LIPOFUSCIN FORMATION IN CULTURED RETINAL-PIGMENT EPITHELIAL-CELLS EXPOSED TO PHOTORECEPTOR OUTER SEGMENT MATERIAL UNDER DIFFERENT OXYGEN CONCENTRATIONS

Lipofuscin accumulates in the course of time in the acidic vacuolar ap paratus of retinal pigment epithelial (RPE) cells and may influence th eir metabolic functions. In order to study the effect of oxidative str ess on lipofuscin accumulation, rabbit RPE cell cultures were kept at an ambient oxygen concentration of either 8% or 40%. To simulate the n ormal phagocytic function of RPE cells, bovine photoreceptor outer seg ments (POS) were added daily. The lipofuscin-specific autofluorescence was measured after 1, 2 and 3 weeks. RPE cells cultured under normoba ric hyperoxic conditions (40% oxygen) showed significantly higher leve ls of lipofuscin-like autofluorescence than those kept under normobari c and probably normooxic conditions (8% oxygen) after 1 (p = 0.0050), 2 (p = 0.0001) as well as 3 (p = 0.0077) weeks. At both oxygen concent rations, the lipofuscin accumulation level was increased after 2 weeks of POS exposure (40% p = 0.0001; 8% p = 0.0037) and even further afte r 3 weeks (40% p = 0.0541; 8% p = 0.0377). The results suggest an invo lvement of oxidative mechanisms in the formation of lipofuscin from ph agocytized POS by RPE cells. The autofluorescence of control cells, no t exposed to POS, was significantly (40%: 1 week p = 0.0011, 2 weeks p = < 0.0001, 3 weeks p = 0.0001; 8%: 1 week p = 0.0036, 2 weeks p = 0. 0063, 3 weeks p = 0.0066) lower than that of the POS-fed cells. The au tofluorescence increased significantly (40% p = 0.0059; 8% p = 0.0034) between week 1 and week 3 in the control cells. This finding may refl ect a contribution to lipofuscin formation by autophagocytized intrace llular material. The present model seems to be useful for further stud ies on the mechanisms behind lipofuscinogenesis of RPE cells as well a s the possible effects of lipofuscin accumulation on cell functions an d viability.