Endosialin-Expressing Pericytes Promote Metastatic Dissemination

Group XIV C-type lectins: emerging targets in tumor angiogenesis

C-type lectins, distinguished by a C-type lectin binding domain (CTLD), are an evolutionarily conserved superfamily of glycoproteins that are implicated in a broad range of physiologic processes. The group XIV subfamily of CTLDs are comprised of CD93, CD248/endosialin, CLEC14a, and thrombomodulin/CD141, and have important roles in creating and maintaining blood vessels, organizing extracellular matrix, and balancing pro- and anti-coagulative processes. As such, dysregulation in the expression and downstream signaling pathways of these proteins often lead to clinically relevant pathology. Recently, group XIV CTLDs have been shown to play significant roles in cancer progression, namely tumor angiogenesis and metastatic dissemination. Interest in therapeutically targeting tumor vasculature is increasing and the search for novel angiogenic targets is ongoing. Group XIV CTLDs have emerged as key moderators of tumor angiogenesis and metastasis, thus offering substantial therapeutic promise for the clinic. Herein, we review our current knowledge of group XIV CTLDs, discuss each's role in malignancy and associated potential therapeutic avenues, briefly discuss group XIV CTLDs in the context of two other relevant lectin families, and offer future direction in further elucidating mechanisms by which these proteins function and facilitate tumor growth.

TCAF2 in Pericytes Promotes Colorectal Cancer Liver Metastasis via Inhibiting Cold-Sensing TRPM8 Channel

Hematogenous metastasis is the main approach for colorectal cancer liver metastasis (CRCLM). However, as the gatekeepers in the tumor vessels, the role of TPCs in hematogenous metastasis remains largely unknown, which may be attributed to the lack of specific biomarkers for TPC isolation. Here, microdissection combined with a pericyte medium-based approach is developed to obtain TPCs from CRC patients. Proteomic analysis reveals that TRP channel-associated factor 2 (TCAF2), a partner protein of the transient receptor potential cation channel subfamily M member 8 (TRPM8), is overexpressed in TPCs from patients with CRCLM. TCAF2 in TPCs is correlated with liver metastasis, short overall survival, and disease-free survival in CRC patients. Gain- and loss-of-function experiments validate that TCAF2 in TPCs promotes tumor cell motility, epithelial-mesenchymal transition (EMT), and CRCLM, which is attenuated in pericyte-conditional Tcaf2-knockout mice. Mechanistically, TCAF2 inhibits the expression and activity of TRPM8, leading to Wnt5a secretion in TPCs, which facilitates EMT via the activation of the STAT3 signaling pathway in tumor cells. Menthol, a TRPM8 agonist, significantly suppresses Wnt5a secretion in TPCs and CRCLM. This study reveals the previously unidentified pro-metastatic effects of TPCs from the perspective of cold-sensory receptors, providing a promising diagnostic biomarker and therapeutic target for CRCLM.

The covert symphony: cellular and molecular accomplices in breast cancer metastasis

Breast cancer has emerged as the most commonly diagnosed cancer and primary cause of cancer-related deaths among women worldwide. Although significant progress has been made in targeting the primary tumor, the effectiveness of systemic treatments to prevent metastasis remains limited. Metastatic disease continues to be the predominant factor leading to fatality in the majority of breast cancer patients. The existence of a prolonged latency period between initial treatment and eventual recurrence in certain patients indicates that tumors can both adapt to and interact with the systemic environment of the host, facilitating and sustaining the progression of the disease. In order to identify potential therapeutic interventions for metastasis, it will be crucial to gain a comprehensive framework surrounding the mechanisms driving the growth, survival, and spread of tumor cells, as well as their interaction with supporting cells of the microenvironment. This review aims to consolidate recent discoveries concerning critical aspects of breast cancer metastasis, encompassing the intricate network of cells, molecules, and physical factors that contribute to metastasis, as well as the molecular mechanisms governing cancer dormancy.

Novel TCF21high pericyte subpopulation promotes colorectal cancer metastasis by remodelling perivascular matrix

OBJECTIVE

Haematogenous dissemination is a prevalent route of colorectal cancer (CRC) metastasis. However, as the gatekeeper of vessels, the role of tumour pericytes (TPCs) in haematogenous metastasis remains largely unknown. Here, we aimed to investigate the heterogeneity of TPCs and their effects on CRC metastasis.

DESIGN

TPCs were isolated from patients with CRC with or without liver metastases and analysed by single-cell RNA sequencing (scRNA-seq). Clinical CRC specimens were collected to analyse the association between the molecular profiling of TPCs and CRC metastasis. RNA-sequencing, chromatin immunoprecipitation-sequencing and bisulfite-sequencing were performed to investigate the TCF21-regulated genes and mechanisms underlying integrin α5 on TCF21 DNA hypermethylation. Pericyte-conditional Tcf21-knockout mice were constructed to investigate the effects of TCF21 in TPCs on CRC metastasis. Masson staining, atomic force microscopy, second-harmonic generation and two-photon fluorescence microscopy were employed to observe perivascular extracellular matrix (ECM) remodelling.

RESULTS

Thirteen TPC subpopulations were identified by scRNA-seq. A novel subset of TCF21^high TPCs, termed 'matrix-pericytes', was associated with liver metastasis in patients with CRC. TCF21 in TPCs increased perivascular ECM stiffness, collagen rearrangement and basement membrane degradation, establishing a perivascular metastatic microenvironment to instigate colorectal cancer liver metastasis (CRCLM). Tcf21 depletion in TPCs mitigated perivascular ECM remodelling and CRCLM, whereas the coinjection of TCF21^high TPCs and CRC cells markedly promoted CRCLM. Mechanistically, loss of integrin α5 inhibited the FAK/PI3K/AKT/DNMT1 axis to impair TCF21 DNA hypermethylation in TCF21^high TPCs.

CONCLUSION

This study uncovers a previously unidentified role of TPCs in haematogenous metastasis and provides a potential diagnostic marker and therapeutic target for CRC metastasis.

CD248 promotes migration and metastasis of osteosarcoma through ITGB1-mediated FAK-paxillin pathway activation

BACKGROUND

Osteosarcoma (OS) is the most common malignant bone tumor with a high incidence in children and adolescents. Frequent tumor metastasis and high postoperative recurrence are the most common challenges in OS. However, detailed mechanism is largely unknown.

