Differential regulation of sinusoidal and canalicular hepatic drug transporter expression by xenobiotics activating drug-sensing receptors in primary human hepatocytes

Sinusoidal and canalicular hepatic drug transporters constitute key factors involved in drug elimination from liver. Regulation of their expression via activation of xenosensors, such as aryl hydrocarbon receptor (AhR), constitutive androstane receptor (CAR), pregnane X receptor (PXR), and nuclear factor E2-related factor 2 (Nrf2), remains incompletely characterized. The present study was therefore designed to carefully analyze expression of major drug transporters in primary human hepatocytes exposed to dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD) (an AhR activator), rifampicin (RIF) (a PXR activator), phenobarbital (PB) (a CAR activator), and oltipraz (OPZ) (a Nrf2 activator), using mainly reverse transcription-real time polymerase chain reaction assays. With a threshold corresponding to a 1.5-fold factor change in mRNA levels, observed in at least three of seven independent human hepatocyte cultures, efflux transporters such as MDR1, MRP2 and BCRP were up-regulated by PB, RIF, and OPZ, whereas MRP3 was induced by OPZ and RIF. MDR1 and BCRP expression was also increased by TCDD- and RIF-augmented mRNA levels of the influx transporter OATP-C. Bile acid transporters, i.e., bile salt export pump and Na(+)-taurocholate cotransporting polypeptide, and the sinusoidal transporter, OAT2, were down-regulated by all the tested chemicals. Influx transporters such as OCT1, OATP-B, and OATP8 were repressed by PB and TCDD. PB also decreased MRP6 expression, whereas mRNA levels of OCT1 and OATP8 were down-regulated by RIF and OPZ, respectively. Taken together, these data establish a complex pattern of transporter regulation by xenobiotics in human hepatocytes, in addition to interindividual variability in responsiveness. This may deserve further attention with respect to drug-drug interactions and adverse effects of hepatic drugs.

Regulation of drug transporter expression in human hepatocytes exposed to the proinflammatory cytokines tumor necrosis factor-alpha or interleukin-6

Tumor necrosis factor (TNF)-alpha and interleukin (IL)-6 are proinflammatory cytokines known to alter expression of drug transporters in rodent liver. However, their effects toward human hepatic transporters remain poorly characterized. Therefore, this study was designed to analyze the effects of these cytokines on drug transporter expression in primary human hepatocytes. Exposure to 100 ng/ml TNF-alpha or 10 ng/ml IL-6 for 48 h was found to down-regulate mRNA levels of major sinusoidal influx transporters, including sodium-taurocholate cotransporting polypeptide (NTCP), organic anion-transporting polypeptide (OATP) 1B1, OATP1B3, OATP2B1, organic cation transporter (OCT) 1, and organic anion transporter 2. TNF-alpha and IL-6 concomitantly reduced NTCP and OATP1B1 protein expression and NTCP, OATP, and OCT1 transport activities. IL-6, but not TNF-alpha, was also found to decrease mRNA expression of the canalicular transporters multidrug resistance 1 gene, multidrug resistance gene-associated protein (MRP) 2, and breast cancer resistance protein (BCRP); it concomitantly decreased MRP2 and BCRP protein expression. TNF-alpha, unlike IL-6, markedly reduced bile salt export pump mRNA levels and increased BCRP protein expression. Expression of the sinusoidal MRP3 efflux pump was found to be up-regulated at protein level by both TNF-alpha and IL-6. Taken together, these data show that TNF-alpha and IL-6 similarly altered expression of sinusoidal drug transporters and rather differentially that of canalicular efflux transporters. Such pronounced changes in hepatic transporter expression are likely to contribute to both cholestasis and alterations of pharmacokinetics caused by inflammation in humans.

