Leaky vessels? Call Ang1!

Aberrant tumor vasculature. Facts and pitfalls

Endothelial cells form a single cell layer lining the inner walls of blood vessels and play critical roles in organ homeostasis and disease progression. Specifically, tumor endothelial cells are heterogenous, and highly permeable, because of specific interactions with the tumor tissue environment and through soluble factors and cell-cell interactions. This review article aims to analyze different aspects of endothelial cell heterogeneity in tumor vasculature, with particular emphasis on vascular normalization, vascular permeability, metabolism, endothelial-to-mesenchymal transition, resistance to therapy, and the interplay between endothelial cells and the immune system.

Engineered clustered myoblast cell injection augments angiogenesis and muscle regeneration in peripheral artery disease

The low survival rate of administered cells due to ischemic and inflammatory environments limits the efficacy of the current regenerative cell therapy in peripheral artery disease (PAD). This study aimed to develop a new method to enhance the efficacy of cell therapy in PAD using cell sheet technology. Clustered cells (CCs) from myoblast cell sheets obtained from C57/BL6 mice were administered into ischemic mouse muscles 7 days after induction of ischemia (defined as day 0). Control groups were administered with single myoblast cells (SCs) or saline. Cell survival, blood perfusion of the limb, angiogenesis, muscle regeneration, and inflammation status were evaluated. The survival of administered cells was markedly improved in CCs compared with SCs at days 7 and 28. CCs showed significantly improved blood perfusion, augmented angiogenesis with increased density of CD31⁺/α-smooth muscle actin⁺ arterioles, and accelerated muscle regeneration, along with the upregulation of associated genes. Additionally, inflammation status was well regulated by CCs administration. CCs administration increased the number of macrophages and then induced polarization into an anti-inflammatory phenotype (CD11c^-/CD206⁺), along with the increased expression of genes associated with anti-inflammatory cytokines. Our findings suggest clinical potential of rescuing severely damaged limbs in PAD using CCs.

Significance of Serum Angiopoietin-2 in Patients with Hemorrhage in Adult-Onset Moyamoya Disease

Background

Moyamoya disease (MMD) is a progressive occlusive cerebrovascular disease that is characterized by abnormal angiogenesis at the base of the brain. This pathological abnormal angiogenesis is susceptible to disturbances, including spontaneous hemorrhage and vasogenic edema. However, the underlying mechanisms of pathological angiogenesis and occurrence of hemorrhage are unclear. Angiopoietins play a fundamental role in the pathophysiology of central nervous system disorders in angiogenesis. This study was aimed at examining whether angiopoietins are associated with formation of abnormal collateral vessels and the occurrence of hemorrhage in adult-onset moyamoya disease (HMMD).

Methods

A total of 27 consecutive adult patients with HMMD were enrolled from June 2011 to May 2017. Serum levels of angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) were examined by enzyme-linked immunosorbent assay. Patients with HMMD were compared with those with spontaneous hemorrhage (controls) and nonhemorrhagic-onset MMD (NHMMD).

Results

Serum Ang-2 levels were significantly higher in patients with adult HMMD than in those with spontaneous hemorrhage and NHMMD. The ROC curve identified that a baseline serum Ang-2 level > 1230 ng/ml may be associated with adult HMMD with 88.39% sensitivity and 70.37% specificity (area under the curve (AUC), 0.89; 95% CI, 0.808-0.973; P < 0.001). Moreover, serum Ang-2 levels were significantly elevated in stages II, III, and IV. In subgroup analysis of a high and low degree of moyamoya vessels, serum Ang-2 levels were significantly higher in the high moyamoya vessel group than in the low moyamoya vessel group. Serum Ang-2 levels were also significantly higher in the low moyamoya vessel group compared with the control group. Serum Ang-1 levels were not significantly different among the groups.

Conclusion

Increased serum Ang-2 levels may contribute to pathological abnormal angiogenesis and/or to the instability of vascular structure and function, thus causing brain hemorrhage in adult HMMD.

VEGF Upregulates EGFR Expression to Stimulate Chemotactic Behaviors in the rMC-1 Model of Müller Glia

Progressive vision loss in adults has become increasingly prevalent worldwide due to retinopathies associated with aging, genetics, and epigenetic factors that damage the retinal microvasculature. Insufficient supply of oxygen and/or nutrients upregulates factors such as vascular endothelial growth factor (VEGF) and epidermal growth factor (EGF), which can induce abnormal angiogenesis and damage the structural arrangement of the retinal blood barrier (BRB). Müller glia (MG) regulate the diffusion of essential compounds across the BRB and respond to retinal insults via reactive gliosis, which includes cell hypertrophy, migration, and/or proliferation near areas of elevated VEGF concentration. Increasing concentrations of exogenous VEGF, upregulated by retinal pigmented epithelium cells, and endogenous epidermal growth factor receptor (EGF-R) stimulation in MG, implicated in MG proliferative and migratory behavior, often lead to progressive and permanent vision loss. Our project examined the chemotactic responses of the rMC-1 cell line, a mammalian MG model, toward VEGF and EGF signaling fields in transwell assays, and within respective concentration gradient fields produced in the glia line (gLL) microfluidic system previously described by our group. rMC-1 receptor expression in defined ligand fields was also evaluated using quantitative polymerase chain reaction (qPCR) and immunocytochemical staining. Results illustrate dramatic increases in rMC-1 chemotactic responses towards EGF gradient fields after pre-treatment with VEGF. In addition, qPCR illustrated significant upregulation of EGF-R upon VEGF pre-treatment, which was higher than that induced by its cognate ligand, EGF. These results suggest interplay of molecular pathways between VEGF and EGF-R that have remained understudied in MG but are significant to the development of effective anti-VEGF treatments needed for a variety of retinopathies.

Angiopoietin-1 accelerates restoration of endothelial cell barrier integrity from nanoparticle-induced leakiness

Nanoparticles (NPs) have been widely used in biomedical field for therapeutic treatments, drug carriers, and bio-imaging agent. Recent studies have highlighted the possibility of utilizing inorganic NPs in inducing endothelial leakiness through endothelial remodeling to promote drug transport across the barrier. However, an uncontrolled and persistent leakiness could lead to promiscuous transport of molecules and cells across the barrier, highlighting the pressing need to control the timely recovery from endothelial cell leakiness. Herein, we show that angiopoietin-1 (Ang1) could promote recovery of human microvascular endothelial cells (HMVECs) from titanium dioxide nanoparticle (TiO₂ NPs)-induced endothelial leakiness. Ang1 is known as an anti-permeability growth factor which forms complexes with its receptor Tie2 at the cell-to-cell junctions. We find that the introduction of Ang1 not only accelerates the recovery of NP-induced endothelial leakiness (NanoEL) but also promotes cell rigidity by increasing tubulin acetylation, thereby remodels the endothelial cells to further mitigate the effects of NP exposure through the activation of the Akt pathway. Using in vitro metastasis model, we further show that HMVECs treated with TiO₂ NPs followed by Ang1 could reduce migration of human skin cancer A431 cells across the endothelial barrier. In summary, Ang1 plays important roles in promoting the recovery of endothelial cell leakiness and endothelial stability through a mechano-transduction pathway and shows great potential as key modulator that allows material scientist to regulate endothelial leakiness induced by NPs.

