Transient expression of MDR-1/P-glycoprotein in a model of partial cortical devascularization

Transporter hypothesis in pharmacoresistant epilepsies Is it at the central or peripheral level?

The multidrug‐resistance (MDR) phenotype is typically observed in patients with refractory epilepsy (RE) whose seizures are not controlled despite receiving several combinations of more than two antiseizure medications (ASMs) directed against different ion channels or neurotransmitter receptors. Since the use of bromide in 1860, more than 20 ASMs have been developed; however, historically ~30% of cases of RE with MDR phenotype remains unchanged. Irrespective of metabolic biotransformation, the biodistribution of ASMs and their metabolites depends on the functional expression of some ATP‐binding cassette transporters (ABC‐t) in different organs, such as the blood‐brain barrier (BBB), bowel, liver, and kidney, among others. ABC‐t, such as P‐glycoprotein (P‐gp), multidrug resistance–associated protein (MRP‐1), and breast cancer–resistance protein (BCRP), are mainly expressed in excretory organs and play a critical role in the pharmacokinetics (PK) of all drugs. The transporter hypothesis can explain pharmacoresistance to a broad spectrum of ASMs, even when administered simultaneously. Since ABC‐t expression can be induced by hypoxia, inflammation, or seizures, a high frequency of uncontrolled seizures increases the risk of RE. These stimuli can induce ABC‐t expression in excretory organs and in previously non‐expressing (electrically responsive) cells, such as neurons or cardiomyocytes. In this regard, an alternative mechanism to the classical pumping function of P‐gp indicates that P‐gp activity can also produce a significant reduction in resting membrane potential (ΔΨ0 = −60 to −10 mV). P‐gp expression in neurons and cardiomyocytes can produce membrane depolarization and participate in epileptogenesis, heart failure, and sudden unexpected death in epilepsy. On this basis, ABC‐t play a peripheral role in controlling the PK of ASMs and their access to the brain and act at a central level, favoring neuronal depolarization by mechanisms independent of ion channels or neurotransmitters that current ASMs cannot control.

The role of P-glycoprotein (P-gp) and inwardly rectifying potassium (Kir) channels in sudden unexpected death in epilepsy (SUDEP)

Sudden unexpected death in epilepsy (SUDEP) is the major cause of death that affects patients with epilepsy. The risk of SUDEP increases according to the frequency and severity of uncontrolled seizures; therefore, SUDEP risk is higher in patients with refractory epilepsy (RE), in whom most antiepileptic drugs (AEDs) are ineffective for both seizure control and SUDEP prevention. Consequently, RE and SUDEP share a multidrug resistance (MDR) phenotype, which is mainly associated with brain overexpression of ABC-transporters such as P-glycoprotein (P-gp). The activity of P-gp can also contribute to membrane depolarization and affect the normal function of neurons and cardiomyocytes. Other molecular regulators of membrane potential are the inwardly rectifying potassium channels (Kir), whose genetic variants have been related to both epilepsy and heart dysfunctions. Although it has been suggested that dysfunctions of the cardiac, respiratory, and brainstem arousal systems are the causes of SUDEP, the molecular basis for explaining its dysfunctions remain unknown. In rats, repetitive seizures or status epilepticus induced high expression of P-gp and loss Kir expression in the brain and heart, and promoted membrane depolarization, malignant bradycardia, and the high rate of mortality. Here we reviewed clinical and experimental evidences suggesting that abnormal expression of depolarizing/repolarizing factors as P-gp and Kir could favor persistent depolarization of membranes without any rapid functional recovery capacity. This condition induced by convulsive stress could be the molecular mechanism leading to acquired severe bradycardia, as an ineffective heart response generating the appropriate scenario for SUDEP development. This article is part of the Special Issue "NEWroscience 2018".

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.

Lithium enhances post-stroke blood-brain barrier integrity, activates the MAPK/ERK1/2 pathway and alters immune cell migration in mice

