The role of notch in modeling and maintaining the vasculature

Therapeutic Targeting Notch2 Protects Bone Micro-Vasculatures from Methotrexate Chemotherapy-Induced Adverse Effects in Rats

Intensive cancer chemotherapy is well known to cause bone vasculature disfunction and damage, but the mechanism is poorly understood and there is a lack of treatment. Using a rat model of methotrexate (MTX) chemotherapy (five once-daily dosses at 0.75 mg/kg), this study investigated the roles of the Notch2 signalling pathway in MTX chemotherapy-induced bone micro-vasculature impairment. Gene expression, histological and micro-computed tomography (micro-CT) analyses revealed that MTX-induced micro-vasculature dilation and regression is associated with the induction of Notch2 activity in endothelial cells and increased production of inflammatory cytokine tumour necrosis factor alpha (TNFα) from osteoblasts (bone forming cells) and bone marrow cells. Blockade of Notch2 by a neutralising antibody ameliorated MTX adverse effects on bone micro-vasculature, both directly by supressing Notch2 signalling in endothelial cells and indirectly via reducing TNFα production. Furthermore, in vitro studies using rat bone marrow-derived endothelial cell revealed that MTX treatment induces Notch2/Hey1 pathway and negatively affects their ability in migration and tube formation, and Notch2 blockade can partially protect endothelial cell functions from MTX damage.

Chronic Thromboembolic Pulmonary Hypertension-Management Strategies and Outcomes

Chronic thromboembolic pulmonary hypertension (CTEPH) is rare but complex pathophysiological disease with hallmark features of chronic thrombotic mechanical obstruction, right ventricular dysfunction, and secondary pulmonary arteriopathy. It increasingly is being understood that chronic infection/inflammation, abnormal fibrinolysis, and cytokines play an important role in pathogenesis such that only a subset of patients with pulmonary embolism develop CTEPH. Diagnosis remains challenging given the lack of early clinical signs and overlap with other cardiopulmonary conditions. Pulmonary endarterectomy is the surgical procedure of choice with good postoperative survival and functional outcomes, especially when done at high-volume centers with a multidisciplinary approach. There has been a resurgence of balloon pulmonary angioplasty (BPA) as salvage therapy for inoperable CTEPH or in its newfound hybrid role for persistent postoperative pulmonary hypertension with excellent 1-year and 3-year survival. Use of riociguat has shown promising improvements in functional outcomes up to 2 years after initiation. Endothelin receptor antagonists serve a supplemental role postoperatively or in inoperable CTEPH. The role of drug therapy preoperatively or in tandem with BPA is currently under investigation.

Current understanding of the pathophysiology of chronic thromboembolic pulmonary hypertension

Chronic thromboembolic pulmonary hypertension (CTEPH) is a unique form of pulmonary hypertension arising from fibrotic obliteration of major pulmonary arteries. Pro-thrombotic states, large clot burden and impaired dissolution are believed to contribute to the occurrence and progression of thrombosis after an acute pulmonary embolic event. Recent data utilizing several models have facilitated the understanding of clot resolution. This review summarizes current knowledge on pathophysiological mechanisms of major vessel occlusion in CTEPH.

The role of Notch signaling in gastric carcinoma: molecular pathogenesis and novel therapeutic targets

Notch signaling, an evolutionarily conserved signaling cascade system, is involved in promoting the progression of different types of cancers. Within the past decades, the Notch signaling pathway has increasingly been shown to have a primary role in deciding the fate of cancer cells and cancer stem cells in the stomach. Most components of Notch signaling are strongly expressed at different levels in gastric carcinoma tissue samples and are associated with a considerable number of clinical parameters. Moreover, crosstalk signaling between the Notch pathway and the Wnt, Ras, and NF-κB pathways promotes the process of gastric carcinogenesis. Consequently, this increases proliferation and prevents apoptosis in gastric cancer cells, and it contributes to the induction of angiogenesis and accelerates the progression of the epithelial-to-mesenchymal transition. Although the Notch signaling pathway presents novel therapeutic targets for cancer therapeutic intervention, there is still a dearth of in-depth understanding of the molecular mechanisms of Notch signaling in gastric carcinoma. In this review, we summarize the landscape of the Notch signaling pathway and recent findings on Notch signaling in gastric cancer. Furthermore, advanced studies and clinical treatments targeting the Notch signaling pathway arediscussed.

Advances in our understanding of mechanisms of venous thrombus resolution

Traditionally, venous thrombosis has been seen as the consequence of a regulated cascade of proteolytic steps leading to the polymerization of fibrinogen and fibrin crosslinking that is facilitated by platelets. A new view of thrombosis is providing a more integrated concept, with components of the vascular wall contributing to the vascular remodeling of thrombosis. Angiogenesis and inflammation are two key mechanisms that safeguard venous thrombus resolution and restitution of vascular patency after thrombosis. Disturbance of these processes leads to thrombus persistence and has potentially severe consequences for affected patients. Examples for clinical conditions associated with recurrent or persisting venous thrombosis are post-thrombotic syndrome or chronic thromboembolic pulmonary hypertension. Recently, studies using animal models of venous thrombosis have contributed to a better understanding of thrombus non-resolution that will eventually lead to modification of current treatment concepts. For example, recent data suggest that innate immunity is involved in the modification of thrombosis.

