The ABCG2 multidrug transporter is a pump gated by a valve and an extracellular lid

Role of the Abcg2 transporter in plasma, milk, and tissue levels of the anthelmintic monepantel in mice

Breast cancer resistance protein/ATP-binding cassette subfamily G2 (BCRP/ABCG2) is an ATP-binding cassette efflux (ABC) transporter expressed in the apical membrane of cells in tissues, such as the liver, intestine, kidney, testis, brain, and mammary gland. It is involved in xenobiotic pharmacokinetics, potentially affecting the efficacy and toxicity of many drugs. In this study, the role of ABCG2 in parasiticide monepantel (MNP) and its primary metabolite, monepantel sulfone (MNPSO2)'s systemic distribution and excretion in milk, was tested using female and male wild-type and Abcg2-/- mice. Liquid chromatography coupled with a tandem mass spectrometer (LC-MS/MS) was used for the analysis in a 10-min run time using positive-mode atmospheric pressure electrospray ionization (ESI+) and multiple reaction monitoring (MRM) scanning. For the primary metabolite tested, milk concentrations were 1.8-fold higher in wild-type mice than Abcg2-/- female lactating mice (P = 0.042) after intravenous administration of MNP. Finally, despite the lack of a difference between groups, we investigated potential differences in MNP and MNPSO2's plasma and tissue accumulation levels between wild-type and Abcg2-/- male mice. In this study, we demonstrated that MNPSO2 milk levels were affected by Abcg2, with potential pharmacological and toxicological consequences, contributing to the undesirable xenobiotic residues in milk.

ABCG2 Transports the Flukicide Nitroxynil and Affects Its Biodistribution and Secretion into Milk

The ABCG2 transporter plays a key role in pharmacological and toxicological processes, affecting bioavailability, tissue accumulation and milk secretion of its substrates. This protein is expressed in several biological barriers acting as a protective mechanism against xenobiotic exposure by pumping out a broad range of compounds. However, its induced expression during lactation in alveolar cells of mammary gland represents a relevant route for active transport of unwanted chemicals into milk. This work aimed to characterize the involvement of ABCG2 in systemic exposure and milk secretion of the flukicide nitroxynil. Using MDCK-II cells overexpressing the transporter, we showed that nitroxynil is an in vitro substrate of different species variants of ABCG2. Moreover, using wild-type and Abcg2-/- mice, we showed that murine Abcg2 clearly affects plasma levels of nitroxynil. We also reported differences in nitroxynil accumulation in several tissues, with almost 2-fold higher concentration in kidney, small intestine and testis of Abcg2-/- mice. Finally, we proved that nitroxynil secretion into milk was also affected by Abcg2, with a 1.9-fold higher milk concentration in wild-type compared with Abcg2-/- mice. We conclude that ABCG2 significantly impacts nitroxynil biodistribution by regulating its passage across biological barriers.

Structural and molecular characterization of lopinavir and ivermectin as breast cancer resistance protein (BCRP/ABCG2) inhibitors

A current clinical challenge in cancer is multidrug resistance (MDR) mediated by ABC transporters. Breast cancer resistance protein (BCRP) or ABCG2 transporter is one of the most important ABC transporters implicated in MDR and the use of inhibitors is a promising approach to overcome the resistance in cancer. This study aimed to characterize the molecular mechanism of ABCG2 inhibitors identified by a repurposing drug strategy using antiviral, anti-inflammatory and antiparasitic agents. Lopinavir and ivermectin can be considered as pan-inhibitors of ABC transporters, since both compounds inhibited ABCG2, P-glycoprotein and MRP1. They inhibited ABCG2 activity showing IC50 values of 25.5 and 23.4 µM, respectively. These drugs were highly cytotoxic and not transported by ABCG2. Additionally, these drugs increased the 5D3 antibody binding and did not affect the mRNA and protein expression levels. Cell-based analysis of the type of inhibition suggested a non-competitive inhibition, which was further corroborated by in silico approaches of molecular docking and molecular dynamics simulations. These results showed an overlap of the lopinavir and ivermectin binding sites on ABCG2, mainly interacting with E446 residue. However, the substrate mitoxantrone occupies a different site, binding to the F436 region, closer to the L554/L555 plug. In conclusion, these results revealed the mechanistic basis of lopinavir and ivermectin interaction with ABCG2. See also the Graphical abstract(Fig. 1).

Cancer chemotherapy resistance: Mechanisms and recent breakthrough in targeted drug delivery

Conventional chemotherapy, one of the most widely used cancer treatment methods, has serious side effects, and usually results in cancer treatment failure. Drug resistance is one of the primary reasons for this failure. The most significant drawbacks of systemic chemotherapy are rapid clearance from the circulation, the drug's low concentration in the tumor site, and considerable adverse effects outside the tumor. Several ways have been developed to boost neoplasm treatment efficacy and overcome medication resistance. In recent years, targeted drug delivery has become an essential therapeutic application. As more mechanisms of tumor treatment resistance are discovered, nanoparticles (NPs) are designed to target these pathways. Therefore, understanding the limitations and challenges of this technology is critical for nanocarrier evaluation. Nano-drugs have been increasingly employed in medicine, incorporating therapeutic applications for more precise and effective tumor diagnosis, therapy, and targeting. Many benefits of NP-based drug delivery systems in cancer treatment have been proven, including good pharmacokinetics, tumor cell-specific targeting, decreased side effects, and lessened drug resistance. As more mechanisms of tumor treatment resistance are discovered, NPs are designed to target these pathways. At the moment, this innovative technology has the potential to bring fresh insights into cancer therapy. Therefore, understanding the limitations and challenges of this technology is critical for nanocarrier evaluation.

Pdr5: A master of asymmetry

Pdr5 is a founding member of a large (pdr) subfamily of clinically and agriculturally significant fungal ABC transporters. The tremendous power of yeast genetics combined with biochemical and structural approaches revealed the astonishing asymmetry of this efflux pump. Asymmetry is manifested in Pdr5's ATP-binding sites, drug binding sites, signal transformation interface, and molecular exit gate. Even its mode of conformational switching is asymmetric with one half of the protein remaining nearly stationary. In the case of its ATP-binding sites, asymmetry is created by replacing a set of highly conserved residues with a characteristic set of deviant ones. This contrasts with the asymmetry of the molecular gate. There, a full complement of canonical residues is present, but structural features in the vicinity prevent some of these from forming a molecular plug during closure. Compared to their canonical-functioning counterparts, the deviant ATP site and these gating residues have different, essential functions. In addition to its remarkable asymmetry, the surprising observation that Pdr5 is a drug / proton co-transporter shines a new light on this remarkable protein.

