Taxol Induces Brk-dependent Prosurvival Phenotypes in TNBC Cells through an AhR/GR/HIF-driven Signaling Axis

The metastatic cascade is a complex process that requires cancer cells to survive despite conditions of high physiologic stress. Previously, cooperation between the glucocorticoid receptor (GR) and hypoxia-inducible factors (HIF) was reported as a point of convergence for host and cellular stress signaling. These studies indicated p38 MAPK-dependent phosphorylation of GR on Ser134 and subsequent p-GR/HIF-dependent induction of breast tumor kinase (PTK6/Brk), as a mediator of aggressive cancer phenotypes. Herein, p-Ser134 GR was quantified in human primary breast tumors (n = 281) and the levels of p-GR were increased in triple-negative breast cancer (TNBC) relative to luminal breast cancer. Brk was robustly induced following exposure of TNBC model systems to chemotherapeutic agents (Taxol or 5-fluorouracil) and growth in suspension [ultra-low attachment (ULA)]. Notably, both Taxol and ULA resulted in upregulation of the Aryl hydrocarbon receptor (AhR), a known mediator of cancer prosurvival phenotypes. Mechanistically, AhR and GR copurified and following chemotherapy and ULA, these factors assembled at the Brk promoter and induced Brk expression in an HIF-dependent manner. Furthermore, Brk expression was upregulated in Taxol-resistant breast cancer (MCF-7) models. Ultimately, Brk was critical for TNBC cell proliferation and survival during Taxol treatment and in the context of ULA as well as for basal cancer cell migration, acquired biological phenotypes that enable cancer cells to successfully complete the metastatic cascade. These studies nominate AhR as a p-GR binding partner and reveal ways to target epigenetic events such as adaptive and stress-induced acquisition of cancer skill sets required for metastatic cancer spread.Implication: Breast cancer cells enlist intracellular stress response pathways that evade chemotherapy by increasing cancer cell survival and promoting migratory phenotypes. Mol Cancer Res; 16(11); 1761-72. ©2018 AACR.

Abstract 3457: Chemotherapy enables Brk/PTK6-dependent survival of triple-negative breast cancer cells via induction of an AhR/GR/HIF signaling axis

The metastatic cascade is a complex process that requires cancer cells to survive despite exposure to conditions of high physiologic stress. We previously showed that breast tumor kinase (Brk; also known as PTK6), a mediator of aggressive breast cancer phenotypes, is induced in breast cancer cells in response to a convergence of cellular and hormonal stress signals mediated by cross talk between hypoxia-inducible factors (HIFs) and glucocorticoid receptors (GR). Specifically, p38-MAPK dependent phosphorylation of GR-Ser134 (p-GR) and p-GR/HIF transcriptional complexes mediated heightened Brk gene expression in response to multiple inputs to stress pathway activation. Following studies in mice to demonstrate that Brk is a GR target gene in the mammary gland, immunohistochemistry (IHC) was performed on human primary breast tumor tissues using total and p-GR antibodies. To model stress-induced p-GR action relevant to tumor progression, molecular markers of stress signaling were measured in triple negative breast cancer (TNBC) cell lines treated with chemotherapy (Taxol and 5-flourouricil) and following growth in suspension (ultra-low attachment (ULA)). Co-immunoprecipitation and ChIP assays were used to demonstrate association of p-GR with novel co-regulatory factors in transcription complexes at the Brk promoter. Cell viability and migration assays were performed following Brk knock-out using CRISPR/Cas9 gene editing. Systemic Dex administration in mice confirmed that Brk is a GR target gene in vivo. In human breast tumor samples, phospho-GR was significantly associated with TNBC relative to luminal cancers. Chemotherapy and ULA induced activation of p38 MAPK, phosphorylation of GR, and upregulation of HIFs as well as the Aryl hydrocarbon receptor (AhR), a known mediator of cancer cell survival under cellular stress. Moreover, AhR and GR co-purified constitutively, and following chemotherapy or ULA culture, these factors assembled at the Brk promoter in a HIF-dependent manner. Brk induction was critical for TNBC cell survival during Taxol treatment or during ULA culture and for cancer cell migration, acquired biological phenotypes that enable cancer cells to successfully complete the metastatic cascade.These studies define AhR as a novel p-GR binding partner and show that increased p-GR/AhR and Brk expression drive a migratory phenotype relevant to TNBC progression. Strategies to target p-GR signaling may ameliorate stress-induced acquisition of aggressive cancer phenotypes required for metastatic cancer spread that are linked to high breast cancer patient mortality.

