Endoglin (CD 105) expression in human lymphoid organs and placenta

Endoglin in the Spotlight to Treat Cancer

A spotlight has been shone on endoglin in recent years due to that fact of its potential to serve as both a reliable disease biomarker and a therapeutic target. Indeed, endoglin has now been assigned many roles in both physiological and pathological processes. From a molecular point of view, endoglin mainly acts as a co-receptor in the canonical TGFβ pathway, but also it may be shed and released from the membrane, giving rise to the soluble form, which also plays important roles in cell signaling. In cancer, in particular, endoglin may contribute to either an oncogenic or a non-oncogenic phenotype depending on the cell context. The fact that endoglin is expressed by neoplastic and non-neoplastic cells within the tumor microenvironment suggests new possibilities for targeted therapies. Here, we aimed to review and discuss the many roles played by endoglin in different tumor types, as well as the strong evidence provided by pre-clinical and clinical studies that supports the therapeutic targeting of endoglin as a novel clinical strategy.

Endoglin: An 'Accessory' Receptor Regulating Blood Cell Development and Inflammation

Transforming growth factor-β1 (TGF-β1) is a pleiotropic factor sensed by most cells. It regulates a broad spectrum of cellular responses including hematopoiesis. In order to process TGF-β1-responses in time and space in an appropriate manner, there is a tight regulation of its signaling at diverse steps. The downstream signaling is mediated by type I and type II receptors and modulated by the ‘accessory’ receptor Endoglin also termed cluster of differentiation 105 (CD105). Endoglin was initially identified on pre-B leukemia cells but has received most attention due to its high expression on activated endothelial cells. In turn, Endoglin has been figured out as the causative factor for diseases associated with vascular dysfunction like hereditary hemorrhagic telangiectasia-1 (HHT-1), pre-eclampsia, and intrauterine growth restriction (IUPR). Because HHT patients often show signs of inflammation at vascular lesions, and loss of Endoglin in the myeloid lineage leads to spontaneous inflammation, it is speculated that Endoglin impacts inflammatory processes. In line, Endoglin is expressed on progenitor/precursor cells during hematopoiesis as well as on mature, differentiated cells of the innate and adaptive immune system. However, so far only pro-monocytes and macrophages have been in the focus of research, although Endoglin has been identified in many other immune system cell subsets. These findings imply a functional role of Endoglin in the maturation and function of immune cells. Aside the functional relevance of Endoglin in endothelial cells, CD105 is differentially expressed during hematopoiesis, arguing for a role of this receptor in the development of individual cell lineages. In addition, Endoglin expression is present on mature immune cells of the innate (i.e., macrophages and mast cells) and the adaptive (i.e., T-cells) immune system, further suggesting Endoglin as a factor that shapes immune responses. In this review, we summarize current knowledge on Endoglin expression and function in hematopoietic precursors and mature hematopoietic cells of different lineages.

Endoglin as an Adhesion Molecule in Mature and Progenitor Endothelial Cells: A Function Beyond TGF-β

Endoglin (ENG) is a transmembrane glycoprotein expressed on endothelial cells that functions as a co-receptor for several ligands of the transforming growth factor beta (TGF-β) family. ENG is also a recognized marker of angiogenesis and mutations in the endoglin gene are responsible for Hereditary Hemorrhagic Telangiectasia (HHT) type 1, a vascular disease characterized by defective angiogenesis, arteriovenous malformations, telangiectasia, and epistaxis. In addition to its involvement in the TGF-β family signaling pathways, several lines of evidence suggest that the extracellular domain of ENG has a role in integrin-mediated cell adhesion via its RGD motif. Indeed, we have described a role for endothelial ENG in leukocyte trafficking and extravasation via its binding to leukocyte integrins. We have also found that ENG is involved in vasculogenic properties of endothelial progenitor cells known as endothelial colony forming cells (ECFCs). Moreover, the binding of endothelial ENG to platelet integrins regulate the resistance to shear during platelet-endothelium interactions under inflammatory conditions. Because of the need for more effective treatments in HHT and the involvement of ENG in angiogenesis, current studies are aimed at identifying novel biological functions of ENG which could serve as a therapeutic target. This review focuses on the interaction between ENG and integrins with the aim to better understand the role of this protein in blood vessel formation driven by progenitor and mature endothelial cells.