METHODS

We examined the expression of CD248 in OS tissue microarrays by immunohistochemistry (IHC) staining. We studied the biological function of CD248 in cell proliferation, invasion and migration of OS cells by CCK8 assay, transwell and wound healing assay. We also studied its function in the metastasis of OS in vivo. At last, we explored the potential mechanism how CD248 promotes OS metastasis by using RNA-seq, western blot, immunofluorescence staining and co-immunoprecipitation using CD248 knockdown OS cells.

RESULTS

CD248 was highly expressed in OS tissues and its high expression was correlated with pulmonary metastasis of OS. Knockdown of CD248 in OS cells significantly inhibited cell migration, invasion and metastasis, while had no obvious effect on cell proliferation. Lung metastasis in nude mice was significantly inhibited when CD248 was knocked down. Mechanistically, we found that CD248 could promote the interaction between ITGB1 and extracellular matrix (ECM) proteins like CYR61 and FN, which activated the FAK-paxillin pathway to promote the formation of focal adhesion and metastasis of OS.

CONCLUSION

Our data showed that high CD248 expression is correlated with the metastatic potential of OS. CD248 may promote migration and metastasis through enhancing the interaction between ITGB1 and certain ECM proteins. Therefore, CD248 is a potential marker for diagnosis and effective target for the treatment of metastatic OS.

Endosialin-positive tumor-derived pericytes promote tumor progression through impeding the infiltration of CD8+ T cells in clear cell renal cell carcinoma

BACKGROUND

Immune checkpoint blockade (ICB) therapy can be effective against clear cell renal cell carcinoma (ccRCC), but many patients show no benefit. Tumor-derived pericytes (TDPs) may promote tumor progression by influencing T cells and are an immunotherapy target; however, they may comprise functionally distinct subtypes. We aimed to identify markers of tumor-promoting TDPs and develop TDP-targeting strategies to enhance ICB therapy effectiveness against ccRCC.

METHODS

We analyzed the relationship between endosialin (EN) expression and cytotoxic T-lymphocyte (CTL) infiltration in ccRCC tumor samples using flow cytometry and in a ccRCC-bearing mice inhibited for EN via knockout or antibody-mediated blockade. The function of EN^high TDPs in CTL infiltration and tumor progression was analyzed using RNA-sequencing (RNA-seq) data from ccRCC tissue-derived TDPs and single-cell RNA-seq (scRNA-seq) data from an online database. The role of EN in TDP proliferation and migration and in CTL infiltration was examined in vitro. Finally, we examined the anti-tumor effect of combined anti-EN and anti-programmed death 1 (PD-1) antibodies in ccRCC-bearing mice.

RESULTS

High EN expression was associated with low CTL infiltration in ccRCC tissues, and inhibition of EN significantly increased CTL infiltration in ccRCC-bearing mice. RNA-seq and scRNA-seq analyses indicated that high EN expression represented the TDP activation state. EN promoted TDP proliferation and migration and impeded CTL infiltration in vitro. Finally, combined treatment with anti-EN and anti-PD-1 antibodies synergistically enhanced anti-tumor efficacy.

CONCLUSION

EN^high TDPs are in an activated state and inhibit CTL infiltration into ccRCC tissues. Combined treatment with anti-EN and anti-PD-1 antibodies may improve ICB therapy effectiveness against ccRCC.

The Tumor Microenvironment in Tumorigenesis and Therapy Resistance Revisited

Tumorigenesis is a complex and dynamic process involving cell-cell and cell-extracellular matrix (ECM) interactions that allow tumor cell growth, drug resistance and metastasis. This review provides an updated summary of the role played by the tumor microenvironment (TME) components and hypoxia in tumorigenesis, and highlight various ways through which tumor cells reprogram normal cells into phenotypes that are pro-tumorigenic, including cancer associated- fibroblasts, -macrophages and -endothelial cells. Tumor cells secrete numerous factors leading to the transformation of a previously anti-tumorigenic environment into a pro-tumorigenic environment. Once formed, solid tumors continue to interact with various stromal cells, including local and infiltrating fibroblasts, macrophages, mesenchymal stem cells, endothelial cells, pericytes, and secreted factors and the ECM within the tumor microenvironment (TME). The TME is key to tumorigenesis, drug response and treatment outcome. Importantly, stromal cells and secreted factors can initially be anti-tumorigenic, but over time promote tumorigenesis and induce therapy resistance. To counter hypoxia, increased angiogenesis leads to the formation of new vascular networks in order to actively promote and sustain tumor growth via the supply of oxygen and nutrients, whilst removing metabolic waste. Angiogenic vascular network formation aid in tumor cell metastatic dissemination. Successful tumor treatment and novel drug development require the identification and therapeutic targeting of pro-tumorigenic components of the TME including cancer-associated- fibroblasts (CAFs) and -macrophages (CAMs), hypoxia, blocking ECM-receptor interactions, in addition to the targeting of tumor cells. The reprogramming of stromal cells and the immune response to be anti-tumorigenic is key to therapeutic success. Lastly, this review highlights potential TME- and hypoxia-centered therapies under investigation.

Interference in melanoma CD248 function reduces vascular mimicry and metastasis

Background

Tumor vascular mimicry is an emerging issue that affects patient survival while having no treatment at the current moment. Despite several factors implicated in vascular mimicry, little is known about stromal factors that modulate tumor microenvironment and shape malignant transformation. CD248, a type-I transmembrane protein dominantly expressed in stromal cells, mediates the interaction between cells and extracellular matrix proteins. CD248 protein expression is associated with the metastatic melanoma phenotype and promotes tumor progression in the stromal cells. This study aimed to explore the cell-autonomous effects of CD248 in melanoma vascular mimicry to aid cancer therapy development.

Methods

Loss-of-function approaches in B16F10 melanoma cells were used to study the cell-autonomous effects of CD248 on cell adhesion, migration, proliferation, and vascular mimicry. A solid-phase binding assay was performed to identify the interaction between CD248 and fibronectin. Horizontal and vertical cell migration assays were performed to analyze cell migration activity, and cell-patterned network formation on Matrigel was used to evaluate vascular mimicry activity. Recombinant CD248 (rCD248) proteins were generated, and whether rCD248 interfered with melanoma CD248 functions was evaluated in vitro. An experimental lung metastasis mouse model was used to investigate the effect of rCD248 treatment in vivo.

