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 IG FBP, presumed to be IGFBP-5. When GC-conditioned medium (GC-CM) was as sessed by Western Ligand Blotting (WLB), FSH appeared to decrease IGFB P-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 i ncubated with pure IGFBP-4 and IGFBP-5, the amount of IGFBP-5 (measura ble by WLB) was decreased. Similarly, when GC-CM from FSH-treated cell s was incubated with iodinated IGFBP-4 and IGFBP-5, IGFBP-5 (but not I GFBP-4) was proteolyzed into fragments of approximately 18 and 14 K. T he ability of FSH-treated GC-CM to proteolyze IGFBP-5 was reduced by t he addition of IGF-I to the rewtion mixture. When the IGFBPs in GC-CM were evaluated by affinity crosslinking, GC-CM from control cultures c ontained one band with an apparent Mr of approximately 34 K, whereas G C-CM from FSH-treated cultures displayed a decrease in the intensity o f the 34 K band, as well as a new band of approximately 24 K. These da ta suggest that rat GC cells produce an FSH-inducible IGFBP-5 protease activity, and reveal that the ability of this protease to cleave IGFB P-5 is blocked by IGFs.