Modulation of Sox10, HIF-1α, Survivin, and YAP by Minocycline in the Treatment of Neurodevelopmental Handicaps following Hypoxic Insult

Premature infants are at an increased risk of developing cognitive and motor handicaps due to chronic hypoxia. Although the current therapies have reduced the incidence of these handicaps, untoward side effects abound. Using a murine model of sublethal hypoxia, we demonstrated reduction in several transcription factors that modulate expression of genes known to be involved in several neural functions. We demonstrate the induction of these genes by minocycline, a tetracycline antibiotic with noncanonical functions, in both in vitro and in vivo studies. Specifically, there was induction of genes, including Sox10, Hif1a, Hif2a, Birc5, Yap1, Epo, Bdnf, Notch1 (cleaved), Pcna, Mag, Mobp, Plp1, synapsin, Adgra2, Pecam1, and reduction in activation of caspase 3, all known to affect proliferation, apoptosis, synaptic transmission, and nerve transmission. Minocycline treatment of mouse pups reared under sublethal hypoxic conditions resulted in improvement in open field testing parameters. These studies demonstrate beneficial effects of minocycline treatment following hypoxic insult, document up-regulation of several genes associated with improved cognitive function, and support the possibility of minocycline as a potential therapeutic target in the treatment of neurodevelopmental handicaps observed in the very premature newborn population. Additionally, these studies may aid in further interpretation of the effects of minocycline in the treatment trials and animal model studies of fragile X syndrome and multiple sclerosis.

Electroacupuncture in the repair of spinal cord injury: inhibiting the Notch signaling pathway and promoting neural stem cell proliferation

Electroacupuncture for the treatment of spinal cord injury has a good clinical curative effect, but the underlying mechanism is unclear. In our experiments, the spinal cord of adult Sprague-Dawley rats was clamped for 60 seconds. Dazhui (GV14) and Mingmen (GV4) acupoints of rats were subjected to electroacupuncture. Enzyme-linked immunosorbent assay revealed that the expression of serum inflammatory factors was apparently downregulated in rat models of spinal cord injury after electroacupuncture. Hematoxylin-eosin staining and immunohistochemistry results demonstrated that electroacupuncture contributed to the proliferation of neural stem cells in rat injured spinal cord, and suppressed their differentiation into astrocytes. Real-time quantitative PCR and western blot assays showed that electroacupuncture inhibited activation of the Notch signaling pathway induced by spinal cord injury. These findings indicate that electroacupuncture repaired the injured spinal cord by suppressing the Notch signaling pathway and promoting the proliferation of endogenous neural stem cells.

VEGF and Notch in tip and stalk cell selection

Sprouting angiogenesis is a dynamic process in which endothelial cells collectively migrate, shape new lumenized tubes, make new connections, and remodel the nascent network into a hierarchically branched and functionally perfused vascular bed. Endothelial cells in the nascent sprout adopt two distinct cellular phenotypes--known as tip and stalk cells--with specialized functions and gene expression patterns. VEGF and Notch signaling engage in an intricate cross talk to balance tip and stalk cell formation and to regulate directed tip cell migration and stalk cell proliferation. In this article, we summarize the current knowledge and implications of the tip/stalk cell concepts and the quantitative and dynamic integration of VEGF and Notch signaling in tip and stalk cell selection.

MEDI0639: a novel therapeutic antibody targeting Dll4 modulates endothelial cell function and angiogenesis in vivo

The Notch signaling pathway has been implicated in cell fate determination and differentiation in many tissues. Accumulating evidence points toward a pivotal role in blood vessel formation, and the importance of the Delta-like ligand (Dll) 4-Notch1 ligand-receptor interaction has been shown in both physiological and tumor angiogenesis. Disruption of this interaction leads to a reduction in tumor growth as a result of an increase in nonfunctional vasculature leading to poor perfusion of the tumor. MEDI0639 is an investigational human therapeutic antibody that targets Dll4 to inhibit the interaction between Dll4 and Notch1. The antibody cross-reacts to cynomolgus monkey but not mouse species orthologues. In vitro MEDI0639 inhibits the binding of Notch1 to Dll4, interacting via a novel epitope that has not been previously described. Binding to this epitope translates into MEDI0639 reversing Notch1-mediated suppression of human umbilical vein endothelial cell growth in vitro. MEDI0639 administration resulted in stimulation of tubule formation in a three-dimensional (3D) endothelial cell outgrowth assay, a phenotype driven by disruption of the Dll4-Notch signaling axis. In contrast, in a two-dimensional endothelial cell-fibroblast coculture model, MEDI0639 is a potent inhibitor of tubule formation. In vivo, MEDI0639 shows activity in a human endothelial cell angiogenesis assay promoting human vessel formation and reducing the number of vessels with smooth muscle actin-positive mural cells coverage. Collectively, the data show that MEDI0639 is a potent modulator of Dll4-Notch signaling pathway.

