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 [3H]folate in human ovary, serous ovarian carcinoma tissue, and ascites is a complex process that has not been well characterized. This study shows changes in binding affinity and mechanism of binding with decreasing receptor concentration, inhibition by folate derivatives, and 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. The gel filtration profile of Triton X-100-solubilized tissue and ascites contained two peaks of radioligand-bound receptor (25 and 100 kDa). Exposure of ascites to cleavage by phosphatidylinositol-specific phospholipase C resulted in a partial conversion of the 100-kDa peak to a 25-kDa peak. This suggests that the receptor may be anchored to the membrane by a glycosylphosphatidyl residue that inserts into Triton X-100 micelles, resulting in a large molecular size on gel filtration. The receptor in ovarian carcinoma tissue 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 radioligand-bound receptors exceed levels found in five normal ovaries. However, only receptors in ovarian carcinoma specimens showed a high degree of immunoreactivity. Hence, even without elevations of the total receptor level, a folate receptor isoform homologous to human milk folate receptor protein seemed to prevail in serous ovarian carcinomas.
Holm
J
J
Department of Clinical Chemistry, Central Hospital, Herning, Denmark.
Hansen
S I
SI
Høier-Madsen
M
M
Birn
H
H
Helkjaer
P E
PE
D001201:Ascites / Q000378:metabolism* / Q000473:pathology; D001665:Binding Sites / Q000187:drug effects; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D018284:Cystadenocarcinoma, Serous / Q000737:chemistry / Q000378:metabolism* / Q000473:pathology; D004591:Electrophoresis, Polyacrylamide Gel; D004797:Enzyme-Linked Immunosorbent Assay; D005260:Female; D058973:Folate Receptors, GPI-Anchored; D005492:Folic Acid / Q000031:analogs & derivatives / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D015151:Immunoblotting; D007477:Ions; D008970:Molecular Weight; D010051:Ovarian Neoplasms / Q000737:chemistry / Q000378:metabolism* / Q000473:pathology; D010053:Ovary / Q000737:chemistry / Q000378:metabolism*; D043265:Phosphatidylinositol Diacylglycerol-Lyase; D054801:Phosphoinositide Phospholipase C; D017434:Protein Structure, Tertiary; D011869:Radioligand Assay; D011956:Receptors, Cell Surface / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010738:Type C Phospholipases / Q000494:pharmacology
D002352:Carrier Proteins; D058973:Folate Receptors, GPI-Anchored; D007477:Ions; D011956:Receptors, Cell Surface; D005492:Folic Acid; D010738:Type C Phospholipases; D054801:Phosphoinositide Phospholipase C; D043265:Phosphatidylinositol Diacylglycerol-Lyase