Mutations at amino-acid 482 in the ABCG2 gene affect substrate and antagonist specificity

BBA, a derivative of 23-hydroxybetulinic acid, potently reverses ABCB1-mediated drug resistance in vitro and in vivo

23-O-(1,4'-Bipiperidine-1-carbonyl)betulinic acid (BBA), a synthetic derivative of 23-hydroxybetulinic acid (23-HBA), shows a reversal effect on multidrug resistance (MDR) in our preliminary screening. Overexpression of ATP-binding cassette (ABC) transporters such as ABCB1, ABCG2, and ABCC1 has been reported in recent studies to be a major factor contributing to MDR. Our study results showed that BBA enhanced the cytotoxicity of ABCB1 substrates and increased the accumulation of doxorubicin or rhodamine123 in ABCB1 overexpressing cells, but had no effect on non ABCB1 substrate, such as cisplatin; what's more, BBA slightly reversed ABCG2-mediated resistance to SN-38, but did not affect the ABCC1-mediated MDR. Further studies on the mechanism indicated that BBA did not alter the expression of ABCB1 at mRNA or protein levels, but affected the ABCB1 ATPase activity by stimulating the basal activity at lower concentrations and inhibiting the activity at higher concentrations. In addition, BBA inhibited the verapamil-stimulated ABCB1 ATPase activity and the photolabeling of ABCB1 with [(125)I] iodoarylazidoprazosin in a concentration-dependent manner, indicating that BBA directly interacts with ABCB1. The docking study confirmed this notion that BBA could bind to the drug binding site(s) on ABCB1, but its binding position was only partially overlapping with that of verapamil or iodoarylazidoprazosin. Importantly, BBA increased the inhibitory effect of paclitaxel in ABCB1 overexpressing KB-C2 cell xenografts in nude mice. Taken together, our findings suggest that BBA can reverse ABCB1-mediated MDR by inhibiting its efflux function of ABCB1, which supports the development of BBA as a novel potential MDR reversal agent used in the clinic.

Nilotinib potentiates anticancer drug sensitivity in murine ABCB1-, ABCG2-, and ABCC10-multidrug resistance xenograft models

A panel of clinically used tyrosine kinase inhibitors were compared and nilotinib was found to most potently sensitize specific anticancer agents by blocking the functions of ABCB1/P-glycoprotein, ABCG2/BCRP and ABCC10/MRP7 transporters involved in multi-drug resistance. Nilotinib appreciably enhanced the antitumor response of (1) paclitaxel in the ABCB1- and novel ABCC10-xenograft models, and (2) doxorubicin in a novel ABCG2-xenograft model. With no apparent toxicity observed in the above models, nilotinib attenuated tumor growth synergistically and increased paclitaxel concentrations in ABCB1-overexpressing tumors. The beneficial actions of nilotinib warrant consideration as viable combinations in the clinic with agents that suffer from MDR-mediated insensitivity.

Axitinib targeted cancer stemlike cells to enhance efficacy of chemotherapeutic drugs via inhibiting the drug transport function of ABCG2

Stemlike cells have been isolated by their ability to efflux Hoechst 33342 dye and are called the side population (SP). We evaluated the effect of axitinib on targeting cancer stemlike cells and enhancing the efficacy of chemotherapeutical agents. We found that axitinib enhanced the cytotoxicity of topotecan and mitoxantrone in SP cells sorted from human lung cancer A549 cells and increased cell apoptosis induced by chemotherapeutical agents. Moreover, axitinib particularly inhibited the function of adenosine triphosphate (ATP)-binding cassette subfamily G member 2 (ABCG2) and reversed ABCG2-mediated multidrug resistance (MDR) in vitro. However, no significant reversal effect was observed in ABCB1-, ABCC1- or lung resistance-related protein (LRP)-mediated MDR. Furthermore, in both sensitive and MDR cancer cells axitinib neither altered the expression of ABCG2 at the mRNA or protein levels nor blocked the phosphorylation of AKT and extracellular signal-regulated kinase (ERK)1/2. In nude mice bearing ABCG2-overexpressing S1-M1-80 xenografts, axitinib significantly enhanced the antitumor activity of topotecan without causing additional toxicity. Taken together, these data suggest that axitinib particularly targets cancer stemlike cells and reverses ABCG2-mediated drug resistance by inhibiting the transporter activity of ABCG2.

Neratinib reverses ATP-binding cassette B1-mediated chemotherapeutic drug resistance in vitro, in vivo, and ex vivo

Neratinib, an irreversible inhibitor of epidermal growth factor receptor and human epidermal receptor 2, is in phase III clinical trials for patients with human epidermal receptor 2-positive, locally advanced or metastatic breast cancer. The objective of this study was to explore the ability of neratinib to reverse tumor multidrug resistance attributable to overexpression of ATP-binding cassette (ABC) transporters. Our results showed that neratinib remarkably enhanced the sensitivity of ABCB1-overexpressing cells to ABCB1 substrates. It is noteworthy that neratinib augmented the effect of chemotherapeutic agents in inhibiting the growth of ABCB1-overexpressing primary leukemia blasts and KBv200 cell xenografts in nude mice. Furthermore, neratinib increased doxorubicin accumulation in ABCB1-overexpressing cell lines and Rhodamine 123 accumulation in ABCB1-overexpressing cell lines and primary leukemia blasts. Neratinib stimulated the ATPase activity of ABCB1 at low concentrations but inhibited it at high concentrations. Likewise, neratinib inhibited the photolabeling of ABCB1 with [(125)I]iodoarylazidoprazosin in a concentration-dependent manner (IC(50) = 0.24 μM). Neither the expression of ABCB1 at the mRNA and protein levels nor the phosphorylation of Akt was affected by neratinib at reversal concentrations. Docking simulation results were consistent with the binding conformation of neratinib within the large cavity of the transmembrane region of ABCB1, which provides computational support for the cross-reactivity of tyrosine kinase inhibitors with human ABCB1. In conclusion, neratinib can reverse ABCB1-mediated multidrug resistance in vitro, ex vivo, and in vivo by inhibiting its transport function.

OSI-930 analogues as novel reversal agents for ABCG2-mediated multidrug resistance

OSI-930, a dual c-Kit and KDR tyrosine kinase inhibitor, is reported to have undergone a Phase I dose escalation study in patients with advanced solid tumors. A series of fifteen pyridyl and phenyl analogues of OSI-930 were designed and synthesized. Extensive screening of these compounds led to the discovery that nitropyridyl and ortho-nitrophenyl analogues, VKJP1 and VKJP3, were effective in reversing ABC subfamily G member 2 (ABCG2) transporter-mediated multidrug resistance (MDR). VKJP1 and VKJP3 significantly sensitized ABCG2-expressing cells to established substrates of ABCG2 including mitoxantrone, SN-38, and doxorubicin in a concentration-dependent manner, but not to the non-ABCG2 substrate cisplatin. However, they were unable to reverse ABCB1- or ABCC1-mediated MDR indicating their selectivity for ABCG2. Western blotting analysis was performed to evaluate ABCG2 expression and it was found that neither VKJP1 nor VKJP3 significantly altered ABCG2 protein expression for up to 72 h. [(3)H]-mitoxantrone accumulation study demonstrated that VKJP1 and VKJP3 increased the intracellular accumulation of [(3)H]-mitoxantrone, a substrate of ABCG2. VKJP1 and VKJP3 also remarkably inhibited the transport of [(3)H]-methotrexate by ABCG2 membrane vesicles. Importantly, both VKJP1 and VKJP3 were efficacious in stimulating the activity of ATPase of ABCG2 and inhibited the photoaffinity labeling of this transporter by its substrate [(125)I]-iodoarylazidoprazosin. The results suggested that VKJP1 and VKJP3, specifically inhibit the function of ABCG2 through direct interaction with its substrate binding site(s). Thus VKJP1 and VKJP3 represent a new class of drugs for reducing MDR in ABCG2 over-expressing tumors.