Fabrication of Biodegradable Synthetic Vascular Networks and Their Use as a Model of Angiogenesis

One of the greatest challenges currently faced in tissue engineering is the incorporation of vascular networks within tissue-engineered constructs. The aim of this study was to develop a technique for producing a perfusable, 3-dimensional, cell-friendly model of vascular structures that could be used to study the factors affecting angiogenesis and vascular biology in engineered systems in more detail. Initially, biodegradable synthetic pseudovascular networks were produced via the combination of robocasting and electrospinning techniques. The internal surfaces of the vascular channels were then recellularized with human dermal microvascular endothelial cells (HDMECs) with and without the presence of human dermal fibroblasts (HDFs) on the outer surface of the scaffold. After 7 days in culture, channels that had been reseeded with HDMECs alone demonstrated irregular cell coverage. However, when using a co-culture of HDMECs inside and HDFs outside the vascular channels, coverage was found to be continuous throughout the internal channel. Using this cell combination, collagen gels loaded with vascular endothelial growth factor were deposited onto the outer surface of the scaffold and cultured for a further 7 days. After this, endothelial cell outgrowth from within the channels into the collagen gel was observed, showing that the engineered vasculature maintains its capacity for angiogenesis. Furthermore, the HDMECs appeared to have formed perfusable tubules within the gel. These results show promising steps towards the development of an in vitro platform for studying angiogenesis and vascular biology in a tissue engineering context.

Renal endothelial injury and microvascular dysfunction in acute kidney injury

The kidney is comprised of heterogeneous cell populations that function together to perform a number of tightly controlled, complex and interdependent processes. Renal endothelial cells contribute to vascular tone, regulation of blood flow to local tissue beds, modulation of coagulation and inflammation, and vascular permeability. Both ischemia and sepsis have profound effects on the renal endothelium, resulting in microvascular dysregulation resulting in continued ischemia and further injury. In recent years, the concept of the vascular endothelium as an organ that is both the source of and target for inflammatory injury has become widely appreciated. Here we revisit the renal endothelium in the light of ever evolving molecular advances.

Virus interactions with endothelial cell receptors: implications for viral pathogenesis

The endothelial lining of the vasculature performs a vital role in maintaining fluid barrier functions despite balancing nutrient and fluid content of tissues, repairing localized damage, coordinating responses of a plethora of factors, immune cells and platelets through a multitude of endothelial cell surface receptors. Viruses that nonlytically cause lethal hemorrhagic or edematous diseases engage receptors on vascular and lymphatic endothelial cells, altering normal cellular responses that control capillary leakage and fluid clearance functions with lethal consequences. Recent studies indicate that receptors directing dengue virus and hantavirus infection of the endothelium contribute to the dysregulation of normal endothelial cell signaling responses that control capillary permeability and immune responses that contribute to pathogenesis. Here we present recent studies of virally altered endothelial functions that provide new insight into targeting barrier functions of the endothelium as a potential therapeutic approach.

Emerging therapeutic strategies to prevent infection-related microvascular endothelial activation and dysfunction

Recent evidence suggests that loss of endothelial barrier function and resulting microvascular leak play important mechanistic roles in the pathogenesis of infection-related end-organ dysfunction and failure. Several distinct therapeutic strategies, designed to prevent or limit infection-related microvascular endothelial activation and permeability, thereby mitigating end-organ injury/dysfunction, have recently been investigated in pre-clinical models. In this review, these potential therapeutic strategies, namely, VEGFR2/Src antagonists, sphingosine-1-phosphate agonists, fibrinopeptide Bβ_15–42, slit2N, secinH3, angiopoietin-1/tie-2 agonists, angiopoietin-2 antagonists, statins, atrial natriuretic peptide, and mesenchymal stromal (stem) cells, are discussed in terms of their translational potential for the management of clinical infectious diseases.

Tumor angiogenesis and anti-angiogenic therapy in malignant gliomas revisited

The cellular and molecular mechanisms of tumor angiogenesis and its prospects for anti-angiogenic cancer therapy are major issues in almost all current concepts of both cancer biology and targeted cancer therapy. Currently, (1) sprouting angiogenesis, (2) vascular co-option, (3) vascular intussusception, (4) vasculogenic mimicry, (5) bone marrow-derived vasculogenesis, (6) cancer stem-like cell-derived vasculogenesis and (7) myeloid cell-driven angiogenesis are all considered to contribute to tumor angiogenesis. Many of these processes have been described in developmental angiogenesis; however, the relative contribution and relevance of these in human brain cancer remain unclear. Preclinical tumor models support a role for sprouting angiogenesis, vascular co-option and myeloid cell-derived angiogenesis in glioma vascularization, whereas a role for the other four mechanisms remains controversial and rather enigmatic. The anti-angiogenesis drug Avastin (Bevacizumab), which targets VEGF, has become one of the most popular cancer drugs in the world. Anti-angiogenic therapy may lead to vascular normalization and as such facilitate conventional cytotoxic chemotherapy. However, preclinical and clinical studies suggest that anti-VEGF therapy using bevacizumab may also lead to a pro-migratory phenotype in therapy resistant glioblastomas and thus actively promote tumor invasion and recurrent tumor growth. This review focusses on (1) mechanisms of tumor angiogenesis in human malignant glioma that are of particular relevance for targeted therapy and (2) controversial issues in tumor angiogenesis such as cancer stem-like cell-derived vasculogenesis and bone-marrow-derived vasculogenesis.

The Role of the Endothelium in HPS Pathogenesis and Potential Therapeutic Approaches

American hantaviruses cause a highly lethal acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). Hantaviruses nonlytically infect endothelial cells and cause dramatic changes in barrier functions of the endothelium without disrupting the endothelium. Instead hantaviruses cause changes in the function of infected endothelial cells that normally regulate fluid barrier functions of capillaries. The endothelium of arteries, veins, and lymphatic vessels is unique and central to the function of vast pulmonary capillary beds, which regulate pulmonary fluid accumulation. The endothelium maintains vascular barrier functions through a complex series of redundant receptors and signaling pathways that serve to both permit fluid and immune cell efflux into tissues and restrict tissue edema. Infection of the endothelium provides several mechanisms for hantaviruses to alter capillary permeability but also defines potential therapeutic targets for regulating acute pulmonary edema and HPS disease. Here we discuss interactions of HPS causing hantaviruses with the endothelium, potential endothelial cell-directed permeability mechanisms, and therapeutic targeting of the endothelium as a means of reducing the severity of HPS disease.

Angiopoietin-1 mediates inhibition of hypertension-induced release of angiopoietin-2 from endothelial cells

AIMS

Adequate endothelial cell stimulation is a prerequisite for the adaptive remodelling of macro- and microvessels. A pivotal autocrine mechanism following endothelial cell activation is the release of angiopoietin-2 (Ang-2), which subsequently antagonizes the binding of Ang-1 to the Tie-2 receptor, thus sensitizing the endothelial cells to pro-angiogenic and/or pro-inflammatory stimuli. Based on the observation that hypertension in mice reduces the abundance of Ang-2 stored in arterial endothelial cells, this study was aimed at testing the hypothesis that an increase in wall stress (WS) or stretch-a hallmark of hypertension-is sufficient to release Ang-2 from endothelial cells.

