Angiogenic role of LYVE-1-positive macrophages in adipose tissue

Here we report the discovery of a characteristic dense vascular network (DVN) in the tip portion of epididymal adipose tissue in adult mice. The DVN is formed by angiogenesis rather than by vasculogenesis, and has functional blood circulation. This DVN and its subsequent branching may provide a new functional route for adipogenesis. The recruitment, infiltration, and accumulation of bone marrow-derived LYVE-1(+) macrophages in the tip region are crucial for the formation of the DVN. Matrix metalloproteinases (MMPs) and the VEGF-VEGFR2 system are responsible not only for the formation of the DVN, but also for the recruitment and infiltration of LYVE-1(+) macrophages into the epididymal adipose tissue tip region. SDF-1, but not the MCP-1-CCR2 system, is a critical factor in recruitment and ongoing retention of macrophages in this area. We also demonstrate that the tip region of epididymal adipose tissue is highly hypoxic, and thus provides a microenvironment conducive to the high expression and enhanced activities of VEGF, VEGFR2, MMPs, and SDF-1 in autocrine and paracrine manners, to create an ideal niche for the recruitment, retention, and angiogenic action of macrophages. These findings shed light on the complex interplay between macrophage infiltration, angiogenesis, and adipogenesis in the tip region of adult epididymal adipose tissue, and provide novel insight into the regulation of alternative outgrowth of adipose tissue.     

Targeted deletion of adipocytes by apoptosis leads to adipose tissue recruitment of alternatively activated M2 macrophages

Obesity is frequently associated with an infiltration of macrophages into adipose tissue. Adipocyte dysfunction causes a phenotypic switch of macrophages from an alternatively activated M2-like phenotype towards a proinflammatory M1 phenotype. The cross talk between adipocytes and infiltrating immune cells, in particular macrophages, is thought to contribute to local and eventually systemic inflammation. Here, we tested the phenotypic impact of a lack of adipocytes on the inflammatory status of macrophages. We took advantage of the fat apoptosis through targeted activation of caspase-8 (FAT-ATTAC) mouse model that allows for the inducible system-wide elimination of adipocytes through a proapoptotic mechanism and followed the degree and type of inflammatory response upon ablation of live adipocytes. Analysis of depots 2 wk after elimination of adipocytes resulted in markedly reduced levels of adipose tissue and a robust down-regulation of circulating adipokines. Quantitative PCR and immunohistochemistry on epididymal and inguinal fat depots revealed an increase of the macrophage markers F4/80 and CD11c. Using polychromatic flow cytometry, we observed an up-regulation of alternatively activated M2 macrophage markers (CD206 and CD301) on the majority of F4/80 positive cells. Apoptosis of adipocytes is sufficient to initiate a large influx of macrophages into the remnant fat pads. However, these macrophages are alternatively activated, antiinflammatory M2 macrophages and not M1 cells. We conclude that adipocyte death is sufficient to initiate macrophage infiltration, and live adipocytes are required to initiate and/or sustain a proinflammatory response within the infiltrating macrophages in adipose tissue.     

Human adipose tissue macrophages are of an anti-inflammatory phenotype but capable of excessive pro-inflammatory mediator production

OBJECTIVE: Obesity is associated with a chronic low-grade inflammation and an increased abundance of macrophages in adipose tissue. Adipose tissue macrophages (ATMs) are assumed to interfere with adipocyte function leading to insulin resistance, thereby contributing to the pathogenesis of type 2 diabetes mellitus. Macrophages exist in separate types of differentiation, but the nature of ATMs is largely unknown. DESIGN AND MEASUREMENTS: Stromal vascular cells (SVCs) and ATMs were isolated from human adipose tissues from different locations. We characterized ATMs phenotypically and functionally by flow cytometry, endocytosis assay and determination of secreted cytokines. For comparison, we used macrophages of the 'classical' (M1) and the 'alternative', anti-inflammatory (M2) type differentiated in vitro from peripheral blood monocytes. RESULTS: Like prototypic M2 macrophages, ATMs expressed considerable amounts of mannose receptor, haemoglobin scavenger receptor CD163 and integrin alphavbeta5. The number of cells expressing these molecules correlated significantly with the donors' body mass indices (BMIs). Notably, SVCs positive for the common monocyte/macrophage marker CD14 contained a considerable fraction of blood monocytes, the abundance of which did not correlate with the BMIs, pointing to the requirement of the surface markers identified here for the identification of ATMs. ATMs showed endocytic activities similar to M2 macrophages and accordingly secreted high amounts of IL-10 and IL-1 receptor antagonist. However, basal and induced secretion of pro-inflammatory mediators TNF-alpha, IL-6, IL-1, MCP-1 and MIP-1alpha was even higher in ATMs than in pro-inflammatory M1 macrophages. CONCLUSION: ATMs comprise a particular macrophage type that is M2-like by surface marker expression, but they are competent to produce extensive amounts of inflammatory cytokines, which could considerably contribute to the development of insulin resistance.     

