Activation of Arginase and the Endothelin System in Models of Ischemic Retinopathy

Abstract

Ischemic retinopathies, such as diabetic retinopathy (DR) and retinopathy of prematurity (ROP) are characterized by microvascular degeneration, followed by an abnormal hypoxia-induced neovascularization (NV). Although the triggering insult varies among the diseases, they share a common end result of capillary loss due to increased oxidative stress, cellular inflammation and vascular injury and dysfunction. We have linked activation of the urea hydrolase enzyme arginase to the latter complications in models of DR. Both arginase and nitric-oxide synthase (NOS) enzymes utilize L-arginine as substrate. NOS dysfunction due to limitations in L-arginine availability has been implicated in the pathogenesis of diabetic complications. Our studies in streptozotocin-induced diabetic mice and high glucose treated retinal ECs have demonstrated signs of retinal vascular activation and injury. These were associated with increased arginase activity and expression, decreased bioavailable nitric oxide (NO), increased superoxide formation and increased leukostasis. Blockade of the arginase pathway prevented these alterations, suggesting a primary role of arginase in retinal vascular dysfunction and injury. Our studies have also shown that endothelium-dependent retinal vasorelaxation was impaired in diabetic mice, however, deletion of arginase improved retinal vessel function and improved blood flow. During ischemic retinopathies, disturbances in retinal blood flow can result in vasoconstriction, ischemia, tissue hypoxia and formation of neovascularization (NV). Such alterations have been linked to development of ROP, a blinding disease that adversely affects premature infants due to oxygen-induced damage of the immature retinal vasculature resulting in pathological NV. Our studies using a mouse model of ROP, the oxygen-induced retinopathy (OIR) model indicate that a potent vasoactive and angiogenic factor endothelin (EDN) is responsible for pathological NV. Our analysis revealed significant increases in EDN1, EDN2 and endothelin A receptor (EDNRA) mRNA and EDN2 protein expression during ischemia. EDN2 was localized to endothelial cells and retinal glia in OIR retinas. Treatment of OIR mice with EDNRA blocker, BQ-123, significantly increased vessel sprouting resulting in enhancement of physiological angiogenesis and decreased pathological NV. OIR triggered a significant increase in STAT3 activation and VEGFA production and increased mRNA expression of angiogenic and inflammatory mediators, which were all reduced by BQ-123 treatment. These studies suggest that EDNRA activation during OIR promotes vessel degeneration and pathological NV. Collectively, both arginase and endothelins are increased in models of ischemic retinopathies. These two pathways could be interrelated through an unknown cross-talk mechanism that needs to be elucidated.