Androgen receptor and estrogen receptor variants in prostate and breast cancers

The androgen receptor (AR) and estrogen receptor alpha (ERα) are steroid receptor transcription factors with critical roles in the development and progression of prostate and breast cancers. Advances in the understanding of mechanisms underlying the ligand-dependent activation of these transcription factors have contributed to the development of small molecule inhibitors that block AR and ERα actions. These inhibitors include competitive antagonists and degraders that directly bind the ligand binding domains of these receptors, luteinizing hormone releasing hormone (LHRH) analogs that suppress gonadal synthesis of testosterone or estrogen, and drugs that block specific enzymes required for biosynthesis of testosterone or estrogen. However, resistance to these therapies is frequent, and is often driven by selection for tumor cells with alterations in the AR or ESR1 genes and/or alternatively spliced AR or ESR1 mRNAs that encode variant forms AR or ERα. While most investigations involving AR have been within the context of prostate cancer, and the majority of investigations involving ERα have been within the context of breast cancer, important roles for AR have been elucidated in breast cancer, and important roles for ERα have been elucidated in prostate cancer. Here, we will discuss the roles of AR and ERα in breast and prostate cancers, outline the effects of gene- and mRNA-level alterations in AR and ESR1 on progression of these diseases, and identify strategies that are being developed to target these alterations therapeutically.

Navigating a plethora of progesterone receptors: Comments on the safety/risk of progesterone supplementation in women with a history of breast cancer or at high-risk for developing breast cancer

Progesterone and progestin agonists are potent steroid hormones. There are at least three major types of progesterone receptor (PR) families that interact with and respond to progesterone or progestin ligands. These receptors include ligand-activated transcription factor isoforms (PR-A and PR-B) encoded by the PGR gene, often termed classical or nuclear progesterone receptor (nPR), membrane-spanning progesterone receptor membrane component proteins known as PGRMC1/2, and a large family of progestin/adipoQreceptors or PAQRs (also called membrane PRs or mPRs). Cross-talk between mPRs and nPRs has also been reported. The complexity of progesterone actions via a plethora of diverse receptors warrants careful consideration of the clinical applications of progesterone, which primarily include birth control formulations in young women and hormone replacement therapy following menopause. Herein, we focus on the benefits and risk of progesterone/progestin supplementation. We conclude that progesterone-only supplementation is considered safe for most reproductive-age women. However, women who currently have ER + breast cancer or have had such cancer in the past should not take sex hormones, including progesterone. Women at high-risk for developing breast or ovarian cancer, either due to their family history or known genetic factors (such as BRCA1/2 mutation) or hormonal conditions, should avoid exogenous sex hormones and proceed with caution when considering using natural hormones to mitigate menopausal symptoms and/or improve quality of life after menopause. These individuals are urged to consult with a qualified OB-GYN physician to thoroughly assess the risks and benefits of sex hormone supplementation. As new insights into the homeostatic roles and specificity of highly integrated rapid signaling and nPR actions are revealed, we are hopeful that the benefits of using progesterone use may be fully realized without an increased risk of women's cancer.

Reevaluating the Role of Progesterone in Ovarian Cancer: Is Progesterone Always Protective?

Ovarian cancer (OC) represents a collection of rare but lethal gynecologic cancers where the difficulty of early detection due to an often-subtle range of abdominal symptoms contributes to high fatality rates. With the exception of BRCA1/2 mutation carriers, OC most often manifests as a post-menopausal disease, a time in which the ovaries regress and circulating reproductive hormones diminish. Progesterone is thought to be a "protective" hormone that counters the proliferative actions of estrogen, as can be observed in the uterus or breast. Like other steroid hormone receptor family members, the transcriptional activity of the nuclear progesterone receptor (nPR) may be ligand dependent or independent and is fully integrated with other ubiquitous cell signaling pathways often altered in cancers. Emerging evidence in OC models challenges the singular protective role of progesterone/nPR. Herein, we integrate the historical perspective of progesterone on OC development and progression with exciting new research findings and critical interpretations to help paint a broader picture of the role of progesterone and nPR signaling in OC. We hope to alleviate some of the controversy around the role of progesterone and give insight into the importance of nPR actions in disease progression. A new perspective on the role of progesterone and nPR signaling integration will raise awareness to the complexity of nPRs and nPR-driven gene regulation in OC, help to reveal novel biomarkers, and lend critical knowledge for the development of better therapeutic strategies.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Receptor for hyaluronan-mediated motility (RHAMM) defines an invasive niche associated with tumor progression and predicts poor outcomes in breast cancer patients

Breast cancer invasion and metastasis result from a complex interplay between tumor cells and the tumor microenvironment (TME). Key oncogenic changes in the TME include aberrant synthesis, processing, and signaling of hyaluronan (HA). Hyaluronan-mediated motility receptor (RHAMM, CD168; HMMR) is an HA receptor enabling tumor cells to sense and respond to this aberrant TME during breast cancer progression. Previous studies have associated RHAMM expression with breast tumor progression; however, cause and effect mechanisms are incompletely established. Focused gene expression analysis of an internal breast cancer patient cohort confirmed that increased RHAMM expression correlates with aggressive clinicopathological features. To probe mechanisms, we developed a novel 27-gene RHAMM-related signature (RRS) by intersecting differentially expressed genes in lymph node (LN)-positive patient cases with the transcriptome of a RHAMM-dependent model of cell transformation, which we validated in an independent cohort. We demonstrate that the RRS predicts for poor survival and is enriched for cell cycle and TME-interaction pathways. Further analyses using CRISPR/Cas9-generated RHAMM-/- breast cancer cells provided direct evidence that RHAMM promotes invasion in vitro and in vivo. Immunohistochemistry studies highlighted heterogeneous RHAMM protein expression, and spatial transcriptomics associated the RRS with RHAMM-high microanatomic foci. We conclude that RHAMM upregulation leads to the formation of 'invasive niches', which are enriched in RRS-related pathways that drive invasion and could be targeted to limit invasive progression and improve patient outcomes. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Glucocorticoid Receptors Drive Breast Cancer Cell Migration and Metabolic Reprogramming via PDK4

Corticosteroids act on the glucocorticoid receptor (GR; NR3C1) to resolve inflammation and are routinely prescribed to breast cancer patients undergoing chemotherapy treatment to alleviate side effects. Triple-negative breast cancers (TNBCs) account for 15% to 20% of diagnoses and lack expression of estrogen and progesterone receptors as well as amplified HER2, but they often express high GR levels. GR is a mediator of TNBC progression to advanced metastatic disease; however, the mechanisms underpinning this transition to more aggressive behavior remain elusive. We previously showed that tissue/cellular stress (hypoxia, chemotherapies) as well as factors in the tumor microenvironment (transforming growth factor β [TGF-β], hepatocyte growth factor [HGF]) activate p38 mitogen-activated protein kinase (MAPK), which phosphorylates GR on Ser134. In the absence of ligand, pSer134-GR further upregulates genes important for responses to cellular stress, including key components of the p38 MAPK pathway. Herein, we show that pSer134-GR is required for TNBC metastatic colonization to the lungs of female mice. To understand the mechanisms of pSer134-GR action in the presence of GR agonists, we examined glucocorticoid-driven transcriptomes in CRISPR knock-in models of TNBC cells expressing wild-type or phospho-mutant (S134A) GR. We identified dexamethasone- and pSer134-GR-dependent regulation of specific gene sets controlling TNBC migration (NEDD9, CSF1, RUNX3) and metabolic adaptation (PDK4, PGK1, PFKFB4). TNBC cells harboring S134A-GR displayed metabolic reprogramming that was phenocopied by pyruvate dehydrogenase kinase 4 (PDK4) knockdown. PDK4 knockdown or chemical inhibition also blocked cancer cell migration. Our results reveal a convergence of GR agonists (ie, host stress) with cellular stress signaling whereby pSer134-GR critically regulates TNBC metabolism, an exploitable target for the treatment of this deadly disease.

Membrane-Initiated Estrogen, Androgen, and Progesterone Receptor Signaling in Health and Disease

Rapid effects of steroid hormones were discovered in the early 1950s, but the subject was dominated in the 1970s by discoveries of estradiol and progesterone stimulating protein synthesis. This led to the paradigm that steroid hormones regulate growth, differentiation, and metabolism via binding a receptor in the nucleus. It took 30 years to appreciate not only that some cellular functions arise solely from membrane-localized steroid receptor (SR) actions, but that rapid sex steroid signaling from membrane-localized SRs is a prerequisite for the phosphorylation, nuclear import, and potentiation of the transcriptional activity of nuclear SR counterparts. Here, we provide a review and update on the current state of knowledge of membrane-initiated estrogen (ER), androgen (AR) and progesterone (PR) receptor signaling, the mechanisms of membrane-associated SR potentiation of their nuclear SR homologues, and the importance of this membrane-nuclear SR collaboration in physiology and disease. We also highlight potential clinical implications of pathway-selective modulation of membrane-associated SR.

Abstract 1351: Lauryn Werner

Clinical studies have linked usage of progestins (synthetic progesterone) to breast cancer risk. However, little is understood regarding the role of native progesterone (P4), signaling through the progesterone receptor (PR), in breast tumor formation. Recently, we reported a link between PR and immune signaling pathways, showing that P4/PR can repress type I interferon signaling pathways. Given these findings, we sought to investigate whether P4/PR drive immunomodulation in the mammary gland and promote tumor formation. To determine the effect of P4 on immune cell populations in the murine mammary gland, mice were treated with P4 or placebo pellets for 21 days. We found that mice treated with P4 exhibited changes in the mammary gland indicative of an inhibited immune response compared to placebo-treated mice. To assess the effect of PR overexpression on mammary gland tumor development as well as immune cell populations in the mammary gland, a transgenic mouse model was used in which PR is overexpressed throughout the entire mouse. Immune cell populations were assessed in the mammary glands by flow cytometry, which revealed decreased numbers of various immune cell populations. Transgenic mice were also monitored for mammary gland tumor development over a 2-year timespan. Following development of mammary gland tumors, immune cell populations in the tumors and spleens of transgenic and control mice were analyzed by flow cytometry. Upon long-term monitoring, we determined that multi-parous PR overexpressing mice developed significantly more mammary gland tumors than control mice. Additionally, tumors from PR overexpressing mice contained fewer infiltrating immune cells, and RNA sequencing analysis of tumor samples revealed that immune-related gene signatures were lower in tumors from PR overexpressing mice as compared to control mice. Finally, we utilized syngeneic mammary gland tumor models to evaluate the effect of P4 on the growth of PR+ mammary gland tumors in vivo, which revealed that P4 promoted tumor growth and decreased immune cell infiltration of PR+ mammary gland tumors. Together, these findings offer a novel mechanism of P4-driven mammary gland tumor development and provide rationale in investigating the usage of anti-progestin therapies to promote immune-mediated elimination of mammary gland tumors.

