Endoglin promotes endothelial cell proliferation and TGF-beta/ALK1 signal transduction

Circulating CD105 shows significant impact in patients of oral cancer and promotes malignancy of cancer cells via CCL20

CD105 is rich in endothelium cells and is involved in angiogenesis. Higher microvascular density of tumor is also related to the prognosis in a variety of cancers. In this present study, patients with positive N classification, advanced T classification, advanced TNM stage, extracapsular spread of lymph nodes (ECS), and perineural invasion had significantly higher levels of peripheral vein (pCD105) and venous return from tumor (tCD105) in 71 patients with OSCC compared to 13 healthy volunteers. Those with higher pCD105 or tCD105 levels had significantly poorer 5-year disease-specific survival rate (DDS) and overall survival rate (OS). The tCD105 and pCD105 levels and ECS were the independent prognostic factors by the multivariate analysis according to the Cox regression model in 5-year DDS and OS rate. SAS and SCC4 cells treated with CD105 showed the increase in migration, invasion, and proliferation in vitro and in vivo. Furthermore, CCL20 expression participated in CD105-elicited cell motility in oral cancer cells. In conclusion, higher level of circulating CD105 is related to adverse pathological features among patients with OSCC. It is also a useful marker for evaluating the prognosis and targeting therapeutics of OSCC.

TGF-β signalling in tumour associated macrophages

Tumour associated macrophages (TAM) represent an important component of tumour stroma. They develop under the influence of tumour microenvironment where transforming growth factor (TGF)β is frequently present. Activities of TAM regulated by TGFβ stimulate proliferation of tumour cells and lead to tumour immune escape. Despite high importance of TGFβ-induction of TAM activities till now our understanding of the mechanism of this induction is limited. We have previously developed a model of type 2 macrophages (M2) resembling certain properties of TAM. We established that in M2 TGFβRII is regulated on the level of subcellular sorting by glucocorticoids. Further studies revealed that in M2 with high levels of TGFβRII on the surface TGFβ activates not only its canonical Smad2/3-mediated signaling, but also Smad1/5-mediated signaling, what is rather typical for bone morphogenetic protein (BMP) stimulation. Complexity of macrophage populations, however, allows assumption that TGFβ signalling may function in different ways depending on the functional state of the cell. To understand the peculiarities of TGFβ signalling in human TAMs experimental systems using primary cells have to be developed and used together with the modern mathematical modelling approaches.

Endothelial Plasticity: Shifting Phenotypes through Force Feedback

The endothelial lining of the vasculature is exposed to a large variety of biochemical and hemodynamic stimuli with different gradients throughout the vascular network. Adequate adaptation requires endothelial cells to be highly plastic, which is reflected by the remarkable heterogeneity of endothelial cells in tissues and organs. Hemodynamic forces such as fluid shear stress and cyclic strain are strong modulators of the endothelial phenotype and function. Although endothelial plasticity is essential during development and adult physiology, proatherogenic stimuli can induce adverse plasticity which contributes to disease. Endothelial-to-mesenchymal transition (EndMT), the hallmark of endothelial plasticity, was long thought to be restricted to embryonic development but has emerged as a pathologic process in a plethora of diseases. In this perspective we argue how shear stress and cyclic strain can modulate EndMT and discuss how this is reflected in atherosclerosis and pulmonary arterial hypertension.

Endothelium-derived microparticles from chronically thromboembolic pulmonary hypertensive patients facilitate endothelial angiogenesis

Background

Increased circulating levels of endoglin⁺ endothelial microparticles (EMPs) have been identified in several cardiovascular disorders, related to severity. Endoglin is an auxilary receptor for transforming growth factor β (TGF-β) important in the regulation of vascular structure.

Results

We quantified the number of microparticles in plasma of six patients with chronic thromboembolic pulmonary hypertension (CTEPH) and age- and sex-matched pulmonary embolic (PE) and healthy controls and investigated the role of microparticle endoglin in the regulation of pulmonary endothelial function in vitro. Results show significantly increased levels of endoglin⁺ EMPs in CTEPH plasma, compared to healthy and disease controls. Co-culture of human pulmonary endothelial cells with CTEPH microparticles increased intracellular levels of endoglin and enhanced TGF-β-induced angiogenesis and Smad1,5,8 phosphorylation in cells, without affecting BMPRII expression. In an in vitro model, we generated endothelium-derived MPs with enforced membrane localization of endoglin. Co-culture of these MPs with endothelial cells increased cellular endoglin content, improved cell survival and stimulated angiogenesis in a manner similar to the effects induced by overexpressed protein.

Conclusions

Increased generation of endoglin⁺ EMPs in CTEPH is likely to represent a protective mechanism supporting endothelial cell survival and angiogenesis, set to counteract the effects of vascular occlusion and endothelial damage.

Electronic supplementary material

The online version of this article (doi:10.1186/s12929-016-0224-9) contains supplementary material, which is available to authorized users.

Smad signal pathway regulates angiogenesis via endothelial cell in an adipose-derived stromal cell/endothelial cell co-culture, 3D gel model

Co-implantation of adipose-derived stromal cells (ASCs) and endothelial cells (ECs) can markedly expedite the formation of functional microvascular beds and provides possible methods for cell-based revascularization therapies to treat various diseases. Furthermore, we investigated the role of TGFβ/Smad signaling pathway for angiogenesis in a three-dimensional (3D) collagen gel model established in vitro with co-culture between ASCs and ECs. We found that angiogenesis was attenuated in the co-culture gels after inhibition of ALK5/Smad2/3 with SB431542. Genes coding for VEGF-A, VEGF-B, VE-ca, FGF-1, PDGF, BMP-4, and BMP-7 were significantly reduced in both mono-cultured and co-cultured ECs. Furthermore, the decrease in co-cultured ECs was prominent relative to mono-cultured ECs. Taken together, these findings suggest that in the co-culture between ASCs and ECs, TGFβ/Smad signal pathway regulates angiogenesis via ECs; moreover, the findings that the co-cultured ECs were regulated more significantly than mono-cultured ECs suggest that suppression of Smad signal pathway may regulate the paracrine secretion of ASCs to further modulate angiogenesis of ECs.

TGF-β & BMP receptors endoglin and ALK1: overview of their functional role and status as antiangiogenic targets

The formation of new blood vessels from existing vasculature, angiogenesis, is facilitated through a host of different signaling processes. Members of the TGF-β superfamily, TGF-β1, TGF-β3, and BMP9, are key propagators of both inhibition and initiation of angiogenesis. HHT, characterized by AVM and capillary bed defects, is caused by germline mutations in the ENG and ACVRL1/ALK1 genes, respectively. Clinical symptoms include epistaxis and GI hemorrhage. The membranous receptors endoglin and ALK1 activate proliferation and migration of endothelial cells during the angiogenic process via the downstream intracellular SMAD signaling pathway. Endothelial cell senescence or activation is dependent on the type of cytokine, ligand concentration, cell-cell interaction, and a multitude of other signaling molecules. Endoglin and ALK1 receptor levels in tumor vasculature correlate inversely with prognosis in humans, whereas in mice, endoglin deficiency decelerates tumor progression. Therefore, endoglin and ALK1 have been identified as potential therapeutic targets for antibody treatment in various cancers. Early phase clinical trials in humans are currently underway to evaluate the efficacy and safety of biological therapy targeting endoglin/ALK1-mediated cells signaling.

