Photodamage to human RPE cells by A2-E, a retinoid component of lipofuscin

PURPOSE. A fluorescent component of lipofuscin, A2-E (N-retinylidene-N-reti nylethanol-amine) has been shown to impair lysosomal function and to increa se the intralysosomal pH of human retinal pigment epithelial (RPE) cells. i n addition to its lysosomotropic properties A2-E is known to be photoreacti ve. The purpose of this study was to determine the phototoxic potential of A2-E on RPE cells. METHODS. A2-E (synthesized by coupling all-trans-retinaldehyde to ethanolam ine) was complexed to low-density lipoprotein (LDL) to allow for specific l oading of the lysosomal compartment. Human RPE cell cultures were loaded wi th the A2-E-LDL, complex four times within 2 weeks. A2-E accumulation was c onfirmed by fluorescence microscopy and flow cytometry analysis. Acridine o range staining allowed assessment of lysosomal integrity and intralysosomal pH. The phototoxic properties of A2-E were determined by exposing A2-E-fre e and A2-E-fed RPE cell cultures to short wavelength visible light (400-500 nm) and assessing cell viability and lysosomal integrity. RESULTS. Fluorescence microscopy and flow cytometry analysis demonstrated t hat the intralysosomal accumulation of A2-E in cultured RPE cells increased with the number of feedings. Acridine orange staining confirmed that the A 2-E was located in the lysosomal compartment and induced an elevation of in tralysosomal pH. Exposure of A2-E-fed cells to light resulted in a signific ant loss of cell viability by 72 hours, which was not observed in either RP E cells maintained in the dark or A2-E-free cultures exposed to light. Toxi city was associated with a loss of lysosomal integrity. CONCLUSIONS. A2-E is detrimental to RPE cell function by a variety of mecha nisms: inhibition of lysosomal degradative capacity, loss of membrane integ rity, and phototoxicity. Such mechanisms could contribute to retinal aging as well as retinal diseases associated with excessive lipofuscin accumulati on-for example, age-related macular degeneration and Stargardt's disease.