High-affinity folate binding in human mammary gland

High-affinity³H-folate binding in Triton X-100 solubilized human mammary gland tissue displayed characteristics, e.g. apparent positive cooperativity and increasing affinity with decreasing concentration of folate binding protein, shown to be typical of specific folate binding. Radioligand dissociation was slow at pH 7.4. A major fraction of the bound radioligand dissociated rapidly at pH 3.5, while a residual binding of 20% persisted even after prolonged dialysis at pH 3.5. Gel chromatography revealed two major folate binding proteins (M_r100 kDa and 25 kDa). However, only one single band was detectable on SDS-PAGE immunoblotting. The highest folate binding activity per g protein was associated with the upper triglyceride-containing layer of the 1000 g supernatant of the homogenate. The folate binding protein extracted from this layer had a low cross-reactivity (<5%) with rabbit antibodies against 25 kDa human milk folate binding protein. The folate binding protein in the 1000 g pellet and the aqueous phase of the 1000 g supernatant was present at a low concentration and had a cross-reactivity of 100%.     

Development of a specific radioimmunoassay for the placental folate receptor and related high-affinity folate binding proteins in human tissues

High-affinity membrane-associated and soluble folate binding proteins (FBPs) from human placenta, milk, and KB cells appear to share antigenic determinants [A. C. Antony et al. (1981) J. Biol. Chem. 256, 9684-9692 and (1985) 260, 14911-14917]. Iodination of a highly purified preparation of placental folate receptor (PFR) by various techniques resulted in significant denaturation of the PFR as evidenced by additional peaks of radioactivity on Sephacryl S-200 gel filtration in 1% Triton X-100. These denatured species had similar molecular weights on sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as radioiodinated and native PFR, and were also recognized, albeit with less efficiency, by specific rabbit antiserum raised against purified PFR. Since these denatured species failed to bind folate, they were specifically excluded from 125I-PFR by their inability to bind pteroylglutamate-Sepharose. This ws accomplished in a single step by iodination of PFR bound to the affinity column and elution of 125I-PFR under identical conditions that the native PFR was purified. The purified 125I-PFR comigrated with unlabeled PFR on SDS-PAGE and its elution profile on Sephacryl S-200 gel filtration was identical to radioligand bound PFR. The resulting radioimmunoassay standard curve using this affinity chromatography purified 125I-PFR, unlabeled PFR, and anti-human PFR serum had a range for measurement between 5 and 500 ng of PFR and was not affected by the concentration of folate in the sample. The practical utility of this radioimmunoassay for measuring cross-reacting material to the PFR was validated by its ability to quantitate the 40,000 and 160,000 Mr FBPs which are the two major forms of high-affinity FBPs in human tissues.     

Megalin binds and mediates cellular internalization of folate binding protein

Folate is an essential vitamin involved in a number of biological processes. High affinity folate binding proteins (FBPs) exist both as glycosylphosphatidylinositol-linked, membrane associated folate binding proteins and as soluble FBPs in plasma and some secretory fluids such as milk, saliva and semen. The function and significance of FBPs are unresolved, however, it has been suggested that they may facilitate folate uptake, e.g. during suckling. The present study shows that megalin, a large, multiligand endocytic receptor and member of the low-density lipoprotein-receptor family, is able to bind and mediate cellular uptake of FBP. Surface plasmon resonance analysis shows binding of bovine and human milk FBP to immobilized megalin, but not to low density lipoprotein receptor related protein. Binding of (125)I-labeled folate binding protein (FBP) to sections of kidney proximal tubule, known to express high levels of megalin, is inhibitable by excess unlabeled FBP and by receptor associated protein, a known inhibitor of binding to megalin. Immortalized rat yolk sac cells, representing an established model for studying megalin-mediated uptake, reveal (125)I-labeled FBP uptake which is inhibited by receptor associated protein and by antimegalin antibodies. Microinjection of (125)I-labeled FBP into renal tubules in vivo shows proximal tubular uptake by endocytosis. Megalin is expressed in several absorptive epithelia, including intestine and kidney proximal tubule, and thus the present findings provide a mechanism for intestinal and renal endocytic uptake of soluble FBP.     

