Plasticity-induced repression of Irf6 underlies acquired resistance to cancer immunotherapy in pancreatic ductal adenocarcinoma

Acquired resistance to immunotherapy remains a critical yet incompletely understood biological mechanism. Here, using a mouse model of pancreatic ductal adenocarcinoma (PDAC) to study tumor relapse following immunotherapy-induced responses, we find that resistance is reproducibly associated with an epithelial-to-mesenchymal transition (EMT), with EMT-transcription factors ZEB1 and SNAIL functioning as master genetic and epigenetic regulators of this effect. Acquired resistance in this model is not due to immunosuppression in the tumor immune microenvironment, disruptions in the antigen presentation machinery, or altered expression of immune checkpoints. Rather, resistance is due to a tumor cell-intrinsic defect in T-cell killing. Molecularly, EMT leads to the epigenetic and transcriptional silencing of interferon regulatory factor 6 (Irf6), rendering tumor cells less sensitive to the pro-apoptotic effects of TNF-α. These findings indicate that acquired resistance to immunotherapy may be mediated by programs distinct from those governing primary resistance, including plasticity programs that render tumor cells impervious to T-cell killing.

SWIP-a stabilized window for intravital imaging of the murine pancreas

Pancreatitis and pancreatic ductal adenocarcinoma (PDAC) are grave illnesses with high levels of morbidity and mortality. Intravital imaging (IVI) is a powerful technique for visualizing physiological processes in both health and disease. However, the application of IVI to the murine pancreas presents significant challenges, as it is a deep, compliant, visceral organ that is difficult to access, easily damaged and susceptible to motion artefacts. Existing imaging windows for stabilizing the pancreas during IVI have unfortunately shown poor stability for time-lapsed imaging on the minutes to hours scale, or are unable to accommodate both the healthy and tumour-bearing pancreata. To address these issues, we developed an improved stabilized window for intravital imaging of the pancreas (SWIP), which can be applied to not only the healthy pancreas but also to solid tumours like PDAC. Here, we validate the SWIP and use it to visualize a variety of processes for the first time, including (1) single-cell dynamics within the healthy pancreas, (2) transformation from healthy pancreas to acute pancreatitis induced by cerulein, and (3) the physiology of PDAC in both autochthonous and orthotopically injected models. SWIP can not only improve the imaging stability but also expand the application of IVI in both benign and malignant pancreas diseases.

MYC levels regulate metastatic heterogeneity in pancreatic adenocarcinoma

The degree of metastatic disease varies widely amongst cancer patients and impacts clinical outcomes. However, the biological and functional differences that drive the extent of metastasis are poorly understood. We analyzed primary tumors and paired metastases using a multi-fluorescent lineage-labeled mouse model of pancreatic ductal adenocarcinoma (PDAC) - a tumor type where most patients present with metastases. Genomic and transcriptomic analysis revealed an association between metastatic burden and gene amplification or transcriptional upregulation of MYC and its downstream targets. Functional experiments showed that MYC promotes metastasis by recruiting tumor associated macrophages (TAMs), leading to greater bloodstream intravasation. Consistent with these findings, metastatic progression in human PDAC was associated with activation of MYC signaling pathways and enrichment for MYC amplifications specifically in metastatic patients. Collectively, these results implicate MYC activity as a major determinant of metastatic burden in advanced PDAC.

Calcium signaling induces a partial EMT

Epithelial plasticity, or epithelial-to-mesenchymal transition (EMT), is a well-recognized form of cellular plasticity, which endows tumor cells with invasive properties and alters their sensitivity to various agents, thus representing a major challenge to cancer therapy. It is increasingly accepted that carcinoma cells exist along a continuum of hybrid epithelial-mesenchymal (E-M) states and that cells exhibiting such partial EMT (P-EMT) states have greater metastatic competence than those characterized by either extreme (E or M). We described recently a P-EMT program operating in vivo by which carcinoma cells lose their epithelial state through post-translational programs. Here, we investigate the underlying mechanisms and report that prolonged calcium signaling induces a P-EMT characterized by the internalization of membrane-associated E-cadherin (ECAD) and other epithelial proteins as well as an increase in cellular migration and invasion. Signaling through Gαq-associated G-protein-coupled receptors (GPCRs) recapitulates these effects, which operate through the downstream activation of calmodulin-Camk2b signaling. These results implicate calcium signaling as a trigger for the acquisition of hybrid/partial epithelial-mesenchymal states in carcinoma cells.

Single-cell lineage tracing of metastatic cancer reveals selection of hybrid EMT states

The underpinnings of cancer metastasis remain poorly understood, in part due to a lack of tools for probing their emergence at high resolution. Here we present macsGESTALT, an inducible CRISPR-Cas9-based lineage recorder with highly efficient single-cell capture of both transcriptional and phylogenetic information. Applying macsGESTALT to a mouse model of metastatic pancreatic cancer, we recover ∼380,000 CRISPR target sites and reconstruct dissemination of ∼28,000 single cells across multiple metastatic sites. We find that cells occupy a continuum of epithelial-to-mesenchymal transition (EMT) states. Metastatic potential peaks in rare, late-hybrid EMT states, which are aggressively selected from a predominately epithelial ancestral pool. The gene signatures of these late-hybrid EMT states are predictive of reduced survival in both human pancreatic and lung cancer patients, highlighting their relevance to clinical disease progression. Finally, we observe evidence for in vivo propagation of S100 family gene expression across clonally distinct metastatic subpopulations.

Abstract 2861: Collateral amplification KRAS-PTHrP drives pancreatic cancer growth and metastasis and reveals a new therapeutic vulnerability

Purpose: Metastasis is the leading cause of cancer-related death in PDAC, yet very little is understood regarding the underlying biology. As a result, targeted therapies to inhibit metastasis are lacking. Whole-genome sequencing has established that the squamous/quasi-mesenchymal/basal-like PDAC subtype, which is characterized by its high metastatic proclivity, is annotated by KRAS gene amplification. Here, we report that the squamous lineage gene parathyroid hormone-related protein (PTHrP encoded by PTHLH) is located directly adjacent to KRAS and is co-amplified in metastatic patients. We hypothesize that this collateral amplification of PTHrP may exert its own oncogenic and pro-metastatic phenotype beyond KRAS and set out to determine if this will confer a novel therapeutic vulnerability.

