Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics
Characterization of soluble folate receptors (folate binding proteins) in humans. Biological roles and clinical potentials in infection and malignancy
Introduction
The high-affinity folate binding protein (FBP) plays a key role in the overall homeostasis of the vitamin folic acid in humans. One can distinguish between a membrane associated FBP which serves as a folate receptor (FR) attached to the outer cell surface by a glycosylphosphatidylinositol (GPI)-anchor, and a soluble form present in plasma and most body fluids [1]. The FR exists in two isoforms, FR α (FOLR1) and FR β (FOLR2). FOLR1 is expressed at the apical surface of normal epithelial tissues, and is overexpressed in several tumor cells where it has become a target for diagnostic imaging and cancer therapeutics [1,2]. FOLR2 is expressed in placental, hematopoietic cells and macrophages [1,2]. A third isoform, termed FR gamma (FOLR3) is secreted from myeloid cells as a soluble protein not possessing a GPI anchor [1,2].
The pathway for cellular internalization of the receptor is complex. The ligand-tagged FR is endocytosed by a non-clathrin and non-caveolar pathway into endosomes via GPI-anchored protein enriched endocytic compartments (GEECs) [2]. The biological function of FRs is to internalize folates into the cells, where the vitamin is of crucial importance to DNA synthesis and repair [1,2]. However, other important functions have recently been assigned to the receptor. Thus, it has been shown that the folate-FOLR1 complex mediates activation of the pro-oncogene STAT3 which contributes to tumor proliferation, angiogenesis and metastasis [3,4].
The presence of soluble forms of the high-affinity FR in milk, body fluids and plasma (serum) has been recognized over the past fifty years. In the earliest studies where neither the complete amino acid sequence nor the existence of different isoforms of FR was known, the concentration of soluble protein was determined indirectly as maximum high-affinity binding of radiolabeled folate (1–1 M stoichiometry). Hence, the concentration of protein was expressed as mass units of bound radiofolate (MW of folate 450 g). Recently the concentrations of soluble FOLR1 and FOLR2 in body fluids have been determined directly as mass concentrations (MW of protein 30,000 g) by immunological methods utilizing both poly- and monoclonal antibodies against FOLR1 and FOLR2.
The current literature has addressed the source of origin, identity (isoform) and nature of the soluble FR in plasma and body fluids, i.e. to what extent individual FR isoforms contribute to the total content of soluble FR in plasma and how physiological processes may regulate their concentrations. Data from recent and earlier studies have been compared disregarding that completely different analytical principles for measurement of soluble FR were applied in the early studies, i.e. mass units referring to bound radiofolate were thus compared to mass units referring to protein [5]. This has caused serious misconceptions and erroneous conclusions as to the identities and origins of the different soluble forms of FR in plasma or serum. The purpose of this review is therefore to address and analyze these topics critically and present a detailed and complete survey of the nature and function of soluble FR isoforms in plasma, body fluids-and secretions. Another issue to be discussed is whether soluble FOLR could serve as a candidate vector for folate conjugates of cytotoxic agents intended to target overexpressed megalin receptors in malignant tissues.
Section snippets
Secretion of soluble membrane-free FOLR1 with or without a retained GPI-anchor into body fluids
GPI-anchored proteins, like the folate receptors FOLR1 and FOLR2, belong to a group of proteins that are attached to the outer cell surface by a GPI-anchor. Their existence was established thirty years ago. They can be released from the intact cell surface into extracellular fluids as free water- soluble proteins devoid of a GPI-anchor after cleavage of the hydrophobic GPI-anchor. Two types of GPI-specific phospholipases in humans cleave GPI on different sites, GPI-phospholipase C and
FOLR isoforms in plasma or serum
The first studies reporting the presence of a high-affinity FBP in plasma/serum appeared more than 40 years ago. The concentration of FBP was determined by titration with tritiated folate to maximum binding. No definite conclusions as to origin, identity and function of the FBP were drawn at that time. The presence of soluble FOLRs in plasma/serum has recently attracted renewed attention after introduction of immunoassays based on antibodies specifically directed against FOLR1 and FOLR2. These
Soluble FOLR in granulocytes
The above evidence indicates that the high-affinity soluble FOLR in serum originates from the granulocyte. Hence, the molecular characteristics, identity and function of the FOLR isoform in neutrophil granulocytes will be addressed.
Distribution, homeostasis and cellular entry of soluble FOLR
The processes involved in distribution and homeostasis of soluble FOLRs, i.e. FOLR3, in humans have only been addressed in very few studies and are by far elucidated. However, evidence achieved in animal studies performed with bovine FBP indicates that the 600 kDa multiligand endocytic receptor megalin could play a crucial role [70]. Megalin binds a great number of structurally and functionally different ligands including several vitamin carrier proteins. Megalin is expressed in many
Conclusion and perspectives
The present review has analyzed the origin, presence and characteristics of the three soluble isoforms of FR in humans with a special focus on their biological functions and putative diagnostic or therapeutic potentials. Studies on high-affinity FOLRs in milk, other body fluids or secretions as well as plasma/serum have appeared over the past 50 years. The earliest studies used high-affinity binding of radiofolate to identify and determine the concentration of soluble FOLR in serum, whereas the
Declaration of Competing Interest
None.
