Retinal and choroidal angiogenesis: pathophysiology and strategies for inhibition

Regulation of VEGF expression in human retinal cells by cytokines: implications for the role of inflammation in age-related macular degeneration

Chronic inflammation is implicated in the pathogenesis of age-related macular degeneration (AMD). Choroidal neovascularization (CNV) observed in exudative form of AMD results in vision loss. Human retinal pigment epithelial cell (HRPE) layer and choroidal tissue are the primary pathological sites in AMD. Pathological and therapeutic evidences have strongly indicated the vascular endothelial growth factor (VEGF) molecules as critical components in CNV pathogenesis. In these studies, we used human primary HRPE and choroidal fibroblast cells (HCHF) prepared from adult donor eyes. The effects of inflammatory cytokine (IFN-γ+ TNF-α+IL-1β) mix (ICM) on global gene expression profiles in HRPE cells, revealed 10- and 9-fold increase in VEGF-A and VEGF-C expression, respectively. The microarray results were validated by quantitative RT-PCR and secretion of VEGFs proteins. IL-1β is the most potent in inducing VEGFs secretion followed by IFN-γ and TNF-α, and the secretion was more effective in the presence of 2 and 3 cytokines. NF-κB and JAK-STAT pathway, but not HIF-1α, Sp-1, Sp-3, and STAT-3, transcription factors were upregulated and translocated to nucleus by ICM treatment. The mRNA levels of VEGF-A and VEGF-C and secretion of these proteins were also significantly enhanced by ICM in HCHF cells. The secretion of other angiogenic molecules, PEDF, SDF-1α, endostatin, and angiopoietins was not affected by ICM. Our results show that the inflammatory cytokines enhance secretion of VEGF-A and VEGF-C by HRPE and HCHF cells. These studies indicate that VEGFs secreted by these cells initiate and promote pathological choroidal and retinal noevascularization processes in AMD.

Comparison of choroidal and retinal endothelial cells: characteristics and response to VEGF isoforms and anti-VEGF treatments

Neovascular eye diseases such as wet age-related macular degeneration and proliferative diabetic retinopathy are two of the most common causes of irreversible visual loss. Although mediated by vascular endothelial growth factor (VEGF), the mechanisms of these diseases are not fully understood. Molecular inhibitors of VEGF including pegaptanib, ranibizumab and bevacizumab are used as treatments for these diseases. However, there have been very few direct comparisons between these agents, and as dose and treatment regimes differ their relative efficacies are hard to determine. In vitro comparisons tend to use cells from different sites or species, which show heterogeneity in their responses. The aim of this study was to compare the characteristics of primary cultures of isolated human choroidal endothelial cells (hCEC) and retinal endothelial cells (hREC), and their proliferation responses to stimulation with VEGF 121 and 165, and to compare the anti-proliferative effects of these three drugs. hCEC and hREC were positive for the cell markers VEGFR1, VEGFR2, CD31, CD34 and von Willebrand's factor (vWF), with greater expression of CD34 on the hREC compared to hCEC. Contrary to previous assumptions VEGF isoforms 121 and 165 were found to be equally potent in stimulating endothelial cell proliferation. However, hREC exhibited higher proliferation with either VEGF isoform compared to hCEC. The anti-VEGF treatments ranibizumab and bevacizumab were effective in decreasing proliferation of hCEC induced by the two VEGF isoforms, individually and in combination, with ranibizumab being moderately more effective, particularly in hREC. Pegaptanib was effective in controlling the proliferation of hCEC stimulated by VEGF 165, but was ineffective against the stimulatory effect of VEGF 121. There were found to be significant differences in microvascular endothelial cells from the retina and choroid, both in the expression of cell markers and their behaviour in response to growth factors and currently available anti-VEGF agents, highlighting the importance of targeting treatments to specific intraocular vascular beds and/or diseases.

A potential therapeutic strategy for inhibition of ocular neovascularization with a new endogenous protein: rhEDI-8t

BACKGROUND

Endogenous angiogenesis inhibitors act as natural negative feedback in the focal area during the neovascularization process, and have less interference on physiological angiogenesis, and thus fewer negative side-effects. These inhibitors are potential candidates to combine with or substitutes for current popular anti-angiogenesis treatments to have synergistic effect. In this study, the effects of recombinant endothelial growth inhibitor protein (rhEDI-8t), a novel endogenous protein originated from collagen VIII, was investigated on ocular neovascularization (NV). Endostatin, a well-identified endogenous angiogenesis inhibitor, was compared in parallel and served as a positive control.

METHODS

The inhibitory effect of rhEDI-8t on vascular endothelial cells was evaluated by a human umbilical vascular endothelial cells (HUVEC) proliferation test and a bovine aortic endothelial cells (BAEC) migration experiment. The effect of rhEDI-8t on ocular NV was further investigated in mice with choroidal neovascularization (choroidal NV) induced by laser, ischemic retinopathy and transgenic mice with expression of VEGF in photoreceptors (rho/VEGF) respectively.

RESULTS

RhEDI-8t inhibited the growth of HUVECs and migration of BAECs stimulated by basic fibroblast growth factor (bFGF). Mice intravitreally treated with rhEDI-8t showed a significant reduction of choroidal NV, retinal NV and subretinal NV.

CONCLUSION

Endogenous angiogenesis inhibitor rhEDI-8t showed a potent anti-angiogenesis effect in both in vitro and in vivo experiments. It contributed to the suppression of ocular NV. The study suggested that rhEDI-8t could be a subsidiary potent therapeutic medicine in addition to anti-VEGF therapy in future clinical anti-angiogenesis treatment.

Effects of green tea fractions on oxygen-induced retinal neovascularization in the neonatal rat

This study aimed to investigate the preventive effects of green tea fractions (GTFs) on rat model of oxygen-induced retinopathy (OIR). Neonatal Sprague-Dawley rats were exposed to daily cycles of 80% oxygen (20.5 h), ambient air (0.5 h), and progressive return to 80% oxygen (3 h) until postnatal day 12 (P12), then the rats were placed in ambient air until P18. The green tea was fractionated by DM-A50, DM-W, M-B, and M-W. The rats were treated once daily from P6 to P17 by gastric gavage of GTFs (0.05 or 0.01 g/ml) or distilled water (DW) at 50 µl/10 g body weight. On P18, the rats were sacrificed and the retinal samples were collected. The retinal neovascularization (NV) was scored and avascular areas (AVAs) were measured as a % of total retinal area (%AVAs) in ADPase stained retinas. The NV scores in 0.01 g/ml M-W were significantly lower than those in DW. The %AVAs in 0.05 g/ml DM-A50 and in 0.05 g/ml and 0.01 g/ml M-W were significantly lower than those in DW. There were less catechins, and less caffeine in M-W fraction compared with other GTFs, suggesting components of green tea except for catechins and caffeine might suppress the neovascularization in rat model of OIR.

