Characterization of the human multidrug resistance protein isoform MRP3 localized to the basolateral hepatocyte membrane

Influence of Slco2b1-knockout and SLCO2B1-humanization on coproporphyrin I and III levels in rats

BACKGROUND AND PURPOSE

Coproporphyrin (CP) I and III are byproducts of haem synthesis currently investigated as biomarkers for drug-drug interactions involving hepatic organic anion transporting polypeptide (OATP) 1B transporters. Another hepatically expressed OATP-member is OATP2B1. The aim of this study was to test the impact of OATP2B1, which specifically transports CPIII, on CP serum levels, applying novel rat models.

EXPERIMENTAL APPROACH

CPIII transport kinetics and the interplay between OATP2B1 and multidrug resistance-associated proteins (MRPs) were determined in vitro using the vTF7 expression system. Novel rSlco2b1-/- and SLCO2B1+/+ rat models were characterized for physiological parameters and for CP serum levels. Hepatic and renal expression of transporters involved in CP disposition were determined by real-time qPCR, Western blot analysis, and immunohistochemistry.

KEY RESULTS

In vitro experiments revealed differences in transport kinetics comparing human and rat OATP2B1 and showed a consistent, species-specific interplay with hMRP3/rMRP3. Deletion of rOATP2B1 was associated with a trend towards lower CPI serum levels compared with wildtype rats, while CPIII remained unchanged. Comparing SLCO2B1+/+ with knockout rats revealed an effect of sex: only in females the genetic modification influenced CP serum levels. Analysis of hepatic and renal transporters revealed marginal, but in part, statistically significant differences in rMRP2 abundance, which may contribute to the observed changes in CP serum levels.

CONCLUSION AND IMPLICATIONS

Our findings support that factors other than OATP1B transporters are of relevance for basal CP levels. Only in female rats, humanization of SLCO2B1 affects basal CPI and CPIII serum levels, despite isomer selectivity of OATP2B1.

Placing steroid hormones within the human ABCC3 transporter reveals a compatible amphiphilic substrate-binding pocket

The human ABC transporter ABCC3 (also known as MRP3) transports a wide spectrum of substrates, including endogenous metabolites and exogenous drugs. Accordingly, it participates in multiple physiological processes and is involved in diverse human diseases such as intrahepatic cholestasis of pregnancy, which is caused by the intracellular accumulation of bile acids and estrogens. Here, we report three cryogenic electron microscopy structures of ABCC3: in the apo-form and in complexed forms bound to either the conjugated sex hormones β-estradiol 17-(β-D-glucuronide) and dehydroepiandrosterone sulfate. For both hormones, the steroid nuclei that superimpose against each other occupy the hydrophobic center of the transport cavity, whereas the two conjugation groups are separated and fixed by the hydrophilic patches in two transmembrane domains. Structural analysis combined with site-directed mutagenesis and ATPase activity assays revealed that ABCC3 possesses an amphiphilic substrate-binding pocket able to hold either conjugated hormone in an asymmetric pattern. These data build on consensus features of the substrate-binding pocket of MRPs and provide a structural platform for the rational design of inhibitors.

A case of focal nodular hyperplasia-like lesion presenting unusual signal intensity on the hepatobiliary phase of gadoxetic acid-enhanced magnetic resonance image

Focal nodular hyperplasia (FNH) or FNH-like lesions of the liver are benign lesions that can be mostly diagnosed by hepatobiliary phase gadoxetic acid-enhanced magnetic resonance imaging (MRI). Accurate imaging diagnosis is based on the fact that most FNHs or FNH-like lesions show characteristic hyper- or isointensity on hepatobiliary phase images. We report a case of an FNH-like lesion in a 73-year-old woman that mimicked a malignant tumor. Dynamic contrast-enhanced computed tomography (CT) and MRI using gadoxetic-acid revealed an ill-defined nodule showing early enhancement in the arterial phase and gradual and prolonged enhancement in the portal and equilibrium/transitional phases. Hepatobiliary phase imaging revealed inhomogeneous hypointensity, accompanied by a slightly isointense area compared to the background liver. Angiography-assisted CT showed a portal perfusion defect of the nodule, inhomogeneous arterial blood supply in the early phase, and less internal enhancement in the late phase, accompanied by irregularly shaped peritumoral enhancement. No central stellate scar was identified in any of the images. Imaging findings could not exclude the possibility of hepatocellular carcinoma, but the nodule was pathologically diagnosed as an FNH-like lesion by partial hepatectomy. In the present case, an unusual inhomogeneous hypointensity on hepatobiliary phase imaging made it difficult to diagnose the FNH-like lesions.

Extracellular Vesicles and Cancer Multidrug Resistance: Undesirable Intercellular Messengers?

Cancer multidrug resistance (MDR) is one of the main mechanisms contributing to therapy failure and mortality. Overexpression of drug transporters of the ABC family (ATP-binding cassette) is a major cause of MDR. Extracellular vesicles (EVs) are nanoparticles released by most cells of the organism involved in cell-cell communication. Their cargo mainly comprises, proteins, nucleic acids, and lipids, which are transferred from a donor cell to a target cell and lead to phenotypical changes. In this article, we review the scientific evidence addressing the regulation of ABC transporters by EV-mediated cell-cell communication. MDR transfer from drug-resistant to drug-sensitive cells has been identified in several tumor entities. This was attributed, in some cases, to the direct shuttle of transporter molecules or its coding mRNA between cells. Also, EV-mediated transport of regulatory proteins (e.g., transcription factors) and noncoding RNAs have been indicated to induce MDR. Conversely, the transfer of a drug-sensitive phenotype via EVs has also been reported. Additionally, interactions between non-tumor cells and the tumor cells with an impact on MDR are presented. Finally, we highlight uninvestigated aspects and possible approaches to exploiting this knowledge toward the identification of druggable processes and molecules and, ultimately, the development of novel therapeutic strategies.

Regulation of ABC transporters by sex steroids may explain differences in drug resistance between sexes

Drug efficacy is dependent on the pharmacokinetics and pharmacodynamics of therapeutic agents. Tight junctions, detoxification enzymes, and drug transporters, due to their localization on epithelial barriers, modulate the absorption, distribution, and the elimination of a drug. The epithelial barriers which control the pharmacokinetic processes are sex steroid hormone targets, and in this way, sex hormones may also control the drug transport across these barriers. Thus, sex steroids contribute to sex differences in drug resistance and have a relevant impact on the sex-related efficacy of many therapeutic drugs. As a consequence, for the further development and optimization of therapeutic strategies, the sex of the individuals must be taken into consideration. Here, we gather and discuss the evidence about the regulation of ATP-binding cassette transporters by sex steroids, and we also describe the signaling pathways by which sex steroids modulate ATP-binding cassette transporters expression, with a focus in the most important ATP-binding cassette transporters involved in multidrug resistance.

