Cloning and Characterization of Two Human Polyspecific Organic Cation Transporters

Abstract

Previously we cloned a polyspecific transporter from rat (rOCT1) that is expressed in renal proximal tubules and hepatocytes and mediates electrogenic uptake of organic cations with different molecular structures. Recently a homologous transporter from rat kidney (rOCT2) was cloned but not characterized in detail. We report cloning and characterization of two homologous transporters from man (hOCT1 and hOCT2) displaying approximately 80% amino acid identity to rOCT1 and rOCT2, respectively. Northern blots showed that hOCT1 is mainly transcribed in liver, while hOCT2 is found in kidney. Using in situ hybridization and immunohistochemistry, expression of hOCT2 was mainly detected in the distal tubule where the transporter is localized at the luminal membrane. After expression in Xenopus laevis oocytes, hOCT1 and hOCT2 mediate tracer influx of N-1-methylnicotinamide (NMN), tetraethylammonium (TEA), and 1-methyl-4-phenylpyridinium (MPP). For cation transport by hOCT2 apparent K(m) and K(i) values were determined in tracer flux measurements. In addition, electrical measurements were performed with voltage-clamped oocytes. Similar to rOCT1, cation transport by hOCT2 was pH independent, electrogenic, and polyspecific; however, the cation specificity was different. In voltage-clamped hOCT2-expressing oocytes, inward currents were induced by superfusion with MPP, TEA, choline, quinine, d-tubocurarine, pancuronium, and cyanine863. Cation transport in distal tubules is indicated for the first time. Here hOCT2 mediates the first step in cation reabsorption. hOCT1 may participate in hepatic excretion of organic cations.

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

“…1A), as expected based on previous studies. 7,19,45 This prompted us to elucidate systematically the interindividual variability of hepatic OCT1 expression and underlying factors. Besides several other tissues, OCT3 transcripts were also detected in human liver using the tissue array (Fig.…”

“…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.…”

OCT1 Genetic Variants are Associated with Postoperative Morphine-Related Adverse Effects in Children

“…However, their expression was not observed in Asian population [30]. Mainly expressed in liver, low-level expression of this transporter has been reported in other tissues such as kidney, brain, placenta, heart, lung, skeletal muscle [31,32]. In brain, weak OCT1 expression was found in endothelial cells [33] and certain regions of corpus callosum and cerebellum [34].…”

Pharmacogenetics of Organic Cation Transporters

“…Близкое физиче-ское расположение всех трех генов ОСТ и их структурное сходство позволяет предполо-жить их возникновение путем дупликации единого гена-предшественника [11].…”