Signalling pathways influencing basal and H2O2-induced P-glycoprotein expression in endothelial cells derived from the blood-brain barrier

Reversal mechanism of multidrug-resistant cancer cells by lectin as chemo-adjuvant and targeted therapy- a systematic review

BACKGROUND

Cancer is characterized as the leading cause of death, and the susceptibility of cancer cells to develop resistance due to long-term exposure to complementary chemotherapeutic treatment is referred to as multidrug resistance cancer cells (MDRC), which is a significant obstacle in the treatment of malignancies. Since complementary medicine lost its effectiveness, the development of potential alternative and novel therapeutic approaches has been elevated to a top priority in recent years. In this context, a bioactive protein lectin from plant and animal sources exhibits an invaluable source of anticancer agents with vast therapeutic potential.

PURPOSE

This manuscript's primary purpose is to enlighten the evidence-based (from 1986 to 2022) possible molecular mechanism of alternative treatment approaches using lectins over the complementary medicines used for cancer treatment.

METHODS

The PRISMA rules have been followed properly and qualitative and quantitative data are synthesized systematically. Articles were identified based on Clinical and preclinical reports published on lectin that investigated the in-depth cellular mechanisms, of reverse drug integrative oncology, as a nano-carried targeted delivery. Articles were systematically screened from 1986 to 2022 and selected based on electronic database searches, Medline (PubMed), Google Scholar, Web of Science, Encyclopaedias, Scopus, and ClinicalTrials.gov database.

RESULTS

The search turned up 4,212 publications from 38 different nations, of which 170 reference articles were used in our analysis, in 16 combination therapy and their mode of action, and 27 clinical trial studies including dosage and mechanism of action were included. Reports from the 30 lectins belonging to 28 different families have been included. The reversal mechanism of lectin and alternative therapy against MDRC is critically screened and according to a few clinical and preclinical reports, lectin can suppress the overexpressing genes like P-53, EGFR, and P-gp, MRP, and ABC transporter proteins associated with intracellular transportation of drugs. Since, the drug efflux mechanism leads to MDRC, in this phenomenon, lectin plays a key role in reversing the efflux mechanism. Few preclinical reports have mentioned that lectin shows synergism in combination with complementary medicine and as a nano drug carrier helps to deliver to the targeted site.

CONCLUSION

We have discussed the alternative therapy using lectin and an in-depth insight into the reversal drug resistance mechanisms to combat MDRC cancer, enhance the efficacy, reduce toxicity and adverse events, and ensure targeted delivery, and their application in the field of cancer diagnosis and prognosis has been discussed. However, further investigation is necessary in drug development and clinical trials which could be helpful to elaborate the reversal mechanism and unlock newer treatment modalities in MDRC cancer.

Cu(ATSM) Increases P-Glycoprotein Expression and Function at the Blood-Brain Barrier in C57BL6/J Mice

P-glycoprotein (P-gp), expressed at the blood-brain barrier (BBB), is critical in preventing brain access to substrate drugs and effluxing amyloid beta (Aβ), a contributor to Alzheimer's disease (AD). Strategies to regulate P-gp expression therefore may impact central nervous system (CNS) drug delivery and brain Aβ levels. As we have demonstrated that the copper complex copper diacetyl bis(4-methyl-3-thiosemicarbazone) (Cu(ATSM)) increases P-gp expression and function in human brain endothelial cells, the present study assessed the impact of Cu(ATSM) on expression and function of P-gp in mouse brain endothelial cells (mBECs) and capillaries in vivo, as well as in peripheral organs. Isolated mBECs treated with Cu(ATSM) (100 nM for 24 h) exhibited a 1.6-fold increase in P-gp expression and a 20% reduction in accumulation of the P-gp substrate rhodamine 123. Oral administration of Cu(ATSM) (30 mg/kg/day) for 28 days led to a 1.5 & 1.3-fold increase in brain microvascular and hepatic expression of P-gp, respectively, and a 20% reduction in BBB transport of [3H]-digoxin. A metallomic analysis showed a 3.5 and 19.9-fold increase in Cu levels in brain microvessels and livers of Cu(ATSM)-treated mice. Our findings demonstrate that Cu(ATSM) increases P-gp expression and function at the BBB in vivo, with implications for CNS drug delivery and clearance of Aβ in AD.

Reduction in ABCG2 mRNA Expression in Human Immortalised Brain Microvascular Endothelial Cells by Ferric Ammonium Citrate is Mediated by Reactive Oxygen Species and Activation of ERK1/2 Signalling

PURPOSE

The ATP-binding cassette (ABC) transport protein ABCG2 (also known as breast cancer resistance protein (BCRP)) is expressed at the luminal face of the blood-brain barrier (BBB), where it limits the brain uptake of a number of therapeutic drugs. We recently reported that the ABC efflux transporter P-glycoprotein (P-gp) was downregulated in human immortalised brain endothelial (hCMEC/D3) cells treated with ferric ammonium citrate (FAC). The aim of the present study, therefore, was to assess whether BCRP expression is also affected by FAC and identify any signalling mechanisms involved.

METHODS

ABCG2 mRNA was assessed by RT-qPCR. Protein levels of BCRP, phosphorylated extracellular-regulated kinases 1 and 2 (p-ERK1/2) and total ERK 1/2 were assessed by Western blot. Reactive oxygen species (ROS) levels were determined using 2',7'-dichlorofluorescin diacetate.

RESULTS

Treatment of hCMEC/D3 cells with FAC (250 µM, 72 h) significantly reduced ABCG2 mRNA levels (32.2 ± 3.7%) without a concomitant reduction in BCRP protein expression. ABCG2 mRNA levels were restored to control levels when co-treated with the antioxidant N-acetylcysteine (NAC), suggesting the effect of FAC was mediated by a ROS-sensitive pathway. We also found that FAC-treatment was associated with increased levels of p-ERK1/2, suggesting involvement of the ERK1/2 signalling pathway in the observed ABCG2 mRNA downregulation. The ERK1/2 signalling pathway inhibitor U0126 restored p-ERK1/2 levels and partially attenuated the FAC-induced reduction in ABCG2 mRNA.

CONCLUSIONS

This study suggests that FAC-induced downregulation of ABCG2 mRNA is driven by ROS and ERK1/2 signalling, mechanisms which may be exploited to modulate BCRP expression at the BBB.

Regulation of P-Glycoprotein in the Brain

Maintenance of the tightly regulated homeostatic environment of the brain is facilitated by the blood-brain barrier (BBB). P-glycoprotein (P-gp), an ATP-binding cassette transporter, is expressed on the luminal surface of the endothelial cells in the BBB, and actively exports a wide variety of substrates to limit exposure of the vulnerable brain environment to waste buildup and neurotoxic compounds. Downregulation of P-gp expression and activity at the BBB have been reported with ageing and in neurodegenerative diseases. Upregulation of P-gp at the BBB contributes to poor therapeutic outcomes due to altered pharmacokinetics of CNS-acting drugs. The regulation of P-gp is highly complex, but unravelling the mechanisms involved may help the development of novel and nuanced strategies to modulate P-gp expression for therapeutic benefit. This review summarises the current understanding of P-gp regulation in the brain, encompassing the transcriptional, post-transcriptional and post-translational mechanisms that have been identified to affect P-gp expression and transport activity.