Advances in the structure, mechanism and targeting of chemoresistance-linked ABC transporters

Cancer cells frequently display intrinsic or acquired resistance to chemically diverse anticancer drugs, limiting therapeutic success. Among the main mechanisms of this multidrug resistance is the overexpression of ATP-binding cassette (ABC) transporters that mediate drug efflux, and, specifically, ABCB1, ABCG2 and ABCC1 are known to cause cancer chemoresistance. High-resolution structures, biophysical and in silico studies have led to tremendous progress in understanding the mechanism of drug transport by these ABC transporters, and several promising therapies, including irradiation-based immune and thermal therapies, and nanomedicine have been used to overcome ABC transporter-mediated cancer chemoresistance. In this Review, we highlight the progress achieved in the past 5 years on the three transporters, ABCB1, ABCG2 and ABCC1, that are known to be of clinical importance. We address the molecular basis of their broad substrate specificity gleaned from structural information and discuss novel approaches to block the function of ABC transporters. Furthermore, genetic modification of ABC transporters by CRISPR-Cas9 and approaches to re-engineer amino acid sequences to change the direction of transport from efflux to import are briefly discussed. We suggest that current information regarding the structure, mechanism and regulation of ABC transporters should be used in clinical trials to improve the efficiency of chemotherapeutics for patients with cancer.

Structure, function, and inhibition of catalytically asymmetric ABC transporters: Lessons from the PDR subfamily

ATP-binding cassette (ABC) superfamily comprises a large group of ubiquitous transmembrane proteins that play a crucial role in transporting a diverse spectrum of substrates across cellular membranes. They participate in a wide array of physiological and pathological processes including nutrient uptake, antigen presentation, toxin elimination, and drug resistance in cancer and microbial cells. ABC transporters couple ATP binding and hydrolysis to undergo conformational changes allowing substrate translocation. Within this superfamily, a set of ABC transporters has lost the capacity to hydrolyze ATP at one of their nucleotide-binding sites (NBS), called the non-catalytic NBS, whose importance became evident with extensive biochemistry carried out on yeast pleiotropic drug resistance (PDR) transporters. Recent single-particle cryogenic electron microscopy (cryo-EM) advances have further catapulted our understanding of the architecture of these pumps. We provide here a comprehensive overview of the structural and functional aspects of catalytically asymmetric ABC pumps with an emphasis on the PDR subfamily. Furthermore, given the increasing evidence of efflux-mediated antifungal resistance in clinical settings, we also discuss potential grounds to explore PDR transporters as therapeutic targets.

The net electrostatic potential and hydration of ABCG2 affect substrate transport

ABCG2 is a medically important ATP-binding cassette transporter with crucial roles in the absorption and distribution of chemically-diverse toxins and drugs, reducing the cellular accumulation of chemotherapeutic drugs to facilitate multidrug resistance in cancer. ABCG2's capacity to transport both hydrophilic and hydrophobic compounds is not well understood. Here we assess the molecular basis for substrate discrimination by the binding pocket. Substitution of a phylogenetically-conserved polar residue, N436, to alanine in the binding pocket of human ABCG2 permits only hydrophobic substrate transport, revealing the unique role of N436 as a discriminator. Molecular dynamics simulations show that this alanine substitution alters the electrostatic potential of the binding pocket favoring hydration of the transport pore. This change affects the contact with substrates and inhibitors, abrogating hydrophilic compound transport while retaining the transport of hydrophobic compounds. The N436 residue is also required for optimal transport inhibition of ABCG2, as many inhibitors are functionally impaired by this ABCG2 mutation. Overall, these findings have biomedical implications, broadly extending our understanding of substrate and inhibitor interactions.

Role of the Abcg2 Transporter in Secretion into Milk of the Anthelmintic Clorsulon: Interaction with Ivermectin

Clorsulon is a benzenesulfonamide drug that is effective in treating helminthic zoonoses such as fascioliasis. When used in combination with the macrocyclic lactone ivermectin, it provides high broad-spectrum antiparasitic efficacy. The safety and efficacy of clorsulon should be studied by considering several factors such as drug-drug interactions mediated by ATP-binding cassette (ABC) transporters due to their potential effects on the pharmacokinetics and drug secretion into milk. The aim of this work was to determine the role of ABC transporter G2 (ABCG2) in clorsulon secretion into milk and the effect of ivermectin, a known ABCG2 inhibitor, on this process. Using in vitro transepithelial assays with cells transduced with murine Abcg2 and human ABCG2, we report that clorsulon was transported in vitro by both transporter variants and that ivermectin inhibited its transport mediated by murine Abcg2 and human ABCG2. Wild-type and Abcg2-/- lactating female mice were used to carry out in vivo assays. The milk concentration and the milk-to-plasma ratio were higher in wild-type mice than in Abcg2-/- mice after clorsulon administration, showing that clorsulon is actively secreted into milk by Abcg2. The interaction of ivermectin in this process was shown after the coadministration of clorsulon and ivermectin to wild-type and Abcg2-/- lactating female mice. Treatment with ivermectin had no effect on the plasma concentrations of clorsulon, but the milk concentrations and milk-to-plasma ratios of clorsulon decreased in comparison to those with treatment without ivermectin, only in wild-type animals. Consequently, the coadministration of clorsulon and ivermectin reduces clorsulon secretion into milk due to drug-drug interactions mediated by ABCG2.

In vitro interaction of the pesticides flupyradifurone, bupirimate and its metabolite ethirimol with the ATP-binding cassette transporter G2 (ABCG2)

ABCG2 is an ATP-binding cassette efflux transporter that is expressed in absorptive and excretory organs such as liver, intestine, kidney, brain and testis where it plays a crucial physiological and toxicological role in protecting cells against xenobiotics, affecting pharmacokinetics of its substrates. In addition, the induction of ABCG2 expression in mammary gland during lactation is related to active secretion of many toxicants into milk. In this study, the in vitro interactions between ABCG2 and three pesticides flupyradifurone, bupirimate and its metabolite ethirimol were investigated to check whether these compounds are substrates and/or inhibitors of this transporter. Using in vitro transepithelial assays with cells transduced with murine, ovine and human ABCG2, we showed that ethirimol and flupyradifurone were transported efficiently by murine Abcg2 and ovine ABCG2 but not by human ABCG2. Bupirimate was not found to be an in vitro substrate of ABCG2 transporter. Accumulation assays using mitoxantrone in transduced MDCK-II cells suggest that none of the tested pesticides were efficient ABCG2 inhibitors, at least in our experimental conditions. Our studies disclose that ethirimol and flupyradifurone are in vitro substrates of murine and ovine ABCG2, opening the possibility of a potential relevance of ABCG2 in the toxicokinetics of these pesticides.

