Effect of follicle stimulating hormone and insulin-like growth factors on proteolysis of insulin-like growth factor binding protein-4 in human granulosa cells

Follicular fluid from dominant follicles, but not atretic follicles, contains insulin-like growth factor binding protein-4 (IGFBP-4) proteolytic activity. The effect of follicle stimulating hormone (FSH) and insulin-like growth factors (IGFs) on IGFBP-4 proteolytic activity was studied using granulosa cell cultures. Proteolytic activity was assessed by the incubation of [I-125]-IGFBP-4 with medium and cleaved products which were analysed by autoradiography. The iodinated IGFBP-4 was cleaved into an 18 kDa fragment when cells were incubated with FSH or IGFs while IGFBP-4 remained intact in the control culture. Inhibition of IGFBP-4 degradation by several protease inhibitors suggests that IGFBP-4 degradation was induced by a metalloserine protease. The degradation of IGFBP-4 was not stimulated when cells were incubated with the IGF-I analogue, LR3-IGF-I, and insulin which binds to IGF-I receptor but has little or no affinity for IGFBPs. Addition of IGFs, but not FSH, to medium from untreated granulosa cell cultures stimulated proteolysis of [I-125]-IGFBP-4. Similarly. exogenously added covalently cross-linked [I-125]-IGF-II-IGFBP-4 complexes were proteolyzed; however, IGFs did not enhance the degradation of these complexes. These results suggest that human granulosa cells produce IGFBP-4 protease and that the increased susceptibility of IGFBP-4 to proteolysis is induced by the binding of IGFs to IGFBP-4. These novel mechanisms may be important in modulating IGF-mediated folliculogenesis in the ovary.