Enhancing chemosensitivity in ABCB1- and ABCG2-overexpressing cells and cancer stem-like cells by an Aurora kinase inhibitor CCT129202

Imidazopyridine CCT129202 is an inhibitor of Aurora kinase activity and displays a favorable antineoplastic effect in preclinical studies. Here, we investigated the enhanced effect of CCT129202 on the cytotoxicity of chemotherapeutic drugs in multidrug resistant (MDR) cells with overexpression of ATP-binding cassette (ABC) transporters and cancer stem-like cells. CCT129202 of more than 90% cell survival concentration significantly enhanced the cytotoxicity of substrate drugs and increased the intracellular accumulations of doxorubicin and rhodamine 123 in ABCB1 and ABCG2 overexpressing cells, while no effect was found on parental sensitive cells. Interestingly, CCT129202 also potentiated the sensitivity of cancer stem-like cells to doxorubicin. Importantly, CCT129202 increased the inhibitory effect of vincristine and paclitaxel on ABCB1 overexpressing KBv200 cell xenografts in nude mice and human esophageal cancer tissue overexpressing ABCB1 ex vivo, respectively. Furthermore, the ATPase activity of ABCB1 was inhibited by CCT129202. Homology modeling predicted the binding conformation of CCT129202 within the large hydrophobic cavity of ABCB1. On the other hand, CCT129202 neither apparently altered the expression levels of ABCB1 and ABCG2 nor inhibited the activity of Aurora kinases in MDR cells under the concentration of reversal MDR. In conclusion, CCT129202 significantly reversed ABCB1- and ABCG2-mediated MDR in vitro, in vivo and ex vivo by inhibiting the function of their transporters and enhanced the eradication of cancer stem-like cells by chemotherapeutic agents. CCT129202 may be a candidate as MDR reversal agent for antineoplastic combination therapy and merits further clinical investigation.

GW583340 and GW2974, human EGFR and HER-2 inhibitors, reverse ABCG2- and ABCB1-mediated drug resistance

The overexpression of ATP binding cassette (ABC) transporters often leads to the development of multidrug resistance (MDR) and results in a suboptimal response to chemotherapy. Previously, we reported that lapatinib (GW572016), a human epidermal growth factor receptor (EGFR) and HER-2 tyrosine kinase inhibitor (TKI), significantly reverses MDR in cancer cells by blocking the efflux function of ABC subfamily B member 1 (ABCB1) and ABC subfamily G member 2 (ABCG2). In the present study, we conducted in vitro experiments to evaluate if GW583340 and GW2974, structural analogues of lapatinib, could reverse ABCB1- and ABCG2-mediated MDR. Our results showed that GW583340 and GW2974 significantly sensitized ABCB1 and ABCG2 overexpressing MDR cells to their anticancer substrates. GW583340 and GW2974 significantly increased the intracellular accumulation of [(3)H]-paclitaxel in ABCB1 overexpressing cells and [(3)H]-mitoxantrone in ABCG2 overexpressing cells respectively. In addition, GW583340 and GW2974 significantly inhibited ABCG2-mediated transport of methotrexate in ABCG2 overexpressing membrane vesicles. There was no significant change in the expression levels of ABCB1 and ABCG2 in the cell lines exposed to 5μM of either GW583340 or GW2974 for 3 days. In addition, a docking model predicted the binding conformation of GW583340 and GW2974 to be within the transmembrane region of homology modeled human ABCB1 and ABCG2. We conclude that GW583340 and GW2974, at clinically achievable plasma concentrations, reverse ABCB1- and ABCG2-mediated MDR by blocking the drug efflux function of these transporters. These findings may be useful in developing combination therapy for cancer treatment with EGFR TKIs.

Apatinib (YN968D1) enhances the efficacy of conventional chemotherapeutical drugs in side population cells and ABCB1-overexpressing leukemia cells

P-glycoprotein (P-gp, ABCB1) overexpression and enrichment of stem-like cells are linked to poor prognosis in tumor patients. In this study, we investigated the effect of apatinib, an oral multi-targeted tyrosine kinase inhibitor (TKI) on enhancing the efficacy of conventional anticancer drugs in side population (SP) cells and ABCB1-overexpressing leukemia cells in vitro, in vivo and ex vivo. Our results showed that apatinib significantly enhanced the cytotoxicity and cell apoptosis induced by doxorubicin in SP cells sorted from K562 cells. Furthermore, apatinib also strongly reversed multidrug resistance (MDR) in K562/ADR cells, and the primary leukemia blasts overexpressing ABCB1 while showed no synergistic interactions with chemotherapeutic agents in MRP1-, MRP4-, MRP7- and LRP-overexpressing cells. Apatinib treatment markedly increased the intracellular accumulation of doxorubicin and rhodamine 123 in K562/ADR cells and the accumulation of rhodamine 123 in the primary leukemia blasts with ABCB1 overexpression. Apatinib stimulated the ATPase activity of P-gp in a dose-dependent manner but did not alter the expression of ABCB1 at both mRNA and protein levels. The phosphorylation level of AKT and ERK1/2 remained unchanged after apatinib treatment in both sensitive and MDR cells. Importantly, apatinib significantly enhanced the antitumor activity of doxorubicin in nude mice bearing K562/ADR xenografts. Taken together, our results suggest that apatinib could target to SP cells and ABCB1-overexpressing leukemia cells to enhance the efficacy of chemotherapeutic drugs. These findings should be useful for the combination of apatinib and chemotherapeutic agents in the clinic.

Breast cancer resistance protein (BCRP/ABCG2): its role in multidrug resistance and regulation of its gene expression

Breast cancer resistance protein (BCRP)/ATP-binding cassette subfamily G member 2 (ABCG2) is an ATP-binding cassette (ABC) transporter identified as a molecular cause of multidrug resistance (MDR) in diverse cancer cells. BCRP physiologically functions as a part of a self-defense mechanism for the organism; it enhances elimination of toxic xenobiotic substances and harmful agents in the gut and biliary tract, as well as through the blood-brain, placental, and possibly blood-testis barriers. BCRP recognizes and transports numerous anticancer drugs including conventional chemotherapeutic and targeted small therapeutic molecules relatively new in clinical use. Thus, BCRP expression in cancer cells directly causes MDR by active efflux of anticancer drugs. Because BCRP is also known to be a stem cell marker, its expression in cancer cells could be a manifestation of metabolic and signaling pathways that confer multiple mechanisms of drug resistance, self-renewal (sternness), and invasiveness (aggressiveness), and thereby impart a poor prognosis. Therefore, blocking BCRP-mediated active efflux may provide a therapeutic benefit for cancers. Delineating the precise molecular mechanisms for BCRP gene expression may lead to identification of a novel molecular target to modulate BCRP-mediated MDR. Current evidence suggests that BCRP gene transcription is regulated by a number of trans-acting elements including hypoxia inducible factor 1α, estrogen receptor, and peroxisome proliferator-activated receptor. Furthermore, alternative promoter usage, demethylation of the BCRP promoter, and histone modification are likely associated with drug-induced BCRP overexpression in cancer cells. Finally, PI3K/AKT signaling may play a critical role in modulating BCRP function under a variety of conditions. These biological events seem involved in a complicated manner. Untangling the events would be an essential first step to developing a method to modulate BCRP function to aid patients with cancer. This review will present a synopsis of the impact of BCRP-mediated MDR in cancer cells, and the molecular mechanisms of acquired MDR currently postulated in a variety of human cancers.

Identification of proline residues in or near the transmembrane helices of the human breast cancer resistance protein (BCRP/ABCG2) that are important for transport activity and substrate specificity

The human breast cancer resistance protein (BCRP/ABCG2) confers multidrug resistance and mediates the active efflux of drugs and xenobiotics. BCRP contains one nucleotide-binding domain (NBD) followed by one membrane-spanning domain (MSD). We investigated whether prolines in or near the transmembrane helices are essential for BCRP function. Six proline residues were substituted with alanine individually, and the mutants were stably expressed in Flp-In(TM)-293 cells at levels comparable to that of wild-type BCRP and predominantly localized on the plasma membrane of the cells. While P392A showed a significant reduction (35-50%) in the efflux activity of mitoxantrone, BODIPY-prazosin, and Hoechst 33342, P485A exhibited a significant decrease of approximately 70% in the efflux activity of only BODIPY-prazosin. Other mutants had no significant changes in the efflux activities of these substrates. Drug resistance profiles of the cells expressing the mutants correlated well with the efflux data. ATPase activity was not substantially affected for P392A or P485A compared to that of wild-type BCRP. These results strongly suggest Pro(392) and Pro(485) are important in determining the overall transport activity and substrate selectivity of BCRP, respectively. Prazosin differentially affected the binding of 5D3, a conformation-sensitive antibody, to wild-type BCRP, P392A, or P485A in a concentration-dependent manner. In contrast, mitoxantrone had no significant effect on 5D3 binding. Homology modeling indicates that Pro(392) may play an important role in the communication between the MSD and NBD as it is predicted to be located at the interface between the two functional domains, and Pro(485) induces flexible hinges that may be essential for the broad substrate specificity of BCRP.

