Estrone and 17beta-estradiol reverse breast cancer resistance protein-mediated multidrug resistance

Nongenomic ERα-AMPK Signaling Regulates Sex-Dependent Bcrp Transport Activity at the Blood-Brain Barrier

The blood-brain barrier (BBB) is an extensive capillary network that protects the brain from environmental and metabolic toxins while limiting drug delivery to the central nervous system (CNS). The ATP-binding cassette transporter breast cancer resistance protein (Bcrp) reduces drug delivery across the BBB by actively transporting its clinical substrates back into peripheral circulation before their entry into the CNS compartment. 17β-Estradiol (E2)-elicited changes in Bcrp transport activity and expression have been documented previously. We report a novel signaling mechanism by which E2 decreases Bcrp transport activity in mouse brain capillaries via rapid nongenomic signaling through estrogen receptor α. We extended this finding to investigate the effects of different endocrine-disrupting compounds (EDCs) and selective estrogen receptor modulators (SERMs) on Bcrp transport function. We also demonstrate sex-dependent expression of Bcrp and E2-sensitive Bcrp transport activity at the BBB ex vivo. This work establishes an explanted tissue-based model by which to interrogate EDCs and SERMs as modulators of nongenomic estrogenic signaling with implications for sex and hormonal regulation of therapeutic delivery into the CNS.

ABCB1 and ABCG2 Regulation at the Blood-Brain Barrier: Potential New Targets to Improve Brain Drug Delivery

The drug efflux transporters ABCB1 and ABCG2 at the blood-brain barrier limit the delivery of drugs into the brain. Strategies to overcome ABCB1/ABCG2 have been largely unsuccessful, which poses a tremendous clinical problem to successfully treat central nervous system (CNS) diseases. Understanding basic transporter biology, including intracellular regulation mechanisms that control these transporters, is critical to solving this clinical problem.In this comprehensive review, we summarize current knowledge on signaling pathways that regulate ABCB1/ABCG2 at the blood-brain barrier. In Section I, we give a historical overview on blood-brain barrier research and introduce the role that ABCB1 and ABCG2 play in this context. In Section II, we summarize the most important strategies that have been tested to overcome the ABCB1/ABCG2 efflux system at the blood-brain barrier. In Section III, the main component of this review, we provide detailed information on the signaling pathways that have been identified to control ABCB1/ABCG2 at the blood-brain barrier and their potential clinical relevance. This is followed by Section IV, where we explain the clinical implications of ABCB1/ABCG2 regulation in the context of CNS disease. Lastly, in Section V, we conclude by highlighting examples of how transporter regulation could be targeted for therapeutic purposes in the clinic. SIGNIFICANCE STATEMENT: The ABCB1/ABCG2 drug efflux system at the blood-brain barrier poses a significant problem to successful drug delivery to the brain. The article reviews signaling pathways that regulate blood-brain barrier ABCB1/ABCG2 and could potentially be targeted for therapeutic purposes.

Genetics of ABCB1 in Cancer

ABCB1, also known as MDR1, is a gene that encodes P-glycoprotein (P-gp), a membrane-associated ATP-dependent transporter. P-gp is widely expressed in many healthy tissues-in the gastrointestinal tract, liver, kidney, and at the blood-brain barrier. P-gp works to pump xenobiotics such as toxins and drugs out of cells. P-gp is also commonly upregulated across multiple cancer types such as ovarian, breast, and lung. Overexpression of ABCB1 has been linked to the development of chemotherapy resistance across these cancers. In vitro work across a wide range of drug-sensitive and -resistant cancer cell lines has shown that upon treatment with chemotherapeutic agents such as doxorubicin, cisplatin, and paclitaxel, ABCB1 is upregulated. This upregulation is caused in part by a variety of genetic and epigenetic mechanisms. This includes single-nucleotide variants that lead to enhanced P-gp ATPase activity without increasing ABCB1 RNA and protein levels. In this review, we summarize current knowledge of genetic and epigenetic mechanisms leading to ABCB1 upregulation and P-gp-enhanced ATPase activity in the setting of chemotherapy resistance across a variety of cancers.

Synthesis of 5-Hydroxy-3',4',7-trimethoxyflavone and Related Compounds and Elucidation of Their Reversal Effects on BCRP/ABCG2-Mediated Anticancer Drug Resistance

3',4',7-Trimethoxyflavone (TMF) has been reported to show a potent reversal effect on drug resistance mediated by breast cancer resistance protein (BCRP)/ATP-binding cassette subfamily G member 2 (ABCG2). In this study, we designed and synthesized five derivatives with either a hydroxy group or a fluorine atom at C-5 and several kinds of capping moiety at the C-7 hydroxy group, on the same 3',4'-dimethoxy-substituted flavone skeleton. We subsequently evaluated the efficacies of these compounds against BCRP-expressing human leukaemia K562/BCRP cells. Reversal of drug resistance was expressed as the concentration of compound causing a twofold reduction in drug sensitivity (RI₅₀ ). Of the synthesized compounds, the reversal effect of 5-hydroxy-3',4',7-trimethoxyflavone (HTMF, RI₅₀ 7.2 nm) towards 7-ethyl-10-hydroxycamptothecin (SN-38) was stronger than that of TMF (RI₅₀ 18 nm). Fluoro-substituted 5-fluoro-3',4',7-trimethoxyflavone (FTMF, RI₅₀ 25 nm) and monoglycosylated 7-(β-glucosyloxy)-5-hydroxy-3',4'-dimethoxyflavone (GOHDMF, 91 nm) also exhibited reversal effects, whereas the di- and triglycoside derivatives did not. TMF, HTMF and FTMF at 0.01-10 μm upregulated the K562/BCRP cellular accumulation of Hoechst 33342 nuclear staining dye. In addition, western blotting revealed that treatment of K562/BCRP cells with 0.1 μm TMF, HTMF or FTMT suppressed the expression of BCRP. HTMF showed the strongest inhibition of BCRP-mediated efflux and suppression of BCRP expression of the three effective synthesized flavones.

New selenosteroids as antiproliferative agents

Starting from natural steroids (diosgenin, hecogenin, smilagenin, estrone), we have prepared a wide panel of selenoderivatives, including benzoselenazolones, selenosemicarbazones, isoselenocyanates, selenoureas, selenocyanates and diselenides, with the aim of developing new families of potential chemotherapeutic agents. The modification of the organoselenium moieties, and their position on the steroid provided valuable information concerning the antiproliferative activities. Among all the families accessed herein, the best profile was achieved for selenoureas on the A ring of estrone, which exhibited GI₅₀ values in the range 2.0-4.1 μM for all the tested tumor cell lines, with increased potency compared with commonly used chemotherapeutic agents, like 5-fluorouracil and cisplatin. Cell cycle analysis revealed that selenoureas induced accumulation of cells in the G₁ phase of the cell cycle in the breast cancer cell lines HBL-100 and T-47D; therefore, a different mechanism than cisplatin, that induces cell cycle accumulation in the S phase as a result of DNA damage, must be involved. In the rest of the tumor cells, a slight increase of the S compartment was observed. Moreover, selenosteoids turned out to be excellent glutathione peroxidase (GPx) mimics for the catalytic removal of deleterious H₂O₂ (t_1/2 8.0-22.5 min) and alkyl peroxides (t_1/2 23.0-38.9 min) when used in substoichiometric amounts (1% molar ratio), thus providing a valuable tool for reducing the intrinsic oxidative stress in tumor progression.