Expression of HES and HEY genes in infantile hemangiomas

Background

Infantile hemangiomas (IHs) are the most common benign tumor of infancy, yet their pathogenesis is poorly understood. IHs are believed to originate from a progenitor cell, the hemangioma stem cell (HemSC). Recent studies by our group showed that NOTCH proteins and NOTCH ligands are expressed in hemangiomas, indicating Notch signaling may be active in IHs. We sought to investigate downstream activation of Notch signaling in hemangioma cells by evaluating the expression of the basic HLH family proteins, HES/HEY, in IHs.

Materials and Methods

HemSCs and hemangioma endothelial cells (HemECs) are isolated from freshly resected hemangioma specimens. Quantitative RT-PCR was performed to probe for relative gene transcript levels (normalized to beta-actin). Immunofluorescence was performed to evaluate protein expression. Co-localization studies were performed with CD31 (endothelial cells) and NOTCH3 (peri-vascular, non-endothelial cells). HemSCs were treated with the gamma secretase inhibitor (GSI) Compound E, and gene transcript levels were quantified with real-time PCR.

Results

HEY1, HEYL, and HES1 are highly expressed in HemSCs, while HEY2 is highly expressed in HemECs. Protein expression evaluation by immunofluorescence confirms that HEY2 is expressed by HemECs (CD31+ cells), while HEY1, HEYL, and HES1 are more widely expressed and mostly expressed by perivascular cells of hemangiomas. Inhibition of Notch signaling by addition of GSI resulted in down-regulation of HES/HEY genes.

Conclusions

HES/HEY genes are expressed in IHs in cell type specific patterns; HEY2 is expressed in HemECs and HEY1, HEYL, HES1 are expressed in HemSCs. This pattern suggests that HEY/HES genes act downstream of Notch receptors that function in distinct cell types of IHs. HES/HEY gene transcripts are decreased with the addition of a gamma-secretase inhibitor, Compound E, demonstrating that Notch signaling is active in infantile hemangioma cells.

Notch receptor and effector expression in von Hippel-Lindau disease-associated central nervous system hemangioblastomas

OBJECT

Central nervous system hemangioblastomas are the most common manifestation of von Hippel-Lindau (VHL) disease, an autosomal dominant tumor suppressor syndrome that results in loss of VHL protein function and continuous upregulation of hypoxia-inducible factors. These tumors are composed of neoplastic stromal cells and abundant vasculature. Stromal cells express markers consistent with multipotent embryonically arrested hemangioblasts, which are precursors for hematopoietic and vascular lineages. Notch receptors are transmembrane signaling molecules that regulate multiple developmental processes including hematopoiesis and vasculogenesis. To investigate the importance of notch signaling in the development of VHL disease-associated CNS hemangioblastomas, the authors examined the presence of the four notch receptors and downstream notch effectors in this setting.

METHODS

The authors used surgical specimens obtained from confirmed VHL-associated hemangioblastomas. Immunohistochemical analysis for the four notch receptors and the downstream effectors was performed on formalin-fixed paraffin-embedded sections. Western blot analysis for HES1 was performed on frozen specimens.

RESULTS

All four notch receptors are present in hemangioblastomas. NOTCH1 and NOTCH4 receptors were widely and prominently expressed in both the stromal and vascular cells, NOTCH2 receptor expression was limited to primarily stromal cells, and NOTCH3 receptor expression was limited to vascular cells. All 4 receptors displayed a nuclear presence. Immunohistochemical analysis also demonstrated that downstream notch effectors, HES1 and HES5, were uniformly expressed in tumor stromal and vascular cells, but HES3, HEY1, and HEY2 were not. Strong HES1 expression was confirmed by Western blot analysis.

CONCLUSIONS

The presence of all four notch receptors and downstream effector molecules suggests that the notch signaling pathway plays a critical role in the maintenance of the undifferentiated pluripotent phenotype of these tumors and in the associated vascular response. Moreover, the prominent expression of notch receptors in VHL-associated CNS hemangioblastomas reveals a new and possibly potent therapeutic target.

Distribution of Notch family proteins in intrauterine growth restriction and hypertension complicated human term placentas

Members of the Notch family have been detected in many developmental and cell specification processes during placental development. However, Notch protein expression in Intrauterine Growth Restriction (IUGR) and Pregnancy Induced Hypertension (PIH) is not clear. In this study we aimed to clarify the immunolocalization of Notch proteins in full-term placentas after IUGR and PIH in comparison with normal placentas. Formalin-fixed, paraffin-embedded term placentas obtained by caesarean operations were processed for immunohistochemical localization of Notch 1, 2, 4 and Jagged 2. Transmission electron microscopy was also performed. In normal term placentas, all Notch proteins were intensely immunostained in the brush border of cells of the syncytiotrophoblast layer of the basal (maternal) side and the chorionic plate (fetal) side. The endothelial cells were also intensely immunostained in both sides for Notch 1. However, in IUGR and PIH placentas, the immunoreactivities of all Notch proteins were decreased significantly in the brush border of cells of the syncytiotrophoblast layer and the reaction was generally observed in the cytoplasm of syncytiotrophoblast cells in the basal and chorionic plate sides. The reactivity in endothelial cells was also significantly decreased. Our results have shown that the immunoreactivity and localization of Notch proteins is altered in pathologic placentas. Therefore, we propose that deregulated expression of Notch proteins may contribute to the disruption of trophoblast differentiation, endothelial cell function and/or feto-maternal traffic down-regulation during pregnancy or vice versa in such pathologic conditions.

