Novel Strategy for RPE Protection: Glutaredoxin-Targeting Natural Products

Purpose: Oxidative stress-induced retinal pigment epithelial (RPE) cell damage is known as an important factor in the pathogenesis of retinopathies, such as age-related macular degeneration (AMD). In our previous study, we identified glutaredoxin 1 (Grx1), a thiol-disulfide oxidoreductase, as a new cytoprotective enzyme in RPE cells. In this study, we searched for small molecule Grx1 inducers from natural products to protect RPE cells from oxidative damage. Methods: Five natural antioxidant phenolics, including Salvianolic acid A (Sal A), Salvianolic acid B (Sal B), total salvianolic acid, curcumin, and epigallocatechin gallate (EGCG) were screened for their Grx1-inducing activities and cytoprotective effects in primary human RPE cells. Grx1 expression was examined by Western blot. Cell viability was evaluated with the WST8 assay. The level of protein glutathionylation (PSSG) was measured by using anti-PSSG antibody. Results: Among all the tested compounds, Sal B was found to be the most potent Grx1 inducer, which upregulated Grx1 by ~3 fold at 50 µM after 24 h. In both a time and dose-dependent manner, Sal B protected cells from H2O2-induced cell viability loss. Sal B also reduced annexin V positive cells, decreased Bax/Bcl-2 ratio, prevented caspase-3 cleavage, and inhibited ROS production. Additionally, H2O2-induced PSSG accumulation was markedly decreased by Sal B treatment. Moreover, knockdown of Grx1 by siRNA significantly reduced the cytoprotective effects of Sal B. Conclusions: Sal B protects primary human RPE cells from oxidative stress-induced damage by upregulating Grx1. Naturally occurring Grx1 inducers may be used as new therapeutic strategies to treat oxidative stress-related retinopathies like AMD.