Methods: We generated a novel genetically engineered mouse model whereby we deleted the cytokine Pthlh in the autochthonous KPCY model. To functionally demonstrate the oncogenic and pro-metastatic roles of PTHrP, we further employed genetic deletion and pharmacological inhibition in orthotopic injection, tail vein metastasis assays, mouse hospital pre-clinical trials, and patient-derived 3D organoid models.

Results: In silico analysis established that PTHLH is co-amplified along with KRAS in TCGA, is specifically enriched in metastatic patients from the COMPASS trial and correlates with significantly decreased overall survival in both cohorts. Further examination revealed that PTHLH is a squamous/quasi-mesenchymal/basal-like lineage marker. We generated KPCY-PthlhCKO mice and showed that they have significantly reduced primary and metastatic tumor burden and dramatically increased overall survival relative to KPCY controls. In parallel experiments, we treated mice with an anti-PTHrP neutralizing monoclonal antibody, which similarly reduced primary and metastatic tumor growth. Finally, RNA-seq revealed a downstream mechanism whereby PTHrP is important for metastatic competency through induction of EMT, thus facilitating entry into the metastatic cascade. Loss of PTHrP reduced the ability of tumor cells to undergo EMT both in vivo and in vitro, resulting in a nearly complete elimination of disseminating cells in KPCY-PthlhCKO mice. Thus, KPCY-PthlhCKO tumors are locked in a well-differentiated epithelial state and are unable to initiate the metastatic process.

Conclusions: This work has demonstrated the importance of the previously unappreciated role for PTHrP signaling in pancreatic cancer cell plasticity and metastasis, and future studies will look to translate anti-PTHrP therapy into clinical trials. In a broader sense, we establish a new paradigm of collateral amplification, where an assumed passenger gene (PTHLH) is co-amplified along with a known oncogene (KRAS) and endows the evolving tumor with its own oncogenic and pro-metastatic phenotype.

Citation Format: Jason Robert Pitarresi, Robert J. Norgard, Anna M. Chiarella, Richard Kremer, Ben Z. Stanger, Anil K. Rustgi. Collateral amplification KRAS-PTHrP drives pancreatic cancer growth and metastasis and reveals a new therapeutic vulnerability [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 2861.

Abstract 1776: Tumor-derived endothelins regulate antitumor immune responses through macrophage endothelin B receptor

Soft tissue sarcomas (STS) comprise a heterogeneous group of solid tumors arising from mesodermal tissues. With few effective systemic therapies, advanced-stage STS is frequently fatal. Immune checkpoint blockade (ICB) is currently under investigation for the treatment of STS. However, STS harbor a highly immunosuppressive tumor microenvironment (TME) that inhibits effective antitumor T cell responses, with only a subset of patients responding to ICB. While the major focus of immunotherapy to date has been on the role of T cells, targeting innate immune cells such as tumor associated macrophages (TAMs) within the TME holds great promise.

TAMs are the most abundant leukocytes in STS, and contribute to the establishment of an immunosuppressive TME through multiple mechanisms. We identified the endothelin B receptor (EDNRB) as a potential novel therapeutic target on TAMs based on an analysis of TCGA data. EDNRB is highly expressed in “immunologically quiet” tumors with high macrophage to T cell ratios and M2-like gene expression. EDNRB encodes a G-protein-coupled receptor that binds the vasoactive peptides endothelin (EDN) 1, 2, or 3. EDNRB is upregulated during macrophage differentiation, and is highly expressed by certain macrophages subsets, including TAMs. However, the role of the EDN-EDNRB axis in regulating macrophage function and antitumor immunity is unknown.

Using a murine syngeneic transplant model with cell lines derived from methylcholanthrene-induced fibrosarcomas, we first genetically deleted or overexpressed the ligand Edn1 in sarcoma cells prior to transplant. Genetic deletion of Edn1 using CRISPR-Cas9 resulted in significantly increased major histocompatibility complex class II (MHCII) expression in TAMs, indicative of a more immunostimulatory phenotype. RNA-seq of sorted TAMs from Edn1 CRISPR tumors revealed enrichment for gene sets involved in antitumor immunity, including interferon gamma and NF-kB. Conversely, overexpression of Edn1 dramatically reduced TAM MHCII expression. Furthermore, knockout of Edn1 or pharmacologic blockade of EDNRB significantly enhanced responses to anti-PD1. To confirm these effects were mediated by macrophage EDNRB, we proceeded to generate a conditional deletion of Ednrb in TAMs by crossing Ednrbflox/flox and LysM-Cre mice. Consistent with our hypothesis, TAMs from sarcomas in LysM-Cre:Ednrbflox/flox mice also took on an immunostimulatory phenotype, with increased MHCII expression and reduced M2 markers.

Taken together, these findings support the existence of a novel regulatory axis, whereby tumor-derived endothelins promote a suppressive TAM phenotype through macrophage EDNRB. Combining endothelin receptor antagonists with immune checkpoint blockade has the potential to enhance antitumor immune responses in STS and other solid tumors by targeting both innate and adaptive immune cells within the TME.

Citation Format: Ian Wesley Folkert, Tsun Ki Jerrick To, Samir Devalaraja, Robert J. Norgard, Malay Haldar. Tumor-derived endothelins regulate antitumor immune responses through macrophage endothelin B receptor [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 1776.

PTHrP Drives Pancreatic Cancer Growth and Metastasis and Reveals a New Therapeutic Vulnerability

Pancreatic cancer metastasis is a leading cause of cancer-related deaths, yet very little is understood regarding the underlying biology. As a result, targeted therapies to inhibit metastasis are lacking. Here, we report that the parathyroid hormone-related protein (PTHrP encoded by PTHLH) is frequently amplified as part of the KRAS amplicon in patients with pancreatic cancer. PTHrP upregulation drives the growth of both primary and metastatic tumors in mice and is highly enriched in pancreatic ductal adenocarcinoma metastases. Loss of PTHrP-either genetically or pharmacologically-dramatically reduces tumor burden, eliminates metastasis, and enhances overall survival. These effects are mediated in part through a reduction in epithelial-to-mesenchymal transition, which reduces the ability of tumor cells to initiate metastatic cascade. Spp1, which encodes osteopontin, is revealed to be a downstream effector of PTHrP. Our results establish a new paradigm in pancreatic cancer whereby PTHrP is a driver of disease progression and emerges as a novel therapeutic vulnerability. SIGNIFICANCE: Pancreatic cancer often presents with metastases, yet no strategies exist to pharmacologically inhibit this process. Herein, we establish the oncogenic and prometastatic roles of PTHLH, a novel amplified gene in pancreatic ductal adenocarcinoma. We demonstrate that blocking PTHrP activity reduces primary tumor growth, prevents metastasis, and prolongs survival in mice.This article is highlighted in the In This Issue feature, p. 1601.

