Consumption of Polyphenol-Rich Zingiber Zerumbet Rhizome Extracts Protects against the Breakdown of the Blood-Retinal Barrier and Retinal Inflammation Induced by Diabetes

The present study investigates the amelioration of diabetic retinopathy (DR) by Zingiber zerumbet rhizome ethanol extracts (ZZRext) in streptozotocin-induced diabetic rats (STZ-diabetic rats). ZZRext contains high phenolic and flavonoid contents. STZ-diabetic rats were treated orally with ZZRext (200, 300 mg/kg per day) for three months. Blood-retinal barrier (BRB) breakdown and increased vascular permeability were found in diabetic rats, with downregulation of occludin, and claudin-5. ZZRext treatment effectively preserved the expression of occludin, and claudin-5, leading to less BRB breakdown and less vascular permeability. Retinal histopathological observation showed that the disarrangement and reduction in thickness of retinal layers were reversed in ZZRext-treated diabetic rats. Retinal gene expression of tumor necrosis factor-α, interleukin (IL)-1β, IL-6, vascular endothelial growth factor, intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 were all decreased in ZZRext-treated diabetic rats. Moreover, ZZRext treatment not only inhibited the nuclear factor κB (NF-κB) activation, but also downregulated the protein expression of p38 mitogen-activated protein kinase (MAPK) in diabetic retina. In conclusion, the results suggest that the retinal protective effects of ZZRext occur through improved retinal structural change and inhibiting retinal inflammation. The antiretinopathy property of ZZRext might be related to the downregulation of p38 MAPK and NF-κB signal transduction induced by diabetes.