Repetitive/temporal hypoxia increased P-glycoprotein expression in cultured rat brain microvascular endothelial cells in vitro

Hypoxia modulates P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) drug transporters in brain endothelial cells of the developing human blood-brain barrier

P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) multidrug resistance (MDR) transporters are localized at the luminal surface of the blood-brain barrier (BBB). They confer fetal brain protection against harmful compounds that may be circulating in the peripheral blood. The fetus develops in low oxygen levels; however, some obstetric pathologies such as pre-eclampsia, placenta accreta/previa may result in even greater fetal hypoxic states. We investigated how hypoxia impacts MDR transporters in human fetal brain endothelial cells (hfBECs) derived from early and mid-stages of pregnancy. Hypoxia decreased BCRP protein and activity in hfBECs derived in early pregnancy. In contrast, in hfBECs derived in mid-pregnancy there was an increase in P-gp and BCRP activity following hypoxia. Results suggest a hypoxia-induced reduction in fetal brain protection in early pregnancy, but a potential increase in transporter-mediated protection at the BBB during mid-gestation. This would modify accumulation of various key physiological and pharmacological substrates of P-gp and BCRP in the developing fetal brain and potentially contribute to the pathogenesis of neurodevelopmental disorders commonly associated with in utero hypoxia.

Calcium Phosphate-Based Nanoformulation Selectively Abolishes Phenytoin Resistance in Epileptic Neurons for Ceasing Seizures

Phenytoin (PHT) is a first-line antiepileptic drug in clinics, which could decrease neuronal bioelectric activity by blocking the voltage-operated sodium channels. However, the intrinsically low blood-brain-barrier (BBB)-crossing capability of PHT and upregulated expression level of the efflux transporter p-glycoprotein (P-gp) coded by the gene Abcb1 in epileptic neurons limit its efficacy in vivo. Herein, a nanointegrated strategy to overcome PHT resistance mechanisms for enhanced antiepileptic efficacy is reported. Specifically, PHT is first incorporated into calcium phosphate (CaP) nanoparticles through biomineralization, followed by the surface modification of the PEGylated BBB-penetrating TAT peptide. The CaP@PHT-PEG-TAT nanoformulation could effectively cross the BBB to be taken in by epileptic neurons. Afterward, the acidic lysosomal environment would trigger their complete degradation to release Ca²⁺ and PHT into the cytosol. Ca²⁺ ions would inhibit mitochondrial oxidative phosphorylation to reverse cellular hypoxia to block hypoxia-inducible factor-1α (Hif1α)-Abcb1-axis, as well as disrupt adenosine triphosphate generation, leading to simultaneous suppression of the expression and drug efflux capacity of P-gp to enhance PHT retention. This study offers an approach for effective therapeutic intervention against drug-resistant epilepsy.

Loss of Blood-Brain Barrier Integrity in an In Vitro Model Subjected to Intermittent Hypoxia: Is Reversion Possible with a HIF-1α Pathway Inhibitor?

Several sleep-related breathing disorders provoke repeated hypoxia stresses, which potentially lead to neurological diseases, such as cognitive impairment. Nevertheless, consequences of repeated intermittent hypoxia on the blood-brain barrier (BBB) are less recognized. This study compared two methods of intermittent hypoxia induction on the cerebral endothelium of the BBB: one using hydralazine and the other using a hypoxia chamber. These cycles were performed on an endothelial cell and astrocyte coculture model. Na-Fl permeability, tight junction protein, and ABC transporters (P-gp and MRP-1) content were evaluated with or without HIF-1 inhibitors YC-1. Our results demonstrated that hydralazine as well as intermittent physical hypoxia progressively altered BBB integrity, as shown by an increase in Na-Fl permeability. This alteration was accompanied by a decrease in concentration of tight junction proteins ZO-1 and claudin-5. In turn, microvascular endothelial cells up-regulated the expression of P-gp and MRP-1. An alteration was also found under hydralazine after the third cycle. On the other hand, the third intermittent hypoxia exposure showed a preservation of BBB characteristics. Furthermore, inhibition of HIF-1α with YC-1 prevented BBB dysfunction after hydralazine treatment. In the case of physical intermittent hypoxia, we observed an incomplete reversion suggesting that other biological mechanisms may be involved in BBB dysfunction. In conclusion, intermittent hypoxia led to an alteration of the BBB model with an adaptation observed after the third cycle.

