The role of wnt signaling in physiological and pathological angiogenesis

Wnt Signaling in Cardiac Remodeling and Heart Failure

Wnt signaling plays an essential role during development, but is also activated in diseases as diverse as neurodegeneration, osteoporosis, and cancer. Accumulating evidence demonstrates that Wnt signaling is also activated during cardiac remodeling and heart failure. In this chapter, we will provide a brief overview of Wnt signaling in all its complexity. Then we will discuss the evidence for its involvement in the development of cardiac hypertrophy, the wound healing after myocardial infarction (MI) and heart failure. Finally, we will provide an overview of the drugs that are available to target Wnt signaling at different levels of the signaling cascade and the results of these pharmacological interventions in cardiac disease.

Increased Wnt5a in squamous cell lung carcinoma inhibits endothelial cell motility

BACKGROUND

Angiogenesis is important both in normal tissue function and disease and represents a key target in lung cancer (LC) therapy. Unfortunately, the two main subtypes of non-small-cell lung cancers (NSCLC) namely, adenocarcinoma (AC) and squamous cell carcinoma (SCC) respond differently to anti-angiogenic e.g. anti-vascular endothelial growth factor (VEGF)-A treatment with life-threatening side effects, often pulmonary hemorrhage in SCC. The mechanisms behind such adverse reactions are still largely unknown, although peroxisome proliferator activator receptor (PPAR) gamma as well as Wnt-s have been named as molecular regulators of the process. As the Wnt microenvironments in NSCLC subtypes are drastically different, we hypothesized that the particularly high levels of non-canonical Wnt5a in SCC might be responsible for alterations in blood vessel growth and result in serious adverse reactions.

METHODS

PPARgamma, VEGF-A, Wnt5a, miR-27b and miR-200b levels were determined in resected adenocarcinoma and squamous cell carcinoma samples by qRT-PCR and TaqMan microRNA assay. The role of PPARgamma in VEGF-A expression, and the role of Wnts in overall regulation was investigated using PPARgamma knock-out mice, cancer cell lines and fully human, in vitro 3 dimensional (3D), distal lung tissue aggregates. PPARgamma mRNA and protein levels were tested by qRT-PCR and immunohistochemistry, respectively. PPARgamma activity was measured by a PPRE reporter system. The tissue engineered lung tissues expressing basal level and lentivirally delivered VEGF-A were treated with recombinant Wnts, chemical Wnt pathway modifiers, and were subjected to PPARgamma agonist and antagonist treatment.

RESULTS

PPARgamma down-regulation and VEGF-A up-regulation are characteristic to both AC and SCC. Increased VEGF-A levels are under direct control of PPARgamma. PPARgamma levels and activity, however, are under Wnt control. Imbalance of both canonical (in AC) and non-canonical (in SCC) Wnts leads to PPARgamma down-regulation. While canonical Wnts down-regulate PPARgamma directly, non-canonical Wnt5a increases miR27b that is known regulator of PPARgamma.

CONCLUSION

During carcinogenesis the Wnt microenvironment alters, which can downregulate PPARgamma leading to increased VEGF-A expression. Differences in the Wnt microenvironment in AC and SCC of NSCLC lead to PPARgamma decrease via mechanisms that differentially alter endothelial cell motility and branching which in turn can influence therapeutic response.

Morphological and Molecular Characterization of Human Dermal Lymphatic Collectors

Millions of patients suffer from lymphedema worldwide. Supporting the contractility of lymphatic collectors is an attractive target for pharmacological therapy of lymphedema. However, lymphatics have mostly been studied in animals, while the cellular and molecular characteristics of human lymphatic collectors are largely unknown. We studied epifascial lymphatic collectors of the thigh, which were isolated for autologous transplantations. Our immunohistological studies identify additional markers for LECs (vimentin, CCBE1). We show and confirm differences between initial and collecting lymphatics concerning the markers ESAM1, D2-40 and LYVE-1. Our transmission electron microscopic studies reveal two types of smooth muscle cells (SMCs) in the media of the collectors with dark and light cytoplasm. We observed vasa vasorum in the media of the largest collectors, as well as interstitial Cajal-like cells, which are highly ramified cells with long processes, caveolae, and lacking a basal lamina. They are in close contact with SMCs, which possess multiple caveolae at the contact sites. Immunohistologically we identified such cells with antibodies against vimentin and PDGFRα, but not CD34 and cKIT. With Next Generation Sequencing we searched for highly expressed genes in the media of lymphatic collectors, and found therapeutic targets, suitable for acceleration of lymphatic contractility, such as neuropeptide Y receptors 1, and 5; tachykinin receptors 1, and 2; purinergic receptors P2RX1, and 6, P2RY12, 13, and 14; 5-hydroxytryptamine receptors HTR2B, and 3C; and adrenoceptors α2A,B,C. Our studies represent the first comprehensive characterization of human epifascial lymphatic collectors, as a prerequisite for diagnosis and therapy.

Fzd7 (Frizzled-7) Expressed by Endothelial Cells Controls Blood Vessel Formation Through Wnt/β-Catenin Canonical Signaling

OBJECTIVE

Vessel formation requires precise orchestration of a series of morphometric and molecular events controlled by a multitude of angiogenic factors and morphogens. Wnt/frizzled signaling is required for proper vascular formation. In this study, we investigated the role of the Fzd7 (frizzled-7) receptor in retinal vascular development and its relationship with the Wnt/β-catenin canonical pathway and Notch signaling.

APPROACH AND RESULTS

Using transgenic mice, we demonstrated that Fzd7 is required for postnatal vascular formation. Endothelial cell (EC) deletion of fzd7 (fzd7^ECKO) delayed retinal plexus formation because of an impairment in tip cell phenotype and a decrease in stalk cell proliferation. Dvl (dishevelled) proteins are a main component of Wnt signaling and play a functionally redundant role. We found that Dvl3 depletion in dvl1^-/- mice mimicked the fzd7^ECKO vascular phenotype and demonstrated that Fzd7 acted via β-catenin activation by showing that LiCl treatment rescued impairment in tip and stalk cell phenotypes induced in fzd7 mutants. Deletion of fzd7 or Dvl1/3 induced a strong decrease in Wnt canonical genes and Notch partners' expression. Genetic and pharmacological rescue strategies demonstrated that Fzd7 acted via β-catenin activation, upstream of Notch signaling to control Dll4 and Jagged1 EC expression.

CONCLUSIONS

Fzd7 expressed by EC drives postnatal angiogenesis via activation of Dvl/β-catenin signaling and can control the integrative interaction of Wnt and Notch signaling during postnatal angiogenesis.

LRP5/canonical Wnt signalling and healing of ischemic myocardium

LRP5 (low-density lipoprotein receptor-related protein 5) activates canonical Wnt signalling. LRP5 plays multiple roles including regulation of lipoprotein and cholesterol homeostasis as well as innate immunity cell function. However, it is not known whether LRP5 has a role in the myocardium. The aim of this study was to investigate LRP5 and Wnt signalling in myocardial remodelling after acute myocardial infarction (MI). Wnt protein levels were determined in a hypercholesterolemic porcine model of MI, in Lrp5 ^-/- C57Bl6 mice, in cultured cardiomyocytes and in human explanted hearts with previous MI episodes. 21 days post-MI, there was upregulation of LRP5 in the ischemic myocardium of hypercholesterolemic pigs as well as an upregulated expression of proteins of the Wnt pathway. We demonstrate via overexpression and silencing experiments that LRP5 induces Wnt pathway activation in isolated cardiomyocytes. Hypoxia and lipid-loading induced the expression of Wnt proteins, whereas this effect is blocked in LRP5-silenced cardiomyocytes. To characterize the function of the LRP5-Wnt axis upregulation in the heart, we induced MI in wild-type and Lrp5 ^-/- mice. Lrp5 ^-/- mice had significantly larger infarcts than Wt mice, indicating a protective role of LRP5 in injured myocardium. The LRP5 upregulation in post-MI hearts seen in pigs and mice was also evident in human hearts as dyslipidemic patients with previous episodes of ischemia have higher expression of LRP5 and Wnt-signalling genes than non-ischemic dilated hearts. We demonstrate an upregulation of LRP5 and the Wnt signalling pathway that it is a prosurvival healing response of cardiomyocytes upon injury.

