Studies on regulation of IGF (insulin-like growth factor)-binding protein (IGFBP) 4 proteolysis by pregnancy-associated plasma protein-A (PAPP-A) in cells treated with phorbol ester

Arun S. Sivanandam, Subburaman Mohan, Hirohito Kita, Sanjay Kapur, Shin Tai Chen, Thomas A. Linkhart, Gyorgy Bagi, David J. Baylink, Xuezhong Qin

Research output: Contribution to journalArticle

8 Scopus citations

Abstract

PAPP-A (pregnancy-associated plasma protein-A) is produced by hSFs (human skin fibroblasts) and hOBs (human osteoblasts) and enhances the mitogenic activity of IGFs (insulin-like growth factors) by degradation of IGFBP-4 (insulin-like growth factor-binding protein 4). PKC (protein kinase C) activation in these cells led to reduction in IGFBP-4 proteolysis. This study was undertaken to determine the mechanism by which activation of PKC suppresses IGFBP-4 proteolysis. Treatment of hSFs/hOBs with TPA (PMA; 100 nM) reduced IGFBP-4 proteolysis without significantly decreasing the PAPP-A level in the CM (conditioned medium). Immunodepletion of the proform of eosinophil major basic protein (proMBP), a known PAPP-A inhibitor, from CM of TPA-treated cells (TPA CM) failed to increase IGFBP-4 proteolytic activity. Transduction of hSFs with proMBP retrovirus increased the concentration of proMBP up to 30 ng/ml and led to a moderate reduction in IGFBP-4 proteolysis. In contrast, TPA treatment blocked IGFBP-4 proteolysis but failed to induce a detectable amount of proMBP in the CM. While proMBP overexpression led to the formation of a covalent proMBP-PAPP-A complex and reduced the migration of PAPP-A on SDS/PAGE, TPA treatment dose- and time-dependently increased the conversion of a ≈ 470 kDa PAPP-A form (PAPP-A470) to a ≈ 400 kDa PAPP-A form (PAPP-A 400). Since unreduced PAPP-A400 co-migrated with the 400 kDa recombinant PAPP-A homodimer and since PAPP-A monomers from reduced PAPP-A470 and PAPP-A400 co-migrated on SDS/PAGE, conversion of PAPP-A470 to PAPP-A400 is unlikely to be caused by proteolytic cleavage of PAPP-A. Consistent with the data showing that the increase in the ratio of PAPP-A400/PAPP-A470 is correlated with the extent of reduction in IGFBP-4 proteolysis, partially purified PAPP-A400 exhibited a 4-fold reduction in IGFBP-4 proteolytic activity compared with PAPP-A470. These data suggest that a novel mechanism, namely conversion of PAPP-A470 to the less-active PAPP-A400, could account for the TPA-induced suppression of PAPP-A activity.

Original languageEnglish (US)
Pages (from-to)57-64
Number of pages8
JournalBiochemical Journal
Volume379
Issue number1
DOIs
StatePublished - Apr 1 2004

Keywords

  • Insulin-like growth factor (IGF)
  • Insulin-like growth factor-binding protein 4 (IGFBP-4)
  • Osteoblast
  • Post-translational modification
  • Pregnancy-associated plasma protein-A (PAPP-A)
  • Protease

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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