METHODS AND RESULTS

In fact, stretching of isolated perfused mouse arteries or human cultured endothelial cells rapidly elicited an increased release of Ang-2. In the cultured endothelial cells, this was preceded by a transient rise in intracellular free calcium, abrogated through calcium chelation and accompanied by a decrease in Tie-2 phosphorylation. Interestingly, Ang-1 abolished the stretch-induced release of Ang-2 from both cultured and native endothelial cells through inhibiting the stretch-dependent mobilization of intracellular calcium.

CONCLUSION

Collectively, these results indicate that increased WS or stretch facilitates the release of Ang-2 from endothelial cell Weibel-Palade bodies, and that Ang-1 can block this by attenuating the stretch-mediated rise in intracellular calcium.

Targeted Delivery of VEGF after a Myocardial Infarction Reduces Collagen Deposition and Improves Cardiac Function

The development of adjunctive therapies which attenuate adverse remodeling and improve LV function post myocardial infarction (MI) is of significant clinical interest. Previously, we have shown that targeted delivery of therapeutic vascular endothelial growth factor (VEGF) to the infarct border zone significantly increases vascular perfusion and results in improvements in LV function. In this study, we tested the hypothesis that improvements in cardiac function observed with this novel targeted drug delivery system strongly correlate with reductions in collagen deposition in the scar tissue after an MI. Rats received anti-P-selectin conjugated immunoliposomes containing VEGF immediately post-MI. Over 4 weeks, evolutionary changes in LV geometry and function were correlated with collagen deposition and infarct size quantified by Gomori's trichrome and picrosirius red staining. Targeted VEGF treated hearts showed a 37% decrease in collagen deposition in the anterior wall, as well as significant improvements in LV filling pressures. Multi-regression analysis showed that the extent of collagen deposition post MI can be predicted by a linear combination of normalized LV mass and ejection fraction. Targeted delivery of VEGF post-MI results in significant decreases in collagen deposition and adverse remodeling. Improvements in cardiac function in this model are related to degree of collagen deposition and extent of scar formation.

Unique angiogenic and vasculogenic properties of renal cell carcinoma in a xenograft model of bone metastasis are associated with high levels of vegf-a and decreased ang-1 expression

Management of various tumor metastases to bone has dramatically improved, but this is not so for renal cell carcinoma (RCC), which is a difficult surgical problem due to its great vascularity. Furthermore, the unique mechanisms that mediate RCC vasculogenesis in bone remain unknown. To understand this process we developed a xenograft model that recapitulates highly vascular RCC versus less vascular tumors that metastasize to bone. Human tumor cell lines of RCC (786-O), prostate cancer (PC3), lung cancer (A549), breast cancer (MDA-MB231), and melanoma (A375) were transduced with firefly luciferase (Luc), injected into the tibiae of nude mice, and differences in growth, osteolysis, and vascularity were assessed by longitudinal bioluminescent imaging, micro-CT for measurement of calcified tissues and vascularity and histology. The results showed that while RCC-Luc has reduced growth and osteolytic potential versus the other tumor lines, it displayed a significant increase in vascular volume (p < 0.05). This expansion was due to 3- and 5-fold increases in small and large vessel numbers respectively. In vitro gene expression profiling revealed that RCC-Luc expresses significantly (p < 0.05) more vegf-a (10-fold) and 20- to 30-fold less ang-1 versus the other lines. These data demonstrate the utility of this model to study the unique vasculogenic properties of RCC bone metastases.

Controlled release of thymosin β4 using collagen-chitosan composite hydrogels promotes epicardial cell migration and angiogenesis

Rapid vascularization at the infarcted site is crucial for cardiac repair following myocardial infarction. Thymosin β4 (Tβ4), a 43-amino acid peptide, is both angiogenic and cardioprotective. Tβ4 in soluble form was previously shown to promote cell migration from quiescent adult cardiac explants. Here we developed a collagen-chitosan hydrogel for the encapsulation of Tβ4, which allowed its controlled release over 28days to elicit localized and prolonged effects. Contrastingly, Tβ4 was fully released over 3days when encapsulated in collagen-only hydrogels due to charge repulsion and lack of interconnected pores as shown by SEM. The charge of encapsulated molecules affected their release from collagen-chitosan hydrogels. While the release of neutral polyalanine was size-controlled diffusion, that of negatively-charged Tβ4 and positively-charged polylysine was affected by electrostatic interactions of peptides with collagen/chitosan molecules. Hydrogels with encapsulated Tβ4 significantly increased cell migration and outgrowth of CD31-positive capillaries from mouse and rat epicardial explants in vitro, compared to Tβ4-free and soluble controls. Potential advantage of Tβ4 over commonly-used angiogenic growth factors is that it can induce recruitment and differentiation of both endothelial and smooth muscle cells necessary for vascular stability. Importantly, Tβ4-encapsulated collagen-chitosan hydrogels promoted angiogenesis in vivo upon subcutaneous injection, compared to collagen-only hydrogels.

Angiogenesis: Future of pharmacological modulation

Angiogenesis is a fundamental biological process that is regulated by a fine balance between pro- and antiangiogenic molecules, and is deranged in various diseases. Historically, angiogenesis was only implicated in few diseases, such as, cancer, arthritis, and psoriasis. However, in recent years, it has been increasingly evident that excessive, insufficient or abnormal angiogenesis contributes to the pathogenesis of many more disorders. Research in angiogenesis offers a potential to cure a variety of diseases such as Alzheimer's and AIDS. Modulation of angiogenesis may have an impact on diseases in the twenty-first century similar to that which the discovery of antibiotics had in the twentieth century.

Angiopoietin-1 is associated with cerebral vasospasm and delayed cerebral ischemia in subarachnoid hemorrhage

Background

Angiopoietin-1 (Ang-1) and -2 (Ang-2) are keyplayers in the regulation of endothelial homeostasis and vascular proliferation. Angiopoietins may play an important role in the pathophysiology of cerebral vasospasm (CVS). Ang-1 and Ang-2 have not been investigated in this regard so far.

Methods

20 patients with subarachnoid hemorrhage (SAH) and 20 healthy controls (HC) were included in this prospective study. Blood samples were collected from days 1 to 7 and every other day thereafter. Ang-1 and Ang-2 were measured in serum samples using commercially available enzyme-linked immunosorbent assay. Transcranial Doppler sonography was performed to monitor the occurrence of cerebral vasospasm.

Results

SAH patients showed a significant drop of Ang-1 levels on day 2 and 3 post SAH compared to baseline and HC. Patients, who developed Doppler sonographic CVS, showed significantly lower levels of Ang-1 with a sustained decrease in contrast to patients without Doppler sonographic CVS, whose Ang-1 levels recovered in the later course of the disease. In patients developing cerebral ischemia attributable to vasospasm significantly lower Ang-1 levels have already been observed on the day of admission. Differences of Ang-2 between SAH patients and HC or patients with and without Doppler sonographic CVS were not statistically significant.

Conclusions

Ang-1, but not Ang-2, is significantly altered in patients suffering from SAH and especially in those experiencing CVS and cerebral ischemia. The loss of vascular integrity, regulated by Ang-1, might be in part responsible for the development of cerebral vasospasm and subsequent cerebral ischemia.