Heterozygous disruption of activin receptor-like kinase 1 is associated with increased arterial pressure in mice

The activin receptor-like kinase 1 (ALK-1) is a type I cell-surface receptor for the transforming growth factor-β (TGF-β) family of proteins. Hypertension is related to TGF-β1, because increased TGF-β1 expression is correlated with an elevation in arterial pressure (AP) and TGF-β expression is upregulated by the renin-angiotensin-aldosterone system. The purpose of this study was to assess the role of ALK-1 in regulation of AP using Alk1 haploinsufficient mice (Alk1(+/-)). We observed that systolic and diastolic AP were significantly higher in Alk1(+/-) than in Alk1(+/+) mice, and all functional and structural cardiac parameters (echocardiography and electrocardiography) were similar in both groups. Alk1(+/-) mice showed alterations in the circadian rhythm of AP, with higher AP than Alk1(+/+) mice during most of the light period. Higher AP in Alk1(+/-) mice is not a result of a reduction in the NO-dependent vasodilator response or of overactivation of the peripheral renin-angiotensin system. However, intracerebroventricular administration of losartan had a hypotensive effect in Alk1(+/-) and not in Alk1(+/+) mice. Alk1(+/-) mice showed a greater hypotensive response to the β-adrenergic antagonist atenolol and higher concentrations of epinephrine and norepinephrine in plasma than Alk1(+/+) mice. The number of brain cholinergic neurons in the anterior basal forebrain was reduced in Alk1(+/-) mice. Thus, we concluded that the ALK-1 receptor is involved in the control of AP, and the high AP of Alk1(+/-) mice is explained mainly by the sympathetic overactivation shown by these animals, which is probably related to the decreased number of cholinergic neurons.

PECAM-1 isoforms, eNOS and endoglin axis in regulation of angiogenesis

Vascular development and maintenance of proper vascular function through various regulatory mechanisms are critical to our wellbeing. Delineation of the regulatory processes involved in development of the vascular system and its function is one of the most important topics in human physiology and pathophysiology. Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31), a cell adhesion molecule with proangiogenic and proinflammatory activity, has been the subject of numerous studies. In the present review, we look at the important roles that PECAM-1 and its isoforms play during angiogenesis, and its molecular mechanisms of action in the endothelium. In the endothelium, PECAM-1 not only plays a role as an adhesion molecule but also participates in intracellular signalling pathways which have an impact on various cell adhesive mechanisms and endothelial nitric oxide synthase (eNOS) expression and activity. In addition, recent studies from our laboratory have revealed an important relationship between PECAM-1 and endoglin expression. Endoglin is an essential molecule during angiogenesis, vascular development and integrity, and its expression and activity are compromised in the absence of PECAM-1. In the present review we discuss the roles that PECAM-1 isoforms may play in modulation of endothelial cell adhesive mechanisms, eNOS and endoglin expression and activity, and angiogenesis.

VEGF, Notch and TGFβ/BMPs in regulation of sprouting angiogenesis and vascular patterning

The blood vasculature is constantly adapting to meet the demand from tissue. In so doing, branches may form, reorganize or regress. These complex processes employ integration of multiple signalling cascades, some of them being restricted to endothelial and mural cells and, hence, suitable for targeting of the vasculature. Both genetic and drug targeting experiments have demonstrated the requirement for the vascular endothelial growth factor (VEGF) system, the Delta-like-Notch system and the transforming growth factor β (TGFβ)/bone morphogenetic protein (BMP) cascades in vascular development. Although several of these signalling cascades in part converge into common downstream components, they differ in temporal and spatial regulation and expression. For example, the pro-angiogenic VEGFA is secreted by cells in need of oxygen, presented to the basal side of the endothelium, whereas BMP9 and BMP10 are supplied via the bloodstream in constant interaction with the apical side to suppress angiogenesis. Delta-like 4 (DLL4), on the other hand, is provided as an endothelial membrane bound ligand. In the present article, we discuss recent data on the integration of these pathways in the process of sprouting angiogenesis and vascular patterning and malformation.

Regulation of TGF-β receptor hetero-oligomerization and signaling by endoglin

Endoglin is a modulator of TGF-β signaling in endothelial cells. We show that it forms stable homodimers serving as a scaffold for binding TβRII, ALK5, and ALK1. ALK1 and ALK5 bind endoglin differentially, with TβRII recruiting ALK5. Signaling data indicate a role for this receptor complex in balancing TGF-β signaling between Smad1/5/8 and Smad2/3.

Complex formation among transforming growth factor-β (TGF-β) receptors and its modulation by coreceptors represent an important level of regulation for TGF-β signaling. Oligomerization of ALK5 and the type II TGF-β receptor (TβRII) has been thoroughly investigated, both in vitro and in intact cells. However, such studies, especially in live cells, are missing for the endothelial cell coreceptor endoglin and for the ALK1 type I receptor, which enables endothelial cells to respond to TGF-β by activation of both Smad2/3 and Smad1/5/8. Here we combined immunoglobulin G–mediated immobilization of one cell-surface receptor with lateral mobility studies of a coexpressed receptor by fluorescence recovery after photobleaching (FRAP) to demonstrate that endoglin forms stable homodimers that function as a scaffold for binding TβRII, ALK5, and ALK1. ALK1 and ALK5 bind to endoglin with differential dependence on TβRII, which plays a major role in recruiting ALK5 to the complex. Signaling data indicate a role for the quaternary receptor complex in regulating the balance between TGF-β signaling to Smad1/5/8 and to Smad2/3.

Oscillating glucose and constant high glucose induce endoglin expression in endothelial cells: the role of oxidative stress

AIM

High glucose-induced oxidative stress has been suggested as one of the mediators of endothelial damage in diabetes. The major endothelial protein, endoglin, has been found overexpressed in the vessels during pathological situations, but little is known about its relation to diabetic vascular complications. To clarify the role of endoglin in endothelial injury, we sought to determine the effects of high and oscillating glucose on its expression.

MATERIALS

Furthermore, the activation of the Krüppel-like factor 6 (KLF-6) and the hypoxia-inducible factor-1α (HIF-1α) as possible regulators of endoglin expression has been evaluated. The possible role of the oxidative stress has been studied evaluating the effects of the antioxidant alpha-lipoic acid (ALA) and the cellular antioxidant response mediated by

NAD(P)H

quinine-oxido-reductase-1 (NQO-1) and heme oxygenase-1 (HO-1).

RESULTS

Primary HUVECs were cultured for 21 days in normal, high and oscillating glucose (5, 25 and 5/25 mmol/l every 24 h, respectively) with/without ALA. In oscillating and high glucose total endoglin, its soluble form (sEng), KLF-6 and HIF-1α were significantly increased. Simultaneously, the oxidative DNA stress markers 8-OHdG and H2A.X were elevated. Moreover, ENG gene transcriptional rate increased during glucose exposures concomitantly with increased KLF-6 nuclear translocations. ALA significantly reduced all these phenomena. Interestingly, during oscillating and chronic high glucose, NQO-1 and HO-1 did not increase, but ALA induced their overexpression.

CONCLUSIONS

Together, these findings provide novel clue about endoglin in the regulation of high glucose-mediated vascular damage in HUVECs and the role of oxidative stress in this regulation.