Fluorescence-based methods for studying activity and drug-drug interactions of hepatic solute carrier and ATP binding cassette proteins involved in ADME-Tox

In humans, approximately 70% of drugs are eliminated through the liver. This process is governed by the concerted action of membrane transporters and metabolic enzymes. Transporters mediating hepatocellular uptake of drugs belong to the SLC (Solute carrier) superfamily of transporters. Drug efflux either toward the portal vein or into the bile is mainly mediated by active transporters of the ABC (ATP Binding Cassette) family. Alteration in the function and/or expression of liver transporters due to mutations, disease conditions, or co-administration of drugs or food components can result in altered pharmacokinetics. On the other hand, drugs or food components interacting with liver transporters may also interfere with liver function (e.g., bile acid homeostasis) and may even cause liver toxicity. Accordingly, certain transporters of the liver should be investigated already at an early stage of drug development. Most frequently radioactive probes are applied in these drug-transporter interaction tests. However, fluorescent probes are cost-effective and sensitive alternatives to radioligands, and are gaining wider application in drug-transporter interaction tests. In our review, we summarize our current understanding about hepatocyte ABC and SLC transporters affected by drug interactions. We provide an update of the available fluorescent and fluorogenic/activable probes applicable in in vitro or in vivo testing of these ABC and SLC transporters, including near-infrared transporter probes especially suitable for in vivo imaging. Furthermore, our review gives a comprehensive overview of the available fluorescence-based methods, not directly relying on the transport of the probe, suitable for the investigation of hepatic ABC or SLC-type drug transporters.

Hepatic Transporters Alternations Associated with Non-alcoholic Fatty Liver Disease (NAFLD): A Systematic Review

BACKGROUND AND OBJECTIVES

Non-alcoholic fatty liver disease (NAFLD) is a progressive liver disorder and is usually accompanied by obesity, metabolic syndrome, and diabetes mellitus. NAFLD progression can lead to impaired functions of hepatocytes such as alternations in expression and function of hepatic transporters. The present study aimed to summarize and discuss the results of clinical and preclinical human studies that investigate the effect of NAFLD on hepatic transporters.

METHODS

The databases of PubMed, Scopus, Embase, and Web of Science were searched systematically up to 1 March 2022. The risk of bias was assessed for cross-sectional studies through the Newcastle-Ottawa Scale score.

RESULTS

Our review included ten cross-sectional studies consisting of 485 participants. Substantial alternations in hepatic transporters were seen during NAFLD progression to non-alcoholic steatohepatitis (NASH) in comparison with control groups. A significant reduction in expression and function of several hepatic uptake transporters, upregulation of many efflux transporters, downregulation of cholesterol efflux transporters, and mislocalization of canalicular transporter ABCC2 are associated with NAFLD progression.

CONCLUSION

Since extensive changes in hepatic transporters could alter the pharmacokinetics of the drugs and potentially affect the safety and efficacy of drugs, close monitoring of drug administration is highly suggested in patients with NASH.

Role of Hepatocyte Transporters in Drug-Induced Liver Injury (DILI)-In Vitro Testing

Bile acids and bile salts (BA/BS) are substrates of both influx and efflux transporters on hepatocytes. Canalicular efflux transporters, such as BSEP and MRP2, are crucial for the removal of BA/BS to the bile. Basolateral influx transporters, such as NTCP, OATP1B1/1B3, and OSTα/β, cooperate with canalicular transporters in the transcellular vectorial flux of BA/BS from the sinusoids to the bile. The blockage of canalicular transporters not only impairs the bile flow but also causes the intracellular accumulation of BA/BS in hepatocytes that contributes to, or even triggers, liver injury. In the case of BA/BS overload, the efflux of these toxic substances back to the blood via MRP3, MRP4, and OST α/β is considered a relief function. FXR, a key regulator of defense against BA/BS toxicity suppresses de novo bile acid synthesis and bile acid uptake, and promotes bile acid removal via increased efflux. In drug development, the early testing of the inhibition of these transporters, BSEP in particular, is important to flag compounds that could potentially inflict drug-induced liver injury (DILI). In vitro test systems for efflux transporters employ membrane vesicles, whereas those for influx transporters employ whole cells. Additional in vitro pharmaceutical testing panels usually include cellular toxicity tests using hepatocytes, as well as assessments of the mitochondrial toxicity and accumulation of reactive oxygen species (ROS). Primary hepatocytes are the cells of choice for toxicity testing, with HepaRG cells emerging as an alternative. Inhibition of the FXR function is also included in some testing panels. The molecular weight and hydrophobicity of the drug, as well as the steady-state total plasma levels, may positively correlate with the DILI potential. Depending on the phase of drug development, the physicochemical properties, dosing, and cut-off values of BSEP IC₅₀ ≤ 25-50 µM or total C_ss,plasma/BSEP IC₅₀ ≥ 0.1 may be an indication for further testing to minimize the risk of DILI liability.

Role and Regulation of Hepatobiliary ATP-Binding Cassette Transporters during Chemical-Induced Liver Injury

Severity of drug-induced liver injury (DILI) ranges from mild, asymptomatic, and transient elevations in liver function tests to irreversible liver damage, often needing transplantation. Traditionally, DILI is classified mechanistically as high-frequency intrinsic DILI, commonly dose dependent or DILI that rarely occurs and is idiosyncratic in nature. This latter form is not dose dependent and has a pattern of histopathological manifestation that is not always uniform. Currently, a third type of DILI called indirect hepatotoxicity has been described that is associated with the pharmacological action of the drug. Historically, DILI was primarily linked to drug metabolism events; however, the impact of transporter-mediated rates of drug uptake and excretion has gained greater prominence in DILI research. This review provides a comprehensive view of the major findings from studies examining the contribution of hepatic ATP-binding cassette transporters as key contributors to DILI and how changes in their expression and function influence the development, severity, and overall toxicity outcome. SIGNIFICANCE STATEMENT: Drug-induced liver injury (DILI) continues to be a focal point in drug development research. ATP-binding cassette (ABC) transporters have emerged as important determinants of drug detoxification, disposition, and safety. This review article provides a comprehensive analysis of the literature addressing: (a) the role of hepatic ABC transporters in DILI, (b) the influence of genetic mutations in ABC transporters on DILI, and (c) new areas of research emphasis, such as the influence of the gut microbiota and epigenetic regulation, on ABC transporters.

Differentiated kidney tubular cell-derived extracellular vesicles enhance maturation of tubuloids

The prevalence of end-stage kidney disease (ESKD) is rapidly increasing with the need for regenerative therapies. Adult stem cell derived kidney tubuloids have the potential to functionally mimic the adult kidney tubule, but still lack the expression of important transport proteins needed for waste removal. Here, we investigated the potential of extracellular vesicles (EVs) obtained from matured kidney tubular epithelial cells to modulate in vitro tubuloids functional maturation. We focused on organic anion transporter 1 (OAT1), one of the most important proteins involved in endogenous waste excretion. First, we show that EVs from engineered proximal tubule cells increased the expression of several transcription factors and epithelial transporters, resulting in improved OAT1 transport capacity. Next, a more in-depth proteomic data analysis showed that EVs can trigger various biological pathways, including mesenchymal-to-epithelial transition, which is crucial in the tubular epithelial maturation. Moreover, we demonstrated that the combination of EVs and tubuloid-derived cells can be used as part of a bioartificial kidney to generate a tight polarized epithelial monolayer with formation of dense cilia structures. In conclusion, EVs from kidney tubular epithelial cells can phenotypically improve in vitro tubuloid maturation, thereby enhancing their potential as functional units in regenerative or renal replacement therapies.

