Abstract 5109: Doxorubicin-induced autophagy functions as a pro-survival pathway in breast cancer cells

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Abstract Breast cancer is the most common malignancy among women with about 220,000 new cases and 41,000 deaths every year in the US. Development of resistance to chemotherapeutics is the main cause of relapse and patient death; thus better understanding of the biology of the disease and mechanisms of drug resistance are urgently needed to enhance efficacy of current therapies and design novel therapeutic strategies. Recently, autophagy attracted a lot of attention because anticancer therapies, such as chemotherapy, antiestrogen therapies (i.e, tamoxifen), and radiation therapy were found to induce autophagy. Autophagy is a highly regulated lysosome-dependent degradation of misfolded proteins, macromolecules and organelles. Autophagy is induced by cellular stress including nutrient deprivation, metabolic, oxidative and hypoxic stress and may lead to either cell survival or caspase-independent autophagic cell death (type II PCD) depending on the duration and severity of the process. We have previously reported that doxorubicin, one of the most commonly used chemotherapeutic agents, induces autophagy in MCF-7 breast cancer cells. However, whether doxorubicin-induced autophagy functions as a pro-survival or pro-death pathway and the mechanism by which doxorubicin induces autopghagy was not understood. We hypothesized that doxorubicin induces autophagy as a protective survival mechanism through induction of reactive oxygen species (ROS). We found that doxorubicin predominantly induced autophagy at lower doses (0.05-0.5 uM) and apoptosis at higher doses (>1uM) in ER(+) MCF-7 and ER(−) MDA-MB-231 cells (p<0.05). Induction of autophagy was demonstrated by acridine orange staining and FACS analysis and induction of LC3-II protein, a marker of autophagy, by Western blotting. Induction of autophagy was also associated with upregulation of Beclin-1, an essential autophagy promoting protein, in both MCF-7 and MDA-231 cells. To determine the role of doxorubicin-induced autophagy in cell survival or death we inhibited autophagy by silencing of beclin-1 by siRNA and found that beclin-1 siRNA led to about 2-fold increase in doxorubicin-induced apoptotic cell death detected by AnnexinV and FACS analysis compared with the non-silencing control siRNA group in MCF-7 cells (p<0.05). We are currently determining whether inhibition of autophagy by beclin-1 siRNA or chemical inhibitors (baffilomycin A and hydrochloroquine) enhance doxorubicin -induced cell death in other breast cancer cell lines and whether chemotherapy-induced autophagy is mediated by the induction of ROS. Overall, our data suggest that doxorubicin-induced autophagy functions as a survival mechanism and may play a role in development of drug resistance. Therefore inhibition of autophagy can reduce cell survival and increase chemotherapy-induced cell death, providing a novel therapeutic strategy against breast cancer. 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 5109.