Endoglin potentiates nitric oxide synthesis to enhance definitive hematopoiesis

During embryonic development, hematopoietic cells develop by a process of endothelial-to hematopoietic transition of a specialized population of endothelial cells. These hemogenic endothelium (HE) cells in turn develop from a primitive population of FLK1(+) mesodermal cells. Endoglin (ENG) is an accessory TGF-β receptor that is enriched on the surface of endothelial and hematopoietic stem cells and is also required for the normal development of hemogenic precursors. However, the functional role of ENG during the transition of FLK1(+) mesoderm to hematopoietic cells is ill defined. To address this we used a murine embryonic stem cell model that has been shown to mirror the temporal emergence of these cells in the embryo. We noted that FLK1(+) mesodermal cells expressing ENG generated fewer blast colony-forming cells but had increased hemogenic potential when compared with ENG non-expressing cells. TIE2(+)/CD117(+) HE cells expressing ENG also showed increased hemogenic potential compared with non-expressing cells. To evaluate whether high ENG expression accelerates hematopoiesis, we generated an inducible ENG expressing ES cell line and forced expression in FLK1(+) mesodermal or TIE2(+)/CD117(+) HE cells. High ENG expression at both stages accelerated the emergence of CD45(+) definitive hematopoietic cells. High ENG expression was associated with increased pSMAD2/eNOS expression and NO synthesis in hemogenic precursors. Inhibition of eNOS blunted the ENG induced increase in definitive hematopoiesis. Taken together, these data show that ENG potentiates the emergence of definitive hematopoietic cells by modulating TGF-β/pSMAD2 signalling and increasing eNOS/NO synthesis.

Critical regulators of endothelial cell functions: for a change being alternative

SIGNIFICANCE

The endothelium regulates vessel dilation and constriction, balances hemostasis, and inhibits thrombosis. In addition, pro- and anti-angiogenic molecules orchestrate proliferation, survival, and migration of endothelial cells. Regulation of all these processes requires fine-tuning of signaling pathways, which can easily be tricked into running the opposite direction when exogenous or endogenous signals get out of hand. Surprisingly, some critical regulators of physiological endothelial functions can turn malicious by mere alternative splicing, leading to the expression of protein isoforms with opposite functions.

RECENT ADVANCES

While reviewing the evidence of alternative splicing on cellular physiology, it became evident that expression of splice factors and their activities are regulated by externally triggered signaling cascades. Furthermore, genome-wide identification of RNA-binding sites of splicing regulatory proteins now offer a glimpse into the splicing code responsible for alternative splicing of molecules regulating endothelial functions.

CRITICAL ISSUES

Due to the constantly growing number of transcript and protein isoforms, it will become more and more important to identify and characterize all transcripts and proteins regulating endothelial cell functions. One critical issue will be a non-ambiguous nomenclature to keep consistency throughout different laboratories.

FUTURE DIRECTIONS

RNA-deep sequencing focusing on exon-exon junction needs to more reliably identify alternative splicing events combined with functional analyses that will uncover more splice variants contributing to or inhibiting proper endothelial functions. In addition, understanding the signals mediating alternative splicing and its regulation might allow us to derive new strategies to preserve endothelial function by suppressing or upregulating specific protein isoforms. Antioxid. Redox Signal. 22, 1212-1229.

Endoglin Regulation of Smad2 Function Mediates Beclin1 Expression and Endothelial Autophagy

Autophagy is the targeted degradation of proteins and organelles critical for homeostasis and cell survival. Transforming growth factor β (TGF-β) differentially regulates autophagy in a context-specific manner, although the precise intracellular mechanisms remain less clear. Importantly, how TGF-β controls autophagic responses in endothelial cells (EC) during angiogenesis is unknown. Here we identified endoglin, an EC-specific TGF-β co-receptor essential for angiogenesis, as a key determinant of autophagy. Among the two opposing TGF-β Smad pathways in the EC system (Smad1/5/8 and Smad2/3), we found Smad2 as the major transcriptional regulator of autophagy that targets beclin1 (BECN1) gene expression. Smad2, but not Smad3, acts as a repressor upstream of the BECN1 promoter region. Overall, endoglin promotes autophagy by impeding Smad2 transcriptional repressor activity. Notably, increased beclin1 levels upon Smad2 knockdown directly correlated with enhanced autophagy during angiogenesis. Taken together, these results establish endoglin as a critical mediator of autophagy and demonstrate a new transcriptional mechanism by which Smad2 inhibits angiogenesis.

BMP9 Crosstalk with the Hippo Pathway Regulates Endothelial Cell Matricellular and Chemokine Responses

Endoglin is a type III TGFβ auxiliary receptor that is upregulated in endothelial cells during angiogenesis and, when mutated in humans, results in the vascular disease hereditary hemorrhagic telangiectasia (HHT). Though endoglin has been implicated in cell adhesion, the underlying molecular mechanisms are still poorly understood. Here we show endoglin expression in endothelial cells regulates subcellular localization of zyxin in focal adhesions in response to BMP9. RNA knockdown of endoglin resulted in mislocalization of zyxin and altered formation of focal adhesions. The mechanotransduction role of focal adhesions and their ability to transmit regulatory signals through binding of the extracellular matrix are altered by endoglin deficiency. BMP/TGFβ transcription factors, SMADs, and zyxin have recently been implicated in a newly emerging signaling cascade, the Hippo pathway. The Hippo transcription coactivator, YAP1 (yes-associated protein 1), has been suggested to play a crucial role in mechanotransduction and cell-cell contact. Identification of BMP9-dependent nuclear localization of YAP1 in response to endoglin expression suggests a mechanism of crosstalk between the two pathways. Suppression of endoglin and YAP1 alters BMP9-dependent expression of YAP1 target genes CCN1 (cysteine-rich 61, CYR61) and CCN2 (connective tissue growth factor, CTGF) as well as the chemokine CCL2 (monocyte chemotactic protein 1, MCP-1). These results suggest a coordinate effect of endoglin deficiency on cell matrix remodeling and local inflammatory responses. Identification of a direct link between the Hippo pathway and endoglin may reveal novel mechanisms in the etiology of HHT.

Cutting the brakes on hematopoietic regeneration by blocking TGFβ to limit chemotherapy-induced myelosuppression

Hematopoietic stressors such as infection, bleeding, or toxic injury trigger a hematopoietic adaptation that sacrifices hematopoietic stem and progenitor cell (HSPC) quiescence to meet an urgent need for new blood cell production. Once the hematopoietic demands are adequately met, homeostasis must be restored. Transforming growth factor β (TGFβ) signaling is a central mediator mandating the return of HSPCs to quiescence after stress. Blockade of TGFβ signaling after hematopoietic stress delays the return of cycling HSPCs to quiescence and in so doing promotes hematopoietic stem cell (HSC) self-renewal and accelerates hematopoietic reconstitution. These findings open the door to new therapeutics that modulate the hematopoietic adaptation to stress. In this review, we will discuss the complex context-dependent activities of TGFβ in hematopoiesis and the potential benefits and limitations of using TGFβ pathway inhibitors to promote multilineage hematopoietic reconstitution after myelosuppressive chemotherapy.

Global gene expression profiling of telangiectasial tissue from patients with hereditary hemorrhagic telangiectasia

Hereditary hemorrhagic telangiectasia (HHT), the most common inherited vascular disorder, is predominantly caused by mutations in ENG and ACVRL1, which are part of the transforming growth factor beta (TGF-β) signaling pathway. HHT is characterized by the presence of mucocutaneous telangiectases and arteriovenous malformations in visceral organs, primarily the lungs, brain and liver. The most common symptom in HHT is epistaxis originating from nasal telangiectasia, which can be difficult to prevent and can lead to severe anemia. The clinical manifestations of HHT are extremely variable, even within family members, and the exact mechanism of how endoglin and ALK1 haploinsufficiency leads to HHT manifestations remains to be identified.