Results

CD248 protein expression in melanoma cells was increased by a fibroblast-conditioned medium. Knockdown of CD248 expression significantly decreased cell adhesion to fibronectin, cell migration, and vascular mimicry in melanoma cells. The lectin domain of CD248 was directly involved in the interaction between CD248 and fibronectin. Furthermore, rCD248 proteins containing its lectin domain inhibited cell adhesion to fibronectin and slowed down cell migration and vascular mimicry. Treatment with rCD248 protein could reduce pulmonary tumor burden, accompanied by a reduction in vascular mimicry in mice with melanoma lung metastasis.

Conclusion

CD248 expression in melanoma cells promotes malignant transformation by increasing the activity of cell adhesion, migration, and vascular mimicry, whereas rCD248 protein functions as a molecular decoy interfering with tumor-promoting effects of CD248 in melanoma cells.

Role of Epithelial-to-Mesenchymal Transition for the Generation of Circulating Tumors Cells and Cancer Cell Dissemination

Tumor-related death is primarily caused by metastasis; consequently, understanding, preventing, and treating metastasis is essential to improving clinical outcomes. Metastasis is mainly governed by the dissemination of tumor cells in the systemic circulation: so-called circulating tumor cells (CTCs). CTCs typically arise from epithelial tumor cells that undergo epithelial-to-mesenchymal transition (EMT), resulting in the loss of cell-cell adhesions and polarity, and the reorganization of the cytoskeleton. Various oncogenic factors can induce EMT, among them the transforming growth factor (TGF)-β, as well as Wnt and Notch signaling pathways. This entails the activation of numerous transcription factors, including ZEB, TWIST, and Snail proteins, acting as transcriptional repressors of epithelial markers, such as E-cadherin and inducers of mesenchymal markers such as vimentin. These genetic and phenotypic changes ultimately facilitate cancer cell migration. However, to successfully form distant metastases, CTCs must primarily withstand the hostile environment of circulation. This includes adaption to shear stress, avoiding being trapped by coagulation and surviving attacks of the immune system. Several applications of CTCs, from cancer diagnosis and screening to monitoring and even guided therapy, seek their way into clinical practice. This review describes the process leading to tumor metastasis, from the generation of CTCs in primary tumors to their dissemination into distant organs, as well as the importance of subtyping CTCs to improve personalized and targeted cancer therapy.

Tumor endothelial marker 1 is upregulated in heart after cardiac injury and participates in cardiac remodeling

Tumor endothelial marker 1 (TEM1) is a transmembrane glycoprotein that appears on mesenchymal lineage-derived cells during embryogenesis, but its expression greatly reduces after birth. Re-upregulation of TEM1 is found in tumor angiogenesis, organ fibrosis and wound healing indicating its potential role in tissue remodeling and repair. The expression level and function of TEM1 in adult heart are unknown. In explanted hearts from heart failure (HF) patients received cardiac transplantation, immunofluorescence staining showed TEM1 was expressed in cardiomyocytes (CMs) and cardiac fibroblasts. Bioinformatics analysis showed TEM1 upregulation in mouse heart after coronary ligation. Cardiac TEM1 expression was reconfirmed in mouse HF induced by coronary ligation or doxorubicin injection. TEM1 expression increased in cultured CMs stimulated with mechanical stretch, doxorubicin and hypoxia. Further studies showed recombinant TEM1 (rTEM1) was a functional protein that influenced cell behaviors of CMs. It directly activated Erk and Akt through interaction with PDGF receptor. TEM1^lacZ/lacZ mice had less collagen deposition and worse cardiac function than wild type mice. These results indicate that TEM1 expression increases in the heart after cardiac injury and works as a functional protein that participates in cardiac remodeling.

Acquired αSMA Expression in Pericytes Coincides with Aberrant Vascular Structure and Function in Pancreatic Ductal Adenocarcinoma

The subpopulations of tumor pericytes undergo pathological phenotype switching, affecting their normal function in upholding structural stability and cross-communication with other cells. In the case of pancreatic ductal adenocarcinoma (PDAC), a significant portion of blood vessels are covered by an α-smooth muscle actin (αSMA)-expressing pericyte, which is normally absent from capillary pericytes. The Desmin^lowαSMA^high phenotype was significantly correlated with intratumoral hypoxia and vascular leakiness. Using an in vitro co-culture system, we demonstrated that cancer cell-derived exosomes could induce ectopic αSMA expression in pericytes. Exosome-treated αSMA⁺ pericytes presented altered pericyte markers and an acquired immune-modulatory feature. αSMA⁺ pericytes were also linked to morphological and biomechanical changes in the pericyte. The PDAC exosome was sufficient to induce αSMA expression by normal pericytes of the healthy pancreas in vivo, and the vessels with αSMA⁺ pericytes were leaky. This study demonstrated that tumor pericyte heterogeneity could be dictated by cancer cells, and a subpopulation of these pericytes confers a pathological feature.

Endosialin/CD248 may be a potential therapeutic target to prevent the invasion and metastasis in osteosarcoma

Endosialin/CD248/tumor endothelial marker 1 is classified as a C-type lectin-like transmembrane receptor, found on the plasma membrane of activated mesenchymal cells, which binds to fibronectin. Although endosialin is expressed at high levels in stem-like cells of sarcomas, its role has not been fully uncovered. The present study aimed to determine whether endosialin expression is associated with tumor progression and metastasis, and whether endosialin has the potential to act as a novel therapeutic target in osteosarcoma (OS) using MORAb-004/ontuxizumab, a humanized monoclonal antibody, which targets the type C lectin domain of endosialin. The results demonstrated that endosialin was highly expressed in OSs with metastatic disease. Furthermore, MORAb-004 had no cytostatic effect on OS cells in vitro and did not change the expression of stem cells and differentiation markers; however, it inhibited migration of OS cells. Taken together, these results suggest that endosialin may play a role in migration, and may be involved in the metastatic process of OSs. Furthermore, MORAb-004 reduces the motility of OS cells, and suppresses invasion and the development of metastatic lesions.

