Transport of Drugs and Endogenous Compounds Mediated by Human OCT1: Studies in Single- and Double-Transfected Cell Models

Organic Cation Transporter 1 (OCT1, gene symbol: SLC22A1) is predominately expressed in human liver, localized in the basolateral membrane of hepatocytes and facilitates the uptake of endogenous compounds (e.g. serotonin, acetylcholine, thiamine), and widely prescribed drugs (e.g. metformin, fenoterol, morphine). Furthermore, exogenous compounds such as MPP+, ASP+ and Tetraethylammonium can be used as prototypic substrates to study the OCT1-mediated transport in vitro. Single-transfected cell lines recombinantly overexpressing OCT1 (e.g., HEK-OCT1) were established to study OCT1-mediated uptake and to evaluate transporter-mediated drug-drug interactions in vitro. Furthermore, double-transfected cell models simultaneously overexpressing basolaterally localized OCT1 together with an apically localized export protein have been established. Most of these cell models are based on polarized grown MDCK cells and can be used to analyze transcellular transport, mimicking the transport processes e.g. during the hepatobiliary elimination of drugs. Multidrug and toxin extrusion protein 1 (MATE1, gene symbol: SLC47A1) and the ATP-driven efflux pump P-glycoprotein (P-gp, gene symbol: ABCB1) are both expressed in the canalicular membrane of human hepatocytes and are described as transporters of organic cations. OCT1 and MATE1 have an overlapping substrate spectrum, indicating an important interplay of both transport proteins during the hepatobiliary elimination of drugs. Due to the important role of OCT1 for the transport of endogenous compounds and drugs, in vitro cell systems are important for the determination of the substrate spectrum of OCT1, the understanding of the molecular mechanisms of polarized transport, and the investigation of potential drug-drug interactions. Therefore, the aim of this review article is to summarize the current knowledge on cell systems recombinantly overexpressing human OCT1.

Progress in the Molecular Characterization of Hepatobiliary Transporters

Over the last 25 years, our understanding of the driving forces for hepatobiliary elimination and knowledge of the molecular basis of uptake and efflux transport in hepatocytes have undergone fundamental changes. This refers to bile acids and many other endogenous substances as well as to drugs that are eliminated on the hepatobiliary route. In this development, not only molecular cloning, functional characterization, and localization of transporters were decisive, but also the discovery of hereditary mutations in genes encoding sinusoidal uptake transporters and canalicular export pumps in humans and rodents. Uptake by passive diffusion and elimination into bile driven by the electrochemical gradient are no longer considered relevant for hepatobiliary elimination in the intact organism. Furthermore, insights into the relative roles of uptake transporters and unidirectional ATP-driven efflux pumps were obtained when we established double-transfected polarized cell lines stably expressing, as an example, the hepatocellular uptake transporter OATP1B3 and the apical (canalicular) efflux pump multidrug resistance protein 2 (MRP2; ABCC2). ATP-dependent efflux transporters localized to the basolateral (sinusoidal) hepatocyte membrane, particularly MRP3 (ABCC3) and MRP4 (ABCC4), pump substances from hepatocytes into sinusoidal blood. Bile acids are substrates for human MRP4 in the presence of physiological concentrations of reduced glutathione, which undergoes co-transport. These efflux pumps have been recognized in recent years to play an important compensatory role in cholestasis and to contribute to the balance between uptake and efflux of bile acids and other organic anions during the vectorial transport from blood into bile. This sinusoidal efflux not only enables subsequent renal elimination but also facilitates the re-uptake of substances into neighboring hepatocytes located more centrally and downstream in the sinusoid.