Targeting microvasculature for neuroprotection after SCI

Spinal cord injury (SCI) is characterized by secondary degeneration, which leads to tissue loss at the epicenter and subsequent functional deficits. This review provides insight into the pathophysiology of microvascular dysfunction and endothelial cell loss, which are among the earliest responses during the first postinjury day. The enigmatic role of the angiogenic response in the penumbra around the lost tissue, which occurs during the first 2 weeks, is also discussed. The importance of stabilizing and rescuing the injured vasculature is now well-recognized, and several pharmacological and genetic treatments have emerged in the past few years. We conclude with suggestions for future experimental research, including development of vascular-selective treatments and exploitation of genetic models. In summary, vascular dysfunction following SCI is an important contributor to neurological deficits, as proposed long ago. However, there now appears to be new and potentially powerful opportunities for treating acute SCI by targeting the vascular responses.

Activating Notch signaling post-SCI modulates angiogenesis in penumbral vascular beds but does not improve hindlimb locomotor recovery

Manipulation of Notch signaling has led to significant tumor shrinkage as well as recovery from several traumatic and ischemic injury models indicating its potential clinical application. We have tested both an agonist and antagonist of Notch signaling to study the effects of Notch-mediated angiogenesis on spinal cord vascular pathology following traumatic injury. Initial neonatal retinal vascularization assays showed their respective bioactivities in vivo. Mice were treated with either the antagonist Jagged1-Fc chimera (Jag1-Fc) or agonist Notch1 antibody (N1 Ab) immediately following a mid-thoracic contusive injury through an initial jugular bolus and tail vein injections for 3 days post-injury. After 14 days, activating Notch signaling decreased the overall vascular density within the penumbral gray matter compared to controls while maintaining the density of perfused vessels. Inhibiting Notch signaling did not change the density or perfusion of microvessels within the lesion penumbra. Furthermore, neither activation nor inhibition of Notch signaling significantly altered inflammation, hypoxia, and lesion volume in the epicenter and penumbra. Importantly, neither treatment changed locomotor function. In postnatal retinal vascular assays, administration of Jag1-Fc and N1 Ab increased and decreased both tip cell numbers and branch points in each treatment, respectively. However, these agents did not modulate primary CNS EC proliferation in vitro in spite of sufficient Notch ligand expression. We conclude that Notch signaling, while an important part of developmental angiogenesis, may play a lesser role in mediating vascular recovery following traumatic injury to the CNS.

Chronic exposure to nicotine and saquinavir decreases endothelial Notch-4 expression and disrupts blood-brain barrier integrity

Since the advent of HAART, there have been substantial improvements in HIV patient survival; however, the prevalence of HIV associated dementia has increased. Importantly, HIV positive individuals who smoke progress to HIV associated neurological conditions faster than those who do not. Recent in vitro data have shown that pharmacological levels of saquinavir causes endothelial oxidative stress and significantly decreases Notch-4 expression, a primary protein involved in maintaining stability of blood-brain barrier (BBB) endothelium. This is concerning as nicotine can also generate reactive oxygen species in endothelium. It is largely unknown if pharmacological doses of these drugs can cause a similar in vivo down-regulation of Notch-4 and if there is a concurrent destabilization of the integrity of the BBB. The data herein show: (i) nicotine and protease inhibitors cause an additive oxidative stress burden in endothelium; (ii) that the integrity of the BBB is disrupted after concurrent chronic nicotine and protease inhibitor administration; and (iii) that BBB endothelial dysfunction is correlated with a decrease in Notch-4 and ZO-1 expression. Considering the high prevalence of smoking in the HIV infected population (3- to 4-fold higher than in the general population) this data must be followed up to determine if all protease inhibitors cause a similar BBB disruption or if there is a safer alternative. In addition, this data may suggest that the induced BBB disruption may allow foreign molecules to gain access to brain and be a contributing factor to the slow progression of HIV associated dementia.

A potential role for notch signaling in the pathogenesis and regulation of hemangiomas