High-affinity folate binding in human prostate

Binding of³H-folate in Triton X-100 solubilized human prostate homogenate was of a high-affinity type and displayed apparent positive cooperativity typical of specific folate binding. Radioligand dissociation was slow at pH 7.4, but rapid at pH 3.5. Gel chromatography reveled two major folate binding proteins (M_r100 and 25kDa), but only one single band (M_r65–70 kDa) was detectable on SDS-PAGE and immunoblotting with rabbit-anti human milk folate binding protein. Concentration of folate binding protein in prostate homogenate expressed as maximum³H-folate binding was 1.10 nmol/g protein, and the cross-reactivity with rabbit-anti human milk folate binding protein serum was 15% as determined by an enzyme-linked immunosorbent assay (median values; n=6).     

A high-affinity folate binding protein in human urine. Radioligand binding characteristics,immunological properties and molecular size

High-affinity binding of [³H]folate in human urine displayed characteristics, e.g. apparent positive cooperativity, which are typical of specific folate binding. By means of a two-site enzyme-linked immunosorbent assay (ELISA) with rabbit antibodies against the low molecular weight folate binding protein from human milk, we measured folate binding protein concentrations in the range of 0.51 to 4.13 nM in urine samples from 16 apparently healthy individuals. Ultrogel AcA 44 chromatography of the urine showed that immunoreactive and radioligand bound folate binding protein coeluted in one large peak (M_r25,000).     

Rabbit antibodies against the folate binding protein from cow's milk. Production,characterization and use for development of an enzyme-linked immunosorbent assay (ELISA)

Rabbit antibodies to purified folate binding protein from cow's milk whey were used for development of a two-site enzyme-linked immunosorbent assay (ELISA) for quantitation of bovine folate binding proteins, The folate binding proteins in human milk and serum showed no cross-reactivity. A partial saturation of purified bovine folate binder with folate gave rise to an increased antigenicity probably due to a ligand (folate)-induced exposure of antigenic sites on the protein.     

Analysis of folate binding protein N-linked glycans by mass spectrometry

The folate binding protein (FBP), also known as the folate receptor (FR), is a glycoprotein which binds the vitamin folic acid and its analogues. FBP contains multiple N-glycosilation sites, is selectively expressed in tissues and body fluids, and mediates targeted therapies in cancer and inflammatory diseases. Much remains to be understood about the structure, composition, and the tissue specificities of N-glycans bound to FBP. Here, we performed structural characterization of N-linked glycans originating from bovine and human milk FBPs. The N-linked glycans were enzymatically released from FBPs, purified, and permethylated. Native and permethylated glycans were further analyzed by matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) mass spectrometry (MS), while tandem MS (MS/MS) was used for their structural characterization. The assignment of putative glycan structures from MS and MS/MS data was achieved using Functional Glycomics glycan database and SimGlycan software, respectively. It was found that FBP from human milk contains putative structures that have composition consistent with high-mannose (Hex(5-6)HexNAc(2)) as well as hybrid and complex N-linked glycans (NeuAc(0-1)Fuc(0-3)Hex(3-6)HexNAc(3-5)). The FBP from bovine milk contains putative structures corresponding to high-mannose (Hex(4-9)HexNAc(2)) as well as hybrid and complex N-linked glycans (Hex(3-6)HexNAc(3-6)), but these glycans mostly do not contain fucose and sialic acid. Glycomic characterization of FBP provides valuable insight into the structure of this pharmacologically important glycoprotein and may have utility in tissue-selective drug targeting and as a biomarker.     

Identification of a Mg2+-dependent protease in human placenta which cleaves hydrophobic folate-binding proteins to hydrophilic forms

Hydrophobic folate-binding proteins (FBPs), which are only 5-10 kDa larger than 40-kDa hydrophilic FBPs, bind significant quantities of Triton X-100 micelles and elute as apparent 160-kDa species on Sephacryl S-200 gel filtration in Triton X-100. Detergent-solubilized placental membranes release a major (greater than 95%) 40-kDa hydrophilic FBP species as well as a minor apparent 160-kDa folate binding species when similarly analyzed. We tested the hypothesis that this recovery of predominantly hydrophilic FBPs was mediated by a putative hydrophobic FBP-specific placental protease. When placenta was solubilized in the presence of increasing concentrations of EDTA, there was a progressive increase in apparent 160-kDa folate binding moieties concomitant with a decrease in 40-kDa FBPs. At 20 mM EDTA, a single apparent 160-kDa folate binding species was recovered and the 40-kDa FBPs could not be detected by radioligand binding or specific radioimmunoassay. The apparent 160-kDa species specifically bound radiolabeled folates and were specifically immunoprecipitated by rabbit anti-40-kDa FBP antiserum. On 15% sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by transfer to nitrocellulose and probing with anti-40-kDa FBP antiserum, the apparent 160-kDa FBPs electrophoresed as 45-kDa species. Detergent binding studies indicated that apparent 160-kDa FBPs were hydrophobic, thus accounting for the molecular weight discrepancy in gel filtration in Triton X-100 versus sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The EDTA-mediated inhibition of conversion of hydrophobic FBPs to hydrophilic FBPs by protease was reversed in a dose-dependent manner by Mg2+. If this protease is physiologically relevant, it could play an important regulatory role in folate transport by influencing the net number of hydrophobic FBPs on the cell surface.     

