Purpose: The retinal pigment epithelium (RPE) regulates the lipid metabolism of the photoreceptors by catalysis of membrane outer segments and via choriocapillary perfusion is also exposed to the regulation of blood lipid levels. Since the uptake the metabolism of cholesterol are mediated by specific low-density lipoprotein (LDL) receptors, expression and regulation of this receptor-type were studied in RPE cultures.
Methods: In vitro experiments were carried out in transformed (SV40) human RPE cells. Human fibroblasts were used as a comparative cell line with known receptor expression. LDL was coupled to a fluorescent marker (Dil); receptor binding was quantified by flow cytometry. Expression and saturation characteristics were determined. LDL metabolism was examined by variation of the temperature (4 and 37 degrees C). LDL and Dil-LDL showed competition at the receptor.
Results: RPE cells demonstrated a higher uptake of Dil-LDL than fibroblasts. Expression could be further stimulated by culture conditions. Uptake kinetics were saturable with complete saturation at 50 micrograms/ml Dil-LDL. LDL uptake was shown to be temperature-dependent, indicating an energy-dependent pathway.
Conclusions: RPE cells exhibit significant expression of receptors for native LDL, possibly mediating the lipid metabolism of the RPE-photoreceptor complex, as well as the uptake of blood lipids. Lack of regulation of the receptor for LDL may lead to intracellular accumulation of lipids, which may play a role in the pathogenesis of AMD.