Protective Effect of DHA In Retinal Vascular Degeneration and Neovascularization Is Through Sphingomyelinase Inhibition

Abstract

Purpose: : Acid (ASMase) and Neutral (NSMase) Sphingomyelinases catalyze the hydrolysis of sphingomyelin to the pro-inflammatory and pro-apoptotic second messenger ceramide. We have previously demonstrated that major polyunsaturated fatty acid in the retina, docosahexaenoic acid (DHA) inhibits ASMase and NSMase activity. In this study we tested the hypothesis that protective role of DHA in retinal vascular inflammation and angiogenesis is through inhibition of SMase activity.

Methods: : STZ-injected male Wistar rat model of Type I diabetes maintained on control or DHA enriched diets for 9 months as well as ischemia-reperfusion (IR) and oxygen induced retinopathy (OIR) animal models in WT and ASMase ^-/- mice were used. Acellular capillaries were analyzed by trypsin digest. ASMase and NSMase activity was determined by sphingomyelinase assay. The mRNA and protein expression levels of ASMase and NSMase were analyzed by real time PCR and Western Blotting.

Results: : ASMase was upregulated in the retinas of streptozotocin (STZ) induced diabetic rats on control diet. DHA rich diet returned ASMase in diabetic retina to the control levels. There was a 2.5 fold increase in the number of acellular capillaries in the retinas from 9 months diabetic rats on control diet. DHA rich diet prevented the development of acellular capillaries in diabetic retinas. To further prove the critical role of ASMase in the developments of retinal vascular pathology, we used ASMase ^-/- mice with IR as an accelerated model of diabetic retinopathy. In the wild type control mice there was a 3.6 fold increase in the number of acellular capillaries in the retinas of IR eyes. This pathology was completely prevented in the retinas of ASMase ^-/- mice. In addition, ASMase ^-/- mice had a 50% reduction in the level of pre-retinal neovascularization in OIR model.

Conclusions: : We demonstrated that ASMase activation is a critical step in pro-inflammatory cytokine signaling in retinal endothelial cells. Moreover, ASMase inhibition by DHA or genetically in ASMase ^-/- mice prevented retinal vascular degeneration and neovascularization in diabetic retinopathy, IR and OIR animal models. Inhibition of ASMase may be of benefit in preventing conditions associated with vascular inflammation, such as diabetic retinopathy and retinopathy of prematurity.

Keywords: diabetic retinopathy • inflammation • lipids