Functional expression of sinusoidal and canalicular hepatic drug transporters in the differentiated human hepatoma HepaRG cell line

Functional expression of both sinusoidal and canalicular hepatic drug transporters was investigated in the highly differentiated human hepatoma HepaRG cell line and also, for comparison, in primary human hepatocytes and in the hepatoma HepG2 cell line. Using RT-qPCR assays, differentiated HepaRG cells were found to display a pattern of transporter expression close to that found in primary human hepatocytes, i.e. they exhibit substantial mRNA levels of the influx transporters OCT1, OATP-B, OATP-C and NTCP, and of the secretion transporters MRP2, MRP3, BSEP and P-glycoprotein. By contrast, expression of influx transporters was not present or very weak in HepG2 cells. Drug transport assays allowed to detect functional activities of OCT1, OATPs/OAT2, NTCP, MRPs and P-glycoprotein in differentiated HepaRG cells as in primary human hepatocytes whereas HepG2 cells only showed notable MRP and P-glycoprotein activities. In addition, expression of canalicular transporters in HepaRG cells was found to be up-regulated by known inducers of transporters such as rifampicin, phenobarbital and chenodeoxycholate acting on P-glycoprotein, MRP2 and BSEP, respectively. HepaRG cells thus exhibit functional expression of both sinusoidal and canalicular drug transporters and have retained regulatory pathways controlling transporter levels. These data, associated with the known high expression of drug metabolizing enzymes in HepaRG cells, highlight the interest of such hepatoma cells for analysing hepatic drug detoxification pathways.

Polycyclic aromatic hydrocarbons inhibit differentiation of human monocytes into macrophages

Polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene (BP) are ubiquitous environmental carcinogenic contaminants exerting deleterious effects toward cells acting in the immune defense such as monocytic cells. To investigate the cellular basis involved, we have examined the consequences of PAH exposure on macrophagic differentiation of human blood monocytes. Treatment by BP markedly inhibited the formation of adherent macrophagic cells deriving from monocytes upon the action of either GM-CSF or M-CSF. Moreover, it reduced expression of macrophagic phenotypic markers such as CD71 and CD64 in GM-CSF-treated monocytic cells, without altering cell viability or inducing an apoptotic process. Exposure to BP also strongly altered functional properties characterizing macrophagic cells such as endocytosis, phagocytosis, LPS-triggered production of TNF-alpha and stimulation of allogeneic lymphocyte proliferation. Moreover, formation of adherent macrophagic cells was decreased in response to PAHs distinct from BP such as dimethylbenz(a)anthracene and 3-methylcholanthrene, which interact, like BP, with the arylhydrocarbon receptor (AhR) known to mediate many PAH effects. In contrast, benzo(e)pyrene, a PAH not activating AhR, had no effect. In addition, AhR was demonstrated to be present and functional in cultured monocytic cells, and the use of its antagonist alpha-naphtoflavone counteracted inhibitory effects of BP toward macrophagic differentiation. Overall, these data demonstrate that exposure to PAHs inhibits functional in vitro differentiation of blood monocytes into macrophages, likely through an AhR-dependent mechanism. Such an effect may contribute to the immunotoxicity of these environmental carcinogens owing to the crucial role played by macrophages in the immune defense.

Down-regulation of organic anion transporter expression in human hepatocytes exposed to the proinflammatory cytokine interleukin 1beta

Interleukin (IL) 1beta is a proinflammatory cytokine known to markedly alter expression of major organic anion transporters in rodent hepatocytes. However, its effects toward human hepatic transporters remain poorly characterized. Therefore, the present study was aimed at determining IL-1beta effects on expression of organic anion transporters in primary human hepatocytes and highly differentiated human hepatoma HepaRG cells. Exposure to 1 ng/ml IL-1beta was first shown to markedly repress mRNA expression of sodium-taurocholate cotransporting polypeptide (NTCP), a major sinusoidal transporter handling bile acids, in both human hepatocytes and HepaRG cells. It concomitantly reduced NTCP protein levels and NTCP-mediated cellular uptake of taurocholate in HepaRG cells. Other transporters such as the influx transporters organic anion transporting polypeptide (OATP)-B, OATP-C, and OATP8 and the efflux pumps multidrug resistance-associated protein (MRP) 2, MRP3, MRP4, and breast cancer resistance protein were also down-regulated at mRNA levels in human hepatocytes treated by IL-1beta for 24 h, and most of these transporters were similarly repressed in IL-1beta-exposed HepaRG cells; the cytokine also reduced bile salt export pump (BSEP) and OATP-C protein expression in human hepatocytes. IL-1beta was further shown to activate the extracellular signal-regulated protein kinase (ERK) in human hepatocytes and HepaRG cells; however, chemical inhibition of this kinase failed to counteract repressing effects of IL-1beta toward NTCP, BSEP, OATP-B, and OATP-C. Taken together, these data indicate that IL-1beta treatment reduced expression of major organic anion transporters in human hepatic cells in an ERK-independent manner. Such IL-1beta effects may likely participate in both cholestasis and alterations of hepatic detoxification pathways caused by inflammation in humans.

