The dipeptide Arg-Gln inhibits retinal neovascularization in the mouse model of oxygen-induced retinopathy.

PURPOSE: Premature infants undergoing intensive care are highly vulnerable to amino acid deprivation. Supplementation of glutamine or arginine has resulted in beneficial effects in human neonates. This study was conducted to examine the effect of the dipeptide arginyl-glutamine (Arg-Gln) on vascular endothelial cell growth factor (VEGF) levels in primary human retinal pigment epithelial (hRPE) cell cultures and on inhibition of neovascularization in the oxygen-induced retinopathy (OIR) model. METHODS: The effects of Arg-Gln on VEGF levels were measured in supernates from hRPE cells by using ELISAs. For in vivo studies, mouse pups received twice-daily intraperitoneal injections of Arg-Gln, a control dipeptide (Ala-Gly) or were not injected. Retinal flatmounts from one cohort were prepared and retinal vessel morphology examined. The contralateral eyes were embedded, sectioned, and stained to count preretinal neovascular nuclei. RNA was isolated from retinas of selected animals and was used to quantify VEGF mRNA by real-time RT-PCR. RESULTS: Treatment of hRPE cells with Arg-Gln decreased VEGF levels in a dose-dependent manner. In the OIR model, Arg-Gln at 5 g/kg per day reduced preretinal neovascularization by 82%+/-7% (P<0.005), when compared with the control dipeptide Ala-Gly, and reduced VEGF mRNA by 64%+/-9% (P<0.001). CONCLUSIONS: Arg-Gln dramatically inhibited retinal neovascularization in the OIR model. This effect was associated with a reduction in retinal VEGF mRNA levels. Similarly the dipeptide reduced VEGF expression in hRPE cells, a cell type likely to respond to retinal hypoxia by expressing VEGF. Arg-Gln appears to be safe and, with future studies in human infants, may prove beneficial in the prevention of ROP.

Effects of VEGFR-1, VEGFR-2, and IGF-IR hammerhead ribozymes on glucose-mediated tight junction expression in cultured human retinal endothelial cells.

PURPOSE: To evaluate whether transfection of human retinal endothelial cells (HRECs) with plasmids expressing ribozymes designed to specifically cleave the mRNA and reduce expression of either vascular endothelial growth factor (VEGF) receptor-1 (VEGFR-1), or VEGF receptor-2 (VEGFR-2), or insulin-like growth factor-I receptor (IGF-IR) modulates occludin expression in high glucose-treated cells. METHODS: Hammerhead ribozymes that specifically cleave the human VEGFR-1, VEGFR-2, and IGF-IR mRNAs were developed and tested in vitro to determine ribozyme kinetics and cleavage specificity. HRECs grown in normal (5.5 mM) and high (25 mM) glucose medium were transfected with plasmids expressing VEGFR-1, VEGFR-2, or IGF-IR hammerhead ribozymes. VEGF and IGF-I levels were measured in conditioned medium of HREC exposed to high glucose conditions, and the effect of varying glucose concentration on VEGFR-1 and VEGFR-2 phosphorylation was examined. The amount of the tight junction protein occludin was determined by western analysis, and the protein was localized by immunohistochemistry. RESULTS: Exposure of HRECs to high glucose resulted in increased VEGF and IGF-I expression as well as VEGFR-2 but not VEGFR-1 phosphorylation. Immunocytochemistry and western analysis revealed that HRECs exposed to high glucose had reduced occludin staining and protein expression, respectively. Transfection of HRECs exposed to high glucose with either VEGFR-1, VEGFR-2, or IGF-IR hammerhead ribozymes prevented the downregulation of occludin protein expression. CONCLUSIONS: Our studies support that activation of VEGFR-1, VEGFR-2, and IGF-IR by high glucose contributes to disruption of tight junctions by decreasing occludin expression and may be important in the pathogenesis of blood-retinal barrier dysfunction in diabetic retinopathy.

Focal adhesion kinase overexpression induces enhanced pathological retinal angiogenesis.

