Trafficking of canalicular ABC transporters in hepatocytes

Gene expression and DNA methylation changes in response to hypoxia in toxicant-adapted Atlantic killifish (Fundulus heteroclitus)

Coastal fish populations are threatened by multiple anthropogenic impacts, including the accumulation of industrial contaminants and the increasing frequency of hypoxia. Some populations of the Atlantic killifish (Fundulus heteroclitus), like those in New Bedford Harbor (NBH), Massachusetts, USA, have evolved a resistance to dioxin-like polychlorinated biphenyls (PCBs) that may influence their ability to cope with secondary stressors. To address this question, we compared hepatic gene expression and DNA methylation patterns in response to mild or severe hypoxia in killifish from NBH and Scorton Creek (SC), a reference population from a relatively pristine environment. We hypothesized that NBH fish would show altered responses to hypoxia due to trade-offs linked to toxicant resistance. Our results revealed substantial differences between populations. SC fish demonstrated dose-dependent changes in gene expression in response to hypoxia, while NBH fish exhibited a muted transcriptional response to severe hypoxia. Interestingly, NBH fish showed significant DNA methylation changes in response to hypoxia, while SC fish did not exhibit notable epigenetic alterations. These findings suggest that toxicant-adapted killifish may face trade-offs in their molecular response to environmental stress, potentially impacting their ability to survive severe hypoxia in coastal habitats. Further research is needed to elucidate the functional implications of these epigenetic modifications and their role in adaptive stress responses.

Gene expression and DNA methylation changes in response to hypoxia in toxicant-adapted Atlantic killifish (Fundulus heteroclitus)

Coastal fish populations are threatened by multiple anthropogenic impacts, including the accumulation of industrial contaminants and the increasing frequency of hypoxia. Some populations of the Atlantic killifish (Fundulus heteroclitus), like those in New Bedford Harbor (NBH), Massachusetts, have evolved a resistance to dioxin-like polychlorinated biphenyls (PCBs) that may influence their ability to cope with secondary stressors. To address this question, we compared hepatic gene expression and DNA methylation patterns in response to mild or severe hypoxia in killifish from NBH and Scorton Creek (SC), a reference population from a relatively pristine environment. We hypothesized that NBH fish would show altered responses to hypoxia due to trade-offs linked to toxicant resistance. Our results revealed substantial differences between populations. SC fish demonstrated a dose-dependent changes in gene expression in response to hypoxia, while NBH fish exhibited a muted transcriptional response to severe hypoxia. Interestingly, NBH fish showed significant DNA methylation changes in response to hypoxia, while SC fish did not exhibit notable epigenetic alterations. These findings suggest that toxicant-adapted killifish may face trade-offs in their molecular response to environmental stress, potentially impacting their ability to survive severe hypoxia in coastal habitats. Further research is needed to elucidate the functional implications of these epigenetic modifications and their role in adaptive stress responses.

Mechanisms and Clinical Significance of Pharmacokinetic Drug Interactions Mediated by FDA and EMA-approved Hepatitis C Direct-Acting Antiviral Agents

The treatment of patients infected with the hepatitis C virus (HCV) has been revolutionised by the development of direct-acting antiviral agents (DAAs) that target specific HCV proteins involved in viral replication. The first DAAs were associated with clinical problems such as adverse drug reactions and pharmacokinetic drug-drug interactions (DDIs). Current FDA/EMA-approved treatments are combinations of DAAs that simultaneously target the HCV N5A-protein, the HCV N5B-polymerase and the HCV NS3/4A-protease. Adverse events and DDIs are less likely with these DAA combinations but several DDIs of potential clinical significance remain. Much of the available information on the interaction of DAAs with CYP drug-metabolising enzymes and influx and efflux transporters is contained in regulatory summaries and is focused on DDIs of likely clinical importance. Important DDIs perpetrated by current DAAs include increases in the pharmacokinetic exposure to statins and dabigatran. Some mechanistic information can be deduced. Although the free concentrations of DAAs in serum are very low, a number of these DDIs are likely mediated by the inhibition of systemic influx transporters, especially OATP1B1/1B3. Other DDIs may arise by DAA-mediated inhibition of intestinal efflux transporters, which increases the systemic concentrations of some coadministered drugs. Conversely, DAAs are victims of DDIs mediated by cyclosporin, ketoconazole, omeprazole and HIV antiretroviral drug combinations, especially when boosted by ritonavir and, to a lesser extent, cobicistat. In addition, concurrent administration of inducers, such as rifampicin, carbamazepine and efavirenz, decreases exposure to some DAAs. Drug-drug interactions that increase the accumulation of HCV N3/4A-protease inhibitors like grazoprevir may exacerbate hepatic injury in HCV patients.

Impact of HCV Infection on Hepatocyte Polarity and Plasticity

The hepatitis C virus (HCV) is an oncogenic virus that alters the cell polarization machinery in order to enter the hepatocyte and replicate. While these alterations are relatively well defined, their consequences in the evolution of the disease remain poorly documented. Since 2012, HCV infection can be effectively cured with the advent of direct acting antivirals (DAA). Nevertheless, patients cured of their HCV infection still have a high risk of developing hepatocellular carcinoma (HCC). Importantly, it has been shown that some of the deregulations induced by HCV are maintained despite a sustained virologic response (SVR), including the down-regulation of some hepatocyte functions such as bile acid metabolism, exemplifying cell dedifferentiation, and the up-regulation of the epithelial–mesenchymal transition (EMT). EMT is a process by which epithelial cells lose their differentiation and their specific polarity to acquire mesenchymal cell properties, including migration and extracellular matrix remodeling capabilities. Of note, epithelial cell polarity acts as a gatekeeper against EMT. Thus, it remains important to elucidate the mechanisms by which HCV alters polarity and promotes EMT that could participate in viral-induced hepatic carcinogenesis. In this review, we define the main steps involved in the polarization process of epithelial cells and recall the essential cellular actors involved. We also highlight the particularities of hepatocyte polarity, responsible for their unique morphology. We then focus on the alterations by HCV of epithelial cell polarity and the consequences of the transformation of hepatocytes involved in the carcinogenesis process.

Oxidative Stress and Localization Status of Hepatocellular Transporters: Impact on Bile Secretion and Role of Signaling Pathways

Significance: Most hepatopathies are primarily or secondarily cholestatic in nature. Oxidative stress (OS) is a frequent trait among them, and impairs the machinery to generate bile by triggering endocytic internalization of hepatocellular transporters, thus causing cholestasis. This is critical, since it leads to accelerated transporter degradation, which could explain the common post-transcriptional downregulation of transporter expression in human cholestatic diseases. Recent Advances: The mechanisms involved in OS-induced hepatocellular transporter internalization are being revealed. Filamentous actin (F-actin) cytoskeleton disorganization and/or detachment of crosslinking actin proteins that afford transporter stability have been characterized as causal factors. Activation of redox-sensitive signaling pathways leading to changes in phosphorylation status of these structures is involved, including Ca²⁺-mediated activation of "classical" and "novel" protein kinase C (PKC) isoforms or redox-signaling cascades downstream of NADPH oxidase. Critical Issues: Despite the well-known occurrence of hepatocellular transporter internalization in human hepatopathies, the cholestatic implications of this phenomenon have been overlooked. Accordingly, no specific treatment has been established in the clinical practice for its prevention/reversion. Future Directions: We need to improve our knowledge on the pro-oxidant triggering factors and the multiple signaling pathways that mediate this oxidative injury in each cholestatic hepatopathy, so as to envisage tailor-made therapeutic strategies for each case. Meanwhile, administration of antioxidants or heme oxygenase-1 induction to elevate the hepatocellular levels of the endogenous scavenger bilirubin are promising alternatives that need to be re-evaluated and implemented. They may complement current treatments in cholestasis aimed to enhance transcriptional carrier expression, by providing membrane stability to the newly synthesized carriers. Antioxid. Redox Signal. 35, 808-831.

