title: Human organic anion transporter 1B1 and 1B3 function as bidirectional carriers and do not mediate GSH-bile acid cotransport.
abstract: Organic anion transporting polypeptides (OATP/SLCO) are generally believed to function as electroneutral anion exchangers, but direct evidence for this contention has only been provided for one member of this large family of genes, rat Oatp1a1/Oatp1 (Slco1a1). In contrast, a recent study has indicated that human OATP1B3/OATP-8 (SLCO1B3) functions as a GSH-bile acid cotransporter. The present study examined the transport mechanism and possible GSH requirement of the two members of this protein family that are expressed in relatively high levels in the human liver, OATP1B3/OATP-8 and OATP1B1/OATP-C (SLCO1B1). Uptake of taurocholate in Xenopus laevis oocytes expressing either OATP1B1/OATP-C, OATP1B3/OATP-8, or polymorphic forms of OATP1B3/OATP-8 (namely, S112A and/or M233I) was cis-inhibited by taurocholate and estrone sulfate but was unaffected by GSH. Likewise, taurocholate and estrone sulfate transport were trans-stimulated by estrone sulfate and taurocholate but were unaffected by GSH. OATP1B3/OATP-8 also did not mediate GSH efflux or GSH-taurocholate cotransport out of cells, indicating that GSH is not required for transport activity. In addition, estrone sulfate uptake in oocytes microinjected with OATP1B3/OATP-8 or OATP1B1/OATP-C cRNA was unaffected by depolarization of the membrane potential or by changes in pH, suggesting an electroneutral transport mechanism. Overall, these results indicate that OATP1B3/OATP-8 and OATP1B1/OATP-C most likely function as bidirectional facilitated diffusion transporters and that GSH is not a substrate or activator of their transport activity.
authors.affiliation: Department of Environmental Medicine, Box EHSC, University of Rochester School of Medicine, 575 Elmwood Avenue, Rochester, NY 14642, USA.
authors.forename: Chitrawina
authors.initials: C
authors.lastname: Mahagita
authors.forename: Steven M
authors.initials: SM
authors.lastname: Grassl
authors.forename: Pawinee
authors.initials: P
authors.lastname: Piyachaturawat
authors.forename: Nazzareno
authors.initials: N
authors.lastname: Ballatori
mesh_terms: D000818:Animals; D001647:Bile Acids and Salts / Q000378:metabolism*; D004970:Estrone / Q000031:analogs & derivatives / Q000378:metabolism; D005978:Glutathione / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D027381:Liver-Specific Organic Anion Transporter 1; D008564:Membrane Potentials / Q000502:physiology; D016297:Mutagenesis, Site-Directed; D009865:Oocytes / Q000378:metabolism; D027361:Organic Anion Transporters / Q000502:physiology*; D029362:Organic Anion Transporters, Sodium-Independent / Q000235:genetics / Q000502:physiology*; D000074542:Solute Carrier Organic Anion Transporter Family Member 1B3; D013656:Taurocholic Acid / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D001647:Bile Acids and Salts; D027381:Liver-Specific Organic Anion Transporter 1; D027361:Organic Anion Transporters; D029362:Organic Anion Transporters, Sodium-Independent; C503999:SLCO1B1 protein, human; C421209:SLCO1B3 protein, human; D000074542:Solute Carrier Organic Anion Transporter Family Member 1B3; D004970:Estrone; D013656:Taurocholic Acid; D005978:Glutathione; C017296:estrone sulfate