PURPOSE: Focal adhesion kinase (FAK) is involved in processes integral to angiogenesis, such as cell growth, survival, and migration. FAK is activated by angiogenic growth factors, such as insulin-like growth factor (IGF)-I, vascular endothelial growth factor (VEGF), and basic fibroblast growth factor (bFGF). The study was conducted to determine whether overexpression of FAK or FAK-related nonkinase (FRNK), an inhibitor of FAK, could influence human retinal endothelial cell (HREC) migration and in vivo angiogenesis. METHODS: Migration in response to a combination of growth factors was examined in transfected HRECs overexpressing FAK or FRNK. The effect of FAK or FRNK overexpression on preretinal neovascularization was examined in a mouse model of oxygen-induced retinopathy. RESULTS: Overexpression of FAK in HRECs resulted in a 102% +/- 13% increase (P = 1.4 x 10(-4)) in cell migration, whereas overexpression of FRNK resulted in a 20% +/- 8% decrease (P = 0.01). Overexpression of FAK in mouse eyes led to formation of numerous large vascular tufts resembling glomeruli and a 57% +/- 7% increase in preretinal neovascularization (P = 3 x 10(-9)), whereas FRNK resulted in a 55% +/- 15% reduction (P = 5 x 10(-5)). CONCLUSIONS: Modulating the FAK/FRNK system may provide a novel approach to inhibiting pathologic retinal angiogenesis.

Decreased expression of the insulin-like growth factor 1 receptor by ribozyme cleavage.

PURPOSE: Insulin-like growth factor (IGF)-1 and its receptor (IGF-1R) are associated with abnormal retinal neovascularization. Ribozymes were designed that selectively decreased the expression of the IGF-1R and these ribozymes were tested in angiogenesis models in vitro and in vivo. METHODS: Two hammerhead ribozymes were designed that cleave the human IGF-1R mRNA. The ribozymes were cloned into recombinant adeno-associated viral vectors (rAAV). The rAAV constructs were transfected into human retinal endothelial cells (HRECs). IGF-1R mRNA and protein levels were examined and the modified Boyden chamber assay used to examine ribozyme effects on cell migration. These constructs were injected intravitreally into mice to determine the effect of the ribozymes on retinal neovascularization in a mouse model of oxygen-induced retinopathy. RESULTS: Relative quantitative RT-PCR analysis showed that IGF-1R Rz1 reduced IGF-1R mRNA levels by 40% +/- 10% (P = 0.003), and Western blot analysis showed a 41% +/- 5% (P = 4.6 x 10(-5)) reduction of IGF-1R protein, confirming that this ribozyme reduces IGF-1R expression. IGF-1R Rz1 also reduced IGF-1-induced cell migration by 90% +/- 5% (P = 2.9 x 10(-9)) showing that IGF-1R Rz1 reduces IGF-1R function in HRECs. IGF-1R Rz1 also reduced the amount of preretinal neovascularization by 65% +/- 6% (P = 2.7 x 10(-5)), as measured by the average number of endothelial preretinal nuclei per section. CONCLUSIONS: These studies demonstrate that the IGF-1R ribozymes are effective at reducing the expression and function of the IGF-1R in vitro and in vivo. Therefore, the IGF-1R ribozymes are an effective method for studying the process of angiogenesis and may ultimately be effective as gene therapy tools for the reduction of pathologic retinal angiogenesis.

Fibronectin fragments promote human retinal endothelial cell adhesion and proliferation and ERK activation through alpha5beta1 integrin and PI 3-kinase.

