In vitro study of the functional expression of organic anion transporting polypeptide 3 at rat choroid plexus epithelial cells and its involvement in the cerebrospinal fluid-to-blood transport of estrone-3-sulfate

The cerebrospinal fluid-to-blood efflux transport of estrone-3-sulfate (E₁S) via the blood-cerebrospinal fluid barrier (BCSFB) may play an important role in regulating E₁S levels in the brain. Here, we investigated the efflux transport of E₁S at the BCSFB using conditionally immortalized rat choroid plexus epithelial cells (TR-CSFB) and identified the responsible transporter. The [³H]E₁S 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 ± 5.1 μM and 12.3 ± 2.3 pmol/min/mg of protein, respectively. [³H]E₁S uptake was inhibited by probenecid, cholate, taurocholate, sulfobromophthalein, dehydroepiandrosterone sulfate, triiodothyronine, thyroxin, and digoxin but not by p-aminohippuric acid, γ-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 [³H]E₁S uptake activity with a K_m of 8.09 ± 2.83 ±M and V_max of 8.02 ± 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 E₁S efflux transport at the BCSFB.