Phosphorylation of pericyte FAK-Y861 affects tumour cell apoptosis and tumour blood vessel regression

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that is overexpressed in many cancer types and in vivo studies have shown that vascular endothelial cell FAK expression and FAK-phosphorylation at tyrosine (Y) 397, and subsequently FAK-Y861, are important in tumour angiogenesis. Pericytes also play a vital role in regulating tumour blood vessel stabilisation, but the specific involvement of pericyte FAK-Y397 and FAK-Y861 phosphorylation in tumour blood vessels is unknown. Using PdgfrβCre + ;FAKWT/WT, PdgfrβCre + ;FAKY397F/Y397F and PdgfrβCre + ;FAKY861F/Y861F mice, our data demonstrate that Lewis lung carcinoma tumour growth, tumour blood vessel density, blood vessel perfusion and pericyte coverage were affected only in late stage tumours in PdgfrβCre + ;FAKY861F/Y861F but not PdgfrβCre + ;FAKY397F/Y397F mice. Further examination indicates a dual role for pericyte FAK-Y861 phosphorylation in the regulation of tumour vessel regression and also in the control of pericyte derived signals that influence apoptosis in cancer cells. Overall this study identifies the role of pericyte FAK-Y861 in the regulation of tumour vessel regression and tumour growth control and that non-phosphorylatable FAK-Y861F in pericytes reduces tumour growth and blood vessel density.

Impairment of a distinct cancer-associated fibroblast population limits tumour growth and metastasis

Profiling studies have revealed considerable phenotypic heterogeneity in cancer-associated fibroblasts (CAFs) present within the tumour microenvironment, however, functional characterisation of different CAF subsets is hampered by the lack of specific markers defining these populations. Here we show that genetic deletion of the Endo180 (MRC2) receptor, predominantly expressed by a population of matrix-remodelling CAFs, profoundly limits tumour growth and metastasis; effects that can be recapitulated in 3D co-culture assays. This impairment results from a CAF-intrinsic contractility defect and reduced CAF viability, which coupled with the lack of phenotype in the normal mouse, demonstrates that upregulated Endo180 expression by a specific, potentially targetable CAF subset is required to generate a supportive tumour microenvironment. Further, characterisation of a tumour subline selected via serial in vivo passage for its ability to overcome these stromal defects provides important insight into, how tumour cells adapt to a non-activated stroma in the early stages of metastatic colonisation.

Endo180, a collagen binding receptor, is highly expressed in a subset of cancer-associated fibroblasts. The authors show, using knockout mice and 3D in vitro assays, that Endo180 depletion impairs tumour fibroblast contractility and viability resulting in reduced tumour growth and metastasis.

The gate to metastasis: key players in cancer cell intravasation

Metastasis is a leading cause of cancer‐related death and consists of a sequence of events including tumor expansion, intravasation of cancer cells into the circulation, survival in the bloodstream, extravasation at distant sites, and subsequent organ colonization. Particularly, intravasation is a process whereby cancer cells transverse the endothelium and leave the primary tumor site, pioneering the metastatic cascade. The identification of those mechanisms that trigger the entry of cancer cells into the bloodstream may reveal fundamentally novel ways to block metastasis at its start. Multiple factors have been implicated in cancer progression, yet, signals that unequivocally provoke the detachment of cancer cells from the primary tumor are still under investigation. Here, we discuss the role of intrinsic properties of cancer cells, tumor microenvironment, and mechanical cues in the intravasation process, outlining studies that suggest the involvement of various factors and highlighting current understanding and open questions in the field.

Capture-free deactivation of CTCs in the bloodstream; a metastasis suppression method by electrostatic stimulation of the peripheral blood

While limited investigations have been reported on CTC elimination and its profits, recently, some new works were reported on detection followed by the destruction of CTCs. Limitations and complications of CTC capturing procedures have highly reduced the chance of selective destruction of CTCs in the bloodstream in the therapeutic guidelines of the patients. Here, we selectively deactivated the invasive function of CTCs during their circulation in the bloodstream by exposing the whole blood to pure positive electrostatic charge stimulation (PPECS). Our treatment suppressed pulmonary metastasis and extended the survival of the mice had been intravenously injected by electrostatically deactivated 4T1 breast cancer CTCs. Moreover, the number of cancerous lung nodules was drastically reduced in the mice injected by treated CTCs in comparison with the non-treated cohort. Evaluating the side effect of the PPECS on the blood components revealed no major effect on the functional properties of the white blood cells, and just a negligible fraction (∼10%) was damaged during this process. This approach does not need any capturing or targeting of CTCs from the blood as it is focused on perturbing the electrical function of negatively-charged tumor cells after being exposed to positive electrostatic charges. Taken together, continuous in-vivo deactivation of CTCs by PPECS with no requirement to complicated capturing protocols may improve the survival of cancer patients.

Construction of Bone Metastasis-Specific Regulation Network Based on Prognostic Stemness-Related Signatures in Breast Invasive Carcinoma

Background

Bone is the most common metastatic site of Breast invasive carcinoma (BRCA). In this study, the bone metastasis-specific regulation network of BRCA was constructed based on prognostic stemness-related signatures (PSRSs), their upstream transcription factors (TFs) and downstream pathways.

Methods

Clinical information and RNA-seq data of 1,080 primary BRCA samples (1,048 samples without bone metastasis and 32 samples with bone metastasis) were downloaded from The Cancer Genome Atlas (TCGA). The edgeR method was performed to identify differential expressed genes (DEGs). Next, mRNA stemness index (mRNAsi) was calculated by one-class logistic regression (OCLR). To analyze DEGs by classification, similar genes were integrated into the same module by weighted gene co-expression network analysis (WGCNA). Then, univariate and multivariate Cox proportional hazard regression were applied to find the PSRSs. Furthermore, PSRSs, 318 TFs obtained from Cistrome database and 50 hallmark pathways quantified by GSVA were integrated into co-expression analysis. Significant co-expression patterns were used to construct the bone metastasis-specific regulation network. Finally, spatial single-cell RNA-seq and chromatin immunoprecipitation sequence (ChIP-seq) data and multi-omics databases were applied to validate the key scientific hypothesis in the regulation network. Additionally, Connectivity Map (CMap) was utilized to select the potential inhibitors of bone metastasis-specific regulation network in BRCA.

Results

Based on edgeR and WGCNA method, 43 PSRSs were identified. In the bone metastasis-specific regulation network, MAF positively regulated CD248 (R = 0.435, P < 0.001), and hallmark apical junction was the potential pathway of CD248 (R = 0.353, P < 0.001). This regulatory pattern was supported by spatial single-cell RNA sequence, ChIP-seq data and multi-omics online databases. Additionally, alexidine was identified as the possible inhibitor for bone metastasis of BRCA by CMap analysis.

Conclusion

PSRSs played important roles in bone metastasis of BRCA, and the prognostic model based on PSRSs showed good performance. Especially, we proposed that CD248 was the most significant PSRS, which was positively regulated by MAF, influenced bone metastasis via apical junction pathway. And this axis might be inhibited by alexidine, which providing a potential treatment strategy for bone metastasis of BRCA.

