Dexamethasone modulates osteogenesis and adipogenesis with regulation of osterix expression in rat calvaria-derived cells.

Osteoblasts and adipocytes originate from common mesenchymal progenitor cells and although a number of compounds can induce osteoblastic and adipogenic differentiation from progenitor cells, the underlying mechanisms have not been elucidated. The present study examined the synergistic effects of dexamethasone (Dex) and bone morphogenetic protein (BMP)-2 on the differentiation of clonal mesenchymal progenitor cells isolated from rat calvaria into osteoblasts and adipocytes, as well as the effects of the timing of treatment. Cells were cultured for various periods of time in the presence of Dex and/or BMP-2. When cells were treated with Dex+BMP-2 during the early phase of differentiation, they differentiated into adipocytes. However, when cells were treated with Dex+BMP-2 during the late phase of differentiation, a synergistic effect on in vitro matrix mineralization was observed. To examine differences between the early and late phases of differentiation, ALP activity was measured in the presence of BMP-2. ALP activity increased markedly on Day 9, corresponding to the onset of the synergistic effect of Dex. Dex treatment inhibited osterix (OSX) expression in cells committed to adipogenic differentiation, but not in cells committed to osteogenic differentiation following BMP-2 treatment. The isoform2 OSX promoter region was found to be involved in the effects of Dex on cells during the early phase of differentiation. Furthermore, cells stably expressing OSX (isoform2) formed mineralized nodules even though they had been treated with Dex+BMP-2 during the early phase of differentiation. It appears that Dex modulates osteogenesis and adipogenesis in mesenchymal stem cells by regulating OSX expression.