Effect of bile acids on the expression of MRP3 and MRP4: An In vitro study in HepG2 cell line

INTRODUCTION AND OBJECTIVES

Free and conjugated bile acids (BA's) cannot cross cell membranes; therefore, a particular transport system is required by the cell. Members of the family of ABC (ATP-binding proteins) transporters transfer bile acids in and out of the cell, preventing their accumulation. High intracellular concentrations of bile acids, such as those observed in cholestasis, have been related to oxidative stress and apoptosis, which in many cases are the leading causes of hepatocyte damage. MRP3 and MRP4 (multidrug resistance-associated protein 3 and 4) proteins belong to the ABC subfamily C, and are transporters of the hepatocyte's basolateral membrane with a compensatory role. Both transporters' increased expression constitutes an essential role in the protective and adaptive responses of bile acid overload, such as cholestasis. This work aimed to analyze both transporters' mRNA and protein expression in an in vitro model of cholestasis using HepG2 cell line treated with main bile acids.

METHODS

The expression of transporters was investigated through confocal microscopy immunofluorescence, Western Blot, and RT-qPCR after the main bile acids in HepG2 line cells.

RESULTS

The results showed the relation between confluence and expression of both transporters in the plasma membrane. MRP3 showed atypical and heterogeneous distribution in this cell line. CDCA (chenodeoxycholic acid) at low concentrations induced the expression of mRNA of both transporters. In contrast, protein expression was induced by CA (cholic acid) at high concentrations.

CONCLUSION

Primary bile acids (CDCA and CA) induce overexpression of the MRP4 and MRP3 transporters in the HepG2 cell line.

The Role of Uptake and Efflux Transporters in the Disposition of Glucuronide and Sulfate Conjugates

Glucuronidation and sulfation are the most typical phase II metabolic reactions of drugs. The resulting glucuronide and sulfate conjugates are generally considered inactive and safe. They may, however, be the most prominent drug-related material in the circulation and excreta of humans. The glucuronide and sulfate metabolites of drugs typically have limited cell membrane permeability and subsequently, their distribution and excretion from the human body requires transport proteins. Uptake transporters, such as organic anion transporters (OATs and OATPs), mediate the uptake of conjugates into the liver and kidney, while efflux transporters, such as multidrug resistance proteins (MRPs) and breast cancer resistance protein (BCRP), mediate expulsion of conjugates into bile, urine and the intestinal lumen. Understanding the active transport of conjugated drug metabolites is important for predicting the fate of a drug in the body and its safety and efficacy. The aim of this review is to compile the understanding of transporter-mediated disposition of phase II conjugates. We review the literature on hepatic, intestinal and renal uptake transporters participating in the transport of glucuronide and sulfate metabolites of drugs, other xenobiotics and endobiotics. In addition, we provide an update on the involvement of efflux transporters in the disposition of glucuronide and sulfate metabolites. Finally, we discuss the interplay between uptake and efflux transport in the intestine, liver and kidneys as well as the role of transporters in glucuronide and sulfate conjugate toxicity, drug interactions, pharmacogenetics and species differences.

Current Models for Predicting Drug-induced Cholestasis: The Role of Hepatobiliary Transport System

Drug-induced cholestasis is the main type of liver disorder accompanied by high morbidity and mortality. Evidence for the role of hepatobiliary pumps in the cholestasis patho-mechanism is constantly increasing. Recognition of the interactions of chemical agents with these transporters at the initial phases of drug discovery can help develop new drug candidates with low cholestasis potential. This review delivers an outline of the role of these transport proteins in bile creation. It addresses the pathophysiological mechanism for drug-induced cholestasis. In-vitro models, including cell-based and membrane-based approaches and In-vivo models such as genetic knockout animals, are considered. The benefits and restrictions of each model are discussed in this review. Current understandings into the cellular and molecular process that control the activity of hepatobiliary pumps have directed to a better understanding of the pathophysiology of drug-induced cholestasis. A combination of in-vitro monitoring for transport interaction, in-silico predicting systems, and consideration of and metabolic and physicochemical properties must cause more effective monitoring of possible liver problems.

Drug resistance: from bacteria to cancer

The phenomenon of drug resistance has been a hindrance to therapeutic medicine since the late 1940s. There is a plethora of factors and mechanisms contributing to progression of drug resistance. From prokaryotes to complex cancers, drug resistance is a prevailing issue in clinical medicine. Although there are numerous factors causing and influencing the phenomenon of drug resistance, cellular transporters contribute to a noticeable majority. Efflux transporters form a huge family of proteins and are found in a vast number of species spanning from prokaryotes to complex organisms such as humans. During the last couple of decades, various approaches in analyses of biochemistry and pharmacology of transporters have led us to understand much more about drug resistance. In this review, we have discussed the structure, function, potential causes, and mechanisms of multidrug resistance in bacteria as well as cancers.

ABCB1, ABCG2, ABCC1, ABCC2, and ABCC3 drug transporter polymorphisms and their impact on drug bioavailability: what is our current understanding?

INTRODUCTION

Interindividual differences in drug response are a frequent clinical challenge partly due to variation in pharmacokinetics. ATP-binding cassette (ABC) transporters are crucial determinants of drug disposition. They are subject of gene regulation and drug-interaction; however, it is still under debate to which extend genetic variants in these transporters contribute to interindividual variability of a wide range of drugs.

AREAS COVERED

This review discusses the current literature on the impact of genetic variants in ABCB1, ABCG2 as well as ABCC1, ABCC2, and ABCC3 on pharmacokinetics and drug response. The aim was to evaluate if results from recent studies would increase the evidence for potential clinically relevant pharmacogenetic effects.

EXPERT OPINION

Although enormous efforts have been made to investigate effects of ABC transporter genotypes on drug pharmacokinetics and response, the majority of studies showed only weak if any associations. Despite few unique results, studies mostly failed to confirm earlier findings or still remained inconsistent. The impact of genetic variants on drug bioavailability is only minor and other factors regulating the transporter expression and function seem to be more critical. In our opinion, the findings on the so far investigated genetic variants in ABC efflux transporters are not suitable as predictive biomarkers.

Multidrug resistance proteins (MRPs): Structure, function and the overcoming of cancer multidrug resistance

ATP-binding cassette (ABC) transporters mediate the ATP-driven translocation of structurally and mechanistically distinct substrates against steep concentration gradients. Among the seven human ABC subfamilies namely ABCA-ABCG, ABCC is the largest subfamily with 13 members. In this respect, 9 of the ABCC members are termed "multidrug resistance proteins" (MRPs1-9) due to their ability to mediate cancer multidrug resistance (MDR) by extruding various chemotherapeutic agents or their metabolites from tumor cells. Furthermore, MRPs are also responsible for the ATP-driven efflux of physiologically important organic anions such as leukotriene C₄, folic acid, bile acids and cAMP. Thus, MRPs are involved in important regulatory pathways. Blocking the anticancer drug efflux function of MRPs has shown promising results in overcoming cancer MDR. As a result, many novel MRP modulators have been developed in the past decade. In the current review, we summarize the structure, tissue distribution, biological and pharmacological functions as well as clinical insights of MRPs. Furthermore, recent updates in MRP modulators and their therapeutic applications in clinical trials are also discussed.