PURPOSE: Extracellular matrix degradation is associated with neovascularization in diabetic retinas. Fibronectin fragments (Fn-fs) are generated during vascular remodeling. The effects of cellular fibronectin (Fn) and selected Fn-fs on adhesion, proliferation, and signal transduction in human retinal endothelial cells (HRECs) were characterized. METHODS: Relative quantitative RT-PCR, flow cytometry, and immunocytochemistry determined integrin expression on HRECs. Adhesion was evaluated by coating plastic with Fn or Fn-fs of 45, 70, 110, or 120 kDa, and MTT conversion was used to measure proliferation and survival. Peptide inhibitors and blocking antibodies determined adhesive sites and integrins used for adhesion. Pharmacologic inhibitors and Western analyses were used to evaluate intracellular signaling. RESULTS: HRECs produced significant levels of alpha(2), alpha(3), alpha(5), alpha(v), beta(1), beta(3), and beta(5) integrin subunit mRNA. Flow cytometry of surface integrin expression revealed high levels of alpha(3), alpha(5), and beta(1) and lower levels of alpha(1), alpha(v), beta(3), and beta(5). These results were confirmed by immunocytochemistry. For adhesion to Fn and Fn-fs. the alpha(5)beta(1) integrin was essential. Pharmacologic inhibitors of PI 3-kinase blocked adhesion to Fn and Fn-fs, whereas the mitogen-activated protein (MAP) kinase kinase (MEK) inhibitor PD98059 blocked phosphorylation. The 110- and 120-kDa Fn-fs showed a concentration-dependent increase in proliferation, whereas 500 ng of the 70 kDa Fn-f-induced proliferation. Addition of III1-C, a matrix assembly domain, increased the proliferative effect of these Fn-fs. CONCLUSIONS: Fn and its Fn-fs modulate HREC adhesion and proliferation through signal-transduction pathways involving coupling of the alpha(5)beta(1) integrin through PI 3-kinase. Mitogenic signals for endothelial cells from degraded extracellular matrix may contribute to the development of diabetic retinopathy.

Expression of IGFBP-3 by human retinal endothelial cell cultures: IGFBP-3 involvement in growth inhibition and apoptosis.

PURPOSE: Growth hormone (GH), insulin-like growth factor (IGF), and somatostatin (SST) modulate each other's actions. SST analogues have been successfully used to treat proliferative diabetic retinopathy (PDR) that is unresponsive to laser therapy and to retard the progression of severe nonproliferative retinopathy to PDR. In this study, the endogenous expression of IGF-binding protein (IGFBP)-3 was examined in human retinal endothelial cells (HRECs), the direct effects of IGFBP-3 on HRECs were evaluated, and the possible involvement of IGFBP-3 in mediating the growth inhibitory effects of SST receptor (SSTR) agonists in HRECs was assessed. METHODS: The cellular localization of IGFBP-3 was examined with anti-IGFBP-3 and fluorescein-conjugated goat anti-rabbit IgG. HRECs were exposed to varying concentrations of human recombinant IGFBP-3, and growth inhibition was evaluated by thiazolyl blue (MTT) conversion. Apoptosis was examined using fluorochrome-annexin V staining. Conditioned media (CM) from SSTR2 agonist (L779976)-treated or SSTR3 agonist (L796778)-treated HRECs were analyzed by ELISA for changes in expression of IGFBP-3. RESULTS: HREC immunostaining showed cell surface and cytoplasmic IGFBP-3. Exogenous IGFBP-3 induced a dose-dependent inhibition of HREC proliferation and staining with fluorochrome-annexin V showed numerous apoptotic HRECs. HRECs exposed to the SSTR2 or SSTR3 agonists expressed IGFBP-3 in a concentration-dependent manner. CONCLUSIONS: Cultured HRECs expressed endogenous IGFBP-3. Exogenous administration of IGFBP-3-induced growth inhibition and apoptosis, supporting a regulatory role for IGFBP-3 in endothelial cells. SSTR agonists mediate their growth-inhibitory effect, in part, by increasing expression of IGFBP-3.

Adult hematopoietic stem cells provide functional hemangioblast activity during retinal neovascularization.

Adults maintain a reservoir of hematopoietic stem cells that can enter the circulation to reach organs in need of regeneration. We developed a novel model of retinal neovascularization in adult mice to examine the role of hematopoietic stem cells in revascularizing ischemic retinas. Adult mice were durably engrafted with hematopoietic stem cells isolated from transgenic mice expressing green fluorescent protein. We performed serial long-term transplants, to ensure activity arose from self-renewing stem cells, and single hematopoietic stem-cell transplants to show clonality. After durable hematopoietic engraftment was established, retinal ischemia was induced to promote neovascularization. Our results indicate that self-renewing adult hematopoietic stem cells have functional hemangioblast activity, that is, they can clonally differentiate into all hematopoietic cell lineages as well as endothelial cells that revascularize adult retina. We also show that recruitment of endothelial precursors to sites of ischemic injury has a significant role in neovascularization.