Metastatic Dissemination Mimicked in a Multicellular Transwell Assay

Tumor progression and metastasis are multistep processes that are critically dependent on the interaction of metastasizing tumor cells with other cells of the local microenvironment. Mimicking the single steps of the metastatic cascade in vitro is therefore challenging when investigating not only tumor cell behavior alone but also cellular crosstalk between different cell populations. In particular, the crosstalk of tumor cells with pericytes and endothelial cells when accessing the bloodstream is of great importance for successful intravasation and metastatic dissemination. To examine metastatic intravasation and analyze the interaction of tumor cells with pericytes, which reside within the basement membrane and endothelial cells, aligning the vessel wall, we have designed a 3D in vitro transwell assay mimicking tumor cell intravasation into a vessel. Modifying the Boyden chamber transwell assay by seeding first an endothelial cell layer onto the transwell membrane and covering it with pericytes before adding the tumor cells allows us to investigate the role of pericytes and endothelial cells on tumor cell intravasation and at the same time to study their crosstalk at the molecular level and how this interaction influences tumor cell behavior. It further allows the manipulation of the system for proof-of-principle experimentation.

TGF-β-induced IGFBP-3 is a key paracrine factor from activated pericytes that promotes colorectal cancer cell migration and invasion

The crosstalk between cancer cells and the tumor microenvironment has been implicated in cancer progression and metastasis. Fibroblasts and immune cells are widely known to be attracted to and modified by cancer cells. However, the role of pericytes in the tumor microenvironment beyond endothelium stabilization is poorly understood. Here, we report that pericytes promoted colorectal cancer (CRC) cell proliferation, migration, invasion, stemness, and chemoresistance in vitro, as well as tumor growth in a xenograft CRC model. We demonstrate that coculture with human CRC cells induced broad transcriptomic changes in pericytes, mostly associated with TGF-β receptor activation. The prognostic value of a TGF-β response signature in pericytes was analyzed in CRC patient data sets. This signature was found to be a good predictor of CRC relapse. Moreover, in response to stimulation by CRC cells, pericytes expressed high levels of TGF-β1, initiating an autocrine activation loop. Investigation of secreted mediators and underlying molecular mechanisms revealed that IGFBP-3 is a key paracrine factor from activated pericytes affecting CRC cell migration and invasion. In summary, we demonstrate that the interplay between pericytes and CRC cells triggers a vicious cycle that stimulates pericyte cytokine secretion, in turn increasing CRC cell tumorigenic properties. Overall, we provide another example of how cancer cells can manipulate the tumor microenvironment.

Emerging Roles of Perivascular Mesenchymal Stem Cells in Synovial Joint Inflammation

In contrast to the significant advances in our understanding of the mesenchymal stem cell (MSC) populations in bone marrow (BM), little is known about the MSCs that are resident in the synovial joint and their possible roles in the tissue homeostasis, chronic inflammation as well as in repair. Neural crest is a transient embryonic structure, generating multipotential MSC capable of migrating along peripheral nerves and blood vessels to colonize most tissue types. In adult, these MSC can provide functional stromal support as a stem cell niche for lymphocyte progenitors for instance in the BM and the thymus. Critically, MSC have major immunoregulatory activities to control adverse inflammation and infection. These MSC will remain associated to vessels (perivascular (p) MSC) and their unique expression of markers such as myelin P0 and transcription factors (e.g. Gli1 and FoxD1) has been instrumental to develop transgenic mice to trace the fate of these cells in health and disease conditions. Intriguingly, recent investigations of chronic inflammatory diseases argue for an emerging role of pMSC in several pathological processes. In response to tissue injuries and with the release of host cell debris (e.g. alarmins), pMSC can detach from vessels and proliferate to give rise to either lipofibroblasts, osteoblasts involved in the ossification of arteries and myofibroblasts contributing to fibrosis. This review will discuss currently available data that suggest a role of pMSC in tissue homeostasis and pathogenesis of the synovial tissue and joints. Graphical abstract.

Interaction with CD68 and Regulation of GAS6 Expression by Endosialin in Fibroblasts Drives Recruitment and Polarization of Macrophages in Hepatocellular Carcinoma

Fibroblasts and macrophages play key roles in the development of hepatocellular carcinoma (HCC). However, cross-talk between these two kinds of cells has not been well studied. Endosialin (CD248/TEM1) is a transmembrane glycoprotein that is expressed in certain cancer cells, tumor stromal cells, and pericytes. In this study, we found that endosialin is mainly expressed in cancer-associated fibroblasts (CAF) in HCC and its expression inversely correlates with patient prognosis. Endosialin interacted with CD68 to recruit macrophages and regulated expression of GAS6 in CAFs to mediate M2 polarization of macrophages. The fully human antibody IgG78 bound glycosylated endosialin and induced its internalization in CAFs, thus weakening the cross-talk between CAFs and macrophages. In subcutaneous and orthotopic xenograft models of HCC in nude mice, treatment with IgG78 significantly inhibited tumor growth. These results indicate that endosialin-positive CAFs promote HCC progression and highlight IgG78 as a promising therapeutic candidate for HCC treatment. SIGNIFICANCE: These findings highlight CAF-expressed endosialin as a primary regulator of macrophage recruitment and polarization and demonstrate endosialin inhibition as a potential treatment strategy for HCC. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/18/3892/F1.large.jpg.

The Tumor Vessel Targeting Strategy: A Double-Edged Sword in Tumor Metastasis

Tumor vessels provide essential paths for tumor cells to escape from the primary tumor and form metastatic foci in distant organs. The vessel targeting strategy has been widely used as an important clinical cancer chemotherapeutic strategy for patients with metastatic tumors. Our review introduces the contribution of angiogenesis to tumor metastasis and summarizes the application of Food and Drug Administration (FDA)-approved vessel targeting drugs for metastatic tumors. We recommend the application and mechanisms of vascular targeting drugs for inhibiting tumor metastasis and discuss the risk and corresponding countermeasures after vessel targeting treatment.