Age related macular degeneration and drusen: neuroinflammation in the retina

Inflammation protects from dangerous stimuli, restoring normal tissue homeostasis. Inflammatory response in the nervous system ("neuroinflammation") has distinct features, which are shared in several diseases. The retina is an immune-privileged site, and the tight balance of immune reaction can be disrupted and lead to age-related macular disease (AMD) and to its peculiar sign, the druse. Excessive activation of inflammatory and immunological cascade with subsequent induction of damage, persistent activation of resident immune cells, accumulation of byproducts that exceeds the normal capacity of clearance giving origin to a chronic local inflammation, alterations in the activation of the complement system, infiltration of macrophages, T-lymphocytes and mast-cells from the bloodstream, participate in the mechanisms which originate the drusen. In addition, aging of the retina and AMD involve also para-inflammation, by which immune cells react to persistent stressful stimuli generating low-grade inflammation, aimed at restoring function and maintaining tissue homeostasis by varying the set point in relation to the new altered conditions. This mechanism is also seen in the normal aging retina, but, in the presence of noxious stimuli as in AMD, it can become chronic and have an adverse outcome. Finally, autophagy may provide new insights to understand AMD pathology, due to its contribution in the removal of defective proteins. Therefore, the AMD retina can represent a valuable model to study neuroinflammation, its mechanisms and therapy in a restricted and controllable environment. Targeting these pathways could represent a new way to treat and prevent both exudative and dry forms of AMD.

CCR3 and choroidal neovascularization

Age-related macular degeneration (AMD) is the leading cause of irreversible blindness in the elderly in industrialized countries. The "wet" AMD, characterized by the development of choroidal neovacularization (CNV), could result in rapid and severe loss of central vision. The critical role of vascular endothelial growth factor A (VEGF-A) in CNV development has been established and VEGF-A neutralization has become the standard care for wet AMD. Recently, CCR3 was reported to play an important role in CNV development and that CCR3 targeting was reported to be superior to VEGF-A targeting in CNV suppression. We investigated the role of CCR3 in CNV development using the Matrigel induced CNV and found that in both rats and mice, CNV was well-developed in the control eyes as well as in eyes treated with CCR3 antagonist SB328437 or CCR3 neutralizing antibodies. No statistically significant difference in CNV areas was found between the control and SB328437 or CCR3-ab treated eyes. Immunostaining showed no specific expression of CCR3 in or near CNV. In contrast, both VEGF-A neutralizing antibodies and rapamycin significantly suppressed CNV. These results indicate that CCR3 plays no significant role in CNV development and question the therapeutic approach of CCR3 targeting to suppress CNV. On the other hand, our data support the therapeutic strategies of VEGF-A and mTOR (mammalian target of rapamycin) targeting for CNV.

Effects of intravitreal bevacizumab and laser in retinopathy of prematurity therapy on the development of peripheral retinal vessels

OBJECTIVE

To investigate laser photocoagulation and intravitreal bevacizumab efficacy and safety in patients with moderate-to-severe stage 3 retinopathy of prematurity (ROP), and to evaluate the effects of treatment on the development of peripheral retinal vessels.

METHODS

A retrospective chart review of 15 premature babies, all of whom were diagnosed with stage 3 ROP, was conducted. Patients with moderate-to-severe stage 3 ROP, thus with a vascular-active ROP, received intravitreal injections of bevacizumab (0.5 mg/0.02 ml) and laser photocoagulation, whereas those with relatively inactive ROP received laser photocoagulation only. Patients were examined 1, 2, 4, and 8 weeks after treatment, or until peripheral retinal vessel growth over the laser scar was noted.

RESULTS

Both eyes of 15 patients diagnosed with moderate-to-severe stage 3 ROP were evaluated. Eight patients (n=16 eyes) received intravitreal bevacizumab injection and laser photocoagulation, and seven patients (n=14 eyes) received laser photocoagulation only. During the follow-up period, regression of plus disease and peripheral retinal vessel development appeared significantly more rapidly in patients who received both intravitreal bevacizumab injection and laser photocoagulation. Peripheral retinal vessel development over the laser scar was identified 1-2 weeks after treatment. No systemic or significant ocular complications, such as vitreous hemorrhage, retinal detachment, or endophthalmitis, were noted during follow-up after treatment.

CONCLUSIONS

A combination of laser photocoagulation and intravitreal bevacizumab injection seems to be a safe and effective therapy in patients with moderate-to-severe stage 3 ROP, promulgating rapid development of peripheral retinal vessels.

Retinopathy of prematurity: understanding ischemic retinal vasculopathies at an extreme of life

Retinopathy of prematurity (ROP) is a major complication of preterm birth. It encompasses a spectrum of pathologies that affect vision, from mild disease that resolves spontaneously to severe disease that causes retinal detachment and subsequent blindness. The pathologies are characterized by an arrest in normal retinal vascular development associated with microvascular degeneration. The resulting ischemia and retinal hypoxia lead to excessive abnormal compensatory blood vessel growth. However, this neovascularization can lead to fibrous scar formation and culminate in retinal detachment. Present therapeutic modalities to limit the adverse consequences of aberrant neovascularization are invasive and/or tissue-destructive. In this Review, we discuss current concepts on retinal microvascular degeneration, neovascularization, and available treatments, as well as present future perspectives toward more profound elucidation of the pathogenesis of ROP.

Therapeutic effect of oral bisphosphonates on choroidal neovascularization in the human eye

Purpose. Choroidal neovascularization (CNV) is often associated with age-related macular degeneration (AMD) and pathological myopia (PM). Bisphosphonates, the drug of choice to treat osteoporosis, have been recently reported to have anti-angiogenic effects. The purpose of this study is to investigate the therapeutic effects of oral bisphosphonates for CNV in humans. Methods. Thirty-six consecutive cases with CNV due to AMD or PM who declined anti-VEGF therapy were recruited. The patients were prescribed 5 mg of oral alendronates daily for 6 months. The best-corrected visual-acuity (BCVA), the lesion size in fundus photographs and fluorescein angiography, foveal thickness and total macular volume in optical coherence tomography were compared between pre- and post-treatment. Results. The mean BCVA of the patients was significantly improved after a months with the treatment in the AMD group. In the PM group, the mean BCVA was maintained up to 6 months with the treatment. The mean lesion size was significantly decreased by 3 months in both groups. The averages of foveal thickness and total macular volume were significantly reduced after 1 month of treatment in the AMD group.Conclusions. Oral bisphosphonate should be further investigated as a possible therapeutic and preventive drug for CNV due to AMD and PM.

A subretinal matrigel rat choroidal neovascularization (CNV) model and inhibition of CNV and associated inflammation and fibrosis by VEGF trap

PURPOSE

The exudative, or the wet form of age-related macular degeneration (AMD) is characterized by choroidal neovascularization (CNV). A subretinal Matrigel (BD Biosciences, Bedford MA) model of CNV is described here, along with the effects of vascular endothelial growth factor (VEGF) neutralization on the development of CNV and associated inflammation and fibrosis.

METHODS

CNV was induced in adult Sprague-Dawley rats by subretinal injection of Matrigel. CNV growth and associated leukocyte infiltration and collagen deposition were examined. VEGF Trap (Regeneron Pharmaceuticals, Tarrytown, NY), a recombinant protein that comprises portions of the extracellular domains of VEGF receptors 1 and 2 and that binds all isoforms of VEGF-A as well as placental growth factor with high affinity, was administered subcutaneously.

