PROTEOLYTIC DEGRADATION OF INSULIN-LIKE GROWTH-FACTOR (IGF)-BINDING PROTEIN-3 BY PORCINE OVARIAN GRANULOSA-CELLS IN CULTURE - REGULATION BYIGF-I

Porcine ovarian granulosa cells in culture secrete glycosylated insuli n-like growth factor (IGF)-binding protein-3 (IGFBP-3), which inhibits gonadotropin and IGF action in the ovary. Synthesis of IGFBP-3 is sti mulated by IGF-I and attenuated by gonadotropin. The purpose of the pr esent study was to determine whether IGFBP-3 levels were also regulate d via proteolysis. Exogenously added nonglycosylated recombinant human IGFBP-3 (rhIGFBP-3) was significantly degraded over time by a soluble serine-specific protease, similar to plasmin, in control cultures and those treated with FSH, insulin, or several other classes of hormones . In contrast, degradation was greatly attenuated by the IGFs. Degrade d rhIGFBP-3 exhibited much reduced affinity for [I-125]IGF-II, suggest ing that degradation could make available IGFs for cellular interactio n. The mechanism of IGFBP-3 protease inhibition by IGFs is unclear. Me diation by IGF receptors is unlikely, as insulin at a dose that activa ted both insulin and type I IGF receptors did not alter intrinsic degr adation of IGFBP-3 (as does IGF). Additionally, IGF-I attenuation of I GFBP-3 degradation was not inhibited by antagonism of receptor action with a tyrosine kinase inhibitor. Further, IGF-I inhibited degradation in cell-free conditioned medium. Direct stabilization of IGFBP-3 via binding of IGFs was suggested from these results. However, long R(3) I GF-I attenuated IGFBP-3 degradation even though it has low affinity fo r IGFBPs. Inhibition of the protease by IGFs is also possible. We conc lude that IGFs inhibit the degradation of exogenous nonglycosylated rh IGFBP-3. If active in vivo, this may serve to increase endogenous IGFB P-3 levels in follicular fluid.