A distinct macrophage population mediates metastatic breast cancer cell extravasation, establishment and growth

The role of the microenvironment in mammary gland development and cancer

The mammary gland is composed of a diverse array of cell types that form intricate interaction networks essential for its normal development and physiologic function. Abnormalities in these interactions play an important role throughout different stages of tumorigenesis. Branching ducts and alveoli are lined by an inner layer of secretory luminal epithelial cells that produce milk during lactation and are surrounded by contractile myoepithelial cells and basement membrane. The surrounding stroma comprised of extracellular matrix and various cell types including fibroblasts, endothelial cells, and infiltrating leukocytes not only provides a scaffold for the organ, but also regulates mammary epithelial cell function via paracrine, physical, and hormonal interactions. With rare exceptions breast tumors initiate in the epithelial compartment and in their initial phases are confined to the ducts but this barrier brakes down with invasive progression because of a combination of signals emitted by tumor epithelial and various stromal cells. In this article, we overview the importance of cellular interactions and microenvironmental signals in mammary gland development and cancer.

NF-κB as a central regulator of macrophage function in tumors

TAMs are usually abundant in the tumor microenvironment and are now known to play an essential role in tumor progression. For example, TAMs influence many aspects of tumorigenesis, such as the growth, survival, invasion, and metastasis of tumor cells, tumor angiogenesis, and the suppression of other tumor-infiltrating immune effector cells. The molecular pathways that regulate these tumor-promoting functions of TAMs are currently under intense investigation. Several recent studies about transgenic murine tumor models have shown that the transcription factor NF-κB is a key player in tumor progression with distinct roles in regulating the functions of macrophages and tumor cells in malignant tumors. Here, we outline the evidence for classical and noncanonical NF-κB signaling pathways driving the tumor-promoting repertoire of TAMs.

An antimicrobial peptide regulates tumor-associated macrophage trafficking via the chemokine receptor CCR2, a model for tumorigenesis

BACKGROUND

Tumor-associated macrophages (TAMs) constitute a significant part of infiltrating inflammatory cells that are frequently correlated with progression and poor prognosis of a variety of cancers. Tumor cell-produced human beta-defensin-3 (hBD-3) has been associated with TAM trafficking in oral cancer; however, its involvement in tumor-related inflammatory processes remains largely unknown.

METHODOLOGY

The relationship between hBD-3, monocyte chemoattractant protein-1 (MCP-1), TAMs, and CCR2 was examined using immunofluorescence microscopy in normal and oral carcinoma in situ biopsy specimens. The ability of hBD-3 to chemoattract host macrophages in vivo using a nude mouse model and analysis of hBD-3 on monocytic cell migration in vitro, applying a cross-desensitization strategy of CCR2 and its pharmacological inhibitor (RS102895), respectively, was also carried out.

CONCLUSIONS/FINDINGS

MCP-1, the most frequently expressed tumor cell-associated chemokine, was not produced by tumor cells nor correlated with the recruitment of macrophages in oral carcinoma in situ lesions. However, hBD-3 was associated with macrophage recruitment in these lesions and hBD-3-expressing tumorigenic cells induced massive tumor infiltration of host macrophages in nude mice. HBD-3 stimulated the expression of tumor-promoting cytokines, including interleukin-1alpha (IL-1alpha), IL-6, IL-8, CCL18, and tumor necrosis factor-alpha (TNF-alpha) in macrophages derived from human peripheral blood monocytes. Monocytic cell migration in response to hBD-3 was inhibited by cross-desensitization with MCP-1 and the specific CCR2 inhibitor, RS102895, suggesting that CCR2 mediates monocyte/macrophage migration in response to hBD-3. Collectively, these results indicate that hBD-3 utilizes CCR2 to regulate monocyte/macrophage trafficking and may act as a tumor cell-produced chemoattractant to recruit TAMs. This novel mechanism is the first evidence of an hBD molecule orchestrating an in vivo outcome and demonstrates the importance of the innate immune system in the development of tumors.

