Possible involvement of elastase in enhanced osteoclast differentiation by neutrophils through degradation of osteoprotegerin

Abstract Neutrophils are one of the most abundant leukocytes in the sites of lesion of inflammatory diseases such as periodontitis and rheumatoid arthritis. These diseases are accompanied by bone loss, which worsens the quality of life of the patients. However, the role of neutrophils in the inflammatory bone loss has not been fully investigated. In the present study, we found that human neutrophils enhanced osteoclast differentiation from mouse bone marrow cells co-cultured with mouse osteoblasts in the presence of active vitamin D3. The enhanced osteoclast differentiation was significantly suppressed by elastatinal, a synthetic inhibitor of neutrophil elastase. Also, we found that human neutrophils degraded human recombinant osteoprotegerin (OPG), a decoy receptor for nuclear factor κB (RANK) ligand (RANKL), the essential osteoclast differentiation-inducing factor, expressed by osteoblasts. Degradation of OPG by neutrophils was suppressed by human α1-protease inhibitor, the major endogenous inhibitor of neutrophil elastase. Recombinant human neutrophil elastase degraded human OPG in its death domain-like region. These results indicated that the degradation of OPG by elastase contributed at least in part to the enhanced osteoclast differentiation by neutrophils. There is a possibility that neutrophils play an important role in inflammatory bone loss.