17-β-Estradiol: a powerful modulator of blood-brain barrier BCRP activity

Hopping the Hurdle: Strategies to Enhance the Molecular Delivery to the Brain through the Blood-Brain Barrier

Modern medicine has allowed for many advances in neurological and neurodegenerative disease (ND). However, the number of patients suffering from brain diseases is ever increasing and the treatment of brain diseases remains an issue, as drug efficacy is dramatically reduced due to the existence of the unique vascular structure, namely the blood-brain barrier (BBB). Several approaches to enhance drug delivery to the brain have been investigated but many have proven to be unsuccessful due to limited transport or damage induced in the BBB. Alternative approaches to enhance molecular delivery to the brain have been revealed in recent studies through the existence of molecular delivery pathways that regulate the passage of peripheral molecules. In this review, we present recent advancements of the basic research for these delivery pathways as well as examples of promising ventures to overcome the molecular hurdles that will enhance therapeutic interventions in the brain and potentially save the lives of millions of patients.

Contribution of Direct Cerebral Vascular Transport in Brain Substance Clearance

The accumulation of harmful substances has long been recognized as a likely cause of many neurodegenerative diseases. The two classic brain clearance pathways are cerebrospinal fluid (CSF) and vascular circulation systems. Since the discovery of the glymphatic system, research on the CSF pathway has gained momentum, and impaired CSF clearance has been implicated in virtually all neurodegenerative animal models. However, the contribution of the direct participation of vascular transport across the blood-brain barrier in clearing substances is often ignored in glymphatic papers. Supportive evidence for the direct involvement of parenchymal vasculature in substance clearance is accumulated. First, multiple mechanisms have been proposed for the vascular drainage of exogenous and endogenous substances across the blood-brain barriers. Second, the "traditional" role of arachnoid villi and granulations as the main site for CSF draining into the vasculature system has been questioned. Third, MRI studies using different CSF tracers indicate that parenchymal vasculature directly participates in tracer efflux, consistent with immunohistochemical findings. Here we will review evidence in the literature that supports the direct participation of the parenchymal vascular system in substance clearance, in addition to the CSF clearance pathways.

Neurovascular Interventions for Neurotrauma: From Treatment of Injured Vessels to Treatment of the Injured Brain?

Traumatic brain injury is often associated with a direct or secondary neurovascular pathology. In this review, we present recent advancements in endovascular neurosurgery that enable accurate and effective vessel reconstruction with emphasis on its role in early diagnosis, the expanding use of flow diversion in pseudoaneurysms, and traumatic arteriovenous fistulas. In addition, future directions in which catheter-based interventions could potentially affect traumatic brain injury are described: targeting blood brain barrier integrity using the advantages of intra-arterial drug delivery of blood brain barrier stabilizers to prevent secondary brain edema, exploring the impact of endovascular venous access as a means to modulate venous outflow in an attempt to reduce intracranial pressure and augment brain perfusion, applying selective intra-arterial hypothermia as a neuroprotection method mitigating some of the risks conferred by systemic cooling, trans-vessel wall delivery of regenerative therapy agents, and shifting attention using multimodal neuromonitoring to post-traumatic vasospasm to further characterize the role it plays in secondary brain injury. Thus, we believe that the potential of endovascular tools can be expanded because they enable access to the "highways" governing perfusion and flow and call for further research focused on exploring these routes because it may contribute to novel endovascular approaches currently used for treating injured vessels, harnessing them for treatment of the injured brain.

Zinc finger proteins and ATP-binding cassette transporter-dependent multidrug resistance

BACKGROUND

Multidrug resistance (MDR) remains a significant challenge in cancer treatment, leading to poor clinical outcomes. Dysregulation of ATP-binding cassette (ABC) transporters has been identified as a key contributor to MDR. Zinc finger proteins (ZNPs) are key regulators of transcription and have emerged as potential contributors to cancer drug resistance. Bridging the knowledge gap between ZNPs and MDR is essential to understand a source of heterogeneity in cancer treatment. This review sought to elucidate how different ZNPs modulate the transcriptional regulation of ABC genes, contributing to resistance to cancer therapies.

METHODS

The search was conducted using PubMed, Google Scholar, EMBASE and Web of Science.

RESULTS

In addition to ABC-blockers, the transcriptional features regulated by ZNP are expected to play a role in reversing ABC-mediated MDR and predicting the efficacy of anticancer treatments. Among the ZNP-induced epithelial to mesenchymal transition, SNAIL, SLUG and Zebs have been identified as important factors in promoting MDR through activation of ATM, NFκB and PI3K/Akt pathways, exposing the metabolism to potential ZNP-MDR interactions. Additionally, nuclear receptors, such as VDR, ER and PXR have been found to modulate certain ABC regulations. Other C2H2-type zinc fingers, including Kruppel-like factors, Gli and Sp also have the potential to contribute to MDR.

CONCLUSION

Besides reviewing evidence on the effects of ZNP dysregulation on ABC-related chemoresistance in malignancies, significant markers of ZNP functions are discussed to highlight the clinical implications of gene-to-gene and microenvironment-to-gene interactions on MDR prospects. Future research on ZNP-derived biomarkers is crucial for addressing heterogeneity in cancer therapy.

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.

Regulation of ABC transporters by sex steroids may explain differences in drug resistance between sexes

Drug efficacy is dependent on the pharmacokinetics and pharmacodynamics of therapeutic agents. Tight junctions, detoxification enzymes, and drug transporters, due to their localization on epithelial barriers, modulate the absorption, distribution, and the elimination of a drug. The epithelial barriers which control the pharmacokinetic processes are sex steroid hormone targets, and in this way, sex hormones may also control the drug transport across these barriers. Thus, sex steroids contribute to sex differences in drug resistance and have a relevant impact on the sex-related efficacy of many therapeutic drugs. As a consequence, for the further development and optimization of therapeutic strategies, the sex of the individuals must be taken into consideration. Here, we gather and discuss the evidence about the regulation of ATP-binding cassette transporters by sex steroids, and we also describe the signaling pathways by which sex steroids modulate ATP-binding cassette transporters expression, with a focus in the most important ATP-binding cassette transporters involved in multidrug resistance.

