The duality of angiogenesis: implications for therapy of human disease

CX3CL1 (Fractalkine)-CX3CR1 Axis in Inflammation-Induced Angiogenesis and Tumorigenesis

The chemotactic cytokine fractalkine (FKN, chemokine CX3CL1) has unique properties resulting from the combination of chemoattractants and adhesion molecules. The soluble form (sFKN) has chemotactic properties and strongly attracts T cells and monocytes. The membrane-bound form (mFKN) facilitates diapedesis and is responsible for cell-to-cell adhesion, especially by promoting the strong adhesion of leukocytes (monocytes) to activated endothelial cells with the subsequent formation of an extracellular matrix and angiogenesis. FKN signaling occurs via CX3CR1, which is the only known member of the CX3C chemokine receptor subfamily. Signaling within the FKN-CX3CR1 axis plays an important role in many processes related to inflammation and the immune response, which often occur simultaneously and overlap. FKN is strongly upregulated by hypoxia and/or inflammation-induced inflammatory cytokine release, and it may act locally as a key angiogenic factor in the highly hypoxic tumor microenvironment. The importance of the FKN/CX3CR1 signaling pathway in tumorigenesis and cancer metastasis results from its influence on cell adhesion, apoptosis, and cell migration. This review presents the role of the FKN signaling pathway in the context of angiogenesis in inflammation and cancer. The mechanisms determining the pro- or anti-tumor effects are presented, which are the cause of the seemingly contradictory results that create confusion regarding the therapeutic goals.

Historical Overview of In Vivo and In Vitro Angiogenesis Assays

Most of angiogenesis assays were designed and developed during Folkman's era. But the growth of new blood vessels in several pathologic conditions as tumor development or inflammation were observed long time ago.The development of new blood vessels was early observed by ancient Egyptians who tried to destroy them by applying empirical methods. From the first observations regarding angiogenesis to a personalized therapy targeting newly formed blood vessels a lot of experimental in vitro and in vivo angiogenesis assays have been developed. The present work will overview the oldest and less known part of angiogenesis assays development, and in addition, it will present the newest data in the experimental field of angiogenesis which is rapidly improved by the needs of new antiangiogenic and antivascular therapy development.

Indigo naturalis and its component tryptanthrin exert anti-angiogenic effect by arresting cell cycle and inhibiting Akt and FAK signaling in human vascular endothelial cells

ETHNOPHARMACOLOGICAL RELEVANCE

Indigo naturalis has been used to treat inflammatory diseases and dermatosis, including psoriasis, since thousands of years in China. It has been proven effective in our previous clinical studies on treating psoriasis, but the active component and the mechanism of how indigo naturalis working still needs to be clarified. Since the dysregulated angiogenesis is known to play an important role in the pathogenesis of psoriasis, the anti-angiogenic effect of indigo naturalis and tryptanthrin, a pure component of indigo naturalis, was investigated.

MATERIALS AND METHODS

The in vivo angiogenesis was studied by chick chorioallantoic membrane assay. The in vitro studies were performed using human vascular endothelial cells. Cell viability was determined by MTT assay. Cell cycle distribution was revealed by flow cytometry. The cellular messenger (m)RNA or protein expression level was analyzed by real-time RT-PCR or Western blot, respectively. Transwell filter migration assay and matrix gel-induced tube formation method were applied to examine the angiogenic potential.

RESULTS

Indigo naturalis significantly inhibited the in vivo vascular endothelial growth factor (VEGF)-induced angiogenesis, as well as tryptanthrin. In vitro studies confirmed that indigo naturalis and tryptanthrin reduced the number of viable vascular endothelial cells. Tryptanthrin resulted in a cell cycle arrest and dose-dependently decreased the expressions of cyclin A, cyclin B, cyclin dependent kinase(CDK) 1 and 2, but not cyclin D and cyclin E, at both the mRNA and protein levels. The migration and tube formation of vascular endothelial cells were significantly inhibited by tryptanthrin in a dose-dependent manner. Result also showed that tryptanthrin could reduce the phosphorylated levels of both protein kinase B (PKB or Akt) and focal adhesion kinase (FAK).

CONCLUSIONS

All together, these results demonstrated the anti-angiogenic effect of tryptanthrin, the acting component of indigo naturalis and revealed the underlying mechanism by inhibiting the cell cycle progression, cell migration and tube formation, likely mediated through blocking the Akt and FAK pathways.

