The urokinase/urokinase receptor system in retinal neovascularization: inhibition by A6 suggests a new therapeutic target.

PURPOSE The objective of the study was to determine the role of urokinase (uPA) and the urokinase receptor (uPAR) in retinal angiogenesis, and whether loss of uPAR or the inhibition of uPA/uPAR interactions could suppress the extent of retinal neovascularization in an animal model of ischemic retinopathy. METHODS Retinal neovascularization was induced by exposing newborn mice to 75% oxygen on postnatal day 7 for 5 days, followed by exposure to room air on days 12 to 17. The expression of uPAR in the retina was investigated by RT-PCR and immunohistochemistry. The role of uPAR in ischemic retinopathy was investigated by quantitating the extent of retinal neovascularization in the uPAR(-/-) mouse. The effects of inhibiting the uPA/uPAR interaction on the development of retinal neovascularization were studied in this animal model with a uPA-derived peptide, A6. Animals were treated with an intraperitoneal injection of A6 at a dose of 5, 10, or 100 mg/kg once a day on days 12 to 16. Control animals included oxygen-exposed mice treated with similar amounts of PBS only on days 12 to 16. The effect of A6 on the expression of uPAR in the retina was examined by real-time RT-PCR. RESULTS The expression of uPAR mRNA was upregulated in experimental animals during the period of angiogenesis and was localized to endothelial cells in the superficial layers of the retina. The uPAR(-/-) mouse demonstrated normal retinal vascular development; however, the absence of functional uPAR resulted in a significant reduction in the extent of retinal neovascularization. Histologic analysis of mice treated with A6 peptide showed significant inhibition of retinal neovascularization, and the response was dose dependent. The RT-PCR analysis of the retinas of the A6-treated animals showed a greater than twofold decrease in uPAR expression. CONCLUSIONS Expression of the urokinase receptor uPAR is essential to the development of retinal neovascularization. Inhibition of the activity of uPAR suppresses retinal neovascularization, possibly through a reduction in cell-associated proteolytic activity, cell signaling, or cell-matrix adhesion necessary for cell migration during angiogenesis. The uPA/uPAR interaction may be an important therapeutic target in the management of proliferative retinopathies.