ERK-dependent induction of TNFalpha expression by the environmental contaminant benzo(a)pyrene in primary human macrophages

Polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene (BP) are toxic environmental contaminants known to enhance production of pro-inflammatory cytokines such as IL-1beta. The present study was designed in order to determine whether TNFalpha, another cytokine acting in inflammation, may also constitute a target for these chemicals. Both TNFalpha mRNA and TNFalpha secretion levels were found to be enhanced in human BP-treated macrophages. Dioxin, a contaminant activating the aryl hydrocarbon receptor (AhR) like PAHs, was also shown to increase TNFalpha expression. BP-mediated TNFalpha induction was however not suppressed by AhR antagonists, making unlikely the involvement of the typical AhR signalling pathway. BP-exposure of macrophages did not enhance NF-kappaB DNA binding activity, but it activated the MAP kinase ERK1/2. In addition, the use of chemical inhibitors of extracellular signal-regulated protein kinase (ERK) activation fully abrogated induction of TNFalpha production in BP-treated macrophages. These data likely indicate that PAHs enhance TNFalpha expression in human macrophages through an ERK-related mechanism. Such a regulation may contribute to confer pro-inflammatory properties to these widely-distributed environmental contaminants.

Functional expression of sinusoidal drug transporters in primary human and rat hepatocytes

Primary hepatocyte cultures are considered as a useful in vitro system for pharmacological/toxicological studies. Although expression of drug-metabolizing enzymes and canalicular drug transporters has been well documented in this cellular model, less information is available about sinusoidal drug transporter activities. This has led us to investigate functional expression of the major sinusoidal transporters in primary human and rat hepatocytes. Using radiolabeled substrates and chemical transporter inhibitors, activities of organic cation transporter 1, organic anion-transporting polypeptides, organic anion transporter 2, and Na(+)-taurocholate cotransporter were detected in cultured human and rat hepatocytes. In parallel, mRNA expression of these transporters was demonstrated using reverse transcriptase-quantitative polymerase chain reaction assays. Functional expression of sinusoidal transport proteins markedly decreased with time in primary rat hepatocyte cultures; by contrast, it remained relatively constant in primary human hepatocytes all along the culture, illustrating the fact that liver-specific functions, including drug-detoxifying pathways, are usually better preserved in cultured human hepatocytes than in their rodent counterparts. Primary hepatocytes, especially human hepatocytes, thus exhibit a pattern of sinusoidal transporter expression close to that found in vivo, highlighting the interest of hepatocyte cultures for drug detoxification studies.

Polarized expression of drug transporters in differentiated human hepatoma HepaRG cells

The HepaRG cell line is a well-differentiated human hepatoma cell line proposed as a surrogate for human hepatocytes, especially for hepatic detoxification studies. Polarized status of drug transporters, i.e., their coordinated location at sinusoidal or canalicular membranes, which represents a key hallmark of hepato-biliary drug transport, remains however incompletely documented in HepaRG cells. The present study was therefore designed to analyze transporter location in HepaRG cells, which exhibit mRNA expressions of most of hepatic transporters. HepaRG cells were demonstrated, through immunofluorescence staining, to express several drug transporters at their sinusoidal pole, especially the influx transporters organic anion transporting polypeptide (OATP) 1B1, OATP2B1 and organic cation transporter (OCT) 1 and the efflux transporter multidrug resistance-associated protein (MRP) 3. In addition, the efflux transporters P-glycoprotein and MRP2 were detected at the canalicular pole of HepaRG cells. Moreover, saturable uptake of reference substrates for the sinusoidal transporters sodium-taurocholate cotransporting polypeptide, OATPs and OCT1 and canalicular secretion of reference substrates for the efflux transporters bile salt export pump and MRP2 were observed. This polarized and functional expression of various sinusoidal and canalicular transporters in HepaRG cells highlights the interest of using these hepatoma cells in xenobiotic transport studies.

