ATP-binding cassette transporter isoform C2 localizes to the apical plasma membrane via interactions with scaffolding protein

Oligomerization of drug transporters: Forms, functions, and mechanisms

Drug transporters are essential players in the transmembrane transport of a wide variety of clinical drugs. The broad substrate spectra and versatile distribution pattern of these membrane proteins infer their pharmacological and clinical significance. With our accumulating knowledge on the three-dimensional structure of drug transporters, their oligomerization status has become a topic of intense study due to the possible functional roles carried out by such kind of post-translational modification (PTM). In-depth studies of oligomeric complexes formed among drug transporters as well as their interactions with other regulatory proteins can help us better understand the regulatory mechanisms of these membrane proteins, provide clues for the development of novel drugs, and improve the therapeutic efficacy. In this review, we describe different oligomerization forms as well as their structural basis of major drug transporters in the ATP-binding cassette and solute carrier superfamilies, summarize our current knowledge on the influence of oligomerization for protein expression level and transport function of these membrane proteins, and discuss the regulatory mechanisms of oligomerization. Finally, we highlight the challenges associated with the current oligomerization studies and propose some thoughts on the pharmaceutical application of this important drug transporter PTM.

A conserved WXXE motif is an apical delivery determinant of ABC transporter C subfamily isoforms

ATP-binding cassette transporter isoform C7 (ABCC7), also designated as cystic fibrosis transmembrane conductance regulator (CFTR), is exclusively targeted to the apical plasma membrane of polarized epithelial cells. Although the apical localization of ABCC7 in epithelia is crucial for the Cl- excretion into lumens, the mechanism regulating its apical localization is poorly understood. In the present study, an apical localization determinant was identified in the N-terminal 80-amino acid long cytoplasmic region of ABCC7 (NT80). In HepG2 cells, overexpression of NT80 significantly disturbed the apical expression of ABCC7 in a competitive manner, suggesting the presence of a sorting determinant in this region. Deletion analysis identified a potential sorting information within a 20-amino acid long peptide (aa 41-60) of NT80. Alanine scanning mutagenesis of this region in full-length ABCC7 further narrowed down the apical localization determinant to four amino acids, W57DRE60. This WDRE sequence was conserved among vertebrate ABCC7 orthologs. Site-directed mutagenesis showed that W57 and E60 were critical for the apical expression of ABCC7, confirming a novel apical sorting determinant of ABCC7. Furthermore, a WXXE motif (tryptophan and glutamic acid residues with two-amino acid spacing) was found to be conserved among the N-terminal regions of apically localized ABCC members with 12-TM configuration. The significance of the WXXE motif was demonstrated for proper trafficking of ABCC4 to the apical plasma membrane.Key words: apical plasma membrane, sorting, ATP-binding cassette transporter, CFTR, MRP4.

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.

Molecular Regulation of Canalicular ABC Transporters

The ATP-binding cassette (ABC) transporters expressed at the canalicular membrane of hepatocytes mediate the secretion of several compounds into the bile canaliculi and therefore play a key role in bile secretion. Among these transporters, ABCB11 secretes bile acids, ABCB4 translocates phosphatidylcholine and ABCG5/G8 is responsible for cholesterol secretion, while ABCB1 and ABCC2 transport a variety of drugs and other compounds. The dysfunction of these transporters leads to severe, rare, evolutionary biliary diseases. The development of new therapies for patients with these diseases requires a deep understanding of the biology of these transporters. In this review, we report the current knowledge regarding the regulation of canalicular ABC transporters' folding, trafficking, membrane stability and function, and we highlight the role of molecular partners in these regulating mechanisms.

