Angiopoietin-2 causes pericyte dropout in the normal retina: evidence for involvement in diabetic retinopathy

Host-derived angiopoietin-2 affects early stages of tumor development and vessel maturation but is dispensable for later stages of tumor growth

The angiopoietin/Tie2 system has been identified as the second vascular-specific receptor tyrosine kinase system controlling vessel assembly, maturation, and quiescence. Angiopoietin-2 (Ang-2) is prominently up-regulated in the host-derived vasculature of most tumors, making it an attractive candidate for antiangiogenic intervention. Yet, the net outcome of Ang-2 functions on tumor angiogenesis is believed to be contextual depending on the local cytokine milieu. Correspondingly, Ang-2 manipulatory therapies have been shown to exert protumorigenic as well as antitumorigenic effects. To clarify the role of Ang-2 for angiogenesis and tumor growth in a definite genetic experimental setting, the present study was aimed at comparatively studying the growth of different tumors in wild-type and Ang-2-deficient mice. Lewis lung carcinomas, MT-ret melanomas, and B16F10 melanomas all grew slower in Ang-2-deficient mice. Yet, tumor growth in wild-type and Ang-2-deficient mice dissociated during early stages of tumor development, whereas tumor growth rates during later stages of primary tumor progression were similar. Analysis of the intratumoral vascular architecture revealed no major differences in microvessel density and perfusion characteristics. However, diameters of intratumoral microvessels were smaller in tumors grown in Ang-2-deficient mice, and the vasculature had an altered pattern of pericyte recruitment and maturation. Ang-2-deficient tumor vessels had higher pericyte coverage indices. Recruited pericytes were desmin and NG2 positive and predominately alpha-smooth muscle actin negative, indicative of a more mature pericyte phenotype. Collectively, the experiments define the role of Ang-2 during tumor angiogenesis and establish a better rationale for combination therapies involving Ang-2 manipulatory therapies.

Endothelial-mural cell signaling in vascular development and angiogenesis

Mural cells are essential components of blood vessels and are necessary for normal development, homeostasis, and organ function. Alterations in mural cell density or the stable attachment of mural cells to the endothelium is associated with several human diseases such as diabetic retinopathy, venous malformation, and hereditary stroke. In addition mural cells are implicated in regulating tumor growth and have thus been suggested as potential antiangiogenic targets in tumor therapy. In recent years our knowledge of mural cell function and endothelial-mural cell signaling has increased dramatically, and we now begin to understand the mechanistic basis of the key signaling pathways involved. This is mainly thanks to sophisticated in vivo experiments using a broad repertoire of genetic technologies. In this review, we summarize the five currently best understood signaling pathways implicated in mural cell biology. We discuss PDGFB/PDGFRbeta- dependent pericyte recruitment, as well as the role of angiopoietins and Tie receptors in vascular maturation. In addition, we highlight the effects of sphingosine-1-phosphate signaling on adherens junction assembly and vascular stability, as well as the role of TGF-beta-signaling in mural cell differentiation. We further reflect recent data suggesting an important function for Notch3 signaling in mural cell maturation.

Increased basement membrane thickness, pericyte ghosts, and loss of retinal thickness and cells in dopamine beta hydroxylase knockout mice

Diabetes can cause damage to sympathetic nerves, and we have previously shown that experimental sympathectomy can produce capillary abnormalities in the retina similar to those seen in early diabetes. We postulate that the diabetes-induced loss of the sympathetic system, and at least in part the sympathetic neurotransmitter norepinephrine (NE), contributes to the development of retinal vascular and neural abnormalities in diabetes. Thus, we predict that non-diabetic animals that lack NE will develop microvascular and neural changes that are similar to those that are characteristic of diabetic retinopathy. To test this, retinas from non-diabetic dopamine beta hydroxylase (Dbh, Dbh(-/-)) knockout mice and their littermate controls were assessed for diabetic-like capillary and neural changes at 5 months of age. Genetic deletion of Dbh resulted in a significant decrease in retinal thickness and number of cells in the retinal ganglion cell layer (central retinal region). In addition, the number of pericyte ghosts and the basement membrane of retinal capillaries were significantly increased in the Dbh(-/-) mice. These results provide evidence that loss of sympathetic neurotransmission may contribute to the microvascular and neural changes of diabetic retinopathy. Restoration of sympathetic neurotransmission may be a new target for therapeutic intervention to inhibit the early phases of diabetic retinopathy.