Hemangiomas are the most common benign tumor of infancy, yet its pathogenesis and the mechanisms governing proliferation and involution are not well understood. It is believed that hemangiomas arise out of clonal, abnormal hemangioma endothelial cells (HemECs). The underlying anomaly of the HemEC is not known, although studies have shown that vascular endothelial growth factor (VEGF) and VEGF signaling may influence HemECs. Moreover, there are numerous subtypes of hemangiomas, with differences in natural history, potential for morbidity, and prognosis, and little is known how this relates to HemEC. The Notch signaling pathway is a highly conserved pathway across species from worms to mammals. Notch signaling has been shown to play a role during embryogenesis in directing vascular patterning and development and arterial and venous cell fate determination. Postnatally, it has been implicated in tumor angiogenesis in multiple malignancies. Notch signaling triggers tumor angiogenesis at least in part to stimulation by VEGF, thus establishing that there is a cross talk between the VEGF and Notch pathways. Given the presence of VEGF and its receptors in hemangiomas and known VEGF-Notch cross talk in tumor angiogenesis, the authors hypothesize that Notch signaling may contribute to hemangioma proliferation and involution. Preliminary studies of resected hemangioma specimens by reverse transcription polymerase chain reaction (RT-PCR) show that all 4 Notch receptors and 2 Notch ligands, Jagged1 and Delta-like ligand 4, are expressed by hemangiomas. These findings support a role for Notch in hemangiomas, meriting further analysis of the functional relevance of Notch signaling in hemangiomas.

Developmental angiogenesis of the central nervous system

The vasculature of the central nervous system (CNS) is highly specialized with a blood-brain-barrier, reciprocal neuroepithelial-endothelial cell interactions and extensive pericyte coverage. Developmentally, numerous important signaling pathways participate in CNS angiogenesis to orchestrate the precise timing and spatial arrangement of the complex CNS vascular network. From a therapeutic standpoint, the CNS vasculature has attracted increased attention since many human ailments, such as stroke, retinopathy, cancer and autoimmune disease are intimately associated with the biology of CNS blood vessels. This review focuses on growth factor pathways that have been shown to be important in developmental CNS vascularization through studies of mouse genetic models and human diseases.

Soluble forms of the Notch ligands Delta1 and Jagged1 promote in vivo tumorigenicity in NIH3T3 fibroblasts with distinct phenotypes

We previously found that soluble forms of the Notch ligands Jagged1 and Delta1 induced fibroblast growth factor receptor-dependent cell transformation in NIH3T3 fibroblasts. However, the phenotypes of these lines differed, indicating distinct functional differences among these Notch ligands. In the present study, we used allografts to test the hypothesis that NIH3T3 fibroblasts that express soluble forms of Delta1 and Jagged1 accelerate tumorigenicity in vivo. With the exception of the full-length Jagged1 transfectant, all other cell lines, including the control, generated tumors when injected subcutaneously in athymic mice. Suppression of Notch signaling by the soluble ligands significantly increased tumor onset and growth, whereas full-length Jagged1 completely suppressed tumor development. In addition, there were striking differences in tumor pathology with respect to growth kinetics, vascularization, collagen content, size and number of necrotic foci, and invasiveness into the underlying tissue. Further, the production of angiogenic factors, including vascular endothelial growth factor, also differed among the tumor types. Lastly, both Jagged1- and Delta1-derived tumors contained phenotypically distinct populations of lipid-filled cells that corresponded with increased expression of adipocyte markers. The divergence of tumor phenotype may be attributed to ligand-specific alterations in Notch receptor responses in exogenous and endogenous cell populations within the allographs. Our findings demonstrate distinct functional properties for these Notch ligands in the promotion of tumorigenicity in vivo.

Notch signaling in cardiac development

The Notch signaling pathway has been demonstrated to play a critical role during mammalian cardiac development based on recent findings from gene-targeted mice. In addition, mutations in the Notch signaling pathway have been associated with human congenital heart defects such as Alagille syndrome, bicuspid aortic valve disease, calcification of the heart valves, and ventricular septal defects. Recently, it was demonstrated that Notch activation in the endocardium regulates ventricular myocardial development and that the Notch downstream target genes Hey1 and Hey2 are required for the establishment of the atrioventricular canal myocardial boundary. The Notch pathway has previously been implicated in regulating endothelial-to-mesenchymal transition during development of the heart valves, and recent reports further dissect the role of individual Notch downstream target genes during this process. In addition, a role for the Notch pathway during cardiac neural crest cell development has been identified, which provides a potential mechanism for the findings seen in Alagille syndrome. This review focuses on recently reported findings that elucidate mechanisms regulated by the Notch pathway during ventricular, atrioventricular canal, and outflow tract development.

The benefit of docosahexanoic acid on the migration of vascular smooth muscle cells is partially dependent on Notch regulation of MMP-2/-9

The Notch pathway is involved in the regulation of the migratory/proliferative phenotype acquired by vascular smooth muscle cells (VSMCs) in the pro-inflammatory context of vascular diseases. Here, we investigated whether docosahexaenoic acid (DHA), a polyunsaturated, omega-3 fatty acid, could reduce fibrinolytic/matrix-metalloproteinase (MMP) activity and whether this reduction occurs through the modulation of Notch signaling. Rat VSMCs were transdifferentiated with interleukin-1beta and then treated with DHA. Migration/proliferation was determined by performing a wound healing assay and measuring MMP-2/-9 activity, type 1 plasminogen activator inhibitor levels, and the expression of these proteins. The involvement of Notch in regulating the fibrinolytic/MMP system was evidenced using Notch pathway inhibitors and the forced expression of Notch1 and Notch3 intracellular domains. DHA significantly decreased VSMC migration/proliferation induced by interleukin-1beta as well as fibrinolytic/MMP activity. Prevention of Notch1 target gene transcription enhanced the interleukin-1beta effects on MMPs and on migration, whereas Notch3 intracellular domain overexpression reduced these effects. Finally, DHA increased Notch3 expression, Hes-1 transcription (a Notch target gene), and enhanced gamma-secretase complex activity. These results suggest that inhibition of the Notch pathway participates in the transition of VSMCs toward a migratory phenotype. These results also suggest that the beneficial inhibitory effects of DHA on fibrinolytic/MMP activity are related in part to the effects of DHA on the expression of Notch pathway components, providing new insight into the mechanisms by which omega-3 fatty acids prevent cardiovascular diseases.

