A new superoxide-generating oxidase in murine osteoclasts

Superoxide production contributes to osteoclastic bone resorption. Evidence strongly indicates that NADPH oxidase is an enzyme system responsible for superoxide generation in osteoclasts. A membrane-bound subunit, p91, is the catalytic domain of NADPH oxidase. However, osteoclasts from p91 knockout mice still produce superoxide at a rate similar to that observed in wild type mice. This unexpected phenomenon prompted us to examine the osteoclasts for an alternative to the p91-containing oxidase. In this study, the cloning of a NADPH oxidase subunit (Nox 4) with 578 amino acids is reported. Nox 4 has 58% similarity in amino acids with the known p91 subunit of NADPH oxidase. Nox 4 is present and active in osteoclasts. Antisense oligonucleotides of Nox 4 reduced osteoclastic superoxide generation as well as resorption pit formation by osteoclasts. This new oxidase complex was present and functional in osteoclasts from p91 knockout mice, explaining the normal resorptive activity seen in the osteoclasts where no p91 is present.