Tumor-conditioned macrophages secrete migration-stimulating factor: a new marker for M2-polarization, influencing tumor cell motility

Tumor-associated macrophages (TAMs) are key orchestrators of the tumor microenvironment directly affecting neoplastic cell growth, neoangiogenesis, and extracellular matrix remodeling. In turn, the tumor milieu strongly influences maturation of TAMs and shapes several of their features. To address the early macrophage (M) differentiation phase in a malignant context, we mimicked a tumor microenvironment by in vitro coculturing human blood monocytes with conditioned media from different cancer cell lines. Only 2 out of 16 tumor cell lines induced M differentiation due to secreted M-CSF isoforms, including high molecular mass species. A global gene profiling of tumor-conditioned M was performed. Comparison with other datasets (polarized M1-M, M2-M, and TAMs isolated from human tumors) highlighted the upregulation of several genes also shared by TAM and M2-polarized M. The most expressed genes were selenoprotein 1, osteoactivin, osteopontin, and, interestingly, migration-stimulating factor (MSF), a poorly studied oncofoetal isoform of fibronectin. MSF (present in fetal/cancer epithelial and stromal cells but not in healthy tissues) was never identified in M. MSF production was confirmed by immunohistochemistry in human TAMs. MSF was induced by M-CSF, IL-4, and TGFbeta but not by proinflammatory stimuli. RNA and protein analysis clearly demonstrated that it is specifically associated with the M2 polarization of M. Tumor-conditioned M-derived MSFs strongly stimulated tumor cell migration, thus contributing to the motile phenotype of neoplastic cells. In conclusion, MSF is a new molecule associated with the M2 polarization of M and expressed by TAMs. Its biological function may contribute to M-mediated promotion of cancer cell invasion and metastasis.

IL-10 regulation of macrophage VEGF production is dependent on macrophage polarisation and hypoxia

Vascular endothelial growth factor A (VEGF) is critical for vascular remodelling during tissue repair subsequent to inflammation or injury, but under pathological conditions, VEGF induces tissue damaging angiogenesis. Macrophages generate VEGF that supports angiogenesis, when they adapt to their environment and respond with a co-ordinated set of signals to promote or resolve inflammation. Depending on the stimulus, the phenotype of macrophage activation is broadly classified into M1 (NOS2(+)) and M2 (arginase-1(+)). In recent studies, IL-10, an anti-inflammatory cytokine that suppresses the M1 phenotype, has been shown to dampen the angiogenic switch and subsequent neovascularisation. However, as we show here, these effects are context dependent. In this study, we have demonstrated that IL-10 inhibits M1 bone marrow-derived macrophages (BMDMs) VEGF, stimulated by LPS/CGS21680 (adenosine A2A receptor agonist), but does not prevent VEGF production from M2 macrophages stimulated with prostaglandin E2 (PGE2). Furthermore, we show that hypoxic-conditioned BMDM generated VEGF was maintained in the presence of IL-10, but was suppressed when concomitantly stimulated with IFN-gamma. Finally, LPS/PGE2 generated an arginase-1(+) M2 macrophage that in addition to generating VEGF produced significant quantities of IL-10. Under these conditions, neither in IL-10 deficient macrophages nor following IL-10 neutralization was VEGF production affected. Our results indicate IL-10 suppressed M1 but not M2 derived VEGF, and that activation signals determined the influence of IL-10 on VEGF production. Consequently, therapies to suppress macrophage activation that as a result generate IL-10, or utilising IL-10 as a potential anti-angiogenic therapy, may result in a paradoxical support of neovascularisation and thus on-going tissue damage or aberrant repair.