Inhibition of blood-brain barrier efflux transporters promotes seizure in pregnant rats: Role of circulating factors

Seizure-provoking factors circulate late in gestation during normal pregnancy, but do not readily gain access to the brain due to the protective nature of the blood-brain barrier. In particular, efflux transporters are powerful ATP-driven pumps that actively prevent unwanted compounds from entering the brain. We hypothesized that acute inhibition of efflux transporters at the blood-brain barrier would result in spontaneous seizures in pregnant rats. We further hypothesized that the blood-brain barrier protects the maternal brain from seizure by increasing expression and/or activity of p-glycoprotein (P-gp), a major efflux transporter. Main blood-brain barrier efflux transporters were inhibited in-vivo in nonpregnant (Nonpreg) and pregnant (Preg; d19) Sprague Dawley rats (n=8/group). Seizures were monitored in conscious animals for 8h via chronically implanted electroencephalography (EEG) electrodes in the hippocampus and motor cortex and time-synced video. P-gp activity was measured via a calcein accumulation assay in freshly isolated cortical and hippocampal capillaries from Preg (d20) and Nonpreg rats (n=8-16/group), to assess regional susceptibility to transporter inhibition. P-gp expression, capillary density, and microglial activation as a measure of neuroinflammation were quantified using immunohistochemistry (n=4-6/group). Efflux transporter inhibition elicited hippocampal seizures within 1h in 100% of Preg rats that was not associated with neuroinflammation or elevated tumor necrosis factor alpha (TNFα) or vascular endothelial growth factor (VEGF), but negatively correlated with levels of estradiol. Hippocampal seizures were considerably less prevalent in Nonpreg rats. However, behavioral seizures in the motor cortex developed of similar severity in both groups of rats, demonstrating regional heterogeneity in response to efflux transporter inhibition. Basal P-gp activity was similar between groups, however, exposure to serum from Preg rats significantly decreased P-gp activity in the hippocampus, but not cortex, compared to serum from Nonpreg rats (0.29±0.1units/s in Preg vs. 0.06±0.02units/s in Nonpreg rats; p<0.05) that was not associated with elevated TNFα or VEGF. Thus, pregnancy differentially increased the susceptibility of the hippocampus to seizures in response to blood-brain barrier efflux transporter inhibition that may be due to the inhibitory effect of circulating factors in pregnancy on P-gp activity in the hippocampus.

Enhanced therapeutic effect of Adriamycin on multidrug resistant breast cancer by the ABCG2-siRNA loaded polymeric nanoparticles assisted with ultrasound

The overexpression of the breast cancer resistance protein (ABCG2) confers resistance to Adriamycin (ADR) in breast cancer. The silencing of ABCG2 using small interfering RNA (siRNA) could be a promising approach to overcome multidrug resistance (MDR) in cancer cells. To deliver ABCG2-siRNA effectively into breast cancer cells, we used mPEG-PLGA-PLL (PEAL) nanoparticles (NPs) with ultrasound-targeted microbubble destruction (UTMD). PEAL NPs were prepared with an emulsion-solvent evaporation method. The NPs size was about 131.5 ± 6.5 nm. The siRNA stability in serum was enhanced. The intracellular ADR concentration increased after the introduction of siRNA-loaded NPs. After intravenous injection of PEAL NPs in tumor-bearing mice, the ABCG2-siRNA-loaded NPs with UTMD efficiently silenced the ABCG2 gene and enhanced the ADR susceptibility of MCF-7/ADR (ADR resistant human breast cancer cells). The siRNA-loaded NPs with UTMD + ADR showed better tumor inhibition effect and good safety in vivo. These results indicate that ADR-chemotherapy in combination with ABCG2-siRNA is an attractive strategy to treat breast cancer.

Placental ABC Transporters: Biological Impact and Pharmaceutical Significance

The human placenta fulfills a variety of essential functions during prenatal life. Several ABC transporters are expressed in the human placenta, where they play a role in the transport of endogenous compounds and may protect the fetus from exogenous compounds such as therapeutic agents, drugs of abuse, and other xenobiotics. To date, considerable progress has been made toward understanding ABC transporters in the placenta. Recent studies on the expression and functional activities are discussed. This review discusses the placental expression and functional roles of several members of ABC transporter subfamilies B, C, and G including MDR1/P-glycoprotein, the MRPs, and BCRP, respectively. Since placental ABC transporters modulate fetal exposure to various compounds, an understanding of their functional and regulatory mechanisms will lead to more optimal medication use when necessary in pregnancy.

siRNA as a tool to improve the treatment of brain diseases: Mechanism, targets and delivery

As the population ages, brain pathologies such as neurodegenerative diseases and brain cancer increase their incidence, being the need to find successful treatments of upmost importance. Drug delivery to the central nervous system (CNS) is required in order to reach diseases causes and treat them. However, biological barriers, mainly blood-brain barrier (BBB), are the key obstacles that prevent the effectiveness of possible treatments due to their ability to strongly limit the perfusion of compounds into the brain. Over the past decades, new approaches towards overcoming BBB and its efflux transporters had been proposed. One of these approaches here reviewed is through small interfering RNA (siRNA), which is capable to specifically target one gene and silence it in a post-transcriptional way. There are different possible functional proteins at the BBB, as the ones responsible for transport or just for its tightness, which could be a siRNA target. As important as the effective silence is the way to delivery siRNA to its anatomical site of action. This is where nanotechnology-based systems may help, by protecting siRNA circulation and providing cell/tissue-targeting and intracellular siRNA delivery. After an initial overview on incidence of brain diseases and basic features of the CNS, BBB and its efflux pumps, this review focuses on recent strategies to reach brain based on siRNA, and how to specifically target these approaches in order to treat brain diseases.

Interaction of Isoflavones with the BCRP/ABCG2 Drug Transporter

This review will provide a comprehensive overview of the interactions between dietary isoflavones and the ATP-binding cassette (ABC) G2 efflux transporter, which is also named the breast cancer resistance protein (BCRP). Expressed in a variety of organs including the liver, kidneys, intestine, and placenta, BCRP mediates the disposition and excretion of numerous endogenous chemicals and xenobiotics. Isoflavones are a class of naturallyoccurring compounds that are found at high concentrations in commonly consumed foods and dietary supplements. A number of isoflavones, including genistein and daidzein and their metabolites, interact with BCRP as substrates, inhibitors, and/or modulators of gene expression. To date, a variety of model systems have been employed to study the ability of isoflavones to serve as substrates and inhibitors of BCRP; these include whole cells, inverted plasma membrane vesicles, in situ organ perfusion, as well as in vivo rodent and sheep models. Evidence suggests that BCRP plays a role in mediating the disposition of isoflavones and in particular, their conjugated forms. Furthermore, as inhibitors, these compounds may aid in reversing multidrug resistance and sensitizing cancer cells to chemotherapeutic drugs. This review will also highlight the consequences of altered BCRP expression and/or function on the pharmacokinetics and toxicity of chemicals following isoflavone exposure.

The xenoestrogens ethinylestradiol and bisphenol A regulate BCRP at the blood-brain barrier of rats

1. Breast cancer resistance protein (BCRP) is an ABC-transporter at the blood-brain barrier (BBB) facilitating efflux of xenobiotics into blood. Expression and function are regulated via estrogen-receptors (ERs). 2. 17α-Ethinylestradiol (EE2) and bisphenol A (BPA) represent two prominent xenoestrogens. We studied whether EE2 and BPA regulate BCRP function and expression upon a 6 h treatment in an ER-dependent manner in a rat BBB-ex-vivo-model. 3. Isolated brain capillaries were incubated with EE2 or BPA. BCRP function and expression were analyzed by confocal microscopy and Western-Blot. ERα-antagonist MPP and ER-antagonist ICI182.780 were used to study involvement of ERs. 4. EE2 and BPA down-regulated BCRP transport function and expression. EE2 effects occurred at pharmacologically relevant doses, BPA exhibited only weak influences. Down-regulation by EE2 was reversed by ICI but not MPP. BPA effects were not reversed by either antagonist. 5. EE2 is a potent regulator of BCRP expression and function acting by ERβ-stimulation. Oral contraception could alter uptake of pharmaceuticals to the brain and might thus be considered as an origin of central nervous system (CNS) side-effects. EE2 could also present a novel co-treatment to improve CNS-pharmacotherapy. BPA is a weak modulator of BCRP expression. Its effects appear not to be caused by ERs.

