The Forkhead box transcription factor FoxM1 is expressed in proliferating cells. Depletion of FoxM1 in mice and various mouse and human cell lines led to cell cycle defects and chromosomal instability. Mouse embryonic fibroblasts derived from knockout mice displayed premature senescence but the underlying cause remained unclear. To investigate the regulatory role of FoxM1 in stress-induced premature senescence, we established NIH3T3 lines with doxycycline-inducible over-expression of FoxM1c. Treatment of these lines with sub-lethal doses (20 µM and 100 µM) of H2O2 induced senescence with β-galactosidase expression and elevated levels of p53 and p21. Interestingly, induction of FoxM1c expression dramatically suppressed senescence and expression of p53 and p21. Consistent with a down-regulation of the p19Arf-p53 pathway, p19Arf levels decreased while expression of the Polycomb group protein Bmi-1 was induced. Bmi-1 as a downstream target of FoxM1c was supported by the dose-dependent induction of Bmi-1 by FoxM1c which occurred at both the protein and RNA levels. Knockdown of FoxM1 expression by RNA interference decreased Bmi-1 expression and both FoxM1c and Bmi-1 reached maximal levels in cells at G2/M phase. Using Bmi-1 promoter-reporters with wildtype and mutated c-Myc sites and shRNAs targeting against c-Myc, we further demonstrated that FoxM1 activated Bmi-1 expression via c-Myc which was recently reported to be a FoxM1 target gene. Our results revealed a functional link between FoxM1c, c-Myc and Bmi-1 which are major regulators of tumorigenesis and have important implication in the regulation of cell proliferation and senescence.

First AACR Centennial Conference on Translational Cancer Medicine-- Nov 4-8, 2007; Singapore