Efficacy of weekly docetaxel and bevacizumab in mesenchymal chondrosarcoma: a new theranostic method combining xenografted biopsies with a mathematical model

The paucity of clinical treatment data on rare tumors, such as mesenchymal chondrosarcoma (MCS), emphasizes the need in theranostic tools for these diseases. We put forward and validated a new theranostic method, combining tumor xenografts and mathematical models, and used it to suggest an improved treatment schedule for a particular MCS patient. Growth curves and gene expression analysis of xenografts, derived from a patient's lung metastasis, served for creating a mathematical model of MCS progression and adapting it to the xenograft setting. The pharmacokinetics and pharmacodynamics of six drugs were modeled, with model variables being adjusted by patient-specific chemosensitivity tests. The xenografted animals were treated by various monotherapy and combination schedules, and the MCS xenograft model was computer simulated under the same treatment scenario. The mathematical model for xenograft growth was then up-scaled to retrieve the MCS patient's tumor progression under different treatment schedules. An average accuracy of 87.1% was obtained when comparing model predictions with the observed tumor growth inhibition in the xenografted animals. Simulation results suggested that a regimen containing bevacizumab applied i.v. in combination with once-weekly docetaxel would be more efficacious in the MCS patient than all other simulated schedules. Weekly docetaxel in the patient resulted in stable metastatic disease and relief of pancytopenia due to tumor infiltration. We suggest that the advantage of weekly docetaxel on the triweekly regimen is directly related to the angiogenesis rate of the tumor. Further validation of this conclusion, and the theranostic method we provide, may facilitate personalization of solid cancer pharmacotherapy.

Pericyte migration: a novel mechanism of pericyte loss in experimental diabetic retinopathy

OBJECTIVE

The mechanism underlying pericyte loss during incipient diabetic retinopathy remains controversial. Hyperglycemia induces angiopoietin-2 (Ang-2) transcription, which modulates capillary pericyte coverage. In this study, we assessed loss of pericyte subgroups and the contribution of Ang-2 to pericyte migration.

RESEARCH DESIGN AND METHODS

Numbers of total pericytes and their subgroups were quantified in retinal digest preparations of spontaneous diabetic XLacZ mice. Pericytes were divided into subgroups according to their localization, their position relative to adjacent endothelial cells, and the expression of LacZ. The contribution of Ang-2 to pericyte migration was assessed in Ang-2 overexpressing (mOpsinhAng2) and deficient (Ang2LacZ) mice.

RESULTS

Pericyte numbers were reduced by 16% (P < 0.01) in XLacZ mice after 6 months of diabetes. Reduction of pericytes was restricted to pericytes on straight capillaries (relative reduction 27%, P < 0.05) and was predominantly observed in LacZ-positive pericytes (-20%, P < 0.01). Hyperglycemia increased the numbers of migrating pericytes (69%; P < 0.05), of which the relative increase due to diabetes was exclusively in LacZ-negative pericytes, indicating reduced adherence to the capillaries (176%; P < 0.01). Overexpression of Ang-2 in nondiabetic retinas mimicked diabetic pericyte migration of wild-type animals (78%; P < 0.01). Ang-2 deficient mice completely lacked hyperglycemia-induced increase in pericyte migration compared with wild-type littermates.

CONCLUSIONS

Diabetic pericyte loss is the result of pericyte migration, and this process is modulated by the Ang-Tie system.

Receptor for advanced glycation end product expression in experimental diabetic retinopathy

The advanced glycation end product (AGE)-receptor for AGE (RAGE) pathway is involved in the pathogenesis of diabetic microvascular damage. The special distribution of RAGE and its engagement has an impact on the development of diabetic retinopathy. In the present study, we used immunofluorescence and confocal laser microscopy to study RAGE expression with special emphasis on Müller glia in Sprague Dawley rats. RAGE expression was low in nondiabetic retinae and was found in ganglion cells and Müller cell end feet. In diabetic retinae, upregulated RAGE was predominantly expressed in retinal glia. Since Müller cells are important in the regulation of important features of early retinal vascular damage, such as vascular permeability, homeostasis, and response to stress, RAGE appears to be a central modulator in diabetic retinopathy.

Disruption of Ang-1/Tie-2 signaling contributes to the impaired myocardial vascular maturation and angiogenesis in type II diabetic mice

OBJECTIVE

Microvascular insufficiency represents a major cause of end-organ failure among diabetics. The current studies were undertaken to determine whether dysregulation of the angiopoietins/Tie-2 system would result in an impairment of smooth muscle cell (SMC) recruitment and vascular maturation, which contributes to impaired angiogenesis in diabetes.

METHODS AND RESULTS

Tie-2 expression was significantly attenuated, whereas angiopoietin-2 (Ang-2) was increased in db/db mice subjected to myocardial ischemia. Our morphological analysis showed that the number of SMC coverage area per neovessel was significantly reduced in db/db mice. This was accompanied by a significant reduction of myocardial capillary density and arteriole formation. Interestingly, Angiopoietin-1(Ang-1)-induced SMC recruitment and vessel outgrowth were severely impaired in db/db mice. Our in vitro studies further demonstrated that exposure of mouse heart endothelial cells to high glucose resulted in a significant upregulation of Ang-2 and a downregulation of Tie-2 expression. These alterations led to a significant impairment of Ang-1-induced Akt and eNOS phosphorylation, along with a remarkable impairment of Ang-1-induced endothelial cell migration and endothelial cell spheroid sprouting. Ang-1 gene transfer restored Tie-2 expression and rescued these abnormalities in diabetes.

CONCLUSIONS

Our findings underscore the important role of Ang-1-Tie-2 signaling in the diabetes-induced impairment of vascular maturation and angiogenesis.