Restriction of access to the central cavity is a major contributor to substrate selectivity in plant ABCG transporters

ABCG46 of the legume Medicago truncatula is an ABC-type transporter responsible for highly selective translocation of the phenylpropanoids, 4-coumarate, and liquiritigenin, over the plasma membrane. To investigate molecular determinants of the observed substrate selectivity, we applied a combination of phylogenetic and biochemical analyses, AlphaFold2 structure prediction, molecular dynamics simulations, and mutagenesis. We discovered an unusually narrow transient access path to the central cavity of MtABCG46 that constitutes an initial filter responsible for the selective translocation of phenylpropanoids through a lipid bilayer. Furthermore, we identified remote residue F562 as pivotal for maintaining the stability of this filter. The determination of individual amino acids that impact the selective transport of specialized metabolites may provide new opportunities associated with ABCGs being of interest, in many biological scenarios.

ABCG2 transporter plays a key role in the biodistribution of melatonin and its main metabolites

The ATP-binding cassette G2 (ABCG2) is an efflux transporter expressed in the apical membrane of cells from a large number of tissues, directly affecting bioavailability, tissue accumulation, and secretion into milk of both xenobiotics and endogenous compounds. The aim of this work was to characterize the role of ABCG2 in the systemic distribution and secretion into milk of melatonin and its main metabolites, 6-hydroxymelatonin, and 6-sulfatoxymelatonin. For this purpose, we first showed that these three molecules are transported by this transporter using in vitro transepithelial assays with MDCK-II polarized cells transduced with different species variants of ABCG2. Second, we tested the in vivo effect of murine Abcg2 in the systemic distribution of melatonin and its metabolites using wild-type and Abcg2^-/- mice. Our results show that after oral administration of melatonin, the plasma concentration of melatonin metabolites in Abcg2^-/-  mice was between 1.5 and 6-fold higher compared to the wild-type mice. We also evaluated in these animals differences in tissue accumulation of melatonin metabolites. The most relevant differences between both types of mice were found for small intestine and kidney (>sixfold increase for 6-sulfatoxymelatonin in Abcg2^-/-  mice). Finally, melatonin secretion into milk was also affected by the murine Abcg2 transporter, with a twofold higher milk concentration in wild-type compared with Abcg2^-/-  lactating female mice. In addition, melatonin metabolites showed a higher milk-to-plasma ratio in wild-type mice. Overall, our results show that the ABCG2 transporter plays a critical role in the biodistribution of melatonin and its main metabolites, thereby potentially affecting their biological and therapeutic activity.

The frequency of rs2231142 in ABCG2 among Native Hawaiian and Pacific Islander subgroups: implications for personalized rosuvastatin dosing

Statins are among the most commonly prescribed medications worldwide. Rosuvastatin is a moderate- to high-intensity statin depending on the prescribed dose. Statin-associated muscle symptoms are the main side effects, contributing to low adherence to statins. The missense variant rs2231142 in ABCG2 affects the functionality of the ABCG2 transporter, altering the pharmacokinetics and pharmacodynamics of rosuvastatin. This special report aims to accentuate the importance of considering the ABCG2 genotype upon prescribing rosuvastatin in high cardiovascular disease risk subgroups, specifically Native Hawaiian and Pacific Islander populations. Based on the reported frequencies of rs2231142 in ABCG2, it may be justifiable to initiate low-dose rosuvastatin in Samoans relative to Marshallese or Native Hawaiians. Interpopulation differences in pharmacogenetic allele frequencies underscore the need to disaggregate broad population categories to achieve health equity in treatment outcomes.

Nucleotide binding is the critical regulator of ABCG2 conformational transitions

ABCG2 is an exporter-type ABC protein that can expel numerous chemically unrelated xeno- and endobiotics from cells. When expressed in tumor cells or tumor stem cells, ABCG2 confers multidrug resistance, contributing to the failure of chemotherapy. Molecular details orchestrating substrate translocation and ATP hydrolysis remain elusive. Here, we present methods to concomitantly investigate substrate and nucleotide binding by ABCG2 in cells. Using the conformation-sensitive antibody 5D3, we show that the switch from the inward-facing (IF) to the outward-facing (OF) conformation of ABCG2 is induced by nucleotide binding. IF-OF transition is facilitated by substrates, and hindered by the inhibitor Ko143. Direct measurements of 5D3 and substrate binding to ABCG2 indicate that the high-to-low affinity switch of the drug binding site coincides with the transition from the IF to the OF conformation. Low substrate binding persists in the post-hydrolysis state, supporting that dissociation of the ATP hydrolysis products is required to reset the high substrate affinity IF conformation of ABCG2.

Snapshots of ABCG1 and ABCG5/G8: A Sterol's Journey to Cross the Cellular Membranes

The subfamily-G ATP-binding cassette (ABCG) transporters play important roles in regulating cholesterol homeostasis. Recent progress in the structural data of ABCG1 and ABCG5/G8 disclose putative sterol binding sites that suggest the possible cholesterol translocation pathway. ABCG1 and ABCG5/G8 share high similarity in the overall molecular architecture, and both transporters appear to use several unique structural motifs to facilitate cholesterol transport along this pathway, including the phenylalanine highway and the hydrophobic valve. Interestingly, ABCG5/G8 is known to transport cholesterol and phytosterols, whereas ABCG1 seems to exclusively transport cholesterol. Ligand docking analysis indeed suggests a difference in recruiting sterol molecules to the known sterol-binding sites. Here, we further discuss how the different and shared structural features are relevant to their physiological functions, and finally provide our perspective on future studies in ABCG cholesterol transporters.

