Abstract 4846: IGF1 attenuates autophagic catabolism and apoptotic cell death in hormonally treated breast cancer cells via a MEK1 pathway
Authors: Takhar Suchreet , Periyasamy-Thandavan Sudharsan , Jackson William Hutch , Singer Adam , Herberg Samuel , Hill William , Huang Shuang , Schoenlein Patricia 
Published: February 2011

In recent studies, we and others showed that autophagy is one of the prosurvival mechanisms during anti-estrogen therapy that plays a significant role in the development of resistance. In this study, we sought to determine if autophagy plays a role in the protective effects of insulin like growth factor 1 (IGF-1)/MEK1 signaling on breast cancer cells treated with hormonal therapy. In initial studies, we established that IGF1 protects estrogen receptor-positive (ER+) breast cancer cells from cell death induced by the anti-estrogen 4-hydroxytamoxifen (4-OHT) and the anti-progestin mifepristone (MIF) and that this protection was mediated by MEK1 signaling. To determine if the mechanism of MEK1-mediated protection involved modulation of autophagy, ER+ breast cancer cells alone or transfected with MEK1-GFP or MEK1 dominant negative (MEK1-DN) expression vectors were treated with 4-OHT and/or MIF, in the presence or absence of IGF1. Effects on autophagy were monitored using immunoblotting with antibodies to key autophagy proteins, electron microscopy, GFP-LC3 reporter-based expression analysis, quantitative PCR, and autophagic proteolysis (turnover of long-lived proteins). Apoptosis was monitored with the JC-1 assay (mitochondrial membrane permeability determination), trypan blue exclusion, cytokeratin 18 cleavage assay, and determination of the levels of cleaved PARP and lamin A. The results showed that IGF1 significantly increased the levels of several autophagy genes, including Atg8, Atg6, and Atg5. However, even with the increased levels of these autophagy proteins, IGF-1 was shown to clearly suppress hormonally-induced long-lived protein turnover or macroautophagy. Thus, the levels of key autophagy proteins were increased, while the catabolic function of the mature autolysosomes was reduced. MEK1 overexpression also significantly protected breast cancer cells from death, while MEK1DN expression significantly enhanced 4-OHT and/or MIF induced apoptotic cell death. Our interpretation of the concomitant attenuation of macroautophagy and cell death by IGF1/MEK signaling is not that autophagy is a prerequisite to cell death, but that autophagic catabolism is a readout of the significant stress experienced by MEK1 inhibition in breast cancer cells. Based on our results, we are now addressing three key questions: (1) will blockade of autophagy enhance the cytotoxicity of MEK1 inhibitors; (2) is the elevated expression of autophagy genes in IGF-1-treated breast cancer cells mediated by MEK1 signaling; and (3) is mitophagy enhanced in breast cancer cells overexpressing MEK1. This knowledge will be used to support the proposed use of MEK1 blockade to enhance the cytotoxic action of hormonal therapy and to circumvent the occurrence of anti-estrogen resistance of breast cancer, particularly in patients with high circulating levels of IGF-1.

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 4846.