Abstract 3497: A novel pathway involving Tcf-driven Bcl2 under regulation of Bmi-1 stem cell factor: Role in chemoresistance
Authors: Hifzur R. Siddique, Lei Wang, Rohinton Tarapore, Yibin Deng, Mohammad Saleem
Journal: Cancer Research (2012)

Abstract Introduction: Bmi-1 a polycomb group (PcG) protein and stem cell factor is reported to play important roles in cell cycle, cell immortalization and senescence. Bmi-1 is reported to be elevated in several tumor types; however its mechanism of action is not clearly understood. Emerging evidences show that the chemoresistance of tumor cells are in part due to the activation of PcG proteins. Objective: In the current study, we investigated the relevance of Bmi-1 in the proliferation of prostatic tumor cells before and after chemotherapy and investigated the underlying mechanism. Methods: Bmi-1 protein was determined in normal, prostate cancer (CaP) cell lines and human prostatic tissues by employing immunoblot and immunohistochemistry analysis. To understand the mechanism a two prong approach (overexpression and silencing) was adopted by stably transfecting normal and CaP cells with either Bmi-1-overexpressing or Bmi-silencing plasmid. Transfected cells were tested for proliferation and sensitivity to various chemotherapeutic agents (casodex, docetaxel and Cisplatin) by employing MTT, 3[H]thymidine uptake and soft-agar assays. Immunoblot, microarray, luciferase-based reporter, TESS and ChIP analysis were used to identify potential targets of Bmi-1. The significance of Bmi-1 in proliferation and its translational significance as a molecular target (for gene therapy and chemotherapy) was evaluated by measuring the tumorigenic growth and sensitivity of CaP cells (harboring varied Bmi-1 expression) to Docetaxel therapy in xenograft mouse models. Results: Bmi-1 protein levels are highly elevated in CaP, colon cancer cell lines and in prostatic tissues of CaP patients. Bmi-1-overexpression was observed to abolish the senescence of normal epithelial cells and increase proliferation of CaP cells even under the presence of chemotherapeutic agents. A reverse effect was observed in Bmi-1-supressed cells. Bmi-1 was observed to regulate the expression of Cyclin D1 and Bcl-2, pro-survival genes and targets of Sonic Hedgehog (SHH) signaling. We provide compelling evidence that regulation of Bcl-2 by Bmi-1 is independent of SHH and is mainly through the activation and binding of TCF-4 transcriptional factor (Wnt signaling component) to promoter regions of Bcl-2. We identified the binding sites of TCF-4 on promoter region of Bcl-2 gene in tumor cells and in human tissues. Docetaxel therapy did not inhibit growth of Bmi-1 overexpressing tumors, however significantly inhibited the growth of Bmi-1-silenced tumors. Finally, we identified small molecule inhibitors of Bmi-1 by using Blue Gene/L Super Computer. Conclusions: For the first time, we identified a novel Tcf/Bcl-2 pathway driven by Bmi-1 that confers chemoresistance to tumor cells. We suggest that Bmi-1 could be developed as a novel therapeutic target to treat chemoresistant tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3497. doi:1538-7445.AM2012-3497