Copper bis(thiosemicarbazone) complexes modulate P-glycoprotein expression and function in human brain microvascular endothelial cells

P-glycoprotein (P-gp) is an efflux transporter at the blood-brain barrier (BBB) that hinders brain access of substrate drugs and clears endogenous molecules such as amyloid beta (Aβ) from the brain. As biometals such as copper (Cu) modulate many neuronal signalling pathways linked to P-gp regulation, it was hypothesised that the bis(thiosemicarbazone) (BTSC) Cu-releasing complex, copper II glyoxal bis(4-methyl-3-thiosemicarbazone) (CuII [GTSM]), would enhance P-gp expression and function at the BBB, while copper II diacetyl bis(4-methyl-3-thiosemicarbazone) (CuII [ATSM]), which only releases Cu under hypoxic conditions, would not modulate P-gp expression. Following treatment with 25-250 nM CuII (BTSC)s for 8-48 h, expression of P-gp mRNA and protein in human brain endothelial (hCMEC/D3) cells was assessed by RT-qPCR and Western blot, respectively. P-gp function was assessed by measuring accumulation of the fluorescent P-gp substrate, rhodamine 123 and intracellular Cu levels were quantified by inductively coupled plasma mass spectrometry. Interestingly, CuII (ATSM) significantly enhanced P-gp expression and function 2-fold and 1.3-fold, respectively, whereas CuII (GTSM) reduced P-gp expression 0.5-fold and function by 200%. As both compounds increased intracellular Cu levels, the effect of different BTSC backbones, independent of Cu, on P-gp expression was assessed. However, only the Cu-ATSM complex enhanced P-gp expression and this was mediated partly through activation (1.4-fold) of the extracellular signal-regulated kinase 1 and 2, an outcome that was significantly attenuated in the presence of an inhibitor of the mitogen-activated protein kinase regulatory pathway. Our findings suggest that CuII (ATSM) and CuII (GTSM) have the potential to modulate the expression and function of P-gp at the BBB to impact brain drug delivery and clearance of Aβ.

ETS1, ELK1, and ETV4 Transcription Factors Regulate Angiopoietin-1 Signaling and the Angiogenic Response in Endothelial Cells

Background

Angiopoietin-1 (Ang-1) is the main ligand of Tie-2 receptors. It promotes endothelial cell (EC) survival, migration, and differentiation. Little is known about the transcription factors (TFs) in ECs that are downstream from Tie-2 receptors.

Objective

The main objective of this study is to identify the roles of the ETS family of TFs in Ang-1 signaling and the angiogenic response.

Methods

In silico enrichment analyses that were designed to predict TF binding sites of the promotors of eighty-six Ang-1-upregulated genes showed significant enrichment of ETS1, ELK1, and ETV4 binding sites in ECs. Human umbilical vein endothelial cells (HUVECs) were exposed for different time periods to recombinant Ang-1 protein and mRNA levels of ETS1, ELK1, and ETV4 were measured with qPCR and intracellular localization of these transcription factors was assessed with immunofluorescence. Electrophoretic mobility shift assays and reporter assays were used to assess activation of ETS1, ELK1, and ETV4 in response to Ang-1 exposure. The functional roles of these TFs in Ang-1-induced endothelial cell survival, migration, differentiation, and gene regulation were evaluated by using a loss-of-function approach (transfection with siRNA oligos).

Results

Ang-1 exposure increased ETS1 mRNA levels but had no effect on ELK1 or ETV4 levels. Immunostaining revealed that in control ECs, ETS1 has nuclear localization whereas ELK1 and ETV4 are localized to the nucleus and the cytosol. Ang-1 exposure increased nuclear intensity of ETS1 protein and enhanced nuclear mobilization of ELK1 and ETV4. Selective siRNA knockdown of ETS1, ELK1, and ETV4 showed that these TFs are required for Ang-1-induced EC survival and differentiation of cells, while ETS1 and ETV4 are required for Ang-1-induced EC migration. Moreover, ETS1, ELK1, and ETV4 knockdown inhibited Ang-1-induced upregulation of thirteen, eight, and nine pro-angiogenesis genes, respectively.

Conclusion

We conclude that ETS1, ELK1, and ETV4 transcription factors play significant angiogenic roles in Ang-1 signaling in ECs.

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.

Pulchinenosides from Pulsatilla Chinensis Increase P-Glycoprotein Activity and Induce P-Glycoprotein Expression

Five pulchinenosides (pulchinenoside B3, pulchinenoside BD, pulchinenoside B7, pulchinenoside B10, and pulchinenoside B11) isolated from Pulsatilla chinensis (Bge) Regel saponins extract exhibited strong antitumor activities but poor gastrointestinal absorption properties. The enteric induction of P-glycoprotein (P-gp) is understood to restrict the oral bioavailability of some pharmaceutical compounds and lead to adverse drug reactions. Therefore, the present investigation was intended to delineate the impacts of pulchinenosides on cellular P-gp function and expression using Sf9 membrane vesicles and LS180 cells as a surrogate of human intestinal epithelial cells. Preliminary cytotoxic studies showed that 10 μM was an acceptable concentration for cytotoxicity and antiproliferation studies for all pulchinenosides using the alamarBlue assay. The cell cycle of LS180 cells detected by flow cytometry was not significantly influenced after 48 hours of coincubation with 10 μM of pulchinenosides. In the presence of pulchinenosides, the ATP-dependent transport of N-methyl-quinidine mediated by P-glycoprotein was stimulated significantly. The upregulation of P-glycoprotein and mRNA levels was found by Western blot and real-time PCR analysis in LS180 cells. Parallel changes indicate that all pulchinenosides are exposed to pulchinenosides-mediated transcriptional regulation. In conclusion, pulchinenosides could induce P-glycoprotein expression and directly increase its functional activity.

Oxidative stress-induced activation of Abl and Src kinases rapidly induces P-glycoprotein internalization via phosphorylation of caveolin-1 on tyrosine-14, decreasing cortisol efflux at the blood-brain barrier

Exposure of the brain to high levels of glucocorticoids during ischemia-reperfusion induces neuronal cell death. Oxidative stress alters blood-brain barrier (BBB) function during ischemia-reperfusion, and so we hypothesized that it might impair P-glycoprotein (P-gp)-mediated efflux transport of glucocorticoids at the BBB. Therefore, the purpose of this study was to clarify the molecular mechanism of this putative decrease of P-gp-mediated efflux function. First, we established that H₂O₂ treatment of a human in vitro BBB model (hCMEC/D3) reduced both P-gp efflux transport activity and protein expression on the plasma membrane within 20 min. These results suggested that the rapid decrease of efflux function might be due to internalization of P-gp. Furthermore, H₂O₂ treatment markedly increased tyrosine-14-phosphorylated caveolin-1, which is involved in P-gp internalization. A brain perfusion study in rats showed that cortisol efflux at the BBB was markedly decreased by H₂O₂ administration, and inhibitors of Abl kinase and Src kinase, which phosphorylate tyrosine-14 in caveolin-1, suppressed this decrease. Overall, these findings support the idea that oxidative stress-induced activation of Abl kinase and Src kinase induces internalization of P-gp via the phosphorylation of tyrosine-14 in caveolin-1, leading to a rapid decrease of P-gp-mediated cortisol efflux at the BBB.

Oxidative stress facilitates cell death by inhibiting Orai1-mediated Ca2+ entry in brain capillary endothelial cells

Brain capillary endothelial cells (BCECs) form the blood-brain barrier (BBB) and play an essential role in the regulation of its functions. Oxidative stress accumulates excessive reactive oxygen species (ROS) and facilitates the death of BCECs, leading to a dysfunctional BBB. However, the mechanisms underlying the death of BCECs under oxidative stress remain unclear. In the present study, the effects of oxidative stress on cell viability, ROS production, intracellular Ca²⁺ concentration, and protein expression were examined using a cell line derived from bovine BCECs, t-BBEC117. When t-BBEC117 cells were exposed to oxidative stress induced by hydrogen peroxide (H₂O₂, 10-100 μM), cell growth was inhibited in a dose-dependent manner. Oxidative stress by 30 μM H₂O₂ increased the production of ROS and its effects were blocked by the ROS scavenger, 10 mM N-acetyl-l-cysteine (NAC). In addition, oxidative stress reduced store-operated Ca²⁺ entry (SOCE) and this decrease was recovered by NAC or the Orai channel activator, 5 μM 2-aminoethyl diphenylborinate (2-APB). The siRNA knockdown of Orai1 revealed that Orai1 was mainly responsible for SOCE channels and its activity was decreased by oxidative stress. However, the protein expression of Orai1 and STIM1 was not affected by oxidative stress. Oxidative stress-induced cell death was rescued by 2-APB, NAC, or the STIM-Orai activating region. In conclusion, oxidative stress reduces Orai1-mediated SOCE and, thus, facilitates the death of BCECs.

