Organic anion transporter 1 (OAT1/SLC22A6) enhances bioluminescence based on d-luciferin-luciferase reaction in living cells by facilitating the intracellular accumulation of d-luciferin

Development and Use of a Kinetical and Real-Time Monitoring System to Analyze the Replication of Hepatitis C Virus

In microbiological research, it is important to understand the time course of each step in a pathogen's lifecycle and changes in the host cell environment induced by infection. This study is the first to develop a real-time monitoring system that kinetically detects luminescence reporter activity over time without sampling cells or culture supernatants for analyzing the virus replication. Subgenomic replicon experiments with hepatitis C virus (HCV) showed that transient translation and genome replication can be detected separately, with the first peak of translation observed at 3-4 h and replication beginning around 20 h after viral RNA introduction into cells. From the bioluminescence data set measured every 30 min (48 measurements per day), the initial rates of translation and replication were calculated, and their capacity levels were expressed as the sums of the measured signals in each process, which correspond to the areas on the kinetics graphs. The comparison of various HuH-7-derived cell lines showed that the bioluminescence profile differs among cell lines, suggesting that both translation and replication capacities potentially influence differences in HCV susceptibility. The effects of RNA mutations within the 5' UTR of the replicon on viral translation and replication were further analyzed in the system developed, confirming that mutations to the miR-122 binding sites primarily reduce replication activity rather than translation. The newly developed real-time monitoring system should be applied to the studies of various viruses and contribute to the analysis of transitions and progression of each process of their life cycle.

A Simple and Rapid Bioluminescence-Based Functional Assay of Organic Anion Transporter 1 as a D-Luciferin Transporter

Organic anion transporter 1 (SLC22A6/OAT1) plays a key role in renal tubular excretion of endo- and exogenous anionic substances including drugs. Since the inhibition of OAT1 function by a concomitant drug may cause pharmacokinetic drug-drug interactions (DDIs) in clinical practice, an in vitro uptake study to evaluate the inhibition potency of OAT1 is useful for the prediction and avoidance of DDIs and recommended for drug candidates in drug development. In this chapter, we describe a rapid and highly sensitive functional assay of OAT1 based on bioluminescence (BL) detection using D-luciferin as a substrate in living cells. The principle of measurement simply relies on the biochemical feature of D-luciferin to be recognized as a substrate of OAT1, and the BL intensity depending on intracellular D-luciferin level and luciferase activity, thereby allowing the quantitative analysis of OAT1-mediated D-luciferin transport. The BL measurement can be completed within 1 min without experimental procedures for removing extracellular uptake solution and washing cells, both of which involve in the conventional uptake studies using isotope-labeled or fluorescent compounds. The present method is applicable to high-throughput screening to identify and avoid potential OAT1 inhibitors in drug development.

Luminogenic D-Luciferin Derivatives as OATP1B1 and 1B3 Substrates in No-wash Assays†

The human hepatic organic ion transporting polypeptides OATP1B1 and -1B3 are uptake transporters that influence the disposition of several small molecule drugs and perpetrate certain adverse drug-drug interactions. To predict these in vivo effects, in vitro systems are used to screen new drug entities as potential transporter substrates or inhibitors. To simplify such studies, we synthesized luminogenic derivatives of the OATP1B1 and -1B3 substrate D-luciferin to test as probe substrates in a rapid, no-wash optical approach for substrate and inhibitor identification and characterization. Each derivative is a pro-luciferin containing a self-immolating trimethyl lock quinone linker that is sensitive to intracellular reducing environments that cause the release of free luciferin in proportion to the amount of probe taken up by the transporter. A subsequent luciferin-limited luciferase reaction produces light in proportion to transporter activity. We tested the derivatives in HEK293 cells that overexpress OATP1B1 or OATP1B3 by transient transfection or viral transduction. Derivatives were identified that showed OATP-dependent uptake that was time and concentration dependent, saturable and sensitive to inhibition by known OATP1B1 and -1B3 substrates and inhibitors. These luminogenic transporter probes enabled an add-only multi-well plate protocol suitable for automation and high throughput screening.

How to Select Firefly Luciferin Analogues for In Vivo Imaging

Bioluminescence reactions are widely applied in optical in vivo imaging in the life science and medical fields. Such reactions produce light upon the oxidation of a luciferin (substrate) catalyzed by a luciferase (enzyme), and this bioluminescence enables the quantification of tumor cells and gene expression in animal models. Many researchers have developed single-color or multicolor bioluminescence systems based on artificial luciferin analogues and/or luciferase mutants, for application in vivo bioluminescence imaging (BLI). In the current review, we focus on the characteristics of firefly BLI technology and discuss the development of luciferin analogues for high-resolution in vivo BLI. In addition, we discuss the novel luciferin analogues TokeOni and seMpai, which show potential as high-sensitivity in vivo BLI reagents.