VEGF-A/VEGFR2 signaling network in endothelial cells relevant to angiogenesis

Vascular endothelial growth factor-A (VEGF-A) is essential for endothelial cell functions associated with angiogenesis. Signal transduction networks initiated by VEGFA/VEGFR2, the most prominent ligand-receptor complex in the VEGF system, leads to endothelial cell proliferation, migration, survival and new vessel formation involved in angiogenesis. Considering its biomedical importance, we have developed the first comprehensive map of endothelial cell-specific signaling events of VEGFA/VEGFR2 system pertaining to angiogenesis. Screening over 20,000 published research articles and following the post-translational modification (PTM) and site specificity of VEGFR2, we have documented 240 proteins and their diverse PTM-dependent reactions involved in VEGFA/VEGFR2 signal transduction. From the ligand-receptor complex, this map has been extended to the level of major transcriptionally regulated genes for which the signaling cascades leading to their transcription factors are reported. We believe that this map would serve as a novel platform for reference, integration, and representation and more significantly, the progressive analysis of dynamic features of VEGF signaling in endothelial cells including their cross-talks with other ligand-receptor systems involved in angiogenesis.

Tumour cell-derived WNT5B modulates in vitro lymphangiogenesis via induction of partial endothelial-mesenchymal transition of lymphatic endothelial cells

Metastasis of the cervical lymph nodes frequently leads to poor survival of patients with oral squamous cell carcinoma (OSCC). The underlying mechanisms of lymph node metastasis are unclear. Wingless-type MMTV integration site family, member 5B (WNT5B), one component of the WNT signal pathway, was markedly up-regulated in OSCC sublines with high potential of lymphatic metastasis compared to that in OSCC cells with low nodal metastasis. Increased WNT5B mRNA was demonstrated in human OSCC tissues in comparison with adjacent non-tumorous tissues. Interestingly, the high level of WNT5B protein in serum was associated with lymph node metastasis in OSCC patients. Knockdown of WNT5B expression in OSCC sublines did not affect tumour growth but impaired lymph node metastasis and tumour lymphangiogenesis of orthotopic transplantation. Conditioned medium from WNT5B knockdown cells reduced the tube formation of lymphatic endothelial cells (LECs). In contrast, recombinant WNT5B enhanced the tube formation, permeability and migration of LECs. In LECs stained with phalloidin, the morphology of those treated with recombinant WNT5B changed from flat to spindle-like. Recombinant WNT5B also increased α-smooth muscle actin and inhibited the expression of vascular endothelial-cadherin but retained characteristics of endothelial cells. The results suggest that WNT5B functions in the partial endothelial-mesenchymal transition (EndoMT). Furthermore, WNT5B-induced tube formation was impaired in the LECs following the knockdown of EndoMT-related transcription factor, SNAIL or SLUG. The WNT5B-induced expression of Snail or Slug was abolished by IWR-1-endo and Rac1 inhibitors, which are involved in the WNT/β-catenin and planar cell polarity pathways, respectively. Collectively, the data suggest that WNT5B induces tube formation by regulating the expression of Snail and Slug proteins through activation of canonical and non-canonical WNT signalling pathways.

Autotaxin-Lysophosphatidic Acid Axis Acts Downstream of Apoprotein B Lipoproteins in Endothelial Cells

OBJECTIVE

As they travel through the blood stream, plasma lipoproteins interact continuously with endothelial cells (ECs). Although the focus of research has mostly been guided by the importance of lipoproteins as risk factors for atherosclerosis, thrombosis, and other cardiovascular diseases, little is known about the mechanisms linking lipoproteins and angiogenesis under physiological conditions, and particularly, during embryonic development. In this work, we performed global mRNA expression profiling of endothelial cells from hypo-, and hyperlipidemic zebrafish embryos with the goal of uncovering novel mediators of lipoprotein signaling in the endothelium.

APPROACH AND RESULTS

Microarray analysis was conducted on fluorescence-activated cell sorting-isolated fli1:EGFP(+) ECs from normal, hypo-, and hyperlipidemic zebrafish embryos. We found that opposed levels of apoprotein B lipoproteins result in differential expression of the secreted enzyme autotaxin in ECs, which in turn affects EC sprouting and angiogenesis. We further demonstrate that the effects of autotaxin in vivo are mediated by lysophosphatidic acid (LPA)-a well-known autotaxin activity product-and that LPA and LPA receptors participate as well in the response of ECs to lipoprotein levels.

CONCLUSIONS

Our findings provide the first in vivo gene expression profiling of ECs facing different levels of plasma apoprotein B lipoproteins and uncover a novel lipoprotein-autotaxin-LPA axis as regulator of EC behavior. These results highlight new roles for lipoproteins as signaling molecules, which are independent of their canonical function as cholesterol transporters.

High serum levels of sclerostin and Dickkopf-1 are associated with acute ischaemic stroke

BACKGROUND AND AIMS

Sclerostin and Dickkopf-1 (Dkk-1) are potent antagonists of Wnt signalling and might therefore play important roles in cardiovascular disease. We investigated whether serum sclerostin and Dkk-1 levels are associated with acute ischaemic stroke and specific stroke subtypes.

METHODS

Serum levels of sclerostin and Dkk-1 were measured by ELISA on day 1 and on day 6 after stroke in 62 patients with large artery atherosclerotic (LAA) stroke, on day 1 after stroke in 62 age- and gender-matched patients with small-artery occlusion (SAO) stroke and on admission in 62 healthy controls. Stroke severity was determined based on the National Institutes of Health Stroke Scale (NIHSS) and by measuring stroke volume on diffusion-weighted imaging. Outcome was measured by the modified Rankin Scale (mRS) on day 90.

RESULTS

Compared with controls, serum sclerostin and Dkk-1 levels were significantly higher in both patients with LAA stroke and with SAO stroke, and no difference was detected between the stroke subtypes. Sclerostin and Dkk-1 levels remained stable between the first and sixth day after stroke in the patients with LAA stroke. Receiver operating characteristic curve analysis was used to evaluate sclerostin and Dkk-1 as markers of a high risk of stroke and produced area under curve values of 0.773 and 0.776. Adjusted logistic regression showed that serum sclerostin and Dkk-1 levels remained as independent markers of stroke. No correlations were found between sclerostin or Dkk-1 levels and stroke severity or stroke outcome.

CONCLUSIONS

High serum levels of sclerostin and Dkk-1 are associated with acute ischaemic stroke.

Pharmacologic Activation of Wnt Signaling by Lithium Normalizes Retinal Vasculature in a Murine Model of Familial Exudative Vitreoretinopathy

Familial exudative vitreoretinopathy (FEVR) is characterized by delayed retinal vascular development, which promotes hypoxia-induced pathologic vessels. In severe cases FEVR may lead to retinal detachment and visual impairment. Genetic studies linked FEVR with mutations in Wnt signaling ligand or receptors, including low-density lipoprotein receptor-related protein 5 (LRP5) gene. Here, we investigated ocular pathologies in a Lrp5 knockout (Lrp5(-/-)) mouse model of FEVR and explored whether treatment with a pharmacologic Wnt activator lithium could bypass the genetic defects, thereby protecting against eye pathologies. Lrp5(-/-) mice displayed significantly delayed retinal vascular development, absence of deep layer retinal vessels, leading to increased levels of vascular endothelial growth factor and subsequent pathologic glomeruloid vessels, as well as decreased inner retinal visual function. Lithium treatment in Lrp5(-/-) mice significantly restored the delayed development of retinal vasculature and the intralaminar capillary networks, suppressed formation of pathologic glomeruloid structures, and promoted hyaloid vessel regression. Moreover, lithium treatment partially rescued inner-retinal visual function and increased retinal thickness. These protective effects of lithium were largely mediated through restoration of canonical Wnt signaling in Lrp5(-/-) retina. Lithium treatment also substantially increased vascular tubular formation in LRP5-deficient endothelial cells. These findings suggest that pharmacologic activation of Wnt signaling may help treat ocular pathologies in FEVR and potentially other defective Wnt signaling-related diseases.

