Wnt but Not BMP Signaling Is Involved in the Inhibitory Action of Sclerostin on BMP-Stimulated Bone Formation

Abstract

Sclerostin is an osteocyte-derived negative regulator of bone formation. It inhibits BMP-stimulated bone formation both in vitro and in vivo but has no direct effect on BMP signaling. Instead, sclerostin inhibits Wnt signaling that is required for BMP-stimulated osteoblastic differentiation. Sclerostin is a member of the Dan family of glycoproteins of which many members have been reported to antagonize BMP activity. Sclerostin has been shown to inhibit BMP-stimulated bone formation, but its mechanism of action seems to be different from classical BMP antagonists. In this study, we investigated the mechanism by which sclerostin inhibits BMP-stimulated bone formation. DNA electroporation of calf muscle of mice using expression plasmids for BMP and sclerostin was used to study the effect of sclerostin on BMP-induced bone formation in vivo. Transcriptional profiling using microarrays of osteoblastic cells treated with BMP in the absence or presence of sclerostin was used to find specific growth factor signaling pathways affected by sclerostin. The affected pathways were further studied using growth factor-specific reporter constructs. BMP-induced ectopic bone formation in calf muscle of mice was prevented by co-expression of sclerostin in vivo. Transcriptional profiling analysis of osteoblastic cultures indicated that sclerostin specifically affects BMP and Wnt signaling out of many other growth signaling pathways. Sclerostin, however, did not inhibit stimulation of direct BMP target genes. Furthermore, we did not obtain any evidence for sclerostin acting as a direct BMP antagonist using a BMP-specific reporter construct. In contrast, sclerostin shared many characteristics with the Wnt antagonist dickkopf-1 in antagonizing BMP-stimulated bone formation and BMP- and Wnt-induced Wnt reporter construct activation. Sclerostin inhibits BMP-stimulated bone formation but does not affect BMP signaling. Instead, it antagonizes Wnt signaling in osteoblastic cells. High bone mass in sclerosteosis and van Buchem disease may, therefore, result from increased Wnt signaling.