Expression and function of ABCG2 in head and neck squamous cell carcinoma and cell lines

Overexpression of breast cancer resistance protein, the ATP-binding cassette, subfamily G, member2 (BCRP/ABCG2), confers multidrug resistance to tumor cells and often limits the efficacy of chemotherapy. The aim of this study was to investigate the expression and functional activity of ABCG2 in head and neck squamous cell carcinoma (HNSCC) and corresponding cell lines. Immunohistochemistry was performed to investigate the presence of the ABCG2 transporter in HNSCC tissues. Expression of ABCG2 in the Hep-2, Hep-2T, CNE and FaDu cell lines was analyzed by real-time quantitative reverse transcription-polymerase chain reaction and Western blotting at the levels of messenger RNA (mRNA) and protein, respectively. The drug sensitivity of the above four cell lines to mitoxantrone was detected using MTT, and the drug accumulation of mitoxantrone was analyzed by flow cytometry. Positive expression of ABCG2 was detected in 52.04% of the laryngeal cancer samples from 98 patients, in 65% of the 40 hypopharyngeal cancer samples and in 58.82% of the 34 nasopharyngeal cancer samples. The level of expression was found to be correlated with tumor TNM stage (P<0.05) and lymph node metastasis (P<0.01). All four HNSCC cell lines expressed ABCG2 at the mRNA and protein levels. The levels of ABCG2 expression in the four cell lines were significantly correlated with the function and sensitivity to mitoxantrone. The addition of fumitremorgin C at a concentration of 5 μM to mitoxantrone treatment caused a varied increase in mitoxantrone accumulation of 1.09-fold, 1.33-fold (P<0.01), 1.4-fold (P<0.01) and 1-fold in the Hep-2, Hep-2T, CNE and FaDu cells, respectively. Expression of ABCG2 varied among the different types of carcinoma tissues and each HNSCC cell line, and it induced multidrug resistance and separation of cancer stem cells attributing to its efflux pump function. Thus, ABCG2 expression may be an unfavorable prognostic factor for HNSCC. Due to the negligible expression and function of ABCG2, we suggest that the FaDu cell line is suitable to be a negative control in studies involving HNSCC. Taken together, ABCG2 is a promising universal biomarker of cancer stem cells and a target gene for HNSCC chemotherapy.

Synthesis and characterization of a BODIPY conjugate of the BCR-ABL kinase inhibitor Tasigna (nilotinib): evidence for transport of Tasigna and its fluorescent derivative by ABC drug transporters

Tasigna (Nilotinib) is a BCR-ABL kinase inhibitor recently approved by the Food and Drug Administration, which is indicated for the treatment of drug-resistant chronic myelogenous leukemia (CML). The efflux of tyrosine kinase inhibitors by ATP-binding cassette (ABC) drug transporters, which actively pump these drugs out of cells utilizing ATP as an energy source, has been linked to the development of drug resistance in CML patients. We report here the synthesis and characterization of a fluorescent derivative of Tasigna to study its interaction with two major ABC transporters, P-glycoprotein (Pgp) and ABCG2, in in vitro and ex vivo assays. A fluorescent derivative of Tasigna, BODIPY FL Tasigna, inhibited the BCR-ABL kinase activity in K562 cells and was also effluxed by Pgp- and ABCG2-expressing cells in both cultured cells and rat brain capillaries expressing Pgp and ABCG2. In addition, [(3)H]-Tasigna was found to be transported by Pgp-expressing polarized LLC-PK1 cells in a transepithelial transport assay. Consistent with these results, both Tasigna and BODIPY FL Tasigna were less effective at inhibiting the phosphorylation of Crkl (a substrate of BCR-ABL kinase) in Pgp- and ABCG2-expressing K562 cells due to their reduced intracellular concentration. Taken together, these data provide evidence that BODIPY FL Tasigna is transported by Pgp and ABCG2, and Tasigna is transported by Pgp. Further, we propose that BODIPY FL Tasigna can potentially be used as a probe for functional analysis of Pgp and ABCG2 in cancer cells and in other preclinical studies.

ATP-binding cassette transporters modulate both coelenterazine- and D-luciferin-based bioluminescence imaging

Bioluminescence imaging (BLI) of luciferase reporters provides a cost-effective and sensitive means to image biological processes. However, transport of luciferase substrates across the cell membrane does affect BLI readout intensity from intact living cells. To investigate the effect of ATP-binding cassette (ABC) transporters on BLI readout, we generated click beetle (cLuc), firefly (fLuc), Renilla (rLuc), and Gaussia (gLuc) luciferase HEK-293 reporter cells that overexpressed different ABC transporters (ABCB1, ABCC1, and ABCG2). In vitro studies showed a significant BLI intensity decrease in intact cells compared to cell lysates, when ABCG2 was overexpressed in HEK-293/cLuc, fLuc, and rLuc cells. Selective ABC transporter inhibitors were also applied. Inhibition of ABCG2 activity increased the BLI intensity more than two-fold in HEK-293/cLuc, fLuc, and rLuc cells; inhibition of ABCB1 elevated the BLI intensity two-fold only in HEK-293/rLuc cells. BLI of xenografts derived from HEK-293/ABC transporter/luciferase reporter cells confirmed the results of inhibitor treatment in vivo. These findings demonstrate that coelenterazine-based rLuc-BLI intensity can be modulated by ABCB1 and ABCG2. ABCG2 modulates d-luciferin-based BLI in a luciferase type-independent manner. Little ABC transporter effect on gLuc-BLI intensity is observed because a large fraction of gLuc is secreted. The expression level of ABC transporters is one key factor affecting BLI intensity, and this may be particularly important in luciferase-based applications in stem cell research.

pH-Dependent transport of pemetrexed by breast cancer resistance protein

Breast cancer resistance protein (BCRP), an ATP-dependent efflux transporter, confers drug resistance to many chemotherapy agents. BCRP is overexpressed in tumors exposed to an acidic environment; therefore, it is important to establish the effect of low pH on BCRP transport activity. It has recently been reported that BCRP transports substrates more efficiently in an acidic microenvironment. In the study presented here, we examine the pH dependence of BCRP using methothrexate (MTX), pemetrexed (PMX), and estrone sulfate (ES) as model substrates. Our study revealed an increase of approximately 40-fold in the BCRP-mediated transport of PMX and MTX when the pH was decreased from 7.4 to 5.5. In contrast, only a 2-fold increase was observed for ES. These results indicate a mechanism of transport that is directly dependent on the effective ionization state of the substrates and BCRP. For ES, which retains a constant ionization state throughout the applied pH, the observed mild increase in activity is attributable to the overall changes in the effective ionization state and conformation of BCRP. For MTX and PMX, the marked increase in BCRP transport activity was likely due to the change in ionization state of MTX and PMX at lowered pH and their intermolecular interactions with BCRP. To further rationalize the molecular basis of the pH dependence, molecular modeling and docking studies were carried out using a homology model of BCRP, which has previously been closely examined in structural and site-directed mutagenesis studies (Am J Physiol Cell Physiol 299:C1100-C1109, 2010). On the basis of docking studies, all model compounds were found to associate with arginine 482 (Arg482) by direct salt-bridge interactions via their negatively charged carboxylate or sulfate groups. However, at lower pH, protonated MTX and PMX formed an additional salt-bridge interaction between their positively charged moieties and the nearby negatively charged aspartic acid 477 (Asp477) carboxylate side chain. The formation of this "salt-bridge triad" is expected to increase the overall electrostatic interactions between MTX and PMX with BCRP, which can form a rational basis for the pH dependence of the observed enhanced binding selectivity and transport activity. Removal of Arg482 in site-directed mutagenesis studies eliminated this pH dependence, which lends further support to our binding model. These results shed light on the importance of electrostatic interactions in transport activity and may have important implications in the design of ionizable chemotherapeutics intended for tumors in the acidic microenvironment.

