A distinguishing gene signature shared by tumor-infiltrating Tie2-expressing monocytes, blood “resident” monocytes, and embryonic macrophages suggests common functions and developmental relationships

CSF1R inhibition delays cervical and mammary tumor growth in murine models by attenuating the turnover of tumor-associated macrophages and enhancing infiltration by CD8+ T cells

Increased numbers of tumor-infiltrating macrophages correlate with poor disease outcome in patients affected by several types of cancer, including breast and prostate carcinomas. The colony stimulating factor 1 receptor (CSF1R) signaling pathway drives the recruitment of tumor-associated macrophages (TAMs) to the neoplastic microenvironment and promotes the differentiation of TAMs toward a pro-tumorigenic phenotype. Twelve clinical trials are currently evaluating agents that target the CSF1/CSF1R signaling pathway as a treatment against multiple malignancies, including breast carcinoma, leukemia, and glioblastoma. The blockade of CSF1R signaling has been shown to greatly decrease the number of macrophages in a tissue-specific manner. However, additional mechanistic insights are needed in order to understand how macrophages are depleted and the global effects of CSF1R inhibition on other tumor-infiltrating immune cells. Using BLZ945, a highly selective small molecule inhibitor of CSF1R, we show that CSF1R inhibition attenuates the turnover rate of TAMs while increasing the number of CD8⁺ T cells that infiltrate cervical and breast carcinomas. Specifically, we find that BLZ945 decreased the growth of malignant cells in the mouse mammary tumor virus-driven polyomavirus middle T antigen (MMTV-PyMT) model of mammary carcinogenesis. Furthermore, we show that BLZ945 prevents tumor progression in the keratin 14-expressing human papillomavirus type 16 (K14-HPV-16) transgenic model of cervical carcinogenesis. Our results demonstrate that TAMs undergo a constant turnover in a CSF1R-dependent manner, and suggest that continuous inhibition of the CSF1R pathway may be essential to maintain efficacious macrophage depletion as an anticancer therapy.

CD13-positive bone marrow-derived myeloid cells promote angiogenesis, tumor growth, and metastasis

Angiogenesis is fundamental to tumorigenesis and an attractive target for therapeutic intervention against cancer. We have recently demonstrated that CD13 (aminopeptidase N) expressed by nonmalignant host cells of unspecified types regulate tumor blood vessel development. Here, we compare CD13 wild-type and null bone marrow-transplanted tumor-bearing mice to show that host CD13(+) bone marrow-derived cells promote cancer progression via their effect on angiogenesis. Furthermore, we have identified CD11b(+)CD13(+) myeloid cells as the immune subpopulation directly regulating tumor blood vessel development. Finally, we show that these cells are specifically localized within the tumor microenvironment and produce proangiogenic soluble factors. Thus, CD11b(+)CD13(+) myeloid cells constitute a population of bone marrow-derived cells that promote tumor progression and metastasis and are potential candidates for the development of targeted antiangiogenic drugs.

Proangiogenic TIE2(+)/CD31 (+) macrophages are the predominant population of tumor-associated macrophages infiltrating metastatic lymph nodes

Tumor-associated macrophages (TAMs) accumulate in various cancers and promote tumor angiogenesis and metastasis, and thus may be ideal targets for the clinical diagnosis of tumor metastasis with high specificity. However, there are few specific markers to distinguish between TAMs and normal or inflammatory macrophages. Here, we show that TAMs localize in green fluorescent protein-labeled tumors of metastatic lymph nodes (MLNs) from B16F1 melanoma cells but not in necrotic tumor regions, suggesting that TAMs may promote the growth of tumor cells and the progression of tumor metastasis. Furthermore, we isolated pure populations of TAMs from MLNs and characterized their gene expression signatures compared to peritoneal macrophages (PMs), and found that TAMs significantly overexpress immunosuppressive cytokines such as IL-4, IL-10, and TGF-β as well as proangiogenic factors such as VEGF, TIE2, and CD31. Notably, immunological analysis revealed that TIE2(+)/CD31(+) macrophages constitute the predominant population of TAMs that infiltrate MLNs, distinct from tissue or inflammatory macrophages. Importantly, these TIE2(+)/CD31(+) macrophages also heavily infiltrated MLNs from human breast cancer biopsies but not reactive hyperplastic LNs. Thus, TIE2(+)/ CD31(+) macrophages may be a unique histopathological biomarker for detecting metastasis in clinical diagnosis, and a novel and promising target for TAM-specific cancer therapy.

On-site education of VEGF-recruited monocytes improves their performance as angiogenic and arteriogenic accessory cells

VEGF-driven neovascularization transiently recruits Ly6C^high monocytes, which subsequently alter their phenotype and exert angiogenic function to enlarge small vessels.

Adult neovascularization relies on the recruitment of monocytes to the target organ or tumor and functioning therein as a paracrine accessory. The exact origins of the recruited monocytes and the mechanisms underlying their plasticity remain unclear. Using a VEGF-based transgenic system in which genetically tagged monocytes are conditionally summoned to the liver as part of a VEGF-initiated angiogenic program, we show that these recruited cells are derived from the abundant pool of circulating Ly6C^hi monocytes. Remarkably, however, upon arrival at the VEGF-induced organ, but not the naive organ, monocytes undergo multiple phenotypic and functional changes, endowing them with enhanced proangiogenic capabilities and, importantly, with a markedly increased capacity to remodel existing small vessels into larger conduits. Notably, monocytes do not differentiate into long-lived macrophages, but rather appear as transient accessory cells. Results from transfers of presorted subpopulations and a novel tandem transfer strategy ruled out selective recruitment of a dedicated preexisting subpopulation or onsite selection, thereby reinforcing active reprogramming as the underlying mechanism for improved performance. Collectively, this study uncovered a novel function of VEGF, namely, on-site education of recruited “standard” monocytes to become angiogenic and arteriogenic professional cells, a finding that may also lend itself for a better design of angiogenic therapies.

Prokineticin receptor-1 is a new regulator of endothelial insulin uptake and capillary formation to control insulin sensitivity and cardiovascular and kidney functions

Background

Reciprocal relationships between endothelial dysfunction and insulin resistance result in a vicious cycle of cardiovascular, renal, and metabolic disorders. The mechanisms underlying these impairments are unclear. The peptide hormones prokineticins exert their angiogenic function via prokineticin receptor‐1 (PKR1). We explored the extent to which endothelial PKR1 contributes to expansion of capillary network and the transcapillary passage of insulin into the heart, kidney, and adipose tissues, regulating organ functions and metabolism in a specific mice model.

