Respiratory viral infection promotes the awakening and outgrowth of dormant metastatic breast cancer cells in lungs

Breast cancer is the second most common cancer globally. Most deaths from breast cancer are due to metastatic disease which often follows long periods of clinical dormancy1. Understanding the mechanisms that disrupt the quiescence of dormant disseminated cancer cells (DCC) is crucial for addressing metastatic progression. Infection with respiratory viruses (e.g. influenza or SARS-CoV-2) is common and triggers an inflammatory response locally and systemically2,3. Here we show that influenza virus infection leads to loss of the pro-dormancy mesenchymal phenotype in breast DCC in the lung, causing DCC proliferation within days of infection, and a greater than 100-fold expansion of carcinoma cells into metastatic lesions within two weeks. Such DCC phenotypic change and expansion is interleukin-6 (IL-6)-dependent. We further show that CD4 T cells are required for the maintenance of pulmonary metastatic burden post-influenza virus infection, in part through attenuation of CD8 cell responses in the lungs. Single-cell RNA-seq analyses reveal DCC-dependent impairment of T-cell activation in the lungs of infected mice. SARS-CoV-2 infected mice also showed increased breast DCC expansion in lungs post-infection. Expanding our findings to human observational data, we observed that cancer survivors contracting a SARS-CoV-2 infection have substantially increased risks of lung metastatic progression and cancer-related death compared to cancer survivors who did not. These discoveries underscore the significant impact of respiratory viral infections on the resurgence of metastatic cancer, offering novel insights into the interconnection between infectious diseases and cancer metastasis.

Targeting cancer cell dormancy

The field of tumour dormancy, originally defined as a clinical phenomenon of late recurrence after a long, apparently disease-free period, has seen significant advances that now allow us to think about monitoring and targeting dormant tumour cells to prevent relapse. In this Viewpoint article, we asked experts to share their views on the steps that are needed to translate dormancy research into the clinic.

Immune evasion by dormant disseminated cancer cells: A Fermi paradox?

Rare disseminated tumor cells (DTCs) can persist after treatment in patients for years, and the immune system does not eliminate them. Goddard et al. propose that immune evasion by rare dormant DTCs is due to an improbability of contact imposed by large distances separating effector T cells and DTCs.

A PERK-Specific Inhibitor Blocks Metastatic Progression by Limiting Integrated Stress Response-Dependent Survival of Quiescent Cancer Cells

PURPOSE

The integrated stress response (ISR) kinase PERK serves as a survival factor for both proliferative and dormant cancer cells. We aim to validate PERK inhibition as a new strategy to specifically eliminate solitary disseminated cancer cells (DCC) in secondary sites that eventually reawake and originate metastasis.

EXPERIMENTAL DESIGN

A novel clinical-grade PERK inhibitor (HC4) was tested in mouse syngeneic and PDX models that present quiescent/dormant DCCs or growth-arrested cancer cells in micro-metastatic lesions that upregulate ISR.

RESULTS

HC4 significantly blocks metastasis, by killing quiescent/slow-cycling ISRhigh, but not proliferative ISRlow DCCs. HC4 blocked expansion of established micro-metastasis that contained ISRhigh slow-cycling cells. Single-cell gene expression profiling and imaging revealed that a significant proportion of solitary DCCs in lungs were indeed dormant and displayed an unresolved ER stress as revealed by high expression of a PERK-regulated signature. In human breast cancer metastasis biopsies, GADD34 expression (PERK-regulated gene) and quiescence were positively correlated. HC4 effectively eradicated dormant bone marrow DCCs, which usually persist after rounds of therapies. Importantly, treatment with CDK4/6 inhibitors (to force a quiescent state) followed by HC4 further reduced metastatic burden. In HNSCC and HER2+ cancers HC4 caused cell death in dormant DCCs. In HER2+ tumors, PERK inhibition caused killing by reducing HER2 activity because of sub-optimal HER2 trafficking and phosphorylation in response to EGF.

CONCLUSIONS

Our data identify PERK as a unique vulnerability in quiescent or slow-cycling ISRhigh DCCs. The use of PERK inhibitors may allow targeting of pre-existing or therapy-induced growth arrested "persister" cells that escape anti-proliferative therapies.

Dormancy in Breast Cancer

The pattern of delayed recurrence in a subset of breast cancer patients has long been explained by a model that incorporates a variable period of cellular or tumor mass dormancy prior to disease relapse. In this review, we critically evaluate existing data to develop a framework for inferring the existence of dormancy in clinical contexts of breast cancer. We integrate these clinical data with rapidly evolving mechanistic insights into breast cancer dormancy derived from a broad array of genetically engineered mouse models as well as experimental models of metastasis. Finally, we propose actionable interventions and discuss ongoing clinical trials that translate the wealth of knowledge gained in the laboratory to the long-term clinical management of patients at a high risk of developing recurrence.

5-Azacytidine- and retinoic-acid-induced reprogramming of DCCs into dormancy suppresses metastasis via restored TGF-β-SMAD4 signaling

Disseminated cancer cells (DCCs) in secondary organs can remain dormant for years to decades before reactivating into overt metastasis. Microenvironmental signals leading to cancer cell chromatin remodeling and transcriptional reprogramming appear to control onset and escape from dormancy. Here, we reveal that the therapeutic combination of the DNA methylation inhibitor 5-azacytidine (AZA) and the retinoic acid receptor ligands all-trans retinoic acid (atRA) or AM80, an RARα-specific agonist, promotes stable dormancy in cancer cells. Treatment of head and neck squamous cell carcinoma (HNSCC) or breast cancer cells with AZA+atRA induces a SMAD2/3/4-dependent transcriptional program that restores transforming growth factor β (TGF-β)-signaling and anti-proliferative function. Significantly, either combination, AZA+atRA or AZA+AM80, strongly suppresses HNSCC lung metastasis formation by inducing and maintaining solitary DCCs in a SMAD4+/NR2F1+ non-proliferative state. Notably, SMAD4 knockdown is sufficient to drive resistance to AZA+atRA-induced dormancy. We conclude that therapeutic doses of AZA and RAR agonists may induce and/or maintain dormancy and significantly limit metastasis development.

Abstract 5131: Pulmonary influenza infection promotes the awakening of dormant metastatic breast cancer cells

Breast cancer is the most common form of cancer and the second cancer-causing death in females. Although remission rates are high if detected early, survival rates drop substantially when breast cancer becomes metastatic. The most common sites of metastatic breast cancer are bone, liver and lung. Respiratory viral infections inflict illnesses on countless people. The latest pandemic caused by the respiratory virus, SARS-CoV-2, has infected more than 600 million worldwide, with documented COVID-related death upward of 1 million in the United States alone. Respiratory viral infections result in increased inflammation with immune cell influx and expansion to facilitate viral clearance. Prior studies have shown that inflammation, including through neutrophils, can contribute to dormant cancer cells reawakening and outgrowth. Moreover, inhibition of IL6 has been shown to decrease breast cancer lung metastasis in mouse models. However, how respiratory viral infections contribute to breast cancer lung metastasis remains to be unraveled. Using MMTV/PyMT and MMTV/NEU mouse models of breast cancer lung metastasis and influenza A virus as a model respiratory virus, we demonstrated that acute influenza infection and the accompanying inflammation and immune cell influx awakens and dramatically increased proliferation and expansion of dormant disseminated cancer cells (DCC) in the lungs. Acute influenza infection leads to immune influx and expansion, including neutrophils and macrophages, with increased proportion of MHCII+ macrophages in early time points, and a sustained decrease in CD206+ macrophages starting 6 days post-infection until 28 days after the initial infection. Additionally, we observed a sustained accumulation of CD4+ T cells around expanding tumor cells for as long as 28 days after the infection. Notably, neutrophil depletion or IL6 knockout reversed the flu-induced dormant cell expansion in the lung. Finally, awakened DCC exhibited downregulation of vimentin immunoreactivity, suggesting a role for phenotypic plasticity in DCC outgrowth following viral infection. In conclusion, we show that respiratory viral infections awaken and increase proliferation of dormant breast cancer cells in the lung, and that depletion of neutrophils or blocking IL6 reverses influenza-induced dormant cell awakening and proliferation.

