Mutational analysis of a highly conserved proline residue in MRP1, MRP2, and MRP3 reveals a partially conserved function

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.

Pharmacogenomics of Drug Metabolizing Enzymes and Transporters: Relevance to Precision Medicine

The interindividual genetic variations in drug metabolizing enzymes and transporters influence the efficacy and toxicity of numerous drugs. As a fundamental element in precision medicine, pharmacogenomics, the study of responses of individuals to medication based on their genomic information, enables the evaluation of some specific genetic variants responsible for an individual’s particular drug response. In this article, we review the contributions of genetic polymorphisms to major individual variations in drug pharmacotherapy, focusing specifically on the pharmacogenomics of phase-I drug metabolizing enzymes and transporters. Substantial frequency differences in key variants of drug metabolizing enzymes and transporters, as well as their possible functional consequences, have also been discussed across geographic regions. The current effort illustrates the common presence of variability in drug responses among individuals and across all geographic regions. This information will aid health-care professionals in prescribing the most appropriate treatment aimed at achieving the best possible beneficial outcomes while avoiding unwanted effects for a particular patient.

Multidrug resistance protein 1 (MRP1, ABCC1), a "multitasking" ATP-binding cassette (ABC) transporter

The multidrug resistance protein 1 (MRP1) encoded by ABCC1 was originally discovered as a cause of multidrug resistance in tumor cells. However, it is now clear that MRP1 serves a broader role than simply mediating the ATP-dependent efflux of drugs from cells. The antioxidant GSH and the pro-inflammatory cysteinyl leukotriene C4 have been identified as key physiological organic anions effluxed by MRP1, and an ever growing body of evidence indicates that additional lipid-derived mediators are also substrates of this transporter. As such, MRP1 is a multitasking transporter that likely influences the etiology and progression of a host of human diseases.

Chalcogenopyrylium dyes as differential modulators of organic anion transport by multidrug resistance protein 1 (MRP1), MRP2, and MRP4

Multidrug resistance proteins (MRPs) mediate the ATP-dependent efflux of structurally diverse compounds, including anticancer drugs and physiologic organic anions. Five classes of chalcogenopyrylium dyes (CGPs) were examined for their ability to modulate transport of [(3)H]estradiol glucuronide (E(2)17βG; a prototypical MRP substrate) into MRP-enriched inside-out membrane vesicles. Additionally, some CGPs were tested in intact transfected cells using a calcein efflux assay. Sixteen of 34 CGPs inhibited MRP1-mediated E(2)17βG uptake by >50% (IC50 values: 0.7-7.6 µM). Of 9 CGPs with IC50 values ≤2 µM, two belonged to class I, two to class III, and five to class V. When tested in the intact cells, only 4 of 16 CGPs (at 10 µM) inhibited MRP1-mediated calcein efflux by >50% (III-1, V-3, V-4, V-6), whereas a fifth (I-5) inhibited efflux by just 23%. These five CGPs also inhibited [(3)H]E(2)17βG uptake by MRP4. In contrast, their effects on MRP2 varied, with two (V-4, V-6) inhibiting E(2)17βG transport (IC(50) values: 2.0 and 9.2 µM) and two (V-3, III-1) stimulating transport (>2-fold), whereas CGP I-5 had no effect. Strikingly, although V-3 and V-4 had opposite effects on MRP2 activity, they are structurally identical except for their chalcogen atom (Se versus Te). This study is the first to identify class V CGPs, with their distinctive methine or trimethine linkage between two disubstituted pyrylium moieties, as a particularly potent class of MRP modulators, and to show that, within this core structure, differences in the electronegativity associated with a chalcogen atom can be the sole determinant of whether a compound will stimulate or inhibit MRP2.