The Reentry Helix Is Potentially Involved in Cholesterol Sensing of the ABCG1 Transporter Protein

ABCG1 has been proposed to play a role in HDL-dependent cellular sterol regulation; however, details of the interaction between the transporter and its potential sterol substrates have not been revealed. In the present work, we explored the effect of numerous sterol compounds on the two isoforms of ABCG1 and ABCG4 and made efforts to identify the molecular motifs in ABCG1 that are involved in the interaction with cholesterol. The functional readouts used include ABCG1-mediated ATPase activity and ABCG1-induced apoptosis. We found that both ABCG1 isoforms and ABCG4 interact with several sterol compounds; however, they have selective sensitivities to sterols. Mutational analysis of potential cholesterol-interacting motifs in ABCG1 revealed altered ABCG1 functions when F571, L626, or Y586 were mutated. L430A and Y660A substitutions had no functional consequence, whereas Y655A completely abolished the ABCG1-mediated functions. Detailed structural analysis of ABCG1 demonstrated that the mutations modulating ABCG1 functions are positioned either in the so-called reentry helix (G-loop/TM5b,c) (Y586) or in its close proximity (F571 and L626). Cholesterol molecules resolved in the structure of ABCG1 are also located close to Y586. Based on the experimental observations and structural considerations, we propose an essential role for the reentry helix in cholesterol sensing in ABCG1.

Structural analysis of cholesterol binding and sterol selectivity by ABCG5/G8

The ATP-binding cassette (ABC) sterol transporters are responsible for maintaining cholesterol homeostasis in mammals by participating in reverse cholesterol transport (RCT) or transintestinal cholesterol efflux (TICE). The heterodimeric ABCG5/G8 carries out selective sterol excretion, preventing the abnormal accumulation of plant sterols in human bodies, while homodimeric ABCG1 contributes to the biogenesis and metabolism of high-density lipoproteins. A sterol-binding site on ABCG5/G8 was proposed at the interface of the transmembrane domain and the core of lipid bilayers. In this study, we have determined the crystal structure of ABCG5/G8 in a cholesterol-bound state. The structure combined with amino acid sequence analysis shows that in the proximity of the sterol-binding site, a highly conserved phenylalanine array supports functional implications for ABCG cholesterol/sterol transporters. Lastly, in silico docking analysis of cholesterol and stigmasterol (a plant sterol) suggests sterol-binding selectivity on ABCG5/G8, but not ABCG1. Together, our results provide a structural basis for cholesterol binding on ABCG5/G8 and the sterol selectivity by ABCG transporters.

Secretion into Milk of the Main Metabolites of the Anthelmintic Albendazole Is Mediated by the ABCG2/BCRP Transporter

Albendazole (ABZ) is an anthelmintic with a broad-spectrum activity, widely used in human and veterinary medicine. ABZ is metabolized in all mammalian species to albendazole sulfoxide (ABZSO), albendazole sulfone (ABZSO₂) and albendazole 2-aminosulphone (ABZSO₂-NH₂). ABZSO and ABZSO₂ are the main metabolites detected in plasma and all three are detected in milk. The ATP-binding cassette transporter G2 (ABCG2) is an efflux transporter that is involved in the active secretion of several compounds into milk. Previous studies have reported that ABZSO was in vitro transported by ABCG2. The aim of this work is to correlate the in vitro interaction between ABCG2 and the other ABZ metabolites with their secretion into milk by this transporter. Using in vitro transepithelial assays with cells transduced with murine Abcg2 and human ABCG2, we show that ABZSO₂ and ABZSO₂-NH₂ are in vitro substrates of both. In vivo assays carried out with wild-type and Abcg2^-/- lactating female mice demonstrated that secretion into milk of these ABZ metabolites was mediated by Abcg2. Milk concentrations and milk-to-plasma ratio were higher in wild-type compared to Abcg2^-/- mice for all the metabolites tested. We conclude that ABZ metabolites are undoubtedly in vitro substrates of ABCG2 and actively secreted into milk by ABCG2.

Observation of the cervical microbiome in the progression of cervical intraepithelial neoplasia

OBJECTIVE

Cervical microbial community in the cervical intraepithelial neoplasia and cervical cancer patients was analysed to study its composition, diversity and signalling pathways by high-throughput 16S rDNA sequencing,and the candidate genes associated with occurrence and progression of cervical intraepithelial neoplasia were screened out and the model was established to predict the evolution of cervical intraepithelial neoplasia malignant transformation from the cervical microbial genes aspect.

METHODS

Cervical tissues of normal, cervical intraepithelial neoplasia and cervical cancer patients without receiving any treatment were collected. The correlation between candidate genes and cervical intraepithelial neoplasia progression was initially determined by analyzing the microbial flora. Real-time fluorescence quantitative PCR was used to detect the expression of candidate genes in different cervical tissues, ROC curve and logistic regression was used to analyse and predict the risk factors related to the occurrence and progression of cervical intraepithelial neoplasia. Finally, the early warning model of cervical intraepithelial neoplasia occurrence and progression is established.

RESULTS

Cervical tissues from normal, cervical intraepithelial neoplasia and cervical cancer patients were collected for microbial community high-throughput 16S rDNA sequencing. The analysis revealed five different pathways related to cervical intraepithelial neoplasia. 10 candidate genes were selected by further bioinformatics analysis and preliminary screening. Real time PCR, ROC curve and Logistic regression analysis showed that human papillomavirus infection, TCT severity, ABCG2, TDG, PCNA were independent risk factors for cervical intraepithelial neoplasia. We used these indicators to establish a random forest model. Seven models were built through different combinations. The model 4 (ABCG2 + PCNA + TDG) was the best early warning model for the occurrence and progression of CIN.

CONCLUSIONS

A total of 5 differential pathways and 10 candidate genes related to occurrence and progression of cervical intraepithelial neoplasia were found in cervical microbial community. This study firstly identified the genes from cervical microbial community that play an important role in the occurrence and progression of cervical intraepithelial neoplasia. At the same time, the early warning model including ABCG2 + PCNA+TDG genes provided a new idea and target for clinical prediction and blocking the evolution of cervical intraepithelial neoplasia malignant transformation from the aspect of cervical microbiological related genes.

