Kaempferol Stimulates WNT/β-catenin Signaling Pathway to Induce Differentiation of Osteoblasts

Flavonoids, a group of natural compounds found in a variety of vegetables and herbal medicines, have been intensively reported on stimulating bone mineral density and bone formation. Among them, kaempferol has been reported to assist bone formation in vitro and in vivo, but its precise mechanism of action for stimulating bone forming abilities of osteoblasts remained elusive. In SaOS-2 osteoblasts, treatment of kaempferol increased early and late osteogenic parameters significantly, including alkaline phosphatase (ALP) activity, collagen synthesis, and mRNA expression levels of Runx2, osterix, osteopontin and bone sialoprotein. Interestingly, kaempferol promoted osteoblastic differentiation via the activation of the WNT signaling pathway. The stimulation of SaOS-2 cells by kaempferol resulted in an increased activity of WNT signaling responsive reporter construct, Axin-2, and, subsequently, stabilization of WNT signaling mediated transcription factor β-catenin, probably leading to the activation of WNT-targeted genes for osteogenesis. In corroboration, the kaempferol-induced ALP activity was fully abolished by FH 535, an inhibitor of WNT signaling pathway. Kaempferol mediated activation of WNT signaling pathway through estrogen signaling pathway, as the application of ICI 182,780 (an inhibitor for estrogen receptors) markedly inhibited kaempferol-induced WNT signaling activation and osteogenic marker like ALP activity in SaOS-2 cells. Immunohistochemical studies in drill-hole defect model showed increased expression of Runx2 and β-catenin staining after kaempferol treatment. Thus, it may be concluded that kaempferol stimulates estrogen signaling followed by WNT signaling pathway activation to achieve its potential for bone-sparing effects.