Identification of gonadotrophin surge-inhibiting factor (GnSIF)/attenuin in human follicular fluid

Evidence from several laboratories suggests that the ovaries of many species produce a non-steroidal factor called gonadotrophin surge-inhibiting or attenuating factor (GnSIF) which may regulate the response of the pituitary to gonadotrophin-releasing hormone (GnRH) and as such, may modulate the timing and/or amplitude of the luteinizing hormone (LH) surge. We have recently isolated a candidate GnSIF from porcine follicular fluid (PFF). Porcine GnSiF is a 69 kDa protein which has undetectable inhibin and follistatin immunological and biological activity. The present study was designed to purify and identify GnSIF from human follicular fluid. GnSIF activity was measured as suppression of GnRH-stimulated LH secretion from rat pituitary cells in primary culture. Human follicular fluid (approximately 500 ml) was recovered from patients undergoing in-vitro fertilization (IVF). GnSIF was purified by heparin-sepharose, Q-sepharose, S-sepharose, and hydrophobic interaction chromatography followed by isoelectric focusing. Gel electrophoresis and Western blot were used to identify human GnSIF and compare it with porcine GnSIF. Using these steps, we obtained a highly-purified preparation of GnSIF that manifests in-vitro bioactivity and chromatography characteristics similar to those observed for porcine GnSIF and that hybridizes with a porcine GnSIF antibody. Following treatment with human chorionic gonadotrophin/human menopausal gonadotrophin (HMG/HCG), human follicular fluid contained roughly 25% of the GnSIF (per mg protein) present in porcine follicular fluid. We conclude that GnSIF is present in human follicular fluid and may participate in the regulation of gonadotrophin secretion in this species.