Functional histology of glioma vasculature by FTIR imaging

Fourier-transform infrared (FTIR) imaging has been used to investigate brain tumor angiogenesis using a mice solid tumor model and bare-gold (∅ 25 nm) or BaSO(4) (∅ 500 nm) nanoparticles (NP) injected into blood vasculature. FTIR images of 20-μm-thick tissue sections were used for chemical histology of healthy and tumor areas. Distribution of BaSO(4)-NP (using the 1,218-1,159 cm(-1) spectral interval) revealed clearly all details of blood vasculature with morphological abnormalities of tumor capillaries, while Au-NP (using the 1,046-1,002 cm(-1) spectral interval) revealed also diffusion properties of leaky blood vessels. Diffusion of Au-NP out of vascular space reached 64 ± 29 μm, showing the fenestration of "leaky" tumor blood vessels, which should allow small NP (<100 nm, as for Au-NP) to diffuse almost freely, while large NP should not (as for BaSO(4)-NP in this study). Therefore, we propose to develop FTIR imaging as a convenient tool for functional molecular histology imaging of brain tumor vasculature, both for identifying blood capillaries and for determining the extravascular diffusion space offered by vessel fenestration.

Myoferlin gene silencing decreases Tie-2 expression in vitro and angiogenesis in vivo

Angiogenesis consists in the growth of new blood vessels from pre-existing ones. Although anti-angiogenesis interventions have been shown to have therapeutic properties in human diseases such as cancer, their effect is only partial and the identification of novel modulators of angiogenesis is warranted. Recently, we reported the unexpected proteomic identification in endothelial cells (EC) of Myoferlin, a member of the Ferlin family of transmembrane proteins. Ferlins are well known to regulate the fusion of lipid vesicles at the plasma membrane in muscle cells, and we showed that Myoferlin gene knockdown not only decreases lipid vesicle fusion in EC but also attenuates Vascular Endothelial Growth Factor (VEGF) Receptor-2 (VEGFR-2) expression. Herein, we show that Myoferlin gene silencing in cultured EC also results in attenuated expression of a second tyrosine kinase receptor, Tie-2, which is another well-described angiogenic receptor. Most importantly, we provide evidence that delivery of a low-volume Myoferlin siRNA preparation in mouse tissues results in attenuated angiogenesis and edema formation. This provides the first evidence that acute Myoferlin knockdown has anti-angiogenic effects and validates Myoferlin as an anti-angiogenesis target. Furthermore, this supports the unexpected but increasingly accepted concept that proper tyrosine kinase receptors expression at the plasma membrane requires Myoferlin.

VEGFR2 and Src kinase inhibitors suppress Andes virus-induced endothelial cell permeability

Hantaviruses predominantly infect human endothelial cells and, in the absence of cell lysis, cause two diseases resulting from increased vascular permeability. Andes virus (ANDV) causes a highly lethal acute pulmonary edema termed hantavirus pulmonary syndrome (HPS). ANDV infection enhances the permeability of endothelial cells in response to vascular endothelial growth factor (VEGF) by increasing signaling responses directed by the VEGFR2-Src-VE-cadherin pathway, which directs adherens junction (AJ) disassembly. Here we demonstrate that inhibiting pathway-specific VEGFR2 and Src family kinases (SFKs) blocks ANDV-induced endothelial cell permeability. Small interfering RNA (siRNA) knockdown of Src within ANDV-infected endothelial cells resulted in an ∼70% decrease in endothelial cell permeability compared to that for siRNA controls. This finding suggested that existing FDA-approved small-molecule kinase inhibitors might similarly block ANDV-induced permeability. The VEGFR2 kinase inhibitor pazopanib as well as SFK inhibitors dasatinib, PP1, bosutinib, and Src inhibitor 1 dramatically inhibited ANDV-induced endothelial cell permeability. Consistent with their kinase-inhibitory concentrations, dasatinib, PP1, and pazopanib inhibited ANDV-induced permeability at 1, 10, and 100 nanomolar 50% inhibitory concentrations (IC(50)s), respectively. We further demonstrated that dasatinib and pazopanib blocked VE-cadherin dissociation from the AJs of ANDV-infected endothelial cells by >90%. These findings indicate that VEGFR2 and Src kinases are potential targets for therapeutically reducing ANDV-induced endothelial cell permeability and, as a result, capillary permeability during HPS. Since the functions of VEGFR2 and SFK inhibitors are already well defined and FDA approved for clinical use, these findings rationalize their therapeutic evaluation for efficacy in reducing HPS disease. Endothelial cell barrier functions are disrupted by a number of viruses that cause hemorrhagic, edematous, or neurologic disease, and as a result, our findings suggest that VEGFR2 and SFK inhibitors should be considered for regulating endothelial cell barrier functions altered by additional viral pathogens.

Pathogenic hantaviruses Andes virus and Hantaan virus induce adherens junction disassembly by directing vascular endothelial cadherin internalization in human endothelial cells

Hantaviruses infect endothelial cells and cause 2 vascular permeability-based diseases. Pathogenic hantaviruses enhance the permeability of endothelial cells in response to vascular endothelial growth factor (VEGF). However, the mechanism by which hantaviruses hyperpermeabilize endothelial cells has not been defined. The paracellular permeability of endothelial cells is uniquely determined by the homophilic assembly of vascular endothelial cadherin (VE-cadherin) within adherens junctions, which is regulated by VEGF receptor-2 (VEGFR2) responses. Here, we investigated VEGFR2 phosphorylation and the internalization of VE-cadherin within endothelial cells infected by pathogenic Andes virus (ANDV) and Hantaan virus (HTNV) and nonpathogenic Tula virus (TULV) hantaviruses. We found that VEGF addition to ANDV- and HTNV-infected endothelial cells results in the hyperphosphorylation of VEGFR2, while TULV infection failed to increase VEGFR2 phosphorylation. Concomitant with the VEGFR2 hyperphosphorylation, VE-cadherin was internalized to intracellular vesicles within ANDV- or HTNV-, but not TULV-, infected endothelial cells. Addition of angiopoietin-1 (Ang-1) or sphingosine-1-phosphate (S1P) to ANDV- or HTNV-infected cells blocked VE-cadherin internalization in response to VEGF. These findings are consistent with the ability of Ang-1 and S1P to inhibit hantavirus-induced endothelial cell permeability. Our results suggest that pathogenic hantaviruses disrupt fluid barrier properties of endothelial cell adherens junctions by enhancing VEGFR2-VE-cadherin pathway responses which increase paracellular permeability. These results provide a pathway-specific mechanism for the enhanced permeability of hantavirus-infected endothelial cells and suggest that stabilizing VE-cadherin within adherens junctions is a primary target for regulating endothelial cell permeability during pathogenic hantavirus infection.

Coadministration of adenoviral vascular endothelial growth factor and angiopoietin-1 enhances vascularization and reduces ventricular remodeling in the infarcted myocardium of type 1 diabetic rats

OBJECTIVE

Hyperglycemia impairs angiogenesis in response to ischemia, leading to ventricular remodeling. Although the effects of overexpressing angiogenic growth factors have been studied in inducing angiogenesis, the formation of functional vessels remains a challenge. The present study evaluates the reversal of diabetes-mediated impairment of angiogenesis in the infarcted diabetic rat myocardium by proangiogenic gene therapy.

RESEARCH DESIGN AND METHODS

Ad*VEGF and Ad*Ang1 were intramyocardially administered in combination immediately after myocardial infarction to nondiabetic and diabetic rats. Ad*LacZ was similarly administered to the respective control groups. The hearts were excised for molecular and immunohistochemical analysis at predetermined time points. The myocardial function was measured by echocardiography 30 days after the intervention.