RESULTS

Initiation of CNV was detected 4 days after Matrigel injection and then increased progressively in size. Systemic administration of VEGF Trap beginning on day 2 and 6 completely prevented development of CNV. When CNV was allowed to develop for 10 days before treatment was initiated, VEGF Trap not only prevented its further progression, but also induced substantial regression of existing lesions. In addition, VEGF Trap treatment reduced the total lesion volume and largely prevented the progressive leukocyte infiltration and fibrosis associated with CNV.

CONCLUSIONS

The subretinal Matrigel CNV model provides a convenient tool for the study of the diverse components of complex CNV lesions. The data not only confirm the critical roles of VEGF in the development and maintenance of CNV, but further demonstrate that VEGF and other VEGF receptor 1 ligands promote CNV-associated inflammation and fibrosis.

Doxycycline's effect on ocular angiogenesis: an in vivo analysis

PURPOSE

To determine the in vivo effect of doxycycline on choroidal angiogenesis and pterygium growth by using a choroidal neovascular (CNV) murine model, a directed in vivo angiogenesis assay (DIVAA) and a pterygium murine model.

DESIGN

Experimental study.

PARTICIPANTS

Three murine models were investigated with 4 mice minimum per group and 22 maximum per group.

METHODS

Mice received water with or without doxycycline. For the CNV, the neovascular lesion volume was determined in choroid-retinal pigment epithelial flat mounts using confocal microscopy 7 days after laser induction. For DIVAA, silicone capsules containing 10,000 human pterygium epithelial cells were implanted in the flanks of mice subcutaneously. After 11 days, neovascularization (NV) was quantified using spectrofluorometry after murine tail-vein injection of fluorescein isothiocyanate-labeled dextran. A pterygium epithelial cell model was developed by injecting 10,000 human pterygium epithelial cells in the nasal subconjunctival space in athymic nude mice. Doxycycline was started on day 6 at 50 mg/kg per day; corneal lesions that resulted from the injections were compared at days 6 and 15.

MAIN OUTCOME MEASURES

The Student t-test was used to evaluate the data for the CNV and DIVAA models and histologic preparations were used to evaluate pterygia lesions.

RESULTS

There was significantly less NV and lesion volume with doxycycline taken in drinking water versus plain water. With doxycycline treatment, the laser-induced CNV showed a maximal 66% decrease in choroidal blood vessel volume (P< or =0.008) and the DIVAA showed a 30% reduction of blood vessel growth and migration (P<0.004). Histologic preparations demonstrated that pterygium cell lesions regressed when mice were administered doxycycline for 9 days.

CONCLUSIONS

Doxycycline significantly inhibited angiogenesis in 3 murine models. The most dramatic effect was found in the CNV model followed by the pterygia epithelial cell DIVAA model. The anterior segment pterygium model also showed regression histologically. This suggests that doxycycline may be successful as an adjunctive treatment for CNV and pterygia in humans; clinical trials would be necessary to determine if there is a benefit.

Sorafenib protects human optic nerve head astrocytes from light-induced overexpression of vascular endothelial growth factor, platelet-derived growth factor, and placenta growth factor

OBJECTIVES

Growth factors, such as vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and placenta growth factor (PlGF) are key players in the development of diabetic retinopathy, age-related macular degeneration, and other retinal neovascular diseases. Glial cells provide a significant source of retinal growth factor production under physiologic and pathologic conditions. Cumulative light exposure has been linked to increased retinal growth factor expression. Previous reports indicate that sorafenib, an oral multikinase inhibitor, might have a beneficial effect on retinal neovascularization. This study was designed to investigate the effects of sorafenib on light-induced overexpression of growth factors in human retinal glial cells.

METHODS

Primary human optic nerve head astrocytes (ONHAs) were exposed to white light and incubated with sorafenib. Viability, expression, and secretion of VEGF-A, PDGF-BB, and PlGF and their mRNA were determined by reverse transcription-polymerase chain reaction, immunohistochemistry, and enzyme-linked immunosorbent assay.

RESULTS

Light exposure decreased cell viability and increased VEGF-A, PDGF-BB, and PlGF expression and secretion. These light-induced effects were significantly reduced when cells were treated with sorafenib at a concentration of 1 microg/ml.

CONCLUSION

Sorafenib significantly reduced light-induced overexpression of VEGF-A, PDGF-BB, and PlGF in primary human ONHAs. Sorafenib has promising properties as a potential supportive treatment for retinal neovascularization.

Clinical safety of ranibizumab in age-related macular degeneration

IMPORTANCE OF THE FIELD

Clinical safety of pharmaceutical products in the elderly is vital because of their increased risk of cardiac and other adverse events.

AREAS COVERED IN THIS REVIEW

Search of the Medline database, including articles and abstracts from 1984 to 2009.

WHAT THE READER WILL GAIN

Knowledge of ocular and systemic risks: The rate of endophthalmitis was 0.05% per injection (MARINA) and <0.1% per injection (ANCHOR), rates confirmed in a retrospective analysis of 14,320 injections. Moderate increases in intraocular pressure were transient, and incidences of intraocular inflammation were rarely serious. Systemic arterial thromboembolic events occurred in 4.6 and 0% of ranibizumab-treated patients and in 3.8 and 0% of sham-treated patients in MARINA (2 years) and PIER (1 year), respectively. In SAILOR, there was a numerically higher rate of cerebrovascular stroke with 0.5 mg ranibizumab compared with 0.3 mg ranibizumab (1.2 vs 0.7%), which was a non-statistically significant trend in patients with a history of stroke.

TAKE HOME MESSAGE

Although further studies to investigate the risk of stroke with ranibizumab therapy are required, repeated intravitreal ranibizumab was well tolerated and not associated with clinically significant safety risks during up to 2 years of treatment.

Retinal vascular leakage occurring in GABA Rho-1 subunit deficient mice

Recent studies demonstrate that GABAergic activity elicits relaxation of retinal arterioles leading to an increase in blood flow. It has also been found that GABAnergic activity in the retina of mice with diabetic retinopathy is suppressed. In this study, we provide further evidence that lack of GABAergic activity significantly alters vasculature development as well as the hypoxia-induced angiogenic response. Using GABA(C) receptor rho(1) subunit-knockout mice (rho-1(-/-)), our results demonstrate that in hypoxia-induced retinas a severe vascular leakage occurred in 2 week-old rho-1(-/-) mice compared with their wildtype counterparts. In addition, our results also showed that all of the rho-1(-/-) mice developed significant retinal vascular leakages by 48 weeks-of-age. Microarray and real-time PCR experiments revealed a unique angiogenesis-related gene expression pattern. This suggests that retinal vascular disorders of rho-1(-/-) mice results from significant up-regulation of angiogenic genes and concomitant down-regulation of anti-angiogenic genes. The study results are consistent with the pathological changes of the retinal vascular leakage seen in diabetic retinopathy. Our data indicate that the GABA(C) rho(1) subunit plays a role in maintaining both homeostasis and balance of retinal neurotransmitter function. Knockout of the retinal GABA(C) rho(1)-subunit leads to changes in vascular permeability similar to the pathological changes induced by retinal hypoxic conditions.