Citation Format: Lauryn Rose Werner, Katelin A. Gibson, Merit L. Goodman, Dominika E. Helm, Katherine R. Walter, Sean M. Holloran, Gloria M. Trinca, Richard C. Hastings, Howard H. Yang, Ying Hu, Junping Wei, Gangjun Lei, Xiao-Yi Yang, Rashna Madan, Alfredo A. Molinolo, Mary A. Markiewicz, Prabhakar Chalise, Margaret L. Axelrod, Justin M. Balko, Kent W. Hunter, Zachary C. Hartman, Carol A. Lange, Christy R. Hagan. Lauryn Werner [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1351.

Abstract P4-04-07: Progesterone promotes immunomodulation and tumor development in the murine mammary gland

Clinical studies have linked usage of progestins (synthetic progesterone) to breast cancer risk. However, little is understood regarding the role of native progesterone (P4), signaling through the progesterone receptor (PR), in breast tumor formation. Recently, we reported a link between PR and immune signaling pathways, showing that P4/PR can repress type I interferon signaling pathways. Given these findings, we sought to investigate whether P4/PR drive immunomodulation in the mammary gland and promote tumor formation. To determine the effect of P4 on immune cell populations in the murine mammary gland, mice were treated with P4 or placebo pellets for 21 days. We found that mice treated with P4 exhibited changes in the mammary gland indicative of an inhibited immune response compared to placebo-treated mice. To assess the effect of PR overexpression on mammary gland tumor development as well as immune cell populations in the mammary gland, a transgenic mouse model was used in which PR is overexpressed throughout the entire mouse. Immune cell populations were assessed in the mammary glands by flow cytometry, which revealed decreased numbers of various immune cell populations. Transgenic mice were also monitored for mammary gland tumor development over a 2-year timespan. Following development of mammary gland tumors, immune cell populations in the tumors and spleens of transgenic and control mice were analyzed by flow cytometry. Upon long-term monitoring, we determined that multi-parous PR overexpressing mice developed significantly more mammary gland tumors than control mice. Additionally, tumors from PR overexpressing mice contained fewer infiltrating immune cells, and RNA sequencing analysis of tumor samples revealed that immune-related gene signatures were lower in tumors from PR overexpressing mice as compared to control mice. Finally, we utilized syngeneic mammary gland tumor models to evaluate the effect of P4 on the growth of PR+ mammary gland tumors in vivo, which revealed that P4 promoted tumor growth and decreased immune cell infiltration of PR+ mammary gland tumors. Together, these findings offer a novel mechanism of P4-driven mammary gland tumor development and provide rationale in investigating the usage of anti-progestin therapies to promote immune-mediated elimination of mammary gland tumors.

Citation Format: Lauryn R Werner, Katelin A Gibson, Merit L Goodman, Dominika E Helm, Katherine R Walter, Sean M Holloran, Gloria M Trinca, Richard C Hastings, Howard H Yang, Ying Hu, Junping Wei, Gangjun Lei, Xiao-Yi Yang, Rashna Madan, Alfred A Molinolo, Mary A Markiewicz, Prabhakar Chalise, Margaret L Axelrod, Justin M Balko, Kent W Hunter, Zachary C Hartman, Carol A Lange, Christy R Hagan. Progesterone promotes immunomodulation and tumor development in the murine mammary gland [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-04-07.

Stress sensing within the breast tumor microenvironment: how glucocorticoid receptors live in the moment

The classification and treatment of breast cancer is largely defined by the expression of steroid hormone receptors (HRs), namely estrogen receptor (ER) and progesterone receptor (PR), and gene amplification/overexpression of human epidermal growth factor receptor 2 (HER2). More recently, studies of androgen receptor (AR), glucocorticoid receptor (GR), and mineralocorticoid receptor (MR) have revealed that targeting these related HRs may be a promising strategy for a more personalized approach to the treatment of specific subtypes of HR+ breast cancer. For example, GR expression is associated with a good prognosis in ER+ breast cancer, but predicts poor prognosis in triple-negative breast cancer (TNBC). GR, like ER, PRs, and AR, is a ligand-activated transcription factor, but also has significant ligand-independent signaling activities. GR transcriptional activity is classically regulated by circulating glucocorticoids (GCs; ligand-dependent). Recent studies demonstrate that GR transcriptional activity is also regulated by a variety of cellular stress stimuli that input to GR Ser134 phosphorylation via rapid activation of the p38 mitogen activated protein kinase (MAPK) signaling pathway (ligand-independent). Furthermore, ligand-independent GR activation promotes feedforward signaling loops that mediate sustained activation of stress signaling pathways to drive advanced cancer biology (i.e. migration, invasion, chemoresistance, survival, and cellular growth). In this review, we will focus on the role of GR as a key sensor and mediator of physiologic and tumor microenvironment (TME)-derived cellular stress signaling in TNBC and discuss how targeting GR and/or associated signaling pathways may provide a strategy to inhibit deadly TNBC progression.

Abstract 3085: Steroid receptor co-activators regulate metabolic kinases to drive therapy resistant ER+ breast cancer

Recurrence of metastatic breast cancer stemming from acquired endocrine and chemotherapy resistance remains a health burden for women with luminal (ER+) breast cancer. Disseminated ER+ tumor cells can remain viable but quiescent for years to decades. Contributing factors to metastatic spread include the maintenance and expansion of breast cancer stem cells (CSCs). Breast CSCs are poorly proliferative and frequently exist as a minority population in therapy resistant tumors. Our objective is to define novel signaling pathways that govern therapy resistance in ER+ breast cancer. In this study, we show that cytoplasmic complexes composed of steroid receptor (SR) co-activators, PELP1 and SRC-3, modulate breast CSC expansion through upregulation of the HIF-activated metabolic target genes PFKFB3 and PFKFB4. Seahorse metabolic assays demonstrated that cytoplasmic PELP1 influences cellular metabolism by increasing both glycolysis and mitochondrial respiration. PELP1 interacts with PFKFB3 and PFKFB4 proteins, and inhibition of PFKFB3 and PFKFB4 kinase activity blocks PELP1-induced tumorspheres and protein-protein interactions with SRC-3. PFKFB4 knockdown inhibited in vivo emergence of circulating tumor cell (CTC) populations in ER+ mammary intraductal (MIND) xenografts. Application of PFKFB inhibitors in combination with ER targeted therapies blocked tumorsphere formation in multiple models of advanced breast cancer, including tamoxifen (TamR) and paclitaxel (TaxR) resistant models and ER+ patient-derived organoids (PDxO). Together, our data suggest that PELP1, SRC-3, and PFKFBs cooperate to drive ER+ tumor cells that include CSCs and CTCs. Identifying non-ER pharmacological targets offers a useful approach to blocking metastatic escape from standard of care ER/estrogen (E2)-targeted strategies to overcome endocrine and chemotherapy resistance in ER+ breast cancer.

Citation Format: Thu Ha Truong, Elizabeth Benner, Kyla M. Hagen, Nuri A. Temiz, Carlos Perez Kerkvliet, Ying Wang, Emilio Cortes-Sanchez, Chieh-Hsiang Yang, Thomas Pengo, Katrin P. Guillen, Bryan E. Welm, Sucheta Telang, Carol A. Lange, Julie H. Ostrander. Steroid receptor co-activators regulate metabolic kinases to drive therapy resistant ER+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3085.

Abstract 744: Progesterone receptors promote quiescence & ovarian cancer cell phenotypes via modulation of DREAM in p53-mutant fallopian tube models

The ability of ovarian steroids to modulate ovarian cancer (OC) risk remains controversial. High grade serous ovarian carcinoma (HGSC), the most aggressive OC subtype, contains abundant estrogen (ER; 76%) and progesterone (PR; 35%) receptors. Progesterone is thought to be a primarily protective factor that reduces OC risk but the relevance of PR actions in HGSC is unknown. Our IHC analyses of human clinical tissues showed robust expression of total and activated phospho-S294 PR within serous tubal intraepithelial carcinomas (STIC), precursor HGSC lesions of fallopian tube epithelia (FTE), suggesting a role for PR signaling in early disease. To explore the molecular mechanisms of PR isoform actions, p53-dominant negative mutant FTE lines expressing the PR-A or PR-B isoform were created. Progestin treatment (R5020 or progesterone) of these models induced spheroid formation and reaggregation, transwell migration and collagen invasion. RNAseq analyses revealed enrichment of cell cycle progression pathways; in particular, the gene targets of the repressive DREAM complex that drives quiescent cell fate. Progestins induced inhibition of proliferation (decreased Ki67; decreased BrdU; increased B-galactosidase) and accumulation of cells in G0/G1 that was reversed following growth factor stimulation. Subsequent immunoprecipitation of DREAM (G0) and B-MYB/MMB complexes (G1/S) confirmed that PR-driven changes in cell fate occur via regulation of the DREAM complex. PR activation enhanced DREAM and suppressed B-MYB/MMB complex formation. PR also induced transcriptional regulation of DREAM/B-MYB/MMB genes (LIN9, MYBL2, DYRK1A, FOXM1) via promoter recruitment and mRNA regulation. Interestingly, DREAM complexes contained PR and PR was co-recruited to DREAM binding sites within DREAM target promoters, along with the DREAM complex proteins p130 and E2F4. The relevance of the quiescent state to early OC phenotypes was examined through the disruption of DREAM/DYRK1s by pharmacological inhibition (harmine) , HPV E6/E7 expression or the depletion of DYRK1A/B kinases that promote DREAM complex formation. These manipulations blocked progestin-induced cell cycle arrest and attenuated PR-driven gene expression and associated OC phenotypes. Our results reveal that activated PRs support quiescence and pro-survival/pro-dissemination cell behaviors that may contribute to early HGSC progression. Taken together, our data suggest an alternative perspective on the tenant that progesterone always confers protection against OC. STICs can reside undetected for decades prior to invasive disease. Our studies reveal clinical opportunities to prevent the ultimate development of HGSC by targeting activated PRs, DREAM complexes, and/or DYRKs (Supported by the Minnesota Ovarian Cancer Alliance to LJM and CAL).

Citation Format: Laura J. Mauro, Megan I. Seibel, Caroline H. Diep, Angela Spartz, Carlos Perez Kerkvliet, Hari Singhal, Elizabeth M. Swisher, Lauren E. Schwartz, Ronny Drapkin, Siddharth Saini, Fatmata Seesay, Larisa Litovchick, Carol A. Lange. Progesterone receptors promote quiescence & ovarian cancer cell phenotypes via modulation of DREAM in p53-mutant fallopian tube models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 744.

Progesterone Receptors Promote Quiescence and Ovarian Cancer Cell Phenotypes via DREAM in p53-Mutant Fallopian Tube Models

CONTEXT

The ability of ovarian steroids to modify ovarian cancer (OC) risk remains controversial. Progesterone is considered to be protective; recent studies indicate no effect or enhanced OC risk. Knowledge of progesterone receptor (PR) signaling during altered physiology that typifies OC development is limited.

OBJECTIVE

This study defines PR-driven oncogenic signaling mechanisms in p53-mutant human fallopian tube epithelia (hFTE), a precursor of the most aggressive OC subtype.

