Soluble ephrin-B2 mediates apoptosis in retinal neovascularization and in endothelial cells

Role and action mechanisms of tPA in CRH-induced apoptosis of mouse oviductal epithelial and mural granulosa cells

Understanding how stress hormones induce apoptosis in oviductal epithelial cells (OECs) and mural granulosa cells (MGCs) can reveal the mechanisms by which female stress impairs embryonic development and oocyte competence. A recent study showed that tissue plasminogen activator (tPA) ameliorates corticosterone-induced apoptosis in MGCs and OECs by acting on its receptors low-density lipoprotein receptor-related protein 1 (LRP1) and Annexin A2 (ANXA2), respectively. However, whether tPA is involved in corticotropin-releasing hormone (CRH)-induced apoptosis and whether it uses the same or different receptors to inhibit apoptosis induced by different hormones in the same cell type remains unknown. This study showed that CRH triggered apoptosis in both OECs and MGCs and significantly downregulated tPA expression. Moreover, tPA inhibits CRH-induced apoptosis by acting on ANXA2 in both OECs and MGCs. While ANXA2 inhibits apoptosis via phosphatidylinositol 3-kinase/protein kinase B (PI3K/Akt) signaling, LRP1 reduces apoptosis via mitogen-activated protein kinase (MAPK) signaling. Thus, tPA used the same receptor to inhibit CRH-induced apoptosis in both OECs and MGCs, however used different receptors to inhibit corticosterone-induced apoptosis in MGCs and OECs. These data helps understand the mechanism by which female stress impairs embryo/oocyte competence and proapoptotic factors trigger apoptosis in different cell types.

Eph Receptors and Ephrins in Retinal Diseases

Retinal diseases are the leading cause of irreversible blindness. They affect people of all ages, from newborns in retinopathy of prematurity, through age-independent diabetic retinopathy and complications of retinal detachment, to age-related macular degeneration (AMD), which occurs mainly in the elderly. Generally speaking, the causes of all problems are disturbances in blood supply, hypoxia, the formation of abnormal blood vessels, and fibrosis. Although the detailed mechanisms underlying them are varied, the common point is the involvement of Eph receptors and ephrins in their pathogenesis. In our study, we briefly discussed the pathophysiology of the most common retinal diseases (diabetic retinopathy, retinopathy of prematurity, proliferative vitreoretinopathy, and choroidal neovascularization) and collected available research results on the role of Eph and ephrins. We also discussed the safety aspect of the use of drugs acting on Eph and ephrin for ophthalmic indications.

The Role of Eph Receptors and Ephrins in Corneal Physiology and Diseases

The cornea, while appearing to be simple tissue, is actually an extremely complex structure. In order for it to retain its biomechanical and optical properties, perfect organization of its cells is essential. Proper regeneration is especially important after injuries and in the course of various diseases. Eph receptors and ephrin are mainly responsible for the proper organization of tissues as well as cell migration and communication. In this review, we present the current state of knowledge on the role of Eph and ephrins in corneal physiology and diseases, in particular, we focused on the functions of the epithelium and endothelium. Since the role of Eph and ephrins in the angiogenesis process has been well established, we also analyzed their influence on conditions with corneal neovascularization.

Ephrin-A5 Is Involved in Retinal Neovascularization in a Mouse Model of Oxygen-Induced Retinopathy

Retinal neovascularization (RNV) is an important pathological feature of vitreoretinopathy that can lead to severe vision loss. The purpose of this study was to identify the role of ephrin-A5 (Efna5) in RNV and to explore its mechanism. The expression pattern and biological significance of Efna5 were investigated in a mouse model of oxygen-induced retinopathy (OIR). The expression of Efna5 and downstream signaling pathway members was determined by RT-PCR, immunofluorescence, immunohistochemistry, and western blot analyses. shRNA was used to knockdown Efna5 in the retina of the OIR mouse model. Retinal flat mounts were performed to evaluate the impact of Efna5 silencing on the RNV process. We found that the Efna5 was greatly upregulated in the retina of OIR mice. Elevated Efna5 mainly colocalized with the retinal vessels and endothelial cells. We then showed that knockdown of Efna5 in OIR mouse retinas using lentivirus-mediated shRNA markedly decreased the expression of Efna5 and reduced the retinal neovascularization and avascular retina area. We further showed hypoxia stimulation dramatically increased both total and phosphorylation levels of ERK1/2 and the phosphorylation levels of Akt in OIR mice. More importantly, knockdown of Efna5 could inhibit the p-Akt and p-ERK signaling pathways. Our results suggested that Efna5 may regulate the RNV. This study suggests that Efna5 was significantly upregulated in the retina of OIR mice and closely involved in the pathological retinal angiogenesis.