Maintaining retinal astrocytes normalizes revascularization and prevents vascular pathology associated with oxygen-induced retinopathy

Astrocytes are well known modulators of normal developmental retinal vascularization. However, relatively little is known about the role of glial cells during pathological retinal neovascularization (NV), a leading contributor to vision loss in industrialized nations. We demonstrate that the loss of astrocytes and microglia directly correlates with the development of pathological NV in a mouse model of oxygen-induced retinopathy (OIR). These two distinct glial cell populations were found to have cooperative survival effects in vitro and in vivo. The intravitreal injection of myeloid progenitor cells, astrocytes, or astrocyte-conditioned media rescued endogenous astrocytes from degeneration that normally occurs within the hypoxic, vaso-obliterated retina following return to normoxia. Protection of the retinal astrocytes and microglia was directly correlated with accelerated revascularization of the normal retinal plexuses and reduction of pathological intravitreal NV normally associated with OIR. Using astrocyte-conditioned media, several factors were identified that may contribute to the observed astrocytic protection and subsequent normalization of the retinal vasculature, including vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). Injection of VEGF or bFGF at specific doses rescued the retinas from developing OIR-associated pathology, an effect that was also preceded by protection of endogenous glia from hypoxia-induced degeneration. Together, these data suggest that vascular-associated glia are also required for normalized revascularization of the hypoxic retina. Methods developed to target and protect glial cells may provide a novel strategy by which normalized revascularization can be promoted and the consequences of abnormal NV in retinal vascular diseases can be prevented.

Chronic whole-body hypoxia induces intussusceptive angiogenesis and microvascular remodeling in the mouse retina

Currently, little is known about the response of the adult retinal microvasculature to hypoxia. To test the hypothesis that chronic systemic hypoxia induces angiogenesis and microvascular remodeling in the adult mouse retina, adult 10-week old female C57Bl/6 mice were exposed to 10% O(2) for 2 or 3 weeks. After hypoxia exposure, retinas were harvested, whole-mounted, and processed for immunohistochemistry. Retinas were stained with lectin, anti-smooth muscle alpha-actin antibody, and anti-NG2 antibody to visualize microvascular networks and their cellular components. Confocal microscopy was used to obtain images of superficial retinal networks. Images were analyzed to assess vessel diameter, vascular length density, branch point density, and the presence of vascular loops, a hallmark of intussusceptive angiogenesis. Both 2 and 3 weeks of hypoxia exposure resulted in a significant increase in the diameters of arterioles and post-arteriole capillaries (p<0.003). After 3 weeks of hypoxia, vascular length density and branch point density were significantly increased in retinas exposed to hypoxia as compared to normoxic controls (p<0.001). The number of vascular loops in the superficial retinal networks was significantly greater in hypoxia-exposed retinas (p < or = 0.001). Our results demonstrate, for the first time, intussusceptive angiogenesis as a tissue-level mechanism of vascular adaptation to chronic systemic hypoxia in the adult mouse retina and contribute to our understanding of hypoxia-induced angiogenesis and microvascular remodeling in the adult animal.

Effect of flavone on the ocular blood flow and formation of choroidal neovascularization

AIM

To investigate the effect of flavone on ocular blood flow in rabbit eyes and the formation of choroidal neovascularization (CNV) in rat model of age-related macular degeneration (AMD).

METHODS

In in vivo studies, colored microsphere technique was used to determine the ocular blood flow in ocular hypertensive rabbit eyes. The rat eyes were treated with 0.5% flavone eye drops 3 times a day for 1 week before and 4 weeks after laser-induced injury of Bruch's membrane. The development of CNV was determined by fluorescein angiography (FA) performed on the 2nd and 4th after injury. In in vitro studies, the effect of flavone on the viability of human umbilical vein endothelial cells HUVECs was measured by MTT assay.

RESULTS

The ocular blood flow in rabbit eyes was significantly increased after flavone instillation. Flavone significantly inhibited the formation of laser induced CNV. In vitro results showed that flavone inhibited the proliferation of HUVECs.

CONCLUSION

Flavone could increase ocular blood flow and inhibit the formation of CNV.

Peptide hormone regulation of angiogenesis

It is now apparent that regulation of blood vessel growth contributes to the classical actions of hormones on development, growth, and reproduction. Endothelial cells are ideally positioned to respond to hormones, which act in concert with locally produced chemical mediators to regulate their growth, motility, function, and survival. Hormones affect angiogenesis either directly through actions on endothelial cells or indirectly by regulating proangiogenic factors like vascular endothelial growth factor. Importantly, the local microenvironment of endothelial cells can determine the outcome of hormone action on angiogenesis. Members of the growth hormone/prolactin/placental lactogen, the renin-angiotensin, and the kallikrein-kinin systems that exert stimulatory effects on angiogenesis can acquire antiangiogenic properties after undergoing proteolytic cleavage. In view of the opposing effects of hormonal fragments and precursor molecules, the regulation of the proteases responsible for specific protein cleavage represents an efficient mechanism for balancing angiogenesis. This review presents an overview of the actions on angiogenesis of the above-mentioned peptide hormonal families and addresses how specific proteolysis alters the final outcome of these actions in the context of health and disease.

Plasminogen activator inhibitor-1 (PAI-1) facilitates retinal angiogenesis in a model of oxygen-induced retinopathy

PURPOSE

Angiogenesis, or the formation of new retinal blood vessels, is a key feature of many proliferative retinal diseases including diabetic retinopathy, retinal vein occlusion, and retinopathy of prematurity. The aim of the present study was to investigate the role of the serine proteinase inhibitor plasminogen activator inhibitor -1 (PAI-1) in facilitating retinal angiogenesis.

METHODS

The temporal expression of PAI-1 was examined by real-time PCR, Western blot analysis, and immunohistochemistry in retinal tissues from mice with oxygen-induced retinopathy. The requirement for PAI-1 in facilitating the retinal angiogenic response in this model was examined by quantitating the angiogenic response with wild-type and PAI-1 null mice. The mechanism by which PAI-1 mediates angiogenesis was further investigated with isolated human retinal vascular endothelial cells.

RESULTS

PAI-1 expression was upregulated in the retinas of mice with oxygen-induced retinopathy, which coincided with a significant increase in the expression of vitronectin in the retina of the experimental mice. There was significant reduction in the angiogenic response of PAI-1(-/-) mice compared with wild-type mice. PAI-1 promotes endothelial cell migration in vitro and facilitates the migration of cells on a vitronectin substrate by regulating alpha v integrin cell surface expression.

CONCLUSIONS

These observations suggest a role for PAI-1 during retinal angiogenesis and point to a potential new therapeutic target in the prevention or treatment of retinal neovascularization seen in many ocular diseases.

Role of Urokinase Inhibitors in Choroidal Neovascularization

Cell migration is a critical step in the angiogenesis cascade that involves proteolysis of the basement membrane and extracellular matrix around existing blood vessels. The urokinase plasminogen activator (uPA) system has been involved in cellular invasion, angiogenesis and tumor growth. Similar expression of urokinase and its receptor (uPAR) is seen in both retinal and choroidal neovascularization. Significant inhibition of choroidal neovascularization (CNV) has been observed when cell surface associated uPA-uPAR activity is prevented with a specific inhibitor of this proteinase system. As the current treatments of CNV are not optimal, the urokinase-uPAR system appears to be an attractive target for alternative pharamacological therapy for CNV.

