Insulin-like growth factors (IGFs) block FSH-induced proteolysis of IGF-binding protein-5 (BP-5) in cultured rat granulosa cells

Rat granulosa cells (GC), in vitro, express IGFBPs under the influence of both FSH and IGF-I. The major IGFBP produced by GC is a 28-29 K IGFBP, presumed to be IGFBP-5. When GC-conditioned medium (GC-CM) was assessed by Western Ligand Blotting (WLB), FSH appeared to decrease IGFBP-5, whereas IGF-I appeared to increase IGFBP-5 and to partially block the effects of FSH treatment. When GC-CM from FSH-treated cells was incubated with pure IGFBP-4 and IGFBP-5, the amount of IGFBP-5 (measurable by WLB) was decreased. Similarly, when GC-CM from FSH-treated cells was incubated with iodinated IGFBP-4 and IGFBP-5, IGFBP-5 (but not IGFBP-4) was proteolyzed into fragments of approximately 18 and 14 K. The ability of FSH-treated GC-CM to proteolyze IGFBP-5 was reduced by the addition of IGF-I to the reaction mixture. When the IGFBPs in GC-CM were evaluated by affinity crosslinking, GC-CM from control cultures contained one band with an apparent Mr of approximately 34 K, whereas GC-CM from FSH-treated cultures displayed a decrease in the intensity of the 34 K band, as well as a new band of approximately 24 K. These data suggest that rat GC cells produce an FSH-inducible IGFBP-5 protease activity, and reveal that the ability of this protease to cleave IGFBP-5 is blocked by IGFs.