Related Experiment Videos
Cloning and functional expression of a human liver organic cation transporter
L Zhang1, M J Dresser, A T Gray
1Department of Biopharmaceutical Sciences, University of California San Francisco 94143, USA.
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)]. In Xenopus laevis oocytes injected with the cRNA of hOCT1, the specific uptake of the organic cation 3H-1-methyl-4-phenylpyridinium (3H-MPP+) was significantly enhanced (8-fold) over that in water-injected oocytes. Uptake of 3H-MPP+ was saturable (K(m) = 14.6 +/- 4.39 microM) and sensitive to membrane potential. Both small monovalent organic cations such as tetraethylammonium and N1-methylnicotinamide and bulkier organic cations (e.g., vecuronium and decynium-22) inhibited the uptake of 3H-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.
Insights
Researchers identified and characterized the first human liver organic cation transporter (hOCT1), crucial for drug and toxin elimination. This discovery aids understanding of drug disposition and potential liver toxicity.
Area of Science:
- Pharmacology
- Molecular Biology
- Hepatology
Background:
- Liver organic cation transporters mediate the elimination of diverse endogenous amines and xenobiotics.
- Understanding organic cation transport mechanisms in the human liver remains limited compared to rat models.
Purpose of the Study:
- To clone, sequence, and functionally characterize the first human polyspecific organic cation transporter from the liver, designated hOCT1.
- To compare the characteristics of hOCT1 with its rat counterpart, rOCT1.
Main Methods:
- Cloning and sequencing of the hOCT1 gene.
- Functional characterization using Xenopus laevis oocyte expression system.
- Northern blot analysis for tissue-specific mRNA expression.
Main Results:
- hOCT1, 78% identical to rOCT1, significantly enhanced 3H-MPP+ uptake in oocytes.
- Uptake of 3H-MPP+ by hOCT1 was saturable, membrane potential-sensitive, and inhibited by various organic cations and taurocholate.
- Northern analysis revealed primary hOCT1 mRNA expression in the human liver, contrasting with rOCT1's broader tissue distribution.
Conclusions:
- hOCT1 is the first identified human liver polyspecific organic cation transporter.
- hOCT1 exhibits distinct kinetic properties and tissue distribution compared to rOCT1.
- Functional expression of hOCT1 provides a vital tool for studying human liver organic cation transport, drug disposition, and hepatotoxicity.