ABC Transporters in Extrahepatic Tissues: Pharmacological Regulation in Heart and Intestine

ATP binding cassette (ABC) transporters are transmembrane proteins expressed in secretory epithelia like the liver, kidneys and intestine, in the epithelia exhibiting barrier function such as the blood-brain barrier and placenta, and to a much lesser extent, in tissues like reproductive organs, lungs, heart and pancreas, among others. They regulate internal distribution of endogenous metabolites and xenobiotics including drugs of therapeutic use and also participate in their elimination from the body. We here describe the function and regulation of ABC transporters in the heart and small intestine, as examples of extrahepatic tissues, in which ABC proteins play clearly different roles. In the heart, they are involved in tissue pathogenesis as well as in protecting this organ against toxic compounds and druginduced oxidative stress. The small intestine is highly exposed to therapeutic drugs taken orally and, consequently, ABC transporters localized on its surface strongly influence drug absorption and pharmacokinetics. Examples of the ABC proteins currently described are Multidrug Resistance-associated Proteins 1 and 2 (MRP1 and 2) for heart and small intestine, respectively, and P-glycoprotein (P-gp) and Breast Cancer Resistance Protein (BCRP) for both organs.

Increase in P-glycoprotein levels in the blood-brain barrier of partial portal vein ligation /chronic hyperammonemia rats is medicated by ammonia/reactive oxygen species/ERK1/2 activation: In vitro and in vivo studies

Liver failure altered P-glycoprotein (P-gp) function and expression at blood-brain barrier (BBB), partly owing to hyperammonemia. We aimed to examine the effects of partial portal vein ligation (PVL) plus chronic hyperammonemia (CHA) on P-gp function and expression at rat BBB. Experimental rats included sham-operation (SH), PVL, CHA and PVL+CHA. The PVL+CHA rats were developed by ammonia-containing diet for 2 weeks after operation. The brain-to-plasma concentration ratios (K_p) and apparent unidirectional influx constants (K_in) of rhodamine123 and sodium fluorescein were measured to assess function of P-gp and BBB integrity, respectively. Human cerebral microvascular endothelial cells (HCMEC/D3) were used to assess effects of ammonia on P-gp expression and function. It was found that PVL+CHA significantly decreased K_p and K_in of rhodamine123 without affecting brain distribution of fluorescein. The P-gp expressions in membrane protein in cortex and hippocampus were significantly increased in CHA and PVL +CHA rats, especially in PVL + CHA rats, while remarkably increased phosphorylated ERK1/2 was only found in PVL +CHA rats. Expressions of tight junction proteins claudin-5 and occluding in rat brain remained unchanged. In vitro data showed that NH₄Cl increased reactive oxygen species, membrane expression and function of P-gp as well as phosphorylated ERK1/2 levels in HCMEC/D3. The NH₄Cl-induced alterations were reversed by reactive oxygen species scavenger N-acetylcysteine and ERK1/2 inhibitor U0126. In conclusion, PVL+CHA increased function and membrane translocation of P-gp at rat BBB partly via ammonia. Reactive oxygen species/ERK1/2 pathway activation may be one of the reasons that ammonia upregulated P-gp expression and function at BBB.

Proteins Regulating Microvesicle Biogenesis and Multidrug Resistance in Cancer

Microvesicles (MV) are emerging as important mediators of intercellular communication. While MVs are important signaling vectors for many physiological processes, they are also implicated in cancer pathology and progression. Cellular activation is perhaps the most widely reported initiator of MV biogenesis, however, the precise mechanism remains undefined. Uncovering the proteins involved in regulating MV biogenesis is of interest given their role in the dissemination of deleterious cancer traits. MVs shed from drug-resistant cancer cells transfer multidrug resistance (MDR) proteins to drug-sensitive cells and confer the MDR phenotype in a matter of hours. MDR is attributed to the overexpression of ABC transporters, primarily P-glycoprotein and MRP1. Their expression and functionality is dependent on a number of proteins. In particular, FERM domain proteins have been implicated in supporting the functionality of efflux transporters in drug-resistant cells and in recipient cells during intercellular transfer by vesicles. Herein, the most recent research on the proteins involved in MV biogenesis and in the dissemination of MV-mediated MDR are discussed. Attention is drawn to unanswered questions in the literature that may prove to be of benefit in ongoing efforts to improve clinical response to chemotherapy and circumventing MDR.

Overcoming the Blood-Brain Barrier: The Role of Nanomaterials in Treating Neurological Diseases

Therapies directed toward the central nervous system remain difficult to translate into improved clinical outcomes. This is largely due to the blood-brain barrier (BBB), arguably the most tightly regulated interface in the human body, which routinely excludes most therapeutics. Advances in the engineering of nanomaterials and their application in biomedicine (i.e., nanomedicine) are enabling new strategies that have the potential to help improve our understanding and treatment of neurological diseases. Herein, the various mechanisms by which therapeutics can be delivered to the brain are examined and key challenges facing translation of this research from benchtop to bedside are highlighted. Following a contextual overview of the BBB anatomy and physiology in both healthy and diseased states, relevant therapeutic strategies for bypassing and crossing the BBB are discussed. The focus here is especially on nanomaterial-based drug delivery systems and the potential of these to overcome the biological challenges imposed by the BBB. Finally, disease-targeting strategies and clearance mechanisms are explored. The objective is to provide the diverse range of researchers active in the field (e.g., material scientists, chemists, engineers, neuroscientists, and clinicians) with an easily accessible guide to the key opportunities and challenges currently facing the nanomaterial-mediated treatment of neurological diseases.

A new approach for Alzheimer's disease treatment through P-gp regulation via ibuprofen

This study was aimed to investigate whether ibuprofen could alter the P-glycoprotein expression and function under Alzheimer's Disease condition and whether this alteration was induced by the inhibition of inflammatory reaction. APP/PS1 mice were used as AD model mice and ibuprofen-treated AD mice were given ibuprofen for 5 months. Then, Abcb1a/1b mRNA levels and P-gp expression were evaluated by qRT-PCR and western blot. Abcb1 mRNA levels were significantly reduced in AD mice compared to control mice, and it could be restored by ibuprofen treatment. Meanwhile, P-gp expression result showed a similar trend. Aβ plaques in cerebral cortices and hippocampus were investigated via immunohistochemical, and the results revealed that Aβ plaques were reduced in ibuprofen-treated AD mice compared with the AD mice, indicated that P-gp function may be recovered by ibuprofen treatment. qRT-PCR and ELISA were used to determined TNF-α, IL-1β, IL-6 and NF-κB levels. The results demonstrated that TNF-α, IL-1β mRNA levels and NF-κB expression were all significantly upregulated in AD mice in comparison with the control mice, and ibuprofen treatment could suppress the increase of inflammatory factors. In conclusion, the P-gp expression and function were suppressed in AD condition by activating inflammatory reaction, and then causing the Aβ efflux decreased. However, upregulating P-gp could increase the Aβ efflux in further to treat AD via inhibiting the inflammatory factors expression.

Mitochondrial expression and activity of P-glycoprotein under oxidative stress in outer blood-retinal barrier

AIM

To investigate the role of oxidative stress in regulating the functional expression of P-glycoprotein (P-gp) in mitochondria of D407 cells.

METHODS

D407 cells were exposed to different ranges of concentrations of H₂O₂. The mitochondrial location of P-gp in the cells subjected to oxidative stress was detected by confocal analysis. Expression of P-gp in isolated mitochondria was assessed by Western blot. The pump activity of P-gp was evaluated by performing the efflux study on isolated mitochondria with Rhodamine 123 (Rho-123) alone and in the presence of P-gp inhibitor (Tariquidar) using flow cytometry analysis. The cells were pretreated with 10 mmol/L N-acetylcysteine (NAC) for 30min before exposing to H₂O₂, and analyzed the mitochondrial extracts by Western blot and flow cytometry.

RESULTS

P-gp was co-localized in the mitochondria by confocal laser scanning microscopy, and it was also detected in the mitochondria of D407 cells using Western blot. Exposure to increasing concentrations of H₂O₂ led to gradually increased expression and location of P-gp in the mitochondria of cells. Rho-123 efflux assay showed higher uptake of Rho-123 on isolated mitochondria in the presence of Tariquidar both in normal and oxidative stress state. H₂O₂ up-regulated P-gp in D407 cells, which could be reversed by NAC treatment.