RESULTS

We observed reduced fibrosis and increased capillary/arteriolar density along with reduced ventricular remodeling, as assessed by echocardiography in the treated diabetic animals compared with the nontreated diabetic controls. We also observed increased phosphorylated mitogen-activated protein kinase-activated protein kinase-2, 2 days after the treatment and increased expression of vascular endothelial growth factor (VEGF), Flk-1, angiopoietin-1 (Ang-1), Tie-2, and survivin, 4 days after treatment in the diabetic animals. Gel shift analysis revealed that the combination gene therapy stimulated the DNA binding activity of nuclear factor-kappaB in the diabetic animals.

CONCLUSIONS

Our preclinical data demonstrate the efficacy of coadministration of adenoviral VEGF and Ang-1 in increasing angiogenesis and reducing ventricular remodeling in the infarcted diabetic myocardium. These unique results call for the initiation of a clinical trial to assess the efficacy of this therapeutic strategy in the treatment of diabetes-related human heart failure.

Implications of vascular endothelial growth factor for postischemic neurovascular remodeling

Neurovascular remodeling has been recently recognized as a promising target for neurologic therapies. Hopes have emerged that, by stimulating vessel growth, it may be possible to stabilize brain perfusion, and at the same time promote neuronal survival, brain plasticity, and neurologic recovery. In this review, we outline the role of vascular endothelial growth factor (VEGF) in the ischemic brain, analyzing how this growth factor contributes to brain remodeling. Studies with therapeutic VEGF administration resulted in quite variable results depending on the route and time point of delivery. Local VEGF administration consistently enhanced neurologic recovery, whereas acute intravenous delivery exacerbated brain infarcts due to enhanced brain edema. Future studies should answer the following questions: (1) whether increased vessel density translates into improvements in blood flow in the hemodynamically compromised brain; (2) how VEGF influences brain plasticity and contributes to motor and nonmotor recovery; (3) what are the actions of VEGF not only in young animals with preserved vasculature, on which previous studies have been conducted, but also in aged animals and in animals with preexisting atherosclerosis; and (4) whether the effects of VEGF can be mimicked by pharmacological compounds or by cell-based therapies. Only on the basis of such information can more definite conclusions be made with regard to whether the translation of therapeutic angiogenesis into clinics is promising.

The role of angiopoietin-1 in kidney disease

Injury to the renal microvasculature and inflammatory process may be major factors in the progression of renal disease, therefore, protection of the renal endothelial cell and regulation of inflammatory process may be an important therapeutic target of renal disease. Thus, we evaluated the protective effect of cartilage oligomeric matrix protein-angiopoietin-1 (COMP-Ang1) in unilateral ureteral obstruction (UUO)-induced renal fibrosis, cyclosporine A (CsA)-induced renal injury, and the diabetic nephropathy model. In the UUO model, morphologic examination indicated less tubular injury and tubulointerstitial fibrosis in mice that received COMP-Ang1 compared to vehicle-treated mice. Interstitial type I collagen, myofibroblast accumulation, renal surface microvasculature and renal blood flow were higher after treatment with COMP-Ang1 compared to vehicle-treated mice. COMP-Ang1 treatment decreased monocyte/macrophage infiltration, tissue levels of transforming growth factor β1, and Smad 2/3 phosphorylation and increased Smad 7 in the obstructed kidney. In CsA-induced renal injury, histologic examination showed significantly decreased CsA-induced tubular damage and tubulointerstitial fibrosis in COMP-Ang1 treated mice. COMP-Ang1 administration also decreased increased macrophage infiltration, adhesion molecule expression, TGF-β1, and Smad 2/3 levels in CsA-treated kidneys, while increasing Smad 7 levels. Laser-Doppler sonographic findings and endothelial factor VIII staining revealed that COMP-Ang1 had a preservative effect on peritubular vasculature. In the diabetic nephropathy model, COMP-Ang1 reduced albuminuria and decreased mesangial expansion, thickening of the glomerular basement membrane and podocyte foot process broadening and effacement. COMP-Ang1 may delay the fibrotic changes in the kidney of diabetic db/db mice through its anti-inflammatory or metabolic effects. In conclusion, COMP-Ang1 may be an endothelium-specific and anti-inflammatory therapeutic modality in fibrotic renal disease.

Additive effect of AAV-mediated angiopoietin-1 and VEGF expression on the therapy of infarcted heart

Vascular endothelial growth factor (VEGF) is a key angiogenic factor and has been used experimentally for induction of neovasculature in ischemic myocardium. However, blood vessels induced by VEGF are immature. Angiopoietin-1 (ang-1) has the ability to recruit and sustain periendothelial support cells and promote vascular maturation. Thus, co-expression of the two may yield a better result than expression of either one alone. Two adeno-associated viral vectors (AAV), CMVVEGF and CMVang-1 with the CMV promoter driving VEGF or ang-1 gene expression, respectively, were injected into ischemic mouse hearts individually or together in different ratios. The results show that co-injected groups had more capillaries than the CMVang-1 group and similar densities of capillaries and alpha-actin positive vessels as the CMVVEGF group. Neovasculature induced by CMVVEGF was leaky. In contrast, neovasculature in CMVang-1-injected or CMVVEGF and CMVang-1 co-injected hearts was less leaky than that in CMVVEGF-injected hearts. The group that received CMVang-1 and CMVVEGF in a 1:1 ratio had the smallest infarct size and best cardiac function and regional wall movement among all the groups. We conclude that ang-1 and VEGF can compensate for each others' shortcomings and yield a better therapeutic effect by acting together.

Combination of VEGF(165)/Angiopoietin-1 gene and endothelial progenitor cells for therapeutic neovascularization

Previous studies have established that vascular endothelial growth factor (VEGF), Angiopoietin-1 (Ang1) and endothelial progenitor cells (EPCs) play important roles in neovascularization, suggesting that combination of them would be a promising therapy for ischemic diseases. So we constructed the adeno-associated virus-2 (AAV-2) vectors simultaneously encoding human VEGF(165) and Ang1 (AAV-Ang1/VEGF), and investigated the combination therapeutic effect of AAV-Ang1/VEGF with EPCs in a rabbit ischemic hindlimb model. In the present study we found that AAV-Ang1/VEGF could successfully and efficiently transfer VEGF(165) and Ang1 gene into bone marrow derived EPCs for gene therapy. Combined administration of AAV-Ang1/VEGF with EPCs had higher blood flow recovery, cellularity, capillary density and smooth muscle alpha-actin positive vessel density than administration of either of them alone. Furthermore, the strategy of pre-intramuscular injection of AAV-Ang1/VEGF followed by EPCs transplantation had a higher therapeutic effect than the strategy of transplantation of AAV-Ang1/VEGF transduced EPCs. It seemed that the former strategy may be a promising therapy for ischemic diseases.

Adeno-associated virus vectors simultaneously encoding VEGF and angiopoietin-1 enhances neovascularization in ischemic rabbit hind-limbs

AIM

Angiopoietin-1 (Ang1) and vascular endothelial growth factor A (VEGF) play important roles in vascular formation and maturation, suggesting a combination of these 2 would be a promising therapy for ischemic diseases. So we constructed an adeno-associated virus (AAV) vector, simultaneously encoding human VEGF(165) and Ang1 (AAV-VEGF/Ang1), and investigated its therapeutic effect in a rabbit ischemic hind-limb model.