Methods and Results

By combining cellular studies and studies in endothelium‐specific loss‐of‐function mouse model (ec‐PKR1^−/−), we showed that a genetically induced PKR1 loss in the endothelial cells causes the impaired capillary formation and transendothelial insulin delivery, leading to insulin resistance and cardiovascular and renal disorders. Impaired insulin delivery in endothelial cells accompanied with defective expression and activation of endothelial nitric oxide synthase in the ec‐PKR1^−/− aorta, consequently diminishing endothelium‐dependent relaxation. Despite having a lean body phenotype, ec‐PKR1^−/− mice exhibited polyphagia, polydipsia, polyurinemia, and hyperinsulinemia, which are reminiscent of human lipodystrophy. High plasma free fatty acid levels and low leptin levels further contribute to the development of insulin resistance at the later age. Peripheral insulin resistance and ectopic lipid accumulation in mutant skeletal muscle, heart, and kidneys were accompanied by impaired insulin‐mediated Akt signaling in these organs. The ec‐PKR1^−/− mice displayed myocardial fibrosis, low levels of capillary formation, and high rates of apoptosis, leading to diastolic dysfunction. Compact fibrotic glomeruli and high levels of phosphate excretion were found in mutant kidneys. PKR1 restoration in ec‐PKR1^−/− mice reversed the decrease in capillary recruitment and insulin uptake and improved heart and kidney function and insulin resistance.

Conclusions

We show a novel role for endothelial PKR1 signaling in cardiac, renal, and metabolic functions by regulating transendothelial insulin uptake and endothelial cell proliferation. Targeting endothelial PKR1 may serve as a therapeutic strategy for ameliorating these disorders.

Tumor-associated macrophages as a paradigm of macrophage plasticity, diversity, and polarization: lessons and open questions

Macrophages are present in all body compartments, including cancerous tissues, and their functions are profoundly affected by signals from the microenvironment under homeostatic and pathological conditions. Tumor-associated macrophages are a major cellular component of cancer-related inflammation and have served as a paradigm for the plasticity and functional polarization of mononuclear phagocytes. Tumor-associated macrophages can exert dual influence of cancer depending on the activation state, with classically activated (M1) and alternatively activated (M2) cells generally exerting antitumoral and protumoral functions, respectively. These are extremes in a continuum of polarization states in a universe of diversity. Tumor-associated macrophages affect virtually all aspects of tumor tissues, including stem cells, metabolism, angiogenesis, invasion, and metastasis. Progress has been made in defining signaling molecules, transcription factors, epigenetic changes, and repertoire of microRNAs underlying macrophage polarization. Preclinical and early clinical data suggest that macrophages may serve as tools for the development of innovative diagnostic and therapeutic strategies in cancer and chronic nonresolving inflammatory diseases.

Macrophages and CSF-1: implications for development and beyond

Recent focus on the diversity of macrophage phenotype and function signifies that these trophic cells are no longer of exclusive interest to the field of immunology. As key orchestrators of organogenesis, the contribution of macrophages to fetal development is worthy of greater attention. This review summarizes the key functions of macrophages and their primary regulator, colony-stimulating factor (CSF)-1, during development; highlighting trophic mechanisms beyond phagocytosis and outlining their roles in a range of developing organ systems. Advances in the understanding of macrophage polarization and functional heterogeneity are discussed from a developmental perspective. In addition, this review highlights the relevance of CSF-1 as a pleiotropic developmental growth factor and summarizes recent experimental evidence and clinical advancements in the area of CSF-1 and macrophage manipulation in reproduction and organogenic settings. Interrogation of embryonic macrophages also has implications beyond development, with recent attention focused on yolk sac macrophage ontogeny and their role in homeostasis and mediating tissue regeneration. The regulatory networks that govern development involve a complex range of growth factors, signaling pathways and transcriptional regulators arising from epithelial, mesenchymal and stromal origins. A component of the organogenic milieu common to the majority of developing organs is the tissue macrophage. These hemopoietic cells are part of the mononuclear phagocyte system regulated primarily by colony-stimulating factor (CSF)-1 (1, 2). There is a resurgence in the field of CSF-1 and macrophage biology; where greater understanding of the heterogeneity of these cells is revealing contributions to tissue repair and regeneration beyond the phagocytic and inflammatory functions for which they were traditionally ascribed (3-6). The accumulation of macrophages during tissue injury is no longer viewed as simply a surrogate for disease severity, with macrophages now known to be vital in governing tissue regeneration in many settings (7-11). In particular it is the influence of CSF-1 in regulating an alternative macrophage activation state that is increasingly linked to organ repair in a range of disease models (12-17). With many similarities drawn between organogenesis and regeneration, it is pertinent to re-examine the role of CSF-1 and macrophages in organ development.

Polarization of tumor-associated macrophage is associated with tumor vascular normalization by endostatin

BACKGROUND

Vascular normalization is an emerging concept in cancer treatment, but its precise mechanisms are not completely understood. The polarization of tumor-associated macrophages (TAMs) is important in tumor angiogenesis and metastasis. However, little is known about the effect of anti-angiogenic agents on the polarization of tumor-associated macrophages. Therefore, we explore the changes of TAMs polarization in the development of tumor vascular normalization induced by endostatin.

METHODS

A murine xenograft model of lung cancer was treated with endostatin for 10 days. The morphology and function of tumor vasculature was examined using various techniques. Flow cytometry was carried out to assess the TAMs, and immunofluorescence was used to examine Tie-2-expressing monocytes (TEMs) in tumors. Levels of the histidine-rich glycoprotein (HRG) in tumors were measured by immunohistochemistry and Western blot.

RESULTS

Tumor vessels became more normal and mature on day six in the endostatin-treated mice. During vascular normalization, the number of M2-like TAMs and TEMs in the tumors was significantly reduced, whereas the number of M1-like TAMs showed an increase on day six after endostatin treatment, although the latter was not statistically significant. The HRG in the tumors accumulated at an early stage after endostatin administration.

CONCLUSIONS

The polarization of TAMs is associated with tumor vascular normalization induced by endostatin. These observations may be useful in the exploration of new strategies for anti-angiogenic treatment.

The irradiated tumor microenvironment: role of tumor-associated macrophages in vascular recovery

Radiotherapy is an important modality used in the treatment of more than 50% of cancer patients in the US. However, despite sophisticated techniques for radiation delivery as well as the combination of radiation with chemotherapy, tumors can recur. Thus, any method of improving the local control of the primary tumor by radiotherapy would produce a major improvement in the curability of cancer patients. One of the challenges in the field is to understand how the tumor vasculature can regrow after radiation in order to support tumor recurrence, as it is unlikely that any of the endothelial cells within the tumor could survive the doses given in a typical radiotherapy regimen. There is now considerable evidence from both preclinical and clinical studies that the tumor vasculature can be restored following radiotherapy from an influx of circulating cells consisting primarily of bone marrow derived monocytes and macrophages. The radiation-induced influx of bone marrow derived cells (BMDCs) into tumors can be prevented through the blockade of various cytokine pathways and such strategies can inhibit tumor recurrence. However, the post-radiation interactions between surviving tumor cells, recruited immune cells, and the remaining stroma remain poorly defined. While prior studies have described the monocyte/macrophage inflammatory response within normal tissues and in the tumor microenvironment, less is known about this response with respect to a tumor after radiation therapy. The goal of this review is to summarize existing research studies to provide an understanding of how the myelomonocytic lineage may influence vascular recovery within the irradiated tumor microenvironment.