CONCLUSION

H₂O₂ could up-regulate the functional expression of P-gp in mitochondria of D407 cells, while antioxidants might suppress oxidative-stress-induced over-expression of functional P-gp. It is indicative that limiting the mitochondrial P-gp transport in retinal pigment epithelium cells would be to improve the effect of mitochondria-targeted antioxidant therapy in age-related macular degeneration-like retinopathy.

Absorption properties and effects of caffeic acid phenethyl ester and its p-nitro-derivative on P-glycoprotein in Caco-2 cells and rats

CONTEXT

Caffeic acid phenethyl ester (CAPE), isolated from honeybee propolis, has pharmacological applications. A synthesized CAPE derivative, p-nitro-caffeic acid phenethyl ester (CAPE-NO₂), showed similar activities with CAPE. The pharmacological activities of CAPE and CAPE-NO₂ are related to their absorption properties.

OBJECTIVE

To understand the pharmacokinetic profiles of CAPE and CAPE-NO₂ in rats and investigate the absorption mechanisms and effects on P-glycoprotein in Caco-2 cells.

MATERIALS AND METHODS

The pharmacokinetic profiles of CAPE and CAPE-NO₂ were obtained after oral administration (10 mg/kg) to rats. Transport studies of CAPE and CAPE-NO₂ (5, 10, 20 μM) were performed in Caco-2 cell model. P-gp activities were assayed by rhodamine 123 cellular retention. Expression of P-gp was determined after the cells were administrated with CAPE and CAPE-NO₂ (5, 20 μM) for 48 and 72 h.

RESULTS

The AUC_(0-t) of CAPE-NO₂ (3239.9 ± 352 ng × h/mL) was two-time greater than CAPE (1659.6 ± 152 ng × h/mL) in rats. The P_app values of CAPE and CAPE-NO₂ were (4.86 ± 0.90) × 10^-6cm/s and (12.34 ± 1.6) × 10^-6cm/s, respectively. The accumulation of rhodamine 123 was increased by 1.3- to 1.9-fold and 1.4- to 2.3-fold in CAPE and CAPE-NO₂ groups after 1 h administration, respectively. However, CAPE and CAPE-NO₂ increased the P-gp levels by 2.1- and 1.7-fold, respectively.

CONCLUSION

The absorption of CAPE-NO₂ can be enhanced in rats and Caco-2 cells compared with CAPE. The two compounds are potential inhibitors of P-gp. The increased P-gp levels generated by CAPE and CAPE-NO₂ played a role as a defense mechanism by limiting intracellular xenobiotic levels.

Doxorubicin and resveratrol co-delivery nanoparticle to overcome doxorubicin resistance

With the extensive application of doxorubicin (DOX), DOX resistance has become one of the main obstacles to the effective treatment of breast cancer. In this paper, DOX and resveratrol (RES) were co-encapsulated in a modified PLGA nanoparticle (NPS) to overcome the DOX resistance. CLSM results indicated that DOX and RES were simultaneously delivered into the nucleus of DOX-resistant human breast cancer cells by DOX/RES-loaded NPS. Consequently, DOX/RES-loaded NPS showed significant cytotoxicity on MDA-MB-231/ADR cells and MCF-7/ADR cells. Furthermore, DOX/RES-loaded NPS could overcome DOX resistance by inhibiting the expression of drug resistance-related protein such as P-gp, MRP-1 and BCRP, and induce apoptosis through down-regulating the expression of NF-κB and BCL-2. In tumor-bearing mice, DOX/RES-loaded NPS mainly delivered DOX and RES to tumor tissue. Compared with free DOX, DOX/RES-loaded NPS significantly inhibited the DOX-resistant tumor growth in tumor-bearing mice without causing significant systemic toxicity. In a word, DOX/RES-loaded NPS could overcome the DOX resistance and had the potential in the treatment of DOX-resistant breast cancer.

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.

Borneol Depresses P-Glycoprotein Function by a NF-κB Signaling Mediated Mechanism in a Blood Brain Barrier in Vitro Model

P-glycoprotein (P-gp) on brain microvascular endothelial cells (BMECs) that form the blood brain barrier (BBB), influences transportation of substances between blood and brain. The objective of this study was to characterize the effects of borneol on P-gp efflux function on BBB and explore the potential mechanisms. We established an in vitro BBB model comprised of rat BMECs and astrocytes to measure the effects of borneol on the known P-gp substrates transport across BBB, and examined the function and expression of P-gp in BMECs and the signaling pathways regulating P-gp expression. Borneol increased intracellular accumulation of Rhodamine 123, enhanced verapamil and digoxin across the BBB in vitro model, and depressed mdr1a mRNA and P-gp expression. Borneol could activate nuclear factor-κB (NF-κB) and inhibition of NF-κB with MG132 (carbobenzoxy-Leu-Leu-leucinal) and SN50 (an inhibitory peptide) obscuring the P-gp decreases induced by borneol. These data suggested that borneol depresses P-gp function in BMECs by a NF-κB signaling medicated mechanism in a BBB in vitro model.

BME, a novel compound of anthraquinone, down regulated P-glycoprotein expression in doxorubicin-resistant human myelogenous leukemia (K562/DOX) cells via generation of reactive oxygen species

P-glycoprotein (P-gp)-mediated multidrug resistance (MDR) in tumor cells is still a main obstacle for the chemotherapeutic treatment of cancers. Thus, development of effective MDR reversing agents is an important approach in the clinic. The present study revealed that BME, a novel compound of anthraquinone, elevated intracellular accumulation of the P-gp substrates and reduced concentration resulting in 50% inhibition of cell growth (IC50) values for doxorubicin (DOX) in doxorubicin-resistant human myelogenous leukemia (K562/DOX) cells. Further more, BME was also reported to down regulated P-gp expression accompanying with generation of nontoxic low level of intracellular reactive oxygen species (iROS) and activation of extracellular signal-regulated kinase (ERK)1/2 as well as c-JUN N-terminal kinase (JNK). However, treatment with N-acetyl-cysteine (NAC), U0216 and SP600125 almost abolished actions of the BME mentioned above. These results indicated that the effect of the BME on the P-gp may be involved in generation of nontoxic low level of iROS and activation of ERK1/2 or JNK, which suggested valuable clues to screen and develop P-gp reversing agents.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Nrf2 upregulates ATP binding cassette transporter expression and activity at the blood-brain and blood-spinal cord barriers

Activation of nuclear factor E2-related factor-2 (Nrf2), a sensor of oxidative stress, is neuroprotective in animal models of cerebral ischemia, traumatic brain injury, subarachnoid hemorrhage, and spinal cord injury. We show here that Nrf2 activation with sulforaphane (SFN) in vivo or in vitro increases expression and transport activity of three ATP-driven drug efflux pumps at the blood-brain barrier [P-glycoprotein, ATP binding cassette b1 (Abcb1); multidrug resistance-associated protein-2 (Mrp2), Abcc2; and breast cancer resistance protein (Bcrp), Abcg2]. Dosing rats with SFN increased protein expression of all three transporters in brain capillaries and decreased by 50% brain accumulation of the P-glycoprotein substrate verapamil. Exposing rat or mouse brain capillaries to SFN increased P-glycoprotein, Bcrp, and Mrp2 transport activity and protein expression; SFN increased P-glycoprotein activity in mouse spinal cord capillaries. Inhibiting transcription or translation abolished upregulation of P-glycoprotein activity. No such effects were seen in brain capillaries from Nrf2-null mice, indicating Nrf2 dependence. Nrf2 signaled indirectly to increase transporter activity/expression. The p53 inhibitor pifithrin abolished the SFN-induced increase in transporter activity/expression, and the p53-activator nutlin-3 increased P-glycoprotein activity. SFN did not alter P-glycoprotein transport activity in brain and spinal cord capillaries from p53-null mice. Inhibitors of p38 MAPK and nuclear factor κB (NF-κB) blocked the effects of SFN and nutlin-3 on P-glycoprotein activity. These results implicate Nrf2, p53, and NF-κB in the upregulation of P-glycoprotein, Bcrp, and Mrp2 at blood-CNS barriers. They imply that the barriers are tightened selectively (efflux transporter upregulation) by oxidative stress, providing increased neuroprotection, but also reduced penetration of many therapeutic drugs.

