Inhibitory effects of beta-tricalciumphosphate wear particles on osteocytes via apoptotic response and Akt inactivation

Wear debris-induced osteolysis, a major contributing factor of orthopedic implant aseptic loosening, affects long-term survival of orthopedic prostheses following joint replacement and revision surgery. Pathogenic effects of wear debris on various cell types including macrophages/monocytes, osteoblasts, and osteoclasts have been well studied. However, the interactions between wear debris particles and osteocytes, which make up over 90% of all bone cells, have not been clearly illustrated. Here, we explored the biological effects of endotoxin-free beta-tricalciumphosphate (beta-TCP) wear particles with the average diameter of 1.997 mu m (range 1.3-3.2 mu m) on osteocytes in vitro. Our results showed that 24 h or 48 h incubation of beta-TCP particles dose-dependently inhibited cell viability of osteocytes MLO-Y4. Alternatively, beta-TCP particles treatment for 24 h significantly increased the osteocytic marker SOST/sclerostin mRNA expression and the release of inflammatory cytokines including TNF-alpha and IL-1 beta into the culture media, but decreased the mRNA expression of another osteocytic marker dentin matrix protein-1 (DMP-1). Furthermore, these osteocytes dysfunctions were accompanied by F-actin disassembly, cell apoptosis, sustained enhancement of intracellular reactive oxygen species (ROS) and mitochondrial injury upon beta-TCP particles stimulation. In addition, beta-TCP particles also caused Akt inactivation at Ser473 resides with a dose- and time-dependent pattern. Taken together, beta-TCP wear particles could cause osteocytes dysfunctions, which may be mediated by apoptotic death and Akt inactivation in MLO-Y4 cells. These findings strongly suggest that osteocytes may play an important role in the beta-TCP wear particles-induced osteolysis, and provide valuable insights for understanding the molecular mechanisms of osteocytes death involved in tissue damage during bone cement and intolerance of cemented prostheses. (C) 2012 Elsevier Ireland Ltd. All rights reserved.