BCRP mRNA expression v. clinical outcome in 40 adult AML patients

Role of breast cancer resistance protein (BCRP/ABCG2) in cancer drug resistance

Since cloning of the ATP-binding cassette (ABC) family member breast cancer resistance protein (BCRP/ABCG2) and its characterization as a multidrug resistance efflux transporter in 1998, BCRP has been the subject of more than two thousand scholarly articles. In normal tissues, BCRP functions as a defense mechanism against toxins and xenobiotics, with expression in the gut, bile canaliculi, placenta, blood-testis and blood-brain barriers facilitating excretion and limiting absorption of potentially toxic substrate molecules, including many cancer chemotherapeutic drugs. BCRP also plays a key role in heme and folate homeostasis, which may help normal cells survive under conditions of hypoxia. BCRP expression appears to be a characteristic of certain normal tissue stem cells termed "side population cells," which are identified on flow cytometric analysis by their ability to exclude Hoechst 33342, a BCRP substrate fluorescent dye. Hence, BCRP expression may contribute to the natural resistance and longevity of these normal stem cells. Malignant tissues can exploit the properties of BCRP to survive hypoxia and to evade exposure to chemotherapeutic drugs. Evidence is mounting that many cancers display subpopulations of stem cells that are responsible for tumor self-renewal. Such stem cells frequently manifest the "side population" phenotype characterized by expression of BCRP and other ABC transporters. Along with other factors, these transporters may contribute to the inherent resistance of these neoplasms and their failure to be cured.

The controversial role of ABC transporters in clinical oncology

The phenomenon of multidrug resistance in cancer is often associated with the overexpression of the ABC (ATP-binding cassette) transporters Pgp (P-glycoprotein) (ABCB1), MRP1 (multidrug resistance-associated protein 1) (ABCC1) and ABCG2 [BCRP (breast cancer resistance protein)]. Since the discovery of Pgp over 35 years ago, studies have convincingly linked ABC transporter expression to poor outcome in several cancer types, leading to the development of transporter inhibitors. Three generations of inhibitors later, we are still no closer to validating the 'Pgp hypothesis', the idea that increased chemotherapy efficacy can be achieved by inhibition of transporter-mediated efflux. In this chapter, we highlight the difficulties and past failures encountered in the development of clinical inhibitors of ABC transporters. We discuss the challenges that remain in our effort to exploit decades of work on ABC transporters in oncology. In learning from past mistakes, it is hoped that ABC transporters can be developed as targets for clinical intervention.

Xanthines down-regulate the drug transporter ABCG2 and reverse multidrug resistance

ABCG2 is an ATP-binding-cassette (ABC) transporter that confers multidrug resistance (MDR) to tumor cells by extruding a broad variety of chemotherapeutic agents, ultimately leading to failure of cancer therapy. Thus, the down-regulation of ABCG2 expression and/or function has been proposed as part of a regimen to improve cancer therapeutic efficacy. In this study, we found that a group of xanthines including caffeine, theophylline, and dyphylline can dramatically decrease ABCG2 protein in cells that have either moderate (BeWo, a placental choriocarcinoma cell line) or high (MCF-7/MX100, a breast cancer drug-resistant cell subline) levels of ABCG2 expression. This down-regulation is time-dependent, dose-dependent, and reversible. Using lysosomal inhibitors, we found that xanthines decreased ABCG2 by inducing its rapid internalization and lysosome-mediated degradation. As a consequence, caffeine treatment significantly increased the retention of an established ABCG2 substrate in MCF-7/MX100 cells but not in parental MCF-7 cells and sensitized the MDR cells to the chemotherapeutic agent mitoxantrone (MX); combination treatment with MX and caffeine decreased the IC(50) of MX ~10-fold and induced a greater degree of apoptotic cell death than MX treatment alone. Taken together, our results describe a novel function for this large class of therapeutically relevant compounds and suggest that a subset of xanthines could be developed as combination therapy to improve the efficacy of anticancer drugs that are ABCG2 substrates.