A high-affinity folate binding protein in human semen

The presence of a folate binding protein of high-affinity type (affinity constant 3.10¹⁰M^–1, maximum folate binding 1.4 nM) in human semen was demonstrated in equilibrium dialysis experiments (37°C, pH 7.4) with the radioligand³H-folate. Radioligand dissociation from the binding protein was slow at pH 7.4, but rapid at pH 3.5. By use of rabbit antibodies against 25 kDa human milk folate binding protein we determined the concentration of folate binding protein in 16 speciments of human semen in an enzyme-linked immunosorbent assay. The concentration of immunoreactive folate binding protein was independent of the number of spermatozoa in individual specimens. Gel filtration showed that immunoreactive and radioligand bound folate binding protein coeluted in two peaks: a major one of 100 kDa and a minor one of 25 kDa.     

A high-affinity folate binding protein in human amniotic fluid. Radioligand binding characteristics,immunological properties and molecular size

The presence of a folate binding protein of high-affinity type (affinity constant 5 · 10⁹M^–1, maximum folate binding 3 nM) in human amniotic fluid was demonstrated in equilibrium dialysis experiments (37°C, pH 7.4) with the radioligand³H-folate. Dissociation of³H-folate from the binding protein was slow at pH 7.4 but rapid at pH 3.5. By use of rabbit antibodies against low molecular weight folate binding protein from human milk we determined the concentration of folate binding protein in 5 amniotic fluids (range 1.5–2.3 nM) in an Enzyme-Linked Immunosorbent Assay (ELISA). ultrogel AcA 44 chromatography of amniotic fluid showed that immunoreactive and radioligand bound folate binding protein coeluted in two peaks: a major one (M _r 25 000) and a minor one (M _r 100 000).     

The isolation and properties of multiple forms of folate binding protein in cultured KB cells

The folate binding proteins (FBPs) of KB cells which were cultured in normal (N) and folate-deficient (D) medium have been characterized. The 200,000 g supernate of lysed cells contained two FBPs which could be separated by DEAE-Bio-Gel A chromatography, indicating that they differ in ionic charge although they could not be separated by gel filtration through Sephadex G-100 (apparent Mr approximately 40,000). Two species of FBP, a major form of apparent Mr approximately 160,000 and a minor form of apparent Mr approximately 40,000, were identified by gel filtration through Sephadex G-150 in the membrane component of the cells after solubilization with Triton X-100. An additional FBP was isolated and purified by affinity chromatography from the medium in which these cells were cultured. By gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the apparent Mr of this FBP was approximately 44,000. The association constants for pteroylglutamic acid of the FBPs in the 200,000 g cell lysate supernate, culture medium, and Triton-solubilized membrane were similar and the relative affinity of folate analogs for the FBP, vis-à-vis pteroylglutamic acid, was similar for all species. An antiserum raised to the purified FBP from the culture medium precipitated the FBPs in the 200,000 g cell lysate supernate, Triton-solubilized membrane, and culture medium, indicating antigenic homology among these FBPs. There was no unsaturated FBP in the 200,000 g cell lysate supernate or medium when KB cells were cultured in N medium. However, when cells were cultured in D medium, the unsaturated FBP of the 200,000 g cell supernate and culture medium was substantial (9.2 and 14.1 pmol/mg protein, respectively). Unsaturated FBP was detected in the membrane of normal cells but this also increased when these cells were cultured in D medium (4.5 to 756 pmol/mg protein), indicating that the FBPs of these cellular compartments are normally saturated by folate. After 16 weeks of culture in D medium, the total folate binding capacity of the membrane-associated FBP was twofold greater than that of normal KB cells, indicating the induction of FBP.     

Folate (pteroylglutamate) uptake in human red blood cells, erythroid precursors and KB cells at high extracellular folate concentrations. Evidence against a role for specific folate-binding and transport proteins.