Citation Format: Shi Biao Chia, Bryan Johnson, Julio A. Aguirre-Ghiso, Mercedes Rincon, James DeGregori. Pulmonary influenza infection promotes the awakening of dormant metastatic breast cancer cells. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5131.

Abstract A005: The aged tumor microenvironment influences tolerance to targeted therapy via NR2F1 overexpression in BRAF-mutant melanoma

Despite the clinical success of targeted inhibitors, tumor responses to these agents are transient, and drug-tolerant residual cells seed resistance. Understanding the role of tumor-intrinsic mechanisms and effects of the tumor microenvironment in mediating drug tolerance will guide and optimize targeted therapies. Given similarities between drug tolerance and cellular dormancy, we studied the role of nuclear receptor subfamily 2 group F member 1 (NR2F1) in response to targeted therapy. We used BRAF-mutant cutaneous melanoma models treated with BRAF and MEK inhibitors (BRAFi + MEKi) since patients treated with this combination typically develop resistance. The aged tumor microenvironment has been shown to increase therapy resistance, and we find that melanoma cells in aged mice express higher levels of NR2F1 than when the same cells are injected into young animals. Transcriptomic analysis of melanoma patient samples treated with BRAFi + MEKi showed increased expression of NR2F1 post-treatment. Similarly, NR2F1 was highly expressed in minimal residual disease collected on BRAFi + MEKi treatment in patient- and xenograft-derived tumors. High expression of NR2F1 promotes tumor survival and invasion in the presence of BRAFi + MEKi in vitro leading to tolerance to BRAFi + MEKi efficacy in vivo. Depletion of NR2F1 in YUMM1.7 allografts grown in aged mice improved response to the combination therapy. Altogether, our findings suggest that NR2F1 promotes drug tolerance leading to minimal residual disease in melanoma and that NR2F1-high cells may be targeted with CDK4/6 inhibitors to improve targeted therapy outcomes in melanoma patients.

Citation Format: Manoela Tiago, Timothy J. Purwin, Mitchell E. Fane, Yash Chhabra, Jessica L. F. Teh, Rama Kadamb, Weijia Cai, Inna Chervoneva, Sheera Rosenbaum, Vivian Chua, Nir Hacohen, Michael A. Davies, Jessie Villanieva, Ashani T. Weeraratna, Claudia Capparelli, Julio A. Aguirre-Ghiso, Andrew E. Aplin. The aged tumor microenvironment influences tolerance to targeted therapy via NR2F1 overexpression in BRAF-mutant melanoma [abstract]. In: Proceedings of the AACR Special Conference: Aging and Cancer; 2022 Nov 17-20; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2022;83(2 Suppl_1):Abstract nr A005.

IGF1R Inhibition Enhances the Therapeutic Effects of Gq/11 Inhibition in Metastatic Uveal Melanoma Progression

Uveal melanoma (UM) is the most common intraocular tumor in adults, and up to 50% of patients develop metastatic disease, which remains uncurable. Because patients with metastatic UM have an average survival of less than 1 year after diagnosis, there is an urgent need to develop new treatment strategies. Although activating mutations in Gαq or Gα11 proteins are major drivers of pathogenesis, the therapeutic intervention of downstream Gαq/11 targets has been unsuccessful in treating UM, possibly due to alternative signaling pathways and/or resistance mechanisms. Activation of the insulin-like growth factor 1 (IGF1) signaling pathway promotes cell growth, metastasis, and drug resistance in many types of cancers, including UM, where expression of the IGF1 receptor (IGF1R) correlates with a poor prognosis. In this article, we show that direct inhibition of Gαq/11 by the cyclic depsipeptide YM-254890 in combination with inhibition of IGF1R by linsitinib cooperatively inhibits downstream signaling and proliferation of UM cells. We further demonstrate that a 2-week combination treatment of 0.3 to 0.4 mg/kg of YM-254890 administered by intraperitoneal injection and 25 to 40 mg/kg linsitinib administered by oral gavage effectively inhibits the growth of metastatic UM tumors in immunodeficient NOD scid gamma (NSG) mice and identifies the IGF1 pathway as a potential resistance mechanism in response to Gαq/11 inhibition in UM. These data suggest that the combination of Gαq/11 and IGF1R inhibition provides a promising therapeutic strategy to treat metastatic UM.

MacroH2A impedes metastatic growth by enforcing a discrete dormancy program in disseminated cancer cells

MacroH2A variants have been linked to inhibition of metastasis through incompletely understood mechanisms. Here, we reveal that solitary dormant disseminated cancer cells (DCCs) display increased levels of macroH2A variants in head and neck squamous cell carcinoma PDX in vivo models and patient samples compared to proliferating primary or metastatic lesions. We demonstrate that dormancy-inducing transforming growth factor-β2 and p38α/β pathways up-regulate macroH2A expression and that macroH2A variant overexpression is sufficient to induce DCC dormancy and suppress metastasis in vivo. Notably, inducible expression of the macroH2A2 variant in vivo suppresses metastasis via a reversible growth arrest of DCCs. This state does not require the dormancy-regulating transcription factors DEC2 and NR2F1; instead, transcriptomic analysis reveals that macroH2A2 overexpression inhibits cell cycle and oncogenic signaling programs, while up-regulating dormancy and senescence-associated inflammatory cytokines. We conclude that the macroH2A2-enforced dormant phenotype results from tapping into transcriptional programs of both quiescence and senescence to limit metastatic outgrowth.

ZFP281 drives a mesenchymal-like dormancy program in early disseminated breast cancer cells that prevents metastatic outgrowth in the lung

Increasing evidence shows that cancer cells can disseminate from early evolved primary lesions much earlier than the classical metastasis models predicted. Here, we reveal at a single-cell resolution that mesenchymal-like (M-like) and pluripotency-like programs coordinate dissemination and a long-lived dormancy program of early disseminated cancer cells (DCCs). The transcription factor ZFP281 induces a permissive state for heterogeneous M-like transcriptional programs, which associate with a dormancy signature and phenotype in vivo. Downregulation of ZFP281 leads to a loss of an invasive, M-like dormancy phenotype and a switch to lung metastatic outgrowth. We also show that FGF2 and TWIST1 induce ZFP281 expression to induce the M-like state, which is linked to CDH1 downregulation and upregulation of CDH11. We found that ZFP281 not only controls the early dissemination of cancer cells but also locks early DCCs in a dormant state by preventing the acquisition of an epithelial-like proliferative program and consequent metastases outgrowth.

Metastasis

Metastasis, the major cause of cancer death, represents one of the major challenges in oncology. Scientists are still trying to understand the biological basis underlying the dissemination and outgrowth of tumor cells, why these cells can remain dormant for years, how they become resistant to the immune system or cytotoxic effects of systemic therapy, and how they interact with their new microenvironment. We asked experts to discuss some of the unknowns, advances, and areas of opportunity related to cancer metastasis.