β-Catenin C-terminal signals suppress p53 and are essential for artery formation

Increased activity of the tumour suppressor p53 is incompatible with embryogenesis, but how p53 is controlled is not fully understood. Differential requirements for p53 inhibitors Mdm2 and Mdm4 during development suggest that these control mechanisms are context-dependent. Artery formation requires investment of nascent endothelial tubes by smooth muscle cells (SMCs). Here, we find that embryos lacking SMC β-catenin suffer impaired arterial maturation and die by E12.5, with increased vascular wall p53 activity. β-Catenin-deficient SMCs show no change in p53 levels, but greater p53 acetylation and activity, plus impaired growth and survival. In vivo, SMC p53 inactivation suppresses phenotypes caused by loss of β-catenin. Mechanistically, β-catenin C-terminal interactions inhibit Creb-binding protein-dependent p53 acetylation and p53 transcriptional activity, and are required for artery formation. Thus in SMCs, the β-catenin C-terminus indirectly represses p53, and this function is essential for embryogenesis. These findings have implications for angiogenesis, tissue engineering and vascular disease.

How p53 is restrained in arterial maturation during embryonic development is unclear. Here, the authors show that β-catenin C-terminal interactions inhibit CREB binding protein-mediated acetylation and activation of p53 in smooth muscle cells, and that this function is essential for artery formation.

Pigment epithelium-derived factor (PEDF) normalizes matrix defects in iPSCs derived from Osteogenesis imperfecta Type VI

Osteogenesis imperfecta (OI) Type VI is characterized by a defect in bone mineralization, which results in multiple fractures early in life. Null mutations in the PEDF gene, Serpinf1, are the cause of OI VI. Whether PEDF restoration in a murine model of OI Type VI could improve bone mass and function was previously unknown. In Belinsky et al, we provided evidence that PEDF delivery enhanced bone mass and improved parameters of bone function in vivo. Further, we demonstrated that PEDF temporally inhibits Wnt signaling to enhance osteoblast differentiation. Here, we demonstrate that generation of induced pluripotent stem cells (iPSCs) from a PEDF null patient provides additional evidence for PEDF's role in regulating extracellular matrix proteins secreted from osteoblasts. PEDF null iPSCs have marked abnormalities in secreted matrix proteins, capturing a key feature of human OI Type VI, which were normalized by exogenous PEDF. Lastly, we place our recent findings within the broader context of PEDF biology and the developmental signaling pathways that are implicated in its actions.

Tie2 Expressing Monocytes in the Spleen of Patients with Primary Myelofibrosis

Primary myelofibrosis (PMF) is a Philadelphia-negative (Ph−) myeloproliferative disorder, showing abnormal CD34⁺ progenitor cell trafficking, splenomegaly, marrow fibrosis leading to extensive extramedullary haematopoiesis, and abnormal neoangiogenesis in either the bone marrow or the spleen. Monocytes expressing the angiopoietin-2 receptor (Tie2) have been shown to support abnormal angiogenic processes in solid tumors through a paracrine action that takes place in proximity to the vessels. In this study we investigated the frequency of Tie2 expressing monocytes in the spleen tissue samples of patients with PMF, and healthy subjects (CTRLs), and evaluated their possible role in favouring spleen angiogenesis. We show by confocal microscopy that in the spleen tissue of patients with PMF, but not of CTRLs, the most of the CD14⁺ cells are Tie2⁺ and are close to vessels; by flow cytometry, we found that Tie2 expressing monocytes were Tie2⁺CD14^lowCD16^brightCDL62⁻CCR2⁻ (TEMs) and their frequency was higher (p = 0.008) in spleen tissue-derived mononuclear cells (MNCs) of patients with PMF than in spleen tissue-derived MNCs from CTRLs undergoing splenectomy for abdominal trauma. By in vitro angiogenesis assay we evidenced that conditioned medium of immunomagnetically selected spleen tissue derived CD14⁺ cells of patients with PMF induced a denser tube like net than that of CTRLs; in addition, CD14⁺Tie2⁺ cells sorted from spleen tissue derived single cell suspension of patients with PMF show a higher expression of genes involved in angiogenesis than that found in CTRLs. Our results document the enrichment of Tie2⁺ monocytes expressing angiogenic genes in the spleen of patients with PMF, suggesting a role for these cells in starting/maintaining the pathological angiogenesis in this organ.

Wnt/β-Catenin Signaling Determines the Vasculogenic Fate of Postnatal Mesenchymal Stem Cells

Vasculogenesis is the process of de novo blood vessel formation observed primarily during embryonic development. Emerging evidence suggest that postnatal mesenchymal stem cells are capable of recapitulating vasculogenesis when these cells are engaged in tissue regeneration. However, the mechanisms underlining the vasculogenic differentiation of mesenchymal stem cells remain unclear. Here, we used stem cells from human permanent teeth (dental pulp stem cells [DPSC]) or deciduous teeth (stem cells from human exfoliated deciduous teeth [SHED]) as models of postnatal primary human mesenchymal stem cells to understand mechanisms regulating their vasculogenic fate. GFP-tagged mesenchymal stem cells seeded in human tooth slice/scaffolds and transplanted into immunodeficient mice differentiate into human blood vessels that anastomize with the mouse vasculature. In vitro, vascular endothelial growth factor (VEGF) induced the vasculogenic differentiation of DPSC and SHED via potent activation of Wnt/β-catenin signaling. Further, activation of Wnt signaling is sufficient to induce the vasculogenic differentiation of postnatal mesenchymal stem cells, while Wnt inhibition blocked this process. Notably, β-catenin-silenced DPSC no longer differentiate into endothelial cells in vitro, and showed impaired vasculogenesis in vivo. Collectively, these data demonstrate that VEGF signaling through the canonical Wnt/β-catenin pathway defines the vasculogenic fate of postnatal mesenchymal stem cells. Stem Cells 2016;34:1576-1587.

Regulation of endothelial homeostasis, vascular development and angiogenesis by the transcription factor ERG

Over the last few years, the ETS transcription factor ERG has emerged as a major regulator of endothelial function. Multiple studies have shown that ERG plays a crucial role in promoting angiogenesis and vascular stability during development and after birth. In the mature vasculature ERG also functions to maintain endothelial homeostasis, by transactivating genes involved in key endothelial functions, while repressing expression of pro-inflammatory genes. Its homeostatic role is lineage-specific, since ectopic expression of ERG in non-endothelial tissues such as prostate is detrimental and contributes to oncogenesis. This review summarises the main roles and pathways controlled by ERG in the vascular endothelium, its transcriptional targets and its functional partners and the emerging evidence on the pathways regulating ERG's activity and expression.

Activation of endothelial β-catenin signaling induces heart failure

Activation of β-catenin-dependent canonical Wnt signaling in endothelial cells plays a key role in angiogenesis during development and ischemic diseases, however, other roles of Wnt/β-catenin signaling in endothelial cells remain poorly understood. Here, we report that sustained activation of β-catenin signaling in endothelial cells causes cardiac dysfunction through suppressing neuregulin-ErbB pathway in the heart. Conditional gain-of-function mutation of β-catenin, which activates Wnt/β-catenin signaling in Bmx-positive arterial endothelial cells (Bmx/CA mice) led to progressive cardiac dysfunction and 100% mortality at 40 weeks after tamoxifen treatment. Electron microscopic analysis revealed dilatation of T-tubules and degeneration of mitochondria in cardiomyocytes of Bmx/CA mice, which are similar to the changes observed in mice with decreased neuregulin-ErbB signaling. Endothelial expression of Nrg1 and cardiac ErbB signaling were suppressed in Bmx/CA mice. The cardiac dysfunction of Bmx/CA mice was ameliorated by administration of recombinant neuregulin protein. These results collectively suggest that sustained activation of Wnt/β-catenin signaling in endothelial cells might be a cause of heart failure through suppressing neuregulin-ErbB signaling, and that the Wnt/β-catenin/NRG axis in cardiac endothelial cells might become a therapeutic target for heart failure.