Expression of membrane transporters and metabolic enzymes involved in estrone-3-sulphate disposition in human breast tumour tissues

Two-thirds of newly diagnosed hormone-dependent (HR?) breast cancers are detected in post-menopausal patients where estrone-3-sulphate (E3S) is the predominant source for tumour estradiol. Understanding intra-tumoral fate of E3S would facilitate in the identification of novel molecular targets for HR? post-menopausal breast cancer patients. Hence this study investigates the clinical expression of (i) organic anion-transporting polypeptides (OATPs), (ii) multidrug resistance protein (MRP-1), breast cancer resistance proteins (BCRP), and (iii) sulphatase (STS), 17β-hydroxysteroid dehydrogenase (17β-HSD-1), involved in E3S uptake, efflux and metabolism, respectively. Fluorescent and brightfield images of stained tumour sections (n = 40) were acquired at 4× and 20× magnification, respectively. Marker densities were measured as the total area of positive signal divided by the surface area of the tumour section analysed and was reported as % area (ImageJ software). Tumour, stroma and non-tumour tissue areas were also quantified (Inform software), and the ratio of optical intensity per histologic area was reported as % area/tumour, % area/stroma and % area/non-tumour. Functional role of OATPs and STS was further investigated in HR? (MCF-7, T47-D, ZR-75) and HR-(MDA-MB-231) cells by transport studies conducted in the presence or absence of specific inhibitors. Amongst all the transporters and enzymes, OATPs and STS have significantly (p < 0.0001) higher expression in HR? tumour sections with highest target signals obtained from the tumour regions of the tissues. Specific OATP-mediated E3S uptake and STS-mediated metabolism were also observed in all HR? breast cancer cells. These observations suggest the potential of OATPs as novel molecular targets for HR? breast cancers.

The role of canalicular ABC transporters in cholestasis

Cholestasis, a hallmark feature of hepatobiliary disease, is characterized by the retention of biliary constituents. Some of these constituents, such as bile acids, inflict damage to hepatocytes and bile duct cells. This damage may lead to inflammation, fibrosis, cirrhosis, and eventually carcinogenesis, sequelae that aggravate the underlying disease and deteriorate clinical outcome. Canalicular ATP-binding cassette (ABC) transporters, which mediate the excretion of individual bile constituents, play a key role in bile formation and cholestasis. The study of these transporters and their regulatory nuclear receptors has revolutionized our understanding of cholestatic disease. This knowledge has served as a template to develop novel treatment strategies, some of which are currently already undergoing phase III clinical trials. In this review we aim to provide an overview of the structure, function, and regulation of canalicular ABC transporters. In addition, we will focus on the role of these transporters in the pathogenesis and treatment of cholestatic bile duct and liver diseases.

Association of functional polymorphism rs2231142 (Q141K) in the ABCG2 gene with serum uric acid and gout in 4 US populations: the PAGE Study

A loss-of-function mutation (Q141K, rs2231142) in the ATP-binding cassette, subfamily G, member 2 gene (ABCG2) has been shown to be associated with serum uric acid levels and gout in Asians, Europeans, and European and African Americans; however, less is known about these associations in other populations. Rs2231142 was genotyped in 22,734 European Americans, 9,720 African Americans, 3,849 Mexican Americans, and 3,550 American Indians in the Population Architecture using Genomics and Epidemiology (PAGE) Study (2008-2012). Rs2231142 was significantly associated with serum uric acid levels (P = 2.37 × 10(-67), P = 3.98 × 10(-5), P = 6.97 × 10(-9), and P = 5.33 × 10(-4) in European Americans, African Americans, Mexican Americans, and American Indians, respectively) and gout (P = 2.83 × 10(-10), P = 0.01, and P = 0.01 in European Americans, African Americans, and Mexican Americans, respectively). Overall, the T allele was associated with a 0.24-mg/dL increase in serum uric acid level (P = 1.37 × 10(-80)) and a 1.75-fold increase in the odds of gout (P = 1.09 × 10(-12)). The association between rs2231142 and serum uric acid was significantly stronger in men, postmenopausal women, and hormone therapy users compared with their counterparts. The association with gout was also significantly stronger in men than in women. These results highlight a possible role of sex hormones in the regulation of ABCG2 urate transporter and its potential implications for the prevention, diagnosis, and treatment of hyperuricemia and gout.

Translational phase I trial of vorinostat (suberoylanilide hydroxamic acid) combined with cytarabine and etoposide in patients with relapsed, refractory, or high-risk acute myeloid leukemia

PURPOSE

To determine the maximum-tolerated dose (MTD) of the histone deacetylase inhibitor vorinostat combined with fixed doses of cytarabine (ara-C or cytosine arabinoside) and etoposide in patients with poor-risk or advanced acute leukemia, to obtain preliminary efficacy data, describe pharmacokinetics, and in vivo pharmacodynamic effects of vorinostat in leukemia blasts.

EXPERIMENTAL DESIGN

In this open-label phase I study, vorinostat was given orally on days one to seven at three escalating dose levels: 200 mg twice a day, 200 mg three times a day, and 300 mg twice a day. On days 11 to 14, etoposide (100 mg/m(2)) and cytarabine (1 or 2 g/m(2) twice a day if ≥65 or <65 years old, respectively) were given. The study used a standard 3+3 dose escalation design.

RESULTS

Eighteen of 21 patients with acute myelogenous leukemia (AML) treated on study completed planned therapy. Dose-limiting toxicities [hyperbilirubinemia/septic death (1) and anorexia/fatigue (1)] were encountered at the 200 mg three times a day level; thus, the MTD was established to be vorinostat 200 mg twice a day. Of 21 patients enrolled, seven attained a complete remission (CR) or CR with incomplete platelet recovery, including six of 13 patients treated at the MTD. The median remission duration was seven months. No differences in percentage S-phase cells or multidrug resistance transporter (MDR1 or BCRP) expression or function were observed in vivo in leukemia blasts upon vorinostat treatment.

CONCLUSIONS

Vorinostat 200 mg twice a day can be given safely for seven days before treatment with cytarabine and etoposide. The relatively high CR rate seen at the MTD in this poor-risk group of patients with AML warrants further studies to confirm these findings.

Characterization of the role of ABCG2 as a bile acid transporter in liver and placenta

ABCG2 is involved in epithelial transport/barrier functions. Here, we have investigated its ability to transport bile acids in liver and placenta. Cholylglycylamido fluorescein (CGamF) was exported by WIF-B9/R cells, which do not express the bile salt export pump (BSEP). Sensitivity to typical inhibitors suggested that CGamF export was mainly mediated by ABCG2. In Chinese hamster ovary (CHO cells), coexpression of rat Oatp1a1 and human ABCG2 enhanced the uptake and efflux, respectively, of CGamF, cholic acid (CA), glycoCA (GCA), tauroCA, and taurolithocholic acid-3-sulfate. The ability of ABCG2 to export these bile acids was confirmed by microinjecting them together with inulin in Xenopus laevis oocytes expressing this pump. ABCG2-mediated bile acid transport was inhibited by estradiol 17β-d-glucuronide and fumitremorgin C. Placental barrier for bile acids accounted for <2-fold increase in fetal cholanemia despite >14-fold increased maternal cholanemia induced by obstructive cholestasis in pregnant rats. In rat placenta, the expression of Abcg2, which was much higher than that of Bsep, was not affected by short-term cholestasis. In pregnant rats, fumitremorgin C did not affect uptake/secretion of GCA by the liver but inhibited its fetal-maternal transfer. Compared with wild-type mice, obstructive cholestasis in pregnant Abcg2(-/-) knockout mice induced similar bile acid accumulation in maternal serum but higher accumulation in placenta, fetal serum, and liver. In conclusion, ABCG2 is able to transport bile acids. The importance of this function depends on the relative expression in the same epithelium of other bile acid exporters. Thus, ABCG2 may play a key role in bile acid transport in placenta, as BSEP does in liver.