Protecting P-glycoprotein at the blood-brain barrier from degradation in an Alzheimer's disease mouse model

BACKGROUND

Failure to clear Aβ from the brain is partly responsible for Aβ brain accumulation in Alzheimer's disease (AD). A critical protein for clearing Aβ across the blood-brain barrier is the efflux transporter P-glycoprotein (P-gp). In AD, P-gp levels are reduced, which contributes to impaired Aβ brain clearance. However, the mechanism responsible for decreased P-gp levels is poorly understood and there are no strategies available to protect P-gp. We previously demonstrated in isolated brain capillaries ex vivo that human Aβ40 (hAβ40) triggers P-gp degradation by activating the ubiquitin-proteasome pathway. In this pathway, hAβ40 initiates P-gp ubiquitination, leading to internalization and proteasomal degradation of P-gp, which then results in decreased P-gp protein expression and transport activity levels. Here, we extend this line of research and present results from an in vivo study using a transgenic mouse model of AD (human amyloid precursor protein (hAPP)-overexpressing mice; Tg2576).

METHODS

In our study, hAPP mice were treated with vehicle, nocodazole (NCZ, microtubule inhibitor to block P-gp internalization), or a combination of NCZ and the P-gp inhibitor cyclosporin A (CSA). We determined P-gp protein expression and transport activity levels in isolated mouse brain capillaries and Aβ levels in plasma and brain tissue.

RESULTS

Treating hAPP mice with 5 mg/kg NCZ for 14 days increased P-gp levels to levels found in WT mice. Consistent with this, P-gp-mediated hAβ42 transport in brain capillaries was increased in NCZ-treated hAPP mice compared to untreated hAPP mice. Importantly, NCZ treatment significantly lowered hAβ40 and hAβ42 brain levels in hAPP mice, whereas hAβ40 and hAβ42 levels in plasma remained unchanged.

CONCLUSIONS

These findings provide in vivo evidence that microtubule inhibition maintains P-gp protein expression and transport activity levels, which in turn helps to lower hAβ brain levels in hAPP mice. Thus, protecting P-gp at the blood-brain barrier may provide a novel therapeutic strategy for AD and other Aβ-based pathologies.

A Link between Intrahepatic Cholestasis and Genetic Variations in Intracellular Trafficking Regulators

Simple Summary

Cholestasis refers to a medical condition in which the liver is not capable of secreting bile. The consequent accumulation of toxic bile components in the liver leads to liver failure. Cholestasis can be caused by mutations in genes that code for proteins involved in bile secretion. Recently mutations in other genes have been discovered in patients with cholestasis of unknown origin. Interestingly, many of these newly discovered genes code for proteins that regulate the intracellular distribution of other proteins, including those involved in bile secretion. This group of genes thus suggests the deregulated intracellular distribution of bile-secreting proteins as an important but still poorly understood mechanism that underlies cholestasis. To expedite a better understanding of this mechanism, we have reviewed these genes and their mutations and we discuss these in the context of cholestasis.

Abstract

Intrahepatic cholestasis is characterized by the accumulation of compounds in the serum that are normally secreted by hepatocytes into the bile. Genes associated with familial intrahepatic cholestasis (FIC) include ATP8B1 (FIC1), ABCB11 (FIC2), ABCB4 (FIC3), TJP2 (FIC4), NR1H4 (FIC5) and MYO5B (FIC6). With advanced genome sequencing methodologies, additional mutated genes are rapidly identified in patients presenting with idiopathic FIC. Notably, several of these genes, VPS33B, VIPAS39, SCYL1, and AP1S1, together with MYO5B, are functionally associated with recycling endosomes and/or the Golgi apparatus. These are components of a complex process that controls the sorting and trafficking of proteins, including those involved in bile secretion. These gene variants therefore suggest that defects in intracellular trafficking take a prominent place in FIC. Here we review these FIC-associated trafficking genes and their variants, their contribution to biliary transporter and canalicular protein trafficking, and, when perturbed, to cholestatic liver disease. Published variants for each of these genes have been summarized in table format, providing a convenient reference for those who work in the intrahepatic cholestasis field.

Bile Acid Toxicity and Protein Kinases

If the bile acids reach to pathological concentrations due to cholestasis, accumulation of hydrophobic bile acids within the hepatocyte may result in cell death. Thus, hydrophobic bile acids induce apoptosis in hepatocytes, while hydrophilic bile acids increase intracellular adenosine 3',5'-monophosphate (cAMP) levels and activate mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) pathways to protect hepatocytes from apoptosis.Two apoptotic pathways have been described in bile acids-induced death. Both are controlled by multiple protein kinase signaling pathways. In mitochondria-controlled pathway, caspase-8 is activated with death domain-independent manner, whereas, Fas-dependent classical pathway involves ligand-independent oligomerization of Fas.Hydrophobic bile acids dose-dependently upregulate the inflammatory response by further stimulating production of inflammatory cytokines. Death receptor-mediated apoptosis is regulated at the cell surface by the receptor expression, at the death-inducing signaling complex (DISC) by expression of procaspase-8, the death receptors Fas-associated death domain (FADD), and cellular FADD-like interleukin 1-beta (IL-1β)-converting enzyme (FLICE) inhibitory protein (cFLIP). Bile acids prevent cFLIP recruitment to the DISC and thereby enhance initiator caspase activation and lead to cholestatic apoptosis. At mitochondria, the expression of B-cell leukemia/lymphoma-2 (Bcl-2) family proteins contribute to apoptosis by regulating mitochondrial cytochrome c release via Bcl-2, Bcl-2 homology 3 (BH3) interacting domain death agonist (Bid), or Bcl-2 associated protein x (Bax). Fas receptor CD95 activation by hydrophobic bile acids is initiated by reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species (ROS) signaling. However, activation of necroptosis by ligands of death receptors requires the kinase activity of receptor interacting protein1 (RIP1), which mediates the activation of RIP3 and mixed lineage kinase domain-like protein (MLKL). In this chapter, mainly the effect of protein kinases signal transduction on the mechanisms of hydrophobic bile acids-induced inflammation, apoptosis, necroptosis and necrosis are discussed.

Structure and Function of Hepatobiliary ATP Binding Cassette Transporters

The liver is beyond any doubt the most important metabolic organ of the human body. This function requires an intensive crosstalk within liver cellular structures, but also with other organs. Membrane transport proteins are therefore of upmost importance as they represent the sensors and mediators that shuttle signals from outside to the inside of liver cells and/or vice versa. In this review, we summarize the known literature of liver transport proteins with a clear emphasis on functional and structural information on ATP binding cassette (ABC) transporters, which are expressed in the human liver. These primary active membrane transporters form one of the largest families of membrane proteins. In the liver, they play an essential role in for example bile formation or xenobiotic export. Our review provides a state of the art and comprehensive summary of the current knowledge of hepatobiliary ABC transporters. Clearly, our knowledge has improved with a breath-taking speed over the last few years and will expand further. Thus, this review will provide the status quo and will lay the foundation for new and exciting avenues in liver membrane transporter research.

Novel Intrinsic Mechanisms of Active Drug Extrusion at the Blood-Brain Barrier: Potential Targets for Enhancing Drug Delivery to the Brain?

The blood-brain barrier (BBB) limits the pharmacotherapy of several brain disorders. In addition to the structural and metabolic characteristics of the BBB, the ATP-driven, drug efflux transporter P-glycoprotein (Pgp) is a selective gatekeeper of the BBB; thus, it is a primary hindrance to drug delivery into the brain. Here, we review the complex regulation of Pgp expression and functional activity at the BBB with an emphasis on recent studies from our laboratory. In addition to traditional processes such as transcriptional regulation and posttranscriptional or posttranslational modification of Pgp expression and functionality, novel mechanisms such as intra- and intercellular Pgp trafficking and intracellular Pgp-mediated lysosomal sequestration in BBB endothelial cells with subsequent disposal by blood neutrophils are discussed. These intrinsic mechanisms of active drug extrusion at the BBB are potential therapeutic targets that could be used to modulate P-glycoprotein activity in the treatment of brain diseases and enhance drug delivery to the brain.