METHODS

Four experimental groups were used to prepare the rabbit ischemic hind-limb model following AAV vectors intramuscular administration as follows: PBS (phosphate buffered solution), AAV-VEGF, AAV-Ang1, AAV-VEGF/Ang1.

RESULTS

Eight weeks after administration, human VEGF(165) and Ang1 were detected by RT-PCR, Western blotting and histochemical staining methods in AAV-VEGF/Ang1 transduced muscles. Group AAV-VEGF/Ang1 showed a significantly increased blood-flow recovery in ischemic hind-limbs compared with the other groups. Histological staining for alkaline phosphatase showed that capillary density of group AAV-VEGF/Ang1 or AAV-VEGF was significantly higher than that of group PBS or AAV-Ang1. Histological immunostaining for smooth muscle alpha-actin (alpha-SMA) revealed that group AAV-VEGF/Ang1 had the highest density of alpha-SMA-positive vessels compared with the other groups. Vascular leakage, one of the major adverse effects induced by VEGF, was very severe in group AAV-VEGF, but the permeability was obviously reduced when VEGF was co-expressed with Ang1 in group AAV-VEGF/Ang1.

CONCLUSION

AAV vectors can simultaneously encode 2 proteins which can be efficiently and stably co-expressed in transduced tissues. AAV-mediated VEGF and Ang1 gene transfer enhances neovascularization, prevents capillary leakage, and improves blood flow in a rabbit hind-limb ischemic model. These findings suggest that intramuscular administration of AAV-VEGF/Ang1 may be useful in the treatment of ischemic diseases.

Imaging the Progression of Intra-tumor Heterogeneity in Prostate and Ovarian Xenografts Using Dynamic Contrast-Enhanced CT

The purpose of this study is to measure the progressive intra-tumor heterogeneous physiological states for prostate (CWRrv) and ovarian (SKOV3x) xenograft mouse models. Dynamic contrast-enhanced CT was used to measure the change in a tumor's physiological state when transitioning from stage I (<7 mm diameter) to stage II (7-20 mm diameter). Images from stage I tumors are in the initial stages of angiogenesis: neovasculature growth. A radial heterogeneity begins to form with signs of angiogenic activity throughout the tumor. Stage II tumors develop both a saccular heterogeneity and a strong radial heterogeneity between periphery and core consistent with the effects of inflammatory and maturation processes. Imaging intra-tumor heterogeneity has the potential to track tumor stage and to be used as predictive factor in determining mortality rates.

Therapeutic angiogenesis: Translating experimental concepts to medically relevant goals

Angiogenesis is central to many physiological and pathological phenomena. In physiological angiogenesis, new vessels are well shaped and their growth is finely tuned to match the metabolic needs of tributary tissues. Accordingly, neovascularization is activated by physical exercise and destabilized by non-use. In contrast, pathological blood vessels that are observed in retinal neovascularization, cancer or in ischemic tissues are leaky, irregularly shaped, and tend to form arterial-venous fistulae. A great deal of attention is focused on new approaches for medical manipulation of vascular growth. These methods are aimed at facilitating the reperfusion of ischemic tissues or eradicating pathological vasculature. In this position paper, we challenge the rationale of therapeutic angiogenesis for the cure of myocardial and peripheral ischemia. Therapeutic angiogenesis aims at combating the insufficiency of, or insensitivity to angiogenic factors in the setting of atherosclerotic-induced arterial occlusion. However, clinical evidence indicates that such a defect is not common among patients with ischemic disease, as a whole. Genetic and environmental factors could account for the great heterogeneity in the expression of the master angiogenic factors. Future improvements in the strategy would require the introduction of in vitro assays and in vivo imaging systems for assessing human angiogenesis. Finally, the promise is to find individualized angiogenesis-based therapies for a genuine cure of ischemia and prevention of organ failure.

Principles and therapeutic implications of angiogenesis, vasculogenesis and arteriogenesis

The vasculature is the first organ to arise during development. Blood vessels run through virtually every organ in the body (except the avascular cornea and the cartilage), assuring metabolic homeostasis by supplying oxygen and nutrients and removing waste products. Not surprisingly therefore, vessels are critical for organ growth in the embryo and for repair of wounded tissue in the adult. Notably, however, an imbalance in angiogenesis (the growth of blood vessels) contributes to the pathogenesis of numerous malignant, inflammatory, ischaemic, infectious and immune disorders. During the last two decades, an explosive interest in angiogenesis research has generated the necessary insights to develop the first clinically approved anti-angiogenic agents for cancer and blindness. This novel treatment is likely to change the face of medicine in the next decade, as over 500 million people worldwide are estimated to benefit from pro- or anti-angiogenesis treatment. In this following chapter, we discuss general key angiogenic mechanisms in health and disease, and highlight recent developments and perspectives of anti-angiogenic therapeutic strategies.

In Vitro Functional Assessment of Human Skeletal Myoblasts After Transduction With Adenoviral Bicistronic Vector Carrying Human VEGF165 and Angiopoietin-1

OBJECTIVES

We report in vitro functional assessment of human skeletal myoblasts with adenoviral bicistronic vector carrying human vascular endothelial growth factor-165 (hVEGF165) and angiopoietin-1 (Ang-1).

METHODS

Myoblasts were assessed for their purity by desmin expression. A replication incompetent adenoviral bicistronic vector (Ad-Bic) carrying both hVEGF165 and Ang-1 was used for transduction of myoblasts. Transduction efficiency was assessed by dual fluorescent immunostaining of the transduced myoblasts. Expression efficiency was analyzed by enzyme linked immunosorbent assay (ELISA), Western blot and reverse transcription polymerase chain reaction (RT-PCR). The biological activity of the secreted human VEGF165 and Ang-1 was determined by human umbilical vein endothelial cells (HUVEC) proliferation assay, Thymidine [H3] incorporation assay and capillary-like structure formation.

RESULTS

The myoblasts preparation was >98% pure. Fluorescent immunostaining showed >95% transduction efficiency. The transduced myoblasts secreted VEGF(165) for up to 30 days after transduction, with peak level (32 +/- 4 ng/ml) at day 8 after transduction as revealed by VEGF ELISA. Western blot further confirmed that both angiogenic factors were actively secreted by transduced myoblasts. The molecular weight was 42 kD for hVEGF165 and 70 kD for Ang-1 respectively. The expression of hVEGF165 and Ang-1 was significantly reduced at day-30 after transduction as seen by RT-PCR. The conditioned medium from bicistronic vector transduced myoblasts stimulated HUVEC to proliferate much faster than other conditioned media (>1.5 folds). Thymidine incorporation assay further confirmed this finding. Matrigel experiment suggested that HUVEC under the condition of both growth factors formed significantly more capillary-like structure.

CONCLUSIONS

The bicistronic vector transduced myoblasts provides a novel strategy for therapeutic angiomyogenesis for cardiac repair.