OBJECTIVES

The purpose of this study was to detect significantly differentially regulated genes in HHT, and try to elucidate the pathways and regulatory mechanisms occurring in the affected tissue of HHT patients, in order to further characterize this disorder and hypothesize on how telangiectases develop. By microarray technology (Agilent G3 Human GE 8x60), we performed global gene expression profiling of mRNA transcripts from HHT nasal telangiectasial (n = 40) and non-telangiectasial (n = 40) tissue using a paired design. Comparing HHT telangiectasial and non-telangiectasial tissue, significantly differentially expressed genes were detected using a paired t-test. Gene set analysis was performed using GSA-SNP. In the group of ENG mutation carriers, we detected 67 differentially expressed mRNAs, of which 62 were down-regulated in the telangiectasial tissue. Gene set analysis identified the gene ontology (GO) terms vasculogenesis, TGF-β signaling, and Wnt signaling as differentially expressed in HHT1. Altered Wnt signaling might be related to HHT pathogenesis and a greater understanding of this may lead to the discovery of therapeutic targets in HHT.

Mononuclear cells and vascular repair in HHT

Hereditary hemorrhagic telangiectasia (HHT) or Rendu–Osler–Weber disease is a rare genetic vascular disorder known for its endothelial dysplasia causing arteriovenous malformations and severe bleedings. HHT-1 and HHT-2 are the most prevalent variants and are caused by heterozygous mutations in endoglin and activin receptor-like kinase 1, respectively. An undervalued aspect of the disease is that HHT patients experience persistent inflammation. Although endothelial and mural cells have been the main research focus trying to unravel the mechanism behind the disease, wound healing is a process with a delicate balance between inflammatory and vascular cells. Inflammatory cells are part of the mononuclear cells (MNCs) fraction, and can, next to eliciting an immune response, also have angiogenic potential. This biphasic effect of MNC can hold a promising mechanism to further elucidate treatment strategies for HHT patients. Before MNC are able to contribute to repair, they need to home to and retain in ischemic and damaged tissue. Directed migration (homing) of MNCs following tissue damage is regulated by the stromal cell derived factor 1 (SDF1). MNCs that express the C-X-C chemokine receptor 4 (CXCR4) migrate toward the tightly regulated gradient of SDF1. This directed migration of monocytes and lymphocytes can be inhibited by dipeptidyl peptidase 4 (DPP4). Interestingly, MNC of HHT patients express elevated levels of DPP4 and show impaired homing toward damaged tissue. Impaired homing capacity of the MNCs might therefore contribute to the impaired angiogenesis and tissue repair observed in HHT patients. This review summarizes recent studies regarding the role of MNCs in the etiology of HHT and vascular repair, and evaluates the efficacy of DPP4 inhibition in tissue integrity and repair.

Genetic variation in the functional ENG allele inherited from the non-affected parent associates with presence of pulmonary arteriovenous malformation in hereditary hemorrhagic telangiectasia 1 (HHT1) and may influence expression of PTPN14

HHT shows clinical variability within and between families. Organ site and prevalence of arteriovenous malformations (AVMs) depend on the HHT causative gene and on environmental and genetic modifiers. We tested whether variation in the functional ENG allele, inherited from the unaffected parent, alters risk for pulmonary AVM in HHT1 mutation carriers who are ENG haploinsufficient. Genetic association was found between rs10987746 of the wild type ENG allele and presence of pulmonary AVM [relative risk = 1.3 (1.0018-1.7424)]. The rs10987746-C at-risk allele associated with lower expression of ENG RNA in a panel of human lymphoblastoid cell lines (P = 0.004). Moreover, in angiogenically active human lung adenocarcinoma tissue, but not in uninvolved quiescent lung, rs10987746-C was correlated with expression of PTPN14 (P = 0.004), another modifier of HHT. Quantitative TAQMAN expression analysis in a panel of normal lung tissues from 69 genetically heterogeneous inter-specific backcross mice, demonstrated strong correlation between expression levels of Eng, Acvrl1, and Ptpn14 (r2 = 0.75-0.9, P < 1 × 10(-12)), further suggesting a direct or indirect interaction between these three genes in lung in vivo. Our data indicate that genetic variation within the single functional ENG gene influences quantitative and/or qualitative differences in ENG expression that contribute to risk of pulmonary AVM in HHT1, and provide correlative support for PTPN14 involvement in endoglin/ALK1 lung biology in vivo. PTPN14 has been shown to be a negative regulator of Yap/Taz signaling, which is implicated in mechanotransduction, providing a possible molecular link between endoglin/ALK1 signaling and mechanical stress. EMILIN2, which showed suggestive genetic association with pulmonary AVM, is also reported to interact with Taz in angiogenesis. Elucidation of the molecular mechanisms regulating these interactions in endothelial cells may ultimately provide more rational choices for HHT therapy.

Role of secreted modular calcium-binding protein 1 (SMOC1) in transforming growth factor β signalling and angiogenesis

AIMS

Secreted modular calcium-binding protein 1 (SMOC1) is a matricellular protein that potentially interferes with growth factor receptor signalling. The aim of this study was to determine how its expression is regulated in endothelial cells and its role in the regulation of endothelial cell function.

METHODS AND RESULTS

SMOC1 was expressed by native murine endothelial cells as well as by cultured human, porcine, and murine endothelial cells. SMOC1 expression in cultured cells was increased by hypoxia via the down-regulation of miR-223, and SMOC1 expression was increased in lungs from miR-223-deficient mice. Silencing SMOC1 (small interfering RNA) attenuated endothelial cell proliferation, migration, and sprouting in in vitro angiogenesis assays. Similarly endothelial cell sprouting from aortic rings ex vivo as well as postnatal retinal angiogenesis in vivo was attenuated in SMOC1(+/-) mice. In endothelial cells, transforming growth factor (TGF)-β signalling via activin-like kinase (ALK) 5 leads to quiescence, whereas TGF-β signalling via ALK1 results in endothelial cell activation. SMOC1 acted as a negative regulator of ALK5/SMAD2 signalling, resulting in altered α2 integrin levels. Mechanistically, SMOC1 associated (immunohistochemistry, proximity ligation assay, and co-immunoprecipitation) with endoglin; an endothelium-specific type III auxiliary receptor for the TGF-β super family and the effects of SMOC1 down-regulation on SMAD2 phosphorylation were abolished by the down-regulation of endoglin.

CONCLUSION

These results indicate that SMOC1 is an ALK5 antagonist produced by endothelial cells that tips TGF-β signalling towards ALK1 activation, thus promoting endothelial cell proliferation and angiogenesis.

Proteoglycans in Normal and Healing Skin

Significance: Proteoglycans have a distinct spatial localization in normal skin and are essential for the correct structural development, organization, hydration, and functional properties of this tissue. The extracellular matrix (ECM) is no longer considered to be just an inert supportive material but is a source of directive, spatial and temporal, contextual information to the cells via components such as the proteoglycans. There is a pressing need to improve our understanding of how these important molecules functionally interact with other matrix structures, cells and cellular mediators in normal skin and during wound healing. Recent Advances: New antibodies to glycosaminoglycan side chain components of skin proteoglycans have facilitated the elucidation of detailed localization patterns within skin. Other studies have revealed important proliferative activities of proteinase-generated fragments of proteoglycans and other ECM components (matricryptins). Knockout mice have further established the functional importance of skin proteoglycans in the assembly and homeostasis of the normal skin ECM. Critical Issues: Our comprehension of the molecular and structural complexity of skin as a complex, dynamic, constantly renewing, layered connective tissue is incomplete. The impact of changes in proteoglycans on skin pathology and the wound healing process is recognized as an important area of pathobiology and is an area of intense investigation. Future Directions: Advanced technology is allowing the development of new artificial skins. Recent knowledge on skin proteoglycans can be used to incorporate these molecules into useful adjunct therapies for wound healing and for maintenance of optimal tissue homeostasis in aging skin.

