven by a TATA-less promoter that uses a canonical Sp1 element for basal transcription. Using nuclear extract from 293 (human embryonic) cells, we mapped a second (non-canonical) Sp1 element to which Sp1 bound with a comparable affinity and which overlaps a functional ets binding site (EBS). Mutagenesis experiments revealed that the binding of ets to the EBS activates the promoter synergistically with Sp1 bound at the downstream site; however, binding of Sp1 to the EBS does not contribute to promoter activity. A further increase in Sp1 by inducible expression in recombinant 293 cells resulted in a small but significant decrease in the hFR-gamma promoter activity, but the decrease was abolished when the EBS was deleted from the promoter. In 293 cells, which do not express hFR-gamma, the Sp1 level was relatively high whereas in the hFR-gamma-positive HL60 leukemia cells, the Sp1 level was low and the EBS predominantly bound an ets protein. To account for the above observations, we propose a model in which when the Sp1 level is low, ets out competes Sp1 for binding to the EBS and synergistically enhances the hFR-gamma promoter activity by interacting with Sp1 bound at the canonical site whereas at higher levels, Sp1 represses the promoter by competitively inhibiting the binding of ets. As a partial extension of this model to the regulation of other ets activated genes, we show that Sp1 can predictably bind to a variety of ets elements including those responsive to Ets1 and Spi.1/Pu.1. A dual concentration-dependent action of Sp1 as an activator or a repressor offers a potential mechanism contributing to tissue-specific regulation of ets-dependent genes by Sp1.

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.

Structure of the genes for porcine endometrial secreted and membrane folate binding proteins

The endometrium of the pig produces two types of folate binding proteins (FBP) which, based on their sequences, are likely to be membrane (m) and secreted (s) forms. A clone containing both a gene coding for the sFBP cDNA and a gene coding for the mFBP was isolated from a yeast artificial chromosome (YAC) library. Each gene was subcloned and sequenced. The gene for sFBP spanned 4.4 kbp and included 5 exons. The mFBP gene spanned 7.0 kbp and also contained 5 exons. Structures of the genes were very similar for the last three exons, and this similarity was shared with other known FBP/folate receptor (FR) gene sequences. Unexpectedly, portions of introns 3 and 4 of both genes were highly homologous, suggesting the possibility that sequences within these introns served some as yet unknown function. In contrast, the structures of the 5' exons differed between the two genes and other known FBP/FR genes. Comparison of putative promoter regions for the two genes with promoter regions for human FBP/FR genes revealed significant sequence homology between sFBP and human gammaFBP and between mFBP and human alphaFR. These regions of homology may play a role in control of transcription of each gene.

Folate, homocysteine and neural tube defects: an overview

Folate administration substantially reduces the risk on neural tube detects (NTD). The interest for studying a disturbed homocysteine (Hcy) metabolism in relation to NTD was raised by the observation of elevated blood Hcy levels in mothers of a NTD child. This observation resulted in the examination of enzymes involved in the folate-dependent Hcy metabolism. Thus far, this has led to the identification of the first and likely a second genetic risk factor for NTD. The C677T and A1298C mutations in the methylenetetrahydrofolate reductase (MTHFR) gene are associated with an increased risk of NTD and cause elevated Hcy concentrations. These levels can be normalized by additional folate intake. Thus, a dysfunctional MTHFR partly explains the observed elevated Hcy levels in women with NTD pregnancies and also, in part, the protective effect of folate on NTD. Although the MTHFR polymorphisms are only moderate risk factors, population-wide they may account for an important part of the observed NTD prevalence.

Differentiation-independent retinoid induction of folate receptor type β, a potential tumor target in myeloid leukemia

Folate receptor (FR) type β is expressed in the myelomonocytic lineage, predominantly during neutrophil maturation and in myeloid leukemias. FR-β expression was elevated up to 20-fold by all-trans retinoic acid (ATRA) in KG-1 myeloid leukemia cells in a dose-dependent and reversible manner in the absence of terminal differentiation or cell growth inhibition. ATRA also increased FR-β expression in vitro in myeloid leukemia cells from patient marrow. FR-β was not up-regulated in KG-1 cells treated with phorbol ester, dexamethasone, 1,25-dihydroxy vitamin D3, or transforming growth factor β. ATRA did not induce FR-β expression in receptor negative cells of diverse origin. The ATRA-induced increase in FR-β expression in KG-1 cells occurred at the level of messenger RNA synthesis, and in 293 cells containing a stably integrated FR-β promoter–luciferase reporter construct, ATRA induced expression of the reporter. From experiments using retinoid agonists and antagonists and from cotransfection studies using the FR-β promoter and expression plasmids for the nuclear receptors retinoic acid receptor (RAR)α, RARβ, or RARγ, it appears that the retinoid effect on FR-β expression could be mediated by ligand binding to RARs α, β, or γ, but not to retinoid X receptors. Furthermore, there was apparent cross-talk between RARα and RARγ selective agonists or antagonists, suggesting a common downstream target for RAR isoforms in inducing FR-β expression. Thus, blocks in the RARα-specific pathway of retinoid-induced differentiation may be bypassed during retinoid induction of FR-β expression. The results suggest that to facilitate FR-targeted therapies, retinoids may be used to modulate FR-β expression in myeloid leukemia cells refractory to retinoid differentiation therapy.

Differentiation-independent retinoid induction of folate receptor type β, a potential tumor target in myeloid leukemia

Folate receptor (FR) type β is expressed in the myelomonocytic lineage, predominantly during neutrophil maturation and in myeloid leukemias. FR-β expression was elevated up to 20-fold by all-trans retinoic acid (ATRA) in KG-1 myeloid leukemia cells in a dose-dependent and reversible manner in the absence of terminal differentiation or cell growth inhibition. ATRA also increased FR-β expression in vitro in myeloid leukemia cells from patient marrow. FR-β was not up-regulated in KG-1 cells treated with phorbol ester, dexamethasone, 1,25-dihydroxy vitamin D3, or transforming growth factor β. ATRA did not induce FR-β expression in receptor negative cells of diverse origin. The ATRA-induced increase in FR-β expression in KG-1 cells occurred at the level of messenger RNA synthesis, and in 293 cells containing a stably integrated FR-β promoter–luciferase reporter construct, ATRA induced expression of the reporter. From experiments using retinoid agonists and antagonists and from cotransfection studies using the FR-β promoter and expression plasmids for the nuclear receptors retinoic acid receptor (RAR)α, RARβ, or RARγ, it appears that the retinoid effect on FR-β expression could be mediated by ligand binding to RARs α, β, or γ, but not to retinoid X receptors. Furthermore, there was apparent cross-talk between RARα and RARγ selective agonists or antagonists, suggesting a common downstream target for RAR isoforms in inducing FR-β expression. Thus, blocks in the RARα-specific pathway of retinoid-induced differentiation may be bypassed during retinoid induction of FR-β expression. The results suggest that to facilitate FR-targeted therapies, retinoids may be used to modulate FR-β expression in myeloid leukemia cells refractory to retinoid differentiation therapy.