Therapeutic angiogenesis and vasculogenesis for tissue regeneration

Therapeutic angiogenesis/vasculogenesis holds promise for the cure of ischaemic disease. The approach postulates the manipulation of spontaneous healing response by supplementation of growth factors or transplantation of vascular progenitor cells. These supplements are intended to foster the formation of arterial collaterals and promote the regeneration of damaged tissues. Angiogenic factors are generally delivered in the form of recombinant proteins or by gene transfer using viral vectors. In addition, new non-viral methods are gaining importance for their safer profile. The association of growth factors with different biological activity might offer distinct advantages in terms of efficacy, yet combined approaches require further optimization. Alternatively, substances with pleiotropic activity might be considered, by virtue of their ability to target multiple mechanisms. For instance, some angiogenic factors not only stimulate the growth of arterioles and capillaries, but also inhibit vascular destabilization triggered by metabolic and oxidative stress. Transplantation of endothelial progenitor cells was recently proposed for the treatment of peripheral and myocardial ischaemia. Progenitor cells can be transplanted either without any preliminary conditioning or after ex vivo genetic manipulation. Delivery of genetically modified progenitor cells eliminates the drawback of immune response against viral vectors and makes feasible repeating the therapeutic procedure in case of injury recurrence. It is envisioned that these new approaches of regenerative medicine will open unprecedented opportunities for the care of life-threatening diseases.

Changing the logic of therapeutic angiogenesis for ischemic disease

The worldwide epidemic of ischemic disease urgently requires innovative treatments. Recently, therapeutic angiogenesis has emerged as a noninvasive supply-side approach, aimed at promoting neovascularization in underperfused tissues through the local delivery of angiogenic growth factors. Successful preclinical studies paved the way for the first clinical trials, with single growth factors given as recombinant proteins or genes. However, clinical results have not matched the initial promises. Our opinion is that the logic of therapeutic angiogenesis needs profound revision. Here, we introduce the concept that pleiotropic agents can stimulate the healing of all the components of ischemic tissue. We also propose prophylactic interventions to delay vascular senescence. The optimization of therapeutic angiogenesis will open unprecedented opportunities for the care of life-threatening ischemic disease.

Manipulating angiogenesis in medicine

Blood vessels nourish organs with vital nutrients and oxygen and, thus, new vessels form when the embryo needs to grow or wounds are to heal. However, forming new blood vessels is a complex and delicate process, which, unfortunately, is often derailed. Thus, when insufficient vessels form, the tissue becomes ischaemic and stops to function adequately. Conversely, when vessels grow excessively, malignant and inflamed tissues grow faster. It is now becoming increasingly evident that abnormal vessel growth contributes to the pathogenesis of numerous malignant, ischaemic, inflammatory, infectious and immune disorders. With an in-depth molecular understanding, we should be better armamented to combat such angiogenic disorders in the future. That such therapeutic strategies might change the face of medicine is witnessed by initial evidence of success in the clinic.

In vivo angiogenic phenotype of endothelial cells and pericytes induced by vascular endothelial growth factor-A

VEGF-A is a major angiogenesis and permeability factor. Its cellular effects, which can be used as targets in anti-angiogenesis therapy, have mainly been studied in vitro using endothelial cell cultures. The purpose of the present study was to further characterize these effects in vivo in vascular endothelial cells and pericytes, in an experimental monkey model of VEGF-A-induced iris neovascularization. Two cynomolgus monkeys (Macaca fascicularis) received four injections of 0.5 microg VEGF-A in the vitreous of one eye and PBS in the other eye. After sacrifice at day 9, eyes were enucleated and iris samples were snap-frozen for immunohistochemistry (IHC) and stained with a panel of antibodies recognizing endothelial and pericyte determinants related to angiogenesis and permeability. After VEGF-A treatment, the pre-existing iris vasculature showed increased permeability, hypertrophy, and activation, as demonstrated by increased staining of CD31, PAL-E, tPA, uPA, uPAR, Glut-1, and alphavbeta3 and alphavbeta5 integrins, VEGF receptors VEGFR-1, -2 and -3, and Tie-2 in endothelial cells, and of NG2 proteoglycan, uPA, uPAR, integrins and VEGFR-1 in pericytes. Vascular sprouts at the anterior surface of the iris were positive for the same antigens except for tPA, Glut-1, and Tie-2, which were notably absent. Moreover, in these sprouts VEGFR-2 and VEGFR-3 expression was very high in endothelial cells, whereas many pericytes were present that were positive for PDGFR-beta, VEGFR-1, and NG2 proteoglycan and negative for alpha-SMA. In conclusion, proteins that play a role in angiogenesis are upregulated in both pre-existing and newly formed iris vasculature after treatment with VEGF-A. VEGF-A induces hypertrophy and loss of barrier function in pre-existing vessels, and induces angiogenic sprouting, characterized by marked expression of VEGFR-3 and lack of expression of tPA and Tie-2 in endothelial cells, and lack of alpha-SMA in pericytes. Our in vivo study indicates a role for alpha-SMA-negative pericytes in early stages of angiogenesis. Therefore, our findings shed new light on the temporal and spatial role of several proteins in the angiogenic cascade in vivo.

Hypoxia and hypoxia inducible factors (HIF) as important regulators of tumor physiology

Regions of low oxygen tension are common findings in malignant tumors and are associated with increased frequency of tumor invasion and metastasis. Indeed, the ability to initiate homeostatic responses and adapt to hypoxia, e.g. by induction of angiogenesis, represents an important and crucial aspect in solid tumor growth. A significant advance in our understanding of the hypoxia response stems from the discovery of the hypoxia inducible factors (HIF) which act as key regulators of hypoxia-induced gene expression. Both, low levels of oxygen, apparently via reduced activity of a recently identified class of 2-oxoglutarate dependent oxygenases, and various tumor specific genetic alterations synergistically act to induce the HIF system. A widespread HIF activation can be observed in a variety of malignant tumors including brain tumors. The HIF system induces adaptive responses including angiogenesis, glycolysis, and pH regulation which confer increased resistance towards the hostile tumor microenvironment. Apart from protumorigenic the wide-ranging HIF pathway is known to harbor antitumorigenic components, which may, however, be disabled by tumor specific genetic alterations. Thus, mounting evidence has identified HIF as a crucial regulator of tumor growth and progression constituting an intriguing and novel target for therapeutic intervention.

Molecular regulation of vessel maturation

The maturation of nascent vasculature, formed by vasculogenesis or angiogenesis, requires recruitment of mural cells, generation of an extracellular matrix and specialization of the vessel wall for structural support and regulation of vessel function. In addition, the vascular network must be organized so that all the parenchymal cells receive adequate nutrients. All of these processes are orchestrated by physical forces as well as by a constellation of ligands and receptors whose spatio-temporal patterns of expression and concentration are tightly regulated. Inappropriate levels of these physical forces or molecules produce an abnormal vasculature--a hallmark of various pathologies. Normalization of the abnormal vasculature can facilitate drug delivery to tumors and formation of a mature vasculature can help realize the promise of therapeutic angiogenesis and tissue engineering.

Angiogenesis in health and disease

Blood vessels constitute the first organ in the embryo and form the largest network in our body but, sadly, are also often deadly. When dysregulated, the formation of new blood vessels contributes to numerous malignant, ischemic, inflammatory, infectious and immune disorders. Molecular insights into these processes are being generated at a rapidly increasing pace, offering new therapeutic opportunities that are currently being evaluated.