Antiangiogenic immunotherapy targeting Flk-1, DNA vaccine and adoptive T cell transfer, inhibits ocular neovascularization

Ocular neovascularization (NV) is the primary cause of blindness in a wide range of ocular diseases. The exact mechanism underlying the pathogenesis of ocular NV is not yet well understood, and so there is no satisfactory therapy for ocular NV. Here, we describe a strategy targeting Flk-1, a self-antigen overexpressed on proliferating endothelial cells in ocular NV, by antiangiogenic immunotherapy-DNA vaccine and adoptive T cell therapy. An oral DNA vaccine encoding Flk-1 carried by attenuated Salmonella typhimurium markedly suppressed development of laser-induced choroidal NV. We further demonstrated that adoptive transfer of vaccine-induced CD8+ T cells reduced pathological preretinal NV, with a concomitant facilitation of physiological revascularization after oxygen-induced retinal vessel obliteration. However, physiological retinal vascular development was unaffected in neonatal mice transferred with vaccine-induced CD8+ T cells. These findings suggested that antiangiogenic immunotherapy targeting Flk-1 such as vaccination and adoptive immunotherapy may contribute to future therapies for ocular NV.

Anti-angiogenic effects of non-peptide integrin alphavbeta3 specific antagonist on laser-induced choroidal neovascularization in mice

BACKGROUND

To evaluate the anti-angiogenic effects of integrin alphavbeta3 specific antagonist BS-1417 on laser-induced choroidal neovascularization (CNV) in mice.

METHODS

Male C57BL/6 mice were treated with daily intraperitoneal injections of BS-1417 or saline starting at the onset (day 0) of experiments. CNV was induced by laser photocoagulation the next day. Fluorescein angiograms (FA) and choroidal flatmount FITC-dextran perfusion were performed on experimental day 8. Histological and immunohistochemical examinations were performed with consecutive cryosections. Sub-confluent human vascular endothelial cells (HUVEC) were grown in vitro under various concentrations (0-10 microg/ml) of BS-1417 and the numbers of cell were measured at 48 hours of incubation. After fixation, immunocytochemistry was performed.

RESULTS

BS-1417 significantly decreased the area of dye leakage determined by FA (40% of control, p = 0.0008) and reduced the CNV size found on choroidal flatmount (18% of control, p = 0.007). In histological findings, BS-1417 apparently suppressed the size of laser-induced CNV. Immunoreactivities for VEGF and integrin alphav were remarkably attenuated with BS-1417 compared to control. BS-1417 inhibited the growth and VEGF expression of HUVEC in vitro.

CONCLUSIONS

The integrin alphavbeta3 may play a key role in the induction of laser-induced CNV. The antagonists for integrin alphavbeta3 may have therapeutic effects in CNV associated diseases such as age-related macular degenerations.

Beta-adrenergic receptor regulation of growth factor protein levels in human choroidal endothelial cells

Remodeling of the choroidal vasculature is a prominent factor in age-related macular degeneration. While many of the growth factors involved in this vascular remodeling are known, their regulation remains much less so. The hypothesis of the present study was that stimulation of human choroidal endothelial cells with the beta-adrenergic receptor agonist isoproterenol would lead to an increase in pigment epithelial derived factor (PEDF) and angiopoietin 1 (Ang1), markers of a stable vasculature. Protein levels of PEDF and Ang1 were significantly increased following stimulation with isoproterenol. However, isoproterenol also significantly increased protein levels of vascular endothelial cell growth factor, which is active during vasculature remodeling. These data suggest that beta-adrenergic receptor agonists are likely upstream of a number of growth factors implicated in ocular disease and have multiple effects on choroidal endothelial cells. Modulation of this signaling in the choroid may offer a new avenue for therapeutics.

Vasoinhibins: novel inhibitors of ocular angiogenesis

Disruption of the quiescent state of blood vessels in the retina leads to aberrant vasopermeability and angiogenesis, the major causes of vision loss in diabetic retinopathy. Prolactin is expressed throughout the retina, where it is proteolytically cleaved to vasoinhibins, a family of peptides (including the 16-kDa fragment of prolactin) with potent antiangiogenic, vasoconstrictive, and antivasopermeability actions. Ocular vasoinhibins act directly on endothelial cells to block blood vessel growth and dilation and to promote apoptosis-mediated vascular regression. Also, vasoinhibins prevent retinal angiogenesis and vasopermeability associated with diabetic retinopathy, and inactivation of endothelial nitric oxide synthase via protein phosphatase 2A is among the various mechanisms mediating their actions. Here, we discuss the potential role of vasoinhibins both in the maintenance of normal retinal vasculature and in the cause and prevention of diabetic retinopathy and other vasoproliferative retinopathies.

Suppression of retinal neovascularization with an antagonist to vascular endothelial cadherin

OBJECTIVES

To examine the role of vascular endothelial cadherin (VE-cadherin) in cellular processes underlying angiogenesis and the effects of VE-cadherin inhibition on retinal angiogenesis.

METHODS

Retinal neovascularization was induced in newborn mice by exposure to 75% oxygen (postnatal days 7-12) followed by room air and quantitated from histological sections. Mice received daily intraperitoneal injections of either a VE-cadherin antagonist or a control peptide from postnatal days 12 to 17. In vitro cell migration, proliferation, and tubule formation were examined in the presence of the VE-cadherin antagonist. The effect of antagonist treatment on the integrity of established cell junctions was examined by fluorescein isothiocyanate-dextran monolayer permeability and VE-cadherin immunocytochemistry.

RESULTS

Treatment with the VE-cadherin antagonist significantly reduced retinal angiogenesis. Inhibition of VE-cadherin function suppressed tubule formation in endothelial cells. The antagonist treatment also decreased cell migration and proliferation. The antagonist treatment did not affect the integrity of existing cell junctions. Immunostaining for VE-cadherin and rates of monolayer permeability were comparable to those in untreated controls.

CONCLUSION

Our study points to a pivotal role played by VE-cadherin in the angiogenic process.

CLINICAL RELEVANCE

Inhibition of VE-cadherin might be an effective strategy for pharmacological inhibition in proliferative retinopathies.

Downregulation of p22phox in retinal pigment epithelial cells inhibits choroidal neovascularization in mice

Choroidal neovascularization (CNV) occurs in a variety of chorioretinal diseases including age-related macular degeneration (AMD), and is the major cause of severe visual loss in patients with AMD. Oxidative stress has been thought to play an important role in the development of CNV. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase is one of the major intracellular sources of reactive oxygen species (ROS) in the vascular system. In this study, we examined the expression of p22phox, an integral subunit in the NADPH oxidase complex, in the mouse eye. We determined that p22phox is expressed in the retinal pigment epithelial (RPE) cells and inner retinal neurons. A small-interfering RNA (siRNA) designed against p22phox efficiently reduced the expression of the protein in the eye when delivered by means of recombinant adeno-associated virus (AAV) vector. Vector treatment inhibited CNV in the mouse when delivered into the subretinal space where RPE cells were transduced. These results suggest that NADPH oxidase-mediated ROS production in RPE cells may play an important role in the pathogenesis of neovascular AMD, and that this pathway may represent a new target for therapeutic intervention in AMD.