Monocytic cells derived from human embryonic stem cells and fetal liver share common differentiation pathways and homeostatic functions

The early emergence of macrophages and their large pattern of tissue distribution during development suggest that they may play a critical role in the initial steps of embryogenesis. In the present study, we show that monocytic cells derived from human embryonic stem cells (hESCs) and from fetal liver follow a differentiation pathway different to that of adult cells, leading to specific functions. Embryonic and fetal monocytic cells differentiated from a CD14(low)CD16(-) precursor to form CD14(high)CD16(+) cells without producing the CD14(high)CD16(-) cell population that predominates in adult peripheral blood. Both demonstrated an enhanced expression of genes encoding tissue-degrading enzymes, chemokines, and scavenger receptors, as was previously reported for M2 macrophages. Compared with adult blood monocytes, embryonic and fetal monocytic cells secreted high amounts of proteins acting on tissue remodeling and angiogenesis, and most of them expressed the Tie2 receptor. Furthermore, they promoted vascular remodeling in xenotransplanted human tumors. These findings suggest that the regulation of human fetal and embryonic monocytic cell differentiation leads to the generation of cells endowed mainly with anti-inflammatory and remodeling functions. Trophic and immunosuppressive functions of M2-polarized macrophages link fetus and tumor development, and hESCs offer a valuable experimental model for in vitro studies of mechanisms sustaining these processes.     

Monocyte chemoattractant protein 1 and CD40 ligation have a synergistic effect on vascular endothelial growth factor production through cyclooxygenase 2 upregulation in gastric cancer

Background Recent studies have reported that expression of monocyte chemoattractant protein 1 (MCP-1) and its receptor (CCR2) and CD40 ligation on mesenchymal cells play important roles in tumor development. Cyclooxygenase 2 (COX-2) has also been shown to contribute to tumor angiogenesis. We examined the interaction between MCP-1 and CD40 ligation in mesenchymal cells in gastric cancer to determine the effect of these factors on vascular endothelial growth factor (VEGF) production via upregulation of COX-2 expression. Methods COX-2, prostaglandin E₂ (PGE₂), and VEGF production were evaluated in CD40 ligand (CD40L)-stimulated macrophages. CD40L and MCP-1 mRNA levels in gastric cancer tissues were evaluated by real-time polymerase chain reaction (PCR). Localizations of MCP-1, CD40L, CD34, CD40, and CCR2 in 34 gastric cancer tissue specimens were evaluated by single-or double-label immunohistochemistry. Results COX-2 expression levels were significantly higher in CD40L-stimulated macrophages and correlated with increased PGE₂ and VEGF production. Addition of MCP-1 to CD40L-stimulated macrophages had a synergistic effect on COX-2 expression and subsequent PGE₂ and VEGF production. CD40L and MCP-1 mRNA levels were significantly higher in poorly differentiated gastric cancers than in H. pylori-infected gastritis patients. High microvessel density was significantly associated with MCP-1 and CCR2 scores and lymph node metastasis. Conclusions MCP-1 and CD40L had a synergistic effect on COX-2 expression and subsequent VEGF production in gastric cancer.     

