Cloning and Functional Expression of a Human Liver Organic Cation Transporter

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.

Cloning and functional expression of a mouse liver organic cation transporter

“…Saturation curves modeled to the Michaelis-Menten equation revealed a K m of 38 µmol/L, which is 2.4 times lower than that observed for rat OCT1, and 4.3 times lower than the K i observed for TEA when human OCT1 was expressed in oocytes. 14,15,28 These discrepancies may be due to species differences, or due to the expression of differing organic cation transporters, with some overlapping substrate specificity.…”

“…Despite several kinetic studies characterizing these transporters in plasma membranes and lysosomes, OCT1 is the only hepatic member of this putative liver-kidney organic cation transporter family that has been cloned and functionally characterized. 14,15,27,28 Other members of the putative organic cation superfamily include OCT2 and rOCT1A, which have also been cloned and functionally characterized, but do not appear to be expressed in the liver. 14,[28][29][30][31] Similarly, homologous cDNAs that encode for a polyspecific renal anion transporter, OAT1 (also known as the paraaminohippurate transporter), have been recently identified, but also appear to be kidney-specific.…”

“…14 A human homologue of rat OCT1 was recently cloned that transports 1-methyl-4-phenylpyridinium (MPP ϩ ), and uptake is also inhibited by the organic cation NMN. 15 In this study, we have isolated a murine cDNA (mOct1/ Slc22a1) from both liver and kidney libraries, and have functionally expressed it in X. laevis oocytes. Although mOct1/Slc22a1 is homologous with OCT1, transport kinetics differed markedly.…”

Influence of CYP3A5 and drug transporter polymorphisms on imatinib trough concentration and clinical response among patients with chronic phase chronic myeloid leukemia

“…17,18 OCT1, which is encoded by SLC22A1, is primarily expressed on hepatocytes, suggesting that it has a role in substrate uptake into the liver. [19][20][21] Although IM is a substrate of OCT1, 15,22,23 no association between IM and SLC22A1 286C4T or 1498G4A, variants not common in the Japanese population, was previously observed. 24,25 In addition, in vitro studies have shown that organic anion-transporting polypeptide 1B3 (OATP1B3), which is encoded by SLCO1B3, contributes to IM uptake into hepatocytes.…”

Expression of organic cation transporters OCT1 (SLC22A1) and OCT3 (SLC22A3) is affected by genetic factors and cholestasis in human liver

“…5 Their cellular uptake may be mediated by organic cation transporters (OCTs) that belong to the solute carrier family 22 (SLC22). 2,6 In human liver, only OCT1 (SLC22A1) [7][8][9] and OCT3 (SLC22A3) 10,11 transcripts are present, whereas OCT2 is primarily expressed in kidney and neither OCT2 (SLC22A2) messenger RNA (mRNA) 7,12 nor OCT2 protein 13 are detectable in liver. OCT1 is localized in the sinusoidal membrane of rat as well as human hepato-cytes.…”