Syngeneic mouse mammary carcinoma cell lines: two closely related cell lines with divergent metastatic behavior

Natural Killer Cell Regulation of Breast Cancer Stem Cells Mediates Metastatic Dormancy

Patients with breast cancer with estrogen receptor-positive tumors face a constant risk of disease recurrence for the remainder of their lives. Dormant tumor cells residing in tissues such as the bone marrow may generate clinically significant metastases many years after initial diagnosis. Previous studies suggest that dormant cancer cells display "stem-like" properties (cancer stem cell, CSC), which may be regulated by the immune system. To elucidate the role of the immune system in controlling dormancy and its escape, we studied dormancy in immunocompetent, syngeneic mouse breast cancer models. Three mouse breast cancer cell lines, PyMT, Met1, and D2.0R, contained CSCs that displayed short- and long-term metastatic dormancy in vivo, which was dependent on the host immune system. Each model was regulated by different components of the immune system. Natural killer (NK) cells were key for the metastatic dormancy phenotype in D2.0R cells. Quiescent D2.0R CSCs were resistant to NK cell cytotoxicity, whereas proliferative CSCs were sensitive. Resistance to NK cell cytotoxicity was mediated, in part, by the expression of BACH1 and SOX2 transcription factors. Expression of STING and STING targets was decreased in quiescent CSCs, and the STING agonist MSA-2 enhanced NK cell killing. Collectively, these findings demonstrate the role of immune regulation of breast tumor dormancy and highlight the importance of utilizing immunocompetent models to study this phenomenon. Significance: The immune system controls disseminated breast cancer cells during disease latency, highlighting the need to utilize immunocompetent models to identify strategies for targeting dormant cancer cells and reducing metastatic recurrence. See related commentary by Cackowski and Korkaya, p. 3319.

Mcam stabilizes a luminal progenitor-like breast cancer cell state via Ck2 control and Src/Akt/Stat3 attenuation

Cell state control is crucial for normal tissue development and cancer cell mimicry of stem/progenitor states, contributing to tumor heterogeneity, therapy resistance, and progression. Here, we demonstrate that the cell surface glycoprotein Mcam maintains the tumorigenic luminal progenitor (LP)-like epithelial cell state, leading to Basal-like mammary cancers. In the Py230 mouse mammary carcinoma model, Mcam knockdown (KD) destabilized the LP state by deregulating the Ck2/Stat3 axis, causing a switch to alveolar and basal states, loss of an estrogen-sensing subpopulation, and resistance to tamoxifen-an effect reversed by Ck2 and Stat3 inhibitors. In vivo, Mcam KD blocked generation of Basal-like tumors and Sox10+Krt14+ cells. In human tumors, MCAM loss was largely exclusive of the Basal-like subtype, linked instead to proliferative Luminal subtypes, including often endocrine-resistant Luminal B cancers. This study has implications for developing therapies targeting MCAM, CK2, and STAT3 and their likely effective contexts.

A novel lipophilic amiloride derivative efficiently kills chemoresistant breast cancer cells

Derivatives of the potassium-sparing diuretic amiloride are preferentially cytotoxic toward tumor cells relative to normal cells, and have the capacity to target tumor cell populations resistant to currently employed therapeutic agents. However, a major barrier to clinical translation of the amilorides is their modest cytotoxic potency, with estimated IC50 values in the high micromolar range. Here we report the synthesis of ten novel amiloride derivatives and the characterization of their cytotoxic potency toward MCF7 (ER/PR-positive), SKBR3 (HER2-positive) and MDA-MB-231 (triple negative) cell line models of breast cancer. Comparisons of derivative structure with cytotoxic potency toward these cell lines underscore the importance of an intact guanidine group, and uncover a strong link between drug-induced cytotoxicity and drug lipophilicity. We demonstrate that our most potent derivative called LLC1 is preferentially cytotoxic toward mouse mammary tumor over normal epithelial organoids, acts in the single digit micromolar range on breast cancer cell line models representing all major subtypes, acts on cell lines that exhibit both transient and sustained resistance to chemotherapeutic agents, but exhibits limited anti-tumor effects in a mouse model of metastatic breast cancer. Nonetheless, our observations offer a roadmap for the future optimization of amiloride-based compounds with preferential cytotoxicity toward breast tumor cells.

An EYA3/NF-κB/CCL2 signaling axis suppresses cytotoxic NK cells in the pre-metastatic niche to promote triple negative breast cancer metastasis

Patients with Triple Negative Breast Cancer (TNBC) exhibit high rates of metastases and poor prognoses. The Eyes absent (EYA) family of proteins are developmental transcriptional cofactors/phosphatases that are re-expressed and/or upregulated in numerous cancers. Herein, we demonstrate that EYA3 correlates with decreased survival in breast cancer, and that it strongly, and specifically, regulates metastasis via a novel mechanism that involves NF-kB signaling and an altered innate immune profile at the pre-metastatic niche (PMN). Remarkably, restoration of NF-kB signaling downstream of Eya3 knockdown (KD) restores metastasis without restoring primary tumor growth, isolating EYA3/NF-kB effects to the metastatic site. We show that secreted CCL2, regulated downstream of EYA3/NF-kB, specifically decreases cytotoxic NK cells in the PMN and that re-expression of Ccl2 in Eya3 -KD cells is sufficient to rescue activation/levels of cytotoxic NK cells in vitro and at the PMN, where EYA3-mediated decreases in cytotoxic NK cells are required for metastatic outgrowth. Importantly, analysis of public breast cancer datasets uncovers a significant correlation of EYA3 with NF-kB/CCL2, underscoring the relevance of EYA3/NF-kB/CCL2 to human disease. Our findings suggest that inhibition of EYA3 could be a powerful means to re-activate the innate immune response at the PMN, inhibiting TNBC metastasis.

Significance

EYA3 promotes metastasis of TNBC cells by promoting NF-kB-mediated CCL2 expression and inhibiting cytotoxic NK cells at the pre-metastatic niche, highlighting a potential therapeutic target in this subset of breast cancer.

The combined treatment with ketogenic diet and metformin slows tumor growth in two mouse models of triple negative breast cancer

Background

Many tumors contain hypoxic microenvironments caused by inefficient tumor vascularization. Hypoxic tumors have been shown to resist conventional cancer therapies. Hypoxic cancer cells rely on glucose to meet their energetic and anabolic needs to fuel uncontrolled proliferation and metastasis. This glucose dependency is linked to a metabolic shift in response to hypoxic conditions.

Methods

To leverage the glucose dependency of hypoxic tumor cells, we assessed the effects of a mild reduction in systemic glucose by controlling both dietary carbohydrates with a ketogenic diet and endogenous glucose production by using metformin on two mouse models of triple-negative breast cancer (TNBC).

Results

Here, we showed that animals with TNBC treated with the combination regimen of ketogenic diet and metformin (a) had their tumor burden lowered by two-thirds, (b) displayed 38% slower tumor growth, and (c) showed 36% longer latency, compared to the animals treated with a ketogenic diet or metformin alone. As a result, lowering systemic glucose by this combined dietary and pharmacologic approach improved overall survival in our mouse TNBC models by 31 days, approximately equivalent to 3 years of life extension in human terms.

Conclusion

This preclinical study demonstrates that reducing systemic glucose by combining a ketogenic diet and metformin significantly inhibits tumor proliferation and increases overall survival. Our findings suggest a possible treatment for a broad range of hypoxic and glycolytic tumor types that can augment existing treatment options to improve patient outcomes.

Apoptotic cells promote circulating tumor cell survival and metastasis

During tumor progression and especially following cytotoxic therapy, cell death of both tumor and stromal cells is widespread. Despite clinical observations that high levels of apoptotic cells correlate with poorer patient outcomes, the physiological effects of dying cells on tumor progression remain incompletely understood. Here, we report that circulating apoptotic cells robustly enhance tumor cell metastasis to the lungs. Using intravenous metastasis models, we observed that the presence of apoptotic cells, but not cells dying by other mechanisms, supports circulating tumor cell (CTC) survival following arrest in the lung vasculature. Apoptotic cells promote CTC survival by recruiting platelets to the forming metastatic niche. Apoptotic cells externalize the phospholipid phosphatidylserine to the outer leaflet of the plasma membrane, which we found increased the activity of the coagulation initiator Tissue Factor, thereby triggering the formation of platelet clots that protect proximal CTCs. Inhibiting the ability of apoptotic cells to induce coagulation by knocking out Tissue Factor, blocking phosphatidylserine, or administering the anticoagulant heparin abrogated the pro-metastatic effect of apoptotic cells. This work demonstrates a previously unappreciated role for apoptotic cells in facilitating metastasis by establishing CTC-supportive emboli, and suggests points of intervention that may reduce the pro-metastatic effect of apoptotic cells.

