Oligodeoxynucleotides inhibit retinal neovascularization in a murine model of proliferative retinopathy

Translational regulation of vascular endothelial growth factor expression in renal epithelial cells by angiotensin II

ANG II regulates growth factor expression in the kidney. We investigated whether ANG II regulated vascular endothelial growth factor (VEGF) synthesis in proximal tubular epithelial (MCT) cells. ANG II (1 nM) increased VEGF protein expression within 5 min, the effect lasting for 30 min. There was no change in VEGF mRNA levels or mRNA stability, and transcription inhibitors did not affect ANG II-induced VEGF expression. Regulation of VEGF translation was investigated. Polyribosomal analysis revealed selective enrichment of heavy ribosomes (polysomes) with VEGF mRNA transcripts compared with light ribosomes in ANG II-treated cells, although distribution of GAPDH was unaltered. In vitro translation of total RNA from polysomal fractions showed selective increase in VEGF protein synthesis in ANG II-treated cells. Preincubation with LY-294002, a PI 3-kinase inhibitor, or expression of dominant-negative Akt prevented ANG II-stimulated increase in VEGF translation. ANG II increased phosphorylation of eukaryotic initiation factor 4E and its binding protein 4E-BP1, critical events that regulate the initiation phase of protein translation. ANG II failed to increase VEGF mRNA translation in cells stably expressing the phosphorylation mutant of 4E-BP1. Our data illustrate that a rapid increase in VEGF protein expression by ANG II is regulated at the initiation phase of translation of VEGF mRNA in renal epithelial cells. Regulation of VEGF translation by ANG II represents a novel pathway of renal response to injury.

AL-3789: a novel ophthalmic angiostatic steroid

Ocular neovascular diseases are a leading cause of blindness in the world. Research is beginning to unravel the complex mechanisms involved in the pathogenesis of ocular neovascular diseases, but currently there are very few methods for the effective treatment of these blinding disorders. AL-3789 (Alcon Laboratories) is an angiostatic steroid which shows significant activity in inhibiting new blood vessel formation in a wide variety of models of neovascularisation, including neovascularisation in ocular tissues. This angiostatic steroid has broad angiostatic activity and is effective in a number of different animal species and tissues, regardless of the angiogenic stimulus. AL-3789 is devoid of conventional steroid hormone activity and does not appear to have any other pharmacological side-effects at the doses and routes of administration tested. In preclinical safety studies, AL-3789 has no apparent ocular or systemic toxicity when dosed chronically by topical ocular, or by systemic administration and therefore it appears to be a relatively safe compound.

Pharmacotherapy for diabetic retinopathy

PURPOSE OF REVIEW

Diabetic retinopathy (DR) is a potentially visually devastating complication of chronic hyperglycemia and other associated systemic abnormalities. Numerous large, prospective, randomized clinical trials have delineated the current standard prevention and treatment protocols including intensive glycemic and blood pressure control and laser photocoagulation for neovascularization and clinically significant macular edema. However, despite standard intervention, vision loss from DR still occurs at an alarming rate. Thus, more recently, researchers have directed their efforts towards better understanding the microscopic changes occurring in DR to develop more effective pharmacologic prevention and treatment strategies.

RECENT FINDINGS

Phase II and III clinical studies involving antivascular endothelial growth factor (VEGF) and protein kinase C (PKC) inhibitors for the management of diabetic macular edema are underway. Researchers recently found elevated pigment endothelium-derived factor (PEDF) associated with active neovascularization, a finding that counteracts prior claims of endogenous anti-angiogenic properties. Other clinical trials are underway to evaluate the efficacy of octreotide, celecoxib, and candesartan on DR. Small clinical studies have suggested beneficial treatment effects for triamcinolone acetonide, interferon alpha-2a, and supplemental oxygen; however, other studies involving losartan, vitamins C and E, and atorvastatin failed to show any benefit.

SUMMARY

Over the past decade, numerous animal models have led to a more thorough understanding of the early microvascular alterations and later neovascularization and edema observed in DR. These discoveries and subsequent human clinical studies involving direct and indirect growth factor modulation, extracellular matrix alteration, vitreolysis, and alternative DR pathways including dyslipidemia, hypoxia, and sorbitol are reviewed in this manuscript.

Upregulation of placental growth factor by vascular endothelial growth factor via a post-transcriptional mechanism

Vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) are key angiogenic stimulators during normal development and wound healing, as well as in a variety of pathological conditions. Recent studies have demonstrated a synergistic effect of VEGF and PlGF in pathological angiogenesis and suggest a role for PlGF in amplifying VEGF action in endothelial cells. We show here in the mouse model of oxygen-induced retinopathy that VEGF is significantly increased (P<0.01) in the retina at both the mRNA and protein levels. In this mouse model, PlGF was significantly upregulated in the retina at the protein level (P<0.01) without a corresponding change in mRNA levels. In cultured human retinal and umbilical vein endothelial cells, VEGF induced the production of PlGF protein by over 10-fold (P<0.01) in a dose-dependent manner through a post-transcriptional mechanism. The increased PlGF expression upon VEGF treatment was significantly reduced by inhibition of the protein kinase C (PKC) and MEK signaling pathways, as well as by treatment with the calcium ionophore A23187. Taken together, our findings demonstrate that VEGF can amplify its effects on endothelial cells by inducing the production of PlGF via a post-transcriptional mechanism in a PKC-dependent manner, and provide a potential link between PKC inhibition and amelioration of vascular complications in the development of angiogenic diseases.

Intravitreal injection of specific receptor tyrosine kinase inhibitor PTK787/ZK222 584 improves ischemia-induced retinopathy in mice

BACKGROUND

Retinal neovascularisation occurs under the influence of angiogenic factors that are induced by hypoxia, like vascular endothelial growth factor (VEGF), which is one of the major mediators. PTK/ZK inhibits VEGF signal transduction by blocking the tyrosine kinase of all three VEGF receptors. PTK/ZK is currently being evaluated in clinical trials for angioinhibitory therapy in tumour patients. To avoid potential systemic side effects, local application would be desirable for the treatment of ischemic retinopathies in humans. We therefore investigated the effect of intravitreally applied PTK/ZK in a mouse model for ischemia-induced retinopathy.

METHODS

C57BL/6J mice were placed in 75% oxygen on postnatal day 7. On day 12, they were treated with an intravitreal injection of PTK/ZK (5 microM or 40 microM) in one eye and buffer solution in the fellow eye. Afterwards, the animals were kept in room air until intracardial perfusion with fluorescein-dextran on day 17. Retinal whole mounts were prepared and ischemic retinopathy was evaluated using a standardised retinopathy score.

RESULTS

A single intravitreal injection of 40 microM PTK/ZK reduced angioproliferative changes compared to the control eye of each animal (n=37). The difference in retinopathy scores was highly significant (P=0.002, Wilcoxon signed-rank test). Injection of 5 microM PTK/ZK did not show a significant antiangiogenic effect.

CONCLUSIONS

Tyrosine kinase inhibitors are promising substances not only in cancer therapy, but also in the treatment of ischemic retinopathies that are mediated by VEGF. We showed that in a mouse model for ischemia-induced retinopathy a single intravitreal injection of 40 microM PTK/ZK is capable of significantly reducing angioproliferative retinopathy. The local application of PTK/ZK could be a new way to treat ischemic ocular diseases such as diabetic retinopathy in humans.

