Functional expression of P-glycoprotein and multidrug resistance-associated protein (Mrp1) in primary cultures of rat astrocytes

CNS Viral Infections-What to Consider for Improving Drug Treatment: A Plea for Using Mathematical Modeling Approaches

Neurotropic viruses may cause meningitis, myelitis, encephalitis, or meningoencephalitis. These inflammatory conditions of the central nervous system (CNS) may have serious and devastating consequences if not treated adequately. In this review, we first summarize how neurotropic viruses can enter the CNS by (1) crossing the blood-brain barrier or blood-cerebrospinal fluid barrier; (2) invading the nose via the olfactory route; or (3) invading the peripheral nervous system. Neurotropic viruses may then enter the intracellular space of brain cells via endocytosis and/or membrane fusion. Antiviral drugs are currently used for different viral CNS infections, even though their use and dosing regimens within the CNS, with the exception of acyclovir, are minimally supported by clinical evidence. We therefore provide considerations to optimize drug treatment(s) for these neurotropic viruses. Antiviral drugs should cross the blood-brain barrier/blood cerebrospinal fluid barrier and pass the brain cellular membrane to inhibit these viruses inside the brain cells. Some antiviral drugs may also require intracellular conversion into their active metabolite(s). This illustrates the need to better understand these mechanisms because these processes dictate drug exposure within the CNS that ultimately determine the success of antiviral drugs for CNS infections. Finally, we discuss mathematical model-based approaches for optimizing antiviral treatments. Thereby emphasizing the potential of CNS physiologically based pharmacokinetic models because direct measurement of brain intracellular exposure in living humans faces ethical restrictions. Existing physiologically based pharmacokinetic models combined with in vitro pharmacokinetic/pharmacodynamic information can be used to predict drug exposure and evaluate efficacy of antiviral drugs within the CNS, to ultimately optimize the treatments of CNS viral infections.

Modulation of Multidrug Resistance Protein 1-mediated Transport Processes by the Antiviral Drug Ritonavir in Cultured Primary Astrocytes

The Multidrug Resistance Protein 1 (Mrp1) is an ATP-dependent efflux transporter and a major facilitator of drug resistance in mammalian cells during cancer and HIV therapy. In brain, Mrp1-mediated GSH export from astrocytes is the first step in the supply of GSH precursors to neurons. To reveal potential mechanisms underlying the drug-induced modulation of Mrp1-mediated transport processes, we investigated the effects of the antiviral drug ritonavir on cultured rat primary astrocytes. Ritonavir strongly stimulated the Mrp1-mediated export of glutathione (GSH) by decreasing the Km value from 200 nmol/mg to 28 nmol/mg. In contrast, ritonavir decreased the export of the other Mrp1 substrates glutathione disulfide (GSSG) and bimane-glutathione. To give explanation for these apparently contradictory observations, we performed in silico docking analysis and molecular dynamics simulations using a homology model of rat Mrp1 to predict the binding modes of ritonavir, GSH and GSSG to Mrp1. The results suggest that ritonavir binds to the hydrophilic part of the bipartite binding site of Mrp1 and thereby differently affects the binding and transport of the Mrp1 substrates. These new insights into the modulation of Mrp1-mediated export processes by ritonavir provide a new model to better understand GSH-dependent detoxification processes in brain cells.

Hydrogen Sulfide Attenuates Lipopolysaccharide-Induced Inflammation via the P-glycoprotein and NF-κB Pathway in Astrocytes

Astrocyte activation is key in neurodegenerative diseases. Hydrogen sulfide (H2S) exhibits neuroprotective effects on astrocytes, although the underlying molecular mechanism remains unclear. Here, we explored the effects of H2S on lipopolysaccharide (LPS)-induced astrocyte activation and astrocyte-mediated neuroinflammation. After inducing primary astrocytes via LPS exposure, H2S levels were altered. The generation and secretion of inflammatory mediators by astrocytes and their interrelation with P-glycoprotein (P-gp), an important transporter belonging to the ABC transporter family, were assessed. Activated astrocytes showed upregulated glial fibrillary acidic protein (GFAP) mRNA expression, and significantly increased proinflammatory factor mRNA/protein expression and release. The secretory capacity of astrocytes was reduced, with significantly decreased proinflammatory factor levels in culture supernatant after P-gp inhibitor verapamil pretreatment. The increase in the intracellular H2S level inhibited LPS-induced GFAP expression and P65 nuclear entry in astrocytes. mRNA expression and release of proinflammatory factors were reduced significantly, with no significant changes in cytoplasmic protein expression. S-sulfhydration levels increased significantly with the increased concentration of sodium hydrosulfide or S-adenosyl-l-methionine addition, with only moderate changes in astrocyte P-gp expression. H2S regulates NF-κB activation, leads to S-sulfhydration of P-gp, and inhibits the biosynthesis and secretion of proinflammatory factors by astrocytes. The regulatory effects of H2S on astrocytes may have clinical value for exploring new therapeutic strategies against neurodegenerative diseases.

ATP-binding cassette (ABC) drug transporters in the developing blood-brain barrier: role in fetal brain protection

The blood-brain barrier (BBB) provides essential neuroprotection from environmental toxins and xenobiotics, through high expression of drug efflux transporters in endothelial cells of the cerebral capillaries. However, xenobiotic exposure, stress, and inflammatory stimuli have the potential to disrupt BBB permeability in fetal and post-natal life. Understanding the role and ability of the BBB in protecting the developing brain, particularly with respect to drug/toxin transport, is key to promoting long-term brain health. Drug transporters, particularly P-gp and BCRP are expressed in early gestation at the developing BBB and have a crucial role in developmental homeostasis and fetal brain protection. We have highlighted several factors that modulate drug transporters at the developing BBB, including synthetic glucocorticoid (sGC), cytokines, maternal infection, and growth factors. Some factors have the potential to increase expression and function of drug transporters and increase brain protection (e.g., sGC, transforming growth factor [TGF]-β). However, others inhibit drug transporters expression and function at the BBB, increasing brain exposure to xenobiotics (e.g., tumor necrosis factor [TNF], interleukin [IL]-6), negatively impacting brain development. This has implications for pregnant women and neonates, who represent a vulnerable population and may be exposed to drugs and environmental toxins, many of which are P-gp and BCRP substrates. Thus, alterations in regulated transport across the developing BBB may induce long-term changes in brain health and compromise pregnancy outcome. Furthermore, a large portion of neonatal adverse drug reactions are attributed to agents that target or access the nervous system, such as stimulants (e.g., caffeine), anesthetics (e.g., midazolam), analgesics (e.g., morphine) and antiretrovirals (e.g., Zidovudine); thus, understanding brain protection is key for the development of strategies to protect the fetal and neonatal brain.