ABCB4/MDR3 in health and disease – at the crossroads of biochemistry and medicine

Several ABC transporters of the human liver are responsible for the secretion of bile salts, lipids and cholesterol. Their interplay protects the biliary tree from the harsh detergent activity of bile salts. Among these transporters, ABCB4 is essential for the translocation of phosphatidylcholine (PC) lipids from the inner to the outer leaflet of the canalicular membrane of hepatocytes. ABCB4 deficiency can result in altered PC to bile salt ratios, which led to intrahepatic cholestasis of pregnancy, low phospholipid associated cholelithiasis, drug induced liver injury or even progressive familial intrahepatic cholestasis type 3. Although PC lipids only account for 30–40% of the lipids in the canalicular membrane, 95% of all phospholipids in bile are PC lipids. We discuss this discrepancy in the light of PC synthesis and bile salts favoring certain lipids. Nevertheless, the in vivo extraction of PC lipids from the outer leaflet of the canalicular membrane by bile salts should be considered as a separate step in bile formation. Therefore, methods to characterize disease causing ABCB4 mutations should be considered carefully, but such an analysis represents a crucial point in understanding the currently unknown transport mechanism of this ABC transporter.

Co-disposition of chitosan nanoparticles by multi types of hepatic cells and their subsequent biological elimination: the mechanism and kinetic studies at the cellular and animal levels

Background: The clearance of nanomaterials (NMs) from the liver is essential for clinical safety, and their hepatic clearance is primarily determined by the co-disposition process of various types of hepatic cells. Studies of this process and the subsequent clearance routes are urgently needed for organic NMs, which are used as drug carriers more commonly than the inorganic ones. Materials and methods: In this study, the co-disposition of chitosan-based nanoparticles (CsNps) by macrophages and hepatocytes at both the cellular and animal levels as well as their subsequent biological elimination were investigated. RAW264.7 and Hepa1-6 cells were used as models of Kupffer cells and hepatocytes, respectively. Results: The cellular studies showed that CsNps released from RAW264.7 cells could enter Hepa1-6 cells through both clathrin- and caveolin-mediated endocytosis. The transport from Kupffer cells to hepatocytes was also studied in mice, and it was observed that most CsNps localized to the hepatocytes after intravenous injection. Following the distribution in hepatocytes, the hepatobiliary-fecal excretion route was shown to be the primary elimination route for CsNps, besides the kidney-urinary excretion route. The elimination of CsNps in mice was a lengthy process, with a half time of about 2 months. Conclusion: The demonstration in this study of the transport of CsNps from macrophages to hepatocytes and the subsequent hepatobiliary-fecal excretion provides basic information for the future development and clinical application of NMs.

Oligomerization of equilibrative nucleoside transporters: a novel regulatory and functional mechanism involving PKC and PP1

Equilibrative nucleoside transporters (ENTs) translocate nucleosides and nucleobases across plasma membranes, as well as a variety of anti-cancer, -viral, and -parasite nucleoside analogs. They are also key members of the purinome complex and regulate the protective and anti-inflammatory effects of adenosine. Despite their important role, little is known about the mechanisms involved in their regulation. We conducted membrane yeast 2-hybrid and coimmunoprecipitation studies and identified, for the first time to our knowledge, the existence of protein-protein interactions between human ENT1 and ENT2 (hENT1 and hENT2) proteins in human cells and the formation of hetero- and homo-oligomers at the plasma membrane and the submembrane region. The use of NanoLuc Binary Technology allowed us to analyze changes in the oligomeric status of hENT1 and hENT2 and how they rapidly modify the uptake profile for nucleosides and nucleobases and allow cells to respond promptly to external signals or changes in the extracellular environment. These changes in hENTs oligomerization are triggered by PKC activation and subsequent action of protein phosphatase 1.-Grañe-Boladeras, N., Williams, D., Tarmakova, Z., Stevanovic, K., Villani, L. A., Mehrabi, P., Siu, K. W. M., Pastor-Anglada, M., Coe, I. R. Oligomerization of equilibrative nucleoside transporters: a novel regulatory and functional mechanism involving PKC and PP1.

Autoimmune BSEP disease: disease recurrence after liver transplantation for progressive familial intrahepatic cholestasis

Severe cholestasis may result in end-stage liver disease with the need of liver transplantation (LTX). In children, about 10 % of LTX are necessary because of cholestatic liver diseases. Apart from bile duct atresia, three types of progressive familial intrahepatic cholestasis (PFIC) are common causes of severe cholestasis in children. The three subtypes of PFIC are defined by the involved genes: PFIC-1, PFIC-2, and PFIC-3 are due to mutations of P-type ATPase ATP8B1 (familial intrahepatic cholestasis 1, FIC1), the ATP binding cassette transporter ABCB11 (bile salt export pump, BSEP), or ABCB4 (multidrug resistance protein 3, MDR3), respectively. All transporters are localized in the canalicular membrane of hepatocytes and together mediate bile salt and phospholipid transport. In some patients with PFIC-2 disease, recurrence has been observed after LTX, which mimics a PFIC phenotype. It could be shown by several groups that inhibitory anti-BSEP antibodies emerge, which most likely cause disease recurrence. The prevalence of severe BSEP mutations (e.g., splice site and premature stop codon mutations) is very high in this group of patients. These mutations often result in the complete absence of BSEP, which likely accounts for an insufficient auto-tolerance against BSEP. Although many aspects of this "new" disease are not fully elucidated, the possibility of anti-BSEP antibody formation has implications for the pre- and posttransplant management of PFIC-2 patients. This review will summarize the current knowledge including diagnosis, pathomechanisms, and management of "autoimmune BSEP disease."

Basolateral sorting and transcytosis define the Cu+-regulated translocation of ATP7B to the bile canaliculus

The Cu+ pump ATP7B plays an irreplaceable role in the elimination of excess Cu+ by the hepatocyte into the bile. The traffic and site of ATP7B action is a subject of controversy. One current proposal is that an increase in intracellular Cu+ results in the translocation of ATP7B to the lysosomes and excretion of excess Cu+ by lysosomal mediated exocytosis at the bile canaliculus. Here we show that ATP7B is transported from the trans-Golgi network to the bile canaliculus by basolateral sorting and endocytosis, and microtubule mediated transcytosis through the subapical compartment. Trafficking ATP7B is not incorporated into lysosomes and addition of Cu+ does not cause relocalization of lysosomes and the appearance of lysosome markers in the bile canaliculus. Our data describes the pathway of the Cu+-mediated transport of ATP7B from the TGN to the bile canaliculus and indicates that the bile canaliculus is the prime site of ATP7B action in the elimination of excess Cu+.

EGFR participates downstream of ERα in estradiol-17β-D-glucuronide-induced impairment of Abcc2 function in isolated rat hepatocyte couplets

Estradiol-17β-D-glucuronide (E17G) induces acute endocytic internalization of canalicular transporters, including multidrug resistance-associated protein 2 (Abcc2) in rat, generating cholestasis. Several proteins organized in at least two different signaling pathways are involved in E17G cholestasis: one pathway involves estrogen receptor alpha (ERα), Ca(2+)-dependent protein kinase C and p38-mitogen activated protein kinase, and the other pathway involves GPR30, PKA, phosphoinositide 3-kinase/AKT and extracellular signal-related kinase 1/2. EGF receptor (EGFR) can potentially participate in both pathways since it interacts with GPR30 and ERα. Hence, the aim of this study was to analyze the potential role of this receptor and its downstream effectors, members of the Src family kinases in E17G-induced cholestasis. In vitro, EGFR inhibition by Tyrphostin (Tyr), Cl-387785 or its knockdown with siRNA strongly prevented E17G-induced impairment of Abcc2 function and localization. Activation of EGFR was necessary but not sufficient to impair the canalicular transporter function, whereas the simultaneous activation of EGFR and GPR30 could impair Abcc2 transport. The protection of Tyr was not additive to that produced by the ERα inhibitor ICI neither with that produced by Src kinase inhibitors, suggesting that EGFR shared the signaling pathway of ERα and Src. Further analysis of ERα, EGFR and Src activations induced by E17G, demonstrated that ERα activation precedes that of EGFR and EGFR activation precedes that of Src. In conclusion, activation of EGFR is a key factor in the alteration of canalicular transporter function and localization induced by E17G and it occurs before that of Src and after that of ERα.

