SYNTHESIS AND SECRETION OF RETINOL-BINDING PROTEIN AND TRANSTHYRETIN BY CULTURED RETINAL-PIGMENT EPITHELIUM

Recent studies indicate that the retinal pigment epithelium (RPE) may serve as an extrahepatic source of retinol-binding protein (RBP) and t ransthyretin (TTR) for the retina by virtue of the fact that this cell layer is the exclusive retinal location for mRNA,coding for these pro teins [Herbert, J., et al. (1991) Invest, Ophthalmol. Vis. Sci. 32, 30 2-309; 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 secret ed like hepatic RBP and TTR. Metabolic labeling of cultured bovine RPE with [S-35] cysteine and [S-35] methionine and subsequent analysis of newly synthesized proteins in the conditioned medium by affinity chro matography, 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 secret ion 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 p roteins in the neurosensory retina remains speculative. They could be involved in the delivery of all-trans-retinol to amacrine and Muller c ells as a precursor for retinoic acid, since these cells are known to contain cellular retinoic acid binding protein [Gaur, V. P., et al. (1 990) Exp. Eye Res. 50, 505-511; Milam et al. (1990) J. Comp. Neurol. 2 96, 123-129]. A role in the visual cycle remains a possibility, althou gh the evidence is stronger that this function may be fulfilled by int erphotoreceptor retinoid-binding protein (IRBP). Finally, since these proteins have been localized by immunocytochemistry to several neurona l cell types in the retina, they may play a role in cell differentiati on.