Subcellular analysis of blood-brain barrier function by micro-impalement of vessels in acute brain slices

The blood-brain barrier (BBB) is a tightly and actively regulated vascular barrier. Answering fundamental biological and translational questions about the BBB with currently available approaches is hampered by a trade-off between accessibility and biological validity. We report an approach combining micropipette-based local perfusion of capillaries in acute brain slices with multiphoton microscopy. Micro-perfusion offers control over the luminal solution and allows application of molecules and drug delivery systems, whereas the bath solution defines the extracellular milieu in the brain parenchyma. Here we show, that this combination allows monitoring of BBB transport at the cellular level, visualization of BBB permeation of cells and molecules in real-time and resolves subcellular details of the neurovascular unit. In combination with electrophysiology, it permits comparison of drug effects on neuronal activity following luminal versus parenchymal application. We further apply micro-perfusion to the human and mouse BBB of epileptic hippocampi highlighting its utility for translational research and analysis of therapeutic strategies.

Nuclear receptor ligand screening in an iPSC-derived in vitro blood-brain barrier model identifies new contributors to leptin transport

BACKGROUND

The hormone leptin exerts its function in the brain to reduce food intake and increase energy expenditure to prevent obesity. However, most obese subjects reflect the resistance to leptin even with elevated serum leptin. Considering that leptin must cross the blood-brain barrier (BBB) in several regions to enter the brain parenchyma, altered leptin transport through the BBB might play an important role in leptin resistance and other biological conditions. Here, we report the use of a human induced pluripotent stem cell (iPSC)-derived BBB model to explore mechanisms that influence leptin transport.

METHODS

iPSCs were differentiated into brain microvascular endothelial cell (BMEC)-like cells using standard methods. BMEC-like cells were cultured in Transwell filters, treated with ligands from a nuclear receptor agonist library, and assayed for leptin transport using an enzyme-linked immune sorbent assay. RNA sequencing was further used to identify differentially regulated genes and pathways. The role of a select hit in leptin transport was tested with the competitive substrate assay and after gene knockdown using CRISPR techniques.

RESULTS

Following a screen of 73 compounds, 17β-estradiol was identified as a compound that could significantly increase leptin transport. RNA sequencing revealed many differentially expressed transmembrane transporters after 17β-estradiol treatment. Of these, cationic amino acid transporter-1 (CAT-1, encoded by SLC7A1) was selected for follow-up analyses due to its high and selective expression in BMECs in vivo. Treatment of BMEC-like cells with CAT-1 substrates, as well as knockdown of CAT-1 expression via CRISPR-mediated epigenome editing, yielded significant increases in leptin transport.

CONCLUSIONS

A major female sex hormone, as well as an amino acid transporter, were revealed as regulators of leptin BBB transport in the iPSC-derived BBB model. Outcomes from this work provide insights into regulation of hormone transport across the BBB.

Characterization and comparison of human glioblastoma models

Glioblastoma (GBM) is one of the deadliest cancers. Treatment options are limited, and median patient survival is only several months. Translation of new therapies is hindered by a lack of GBM models that fully recapitulate disease heterogeneity. Here, we characterize two human GBM models (U87-luc2, U251-RedFLuc). In vitro, both cell lines express similar levels of luciferase and show comparable sensitivity to temozolomide and lapatinib exposure. In vivo, however, the two GBM models recapitulate different aspects of the disease. U87-luc2 cells quickly grow into large, well-demarcated tumors; U251-RedFLuc cells form small, highly invasive tumors. Using a new method to assess GBM invasiveness based on detecting tumor-specific anti-luciferase staining in brain slices, we found that U251-RedFLuc cells are more invasive than U87-luc2 cells. Lastly, we determined expression levels of ABC transporters in both models. Our findings indicate that U87-luc2 and U251-RedFLuc GBM models recapitulate different aspects of GBM heterogeneity that need to be considered in preclinical research.

Multidrug efflux transporter ABCG2: expression and regulation

The adenosine triphosphate (ATP)-binding cassette efflux transporter G2 (ABCG2) was originally discovered in a multidrug-resistant breast cancer cell line. Studies in the past have expanded the understanding of its role in physiology, disease pathology and drug resistance. With a widely distributed expression across different cell types, ABCG2 plays a central role in ATP-dependent efflux of a vast range of endogenous and exogenous molecules, thereby maintaining cellular homeostasis and providing tissue protection against xenobiotic insults. However, ABCG2 expression is subjected to alterations under various pathophysiological conditions such as inflammation, infection, tissue injury, disease pathology and in response to xenobiotics and endobiotics. These changes may interfere with the bioavailability of therapeutic substrate drugs conferring drug resistance and in certain cases worsen the pathophysiological state aggravating its severity. Considering the crucial role of ABCG2 in normal physiology, therapeutic interventions directly targeting the transporter function may produce serious side effects. Therefore, modulation of transporter regulation instead of inhibiting the transporter itself will allow subtle changes in ABCG2 activity. This requires a thorough comprehension of diverse factors and complex signaling pathways (Kinases, Wnt/β-catenin, Sonic hedgehog) operating at multiple regulatory levels dictating ABCG2 expression and activity. This review features a background on the physiological role of transporter, factors that modulate ABCG2 levels and highlights various signaling pathways, molecular mechanisms and genetic polymorphisms in ABCG2 regulation. This understanding will aid in identifying potential molecular targets for therapeutic interventions to overcome ABCG2-mediated multidrug resistance (MDR) and to manage ABCG2-related pathophysiology.

Low oxygen tension differentially regulates the expression of placental solute carriers and ABC transporters

Low oxygen concentration, or hypoxia, is an important physiological regulator of placental function including chemical disposition. Here, we compared the ability of low oxygen tension to alter the expression of solute carriers (SLC) and ABC transporters in two human placental models, namely BeWo cells and term placental explants. We found that exposure to low oxygen concentration differentially regulates transporter expression in BeWo cells, including downregulation of ENT1, OATP4A1, OCTN2, BCRP, and MRP2/3/5, and upregulation of CNT1, OAT4, OATP2B1, SERT, SOAT, and MRP1. Similar upregulation of MRP1 and downregulation of MRP5 and BCRP were observed in explants, whereas uptake transporters were decreased or unchanged. Furthermore, a screening of transcriptional regulators of transporters revealed that hypoxia leads to a decrease in the mRNA levels of aryl hydrocarbon receptor, nuclear factor erythroid 2-related factor 2, and retinoid x receptor alpha in both human placental models. These data suggest that transporter expression is differentially regulated by oxygen concentration across experimental human placental models.