Defective remodeling and maturation of the lymphatic vasculature in Angiopoietin-2 deficient mice

Molecular mechanisms regulating the remodeling of the lymphatic vasculature from an immature plexus of vessels to a hierarchal network of initial and collecting lymphatics are not well understood. One gene thought to be important for this process is Angiopoietin-2 (Ang-2). Ang2(-/-) mice have previously been reported to exhibit an abnormal lymphatic phenotype but the precise nature of the lymphatic defects and the underlying mechanisms have yet to be defined. Here we demonstrate by whole-mount immunofluorescence staining of ear skin and mesentery that lymphatic vessels in Ang2(-/-) mice fail to mature and do not exhibit a collecting vessel phenotype. Furthermore, dermal lymphatic vessels in Ang2(-/-) pups prematurely recruit smooth muscle cells and do not undergo proper postnatal remodeling. In contrast, Ang2 knock-out Ang1 knock-in mice do develop a hierarchal lymphatic vasculature, suggesting that activation of Tie-2 is required for normal lymphatic development. Taken together, this work pinpoints a specific lymphatic defect of Ang2(-/-) mice and further defines the sequential steps in lymphatic vessel remodeling.

Intra and extravascular transmembrane signalling of angiopoietin-1-Tie2 receptor in health and disease

Angiopoietin-1 (Ang-1) is the primary agonist for Tie2 tyrosine kinase receptor (Tie2), and the effect of Ang-1-Tie2 signalling is context-dependent. Deficiency in either Ang-1 or Tie2 protein leads to severe microvascular defects and subsequent embryonic lethality in murine model. Tie2 receptors are expressed in several cell types, including endothelial cells, smooth muscle cells, fibroblasts, epithelial cells, monocytes, neutrophils, eosinophils and glial cells. Ang-1-Tie2 signalling induces a chemotactic effect in smooth muscle cells, neutrophils and eosinophils, and induces differentiation of mesenchymal cells to smooth muscle cells. Additionally, this signalling pathway induces the secretion of serotonin, matrix metalloproteinases (MMPs) and plasmin. Ang-1 inhibits the secretion of tissue inhibitor of matrix metalloproteinase (TIMPs). Aberrant expression and activity of Tie2 in vascular and non-vascular cells may result in the development of rheumatoid arthritis, cancer, hypertension and psoriasis. Ang-1 has an anti-inflammatory effect, when co-localized with vascular endothelial growth factor (VEGF) in the vasculature. Thus, Ang-1 could be potentially important in the therapy of various pathological conditions such as pulmonary hypertension, arteriosclerosis and diabetic retinopathy. In this article, we have summarized and critically reviewed the pathophysiological role of Ang-1-Tie2 signalling pathway.

Why targeted therapy hasn't worked in advanced cancer

In this issue of the JCI, Nissen et al. report that a reciprocal interaction exists between the growth factors FGF2 and PDGF-BB, causing tumors to exhibit increased angiogenesis and metastatic potential. Both FGF2 and PDGF-BB signal through tyrosine kinase receptors, which have been the target of tyrosine kinase inhibitors for cancer therapies. These inhibitors are usually small molecules that inhibit the kinase activity of a receptor or nonreceptor tyrosine kinase, preventing downstream signaling. The results of this study shed light on why tyrosine kinase inhibitors have been useful for the treatment of only a small number of advanced cancers. Currently, a major focus of pharmaceutical companies is to develop ever more potent and specific tyrosine kinases. The results presented here suggest that this approach may not be successful.

Pericytes and ocular diseases

Pericytes are vascular mural cells which play multiple roles in angiogenesis and maintenance of blood vessel morphology and stability. In this review, we analyze recent data on the participation of pericytes in the pathogenesis of proliferative and non-proliferative diabetic retinopathy, as well as an emerging role in other angiogenic ocular diseases such as retinopathy of prematurity. Ways to exploit pericytes as targets for treatment of ocular diseases are discussed.

Role of PTEN/PI3K pathway in endothelial cells

PTEN (phosphatase and tensin homologue deleted on chromosome 10) is an important tumour-suppressor gene that encodes a 3-phosphatase. The major substrate of PTEN is PIP(3) (phosphatidylinositol 3,4,5-trisphosphate) generated by the action of PI3Ks (phosphoinositide 3-kinases). Hereditary mutation of PTEN causes tumour-susceptibility diseases such as Cowden disease. We used the Cre-loxP system to generate an endothelial cell-specific mutation of PTEN in mice. Heterozygous mutation of PTEN in endothelial cells enhances postnatal neovascularization, including tumour angiogenesis necessary for tumour growth. This observation suggests that Cowden disease patients are not only at risk for additional tumorigenic mutations due to complete loss of PTEN function, but may also experience accelerated growth of incipient tumours due to enhanced angiogenesis. Homozygous mutation of Pten in murine endothelial cells impairs cardiovascular morphogenesis and is embryonic lethal due to endothelial cell hyperproliferation and impaired vascular remodelling. Additional homozygous mutation of p85alpha, the regulatory subunit of class IA PI3Ks, or p110gamma, the catalytic subunit of the sole class IB PI3K, led to a partial rescue of all phenotypes in our PTEN-deficient mice. Thus inhibition of the PI3K pathway, including the targeting of PI3Kgamma, may be an attractive therapeutic strategy for the treatment of various malignancies.