One ring is not enough to rule them all. Albumin-dependent ABCG2-mediated transport of chlorophyll-derived photosensitizers

Breast cancer resistance protein (BCRP, ABCG2) is a member of the ATP-binding-cassette (ABC) superfamily of membrane transporters. It is involved in the efflux of a broad range of xenobiotics of highly diverse structures. BCRP activity greatly influences drug distribution in vivo and is often associated with cancer multidrug resistance, which is observed in the case of both chemotherapy and photodynamic therapy. The set of ABCG2 substrates includes porphyrins and chlorins such as heme, hemin, protoporphyrin IX, chlorin e6, pheophorbide a, and their derivatives. Here we provide an evidence that magnesium- and zinc-substituted derivatives of pheophorbide a, which are very promising photosensitizers for use in photodynamic therapy, are also recognized and transported by ABCG2. Interestingly, despite minor structural differences, they clearly differ in the transport rate, both between each other and compared to pheophorbide a. In addition, their transport rate, like those of other structurally similar compounds, is strictly dependent on the level of serum albumin in the extracellular environment. The results that we present here are crucial for the use of metal-substituted pheophorbides in clinical practice but also provide an important insight into the mechanism of porphyrin transport by ABCG2.

Structure and efflux mechanism of the yeast pleiotropic drug resistance transporter Pdr5

Pdr5, a member of the extensive ABC transporter superfamily, is representative of a clinically relevant subgroup involved in pleiotropic drug resistance. Pdr5 and its homologues drive drug efflux through uncoupled hydrolysis of nucleotides, enabling organisms such as baker’s yeast and pathogenic fungi to survive in the presence of chemically diverse antifungal agents. Here, we present the molecular structure of Pdr5 solved with single particle cryo-EM, revealing details of an ATP-driven conformational cycle, which mechanically drives drug translocation through an amphipathic channel, and a clamping switch within a conserved linker loop that acts as a nucleotide sensor. One half of the transporter remains nearly invariant throughout the cycle, while its partner undergoes changes that are transmitted across inter-domain interfaces to support a peristaltic motion of the pumped molecule. The efflux model proposed here rationalises the pleiotropic impact of Pdr5 and opens new avenues for the development of effective antifungal compounds.

Pdr5 is an ABC transporter conferring multidrug resistance to pathogenic fungi. Here, structural analysis of Pdr5 provides insights into the transport mechanism featuring asymmetric movements of Pdr5 domain and enabling efflux of a broad spectrum of compounds.

Tetrahydroquinoline/4,5-Dihydroisoxazole Molecular Hybrids as Inhibitors of Breast Cancer Resistance Protein (BCRP/ABCG2)

Multidrug resistance (MDR) is one of the major factors in the failure of many chemotherapy approaches. In cancer cells, MDR is mainly associated with the expression of ABC transporters such as P-glycoprotein, MRP1 and ABCG2. Despite major efforts to develop new selective and potent inhibitors of ABC drug transporters, no ABCG2-specific inhibitors for clinical use are yet available. Here, we report the evaluation of sixteen tetrahydroquinoline/4,5-dihydroisoxazole derivatives as a new class of ABCG2 inhibitors. The affinity of the five best inhibitors was further investigated by the vanadate-sensitive ATPase assay. Molecular modelling data, proposing a potential binding mode, suggest that they can inhibit the ABCG2 activity by binding on site S1, previously reported as inhibitors binding region, as well targeting site S2, a selective region for substrates, and by specifically interacting with residues Asn436, Gln398, and Leu555. Altogether, this study provided new insights into THQ/4,5-dihydroisoxazole molecular hybrids, generating great potential for the development of novel most potent ABCG2 inhibitors.

Role of the Abcg2 transporter in plasma levels and tissue accumulation of the anti-inflammatory tolfenamic acid in mice

The Breast Cancer Resistance Protein (BCRP/ABCG2) is an ATP-binding cassette efflux transporter that is expressed in the apical membrane of cells from relevant tissues involved in drug pharmacokinetics such as liver, intestine, kidney, testis, brain and mammary gland, among others. Tolfenamic acid is an anti-inflammatory drug used as an analgesic and antipyretic in humans and animals. Recently, tolfenamic acid has been repurposed as an antitumoral drug and for use in chronic human diseases such as Alzheimer. The aim of this work was to study whether tolfenamic acid is an in vitro Abcg2 substrate, and to investigate the potential role of Abcg2 in plasma exposure, secretion into milk and tissue accumulation of this drug. Using in vitro transepithelial assays with cells transduced with Abcg2, we showed that tolfenamic acid is an in vitro substrate of Abcg2. The in vivo effect of this transporter was tested using wild-type and Abcg2^-/- mice, showing that after oral and intravenous administration of tolfenamic acid, its area under the plasma concentration-time curve in Abcg2^-/- mice was between 1.7 and 1.8-fold higher compared to wild-type mice. Abcg2^-/- mice also showed higher liver and testis accumulation of tolfenamic acid after intravenous administration. In this study, we demonstrate that tolfenamic acid is transported in vitro by Abcg2 and that its plasma levels as well as its tissue distribution are affected by Abcg2, with potential pharmacological and toxicological consequences.

Multidrug Resistance in Mammals and Fungi-From MDR to PDR: A Rocky Road from Atomic Structures to Transport Mechanisms

Multidrug resistance (MDR) can be a serious complication for the treatment of cancer as well as for microbial and parasitic infections. Dysregulated overexpression of several members of the ATP-binding cassette transporter families have been intimately linked to MDR phenomena. Three paradigm ABC transporter members, ABCB1 (P-gp), ABCC1 (MRP1) and ABCG2 (BCRP) appear to act as brothers in arms in promoting or causing MDR in a variety of therapeutic cancer settings. However, their molecular mechanisms of action, the basis for their broad and overlapping substrate selectivity, remains ill-posed. The rapidly increasing numbers of high-resolution atomic structures from X-ray crystallography or cryo-EM of mammalian ABC multidrug transporters initiated a new era towards a better understanding of structure-function relationships, and for the dynamics and mechanisms driving their transport cycles. In addition, the atomic structures offered new evolutionary perspectives in cases where transport systems have been structurally conserved from bacteria to humans, including the pleiotropic drug resistance (PDR) family in fungal pathogens for which high resolution structures are as yet unavailable. In this review, we will focus the discussion on comparative mechanisms of mammalian ABCG and fungal PDR transporters, owing to their close evolutionary relationships. In fact, the atomic structures of ABCG2 offer excellent models for a better understanding of fungal PDR transporters. Based on comparative structural models of ABCG transporters and fungal PDRs, we propose closely related or even conserved catalytic cycles, thus offering new therapeutic perspectives for preventing MDR in infectious disease settings.

