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.

Serum-soluble folate receptor ß as a biomarker for the activity of rheumatoid arthritis synovitis and the response to anti-TNF agents.

This study aims to develop a sandwich ELISA system for the measurement of soluble folate receptor ß (sFRß) and evaluate whether base line levels of serum sFRß are a biomarker for the activity of RA synovitis and the response to anti-TNF agents. Serum sFRß from normal controls (41 samples), patients with OA (29 samples), and patients with RA (27 samples) and synovial fluid sFRß from patients with RA (17 samples) were measured by sandwich ELISA, using anti-FRαß and anti-FRß antibodies as capture and detection antibodies, respectively. Baseline levels of serum sFRß before therapy were evaluated in relation with DAS28-CRP or CRP and response to anti-TNF agents at 3-month follow-up. sFRß levels in RA synovial fluids were higher than those in RA sera, and sFRß levels in RA sera were higher than those in osteoarthritis and normal control sera. A significant relationship was observed between serum sFRß levels and the DAS28-CRP scores or CRP values. The area under curve (AUC) values for receiver-operating characteristic curves defined using the serum sFRß levels of RA patients before therapy had a higher predictive capacity than DAS28-CRP and CRP for the effective response of anti-TNF agents. The high serum sFRß levels with a cutoff value of 8 ng/mL were 100% specificity for the effective response of anti-TNF agents. The findings support that the serum sFRß levels in patients with RA act as a disease activation biomarker and that high serum sFRß levels act as a predictive biomarker for the response to anti-TNF agents.