Targets for pharmacological intervention of endothelial hyperpermeability and barrier function

Many diseases share the common feature of vascular leakage, and endothelial barrier dysfunction is often the underlying cause. The subsequent stages of endothelial barrier dysfunction contribute to endothelial hyperpermeability. Vasoactive agents induce loss of junctional integrity, a process that involves actin-myosin interaction. Subsequently, the interaction of leukocytes amplifies leakage by the leukocyte-derived mediators. The processes mainly occur at the postcapillary venules. The whole microvascular bed, including the capillaries, becomes involved in vascular leakage by the induction of angiogenesis. Plasma leakage results from gaps between endothelial cells as well as by the induction of transcellular transport pathways. Several mechanisms can improve endothelial barrier function, depending on the tissue affected and the cause of hyperpermeability. They include blockade of specific receptors and elevation of cyclic AMP (cAMP) by agents such as beta(2)-adrenergic agents. However, current therapies based on these principles often fail. Recent research has identified several new promising targets for pharmacological therapy. Endogenous compounds were also found with barrier-improving characteristics. Important insights were obtained in the different pathways involved in barrier dysfunction. Such insights regard the regulation of endothelial contraction and endothelial junction integrity: inhibitors of RhoA activation and Rho kinase represent a potentially valuable group of agents with endothelial hyperpermeability reducing properties, and strategies to target vascular endothelial growth factor (VEGF)-mediated edema are under current investigation. In clinical practice, not only tools to improve an impaired endothelial barrier function are necessary. Sometimes, a controlled, temporal, and local increase in permeability can also be desired, for example, with the aim to enhance drug delivery. Therefore, vessel leakiness is also being exploited to enable tissue access of liposomes, viral vectors, and other therapeutic agents that do not readily cross healthy endothelium. This review discusses strategies for targeting signaling molecules in therapies for diseases involving altered endothelial permeability.

Selecting protein tyrosine phosphatases as drug targets

Protein tyrosine phosphatases (PTPs) have emerged as a new and promising class of signaling targets, since the discovery of PTP1B as a major drug target for diabetes and obesity. Blocking individual PTPs results in the activation of specific tyrosine phosphorylation events, but matching PTPs with such pathways and therapeutic indications is a complex undertaking. The history of PTP1B shows that its unusual knockout phenotype and observations with generic and antisense inhibitors in vivo, but not its classical molecular biology, triggered the rapid development of inhibitors that are today being developed for the clinic.

Gene therapy by adenovirus-mediated vascular endothelial growth factor and angiopoietin-1 promotes perfusion of muscle flaps

An experimental study was conducted to investigate the potential use of intravascular gene therapy with adenovirus-mediated (Ad) vascular endothelial growth factor (VEGF) or angiopoietin-1 (Ang-1) for the enhancement of muscle flap perfusion and to evaluate the effect of therapy on microcirculatory hemodynamics and microvascular permeability in vivo by using a cremaster muscle flap model in the rat. The cremaster tube flap was left intact after isolation of the pudo-epigastric pedicle. A total of 90 male Sprague-Dawley rats were divided into five groups of 18 each, according to the type of intraarterial treatment. Control flaps received phosphate-buffered saline. Group 2 (the control gene encoding green fluorescent protein, Ad-GFP) served as the adenovirus control. In Groups 3, 4, and 5, flaps were pretreated with Ad-VEGF, Ad-Ang-1, and Ad-Ang-1 + Ad-VEGF, respectively. Flaps were preserved in a subcutaneous pocket in the hindlimb for evaluation of functional capillary density and microvascular permeability indices at 3, 7, and 14 days by intravital microscopy system. At day 7 and 14, Ad-VEGF, Ad-Ang-1, and combined treatment groups showed significantly higher numbers of capillary densities when compared with control and Ad-GFP groups (p < 0.05). At day 14, Ad-VEGF was the superior treatment group compared with Ad-Ang-1 and Ad-VEGF + Ad-Ang-1 (p < 0.05). Overall, there was a linear increase in the number of functional capillaries in all treatment groups (p < 0.05). At day 3 after Ad-Ang-1 therapy, a significantly lower permeability index was found when compared with Ad-VEGF + Ad-Ang-1 and Ad-VEGF alone treatment (p < 0.05). At day 7, the Ad-VEGF group had the highest score of permeability index compared with control, combined, and Ad-Ang-1 groups (p < 0.05). Histologic evaluation of muscle flaps demonstrated mild focal inflammation. There was evidence of mild vasculitis in all flaps except control muscles. Intravascular angiogenic therapy with Ad-VEGF or Ad-Ang-1 was technically feasible, as demonstrated by expression of the control gene, GFP, along the vascular tree. All treatment groups increased perfusion of the muscle flap over a period of 14 days, indicating a long-lasting effect of gene therapy. Ang-1 alone or in combination with VEGF was as effective as VEGF alone in augmenting muscle perfusion with more stable vessels 1 week after gene therapy.

Angiogenesis induction and regression in human surgical wounds

Angiogenesis in human wound healing is not well characterized, with only sparse information available regarding the maturation and fate of vessels formed as a consequence of human tissue repair. Therefore, this study aimed to establish the temporal profile of angiogenesis in human dermal wounds. Punch biopsies were obtained under local anesthesia from 45 patients following breast surgery. Scars were predominantly between 2 and 52 weeks after surgery but in five patients were > 52 weeks. Control samples were taken from breast skin peroperatively (n = 24). Quantification of vascular density was performed using the Chalkley grid, following antibody staining for platelet endothelial cell adhesion molecule. Vascular patterns, wound cellularity and morphology were also determined. Cumulative microvessel density was increased in all samples when compared to controls (p < 0.05). This was greatest 2 to 24 weeks following surgery 17 (15-21) median (range), decreased thereafter, but remained elevated compared to controls even in the mature scars > 52 weeks. Control tissue showed an ordered morphological arrangement of dermal structures, collagen, and elastic fibers. However, wounding resulted in marked structural distortion for up to 15 weeks. In conclusion, this study shows for the first time the prolonged persistence of both microvessels and cellularity (fibroblastic cells), in addition to structural distortion in human dermal wounds, which is in contrast to previous in vitro and in vivo studies.

Melphalan availability in hypoxia-inducible factor-1alpha+/+ and factor-1alpha-/- tumors is independent of tumor vessel density and correlates with melphalan erythrocyte transport

Many tumors are impervious to anticancer agents. Resistance due to lack of cytotoxic penetration into the tumor is often overlooked but can play a significant role in undermining therapy. Insufficient and irregular vascularization of tumors is a possible barrier to drug delivery, but there are others, e.g., impaired vessel permeability and poor interstitial transport. We evaluated the importance of tumor vessel density in the availability of melphalan to tumors. A nude mouse tumor model with different vascularization due to a single-gene deletion of hypoxia-inducible factor 1alpha (HIF-1alpha(+/+) and HIF-1alpha(-/-) embryonic stem cell-derived tumors) was used. The availability of melphalan to HIF-1alpha(+/+) (n = 20) and HIF-1alpha(-/-) (n = 23) tumors was not significantly different (p = 0.12). Furthermore, in the various subgroup analyses accentuating the difference in vessel density, no significant correlation between vessel density and melphalan availability was found. In the second part of the study, melphalan, was demonstrated to be transported in blood in mice with a distribution of 24% in erythrocytes vs. 76% in plasma. A strong correlation (r = 0.93, p < 0.000001) between melphalan concentrations in plasma and erythrocytes was found, indicating an equilibrium between these 2 compartments. Plasma and erythrocyte concentrations of melphalan are correlated with the tumor availability of melphalan (r = 0.66 and 0.64, respectively, both p < 0.001). These data suggest that tumor vessel density is not an important predictor of the tumor availability of small cytotoxic drugs such as melphalan and indicate the importance of erythrocytes in the transport of melphalan.