Implication of the hypoxia response element of the vegf promoter in mouse models of retinal and choroidal neovascularization, but not retinal vascular development

Retinal neovascularization (NV) and macular edema, resulting from blood-retinal barrier (BRB) breakdown, are major causes of visual loss in ischemic retinopathies. Choroidal NV (CNV) occurs in diseases of the retinal pigmented epithelium/Bruch's membrane complex and is another extremely prevalent cause of visual loss. We used mice in which the hypoxia response element (HRE) is deleted from the vascular endothelial growth factor (vegf) promoter (Vegf(delta/delta) mice) to explore the role of induction of VEGF through the HRE in these disease processes. Compared to wild type (Vegf+/+) mice with oxygen-induced ischemic retinopathy (OIR) in which vegf mRNA levels were increased and prominent retinal NV and BRB breakdown occurred, Vegf(delta/delta) littermates with OIR failed to increase vegf mRNA levels in the retina and had significantly less retinal NV and BRB breakdown, but showed prominent dilation of some superficial retinal vessels. Vegf(+/delta) littermates with ischemic retinopathy developed comparable retinal NV to Vegf+/+ mice, exhibited intermediate levels of BRB breakdown, and did not show vasodilation. In a mouse model of CNV, due to laser-induced rupture of Bruch's membrane, the area of CNV at Bruch's membrane rupture sites was more than tenfold greater in Vegf+/+ mice than in Vegf(delta/delta) littermates. In contrast to these dramatic differences in pathologic ocular NV, Vegf(delta/delta) mice showed subtle differences in retinal vascular development compared to Vegf+/+ mice; it was slightly delayed, but otherwise normal. These data suggest that induction of VEGF through the HRE in its promoter is critical for retinal and CNV, but not for retinal vascular development.

Setting sights on the treatment of ocular angiogenesis using antisense oligonucleotides

The application of antisense technology to study physiological and disease processes continues to mature. Antisense approaches are among the most direct means to use genomic sequence information. When developing therapeutics, applications range from early target validation in discovery to the therapeutic product. In this review, we describe the application of antisense oligonucleotides (ASOs) to identify genes that are important in controlling angiogenesis. High-throughput assays in vitro have been used to evaluate many gene targets. Genes that appear to be important in angiogenesis are then evaluated further in animal models of ocular angiogenesis. The ability of ASOs to reduce target-gene expression in the appropriate cells in the eye raises the possibility that this class of compounds could be used for target validation in vivo, and also be developed as a novel class of therapeutics in their own right.

Scleral permeability of a small, single-stranded oligonucleotide

Developing more effective ocular drug delivery systems is essential to improving the treatment of posterior segment eye disease. The large target area provided by the sclera and potentially less vision threatening complications are advantages of transscleral administration compared to more traditional modalities of drug delivery to the posterior segment. We aimed to determine the permeability coefficient for the in vitro diffusion of a small, single-stranded, oligonucleotide across human sclera. Transscleral permeability was measured by placing 100 microL of 2.96 x 10(-4) mol single-stranded, fluorescein-labeled oligonucleotide (MW = 7998.3) on the episcleral surface of sclera mounted in a perfusion chamber. Fractions of choroidal perfusate were collected hourly for 24 hours. The permeability constant or K(trans) for the transscleral diffusion of the naked, single-stranded, fluorescein-labeled oligonucleotide was 7.67 +/- 1.8 x 10(-7) cm/s (mean +/- SEM, N = 7). The permeability constant or K(trans) after intrascleral injection of the same fluorescein-labeled oligonucleotide was 1.32 +/- 0.42 x 10(-7) (mean +/- SEM, N = 4). This analysis demonstrates that diffusion of a naked, 24-base, single-stranded, fluorescein-labeled oligonucleotide can be accomplished by both of the described methods. The ability to deliver single-stranded oligonucleotides across the sclera may prove to be advantageous given the development of several novel therapeutic strategies that use similar molecules.

Vascular endothelial growth factor: basic science and clinical progress

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. Hypoxia has been shown to be a major inducer of VEGF gene transcription. The tyrosine kinases Flt-1 (VEGFR-1) and Flk-1/KDR (VEGFR-2) are high-affinity VEGF receptors. The role of VEGF in developmental angiogenesis is emphasized by the finding that loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF is critical also for reproductive and bone angiogenesis. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Anti-VEGF monoclonal antibodies and other VEGF inhibitors block the growth of several tumor cell lines in nude mice. Clinical trials with various VEGF inhibitors in a variety of malignancies are ongoing. Very recently, an anti-VEGF monoclonal antibody (bevacizumab; Avastin) has been approved by the Food and Drug Administration as a first-line treatment for metastatic colorectal cancer in combination with chemotherapy. Furthermore, VEGF is implicated in intraocular neovascularization associated with diabetic retinopathy and age-related macular degeneration.

Angiopoietin-2 enhances retinal vessel sensitivity to vascular endothelial growth factor

Increased expression of vascular endothelial growth factor (VEGF) in the retina starting after postnatal day (P)7 results in neovascularization originating from deep retinal capillaries, but not those in the superficial capillary bed. Doxycycline was administered starting P0 to double transgenic mice with inducible expression of VEGF in the retina. These mice showed proliferation and dilation of superficial retinal capillaries, indicating that at this stage of development, the superficial capillaries are sensitive to the effects of VEGF. Angiopoietin-2 (Ang2) is expressed along the surface of the retina for several days after birth, but by P7 and later, Ang2 is only expressed in the region of the deep capillary bed. In mice with ubiquitous doxycycline-inducible expression of Ang2, in the absence of doxycycline, intravitreous injection of a gutless adenoviral vector expressing VEGF (AGV.VEGF) resulted in neovascularization of the cornea and iris, but no retinal neovascularization. After treatment with doxycycline to induce Ang2 expression, intravitreous injection of AGV.VEGF caused retinal neovascularization in addition to corneal and iris neovascularization. The retinal neovascularization originated from both the superficial and deep capillary beds. These data suggest that Ang2 promotes sensitivity to the angiogenic effects of VEGF in retinal vessels.

Pathogenesis of retinopathy of prematurity

Retinopathy of prematurity (ROP) is a major cause of blindness in children in developed countries. ROP is a two-phase disease, beginning with delayed retinal vascular growth after premature birth (Phase I). Phase II follows when Phase I-induced hypoxia releases factors to stimulate new blood vessel growth. Both oxygen-regulated and non-oxygen-regulated factors contribute to normal vascular development and retinal neovascularization. Vascular endothelial growth factor (VEGF) is an important oxygen-regulated factor. A critical non-oxygen-regulated growth factor is insulin-like growth factor-I (IGF-I). In knockout mice, lack of IGF-I prevents normal retinal vascular growth, despite the presence of VEGF, important to vessel development. In vitro, low IGF-I levels prevent VEGF-induced activation of Akt, a kinase critical for vascular endothelial cell survival. We found that premature infants who develop ROP have low levels of serum IGF-I compared to age-matched infants without disease. IGF-I is critical to normal vascular development. Low IGF-I predicts ROP in premature infants, and restoration of IGF-I to normal levels might prevent ROP.

