Diabetes Mellitus, Experimental/drug therapy; Diabetic Retinopathy/drug therapy; Eucommiaceae/chemistry; Glycation End Products, Advanced/pharmacology; Hyperglycemia/drug therapy; Lignans/pharmacology; Animals; Blood-Retinal Barrier/drug effects; Blood-Retinal Barrier/metabolism; Cell Line; Cell Survival/drug effects; Diabetes Mellitus, Experimental/chemically induced; Diabetes Mellitus, Experimental/genetics; Diabetes Mellitus, Experimental/pathology; Diabetic Retinopathy/chemically induced; Diabetic Retinopathy/genetics; Diabetic Retinopathy/pathology; Endothelial Cells/drug effects; Endothelial Cells/metabolism; Endothelial Cells/pathology; Evans Blue/metabolism; Gene Expression Regulation; Heme Oxygenase-1/genetics; Heme Oxygenase-1/metabolism; Hyperglycemia/chemically induced; Hyperglycemia/genetics; Hyperglycemia/pathology; Lignans/isolation & purification; Male; Membrane Proteins/genetics; Membrane Proteins/metabolism; Mice; Mice, Inbred C57BL; NF-E2-Related Factor 2/genetics; NF-E2-Related Factor 2/metabolism; Plant Extracts/chemistry; Plant Leaves/chemistry; Reactive Oxygen Species/antagonists & inhibitors; Reactive Oxygen Species/metabolism; Retina/drug effects; Retina/metabolism; Retina/pathology; Signal Transduction; Streptozocin