METHODS

PR expression in clinical samples of serous tubal intraepithelial carcinoma (STIC) lesions and high-grade serous OC (HGSC) tumors was analyzed. Novel PR-A and PR-B isoform-expressing hFTE models were characterized for gene expression and cell cycle progression, emboli formation, and invasion. PR regulation of the DREAM quiescence complex and DYRK1 kinases was established.

RESULTS

STICs and HGSC express abundant activated phospho-PR. Progestin promoted reversible hFTE cell cycle arrest, spheroid formation, and invasion. RNAseq/biochemical studies revealed potent ligand-independent/-dependent PR actions, progestin-induced regulation of the DREAM quiescence complex, and cell cycle target genes through enhanced complex formation and chromatin recruitment. Disruption of DREAM/DYRK1s by pharmacological inhibition, HPV E6/E7 expression, or DYRK1A/B depletion blocked progestin-induced cell arrest and attenuated PR-driven gene expression and associated OC phenotypes.

CONCLUSION

Activated PRs support quiescence and pro-survival/pro-dissemination cell behaviors that may contribute to early HGSC progression. Our data support an alternative perspective on the tenet that progesterone always confers protection against OC. STICs can reside undetected for decades prior to invasive disease; our studies reveal clinical opportunities to prevent the ultimate development of HGSC by targeting PRs, DREAM, and/or DYRKs.

PELP1/SRC-3-dependent regulation of metabolic PFKFB kinases drives therapy resistant ER+ breast cancer

Recurrence of metastatic breast cancer stemming from acquired endocrine and chemotherapy resistance remains a health burden for women with luminal (ER+) breast cancer. Disseminated ER+ tumor cells can remain viable but quiescent for years to decades. Contributing factors to metastatic spread include the maintenance and expansion of breast cancer stem cells (CSCs). Breast CSCs frequently exist as a minority population in therapy resistant tumors. In this study, we show that cytoplasmic complexes composed of steroid receptor (SR) co-activators, PELP1 and SRC-3, modulate breast CSC expansion through upregulation of the HIF-activated metabolic target genes PFKFB3 and PFKFB4. Seahorse metabolic assays demonstrated that cytoplasmic PELP1 influences cellular metabolism by increasing both glycolysis and mitochondrial respiration. PELP1 interacts with PFKFB3 and PFKFB4 proteins, and inhibition of PFKFB3 and PFKFB4 kinase activity blocks PELP1-induced tumorspheres and protein-protein interactions with SRC-3. PFKFB4 knockdown inhibited in vivo emergence of circulating tumor cell (CTC) populations in mammary intraductal (MIND) models. Application of PFKFB inhibitors in combination with ER targeted therapies blocked tumorsphere formation in multiple models of advanced breast cancer including tamoxifen (TamR) and paclitaxel (TaxR) resistant models, murine tumor cells, and ER+ patient-derived organoids (PDxO). Together, our data suggest that PELP1, SRC-3, and PFKFBs cooperate to drive ER+ tumor cell populations that include CSCs and CTCs. Identifying non-ER pharmacological targets offers a useful approach to blocking metastatic escape from standard of care ER/estrogen (E2)-targeted strategies to overcome endocrine and chemotherapy resistance.

Progesterone promotes immunomodulation and tumor development in the murine mammary gland

BACKGROUND

Clinical studies have linked usage of progestins (synthetic progesterone [P4]) to breast cancer risk. However, little is understood regarding the role of native P4, signaling through the progesterone receptor (PR), in breast tumor formation. Recently, we reported a link between PR and immune signaling pathways, showing that P4/PR can repress type I interferon signaling pathways. Given these findings, we sought to investigate whether P4/PR drive immunomodulation in the mammary gland and promote tumor formation.

METHODS

To determine the effect of P4 on immune cell populations in the murine mammary gland, mice were treated with P4 or placebo pellets for 21 days. Immune cell populations in the mammary gland, spleen, and inguinal lymph nodes were subsequently analyzed by flow cytometry. To assess the effect of PR overexpression on mammary gland tumor development as well as immune cell populations in the mammary gland, a transgenic mouse model was used in which PR was overexpressed throughout the entire mouse. Immune cell populations were assessed in the mammary glands, spleens, and inguinal lymph nodes of 6-month-old transgenic and control mice by flow cytometry. Transgenic mice were also monitored for mammary gland tumor development over a 2-year time span. Following development of mammary gland tumors, immune cell populations in the tumors and spleens of transgenic and control mice were analyzed by flow cytometry.

RESULTS

We found that mice treated with P4 exhibited changes in the mammary gland indicative of an inhibited immune response compared with placebo-treated mice. Furthermore, transgenic mice with PR overexpression demonstrated decreased numbers of immune cell populations in their mammary glands, lymph nodes, and spleens. On long-term monitoring, we determined that multiparous PR-overexpressing mice developed significantly more mammary gland tumors than control mice. Additionally, tumors from PR-overexpressing mice contained fewer infiltrating immune cells. Finally, RNA sequencing analysis of tumor samples revealed that immune-related gene signatures were lower in tumors from PR-overexpressing mice as compared with control mice.

CONCLUSION

Together, these findings offer a novel mechanism of P4-driven mammary gland tumor development and provide rationale in investigating the usage of antiprogestin therapies to promote immune-mediated elimination of mammary gland tumors.

Steroid Receptor Co-Activators Regulate Metabolic Kinases to Drive Therapy Resistant ER+ Breast Cancer

Recurrence of metastatic breast cancer stemming from acquired endocrine and chemotherapy resistance remains a health burden for women with luminal (ER+) breast cancer. Disseminated ER+ tumor cells can remain viable but quiescent for years to decades. Contributing factors to metastatic spread include the maintenance and expansion of breast cancer stem cells (CSCs). Breast CSCs are poorly proliferative and frequently exist as a minority population in therapy resistant tumors. Our objective is to define novel signaling pathways that govern therapy resistance in ER+ breast cancer. In this study, we show that cytoplasmic complexes composed of steroid receptor (SR) co-activators, PELP1 and SRC-3, modulate breast CSC expansion through upregulation of the HIF-activated metabolic target genes PFKFB3 and PFKFB4. Seahorse metabolic assays demonstrated that cytoplasmic PELP1 influences cellular metabolism by increasing both glycolysis and mitochondrial respiration. PELP1 interacts with PFKFB3 and PFKFB4 proteins, and inhibition of PFKFB3 and PFKFB4 kinase activity blocks PELP1-induced tumorspheres and protein-protein interactions with SRC-3. PFKFB4 knockdown inhibited in vivo emergence of circulating tumor cell (CTC) populations in ER+ mammary intraductal (MIND) xenografts. Application of PFKFB inhibitors in combination with ER targeted therapies blocked tumorsphere formation in multiple models of advanced breast cancer, including tamoxifen (TamR) and paclitaxel (TaxR) resistant models and ER+ patient-derived organoids (PDxO). Together, our data suggest that PELP1, SRC-3, and PFKFBs cooperate to drive ER+ tumor cells that include CSCs and CTCs. Identifying non-ER pharmacological targets offers a useful approach to blocking metastatic escape from standard of care ER/estrogen (E2)-targeted strategies to overcome endocrine and chemotherapy resistance in ER+ breast cancer.

Publish or Perish: Five Steps to Navigating a Less Painful Peer Review

This Perspective presents comments intended for junior researchers by Carol A. Lange, Editor-in-Chief, Endocrinology, and Stephen R. Hammes, former Editor-in-Chief, Molecular Endocrinology, and former co-Editor-in-Chief, Endocrinology.

PRINCIPAL POINTS

1. Know when you are ready and identify your target audience.2. Select an appropriate journal.3. Craft your title and abstract to capture your key words and deliver your message.4. Tell a clear and impactful story.5. Review, polish, and perfect your manuscript.

Breast Tumor Kinase (Brk/PTK6) Mediates Advanced Cancer Phenotypes via SH2-Domain Dependent Activation of RhoA and Aryl Hydrocarbon Receptor (AhR) Signaling

Protein tyrosine kinase 6 (PTK6; also called Brk) is overexpressed in 86% of patients with breast cancer; high PTK6 expression predicts poor outcome. We reported PTK6 induction by HIF/GR complexes in response to either cellular or host stress. However, PTK6-driven signaling events in the context of triple-negative breast cancer (TNBC) remain undefined. In a mouse model of TNBC, manipulation of PTK6 levels (i.e., via knock-out or add-back) had little effect on primary tumor volume, but altered lung metastasis. To delineate the mechanisms of PTK6 downstream signaling, we created kinase-dead (KM) and kinase-intact domain structure mutants of PTK6 via in-frame deletions of the N-terminal SH3 or SH2 domains. While the PTK6 kinase domain contributed to soft-agar colony formation, PTK6 kinase activity was entirely dispensable for cell migration. Specifically, TNBC models expressing a PTK6 variant lacking the SH2 domain (SH2-del PTK6) were unresponsive to growth factor-stimulated cell motility relative to SH3-del, KM, or wild-type PTK6 controls. Reverse-phase protein array revealed that while intact PTK6 mediates spheroid formation via p38 MAPK signaling, the SH2 domain of PTK6 limits this biology, and instead mediates TNBC cell motility via activation of the RhoA and/or AhR signaling pathways. Inhibition of RhoA and/or AhR blocked TNBC cell migration as well as the branching/invasive morphology of PTK6⁺/AhR⁺ primary breast tumor tissue organoids. Inhibition of RhoA also enhanced paclitaxel cytotoxicity in TNBC cells, including in a taxane-refractory TNBC model. IMPLICATIONS: The SH2-domain of PTK6 is a potent effector of advanced cancer phenotypes in TNBC via RhoA and AhR, identified herein as novel therapeutic targets in PTK6⁺ breast tumors.