Inhibition of Xenobiotics Transporters’ Efflux Ability after Nanoplastics Exposure in Larval Japanese Medaka

Nanoplastics can enter into the aquatic environment as primary nano-sized or fragmented from larger-sized plastic particles, and their ecological effects and environmental fate have aroused increasing public concerns. Here, we identified the disruption of ATP-binding cassette (ABC) efflux after polystyrene (PS) nanoplastics (76 ± 7 nm) exposure in larval Japanese medaka (Oryzias latipes). Nanoplastics (0.001–10 μg/mL) caused 3–6-fold higher lipid peroxidation in fish larvae than the control, with concomitant downregulated expression of efflux transporter-related genes (abcb6a, abcc2, abcg2). Two probes of rhodamine (indicative of p-glycoprotein function for parent compounds’ efflux, P-gp) and fluorescein (indicative of multidrug resistance-associated protein function for metabolites’ efflux, MRP) were further used to verify the inhibited ABC efflux ability, via rhodamine and fluorescein bioaccumulation results. Three-fold higher accumulation of rhodamine was observed following treatment with 10 μg/mL of nanoplastics. Excessive accumulation also occurred for fluorescein, with 1.7–1.8-fold higher concentrations than controls in larvae treated with 0.01–0.1 μg/mL of nanoplastics. Although the inhibition of ABC transporters diminished after two hours of depuration, the co-existence of nanoplastics and other contaminants still raises concerns. Collectively, this study suggests that nanoplastics can negatively impact ABC transporters’ efflux ability and could cause unanticipated accumulation of co-existing organic pollutants in aquatic organisms.

The Interplay of ABC Transporters in Aβ Translocation and Cholesterol Metabolism: Implicating Their Roles in Alzheimer's Disease

The occurrence of Alzheimer's disease (AD) worldwide has been progressively accelerating at an alarming rate, without any successful therapeutic strategy for the disease mitigation. The complexity of AD pathogenesis needs to be targeted with an alternative approach, as provided by the superfamily of ATP-binding cassette (ABC) transporters, which constitutes an extensive range of proteins, capable of transporting molecular entities across biological membranes. These protein moieties have been implicated in AD, based upon their potential in lipid transportation, resulting in maintenance of cholesterol homeostasis. These transporters have been reported to target the primary hallmark of AD pathogenesis, namely, beta-amyloid hypothesis, which is associated with accumulation of beta-amyloid (Aβ) plaques in AD patients. The ABC transporters have been observed to be localized to the capillary endothelial cells of the blood-brain barrier and neural parenchymal cells, where they exhibit different roles, consequently influencing the neuronal expression of Aβ peptides. The review highlights different families of ABC transporters, ABCB1 (P-glycoprotein), ABCA (ABCA1, ABCA2, and ABCA7), ABCG2 (BCRP; breast cancer resistance protein), ABCG1 and ABCG4, as well as ABCC1 (MRP; multidrug resistance protein) in the CNS, and their interplay in regulating cholesterol metabolism and Aβ peptide load in the brain, simultaneously exerting protective effects against neurotoxic substrates and xenobiotics. The authors aim to establish the significance of this alternative approach as a novel therapeutic target in AD, to provide the researchers an opportunity to evaluate the potential aspects of ABC transporters in AD treatment.

ABC Transporters at the Blood-Brain Interfaces, Their Study Models, and Drug Delivery Implications in Gliomas

Drug delivery into the brain is regulated by the blood-brain interfaces. The blood-brain barrier (BBB), the blood-cerebrospinal fluid barrier (BCSFB), and the blood-arachnoid barrier (BAB) regulate the exchange of substances between the blood and brain parenchyma. These selective barriers present a high impermeability to most substances, with the selective transport of nutrients and transporters preventing the entry and accumulation of possibly toxic molecules, comprising many therapeutic drugs. Transporters of the ATP-binding cassette (ABC) superfamily have an important role in drug delivery, because they extrude a broad molecular diversity of xenobiotics, including several anticancer drugs, preventing their entry into the brain. Gliomas are the most common primary tumors diagnosed in adults, which are often characterized by a poor prognosis, notably in the case of high-grade gliomas. Therapeutic treatments frequently fail due to the difficulty of delivering drugs through the brain barriers, adding to diverse mechanisms developed by the cancer, including the overexpression or expression de novo of ABC transporters in tumoral cells and/or in the endothelial cells forming the blood-brain tumor barrier (BBTB). Many models have been developed to study the phenotype, molecular characteristics, and function of the blood-brain interfaces as well as to evaluate drug permeability into the brain. These include in vitro, in vivo, and in silico models, which together can help us to better understand their implication in drug resistance and to develop new therapeutics or delivery strategies to improve the treatment of pathologies of the central nervous system (CNS). In this review, we present the principal characteristics of the blood-brain interfaces; then, we focus on the ABC transporters present on them and their implication in drug delivery; next, we present some of the most important models used for the study of drug transport; finally, we summarize the implication of ABC transporters in glioma and the BBTB in drug resistance and the strategies to improve the delivery of CNS anticancer drugs.

P-glycoprotein (ABCB1) and Oxidative Stress: Focus on Alzheimer's Disease

ATP-binding cassette (ABC) transporters, in particular P-glycoprotein (encoded by ABCB1), are important and selective elements of the blood-brain barrier (BBB), and they actively contribute to brain homeostasis. Changes in ABCB1 expression and/or function at the BBB may not only alter the expression and function of other molecules at the BBB but also affect brain environment. Over the last decade, a number of reports have shown that ABCB1 actively mediates the transport of beta amyloid (Aβ) peptide. This finding has opened up an entirely new line of research in the field of Alzheimer's disease (AD). Indeed, despite intense research efforts, AD remains an unsolved pathology and effective therapies are still unavailable. Here, we review the crucial role of ABCB1 in the Aβ transport and how oxidative stress may interfere with this process. A detailed understanding of ABCB1 regulation can provide the basis for improved neuroprotection in AD and also enhanced therapeutic drug delivery to the brain.

Reactive astrocytes increase the expression of P‑gp and Mrp1 via TNF‑α and NF‑κB signaling

To understand multidrug resistance gene expression in reactive astrocytes, the present study involved stimulated astrocytes with tumor necrosis factor (TNF)‑α and determined gene expression by reverse transcription‑quantitative polymerase chain reaction and western blot analysis. Stimulation induced P‑glycoprotein (gp) and multidrug resistance‑associated protein (MRP0) 1 expression, which peaked by 24 h. Interestingly, the expression of P‑gp and Mrp1 correlated with cell proliferation. Therefore, reactivation of astrocytes may increase P‑gp and Mrp1 expression through TNF‑α and nuclear factor (NF)‑κB signaling. To the best of the authors' knowledge, the present study is the first to demonstrate that in vitro activated astrocytes likely induced P‑gp and Mrp1 expression by increasing NF‑κB expression. The results of the present study provide a novel insight into the mechanism of refractory epilepsy and suggest inhibiting cytokine signaling may block multidrug resistance.