Role of macrophage infiltration in the orbital fat of patients with Graves' ophthalmopathy

Objective  Infiltration of the retro-ocular space by inflammatory cells, accumulation of glycosaminoglycans, and the overabundance of orbital adipose tissue are characteristic findings in Graves' ophthalmopathy (GO). The cause of macrophage infiltration in the orbital adipose tissue of patients with GO remains to be elucidated.
Design  Immunohistochemistry of orbital adipose tissues with anti-CD68 was used for determining macrophage infiltration pattern and cell counts. Quantitative real-time PCR was used for analysing mRNA expression. Correlation of macrophage infiltration with the duration of GO and mRNA expression were also determined.
Patients  Fifteen subjects with GO who underwent orbital decompression were recruited. Six patients without thyroid history who underwent elective orbital surgery were enrolled as controls.
Measurements  Histological distribution of macrophages, macrophage cell counts, CD68 and monocyte chemoattractant protein-1 (MCP-1) mRNA levels, and duration of GO.
Results  We demonstrated that macrophage infiltration in orbital fat from patients with GO was higher than controls ( P  = 0·005). The infiltration of macrophages was located primarily around blood vessels and between mature adipocytes. Macrophage infiltration did not attenuate in GO of long duration. We also found that the expression of MCP-1 was higher in GO orbital fat than that in the orbital fat of controls ( P =  0·047) and the infiltration of macrophages in adipose tissue from patients with GO was positively correlated with expression of MCP-1 mRNA ( r  = 0·546, P  = 0·035).
Conclusion  Macrophage infiltration may play an important role in the pathogenesis of GO via over-expression of MCP-1.     

Monocyte chemoattractant protein-1 release is higher in visceral than subcutaneous human adipose tissue (AT): implication of macrophages resident in the AT

Human adipose tissue (AT) produces several adipokines including monocyte chemoattractant protein (MCP)-1, involved in the pathogenesis of atherosclerosis. OBJECTIVE: Human AT cultures, isolated adipocytes, and stromal-vascular cells were used to investigate the relationship among AT-resident macrophages, MCP-1, and adiposity and the regulation of MCP-1. RESULTS: mRNA levels of specific macrophage markers (CD68 and CD14) are correlated with adiposity in sc AT and visceral AT (P < 0.05). MCP-1 production is higher in stromal-vascular cells vs. adipocytes (P < 0.01) and correlates with macrophage markers in both AT compartments (P < 0.05). MCP-1 release is higher in obese subjects (P < 0.05) and in VAT (P < 0.01), but after adjusting for AT-resident macrophages, the differences disappear. MCP-1 is stimulated by IL-1beta, TNF-alpha, IL-8, IL-4, and IL-6 + IL-6-soluble receptor and is decreased by dexamethasone, IL-10, metformin, and thiazolidinediones. DISCUSSION: MCP-1 is correlated with specific macrophage markers, adiposity, and AT localization, but the relationship seems to be related to the number of AT-resident macrophages. Despite this, MCP-1 may be involved in obesity-related health complications, and the decrease of MCP-1 by metformin and thiazolidinediones suggests that these antidiabetic compounds have antiinflammatory properties improving the low-grade inflammatory state observed in obesity.     

The Role of the Hemostatic System in Tumor Growth, Metastasis, and Angiogenesis: Tissue factor is a Bifunctional Molecule Capable of Inducing Both Fibrin Deposition and Angiogenesis in Cancer

Cancer patients are prone to venous thromboembolism (VTE), and this hypercoagulability favors tumor growth and metastasis. After a brief review of the clinical aspects of VTE and cancer, we discuss the pathogenesis of hypercoagulability with an emphasis on the role of tissue factor (TF). The discovery that, in addition to tumor cells, TF is expressed by tumor-associated macrophages and tumor-associated endothelial cells led to studies of the role of TF in the regulation of tumor angiogenesis. In human lung cancer, melanoma, and breast cancer, TF and vascular endothelial growth factor (VEGF) co-localize in tumor cells; a close correlation exists between TF and VEGF synthesis (P = .001) in tumor cell lines and with angiogenesis in vivo in a severe, combined immunodeficient mouse model. Transfection of a TF/VEGF low-producing human tumor cell line with full length TF complementary DNA (cDNA) results in conversion to a high producer of TF and VEGF; transfection of a deletion-mutant TF cDNA lacking cytoplasmic serine residues restores full TF procoagulant activity but not VEGF synthesis to the cells. These results suggest that the cytoplasmic tail of TF is necessary for tumor cell VEGF synthesis. Targeting of TF in tumors and tumor-associated blood vessels is discussed as a strategy for drug delivery and rational anti-cancer and anti-angiogenesis drug design.     