GRAPHICAL ABSTRACT

Mcam stabilizes luminal progenitor breast cancer phenotypes via Ck2 control and Src/Akt/Stat3 attenuation

Breast cancers are categorized into subtypes with distinctive therapeutic vulnerabilities and prognoses based on their expression of clinically targetable receptors and gene expression patterns mimicking different cell types of the normal gland. Here, we tested the role of Mcam in breast cancer cell state control and tumorigenicity in a luminal progenitor-like murine tumor cell line (Py230) that exhibits lineage and tumor subtype plasticity. Mcam knockdown Py230 cells show augmented Stat3 and Pi3K/Akt activation associated with a lineage state switch away from a hormone-sensing/luminal progenitor state toward alveolar and basal cell related phenotypes that were refractory to growth inhibition by the anti-estrogen therapeutic, tamoxifen. Inhibition of Stat3, or the upstream activator Ck2, reversed these cell state changes. Mcam binds Ck2 and acts as a regulator of Ck2 substrate utilization across multiple mammary tumor cell lines. In Py230 cells this activity manifests as increased mesenchymal morphology, migration, and Src/Fak/Mapk/Paxillin adhesion complex signaling in vitro, in contrast to Mcam's reported roles in promoting mesenchymal phenotypes. In vivo, Mcam knockdown reduced tumor growth and take rate and inhibited cell state transition to Sox10+/neural crest like cells previously been associated with tumor aggressiveness. This contrasts with human luminal breast cancers where MCAM copy number loss is highly coupled to Cyclin D amplification, increased proliferation, and the more aggressive Luminal B subtype. Together these data indicate a critical role for Mcam and its regulation of Ck2 in control of breast cancer cell state plasticity with implications for progression, evasion of targeted therapies and combination therapy design.

Nonlinear Frequency Mixing Ultrasound Imaging of Nanoscale Contrast Agents

OBJECTIVE

Nanoscale ultrasound contrast agents show promise as alternatives for diagnostics and therapies due to their enhanced stability and ability to traverse blood vessels. Nonetheless, their reduced size limits echogenicity. This study introduces an enhanced nanobubble frequency mixing ultrasound imaging method, by capitalizing on their nonlinear acoustic response to dual-frequency excitation.

METHODS

A single broadband transducer (L12-3v) controlled by a programmable ultrasound system was used to transmit a dual-frequency single-cycle wavefront. The frequency mixing effect enabled simultaneous transducer capture of nanobubble-generated sum and difference frequencies in real time without the need for additional hardware or post-processing, by substituting the single-frequency wavefront in a standard contrast harmonic pulse inversion imaging protocol, with the dual-frequency wavefront.

RESULTS

Optimization experiments were conducted in tissue mimicking phantoms. Among the dual-frequency combinations that were tested, the highest contrast was obtained using 4&8 MHz. The nanobubble contrast improved with increased mechanical index, and achieved a maximal contrast improvement of 8.4 ± 0.5 dB compared to 4 MHz pulse inversion imaging. In imaging of a breast cancer tumor mouse model, after a systemic nanobubble injection, the contrast was improved by 3.4 ± 1.7, 4.8 ± 1.8, and 6.3 ± 1.6 dB for mechanical indices of 0.04, 0.08, and 0.1, respectively.

CONCLUSION

Nonlinear frequency mixing significantly improved the nanobubble contrast, which facilitated their imaging in-vivo.

SIGNIFICANCE

This study offers a new avenue to enhance ultrasound imaging utilizing nanobubbles, potentially leading to advancements in other diagnostic applications.

The Combined Treatment with Ketogenic Diet and Metformin Slows Tumor Growth in Two Mouse Models of Triple Negative Breast Cancer

BACKGROUND

Many tumors contain hypoxic microenvironments caused by inefficient tumor vascularization. Hypoxic tumors have been shown to resist conventional cancer therapies. Hypoxic cancer cells rely on glucose to meet their energetic and anabolic needs to fuel uncontrolled proliferation and metastasis. This glucose dependency is linked to a metabolic shift in response to hypoxic conditions.

METHODS

To leverage the glucose dependency of hypoxic tumor cells, we assessed the effects of a controlled reduction in systemic glucose by combining dietary carbohydrate restriction, using a ketogenic diet, with gluconeogenesis inhibition, using metformin, on two mouse models of triple-negative breast cancer (TNBC).

RESULTS

We confirmed that MET - 1 breast cancer cells require abnormally high glucose concentrations to survive in a hypoxic environment in vitro. Then, we showed that, compared to a ketogenic diet or metformin alone, animals treated with the combination regimen showed significantly lower tumor burden, higher tumor latency and slower tumor growth. As a result, lowering systemic glucose by this combined dietary and pharmacologic approach improved overall survival in our mouse model by 31 days, which is approximately equivalent to 3 human years.

CONCLUSION

This is the first preclinical study to demonstrate that reducing systemic glucose by combining a ketogenic diet and metformin significantly inhibits tumor proliferation and increases overall survival. Our findings suggest a possible treatment for a broad range of hypoxic and glycolytic tumor types, one that can also augment existing treatment options to improve patient outcomes.

Dose-Specific Intratumoral GM-CSF Modulates Breast Tumor Oxygenation and Antitumor Immunity

GM-CSF has been employed as an adjuvant to cancer immunotherapy with mixed results based on dosage. We previously showed that GM-CSF regulated tumor angiogenesis by stimulating soluble vascular endothelial growth factor (VEGF) receptor-1 from monocytes/macrophages in a dose-dependent manner that neutralized free VEGF, and intratumoral injections of high-dose GM-CSF ablated blood vessels and worsened hypoxia in orthotopic polyoma middle T Ag (PyMT) triple-negative breast cancer (TNBC). In this study, we assessed both immunoregulatory and oxygen-regulatory components of low-dose versus high-dose GM-CSF to compare effects on tumor oxygen, vasculature, and antitumor immunity. We performed intratumoral injections of low-dose GM-CSF or saline controls for 3 wk in FVB/N PyMT TNBC. Low-dose GM-CSF uniquely reduced tumor hypoxia and normalized tumor vasculature by increasing NG2+ pericyte coverage on CD31+ endothelial cells. Priming of "cold," anti-PD1-resistant PyMT tumors with low-dose GM-CSF (hypoxia reduced) sensitized tumors to anti-PD1, whereas high-dose GM-CSF (hypoxia exacerbated) did not. Low-dose GM-CSF reduced hypoxic and inflammatory tumor-associated macrophage (TAM) transcriptional profiles; however, no phenotypic modulation of TAMs or tumor-infiltrating lymphocytes were observed by flow cytometry. In contrast, high-dose GM-CSF priming increased infiltration of TAMs lacking the MHC class IIhi phenotype or immunostimulatory marker expression, indicating an immunosuppressive phenotype under hypoxia. However, in anti-PD1 (programmed cell death 1)-susceptible BALB/c 4T1 tumors (considered hot versus PyMT), high-dose GM-CSF increased MHC class IIhi TAMs and immunostimulatory molecules, suggesting disparate effects of high-dose GM-CSF across PyMT versus 4T1 TNBC models. Our data demonstrate a (to our knowledge) novel role for low-dose GM-CSF in reducing tumor hypoxia for synergy with anti-PD1 and highlight why dosage and setting of GM-CSF in cancer immunotherapy regimens require careful consideration.

Breast Cancer-Secreted Factors Promote Lung Metastasis by Signaling Systemically to Induce a Fibrotic Premetastatic Niche

Metastatic cancer is largely incurable and is the main cause of cancer-related deaths. The metastatic microenvironment facilitates formation of metastases. Cancer-associated fibroblasts (CAF) are crucial players in generating a hospitable metastatic niche by mediating an inflammatory microenvironment. Fibroblasts also play a central role in modifying the architecture and stiffness of the extracellular matrix (ECM). Resolving the early changes in the metastatic niche could help identify approaches to inhibit metastatic progression. Here, we demonstrate in mouse models of spontaneous breast cancer pulmonary metastasis that fibrotic changes and rewiring of lung fibroblasts occurred at premetastatic stages, suggesting systemic influence by the primary tumor. Activin A (ActA), a TGFβ superfamily member, was secreted from breast tumors and its levels in the blood were highly elevated in tumor-bearing mice. ActA upregulated the expression of profibrotic factors in lung fibroblasts, leading to enhanced collagen deposition in the lung premetastatic niche. ActA signaling was functionally important for lung metastasis, as genetic targeting of ActA in breast cancer cells significantly attenuated lung metastasis and improved survival. Moreover, high levels of ActA in human patients with breast cancer were associated with lung metastatic relapse and poor survival. This study uncovers a novel mechanism by which breast cancer cells systemically rewire the stromal microenvironment in the metastatic niche to facilitate pulmonary metastasis.

SIGNIFICANCE

ActA mediates cross-talk between breast cancer cells and cancer-associated fibroblasts in the lung metastatic niche that enhances fibrosis and metastasis, implicating ActA as a potential therapeutic target to inhibit metastatic relapse.

