Inhibition of GSK3β Stimulates BMP Signaling and Decreases SOST Expression Which Results in Enhanced Osteoblast Differentiation

Both bone morphogenetic protein (BMP) and Wnt signaling have significant roles in osteoblast differentiation and the interaction between BMP and Wnt signaling is well known. Sclerostin is an important inhibitor of bone formation, inhibiting Wnt signaling and downstream effects of BMP such as alkaline phosphatase activity and matrix mineralization in vitro. However, little is known about the effect of BMP and Wnt signaling interaction on the regulation of SOST, the gene encoding sclerostin. Possibly, uncoupling of osteoblast differentiation regulators and SOST expression could increase osteoblast differentiation. Therefore, we investigated the effect of BMP and Wnt signaling interaction on the expression of SOST and the subsequent effect on osteoblast differentiation. Human osteosarcoma cells (SaOS-2) and murine pre-osteoblast cells (KS483) were treated with different concentrations of Wnt3a, a specific GSK3 inhibitor (GIN) and BMP4. Both Wnt3a and GIN increased BMP4-induced BMP signaling and BMP4 increased Wnt3a and GIN-induced Wnt signaling. However, the effect of GIN was much stronger. Quantitative RT-PCR analysis showed that SOST expression dose-dependently decreased with increasing Wnt signaling, while BMP4 induced SOST expression. GIN significantly decreased the BMP4-induced SOST expression. This resulted in an increased osteoblast differentiation as measured by ALP activity in the medium and matrix mineralization. We conclude that GSK3 inhibition by GIN caused an uncoupling of BMP signaling and SOST expression, resulting in an increased BMP4-induced osteoblast differentiation. This effect can possibly be used in clinical practice to induce local bone formation, for example, fracture healing or osseointegration of implants. J. Cell. Biochem. 116: 2938-2946, 2015. (c) 2015 Wiley Periodicals, Inc.