Effect of vitamin K-2 on osteoblast apoptosis: Vitamin K-2 inhibits apoptotic cell death of human osteoblasts induced by Fas, proteasome inhibitor, etoposide, and staurosporine

Vitamin K-2 is used for the treatment of osteoporosis, but the precise mode of action is still not clear. We investigated the effects of vitamin K-2 o n apoptosis of human osteoblasts. Human osteoblastic cell line MG63 cells a nd human primary osteoblast-like cells obtained from bone fragments in corr ective surgery were used as human osteoblasts. Cells were cultured with or without various concentrations of vitamin K-2 and tumor necrosis factor-alp ha (TNF-alpha). We then determined the proliferative response, expression o f Fas and Bcl-2-related proteins, and Fas-mediated apoptosis of these cells induced by anti-Fas immunoglobulin M (IgM). In addition, the effect of vit amin K-2 in osteoblast apoptosis induced by Z-Leu-Leu-Leu-aldehyde (LLL-CHO ), etoposide, or staurosporine was also examined. Human osteoblasts did not show spontaneous apoptosis in culture, even in the presence of vitamin K-2 or TNF-alpha. Furthermore, proliferation of the cells was not influenced b y vitamin K-2 or TNF-alpha. Fas was functionally expressed on human osteobl asts, and the treatment with TNF-alpha significantly enhanced both Fas expr ession and Fas-mediated apoptosis of osteoblasts. The addition of vitamin K -2 to the culture resulted in a dose-dependent inhibition of functional Fas expression on osteoblasts, in the presence or absence of TNF-alpha. Treatm ent of human osteoblasts with vitamin K-2 clearly suppressed Bax expression of the cells, although the expression of Bcl-2 was not influenced by vitam in K-2. Fas ligand (FasL) cDNA transformants were cytotoxic against osteobl asts, and the cytotoxicity was increased when osteoblasts were treated with TNF-alpha. The addition of vitamin K-2 to osteoblasts significantly decrea sed the cytotoxic effects of FasL cDNA transformants. Furthermore, apoptosi s of human osteoblasts induced by LLL-CHO, etoposide, or staurosporine was also clearly suppressed in vitamin K-2-treated osteoblasts. Our results sug gest that vitamin K-2 inhibits apoptotic cell death of osteoblasts and main tains the number of osteoblasts. These actions may explain the therapeutic efficacy of vitamin K-2 in osteoporosis.