Protein synthesis and secretion by cultured retinal pigment epithelia

Protein synthesis and secretion by post-natal sheep and calf retinal pigment epithelial (RPE) cells was investigated following labelling of choroidal pieces, isolated RPE cells and RPE cells in tissue culture with l-[U-¹⁴C] leucine. We show that RPE cells secrete a specific set of proteins that includes retinol binding protein (RBP) and transthyretin (TTR), which are both involved in retinol transport in blood. Using a two-chambered culture system we show that protein secretion by the post-natal RPE cells occurs predominantly across the apical pole of the cells, i.e., across the surface of the cells which, in vivo, faces the retina. In agreement with results of others using foetal RPE cells (Ong, D.E., Davis, J.T., O'Day, W.T. and Bok, D. (1994) Biochemistry 33, 1835-1842) we show that RBP and, to a lesser extent, TTR are also secreted predominantly across the apical pole of the cell. We have developed a cell culture model for the RPE that may be used as an in vitro model for studying transport across the blood-retinal barrier.