Tumor angiogenesis: causes, consequences, challenges and opportunities

Tumor vascularization occurs through several distinct biological processes, which not only vary between tumor type and anatomic location, but also occur simultaneously within the same cancer tissue. These processes are orchestrated by a range of secreted factors and signaling pathways and can involve participation of non-endothelial cells, such as progenitors or cancer stem cells. Anti-angiogenic therapies using either antibodies or tyrosine kinase inhibitors have been approved to treat several types of cancer. However, the benefit of treatment has so far been modest, some patients not responding at all and others acquiring resistance. It is becoming increasingly clear that blocking tumors from accessing the circulation is not an easy task to accomplish. Tumor vessel functionality and gene expression often differ vastly when comparing different cancer subtypes, and vessel phenotype can be markedly heterogeneous within a single tumor. Here, we summarize the current understanding of cellular and molecular mechanisms involved in tumor angiogenesis and discuss challenges and opportunities associated with vascular targeting.

Normalization of Tumor Vasculature by Oxygen Microbubbles with Ultrasound

Tumor microenvironment influences the efficacy of anti-cancer therapies. The dysfunctional tumor vasculature limits the efficiency of oxygenation and drug delivery to reduce treatment outcome. A concept of tumor vascular normalization (VN), which inhibits angiogenesis to improve vessel maturity, blood perfusion, and oxygenation, has been demonstrated under the anti-angiogenic therapy. The efficiency of drug delivery and penetration is increased by enhancing perfusion and reducing interstitial fluid pressure during the time window of VN. However, anti-angiogenic agents only induce transient VN and then prune vessels to aggravate tumor hypoxia. To repair tumor vessels without altering vessel density, we proposed to induce tumor VN by local oxygen release via oxygen microbubbles with ultrasound. With tumor perfusion enhancement under ultrasound contrast imaging tracing, the time window of VN was defined as 2-8 days after a single oxygen microbubble treatment. The enhanced tumor oxygenation after oxygen microbubble treatment inhibited hypoxia inducible factor-1 alpha (HIF-1α)/vascular endothelial growth factor (VEGF) pathway to improve the morphology and function of tumor vasculature. The pericyte coverage and Hoechst penetration of tumor vessels increased without any changes to the vessel density. Finally, the intratumoral accumulation of anti-cancer drug doxorubicin could be increased 3-4 folds during tumor VN. These findings demonstrate that regulating tumor oxygenation by oxygen microbubbles could normalize dysfunctional vessels to enhance vascular maturity, blood perfusion, and drug penetration. Furthermore, ultrasound perfusion imaging provides a simple and non-invasive way to detect the VN time window, which increases the feasibility of VN in clinical cancer applications.

Pericytes in Metastasis

Pericytes have long been known to contribute indirectly to tumour growth by regulating angiogenesis. Thus, remodelling tumour blood vessels to maintain blood supply is critical for continued tumour growth. A role for pericytes in restricting leakage of tumour cells through blood vessels has also become evident given that adequate pericyte coverage of these blood vessels is critical for maintaining vascular permeability. Interestingly, the relocation of pericytes from blood vessels to the tumour microenvironment results in the emergence of different properties in these cells that actively promote tumour growth and metastasis-functions not associated with their well-studied role in vascular stability and permeability. These form the focus of this review.

C-type lectin domain group 14 proteins in vascular biology, cancer and inflammation

The C‐type lectin domain (CTLD) group 14 family of transmembrane glycoproteins consist of thrombomodulin, CD93, CLEC14A and CD248 (endosialin or tumour endothelial marker‐1). These cell surface proteins exhibit similar ectodomain architecture and yet mediate a diverse range of cellular functions, including but not restricted to angiogenesis, inflammation and cell adhesion. Thrombomodulin, CD93 and CLEC14A can be expressed by endothelial cells, whereas CD248 is expressed by vasculature associated pericytes, activated fibroblasts and tumour cells among other cell types. In this article, we review the current literature of these family members including their expression profiles, interacting partners, as well as established and speculated functions. We focus primarily on their roles in the vasculature and inflammation as well as their contributions to tumour immunology. The CTLD group 14 family shares several characteristic features including their ability to be proteolytically cleaved and engagement of some shared extracellular matrix ligands. Each family member has strong links to tumour development and in particular CD93, CLEC14A and CD248 have been proposed as attractive candidate targets for cancer therapy.

Linking myofibroblast generation and microvascular alteration: The role of CD248 from pathogenesis to therapeutic target (Review)

Fibrosis is characterized by excessive extracellular matrix (ECM) deposition, and is the pathological outcome of tissue injury in a number of disorders. Accumulation of the ECM may disrupt the structure and function of native tissues and organs, including the lungs, heart, liver and skin, resulting in significant morbidity and mortality. On this basis, multiple lines of evidence have focused on the molecular pathways and cellular mechanisms involved in fibrosis, which has led to the development of novel antifibrotic therapies. CD248 is one of several proteins identified to be localized to the stromal compartment in cancers and fibroproliferative disease, and may serve a key role in myofibroblast generation and accumulation. Numerous studies have supported the contribution of CD248 to tumour growth and fibrosis, stimulating interest in this molecule as a therapeutic target. In addition, it has been revealed that CD248 may be involved in pathological angiogenesis. The present review describes the current understanding of the structure and function of CD248 during angiogenesis and fibrosis, supporting the hypothesis that blocking CD248 signalling may prevent both myofibroblast generation and microvascular alterations during tissue fibrosis.

Epithelial Mesenchymal Transition in Tumor Metastasis

Metastasis is the major cause of cancer-related deaths; therefore, the prevention and treatment of metastasis are fundamental to improving clinical outcomes. Epithelial mesenchymal transition (EMT), an evolutionarily conserved developmental program, has been implicated in carcinogenesis and confers metastatic properties upon cancer cells by enhancing mobility, invasion, and resistance to apoptotic stimuli. Furthermore, EMT-derived tumor cells acquire stem cell properties and exhibit marked therapeutic resistance. Given these attributes, the complex biological process of EMT has been heralded as a key hallmark of carcinogenesis, and targeting EMT pathways constitutes an attractive strategy for cancer treatment. However, demonstrating the necessity of EMT for metastasis in vivo has been technically challenging, and recent efforts to demonstrate a functional contribution of EMT to metastasis have yielded unexpected results. Therefore, determining the functional role of EMT in metastasis remains an area of active investigation. Studies using improved lineage tracing systems, dynamic in vivo imaging, and clinically relevant in vivo models have the potential to uncover the direct link between EMT and metastasis. This review focuses primarily on recent advances in and emerging concepts of the biology of EMT in metastasis in vivo and discusses future directions in the context of novel diagnostic and therapeutic opportunities.

