Polyspecific substrate uptake by the hepatic organic anion transporter Oatp1 in stably transfected CHO cells

The rat liver organic anion transporting polypeptide (Oatp1) has been extensively characterized mainly in the Xenopus laevis expression system as a polyspecific carrier transporting organic anions (bile salts), neutral compounds, and even organic cations. In this study, we extended this characterization using a mammalian expression system and confirm the basolateral hepatic expression of Oatp1 with a new antibody. Besides sulfobromophthalein [Michaelis-Menten constant (K-m) of similar to 3 mu M], taurocholate (K-m of similar to 32 mu M), and estradiol-17 beta-glucuronide (K-m of similar to 4 mu M), substrates previously shown to be transported by Oatp1 in transfected HeLa cells, we determined the kinetic parameters for cholate (K-m of similar to 54 mu M), glycocholate (K-m of similar to 54 mu M), estrone-3-sulfate (K-m of similar to 11 mu M), CRC-220 (K-m of similar to 57 mu M), ouabain (K-m of similar to 3,000 mu M), and ochratoxin A (K-m of similar to 29 mu M) in stably transfected Chinese hamster ovary (CHO) cells. In addition, three new substrates, taurochenodeoxycholate (K-m of similar to 7 mu M), tauroursodeoxycholate (K-m of similar to 13 mu M), and dehydroepiandrosterone sulfate (K-m of similar to 5 mu M), were also investigated. The results establish the polyspecific nature of Oatp1 in a mammalian expression system and definitely identify conjugated dihydroxy bile salts and steroid conjugates as high-affinity endogenous substrates of Oatp1.