Abstract PO015: Tumor-cell-intrinsic epigenetic factors underlie the heterogeneity of immune infiltration and response to immunotherapy in pancreatic cancer

Resistance to immunotherapy is a major problem of current clinical care for cancer patients. While T cell abundance is essential for tumor responsiveness to immunotherapy, factors that dictate T cell infiltration in tumor microenvironments are not fully understood. To understand the tumor-cell-intrinsic factors underlying the heterogeneity of tumor immunity and sensitivity to immunotherapy, we established a new experimental system by generating a library of congenic pancreatic tumor cell clones from a genetic mouse model driven by mutant Kras and p53. These tumor cell clones robustly formed implanted tumors that recapitulated T-cell-inflamed and non-T-cell-inflamed tumor microenvironments, associated with distinct patterns of infiltration by T cells and myeloid cells. We found that the non-T-cell-inflamed phenotype was dominant over the T-cell-inflamed phenotype in the local tumor microenvironment. An integrated transcriptomic and epigenetic analysis revealed that tumor-cell-intrinsic expression of CXCL1, EPHA2, PTGS2, and USP22 as determinants of the non-T-cell-inflamed microenvironment, and ablation of tumor-cell-intrinsic CXCL1, EPHA2, PTGS2, or USP22 promoted T cell infiltration and sensitivity to a combination of chemotherapies, CD40 agonist, and checkpoint blockades. Furthermore, we performed an in vivo CRISPR-based genetic screen to identify tumor cell intrinsic epigenetic regulators of anti-tumor immunity and discovered novel therapeutic opportunities to improve the efficacy of currently developed immunotherapy for pancreatic cancer. These results demonstrated that heterogeneity of tumor immune phenotypes is driven by tumor-cell-intrinsic factors, including epigenetic factors, that can be manipulated to influence the outcome of immunotherapies.

Citation Format: Jinyang Li, Salina Yuan, Robert J. Norgard, Fangxue Yan, Robert H. Vonderheide, Andres Blanco, Ben Z. Stanger. Tumor-cell-intrinsic epigenetic factors underlie the heterogeneity of immune infiltration and response to immunotherapy in pancreatic cancer [abstract]. In: Abstracts: AACR Virtual Special Conference: Tumor Immunology and Immunotherapy; 2020 Oct 19-20. Philadelphia (PA): AACR; Cancer Immunol Res 2021;9(2 Suppl):Abstract nr PO015.

Abstract PO-057: Collateral amplification of the PTHrP gene drives pancreatic cancer growth and metastasis and reveals a new therapeutic vulnerability

Purpose: Metastasis is the leading cause of cancer-related death in PDAC, yet very little is understood regarding the underlying biology. As a result, targeted therapies to inhibit metastasis are lacking. Whole-genome sequencing has established that the squamous/quasi-mesenchymal/basal-like PDAC subtype, which is characterized by its high metastatic proclivity, is annotated by KRAS gene amplification. Here, we report that the squamous lineage gene parathyroid hormone-related protein (PTHrP encoded by PTHLH) is located directly adjacent to KRAS and is co-amplified in metastatic PDAC patients. We hypothesize that this collateral amplification of PTHrP may exert its own oncogenic and pro-metastatic phenotype beyond KRAS and set out to determine if this will confer a novel therapeutic vulnerability.

Methods: We generated a novel genetically engineered mouse model whereby we deleted the cytokine Pthlh in the autochthonous KPCY model. To functionally demonstrate the oncogenic and pro-metastatic roles of PTHrP, we further employed genetic deletion and pharmacological inhibition in orthotopic injection, tail vein metastasis assays, mouse hospital pre-clinical trials, and patient-derived 3D organoid models.

Results: In silico analysis established that PTHLH is co-amplified along with KRAS in TCGA, is specifically enriched in metastatic patients from the COMPASS trial and correlates with significantly decreased overall survival in both cohorts. Further examination revealed that PTHLH is a squamous/quasi-mesenchymal/basal-like lineage marker. We generated KPCY-PthlhCKO mice and showed that they have significantly reduced primary and metastatic tumor burden and dramatically increased overall survival relative to KPCY controls. In parallel experiments, we treated mice with an anti-PTHrP neutralizing monoclonal antibody, which similarly reduced primary and metastatic tumor growth. Finally, RNA-seq revealed a downstream mechanism whereby PTHrP is important for metastatic competency through induction of EMT, thus facilitating entry into the metastatic cascade. Loss of PTHrP reduced the ability of tumor cells to undergo EMT, resulting in a nearly complete elimination of disseminating cells in KPCY-PthlhCKO mice. Thus, KPCY-PthlhCKO tumors are locked in a well-differentiated epithelial state and are unable to initiate the metastatic process.

Conclusions: This work has demonstrated the importance of the previously unappreciated role for PTHrP signaling in pancreatic cancer cell plasticity and metastasis, and future studies will look to translate anti-PTHrP therapy into clinical trials. In a broader sense, we establish a new paradigm of collateral amplification, where an assumed passenger gene (PTHLH) is co-amplified along with a known oncogene (KRAS) and endows the evolving tumor with its own oncogenic and pro-metastatic phenotype.

Citation Format: Jason R. Pitarresi, Robert J. Norgard, Anna M. Chiarella, Richard Kremer, Ben Z. Stanger, Anil K. Rustgi. Collateral amplification of the PTHrP gene drives pancreatic cancer growth and metastasis and reveals a new therapeutic vulnerability [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2020 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2020;80(22 Suppl):Abstract nr PO-057.