Mechanisms linking obesity,inflammation and altered metabolism to colon carcinogenesis

Due to its prevalence, obesity is now considered a global epidemic. It is linked to increased risk of colorectal cancer, the third most common cancer and the second leading cause of death among adults in Western countries. Obese adipose tissue differs from lean adipose tissue in its immunogenic profile, body fat distribution and metabolic profile. Obese adipose tissue releases free fatty acids, adipokines and many pro‐inflammatory chemokines. These factors are known to play a key role in regulating malignant transformation and cancer progression. Obese adipose tissue is infiltrated by macrophages that participate in inflammatory pathways activated within the tissue. Adipose tissue macrophages consist of two different phenotypes. M1 macrophages reside in obese adipose tissue and produce pro‐inflammatory cytokines, and M2 macrophages reside in lean adipose tissue and produce anti‐inflammatory cytokines, such as interleukin‐10 (IL‐10). The metabolic networks that confer tumour cells with their oncogenic properties, such as increased proliferation and the ability to avoid apoptosis are still not well understood. We review the interactions between adipocytes and immune cells that may alter the metabolism towards promotion of colorectal cancer.     

Plasma monocyte chemoattractant protein-1 and pulmonary vascular resistance in chronic thromboembolic pulmonary hypertension.

The pathogenesis of severe pulmonary hypertension seems to be related to inflammatory response in diseased sites. Monocyte chemoattractant protein-1 (MCP-1) has been reported to play a role in the development of congestive heart failure. In this immunological response, activation and migration of leukocytes including macrophages to the inflammatory region are important factors. We hypothesized that the severity of pulmonary hypertension may be related to MCP-1, which is thought to be upregulated by blood pressure or shear stress in pulmonary vasculature as well as by immunological and inflammatory reactions in chronic thromboembolic pulmonary hypertension (CTEPH). Circulating levels of MCP-1, interleukin-1beta (IL-1beta), and tumor necrosis factor-alpha (TNF-alpha) were measured by sandwich ELISA in 14 patients with CTEPH. The plasma level of MCP-1 was significantly correlated with pulmonary vascular resistance. In IL-1beta and TNF-alpha, on the other hand, there was no correlation between cytokines and pulmonary hemodynamics. Pathological specimens obtained from the patients with CTEPH undergoing thromboendarterectomy demonstrated immunoreactivity of MCP-1 in endothelium, smooth muscle cells, and macrophages within neointima in the hypertensive large elastic pulmonary artery. We conclude that MCP-1 is upregulated in the remodeling of pulmonary arteries in close association with increased pulmonary vascular resistance in CTEPH.     

Wnt2 acts as an angiogenic growth factor for non-sinusoidal endothelial cells and inhibits expression of stanniocalcin-1

Recently, we have shown that Wnt2 is an autocrine growth and differentiation factor for hepatic sinusoidal endothelial cells. As Wnt signaling has become increasingly important in vascular development and cancer, we analyzed Wnt signaling in non-sinusoidal endothelial cells of different vascular origin (HUVEC, HUAEC, HMVEC-LLy). Upon screening the multiple components of the Wnt pathway, we demonstrated lack of Wnt2 expression, but presence of Frizzled-4, one of its receptors, in cultured non-sinusoidal endothelial cells. Treatment of these cells by exogenous Wnt2 induced endothelial proliferation and sprouting angiogenesis in vitro. Upon analysis of Wnt2 tissue expression as a basis for paracrine Wnt2 effects on non-sinusoidal endothelial cells in vivo, Wnt2 was found to be expressed in densely vascularized murine malignant tumors and in wound healing tissues in close proximity to CD31+ endothelial cells. By gene profiling, stanniocalcin-1 (STC1), a known regulator of angiogenesis, was identified as a target gene of Wnt2 signaling in HUVEC down-regulated by Wnt2 treatment. Tumor-conditioned media counter-acted Wnt2 and up-regulated STC1 expression in HUVEC. In conclusion, we provide evidence that Wnt2 acts as an angiogenic factor for non-sinusoidal endothelium in vitro and in vivo whose target genes undergo complex regulation by the tissue microenvironment.     