Lithium prevents rat steroid-related osteonecrosis of the femoral head by β-catenin activation

This study explored the use of lithium to prevent rat steroid-related osteonecrosis of the femoral head (ONFH) through the modulation of the β-catenin pathway. ONFH was induced by methylprednisolone combined with lipopolysaccharide, and serum lipids were analyzed. ONFH was detected by hematoxylin-eosin staining. Micro-CT-based angiography and bone scanning were performed to analyze vessels and bone structure, respectively. Immunohistochemical staining for peroxisome proliferator-activated receptor gamma (PPARγ), bone morphogenetic protein-2 (BMP-2), and vascular endothelial growth factor (VEGF) was analyzed. Protein levels of phospho-glycogen synthase kinase-3β at Tyr-216 (p-Tyr(216) GSK-3β), total glycogen synthase kinase-3β (GSK-3β) and β-catenin, as well as mRNA levels of GSK-3β and β-catenin in femoral heads, were assessed. The rate of empty bone lacunae in the femoral heads was lower in the lithium and control groups than in the model group. The lithium group showed preventive effects against steroid-related vessel loss by micro-CT-based angiography and VEGF staining. Lithium treatment improved hyperlipidemia and reduced PPARγ expression. Moreover, lithium improved steroid-related bone loss in micro-CT bone scans and BMP-2 staining analyses. Furthermore, local β-catenin was reduced in steroid-related ONFH, and lithium treatment increased β-catenin expression while reducing p-Tyr(216) GSK-3β levels. The local β-catenin pathway was inhibited during steroid-related ONFH. Lithium may enhance angiogenesis and stabilize osteogenic/adipogenic homeostasis during steroid-related ONFH in rats by activating the β-catenin pathway.

An Intronic Flk1 Enhancer Directs Arterial-Specific Expression via RBPJ-Mediated Venous Repression

Supplemental Digital Content is available in the text.

Objective—

The vascular endothelial growth factor (VEGF) receptor Flk1 is essential for vascular development, but the signaling and transcriptional pathways by which its expression is regulated in endothelial cells remain unclear. Although previous studies have identified 2 Flk1 regulatory enhancers, these are dispensable for Flk1 expression, indicating that additional enhancers contribute to Flk1 regulation in endothelial cells. In the present study, we sought to identify Flk1 enhancers contributing to expression in endothelial cells.

Approach and Results—

A region of the 10th intron of the Flk1 gene (Flk1in10) was identified as a putative enhancer and tested in mouse and zebrafish transgenic models. This region robustly directed reporter gene expression in arterial endothelial cells. Using a combination of targeted mutagenesis of transcription factor–binding sites and gene silencing of transcription factors, we found that Gata and Ets factors are required for Flk1in10 enhancer activity in all endothelial cells. Furthermore, we showed that activity of the Flk1in10 enhancer is restricted to arteries through repression of gene expression in venous endothelial cells by the Notch pathway transcriptional regulator Rbpj.

Conclusions—

This study demonstrates a novel mechanism of arterial–venous identity acquisition, indicates a direct link between the Notch and VEGF signaling pathways, and illustrates how cis-regulatory diversity permits differential expression outcomes from a limited repertoire of transcriptional regulators.

Microarray analysis in pulmonary hypertension

Microarrays are a powerful and effective tool that allows the detection of genome-wide gene expression differences between controls and disease conditions. They have been broadly applied to investigate the pathobiology of diverse forms of pulmonary hypertension, namely group 1, including patients with idiopathic pulmonary arterial hypertension, and group 3, including pulmonary hypertension associated with chronic lung diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. To date, numerous human microarray studies have been conducted to analyse global (lung homogenate samples), compartment-specific (laser capture microdissection), cell type-specific (isolated primary cells) and circulating cell (peripheral blood) expression profiles. Combined, they provide important information on development, progression and the end-stage disease. In the future, system biology approaches, expression of noncoding RNAs that regulate coding RNAs, and direct comparison between animal models and human disease might be of importance.

Comprehensive overview of compartment-specific microarray studies of material from pulmonary hypertension patientshttp://ow.ly/YEFO2

The relationship between beta-catenin and apoptosis: A cytological and immunocytochemical examination

Disruption of the adhesive role of beta-catenin by caspases has been reported; however, the relationship between the Wnt/beta-catenin signaling pathway and apoptosis remains unclear. Therefore, we aimed to evaluate squamous epithelial cells in cervicovaginal smears by using cytological and immunocytochemical methods to observe changes in the presence and localization of beta-catenin during apoptosis, death receptor-, and mitochondria-mediated apoptosis. We investigated 224 cervicovaginal smears using the Papanicolaou method. Anti-beta-catenin and anti-cleaved caspase 3, 8, and 9 antibodies were used for immunocytochemical staining. Apoptotic cells were negative for beta-catenin. This showed that the Wnt/beta-catenin signaling pathway was inactive in apoptotic cells. However, beta-catenin showed intense positivity in the membrane, cytoplasm, and nucleus of non-apoptotic epithelial cells around these apoptotic cells. Therefore, the Wnt/beta-catenin signaling pathway was active in non-apoptotic epithelial cells, and this activity in non-apoptotic cells may have been induced by apoptotic cells. A highly significant association between the presence of death receptor-mediated apoptosis and the activity of the Wnt/beta-catenin signaling pathway was also found (P<0.001). In conclusion, the Wnt/beta-catenin signaling pathway was found to be inactive in apoptotic cells, but apoptotic cells may induce the Wnt/beta-catenin signaling pathway in non-apoptotic cells to compensate for a decrease in epithelial cells because of apoptosis in order to maintain epithelial tissue integrity.

Wnt Signaling in Cardiac Disease

Wnt signaling encompasses multiple and complex signaling cascades and is involved in many developmental processes such as tissue patterning, cell fate specification, and control of cell division. Consequently, accurate regulation of signaling activities is essential for proper embryonic development. Wnt signaling is mostly silent in the healthy adult organs but a reactivation of Wnt signaling is generally observed under pathological conditions. This has generated increasing interest in this pathway from a therapeutic point of view. In this review article, the involvement of Wnt signaling in cardiovascular development will be outlined, followed by its implication in myocardial infarct healing, cardiac hypertrophy, heart failure, arrhythmias, and atherosclerosis. The initial experiments not always offer consensus on the effects of activation or inactivation of the pathway, which may be attributed to (i) the type of cardiac disease, (ii) timing of the intervention, and (iii) type of cells that are targeted. Therefore, more research is needed to determine the exact implication of Wnt signaling in the conditions mentioned above to exploit it as a powerful therapeutic target.

Wnt5a Regulates the Assembly of Human Adipose Derived Stromal Vascular Fraction-Derived Microvasculatures

Human adipose-derived stromal vascular fraction (hSVF) cells are an easily accessible, heterogeneous cell system that can spontaneously self-assemble into functional microvasculatures in vivo. However, the mechanisms underlying vascular self-assembly and maturation are poorly understood, therefore we utilized an in vitro model to identify potential in vivo regulatory mechanisms. We utilized passage one (P1) hSVF because of the rapid UEA1+ endothelium (EC) loss at even P2 culture. We exposed hSVF cells to a battery of angiogenesis inhibitors and found that the pan-Wnt inhibitor IWP2 produced the most significant hSVF-EC networking decrease (~25%). To determine which Wnt isoform(s) and receptor(s) may be involved, hSVF was screened by PCR for isoforms associated with angiogenesis, with only WNT5A and its receptor, FZD4, being expressed for all time points observed. Immunocytochemistry confirmed Wnt5a protein expression by hSVF. To see if Wnt5a alone could restore IWP2-induced EC network inhibition, recombinant human Wnt5a (0–150 ng/ml) was added to IWP2-treated cultures. The addition of rhWnt5a significantly increased EC network area and significantly decreased the ratio of total EC network length to EC network area compared to untreated controls. To determine if Wnt5a mediates in vivo microvascular self-assembly, 3D hSVF constructs containing an IgG isotype control, anti-Wnt5a neutralizing antibody or rhWnt5a were implanted subcutaneously for 2w in immune compromised mice. Compared to IgG controls, anti-Wnt5a treatment significantly reduced vessel length density by ~41%, while rhWnt5a significantly increased vessel length density by ~62%. However, anti-Wnt5a or rhWnt5a did not significantly affect the density of segments and nodes, both of which measure vascular complexity. Taken together, this data demonstrates that endogenous Wnt5a produced by hSVF plays a regulatory role in microvascular self-assembly in vivo. These findings also suggest that manipulating Wnt signaling could enhance control of hSVF vascularization in tissue engineering applications.