Abstract PO-053: MYC Influences metastatic heterogeneity in pancreatic cancer

Tumor heterogeneity - resulting from genetic and epigenetic alterations acquired during tumor progression - is a critical driver of phenotypic diversity in most cancers. A lethal consequence of tumor heterogeneity is the acquisition of metastatic traits by tumor cells, leading to poor clinical outcomes. This remains a major problem in pancreatic ductal adenocarcinoma (PDAC), which continues to have the worst prognosis of any major cancer type. While most cases of PDAC present with metastatic disease at the time of diagnosis, the patterns and burden of metastasis can vary widely, with some patients exhibiting a limited metastatic burden while others have more extensive spread, which impacts clinical outcomes. However, the biological and functional differences that drive metastatic heterogeneity are poorly understood. One barrier to understanding metastatic heterogeneity has been a paucity of model systems that capture this natural variation and allow for direct assessment of paired primary tumors and metastases. We previously developed an autochthonous model of PDAC – the KPCX model – that employs multiplexed fluorescence-based labeling to track the contribution of multiple distinct tumor populations to metastasis. Importantly, this technique allows for ascertainment of primary-metastatic lineage relationships in vivo, so that primary tumor clones with substantial metastatic potential can be distinguished with those having poor metastatic potential. To understand the factors underlying differences in metastatic potential, we analyzed paired primary tumors and metastases in the KPCX model and from a cohort of 398 PDAC patients. Genomic and transcriptomic analysis of murine and human metastatic PDAC revealed an association between the highly metastatic state and gene amplification or transcriptional upregulation of MYC and its transcriptional targets. Functional assessments showed that MYC promotes metastasis by recruiting tumor associated macrophages (TAMs), leading to greater bloodstream intravasation. Consistent with these findings, metastatic progression in human PDAC was also associated of MYC signaling pathways and enrichment for MYC amplification in metastasis. Collectively, these results implicate MYC activity as a major determinant of metastatic burden and heterogeneity in advanced PDA.

Citation Format: Ravikanth Maddipati, Robert J. Norgard, Timour Baslan, Komal S. Rathi, Amy Zhang, Pichai Raman, Max D. Wengyn, Taiji Yamazoe, Jinyang Li, David Balli, Michael J. LaRiviere, Ian W. Folkert, Ian D. Millstein, Jonathan Bermeo, Erica L. Carpenter, Scott Lowe, Christine Iacobuzio-Donahue, Faiyaz Notta, Ben Z. Stanger. MYC Influences metastatic heterogeneity in pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2020 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2020;80(22 Suppl):Abstract nr PO-053.

Abstract PR-003: Tumor-cell-intrinsic epigenetic factors underlie the heterogeneity of immune infiltration and response to immunotherapy in pancreatic cancer

Resistance to immunotherapy is a major problem of current clinical care for cancer patients. While T cell abundance is essential for tumor responsiveness to immunotherapy, factors that dictate T cell infiltration in tumor microenvironments are not fully understood. To understand the tumor-cell-intrinsic factors underlying the heterogeneity of tumor immunity and sensitivity to immunotherapy, we established a new experimental system by generating a library of congenic pancreatic tumor cell clones from a genetic mouse model driven by mutant Kras and p53. These tumor cell clones robustly formed implanted tumors that recapitulated T-cell-inflamed and non-T-cell-inflamed tumor microenvironments, associated with distinct patterns of infiltration by T cells and myeloid cells. We found that the non-T-cell-inflamed phenotype was dominant over the T-cell-inflamed phenotype in the local tumor microenvironment. An integrated transcriptomic and epigenetic analysis revealed that tumor-cell-intrinsic expression of CXCL1, EPHA2 and PTGS2 as determinants of the non-T-cell-inflamed microenvironment, and ablation of tumor-cell-intrinsic CXCL1, EPHA2 or PTGS2 promoted T cell infiltration and sensitivity to a combination of chemotherapies, CD40 agonist, and checkpoint blockades. Similarly, we identified tumor cell-intrinsic epigenetic factor, USP22, as a key regulator of immune infiltration and immunotherapy response in pancreatic cancer. Ablation of tumor-cell-intrinsic USP22 enhanced T cell infiltration and suppressed myeloid cell infiltration in implanted pancreatic tumors as well as increased sensitivities of tumors to the combined immunotherapy. These results demonstrated that heterogeneity of tumor immune phenotypes is driven by tumor-cell-intrinsic factors, including epigenetic factors, that can be manipulated to influence the outcome of immunotherapies.

Citation Format: Jinyang Li, Salina Yuan, Robert J. Norgard, Fangxue Yan, Robert H. Vonderheide, Andres Blanco, Ben Z. Stanger. Tumor-cell-intrinsic epigenetic factors underlie the heterogeneity of immune infiltration and response to immunotherapy in pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Pancreatic Cancer; 2020 Sep 29-30. Philadelphia (PA): AACR; Cancer Res 2020;80(22 Suppl):Abstract nr PR-003.

Epigenetic and Transcriptional Control of the Epidermal Growth Factor Receptor Regulates the Tumor Immune Microenvironment in Pancreatic Cancer

Although immunotherapy has revolutionized cancer care, patients with pancreatic ductal adenocarcinoma (PDA) rarely respond to these treatments, a failure that is attributed to poor infiltration and activation of T cells in the tumor microenvironment (TME). We performed an in vivo CRISPR screen and identified lysine demethylase 3A (KDM3A) as a potent epigenetic regulator of immunotherapy response in PDA. Mechanistically, KDM3A acts through Krueppel-like factor 5 (KLF5) and SMAD family member 4 (SMAD4) to regulate the expression of the epidermal growth factor receptor (EGFR). Ablation of KDM3A, KLF5, SMAD4, or EGFR in tumor cells altered the immune TME and sensitized tumors to combination immunotherapy, whereas treatment of established tumors with an EGFR inhibitor, erlotinib, prompted a dose-dependent increase in intratumoral T cells. This study defines an epigenetic-transcriptional mechanism by which tumor cells modulate their immune microenvironment and highlights the potential of EGFR inhibitors as immunotherapy sensitizers in PDA. SIGNIFICANCE: PDA remains refractory to immunotherapies. Here, we performed an in vivo CRISPR screen and identified an epigenetic-transcriptional network that regulates antitumor immunity by converging on EGFR. Pharmacologic inhibition of EGFR is sufficient to rewire the immune microenvironment. These results offer a readily accessible immunotherapy-sensitizing strategy for PDA.This article is highlighted in the In This Issue feature, p. 521.

Abstract PO-071: MYC influences metastatic heterogeneity in pancreatic cancer

Tumor heterogeneity - resulting from genetic and epigenetic alterations acquired during tumor progression - is a critical driver of phenotypic diversity in most cancers. A lethal consequence of tumor heterogeneity is the acquisition of metastatic traits by tumor cells, leading to poor clinical outcomes. This remains a major problem in pancreatic ductal adenocarcinoma (PDAC), which continues to have the worst prognosis of any major cancer type. While most cases of PDAC present with metastatic disease at the time of diagnosis, the patterns and burden of metastasis can vary widely, with some patients exhibiting a limited metastatic burden while others have more extensive spread, which impacts clinical outcomes. However, the biological and functional differences that drive metastatic heterogeneity are poorly understood.

