Insulin-like growth factor family in Graafian follicle development and function.

The insulin-like growth factor (IGF) family plays an important role in follicle development, dominant follicle growth and steroidogenesis, and follicular atresia. Insulin-like growth factor (IGF)-II is the primary IGF in human ovary, acting as a mediator of gonadotropin action. IGF-II stimulates granulosa steroidogenesis, and its actions, along with those of follicle-stimulating hormone (FSH), are inhibited by IGF binding protein (IGFBP)-4. At the time of follicle selection, in the estrogen-dominant follicle, granulosa IGF-II synthesis increases dramatically. Simultaneously, in the selected follicle, IGFBP-4, an inhibitor of IGF-II action and abundant within androgen-dominant follicles, is proteolyzed by a specific IGFBP-4 protease, resulting in decreased affinity of IGFBP-4 for IGF-II. The IGFBP-4 protease has recently been identified in human ovary as pregnancy-associated plasma protein-A (PAPP-A). In the Graafian follicle, IGF-II bioavailability is increased to act as a mediator of FSH action, by increased synthesis of this peptide and concomitant decrease in an inhibitor of its actions by proteolysis of IGFBP-4.