Cloning and Functional Expression of a Human Liver Organic Cation Transporter

  1. Lei Zhang1,
  2. Mark J. Dresser1,
  3. Andrew T. Gray2,
  4. Spencer C. Yost2,
  5. Shigeyuki Terashita1 and
  6. Kathleen M. Giacomini1
  1. Departments of 1Biopharmaceutical Sciences (L.Z., M.J.D., S.T., K.M.G.) and 2Anesthesia (A.T.G., S.C.Y.), University of California San Francisco, San Francisco, California 94143

    Abstract

    Polyspecific organic cation transporters in the liver mediate the elimination of a wide array of endogenous amines and xenobiotics. In contrast to our understanding of the mechanisms of organic cation transport in rat liver, little is known about the mechanisms of organic cation transport in the human liver. We report the cloning, sequencing, and functional characterization of the first human polyspecific organic cation transporter from liver (hOCT1). hOCT1 (554 amino acids) is 78% identical to the previously cloned organic cation transporter from rat, rOCT1 [Nature (Lond.) 372:549–552 (1994)]. InXenopus laevis oocytes injected with the cRNA of hOCT1, the specific uptake of the organic cation3H-1-methyl-4-phenylpyridinium (3H-MPP+) was significantly enhanced (8-fold) over that in water-injected oocytes. Uptake of3H-MPP+ was saturable (Km = 14.6 ± 4.39 μm) and sensitive to membrane potential. Both small monovalent organic cations such as tetraethylammonium andN1-methylnicotinamide and bulkier organic cations (e.g., vecuronium and decynium-22) inhibited the uptake of3H-MPP+. In addition, the bile acid taurocholate inhibited the uptake of 3H-MPP+ in oocytes expressing hOCT1. Northern analysis demonstrated that the mRNA transcript of hOCT1 is expressed primarily in the human liver, whereas the mRNA transcript of rOCT1 is found in rat kidney, liver, intestine, and colon [Nature (Lond.) 372:549–552 (1994)]. In comparison to rOCT1, hOCT1 exhibits notable differences in its kinetic characteristics and tissue distribution. The functional expression of hOCT1 will provide a powerful tool for elucidation of the mechanisms of organic cation transport in the human liver and understanding of the mechanisms involved in the disposition and hepatotoxicity of drugs.

    Footnotes

    • Send reprint requests to: Kathleen M. Giacomini, Ph.D., University of California, San Francisco, School of Pharmacy, Department of Biopharmaceutical Sciences, San Francisco, CA 94143. E-mail:kmg{at}itsa.ucsf.edu

    • ↵1 The sequence reported here was submitted to GenBank with the accession number U77086.

    • This study was supported by grants from the National Institutes of Health (GM36780 and GM26691). M.D. was supported in part by a grant from the ARCS Foundation, and S.T. was supported by Fujisawa Pharmaceutical Company.

    • Abbreviations:
      TEA
      tetraethylammonium
      MPP+
      1-methyl-4-phenylpyridinium
      hOCT1
      human organic cation transporter
      rOCT1
      rat organic cation transporter
      PAH
      para-aminohippurate
      RT
      reverse transcriptase
      TMD
      transmembrane domain
      RACE
      rapid amplification of cDNA ends
      PCR
      polymerase chain reaction
      HEPES
      4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
      • Received February 7, 1997.
      • Accepted March 6, 1997.
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    1. Molecular Pharmacology June 1, 1997 vol. 51 no. 6 913-921
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