Comparative Localization and Functional Activity of the Main Hepatobiliary Transporters in HepaRG Cells and Primary Human Hepatocytes

The role of hepatobiliary transporters in drug-induced liver injury remains poorly understood. Various in vivo and in vitro biological approaches are currently used for studying hepatic transporters; however, appropriate localization and functional activity of these transporters are essential for normal biliary flow and drug transport. Human hepatocytes (HHs) are considered as the most suitable in vitro cell model but erratic availability and inter-donor functional variations limit their use. In this work, we aimed to compare localization of influx and efflux transporters and their functional activity in differentiated human HepaRG hepatocytes with fresh HHs in conventional (CCHH) and sandwich (SCHH) cultures. All tested influx and efflux transporters were correctly localized to canalicular [bile salt export pump (BSEP), multidrug resistance-associated protein 2 (MRP2), multidrug resistance protein 1 (MDR1), and MDR3] or basolateral [Na(+)-taurocholate co-transporting polypeptide (NTCP) and MRP3] membrane domains and were functional in all models. Contrary to other transporters, NTCP and BSEP were less abundant and active in HepaRG cells, cellular uptake of taurocholate was 2.2- and 1.4-fold and bile excretion index 2.8- and 2.6-fold lower, than in SCHHs and CCHHs, respectively. However, when taurocholate canalicular efflux was evaluated in standard and divalent cation-free conditions in buffers or cell lysates, the difference between the three models did not exceed 9.3%. Interestingly, cell imaging showed higher bile canaliculi contraction/relaxation activity in HepaRG hepatocytes and larger bile canaliculi networks in SCHHs. Altogether, our results bring new insights in mechanisms involved in bile acids accumulation and excretion in HHs and suggest that HepaRG cells represent a suitable model for studying hepatobiliary transporters and drug-induced cholestasis.

MicroRNAs and their relevance to ABC transporters

MicroRNAs (miRNAs) are small noncoding RNAs, which regulate the expression of their target genes post-transcriptionally by RNA interference. They are involved in almost all cellular processes, including proliferation, differentiation, apoptosis, cell survival and the maintenance of tissue specificity. Recent findings also suggest that efflux pumps of the ABC (ATP-binding cassette) transporter family are subject to miRNA-mediated gene regulation. Moreover, it seems that ABC transporters are embedded in a concerted and miRNA-guided network of concurrently regulated proteins that mediate altered drug transport and cell survival in changing environmental conditions. In this review, we summarize recent findings of miRNAs interacting with ABC transporters, which have been connected with drug distribution as well as with drug resistance. Additionally, we specify findings of complex miRNA-protein pathways conferring increased drug export and cell survival.

ABCB4: Insights from pathobiology into therapy

Adenosine triphosphate (ATP)-binding cassette, sub-family B, member 4 (ABCB4), also called multidrug resistance 3 (MDR3), is a member of the ATP-binding cassette transporter superfamily, which is localized at the canalicular membrane of hepatocytes, and mediates the translocation of phosphatidylcholine into bile. Phosphatidylcholine secretion is crucial to ensure solubilization of cholesterol into mixed micelles and to prevent bile acid toxicity towards hepatobiliary epithelia. Genetic defects of ABCB4 may cause progressive familial intrahepatic cholestasis type 3 (PFIC3), a rare autosomic recessive disease occurring early in childhood that may be lethal in the absence of liver transplantation, and other cholestatic or cholelithiasic diseases in heterozygous adults. Development of therapies for these conditions requires understanding of the biology of this transporter and how gene variations may cause disease. This review focuses on our current knowledge on the regulation of ABCB4 expression, trafficking and function, and presents recent advances in fundamental research with promising therapeutic perspectives.

Current advances on ABC drug transporters in fish

Most members of the large ATP-binding cassette (ABC) gene family are transporters involved in substrate translocation across biological membranes. In eukaryotes, ABC proteins functioning as drug transporters are located in the plasma membrane and mediate the cellular efflux of a wide range of organic chemicals, with some transporters also transporting certain metals. As the enhanced expression of ABC drug transporters can confer multidrug resistance (MDR) to cancers and multixenobiotic resistance (MXR) to organisms from polluted habitats, these ABC family members are also referred to as MDR or MXR proteins. In mammals, ABC drug transporters show predominant expression in tissues involved in excretion or constituting internal or external body boundaries, where they facilitate the excretion of chemicals and their metabolites, and limit chemical uptake and penetration into "sanctuary" sites of the body. Available knowledge about ABC proteins is still limited in teleost fish, a large vertebrate group of high ecological and economic importance. Using transport activity measurements and immunochemical approaches, early studies demonstrated similarities in the tissue distribution of ABC drug transporters between teleosts and mammals, suggesting conserved roles of the transporters in the biochemical defence against toxicants. Recently, the availability of teleost genome assemblies has stimulated studies of the ABC family in this taxon. This review summarises the current knowledge regarding the genetics, functional properties, physiological function, and ecotoxicological relevance of teleostean ABC transporters. The available literature is reviewed with emphasis on recent studies addressing the tissue distribution, substrate spectrum, regulation, physiological function and phylogenetic origin of teleostean ABC transporters.

Inherited disorders of bilirubin transport and conjugation: new insights into molecular mechanisms and consequences

Inherited disorders of bilirubin metabolism might reduce bilirubin uptake by hepatocytes, bilirubin conjugation, or secretion of bilirubin into bile. Reductions in uptake could increase levels of unconjugated or conjugated bilirubin (Rotor syndrome). Defects in bilirubin conjugation could increase levels of unconjugated bilirubin; the effects can be benign and frequent (Gilbert syndrome) or rare but severe, increasing the risk of bilirubin encephalopathy (Crigler-Najjar syndrome). Impairment of bilirubin secretion leads to accumulation of conjugated bilirubin (Dubin-Johnson syndrome). We review the genetic causes and pathophysiology of disorders of bilirubin transport and conjugation as well as clinical and therapeutic aspects. We also discuss the possible mechanisms by which hyperbilirubinemia protects against cardiovascular disease and the metabolic syndrome and the effects of specific genetic variants on drug metabolism and cancer development.

Drug-induced trafficking of p-glycoprotein in human brain capillary endothelial cells as demonstrated by exposure to mitomycin C

P-glycoprotein (Pgp; ABCB1/MDR1) is a major efflux transporter at the blood-brain barrier (BBB), restricting the penetration of various compounds. In other tissues, trafficking of Pgp from subcellular stores to the cell surface has been demonstrated and may constitute a rapid way of the cell to respond to toxic compounds by functional membrane insertion of the transporter. It is not known whether drug-induced Pgp trafficking also occurs in brain capillary endothelial cells that form the BBB. In this study, trafficking of Pgp was investigated in human brain capillary endothelial cells (hCMEC/D3) that were stably transfected with a doxycycline-inducible MDR1-EGFP fusion plasmid. In the presence of doxycycline, these cells exhibited a 15-fold increase in Pgp-EGFP fusion protein expression, which was associated with an increased efflux of the Pgp substrate rhodamine 123 (Rho123). The chemotherapeutic agent mitomycin C (MMC) was used to study drug-induced trafficking of Pgp. Confocal fluorescence microscopy of single hCMEC/D3-MDR1-EGFP cells revealed that Pgp redistribution from intracellular pools to the cell surface occurred within 2 h of MMC exposure. Pgp-EGFP exhibited a punctuate pattern at the cell surface compatible with concentrated regions of the fusion protein in membrane microdomains, i.e., lipid rafts, which was confirmed by Western blot analysis of biotinylated cell surface proteins in Lubrol-resistant membranes. MMC exposure also increased the functionality of Pgp as assessed in three functional assays with Pgp substrates (Rho123, eFluxx-ID Gold, calcein-AM). However, this increase occurred with some delay after the increased Pgp expression and coincided with the release of Pgp from the Lubrol-resistant membrane complexes. Disrupting rafts by depleting the membrane of cholesterol increased the functionality of Pgp. Our data present the first direct evidence of drug-induced Pgp trafficking at the human BBB and indicate that Pgp has to be released from lipid rafts to gain its full functionality.