The role of vascular endothelial growth factor and other endogenous interplayers in age-related macular degeneration

Age-related macular degeneration (AMD) is a multifaceted disease characterized by early subclinical changes at the choroidea-retinal pigment epithelium interface. Both the causal and formal pathogenesis of the disease is still puzzling. Similarly, the reason for progression into two distinct late forms which are "geographic atrophy" and "choroidal neovascularization" remains enigmatic. Late changes are usually responsible for the dramatic loss in central function that has a devastating effect on quality of life. In industrialized countries the disease is a major cause for visual disability among persons over 60 years of age. Due to demographic right-shift and increased life expectancy, AMD is not only a medical problem but will have a pronounced socio-economic effect. Neovascular AMD with the development of choroidal neovascularization in the macular area accounts for 80% of the severe loss of visual acuity due to AMD. In the last decades, treatment modes were merely based on the destruction or surgical removal of the neovascular complex. In the present, however, the philosophical approach to treat the disease is changing to a pathology modifying manner. Intelligent targeting of the involved relevant factors and pathways should stop disease progression, reduce complications and improve vision. The first step into this new era has been accomplished with the introduction of antiangiogenic agents. The new agents act either directly on vascular endothelial growth factor (VEGF) or indirectly on its functional cascade. VEGF makes a fundamental contribution to neovascular processes but it also acts in physiological pathways. The main purpose of this review is to summarize its physiological role especially within the eye, the role in the development of AMD and to understand and foresee both the benefits and potential side-effects of the anti-VEGF-based therapy.

MCP-1 deficiency delays regression of pathologic retinal neovascularization in a model of ischemic retinopathy

PURPOSE

The present study investigates whether retinal neovascularization (NV) and apoptosis are altered in MCP-1-deficient ((-/-)) mice in the OIR model.

METHODS

Postnatal day (P) 7 MCP-1(-/-) and C57BL/6 (B6) mice were exposed to 75% oxygen for 5 days and then recovered in room air. Immunostaining was performed to localize macrophages/microglia within retinal whole mounts and cross-sections. Retinopathy was qualitatively assessed in FITC-dextran-perfused retinas, and preretinal NV was quantified on P17, P21, and P24. TUNEL analysis was used to compare apoptosis between B6 and MCP-1(-/-) mice.

RESULTS

MCP-1(-/-) and B6 mice revealed normal vascular development in room air controls and similar vaso-obliteration in oxygen-exposed mice on P12. MCP-1(-/-) mice exhibited significantly reduced vascular tuft-associated F4/80(+) cells compared with B6 mice. FITC-dextran-perfused retinas exhibited prominent neovascular tufts on P17, and quantification of preretinal nuclei revealed no significant differences between MCP-1(-/-) and B6 mice. In contrast, on P21 and P24, MCP-1(-/-) mice exhibited significant increases in preretinal neovascular nuclei compared with B6 controls. These increases in NV in the MCP-1(-/-) mice were associated with a significant reduction in vascular tuft apoptosis.

CONCLUSIONS

The results demonstrate that the absence of MCP-1 does not alter normal retinal vascular development. Furthermore, MCP-1(-/-) mice exhibit a similar neovascular response on P17. However, the reduction in tuft-associated macrophages/microglia in the MCP-1(-/-) mice correlates with reduced vascular tuft apoptosis and delayed regression of retinal NV. These findings suggest that macrophages/microglia may contribute to tuft regression through their proapoptotic properties.

Insulin-like growth factors, angiopoietin-2, and pigment epithelium-derived growth factor in the hypoxic retina

As the response of the adult retina to hypoxia is likely to differ from that already established in the neonatal animal, this study was undertaken to examine the expression patterns of insulin-like growth factor-I (IGF-I) and -II (IGF-II), angiopoietin-2 (Ang-2), and pigment epithelium-derived growth factor (PEDF) in normal and hypoxic retinas of adult rats. In the latter, the retinas were examined from 3 hr to 14 days after hypoxic exposure. The mRNA and protein expression of IGF-I, IGF-II, Ang-2, and PEDF in the retina was determined by real-time RT-PCR, Western blotting, and immunohistochemistry. The results showed up-regulated expression of IGF-I, IGF-II, and Ang-2 mRNA and protein in response to hypoxia, whereas PEDF expression was drastically reduced, suggesting that increased expression of IGF-I and IGF-II may be involved not only in neovascularization but also in neuroprotection in hypoxic conditions. The up-regulation of Ang-2, a proangiogenic factor, and the down-regulation of PEDF, an antiangiogenic factor, is indicative of an imbalance between pro- and antiangiogenic factors in the hypoxic retina that may favor neovascularization. This was supported by the increased density of rat endothelial cell antigen-1 (RECA-1) protein quantification and RECA-1-stained blood vessels in the inner retina.

Evaluation of the vascular targeting agent combretastatin a-4 prodrug on retinal neovascularization in the galactose-fed dog

PURPOSE

Combretastatin A-4 (CA-4) is a vascular targeting agent known to rapidly shut off blood flow in new vessels and, as a result, regress neovascularization. In this pilot study, the ability of CA-4 to modify retinal neovascularization, which results in altered retinal vessel blood flow and retinal permeability, was evaluated in aphakic long-term galactose-fed beagles, an animal model that develops diabetes-like retinal neovascularization.

METHODS

Two (2) groups of aphakic dogs, each group comprised of 4 galactose-fed dogs and 2 age-matched controls dogs, were utilized. Each group initially received the combretastatin A-4-phosphate prodrug (CA-4P) as either sub-Tenon's injections, administered at the corneoscleral junction, or intravitreal injections. Six (6) weeks after this treatment, all dogs also received systemic (intravenous) injections of CA-4P. Retinal vascular changes were monitored at 2-week intervals by fluorescein angiography.

RESULTS

All galactose-fed dogs demonstrated the presence of retinal neovascular lesions by fluorescein angiograms. Fluorescein leakage or perfusion through neovascular vessels was not altered by either sub-Tenon's, intravitreal, or systemic CA-4P administration. Whereas CA-4P was well tolerated by the healthy eyes of the control animals, its administration to some galactose-fed dogs was associated with corneal edema and increases in intraocular pressure following sub-Tenon's and intraocular injections.

CONCLUSIONS

Neovascularization in the galactose-fed dog progresses over a period of years, similar to that observed with clinical diabetic retinopathy. The failure of CA-4P to ameliorate neovascularization suggests that chronic, long-term administration may be required to destroy the slowly growing retinal endothelial cells.

AQ4, an antitumor anthracenedione, inhibits endothelial cell proliferation and vascular endothelial growth factor secretion: implications for the therapy of ocular neovascular disorders

Current clinical options for the treatment of neovascular disorders of the posterior segment of the eye have several drawbacks. Photocoagulation lasers can impair peripheral and night vision. Photodynamic therapies as well as intravitreal macromolecule injections (Macugen and Lucentis) require frequent, invasive administrations. Above all, only modest improvement in vision is achieved with any of the existing treatments. In order to overcome these limitations in the long run, this study investigated the antiangiogenic potential of AQ4, a low molecular weight anthracenedione. The results indicate that AQ4 enters the cell nucleus and inhibits proliferation of choroid-retina endothelial (RF/6A) cells and human retinal pigment epithelial (ARPE-19) cells under hypoxic (1% O(2)) as well as normoxic (21% O(2)) conditions. The IC(50) for these effects ranges from 5.5 to 6.9 muM. AQ4 does not affect the viability of non-dividing RF/6A or ARPE-19 cells up to 0.1 mM. Further, AQ4 (20 muM) reduces vascular endothelial growth factor (VEGF) protein secretion by about 50% in ARPE-19 cells under normoxia as well as hypoxia, possibly by reducing VEGF transcription. AQ4 arrests the growth of endothelial cells in S phase, consistent with interference of AQ4 with DNA replication. These results for the first time suggest that AQ4 can potentially alleviate the neovascularization of choroid/retina by a dual mechanism of inhibiting the proliferation of endothelial cells and by reducing mitogenic VEGF stimulus from retinal pigment epithelial cells.

