Selenium-induced Apoptosis of Osteoclasts by Mitochondrial Dysfunction and Caspase-3 Activation

The molecular mechanisms underlying selenium toxicity in bone-related cells are poorly understood. To date, the relation between selenium, which is an essential trace element in mammals, and bone metabolism remains unknown. Objectives: To gain some insights into the mechanisms underlying acute selenite-induced cytotoxicity in bone marrow-derived monocytes (BMMs), which are able to differentiate into bone-resorbing osteoclasts, and in mature osteoclasts. Methods: We injected mice intraperitoneally with selenite and applied selenite treatments to both osteoclasts and mitochondria isolated from osteoclasts. Results: Differentiation of the BMMs into osteoclasts was inhibited by selenite, in a dose-dependent fashion, in both the BMMs isolated from the selenite-exposed mice, and the BMMs that were directly treated with selenite. Moreover, selenite-induced apoptotic patterns in the osteoclast-forming cultures were detected by DAPI staining and DNA fragmentation. The selenite-treated osteoclasts evidenced an increase in generation of the superoxide anion, and a decrease of free thiol groups, suggesting that selenite is able to shift the cells, during apoptosis, to more oxidizing environments. In addition, selenite induced protein aggregates formed by thiol cross-linking, loss of mitochondrial membrane potential, and release of cytochrome C in isolated mitochondria, and the activation of caspase-3 in the osteoclasts. The selenite-induced DNA fragmentation was blocked by an inhibitor of mitochondrial permeability transition pores, cyclosporin A, or by a capase-3 inhibitor, DEVD-CHO. Conclusion: These findings suggest that selenite-induced apoptosis in osteoclast is mediated by mitochondrial dysfunction, caspase-3 activation, and, consequently, DNA fragmentation. This is the first report that selenite directly attacks BMMs and osteoclasts as a major target and induces apoptosis through a mitochondria-dependent signaling pathway and the activation of caspase-3. Supported by a grant 01-PJ5-PG1-01CH12-0002 from the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea (B.-M. M.) and by a grant 2000-2-20900-008-5 from the KOSEF (I.Y. K.).