The insulin-like growth factor (IGF)-dependent IGF binding protein-4 protease secreted by human fibroblasts is pregnancy-associated plasma protein-A

TLDR

Proteolytic cleavage of IGFBPs, especially IGFBP‑4, rapidly modifies their structure and function, and IGFBP‑4 proteolysis removes its inhibitory effect on IGF signaling, acting as a positive regulator of IGF bioavailability. The study aimed to isolate and identify the IGF‑dependent, IGFBP‑4‑specific protease secreted by human fibroblasts. The protease was identified as pregnancy‑associated plasma protein‑A (PAPP‑A), whose antibodies inhibited its activity, whose purified form from pregnancy sera retained IGF‑dependent IGFBP‑4 protease activity, and whose mRNA and protein are expressed and secreted by fibroblasts and osteoblasts, linking PAPP‑A to IGF regulation.

Abstract

Proteolytic cleavage of the six known insulin-like growth factor binding proteins (IGFBPs) is a powerful means of rapid structure and function modification of these important growth-regulatory proteins. Intact IGFBP-4 is a potent inhibitor of IGF action in vitro, and cleavage of IGFBP-4 has been shown to abolish its ability to inhibit IGF stimulatory effects in a variety of systems, suggesting that IGFBP-4 proteolysis acts as a positive regulator of IGF bioavailability. Here we report the isolation of an IGF-dependent IGFBP-4-specific protease from human fibroblast-conditioned media and its identification by mass spectrometry microsequencing as pregnancy-associated plasma protein-A (PAPP-A), a protein of unknown function found in high concentrations in the maternal circulation during pregnancy. Antibodies raised against PAPP-A both inhibited and immunodepleted IGFBP-4 protease activity in human fibroblast-conditioned media. Moreover, PAPP-A purified from pregnancy sera had IGF-dependent IGFBP-4 protease activity. PAPP-A mRNA was expressed by the human fibroblasts and osteoblasts, and PAPP-A protein was secreted into the culture medium. In conclusion, we have identified an IGF-dependent IGFBP protease and at the same time assigned a function to PAPP-A. This represents an unanticipated union of two areas of research that were not linked in any way before this report.