ABC transporter functions as a pacemaker for sequestration of plant glucosides in leaf beetles

Plant-herbivore interactions dominate the planet’s terrestrial ecology. When it comes to host–plant specialization, insects are among the most versatile evolutionary innovators, able to disarm multiple chemical plant defenses. Sequestration is a widespread strategy to detoxify noxious metabolites, frequently for the insect’s own benefit against predation. In this study, we describe the broad-spectrum ATP-binding cassette transporter CpMRP of the poplar leaf beetle, Chrysomela populi as the first candidate involved in the sequestration of phytochemicals in insects. CpMRP acts in the defensive glands of the larvae as a pacemaker for the irreversible shuttling of pre-selected metabolites from the hemolymph into defensive secretions. Silencing CpMRP in vivo creates a defenseless phenotype, indicating its role in the secretion process is crucial. In the defensive glands of related leaf beetle species, we identified sequences similar to CpMRP and assume therefore that exocrine gland-based defensive strategies, evolved by these insects to repel their enemies, rely on ABC transporters as a key element.

DOI:http://dx.doi.org/10.7554/eLife.01096.001

For millions of years, plant feeding insects have been locked in an arms race with the plants they consume. Plants have evolved defensive strategies such as the ability to produce noxious chemicals that deter insects, while many insects have evolved the means to thwart this defense and even turn it to their own advantage. The larvae of the poplar leaf beetle, Chrysomela populi, sequester toxic plant compounds in specialized glands on their backs and use these compounds to defend themselves against predators. The glands are lined with chemically inert chitin, the substance that makes up the insect exoskeleton, and the deterrent chemicals are released whenever the insect is threatened.

Now, Strauss et al. have identified a key transport protein used by the larvae to move toxic plant compounds to these glands. This transport protein belongs to a family of membrane proteins called ABC transporters, which help to shuttle substances out of cells or into cell organelles using energy produced by the hydrolysis of ATP molecules. The gene for this transporter is expressed in the glands of the leaf beetles at levels 7,000 times higher than elsewhere in the larvae.

Larvae that lack a functional version of the transporter gene continue to grow, but are unable to defend themselves against predators. Similar genes are found in other species of leaf beetle, suggesting that this type of transporter has been retained throughout evolution. Moreover, the transporter is not specific to a particular plant toxin; this enables leaf beetles to eat many different types of plants and boosts their chances of survival should a previous food source disappear.

DOI:http://dx.doi.org/10.7554/eLife.01096.002

Bile formation and secretion

Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions.

Role of hepatic efflux transporters in regulating systemic and hepatocyte exposure to xenobiotics

Hepatic efflux transporters include numerous well-known and emerging proteins localized to the canalicular or basolateral membrane of the hepatocyte that are responsible for the excretion of drugs into the bile or blood, respectively. Altered function of hepatic efflux transporters due to drug-drug interactions, genetic variation, and/or disease states may lead to changes in xenobiotic exposure in the hepatocyte and/or systemic circulation. This review focuses on transport proteins involved in the hepatocellular efflux of drugs and metabolites, discusses mechanisms of altered transporter function as well as the interplay between multiple transport pathways, and highlights the importance of considering intracellular unbound concentrations of transporter substrates and/or inhibitors. Methods to evaluate hepatic efflux transport and predict the effects of impaired transporter function on systemic and hepatocyte exposure are discussed, and the sandwich-cultured hepatocyte model to evaluate comprehensively the role of hepatic efflux in the hepatobiliary disposition of xenobiotics is characterized.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Hormonal modulation of hepatic cAMP prevents estradiol 17β-D-glucuronide-induced cholestasis in perfused rat liver

BACKGROUND

Estradiol-17β-D-glucuronide (E17G) induces cholestasis in vivo, endocytic internalization of the canalicular transporters multidrug resistance-associated protein 2 (Abcc2) and bile salt export pump (Abcb11) being a key pathomechanism. Cyclic AMP (cAMP) prevents cholestasis by targeting these transporters back to the canalicular membrane. In hepatocyte couplets, glucagon and salbutamol, both of which increase cAMP, prevented E17G action by stimulating the trafficking of these transporters by different mechanisms, namely: glucagon activates a protein kinase A-dependent pathway, whereas salbutamol activates an exchange-protein activated by cAMP (Epac)-mediated, microtubule-dependent pathway.

METHODS

The present study evaluated whether glucagon and salbutamol prevent E17G-induced cholestasis in a more physiological model, i.e., the perfused rat liver (PRL). Additionally, the preventive effect of in vivo alanine administration, which induces pancreatic glucagon secretion, was evaluated.

RESULTS

In PRLs, glucagon and salbutamol prevented E17G-induced decrease in both bile flow and the secretory activity of Abcc2 and Abcb11. Salbutamol prevention fully depended on microtubule integrity. On the other hand, glucagon prevention was microtubule-independent only at early time periods after E17G administration, but it was ultimately affected by the microtubule disrupter colchicine. Cholestasis was associated with endocytic internalization of Abcb11 and Abcc2, the intracellular carriers being partially colocalized with the endosomal marker Rab11a. This effect was completely prevented by salbutamol, whereas some transporter-containing vesicles remained colocalized with Rab11a after glucagon treatment. In vivo, alanine administration increased hepatic cAMP and accelerated the recovery of bile flow and Abcb11/Abcc2 transport function after E17G administration. The initial recovery afforded by alanine was microtubule-independent, but microtubule integrity was required to sustain this protective effect.

CONCLUSION

We conclude that modulation of cAMP levels either by direct administration of cAMP modulators or by physiological manipulations leadings to hormone-mediated increase of cAMP levels (alanine administration), prevents estrogen-induced cholestasis in models with preserved liver architecture, through mechanisms similar to those arisen from in vitro studies.

Coordinated elevation of mitochondrial oxidative phosphorylation and autophagy help drive hepatocyte polarization

Cell polarization requires increased cellular energy and metabolic output, but how these energetic demands are met by polarizing cells is unclear. To address these issues, we investigated the roles of mitochondrial bioenergetics and autophagy during cell polarization of hepatocytes cultured in a collagen sandwich system. We found that as the hepatocytes begin to polarize, they use oxidative phosphorylation to raise their ATP levels, and this energy production is required for polarization. After the cells are polarized, the hepatocytes shift to become more dependent on glycolysis to produce ATP. Along with this central reliance on oxidative phosphorylation as the main source of ATP production in polarizing cultures, several other metabolic processes are reprogrammed during the time course of polarization. As the cells polarize, mitochondria elongate and mitochondrial membrane potential increases. In addition, lipid droplet abundance decreases over time. These findings suggest that polarizing cells are reliant on fatty acid oxidation, which is supported by pharmacologic inhibition of β-oxidation by etomoxir. Finally, autophagy is up-regulated during cell polarization, with inhibition of autophagy retarding cell polarization. Taken together, our results describe a metabolic shift involving a number of coordinated metabolic pathways that ultimately serve to increase energy production during cell polarization.

Actin polymerization controls the activation of multidrug efflux at fertilization by translocation and fine-scale positioning of ABCB1 on microvilli

Multidrug efflux is activated at fertilization in sea urchin eggs, but it is unclear how cortical reorganization initiates transport. Using structured illumination microscopy, we found that the multidrug transporter ABCB1a translocates along polymerizing actin filaments to the microvillar tips. This short-range (micrometer scale) translocation is necessary for up-regulation of efflux activity.

