Human brain microvascular endothelial cells release different types of P-glycoprotein-containing extracellular vesicles upon exposure to doxorubicin

In the brain, the efflux transporter P-glycoprotein (Pgp) is predominantly located on the luminal membrane of microvascular endothelial cells (BMECs) that form the blood-brain barrier. In addition, Pgp is localized in intracellular organelles involved in Pgp traffic and cycling and, by the release of extracellular vesicles (EVs), in intercellular Pgp transfer to cells with low Pgp expression. We recently described that drug exposure of a human BMEC line (hCMEC/D3) induces the release of Pgp-EGFP-containing EVs; however, the nature of the Pgp-enriched vesicles was not characterized. The two main categories of EVs are exosomes and microvesicles, which differ in origin, size, and molecular cargo. In the present study, we performed similar experiments with hCMEC/D3 cells in the absence and presence of doxorubicin and isolated and characterized the EVs released by the cells during the experiments by differential ultracentrifugation with/without subsequent sucrose gradient fractionation of EV pellets, proteomic profiling, EV size analysis, and confocal fluorescence microscopy. Using cocultures of hCMEC/D3 wildtype cells and cells transduced with MDR1-EGFP or monocultures of hCMEC/D3-MDR1-EGFP cells, we found release of both Pgp-enriched exosomes and microvesicles but analysis of the exosomal marker protein Rab7 indicated that doxorubicin increased particularly the release of exosomes. Transfer experiments with isolated EVs demonstrated EV endocytosis by recipient cells. EV release from BMECs in response to anticancer drugs such as doxorubicin likely serves different functions, including non-genetic intercellular transfer of a resistance phenotype to neighboring BMECs and a mechanism of drug extrusion that contributes to brain protection against potentially toxic chemotherapeutic drugs.

Efflux transport proteins of Tetrahymena thermophila play important roles in resistance to perfluorooctane sulfonate exposure

The biotoxicity of perfluorooctane sulfonate (PFOS) has been a concern. However, the effects of PFOS on Tetrahymena thermophila, a unicellular model organism, remain unclear. This study aimed to investigate the toxicity and detoxification mechanism of PFOS in this protozoan. PFOS did not show prominent toxic effects on T. thermophila. Cell viability of T. thermophila can be concentration-dependently increased by PFOS. PFOS also increased the stability of cell membranes and the activity of lysosomes. However, PFOS inhibited efflux transporter activities. Most of the PFOS amount remained in the culture medium during the culture periods. Only a low amount of PFOS was absorbed by cells, where PFOS molecules were mainly combined with membrane proteins. The expressions of four membrane protein genes involved in transporting xenobiotics were analyzed by real time-PCR. The gene abcg25 was significantly up-regulated. The growth of abcg25 gene knockout protozoans under PFOS treatment was slightly inhibited. However, the amount of PFOS adsorbed by the knockout protozoans showed no significant difference from the Wild-type protozoans. We concluded that the ABCG25 protein might play a key role in preventing PFOS from entering the cell or being exported from the cells to protect T. thermophila against PFOS. However, ABCG25 was not the only membrane protein able to bind with PFOS.

Imaging the impact of blood-brain barrier disruption induced by focused ultrasound on P-glycoprotein function

The P-glycoprotein (P-gp/ABCB1) is a major efflux transporter which impedes the brain delivery of many drugs across the blood-brain barrier (BBB). Focused ultrasound with microbubbles (FUS) enables BBB disruption, which immediate and delayed impact on P-gp function remains unclear. Positron emission tomography (PET) imaging using the radiolabeled substrate [11C]metoclopramide provides a sensitive and translational method to study P-gp function at the living BBB. A FUS protocol was devised in rats to induce a substantial and targeted disruption of the BBB in the left hemisphere. BBB disruption was confirmed by the Evan's Blue extravasation test or the minimally-invasive contrast-enhanced MRI. The expression of P-gp was measured 24 h or 48 h after FUS using immunostaining and fluorescence microscopy. The brain kinetics of [11C]metoclopramide was studied by PET at baseline, and both immediately or 24 h after FUS, with or without half-maximum P-gp inhibition (tariquidar 1 mg/kg). In each condition (n = 4-5 rats per group), brain exposure of [11C]metoclopramide was estimated as the area-under-the-curve (AUC) in regions corresponding to the sonicated volume in the left hemisphere, and the contralateral volume. Kinetic modeling was performed to estimate the uptake clearance ratio (R1) of [11C]metoclopramide in the sonicated volume relative to the contralateral volume. In the absence of FUS, half-maximum P-gp inhibition increased brain exposure (+135.0 ± 12.9%, p < 0.05) but did not impact R1 (p > 0.05). Immediately after FUS, BBB integrity was selectively disrupted in the left hemisphere without any detectable impact on the brain kinetics of [11C]metoclopramide compared with the baseline group (p > 0.05) or the contralateral volume (p > 0.05). 24 h after FUS, BBB integrity was fully restored while P-gp expression was maximally down-regulated (-45.0 ± 4.5%, p < 0.001) in the sonicated volume. This neither impacted AUC nor R1 in the FUS + 24 h group (p > 0.05). Only when P-gp was inhibited with tariquidar were the brain exposure (+130 ± 70%) and R1(+29.1 ± 15.4%) significantly increased in the FUS + 24 h/tariquidar group, relative to the baseline group (p < 0.001). We conclude that the brain kinetics of [11C]metoclopramide specifically depends on P-gp function rather than BBB integrity. Delayed FUS-induced down-regulation of P-gp function can be detected. Our results suggest that almost complete down-regulation is required to substantially enhance the brain delivery of P-gp substrates.

