Quercetin, a flavonoid, inhibits the proliferation, differentiation, and mineralization of osteoblasts in vitro.

It is possible that the flavonoids that are found in many foods might have a protective effect against osteoclastic activity. However, little information is available about the effects of flavonoids on osteoblastogenesis. Therefore, we investigated the effects of quercetin, a flavonoid, on the metabolism of rat calvarial osteoblast-like cells (ROB cells) in culture. The proliferation of cells was markedly inhibited upon exposure of cells to quercetin at 5 x 10(-6) to 1 x 10(-5) M. Quercetin at 1 x 10(-5) M did not induce apoptosis in ROB cells but arrested cells at the G1 phase of the cell cycle. In addition, quercetin stimulated the expression of mRNA for p21(waf1/cip1), which inhibits the activity of cyclin-dependent kinases, and inhibited the phosphorylation of histone H1. Furthermore, after cells had ceased to proliferate, quercetin reduced the activity of alkaline phosphatase, the level of expression of mRNA for osteocalcin, the rate of deposition of Ca(2+), and the formation of mineralized nodules, all of which are markers of osteoblast differentiation. These findings indicate that quercetin inhibits the proliferation, differentiation, and mineralization of osteoblastic cells.