The nuclear receptors PXR and LXR are regulators of the scaffold protein PDZK1

PDZK1 (NHERF3) interacts with membrane proteins whereby modulating their spatial arrangement, membrane stability, and function. One of the membrane proteins shown to be stabilized by interaction with PDZK1 is the HDL-receptor SR-BI (SCARB1). Testing the influence of TO 901317, a known activator of liver X receptor alpha (LXRα, NR1H3) which is a central regulator of the lipid homeostasis, Grefhorst et al. reported in 2012 that administration of TO 901317 did not affect PDZK1 expression and reduced the amount of SR-BI protein in mouse liver. Considering that TO 901317 also activates the xenosensor pregnane X receptor (PXR, NR1I2), it was aim of this study to further investigate the influence of LXRα and PXR activation on transcription of PDZK1. First, we tested the transactivation of PDZK1 by LXRα or PXR in cell-based reporter gene assays comparing the effect of prototypical ligands to that of TO 901317. Ligand mediated activation of LXRα increased, while that of PXR lowered luciferase activity. Further, we located the most likely binding site for LXRα and PXR on the PDZK1 promoter between -85 bp and -54 bp. The transcriptional regulation by LXRα was further supported showing enhanced mRNA expression of PDZK1 in HepG2 cells treated with the selective LXRα-agonist GW3965, while treatment with TO 901317 reduced the protein amount of PDZK1. Taken together, we provide evidence that both LXRα and PXR are transcriptional regulators of PDZK1 supporting the previous notion that the scaffold protein is part of cholesterol homeostasis and drug metabolism.

Beyond Competitive Inhibition: Regulation of ABC Transporters by Kinases and Protein-Protein Interactions as Potential Mechanisms of Drug-Drug Interactions

ATP-binding cassette (ABC) transporters are transmembrane efflux transporters mediating the extrusion of an array of substrates ranging from amino acids and lipids to xenobiotics, and many therapeutic compounds, including anticancer drugs. The ABC transporters are also recognized as important contributors to pharmacokinetics, especially in drug-drug interactions and adverse drug effects. Drugs and xenobiotics, as well as pathologic conditions, can influence the transcription of ABC transporters, or modify their activity or intracellular localization. Kinases can affect the aforementioned processes for ABC transporters as do protein interactions. In this review, we focus on the ABC transporters ABCB1, ABCB11, ABCC1, ABCC4, and ABCG2 and illustrate how kinases and protein-protein interactions affect these transporters. The clinical relevance of these factors is currently unknown; however, these examples suggest that our understanding of drug-drug interactions will benefit from further knowledge of how kinases and protein-protein interactions affect ABC transporters.

Biochemical studies on the structure-function relationship of major drug transporters in the ATP-binding cassette family and solute carrier family

Human drug transporters often play key roles in determining drug accumulation within cells. Their activities are often directly related to therapeutic efficacy, drug toxicity as well as drug-drug interactions. However, the progress for interpretation of their crystal structures is relatively slow. Hence, conventional biochemical studies together with computer modeling became useful manners to reveal essential structures of these membrane proteins. Over the years, quite a few structure-function relationship information had been obtained for members of the two major transporter families: the ATP-binding cassette family and the solute carrier family. Critical structural features of drug transporters include transmembrane domains, post-translational modification sites and domains for cell surface assembly and protein-protein interactions. Alterations at these important sites may affect protein stability, trafficking to the plasma membrane and/or ability of transporters to interact with substrates.

Upregulation of PDZK1 by Calculus Bovis Sativus May Play an Important Role in Restoring Biliary Transport Function in Intrahepatic Cholestasis

Intrahepatic cholestasis is a main cause of hepatic accumulation of bile acids leading to liver injury, fibrosis, and liver failure. Our previous studies proved that Calculus Bovis Sativus (CBS) can restore biliary transport function through upregulating the multidrug resistance-associated protein 2 (MRP2) and breast cancer resistance protein (BCRP) in 17α-ethynylestradiol- (EE-) induced intrahepatic cholestasis rats. The regulation mechanism of CBS on these transporters, however, remains unclear. This study was designed to evaluate the possible relationship between the effect of CBS on transport activities and the regulation of CBS on the expression of PDZK1, a mainly scaffold protein which can regulate MRP2 and BCRP. Intrahepatic cholestasis model was induced in rats with injection of EE for five consecutive days and then the biliary excretion rates and cumulative biliary excretions were measured. The mRNA and protein expression levels of PDZK1 were detected by reverse transcription-quantitative real-time polymerase chain reaction, western blot, and immunohistochemical analysis. When treated with CBS, cumulative biliary excretions and mRNA and protein expressions of PDZK1 were significantly increased in intrahepatic cholestasis rats. This study demonstrated that CBS exerted a beneficial effect on EE-induced intrahepatic cholestasis rats by restoring biliary transport function, which may result from the upregulation of PDZK1 expression.

