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 hormo nes signal through a unique class of G-protein-coupled receptors (GPCRs) th at have a long extracellular domain (ECD), which is the primary site for ho rmone 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. Coexpr ession of hFSHR-ECD with FSH beta or FSH alpha/beta increased the secretion of the truncated receptor. Pharmacological studies to assess ligand-recept or interactions without the transmembrane domains showed higher affinity va lues (K(D)s) for ([125I])hFSH using mammalian-expressed full-length recepto r, secreted hFSHR-ECD, or secreted hFSHR-ECD coexpressed with FSH beta, whe reas the K-D value for hFSHR-ECD coexpressed with FSH alpha/beta subunits s howed lower affinity. Competition of other glycoprotein hormones for secret ed hFSHR-ECD coexpressed with FSH beta or mammalian full-length hFSHR resul ted in similar binding profiles, indicating analogous pharmacology. Finally , we have demonstrated that a small molecule, suramin, which has been repor ted to interact with the mammalian full-length FSHR, competes for the bindi ng of ([125I])hFSH by interacting directly at the hFSHR-ECD.