Mammalian multidrug-resistance proteins (MRPs)

Subfamily C of the human ABC (ATP-binding cassette) superfamily contains nine proteins that are often referred to as the MRPs (multidrug-resistance proteins). The 'short' MRP/ABCC transporters (MRP4, MRP5, MRP8 and ABCC12) have a typical ABC structure with four domains comprising two membrane-spanning domains (MSD1 and MSD2) each followed by a nucleotide-binding domain (NBD1 and NBD2). The 'long' MRP/ABCCs (MRP1, MRP2, MRP3, ABCC6 and MRP7) have five domains with the extra domain, MSD0, at the N-terminus. The proteins encoded by the ABCC6 and ABCC12 genes are not known to transport drugs and are therefore referred to as ABCC6 and ABCC12 (rather than MRP6 and MRP9) respectively. A large number of molecules are transported across the plasma membrane by the MRPs. Many are organic anions derived from exogenous sources such as conjugated drug metabolites. Others are endogenous metabolites such as the cysteinyl leukotrienes and prostaglandins which have important signalling functions in the cell. Some MRPs share a degree of overlap in substrate specificity (at least in vitro), but differences in transport kinetics are often substantial. In some cases, the in vivo substrates for some MRPs have been discovered aided by studies in gene-knockout mice. However, the molecules that are transported in vivo by others, including MRP5, MRP7, ABCC6 and ABCC12, still remain unknown. Important differences in the tissue distribution of the MRPs and their membrane localization (apical in contrast with basolateral) in polarized cells also exist. Together, these differences are responsible for the unique pharmacological and physiological functions of each of the nine ABCC transporters known as the MRPs.

ABCC2/Abcc2: a multispecific transporter with dominant excretory functions

ABCC2/Abcc2 (MRP2/Mrp2) is expressed at major physiological barriers, such as the canalicular membrane of liver cells, kidney proximal tubule epithelial cells, enterocytes of the small and large intestine, and syncytiotrophoblast of the placenta. ABCC2/Abcc2 always localizes in the apical membranes. Although ABCC2/Abcc2 transports a variety of amphiphilic anions that belong to different classes of molecules, such as endogenous compounds (e.g., bilirubin-glucuronides), drugs, toxic chemicals, nutraceuticals, and their conjugates, it displays a preference for phase II conjugates. Phenotypically, the most obvious consequence of mutations in ABCC2 that lead to Dubin-Johnson syndrome is conjugate hyperbilirubinemia. ABCC2/Abcc2 harbors multiple binding sites and displays complex transport kinetics.

Identification of novel specific and general inhibitors of the three major human ATP-binding cassette transporters P-gp, BCRP and MRP2 among registered drugs

PURPOSE

To study the inhibition patterns of the three major human ABC transporters P-gp (ABCB1), BCRP (ABCG2) and MRP2 (ABCC2), using a dataset of 122 structurally diverse drugs.

METHODS

Inhibition was investigated in cellular and vesicular systems over-expressing single transporters. Computational models discriminating either single or general inhibitors from non-inhibitors were developed using multivariate statistics.

RESULTS

Specific (n = 23) and overlapping (n = 19) inhibitors of the three ABC transporters were identified. GF120918 and Ko143 were verified to specifically inhibit P-gp/BCRP and BCRP in defined concentration intervals, whereas the MRP inhibitor MK571 was revealed to inhibit all three transporters within one log unit of concentration. Virtual docking experiments showed that MK571 binds to the ATP catalytic site, which could contribute to its multi-specific inhibition profile. A computational model predicting general ABC inhibition correctly classified 80% of both ABC transporter inhibitors and non-inhibitors in an external test set.

CONCLUSIONS

The inhibitor specificities of P-gp, BCRP and MRP2 were shown to be highly overlapping. General ABC inhibitors were more lipophilic and aromatic than specific inhibitors and non-inhibitors. The identified specific inhibitors can be used to delineate transport processes in complex experimental systems, whereas the multi-specific inhibitors are useful in primary ABC transporter screening in drug discovery settings.

Pharmacogenomics of MRP transporters (ABCC1-5) and BCRP (ABCG2)

Elucidation of the key mechanisms that confer interindividual differences in drug response remains an important focus of drug disposition and clinical pharmacology research. We now know both environmental and host genetic factors contribute to the apparent variability in drug efficacy or in some cases, toxicity. In addition to the widely studied and recognized genes involved in the metabolism of drugs in clinical use today, we now recognize that membrane-bound proteins, broadly referred to as transporters, may be equally as important to the disposition of a substrate drug, and that genetic variation in drug transporter genes may be a major contributor of the apparent intersubject variation in drug response, both in terms of attained plasma and tissue drug level at target sites of action. Of particular relevance to drug disposition are members of the ATP Binding Cassette (ABC) superfamily of efflux transporters. In this review a comprehensive assessment and annotation of recent findings in relation to genetic variation in the Multidrug Resistance Proteins 1-5 (ABCC1-5) and Breast Cancer Resistance Protein (ABCG2) are described, with particular emphasis on the impact of such transporter genetic variation to drug disposition or efficacy.