Increased Ephrin-B2 expression in pericytes contributes to retinal vascular death in rodents

AIMS

Diabetes-induced retinal vascular cell death aggravates diabetic retinopathy (DR) to the proliferative stage and blindness. Pericytes play a crucial role in retinal capillaries survival, stability, and angiogenesis. Ephrin-B2 is a tyrosine kinase that regulates pericytes/endothelial cells communication during angiogenesis; yet, its role in DR is still unclear. We hypothesize that diabetes increases Ephrin-B2 signaling in pericytes, which contributes to inflammation and retinal vascular cell death.

METHODS

Selective inhibition of the Ephrin-B2 expression in the retinal pericytes was achieved using an intraocular injection of adeno-associated virus (AAV) with a specific pericyte promotor. Vascular death was determined by retinal trypsin digest. Pathological angiogenesis was assessed using the oxygen-induced retinopathy model in pericyte-Ephrin-B2 knockout mice, wild type, and wild type injected with AAV. Angiogenic properties, inflammatory, and apoptotic markers were measured in human retinal pericytes (HRP) grown under diabetic conditions.

KEY FINDING

Diabetes significantly increased the expression of Ephrin-B2, inflammatory, and apoptotic markers in the diabetic retinas and HRP. These effects were prevented by silencing Ephrin-B2 in HRP. Moreover, Ephrin-B2 silencing in retinal pericytes decreased pathological angiogenesis and acellular capillaries formation in diabetic retinas.

SIGNIFICANCE

Increased Ephrin-B2 expression in the pericytes contributed to diabetes-induced retinal inflammation and vascular death. These results identify pericytes-Ephrin-B2 as a therapeutic target for DR.

The critical role of the interplays of EphrinB2/EphB4 and VEGF in the induction of angiogenesis

The significant role of VEGF (vascular endothelial growth factor) as an angiogenesis inducer is well recognized. Besides VEGF, EphrinB2/EphB4 also plays essential roles in vascular development and postnatal angiogenesis. Compared with classical proangiogenic factors, not only does EphrinB2/EphB4 promote sprouting of new vessels, it is also involved in the vessel maturation. Given their involvement in many physiologic and pathological conditions, EphB4 and EphrinB2 are increasingly recognized as attractive therapeutic targets for angiogenesis-related diseases through modulating their expression and function. Previous works mainly focused on the individual role of VEGF and EphrinB2/EphB4 in angiogenesis, respectively, but the correlation between EphrinB2/EphB4 and VEGF in angiogenesis has not been fully disclosed. Here, we summarize the structure and bidirectional signaling of EphrinB2/EphB4, provide an overview on the relationship between EphrinB2/EphB4 signaling and VEGF pathway in angiogenesis and highlight the associated potential usefulness in anti-angiogenetic therapy.

HMQ-T-B10 induces human liver cell apoptosis by competitively targeting EphrinB2 and regulating its pathway

