Blood—Brain Barrier Is Involved in the Efflux Transport of a Neuroactive Steroid, Dehydroepiandrosterone Sulfate, via Organic Anion Transporting Polypeptide 2

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Abstract
We have investigated the transport characteristics of dehydroepiandrosterone sulfate (DHEAS), a neuroactive steroid, at the blood—brain barrier (BBB) in a series of functional in vivo and in vitro studies. The apparent BBB efflux rate constant of [3H]DHEAS evaluated by the brain efflux index method was 2.68 × 10‐2 min‐1. DHEAS efflux transport was a saturable process with a Michaelis constant (Km) of 32.6 μM. Significant amounts of [3H]DHEAS were determined in the jugular venous plasma by HPLC, providing direct evidence that most of the DHEAS is transported in intact form from brain to the circulating blood across the BBB. This efflux transport of [3H]DHEAS was significantly inhibited by common rat organic anion‐transporting polypeptide (oatp) substrates such as taurocholate, cholate, sulfobromophthalein, and estrone‐3‐sulfate. Moreover, the apparent efflux clearance of [3H]DHEAS across the BBB (118 μl/min‐g of brain) was 10.4‐fold greater than its influx clearance estimated by the in situ brain perfusion technique (11.4 μl/min‐g of brain), suggesting that DHEAS is predominantly transported from the brain to blood across the BBB. In cellular uptake studies using a conditionally immortalized mouse brain capillary endothelial cell line (TM‐BBB4), [3H]DHEAS uptake by TM‐BBB4 cells exhibited a concentration dependence with a Km of 34.4 μM and was significantly inhibited by the oatp2‐specific substrate digoxin. Conversely, [3H]digoxin uptake by TM‐BBB4 cells was significantly inhibited by DHEAS. Moreover, the net uptake of [3H]DHEAS at 30 min was significantly increased under ATP‐depleted conditions, suggesting that an energy‐dependent efflux process may also be involved in TM‐BBB4. RT‐PCR and sequence analysis suggest that an oatp2 is expressed in TM‐BBB4 cells. In conclusion, DHEAS efflux transport takes place across the BBB, and studies involving in vitro DHEAS uptake and RT‐PCR suggest that there is oatp2‐mediated DHEAS transport at the BBB.

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