Downregulation of vascular endothelial growth factor receptor-2 under oxidative stress conditions is mediated by β-transduction repeat-containing protein via glycogen synthase kinase-3β signaling

Vascular endothelial growth factor receptor-2 (VEGFR-2), which is a key determinant of the angiogenecic response, is decreased in diabetic mice under oxidative stress. β-transduction repeat-containing protein (β-TrCP) has been reported to participate in VEGFR-2 degradation in thyroid cancer cells. Additionally, glycogen synthase kinase-3β (GSK-3β) acts as a mediator in the β-TrCP-induced degradation of several proteins. However, the role played by β-TrCP and GSK-3β in the degradation of VEGFR-2 in endothelial cells where hyperglycemia had been induced was not fully understood. In the present study, we aimed to analyze the mechanisms of VEGFR-2 degradation by studying excess reactive oxygen species (ROS) induced by hyperglycemia or glucose oxidase (GO). Human umbilical vein endothelial cells (HUVECs) were treated with different concentrations of glucose (6.6, 19.8 and 33 mM), mannitol (33 mM) and GO (1 U/ml). Angiogenic function, ROS levels, the co-location of VEGFR-2 and β-TrCP were evaluated. Cells were collected for RT-qPCR and western blot analysis. We noted that angiogenesis was impaired upon increasing the glucose concentration. When HUVECs were in a hyperglycemic state, ROS production increased, comparable to exposure to GO; GO catalyzes oxidation of glucose into H₂O₂ and D-glucono-δ-lactone. Phosphorylated VEGFR-2 was reduced by hyperglycemia while total VEGFR-2 was almost unaltered. However, VEGFR-2 was reduced when directly exposed to ROS, with resultant co-location of β-TrCP and VEGFR-2. Through a co-immunoprecipitation assay, we noted that ubiquitinated VEGFR-2 was significantly augmented by excess ROS. Decreased VEGFR-2 caused by ROS was ameliorated by β-TrCP siRNA, proteasome inhibitor MG132 and GSK-3β activity inhibitor (lithium chloride and SB216763). We suggest that redundant ROS reduces VEGFR-2 through β-TrCP-mediated VEGFR-2 degradation, which is postulated to be regulated by GSK-3β.

From skeletal to cardiovascular disease in 12 steps-the evolution of sclerostin as a major player in CKD-MBD

Canonical Wnt signaling activity contributes to physiological and adaptive bone mineralization and is an essential player in bone remodeling. Sclerostin is a prototypic soluble canonical Wnt signaling pathway inhibitor that is produced in osteocytes and blocks osteoblast differentiation and function. Therefore, sclerostin is a potent inhibitor of bone formation and mineralization. Accordingly, rodent sclerostin-deficiency models exhibit a strong bone phenotype. Moreover, blocking sclerostin represents a promising treatment perspective against osteoporosis. Beyond the bone field novel data definitely associate Wnt signaling in general and sclerostin in particular with ectopic extraosseous mineralization processes, as is evident in cardiovascular calcification or calciphylaxis. Uremia is characterized by parallel occurrence of disordered bone mineralization and accelerated cardiovascular calcification (chronic kidney disease - mineral and bone disorder, CKD-MBD), linking skeletal and cardiovascular disease-the so-called bone-vascular calcification paradox. In consequence, sclerostin may qualify as an emerging player in CKD-MBD. We present a stepwise review approach regarding the rapidly evolving field sclerostin participation in CKD-MBD. Starting from data originating in the classical bone field we look separately at three major areas of CKD-MBD: disturbed mineral metabolism, renal osteodystrophy, and uremic cardiovascular disease. Our review is intended to help the nephrologist revise the potential importance of sclerostin in CKD by focusing on how sclerostin research is gradually evolving from the classical osteoporosis niche into the area of CKD-MBD. In particular, we integrate the limited amount of available data in the context of pediatric nephrology.

Definitive Hematopoiesis in the Yolk Sac Emerges from Wnt-Responsive Hemogenic Endothelium Independently of Circulation and Arterial Identity

Adult-repopulating hematopoietic stem cells (HSCs) emerge in low numbers in the midgestation mouse embryo from a subset of arterial endothelium, through an endothelial-to-hematopoietic transition. HSC-producing arterial hemogenic endothelium relies on the establishment of embryonic blood flow and arterial identity, and requires β-catenin signaling. Specified prior to and during the formation of these initial HSCs are thousands of yolk sac-derived erythro-myeloid progenitors (EMPs). EMPs ensure embryonic survival prior to the establishment of a permanent hematopoietic system, and provide subsets of long-lived tissue macrophages. While an endothelial origin for these HSC-independent definitive progenitors is also accepted, the spatial location and temporal output of yolk sac hemogenic endothelium over developmental time remain undefined. We performed a spatiotemporal analysis of EMP emergence, and document the morphological steps of the endothelial-to-hematopoietic transition. Emergence of rounded EMPs from polygonal clusters of Kit(+) cells initiates prior to the establishment of arborized arterial and venous vasculature in the yolk sac. Interestingly, Kit(+) polygonal clusters are detected in both arterial and venous vessels after remodeling. To determine whether there are similar mechanisms regulating the specification of EMPs with other angiogenic signals regulating adult-repopulating HSCs, we investigated the role of embryonic blood flow and Wnt/β-catenin signaling during EMP emergence. In embryos lacking a functional circulation, rounded Kit(+) EMPs still fully emerge from unremodeled yolk sac vasculature. In contrast, canonical Wnt signaling appears to be a common mechanism regulating hematopoietic emergence from hemogenic endothelium. These data illustrate the heterogeneity in hematopoietic output and spatiotemporal regulation of primary embryonic hemogenic endothelium.

Loss of PPARγ in endothelial cells leads to impaired angiogenesis

Tie2-promoter-mediated loss of peroxisome proliferator-activated receptor gamma (PPARγ, also known as PPARG) in mice leads to osteopetrosis and pulmonary arterial hypertension. Vascular disease is associated with loss of PPARγ in pulmonary microvascular endothelial cells (PMVEC); we evaluated the role of PPARγ in PMVEC functions, such as angiogenesis and migration. The role of PPARγ in angiogenesis was evaluated in Tie2CrePPARγ(flox/flox) and wild-type mice, and in mouse and human PMVECs. RNA sequencing and bioinformatic approaches were utilized to reveal angiogenesis-associated targets for PPARγ. Tie2CrePPARγ(flox/flox) mice showed an impaired angiogenic capacity. Analysis of endothelial progenitor-like cells using bone marrow transplantation combined with evaluation of isolated PMVECs revealed that loss of PPARγ attenuates the migration and angiogenic capacity of mature PMVECs. PPARγ-deficient human PMVECs showed a similar migration defect in culture. Bioinformatic and experimental analyses newly revealed E2F1 as a target of PPARγ in the regulation of PMVEC migration. Disruption of the PPARγ-E2F1 axis was associated with a dysregulated Wnt pathway related to the GSK3B interacting protein (GSKIP). In conclusion, PPARγ plays an important role in sustaining angiogenic potential in mature PMVECs through E2F1-mediated gene regulation.

