GROWTH-HORMONE, BUT NOT INSULIN-LIKE GROWTH FACTOR-I, INDUCES A SERUMPROTEASE ACTIVITY FOR INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-3 INHYPOPHYSECTOMIZED RATS IN-VIVO

Insulin-like growth factor binding proteins (IGFBPs) modulate IGF acti on. Proteolytic cleavage of IGFBPs yields lower molecular forms with r educed ability to bind IGFs, thereby increasing IGF bioavailability. I n serum from normal adult rats, we found a proteolytic activity for IG FBP-3, presumably a cation-dependent serine protease. It is lacking in serum from hypophysectomized rats and restored by infusion of growth hormone (GH), but not IGF I. Thus, IGF I does not appear to mediate th e GH effect on IGFBP-3 proteolysis. Rather, GH seems to modulate IGF a ction indirectly via alteration of IGFBP-3 structure.