Abstract 2100: Regulation of breast tumor kinase (Brk) expression in triple negative breast cancer integrates cell stress and cortisol signaling pathways

Triple negative breast cancers (TNBC) lack expression of the molecules currently exploited for targeted therapy (estrogen receptor, progesterone receptor, and HER2). These tumors frequently respond well to chemotherapy, but emergence of chemotherapy-resistance is a common clinical problem. Thus, identification of drivers of TNBC that may be exploited for targeted therapy is of great importance. Breast tumor kinase (Brk), also known as protein tyrosine kinase 6 (PTK6), is a soluble tyrosine kinase that is absent from normal mammary epithelial cells but inappropriately expressed in 85% of breast cancers. Previous studies from our lab have shown Brk to be a critical regulator of breast cancer cell migration in vitro and a potent driver of basal-type mammary tumors in vivo. Notably, we recently identified a novel mechanism of upregulation of Brk expression in TNBC cells. Brk expression is induced following exposure to cell stress, including hypoxia, oxidative damage, and nutrient starvation. Moreover, we found that Brk is a direct target gene of both hypoxia-inducible factor 1 alpha (HIF-1α) and HIF-2α. It is becoming increasingly evident that cortisol signaling via activation of the glucocorticoid receptor (GR) leads to enhanced survival and chemoresistance in tumors of epithelial origin, such as breast cancer. In fact, GR expression in TNBC predicts poor outcome. Herein, we sought to investigate crosstalk between cell stress pathways (HIFs) and cortisol signaling (GR) that may influence expression of Brk in TNBC. Brk mRNA and protein were induced in response to the GR ligand, dexamethasone (dex), in both normoxia and hypoxia, and blocked following treatment with RU486, a GR antagonist. Brk mRNA and protein failed to be induced following dex treatment in cells lacking HIF-1α and HIF-2α, suggesting that even in normoxia, GR regulation of Brk requires HIF-1α/2α. Chromatin immunoprecipitation (ChIP) assays showed HIF and GR recruitment to multiple regions of the Brk promoter in response to either hypoxia (a ligand-independent GR action) or treatment with dex, indicating that Brk is a direct GR/HIF target gene. These data show that cortisol signaling via GR is a novel mechanism of upregulation of Brk expression in TNBC. Additionally, GR upregulation of Brk is HIF-dependent, revealing molecular linkage between cell stress pathways and stress hormone signaling. Collectively, our studies suggest that GR and HIF cross talk may promote aggressive tumor behavior, in part via upregulation of Brk. Breast cancer patients are routinely given dex (a GR agonist) to alleviate the inflammatory side effects of chemotherapy. This treatment may inadvertently promote tumor progression via induction of Brk expression. Citation Format: Tarah M. Regan Anderson, Andrea R. Daniel, Carol A. Lange. Regulation of breast tumor kinase (Brk) expression in triple negative breast cancer integrates cell stress and cortisol signaling pathways. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2100. doi:10.1158/1538-7445.AM2014-2100