Regulation of multiple angiogenic pathways by Dll4 and Notch in human umbilical vein endothelial cells

The Notch ligand, Dll4, is essential for angiogenesis during embryonic vascular development and is involved in tumour angiogenesis. Several recent publications demonstrated that blockade of Dll4 signalling inhibits tumour growth, suggesting that it may constitute a good candidate for anti-cancer therapy. In order to understand the role of Dll4 at the cellular level, we performed an analysis of Dll4-regulated genes in HUVECs. The genes identified included several angiogenic signalling pathways, such as VEGF, FGF and HGF. In particular we identified downregulation (VEGFR2, placenta growth factor PlGF) of VEGF pathway components resulting in the overall effect of limiting the response of HUVEC to VEGF. However extensive upregulation of VEGFR1 was observed allowing continued response to its ligand PlGF but the soluble form of the VEGFR1, sVEGFR1 was also upregulated. PlGF enhanced tubulogenesis of HUVEC suggesting that downregulation of PlGF and upregulation of VEGFR1 including sVEGFR1 are important mechanisms by which Dll4 attenuates PlGF and VEGF signalling. Dll4-stimulated HUVECs had impaired ERK activation in response to VEGF and HGF indicating that Dll4 signalling negatively regulates these pathways. Dll4 expression reduced vessel sprout length in a 3D tubulogenesis assay confirming that Dll4 signalling inhibits angiogenesis. Altogether, our data suggest that Dll4 expression acts as a switch from the proliferative phase of angiogenesis to the maturation and stabilisation phase by blocking endothelial cell proliferation and allowing induction of a more mature, differentiated phenotype. The regulation of sVEGFR1 provides a novel mechanism for Dll4 signalling to regulate cells at distance, not just in adjacent cells.

Control of blood vessel identity: from embryo to adult

Arteries and veins have been historically defined by the direction of blood flow and oxygen tension within the vessel, in addition to their functional, hemodynamic, and anatomical differences. It is now known that the molecular identity of these vessels is genetically predetermined, with specific molecular pathways activated during the development of arteries and veins. Eph-B4 is a determinant of venous differentiation and Ephrin-B2 is a determinant of arterial differentiation. Placement of a vein into the higher pressure and flow of the arterial circulation results in adaptation of the vein to the arterial environment. There is selective loss of Eph-B4 expression without induction of Ephrin-B2 expression during vein graft adaptation. These findings suggest that loss of venous identity is the crucial mechanism in vein graft adaptation and that developmentally critical determinants of vessel identity are plastic during adult life.

Notch and integrin affinity: a sticky situation

The Notch pathway is a conserved signal transduction system that mediates intercellular signaling to regulate cell fate decisions in various tissues. Dysregulation of Notch activity results in various disorders, including cardiovascular diseases and cancer. Notch regulates cell fate through a number of mechanisms that include control of cell proliferation, survival, migration, and differentiation. Notch activation increases vascular endothelial cell adhesion through the enhancement of beta(1) integrin affinity for fibronectin, collagens I and IV, and vitronectin without altering the abundance of beta(1) integrin at the cell surface. A study now suggests that this Notch-dependent increase in beta(1) integrin affinity occurs through the activation of the small guanosine triphosphate (GTP)-binding protein, R-Ras. It is proposed that Notch-dependent activation of R-Ras reverses H-Ras-mediated suppression of integrin affinity. Activation of R-Ras by Notch may be triggered by a noncanonical CSL (CBF1 or RBP-Jkappa in vertebrates, Suppressor of Hairless in Drosophila, Lag-1 in Caenorhabditis elegans)-independent pathway. Because R-Ras is selectively distributed in vascular cells, these findings are of particular importance in understanding the effector functions of Notch in the vascular system.

RBP-J, the transcription factor downstream of Notch receptors, is essential for the maintenance of vascular homeostasis in adult mice

In adults, angiogenic abnormalities are involved in not only tumor growth but several human inherited diseases as well. It is unclear, however, concerning how the normal vascular structure is maintained and how angiogenesis is initiated in normal adults. Using the Cre-LoxP-mediated conditional gene deletion, we show in the present study that in adult mice disruption of the transcription factor recombination signal-binding protein Jkappa (RBP-J) in endothelial cells strikingly induced spontaneous angiogenesis in multiple tissues, including retina and cornea, as well as in internal organs, such as liver and lung. In a choroidal neovascularization model, which mimics the angiogenic process in tumor growth and age-related macular degeneration, RBP-J deficiency induced a more intensive angiogenic response to injury. This could be transmitted by bone marrow, indicating that RBP-J could modulate bone marrow-derived endothelial progenitor cells in adult angiogenesis. In addition, in the absence of RBP-J, proliferation of endothelial cells increased significantly, leading to accumulative vessel outgrowth. These findings suggest that in adults RBP-J-mediated Notch signaling may play an essential role in the maintenance of vascular homeostasis by repressing endothelial cell proliferation.