Abstract 3782: Steroid receptor co-activator complexes cooperate to reprogram metabolic pathways and drive therapy resistance in luminal breast cancer

Late recurrence of metastatic disease stemming from acquired therapy resistance remains a significant health burden for women with estrogen receptor (ER) positive breast cancer. Disseminated ER+ tumor cell populations can remain quiescent for years to decades, and contributing factors include breast cancer stem cells (CSCs), which are non-proliferative and frequently exist as a minority population in recurrent therapy-resistant tumors. Progesterone receptors (PR) are known drivers of both normal stem and breast CSC outgrowth. Our objective was to define novel signaling pathways governing cell fate transitions involved in driving therapy resistance in ER+ breast cancer. We reported that cytoplasmic complexes composed of steroid receptor (SR) co-activators, PELP1 and SRC-3, drive breast CSC outgrowth. SRC-3 knockdown abrogated cytoplasmic PELP1-induced CSC expansion and target genes required for cell survival, suggesting an essential role for PELP1/SRC-3 complexes in breast CSC outgrowth. PELP1 also forms a signaling and transcriptional complex with ERα and PR-B. Phospho-PR species are key mediators of stemness in ER+ breast cancer models. Accordingly, PR knockdown and antiprogestins disrupted PELP1/SRC-3 complexes and blocked PELP1-induced breast CSC outgrowth. Moreover, mammary stem cell (MaSC) populations were increased in vivo in MMTV-tTA;TRE-cyto-PELP1 transgenic mouse models as well as in MMTV-tTA;TRE-hPR-B mice. To better understand PELP1-mediated pathways, we performed RNA-seq on MCF-7 PELP1+ models grown in tumorsphere conditions to enrich for breast CSC populations (ALDH+, CD44+/CD24-). Cytoplasmic PELP1-expressing cells had a different global gene profile relative to cells expressing WT PELP1 (i.e. nuclear). Gene sets associated with stem cell biology, hypoxic stress, and cancer metabolism were differentially regulated, including members of the glycolytic bi-functional kinase/phosphatase PFKFB family. Seahorse metabolic phenotyping demonstrated that cytoplasmic PELP1 influences cellular metabolism by increasing both glycolysis and mitochondrial respiration. Cytoplasmic PELP1 interacted strongly with PFKFB3 and PFKFB4, and inhibition of PFKFB3 or PFKFB4 kinase activity blocked PELP1-induced tumorsphere formation and protein-protein interactions with SRC-3. Additionally, antiprogestin and PFKFB inhibitors were synergistic when combined with ER+ targeted therapies. These aspects of PELP1/SRC-3 biology were phenocopied in therapy resistant models (tamoxifen resistant [TamR], paclitaxel resistant [TaxR]). Together, our data suggest that PELP1, SRC-3, PR, and PFKFBs form signaling complexes that reprogram cellular metabolism to drive expansion of breast CSCs. Identifying the molecular mechanisms that regulate recurrent ER+ tumor cell populations will enable specific targeting within heterogeneous breast tumors. Our work may lead to the development of non-ER targets that can be used in combination with current endocrine treatments and PR-targeting strategies to overcome therapy resistance in ER+ breast cancer.

Citation Format: Julie Hanson Ostrander, Thu H. Truong, Elizabeth Benner, Amy R. Dwyer, Carol A. Lange. Steroid receptor co-activator complexes cooperate to reprogram metabolic pathways and drive therapy resistance in luminal breast cancer [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3782.

90 YEARS OF PROGESTERONE: Steroid receptors as MAPK signaling sensors in breast cancer: let the fates decide

Steroid hormone receptors (SRs) are classically defined as ligand-activated transcription factors that function as master regulators of gene programs important for a wide range of processes governing adult physiology, development, and cell or tissue homeostasis. A second function of SRs includes the ability to activate cytoplasmic signaling pathways. Estrogen (ER), androgen (AR), and progesterone (PR) receptors bind directly to membrane-associated signaling molecules including mitogenic protein kinases (i.e. c-SRC and AKT), G-proteins, and ion channels to mediate context-dependent actions via rapid activation of downstream signaling pathways. In addition to making direct contact with diverse signaling molecules, SRs are further fully integrated with signaling pathways by virtue of their N-terminal phosphorylation sites that act as regulatory hot-spots capable of sensing the signaling milieu. In particular, ER, AR, PR, and closely related glucocorticoid receptors (GR) share the property of accepting (i.e. sensing) ligand-independent phosphorylation events by proline-directed kinases in the MAPK and CDK families. These signaling inputs act as a 'second ligand' that dramatically impacts cell fate. In the face of drugs that reliably target SR ligand-binding domains to block uncontrolled cancer growth, ligand-independent post-translational modifications guide changes in cell fate that confer increased survival, EMT, migration/invasion, stemness properties, and therapy resistance of non-proliferating SR+ cancer cell subpopulations. The focus of this review is on MAPK pathways in the regulation of SR+ cancer cell fate. MAPK-dependent phosphorylation of PR (Ser294) and GR (Ser134) will primarily be discussed in light of the need to target changes in breast cancer cell fate as part of modernized combination therapies.

SAT-133 Breast Tumor Kinase (Brk/PTK6) Mediates Triple Negative Breast Cancer Cell Migration and Taxol Resistance via SH2 Domain-Dependent Activation of RhoA and AhR

Triple negative breast cancer (TNBC) patients have higher recurrence rates and a worse prognosis relative to patients diagnosed with other breast cancer subtypes. Protein tyrosine kinase 6 (PTK6; also called Brk), a soluble tyrosine kinase, is overexpressed in 86% of breast cancer patients, however its precise function in the context of TNBC is poorly defined. PTK6 expression is elevated in TNBC models in response to both cellular and endocrine stress, coordinated transcriptionally by the Hypoxia-Inducible Factors (HIFs) and glucocorticoid receptor (GR). We showed previously that PTK6 expression, but not its intrinsic kinase activity, is required for breast cancer cell motility. To further delineate the mechanisms of PTK6 signaling, we created kinase-intact domain structure mutants of PTK6 via in frame deletions of the N-terminal SH3 or SH2 domains. MDA-MB-231 cells expressing a PTK6 variant lacking the SH2 domain (SH2-del PTK6) were less responsive to growth factor-stimulated cell motility relative to wild type or kinase dead (KM) controls. To identify signal transduction pathways activated in TNBC cells harboring PTK6 domain mutants, we used a reverse phase protein array (RPPA), which revealed that the SH2 domain of PTK6 mediates TNBC cell motility via activation of the RhoA and/or AhR signaling pathways. Moreover, in TNBC cells, including a taxane-refractory TNBC model, addition of AhR or Rho inhibitors to paclitaxel (Taxol) enhanced cytotoxicity. Together, these studies reveal that the SH2-domain of PTK6 is an effector of advanced cancer phenotypes in GR+ TNBC cells and identify RhoA and AhR as novel therapeutic targets in PTK6+ tumors.

OR05-04 Steroid Receptor Co-Activators Complexes Cooperate with Progesterone Receptors (PR) to Reprogram Metabolic Pathways that Drive Therapy Resistant Populations in ER+ Breast Cancer

Late recurrence of metastatic disease stemming from acquired therapy resistance remains a significant health burden for women with ER+ breast cancer. Disseminated ER+ tumor cell populations can remain quiescent for years to decades, and contributing factors include breast cancer stem cells (CSCs), which are non-proliferative and frequently exist as a minority population in recurrent therapy-resistant tumors. Progesterone receptors (PR) are known drivers of normal stem and breast CSCs. Our objective was to define novel signaling pathways governing cell fate transitions involved in driving therapy resistance in ER+ breast cancer. We reported that cytoplasmic complexes composed of steroid receptor (SR) co-activators, PELP1 and SRC-3, drive breast CSC outgrowth. SRC-3 knockdown abrogated PELP1-induced CSC expansion and target genes required for cell survival, suggesting an essential role for PELP1/SRC-3 complexes. PELP1 also forms a signaling and transcriptional complex with ER and PR-B. Phospho-PR species are key mediators of stemness in ER+ breast cancer models. Accordingly, PR knockdown and antiprogestins disrupted PELP1/SRC-3 complexes and blocked PELP1-induced breast CSC outgrowth. Mammary stem cell (MaSC) populations were increased in vivo in MMTV-tTA;TRE-cyto-PELP1 transgenic mice as well as in MMTV-tTA;TRE-hPR-B mice. To better understand PELP1-mediated pathways, we performed RNA-seq on MCF-7 PELP1+ models grown in tumorsphere conditions to enrich for CSC populations (ALDH+, CD44+/CD24-). Cytoplasmic PELP1-expressing cells had a different global gene profile relative to WT PELP1 (i.e. nuclear). Gene sets associated with stem cell biology, hypoxic stress, and cancer metabolism were differentially regulated, including members of the glycolytic bi-functional kinase/phosphatase PFKFB family. Seahorse metabolic phenotyping demonstrated cytoplasmic PELP1 influences metabolism by increasing both glycolysis and mitochondrial respiration. Cytoplasmic PELP1 interacted strongly with PFKFB3 and PFKFB4, and inhibition of PFKFB3 or PFKFB4 kinase activity blocked PELP1-induced tumorspheres and protein-protein interactions with SRC-3. Additionally, antiprogestin and PFKFB inhibitors were synergistic when combined with ER+ targeted therapies. These aspects of PELP1/SRC-3 biology were phenocopied in therapy resistant models (tamoxifen resistant [TamR], paclitaxel resistant [TaxR]). Together, our data suggest that PELP1, SRC-3, PR, and PFKFBs form complexes that reprogram cellular metabolism to drive breast CSC expansion. Identifying the mechanisms that regulate recurrent ER+ tumor cell populations will enable specific targeting within heterogeneous breast tumors and may lead to the development of non-ER targets that can be used in combination with endocrine treatments to overcome therapy resistance.

Glucocorticoid receptors are required effectors of TGFβ1-induced p38 MAPK signaling to advanced cancer phenotypes in triple-negative breast cancer

BACKGROUND

Altered signaling pathways typify breast cancer and serve as direct inputs to steroid hormone receptor sensors. We previously reported that phospho-Ser134-GR (pS134-GR) species are elevated in triple-negative breast cancer (TNBC) and cooperate with hypoxia-inducible factors, providing a novel avenue for activation of GR in response to local or cellular stress.

METHODS

We probed GR regulation by factors (cytokines, growth factors) that are rich within the tumor microenvironment (TME). TNBC cells harboring endogenous wild-type (wt) or S134A-GR species were created by CRISPR/Cas knock-in and subjected to transwell migration, invasion, soft-agar colony formation, and tumorsphere assays. RNA-seq was employed to identify pS134-GR target genes that are regulated both basally (intrinsic) or by TGFβ1 in the absence of exogenously added GR ligands. Regulation of selected basal and TGFβ1-induced pS134-GR target genes was validated by qRT-PCR and chromatin immunoprecipitation assays. Bioinformatics tools were used to probe public data sets for expression of pS134-GR 24-gene signatures.

RESULTS

In the absence of GR ligands, GR is transcriptionally activated via p38-dependent phosphorylation of Ser134 as a mechanism of homeostatic stress-sensing and regulated upon exposure of TNBC cells to TME-derived agents. The ligand-independent pS134-GR transcriptome encompasses TGFβ1 and MAPK signaling gene sets associated with TNBC cell survival and migration/invasion. Accordingly, pS134-GR was essential for TNBC cell anchorage-independent growth in soft-agar, migration, invasion, and tumorsphere formation, an in vitro readout of cancer stemness properties. Both pS134-GR and expression of the MAPK-scaffolding molecule 14-3-3ζ were essential for a functionally intact p38 MAPK signaling pathway downstream of MAP3K5/ASK1, indicative of a feedforward signaling loop wherein self-perpetuated GR phosphorylation enables cancer cell autonomy. A 24-gene pS134-GR-dependent signature induced by TGFβ1 predicts shortened overall survival in breast cancer patients.

CONCLUSIONS

Phospho-S134-GR is a critical downstream effector of p38 MAPK signaling and TNBC migration/invasion, survival, and stemness properties. Our studies define a ligand-independent role for GR as a homeostatic "sensor" of intrinsic stimuli as well as extrinsic factors rich within the TME (TGFβ1) that enable potent activation of the p38 MAPK stress-sensing pathway and nominate pS134-GR as a therapeutic target in aggressive TNBC.

