Further pharmacological and genetic evidence for the efficacy of PlGF inhibition in cancer and eye disease.

Our findings that PlGF is a cancer target and anti-PlGF is useful for anticancer treatment have been challenged by Bais et al. Here we take advantage of carcinogen-induced and transgenic tumor models as well as ocular neovascularization to report further evidence in support of our original findings of PlGF as a promising target for anticancer therapies. We present evidence for the efficacy of additional anti-PlGF antibodies and their ability to phenocopy genetic deficiency or silencing of PlGF in cancer and ocular disease but also show that not all anti-PlGF antibodies are effective. We also provide additional evidence for the specificity of our anti-PlGF antibody and experiments to suggest that anti-PlGF treatment will not be effective for all tumors and why. Further, we show that PlGF blockage inhibits vessel abnormalization rather than density in certain tumors while enhancing VEGF-targeted inhibition in ocular disease. Our findings warrant further testing of anti-PlGF therapies.

The SDF-1/CXCR4 ligand/receptor pair is an important contributor to several types of ocular neovascularization.

Hypoxia causes increased expression of several proteins that have the potential to promote neovascularization. Vascular endothelial growth factor (VEGF) is up-regulated by hypoxia in the retina and plays a central role in the development of several types of ocular neovascularization, but the effects of other hypoxia-regulated proteins are less clear. Stromal-derived factor-1 (SDF-1) and its receptor, CXCR4, have hypoxia response elements in the promoter regions of their genes and are increased in hypoxic liver and heart. In this study, we found that SDF-1 and CXCR4 are increased in hypoxic retina, with SDF-1 localized in glial cells primarily near the surface of the retina and CXCR4 localized in bone marrow-derived cells. Glial cells also expressed CXCR4, which suggested the possibility of autocrine stimulation, but influx of bone marrow-derived cells is the major source of increased levels of CXCR4. High levels of VEGF in the retina in the absence of hypoxia also increased levels of Cxcr4 and Sdf1 mRNA. CXCR4 antagonists reduced influx of bone marrow-derived cells into ischemic retina and strongly suppressed retinal neovascularization, VEGF-induced subretinal neovascularization, and choroidal neovascularization. These data suggest that SDF-1 and CXCR4 contribute to the involvement of bone marrow-derived cells and collaborate with VEGF in the development of several types of ocular neovascularization. They provide new targets for therapeutic intervention that may help to bolster and supplement effects obtained with VEGF antagonists.

TNF-alpha is critical for ischemia-induced leukostasis, but not retinal neovascularization nor VEGF-induced leakage.

Vascular endothelial growth factor (VEGF) and tumor necrosis factor-alpha (TNF-alpha) show significant overlap with regard to their effects in the eye. It has been postulated that VEGF-induced leukostasis, breakdown of the blood-retinal barrier, and ischemia-induced retinal neovascularization may be mediated, at least in part, through TNF-alpha. In this study, we used mice deficient in TNF-alpha to test our hypothesis. Compared to wild type mice, TNF-alpha-deficient mice showed an 80% reduction in leukocyte accumulation in retinal vessels after intravitreous injection of VEGF, and 100% reductions after intravitreous injections of interleukin-1beta (IL-1beta) or platelet-activating factor (PAF). The increase in retinal vascular permeability induced by injection of PAF was significantly reduced in mice lacking TNF-alpha, but VEGF- and IL-1beta-induced leakage was unaffected. Compared to wild type mice with oxygen-induced ischemic retinopathy, TNF-alpha-deficient mice with ischemic retinopathy showed significantly reduced leukostasis and mild reduction in vascular leakage, but no significant difference in retinal neovascularization. These data suggest that TNF-alpha mediates VEGF-, IL-1beta-, and PAF-induced leukostasis and vascular leakage mediated by PAF, but not leakage caused by VEGF or IL-1beta. Ischemia-induced retinal neovascularization, which has previously been shown to require VEGF, does not require TNF-alpha and is unaffected by attenuation of leukostasis.

Vasohibin is up-regulated by VEGF in the retina and suppresses VEGF receptor 2 and retinal neovascularization.