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.

Glucocorticoid regulation of placental breast cancer resistance protein (Bcrp1) in the mouse

Placental breast cancer resistance protein (Bcrp1; encoded by the Abcg2 gene) limits maternal-fetal transplacental transfer of numerous endogenous and exogenous substrates; however, the regulation of placental Abcg2 and Bcrp1 and is not well understood. Placental Abcg2 messenger RNA (mRNA) levels decrease with advancing gestation in the mouse, and this corresponds to increasing levels of maternal and fetal plasma glucocorticoid. Glucocorticoids, including dexamethasone (DEX), downregulate Bcrp1 expression and function in both breast cancer cell lines and the blood-brain barrier in vitro; whether this occurs in the placenta is not known. The potential regulatory role of synthetic glucocorticoids on placental Bcrp1 is of interest, given that approximately 10% of pregnant women are treated with synthetic glucocorticoid for threatened preterm labor. We hypothesized that (1) exposure of pregnant mice to DEX will downregulate placental Abcg2 mRNA and Bcrp1 protein, and (2) results in increased fetal accumulation of [(3)H]mitoxantrone. Pregnant mice were treated with DEX (low-dose: 0.1 mg/kg or high-dose: 1 mg/kg) or vehicle (saline) from embryonic day (E) E9.5 to E15.5 or E12.5 to E18.5. In placentae derived from female fetuses, high-dose DEX significantly downregulated Abcg2 mRNA expression on E15.5 (P < .05) and significantly inhibited Bcrp1 function (P < .05). Similarly, high-dose DEX significantly inhibited Bcrp1 function in the placentae derived from male fetuses (P < .05). In conclusion, there is a dose-dependent regulatory effect of synthetic glucocorticoid on placental Abcg2 mRNA and Bcrp1 function in vivo. Further, it appears that, at the level of Abcg2 gene expression, the female-derived placentae are more susceptible to the effects of DEX than male placentae.

Breast cancer-resistance protein (BCRP1) in the fetal mouse brain: development and glucocorticoid regulation

Breast cancer-resistance protein (BCRP1), encoded by Abcg2 mRNA, limits the penetration of a spectrum of compounds into the brain. The fetal brain is a primary target for many BCRP1 substrates; however, the developmental expression, function, and regulation of Abcg2/BCRP1 in the mouse fetal brain are unknown. Synthetic glucocorticoids (e.g., dexamethasone [DEX]) increase Abcg2/BCRP1 expression and function in vitro in endothelial cells derived from brain microvessels. A regulatory role of glucocorticoids on Abcg2/BCRP1 in the fetal brain is of importance given that approximately 10% of pregnant women are treated with synthetic glucocorticoid for threatened preterm labor. We hypothesized the following: 1) Abcg2 mRNA and BCRP1 protein expression increases with development (from Embryonic Day [E] 15.5 to E18.5), corresponding to decreased accumulation of BCRP1 substrate in the fetal brain. 2) Maternal treatment with DEX will up-regulate Abcg2 mRNA and BCRP1 protein expression in the fetal brain, resulting in decreased BCRP1 substrate accumulation. Pregnant FVB dams were euthanized on E15.5 or E18.5, and fetal brains were collected and analyzed for [(3)H]mitoxantrone (BCRP1-specific substrate) accumulation and Abcg2/BCRP1 expression. In another six groups (n = 4-5/group), pregnant mice were treated with DEX (0.1 or 1 mg/kg) or vehicle (saline) from either E9.5 to E15.5 (midgestation) or E12.5 to E18.5 (late gestation) and then injected with [(3)H]mitoxantrone. In conclusion, Abcg2 mRNA expression significantly decreases with advancing gestation, while BCRP1-mediated neuroprotection increases. Furthermore, there is a dose-, sex-, and age-dependent effect of DEX on Abcg2 mRNA in the fetal brain in vivo, indicating a complex regulatory role of glucocorticoid during development.

Therapeutic promises of 2-methoxyestradiol and its drug disposition challenges

2-Methoxyestradiol (2MeO-E2) is an endogenous metabolite of estrogen which was initially considered to be inactive. During the last few decades it has been shown that 2MeO-E2 is a promising anticancer drug. In vitro experiments have demonstrated that it has several anticancer activities, and potential to alleviate hypertension, glomerulosclerosis, hypercholesterolemia, and other disorders. However, due to its low solubility and extensive glucuronidation, to achieve effective concentrations large doses of 2MeO-E2 would be required. Clinical studies reflected very high inter- and intrapatient variability and oral bioavailability of 1 to 2%. Thus, this review paper highlights the origin of this compound, its therapeutic promises, and possible mechanisms of action. It also discusses the pharmacokinetic properties of 2MeO-E2 as well as current developments to overcome low drug solubility and its extensive first pass metabolism.

Transcriptional modulation of BCRP gene to reverse multidrug resistance by toremifene in breast adenocarcinoma cells

Breast cancer resistance protein (BCRP/ABCG2), an ATP-binding cassette half transporter, confers multidrug resistance (MDR) to a series of antitumor agents such as mitoxantrone, daunorubicin, SN-38, and topotecan, and often limits the efficacy of chemotherapy. Recent studies have indicated that a putative estrogen response element (ERE) is located in the promoter region of the BCRP gene. However, whether and how BCRP is regulated transcriptionally by toremifene (TOR) remains unknown. In the present study, two plasmid vectors have been designed to express the wild-type full-length BCRP cDNA enforced driven by its endogenous promoter containing a functional ERE and a constitutive cytomegalovirus (CMV) promoter as control, respectively, which were transfected into estrogen-responsive MCF-7 and estrogen-independent MDA-MB-231 human breast adenocarcinoma cell lines. We showed that toremifene alone significantly downregulated BCRP mRNA and protein levels in estrogen receptor α (ERα)-positive MCF-7 cells in a dose-dependent manner, and the inhibitory effect was partially reversed by estrone (E(1)). Furthermore, gel shift assays demonstrated that specific binding of ERα to the ERE in the BCRP promoter is essential for transcriptional inhibition of BCRP by toremifene. Interestingly, toremifene alone increased the cellular accumulation of mitoxantrone in BCRP-transfected cells, suggesting that TOR indeed inhibits BCRP-mediated drug efflux and overcome drug resistance. To the best of our knowledge, this is the first report describing a direct effect of toremifene on BCRP. Our results thus indicate that toremifene by itself downregulates BCRP expression to reverse BCRP-mediated atypical multidrug resistance via a novel transcriptionally mechanism, which might be involved in TOR-ER complexes binding to the ERE of BCRP promoter to repress transcription of BCRP gene.

Structure-activity relationships and quantitative structure-activity relationships for breast cancer resistance protein (ABCG2)

Breast cancer resistance protein (ABCG2), the newest ABC transporter, was discovered independently by three groups in the late 1990s. ABCG2 is widely distributed in the body with expression in the brain, intestine, and liver, among others. ABCG2 plays an important role by effluxing drugs at the blood-brain, blood-testis, and maternal-fetal barriers and in the efflux of xenobiotics at the small intestine and kidney proximal tubule brush border and liver canalicular membranes. ABCG2 transports a wide variety of substrates including HMG-CoA reductase inhibitors, antibiotics, and many anticancer agents and is one contributor to multidrug resistance in cancer cells. Quantitative structure-activity relationship (QSAR) models and structure-activity relationships (SARs) are often employed to predict ABCG2 substrates and inhibitors prior to in vitro and in vivo studies. QSAR models correlate in vivo biological activity to physicochemical properties of compounds while SARs attempt to explain chemical moieties or structural features that contribute to or are detrimental to the biological activity. Most ABCG2 datasets available for in silico modeling are comprised of congeneric series of compounds; the results from one series usually cannot be applied to another series of compounds. This review will focus on in silico models in the literature used for the prediction of ABCG2 substrates and inhibitors.