Intravitreal triamcinolone does not alter basal vascular endothelial growth factor mRNA expression in rat retina

Intravitreal triamcinolone inhibits choroidal neovascularization. To investigate if vascular endothelial growth factor (VEGF) is affected, we examined VEGF expression after intravitreal triamcinolone administration in rat retina. Using in situ hybridization, we have found dense clustered VEGF mRNA signals in the retinal pigment epithelium, moderate patchy signals in the inner nuclear layer, and positive labeling in a sub-population of ganglion cells. Densitometry and northern blot analysis revealed no significant alteration of VEGF mRNA expression pattern and level 3-21 days after triamcinolone injection. Our data indicate that intravitreal triamcinolone does not affect basal VEGF mRNA expression in normal adult rat retina.

Pathogenesis of retinopathy of prematurity

Retinopathy of prematurity (ROP) is a major cause of blindness in children in developed countries. ROP, a two-phase disease, is initiated with delayed retinal vascular growth after premature birth (phase I). Insufficient vascularization of the developing retina creates hypoxia, which precipitates the release of factors stimulating new and abnormal blood vessel growth (phase II). ROP develops because of abnormalities in both oxygen-regulated and non-oxygen-regulated factors, which affect both phases of the disease. Vascular endothelial growth factor (VEGF) is an important oxygen-regulated factor that, if suppressed, inhibits normal vessel growth, but in excess, precipitates retinal neovascularization. A critical non-oxygen-regulated growth factor is insulin-like growth factor (IGF-1). Similar to VEGF, low levels of IGF-1 prevent normal vessel growth (phase I), and higher levels allow neovascularization (phase II). We found that premature infants who develop ROP have low levels of serum IGF-1 compared with age-matched infants without disease. IGF-1 is critical to normal vascular development. Low IGF-1 predicts ROP, and restoration of IGF-1 to normal levels might prevent ROP.

Vitreous levels of pigment epithelium-derived factor and vascular endothelial growth factor: implications for ocular angiogenesis

PURPOSE

Pigment epithelium-derived factor (PEDF) has been demonstrated to suppress ocular angiogenesis in several animal models. In this study, we sought to measure the levels of PEDF and vascular endothelial growth factor (VEGF) in the vitreous of patients with and without ocular neovascular disorders.

DESIGN

Case-control study of patients undergoing intraocular surgery for a variety of neovascular and nonneovascular conditions.

METHODS

Vitreous samples were collected from 65 eyes of 65 patients with no neovascular disorder (n = 24), choroidal neovascularization (n = 9), active proliferative diabetic retinopathy (n = 16), and inactive proliferative diabetic retinopathy (n = 16). The levels of VEGF and PEDF in these vitreous samples were determined by enzyme-linked immunosorbent assay.

RESULTS

The VEGF levels were at or below the level of detectability in the reference and choroidal neovascularization groups. The VEGF levels were significantly elevated in both the active and inactive PDR groups, and significantly higher in the active PDR group as compared with the inactive PDR group. The PEDF levels, which were present at relatively high concentrations in all groups, were higher in patients with active PDR compared with the control and choroidal neovascularization groups.

CONCLUSIONS

High levels of immunoreactive PEDF are present in the vitreous of individuals with or without ocular neovascularization, but PEDF levels are significantly higher in patients with active PDR compared with patients with choroidal neovascularization or nonneovascular retinal diseases. Although these results do not preclude the possibility that endogenous PEDF helps to modulate ocular neovascularization, they do not support ischemia-induced downregulation of PEDF as a mechanism for such modulation.

Vascular endothelial growth factor antisense oligodeoxynucleotides with lipiodol in arterial embolization of liver cancer in rats

AIM: Transcatheter arterial embolization (TAE) of the hepatic artery has been accepted as an effective treatment for unresectable hepatocellular carcinoma (HCC). However, embolized vessel recanalization and collateral circulation formation are the main factors of HCC growth and recurrence and metastasis after TAE. Vascular endothelial growth factor (VEGF) plays an important role in tumor angiogenesis. This study was to explore the inhibitory effect of VEGF antisense oligodeoxynucleotides (ODNs) on VEGF expression in cultured Walker-256 cells and to observe the anti-tumor effect of intra-arterial infusion of antisense ODNs mixed with lipiodol on rat liver cancer.

METHODS: VEGF antisense ODNs and sense ODNs were added to the media of non-serum cultured Walker-256 cells. Forty-eight hours later, VEGF concentrations of supernatants were detected by ELISA. Endothelial cell line ECV-304 cells were cultured in the supernatants. Seventy-two hours later, growth of ECV-304 cells was analyzed by MTT method. Thirty Walker-256 cell implanted rat liver tumor models were divided into 3 groups. 0.2 mL lipiodol (LP group, n = 10), 3OD antisense ODNs mixed with 0.2 mL lipiodol (LP+ODNs group, n = 10) and 0.2 mL normal saline (control group, n = 10) were infused into the hepatic artery. Volumes of tumors were measured by MRI before and 7 d after the treatment. VEGF mRNA in cancerous and peri-cancerous tissues was detected by RT-PCR. Microvessel density (MVD) and VEGF expression were observed by immunohistochemistry.

RESULTS: Antisense ODNs inhibited Walker-256 cells’ VEGF expression. The tumor growth rate was significantly lower in LP+ODNs group than that in LP and control groups (140.1 ± 33. 8%, 177. 9 ± 64. 9% and 403.9 ± 69.4% respectively, F = 60.019, P < 0.01). VEGF mRNAs in cancerous and peri-cancerous tissues were expressed highest in LP group and lowest in LP+ODNs group. The VEGF positive rates showed no significant difference among LP, control and LP+ODNs groups (90%, 70% and 50%, H = 3.731, P>0.05). The MVD in LP+ODNs group (53.1 ± 18.4) was significantly less than that in control group (73.2 ± 20.4) and LP group (80.3 ± 18.5) (F = 5.44, P < 0.05)

CONCLUSION: VEGF antisense ODNs can inhibit VEGF expression of Walker-256 cells. It maybe an antiangiogenesis therapy agent for malignant tumors. VEGF antisense ODNs mixed with lipiodol embolizing liver cancer is better in inhibiting liver cancer growth, VEGF expression and microvessel density than lipiodol alone.

Annexin II regulates fibrin homeostasis and neoangiogenesis in vivo

A central tenet of fibrinolysis is that tissue plasminogen activator-dependent (t-PA- dependent) conversion of plasminogen to active plasmin requires the presence of the cofactor/substrate fibrin. However, previous in vitro studies have suggested that the endothelial cell surface protein annexin II can stimulate t-PA-mediated plasminogen activation in the complete absence of fibrin. Here, homozygous annexin II-null mice displayed deposition of fibrin in the microvasculature and incomplete clearance of injury-induced arterial thrombi. While these animals demonstrated normal lysis of a fibrin-containing plasma clot, t-PA-dependent plasmin generation at the endothelial cell surface was markedly deficient. Directed migration of annexin II-null endothelial cells through fibrin and collagen lattices in vitro was also reduced, and an annexin II peptide mimicking sequences necessary for t-PA binding blocked endothelial cell invasion of Matrigel implants in wild-type mice. In addition, annexin II-deficient mice displayed markedly diminished neovascularization of fibroblast growth factor-stimulated cornea and of oxygen-primed neonatal retina. Capillary sprouting from annexin II-deficient aortic ring explants was markedly reduced in association with severe impairment of activation of metalloproteinase-9 and -13. These data establish annexin II as a regulator of cell surface plasmin generation and reveal that impaired endothelial cell fibrinolytic activity constitutes a barrier to effective neoangiogenesis.

