High-affinity folate receptor in human ovary, serous ovarian adenocarcinoma, and ascites: Radioligand binding mechanism, molecular size, ionic properties, hydrophobic domain, and immunoreactivity

High-affinity folate receptors are expressed in normal ovaries and ovarian carcinomas. Binding of [H-3]folate in human ovary, serous ovarian carcinoma tissue, and ascites is a complex process that has not been well characteri zed. This study shows changes in binding affinity and mechanism of binding with decreasing receptor concentration, inhibition by folate derivatives, a nd a slow radioligand dissociation at pH 7.4 becoming rapid and complete at pH 3.5, The receptor seems to be positively charged since it elutes in the front effluent of a DEAE-Sepharose CL-6B ion-exchange column at pH 6.3, Th e gel filtration profile of Triton X-100-solubilized tissue and ascites con tained two peaks of radioligand-bound receptor (25 and 100 kDa), Exposure o f ascites to cleavage by phosphati-dylinositol-specific phospholipase C res ulted in a partial conversion of the 100-kDa peak to a 25-kDa peak. This su ggests that the receptor may be anchored to the membrane by a glycosylphosp hatidyl residue that inserts into Triton X-100 micelles, resulting in a lar ge molecular size on gel filtration. The receptor in ovarian carcinoma tiss ue immunoreacts with antibodies against purified human milk folate receptor protein as shown by enzyme-linked immunosorbent assay, immunoprecipitation , sodium dodecyl sulfate-polyacrylamide gel electrophoresis immunoblotting (a single band of 45 kDa), and immunohistochemistry. In only three of seven ovarian carcinomas did expression of radio-ligand-bound receptors exceed l evels found in five normal ovaries, However, only receptors in ovarian carc inoma specimens showed a high degree of immunoreactivity, Hence, even witho ut elevations of the total receptor level, a folate receptor isoform homolo gous to human milk folate receptor protein seemed to prevail in serous ovar ian carcinomas. (C) 1999 Academic Press.