Breast cancer resistance protein/ABCG2 is differentially regulated downstream of extracellular signal-regulated kinase

Breast cancer resistance protein (BCRP)/ABCG2 is a drug efflux pump responsible for multidrug resistance in cancer cells. We report that dephosphorylation of extracellular signal-regulated kinase (ERK) by treatment with mitogen-activated protein kinase/ERK kinase (MEK) inhibitors causes two opposing effects, transcriptional upregulation and prompted protein degradation of endogenous BCRP in breast cancer MCF-7 cells. Endogenous BCRP was eventually found to be upregulated. Conversely, treatment with epidermal growth factor was associated with its downregulation in the cells. MEK inhibitors also caused prompted degradation of exogenous BCRP in MCF-7 and gastric cancer NCI-N87 cells that express exogenous BCRP without affecting its transcriptional levels, and potentiated anticancer agents in the cells. A lysosomal inhibitor abolished this prompted degradation of exogenous BCRP, but a proteasome inhibitor did not. Inhibition of p90 ribosomal protein S6 kinase (RSK), one of the downstream effectors of ERK, resulted in transcriptional upregulation of endogenous BCRP but did not affect the protein degradation of exogenous BCRP. The data suggest that BCRP expression is differentially regulated downstream of the MEK-ERK pathway, transcriptionally upregulated through the inhibition of the MEK-ERK-RSK pathway, and posttranscriptionally downregulated through the inhibition of the MEK-ERK-non-RSK pathway. Although the immediate downstream effector of ERK remains to be elucidated, the data provide new insights into regulatory mechanisms of BCRP activity and may assist the development of BCRP-specific expression modulators.

BCRP expression does not result in resistance to STX140 in vivo, despite the increased expression of BCRP in A2780 cells in vitro after long-term STX140 exposure

The anti-proliferative and anti-angiogenic properties of the endogenous oestrogen metabolite, 2-methoxyoestradiol (2-MeOE2), are enhanced in a series of sulphamoylated derivatives of 2-MeOE2. To investigate possible mechanisms of resistance to these compounds, a cell line, A2780.140, eightfold less sensitive to the 3,17-O,O-bis-sulphamoylated derivative, STX140, was derived from the A2780 ovarian cancer cell line by dose escalation. Other cell lines tested did not develop STX140 resistance. RT-PCR and immunoblot analysis demonstrated that breast cancer resistance protein (BCRP) expression is dramatically increased in A2780.140 cells. The cells are cross-resistant to the most structurally similar bis-sulphamates, and to BCRP substrates, mitoxantrone and doxorubicin; but they remain sensitive to taxol, an MDR1 substrate, and to all other sulphamates tested. Sensitivity can be restored using a BCRP inhibitor, and this pattern of resistance is also seen in a BCRP-expressing MCF-7-derived cell line, MCF-7.MR. In mice bearing wild-type (wt) and BCRP-expressing tumours on either flank, both STX140 and mitoxantrone inhibited the growth of the MCF-7wt xenografts, but only STX140 inhibited growth of the MCF-7.MR tumours. In conclusion, STX140, a promising orally bioavailable anti-cancer agent in pre-clinical development, is highly efficacious in BCRP-expressing xenografts. This is despite an increase in BCRP expression in A2780 cells in vitro after chronic dosing with STX140.

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.

Expression and localisation of breast cancer resistance protein (BCRP) in human fetal membranes and decidua and the influence of labour at term

Breast cancer resistance protein (BCRP) is a multidrug resistant ABC transport protein (ABCG-2). It extrudes a wide range of substrates, including many chemotherapy drugs, steroids and folate. It is present in many cancers, as well as normal tissues, in particular barrier tissues such as the blood-brain barrier, the intestine, blood vessels and the human placenta. Human fetal membranes (amnion and chorion laeve) provide the barrier between the maternal uterine environment and the fetus. In the present study, we defined the expression and localisation of BCRP mRNA and protein in human fetal membranes (amnion and chorion) and attached decidua obtained before and following labour at term. BCRP protein and mRNA was expressed in all tissues examined and the levels of expression were not altered by labour. BCRP was localised to the amnion epithelial cells, chorion trophoblast cells and decidua stromal cells, as well as the endothelial cells of maternal blood vessels in the decidua, but was absent from mesenchymal cells. In the amnion epithelium, BCRP protein was localised to the apical surface, cytoplasm and membrane between cells. In the chorion trophoblast and decidua stromal cells, BCRP protein was localised to the plasma membrane. However, in the chorion trophoblast, BCRP protein was also highly expressed in the nucleus. The level of BCRP protein in the membranes was comparable to that in the placenta. These high levels raise the possibility that this transporter plays an important role in the physiological function of the tissues.

Development of inhibitors of ATP-binding cassette drug transporters: present status and challenges

BACKGROUND

Multi-drug resistance (MDR) of cancer cells is an obstacle to effective chemotherapy of cancer. The ATP-binding cassette (ABC) transporters, including P-glycoprotein (ABCB1), MRP1 (ABCC1) and ABCG2, play an important role in the development of this resistance. An attractive approach to overcoming MDR is the inhibition of the pumping action of these transporters. Several inhibitors/modulators of ABC transporters have been developed, but cytotoxic effects and adverse pharmacokinetics have prohibited their use. The ongoing search for such inhibitors/modulators that can be applied in the clinic has led to three generations of compounds. The most recent inhibitors are more potent and less toxic than first-generation compounds, yet some are still prone to adverse effects, poor solubility and unfavorable changes in the pharmacokinetics of the anticancer drugs.

OBJECTIVE

This review provides an update of the published work on the development of potent modulators to overcome MDR in cancer cells, their present status in clinical studies and suggestions for further improvement to obtain better inhibitors.

METHODS

This review summarizes recent advances in the development of less toxic modulators, including small molecules and natural products. In addition, a brief overview of other novel approaches that can be used to inhibit ABC drug transporters mediating MDR has also been provided.

CONCLUSION

The multifactorial nature of MDR indicates that it may be important to develop modulators that can simultaneously inhibit both the function of the drug transporters and key signaling pathways, which are responsible for development of this phenomenon.

ABCG2: structure, function and role in drug response

ABCG2 was discovered in multi-drug-resistant cancer cells, with the identification of chemotherapeutic agents, such as mitoxantrone, flavopiridol, methotrexate and irinotecan as substrates. Later, drugs from other therapeutic groups were also described as substrates, including antibiotics, antivirals, HMG-CoA reductase inhibitors and flavonoids. An expanding list of compounds inhibiting ABCG2 has also been generated. The wide variety of drugs transported by ABCG2 and its normal tissue distribution with highest levels in the placenta, intestine and liver, suggest a role in protection against xenobiotics. ABCG2 also has an important role in the pharmacokinetics of its substrates. Single nucleotide polymorphisms of the gene were shown to alter either plasma concentrations of substrate drugs or levels of resistance against chemotherapeutic agents in cell lines. ABCG2 was also described as the determinant of the side population of stem cells. All these aspects of the transporter warrant further research aimed at understanding ABCG2's structure, function and regulation of expression.