Annexin II regulates fibrin homeostasis and neoangiogenesis in vivo

A central tenet of fibrinolysis is that tissue plasminogen activator-dependent (t-PA- dependent) conversion of plasminogen to active plasmin requires the presence of the cofactor/substrate fibrin. However, previous in vitro studies have suggested that the endothelial cell surface protein annexin II can stimulate t-PA-mediated plasminogen activation in the complete absence of fibrin. Here, homozygous annexin II-null mice displayed deposition of fibrin in the microvasculature and incomplete clearance of injury-induced arterial thrombi. While these animals demonstrated normal lysis of a fibrin-containing plasma clot, t-PA-dependent plasmin generation at the endothelial cell surface was markedly deficient. Directed migration of annexin II-null endothelial cells through fibrin and collagen lattices in vitro was also reduced, and an annexin II peptide mimicking sequences necessary for t-PA binding blocked endothelial cell invasion of Matrigel implants in wild-type mice. In addition, annexin II-deficient mice displayed markedly diminished neovascularization of fibroblast growth factor-stimulated cornea and of oxygen-primed neonatal retina. Capillary sprouting from annexin II-deficient aortic ring explants was markedly reduced in association with severe impairment of activation of metalloproteinase-9 and -13. These data establish annexin II as a regulator of cell surface plasmin generation and reveal that impaired endothelial cell fibrinolytic activity constitutes a barrier to effective neoangiogenesis.

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Growth factors and protein kinase C inhibitors as novel therapies for the medical management diabetic retinopathy

Diabetic retinopathy is a leading cause of acquired visual loss. Current treatment modalities are not effective in all cases and may have side effects. Investigation of the biochemical basis of diabetic retinopathy suggests that future treatments may reverse or halt the progression of diabetic retinopathy, or actually prevent the development of diabetic retinopathy. Pharmacological manipulation of protein kinase C and various growth factors may form the basis of future treatments for diabetic retinopathy.

Pharmacologic therapy for diabetic retinopathy

Diabetic retinopathy remains one of the major causes of acquired blindness in developed nations. This is true despite the development of laser treatment, which can prevent blindness in the majority of those who develop macular oedema (ME) or proliferative diabetic retinopathy (PDR). ME is manifest by retinal vascular leakage and thickening of the retina. The hallmark of PDR is neovascularisation (NV)--abnormal angiogenesis that may ultimately cause severe vitreous cavity bleeding and/or retinal detachment. Pharmacologic therapy aimed specifically at preventing vascular leakage and NV would be a welcome addition to the armamentarium. PDR and ME could be prevented by improved metabolic control or by pharmacologically blunting the biochemical consequences of hyperglycaemia (e.g., with aldose reductase inhibitors, inhibitors of non-enzymatic glycation or by protein kinase C [PKC] inhibition). The angiogenesis in PDR could be treated via growth factor (e.g., vascular endothelial growth factor [VEGF], insulin like growth factor-1 [IGF-1]) blockade, integrin (e.g., alpha-v beta-3) blockade, extracellular matrix alteration (e.g., with steroid compounds) or interference with intracellular signal transduction pathways (e.g., PKC and mitogen activated protein kinase [MAPK] pathway proteins). Some of these antiangiogenic agents may also prove useful for treating or preventing ME. Numerous potentially useful antiangiogenic compounds are in development; two drugs are presently in clinical trials for treatment of the preproliferative stage of PDR, while two are in clinical trials for treatment of ME.

Neuropeptide Y and Y2-receptor are involved in development of diabetic retinopathy and retinal neovascularization

BACKGROUND

Neuropeptide Y is a sympathetic neurotransmitter, a potent endothelium-derived angiogenic factor and a vascular mitogen. We have studied the role of the functional leucine7 to proline7 polymorphism of the signal peptide region of preproneuropeptide Y (prepro-NPY) as a genetic susceptibility factor for diabetic retinopathy. In addition, we investigated the role of the NPY Y2-receptor as a putative mediator of angiogenic NPY signaling in the retina.

METHODS

Frequencies of proline7 (Pro7) carriers in the prepro-NPY were determined in type 1 and type 2 diabetes patients having retinopathy, in type 2 diabetes patients without retinopathy and in healthy control subjects. The role of Y2-receptor in hyperoxemia-induced retinal neovascularization was investigated in Y2-receptor knockout mice (Y2-/-) and in rats administered Y2-receptor mRNA antisense oligonucleotide.

RESULTS

The carriers having Pro7 in the preproNPY are markedly over-represented among type 2 diabetes patients with retinopathy compared to type 2 diabetes patients without retinopathy and to the population control. Neonatal exposure to hyperoxia resulted in development of retinal neovascularization that was prevented in Y2(-1-) -mice, and significantly inhibited in rats treated with the Y2-receptor antisense oligonucleotide.

CONCLUSIONS

NPY and Y2-receptor play important roles in diabetic retinopathy and retinal neovascularization and are thus potential new targets for drug molecules for treatment of retinopathy.

Angiopoietin/Tek interactions regulate mmp-9 expression and retinal neovascularization

The objective of the study was to determine the role of the angiopoietins in the regulation of gelatinase expression during angiogenesis, and whether inhibition of the angiopoietin/Tek interaction in vivo can suppress the extent of retinal neovascularization. Retinal microvascular endothelial cells were treated with angiopoietins and examined for the production of gelatinases. The effects of inhibiting angiopoietin binding to the Tie-2 receptor was studied in newborn mice with experimentally induced retinal neovascularization. Animals were treated with an ip injection of the Tie-2 antagonist, muTek delta Fc, while oxygen-exposed mice treated with similar concentrations of murine IgG were used as controls. The effect of muTek delta Fc on the gelatinase expression in the retina was examined by real-time RT-PCR analysis. The stimulation of cultured retinal endothelial cells with Ang-1 and -2 resulted in the increased expression of matrix metalloproteinase (MMP)-9. Ang-2 expression was up-regulated in experimental animals during the period of angiogenesis and was the greatest on Day 17 (the time of maximal angiogenic response). Histologic analysis of mice treated with the Tie-2 antagonist, muTek delta Fc, showed significant (87%; p = 0.001) inhibition of retinal neovascularization, and the response was dose-dependent. In vitro binding data support the fact that both Ang-1 and Ang-2 bind with high avidity to muTek delta Fc. The RT-PCR analysis of the retinas of the Tek-treated animals showed a similar (80%; p = 0.001) inhibition of the MMP-9 expression, which correlated with the decrease in angiogenesis. The up-regulation of gelatinases in microvascular endothelial cells by Ang-2 may be an important early response during the development of retinal neovascularization. Inhibition of the binding activity of the angiopoietins in vivo suppressed retinal neovascularization concomitant with a reduction in the expression of MMP-9.