References (78)
- et al.
Folate receptor endocytosis and trafficking
Adv. Drug Deliv. Rev.
(2004) - et al.
Folic acid mediates activation of the pro-oncogene STAT3 via the folate receptor
Alpha, Cell. Signal.
(2015) Release and extracellular transit of glycosylphosphatidylinositol proteins
J. Lab. Clin. Med.
(1998)- et al.
Shedding and uptake of gangliosides and glycosylphosphatidylinositol- anchored proteins
Biochim. Biophys. Acta
(2006) - et al.
Isolation, characterization, and comparison of the solubilized particulate and soluble folate binding proteins from human milk
J. Biol. Chem.
(1982) - et al.
The complete amino acid sequence of a human folate binding protein from KB cells determined from the cDNA
J. Biol. Chem.
(1989) - et al.
Folate-binding protein and the absorption of folic acid in the small intestine of the suckling rat
Am. J. Clin. Nutr.
(1988) - et al.
High-affinity folate receptor in human ovary, serous ovarian carcinoma, and ascites: radioligand binding mechanism, molecular size, ionic properties, hydrophobic domain, and immunoreactivity
Arch. Biochem. Biophys.
(1999) - et al.
Lipids in human saliva
Arch. Oral Biol.
(1996) - et al.
The complex interplay between ligand binding and conformational structure of the folate binding protein (folate receptor): biological perspectives
Biochim.Biophys. Acta
(2015)
Folate-receptor 1 (FOLR1) protein is elevated in the serum of ovarian cancer patients
Clin. Biochem.
Total folate binding capacity of normal human plasma, and variations in uremia , cirrhosis, and pregnancy
Blood
High and low affinity binding of folate to proteins in serum of pregnant women
Biochim. Biophys. Acta
High-affinity binding of folate to a protein in serum of male subjects
Clin. Chim. Acta
Change in binding properties of folate binding protein in cow's whey due to removal of a cofactor during affinity chromatographic purification
Biochim. Biophys. Acta
The heterogeneity and properties of folate binding proteins from chronic myelogenous leukemia cells
Blood
Properties of purified folate-binding proteins from chronic myelogenous leukemia cells
Biochim. Biophys. Acta
Chin. Chem. Lett.
Targeting megalin to enhance delivery of anti-clusterin small-interfering RNA nanomedicine to chemotreated breast cancer
Eur. J. Pharm. Biopharm.
Folate receptors
Annu.Rev.Nutr.
High folic acid diet enhances tumour growth in PyMT-induced breast cancer
Br. J. Cancer
Conditions associated with circulating tumor-associated folate receptor 1 protein in healthy men and women
PLoS One
GPI-anchored complement regulatory proteins in seminal plasma
J. Clin. Invest.
Amino-acid sequence homology between human and bovine low molecular weight FBP isolated from milk
Carlsb. Res. Commun.
A combination of cat-ion exchange and ligand-affinity for purification of two molecular species of the folate binding protein in human milk, one equipped with a hydrophobic glycosyl phosphatidylinositol tail: characterization of hydrophobicity and electrical charge
Biosci. Rep.
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
Biosci. Rep.
Cubilin and megalin expression and their interaction in the rat intestine: effect of thyroidectomy
Am. J. Physiol. Endocrinol. Metab.
A high-affinity folate binding protein in human semen
Biosci. Rep.
A minor fraction of a high-affinity folate binding protein from the epididymis is associated with membranous vesicles and spermatozoa in human semen
Int. J. Androl.
A high-affinity soluble folate receptor in fluids of non-neoplastic ovarian cysts: radioligand binding, molecular size, hydrophobic residue, and immunological properties
APMIS
A high-affinity folate binding protein in fluid of benign cysts of human liver and mammary gland
Int. J. Vit. Nutr.Res.
Quantitation of the folate binding protein in human cerebrospinal fluid by an enzyme-linked immunosorbent assay (ELISA)
Pteridines
High affinity folate binding in human choroid plexus. Characterization of radioligand binding, immunoreactivity, molecular heterogeneity and hydrophobic domain of the binding protein
Biochem. J.
Characterization of a folate receptor in parotid gland and a folate binding protein in saliva from humans
APMIS
Immunoreactive folate-binding proteins from human saliva
Biochem. J.
Serum folate receptor alpha, mesothelin and megakaryocyte potentiating factor in ovarian cancer: association to disease stage and grade and comparison to CA125 and HE4
J Ovarian Res.
Serum folate receptor alpha as a biomarker for ovarian cancer: implications for diagnosis, prognosis and predicting its local tumor expression
Int. J. Cancer
Serum folate receptor a (sFR) in ovarian cancer diagnosis and surveillance
Cancer Med.
alpha isoforms of soluble and membrane-linked folate-binding protein in human blood
Biosci.Rep.
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