Osteoclasts, the bone resorbing cells of the body, form when osteoclast precursors are exposed to the key osteoclastogenic cytokine receptor activator of NF-κB ligand (RANKL), a process requiring induction of NF-κB signaling. Quercetin is a ubiquitous plant-derived flavonoid with well documented anti-inflammatory properties, in part, a consequence of its capacity to downmodulate the NF-κB signal transduction pathway. Consistent with this mechanism of action quercetin is reported to suppress osteoclastogenesis in vitro and prevent bone loss in ovariectomized mice in vivo. By contrast, the effect of quercetin on osteoblasts, the cells responsible for bone formation, is contradictory with conflicting reports of inhibition as well as stimulation. Given our previous reports that NF-κB antagonists promote osteoblast differentiation and activity, we compared the effects of quercetin on osteoclast and osteoblast differentiation and on NF-κB signal transduction in vitro. As expected, quercetin potently suppressed osteoclastogenesis and NF-κB activation induced by RANKL in osteoclast precursors. However, the same doses of quercetin had no effect on osteoblast mineralization, and failed to significantly alleviate the inhibitory effect of NF-κB-induced by TNFα, even though quercetin potently suppressed NF-κB activation in these cells. This apparent contradiction was explained by the fact that addition to its anti-NF-κB activity, quercetin also potently antagonized both TGFβ and BMP-2-induced Smad activation in osteoblast precursors. Taken together our data suggest that multiple competing actions of quercetin mediate both stimulatory and inhibitory actions on osteoblasts with the final physiological effect likely a function of the net balance between these stimulatory and inhibitory effects.

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