Multidrug resistance of Rhizoctonia solani determined by enhanced efflux for fungicides

Plant pathogens can develop multidrug resistance (MDR) through metabolomic and efflux activities. Although MDR has been observed in the field, its mechanisms are yet to be further studied. MDR in Rhizoctonia solani induced by the uncoupler SYP-14288, which involved efflux transporters including ATP binding cassette (ABC) and major facilitator superfamily (MFS) have been reported in our previous study. To confirm this, corresponding genes of the wild-type R. solani X19 and its derived MDR mutant X19-7 were compared through transcriptomics, RNA-Seq data validation, and heterologous expression. Genes encoding six ABC transporters and seven MFS transporters were identified to be associated with MDR and mostly showed a constitutively higher expression in X19-7 than in X19 regardless of SYP-14288 treatment. Eight ABC transporter-encoding genes and eight MFS transporter-encoding genes were further characterized by transferring into Saccharomyces cerevisiae. The sensitivity of transformants containing either ABC transporter-encoding gene AG1IA_06082 and MFS transporter-encoding gene AG1IA_08645 was significantly decreased in responses to fungicides having various modes of action including SYP-14288, fluazinam, chlorothalonil, and difenoconazole, indicating that these two genes were related to MDR. The roles of two genes were further confirmed by successfully detecting their protein products and high accumulation of SYP-14288 in yeast transformants. Thus, ABC and MFS transporters contributed to the development of MDR in R. solani. The result helps to understand the cause and mechanisms that influence the efficient use of fungicide.

Protective role of the placental efflux transporter BCRP/ABCG2 in the relationship between prenatal cadmium exposure, placenta weight, and size at birth

BACKGROUND AND AIM

Placental efflux transporter proteins, such as BCRP, reduce the placental and fetal toxicity of environmental contaminants but have received little attention in perinatal environmental epidemiology. Here, we evaluate the potential protective role of BCRP following prenatal exposure to cadmium, a metal that preferentially accumulates in the placenta and adversely impacts fetal growth. We hypothesized that individuals with a reduced function polymorphism in ABCG2, the gene encoding BCRP, would be most vulnerable to the adverse impacts of prenatal cadmium exposure, notably, smaller placental and fetal size.

METHODS

We measured cadmium in maternal urine samples at each trimester and in term placentas from UPSIDE-ECHO study participants (NY, USA; n = 269). We fit adjusted multivariable linear regression and generalized estimating equation models to examine log-transformed urinary and placental cadmium concentrations in relation to birthweight, birth length, placental weight, and fetoplacental weight ratio (FPR) and stratified models by ABCG2 Q141K (C421A) genotype.

RESULTS

Overall 17% of participants expressed the reduced-function ABCG2 C421A variant (AA or AC). Placental cadmium concentrations were inversely associated with placental weight (β = -19.55; 95%CI: -37.06, -2.04) and trended towards higher FPR (β = 0.25; 95%CI: -0.01, 0.52) with stronger associations in 421A variant infants. Notably, higher placental cadmium concentrations in 421A variant infants were associated with reduced placental weight (β = -49.42; 95%CI: 98.87, 0.03), and higher FPR (β = 0.85, 95%CI: 0.18, 1.52), while higher urinary cadmium concentration was associated with longer birth length (β = 0.98; 95%CI: 0.37, 1.59), lower ponderal index (β = -0.09; 95%CI: 0.15, -0.03), and higher FPR (β = 0.42; 95%CI: 0.14, 0.71).

CONCLUSIONS

Infants with reduced function ABCG2 polymorphisms may be particularly vulnerable to the developmental toxicity of cadmium as well as other xenobiotics that are BCRP substrates. Additional work examining the influence of placental transporters in environmental epidemiology cohorts is warranted.

Effects of whole-brain radiation therapy on the blood-brain barrier in immunocompetent and immunocompromised mouse models

BACKGROUND

Approximately 20% of all cancer patients will develop brain metastases in their lifespan. The standard of care for patients with multiple brain metastases is whole-brain radiation therapy, which disrupts the blood-brain barrier. Previous studies have shown inflammatory mediators play a role in the radiation-mediated increase in permeability. Our goal was to determine if differential permeability post-radiation occurs between immunocompetent and immunocompromised mice.

METHODS

We utilized a commissioned preclinical irradiator to irradiate brains of C57Bl/6J wild-type and athymic nude mice. Acute (3-24 h) effects on blood-brain barrier integrity were evaluated with our in-situ brain perfusion technique and quantitative fluorescent and phosphorescent microscopy. The presence of inflammatory mediators in the brain and serum was determined with a proinflammatory cytokine panel.

RESULTS

Blood-brain barrier integrity and efflux transporter activity were altered in the immunocompetent mice 12 h following irradiation without similar observations in the immunocompromised mice. We observed increased TNF-α concentrations in the serum of wild-type mice immediately post-radiation and nude mice 12 h post-radiation. The brain concentration of CXCL1 was also increased in both mouse strains at the 12-h time point.

CONCLUSIONS

The immune response plays a role in the magnitude of blood-brain barrier disruption following irradiation in a time- and size-dependent manner.