Mouse models of hereditary hemorrhagic telangiectasia: recent advances and future challenges

Hereditary hemorrhagic telangiectasia (HHT) is a genetic disorder characterized by a multi-systemic vascular dysplasia and hemorrhage. The precise factors leading to these vascular malformations are not yet understood and robust animal models of HHT are essential to gain a detailed understanding of the molecular and cellular events that lead to clinical symptoms, as well as to test new therapeutic modalities. Most cases of HHT are caused by mutations in either endoglin (ENG) or activin receptor-like kinase 1 (ACVRL1, also known as ALK1). Both genes are associated with TGFβ/BMP signaling, and loss of function mutations in the co-receptor ENG are causal in HHT1, while HHT2 is associated with mutations in the signaling receptor ACVRL1. Significant advances in mouse genetics have provided powerful ways to study the function of Eng and Acvrl1 in vivo, and to generate mouse models of HHT disease. Mice that are null for either Acvrl1 or Eng genes show embryonic lethality due to major defects in angiogenesis and heart development. However mice that are heterozygous for mutations in either of these genes develop to adulthood with no effect on survival. Although these heterozygous mice exhibit selected vascular phenotypes relevant to the clinical pathology of HHT, the phenotypes are variable and generally quite mild. An alternative approach using conditional knockout mice allows us to study the effects of specific inactivation of either Eng or Acvrl1 at different times in development and in different cell types. These conditional knockout mice provide robust and reproducible models of arteriovenous malformations, and they are currently being used to unravel the causal factors in HHT pathologies. In this review, we will summarize the strengths and limitations of current mouse models of HHT, discuss how knowledge obtained from these studies has already informed clinical care and explore the potential of these models for developing improved treatments for HHT patients in the future.

Pericytes as targets in hereditary hemorrhagic telangiectasia

Defective paracrine Transforming Growth Factor-β (TGF-β) signaling between endothelial cells and the neighboring mural cells have been thought to lead to the development of vascular lesions that are characteristic of Hereditary Hemorrhagic Telangiectasia (HHT). This review highlights recent progress in our understanding of TGF-β signaling in mural cell recruitment and vessel stabilization and how perturbed TGF-β signaling might contribute to defective endothelial-mural cell interaction affecting vessel functionalities. Our recent findings have provided exciting insights into the role of thalidomide, a drug that reduces both the frequency and the duration of epistaxis in individuals with HHT by targeting mural cells. These advances provide opportunities for the development of new therapies for vascular malformations.

The role of TGFβ1 and LRG1 in cardiac remodelling and heart failure

Heart failure is a life-threatening condition that carries a considerable emotional and socio-economic burden. As a result of the global increase in the ageing population, sedentary life-style, increased prevalence of risk factors, and improved survival from cardiovascular events, the incidence of heart failure will continue to rise. Despite the advances in current cardiovascular therapies, many patients are not suitable for or may not benefit from conventional treatments. Thus, more effective therapies are required. Transforming growth factor (TGF) β family of cytokines is involved in heart development and dys-regulated TGFβ signalling is commonly associated with fibrosis, aberrant angiogenesis and accelerated progression into heart failure. Therefore, a potential therapeutic pathway is to modulate TGFβ signalling; however, broad blockage of TGFβ signalling may cause unwanted side effects due to its pivotal role in tissue homeostasis. We found that leucine-rich α-2 glycoprotein 1 (LRG1) promotes blood vessel formation via regulating the context-dependent endothelial TGFβ signalling. This review will focus on the interaction between LRG1 and TGFβ signalling, their involvement in the pathogenesis of heart failure, and the potential for LRG1 to function as a novel therapeutic target.

Lung Cancer Biomarkers

Lung cancer is the most frequently occurring cancer in the world and continually leads in mortality among cancers. The overall 5-year survival rate for lung cancer has risen only 4% (from 12% to 16%) over the past 4 decades, and late diagnosis is a major obstacle in improving lung cancer prognosis. Survival of patients undergoing lung resection is greater than 80%, suggesting that early detection and diagnosis of cancers before they become inoperable and lethal will greatly improve mortality. Lung cancer biomarkers can be used for screening, detection, diagnosis, prognosis, prediction, stratification, therapy response monitoring, and so on. This review focuses on noninvasive diagnostic and prognostic biomarkers. For that purpose, our discussion in this review will focus on biological fluid-based biomarkers. The body fluids include blood (serum or plasma), sputum, saliva, BAL, pleural effusion, and VOC. Since it is rich in different cellular and molecular elements and is one of the most convenient and routine clinical procedures, serum or plasma is the main source for the development and validation of many noninvasive biomarkers. In terms of molecular aspects, the most widely validated ones are proteins, some of which are used in the clinical sector, though in limited accessory purposes. We will also discuss the lung cancer (protein) biomarkers in clinical trials and currently in the validation phase with hundreds of samples. After proteins, we will discuss microRNAs, methylated DNA, and circulating tumor cells, which are being vigorously developed and validated as potential lung cancer biomarkers. The main aim of this review is to provide researchers and clinicians with an understanding of the potential noninvasive lung cancer biomarkers in biological fluids that have recently been discovered.

The great escape; the hallmarks of resistance to antiangiogenic therapy

The concept of antiangiogenic therapy in cancer treatment has led to the approval of different agents, most of them targeting the well known vascular endothelial growth factor pathway. Despite promising results in preclinical studies, the efficacy of antiangiogenic therapy in the clinical setting remains limited. Recently, awareness has emerged on resistance to antiangiogenic therapies. It has become apparent that the intricate complex interplay between tumors and stromal cells, including endothelial cells and associated mural cells, allows for escape mechanisms to arise that counteract the effects of these targeted therapeutics. Here, we review and discuss known and novel mechanisms that contribute to resistance against antiangiogenic therapy and provide an outlook to possible improvements in therapeutic approaches.

Embelin inhibits endothelial mitochondrial respiration and impairs neoangiogenesis during tumor growth and wound healing

In the normal quiescent vasculature, only 0.01% of endothelial cells (ECs) are proliferating. However, this proportion increases dramatically following the angiogenic switch during tumor growth or wound healing. Recent evidence suggests that this angiogenic switch is accompanied by a metabolic switch. Here, we show that proliferating ECs increasingly depend on mitochondrial oxidative phosphorylation (OxPhos) for their increased energy demand. Under growth conditions, ECs consume three times more oxygen than quiescent ECs and work close to their respiratory limit. The increased utilization of the proton motif force leads to a reduced mitochondrial membrane potential in proliferating ECs and sensitizes to mitochondrial uncoupling. The benzoquinone embelin is a weak mitochondrial uncoupler that prevents neoangiogenesis during tumor growth and wound healing by exhausting the low respiratory reserve of proliferating ECs without adversely affecting quiescent ECs. We demonstrate that this can be exploited therapeutically by attenuating tumor growth in syngenic and xenograft mouse models. This novel metabolic targeting approach might be clinically valuable in controlling pathological neoangiogenesis while sparing normal vasculature and complementing cytostatic drugs in cancer treatment.