Intra-Amniotic Soluble Endoglin Impairs Lung Development in Neonatal Rats

Soluble endoglin (sENG) is increased in the amniotic fluid of women with preeclampsia and chorioamnionitis. Preterm infants born to women with these disorders have an increased risk of aberrant lung development. Whether this increased risk is secondary to elevated sENG levels is unclear. The objective of this study was to determine whether intrauterine exposure to an adenovirus overexpressing sENG impairs neonatal lung angiogenesis by modulating lung endothelial nitric oxide synthase (eNOS) signaling. Pregnant Sprague-Dawley rats were randomly assigned to receive ultrasound-guided intra-amniotic injections of adenovirus overexpressing sENG (Ad-sENG) or control adenovirus (Ad-control) on embryonic day 17. After this exposure, rat pups were maintained in normoxia and evaluated on postnatal day 14. Intra-amniotic Ad-sENG decreased lung vascular endothelial growth factor receptor 2 and eNOS expression in rat pups. This was accompanied by a marked decrease in lung angiogenesis and alveolarization. Ad-sENG-exposed pups also had an increase in right ventricular systolic pressure, weight ratio of right ventricle to left ventricle plus septum, and pulmonary vascular remodeling. In addition, exposure of human pulmonary artery endothelial cells to recombinant sENG reduced endothelial tube formation and protein levels of eNOS, phosphorylated eNOS, and phosphorylated Smad1/5. Together, our findings demonstrate that intrauterine exposure to an adenovirus overexpressing sENG disrupts lung development by impairing Smad1/5-eNOS signaling. We speculate that sENG-mediated dysregulation of Smad1/5-eNOS signaling contributes to impaired lung development and potentially primes the developing lung for further postnatal insults. Further studies exploring the relationship between amniotic fluid sENG levels and preterm respiratory outcomes will be necessary.

Endothelial endoglin is involved in inflammation: role in leukocyte adhesion and transmigration

Human endoglin is an RGD-containing transmembrane glycoprotein identified in vascular endothelial cells. Although endoglin is essential for angiogenesis and its expression is up-regulated in inflammation and at sites of leukocyte extravasation, its role in leukocyte trafficking is unknown. This function was tested in endoglin heterozygous mice (Eng(+/-)) and their wild-type siblings Eng(+/+) treated with carrageenan or LPS as inflammatory agents. Both stimuli showed that inflammation-induced leukocyte transendothelial migration to peritoneum or lungs was significantly lower in Eng(+/-) than in Eng(+/+) mice. Leukocyte transmigration through cell monolayers of endoglin transfectants was clearly enhanced in the presence of endoglin. Coating transwells with the RGD-containing extracellular domain of endoglin, enhanced leukocyte transmigration, and this increased motility was inhibited by soluble endoglin. Leukocytes stimulated with CXCL12, a chemokine involved in inflammation, strongly adhered to endoglin-coated plates and to endoglin-expressing endothelial cells. This endoglin-dependent adhesion was abolished by soluble endoglin, RGD peptides, the anti-integrin α5β1 inhibitory antibody LIA1/2 and the chemokine receptor inhibitor AMD3100. These results demonstrate for the first time that endothelial endoglin interacts with leukocyte integrin α5β1 via its RGD motif, and this adhesion process is stimulated by the inflammatory chemokine CXCL12, suggesting a regulatory role for endoglin in transendothelial leukocyte trafficking.