Endogenous Coproporphyrin I and III are Altered in Multidrug Resistance-Associated Protein 2-Deficient (TR-) Rats

Recent studies have focused on coproporphyrin (CP)-I and CP-III (CPs) as endogenous biomarkers for organic anion transporting polypeptides (OATPs). Previous data showed that CPs are also substrates of multidrug resistance-associated protein (MRP/Mrp) 2 and 3. This study was designed to examine the impact of loss of Mrp2 function on the routes of excretion of endogenous CPs in wild-type (WT) Wistar compared to Mrp2-deficient TR^- rats. To exclude possible confounding effects of rat Oatps, the transport of CPs was investigated in Oatp-overexpressing HeLa cells. Results indicated that CPs are substrates of rodent Oatp1b2, and that CP-III is a substrate of Oatp2b1. Quantitative targeted absolute proteomic (QTAP) analysis revealed no differences in Oatps, but an expected significant increase in Mrp3 protein levels in TR^- compared to WT rat livers. CP-I and CP-III concentrations measured by LC-MS/MS were elevated in TR^- compared to WT rat liver, while CP-I and CP-III estimated biliary clearance was decreased 75- and 840-fold in TR^- compared to WT rats, respectively. CP-III concentrations were decreased 14-fold in the feces of TR^- compared to WT rats, but differences in CP-I were not significant. In summary, the disposition of CPs was markedly altered by loss of Mrp2 and increased Mrp3 function as measured in TR^- rats.

Prediction of Drug-Induced Hyperbilirubinemia by In Vitro Testing

Bilirubin, the end product of heme catabolism, is produced continuously in the body and may reach toxic levels if accumulates in the serum and tissues; therefore, a highly efficient mechanism evolved for its disposition. Normally, unconjugated bilirubin enters hepatocytes through the uptake transporters organic anion transporting polypeptide (OATP) 1B1 and 1B3, undergoes glucuronidation by the Phase II enzyme UDP glucuronosyltransferase 1A1 (UGT1A1), and conjugated forms are excreted into the bile by the canalicular export pump multidrug resistance protein 2 (MRP2). Any remaining conjugated bilirubin is transported back to the blood by MRP3 and passed on for uptake and excretion by downstream hepatocytes or the kidney. The bile salt export pump BSEP as the main motor of bile flow is indirectly involved in bilirubin disposition. Genetic mutations and xenobiotics that interfere with this machinery may impede bilirubin disposition and cause hyperbilirubinemia. Several pharmaceutical compounds are known to cause hyperbilirubinemia via inhibition of OATP1Bs, UGT1A1, or BSEP. Herein we briefly review the in vitro prediction methods that serve to identify drugs with a potential to induce hyperbilirubinemia. In vitro assays can be deployed early in drug development and may help to minimize late-stage attrition. Based on current evidence, drugs that behave as mono- or multispecific inhibitors of OATP1B1, UGT1A1, and BSEP in vitro are at risk of causing clinically significant hyperbilirubinemia. By integrating inhibition data from in vitro assays, drug serum concentrations, and clinical reports of hyperbilirubinemia, predictor cut-off values have been established and are provisionally suggested in this review. Further validation of in vitro readouts to clinical outcomes is expected to enhance the predictive power of these assays.

Comparison of Various Cell Lines and Three-Dimensional Mucociliary Tissue Model Systems to Estimate Drug Permeability Using an In Vitro Transport Study to Predict Nasal Drug Absorption in Rats

Recently, various types of cultured cells have been used to research the mechanisms of transport and metabolism of drugs. Although many studies using cultured cell systems have been published, a comparison of different cultured cell systems has never been reported. In this study, Caco-2, Calu-3, Madin–Darby canine kidney (MDCK), EpiAirway and MucilAir were used as popular in vitro cell culture systems, and the permeability of model compounds across these cell systems was evaluated to compare barrier characteristics and to clarify their usefulness as an estimation system for nasal drug absorption in rats. MDCK unexpectedly showed the best correlation (r = 0.949) with the fractional absorption (F_n) in rats. Secondly, a high correlation was observed in Calu-3 (r = 0.898). Also, Caco-2 (r = 0.787) and MucilAir (r = 0.750) showed a relatively good correlation with F_n. The correlation between F_n and permeability to EpiAirway was the poorest (r = 0.550). Because EpiAirway forms leakier tight junctions than other cell culture systems, the paracellular permeability was likely overestimated with this system. On the other hand, because MDCK formed such tight cellular junctions that compounds of paracellular model were less likely permeated, the paracellular permeability could be underestimated. Calu-3, Caco-2 and MucilAir form suitable cellular junctions and barriers, indicating that those cell systems enable the precise estimation of nasal drug absorption.

Fetal androgen exposure is a determinant of adult male metabolic health

Androgen signalling is a critical driver of male development. Fetal steroid signalling can be dysregulated by a range of environmental insults and clinical conditions. We hypothesised that poor adult male health was partially attributable to aberrant androgen exposure during development. Testosterone was directly administered to developing male ovine fetuses to model excess prenatal androgenic overexposure associated with conditions such as polycystic ovary syndrome (PCOS). Such in utero androgen excess recreated the dyslipidaemia and hormonal profile observed in sons of PCOS patients. 1,084 of 15,134 and 408 of 2,766 quantifiable genes and proteins respectively, were altered in the liver during adolescence, attributable to fetal androgen excess. Furthermore, prenatal androgen excess predisposed to adolescent development of an intrahepatic cholestasis-like condition with attendant hypercholesterolaemia and an emergent pro-fibrotic, pro-oxidative stress gene and protein expression profile evident in both liver and circulation. We conclude that prenatal androgen excess is a previously unrecognised determinant of lifelong male metabolic health.

Prognostic significance and molecular mechanisms of adenosine triphosphate-binding cassette subfamily C members in gastric cancer

Gastric cancer (GC) is one of the major leading causes of tumor-related deaths worldwide. Adenosine triphosphate-binding cassette subfamily C (ABCC) consists of 13 members, ABCC1 to 13, which were examined for their associations with GC.The online Kaplan-Meier Plotter database was used to determine the prognostic significance of ABCC subfamily members in GC. Stratified analyses were performed using gender, disease stage, degree of tumor differentiation, expression of human epidermal growth factor receptor 2 (HER2), and Lauren classification. Molecular mechanisms were examined using the database for annotation, visualization, and integrated discovery database.ABCC1, ABCC3, ABCC7, ABCC8, ABCC9, and ABCC10 expression showed prognostic significance in the whole population and in male and female subpopulations (all P ≤ .05). Furthermore, high expression of most ABCC family members always suggested a poor prognosis, except for ABCC7 (P > .05). Stratified analyses revealed that ABCC1, ABCC3, ABCC7, ABCC8, ABCC9, and ABCC10 expression showed prognostic significance for the whole population, as well as male and female populations. ABCC2 and ABCC9 were significantly correlated with all disease stages, while ABCC2 and ABCC6 were significantly correlated with all Lauren classifications. Expression of ABCC1, ABCC3, ABCC5, ABCC7, ABCC8, ABCC9, and ABCC10 was significantly correlated with either negative or positive of HER2 status (all P ≤ .05). Enrichment analysis indicated that these genes were involved in ATPase activity, transmembrane transport, or were ABC transporters (all P ≤ .05).ABCC1, ABCC3, ABCC7, ABCC8, ABCC9, and ABCC10 may be potential prognosis biomarkers for GC, acting as ABC transporters and via ATPase activity.