Citation Format: Ravikanth Maddipati, Robert J. Norgard, Timour Baslan, Komal S. Rathi, Amy Zhang, Pichai Raman, Max D. Wengyn, Taiji Yamazoe, Jinyang Li, David Balli, Michael LaRiviere, Ian W. Folkert, Ian D. Millstein, Jonathan Bermeo, Erica L. Carpenter, Scott Lowe, Christine Iacobuzio-Donahue, Faiyaz Notta, Ben Z. Stanger. MYC influences metastatic heterogeneity in pancreatic cancer [abstract]. In: Proceedings of the AACR Virtual Special Conference on Tumor Heterogeneity: From Single Cells to Clinical Impact; 2020 Sep 17-18. Philadelphia (PA): AACR; Cancer Res 2020;80(21 Suppl):Abstract nr PO-071.

Pharmacologic Activation of the G Protein-Coupled Estrogen Receptor Inhibits Pancreatic Ductal Adenocarcinoma

BACKGROUND & AIMS

Female sex is associated with lower incidence and improved clinical outcomes for most cancer types including pancreatic ductal adenocarcinoma (PDAC). The mechanistic basis for this sex difference is unknown. We hypothesized that estrogen signaling may be responsible, despite the fact that PDAC lacks classic nuclear estrogen receptors.

METHODS

Here we used murine syngeneic tumor models and human xenografts to determine that signaling through the nonclassic estrogen receptor G protein-coupled estrogen receptor (GPER) on tumor cells inhibits PDAC.

RESULTS

Activation of GPER with the specific, small molecule, synthetic agonist G-1 inhibited PDAC proliferation, depleted c-Myc and programmed death ligand 1 (PD-L1), and increased tumor cell immunogenicity. Systemically administered G-1 was well-tolerated in PDAC bearing mice, induced tumor regression, significantly prolonged survival, and markedly increased the efficacy of PD-1 targeted immune therapy. We detected GPER protein in a majority of spontaneous human PDAC tumors, independent of tumor stage.

CONCLUSIONS

These data, coupled with the wide tissue distribution of GPER and our previous work showing that G-1 inhibits melanoma, suggest that GPER agonists may be useful against a range of cancers that are not classically considered sex hormone responsive and that arise in tissues outside of the reproductive system.

Senescence-Induced Vascular Remodeling Creates Therapeutic Vulnerabilities in Pancreas Cancer

KRAS mutant pancreatic ductal adenocarcinoma (PDAC) is characterized by a desmoplastic response that promotes hypovascularity, immunosuppression, and resistance to chemo- and immunotherapies. We show that a combination of MEK and CDK4/6 inhibitors that target KRAS-directed oncogenic signaling can suppress PDAC proliferation through induction of retinoblastoma (RB) protein-mediated senescence. In preclinical mouse models of PDAC, this senescence-inducing therapy produces a senescence-associated secretory phenotype (SASP) that includes pro-angiogenic factors that promote tumor vascularization, which in turn enhances drug delivery and efficacy of cytotoxic gemcitabine chemotherapy. In addition, SASP-mediated endothelial cell activation stimulates the accumulation of CD8⁺ T cells into otherwise immunologically "cold" tumors, sensitizing tumors to PD-1 checkpoint blockade. Therefore, in PDAC models, therapy-induced senescence can establish emergent susceptibilities to otherwise ineffective chemo- and immunotherapies through SASP-dependent effects on the tumor vasculature and immune system.

Tumor Cell-Intrinsic USP22 Suppresses Antitumor Immunity in Pancreatic Cancer

Although immune checkpoint blockade (ICB) improves clinical outcome in several types of malignancies, pancreatic ductal adenocarcinoma (PDA) remains refractory to this therapy. Preclinical studies have demonstrated that the relative abundance of suppressive myeloid cells versus cytotoxic T cells determines the efficacy of combination immunotherapies, which include ICB. Here, we evaluated the role of the ubiquitin-specific protease 22 (USP22) as a regulator of the immune tumor microenvironment (TME) in PDA. We report that deletion of USP22 in pancreatic tumor cells reduced the infiltration of myeloid cells and promoted the infiltration of T cells and natural killer (NK) cells, leading to an improved response to combination immunotherapy. We also showed that ablation of tumor cell-intrinsic USP22 suppressed metastasis of pancreatic tumor cells in a T-cell-dependent manner. Finally, we provide evidence that USP22 exerted its effects on the immune TME by reshaping the cancer cell transcriptome through its association with the deubiquitylase module of the SAGA/STAGA transcriptional coactivator complex. These results indicated that USP22 regulates immune infiltration and immunotherapy sensitivity in preclinical models of pancreatic cancer.

Abstract PR01: Senescence induction triggers vascular remodeling and new vulnerabilities to chemo- and immunotherapy in pancreas cancer

Background: KRAS mutant pancreatic ductal adenocarcinoma (PDAC) is characterized by a desmoplastic response that promotes hypovascularity, poor drug delivery, immunosuppression, and de novo resistance to chemo- and immunotherapies. Recently, we demonstrated that a combination of MEK and CDK4/6 inhibitors can potently suppress PDAC tumor cell proliferation through induction of RB-mediated senescence and trigger a senescence-associated secretory phenotype (SASP) capable of remodeling the tumor microenvironment (TME) (Ruscetti et al., Science 2018). Here, we set out to explore how senescence induction could remodel the PDAC TME and alter the treatment landscape of this disease.

Methods: The Pdx1-Cre;LSL-KRASG12D;Trp53fl/wt (KPC) genetically engineered mouse model (GEMM) of PDAC, as well as immunocompetent C57BL/6 mice transplanted with PDAC organoids derived from this model, were treated for 2 weeks with the MEK inhibitor trametinib and CDK4/6 inhibitor palbociclib. Induction of senescence was determined by SA-β-gal staining, and secretion of SASP factors was determined by qPCR and cytokine array. The impact on vascularization and vascular function, as well as the immune system, was determined by immunohistochemistry and flow cytometry analysis. shRNAs targeting the p65 subunit of NF-KB were used to assess the effect of SASP knockdown on treatment responses, and high doses of a VEGFR2 blocking antibody were used to assess the effects of inhibiting neovascularization on these SASP-dependent phenotypes. Trametinib and palbociclib treatment was combined with the chemotherapeutic agent gemcitabine or PD-1 checkpoint blockade immunotherapy to study the impact on tumor responses and long-term survival of PDAC tumor-bearing animals.