CD248: A therapeutic target in cancer and fibrotic diseases

CD248/endosialin/TEM1 is a type 1 transmembrane glycoprotein found on the plasma membrane of activated mesenchymal cells. CD248 functions during embryo development and is either not expressed or found at very low levels in adult tissues. CD248 is expressed at high levels by malignant sarcoma cells, by the pericyte component of tumor vasculature and by mesenchymal cells in some fibrotic diseases. CD248 is being targeted by several experimental therapeutics including antibodies, antibody drug conjugates, as an antigen for CART cells and in therapeutic vaccines. Although the function of CD248 has yet to be fully elucidated, this protein is a potential broad scope therapeutic target.

Pericytes in Breast Cancer

Breast cancer is a heterogeneous disease driven not only by evolutionally diverse cancer cell themselves but also by highly dynamic microenvironment. At the center of the tumor microenvironment, tumor vasculature plays multiple roles from supporting tumor growth to providing a route for metastasis to the distant organ sites. Blood vessels in breast cancer present with perfusion defects associated with vessel dilation, tortuosity, and poor perivascular coverage (Li et al., Ultrasound Med 32:1145-1155, 2013; Eberhard et al., Cancer Res 60:1388-1393, 2000; Cooke et al., Cancer Cell 21:66-81, 2012). Such abnormal vascular system is partly due to the morphological and molecular alteration of pericytes that is accompanied by a significant heterogeneity within the populations (Kim et al., JCI Insight 1:e90733, 2016). While pericytes are implicated for their controversial roles in breast cancer metastasis (Cooke et al., Cancer Cell 21:66-81, 2012; Gerhardt and Semb, J Mol Med (Berl) 86:135-144, 2008; Keskin et al., Cell Rep 10:1066-1081, 2015; Meng et al., Future Oncol 11:169-179, 2015; Xian et al., J Clin Invest 116:642-651, 2006), the impact of their heterogeneity on breast cancer progression, metastasis, intratumoral immunity, and response to chemotherapy are largely unknown. Due to the complexity of angiogenic programs of breast cancer, the anti-angiogenic or anti-vascular treatment has been mostly unsuccessful (Tolaney et al., Proc Natl Acad Sci U S A 112:14325-14330, 2015; Mackey et al., Cancer Treat Rev 38:673-688, 2012; Sledge, J Clin Oncol 33:133-135, 2015) and requires much in-depth knowledge on different components of tumor microenvironment and how these stromal cells are interacting and communicating to each other. Therefore, understanding pericyte heterogeneity and their differential functional contribution will shed light on new potential approaches to treat breast cancer.

Microvascular Mural Cells in Cancer

Microvascular mural cells (MMCs) are important regulators of tumor vessel properties, such as endothelial cell differentiation and vessel permeability, and are recognized as modulators of tumor angiogenesis and growth. Emerging experimental studies suggest impact of MMCs on additional aspects of tumor biology, exerted by functionally distinct subsets. These have been shown to control metastasis both in primary tumors and in the premetastatic niche. Other studies link marker-defined MMCs to tumor immune surveillance and drug sensitivity. In parallel, recent efforts to profile MMCs in clinical samples are confirming the existence of clinically relevant marker-defined MMC subsets which show marker- and tumor-type- specific associations with prognosis and response to treatment. Collectively, findings encourage to continued analyses of MMC subsets as candidate biomarkers and drug targets.

FAP Delineates Heterogeneous and Functionally Divergent Stromal Cells in Immune-Excluded Breast Tumors

Cancer-associated fibroblasts (CAFs) are generally associated with poor clinical outcome. CAFs support tumor growth in a variety of ways and can suppress antitumor immunity and response to immunotherapy. However, a precise understanding of CAF contributions to tumor growth and therapeutic response is lacking. Discrepancies in this field of study may stem from heterogeneity in the composition and function of fibroblasts in the tumor microenvironment. Furthermore, it remains unclear whether CAFs directly interact with and suppress T cells. Here, mouse and human breast tumors were used to examine stromal cells expressing fibroblast activation protein (FAP), a surface marker for CAFs. Two discrete populations of FAP⁺ mesenchymal cells were identified on the basis of podoplanin (PDPN) expression: a FAP⁺PDPN⁺ population of CAFs and a FAP⁺PDPN^- population of cancer-associated pericytes (CAPs). Although both subsets expressed extracellular matrix molecules, the CAF transcriptome was enriched in genes associated with TGFβ signaling and fibrosis compared with CAPs. In addition, CAFs were enriched at the outer edge of the tumor, in close contact with T cells, whereas CAPs were localized around vessels. Finally, FAP⁺PDPN⁺ CAFs suppressed the proliferation of T cells in a nitric oxide-dependent manner, whereas FAP⁺PDPN^- pericytes were not immunosuppressive. Collectively, these findings demonstrate that breast tumors contain multiple populations of FAP-expressing stromal cells of dichotomous function, phenotype, and location.

EACR-MRS conference on Seed and Soil: In Vivo Models of Metastasis

New experimental tools are urgently required to better understand the metastatic process. The importance of such tools is underscored by the fact that many anti-cancer therapies are generally ineffective against established metastases. This makes a major contribution to the fact that metastatic spread is responsible for over 90% of cancer patient deaths. It was therefore timely that the recent "Seed and Soil: In Vivo Models of Metastasis" conference held in Berlin, Germany (27-29 of November 2017) aimed to give an in-depth overview of the latest research models and tools for studying metastasis, and to showcase recent findings from world-leading metastasis researchers. This Meeting Report summarises the major themes of this ground-breaking conference.

Hepatic stellate cells limit hepatocellular carcinoma progression through the orphan receptor endosialin

Hepatocellular carcinoma (HCC) is among the most common and deadliest cancers worldwide. A major contributor to HCC progression is the cross talk between tumor cells and the surrounding stroma including activated hepatic stellate cells (HSC). Activation of HSC during liver damage leads to upregulation of the orphan receptor endosialin (CD248), which contributes to regulating the balance of liver regeneration and fibrosis. Based on the established role of endosialin in regulating HSC/hepatocyte cross talk, we hypothesized that HSC‐expressed endosialin might similarly affect cell proliferation during hepatocarcinogenesis. Indeed, the histological analysis of human HCC samples revealed an inverse correlation between tumor cell proliferation and stromal endosialin expression. Correspondingly, global genetic inactivation of endosialin resulted in accelerated tumor growth in an inducible mouse HCC model. A candidate‐based screen of tumor lysates and differential protein arrays of cultured HSC identified several established hepatotropic cytokines, including IGF2, RBP4, DKK1, and CCL5 as being negatively regulated by endosialin. Taken together, the experiments identify endosialin‐expressing HSC as a negative regulator of HCC progression.