Mechanistic Modeling of the Hepatic Disposition of Estradiol-17β-Glucuronide in Sandwich-Cultured Human Hepatocytes

Estradiol-17β-glucuronide (E₂17G) is an estrogen metabolite that has cholestatic properties. In humans, circulating E₂17G is transported into hepatocytes by organic anion transporting polypeptides (OATPs) and is excreted into bile by multidrug-resistance associated protein 2 (MRP2). E₂17G is also a substrate of the basolateral efflux transporters MRP3 and MRP4, which translocate E₂17G from hepatocytes to blood. However, the contribution of basolateral efflux to hepatocyte disposition of E₂17G has not been evaluated previously. To address this question, E₂17G disposition was studied in sandwich-cultured human hepatocytes and mechanistic modeling was applied to calculate clearance values (mean ± S.D.) for uptake, intrinsic biliary excretion (CL_int,bile) and intrinsic basolateral efflux (CL_int,BL). The biliary excretion index of E₂17G was 45% ± 6%. The CL_int,BL of E₂17G [0.18 ± 0.03 (ml/min)/g liver) was 1.6-fold higher than CL_int,bile [0.11 ± 0.06 (ml/min)/g liver]. Simulations were performed to study the effects of increased CL_int,BL and a concomitant decrease in CL_int,bile on hepatic E₂17G exposure. Results demonstrated that increased CL_int,BL can effectively reduce hepatocellular and biliary exposure to this potent cholestatic agent. Simulations also revealed that basolateral efflux can compensate for impaired biliary excretion and, vice versa, to avoid accumulation of E₂17G in hepatocytes. However, when both clearance processes are impaired by 90%, hepatocyte E₂17G exposure increases up to 10-fold. These data highlight the contribution of basolateral efflux transport, in addition to MRP2-mediated biliary excretion, to E₂17G disposition in human hepatocytes. This elimination route could be important, especially in cases where basolateral efflux is induced, such as cholestasis. SIGNIFICANCE STATEMENT: The disposition of the cholestatic estrogen metabolite estradiol-17β-glucuronide (E₂17G) was characterized in sandwich-cultured human hepatocytes. The intrinsic basolateral efflux clearance was estimated to be 1.6-fold higher than the intrinsic biliary excretion clearance, emphasizing the contribution of basolateral elimination in addition to biliary excretion. Simulations highlight how hepatocytes can effectively cope with increased E₂17G through the regulation of both basolateral and biliary transporters.

Highly Combinatorial Genetic Interaction Analysis Reveals a Multi-Drug Transporter Influence Network

Many traits are complex, depending non-additively on variant combinations. Even in model systems, such as the yeast S. cerevisiae, carrying out the high-order variant-combination testing needed to dissect complex traits remains a daunting challenge. Here, we describe “X-gene” genetic analysis (XGA), a strategy for engineering and profiling highly combinatorial gene perturbations. We demonstrate XGA on yeast ABC transporters by engineering 5,353 strains, each deleted for a random subset of 16 transporters, and profiling each strain’s resistance to 16 compounds. XGA yielded 85,648 genotype-to-resistance observations, revealing high-order genetic interactions for 13 of the 16 transporters studied. Neural networks yielded intuitive functional models and guided exploration of fluconazole resistance, which was influenced non-additively by five genes. Together, our results showed that highly combinatorial genetic perturbation can functionally dissect complex traits, supporting pursuit of analogous strategies in human cells and other model systems.

Celaj et al. introduce “X-gene” genetic analysis (XGA), a strategy for modeling complex systems by engineering and profiling highly combinatorial genetic perturbations. They apply XGA to 16 yeast ABC transporters, revealing many high-order genetic interactions. Neural network models yielded intuitive functional models and illuminated an ABC transporter influence network, supporting application of XGA to other organisms and processes.

ATP-binding Cassette Exporters: Structure and Mechanism with a Focus on P-glycoprotein and MRP1

BACKGROUND

Proteins that belong to the ATP-binding cassette superfamily include transporters that mediate the efflux of substrates from cells. Among these exporters, P-glycoprotein and MRP1 are involved in cancer multidrug resistance, protection from endo and xenobiotics, determination of drug pharmacokinetics, and the pathophysiology of a variety of disorders.

OBJECTIVE

To review the information available on ATP-binding cassette exporters, with a focus on Pglycoprotein, MRP1 and related proteins. We describe tissue localization and function of these transporters in health and disease, and discuss the mechanisms of substrate transport. We also correlate recent structural information with the function of the exporters, and discuss details of their molecular mechanism with a focus on the nucleotide-binding domains.

METHODS

Evaluation of selected publications on the structure and function of ATP-binding cassette proteins.

CONCLUSIONS

Conformational changes on the nucleotide-binding domains side of the exporters switch the accessibility of the substrate-binding pocket between the inside and outside, which is coupled to substrate efflux. However, there is no agreement on the magnitude and nature of the changes at the nucleotide- binding domains side that drive the alternate-accessibility. Comparison of the structures of Pglycoprotein and MRP1 helps explain differences in substrate selectivity and the bases for polyspecificity. P-glycoprotein substrates are hydrophobic and/or weak bases, and polyspecificity is explained by a flexible hydrophobic multi-binding site that has a few acidic patches. MRP1 substrates are mostly organic acids, and its polyspecificity is due to a single bipartite binding site that is flexible and displays positive charge.

ABC Transporters: Regulation and Association with Multidrug Resistance in Hepatocellular Carcinoma and Colorectal Carcinoma

For most cancers, the treatment of choice is still chemotherapy despite its severe adverse effects, systemic toxicity and limited efficacy due to the development of multidrug resistance (MDR). MDR leads to chemotherapy failure generally associated with a decrease in drug concentration inside cancer cells, frequently due to the overexpression of ABC transporters such as P-glycoprotein (P-gp/MDR1/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2), which limits the efficacy of chemotherapeutic drugs. The aim of this review is to compile information about transcriptional and post-transcriptional regulation of ABC transporters and discuss their role in mediating MDR in cancer cells. This review also focuses on drug resistance by ABC efflux transporters in cancer cells, particularly hepatocellular carcinoma (HCC) and colorectal carcinoma (CRC) cells. Some aspects of the chemotherapy failure and future directions to overcome this problem are also discussed.

Modulation of Hepatic MRP3/ABCC3 by Xenobiotics and Pathophysiological Conditions: Role in Drug Pharmacokinetics

Liver transporters play an important role in the pharmacokinetics and disposition of pharmaceuticals, environmental contaminants, and endogenous compounds. Among them, the family of ATP-Binding Cassette (ABC) transporters is the most important due to its role in the transport of endo- and xenobiotics. The ABCC sub-family is the largest one, consisting of 13 members that include the cystic fibrosis conductance regulator (CFTR/ABCC7); the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) and the multidrug resistanceassociated proteins (MRPs). The MRP-related proteins can collectively confer resistance to natural, synthetic drugs and their conjugated metabolites, including platinum-containing compounds, folate anti-metabolites, nucleoside and nucleotide analogs, among others. MRPs can be also catalogued into "long" (MRP1/ABCC1, -2/C2, -3/C3, -6/C6, and -7/C10) and "short" (MRP4/C4, -5/C5, -8/C11, -9/C12, and -10/C13) categories. While MRP2/ABCC2 is expressed in the canalicular pole of hepatocytes, all others are located in the basolateral membrane. In this review, we summarize information from studies examining the changes in expression and regulation of the basolateral hepatic transporter MPR3/ABCC3 by xenobiotics and during various pathophysiological conditions. We also focus, primarily, on the consequences of such changes in the pharmacokinetic, pharmacodynamic and/or toxicity of different drugs of clinical use transported by MRP3.