Extracts from Argentinian native plants reverse fluconazole resistance in Candida species by inhibiting the efflux transporters Mdr1 and Cdr1

BACKGROUND

The development of multidrug resistance (MDR) associated with the overexpression of the efflux transporters Mdr1 and Cdr1 in Candida species impedes antifungal therapies. The urgent need for novel agents able to inhibit the function of both pumps, led us to evaluate this property in 137 extracts obtained from Argentinian plants.

METHODS

The ability of the extracts to reverse efflux pump-mediated MDR was determined with an agar chemosensitization assay using fluconazole (FCZ) resistant Mdr1- and Cdr1-overexpressing clinical isolates of Candida albicans and Candida glabrata as well as Saccharomyces cerevisiae strains selectively expressing Mdr1 (AD/CaMDR1) or Cdr1 (AD/CaCDR1). The resistance-reversing activity of the most potent extracts was further confirmed using a Nile Red accumulation assay.

RESULTS

Fifteen plant extracts overcame the FCZ resistance of Candida albicans 1114, which overexpresses CaMdr1 and CaCdr1, and AD/CaMDR1, with those from Acalypha communis and Solanum atriplicifolium being the most effective showing 4- to 16-fold reversal of resistance at concentrations ≥ 25 µg/mL. Both extracts, and to a lesser extent that from Pterocaulon alopecuroides, also restored FCZ sensitivity in CgCdr1-overexpressing C. glabrata 109 and in AD/CaCDR1 with fold reversal values ranging from 4 to 32 and therefore demonstrating a dual effect against Mdr1 and Cdr1. Both, A. communis and S. atriplicifolium extracts at concentrations ≥ 12.5 and ≥ 25 µg/mL, respectively, increased the intracellular Nile Red accumulation in all yeast strains overexpressing efflux pumps.

CONCLUSIONS

The non-toxic and highly active extracts from A. communis and S. atripicifolium, provide promising sources of compounds for potentiating the antifungal effect of FCZ by blocking the efflux function of Mdr1 and Cdr1 transporters.

Exploration of novel phthalazinone derivatives as potential efflux transporter inhibitors for reversing multidrug resistance and improving the oral absorption of paclitaxel

Chemotherapy is an important means of cancer treatment. However, overexpression of efflux transporters (including but not limited to P-gp and BCRP) can lead to resistance to cancer chemotherapy. Multiple-target inhibitors of efflux transporter can be overcome the resistance and improve the oral bioavailability of chemotherapy drugs. Therefore, we designed and synthesized a series of phthalazinone ring derivatives (1-20) with different aromatic heterocycles substituents on the amide bond for dual inhibition of P-gp and BCRP. Most target compounds significantly increased the accumulation of P-gp substrates in the chemo-resistant cancer cell lines by inhibiting the efflux of transporters. Compound 19 in particular showed stronger MDR reversal compared to Gefitinib and Verapamil, and comparable to that of the BCRP inhibitor Ko143. In addition, compound 19 improved intestinal absorption of paclitaxel (PTX) and enhanced the bioavailability of the orally administered drug in vivo.

Multidrug efflux transporter ABCG2: expression and regulation

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

Head-to-head comparison of (R)-[11C]verapamil and [18F]MC225 in non-human primates, tracers for measuring P-glycoprotein function

Purpose

P-glycoprotein (P-gp) function is altered in several brain disorders; thus, it is of interest to monitor the P-gp function in vivo using PET. (R)-[¹¹C]verapamil is considered the gold standard tracer to measure the P-gp function; however, it presents some drawbacks that limit its use. New P-gp tracers have been developed with improved properties, such as [¹⁸F]MC225. This study compares the characteristics of (R)-[¹¹C]verapamil and [¹⁸F]MC225 in the same subjects.

Methods

Three non-human primates underwent 4 PET scans: 2 with (R)-[¹¹C]verapamil and 2 with [¹⁸F]MC225, at baseline and after P-gp inhibition. The 30-min PET data were analyzed using 1-Tissue Compartment Model (1-TCM) and metabolite-corrected plasma as input function. Tracer kinetic parameters at baseline and after inhibition were compared. Regional differences and simplified methods to quantify the P-gp function were also assessed.

Results

At baseline, [¹⁸F]MC225 V_T values were higher, and k₂ values were lower than those of (R)-[¹¹C]verapamil, whereas K₁ values were not significantly different. After inhibition, V_T values of the 2 tracers were similar; however, (R)-[¹¹C]verapamil K₁ and k₂ values were higher than those of [¹⁸F]MC225. Significant regional differences between tracers were found at baseline, which disappeared after inhibition. The positive slope of the SUV-TAC was positively correlated to the K₁ and V_T of both tracers.

Conclusion

[¹⁸F]MC225 and (R)-[¹¹C]verapamil show comparable sensitivity to measure the P-gp function in non-human primates. Moreover, this study highlights the 30-min V_T as the best parameter to measure decreases in the P-gp function with both tracers. [¹⁸F]MC225 may become the first radiofluorinated tracer able to measure decreases and increases in the P-gp function due to its higher baseline V_T.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-021-05411-2.

