Abstract
We identified a novel prostaglandin (PG)-specific organic anion transporter (OAT) in the OAT group of the SLC22 family. The transporter designated OAT-PG from mouse kidney exhibited Na(+)-independent and saturable transport of PGE(2) when expressed in a proximal tubule cell line (S(2)). Unusual for OAT members, OAT-PG showed narrow substrate selectivity and high affinity for a specific subset of PGs, including PGE(2), PGF(2alpha), and PGD(2). Similar to PGE(2) receptor and PGT, a structurally distinct PG transporter, OAT-PG requires for its substrates an alpha-carboxyl group, with a double bond between C13 and C14 as well as a (S)-hydroxyl group at C15. Unlike the PGE(2) receptor, however, the hydroxyl group at C11 in a cyclopentane ring is not essential for OAT-PG substrates. Addition of a hydroxyl group at C19 or C20 impairs the interaction with OAT-PG, whereas an ethyl group at C20 enhances the interaction, suggesting the importance of hydrophobicity around the omega-tail tip forming a "hydrophobic core" accompanied by a negative charge, which is essential for substrates of OAT members. OAT-PG-mediated transport is concentrative in nature, although OAT-PG mediates both facilitative and exchange transport. OAT-PG is kidney-specific and localized on the basolateral membrane of proximal tubules where a PG-inactivating enzyme, 15-hydroxyprostaglandin dehydrogenase, is expressed. Because of the fact that 15-keto-PGE(2), the metabolite of PGE(2) produced by 15-hydroxyprostaglandin dehydrogenase, is not a substrate of OAT-PG, the transport-metabolism coupling would make unidirectional PGE(2) transport more efficient. By removing extracellular PGE(2), OAT-PG is proposed to be involved in the local PGE(2) clearance and metabolism for the inactivation of PG signals in the kidney cortex.