Abstract 1677: Inhibition of autophagy proteins ATG7 and BECN1 correlates with increased interferon regulatory factor-1 (IRF1) protein expression through STAT1 phosphorylation.
Authors: Schwartz-Roberts Jessica L. , Cook Katherine L. , Shajahan Ayesha N. , Axelrod Margaret , Warri Anni , Clarke Robert 
Published: August 2014

Approximately two-thirds of newly diagnosed invasive breast tumors express the estrogen receptor-α protein (ER+) and most will be treated with an endocrine therapy such as an antiestrogen or aromatase inhibitor. Despite the widespread clinical efficacy of antiestrogens in the treatment of ER+ breast cancers, approximately half of these women will exhibit de novo or acquired resistance to endocrine therapies. One mechanism by which breast cancer cells evade antiestrogen-induced cell death is through stimulation of autophagy. Autophagy is a catabolic process in which cells digest subcellular organelles in an attempt to maintain homeostasis. Evidence for increased autophagy has been shown to protect cancer cells against various therapies, including radiotherapy and the antiestrogen, tamoxifen. Previous studies from our lab have also established interferon regulatory factor-1 (IRF1) as a tumor suppressor gene and mediator of antiestrogen sensitivity. High mRNA levels of IRF1 in breast cancer core biopsies correlated significantly with a pathologic complete response after neoadjuvant systemic treatment. However, the link between autophagy and this immune function gene is largely unknown. Here, we show that reduced expression of autophagy-related protein, ATG7, correlates with increased nuclear IRF1 protein in 107 human breast cancer core biopsies. Knockout of a single ATG7 allele in mice induced IRF1 activation in both mammary and spleen tissue compared to wild-type mice. In antiestrogen sensitive and resistant breast cancer cells, knockdown of ATG7 and BECN1, but not ATG5, using siRNA resulted in increased IRF1 protein expression. We go on to show that increased IRF1 expression is an effect of increased phosphorylation of STAT1, suggesting that autophagy inhibits the interferon signaling pathway. Using the contrary approach, we found that inhibition of IRF1 using siRNA inhibits the lipidation of LC3 and autophagosome formation. Furthermore, antiestrogen sensitive LCC1 cells become resistant to the growth inhibitory effects of faslodex (Fulvestrant; ICI182780;ICI), 3-methyladenine, and doxorubicin following knockdown of IRF1. Overexpression of IRF1 in antiestrogen resistant LCC9 cells inhibits ATG7 protein expression and induces mitochondrial outer membrane permeability. Thus, the disruption of autophagy machinery in breast cancer cells activates the interferon signaling pathway to induce apoptosis and breast cancer cell death.

Citation Format: Jessica L. Schwartz-Roberts, Katherine L. Cook, Ayesha N. Shajahan, Margaret Axelrod, Anni Warri, Robert Clarke. Inhibition of autophagy proteins ATG7 and BECN1 correlates with increased interferon regulatory factor-1 (IRF1) protein expression through STAT1 phosphorylation. [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 1677. doi:10.1158/1538-7445.AM2013-1677