Abstract SY31-01: Tissue resident macrophages and a mesenchymal-like program control early disseminated cancer cell dormancy

Tissue resident macrophages and a mesenchymal-like program control early disseminated cancer cell dormancy. Julio A. Aguirre-GhisoDepartment of Cell Biology, Department of Medicine-Oncology, Cancer Dormancy and Tumor Microenvironment Institute, Albert Einstein Cancer Center, Gruss-Lipper Biophotonics Center, Albert Einstein College of Medicine, Bronx, NY 10461, USA. Increasing evidence shows that cancer cells can disseminate from early-evolved primary lesions much earlier than the classical metastasis models predicted. The current paradigm supports that the tissue microenvironment where disseminated cancer cells (DCCs) lodge is a critical determinant of the timing of metastasis. However, how tissue-resident myeloid cells control this process is largely unclear. The lung tissue contains a resident population of alveolar macrophages (AMs), whose function in metastatic dormancy and growth is largely unexplored. Here, we used various transgenic mouse models, human sample analysis and single cell RNA-sequencing to reveal DCC heterogeneity and plasticity in the lung across disease evolution. We found a previously unrecognized role of mesenchymal- and pluripotency-like (M-like) programs driven by the transcription factor ZFP281 in coordinating early cancer cell spread and a long-lived dormancy program in early DCCs. The gene profiles revealed molecules that would allow early DCCs to crosstalk with AMs. Using a 3D Matrigel assay, we found that AMs are responsible for inducing an M-like phenotype in cancer cells (of both early and late evolved cancer cells) and exclusively suppressed growth of early lesion (EL) HER2+ cancer cells. The conditioned media (CM) from AMs was also found to induce an M-like phenotype and growth arrest of EL cells. RNA-sequencing profiling of pure EL cells and AMs that had been co-cultured revealed a reciprocal influence of the two cell types: increased ECM organization and induction of invasiveness in EL cells, as well as changes in inflammatory response and immune programs in AMs. Prediction of ligand-receptor interactions using the NATMI tool allowed for identification of signaling axis that would allow AMs to induce an M-like and growth program in EL cells. To test whether indeed AMs are inducing an M-like and growth arrest phenotype in early DCCs in lungs, we depleted AMs in mice bearing early MMTV-HER2 or late evolved E0771 breast cancer cells in lungs via intranasal instillation of clodronate liposomes or via the use of a AM=specific CD169-DTR targeting system. Following AM depletion, dormant DCCs began to proliferate and formed clusters and overt metastases. Our results support that tissue resident macrophages play a key role in limiting metastatic expansion of breast cancer cells via the induction of a DCC mesenchymal-like dormancy program.

Citation Format: Julio A. Aguirre-Ghiso. Tissue resident macrophages and a mesenchymal-like program control early disseminated cancer cell dormancy [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 SY31-01.

Stromal changes in the aged lung induce an emergence from melanoma dormancy

Disseminated cancer cells from primary tumours can seed in distal tissues, but may take several years to form overt metastases, a phenomenon that is termed tumour dormancy. Despite its importance in metastasis and residual disease, few studies have been able to successfully characterize dormancy within melanoma. Here we show that the aged lung microenvironment facilitates a permissive niche for efficient outgrowth of dormant disseminated cancer cells-in contrast to the aged skin, in which age-related changes suppress melanoma growth but drive dissemination. These microenvironmental complexities can be explained by the phenotype switching model, which argues that melanoma cells switch between a proliferative cell state and a slower-cycling, invasive state1-3. It was previously shown that dermal fibroblasts promote phenotype switching in melanoma during ageing4-8. We now identify WNT5A as an activator of dormancy in melanoma disseminated cancer cells within the lung, which initially enables the efficient dissemination and seeding of melanoma cells in metastatic niches. Age-induced reprogramming of lung fibroblasts increases their secretion of the soluble WNT antagonist sFRP1, which inhibits WNT5A in melanoma cells and thereby enables efficient metastatic outgrowth. We also identify the tyrosine kinase receptors AXL and MER as promoting a dormancy-to-reactivation axis within melanoma cells. Overall, we find that age-induced changes in distal metastatic microenvironments promote the efficient reactivation of dormant melanoma cells in the lung.

Altered BAF occupancy and transcription factor dynamics in PBAF-deficient melanoma

ARID2 is the most recurrently mutated SWI/SNF complex member in melanoma; however, its tumor-suppressive mechanisms in the context of the chromatin landscape remain to be elucidated. Here, we model ARID2 deficiency in melanoma cells, which results in defective PBAF complex assembly with a concomitant genomic redistribution of the BAF complex. Upon ARID2 depletion, a subset of PBAF and shared BAF-PBAF-occupied regions displays diminished chromatin accessibility and associated gene expression, while BAF-occupied enhancers gain chromatin accessibility and expression of genes linked to the process of invasion. As a function of altered accessibility, the genomic occupancy of melanoma-relevant transcription factors is affected and significantly correlates with the observed transcriptional changes. We further demonstrate that ARID2-deficient cells acquire the ability to colonize distal organs in multiple animal models. Taken together, our results reveal a role for ARID2 in mediating BAF and PBAF subcomplex chromatin dynamics with consequences for melanoma metastasis.

Primary tumor associated macrophages activate programs of invasion and dormancy in disseminating tumor cells

Metastases are initiated by disseminated tumor cells (DTCs) that colonize distant organs. Growing evidence suggests that the microenvironment of the primary tumor primes DTCs for dormant or proliferative fates. However, the manner in which this occurs remains poorly understood. Here, using the Window for High-Resolution Intravital Imaging of the Lung (WHRIL), we study the live lung longitudinally and follow the fate of individual DTCs that spontaneously disseminate from orthotopic breast tumors. We find that spontaneously DTCs have increased levels of retention, increased speed of extravasation, and greater survival after extravasation, compared to experimentally metastasized tumor cells. Detailed analysis reveals that a subset of macrophages within the primary tumor induces a pro-dissemination and pro-dormancy DTC phenotype. Our work provides insight into how specific primary tumor microenvironments prime a subpopulation of cells for expression of proteins associated with dissemination and dormancy.

An NR2F1-specific agonist suppresses metastasis by inducing cancer cell dormancy

We describe the discovery of an agonist of the nuclear receptor NR2F1 that specifically activates dormancy programs in malignant cells. The agonist led to a self-regulated increase in NR2F1 mRNA and protein and downstream transcription of a novel dormancy program. This program led to growth arrest of an HNSCC PDX line, human cell lines, and patient-derived organoids in 3D cultures and in vivo. This effect was lost when NR2F1 was knocked out by CRISPR-Cas9. RNA sequencing revealed that agonist treatment induces transcriptional changes associated with inhibition of cell cycle progression and mTOR signaling, metastasis suppression, and induction of a neural crest lineage program. In mice, agonist treatment resulted in inhibition of lung HNSCC metastasis, even after cessation of the treatment, where disseminated tumor cells displayed an NR2F1hi/p27hi/Ki-67lo/p-S6lo phenotype and remained in a dormant single-cell state. Our work provides proof of principle supporting the use of NR2F1 agonists to induce dormancy as a therapeutic strategy to prevent metastasis.

A tumor-derived type III collagen-rich ECM niche regulates tumor cell dormancy

Cancer cells disseminate and seed in distant organs, where they can remain dormant for many years before forming clinically detectable metastases. Here we studied how disseminated tumor cells sense and remodel the extracellular matrix (ECM) to sustain dormancy. ECM proteomics revealed that dormant cancer cells assemble a type III collagen-enriched ECM niche. Tumor-derived type III collagen is required to sustain tumor dormancy, as its disruption restores tumor cell proliferation through DDR1-mediated STAT1 signaling. Second-harmonic generation two-photon microscopy further revealed that the dormancy-to-reactivation transition is accompanied by changes in type III collagen architecture and abundance. Analysis of clinical samples revealed that type III collagen levels were increased in tumors from patients with lymph node-negative head and neck squamous cell carcinoma compared to patients who were positive for lymph node colonization. Our data support the idea that the manipulation of these mechanisms could serve as a barrier to metastasis through disseminated tumor cell dormancy induction.

Translating the Science of Cancer Dormancy to the Clinic

The paradigm of metastasis has been significantly remodeled by the incorporation of cancer dormancy as a mechanism to explain long-term remission intervals followed by relapse. There is overall consensus on the potential impact of better understanding dormancy. Key cancer-cell autonomous and microenvironmental mechanisms might explain this biology and, in turn, the timing of metastasis. However, the approach and feasibility to apply this biology to clinical trials has been controversial. The discussion here provides insight into how these controversies are being resolved by the development of active clinical trials, thus bringing to reality opportunities to target cancer dormancy.