Transcriptional induction of CYP1A1 by oltipraz in human Caco-2 cells is aryl hydrocarbon receptor- and calcium-dependent

Oltipraz, a synthetic derivative of the cruciferous vegetable product 1,2-dithiole-3-thione, is considered as one of the most potent chemoprotectants. It modulates both cytochrome P-450 (CYP) and glutathione S-transferase expression and activities in rat tissues. Its effects, however, are variable according to the enzyme, tissue, and species. We show here that, as previously found in rat lung and kidney, CYP1A1 is inducible by oltipraz in both rat intestine and Caco-2 cells, a cell line originated from a human colon adenocarcinoma. In these cells, a 50 microm oltipraz treatment increased CYP1A1 mRNA ( approximately 30-fold), protein and activity. mRNA level was augmented as early as 2 h after the beginning of treatment, suggesting a transcriptional activation, and was maximal between 8 and 12 h. Transient transfection of Caco-2 cells with constructs containing different sizes of the 5'-flanking region of the CYP1A1 gene upstream of the luciferase reporter gene showed an increase in luciferase activity in oltipraz-treated cells, which correlates with the presence of the xenobiotic responsive element (XRE). Furthermore we demonstrated that resveratrol, an antagonist of the aryl hydrocarbon (Ah) receptor, inhibited the induction of both CYP1A1 promoter activity and mRNA by oltipraz, supporting the involvement of the Ah receptor in this induction. In an attempt to further characterize the mechanism of CYP1A1 induction, we showed a rapid increase in intracellular calcium concentration upon treatment of Caco-2 cells with oltipraz. Moreover, the effect of this compound on CYP1A1 was strongly abolished in the presence of BAPTA-AM, a well known chelator of intracellular calcium, and 2-aminoethyl diphenylborate, an inhibitor of store-operated calcium channels. These results bring the first demonstration that oltipraz activates transcription of the CYP1A1 gene through the Ah receptor-XRE pathway in Caco-2 cells and that CYP1A1 induction relies upon an increase of intracellular calcium concentration.

Physiological, pharmacological and clinical features of the multidrug resistance protein 2

Multidrug resistance protein 2 (MRP2, ABCC2) is a drug efflux pump belonging to the ATP-binding cassette (ABC) transporter superfamily. MRP2 is present predominantly at the biliary pole of hepatocytes and is also expressed in the kidney and intestine. It plays a major role in hepato-biliary elimination of many structurally diverse xenobiotics, including organic anions and drug conjugates, and therefore most likely contributes to pharmacokinetic parameters of these compounds. MRP2 also handles endogenous molecules such as bilirubin, and its overexpression has been shown to confer a multidrug resistance phenotype to tumoral cells. MRP2 expression can be regulated by endogenous substances such as inflammatory cytokines and biliary acids. The MRP2 levels and activity can also be affected by a large panel of xenobiotics, including chemopreventive agents and ligands of the pregnane X receptor, which may be a potential source of drug-drug interactions and drug adverse effects. MRP2 appears therefore as one of the major drug efflux pumps of the organism, whose functional and regulatory features are important to consider, notably for drug disposition.

Cytochrome P450-dependent toxicity of environmental polycyclic aromatic hydrocarbons towards human macrophages

Polycyclic aromatic hydrocarbons (PAHs) such as benzo(a)pyrene (BP) are potent immunosuppressive environmental contaminants acting on lymphocytes and monocytes. To establish whether differentiated macrophages, which play a crucial role in innate and acquired immunity, can also constitute major cellular targets, we have characterized PAH effects towards primary human macrophages. BP-treatment was found to dramatically alter their functional capacities and to trigger a caspase- and mitochondrion-related apoptosis, associated with down-regulation of the survival factors c-FLIP(L) and Bcl-X(L) and up-regulation of the pro-apoptotic factor p53. Such deleterious effects were associated with BP metabolite production, whose inhibition by the cytochrome P-450 1A1 inhibitor alpha-naphthoflavone fully abolished BP toxicity. In contrast to BP, the related halogenated arylhydrocarbon 2,3,7,8-tetrachlorodibenzo-p-dioxin, known to be poorly metabolized if any, only minimally affected macrophages. Overall, these data provide evidence for a cytochrome P-450-dependent toxicity of PAHs towards human differentiated macrophages, which may contribute to their immunosuppressive effects.