Dysfunction of ABC transporters at the blood-brain barrier: Role in neurological disorders

ABC (ATP-binding cassette) transporters represent one of the largest and most diverse superfamily of proteins in living species, playing an important role in many biological processes such as cell homeostasis, cell signaling, drug metabolism and nutrient uptake. Moreover, using the energy generated from ATP hydrolysis, they mediate the efflux of endogenous and exogenous substrates from inside the cells, thereby reducing their intracellular accumulation. At present, 48 ABC transporters have been identified in humans, which were classified into 7 different subfamilies (A to G) according to their phylogenetic analysis. Nevertheless, the most studied members with importance in drug therapeutic efficacy and toxicity include P-glycoprotein (P-gp), a member of the ABCB subfamily, the multidrug-associated proteins (MPRs), members of the ABCC subfamily, and breast cancer resistance protein (BCRP), a member of the ABCG subfamily. They exhibit ubiquitous expression throughout the human body, with a special relevance in barrier tissues like the blood-brain barrier (BBB). At this level, they play a physiological function in tissue protection by reducing or limiting the brain accumulation of neurotoxins. Furthermore, dysfunction of ABC transporters, at expression and/or activity level, has been associated with many neurological diseases, including epilepsy, multiple sclerosis, Alzheimer's disease, and amyotrophic lateral sclerosis. Additionally, these transporters are strikingly associated with the pharmacoresistance to central nervous system (CNS) acting drugs, because they contribute to the decrease in drug bioavailability. This article reviews the signaling pathways that regulate the expression and activity of P-gp, BCRP and MRPs subfamilies of transporters, with particular attention at the BBB level, and their mis-regulation in neurological disorders.

Hypoxic Stress Induced by Hydralazine Leads to a Loss of Blood-Brain Barrier Integrity and an Increase in Efflux Transporter Activity

Understanding cellular and molecular mechanisms induced by hypoxic stress is crucial to reduce blood-brain barrier (BBB) disruption in some neurological diseases. Since the brain is a complex organ, it makes the interpretation of in vivo data difficult, so BBB studies are often investigated using in vitro models. However, the investigation of hypoxia in cellular pathways is complex with physical hypoxia because HIF-1α (factor induced by hypoxia) has a short half-life. We had set up an innovative and original method of induction of hypoxic stress by hydralazine that was more reproducible, which allowed us to study its impact on an in vitro BBB model. Our results showed that hydralazine, a mimetic agent of the hypoxia pathway, had the same effect as physical hypoxia, with few cytotoxicity effects on our cells. Hypoxic stress led to an increase of BBB permeability which corresponded to an opening of our BBB model. Study of tight junction proteins revealed that this hypoxic stress decreased ZO-1 but not occludin expression. In contrast, cells established a defence mechanism by increasing expression and activity of their efflux transporters (Pgp and MRP-1). This induction method of hypoxic stress by hydralazine is simple, reproducible, controllable and suitable to understand the cellular and molecular mechanisms involved by hypoxia on the BBB.

Carbonic anhydrase IX-directed immunoliposomes for targeted drug delivery to human lung cancer cells in vitro

Targeted drug delivery to cancer cells by use of antibody-conjugated liposomes (immunoliposomes) has attracted considerable interest in recent years. Despite increasing efforts in developing immunoliposomes as drug carriers, the investigation of useful tumor-associated antigen targets is far from complete. Carbonic anhydrase IX (CA IX) is a cell surface antigen characterized by hypoxia-induced expression in many solid tumors. This study investigated the feasibility of CA IX-directed immunoliposomes for targeted delivery of docetaxel to human lung cancer cells in vitro. Docetaxel-loaded immunoliposomes targeting CA IX were developed with an encapsulation efficiency of 84.4±3.9% and an average particle size of 143.9±11.1 nm. Using fluorescence-based flow cytometry, the in vitro binding activity of the immunoliposomes was found to be significantly higher (by 1.65-fold) than that of the nontargeted liposomes in CA IX-positive lung cancer cells, whereas no such difference was observed between the two groups when CA IX was not expressed. Furthermore, immunoliposomal docetaxel exhibited the strongest growth inhibitory effect against CA IX-positive lung cancer cells when compared with nontargeted liposomal docetaxel or free docetaxel solution. These data suggested that CA IX-directed immunoliposomes could serve as a promising drug delivery system for targeted killing of lung cancer cells.

ATP-binding cassette transporters in immortalised human brain microvascular endothelial cells in normal and hypoxic conditions

Background

Rapid reperfusion following ischemia is the most effective therapy in stroke therapy. However, the success may be compromised by ischemia & reperfusion (I/R) injury and at the human blood–brain barrier (BBB), therefore the effects on transendothelial transport are of special interest. Current studies suggest the ATP-binding cassette (ABC) transporters to be regulated upon ischemic stroke in a way that impedes the effects of drug therapy. The immortalised human brain microvascular endothelial cell line hCMEC/D3 provides most of the unique properties of the BBB with respect to transport and might be a reliable in vitro model to study transendothelial transport after I/R.

Methods

We exposed hCMEC/D3 cells to 24 hours of hypoxia alone and to hypoxia followed by 60 min of reoxygenisation as an in vitro model for I/R. Western blot showed mild upregulation of hypoxia inducible factor (HIF-1α) after hypoxia alone and RNA lysates were analysed with a well-established real-time RT-PCR-based TaqMan low-density array detecting 47 of 48 known human ABC transporters.

Results

No significant increases of ABC mRNA expression levels were detected neither in hypoxic nor in I/R samples. However, slight decrease of ABCC1 in hypoxic and I/R samples and of ABCA10 and ABCD3 in I/R samples was observed.