Expression of genes related to multiple drug resistance and apoptosis in acute leukemia: response to induction chemotherapy

Resistance to chemotherapy is a major impediment to the successful treatment of acute leukemia (AL). Expression of genes involved in drug resistance and apoptosis may be responsible for this. This study aimed to investigate the expression of drug resistance (MDR1, MRP1, LRP, BCRP, GSTP1, DHFR) and apoptotic genes (p53, BCL-2, Survivin) in adult acute leukemias and compare them with clinical and hematological findings and response to induction chemotherapy. Eighty-five patients with AL [45 with acute myeloid leukemia (AML) and 40 with acute lymphoblastic leukemia (ALL)] were used as a study group. Real-time PCR results showed that expression level of MDR1 was significantly higher in AML whereas expression of DHFR, BCRP and Survivin was significantly higher in ALL patients. In AML, significant correlation was observed between LRP and MRP1 (r(s)=0.44, p=0.016), LRP and DHFR (r(s)=0.41, p=0.02), MDR1 and BCL-2 (r(s)=0.38, p=0.03). Expression of GSTP1 and LRP correlated with high white blood count (p=0.03 and p=0.03) and BCL-2 with high peripheral blast count (p=0.009). MDR1 expression was significantly associated with the expression of immature stem cell marker CD34 (p=0.002). In ALL, significant association was found between LRP gene and female sex (p<0.0001), LRP and B-ALL patients (p=0.04) and LRP and BCR/ABL positive patients (p=0.004). High expression of MDR1 and BCL-2 in AML and MRP1 gene in ALL was associated with response to induction chemotherapy (p=0.001, p=0.02 and p=0.007 respectively). These results showed the potential clinical relevance of MDR1, MRP1 and BCL-2 in adult patients with acute leukemia in the context of induction chemotherapy.

The challenge of exploiting ABCG2 in the clinic

ABCG2, or breast cancer resistance protein (BCRP), is an ATP-binding cassette half transporter that has been shown to transport a wide range of substrates including chemotherapeutics, antivirals, antibiotics and flavonoids. Given its wide range of substrates, much work has been dedicated to developing ABCG2 as a clinical target. But where can we intervene clinically and how can we avoid the mistakes made in past clinical trials targeting P-glycoprotein? This review will summarize the normal tissue distribution, cancer tissue expression, substrates and inhibitors of ABCG2, and highlight the challenges presented in exploiting ABCG2 in the clinic. We discuss the possibility of inhibiting ABCG2, so as to increase oral bioavailability or increase drug penetration into sanctuary sites, especially the central nervous system; and at the other end of the spectrum, the possibility of improving ABCG2 function, in the case of gout caused by a single nucleotide polymphism. Together, these aspects of ABCG2/BCRP make the protein a target of continuing interest for oncologists, biologists, and pharmacologists.

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.

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.

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.

Drug transport by breast cancer resistance protein

IMPORTANCE OF THE FIELD

The ATP-binding cassette transporter ABCG2 is a well-known major mediator of multi-drug resistance in cancers. Beyond multi-drug resistance, experimental and recent clinical studies demonstrate a role for ABCG2 as a determinant of drug pharmacokinetic, safety and efficacy profiles.

AREAS COVERED IN THIS REVIEW

The clinical evidence of the role of ABCG2 in pharmacokinetics and pharmacodynamics is reviewed. Key questions that arise from the perspective of preclinical drug evaluation are addressed, including the structure of ABCG2 and mechanisms of drug-transporter interactions, mechanisms responsible for the polyspecificity of ABCG2, methods suitable for studying drug-ABCG2 interactions in vitro and in silico prediction of ABCG2 substrates and inhibitors.

WHAT THE READER WILL GAIN

An update on current knowledge of the importance of ABCG2 in drug disposition with special emphasis on drug development.

TAKE HOME MESSAGE

The field of drug-ABCG2 interaction is rapidly advancing and beginning to expand into clinical practice. However, the structural understanding of drug binding and transport by ABCG2 is still incomplete. Incorporation of novel concepts of drug-transporter interactions such as electrostatic funneling might help explain the multispecificity of ABCG2 and enable in silico predictions.

Synergism of hydroxyapatite nanoparticles and recombinant mutant human tumour necrosis factor-alpha in chemotherapy of multidrug-resistant hepatocellular carcinoma

BACKGROUND/AIMS

Locoregional chemotherapy continues to be the mainstay for the treatment of unresectable hepatocellular carcinoma (HCC). One of the principal obstacles implicated in its unsuccessful therapy is multidrug resistance (MDR). Former studies have identified the multidrug-resistant nature and possible mechanisms of hepatoma cells both in vitro and in vivo. This work aimed to develop an effective strategy for the treatment of HCC with MDR.

