Activated protein C stimulates osteoblast proliferation via endothelial protein C receptor

Bone is continually remodeled by the action of osteoblasts, osteocytes, and osteoclasts. Resting osteoblasts are able to proliferate and differentiate into mature osteoblasts when physiologically required, as after tissue injury. Activated protein C (APC) is a serine protease that functions in anticoagulation, anti-inflammation, anti-apoptosis, cell proliferation, and wound repair. In this study, we examined the effect of APC on osteoblast proliferation and differentiation.

We examined the presence of protein C in human fracture hematoma by immunohistochemical staining. We then evaluated the effect of APC, diisopropyl fluorophosphate-inactivated APC (DIP-APC) or protein C zymogen on normal human osteoblast (NHOst) proliferation using tetrazolium salt assay in the presence or absence of aprotinin, hirudin, protein C, antibody against protein C, endothelial protein C receptor (EPCR) or protease-activated receptor (PAR)-1. Finally, activation of p44/42 MAP kinase was evaluated by Western blot analysis.

Both APC and DIP-APC increased osteoblast proliferation in a dose-dependent manner, while protein C did not. The APC-induced increased proliferation of osteoblast was not affected by aprotinin, hirudin, and anti-protein C antibody which inhibits the protease activity of APC. Treatment with protein C or anti-EPCR antibody which inhibits APC binding to EPCR inhibited APC-mediated osteoblast proliferation, while treatment with anti-PAR-1 antibody did not. APC promoted the phosphorylation of p44/42 MAP kinase within osteoblasts; this effect was inhibited by the anti-EPCR antibody.

APC stimulates osteoblast proliferation by activating p44/42 MAP kinase through a mechanism that requires EPCR but not PAR-1 or the proteolytic activity of APC. APC generated at fracture sites may contribute to fracture healing by promoting osteoblast proliferation.