PXR-mediated P-glycoprotein induction by small molecule tyrosine kinase inhibitors

The rapid development of drug resistance as a result of exposure to small molecule tyrosine kinase inhibitors (TKIs) is an important drawback to the successful use of these agents in the clinic. Although one of the most established mechanisms by which cells acquire drug resistance to anticancer drugs is the up regulation of drug efflux transporters such as P-glycoprotein (PGP), it is currently still unknown whether TKIs have the propensity to induce PGP. The effect of TKIs on the protein expression and activity of PGP was assessed after treatment of LS180 cells with clinically relevant concentrations of the TKIs. In addition, the involvement of the nuclear pregnane X receptor (PXR), a known regulator of PGP expression, was determined. At least five out of the nine tested TKIs (erlotinib, gefitinib, nilotinib, sorafenib, vandetanib) were able to induce the expression of PGP within 48 h in LS180 cells. Accordingly, these TKIs were also shown to affect the accumulation of a P-glycoprotein specific probe substrate. Furthermore, we showed that the pregnane X receptor (PXR), which is an important regulator of PGP induction, is involved in the upregulation of PGP protein expression following exposure to these TKIs. Our data show that PXR-mediated upregulation of PGP expression by TKIs might be a possible mechanism underlying acquired drug resistance in cancer cells.

Susceptibility of juvenile and adult blood-brain barrier to endothelin-1: regulation of P-glycoprotein and breast cancer resistance protein expression and transport activity

Background

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) play a critical role in keeping neurotoxic substances from entering the brain. We and others have previously reported an impact of inflammation on the regulation of adult blood–brain barrier (BBB) efflux transporters. However, studies in children have not been done. From the pediatric clinical perspective, it is important to understand how the central nervous system (CNS) and BBB drug efflux transporters differ in childhood from those of adults under normal and inflammatory conditions. Therefore, we examined and compared the regulation of P-gp and BCRP expression and transport activity in young and adult BBB and investigated the molecular mechanisms underlying inflammatory responses.

Methods

Rats at postnatal day (P) P21 and P84, corresponding to the juvenile and adult stages of human brain maturation, respectively, were treated with endothelin-1 (ET-1) given by the intracerebroventricular (icv) route. Twenty-four hours later, we measured P-gp and BCRP protein expression in isolated brain capillary by immunoblotting as well as by transport activity in vivo by measuring the unbound drug partitioning coefficient of the brain (K_p,uu,brain) of known efflux transporter substrates administered intravenously. Glial activation was measured by immunohistochemistry. The release of cytokines/chemokines (interleukins-1α, 1-β (IL-1β), -6 (IL-6), -10 (IL-10), monocyte chemoattractant protein (MCP-1/CCL2), fractalkine and tissue inhibitor of metalloproteinases-1 (TIMP-1)) were simultaneously measured in brain and serum samples using the Agilent Technology cytokine microarray.

Results

We found that juvenile and adult BBBs exhibited similar P-gp and BCRP transport activities in the normal physiological conditions. However, long-term exposure of the juvenile brain to low-dose of ET-1 did not change BBB P-gp transport activity but tended to decrease BCRP transport activity in the juvenile brain, while a significant increase of the activity of both transporters was evidenced at the BBB in the adult brain. Moreover, juvenile and adult brain showed differences in their expression profiles of cytokines and chemokines mediated by ET-1.

Conclusions

BBB transporter activity during neuroinflammation differs between the juvenile and adult brains. These findings emphasize the importance of considering differential P-gp and BCRP transport regulation mechanisms between adult and juvenile BBB in the context of neuroinflammation.

Pro-inflammatory cytokine regulation of P-glycoprotein in the developing blood-brain barrier

Placental P-glycoprotein (P-gp) acts to protect the developing fetus from exogenous compounds. This protection declines with advancing gestation leaving the fetus and fetal brain vulnerable to these compounds and potential teratogens in maternal circulation. This vulnerability may be more pronounced in pregnancies complicated by infection, which is common during pregnancy. Pro-inflammatory cytokines (released during infection) have been shown to be potent inhibitors of P-gp, but nothing is known regarding their effects at the developing blood-brain barrier (BBB). We hypothesized that P-gp function and expression in endothelial cells of the developing BBB will be inhibited by pro-inflammatory cytokines. We have derived brain endothelial cell (BEC) cultures from various stages of development of the guinea pig: gestational day (GD) 50, 65 (term ∼68 days) and postnatal day (PND) 14. Once these cultures reached confluence, BECs were treated with various doses (10⁰–10⁴ pg/mL) of pro-inflammatory cytokines: interleukin-1β (IL-1β), interleukin-6 (IL-6) or tumor necrosis factor- α (TNF-α). P-gp function or abcb1 mRNA (encodes P-gp) expression was assessed following treatment. Incubation of GD50 BECs with IL-1β, IL-6 or TNF-α resulted in no change in P-gp function. GD65 BECs displayed a dose-dependent decrease in function with all cytokines tested; maximal effects at 42%, 65% and 34% with IL-1β, IL-6 and TNF-α treatment, respectively (P<0.01). Inhibition of P-gp function by IL-1β, IL-6 and TNF-α was even greater in PND14 BECs; maximal effects at 36% (P<0.01), 84% (P<0.05) and 55% (P<0.01), respectively. Cytokine-induced reductions in P-gp function were associated with decreased abcb1 mRNA expression. These data suggest that BBB P-gp function is increasingly responsive to the inhibitory effects of pro-inflammatory cytokines, with increasing developmental age. Thus, women who experience infection and take prescription medication during pregnancy may expose the developing fetal brain to greater amounts of exogenous compounds – many of which are considered potentially teratogenic.

Nitric oxide contributes to hypoxia-reoxygenation-induced P-glycoprotein expression in rat brain endothelial cells

Ischemia-reperfusion leads to increased levels at the blood-brain barrier of the multidrug efflux transporter, P-glycoprotein that provides protection to the brain by limiting access of unwanted substances. This is coincident with the production of nitric oxide. This present study using immortalized rat brain endothelial cells (GPNTs) examines whether following hypoxia-reoxygenation, nitric oxide contributes to the alterations in P-glycoprotein levels. After 6 h of hypoxia, both nitric oxide and reactive oxygen species, detected intracellularly using fluorescent monitoring dyes, were produced in the subsequent reoxygenation phase coincident with increased P-glycoprotein. The evidence that nitric oxide can directly affect P-glycoprotein expression was sought by applying S-nitroso-N-acetyl-DL: -penicillamine that as shown increased the nitric oxide generation. Sodium nitroprusside, though more effective at increasing P-glycoprotein expression, appeared to produce different reactive species. Real time RT-PCR analysis revealed the predominant form of nitric oxide synthase in these cells to be endothelial, inhibition of which partially prevented the increase in P-glycoprotein during reoxygenation. These data indicate that the production of nitric oxide by endothelial nitric oxide synthase during reoxygenation can influence P-glycoprotein expression in cells of the blood-rat brain barrier, highlighting another route by which nitric oxide may protect the brain.

Resistance to the translation initiation inhibitor silvestrol is mediated by ABCB1/P-glycoprotein overexpression in acute lymphoblastic leukemia cells

Protein synthesis is a powerful therapeutic target in leukemias and other cancers, but few pharmacologically viable agents are available that affect this process directly. The plant-derived agent silvestrol specifically inhibits translation initiation by interfering with eIF4A/mRNA assembly with eIF4F. Silvestrol has potent in vitro and in vivo activity in multiple cancer models including acute lymphoblastic leukemia (ALL) and is under pre-clinical development by the US National Cancer Institute, but no information is available about potential mechanisms of resistance. In a separate report, we showed that intraperitoneal silvestrol is approximately 100% bioavailable systemically, although oral doses were only 1% bioavailable despite an apparent lack of metabolism. To explore mechanisms of silvestrol resistance and the possible role of efflux transporters in silvestrol disposition, we characterized multi-drug resistance transporter expression and function in a silvestrol-resistant ALL cell line generated via culture of the 697 ALL cell line in gradually increasing silvestrol concentrations. This resistant cell line, 697-R, shows significant upregulation of ABCB1 mRNA and P-glycoprotein (Pgp) as well as cross-resistance to known Pgp substrates vincristine and romidepsin. Furthermore, 697-R cells readily efflux the fluorescent Pgp substrate rhodamine 123. This effect is prevented by Pgp inhibitors verapamil and cyclosporin A, as well as siRNA to ABCB1, with concomitant re-sensitization to silvestrol. Together, these data indicate that silvestrol is a substrate of Pgp, a potential obstacle that must be considered in the development of silvestrol for oral delivery or targeting to tumors protected by Pgp overexpression.