Hepatocellular carcinoma (HCC) is a highly prevalent cancer worldwide and it is necessary to discover and develop novel preventive strategies and therapeutic approaches for HCC. Herein, we report that EphrinB2 expression is correlated with liver cancer progression. Moreover, by using phosphorylated proteomics array, we reveal a pro-apoptosis protein whose phosphorylation and activation levels are up-regulated upon EphrinB2 knockdown. These results suggest that EphrinB2 may act as an anti-apoptotic protein in liver cancer cells. We also explored the therapeutic potential of HMQ-T-B10 (B10), which was designed and synthesized in our laboratory, for HCC and its underlying mechanisms in vitro and in vivo. Our data demonstrate that B10 could bind EphrinB2 and show inhibitory activity on human liver cancer cells. Moreover, induction of human liver cancer cell apoptosis by B10 could be augmented upon EphrinB2 knockdown. B10 inhibited HCC cell growth and induced HCC cell apoptosis by repressing the EphrinB2 and VEGFR2 signalling pathway. Growth of xenograft tumours derived from Hep3B in nude mice was also significantly inhibited by B10. Collectively, these findings highlight the potential molecular mechanisms of B10 and its potential as an effective antitumour agent for HCC.

Asymmetric dimethylarginine contributes to retinal neovascularization of diabetic retinopathy through EphrinB2 pathway

Diabetic retinopathy (DR) is a leading cause of vision loss with retinal neovascularization. This study aims to investigate whether Asymmetric dimethylarginine (ADMA) impacts the pathogenesis of DR via focusing on promoting retinal neovascularization and its underlying molecular mechanisms. Diabetic rats were induced by a single intraperitoneal injection of streptozotocin (STZ) for 20 weeks. ADMA levels in aqueous and the influence of hypoxia on ADMA and angiogenesis in RF/6A cells were examined. The effects and underlying molecular mechanisms of ADMA on neovascularization of RF/6A cells were further evaluated by administration of ADMA, DDAH siRNA or ephrinB2 siRNA. Results showed that ADMA levels were elevated in both aqueous from diabetic rats and culture medium in RF/6A cells pretreated with hypoxia. Administration of ADMA directly promoted proliferation, migration, adhesion and tube formation of RF/6A cells, which was further confirmed by DDAH1 siRNA or DDAH2 siRNA. In addition, ephrinB2 expression was increased under diabetic conditions, and the angiogenic effects of ADMA were blocked by ephrinB2 siRNA. In conclusion, ADMA contributes to the neovascularization of retina in diabetic mellitus, which is regulated by ephrinB2.

Role of EFNB2/EPHB4 signaling in spiral artery development during pregnancy: An appraisal

EFNB2 and EPHB4, which belong to a large tyrosine kinase receptor superfamily, are molecular markers of arterial and venous blood vessels, respectively. EFNB2/EPHB4 signaling plays an important role in physiological and pathological angiogenesis, and its role in tumor vessel development has been extensively studied. Pregnancy and tumors share similar features, including continuous cell proliferation and increased demand for a blood supply. Our previous studies showed that Efnb2 and Ephb4 were expressed dynamically in the spiral arteries, uterine natural killer cells, and trophoblasts during mouse gestation Days 6.5-12.5. Moreover, uterine natural killer cells and trophoblasts are required for the modification of spiral arteries. Oxygen tension within the pregnant uterus, which contributes to the vascular development, also affects EFNB2 and EPHB4 expression. Considering the role of EFNB2/EPHB4 signaling in embryonic and tumor vascular development, and its dynamic expression in the decidua and placenta, we hypothesize that EFNB2 and EPHB4 are involved in the regulation of spiral artery remodeling. Investigating this hypothesis will help clarify the mechanisms of pathological pregnancy that may underlie abnormal spiral artery development.

Erythropoietin Stimulates Tumor Growth via EphB4

While recombinant human erythropoietin (rhEpo) has been widely used to treat anemia in cancer patients, concerns about its adverse effects on patient survival have emerged. A lack of correlation between expression of the canonical EpoR and rhEpo's effects on cancer cells prompted us to consider the existence of an alternative Epo receptor. Here, we identified EphB4 as an Epo receptor that triggers downstream signaling via STAT3 and promotes rhEpo-induced tumor growth and progression. In human ovarian and breast cancer samples, expression of EphB4 rather than the canonical EpoR correlated with decreased disease-specific survival in rhEpo-treated patients. These results identify EphB4 as a critical mediator of erythropoietin-induced tumor progression and further provide clinically significant dimension to the biology of erythropoietin.