Macrophages regulate corpus luteum development during embryo implantation in mice

Macrophages are prominent in the uterus and ovary at conception. Here we utilize the Cd11b-Dtr mouse model of acute macrophage depletion to define the essential role of macrophages in early pregnancy. Macrophage depletion after conception caused embryo implantation arrest associated with diminished plasma progesterone and poor uterine receptivity. Implantation failure was alleviated by administration of bone marrow-derived CD11b+F4/80+ monocytes/macrophages. In the ovaries of macrophage-depleted mice, corpora lutea were profoundly abnormal, with elevated Ptgs2, Hif1a, and other inflammation and apoptosis genes and with diminished expression of steroidogenesis genes Star, Cyp11a1, and Hsd3b1. Infertility was rescued by exogenous progesterone, which confirmed that uterine refractoriness was fully attributable to the underlying luteal defect. In normally developing corpora lutea, macrophages were intimately juxtaposed with endothelial cells and expressed the proangiogenic marker TIE2. After macrophage depletion, substantial disruption of the luteal microvascular network occurred and was associated with altered ovarian expression of genes that encode vascular endothelial growth factors. These data indicate a critical role for macrophages in supporting the extensive vascular network required for corpus luteum integrity and production of progesterone essential for establishing pregnancy. Our findings raise the prospect that disruption of macrophage-endothelial cell interactions underpinning corpus luteum development contributes to infertility in women in whom luteal insufficiency is implicated.

Lipoprotein lipase in non-small cell lung cancer tissue is highly expressed in a subpopulation of tumor-associated macrophages

High lipoprotein lipase (LPL) activity in non-small cell lung cancer (NSCLC) tissue strongly predicts shorter patient survival. We tested the hypothesis that in NSCLC tissue, macrophages are the major site of LPL expression. LPL expression in the entire NSCLC tissue and in the adjacent non-cancer lung tissue was compared to the expression of genes preferentially expressed in macrophages. LPL expression at the cellular level was analyzed by mRNA fluorescence in situ hybridization. In the whole cancer tissue (but not in the adjacent non-cancer tissue), expression of LPL correlated with expression of genes preferentially expressed in macrophages (MSR1, CD163, FOLR2), but not with expression of genes preferentially expressed in tumor cells. All cells in the cancer and adjacent non-cancer tissue exhibit low LPL expression. However, in cancer tissue only, there were individual highly LPL-expressing cells which were macrophages. These LPL-overexpressing cells were approximately 10 times less abundant than anti-CD163-stained, tumor-associated macrophages. To conclude, in NSCLC tissue, a subpopulation of tumor-associated macrophages highly expresses LPL. Because tumor-associated macrophages are pro-tumorigenic, these cells should be further characterized to better understand the underlying nature of the close relationship between high LPL activity in NSCLC tissue and shorter patient survival.

Macrophage-tumor cell interactions regulate the function of nitric oxide

Tumor cell-macrophage interactions change as the tumor progresses, and the generation of nitric oxide (NO) by the inducible nitric oxide synthase (iNOS) plays a major role in this interplay. In early stages, macrophages employ their killing mechanisms, particularly the generation of high concentrations of NO and its derivative reactive nitrogen species (RNS) to initiate tumor cell apoptosis and destroy emerging transformed cells. If the tumor escapes the immune system and grows, macrophages that infiltrate it are reprogramed in situ by the tumor microenvironment. Low oxygen tensions (hypoxia) and immunosuppressive cytokines inhibit iNOS activity and lead to production of low amounts of NO/RNS, which are pro-angiogenic and support tumor growth and metastasis by inducing growth factors (e.g., VEGF) and matrix metalloproteinases (MMPs). We review here the different roles of NO/RNS in tumor progression and inhibition, and the mechanisms that regulate iNOS expression and NO production, highlighting the role of different subtypes of macrophages and the microenvironment. We finally claim that some tumor cells may become resistant to macrophage-induced death by increasing their expression of microRNA-146a (miR-146a), which leads to inhibition of iNOS translation. This implies that some cooperation between tumor cells and macrophages is required to induce tumor cell death, and that tumor cells may control their fate. Thus, in order to induce susceptibility of tumors cells to macrophage-induced death, we suggest a new therapeutic approach that couples manipulation of miR-146a levels in tumors with macrophage therapy, which relies on ex vivo stimulation of macrophages and their re-introduction to tumors.

Mast cells and macrophages exert beneficial and detrimental effects on tumor progression and angiogenesis

Mast cells and macrophages are critical regulators of inflammation and immunological response in the tumor microenvironment. Increased number of mast cells and macrophages have been reported to correlate with poor prognosis in numerous solid and hematological tumors. In contrast to their pro-tumorigenic role, mast cells and macrophages have shown also anti-tumorigenic effect in certain malignancies, for example by supporting cancer rejection. Thus, mast cells and macrophages can exert both detrimental and beneficial effects on tumor progression. Mast cell- and macrophages-derived growth factors able to promote tumor development and angiogenesis include TNF-α, TGF-β1, FGF-2, VEGF, PDGF, IL-8, osteopontin, and NGF. On the contrary, mast cell- and macrophages-produced cytokines that may participate in anti-tumor response include IL-1, IL-2, IL-4, IL-10, and IFN-γ. It is to note that mast cells and macrophages may also show beneficial and detrimental effects in the same cancer depending on the tumor stage.

Tumor-associated macrophages: functional diversity, clinical significance, and open questions

Inflammation is now a well-recognized hallmark of cancer progression. Tumor-associated macrophages (TAMs) are one of the major inflammatory cells that infiltrate murine and human tumors. While epidemiological studies indicate a clear correlation between TAM density and poor prognosis in a number of human cancers, transgenic studies and transcriptome profiling of TAMs in mice have established their crucial role in cancer progression. In fact, TAMs affect diverse aspects of cancer progression including tumor cell growth and survival, invasion, metastasis, angiogenesis, inflammation, and immunoregulation. New evidences have extended the repertoire of these cells to other tumor promoting activities like interactions with cancer stem cells, response to chemotherapy, and tumor relapse. These findings have triggered efforts to target TAMs and their associated molecules to modulate tumor progression. In particular, "re-education" to activate their anti-tumor potential or elimination of tumor promoting TAMs are strategies undergoing preclinical and clinical evaluation. Proof-of-principle studies indicate that TAM-centered therapeutic strategies may contribute to cancer therapy.

