Zinc protects against oxidative damage in cultured human retinal pigment epithelial cells

This study was undertaken to determine whether bioavailable zinc can influence the effects of oxidative stress on cultured human retinal pigment epithelial (RPE) cells. RPE cells were maintained for 7 d in culture medium containing 14 μM total zinc, or in medium containing 0.55 μM total zinc. After 1 week, MTT assays were performed to determine the relative cytotoxicity of H202 or paraquat on RPE cells. Conjugated dienes and thiobarbituric acid reactive substances (TBARS) were measured in RPE cells treated with 0, 0.5 mM H2O2, 10 μM FeSO4 + 0.5 mM H2O2 or 10 μM FeSO4 + xanthine/xanthine oxidase for 24 h or paraquat for 7 d. Oxidized proteins were determined by the formation of carbonyl residues. The antioxidants metallothionein, catalase, superoxide dismutase, and glutathione peroxidase were also measured. The MTT assays showed that zinc protected cultured RPE from the toxicity of H2O2 and paraquat. RPE cells in 0.55 μM zinc medium contained higher levels of TBARS, conjugated dienes and protein carbonyls due to the oxidative stresses, compared to cells in 14 μM zinc. Catalase and MT content were reduced in cells cultured in 0.55 μM zinc medium and were reduced additionally when treated with above stresses. Superoxide dismutase activity increased in 0.55 μM zinc medium in response to these stresses. Our results show RPE cells cultured in zinc-reduced medium are more susceptible to oxidative insult.