Effects of genistein and structurally related phytoestrogens on cell cycle kinetics and apoptosis in MDA-MB-468 human breast cancer cells

We have studied the effects of phytoestrogens (genistein, quercetin, daidzein, biochanin A and kaempferol) on proliferation, cell cycle kinetics, and apoptosis of MDAMB-468 breast cancer cells. Genistein and quercetin inhibited cell growth with IC₅₀ values of 8.8 and 18.1 μM, respectively, while the other phytoestrogens were less effective. Flow cytometric analysis showed G₂/M cell cycle arrest with 25 μM and higher concentrations of genistein. At 100 μM, genistein, quercetin and kaempferol caused accumulation of 70, 60 and 35% of cells, respectively, in G₂/M phase by 24 h. In contrast, biochanin A and daidzein were ineffective. APO-BRDU analysis revealed apoptosis with 10 μM genistein (19.5%), reaching 86% at 100 μM. Apoptosis by genistein was confirmed by Hoechst 33342 staining and fluorescence microscopy. With 100 μM quercetin, 47% of the cells were apoptotic, while the other bioflavonoids had little effect. Genistein treatment resulted in a biphasic response on cyclin B1: 70% increase in cyclin B1 level at 25 μM, and 50 and 70% decrease at 50 and 100 μM, respectively. In contrast, the action of quercetin involved an increase in cyclin B1 level. Genistein had no effect on cdc2 level up to 50 μM concentration; however, there was a decrease in the phosphorylated form of the protein at 100 μM. Quercetin had no effect on cdc2 levels. Our results suggest that the action of genistein and quercetin involves G₂/M arrest and apoptosis in MDA-MB-468 cells. Biochanin A and daidzein, although structurally related to genistein, did not share this mechanism. Thus, structurally related phytoestrogens have discrete target sites and mechanisms in their growth inhibitory action on breast cancer cells.