Regulation of the Axillary Osmidrosis-Associated ABCC11 Protein Stability by N-Linked Glycosylation: Effect of Glucose Condition

ATP-binding cassette C11 (ABCC11) is a plasma membrane protein involved in the transport of a variety of lipophilic anions. ABCC11 wild-type is responsible for the high-secretion phenotypes in human apocrine glands, such as that of wet-type ear wax, and the risk of axillary osmidrosis. We have previously reported that mature ABCC11 is a glycoprotein containing two N-linked glycans at Asn838 and Asn844. However, little is known about the role of N-linked glycosylation in the regulation of ABCC11 protein. In the current study, we investigated the effects of N-linked glycosylation on the protein level and localization of ABCC11 using polarized Madin-Darby canine kidney II cells. When the N-linked glycosylation in ABCC11-expressing cells was chemically inhibited by tunicamycin treatment, the maturation of ABCC11 was suppressed and its protein level was significantly decreased. Immunoblotting analyses demonstrated that the protein level of the N-linked glycosylation-deficient mutant (N838Q and N844Q: Q838/844) was about half of the ABCC11 wild-type level. Further biochemical studies with the Q838/844 mutant showed that this glycosylation-deficient ABCC11 was degraded faster than wild-type probably due to the enhancement of the MG132-sensitive protein degradation pathway. Moreover, the incubation of ABCC11 wild-type-expressing cells in a low-glucose condition decreased mature, glycosylated ABCC11, compared with the high-glucose condition. On the other hand, the protein level of the Q838/844 mutant was not affected by glucose condition. These results suggest that N-linked glycosylation is important for the protein stability of ABCC11, and physiological alteration in glucose may affect the ABCC11 protein level and ABCC11-related phenotypes in humans, such as axillary osmidrosis.

A PDZ-Like Motif in the Biliary Transporter ABCB4 Interacts with the Scaffold Protein EBP50 and Regulates ABCB4 Cell Surface Expression

ABCB4/MDR3, a member of the ABC superfamily, is an ATP-dependent phosphatidylcholine translocator expressed at the canalicular membrane of hepatocytes. Defects in the ABCB4 gene are associated with rare biliary diseases. It is essential to understand the mechanisms of its canalicular membrane expression in particular for the development of new therapies. The stability of several ABC transporters is regulated through their binding to PDZ (PSD95/DglA/ZO-1) domain-containing proteins. ABCB4 protein ends by the sequence glutamine-asparagine-leucine (QNL), which shows some similarity to PDZ-binding motifs. The aim of our study was to assess the potential role of the QNL motif on the surface expression of ABCB4 and to determine if PDZ domain-containing proteins are involved. We found that truncation of the QNL motif decreased the stability of ABCB4 in HepG2-transfected cells. The deleted mutant ABCB4-ΔQNL also displayed accelerated endocytosis. EBP50, a PDZ protein highly expressed in the liver, strongly colocalized and coimmunoprecipitated with ABCB4, and this interaction required the QNL motif. Down-regulation of EBP50 by siRNA or by expression of an EBP50 dominant-negative mutant caused a significant decrease in the level of ABCB4 protein expression, and in the amount of ABCB4 localized at the canalicular membrane. Interaction of ABCB4 with EBP50 through its PDZ-like motif plays a critical role in the regulation of ABCB4 expression and stability at the canalicular plasma membrane.