Pericytes: gatekeepers in tumour cell metastasis?

Tumour cells use two major routes to spread during metastasis, e.g. lymph vessels and blood vessels within or surrounding the primary tumour. The growth rate of the primary tumour often correlates with the quantity of new blood vessels that form within the tumour. However, qualitative abnormalities of the tumour vasculature profoundly affect the perfusion of the primary tumour and the escape of tumour cells into the circulation. In this paper, we review recent evidence for a novel role of the supporting mural cells in limiting blood-borne metastasis.

Recent advancement of understanding pathogenesis of type 1 diabetes and potential relevance to diabetic nephropathy

Type 1 diabetes mellitus is an autoimmune disease characterized by progressive destruction of pancreatic beta cells by genetic and environmental factors which leads to an absolute dependence of insulin for survival and maintenance of health. Although the majority of mechanisms of beta cell destruction remain unclear, many molecules, including proinflammatory cytokines and chemokines such as tumor necrosis factor alpha and monocyte chemoattractant protein-1, are implicated in the development of beta cell damage. Furthermore, beta cell destruction is enhanced by the Th1 and Th17 subsets of CD4+ T cells. In contrast, there are mechanisms involved in the maintenance of peripheral tolerance by regulatory T cells, the function of which depends on the pleiotropic cytokine transforming growth factor beta. Development and progression of renal injuries in patients with diabetic nephropathy are also associated with several growth factors and proinflammatory cytokines, including tumor necrosis factor alpha, insulin-like growth factor-1, monocyte chemoattractant protein-1, vascular endothelial growth factor, and transforming growth factor beta. Although the pathogenic mechanisms underlying type 1 diabetes and diabetic nephropathy are principally different, i.e., autoimmunity and inflammation, some common factors, including susceptibility genes and proinflammatory cytokines, are involved in both mechanisms, including infiltrating cell recruitment, upregulation of other cytokines and chemokines, or apoptosis.

Physiologic angiodynamics in the brain

Hypoxic acclimatization includes increased brain capillary density. Adaptive angiogenesis, which occurs over a 3-week period, is mediated by upregulation of vascular endothelial growth factor induced by hypoxia-inducible factor-1 in concert with the capillary remodeling molecule angiopoietin-2, which is upregulated through cyclooxygenase-2 production of prostaglandin E2. The process is apparently orchestrated by pericytes, which regulate the microvascular milieu and coordinate the interactions within the neurovascular unit. The return to normoxia is accompanied by microvascular regression and decreasing numbers of capillaries to prehypoxic densities. Regression is the result of endothelial cell apoptosis, suggesting the existence of physiologic mechanisms for adjusting capillary density to balance oxygen availability and oxygen consumption. The capacity for adaptation is diminished in older rats because of the attenuation of the hypoxia-inducible factor-1 response.

Arginine-specific gingipain A from Porphyromonas gingivalis induces Weibel-Palade body exocytosis and enhanced activation of vascular endothelial cells through protease-activated receptors

Gingipains, cysteine proteases derived from Porphyromonas gingivalis, are important virulence factors in periodontal diseases. We found that arginine-specific gingipain A (RgpA) increased the responsiveness of vascular endothelial cells to P. gingivalis lipopolysaccharides (LPS) and P. gingivalis whole cells to induce enhanced IL-8 production through protease-activated receptors (PARs) and phospholipase C (PLC) gamma. We therefore investigated whether RgpA-induced enhanced cell activation is mediated through exocytosis of Weibel-Palade bodies (WPBs) because they store vasoactive substances. RgpA rapidly activated PAR- and PLCgamma-dependent WPB exocytosis. In addition, angiopoietin (Ang)-2, a substance of WPB, enhanced IL-8 production by P. gingivalis LPS, suggesting that Ang-2 mediates the RgpA-induced enhanced cell responses. Thus, we propose a novel role for RgpA in induction of a proinflammatory event through PAR-mediated WPB exocytosis, which may be an important step for enhanced endothelial responses to P. gingivalis.

High glucose increases angiopoietin-2 transcription in microvascular endothelial cells through methylglyoxal modification of mSin3A

Methylglyoxal is a highly reactive dicarbonyl degradation product formed from triose phosphates during glycolysis. Methylglyoxal forms stable adducts primarily with arginine residues of intracellular proteins. The biologic role of this covalent modification in regulating cell function is not known. Here we report that in mouse kidney endothelial cells, high glucose causes increased methylglyoxal modification of the corepressor mSin3A. Methylglyoxal modification of mSin3A results in increased recruitment of O-GlcNAc-transferase, with consequent increased modification of Sp3 by O-linked N-acetylglucosamine. This modification of Sp3 causes decreased binding to a glucose-responsive GC-box in the angiopoietin-2 (Ang-2) promoter, resulting in increased Ang-2 expression. Increased Ang-2 expression induced by high glucose increased expression of intracellular adhesion molecule 1 and vascular cell adhesion molecule 1 in cells and in kidneys from diabetic mice and sensitized microvascular endothelial cells to the proinflammatory effects of tumor necrosis factor alpha. This novel mechanism for regulating gene expression may play a role in the pathobiology of diabetic vascular disease.