Anti-Dll4 therapy: can we block tumour growth by increasing angiogenesis?

Since the early 1970s, the dogma postulating that blocking tumour angiogenesis can inhibit tumour growth has been accepted widely and has resulted in the generation of a variety of successful anti-angiogenic therapies. More recently, new signalling pathways, such as the Dll4-Notch signalling pathway, have been shown to regulate angiogenesis during development. In pathological conditions, such as cancer, Dll4 is up-regulated strongly in the tumour vasculature. Based on this expression pattern, different molecules have been generated to block Dll4 signalling. Unexpectedly, these blocking agents inhibited tumour growth in vivo by triggering excessive but nonfunctional angiogenesis. Altogether, these molecules constitute a new category of pro-angiogenic yet anticancer agents and offer an exciting alternative to previously described vascular targeting molecules.

Notch signaling in blood vessels: who is talking to whom about what?

It has become increasingly clear that the Notch signaling pathway plays a critical role in the development and homeostasis of the cardiovascular system. This notion has emerged from loss- and gain-of-function analysis and from the realization that several hereditary cardiovascular disorders originate from gene mutations that have a direct impact on Notch signaling. Current research efforts are focused on determining the specific cellular and molecular effects of Notch signaling. The rationale for this has stemmed from the clinical importance and therapeutic potential of modulating vascular formation during various disease states. A more complete appreciation of Notch signaling, as it relates to vascular morphogenesis, requires an in-depth knowledge of expression patterns of the various signaling components and a comprehensive understanding of downstream targets. The goal of this review is to summarize current knowledge regarding Notch signaling during vascular development and within the adult vascular wall. Our focus is on the genetic analysis and cellular experiments that have been performed with Notch ligands, receptors, and downstream targets. We also highlight questions and controversies regarding the contribution of this pathway to vascular development.

Blockade of Dll4 inhibits tumour growth by promoting non-productive angiogenesis

Tumour growth requires accompanying expansion of the host vasculature, with tumour progression often correlated with vascular density. Vascular endothelial growth factor (VEGF) is the best-characterized inducer of tumour angiogenesis. We report that VEGF dynamically regulates tumour endothelial expression of Delta-like ligand 4 (Dll4), which was previously shown to be absolutely required for normal embryonic vascular development. To define Dll4 function in tumour angiogenesis, we manipulated this pathway in murine tumour models using several approaches. Here we show that blockade resulted in markedly increased tumour vascularity, associated with enhanced angiogenic sprouting and branching. Paradoxically, this increased vascularity was non-productive-as shown by poor perfusion and increased hypoxia, and most importantly, by decreased tumour growth-even for tumours resistant to anti-VEGF therapy. Thus, VEGF-induced Dll4 acts as a negative regulator of tumour angiogenesis; its blockade results in a striking uncoupling of tumour growth from vessel density, presenting a novel therapeutic approach even for tumours resistant to anti-VEGF therapies.

Notch expression patterns in the retina: An eye on receptor-ligand distribution during angiogenesis

The critical contribution of the Notch signaling pathway to vascular morphogenesis has been underscored by loss-of-function studies in mouse and zebrafish. Nonetheless, a comprehensive understanding as to how this signaling system influences the formation of blood vessels at the cellular and molecular level is far from reached. Here, we provide a detailed analysis of the distribution of active Notch1 in relation to its DSL (Delta, Serrate, Lag2) ligands, Jagged1, Delta-like1, and Delta-like4, during progressive stages of vascular morphogenesis and maturation. Important differences in the cellular distribution of Notch ligands were found. Jagged1 (Jag1) was detected in "stalk cells" of the leading vasculature and at arterial branch points, a site where Delta-like4 (Dll4) was clearly absent. Dll4 was the only ligand expressed in "tip cells" at the end of the growing vascular sprouts. It was also present in stalk cells, capillaries, arterial endothelium, and in mural cells of mature arteries in a homogenous manner. Delta-like1 (Dll1) was observed in both arteries and veins of the developing network, but was also excluded from mature arterial branch points. These findings support alternative and distinct roles for Notch ligands during the angiogenic process.