Peroxisome proliferator-activated receptor gamma (PPARgamma) ligands reverse CTL suppression by alternatively activated (M2) macrophages in cancer

Tumors may escape from immune control by the induction of CD11b(+)Gr-1(+) myeloid suppressor cells in the spleen. In this study, we demonstrate that this cell population can be subdivided into a CD11b(hi)Gr-1(int)SSC(lo)Ly6G(neg)M-CSFR(int) immature monocytic fraction and a CD11b(hi+)Gr-1(hi)SSC(hi)Ly6G(hi)M-CSFR(neg) granulocytic fraction. Upon in vitro culture, the monocytic CD11b(+)Gr-1(+) cell fraction is sufficient for cytotoxic T lymphocyte (CTL) suppression, which is linked to the gradual differentiation of these monocytic cells into mature F4/80(+) CD68(+) macrophages. These CTL-suppressive macrophages are alternatively activated (M2), as demonstrated by the expression of known and novel M2 signature genes. In search of M2-associated genes involved in the suppressive activity, it is shown that stimulation of peroxisome proliferator-activated receptor gamma (PPARgamma) and inhibition of phospholipase A(2) (PLA(2)) activity cooperate to alleviate CTL suppression. Of importance, purified tumor-associated macrophages display a similar M2 phenotype and are suppressive for antitumor CTLs, via a mechanism that can be almost completely reversed by PPARgamma ligands. Overall, our data identify PLA(2) and especially PPARgamma as new potential therapeutic targets to subvert macrophage-mediated CTL suppression in cancer.     

5-Lipoxygenase-activating protein: a potential link between innate and adaptive immunity in atherosclerosis and adipose tissue inflammation

Transforming growth factor-beta (TGF-beta) is a major antiinflammatory mediator in atherosclerosis. Transgenic ApoE(-/-) mice with a dominant-negative TGFbeta type II receptor (dnTGFbetaRII) on CD4(+) and CD8(+) T cells display aggravated atherosclerosis. The aim of the present study was to elucidate the mechanisms involved in this enhanced inflammatory response. Gene array analyses identified the 5-lipoxygenase-activating protein (FLAP) among the most upregulated genes in both the aorta and adipose tissue of dnTGFbetaRII transgenic ApoE(-/-) mice compared with their ApoE(-/-) littermates, a finding that was confirmed by real-time quantitative RT-PCR. Aortas from the former mice in addition produced increased amounts of the lipoxygenase product leukotriene B(4) after ex vivo stimulation. FLAP protein expression in both the aorta and adipose tissue was detected in macrophages, but not in T cells. Four weeks of treatment with the FLAP inhibitor MK-886 (10 mg/kg in 1% tylose delivered by osmotic pumps) significantly reduced atherosclerotic lesion size and T-cell content. Finally, FLAP mRNA levels were upregulated approximately 8-fold in adipose tissue derived from obese ob/ob mice. In conclusion, the results of the present study suggest a key role for mediators of the 5-lipoxygenase pathway in inflammatory reactions of atherosclerosis and metabolic disease.     