ABCG2 is a direct transcriptional target of hedgehog signaling and involved in stroma-induced drug tolerance in diffuse large B-cell lymphoma

Successful treatment of diffuse large B-cell lymphoma (DLBCL) is frequently hindered by development of resistance to conventional chemotherapy resulting in disease relapse and high mortality. High expression of anti-apoptotic and/or drug transporter proteins induced by oncogenic signaling pathways has been implicated in the development of chemoresistance in cancer. Previously, our studies showed high expression of ATP-binding cassette drug transporter ABCG2 in DLBCL correlated inversely with disease-free and failure-free survival. In this study, we have implicated activated hedgehog (Hh) signaling pathway as a key factor behind high ABCG2 expression in DLBCL through direct upregulation of ABCG2 gene transcription. We have identified a single binding site for GLI transcription factors in the ABCG2 promoter and established its functionality using luciferase reporter, site-directed mutagenesis and chromatin-immunoprecipitation assays. Furthermore, in DLBCL tumor samples, significantly high ABCG2 and GLI1 levels were found in DLBCL tumors with lymph node involvement in comparison to DLBCL tumor cells collected from pleural and/or peritoneal effusions. This suggests a role for the stromal microenvironment in maintaining high levels of ABCG2 and GLI1. Accordingly, in vitro co-culture of DLBCL cells with HS-5 stromal cells increased ABCG2 mRNA and protein levels by paracrine activation of Hh signaling. In addition to ABCG2, co-culture of DLBCL cells with HS-5 cells also resulted in increase expression of the antiapoptotic proteins BCL2, BCL-xL and BCL2A1 and in induced chemotolerance to doxorubicin and methotrexate, drugs routinely used for the treatment of DLBCL. Similarly, activation of Hh signaling in DLBCL cell lines with recombinant Shh N-terminal peptide resulted in increased expression of BCL2 and ABCG2 associated with increased chemotolerance. Finally, functional inhibition of ABCG2 drug efflux activity with fumitremorgin (FTC) or inhibition of Hh signaling with cyclopamine-KAAD abrogated the stroma-induced chemotolerance suggesting that targeting ABCG2 and Hh signaling may have therapeutic value in overcoming chemoresistance in DLBCL.

The phosphodiesterase-5 inhibitor vardenafil is a potent inhibitor of ABCB1/P-glycoprotein transporter

One of the major causes of chemotherapy failure in cancer treatment is multidrug resistance (MDR) which is mediated by the ABCB1/P-glycoprotein. Previously, through the use of an extensive screening process, we found that vardenafil, a phosphodiesterase 5 (PDE-5) inhibitor significantly reverses MDR in ABCB1 overexpressing cancer cells, and its efficacy was greater than that of tadalafil, another PDE-5 inhibitor. The present study was designed to determine the reversal mechanisms of vardenafil and tadalafil on ABC transporters-mediated MDR. Vardenafil or tadalafil alone, at concentrations up to 20 µM, had no significant toxic effects on any of the cell lines used in this study, regardless of their membrane transporter status. However, vardenafil when used in combination with anticancer substrates of ABCB1, significantly potentiated their cytotoxicity in ABCB1 overexpressing cells in a concentration-dependent manner, and this effect was greater than that of tadalafil. The sensitivity of the parenteral cell lines to cytotoxic anticancer drugs was not significantly altered by vardenafil. The differential effects of vardenafil and tadalafil appear to be specific for the ABCB1 transporter as both vardenafil and tadalafil had no significant effect on the reversal of drug resistance conferred by ABCC1 (MRP1) and ABCG2 (BCRP) transporters. Vardenafil significantly increased the intracellular accumulation of [³H]-paclitaxel in the ABCB1 overexpressing KB-C2 cells. In addition, vardenafil significantly stimulated the ATPase activity of ABCB1 and inhibited the photolabeling of ABCB1 with [¹²⁵I]-IAAP. Furthermore, Western blot analysis indicated the incubation of cells with either vardenafil or tadalafil for 72 h did not alter ABCB1 protein expression. Overall, our results suggest that vardenafil reverses ABCB1-mediated MDR by directly blocking the drug efflux function of ABCB1.

Human ABCG2: structure, function, and its role in multidrug resistance

Human ABCG2 is a member of the ATP-binding cassette (ABC) transporter superfamily and is known to contribute to multidrug resistance (MDR) in cancer chemotherapy. Among ABC transporters that are known to cause MDR, ABCG2 is particularly interesting for its potential role in protecting cancer stem cells and its complex oligomeric structure. Recent studies have also revealed that the biogenesis of ABCG2 could be modulated by small molecule compounds. These modulators, upon binding to ABCG2, accelerate the endocytosis and trafficking to lysosome for degradation and effectively reduce the half-life of ABCG2. Hence, targeting ABCG2 stability could be a new venue for therapeutic discovery to sensitize drug resistant human cancers. In this report, we review recent progress on understanding the structure, function, biogenesis, as well as physiological and pathophysiological functions of ABCG2.

Sildenafil reverses ABCB1- and ABCG2-mediated chemotherapeutic drug resistance

Sildenafil is a potent and selective inhibitor of the type 5 cGMP (cyclic guanosine 3',5'-monophosphate)-specific phosphodiesterase that is used clinically to treat erectile dysfunction and pulmonary arterial hypertension. Here, we report that sildenafil has differential effects on cell surface ABC transporters such as ABCB1, ABCC1, and ABCG2 that modulate intracompartmental and intracellular concentrations of chemotherapeutic drugs. In ABCB1-overexpressing cells, nontoxic doses of sildenafil inhibited resistance and increased the effective intracellular concentration of ABCB1 substrate drugs such as paclitaxel. Similarly, in ABCG2-overexpressing cells, sildenafil inhibited resistance to ABCG2 substrate anticancer drugs, for example, increasing the effective intracellular concentration of mitoxantrone or the fluorescent compound BODIPY-prazosin. Sildenafil also moderately inhibited the transport of E(2)17βG and methotrexate by the ABCG2 transporter. Mechanistic investigations revealed that sildenafil stimulated ABCB1 ATPase activity and inhibited photolabeling of ABCB1 with [(125)I]-iodoarylazidoprazosin (IAAP), whereas it only slightly stimulated ABCG2 ATPase activity and inhibited photolabeling of ABCG2 with [(125)I]-IAAP. In contrast, sildenafil did not alter the sensitivity of parental, ABCB1-, or ABCG2-overexpressing cells to non-ABCB1 and non-ABCG2 substrate drugs, nor did sildenafil affect the function of another ABC drug transporter, ABCC1. Homology modeling predicted the binding conformation of sildenafil within the large cavity of the transmembrane region of ABCB1. Overall, we found that sildenafil inhibits the transporter function of ABCB1 and ABCG2, with a stronger effect on ABCB1. Our findings suggest a possible strategy to enhance the distribution and potentially the activity of anticancer drugs by jointly using a clinically approved drug with known side effects and drug-drug interactions.

14-Aminocamptothecins: their synthesis, preclinical activity, and potential use for cancer treatment

14-Aminocamptothecins were synthesized in good yields by treating camptothecin (1a) and 7-ethylcamptothecin (1b) with 90% fuming nitric acid either neat or in acetic anhydride and then followed by reduction of the resulting 14-nitrocamptothecins (2). 14-Aminocamptothecin (3a) and 7-ethyl-14-aminocamptothecin (3b) demonstrated excellent cytotoxic potency against human tumor cell lines in vitro, and they are not substrates for any of the major clinically relevant efflux pumps (MDR1, MRP1, and BCRP). 3a and 3b showed similar cytotoxicity against human and mouse bone marrow progenitor cells. This is in contrast to many camptothecin analogues, which are substrates for efflux pumps and are dramatically more toxic to human marrow cells relative to murine. 3a and 3b demonstrated significant brain penetration when dosed orally in mice. 3b showed significantly better efficacy relative to topotecan when dosed orally in the three ectopic xenograft models, H460, HT29, and PC-3. On the basis of its favorable in vitro and in vivo profile, 3b warrants future development.