Curtailing endothelial TGF-β signaling is sufficient to reduce endothelial-mesenchymal transition and fibrosis in CKD

Excessive TGF-β signaling in epithelial cells, pericytes, or fibroblasts has been implicated in CKD. This list has recently been joined by endothelial cells (ECs) undergoing mesenchymal transition. Although several studies focused on the effects of ablating epithelial or fibroblast TGF-β signaling on development of fibrosis, there is a lack of information on ablating TGF-β signaling in the endothelium because this ablation causes embryonic lethality. We generated endothelium-specific heterozygous TGF-β receptor knockout (TβRII(endo+/-)) mice to explore whether curtailed TGF-β signaling significantly modifies nephrosclerosis. These mice developed normally, but showed enhanced angiogenic potential compared with TβRII(endo+/+) mice under basal conditions. After induction of folic acid nephropathy or unilateral ureteral obstruction, TβRII(endo+/-) mice exhibited less tubulointerstitial fibrosis, enhanced preservation of renal microvasculature, improvement in renal blood flow, and less tissue hypoxia than TβRII(endo+/+) counterparts. In addition, partial deletion of TβRII in the endothelium reduced endothelial-to-mesenchymal transition (EndoMT). TGF-β-induced canonical Smad2 signaling was reduced in TβRII(+/-) ECs; however, activin receptor-like kinase 1 (ALK1)-mediated Smad1/5 phosphorylation in TβRII(+/-) ECs remained unaffected. Furthermore, the S-endoglin/L-endoglin mRNA expression ratio was significantly lower in TβRII(+/-) ECs compared with TβRII(+/+) ECs. These observations support the hypothesis that EndoMT contributes to renal fibrosis and curtailing endothelial TGF-β signals favors Smad1/5 proangiogenic programs and dictates increased angiogenic responses. Our data implicate endothelial TGF-β signaling and EndoMT in regulating angiogenic and fibrotic responses to injury.

The role of endoglin in kidney fibrosis

Tubulointerstitial fibrosis and glomerulosclerosis, are a major feature of end stage chronic kidney disease (CKD), characterised by an excessive accumulation of extracellular matrix (ECM) proteins. Transforming growth factor beta-1 (TGF-β1) is a cytokine with an important role in many steps of renal fibrosis such as myofibroblast activation and proliferation, ECM protein synthesis and inflammatory cell infiltration. Endoglin is a TGF-β co-receptor that modulates TGF-β responses in different cell types. In numerous cells types, such as mesangial cells or myoblasts, endoglin regulates negatively TGF-β-induced ECM protein expression. However, recently it has been demonstrated that 'in vivo' endoglin promotes fibrotic responses. Furthermore, several studies have demonstrated an increase of endoglin expression in experimental models of renal fibrosis in the kidney and other tissues. Nevertheless, the role of endoglin in renal fibrosis development is unclear and a question arises: Does endoglin protect against renal fibrosis or promotes its development? The purpose of this review is to critically analyse the recent knowledge relating to endoglin and renal fibrosis. Knowledge of endoglin role in this pathology is necessary to consider endoglin as a possible therapeutic target against renal fibrosis.

Functional analysis of endoglin mutations from hereditary hemorrhagic telangiectasia type 1 patients reveals different mechanisms for endoglin loss of function

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant inheritable vascular dysplasia caused by mutations in genes encoding either endoglin or activin receptor-like kinase-1 (ALK1). Functional significance of endoglin missense mutations remains largely unknown leading to a difficult discrimination between polymorphisms and pathogenic mutations. In order to study the functional significance of endoglin mutations and to help HHT1 diagnosis, we developed a cellular assay based on the ability of endoglin to enhance ALK1 response to bone morphogenetic protein 9 (BMP9). We generated and characterized 31 distinct ENG mutants reproducing human HHT1 missense mutations identified in patients of the Molecular Genetics Department in Lyon. We found that 16 mutants behaved like wild-type (WT) endoglin, and thus corresponded to benign rare variants. The 15 other variants showed defects in BMP9 response and were identified as pathogenic mutations. Interestingly, two mutants (S278P and F282V) had lost their ability to bind BMP9, identifying two crucial amino acids for BMP9 binding to endoglin. For all the others, the functional defect was correlated with a defective trafficking to the cell surface associated with retention in the endoplasmic reticulum. Further, we demonstrated that some intracellular mutants dimerized with WT endoglin and impaired its cell-surface expression thus acting as dominant-negatives. Taken together, we show that endoglin loss-of-function can result from different mechanisms in HHT1 patients. We also provide a diagnostic tool helping geneticists in screening for novel or conflicting ENG mutations.

ALK5 and ALK1 play antagonistic roles in transforming growth factor β-induced podosome formation in aortic endothelial cells

Transforming growth factor β (TGF-β) and related cytokines play a central role in the vascular system. In vitro, TGF-β induces aortic endothelial cells to assemble subcellular actin-rich structures specialized for matrix degradation called podosomes. To explore further this TGF-β-specific response and determine in which context podosomes form, ALK5 and ALK1 TGF-β receptor signaling pathways were investigated in bovine aortic endothelial cells. We report that TGF-β drives podosome formation through ALK5 and the downstream effectors Smad2 and Smad3. Concurrent TGF-β-induced ALK1 signaling mitigates ALK5 responses through Smad1. ALK1 signaling induced by BMP9 also antagonizes TGF-β-induced podosome formation, but this occurs through both Smad1 and Smad5. Whereas ALK1 neutralization brings ALK5 signals to full potency for TGF-β-induced podosome formation, ALK1 depletion leads to cell disturbances not compatible with podosome assembly. Thus, ALK1 possesses passive and active modalities. Altogether, our results reveal specific features of ALK1 and ALK5 signaling with potential clinical implications.

The role of pericyte detachment in vascular rarefaction

BACKGROUND

Pericytes surround endothelial cells at the perivascular interface. Signaling between endothelial cells and pericytes is crucial for capillary homeostasis, as pericytes stabilize vessels and regulate many microvascular functions. Recently it has been shown that pericytes are able to detach from the vascular wall and contribute to fibrosis by becoming scar-forming myofibroblasts in many organs including the kidney. At the same time, the loss of pericytes within the perivascular compartment results in vulnerable capillaries which are prone to instability, pathological angiogenesis, and, ultimately, rarefaction.

AIMS

This review will give an overview of pericyte-endothelial cell interactions, summarize the signaling pathways that have been identified to be involved in pericyte detachment from the vascular wall, and present pathological endothelial responses in the context of disease of the kidney.

Endoglin promotes angiogenesis in cell- and animal-based models of retinal neovascularization

PURPOSE

We investigated endoglin expression in hypoxic microvascular endothelial cells and retinal endoglin expression in rats that develop experimental oxygen-induced retinopathy (OIR). We also tested neutralizing antibodies (Abs) against endoglin (anti-CD105 Ab) and VEGF (anti-VEGF Ab) either alone or in combination for efficacy against serum-induced retinal microvascular endothelial cell proliferation and retinal neovascularization (NV) in OIR rats. To our knowledge, this marks the first time that a biologic agent has been used to target retinal endoglin and modulate retinal neovascularization.

METHODS

Induction of endoglin by hypoxia was measured by immunohistochemical analysis and ELISA. Proliferation was quantified using a colorimetric 5-bromo-2-deoxyuridine ELISA. Western blots were used to measure endoglin levels in retinas of OIR rats. Immunohistochemical staining was also preformed in OIR rats using anti-CD105 and fluorescein isothiocyanate-conjugated isolectin B4 antibodies.

RESULTS

Anti-CD105 Ab and Anti-VEGF Ab, administered alone or in combination, reduced serum-induced retinal microvascular endothelial cell proliferation. Additionally, in a rat model of oxygen-induced retinopathy, retinal endoglin was significantly increased at 14(2), 14(3), 14(4) and 14(6) compared with retinal levels in control rats. At 14(2), immunohistochemical analysis demonstrated that endoglin was elevated in newly developed vessels at the peripheral extent of major veins, precisely where NV is expected to develop in OIR rats. Neutralizing anti-CD105 reduced retinal NV in OIR rats.

CONCLUSIONS

Our data support other studies showing that reduction of endoglin expression inhibits retinal NV. Our findings demonstrate that retinal endoglin immunolocalization overlaps with nascent neovascular structures in OIR rats. Therefore, endoglin may serve as a useful predictor of incipient neovascular disease.