Abstract P5-05-06: Progesterone receptor (PR) isoform-specific expansion of breast cancer stem-like cells occurs through a phospho-PR/FOXO1 driven signaling axis

Luminal breast cancers account for ˜75% of cases and are positive for progesterone receptor (PR) and estrogen receptor (ER) expression. PR is a classical ER-target gene and is used as a biomarker of ER activity; however, a growing body of evidence supports the role of PR as an important modifier of ER actions and a key driver of luminal breast cancer progression. Progesterone signaling is mediated by two PR isoforms: full-length PR-B and truncated PR-A, which lacks the N-terminal 164 amino acids. Little is known about PR isoform-specific actions in PR-expressing breast tumors, given that total PR expression is measured clinically. Herein, we sought to identify phenotypic differences in luminal breast cancer cells (T47D) overexpressing PR-A (T47D-YA) or PR-B (T47D-YB). We demonstrate that PR-B expression is required for anchorage-independent colony formation, while PR-A expressing cells fail to proliferate in soft agar. PR-B driven proliferation has been mapped to PR phosphorylation events, which include MAPK or CDK consensus sites such as Ser294. We demonstrate that in contrast to previous reports, PR-A is well phosphorylated at Ser294 in response to progestins (e.g. R5020) using our custom phospho-PR (Ser294) polyclonal antibody. Interestingly, Ser294 phosphorylation of PR-A occurs more rapidly and robustly following hormone treatment compared to PR-B expressing cells. Our findings indicate that PR-A is a dominant driver of stem-like expansion in breast cancer cells. PR-A tumorspheres exhibited enriched ALDH+ and CD44+/CD24- populations compared to PR-B and promoted heightened gene expression of stem-like genes (e.g. FOXO1). We demonstrate that the PR target gene and co-activator FOXO1 promotes both PR-A and PR-B phosphorylation at Ser294 and augments tumorsphere formation. Direct inhibition of FOXO1 levels abrogates phospho-PR (Ser294) levels and tumorsphere formation in PR expressing cells. Finally, we show that Ser294 is required for PR-A induced expression of stem-like genes (e.g. FOXO1) and stem-like behavior as measured by ALDH+/CD44+ tumorspheres. Taken together, our data reveal unique functions of PR isoforms as modulators of distinct and opposing pathways (i.e. proliferation versus stem-like expansion) in luminal breast cancer models. A clear understanding of PR isoform-specific actions, phosphorylation events, and the role of co-factors such as FOXO1 may lead to novel biomarkers of advanced tumor behavior and reveal new approaches to pharmacologically target PR isoforms in luminal breast cancers.

Citation Format: Truong TH, Dwyer AR, Diep CH, Lange CA. Progesterone receptor (PR) isoform-specific expansion of breast cancer stem-like cells occurs through a phospho-PR/FOXO1 driven signaling axis [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-05-06.

Phosphorylated Progesterone Receptor Isoforms Mediate Opposing Stem Cell and Proliferative Breast Cancer Cell Fates

Progesterone receptors (PRs) are key modifiers of estrogen receptor (ER) target genes and drivers of luminal breast cancer progression. Total PR expression, rather than isoform-specific PR expression, is measured in breast tumors as an indicator of functional ER. We identified phenotypic differences between PR-A and PR-B in luminal breast cancer models with a focus on tumorsphere biology. Our findings indicated that PR-A is a dominant driver of cancer stem cell (CSC) expansion in T47D models, and PR-B is a potent driver of anchorage-independent proliferation. PR-A+ tumorspheres were enriched for aldehyde dehydrogenase (ALDH) activity, CD44+/CD24-, and CD49f+/CD24- cell populations relative to PR-B+ tumorspheres. Progestin promoted heightened expression of known CSC-associated target genes in PR-A+ but not PR-B+ cells cultured as tumorspheres. We report robust phosphorylation of PR-A relative to PR-B Ser294 and found that this residue is required for PR-A-induced expression of CSC-associated genes and CSC behavior. Cells expressing PR-A S294A exhibited impaired CSC phenotypes but heightened anchorage-independent cell proliferation. The PR target gene and coactivator, FOXO1, promoted PR phosphorylation and tumorsphere formation. The FOXO1 inhibitor (AS1842856) alone or combined with onapristone (PR antagonist), blunted phosphorylated PR, and tumorsphere formation in PR-A+ and PR-B+ T47D, MCF7, and BT474 models. Our data revealed unique isoform-specific functions of phosphorylated PRs as modulators of distinct and opposing pathways relevant to mechanisms of late recurrence. A clear understanding of PR isoforms, phosphorylation events, and the role of cofactors could lead to novel biomarkers of advanced tumor behavior and reveal new approaches to pharmacologically target CSCs in luminal breast cancer.

Deciphering Steroid Receptor Crosstalk in Hormone-Driven Cancers

Steroid hormone receptors (SRs) have a multitude of functions in human biology and disease progression. The SR family of related ligand-activated transcription factors includes androgen, estrogen, glucocorticoid, mineralocorticoid, and progesterone receptors. Antiestrogen or estrogen receptor (ER)-targeted therapies to block ER action remain the primary treatment of luminal breast cancers. Although this strategy is successful, ∼40% of patients eventually relapse due to endocrine resistance. The majority of hormone-independent tumors retain some level of SR expression, but sidestep hormone ablation treatments. SRs are known to crosstalk extensively with kinase signaling pathways, and this interplay has been shown to bypass ER-targeted therapies in part by providing alternative proliferation and survival signals that enable hormone independence. Modified receptors adopt alternate conformations that resist antagonism or promote agonism. SR-regulated transcription and SR-binding events have been classically studied as single receptor events using single hormones. However, it is becoming increasingly evident that individual steroids and SRs rarely act alone. Emerging evidence shows that coexpressed SRs crosstalk with each other in hormone-driven cancers, such as breast and prostate. Crosstalk between related SRs allows them to modulate signaling and transcriptional responses to noncognate ligands. This flexibility can lead to altered genomic binding and subsequent changes in SR target gene expression. This review will discuss recent mechanistic advances in elucidating SR crosstalk and the implications for treating hormone-driven cancers. Understanding this crosstalk (i.e., both opposing and collaborative) is a critical step toward expanding and modernizing endocrine therapies and will ultimately improve patient outcomes.

NOTCH3 expression is linked to breast cancer seeding and distant metastasis

Background

Development of distant metastases involves a complex multistep biological process termed the invasion-metastasis cascade, which includes dissemination of cancer cells from the primary tumor to secondary organs. NOTCH developmental signaling plays a critical role in promoting epithelial-to-mesenchymal transition, tumor stemness, and metastasis. Although all four NOTCH receptors show oncogenic properties, the unique role of each of these receptors in the sequential stepwise events that typify the invasion-metastasis cascade remains elusive.

Methods

We have established metastatic xenografts expressing high endogenous levels of NOTCH3 using estrogen receptor alpha-positive (ERα⁺) MCF-7 breast cancer cells with constitutive active Raf-1/mitogen-associated protein kinase (MAPK) signaling (vMCF-7^Raf-1) and MDA-MB-231 triple-negative breast cancer (TNBC) cells. The critical role of NOTCH3 in inducing an invasive phenotype and poor outcome was corroborated in unique TNBC cells resulting from a patient-derived brain metastasis (TNBC-M25) and in publicly available claudin-low breast tumor specimens collected from participants in the Molecular Taxonomy of Breast Cancer International Consortium database.

Results

In this study, we identified an association between NOTCH3 expression and development of metastases in ERα⁺ and TNBC models. ERα⁺ breast tumor xenografts with a constitutive active Raf-1/MAPK signaling developed spontaneous lung metastases through the clonal expansion of cancer cells expressing a NOTCH3 reprogramming network. Abrogation of NOTCH3 expression significantly reduced the self-renewal and invasive capacity of ex vivo breast cancer cells, restoring a luminal CD44^low/CD24^high/ERα^high phenotype. Forced expression of the mitotic Aurora kinase A (AURKA), which promotes breast cancer metastases, failed to restore the invasive capacity of NOTCH3-null cells, demonstrating that NOTCH3 expression is required for an invasive phenotype. Likewise, pharmacologic inhibition of NOTCH signaling also impaired TNBC cell seeding and metastatic growth. Significantly, the role of aberrant NOTCH3 expression in promoting tumor self-renewal, invasiveness, and poor outcome was corroborated in unique TNBC cells from a patient-derived brain metastasis and in publicly available claudin-low breast tumor specimens.

Conclusions

These findings demonstrate the key role of NOTCH3 oncogenic signaling in the genesis of breast cancer metastasis and provide a compelling preclinical rationale for the design of novel therapeutic strategies that will selectively target NOTCH3 to halt metastatic seeding and to improve the clinical outcomes of patients with breast cancer.

Electronic supplementary material

The online version of this article (10.1186/s13058-018-1020-0) contains supplementary material, which is available to authorized users.

Abstract B58: Progesterone receptor (PR) isoforms drive distinct cell-cell interactions and gene expression programs in human fallopian tube models of early HGSOC

High-grade serous ovarian carcinoma (HGSOC), the most prevalent and aggressive form of ovarian cancer (OC), contains abundant receptors for the ovarian steroid hormones, estrogen (ER; 76%) and progesterone (PR; 35%). These receptors contribute to the development of breast and reproductive tract cancers via complex mechanisms. Estrogen, acting through ERalpha, is well accepted as a stimulatory hormone while progesterone is thought to be a protective factor that reduces OC risk. Our understanding, however, of the molecular and cell-type specific mechanisms of PR isoform action in the initiation and progression of OC is limited. Therefore, to investigate altered PR isoform signaling in early-stage HGSOC, we utilized p53-dominant negative mutant fallopian tube epithelial (FTE) cells to generate stable cell lines expressing either PR-A or PR-B. Progestin (R5020) treatment of 2D adherent cultures revealed functional PR signaling through MAPK-dependent PR S294 phosphorylation as well as isoform specific expression of PR target genes encoding adhesion molecules (e.g., HEF1), cell cycle regulators (e.g., FOXO1), and glucocorticoid signaling proteins (e.g., CRISPLD2, NDRG1). In order to mimic early dissemination of FTE cells from serous tubal intraepithelial carcinoma (STIC) lesions, a precursor for the majority of HGSOCs, we established a 3D spheroid culture model. Interestingly, we observed that PR expression greatly increased the formation, size, and number of total spheroids compared to PR-negative controls. Progestin treatment resulted in additional enhancement of spheroid size with distinct morphologic differences observed between PR-A and PR-B expressing lines. Furthermore, progestin-treated cells grown as 3D spheroids exhibited increased basal PR protein and PR S294 phosphorylation levels relative to cells maintained in 2D adherent cultures, suggesting that hyperactivation of PRs occurs via cell-cell interactions, in part conferred by 3D architecture. Both PR-A and PR-B spheroids secreted matrix proteins such as fibronectin and other EMT biomarkers (e.g., integrin alpha 5) and progestin treatment modulated the PR isoform-specific expression of these markers. Preliminary RNA-seq analyses, comparing PR-A and PR-B spheroids, revealed differential expression of genes encoding pathways that regulate cell adhesion and cell movement as well as cytokine signaling and cell metabolism. Our results highlight the importance of cell context in understanding PR action and suggest that activation of PR signaling promotes and enhances the formation of nonadherent 3D structures in an isoform-specific manner. Such effects of progesterone are predicted to influence the shedding, aggregation, and dissemination of early FTE lesions. These preliminary findings demonstrate the importance of understanding the impact of steroid hormone receptors, including PR isoforms, on early OC development.