Transcriptional upregulation of breast cancer resistance protein by 17beta-estradiol in ERalpha-positive MCF-7 breast cancer cells

OBJECTIVES

Breast cancer resistance protein (BCRP) confers resistance to certain anticancer drugs such as mitoxantrone, topotecan and SN-38. A putative estrogen response element (ERE) was located in the promoter region of the BCRP gene. The present study aimed to investigate whether human BCRP expression is regulated pretranscriptionally by 17beta-estradiol.

METHODS

Two recombinant plasmids (pcDNA3-promoter-BCRP and pcDNA3-CMV-BCRP) were designed to express the full-length BCRP cDNA enforced driven by its endogenous promoter containing a functional ERE and a control constitutive cytomegalovirus (CMV) promoter, respectively, which were transfected into estrogen receptor alpha (ERalpha)-positive MCF-7 and ERalpha-negative MDA-MB-231 breast cancer cell lines.

RESULTS

17beta-estradiol significantly upregulated BCRP mRNA and protein expression in a dose-dependent manner, and the effect was abolished by the antiestrogen tamoxifen. Furthermore, electrophoretic mobility shift assays demonstrated that the putative ERE in the promoter region of the BCRP gene and ERalpha are essential for transcriptional activation of BCRP by 17beta-estradiol.

CONCLUSIONS

Taken together, our findings indicate that BCRP expression is upregulated by 17beta-estradiol via a novel pretranscriptional mechanism which might be involved in 17beta-estradiol-ER complexes binding to the ERE of BCRP promoter via the classical pathway to activate transcription of the BCRP gene.

Cholesterol gallstones and cancer of gallbladder (CAGB): molecular links

There is a known association between cholesterol gallstones and cancer of gall bladder (CAGB). However, the exact relation is not clear. It is proposed they are linked at molecular level by the activity of the orphan nuclear receptors (ONRs) and ABC transporter pumps involved in cholesterol and xenobiotic efflux from the liver into bile. There is evidence that these two pathways are closely interlinked and influence each other. Genetic and environmental factors that upregulate these systems can lead to the simultaneous pumping of cholesterol (which precipitate as gallstones) and a food carcinogen into the bile in gall bladder; the latter causes malignant transformation. Aflatoxin B, a potent hepatocarcinogen, could be the culprit in endemic regions such as South America and North India.

Progesterone Inhibits Folic Acid Transport in Human Trophoblasts

The aim of this work was to test the putative involvement of members of the ABC superfamily of transporters on folic acid (FA) cellular homeostasis in the human placenta. [(3)H]FA uptake and efflux in BeWo cells were unaffected or hardly affected by multidrug resistance 1 (MDR1) inhibition (with verapamil), multidrug resistance protein (MRP) inhibition (with probenecid) or breast cancer resistance protein (BCRP) inhibition (with fumitremorgin C). However, [(3)H]FA uptake and efflux were inhibited by progesterone (200 microM). An inhibitory effect of progesterone upon [(3)H]FA uptake and efflux was also observed in human cytotrophoblasts. Moreover, verapamil and ss-estradiol also reduced [(3)H]FA efflux in these cells. Inhibition of [(3)H]FA uptake in BeWo cells by progesterone seemed to be very specific since other tested steroids (beta-estradiol, corticosterone, testosterone, aldosterone, estrone and pregnanediol) were devoid of effect. However, efflux was also inhibited by beta-estradiol and corticosterone and stimulated by estrone. Moreover, the effect of progesterone upon the uptake of [(3)H]FA by BeWo cells was concentration-dependent (IC(50 )= 65 [range 9-448] microM) and seems to involve competitive inhibition. Also, progesterone (1-400 microM) did not affect either [(3)H]FA uptake or efflux at an external acidic pH. Finally, inhibition of [(3)H]FA uptake by progesterone was unaffected by either 4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid (SITS), a known inhibitor of the reduced folate carrier (RFC), or an anti-RFC antibody. These results suggest that progesterone inhibits RFC. In conclusion, our results show that progesterone, a sterol produced by the placenta, inhibits both FA uptake and efflux in BeWo cells and primary cultured human trophoblasts.

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.

The emerging pharmacotherapeutic significance of the breast cancer resistance protein (ABCG2)

The breast cancer resistance protein (also termed ABCG2) is an ATP-binding cassette transporter, which mediates the extrusion of toxic compounds from the cell, and which was originally identified in relation to the development of multidrug resistance of cancer cells. ABCG2 interacts with a range of substrates including clinical drugs but also substances such as sterols, porphyrins and a variety of dietary compounds. Physiological functions of ABCG2 at both cellular and systemic levels are reviewed. For example, ABCG2 expression in erythrocytes may function in porphyrin homeostasis. In addition, ABCG2 expression at apical membranes of cells such as hepatocytes, enterocytes, endothelial and syncytiotrophoblast cells may correlate to protective barrier or secretory functions against environmental or clinically administered substances. ABCG2 also appears influential in the inter-patient variation and generally poor oral bioavailability of certain chemotherapeutic drugs such as topotecan. As this often precludes an oral drug administration strategy, genotypic and environmental factors altering ABCG2 expression and activity are considered. Finally, clinical modulation of ABCG2 activity is discussed. Some of the more recent strategies include co-administered modulating agents, hammerhead ribozymes or antisense oligonucleotides, and with specificity in cell targeting, these may be used to reduce drug resistance and increase drug bioavailability to improve the profile of chemotherapeutic efficacy versus toxicity. While many such strategies remain in relative infancy at present, increased knowledge of modulators of ABCG2 could hold the key to novel approaches in medical treatment.

Flavonoids inhibit breast cancer resistance protein-mediated drug resistance: transporter specificity and structure–activity relationship

PURPOSE

ATP-binding cassette (ABC) transporters, such as P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug resistance-related protein 1 (MRP1), confer resistance to various anticancer agents. We previously reported that some flavonoids have BCRP-inhibitory activity. Here we show the reversal effects of an extensive panel of flavonoids upon BCRP-, P-gp-, and MRP1-mediated drug resistance.

METHODS

Reversal effects of flavonoids upon BCRP-, P-gp-, or MRP1-mediated drug resistance were examined in the BCRP- or MDR1-transduced human leukemia K562 cells or in the MRP1-transfected human epidermoid carcinoma KB-3-1 cells using cell growth inhibition assays. The IC(50) values were determined from the growth inhibition curves. The RI(50) values were then determined as the concentration of inhibitor that causes a twofold reduction of the IC(50) in each transfectant. The reversal of BCRP activity was tested by measuring the fluorescence of intracellular topotecan.

RESULTS

The BCRP-inhibitory activity of 32 compounds was screened, and 20 were found to be active. Among these active compounds, 3',4',7-trimethoxyflavone showed the strongest anti-BCRP activity with RI(50) values of 0.012 microM for SN-38 and 0.044 muM for mitoxantrone. We next examined the effects of a panel of 11 compounds on P-gp- and MRP1-mediated drug resistance. Two of the flavones, 3',4',7-trimethoxyflavone and acacetin, showed only low anti-P-gp activity, with the remainder displaying no suppressive effects against P-gp. None of the flavonoids that we tested inhibited MRP1.

CONCLUSION

Our present results thus indicate that many flavonoids selectively inhibit BCRP only. Moreover, we examined the structure-BCRP inhibitory activity relationship from our current study.