Citation Format: Tarah M. Regan Anderson, Shihong Ma, Carlos J. Perez Kerkvliet, Taylor M. Helle, Raisa Krutilina, Ganesh V. Raj, John A. Cidlowski, Kathryn L. Schwertfeger, Tiffany N. Seagroves, Carol A. Lange. Chemotherapy enables Brk/PTK6-dependent survival of triple-negative breast cancer cells via induction of an AhR/GR/HIF signaling axis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3457.

Modifications to glucocorticoid and progesterone receptors alter cell fate in breast cancer

Steroid hormone receptors (SRs) are heavily posttranslationally modified by the reversible addition of a variety of molecular moieties, including phosphorylation, acetylation, methylation, SUMOylation, and ubiquitination. These rapid and dynamic modifications may be combinatorial and interact (i.e. may be sequential, complement, or oppose each other), creating a vast array of uniquely modified receptor subspecies that allow for diverse receptor behaviors that enable highly sensitive and context-dependent hormone action. For example, in response to hormone or growth factor membrane-initiated signaling events, posttranslational modifications (PTMs) to SRs alter protein-protein interactions that govern the complex process of promoter or gene-set selection coupled to transcriptional repression or activation. Unique phosphorylation events allow SRs to associate or disassociate with specific cofactors that may include pioneer factors and other tethering partners, which specify the resulting transcriptome and ultimately change cell fate. The impact of PTMs on SR action is particularly profound in the context of breast tumorigenesis, in which frequent alterations in growth factor-initiated signaling pathways occur early and act as drivers of breast cancer progression toward endocrine resistance. In this article, with primary focus on breast cancer relevance, we review the mechanisms by which PTMs, including reversible phosphorylation events, regulate the closely related SRs, glucocorticoid receptor and progesterone receptor, allowing for precise biological responses to ever-changing hormonal stimuli.

Abstract A56: Regulation of breast tumor kinase (Brk) expression in triple-negative breast cancer integrates cellular (HIF-2alpha) and hormonal (cortisol) stress signaling

Triple-negative breast cancers (TNBC) have a worse prognosis relative to other breast cancer subtypes, underscoring the urgent need for identification of driver molecules or pathways for targeted therapies. Breast tumor kinase (Brk) is a soluble tyrosine kinase that is aberrantly elevated and active in 86% of breast cancers. Our lab has shown Brk to be a potent driver of basal-type mammary tumors. Mechanisms through which Brk overexpression is acquired in breast cancer cells are largely unknown. We recently reported that Brk is a direct target gene of hypoxia-inducible factor 1 alpha (HIF-1alpha) and HIF-2alpha, activated in response to cellular stresses such as hypoxia, low glucose, or nutrient starvation. It is becoming increasingly evident that the stress sensing hormone, cortisol, via activation of the glucocorticoid receptor (GR), leads to cell 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 and GR signaling that may influence expression of Brk in TNBC. An explant model of primary human TNBC demonstrated robust induction of Brk mRNA and protein with the GR ligand, dexamethasone (dex). Brk mRNA and protein were also induced in response to dex in TNBC cell line models. MDA-MB-231 cells with HIF-1a/2a knockdown (DKD), failed to induce Brk expression following dex treatment, suggesting that GR regulation of Brk requires HIF-1a/2a. Chromatin immunoprecipitation (ChIP) assays showed HIF and GR co-recruitment to the Brk promoter in response to either hypoxia or dex, indicating that Brk is a direct GR/HIF target gene. HIF-2a mRNA and protein were also directly regulated by GR in response to dex treatment. Notably, expression of Proline, glutamate and leucine rich protein 1 (PELP1), an important steroid receptor coactivator, was significantly induced by hypoxic cell stress, while DKD cells (lacking HIFs) exhibited markedly reduced PELP1 protein levels relative to control cells. Co-immunoprecipitation (co-IP) assays showed that PELP1 and GR interact basally and in response to dex treatment in multiple TNBC cell lines. Moreover, PELP1 was recruited to the Brk promoter with HIF2a and GR following dex treatment. Inhibition of PELP1 with the peptidomemtic, D2, blocked dex induction of Brk mRNA. Physiologic cell stress resulted in phosphorylation of GR at serine 134 (S134) and this event was required for the GR and PELP1 interaction. Enhanced phosphorylation at this site via H2O2 treatment increased GR recruitment to the Brk promoter, while blockade of this site via the p38 MAPK inhibitor SB203580 diminished GR recruitment to the Brk promoter and blocked Brk induction. Notably, mutant GR in which S134 was mutated to an alanine (S134A) was not recruited to the Brk promoter basally or in response to dex treatment, highlighting the importance of this phosphorylation event in the GR regulation of Brk expression. Our data show that GR initiates a feed-forward signaling loop leading to upregulation of Brk in TNBC and reveal molecular linkage between cell stress and stress hormone signaling in driving aggressive phenotypes in breast cancer. Collectively, our studies suggest that GR, HIF, PELP1 cross talk may promote aggressive tumor behavior, in part via upregulation of Brk. Breast cancer patients are routinely given high doses of dex to alleviate the inflammatory side effects of chemotherapy. This treatment may inadvertently promote chemoresistance and tumor progression via robust induction of Brk expression. Targeting the GR/HIF/PELP1 complex may provide a means of blocking Brk-dependent tumor progression and metastasis in patients with TNBC.

