The role of cyclooxygenase-2 (COX-2) in inflammatory bone resorption

Prostaglandin E2 (PGE(2)) is an important inflammatory mediator that plays an essential role in the development and progression of periradicular diseases. Cyclooxygenase-2 (COX-2) is the inducible enzyme responsible for increased PGE(2) levels during inflammation and other pathologic processes. The purpose of this study was to determine the role of COX-2-mediated PGE(2) synthesis in osteoclast formation in response to endodontic pathogens and materials. Primary osteoblast cultures and osteoclast cultures were prepared from COX-2 knockout (K/O) and wild-type (WT) littermates. These cultured cells were exposed to lipopolysaccharide (LPS) or root canal obturation materials including gutta-percha (GP), Resilon (RS), mineral trioxide aggregates (MTAs), and AH Plus (AH+). Osteoclast formation was evaluated using tartrate-reststant acid phosphatase (TRAP) staining. The expression of receptor activator of NF-kappa B ligand (RANKL) and osteoprotegerin (OPG) was determined by real-time polymerase chain reaction (PCR) analysis. It was found that in both WT and K/O cultures, treatment with LPS led to a marked increase in osteoclast formation. The number of osteoclasts formed was significantly lower in K/O cultures compared to WT cultures. Exposure to endodontic materials did not lead to any significant osteoclast formation. LPS and endodontic materials caused a decrease in both RANKL and OPG expression in WT cells. In K/O cells, the baseline levels of RANKL and OPG expression were dramatically decreased compared to the WT cells. In conclusion, COX-2-mediated PGE(2) expression is required for LPS-induced inflammatory bone resorption and maintaining the baseline level of RANKL and OPG expression. LPS-induced osteoclast formation may be independent of the RANKL pathway.