Altered Expression and Function of Hepatic Transporters in a Rodent Model of Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common form of inherited polycystic kidney disease (PKD) and is a leading cause of kidney failure. Fluid-filled cysts develop in the kidneys of patients with ADPKD, and cysts often form in their liver and other organs. Previous data have shown that bile acids are increased in the liver of polycystic kidney (PCK) rats, a rodent model of PKD; these changes may be associated with alterations in liver transporter expression and function. However, the impact of PKD on hepatic transporters has not been characterized. Therefore, this preclinical study was designed to investigate hepatic transporter expression and function in PCK compared with wild-type (WT) Sprague-Dawley rats. Transporter gene expression was measured by quantitative polymerase chain reaction, and protein levels were quantified by Western blot and liquid chromatography-tandem mass spectroscopy (LC-MS/MS)-based proteomic analysis in rat livers. Transporter function was assessed in isolated perfused livers (IPLs), and biliary and hepatic total glutathione content was measured. Protein expression of Mrp2 and Oatp1a4 was decreased 3.0-fold and 2.9-fold, respectively, in PCK rat livers based on Western blot analysis. Proteomic analysis confirmed a decrease in Mrp2 and a decrease in Oatp1a1 expression (PCK/WT ratios, 0.368 ± 0.098 and 0.563 ± 0.038, respectively; mean ± S.D.). The biliary excretion of 5(6)-carboxy-2',7'-dichlorofluorescein, a substrate of Oatp1a1, Mrp2, and Mrp3, was decreased 28-fold in PCK compared with WT rat IPLs. Total glutathione was significantly reduced in the bile of PCK rats. Differences in hepatic transporter expression and function may contribute to altered disposition of Mrp2 and Oatp substrates in PKD.

ABC Family Transporters

The transport of specific molecules across lipid membranes is an essential function of all living organisms. The processes are usually mediated by specific transporters. One of the largest transporter families is the ATP-binding cassette (ABC) family. More than 40 ABC transporters have been identified in human, which are divided into 7 subfamilies (ABCA to ABCG) based on their gene structure, amino acid sequence, domain organization, and phylogenetic analysis. Of them, at least 11 ABC transporters including P-glycoprotein (P-GP/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2) are involved in multidrug resistance (MDR) development. These ABC transporters are expressed in various tissues such as the liver, intestine, kidney, and brain, playing important roles in absorption, distribution, and excretion of drugs. Some ABC transporters are also involved in diverse cellular processes such as maintenance of osmotic homeostasis, antigen processing, cell division, immunity, cholesterol, and lipid trafficking. Several human diseases such as cystic fibrosis, sitosterolemia, Tangier disease, intrahepatic cholestasis, and retinal degeneration are associated with mutations in corresponding transporters. This chapter will describe function and expression of several ABC transporters (such as P-GP, BCRP, and MRPs), their substrates and inhibitors, as well as their clinical significance.

Enantioselective Drug Recognition by Drug Transporters

Drug transporters mediate the absorption, tissue distribution, and excretion of drugs. The cDNAs of P-glycoprotein, multidrug resistance proteins (MRPs/ABCC), breast cancer resistance protein (BCRP/ABCG2), peptide transporters (PEPTs/SLC15), proton-coupled folate transporters (PCFT/SLC46A1), organic anion transporting polypeptides (OATPs/SLCO), organic anion transporters (OATs/SLC22), organic cation transporters (OCTs/SLC22), and multidrug and toxin extrusions (MATEs/SLC47) have been isolated, and their functions have been elucidated. Enantioselectivity has been demonstrated in the pharmacokinetics and efficacy of drugs, and is important for elucidating the relationship with recognition of drugs by drug transporters from a chiral aspect. Enantioselectivity in the transport of drugs by drug transporters and the inhibitory effects of drugs on drug transporters has been summarized in this review.

New Insights in Genetic Cholestasis: From Molecular Mechanisms to Clinical Implications

Cholestasis is characterised by impaired bile secretion and accumulation of bile salts in the organism. Hereditary cholestasis is a heterogeneous group of rare autosomal recessive liver disorders, which are characterised by intrahepatic cholestasis, pruritus, and jaundice and caused by defects in genes related to the secretion and transport of bile salts and lipids. Phenotypic manifestation is highly variable, ranging from progressive familial intrahepatic cholestasis (PFIC)-with onset in early infancy and progression to end-stage liver disease-to a milder intermittent mostly nonprogressive form known as benign recurrent intrahepatic cholestasis (BRIC). Cases have been reported of initially benign episodic cholestasis that subsequently transitions to a persistent progressive form of the disease. Therefore, BRIC and PFIC seem to represent two extremes of a continuous spectrum of phenotypes that comprise one disease. Thus far, five representatives of PFIC (named PFIC1-5) caused by pathogenic mutations present in both alleles of ATP8B1, ABCB11, ABCB4, TJP2, and NR1H4 have been described. In addition to familial intrahepatic cholestasis, partial defects in ATP8B1, ABCB11, and ABCB4 predispose patients to drug-induced cholestasis and intrahepatic cholestasis in pregnancy. This review summarises the current knowledge of the clinical manifestations, genetics, and molecular mechanisms of these diseases and briefly outlines the therapeutic options, both conservative and invasive, with an outlook for future personalised therapeutic strategies.

Influence of Morbid Obesity on the Pharmacokinetics of Morphine, Morphine-3-Glucuronide, and Morphine-6-Glucuronide

Introduction

Obesity is associated with many pathophysiological changes that may result in altered drug metabolism. The aim of this study is to investigate the influence of obesity on the pharmacokinetics of morphine, morphine-3-glucuronide (M3G), and morphine-6-glucuronide (M6G) through a combined analysis in morbidly obese patients and non-obese healthy volunteers.

Methods

In this analysis, data from 20 morbidly obese patients [mean body mass index 49.9 kg/m² (range 37.6–78.6 kg/m²) and weight 151.3 kg (range 112–251.9 kg)] and 20 healthy volunteers [mean weight 70.6 kg (range 58–85 kg)] were included. Morbidly obese patients received 10 mg of intravenous (I.V.) morphine after gastric bypass surgery, with additional morphine I.V. doses as needed. Healthy volunteers received an I.V. bolus of morphine of 0.1 mg/kg followed by an infusion of 0.030 mg kg⁻¹ h⁻¹ for 1 h. Population pharmacokinetic modeling was performed using NONMEM 7.2.

Results

In morbidly obese patients, elimination clearance of M3G and M6G was decreased substantially compared with healthy volunteers (p < 0.001). Regarding glucuronidation, only a slight decrease in the formation of M6G and a delay in the formation of M3G was found (both p < 0.001). Obesity was also identified as a covariate for the peripheral volume of distribution of morphine (p < 0.001).

Conclusion

Metabolism of morphine is not altered in morbidly obese patients. However, decreased elimination of both M3G and M6G is evident, resulting in a substantial increase in exposure to these two metabolites. A rational explanation of this finding is that it results from alterations in membrane transporter function and/or expression in the liver.

ClinicalTrials.gov identifier: NCT01097148.

