The cerebrospinal fluid-to-blood efflux transport of estrone-3-sulfate (E 1 S) via the blood-cerebrospinal fluid barrier (BCSFB) may play an important role in regulating E 1 S levels in the brain. Here, we investigated the efflux transport of E 1 S at the BCSFB using conditionally immortalized rat choroid plexus epithelial cells (TR-CSFB) and identified the responsible transporter. The [ 3 H]E 1 S uptake by TR-CSFB cells was composed of saturable and nonsaturable components, and the K m and V max values of the saturable component were determined to be 16.8 Ϯ 5.1 M and 12.3 Ϯ 2.3 pmol/min/mg of protein, respectively. [ 3 H]E 1 S uptake was inhibited by probenecid, cholate, taurocholate, sulfobromophthalein, dehydroepiandrosterone sulfate, triiodothyronine, thyroxin, and digoxin but not by p-aminohippuric acid, ␥-aminobutyric acid, or methotrexate, suggesting the involvement of organic anion transporting polypeptide (oatp) in the uptake. Reverse transcription-polymerase chain reaction analysis revealed that oatp3 was expressed in TR-CSFB cells and isolated rat choroid plexus, although oatp1 was not detected in either. Xenopus laevis oocytes expressing oatp3 exhibited [ 3 H]E 1 S uptake activity with a K m of 8.09 Ϯ 2.83 M and V max of 8.02 Ϯ 0.87 pmol/h/oocyte. Moreover, oatp3 is localized at the brush-border membrane of choroid plexus epithelial cells. These results suggest that oatp3 is involved in the E 1 S efflux transport at the BCSFB.

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Immortalized choroid plexus epithelial cell lines were successfully established from Tg rats and have the properties of choroid plexus epithelial cells, and amino acid transport activity was observed in vivo.