Efflux proteins at the blood-brain barrier: review and bioinformatics analysis

1. Efflux proteins at the blood-brain barrier provide a mechanism for export of waste products of normal metabolism from the brain and help to maintain brain homeostasis. They also prevent entry into the brain of a wide range of potentially harmful compounds such as drugs and xenobiotics. 2. Conversely, efflux proteins also hinder delivery of therapeutic drugs to the brain and central nervous system used to treat brain tumours and neurological disorders. For bypassing efflux proteins, a comprehensive understanding of their structures, functions and molecular mechanisms is necessary, along with new strategies and technologies for delivery of drugs across the blood-brain barrier. 3. We review efflux proteins at the blood-brain barrier, classified as either ATP-binding cassette (ABC) transporters (P-gp, BCRP, MRPs) or solute carrier (SLC) transporters (OATP1A2, OATP1A4, OATP1C1, OATP2B1, OAT3, EAATs, PMAT/hENT4 and MATE1). 4. This includes information about substrate and inhibitor specificity, structural organisation and mechanism, membrane localisation, regulation of expression and activity, effects of diseases and conditions and the principal technique used for in vivo analysis of efflux protein activity: positron emission tomography (PET). 5. We also performed analyses of evolutionary relationships, membrane topologies and amino acid compositions of the proteins, and linked these to structure and function.

Spatiotemporal Changes in P-glycoprotein Levels in Brain and Peripheral Tissues Following Ischemic Stroke in Rats

P-glycoprotein (P-gp) is known to transport a diverse array of xenobiotics, including therapeutic drugs. A member of the ATP-binding cassette (ABC) transporter family, P-gp is a protein encoded by the gene Mdr1 in humans and Abcb1 in rodents (represented by 2 isoforms Abcb1a and Abcb1b). Lining the luminal and abluminal membrane of brain capillary endothelial cells, P-gp is a promiscuous efflux pump extruding a variety of exogenous toxins and drugs. In this study, we measured dynamic changes in Abcb1a and Abcb1b transcripts and P-gp protein in the brain, liver, and kidney after experimental stroke. P-glycoprotein has been shown to increase in brain endothelial cells following hypoxia in vitro or after exposure to proinflammatory cytokines. Using a rat model of ischemic stroke, we hypothesized that P-gp expression will be increased in the brain, liver, and kidney in response to neuroinflammation following ischemic stroke. Adult Sprague Dawley rats underwent middle cerebral artery occlusion (MCAO) for 90 minutes and were killed at 4, 14, 24, and 48 hours postreperfusion onset to determine the time course of P-gp expression. To mimic ischemia occurring at the blood-brain barrier, rat brain endothelial (RBE4) cells were subjected to hypoxia and low glucose (HLG) for 16 hours. Immunoblotting analyses showed P-gp increases in brain and liver following 90-minute MCAO, as well as in cultured RBE4 cells after 16-hour HLG treatment, but fluctuated in the kidney depending on the time point. The relative roles of each isoform in the protein expression were analyzed with quantitative reverse transcriptase polymerase chain reaction. Ischemic stroke leads to significant increases in P-gp levels not only in the brain but also in the liver. The increase in P-gp could dramatically reduce the bioavailability and efficacy of neuroprotective drugs. Therefore, P-gp represents a big hurdle to drug delivery to the ischemic brain.

Co-delivery nanoparticles with characteristics of intracellular precision release drugs for overcoming multidrug resistance

Combination chemotherapy in clinical practice has been generally accepted as a feasible strategy for overcoming multidrug resistance (MDR). Here, we designed and successfully prepared a co-delivery system named S-D1@L-D2 NPs, where denoted some smaller nanoparticles (NPs) carrying a drug doxorubicin (DOX) were loaded into a larger NP containing another drug (vincristine [VCR]) via water-in-oil-in-water double-emulsion solvent diffusion-evaporation method. Chitosan-alginate nanoparticles carrying DOX (CS-ALG-DOX NPs) with a smaller diameter of about 20 nm formed S-D1 NPs; vitamin E D-α-tocopheryl polyethylene glycol 1000 succinate-modified poly(lactic-co-glycolic acid) nanoparticles carrying VCR (TPGS-PLGA-VCR NPs) with a larger diameter of about 200 nm constituted L-D2 NPs. Some CS-ALG-DOX NPs loaded into TPGS-PLGA-VCR NPs formed CS-ALG-DOX@TPGS-PLGA-VCR NPs. Under the acidic environment of cytosol and endosome or lysosome in MDR cell, CS-ALG-DOX@TPGS-PLGA-VCR NPs released VCR and CS-ALG-DOX NPs. VCR could arrest cell cycles at metaphase by inhibiting microtubule polymerization in the cytoplasm. After CS-ALG-DOX NPs escaped from endosome, they entered the nucleus through the nuclear pore and released DOX in the intra-nuclear alkaline environment, which interacted with DNA to stop the replication of MDR cells. These results indicated that S-D1@L-D2 NPs was a co-delivery system of intracellular precision release loaded drugs with pH-sensitive characteristics. S-D1@L-D2 NPs could obviously enhance the in vitro cytotoxicity and the in vivo anticancer efficiency of co-delivery drugs, while reducing their adverse effects. Overall, S-D1@L-D2 NPs can be considered an innovative platform for the co-delivery drugs of clinical combination chemotherapy for the treatment of MDR tumor.

P2X Receptor-Mediated Modulation of Sensory Transmission to the Spinal Cord Dorsal Horn

In the somatosensory system, P2X receptors are expressed on both peripheral and central terminals of primary afferent neurons. Those expressed on peripheral terminals are activated in response to both nociceptive and innocuous stimuli, whereas those at central terminals (“central terminal P2X receptors”) play an important role in modulating sensory transmission to the spinal cord dorsal horn. The author reviews recent studies on the central terminal P2X receptors. It is proposed that central terminal P2X receptors, once activated, may be involved in both central sensitization and initiation of pain. Thus, these receptors may repesent a promising target for therapeutic management of pathological pain.

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.