Lithium induces neuroprotection against cerebral ischemia, although the underlying mechanisms remain elusive. We have previously suggested a role for lithium in calcium regulation and (extra)cerebral vessel relaxation under non-ischemic conditions. Herein, we aimed to investigate whether or not lithium contributes to post-stroke stabilization of the blood-brain barrier (BBB) in mice. Using an oxygen-glucose-deprivation (OGD) model, we first analyzed the impact of lithium treatment on endothelial cells (EC) in vitro. Indeed, such treatment of EC exposed to OGD resulted in increased cell survival as well as in enhanced expression of tight junction proteins and P-glycoprotein. Additional in vivo studies demonstrated an increased stabilization of the BBB upon lithium treatment in stroke mice, as shown by a reduced Evans blue extravasation and an elevation of tight junction protein expression. Furthermore, stabilization of the BBB as a consequence of lithium treatment was associated with an inhibition of matrix metalloproteinase-9 activity, independent of calveolin-1 regulation. In line with this, flow cytometry analysis revealed that lithium treatment led to a decreased neutrophil invasion and an increased T cell extravasation from the blood compartment towards the brain parenchyma. We finally identified the pro-survival MAPK/ERK1/2 pathway as the key regulator of the impact of lithium on the BBB. In conclusion, we demonstrate for the first time that lithium is able to enhance post-stroke BBB integrity. Importantly, our work delivers novel insights into the exact mechanism of lithium-induced acute neuroprotection, providing critical information for future clinical trials involving lithium treatment in stroke patients.

Convulsive Stress Mimics Brain Hypoxia and Promotes the P-Glycoprotein (P-gp) and Erythropoietin Receptor Overexpression. Recombinant Human Erythropoietin Effect on P-gp Activity

Erythropoietin (EPO) is not only a hormone that promotes erythropoiesis but also has a neuroprotective effect on neurons attributed to its known anti-apoptotic action. Previously, our group has demonstrated that recombinant-human EPO (rHu-EPO) can protect neurons and recovery motor activity in a chemical focal brain hypoxia model (Merelli et al., 2011). We and others also have reported that repetitive seizures can mimic a hypoxic- like condition by HIF-1α nuclear translocation and high neuronal expression P-gp. Here, we report that a single 20-min status epilepticus (SE) induces P-gp and EPO-R expression in cortical pyramidal neurons and only P-gp expression in astrocytes. In vitro, excitotoxic stress (300 μM glutamate, 5 min), can also induce the expression of EPO-R and P-gp simultaneously with both HIF-1α and NFkB nuclear translocation in primary cortical neurons. Primary astrocytes exposed to chemical hypoxia with CoCl₂ (0.3 mM, 6 h) increased P-gp expression as well as an increased efflux of Rhodamine 123 (Rho123) that is a P-gp substrate. Tariquidar, a specific 3^er generation P-gp-blocker was used as an efflux inhibitor control. Astrocytes treated with rHu-EPO showed a significant recovery of the Rho123 retention in a similar way as seen by Tariquidar, demonstrating for first time that rHu-EPO can inhibit the P-gp-dependent efflux activity. Taking together, these data suggest that stimulation of EPO depending signaling system could not only play a central role in brain cell protection, but this system could be a new tool for reverse the pharmacoresistant phenotype in refractory epilepsy as well as in other pharmacoresistant hypoxic brain diseases expressing P-gp.

Pilocarpine-Induced Status Epilepticus Is Associated with P-Glycoprotein Induction in Cardiomyocytes, Electrocardiographic Changes, and Sudden Death

Sudden unexpected death in epilepsy (SUDEP) is the major cause of death in those patients suffering from refractory epilepsy (RE), with a 24-fold higher risk relative to the normal population. SUDEP risk increases with seizure frequency and/or seizure-duration as in RE and Status Epilepticus (SE). P-glycoprotein (P-gp), the product of the multidrug resistant ABCB1-MDR-1 gene, is a detoxifying pump that extrudes drugs out of the cells and can confer pharmacoresistance to the expressing cells. Neurons and cardiomyocytes normally do not express P-gp, however, it is overexpressed in the brain of patients or in experimental models of RE and SE. P-gp was also detected after brain or cardiac hypoxia. We have previously demonstrated that repetitive pentylenetetrazole (PTZ)-induced seizures increase P-gp expression in the brain, which is associated with membrane depolarization in the hippocampus, and in the heart, which is associated with fatal SE. SE can produce hypoxic-ischemic altered cardiac rhythm (HIACR) and severe arrhythmias, and both are related with SUDEP. Here, we investigate whether SE induces the expression of hypoxia-inducible transcription factor (HIF)-1α and P-gp in cardiomyocytes, which is associated with altered heart rhythm, and if these changes are related with the spontaneous death rate. SE was induced in Wistar rats once a week for 3 weeks, by lithium-pilocarpine-paradigm. Electrocardiograms, HIF-1α, and P-gp expression in cardiomyocytes, were evaluated in basal conditions and 72 h after SE. All spontaneous deaths occurred 48 h after each SE was registered. We observed that repeated SE induced HIF-1α and P-gp expression in cardiomyocytes, electrocardiographic (ECG) changes, and a high rate of spontaneous death. Our results suggest that the highly accumulated burden of convulsive stress results in a hypoxic heart insult, where P-gp expression may play a depolarizing role in cardiomyocyte membranes and in the development of the ECG changes, such as QT interval prolongation, that could be related with SUDEP. We postulate that this mechanism could explain, in part, the higher SUDEP risk in patients with RE or SE.