The role of autocrine TGFbeta1 in endothelial cell activation induced by phagocytosis of necrotic trophoblasts: a possible role in the pathogenesis of pre-eclampsia

Pre-eclampsia is a disorder of pregnancy characterized by hypertension and endothelial cell dysfunction. The causes of pre-eclampsia are unclear but it is proposed that a factor released from the placenta triggers the maternal symptoms. One possible triggering factor is dead trophoblasts that are shed from the placenta, then deported to become trapped in the maternal pulmonary capillaries. It is hypothesized that trophoblasts die by apoptosis in normal pregnancy, but by necrosis in pre-eclampsia. Deported trophoblasts may be phagocytosed by the pulmonary endothelial cells and we have previously shown that phagocytosis of necrotic trophoblasts leads to the activation of endothelial cells, accompanied by the release of interleukin-6 from these cells. However, the mechanistic pathway linking phagocytosis of necrotic trophoblasts and endothelial cell activation is unknown. Here we show that, after phagocytosis of necrotic, but not apoptotic, trophoblasts, endothelial cells secrete TGFbeta1. Using recombinant endoglin to inhibit the function of TGFbeta1 we have shown that the TGFbeta1 does not directly activate endothelial cells but rather it induces endothelial IL-6 secretion. The IL-6 then induces endothelial cell activation. Inhibiting either TGFbeta1 or IL-6 prevented endothelial cell activation in response to phagocytosing necrotic trophoblasts, but inhibiting IL-6 did not prevent secretion of TGFbeta1, confirming the order of signalling. IL-6 also reduced endothelial cell-surface endoglin but increased the amount of soluble endoglin released from placental explants. These interactions between the IL-6 and TGFbeta1 pathways in both the endothelium and placenta may help to regulate the maternal response to deported trophoblasts in pregnancy.

The change in concentrations of angiogenic and anti-angiogenic factors in maternal plasma between the first and second trimesters in risk assessment for the subsequent development of preeclampsia and small-for-gestational age

INTRODUCTION

An imbalance between angiogenic and anti-angiogenic factors has been proposed as central to the pathophysiology of preeclampsia (PE). Indeed, patients with PE and those delivering small-for-gestational age (SGA) neonates have higher plasma concentrations of soluble vascular endothelial growth factor receptor-1 (sVEGFR-1) and the soluble form of endoglin (s-Eng), as well as lower plasma concentrations of vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) than do patients with normal pregnancies. Of note, this imbalance has been observed before the clinical presentation of PE or the delivery of an SGA neonate. The objective of this study was to determine if changes in the profile of angiogenic and anti-angiogenic factors in maternal plasma between the first and second trimesters are associated with a high risk for the subsequent development of PE and/or delivery of an SGA neonate.

METHODS

This longitudinal case-control study included 402 singleton pregnancies in the following groups: (1) normal pregnancies with appropriate for gestational age (AGA) neonates (n = 201); (2) patients who delivered an SGA neonate (n = 145); and (3) patients who developed PE (n = 56). Maternal plasma samples were obtained at the time of each prenatal visit, scheduled at 4-week intervals from the first or early second trimester until delivery. In this study, we included two samples per patient: (1) first sample obtained between 6 and 15 weeks of gestation ('first trimester' sample), and (2) second sample obtained between 20 and 25 weeks of gestation ('second trimester' sample). Plasma concentrations of s-Eng, sVEGFR-1, and PlGF were determined by specific and sensitive immunoassays. Changes in the maternal plasma concentrations of these angiogenesis-related factors were compared among normal patients and those destined to develop PE or deliver an SGA neonate while adjusting for maternal age, nulliparity, and body mass index. General linear models and polytomous logistic regression models were used to relate the analyte concentrations, ratios, and product to the subsequent development of PE and SGA.

RESULTS

(1) An increase in the maternal plasma concentration of s-Eng between the first and second trimesters conferred risk for the development of preterm PE and SGA (OR 14.9, 95% CI 4.9-45.0 and OR 2.9, 95% CI 1.5-5.6, respectively). (2) An increase in the maternal plasma concentration of sVEGFR-1 between the first and second trimester conferred risk for the development of preterm PE (OR 3.9, 95% CI 1.2-12.6). (3) A subnormal increase in maternal plasma PlGF concentration between the first and the second trimester was a risk factor for the subsequent development of preterm and term PE (OR 4.3, 95% CI 1.2-15.5 and OR 2.7, 95% CI 1.2-5.9, respectively). (4) In addition, the combination of the three analytes into a pro-angiogenic versus anti-angiogenic ratio (PlGF/(s-Eng x VEGFR-1)) conferred risk for the subsequent development of preterm PE (OR 3.7, 95% CI 1.1-12.1). (5) Importantly, patients with a high change in the s-Eng x sVEGFR-1 product had an OR of 10.4 (95% CI 3.2-33.8) for the development of preterm PE and 1.6 (95% CI 1.0-2.6) for the development of SGA.