Triggering of protection mechanism against Phoneutria nigriventer spider venom in the brain

Severe accidents caused by the "armed" spider Phoneutria nigriventer cause neurotoxic manifestations in victims. In experiments with rats, P. nigriventer venom (PNV) temporarily disrupts the properties of the BBB by affecting both the transcellular and the paracellular route. However, it is unclear how cells and/or proteins participate in the transient opening of the BBB. The present study demonstrates that PNV is a substrate for the multidrug resistance protein-1 (MRP1) in cultured astrocyte and endothelial cells (HUVEC) and increases mrp1 and cx43 and down-regulates glut1 mRNA transcripts in cultured astrocytes. The inhibition of nNOS by 7-nitroindazole suggests that NO derived from nNOS mediates some of these effects by either accentuating or opposing the effects of PNV. In vivo, MRP1, GLUT1 and Cx43 protein expression is increased differentially in the hippocampus and cerebellum, indicating region-related modulation of effects. PNV contains a plethora of Ca(2+), K(+) and Na(+) channel-acting neurotoxins that interfere with glutamate handling. It is suggested that the findings of the present study are the result of a complex interaction of signaling pathways, one of which is the NO, which regulates BBB-associated proteins in response to PNV interference on ions physiology. The present study provides additional insight into PNV-induced BBB dysfunction and shows that a protective mechanism is activated against the venom. The data shows that PNV has qualities for potential use in drug permeability studies across the BBB.

Development of purine-derived 18F-labeled pro-drug tracers for imaging of MRP1 activity with PET

Multidrug resistance-associated protein 1 (MRP1) is a drug efflux transporter that has been implicated in the pathology of several neurological diseases and is associated with development of multidrug resistance. To enable measurement of MRP1 function in the living brain, a series of 6-halopurines decorated with fluorinated side chains have been synthesized and evaluated as putative pro-drug tracers. The tracers were designed to undergo conjugation with glutathione within the brain and hence form the corresponding MRP1 substrate tracers in situ. 6-Bromo-7-(2-[¹⁸F]fluoroethyl)purine showed good brain uptake and rapid metabolic conversion. Dynamic PET imaging demonstrated a marked difference in brain clearance rates between wild-type and mrp1 knockout mice, suggesting that the tracer can allow noninvasive assessment of MRP1 activity in vivo.

Cyclooxygenase-2 in epilepsy

Epilepsy is one of the more prevalent neurologic disorders in the world, affecting approximately 50 million people of different ages and backgrounds. Epileptic seizures propagating through both lobes of the forebrain can have permanent debilitating effects on a patient's cognitive and somatosensory brain functions. Epilepsy, defined by the sporadic occurrence of spontaneous recurrent seizures (SRS), is often accompanied by inflammation of the brain. Pronounced increases in the expression of key inflammatory mediators (e.g., interleukin -1β [IL-1β], tumor necrosis factor alpha [TNFα], cyclooxygenase-2 [COX-2], and C-X-C motif chemokine 10 [CXCL10]) after seizures may cause secondary damage in the brain and increase the likelihood of repetitive seizures. The COX-2 enzyme is induced rapidly during seizures. The increased level of COX-2 in specific areas of the epileptic brain can help to identify regions of seizure-induced brain inflammation. A good deal of effort has been expended to determine whether COX-2 inhibition might be neuroprotective and represent an adjunct therapeutic strategy along with antiepileptic drugs used to treat epilepsy. However, the effectiveness of COX-2 inhibitors on epilepsy animal models appears to depend on the timing of administration. With all of the effort placed on making use of COX-2 inhibitors as therapeutic agents for the treatment of epilepsy, inflammation, and neurodegenerative diseases there has yet to be a selective and potent COX-2 inhibitor that has shown a clear therapeutic outcome with acceptable side effects.

Glutathione metabolism and Parkinson's disease

It has been established that oxidative stress, defined as the condition in which the sum of free radicals in a cell exceeds the antioxidant capacity of the cell, contributes to the pathogenesis of Parkinson disease. Glutathione is a ubiquitous thiol tripeptide that acts alone or in concert with enzymes within cells to reduce superoxide radicals, hydroxyl radicals, and peroxynitrites. In this review, we examine the synthesis, metabolism, and functional interactions of glutathione and discuss how these relate to the protection of dopaminergic neurons from oxidative damage and its therapeutic potential in Parkinson disease.

The impact of P-gp functionality on non-steady state relationships between CSF and brain extracellular fluid

In the development of central nervous system (CNS)-targeted drugs, the prediction of human CNS target exposure is a big challenge. Cerebrospinal fluid (CSF) concentrations have often been suggested as a ‘good enough’ surrogate for brain extracellular fluid (brain_ECF, brain target site) concentrations in humans. However, brain anatomy and physiology indicates prudence. We have applied a multiple microdialysis probe approach in rats, for continuous measurement and direct comparison of quinidine kinetics in brain_ECF, CSF, and plasma. The data obtained indicated important differences between brain_ECF and CSF kinetics, with brain_ECF kinetics being most sensitive to P-gp inhibition. To describe the data we developed a systems-based pharmacokinetic model. Our findings indicated that: (1) brain_ECF- and CSF-to-unbound plasma AUC_0–360 ratios were all over 100 %; (2) P-gp also restricts brain intracellular exposure; (3) a direct transport route of quinidine from plasma to brain cells exists; (4) P-gp-mediated efflux of quinidine at the blood–brain barrier seems to result of combined efflux enhancement and influx hindrance; (5) P-gp at the blood–CSF barrier either functions as an efflux transporter or is not functioning at all. It is concluded that in parallel obtained data on unbound brain_ECF, CSF and plasma concentrations, under dynamic conditions, is a complex but most valid approach to reveal the mechanisms underlying the relationship between brain_ECF and CSF concentrations. This relationship is significantly influenced by activity of P-gp. Therefore, information on functionality of P-gp is required for the prediction of human brain target site concentrations of P-gp substrates on the basis of human CSF concentrations.

Brain metastases from renal cell carcinoma in the era of tyrosine kinase inhibitors

BACKGROUND

The effectiveness of tyrosine kinase inhibitors (TKI) in preventing brain metastases in patients with renal cell carcinoma is unclear.

METHODS

Preclinical studies were conducted to determine the steady-state brain and plasma concentrations of sorafenib and sunitinib in mice deficient in the drug efflux transporters; p-glycoprotein, and breast cancer resistance protein. A single-institution retrospective analysis of patients treated from 2008 to 2010 was conducted to assess the incidence of brain metastases before and during TKI treatment.

RESULTS

Transport of sorafenib and sunitinib across the blood-brain barrier was restricted. Retrospective analysis revealed that the median time to develop metastatic brain disease was 28 months (range, 1-108 months) while on TKI therapy and 11.5 months (range, 0-64 months) in patients who did not receive TKI therapy. The incidence of brain metastases per month in patients not treated with TKI therapy was 1.6 higher than the incidence in patients treated with TKI therapy.

CONCLUSIONS

Penetration of sorafenib or sunitinib through an intact blood-brain barrier to brain tissue is limited; however, the incidence of brain metastases per unit time is decreased in patients on TKI therapy in comparison with the "cytokine" era.