Vasohibin is a recently identified protein that is up-regulated in cultured vascular endothelial cells by vascular endothelial growth factor and fibroblast growth factor 2. It inhibits endothelial cell migration, proliferation, and tube formation, and suppresses angiogenesis in chick chorioallantoic membrane, after subcutaneous implantation of matrigel, and in a tumor xenograft model. This has led to the hypothesis that vasohibin functions as a negative feedback inhibitor of angiogenesis. In this study, we tested that hypothesis in a well-characterized model of retinal neovascularization. In ischemic retina, increased expression of VEGF was accompanied by elevation of vasohibin mRNA and blocking of the increase in vegf mRNA with vegf siRNA significantly attenuated the rise in vasohibin mRNA. In transgenic mice in which the rhodopsin promoter drives expression of VEGF in the retina, there was also a significant increase in vasohibin mRNA. In mice with ischemic retinopathy, there was increased expression of vasohibin in vascular endothelial cells, and vasohibin knockdown caused an increase in neovascularization. Conversely, intraocular injection of recombinant vasohibin or an adenoviral vector containing a vasohibin expression cassette strongly suppressed retinal neovascularization in mice with ischemic retinopathy. Knockdown of vasohibin mRNA in ischemic retina had no significant effect on vegf or vegf receptor 1 mRNA levels but caused a significant elevation in the level of vegf receptor 2 mRNA. These data support the hypothesis that vasohibin acts as a negative feedback regulator of neovascularization in the retina and suggest that suppression of VEGF receptor 2 may play some role in mediating its activity.

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.

Mouse model of post-surgical breakdown of the blood-retinal barrier.

PURPOSE: Post-surgical macular edema is an important clinical problem resulting from breakdown of the blood-retinal barrier (BRB) after surgery. This study was designed to develop a mouse model of post-surgical BRB breakdown. METHODS: Two 25-gauge needles, one for infusion and one for aspiration, were inserted through the limbus and into the lens of one eye of adult male C57BL/6 mice. The anterior portion of the lens was aspirated and the fellow eye was untreated. At several time points after surgery, the integrity of the BRB was assessed quantitatively, using [3H]mannitol as a tracer, or qualitatively, using immunohistochemical staining for albumin. RESULTS: Eyes with partial lens extraction had a significant increase in retinal vascular leakage one day after surgery, which persisted two and three days after surgery, but by five days, was not significantly different from controls. Immunohistochemical staining for albumin demonstrated that the breech in the barrier was sufficient to allow passage of a 60kDa protein into the retina, and was localized predominantly to retinal vessels. CONCLUSIONS: Partial lens extraction in mice results in BRB breakdown (primarily the inner BRB) that is highly reproducible in the severity of leakage and its time course. This provides a valuable tool for investigation of the molecular pathogenesis and new treatment approaches for post-surgical breakdown of the BRB.

Periocular gene transfer of sFlt-1 suppresses ocular neovascularization and vascular endothelial growth factor-induced breakdown of the blood-retinal barrier.

Vascular endothelial growth factor (VEGF) is a critical stimulus for both retinal and choroidal neovascularization, and for diabetic macular edema. We used mouse models for these diseases to explore the potential of gene transfer of soluble VEGF receptor-1 (sFlt-1) as a treatment. Intravitreous or periocular injection of an adenoviral vector encoding sFlt-1 (AdsFlt-1.10) markedly suppressed choroidal neovascularization at rupture sites in Bruch's membrane. Periocular injection of AdsFlt-1.10 also caused significant reduction in VEGF-induced breakdown of the blood-retinal barrier, but failed to significantly inhibit ischemia-induced retinal neovascularization. Periocular delivery of an adenoviral vector encoding pigment epithelium-derived factor (PEDF), another secreted protein, resulted in high levels of PEDF in the retinal pigmented epithelium and choroid, but not in the retina. This may explain why periocular injection of AdsFlt-1.10 inhibited choroidal, but not retinal neovascularization. Periocular delivery offers potential advantages over other routes of delivery and the demonstration that sFlt-1 enters the eye from the periocular space in sufficient levels to achieve efficacy in treating choroidal neovascularization and retinal vascular permeability is a novel finding that has important clinical implications. These data suggest that periocular gene transfer of sFlt-1 should be considered for treatment of choroidal neovascularization and diabetic macular edema.

Quantitative assessment of the integrity of the blood-retinal barrier in mice.