Incipient diabetic retinopathy--insights from an experimental model

Vascular complications of chronic hyperglycemia including diabetic retinopathy are an increasing therapeutic and socioeconomic challenge. The epidemiology of diabetic eye disease has been well described, and there is as yet no clear indication for a reduction of incidence of blindness. Due to the complex multifactorial nature of the damage to diabetic vessels, it had been difficult to identify key targets for treatment and prevention. Novel techniques to study molecules and mechanisms involved in retinal vessel development and vascular cell interactions improved the understanding of retinal cell biology and pathobiology. A unifying concept has been proposed which links hyperglycemia-induced mitochondrial overproduction of reactive oxygen species with long-known biochemical alterations such as the formation of advanced glycation end products or the activation of the protein kinase C pathway. Specific inhibitors were identified that inhibited multiple biochemical abnormalities downstream of oxidative stress induced by high glucose.

Bioinformatics analysis of diabetic retinopathy using functional protein sequences

Diabetic retinopathy is the leading cause of blindness among patients with diabetes mellitus. We evaluated the role of several proteins that are likely to be involved in diabetic retinopathy by employing multiple sequence alignment using ClustalW tool and constructed a phylogram tree using functional protein sequences extracted from NCBI. Phylogram was constructed using Neighbor-Joining Algorithm in bioinformatics approach. It was observed that aldose reductase and nitric oxide synthase are closely associated with diabetic retinopathy. It is likely that vascular endothelial growth factor, pro-inflammatory cytokines, advanced glycation end products, and adhesion molecules that also play a role in diabetic retinopathy may do so by modulating the activities of aldose reductase and nitric oxide synthase. These results imply that methods designed to normalize aldose reductase and nitric oxide synthase activities could be of significant benefit in the prevention and treatment of diabetic retinopathy.

Systemic Overexpression of Angiopoietin-2 Promotes Tumor Microvessel Regression and Inhibits Angiogenesis and Tumor Growth

Angiopoietin-2 (Ang-2) is a conditional antagonist and agonist for the endothelium-specific Tie-2 receptor. Although endogenous Ang-2 cooperates with vascular endothelial growth factor (VEGF) to protect tumor endothelial cells, the effect on tumor vasculature of high levels of exogenous Ang-2 with different levels of VEGF has not been studied in detail. Here, we report that systemic overexpression of Ang-2 leads to unexpected massive tumor vessel regression within 24 h, even without concomitant inhibition of VEGF. By impairing pericyte coverage of the tumor vasculature, Ang-2 destabilizes the tumor vascular bed while improving perfusion in surviving tumor vessels. Ang-2 overexpression transiently exacerbates tumor hypoxia without affecting ATP levels. Although sustained systemic Ang-2 overexpression does not affect tumor hypoxia and proliferation, it significantly inhibits tumor angiogenesis, promotes tumor apoptosis, and suppresses tumor growth. The similar antitumoral, antiangiogenic efficacy of systemic overexpression of Ang-2, soluble VEGF receptor-1, and the combination of both suggests that concomitant VEGF inhibition is not required for Ang-2-induced tumor vessel regression and growth delay. This study shows the important roles of Ang-2-induced pericyte dropout during tumor vessel regression. It also reveals that elevated Ang-2 levels have profound pleiotropic effects on tumor vessel structure, perfusion, oxygenation, and apoptosis.

Angiogenic inhibition reduces germinal matrix hemorrhage

The germinal matrix of premature infants is selectively vulnerable to hemorrhage within the first 48 h of life. To assess the role of vascular immaturity in germinal matrix hemorrhage (GMH), we evaluated germinal matrix angiogenesis in human fetuses and premature infants, as well as in premature rabbit pups, and noted active vessel remodeling in all three. Vascular endothelial growth factor (VEGF), angiopoietin-2 and endothelial cell proliferation were present at consistently higher levels in the germinal matrix relative to the white matter anlagen and cortical mantle. On that basis, we asked whether prenatal treatment with either of two angiogenic inhibitors, the COX-2 inhibitor celecoxib, or the VEGFR2 inhibitor ZD6474, could suppress the incidence of GMH in premature rabbit pups. Celecoxib treatment decreased angiopoietin-2 and VEGF levels as well as germinal matrix endothelial proliferation. Furthermore, treatment with celecoxib or ZD6474 substantially decreased the incidence of GMH. Thus, by suppressing germinal matrix angiogenesis, prenatal celecoxib or ZD6474 treatment may be able to reduce both the incidence and severity of GMH in susceptible premature infants.