The acridone derivative MBLI-87 sensitizes breast cancer resistance protein-expressing xenografts to irinotecan

The breast cancer resistance protein ABCG2 confers cellular resistance to irinotecan (CPT-11) and its active metabolite SN-38. We utilised ABCG2-expressing xenografts as a model to evaluate the ability of a non-toxic ABCG2 inhibitor to increase intracellular drug accumulation. We assessed the activity of irinotecan in vivo in SCID mice: irinotecan completely inhibited the development of control pcDNA3.1 xenografts, whilst only delaying the growth of ABCG2-expressing xenografts. Addition of MBLI-87, an acridone derivative inhibitor, significantly increased the irinotecan effect against the growth of ABCG2-expressing xenografts. In vitro, MBLI-87 was as potent as GF120918 against ABCG2-mediated irinotecan efflux, and additionally was specific for ABCG2. A significant sensitisation to irinotecan was achieved despite the fact that doses remained well below the maximum tolerated dose (due to the rather limited solubility of MBLI-87). This suggested that MBLI-87 is an excellent candidate to prevent drug efflux by ABCG2, without altering plasma concentrations of irinotecan and SN-38 after IP (intra-peritoneal) injections. This could constitute a useful strategy to improve drug pharmacology, to facilitate drug penetration into normal tissue compartments protected by ABCG2, and potentially to reverse drug resistance in cancer cells.

In vitro and in vivo evidence for the importance of breast cancer resistance protein transporters (BCRP/MXR/ABCP/ABCG2)

The breast cancer resistance protein (BCRP/ABCG2) is a member of the G-subfamiliy of the ATP-binding cassette (ABC)-transporter superfamily. This half-transporter is assumed to function as an important mechanism limiting cellular accumulation of various compounds. In context of its tissue distribution with localization in the sinusoidal membrane of hepatocytes, and in the apical membrane of enterocytes ABCG2 is assumed to function as an important mechanism facilitating hepatobiliary excretion and limiting oral bioavailability, respectively. Indeed functional assessment performing mouse studies with genetic deletion or chemical inhibition of the transporter, or performing pharmacogenetic studies in humans support this assumption. Furthermore the efflux function of ABCG2 has been linked to sanctuary blood tissue barriers as described for placenta and the central nervous system. However, in lactating mammary glands ABCG2 increases the transfer of substrates into milk thereby increasing the exposure to potential noxes of a breastfed newborn. With regard to its broad substrate spectrum including various anticancer drugs and environmental carcinogens the function of ABCG2 has been associated with multidrug resistance and tumor development/progression. In terms of cancer biology current research is focusing on the expression and function of ABCG2 in immature stem cells. Recent findings support the notion that the physiological function of ABCG2 is involved in the elimination of uric acid resulting in higher risk for developing gout in male patients harboring genetic variants. Taken together ABCG2 is implicated in various pathophysiological and pharmacological processes.

Rapid detection of ABC transporter interaction: potential utility in pharmacology

INTRODUCTION

The ATP-binding cassette (ABC) transporters P-glycoprotein (P-gp/ABCB1), multidrug resistance-associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) are known to transport a wide range of structurally diverse compounds. Their high level of expression at the blood-brain, maternal-fetal, and blood-testis barriers as well as their purported roles in oral absorption suggests that ABC transporters play important pharmacologic roles.

METHODS

We have developed a method to characterize the function and inhibition of ABC transporters using an automated cell counter with fluorescence detection capability. The assay was performed using stably-transfected HEK293 cells expressing P-gp, MRP1, or ABCG2 and examining transport of fluorescent substrates in the presence or absence of known inhibitors and compared to results obtained with a flow cytometer. Fold increase in intracellular fluorescence was then calculated for cells incubated with fluorescent substrate in the absence of inhibitor versus in the presence of inhibitor.

RESULTS

Fold increase values obtained either with the cell counter or flow cytometer were comparable for cells expressing either MRP1 or ABCG2; slightly higher fold increase values were observed when cells expressing P-gp were read on a flow cytometer compared to the cell counter.

DISCUSSION

The assay described provides an inexpensive detection method to aid in the development of novel ABC transporter inhibitors or to characterize potential drug-drug interactions.

Apatinib (YN968D1) reverses multidrug resistance by inhibiting the efflux function of multiple ATP-binding cassette transporters

Apatinib, a small-molecule multitargeted tyrosine kinase inhibitor, is in phase III clinical trial for the treatment of patients with non-small-cell lung cancer and gastric cancer in China. In this study, we determined the effect of apatinib on the interaction of specific antineoplastic compounds with P-glycoprotein (ABCB1), multidrug resistance protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2). Our results showed that apatinib significantly enhanced the cytotoxicity of ABCB1 or ABCG2 substrate drugs in KBv200, MCF-7/adr, and HEK293/ABCB1 cells overexpressing ABCB1 and in S1-M1-80, MCF-7/FLV1000, and HEK293/ABCG2-R2 cells overexpressing ABCG2 (wild-type). In contrast, apatinib did not alter the cytotoxicity of specific substrates in the parental cells and cells overexpressing ABCC1. Apatinib significantly increased the intracellular accumulation of rhodamine 123 and doxorubicin in the multidrug resistance (MDR) cells. Furthermore, apatinib significantly inhibited the photoaffinity labeling of both ABCB1 and ABCG2 with [(125)I]iodoarylazidoprazosin in a concentration-dependent manner. The ATPase activity of both ABCB1 and ABCG2 was significantly increased by apatinib. However, apatinib, at a concentration that produced a reversal of MDR, did not significantly alter the ABCB1 or ABCG2 protein or mRNA expression levels or the phosphorylation of AKT and extracellular signal-regulated kinase 1/2 (ERK1/2). Importantly, apatinib significantly enhanced the effect of paclitaxel against the ABCB1-resistant KBv200 cancer cell xenografts in nude mice. In conclusion, apatinib reverses ABCB1- and ABCG2-mediated MDR by inhibiting their transport function, but not by blocking the AKT or ERK1/2 pathway or downregulating ABCB1 or ABCG2 expression. Apatinib may be useful in circumventing MDR to other conventional antineoplastic drugs.

Apatinib (YN968D1) reverses multidrug resistance by inhibiting the efflux function of multiple ATP-binding cassette transporters

Apatinib, a small-molecule multitargeted tyrosine kinase inhibitor, is in phase III clinical trial for the treatment of patients with non-small-cell lung cancer and gastric cancer in China. In this study, we determined the effect of apatinib on the interaction of specific antineoplastic compounds with P-glycoprotein (ABCB1), multidrug resistance protein 1 (MRP1, ABCC1), and breast cancer resistance protein (BCRP, ABCG2). Our results showed that apatinib significantly enhanced the cytotoxicity of ABCB1 or ABCG2 substrate drugs in KBv200, MCF-7/adr, and HEK293/ABCB1 cells overexpressing ABCB1 and in S1-M1-80, MCF-7/FLV1000, and HEK293/ABCG2-R2 cells overexpressing ABCG2 (wild-type). In contrast, apatinib did not alter the cytotoxicity of specific substrates in the parental cells and cells overexpressing ABCC1. Apatinib significantly increased the intracellular accumulation of rhodamine 123 and doxorubicin in the multidrug resistance (MDR) cells. Furthermore, apatinib significantly inhibited the photoaffinity labeling of both ABCB1 and ABCG2 with [(125)I]iodoarylazidoprazosin in a concentration-dependent manner. The ATPase activity of both ABCB1 and ABCG2 was significantly increased by apatinib. However, apatinib, at a concentration that produced a reversal of MDR, did not significantly alter the ABCB1 or ABCG2 protein or mRNA expression levels or the phosphorylation of AKT and extracellular signal-regulated kinase 1/2 (ERK1/2). Importantly, apatinib significantly enhanced the effect of paclitaxel against the ABCB1-resistant KBv200 cancer cell xenografts in nude mice. In conclusion, apatinib reverses ABCB1- and ABCG2-mediated MDR by inhibiting their transport function, but not by blocking the AKT or ERK1/2 pathway or downregulating ABCB1 or ABCG2 expression. Apatinib may be useful in circumventing MDR to other conventional antineoplastic drugs.

