Polycomb group protein Bmi1 regulates proliferation and senescence of breast cancer cells and is transcriptionally repressed by TP53.

2798 The Polycomb group (PcG) gene Bmi1 was first identified as a collaborating oncogene in the induction of mouse pre-B-cell lymphoma, and was subsequently reported overexpressed in various human malignancies including breast cancer. Bmi1 regulates cellular proliferation and senescence, and self-renewal of specific normal and malignant stem cell types via its repressive effect on the Ink4a/Arf tumor suppressor locus. Here we show that Bmi1 is overexpressed in a number of human breast cancer cell lines and breast tumors. Moreover, published and in-house microarray data suggest that Bmi1 expression negatively correlates with that of Ink4a/p16, and positively correlates with that of PTEN in human breast tumors; these correlations are statistically significant and consistent across multiple datasets. To determine the biological importance of deregulated Bmi1 expression in breast carcinogenesis, we repressed expression of the protein using a RNA interference (RNAi) strategy. Knockdown of Bmi1 led to induction of cell death and senescence in a number of breast cancer cell lines as determined by annexin V/propidium iodide (PI) and senescence-associated beta-galactosidase (SA-β-gal) staining, respectively. Consistent with this, overexpression of Bmi1 led to increased cell proliferation of MCF-7 breast cancer cells. Additionally, depletion of Bmi1 inhibited anchorage-independent growth of T-47D breast cancer cells, indicating a decrease in transformed phenotype of these cells. Interestingly, we observed that Bmi1 expression positively correlates with TP53 mutation status in a number of breast cancer cell lines, and was significantly higher in the colon cancer cell line, HCT116 compared to its isogenic TP53-/- subcell line. Knockdown of TP53 in HCT116 cells resulted in a decrease in Bmi1 expression, suggesting that TP53 might regulate Bmi1 levels. To further examine this possibility, we utilized tetracycline-repressible DLD-1 colon cancer cell lines carrying either wild-type TP53 or a tumor-derived mutant form of TP53 (R175H), which is incapable of activating gene transcription. Quantitative real-time PCR and Western blot analysis demonstrated that overexpression of wild-type, but not mutant, TP53 results in repression of Bmi1. These data suggest that Bmi1 and TP53 might act in an antagonistic manner. Future studies will be aimed at determining whether an imbalance between these proliferation-regulating signals contributes to oncogenesis.