Fertilization changes the structure and function of the cell surface. In sea urchins, these changes include polymerization of cortical actin and a coincident, switch-like increase in the activity of the multidrug efflux transporter ABCB1a. However, it is not clear how cortical reorganization leads to changes in membrane transport physiology. In this study, we used three-dimensional superresolution fluorescence microscopy to resolve the fine-scale movements of the transporter along polymerizing actin filaments, and we show that efflux activity is established after ABCB1a translocates to the tips of the microvilli. Inhibition of actin poly­merization or bundle formation prevents tip localization, resulting in the patching of ABCB1a at the cell surface and decreased efflux activity. In contrast, enhanced actin polymerization promotes tip localization. Finally, interference with Rab11, a regulator of apical recycling, inhibits activation of efflux activity in embryos. Together our results show that actin-mediated, short-range traffic and positioning of transporters at the cell surface regulates multidrug efflux activity and highlight the multifaceted roles of microvilli in the spatial distribution of membrane proteins.

Endogenous bile acid disposition in rat and human sandwich-cultured hepatocytes

Sandwich-cultured hepatocytes (SCH) are used commonly to investigate hepatic transport protein-mediated uptake and biliary excretion of substrates. However, little is known about the disposition of endogenous bile acids (BAs) in SCH. In this study, four endogenous conjugated BAs common to rats and humans [taurocholic acid (TCA), glycocholic acid (GCA), taurochenodeoxycholic acid (TCDCA), and glycochenodeoxycholic acid (GCDCA)], as well as two BA species specific to rodents (α- and β-tauromuricholic acid; α/β TMCA), were profiled in primary rat and human SCH. Using B-CLEAR® technology, BAs were measured in cells+bile canaliculi, cells, and medium of SCH by LC-MS/MS. Results indicated that, just as in vivo, taurine-conjugated BA species were predominant in rat SCH, while glycine-conjugated BAs were predominant in human SCH. Total intracellular BAs remained relatively constant over days in culture in rat SCH. Total BAs in control (CTL) cells+bile, cells, and medium were approximately 3.4, 2.9, and 8.3-fold greater in human than in rat. The estimated intracellular concentrations of the measured total BAs were 64.3±5.9 μM in CTL rat and 183±56 μM in CTL human SCH, while medium concentrations of the total BAs measured were 1.16±0.21 μM in CTL rat SCH and 9.61±6.36 μM in CTL human SCH. Treatment of cells for 24h with 10 μM troglitazone (TRO), an inhibitor of the bile salt export pump (BSEP) and the Na⁺-taurocholate cotransporting polypeptide (NTCP), had no significant effect on endogenous BAs measured at the end of the 24-h culture period, potentially due to compensatory mechanisms that maintain BA homeostasis. These data demonstrate that BAs in SCH are similar to in vivo, and that SCH may be a useful in vitro model to study alterations in BA disposition if species differences are taken into account.

Conjugated bile acids activate the sphingosine-1-phosphate receptor 2 in primary rodent hepatocytes

Bile acids have been shown to be important regulatory molecules for cells in the liver and gastrointestinal tract. They can activate various cell signaling pathways including extracellular regulated kinase (ERK)1/2 and protein kinase B (AKT) as well as the G-protein-coupled receptor (GPCR) membrane-type bile acid receptor (TGR5/M-BAR). Activation of the ERK1/2 and AKT signaling pathways by conjugated bile acids has been reported to be sensitive to pertussis toxin (PTX) and dominant-negative Gα(i) in primary rodent hepatocytes. However, the GPCRs responsible for activation of these pathways have not been identified. Screening GPCRs in the lipid-activated phylogenetic family (expressed in HEK293 cells) identified sphingosine-1-phosphate receptor 2 (S1P(2) ) as being activated by taurocholate (TCA). TCA, taurodeoxycholic acid (TDCA), tauroursodeoxycholic acid (TUDCA), glycocholic acid (GCA), glycodeoxycholic acid (GDCA), and S1P-induced activation of ERK1/2 and AKT were significantly inhibited by JTE-013, a S1P(2) antagonist, in primary rat hepatocytes. JTE-013 significantly inhibited hepatic ERK1/2 and AKT activation as well as short heterodimeric partner (SHP) mRNA induction by TCA in the chronic bile fistula rat. Knockdown of the expression of S1P(2) by a recombinant lentivirus encoding S1P(2) shRNA markedly inhibited the activation of ERK1/2 and AKT by TCA and S1P in rat primary hepatocytes. Primary hepatocytes prepared from S1P(2) knock out (S1P(2) (-/-) ) mice were significantly blunted in the activation of the ERK1/2 and AKT pathways by TCA. Structural modeling of the S1P receptors indicated that only S1P(2) can accommodate TCA binding. In summary, all these data support the hypothesis that conjugated bile acids activate the ERK1/2 and AKT signaling pathways primarily through S1P(2) in primary rodent hepatocytes.

Prevention of estradiol 17beta-D-glucuronide-induced canalicular transporter internalization by hormonal modulation of cAMP in rat hepatocytes

In estradiol 17β-d-glucuronide (E17G)-induced cholestasis, the canalicular hepatocellular transporters bile salt export pump (Abcb11) and multidrug-resistance associated protein 2 (Abcc2) undergo endocytic internalization. cAMP stimulates the trafficking of transporter-containing vesicles to the apical membrane and is able to prevent internalization of these transporters in estrogen-induced cholestasis. Hepatocyte levels of cAMP are regulated by hormones such as glucagon and adrenaline (via the β2 receptor). We analyzed the effects of glucagon and salbutamol (a β2 adrenergic agonist) on function and localization of Abcb11 and Abcc2 in isolated rat hepatocyte couplets exposed to E17G and compared the mechanistic bases of their effects. Glucagon and salbutamol partially prevented the impairment in Abcb11 and Abcc2 transport capacity. E17G also induced endocytic internalization of Abcb11 and Abcc2, which partially colocalized with the endosomal marker Rab11a. This effect was completely prevented by salbutamol, whereas some transporter-containing vesicles remained internalized and mainly colocalizing with Rab11a in the perinuclear region after incubation with glucagon. Glucagon prevention was dependent on cAMP-dependent protein kinase (PKA) and independent of exchange proteins activated directly by cAMP (Epac) and microtubules. In contrast, salbutamol prevention was PKA independent and Epac/MEK and microtubule dependent. Anticholestatic effects of glucagon and salbutamol were additive in nature. Our results show that increases in cAMP could activate different anticholestatic signaling pathways, depending on the hormonal mediator involved.

Down-regulation of ATP-binding cassette C2 protein expression in HepG2 cells after rifampicin treatment is mediated by microRNA-379

microRNAs (miRNAs), which contribute to the post-transcriptional processing through 3'-untranslated region-interference, have been shown to be involved in the regulation of ATP-binding cassette (ABC) membrane transporters. The aim of this study was to investigate whether ABCC2, an important efflux transporter for various endogenous and exogenous compounds at several compartment barriers, is subject to miRNA-mediated post-transcriptional gene regulation. We screened the expression of 377 human miRNAs in HepG2 cells after 48 h of treatment with 5 μM rifampicin [a pregnane X receptor (PXR) ligand] or vehicle using reverse transcription-polymerase chain reaction-based low-density arrays. Specific miRNA, ABCC2 mRNA, and protein expression were monitored in HepG2 cells undergoing rifampicin treatment for 72 h. Loss- and gain-of-function experiments and reporter gene assays were performed for further confirmation. Highly deregulated miRNAs compared with in silico data revealed miRNA (miR) 379 as candidate miRNA targeting ABCC2 mRNA. Under rifampicin treatment, ABCC2 mRNA increased significantly, with a maximal fold change of 1.56 ± 0.43 after 24 h. In addition, miR-379 increased (maximally 4.10 ± 1.33-fold after 48 h), whereas ABCC2 protein decreased with a maximal fold change of 0.47 ± 0.08 after 72 h. In contrast, transfection of miR-379 inhibitor led to an elevation of ABCC2 protein expression after rifampicin incubation for 48 h. We identify a miRNA negatively regulating ABCC2 on the post-transcriptional level and provide evidence that this miRNA impedes overexpression of ABCC2 protein after a PXR-mediated external transcriptional stimulus in HepG2 cells.

