Radicicol but not geldanamycin evokes oxidative stress response and efflux protein inhibition in ARPE-19 human retinal pigment epithelial cells

Drug delivery to retinal cells has represented a major challenge for ophthalmologists for many decades. However, drug targeting to the retina is essential in therapies against retinal diseases such as age-related macular degeneration, the most common reason of blindness in the developed countries. Retinal cells are chronically exposed to oxidative stress that contributes to cellular senescence and may cause neovascularization in the most severe age-related macular degeneration cases. Various pre- and clinical studies have revealed that heat shock protein 90 (HSP90) inhibitors, such as geldanamycin and radicicol, are promising drugs in the treatment of different malignant processes. In this study, our goal was to compare the effects of 0.1 µM, 1 µM or 5 µM geldanamycin or radicicol on the oxidative stress response, cytotoxicity, and efflux protein activity (a protein pump which removes drugs from cells) in ARPE-19 (human retinal pigment epithelial, RPE) cells. Our findings indicate that geldanamycin and radicicol increased HSP70 and HSP27 expression analyzed by western blotting. Cellular levels of protein carbonyls were increased in response to 0.1 µM (P = 0.048 for 24 h, P = 0.018 for 48 h) or 5 µM (P = 0.030 for 24 h, P = 0.046 for 48 h) radicicol but not to geldanamycin analyzed by ELISA assay. In addition, HNE-protein adducts were accumulated in the RPE cells exposed to 0.1 µM or 5 µM radicicol but not to geldanamycin analyzed by western blotting. However, MTT assay revealed that 5 µM geldanamycin reduced cellular viability 20–30% (P < 0.05 for 24 h, P < 0.01 for 48 h), but this was not observed at any radicicol concentration in RPE cells. Interestingly, the increased oxidative stress response was associated with efflux protein inhibition (20–30%) when the cells were exposed to 1 µM or 5 µM (P < 0.05) radicicol, but not in geldanamycin-treated RPE cells. These novel findings help in understanding the influence of HSP90 inhibition and regulatory mechanisms of drug delivery to retinal cells.