Endocytosis of ABCG2 drug transporter caused by binding of 5D3 antibody: trafficking mechanisms and intracellular fate

ABCG2, a metabolite and xenobiotic transporter located at the plasma membrane (predominantly in barrier tissues and progenitor cells), undergoes a direct progressive endocytosis process from plasma membrane to intracellular compartments upon binding of 5D3 monoclonal antibody. This antibody is specific to an external epitope on the protein molecule and locks it in a discrete conformation within its activity cycle, presumably providing a structural trigger for the observed internalization phenomenon. Using routine and novel assays, we show that ABCG2 is endocytosed by a mixed mechanism: partially via a rapid, clathrin-dependent pathway and partially in a cholesterol-dependent, caveolin-independent manner. While the internalization process is entirely dynamin-dependent and converges initially at the early endosome, subsequent intracellular fate of ABCG2 is again twofold: endocytosis leads to only partial lysosomal degradation, while a significant fraction of the protein is retained in a post-endosomal compartment with the possibility of at least partial recycling back to the cell surface. This externally triggered, conformation-related trafficking pathway may serve as a general regulatory paradigm for membrane transporters, and its discovery was made possible thanks to consistent application of quantitative methods.

Distribution dynamics and functional importance of NHERF1 in regulation of Mrp-2 trafficking in hepatocytes

Na(+)/H(+) exchanger regulatory factor 1 (NHERF1) is a multifunctional scaffolding protein that interacts with receptors and ion transporters in its PDZ domains and with the ezrin-radixin-moesin (ERM) family of proteins in its COOH terminus. The role of NHERF1 in hepatocyte function remains largely unknown. We examine the distribution and physiological significance of NHERF1 and multidrug resistance-associated protein 2 (Mrp-2) in hepatocytes. A WT radixin binding site mutant (F355R) and NHERF1 PDZ1 and PDZ2 domain adenoviral mutant constructs were tagged with yellow fluorescent protein and expressed in polarized hepatocytes to study localization and function of NHERF1. Cellular distribution of NHERF1 and radixin was visualized by fluorescence microscopy. A 5-chloromethylfluorescein diacetate (CMFDA) assay was used to characterize Mrp-2 function. Similar to Mrp-2, WT NHERF1 and the NHERF1 PDZ2 deletion mutant were localized to the canalicular membrane. In contrast, the radixin binding site mutant (F355R) and the NHERF1 PDZ1 deletion mutant, which interacts poorly with Mrp-2, were rarely associated with the canalicular membrane. Knockdown of NHERF1 led to dramatically impaired CMFDA secretory response. Use of CMFDA showed that the NHERF1 PDZ1 and F355R mutants were devoid of a secretory response, while WT NHERF1-infected cells exhibited increased secretion of glutathione-methylfluorescein. The data indicate that NHERF1 interacts with Mrp-2 via the PDZ1 domain of NHERF1 and, furthermore, that NHERF1 is essential for maintaining the localization and function of Mrp-2.

Regulation of ABCC6 trafficking and stability by a conserved C-terminal PDZ-like sequence

Mutations in the ABCC6 ABC-transporter are causative of pseudoxanthoma elasticum (PXE). The loss of functional ABCC6 protein in the basolateral membrane of the kidney and liver is putatively associated with altered secretion of a circulatory factor. As a result, systemic changes in elastic tissues are caused by progressive mineralization and degradation of elastic fibers. Premature arteriosclerosis, loss of skin and vascular tone, and a progressive loss of vision result from this ectopic mineralization. However, the identity of the circulatory factor and the specific role of ABCC6 in disease pathophysiology are not known. Though recessive loss-of-function alleles are associated with alterations in ABCC6 expression and function, the molecular pathologies associated with the majority of PXE-causing mutations are also not known. Sequence analysis of orthologous ABCC6 proteins indicates the C-terminal sequences are highly conserved and share high similarity to the PDZ sequences found in other ABCC subfamily members. Genetic testing of PXE patients suggests that at least one disease-causing mutation is located in a PDZ-like sequence at the extreme C-terminus of the ABCC6 protein. To evaluate the role of this C-terminal sequence in the biosynthesis and trafficking of ABCC6, a series of mutations were utilized to probe changes in ABCC6 biosynthesis, membrane stability and turnover. Removal of this PDZ-like sequence resulted in decreased steady-state ABCC6 levels, decreased cell surface expression and stability, and mislocalization of the ABCC6 protein in polarized cells. These data suggest that the conserved, PDZ-like sequence promotes the proper biosynthesis and trafficking of the ABCC6 protein.