Significance of macrophage chemoattractant protein-1 expression and macrophage infiltration in squamous cell carcinoma of the esophagus

OBJECTIVES: Macrophage chemoattractant protein-1 (MCP-1) is a chemokine-inducing infiltration of macrophages, which can play several roles in tumor growth and metastasis. We have attempted to clarify the relationship between MCP-1 expression and macrophage infiltration in esophageal squamous cell carcinoma (SCC). METHODS: Paraffin-embedded sections of tissue samples taken from 56 patients with esophageal SCC after curative surgery were immunohistochemically stained for MCP-1, CC chemokine receptor 2 (CCR-2), and thymidine phosphorylase (TP). Macrophage recruitment in SCC was evaluated by monocytic count based on CD68 immunostaining. Microvessels immunostained for Factor VIII-related antigen were counted in SCC, and microvessel density (MVD) was determined. Ki-67 labeling index was calculated based on Ki-67 immunostaining, and an apoptotic index was calculated based on the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate biotin nick end labeling. RESULTS: MCP-1 was expressed in cancer cells of 31 SCC (55.4%) and in stromal cells mainly identified as macrophages of 16 SCC (28.6%). CCR-2 was expressed in stromal cells of all SCC and in vascular endothelial cells of 15 SCC (26.8%). There was a significant correlation between the expression of MCP-1 in cancer cells and of CCR-2 in stromal cells. TP was expressed in stromal cells in 76.7% of the SCC. Monocytic count, MVD, and Ki-67 LI in SCC with MCP-1 expression in cancer cells were higher than that without, and apoptotic index in SCC with MCP-1 expression in cancer cells were lower than that without. Furthermore, the monocytic count was positively correlated with MVD, while it was inversely correlated with apoptotic index. Clinicopathologically, MCP-1 expression in cancer cells was correlated with venous invasion, distant metastasis, and lymph node metastasis. Monocytic count in SCC with venous invasion, distant metastasis, or lymph node metastasis was higher than that without them. Five-year survival rate in the patients with high monocytic count or MCP-1 expression was worse than that with a low monocytic count or without MCP-1 expression. CONCLUSIONS: These results suggest that MCP-1 expression and macrophage infiltration is associated with angiogenic promotion in esophageal SCC. MCP-1 expression may be interactively associated with macrophage infiltration in esophageal SCC; MCP-1 may play an important role in tumor angiogenesis through production of angiogenic factors, such as TP, by recruited macrophages in esophageal SCC. Furthermore, CCR-2 expression in vascular endothelial cells may participate partially in angiogenesis. Clinicopathologically, esophageal SCC patients with MCP-1 expression have no favorable prognosis.     

Neutrophils and macrophages promote angiogenesis in the early stage of endometriosis in a mouse model

Substantial evidence suggests that inflammatory cytokines, immune cells, and angiogenesis are important for endometriosis. In this study, we investigated the role of the sequential events in the development of endometriosis in a mouse model. Uterine tissue was transplanted into the peritoneum of ovariectomized mice and then supplemented with estrogen or vehicle. On different days after transplantation, cell proliferation, angiogenesis, and infiltrated immune cells in ectopic tissue were examined using immunochemical staining. Many disintegrated blood vessels but no bromodeoxyuridine-positive cells in ectopic tissue were observed in the estrogen-treated group on posttransplantation d 1 and 2. On d 4-7, bromodeoxyuridine-positive cells were detected in the blood vessels of ectopic tissue, indicating that angiogenesis was initiated in this stage. Angiogenesis also occurred in ectopic tissue in the vehicle-treated group. Profound infiltration of neutrophils in ectopic tissue occurred on d 1-4, when the number of neutrophils and levels of macrophage inflammatory protein (MIP)-1alpha and MIP-2 chemokines in peritoneal fluids also reached their peak. Peritoneal macrophage numbers did not change, but secretions of TNFalpha, IL-6, MIP-1alpha, and MIP-2 from macrophages isolated on d 2 were higher than on d 0. In vitro studies showed that peritoneal neutrophils and macrophages secreted vascular endothelial growth factor, which was up-regulated by TNFalpha and IL-6. Our results suggest that neutrophils and macrophages may promote angiogenesis in the early stage of endometriosis and that chemokines and cytokines amplify the angiogenic signal for the growth of endometriotic tissue.     

