title: Pregnancy-associated plasma protein-A (PAPP-A) cleaves insulin-like growth factor binding protein (IGFBP)-5 independent of IGF: implications for the mechanism of IGFBP-4 proteolysis by PAPP-A.
abstract: Pregnancy-associated plasma protein-A (PAPP-A) has recently been identified as the proteinase responsible for cleavage of insulin-like growth factor binding protein (IGFBP)-4, an inhibitor of IGF action, in several biological fluids. Cleavage of IGFBP-4 by PAPP-A is believed to occur only in the presence of IGF. We here report that in addition to IGFBP-4, PAPP-A also cleaves IGFBP-5. Cleavage occurs at one site, between Ser-143 and Lys-144 of IGFBP-5. In the presence of IGF, IGFBP-4 and -5 are cleaved with similar rates by PAPP-A. Interestingly, cleavage of IGFBP-5 by PAPP-A does not require the presence of IGF, but is slightly inhibited by IGF. These findings have implications for the mechanism of proteolysis of IGFBP-4 by PAPP-A, suggesting that IGFBP-4 binds IGF, which then becomes a PAPP-A substrate. Using highly purified, recombinant proteins, we establish that (1) PAPP-A cleavage of IGFBP-4 can occur in the absence of IGF, although the rate of hydrolysis is very slow, and (2) IGF is unable to bind to PAPP-A. We thus conclude that IGF enhances proteolysis by binding to IGFBP-4, not by interaction with PAPP-A, which could not previously be ruled out.
authors.affiliation: Department of Molecular and Structural Biology, University of Aarhus, Science Park, Denmark.
authors.forename: L S
authors.initials: LS
authors.lastname: Laursen
authors.forename: M T
authors.initials: MT
authors.lastname: Overgaard
authors.forename: R
authors.initials: R
authors.lastname: Søe
authors.forename: H B
authors.initials: HB
authors.lastname: Boldt
authors.forename: L
authors.initials: L
authors.lastname: Sottrup-Jensen
authors.forename: L C
authors.initials: LC
authors.lastname: Giudice
authors.forename: C A
authors.initials: CA
authors.lastname: Conover
authors.forename: C
authors.initials: C
authors.lastname: Oxvig
mesh_terms: D000595:Amino Acid Sequence; D002460:Cell Line; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D006868:Hydrolysis; D018974:Insulin-Like Growth Factor Binding Protein 4 / Q000737:chemistry / Q000378:metabolism*; D018975:Insulin-Like Growth Factor Binding Protein 5 / Q000378:metabolism*; D011266:Pregnancy-Associated Plasma Protein-A / Q000378:metabolism*; D011485:Protein Binding; D013002:Somatomedins / Q000378:metabolism*
chemical_list: D018974:Insulin-Like Growth Factor Binding Protein 4; D018975:Insulin-Like Growth Factor Binding Protein 5; D013002:Somatomedins; D011266:Pregnancy-Associated Plasma Protein-A