David S. Miller: Scientist, Mentor, Friend-a tribute and thank you

David S. Miller was Acting Scientific Director of the Division of Intramural Research at the National Institute of Environmental Health Sciences, National Institutes of Health, and Head of the Intracellular Regulation Group in the Laboratory of Toxicology and Pharmacology before he retired in 2016. David received his Ph.D. in biochemistry from the University of Maine in 1973. David was a Group Leader at the Michigan Cancer Foundation before joining the NIEHS in 1985. His research covered a wide range from renal excretory transport mechanisms to regulation of transporters at the blood–CSF and blood–brain barriers, from fish, amphibians and birds to mammals. David was an outstanding scientist with irresistible enthusiasm for science and an incredible ability to think outside the box while being an exceptional mentor and friend.

Drug efflux transporters and metabolic enzymes in human circulating and testicular T-cell subsets: relevance to HIV pharmacotherapy

OBJECTIVES

ATP-binding cassette (ABC) drug efflux transporters and drug metabolic enzymes could reduce antiretroviral concentrations in HIV target cells. The testis has been demonstrated to be a sanctuary site, displaying suboptimal antiretroviral concentrations and persistent HIV infection. Therefore, we compared the expression and function of ABC transporters and metabolic enzymes in CD4 and CD8 T cells isolated from human testis and peripheral blood mononuclear cells (PBMCs), and assessed their expression in circulating naive and memory CD4 T-cell phenotypes.

DESIGN

Testicular tissue and blood were collected from 15 uninfected donors undergoing gender affirmation surgery. Testicular interstitial cells were isolated by enzymatic digestion, whereas PBMCs were isolated from blood by density gradient centrifugation. The expression and/or function of ABC transporters and metabolic enzymes were examined in blood and testicular T-cell subsets by flow cytometry.

RESULTS

ABC transporters (P-gp, BCRP, MRP1) and metabolic enzymes (CYP3A4, UGT1A1) were expressed in testicular and circulating CD4 and CD8 T cells, as well as in circulating naive, central, transitional, and effector memory T-cell phenotypes. MRP1 demonstrated lower frequencies in T cells from testis compared with PBMCs, as well as in circulating naive T cells compared with the memory T-cell phenotypes. Functional activity of P-gp and BCRP was detected in T-cell subsets from testis and PBMCs.

CONCLUSION

Our findings demonstrate for the first time that antiretroviral drug efflux transporters and metabolic enzymes are functionally expressed in T-cell subsets infiltrating the human testis. These transporters and enzymes can reduce antiretroviral intracellular concentrations, potentially contributing to residual HIV replication in the testis, and negatively impact HIV cure strategies.

A Combination of Clioquinol, Zinc and Copper Increases the Abundance and Function of Breast Cancer Resistance Protein in Human Brain Microvascular Endothelial Cells

Modulating the abundance of the blood-brain barrier (BBB) efflux transporter breast cancer resistance protein (BCRP) has the potential to impact brain levels of drugs and endogenous substrates. Studies have demonstrated that the metal ionophore clioquinol (CQ) increases BBB abundance of P-glycoprotein (P-gp), an effect associated with increased endothelial cell levels of Cu²⁺. This study therefore assessed whether human brain endothelial (hCMEC/D3) cell abundance and function of BCRP is modulated by CQ. hCMEC/D3 cells were treated with CQ, Zn²⁺ and Cu²⁺ (CZC) (0.5 μM, 0.5 μM, 0.1 μM, respectively) for 24 h and BCRP mRNA and protein abundance was determined by Western blot and qPCR, respectively. After a series of optimisation studies assessing specificity of bodipy prazosin (BP) and Ko143 as a substrate and inhibitor of BCRP, respectively, the impact of CZC on BP uptake was assessed. While CZC did not increase mRNA expression of BCRP, BCRP abundance was increased 1.8 ± 0.1-fold; this was associated with a 68.1 ± 3.3% reduction in accumulation of BP in hCMEC/D3 cells. This is the first study to demonstrate that augmenting metal ion availability enhances protein abundance and function of BCRP at the BBB, which may be exploited to modulate CNS access of therapeutics and endogenous substrates.

Transcription factor-mediated regulation of the BCRP/ABCG2 efflux transporter: a review across tissues and species

Introduction: The breast cancer resistance protein (BCRP/ABCG2) is a member of the ATP-binding cassette superfamily of transporters. Using the energy garnered from the hydrolysis of ATP, BCRP actively removes drugs and endogenous molecules from the cell. With broad expression across the liver, kidney, brain, placenta, testes, and small intestines, BCRP can impact the pharmacokinetics and pharmacodynamics of xenobiotics.Areas covered: The purpose of this review is to summarize the transcriptional signaling pathways that regulate BCRP expression across various tissues and mammalian species. We will cover the endobiotic- and xenobiotic-activated transcription factors that regulate the expression and activity of BCRP. These include the estrogen receptor, progesterone receptor, peroxisome proliferator-activated receptor, constitutive androstane receptor, pregnane X receptor, nuclear factor e2-related factor 2, and aryl hydrocarbon receptor.Expert opinion: Key transcription factors regulate BCRP expression and function in response to hormones and xenobiotics. Understanding this regulation provides an opportunity to improve pharmacotherapeutic outcomes by enhancing the efficacy and reducing the toxicity of drugs that are substrates of this efflux transporter.

Cytosolic phospholipase A2 is a key regulator of blood-brain barrier function in epilepsy

Blood-brain barrier dysfunction in epilepsy contributes to seizures and resistance to antiseizure drugs. Reports show that seizures increase brain glutamate levels, leading to barrier dysfunction. One component of barrier dysfunction is overexpression of the drug efflux transporters P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP). Based on our previous studies, we hypothesized that glutamate released during seizures activates cytosolic phospholipase A2 (cPLA2), resulting in P-gp and BCRP overexpression. We exposed isolated rat brain capillaries to glutamate ex vivo and used an in vivo-ex vivo approach of isolating brain capillaries from rats after status epilepticus (SE) and in chronic epileptic (CE) rats. Glutamate increased cPLA2, P-gp, and BCRP protein and activity levels in isolated brain capillaries. We confirmed the role of cPLA2 in the signaling pathway in brain capillaries from male and female mice lacking cPLA2. We also demonstrated, in vivo, that cPLA2 inhibition prevents overexpression of P-gp and BCRP at the blood-brain barrier in rats after status epilepticus and in CE rats. Our data support the hypothesis that glutamate signals cPLA2 activation, resulting in overexpression of blood-brain barrier P-gp and BCRP.-Hartz, A. M. S., Rempe, R. G., Soldner, E. L. B., Pekcec, A., Schlichtiger, J., Kryscio, R., Bauer, B. Cytosolic phospholipase A2 is a key regulator of blood-brain barrier function in epilepsy.

