Formation of lipofuscin in cultured retinal pigment epithelial cells exposed to pre-oxidized photoreceptor outer segments

Accumulation of lipofuscin in the retinal pigment epithelium (RPE) with increasing age may affect essential supportive functions for the photoreceptors. Earlier, we described a model system for the study of lipofuscinogenesis in RPE cell cultures and showed that mild oxidative stress enhances lipofuscin formation from phagocytized photoreceptor outer segments (POS). In the present study, bovine POS were photo-oxidized, and turned into a lipofuscin-like material, by irradiation with UV light. Transmission electron microscopy of irradiated POS showed loss of the normal stacks of the disk membranes with conversion into an amorphous osmiophilic electron-dense mass. The formation of thiobarbituric acid reactive substances (TEARS), estimated during the irradiation process, indicated lipid peroxidation. Irradiated POS also showed a strong granular yellow autofluorescence. RPE cell cultures, kept at 21% ambient oxygen, were fed daily for 3, 5 or 7 days with either (i) UV-peroxidized POS, (ii) native POS or (iii) culture medium only. RPE cells fed irradiated POS showed significantly higher levels of lipofuscin-specific autofluorescence compared to cells exposed to native POS after 3 days (p = 0.0056), 5 days (p = 0.0037) and 7 days (p = 0.0020), and to the non-exposed control cells (3 days: p = 0.005, 5 days: p = 0.0037, 7 days: p = 0.0094). The lipofuscin content of cells exposed to irradiated POS increased significantly between days 3 and 7 (p = 0.0335). Ultrastructural studies showed much more numerous and larger lipofuscin-like inclusions in RPE cells fed irradiated POS compared to cells exposed to native POS. In the control cells, lipofuscin-like granules were small and sparse. It appears that exposing RPE cells to previously peroxidized POS, thus artificially converted to lipofuscin and obviously not digestible by the lysosomal enzymes, accelerates the formation of severely lipofuscin-loaded cells. The results will be useful for further studies of possible harmful effects of lipofuscin in heavily loaded RPE cells.