Notch-independent regulation of Hes-1 expression by c-Jun N-terminal kinase signaling in human endothelial cells

Our laboratory has recently demonstrated constitutive activation of the Notch signaling pathway in Kaposi's sarcoma tumor cells. As endothelial cells (EC) are believed to be the progenitor of these tumor cells, this study was designed to examine the effect of Notch activation on normal human EC. Recent reports suggest Notch activation induces EC growth arrest, and that this growth arrest may be linked to the establishment or maintenance of EC quiescence, the phenotype seen in contact-inhibited EC lining the vasculature. To gain further insight into Notch activation and quiescence, we first confirmed that Notch activation induced EC growth arrest. Next, we examined Notch activation in confluent, growth arrested EC (mimicking the cells lining the vasculature). In contrast to previous reports, we found confluent EC possess lower levels of activated Notch compared to proliferating control cells. Interestingly, these cells express elevated levels of Hes-1 protein (an immediate downstream target of Notch signaling) despite decreased Notch activation. Under these conditions, Hes-1 expression was mediated, at least in part, by a Notch-independent mechanism involving c-jun N-terminal protein kinase (JNK) signaling. This is the first report, to our knowledge, that JNK signaling can modulate Hes-1 expression in a Notch-independent manner.

Smooth Muscle alpha-actin is a direct target of Notch/CSL

Intercellular signaling mediated by Notch receptors is essential for proper cardiovascular development and homeostasis. Notch regulates cell fate decisions that affect proliferation, survival, and differentiation of endothelial and smooth muscle cells. It has been reported that Jagged1-Notch interactions may participate in endocardial cushion formation by inducing endothelial-to-mesenchymal transformation. Here, we show that Notch directly regulates expression of the mesenchymal and smooth muscle cell marker smooth muscle alpha-actin (SMA) in endothelial and vascular smooth muscle cells via activation of its major effector, CSL. Notch/CSL activation induces SMA expression during endothelial-to-mesenchymal transformation, and Notch activation is required for expression of SMA in vascular smooth muscle cells. CSL directly binds a conserved cis element in the SMA promoter, and this consensus sequence is required for Notch-mediated SMA induction. This is the first evidence of the requirement for Notch activation in the regulation of SMA expression.

Hypoxia-regulated differentiation: let's step it up a Notch

Although hypoxia is widely associated with adult pathologies such as cancer, it is also a physiological process that regulates cell differentiation during organogenesis. In an attempt to characterize the molecular mechanisms that are involved in hypoxia-regulated cell fate, a recent publication by Gustafsson and colleagues elegantly demonstrated that hypoxia blocks cell differentiation through the regulation of Notch signalling. This study showed that hypoxia-inducible factor (HIF)-1alpha interacts and acts in synergy with the Notch intracellular domain (NIC) and subsequently activates transcription of Notch targets. The identification of this crosstalk between pathways that are often deregulated in cancer is groundbreaking and opens up new areas for cancer research.

Structural model of an antistasin/notch-like fusion protein from the cocoon wall of the aquatic leech, Theromyzon tessulatum

The aquatic leech, Theromyzon tessulatum, secretes a proteinaceous cocoon with extraordinary physical properties (e.g., proteolytic, thermal resiliency). The deduced amino acid sequence of a major protein (Tcp-Theromyzon cocoon protein) from the T. tessulatum cocoon wall has been used to model the endogenous structure of the Tcp protein. The Tcp protein sequence comprises six internal repeats, each containing 12 ordered Cys residues. Amino acid alignments suggest that the region Cys1-->6 is homologous to antistasin, a leech anticoagulant, and Cys7-->12 is homologous to an epidermal growth factor-like domain found in notch-class proteins, which play critical roles in development, signaling, and adhesion throughout the Animalia. Modeling of individual domains (i.e., antistasin and notch) positions multiple hydrophobic and charged residues on the surface. When the antistasin and notch domains were fused, hydrophobic pockets appeared that may facilitate a polymerization mechanism. Collectively, the predicted features of our Tcp model are consistent with the physical properties of the leech cocoon wall.

Notch signaling and its integration with other signaling mechanisms

An improved understanding of stem cell differentiation is critical for progress in regenerative medicine. It is an emerging view that a relatively small number of intracellular signaling mechanisms play particularly important roles in differentiation control. As one may expect, these pathways are highly evolutionarily conserved, used in many tissues and iteratively during differentiation of a particular tissue. The Notch signaling system is one pathway meeting these criteria. In many cases, Notch signaling keeps stem/progenitor cells undifferentiated, although it can in some cellular contexts be instructive for differentiation toward a particular fate. Here, we review our current understanding of how Notch controls cellular differentiation in various organs and how Notch integrates with other major signaling pathways, primarily focusing on Notch signaling in mammals. Given the importance of Notch in many stem cell fate decisions, the possibility of experimentally manipulating Notch signaling opens up new avenues to control stem cell differentiation.

Recent insights into the role of Notch signaling in tumorigenesis

Members of the Notch family of transmembrane receptors play an important role in cell fate determination. Over the past decade, a role for Notch in the pathogenesis of hematologic and solid malignancies has become apparent. Numerous cellular functions and microenvironmental cues associated with tumorigenesis are modulated by Notch signaling, including proliferation, apoptosis, adhesion, epithelial-to-mesenchymal transition, and angiogenesis. It is becoming increasingly evident that Notch signaling can be both oncogenic and tumor suppressive. This review highlights recent findings regarding the molecular and functional aspects of Notch-mediated neoplastic transformation. In addition, cellular mechanisms that potentially explain the complex role of Notch in tumorigenesis are discussed.