ABC Transporters: Regulation and Association with Multidrug Resistance in Hepatocellular Carcinoma and Colorectal Carcinoma

For most cancers, the treatment of choice is still chemotherapy despite its severe adverse effects, systemic toxicity and limited efficacy due to the development of multidrug resistance (MDR). MDR leads to chemotherapy failure generally associated with a decrease in drug concentration inside cancer cells, frequently due to the overexpression of ABC transporters such as P-glycoprotein (P-gp/MDR1/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2), which limits the efficacy of chemotherapeutic drugs. The aim of this review is to compile information about transcriptional and post-transcriptional regulation of ABC transporters and discuss their role in mediating MDR in cancer cells. This review also focuses on drug resistance by ABC efflux transporters in cancer cells, particularly hepatocellular carcinoma (HCC) and colorectal carcinoma (CRC) cells. Some aspects of the chemotherapy failure and future directions to overcome this problem are also discussed.

Interactions between serum urate-associated genetic variants and sex on gout risk: analysis of the UK Biobank

BACKGROUND

Sex-specific differences in the effect of genetic variants on serum urate levels have been described. The aim of this study was to systematically examine whether serum urate-associated genetic variants differ in their influence on gout risk in men and women.

METHODS

This research was conducted using the UK Biobank Resource. Thirty single nucleotide polymorphisms (SNPs) associated with serum urate were tested for their association with gout in men and women of European ancestry, aged 40-69 years. Gene-sex interactions for gout risk were analysed using an interaction analysis in logistic regression models.

RESULTS

Gout was present in 6768 (4.1%) men and 574 (0.3%) women, with an odds ratio (95% confidence interval) for men 13.42 (12.32-14.62) compared with women. In men, experiment-wide association with gout was observed for 21 of the 30 serum urate-associated SNPs tested, and in women for three of the 30 SNPs. Evidence for gene-sex interaction was observed for ABCG2 (rs2231142) and PDZK1 (rs1471633), with the interaction in ABCG2 driven by an amplified effect in men and in PDZK1 by an absence of effect in women. Similar findings were observed in a sensitivity analysis which excluded pre-menopausal women. For the other SNPs tested, no significant gene-sex interactions were observed.

CONCLUSIONS

In a large population of European ancestry, ABCG2 and PDZK1 gene-sex interactions exist for gout risk, with the serum urate-raising alleles exerting a greater influence on gout risk in men than in women. In contrast, other serum urate-associated genetic variants do not demonstrate significant gene-sex interactions for gout risk.

Plasma and brain pharmacokinetics of letrozole and drug interaction studies with temozolomide in NOD-scid gamma mice and sprague dawley rats

PURPOSE

The aromatase inhibitor, letrozole, is being investigated in experimental animal models as a novel treatment for high-grade gliomas (HGGs). To facilitate optimal dosing for such studies, we evaluated the plasma and brain pharmacokinetics (PK) of letrozole in NOD-scid gamma (NSG) mice, which are frequently employed for assessing efficacy against patient-derived tumor cells. Furthermore, we evaluated the potential PK interactions between letrozole and temozolomide (TMZ) in Sprague-Dawley rats.

METHODS

NSG mice were administered letrozole (8 mg/kg; i.p) as a single or multiple dose (b.i.d, 10 days). Brain tissue and blood samples were collected over 24 h. Letrozole and TMZ interaction study employed jugular vein-cannulated rats (three groups; TMZ alone, letrozole alone and TMZ + letrozole). Intracerebral microdialysis was performed for brain extracellular fluid (ECF) collection simultaneously with venous blood sampling. Drug levels were measured employing HPLC and PK analysis was conducted using Phoenix WinNonlin^®.

RESULTS

In NSG mice, peak plasma and brain tissue letrozole concentrations (C_max) were 3-4 and 0.8-0.9 µg/ml, respectively. The elimination half-life was 2.6 h with minimal accumulation following multiple dosing. In the drug interaction study, no PK changes were evident when TMZ and letrozole were given in combination. For instance, peak plasma and brain ECF TMZ levels when given alone were 14.7 ± 1.1 and 4.6 ± 0.6 µg/ml, respectively, and 12.6 ± 2.4 and 3.4 ± 0.8 µg/ml, respectively, when given with letrozole.

CONCLUSIONS

These results will guide the optimization of dosing regimen for further development of letrozole for HGG treatment.

Isolation of Cerebral Capillaries from Fresh Human Brain Tissue

Understanding blood-brain barrier function under physiological and pathophysiological conditions is critical for the development of new therapeutic strategies that hold the promise to enhance brain drug delivery, improve brain protection, and treat brain disorders. However, studying the human blood-brain barrier function is challenging. Thus, there is a critical need for appropriate models. In this regard, brain capillaries isolated from human brain tissue represent a unique tool to study barrier function as close to the human in vivo situation as possible. Here, we describe an optimized protocol to isolate capillaries from human brain tissue at a high yield and with consistent quality and purity. Capillaries are isolated from fresh human brain tissue using mechanical homogenization, density-gradient centrifugation, and filtration. After the isolation, the human brain capillaries can be used for various applications including leakage assays, live cell imaging, and immune-based assays to study protein expression and function, enzyme activity, or intracellular signaling. Isolated human brain capillaries are a unique model to elucidate the regulation of the human blood-brain barrier function. This model can provide insights into central nervous system (CNS) pathogenesis, which will help the development of therapeutic strategies for treating CNS disorders.