Overexpression of miR‑145 in U87 cells reduces glioma cell malignant phenotype and promotes survival after in vivo implantation

In the present study, we sought to elucidate the effect of miR‑145 on glioma cell progression and its mechanisms of action. We examined the effects of miR‑145 on proliferation and invasion of U87 glioma cells and on capillary tube formation. Our data show that restoration of miR‑145 in U87 glioma cells significantly reduced their in vitro proliferation, invasion and angiogenesis. However, decreased miR‑145 expression promoted U87 glioma cell proliferation, invasion and angiogenesis, and reduced-expression of miR‑145 increased ADAM17 and EGFR expression in U87 cells. Overexpression of miR‑145 reduced ADAM17 and EGFR expression. VEGF secretion and VEGF expression were decreased by increased miR‑145 expression in U87 cells and were reversed by miR‑145 downregulation in vitro. Nude mice with intracerebral implantation of U87 overexpressing miR‑145 cells exhibited significantly reduced tumor growth and promoted survival compared with control groups. Taken together, these results suggest a role for miR‑145 as a tumor suppressor which inhibits glioma cell proliferation, invasion and angiogenesis in vitro and reduces glioma growth in vivo.

In vivo microscopy of microvessel oxygenation and network connections

Abnormal or compromised microvascular function is a key component of various diseases. In vivo microscopy of microvessel function in preclinical models can be useful for the study of a disease state and effects of new treatments. Wide-field imaging of microvascular oxygenation via hemoglobin (Hb) saturation measurements has been applied in various applications alone and in combination with other measures of microvessel function, such as blood flow. However, most current combined imaging methods of microvessel function do not provide direct information on microvessel network connectivity or changes in connections and blood flow pathways. First-pass fluorescence (FPF) imaging of a systemically administered fluorescent contrast agent can be used to directly image blood flow pathways and connections relative to a local supplying arteriole in a quantitative manner through measurement of blood supply time (BST). Here, we demonstrate the utility of information produced by the combination of Hb saturation measurements via spectral imaging with BST measurements via FPF imaging for correlation of microvessel oxygenation with blood flow pathways and connections throughout a local network. Specifically, we show network pathway effects on oxygen transport in normal microvessels, dynamic changes associated with wound healing, and pathological effects of abnormal angiogenesis in tumor growth and development.

Tumor angiogenesis is caused by single melanoma cells in a manner dependent on reactive oxygen species and NF-κB

Melanomas have a high angiogenic potential, but respond poorly to medical treatment and metastasize very early. To understand the early events in tumor angiogenesis, animal models with high tumor resolution and blood vessel resolution are required, which provide the opportunity to test the ability of small molecule inhibitors to modulate the angiogenic tumor program. We have established a transgenic melanoma angiogenesis model in the small laboratory fish species Japanese medaka. Here, pigment cells are transformed by an oncogenic receptor tyrosine kinase in fish expressing GFP throughout their vasculature. We show that angiogenesis occurs in a reactive oxygen species (ROS)- and NF-κB-dependent, but hypoxia-independent manner. Intriguingly, we observed that blood vessel sprouting is induced even by single transformed pigment cells. The oncogenic receptor as well as human melanoma cells harboring other oncogenes caused the production of pro-angiogenic factors, most prominently angiogenin, through NF-κB signaling. Inhibiting NF-κB prevented tumor angiogenesis and led to the regression of existing tumor blood vessels. In conclusion, our high-resolution medaka melanoma model discloses that ROS and NF-κB signaling from single tumor cells causes hypoxia-independent angiogenesis, thus, demonstrating that the intrinsic malignant tumor cell features are sufficient to initiate and maintain a pro-angiogenic signaling threshold.

Enhanced antitumor efficacy by blocking activation of the phosphatidylinositol 3-kinase/Akt pathway during anti-angiogenesis therapy