Pubertal exposure to dietary advanced glycation end products disrupts ductal morphogenesis and induces atypical hyperplasia in the mammary gland

BACKGROUND

Advanced glycation end products (AGEs) are reactive metabolites intrinsically linked with modern dietary patterns. Processed foods, and those high in sugar, protein and fat, often contain high levels of AGEs. Increased AGE levels are associated with increased breast cancer risk, however their significance has been largely overlooked due to a lack of direct cause-and-effect relationship.

METHODS

To address this knowledge gap, FVB/n mice were fed regular, low AGE, and high AGE diets from 3 weeks of age and mammary glands harvested during puberty (7 weeks) or adulthood (12 weeks and 7 months) to determine the effects upon mammary gland development. At endpoint mammary glands were harvested and assessed histologically (n ≥ 4). Immunohistochemistry and immunofluorescence were used to assess cellular proliferation and stromal fibroblast and macrophage recruitment. The Kruskal-Wallis test were used to compare continuous outcomes among groups. Mammary epithelial cell migration and invasion in response to AGE-mediated fibroblast activation was determined in two-compartment co-culture models. In vitro experiments were performed in triplicate. The nonparametric Wilcoxon rank sum test was used to compare differences between groups.

RESULTS

Histological analysis revealed the high AGE diet delayed ductal elongation, increased primary branching, as well as increased terminal end bud number and size. The high AGE diet also led to increased recruitment and proliferation of stromal cells to abnormal structures that persisted into adulthood. Atypical hyperplasia was observed in the high AGE fed mice. Ex vivo fibroblasts from mice fed dietary-AGEs retain an activated phenotype and promoted epithelial migration and invasion of non-transformed immortalized and tumor-derived mammary epithelial cells. Mechanistically, we found that the receptor for AGE (RAGE) is required for AGE-mediated increases in epithelial cell migration and invasion.

CONCLUSIONS

We observed a disruption in mammary gland development when mice were fed a diet high in AGEs. Further, both epithelial and stromal cell populations were impacted by the high AGE diet in the mammary gland. Educational, interventional, and pharmacological strategies to reduce AGEs associated with diet may be viewed as novel disease preventive and/or therapeutic initiatives during puberty.

Natural killer cell regulation of breast cancer stem cells mediates metastatic dormancy

Breast cancer patients with estrogen receptor positive tumors face a constant risk of disease recurrence for the remainder of their lives. Dormant tumor cells residing in tissues such as the bone marrow may generate clinically significant metastases many years after initial diagnosis. Previous studies suggest that dormant cells display "stem like" properties (CSCs), which may be regulated by the immune system. Although many studies have examined tumor cell intrinsic characteristics of dormancy, the role of the immune system in controlling dormancy and its escape is not well understood. This scientific gap is due, in part, to a lack of immunocompetent mouse models of breast cancer dormancy with many studies involving human xenografts in immunodeficient mice. To overcome this limitation, we studied dormancy in immunocompetent, syngeneic mouse breast cancer models. We find that PyMT, Met-1 and D2.0R cell lines contain CSCs that display both short- and long-term metastatic dormancy in vivo, which is dependent on the host immune system. Natural killer cells were key for the metastatic dormancy phenotype observed for D2.0R and the role of NK cells in regulating CSCs was further investigated.Quiescent D2.0R CSC are resistant to NK cytotoxicity, while proliferative D2.0R CSC were sensitive to NK cytotoxicity both in vitro and in vivo. This resistance was mediated, in part, by the expression of Bach1 and Sox2 transcription factors. NK killing was enhanced by the STING agonist MSA-2. Collectively, our findings demonstrate the important role of immune regulation of breast tumor dormancy and highlight the importance of utilizing immunocompetent models to study this phenomenon.

The chemokine monocyte chemoattractant protein-1/CCL2 is a promoter of breast cancer metastasis

Breast cancer is the most prevalent cancer worldwide, and metastasis is the leading cause of death in cancer patients. Human monocyte chemoattractant protein-1 (MCP-1/CCL2) was isolated from the culture supernatants of not only mitogen-activated peripheral blood mononuclear leukocytes but also malignant glioma cells based on its in vitro chemotactic activity toward human monocytes. MCP-1 was subsequently found to be identical to a previously described tumor cell-derived chemotactic factor thought to be responsible for the accumulation of tumor-associated macrophages (TAMs), and it became a candidate target of clinical intervention; however, the role of TAMs in cancer development was still controversial at the time of the discovery of MCP-1. The in vivo role of MCP-1 in cancer progression was first evaluated by examining human cancer tissues, including breast cancers. Positive correlations between the level of MCP-1 production in tumors and the degree of TAM infiltration and cancer progression were established. The contribution of MCP-1 to the growth of primary tumors and metastasis to the lung, bone, and brain was examined in mouse breast cancer models. The results of these studies strongly suggested that MCP-1 is a promoter of breast cancer metastasis to the lung and brain but not bone. Potential mechanisms of MCP-1 production in the breast cancer microenvironment have also been reported. In the present manuscript, we review studies in which the role of MCP-1 in breast cancer development and progression and the mechanisms of its production were examined and attempt to draw a consensus and discuss the potential use of MCP-1 as a biomarker for diagnosis.

Vangl-dependent Wnt/planar cell polarity signaling mediates collective breast carcinoma motility and distant metastasis

BACKGROUND

In light of the growing appreciation for the role of collective cell motility in metastasis, a deeper understanding of the underlying signaling pathways will be critical to translating these observations to the treatment of advanced cancers. Here, we examine the contribution of Wnt/planar cell polarity (Wnt/PCP), one of the non-canonical Wnt signaling pathways and defined by the involvement of the tetraspanin-like proteins Vangl1 and Vangl2, to breast tumor cell motility, collective cell invasiveness and mammary tumor metastasis.

METHODS

Vangl1 and Vangl2 knockdown and overexpression and Wnt5a stimulation were employed to manipulate Wnt/PCP signaling in a battery of breast cancer cell lines representing all breast cancer subtypes, and in tumor organoids from MMTV-PyMT mice. Cell migration was assessed by scratch and organoid invasion assays, Vangl protein subcellular localization was assessed by confocal fluorescence microscopy, and RhoA activation was assessed in real time by fluorescence imaging with an advanced FRET biosensor. The impact of Wnt/PCP suppression on mammary tumor growth and metastasis was assessed by determining the effect of conditional Vangl2 knockout on the MMTV-NDL mouse mammary tumor model.

RESULTS

We observed that Vangl2 knockdown suppresses the motility of all breast cancer cell lines examined, and overexpression drives the invasiveness of collectively migrating MMTV-PyMT organoids. Vangl2-dependent RhoA activity is localized in real time to a subpopulation of motile leader cells displaying a hyper-protrusive leading edge, Vangl protein is localized to leader cell protrusions within leader cells, and actin cytoskeletal regulator RhoA is preferentially activated in the leader cells of a migrating collective. Mammary gland-specific knockout of Vangl2 results in a striking decrease in lung metastases in MMTV-NDL mice, but does not impact primary tumor growth characteristics.

CONCLUSIONS

We conclude that Vangl-dependent Wnt/PCP signaling promotes breast cancer collective cell migration independent of breast tumor subtype and facilitates distant metastasis in a genetically engineered mouse model of breast cancer. Our observations are consistent with a model whereby Vangl proteins localized at the leading edge of leader cells in a migrating collective act through RhoA to mediate the cytoskeletal rearrangements required for pro-migratory protrusion formation.

Pleiotrophin drives a prometastatic immune niche in breast cancer

Metastatic cancer cells adapt to thrive in secondary organs. To investigate metastatic adaptation, we performed transcriptomic analysis of metastatic and non-metastatic murine breast cancer cells. We found that pleiotrophin (PTN), a neurotrophic cytokine, is a metastasis-associated factor that is expressed highly by aggressive breast cancers. Moreover, elevated PTN in plasma correlated significantly with metastasis and reduced survival of breast cancer patients. Mechanistically, we find that PTN activates NF-κB in cancer cells leading to altered cytokine production, subsequent neutrophil recruitment, and an immune suppressive microenvironment. Consequently, inhibition of PTN, pharmacologically or genetically, reduces the accumulation of tumor-associated neutrophils and reverts local immune suppression, resulting in increased T cell activation and attenuated metastasis. Furthermore, inhibition of PTN significantly enhanced the efficacy of immune checkpoint blockade and chemotherapy in reducing metastatic burden in mice. These findings establish PTN as a previously unrecognized driver of a prometastatic immune niche and thus represents a promising therapeutic target for the treatment of metastatic breast cancer.

