Genistein, isoflavonoids in soybeans, prevents the formation of excess radiation-induced centrosomes via p21 up-regulation.

The centrosome is a cytoplasmic organelle which duplicates once during each cell cycle, and the presence of excess centrosomes promote chromosome instability through chromosome missegregation following cytokinesis. Ionizing radiation (IR) can induce extra centrosomes by permitting the continuation of CDK2/Cyclin-A/E-mediated centrosome duplication when cells are arrested in the cell cycle after irradiation. The work described here shows that, in addition to IR, extra centrosomes were induced in human U2OS and mouse NIH3T3 cells after treatment with agents which include DNA adduct-forming chemicals: benzopyrene (BP), 4-nitroquinoline 1-oxide (4NQO), a DNA cross linker: cis-diamminedichloro-platinum (cisplatin), topoisomerase inhibitors: camptothecin, etoposide, genistein, and ultra-violet light (UV). These agents were divided into two categories with respect to the regulation of p21, which is an inhibitor of CDK2/Cyclin-A/E: specifically, p21 was up-regulated by an IR exposure and treatment with topoisomerase inhibitors. However, UV, BP, 4NQO and cisplatin down-regulated p21 below basal levels. When cells were irradiated with IR in combination with all of these agents, except genistein, enhanced induction of extra centrosomes was observed, regardless of the nature of p21 expression. Genistein significantly suppressed the frequency of IR-induced extra centrosomes in a dose-dependent manner, and 20μg/ml of genistein reduced this frequency to 66%. Consistent with this, genistein substantially up-regulated p21 expression over the induction caused by IR alone, while other agents down-regulated or marginally affected this. This suggests the inhibitory effect of genistein on the induction of extra centrosomes occurs through the inactivation of CDK2/Cyclin-A/E via p21 up-regulation. This hypothesis is supported by the observation that p21 knockdown with siRNA reduced the activity of CDK2/Cyclin-A/E and restored the enhanced effect of a combined treatment with genistein and IR. These results demonstrate the preventive effect of genistein and a crucial role for p21 in IR-induced excess centrosomes.