Repression of adenosine triphosphate-binding cassette transporter ABCG2 by estrogen increases intracellular glutathione in brain endothelial cells following ischemic reperfusion injury

The adenosine triphosphate-binding cassette efflux transporter ABCG2, which is located in the blood-brain barrier limits the entry of endogenous compounds and xenobiotics into the brain, and its expression and activity are regulated by estrogen. This study was aimed to define the role of ABCG2 in estrogen-mediated neuroprotection against ischemic injury. ABCG2 protein levels before and after ischemic stroke were increased in the brain of female mice by ovariectomy, which were reversed by estrogen replacement. In brain endothelial cell line bEnd.3, estrogen reduced the basal ABCG2 protein level and efflux activity and protected cells from ischemic injury without inducing ABCG2 expression. When bEnd.3 cells were transfected with ABCG2 small interfering RNA, ischemia-induced cell death was reduced, and the intracellular concentration of glutathione, an antioxidant that is transported by ABCG2, was increased. In addition, after ischemic stroke in ovariectomized mice, estrogen prevented the reduction of intracellular glutathione level in brain microvessels. These data suggested that the suppression of ABCG2 by estrogen is involved in neuroprotection against ischemic injury by increasing intracellular glutathione, and that the modulation of ABCG2 activity offers a therapeutic target for brain diseases in estrogen-deficient aged women.

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.

RNA sequencing identifies gene expression profile changes associated with β-estradiol treatment in U2OS osteosarcoma cells

This study was conducted to identify gene expression profile changes associated with β-estradiol (E2) treatment in U2OS osteosarcoma cells by high-throughput RNA sequencing (RNA-seq). Two U2OS cell samples treated with E2 (15 μmol/L) and two untreated control U2OS cell samples were subjected to RNA-seq. Differentially expressed genes (DEGs) between the groups were identified, and main biological process enrichment was performed using gene ontology (GO) analysis. A protein–protein interaction (PPI) network was constructed using Cytoscape based on the Human Protein Reference Database. Finally, NFKB1 expression was confirmed by quantitative real-time polymerase chain reaction (qRT-PCR). The map ratios of the four sequenced samples were >65%. In total, 128 upregulated and 92 downregulated DEGs were identified in E2 samples. After GO enrichment, the downregulated DEGs, such as AKT1, were found to be mainly enriched in cell cycle processes, whereas the upregulated DEGs, such as NFKB1, were involved in the regulation of gene expression. Moreover, AKT1 (degree =117) and NFKB1 (degree =72) were key nodes with the highest degrees in the PPI network. Similarly, the results of qRT-PCR confirmed that E2 upregulated NFKB1 expression. The results suggest that E2 upregulates the expression of NFKB1, ATF7IP, and HDAC5, all of which are involved in the regulation of gene expression and transcription, but downregulates that of TCF7L2, ALCAM, and AKT, which are involved in Wnt receptor signaling through β-catenin and morphogenesis in U2OS osteosarcoma cells.

ABCG2/BCRP: Specific and Nonspecific Modulators

Multidrug resistance (MDR) in cancer cells is the development of resistance to a variety of structurally and functionally nonrelated anticancer drugs. This phenomenon has become a major obstacle to cancer chemotherapy seriously affecting the clinical outcome. MDR is associated with increased drug efflux from cells mediated by an energy-dependent mechanism involving the ATP-binding cassette (ABC) transporters, mainly P-glycoprotein (ABCB1), the MDR-associated protein-1 (ABCC1), and the breast cancer resistance protein (ABCG2). The first two transporters have been widely studied already and reviews summarized the results. The ABCG2 protein has been a subject of intense study since its discovery as its overexpression has been detected in resistant cell lines in numerous types of human cancers. To date, a long list of modulators of ABCG2 exists and continues to increase. However, little is known about the clinical consequences of ABCG2 modulation. This makes the design of novel, potent, and nontoxic inhibitors of this efflux protein a major challenge to reverse MDR and thereby increase the success of chemotherapy. The aim of the present review is to describe and highlight specific and nonspecific modulators of ABCG2 reported to date based on the selectivity of the compounds, as many of them are effective against one or more ABC transport proteins.

Phytochemical mediated-modulation of the expression and transporter function of breast cancer resistance protein at the blood-brain barrier: An in-vitro study

Clinical translation of BCRP inhibitors have failed due to neurotoxicity and novel approaches are required to identify suitable modulators of BCRP to enhance CNS drug delivery. In this study we examine 18 compounds, primarily phytochemicals, as potential novel modulators of AhR-mediated regulation of BCRP expression and function in immortalised and primary porcine brain microvascular endothelial cells as a mechanism to enhance CNS drug delivery. The majority of modulators possessed a cellular viability IC₅₀ >100µm in both cell systems. BCRP activity, when exposed to modulators for 1h, was diminished for most modulators through significant increases in H33342 accumulation at <10µm with 2,6,4-trimethoflavone increasing H33342 intracellular accumulation by 3.7-6.6 fold over 1-100µm. Western blotting and qPCR identified two inducers of BCRP (quercetin and naringin) and two down-regulators (17-β-estradiol and curcumin) with associated changes in BCRP efflux transport function further confirmed in both cell lines. siRNA downregulation of AhR resulted in a 1.75±0.08 fold change in BCRP expression, confirming the role of AhR in the regulation of BCRP. These findings establish the regulatory role AhR of in controlling BCRP expression at the BBB and confirm quercetin, naringin, 17-β-estradiol, and curcumin as novel inducers and down-regulators of BCRP gene, protein expression and functional transporter activity and hence potential novel target sites and candidates for enhancing CNS drug delivery.

The restorative role of annexin A1 at the blood-brain barrier

Annexin A1 is a potent anti-inflammatory molecule that has been extensively studied in the peripheral immune system, but has not as yet been exploited as a therapeutic target/agent. In the last decade, we have undertaken the study of this molecule in the central nervous system (CNS), focusing particularly on the primary interface between the peripheral body and CNS: the blood-brain barrier. In this review, we provide an overview of the role of this molecule in the brain, with a particular emphasis on its functions in the endothelium of the blood-brain barrier, and the protective actions the molecule may exert in neuroinflammatory, neurovascular and metabolic disease. We focus on the possible new therapeutic avenues opened up by an increased understanding of the role of annexin A1 in the CNS vasculature, and its potential for repairing blood-brain barrier damage in disease and aging.