Regulation of P-glycoprotein and other ABC drug transporters at the blood-brain barrier

ATP-binding cassette (ABC) transporters are important selective elements of the blood-brain barrier. They line the luminal plasma membrane of the brain capillary endothelium, facing the vascular space, and both protect the central nervous system from entry of neurotoxicants and limit the access of therapeutic drugs to the brain parenchyma. Recent studies highlight the multiple signaling pathways through which the expression and activity of P-glycoprotein and other ABC transporters are modulated in response to xenobiotics, stress and disease. The results show that increased transporter expression occurs in response to signals that activate specific transcription factors, including pregnane-X receptor, constitutive androstane receptor, nuclear factor-kappaB and activator protein-1, and that reduced transporter activity occurs rapidly and reversibly in response to signaling through Src kinase, protein kinase C and estrogen receptors. A detailed understanding of such regulation can provide the basis for improved neuroprotection and enhanced therapeutic drug delivery to the brain.

P-glycoprotein expression in immortalised rat brain endothelial cells: comparisons following exogenously applied hydrogen peroxide and after hypoxia-reoxygenation

Levels of multidrug efflux transporter P-glycoprotein (P-gp) on endothelial cells lining brain blood vessels are important for limiting access of many compounds to the brain. In vivo studies have indicated that ischaemia-reperfusion that generates reactive oxygen species also increases P-gp levels in brain endothelial cells. To investigate possible mechanisms, in vitro studies were performed on immortalised (GPNT) and primary rat brain endothelial cells. Exposure to hydrogen peroxide (200 microM) resulted in intracellular oxidative stress as detected from higher levels of dichlorofluorescein fluorescence and raised levels of P-gp protein, mdr1a and mdr1b transcripts and, in GPNT cells, increased mdr1a and mdr1b promoter activity. The P-gp protein increases were abolished by pre-treatment with polyethylene glycol-catalase and were curtailed by co-culture with primary rat astrocytes. Exposure of GPNT cells to 6 h hypoxia followed by 24 h reoxygenation produced less intracellular oxidative stress as judged from smaller increments in dichlorofluorescein fluorescence but still resulted in raised levels of P-gp protein, an effect partially abolished by pre-treatment with polyethylene glycol-catalase. However, transcript levels and promoter activities were not significantly increased. These data suggest that hydrogen peroxide contributes to P-gp up-regulation following hypoxia-reoxygenation but the underlying mechanisms of its actions differ from those occurring after direct hydrogen peroxide application.

Regulation of major efflux transporters under inflammatory conditions at the blood-brain barrier in vitro

ATP-driven efflux transport proteins at the blood-brain barrier protect the healthy brain but impede pharmacotherapy of the disordered CNS. To investigate the question how ATP-binding cassette (ABC)-transporters are regulated during inflammation or infection we analysed the effects of the cytokines tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) on the expression of brain multidrug resistance proteins in primary cultures of porcine brain capillary endothelial cells. We found that TNF-alpha and IL-1beta rapidly decrease Abcg2 (BMDP/BCRP) mRNA expression within 6 h. After 24 and 48 h the mRNA level came back to control values. The mRNA reduction at 6 h was counter-regulated by the anti-inflammatory glucocorticoid hydrocortisone. Abcg2 protein levels were suppressed at prolonged stimulations but not after 6 h of stimulation which correlates with Abcg2 specific substrate uptake measurements. Abcb1 (p-glycoprotein) protein expression was transiently increased after TNF-alpha addition within 6 h of incubation followed by a reduction after 24 and 48 h whereas the Abcb1 mRNA levels were not changed. IL-1beta caused a continuous decrease in protein expression of both ABC-transporters. Long-term treatment with an assumed TNF-alpha-downstream agent, the vasoconstrictor endothelin-1, induced Abcg2 protein expression but suppressed Abcb1. Other efflux pumps like multidrug resistance-associated proteins/Abcc were rarely affected. The present results imply a complex regulation of the two most abundant ABC-transporters at the blood-brain barrier during early inflammation stages suggesting that Abcb1 (p-glycoprotein) is an early target of TNF-alpha-signalling counterbalanced by Abcg2.

Increased p38-MAPK is responsible for chemotherapy resistance in human gastric cancer cells

Background

Chemoresistance is one of the main obstacles to successful cancer therapy and is frequently associated with Multidrug resistance (MDR). Many different mechanisms have been suggested to explain the development of an MDR phenotype in cancer cells. One of the most studied mechanisms is the overexpression of P-glycoprotein (P-gp), which is a product of the MDR1 gene. Tumor cells often acquire the drug-resistance phenotype due to upregulation of the MDR1 gene. Overexpression of MDR1 gene has often been reported in primary gastric adenocarcinoma.

Methods

This study investigated the role of p38-MAPK signal pathway in vincristine-resistant SGC7901/VCR cells. P-gp and MDR1 RNA were detected by Western blot analysis and RT-PCR amplification. Mitgen-activated protein kinases and function of P-gp were demonstrated by Western blot and FACS Aria cytometer analysis. Ap-1 activity and cell apoptosis were detected by Dual-Luciferase Reporter Assay and annexin V-PI dual staining.

Results

The vincristine-resistant SGC7901/VCR cells with increased expression of the multidrug-resistance 1 (MDR1) gene were resistant to P-gp-related drug and P-gp-unrelated drugs. Constitutive increases of phosphorylated p38-MAPK and AP-1 activities were also found in the drug-resistant cells. Inhibition of p38-MAPK by SB202190 reduced activator protein-1 (AP-1) activity and MDR1 expression levels and increased the sensitivity of SGC7901/VCR cells to chemotherapy.

Conclusion

Activation of the p38-MAPK pathway might be responsible for the modulation of P-glycoprotein-mediated and P-glycoprotein-unmediated multidrug resistance in the SGC7901/VCR cell line.

Modulation of P-glycoprotein at the blood-brain barrier: opportunities to improve central nervous system pharmacotherapy

Pharmacotherapy of central nervous system (CNS) disorders (e.g., neurodegenerative diseases, epilepsy, brain cancer, and neuro-AIDS) is limited by the blood-brain barrier. P-glycoprotein, an ATP-driven, drug efflux transporter, is a critical element of that barrier. High level of expression, luminal membrane location, multispecificity, and high transport potency make P-glycoprotein a selective gatekeeper of the blood-brain barrier and thus a primary obstacle to drug delivery into the brain. As such, P-glycoprotein limits entry into the CNS for a large number of prescribed drugs, contributes to the poor success rate of CNS drug candidates, and probably contributes to patient-to-patient variability in response to CNS pharmacotherapy. Modulating P-glycoprotein could therefore improve drug delivery into the brain. Here we review the current understanding of signaling mechanisms responsible for the modulation of P-glycoprotein activity/expression at the blood-brain barrier with an emphasis on recent studies from our laboratories. Using intact brain capillaries from rats and mice, we have identified multiple extracellular and intracellular signals that regulate this transporter; several signaling pathways have been mapped. Three pathways are triggered by elements of the brain's innate immune response, one by glutamate, one by xenobiotic-nuclear receptor (pregnane X receptor) interactions, and one by elevated beta-amyloid levels. Signaling is complex, with several pathways sharing common signaling elements [tumor necrosis factor (TNF) receptor 1, endothelin (ET) B receptor, protein kinase C, and nitric-oxide synthase), suggesting a regulatory network. Several pathways include autocrine/paracrine elements, involving release of the proinflammatory cytokine, TNF-alpha, and the polypeptide hormone, ET-1. Finally, several steps in signaling are potential therapeutic targets that could be used to modulate P-glycoprotein activity in the clinic.