Conclusion

Our data suggests that hCMEC/D3 cell line and – at the moment – in vitro models in general are a poor basis for stroke research but may be enhanced by co-culturing more cells of the neurovascular unit inducing an overall ischemic response at the BBB.

The ATP-binding cassette transporters ABCB1 and ABCC1 are not regulated by hypoxia in immortalised human brain microvascular endothelial cells

BACKGROUND

ATP-binding cassette transporters at the blood-brain barrier are actively regulated upon ischemic stroke in a way that impedes the access of pharmacological compounds to the brain tissue. The luminal endothelial transporter ABCB1 was recently shown to be increased, whereas the abluminal transporter ABCC1 was decreased on ischemic brain capillaries. In vitro studies using epithelial cells suggested that ABCB1 is regulated during hypoxia in a hypoxia-inducible factor (HIF)-1α-dependent way.

METHODS

In order to investigate whether hypoxia might be responsible for the expression changes of ABCB1 and ABCC1 in the ischemic brain, the immortalised human brain microvascular endothelial cell line hCMEC/D3 was exposed to hypoxia (1%) or anoxia (0%). Cell lysates were analysed by Western blot to detect the protein expression of ABCB1, ABCC1, HIF-1α and HIF-2α.

RESULTS

During hypoxia, an accumulation of HIF-1α and HIF-2α was noticed in hCMEC/D3 cells that followed different time kinetics. Both HIF-1α and HIF-2α abundance increased within 4 h of hypoxia. HIF-1α levels decreased to below detection levels within 16 h of hypoxia, whereas HIF-2α remained elevated even after 48 h. No changes of ABCB1 and ABCC1 expression were detected, neither on the mRNA nor protein level.

CONCLUSION

Our data suggests that other factors than hypoxia may be responsible for the expression changes of ATP-binding cassette transporters in the ischemic brain.

Polymorphisms in the ABCB1 gene in phenobarbital responsive and resistant idiopathic epileptic Border Collies

BACKGROUND

Variation in the ABCB1 gene is believed to play a role in drug resistance in epilepsy.

HYPOTHESIS/OBJECTIVES

Variation in the ABCB1 gene encoding the permeability-glycoprotein could have an influence on phenobarbital (PB) resistance, which occurs with high frequency in idiopathic epileptic Border Collies (BCs).

ANIMALS

Two hundred and thirty-six client-owned BCs from Switzerland and Germany including 25 with idiopathic epilepsy, of which 13 were resistant to PB treatment.

METHODS

Prospective and retrospective case-control study. Data were collected retrospectively regarding disease status, antiepileptic drug (AED) therapy, and drug responsiveness. The frequency of a known mutation in the ABCB1 gene (4 base-pair deletion in the ABCB1 gene [c.296_299del]) was determined in all BCs. Additionally, the ABCB1 coding exons and flanking sequences were completely sequenced to search for additional variation in 41 BCs. Association analyses were performed in 2 case-control studies: idiopathic epileptic and control BCs and PB-responsive and resistant idiopathic epileptic BCs.

RESULTS

One of 236 BCs (0.4%) was heterozygous for the mutation in the ABCB1 gene (c.296_299del). A total of 23 variations were identified in the ABCB1 gene: 4 in exons and 19 in introns. The G-allele of the c.-6-180T > G variation in intron 1 was significantly more frequent in epileptic BCs resistant to PB treatment than in epileptic BCs responsive to PB treatment (P(raw) = .0025).

CONCLUSIONS AND CLINICAL IMPORTANCE

A variation in intron 1 of the ABCB1 gene is associated with drug responsiveness in BCs. This might indicate that regulatory mutations affecting the expression level of ABCB1 could exist, which may influence the reaction of a dog to AEDs.

Platelet activating factor induces blood brain barrier permeability alteration in vitro

The purposes of this article were to investigate whether blood brain barrier (BBB) permeability is altered after platelet activating factor (PAF) induced injury in vitro and elucidate the preliminary possible mechanisms of it. MTT method was used to observe cell damage after PAF incubation with rat brain microvessel endothelial cells (RBMECs). Intracellular concentrations of Nimodipine in normal and PAF injured RBMECs were estimated by LC-MS/MS analytical method to estimate BBB permeability. Accumulation of P-glycoprotein (P-gp) substrate rhodamine 123 in normal or PAF injured RBMECs was measured with Poly Immune Analysis System-1420 to evaluate the function of P-gp on RBMECs. Intercellular adhesion molecule-1 (ICAM-1) mRNA and protein expression levels in RBMECs were assayed by RT-PCR and flow cytometry respectively. Results showed that after RBMECs were incubated with 1 μM PAF for 24h, cell survival rate was decreased, and intracellular concentrations of Nimodipine were increased evidently. Rhodamine 123 accumulation between normal and PAF injured cells has no significant difference, but ICAM-1 mRNA and protein expression were increased remarkably in PAF injured cells, which could be inhibited by PAF antagonists. In conclusion, the present study demonstrated that BBB permeability was increased after PAF incubation, and which may be due to ICAM-1 up-regulating but not P-glycoprotein function alteration.

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).