Membrane-associated folate (pteroylglutamate, PteGlu)-binding proteins (FBPs) play an important role as PteGlu-transport proteins in malignant and normal human cells. Since high extracellular folate (PteGlu) concentrations (EFC) profoundly influenced uptake and toxicity of the anti-PteGlu methotrexate in malignant KB cells, we studied human cells to determine additional mechanisms for PteGlu uptake when the EFC was varied. At low EFC (less than 10 nM), the predominant mechanism for folate uptake in mature erythrocytes was through binding to externally oriented FBPs which were quantitatively insignificant (4-6 orders of magnitude lower) and of no apparent physiological relevance when compared with KB cells. However, the predominant mechanism of PteGlu accumulation at high EFC [10-250 nM] in intact erythrocytes and sealed right-side-out (RSO) ghosts was not FBP-mediated and non-specific. This conclusion was based on the findings that radiolabelled PteGlu uptake: (i) continued even in the presence of a 1000-fold excess of unlabelled PteGlu and was linear and not saturable up to 250 nM; (ii) was two-fold higher at pH 4.5 than 7.5; (iii) was less than 2-fold increased at 37 degrees C compared with 4 degrees C; and (iv) was unaffected after trypsin-mediated proteolysis of greater than 75% FBPs. The [3H]PteGlu and 125I-PteGlu (histamine derivative) accumulated intracellularly through the non-specific PteGlu-uptake mechanism was unaltered biochemically and in a soluble compartment. Raising the EFC 500-fold higher than controls during erythropoiesis in vitro resulted in reversal of the expected anti-(placental folate-receptor)-antiserum-induced megaloblastic changes in orthochromatic normoblasts derived from burst-forming unit-erythroid colonies. Furthermore, at EFC greater than 0.1 microM, KB-cell accumulation of [3H]PteGlu was also predominantly through a mechanism that did not involve specific FBPs. Thus, at high EFC, a major component of PteGlu transport in human cells is not mediated through FBPs and is likely to be a passive diffusion process.     

Role of the membrane-associated folate binding protein (folate receptor) in methotrexate transport by human KB cells

The uptake of methotrexate by KB cells was observed to be dependent on time, temperature, and concentration of extracellular methotrexate. The Kd for methotrexate surface binding to KB cells was approximately 200 nM. Following exposure of KB cells to trace quantities of [3H]methotrexate for periods ranging from 6 min to 24 h, the cellular methotrexate was progressively formed into methotrexate polyglutamates and was bound to dihydrofolate reductase as well as to a particulate folate binding protein. To further study the mechanism of methotrexate uptake in KB cells, the N-hydroxysuccinimide ester of methotrexate was used to covalently label the surface of KB cells and to inhibit transport of methotrexate. The N-hydroxysuccinimide ester of methotrexate was bound to a species of protein with an apparent molecular weight of 160,000 in 1% (v/v) Triton X-100 that bound folic acid and was specifically precipitated by antiserum raised against the previously purified high-affinity folate binding protein (the folate receptor) from human KB cells. In addition, trypsin was utilized to remove surface-accessible covalently bound methotrexate. The amount of covalently bound methotrexate that could be released by trypsin initially decreased on incubation at 37 degrees C, suggesting that the methotrexate and binding protein were internalized. However, with time, trypsin could again release the covalently bound methotrexate, suggesting that the binding protein cycles from the external cell surface to the inside of the cell and out again.     

Homologous membrane folate binding proteins in human placenta: cloning and sequence of a cDNA

A preparation of folate binding protein purified from human placental membranes in the presence of a variety of protease inhibitors followed by deglycosylation with N-glycanase gave a sharp band at Mr approximately 28,000 following SDS-polyacrylamide gel electrophoresis. The deglycosylated protein bound [3H]folic acid as tightly as the native protein. Peptides obtained following digestion of the purified protein with staphylococcal V8 protease and HPLC purification were sequenced. Polyclonal antibodies against the protein preparation were affinity purified and used to screen a placental cDNA expression library. A full-length cDNA for a placental folate binding protein was thus obtained and the corresponding protein sequence deduced. This result, taken together with the peptide sequence data, indicates the expression of at least two homologous folate binding proteins in placenta, one of which appears to be identical with the folate binding protein from human milk and nasopharyngeal epidermoid carcinoma (KB) cells; the cDNA sequence obtained corresponds to the other protein. The deduced protein sequence is characterized by a putative 16-residue amino-terminal signal peptide that is cleaved, resulting in a 239-residue polypeptide. The mature protein exhibits two potential sites for N-linked glycosylation at Asn-99 and Asn-179, eight potential intramolecular disulfide bonds, and a stretch of hydrophobic residues at the carboxyl terminus that could form a transmembrane domain. The protein bears a 68% sequence homology with the KB cell folate binding protein and may represent a fetal folate transport protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ligand binding characteristics of two molecular forms,one equipped with a hydrophobic glycosyl phosphatidylinositol tail,of the folate binding protein purified from human milk

