Antioxidant Up–regulation and Nuclear DNA Repair, but Not Mitochondrial DNA Repair, Play a Key Role in Adaptation to Oxidative Stress in Retinal Pigment Epithelial Cells

Abstract

Abstract: : Purpose: To investigate the role of the adaptive response in preventing oxidative stress–induced damage to the retinal pigment epithelium (RPE). Methods: An adaptive response was generated by exposing cultured RPE cells to non–lethal treatments of H₂O₂ for 5 days. A comparison in the response of the adapted and non–adapted RPE towards toxic H₂O₂ doses was then undertaken. Cell viability was determined by the MTT assay, the antioxidant catalytic activity was analyzed via enzyme assays and the nuclear (nDNA) and mitochondrial DNA (nDNA) susceptibility and repair efficiencies were measured by QPCR. Results: Prior exposure to sub–lethal H₂O₂ confirmed an adaptive response, resulting in a greater cellular resistance to subsequent toxic exposures compared to non–adapted RPE (P<0.05). A greater catalase (CAT), glutathione peroxidase (GPX) and copper zinc superoxide dismutase (CuZnSOD) enzymatic activity (P<0.05), increased nDNA protection (P<0.05) and up–regulated nDNA repair (P<0.05) was also observed. However, there was no adaptive benefit for mtDNA protection or repair in response to oxidative stress. Conclusions: This study confirms a role for the adaptive response as an important antioxidant defence for cells located in inherently oxidizing micro–environments, such as the RPE. Furthermore, it identifies that the mitochondria are a weak link in otherwise efficient oxidative stress defences and that this may contribute to ageing and age–related diseases such as age–related macular degeneration.

Keywords: oxidation/oxidative or free radical damage • mitochondria