High density lipoprotein mediated lipid efflux from retinal pigment epithelial cells in culture

Backgound/aim: The transport of radiolabelled photoreceptor outer segments (POS) lipids was investigated by cultured retinal pigment epithelial cells (RPE). Phagocytosis of POS by the RPE is essential to maintain the health and function of the photoreceptors in vivo. POS are phagocytised at the apical cell surface of RPE cells. Phagocytised POS lipids may be either recycled to the photoreceptors for reincorporation into new POS or they may be transported to the basolateral surface for efflux into the circulation. Results: The authors have demonstrated that high density lipoprotein (HDL) stimulates efflux of radiolabelled lipids, of POS origin, from the basal surface of RPE cells in culture. Effluxed lipids bind preferentially to HDL species of low and high molecular weight. Effluxed radiolabelled phosphotidyl choline was the major phospholipid bound to HDL, with lesser amounts of phosphatidyl ethanolamine, phosphatidyl inosotol. Effluxed radiolabelled triglycerides, cholesterol, and cholesterol esters also bound to HDL. Lipid free apolipoprotein A-I (apoA-I) and apoA-I containing vesicles also stimulate lipid efflux. Conclusion: The findings suggest a role for HDL and apoA-I in regulating lipid and cholesterol transport from RPE cells that may influence the pathological lipid accumulation associated with age related macular degeneration.