Two molecular forms of the folate binding protein were isolated and purified from human milk by a combination of cation exchange- and affinity chromatography. One protein (27 kDa) was a cleavage product of the other 100 kDa protein as evidenced by N-terminal amino acid sequence homology and a reduction in the molecular size of the latter protein to 27 kDa after cleavage of its hydrophobic glycosylphosphatidylinositol tail by phosphatidylinositol-specific phospholipase C. High-affinity binding of [³H]folate was characterized by upward convex Scatchard plots and increasing ligand binding affinity with decreasing concentrations of both proteins. Downward convex Scatchard plots and binding affinities showing no dependence on the protein concentration were, however, observed in highly diluted solutions of both proteins. Radioligand binding was inhibited by folate analogs, and dissociation of radioligand was slow at pH 7.4 but rapid and complete at pH 5.0 and 3.5. Ligand binding quenched the tryptophan fluorescence of the 27 kDa protein suggesting that tryptophan is present at the binding site and/or ligand binding induces a conformation change that affects tryptophan environment in the protein. The 27 kDa protein representing soluble folate binding protein exhibited a greater affinity for ligand binding than the 100 kDa protein which possesses a hydrophobic tail identical to the one that anchors the folate receptor to the cell membrane.     

Isolation and characterization of a folate receptor from human placenta

While folate binding proteins have been described in serum and a variety of tissues, the function of these proteins is unknown. A particulate folate binding protein from human placenta has been isolated and characterized following solubilization with Triton X-100. The protein was purified 61,000-fold using affinity chromatography on pteroylglutamic acid-Sepharose as the major purification step. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis the purified protein gave a single band with a Mr = 38,500. Stoichiometry of binding indicated that 1 mol of folate was bound per mol of protein. The protein was a glycoprotein that contained 12% carbohydrate. Antiserum was raised in a rabbit, and on immunodiffusion, gave a single precipitin line with the purified placental folate binding protein. Immunoprecipitation studies using this antiserum indicated that the purified placental folate binding protein shared antigenic determinants with both the large Mr and small Mr folate binding proteins from human milk. Immunofluorescent studies with this antiserum and human erythrocytes revealed the presence of an immunologically similar protein on the plasma membrane of these cells suggesting that this protein may function as a folate receptor.     

Characterization of the folate receptor in human molar placenta

We have characterized a high-affinity folate receptor in human molar placenta tissue. Radioligand binding exhibited characteristics typical of other high-affinity folate binding proteins. Those included, positive cooperativity, a tendency to increased binding affinity with decreasing receptor concentration, a slow ligand dissociation at pH 7.4 becoming rapid at pH 3.5, and inhibition by folate analogues. The folate receptor cross-reacted with antibodies against human milk folate binding protein, e.g. the syncytothrophoblastic layer of molar placenta tissue sections showed strongly positive immunostaining. The gel filtration profile contained two radioligand-bound peaks (25 and 100 kDa), however, with considerable overlap. Only a single band of 70 kDa was seen on SDS-PAGE immunoblotting. The folate receptor in placental tissue may play a crucial role in the transfer of folate from maternal circulation to the fetus.     

Conversion of an apparent 100 kDa folate binding protein from human milk,choroid plexus and semen to a 25 kDa molecular species by phosphatidylinositol-specific phospholipase C

Gel filtration studies in the presence of Triton X-100 showed that treatment with phosphatidylinositol-specific phospholipase C reduced the apparent molecular size of the 100 kDa folate binding protein from human milk, choroid plexus and semen to 25 kDa. Cleavage of a hydrophobic glycosly phosphatidylinositol domain (a membrane anchor) inserting the protein into Triton X-100 micelles could account for this phenomenon.     

High-affinity folate binding in human liver membranes

High-affinity binding of³H-folate in Triton X-100 solubilized membranes of human liver displayed characteristics, e.g. apparent positive cooperativity, which are typical of specific folate binding. Ultrogel^® AcA 44 chromatography of solubilized membranes saturated with³H-folate revealed a major peak of 100 kDa and a minor peak of 25 kDa. The 100 kDa peak could represent a hydrophobic membrane associated molecular form of the protein. This notion was supported by the fact that the two peaks had identical molecular weights as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis with immunoblotting.