Kindlin-1 regulates IL-6 secretion and modulates the immune environment in breast cancer models

The adhesion protein Kindlin-1 is over-expressed in breast cancer where it is associated with metastasis-free survival; however, the mechanisms involved are poorly understood. Here, we report that Kindlin-1 promotes anti-tumor immune evasion in mouse models of breast cancer. Deletion of Kindlin-1 in Met-1 mammary tumor cells led to tumor regression following injection into immunocompetent hosts. This was associated with a reduction in tumor infiltrating Tregs. Similar changes in T cell populations were seen following depletion of Kindlin-1 in the polyomavirus middle T antigen (PyV MT)-driven mouse model of spontaneous mammary tumorigenesis. There was a significant increase in IL-6 secretion from Met-1 cells when Kindlin-1 was depleted and conditioned media from Kindlin-1-depleted cells led to a decrease in the ability of Tregs to suppress the proliferation of CD8+ T cells, which was dependent on IL-6. In addition, deletion of tumor-derived IL-6 in the Kindlin-1-depleted tumors reversed the reduction of tumor-infiltrating Tregs. Overall, these data identify a novel function for Kindlin-1 in regulation of anti-tumor immunity, and that Kindlin-1 dependent cytokine secretion can impact the tumor immune environment.

Current methods for studying metastatic potential of tumor cells

Cell migration and invasiveness significantly contribute to desirable physiological processes, such as wound healing or embryogenesis, as well as to serious pathological processes such as the spread of cancer cells to form tumor metastasis. The availability of appropriate methods for studying these processes is essential for understanding the molecular basis of cancer metastasis and for identifying suitable therapeutic targets for anti-metastatic treatment. This review summarizes the current status of these methods: In vitro methods for studying cell migration involve two-dimensional (2D) assays (wound-healing/scratch assay), and methods based on chemotaxis (the Dunn chamber). The analysis of both cell migration and invasiveness in vitro require more complex systems based on the Boyden chamber principle (Transwell migration/invasive test, xCELLigence system), or microfluidic devices with three-dimensional (3D) microscopy visualization. 3D culture techniques are rapidly becoming routine and involve multicellular spheroid invasion assays or array chip-based, spherical approaches, multi-layer/multi-zone culture, or organoid non-spherical models, including multi-organ microfluidic chips. The in vivo methods are mostly based on mice, allowing genetically engineered mice models and transplant models (syngeneic mice, cell line-derived xenografts and patient-derived xenografts including humanized mice models). These methods currently represent a solid basis for the state-of-the art research that is focused on understanding metastatic fundamentals as well as the development of targeted anti-metastatic therapies, and stratified treatment in oncology.

Diabetic hyperglycemia promotes primary tumor progression through glycation-induced tumor extracellular matrix stiffening

Diabetes mellitus is a complex metabolic disorder that is associated with an increased risk of breast cancer. Despite this correlation, the interplay between tumor progression and diabetes, particularly with regard to stiffening of the extracellular matrix, is still mechanistically unclear. Here, we established a murine model where hyperglycemia was induced before breast tumor development. Using the murine model, in vitro systems, and patient samples, we show that hyperglycemia increases tumor growth, extracellular matrix stiffness, glycation, and epithelial-mesenchymal transition of tumor cells. Upon inhibition of glycation or mechanotransduction in diabetic mice, these same metrics are reduced to levels comparable with nondiabetic tumors. Together, our study describes a novel biomechanical mechanism by which diabetic hyperglycemia promotes breast tumor progression via glycating the extracellular matrix. In addition, our work provides evidence that glycation inhibition is a potential adjuvant therapy for diabetic cancer patients due to the key role of matrix stiffening in both diseases.

Cationic Polyethyleneimine (PEI)–Gold Nanocomposites Modulate Macrophage Activation and Reprogram Mouse Breast Triple-Negative MET-1 Tumor Immunological Microenvironment

Nanomedicines based on inorganic nanoparticles have grown in the last decades due to the nanosystems’ versatility in the coating, tuneability, and physical and chemical properties. Nonetheless, concerns have been raised regarding the immunotropic profile of nanoparticles and how metallic nanoparticles affect the immune system. Cationic polymer nanoparticles are widely used for cell transfection and proved to exert an adjuvant immunomodulatory effect that improves the efficiency of conventional vaccines against infection or cancer. Likewise, gold nanoparticles (AuNPs) also exhibit diverse effects on immune response depending on size or coatings. Photothermal or photodynamic therapy, radiosensitization, and drug or gene delivery systems take advantage of the unique properties of AuNPs to deeply modify the tumoral ecosystem. However, the collective effects that AuNPs combined with cationic polymers might exert on their own in the tumor immunological microenvironment remain elusive. The purpose of this study was to analyze the triple-negative breast tumor immunological microenvironment upon intratumoral injection of polyethyleneimine (PEI)–AuNP nanocomposites (named AuPEI) and elucidate how it might affect future immunotherapeutic approaches based on this nanosystem. AuPEI nanocomposites were synthesized through a one-pot synthesis method with PEI as both a reducing and capping agent, resulting in fractal assemblies of about 10 nm AuNPs. AuPEI induced an inflammatory profile in vitro in the mouse macrophage-like cells RAW264.7 as determined by the secretion of TNF-α and CCL5 while the immunosuppressor IL-10 was not increased. However, in vivo in the mouse breast MET-1 tumor model, AuPEI nanocomposites shifted the immunological tumor microenvironment toward an M2 phenotype with an immunosuppressive profile as determined by the infiltration of PD-1-positive lymphocytes. This dichotomy in AuPEI nanocomposites in vitro and in vivo might be attributed to the highly complex tumor microenvironment and highlights the importance of testing the immunogenicity of nanomaterials in vitro and more importantly in vivo in relevant immunocompetent mouse tumor models to better elucidate any adverse or unexpected effect.

Pro-inflammatory megakaryocyte gene expression in murine models of breast cancer

Despite abundant research demonstrating that platelets can promote tumor cell metastasis, whether primary tumors affect platelet-producing megakaryocytes remains understudied. In this study, we used a spontaneous murine model of breast cancer to show that tumor burden reduced megakaryocyte number and size and disrupted polyploidization. Single-cell RNA sequencing demonstrated that megakaryocytes from tumor-bearing mice exhibit a pro-inflammatory phenotype, epitomized by increased Ctsg, Lcn2, S100a8, and S100a9 transcripts. Protein S100A8/A9 and lipocalin-2 levels were also increased in platelets, suggesting that tumor-induced alterations to megakaryocytes are passed on to their platelet progeny, which promoted in vitro tumor cell invasion and tumor cell lung colonization to a greater extent than platelets from wild-type animals. Our study is the first to demonstrate breast cancer-induced alterations in megakaryocytes, leading to qualitative changes in platelet content that may feedback to promote tumor metastasis.

Low frequency nanobubble-enhanced ultrasound mechanotherapy for noninvasive cancer surgery

Scaling down the size of microbubble contrast agents to the nanometer level holds the promise for noninvasive cancer therapy. However, the small size of nanobubbles limits the obtained bioeffects as a result of ultrasound cavitation, when operating near the nanobubble resonance frequency. Here we show that coupled with low energy insonation at a frequency of 80 kHz, well below the resonance frequency of these agents, nanobubbles serve as noninvasive therapeutic warheads that trigger potent mechanical effects in tumors following a systemic injection. We demonstrate these capabilities in tissue mimicking phantoms, where a comparison of the acoustic response of micro- and nano-bubbles after insonation at a frequency of 250 or 80 kHz revealed that higher pressures were needed to implode the nanobubbles compared to microbubbles. Complete nanobubble destruction was achieved at a mechanical index of 2.6 for the 250 kHz insonation vs. 1.2 for the 80 kHz frequency. Thus, the 80 kHz insonation complies with safety regulations that recommend operation below a mechanical index of 1.9. In vitro in breast cancer tumor cells, the cell viability was reduced to 17.3 ± 1.7% of live cells. In vivo, in a breast cancer tumor mouse model, nanobubble tumor distribution and accumulation were evaluated by high frequency ultrasound imaging. Finally, nanobubble-mediated low frequency insonation of breast cancer tumors resulted in effective mechanical tumor ablation and tumor tissue fractionation. This approach provides a unique theranostic platform for safe, noninvasive and low energy tumor mechanotherapy.

The Sympathetic Nervous System Contributes to the Establishment of Pre-Metastatic Pulmonary Microenvironments

Emerging evidence suggests that neural activity contributes to tumor initiation and its acquisition of metastatic properties. More specifically, it has been reported that the sympathetic nervous system regulates tumor angiogenesis, tumor growth, and metastasis. The function of the sympathetic nervous system in primary tumors has been gradually elucidated. However, its functions in pre-metastatic environments and/or the preparation of metastatic environments far from the primary sites are still unknown. To investigate the role of the sympathetic nervous system in pre-metastatic environments, we performed chemical sympathectomy using 6-OHDA in mice and observed a decrease in lung metastasis by attenuating the recruitment of myeloid-derived suppressor cells. Furthermore, we note that neuro-immune cell interactions could be observed in tumor-bearing mouse lungs in conjunction with the decreased expression of Sema3A. These data indicate that the sympathetic nervous system contributes to the preparation of pre-metastatic microenvironments in the lungs, which are mediated by neuro-immune cell interactions.

