[Role of free radicals in bone resorption]

In order to examine the role of the oxygen-derived free radicals in bone metabolism, the mouse calvaria organ culture system and the mouse bone marrow culture system were used. The activity of bone resorption stimulated by 1 alpha, 25(OH)2 vitamin D3 was inhibited by the addition of catalase, an eliminating agent of H2O2. Catalase also significantly suppressed the osteoclast-like cell formation induced by 1 alpha, 25(OH)2 vitamin D3 in a dose-dependent manner, whereas the agent had no effect on the mitogenic activity of the bone marrow-derived stromal cells. This suppression was reversed by adding H2O2 to the system. The effect of catalase was limited to the early stage precursor of the osteoclast-like cells. Moreover, when superoxide dismutase (SOD), which activates the production of H2O2, was added to the bone marrow culture, the number of osteoclast-like cells increased significantly in a dose-dependent manner. Finally, the source of the oxygen-derived free radicals was examined by measuring the fluorescence of 2.7-dichlorofluorescein (DCF), the obtained result indicated that the osteoclast-like cells could produce almost the equivalent amount of H2O2 as could the macrophages. These results strongly suggest that the oxygen-derived free radicals, particularly H2O2, may regulate bone resorption, promoting the differentiation of osteoclast precursor cells to osteoclasts.