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Y. Han, J. G. Hu, T. V. Bui, R. A. Radu, M. Marinò, D. Bok, N. L. Mata; Megalin and Related Endocytic Receptors Mediate Uptake of Retinoid Binding Proteins in the Retinal Pigment Epithelium. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3245.
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Megalin is a ~ 600 kDa low density lipoprotein receptor (LDLR) which resides at the apical processes of proximal convoluted tubules in kidney and functions to endocytose retinol binding protein (RBP) from the glomerular filtrate into the circulation. In the present investigation, we have explored the possibility that megalin and related endocytic receptors mediate uptake of RBP and interphotoreceptor retinoid binding protein (IRBP) into the retinal pigment epithelium (RPE).
Human fetal RPE cells were grown on Millicell chambers which allowed the separation of apical and basal compartments. Expression and cytolocalization of endocytic receptors were determined by Western blot and immunocytochemistry. Protein identity was confirmed by peptide sequencing. Retinoid uptake was measured by HPLC and protein uptake was monitored using immunocytochemical techniques.
Megalin was detected within apical processes of human RPE and efficiently endocytosed both RBP- and IRBP-retinol. Lipoprotein receptor-related protein (LRP) was also found on the apical RPE cell surface and is shown to be principally involved in uptake of IRBP-retinol. Treatment of human RPE cell cultures with receptor-associated protein (RAP, an LDLR ligand-binding antagonist) suppressed endocytosis of RBP and IRBP and led to the identification of a truncated megalin isoform on basolateral RPE membranes. Inclusion of RAP in the basal media of RPE cell cultures profoundly reduced uptake of RBP-retinol from this compartment, thereby implicating the involvement of LDLRs in this process.
The studies described in this report provide compelling evidence which supports a role for megalin and related LDLRs in uptake of RBP and IRBP. These findings are significant as they resolve long-standing issues concerning retinoid uptake by the RPE.
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