Involvement of reduced folate carrier 1 in the inner blood-retinal barrier transport of methyltetrahydrofolate

The purpose of this study was to elucidate the mechanism of methyltetrahydrofolate (MTF) transport at the inner blood-retinal barrier (inner BRB). The characteristics and function of MTF transport at the inner BRB were examined using a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2) as an in vitro model of the inner BRB. The 3H]MTF uptake by TR-iBRB2 cells increased with lowering extracellular pH and was Na+- and Cl--independent. The 3H]MTF uptake was concentration-dependent with a K(m) of 5.1 microM. This process was inhibited by reduced folate carrier 1 (RFC1) substrates, such as methotrexate and formyltetrahydrofolate, in a concentration-dependent manner with an IC50 of 8.7 and 2.8 microM, respectively, suggesting that RFC1 mediates MTF uptake in TR-iBRB2 cells. Although both RFC1 and proton-coupled folate transporter (PCFT) mRNA, which are pH-sensitive folate transporters, are expressed in TR-iBRB2 cells and isolated rat retinal vascular endothelial cells, the expression level of RFC1 mRNA was 83- and 49-fold greater than that of PCFT, respectively. Taken together, the above findings are consistent with the involvement of RFC1 in the inner BRB transport of MTF.