Drug transporter expression in human macrophages

Macrophages represent major cellular targets of various drugs, especially antibiotics and anti-viral drugs. Factors that may govern intracellular accumulation of drugs in these cells, especially those related to activity of drug transporters, are consequently likely important to consider. The present study was therefore designed to extensively characterize expression of solute carrier (SLC) and ATP-binding cassette (ABC) transporters in primary human macrophages generated from blood monocytes. Using quantitative polymerase chain reaction assays, these cells were found to exhibit very high or high levels of mRNA expression of concentrative nucleoside transporter (CNT) 3, equilibrative nucleoside transporter 3, monocarboxylate transporter (MCT) 1, MCT4, peptide/histidine transporter (PHT) 1, PHT2, organic anion transporting polypeptide transporter 2B1 and ABC pumps multidrug resistance protein (MRP) 1/ABCC1 and MRP3/ABCC3. By contrast, other transporters, including the efflux pump ABCB1/P-glycoprotein, were found at lower levels or were not expressed. Concomitantly, human macrophages displayed notable uptake of the MCT substrate lactate and of the CNT substrate uridine and also exhibited cellular efflux of the MRP substrate carboxy-2',7'-dichlorofluorescein. Such a functional expression of these transporters has likely to be considered with respect to cellular pharmacokinetics of drugs targeting macrophages.

Differential regulation of drug transporter expression by hepatocyte growth factor in primary human hepatocytes

Hepatocyte growth factor (HGF) is known to down-regulate expression of drug-detoxifying proteins such as cytochromes P450 (P450s) in human hepatocytes. The present study was designed to determine whether HGF may also impair expression of uptake and efflux drug transporters, which constitute important determinants of the liver detoxification pathway, such as P450s. Exposure of primary human hepatocytes to 20 ng/ml HGF for 48 h was found to down-regulate mRNA levels of major sinusoidal uptake transporters, including sodium taurocholate-cotransporting polypeptide (NTCP), organic anion-transporting polypeptide (OATP) 2B1, OATP1B1, organic cation transporter (OCT) 1, and organic anion transporter 2. HGF concomitantly reduced NTCP, OATP2B1, and OATP1B1 protein expression and NTCP, OATP, and OCT1 transport activities. With respect to efflux pumps, HGF decreased mRNA expression of the canalicular bile salt export pump, whereas that of the multidrug resistance (MDR) 1 gene was transiently increased. Moreover, Western blot analysis indicated that HGF up-regulated expressions of MDR1/P-glycoprotein and breast cancer resistance protein in human hepatocytes, whereas those of multidrug resistance gene-associated protein (MRP) 2 and MRP3 were unchanged. However, HGF prevented constitutive androstane receptor-related up-regulation of MRP2 occurring in phenobarbital-treated hepatocytes. Taken together, these data demonstrate that HGF differentially regulates transporter expression in human hepatocytes, i.e., it represses most of the sinusoidal uptake transporters, whereas expression of most of the efflux transporters is unchanged or increased. Such changes probably contribute to alterations of pharmacokinetics in patients with diseases associated with increased plasma levels of HGF such as fulminant hepatitis.

Regulation of drug transporter mRNA expression by interferon-γ in primary human hepatocytes

Interferon (IFN)-γ is known to downregulate expression of drug detoxifying proteins such as cytochromes P-450 (CYPs) in human hepatocytes. The present study was designed to determine whether IFN-γ may also impair expression of influx and efflux drug transporters, which constitute important determinants of the liver detoxification pathway. Exposure of primary human hepatocytes to 10 ng/mL IFN-γ was found to downregulate mRNA levels of sinusoidal influx transporters such as sodium-taurocholate cotransporting polypeptide, organic anion transporting polypeptide (OATP) 2B1, OATP1B1, and OATP1B3. IFN-γ concomitantly reduced mRNA expression of drug efflux pumps such as multidrug resistance gene 1, multidrug resistance protein (MRP) 2, MRP3, breast cancer resistance protein and bile salt export pump. Such IFN-γ-mediated repression of major hepatic drug transporters may contribute to impaired liver clearance of drugs administrated to patients suffering from inflammation or viral infections associated with increased secretion of IFN-γ.

Nature and uses of fluorescent dyes for drug transporter studies

INTRODUCTION

Drug transporters are now recognized as major players involved in pharmacokinetics and toxicology. Methods for assessing their activity are important to consider, particularly owing to regulatory requirements with respect to inhibition of drug transporter activity and prediction of drug-drug interactions. In this context, the use of fluorescent-dye-based transport assays is likely to deserve attention.