Role of ABC transporters in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder and the most common form of age-related dementia that begins with memory loss and progresses to include severe cognitive impairment. A major pathological hallmark of AD is the accumulation of beta amyloid peptide (Aβ) in senile plaques in the brain of AD patients. The exact mechanism by which AD takes place remains unknown. However, an increasing number of studies suggests that ATP-binding cassette (ABC) transporters, which are localized on the surface of brain endothelial cells of the blood-brain barrier (BBB) and brain parenchyma, may contribute to the pathogenesis of AD. Recent studies have unraveled important roles of ABC transporters including ABCB1 (P-glycoprotein, P-gp), ABCG2 (breast cancer resistant protein, BCRP), ABCC1 (multidrug resistance protein 1, MRP1), and the cholesterol transporter ABCA1 in the pathogenesis of AD and Aβ peptides deposition inside the brain. Therefore, understanding the mechanisms by which these transporters contribute to Aβ deposition in the brain is important for the development of new therapeutic strategies against AD. This review summarizes and highlights the accumulating evidence in the literature which describe the role of altered function of various ABC transporters in the pathogenesis and progression of AD and the implications of modulating their functions for the treatment of AD.

Extracellular ATP signaling via P2X(4) receptor and cAMP/PKA signaling mediate ATP oscillations essential for prechondrogenic condensation

Prechondrogenic condensation is the most critical process in skeletal patterning. A previous study demonstrated that ATP oscillations driven by Ca(2+) oscillations play a critical role in prechondrogenic condensation by inducing oscillatory secretion. However, it remains unknown what mechanisms initiate the Ca(2+)-driven ATP oscillations, mediate the link between Ca(2+) and ATP oscillations, and then result in oscillatory secretion in chondrogenesis. This study has shown that extracellular ATP signaling was required for both ATP oscillations and prechondrogenic condensation. Among P2 receptors, the P2X(4) receptor revealed the strongest expression level and mediated ATP oscillations in chondrogenesis. Moreover, blockage of P2X(4) activity abrogated not only chondrogenic differentiation but also prechondrogenic condensation. In addition, both ATP oscillations and secretion activity depended on cAMP/PKA signaling but not on K(ATP) channel activity and PKC or PKG signaling. This study proposes that Ca(2+)-driven ATP oscillations essential for prechondrogenic condensation is initiated by extracellular ATP signaling via P2X(4) receptor and is mediated by cAMP/PKA signaling and that cAMP/PKA signaling induces oscillatory secretion to underlie prechondrogenic condensation, in cooperation with Ca(2+) and ATP oscillations.

Cerebral expression of drug transporters in epilepsy

Over-expression of drug efflux transporters at the level of the blood-brain barrier (BBB) has been proposed as a mechanism responsible for multidrug resistance. Drug transporters in epileptogenic tissue are not only expressed in endothelial cells at the BBB, but also in other brain parenchymal cells, such as astrocytes, microglia and neurons, suggesting a complex cell type-specific regulation under pathological conditions associated with epilepsy. This review focuses on the cerebral expression patterns of several classes of well-known membrane drug transporters such as P-glycoprotein (Pgp), and multidrug resistance-associated proteins (MRPs) in the epileptogenic brain. Both experimental and clinical evidence of epilepsy-associated cerebral drug transporter regulation and the possible mechanisms underlying drug transporter regulation are discussed. Knowledge of the cerebral expression patterns of drug transporters in normal and epileptogenic brain will provide relevant information to guide strategies attempting to overcome drug resistance by targeting specific transporters.

Transport and metabolism at blood-brain interfaces and in neural cells: relevance to bilirubin-induced encephalopathy

Bilirubin, the end-product of heme catabolism, circulates in non-pathological plasma mostly as a protein-bound species. When bilirubin concentration builds up, the free fraction of the molecule increases. Unbound bilirubin then diffuses across blood-brain interfaces (BBIs) into the brain, where it accumulates and exerts neurotoxic effects. In this classical view of bilirubin neurotoxicity, BBIs act merely as structural barriers impeding the penetration of the pigment-bound carrier protein, and neural cells are considered as passive targets of its toxicity. Yet, the role of BBIs in the occurrence of bilirubin encephalopathy appears more complex than being simple barriers to the diffusion of bilirubin, and neural cells such as astrocytes and neurons can play an active role in controlling the balance between the neuroprotective and neurotoxic effects of bilirubin. This article reviews the emerging in vivo and in vitro data showing that transport and metabolic detoxification mechanisms at the blood-brain and blood-cerebrospinal fluid barriers may modulate bilirubin flux across both cellular interfaces, and that these protective functions can be affected in chronic unconjugated hyperbilirubinemia. Then the in vivo and in vitro arguments in favor of the physiological antioxidant function of intracerebral bilirubin are presented, as well as the potential role of transporters such as ABCC1 and metabolizing enzymes such as cytochromes P-450 in setting the cerebral cell- and structure-specific toxicity of bilirubin following hyperbilirubinemia. The relevance of these data to the pathophysiology of bilirubin-induced neurological diseases is discussed.

Circadian regulation of ATP release in astrocytes

Circadian clocks sustain daily oscillations in gene expression, physiology, and behavior, relying on transcription-translation feedback loops of clock genes for rhythm generation. Cultured astrocytes display daily oscillations of extracellular ATP, suggesting that ATP release is a circadian output. We hypothesized that the circadian clock modulates ATP release via mechanisms that regulate acute ATP release from glia. To test the molecular basis for circadian ATP release, we developed methods to measure in real-time ATP release and Bmal1::dLuc circadian reporter expression in cortical astrocyte cultures from mice of different genotypes. Daily rhythms of gene expression required functional Clock and Bmal1, both Per1 and Per2, and both Cry1 and Cry2 genes. Similarly, high-level, circadian ATP release also required a functional clock mechanism. Whereas blocking IP(3) signaling significantly disrupted ATP rhythms with no effect on Bmal1::dLuc cycling, blocking vesicular release did not alter circadian ATP release or gene expression. We conclude that astrocytes depend on circadian clock genes and IP(3) signaling to express daily rhythms in ATP release.

Multi-walled carbon nanotubes affect drug transport across cell membrane in rat astrocytes

The impact of carbon nanotubes on the cell membrane is an aspect of particular importance and interest in the study of carbon nanotubes' interactions with living systems. One of the many functions of the cell membrane is to execute substance transport into and out of the cell. We investigated the influence of multi-walled carbon nanotubes (MWCNTs) on the transport of several compounds across in the cell membrane of rat astrocytes using flow cytometry. These compounds are fluorescein diacetate, carboxyfluorescein diacetate, rhodamine 123 and doxorubicin, which are prosubstrate/substrates of multidrug transporter proteins. Results showed that MWCNTs significantly inhibited cellular uptake of doxorubicin but not the other drugs and the mode of loading made a significant difference in doxorubicin uptake. Retention of fluorescein, carboxyfluorescein and rhodamine 123 was remarkably higher in MWCNT-exposed cells after an efflux period. A kinetics study also demonstrated slower efflux of intracellular fluorescein and rhodamine 123. Data presented in this paper suggest that MWCNTs could affect drug transport across cell membranes. The implications of the findings are discussed.