TIE2-expressing monocytes/macrophages regulate revascularization of the ischemic limb

A third of patients with critical limb ischemia (CLI) will eventually require limb amputation. Therapeutic neovascularization using unselected mononuclear cells to salvage ischemic limbs has produced modest results. The TIE2-expressing monocytes/macrophages (TEMs) are a myeloid cell subset known to be highly angiogenic in tumours. This study aimed to examine the kinetics of TEMs in patients with CLI and whether these cells promote neovascularization of the ischemic limb. Here we show that there are 10-fold more circulating TEMs in CLI patients, and removal of ischemia reduces their numbers to normal levels. TEM numbers in ischemic muscle are two-fold greater than normoxic muscle from the same patient. TEMs from patients with CLI display greater proangiogenic activity than TIE2-negative monocytes in vitro. Using a mouse model of hindlimb ischemia, lentiviral-based Tie2 knockdown in TEMs impaired recovery from ischemia, whereas delivery of mouse macrophages overexpressing TIE2, or human TEMs isolated from CLI patients, rescued limb ischemia. These data suggest that enhancing TEM recruitment to the ischemic muscle may have the potential to improve limb neovascularization in CLI patients.

Exploiting microRNA regulation for genetic engineering

RNA interference (RNAi) has been a landmark discovery in science. A typical application is to knock down the expression of endogenous genes by delivering small interfering RNA (siRNA) into cells triggering the degradation of complementary mRNA. However, RNAi can also be exploited the other way round: making use of the huge diversity of endogenous microRNAs (miRNA), the expression of exogenously introduced genes tagged with artificial miRNA target sequences can be negatively regulated according to the activity of a given miRNA which can be tissue-, lineage-, activation- or differentiation stage specific. This has significantly expanded the regulatory potential of gene transfer vectors and will benefit both basic science and therapeutic applications. This review briefly introduces the reader to the technical basis for exploiting miRNA regulation, followed by a discussion of specific applications for miRNA-regulated vectors/viruses in basic research, gene- and virotherapy.

Circulating mononuclear progenitor cells: differential roles for subpopulations in repair of retinal vascular injury

PURPOSE

We examined effect on retinal vascular homing of exogenous CD34(+) and CD14(+) progenitor cells using mouse models of chronic (streptozotocin [STZ]-induced diabetes) and acute (ischemia-reperfusion [I/R]) ocular vascular injury.

METHODS

STZ-treated mice of short or long duration (≤4, ≥11 months) diabetes, along with age- and sex-matched controls, were given intravitreous injections of human CD34(+) and CD14(+) cells isolated from healthy or diabetic donors alone or in combination. I/R injured mice were given diabetic or nondiabetic CD34(+) cells with mesenchymal stem cells (MSCs) or diabetic CD34(+) cells manipulated by ex vivo fucosylation with ASC-101. Injected cells were localized by fluorescent immunocytochemistry, and the degree of retinal vascular colocalization quantified morphometrically. Permeability was assessed by fluorescent albumin leakage.

RESULTS

Diabetic CD14(+) cells associated with vessels to a greater degree than diabetic CD34(+) cells. Vascular permeability was reduced only by nondiabetic cells and only at the highest number of cells tested. Diabetic CD34(+) cells consistently demonstrated reduced migration. There was a 2-fold or 4-fold increase over control in the specific localization of diabetic CD34(+) cells within the vasculature when these cells were co-administered with MSCs or ex vivo fucosylated prior to injection, respectively.

CONCLUSIONS

Diabetic CD14(+) cells, unlike diabetic CD34(+) cells, retain robust homing characteristics. CD34(+) or CD14(+) subsets rather than whole bone marrow or peripheral blood cells may prove more beneficial in autologous cell therapy for diabetics. Co-administration with MSCs or ex vivo fucosylation may enhance utility of CD34(+) cells in cell therapy for diabetic ocular conditions like macular ischemia and retinal nonperfusion.

PHD2 regulates arteriogenic macrophages through TIE2 signalling

Occlusion of the main arterial route redirects blood flow to the collateral circulation. We previously reported that macrophages genetically modified to express low levels of prolyl hydroxylase domain protein 2 (PHD2) display an arteriogenic phenotype, which promotes the formation of collateral vessels and protects the skeletal muscle from ischaemic necrosis. However, the molecular mechanisms underlying this process are unknown. Here, we demonstrate that femoral artery occlusion induces a switch in macrophage phenotype through angiopoietin-1 (ANG1)-mediated Phd2 repression. ANG blockade by a soluble trap prevented the downregulation of Phd2 expression in macrophages and their phenotypic switch, thus inhibiting collateral growth. ANG1-dependent Phd2 repression initiated a feed-forward loop mediated by the induction of the ANG receptor TIE2 in macrophages. Gene silencing and cell depletion strategies demonstrate that TIE2 induction in macrophages is required to promote their proarteriogenic functions, enabling collateral vessel formation following arterial obstruction. These results indicate an indispensable role for TIE2 in sustaining in situ programming of macrophages to a proarteriogenic, M2-like phenotype, suggesting possible new venues for the treatment of ischaemic disorders.

Diabetes impairs stem cell and proangiogenic cell mobilization in humans

OBJECTIVE

Diabetes mellitus (DM) increases cardiovascular risk, at least in part, through shortage of vascular regenerative cells derived from the bone marrow (BM). In experimental models, DM causes morphological and functional BM alterations, but information on BM function in human DM is missing. Herein, we sought to assay mobilization of stem and proangiogenic cells in subjects with and without DM.

RESEARCH DESIGN AND METHODS

In a prospective trial (NCT01102699), we tested BM responsiveness to 5 μg/kg human recombinant granulocyte colony-stimulating factor (hrG-CSF) in 24 individuals with DM (10 type 1 and 14 type 2) and 14 individuals without DM. Before and 24 h after hrG-CSF, we quantified circulating stem/progenitor cells and total and differential white blood cell counts. We also evaluated in vivo the proangiogenic capacity of peripheral blood mononuclear cells using the Matrigel plug assay.

RESULTS

In response to hrG-CSF, levels of CD34(+) cells and other progenitor cell phenotypes increased in subjects without DM. Patients with DM had significantly impaired mobilization of CD34(+), CD133(+), and CD34(+)CD133(+) hematopoietic stem cells and CD133(+)KDR(+) endothelial progenitors, independently of potential confounders. The in vivo angiogenic capacity of peripheral blood mononuclear cells significantly increased after hrG-CSF in control subjects without DM, but not in patients with DM. DM was also associated with the inability to upregulate CD26/DPP-4 on CD34(+) cells, which is required for the mobilizing effect of granulocyte colony-stimulating factor.

CONCLUSIONS

Stem and proangiogenic cell mobilization in response to hrG-CSF is impaired in DM, possibly because of maladaptive CD26/DPP-4 regulation. These alterations may hamper tissue repair and favor the development of cardiovascular complications.