Mechanism of action of the angiostatic cortisene anecortave acetate

Anecortave acetate is a unique angiostatic agent. Although derived from the glucocorticoid cortisol acetate, it was designed to be devoid of glucocorticoid activity while retaining efficacious anti-angiogenic activity. These modifications have led to a new class of anti-angiogenic agents, the angiostatic cortisenes. Anecortave acetate has broad based angiostatic activity, inhibiting neovascularization at multiple steps. Unlike several other angiostatic agents that selectively target only one angiogenic factor, anecortave acetate inhibits neovascularization induced by many different angiogenic factors.

Transplantation of the RPE in AMD

The retinal pigment epithelium (RPE) maintains retinal function as the metabolic gatekeeper between photoreceptors (PRs) and the choriocapillaries. The RPE and Bruch's membrane (BM) suffer cumulative damage over lifetime, which is thought to induce age-related macular degeneration (AMD) in susceptible individuals. Unlike palliative pharmacologic treatments, replacement of the RPE has a curative potential for AMD. This article reviews mechanisms leading to RPE dysfunction in aging and AMD, laboratory studies on RPE transplantation, and surgical techniques used in AMD patients. Future strategies using ex vivo steps prior to transplantation, BM prosthetics, and stem cell applications are discussed. The functional peculiarity of the macular region, epigenetic phenomena leading to an age-related shift in protein expression, along with the accumulation of lipofuscin may affect the metabolism in the central RPE. Thickening of BM with age decreases its hydraulic conductivity. Drusen are deposits of extracellular material and formed in part by activation of the alternative complement pathway in individuals carrying a mutant allele of complement factor H. AMD likely represents an umbrella term for a disease entity with multifactorial etiology and manifestations. Presently, a slow progressing (dry) non-neovascular atrophic form and a rapidly blinding neovascular (wet) form are discerned. No therapy is currently available for the former, while RPE transplantation and promising (albeit non-causal) anti-angiogenic therapies are available for the latter. The potential of RPE transplantation was demonstrated in animal models. Rejection of allogeneic homologous transplants in patients focused further studies on autologous sources. In vitro studies elucidated cell adhesion and wound healing mechanisms on aged human BM. Currently, autologous RPE, harvested from the midperiphery, is being transplanted as a cell suspension or a patch of RPE and choroid in AMD patients. These techniques have been evaluated from several groups. Autologous RPE transplants may have the disadvantage of carrying the same genetic information that may have led to AMD manifestation. An intermittent culturing step would allow for in vitro therapy of the RPE, its rejuvenation and prosthesis of BM to improve the success RPE transplants. Recent advances in stem cell biology when combined with lessons learned from studies of RPE transplantation are intriguing future therapeutic modalities for AMD patients.

Fibrosis and diseases of the eye

Most diseases that cause catastrophic loss of vision do so as a result of abnormal angiogenesis and wound healing, often in response to tissue ischemia or inflammation. Disruption of the highly ordered tissue architecture in the eye caused by vascular leakage, hemorrhage, and concomitant fibrosis can lead to mechanical disruption of the visual axis and/or biological malfunctioning. An increased understanding of inflammation, wound healing, and angiogenesis has led to the development of drugs effective in modulating these biological processes and, in certain circumstances, the preservation of vision. Unfortunately, such pharmacological interventions often are too little, too late, and progression of vision loss frequently occurs. The recent development of progenitor and/or stem cell technologies holds promise for the treatment of currently incurable ocular diseases.

Combination angiostatic therapy completely inhibits ocular and tumor angiogenesis

Angiostatic therapies designed to inhibit neovascularization associated with multiple pathological conditions have only been partially successful; complete inhibition has not been achieved. We demonstrate synergistic effects of combining angiostatic molecules that target distinct aspects of the angiogenic process, resulting in the complete inhibition of neovascular growth associated with development, ischemic retinopathy, and tumor growth, with little or no effect on normal, mature tissue vasculature. Tumor vascular obliteration using combination angiostatic therapy was associated with reduced tumor mass and increased survival in a rat 9L gliosarcoma model, whereas individual monotherapies were ineffective. Significant compensatory up-regulation of several proangiogenic factors was observed after treatment with a single angiostatic agent. In contrast, treatment with combination angiostatic therapy significantly reduced compensatory up-regulation. Therapies that combine angiostatic molecules targeting multiple, distinct aspects of the angiogenic process may represent a previously uncharacterized paradigm for the treatment of many devastating diseases with associated pathological neovascularization.

Ocular neovascularization: Implication of endogenous angiogenic inhibitors and potential therapy

Ocular neovascularization (NV) is the primary cause of blindness in a wide range of ocular diseases, such as diabetic retinopathy (DR) and age-related macular degeneration (AMD). The exact mechanism underlying the pathogenesis of ocular NV is not yet well understood, and as a consequence, there is no satisfactory therapy for ocular NV. In the last 10 years, a number of studies provided increasing evidence demonstrating that the imbalance between angiogenic stimulating factors and angiogenic inhibitors is a major contributor to the angiogenesis induced by various insults, such as hypoxia or ischemia, inflammation and tumor. The angiogenic inhibitors alone or in combination with other existing therapies are, therefore, believed to be promising in the treatment of ocular NV in the near future. This article reviews recent progress in studies on the mechanisms and treatment of ocular NV, focusing on the implication and therapeutic potential of endogenous angiogenic inhibitors in ocular NV.

Vitreous fluid biomarkers

Interactions between cells and the network of secreted proteins are associated with the ocular disease. In most cases, clinical appearcance is sufficiently diagnostic. However, in cases of nonspecific or atypical clinical presentation, diagnostic sampling of vitreous fluid can aid diagnosis and treatment for ocular disease. Progresses in the basic sciences, particularly molecular biology, and advances in surgical instrumentation have greatly enhanced the diagnostic armamentarium. These developments also have led to a better understanding of the pathophysiological processes involved in ocular diseases and have prompted evolution of new therapeutic modalities. In this chapter, we review techniques for vitreous fluid sampling and biomarker quantitation thereof. The molecular biology of bioactive vitreous fluid factors is also discussed with respect to their clinical involvement in the development of ocular disease.