Antibody validation and scoring guidelines for ABCG2 immunohistochemical staining in formalin-fixed paraffin-embedded colon cancer tissue

Overexpression of the ATP-dependent drug efflux pump ABCG2 is a major molecular mechanism of multidrug resistance in cancer and might be a predictive biomarker for drug response. Contradictory results have been reported for immunohistochemical studies of ABCG2 protein expression in colorectal cancer (CRC), probably because of the use of different antibodies and scoring approaches. In this study, we systematically studied six commercially available anti-ABCG2 antibodies, using cell lines with up-regulation of ABCG2, and selected one antibody for validation in CRC tissue. Furthermore, we established scoring guidelines for ABCG2 expression based on the clinically used guidelines for HER2 immunohistochemistry assessment in gastric cancer. The guidelines provide a semi-quantitative measure of the basolateral membrane staining of ABCG2 and disregard the apical membrane staining and the cytoplasmic signal. Intra-tumor heterogeneity in ABCG2 immunoreactivity was observed; however, statistical analyses of tissue microarrays (TMAs) and the corresponding whole sections from primary tumors of 57 metastatic CRC patients revealed a strong positive correlation between maximum TMA scores and whole sections, especially when more than one core was used. In conclusion, here, we provide validated results to guide future studies on the associations between ABCG2 immunoreactivity in tumor cells and the benefits of chemotherapeutic treatment in patients with CRC.

Antiretroviral drug transporters and metabolic enzymes in human testicular tissue: potential contribution to HIV-1 sanctuary site

OBJECTIVES

The testes are a potential viral sanctuary site for HIV-1 infection. Our study aims to provide insight into the expression and localization of key drug transporters and metabolic enzymes relevant to ART in this tissue compartment.

METHODS

We characterized gene and protein expression of 12 representative drug transporters and two metabolic enzymes in testicular tissue samples obtained from uninfected (n = 8) and virally suppressed HIV-1-infected subjects on ART (n = 5) and quantified antiretroviral drug concentrations in plasma and testicular tissues using LC/MS/MS from HIV-1-infected subjects.

RESULTS

Our data demonstrate that key ABC drug transporters (permeability glycoprotein, multidrug-resistance protein 1, 2 and 4, and breast cancer resistance protein), solute carrier transporters (organic anion transporting polypeptides 1B1 and 2B1, organic anion transporter 1, concentrative nucleoside transporter 1, equilibrative nucleoside transporter 2) and cytochrome P450 metabolic enzymes (CYP3A4 and CYP2D6) previously shown to interact with many commonly used antiretroviral drugs are expressed at the mRNA and protein level in the testes of both subject groups and localize primarily at the blood-testis barrier, with no significant differences between the two groups. Furthermore, we observed that PIs known to be substrates for ATP-binding cassette membrane transporters, displayed variable testicular tissue penetration, with darunavir concentrations falling below therapeutic values. In contrast, the NRTIs emtricitabine, lamivudine and tenofovir displayed favourable tissue penetration, reaching concentrations comparable to plasma levels. We also demonstrated that nuclear receptors, peroxisome proliferator-activated receptors α and γ exhibited higher gene expression in the testicular tissue compared with pregnane X receptor and constitutive androstane receptor, suggesting a potential regulatory pathway governing drug transporter and metabolic enzyme expression in this tissue compartment.

CONCLUSIONS

Our data suggest the testes are a complex pharmacological compartment that can restrict the distribution of certain antiretroviral drugs and potentially contribute to HIV-1 persistence.

Apicobasal polarity of brain endothelial cells

Normal brain homeostasis depends on the integrity of the blood-brain barrier that controls the access of nutrients, humoral factors, and immune cells to the CNS. The blood-brain barrier is composed mainly of brain endothelial cells. Forming the interface between two compartments, they are highly polarized. Apical/luminal and basolateral/abluminal membranes differ in their lipid and (glyco-)protein composition, allowing brain endothelial cells to secrete or transport soluble factors in a polarized manner and to maintain blood flow. Here, we summarize the basic concepts of apicobasal cell polarity in brain endothelial cells. To address potential molecular mechanisms underlying apicobasal polarity in brain endothelial cells, we draw on investigations in epithelial cells and discuss how polarity may go awry in neurological diseases.

Regulation of ABC efflux transporters at blood-brain barrier in health and neurological disorders

The strength of the blood-brain barrier (BBB) in providing protection to the central nervous system from exposure to circulating chemicals is maintained by tight junctions between endothelial cells and by a broad range of transporter proteins that regulate exchange between CNS and blood. The most important transporters that restrict the permeability of large number of toxins as well as therapeutic agents are the ABC transporters. Among them, P-gp, BCRP, MRP1 and MRP2 are the utmost studied. These efflux transporters are neuroprotective, limiting the brain entry of neurotoxins; however, they could also restrict the entry of many therapeutics and contribute to CNS pharmacoresistance. Characterization of several regulatory pathways that govern expression and activity of ABC efflux transporters in the endothelium of brain capillaries have led to an emerging consensus that these processes are complex and contain several cellular and molecular elements. Alterations in ABC efflux transporters expression and/or activity occur in several neurological diseases. Here, we review the signaling pathways that regulate expression and transport activity of P-gp, BCRP, MRP1 and MRP2 as well as how their expression/activity changes in neurological diseases. This article is part of a Special Issue entitled SI: Neuroprotection.

In vivo and ex vivo regulation of breast cancer resistant protein (Bcrp) by peroxisome proliferator-activated receptor alpha (Pparα) at the blood-brain barrier