Reciprocal interactions between endothelial cells and macrophages in angiogenic vascular niches

The ability of macrophages to promote vascular growth has been associated with the secretion and local delivery of classic proangiogenic factors (e.g., VEGF-A and proteases). More recently, a series of studies have also revealed that physical contact of macrophages with growing blood vessels coordinates vascular fusion of emerging sprouts. Interestingly, the interactions between macrophages and vascular endothelial cells (ECs) appear to be bidirectional, such that activated ECs also support the expansion and differentiation of proangiogenic macrophages from myeloid progenitors. Here, we discuss recent findings suggesting that dynamic angiogenic vascular niches might also exist in vivo, e.g. in tumors, where sprouting blood vessels and immature myeloid cells like monocytes engage in heterotypic interactions that are required for angiogenesis. Finally, we provide an account of emerging mechanisms of cell-to-cell communication that rely on secreted microvesicles, such as exosomes, which can offer a vehicle for the rapid exchange of molecules and genetic information between macrophages and ECs engaged in angiogenesis.

Myeloid-derived suppressor cells as a vehicle for tumor-specific oncolytic viral therapy

One of the several impediments to effective oncolytic virus therapy of cancer remains a lack of tumor-specific targeting. Myeloid-derived suppressor cells (MDSC) are immature myeloid cells induced by tumor factors in tumor-bearing hosts. The biodistribution kinetics of MDSC and other immune cell types in a murine hepatic colon cancer model was investigated through the use of tracking markers and MRI. MDSCs were superior to other immune cell types in preferential migration to tumors in comparison with other tissues. On the basis of this observation, we engineered a strain of vesicular stomatitis virus (VSV), an oncolytic rhabdovirus that bound MDSCs and used them as a delivery vehicle. Improving VSV-binding efficiency to MDSCs extended the long-term survival of mice bearing metastatic colon tumors compared with systemic administration of wild-type VSV alone. Survival was further extended by multiple injections of the engineered virus without significant toxicity. Notably, direct tumor killing was accentuated by promoting MDSC differentiation towards the classically activated M1-like phenotype. Our results offer a preclinical proof-of-concept for using MDSCs to facilitate and enhance the tumor-killing activity of tumor-targeted oncolytic therapeutics.

Impact of notch signaling on inflammatory responses in cardiovascular disorders

Notch signaling is a major pathway in cell fate decisions. Since the first reports showing the major role of Notch in embryonic development, a considerable and still growing literature further highlights its key contributions in various pathological processes during adult life. In particular, Notch is now considered as a major player in vascular homeostasis through the control of key cellular functions. In parallel, confounding evidence emerged that inflammatory responses regulate Notch signaling in vitro in endothelial cells, smooth muscle cells or vascular infiltrating cells and in vivo in vascular and inflammatory disorders and in cardiovascular diseases. This review presents how inflammation influences Notch in vascular cells and, reciprocally, emphasizes the functional role of Notch on inflammatory processes, notably by regulating key cell functions (differentiation, proliferation, apoptosis/survival, activation). Understanding how the disparity of Notch receptors and ligands impacts on vasculature biology remains critical for the design of relevant and adequate therapeutic strategies targeting Notch in this major pathological context.

Macrophage regulation of tumor responses to anticancer therapies

Tumor-associated macrophages (TAMs) promote key processes in tumor progression, like angiogenesis, immunosuppression, invasion, and metastasis. Increasing studies have also shown that TAMs can either enhance or antagonize the antitumor efficacy of cytotoxic chemotherapy, cancer-cell targeting antibodies, and immunotherapeutic agents--depending on the type of treatment and tumor model. TAMs also drive reparative mechanisms in tumors after radiotherapy or treatment with vascular-targeting agents. Here, we discuss the biological significance and clinical implications of these findings, with an emphasis on novel approaches that effectively target TAMs to increase the efficacy of such therapies.

MicroRNA-mediated control of macrophages and its implications for cancer

Deregulation of microRNAs (miRNAs) can drive oncogenesis, tumor progression, and metastasis by acting cell-autonomously in cancer cells. However, solid tumors are also infiltrated by large amounts of non-neoplastic stromal cells, including macrophages, which express several active miRNAs. Tumor-associated macrophages (TAMs) enhance angiogenic, immunosuppressive, invasive, and metastatic programming of neoplastic tissue and reduce host survival. Here, we review the role of miRNAs (including miR-155, miR-146, and miR-511) in the control of macrophage production and activation, and examine whether reprogramming miRNA activity in TAMs and/or their precursors might be effective for controlling tumor progression.

A role for miR-155 in enabling tumor-infiltrating innate immune cells to mount effective antitumor responses in mice

A productive immune response requires transient upregulation of the microRNA miR-155 in hematopoietic cells mediating innate and adaptive immunity. In order to investigate miR-155 in the context of tumor-associated immune responses, we stably knocked down (KD) miR-155 in the myeloid compartment of MMTV-PyMT mice, a mouse model of spontaneous breast carcinogenesis that closely mimics tumor-host interactions seen in humans. Notably, miR-155/KD significantly accelerated tumor growth by impairing classic activation of tumor-associated macrophages (TAMs). This created an imbalance toward a protumoral microenvironment as evidenced by a lower proportion of CD11c(+) TAMs, reduced expression of activation markers, and the skewing of immune cells within the tumor toward an macrophage type 2/T helper 2 response. This study highlights the importance of tumor-infiltrating hematopoietic cells in constraining carcinogenesis and establishes an antitumoral function of a prototypical oncomiR.

Semaphorins in cancer: biological mechanisms and therapeutic approaches

The hallmarks of cancer include multiple alterations in the physiological processes occurring in normal tissues, such as cell proliferation, apoptosis, and restricted cell migration. These aberrant behaviors are due to genetic and epigenetic changes that affect signaling pathways controlling cancer cells, as well as the surrounding "normal" cells in the tumor microenvironment. Semaphorins and their receptors (mainly plexins and neuropilins) are aberrantly expressed in human tumors, and multiple family members are emerging as pivotal signals deregulated in cancer. Notably, different semaphorins can promote or inhibit tumor progression, depending on the implicated receptor complexes and responsive cell type. The important role of semaphorin signals in the regulation of tumor angiogenesis, invasion and metastasis has initiated multiple experimental approaches aimed at targeting these pathways to inhibit cancer.

The role of semaphorins and their receptors in vascular development and cancer

Semaphorins (Semas) are a large family of traditional axon guidance molecules. Through interactions with their receptors, Plexins and Neuropilins, Semas play critical roles in a continuously growing list of diverse biological systems. In this review, we focus on their function in regulating vascular development. In addition, over the past few years a number of findings have shown the crucial role that Semas and their receptors play in the regulation of cancer progression and tumor angiogenesis. In particular, Semas control tumor progression by directly influencing the behavior of cancer cells or, indirectly, by modulating angiogenesis and the function of other cell types in the tumor microenvironment (i.e., inflammatory cells and fibroblasts). Some Semas can activate or inhibit tumor progression and angiogenesis, while others may have the opposite effect depending on specific post-translational modifications. Here we will also discuss the diverse biological effects of Semas and their receptor complexes on cancer progression as well as their impact on the tumor microenvironment.

