PLASMINOGEN BINDS THE HEPARIN-BINDING DOMAIN OF INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN-3

Limited proteolysis lowers affinity of insulin-like growth factor (IGF )-binding protein (IGFBP)-3 for bound IGFs, resulting in greater IGF b ioavailability. Plasmin is one of many proteases that cleave IGFBP-3, and the plasmin system may regulate IGFBP-3 proteolysis and IGF bioava ilability in cultured cells in vitro. A role for the plasmin system in IGFBP-3 proteolysis in vivo is suggested by data presented here showi ng that IGFBP-3 binds plasminogen (Pg; Glu-Pg) with a dissociation con stant (K-d) ranging from 1.43 to 3.12 nM. IGF-I and Glu-Pg do not comp ete for IGFBP-3 binding; instead, the binary IGFBP-3/Glu-Pg complex bi nds IGF-I with high affinity (K-d = 0.47 nM) to form a ternary complex . Competitive binding studies suggest that the kringle 1, 4, and 5 dom ains of Glu-Pg and the heparin-binding domain of IGFBP-3 participate i n forming the IGFBP-3/Glu-Pg complex, and other studies show that Glu- Pg in this complex is activated at a normal rate by tissue Pg activato r. Importantly, IGFBP-3/Glu-Pg complexes were detected in both human c itrate plasma and serum, indicating that these complexes exist in vivo . Binding of IGFBP-3 to Glu-Pg in vivo suggests how Glu-Pg activation can specifically lead to IGFBP-3 proteolysis with subsequent release o f IGFs to local target tissues.