Citation Format: Megan I. Seibel, Angela Spartz, Nuri Temiz, Laura J. Mauro, Carol A. Lange. Progesterone receptor (PR) isoforms drive distinct cell-cell interactions and gene expression programs in human fallopian tube models of early HGSOC. [abstract]. In: Proceedings of the AACR Conference: Addressing Critical Questions in Ovarian Cancer Research and Treatment; Oct 1-4, 2017; Pittsburgh, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(15_Suppl):Abstract nr B58.

Abstract A30: PELP1 and AIB1 cooperate to promote breast cancer progression in ER+ breast cancer models

Luminal breast cancers account for ~75% of newly diagnosed cases and express estrogen receptor (ER) and a range of progesterone receptor (PR)-positive cells. Although adjuvant hormone therapies that target ER actions (e.g., tamoxifen or aromatase inhibitors) have improved overall patient survival, up to ~40% of luminal breast tumors eventually progress to ER+, but endocrine-resistant disease. Therefore, there is a critical need to delineate the processes driving ER+ breast cancer progression and identify new biomarkers that can be targeted in combination with ER-targeted therapies. An emerging biomarker of increased breast cancer risk and aggressive tumor behavior is oncogenic PELP1 (i.e., partially localized to the cytoplasm; cyto PELP1). PELP1 is typically located in the nucleus in normal breast tissue; however, partial to complete localization of PELP1 to the cytoplasm has been observed in up to 50% of PELP1+ breast tumors. Although a number of studies have implicated oncogenic PELP1 in luminal breast cancer biology, the mechanisms underlying oncogenic PELP1 actions in cancer remain poorly defined. To elucidate the impact of oncogenic PELP1 on steroid hormone receptor (SR) signaling pathways and transcription programs, we generated SR+ breast cancer cell models expressing vector control, wild-type (nuclear) PELP1, and oncogenic (cyto) PELP1. Herein, we identified AIB1 (amplified in breast cancer 1; ER coactivator) as a preferential binding partner of oncogenic PELP1. In particular, our data demonstrate that oncogenic PELP1 overexpression increases activation (i.e., phosphorylation) of AIB1, enhances tumorsphere formation in SR+ breast cancer models, and upregulates specific target genes identified through RNA-Seq analysis that are related to cell survival, breast cancer progression, and stem/progenitor formation independent of hormone stimulation. Knockdown of AIB1 inhibits oncogenic PELP1-induced tumorsphere formation and downregulates oncogenic PELP1 target genes. Moreover, knockdown of PELP1 in AIB1-mouse derived tumor cells results in decreased tumor growth in vivo. To our knowledge, our findings are the first to directly link oncogenic PELP1-induced phenotypes to AIB1 in breast cancer. Our studies suggest that directly targeting PELP1 may halt tumor progression, particularly in the context of AIB1-mediated tumorigenesis. Taken together, our data highlight the oncogenic PELP1/AIB1 interaction as an important hormone-independent mechanism of increased breast tumor cell survival and altered cell fate, and as an important mediator of disease progression. In sum, we conclude that oncogenic PELP1 and AIB1 could be used as biomarkers in conjunction with each other to identify breast cancer patients likely to respond to therapeutic strategies designed to selectively target PELP1, AIB1, or the oncogenic PELP1/AIB1 signaling and transcriptional complex.

Citation Format: Thu H. Truong, Hsiangyu Hu, Julie H. Ostrander, Carol A. Lange. PELP1 and AIB1 cooperate to promote breast cancer progression in ER+ breast cancer models [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr A30.

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.

Abstract 532: PELP1 and AIB1 (SRC-3) complexes promote cancer stem cell-associated phenotypes in ER+ breast cancer models

Proline, glutamic acid, and leucine rich protein 1 (PELP1) is overexpressed in approximately 80% of invasive breast tumors. PELP1 dynamically shuttles between the nucleus and cytoplasm, but is primarily nuclear in normal breast tissue. However, altered localization of PELP1 to the cytoplasm is an oncogenic event that promotes breast cancer initiation and progression. Herein, we sought to identify interacting partners unique to cytoplasmic PELP1 and determine the mechanisms by which these interactions promote oncogenic PELP1 signaling. We identified AIB1 (amplified in breast cancer 1; also known as SRC-3 or NCOA3) as a novel binding partner of cytoplasmic PELP1 in both estrogen receptor-positive (ER+) and ER-negative cell lines. Cytoplasmic PELP1 expression elevated basal phosphorylation levels (i.e., activation) of AIB1, enhanced ALDH+ tumorsphere formation, and upregulated specific target genes independently of hormone stimulation. Direct manipulation of AIB1 levels using shRNA abrogated cytoplasmic PELP1-induced tumorsphere formation and downregulated cytoplasmic PELP1-specific target genes. SI-2, an AIB1 inhibitor, inhibited the PELP1/AIB1 interaction and decreased cytoplasmic PELP1-induced tumorsphere formation. Similar results were observed in a murine-derived MMTV-AIB1 tumor cell line (J110). Furthermore, in vivo syngeneic tumor studies showed that PELP1 knockdown in J110 cells resulted in increased survival of tumor-bearing mice as compared to mice injected with control cells. These studies suggest that manipulating PELP1 location or levels has the potential to mitigate tumor progression, particularly in the context of AIB1-mediated tumorigenesis. Taken together, our data demonstrate that cytoplasmic PELP1/AIB1-containing complexes function to promote advanced cancer phenotypes, including outgrowth of stem-like cells, associated with estrogen-independent breast cancer progression.

Citation Format: Thu H. Truong, Hsiangyu Hu, Nuri A. Temiz, Kyla M. Hagen, Brian J. Girard, Nicholas J. Brady, Kathryn L. Schwertfeger, Carol A. Lange, Julie H. Ostrander. PELP1 and AIB1 (SRC-3) complexes promote cancer stem cell-associated phenotypes in ER+ breast cancer models [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 532.

Abstract 949: Progesterone receptor/IRS-1 cooperation promotes stem cell outgrowth and endocrine resistance in estrogen receptor-positive luminal breast cancer

Luminal breast cancers account for ~75% of all cases and while adjuvant hormone therapy targeting ERα has significantly improved overall survival for patients with ER+ tumors, acquired resistance remains a major clinical problem. Tamoxifen-resistant (TamR) breast cancer models show loss of IGF1R concomitant with increased insulin-induced growth, underpinned by insulin receptor (InsR) compensation for IGF1R loss. PR (gene name PGR) is an estrogen-regulated target gene whose expression is used as a clinical marker of ER activity. However, its de novo relevance to breast cancer is unclear. We have shown previously that post-translational modifications create unique PR species whose altered behavior drives an endocrine-resistant gene signature, in part by crosstalk with the IGFR pathway. We propose that phospho-PR target gene selectivity is mediated by cooperation between PR-B and InsR/IGF1R pathway components. Herein we show that phospho-PR-expressing T47D cells lose expression of IGF1R compared to control cells expressing WT PR-B. Furthermore, expression of the adapter protein IRS-1 requires PRB expression and these IRS-1+ cells were more sensitive to insulin in anchorage-dependent growth assays. Phospho-PR T47D cells exhibit increased ALDH+ and CD24-/CD44+ tumorsphere formation compared to wt PRB-expressing cells. Inhibitors targeting IRS-1 and InsR were used to test their requirement for tumorsphere growth and, surprisingly, IRS-1 perturbation reduced phospho-PRB but not wt PRB tumorsphere growth. Interestingly, IRS-1 replaces IGF1R in phospho-PRB-containing transcriptional complexes that are recruited to the CTSD promoter, which we previously identified as a phospho-PR target gene. Finally, breast cancer cells expressing phospho-PR species exhibited tamoxifen-resistant growth. Collectively our data suggest that phospho-PRB cooperates with IRS-1 downstream of the Ins/IGF1R system to promote outgrowth of endocrine-resistant cells that include ALDH+ stem-like cells capable of forming secondary tumorspheres in vitro. Targeting phospho-PR species in addition to the signaling components of PR-complexes may provide a means to block emergence of endocrine-resistant cancer cells during breast cancer progression.

Citation Format: Amy R. Dwyer, Deepali Sachdev, Carol A. Lange. Progesterone receptor/IRS-1 cooperation promotes stem cell outgrowth and endocrine resistance in estrogen receptor-positive luminal breast cancer [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 949.

Cancer Stem Cell Phenotypes in ER+ Breast Cancer Models Are Promoted by PELP1/AIB1 Complexes

Proline, glutamic acid, leucine-rich protein 1 (PELP1) is overexpressed in approximately 80% of invasive breast tumors. PELP1 dynamically shuttles between the nucleus and cytoplasm, but is primarily nuclear in normal breast tissue. However, altered localization of PELP1 to the cytoplasm is an oncogenic event that promotes breast cancer initiation and progression. Herein, interacting partners unique to cytoplasmic PELP1 and the mechanisms by which these interactions promote oncogenic PELP1 signaling were sought. AIB1 (amplified in breast cancer 1; also known as SRC-3 or NCOA3) was identified as a novel binding partner of cytoplasmic PELP1 in both estrogen receptor-positive (ER+) and ER-negative cell lines. Cytoplasmic PELP1 expression elevated basal phosphorylation levels (i.e., activation) of AIB1 at Thr24, enhanced ALDH+ tumorsphere formation, and upregulated specific target genes independently of hormone stimulation. Direct manipulation of AIB1 levels using shRNA abrogated cytoplasmic PELP1-induced tumorsphere formation and downregulated cytoplasmic PELP1-specific target genes. SI-2, an AIB1 inhibitor, limited the PELP1/AIB1 interaction and decreased cytoplasmic PELP1-induced tumorsphere formation. Similar results were observed in a murine-derived MMTV-AIB1 tumor cell line. Furthermore, in vivo syngeneic tumor studies revealed that PELP1 knockdown resulted in increased survival of tumor-bearing mice as compared with mice injected with control cells.Implications: These data demonstrate that cytoplasmic PELP1/AIB1-containing complexes function to promote advanced cancer phenotypes, including outgrowth of stem-like cells, associated with estrogen-independent breast cancer progression. Mol Cancer Res; 16(4); 707-19. ©2018 AACR.