Expression and function of p-glycoprotein in normal tissues: effect on pharmacokinetics

ATP-binding cassette (ABC) drug efflux transporters limit intracellular concentration of their substrates by pumping them out of cell through an active, energy dependent mechanism. Several of these proteins have been originally associated with the phenomenon of multidrug resistance; however, later on, they have also been shown to control body disposition of their substrates. P-glycoprotein (Pgp) is the first detected and the best characterized of ABC drug efflux transporters. Apart from tumor cells, its constitutive expression has been reported in a variety of other tissues, such as the intestine, brain, liver, placenta, kidney, and others. Being located on the apical site of the plasma membrane, Pgp can remove a variety of structurally unrelated compounds, including clinically relevant drugs, their metabolites, and conjugates from cells. Driven by energy from ATP, it affects many pharmacokinetic events such as intestinal absorption, brain penetration, transplacental passage, and hepatobiliary excretion of drugs and their metabolites. It is widely believed that Pgp, together with other ABC drug efflux transporters, plays a crucial role in the host detoxication and protection against xenobiotic substances. On the other hand, the presence of these transporters in normal tissues may prevent pharmacotherapeutic agents from reaching their site of action, thus limiting their therapeutic potential. This chapter focuses on P-glycoprotein, its expression, localization, and function in nontumor tissues and the pharmacological consequences hereof.

RNAi inhibits Coriaria lactone-induced MDR1b overexpression in rat brain microvascular endothelial cells

P-glyprotein (multidrug resistance gene 1, MDR1) was the first isolated and most extensively investigated refractory epilepsy-related drug-resistant protein. Coriaria lactone (CL) can induce P-glyprotein expression of brain capillary and astrocytes in vivo. We established the primary rat brain microvascular endothelial cell (BMECs) models overexpressing P-glycoprotein induced by CL successfully and infected them with adenovirus vector, which carry small interfering RNA (siRNA) designed to target MDR1b. MDR1b mRNA levels and P-glyprotein expression in experimental group decreased significantly 72 h after infection, and the fluorescence intensity of rhodamine within the cells of experimental group increased significantly. So, CL can induce P-glyprotein overexpression in BMECs in vitro, and siRNA can effectively inhibit expression of P-glyprotein and decrease its "pumping out" function. This may offer a new method and rationale for studying the mechanism of refractory epilepsy and relevant gene therapy.

Beta-amyloid peptide stimulates endozepine release in cultured rat astrocytes through activation of N-formyl peptide receptors

Astroglial cells synthesize and release endozepines, a family of neuropeptides derived from diazepam-binding inhibitor (DBI). The authors have recently shown that beta-amyloid peptide (Abeta) stimulates DBI gene expression and endozepine release. The purpose of this study was to determine the mechanism of action of Abeta in cultured rat astrocytes. Abeta(25-35) and the N-formyl peptide receptor (FPR) agonist N-formyl-Met-Leu-Phe (fMLF) increased the secretion of endozepines in a dose-dependent manner with EC(50) value of approximately 2 microM. The stimulatory effects of Abeta(25-35) and the FPR agonists fMLF and N-formyl-Met-Met-Met (fMMM) on endozepine release were abrogated by the FPR antagonist N-t-Boc-Phe-Leu-Phe-Leu-Phe. In contrast, Abeta(25-35) increased DBI mRNA expression through a FPR-independent mechanism. Abeta(25-35) induced a transient stimulation of cAMP formation and a sustained activation of polyphosphoinositide turnover. The stimulatory effect of Abeta(25-35) on endozepine release was blocked by the adenylyl cyclase inhibitor somatostatin, the protein kinase A (PKA) inhibitor H89, the phospholipase C inhibitor U73122, the protein kinase C (PKC) inhibitor chelerythrine and the ATP binding cassette transporter blocker glyburide. Taken together, these data demonstrate for the first time that Abeta(25-35) stimulates endozepine release from rat astrocytes through a FPR receptor positively coupled to PKA and PKC.

The cytotoxic effect of unconjugated bilirubin in human neuroblastoma SH-SY5Y cells is modulated by the expression level of MRP1 but not MDR1

In vitro and in vivo studies have demonstrated that UCB (unconjugated bilirubin) is neurotoxic. Although previous studies suggested that both MRP1 (multidrug resistance-associated protein 1) and MDR1 (multidrug resistance protein 1) may protect cells against accumulation of UCB, direct comparison of their role in UCB transport was never performed. To this end, we used an inducible siRNA (small interfering RNA) expression system to silence the expression of MRP1 and MDR1 in human neuroblastoma SH-SY5Y cells. The effects of in vitro exposure to clinically-relevant levels of unbound UCB were compared between unsilenced (control) cells and cells with similar reductions in the expression of MRP1 or MDR1, documented by RT–PCR (reverse transcription–PCR) (mRNA), immunoblotting (protein), and for MDR1, the enhanced net uptake of a specific fluorescent substrate. Cytotoxicity was assessed by the MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide] test. MRP1-deficient cells accumulated significantly more UCB and suffered greater cytotoxicity than controls. By contrast, MDR1-deficient cells exhibited UCB uptake and cytotoxicity comparable with controls. At intermediate levels of silencing, the increased susceptibility to UCB toxicity closely correlated with the decrease in the expression of MRP1, but not of MDR1. These data support the concept that limitation of cellular UCB accumulation, due to UCB export mediated by MRP1, but not MDR1, plays an important role in preventing bilirubin encephalopathy in the newborn.

Transcriptional profiles of receptors and transporters involved in brain cholesterol homeostasis at the blood-brain barrier: use of an in vitro model

Brain is the most cholesterol rich organ of the whole body and recent studies suggest a role for the blood-brain barrier (BBB) in cerebral cholesterol homeostasis. Low density lipoprotein receptor (LDLR)-related receptors and ATP-binding Cassette (ABC) transporters play an important role in peripheral sterol homeostasis. The purpose of this study was to determine the mRNA expression profiles of ABC transporters (ABCA1, 2, 3, 7 and ABCG1) and low density lipoprotein receptor (LDLR)-related receptors (LDLR, vLDLR, LRP1, LRP2 and LRP8) in BBB endothelium using an in vitro co-culture model of bovine brain capillary endothelial cells (BCECs) and rat glial cells. All transcripts tested are expressed by BCECs and in capillary extract, except vLDLR. Glial cells influence ABCG1, A1, 2, 7 and LRP1 transcription, suggesting a role in cerebral lipid supply/elimination through the modulation of the expression of these transporters and receptors by these cells. Altogether, these results highlight the importance of glial input in the BBB transport phenotype for sterol homeostasis in the central nervous system, and confirm the importance of the BBB in this process.