Common Protective and Diverse Smooth Muscle Cell Effects of AAV-Mediated Angiopoietin-1 and -2 Expression in Rat Cardiac Allograft Vasculopathy

Angiopoietin-1 (Ang1) and Ang2 regulate the maintenance of normal vasculature by direct endothelial and indirect smooth muscle cell (SMC) effects. Dysfunction of vascular wall cells is considered central in cardiac allograft vasculopathy (CAV), where inflammation and arterial injury initiate subsequent intimal SMC proliferation. In this study, we investigated the effect of exogenous Ang1 and Ang2 in chronically rejecting rat cardiac allografts by intracoronary adeno-associated virus (AAV)-mediated gene transfer. Bioluminescent imaging of AAV-transfected syngeneic grafts revealed gradual and stable transgene expression in graft cardiomyocytes. In cardiac allografts, both AAV-Ang1 and AAV-Ang2 decreased inflammation and increased antiapoptotic Bcl-2 mRNA and Bcl-2/Bax ratio at 8 weeks. Only AAV-Ang2 decreased the development of CAV, whereas AAV-Ang1 activated arterial SMC and increased PDGF-A mRNA in the allograft. Collectively, our results show that exogenous Ang1 and Ang2 have similar antiinflammatory and antiapoptotic effects in cardiac allografts. Prolonged AAV-mediated Ang1 transgene expression also induced SMC activation, whereas AAV-Ang2 lacked the SMC activating effects and decreased CAV. Our results thus highlight the common protective and diverse SMC effects of Ang1 and Ang2 in cardiac allograft microenvironment and the importance of timing of angiopoietins to achieve therapeutic effects.

Effect of R-(+)-alpha-lipoic acid on experimental diabetic retinopathy

AIMS/HYPOTHESIS

Hyperglycaemia-induced mitochondrial overproduction of reactive oxygen species (ROS) is central to the pathogenesis of endothelial damage in diabetes. R-(+)-alpha-lipoic acid has advantages over classic antioxidants, as it distributes to the mitochondria, is regenerated by glycolytic flux, and has a low redox potential.

METHODS

To assess the effect of R-(+)-alpha-lipoic acid on experimental diabetic retinopathy, three groups of male Wistar rats were studied: non-diabetic controls, untreated diabetic controls, and diabetic rats treated with 60 mg/kg bodyweight R-(+)-alpha-lipoic acid i.p. for 30 weeks. Quantitative retinal morphometry included acellular occluded capillaries and pericyte numbers. The effects of R-(+)-alpha-lipoic acid on parameters of oxidative and nitrative stress, AGE and its receptor and nuclear factor kappa B (NFkappaB) were assessed by immunoblotting, and NFkappaB activation by electrophoretic mobility shift assay. Angiopoietin-2 and vascular endothelial growth factors were also determined by immunoblotting.

RESULTS

After 30 weeks of diabetes, the number of acellular capillaries was significantly elevated in diabetic rats (57.1+/-10.6 acellular capillary segments [ac]/mm(2) of retinal area) compared with non-diabetic (19.8+/-5.1 ac/mm(2); p<0.001). Treatment with 60 mg/kg R-(+)-alpha-lipoic acid reduced the numbers by 88% (p<0.001 vs diabetic). Pericyte loss was also significantly inhibited in diabetic rats treated with R-(+)-alpha-lipoic acid (non-diabetic: 1,940+/-137 pericytes/mm(2)capillary area; untreated diabetic: 1,294+/-94 pericytes/mm(2)capillary area vs treated diabetic: 1,656+/-134 pericytes/mm(2); p<0.01). R-(+)-alpha-lipoic acid treatment reduced oxidative stress, normalised NFkappaB activation and angiopoietin-2 expression, and reduced vascular endothelial growth factor in the diabetic retina by 43% (p<0.0001).

CONCLUSIONS/INTERPRETATION

R-(+)-alpha-lipoic acid prevents microvascular damage through normalised pathways downstream of mitochondrial overproduction of ROS, and preserves pericyte coverage of retinal capillaries, which may provide additional endothelial protection.

Increased serum levels of the specific advanced glycation end product methylglyoxal-derived hydroimidazolone are associated with retinopathy in patients with type 2 diabetes mellitus

Advanced glycation end products (AGEs) are thought to play a major pathogenic role in diabetic retinopathy. The most important AGE is unknown, but as increased serum methylglyoxal-derived hydroimidazolone has been demonstrated in patients with type 2 diabetes mellitus, the aim of the present study was to elucidate possible associations between serum levels of hydroimidazolone and retinopathy in patients with type 2 diabetes mellitus. We recruited 227 patients with type 2 diabetes mellitus and retinopathy ranging from none to proliferative. Level of retinopathy was determined from 7 standard field stereo photographs per eye according to the Early Treatment Diabetic Retinopathy Study. The patients were 66 +/- 11 years old, with a known diabetes duration of 14 +/- 9 years. Serum levels of hydroimidazolone were determined with a competitive immunoassay. Serum levels of hydroimidazolone were increased in nonproliferative (median, 4.50 U/mL; interquartile range, 3.69-5.77 U/mL) and proliferative retinopathy (median, 4.88 U/mL; interquartile range, 3.70-6.52 U/mL) compared with patients without retinopathy (median, 4.02 U/mL; interquartile range, 3.47-4.88 U/mL) (P = .008 and .002, respectively). There was no association between hydroimidazolone and hemoglobin A1c (r = 0.04, P = .57). In addition, patients with proliferative retinopathy and a relatively short known duration of diabetes, that is, less than the median of 14 years, had increased serum levels of hydroimidazolone (median, 6.91 U/mL; interquartile range, 4.70-8.91 U/mL) compared with those with nonproliferative retinopathy (median, 4.34; interquartile range, 3.86-5.53U/mL, P = .015). Serum levels of hydroimidazolone are increased in type 2 diabetic patients with retinopathy. This association is independent of hitherto known associated factors, such as hemoglobin A1c.