Retinal and choroidal angiogenesis: pathophysiology and strategies for inhibition

Retinal angiogenesis and choroidal angiogenesis are major causes of vision loss, and the pathogenesis of this angiogenesis process is still uncertain. However, several key steps of the angiogenic cascade have been elucidated. In retinal angiogenesis, hypoxia is the initial stimulus that causes up regulation of growth factors, integrins and proteinases, which result in endothelial cell proliferation and migration that are critical steps in this process. Once the endothelial tube is formed from the existing blood vessels, maturation starts with recruitment of mural cell precursors and formation of the basement membrane. Normally, there is a tight balance between angiogenic factors and endogenous angiogenesis inhibitors that help to keep the angiogenic process under control. Although the steps of choroidal angiogenesis seem to be similar to those of retinal angiogenesis, there are some major differences between these two processes. Several anti-angiogenic approaches are being developed in animal models to prevent ocular angiogenesis by blocking the key steps of the angiogenic cascade. Based on these pre-clinical studies, several anti-angiogenic clinical trials are ongoing in patients with diabetic retinopathy and age-related macular degeneration. This review discusses the pathogenesis of retinal and choroidal angiogenesis, and alternative pharmacological approaches to inhibit angiogenesis in ocular diseases.

Vascular endothelial growth factor and diabetic retinopathy: pathophysiological mechanisms and treatment perspectives

Retinal neovascularization and macular edema are central features of diabetic retinopathy, the major cause of blindness in the developed world. Current treatments are limited in their efficacy and are associated with significant adverse effects. Characterization of the molecular and cellular processes involved in vascular growth and permeability has led to the recognition that the angiogenic growth factor and vascular permeability factor vascular endothelial growth factor (VEGF) plays a pivotal role in the retinal microvascular complications of diabetes. Therefore, VEGF represents an exciting target for therapeutic intervention in diabetic retinopathy. This review highlights the current understanding of the mechanisms that regulate VEGF gene expression and mediate its biological effects and how these processes may become altered during diabetes. The cellular and molecular alterations that characterize experimental models of diabetes are considered in relation to the influence of high glucose-mediated oxidative stress on VEGF expression and on the mechanisms of VEGF's actions under hyperglycemic induction. Finally, potential therapeutic strategies for preventing VEGF overexpression or blocking its pathological effects in the diabetic retina are considered.

Structure and inhibitory effects on angiogenesis and tumor development of a new vascular endothelial growth inhibitor

Blocking angiogenesis is an attractive strategy to inhibit tumor growth, invasion, and metastasis. We describe here the structure and the biological action of a new cyclic peptide derived from vascular endothelial growth factor (VEGF). This 17-amino acid molecule designated cyclopeptidic vascular endothelial growth inhibitor (cyclo-VEGI, CBO-P11) encompasses residues 79-93 of VEGF which are involved in the interaction with VEGF receptor-2. In aqueous solution, cyclo-VEGI presents a propensity to adopt a helix conformation that was largely unexpected because only beta-sheet structures or random coil conformations have been observed for macrocyclic peptides. Cyclo-VEGI inhibits binding of iodinated VEGF165 to endothelial cells, endothelial cells proliferation, migration, and signaling induced by VEGF165. This peptide also exhibits anti-angiogenic activity in vivo on the differentiated chicken chorioallantoic membrane. Furthermore, cyclo-VEGI significantly blocks the growth of established intracranial glioma in nude and syngeneic mice and improves survival without side effects. Taken together, these results suggest that cyclo-VEGI is an attractive candidate for the development of novel angiogenesis inhibitor molecules useful for the treatment of cancer and other angiogenesis-related diseases.

A selective and oral small molecule inhibitor of vascular epithelial growth factor receptor (VEGFR)-2 and VEGFR-1 inhibits neovascularization and vascular permeability

Vascular endothelial growth factor (VEGF) is a key driver of the neovascularization and vascular permeability that leads to the loss of visual acuity in diabetic retinopathy and neovascular age-related macular degeneration. Our aim was to identify an orally active, selective small molecule kinase inhibitor of vascular endothelial growth factor receptor (VEGFR)-2 with activity against both VEGF-induced angiogenesis and vascular permeability. We used a biochemical assay to identify 3-[5-methyl-2- (2-oxo-1,2-dihydro-indol-3-ylidenemethyl)-1H-pyrrol-3-yl]-proprionic acid (SU10944), a pyrrole indolinone, which is a potent ATP-competitive inhibitor of VEGFR-2 (Ki of 21 +/- 5 nM). In cellular assays, SU10944 inhibited VEGF-induced receptor autophosphorylation (IC50 of 227 +/- 80 nM) as well as downstream signaling (IC50 of 102 +/- 27 nM). In biochemical assays, SU10944 exhibits potent inhibitory activity against VEGFR-1; weak activity against other related subgroup members, including stem cell factor receptor (SCFR), platelet-derived growth factor receptor beta (PDGFRbeta), and fibroblast growth factor receptor-1 (FGFR-1); and no detectable activity against other protein tyrosine kinases such as epidermal growth factor receptor (EGFR), Src, and hepatocyte growth factor receptor. In cellular assays, the selectivity for SU10944 to inhibit VEGFR is maintained compared with other tyrosine kinases (IC50 for SCFR of 1.6 +/- 0.3 microM, for PDGFRbeta of 30.6 +/- 13.3 microM, for FGFR-1 of >50 microM, and for EGFR of >50 microM). Upon oral administration, SU10944 gave a clear dose response in the corneal micropocket model with an ED50 value for inhibition of neovascularization of approximately 30 mg/kg and a maximum inhibition of 95% at 300 mg/kg. Similarly, upon oral administration in the Miles assay, SU10944 potently inhibited VEGF-induced vascular permeability. Our data indicate that small molecule inhibitors of VEGFR signaling have the potential to ameliorate VEGF-induced neovascularization as well as vascular permeability.

Vascular endothelial growth factor induces protein synthesis in renal epithelial cells: a potential role in diabetic nephropathy

BACKGROUND

Vascular endothelial growth factor (VEGF) is an important determinant of ocular complications of diabetes. Its potential role in diabetic renal disease has not been extensively studied.

METHODS

We employed mice with streptozotocin-induced type 1 diabetes and db/db mice with type 2 diabetes to study the regulation of renal VEGF. Studies of VEGF regulation of protein synthesis were performed using proximal tubular epithelial (MCT) cells in culture.

RESULTS

A nearly three-fold increase of VEGF165 expression in the renal cortex was seen, coinciding with renal hypertrophy in mice with either type 1 or type 2 diabetes. VEGF increased de novo protein synthesis and induced significant hypertrophy in MCT cells. VEGF stimulation of protein synthesis was dependent on tyrosine phosphorylation of the type 2 VEGF receptor and phosphatidylinositol 3-kinase (PI 3-kinase) activity. Activity of Akt was increased two- to three-fold by VEGF. Expression of dominant-negative Akt showed that Akt activation was also needed for VEGF-induced protein synthesis and cell hypertrophy. As PI 3-kinase-Akt axis regulates initial events in protein translation, these events were examined in the context of VEGF regulation of protein synthesis. VEGF stimulated eukaryotic initiation factor 4E-binding protein (4E-BP1) phosphorylation, which was dependent on activation of PI 3-kinase and Akt. Stable transfection with 4E-BP1 Thr37,46-Ala37,46 mutant abolished the VEGF-induced de novo protein synthesis and cell hypertrophy.