Class 3 semaphorins: physiological vascular normalizing agents for anti-cancer therapy

Findings from preclinical and clinical studies show that vascular normalization represents a novel strategy to enhance the efficacy of and overcome the acquired resistance to anti-angiogenic therapies in cancer. Several mechanisms of tumour vessel normalization have been revealed. Amongst them, secreted class 3 semaphorins (Sema3), which regulate axon guidance and angiogenesis, have been recently identified as novel vascular normalizing agents that inhibit metastatic dissemination by restoring vascular function. Here, we discuss the different biological functions and mechanisms of action of Sema3 in the context of tumour vascular normalization, and their impact on the different cellular components of the tumour microenvironment.

Interactions of human monocytes with TMVs (tumour-derived microvesicles)

The tumour microenvironment represents a dynamic complex milieu, which includes tumour cells, cells of the immune system and other (cellular and non-cellular) components. The role of these particular ‘puzzle pieces’ may change substantially due to their mutual interactions. The present review concerns different opinions on interactions that occur between monocytes, tumour cells and TMVs (tumour-derived microvesicles).

Endometriosis, a disease of the macrophage

Endometriosis, a common cause of pelvic pain and female infertility, depends on the growth of vascularized endometrial tissue at ectopic sites. Endometrial fragments reach the peritoneal cavity during the fertile years: local cues decide whether they yield endometriotic lesions. Macrophages are recruited at sites of hypoxia and tissue stress, where they clear cell debris and heme-iron and generate pro-life and pro-angiogenesis signals. Macrophages are abundant in endometriotic lesions, where are recruited and undergo alternative activation. In rodents macrophages are required for lesions to establish and to grow; bone marrow-derived Tie-2 expressing macrophages specifically contribute to lesions neovasculature, possibly because they concur to the recruitment of circulating endothelial progenitors, and sustain their survival and the integrity of the vessel wall. Macrophages sense cues (hypoxia, cell death, iron overload) in the lesions and react delivering signals to restore the local homeostasis: their action represents a necessary, non-redundant step in the natural history of the disease. Endometriosis may be due to a misperception of macrophages about ectopic endometrial tissue. They perceive it as a wound, they activate programs leading to ectopic cell survival and tissue vascularization. Clearing this misperception is a critical area for the development of novel medical treatments of endometriosis, an urgent and unmet medical need.

G-CSF rescues tumor growth and neo-angiogenesis during liver metastasis under host angiopoietin-2 deficiency

Suppression of neo-angiogenesis is a clinically used anti-tumor strategy with new targets such as angiopoietin-2 (Ang2) being proposed. However, the functions of Ang2 in vascular remodeling, inflammation and tumor growth are not consistent. We examined effect of depletion of host Ang2 on liver colony formation using Ang2 deficient (Ang2(-/-)) mice. Surprisingly, the metastatic colonies formed in Ang2(-/-) mice were larger than those in the wild type. These colonies had greater vascular density with more pericyte coverage than the vessels in liver colonies in the wild type. Liver VEGF concentration in both genotypes was equivalent, and thus, the differences appeared VEGF independent. However, after colony formation, the serum concentration of granulocyte-colony stimulating factor (G-CSF) and CXCL1 in Ang2(-/-) mice was 12 and 6 times greater than after colony formation in wild type. Increase of these two cytokines was associated with two times greater numbers of neutrophils recruited to the liver. Two times more Tie2+/CD11b+/CD31- cells were present in the tumors in Ang2(-/-) than in the wild type livers. These results suggest that the depletion of host Ang2 induced compensatory VEGF-independent angiogenic mechanisms and thus enhanced liver metastatic colony growth and colony vascularity. They further indicate organotypic differences in response to tumor metastasis. In contrast, Ang2 deficiency inhibited tumor growth during metastatic colony formation in the lung, consistent with the reports of decreased pulmonary seeding of tumor cells after pharmacological inhibition of Ang2. Further studies are thus required to assess the effects of pharmacological Ang2 blockade for cancer patients particularly in the liver.

NRP1 acts cell autonomously in endothelium to promote tip cell function during sprouting angiogenesis

Neuropilin (NRP) 1 is a receptor for the vascular endothelial growth factor (VEGF)-A and is essential for normal angiogenesis. Previous in vitro experiments identified NRP1 interactions with VEGF-A's main signaling receptor VEGFR2 within endothelial cells, but also between nonendothelial NRP1 and endothelial VEGFR2. Consistent with an endothelial role for NRP1 in angiogenesis, we found that VEGFR2 and NRP1 were coexpressed in endothelial tip and stalk cells in the developing brain. In addition, NRP1 was expressed on two cell types that interact with growing brain vessels-the neural progenitors that secrete VEGF-A to stimulate tip cell activity and the pro-angiogenic macrophages that promote tip cell anastomosis. Selective targeting of Nrp1 in each of these cell types demonstrated that neural progenitor- and macrophage-derived NRP1 were dispensable, whereas endothelial NRP1 was essential for normal brain vessel growth. NRP1 therefore promotes brain angiogenesis cell autonomously in endothelium, independently of heterotypic interactions with nonendothelial cells. Genetic mosaic analyses demonstrated a key role for NRP1 in endothelial tip rather than stalk cells during vessel sprouting. Thus, NRP1-expressing endothelial cells attained the tip cell position when competing with NRP1-negative endothelial cells in chimeric vessel sprouts. Taken together, these findings demonstrate that NRP1 promotes endothelial tip cell function during angiogenesis.

Recruitment of monocytes/macrophages in different tumor microenvironments

After emigration from the bone marrow into the peripheral blood, monocytes enter tissues and differentiate into macrophages. Monocytes/macrophages have many roles in immune regulation, angiogenesis, and tumor metastasis and invasion. In addition, studies have revealed that these cells are essential to tumor progression. Recently, an accumulation of evidence has indicated that macrophages in distinct regions of tumor masses have distinct origins. For instance, classical monocytes appear to be a major source of macrophages in tumor epithelial, perivascular, and hypoxic regions. In contrast, non-classical monocytes are an important source of macrophages in the tumor perivascular region. During the past century, it has been demonstrated that several chemoattractants can regulate the recruitment of monocytes/macrophages to tumor sites. Despite the importance of monocytes/macrophages in tumor progression, there had been, until recently, no efforts to summarize receptor-ligand pairs between tumor-derived chemokines and corresponding receptors in monocytes in different microenvironments. In this review, we present a cohesive view of the distinct expression patterns of chemokine receptors in two different monocyte subsets (classical and non-classical monocytes) and describe their roles in monocyte/macrophage recruitment into distinct tumor microenvironments. This review provides insight into the behavior of monocytes/macrophages in different tumor microenvironments.

