Abstract 4845: TONDU, a transcription factor that regulates prosurvival autophagy and the development of antiestrogen resistance in breast cancer
Authors: Periyasamy-Thandavan Sudharsan , Singer Adam , Takhar Suchreet , Jackson William Hutch , Samaddar Julia , Browning Darren , Schoenlein Patricia 
Published: February 2011

In recent studies, we have demonstrated that autophagy is essential for the survival of estrogen receptor positive (ER+) breast cancer cells during anti-estrogen therapy and facilitates the development of antiestrogen resistance. For these studies, we subjected ER+ MCF-7 cells to a step-wise drug selection protocol, with 4 hydroxytamoxifen (4-OHT) as the selecting drug and established an antiestrogen resistant subline, designated TR5 (4-OHT resistant to 5μM). TR5 cells, unlike the parent MCF-7 cells, do not die in response to 4-OHT treatment and tolerate high levels of antiestrogen-induced autophagosome formation. Microarray analysis of 4-OHT-treated TR5 cells shows an approximate 5-10 -fold increase in TONDU (TDU) mRNA compared to that of the parent MCF-7 cells. TDU, also known as vestigial like gene-1, was initially identified as a transcription factor required for wing development in Drosophilia, but specific roles for TDU in mammalian cells have not been identified. Based on the upregulation of TDU in antiestrogen-resistant TR5 cells, we hypothesized that TDU played a role in pro-survival autophagy. To test this hypothesis, we analyzed the levels of TDU in cultured MCF-7 and TR5 cells under conditions that induce autophagy and further determined how modulation of TDU levels (TDU cDNA overexpression and TDU attenuation by RNAi targeting) affected autophagy and death. 4-OHT and rapamycin, an mTOR inhibitor that induces macroautophagy, increased TDU expression within 24 hours of treatment and increased TDU expression persisted for 96 hours. Further, quantitative Real-Time (RT) PCR analysis of cells overexpressing TDU revealed that TDU upregulated the transcription of autophagy genes (i.e. upregulation of LC3, Atg5 and Atg6). Importantly, the TDU-mediated increased expression of autophagy genes was blocked by TDU RNAi. Consistent with this apparent TDU-induced autophagy, TDU overexpression significantly increased autophagic proteolysis (long-lived protein turnover). In addition, down regulation of TDU by RNAi induced cell death in E2- and 4-OHT-treated MCF-7 and TR5 cells, with increased mitochondrial membrane permeability, cleavage of PARP, and cleavage of lamin-A as read-outs of cell death. To our knowledge, these studies uniquely identify TDU as a transcription factor involved in the regulation of autophagy in ER+ breast cancer cells and as a potential molecular target to block pro-survival autophagy and enhance the cytotoxicity of antiestrogen therapy. Our current studies are aimed at determining whether TDU expression levels can serve as a prognostic indicator of breast cancer response to hormonal therapy.

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 4845.