PURPOSE: The purpose of this study was to develop and characterize a quantitative assay of blood-retinal barrier (BRB) function in mice and to determine the effect of several purported vasopermeability factors on the BRB. METHODS: Adult C57BL/6J mice were treated with three regimens of increasingly extensive retinal cryopexy and subsequently were given an intraperitoneal injection of 1 microCi/g body weight of [(3)H]mannitol. At several time points, the amount of radioactivity per milligram tissue was compared in retina, lung, and kidney. Time points that maximize signal-to-background differential in the retina were identified, and the ratio of counts per minute (CPM) per milligram retina to CPM per milligram lung (retina-to-lung leakage ratio, RLLR) or kidney (retina-to-renal leakage ratio, RRLR) were calculated. This technique was then used to compare the amount of BRB breakdown that occurs after intravitreous injection of vascular endothelial growth factor (VEGF), insulin-like growth factor (IGF)-1, prostaglandin (PG) E(1), PGE(2), interleukin (IL)-1beta, or tumor necrosis factor (TNF)-alpha. RESULTS: Twenty-four hours after retinal cryopexy, there was a higher level of radioactivity in treated than in control retinas, and the signal-to-background difference was optimal when measurements were obtained 1 hour after injection of [(3)H]mannitol. In untreated mice, the RLLR was 0.30 +/- 0.02 and the RRLR was 0.22 +/- 0.01. Twenty-four hours after one 5-second application of retinal cryopexy, the RLLR was 0.73 +/- 0.20 and the RRLR was 0.71 +/- 0.23. With increasing amounts of cryopexy, there was an increase in the RLLR and RRLR, so that after two 10-second applications, the RLLR was 1.66 +/- 0.31 and the RRLR was 1.47 +/- 0.20. Intravitreous injection of VEGF, IGF-1, PGE(1), PGE(2), IL-1beta, or TNF-alpha each caused significant increases in the RLLR and RRLR, but there were some differences in potency and time course. VEGF caused prominent BRB breakdown at 6 hours that returned to near normal by 24 hours. IL-1beta also caused relatively rapid breakdown of the BRB, but its effect was more prolonged than that caused by VEGF. There was delayed, but substantial breakdown of the BRB after injection of TNF-alpha. IGF-1, PGE(2), and PGE(1) caused less severe, relatively delayed, and more prolonged BRB breakdown. CONCLUSIONS: Measurement of the RLLR or RRLR after intraperitoneal injection of [(3)H]mannitol in mice provides a quantitative assessment of BRB function that is normalized and can therefore be compared from assay to assay. Comparison of the extent and duration of BRB breakdown after intravitreous injection of vasoactive substances shows that agents can be grouped by resultant extent and time course of leakage. Additional studies are needed to determine whether this grouping has its basis in shared mechanisms of BRB disruption.

Unlabelled iododeoxyuridine increases the rate of uptake of [125I]iododeoxyuridine in human xenografted glioblastomas.

5-Iodo-2'-deoxyuridine (IdUrd), a thymidine (TdR) analogue, can be radiolabelled with iodine-125, an Auger radiation emitter, to provoke double-strand breaks once incorporated into DNA of cancer cells. We have previously shown that co-incubation of [125I]IdUrd with unlabelled IdUrd provided an additive cytotoxicity in two human glioblastoma cell lines. This observation was unexpectedly correlated with an increase in the rate of DNA incorporation of [125I]IdUrd. Here, we further evaluated the effects of unlabelled IdUrd on the uptake of [125I]IdUrd in vitro and in vivo in mice xenografted with three human glioblastoma lines. The results showed that, in these three glioblastoma lines, unlabelled IdUrd increased the rate of uptake of [125I]IdUrd in vitro by 2- to 4.4-fold and in vivo by 1.5- to 2.8-fold. The rate of uptake of [125I]IdUrd in normal rapidly dividing tissues was also increased by 1.3- to 2.8-fold. TdR completely blocked [125I]IdUrd uptake in tumours and tissues. Analogues of IdUrd, such as deoxyuridine and 5-iodo-1,3-dimethyuracil, did not reproduce the effect of IdUrd on the uptake of [125I]IdUrd, suggesting that it is not related to protection against [125I]IdUrd degradation. It is concluded that combined administration of unlabelled IdUrd may improve the use of radiolabelled IdUrd for cancer diagnosis or therapy.