ACCUMULATION OF LIPOFUSCIN WITHIN RETINAL-PIGMENT EPITHELIAL-CELLS RESULTS IN ENHANCED SENSITIVITY TO PHOTOOXIDATION

Retinal pigment epithelial (RPE) cells are largely postmitotic. They c ontinuously phagocytose the outer tips of the photoreceptor outer segm ents (POS). Over the life span of an individual, this activity results , although surprisingly slowly, in the intralysomal accumulation of li pofuscin, or age-pigment. Native lipofuscin shows orange-red autofluor escence when exposed to blue light. The loss of energy resulting from the conversion of excitatory blue light into emitted orange-red light may induce photooxidative reactions. We exposed neonatal rabbit RPE ce lls in culture to purified POS from cow eyes. The material were either native or peroxidized by irradiation with UV-light before being added to the RPE cultures. Lipofuscin accumulation was studied by transmiss ion electron microscopy and measured by microfluorometric registration of its autofluorescence. Cells exposed to peroxidized POS accumulated much more lipofuscin than those exposed to native POS, indicating tha t peroxidized outer segements are not digestable by lysosomal enzymes. Furthermore, lipofuscin-loaded RPE cells were considerable more sensi tive to visible blue light than unloaded control cells. The former one s showed lysosomal membrane destabilization with ensuing leakage of ly tic enzymes and eventually cell death. We suggest that photo-oxidation of lysosomal membranes surrounding accumulated lipofuscin may be of i mportance for the development of age-related macular degeneration.