CONCLUSION

VEGF augments protein synthesis and induces hypertrophy in MCT cells in a PI 3-kinase- and Akt-dependent manner. Phosphorylation of Thr37,46 in 4E-BP1 is required for VEGF-induced protein synthesis and hypertrophy in MCT cells. These data suggest a role for VEGF in the pathogenesis of diabetic renal disease.

Ophthalmic drug discovery

Millions of people suffer from a wide variety of ocular diseases, many of which lead to irreversible blindness. The leading causes of irreversible blindness in the elderly--age-related macular degeneration and glaucoma--will continue to effect more individuals as the worldwide population continues to age. Although there are therapies for treating glaucoma, as well as ongoing clinical trials of treatments for age-related macular degeneration, there still is a great need for more efficacious treatments that halt or even reverse ocular diseases. The eye has special attributes that allow local drug delivery and non-invasive clinical assessment of disease, but it is also a highly complex and unique organ, which makes understanding disease pathogenesis and ocular drug discovery challenging. As we learn more about the cellular mechanisms involved in age-related macular degeneration and glaucoma, potentially, new drug targets will emerge. This review provides insight into some of the new approaches to therapy.

Downregulation of fibronectin overexpression reduces basement membrane thickening and vascular lesions in retinas of galactose-fed rats

Overexpression of extracellular matrix (ECM) components is closely associated with the development of vascular basement membrane (BM) thickening, a histological hallmark of diabetic microangiopathy. To determine whether BM thickening of retinal capillaries could be prevented by down regulating synthesis of fibronectin, an ECM component, we used antisense oligos targeted against translation initiation site of the fibronectin transcript in galactose-fed rat, an animal model of diabetic retinopathy. After 2 months of galactose-feeding, intravitreal administration of 3 micro mol/l antisense fibronectin oligos was initiated at monthly intervals for 3 months. The antisense strategy significantly reduced fibronectin mRNA and protein level in the retinas of treated eyes compared with untreated eyes of galactose-fed rats (130 +/- 16 vs. 179 +/- 18% of control, P < 0.01, and 144 +/- 28 vs. 204 +/- 22% of control, respectively, r = 0.9) and resulted in partial reduction of retinal capillary BM width (123 +/- 16 vs. 201 +/- 12 nm, P < 0.03). In eyes treated with antisense fibronectin oligos, approximately 35% reduction in both pericyte loss and acellular retinal capillaries was observed (P < 0.04 and P < 0.03, respectively). Glycohemoglobin level was consistently elevated in the treated (6.9 +/- 0.6%) and untreated (6.5 +/- 0.7%) galactose-fed rats compared with control rats (4.5 +/- 0.8%). Overall, these results indicate that downregulation of fibronectin synthesis reduces BM thickening in retinal capillaries with beneficial effect to retinal lesions. The antisense fibronectin oligos may provide a useful approach for reducing vascular lesions in diabetic retinopathy. The thickened vascular BM may be a potential therapeutic target for preventing retinal lesions in diabetic retinopathy.

A peptide encoded by exon 6 of VEGF (EG3306) inhibits VEGF-induced angiogenesis in vitro and ischaemic retinal neovascularisation in vivo

VEGF is an important mediator of pathological angiogenesis in the eye and is a target for the development of novel anti-angiogenic molecules. In a previous study we identified 12-amino acid peptides derived from exon 6 of VEGF that inhibited VEGF binding to its receptors in HUVECs, endothelial cell functions, and in vitro angiogenesis. Screening of a series of truncated peptides corresponding to the inhibitory region of exon 6 identified a seven amino acid residue peptide, RKRKKSR, as the minimum exon 6-encoded sequence which retains the ability to inhibit VEGF receptor binding and angiogenesis in vitro. The effect of the seven-residue peptide was examined in a mouse model of ischaemic retinal neovascularisation. Administration of the peptide caused a 50% inhibition of retinal neovascularisation and was as effective in inhibiting ischaemic angiogenesis as soluble Flt-1 adenovirus. These results demonstrate that a seven amino acid VEGF exon 6-derived peptide is an effective inhibitor of ocular neovascularisation in vivo, and may have applications in the treatment of pathophysiological ocular neovascularisation in human disease.

Pathogenesis of retinopathy of prematurity

Retinopathy of prematurity (ROP) is a blinding disease, initiated by delayed retinal vascular growth after premature birth. There are both oxygen-regulated and non-oxygen-regulated factors, which contribute to both normal vascular development and retinal neovascularization. One important oxygen-regulated factor, critical to both phases of ROP, is vascular endothelial growth factor (VEGF). A critical non oxygen-regulated growth factor is insulin-like growth factor (IGF-1). In knockout mice, lack of IGF-1 prevents normal retinal vascular growth, despite the presence of VEGF, important to vessel development. In vitro, low IGF-1 prevents vascular endothelial growth factor-induced activation of Akt, a kinase critical for vascular endothelial cell survival. Premature infants who develop ROP have lower levels of serum IGF-1 than age-matched infants without disease.

CONCLUSION

IGF-1 is critical to normal vascular development. Low IGF-1 predicts ROP and restoration of IGF-1 to normal levels may prevent ROP.

Intravitreous anti-raf-1 kinase antisense oligonucleotide as an angioinhibitory agent in porcine preretinal neovascularization

PURPOSE

To test the efficacy of a synthetic antisense oligonucleotide inhibitor of an intracellular signal transduction protein, C-raf-1 kinase, as an inhibitor of ocular neovascularization.

METHODS

A 2'methoxyethyl, 2'deoxy chimeric 20-nucleotide sequence containing a uniform phosphorothioate backbone was synthesized which targets the 3'untranslated region of porcine C-raf-1 mRNA (ISIS 107189). Efficacy of mRNA inhibition was tested in vitro in porcine vascular endothelial cells and treated pigs by Northern blotting. In a pig model of intraocular neovascularization induced by branch retinal vein occlusion, intravitreal injection of ISIS 107189 compound (8 microM calculated intraocular concentration) at baseline and on days 14, 42, and 70, was tested against vehicle as control for inhibition of neovascularization. After enucleation on day 84, ocular tissues were analyzed for ISIS 107189 content by solid-phase extraction and capillary gel electrophoresis. Cryostat sections were immunostained for C-raf-1 kinase protein.

RESULTS

The antisense oligonucleotide demonstrated high potency for inhibition of C-raf-1 kinase in the porcine cells lines. Levels of C-raf-1 kinase were also decreased in the retina of pigs following a single 180 microg dose. Pig eyes injected with the multiple doses of 180 microg oligonucleotide demonstrated a marked decrease in neovascularization due to branch retinal vein occlusion 12 weeks after treatment (p = 0.05, Mann-Whitney U-test). Posterior subcapsular cataracts were noted in the treated eyes. Concentrations of oligonucleotide in retina ranged from 2-12 microM in the treated eyes. Qualitative assessment of the expression of C-raf-1 kinase via immunohistostaining of frozen sections demonstrated inhibition of expression in the treated eyes compared to controls.

CONCLUSIONS

ISIS 107189 successfully inhibited neovascularization in this model, which was correlated with an inhibition of expression of C-raf-1 kinase. While not proven in these studies, these results suggest that C-raf kinase may be important in angiogenesis. Antisense therapy has potential applicability in the therapy of ocular neovascular diseases.