The role of the sodium-taurocholate cotransporting polypeptide (NTCP) and of the bile salt export pump (BSEP) in physiology and pathophysiology of bile formation

Bile formation is an important function of the liver. Bile salts are a major constituent of bile and are secreted by hepatocytes into bile and delivered into the small intestine, where they assist in fat digestion. In the small intestine, bile salts are almost quantitatively reclaimed and transported back via the portal circulation to the liver. In the liver, hepatocytes take up bile salts and secrete them again into bile for ongoing enterohepatic circulation. Uptake of bile salts into hepatocytes occurs largely in a sodium-dependent manner by the sodium taurocholate cotransporting polypeptide NTCP. The transport properties of NTCP have been extensively characterized. It is an electrogenic member of the solute carrier family of transporters (SLC10A1) and transports predominantly bile salts and sulfated compounds, but is also able to mediate transport of additional substrates, such as thyroid hormones, drugs and toxins. It is highly regulated under physiologic and pathophysiologic conditions. Regulation of NTCP copes with changes of bile salt load to hepatocytes and prevents entry of cytotoxic bile salts during liver disease. Canalicular export of bile salts is mediated by the ATP-binding cassette transporter bile salt export pump BSEP (ABCB11). BSEP constitutes the rate limiting step of hepatocellular bile salt transport and drives enterohepatic circulation of bile salts. It is extensively regulated to keep intracellular bile salt levels low under normal and pathophysiologic situations. Mutations in the BSEP gene lead to severe progressive familial intrahepatic cholestasis. The substrates of BSEP are practically restricted to bile salts and their metabolites. It is, however, subject to inhibition by endogenous metabolites or by drugs. A sustained inhibition will lead to acquired cholestasis, which can end in liver injury.

Improved extrapolation of hepatobiliary clearance from in vitro sandwich cultured rat hepatocytes through absolute quantification of hepatobiliary transporters

Previously we have reported that hepatobiliary transporter expressions in sandwich cultured hepatocytes (SCH) are altered 2- to 5-fold. This change could limit the model's predictive power for in vivo biliary clearance. The present study was designed to better establish in vitro to in vivo correlation (IVIVC) of biliary clearance. Eleven compounds representing the substrates of Mrp2/Abcc2, Bcrp/Abcg2 and Bsep/Abcb11 were tested in the sandwich cultured rat hepatocyte (SCRH) model. Simultaneously, the absolute difference of hepatobiliary transporters between rat livers and SCRH at day 5 post culture was determined by LC-MS/MS. This difference was integrated into the well-stirred hepatic prediction model. A correction factor named "g_factor" was mathematically defined to reflect the difference in hepatobiliary transporter expressions between the SCRH model and in vivo models, as well as the contribution of multiple transporters. When the g_factor correction was applied, the in vivo biliary clearance prediction was significantly improved. In addition, for those compounds which are poorly permeable and/or undergo transporter-dependent active uptake, the known intracellular concentrations of substrates were used to estimate intrinsic bile clearance. This led to further improvement in the prediction of in vivo bile secretion. While the rate-limiting processes of uptake transporters in the SCRH model remain to be further determined, we showed that integration of the absolute difference of hepatobiliary transporter proteins and transport contributions could improve the predictability of SCRH model. This integration is fundamental for increased confidence in the IVIVC of human biliary clearance.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Dynamic localization of hepatocellular transporters in health and disease

Vesicle-based trafficking of hepatocellular transporters involves delivery of the newly-synthesized carriers from the rough endoplasmic reticulum to either the plasma membrane domain or to an endosomal, submembrane compartment, followed by exocytic targeting to the plasma membrane. Once delivered to the plasma membrane, the transporters usually undergo recycling between the plasma membrane and the endosomal compartment, which usually serves as a reservoir of pre-existing transporters available on demand. The balance between exocytic targeting and endocytic internalization from/to this recycling compartment is therefore a chief determinant of the overall capability of the liver epithelium to secrete bile and to detoxify endo and xenobiotics. Hence, it is a highly regulated process. Impaired regulation of this balance may lead to abnormal localization of these transporters, which results in bile secretory failure due to endocytic internalization of key transporters involved in bile formation. This occurs in several experimental models of hepatocellular cholestasis, and in most human cholestatic liver diseases. This review describes the molecular bases involved in the biology of the dynamic localization of hepatocellular transporters and its regulation, with a focus on the involvement of signaling pathways in this process. Their alterations in different experimental models of cholestasis and in human cholestatic liver disease are reviewed. In addition, the causes explaining the pathological condition (e.g. disorganization of actin or actin-transporter linkers) and the mediators involved (e.g. activation of cholestatic signaling transduction pathways) are also discussed. Finally, several experimental therapeutic approaches based upon the administration of compounds known to stimulate exocytic insertion of canalicular transporters (e.g. cAMP, tauroursodeoxycholate) are described.

Physiology of bile secretion

The formation of bile depends on the structural and functional integrity of the bile-secretory apparatus and its impairment, in different situations, results in the syndrome of cholestasis. The structural bases that permit bile secretion as well as various aspects related with its composition and flow rate in physiological conditions will first be reviewed. Canalicular bile is produced by polarized hepatocytes that hold transporters in their basolateral (sinusoidal) and apical (canalicular) plasma membrane. This review summarizes recent data on the molecular determinants of this primary bile formation. The major function of the biliary tree is modification of canalicular bile by secretory and reabsorptive processes in bile-duct epithelial cells (cholangiocytes) as bile passes through bile ducts. The mechanisms of fluid and solute transport in cholangiocytes will also be discussed. In contrast to hepatocytes where secretion is constant and poorly controlled, cholangiocyte secretion is regulated by hormones and nerves. A short section dedicated to these regulatory mechanisms of bile secretion has been included. The aim of this revision was to set the bases for other reviews in this series that will be devoted to specific issues related with biliary physiology and pathology.

Analysis of ABCG2 expression and side population identifies intrinsic drug efflux in the HCC cell line MHCC-97L and its modulation by Akt signaling

Active drug efflux by the adenosine triphosphate-binding cassette (ABC) transporter ABCG2 is one of the common mechanisms causing multiple drug resistance in various human cancers. In the intrinsic drug resistance of hepatocellular carcinoma (HCC), the role of ABCG2 is closely associated with 'side population (SP)', a minor subset of cancer stem-like cells with unique capacity to extrude lipophilic dye Hoechst 33342 and many chemotherapeutic agents. In this study, we showed that ABCG2 was intrinsically expressed in a subgroup of HCC tissues and its expression pattern significantly influenced the levels of drug efflux from HCC cell lines. In MHCC-97L HCC cell line with intrinsic ABCG2 expression, we confirmed the importance of SP cells to the drug efflux-related chemotherapy resistance and found that the SP analysis provided an efficient method to evaluate the functional activity of ABCG2 transporter. In this cell line, we discovered that the SP proportion was modulated by the treatments of Akt signaling inhibitors and serum supplement, which led to the finding that Akt signaling was able to regulate the SP cells' efflux activity via altering the subcellular localization of ABCG2 transporter. We further demonstrated that the Akt signaling inhibition attenuated the doxorubicin efflux from MHCC-97L cells and increased the drug efficacy. Our results indicate the protective role of intrinsic ABCG2 expression in HCC cells and suggest that suppressing Akt signaling could help overcome the drug efflux by ABCG2 transporter.

Which in vitro models could be best used to study hepatocyte polarity?

The correct functioning of the liver is ensured by the setting and the maintenance of hepatocyte polarity. The complex polarity of the hepatocyte is characterized by the existence of several basolateral and apical poles per cell. Many in vitro models are available for studying hepatocyte polarity, but which are the more suitable? To answer this question, we aimed to identify criteria which determine the typical hepatocyte polarity. Therefore, we compiled a range of protein markers of membrane domains in rat hepatocytes and investigated their involvement in hepatocytic functions. Then, we focused on the relationship between hepatic functions and the cytoskeleton, Golgi apparatus and endoplasmic reticulum. Subsequently, we compared different cell lines expressing hepatocyte polarity. Finally, to demonstrate the usefulness of some of these lines, we presented new data on endoplasmic reticulum organization in relation to polarity.