CONCLUSIONS

Changes in the maternal plasma concentrations of s-Eng, sVEGFR-1, PlGF or their ratios between the first and second trimesters of pregnancy confer an increased risk to deliver an SGA neonate and/or develop PE.

The role of TGF-beta in allergic inflammation

The transforming growth factor beta (TGF-beta) plays a dual role in allergic disease. It is important in suppressing T cells and also mediates repair responses that lead to unwanted remodeling of tissues. Advances in the immunology of allergy indicate that allergens cause overreactions in the lymphocyte compartment because of the lack or decreased number of suppressive, regulatory T cells. TGF-beta was shown to induce regulatory T cells and participate directly in suppression of effector T cells. Therefore, TGF-beta may help return reactivity to allergens to normal subsymptomatic activity. Whether chronic inflammatory diseases such as asthma profit from TGF-beta-mediated suppression of specific immune responses or whether the TGF-beta-mediated tissue remodeling aggravates diseases more than it helps control immune reactions is unclear. This article addresses these issues and future strategies in this field.

FGF-1 and S100A13 possibly contribute to angiogenesis in endometriosis

Endometriosis is referred often as an angiogenic disease. The pivotal role of angiogenesis in the pathophysiology of this disease has been confirmed by many studies. This process has several steps, and VEGF is probably the most important in its initiation. There are others involved in its continuation and maintenance of the tight balance between a quiescent and activated blood vessel state. In the process of formation of new blood capillaries and arterioles, many different factors are involved in sometimes distinct pathways. Such factors are TGF-beta and endoglin--the latter being one of the main modulators of the TGF-beta signaling pathway. Endoglin is now not only established as a marker of active neo-angiogenesis and activated endothelium, but also turns to be an active player in the very process of endometriotic angiogenesis. Its signaling pathway of hypoxic activation is tightly interconnected with that of VEGF, and also some of the FGFs. FGF-1 and S100A13 are members of two distinct families of proteins -- the FGFs, growth and angiogenic factors, and that of the S100 proteins, -- Ca(2+)-binding proteins involved in cell function regulation, motility and signaling. These two particular members are quite unique in having no signal peptide sequence and being involved in common export pathway. Our hypothesis is that these two factors are involved in vascular remodeling in endometriotic angiogenesis, playing a role in vascular wall formation and migration of endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). We believe also that endoglin is tightly involved in the new arteriolar formation in endometriosis, being expressed in VSMCs but not on the ECs of the middle-sized vessels.

TGF-{beta} signaling of human T cells is modulated by the ancillary TGF-{beta} receptor endoglin

Transforming growth factor beta (TGF-beta) inhibits T cell activation and alters differentiation of naive T cells into effector cells. Although four main cell-surface proteins can interact with TGF-beta, only the signaling receptors type I (TGF-betaR type I) and type II (TGF-betaR type II) have so far been described on T cells. The aim of the present study was to investigate the expression of the ancillary receptor endoglin (CD105) by T cells and its role in TGF-beta-mediated signal transduction and function. CD105 expression was analyzed on resting and activated human CD4(+) T cells by flow cytometry, western blot, immunoprecipitation, proliferation and SMAD-responsive reporter gene assays. CD4(+) T cells constitutively expressed CD105 in memory T cells and partially also in naive T cells; however, surface expression is regulated and is increased following TCR engagement, which induced serine/threonine phosphorylation of CD105. In contrast to the suppressive signal mediated by the TGF-beta, cross-linking of CD105 substantially enhanced T cell proliferation, indicating that CD105 by itself mediates signal transduction. Furthermore, CD105 cross-linking induced SMAD-independent signaling via ERK kinase phosphorylation. The present study demonstrates that CD105 is expressed on the surface by activated CD4(+) T cells and CD3 regulated by post-translational means. Furthermore, CD105 acts as a regulatory receptor, counteracting TGF-beta-mediated suppression.