Endothelial/pericyte interactions

Interactions between endothelial cells and mural cells (pericytes and vascular smooth muscle cells) in the blood vessel wall have recently come into focus as central processes in the regulation of vascular formation, stabilization, remodeling, and function. Failure of the interactions between the 2 cell types, as seen in numerous genetic mouse models, results in severe and often lethal cardiovascular defects. Abnormal interactions between the 2 cell types are also implicated in a number of human pathological conditions, including tumor angiogenesis, diabetic microangiopathy, ectopic tissue calcification, and stroke and dementia syndrome CADASIL. In the present review, we summarize current knowledge concerning the identity, characteristics, diversity, ontogeny, and plasticity of pericytes. We focus on the advancement in recent years of the understanding of intercellular communication between endothelial and mural cells with a focus on transforming growth factor beta, angiopoietins, platelet-derived growth factor, spingosine-1-phosphate, and Notch ligands and their respective receptors. We finally highlight recent important data contributing to the understanding of the role of pericytes in tumor angiogenesis, diabetic retinopathy, and hereditary lymphedema.

Endothelial survival factors and spatial completion, but not pericyte coverage of retinal capillaries determine vessel plasticity

Pericyte loss and capillary regression are characteristic for incipient diabetic retinopathy. Pericyte recruitment is involved in vessel maturation, and ligand-receptor systems contributing to pericyte recruitment are survival factors for endothelial cells in pericyte-free in vitro systems. We studied pericyte recruitment in relation to the susceptibility toward hyperoxia-induced vascular remodeling using the pericyte reporter X-LacZ mouse and the mouse model of retinopathy of prematurity (ROP). Pericytes were found in close proximity to vessels, both during formation of the superficial and the deep capillary layers. When exposure of mice to the ROP was delayed by 24 h, i.e., after the deep retinal layer had formed [at postnatal (p) day 8], preretinal neovascularizations were substantially diminished at p18. Mice with a delayed ROP exposure had 50% reduced avascular zones. Formation of the deep capillary layers at p8 was associated with a combined up-regulation of angiopoietin-1 and PDGF-B, while VEGF was almost unchanged during the transition from a susceptible to a resistant capillary network. Inhibition of Tie-2 function either by soluble Tie-2 or by a sulindac analog, an inhibitor of Tie-2 phosphorylation, resensitized retinal vessels to neovascularizations due to a reduction of the deep capillary network. Inhibition of Tie-2 function had no effect on pericyte recruitment. Our data indicate that the final maturation of the retinal vasculature and its resistance to regressive signals such as hyperoxia depend on the completion of the multilayer structure, in particular the deep capillary layers, and are independent of the coverage by pericytes.

The PTEN/PI3K pathway governs normal vascular development and tumor angiogenesis

PTEN is an important tumor suppressor gene. Hereditary mutation of PTEN causes tumor-susceptibility diseases such as Cowden disease. We used the Cre-loxP system to generate an endothelial cell-specific mutation of Pten (Tie2CrePten) in mice. Tie2CrePten(flox/+) mice displayed enhanced tumorigenesis due to an increase in angiogenesis driven by vascular growth factors. This effect was partially dependent on the PI3K subunits p85alpha and p110gamma. In vitro, Tie2CrePten(flox/+) endothelial cells showed enhanced proliferation/migration. Tie2CrePten(flox/flox) mice died before embryonic day 11.5 (E11.5) due to bleeding and cardiac failure caused by impaired recruitment of pericytes and vascular smooth muscle cells to blood vessels, and of cardiomyocytes to the endocardium. These phenotypes depend strongly on p110gamma rather than on p85alpha and were associated with decreased expression of Ang-1, VCAM-1, connexin 40, and ephrinB2 but increased expression of Ang-2, VEGF-A, VEGFR1, and VEGFR2. Pten is thus indispensable for normal cardiovascular morphogenesis and post-natal angiogenesis, including tumor angiogenesis.

