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, c ontains insulin-like growth factor binding protein-4 (IGFBP-4) proteol ytic activity. The effect of follicle stimulating hormone (FSH) and in sulin-like growth factors (IGFs) on IGFBP-4 proteolytic activity was s tudied using granulosa cell cultures. Proteolytic activity was assesse d by the incubation of [I-125]-IGFBP-4 with medium and cleaved product s which were analysed by autoradiography. The iodinated IGFBP-4 was cl eaved into an 18 kDa fragment when cells were incubated with FSH or IG Fs while IGFBP-4 remained intact in the control culture. Inhibition of IGFBP-4 degradation by several protease inhibitors suggests that IGFB P-4 degradation was induced by a metalloserine protease. The degradati on of IGFBP-4 was not stimulated when cells were incubated with the IG F-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 proteolysi s of [I-125]-IGFBP-4. Similarly. exogenously added covalently cross-li nked [I-125]-IGF-II-IGFBP-4 complexes were proteolyzed; however, IGFs did not enhance the degradation of these complexes. These results sugg est that human granulosa cells produce IGFBP-4 protease and that the i ncreased susceptibility of IGFBP-4 to proteolysis is induced by the bi nding of IGFs to IGFBP-4. These novel mechanisms may be important in m odulating IGF-mediated folliculogenesis in the ovary.