CCL21 chemokine regulates chemokine receptor CCR7 bearing malignant melanoma cells

PURPOSE

The chemokine CC-ligand 21/secondary lymphoid tissue chemokine (CCL21/SLC) regulates the homing of naïve T cells and dendritic cells that express CC-chemokine receptor 7 (CCR7) from distant sites to lymphoid tissue such as lymph nodes. We hypothesized that CCL21/SLC regulates the migration of CCR7-bearing melanoma cells from a primary lesion to regional tumor-draining lymph nodes.

EXPERIMENTAL DESIGN

Quantitative real-time reverse transcriptase-PCR (qRT) assay and immunohistochemistry (IHC) were used to assess the level of CCR7 expression in melanoma cell lines and in primary and metastatic melanoma tumors. Cell migration assay using melanoma cell lines was performed under the induction of CCL21/SLC. The CCL21/SLC expression level in tumor-draining sentinel lymph nodes (SLNs) was assessed by both qRT assay and IHC.

RESULTS

Melanoma cell lines and tumors demonstrated heterogeneous expression of CCR7 mRNA by qRT assay. There was strong functional correlation between CCR7 mRNA expression and cell migration induced by CCL21/SLC. IHC evidence of CCR7 expression in primary melanomas significantly (P = 0.02) correlated with Breslow thickness. Assessment of SLN from 55 melanoma patients by qRT assay demonstrated that CCL21/SLC mRNA expression level was significantly (P = 0.008) higher in pathologically melanoma-negative SLNs than in melanoma-positive SLNs.

CONCLUSIONS

This report demonstrates a potential mechanism for recruitment and homing of CCR7(+) metastatic melanoma cells to tumor-draining lymph nodes, which express CCL21/SLC. The study also suggests that lymph nodes bearing metastasis may suppress CCL21/SLC production.

Lumican is down-regulated in cells expressing endoglin. Evidence for an inverse correlationship between Endoglin and Lumican expression

Endoglin (CD105) is a homodimeric membrane glycoprotein, which acts as a TGF-beta coreceptor in the vasculature and plays an important role in cardiovascular development and vascular remodelling. To isolate putative genes regulated by endoglin expression, a PCR-based RNA fingerprinting technique was carried out. Myoblasts stably transfected with endoglin showed a decrease in the expression of lumican both at the RNA and protein levels. Lumican is a proteoglycan of the extracellular matrix, belonging to the SLRP (Small Leucine-Rich Repeat Proteoglycans) family. Lumican down-regulation by endoglin appeared to be controlled, at least in part, at the transcriptional level, as indicated by RT-PCR, and transient transfection experiments using a lumican promoter reporter based vector. This inverse correlation between endoglin and lumican expression was substantiated by immunohistochemical staining of vessels from human tissues. Thus, cells belonging to the high endothelia, such as tonsil, express a large amount of endoglin, and the lumican content of their matrix is considerably reduced. Conversely, in resting endothelia, such as that of large vessels, the expression of endoglin is reduced whereas the amount of lumican is greatly increased. The inverse regulation in the expression of endoglin and lumican was also evident after TGF-beta treatments since endoglin was up-regulated, whereas lumican was down-regulated by this cytokine. This report describes for the first time a relationship between endoglin and lumican expression.