Structure and function of the human breast cancer resistance protein (BCRP/ABCG2)

The human breast cancer resistance protein (BCRP/ABCG2) is the second member of the G subfamily of the large ATP-binding cassette (ABC) transporter superfamily. BCRP was initially discovered in multidrug resistant breast cancer cell lines where it confers resistance to chemotherapeutic agents such as mitoxantrone, topotecan and methotrexate by extruding these compounds out of the cell. BCRP is capable of transporting non-chemotherapy drugs and xenobiotiocs as well, including nitrofurantoin, prazosin, glyburide, and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine. BCRP is frequently detected at high levels in stem cells, likely providing xenobiotic protection. BCRP is also highly expressed in normal human tissues including the small intestine, liver, brain endothelium, and placenta. Therefore, BCRP has been increasingly recognized for its important role in the absorption, elimination, and tissue distribution of drugs and xenobiotics. At present, little is known about the transport mechanism of BCRP, particularly how it recognizes and transports a large number of structurally and chemically unrelated drugs and xenobiotics. Here, we review current knowledge of structure and function of this medically important ABC efflux drug transporter.

Substrate affinity of photosensitizers derived from chlorophyll-a: the ABCG2 transporter affects the phototoxic response of side population stem cell-like cancer cells to photodynamic therapy

Photosensitizers (PS) synthesized with the aim of optimizing photodynamic therapy (PDT) of tumors do not always fulfill their potential when tested in vitro and in vivo in different tumor models. The ATP-dependent transporter ABCG2, a multidrug resistant pump expressed at variable levels in cancerous cells, can bind and efflux a wide range of structurally different classes of compounds including several PS used preclinically and clinically such as porphyrins and chlorins. ABCG2 may lower intracellular levels of substrate PS below the threshold for cell death in tumors treated by PDT, leaving resistant cells to repopulate the tumor. To determine some of the structural factors that affect substrate affinity of PS for ABCG2, we used an ABCG2-expressing cell line (HEK 293 482R) and its nonexpressing counterpart, and tyrosine kinase ABCG2 inhibitors in a simple flow cytometric assay to identify PS effluxed by the ABCG2 pump. We tested a series of conjugates of substrate PS with different groups attached at different positions on the tetrapyrrole macrocycle to examine whether a change in affinity for the pump occurred and whether such changes depended on the position or the structure/type of the attached group. PS without substitutions including pyropheophorbides and purpurinimides were generally substrates for ABCG2, but carbohydrate groups conjugated at positions 8, 12, 13, and 17 but not at position 3 abrogated ABCG2 affinity regardless of structure or linking moiety. At position 3, affinity was retained with the addition of iodobenzene, alkyl chains and monosaccharides, but not with disaccharides. This suggests that structural characteristics at position 3 may offer important contributions to requirements for binding to ABCG2. We examined several tumor cell lines for ABCG2 activity, and found that although some cell lines had negligible ABCG2 activity in bulk, they contained a small ABCG2-expressing side population (SP) thought to contain cells which are responsible for initiating tumor regrowth. We examined the relevance of the SP to PDT resistance with ABCG2 substrates in vitro and in vivo in the murine mammary tumor 4T1. We show for the first time in vivo that the substrate PS HPPH (2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a) but not the nonsubstrate PS HPPH-Gal (a galactose conjugate of HPPH) selectively preserved the SP which was primarily responsible for regrowth in vitro. The SP could be targeted by addition of imatinib mesylate, a tyrosine kinase inhibitor which inhibits the ATPase activity of ABCG2, and prevents efflux of substrates. A PDT resistant SP may be responsible for recurrences observed both preclinically and clinically. To prevent ABCG2 mediated resistance, choosing nonsubstrate PS or administering an ABCG2 inhibitor alongside a substrate PS might be advantageous when treating ABCG2-expressing tumors with PDT.

Transmembrane helices 1 and 6 of the human breast cancer resistance protein (BCRP/ABCG2): identification of polar residues important for drug transport

The human breast cancer resistance protein (BCRP/ABCG2) mediates efflux of drugs and xenobiotics. In this study, we investigated the role of polar residues within or near the predicted transmembrane α-helices 1 and 6 of BCRP in drug transport. We substituted Asn(387), Gln(398), Asn(629), and Thr(642) with Ala, Thr(402) with Ala and Arg, and Tyr(645) with Phe, and the mutants were stably expressed in human embryonic kidney-293 or Flp-In-293 cells. Immunoblotting and confocal microscopy analysis revealed that all of the mutants were well expressed and predominantly targeted to the plasma membrane. While T402A and T402R showed a significant global reduction in the efflux of mitoxantrone, Hoechst 33342, and BODIPY-prazosin, N629A exhibited significantly increased efflux activities for all of the substrates. N387A and Q398A displayed significantly impaired efflux for mitoxantrone and Hoechst 33342, but not for BODIPY-prazosin. In contrast, T642A and Y645F showed a moderate reduction in Hoechst 33342 efflux only. Drug resistance profiles of human embryonic kidney-293 cells expressing the mutants generally correlated with the efflux data. Furthermore, N629A was associated with a marked increase, and N387A and T402A with a significant reduction, in BCRP ATPase activity. Mutations of some of the polar residues may cause conformational changes, as manifested by the altered binding of the 5D3 antibody to BCRP in the presence of prazosin. The inward-facing homology model of BCRP indicated that Thr(402) within transmembrane 1 may be important for helical interactions, and Asn(629) may be involved in BCRP-substrate interaction. In conclusion, we have demonstrated the functional importance of some of these polar residues in BCRP activity.

Marine sponge-derived sipholane triterpenoids reverse P-glycoprotein (ABCB1)-mediated multidrug resistance in cancer cells

Previously, we reported sipholenol A, a sipholane triterpenoid from the Red Sea sponge Callyspongia siphonella, as a potent reversal of multidrug resistance (MDR) in cancer cells that overexpressed P-glycoprotein (P-gp). Through extensive screening of several related sipholane triterpenoids that have been isolated from the same sponge, we identified sipholenone E, sipholenol L and siphonellinol D as potent reversals of MDR in cancer cells. These compounds enhanced the cytotoxicity of several P-gp substrate anticancer drugs, including colchicine, vinblastine and paclitaxel, and significantly reversed the MDR-phenotype in P-gp-overexpressing MDR cancer cells KB-C2 in a dose-dependent manner. Moreover, these three sipholanes had no effect on the response to cytotoxic agents in cells lacking P-gp expression or expressing MRP1 (ABCC1) or MRP7 (ABCC10) or breast cancer resistance protein (BCRP/ABCG2). All three sipholanes (IC(50) >50 μM) were not toxic to all the cell lines that were used. [(3)H]-Paclitaxel accumulation and efflux studies demonstrated that all three triterpenoids time-dependently increased the intracellular accumulation of [(3)H]-paclitaxel by directly inhibiting P-gp-mediated drug efflux. Sipholanes also inhibited calcein-AM transport from P-gp-overexpressing cells. The Western blot analysis revealed that these three triterpenoids did not alter the expression of P-gp. However, they stimulated P-gp ATPase activity in a concentration-dependent manner and inhibited the photolabeling of this transporter with its transport substrate [(125)I]-iodoarylazidoprazosin. In silico molecular docking aided the virtual identification of ligand binding sites of these compounds. In conclusion, sipholane triterpenoids efficiently inhibit the function of P-gp through direct interactions and may represent potential reversal agents for the treatment of MDR.

Interaction of drugs of abuse and maintenance treatments with human P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2)

Drug interaction with P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) may influence its tissue disposition including blood-brain barrier transport and result in potent drug-drug interactions. The limited data obtained using in-vitro models indicate that methadone, buprenorphine, and cannabinoids may interact with human P-gp; but almost nothing is known about drugs of abuse and BCRP. We used in vitro P-gp and BCRP inhibition flow cytometric assays with hMDR1- and hBCRP-transfected HEK293 cells to test 14 compounds or metabolites frequently involved in addiction, including buprenorphine, norbuprenorphine, methadone, ibogaine, cocaine, cocaethylene, amphetamine, N-methyl-3,4-methylenedioxyamphetamine, 3,4-methylenedioxyamphetamine, nicotine, ketamine, Delta9-tetrahydrocannabinol (THC), naloxone, and morphine. Drugs that in vitro inhibited P-gp or BCRP were tested in hMDR1- and hBCRP-MDCKII bidirectional transport studies. Human P-gp was significantly inhibited in a concentration-dependent manner by norbuprenorphine>buprenorphine>methadone>ibogaine and THC. Similarly, BCRP was inhibited by buprenorphine>norbuprenorphine>ibogaine and THC. None of the other tested compounds inhibited either transporter, even at high concentration (100 microm). Norbuprenorphine (transport efflux ratio approoximately 11) and methadone (transport efflux ratio approoximately 1.9) transport was P-gp-mediated; however, with no significant stereo-selectivity regarding methadone enantiomers. BCRP did not transport any of the tested compounds. However, the clinical significance of the interaction of norbuprenorphine with P-gp remains to be evaluated.