Role of proline 1150 in functional interactions between the membrane spanning domains and nucleotide binding domains of the MRP1 (ABCC1) transporter

The ATP-binding cassette multidrug resistance protein 1 (MRP1) mediates ATP-dependent cellular efflux of drugs and organic anions. We previously described a mutant, MRP1-Pro1150Ala, which exhibits selectively increased estradiol glucuronide (E217betaG) and methotrexate transport as well as altered interactions with ATP. We have now further explored the functional importance of MRP1-Pro1150 at the interface of transmembrane helix 15 and cytoplasmic loop 7 (CL7) by replacing it with Gly, Ile, Leu and Val. All four mutants exhibited a phenotype similar to MRP1-Pro1150Ala with respect to organic anion transport and [gamma32P]8N3ATP photolabeling. They also displayed very low levels of substrate-independent vanadate-induced trapping of [alpha32P]8N3ADP. To better understand the relationship between the altered nucleotide interactions and transport activity of these mutants, [alpha32P]8N3ADP trapping experiments were performed under different conditions. Unlike leukotriene C4, E217betaG decreased [alpha32P]8N3ADP trapping by both wild-type and mutant MRP1. [alpha32P]8N3ADP trapping by MRP1-Pro1150Ala could be increased by using Ni2+ instead of Mg2+, and by decreasing temperature; however, the transport properties of the mutant remained unchanged. We conclude that the reduced [alpha32P]8N3ADP trapping associated with loss of Pro1150, or the presence of E217betaG, is due to enhanced ADP release following ATP hydrolysis rather than a reduction in ATP hydrolysis itself. We hypothesize that loss of Pro1150 alters the role of CL7 as a coupling helix that mediates signaling between the nucleotide binding domains and some substrate binding sites in the membrane spanning domains of MRP1.

Modulation of human multidrug resistance protein (MRP) 1 (ABCC1) and MRP2 (ABCC2) transport activities by endogenous and exogenous glutathione-conjugated catechol metabolites

Members of the multidrug resistance protein (MRP/ABCC) subfamily of ATP-binding cassette proteins transport a wide array of anionic compounds, including sulfate, glucuronide, and glutathione (GSH) conjugates. The present study tested the ATP-dependent vesicular transport of leukotriene C(4) and 17beta-estradiol 17-(beta-d-glucuronide) (E(2)17betaG) mediated by the MRP1 and MRP2 transporters in the presence of six potential modulators from three different classes of GSH-conjugated catechol metabolites: the ecstasy metabolite 5-(glutathion-S-yl)-N-methyl-alpha-methyldopamine (5-GS-N-Me-alpha-MeDA), the caffeic acid metabolite 2-(glutathion-S-yl)-caffeic acid (2-GS-CA), and four GSH conjugates of 2-hydroxy (OH) and 4-OH estrogens (GS estrogens). MRP1-mediated E(2)17betaG transport was inhibited in a competitive manner with a relative order of potency of GS estrogens (IC(50) <1 microM) > 2-GS-CA (IC(50) 3 microM) > 5-GS-N-Me-alpha-MeDA (IC(50) 31 microM). MRP2-mediated transport was inhibited with a similar order of potency, except the 2-hydroxy-4-(glutathion-S-yl)-estradiol and 4-hydroxy-2-(glutathion-S-yl)-estradiol conjugates were approximately 50- and 300-fold less potent, respectively. Transport activity was unaffected by N-acetylcysteine conjugates of N-Me-alpha-MeDA and CA. The position of GSH conjugation appears important as all four GS estrogen conjugates tested were potent inhibitors of MRP1 transport, but only the 2-hydroxy-1-(glutathion-S-yl)-estradiol and 2-hydroxy-1-(glutathion-S-yl)-estrone conjugates were potent inhibitors of MRP2-mediated transport. In conclusion, we have identified three new classes of MRP1 and MRP2 modulators and demonstrated that one of these, the estrogen conjugates, shows unanticipated differences in their interactions with the two transporters.