Prostate Cancer Dormancy and Reactivation in Bone Marrow

Prostate cancer has a variable clinical course, ranging from curable local disease to lethal metastatic spread. Eradicating metastatic cells is a unique challenge that is rarely met with the available therapies. Thus, targeting prostate cancer cells in earlier disease states is a crucial window of opportunity. Interestingly, cancer cells migrate from their primary site during pre-cancerous and malignant phases to seed secondary organs. These cells, known as disseminated cancer cells (DCCs), may remain dormant for months or decades before activating to form metastases. Bone marrow, a dormancy-permissive site, is the major organ for housed DCCs and eventual metastases in prostate cancer. The dynamic interplay between DCCs and the primary tumor microenvironment (TME), as well as that between DCCs and the secondary organ niche, controls the conversion between states of dormancy and activation. Here, we discuss recent discoveries that have improved our understanding of dormancy signaling and the role of the TME in modulating the epigenetic reprogramming of DCCs. We offer potential strategies to target DCCs in prostate cancer.

The State of Melanoma: Emergent Challenges and Opportunities

Five years ago, the Melanoma Research Foundation (MRF) conducted an assessment of the challenges and opportunities facing the melanoma research community and patients with melanoma. Since then, remarkable progress has been made on both the basic and clinical research fronts. However, the incidence, recurrence, and death rates for melanoma remain unacceptably high and significant challenges remain. Hence, the MRF Scientific Advisory Council and Breakthrough Consortium, a group that includes clinicians and scientists, reconvened to facilitate intensive discussions on thematic areas essential to melanoma researchers and patients alike, prevention, detection, diagnosis, metastatic dormancy and progression, response and resistance to targeted and immune-based therapy, and the clinical consequences of COVID-19 for patients with melanoma and providers. These extensive discussions helped to crystalize our understanding of the challenges and opportunities facing the broader melanoma community today. In this report, we discuss the progress made since the last MRF assessment, comment on what remains to be overcome, and offer recommendations for the best path forward.

Abstract 2986: NR2F1 underlies persistence of residual disease in melanoma

Despite the clinical success of targeted therapy and checkpoint inhibitors in melanoma, therapeutic responses are transient, followed by relapse that may be driven by a small subpopulation of residual or drug-tolerant cells. Understanding residual disease and metastasis mechanisms can provide clues both for developing improved versions of a drug and for guiding the selection of appropriate drug combinations for melanoma therapy. Here, we found that the well-known marker of tumor dormancy, Nuclear Receptor Subfamily 2 Group F Member 1 (NR2F1), was overexpressed in minimal disease residual cells following CDK4/6 and MEK inhibitors (CDK4/6i+MEKi) treatment in vivo. Furthermore, melanoma cells overexpressing NR2F1 were less sensitive to (CDK4/6i+MEKi) or BRAF and MEK inhibitors (BRAFi+MEKi) treatment in vitro and in vivo models, inhibiting apoptosis. Surprisingly, we did not find any evidence of decreased cell growth in our model. Using a three-dimensional tumor spheroid assay in vitro, we found the NR2F1 expression enhanced melanoma invasion following CDK4/6i+MEKi or BRAFi+MEKi treatments. The use of published RNA Seq data sets that were gathered from the GEO database and Single Cell Seq data sets from PDX melanoma samples showed that high expression of NR2F1 is enriched in the undifferentiated cell state and invasive cells, respectively. Furthermore, BRAF mutant patient sample with an acquired mutation in NRAS Q61R following BRAFi+MEKi+CDKi presented a high expression of NR2F1. Altogether, these findings suggest that NR2F1 may play a role in residual disease persistence besides known features of tumor dormancy, especially important in determining responses to dramatic changes in the environment, such as changes induced by anti-cancer therapy.

Citation Format: Manoela Tiago, Jessica L. Teh, Timothy J. Purwin, Weijia Cai, Connor Hollingworth, Melisa Lopez-Anton, Julio A. Aguirre-Ghiso, Andrew E. Aplin. NR2F1 underlies persistence of residual disease in melanoma [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 2986.

The Different Routes to Metastasis via Hypoxia-Regulated Programs

Hypoxia is linked to metastasis; however, how it affects metastatic progression is not clear due to limited consensus in the literature. We posit that this lack of consensus is due to hypoxia being studied using different approaches, such as in vitro, primary tumor, or metastasis assays in an isolated manner. Here, we review the pros and cons of in vitro hypoxia assays, highlight in vivo studies that inform on physiological hypoxia, and review the evidence that primary tumor hypoxia might influence the fate of disseminated tumor cells (DTCs) in secondary organs. Our analysis suggests that consensus can be reached by using in vivo methods of study, which also allow better modeling of how hypoxia affects DTC fate and metastasis.

NR2F1 stratifies dormant disseminated tumor cells in breast cancer patients

BACKGROUND

The presence of disseminated tumor cells (DTCs) in bone marrow (BM) is an independent prognostic factor in early breast cancer but does not uniformly predict outcome. Tumor cells can persist in a quiescent state over time, but clinical studies of markers predicting the awakening potential of DTCs are lacking. Recently, experiments have shown that NR2F1 (COUP-TF1) plays a key role in dormancy signaling.

METHODS

We analyzed the NR2F1 expression in DTCs by double immunofluorescence (DIF) staining of extra cytospins prepared from 114 BM samples from 86 selected DTC-positive breast cancer patients. Samples collected at two or more time points were available for 24 patients. Fifteen samples were also analyzed for the proliferation marker Ki67.

RESULTS

Of the patients with detectable DTCs by DIF, 27% had ≥ 50% NR2F1high DTCs, chosen a priori as the cut-off for "dormant profile" classification. All patients with systemic relapse within 12 months after BM aspiration carried ≤ 1% NR2F1high DTCs, including patients who transitioned from having NR2F1high-expressing DTCs in previous BM samples. Of the patients with serial samples, half of those with no relapse at follow-up had ≥ 50% NR2F1high DTCs in the last BM aspiration analyzed. Among the 18 relapse-free patients at the time of the last DTC-positive BM aspiration with no subsequent BM analysis performed, distant disease-free intervals were favorable for patients carrying ≥ 50% NR2F1high DTCs compared with those with predominantly NR2F1low DTCs (p = 0.007, log-rank). No survival difference was observed by classification according to Ki67-expressing DTCs (p = 0.520).

CONCLUSIONS

Our study translates findings from basic biological analysis of DTC dormancy to the clinical situation and supports further clinical studies of NR2F1 as a marker of dormancy.

Abstract IA16: Macrophages orchestrate early dissemination and metastasis

Cancer cell dissemination can occur during very early stages of breast cancer, but the mechanisms controlling this process are unclear. Here we show that a previously reported early MMTV-HER2+/P-p38lo/TWISThi/E-cadherinlo cancer cell subpopulation depends on macrophages for early dissemination. Depletion of macrophages before overt tumor detection drastically reduced early dissemination and diminished the late metastatic burden. CD206+/Tie2+ macrophages were attracted into early lesions in part by CCL2 produced by early HER2+ cancer cells and myeloid cells. Upregulation of Wnt-1 by macrophages could be stimulated by CCL2 and correlated with loss of E-cadherin in HER2+ early cancer cells. Both MMTV-PyMT and MMTV-HER2 early lesions showed macrophage-containing tumor microenvironments of metastasis (TMEM) structures, and PyMT early cancer cells also showed a reduction in early lesion E-cadherin junctions. Intraepithelial macrophages and loss of E-cadherin junctions was also found more frequently in high-grade human DCIS than in low-grade and normal breast tissue, but no association was found with HER2 status. We reveal a previously unrecognized mechanism by which macrophages play a causal role in early dissemination, impacting long-term metastasis development.

Citation Format: Nina Linde, Maria Casanova-Acebes, Maria Soledad Sosa, Arthur Mortha, Adeeb Rahman, Eduardo F. Farias, Kathryn Harper, Ethan Tardio, Ivan Reyes-Torres, Joan G. Jones, John S. Condeelis, Miriam Merad, Julio A. Aguirre-Ghiso. Macrophages orchestrate early dissemination and metastasis [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2017 Oct 1-4; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2018;6(9 Suppl):Abstract nr IA16.