EphB4 cellular kinase activity assayed using an enzymatic protein interaction system

Receptor tyrosine kinases (RTKs) are important players in various cellular processes, including proliferation, migration, metabolism, and neuronal development. EphB4 RTK is essential for the development of a functional arterial-venous network in embryonic and adult neoangiogenesis. To develop novel inhibitors of EphB4 that might have applications in severe diseases like cancer and retinopathies, assays need to be in place that resemble, in a most physiological fashion, the activation and downstream function of the kinase. In addition, such assays need to be amenable to high-throughput screening to serve efficiently the modern drug discovery processes in the pharmaceutical industry. The authors have developed an enzyme fragment complementation assay that measures the interaction of a downstream docking protein to the activated and phosphorylated full-length EphB4 kinase in cells. The assay is specific, robust, and amenable to miniaturization and high-throughput screening. It covers most steps in the activation process of EphB4, including ligand binding, autophosphorylation, and docking of a downstream interactor. This assay format can be transferred to other RTKs and adds an important cell-based kinase assay option to researchers in the field.

Endothelial tip cells in ocular angiogenesis: potential target for anti-angiogenesis therapy

Endothelial tip cells are leading cells at the tips of vascular sprouts coordinating multiple processes during angiogenesis. In the developing retina, tip cells play a tightly controlled, timely role in angiogenesis. In contrast, excessive numbers of tip cells are a characteristic of the chaotic pathological blood vessels in proliferative retinopathies. Tip cells control adjacent endothelial cells in a hierarchical manner to form the stalk of the sprouting vessel, using, among others, the VEGF-DLL-Notch signaling pathway, and recruit pericytes. Tip cells are guided toward avascular areas by signals from the local extracellular matrix that are released by cells from the neuroretina such as astrocytes. Recently, tip cells were identified in endothelial cell cultures, enabling identification of novel molecular markers and mechanisms involved in tip cell biology. These mechanisms are relevant for understanding proliferative retinopathies. Agents that primarily target tip cells can block pathological angiogenesis in the retina efficiently and safely without adverse effects. A striking example is platelet-derived growth factor, which was recently shown to be an efficacious additional target in the treatment of retinal neovascularization. Here we discuss these and other tip cell-based strategies with respect to their potential to treat patients with ocular diseases dominated by neovascularization.

Ephrin regulation of palate development

Studies of palate development are motivated by the all too common incidence of cleft palate, a birth defect that imposes a tremendous health burden and can leave lasting disfigurement. Although, mechanistic studies of palate growth and fusion have focused on growth factors such as Transforming Growth Factor ß-3 (Tgfß3), recent studies have revealed that the ephrin family of membrane bound ligands and their receptors, the Ephs, play central roles in palatal morphogenesis, growth, and fusion. In this mini-review, we will discuss the recent findings by our group and others on the functions of ephrins in palatal development.

[Eph family receptors as therapeutic targets]

Anti-angiogenic therapy is currently a commonly accepted and rapidly developing approach in oncology and other pathologies linked to aberrant neovascularization. Discovery and validation of additional molecular targets in angiogenesis is needed due to the limitations of the existing clinical therapeutics inhibiting activity of vascular endothelial growth factor (VEGF) and its receptors. A brief review of normal and pathological biological functions of the Eph family of receptor tyrosine kinases and their ephrin ligands is presented, and the approaches to developing therapeutics with anti- and pro-angiogenic and anti-tumor activity based on selective molecular modulation of Eph-ephrin signaling pairs are discussed. Functional roles of Eph-kinases and ephrins in such mechanisms of cancerogenesis as cell proliferation and invasion are also addressed.

Eph receptors and ephrin ligands: important players in angiogenesis and tumor angiogenesis

Eph receptors and their ephrin ligands were identified in the late 1980's. Subsequently, they were linked to different physiological and pathophysiological processes like embryonic development, angiogenesis, and tumorigenesis. In this regard, recent work focused on the distribution and effects of Eph receptors and ephrins on tumor cells and tumor microenvironment. The purpose of this review is to outline the role of these molecules in physiological angiogenesis and pathophysiological tumor angiogenesis. Furthermore, novel therapeutical approaches are discussed as Eph receptors and ephrins represent attractive targets for antiangiogenic therapy.