Synthesis and secretion of retinol-binding protein and transthyretin by cultured retinal pigment epithelium

• Published in: Biochemistry (Easton) 1994-02, Vol.33 (7), p.1835-1842
• Main Authors: Ong, David E, Davis, James T, O'Day, William T, Bok, Dean
• Format: Article
• Language: English
• Subjects: Analytical, structural and metabolic biochemistry; Animal cells; Animals; Binding and carrier proteins; Biological and medical sciences; Cattle; Cell cultures. Hybridization. Fusion; Cells, Cultured; Chromatography, Affinity; Chromatography, Gel; Cysteine - metabolism; Fundamental and applied biological sciences. Psychology; Humans; Methionine - metabolism; Molecular and cellular biology; Pigment Epithelium of Eye - metabolism; Prealbumin - biosynthesis; Prealbumin - chemistry; Prealbumin - metabolism; Proteins; Retinol-Binding Proteins - biosynthesis; Retinol-Binding Proteins - chemistry; Retinol-Binding Proteins - metabolism; Sequence Analysis; Sulfur Radioisotopes
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00173a029

Recent studies indicate that the retinal pigment epithelium (RPE) may serve as an extrahepatic source of retinol-binding protein (RBP) and transthyretin (TTR) for the retina by virtue of the fact that this cell layer is the exclusive retinal location for mRNA coding for these proteins [Herbert, J., et al. (1991) Invest. Ophthalmol. Vis. Sci. 32, 302-309; Cavallaro, T., et al. (1990) Invest. Ophthalmol. Vis. Sci. 31, 497-501], although the proteins themselves are present in a variety of retinal neurons. It is therefore necessary to determine whether these mRNAs are translated and whether their translated products are secreted like hepatic RBP and TTR. Metabolic labeling of cultured bovine RPE with [35S]cysteine and [35S]methionine and subsequent analysis of newly synthesized proteins in the conditioned medium by affinity chromatography, gel filtration, partial amino acid sequence analysis, and autoradiography of electrophoretograms indicate that both RBP and TTR are synthesized and secreted by the RPE. Moreover, for cells grown in chambers with permeable supports, the predominant direction for secretion was into the apical medium. The mean apical:basal ratio after 72 h of incubation was 9.2 for TTR and 4.5 for RBP. A function for these proteins in the neurosensory retina remains speculative. They could be involved in the delivery of all-trans-retinol to amacrine and Müller cells as a precursor for retinoic acid, since these cells are known to contain cellular retinoic acid binding protein [Gaur, V.P., et al. (1990) Exp. Eye Res. 50, 505-511; Milam et al. (1990) J. Comp. Neurol. 296, 123-129].