Interleukin 4 Controls the Pro-Tumoral Role of Macrophages in Mammary Cancer Pulmonary Metastasis in Mice

Metastasis is the systemic manifestation of cancer and the main cause of death from breast cancer. In mouse models of lung metastases, recruitment of classical monocytes from blood to the lung and their differentiation to metastasis-associated macrophages (MAMs) facilitate cancer cell extravasation, survival and growth. Ablation of MAMs or their monocytic progenitors inhibits metastasis. We hypothesized that factors controlling macrophage polarization modulate tumor cell extravasation in the lung. We evaluated whether signaling by Th1 or Th2 cytokines in macrophages affected transendothelial migration of tumor cells in vitro. Interferon gamma and LPS inhibited macrophage-dependent tumor cell extravasation while the Th2 cytokine interleukin-4 (IL4) enhanced this process. We demonstrated that IL4 receptor (IL4rα)-null mice developed fewer and smaller lung metastasis in E0771-LG mammary cancer models of this disease. Adoptive transfer of wild-type monocytes to IL4rα-deficient mice partially rescued this phenotype. IL4 signaling in macrophages controlled the expression of the chemokine receptor CXCR2, necessary for IL4-mediated tumor cell extravasation in vitro. Furthermore, IL4 signaling in macrophages regulated the transcript abundance of several other genes already causally associated with mammary cancer lung metastasis including Ccl2, Csf1, Ccr1, Hgf and Flt1. The central role of IL4 signaling in MAMs was confirmed by high-resolution intravital imaging of the lung in mice at the time of metastatic seeding, which showed reduced physical interaction between tumor cells and IL4rα-deficient macrophages. This interaction with wild-type MAMs enhanced tumor cell survival and seeding, which was lost in the IL4rα mice. These data indicate that IL4 signaling in monocytes and macrophages is key during seeding and growth of breast metastasis in the lung, as it regulates pro-tumoral paracrine signaling between cancer cells and macrophages.

Low lamin A levels enhance confined cell migration and metastatic capacity in breast cancer

Aberrations in nuclear size and shape are commonly used to identify cancerous tissue. However, it remains unclear whether the disturbed nuclear structure directly contributes to the cancer pathology or is merely a consequence of other events occurring during tumorigenesis. Here, we show that highly invasive and proliferative breast cancer cells frequently exhibit Akt-driven lower expression of the nuclear envelope proteins lamin A/C, leading to increased nuclear deformability that permits enhanced cell migration through confined environments that mimic interstitial spaces encountered during metastasis. Importantly, increasing lamin A/C expression in highly invasive breast cancer cells reflected gene expression changes characteristic of human breast tumors with higher LMNA expression, and specifically affected pathways related to cell-ECM interactions, cell metabolism, and PI3K/Akt signaling. Further supporting an important role of lamins in breast cancer metastasis, analysis of lamin levels in human breast tumors revealed a significant association between lower lamin A levels, Akt signaling, and decreased disease-free survival. These findings suggest that downregulation of lamin A/C in breast cancer cells may influence both cellular physical properties and biochemical signaling to promote metastatic progression.

Clonal barcoding with qPCR detection enables live cell functional analyses for cancer research

Single-cell analysis methods are valuable tools; however, current approaches do not easily enable live cell retrieval. That is a particular issue when further study of cells that were eliminated during experimentation could provide critical information. We report a clonal molecular barcoding method, called SunCatcher, that enables longitudinal tracking and live cell functional analysis. From complex cell populations, we generate single cell-derived clonal populations, infect each with a unique molecular barcode, and retain stocks of individual barcoded clones (BCs). We develop quantitative PCR-based and next-generation sequencing methods that we employ to identify and quantify BCs in vitro and in vivo. We apply SunCatcher to various breast cancer cell lines and combine respective BCs to create versions of the original cell lines. While the heterogeneous BC pools reproduce their original parental cell line proliferation and tumor progression rates, individual BCs are phenotypically and functionally diverse. Early spontaneous metastases can also be identified and quantified. SunCatcher thus provides a rapid and sensitive approach for studying live single-cell clones and clonal evolution, and performing functional analyses.

The Cationic Amphiphilic Drug Hexamethylene Amiloride Eradicates Bulk Breast Cancer Cells and Therapy-Resistant Subpopulations with Similar Efficiencies

The resistance of cancer cell subpopulations, including cancer stem cell (CSC) populations, to apoptosis-inducing chemotherapeutic agents is a key barrier to improved outcomes for cancer patients. The cationic amphiphilic drug hexamethylene amiloride (HMA) has been previously demonstrated to efficiently kill bulk breast cancer cells independent of tumor subtype or species but acts poorly toward non-transformed cells derived from multiple tissues. Here, we demonstrate that HMA is similarly cytotoxic toward breast CSC-related subpopulations that are resistant to conventional chemotherapeutic agents, but poorly cytotoxic toward normal mammary stem cells. HMA inhibits the sphere-forming capacity of FACS-sorted human and mouse mammary CSC-related cells in vitro, specifically kills tumor but not normal mammary organoids ex vivo, and inhibits metastatic outgrowth in vivo, consistent with CSC suppression. Moreover, HMA inhibits viability and sphere formation by lung, colon, pancreatic, brain, liver, prostate, and bladder tumor cell lines, suggesting that its effects may be applicable to multiple malignancies. Our observations expose a key vulnerability intrinsic to cancer stem cells and point to novel strategies for the exploitation of cationic amphiphilic drugs in cancer treatment.

gp130 Cytokines Activate Novel Signaling Pathways and Alter Bone Dissemination in ER+ Breast Cancer Cells

Breast cancer cells frequently home to the bone marrow, where they encounter signals that promote survival and quiescence or stimulate their proliferation. The interleukin-6 (IL-6) cytokines signal through the co-receptor glycoprotein130 (gp130) and are abundantly secreted within the bone microenvironment. Breast cancer cell expression of leukemia inhibitory factor (LIF) receptor (LIFR)/STAT3 signaling promotes tumor dormancy in the bone, but it is unclear which, if any of the cytokines that signal through LIFR, including LIF, oncostatin M (OSM), and ciliary neurotrophic factor (CNTF), promote tumor dormancy and which signaling pathways are induced. We first confirmed that LIF, OSM, and CNTF and their receptor components were expressed across a panel of breast cancer cell lines, although expression was lower in estrogen receptor-negative (ER^- ) bone metastatic clones compared with parental cell lines. In estrogen receptor-positive (ER⁺ ) cells, OSM robustly stimulated phosphorylation of known gp130 signaling targets STAT3, ERK, and AKT, while CNTF activated STAT3 signaling. In ER^- breast cancer cells, OSM alone stimulated AKT and ERK signaling. Overexpression of OSM, but not CNTF, reduced dormancy gene expression and increased ER⁺ breast cancer bone dissemination. Reverse-phase protein array revealed distinct and overlapping pathways stimulated by OSM, LIF, and CNTF with known roles in breast cancer progression and metastasis. In breast cancer patients, downregulation of the cytokines or receptors was associated with reduced relapse-free survival, but OSM was significantly elevated in patients with invasive disease and distant metastasis. Together these data indicate that the gp130 cytokines induce multiple signaling cascades in breast cancer cells, with a potential pro-tumorigenic role for OSM and pro-dormancy role for CNTF. © 2021 American Society for Bone and Mineral Research (ASBMR).

In situ T-cell transfection by anti-CD3-conjugated lipid nanoparticles leads to T-cell activation, migration, and phenotypic shift

Ex vivo programming of T cells can be efficacious but is complex and expensive; therefore, the development of methods to transfect T cells in situ is important. We developed and optimized anti-CD3-targeted lipid nanoparticles (aCD3-LNPs) to deliver tightly packed, reporter gene mRNA specifically to T cells. In vitro, targeted LNPs efficiently delivered mCherry mRNA to Jurkat T cells, and T-cell activation and depletion were associated with aCD3 antibody coating on the surface of LNPs. aCD3-LNPs, but not non-targeted LNPs, accumulated within the spleen following systemic injection, with mCherry and Fluc signals visible within 30 min after injection. At 24 h after aCD3-LNP injection, 2-4% of all splenic T cells and 2-7% of all circulating T cells expressed mCherry, and this was dependent on aCD3 coating density. Targeting and transfection were accompanied by systemic CD25⁺, OX40⁺, and CD69⁺ T-cell activation with temporary CD3e ligand loss and depletion of splenic and circulating subsets. Migration of splenic CD8a⁺ T cells from the white-pulp to red-pulp, and differentiation from naïve to memory and effector phenotypes, followed upon aCD3-LNP delivery. Additionally, aCD3-LNP injection stimulated the secretion of myeloid-derived chemokines and T-helper cytokines into plasma. Lastly, we administered aCD3-LNPs to tumor bearing mice and found that transfected T cells localized within tumors and tumor-draining lymph nodes following immunotherapy treatment. In summary, we show that CD3-targeted transfection is feasible, yet associated with complex immunological consequences that must be further studied for potential therapeutic applications.