Effect of chronic exposure to morphine on the rat blood-brain barrier: focus on the P-glycoprotein

Morphine may affect the properties of the blood-brain barrier (BBB) by modifying the expression of certain BBB markers. We have determined the effect of chronic morphine treatment on the expression and function of some BBB markers in the rat. The mRNAs of 19 selected genes encoding caveolins, endothelial transporters, receptors and tight junctions proteins in the total RNA of isolated cortex microvessels were assayed by quantitative RT-PCR (qRT-PCR). The expression of genes Mdr1a, Mrp1, Bcrp, Glut-1 and Occludin, was slightly increased, while that of Flk-1 was decreased in microvessels from morphine-treated rats. The expression of the Mrd1a and Mdr1b genes encoding the P-glycoprotein (P-gp) also increased in the whole hippocampus and cortex of morphine-treated rats. The Mdr1a gene induction (1.38-fold) observed by qRT-PCR was also confirmed using in situ hybridization technique (1.40-fold). Immunoblotting revealed an increase in P-gp expression in the hippocampus (1.8-fold) and cortex (1.36-fold) of morphine-treated rats, but no effect in isolated microvessels. In contrast, morphine treatment increased by 1.48-fold the expression of P-gp in a large vessel-enriched fraction. The integrity of the BBB, measured by in situ brain perfusion of [(14)C]-sucrose, and the activity of P-gp at the BBB, measured with the P-gp substrate [(3)H]-colchicine, were not modified by morphine. Immunohistofluorescence experiments revealed that P-gp expression is restricted to large vessels and microvessels in control rats and that morphine treatment did not induce the expression of P-gp in the brain parenchyma (astrocytes or neurons). Taken together, our results showed that chronic morphine treatment does not significantly alter BBB integrity or P-gp activity. The impact of morphine-mediated P-gp induction observed in large vessels remains to be determined in terms of brain disposition of drugs that are P-gp substrates.

ABC transporter (P-gp/ABCB1, MRP1/ABCC1, BCRP/ABCG2) expression in the developing human CNS

P-glycoprotein (P-gp/ABCB1), multidrug resistance associated protein 1 (MRP1/ABCC1), and breast cancer resistance protein (BCRP/ABCG2) are plasma membrane efflux pumps that limit the intracellular uptake and retention of numerous lipophilic, amphipathic xeno- and endobiotics. Little is known about the neonatal and developmental expression of P-gp/ABCB1, MRP1/ABCC1, and BCRP/ABCG2 in the human central nervous system (CNS), therefore post-mortem CNS tissue from infants born at 22 (0/7)-42 (0/7) weeks of gestation and adults was immunostained to determine their ontogeny and cellular localization. P-gp/ABCB1 immunostaining was observed in microvessel endothelial cells as early as 22 (0/7) weeks, increasing in prevalence and intensity with maturation, and later in gestation in large pyramidal neurons. MRP1/ABCC1 immunostaining was prominent early in the choroid plexus and ventricular ependyma, and noted later in large pyramidal neurons. BCRP/ABCG2 expression was limited to microvessel endothelial cells. P-gp/ABCB1, MRP1/ABCC1 and BCRP/ABCG2 in adult brain matched term newborn CNS but with more intense immunostaining. We conclude that P-gp/ABCB1, MRP1/ABCC1, and BCRP/ABCG2 are expressed in a developmental, cell specific, fashion in the human CNS. The complementary pattern of P-gp/ABCB1 and BCRP/ABCG2 at the blood-brain with MRP1/ABCC1 at the blood-CSF barriers may limit CNS uptake and retention of drugs and toxins in neonates.

HIV-1 viral envelope glycoprotein gp120 produces oxidative stress and regulates the functional expression of multidrug resistance protein-1 (Mrp1) in glial cells

Brain human immunodeficiency virus type-1 (HIV-1) infection is associated with oxidative stress, which may lead to HIV-1 encephalitis, a chronic neurodegenerative condition. In vitro, oxidative stress can be induced in glial cells by exposure to HIV-1 envelope protein glycoprotein (gp120). Multidrug resistance proteins (Mrps) are known to efflux endogenous substrates (i.e. GSH and GSSG) involved in cellular defense against oxidative stress. Altered GSH/GSSG export may contribute to oxidative damage during HIV-1 encephalitis. At present, it is unknown if gp120 exposure can alter the functional expression of Mrp isoforms. Heat-shock protein 70, inducible nitric oxide synthase, intracellular GSSG, 2',7'-dichlorofluorescein fluorescence, and extracellular nitrite were increased in primary cultures of rat astrocytes triggered with gp120, suggesting an oxidative stress response. RT-PCR and immunoblot analysis demonstrated increased Mrp1 mRNA (2.3-fold) and protein (2.2-fold), respectively, in gp120 treated astrocytes while Mrp4 mRNA or protein expression was not changed. Cellular retention of 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, an established Mrp substrate, was reduced (twofold) in gp120-treated astrocytes, suggesting increased Mrp-mediated transport. In addition, GSH and GSSG export were enhanced in gp120-triggered cells. These data suggest that gp120 can up-regulate Mrp1, but not Mrp4, functional expression in cultured astrocytes. Our observation of increased GSH/GSSG efflux in response to gp120 treatment implies that Mrp isoforms may be involved in regulating the oxidative stress response in glial cells.

Functional induction of P-glycoprotein in the blood-brain barrier of streptozotocin-induced diabetic rats: evidence for the involvement of nuclear factor-kappaB, a nitrosative stress-sensitive transcription factor, in the regulation

The objective of this study was to investigate the transport kinetics of cyclosporin A, a well known substrate for P-glycoprotein (P-gp), across the blood-brain barrier (BBB), and the expression of the transporter in the brain of streptozotocin-induced diabetic rats. The in vivo transport clearance of cyclosporin A was significantly reduced in diabetic rats compared with that in the control. The decreased transport was associated with the increased level of mRNA and the protein for P-glycoprotein in the rat brain. The functional activity of the efflux transporter in mouse brain capillary endothelial (MBEC4) cells, an in vitro model of the BBB, was also stimulated when slow nitric oxide (NO)-releasing donors were present, whereas the stimulation was absent in the case of rapid NO-releasing donors (e.g., S-nitroso-N-acetyl-dl-penicillamine and diethylenetriamine). The stimulatory effect was highest for sodium nitroprusside (SNP) and the functional induction associated with the increased mRNA and protein level of the transporter. The pretreatment of the cell with SNP along with ascorbate, methylene blue, or superoxide dismutase attenuated the induction of function and expression for P-glycoprotein, suggesting that the reaction product between superoxide and NO is involved in the induction of function and expression. The level of nuclear translocation of nuclear factor-kappaB (NF-kappaB) and DNA binding activity of nuclear extracts to the NF-kappaB consensus oligonucleotide was increased in MBEC4 cells pretreated with SNP. Taken together, these observations suggest that nitrosative stress leads to the up-regulation of the message for the efflux transporter and, ultimately, to the enhanced function, probably via a NF-kappaB-dependent mechanism.