Identification of novel inhibitors of rat Mrp3

Multidrug resistance-associated protein (MRP; ABCC gene family) mediated efflux transport plays an important role in the systemic and tissue exposure profiles of many drugs and their metabolites, and also of endogenous compounds like bile acids and bilirubin conjugates. However, potent and isoform-selective inhibitors of the MRP subfamily are currently lacking. Therefore, the purpose of the present work was to identify novel rat Mrp3 inhibitors. Using 5(6)-carboxy-2',7'-dichlorofluorescein diacetate (CDFDA) as a model-(pro)substrate for Mrp3 in an oil-spin assay with primary rat hepatocytes, the extent of inhibition of CDF efflux was determined for 1584 compounds, yielding 59 hits (excluding the reference inhibitor) that were identified as new Mrp3 inhibitors. A naive Bayesian prediction model was constructed in Pipeline Pilot to elucidate physicochemical and structural features of compounds causing Mrp3 inhibition. The final Bayesian model generated common physicochemical properties of Mrp3 inhibitors. For instance, more than half of the hits contain a phenolic structure. The identified compounds have an AlogP between 2 and 4.5, between 5 to 8 hydrogen bond acceptor atoms, a molecular weight between 260 and 400, and 2 or more aromatic rings. Compared to the depleted dataset (i.e. 90% remaining compounds), the Mrp3 hit rate in the enriched set was 7.5-fold higher (i.e. 17.2% versus 2.3%). Several hits from this first screening approach were confirmed in an additional study using Mrp3 transfected inside-out membrane vesicles. In conclusion, several new and potent inhibitors of Mrp3 mediated efflux were identified in an optimized in vitro rat hepatocyte assay and confirmed using Mrp3 transfected inside-out membrane vesicles. A final naive Bayesian model was developed in an iterative way to reveal common physicochemical and structural features for Mrp3 inhibitors. The final Bayesian model will enable in silico screening of larger libraries and in vitro identification of more potent Mrp3 inhibitors.

Dynamic disequilibrium of macromolecular transport as possible mechanism for hydrocephalus associated with long-term spaceflight

Hydrocephalus associated with long term spaceflight (HALS) for missions lasting over five months is well described but poorly understood. While structural changes of the brain due to microgravitational forces affecting the circulation of cerebrospinal fluid (CSF) have been described as one potential cause, we propose an alternative hypothesis based on dynamic disequilibrium of macromolecular transport across the blood brain barrier. We propose that factors altering physiology under conditions of spaceflight such as microgravity, hypercapnia, venous hypertension, medications, and dietary substances contribute to increased protein load in the ventricles and/or contribute to impairment of transport out of the ventricles that results in HALS. Individual variation in the genetic expression of efflux transporters (p-glycoprotein) has been shown to correlate with the presence and degree of hydrocephalus in animal studies. We describe the evidence behind this concept and propose how these factors can be studied in order to determine the underlying pathogenesis which is imperative in order to cure or prevent HALS.

Blocking of efflux transporters in rats improves translational validation of brain radioligands

Background

Positron emission tomography (PET) is a molecular imaging technique that can be used to investigate the in vivo pharmacology of drugs. Initial preclinical evaluation of PET tracers is often conducted in rodents due to the accessibility of disease models as well as economic considerations. Compared to larger species, rodents display a higher expression and/or activity of efflux transporters such as the P-glycoprotein (P-gp). Low brain uptake could, therefore, be species-specific and uptake in rodents not be predictive for that in humans. We hypothesized that a better prediction from rodent data could be achieved when a tracer is evaluated under P-gp inhibition. Consequently, we compared the performance of eight neuroreceptor tracers in rats with and without P-gp inhibition including a specific binding blockade. This data set was then used to predict the binding of these eight tracers in pigs.

Methods

PET tracers targeting serotonin 5-HT_2A receptors ([¹⁸F]MH.MZ, [¹⁸F]Altanserin, [¹¹C]Cimbi-36, [¹¹C]Pimavanserin), serotonin 5-HT₇ receptors ([¹¹C]Cimbi-701, [¹¹C]Cimbi-717 and [¹¹C]BA-10) and dopamine D_2/3 receptors ([¹⁸F]Fallypride) were used in the study. The brain uptake and target-specific binding of these PET radiotracers were evaluated in rats with and without inhibition of P-gp. Rat data were subsequently compared to the results obtained in pigs.

Results

Without P-gp inhibition, the amount of target-specific binding in the rat brain was sufficient to justify further translation for three out of eight evaluated tracers. With P-gp inhibition, results for five out of eight tracers justified further translation. The performance in pigs could correctly be predicted for six out of eight tracers when rat data obtained under P-gp inhibition were used, compared to four out of eight tracers without P-gp inhibition.

Conclusions

P-gp strongly affects the uptake of PET tracers in rodents, but false prediction outcomes can be reduced by evaluating a tracer under P-gp inhibition.

[131I]MIBG exports via MRP transporters and inhibition of the MRP transporters improves accumulation of [131I]MIBG in neuroblastoma

INTRODUCTION

¹³¹I-labeled m-iodobenzylguanidine ([¹³¹I]MIBG) has been used to treat neuroblastoma patients, but [¹³¹I]MIBG may be immediately excreted from the cancer cells by the adenosine triphosphate binding cassette transporters, similar to anticancer drugs. The purpose of this study was to clarify the efflux mechanism of [¹³¹I]MIBG in neuroblastomas and improve accumulation by inhibition of the transporter in neuroblastomas.