Modulation of P-glycoprotein at the blood-brain barrier: opportunities to improve central nervous system pharmacotherapy

Pharmacotherapy of central nervous system (CNS) disorders (e.g., neurodegenerative diseases, epilepsy, brain cancer, and neuro-AIDS) is limited by the blood-brain barrier. P-glycoprotein, an ATP-driven, drug efflux transporter, is a critical element of that barrier. High level of expression, luminal membrane location, multispecificity, and high transport potency make P-glycoprotein a selective gatekeeper of the blood-brain barrier and thus a primary obstacle to drug delivery into the brain. As such, P-glycoprotein limits entry into the CNS for a large number of prescribed drugs, contributes to the poor success rate of CNS drug candidates, and probably contributes to patient-to-patient variability in response to CNS pharmacotherapy. Modulating P-glycoprotein could therefore improve drug delivery into the brain. Here we review the current understanding of signaling mechanisms responsible for the modulation of P-glycoprotein activity/expression at the blood-brain barrier with an emphasis on recent studies from our laboratories. Using intact brain capillaries from rats and mice, we have identified multiple extracellular and intracellular signals that regulate this transporter; several signaling pathways have been mapped. Three pathways are triggered by elements of the brain's innate immune response, one by glutamate, one by xenobiotic-nuclear receptor (pregnane X receptor) interactions, and one by elevated beta-amyloid levels. Signaling is complex, with several pathways sharing common signaling elements [tumor necrosis factor (TNF) receptor 1, endothelin (ET) B receptor, protein kinase C, and nitric-oxide synthase), suggesting a regulatory network. Several pathways include autocrine/paracrine elements, involving release of the proinflammatory cytokine, TNF-alpha, and the polypeptide hormone, ET-1. Finally, several steps in signaling are potential therapeutic targets that could be used to modulate P-glycoprotein activity in the clinic.

Adaptive responses of renal organic anion transporter 3 (OAT3) during cholestasis

During cholestasis, bile acids are mainly excreted into the urine, but adaptive renal responses to cholestasis, especially molecular mechanisms for renal secretion of bile acids, have not been well understood. Organic anion transporters (OAT1 and OAT3) are responsible for membrane transport of anionic compounds at the renal basolateral membranes. In the present study, we investigated the pathophysiological roles of OAT1 and OAT3 in terms of renal handling of bile acids. The Eisai hyperbilirubinemic rats (EHBR), mutant rats without multidrug resistance-associated protein 2, showed higher serum and urinary concentrations of bile acids, compared with Sprague-Dawley (SD) rats (wild type). The protein expression level of rat OAT3 was significantly increased in EHBR compared with SD rats, whereas the expression of rat OAT1 was unchanged. The transport activities of rat and human OAT3, but not OAT1, were markedly inhibited by various bile acids such as chenodeoxycholic acid and cholic acid. Cholic acid, glycocholic acid, and taurocholic acid, which mainly increased during cholestasis, are transported by OAT3. The plasma concentration of beta-lactam antibiotic cefotiam, a specific substrate for OAT3, was more increased in EHBR than in SD rats despite upregulation of OAT3 protein. This may be due to the competitive inhibition of cefotiam transport by bile acids via OAT3. In conclusion, the present study clearly demonstrated that OAT3 is responsible for renal secretion of bile acids during cholestasis and that the pharmacokinetic profile of OAT3 substrates may be affected by cholestasis.

Ezrin and radixin both regulate the apical membrane localization of ABCC2 (MRP2) in human intestinal epithelial Caco-2 cells

Multidrug resistance-associated protein ABCC2 (MRP2) is widely expressed in mammalian tissues including intestine, liver and kidney, and it has been shown to be located exclusively on the apical membrane of polarized cells. Recently, several reports suggest that apical membrane localization of ABCC2 (Mrp2) was regulated by radixin in rodent liver. To investigate the mechanism underlying this apical membrane targeting of MRP2 in human intestine, we chose Caco-2 cells as a model to examine the unique roles of ezrin and radixin. Following immunostaining, radixin and ezrin were found to be concentrated at the apical membrane of Caco-2 cells. Using the RNAi method, radixin and ezrin stable knockdown Caco-2 cells were constructed. A cell surface biotinylation experiment with radixin or ezrin stable knockdown Caco-2 cells showed that radixin or ezrin deficiency caused the loss of ABCC2 (MRP2) from the cell surface. An immunoprecipitation assay showed that radixin and ezrin were associated with ABCC2 (MRP2). These findings indicate that both ezrin and radixin are independently required for the apical membrane localization of ABCC2 (MRP2) in Caco-2 cells. Radixin and ezrin play similar roles in the apical membrane localization of ABCC2 (MRP2) and their expression level and subcellular distribution are important factors in the regulation of ABCC2 (MRP2) at the post-transcriptional level.

Trafficking of the copper-ATPases, ATP7A and ATP7B: Role in copper homeostasis

Copper is essential for human health and copper imbalance is a key factor in the aetiology and pathology of several neurodegenerative diseases. The copper-transporting P-type ATPases, ATP7A and ATP7B are key molecules required for the regulation and maintenance of mammalian copper homeostasis. Their absence or malfunction leads to the genetically inherited disorders, Menkes and Wilson diseases, respectively. These proteins have a dual role in cells, namely to provide copper to essential cuproenzymes and to mediate the excretion of excess intracellular copper. A unique feature of ATP7A and ATP7B that is integral to these functions is their ability to sense and respond to intracellular copper levels, the latter manifested through their copper-regulated trafficking from the transGolgi network to the appropriate cellular membrane domain (basolateral or apical, respectively) to eliminate excess copper from the cell. Research over the last decade has yielded significant insight into the enzymatic properties and cell biology of the copper-ATPases. With recent advances in elucidating their localization and trafficking in human and animal tissues in response to physiological stimuli, we are progressing rapidly towards an integrated understanding of their physiological significance at the level of the whole animal. This knowledge in turn is helping to clarify the biochemical and cellular basis not only for the phenotypes conferred by individual Menkes and Wilson disease patient mutations, but also for the clinical variability of phenotypes associated with each of these diseases. Importantly, this information is also providing a rational basis for the applicability and appropriateness of certain diagnostic markers and therapeutic regimes. This overview will provide an update on the current state of our understanding of the localization and trafficking properties of the copper-ATPases in cells and tissues, the molecular signals and posttranslational interactions that govern their trafficking activities, and the cellular basis for the clinical phenotypes associated with disease-causing mutations.

Cell polarity development and protein trafficking in hepatocytes lacking E-cadherin/beta-catenin-based adherens junctions

Using a mutant hepatocyte cell line in which E-cadherin and beta-catenin are completely depleted from the cell surface, and, consequently, fail to form adherens junctions, we have investigated adherens junction requirement for apical-basolateral polarity development and polarized membrane trafficking. It is shown that these hepatocytes retain the capacity to form functional tight junctions, develop full apical-basolateral cell polarity, and assemble a subapical cortical F-actin network, although with a noted delay and a defect in subsequent apical lumen remodeling. Interestingly, whereas hepatocytes typically target the plasma membrane protein dipeptidyl peptidase IV first to the basolateral surface, followed by its transcytosis to the apical domain, hepatocytes lacking E-cadherin-based adherens junctions target dipeptidyl peptidase IV directly to the apical surface. Basolateral surface-directed transport of other proteins or lipids tested was not visibly affected in hepatocytes lacking E-cadherin-based adherens junctions. Together, our data show that E-cadherin/beta-catenin-based adherens junctions are dispensable for tight junction formation and apical lumen biogenesis but not for apical lumen remodeling. In addition, we suggest a possible requirement for E-cadherin/beta-catenin-based adherens junctions with regard to the indirect apical trafficking of specific proteins in hepatocytes.