Reck enables cerebrovascular development by promoting canonical Wnt signaling

The cerebral vasculature provides the massive blood supply that the brain needs to grow and survive. By acquiring distinctive cellular and molecular characteristics it becomes the blood-brain barrier (BBB), a selectively permeable and protective interface between the brain and the peripheral circulation that maintains the extracellular milieu permissive for neuronal activity. Accordingly, there is great interest in uncovering the mechanisms that modulate the formation and differentiation of the brain vasculature. By performing a forward genetic screen in zebrafish we isolated no food for thought (nft (y72)), a recessive late-lethal mutant that lacks most of the intracerebral central arteries (CtAs), but not other brain blood vessels. We found that the cerebral vascularization deficit of nft (y72) mutants is caused by an inactivating lesion in reversion-inducing cysteine-rich protein with Kazal motifs [reck; also known as suppressor of tumorigenicity 15 protein (ST15)], which encodes a membrane-anchored tumor suppressor glycoprotein. Our findings highlight Reck as a novel and pivotal modulator of the canonical Wnt signaling pathway that acts in endothelial cells to enable intracerebral vascularization and proper expression of molecular markers associated with BBB formation. Additional studies with cultured endothelial cells suggest that, in other contexts, Reck impacts vascular biology via the vascular endothelial growth factor (VEGF) cascade. Together, our findings have broad implications for both vascular and cancer biology.

Sustained proliferation in cancer: Mechanisms and novel therapeutic targets

Proliferation is an important part of cancer development and progression. This is manifest by altered expression and/or activity of cell cycle related proteins. Constitutive activation of many signal transduction pathways also stimulates cell growth. Early steps in tumor development are associated with a fibrogenic response and the development of a hypoxic environment which favors the survival and proliferation of cancer stem cells. Part of the survival strategy of cancer stem cells may manifested by alterations in cell metabolism. Once tumors appear, growth and metastasis may be supported by overproduction of appropriate hormones (in hormonally dependent cancers), by promoting angiogenesis, by undergoing epithelial to mesenchymal transition, by triggering autophagy, and by taking cues from surrounding stromal cells. A number of natural compounds (e.g., curcumin, resveratrol, indole-3-carbinol, brassinin, sulforaphane, epigallocatechin-3-gallate, genistein, ellagitannins, lycopene and quercetin) have been found to inhibit one or more pathways that contribute to proliferation (e.g., hypoxia inducible factor 1, nuclear factor kappa B, phosphoinositide 3 kinase/Akt, insulin-like growth factor receptor 1, Wnt, cell cycle associated proteins, as well as androgen and estrogen receptor signaling). These data, in combination with bioinformatics analyses, will be very important for identifying signaling pathways and molecular targets that may provide early diagnostic markers and/or critical targets for the development of new drugs or drug combinations that block tumor formation and progression.

Isolation and characterization of circulating lymphatic endothelial colony forming cells

RATIONALE

The identification of circulating endothelial progenitor cells has led to speculation regarding their origin as well as their contribution to neovascular development. Two distinct types of endothelium make up the blood and lymphatic vessel system. However, it has yet to be determined whether there are distinct lymphatic-specific circulating endothelial progenitor cells.

OBJECTIVE

This study aims to isolate and characterize the cellular properties and global gene expression of lymphatic-specific endothelial progenitor cells.

METHODS AND RESULTS

We isolated circulating endothelial colony forming cells (ECFCs) from whole peripheral blood. These cells are endothelial in nature, as defined by their expression of endothelial markers and their ability to undergo capillary morphogenesis in three-dimensional culture. A subset of isolated colonies express markers of lymphatic endothelium, including VEGFR-3 and Prox-1, with low levels of VEGFR-1, a blood endothelial marker, while the bulk of the isolated cells express high VEGFR-1 levels with low VEGFR-3 and Prox-1 expression. The different isolates have differential responses to VEGF-C, a lymphatic endothelial specific cytokine, strongly suggesting that there are lymphatic specific and blood specific ECFCs. Global analysis of gene expression revealed key differences in the regulation of pathways involved in cellular differentiation between blood and lymphatic-specific ECFCs.

CONCLUSION

These data indicate that there are two distinguishable circulating ECFC types, blood and lymphatic, which are likely to have discrete functions during neovascularization.

MARCKSL1 exhibits anti-angiogenic effects through suppression of VEGFR-2-dependent Akt/PDK-1/mTOR phosphorylation

Myristoylated alanine-rich C kinase substrate-like 1 (MARCKSL1) plays a pivotal role in the regulation of apoptosis and has been shown to maintain antitumor and metastasis-suppressive properties. In the present study, we examined the effects of MARCKSL1 as a novel anti-angiogenic agent on the inhibition of angiogenesis-mediated cell migration. MARCKSL1 also reduced vascular endothelial growth factor (VEGF)-induced human umbilical vein endothelial cell (HUVEC) proliferation, as well as capillary-like tubular structure formation in vitro. MARCKSL1 disrupted phosphorylation of vascular endothelial growth factor receptor-2 (VEGFR-2) in ovarian tumorigenesis. In addition, MARCKSL1 showed potent anti-angiogenic activity and reduced the levels of VEGF and hypoxia-inducible factor 1α (HIF-1α) expression, an essential regulator of angiogenesis. Consistently, MARCKSL1 decreased VEGF‑induced phosphorylation of the PI3K/Akt signaling pathway components, including phosphoinositide-dependent protein kinase 1 (PDK-1), mammalian target of rapamycin (mTOR), tuberous sclerosis complex 2 (TSC-2), p70 ribosomal protein S6 kinase (p70S6K), and glycogen synthase kinase 3β (GSK-3β) protein. Collectively, our results provide evidence for the physiological/biological function of an endothelial cell system involved in angiogenesis through suppression of Akt/PDK-1/mTOR phosphorylation by interaction with VEGFR-2.

Endostatin combined with radiotherapy suppresses vasculogenic mimicry formation through inhibition of epithelial-mesenchymal transition in esophageal cancer

The growth of solid tumors requires angiogenesis to provide oxygen and nutrients and to support cell proliferation. The switch from an avascular to a vascular phenotype is typically related to acceleration of tumor growth. Anti-angiogenic therapy is becoming a very promising way for malignant tumors. Meanwhile, malignant tumor cells themselves were able to develop the formation of cell-lined vessels that contribute to tumor neovascularization and supply the nutrients and oxygen, which is called vasculogenic mimicry (VM). However, the molecular mechanism of VM remains unclear. The purpose of this study was to investigate the efficacy of the novel recombinant human endostatin (rh-Endo) protein combined with radiotherapy on human esophageal squamous cell carcinoma (ESCC) cell lines Eca-109 and TE13. Our results showed that rh-Endo combined with radiotherapy significantly inhibited the proliferation, migration, invasion, and VM of human esophageal cancer cells in a dose-dependent manner; however, it has no direct effect on apoptosis of carcinoma cells, which indicated that rh-Endo combined with radiotherapy significantly changed the microenvironment of esophageal carcinoma, and played an important role in preventing distant metastasis. Our findings suggested that rh-Endo inhibited the metastasis of esophageal cancer and the activation of AKT pathway, and the down-regulation of epithelial-mesenchymal transition (EMT) may be associated with such effect of rh-Endo. These results also supported the bright prospect of rh-Endo combined with radiotherapy for clinical applications in the future.

Participation of WNT and β-Catenin in Physiological and Pathological Endometrial Changes: Association with Angiogenesis

WNT proteins are involved in embryonic development, sex determination, stem cell recruitment, angiogenesis, and cancer. They take part in morphological changes in the endometrium during development, regulate processes of endometrial proliferation and differentiation. This review presents current knowledge about implication of WNT proteins and β-catenin in physiological endometrial functions as well as their involvement in uterine carcinogenesis. Influence of WNT proteins on the formation of blood vessel, taking place both under healthy and pathological conditions, is also considered. Participation of WNT proteins, β-catenin, and inhibitors and inducers of WNT signaling in the process of endometrial angiogenesis is largely unknown. Thus, confirmation of their local and systemic participation in the process of endometrial angiogenesis may in the long term help to establish new diagnostic and therapeutic approaches in conditions associated with the pathology of the female reproductive system.