Neuropilin-1 identifies a subset of bone marrow Gr1- monocytes that can induce tumor vessel normalization and inhibit tumor growth

Improving tumor perfusion, thus tempering tumor-associated hypoxia, is known to impair cancer progression. Previous work from our laboratory has shown that VEGF-A165 and semaphorin 3A (Sema3A) promote vessel maturation through the recruitment of a population of circulating monocytes expressing the neuropilin-1 (Nrp1) receptor (Nrp1-expressing monocytes; NEM). Here, we define the characteristics of bone marrow NEMs and assess whether these cells might represent an exploitable tool to induce tumor vessel maturation. Gene expression signature and surface marker analysis have indicated that NEMs represent a specific subset of CD11b+ Nrp1+ Gr1- resident monocytes, distinctively recruited by Sema3A. NEMs were found to produce several factors involved in vessel maturation, including PDGFb, TGF-β, thrombospondin-1, and CXCL10; consistently, they were chemoattractive for vascular smooth muscle cells in vitro. When directly injected into growing tumors, NEMs, isolated either from the bone marrow or from Sema3A-expressing muscles, exerted antitumor activity despite having no direct effects on the proliferation of tumor cells. NEM inoculation specifically promoted mural cell coverage of tumor vessels and decreased vascular leakiness. Tumors treated with NEMs were smaller, better perfused and less hypoxic, and had a reduced level of activation of HIF-1α. We conclude that NEMs represent a novel, unique population of myeloid cells that, once inoculated into a tumor, induce tumor vessel normalization and inhibit tumor growth.

Imaging interactions between macrophages and tumour cells that are involved in metastasis in vivo and in vitro

Tumour-associated macrophages participate in several protumour functions including tumour growth and angiogenesis, and facilitate almost every step of the metastatic cascade. Interfering with macrophage functions may therefore provide an important strategy in the clinical management of cancer and metastatic disease. Our understanding of macrophage functions has been greatly expanded by direct observations of macrophage-carcinoma cell interactions using light microscopy. Imaging approaches include intravital microscopy of tumours in mouse models of cancer and visualization of macrophage-carcinoma cell interactions in in vitro assays; whether atop 2D substrates, embedded in 3D matrices or in more complex assemblies of multiple cell types that mimic specific topologies of the tumour microenvironment. Such imaging and reconstitution approaches have provided us with a wealth of information on the motile behaviour and physical associations between macrophages and carcinoma cells and the role of the tumour microenvironment in influencing the movement of these cells. Finally, high-resolution imaging techniques have permitted researchers to correlate motility patterns with specific gene signatures and biochemical pathways in cells, pointing to potential targets for intervention. Here, we review experimental approaches employed in the study of macrophage interactions with carcinoma cells with an emphasis on imaging invasive and metastatic cell motility in breast carcinomas.

Nichotherapy for stem cells: there goes the neighborhood

Stem cells and their malignant counterparts require the support of a specific microenvironment or "niche". While various anti-cancer therapies have been broadly successful, there are growing opportunities to target the environment in which these cells reside to further improve therapeutic efficacy and outcome. This is particularly true when the aim is to target normal or malignant stem cells. The field aiming to target or use the niches that harbor, protect, and support stem cells could be designated as "nichotherapy". In this essay, we provide a few examples of nichotherapies. Some have been employed for decades, such as hematopoietic stem cell mobilization, whereas others are emerging, such as chemosensitization of leukemia stem cells by targeting their niche.

Phenotypic characterisation of pro-inflammatory monocytes and dendritic cells in peripheral arterial disease

Atherosclerosis is a chronic inflammatory process involving antigen-presenting cells like monocytes and dendritic cells (DC). The aim of this study was to perform a phenotypic characterisation of these cell types in patients with different degrees of peripheral arterial disease (PAD). Sixty patients with PAD [N= 30 intermittent claudication (IC), N= 30 critical limb ischemia (CLI)] and 30 controls were included. Peripheral blood leucocytes were analysed from peripheral blood by flow cytometry using different gating strategies to directly identify and analyse monocytes, myeloid DC, (mDC) and plasmacytoid DC (pDC). PAD patients showed a significantly higher proportion of proinflammatory CD14++CD16+ monocytes (p<0.0001) compared with healthy individuals. We found an increased number of mDC/ml and a reduced number of pDC/ml (both p<0.01) in PAD patients, leading to a shift in the mDC/pDC ratio (p<0.01). As compared to patients with intermittent claudication, CLI patients presented a reduced expression of HLA-DR (p<0.01), CD86 and CD40 on both mDCs and pDCs (p<0.01). Peripheral blood monocytes show a proinflammatory phenotype in PAD patients compared to controls. In contrast, CLI patients show a reduced expression of proinflammatory markers. We hypothesise that severe ischaemia and/or prolonged inflammation in CLI might lead to a paradoxical attenuation in the proinflammatory membrane pattern of circulating mononuclear cells, possibly hindering an adequate regulatory function of mDCs and pDCs and favouring the progression of disease.

Instruction of myeloid cells by the tumor microenvironment: Open questions on the dynamics and plasticity of different tumor-associated myeloid cell populations

The versatility and plasticity of myeloid cell polarization/differentiation has turned out to be crucial in health and disease, and has become the subject of intense investigation during the last years. On one hand, myeloid cells provide a critical contribution to tissue homeostasis and repair. On the other hand, myeloid cells not only play an important role as first line defense against pathogens but also they are involved in a broad array of inflammation-related diseases such as cancer. Recent studies show that macrophages can exist in different activation states within the same tumor, underlining their plasticity and heterogeneity. In this review, we will discuss recent evidence on how the tumor microenvironment, as it evolves, shapes the recruitment, function, polarization and differentiation of the myeloid cell compartment, leading to the selection of myeloid cells with immunosuppressive and angiogenic functions that facilitate tumor progression and dissemination.

In vivo inhibition of c-MYC in myeloid cells impairs tumor-associated macrophage maturation and pro-tumoral activities

Although tumor-associated macrophages (TAMs) are involved in tumor growth and metastasis, the mechanisms controlling their pro-tumoral activities remain largely unknown. The transcription factor c-MYC has been recently shown to regulate in vitro human macrophage polarization and be expressed in macrophages infiltrating human tumors. In this study, we exploited the predominant expression of LysM in myeloid cells to generate c-Myc^fl/fl LysM^cre/+ mice, which lack c-Myc in macrophages, to investigate the role of macrophage c-MYC expression in cancer. Under steady-state conditions, immune system parameters in c-Myc^fl/fl LysM^cre/+ mice appeared normal, including the abundance of different subsets of bone marrow hematopoietic stem cells, precursors and circulating cells, macrophage density, and immune organ structure. In a model of melanoma, however, TAMs lacking c-Myc displayed a delay in maturation and showed an attenuation of pro-tumoral functions (e.g., reduced expression of VEGF, MMP9, and HIF1α) that was associated with impaired tissue remodeling and angiogenesis and limited tumor growth in c-Myc^fl/fl LysM^cre/+ mice. Macrophage c-Myc deletion also diminished fibrosarcoma growth. These data identify c-Myc as a positive regulator of the pro-tumoral program of TAMs and suggest c-Myc inactivation as an attractive target for anti-cancer therapy.