VEGF and pleiotrophin modulate the immune profile of breast cancer

Angiogenesis, the sprouting of the existing vascular network to form new vessels, is required for the growth of solid tumors. For this reason, the primary stimulant of angiogenesis, vascular endothelial growth factor-A (VEGF), is an attractive target for tumor therapy. In fact, there are currently numerous anti-VEGF therapies in clinical development for the treatment of various cancers, including breast cancer. VEGF signals through two primary VEGF receptors, VEGFR1 and VEGFR2. VEGFR2 is the primary angiogenic receptor, and VEGFR1 has been implicated in macrophage chemotaxis and tumor cell survival and invasion. It has only been appreciated recently that the VEGFRs are expressed not only on endothelial cells and tumor cells but also on many host immune cells. Therefore, to better understand the effects of anti-VEGF therapy it is important to consider the effects of VEGF on all cells in the tumor microenvironment, including immune cells. Bevacizumab (Avastin^®, Genetech), which binds VEGF and inhibits interaction with VEGFR1 and VEGFR2, was approved for the treatment of metastatic HER2/NEU-negative breast cancer in 2008, however, the majority of human mammary tumors are either innately resistant or will acquire resistance to anti-VEGF therapy. This suggests that these tumors activate alternate angiogenesis pathways. Pleiotrophin (PTN) is an important angiogenic cytokine in breast cancer and is expressed at high levels in approximately 60% of human breast tumors. PTN functions as an angiogenic factor and promotes remodeling of the tumor microenvironment as well as epithelial-mesenchymal transition (EMT). In addition, PTN can have profound effects on macrophage phenotype. The present review focuses on the functions of VEGF and PTN on immune cell infiltration and function in breast cancer. Furthermore, we will discuss how anti-VEGF therapy modulates the immune cell profile.

Selectins promote tumor metastasis

Cancer metastasis is facilitated by cell-cell interactions between cancer cells and endothelial cells in distant tissues. In addition, cancer cell interactions with platelets and leukocytes contribute to cancer cell adhesion, extravasation, and the establishment of metastatic lesions. Selectins are carbohydrate-binding molecules that bind to sialylated, fucosylated glycan structures, and are found on endothelial cells, platelets and leukocytes. There are three members of the selectin family: P-selectin expressed on activated platelets and endothelial cells, L-selectin present on leukocytes and E-selectin expressed on activated endothelial cells. Besides the accepted roles of selectins in physiological processes, such as inflammation, immune response and hemostasis, there is accumulating evidence for the potential of selectins to contribute to a number of pathophysiological processes, including cancer metastasis. Cancer cell interactions with selectins are possible due to a frequent presence of carbohydrate determinants--selectin ligands on the cell surface of tumor cells from various type of cancer. The degree of selectin ligand expression by cancer cells is well correlated with metastasis and poor prognosis for cancer patients. Initial adhesion events of cancer cells facilitated by selectins result in activation of integrins, release of chemokines and are possibly associated with the formation of permissive metastatic microenvironment. While E-selectin has been evaluated as one of the initiating adhesion events during metastasis, it is becoming apparent that P-selectin and L-selectin-mediated interactions significantly contribute to this process as well. In this review we discuss the current evidence for selectins as potential facilitators of metastasis.

Macrophage diversity enhances tumor progression and metastasis

There is persuasive clinical and experimental evidence that macrophages promote cancer initiation and malignant progression. During tumor initiation, they create an inflammatory environment that is mutagenic and promotes growth. As tumors progress to malignancy, macrophages stimulate angiogenesis, enhance tumor cell migration and invasion, and suppress antitumor immunity. At metastatic sites, macrophages prepare the target tissue for arrival of tumor cells, and then a different subpopulation of macrophages promotes tumor cell extravasation, survival, and subsequent growth. Specialized subpopulations of macrophages may represent important new therapeutic targets.

Shigella mediated depletion of macrophages in a murine breast cancer model is associated with tumor regression

A tumor promoting role of macrophages has been described for a transgenic murine breast cancer model. In this model tumor-associated macrophages (TAMs) represent a major component of the leukocytic infiltrate and are associated with tumor progression. Shigella flexneri is a bacterial pathogen known to specificly induce apotosis in macrophages. To evaluate whether Shigella-induced removal of macrophages may be sufficient for achieving tumor regression we have developed an attenuated strain of S. flexneri (M90TΔaroA) and infected tumor bearing mice. Two mouse models were employed, xenotransplantation of a murine breast cancer cell line and spontanous breast cancer development in MMTV-HER2 transgenic mice. Quantitative analysis of bacterial tumor targeting demonstrated that attenuated, invasive Shigella flexneri primarily infected TAMs after systemic administration. A single i.v. injection of invasive M90TΔaroA resulted in caspase-1 dependent apoptosis of TAMs followed by a 74% reduction in tumors of transgenic MMTV-HER-2 mice 7 days post infection. TAM depletion was sustained and associated with complete tumor regression.