Wnt3a Promotes the Vasculogenic Mimicry Formation of Colon Cancer via Wnt/β-Catenin Signaling

Our previous study provided evidence that non-canonical Wnt signaling is involved in regulating vasculogenic mimicry (VM) formation. However, the functions of canonical Wnt signaling in VM formation have not yet been explored. In this study, we found the presence of VM was related to colon cancer histological differentiation (p < 0.001), the clinical stage (p < 0.001), and presence of metastasis and recurrence (p < 0.001). VM-positive colon cancer samples showed increased Wnt3a expression (p < 0.001) and β-catenin nuclear expression (p < 0.001) compared with the VM-negative samples. In vitro, over-regulated Wnt3a expression in HT29 colon cancer cells promoted the capacity to form tube-like structures in the three-dimensional (3-D) culture together with increased expression of endothelial phenotype-associated proteins such as VEGFR2 and VE-cadherin. The mouse xenograft model showed that Wnt3a-overexpressing cells grew into larger tumor masses and formed more VM than the control cells. In addition, the Wnt/β-catenin signaling antagonist Dickkopf-1(Dkk1) can reverse the capacity to form tube-like structures and can decrease the expressions of VEGFR2 and VE-cadherin in Wnt3a-overexpressing cells. Taken together, our results suggest that Wnt/β-catenin signaling is involved in VM formation in colon cancer and might contribute to the development of more accurate treatment modalities aimed at VM.

Genetics, diagnosis and management of colorectal cancer (Review)

Colorectal cancer (CRC) is the third most common type of cancer worldwide and a leading cause of cancer death. Surgery represents the mainstay of treatment in early cases but often patients are primarily diagnosed in an advanced stage of disease and sometimes also distant metastases are present. Neoadjuvant therapy is therefore needed but drug resistance may influence response and concur to recurrent disease. At molecular level, it is a very heterogeneous group of diseases with about 30% of hereditary or familial cases. During colorectal adenocarcinomas development, epithelial cells from gastrointestinal trait acquire sequential genetic and epigenetic mutations in specific oncogenes and/or tumour suppressor genes, causing CRC onset, progression and metastasis. Molecular characterization of cancer associated mutations gives valuable information about disease prognosis and response to the therapy. Very early diagnosis and personalized care, as well as a better knowledge of molecular basis of its onset and progression, are therefore crucial to obtain a cure of CRC. In this review, we describe updated genetics, current diagnosis and management of CRC pointing out the extreme need for a multidisciplinary approach to achieve the best results in patient outcomes.

PEDF and its roles in physiological and pathological conditions: implication in diabetic and hypoxia-induced angiogenic diseases

Pigment epithelium-derived factor (PEDF) is a broadly expressed multifunctional member of the serine proteinase inhibitor (serpin) family. This widely studied protein plays critical roles in many physiological and pathophysiological processes, including neuroprotection, angiogenesis, fibrogenesis and inflammation. The present review summarizes the temporal and spatial distribution patterns of PEDF in a variety of developing and adult organs, and discusses its functions in maintaining physiological homoeostasis. The major focus of the present review is to discuss the implication of PEDF in diabetic and hypoxia-induced angiogenesis, and the pathways mediating PEDF's effects under these conditions. Furthermore, the regulatory mechanisms of PEDF expression, function and degradation are also reviewed. Finally, the therapeutic potential of PEDF as an anti-angiogenic drug is briefly summarized.

Origin and differentiation of vascular smooth muscle cells

Vascular smooth muscle cells (SMCs), a major structural component of the vessel wall, not only play a key role in maintaining vascular structure but also perform various functions. During embryogenesis, SMC recruitment from their progenitors is an important step in the formation of the embryonic vascular system. SMCs in the arterial wall are mostly quiescent but can display a contractile phenotype in adults. Under pathophysiological conditions, i.e. vascular remodelling after endothelial dysfunction or damage, contractile SMCs found in the media switch to a secretory type, which will facilitate their ability to migrate to the intima and proliferate to contribute to neointimal lesions. However, recent evidence suggests that the mobilization and recruitment of abundant stem/progenitor cells present in the vessel wall are largely responsible for SMC accumulation in the intima during vascular remodelling such as neointimal hyperplasia and arteriosclerosis. Therefore, understanding the regulatory mechanisms that control SMC differentiation from vascular progenitors is essential for exploring therapeutic targets for potential clinical applications. In this article, we review the origin and differentiation of SMCs from stem/progenitor cells during cardiovascular development and in the adult, highlighting the environmental cues and signalling pathways that control phenotypic modulation within the vasculature.

Wnt signaling in atherosclerosis

Atherosclerosis is a disease of the vascular wall that forms the basis for a large spectrum of pathologies of various organs and tissues. Although massive research efforts in the last decades have yielded valuable information about its underlying molecular mechanisms, this has not led to a translation into effective therapeutic interventions that can stop the progression or even can induce regression of atherosclerosis. This underscores the importance of investigations on the involvement of novel signaling pathways in the development and progression of this condition. In this review we focus on the role of Wnt signaling in atherosclerosis. Experimental evidence is presented that Wnt signaling is involved in many aspects of the development and progression of vascular lesions including endothelial dysfunction, macrophage activation and the proliferation and migration of vascular smooth muscle cells. Subsequently, we will discuss the role of Wnt signaling in myocardial infarction and stroke, two common pathologies resulting from the progression of atherosclerotic lesions towards an unstable phenotype. Despite the fact that the published data sometimes are ambiguous or even conflicting, a picture is emerging that an attenuation of Wnt signaling is beneficial for the cardiovascular system that is compromised by atherosclerosis.

Wnt5a attenuates hypoxia-induced pulmonary arteriolar remodeling and right ventricular hypertrophy in mice

Hypoxic pulmonary hypertension (HPH), which is characterized by pulmonary arteriolar remodeling and right ventricular hypertrophy, is still a life-threatening disease with the current treatment strategies. The underlying molecular mechanisms of HPH remain unclear. Our previously published study showed that Wnt5a, one of the ligands in the Wnt family, was critically involved in the inhibition of hypoxia-induced pulmonary arterial smooth muscle cell proliferation by downregulation of β-catenin/cyclin D1 in vitro. In this study, we investigated the possible functions and mechanisms of Wnt5a in HPH in vivo. Recombinant mouse Wnt5a (rmWnt5a) or phosphate buffered saline (PBS) was administered to male C57/BL6 mice weekly from the first day to the end of the two or four weeks after exposed to hypoxia (10% O2). Hypoxia-induced pulmonary hypertension was associated with a marked increase in β-catenin/cyclin D1 expression in lungs. Right ventricular systolic pressure and right ventricular hypertrophy index were reduced in animals treated with rmWnt5a compared with PBS. Histology showed less pulmonary vascular remodeling and right ventricular hypertrophy in the group treated with rmWnt5a than with PBS. Treatment with rmWnt5a resulted in a concomitant reduction in β-catenin/cyclin D1 levels in lungs. These data demonstrate that Wnt5a exerts its beneficial effects on HPH by regulating pulmonary vascular remodeling and right ventricular hypertrophy in a manner that is associated with reduction in β-catenin/cyclin D1 signaling. A therapy targeting the β-catenin/cyclin D1 signaling pathway might be a potential strategy for HPH treatment.

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.

Blood cells and endothelial barrier function

The barrier properties of endothelial cells are critical for the maintenance of water and protein balance between the intravascular and extravascular compartments. An impairment of endothelial barrier function has been implicated in the genesis and/or progression of a variety of pathological conditions, including pulmonary edema, ischemic stroke, neurodegenerative disorders, angioedema, sepsis and cancer. The altered barrier function in these conditions is often linked to the release of soluble mediators from resident cells (e.g., mast cells, macrophages) and/or recruited blood cells. The interaction of the mediators with receptors expressed on the surface of endothelial cells diminishes barrier function either by altering the expression of adhesive proteins in the inter-endothelial junctions, by altering the organization of the cytoskeleton, or both. Reactive oxygen species (ROS), proteolytic enzymes (e.g., matrix metalloproteinase, elastase), oncostatin M, and VEGF are part of a long list of mediators that have been implicated in endothelial barrier failure. In this review, we address the role of blood borne cells, including, neutrophils, lymphocytes, monocytes, and platelets, in the regulation of endothelial barrier function in health and disease. Attention is also devoted to new targets for therapeutic intervention in disease states with morbidity and mortality related to endothelial barrier dysfunction.