Understanding the Role of Hypoxia Inducible Factor During Neurodegeneration for New Therapeutics Opportunities

Neurodegeneration (NDG) is linked with the progressive loss of neural function with intellectual and/or motor impairment. Several diseases affecting older individuals, including Alzheimer's disease, Amyotrophic Lateral Sclerosis, Huntington's disease, Parkinson's disease, stroke, Multiple Sclerosis and many others, are the most relevant disorders associated with NDG. Since other pathologies such as refractory epilepsy, brain infections, or hereditary diseases such as "neurodegeneration with brain iron accumulation", also lead to chronic brain inflammation with loss of neural cells, NDG can be said to affect all ages. Owing to an energy and/or oxygen supply imbalance, different signaling mechanisms including MAPK/PI3K-Akt signaling pathways, glutamatergic synapse formation, and/or translocation of phosphatidylserine, might activate some central executing mechanism common to all these pathologies and also related to oxidative stress. Hypoxia inducible factor 1-α (HIF-1α) plays a twofold role through gene activation, in the sense that this factor has to "choose" whether to protect or to kill the affected cells. Most of the afore-mentioned processes follow a protracted course and are accompanied by progressive iron accumulation in the brain. We hypothesize that the neuroprotective effects of iron chelators are acting against the generation of free radicals derived from iron, and also induce sufficient -but not excessive- activation of HIF-1α, so that only the hypoxia-rescue genes will be activated. In this regard, the expression of the erythropoietin receptor in hypoxic/inflammatory neurons could be the cellular "sign" to act upon by the nasal administration of pharmacological doses of Neuro-EPO, inducing not only neuroprotection, but eventually, neurorepair as well.

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.

Cardiomyocytes of Chronically Ischemic Pig Hearts Express the MDR-1 Gene-encoded P-glycoprotein

The multidrug-resistant (MDR)-1 gene-encoded P-glycoprotein (Pgp-170) is not normally present in the cardiomyocyte. Given that in other tissues Pgp-170 is not found under normoxic conditions but is expressed during hypoxia, we searched for Pgp-170 in chronically ischemic porcine cardiomyocytes. Pgp-170 was detected and localized via immunohistochemistry in ischemic and nonischemic cardiomyocytes of eight adult pigs 8 weeks after placement of an Ameroid constrictor at the origin of the left circumflex artery (Cx). Regional myocardial ischemia in the Cx bed was documented with nuclear perfusion scans. Pgp-170 mass was quantified using Western blot analysis. In all pigs, Pgp-170 was consistently present in the sarcolemma and T invaginations of the cardiomyocytes of the ischemic zone. Pgp-170 expression decreased toward the border of the ischemic zone and was negative in nonischemic regions as well as in the myocardium of sham-operated animals. Western blot analysis yielded significantly higher Pgp-170 mass in ischemic than in nonischemic areas. We conclude that Pgp-170 is consistently expressed in the cardiomyocytes of chronically ischemic porcine myocardium. Its role in the ischemic heart as well as in conditions such as myocardial hibernation, stunning, and preconditioning may have potentially relevant clinical implications and merits further investigation.

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.

Reduction of brain infarction induced by a transient brain ischemia in mdr1a knockout mice

In order to evaluate the functional role of P-glycoprotein (P-gp) in cerebral ischemia, both multidrug resistance 1a knockout (KO) mice and wild-type mice were subjected to transient focal ischemia under a constant body and brain temperature about 37 degrees C. The results showed that the volume of brain infarction induced by middle cerebral artery occlusion in KO mice was significantly smaller than that seen in wild-type mice, although there were no significant differences in cerebral blood flow, physiological data and on anatomical analysis of cerebrovasculature between both groups. We suggest that multidrug resistance 1a P-gp plays a role for adjusting the expressions of endogenous neuronal cell modulating substances, such as cytokines, neuronal peptides, and others, in the brain, which is consistent with a previous paper (Bobrov et al. Neurosci Lett 24: 6-11, 2008).

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.

