Dexamethasone Affects FAS-and Serum Deprivation-Induced Cell Death of Human Osteoblastic Cells through Survivin Regulation

Abstract

Glucocorticoid-induced bone loss is the most prevalent form of secondary osteoporosis. Such loss could be due to the alteration of osteoclast and osteoblast lifespan through regulated apoptosis. The current study investigated the effect of dexamethasone on Fas- and starvation-induced apoptosis of mature osteoblasts and their precursors. Using the human osteoblastic hFOB1.19 and the MG63 osteosarcoma cell lines, we found that sub-lethal doses of dexamethasone act on pre-osteoblasts but not on mature cells by increasing their susceptibility to apoptosis. Apoptosis occurs in a caspase-dependent manner as both DNA fragmentation and mitochondrial transmembrane potential dissipation (ΔΨm) are inhibited by the pan-caspase inhibitor zVAD. The increased susceptibility of osteoblast precursors to apoptosis could be due to dexamethasonemediated down-regulation of survivin expression. Dexamethasone can up-regulate survivin, and to a lesser extent Bcl-2, in mature cells but not in pre-osteoblasts. In addition, it can induce FLIP over-expression in osteosarcoma cells. All these effects are inhibited by the glucocorticoid antagonist RU486, indicating that dexamethasone action is specific and, furthermore, that it depends on glucocorticoid receptor. Finally, we have found that survivin and Bcl-2 are essential for pre- and mature osteoblast survival as their silencing is sufficient to induce spontaneous apoptosis in both cell types. In conclusion, our data outline a new molecular mechanism of glucocorticoid-mediated bone loss due to the enhanced apoptosis of precursors compared to mature osteoblasts. Furthermore, the data suggest a mechanism of dexamethasone-induced resistance of osteosarcoma cells to Fas- and stress-induced apoptosis.