Electronic supplementary material

The online version of this article (doi:10.1007/s40262-017-0544-2) contains supplementary material, which is available to authorized users.

Cell Migration Related to MDR-Another Impediment to Effective Chemotherapy?

Multidrug resistance, mediated by members of the ATP-binding cassette (ABC) proteins superfamily, has become one of the biggest obstacles in conquering tumour progression. If the chemotherapy outcome is considered successful, when the primary tumour volume is decreased or completely abolished, modulation of ABC proteins activity is one of the best methods to overcome drug resistance. However, if a positive outcome is represented by no metastasis or, at least, elongation of remission-free time, then the positive effect of ABC proteins inhibition should be compared with the several side effects it causes, which may inflict cancer progression and decrease overall patient health. Clinical trials conducted thus far have shown that the tested ABC modulators add limited or no benefits to cancer patients, as some of them are merely toxic and others induce unwanted drug–drug interactions. Moreover, the inhibition of certain ABC members has been recently indicated as potentially responsible for increased fibroblasts migration. A better understanding of the complex role of ABC proteins in relation to cancer progression may offer novel strategies in cancer therapy.

Beyond drug-drug interactions: effects of transporter inhibition on endobiotics, nutrients and toxins

INTRODUCTION

Membrane transport proteins play a central role in regulating the disposition of endobiotics, dietary nutrients and environmental toxins. The inhibition of transporters by drugs has potential physiologic consequences. The full extent of the effect of drugs on the function of transporters is poorly understood because only a small subset of the hundreds of transporters expressed in humans - primarily those mediating the rate-determining step in the elimination of specific drugs - are assessed during clinical development. Areas covered: We provide a comprehensive overview of literature reports implicating the inhibition of transporters as the mechanism for off-target effects of drugs. Expert opinion: Transporter inhibition, the mechanism of action of many marketed drugs, appears to play an underappreciated role in a number of side effects including vitamin deficiency, edema, dyslipidemia, cholestasis and gout. Cell systems more broadly expressing transporter networks and methods like unbiased metabolomics should be incorporated into the screening paradigm to expand our understanding of the impact of drugs on the physiologic function of transporters and to allow for these effects to be taken into account in drug discovery and clinical practice.

Importance of Hepatic Transporters in Clinical Disposition of Drugs and Their Metabolites

This review provides a practical clinical perspective on the relevance of hepatic transporters in pharmacokinetics and drug-drug interactions (DDIs). Special emphasis is placed on transporters with clear relevance to clinical DDIs, efficacy, and safety. Basolateral OATP1B1 and 1B3 emerged as important hepatic drug uptake pathways, sites for systemic DDIs, and sources of pharmacogenetic variability. As the first step in hepatic drug removal from the circulation, OATPs are an important determinant of systemic pharmacokinetics, specifically influencing systemic absorption, clearance, and hepatic distribution for subsequent metabolism and/or excretion. Biliary excretion of parent drugs is a less prevalent clearance pathway than metabolism or urinary excretion, but BCRP and MRP2 are critically important to biliary/fecal elimination of drug metabolites. Inhibition of biliary excretion is typically not apparent at the level of systemic pharmacokinetics but can markedly increase liver exposure. Basolateral efflux transporters MRP3 and MRP4 mediate excretion of parent drugs and, more commonly, polar metabolites from hepatocytes into blood. Basolateral excretion is an area in need of further clinical investigation, which will necessitate studies more complex than just systemic pharmacokinetics. Clinical relevance of hepatic uptake is relatively well appreciated, and clinical consequences of hepatic excretion (biliary and basolateral) modulation remain an active research area.

7-ketocholesterol and 27-hydroxycholesterol decreased doxorubicin sensitivity in breast cancer cells: estrogenic activity and mTOR pathway

Hypercholesterolemia is one of the risk factors for poor outcome in breast cancer therapy. To elucidate the influence of the main circulating oxysterols, cholesterol oxidation products, on the cell-killing effect of doxorubicin, cells were exposed to oxysterols at a subtoxic concentration. When cells were exposed to oxysterols in fetal bovine serum-supplemented medium, 7-ketocholesterol (7-KC), but not 27-hydroxycholesterol (27-HC), decreased the cytotoxicity of doxorubicin in MCF-7 (high estrogen receptor (ER)α/ERβ ratio) cells and the decreased cytotoxicity was restored by the P-glycoprotein inhibitor verapamil. 7-KC stimulated the efflux function of P-glycoprotein and reduced intracellular doxorubicin accumulation in MCF-7 but not in ERα(-) MDA-MB-231 and the resistant MCF-7/ADR cells. In MCF-7 cells, 7-KC increased the mRNA and protein levels of P-glycoprotein. The 7-KC-suppressed doxorubicin accumulation was restored by the fluvestrant and ERα knockdown. In a yeast reporter assay, the ERα activation by 7-KC was more potent than 27-HC. 7-KC, but not 27-HC, stimulated the expression of an ER target, Trefoil factor 1 in MCF-7 cells. When charcoal-stripped fetal bovine serum was used, both 7-KC and 27-HC induced Trefoil factor 1 expression and reduced doxorubicin accumulation in MCF-7 cells. 7-KC-reduced doxorubicin accumulation could be reversed by inhibitors of phosphatidylinositol 3-kinase, Akt, and mammalian target of rapamycin (mTOR). These findings demonstrate that 7-KC decreases the cytotoxicity of doxorubicin through the up-regulation of P-glycoprotein in an ERα- and mTOR-dependent pathway. The 7-KC- and 27-HC-elicited estrogenic effects are crucial in the P-glycoprotein induction in breast cancer cells.

Identification of novel MRP3 inhibitors based on computational models and validation using an in vitro membrane vesicle assay

INTRODUCTION

Multidrug resistance-associated protein 3 (MRP3), an efflux transporter on the hepatic basolateral membrane, may function as a compensatory mechanism to prevent the accumulation of anionic substrates (e.g., bile acids) in hepatocytes. Inhibition of MRP3 may disrupt bile acid homeostasis and is one hypothesized risk factor for the development of drug-induced liver injury (DILI). Therefore, identifying potential MRP3 inhibitors could help mitigate the occurrence of DILI.

METHODS

Bayesian models were developed using MRP3 transporter inhibition data for 86 structurally diverse drugs. The compounds were split into training and test sets of 57 and 29 compounds, respectively, and six models were generated based on distinct inhibition thresholds and molecular fingerprint methods. The six Bayesian models were validated against the test set and the model with the highest accuracy was utilized for a virtual screen of 1470 FDA-approved drugs from DrugBank. Compounds that were predicted to be inhibitors were selected for in vitro validation. The ability of these compounds to inhibit MRP3 transport at a concentration of 100μM was measured in membrane vesicles derived from stably transfected MRP3-over-expressing HEK-293 cells with [³H]-estradiol-17β-d-glucuronide (E₂17G; 10μM; 5min uptake) as the probe substrate.