Targeting gene expression to endothelium in transgenic animals: a comparison of the human ICAM-2, PECAM-1 and endoglin promoters

It is highly likely that successful pig-to-human xenotransplantation of vascularized organs will require genetic modification of the donor pig, and in particular of donor vascular endothelium. Promoters are generally tested in transgenic mice before generating transgenic pigs. Several promoters have been used to drive endothelial cell-specific expression in mice but none have yet been tested in pigs. We compared the promoters of three human genes that are predominantly expressed in vascular endothelium: intercellular adhesion molecule 2 (ICAM-2), platelet endothelial cell adhesion molecule 1 (PECAM-1) and endoglin. Expression of human complement regulatory proteins (hCRPs), directed by each of the promoters in mice, was largely restricted to vascular endothelium and leukocyte subpopulations. However, expression from the PECAM-1 promoter was weak in liver and non-uniform in the small vessels of heart, kidney, and lung. Conversely, expression from the endoglin promoter was consistently strong in the small vessels of these organs but was absent in larger vessels. The ICAM-2 promoter, which produced strong and uniform endothelial expression in all organs examined, was therefore used to generate hCRP transgenic pigs. Leukocytes from 57 pigs containing at least one intact transgene were tested for transgene expression by flow cytometry. Forty-seven of these transgenic pigs were further analyzed by immunohistochemical staining of liver biopsies, and 18 by staining of heart and kidney sections. Only two of the pigs showed expression, which appeared to be restricted to vascular endothelium in heart and kidney but was markedly weaker than in transgenic mice produced with the same batch of DNA. Thus, in this case, promoter performance in mice and pigs was not equivalent. The weak expression driven by the human ICAM-2 promoter in pigs relative to mice suggests the need for additional regulatory elements to achieve species-specific gene expression in pigs.

Immunophenotypic analysis of human spleen compartments

Microanatomical compartments of the human spleen are yet under evaluation as most of the present information comes from experiments on animals with different anatomical structures. Immune staining of stromal and blood-born cells by cell surface antigens facilitates the differentiation of functional microanatomical compartmentalization of immune organs, including the spleen. Twenty-two specimens from healthy adult subjects with the average age of 35.6 +/- 13.8 (Range 17 to 58) years were included in this study. Monoclonal antibodies used in this study were supplied from the 5th, 6th and 7th International Workshops and Conferences on Human Leukocyte Differentiation Antigens. Tetraspan antigens presented a rather unique staining pattern in the human spleen, suggesting special roles for each (CD9, CD53, CD63, CD151 and CD231) in certain locations. Sinus lining cells presented a distinctive antigenic profile, sharing both endothelial cell (CD31, CD36, CD54, CD62P, CD102, CD105, CD106 and CD146) and macrophage lineage characteristics. The sheathed capillaries were not restricted to the perifollicular zone alone. Extracellular matrix receptors (CD49 a, CD49 b, CD49 c, CD49 e, CD49f, CD29 and CD44) stained the penicillary arterioles and vascular smooth muscle. These molecules were also found on the vascular endothelium. Leukocyte antigens (CD11a, CD11b, CD22, CD43, CD45, CD45RB, CD45RO and CD50) were mainly expressed in the white and red pulp of the spleen at different intensities, excluding the penicillary arterioles. Activation antigens (CD26, CD71 and CD98) presented a diffuse and broad staining pattern. In conclusion, microanatomical compartmentalization, microcirculation and function of the human spleen were evaluated using a wide panel of monoclonal antibodies.

Vasculogenesis in early human placental villi: an ultrastructural study

In this study we examined the chorionic villi of 5 normal human placentas at 12-14 weeks of gestation ultrastructurally with regard to differentiation of the vascular components. The aim of the present report is to discuss the factors influencing vasculogenesis (in situ formation of blood vessels) at the ultrastructural level. Our observations have led us to think that the cytotrophoblast influences vasculogenesis in human chorionic villi. Mesenchymal-preendothelial cell groups were always found in very close association with the cytotrophoblast at the periphery of the villi, forming blood vessels. The cytotrophoblast probably attracts mesenchymal cells towards the margin of the villi by secreting vascular endothelial growth factor (VEGF). Once cells attach to the trophoblastic basement membrane they begin to differentiate into endothelial cells. This close structural relation between two cell types (cytotrophoblast and mesenchymal cells) may not be the only mechanism controlling vasculogenesis, but it seems to be one of the factors influencing the differentiation of mesenchymal cells into the endothelial cells of blood vessels in early human chorionic villi.