METHODS

[¹³¹I]MIBG was incubated in human embryonic kidney (HEK)293 cells expressing human organic anion transporting polypeptide (OATP)1B1, OATP1B3, OATP2B1, organic anion transporter (OAT)1 and OAT2, organic cation transporter (OCT)1 and OCT2, and sodium taurocholate cotransporting polypeptide, and in vesicles expressing P-glycoprotein (MDR1), multidrug resistance associated protein (MRP)1-4, or breast cancer resistance protein with and without MK-571 and probenecid (MRP inhibitors). Time activity curves of [¹³¹I]MIBG with and without MK-571 and probenecid were established using an SK-N-SH neuroblastoma cell line, and transporter expression of multiple drug resistance was measured. Biodistribution and SPECT imaging examinations were conducted using [¹²³I]MIBG with and without probenecid in SK-N-SH-bearing mice.

RESULTS

[¹³¹I]MIBG uptake was significantly higher in OAT1, OAT2, OCT1, and OCT2 than in mock cells. Uptake via OCT1 and OCT2 was little inhibited by MK-571 and probenecid. [¹³¹I]MIBG uptake into vesicles that highly expressed MRP1 or MRP4 was significantly higher in ATP than in AMP, and these inhibitors restored uptake to levels similar to that in AMP. Examining the time activity curves for [¹³¹I]MIBG in SK-N-SH cells, higher expressions of MDR1, MRP1, MRP4, and MK-571, or probenecid loading produced significantly higher uptake than in control at most incubation times. The ratios of tumors to blood or muscle in SK-N-SH-bearing mice were significantly increased by probenecid loading in comparison with normal mice.

CONCLUSIONS

[¹³¹I]MIBG exports via MRP1 and MRP4 in neuroblastoma. The accumulation and tumor-to-blood or muscle ratios of [¹³¹I]MIBG are improved by inhibition of MRPs with probenecid in neuroblastoma. ADVANCES IN KNOWLEDGE: [¹³¹I]MIBG, widely used for treatment of neuroendocrine tumors including neuroblastoma, is excreted via MRP1 and MRP4 in neuroblastoma.

IMPLICATIONS FOR PATIENT CARE

Loading with probenecid, OAT, and MRP inhibitors improves [¹³¹I]MIBG accumulation.

Models used to screen for the treatment of multidrug resistant cancer facilitated by transporter-based efflux

PURPOSE

Efflux transporters of the adenosine triphosphate-binding cassette (ABC)-superfamily play an important role in the development of multidrug resistance (multidrug resistant; MDR) in cancer. The overexpression of these transporters can directly contribute to the failure of chemotherapeutic drugs. Several in vitro and in vivo models exist to screen for the efficacy of chemotherapeutic drugs against MDR cancer, specifically facilitated by efflux transporters.

RESULTS

This article reviews a range of efflux transporter-based MDR models used to test the efficacy of compounds to overcome MDR in cancer. These models are classified as either in vitro or in vivo and are further categorised as the most basic, conventional models or more complex and advanced systems. Each model's origin, advantages and limitations, as well as specific efflux transporter-based MDR applications are discussed. Accordingly, future modifications to existing models or new research approaches are suggested to develop prototypes that closely resemble the true nature of multidrug resistant cancer in the human body.

CONCLUSIONS

It is evident from this review that a combination of both in vitro and in vivo preclinical models can provide a better understanding of cancer itself, than using a single model only. However, there is still a clear lack of progression of these models from basic research to high-throughput clinical practice.

The discovery of novel 3-aryl-indazole derivatives as peripherally restricted pan-Trk inhibitors for the treatment of pain

The design, synthesis, and biological evaluation of novel 3-aryl-indazole derivatives as peripherally selective pan-Trk inhibitors are described. Three strategies were used to obtain a potent compound exhibiting low central nervous system (CNS) penetration and high plasma exposure: 1) a structure-based drug design (SBDD) approach was used to improve potency; 2) a substrate for an efflux transporter for lowering brain penetration was explored; and 3) the most basic pKa (pKa-MB) value was used as an indicator to identify compounds with good membrane permeability. This enabled the identification of the peripherally targeted 17c with the potency, kinase-selectivity, and plasma exposure required to demonstrate in vivo efficacy in a Complete Freund's adjuvant (CFA)-induced thermal hypersensitivity model.

ABC transporters in drug-resistant epilepsy: mechanisms of upregulation and therapeutic approaches

Drug-resistant epilepsy (DRE) affects approximately one third of epileptic patients. Among various theories that try to explain multidrug resistance, the transporter hypothesis is the most extensively studied. Accordingly, the overexpression of efflux transporters in the blood-brain barrier (BBB), mainly from the ATP binding cassette (ABC) superfamily, may be responsible for hampering the access of antiepileptic drugs into the brain. P-glycoprotein and other efflux transporters are known to be upregulated in endothelial cells, astrocytes and neurons of the neurovascular unit, a functional barrier critically involved in the brain penetration of drugs. Inflammation and oxidative stress involved in the pathophysiology of epilepsy together with uncontrolled recurrent seizures, drug-associated induction and genetic polymorphisms are among the possible causes of ABC transporters overexpression in DRE. The aforementioned pathological mechanisms will be herein discussed together with the multiple strategies to overcome the activity of efflux transporters in the BBB - from direct transporters inhibition to down-regulation of gene expression resorting to RNA interference (RNAi), or by targeting key modulators of inflammation and seizure-mediated signalling.

Potential detoxification effect of active ingredients in liquorice by upregulating efflux transporter

BACKGROUND

As one of most widely used herbal medicine, liquorice exhibits diverse pharmacological activities, for instance, analgesic, antitussive, antiarrhythmic, anti-inflammatory, and immune regulation. Additionally, detoxification effects were observed in combination of liquorice with other herbal drugs. The mechanism of detoxification of liquorice has been extensively investigated through material basis and interference with CYPs though, investigations of its effect on transporters were very limited, according to the literature.