Estrogen-mediated post transcriptional down-regulation of P-glycoprotein in MDR1-transduced human breast cancer cells

The human multidrug resistance gene 1 (MDR1) encodes the plasma membrane P-glycoprotein (P-gp/ABCB1) that functions as an efflux pump for various anticancer agents. We recently reported that estrogens down-regulate the expression of breast cancer resistance protein (BCRP/ABCG2). In our present study we demonstrate that estrogens also down-regulate P-gp expression in the MDR1-transduced, estrogen receptor alpha (ER-alpha)-positive human breast cancer cells, MCF-7/MDR and T-47D/MDR. The P-gp expression levels in MCF-7/MDR cells treated with 100 pM estradiol were found to be 10-20-fold lower than the levels in these same cells that were cultured without estradiol. In contrast, estradiol did not affect the P-gp expression levels in the ER-alpha-negative cancer cells, MDA-MB-231/MDR and NCI/ADR-RES. Estrone and diethylstilbestrol were also found to down-regulate P-gp in MCF-7/MDR cells, but progesterone treatment did not produce this effect. Tamoxifen reversed the estradiol-mediated down-regulation of P-gp in MCF-7/MDR cells, suggesting that ER-alpha activity is necessary for the effects of estradiol upon P-gp. However, estradiol was found not to alter the MDR1 transcript levels in either MCF-7/MDR and T-47D/MDR cells, suggesting that post-transcriptional mechanisms underlie its effects upon P-gp down-regulation. MCF-7/MDR cells also showed eight-fold higher sensitivity to vincristine when treated with 100 pM estradiol, than when treated with 1 pM estradiol. These results may serve to provide a better understanding of the expression control of ABC transporters, and possibly allow for the establishment of new cancer chemotherapy strategies that would control P-gp expression in breast cancer cells and thereby increase their sensitivity to MDR1-related anticancer agents.

Metabolites of ginsenosides as novel BCRP inhibitors

We have previously shown ginsenosides derived from Panax ginseng exert opposing effects on angiogenesis. Here, we examined protopanaxadiol-containing ginsenosides (Rg3, Rh2, and PPD) and protopanaxatriol-containing ginsenosides (Rg1, Rh1, and PPT) as potential inhibitors of breast cancer resistance protein (BCRP). Among these ginsenosides, metabolites Rh2, PPD, and PPT significantly enhanced the cytotoxicity of mitoxantrone (MX) to human breast carcinoma MCF-7/MX cells which overexpress BCRP. PPD was the most potent followed by Rh2 and PPT. This effect was not seen in sensitive MCF-7 cells. Rg3, Rg1, and Rh1 were ineffective in either MCF-7 or MCF-7/MX cells. PPD, Rh2, and PPT were able to inhibit MX efflux in MCF-7/MX cells. PPD and Rh2 also increased MX uptake. In inside out membrane vesicles from Lactococcus lactis cells expressing BCRP, only PPD was found to significantly inhibit BCRP-associated vanadate sensitive ATPase activity. These results indicate that metabolites PPD, Rh2, and PPT were inhibitors of BCRP.

Potential role of trans-inhibition of the bile salt export pump by progesterone metabolites in the etiopathogenesis of intrahepatic cholestasis of pregnancy

BACKGROUND/AIMS

Progesterone metabolites such as 5alpha-pregnan-3alpha-ol-20-one (PM4) are elevated in serum of women with intrahepatic cholestasis of pregnancy (ICP).

METHODS/RESULTS

When assayed in isolated perfused rat liver, PM4 did not induce cholestasis, whereas sulfated PM4 (PM4-S), which unlike PM4 is secreted into bile, reduced bile flow and bile acid (BA) output. Whether PM4-S inhibited the bile salt export pump (BSEP) was investigated. Radiolabeled methylesters (ME) of cholic acid and chenodeoxycholic acid were taken up by Xenopus laevis oocytes co-expressing rat BSEP and human carboxylesterase-1 (CES1), efficiently hydrolyzed to free BAs by CES1 and subsequently exported by BSEP. Rifampicin or cyclosporin A in the extracellular medium had no effect on BA efflux. In contrast, estradiol 17beta-D-glucuronide and several progesterone metabolites, including PM4-S, induced a marked non-competitive trans-inhibition of BSEP-mediated BA efflux (Ki=20-60 microM). Mitochondrial activity was markedly impaired in oocytes incubated with BA-MEs, but not with free BAs. Co-expression of CES1 and BSEP partly protected oocytes from this toxic effect. Trans-inhibition of BSEP abolished this protection.

CONCLUSIONS

Several estrogens and progesterone metabolites are able to induce trans-inhibition of BSEP and the subsequent toxicity induced by the accumulation of BAs, which may play a role in the etiopathogenesis of ICP.

Complex interaction of BCRP/ABCG2 and imatinib in BCR-ABL-expressing cells: BCRP-mediated resistance to imatinib is attenuated by imatinib-induced reduction of BCRP expression

Imatinib, a potent tyrosine kinase inhibitor, is effluxed from cells by the breast cancer resistance protein (BCRP/ABCG2), yet published studies to date fail to demonstrate resistance to imatinib cytotoxicity in BCRP-overexpressing cells in vitro. We investigated cellular resistance to imatinib in BCR-ABL-expressing cells transduced and selected to overexpress BCRP (K562/BCRP-MX10). These cells exhibited a 2- to 3-fold increase in resistance to imatinib (P < .05) and a 7- to 12-fold increase in resistance to mitoxantrone, a known BCRP substrate. Resistance to imatinib was completely abolished by the specific BCRP inhibitor fumitremorgin C. Studies of the mechanism of the diminished resistance to imatinib compared with mitoxantrone revealed that imatinib decreased the expression of BCRP in K562/BCRP-MX10 cells without affecting mRNA levels. BCRP levels in cells that do not express BCR-ABL were not affected by imatinib. Loss of BCRP expression was accompanied by imatinib-induced reduction of phosphorylated Akt in the BCRP-expressing K562 cells. The phosphoinositol-3 kinase (PI3K) inhibitor LY294002 also decreased BCRP levels in K562/BCRP-MX10 cells. These studies show that BCRP causes measurable imatinib resistance, but this effect is attenuated by imatinib-mediated inhibition of BCR-ABL, which in turn downregulates overall BCRP levels posttranscriptionally via the PI3K-Akt pathway.

Inhibitors of cancer cell multidrug resistance mediated by breast cancer resistance protein (BCRP/ABCG2)

Breast cancer resistance protein (BCRP/ABCG2) belongs to the ATP-binding cassette (ABC) transporter superfamily. It is able to efflux a broad range of anti-cancer drugs through the cellular membrane, thus limiting their anti-proliferative effects. Due to its relatively recent discovery in 1998, and in contrast to the other ABC transporters P-glycoprotein (MDR1/ABCB1) and multidrug resistance-associated protein (MRP1/ABCC1), only a few BCRP inhibitors have been reported. This review summarizes the known classes of inhibitors that are either specific for BCRP or also inhibit the other multidrug resistance ABC transporters. Information is presented on structure-activity relationship aspects and how modulators may interact with BCRP.

Functional SNPs of the breast cancer resistance protein ‐ therapeutic effects and inhibitor development

Breast cancer resistance protein (BCRP) is a half-molecule ATP-binding cassette transporter that pumps out various anticancer agents such as 7-ethyl-10-hydroxycamptothecin, topotecan and mitoxantrone. We have previously identified three polymorphisms within the BCRP gene, G34A (substituting Met for Val-12), C376T (substituting a stop codon for Gln-126) and C421A (substituting Lys for Gln-141). C421A BCRP-transfected murine fibroblast PA317 cells showed markedly decreased protein expression and low-level drug resistance when compared with wild-type BCRP-transfected cells. In contrast, G34A BCRP-transfected PA317 cells showed a similar protein expression and drug resistance profile to wild-type. The C376T polymorphism would be expected to have a considerable impact as active BCRP protein will not be expressed from a T376 allele. Hence, people with C376T and/or C421A polymorphisms may express low levels of BCRP, resulting in hypersensitivity of normal cells to BCRP-substrate anticancer agents. Estrogens, estrone and 17beta-estradiol, were previously found to restore drug sensitivity levels in BCRP-transduced cells by increasing the cellular accumulation of anticancer agents. BCRP transports sulfated estrogens but not free estrogens and in a series of screening experiments for synthesized and natural estrogenic compounds, several tamoxifen derivatives and phytoestrogens/flavonoids were identified that effectively circumvent BCRP-mediated drug resistance. The kinase inhibitors gefitinib and imatinib mesylate also interact with BCRP. Gefitinib, an inhibitor of epidermal growth factor receptor-tyrosine kinase, inhibits its transporter function and reverses BCRP-mediated drug resistance both in vitro and in vivo. BCRP-transfected human epidermoid carcinoma A431 cells and BCRP-transfected human non-small cell lung cancer PC-9 cells show gefitinib resistance. Imatinib, an inhibitor of BCR-ABL tyrosine kinase, also inhibits BCRP-mediated drug transport. Hence, both functional SNPs and inhibitors of BCRP reduce its transporter function and thus modulate substrate pharmacokinetics and pharmacodynamics.