Results: We find that therapy-induced senescence following trametinib and palbociclib treatment produces a SASP rich in proangiogenic factors, culminating in increased vascular density and perfusion in hypovascular PDAC tumors. This SASP-dependent vascular remodeling leads to enhanced drug uptake of the chemotherapeutic agent gemcitabine, and combining our senescence-inducing therapy with gemcitabine drives tumor regressions and prolonged survival in gemcitabine-refractory PDAC GEMMs and PDXs. In addition, increased antigen presentation and SASP-mediated vascular remodeling upon treatment mediates CD8+ T cell accumulation and activation within the PDAC TME, sensitizing these tumors to PD-1 checkpoint blockade.

Conclusions: These results demonstrate that therapy-induced senescence can establish emergent susceptibilities to otherwise ineffective chemo- and immunotherapies in PDAC through SASP-dependent, non-cell autonomous effects on the tumor vasculature and immune system.

This abstract is also being presented as Poster A46.

Citation Format: Marcus Ruscetti, John P. Morris, IV, Riccardo Mezzadra, James Russell, Josef Leibold, Paul B. Romesser, Janelle Simon, Amanda Kulick, Yu-jui Ho, Myles Fennell, Jinyang Li, Robert J. Norgard, John E. Wilkinson, Direna Alonso-Curbelo, Ramya Sridharan, Xiang Li, Daniel Heller, Elisa de Stanchina, Ben Z. Stanger, Charles J. Sherr, Scott W. Lowe. Senescence induction triggers vascular remodeling and new vulnerabilities to chemo- and immunotherapy in pancreas cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr PR01.

Abstract B43: p120 catenin loss drives pancreatic cancer EMT and metastasis through activation of PTHrP-mediated calcium signaling

Purpose: An unbiased approach to discover candidate cancer genes in pancreatic ductal adenocarcinoma (PDAC) identified the p120 catenin gene (Ctnnd1) as one of the top 20 driver genes, and further analysis revealed that P120CTN loss correlated with reduced survival. Results presented herein used genetically engineered mouse models (GEMMs) to show that P120CTN is a potent metastasis suppressor in PDAC.

Methods: We employed orthotopic injection, tail vein metastasis assays, and mouse hospital preclinical trials to show that deletion or pharmacologic inhibition of Parathyroid Hormone Related Protein (PTHrP/PTHLH), a signaling element downstream of P120CTN, delayed tumor development and metastatic outgrowth. Finally, we generated a novel GEMM of Pthlh deletion to demonstrate in the autochthonous KPC model that loss of PTHrP phenocopies anti-PTHrP therapy by blocking both primary and metastatic tumor growth.

Results: We have generated a mouse model to delete the gene Ctnnd1, whose gene product p120 catenin (P120CTN) is necessary for E-cadherin stability, resulting in enhanced epithelial-to-mesenchymal transition (EMT) and metastasis in KPC animals. Specifically, we show that KPC-p120ctncKO mice have a dramatically enhanced metastatic phenotype relative to KPC controls, suggesting that P120CTN is a critical factor in metastatic cell dissemination. An unbiased screen of tumor cells isolated from these mice identified misregulated calcium signaling through the Parathyroid Hormone Related Protein (PTHrP) as a previously unappreciated contributor to EMT and metastasis. Genetic deletion of the gene that codes for PTHrP in orthotopic transplantation experiments showed significantly reduced tumor growth and metastasis, establishing PTHrP as an oncogenic and prometastatic secreted peptide. Furthermore, treatment with anti-PTHrP monoclonal antibodies reduced tumor cell proliferation and migration in vitro, demonstrating that anti-PTHrP therapies may be of clinical benefit. Importantly, we generated KPC-PthlhcKO mice and showed that they have significantly reduced primary and metastatic tumor burden and increased survival relative to KPC controls. In parallel experiments, we treated KPC mice in a preclinical trial with anti-PTHrP neutralizing antibodies, which delayed both primary and metastatic tumor growth. Finally, analysis of human samples demonstrated that increased PTHLH expression is associated with significantly decreased survival, and that a subset of patients have PTHLH genomic amplifications.

Conclusions: This novel work has demonstrated the importance of the previously unappreciated role that PTHrP-mediated calcium signaling plays in pancreatic cancer cellular plasticity and metastasis, and future studies will look to determine the efficacy of anti-PTHrP monoclonal antibodies with a view towards translation in human clinical trials.

Citation Format: Jason R. Pitarresi, Robert J. Norgard, Ben Z. Stanger, Anil K. Rustgi. p120 catenin loss drives pancreatic cancer EMT and metastasis through activation of PTHrP-mediated calcium signaling [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr B43.

Abstract A28: Investigation of tumor-cell-intrinsic factors regulating immune infiltration and response to immunotherapy in pancreatic cancer

Resistance to immunotherapy is one major problem of current clinical care for cancer patients. While T-cell abundance is essential for tumor responsiveness to immunotherapy, factors that dictate T-cell infiltration in tumor microenvironments are not fully understood. To understand the tumor cell-intrinsic factors underlying the heterogeneity of tumor immunity and sensitivity to immunotherapy, we established a new experimental system by generating a library of congenic pancreatic tumor cell clones from a genetic mouse model driven by mutant Kras and p53. These tumor cell clones robustly formed implanted tumors that recapitulated the T cell-inflamed and non-T cell-inflamed tumor microenvironments in human patients, associated with distinct patterns of infiltration by T cells and myeloid cells. We found that the non-T cell-inflamed phenotype was dominant over the T cell-inflamed phenotype in the local tumor microenvironment. Both quantitative and qualitative features, specifically expression of markers of prior TCR activation, of intratumoral CD8+ T cells predicted the response to immunotherapies. An integrated transcriptomic and epigenetic analysis revealed that tumor cell-intrinsic expression of the chemokine CXCL1 as a major determinant of the non-T cell-inflamed microenvironment, and ablation of tumor cell-intrinsic CXCL1 promoted T-cell infiltration and sensitivity to a combination of chemotherapies, CD40 agonist, and checkpoint blockades. Similarly, we identified tumor cell-intrinsic EPHA2 and PTGS2 as key regulators of immune infiltration and immunotherapy response in our experimental system. Ablation of tumor cell-intrinsic EPHA2 or PTGS2 enhanced T-cell infiltration and suppressed myeloid cell infiltration in implanted pancreatic tumors, and increased sensitivities of tumors to the combined immunotherapy. These results demonstrated that heterogeneity of tumor immune phenotypes is driven by tumor cell-intrinsic factors that can be manipulated to influence the outcome of immunotherapies. The observation that non-T cell-inflamed phenotype is dominant emphasized the importance of targeting mechanisms driving T-cell low phenotype for improving immunotherapy response.