Abstract 78: p38 kinase is a negative regulator of tumorigenesis and recurrence after surgery in a mammary carcinoma model

The mitogen activated protein kinase (MAPK) p38 is a kinase involved in the response to different external stimuli, extremely diverse such as oxidative stress or growth factors. Its activation triggers different responses which include from promoting cell apoptosis or cell cycle arrest, to cell differentiation and the activation of survival pathways. Although the role of p38 in cancer is a well studied field, there are still multiple unanswered questions, given the large amount of evidence supporting both a tumorigenic but also a tumor suppressor function for p38 in cancer development and progression. In this study, the effects produced by p38 negative modulation using the chemical inhibitor SB203580 in different aspects of tumor biology are analyzed in vitro and in vivo. As a study model, F3II cell line was used, which is an aggressive murine mammary carcinoma. We found that p38 inhibition provokes contradictory effects in our mammary carcinoma model. On one hand, p38 inhibition provoked the reduction of cell viability and colony formation. On the other hand, SB203580 treatment increased cell adhesion and proliferation in matrix coated surfaces. In vivo we found that SB203580 treatment increases tumor aggressiveness. The shortened latency times, higher number of lung metastasis and larger local recurrences are a reflection of the aforementioned effect. Taking together, the results presented in this work position p38 as a tumor suppressor kinase, in the context of cells adapting to a new environment and developing the first stages of growth. P38 inhibition represented an advantage for cells newly inoculated in vivo. We consider that this phenotypic adaptation is probably a consequence of ERK activation and integrin α5 increased expression. Data presented here is of importance to show how determinant is the microenvironment in the responses elicited by the modulation of p38 kinase. We propose a differential role according to the environment conditions, which reflects the complexity of the pathway and the importance of the extracellular stimuli when evaluating the implications of treatment with p38 MAP kinase inhibitors.

Citation Format: Carla S. Capobianco, Johanna E. Sidabra, Maria F. Gottardo, Julio A. Aguirre-Ghiso, Daniel F. Alonso, Hernan G. Farina. p38 kinase is a negative regulator of tumorigenesis and recurrence after surgery in a mammary carcinoma model [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 78.

Macrophages orchestrate breast cancer early dissemination and metastasis

Cancer cell dissemination during very early stages of breast cancer proceeds through poorly understood mechanisms. Here we show, in a mouse model of HER2⁺ breast cancer, that a previously described sub-population of early-evolved cancer cells requires macrophages for early dissemination. Depletion of macrophages specifically during pre-malignant stages reduces early dissemination and also results in reduced metastatic burden at end stages of cancer progression. Mechanistically, we show that, in pre-malignant lesions, CCL2 produced by cancer cells and myeloid cells attracts CD206⁺/Tie2⁺ macrophages and induces Wnt-1 upregulation that in turn downregulates E-cadherin junctions in the HER2⁺ early cancer cells. We also observe macrophage-containing tumor microenvironments of metastasis structures in the pre-malignant lesions that can operate as portals for intravasation. These data support a causal role for macrophages in early dissemination that affects long-term metastasis development much later in cancer progression. A pilot analysis on human specimens revealed intra-epithelial macrophages and loss of E-cadherin junctions in ductal carcinoma in situ, supporting a potential clinical relevance.

Abstract 3051: Mechanism of early dissemination and metastasis in Her2+ mammary cancer

Metastasis is the leading cause of cancer related deaths and these lesions develop from disseminated cancer cells (DCC) that can remain dormant. Metastasis initiating cells are thought to originate from a subpopulation present in progressed invasive tumors. However, DCCs detected in patients before the manifestation of breast cancer metastasis contain fewer genetic abnormalities than primary tumors or than DCCs from patients with metastases. These findings and those in pancreatic cancer and melanoma models argued that dissemination might occur during early stages of tumor evolution. Yet, the mechanisms that might allow early-disseminated cancer cells (eDCC) to complete all steps of metastasis were unknown. Here we show that in early lesions (EL), before any overt primary tumor (PT) masses are detected, there is a sub-population of Her2+/P-p38lo/P-ATF2lo/TWISThi/E-cadherinlo early cancer cells that are invasive and disseminate to target organs. Intra-vital imaging and organoid studies of early lesions revealed that Her2+ eDCC precursors locally invaded, intravasated and lodged in target organs. Her2+ eDCCs activated a Wnt-dependent EMT-like dissemination program but without complete loss of epithelial phenotype that was reversed by Her2 or Wnt inhibition. Surprisingly, while the majority of eDCCs are TWISThi/E-cadherinlo and dormant, they eventually initiate metastasis. Our work identifies a mechanism for early dissemination whereby Her2 aberrantly activates a program similar to mammary ductal branching that spawns eDCCs capable of forming metastasis after a dormancy phase.

Citation Format: Kathryn Harper, Maria Soledad Sosa, David Entenberg, Hedayatollah Hosseini, Julie Cheung, Rita Nobre, Alvaro Avivar-Valderas, Chandandaneep Nagi, Nomeda Girnius, Roger Davis, Eduardo Farias, John Condeelis, Christoph A. Klein, Julio A. Aguirre-Ghiso. Mechanism of early dissemination and metastasis in Her2+ mammary cancer [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 3051. doi:10.1158/1538-7445.AM2017-3051

Phenotypic heterogeneity of disseminated tumour cells is preset by primary tumour hypoxic microenvironments

Hypoxia is a poor-prognosis microenvironmental hallmark of solid tumours, but it is unclear how it influences the fate of disseminated tumour cells (DTCs) in target organs. Here we report that hypoxic HNSCC and breast primary tumour microenvironments displayed upregulation of key dormancy (NR2F1, DEC2, p27) and hypoxia (GLUT1, HIF1α) genes. Analysis of solitary DTCs in PDX and transgenic mice revealed that post-hypoxic DTCs were frequently NR2F1^hi/DEC2^hi/p27^hi/TGFβ2^hi and dormant. NR2F1 and HIF1α were required for p27 induction in post-hypoxic dormant DTCs, but these DTCs did not display GLUT1^hi expression. Post-hypoxic DTCs evaded chemotherapy and, unlike ER^- breast cancer cells, post-hypoxic ER⁺ breast cancer cells were more prone to enter NR2F1-dependent dormancy. We propose that primary tumour hypoxic microenvironments give rise to a subpopulation of dormant DTCs that evade therapy. These post-hypoxic dormant DTCs may be the source of disease relapse and poor prognosis associated with hypoxia.

Early dissemination seeds metastasis in breast cancer

Accumulating data suggest that metastatic dissemination often occurs early during tumour formation but the mechanisms of early metastatic spread have not yet been addressed. Here, we studied metastasis in a HER2-driven mouse breast cancer model and found that progesterone-induced signalling triggered migration of cancer cells from early lesions shortly after HER2 activation, but promoted proliferation in advanced primary tumour cells. The switch from migration to proliferation was regulated by elevated HER2 expression and increased tumour cell density involving miRNA-mediated progesterone receptor (PGR) down-regulation and was reversible. Cells from early, low-density lesions displayed more stemness features than cells from dense, advanced tumours, migrated more and founded more metastases. Strikingly, we found that at least 80% of metastases were derived from early disseminated cancer cells (DCC). Karyotypic and phenotypic analysis of human disseminated cancer cells and primary tumours corroborated the relevance of these findings for human metastatic dissemination.

Mer Tyrosine Kinase Regulates Disseminated Prostate Cancer Cellular Dormancy

Many prostate cancer (PCa) recurrences are thought to be due to reactivation of disseminated tumor cells (DTCs). We previously found a role of the TAM family of receptor tyrosine kinases TYRO3, AXL, and MERTK in PCa dormancy regulation. However, the mechanism and contributions of the individual TAM receptors is largely unknown. Knockdown of MERTK, but not AXL or TYRO3 by shRNA in PCa cells induced a decreased ratio of P-Erk1/2 to P-p38, increased expression of p27, NR2F1, SOX2, and NANOG, induced higher levels of histone H3K9me3 and H3K27me3, and induced a G1/G0 arrest, all of which are associated with dormancy. Similar effects were also observed with siRNA. Most importantly, knockdown of MERTK in PCa cells increased metastasis free survival in an intra-cardiac injection mouse xenograft model. MERTK knockdown also failed to inhibit PCa growth in vitro and subcutaneous growth in vivo, which suggests that MERTK has specificity for dormancy regulation or requires a signal from the PCa microenvironment. The effects of MERTK on the cell cycle and histone methylation were reversed by p38 inhibitor SB203580, which indicates the importance of MAP kinases for MERTK dormancy regulation. Overall, this study shows that MERTK stimulates PCa dormancy escape through a MAP kinase dependent mechanism, also involving p27, pluripotency transcription factors, and histone methylation. J. Cell. Biochem. 118: 891-902, 2017. © 2016 Wiley Periodicals, Inc.