Semaphorin signals tweaking the tumor microenvironment

Solid tumors not only comprise malignant cells but also other nonmalignant cell types, forming a unique microenvironment that can strongly influence the behavior of tumor cells. Recent advances in the understanding of cancer biology have highlighted the functional role of semaphorins. In fact, semaphorins form a family of molecular signals known to guide and control cell migration during embryo development and in adults. Tumor cells express semaphorins as well as their receptors, plexins and neuropilins. It has been shown that semaphorin signaling can regulate tumor cell behavior. Moreover, semaphorins are important regulators of tumor angiogenesis. Conversely, very little is known about the functional relevance of semaphorin signals for tumor-infiltrating stromal cells, such as leukocytes. In this chapter, we review the current knowledge on the functional role of semaphorins in cancer progression, and we focus on the emerging role of semaphorins in mediating the cross talk between tumor cells and different tumor stromal cells.

Deciphering the roles of macrophages in developmental and inflammation stimulated lymphangiogenesis

Lymphatic vessels share an intimate relationship with hematopoietic cells that commences during embryogenesis and continues throughout life. Lymphatic vessels provide a key conduit for immune cell trafficking during immune surveillance and immune responses and in turn, signals produced by immune lineage cells in settings of inflammation regulate lymphatic vessel growth and activity. In the majority of cases, the recruitment and activation of immune cells during inflammation promotes the growth and development of lymphatic vessels (lymphangiogenesis) and enhances lymph flow, effects that amplify cell trafficking to local lymph nodes and facilitate the mounting of effective immune responses. Macrophages comprise a major, heterogeneous lineage of immune cells that, in addition to key roles in innate and adaptive immunity, perform diverse tasks important for tissue development, homeostasis and repair. Here, we highlight the emerging roles of macrophages in lymphangiogenesis, both during development and in settings of pathology. While much attention has focused on the production of pro-lymphangiogenic stimuli including VEGF-C and VEGF-D by macrophages in models of inflammation including cancer, there is ample evidence to suggest that macrophages provide additional signals important for the regulation of lymphatic vascular growth, morphogenesis and function.

Macrophages and angiogenesis: a role for Wnt signaling

Macrophages regulate many developmental and pathological processes in both embryonic and adult tissues, and recent studies have shown a significant role in angiogenesis. Similarly, Wnt signaling is fundamental to tissue morphogenesis and also has a role in vascular development. In this review, we summarize recent advances in the field of macrophage-regulated angiogenesis, with a focus on the role of macrophage-derived Wnt ligands. We review data that provide both direct and indirect evidence for macrophage-derived Wnt regulation of physiologic and pathologic angiogenesis. Finally, we propose that Wnt signaling plays a central role in differentiation of tumor associated and wound infiltrating macrophages to a proangiogenic phenotype.

Endothelial cells provide an instructive niche for the differentiation and functional polarization of M2-like macrophages

Endothelial cells and macrophages are known to engage in tight and specific interactions that contribute to the modulation of vascular function. Here we show that adult endothelial cells provide critical signals for the selective growth and differentiation of macrophages from several hematopoietic progenitors. The process features the formation of well-organized colonies that exhibit progressive differentiation from the center to the periphery and toward an M2-like phenotype, characterized by enhanced expression of Tie2 and CD206/Mrc1. These colonies are long-lived depending on the contact with the endothelium; removal of the endothelial monolayer results in rapid colony dissolution. We further found that Csf1 produced by the endothelium is critical for the expansion of the macrophage colonies and that blockade of Csf1 receptor impairs colony growth. Functional analyses indicate that these macrophages are capable of accelerating angiogenesis, promoting tumor growth, and effectively engaging in tight associations with endothelial cells in vivo. These findings uncover a critical role of endothelial cells in the induction of macrophage differentiation and their ability to promote further polarization toward a proangiogenic phenotype. This work also highlights some of the molecules underlying the M2-like differentiation, a process that is relevant to the progression of both developmental and pathologic angiogenesis.

Estrogen represses hepatocellular carcinoma (HCC) growth via inhibiting alternative activation of tumor-associated macrophages (TAMs)

BACKGROUND

Hepatocarcinoma cancer (HCC) occurs more often in men than in women, and little is known about its underlying molecular mechanisms.

RESULTS

We identify that 17β-estradiol (E2) could suppress tumor growth via regulating the polarization of macrophages.

CONCLUSION

Estrogen functions as a suppressor for macrophage alternative activation.

SIGNIFICANCE

These studies introduce a novel mechanism for suppressing male-predominant HCC. Hepatocarcinoma cancer (HCC), one of the most malignant cancers, occurs significantly more often in men than in women; however, little is known about its underlying molecular mechanisms. Here we identified that 17β-estradiol (E2) could suppress tumor growth via regulating the polarization of macrophages. We showed that E2 re-administration reduced tumor growth in orthotopic and ectopic mice HCC models. E2 functioned as a suppressor for macrophage alternative activation and tumor progression by keeping estrogen receptor β (ERβ) away from interacting with ATP5J (also known as ATPase-coupling factor 6), a part of ATPase, thus inhibiting the JAK1-STAT6 signaling pathway. These studies introduce a novel mechanism for suppressing male-predominant HCC.

Recent advances in vascular development

PURPOSE OF REVIEW

This review offers a concise summary of the most recent experimental advances in vascular development using the mouse as a model organism.

RECENT FINDINGS

Recent mouse studies have revealed a spread of phenotypic diversity between endothelia of distinct developmental origins and organs. For example, expression of unique transcription factors distinguishes hemogenic from nonhemogenic endothelium within the same vessel. Vasculature of the brain is particularly susceptible to endothelial malformations due to combinatorial germline and somatic mutations; surprisingly these mutations can afflict the endothelium by either cell autonomous or paracrine effects. Mutant mice have been used to understand how multiple signaling pathways integrate and refine cellular responses. In particular, we learned how VEGFR3 regulates Notch signaling and EphrinB2 coordinates VEGFR2 responses. The regulation of Prox1 by miR181 highlighted the contribution of microRNAs in the induction of lymphatic endothelium. Information gained on heterotypic interactions has further clarified the influence of blood vessels on the morphogenesis of parenchyma and contributed to our understanding of organ-specific endothelial differentiation. Finally, mouse models have uncovered endothelial cell polarity as a keystone for successful vascular lumenization.

SUMMARY

Our understanding of the process of vascular development has gained significant refinement in the last two years and has clarified the origin of several disorders rooted in development.