Transforming growth factor β--at the centre of systemic sclerosis

Transforming growth factor β (TGF-β) has long been implicated in fibrotic diseases, including the multisystem fibrotic disease systemic sclerosis (SSc). Expression of TGF-β-regulated genes in fibrotic skin and lungs of patients with SSc correlates with disease activity, which points to this cytokine as the central mediator of pathogenesis. Patients with SSc often develop pulmonary arterial hypertension (PAH), a particularly lethal complication caused by vascular dysfunction. Several genetic diseases with vascular features related to SSc, such as familial PAH and hereditary haemorrhagic telangiectasia, are caused by mutations in the TGF-β-sensing ALK-1 signalling pathway. These observations suggest that increased TGF-β signalling causes both vascular and fibrotic features of SSc. The question of how latent TGF-β becomes activated in local SSc tissues is, therefore, central to the understanding of SSc. Both TGF-β1 and TGF-β3 can be activated by integrins αvβ6 and αvβ8, whose upregulation in bronchial epithelial cells can activate TGF-β in SSc lungs. Other αv integrins, thrombospondin-1 or altered TGF-β sequestration by matrix proteins might be important in other target tissues. How the immune system triggers this process remains unclear, although links between inflammation and TGF-β activation are emerging. Together, these observations provide an increasingly secure framework for understanding TGF-β in SSc pathogenesis.

Activin A is anti-lymphangiogenic in a melanoma mouse model

Melanoma spreads primarily to the sentinel lymph nodes, and its risk correlates with lymphangiogenesis, which is mainly driven by vascular endothelial growth factor (VEGF)-C. However, anti-lymphangiogenic factors are poorly characterized. We have shown in a melanoma model that Wnt1 reduces lymphangiogenesis by reducing VEGF-C expression. Screening this model for additional potentially anti-lymphangiogenic factors identified increased activin A expression and reduced expression of the antagonist, follistatin (FST), in Wnt1(+) cells. Activin A is known to reduce blood vessel formation, but the effects on lymphangiogenesis are unknown. Here we show that human primary melanoma expresses significantly higher levels of activin A and lower levels of FST compared with nevi and melanoma metastasis. Using our mouse model with melanoma cells overexpressing Wnt1, FST, Wnt1/FST, or the inhibin βA subunit (INHBA, resulting in activin A expression), we found both activin A and Wnt1 to reduce lymphangiogenesis. Whereas Wnt1 also reduced metastasis, this was not seen with activin A. In vitro, activin A phosphorylated SMAD2 in both melanoma and lymphatic endothelium but, although it reduced sprouting of lymphatic endothelium, it enhanced the migration of melanoma cells. In conclusion, activin A is an anti-lymphangiogenic factor, but because of its pleiotropic effects on cell mobility it appears not suitable as a pharmacological target.

Investigation of endoglin wild-type and missense mutant protein heterodimerisation using fluorescence microscopy based IF, BiFC and FRET analyses

The homodimeric transmembrane receptor endoglin (CD105) plays an important role in angiogenesis. This is highlighted by mutations in its gene, causing the vascular disorder HHT1. The main role of endoglin function has been assigned to the modulation of transforming growth factor β and bone morphogenetic protein signalling in endothelial cells. Nevertheless, other functions of endoglin have been revealed to be involved in different cellular functions and in other cell types than endothelial cells. Compared to the exploration of its natural function, little experimental data have been gathered about the mode of action of endoglin HHT mutations at the cellular level, especially missense mutations, and to what degree these might interfere with normal endoglin function. In this paper, we have used fluorescence-based microscopic techniques, such as bimolecular fluorescence complementation (BiFC), immunofluorescence staining with the endoglin specific monoclonal antibody SN6, and protein interaction studies by Förster Resonance Energy Transfer (FRET) to investigate the formation and cellular localisation of possible homo- and heterodimers composed of endoglin wild-type and endoglin missense mutant proteins. The results show that all of the investigated missense mutants dimerise with themselves, as well as with wild-type endoglin, and localise, depending on the position of the affected amino acid, either in the rough endoplasmic reticulum (rER) or in the plasma membrane of the cells. We show that the rER retained mutants reduce the amount of endogenous wild-type endoglin on the plasma membrane through interception in the rER when transiently or stably expressed in HMEC-1 endothelial cells. As a result of this, endoglin modulated TGF-β1 signal transduction is also abrogated, which is not due to TGF-β receptor ER trafficking interference. Protein interaction analyses by FRET show that rER located endoglin missense mutants do not perturb protein processing of other membrane receptors, such as TβRII, ALK5 or ALK1.

Src-mediated post-translational regulation of endoglin stability and function is critical for angiogenesis

Endoglin is a transforming growth factor β (TGF-β) co-receptor essential for angiogenesis and tumor vascularization. Endoglin modulates the crucial balance between pro- and anti-angiogenic signaling by activin receptor-like kinase (ALK) 1, 5, and TGF-β type II (TβRII) receptors. Despite its established role in physiology and disease, the mechanism of endoglin down-regulation remains unknown. Here we report that the conserved juxtamembrane cytoplasmic tyrosine motif ((612)YIY(614)) is a critical determinant of angiogenesis. Src directly phosphorylates this motif to induce endoglin internalization and degradation via the lysosome. We identified epidermal growth factor (EGF) and vascular endothelial growth factor (VEGF) as Src-activators that induce endoglin turnover following (612)YIY(614) phosphorylation. Interestingly, Src phosphorylation of endoglin-(612)YIY(614) was also an important process for receptor down-regulation by TRACON105 (TRC105), an endoglin-targeting antibody currently in clinical trials. The regulation of (612)YIY(614) phosphorylation was critical for angiogenesis, as both the phosphomimetic and unphosphorylatable mutants impaired endothelial functions including proliferation, migration, and capillary tube formation. Collectively, these findings establish Src and pro-angiogenic mitogens as critical mediators of endoglin stability and function.

Effects of the combination of TRC105 and bevacizumab on endothelial cell biology

Endoglin, or CD105, is a cell membrane glycoprotein that is overexpressed on proliferating endothelial cells (EC), including those found in malignancies and choroidal neovascularization. Endoglin mediates the transition from quiescent endothelium, characterized by the relatively dominant state of Smad 2/3 phosphorylation, to active angiogenesis by preferentially phosphorylating Smad 1/5/8. The monoclonal antibody TRC105 binds endoglin with high avidity and is currently being tested in phase 1b and phase 2 clinical trials. In this report, we evaluated the effects of TRC105 on primary human umbilical vascular endothelial cells (HUVEC) as a single agent and in combination with bevacizumab. As single agents, both TRC105 and bevacizumab efficiently blocked HUVEC tube formation, and the combination of both agents achieved even greater levels of inhibition. We further assessed the effects of each drug on various aspects of HUVEC function. While bevacizumab was observed to inhibit HUVEC viability in nutrient-limited medium, TRC105 had little effect on HUVEC viability, either alone or in combination with bevacizumab. Additionally, both drugs inhibited HUVEC migration and induced apoptosis. At the molecular level, TRC105 treatment of HUVEC lead to decreased Smad 1/5/8 phosphorylation in response to BMP-9, a primary ligand for endoglin. Together, these results indicate that TRC105 acts as an effective anti-angiogenic agent alone and in combination with bevacizumab.