TMEM176B Regulates AKT/mTOR Signaling and Tumor Growth in Triple-Negative Breast Cancer

TMEM176B is a member of the membrane spanning 4-domains (MS4) family of transmembrane proteins, and a putative ion channel that is expressed in immune cells and certain cancers. We aimed to understand the role of TMEM176B in cancer cell signaling, gene expression, cell proliferation, and migration in vitro, as well as tumor growth in vivo. We generated breast cancer cell lines with overexpressed and silenced TMEM176B, and a therapeutic antibody targeting TMEM176B. Proliferation and migration assays were performed in vitro, and tumor growth was evaluated in vivo. We performed gene expression and Western blot analyses to identify the most differentially regulated genes and signaling pathways in cells with TMEM176B overexpression and silencing. Silencing TMEM176B or inhibiting it with a therapeutic antibody impaired cell proliferation, while overexpression increased proliferation in vitro. Syngeneic and xenograft tumor studies revealed the attenuated growth of tumors with TMEM176B gene silencing compared with controls. We found that the AKT/mTOR signaling pathway was activated or repressed in cells overexpressing or silenced for TMEM176B, respectively. Overall, our results suggest that TMEM176B expression in breast cancer cells regulates key signaling pathways and genes that contribute to cancer cell growth and progression, and is a potential target for therapeutic antibodies.

Reversing an Oncogenic Epithelial-to-Mesenchymal Transition Program in Breast Cancer Reveals Actionable Immune Suppressive Pathways

Approval of checkpoint inhibitors for treatment of metastatic triple negative breast cancer (mTNBC) has opened the door for the use of immunotherapies against this disease. However, not all patients with mTNBC respond to current immunotherapy approaches such as checkpoint inhibitors. Recent evidence demonstrates that TNBC metastases are more immune suppressed than primary tumors, suggesting that combination or additional immunotherapy strategies may be required to activate an anti-tumor immune attack at metastatic sites. To identify other immune suppressive mechanisms utilized by mTNBC, our group and others manipulated oncogenic epithelial-to-mesenchymal transition (EMT) programs in TNBC models to reveal differences between this breast cancer subtype and its more epithelial counterpart. This review will discuss how EMT modulation revealed several mechanisms, including tumor cell metabolism, cytokine milieu and secretion of additional immune modulators, by which mTNBC cells may suppress both the innate and adaptive anti-tumor immune responses. Many of these pathways/proteins are under preclinical or clinical investigation as therapeutic targets in mTNBC and other advanced cancers to enhance their response to chemotherapy and/or checkpoint inhibitors.

Harnessing the predictive power of preclinical models for oncology drug development

Recent progress in understanding the molecular basis of cellular processes, identification of promising therapeutic targets and evolution of the regulatory landscape makes this an exciting and unprecedented time to be in the field of oncology drug development. However, high costs, long development timelines and steep rates of attrition continue to afflict the drug development process. Lack of predictive preclinical models is considered one of the key reasons for the high rate of attrition in oncology. Generating meaningful and predictive results preclinically requires a firm grasp of the relevant biological questions and alignment of the model systems that mirror the patient context. In doing so, the ability to conduct both forward translation, the process of implementing basic research discoveries into practice, as well as reverse translation, the process of elucidating the mechanistic basis of clinical observations, greatly enhances our ability to develop effective anticancer treatments. In this Review, we outline issues in preclinical-to-clinical translatability of molecularly targeted cancer therapies, present concepts and examples of successful reverse translation, and highlight the need to better align tumour biology in patients with preclinical model systems including tracking of strengths and weaknesses of preclinical models throughout programme development.

Calcium/Calmodulin Dependent Protein Kinase Kinase 2 Regulates the Expansion of Tumor-Induced Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) are a hetero geneous group of cells, which can suppress the immune response, promote tumor progression and impair the efficacy of immunotherapies. Consequently, the pharmacological targeting of MDSC is emerging as a new immunotherapeutic strategy to stimulate the natural anti-tumor immune response and potentiate the efficacy of immunotherapies. Herein, we leveraged genetically modified models and a small molecule inhibitor to validate Calcium-Calmodulin Kinase Kinase 2 (CaMKK2) as a druggable target to control MDSC accumulation in tumor-bearing mice. The results indicated that deletion of CaMKK2 in the host attenuated the growth of engrafted tumor cells, and this phenomenon was associated with increased antitumor T cell response and decreased accumulation of MDSC. The adoptive transfer of MDSC was sufficient to restore the ability of the tumor to grow in Camkk2-/- mice, confirming the key role of MDSC in the mechanism of tumor rejection. In vitro studies indicated that blocking of CaMKK2 is sufficient to impair the yield of MDSC. Surprisingly, MDSC generated from Camkk2-/- bone marrow cells also showed a higher ability to terminally differentiate toward more immunogenic cell types (e.g inflammatory macrophages and dendritic cells) compared to wild type (WT). Higher intracellular levels of reactive oxygen species (ROS) accumulated in Camkk2-/- MDSC, increasing their susceptibility to apoptosis and promoting their terminal differentiation toward more mature myeloid cells. Mechanistic studies indicated that AMP-activated protein kinase (AMPK), which is a known CaMKK2 proximal target controlling the oxidative stress response, fine-tunes ROS accumulation in MDSC. Accordingly, failure to activate the CaMKK2-AMPK axis can account for the elevated ROS levels in Camkk2-/- MDSC. These results highlight CaMKK2 as an important regulator of the MDSC lifecycle, identifying this kinase as a new druggable target to restrain MDSC expansion and enhance the efficacy of anti-tumor immunotherapy.

Modeling metastasis in mice: a closer look

Unraveling the multifaceted cellular and physiological processes associated with metastasis is best achieved by using in vivo models that recapitulate the requisite tumor cell-intrinsic and -extrinsic mechanisms at the organismal level. We discuss the current status of mouse models of metastasis. We consider how mouse models can refine our understanding of the underlying biological and molecular processes that promote metastasis, and we envisage how the application of new technologies will further enhance investigations of metastasis at single-cell resolution in the context of the whole organism. Our view is that investigations based on state-of-the-art mouse models can propel a holistic understanding of the biology of metastasis, which will ultimately lead to the discovery of new therapeutic opportunities.

Acoustically Detonated Microbubbles Coupled with Low Frequency Insonation: Multiparameter Evaluation of Low Energy Mechanical Ablation

Noninvasive ultrasound surgery can be achieved using focused ultrasound to locally affect the targeted site without damaging intervening tissues. Mechanical ablation and histotripsy use short and intense acoustic pulses to destroy the tissue via a purely mechanical effect. Here, we show that coupled with low-frequency excitation, targeted microbubbles can serve as mechanical therapeutic warheads that trigger potent mechanical effects in tumors using focused ultrasound. Upon low frequency excitation (250 kHz and below), high amplitude microbubble oscillations occur at substantially lower pressures as compared to higher MHz ultrasonic frequencies. For example, inertial cavitation was initiated at a pressure of 75 kPa for a center frequency of 80 kHz. Low frequency insonation of targeted microbubbles was then used to achieve low energy tumor cell fractionation at pressures below a mechanical index of 1.9, and in accordance with the Food and Drug Administration guidelines. We demonstrate these capabilities in vitro and in vivo. In cell cultures, cell viability was reduced to 16% at a peak negative pressure of 800 kPa at the 250 kHz frequency (mechanical index of 1.6) and to 10% at a peak negative pressure of 250 kPa at a frequency of 80 kHz (mechanical index of 0.9). Following an intratumoral injection of targeted microbubbles into tumor-bearing mice, and coupled with low frequency ultrasound application, significant tumor debulking and cancer cell death was observed. Our findings suggest that reducing the center frequency enhances microbubble-mediated mechanical ablation; thus, this technology provides a unique theranostic platform for safe low energy tumor fractionation, while reducing off-target effects.

Evolution of fibroblasts in the lung metastatic microenvironment is driven by stage-specific transcriptional plasticity

Mortality from breast cancer is almost exclusively a result of tumor metastasis, and lungs are one of the main metastatic sites. Cancer-associated fibroblasts are prominent players in the microenvironment of breast cancer. However, their role in the metastatic niche is largely unknown. In this study, we profiled the transcriptional co-evolution of lung fibroblasts isolated from transgenic mice at defined stage-specific time points of metastases formation. Employing multiple knowledge-based platforms of data analysis provided powerful insights on functional and temporal regulation of the transcriptome of fibroblasts. We demonstrate that fibroblasts in lung metastases are transcriptionally dynamic and plastic, and reveal stage-specific gene signatures that imply functional tasks, including extracellular matrix remodeling, stress response, and shaping the inflammatory microenvironment. Furthermore, we identified Myc as a central regulator of fibroblast rewiring and found that stromal upregulation of Myc transcriptional networks is associated with disease progression in human breast cancer.