Transcriptomic Analysis Reveals Novel Mechanisms Mediating Islet Dysfunction in the Intrauterine Growth-Restricted Rat

Intrauterine growth restriction (IUGR) increases the risk of type 2 diabetes developing in adulthood. In previous studies that used bilateral uterine artery ligation in a rat model of IUGR, age-associated decline in glucose homeostasis and islet function was revealed. To elucidate mechanisms contributing to IUGR pathogenesis, the islet transcriptome was sequenced from 2-week-old rats, when in vivo glucose tolerance is mildly impaired, and at 10 weeks of age, when rats are hyperglycemic and have reduced β-cell mass. RNA sequencing and functional annotation with Ingenuity Pathway Analysis revealed temporal changes in IUGR islets. For instance, gene expression involving amino acid metabolism was significantly reduced primarily at 2 weeks of age, but ion channel expression, specifically that involved in cell-volume regulation, was more disrupted in adult IUGR islets. Additionally, we observed alterations in the microenvironment of IUGR islets with extracellular matrix genes being significantly increased at 2 weeks of age and significantly decreased at 10 weeks. Specifically, hyaluronan synthase 2 expression and hyaluronan staining were increased in IUGR islets at 2 weeks of age (P < 0.05). Mesenchymal stromal cell-derived factors that have been shown to preserve islet allograft function, such as Anxa1, Cxcl12, and others, also were increased at 2 weeks and decreased in adult islets. Finally, comparisons of differentially expressed genes with those of type 2 diabetic human islets support a role for these pathways in human patients with diabetes. Together, these data point to new mechanisms in the pathogenesis of IUGR-mediated islet dysfunction in type 2 diabetes.

Generation and characterisation of two D2A1 mammary cancer sublines to model spontaneous and experimental metastasis in a syngeneic BALB/c host

Studying the complex mechanisms underlying breast cancer metastasis and therapy response necessitates relevant in vivo models, particularly syngeneic models with an intact immune system. Two syngeneic spontaneously metastatic sublines, D2A1-m1 and D2A1-m2, were generated from the poorly metastasising BALB/c-derived D2A1 cell line by serial in vivo passaging. In vivo and in vitro analyses revealed distinct and shared characteristics of the metastatic D2A1-m1 and D2A1-m2 sublines. In particular, D2A1-m1 cells are more aggressive in experimental metastasis assays, while D2A1-m2 cells are more efficient at disseminating from the primary tumour in spontaneous metastasis assays. Surprisingly, classical metastasis-associated in vitro phenotypes, such as enhanced proliferation, migration and invasion, are reduced in the sublines compared to the parental cell line. Further, evasion of immune control cannot fully explain their enhanced metastatic properties. By contrast, both sublines show increased resistance to apoptosis when cultured in non-adherent conditions and, for the D2A1-m2 subline, increased 3D tumour spheroid growth. Moreover, the enhanced spontaneous metastatic phenotype of the D2A1-m2 subline is associated with an increased ability to recruit an activated tumour stroma. The metastatic D2A1-m1 and D2A1-m2 cell lines provide additional syngeneic models for investigating the different steps of the metastatic cascade and thereby represent valuable tools for breast cancer researchers. Finally, this study highlights that morphology and cell behaviour in 2D cell-based assays cannot be used as a reliable predictor of metastatic behaviour in vivo.

Summary: We describe two D2A1 mouse mammary cancer sublines with enhanced spontaneous metastasis in a syngeneic host, and highlight the limitations of in vitro assays to predict in vivo metastatic behaviour.

Physiologically activated mammary fibroblasts promote postpartum mammary cancer

Women diagnosed with breast cancer within 5 years of childbirth have poorer prognosis than nulliparous or pregnant women. Weaning-induced breast involution is implicated, as the collagen-rich, immunosuppressive microenvironment of the involuting mammary gland is tumor promotional in mice. To investigate the role of mammary fibroblasts, isolated mammary PDGFRα⁺ cells from nulliparous and postweaning mice were assessed for activation phenotype and protumorigenic function. Fibroblast activation during involution was evident by increased expression of fibrillar collagens, lysyl oxidase, Tgfb1, and Cxcl12 genes. The ability of mammary tumors to grow in an isogenic, orthotopic transplant model was increased when tumor cells were coinjected with involution-derived compared with nulliparous-derived mammary fibroblasts. Mammary tumors in the involution-fibroblast group had increased Ly6C⁺ monocytes at the tumor border, and decreased CD8⁺ T cell infiltration and tumor cell death. Ibuprofen treatment suppressed involution-fibroblast activation and tumor promotional capacity, concurrent with decreases in tumor Ly6C⁺ monocytes, and increases in intratumoral CD8⁺ T cell infiltration, granzyme levels, and tumor cell death. In total, our data identify a COX/prostaglandin E2 (PGE2)-dependent activated mammary fibroblast within the involuting mammary gland that displays protumorigenic, immunosuppressive activity, identifying fibroblasts as potential targets for the prevention and treatment of postpartum breast cancer.

Macrophage-dependent tumor cell transendothelial migration is mediated by Notch1/MenaINV-initiated invadopodium formation

The process of intravasation involving transendothelial migration is a key step in metastatic spread. How the triple cell complex composed of a macrophage, Mena over-expressing tumor cell and endothelial cell, called the tumor microenvironment of metastasis (TMEM), facilitates tumor cell transendothelial migration is not completely understood. Previous work has shown that the physical contact between a macrophage and tumor cell results in the formation of invadopodia, actin-rich matrix degrading protrusions, important for tumor cell invasion and transendothelial migration and tumor cell dissemination. Herein, we show that the macrophage-induced invadopodium is formed through a Notch1/Mena^INV signaling pathway in the tumor cell upon macrophage contact. This heterotypic tumor cell – macrophage interaction results in the upregulation of Mena^INV through the activation of MENA transcription. Notch1 and Mena^INV expression are required for tumor cell transendothelial migration, a necessary step during intravasation. Inhibition of the Notch signaling pathway blocked macrophage-induced invadopodium formation in vitro and the dissemination of tumor cells from the primary tumor in vivo. Our findings indicate a novel role for Notch1 signaling in the regulation of Mena^INV expression and transendothelial migration and provide mechanistic information essential to the use of therapeutic inhibitors of metastasis.