The role of the Angiopoietins in vascular morphogenesis

The Angiopoietin/Tie system acts as a vascular specific ligand/receptor system to control endothelial cell survival and vascular maturation. The Angiopoietin family includes four ligands (Angiopoietin-1, Angiopoietin-2 and Angiopoietin-3/4) and two corresponding tyrosine kinase receptors (Tie1 and Tie2). Ang-1 and Ang-2 are specific ligands of Tie2 binding the receptor with similar affinity. Tie2 activation promotes vessel assembly and maturation by mediating survival signals for endothelial cells and regulating the recruitment of mural cells. Ang-1 acts in a paracrine agonistic manner inducing Tie2 phosphorylation and subsequent vessel stabilization. In contrast, Ang-2 is produced by endothelial cells and acts as an autocrine antagonist of Ang-1-mediated Tie2 activation. Ang-2 thereby primes the vascular endothelium to exogenous cytokines and induces vascular destabilization at higher concentrations. Ang-2 is strongly expressed in the vasculature of many tumors and it has been suggested that Ang-2 may act synergistically with other cytokines such as vascular endothelial growth factor to promote tumor-associated angiogenesis and tumor progression. The better mechanistic understanding of the Ang/Tie system is gradually paving the way toward the rationale exploitation of this vascular signaling system as a therapeutic target for neoplastic and non-neoplastic diseases.     

Endoglin in angiogenesis and vascular diseases

Endoglin is a transmembrane auxillary receptor for transforming growth factor-β (TGF-β) that is predominantly expressed on proliferating endothelial cells. Endoglin deficient mice die during midgestation due to cardiovascular defects. Mutations in endoglin and activin receptor-like kinase 1 (ALK1), an endothelial specific TGF-β type I receptor, have been linked to hereditary hemorrhagic telangiectasia (HHT), an autosomal dominant vascular dysplasia characterized by telangiectases and arteriovenous malformations. Endoglin heterozygote mice develop HHT-like vascular abnormalities, have impaired tumor and post-ischemic angiogenesis and demonstrate an endothelial nitric oxide synthase-dependent deterioration in the regulation of vascular tone. In pre-eclampsia, placenta-derived endoglin has been shown to be strongly upregulated and high levels of soluble endoglin are released into the circulation. Soluble endoglin was found to cooperate with a soluble form of vascular endothelial growth factor receptor 1 in the pathogenesis of pre-eclampsia by inducing endothelial cell dysfunction. Endoglin is highly expressed in tumor-associated endothelium, and endoglin antibodies have been successfully used to target activated endothelial cells and elicit anti-angiogenic effects in tumor mouse models. These exciting advances provide opportunities for the development of new therapies for diseases with vascular abnormalities.     

Bone marrow-derived cells do not engraft into skeletal muscle microvasculature but promote angiogenesis after acute injury

The skeletal muscle is supported by a vast network of microvessels with the capacity to regenerate in response to injury. However, the dynamics of microvascular repair and the origin of reconstituted endothelial cells in the skeletal muscle are poorly understood. A growing body of literature exists to indicate bone marrow (BM)-derived cells engraft into regenerating vascular endothelium and muscle macrovasculature. Therefore, we investigated the extent of BM contribution to skeletal muscle microvasculature after acute injury. Because reporters and markers commonly used to trace donor BM cells are not endothelial specific but are also expressed by leukocytes, we generated novel BM chimeras utilizing Tie2-green fluorescent protein BM cells transplanted into CD31 and Caveolin-1 knockout recipients. In turn, we surveyed BM vascular contribution, not just by the presence of green fluorescent protein, but also CD31 and Caveolin-1, respectively. After stable BM reconstitution, chimera limb muscles were cardiotoxin (CTX) injured and examined 21 days post-injury for the presence of green fluorescent protein, CD31, and Caveolin-1. Acute muscle injury by CTX is characterized by initial microvasculature death followed by rapid endothelial regeneration within 14 days post-damage. Histological analysis of injured and uninjured contralateral limb muscles revealed a complete absence of BM engraftment in the muscle vasculature of wild-type and CD31/Caveolin-1 knockout chimeras. In contrast, F4/80(+) cells isolated from CTX-injured muscle, expressed endothelial-related markers and promoted angiogenesis in?vitro. Therefore, despite the absence of BM engraftment to regenerated skeletal muscle microvasculature, macrophages recruited after injury promote angiogenesis and, in turn, vascular regeneration.