Abstract 1727: Evidence that E2F1-mediated upregulation of Bim, a proapoptotic BH3-only protein, functions as a prosurvival molecule in cancer.

Proapoptotic BH3-only protein Bim plays an important role in Bax/Bak-mediated cytochrome c release and apoptosis. Here we provide evidence for a novel prosurvival function of Bim in cancer cells. Bim was constitutively overexpressed in multiple prostate and breast cancer cells as well as in primary tumor cells. Quantitative real time PCR analysis showed that Bim was transcriptionally upregulated. We have identified eight endogenous E2F1-binding sites on the Bim promoter using in silico analysis. Luciferase assay demonstrated that Bim expression was E2F1-dependent as mutation of E2F1-binding sites on the Bim promoter inhibited luciferase activities. In support, E2F1 silencing led to the loss of Bim expression in cancer cells. Bim primarily localized to mitochondrial and cytoskeleton-associated fractions. Bim-silencing or microinjection of anti-Bim antibodies into the cell cytoplasm resulted in cell rounding, detachment, and subsequent apoptosis. We observed upregulation of prosurvival proteins Bcl-xL and Mcl-1, which sequester Bim in cancer cells. In addition, phosphorylated form of Bim was also elevated in cancer cells. These findings suggest that the constitutively overexpressed Bim may function as a prosurvival molecule in epithelial cancer cells, and phosphorylation and association with Bcl-xL/Mcl-1 block its proapoptotic functions.

The prosurvival functions associated with multiple proapoptotic proteins raise the possibility that anticancer agents that only harness proapoptotic function of Bim or other BH3-only proteins may not be an efficient approach for the permeabilization of mitochondria. Our findings demonstrate that in order to induce efficient apoptosis, inhibition of prosurvival function of Bim may provide a new approach for efficient cancer therapy.

Citation Format: Neelu Yadav, Raghu Gogada, Junwei Liu, Shaohua Tang, Dianmu Zhang, C Marcelo Aldaz, Dean Tang, Dhyan Chandra. Evidence that E2F1-mediated upregulation of Bim, a proapoptotic BH3-only protein, functions as a prosurvival molecule in cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1727. doi:10.1158/1538-7445.AM2013-1727