METHODS

The treatment was exploited to inhibit the MDR cells by co-administration of the recombinant mutant human tumour necrosis factor-alpha (rmhTNF-alpha), a sublethal dose of chemicals [adriamycin (ADM), mitomycin and 5-FU] and hydroxyapatite nanoparticles (nHAPs). Real-time quantitative reverse transcriptase-polymerase chain reaction and electrochemiluminescence Western blot were used to detect the expression of several related genes.

RESULTS

The chemicals acted synergistically with rmhTNF-alpha and nHAP in suppressing the growth of hepatoma cells and inducing apoptosis of the cells, with the MDR phenotype reversed, as measured by intracellular ADM retention. Analysis of mRNA and protein revealed that rmhTNF-alpha inhibited the gene expression of XIAP, survivin, Ki67, PCNA, MDR1 and BCRP to some extent. Moreover, the inhibitory effects mentioned above could be as good or better than when nHAP is incorporated into the regimens.

CONCLUSIONS

rmhTNF-alpha was not only able to restore the chemotherapeutic sensitivity to HepG2/ADM, its xenograft model and clinical samples but also further inhibited the growth of these tumours by a combination of nHAP. These results strongly suggested that chemicals in combination with rmhTNF-alpha and nHAP may be beneficial for the local treatment of advanced HCC.

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.

ABCB1 and ABCG2 proteins, their functional activity and gene expression in concert with drug sensitivity of leukemia cells

Resistance to chemotherapy is an obstacle to the successful treatment of oncohematological malignances. Failure of therapeutic treatment may be due to the development of multidrug resistance (MDR), the mechanisms of which include upregulation of membrane-resident transporters that efflux chemotherapeutic drugs from tumor cells. Deregulation may occur at different levels: gene or protein expression or function depletion. Childhood acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) cells and chronic lymphocytic leukemia (CLL) cells of adults were studied. ABCB1 (P-gp) and ABCG2 (BCRP) expression were determined by flow cytometry, rhodamine 123 (Rho123) and mitoxantrone were used for functional activity study of MDR proteins, sensitization of leukemic cells to drugs was quantified by methyl thiazolyl tetrazolium (MTT) assays. Appropriate gene expression was determined using semi-quantitative RT-PCR. No differences between expression of P-gp and BCRP and genes in primary and relapsed acute leukemia (AL) cells as well as in de novo and treated CLL samples were established. Higher expression of P-gp and BCRP proteins was detected in CLL lymphocytes compared to blast cells. Increased P-gp protein expression and function was detected in cells of CLL patients who had more aggressive therapy regimen. Doxorubicine, rubomycinum and L-asparaginase resistance correlates with P-gp overexpression and increased function in pediatric AL whereas vincristine resistance might be associated with P-gp protein expression in AL samples and impared P-gp function in CLL lymphocytes only. A tendency for the decreased doxorubicin cytotoxic activity was shown in BCRP-overexpressing cells both in children and adults leukemia. Multifactorial ANOVA showed that P-gp/MDR1 and BCRP as well as their function could not be used as unconditional and universal predictors of leukemia cell drug resistance in vitro. These results suggest that studied MDR transporter-proteins have a limited role per se in vitro and admittedly in vivo drug resistance estimated in leukemia patients or it is not yet fully understood unless would not be studied in aggregate. In any event, the expression and function studies of the proteins under investigation when singularly considered do not have a crucial significance for impact on drug resistance evaluation in all leukemia patients.