ABC Transporters during Epilepsy and Mechanisms Underlying Multidrug Resistance in Refractory Epilepsy

It is estimated 20-25% of the epileptic patients fails to achieve good control with the different antiepileptic drugs (AEDs) treatments, developing refractory epilepsy (RE). Discovered first in cancer, the activity of P-glycoprotein (P-gp) and others ABC transporters as multidrug-resistance-associated proteins (MRPs) and breast cancer resistant protein (BCRP) are directly related with the refractoriness. We have observed the overexpression of these all transporters in the brain of patients with RE, and according with other authors, all these data suggests an active drug efflux from brain. Both constitutive and seizure induced brain P-gp overexpression was also suggested. As confirmation of these clinical evidences, different models of experimental epilepsy have demonstrated P-gp overexpression on blood brain barrier (BBB) and brain parenchyma cells, as astrocytes and neurons. In our model, early P-pg detection in vessel-related cells and later additional P-gp detection in neurons, correlated with the gradual loss of protective effect of phenytoin. The progressive neuronal P-gp expression, depending on intensity and time-constancy of seizure-injury, was in agreement with the development of "P-gp-positive seizure-axis" proposed by Kwan & Brodie, who also showed that the development of RE directly correlated with the number and frequency of seizures before initiation of drug therapy. P-gp expression in excretory organs suggests that P-gp have a central role in drug elimination. Persistent low levels of AEDs in plasma and P-gp brain overexpression in several RE pediatric patients were reported. We also observed in adult RE patients, an increased liver clearance of 99mTc-hexakis-2-methoxyisobutylisonitrile (99mTc-MIBI) (a P-gp substrate), and the surgically treated cases showed P-gp brain overexpression. These results suggest the systemic hyperactivity of P-gp in RE patients, including brain P-gp over-expression should be suspected when persistent subtherapeutic levels of AEDs in plasma are detected. P-gp neuronal expression described in both clinical and experimental reports indicates that additional mechanisms could be operative from seizure-affected P-gp-positive neurons, due to AEDs targets are expressed at membrane level. An alternative mechanism was demonstrated in P-gp-expressed cells that exhibit lower membrane potential (Deltapsi(0)=-10 to -20) compared to normal physiological Deltapsi(0) of -60 mV. Under this situation and irrespective to the P-gp pharmacoresistant property or type of drug treatment selected, P-gp-expressed neurons could increase their sensitivity to new seizures perhaps as an epileptogenic mechanism. The understanding of properties of these ABC transporters can offer new tools for better selection of more effective preventive or therapeutic strategies and avoid the invasive surgical treatments for RE.

Neuronal mdr-1 gene expression after experimental focal hypoxia: A new obstacle for neuroprotection?

Neuronal damage after stroke-associated brain hypoxia is a leading cause of long-term disability and death. The refractoriness to therapeutic strategies for neuroprotection after 3 h post brain ischemia is poorly understood. P-glycoprotein (P-gp), the multidrug resistance gene (MDR-1) product is normally expressed at blood-brain-barrier. P-gp neuronal expression has been demonstrated in refractory epilepsy and after brain ischemia. In this report we investigated the hypoxia-induced neuronal P-gp expression after local injection of CoCl(2) (1-200 mM) in the fronto-parietal cortex of male adult rats (Bregma -1.30 mm) by stereotaxic surgery. P-gp immunostaining of brain slides was analyzed using specific monoclonal antibodies and double immunolabeling was done with specific astrocytic and neuronal markers. Five days after injection of 1 mM CoCl(2), P-gp expression surrounding the lesion site was observed in neurons, astrocytic end-foot on capillary blood vessels and endothelial cells on blood vessels. Higher CoCl(2) doses (200 mM) resulted in additional P-gp immunostaining of the entire astrocytic and neuronal cytoplasm. Electron microscopy (EM) studies showed alterations in neurons as early as 6 h after the CoCl(2) injection. P-gp expression in hypoxic neurons and astrocytic end-foot could potentially impair of drugs access to the brain parenchyma thus suggesting the presence of two P-gp-based pumping systems (one in astrocytes and other in the hypoxic neurons) that are able to behave as a previously unnoticed obstacle for pharmacological strategies of neuroprotection.