P-glycoprotein: so many ways to turn it on

Expression of the ABC transporter P-glycoprotein (P-gp or ABCB1) is associated with resistance to chemotherapy in cancer. However, early investigations into the regulation of ABCB1 expression revealed that the process is not a classical induction as observed for certain metabolizing enzymes. The process involves the cellular stress response pathway initiated by either inflicted (e.g., chemotherapy damage) or endogenous (e.g., hypoxia) factors. However, ABCB1 is also expressed in a number of noncancerous tissues. In particular, the protein is found at tissues providing a barrier or secretory function. The localization of ABCB1 in normal tissues will impact significantly on drug pharmacokinetics, in particular the absorption and elimination processes. This review also describes the mechanism underlying ABCB1 expression in noncancerous tissue, a process that does not involve the stress response.

Peripheral inflammatory hyperalgesia modulates morphine delivery to the brain: a role for P-glycoprotein

P-glycoprotein (Pgp, ABCB1) is a critical efflux transporter at the blood-brain barrier (BBB) where its luminal location and substrate promiscuity limit the brain distribution of numerous therapeutics. Moreover, Pgp is known to confer multi-drug resistance in cancer chemotherapy and brain diseases, such as epilepsy, and is highly regulated by inflammatory mediators. The involvement of inflammatory processes in neuropathological states has led us to investigate the effects of peripheral inflammatory hyperalgesia on transport properties at the BBB. In the present study, we examined the effects of lambda-carrageenan-induced inflammatory pain (CIP) on brain endothelium regulation of Pgp. Western blot analysis of enriched brain microvessel fractions showed increased Pgp expression 3 h post-CIP. In situ brain perfusion studies paralleled these findings with decreased brain uptake of the Pgp substrate and opiate analgesic, [(3)H] morphine. Cyclosporin A-mediated inhibition of Pgp enhanced the uptake of morphine in lambda-carrageenan and control animals. This indicates that the CIP induced decrease in morphine transport was the result of an increase in Pgp activity at the BBB. Furthermore, antinociception studies showed decreased morphine analgesia following CIP. The observation that CIP modulates Pgp at the BBB in vivo is critical to understanding BBB regulation during inflammatory disease states.

Neural precursor cell influences on blood-brain barrier characteristics in rat brain endothelial cells

This study explores the effects of neural precursor cells (NPCs) on barrier characteristics in brain vasculature. Primary rat brain endothelial cells were exposed to conditioned medium from NPCs isolated from day 14 embryonic rat brains and maintained as free-floating undifferentiated neurospheres. Such exposure increased brain endothelial transcript levels of the mdr1a but not mdr1b gene encoding P-glycoprotein (Pgp) and reduced proliferation but did not alter transendothelial resistance (TER). These effects were compared to those seen following co-culture with differentiating NPCs or with primary astrocytes. NPCs, if grown adherent, differentiate into glial and neuronal cells as assessed by immunocytochemical and mRNA analysis. Brain endothelial cells when co-cultured with these cells also showed reduced proliferation and enhanced mdr1a expression, but in addition increased TER. Similar increases were observed in co-culture with astrocytes. These results suggest that undifferentiated NPCs produce factors that influence Pgp expression whereas their progeny also affect tight junction integrity.

Tumor necrosis factor alpha and endothelin-1 increase P-glycoprotein expression and transport activity at the blood-brain barrier

The ATP-driven drug efflux pump, P-glycoprotein, is a critical and selective element of the blood-brain barrier and a primary impediment to pharmacotherapy of central nervous system (CNS) disorders. Thus, an understanding of how P-glycoprotein function is regulated has the potential to improve CNS therapy. We recently demonstrated rapid (minutes) and reversible inactivation of P-glycoprotein in rat brain capillaries signaled through tumor necrosis factor-alpha (TNF-alpha) and endothelin-1 (ET-1), components of the brain's innate immune response. In this study, we examined the longer-term consequences of continuous exposure of rat brain capillaries to low levels of TNF-alpha and ET-1. Exposing brain capillaries to TNF-alpha or ET-1 caused a rapid decrease in P-glycoprotein transport activity with no change in transporter protein expression. This was followed by a 2- to 3-h plateau at the low activity level and then by a sharp increase in both transport activity and protein expression. After 6 h, transport activity and transporter protein expression was double that of control samples. TNF-alpha signaled through TNF-R1, which in turn caused ET release and action through ETA and ETB receptors, nitric-oxide synthase, protein kinase C and nuclear factor-kappaB (NF-kappaB) and finally increased P-glycoprotein expression and transport activity. Assuming similar effects occur in vivo, the present results imply a tightening of the selective blood-brain barrier with chronic inflammation and thus reduced efficacy of CNS-acting drugs that are P-glycoprotein substrates. Moreover, involvement of NF-kappaB raises the possibility that other effectors acting through this transcription factor may have similar effects on this key blood-brain barrier transporter.

Changes in fluid secretion rate alter net transepithelial transport of MRP2 and P-glycoprotein substrates in Malpighian tubules of Drosophila melanogaster

The effects of stimulants of fluid secretion on net transepithelial transport of the MRP2 substrate Texas Red and the p-glycoprotein substrate daunorubicin were examined in Malpighian tubules of Drosophila melanogaster. Fluid secretion rates were determined using the Ramsay assay and secreted fluid concentrations of Texas Red and daunorubicin were determined using a microfluorometric technique. Nanoliter droplets of secreted fluid were collected in optically flat glass capillaries and dye concentration was determined from fluorescence intensity measured by confocal laser scanning microscopy. Net transepithelial flux of each compound was then calculated as the product of its concentration in the secreted fluid and the fluid secretion rate. Net transepithelial flux of Texas Red increased when fluid secretion was stimulated by tyramine, cyclic AMP or hypoosmotic saline. Net flux decreased when fluid secretion rate of cAMP-stimulated tubules was reduced by elevating saline osmolality with sucrose. Net transepithelial flux of daunorubicin increased when fluid secretion was stimulated by cAMP. Significant increases in dye flux were seen only when the dyes were present at concentrations close to or greater than the concentration required for half maximal transport. Regression analyses showed that 57- 88% of the change in dye flux was attributable to the change in fluid secretion rate when tubules were stimulated with cAMP, cGMP, or tyramine. The results do not suggest that the effects of tyramine and cAMP are mediated through changes in transepithelial potential, nor do they indicate the direct effects of the stimulants on MRP2-like or p-glycoprotein-like transporters (e.g., via protein kinases). Instead, the results suggest that increases in fluid secretion rate minimize diffusive backflux of these dyes and, thus, facilitate higher rates of net transepithelial transport indirectly.

Up-regulation of P-glycoprotein expression by glutathione depletion-induced oxidative stress in rat brain microvessel endothelial cells

Glutathione (GSH) depletion has been implicated in the pathogenesis of neurological diseases. During GSH depletion, cells of the blood-brain barrier (BBB) are subjected to chronic oxidative stress. In this study, we investigated the effect of such stress, produced with the GSH synthesis inhibitor l-buthionine-(S,R)-sulfoximine (BSO), on expression of P-glycoprotein (Pgp) in primary cultured rat brain microvessel endothelial cells that comprise the blood-brain barrier (BBB). Application of BSO to cell monolayers at concentrations up to 800 microm caused increases in expression of Pgp. Concentrations >or= 400 microm BSO decreased cell viability. Application of 200 microm BSO caused a significant increase in Pgp function activity, as assessed by rhodamine 123 (Rh123) accumulation experiments. At this concentration, BSO produced time-dependent decreases in levels of intracellular GSH and increases in levels of intracellular reactive oxygen species (iROS). The increases were also observed within 48 h following BSO treatment in mdr1a and mdr1b mRNA. Exposure of cells to BSO for 24 h produced maximal effects in the accumulation of iROS, and in expression and function of Pgp. The ROS scavenger N-acetylcysteine prevented ROS generation and attenuated the changes of both expression and activity of Pgp induced by BSO. Therefore, the transport of Pgp substrates may be affected by changing Pgp expression under conditions of chronic oxidative stress induced by GSH depletion.