AREAS COVERED

This review provides an overview of the nature of fluorescent dye substrates for ATP-binding cassette and solute carrier drug transporters. Their use for investigating drug transporter activity in cultured cells and clinical hematological samples, drug transporter inhibition, drug transporter imaging and drug transport at the organ level are summarized.

EXPERT OPINION

A wide range of fluorescent dyes is now available for use in various aspects of drug transporter studies. The use of these dyes for transporter analyses may, however, be hampered by classic pitfalls of fluorescence technology, such as quenching. Transporter-independent processes such as passive diffusion of dyes through plasma membrane or dye sequestration into subcellular compartments must also be considered, as well as the redundant handling by various distinct transporters of some fluorescent probes. Finally, standardization of dye-based transport assays remains an important on-going issue.

Potent inhibition of carcinogen-bioactivating cytochrome P450 1B1 by the p53 inhibitor pifithrin α

Pifithrin alpha (PFTalpha) is a chemical compound that inhibits p53-mediated gene activation and apoptosis. It has also been recently shown to alter metabolism of carcinogenic polycyclic aromatic hydrocarbons (PAHs). This has led us to examine the effect of PFTalpha on the activity of cytochrome P-450 (CYP) 1 isoforms, known to metabolize PAHs, such as benzo(a)pyrene (BP), into mutagenic metabolites. We report that PFTalpha caused a potent inhibition of CYP1-related activity as measured by ethoxyresorufin O-deethylase activity in CYP1-containing MCF-7 cells and liver microsomes. It also directly affected the catalytic activity of human recombinant CYP1A1, CYP1A2 and CYP1B1 isoforms, with a potent inhibitory effect towards CYP1B1. The nature of this CYP1B1 inhibition by PFTalpha was mixed-type with an apparent K(i) of 4.38 nM. Blockage of CYP1 activity by PFTalpha was associated with a decreased metabolism of BP, a reduced formation of BP-derived adducts and a diminished BP-induced apoptosis in human cultured cells targets for PAHs like primary human macrophages and p53-negative KG1a leukaemia cells. These data further substantiate an unexpected and p53-independent action of PFTalpha for preventing toxicity of chemical carcinogens such as PAHs, through inhibition of CYP1 enzyme activities, especially that of CYP1B1.

AhR- and c-maf-dependent induction of beta7-integrin expression in human macrophages in response to environmental polycyclic aromatic hydrocarbons

In order to identify molecular targets of environmental polycyclic aromatic hydrocarbons (PAHs), we have analysed regulation of integrin (ITG) expression in PAH-exposed human macrophages. Among ITG subunits, beta7 ITG was found to be markedly up-regulated at both mRNA and protein levels in response to the prototypical PAH benzo(a)pyrene (BP). Knock-down of the transcription factor c-maf, known to control beta7 ITG expression, markedly impaired BP-mediated beta7 ITG induction. Moreover, chromatin immunoprecipitation and electrophoretic mobility shift assays showed BP-triggered binding of c-maf to a specific maf-responsive element found in beta7 ITG promoter. Such a binding, and also beta7 ITG induction, were however abolished in response to chemical inhibition of the aryl hydrocarbon receptor (AhR), to which PAHs bind. Taken together, these data establish beta7 ITG as a new molecular target of PAHs, whose up-regulation by these environmental contaminants most likely requires activation of co-operative pathways involving both AhR and c-maf.

Regulation of human hepatic drug transporter activity and expression by diesel exhaust particle extract

Diesel exhaust particles (DEPs) are common environmental air pollutants primarily affecting the lung. DEPs or chemicals adsorbed on DEPs also exert extra-pulmonary effects, including alteration of hepatic drug detoxifying enzyme expression. The present study was designed to determine whether organic DEP extract (DEPe) may target hepatic drug transporters that contribute in a major way to drug detoxification. Using primary human hepatocytes and transporter-overexpressing cells, DEPe was first shown to strongly inhibit activities of the sinusoidal solute carrier (SLC) uptake transporters organic anion-transporting polypeptides (OATP) 1B1, 1B3 and 2B1 and of the canalicular ATP-binding cassette (ABC) efflux pump multidrug resistance-associated protein 2, with IC50 values ranging from approximately 1 to 20 μg/mL and relevant to environmental exposure situations. By contrast, 25 μg/mL DEPe failed to alter activities of the SLC transporter organic cation transporter (OCT) 1 and of the ABC efflux pumps P-glycoprotein and bile salt export pump (BSEP), whereas it only moderately inhibited those of sodium taurocholate co-transporting polypeptide and of breast cancer resistance protein (BCRP). Treatment by 25 μg/mL DEPe was next demonstrated to induce expression of BCRP at both mRNA and protein level in cultured human hepatic cells, whereas it concomitantly repressed mRNA expression of various transporters, including OATP1B3, OATP2B1, OCT1 and BSEP. Such changes in transporter expression were found to be highly correlated to those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a reference activator of the aryl hydrocarbon receptor (AhR) pathway. This suggests that DEPe, which is enriched in known ligands of AhR like polycyclic aromatic hydrocarbons, alters drug transporter expression via activation of the AhR cascade. Taken together, these data established human hepatic transporters as targets of organic chemicals containing in DEPs, which may contribute to their systemic effects through impairing hepatic transport of endogenous compound or drug substrates of these transporters.