Tumor stiffening reversion through collagen crosslinking inhibition improves T cell migration and anti-PD-1 treatment

Only a fraction of cancer patients benefits from immune checkpoint inhibitors. This may be partly due to the dense extracellular matrix (ECM) that forms a barrier for T cells. Comparing five preclinical mouse tumor models with heterogeneous tumor microenvironments, we aimed to relate the rate of tumor stiffening with the remodeling of ECM architecture and to determine how these features affect intratumoral T cell migration. An ECM-targeted strategy, based on the inhibition of lysyl oxidase, was used. In vivo stiffness measurements were found to be strongly correlated with tumor growth and ECM crosslinking but negatively correlated with T cell migration. Interfering with collagen stabilization reduces ECM content and tumor stiffness leading to improved T cell migration and increased efficacy of anti-PD-1 blockade. This study highlights the rationale of mechanical characterizations in solid tumors to understand resistance to immunotherapy and of combining treatment strategies targeting the ECM with anti-PD-1 therapy.

Breast cancer dormancy: need for clinically relevant models to address current gaps in knowledge

Breast cancer is the most commonly diagnosed cancer in the USA. Although advances in treatment over the past several decades have significantly improved the outlook for this disease, most women who are diagnosed with estrogen receptor positive disease remain at risk of metastatic relapse for the remainder of their life. The cellular source of late relapse in these patients is thought to be disseminated tumor cells that reactivate after a long period of dormancy. The biology of these dormant cells and their natural history over a patient's lifetime is largely unclear. We posit that research on tumor dormancy has been significantly limited by the lack of clinically relevant models. This review will discuss existing dormancy models, gaps in biological understanding, and propose criteria for future models to enhance their clinical relevance.

MicroRNA-200c restoration reveals a cytokine profile to enhance M1 macrophage polarization in breast cancer

Many immune suppressive mechanisms utilized by triple negative breast cancer (TNBC) are regulated by oncogenic epithelial-to-mesenchymal transition (EMT). How TNBC EMT impacts innate immune cells is not fully understood. To determine how TNBC suppresses antitumor macrophages, we used microRNA-200c (miR-200c), a powerful repressor of EMT, to drive mesenchymal-like mouse mammary carcinoma and human TNBC cells toward a more epithelial state. MiR-200c restoration significantly decreased growth of mouse mammary carcinoma Met-1 cells in culture and in vivo. Cytokine profiling of Met-1 and human BT549 cells revealed that miR-200c upregulated cytokines, such as granulocyte-macrophage colony-stimulating factor (GM-CSF), promoted M1 antitumor macrophage polarization. Cytokines upregulated by miR-200c correlated with an epithelial gene signature and M1 macrophage polarization in BC patients and predicted a more favorable overall survival for TNBC patients. Our findings demonstrate that immunogenic cytokines (e.g., GM-CSF) are suppressed in aggressive TNBC, warranting further investigation of cytokine-based therapies to limit disease recurrence.

Targeting the Aryl Hydrocarbon Receptor Signaling Pathway in Breast Cancer Development

Activation of the aryl hydrocarbon receptor (AhR) through environmental exposure to known human carcinogens including dioxins can lead to the promotion of breast cancer. While the repressor protein of the AhR (AhRR) blocks the canonical AhR pathway, the function of AhRR in the development of breast cancer is not well-known. In the current study we examined the impact of suppressing AhR activity using its dedicated repressor protein AhRR. AhRR is a putative tumor suppressor and is silenced in several cancer types, including breast, where its loss correlates with shorter patient survival. Using the AhRR transgenic mouse, we demonstrate that AhRR overexpression opposes AhR-driven and inflammation-induced growth of mammary tumors in two different murine models of breast cancer. These include a syngeneic model using E0771 mammary tumor cells as well as the Polyoma Middle T antigen (PyMT) transgenic model. Further AhRR overexpression or knockout of AhR in human breast cancer cells enhanced apoptosis induced by chemotherapeutics and inhibited the growth of mouse mammary tumor cells. This study provides the first in vivo evidence that AhRR suppresses mammary tumor development and suggests that strategies which lead to its functional restoration and expression may have therapeutic benefit.

Obesity-Activated Lung Stromal Cells Promote Myeloid Lineage Cell Accumulation and Breast Cancer Metastasis

Obesity is correlated with increased incidence of breast cancer metastasis; however, the mechanisms underlying how obesity promotes metastasis are unclear. In a diet-induced obese mouse model, obesity enhanced lung metastasis in both the presence and absence of primary mammary tumors and increased recruitment of myeloid lineage cells into the lungs. In the absence of tumors, obese mice demonstrated increased numbers of myeloid lineage cells and elevated collagen fibers within the lung stroma, reminiscent of premetastatic niches formed by primary tumors. Lung stromal cells isolated from obese tumor-naïve mice showed increased proliferation, contractility, and expression of extracellular matrix, inflammatory markers and transforming growth factor beta-1 (TGFβ1). Conditioned media from lung stromal cells from obese mice promoted myeloid lineage cell migration in vitro in response to colony-stimulating factor 2 (CSF2) expression and enhanced invasion of tumor cells. Together, these results suggest that prior to tumor formation, obesity alters the lung microenvironment, creating niches conducive to metastatic growth.

Development of thermosensitive resiquimod-loaded liposomes for enhanced cancer immunotherapy

Resiquimod (R848) is a toll-like receptor 7 and 8 (TLR7/8) agonist with potent antitumor and immunostimulatory activity. However, systemic delivery of R848 is poorly tolerated because of its poor solubility in water and systemic immune activation. In order to address these limitations, we developed an intravenously-injectable formulation with R848 using thermosensitive liposomes (TSLs) as a delivery vehicle. R848 was remotely loaded into TSLs composed of DPPC: DSPC: DSPE-PEG2K (85:10:5, mol%) with 100 mM FeSO4 as the trapping agent inside. The final R848 to lipid ratio of the optimized R848-loaded TSLs (R848-TSLs) was 0.09 (w/w), 10-fold higher than the previously-reported values. R848-TSLs released 80% of R848 within 5 min at 42 °C. These TSLs were then combined with αPD-1, an immune checkpoint inhibitor, and ultrasound-mediated hyperthermia in a neu deletion (NDL) mouse mammary carcinoma model (Her2+, ER/PR negative). Combined with αPD-1, local injection of R848-TSLs showed superior efficacy with complete NDL tumor regression in both treated and abscopal sites achieved in 8 of 11 tumor bearing mice over 100 days. Immunohistochemistry confirmed enhanced CD8+ T cell infiltration and accumulation by R848-TSLs. Systemic delivery of R848-TSLs, combined with local hyperthermia and αPD-1, inhibited tumor growth and extended median survival from 28 days (non-treatment control) to 94 days. Upon re-challenge with reinjection of tumor cells, none of the previously cured mice developed tumors, as compared with 100% of age-matched control mice. The dose of R848 (10 μg for intra-tumoral injection or 6 mg/kg for intravenous injection delivered up to 4 times) was well-tolerated without weight loss or organ hypertrophy. In summary, we developed R848-TSLs that can be administered locally or systematically, resulting in tumor regression and enhanced survival when combined with αPD-1 in mouse models of breast cancer.

VEGF-C mediates tumor growth and metastasis through promoting EMT-epithelial breast cancer cell crosstalk

It is well established that a subset of cells within primary breast cancers can undergo an epithelial-to-mesenchymal transition (EMT), although the role of EMT in metastasis remains controversial. We previously demonstrated that breast cancer cells that had undergone an oncogenic EMT could increase metastasis of neighboring cancer cells via non-canonical paracrine-mediated activation of GLI activity that is dependent on SIX1 expression in the EMT cancer cells. However, the mechanism by which these SIX1-expressing EMT cells activate GLI signaling remained unclear. In this study, we demonstrate a novel mechanism for activation of GLI-mediated signaling in epithelial breast tumor cells via EMT cell-induced production and secretion of VEGF-C. We show that VEGF-C, secreted by breast cancer cells that have undergone an EMT, promotes paracrine-mediated increases in proliferation, migration, and invasion of epithelial breast cancer cells, via non-canonical activation of GLI-signaling. We further show that the aggressive phenotypes, including metastasis, imparted by EMT cells on adjacent epithelial cancer cells can be disrupted by either inhibiting VEGF-C in EMT cells or by knocking down NRP2, a receptor which interacts with VEGF-C, in neighboring epithelial cancer cells. Interrogation of TCGA and GEO public datasets supports the relevance of this pathway in human breast cancer, demonstrating that VEGF-C strongly correlates with activation of Hedgehog signaling and EMT in the human disease. Our study suggests that the VEGF-C/NRP2/GLI axis is a novel and conserved paracrine means by which EMT cells enhance metastasis, and provides potential targets for therapeutic intervention in this heterogeneous disease.