Functions of the breast cancer resistance protein (BCRP/ABCG2) in chemotherapy

The breast cancer resistance protein, BCRP/ABCG2, is a half-molecule ATP-binding cassette transporter that facilitates the efflux of various anticancer agents from the cell, including 7-ethyl-10-hydroxycamptothecin, topotecan and mitoxantrone. The expression of BCRP can thus confer a multidrug resistance phenotype in cancer cells, and its transporter activity is involved in the in vivo efficacy of chemotherapeutic agents. Thus, the elucidation of the substrate preferences and structural relationships of BCRP is essential to understanding its in vivo functions during chemotherapeutic treatments. Single nucleotide polymorphisms (SNPs) have also been found to be key factors in determining the efficacy of chemotherapeutics, and those therapeutics that inhibit BCRP activity, such as the SNP that results in a C421A mutant, may result in unexpected side effects of the BCRP- anticancer drugs interaction even at normal dosages. In order to modulate the BCRP activity during chemotherapy, various compounds have been tested as inhibitors of this protein. Estrogenic compounds including estrone, several tamoxifen derivatives in addition to phytoestrogens and flavonoids have been shown to reverse BCRP-mediated drug resistance. Intriguingly, recently developed molecular targeted cancer drugs, such as the tyrosine kinase inhibitors imatinib mesylate, gefitinib and others, can also interact with BCRP. Since both functional SNPs and inhibitory agents of BCRP modulate the in vivo pharmacokinetics and pharmacodynamics of its substrate drugs, BCRP activity is an important consideration in the development of molecular targeted chemotherapeutics.

Synthesis and structure-activity relationship studies on tryprostatin A, an inhibitor of breast cancer resistance protein

Tryprostatin A is an inhibitor of breast cancer resistance protein, consequently a series of structure-activity studies on the cell cycle inhibitory effects of tryprostatin A analogues as potential antitumor antimitotic agents have been carried out. These analogues were assayed for their growth inhibition properties and their ability to perturb the cell cycle in tsFT210 cells. SAR studies resulted in the identification of the essential structural features required for cytotoxic activity. The absolute configuration L-Tyr-L-pro in the diketopiperazine ring along with the presence of the 6-methoxy substituent on the indole moiety of 1 was shown to be essential for dual inhibition of topoisomerase II and tubulin polymerization. Biological evaluation also indicated the presence of the 2-isoprenyl moiety on the indole scaffold of 1 was essential for potent inhibition of cell proliferation. Substitution of the indole N(a)-H in 1 with various alkyl or aryl groups, incorporation of various L-amino acids into the diketopiperazine ring in place of L-proline, and substitution of the 6-methoxy group in 1 with other functionality provided active analogues. The nature of the substituents present on the indole N(a)-H or the indole C-2 position influenced the mechanism of action of these analogues. Analogues 68 (IC(50)=10 microM) and 67 (IC(50)=19 microM) were 7-fold and 3.5-fold more potent, respectively, than 1 (IC(50)=68 microM) in the inhibition of the growth of tsFT210 cells. Diastereomer-2 of tryprostatin B 8 was a potent inhibitor of the growth of three human carcinoma cell lines: H520 (IC(50)=11.9 microM), MCF-7 (IC(50)=17.0 microM) and PC-3 (IC(50)=11.1 microM) and was equipotent with etoposide, a clinically used anticancer agent. Isothiocyanate analogue 71 and 6-azido analogue 72 were as potent as 1 in the tsFT210 cell proliferation and may be useful tools in labeling BCRP.

New inhibitors of ABCG2 identified by high-throughput screening

In order to identify novel inhibitors of the ATP-binding cassette transporter, ABCG2, a high-throughput assay measuring the accumulation of the ABCG2 substrate pheophorbide a in ABCG2-overexpressing NCI-H460 MX20 cells was used to screen libraries of compounds. Out of a library of 7,325 natural products and synthetic compounds from the National Cancer Institute/Developmental Therapeutics Program collection, 18 were found to inhibit ABCG2 at 10 micromol/L. After eliminating flavonoids and compounds of limited availability from the 18 original compounds, 10 of the 11 remaining compounds reversed mitoxantrone resistance in NCI-H460/MX20 cells and prevented ABCG2-mediated BODIPY-prazosin transport in ABCG2-transfected HEK293 cells, confirming an interaction with ABCG2. Based on the activity profiles and the availability of materials, five inhibitors were examined for their ability to compete with [(125)I]iodoarylazidoprazosin labeling of ABCG2, increase binding of the anti-ABCG2 antibody 5D3, and prevent P-glycoprotein or multidrug resistance protein 1-mediated transport. At a concentration of 20 micromol/L, all of the compounds reduced iodoarylazidoprazosin labeling by 50% to 80% compared with controls. All five compounds also increased 5D3 labeling of ABCG2, indicating that these compounds are inhibitors but not substrates of ABCG2. None of the compounds affected P-glycoprotein-mediated rhodamine 123 transport, whereas three affected multidrug resistance protein-1-mediated calcein transport at 25 mumol/L, suggesting that the compounds are relatively specific for ABCG2. These five novel inhibitors of ABCG2 activity may provide a basis for further investigation of ABCG2 function and its relevance in multidrug resistance.