ABC transporters, neural stem cells and neurogenesis – a different perspective

Stem cells intrigue. They have the ability to divide exponentially, recreate the stem cell compartment, as well as create differentiated cells to generate tissues. Therefore, they should be natural candidates to provide a renewable source of cells for transplantation applied in regenerative medicine. Stem cells have the capacity to generate specific tissues or even whole organs like the blood, heart, or bones. A subgroup of stem cells, the neural stem cells (NSCs), is characterized as a self-renewing population that generates neurons and glia of the developing brain. They can be isolated, genetically manipulated and differentiated in vitro and reintroduced into a developing, adult or a pathologically altered central nervous system. NSCs have been considered for use in cell replacement therapies in various neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. Characterization of genes with tightly controlled expression patterns during differentiation represents an approach to understanding the regulation of stem cell commitment. The regulation of stem cell biology by the ATP-binding cassette (ABC) transporters has emerged as an important new field of investigation. As a major focus of stem cell research is in the manipulation of cells to enable differentiation into a targeted cell population; in this review, we discuss recent literatures on ABC transporters and stem cells, and propose an integrated view on the role of the ABC transporters, especially ABCA2, ABCA3, ABCB1 and ABCG2, in NSCs' proliferation, differentiation and regulation, along with comparisons to that in hematopoietic and other stem cells.

Chemopreventive effects of aloe against genotoxicity induced by benzo[a]pyrene

Chemopreventive effects of aloe against benzo[a]pyrene (BaP) mutagenicity were investigated in the Salmonella typhimurium bacterial mutation assay, the chromosome aberration assay using Chinese hamster ovary (CHO) cells, and the mouse micronuclei test using bone-marrow cells. In the bacterial assay, aloe produced a concentration-dependent decrease in the number of mutant colonies induced by BaP. The chromosome-damaging responses of BaP in CHO cells were abolished by treatment with aloe, approximately to the level seen in control. In the in vivo mouse bone-marrow micronuclei test, pretreatment of aloe 24 h prior to BaP treatment reduced the frequency of micronucleated polychromatic erythrocytes. In the cells of CHO and bone marrow treated with aloe, glutathione (GSH) levels were shown to be higher and extracellular discharge rate increased as incubation time with aloe rose. MDR1 and MRP2 gene were more expressed in Hepa c cells than in NTCC cells, but there was no change in BCRP gene expression. The antimutagenic effects of aloe were statistically significant and concentration dependent. These results demonstrated that aloe might exert chemopreventive effects against BaP-induced mutagenicity.

Role of drug efflux transporters in the brain for drug disposition and treatment of brain diseases

The blood-brain barrier (BBB) serves as a protective mechanism for the brain by preventing entry of potentially harmful substances from free access to the central nervous system (CNS). Tight junctions present between the brain microvessel endothelial cells form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the barrier properties of the brain capillary endothelial cells. Because of these properties, the BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for drug efflux transporters such as P-glycoprotein (Pgp), multidrug resistance proteins (MRPs) or organic anion transporting polypeptides (OATPs) that are expressed at brain capillary endothelial cells and/or astrocytic end-feet and are key elements of the molecular machinery that confers the special permeability properties to the BBB. The combined action of these carrier systems results in rapid efflux of xenobiotics from the CNS. The objective of this review is to summarize transporter characteristics (cellular localization, specificity, regulation, and potential inhibition) for drug efflux transport systems identified in the BBB and blood-cerebrospinal fluid (CSF) barrier. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on brain access of therapeutic drugs also are described and critically discussed. The potential impact of efflux transport on the pharmacodynamics of agents acting in the CNS is illustrated. Furthermore, the current knowledge about drug efflux transporters as a major determinant of multidrug resistance of brain diseases such as epilepsy is reviewed. Finally, we summarize strategies for modulating or by-passing drug efflux transporters at the BBB as novel therapeutic approaches to drug-resistant brain diseases.

A proposal for the physiological significance of mdr1 and Bcrp1/Abcg2 gene expression in normal tissue regeneration and after cancer therapy

Cellular multi-drug resistance (MDR), which often develops in cancer cells of patients subjected to anti-cancer treatment, remains a significant barrier to successful cancer therapy. One of the principal causes of cellular MDR development is an increased expression of ABC-transporter genes such as mdr1 and Bcrp1/Abcg2. Despite many years of intensive research, the natural biological role of mdr1 in the context of cancer has remained elusive. Some hints about this role came, however, from an observation that P-gp, the mdr1 encoded protein, is expressed widely in stem cells and from the discovery that P-gp possesses an anti-apoptotic activity independently of exogenous drug application. Here, we discuss our own and other groups' recently published works and propose an integrated view of mdr1 and Bcrp1/Abcg2 activity during tissue regeneration in normal tissues as part of a stress-induced regeneration genetic program and in cancerous tissues in response to cancer therapy.