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.
BACKGROUND
During pregnancy, circulating IGF binding protein-5 (IGFBP-5) undergoes substantial molecular redistribution from ternary complexes to either binary complexes or the uncomplexed protein.


OBJECTIVE
This study aimed to characterize the proteolysis of circulating IGFBP-5 during pregnancy and to determine whether it can increase IGF bioavailability.


METHODS
Biochemical methods were used to purify and characterize IGFBP-5 fragments and IGFBP-5-specific proteolytic activity from pregnancy plasma.


RESULTS
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.


CONCLUSIONS
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.
Yan
Xiaolang
X
Kolling Institute of Medical Research, Royal North Shore Hospital, St. Leonards, New South Wales 2065, Australia. sfirth@med.usyd.edu.au
Baxter
Robert C
RC
Firth
Sue M
SM
D015153:Blotting, Western; D002850:Chromatography, Gel; D005260:Female; D006801:Humans; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000378:metabolism*; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000494:pharmacology; D010446:Peptide Fragments / Q000378:metabolism*; D010766:Phosphorylation; D011247:Pregnancy; D011266:Pregnancy-Associated Plasma Protein-A / Q000378:metabolism*; D011485:Protein Binding; D017526:Receptor, IGF Type 1 / Q000378:metabolism
D018975:Insulin-Like Growth Factor Binding Protein 5; D010446:Peptide Fragments; D007334:Insulin-Like Growth Factor I; D017526:Receptor, IGF Type 1; C430889:PAPPA2 protein, human; D011266:Pregnancy-Associated Plasma Protein-A