Anti-angiogenesis has been a promising strategy for cancer therapy. However, many signal pathways are activated during anti-angiogenic treatment to counteract the therapeutic efficacy. Among these pathways, evidence has directly pointed to the phosphatidylinositol 3-kinase/Akt (PI3K/Akt) pathway, whose activation resulted in tolerance to the absence of nutrients and oxygen when tumor angiogenesis has been inhibited. In the present study, we investigated the effects of blocking activation of the PI3K/Akt pathway on cell survival in vitro and tumor growth in vivo during anti-angiogenesis therapy. In modeled microenvironments in vitro, we observed that the phosphorylation of Akt in tumor cells was increased gradually in the absence of serum and oxygen in a time-dependent manner. The specific inhibitors of PI3K inhibited the proliferation of tumor cells in a dose-dependent manner in vitro. Moreover, inhibition was enhanced gradually with increased serum deprivation and/or hypoxia. In a mouse tumor model, we found the phosphorylation of Akt obviously increased following anti-angiogenic therapy using plasmids encoding soluble vascular endothelial growth factor receptor-2, but significantly reduced after treatment with LY294002. Consequently, the combinational treatment exhibited better antitumor effects compared with single treatments, presenting larger necrosis-like areas, more apoptotic cells, less microvessel density and less phosphorylated Akt in tumors. These results suggest that blocking activation of the PI3K/Akt pathway during anti-angiogenesis therapy could enhance antitumor efficacy. Thus, targeting the PI3K/Akt pathway might be a promising strategy to reverse tumor resistance to anti-angiogenesis therapy.

Effect of exogenous administration of vascular endothelial growth factor on hepatic microvessel density in rats with liver cirrhosis

AIM: To investigate the effect of portal vein administration of vascular endothelial growth factor on the density of hepatic microvessels in rats with liver cirrhosis.

METHODS: Twenty-five male Sprague-Dawley rats with portal hypertension were randomly divided into two groups: treatment group (n = 15) and model group (n = 10). The treatment group was given recombinant rat vascular endothelial growth factor 165 (30 ng/d) via the portal vein for 2 weeks. The model group underwent only sham operation. Ten normal rats were used as normal controls. After treatment, liver histopathology was examined by light microscopy and sinusoidal ultrastructure was observed by transmission electron microscopy. The density of hepatic microvessels was detected by immunostaining of von Willebrand factor (vWF).

RESULTS: Light microscopy examination showed that hepatic fibrosis improved in the treatment group compared with the model group. Electron microscopy analysis showed decreased number of fenestrations in sinusoidal endothelial cells, basement membrane formation, and irregular hepatic sinus endothelial cells. These pathological changes were more severe in the model group than in the treatment group. Microvessel density was higher in fibrotic stroma (2.04 ± 0.61 vs 1.26 ± 0.30, P < 0.01) but lower in liver parenchyma (0.74 ± 0.05 vs 1.32 ± 0.48, P < 0.01) in the treatment group than in the model group. However, microvessel density in both liver stroma and parenchyma was higher in the treatment and model groups than in the normal control group (1.26 ± 0.30, 2.04 ± 0.61 vs 0.70 ± 0.07; 1.32 ± 0.48, 0.74 ± 0.05 vs 0.28 ± 0.08, all P < 0.05).

CONCLUSION: Exogenous administration of vascular endothelial growth factor is capable of increasing the number of microvessels in fibrotic stroma but decreasing that in liver parenchyma.

Context-dependent role of angiopoietin-1 inhibition in the suppression of angiogenesis and tumor growth: implications for AMG 386, an angiopoietin-1/2-neutralizing peptibody

AMG 386 is an investigational first-in-class peptide-Fc fusion protein (peptibody) that inhibits angiogenesis by preventing the interaction of angiopoietin-1 (Ang1) and Ang2 with their receptor, Tie2. Although the therapeutic value of blocking Ang2 has been shown in several models of tumorigenesis and angiogenesis, the potential benefit of Ang1 antagonism is less clear. To investigate the consequences of Ang1 neutralization, we have developed potent and selective peptibodies that inhibit the interaction between Ang1 and its receptor, Tie2. Although selective Ang1 antagonism has no independent effect in models of angiogenesis-associated diseases (cancer and diabetic retinopathy), it induces ovarian atrophy in normal juvenile rats and inhibits ovarian follicular angiogenesis in a hormone-induced ovulation model. Surprisingly, the activity of Ang1 inhibitors seems to be unmasked in some disease models when combined with Ang2 inhibitors, even in the context of concurrent vascular endothelial growth factor inhibition. Dual inhibition of Ang1 and Ang2 using AMG 386 or a combination of Ang1- and Ang2-selective peptibodies cooperatively suppresses tumor xenograft growth and ovarian follicular angiogenesis; however, Ang1 inhibition fails to augment the suppressive effect of Ang2 inhibition on tumor endothelial cell proliferation, corneal angiogenesis, and oxygen-induced retinal angiogenesis. In no case was Ang1 inhibition shown to (a) confer superior activity to Ang2 inhibition or dual Ang1/2 inhibition or (b) antagonize the efficacy of Ang2 inhibition. These results imply that Ang1 plays a context-dependent role in promoting postnatal angiogenesis and that dual Ang1/2 inhibition is superior to selective Ang2 inhibition for suppression of angiogenesis in some postnatal settings.

