Transcellular transfer of folate across the retinal pigment epithelium

The differential polarized distribution of the folate receptor alpha (FR alpha) in the basal membrane and reduced-folate transporter (RFT-1) in the apical membrane was demonstrated previously in the retinal pigment epithelium (RPE). Based on this, we hypothesized that folate would enter the RPE via FR alpha and exit the cell via RFT-1. To test this, we performed in vitro transport assays using ARPE-19 cells cultured on permeable supports. The cells were grown for 4 weeks and electron microscopic analysis indicated that the cells have the phenotypic features of normal RPE cells including apical microvillous processes and junctional complexes. Measurement of transepithelial resistance showed that the resistance increases in the cells as they differentiate over several weeks. Transport assays showed that ARPE-19 cells transport folate in a basal-to-apical direction, but do not transport this vitamin significantly in the apical-to basal-direction. This was not a diffusional process, as the paracellular markers inulin and sucrose were transferred across the cell monolayer at a much lower level. The presence of FR alpha in the basal membrane was demonstrable by folate binding and that of RFT-1 in the apical membrane by blockade of folate transport by RFT-1-specific antibody. This study represents the first in vitro demonstration of transcellular transfer of folate across RPE and suggests that folate is transported from the choriocapillaris to the adjacent photoreceptor cells in vivo by the concerted action of FR alpha in the basal membrane and RFT-1 in the apical membrane.