Inhibition of Human Drug Transporter Activities by the Pyrethroid Pesticides Allethrin and Tetramethrin

Pyrethroids are widely-used chemical insecticides, to which humans are commonly exposed, and known to alter functional expression of drug metabolizing enzymes. Limited data have additionally suggested that drug transporters, that constitute key-actors of the drug detoxification system, may also be targeted by pyrethroids. The present study was therefore designed to analyze the potential regulatory effects of these pesticides towards activities of main ATP-binding cassette (ABC) and solute carrier (SLC) drug transporters, using transporter-overexpressing cells. The pyrethroids allethrin and tetramethrin were found to inhibit various ABC and SLC drug transporters, including multidrug resistance-associated protein (MRP) 2, breast cancer resistance protein (BCRP), organic anion transporter polypeptide (OATP) 1B1, organic anion transporter (OAT) 3, multidrug and toxin extrusion transporter (MATE) 1, organic cation transporter (OCT) 1 and OCT2, with IC50 values however ranging from 2.6 μM (OCT1 inhibition by allethrin) to 77.6 μM (OAT3 inhibition by tetramethrin) and thus much higher than pyrethroid concentrations (in the nM range) reached in environmentally pyrethroid-exposed humans. By contrast, allethrin and tetramethrin cis-stimulated OATP2B1 activity and failed to alter activities of OATP1B3, OAT1 and MATE2-K, whereas P-glycoprotein activity was additionally moderately inhibited. Twelve other pyrethoids used at 100 μM did not block activities of the various investigated transporters, or only moderately inhibited some of them (inhibition by less than 50%). In silico analysis of structure-activity relationships next revealed that molecular parameters, including molecular weight and lipophilicity, are associated with transporter inhibition by allethrin/tetramethrin and successfully predicted transporter inhibition by the pyrethroids imiprothrin and prallethrin. Taken together, these data fully demonstrated that two pyrethoids, i.e., allethrin and tetramethrin, can act as regulators of the activity of various ABC and SLC drug transporters, but only when used at high and non-relevant concentrations, making unlikely any contribution of these transporter activity alterations to pyrethroid toxicity in environmentally exposed humans.

Polarized location of SLC and ABC drug transporters in monolayer-cultured human hepatocytes

Human hepatocytes cultured in a monolayer configuration represent a well-established in vitro model in liver toxicology, notably used in drug transporter studies. Polarized status of drug transporters, i.e., their coordinated location at sinusoidal or canalicular membranes, remains however incompletely documented in these cultured hepatocytes. The present study was therefore designed to analyze transporter expression and location in such cells. Most of drug transporters were first shown to be present at notable mRNA levels in monolayer-cultured human hepatocytes. Cultured human hepatocytes, which morphologically exhibited bile canaliculi-like structures, were next demonstrated, through immunofluorescence staining, to express the influx transporters organic anion transporting polypeptide (OATP) 1B1, OATP2B1 and organic cation transporter (OCT) 1 and the efflux transporter multidrug resistance-associated protein (MRP) 3 at their sinusoidal pole. In addition, the efflux transporters P-glycoprotein and MRP2 were detected at the canalicular pole of monolayer-cultured human hepatocytes. Moreover, canalicular secretion of reference substrates for the efflux transporters bile salt export pump, MRP2 and P-glycoprotein as well as sinusoidal drug transporter activities were observed. This polarized and functional expression of drug transporters in monolayer-cultured human hepatocytes highlights the interest of using this human in vitro cell model in xenobiotic transport studies.