PURPOSE

The objective of this study was attempt to investigate the effect of active ingredients existing in liquorice on the efflux transporters as to clarify the potential mechanism of detoxification of liquorice.

METHODS

Multiple analytical approaches have been explored, including flow cytometry, fluorescent detection, RT-PCR, Western blot to measure the function, activity as well as mRNA/protein expression of efflux transporters on LS-180 cell model after treatments with active compounds of liquorice. Additionally, Caco-2 cell model was utilized to further investigate the potential impact of those ingredients on efflux transporter.

RESULTS

The resulting data indicated that those active ingredients, including flavonoids (liquiritin, liquiritigenin, isoliquiritin, isoliquiritigenin and licochalcone A) and pentacyclic triterpene saponin (glycyrrhetinic acid) were able to upregulate the expression of efflux transporters, for example P-gp, BCRP and MRP2. The gene expressions were approximately over 2.5 folds by comparison with that of control, and up to 13 folds and 16 folds for BCRP by isoliquiritin and isoliquiritigenin, and further confirmed by Western blot. The functional assay also supported up-regulation of efflux transporter by those ingredients. Flow cytometry study showed that the level of rhodamine123 as probe substrate in LS-180 cells decreased to approximately 50% after treatment with active ingredients of liquorice, compared with that of control. The fluorescent assay confirmed that change of rhodamine 123 was correlated with the concentrations of active ingredients given. The efflux transport of rhodamine 123 was enhanced in Caco-2 cell models as well.

CONCLUSION

The study clarified potential detoxification mechanism of liquorice by up-regulating efflux transporter as to reduce absorption of xenobiotics across small intestinal membrane, which provided a new insight into pharmacological function of liquorice.

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.

A potential efflux boron transporter gene GsBOR2, positively regulates Arabidopsis bicarbonate tolerance

Soil alkalization severely restricts agricultural production and economic development worldwide, this problem is far more serious in Songnen Plain, the largest commodity grain base of China. However, little research has been done concerning the mechanisms of plant responses to alkaline stress to date. In this study, we isolated an alkali inducible gene GsBOR2 from Glycine soja on the basis of RNA seq data. GsBOR2 sh high protein sequence similarity with the known boron transporters in other species. The expression of GsBOR2 was highly up-regulated by 50 mM NaHCO₃ treatment and displayed tissue specificity. We then generated the transgenic Arabidopsis overexpressing GsBOR2 and found that the transgenic lines exhibited enhanced alkaline tolerance compared to wild type plants, as illustrated by longer roots and greater shoot biomass. Moreover, GsBOR2 overexpression was also capable of increasing plant resistance to KHCO₃ treatment but not to high-pH stress. Functional complementation of Scbor1 mutant yeasts suggested that GsBOR2 could likely mediate the efflux of boron from cells. Taken together, the alkali responsive gene GsBOR2 is a positive regulator of plant bicarbonate tolerance.

Sulfation disposition of liquiritigenin in SULT1A3 overexpressing HEK293 cells: The role of breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 4 (MRP4) in sulfate efflux of liquiritigenin

This study aimed to investigate the cellular disposition of liquiritigenin via the sulfonation pathway and the role of efflux transporters in liquiritigenin sulfate excretion. The sulfonation disposition of liquiritigenin was investigated using SULT1A3 overexpressed HEK293 cells (HEK-SULT1A3 cells). Liquiritigenin generated one mono-sulfate metabolite (7-O-sulfate) in HEK-SULT1A3 cell lysate. And the sulfonation followed the Michaelis-Menten kinetic (V_max = 0.84 nmol/min/mg and K_m = 7.12 μM). Expectedly, recombinant SULT1A3 (hSULT1A3) showed a highly similar kinetic profile with cell lysate. Furthermore, 7-O-sulfate was rapidly generated and excreted in HEK-SULT1A3 cells. Ko143 (a BCRP-selective inhibitor) at 20 μM significantly decreased the excretion rate of liquiritigenin sulfate (>42.5%, p < 0.001). Moreover, the pan-MRPs inhibitor MK-571 at 20 μM essentially abolished the liquiritigenin sulfate effluxion, resulting in the marked reduction of excretion rate (>97.4%, p < 0.001). Furthermore, knockdown of BCRP led to moderate reduction in sulfate excretion (15.9%-16.9%, p < 0.05). Silencing of MRP4 caused significant decreased in sulfate excretion (20.2%-32.5%, p < 0.01). In conclusion, one sulfate metabolite was generated from liquiritigenin in HEK-SULT1A3 cells. BCRP and MRP4 should be the key factors for the cellular excretion of liquiritigenin sulfate.

Taxane resistance in castration-resistant prostate cancer: mechanisms and therapeutic strategies

Despite its good initial response and significant survival benefit in patients with castration-resistant prostate cancer (CRPC), taxane therapy inevitably encounters drug resistance in all patients. Deep understandings of taxane resistant mechanisms can significantly facilitate the development of new therapeutic strategies to overcome taxane resistance and improve CRPC patient survival. Multiple pathways of resistance have been identified as potentially crucial areas of intervention. First, taxane resistant tumor cells typically have mutated microtubule binding sites, varying tubulin isotype expression, and upregulation of efflux transporters. These mechanisms contribute to reducing binding affinity and availability of taxanes. Second, taxane resistant tumors have increased stem cell like characteristics, indicating higher potential for further mutation in response to therapy. Third, the androgen receptor pathway is instrumental in the proliferation of CRPC and multiple hypotheses leading to this pathway reactivation have been reported. The connection of this pathway to the AKT pathway has received significant attention due to the upregulation of phosphorylated AKT in CRPC. This review highlights recent advances in elucidating taxane resistant mechanisms and summarizes potential therapeutic strategies for improved treatment of CRPC.