Clinically-Relevant ABC Transporter for Anti-Cancer Drug Resistance

Multiple drug resistance (MDR), referring to the resistance of cancer cells to a broad spectrum of structurally and mechanistically unrelated drugs across membranes, severely impairs the response to chemotherapy and leads to chemotherapy failure. Overexpression of ATP binding cassette (ABC) transporters is a major contributing factor resulting in MDR, which can recognize and mediate the efflux of diverse drugs from cancer cells, thereby decreasing intracellular drug concentration. Therefore, modulators of ABC transporter could be used in combination with standard chemotherapeutic anticancer drugs to augment the therapeutic efficacy. This review summarizes the recent advances of important cancer-related ABC transporters, focusing on their physiological functions, structures, and the development of new compounds as ABC transporter inhibitors.

ABCG2 Is Overexpressed on Red Blood Cells in Ph-Negative Myeloproliferative Neoplasms and Potentiates Ruxolitinib-Induced Apoptosis

Myeloproliferative neoplasms (MPNs) are a group of disorders characterized by clonal expansion of abnormal hematopoietic stem cells leading to hyperproliferation of one or more myeloid lineages. The main complications in MPNs are high risk of thrombosis and progression to myelofibrosis and leukemia. MPN patients with high risk scores are treated by hydroxyurea (HU), interferon-α, or ruxolitinib, a tyrosine kinase inhibitor. Polycythemia vera (PV) is an MPN characterized by overproduction of red blood cells (RBCs). ABCG2 is a member of the ATP-binding cassette superfamily transporters known to play a crucial role in multidrug resistance development. Proteome analysis showed higher ABCG2 levels in PV RBCs compared to RBCs from healthy controls and an additional increase of these levels in PV patients treated with HU, suggesting that ABCG2 might play a role in multidrug resistance in MPNs. In this work, we explored the role of ABCG2 in the transport of ruxolitinib and HU using human cell lines, RBCs, and in vitro differentiated erythroid progenitors. Using stopped-flow analysis, we showed that HU is not a substrate for ABCG2. Using transfected K562 cells expressing three different levels of recombinant ABCG2, MPN RBCs, and cultured erythroblasts, we showed that ABCG2 potentiates ruxolitinib-induced cytotoxicity that was blocked by the ABCG2-specific inhibitor KO143 suggesting ruxolitinib intracellular import by ABCG2. In silico modeling analysis identified possible ruxolitinib-binding site locations within the cavities of ABCG2. Our study opens new perspectives in ruxolitinib efficacy research targeting cell types depending on ABCG2 expression and polymorphisms among patients.

Cabozantinib Reverses Topotecan Resistance in Human Non-Small Cell Lung Cancer NCI-H460/TPT10 Cell Line and Tumor Xenograft Model

Cabozantinib (CBZ) is a small molecule tyrosine kinase receptor inhibitor, which could also inhibit the ABCG2 transporter function. Therefore, CBZ could re-sensitize cancer cells that are resistant to ABCG2 substrate drugs including topotecan (TPT). However, its reversal effect against TPT resistance has not been tested in a TPT-induced resistant cancer model. In this study, a new TPT selected human non-small cell lung cancer (NSCLC)-resistant cell model NCI-H460/TPT10 with ABCG2 overexpression and its parental NCI-H460 cells were utilized to investigate the role of CBZ in drug resistance. The in vitro study showed that CBZ, at a non-toxic concentration, could re-sensitize NCI-H460/TPT10 cells to TPT by restoring intracellular TPT accumulation via inhibiting ABCG2 function. In addition, the increased cytotoxicity by co-administration of CBZ and TPT may be contributed by the synergistic effect on downregulating ABCG2 expression in NCI-H460/TPT10 cells. To further verify the applicability of the NCI-H460/TPT10 cell line to test multidrug resistance (MDR) reversal agents in vivo and to evaluate the in vivo efficacy of CBZ on reversing TPT resistance, a tumor xenograft mouse model was established by implanting NCI-H460 and NCI-H460/TPT10 into nude mice. The NCI-H460/TPT10 xenograft tumors treated with the combination of TPT and CBZ dramatically reduced in size compared to tumors treated with TPT or CBZ alone. The TPT-resistant phenotype of NCI-H460/TPT10 cell line and the reversal capability of CBZ in NCI-H460/TPT10 cells could be extended from in vitro cell model to in vivo xenograft model. Collectively, CBZ is considered to be a potential approach in overcoming ABCG2-mediated MDR in NSCLC. The established NCI-H460/TPT10 xenograft model could be a sound clinically relevant resource for future drug screening to eradicate ABCG2-mediated MDR in NSCLC.

Medically Important Alterations in Transport Function and Trafficking of ABCG2

Several polymorphisms and mutations in the human ABCG2 multidrug transporter result in reduced plasma membrane expression and/or diminished transport function. Since ABCG2 plays a pivotal role in uric acid clearance, its malfunction may lead to hyperuricemia and gout. On the other hand, ABCG2 residing in various barrier tissues is involved in the innate defense mechanisms of the body; thus, genetic alterations in ABCG2 may modify the absorption, distribution, excretion of potentially toxic endo- and exogenous substances. In turn, this can lead either to altered therapy responses or to drug-related toxic reactions. This paper reviews the various types of mutations and polymorphisms in ABCG2, as well as the ways how altered cellular processing, trafficking, and transport activity of the protein can contribute to phenotypic manifestations. In addition, the various methods used for the identification of the impairments in ABCG2 variants and the different approaches to correct these defects are overviewed.

The Novel Benzamide Derivative, VKNG-2, Restores the Efficacy of Chemotherapeutic Drugs in Colon Cancer Cell Lines by Inhibiting the ABCG2 Transporter

The overexpression of ATP-binding cassette transporter, ABCG2, plays an important role in mediating multidrug resistance (MDR) in certain types of cancer cells. ABCG2-mediated MDR can significantly attenuate or abrogate the efficacy of anticancer drugs by increasing their efflux from cancer cells. In this study, we determined the efficacy of the novel benzamide derivative, VKNG-2, to overcome MDR due to the overexpression of the ABCG2 transporter in the colon cancer cell line, S1-M1-80. In vitro, 5 μM of VKNG-2 reversed the resistance of S1-M1-80 cell line to mitoxantrone (70-fold increase in efficacy) or SN-38 (112-fold increase in efficacy). In contrast, in vitro, 5 μM of VKNG-2 did not significantly alter either the expression of ABCG2, AKT, and PI3K p110β protein or the subcellular localization of the ABCG2 protein compared to colon cancer cells incubated with the vehicle. Molecular docking data indicated that VKNG-2 had a high docking score (-10.2 kcal/mol) for the ABCG2 transporter substrate-drug binding site whereas it had a low affinity on ABCB1 and ABCC1 transporters. Finally, VKNG-2 produced a significant concentration-dependent increase in ATPase activity (EC50 = 2.3 µM). In conclusion, our study suggests that in vitro, VKNG-2 reverses the resistance of S1-M1-80, a cancer cell line resistant to mitoxantrone and SN-38, by inhibiting the efflux function of the ABCG2 transporter.