Distribution of vascular endothelial growth factor receptor in hepatic arteries and veins of rats with portal hypertension

AIM: To observe the distribution of vascular endothelial growth factor receptor-1 and -2 (VEGFR-1 and -2) in both hepatic arteries and veins of rats with portal hypertension (PHT).

METHODS: Thirty male Sprague-Dawley rats, weighing 180-220 g, were randomly divided into two groups: normal control group (n = 10), and model group (n = 20). The normal control group was given normal drinking water, while the model group was given drinking water containing thioacetamide (TAA). Twelve weeks later, 12 rats who had a portal pressure > 1.57 kPa and obvious pseudolobules were selected for subsequent experiments. The distribution of VEGFR-1 and -2 in both hepatic arteries and veins of these rats was determined by immunohistochemistry.

RESULTS: VEGFR-2 expression in both hepatic arteries and veins of PHT rats significantly increased (t = 24.306, 54.776, both P < 0.05). VEGFR-1 expression in both hepatic arteries and portal veins of PHT models also significantly increased (t = 20.669, 33.210, both P < 0.05). VEGFR-2 expression in hepatic veins was markedly higher than that in hepatic arteries in PHT rats (t = 23.424, P < 0.05). There is no difference in VEGFR-1 expression between hepatic arteries and veins in PHT models (t = 1.434, P > 0.05).

CONCLUSION: VEGFR-2 can decrease the pressure of portal veins possibly by promoting the neovascularization of hepatic veins.

Spectral imaging reveals microvessel physiology and function from anastomoses to thromboses

Abnormal microvascular physiology and function is common in many diseases. Numerous pathologies include hypervascularity, aberrant angiogenesis, or abnormal vascular remodeling among the characteristic features of the disease, and quantitative imaging and measurement of microvessel function can be important to increase understanding of these diseases. Several optical techniques are useful for direct imaging of microvascular function. Spectral imaging is one such technique that can be used to assess microvascular oxygen transport function with high spatial and temporal resolution in microvessel networks through measurements of hemoglobin saturation. We highlight novel observation made with our intravital microscopy spectral imaging system employed with mouse dorsal skin-fold window chambers for imaging hemoglobin saturation in microvessel networks. Specifically, we image acute oxygenation fluctuations in a tumor microvessel network, the development of arteriovenous malformations in a mouse model of hereditary hemorrhagic telangiectasia, and the formation of spontaneous and induced microvascular thromboses and occlusions.

Angiomotin-Like Protein 1 Controls Endothelial Polarity and Junction Stability During Sprouting Angiogenesis

Rationale: We have previously shown that angiomotin (Amot) is essential for endothelial cell migration during mouse embryogenesis. However, ≈5% of Amot knockout mice survived without any detectable vascular defects. Angiomotin-like protein 1 (AmotL1) potentially compensates for the absence of Amot as it is 62% homologous to Amot and exhibits similar expression pattern in endothelial cells.

Objective: Here, we report the identification of a novel isoform of AmotL1 that controls endothelial cell polarization and directional migration.

Methods and Results: Small interfering RNA–mediated silencing of AmotL1 in mouse aortic endothelial cells caused a significant reduction in migration. In confluent mouse pancreatic islet endothelial cells (MS-1), AmotL1 colocalized with Amot to tight junctions. Small interfering RNA knockdown of both Amot and AmotL1 in MS-1 cells exhibited an additive effect on increasing paracellular permeability compared to that of knocking down either Amot or AmotL1, indicating both proteins were required for proper tight junction activity. Moreover, as visualized using high-resolution 2-photon microscopy, the morpholino-mediated knockdown of amotl1 during zebrafish embryogenesis resulted in vascular migratory defect of intersegmental vessels with strikingly decreased junction stability between the stalk cells and the aorta. However, the phenotype was quite distinct from that of amot knockdown which affected polarization of the tip cells of intersegmental vessels. Double knockdown resulted in an additive phenotype of depolarized tip cells with no or decreased connection of the stalk cells to the dorsal aorta.

