Ischemia Induces Blood-retinal Barrier (BRB) Breakdown through HIF-1 Dependent VEGF Upregulation and VEGF Mediates BRB Breakdown through ICAM-1 and Adenosine

Abstract

Abstract: : Purpose:To determine whether ischemia induces BRB breakdown and neovascularization (NV) through HIF-1 dependent upregulation of VEGF, and to investigate the mechanism of VEGF-mediated BRB breakdown. Methods:Oxygen-induced ischemic retinopathy was generated in mice homozygous for a deletion of the hypoxia-response element in the VEGF promoter and thus incapable of producing VEGF through HIF-1 activation (Vegf^δδ mice) and in wild-type littermates by exposing 7-day old mice to 75% oxygen for 5 days and then transferring them to the relative hypoxia of room air for 5 days. VEGF was administered by intravitreal injection (10^-6M) to normal and ICAM-1 knockout mice. Some VEGF-injected mice were treated with the adenosine antagonist, XAC (30mg/kg i.p.). BRB breakdown was quantitatively evaluated by administering [³H]-mannitol as a tracer and determining the retina to lung and retina to renal leakage ratios. BRB breakdown sites were localized by immunohistochemical staining for albumin. Results:Widespread, diffuse BRB breakdown occurred in oxygen-induced ischemic retinopathy, but it was blocked in Vegf^δδ mice compared to littermate controls. The BRB was not significantly different in Vegf^δδ mice with oxygen-induced ischemic retinopathy than in untreated, age-matched, wild type controls. Also in this model, Vegf^δδ mice had dilation of inner retinal vessels, but no NV. VEGF-mediated BRB breakdown was eliminated in ICAM-1 knockout mice and significantly reduced by XAC. Conclusions:These data suggest that HIF-1 dependent VEGF induces BRB breakdown and retinal NV in oxygen-induced ischemic retinopathy. Mice are incapable of developing retinal NV in this model without HIF-1 stimulation of VEGF expression, but vasodilation occurs. VEGF mediates BRB breakdown through ICAM-1, which stimulates an influx of inflammatory cells, and through adenosine. Inhibition of ICAM-1 or adenosine may suppress BRB breakdown leading to macular edema in ischemic retinopathies.

Keywords: pump/barrier function • ischemia • growth factors/growth factor receptors