RESULTS

A predictive Bayesian model was developed with a sensitivity of 73% and specificity of 71% against the test set used to evaluate the six models. The area under the Receiver Operating Characteristic (ROC) curve was 0.710 against the test set. The final selected model was based on compounds that inhibited substrate transport by at least 50% compared to the negative control, and functional-class fingerprints (FCFP) with a circular diameter of six atoms, in addition to one-dimensional physicochemical properties. The in vitro screening of predicted inhibitors and non-inhibitors resulted in similar model performance with a sensitivity of 64% and specificity of 70%. The strongest inhibitors of MRP3-mediated E₂17G transport were fidaxomicin, suramin, and dronedarone. Kinetic assessment revealed that fidaxomicin was the most potent of these inhibitors (IC₅₀=1.83±0.46μM). Suramin and dronedarone exhibited IC₅₀ values of 3.33±0.41 and 47.44±4.41μM, respectively.

CONCLUSION

Bayesian models are a useful screening approach to identify potential inhibitors of transport proteins. Novel MRP3 inhibitors were identified by virtual screening using the selected Bayesian model, and MRP3 inhibition was confirmed by an in vitro transporter inhibition assay. Information generated using this modeling approach may be valuable in predicting the potential for DILI and/or MRP3-mediated drug-drug interactions.

MRP3 as a novel resistance factor for sorafenib in hepatocellular carcinoma

The mechanism of resistance of hepatocellular carcinoma (HCC) to sorafenib is unknown and no useful predictive biomarker for sorafenib treatment has been reported. Accordingly, we established sorafenib-resistant HCC cells and investigated the underlying mechanism of resistance to sorafenib. Sorafenib-resistant cell lines were established from the HCC cell line PLC/PRF5 by cultivation under continuous exposure to increasing concentration of sorafenib. The IC50 values of the 2 resistant clones PLC/PRF5-R1 and PLC-PRF5-R2 were 9.2±0.47 μM (1.8-fold) and 25±5.1 μM (4.6-fold) respectively, which were significantly higher than that of parental PLC/PRF5 cells (5.4±0.17 μM) (p < 0.01 respectively), as determined by MTT assay. Western blot analysis of signal transduction-related proteins showed no significant differences in expression of AKT/pAKT, mTOR/pmTOR, or ERK/pERK between the 2 resistant clones versus parent cells, suggesting no activation of an alternative signal transduction pathway. Likewise, when expression of membrane transporter proteins was determined, there were no significant differences in expression levels of BSEP, MDR1, MRP2, BCRP, MRP4 and OCT1 between resistant clones and parent cells. However, the expression levels of MRP3 in the 2 resistant clones were significantly higher than that of parent cells. When MRP3 gene was knocked down by siRNA in PLC-PRF5-R2 cells, the sensitivity of the cells to sorafenib was restored. In the analysis of gene mutation, there was no mutation in the activation segment of Raf1 kinase in the resistant clones. Our data clearly demonstrate that the efflux transporter MRP3 plays an important role in resistance to sorafenib in HCC cells.

Benign Hepatocellular Nodules: Hepatobiliary Phase of Gadoxetic Acid-enhanced MR Imaging Based on Molecular Background

Gadoxetic acid is a contrast agent for magnetic resonance (MR) imaging with hepatocyte-specific properties and is becoming increasingly important in detection and characterization of hepatocellular carcinoma and benign hepatocellular nodules, including focal nodular hyperplasia (FNH), nodular regenerative hyperplasia (NRH), hepatocellular adenoma (HCA), and dysplastic nodule. In these hepatocellular nodules, a positive correlation between the grade of membranous uptake transporter organic anion-transporting polypeptide (OATP) 1B3 expression and signal intensity in the hepatobiliary (HB) phase has been verified. In addition, it has been clarified that OATP1B3 expression is regulated by activation of β-catenin and/or hepatocyte nuclear factor 4α. On the other hand, recent studies have also revealed some of the background molecular mechanisms of benign hepatocellular nodules. FNH commonly shows iso- or hyperintensity in the HB phase with equal or stronger OATP1B3 expression, with map-like distribution of glutamine synthetase (a target of Wnt/β-catenin signaling) and OATP1B3 expression. NRH shows doughnut-like enhancement with hypointensity in the central portion in the HB phase with OATP1B3 expression. The majority of HCAs show hypointensity in the HB phase, but β-catenin-activated HCA exclusively demonstrates iso- or hyperintensity with increased expression of nuclear β-catenin, glutamine synthetase, and OATP1B3. Dysplastic nodule commonly shows iso- or hyperintensity in the HB phase with similar to increased OATP1B3 expression, but one-third of high-grade dysplastic nodules can be demonstrated as a hypointense nodule with decreased OATP1B3 expression. Knowledge of these background molecular mechanisms of gadoxetic acid-enhanced MR imaging is important not only for precise imaging diagnosis but also understanding of the pathogenesis of benign hepatocellular nodules. ^©RSNA, 2016.

Activation of p38 Mitogen-Activated Protein Kinase by Clotrimazole Induces Multidrug Resistance-Associated Protein 3 Activation through a Novel Transcriptional Element

Multidrug resistance-associated protein 3 (MRP3) is a basolaterally localized transporter in the liver and contributes to the transport of various metabolites such as conjugates of endogenous compounds and drugs from hepatocytes. MRP3 expression in the human liver is low under normal physiologic conditions but is induced by drug treatment. Although several studies have identified a region necessary for the basal transcription of MRP3, no region that responds to drugs has been reported. To identify the xenobiotic-responsive elements of MRP3, we constructed a luciferase reporter plasmid containing the MRP3 5'-flanking region up to -10 kb upstream from the transcription start site. Among typical nuclear receptor ligands, clotrimazole dramatically enhanced MRP3 reporter activity in HepG2 cells, whereas rifampicin had no effect. We then conducted MRP3 reporter assays with deletion or mutation constructs to identify a clotrimazole-responsive element. The element was located approximately -6.8 kb upstream from the MRP3 transcription start site. Overexpression of the pregnane X receptor did not enhance clotrimazole-mediated transcription. We found that clotrimazole was toxic to HepG2 cells and we therefore investigated whether mitogen-activated protein kinase (MAPK) activation is involved in the transactivation of MRP3 by clotrimazole. p38 MAPK inhibitor SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole] suppressed MRP3 mRNA expression induced by clotrimazole, whereas c-Jun N-terminal kinase inhibitor SP600125 (1,9-pyrazoloanthrone) and extracellular signal-regulated kinase inhibitor PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] did not. Phosphorylated p38 MAPK was detected in HepG2 cells treated with clotrimazole. These results suggest that activation of the p38 MAPK pathway induces the transcriptional activation of MRP3.

Mechanisms involved in the transport of mercuric ions in target tissues

Mercury exists in the environment in various forms, all of which pose a risk to human health. Despite guidelines regulating the industrial release of mercury into the environment, humans continue to be exposed regularly to various forms of this metal via inhalation or ingestion. Following exposure, mercuric ions are taken up by and accumulate in numerous organs, including brain, intestine, kidney, liver, and placenta. In order to understand the toxicological effects of exposure to mercury, a thorough understanding of the mechanisms that facilitate entry of mercuric ions into target cells must first be obtained. A number of mechanisms for the transport of mercuric ions into target cells and organs have been proposed in recent years. However, the ability of these mechanisms to transport mercuric ions and the regulatory features of these carriers have not been characterized completely. The purpose of this review is to summarize the current findings related to the mechanisms that may be involved in the transport of inorganic and organic forms of mercury in target tissues and organs. This review will describe mechanisms known to be involved in the transport of mercury and will also propose additional mechanisms that may potentially be involved in the transport of mercuric ions into target cells.