Comparison of ATP-binding cassette transporter interactions with the tyrosine kinase inhibitors imatinib, nilotinib, and dasatinib

Although the development of tyrosine kinase inhibitors (TKIs) to control the unregulated activity of BCR-ABL revolutionized the therapy of chronic myeloid leukemia, resistance to TKIs is a clinical reality. Among the postulated mechanisms of resistance is the overexpression of ATP-binding cassette (ABC) transporters, such as P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2), which mediate reduced intracellular drug accumulation. We compared the interactions of the TKIs imatinib, nilotinib, and dasatinib with ABCB1 and ABCG2 in ex vivo and in vitro systems. The TKIs inhibited rhodamine 123 and Hoechst 33342 efflux mediated by endogenous expression of the transporters in murine and human hematopoietic stem cells with potency order nilotinib >> imatinib >> dasatinib. Studies with ABCB1-, ABCG2-, and ABCC1-transfected human embryonic kidney 293 cells verified that nilotinib was the most potent inhibitor of ABCB1 and ABCG2. Cytotoxicity assays in stably transduced K562-ABCG2 and K562-ABCB1 cells confirmed that the TKIs were also substrates for the two transporters. Like imatinib, both nilotinib and dasatinib decreased ABCG2 surface expression in K562-ABCG2 cells. Finally, we found that all TKIs were able to compete labeling of ABCB1 and ABCG2 by the photo-cross-linkable prazosin analog [(125)I]iodoarylazidoprazosin, suggesting interaction at the prazosin-binding site of both proteins. Our experiments support the hypothesis that all three TKIs are substrates of ABC transporters and that, at higher concentrations, TKIs overcome transporter function. Taken together, the results suggest that therapeutic doses of imatinib and nilotinib may diminish the potential of ABCB1 and ABCG2 to limit oral absorption or confer resistance. Clinical data are required to definitively answer the latter question.

Organic cation uptake is enhanced in bcrp1-transfected MDCKII cells

Stably transfected cell models are routinely used to examine drug-transporter interactions. In one such model of bcrp1-transfected MDCKII cells, we observed a significant enhancement of organic cation intracellular accumulation. Therefore, our goal was to further explore the expression and functional consequences of this cation transport system. Transport assays were carried out in wild-type and bcrp1-transfected MDCKII cells to examine uptake of [3H]-prazosin (bcrp1 positive control), [3H]-agmatine, [3H]-TEA, and [14C]-choline. RT-PCR was employed to determine the mRNA levels of bcrp1 and OCT2/OCT3. Western blots were used to evaluate corresponding protein levels. Accumulation studies determined a significant increase in the uptake of the organic cations agmatine, TEA, and choline in bcrp1-transfected cells when compared to wild-type cells. Directional transport of [3H]-agmatine showed a significantly greater apical (A) to basolateral (B) than B-to-A flux in both cell types. In spite of this, the A-to-B flux was significantly lower in bcrp1-transfected cells. RT-PCR revealed 10-fold higher OCT2 mRNA levels in bcrp1-transfected cells, with no changes in OCT3. OCT2 protein expression was approximately 3.5-fold higher in bcrp1-transfected cells. The upregulation of OCT2 in bcrp1-transfected MDCKII cells contributed to a significant enhancement in the uptake of several organic cations. These results are consistent with the endogenous expression of OCT2 in the kidney tubule, and may be related to the expression and function of bcrp1. Our findings illustrate the importance of understanding how endogenous transporters, which may compete for common substrates, may be influenced by the overexpression and enhanced function of recombinant transport systems.

Role of basic residues within or near the predicted transmembrane helix 2 of the human breast cancer resistance protein in drug transport

The human breast cancer resistance protein (BCRP/ABCG2) mediates efflux of drugs and xenobiotics out of cells. In this study, we investigated the role of five basic residues within or near transmembrane (TM) 2 of BCRP in transport activity. Lys(452), Lys(453), His(457), Arg(465), and Lys(473) were replaced with Ala or Asp. K452A, K453D, H457A, R465A, and K473A were stably expressed in human embryonic kidney (HEK) cells, and their plasma membrane expression and transport activities were examined. All of the mutants were expressed predominantly on the plasma membrane of HEK cells. After normalization to BCRP levels, the activities of K452A and H457A in effluxing mitoxantrone, boron-dipyrromethene-prazosin, and Hoechst33342 were increased approximately 2- to 6-fold compared with those of wild-type BCRP, whereas the activities of K453D and R465A were decreased by 40 to 60%. Likewise, K452A and H457A conferred increased resistance to mitoxantrone and 7-ethyl-10-hydroxy-camptothecin (SN-38), and K453D and R465A exhibited lower resistance. The transport activities and drug-resistance profiles of K473A were not changed. These mutations also differentially affected BCRP ATPase activities with a 2- to 4-fold increase in V(max)/K(m) for K452A and H457A and a 40 to 70% decrease for K453D and R465A. These mutations may induce conformational changes as manifested by the altered binding of the 5D3 antibody to BCRP in the presence of prazosin and altered trypsin digestion. Molecular modeling and docking calculations indicated that His(457) and Arg(465) might be directly involved in substrate binding. In conclusion, we have identified several basic residues within or near TM2 that may be important for interaction of substrates with BCRP.

ABCG2: a potential marker of stem cells and novel target in stem cell and cancer therapy

ABCG2 is a member of the ATP binding cassette (ABC) transporters, which can pump a wide variety of endogenous and exogenous compounds out of cells. Widely expressed in stem cells, ABCG2 is also found to confer the side population phenotype and is recognized as a universal marker of stem cells. Although the precise physiological role of ABCG2 in stem cells is still unclear, existing data strongly suggest that ABCG2 plays an important role in promoting stem cell proliferation and the maintenance of the stem cell phenotype. In addition, ABCG2 is also found to be expressed in a number of cancer cells and appears to be a marker of cancer stem cells. Moreover, ABCG2 expression in tumors may contribute to their formation and progression. Thus, ABCG2 has potential applications in stem cell and tumor therapy.

ABCG2: the key to chemoresistance in cancer stem cells?

Multi-drug chemoresistance remains one of the most common reasons for chemotherapy failure. The membrane transporter protein ABCG2/BCRP1 has been shown in vitro to effectively reduce the intracellular concentrations of several prominent anticancer chemotherapeutic agents such as mitoxantrone and doxorubicin. Intriguingly, cancer stem cells are known to be characterized by multi-drug chemoresistance. Taking into account that the ABCG2(+) subset of tumor cells are often enriched with cells with cancer stem-like phenotypes, it has been proposed that ABCG2 activity underlies the ability of cancer cells to regenerate post-chemotherapy. Furthermore, we also review evidence suggesting that tyrosine kinase inhibitors, including imatinib and gefitinib, are both direct and downstream inactivators of ABCG2 and, therefore, serve as candidates to reverse cancer stem cell chemoresistance and potentially target cancer stem cells.

Pharmacogenetics of membrane transporters: an update on current approaches

This review provides an overview of the pharmacogenetics of membrane transporters including selected ABC transporters (ABCB1, ABCC1, ABCC2, and ABCG2) and OATPs (OATP1B1 and OATP1B3). Membrane transporters are heavily involved in drug clearance and alters drug disposition by actively transporting substrate drugs between organs and tissues. As such, polymorphisms in the genes encoding these proteins may have significant effects on the absorption, distribution, metabolism and excretion of compounds, and may alter pharmacodynamics of many agents. This review discusses the techniques used to identify substrates and inhibitors of these proteins and subsequently to assess the effect of genetic mutation on transport, both in vitro and in vivo. A comprehensive list of substrates for the major drug transporters is included. Finally, studies linking transporter genotype with clinical outcomes are discussed.