These data support TAMs as useful targets for antitumor therapy and highlight attenuated bacterial pathogens as potential tools.

Selectins as mediators of lung metastasis

Lung metastasis remains the major cause of cancer related mortality in patients with breast, gastrointestinal, sarcoma, melanoma and kidney cancer. Here we characterize the expression of selectins during metastatic lung colonization and analyzed their function in the formation of pulmonary metastasis. E-selectin, together with VCAM-1, were detected 6 h after the microvascular arrest of tumor cells indicating an inflammatory activation of the local endothelial cells. No E-selectin expression was detected in pre-metastatic lungs of mice carrying primary tumors. P- and L-selectin were present during initiating steps of lung colonization and correlated with the recruitment of platelets and leukocytes to metastatic tumor cells. Experimental metastasis was significantly reduced in the absence of P- or L-selectin while no attenuation of metastasis was observed in E-selectin-deficient mice. Collectively, selectins are upregulated within the metastatic microenvironment of tumor cells and the formation of a permissive metastatic microenvironment is facilitated by P- and L-selectin mediated interactions between tumor cells and blood components. E-selectin does not affect metastatic initiation in the lung tissue and its expression rather indicates a local activation of lung microvascular endothelial cells.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s12307-010-0043-6) contains supplementary material, which is available to authorized users.

Elusive identities and overlapping phenotypes of proangiogenic myeloid cells in tumors

It is now established that bone marrow-derived myeloid cells regulate tumor angiogenesis. This was originally inferred from studies of human tumor biopsies in which a positive correlation was seen between the number of tumor-infiltrating myeloid cells, such as macrophages and neutrophils, and tumor microvessel density. However, unequivocal evidence was only provided once mouse models were used to examine the effects on tumor angiogenesis by genetically or pharmacologically targeting myeloid cells. Since then, identifying the exact myeloid cell types involved in this process has proved challenging because of myeloid cell heterogeneity and the expression of overlapping phenotypic markers in tumors. As a result, investigators often simply refer to them now as "bone marrow-derived myeloid cells." Here we review the findings of various attempts to phenotype the myeloid cells involved and discuss the therapeutic implications of correctly identifying-and thus being able to target-this proangiogenic force in tumors.

Nuclear factor-kappaB and tumor-associated macrophages

Tumor-associated macrophages (TAM) have been linked with the progression of cancer by favoring tumor angiogenesis, growth, and metastasis. The precise mechanisms that maintain the protumor phenotype of TAM are poorly understood, but recent research has highlighted a number of signaling pathways that are important in TAM phenotype and function. Nuclear factor-kappaB (NF-kappaB) is considered the master regulator of inflammatory and immune responses. Recently several genetic studies have indicated NF-kappaB is an important pathway in TAM for the integration of signals from the tumor microenvironment that promote carcinogenesis. This review will focus on the role of NF-kappaB in TAM and the potential of targeting this pathway as a novel therapeutic strategy against cancer.

Macrophages and NF-κB in cancer

Macrophages are tissue resident phagocytes with important roles in development, wound healing, and inflammation. There is enormous heterogeneity in macrophage phenotype, from 'classically' activated macrophages that have important roles in inflammation and innate immunity, to 'alternative' macrophage activation that is associated with wound healing, angiogenesis, and immune-suppression. Most, if not all, solid tumors have a significant macrophage population, clinical and experimental evidence suggests tumor-associated macrophages (TAM) are linked with tumor progression. The trophic functions of TAM are associated with increased angiogenesis, malignant cell invasion, and metastasis. NF-κB is s central regulator of inflammation and NF-κB activation particularly in TAM is linked with promotion of carcinogenesis in various experimental models of inflammation-associated cancer. NF-κB activation in TAM has, therefore, been suggested to represent a molecular link between inflammation and cancer. However, TAM frequently display an anti-inflammatory phenotype linked with immune-suppression that is not easily reconciled with a pro-inflammatory function for NF-κB in TAM. Here, I review the form and function of TAM and discuss the role of NF-κB activation in TAM in carcinogenesis.

