Quercetin as a Modulator of the Cellular Neoplastic Phenotype

Quercetin (3,3′,4′,5,7-pentahydroxyflavone) is a widely distributed plant-derived flavonoid present in most vegetables and fruits, and it is therefore a common component of the human diet.1 Quercetin has been shown to exert multiple biochemical effects in mammalian cells, including the increase of cAMP levels,2 the inhibition of enzymatic activities such as protein kinase C,3,4 protein tyrosine kinases,5–7 and cAMP and cGMP phosphodiesterases,8,9 as well as the interaction with estrogen type II binding sites.10 These biological actions of quercetin may explain its predominantly inhibitory effect of tumor-derived cell lines,10–14 and its ability to arrest tumor cells in the G1 phase13,15 or, less frequently, in the G2 -M phase16 of the cell cycle. Although some reports17,18 indicate that quercetin could be carcinogenic under certain experimental conditions, the overwhelming evidence demonstrating its ability to inhibit the growth of tumor cells identifies quercetin as an anticarcinogenic phytochemical. Furthermore, the dosage of quercetin could be easily modified in the human diet to attain the appropriate levels for optimum cancer chemopreventive action.