Nitroblue tetrazolium reduction and bone resorption by osteoclasts in vitro inhibited by a manganese-based superoxide dismutase mimic

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Abstract
Oxygen-derived free radicals are produced by osteoclasts. Oxygen radical formation occurs at the osteoclast/bone surface interface. This location next to bone implies that oxygen radicals, including but not limited to superoxide, are needed for bone resorption. Compounds that scavenge superoxide are being developed as pharmaceutical agents to inhibit the damaging effects of oxygen radical formation on tissues. One such scavenger is the Desferal-manganese complex (DMnC). DMnC reduced the amount of formazan staining produced by the interaction of oxygen radicals with nitroblue tetrazolium (NBT) in both individual mouse calvarial osteoclasts in tissue explants and isolated osteoclasts. As a result of the reduced concentrations of oxygen radicals, DMnC inhibited bone resorption by calvarial explants and isolated osteoclasts. Superoxide dismutase (SOD) inhibited NBT reduction and bone resorption by isolated osteoclasts but to a lesser degree than DMnC. Inhibition of bone resorption in the isolated osteoclast system increased in parallel to the concentration of DMnC in cultures. Desferal without Mn had no effect on bone resorption by isolated osteoclasts. These results support the hypothesis that osteoclasts produce oxygen radicals as part of the process of bone resorption.