Assessment of the WAP-Myc mouse mammary tumor model for spontaneous metastasis

Breast cancer is the most common form of cancer in women. Despite significant therapeutic advances in recent years, breast cancer also still causes the greatest number of cancer-related deaths in women, the vast majority of which (> 90%) are caused by metastases. However, very few mouse mammary cancer models exist that faithfully recapitulate the multistep metastatic process in human patients. Here we assessed the suitability of a syngrafting protocol for a Myc-driven mammary tumor model (WAP-Myc) to study autochthonous metastasis. A moderate but robust spontaneous lung metastasis rate of around 25% was attained. In addition, increased T cell infiltration was observed in metastatic tumors compared to donor and syngrafted primary tumors. Thus, the WAP-Myc syngrafting protocol is a suitable tool to study the mechanisms of metastasis in MYC-driven breast cancer.

Nephronectin is a prognostic biomarker and promotes gastric cancer cell proliferation, migration and invasion

Gastric cancer (GC) is a malignant disease with high incidence and mortality rates worldwide. Nephronectin (NPNT) was found to be dysregulated in some kinds of cancer. The goal of our study was to explore the expression profile of NPNT based on large numbers of GC samples with detailed clinicopathological and prognostic data from our institution and the data from a public database. A total of 117 GC samples and 73 corresponding non-tumorous adjacent tissues (NATs) were obtained from GC patients and used to detect expression of NPNT through immunohistochemistry. Western blot and qRT-PCR were performed to examine expression of NPNT in GC cell lines. Our results found that the positive expression ratio of NPNT in GC tissues is significantly higher than that in NATs (p<0.001). Chi-squared analysis results showed positive expression ratio of NPNT was significantly associated with depth of tumor invasion (p=0.049) and TNM stage (p=0.017). Kaplan-Meier survival and cox analysis results showed that patients with positive NPNT protein expression tend to have poorer prognosis than those with negative NPNT expression (p=0.0032) and NPNT expression was independent prognostic factor. High expression level was seen in GC cell lines. Furthermore, through a series of cancer cell proliferation, invasion and migration associated experiments, we found that NPNT could evidently promote GC cell proliferation, invasion and migration, as well as epithelial-mesenthymal transition. In summary, NPNT was evidently overexpressed in GC and had an oncogenic role. In the future, NPNT could serve as a promising therapeutic target for treating GC patients.

Stromal Platelet-Derived Growth Factor Receptor-β Signaling Promotes Breast Cancer Metastasis in the Brain

Platelet-derived growth factor receptor-beta (PDGFRβ) is a receptor tyrosine kinase found in cells of mesenchymal origin such as fibroblasts and pericytes. Activation of this receptor is dependent on paracrine ligand induction, and its preferred ligand PDGFB is released by neighboring epithelial and endothelial cells. While expression of both PDGFRβ and PDGFB has been noted in patient breast tumors for decades, how PDGFB-to-PDGFRβ tumor-stroma signaling mediates breast cancer initiation, progression, and metastasis remains unclear. Here we demonstrate this paracrine signaling pathway that mediates both primary tumor growth and metastasis, specifically, metastasis to the brain. Elevated levels of PDGFB accelerated orthotopic tumor growth and intracranial growth of mammary tumor cells, while mesenchymal-specific expression of an activating mutant PDGFRβ (PDGFRβ^D849V) exerted proproliferative signals on adjacent mammary tumor cells. Stromal expression of PDGFRβ^D849V also promoted brain metastases of mammary tumor cells expressing high PDGFB when injected intravenously. In the brain, expression of PDGFRβ^D849V was observed within a subset of astrocytes, and aged mice expressing PDGFRβ^D849V exhibited reactive gliosis. Importantly, the PDGFR-specific inhibitor crenolanib significantly reduced intracranial growth of mammary tumor cells. In a tissue microarray comprised of 363 primary human breast tumors, high PDGFB protein expression was prognostic for brain metastases, but not metastases to other sites. Our results advocate the use of mice expressing PDGFRβ^D849V in their stromal cells as a preclinical model of breast cancer-associated brain metastases and support continued investigation into the clinical prognostic and therapeutic use of PDGFB-to-PDGFRβ signaling in women with breast cancer. SIGNIFICANCE: These studies reveal a previously unknown role for PDGFB-to-PDGFRβ paracrine signaling in the promotion of breast cancer brain metastases and support the prognostic and therapeutic clinical utility of this pathway for patients.See related article by Wyss and colleagues, p. 594.

Chemokine releasing particle implants for trapping circulating prostate cancer cells

Prostate cancer (PCa) is the most prevalent cancer in U.S. men and many other countries. Although primary PCa can be controlled with surgery or radiation, treatment options of preventing metastatic PCa are still limited. To develop a new treatment of eradicating metastatic PCa, we have created an injectable cancer trap that can actively recruit cancer cells in bloodstream. The cancer trap is composed of hyaluronic acid microparticles that have good cell and tissue compatibility and can extend the release of chemokines to 4 days in vitro. We find that erythropoietin (EPO) and stromal derived factor-1α can attract PCa in vitro. Animal results show that EPO-releasing cancer trap attracted large number of circulating PCa and significantly reduced cancer spreading to other organs compared with controls. These results support that cancer trap may serve as a unique device to sequester circulating PCa cells and subsequently reduce distant metastasis.

Novel roles of PRK1 and PRK2 in cilia and cancer biology

PRK1 and PRK2 are two closely related AGC-family serine/threonine protein kinases. Here we demonstrate novel roles for them at cilia and in cancer biology. In both instances serum withdrawal leads to increased activating PRK1 and PRK2 phosphorylation (pPRK1/pPRK2) and their depletion results in reduced spheroid growth. pPRK1/pPRK2 localise to the transition zone of cilia and their co-depletion results in reduced cilia size, impaired planer polarity and impaired cilia associated signalling. High PRK2 (but not PRK1) expression correlates with poor outcome in patients with basal-like/Triple Negative (TN) Breast Cancer (BC) where there is also higher expression relative to other BC tumour subtypes. In agreement, depletion of PRK1 and PRK2 in mouse TNBC cells, or CRISPR/Cas9 mediated deletion of PRK2 alone, significantly reduces cell proliferation and spheroid growth. Finally proteomic analysis to identify PRK2 binding partners in mouse TNBC cells revealed proteins that are important for both cilia and BC biology. Taken together these data demonstrate novel roles for PRK1 and PRK2 at cilia and in BC biology and in the case of PRK2 in particular, identifies it as a novel TNBC therapeutic target.

Obesity Promotes Cooperation of Cancer Stem-Like Cells and Macrophages to Enhance Mammary Tumor Angiogenesis

Obesity is correlated with worsened prognosis and treatment resistance in breast cancer. Macrophage-targeted therapies are currently in clinical trials, however, little is known about how obesity may impact treatment efficacy. Within breast adipose tissue, obesity leads to chronic, macrophage-driven inflammation, suggesting that obese breast cancer patients may benefit from these therapies. Using a high fat diet model of obesity, we orthotopically transplanted cancer cell lines into the mammary glands of obese and lean mice. We quantified changes in tumor invasiveness, angiogenesis and metastasis, and examined the efficacy of macrophage depletion to diminish tumor progression in obese and lean mice. Mammary tumors from obese mice grew significantly faster, were enriched for cancer stem-like cells (CSCs) and were more locally invasive and metastatic. Tumor cells isolated from obese mice demonstrated enhanced expression of stem cell-related pathways including Sox2 and Notch2. Despite more rapid growth, mammary tumors from obese mice had reduced necrosis, higher blood vessel density, and greater macrophage recruitment. Depletion of macrophages in obese tumor-bearing mice resulted in increased tumor necrosis, reduced endothelial cells, and enhanced recruitment of CD8⁺ T cells compared to IgG-treated controls. Macrophages may be an important clinical target to improve treatment options for obese breast cancer patients.

RORγ is a targetable master regulator of cholesterol biosynthesis in a cancer subtype

Tumor subtype-specific metabolic reprogrammers could serve as targets of therapeutic intervention. Here we show that triple-negative breast cancer (TNBC) exhibits a hyper-activated cholesterol-biosynthesis program that is strongly linked to nuclear receptor RORγ, compared to estrogen receptor-positive breast cancer. Genetic and pharmacological inhibition of RORγ reduces tumor cholesterol content and synthesis rate while preserving host cholesterol homeostasis. We demonstrate that RORγ functions as an essential activator of the entire cholesterol-biosynthesis program, dominating SREBP2 via its binding to cholesterol-biosynthesis genes and its facilitation of the recruitment of SREBP2. RORγ inhibition disrupts its association with SREBP2 and reduces chromatin acetylation at cholesterol-biosynthesis gene loci. RORγ antagonists cause tumor regression in patient-derived xenografts and immune-intact models. Their combination with cholesterol-lowering statins elicits superior anti-tumor synergy selectively in TNBC. Together, our study uncovers a master regulator of the cholesterol-biosynthesis program and an attractive target for TNBC.