Polymorphisms in transporter and phase II metabolism genes as potential modifiers of the predisposition to and treatment outcome of de novo acute myeloid leukemia in Israeli ethnic groups

Drug metabolism/disposition and transporter genes may influence predisposition or prognosis of AML (acute myeloid leukemia) patients. We analyzed polymorphisms in 3 transporters and 4 drug metabolism genes in 293 Israeli individuals (112 AML patients and 181 controls). We analyzed: ABCC3 (MRP3) C-211T; ABCG2 (BCRP) C421A; CNT1 (SLC28A1) G565A and NAT1, NAT2, and GSTT1 and GSTM1 null alleles for influence on predisposition, as well as treatment response and survival. We found that the ABCC3 C-211T polymorphism and GSTM1 null genotype have adverse prognostic significance in AML. None of the other polymorphisms studied were found to influence either predisposition or prognosis in Israeli AML patients.

ABCG2 expression is an independent unfavorable prognostic factor in esophageal squamous cell carcinoma

OBJECTIVE

ATP-binding cassette, subfamily G, member 2 (ABCG2) is a new member of the superfamily of ATP-binding cassette transporter proteins and known to be not only a member of the membrane transporters implicated in multidrug resistance, but also a molecular determinant of the side population phenotype, characteristics of which are reminiscent of stem cells. The aim of the current study was to clarify the significance of ABCG2 expression in esophageal squamous cell carcinoma (ESCC).

METHODS

We immunohistochemically investigated paraffin sections of 100 ESCC tumors and assessed the expression level of ABCG2 mRNA in 33 specimens by quantitative RT-PCR.

RESULTS

In the immunohistochemical study, ABCG2 expression was detectable in 61% of patients and the proportion of ABCG2-positive cells was variable (0-100%). Interestingly, the presence of ABCG2-positive cells in the tumor, regardless of their amount, was associated with poorer survival (p = 0.0088). Moreover, it was revealed to be an independent prognostic factor along with the extent of the primary tumor and positive lymph node metastasis in multivariate analysis using Cox's regression model. In the quantitative RT-PCR study, higher tumor ABCG2 mRNA expression was associated with poorer survival (p = 0.017).

CONCLUSIONS

The absolute presence of ABCG2-positive cells in the tumor is a single independent prognostic factor, suggesting the underlying roles in malignant characteristics of ESCC other than drug resistance.

ABCG2: determining its relevance in clinical drug resistance

Multidrug resistance is a major obstacle to successful cancer treatment. One mechanism by which cells can become resistant to chemotherapy is the expression of ABC transporters that use the energy of ATP hydrolysis to transport a wide variety of substrates across the cell membrane. There are three human ABC transporters primarily associated with the multidrug resistance phenomenon, namely Pgp, MRP1, and ABCG2. All three have broad and, to a certain extent, overlapping substrate specificities, transporting the major drugs currently used in cancer chemotherapy. ABCG2 is the most recently described of the three major multidrug-resistance pumps, and its substrates include mitoxantrone, topotecan, irinotecan, flavopiridol, and methotrexate. Despite several studies reporting ABCG2 expression in normal and malignant tissues, no trials have thus far addressed the role of ABCG2 in clinical drug resistance. This gives us an opportunity to critically review the disappointing results of past clinical trials targeting Pgp and to propose strategies for ABCG2. We need to know in which tumor types ABCG2 contributes to the resistance phenotype. We also need to develop standardized assays to detect ABCG2 expression in vivo and to carefully select the chemotherapeutic agents and clinical trial designs. This review focuses on our current knowledge about normal tissue distribution, tumor expression profiles, and substrates and inhibitors of ABCG2, together with lessons learned from clinical trials with Pgp inhibitors. Implications of SNPs in the ABCG2 gene affecting the pharmacokinetics of substrate drugs, including many non-chemotherapy agents and ABCG2 expression in the SP population of stem cells are also discussed.