Citation Format: Jinyang Li, Katelyn T Byrne, Nune Markosyan, Taiji Yamazoe, Fangxue Yan, Zeyu Chen, Yu H. Sun, Jeffrey Lin, Yogev Sela, Robert J. Norgard, Salina Yuan, Allyson J. Merrell, John W. Tobias, Robert H. Vonderheide, Ben Z. Stanger. Investigation of tumor-cell-intrinsic factors regulating immune infiltration and response to immunotherapy in pancreatic cancer [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr A28.

Abstract B38: Calcium signaling induces a partial EMT in pancreatic ductal adenocarcinoma

Metastasis and chemoresistance—the two main reasons for the high mortality of cancer—are associated with a form of cellular plasticity known as epithelial-to-mesenchymal transition (EMT). Cancer cells undergoing EMT become invasive, facilitating metastasis, and undergo a shift in their vulnerability to antineoplastic drugs. In recent work, it has been shown that EMT does not involve a single mechanism but rather a diversity of programs, yielding a continuum of cell phenotypes along the epithelial-mesenchymal spectrum. We previously developed a lineage-traced model of pancreatic ductal adenocarcinoma (PDA) to study EMT in the context of stochastically-arising tumors. As expected, epithelial-mesenchymal plasticity in some tumors involves transcriptional repression of the epithelial state, resulting in a “classical EMT” (C-EMT) phenotype. Surprisingly, however, epithelial-mesenchymal plasticity in the majority of tumors involves post-transcriptional repression of the epithelial state, resulting in a “partial EMT” (P-EMT) phenotype. These two plasticity programs are associated with other aspects of tumor biology as well, including distinct modes of cellular invasion. Here, we identify calcium signaling in pancreatic cancer cells as a regulator of the P-EMT phenotype. Prolonged calcium flux induces PDA cells to remove E-cadherin (ECAD) and other epithelial proteins from the surface and relocalize it intracellularly. This loss of the epithelial phenotype occurs without changes in the abundance of mRNAs for these proteins, reminiscent of the P-EMT phenotype observed in tumors in vivo. In addition, inhibition of the calcium-signaling protein calmodulin blunts this EMT-inducing effect. These results implicate calcium signaling as a mediator of partial EMT phenotypes.

Citation Format: Robert J. Norgard, Ravikanth Maddipati, Nicole M. Aiello, David Balli, Jason R. Pitarresi, Derick N. Rosario-Berrios, Jinyang Li, Salina Yuan, Taiji Yamazoe, Yogev Sela, Allyson J. Merrell, Maximilian D. Wengyn, Kathryn Sun, Anil K. Rustgi, Ben Z. Stanger. Calcium signaling induces a partial EMT in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr B38.

A biomimetic pancreatic cancer on-chip reveals endothelial ablation via ALK7 signaling

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive, lethal malignancy that invades adjacent vasculatures and spreads to distant sites before clinical detection. Although invasion into the peripancreatic vasculature is one of the hallmarks of PDAC, paradoxically, PDAC tumors also exhibit hypovascularity. How PDAC tumors become hypovascular is poorly understood. We describe an organotypic PDAC-on-a-chip culture model that emulates vascular invasion and tumor-blood vessel interactions to better understand PDAC-vascular interactions. The model features a 3D matrix containing juxtaposed PDAC and perfusable endothelial lumens. PDAC cells invaded through intervening matrix, into vessel lumen, and ablated the endothelial cells, leaving behind tumor-filled luminal structures. Endothelial ablation was also observed in in vivo PDAC models. We also identified the activin-ALK7 pathway as a mediator of endothelial ablation by PDAC. This tumor-on-a-chip model provides an important in vitro platform for investigating the process of PDAC-driven endothelial ablation and may provide a mechanism for tumor hypovascularity.

Cellular Plasticity in Cancer

During cancer progression, tumor cells undergo molecular and phenotypic changes collectively referred to as cellular plasticity. Such changes result from microenvironmental cues, stochastic genetic and epigenetic alterations, and/or treatment-imposed selective pressures, thereby contributing to tumor heterogeneity and therapy resistance. Epithelial-mesenchymal plasticity is the best-known case of tumor cell plasticity, but recent work has uncovered other examples, often with functional consequences. In this review, we explore the nature and role(s) of these diverse cellular plasticity programs in premalignant progression, tumor evolution, and adaptation to therapy and consider ways in which targeting plasticity could lead to novel anticancer treatments. SIGNIFICANCE: Changes in cell identity, or cellular plasticity, are common at different stages of tumor progression, and it has become clear that cellular plasticity can be a potent mediator of tumor progression and chemoresistance. Understanding the mechanisms underlying the various forms of cell plasticity may deliver new strategies for targeting the most lethal aspects of cancer: metastasis and resistance to therapy.

Acetyl-CoA Metabolism Supports Multistep Pancreatic Tumorigenesis

Pancreatic ductal adenocarcinoma (PDA) has a poor prognosis, and new strategies for prevention and treatment are urgently needed. We previously reported that histone H4 acetylation is elevated in pancreatic acinar cells harboring Kras mutations prior to the appearance of premalignant lesions. Because acetyl-CoA abundance regulates global histone acetylation, we hypothesized that altered acetyl-CoA metabolism might contribute to metabolic or epigenetic alterations that promote tumorigenesis. We found that acetyl-CoA abundance is elevated in KRAS-mutant acinar cells and that its use in the mevalonate pathway supports acinar-to-ductal metaplasia (ADM). Pancreas-specific loss of the acetyl-CoA-producing enzyme ATP-citrate lyase (ACLY) accordingly suppresses ADM and tumor formation. In PDA cells, growth factors promote AKT-ACLY signaling and histone acetylation, and both cell proliferation and tumor growth can be suppressed by concurrent BET inhibition and statin treatment. Thus, KRAS-driven metabolic alterations promote acinar cell plasticity and tumor development, and targeting acetyl-CoA-dependent processes exerts anticancer effects. SIGNIFICANCE: Pancreatic cancer is among the deadliest of human malignancies. We identify a key role for the metabolic enzyme ACLY, which produces acetyl-CoA, in pancreatic carcinogenesis. The data suggest that acetyl-CoA use for histone acetylation and in the mevalonate pathway facilitates cell plasticity and proliferation, suggesting potential to target these pathways.See related commentary by Halbrook et al., p. 326.This article is highlighted in the In This Issue feature, p. 305.

