Objective-Tissue kallikrein, a widely used vasodilator for the treatment of hypertension and peripheral circulatory disorder, acts by releasing kinin, a potent vasodilator peptide. To identify the role of tissue kallikrein in retinal neovascularization, we investigated the antiangiogenic effect by using an in vitro and in vivo angiogenesis model. Methods and Results-Tissue kallikrein in vitreous fluid was markedly elevated in proliferative diabetic retinopathy patients compared with that in

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... rol patients with macular hole and epiretinal membrane. Tissue kallikrein inhibited vascular endothelial growth factor-165 (VEGF 165 )-induced tube formation, proliferation, and migration in vitro angiogenesis model via suppression of the VEGF 165 -induced phosphorylation of VEGF receptor-2. Furthermore, tissue kallikrein cleavage of VEGF 165 was on the C-terminal side, which was analyzed by Western blotting and mass spectrometry. When administered subcutaneously, tissue kallikrein reduced the pathological vascular changes in retinal neovascularization induced in neonatal mice by returning the retina to normoxia after exposure to hyperoxia. Conclusion-These findings indicate that tissue kallikrein is partly involved in pathogenesis of proliferative diabetic retinopathy and may be a promising therapeutic agent that could cleave VEGF 165 itself when administered by a peripheral route. (Arterioscler Thromb Vasc Biol. 2011;31:1041-1048.)