Plasmin degradation of insulin-like growth factor binding protein-5 (IGFBP-5): regulation by IGFBP-5-(201-218)

Using the major bone insulin-like growth factor-binding protein (IGFBP) IGFBP-5, we took a mechanistic approach in evaluating the role of the heparin-binding domain of IGFBP-5 in regulating plasmin (Pm) proteolysis of IGFBP-5. Using synthetic IGFBP-5 peptide fragments, we determined that the heparin-binding domain, IGFBP-5-(208-218), inhibits Pm proteolysis of intact IGFBP-5. The mechanism of action of IGFBP-5-(201-218) was by inhibiting Pm binding to substrate IGFBP-5. IGFBP-5-(201-218) action was independent of site of proteolysis, fluid, or solid phase interaction. In addition, IGFBP-5-(201-218) was found to inhibit plasminogen (Pg) activation to Pm. IGFBP-5-(201-218) did not directly inhibit the activity of Pm, urokinase Pg activator (PA), or tissue-type PA but acted as a competitive inhibitor of Pg activation by PA, which is in contrast to the stimulating effect of heparin on Pg activation. These data indicate that the heparin-binding domain contains the serine protease (Pg-to-Pm) binding site region of IGFBP-5, and that this region, which is presumed to represent a Pm-induced proteolytic product of IGFBP-5, is capable of regulating Pm action.