Inhibition of ABCG2-mediated transport by protein kinase inhibitors with a bisindolylmaleimide or indolocarbazole structure

ABCG2 is a transporter with potential importance in cancer drug resistance, drug oral absorption, and stem cell biology. In an effort to identify novel inhibitors of ABCG2, we examined the ability of commercially available bisindolylmaleimides (BIM) and indolocarbazole protein kinase inhibitors (PKI) to inhibit ABCG2, given the previous demonstration that the indolocarbazole PKI UCN-01 interacted with the transporter. At a concentration of 10 micromol/L, all of the compounds tested increased intracellular fluorescence of the ABCG2-specific substrate pheophorbide a in ABCG2-transfected HEK-293 cells by 1.3- to 6-fold as measured by flow cytometry; the ABCG2-specific inhibitor fumitremorgin C increased intracellular fluorescence by 6.6-fold. In 4-day cytotoxicity assays, wild-type ABCG2-transfected cells were not more than 2-fold resistant to any of the compounds, suggesting that the PKIs are not significantly transported by ABCG2. BIMs I, II, III, IV, and V, K252c, and arcyriaflavin A were also able to inhibit [(125)I]iodoarylazidoprazosin labeling of ABCG2 by 65% to 80% at 20 micromol/L, compared with a 50% to 70% reduction by 20 micromol/L fumitremorgin C. K252c and arcyriaflavin A were the most potent compounds, with IC(50) values for inhibition of [(125)I]iodoarylazidoprazosin labeling of 0.37 and 0.23 micromol/L, respectively. K252c and arcyriaflavin A did not have any effect on the ATPase activity of ABCG2. Four minimally toxic compounds--BIM IV, BIM V, arcyriaflavin A, and K252c-reduced the relative resistance of ABCG2-transfected cells to SN-38 in cytotoxicity assays. We find that indolocarbazole and BIM PKIs directly interact with the ABCG2 protein and may thus increase oral bioavailability of ABCG2 substrates.

ABC transporters and drug resistance in leukemia: was P-gp nothing but the first head of the Hydra?

More than 30 years ago it was discovered that permeability glycoprotein (P-gp) can cause drug resistance. Over the following decades numerous studies showed that high expression of P-gp is associated with poor prognosis in acute myeloid leukemia in adults and that it causes multidrug resistance via ATP-dependent drug efflux. It was hoped that an inhibition of P-gp could sensitize resistant leukemic cells to chemotherapy and thus improve treatment results. Today we know that the family of ATP-binding cassette transporters (ABC transporters) comprises 48 different proteins. Some of them seem to be able to cause drug resistance as well as P-gp. This review focuses on emerging data on the clinical relevance of other ABC transporters, such as BCRP, MRP3, and ABCA3. When Heracles fought the ancient Hydra, he had to fight all the heads at ones but only one head was vital for the beast. Can we block all the relevant ABC transporters at once? Is there one transporter that is more important than the others?

The roles of four multi-drug resistance proteins in hepatocellular carcinoma multidrug resistance

The roles of multi-drug resistance protein 1 (MDR1), multi-drug resistance related protein 1 (MRP1), lung resistance protein (LRP) and breast cancer resistance protein (BCRP) in the multi-drug resistance (MDR) of hepatocellular carcinoma (HCC) were studied. By exposing HepG2 cell line to progressively increased concentrations of adriamycin (ADM), HepG2 multi-drug resistant subline (HepG2/ADM) was induced. The MDR index of HepG2/ADM was detected by using MTT. The expressions of the four MDR proteins in the three cell lines (L02, HepG2, HepG2/ADM) were investigated at mRNA and protein levels by real-time RT-PCR and Western blot respectively. Our results showed that when the ADM concentration was under 100 microg/L, HepG2 could easily be induced to be drug-resistant. The IC(50) of the HepG2/ADM to ADM was 282 times that of HepG2. The expression of MDR1 and BCRP mRNA in HepG2/ADM cells were 400 and 9 times that of HepG2 cells respectively while there was no difference in the mRNA expressions of MRP1 and LRP. There was no difference between HepG2 and L02 cells in the mRNA expressions of the four genes. At the protein level, the expressions of MDR1, BCRP and LRP but MRP1 in HepG2/ADM were significantly higher than those of HepG2 and L02. Between HepG2 and L02, there was no difference in the expressions of four genes at the protein level. HepG2/ADM is a good model for the study of MDR. The four genes are probably the normally expressed gene in liver. The expressions of MDR1 and BCRP could be up-regulated by anti-cancer agents in vitro. The MDR of HCC was mainly due to the up-regulation of MDR1 and BCRP but MRP1 and LRP. These findings suggest they may serve as targets for the reversal of MDR of HCC.