Collagen Matrix Density Drives the Metabolic Shift in Breast Cancer Cells

Increased breast density attributed to collagen I deposition is associated with a 4-6 fold increased risk of developing breast cancer. Here, we assessed cellular metabolic reprogramming of mammary carcinoma cells in response to increased collagen matrix density using an in vitro 3D model. Our initial observations demonstrated changes in functional metabolism in both normal mammary epithelial cells and mammary carcinoma cells in response to changes in matrix density. Further, mammary carcinoma cells grown in high density collagen matrices displayed decreased oxygen consumption and glucose metabolism via the tricarboxylic acid (TCA) cycle compared to cells cultured in low density matrices. Despite decreased glucose entry into the TCA cycle, levels of glucose uptake, cell viability, and ROS were not different between high and low density matrices. Interestingly, under high density conditions the contribution of glutamine as a fuel source to drive the TCA cycle was significantly enhanced. These alterations in functional metabolism mirrored significant changes in the expression of metabolic genes involved in glycolysis, oxidative phosphorylation, and the serine synthesis pathway. This study highlights the broad importance of the collagen microenvironment to cellular expression profiles, and shows that changes in density of the collagen microenvironment can modulate metabolic shifts of cancer cells.

The Relationship Between Dormant Cancer Cells and Their Microenvironment

The majority of cancer deaths are due to metastases that can occur years or decades after primary tumor diagnosis and treatment. Disseminated tumor cells (DTCs) surviving in a dormant state in target organs appear to explain the timing of this phenomenon. Knowledge on this process is important as it might provide a window of opportunity to prevent recurrences by eradicating dormant DTCs and/or by maintaining DTCs in a dormant state. Importantly, this research might offer markers of dormancy for early monitoring of metastatic relapse. However, our understanding of the mechanisms underlying the regulation of entry into and exit from dormancy is still limited and crippling any therapeutic opportunity. While cancer cell-intrinsic signaling pathways have been linked to dormancy regulation, it is likely that these pathways and the switch controlling reactivation from dormancy are regulated by microenvironmental cues. Here we review and discuss recent findings on how the microenvironment regulates cancer dormancy and raise new questions that may help advance the field.

Origin and interpretation of cancer transcriptome profiling: the essential role of the stroma in determining prognosis and drug resistance

Mesenchymal gene expression in tumors has been implicated in cancer recurrence, metastasis, and poor prognosis of patients. The source of these mesenchymal signals has been mostly attributed to the epithelial-to-mesenchymal transition-like phenotype of epithelial tumor cells. However, recent evidence from colorectal and other cancer transcriptome studies clearly shows that the mesenchymal gene expression likely originates from stromal cells in and around the tumor and that this microenvironment specifically confers tumor aggressiveness. These findings highlight the need to move away from tumor-centric interpretations and to better establish the complementary role of the stromal microenvironment in fueling aggressive traits of cancer cells. This observation also suggests that future attempts at transcriptome profiling of whole tumor tissue must take into account the origin of mesenchymal gene expression profiles to better guide development of diagnostic and therapeutic strategies for cancer.

Abstract 4394: Phenotypic heterogeneity of disseminated tumor cells is predetermined by primary tumor hypoxic microenvironments

Heterogeneity of primary tumors (PTs) is reflected by genetic and epigenetic diversity, as well as diverse PT microenvironments. Whether PT microenvironments might influence the fate of disseminating tumor cells (DTC) has never been explored in situ. We previously defined, in breast cancer, a dormancy signature (DS) associated with longer metastasis-free periods. Key genes in the DS induce quiescence and are also regulated by hypoxia. Interestingly, a main response of tumor cells to hypoxia is growth arrest, while clinical evidence links hypoxic tumors to increased therapy resistance and a worse outcome. We hypothesized that hypoxic PT microenvironments may spawn a subpopulation of DTCs that, by virtue of becoming dormant, might escape therapies and eventually fuel incurable metastasis. We used H2B-GFP inducible HEp3 HNSCC and photo-switchable H2B-Dendra2 (green-to-red fluorescence) expressing MDA-MB-231 and ZR-75-1 human breast cancer cell lines to identify cells from hypoxic microenvironments. To initiate spatially defined hypoxic microenvironments in vivo in primary tumors we implanted induction NANo IntraVItal Devices (iNANIVIDs) carrying a hypoxia-mimetic agent (desferrioxamine - DFOM) in T-HEp3 tumors or exposed cultured MDA-MB-231 or ZR-75-1 cells in vitro to 21% or 1% O2. The regions influenced by the DFOM-iNANIVID displayed significant upregulation of p27, NR2F1 and DEC2 (dormancy genes), as well as induction of hypoxia markers (GLUT1, HIF1α). Human HNSCC PT samples showed the same link between spontaneous hypoxic regions and upregulation of dormancy markers. We found a significant increase in quiescent lung DTCs of hypoxia induced T-HEp3 or MDA-MB-231 cells, traceable >2 weeks after extravasation by fluorescent label retention. Significantly more single, non-proliferating HEp3 DTCs originating from the iNANIVID induced hypoxic regions showed a dormant profile compared to DTCs originating from a normoxic milieu. Only the hypoxic pre-treated group was able to form micrometastases at 10 days after injection, suggesting the presence of a more aggressive sub clone in this group. Analysis in 3D culture models revealed that ZR-75-1 cells (ER+) were more prone to enter a prolonged quiescent state after exposure to hypoxia (1% O2) while this response was not observed in MDA-MB-231 (TN). The induction of quiescence in ZR-75-1 was NR2F1 dependent. Lastly, using a spontaneously metastatic PyMT driven Dendra2-tagged breast cancer model in immunocompetent mice, we found that ∼75% of dormant DTCs upregulate the dormancy marker NR2F1 at or soon after reaching the lung, suggesting a rapid induction of dormancy upon reaching target organs. We propose that hypoxic PT microenvironments increase phenotypic heterogeneity of DTCs and lead to the expression of the DS. These DTCs may be more prone to enter dormancy, evade anti-proliferative therapies and eventually fuel metastasis.

Citation Format: Georg Fluegen, Alvaro Avivar-Valderas, Yarong Wang, Michael R. Padgen, James K. Williams, Vladislav V. Verkhusha, David Entenberg, Kevin W. Eliceiri, James Castracane, Patricia J. Keely, John S. Condeelis, Julio A. Aguirre-Ghiso. Phenotypic heterogeneity of disseminated tumor cells is predetermined by primary tumor hypoxic microenvironments. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4394.

Validation of a device for the active manipulation of the tumor microenvironment during intravital imaging

The tumor microenvironment is recognized as playing a significant role in the behavior of tumor cells and their progression to metastasis. However, tools to manipulate the tumor microenvironment directly, and image the consequences of this manipulation with single cell resolution in real time in vivo, are lacking. We describe here a method for the direct, local manipulation of microenvironmental parameters through the use of an implantable Induction Nano Intravital Device (iNANIVID) and simultaneous in vivo visualization of the results at single-cell resolution. As a proof of concept, we deliver both a sustained dose of EGF to tumor cells while intravital imaging their chemotactic response as well as locally induce hypoxia in defined microenvironments in solid tumors.