The potential role of PKC beta in diabetic retinopathy and macular edema

Diabetic retinopathy is one of the most debilitating complications of diabetes mellitus. Despite major advances in understanding the pathogenesis of this disease and the efficacy of current therapies, diabetic retinopathy remains the leading cause of new-onset blindness among working-age people. The mainstay of current therapy, laser photocoagulation, is useful in preventing blindness and severe vision loss but is not often effective in restoring lost visual acuity. In addition, troublesome side effects and potentially serious complications may occur. Diabetic retinopathy is characterized by a progression of abnormalities. Nonproliferative retinopathy results from a series of biochemical and cellular changes that ultimately cause progressive retinal ischemia. The subsequent elaboration of growth factors in response to ischemia leads to the development of proliferative retinopathy, which is characterized by aberrant neovacularization of the retina-potentially leading to severe, irreversible visual loss. Increased retinal vascular leakage may also occur at any stage in this process, resulting in macular edema and possible progressive visual impairment. Although numerous biochemical factors are thought to play a role in the development of retinopathy, activation of protein kinase C (PKC), specifically the beta isoform of PKC (PKC beta), is implicated for both the early and late-stage manifestations of retinopathy. Studies suggest that orally administered LY333531, a beta-isoform specific PKC inhibitor, may be effective in ameliorating retinopathy progression, proliferation, and retinal vascular leakage. The status of ongoing clinical trials aimed at addressing the efficacy of PKC beta with regard to diabetes-induced retinal complications and perspectives on the role of PKC beta are presented.

Inhibitory mechanism of vascular endothelial growth factor (VEGF) by bucillamine

1. Vascular endothelial growth factor (VEGF) plays an important role in the neovascularization of ischaemic retinal diseases such as proliferative diabetic retinopathy. We determined that bucillamine, an anti-rheumatic drug, inhibits the VEGF production induced by hypoxia in bovine retinal microcapillary endothelial cells (BREC). To further clarify the inhibitory mechanism, we investigated the possible mechanism by which bucillamine exerts this inhibitory effect. 2. Bucillamine (100 micro M) decreased the hypoxia-induced increase of VEGF mRNA by 54.5% (P<0.001). Bucillamine (100 micro M) reduced the hypoxia-induced VEGF content in culture media by 29.0% (P<0.001), while monosulfydryl drugs, N-acetylcysteine and D-penicillamine, did not. 3. Bucillamine (100 micro M) did not affect VEGF mRNA half-life (hypoxia, 4.3 h; hypoxia+bucillamine, 3.9 h; normoxia, 2.7 h; normoxia+bucillamine, 2.7 h). 4. Reporter gene studies revealed that bucillamine reduced transcriptional activity in the 5'-flanking region of the VEGF gene by 74.0%. Hypoxia stimulated binding activity of BREC nuclear protein to a hypoxia responsive element (HRE), which was decreased by bucillamine. 5. Bucillamine inhibited hypoxic-induction of HIF-1alpha mRNA by 73.1% (P<0.001). Bucillamine also inhibited spontaneous VEGF mRNA expression by 26.6%. Furthermore, it inhibited activity of VEGF promoter and decreased binding activity to Sp1 and HRE, but did not alter AP1 and AP2 activity in normoxia. 6. These data suggest that bucillamine inhibits hypoxic induction of VEGF through inhibition of HIF-1 induction and binding activity in BREC. Bucillamine also inhibits the spontaneous expression of VEGF mRNA by its effect on Sp1 and HRE binding.

Effective transfection of a cis element "decoy" of the nuclear factor-kappaB binding site into the experimental choroidal neovascularization

PURPOSE

To evaluate the efficacy of the gene transfer of a double-stranded phosphorothioate oligonucleotides (ODNs), called a "decoy", against the NF-kappaB binding site into cells of an experimentally-induced choroidal neovascularization.

METHODS

FITC-labeled decoy was injected into the subretinal space of rat eyes by the HVJ-liposome delivery system, and 3 days later, choroidal neovascularization was induced by laser photocoagulation. The eyes were removed and the transfected cells were detected by fluorescence microscopy and also detected by immunohistochemistry. The degree of neovascularization was evaluated by fluorescein angiography.

RESULTS

The decoy was transfected into the retinal pigment epithelial (RPE) cells, inner and outer segment of the photoreceptors at 3 days after the injection. When choroidal neovascularization was induced, highly effective transfection of the decoy was observed 3 to 14 days after photocoagulation, after which the level decreased. Decoys were transfected into the RPE cells and macrophages in the choroidal neovascularization. The eyes transfected with NF-kappaB decoy showed a weaker leakage in fluorescein angiograms than that of the control eyes transfected with scrambled decoy.

CONCLUSIONS

A decoy can be transfected into retinal cells and cells within a choroidal neovascularization by the HVJ-liposome method. The transferred NF-kappaB decoy reduced the degree of choroidal neovascularization. Decoy targeted against NF-kappaB may be considered as a potential therapy for neovascularization.

Potential long-term inhibition of ocular neovascularisation by recombinant adeno-associated virus-mediated secretion gene therapy

Neovascularisation (NV) within the eye often results in visual loss. Vascular endothelial growth factor (VEGF) has been implicated in the development of ocular NV. Previous studies have shown that VEGF antagonists successfully suppressed retinal and choroidal NV in animal models. However, the systemic approach and transient nature of the delivery systems used in these studies hinder therapeutic application. To achieve stable and localised ocular anti-angiogenic therapy, we explored the use of recombinant adeno-associated virus (rAAV)-mediated secretion gene therapy (SGT). In this study, we generated a rAAV vector encoding soluble VEGF receptor 1, sFlt-1 (AAV-CMV.sflt) and determined its ability to inhibit cautery-induced corneal NV and laser-induced choroidal NV. Delivery of AAV-CMV.sflt into the anterior chamber resulted in transgene expression in the iris pigment epithelium and corneal endothelium, which reduced the development of corneal NV in the stroma of cauterised rats by 36% compared with cauterised control groups (P = 0.009). Subretinal delivery of AAV-CMV.sflt near the equator of the eye also suppressed choroidal NV at the laser lesions around the optic nerve by 19% (P = 0.002), indicating that there was diffusion of the secreted anti-angiogenic protein across the retina. Both results suggest that the long-term suppression of ocular NV is possible through the use of stable rAAV-mediated SGT.

Nucleotide sequence and expression of the feline vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is an angiogenic factor which targets vascular endothelial cells. In this study, cDNA encoding a feline VEGF (fVEGF) isoform was cloned from a feline lymphoid tumor cell line and sequenced. The fVEGF cDNA contained an open reading frame of 567 nucleotides coding for a polypeptide of 163 amino acids with a putative signal peptide of 26 amino acids. The predicted fVEGF amino acid sequence shared 98.4, 94.2 and 94.2% homology with the sequences of canine, bovine and human VEGF, respectively. Though predicted fVEGF polypeptide was two amino acid residues shorter than human VEGF165, a potential glycosylation site and regions critical for receptor binding were conserved in all the species examined. Transient expression of fVEGF in mammalian cells resulted in secretion of VEGF which could be detected by antibodies against human VEGF165. Furthermore, wide expression of fVEGF mRNA was observed in various feline tissues using RT-PCR methods.

Potential new medical therapies for diabetic retinopathy: protein kinase C inhibitors

PURPOSE

To review the evidence supporting a role for the beta (beta) isoform of protein kinase C (PKC) in the pathogenesis of diabetic retinopathy and the possible therapeutic benefit of inhibiting this enzyme.