Breast cancer resistance protein (Bcrp/Abcg2) localized at the blood-brain barrier (BBB) limits permeability into the brain of many xenobiotics, including pharmacological agents. Peroxisome proliferator-activated receptor α (Pparα), a ligand-activated transcription factor, primarily involved in lipid metabolism, has been shown to regulate the functional expression of Bcrp in human cerebral microvascular endothelial cells (hCMEC/D3). The aim of this study was to investigate ex vivo and in vivo, the regulation of Bcrp by Pparα in an intact BBB. Ex vivo quantitative real-time PCR and immunoblot analyses showed significant up-regulation of Abcg2/Bcrp mRNA and protein levels in CD-1 mouse brain capillaries incubated with clofibrate, a Pparα ligand. Fluorescence-based transport assays in CD-1 and C57BL/6 brain capillaries showed that exposure to clofibrate significantly increased Bcrp transport activity. This increase was not observed in capillaries isolated from Pparα knockout mice. In vivo, we found: i) significant Bcrp protein up-regulation in clofibrate-dosed CD-1 and C57BL/6 capillary lysates, but no effect in Pparα knockout capillary lysates, and ii) significantly increased Bcrp transport activity in capillaries isolated from clofibrate-treated mice. These results demonstrate an increase in Bcrp functional expression by Pparα in brain capillaries, and suggest that Pparα is another nuclear receptor that can contribute to the regulation of membrane efflux transporters and drug permeability at the BBB. We propose the involvement of the following pathways in clofibrate-mediated induction of the drug transporter Abcg2/Bcrp mRNA, protein expression and function by the nuclear receptor Pparα, in mouse brain capillary endothelial cells. Upon activation with clofibrate (Pparα, ligand), Pparα complex translocates from the cytoplasm into the nucleus and further recruits coactivators and transcription machinery which induce the transcription of Abcg2 gene and ultimately results in upregulation of Bcrp protein expression and function. These findings have significant implications since Bcrp is known to play an important role at the BBB in preventing the permeability of several xenobiotics and drugs into the brain.

Research Progress on the Role of ABC Transporters in the Drug Resistance Mechanism of Intractable Epilepsy

The pathogenesis of intractable epilepsy is not fully clear. In recent years, both animal and clinical trials have shown that the expression of ATP-binding cassette (ABC) transporters is increased in patients with intractable epilepsy; additionally, epileptic seizures can lead to an increase in the number of sites that express ABC transporters. These findings suggest that ABC transporters play an important role in the drug resistance mechanism of epilepsy. ABC transporters can perform the funcions of a drug efflux pump, which can reduce the effective drug concentration at epilepsy lesions by reducing the permeability of the blood brain barrier to antiepileptic drugs, thus causing resistance to antiepileptic drugs. Given the important role of ABC transporters in refractory epilepsy drug resistance, antiepileptic drugs that are not substrates of ABC transporters were used to obtain ABC transporter inhibitors with strong specificity, high safety, and few side effects, making them suitable for long-term use; therefore, these drugs can be used for future clinical treatment of intractable epilepsy.

Regulation of ABC transporters at the blood-brain barrier

ATP binding cassette (ABC) transporters at the blood-brain barrier function as ATP-driven xenobiotic efflux pumps and limit delivery of small molecule drugs to the brain. Here I review recent progress in understanding the regulation of the expression and transport activity of these transporters and comment on how this new information might aid in improving drug delivery to the brain.

Regulation of ABC transporters blood-brain barrier: the good, the bad, and the ugly

The brain capillary endothelial cells that constitute the blood-brain barrier express multiple ABC transport proteins on the luminal, blood-facing, plasma membrane. These transporters function as ATP-driven efflux pumps for xenobiotics and endogenous metabolites. High expression of these ABC transporters at the barrier is a major obstacle to the delivery of therapeutics, including chemotherapeutics, to the CNS. Here, I review the signals that alter ABC transporter expression and transport function with an emphasis on P-glycoprotein, Mrp2, and breast cancer resistance protein (BCRP), the efflux transporters for which we have the most detailed picture of regulation. Recent work shows that transporter protein expression can be upregulated in response to inflammatory and oxidative stress, therapeutic drugs, diet, and persistent environmental pollutants; as a consequence, drug delivery to the brain is reduced (potentially bad and ugly). In contrast, basal transport activity of P-glycoprotein and BCRP can be reduced through complex signaling pathways that involve events in and on the brain capillary endothelial cells. Targeting these signaling events provides opportunities to rapidly and reversibly increase brain accumulation of drugs that are substrates for the transporters (potentially good). The clinical usefulness of targeting signaling to reduce efflux transporter activity and improve drug delivery to the CNS remains to be established.

Role of the breast cancer resistance protein (BCRP/ABCG2) in drug transport--an update

The human breast cancer resistance protein (BCRP, gene symbol ABCG2) is an ATP-binding cassette (ABC) efflux transporter. It was so named because it was initially cloned from a multidrug-resistant breast cancer cell line where it was found to confer resistance to chemotherapeutic agents such as mitoxantrone and topotecan. Since its discovery in 1998, the substrates of BCRP have been rapidly expanding to include not only therapeutic agents but also physiological substances such as estrone-3-sulfate, 17β-estradiol 17-(β-D-glucuronide) and uric acid. Likewise, at least hundreds of BCRP inhibitors have been identified. Among normal human tissues, BCRP is highly expressed on the apical membranes of the placental syncytiotrophoblasts, the intestinal epithelium, the liver hepatocytes, the endothelial cells of brain microvessels, and the renal proximal tubular cells, contributing to the absorption, distribution, and elimination of drugs and endogenous compounds as well as tissue protection against xenobiotic exposure. As a result, BCRP has now been recognized by the FDA to be one of the key drug transporters involved in clinically relevant drug disposition. We published a highly-accessed review article on BCRP in 2005, and much progress has been made since then. In this review, we provide an update of current knowledge on basic biochemistry and pharmacological functions of BCRP as well as its relevance to drug resistance and drug disposition.

Steroids and the blood-brain barrier: therapeutic implications

Steroids have a wide spectrum of impact, serving as fundamental regulators of nearly every physiological process within the human body. Therapeutic applications of steroids are equally broad, with a diverse range of medications and targets. Within the central nervous system (CNS), steroids influence development, memory, behavior, and disease outcomes. Moreover, steroids are well recognized as to their impact on the vascular endothelium. The blood-brain barrier (BBB) at the level of the brain microvascular endothelium serves as the principle interface between the peripheral circulation and the brain. Steroids have been identified to impact several critical properties of the BBB, including cellular efflux mechanisms, nutrient uptake, and tight junction integrity. Such actions not only influence brain homeostasis but also the delivery of CNS-targeted therapeutics. A greater understanding of the respective steroid-BBB interactions may shed further light on the differential treatment outcomes observed across CNS pathologies. In this chapter, we examine the current therapeutic implications of steroids respective to BBB structure and function, with emphasis on glucocorticoids and estrogens.

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.

