Involvement of Pregnancy-Associated Plasma Protein-A2 in Insulin-Like Growth Factor (IGF) Binding Protein-5 Proteolysis during Pregnancy: A Potential Mechanism for Increasing IGF Bioavailability

Abstract

During pregnancy, circulating IGF binding protein-5 (IGFBP-5) undergoes substantial molecular redistribution from ternary complexes to either binary complexes or the uncomplexed protein. This study aimed to characterize the proteolysis of circulating IGFBP-5 during pregnancy and to determine whether it can increase IGF bioavailability. Biochemical methods were used to purify and characterize IGFBP-5 fragments and IGFBP-5-specific proteolytic activity from pregnancy plasma. Circulating IGFBP-5 was fully proteolyzed at all stages of pregnancy. Cleavage after either Ser143 or Lys144 resulted in two complementary fragments. Of two pools of proteolytic activity (>150 kDa and approximately 40 kDa) identified in pregnancy plasma, only the greater than 150-kDa proteolytic activity was specific to pregnancy. The approximately 40-kDa proteolytic activity, also present in nonpregnancy plasma, appeared largely inactive against IGF-I-complexed IGFBP-5. The greater than 150-kDa proteolytic activity was inhibited by alpha-PAPP-A2 but not alpha-PAPP-A1 antibody, cleaved recombinant IGFBP-5 at Ser143-Lys144 similar to PAPP-A2, and was inactive against IGFBP-5 (Ala128), a PAPP-A2-resistant analog. Compared to nonpregnancy plasma, incubation with pregnancy plasma resulted in release of more bioactive IGF-I from IGF-I-IGFBP-5 complexes as measured by stimulation of IGF-I receptor phosphorylation. Circulating IGFBP-5 is proteolyzed by PAPP-A2 during pregnancy, resulting in increased IGF bioavailability, which may have important consequences for the development of the fetus and/or the well-being of the mother.