Identification of Two Dysfunctional Variants in the ABCG2 Urate Transporter Associated with Pediatric-Onset of Familial Hyperuricemia and Early-Onset Gout

The ABCG2 gene is a well-established hyperuricemia/gout risk locus encoding a urate transporter that plays a crucial role in renal and intestinal urate excretion. Hitherto, p.Q141K—a common variant of ABCG2 exhibiting approximately one half the cellular function compared to the wild-type—has been reportedly associated with early-onset gout in some populations. However, compared with adult-onset gout, little clinical information is available regarding the association of other uricemia-associated genetic variations with early-onset gout; the latent involvement of ABCG2 in the development of this disease requires further evidence. We describe a representative case of familial pediatric-onset hyperuricemia and early-onset gout associated with a dysfunctional ABCG2, i.e., a clinical history of three generations of one Czech family with biochemical and molecular genetic findings. Hyperuricemia was defined as serum uric acid (SUA) concentrations 420 μmol/L for men or 360 μmol/L for women and children under 15 years on two measurements, performed at least four weeks apart. The proband was a 12-year-old girl of Roma ethnicity, whose SUA concentrations were 397–405 µmol/L. Sequencing analyses focusing on the coding region of ABCG2 identified two rare mutations—c.393G>T (p.M131I) and c.706C>T (p.R236X). Segregation analysis revealed a plausible link between these mutations and hyperuricemia and the gout phenotype in family relatives. Functional studies revealed that p.M131I and p.R236X were functionally deficient and null, respectively. Our findings illustrate why genetic factors affecting ABCG2 function should be routinely considered in clinical practice as part of a hyperuricemia/gout diagnosis, especially in pediatric-onset patients with a strong family history.

Conserved amino acids in the region connecting membrane spanning domain 1 to nucleotide binding domain 1 are essential for expression of the MRP1 (ABCC1) transporter

Multidrug resistance protein 1 (MRP1) (gene symbol ABCC1) is an ATP-binding cassette (ABC) transporter which effluxes xeno- and endobiotic organic anions including estradiol glucuronide and the pro-inflammatory leukotriene C₄. MRP1 also confers multidrug resistance by reducing intracellular drug accumulation through active efflux. MRP1 has three membrane spanning domains (MSD), and two nucleotide binding domains (NBD). MSD1 and MSD2 are linked to NBD1 and NBD2 by connecting regions (CR) 1 and CR2, respectively. Here we targeted four residues in CR1 (Ser⁶¹², Arg⁶¹⁵, His⁶²², Glu⁶²⁴) for alanine substitution and unexpectedly, found that cellular levels of three mutants (S612A, R615A, E624A) in transfected HEK cells were substantially lower than wild-type MRP1. Whereas CR1-H622A properly trafficked to the plasma membrane and exhibited organic anion transport activity comparable to wild-type MRP1, the poorly expressing R615A and E624A (and to a lesser extent S612A) mutant proteins were retained intracellularly. Analyses of cryogenic electron microscopic and atomic homology models of MRP1 indicated that Arg⁶¹⁵ and Glu⁶²⁴ might participate in bonding interactions with nearby residues to stabilize expression of the transporter. However, this was not supported by double exchange mutations E624K/K406E, R615D/D430R and R615F/F619R which failed to improve MRP1 levels. Nevertheless, these experiments revealed that the highly conserved CR1-Phe⁶¹⁹ and distal Lys⁴⁰⁶ in the first cytoplasmic loop of MSD1 are also essential for expression of MRP1 protein. This study is the first to demonstrate that CR1 contains several highly conserved residues critical for plasma membrane expression of MRP1 but thus far, currently available structures and models do not provide any insights into the underlying mechanism(s). Additional structures with rigorous biochemical validation data are needed to fully understand the bonding interactions critical to stable expression of this clinically important ABC transporter.

Molecular Simulations of Hydrophobic Gating of Pentameric Ligand Gated Ion Channels: Insights into Water and Ions

Ion channels are proteins which form gated nanopores in biological membranes. Many channels exhibit hydrophobic gating, whereby functional closure of a pore occurs by local dewetting. The pentameric ligand gated ion channels (pLGICs) provide a biologically important example of hydrophobic gating. Molecular simulation studies comparing additive vs polarizable models indicate predictions of hydrophobic gating are robust to the model employed. However, polarizable models suggest favorable interactions of hydrophobic pore-lining regions with chloride ions, of relevance to both synthetic carriers and channel proteins. Electrowetting of a closed pLGIC hydrophobic gate requires too high a voltage to occur physiologically but may inform designs for switchable nanopores. Global analysis of ∼200 channels yields a simple heuristic for structure-based prediction of (closed) hydrophobic gates. Simulation-based analysis is shown to provide an aid to interpretation of functional states of new channel structures. These studies indicate the importance of understanding the behavior of water and ions within the nanoconfined environment presented by ion channels.

One ring to bring them all and in the darkness bind them: The trafficking of heme without deliverers

Heme, as a hydrophobic iron-containing organic ring, is lipid soluble and can interact with biological membranes. The very same properties of heme that nature exploits to support life also renders heme potentially cytotoxic. In order to utilize heme, while also mitigating its toxicity, cells are challenged to tightly control the concentration and bioavailability of heme. On the bright side, it is reasonable to envision that, analogous to other transition metals, a combination of membrane-bound transporters, soluble carriers, and chaperones coordinate heme trafficking to subcellular compartments. However, given the dual properties exhibited by heme as a transition metal and lipid, it is compelling to consider the dark side: the potential role of non-proteinaceous biomolecules including lipids and nucleic acids that bind, sequester, and control heme trafficking and bioavailability. The emergence of inter-organellar membrane contact sites, as well as intracellular vesicles derived from various organelles, have raised the prospect that heme can be trafficked through hydrophobic channels. In this review, we aim to focus on heme delivery without deliverers - an alternate paradigm for the regulation of heme homeostasis through chaperone-less pathways for heme trafficking.