Conclusions: These results cumulatively validate that Amot and AmotL1 have similar effects on endothelial migration and tight junction formation in vitro. However, in vivo Amot appears to control the polarity of vascular tip cells whereas AmotL1 mainly affects the stability of cell–cell junctions of the stalk cells.

Epidermal vascular endothelial growth factor production is required for permeability barrier homeostasis, dermal angiogenesis, and the development of epidermal hyperplasia: implications for the pathogenesis of psoriasis

Primary abnormalities in permeability barrier function appear to underlie atopic dermatitis and epidermal trauma; a concomitant barrier dysfunction could also drive other inflammatory dermatoses, including psoriasis. Central to this outside-inside view of disease pathogenesis is the epidermal generation of cytokines/growth factors, which in turn signal downstream epidermal repair mechanisms. Yet, this cascade, if sustained, signals downstream epidermal hyperplasia and inflammation. We found here that acute barrier disruption rapidly stimulates mRNA and protein expression of epidermal vascular endothelial growth factor-A (VEGF-A) in normal hairless mice, a specific response to permeability barrier requirements because up-regulation is blocked by application of a vapor-impermeable membrane. Moreover, epidermal vegf(-/-) mice display abnormal permeability barrier homeostasis, attributable to decreased VEGF signaling of epidermal lamellar body production; a paucity of dermal capillaries with reduced vascular permeability; and neither angiogenesis nor epidermal hyperplasia in response to repeated tape stripping (a model of psoriasiform hyperplasia). These results support a central role for epidermal VEGF in the maintenance of epidermal permeability barrier homeostasis and a link between epidermal VEGF production and both dermal angiogenesis and the development of epidermal hyperplasia. Because psoriasis is commonly induced by external trauma [isomorphic (Koebner) phenomenon] and is associated with a prominent permeability barrier abnormality, excess VEGF production, prominent angiogenesis, and epidermal hyperplasia, these results could provide a potential outside-inside mechanistic basis for the development of psoriasis.

The extracellular matrix in wound healing: a closer look at therapeutics for chronic wounds

Disappointing results with the use of exogenous recombinant growth factors in chronic wounds have redirected the focus to the extracellular matrix (ECM). Newer research has clearly changed our view on the role of the ECM in tissue repair and dismissed the dogma that the sole function of ECM is a passive physical support for cells. It is now clear that intact or fragmented ECM molecules are capable of transducing signals pivotal for cell processes in wound healing primarily via integrin interactions in concert with growth factor activation. In addition, our knowledge about ECM molecules in minute concentrations with biological activity, but devoid of significant structural influence, is increasing. This article reviews the multifaceted molecular roles of ECM in the normal wound-healing process and some molecular abnormalities in chronic wounds, and touches on potential therapies based on the developments of tissue biology.

Expression of vascular endothelial growth factor in cultured human dental follicle cells and its biological roles

AIM

To investigate the expression of vascular endothelial growth factor (VEGF) in cultured human dental follicle cells (HDFC), and to examine the roles of VEGF in the proliferation, differentiation, and apoptosis of HDFC in vitro.

METHODS

Immunocytochemistry, ELISA, and RT-PCR were used to detect the expression and transcription of VEGF in cultured HDFC. The dose-dependent and the time-course effect of VEGF on cell proliferation and alkaline phosphatase (ALP) activity in cultured HDFC were determined by MTT assay and colorimetric ALP assay, respectively. The effect of specific mitogen-activated protein kinase (MAPK) inhibitors (PD98059 and U0126) on the VEGF-mediated HDFC proliferation was also determined by MTT assay. The effect of VEGF on HDFC apoptosis was measured by flow cytometry.

RESULTS

VEGF was transcribed and expressed in cultured HDFC. VEGF at 10-300 microg/L significantly increased HDFC proliferation and ALP activity compared to the control. Following 1, 3, 5, or 7 d of stimulation, VEGF induced a significant increase in HDFC proliferation compared with the corresponding control, while VEGF was effective at increasing ALP activity at the incubation time point of 3, 5, or 7 d. PD98059 and U0126 could attenuate the VEGF-mediated HDFC proliferation. Fewer apoptotic cells were observed in the VEGF-treated groups compared to the controls, although the difference was not statistically significant.

CONCLUSION

VEGF is expressed in cultured HDFC, and at a proper concentration range can stimulate HDFC proliferation, induce HDFC to differentiate in a "cementoblast/osteoblast" pathway and protect HDFC from apoptosis. The MAPK signaling pathway might be involved in the VEGF-mediated HDFC proliferation.