Modulation of p-glycoprotein and breast cancer resistance protein by some prescribed corticosteroids

Efflux transporters, p-glycoprotein and breast cancer resistance protein (BCRP), located at barrier sites such as the blood-brain barrier may affect distribution of steroids used for treating chronic inflammatory conditions and thus the extent to which they may perturb the hypothalamic-pituitary-adrenal axis. In the present study, six different glucocorticoids were investigated for their possible interactions with these efflux transporters. Beclomethasone dipropionate, mometasone furoate and ciclesonide active principle but not fluticasone propionate or triamcinolone, (all at 0.1 to 10 microM) caused inhibition of efflux, resulting in increased accumulation of mitoxantrone in BCRP-expressing MCF7/MR breast cancer cells and of calcein in p-glycoprotein-expressing SW620/R colon carcinoma cell. At 5 microM the same three increased sensitivity of p-glycoprotein-expressing SW620/R to doxorubicin and stimulated ATPase activity associated with BCRP expressed in bacterial membrane vesicles. Budesonide also stimulated ATPase activity. These data demonstrate the capacity of some clinically used glucocorticoids to interact with efflux transporters.

Breast cancer resistance protein: molecular target for anticancer drug resistance and pharmacokinetics/pharmacodynamics

Breast cancer resistance protein (BCRP) is a half-molecule ATP-binding cassette transporter that forms a functional homodimer and pumps out various anticancer agents, such as 7-ethyl-10-hydroxycamptothecin, topotecan, mitoxantrone and flavopiridol, from cells. Estrogens, such as estrone and 17beta-estradiol, have been found to restore drug sensitivity levels in BCRP-transduced cells by increasing the cellular accumulation of such agents. Furthermore, synthetic estrogens, tamoxifen derivatives and phytoestrogens/flavonoids have now been identified that can effectively circumvent BCRP-mediated drug resistance. Transcellular transport experiments have shown that BCRP transports sulfated estrogens and various sulfated steroidal compounds, but not free estrogens. The kinase inhibitor gefitinib inhibited the transporter function of BCRP and reversed BCRP-mediated drug resistance both in vitro and in vivo. BCRP-transduced human epidermoid carcinoma A431 (A431/BCRP) and BCRP-transduced human non-small cell lung cancer PC-9 (PC-9/BCRP) cells showed gefitinib resistance. Physiological concentrations of estrogens (10-100 pM) reduced BCRP protein expression without affecting its mRNA levels. Two functional polymorphisms of the BCRP gene have been identified. The C376T (Q126Stop) polymorphism has a dramatic phenotype as active BCRP protein cannot be expressed from a C376T allele. The C421A (Q141K) polymorphism is also significant as Q141K-BCRP-transfected cells show markedly low protein expression levels and low-level drug resistance. Hence, individuals with C376T or C421A polymorphisms may express low levels of BCRP or none at all, resulting in hypersensitivity of normal cells to BCRP-substrate anticancer agents. In summary, both modulators of BCRP and functional single nucleotide polymorphisms within the BCRP gene affect the transporter function of the protein and thus can modulate drug sensitivity and substrate pharmacokinetics and pharmacodynamics in affected cells and individuals.

Effects of dihydropyridines and pyridines on multidrug resistance mediated by breast cancer resistance protein: in vitro and in vivo studies

Breast cancer resistance protein (BCRP, ABCG2) is a recently identified member of the ATP-binding cassette family of cell surface transport proteins. This study was conducted to investigate the effect of a series of newly synthesized 1,4-dihydropyridines and pyridines, designed as potent P-glycoprotein inhibitors, on BCRP-mediated drug efflux both in vitro and in vivo. The effects of 25 synthesized dihydropyridines and corresponding pyridines along with 4 commercially available dihydropyridines (niguldipine, nicardipine, nifedipine, and nitrendipine) on the intracellular accumulation of the BCRP substrate mitoxantrone were evaluated in BCRP-expressing human breast cancer MCF-7/MX100 and human non-small cell lung cancer H460/MX20 cells. At a 2.5 microM concentration, 24 of 25 newly synthesized dihydropyridines and pyridines produced a significant increase of mitoxantrone accumulation in both cell lines. The most potent compound was able to enhance mitoxantrone accumulation approximately 4.5-fold, greater than that obtained with 10 microM fumitremorgin C, which is a specific BCRP inhibitor. The results from the two cell lines showed good correlation (r(2) = 0.71, p < 0.01). Niguldipine, nicardipine, and nitrendipine also demonstrated potent BCRP inhibition, whereas nifedipine had no effect. The effects of the dihydropyridine and pyridine compounds on mitoxantrone cytotoxicity paralleled their effects on mitoxantrone accumulation. Coadministration of a selected dihydropyridine compound, I(m) [DHP-014; 3-(3-(4,4-diphenylpiperidin-1-yl)propyl) 5-methyl 4-(3,4-dimethoxyphenyl)-2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylate)] with topotecan, a good BCRP substrate and a moderate to poor P-glycoprotein substrate, resulted in significant increases in the systemic exposure and peak concentration of topotecan in Sprague-Dawley rats when oral topotecan (2 mg/kg) was combined with 20 mg/kg DHP-014. The observed increase of topotecan exposure provides proof-of-concept for in vivo inhibition of BCRP by these agents.

N-Linked glycosylation of the human ABC transporter ABCG2 on asparagine 596 is not essential for expression, transport activity, or trafficking to the plasma membrane

The human ATP-binding cassette half-transporter ABCG2 is a 72 kDa plasma membrane protein that can confer multidrug resistance to cells in culture when overexpressed. Both transiently and stably expressed ABCG2 are glycosylated, and treatment with peptide N-glycosidase F reduces the apparent molecular mass on SDS-PAGE gels to approximately 60 kDa. Sequence analysis revealed three potential N-linked glycosylation sites in human ABCG2 at amino acids 418, 557, and 596. Site-directed mutagenesis experiments, in which each Asn was changed to Gln independently, revealed that only asparagine 596 is N-linked glycosylated. These data provide the first direct identification of the modified residue in ABCG2 and evidence for the localization of loop 5 to the extracellular space, previously only predicted from hydropathy analysis. Immunoblot and pulse-chase analyses revealed that the glycosylation-deficient ABCG2 (N596Q) variant and the glycosylated parent transporter are expressed equivalently at steady state and have similar half-lives. Cell surface analysis of ABCG2 expression showed comparable amounts of the N596Q variant present at the plasma membrane compared to the glycosylated ABCG2 protein. The ABCG2 (N596Q) variant is also functional, demonstrating rhodamine 123 transport in intact cells comparable to that in cells expressing glycosylated ABCG2. Furthermore, in crude membrane preparations, neither the basal nor the prazosin-stimulated ( approximately 2-fold) ATPase activities of ABCG2 (N596Q) were affected compared to glycosylated ABCG2. Although subtle defects in transporter trafficking and function may exist, these data taken together suggest that N-glycosylation at arginine 596 is not essential for the expression, trafficking to the plasma membrane, or the overall function of ABCG2.