High levels of pigment epithelium-derived factor in diabetes impair wound healing through suppression of Wnt signaling

Diabetic foot ulcer (DFU) caused by impaired wound healing is a common vascular complication of diabetes. The current study revealed that plasma levels of pigment epithelium-derived factor (PEDF) were elevated in type 2 diabetic patients with DFU and in db/db mice. To test whether elevated PEDF levels contribute to skin wound-healing delay in diabetes, endogenous PEDF was neutralized with an anti-PEDF antibody in db/db mice. Our results showed that neutralization of PEDF accelerated wound healing, increased angiogenesis in the wound skin, and improved the functions and numbers of endothelial progenitor cells (EPCs) in the diabetic mice. Further, PEDF-deficient mice showed higher baseline blood flow in the skin, higher density of cutaneous microvessels, increased skin thickness, improved numbers and functions of circulating EPCs, and accelerated wound healing compared with wild-type mice. Overexpression of PEDF suppressed the Wnt signaling pathway in the wound skin. Lithium chloride-induced Wnt signaling activation downstream of the PEDF interaction site attenuated the inhibitory effect of PEDF on EPCs and rescued the wound-healing deficiency in diabetic mice. Taken together, these results suggest that elevated circulating PEDF levels contribute to impaired wound healing in the process of angiogenesis and vasculogenesis through the inhibition of Wnt/β-catenin signaling.

The c-Cbl ubiquitin ligase regulates nuclear β-catenin and angiogenesis by its tyrosine phosphorylation mediated through the Wnt signaling pathway

Wnt signaling plays important roles in both the tumor-induced angiogenesis and tumorigenesis through the transcriptionally active nuclear β-catenin. Recently, c-Cbl was identified as a unique E3 ubiquitin ligase targeting the active nuclear β-catenin. However, little is known about the molecular mechanisms by which c-Cbl regulates ubiquitination and degradation of active β-catenin. Here, we demonstrate that Wnt activation promotes the phosphorylation of c-Cbl at tyrosine 731(Tyr-731), which increases c-Cbl dimerization and binding to β-catenin. Tyr-731 phosphorylation and dimerization mediate c-Cbl nuclear translocation and lead to the degradation of nuclearly active β-catenin in the Wnt-on phase. c-Cbl activation also inhibits expression of the pro-angiogenic Wnt targets, IL-8 and VEGF. Phospho-Tyr-731-inactive mutant c-Cbl (Y731F) enhances and phosphomimetic mutant c-Cbl (Y731E) suppresses angiogenesis in zebrafish. Taken together, we have identified a novel mechanism for the regulation of active nuclear β-catenin by c-Cbl and its critical role in angiogenesis. This mechanism can be further explored to modulate both the pathological angiogenesis and the tumorigenesis.

Blood and lymphatic vessel formation

Blood and lymphatic vessels deliver oxygen and nutrients, remove waste and CO2, and regulate interstitial pressure in tissues and organs. These vessels begin life early in embryogenesis using transcription factors and signaling pathways that regulate differentiation, morphogenesis, and proliferation. Here we describe how these vessels develop in the mouse embryo, and the signals that are important to their development.

Complement C1q-induced activation of β-catenin signalling causes hypertensive arterial remodelling

Hypertension induces structural remodelling of arteries, which leads to arteriosclerosis and end-organ damage. Hyperplasia of vascular smooth muscle cells (VSMCs) and infiltration of immune cells are the hallmark of hypertensive arterial remodelling. However, the precise molecular mechanisms of arterial remodelling remain elusive. We have recently reported that complement C1q activates β-catenin signalling independent of Wnts. Here, we show a critical role of complement C1-induced activation of β-catenin signalling in hypertensive arterial remodelling. Activation of β-catenin and proliferation of VSMCs were observed after blood-pressure elevation, which were prevented by genetic and chemical inhibition of β-catenin signalling. Macrophage depletion and C1qa gene deletion attenuated the hypertension-induced β-catenin signalling, proliferation of VSMCs and pathological arterial remodelling. Our findings unveil the link between complement C1 and arterial remodelling and suggest that C1-induced activation of β-catenin signalling becomes a novel therapeutic target to prevent arteriosclerosis in patients with hypertension.

The role of macrophages in hypertension-induced arterial remodeling is poorly understood. Here, Sumida et al. show that high blood pressure drives the alternatively activated macrophages to secrete complement C1q protein, which in turn elicits proliferative β-catenin signalling in the arterial smooth muscle cells.

Nanoparticle-mediated expression of a Wnt pathway inhibitor ameliorates ocular neovascularization

OBJECTIVE

The deficiency of very low-density lipoprotein receptor resulted in Wnt signaling activation and neovascularization in the retina. The present study sought to determine whether the very low-density lipoprotein receptor extracellular domain (VLN) is responsible for the inhibition of Wnt signaling in ocular tissues.

APPROACH AND RESULTS

A plasmid expressing the soluble VLN was encapsulated with poly(lactide-co-glycolide acid) to form VLN nanoparticles (VLN-NP). Nanoparticles containing a plasmid expressing the low-density lipoprotein receptor extracellular domain nanoparticle were used as negative control. MTT, modified Boyden chamber, and Matrigel (™) assays were used to evaluate the inhibitory effect of VLN-NP on Wnt3a-stimulated endothelial cell proliferation, migration, and tube formation. Vldlr(-/-) mice, oxygen-induced retinopathy, and alkali burn-induced corneal neovascularization models were used to evaluate the effect of VLN-NP on ocular neovascularization. Wnt reporter mice (BAT-gal), Western blotting, and luciferase assay were used to evaluate Wnt pathway activity. Our results showed that VLN-NP specifically inhibited Wnt3a-induced endothelial cell proliferation, migration, and tube formation. Intravitreal injection of VLN-NP inhibited abnormal neovascularization in Vldlr(-/-), oxygen-induced retinopathy, and alkali burn-induced corneal neovascularization models, compared with low-density lipoprotein receptor extracellular domain nanoparticle. VLN-NP significantly inhibited the phosphorylation of low-density lipoprotein receptor-related protein 6, the accumulation of β-catenin, and the expression of vascular endothelial growth factor in vivo and in vitro.

CONCLUSIONS

Taken together, these results suggest that the soluble VLN is a negative regulator of the Wnt pathway and has antiangiogenic activities. Nanoparticle-mediated expression of VLN may thus represent a novel therapeutic approach to treat pathological ocular angiogenesis and potentially other vascular diseases affected by Wnt signaling.

The endothelial transcription factor ERG promotes vascular stability and growth through Wnt/β-catenin signaling

Blood vessel stability is essential for embryonic development; in the adult, many diseases are associated with loss of vascular integrity. The ETS transcription factor ERG drives expression of VE-cadherin and controls junctional integrity. We show that constitutive endothelial deletion of ERG (Erg(cEC-KO)) in mice causes embryonic lethality with vascular defects. Inducible endothelial deletion of ERG (Erg(iEC-KO)) results in defective physiological and pathological angiogenesis in the postnatal retina and tumors, with decreased vascular stability. ERG controls the Wnt/β-catenin pathway by promoting β-catenin stability, through signals mediated by VE-cadherin and the Wnt receptor Frizzled-4. Wnt signaling is decreased in ERG-deficient endothelial cells; activation of Wnt signaling with lithium chloride, which stabilizes β-catenin levels, corrects vascular defects in Erg(cEC-KO) embryos. Finally, overexpression of ERG in vivo reduces permeability and increases stability of VEGF-induced blood vessels. These data demonstrate that ERG is an essential regulator of angiogenesis and vascular stability through Wnt signaling.