Abstract B35: NR2F1 limits early dissemination and phenotypic plasticity of early-progressed ErbB2+ tumor cells by maintaining luminal differentiation

Tumor cells derived from MMTV-Neu primary tumors (tumor-derived cells, TDC) are known to be able to complete all steps of the metastatic cascade. However, new evidence shows that cells derived from early stages of tumor progression (before detection of palpable tumors, ErbB2+ early-progressed tumor cells, EPTC) can disseminate and, as disseminated tumor cells (early DTCs), enter a non-proliferative state for prolonged periods at secondary organs. When EPTC are co-cultured with TDC, the tumor sphere formation capacity of the later is increased suggesting a synergistic contribution of early-progressed tumor cells to the growth of TDC. Thus, early DTCs are proposed to contribute to metastasis, but the mechanisms that would allow these cells, considered sessile and with few genetic alterations, to complete all steps of metastasis are unknown. Here we report that the orphan nuclear receptor NR2F1 is downregulated in human pre-malignant and malignant lesions and mouse ErbB2+ EPTC and TDC when compared with normal mammary tissue. In ErBb2+ EPTC further downregulation of NR2F1 activates a motile phenotype and this is coincident with the detection of circulating tumor cells (CTCs) and DTCs in lungs and bone marrow. Knock down of basal NR2F1 levels in ErBb2+ EPTC induced cell motility, loss of laminin-V deposition, β-catenin delocalization from the membrane and dramatic loss of E-cadherin junctions. Interestingly, TWIST, SMAD2, CK14 and ROR1 levels became upregulated upon NR2F1 depletion. This anti-EMT NR2F1-regulated program is ErbB2 oncogen-dependent. These results suggest that NR2F1 expression maintains epithelial identity and suppresses epithelial-mesenchymal transition (EMT), possibly by blocking WNT signaling. Knock down of NR2F1 in ErBb2+ EPTC also enhanced mammosphere formation efficiency and this was accompanied by upregulation of the pluripotency transcription factor NANOG and TWIST. These findings suggest that NR2F1 limits the spreading of ErBb2+ EPTC cells and it promotes a luminal differentiation-associated program by limiting the activation of a stem cell-like program. Our findings provide for the first time evidence that NR2F1 functions in mammary epithelial cells as a suppressor of pluripotency and dissemination during early stages of tumor progression. We propose that therapies that might restore expression of NR2F1 might limit early dissemination and the progression to metastasis of already disseminated tumor cells.

Citation Format: Maria Soledad Sosa, Julie Cheung, Julio A. Aguirre-Ghiso. NR2F1 limits early dissemination and phenotypic plasticity of early-progressed ErbB2+ tumor cells by maintaining luminal differentiation. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr B35.

Abstract A45: PERK-Inhibition as a possible therapy for hypoxia-induced solitary dormant tumor cells

Disseminated tumor cells (DTCs) travel to secondary organs, where they may remain viable for extended amounts of time (years to decades). This state, known as cellular dormancy, is defined as a state of quiescence and cells remain solitary. Preliminary data from our group strongly suggests that single DTCs originating from hypoxic microenvironments within the primary tumor may be more prone to enter a prolonged cellular dormancy at secondary sites. Hypoxic stress also leads to the activation of the endoplasmic reticulum (ER) stress sensor PKR-like ER Kinase (PERK), which promotes the activation of survival genes through the unfolded protein response (UPR). We hypothesize that hypoxia-induced dormant cells require PERK for survival and that through PERK inhibition we can target dormancy. Here, we evaluate PERK inhibition as a way to selectively eliminate quiescent head and neck squamous cell carcinoma (HNSCC) tumor cells. To determine whether hypoxia induces dormancy and PERK activation we cultured HNSCC tumorigenic HEp3 (T-HEp3) cells in normoxic (21% O2) and hypoxic (1% O2) conditions for 8 and 24 hours, and measured the expression of quiescence markers (p27), dormancy markers (DEC2 & NR2F1), and PERK activation (ATF4) via western blot. We found that only at 8 hours p27, DEC2 and NR2F1 are higher expressed in HNSCC in hypoxic conditions in comparison to normoxic conditions. NR2F1 and DEC2 have been shown to promote dormancy by limiting proliferation via p27 induction and mediating the expression of pluripotency genes. After 8 and 24 hours, ATF4, a downstream target of PERK, was upregulated in hypoxic over normoxic cells. These findings support that hypoxic cells express dormancy markers simultaneously with active PERK signaling. To test HNSCC cell sensitivity to PERK inhibition, we seeded T-HEp3-TET-On-H2B-GFP cells at single cell level in a 3D Matrigel. These cells express a green fluorescent protein (GFP) in the presence of doxycycline (Dox). By seeding at a single cell level the solitary cells enter a quiescent phenotype. H2B-GFP was induced with Dox in 2D tissue culture for 3 days, prior to seeding in the Matrigel. Cells were then cultured in Dox free Matrigel in normoxic conditions for 10 days, daily treatment with PERK inhibitor LY4 was started at day 4. As a marker for quiescence, we monitored over time the retention of the H2B-GFP label in the nucleosomes. Treatment with 2μM and 6μM LY4 led to a significant decrease (p = 0.03, 0.0003, respectively) in the percentage of green label retaining cells at day 10 and 7, respectively. The total number of live cells did not increase over time (p = 0.17), arguing that the loss of H2B-GFP+ cells was not due to an increase in proliferation and dilution of the label but rather due to cell death of the quiescent, GFP positive cells. Further experiments will confirm the effect of such treatment on the hypoxic cells and the rate of death. These results indicate that dormant cells, either induced through hypoxic conditioning or single cell state, upregulate a set of dormancy markers as well as PERK. Quiescent cells seem to be dependent on PERK activity for their survival. The PERK inhibition selectively affected the survival of the quiescent, dormant-like H2B-GFP positive cells. This initial data suggests that PERK might represent a novel therapeutic target against disseminated DTCs, for which no therapeutic strategy is currently available.

Citation Format: Miguel Vizarreta Sandoval, Georg Fluegen, Kirk A. Staschke, Veronica Calvo-Vidal, Julio A. Aguirre-Ghiso. PERK-Inhibition as a possible therapy for hypoxia-induced solitary dormant tumor cells. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr A45.

Abstract A43: Phenotypic heterogeneity of disseminated tumor cells is predetermined by primary tumor hypoxic microenvironments

Heterogeneity within and between primary tumors (PTs) is reflected by genetic and epigenetic diversity and varying PT microenvironments. Further, whether PT microenvironments might influence the fate of disseminating tumor cells (DTC) has never been explored. We found that breast tumors enriched for a specific dormancy signature (DS+) displayed longer metastasis-free periods than those poor (DS-) for the signature. Key genes in the DS induce quiescence and are also regulated by hypoxia. Clinical evidence links hypoxic tumors to increased therapy resistance and a worse outcome. However, a main response of tumor cells to hypoxia is growth arrest, but how this response is linked to the clinical outcome is unknown. We hypothesized that hypoxic PT microenvironments may spawn a subpopulation of DTCs that, by virtue of becoming dormant, might escape therapies and eventually fuel incurable metastasis. We used H2B-GFP inducible HEp3 HNSCC and photo-switchable (green-to-red fluorescence) H2B-Dendra2 expressing MDA-MB-231 and ZR-75-1 human breast cancer cell lines to identify cells from hypoxic microenvironments. To initiate spatially defined hypoxic microenvironments in primary tumors we implanted induction NANo IntraVItal Devices (iNANIVIDs) carrying a hypoxia-mimetic agent (desferrioxamine - DFOM) in T-HEp3 tumors in vivo or exposed cultured MDA-MB-231 or ZR-75-1 cells in vitro to either 21% or 1% O2. The regions influenced by the DFOM-iNANIVID displayed significant upregulation of p27, NR2F1 and DEC2 (dormancy genes), as well as induction of hypoxia markers (GLUT1, HIF1α). Human HNSCC PT samples showed the same link between spontaneous hypoxic regions and up regulation of dormancy markers. We further found a significant increase in quiescent lung DTCs of hypoxia induced H2B-GFP T-HEp3 or H2B-Dendra2 MDA-MB-231 cells, traceable > 2 weeks after extravasation using H2B-GFP and H2B-Dendra2-RED label retention. Using human Vimentin to screen for HEp3 tumor cells in lungs, we found that single, unproliferating DTCs originating from the iNANIVID induced hypoxic regions showed a dormant profile (upregulation of p27, NR2F1, DEC2 and TGFβ2) compared to DTCs originating from a normoxic milieu. Simultaneously, only the hypoxic pre-treated group was able to form micro-metastasis at 10 days after injection, suggesting the presence of a more aggressive sub clone in this group. Further, analysis in 3D culture models revealed that ER+/DS+ breast cancer cells (ZR-75-1) are more prone to enter a prolonged quiescent state after a brief exposure to hypoxia (1% O2) in an NR2F1-dependent manner. This response is not observed in triple negative/DS- breast cancer cells. Lastly, using a spontaneously metastatic PyMT driven Dendra2-tagged breast cancer model, we found that ~75% of dormant DTCs up regulate the dormancy marker NR2F1 at or soon after reaching the lung, suggesting a rapid induction of dormancy upon reaching target organs. We propose that hypoxic primary tumor stress microenvironments increase phenotypic heterogeneity of DTCs and leads to the expression of the DS. Upon spreading, these DTCs may be more prone to enter dormancy, evade anti-proliferative therapies and eventually fuel metastasis.