Low-density lipoprotein receptor-related protein-1 (LRP-1) expression in a rat model of oxygen-induced retinal neovascularization

The low-density lipoprotein receptor-related protein-1 (LRP-1) is a high-molecular weight receptor of the LDL receptor gene family. Its ability to bind and internalize both proteinases and proteinase-inhibitor complexes from the extracellular space suggests that it has a major role in modulating uncontrolled retinal cell proliferation. In order to test this assumption, we investigated the expression of LRP-1 and receptor-associated ligands in a rat model of oxygen-induced retinal neovascularization. Wistar albino rats were placed into incubators at birth and exposed to an atmosphere alternating between 50% and 10% of oxygen every 24 h. After 14 days, the animals were allowed to recover in room air and sacrificed at postnatal day 20 (P20). The protein expression of LRP-1 and alpha2-macroglobulin (alpha2M) in the retina from unexposed and hyperoxia-exposed rats was investigated by Western blot. The localization of LRP-1 after neovascularization was assessed by immunohistochemical staining. The activity of metalloproteinases (MMPs) was determined by zymography. Histological analysis was done to quantitate the neovascular response in these animals. Western blot analysis showed that LRP-1 was expressed, along with alpha2M, in the retina of rats with oxygen-induced neovascularization at P20. By immunohistochemical analysis, positive staining for LRP-1 appeared in cells extending from the inner limiting membrane (ILM) to the outer limiting membrane (OLM). The cells of the retina that expressed LRP-1 were identified by immunofluorescence as Müller cells. Zymographic analysis demonstrated increased activity of MMP-2 and MMP-9 under neovascular conditions. This is the first demonstration of the involvement of LRP-1 in retinal neovascularization. In retinas of rats with oxygen-induced neovascularization, the expression of LRP-1 and alpha2M was increased along with an enhanced activity of MMPs, suggesting that LRP-1 expression may play a role in modulating retinal neovascularization by regulating proteolytic activity.

Myeloid progenitors differentiate into microglia and promote vascular repair in a model of ischemic retinopathy

Vision loss associated with ischemic diseases such as retinopathy of prematurity and diabetic retinopathy are often due to retinal neovascularization. While significant progress has been made in the development of compounds useful for the treatment of abnormal vascular permeability and proliferation, such therapies do not address the underlying hypoxia that stimulates the observed vascular growth. Using a model of oxygen-induced retinopathy, we demonstrate that a population of adult BM-derived myeloid progenitor cells migrated to avascular regions of the retina, differentiated into microglia, and facilitated normalization of the vasculature. Myeloid-specific hypoxia-inducible factor 1alpha (HIF-1alpha) expression was required for this function, and we also demonstrate that endogenous microglia participated in retinal vascularization. These findings suggest what we believe to be a novel therapeutic approach for the treatment of ischemic retinopathies that promotes vascular repair rather than destruction.

Pharmacologic manipulation of sphingosine kinase in retinal endothelial cells: implications for angiogenic ocular diseases

PURPOSE

The increased vascular permeability and pathogenic angiogenesis observed in diabetic retinopathy are induced, at least in part, by local inflammation and vascular endothelial growth factor (VEGF). Therefore, inhibition of signaling from VEGF and tumor necrosis factor-alpha (TNFalpha) is a promising approach to the treatment of this disease, as well as ocular diseases with similar etiologies, including age-related macular degeneration. A growing body of evidence demonstrates that sphingosine kinase (SK) plays an important role in cellular proliferation and angiogenesis. This study was undertaken to examine the effects of SK inhibitors on the responses of retinal endothelial cells (RECs) to VEGF and TNFalpha and their therapeutic efficacy in a diabetic retinopathy model.

METHODS

The expression and function of SK in bovine and human RECs were examined by immunoblot analysis. The involvement of SK in mediating responses to VEGF and TNFalpha was examined by using pharmacologic inhibitors of SK in cellular and in vivo assays, including a 3-month streptozotocin-induced diabetic retinopathy model in rats.

RESULTS

SK was present and active in human and bovine RECs, and SK activity in these cells was stimulated by VEGF. Inhibitors of SK blocked VEGF-induced production of sphingosine 1-phosphate and markedly attenuated VEGF-induced proliferation and migration of RECs. In addition, SK inhibitors were shown to block TNFalpha-induced expression of adhesion proteins, suppress VEGF-induced vascular leakage in an in vivo mouse model, and reduce retinal vascular leakage in the rat diabetic retinopathy model.

CONCLUSIONS

Overall, these studies demonstrate that inhibitors of SK attenuate the effects of proliferative and inflammatory stimuli on RECs both in vitro and in vivo, and so could be significant therapeutics in the treatment of diabetic retinopathy.

Overexpression of VEGF-A induces neovascularization and increased vascular leakage in rabbit eye after intravitreal adenoviral gene transfer

AIM

The aim of this study was to determine dose-response effects of vascular endothelial growth factor A as delivered using an adenoviral vector on vascular growth and pathological changes in the rabbit eye. Moreover, we wanted to develop a large animal model for angioproliferative diseases in the eye.

METHODS

Seventeen New Zealand White rabbits were injected with adenoviral vascular endothelial growth factor-A (AdVEGF-A) intravitreally with different doses (10(9)-10(11) vp). Controls were injected with an empty virus (AdCMV). Some animals had a combination of AdVEGF-A and AdsKDR (a soluble form of the VEGF receptor-2). Animals were killed 6 days after the gene transfer. On the basis of these results, 14 rabbits were injected intravitreally with AdVEGF-A or adenoviral LacZ (AdLacZ) with 10(10) vp in a volume of 0.1 mL. Animals were killed 3, 6, 14 and 28 days after the gene transfer, eyes were removed and analysed histologically.

RESULTS

In enzyme-linked immunosorbent assay (ELISA) analysis, human VEGF-A was present in vitreous humour in all VEGF-A transduced eyes. The amount of VEGF-A showed a dose-dependent increase with the AdVEGF-A dose and was the highest 6 days after the gene transfer. Histologic analyses revealed an increased capillary area and density in the AdVEGF-A eyes when compared with the AdLacZ eyes (P < 0.05). In the AdVEGF-A/AdsKDR eyes the average capillary area was not increased compared with AdLacZ eyes.

CONCLUSION

This model could be useful for large animal studies regarding the pathogenesis of neoangiogenesis and for the development of new therapeutic strategies for angioproliferative diseases of the eye. Our results establish the key role of VEGF-A in the induction of neovascularization and pathological changes in the rabbit eye.

MMP-2 and MMP-9 secretion by rpe is stimulated by angiogenic molecules found in choroidal neovascular membranes

PURPOSE

Matrix metalloproteinases (MMP)-2 and -9 play an important role in the pathogenesis of choroidal neovascularization (CNV). Retinal pigment epithelial cells (RPE) are an important source of MMPs in the outer retinal environment, however little is known about the local factors that modulate MMP secretion in these cells. The purpose of this study was to determine the effects of CNV involved growth factors and the extracellular matrix molecule fibronectin on MMP-2 and -9 secretion by cultured human RPE.

METHODS

MMP-2 and -9 secretion was studied using gelatin zymography, Western blot, and ELISA assay of RPE culture supernatants. The effects of stimulating the cells for 36 hours with vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bGFG), tumor necrosis factor-alpha (TNF-alpha), or fibronectin (FN), all angiogenic factors found in CNV membranes, was determined.

RESULTS

Resting RPE cells secreted MMP-2 but not MMP-9. Stimulation with TNF-alpha induced secretion of MMP-9 and increased the secretion of MMP-2. MMP-2 secretion was also increased by stimulation with FN and VEGF, but not bFGF.

CONCLUSION

The results indicated that the angiogenic molecules VEGF, FN, and TNF-alpha stimulate MMP-2 and -9 secretion from RPE and thus further promote CNV.