Li+-induced release of the folate binding protein from human granulocytes

Characterization of soluble folate receptors (folate binding proteins) in humans. Biological roles and clinical potentials in infection and malignancy

This review surveys soluble Folate Receptors (FOLRs) in humans. FOLR1 and FOLR2 are equipped with cellular glycosylphosphatidylinositol (GPI) anchors. FOLR1 is secreted from epithelia with or without a micelle-encapsulated GPI-anchor into milk and other body fluids/secretions, e.g. semen where its interaction with spermatozoa indicates a role in male fertility. FOLR1 and FOLR2 serve as serum biomarkers of various diseases. FOLR3 possesses no GPI-anchor and originates from secretory granules of neutrophil granulocytes; its concentration in serum correlates to the FOLR3 content in leukocytes and rises with increased leukocyte counts (infection, malignancy and pregnancy). FOLR3 exerts anti-microbial and anti-tumor effects by depriving bacteria and tumor cells of natural folates. Megalin receptors mediate reabsorption of ultrafiltered folate-bound FOLR into cells of proximal kidney tubules and of folate-bound FOLR uptake in growing embryos. Megalin receptors overexpressed in malignant tumors could be suitable therapeutic targets for folate-conjugated cytotoxic agents utilizing soluble FOLRs as vectors.

alpha Isoforms of soluble and membrane-linked folate-binding protein in human blood

The high-affinity FBP/FR (folate-binding protein/folate receptor) is expressed in three isoforms. FRalpha and FRbeta are attached to cell membranes by hydrophobic GPI (glycosylphosphatidylinositol) anchors, whereas FBPgamma is a secretory protein. Mature neutrophil granulocytes contain a non-functional FRbeta on the surface, and, in addition, nanomolar concentrations of a secretory functional FBP (29 kDa) can be present in the secondary granules. A statistically significant correlation between the concentrations of functional FBP, probably a gamma isoform, in granulocytes and serum supported the hypothesis that serum FBP (29 kDa) mainly originates from neutrophils. The presence of FBP/FRalpha isoforms were established for the first time in human blood using antibodies specifically directed against human milk FBPalpha. The alpha isoforms identified on erythrocyte membranes, and in granulocytes and serum, only constituted an almost undetectable fraction of the functional FBP. The FBPalpha in neutrophil granulocytes was identified as a cytoplasmic component by indirect immunofluorescence. Gel filtration of serum revealed a peak of FBPalpha (>120 kDa), which could represent receptor fragments from decomposed erythrocytes and granulocytes. The soluble FBPs may exert bacteriostatic effects and protect folates in plasma from biological degradation, whereas FRs on the surface of blood cells could be involved in intracellular folate uptake or serve as signal proteins. The latter receptors have also been used for therapeutic targeting in malignancy.