Citation Format: Tarah M. Regan Anderson, Shihong Ma, Ganesh V. Raj, Carol A. Lange. Regulation of breast tumor kinase (Brk) expression in triple-negative breast cancer integrates cellular (HIF-2alpha) and hormonal (cortisol) stress signaling. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research; Oct 17-20, 2015; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Res 2016;14(2_Suppl):Abstract nr A56.

Breast Tumor Kinase (Brk/PTK6) Is Induced by HIF, Glucocorticoid Receptor, and PELP1-Mediated Stress Signaling in Triple-Negative Breast Cancer

Cancer cells use stress response pathways to sustain their pathogenic behavior. In breast cancer, stress response-associated phenotypes are mediated by the breast tumor kinase, Brk (PTK6), via the hypoxia-inducible factors HIF-1α and HIF-2α. Given that glucocorticoid receptor (GR) is highly expressed in triple-negative breast cancer (TNBC), we investigated cross-talk between stress hormone-driven GR signaling and HIF-regulated physiologic stress. Primary TNBC tumor explants or cell lines treated with the GR ligand dexamethasone exhibited robust induction of Brk mRNA and protein that was HIF1/2-dependent. HIF and GR coassembled on the BRK promoter in response to either hypoxia or dexamethasone, indicating that Brk is a direct GR/HIF target. Notably, HIF-2α, not HIF-1α, expression was induced by GR signaling, and the important steroid receptor coactivator PELP1 was also found to be induced in a HIF-dependent manner. Mechanistic investigations showed how PELP1 interacted with GR to activate Brk expression and demonstrated that physiologic cell stress, including hypoxia, promoted phosphorylation of GR serine 134, initiating a feed-forward signaling loop that contributed significantly to Brk upregulation. Collectively, our findings linked cellular stress (HIF) and stress hormone (cortisol) signaling in TNBC, identifying the phospho-GR/HIF/PELP1 complex as a potential therapeutic target to limit Brk-driven progression and metastasis in TNBC patients.