Abstract ES9-2: Tracking progesterone receptor actions in breast cancer progression: Jekyll and Hyde

Recent work on the actions for progesterone receptors (PRs) in breast cancer has raised the profile of PRs as players rather than simply markers of estrogen (E)-responsive luminal disease. Indeed, estrogen receptors (ERs) and PRs are intimate partners at the genome regulatory level rather than having a simple driver/passenger relationship. PRs alter breast cancer cell proliferation in complex ways and can be either growth inhibitory or stimulatory depending on the cell and hormonal context. Additionally, PRs are important drivers of normal mammary stem cell as well as cancer stem cell expansion. Along with ERs, targeting PRs as part of modernized endocrine therapies may provide a rational treatment alternative for women with ER- and PR-positive breast cancers. However, should PR agonists or antagonists be paired with anti-estrogens or estrogen-ablative therapies and when? This lecture will review historical and current literature on the complex topic of PR action in luminal breast cancer, with focus on key knowledge gaps and future steps that could speed translation of exciting research findings into useful clinical therapeutic applications.

Citation Format: Lange CA. Tracking progesterone receptor actions in breast cancer progression: Jekyll and Hyde [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr ES9-2.

Progesterone receptors (PR) mediate STAT actions: PR and prolactin receptor signaling crosstalk in breast cancer models

Estrogen is the major mitogenic stimulus of mammary gland development during puberty wherein ER signaling acts to induce abundant PR expression. PR signaling, in contrast, is the primary driver of mammary epithelial cell proliferation in adulthood. The high circulating levels of progesterone during pregnancy signal through PR, inducing expression of the prolactin receptor (PRLR). Cooperation between PR and prolactin (PRL) signaling, via regulation of downstream components in the PRL signaling pathway including JAKs and STATs, facilitates the alveolar morphogenesis observed during pregnancy. Indeed, these pathways are fully integrated via activation of shared signaling pathways (i.e. JAKs, MAPKs) as well as by the convergence of PRs and STATs at target genes relevant to both mammary gland biology and breast cancer progression (i.e. proliferation, stem cell outgrowth, tissue cell type heterogeneity). Thus, rather than a single mediator such as ER, transcription factor cascades (ER>PR>STATs) are responsible for rapid proliferative and developmental programming in the normal mammary gland. It is not surprising that these same mediators typify uncontrolled proliferation in a majority of breast cancers, where ER and PR are most often co-expressed and may cooperate to drive malignant tumor progression. This review will primarily focus on the integration of PR and PRL signaling in breast cancer models and the importance of this cross-talk in cancer progression in the context of mammographic density. Components of these PR/PRL signaling pathways could offer alternative drug targets and logical complements to anti-ER or anti-estrogen-based endocrine therapies.

Pathway-based discovery of genetic interactions in breast cancer

Breast cancer is the second largest cause of cancer death among U.S. women and the leading cause of cancer death among women worldwide. Genome-wide association studies (GWAS) have identified several genetic variants associated with susceptibility to breast cancer, but these still explain less than half of the estimated genetic contribution to the disease. Combinations of variants (i.e. genetic interactions) may play an important role in breast cancer susceptibility. However, due to a lack of statistical power, the current tests for genetic interactions from GWAS data mainly leverage prior knowledge to focus on small sets of genes or SNPs that are known to have an association with breast cancer. Thus, many genetic interactions, particularly among novel variants, remain understudied. Reverse-genetic interaction screens in model organisms have shown that genetic interactions frequently cluster into highly structured motifs, where members of the same pathway share similar patterns of genetic interactions. Based on this key observation, we recently developed a method called BridGE to search for such structured motifs in genetic networks derived from GWAS studies and identify pathway-level genetic interactions in human populations. We applied BridGE to six independent breast cancer cohorts and identified significant pathway-level interactions in five cohorts. Joint analysis across all five cohorts revealed a high confidence consensus set of genetic interactions with support in multiple cohorts. The discovered interactions implicated the glutathione conjugation, vitamin D receptor, purine metabolism, mitotic prometaphase, and steroid hormone biosynthesis pathways as major modifiers of breast cancer risk. Notably, while many of the pathways identified by BridGE show clear relevance to breast cancer, variants in these pathways had not been previously discovered by traditional single variant association tests, or single pathway enrichment analysis that does not consider SNP-SNP interactions.

Abstract 3618: PELP1 cytoplasmic mislocalization favors estrogen-induced signaling via AIB1 in ER+ breast cancer models

Luminal breast cancers account for ~75% of cases and express (ER) and frequently contain a range of progesterone receptor (PR) positive cells. ER and PR are steroid hormone receptors (SR) that rapidly and dynamically shuttle between the cytoplasm and the nucleus, a process that allows for essential crosstalk with cytoplasmic signaling pathways that serve as direct inputs to SR transcriptional functions. During disease progression, aberrant hormone signaling often leads to expression of altered SR gene programs and endocrine resistance. An emerging biomarker of increased risk and aggressive cancer behavior is cytoplasmic PELP1. Our lab was the first to show that PELP1 forms a functional signaling and transcriptional complex with ER and PR to regulate novel estrogen-driven SR target genes associated with endocrine resistance. PELP1 is primarily nuclear in normal breast tissue, but cytoplasmic localization of PELP1 (cyto-PELP1) is observed in 40-58% of invasive tumors. To elucidate the impact of PELP1 mislocalization on SR signaling pathways and transcription programs, we have generated SR+ breast cancer cell models expressing vector, wild-type (nuclear) PELP1, and cyto-PELP1. Our studies show that cells expressing cyto-PELP1 exhibit increased mammosphere formation relative to WT-PELP1 or vector control cells in response to estrogen. Moreover, we have identified AIB1 (ER co-activator) as a robust binding partner for cytoplasmic, but not nuclear PELP1, through mass spectrometry. This interaction occurs in a hormone-dependent manner as shown by co-immunoprecipitation assays. AIB1 activation (i.e. phosphorylation levels) is increased in cyto-PELP1 cells relative to WT-PELP1 or control cells. Knockdown of AIB1 reduces cyto-PELP1 protein levels and leads to decreased gene expression of key pro-inflammatory and immune regulatory cyto-PELP1 target genes. Our results reveal novel cytoplasmic actions of AIB1 and highlight the significance of delineating the impact of dynamic PELP1 shuttling/mislocalization with respect to altered hormone/SR-driven target gene expression in luminal breast cancer progression. We conclude direct signaling inputs to SR complexes are profoundly altered in the context of PELP1 mislocalization and interaction with AIB1. Our findings are relevant to mechanisms of endocrine resistance that may be targeted by inhibitors of the PELP1/SR interaction.

Citation Format: Thu H. Truong, Julie H. Ostrander, Carol A. Lange. PELP1 cytoplasmic mislocalization favors estrogen-induced signaling via AIB1 in ER+ breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3618. doi:10.1158/1538-7445.AM2017-3618

Abstract 1331: TGFβ1 induces breast tumor kinase overexpression in triple negative breast cancer via p38 MAPK signaling to glucocorticoid receptors

Triple negative breast cancer (TNBC) is the deadliest breast cancer (BC) subtype, accounting for 20-30% of all BCs. It has a heterogeneous pathology and pathogenicity, but it is defined by the lack of estrogen receptor, progesterone receptor, and Her2 epidermal growth factor receptor expression. Because targeted therapy through these receptors is not possible, treatment relies on chemotherapy and surgery, which are often inadequate. Thus, the identification of possible molecular targets is critically important in TNBC. Breast tumor kinase (Brk) is a soluble tyrosine kinase that is overexpressed in 85% of BCs and a driver of aggressive and metastatic phenotypes. Overexpression of Brk mRNA and protein occurs in TNBC by unknown mechanisms. The glucocorticoid receptor (GR), a very potent modulator of cytokine mediated actions of the immune system, is emerging as a mediator of chemoresistance and recurrence in TNBC. We previously demonstrated that GR signaling cooperates with physiologic stress signaling mediated by hypoxia inducible factors HIF-1a and HIF-2 to modulate the expression of Brk mRNA and protein in TNBC cells. Moreover, phosphorylation of GR at ser134 by p38 MAPK is essential for GR regulation of Brk expression. P38 is an essential Ser/Thr kinase that regulates cellular transduction of growth factors, such as Hepatocyte Growth Factor (HGF), and cytokines (e.g. TGFβ1) in TNBC cells, and was previously shown to be co-expressed with Brk in human breast tumors. Herein, we further probed mechanisms of crosstalk between key cytokines, GR, and p38 signaling in the regulation of Brk overexpression. We hypothesize that TGFβ1 signaling modulates EMT and metastasis in part by increasing the expression of Brk in TNBC. Treatment of MDA-MB-231 cells with TGFβ1 for 1, 2, 24 and 48 hours increased Brk protein expression relative to vehicle controls. Additionally, TGFβ1 increased both HIF1 and HIF2 protein levels (at 24 and 48 hours respectively). TGFβ1 regulated Brk expression at the level of mRNA, as measured using RT-PCR. Moreover, TGFβ1 synergized with activated GR to further increase Brk mRNA levels. In contrast, mRNA levels of HIF1 and HIF2 were not modulated by TGFβ1, suggesting that the observed protein increases are due to stabilization of HIFs. Finally, TGFβ1 robustly induced p38-dependent phosphorylation of GR at serine 134. This phosphorylation event promoted ligand-independent GR transcriptional activity at the Brk promoter. Human breast tumors significantly co-express active p38 MAPK and Brk. Our molecular model implicates TGFβ1 signaling (via p38 MAPKs) to phospho-GR in the aberrant overexpression of Brk in TNBC. We conclude that blocking of the TGFβ1 pathway may provide a strategy to inhibit Brk mediated TNBC tumor progression. This work was supported by NIH/NCI R01 CA192178 (to CAL) and T32 GM008244-24.

Citation Format: Carlos J. Santos Perez, Tarah Regan Anderson, Carol A. Lange. TGFβ1 induces breast tumor kinase overexpression in triple negative breast cancer via p38 MAPK signaling to glucocorticoid receptors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1331. doi:10.1158/1538-7445.AM2017-1331

Posttranslationally modified progesterone receptors direct ligand-specific expression of breast cancer stem cell-associated gene programs

Background

Estrogen and progesterone are potent breast mitogens. In addition to steroid hormones, multiple signaling pathways input to estrogen receptor (ER) and progesterone receptor (PR) actions via posttranslational events. Protein kinases commonly activated in breast cancers phosphorylate steroid hormone receptors (SRs) and profoundly impact their activities.

Methods

To better understand the role of modified PRs in breast cancer, we measured total and phospho-Ser294 PRs in 209 human breast tumors represented on 2754 individual tissue spots within a tissue microarray and assayed the regulation of this site in human tumor explants cultured ex vivo. To complement this analysis, we assayed PR target gene regulation in T47D luminal breast cancer models following treatment with progestin (promegestone; R5020) and antiprogestins (mifepristone, onapristone, or aglepristone) in conditions under which the receptor is regulated by Lys388 SUMOylation (K388 intact) or is SUMO-deficient (via K388R mutation to mimic persistent Ser294 phosphorylation). Selected phospho-PR-driven target genes were validated by qRT-PCR and following RUNX2 shRNA knockdown in breast cancer cell lines. Primary and secondary mammosphere assays were performed to implicate phospho-Ser294 PRs, epidermal growth factor signaling, and RUNX2 in breast cancer stem cell biology.

