Pregnancy-associated plasma protein-A accounts for the insulin-like growthfactor (IGF)-binding protein-4 (IGFBP-4) proteolytic activity in human pregnancy serum and enhances the mitogeneic activity of IGF by degrading IGPBP-4 in vitro

Pregnancy-associated plasma protein-A (PAPP-A) has been identified as the i nsulin-like growth factor (IGF)-dependent IGF-binding protein-4 (IGFBP-4) p rotease produced by human fibroblasts. Recently, we found that serum protea ses induced during human pregnancy cleaved IGFBP-4 in both an IGF-II-depend ent and an IGF-II-independent fashion. This study sought to determine wheth er PAPP-A is the predominant IGFBP-I, protease in human pregnancy serum (PS ) and to assess the in vitro role of serum PAPP-A. Immunoprecipitation with PAPP-A antibody effectively depleted PAPP-A from the PS and completely abo lished both IGF-LI-dependent and IGF-II-independent IGFBP-4 proteolytic act ivity in PS. Direct addition of PAPP-A antibody to PS completely blocked IG FBP-4 proteolysis and partially blocked IGFBP-5 proteolysis, but had no eff ect on IGFBP-3 proteolysis. To evaluate the role of serum PAPP-A, we tested whether PAPP-A in PS modulated the inhibitory activity of IGFBP-4 on IGF-I I-induced cell proliferation in human osteosarcoma MG63 cells. The wild-typ e IGFBP-4 (WTBP-4; 200 ng/mL) failed to inhibit proliferation of the cells treated with PS (0.1% or 0.3%) alone or in combination with IGF-II (40 ng/m L), whereas the inhibitory effect of WTBP-4 was observed in the cells treat ed with nonpregnancy serum alone or in combination with IGF-II (P < 0.05). In contrast to WTBP-4, a protease-resistant IGFBP-4 was able to inhibit pro liferation of the cells treated with PS alone or in combination with IGF-II (P < 0.05). In the presence of PAPP-A neutralizing antibody, the inhibitor y effect of WTBP-4 on proliferation of the cells treated with IGF-II and PS was restored. In summary, these data demonstrate 1) that PAPP-A represents the predominant IGFBP-4 protease in PS; 2) that PAPP-A may in part contrib ute to IGFBP-5, but not IGFBP-3, proteolytic activity in PS; and 3) that PA PP-A enhances the bioactivity of IGFs in vitro by degrading IGFBP-4.