Abstract 2107: Aurora A kinase and progesterone receptor cross talk in breast cancer

Breast cancer is the number one diagnosed cancer in women. Luminal breast cancers express steroid hormone receptors (SR) and these cases can be effectively treated with endocrine therapies that block estrogen receptor (ER) activity or estrogen synthesis. Unfortunately, at least 40% of women develop resistance to anti-estrogen treatments and progress to metastatic disease. One of the important transitions from hormone responsive (ER+/PR+ luminal A) to refractory (ER+ luminal B) disease involves the loss of progesterone receptor (PR) protein expression. However, data from large clinical trials showed that exposure to a synthetic progestin (MPA) and estrogen, but not estrogen alone, increased breast cancer incidence and tumor grade. These data, while somewhat controversial, implicate PR action in breast cancer development and progression. PR isoforms (A and B) are ligand-activated transcription factors, though they can be activated in the absence of progestin via signaling pathways commonly elicited downstream of growth factor receptors. We have shown that growth factor signaling (EGF or heregulin) through PR-B can initiate rapid proliferation and survival of breast cancer cells growing in soft-agar. PR-B is phosphorylated and hyperactived by mitogenic protein kinases, including MAPK, CDK2, and CK2. Notably, over-expression and activation of Aurora A kinase (downstream of MAPKs) is associated with invasive breast cancer and poor prognosis. We hypothesize that activation of protein kinases commonly overexpressed in breast cancer provide a context for aberrant PR activity, resulting in upregulation of Her2/erbB2 signaling, EMT, and increased expression of mammary stem-cell mediators (stemness). To model this process, we over-expressed oncogenic Raf1 in ER+/PR+ MCF-7 breast cancer cells. MCF-7-Raf1 cells exhibited constitutive activation of the MAPK pathway and upregulation of both ER and PR mRNA and protein relative to parental controls. Addition of progestin to MCF-7-Raf1 cells increased PR phosphorylation and soft agar growth that was blocked by MLN8237, an Aurora kinase inhibitor. Interestingly, passage of MCF-7-Raf1 cells as mouse xenografts (1GX) led to rapid tumor progression that was associated with increased Aurora A phosphorylation, loss of PR mRNA and protein, expression of epithelial-to-mesenchymal (EMT) markers, increased stemness, and tamoxifen resistance. These data suggest that Aurora A kinase and PR cross-talk may drive early breast cancer progression in response to growth promoting signals. Targeting this cross-talk with Aurora kinase inhibitors and anti-progestins, in conjunction with conventional estrogen-blocking therapies, may improve survival outcomes by preventing progression to endocrine failure. MCF-7-Raf1 and MCF-7-Raf-1 (1GX) cells may provide a useful model for the study of SR-loss during luminal A to luminal B transition.

Citation Format: Katherine A. Leehy, Tarah M. Regan Anderson, Andrea R. Daniel, Antonino B. D'Assoro, Carol A. Lange. Aurora A kinase and progesterone receptor cross talk in breast cancer. [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 2107. doi:10.1158/1538-7445.AM2014-2107

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

Breast tumor kinase (Brk/PTK6) is a mediator of hypoxia-associated breast cancer progression

Basal-type triple-negative breast cancers (TNBC) are aggressive and difficult to treat relative to luminal-type breast cancers. TNBC often express abundant Met receptors and are enriched for transcriptional targets regulated by hypoxia-inducible factor-1α (HIF-1α), which independently predict cancer relapse and increased risk of metastasis. Brk/PTK6 is a critical downstream effector of Met signaling and is required for hepatocyte growth factor (HGF)-induced cell migration. Herein, we examined the regulation of Brk by HIFs in TNBC in vitro and in vivo. Brk mRNA and protein levels are upregulated strongly in vitro by hypoxia, low glucose, and reactive oxygen species. In HIF-silenced cells, Brk expression relied upon both HIF-1α and HIF-2α, which we found to regulate BRK transcription directly. HIF-1α/2α silencing in MDA-MB-231 cells diminished xenograft growth and Brk reexpression reversed this effect. These findings were pursued in vivo by crossing WAP-Brk (FVB) transgenic mice into the MET(Mut) knockin (FVB) model. In this setting, Brk expression augmented MET(Mut)-induced mammary tumor formation and metastasis. Unexpectedly, tumors arising in either MET(Mut) or WAP-Brk × MET(Mut) mice expressed abundant levels of Sik, the mouse homolog of Brk, which conferred increased tumor formation and decreased survival. Taken together, our results identify HIF-1α/2α as novel regulators of Brk expression and suggest that Brk is a key mediator of hypoxia-induced breast cancer progression. Targeting Brk expression or activity may provide an effective means to block the progression of aggressive breast cancers.