LY294,002, a specific inhibitor of PI3K/Akt kinase pathway, antagonizes P-glycoprotein-mediated multidrug resistance

The transmembrane transport pump P-glycoprotein (P-gp) causes the efflux of chemotherapeutic agents from cells and is an important system that secures multidrug resistance (MDR) of neoplastic cells. In the present study drug sensitive L1210 and multidrug resistant L1210/VCR mouse leukemic cell lines were used as an experimental model. We found that LY 294,002, a specific inhibitor of PI3K/Akt kinase pathway, reduced the degree of vincristine resistance in L1210/VCR cells significantly and in a concentration-dependent manner. This was accompanied by decrease in IC(50) value to vincristine from 3.195+/-0.447 to 1.898+/-0.676 micromol/l for 2 micromol/l, to 0.947+/-0.419 micromol/l for 4 micromol/l, and to 0.478+/-0.202 micromol/l for 8 micromol/l LY294,002. The IC(50) value of sensitive cells for vincristine was about 0.010 micromol/l. FACS analysis of the proportion of cells in apoptosis or necrosis by annexin-V apoptosis kit showed the following: (i) vincristine-induced apoptosis in resistant cell to a much lower extent than in sensitive cells; (ii) LY294,002 alone did not induce apoptosis or necrosis in both sensitive and resistant cells; (iii) LY294,002 applied together with vincristine significantly increased the number of apoptotic cells. Transport activity of P-gp in resistant cells was monitored using calcein/AM as substrate and was depressed by LY294,002 in a concentration dependent manner. Significant differences in calcein retention were not observed when cells were preincubated with LY294,002 at different times from 0.5 to 24h. Sensitive and resistant cells contain similar amounts of uncleaved (i.e., unactivated) caspase-3 but in latter cells the activation of caspase-3 by proteolytic cleavage was decreased. The reversal of vincristine resistance by LY294,002 was associated with marked activation of caspase-3. Western blot analysis revealed that the development of MDR phenotype in L1210/VCR cells was also associated with increased level of Bcl-2 protein. All the above findings point to the possible involvement of PI3K/Akt kinase pathway in modulation of P-gp mediated multidrug resistance in L1210/VCR mouse leukemic cell line. MDR reversal effect of LY294,002 is accompanied with this compound's influence on vincristine-induced apoptosis.

Effects of capsaicin on P-gp function and expression in Caco-2 cells

Capsaicin is the pungent component of hot chilli, a popular spice in many populations. The aim of the present study was to evaluate the chronicity and reversibility of the modulating effect of capsaicin on both the P-gp expression and activity in the Caco-2 cell monolayers. Capsaicin at concentrations ranging from 10 to 100 microM, which were found to be non-cytotoxic towards the Caco-2 cells, were observed to inhibit P-gp mediated efflux transport of [3H]-digoxin in the cells. The acute inhibitory effect was dependent on the capsaicin concentration and duration of exposure, with abolishment of polarity of [3H]-digoxin transport attained at 50 microM of capsaicin. In contrast, longer term (48 and 72 h) co-incubation of the Caco-2 cells with capsaicin (50 and 100 microM) increased P-gp activity through an up-regulation of cellular P-gp protein and MDR1 mRNA levels. The up-regulated protein was functionally active, as demonstrated by higher degree of [3H]-digoxin efflux across the cell monolayers, but the induction was readily reversed by the removal of the spice from the culture medium. The induction of P-gp protein and mRNA levels was also influenced by capsaicin concentration and duration of exposure, with higher expression levels, in particular of the mRNA, seen at higher spice concentrations over prolonged period of incubation. Our data suggest that caution should be exercised when capsaicin is to be consumed with drugs that are P-gp substrates. In particular, the oral bioavailability of these drugs may be influenced by the P-gp status of populations that rely heavily on hot chilli in their diets.

Astrocyte-endothelial interactions at the blood-brain barrier

The blood-brain barrier, which is formed by the endothelial cells that line cerebral microvessels, has an important role in maintaining a precisely regulated microenvironment for reliable neuronal signalling. At present, there is great interest in the association of brain microvessels, astrocytes and neurons to form functional 'neurovascular units', and recent studies have highlighted the importance of brain endothelial cells in this modular organization. Here, we explore specific interactions between the brain endothelium, astrocytes and neurons that may regulate blood-brain barrier function. An understanding of how these interactions are disturbed in pathological conditions could lead to the development of new protective and restorative therapies.

Modulation of p-glycoprotein transport function at the blood-brain barrier

The central nervous system (CNS) effects of many therapeutic drugs are blunted because of restricted entry into the brain. The basis for this poor permeability is the brain capillary endothelium, which comprises the blood-brain barrier. This tissue exhibits very low paracellular (tight-junctional) permeability and expresses potent, multispecific, drug export pumps. Together, these combine to limit use of pharmacotherapy to treat CNS disorders such as brain cancer and bacterial or viral infections. Of all the xenobiotic efflux pumps highly expressed in brain capillary endothelial cells, p-glycoprotein handles the largest fraction of commonly prescribed drugs and thus is an obvious target for manipulation. Here we review recent studies focused on understanding the mechanisms by which p-glycoprotein activity in the blood-brain barrier can be modulated. These include (i) direct inhibition by specific competitors, (ii) functional modulation, and (iii) transcriptional modulation. Each has the potential to specifically reduce p-glycoprotein function and thus selectively increase brain permeability of p-glycoprotein substrates.

Rapid regulation of P-glycoprotein at the blood-brain barrier by endothelin-1

The ATP-driven xenobiotic transporter P-glycoprotein is a critical element of the blood-brain barrier. To study regulation of P-glycoprotein function, we measured specific transport [(3'-oxo-4-butenyl-4-methyl-threonine(1), (valine(2)) cyclosporin (PSC833)-sensitive] of the fluorescent cyclosporin A derivative [N-epsilon(4-nitrobenzofurazan-7-yl)-D-Lys(8)]-cyclosporin A (NBDL-CSA) into the lumens of isolated rat brain capillaries using confocal microscopy and quantitative image analysis. Luminal NBDL-CSA accumulation was rapidly and reversibly reduced in a concentration-dependent manner by 0.1 to 100 nM endothelin-1 (ET-1). In this concentration range, ET-1 did not affect junctional permeability. The ET(B) receptor agonist sarafotoxin 6c also reduced transport. An ET(B) receptor antagonist blocked effects of ET-1 and sarafotoxin 6c; an ET(A) receptor antagonist was without effect. Consistent with this, immunostaining and Western blotting showed expression of the ET(B) receptor in brain capillary membranes. NBDL-CSA transport was also reduced by sodium nitroprusside, a NO donor, and by phorbol ester, a protein kinase C (PKC) activator. Inhibition of NO synthase (NOS) or PKC abolished the ET-1 effects. Thus, ET-1, acting through an ET(B) receptor, NOS, and PKC rapidly and reversibly reduced transport mediated by P-glycoprotein at the blood-brain barrier.

Rapid induction of P-glycoprotein expression by high permeability compounds in colonic cells in vitro: a possible source of transporter mediated drug interactions?

P-glycoprotein (PGP) substrates with high membrane permeability, such as propranolol and verapamil, are considered to be essentially "transparent" to PGP since the transporter does not significantly limit their absorption or elimination. However, the question of whether such compounds can modulate PGP expression in epithelial cells following short-term exposure, with potential consequences for drug interactions, has not been addressed. LS180 colonic epithelial cells were exposed to propranolol or verapamil at concentrations (50-300 microM) consistent with those likely to be present in the gut lumen during oral dosing. Both compounds stimulated four to six-fold increases in MDR1 mRNA and PGP protein expression measured by quantitative real-time PCR and immunoblotting, respectively. These changes were accompanied by an induction in transporter activity measured by rhodamine 123 efflux. In contrast, metoprolol, a compound with similar permeability but no affinity for PGP had no effect on PGP expression. The induction of PGP by propranolol and verapamil was rapid with significant increases occurring within 3h with maximal stimulation after 6h exposure. Rifampicin, shown to cause clinical drug interactions via a PXR-mediated increase in PGP expression, exhibited a very similar time-course and extent of induction. In conclusion, verapamil and propranolol, whose trans-epithelial permeability are unaffected by PGP, appear to be effective inducers of PGP expression in gut epithelial cells in vitro. While the in vivo significance of these observations is unknown, this questions whether high permeability, "PGP-transparent" compounds, currently favoured in drug selection strategies, should be evaluated in terms of their potential for transporter-mediated drug interactions.