A retinoic acid-enhanced, multicellular human blood-brain barrier model derived from stem cell sources

Blood-brain barrier (BBB) models are often used to investigate BBB function and screen brain-penetrating therapeutics, but it has been difficult to construct a human model that possesses an optimal BBB phenotype and is readily scalable. To address this challenge, we developed a human in vitro BBB model comprising brain microvascular endothelial cells (BMECs), pericytes, astrocytes and neurons derived from renewable cell sources. First, retinoic acid (RA) was used to substantially enhance BBB phenotypes in human pluripotent stem cell (hPSC)-derived BMECs, particularly through adherens junction, tight junction, and multidrug resistance protein regulation. RA-treated hPSC-derived BMECs were subsequently co-cultured with primary human brain pericytes and human astrocytes and neurons derived from human neural progenitor cells (NPCs) to yield a fully human BBB model that possessed significant tightness as measured by transendothelial electrical resistance (~5,000 Ωxcm(2)). Overall, this scalable human BBB model may enable a wide range of neuroscience studies.

Harmful effect of ERβ on BCRP-mediated drug resistance and cell proliferation in ERα/PR-negative breast cancer

The role of estrogen receptor β (ERβ) in breast cancer is still under investigation. Various studies have provided evidence that ERβ behaves as a tumor suppressor in breast cancer, whereas some studies of estrogen receptor α (ERα) negative breast cancer reported a positive correlation between high ERβ expression and poor prognostic phenotypes, such as induced proliferation, invasion and metastasis. In the present immunohistochemistry study of 99 ERα/progesterone receptor (PR)-negative breast cancer samples, nuclear expression of ERβ was positively associated with membranous expression of breast cancer resistance protein (BCRP), Ki67 (proliferation marker) and tumor size. Moreover, both endogenous and exogenous ERβ upregulated BCRP expression which induced BCRP-mediated drug resistance and enhanced proliferation of ERα-/PR- breast cancer cells in the presence of 17β-estradiol, whereas these effects were reversed by additional use of tamoxifen (TAM). In addition, the regulation of BCRP via specific binding between ERβ and estrogen response element (ERE) was demonstrated in an electrophoretic mobility shift assay. Overall, our findings manifest that ERβ might act as a tumor promoter of cell proliferation and BCRP-mediated drug resistance in ERα-/PR- breast cancer. TAM routinely used for patients with ERα+/PR+, ERα+/PR- and ERα-/PR+ breast cancer might also be effective in ERα-/PR- but ERβ+ breast cancer. Therefore, the detection of ERβ in clinic is valuable and should not be neglected in breast cancer, especially for the ERα-/PR- phenotype.

Role of nuclear receptors in the regulation of drug transporters in the brain

ATP-binding cassette membrane-associated drug efflux transporters and solute carrier influx transporters, expressed at the blood-brain barrier, blood-cerebrospinal fluid barrier, and in brain parenchyma, are important determinants of drug disposition in the central nervous system. Targeting the regulatory pathways that govern the expression of these transporters could provide novel approaches to selectively alter drug permeability into the brain. Nuclear receptors are ligand-activated transcription factors which regulate the gene expression of several metabolic enzymes and drug efflux/influx transporters. Although efforts have primarily been focused on investigating these regulatory pathways in peripheral organs (i.e., liver and intestine), recent findings demonstrate their significance in the brain. This review addresses the role of nuclear receptors in the regulation of drug transporter functional expression in the brain. An in-depth understanding of these pathways could guide the development of novel pharmacotherapy with either enhanced efficacy in the central nervous system or minimal associated neurotoxicity.

Breast cancer resistance protein (Bcrp) and the testis--an unexpected turn of events

Breast cancer resistance protein (Bcrp) is an ATP-dependent efflux drug transporter. It has a diverse spectrum of hydrophilic and hydrophobic substrates ranging from anticancer, antiviral and antihypertensive drugs, to organic anions, antibiotics, phytoestrogens (e.g., genistein, daidzein, coumestrol), xenoestrogens and steroids (e.g., dehydroepiandrosterone sulfate). Bcrp is an integral membrane protein in cancer and normal cells within multiple organs (e.g., brain, placenta, intestine and testis) that maintains cellular homeostasis by extruding drugs and harmful substances from the inside of cells. In the brain, Bcrp is a major component of the blood-brain barrier located on endothelial cells near tight junctions (TJs). However, Bcrp is absent at the Sertoli cell blood-testis barrier (BTB); instead, it is localized almost exclusively to the endothelial TJ in microvessels in the interstitium and the peritubular myoid cells in the tunica propria. Recent studies have shown that Bcrp is also expressed stage specifically and spatiotemporally by Sertoli and germ cells in the seminiferous epithelium of rat testes, limited only to a testis-specific cell adhesion ultrastructure known as the apical ectoplasmic specialisation (ES) in stage VI-early VIII tubules. These findings suggest that Bcrp is equipped by late spermatids and Sertoli cells to protect late-stage spermatids completing spermiogenesis. Furthermore, Bcrp was found to be associated with F (filamentous)-actin and several actin regulatory proteins at the apical ES and might be involved in the organisation of actin filaments at the apical ES in stage VII-VIII tubules. These findings will be carefully evaluated in this brief review.

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.

Assessing blood-brain barrier function using in vitro assays

The impermeability of the blood-brain barrier (BBB) is due to a number of properties including tight junctions on adjoining endothelial cells, absence of pinocytic vesicles, and expression of multidrug transporters. Although the permeability of many chemicals can be predicted by their polarity, or oil/water partition coefficient, many lipophilic chemicals are not permeable because of multidrug transporters at the luminal and abluminal membranes. In contrast, many nutrients, which are usually polar, cross the BBB more readily than predicted by their oil/water partition coefficients due to the expression of specific nutrient transporters. In vitro models are being developed because rodent models are of low input and relatively expensive. Isolated brain microvessels and cell culture models each offers certain advantages and disadvantages. Isolated brain microvessels are useful in measuring multidrug drug transporters and tight junction integrity, whereas cell culture models allow the investigator to measure directional transport and can be genetically manipulated. In this chapter, we describe how to isolate large batches of brain microvessels from freshly slaughtered cows. The different steps in the isolation procedure include density gradient centrifugations and filtering. Purity is determined microscopically and by marker enzymes. Permeability is assessed by measuring the uptake of fluorescein-labeled dextran in an assay that has been optimized to have a large dynamic range and low inter-day variability. We also describe how to evaluate transendothelial cell electrical resistance and paracellular transport in cell culture models.