Abstract 11: Bcl-2 overexpression in A549 cells enhances unusually sensitivity to microtubule-interfering agents through an increase in mitochondrial fragmentation mediated by Bim
Authors: Savry Amandine , Carre Manon , Rovini Amandine , Chacon Christine , Braguer Diane , Bourgarel-Rey Véronique 
Published: June 2012

Abstract
Apoptosis or programmed cell death is defined as a mechanism of cellular suicide involved in the regulation of tissue homeostasis. A disruption of apoptosis is implicated in the development of cancer. Therefore, the ability of tumor cells to evade engagement of apoptosis can play a significant role in their resistance to conventional therapeutic drugs. The members of Bcl-2 family are central regulators of apoptosis. Among them, Bcl-2, an anti-apoptotic member, is commonly overexpressed in various tumors and often associated with unfavorable outcome. However, we and others have previously reported that a low Bcl-2 expression could be associated with a paradoxical resistance to microtubule-interfering agents (MIAs). Thus, the role of Bcl-2 in the anticancer activity of MIAs, never fully evaluated, remains unclear. To further investigate this role, we generated various cellular models overexpressing Bcl-2. We first showed that SHEP Bcl-2, neuroblastoma cells transfected with a Bcl-2 expression vector, exhibit classical resistance to MIAs. In contrast, A549 Bcl-2, human lung carcinoma cells, were paradoxically more sensitive (2-fold) to MIAs than control cells. This enhanced sensitivity, specific of MIAs, is confirmed by a more important mitotic block. Next, we studied the mitochondrial network structure, essential organelle for MIAs-induced apoptosis. After paclitaxel treatment, a classical mitochondrial fragmentation is observed in different cell lines. Interestingly in A549 cell line, the mitochondrial fragmentation is 3 fold more important in cells overexpressing Bcl-2 than in control cells. Thus, this enhanced mitochondrial fragmentation was correlated to the highest cytotoxicity of paclitaxel. To understand this unusual sensitivity, we realized DNA microarray analysis across both A549 cell lines. We showed an induction of the mitochondrial pro-apoptotic Bim. This increase, confirmed at protein (total and mitochondrial) levels only in A549 Bcl-2 is also associated with the increased sensitivity to MIAs. Moreover, the microtubular localization of Bim suggests its critical role in MIAs apoptosis. We then showed that the silencing of Bim by SiRNA prevents mitochondrial fragmentation and reverses the MIAs sensitivity. Therefore, we highlighted Bim involvement in enhanced sensitivity of A549 Bcl-2 cells, through a decrease of mitochondrial fragmentation. Indeed, Bim protein level seemed to be a better determinant of MIAs sensitivity than Bcl-2 status in pulmonary epithelial tumor. Thus, it appeared that Bim expression may be an effective biomarker in predicting MIAs treatment's efficacy. Its involvement in the MIAs-induced mitochondrial fragmentation strengthens the mitochondrial major role in the apoptotic pathway.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 11. doi:10.1158/1538-7445.AM2011-11