Functional differences in steroid sulfate uptake of organic anion transporter 4 (OAT4) and organic anion transporting polypeptide 2B1 (OATP2B1) in human placenta

Human trophoblasts depend on the supply of external precursors such as dehydroepiandrosterone-3-sulfate (DHEA-S) and 16α-OH-DHEA-S for synthesis of estrogens. Recently, we have characterized the uptake of DHEA-S by isolated mononucleated trophoblasts and identified different transporter polypeptides involved in this process. Immunohistochemistry of 1st and 3rd trimester placenta detected organic anion transporter 4 (OAT4) and organic anion transporting polypeptide 2B1 (OATP2B1, former name OATP-B) in cytotrophoblast membranes and at the basal surface of the syncytiotrophoblast, indicating that both transporter polypeptides are involved in placental uptake of foetal derived steroid sulfates. In the present study we have characterized and compared the kinetics of DHEA-S and estrone sulfate (E1S) uptake by these transporters stably expressed in FlpIn™-HEK293 cells using the Flp recombinase-mediated site-specific recombination. Uptake of E1S by OAT4- and OATP2B1-transfected cells was highly increased compared to the non-transfected cells. In contrast, DHEA-S uptake was only highly increased in OAT4 (40 times), but only weakly enhanced in OATP2B1 cells. The uptake of DHEA-S and E1S by OAT4 was partly Na+-dependent (about 50%), whereas uptake of DHEA-S by OATP2B1 was Na+-independent. Kinetic analysis of the initial uptake rates of E1S by OAT4 and OATP2B1 gave very similar values for Km (about 20 μM) and Vmax (about 600 pmol/(min × mg protein)). In contrast, the affinity of DHEA-S towards OATP2B1 was about 10 times lower (Km > 200 μM) then for OAT4 (Km = 29 μM). Our results suggest different physiological roles of the two transporter polypeptides in placental uptake of foetal derived steroid sulfates. OATP2B1 seems not to be involved in de novo synthesis of placental estrogens but may contribute to the clearance of estrogen sulfates from foetal circulation.