Results

Phospho-Ser294 PR species were abundant in a majority (54%) of luminal breast tumors, and PR promoter selectivity was exquisitely sensitive to posttranslational modifications. Phospho-PR expression and target gene programs were significantly associated with invasive lobular carcinoma (ILC). Consistent with our finding that activated phospho-PRs undergo rapid ligand-dependent turnover, unique phospho-PR gene signatures were most prevalent in breast tumors clinically designated as PR-low to PR-null (luminal B) and included gene sets associated with cancer stem cell biology (HER2, PAX2, AHR, AR, RUNX). Validation studies demonstrated a requirement for RUNX2 in the regulation of selected phospho-PR target genes (SLC37A2). In vitro mammosphere formation assays support a role for phospho-Ser294-PRs via growth factor (EGF) signaling as well as RUNX2 as potent drivers of breast cancer stem cell fate.

Conclusions

We conclude that PR Ser294 phosphorylation is a common event in breast cancer progression that is required to maintain breast cancer stem cell fate, in part via cooperation with growth factor-initiated signaling pathways and key phospho-PR target genes including SLC37A2 and RUNX2. Clinical measurement of phosphorylated PRs should be considered a useful marker of breast tumor stem cell potential. Alternatively, unique phospho-PR target gene sets may provide useful tools with which to identify patients likely to respond to selective PR modulators that block PR Ser294 phosphorylation as part of rational combination (i.e., with antiestrogens) endocrine therapies designed to durably block breast cancer recurrence.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-017-0462-7) contains supplementary material, which is available to authorized users.

Development of a test that measures real-time HER2 signaling function in live breast cancer cell lines and primary cells

Background

Approximately 18–20% of all human breast cancers have overexpressed human epidermal growth factor receptor 2 (HER2). Standard clinical practice is to treat only overexpressed HER2 (HER2+) cancers with targeted anti-HER2 therapies. However, recent analyses of clinical trial data have found evidence that HER2-targeted therapies may benefit a sub-group of breast cancer patients with non-overexpressed HER2. This suggests that measurement of other biological factors associated with HER2 cancer, such as HER2 signaling pathway activity, should be considered as an alternative means of identifying patients eligible for HER2 therapies.

Methods

A new biosensor-based test (CELx^TM HSF) that measures HER2 signaling activity in live cells is demonstrated using a set of 19 human HER2+ and HER2– breast cancer reference cell lines and primary cell samples derived from two fresh patient tumor specimens. Pathway signaling is elucidated by use of highly specific agonists and antagonists. The test method relies upon well-established phenotypic, adhesion-related, impedance changes detected by the biosensor.

Results

The analytical sensitivity and analyte specificity of this method was demonstrated using ligands with high affinity and specificity for HER1 and HER3. The HER2-driven signaling quantified ranged 50-fold between the lowest and highest cell lines. The HER2+ cell lines were almost equally divided into high and low signaling test result groups, suggesting that little correlation exists between HER2 protein expression and HER2 signaling level. Unexpectedly, the highest HER2-driven signaling level recorded was with a HER2– cell line.

Conclusions

Measurement of HER2 signaling activity in the tumor cells of breast cancer patients is a feasible approach to explore as a biomarker to identify HER2-driven cancers not currently diagnosable with genomic techniques. The wide range of HER2-driven signaling levels measured suggests it may be possible to make a distinction between normal and abnormal levels of activity. Analytical validation studies and clinical trials treating HER2- patients with abnormal HER2-driven signaling would be required to evaluate the analytical and clinical validity of using this functional biomarker as a diagnostic test to select patients for treatment with HER2 targeted therapy. In clinical practice, this method would require patient specimens be delivered to and tested in a central lab.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3181-0) contains supplementary material, which is available to authorized users.

Progesterone induces progesterone receptor gene (PGR) expression via rapid activation of protein kinase pathways required for cooperative estrogen receptor alpha (ER) and progesterone receptor (PR) genomic action at ER/PR target genes

Progesterone Receptors (PRs) are critical effectors of estrogen receptor (ER) signaling required for mammary gland development and reproductive proficiency. In breast and reproductive tract malignancies, PR expression is a clinical prognostic marker of ER action. While estrogens primarily regulate PR expression, other factors likely contribute to a dynamic range of receptor expression across diverse tissues. In this study, we identified estrogen-independent but progestin (R5020)-dependent regulation of ER target genes including PGR in ER+/PR+ cancer cell lines. R5020 (10nM-10μM range) induced dose-dependent PR mRNA and protein expression in the absence of estrogen but required both PR and ERα. Antagonists of either PR (RU486, onapristone) or ERα (ICI 182,780) attenuated R5020 induction of TFF1, CTSD, and PGR. Chromatin immunoprecipitation (ChIP) assays performed on ER+/PR+ cells demonstrated that both ERα and PR were recruited to the same ERE/Sp1 site-containing region of the PGR proximal promoter in response to high dose progestin (10μM). Recruitment of ERα and PR to chromatin and subsequent PR mRNA induction were dependent upon rapid activation of MAPK/ERK and AKT; inhibition of these kinase pathways via U0126 or LY294002 blocked these events. Overall, we have identified a novel mechanism of ERα activation initiated by rapid PR-dependent kinase pathway activation and associated with phosphorylation of ERα Ser118 for estrogen-independent but progestin-dependent ER/PR cross talk. These studies may provide insight into mechanisms of persistent ER-target gene expression during periods of hormone (i.e. estrogen) ablation and suggest caution following prolonged treatment with aromatase or CYP17 inhibitors (i.e. contexts when progesterone levels may be abnormally elevated).

Tumour and cellular distribution of activated forms of PR in breast cancers: a novel immunohistochemical analysis of a large clinical cohort

Background

The progesterone receptor (PR) is expressed by ∼70% of early breast tumours and is implicated in the progression of breast cancer. In cancerous tissues PR may be activated in the absence of a ligand, or when ligand concentrations are very low, resulting in aberrantly activated PR (APR). The presence of APR may indicate that patients with breast cancer are more likely to respond to antiprogestins. The aims of this study were to describe and classify the histological subnuclear morphology of active and inactive PR in archival breast cancer samples.

Methods

Archived tumour specimens from 801 women with invasive breast cancer were collected. Tissue samples (n=789) were analysed for PR isoforms A and B (PRA and PRB), Ki67 and estrogen receptors (ERα) status, using immunohistochemistry. Medical records were used to determine human epidermal growth factor 2 (HER2) status, tumour stage and grade.

Results

A total of 79% of tumours stained positive for either PRA or PRB, and of these 25% of PRA-positive and 23% of PRB-positive tumours had PR present in the activated form. APRA was associated with higher tumour grade (p=0.001). APRB was associated with a higher tumour grade (p=0.046) and a trend for a more advanced stage. Patients with PR-positive tumours treated with antiestrogens had better disease-free survival (DFS) than those with PR-negative tumours (p<0.0001). Cumulative progression rate and DFS were similar irrespective of APR status. Both APRA and APRB were independent of HER2, ERα and Ki67 expression.

Conclusions

APR had a binary mode of expression in the breast cancer specimens tested, allowing separation into two tumour subsets. APR is an independent target at the cellular and tumour level and may therefore be a suitable predictive marker for antiprogestins, such as onapristone. Using the described technique, a companion diagnostic is under development to identify APR in solid tumours.

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 A39: FOXO1 is a key determinant of progesterone receptor isoform-specific senescence programming in ovarian cancer cells

Progesterone promotes proliferation and pro-survival in the breast, but inhibits growth in the reproductive tract and ovaries. Herein, using progesterone receptor (PR) isoform-specific ovarian cancer models, we show that PR-A and PR-B promote expression of distinct gene sets that differ from PR-driven genes in breast cancer cells. In ovarian cancer models, PR-A primarily regulates genes independently of progestin, while PR-B is the dominant ligand-dependent isoform. Notably, FOXO1 and the PR/FOXO1 target-gene p21 are repressed by PR-A, but induced by PR-B. In the presence of progestin, PR-B, but not PR-A, robustly induced cellular senescence via FOXO1-dependent induction of p21 and p15. ChIP assays performed on PR-isoform specific cells demonstrated that while each isoform is recruited to the same PRE-containing region of the p21 promoter in response to progestin, only PR-B elicits active chromatin. Overexpression of constitutively active FOXO1 (FOXO1-AAA) in PR-A-only expressing cells conferred robust ligand-dependent upregulation of the PR-B-target genes GZMA, IGFBP1, and p21, and induced cellular senescence. Similar results were observed when PR-A was titrated into PR-B-containing cells. PR isoform-specific regulation of the FOXO1/p21 axis recapitulated in human primary ovarian tumor explants treated with progestin. Our data indicate a key requirement for FOXO1 (a PR-B-induced, but PR-A-repressed gene) in progesterone signaling to cellular senescence in ovarian cancer cells and reveal a novel mechanism for PR-target gene (FOXO1) control of hormone sensitivity. Ultimately, harnessing PR isoform-specific gene regulation in hormone-driven cancers may provide a means to therapeutically induce the protective actions of progesterone while blocking unwanted proliferative and pro-survival effects.

Citation Format: Caroline H. Diep, Todd P. Knutson, Laura J. Mauro, Carol A. Lange. FOXO1 is a key determinant of progesterone receptor isoform-specific senescence programming in ovarian cancer cells. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr A39.

Active FOXO1 Is a Key Determinant of Isoform-Specific Progesterone Receptor Transactivation and Senescence Programming

Progesterone promotes differentiation coupled to proliferation and prosurvival in the breast, but inhibits estrogen-driven growth in the reproductive tract and ovaries. Herein, it is demonstrated, using progesterone receptor (PR) isoform-specific ovarian cancer model systems, that PR-A and PR-B promote distinct gene expression profiles that differ from PR-driven genes in breast cancer cells. In ovarian cancer models, PR-A primarily regulates genes independently of progestin, while PR-B is the dominant ligand-dependent isoform. Notably, FOXO1 and the PR/FOXO1 target gene p21 (CDKN1A) are repressed by PR-A, but induced by PR-B. In the presence of progestin, PR-B, but not PR-A, robustly induced cellular senescence via FOXO1-dependent induction of p21 and p15 (CDKN2B). Chromatin immunoprecipitation (ChIP) assays performed on PR isoform-specific cells demonstrated that while each isoform is recruited to the same PRE-containing region of the p21 promoter in response to progestin, only PR-B elicits active chromatin marks. Overexpression of constitutively active FOXO1 in PR-A-expressing cells conferred robust ligand-dependent upregulation of the PR-B target genes GZMA, IGFBP1, and p21, and induced cellular senescence. In the presence of endogenous active FOXO1, PR-A was phosphorylated on Ser294 and transactivated PR-B at PR-B target genes; these events were blocked by the FOXO1 inhibitor (AS1842856). PR isoform-specific regulation of the FOXO1/p21 axis recapitulated in human primary ovarian tumor explants treated with progestin; loss of progestin sensitivity correlated with high AKT activity.

IMPLICATIONS

This study indicates FOXO1 as a critical component for progesterone signaling to promote cellular senescence and reveals a novel mechanism for transcription factor control of hormone sensitivity.