Effects of increased accumulation of doxorubicin due to emodin on efflux transporter and LRP1 expression in lung adenocarcinoma and colorectal carcinoma cells

Treatment with doxorubicin (dox) and emodin, separately and together, under normoxic and hypoxia-like conditions induced by CoCl₂, led to greater intracellular compound accumulation over 10 h post-addition in the presence of CoCl₂ in lung adenocarcinoma (A549) and colorectal carcinoma (HCT-15) cell lines. Confocal microscopy revealed that emodin, by itself, showed high cytosolic distribution in both cell lines, at 40 min post-addition but had entered the nuclei by 2 h, while dox entered the nuclei by 40 min. Both compounds modulated the expression of the efflux transporters (PgP, ABCG2, or MRP1-4) and the endocytic receptor, low-density lipoprotein receptor-related protein 1 (LRP1), to different extents under the study conditions. Efflux transporter upregulation was linked to lower intracellular compound levels due to greater efflux. Increased dox accumulation was accompanied by unaltered expression or upregulation of LRP1 in A549 cells. In both cell lines, increased accumulation of dox and emodin was observed whenever LRP1 and the efflux transporters known to transport dox and emodin were all up- or downregulated concomitantly. Increased growth inhibition was linked to co-treatment with dox and emodin and with increased ligand accumulation. The results presented in this study raise the hypothesis that higher production of LRP1 protein may be associated with higher endocytosis of upregulated transporter proteins at the cell surface, and hence, increased dox and emodin accumulation and growth inhibition. If so, elevation of LRP1 expression may be a useful target for interventions to promote the efficacy of these and other anticancer drugs.

The transepithelial transport mechanism of polybrominated diphenyl ethers in human intestine determined using a Caco-2 cell monolayer

Oral ingestion plays an important role in human exposure to polybrominated diphenyl ethers (PBDEs). The uptake of PBDEs primarily occurs in the small intestine. The aim of the present study is to investigate the transepithelial transport characteristics and mechanisms of PBDEs in the small intestine using a Caco-2 cell monolayer model. The apparent permeability coefficients of PBDEs indicated that tri- to hepta-BDEs were poorly absorbed compounds. A linear increase in transepithelial transport was observed with various concentrations of PBDEs, which suggested that passive diffusion dominated their transport at the concentration range tested. In addition, the pseudo-first-order kinetics equation can be applied to the transepithelial transport of PBDEs. The rate-determining step in transepithelial transport of PBDEs was trans-cell transport including the trans-pore process. The significantly lower transepithelial transport rates at low temperature for bidirectional transepithelial transport suggested that an energy-dependent transport mechanism was involved. The efflux transporters (P-glycoprotein, multidrug resistance-associated protein, and breast cancer resistance protein) and influx transporters (organic cation transporters) participated in the transepithelial transport of PBDEs. In addition, the transepithelial transport of PBDEs was pH sensitive; however, more information is required to understand the influence of pH.

Identification and characterization of silicon efflux transporters in horsetail (Equisetum arvense)

Silicon (Si) is a beneficial element to plants, and its absorption via transporters leads to protective effects against biotic and abiotic stresses. In higher plants, two groups of root transporters for Si have been identified: influx transporters (Lsi1) and efflux transporters (Lsi2). Lsi1 transporters belong to the NIPIII aquaporins, and functional Lsi1s have been found in many plants species. Much less is known about Lsi2s that have been characterized in only a few species. Horsetail (Equisetum arvense), known among the highest Si accumulators in the plant kingdom, is a valuable model to study Si absorption and deposition. In this study, we first analyzed discrete Si deposition patterns in horsetail shoots, where ubiquitous silicification differs markedly from that of higher plants. Then, using the sequenced horsetail root transcriptome, two putative Si efflux transporter genes, EaLsi2-1 and EaLsi2-2, were identified. These genes share low sequence similarity with their homologues in higher plants. Further characterisation of EaLsi2-1 in transient expression assay using Nicotiana benthamiana epidermal cells confirmed transmembrane localization. In order to determine their functionality, the EaLsi2-1 was expressed in Xenopus oocytes, confirming that the translated protein was efficient for Si efflux. Both genes were equally expressed in roots and shoots, but interestingly, showed a much higher expression in the shoots than in the roots in contrast to Lsi2s found in other plants, a result consistent with the specific anatomy of horsetail and its rank as one of the highest Si accumulators among plant species.

Transporters involved in mineral nutrient uptake in rice

One of the most important roles of plant roots is to take up essential mineral nutrients from the soil for use in plant growth and development. The uptake of mineral elements is mediated by various transporters belonging to different transporter families. Here we reviewed transporters for the uptake of macronutrients and micronutrients identified in rice, an important staple food for half of the world's population. Rice roots are characterized by having two Casparian strips on the exodermis and endodermis and by the formation of aerenchyma in the mature root zone. This distinct anatomical structure dictates that a pair of influx and efflux transporters at both the exodermis and endodermis is required for the radial transport of a mineral element from the soil solution to the stele. Some transporters showing polar localization at the distal and proximal sides of the exodermis and endodermis have been identified for silicon and manganese, forming an efficient uptake system. However, transporters for the uptake of most mineral elements remain to be identified.

