The polycomb group family protein BMI-1 is frequently deregulated in cancer. BMI-1 has been shown to promote stemness and tumorigenicity largely through epigenetic repression of the CDKN2A locus, inhibiting the expression of cell-cycle inhibitors p16INK4A and p14ARF. We have previously shown that BMI-1 functions as an oncoprotein in Ewing sarcoma, promoting anchorage independent growth in vitro and tumorigenicity in vivo independently of CDKN2A repression. In the current study we have investigated the potential contribution of BMI-1 to loss of cell contact inhibition, a fundamental transforming property of cancer cells. Using cells that stably express an shRNA against BMI-1, we discovered that loss of BMI-1 expression restores contact inhibition to CDKN2A-null Ewing sarcoma cells. Significantly, although proliferation of cells in log phase growth is unaffected by loss of BMI-1, at high cell density BMI-1 knockdown cells undergo cell cycle arrest and death. In contrast, control vector-transduced cells continue to enter cell cycle and proliferate, avoiding contact inhibition. Although many signaling pathways are involved in mediating the cell contact inhibition response, inactivation of the Yes-Associated Protein (YAP), a key downstream target of the Hippo pathway, has been implicated in both Drosophila and mammalian cells. Intriguingly, our data show that, although the Hippo pathway is activated in response to increasing cell density, as evidenced by increasing YAP phosphorylation, YAP is not degraded in control cells that express high levels of BMI-1. In contrast, silencing BMI-1 expression results in loss of YAP protein at high cell density coincident with induction of contact inhibition. Together, these findings suggest that YAP is a novel downstream target of BMI-1 and that BMI-1-mediated stabilization of YAP renders cancer cells impervious to cell contact inhibition. Experiments are currently ongoing to elucidate the precise molecular mechanism linking BMI-1 to YAP; however, preliminary studies show that knockdown of BMI-1 has no effect on YAP1 transcript expression at either low or high cell density indicating that the effects of BMI-1 on YAP stabilization are most likely to be post-translational and indirect. Cancer cells escape cell contact inhibition to achieve unrestrained proliferation and enhanced invasive and metastatic properties. Our findings suggest that BMI-1 may be a key upstream mediator of this critical cancer-associated phenotype.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5169.