The role of membrane transporters in drug delivery to brain tumors

Most brain tumors are highly resistant to chemotherapy because many chemotherapeutic drugs poorly cross the blood-brain barrier, the blood-cerebrospinal-fluid barrier, and the plasma membrane of the tumor cells. This restricted drug delivery is largely due to the presence of integral plasma membrane proteins belonging to the solute carriers (SLCs) and to the ATP-binding cassette (ABC) superfamily of transporters that decisively determine substance uptake and efflux, respectively, by the barrier-forming cells and the tumor cells. This review focuses on the localization and function of drug-transporting members of both transporter groups in human brain.

Multidrug resistance protein 1 (MRP1) in rabbit conjunctival epithelial cells: its effect on drug efflux and its regulation by adenoviral infection

PURPOSE

To evaluate the expression, localization, function, and regulation of multidrug resistance protein (MRP1) in rabbit conjunctival epithelial cells (RCEC).

MATERIALS AND METHODS

MRP1 gene expression in RCEC was determined by reverse transcription-polymerase chain reaction (RT-PCR), and MRP1 protein expression and its localization were determined by Western blot analysis and immunofluorescence using an anti-MRP1 monoclonal antibody, MRPr1. The effect of MRP1 on the transport and uptake of fluorescein was evaluated in RCEC grown on Transwell filters. Moreover, the effect of adenovirus type 5 (Ad5)-infected RCEC, and cytokines (Interleukin 1 (IL-1), IL-6, and tumor necrosis factor alpha (TNFalpha)) on MRP1 expression and leukotriene C4 (LTC4) uptake were investigated.

RESULTS

A 652 bp RT-PCR product from rabbit conjunctiva showed a 87% homology to human MRP1. Immunostaining with MRPr1 revealed a predominant basolateral localization of MRP1 in RCEC. Uptake of fluorescein, a MRP1 substrate, was increased (203-290%) in the presence of uricosuric drug probenecid at 100 microM, anti-inflammatory drug indomethacin at 10 microM and diclofenac, flurbiprofen, and ofloxacin at 1 mM, and by ATP depletion, but not influenced by the depletion of GSH, and the presence of antiviral cidofovir and anti-inflammatory drug cromolyn and prednisolone. Apical-to-basolateral facilitated transport of LTC4 was abolished in the presence of probenecid. Western blot analysis with MRPr1 revealed a distinct band at approximately 190 kDa for freshly isolated and cultured RCEC. Both Ad5 and cytokines (IL-1, IL-6, and TNF-alpha) up-regulated MRP1 expression, thereby reducing LTC4 uptake.

CONCLUSIONS

MRP1 appears to be primarily localized in the basolateral membrane of RCEC and function in the efflux of certain organic anions and inflammatory factors out of cells from the basolateral membrane. The upregulation in the expression of MRP1 by Ad5-infection and cytokines suggests a role of MRP1 in the transport of inflammatory factors during ocular inflammation. Supported by NIH grants EY12578, EY10421, and EY12356.

Expression of the ATP-binding cassette membrane transporter, ABCG2, in human and rodent brain microvessel endothelial and glial cell culture systems

PURPOSE

The function of ABCG2 (BCRP), a member of the ATP-binding cassette (ABC) superfamily of membrane-associated drug transporters, at the blood-brain barrier remains highly controversial. This project investigates the functional expression of endogenous ABCG2 in cultures of human and rodent brain cellular compartments.

MATERIALS AND METHODS

RT-PCR, western blot and fluorescent immunocytochemical analyses were performed on ABCG2-overexpressing human breast cancer (MCF-MX100) cells, human and rat brain microvessel endothelial (HBEC and RBE4, respectively), and rat glial cells.

RESULTS

RT-PCR analysis detected ABCG2 mRNA in all the cell culture systems. Western blot analysis with anti-ABCG2 monoclonal BXP-21 antibody detected a robust band at approximately 72 kDa in the ABCG2-overexpressing MCF-MX100 cell line, whereas low expression was found in human and rat brain cell systems. Immunofluorescence microscopy detected predominant plasma membrane localization of ABCG2 in MCF-MX100 cells but weak signal in all brain cellular compartments. In the presence of ABCG2 inhibitors, the accumulation of (3)H-mitoxantrone and pheophorbide A, two established ABCG2 substrates, was significantly increased in MCF-MX100 cells but not in the human and rodent brain cell culture systems.

CONCLUSIONS

Our data show low endogenous ABCG2 protein expression, localization and activity in cultures of human and rat brain microvessel endothelial and glial cells.

Role of multidrug resistance-associated protein 1 expression in the in vitro susceptibility of rat nerve cell to unconjugated bilirubin

Nerve cell injury by unconjugated bilirubin (UCB) has been implicated in brain damage during neonatal hyperbilirubinemia, particularly in the preterm newborn. Recently, it was shown that UCB is a substrate for the multidrug resistance-associated protein 1 (Mrp1), an ATP-dependent efflux pump, which may decrease UCB intracellular levels. To obtain a further insight into the role of Mrp1 in the increased vulnerability of immature cells to UCB, we evaluated the mRNA and the protein levels of Mrp1 throughout differentiation in primary cultures of rat neurons and astrocytes. Furthermore, in order to provide supportive evidence for the role of Mrp1 in the protection of nerve cells from UCB-induced effects, we evaluated cell susceptibility to UCB when Mrp1 was inhibited with MK571 ((E)-3-[[[3-[2-(7-chloro-2-quinolinyl) ethenyl]phenyl]-[[3-dimethylamino)-3-oxopropyl]thio]methyl]thio]-propanoic acid). The results are the first to demonstrate that Mrp1 is expressed in neurons and that both mRNA and protein levels of Mrp1 increase with cell differentiation. Additionally, inhibition of Mrp1 was associated with an increase in UCB toxic effects, namely cell death, cell dysfunction, and secretion of interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, as well as of glutamate. These results point to a novel role of Mrp1 in the susceptibility of premature babies to UCB encephalopathy, and provide a startup point for the development of a new therapeutic strategy.