Wnt but not BMP signaling is involved in the inhibitory action of sclerostin on BMP-stimulated bone formation.
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.


BACKGROUND
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
van Bezooijen
Rutger L
RL
Department of Endocrinology and Metabolic Diseases, Leiden Univerity Medical Center, Ther Netherlands. R.L.van_Bezooyen@lumc.nl
Svensson
J Peter
JP
Eefting
Daniël
D
Visser
Annemieke
A
van der Horst
Geertje
G
Karperien
Marcel
M
Quax
Paul H A
PH
Vrieling
Harry
H
Papapoulos
Socrates E
SE
ten Dijke
Peter
P
Löwik
Clemens W G M
CW
D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D001846:Bone Development / Q000187:drug effects / Q000502:physiology*; D055415:Bone Morphogenetic Protein 4; D019485:Bone Morphogenetic Proteins / Q000235:genetics / Q000494:pharmacology / Q000502:physiology*; D002478:Cells, Cultured; D018274:Electroporation; D005786:Gene Expression Regulation; D017930:Genes, Reporter; D005819:Genetic Markers / Q000502:physiology*; D006023:Glycoproteins; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins; D008648:Mesoderm / Q000166:cytology / Q000502:physiology; D051379:Mice; D018482:Muscle, Skeletal / Q000502:physiology; D020411:Oligonucleotide Array Sequence Analysis; D011994:Recombinant Proteins / Q000494:pharmacology; D015398:Signal Transduction; D014162:Transfection; D051153:Wnt Proteins / Q000502:physiology*
D048868:Adaptor Proteins, Signal Transducing; C525809:BMP4 protein, human; C525810:Bmp4 protein, mouse; D055415:Bone Morphogenetic Protein 4; D019485:Bone Morphogenetic Proteins; D005819:Genetic Markers; D006023:Glycoproteins; D036341:Intercellular Signaling Peptides and Proteins; D011994:Recombinant Proteins; C486030:Sost protein, mouse; D051153:Wnt Proteins