Elevated Dickkopf-2 levels contribute to the abnormal phenotype of human osteoarthritic osteoblasts.

The Wnt signaling pathway is crucial for osteogenesis and regulates terminal osteoblast differentiation. Although osteoarthritic (OA) osteoblasts show an abnormal phenotype and poor in vitro mineralization, the mechanism leading to this situation still remains unknow. Recent evidence indicates that Wnt signaling may be altered in OA osteoblasts. In this study we determined whether an alteration of the Wnt/β-catenin signaling pathway is responsible for the abnormal phenotype of OA osteoblasts. Expression of the Wnt signaling antagonist Dickkopf-1 (DKK1) was similar in normal and OA osteoblasts, whereas DKK2 expression was higher in OA osteoblasts than in normal osteoblasts. OA osteoblasts showed a decrease of Wnt3a-dependent Wnt/β-catenin signaling, measured by the TOPflash reporter assay and by Western blot analysis, compared with normal osteoblasts. Correcting DKK2 levels in OA osteoblasts by siRNA techniques enhanced Wnt/β-catenin signaling. Elevated DKK2 levels could be explained by elevated transforming growth factor β1 (TGF-β1) in OA osteoblasts, and exogenous TGF-β1 increased DKK2 expression in normal osteoblasts, whereas ablating TGF-β1 expression in OA osteoblasts reduced DKK2 expression. Inhibiting TGF-β1 or DKK2 expression corrected the abnormal phenotype of OA osteoblasts. In vitro mineralization of OA osteoblasts also was increased by DKK2 siRNA. We conclude that elevated TGF-β1 levels in OA osteoblasts can stimulate DKK2 expression, which, in turn, is responsible, at least in part, for their abnormal phenotype.