DESIGN

Brief literature review of research suggesting the potential use for systemic inhibitors of the beta isoform of PKC as a medical therapy to prevent the progression of diabetic retinopathy. Brief consideration is given to previous, primarily clinical, studies dealing with other therapies for this disease.

RESULTS

Kinases transfer the terminal, "high energy," phosphate group of ATP to a site on a target protein, thereby activating the protein, which may be an enzyme, cell membrane receptor, or ion transport channel. The PKC family is a group of such enzymes that require specific activator molecules, including diacylglycerol, whose intracellular concentration is substantially increased during the hyperglycemia of diabetes. Protein kinase Cbeta is present at high levels in the retina. Increased activation of this enzyme, perhaps by producing tissue hypoxia, leads to increased expression of vascular endothelial growth factor, a mitogen that increases proliferation of vascular endothelial cells leading to neovascularization and enhances breakdown of the blood-retinal barrier, perhaps resulting in macular edema.

CONCLUSIONS

By interfering with the above biochemical pathways, PKC inhibitors may retard or prevent the development and progression of diabetic retinopathy. Because members of the PKC family are found throughout the body, a generalized inhibitor is likely to be toxic. However, an inhibitor specific for PKCbeta may act effectively within the retina and have a favorable toxicity profile. Two phase III randomized controlled clinical trials of such an inhibitor are now in progress, attempting to evaluate the efficacy of this approach to preventing the progression, or inducing regression, of "nonclinically significant" diabetic macular edema and of severe nonproliferative diabetic retinopathy.

The pathogenesis of diabetic retinopathy: old concepts and new questions

Hyperglycaemia appears to be a critical factor in the aetiology of diabetic retinopathy and initiates downstream events including: basement membrane thickening, pericyte drop out and retinal capillary non-perfusion. More recently, focus has been directed to the molecular basis of the disease process in diabetic retinopathy. Of particular importance in the development and progression of diabetic retinopathy is the role of growth factors (eg vascular endothelial growth factor, placenta growth factor and pigment epithelium-derived factor) together with specific receptors and obligate components of the signal transduction pathway needed to support them. Despite these advances there are still a number of important questions that remain to be answered before we can confidently target pathological signals. How does hyperglycaemia regulate retinal vessels? Which growth factors are most important and at what stage of retinopathy do they operate? What is the preferred point in the growth factor signalling cascade for therapeutic intervention? Answers to these questions will provide the basis for new therapeutic interventions in a debilitating ocular condition.

Inhibition of proliferative retinopathy by the anti-vascular agent combretastatin-A4

Retinal neovascularization occurs in a variety of diseases including diabetic retinopathy, the most common cause of blindness in the developed world. There is accordingly considerable incentive to develop drugs that target the aberrant angiogenesis associated with these conditions. Previous studies have shown that a number of anti-angiogenic agents can inhibit retinal neovascularization in a well-characterized murine model of ischemia-induced proliferative retinopathy. Combretastatin-A4 (CA-4) is an anti-vascular tubulin-binding agent currently undergoing clinical evaluation for the treatment of solid tumors. We have recently shown that CA-4 is not tumor-specific but elicits anti-vascular effects in nonneoplastic angiogenic vessels. In this study we have examined the capacity of CA-4 to inhibit retinal neovascularization in vivo. CA-4 caused a dose-dependent inhibition of neovascularization with no apparent side effects. The absence of vascular abnormalities or remnants of disrupted neovessels in retinas of CA-4-treated mice suggests an anti-angiogenic mechanism in this model, in contrast to the anti-vascular effects observed against established tumor vessels. Importantly, histological and immunohistochemical analyses indicated that CA-4 permitted the development of normal retinal vasculature while inhibiting aberrant neovascularization. These data are consistent with CA-4 eliciting tissue-dependent anti-angiogenic effects and suggest that CA-4 has potential in the treatment of nonneoplastic diseases with an angiogenic component.

Inhibition of retinal neovascularisation by gene transfer of soluble VEGF receptor sFlt-1

Retinal angiogenesis is a central feature of the leading causes of blindness. Current treatments for these conditions are of limited efficacy and cause significant adverse effects. In this study, we evaluated the angiostatic effect of gene transfer of the soluble VEGF receptor sFlt-1 in a mouse model of ischaemia-induced retinal neovascularisation using adenovirus and adeno-associated virus (AAV) vectors. We induced proliferative retinopathy in mice by exposure to 75% oxygen from postnatal day 7 (p7) to p12 and injected intravitreally recombinant viral vectors expressing the reporter green fluorescent protein (GFP) or vectors expressing the VEGF inhibitor sFlt-1. Efficient adenovirus-mediated GFP expression was evident in cells of the corneal endothelium and iris pigment epithelium. AAV-mediated GFP expression was evident in ganglion cells and cells of the inner nuclear layer of the retina. Vector-mediated sFlt-1 expression was confirmed by ELISA of pooled homogenised whole eyes. Injection of either vector expressing sFlt-1 resulted in a reduction in the number of neovascular endothelial cells by 56% and 52% for adenovirus and AAV vectors, respectively (P < 0.05). Local gene transfer of sFlt-1 consistently inhibits experimental retinal neovascularisation by approximately 50% and offers a powerful novel approach to the clinical management of retinal neovascular disorders.

Inducible expression of vascular endothelial growth factor in adult mice causes severe proliferative retinopathy and retinal detachment

Transgenic mice with vascular endothelial growth factor (VEGF) driven by the rhodopsin promoter (rho/VEGF mice) develop neovascularization that originates from the deep capillary bed of the retina and grows into the subretinal space. In rho/VEGF mice, VEGF expression in photoreceptors begins between postnatal days 5 and 7, the period when the deep capillary bed is developing. An important question is whether or not the developmental stage of the deep capillary bed is critical for occurrence of neovascularization. Also, although rho/VEGF mice are extremely useful for the study of ocular neovascularization, there are some applications for which the early onset of VEGF expression is a disadvantage. In this study, we used the reverse tetracycline transactivator (rtTA) inducible promoter system coupled to either the rhodopsin or interphotoreceptor retinoid-binding protein (IRBP) promoter to control the time of onset of VEGF transgene expression in photoreceptors. In the absence of doxycycline, adult double-transgenic rho/rtTA-TRE/VEGF or IRBP/rtTA-TRE/VEGF mice showed little VEGF transgene expression and no phenotype. The addition of doxycycline to the drinking water resulted in prominent transgene expression and evidence of neovascularization within 3 to 4 days. Like rho/VEGF mice, the neovascularization originated from the deep capillary bed of the retina, but it was more extensive and caused outer retinal folds followed by total retinal detachment. Real-time polymerase chain reaction and enzyme-linked immunosorbent assay demonstrated that the mice with inducible expression of VEGF that developed retinal detachment had much higher ocular levels of VEGF mRNA and protein compared to rho/VEGF mice that manifest a much milder phenotype. These data demonstrate that regardless of developmental stage of the vascular bed, increased expression of VEGF in the retina is sufficient to cause neovascularization, and high levels of expression cause severe neovascularization and traction retinal detachment. Mice with inducible expression of VEGF in the retina provide a valuable new model of ocular neovascularization.