Angiopoietin-1 attenuates H2O2-induced SEK1/JNK phosphorylation through the phosphatidylinositol 3-kinase/Akt pathway in vascular endothelial cells

Oxidative stress activates various signal transduction pathways, including Jun N-terminal kinase (JNK) and its substrates, that induce apoptosis. We reported here the role of angiopoietin-1 (Ang1), which is a prosurvival factor in endothelial cells, during endothelial cell damage induced by oxidative stress. Hydrogen peroxide (H2O2) increased apoptosis of endothelial cells through JNK activation, whereas Ang1 inhibited H2O2-induced apoptosis and concomitant JNK phosphorylation. The inhibition of H2O2-induced JNK phosphorylation was reversed by inhibitors of phosphatidylinositol (PI) 3-kinase and dominant-negative Akt, and constitutively active-Akt attenuated JNK phosphorylation without Ang1. These data suggested that Ang1-dependent Akt phosphorylation through PI 3-kinase leads to the inhibition of JNK phosphorylation. H2O2-induced phosphorylation of SAPK/Erk kinase (SEK1) at Thr261, which is an upstream regulator of JNK, was also attenuated by Ang1-dependent activation of the PI 3-kinase/Akt pathway. In addition, Ang1 induced SEK1 phosphorylation at Ser80, suggesting the existence of an additional signal transduction pathway through which Ang1 attenuates JNK phosphorylation. These results demonstrated that Ang1 attenuates H2O2-induced SEK1/JNK phosphorylation through the PI 3-kinase/Akt pathway and inhibits the apoptosis of endothelial cells to oxidative stress.

Local delivery of angiopoietin-2 into the preovulatory follicle terminates the menstrual cycle in rhesus monkeys

The angiopoietin (ANGPT)-receptor (TEK) system plays a crucial role in blood vessel formation and stability. Because the endogenous agonist ANGPT1, antagonist ANGPT2, and TEK are expressed in the primate ovary, experiments were designed to investigate their role at a critical time during tissue remodeling/ angiogenesis in the menstrual cycle (i.e., at midcycle during maturation, ovulation, and luteinization of the dominant follicle). Either vehicle, 20 microg of ANGPT1, 2 microg of ANGPT2 (low-dose), or 20 microg of ANGPT2 (high-dose) was injected directly into the preovulatory follicle of monkeys around the day (-1 to 0) of the midcycle estradiol (E2)/LH peak. Ovaries were evaluated on Day 3 postinjection for follicle rupture, and serum samples were analyzed for levels of E2 and progesterone. Similar to controls, ANGPT1 treatment was followed by ovulation, and elevated progesterone levels during the luteal phase. In contrast, high-dose ANGPT2 treatment prevented follicle rupture, and progesterone levels never rose above baseline in the subsequent 12 days. However, an E2 peak typically occurred 12 days postinjection. Laparoscopy detected a preovulatory follicle on the contralateral (noninjected) ovary. Progesterone levels subsequently increased above baseline in these animals. Thus, exogenous ANGPT2 disrupted maturation of the preovulatory follicle, preventing its ovulation and conversion into the corpus luteum. ANGPT antagonism eliminated the dominant structure, thereby resetting the ovarian cycle, with selection and maturation of the next preovulatory follicle occurring in a timely manner. The data are consistent with a critical role of the ANGPT-TIE1/TEK system in the ovary, notably at the late stages of follicle maturation during the menstrual cycle.

The Tie-2 ligand angiopoietin-2 destabilizes quiescent endothelium through an internal autocrine loop mechanism

The angiopoietins Ang-1 and Ang-2 have been identified as ligands of the endothelial receptor tyrosine kinase Tie-2, which controls vascular assembly and endothelial quiescence. The largely complementary phenotypes of Ang-1-deficient mice and Ang-2-overexpressing mice have led to an antagonistic model in which Ang-1 acts as Tie-2-activating agonist and Ang-2 acts as a Tie-2-inhibiting antagonist. To date, no mechanistic equivalent of the antagonistic Ang-1/Ang-2 model has been established and the mechanisms of Ang-2 function in particular remain mysterious. We have studied the effector functions of Ang-1 and Ang-2 on quiescent endothelial cells using a three-dimensional co-culture model of endothelial cells and smooth-muscle cells. Endothelial-cell monolayer integrity in this model is dependent on Tie-2 signaling, as evidenced by detaching endothelial cells following exposure to the small molecular weight Tie-2 inhibitor A-422885.66, which cannot be overcome by exogenous Ang-1. Accordingly, exogenous Ang-2 rapidly destabilizes the endothelial layer, which can be observed within 30-60 minutes and leads to prominent endothelial-cell detachment within 4 hours. Exogenous Ang-2-mediated endothelial-cell detachment can be rescued by Ang-1, soluble Tie-2 and vascular endothelial growth factor. Similar findings were obtained in an umbilical-vein explant model. Ang-2 is mainly produced by endothelial cells and therefore acts primarily in an autocrine manner. Thus, stimulated release of endogenous Ang-2 or overexpression of Ang-2 in endothelial cells perturbs co-culture spheroid integrity, which can be rescued by exogenous Ang-1 and vascular endothelial growth factor. However, autocrine Ang-2-mediated endothelial-cell detachment cannot be blocked by soluble Tie-2. Taken together, the data demonstrate for the first time the antagonistic Ang-1/Ang-2 concept in a defined cellular model and identify Ang-2 as a rapidly acting autocrine regulator of the endothelium that acts through an internal autocrine loop mechanism.