Apigenin inhibits growth and induces G2/M arrest by modulating cyclin-CDK regulators and ERK MAP kinase activation in breast carcinoma cells

We have previously reported that apigenin inhibits the growth of thyroid ca ncer cells by attenuating epidermal growth factor receptor (EGF-R) tyrosine phosphorylation and phosphorylation of ERK mitogen-activated protein (MAP) kinase. In this study we assessed the growth inhibitory effect of apigenin on MCF-7 breast carcinoma cells that express two key cell cycle regulators , wild-type p53 and the retinoblastoma tumor suppressor protein (Rb), and M DA-MB-468 breast carcinoma cells that are mutant for p53 and Rb negative. W e found that apigenin potently inhibited growth of both MCF-7 and MDA-MB-46 8 breast carcinoma cells. The approximate IC50 values determined after 3 da ys incubation, were 7.8 mug/ml for MCF-7 cells, and 8.9 mug/ml for MDA-MB-4 68 cells, respectively. Because the cells cycle studies using FACS showed t hat both MCF-7 and MDA-MB-468 cells were arrested in G2/M phase after apige nin treatment, we studied the effects of apigenin on cell cycle regulatory molecules. We observed that G2/M arrest by apigenin involved a significant decrease in cyclin B1 and CDK1 protein levels, resulting in a marked inhibi tion of CDK1 kinase activity. Apigenin reduced the protein levels of CDK4, cyclins D1 and A, but did not affect cyclin E, CDK2 and CDK6 protein expres sion. In MCF-7 cells, apigenin markedly reduced Rb phosphorylation after 12 h. We also found that apigenin treatment resulted in a dose- and time-depen dent inhibition of ERK MAP kinase phosphorylation and activation in MDA-MB- 468 cells. These results suggest that apigenin is a promising anti-breast c ancer agent and its growth inhibitory effects are mediated by targeting dif ferent signal transduction pathways in MCF-7 and MDA-MB-468 breast carcinom a cells.