Functional characterization of monocarboxylate transporter 12 (SLC16A12/MCT12) as a facilitative creatine transporter

SLC16A12/MCT12 has been recently identified as a creatine transporter in a Xenopus oocyte expression system; however, the mechanism, by which MCT12 transports creatine, remains unclear. This study was performed to determine the functional and molecular characteristics of MCT12 in mammalian cells. The results showed that the uptake of [¹⁴C]creatine was not significantly increased in HEK293 cells transiently expressing MCT12 with or without CD147, a molecular chaperone, compared with mock cells. When [¹⁴C]creatine was accumulated in the cells with the aid of SLC6A8/CRT1, a concentrative creatine transporter, followed by assessing the remaining intracellular [¹⁴C]creatine after initiating efflux, coexpression of MCT12 resulted in a decrease in the intracellular [¹⁴C]creatine and remarkably enhanced the efflux of [¹⁴C]creatine from the cells in a time-dependent manner. This activity was not affected by extracellular pH. The creatine efflux activity involved dissipation by the mutations of conserved charged amino acids such as Arg37, Asp65 and Asp299 in the transmembrane domains, indicating direct involvement of MCT12 in the creatine efflux. These results suggest that MCT12 mediates facilitative diffusion of creatine, depending on the concentration gradient across the plasma membrane in mammalian cells. The finding may provide important clues to understanding the disposition kinetics of creatine and its derivatives.

Isolation of a novel glycyrrhizin metabolite as a causal candidate compound for pseudoaldosteronism

Pseudoaldosteronism is a common adverse effect associated with traditional Japanese Kampo medicines. The pathogenesis is mainly caused by 3-monoglucuronyl glycyrrhetinic acid (3MGA), one of the metabolites of glycyrrhizin (GL) contained in licorice. We developed an anti-3MGA monoclonal antibody (MAb) and an ELISA system to easily detect 3MGA in the plasma and urine of the patients. However, we found that some metabolites of GL cross-reacted with this MAb. Mrp2-deficient Eisai Hyperbilirubinemia rats (EHBRs) were administered glycyrrhetinic acid (GA), and we isolated 22α-hydroxy-18β-glycyrrhetyl-3-O-sulfate-30-glucuronide (1) from the pooled urine with the guidance of positive immunostaining of eastern blot as the new metabolite of GL. The IC₅₀ of 1 for type 2 11β-hydroxysteroid dehydrogenase (11β-HSD2) was 2.0 µM. Similar plasma concentrations of 1 and GA were observed 12 h after oral administration of GA to EHBR. Compound 1 was eliminated via urine, whereas GA was not. In Sprague–Dawley (SD) rats orally treated with GA, compound 1 was absent from both the plasma and the urine. Compound 1 was actively transported into cells via OAT1 and OAT3, whereas GA was not. Compound 1, when produced in Mrp2-deficiency, represents a potential causative agent of pseudoaldosteronism, and might be used as a biomarker to prevent the adverse effect.

Urate transport function of rat sodium-dependent nucleobase transporter 1

Sodium-dependent nucleobase transporter 1 (SNBT1) is a nucleobase-specific transporter identified in our recent study. In an attempt to search for its potential substrates other than nucleobases in this study, we could successfully find urate, a metabolic derivative of purine nucleobases, as a novel substrate, as indicated by its specific Na+ -dependent and saturable transport, with a Michaelis constant of 433 μmol/L, by rat SNBT1 (rSNBT1) stably expressed in Madin-Darby canine kidney II cells. However, urate uptake was observed only barely in the everted tissue sacs of the rat small intestine, in which rSNBT1 operates for nucleobase uptake. These findings suggested that urate undergoes a futile cycle, in which urate transported into epithelial cells is immediately effluxed back by urate efflux transporters, in the small intestine. In subsequent attempts to examine that possibility, such a futile urate cycle was demonstrated in the human embryonic kidney 293 cell line as a model cell system, where urate uptake induced by transiently introduced rSNBT1 was extensively reduced by the co-introduction of rat breast cancer resistance protein (rBCRP), a urate efflux transporter present in the small intestine. However, urate uptake was not raised in the presence of Ko143, a BCRP inhibitor, in the everted intestinal tissue sacs, suggesting that some other transporter might also be involved in urate efflux. The newly found urate transport function of SNBT1, together with the suggested futile urate cycle in the small intestine, should be of interest for its evolutional and biological implications, although SNBT1 is genetically deficient in humans.