Impact of breast cancer resistance protein on cancer treatment outcomes

Breast cancer resistance protein (BCRP/ABCG2) was discovered in multidrug resistant breast cancer cells having an ATP-dependent transport-based resistance phenotype. This ABC transporter functions (at least in part) as a xenobiotic protective mechanism for the organism: in the gut and biliary tract, it prevents absorption and enhances elimination of potentially toxic substances. As a placental barrier, it protects the fetus; similarly, it serves as a component of blood-brain and blood-testis barrier; BCRP is expressed in stem cells and may protect them from potentially harmful agents. Therefore, BCRP could influence cancer outcomes by (a) endogenous BCRP affecting the absorption, distribution, metabolism, and elimination of anticancer drugs; (b) BCRP expression in cancer cells may directly cause resistance by active efflux of anticancer drugs; (c) BCRP expression in cancer cells could be a manifestation of the activity of metabolic and signaling pathways that impart multiple mechanisms of drug resistance, self-renewal (stemness), and invasiveness (aggressiveness)--i.e. impart a poor prognosis--to cancers. This chapter presents a synopsis of translational clinical studies relating BCRP expression in leukemias, lymphomas, and a variety of solid tumors with clinical outcome. Data are emerging that expression of BCRP, like P-glycoprotein/ABCB1, is associated with adverse outcomes in a variety of human cancers. Whether this adverse prognostic effect results from resistance imparted to the cancer cells as the direct result of BCRP efflux of anticancer drugs, or whether BCRP expression (and also Pgp expression - coexpression of these transporters is common among poor risk cancers) serves as indicators of the activity of signaling pathways that enhance cancer cellular proliferation, metastases, genomic instability, enhance drug resistance, and oppose programmed cell death mechanisms is yet unknown.

Botryllamides: natural product inhibitors of ABCG2

ABCG2 is a membrane-localized, human transporter protein that has been demonstrated to reduce the intracellular accumulation of substrates through ATP-dependent efflux. Highly expressed in placental syncytiotrophoblasts, brain microvasculature, and the gastrointestinal tract, ABCG2 has been shown to mediate normal tissue protection as well as limit oral bioavailability of substrate compounds. Development of ABCG2 inhibitors for clinical use may allow increased penetration of therapeutic agents into sanctuary sites and increased gastrointestinal absorption. Previously identified inhibitors have lacked potency or specificity or were toxic at concentrations needed to inhibit ABCG2; none are in clinical development. A previously developed high-throughput assay measuring inhibition of ABCG2-mediated pheophorbide a transport was applied to natural product extract libraries. Among the active samples were extracts from the marine ascidian Botryllus tyreus. Bioassay-guided fractionation resulted in purification of a series of botryllamides. Ten botryllamides were obtained, two of which (designated I and J) were novel. Activity against ABCG2 was confirmed by assessing the ability of the compounds to inhibit ABCG2-mediated BODIPY-prazosin transport in ABCG2-transfected HEK293 cells, compete with [(125)I]-iodoarylazidoprazosin (IAAP) labeling of ABCG2, stimulate ABCG2-associated ATPase activity, and reverse ABCG2-mediated resistance.

Identification of compounds that correlate with ABCG2 transporter function in the National Cancer Institute Anticancer Drug Screen

ABCG2 is an ATP-binding cassette transporter that counts multiple anticancer compounds among its substrates and is believed to regulate oral bioavailability as well as serve a protective role in the blood-brain barrier, the maternal-fetal barrier, and hematopoietic stem cells. We sought to determine whether novel compounds that interact with the transporter could be identified through analysis of cytotoxicity profiles recorded in the NCI Anticancer Drug Screen database. A flow cytometric assay was used to measure ABCG2 function in the 60 cell lines and generate a molecular profile for COMPARE analysis. This strategy identified >70 compounds with Pearson correlation coefficients (PCCs) >0.4, where reduced drug sensitivity correlated with ABCG2 expression, as well as >120 compounds with PCCs < -0.4, indicating compounds to which ABCG2 expression conferred greater sensitivity. Despite identification of known single nucleotide polymorphisms in the ABCG2 gene in a number of the cell lines, omission of these lines from the COMPARE analysis did not affect PCCs. Available compounds were subjected to validation studies to confirm interaction with the transporter, including flow cytometry, [(125)I]IAAP binding, and cytotoxicity assays, and interaction was documented in 20 of the 27 compounds studied. Although known substrates of ABCG2 such as mitoxantrone or topotecan were not identified, we characterized three novel substrates-5-hydroxypicolinaldehyde thiosemicarbazone (NSC107392), (E)-N-(1-decylsulfanyl-3-hydroxypropan-2-yl)-3-(6-methyl-2,4-dioxo-1H-pyrimidin-5-yl)prop-2-enamide (NSC265473), and 1,2,3,4,7-pentahydroxy-1,3,4,4a,5,11b-hexahydro[1,3]dioxolo[4,5-j]phenanthridin-6(2H)-one [NSC349156 (pancratistatin)]-and four compounds that inhibited transporter function-2-[methyl(2-pyridin-2-ylethyl)-amino]fluoren-9-one hydroiodide (NSC24048), 5-amino-6-(7-amino-5,8-dihydro-6-methoxy-5,8-dioxo-2-quinolinyl)-4-(2-hydroxy-3,4-dimethoxyphenyl)-3-methyl-2-pyridinecarboxylic acid, methyl ester (NSC45384), (17beta)-2,4-dibromo-estra-1,3,5(10)-triene-3,17-diol (NSC103054), and methyl N-(pyridine-4-carbonylamino)carbamodithioate (NSC636795). In summary, COMPARE analysis of the NCI drug screen database using the ABCG2 functional profile was able to identify novel substrates and transporter-interacting compounds.

Evaluation of current methods used to analyze the expression profiles of ATP-binding cassette transporters yields an improved drug-discovery database

The development of multidrug resistance (MDR) to chemotherapy remains a major challenge in the treatment of cancer. Resistance exists against every effective anticancer drug and can develop by multiple mechanisms. These mechanisms can act individually or synergistically, leading to MDR, in which the cell becomes resistant to a variety of structurally and mechanistically unrelated drugs in addition to the drug initially administered. Although extensive work has been done to characterize MDR mechanisms in vitro, the translation of this knowledge to the clinic has not been successful. Therefore, identifying genes and mechanisms critical to the development of MDR in vivo and establishing a reliable method for analyzing highly homologous genes from small amounts of tissue is fundamental to achieving any significant enhancement in our understanding of MDR mechanisms and could lead to treatments designed to circumvent it. In this study, we use a previously established database that allows the identification of lead compounds in the early stages of drug discovery that are not ATP-binding cassette (ABC) transporter substrates. We believe this can serve as a model for appraising the accuracy and sensitivity of current methods used to analyze the expression profiles of ABC transporters. We found two platforms to be superior methods for the analysis of expression profiles of highly homologous gene superfamilies. This study also led to an improved database by revealing previously unidentified substrates for ABCB1, ABCC1, and ABCG2, transporters that contribute to MDR.

Identification of inhibitors of ABCG2 by a bioluminescence imaging-based high-throughput assay

ABCG2 is a member of the ATP-binding cassette (ABC) family of transporters, the overexpression of which is associated with tumor resistance to a variety of chemotherapeutic agents. Accordingly, combining ABCG2 inhibitor(s) with chemotherapy has the potential to improve treatment outcome. To search for clinically useful ABCG2 inhibitors, a bioluminescence imaging (BLI)-based assay was developed to allow high-throughput compound screening. This assay exploits our finding that d-luciferin, the substrate of firefly luciferase (fLuc), is a specific substrate of ABCG2, and ABCG2 inhibitors block the export of d-luciferin and enhance bioluminescence signal by increasing intracellular d-luciferin concentrations. HEK293 cells, engineered to express ABCG2 and fLuc, were used to screen the Hopkins Drug Library that includes drugs approved by the Food and Drug Administration (FDA) as well as drug candidates that have entered phase II clinical trials. Forty-seven compounds showed BLI enhancement, a measure of anti-ABCG2 activity, of > or =5-fold, the majority of which were not previously known as ABCG2 inhibitors. The assay was validated by its identification of known ABCG2 inhibitors and by confirming previously unknown ABCG2 inhibitors using established in vitro assays (e.g., mitoxantrone resensitization and BODIPY-prazosin assays). Glafenine, a potent new inhibitor, also inhibited ABCG2 activity in vivo. The BLI-based assay is an efficient method to identify new inhibitors of ABCG2. As they were derived from a FDA-approved compound library, many of the inhibitors uncovered in this study are ready for clinical testing.