Tumor Cell-Intrinsic Factors Underlie Heterogeneity of Immune Cell Infiltration and Response to Immunotherapy

The biological and functional heterogeneity between tumors-both across and within cancer types-poses a challenge for immunotherapy. To understand the factors underlying tumor immune heterogeneity and immunotherapy sensitivity, we established a library of congenic tumor cell clones from an autochthonous mouse model of pancreatic adenocarcinoma. These clones generated tumors that recapitulated T cell-inflamed and non-T-cell-inflamed tumor microenvironments upon implantation in immunocompetent mice, with distinct patterns of infiltration by immune cell subsets. Co-injecting tumor cell clones revealed the non-T-cell-inflamed phenotype is dominant and that both quantitative and qualitative features of intratumoral CD8+ T cells determine response to therapy. Transcriptomic and epigenetic analyses revealed tumor-cell-intrinsic production of the chemokine CXCL1 as a determinant of the non-T-cell-inflamed microenvironment, and ablation of CXCL1 promoted T cell infiltration and sensitivity to a combination immunotherapy regimen. Thus, tumor cell-intrinsic factors shape the tumor immune microenvironment and influence the outcome of immunotherapy.

EMT Subtype Influences Epithelial Plasticity and Mode of Cell Migration

Epithelial-mesenchymal transition (EMT) is strongly implicated in tumor cell invasion and metastasis. EMT is thought to be regulated primarily at the transcriptional level through the repressive activity of EMT transcription factors. However, these classical mechanisms have been parsed out almost exclusively in vitro, leaving questions about the programs driving EMT in physiological contexts. Here, using a lineage-labeled mouse model of pancreatic ductal adenocarcinoma to study EMT in vivo, we found that most tumors lose their epithelial phenotype through an alternative program involving protein internalization rather than transcriptional repression, resulting in a "partial EMT" phenotype. Carcinoma cells utilizing this program migrate as clusters, contrasting with the single-cell migration pattern associated with traditionally defined EMT mechanisms. Moreover, many breast and colorectal cancer cell lines utilize this alternative program to undergo EMT. Collectively, these results suggest that carcinoma cells have different ways of losing their epithelial program, resulting in distinct modes of invasion and dissemination.

Metastatic progression is associated with dynamic changes in the local microenvironment

Most cancer-associated deaths result from metastasis. However, it remains unknown whether the size, microenvironment or other features of a metastatic lesion dictate its behaviour or determine the efficacy of chemotherapy in the adjuvant (micrometastatic) setting. Here we delineate the natural history of metastasis in an autochthonous model of pancreatic ductal adenocarcinoma (PDAC), using lineage tracing to examine the evolution of disseminated cancer cells and their associated microenvironment. With increasing size, lesions shift from mesenchymal to epithelial histology, become hypovascular and accumulate a desmoplastic stroma, ultimately recapitulating the primary tumours from which they arose. Moreover, treatment with gemcitabine and nab-paclitaxel significantly reduces the overall number of metastases by inducing cell death in lesions of all sizes, challenging the paradigm that PDAC stroma imposes a critical barrier to drug delivery. These results illuminate the cellular dynamics of metastatic progression and suggest that adjuvant chemotherapy affords a survival benefit by directly targeting micrometastases.

Abstract 4891: Dormancy and growth of metastatic breast cancer cells in a bone-like microenvironment

We have developed a three dimensional cell culture model (bioreactor) in which cancer cells are cultured in a bone-like microenvironment. When co-cultured with MC3T3-E1 osteoblasts and the resulting bone-like matrix, MDA-MB-231 (231) human metastatic breast cancer cells attach, form “single cell files”, develop invadapodia, infiltrate and partially degrade the thick matrix. Eventually, the cells form large colonies. By contrast, the metastasis-suppressed variant, MDA-MB-231BRMS1, (231BRMS1) demonstrates a dormant phenotype in the same system. They loosely attach, and are slow to proliferate. This pattern of growth is also observed in femurs of mice inoculated with these cells. We hypothesized that changes in the cytokine microenvironment or the matrix structure would either cause the 231 cells to become dormant or the 231BRMS1 cells to proliferate. Because of anecdotal evidence that bone trauma or breakage is associated with latent metastasis, we added either a cocktail of inflammatory cytokines (IL-6, IL-8, MCP-1, VEGF, GROα ) or bone remodeling cytokines (IL-6, TNFα, IL-1β, PGE-2 ). The inflammatory cytokines had little effect on the morphology or proliferation of either cell type. In contrast, bone remodeling cytokines stimulated the 231BRMS1 cells to grow vigorously. Of the remodeling cytokines, we determined that TNFα and IL-1β were sufficient to cause the change in growth. However, we were able to block the response of the 231BRMS1 cells with indomethecin, a cyclooxygenase inhibitor or with an inhibitor to the receptor for PGE2. These data suggest that the additional cytokines lead to prostaglandin production.

We also modulated the matrix by growing the osteoblasts with charcoal-stripped serum to reduced estradiol. The osteoblasts differentiated under these conditions (alkaline phosphatase positive) but did not fully mineralize the matrix (minimal von Kossa staining ). In the presence of the modified matrix, both the 231(ER-) and the 231BRMS1(ER-) cells showed increased proliferation. In contrast, MCF-7 cells (ER+) maintained a dormant phenotype and did not grow. In conclusion, our data highlight the importance of a bone-like microenvironment in maintaining breast cancer dormancy. Disrupting the microenvironment could result in dormant cells re-entering the cell cycle. We are currently determining the role of the cytokine microenvironment and the matrix alteration in the modulation of the cancer cells dormancy or growth in the bone.

This work was supported by the U.S. Army Medical Research and Materiel Command under W81XWH-12-1-0127, and by the Metavivor Research Foundation.

Citation Format: Yu-Chi Chen, Andrea M. Mastro, Donna M. Sosnoski, Robert J. Norgard, Cassidy D. Grove, Erwin A. Vogler. Dormancy and growth of metastatic breast cancer cells in a bone-like microenvironment. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4891. doi:10.1158/1538-7445.AM2014-4891