Heparin as an inhibitor of cancer progression

Heparin is frequently used in the treatment of cancer-associated thromboembolism. Accumulating clinical evidence indicates that cancer patients treated with unfractionated and low-molecular weight heparin (LMWH) survive longer than patients treated by other anticoagulants, especially patients in the early stage of the disease. Experimental analysis from a number of animal models constantly provides evidence for the ability of heparin to attenuate metastasis. The non-anticoagulant activity of heparin on metastasis includes the ability to inhibit cell-cell-interaction through blocking of P- and L-selectin, to inhibit extracellular matrix protease heparanase, and to inhibit angiogenesis. This chapter summarizes current experimental evidence on the biology of heparin during cancer progression, with the focus on potential mechanism of heparin antimetastatic activity.

Gene expression analysis of macrophages that facilitate tumor invasion supports a role for Wnt-signaling in mediating their activity in primary mammary tumors

The tumor microenvironment modifies the malignancy of tumors. In solid tumors, this environment is populated by many macrophages that, in genetic studies that depleted these cells from mouse models of breast cancer, were shown to promote tumor progression to malignancy and increase metastatic potential. Mechanistic studies showed that these tumor-promoting effects of macrophages are through the stimulation of tumor cell migration, invasion, intravasation, and enhancement of angiogenesis. Using an in vivo invasion assay, it was demonstrated that invasive carcinoma cells are a unique subpopulation of tumor cells whose invasion and chemotaxis is dependent on the comigration of tumor-associated macrophages (TAMs) with obligate reciprocal signaling through an epidermal growth factor-CSF-1 paracrine loop. In this study, these invasion-promoting macrophages were isolated and subjected to analysis of their transcriptome in comparison with TAMs isolated indiscriminately to function using established macrophage markers. Unsupervised analysis of transcript patterns showed that the invasion-associated TAMs represent a unique subpopulation of TAMs that, by gene ontology criteria, have gene expression patterns related to tissue and organ development. Gene set enrichment analysis showed that these macrophages are also specifically enriched for molecules involved in Wnt-signaling. Previously, it was shown that macrophage-derived Wnt molecules promote vascular remodeling and that tumor cells are highly motile and intravasate around perivascular TAM clusters. Taken together, we conjecture that invasive TAMs link angiogenesis and tumor invasion and that Wnt-signaling plays a role in mediating their activity.

Analysis of stromal signatures in the tumor microenvironment of ductal carcinoma in situ

Recent advances in the study of the tumor microenvironment have revealed significant interaction between tumor cells and their surrounding stroma in model systems. We have previously shown that two distinct stromal signatures derived from a macrophage (CSF1) response and a fibroblastic (DTF-like) response are present in subsets of invasive breast cancers and show a correlation with clinical outcome. In the present study we explore whether these signatures also exist in the stroma of ductal carcinoma in situ (DCIS). We studied the signatures by both gene expression profile analysis of a publically available data set of DCIS and by immunohistochemistry (IHC) on a tissue microarray of DCIS and invasive breast cancer cases. Both the gene expression and immunohistochemical data show that the macrophage response and fibroblast expression signatures are present in the stroma of subsets of DCIS cases. The incidence of the stromal signatures in DCIS is similar to the incidence in invasive breast cancer that we have previously reported. We also find that the macrophage response signature is associated with higher grade DCIS and cases which are ER and PR negative, whereas the fibroblast signature was not associated with any clinicopathologic features in DCIS. A comparison of 115 matched cases of DCIS and invasive breast cancer found a correlation between the type of stromal response in DCIS and invasive ductal carcinoma (IDC) within the same patient for both the macrophage response and the fibroblast stromal signatures (P = 0.03 and 0.08, respectively). This study is a first characterization of these signatures in DCIS. These signatures have significant clinicopathologic associations and tend to be conserved as the tumor progresses from DCIS to invasive breast cancer.