COX-2 Promotes Intravitreal Neovascularization via a PGE2 / EP3 Receptor-dependent Mechanism

Abstract

Abstract: : Purpose: Inflammatory mechanisms influence vitreal neovascularization and cyclooxygenase (COX)-2 promotes tumor angiogenesis. We investigated the role of COX-2 and specifically PGE2 and its receptors in ischemic proliferative retinopathy. Methods: The mouse and rat model of oxygen induced ischemic proliferative retinopathy were used. COX-2 and EP receptor expression were analysed by RT-PCR and Western blot analysis and their localization was studied by immunohistochemistry in the ischemic retina. Mice and rat pups undergoing the ischemic phase of the model were treated respectively with selective COX-2 inhibitors APHS and etodolac, alone or in conjunction with butaprost (EP2 agonist) or M&B;28767 (EP3 agonist) or treated with THG 213.15 (EP4 antagonist). Neovascularization was analyzed on flatmounts after 5 days of ischemia. VEGF and eNOS expression were analyzed in vehicle and COX-2 inhibitor-treated retinas by Western blot analysis. Results: COX-2 and EP 2, 3 and 4 (but not EP1) were equivalently expressed in whole retinas of normoxic and ischemic rodents. Immunohistochemical localization revealed increased COX-2 expression in retinal astrocytes during ischemia. EP3 was localized at the vascular endothelium. VEGF, known to be affected by PGE2, increased 8 h post-ischemia and returned to basal values by 24 h. APHS and etodolac effectively inhibited intravitreal neovascularization but not the percent of avascular area; COX-1 inhibitor SC560 was ineffective. Effects of COX-2 inhibition were reversed in part by simultaneous PGE2 injection and markedly by EP3 agonist M&B;28767, but not by modulation of EP2 and EP4 activity. eNOS, which mediates some of the effects of VEGF, was downregulated by 24 h treatment with etodolac. Conclusions: The proangiogenic effect of COX-2 in ischemic proliferative retinopathy is mainly mediated by PGE2 via activation of its EP3 receptor, which induces eNOS expression. COX-2 inhibitors may exert beneficial antiangiogenic effects in the ischemic proliferative retinopathy.

Keywords: neovascularization • ischemia • retinopathy of prematurity