Neural deletion of Tgfbr2 impairs angiogenesis through an altered secretome

Simultaneous generation of neural cells and that of the nutrient-supplying vasculature during brain development is called neurovascular coupling. We report on a transgenic mouse with impaired transforming growth factor β (TGFβ)-signalling in forebrain-derived neural cells using a Foxg1-cre knock-in to drive the conditional knock-out of the Tgfbr2. Although the expression of FOXG1 is assigned to neural progenitors and neurons of the telencephalon, Foxg1^cre/+;Tgfbr2^flox/flox (Tgfbr2-cKO) mutants displayed intracerebral haemorrhage. Blood vessels exhibited an atypical, clustered appearance were less in number and displayed reduced branching. Vascular endothelial growth factor (VEGF) A, insulin-like growth factor (IGF) 1, IGF2, TGFβ, inhibitor of DNA binding (ID) 1, thrombospondin (THBS) 2, and a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) 1 were altered in either expression levels or tissue distribution. Accordingly, human umbilical vein endothelial cells (HUVEC) displayed branching defects after stimulation with conditioned medium (CM) that was derived from primary neural cultures of the ventral and dorsal telencephalon of Tgfbr2-cKO. Supplementing CM of Tgfbr2-cKO with VEGFA rescued these defects, but application of TGFβ aggravated them. HUVEC showed reduced migration towards CM of mutants compared with controls. Supplementing the CM with growth factors VEGFA, fibroblast growth factor (FGF) 2 and IGF1 partially restored HUVEC migration. In contrast, TGFβ supplementation further impaired migration of HUVEC. We observed differences along the dorso-ventral axis of the telencephalon with regard to the impact of these factors on the phenotype. Together these data establish a TGFBR2-dependent molecular crosstalk between neural and endothelial cells during brain vessel development. These findings will be useful to further elucidate neurovascular interaction in general and to understand pathologies of the blood vessel system such as intracerebral haemorrhages, hereditary haemorrhagic telangiectasia, Alzheimeŕs disease, cerebral amyloid angiopathy or tumour biology.

Endodontic therapy of a mandibular canine tooth with irreversible pulpitis secondary to dentigerous cyst

Dentigerous cysts are uncommon, yet are being reported with increasing frequency in the veterinary literature. Dentigerous cysts are a type of benign odontogenic cyst associated with impacted teeth, most commonly the mandibular first premolar tooth. Significant bone destruction can occur secondary to the expansion of a dentigerous cyst. The expanding cyst can lead to pathology of neighboring teeth, which can include external root resorption or pulpitis. Intraoral dental radiographs are imperative to properly assess the presence and extent of a dentigerous cyst, as well as the status of the neighboring teeth. This case report describes treatment for dentigerous cyst including cyst lining curettage, mandibular bone regeneration, and endodontic therapy for a canine tooth with irreversible pulpitis.

Key role of the endothelial TGF-β/ALK1/endoglin signaling pathway in humans and rodents pulmonary hypertension

Mutations affecting transforming growth factor-beta (TGF-β) superfamily receptors, activin receptor-like kinase (ALK)-1, and endoglin (ENG) occur in patients with pulmonary arterial hypertension (PAH). To determine whether the TGF-β/ALK1/ENG pathway was involved in PAH, we investigated pulmonary TGF-β, ALK1, ALK5, and ENG expressions in human lung tissue and cultured pulmonary-artery smooth-muscle-cells (PA-SMCs) and pulmonary endothelial cells (PECs) from 14 patients with idiopathic PAH (iPAH) and 15 controls. Seeing that ENG was highly expressed in PEC, we assessed the effects of TGF-β on Smad1/5/8 and Smad2/3 activation and on growth factor production by the cells. Finally, we studied the consequence of ENG deficiency on the chronic hypoxic-PH development by measuring right ventricular (RV) systolic pressure (RVSP), RV hypertrophy, and pulmonary arteriolar remodeling in ENG-deficient (Eng+/-) and wild-type (Eng+/+) mice. We also evaluated the pulmonary blood vessel density, macrophage infiltration, and cytokine expression in the lungs of the animals. Compared to controls, iPAH patients had higher serum and pulmonary TGF-β levels and increased ALK1 and ENG expressions in lung tissue, predominantly in PECs. Incubation of the cells with TGF-β led to Smad1/5/8 phosphorylation and to a production of FGF2, PDGFb and endothelin-inducing PA-SMC growth. Endoglin deficiency protected mice from hypoxic PH. As compared to wild-type, Eng+/- mice had a lower pulmonary vessel density, and no change in macrophage infiltration after exposure to chronic hypoxia despite the higher pulmonary expressions of interleukin-6 and monocyte chemoattractant protein-1. The TGF-β/ALK1/ENG signaling pathway plays a key role in iPAH and experimental hypoxic PH via a direct effect on PECs leading to production of growth factors and inflammatory cytokines involved in the pathogenesis of PAH.

Facilitation of endoglin-targeting cancer therapy by development/utilization of a novel genetically engineered mouse model expressing humanized endoglin (CD105)

Endoglin (ENG) is a TGF-β coreceptor and essential for vascular development and angiogenesis. A chimeric antihuman ENG (hENG) monoclonal antibody (mAb) c-SN6j (also known as TRC105) shows promising safety and clinical efficacy features in multiple clinical trials of patients with various advanced solid tumors. Here we developed a novel genetically engineered mouse model to optimize the ENG-targeting clinical trials. We designed a new targeting vector that contains exons 4-8 of hENG gene to generate novel genetically engineered mice (GEMs) expressing functional human/mouse chimeric (humanized) ENG with desired epitopes. Genotyping of the generated mice confirmed that we generated the desired GEMs. Immunohistochemical analysis demonstrated that humanized ENG protein of the GEMs expresses epitopes defined by 7 of our 8 anti-hENG mAbs tested. Surprisingly the homozygous GEMs develop normally and are healthy. Established breast and colon tumors as well as metastasis and tumor microvessels in the GEMs were effectively suppressed by systemic administration of anti-hENG mAbs. Additionally, test result indicates that synergistic potentiation of antitumor efficacy can be induced by simultaneous targeting of two distinct epitopes by anti-hENG mAbs. Sorafenib and capecitabine also showed antitumor efficacy in the GEMs. The presented novel GEMs are the first GEMs that express the targetable humanized ENG. Test results indicate utility of the GEMs for the clinically relevant studies. Additionally, we generated GEMs expressing a different humanized ENG containing exons 5-6 of hENG gene, and the homozygous GEMs develop normally and are healthy.

Endothelial depletion of Acvrl1 in mice leads to arteriovenous malformations associated with reduced endoglin expression

Rare inherited cardiovascular diseases are frequently caused by mutations in genes that are essential for the formation and/or function of the cardiovasculature. Hereditary Haemorrhagic Telangiectasia is a familial disease of this type. The majority of patients carry mutations in either Endoglin (ENG) or ACVRL1 (also known as ALK1) genes, and the disease is characterized by arteriovenous malformations and persistent haemorrhage. ENG and ACVRL1 encode receptors for the TGFβ superfamily of ligands, that are essential for angiogenesis in early development but their roles are not fully understood. Our goal was to examine the role of Acvrl1 in vascular endothelial cells during vascular development and to determine whether loss of endothelial Acvrl1 leads to arteriovenous malformations. Acvrl1 was depleted in endothelial cells either in early postnatal life or in adult mice. Using the neonatal retinal plexus to examine angiogenesis, we observed that loss of endothelial Acvrl1 led to venous enlargement, vascular hyperbranching and arteriovenous malformations. These phenotypes were associated with loss of arterial Jag1 expression, decreased pSmad1/5/8 activity and increased endothelial cell proliferation. We found that Endoglin was markedly down-regulated in Acvrl1-depleted ECs showing endoglin expression to be downstream of Acvrl1 signalling in vivo. Endothelial-specific depletion of Acvrl1 in pups also led to pulmonary haemorrhage, but in adult mice resulted in caecal haemorrhage and fatal anaemia. We conclude that during development, endothelial Acvrl1 plays an essential role to regulate endothelial cell proliferation and arterial identity during angiogenesis, whilst in adult life endothelial Acvrl1 is required to maintain vascular integrity.