Induction of G(1)/S phase arrest and apoptosis by quercetin in human osteosarcoma cells. Quercetin (3,3',4',5,7-pentahydroxyflavone) is a polyphenolic flavonoid compound and is found in a variety of plants. Potential biological activities including antioxidant and anticarcinogenesis have been reported. The antiproliferative effect and apoptosis inducing effect of quercetin in human osteosarcoma cells was evaluated in this study. The IC(50) values were 290 microM and 160 microM at 24 h and 48 h incubation, respectively. Antiproliferative action of quercetin appeared to be linked to apoptotic cell death based on increase in the sub-G(1) apoptotic cell population analyzed by flow cytometric analysis. Prior to apoptosis induction, quercetin caused cell cycle arrest at G(1)/S phase. The G(1)/S phase arrest was accompanied by down regulation of cyclin D1, one of the cyclins required for advance from G(1) to S. Subsequent apoptosis was induced by the gradual activation of caspase-3 and the cleavage of PARP.