Pharmacological characterization of soluble human FSH receptor extracellular domain: facilitated secretion by coexpression with FSH.

Follicle-stimulating hormone (FSH) is a member of the glycoprotein hormone family that regulates gametogenesis and steroidogenesis. Glycoprotein hormones signal through a unique class of G-protein-coupled receptors (GPCRs) that have a long extracellular domain (ECD), which is the primary site for hormone binding. The hFSHR-ECD was expressed in insect cells as a C-terminal, epitope-tagged protein resulting in production of soluble active receptor in the intracellular compartment and in the secreted culture medium. Coexpression of hFSHR-ECD with FSHbeta or FSHalpha/beta increased the secretion of the truncated receptor. Pharmacological studies to assess ligand-receptor interactions without the transmembrane domains showed higher affinity values (K(D)S) for [125I]hFSH using mammalian-expressed full-length receptor, secreted hFSHR-ECD, or secreted hFSHR-ECD coexpressed with FSHbeta, whereas the K(D) value for hFSHR-ECD coexpressed with FSHalpha/beta subunits showed lower affinity. Competition of other glycoprotein hormones for secreted hFSHR-ECD coexpressed with FSHbeta or mammalian full-length hFSHR resulted in similar binding profiles, indicating analogous pharmacology. Finally, we have demonstrated that a small molecule, suramin, which has been reported to interact with the mammalian full-length FSHR, competes for the binding of [125I]hFSH by interacting directly at the hFSHR-ECD.