Products of lipid peroxidation induce missorting of the principal lysosomal protease in retinal pigment epithelium

Phagocytosis of photoreceptor outer segments (OS) by retinal pigment epithelium (RPE) is essential for OS renewal and survival of photoreceptors. Internalized, oxidatively modified macromolecules perturb the lysosomal function of the RPE and can lead to impaired processing of photoreceptor outer segments. In this study, we sought to investigate the impact of intracellular accumulation of oxidatively damaged lipid-protein complexes on maturation and distribution of cathepsin D, the major lysosomal protease in the RPE. Primary cultures of human RPE cells were treated with copper-oxidized low density lipoprotein (LDL) and then challenged with serum-coated latex beads to stimulate phagocytosis. Three observations were noted to occur in this experimental system. First, immature forms of cathepsin D (52 and 46 kDa) were exclusively associated with latex-containing phagosomes. Second, maturation of cathepsin D was severely impaired in RPE cells loaded with oxidized LDL (oxLDL) prior to the phagocytic challenge. Third, pre-treatment with oxLDL caused sustained secretion of pro-cathepsin D and the latent form of gelatinase A into the extracellular space in a dose-dependent manner. These data stimulate the hypothesis that intracellular accumulation of poorly degradable, oxidized lipid-protein cross-links, may alter the turnover of cathepsin D, causing its mistargeting into the extracellular space together with the enhanced secretion of a gelatinase.