CD34-related coexpression of MDR1 and BCRP indicates a clinically resistant phenotype in patients with acute myeloid leukemia (AML) of older age

Clinical resistance to chemotherapy in acute myeloid leukemia (AML) is associated with the expression of the multidrug resistance (MDR) proteins P-glycoprotein, encoded by the MDR1/ABCB1 gene, multidrug resistant-related protein (MRP/ABCC1), the lung resistance-related protein (LRP), or major vault protein (MVP), and the breast cancer resistance protein (BCRP/ABCG2). The clinical value of MDR1, MRP1, LRP/MVP, and BCRP messenger RNA (mRNA) expression was prospectively studied in 154 newly diagnosed AML patients ≥60 years who were treated in a multicenter, randomized phase 3 trial. Expression of MDR1 and BCRP showed a negative whereas MRP1 and LRP showed a positive correlation with high white blood cell count (respectively, p < 0.05, p < 0.001, p < 0.001 and p < 0.001). Higher BCRP mRNA was associated with secondary AML (p < 0.05). MDR1 and BCRP mRNA were highly significantly associated (p < 0.001), as were MRP1 and LRP mRNA (p < 0.001) expression. Univariate regression analyses revealed that CD34 expression, increasing MDR1 mRNA as well as MDR1/BCRP coexpression, were associated with a lower complete response (CR) rate and with worse event-free survival and overall survival. When adjusted for other prognostic actors, only CD34-related MDR1/BCRP coexpression remained significantly associated with a lower CR rate (p = 0.03), thereby identifying a clinically resistant subgroup of elderly AML patients.

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

ABCG2 expression, function, and promoter methylation in human multiple myeloma

We investigated the role of the breast cancer resistance protein (BCRP/ABCG2) in drug resistance in multiple myeloma (MM). Human MM cell lines, and MM patient plasma cells isolated from bone marrow, were evaluated for ABCG2 mRNA expression by quantitative polymerase chain reaction (PCR) and ABCG2 protein, by Western blot analysis, immunofluorescence microscopy, and flow cytometry. ABCG2 function was determined by measuring topotecan and doxorubicin efflux using flow cytometry, in the presence and absence of the specific ABCG2 inhibitor, tryprostatin A. The methylation of the ABCG2 promoter was determined using bisulfite sequencing. We found that ABCG2 expression in myeloma cell lines increased after exposure to topotecan and doxorubicin, and was greater in logphase cells when compared with quiescent cells. Myeloma patients treated with topotecan had an increase in ABCG2 mRNA and protein expression after treatment with topotecan, and at relapse. Expression of ABCG2 is regulated, at least in part, by promoter methylation both in cell lines and in patient plasma cells. Demethylation of the promoter increased ABCG2 mRNA and protein expression. These findings suggest that ABCG2 is expressed and functional in human myeloma cells, regulated by promoter methylation, affected by cell density, up-regulated in response to chemotherapy, and may contribute to intrinsic drug resistance.

Role of ABCG2/BCRP in biology and medicine

The protein variously named ABCG2/BCRP/MXR/ABCP is a recently described ATP-binding cassette (ABC) transporter originally identified by its ability to confer drug resistance that is independent of Mrp1 (multidrug-resistance protein 1) and Pgp (P-glycoprotein). Unlike Mrp1 and Pgp, ABCG2 is a half-transporter that must homodimerize to acquire transport activity. ABCG2 is found in a variety of stem cells and may protect them from exogenous and endogenous toxins. ABCG2 expression is upregulated under low-oxygen conditions, consistent with its high expression in tissues exposed to low-oxygen environments. ABCG2 interacts with heme and other porphyrins and protects cells and/or tissues from protoporphyrin accumulation under hypoxic conditions. Individuals who carry ABCG2 alleles that have impaired function may be more susceptible to porphyrin-induced toxicity. Abcg2 knock-out models have allowed in vivo studies of Abcg2 function in host and cellular defense. In combination with immunohistochemical analyses, these studies have revealed how ABCG2 influences the absorption, distribution, and excretion of drugs and cytotoxins.