Involvement of a di-leucine motif in targeting of ABCC1 to the basolateral plasma membrane of polarized epithelial cells

Localization of ATP-binding cassette transporter isoform C1 (ABCC1) to the basolateral membrane of polarized cells is crucial for export of a variety of cellular metabolites; however, the mechanism regulating basolateral targeting of the transporter is poorly understood. Here we describe identification of a basolateral targeting signal in the first cytoplasmic loop domain (CLD1) of human ABCC1. Comparison of the CLD1 amino acid sequences from ABCC1 to ABCC2 revealed that ABCC1 possesses a characteristic sequence, E(295)EVEALI(301), which is comprised of a cluster of acidic glutamate residues followed by a di-leucine motif. This characteristic sequence is highly conserved among vertebrate ABCC1 orthologs and is positioned at a site that is structurally equivalent to the apical targeting signal previously described in ABCC2. Alanine scanning mutagenesis of this sequence in full-length human ABCC1 showed that both L(300) and I(301) residues were required for basolateral targeting of ABCC1 in polarized HepG2 and MDCK cells. Conversely, E(295), E(296), and E(298) residues were not required for basolateral localization of the transporter. Therefore, a di-leucine motif within the CLD1 is a basolateral targeting determinant of ABCC1.

A cis-acting five-amino-acid motif controls targeting of ABCC2 to the apical plasma membrane domain

ATP-binding cassette transporter isoform C2 (ABCC2) is exclusively targeted to the apical plasma membrane of polarized cells. Although apical localization of ABCC2 in hepatocytes is crucial for the biliary excretion of a variety of metabolites, the mechanism regulating its apical targeting is poorly understood. In the present study, an apical targeting signal was identified in the first cytoplasmic loop domain (CLD1) of ABCC2 in HepG2 cells. Overexpression of CLD1 significantly disturbed the apical targeting of FLAG-ABCC2 in a competitive manner, suggesting the presence of a saturable sorting machinery in HepG2 cells. Next, deletion analysis identified a potential targeting sequence within a 20-amino-acid long peptide (aa 272-291) of CLD1. Alanine scanning mutagenesis of this region in full-length ABCC2 further narrowed down the apical targeting determinant to five amino acids, S(283)QDAL(287). Of these, S(283) and L(287) were found to be conserved among vertebrate ABCC2 orthologs. Site-directed mutagenesis showed that both S(283) and L(287) were crucial for the targeting specificity of ABCC2. Introducing this apical targeting sequence into the corresponding region of ABCC1, an exclusively basolateral protein, caused the hybrid ABCC1 to partially localize in the apical membrane. Thus, the CLD1 of ABCC2 contains a novel apical sorting determinant, and a saturable sorting machinery is present in polarized HepG2 cells.

PDZ adaptors: their regulation of epithelial transporters and involvement in human diseases

Homeostasis in the body is at least partially maintained by mechanisms that control membrane permeability, and thereby serve to control the uptake of essential substances (e.g., nutrients) and the efflux of unwanted substances (e.g., xenobiotics and metabolites) in epithelial cells. Various transporters play fundamental roles in such bidirectional transport, but little is known about how they are organized on plasma membranes. Protein-protein interactions may play a key role: several transporters in epithelial cells interact with the so-called adaptor proteins, which are membrane anchored and interact with both transporters and other membranous proteins. Although most of the evidences for transporter-adaptor interaction has been obtained in vitro, recent studies suggest that adaptor-mediated transporter regulation does occur in vivo and could be relevant to human diseases. Thus, protein-protein interaction is not only associated with the formation of macromolecular complexes but is also involved in various cellular events, and may provide transporters with additional functionality by forming transporter networks on plasma membranes. Interactions between xenobiotic transporters and PSD95/Dlg/ZO1 (PDZ) adaptors were previously reviewed by Kato and Tsuji (2006. Eur J Pharm Sci 27:487-500); the present review focuses on the latest findings about PDZ adaptors as regulators of transporter networks and their potential role in human diseases.