Why are retinal pigment epithelial cells so resistant to oxidative stress?

Abstract

Abstract Purpose The functional integrity of postmitotic retinal pigment epithelial (RPE) cells is crucial for photoreceptor survival, and RPE malfunction is considered an important factor behind age‐related macular degeneration (AMD). Although RPE cells daily phagocytose large amounts of fatty acid‐rich distal photoreceptor material, their lysosomal accumulation of lipofuscin (LF) is nevertheless slow. Only late in life do humans have LF‐loaded RPE cells, which then not only have a reduced capacity to endocytose photoreceptor material but also are less capable of keeping up with normal autophagy. The latter activity is important for normal turnover of the cell’s own components. Methods ARPE‐19 cells in culture were evaluated with respect to lysosomal stability at oxidative stress. Redox‐active iron, autophagic activity (evaluation of LC3‐I and LC3‐II protein), and anti‐oxidative proteins were assayed using cytochemistry (Sulphide‐Silver Method), electron spin resonance (ESR) and western blotting for a variety of stress proteins. Results The lysosomal compartment of the ARPE‐19 cells was found remarkably resistant to oxidative stress. The SSM and ESR methods suggested low lysosomal amounts of redox‐active iron and no enhanced autophagy following oxidative stress. The normal autophagy rate was higher than in other professional scavengers. Conclusion The found low amount of lysosomal redox‐active iron explains the slow formation of lipofuscin over time and the remarkable stability that ARPE‐19 cells and their lysosomes show even under severe oxidative stress. The molecular mechanisms behind this phenomenon may be an unusual capacity to quickly export low mass iron from lysosomes to the cytosol and/or a high normal degree of autophagy of iron‐binding stress proteins.