ABCG: a new fold of ABC exporters and a whole new bag of riddles!

ATP-binding cassette (ABC) superfamily comprises membrane transporters that power the active transport of substrates across biological membranes. These proteins harness the energy of nucleotide binding and hydrolysis to fuel substrate translocation via an alternating-access mechanism. The primary structural blueprint is relatively conserved in all ABC transporters. A transport-competent ABC transporter is essentially made up of two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). While the NBDs are conserved in their primary sequence and form at their interface two nucleotide-binding sites (NBSs) for ATP binding and hydrolysis, the TMDs are variable among different families and form the translocation channel. Transporters catalyzing the efflux of substrates from the cells are called exporters. In humans, they range from A to G subfamilies, with the B, C and G subfamilies being involved in chemoresistance. The recently elucidated structures of ABCG5/G8 followed by those of ABCG2 highlighted a novel structural fold that triggered extensive research. Notably, suppressor genetics in the orthologous yeast Pleiotropic Drug Resistance (PDR) subfamily proteins have pointed to a crosstalk between TMDs and NBDs modulating substrate export. Considering the structural information provided by their neighbors from the G subfamily, these studies provide mechanistic keys and posit a functional role for the non-hydrolytic NBS found in several ABC exporters. The present chapter provides an overview of structural and functional aspects of ABCG proteins with a special emphasis on the yeast PDR systems.

The first intracellular loop is essential for the catalytic cycle of the human ABCG2 multidrug resistance transporter

The human multidrug transporter ABCG2 is required for physiological detoxification and mediates anticancer drug resistance. Here, we identify pivotal residues in the first intracellular loop (ICL1), constituting an intrinsic part of the transmission interface. The architecture includes a triple helical bundle formed by the hot spot helix of the nucleotide‐binding domain, the elbow helix, and ICL1. We show here that the highly conserved ICL1 residues G462, Y463, and Y464 are essential for the proper cross talk of the closed nucleotide‐binding domain dimer with the transmembrane domains. Hence, ICL1 acts as a molecular spring, triggering the conformational switch of ABCG2 before substrate extrusion. These data suggest that the ABCG2 transmission interface may offer therapeutic options for the treatment of drug‐resistant malignancies.

Evolutionary history of ATP-binding cassette proteins

ATP-binding cassette (ABC) proteins are found in every sequenced genome and evolved deep in the phylogenetic tree of life. ABC proteins form one of the largest homologous protein families, with most being involved in substrate transport across biological membranes, and a few cytoplasmic members regulating in essential processes like translation. The predominant ABC protein classification scheme is derived from human members, but the increasing number of fully sequenced genomes permits to reevaluate this paradigm in the light of the evolutionary history the ABC-protein superfamily. As we study the diversity of substrates, mechanisms, and physiological roles of ABC proteins, knowledge of the evolutionary relationships highlights similarities and differences that can be attributed to specific branches in protein divergence. While alignments and trees built on natural sequence variation account for the evolutionary divergence of ABC proteins, high-throughput experiments and next-generation sequencing creating experimental sequence variation are instrumental in identifying functional constraints. The combination of natural and experimentally produced sequence variation allows a broader and more rational study of the function and physiological roles of ABC proteins.

The ABCG2/BCRP transporter and its variants - from structure to pathology

The ABCG2 protein has a key role in the transport of a wide range of structurally dissimilar endo- and xenobiotics in the human body, especially in the tissue barriers and the metabolizing or secreting organs. The human ABCG2 gene harbors a high number of polymorphisms and mutations, which may significantly modulate its expression and function. Recent high-resolution structural data, complemented with molecular dynamic simulations, may significantly help to understand intramolecular movements and substrate handling, as well as the effects of mutations on the membrane transporter function of ABCG2. As reviewed here, structural alterations may result not only in direct alterations in drug binding and transporter activity, but also in improper folding or problems in the carefully regulated process of trafficking, including vesicular transport, endocytosis, recycling, and degradation. Here, we also review the clinical importance of altered ABCG2 expression and function in general drug metabolism, cancer multidrug resistance, and impaired uric acid excretion, leading to gout.

Picky ABCG5/G8 and promiscuous ABCG2 - a tale of fatty diets and drug toxicity

Structural data on ABCG5/G8 and ABCG2 reveal a unique molecular architecture for subfamily G ATP‐binding cassette (ABCG) transporters and disclose putative substrate‐binding sites. ABCG5/G8 and ABCG2 appear to use several unique structural motifs to execute transport, including the triple helical bundles, the membrane‐embedded polar relay, the re‐entry helices, and a hydrophobic valve. Interestingly, ABCG2 shows extreme substrate promiscuity, whereas ABCG5/G8 transports only sterol molecules. ABCG2 structures suggest a large internal cavity, serving as a binding region for substrates and inhibitors, while mutational and pharmacological analyses support the notion of multiple binding sites. By contrast, ABCG5/G8 shows a collapsed cavity of insufficient size to hold substrates. Indeed, mutational analyses indicate a sterol‐binding site at the hydrophobic interface between the transporter and the lipid bilayer. In this review, we highlight key differences and similarities between ABCG2 and ABCG5/G8 structures. We further discuss the relevance of distinct and shared structural features in the context of their physiological functions. Finally, we elaborate on how ABCG2 and ABCG5/G8 could pave the way for studies on other ABCG transporters.

The transport pathway in the ABCG2 protein and its regulation revealed by molecular dynamics simulations

Atomic-level structural insight on the human ABCG2 membrane protein, a pharmacologically important transporter, has been recently revealed by several key papers. In spite of the wealth of structural data, the pathway of transmembrane movement for the large variety of structurally different ABCG2 substrates and the physiological lipid regulation of the transporter has not been elucidated. The complex molecular dynamics simulations presented here may provide a breakthrough in understanding the steps of the substrate transport process and its regulation by cholesterol. Our analysis revealed drug binding cavities other than the central binding site and delineated a putative dynamic transport pathway for substrates with variable structures. We found that membrane cholesterol accelerated drug transport by promoting the closure of cytoplasmic protein regions. Since ABCG2 is present in all major biological barriers and drug-metabolizing organs, influences the pharmacokinetics of numerous clinically applied drugs, and plays a key role in uric acid extrusion, this information may significantly promote a reliable prediction of clinically important substrate characteristics and drug-drug interactions.