(18)F-FCWAY, a serotonin 1A receptor radioligand, is a substrate for efflux transport at the human blood-brain barrier

Efflux transporters at the blood-brain barrier can decrease the entry of drugs and increase the removal of those molecules able to bypass the transporter. We previously hypothesized that (18)F-FCWAY, a radioligand for the serotonin 5-HT1A receptor, is a weak substrate for permeability glycoprotein (P-gp) based on its very early peak and rapid washout from human brain. To determine whether (18)F-FCWAY is a substrate for P-gp, breast cancer resistance protein (BCRP), and multidrug resistance protein (MRP1) - the three most prevalent efflux transporters at the blood-brain barrier - we performed three sets of experiments. In vitro, we conducted fluorescence-activated cell sorting (FACS) flow cytometry studies in cells over-expressing P-gp, BCRP, and MRP1 treated with inhibitors specific to each transporter and with FCWAY. Ex vivo, we measured (18)F-FCWAY concentration in plasma and brain homogenate of transporter knockout mice using γ-counter and radio-HPLC. In vivo, we conducted positron emission tomography (PET) studies to assess changes in humans who received (18)F-FCWAY during an infusion of tariquidar (2-4mg/kg iv), a potent and selective P-gp inhibitor. In vitro studies showed that FCWAY allowed fluorescent substrates to get into the cell by competitive inhibition of all three transporters at the cell membrane. Ex vivo measurements in knockout mice indicate that (18)F-FCWAY is a substrate only for P-gp and not BCRP. In vivo, tariquidar increased (18)F-FCWAY brain uptake in seven of eight subjects by 60-100% compared to each person's baseline. Tariquidar did not increase brain uptake via some peripheral mechanism, given that it did not significantly alter concentrations in plasma of the parent radioligand (18)F-FCWAY or its brain-penetrant radiometabolite (18)F-FC. These results show that (18)F-FCWAY is a weak substrate for efflux transport at the blood-brain barrier; some radioligand can enter brain, but its removal is hastened by P-gp. Although (18)F-FCWAY is not ideal for measuring 5-HT1A receptors, it demonstrates that weak substrate radioligands can be useful for measuring both increased and decreased function of efflux transporters, which is not possible with currently available radioligands such as (11)C-loperamide and (11)C-verapamil that are avid substrates for transporters.

Zebrafish Abcb4 is a potential efflux transporter of microcystin-LR

Microcystin-LR (MC-LR) is one of the most common microcystins (MCs), which are hepatotoxic and released into a water body during a period of cyanobacterial blooms. These toxicants can be accumulated in aquatic animals and transferred along the food chain and thus pose adverse effects on aquatic environment and public health. Zebrafish Abcb4 is reported to mediate the cellular efflux of ecotoxicologically relevant compounds including galaxolide, tonalide and phenanthrene; however, it remains unclear whether Abcb4 functions in the detoxification of MC-LR. Here, we demonstrated the role of zebrafish Abcb4 in cellular efflux of MC-LR. Transcripts of zebrafish abcb4 were detected in all of adult tissues examined. MC-LR was able to induce the expression of abcb4 gene and overexpression of Abcb4 significantly decreased the cytotoxicity and accumulation of MC-LR in LLC-PK1 cells and developing embryos. In contrast, overexpression of an Abcb4-G1177D mutant abolished its transporter function but not substrate binding activity, and sensitized LLC-PK1 cells and developing embryos to this cyanobacterial toxin. Moreover, ATPase activity in developing embryos can be induced by MC-LR. Thus, zebrafish Abcb4 plays crucial roles in cellular efflux of MC-LR and is a potential molecular marker for the monitoring of cyanobacteria contamination in the aquatic environment.

Membrane transport of nobilin conjugation products and use of the extract of Chamomillae romanae flos influence absorption of nobilin in the Caco-2 model

The purpose of this work was to investigate the role of bioconjugation and carrier mediated efflux of conjugation products in the absorption mechanism of the sesquiterpene lactone nobilin in the Caco-2 model in vitro and to elucidate the impact of the extract of Chamomillae romanae flos and its ingredients on absorption. Transport experiments with inhibitors of P-gp, BCRP, and MRPs were performed to detect efflux and its connection to bioconversion and the effect of different ingredients of the plant extract on absorption processes was determined. Permeability, transport and bioconversion parameter values were deduced by kinetic multi-compartment modeling. Nobilin exhibited high permeability, low relative absorption and fast bioconversion producing glucuronide, cysteine conjugate, and glutathione conjugate that were transported by P-gp (the first two), apical MRP2 and basal MRP3 and possibly MRP1 out of the cell. Inhibition of efflux resulted in diminished bioconjugation and improved absorption. The extract increased the relative fraction absorbed primarily by directly inhibiting bioconversion, and by reducing efflux. Individual extract ingredients could only partly explain this effect. Extensive bioconversion, hence, limited absorption of nobilin in the Caco-2 model and the interplay between conjugation and efflux was shown to provide a possible mechanism for absorption increase. Plant extract increased absorption by this mechanism in addition to metabolic enzyme inhibition.