Citation Format: Georg Fluegen, Alvaro Avivar-Valderas, Yarong Wang, Michael R. Padgen, James K. Williams, Vladislav Verkhusha, Julie F. Cheung, David Entenberg, James Castracane, Patricia J. Keely, John Condeelis, Julio A. Aguirre-Ghiso. Phenotypic heterogeneity of disseminated tumor cells is predetermined by primary tumor hypoxic microenvironments. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Metastasis; 2015 Nov 30-Dec 3; Austin, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(7 Suppl):Abstract nr A43.

The In Ovo Chick Chorioallantoic Membrane (CAM) Assay as an Efficient Xenograft Model of Hepatocellular Carcinoma

The chick chorioallantoic membrane (CAM) begins to develop by day 7 after fertilization and matures by day 12. The CAM is naturally immunodeficient and highly vascularized, making it an ideal system for tumor implantation. Furthermore, the CAM contains extracellular matrix proteins such as fibronectin, laminin, collagen, integrin alpha(v)beta3, and MMP-2, making it an attractive model to study tumor invasion and metastasis. Scientists have long taken advantage of the physiology of the CAM by using it as a model of angiogenesis. More recently, the CAM assay has been modified to work as an in vivo xenograft model system for various cancers that bridges the gap between basic in vitro work and more complex animal cancer models. The CAM assay allows for the study of tumor growth, anti-tumor therapies, and pro-tumor molecular pathways in a biologically relevant system that is both cost- and time-effective. Here, we describe the development of CAM xenograft model of hepatocellular carcinoma (HCC) with embryonic survival rates of up to 93% and reliable tumor take leading to growth of three-dimensional, vascularized tumors.

Characterization of single disseminated prostate cancer cells reveals tumor cell heterogeneity and identifies dormancy associated pathways

Cancer dormancy refers to the prolonged clinical disease-free time between removal of the primary tumor and recurrence, which is common in prostate cancer (PCa), breast cancer, esophageal cancer, and other cancers. PCa disseminated tumor cells (DTC) are detected in both patients with no evidence of disease (NED) and advanced disease (ADV). However, the molecular and cellular nature of DTC is unknown. We performed a first-in-field study of single DTC transcriptomic analyses in cancer patients to identify a molecular signature associated with cancer dormancy. We profiled eighty-five individual EpCAM⁺/CD45⁻ cells from the bone marrow of PCa patients with NED or ADV. We analyzed 44 DTC with high prostate-epithelial signatures, and eliminated 41 cells with high erythroid signatures and low prostate epithelial signatures. DTC were clustered into 3 groups: NED, ADV_1, and ADV_2, in which the ADV_1 group presented a distinct gene expression pattern associated with the p38 stress activated kinase pathway. Additionally, DTC from the NED group were enriched for a tumor dormancy signature associated with head and neck squamous carcinoma and breast cancer. This study provides the first clinical evidence of the p38 pathway as a potential biomarker for early recurrence and an attractive target for therapeutic intervention.

NR2F1 controls tumour cell dormancy via SOX9- and RARβ-driven quiescence programmes

Metastases can originate from disseminated tumour cells (DTCs), which may be dormant for years before reactivation. Here we find that the orphan nuclear receptor NR2F1 is epigenetically upregulated in experimental head and neck squamous cell carcinoma (HNSCC) dormancy models and in DTCs from prostate cancer patients carrying dormant disease for 7-18 years. NR2F1-dependent dormancy is recapitulated by a co-treatment with the DNA-demethylating agent 5-Aza-C and retinoic acid across various cancer types. NR2F1-induced quiescence is dependent on SOX9, RARβ and CDK inhibitors. Intriguingly, NR2F1 induces global chromatin repression and the pluripotency gene NANOG, which contributes to dormancy of DTCs in the bone marrow. When NR2F1 is blocked in vivo, growth arrest or survival of dormant DTCs is interrupted in different organs. We conclude that NR2F1 is a critical node in dormancy induction and maintenance by integrating epigenetic programmes of quiescence and survival in DTCs.

Mechanisms of disseminated cancer cell dormancy: an awakening field

Metastases arise from residual disseminated tumour cells (DTCs). This can happen years after primary tumour treatment because residual tumour cells can enter dormancy and evade therapies. As the biology of minimal residual disease seems to diverge from that of proliferative lesions, understanding the underpinnings of this new cancer biology is key to prevent metastasis. Analysis of approximately 7 years of literature reveals a growing focus on tumour and normal stem cell quiescence, extracellular and stromal microenvironments, autophagy and epigenetics as mechanisms that dictate tumour cell dormancy. In this Review, we attempt to integrate this information and highlight both the weaknesses and the strengths in the field to provide a framework to understand and target this crucial step in cancer progression.

Stress signaling and the shaping of the mammary tissue in development and cancer

The postnatal mammary gland develops extensively through cycles of proliferation, branching, involution and remodeling. We review recent advances made in the field of stress signaling pathways and its roles in mammary gland organogenesis, how they contribute to normal organ specification and homeostasis and how its subversion by oncogenes leads to cancer. We analyze stress signaling in mammary gland biology taking into account the interrelationship with the extracellular matrix and adhesion signaling during morphogenesis. By integrating the information gathered from in vivo and three dimensional in vitro organogenesis studies, we review the novel contribution of p38(SAPK), c-Jun NH2-terminal kinase and PKR-like endoplasmic reticulum kinase (PERK) signaling pathways to the timely activation of cell death, correct establishment of polarity and growth arrest and autophagy, respectively. We also review the evidence supporting that the activation of the aforementioned stress kinases maintain breast acinar structures as part of a tumor suppressive program and that its deregulation is commonplace during breast cancer initiation.

TGF-β2 dictates disseminated tumour cell fate in target organs through TGF-β-RIII and p38α/β signalling

In patients non-proliferative disseminated tumour cells (DTCs) can persist in the bone marrow (BM) while other organs (i.e. lung) present growing metastasis. This suggested that the BM might be a metastasis “restrictive soil” by encoding dormancy-inducing cues in DTCs. Here we show in a HNSCC model that strong and specific TGFβ2 signalling in the BM activates p38α/β, inducing a [ERK/p38]^low signalling ratio. This results in induction of DEC2/SHARP1 and p27, downregulation of CDK4 and dormancy of malignant DTCs. TGFβ2-induced dormancy required TGFβ-receptor-I, TGFβ-receptor-III and SMAD1/5 activation to induce p27. In lungs, a metastasis “permissive soil” with low TGFβ2 levels, DTC dormancy was short lived and followed by metastatic growth. Importantly, systemic inhibition of TGFβ-receptor-I or p38α/β activities awakened dormant DTCs fueling multi-organ metastasis. Our work reveals a “seed and soil” mechanism where TGFβ2 and TGFβRIII signalling through p38α/β regulates DTC dormancy and defines restrictive (BM) and -permissive (lung) microenvironments for HNSCC metastasis.