Gastroenterology

Gastroenterology

Volume 117, Issue 3, September 1999, Pages 688-695
Gastroenterology

Liver, Pancreas, and Biliary Tract
Localization and function of the organic anion–transporting polypeptide Oatp2 in rat liver,☆☆

https://doi.org/10.1016/S0016-5085(99)70463-4Get rights and content

Abstract

Background & Aims: Multispecific organic anion–transporting polypeptides (Oatps) are involved in the transcellular movement of amphipathic compounds in many tissues including the liver, kidney, and blood–brain barrier. Recently, a high-affinity digoxin transporter (Oatp2) was cloned from rat brain and shown to be also expressed in the liver. Methods: We investigated the cellular and subcellular distribution of Oatp2 in rat liver by in situ hybridization technology and immunofluorescence microscopy and compared its substrate specificity with that of Oatp1 in complementary RNA–injected Xenopus laevis oocytes. Results: The results show a selective basolateral (sinusoidal) expression of Oatp2 in midzonal to perivenous hepatocytes, but not in periportal or the innermost layer of perivenous hepatocytes. Common substrates of both Oatp1 and Oatp2 include bile salts, steroid conjugates, thyroid hormones (T3, T4), ouabain, and the endothelin receptor antagonist BQ-123 (Michaelis constants: Oatp1, ~600 μmol/L; Oatp2, ~30 μmol/L). Other organic anions including sulfolithotaurocholate, bilirubin monoglucuronide, and sulfobromophthalein were transported only by Oatp1. Conclusions: These results provide definite evidence for the partially overlapping and partially selective substrate specificities of Oatp1 and Oatp2. The unique acinar distribution of Oatp2 might indicate that it represents a high-affinity “backup” system for complete hepatocellular removal of certain cholephilic substances from portal blood plasma.

GASTROENTEROLOGY 1999;117:688-695

Section snippets

Chemicals

[Prolyl-3,4(n)-3H]BQ-123 (43.0 Ci/mmol) was obtained from Amersham Life Science (Amersham, England). Unlabeled BQ-123 was purchased from Research Biochemicals Internet (Natick, MA). [3H]LTC4 (165 Ci/mmol), [3H]dehydroepiandrosterone sulfate (DHEAS) (16 Ci/mmol), [3H]α-ketoglutarate (280.8 Ci/mmol), and [3H]p-aminohippurate (5 Ci/mmol) were purchased from DuPont–New England Nuclear (Boston, MA). [14C] Dinitrophenylglutathione (10 mCi/mmol) was synthesized as described by Ishikawa.12 Uridine

Cellular and subcellular distribution of Oatp2

The reactivity of the polyclonal antibodies with basolateral rat liver plasma membrane proteins is illustrated in Figure 1.

. Specificity of the Oatp1 and Oatp2 antibodies. Antibodies against the C-terminal ends of Oatp1 and Oatp2 were raised in rabbits and used for Western blotting of basolateral liver plasma membrane proteins as described in Materials and Methods. Initial SDS–polyacrylamide gel electrophoresis was performed with 100 μg of basolateral liver plasma membrane protein in each lane.

Discussion

The present study shows a unique compartmentalized basolateral expression of Oatp2 in parenchymal liver cells of normal rats on both the mRNA (Figure 2) and protein (Fig. 3, Fig. 3) levels. Recently, other investigators reported similar results on the immunohistochemical level.24 Our data suggest that Oatp2 is not expressed in the periportal proliferative compartment and requires either aging and differentiation of hepatocytes within the individual liver cell plates27 and/or is dependent on

Acknowledgements

The authors thank Dr. D. Keppler, Deutsches Krebsforschungszentrum, Abteilung Tumorbiochemie, Heidelberg, Germany, for making the radiolabeled bilirubin monoglucuronide available.

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    Address requests for reprints to: Peter J. Meier-Abt, M.D., Division of Clinical Pharmacology and Toxicology, Department of Internal Medicine, University Hospital, CH-8091 Zurich, Switzerland. e-mail: [email protected]; fax: (41) 1-255-4411.

    ☆☆

    Supported by the Swiss National Science Foundation (grants 31-045536.95 to P.J.M. and 31-045677.95 to B.H.); the Olga Mayenfisch Foundation (to B.G.), Zurich, Switzerland; the Cloetta Foundation (to B.H.), Zurich, Switzerland; the Deutsche Forschungs- gemeinschaft (SFB 601/A2), Heidelberg, Germany; and the Doerenkamp Foundation (to C.R.), Germany. Dr. van Montfoort was supported by an Ubbo Emmius scholarship from the University of Groningen, The Netherlands. Dr. Kamisako was supported by the Alexander von Humboldt Foundation, Bonn, Germany.

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